Synthesis and reactivity of
[(C2F5)2PCH2CH2P(C2F5)]Fe(CO)3

Maurice Brookhart, Wayne Chandler, Anne Pfister, Catherine Santini, Peter White

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Maurice Brookhart, Wayne Chandler, Anne Pfister, Catherine Santini, Peter White. Synthesis and reactivity of [(C2F5)2PCH2CH2P(C2F5)]Fe(CO)3. Organometallics, 1992, 11, pp.1263-1274. hal-00006225

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Synthesis and Reactivity of [(C₂F₅)₂PCH₂CH₂P(C₂F₅)₂]Fe(CO)₃

Maurice Brookhart,* Wayne A. Chandler, Anne C. Pfister, Catherine C. Santini, and Peter S. White

Department of Chemistry, The University of North Carolina, Chapel Hill, North Carolina 27599-3290

Reaction of (C₂F₅)₂PCH₂CH₂P(C₂F₅)₂ (1) with (benzylideneacetone)Fe(CO)₃ gives [(C₂F₅)₂PCH₂CH₂P-(C₂F₅)₂]Fe(CO)₃ ((dfepe)Fe(CO)₃ (2)) in 59% yield. Photosubstitution of CO by L yields (dfepe)Fe(CO)₂L (L = THF, P(OCH₃)₃, C₂H₄). The ethylene complex has been characterized by single-crystal X-ray analysis (C₁₄H₈F₂₀FeO₂P₂, monoclinic, P2₁/c, *a* = 10.806 (5) Å, *b* = 14.069 (5) Å, *c* = 15.415 (3) Å, *β* = 101.61 (2)°, *V* = 2295.4 (14) Å³, and *Z* = 4) and shows an unusually short carbon-carbon double bond (1.362 (7) Å) indicating weak π-back-bonding. Photolysis of 2 under 1 atm of acetylene yields the cyclopentadienone complex (η⁴-C₅H₄O)Fe(CO)(dfepe) while irradiation in the presence of butadiene yields (η⁴-C₄H₆)Fe(CO)(dfepe). Three styrene complexes (η⁶-C₈H₈)Fe(CO)(dfepe), (η²-C₈H₈)Fe(CO)₂(dfepe), and (η⁴-C₈H₈)Fe(CO)(dfepe) were isolated when 2 was photolyzed in the presence of styrene. The structure of the η⁴-styrene complex was verified by a single-crystal X-ray analysis (C₁₉H₁₂F₂₀FeOP₂, triclinic, P1, *a* = 8.7382 (16) Å, *b* = 10.789 (3) Å, *c* = 15.652 (6) Å, *α* = 87.382 (16)°, *β* = 101.61 (2)°, *γ* = 73.857 (17)°, *V* = 1262.9 (6) Å³, and *Z* = 2). The η⁴-styrene complex can be used as an efficient (dfepe)Fe(CO)-transfer agent. Photolysis of 2 under 1 atm of H₂ gave the fluxional cis-dihydride complex (dfepe)(CO)₂Fe(H)₂ (17). Attempted oxidative addition of Et₃SiH to photogenerated (dfepe)Fe(CO)₂ gave dihydride complex 17 and (Et₃Si)₂O. Photolysis of 2 in alkane solvents at temperatures from -50 to +25 °C gave no evidence for oxidative addition adducts.

## Introduction

There are a limited number of bidentate ligands with strong π-accepting properties available for use in synthesis and stabilization of low-valent transition-metal complexes.¹⁻⁷ An attractive class of such ligands is the per-

fluorinated bidentate alkyl phosphines, (R<sub>f</sub>)₂PCF₂CF₂P-(R<sub>f</sub>)₂ (R<sub>f</sub> = perfluoroalkyl). It is well-established that (CF₃)₃P is similar to CO in its π-acceptor properties, and thus (R<sub>f</sub>)₂PCF₂CF₂P(R<sub>f</sub>)₂ complexes represent chelates which should mimic the electronic properties of two cis carbonyl ligands. Aside from the usual advantages of chelates, several other features of these ligands are of interest: (1) The steric bulk of the ligands can be varied by changing R<sub>f</sub>. (2) The presence of highly lipophilic R<sub>f</sub>

<footnote>(1) (a) Silverthorn, W. E. In *Advances in Organometallic Chemistry*; Stone, F. G. A., West, R., Eds.; Academic Press: New York, 1975; Vol. 13, p 47 (see also references therein). (b) King, R. B. *Acc. Chem. Res.* **1980**, *13*, 243. (c) Johnson, T. R.; Nixon, J. F. *J. Chem. Soc. A* 1969, 2518.</footnote>

<footnote>(2) (a) Burg, A. B. *J. Am. Chem. Soc.* **1961**, *83*, 2226. (b) Cooper, P.; Fields, R.; Haszeldine, R. N. *J. Chem. Soc. C* 1971, 3031. (c) Udovich, C. A.; Clark, R. J.; Haas, H. *Inorg. Chem.* **1969**, *8*, 1066.</footnote>

<footnote>(3) (a) Burg, A. B.; Street, G. B. *Inorg. Chem.* **1966**, *5*, 1532. (b) Burg, A. B.; Street, G. B. *J. Am. Chem. Soc.* **1963**, *85*, 3522.</footnote>

<footnote>(4) Phillips, I. G.; Ball, R. G.; Cavell, R. G. *Inorg. Chem.* **1988**, *27*, 4038.</footnote>

<footnote>(5) Ernst, M. F.; Roddick, D. M. *Inorg. Chem.* **1989**, *28*, 1624.</footnote>

<footnote>(6) Ernst, M. F.; Roddick, D. M. *Organometallics* **1990**, *9*, 1586.</footnote>

<footnote>(7) Koola, J. D.; Roddick, D. M. *J. Am. Chem. Soc.* **1991**, *113*, 1450.</footnote>
<table><caption>Table I. ν(CO) Stretching Frequencies for Iron Carbonyl Complexes of the Type L₂Fe(CO)₃</caption><thead><tr><th>compd</th><th>phase</th><th></th><th>freq, cm<sup>-1</sup></th><th></th><th>ref</th></tr></thead><tbody><tr><td>(Ph₂PCH₂CH₂PPH₂)Fe(CO)₃</td><td>CH₂Cl₂</td><td>1992 (s)</td><td>1923 (m)</td><td>1901 (s)</td><td>a</td></tr><tr><td>[(CH₃)₂PCF₂C₂P(CH₃)₂]Fe(CO)₃</td><td>CH₂Cl₂</td><td>1994 (s)</td><td>1925 (m)</td><td>1910 (vs)</td><td>b</td></tr><tr><td>(F₃P)₂Fe(CO)₂ C₂v</td><td>gas</td><td>2079 (m)</td><td></td><td>2026 (vs)</td><td>c</td></tr><tr><td>(F₃P)₂Fe(CO)₃ C₈</td><td>gas</td><td>2084 (m)</td><td></td><td>2001 (s)</td><td>c</td></tr><tr><td>(F₃P)₂Fe(CO)₃ D₃h</td><td>gas</td><td>2005 (s)</td><td></td><td></td><td>c</td></tr><tr><td>[(CF₃)₃P]₂Fe(CO)₃ C₂v</td><td>gas</td><td>2080 (m)</td><td></td><td>2025 (s)</td><td>d</td></tr><tr><td>(dfepe)Fe(CO)₃</td><td>pentane</td><td>2055 (s)</td><td>2004 (m)</td><td>1991 (s)</td><td>this work</td></tr></tbody></table>

ᵃ Manuel, T. A. *Inorg. Chem.* **1963**, *2*, 854. ᵇ Langford, B. R.; Akhtar, M.; Ellis, P. D.; MacDiarmid, A. G.; Odom, J. D. *Inorg. Chem.* **1975**, *14*, 2937. ᶜ Tripathi, J. B. PD.; Bigorgne, M. *J. Organomet. Chem.* **1967**, *9*, 307. ᵈ Burg, A. B. *Inorg. Chem.* **1986**, *25*, 4751.

groups should enhance the solubility of complexes in inert, nonpolar solvents (including fluorocarbons). (3) No C–H bonds or other reactive functional groups are present which could deactivate reactive unsaturated intermediates by coordination or oxidative addition. (4) The P–Rf bond is substantially more inert than the P–F bond, and thus such species offer advantages over complexes of the type F₂PCX₂CX₂PF₂.²

Burg, in the early 1960s, prepared (CF₃)₂PCX₂CX₂P-(CF₃)₂ (X = H, F), the first examples of this class of ligand, together with the nickel carbonyl complexes (CF₃)₂PCX₂CX₂P(CF₃)₂Ni(CO)₂ (X = H, F).³ The difficult and specialized procedures involved in preparing these ligands limited their further development. Recently, however, more general routes to these ligands have become available. Cavell⁴ has prepared (CF₃)₂P(CH₂)nP(CF₃)₂ (n = 1–3), (CF₃)₂PCH(CH₃)P(CF₃)₂, (CF₃)₂PC(CH₃)₂P(CF₃)₂, and (CF₃)₂PCF₂CF₂P(CF₃)₂ by reaction of (CF₃)₂P–P(CF₃)₂ with the appropriate alkyl or perfluoroalkyl diiodides. Pt(II) derivatives of several of these ligands were prepared along with molybdenum derivatives (CO)₄Mo-[(CF₃)₂PCX₂CX₂P(CF₃)₂] (X = H, F) and (CO)₄Mo-[(CF₃)₂P(CH₃)₂P(CF₃)₂]. Roddick⁵ has reported a convenient synthesis of (C₂F₅)₂PCH₂CH₂P(C₂F₅)₂ (termed dfepe) via conversion of C₂F₅Cl to C₂F₅Li with n-BuLi at –100 °C followed by reaction with Cl₂PCH₂CH₂PCl₂. This ligand has been used to prepare (dfepe)Cr(CO)₄ and (dfepe)Mo-(CO)₄,⁶ as well as (η⁶-C₆H₅CH₃)Mo(dfepe)(L)⁶ where L = CO, H₂, CH₃CN, N₂, and THF and Ru(dfepe)₂(H)₂.⁷ Analysis of the CO-stretching frequencies of the nickel, molybdenum, and chromium carbonyl derivatives cited above show clearly that these bidentate ligands are strong π-acceptors.

In this paper we report our investigations of the synthesis, structure, and reactivity of (C₂F₅)₂PCH₂CH₂P(C₂F₅)₂ (1) derivatives of iron(0) complexes.

## Results and Discussion

**Synthesis and Characterization of (dfepe)Fe(CO)₃**
(2). Complex 2 was prepared by displacement of benzyldieneacetone (BDA) from (BDA)Fe(CO)₃ by 1 in toluene at 110 °C. The tricarbonyl 2 is a yellow oil at 25 °C which is highly soluble in hydrocarbon and perfluorocarbon solvents. The infrared spectrum in pentane shows three strong νCO bands at 2055, 2004, and 1991 cm⁻¹. Roddick⁷ has recently reported the synthesis of the ruthenium analogue, Ru(dfepe)(CO)₃, by reaction of Ru(dfepe)₂(H)₂ with CO (νCO bands at 2068, 2019, 1996 cm⁻¹). The infrared data for 2 are compared to νCO data for other complexes of the type L₂Fe(CO)₃ in Table I. Qualitative comparison of these data shows, as noted earlier for such ligands, that dfepe is a strong π-acceptor bidentate ligand comparable to two cis PF₃ or two cis CO ligands. For example, the stretching frequencies of 2 are substantially higher than the diphos analogue [(C₆H₅)₂PCH₂CH₂P(C₆H₅)₂]Fe(CO)₃ and comparable to (F₃P)₂Fe(CO)₃ derivatives. While several structures are possible for 2 we favor a trigonal-

bipyramidal geometry in which the bidentate phosphine spans the axial–equatorial sites. In a five-membered ring,

<figure><img src="image_2.png" /></figure>

2

axial–equatorial substitution minimizes the ring strain,⁸ as found in X-ray structures of [(C₆H₅)₂PCH₂CH₂P(C₆H₅)₂]Fe(CO)₃⁹ and [(C₆H₁₁)₂PC≡C[P(C₆H₁₁)₂]CF₂CF₂]-Fe(CO)₃.¹⁰ The ethylene derivative (dfepe)Fe(CO)₂(C₂H₄) adopts such a geometry in which an equatorial CO has been replaced by C₂H₄ (see below). The infrared data are consistent with this structure. The NMR data indicate a rapidly fluxional molecule typical of five-coordinate systems of the type Fe(CO)₃L₂. The ¹³C spectrum shows a single carbonyl resonances at 211 ppm (broadened due to ³¹P coupling), and the ³¹P spectrum exhibits a single band at 140 ppm (quintet, ²J<sub>PF</sub> = 66 Hz). In the ¹⁹F spectrum the CF₃ groups appear as a singlet at δ –13.3 ppm while the diastereotopic fluorines of the CF₂ groups appear as an AB pattern centered at –46.2 ppm (²J<sub>FF</sub> = 314 Hz, ²J<sub>PFa</sub> = 65 Hz, ²J<sub>PFb</sub> = 61 Hz). The exchange process likely occurs via a Berry pseudorotation mechanism as proposed for complexes of similar structures. The ¹³C and ³¹P spectra show no line broadening down to –80 °C, which indicates a barrier to isomerization of less than ca. 7–8 kcal/mol.

**Photochemistry of [Bis[bis(pentafluoroethyl)-phosphino]ethane]tricarbonyliron, Fe(dfepe)(CO)₃**
(2). In order to access a variety of derivatives of (dfepe)Fe⁰, methods were sought to replace the carbonyl ligands of 2. Thermal substitution of CO in Fe(CO)₅ requires high temperatures,¹¹ and we were concerned about the thermal lability of dfepe in 2 under such conditions (see below). Thus, initial efforts were directed at photochemical substitutions with the goal of directly introducing various ligands or replacing CO with a labile molecule

<footnote>(8) (a) Akhtar, M.; Ellis, P. D.; MacDiarmid, A. G.; Odom, J. D. *Inorg. Chem.* **1972**, *11*, 2917. (b) Muetterties, E. L. *Acc. Chem. Res.* **1970**, *3*, 266. (c) Muetterties, E. L.; Mahler, W.; Schmutzler, R. *Inorg. Chem.* **1963**, *2*, 613. (d) Gorenstein, D.; Westheimer, F. H. *J. Am. Chem. Soc.* **1970**, *92*, 634. (e) Gorenstein, D.; Westheimer, F. H. *J. Am. Chem. Soc.* **1970**, *92*, 644.</footnote>

<footnote>(9) Battaglia, L. P.; Delledonne, D.; Nardelli, M.; Pelizzi, C.; Predieri, G.; Chiusoli, G. P. *J. Organomet. Chem.* **1987**, *330*, 101.</footnote>

<footnote>(10) Einstein, F. W. B.; Huang, C.-H. *Acta Crystallogr.* **1978**, *B34*, 1486.</footnote>

<footnote>(11) Shriver, D. F.; Whitmire, K. H. In *Comprehensive Organometallics Chemistry: Wilkinson, G., Ed.; Pergamon Press: Oxford, U.K.*, 1982; Vol. 4, Chapter 31.1.</footnote>
which could subsequently be readily replaced.

**Photolysis of 2 in THF.** Photolysis of 2 in THF solution, at -45 °C, leads to formation of a new species with $ν_{CO}$ at 1988 and 1931 cm⁻¹ (eq 1). This species is assigned

$$ (\text{dfepe})\text{Fe(CO)}_3 \xrightarrow[\text{THF / -45° C}]{\text{hν}} (\text{dfepe})\text{Fe(CO)}_2(\text{THF}) \xrightarrow[\text{ν}_{\text{CO}} = 1988, 1931 \text{ cm}^{-1}]{\text{P(OCH}_3)_3} $$

2

3

$$ (\text{dfepe})\text{Fe(CO)}_2(\text{P(OCH}_3)_3) \quad (1) $$

4

to the THF adduct $Fe(dfepe)(CO)_2(THF)$ (3). A similar THF adduct was reported during the photolysis of $(C_4H_5)Fe(CO)_3$ in THF.¹² Complex 3 is too unstable to isolate (decomposition in solution occurs over 5 h at -30 °C); however, evidence for the presence of 3 comes from reaction with $P(OCH_3)_3$. Addition of $P(OCH_3)_3$ to a THF solution of 3 at -45 °C in the dark affords exclusively $Fe(dfepe)(CO)_2[P(OCH_3)_3]$ (4), which has been isolated and characterized (see below). $(P(OCH_3)_3)$ does not react in the dark with 2 in THF. The ready generation and high reactivity of 3 makes it a convenient thermal source of “$(dfepe)Fe(CO)_2$”.

**Photosubstitution by $P(OCH_3)_3$.** Photolysis of 2 in diethyl ether at -45 °C in the presence of $P(OCH_3)_3$ gives the phosphite adduct 4 (yellow crystals, 80% yield), which exhibits two $ν_{CO}$ bands at 1988 and 1973 cm⁻¹. In the ¹³C NMR spectrum, the carbonyl carbons show a single resonance at δ 212 ppm (d, $^2J_{PC}$ = 50 Hz), while the ³¹P spectrum exhibits a triplet at δ 173 ppm for $P(OMe)_3$ ($^2J_{PP}$ = 8 Hz) and a broad quintet at δ 133 ppm ($^2J_{FP}$ = 59 Hz) for both bidentate ³¹P nuclei. The structure most consistent with the IR and NMR data is one in which phosphite ligand occupies an equatorial site and the bidentate ligand spans the normally preferred axial-equatorial positions. This small $^2J_{PP}$ is indicative of a cis relationship

<figure><img src="image_3.png" /></figure>

4

between the phosphite phosphorus atom and both bidentate phosphorus atoms and supports the assignment of structure 4.¹³ Analysis of the relative CO intensities gives a calculated OC-Fe-CO angle of ca. 99°.¹⁴ The observation of a single ¹³CO resonances and a single ³¹P resonance for the bidentate ligand reflects the expected highly fluxional character of 4.

**Photosubstitution by $C_2H_4$.** Photolysis of 2 in a diethyl ether solution saturated with ethylene gives the ethylene complex $(dfepe)Fe(CO)_2(C_2H_4)$ (5), which can be isolated as a crystalline solid in good yields (70%). The infrared spectrum shows two strong $ν_{CO}$ bands at 2020 and 1979 cm⁻¹ consistent with cis carbonyl groups. Key

<table><caption>**Table II.** Crystallographic Data and Collection Parameters and Refinement Parameters for $(CO)_2(dfepe)Fe(η^2-C_2H_5)CH=CH_2$ (5) and $(CO)(dfepe)Fe(η^4-C_6H_5CH=CH_2)$ (13)</caption><thead><tr><th></th><th>5</th><th>13</th></tr></thead><tbody><tr><td>molecular formula</td><td>C<sub>14</sub>H<sub>8</sub>F<sub>20</sub>FeO<sub>2</sub>P<sub>2</sub></td><td>C<sub>19</sub>H<sub>12</sub>F<sub>20</sub>FeOP<sub>2</sub></td></tr><tr><td>fw</td><td>705.97</td><td>754.06</td></tr><tr><td>cryst dimens, mm</td><td>0.25 × 0.30 × 0.50</td><td>0.30 × 0.30 × 0.20</td></tr><tr><td>space group</td><td>P2<sub>1</sub>/c</td><td>P<span style="text-decoration: overline">I</span></td></tr><tr><td>cell params</td><td></td><td></td></tr><tr><td>α, Å</td><td>10.806 (5)</td><td>8.7382 (16)</td></tr><tr><td>b, Å</td><td>14.069 (5)</td><td>10.789 (3)</td></tr><tr><td>c, Å</td><td>15.415 (3)</td><td>15.652 (6)</td></tr><tr><td>α, deg</td><td></td><td>87.382 (16)</td></tr><tr><td>β, deg</td><td>101.6 (2)</td><td>69.432 (24)</td></tr><tr><td>γ, deg</td><td></td><td>73.857 (17)</td></tr><tr><td>V, Å<sup>3</sup></td><td>2295.4 (14)</td><td>1262.9 (6)</td></tr><tr><td>Z</td><td>4</td><td>2</td></tr><tr><td>d(calcd), Mg/m<sup>3</sup></td><td>2.043</td><td>1.983</td></tr><tr><td>F(000)</td><td>1136</td><td>739.92</td></tr><tr><td>radiation (wavelength, Å)</td><td>Mo Kα (0.71073)</td><td>Mo Kα (0.71073)</td></tr><tr><td>monochromator</td><td>graphite</td><td>graphite</td></tr><tr><td>linear abs coeff, mm<sup>-1</sup></td><td>0.94</td><td>0.88</td></tr><tr><td>scan mode</td><td>ω-2θ</td><td>ω-2θ</td></tr><tr><td>background</td><td>25% of full scan<br>width on<br>both sides</td><td>25% of full scan<br>width on<br>both sides</td></tr><tr><td>2θ limits, deg</td><td>2 &lt; 2θ &lt; 45</td><td>2 &lt; 2θ &lt; 50</td></tr><tr><td>octants collected</td><td>±h,k,l</td><td>±h,k,l</td></tr><tr><td>total no. unique reflns</td><td>2975</td><td>4409</td></tr><tr><td>data with I ≥ 2.5σ(I)</td><td>2209</td><td>3630</td></tr><tr><td>R (including unobserveds)</td><td>0.035 (0.053)</td><td>0.044 (0.054)</td></tr><tr><td>R<sub>w</sub> (including unobserveds)</td><td>0.033 (0.033)</td><td>0.049 (0.049)</td></tr><tr><td>GoF</td><td>1.84</td><td>2.82</td></tr><tr><td>no. of params</td><td>385</td><td>437</td></tr><tr><td>largest param shift/error</td><td>0.28</td><td>0.308</td></tr><tr><td>max density in<br>final difference<br>synthesis, e/Å<sup>3</sup></td><td>0.320</td><td>0.680</td></tr></tbody></table>

NMR spectral parameters of the $C_2H_4$ ligand and ¹H δ 2.4 ppm (t, $^3J_{PH}$ = 3 Hz); ¹³C δ 39.9 ppm ($J_{CH}$ = 160 Hz). The data are similar to those for the parent complex¹⁵ $(CO)_4Fe(η^2-C_2H_5)$: ¹H δ 2.45 ppm; ¹³C δ 35.3 ppm. No decoalescence of the ¹H signal of the $C_2H_4$ ligand occurred down to -60 °C in $CD_2Cl_2$. Below -60 °C the complex precipitated from the solution. While the parent complex $(CO)_4Fe(C_2H_4)$ is an oil unstable above -10 °C, the dfepe derivative 5 is quite stable at 25 °C for extended periods of time.

<figure><img src="image_4.png" /></figure>

5

**X-ray Structure Determination of 5.** The structure of 5 was confirmed by a single-crystal X-ray structure determination. Single crystals for X-ray analysis were grown from a saturated solution of hexane at -30 °C. Complex 5 crystallizes in the monoclinic system with space group $P2_1/c$. The unit cell contains four molecules separated by the normal van der Waals distances. Crystallographic data are collected in Table II, and atomic coordinates are listed in Table III. An ORTEP diagram of 5

<footnote>(12) Fischler, I.; Hildenbrand, K.; von Gustorf, E. K. *Angew. Chem., Int. Ed. Engl.* **1975**, *14*, 54.</footnote>

<footnote>(13) Tolman, C. A.; English, A. D.; Ittel, S. D.; Jesson, J. P. *Inorg. Chem.* **1978**, *17*, 2374.</footnote>

<footnote>(14) Braterman, P. S.; Bau, R.; Kaesz, H. D. *Inorg. Chem.* **1967**, *6*, 2097.</footnote>

<footnote>(15) (a) Lindner, E.; Schauss, E.; Hiller, W.; Fawzi, R. *Chem. Ber.* **1985**, *118*, 3915. (b) Lindner, E.; Schauss, E.; Hiller, W.; Fawzi, R. *Angew. Chem., Int. Ed.* **1984**, *23*, 711.</footnote>
<table><caption>Table III. Fractional Atomic Coordinates and Isotropic Thermal Parameters for (CO)₂(dfepe)Fe(η²-CH₂=CH₂) (5)</caption><thead><tr><th>atom</th><th>x</th><th>y</th><th>z</th><th>B<sub>iso</sub> Å<sup>2</sup></th></tr></thead><tbody><tr><td>Fe(1)</td><td>0.70629 (6)</td><td>0.89945 (4)</td><td>0.17854 (4)</td><td>3.24 (3)</td></tr><tr><td>P(1)</td><td>0.54942 (10)</td><td>0.84621 (7)</td><td>0.23735 (7)</td><td>3.10 (5)</td></tr><tr><td>P(2)</td><td>0.81324 (10)</td><td>0.78986 (8)</td><td>0.25586 (7)</td><td>3.42 (5)</td></tr><tr><td>C(1)</td><td>0.5849 (4)</td><td>0.7254 (3)</td><td>0.2807 (3)</td><td>3.66 (20)</td></tr><tr><td>C(2)</td><td>0.7238 (4)</td><td>0.7181 (3)</td><td>0.3212 (3)</td><td>4.02 (21)</td></tr><tr><td>C(3)</td><td>0.7417 (4)</td><td>1.0270 (3)</td><td>0.2532 (3)</td><td>4.71 (23)</td></tr><tr><td>C(4)</td><td>0.6847 (4)</td><td>1.0444 (3)</td><td>0.1778 (3)</td><td>5.1 (3)</td></tr><tr><td>C(5)</td><td>0.6296 (4)</td><td>0.8670 (3)</td><td>0.0700 (3)</td><td>4.50 (22)</td></tr><tr><td>O(6)</td><td>0.5891 (3)</td><td>0.84889 (24)</td><td>-0.00290 (19)</td><td>6.66 (19)</td></tr><tr><td>C(7)</td><td>0.8385 (4)</td><td>0.9323 (3)</td><td>0.1332 (3)</td><td>4.70 (24)</td></tr><tr><td>O(8)</td><td>0.9219 (3)</td><td>0.9551 (3)</td><td>0.10288 (23)</td><td>7.43 (21)</td></tr><tr><td>C(11)</td><td>0.3384 (4)</td><td>0.8205 (3)</td><td>0.1655 (3)</td><td>4.08 (21)</td></tr><tr><td>C(12)</td><td>0.3100 (5)</td><td>0.9024 (4)</td><td>0.1178 (3)</td><td>5.8 (3)</td></tr><tr><td>F(13)</td><td>0.31410 (22)</td><td>0.77461 (18)</td><td>0.21238 (17)</td><td>5.48 (13)</td></tr><tr><td>F(14)</td><td>0.40927 (25)</td><td>0.75959 (18)</td><td>0.10210 (16)</td><td>5.79 (13)</td></tr><tr><td>F(15)</td><td>0.3664 (3)</td><td>0.93989 (25)</td><td>0.05779 (21)</td><td>9.47 (20)</td></tr><tr><td>F(16)</td><td>0.1995 (3)</td><td>0.87678 (25)</td><td>0.07865 (25)</td><td>10.28 (22)</td></tr><tr><td>F(17)</td><td>0.3001 (3)</td><td>0.97142 (21)</td><td>0.17208 (20)</td><td>9.12 (20)</td></tr><tr><td>C(21)</td><td>0.5089 (4)</td><td>0.9094 (3)</td><td>0.3394 (3)</td><td>3.71 (21)</td></tr><tr><td>C(22)</td><td>0.4011 (4)</td><td>0.8804 (3)</td><td>0.3853 (3)</td><td>4.82 (25)</td></tr><tr><td>F(23)</td><td>0.49300 (24)</td><td>1.00281 (15)</td><td>0.32178 (16)</td><td>5.03 (13)</td></tr><tr><td>F(24)</td><td>0.61600 (22)</td><td>0.89863 (18)</td><td>0.40158 (14)</td><td>4.99 (13)</td></tr><tr><td>F(25)</td><td>0.29103 (23)</td><td>0.90353 (20)</td><td>0.33753 (19)</td><td>6.49 (16)</td></tr><tr><td>F(26)</td><td>0.4141 (3)</td><td>0.92602 (21)</td><td>0.46210 (18)</td><td>7.51 (17)</td></tr><tr><td>F(27)</td><td>0.4010 (3)</td><td>0.78798 (18)</td><td>0.40136 (17)</td><td>6.28 (15)</td></tr><tr><td>C(31)</td><td>0.8974 (5)</td><td>0.6947 (3)</td><td>0.2002 (3)</td><td>5.5 (3)</td></tr><tr><td>C(32)</td><td>0.8254 (6)</td><td>0.6525 (4)</td><td>0.1170 (3)</td><td>7.0 (3)</td></tr><tr><td>F(33)</td><td>0.9362 (3)</td><td>0.62229 (19)</td><td>0.25679 (18)</td><td>6.78 (15)</td></tr><tr><td>F(34)</td><td>1.0054 (3)</td><td>0.73566 (21)</td><td>0.17708 (20)</td><td>7.35 (18)</td></tr><tr><td>F(35)</td><td>0.8889 (4)</td><td>0.57970 (21)</td><td>0.09237 (21)</td><td>10.10 (23)</td></tr><tr><td>F(36)</td><td>0.8103 (3)</td><td>0.71482 (21)</td><td>0.05109 (17)</td><td>8.11 (19)</td></tr><tr><td>F(37)</td><td>0.7150 (3)</td><td>0.62479 (22)</td><td>0.13282 (22)</td><td>8.37 (19)</td></tr><tr><td>C(41)</td><td>0.9528 (4)</td><td>0.8316 (3)</td><td>0.3440 (3)</td><td>4.8 (24)</td></tr><tr><td>C(42)</td><td>10.439 (5)</td><td>0.7658 (4)</td><td>0.4051 (3)</td><td>6.7 (3)</td></tr><tr><td>F(43)</td><td>1.02401 (23)</td><td>0.89057 (20)</td><td>0.30436 (17)</td><td>6.35 (15)</td></tr><tr><td>F(44)</td><td>0.89968 (24)</td><td>0.88582 (19)</td><td>0.39898 (15)</td><td>5.78 (14)</td></tr><tr><td>F(45)</td><td>1.1220 (3)</td><td>0.72225 (23)</td><td>0.36109 (21)</td><td>8.36 (18)</td></tr><tr><td>F(46)</td><td>1.1137 (3)</td><td>0.8144 (3)</td><td>0.46928 (20)</td><td>9.25 (19)</td></tr><tr><td>F(47)</td><td>0.9807 (3)</td><td>0.69946 (22)</td><td>0.43860 (19)</td><td>7.67 (17)</td></tr></tbody></table>

ªB<sub>iso</sub> is the mean of the principal axes of the thermal ellipsoid.

<table><caption>Table IV. Selected Bond Distances (Å) and Angles (deg) for (dfepe)(CO)₂Fe(η²-CH₂=CH²) (5)</caption><tr><td>C(3)-C(4)</td><td>1.362 (7)</td><td>C(5)-Fe</td><td>1.771 (4)</td></tr><tr><td>C(3)-Fe</td><td>2.125 (4)</td><td>C(7)-Fe</td><td>1.773 (5)</td></tr><tr><td>P(1)-Fe</td><td>2.2078 (15)</td><td>C(4)-Fe</td><td>2.113 (4)</td></tr><tr><td>P(2)-Fe</td><td>2.1398 (13)</td><td></td><td></td></tr><tr><td>P(2)-Fe-P(1)</td><td>84.27 (5)</td><td>P(1)-Fe-C(3)</td><td>97.68 (12)</td></tr><tr><td>Fe-C(3)-C(4)</td><td>70.8 (3)</td><td>P(1)-Fe-C(4)</td><td>96.10 (13)</td></tr><tr><td>Fe-C(4)-C(3)</td><td>71.7 (3)</td><td>C(1)-Fe-C(3)</td><td>85.53 (19)</td></tr><tr><td>P(1)-Fe-C(5)</td><td>92.76 (14)</td><td>C(1)-Fe-C(4)</td><td>88.81 (19)</td></tr><tr><td>P(1)-Fe-C(7)</td><td>184.97 (14)</td><td>C(2)-Fe-C(4)</td><td>99.81 (19)</td></tr><tr><td>C(7)-Fe-C(5)</td><td>87.47 (20)</td><td>P(2)-Fe-C(3)</td><td>106.79 (13)</td></tr><tr><td>P(2)-Fe-C(5)</td><td>115.99 (14)</td><td>C(3)-Fe-C(4)</td><td>37.49 (18)</td></tr><tr><td>P(2)-Fe-C(7)</td><td>91.11 (14)</td><td></td><td></td></tr></table>

is shown in Figure 1, and selected bond distances and angles are summarized in Table IV. The configuration around the central iron atom approximates a trigonal bipyramid. One carbonyl group, the ethylene ligand and P(2) of the bidentate ligand occupy the three equatorial positions; P(1) and the remaining carbonyl group occupy the two axial sites [P(1)-Fe-C(7) = 175°]. The P(1)-Fe-P(2) angle of 84° is similar to values found in other ethano-bridged bidentate phosphine complexes.⁶,⁷,⁹,¹⁰ The angles between the midpoint of C(3) and C(4) of the ethylene ligand, iron, and P(2) and C(5), respectively are 126 and 119°. Similarly, P(1)-Fe-midpoint (C(4)C(3)) and C(7)-Fe-midpoint (C(4)C(3)) are 98 and 87°.

For maximum back-bonding, the ethylene carbons should lie in the equatorial plane. This geometry is observed in 5. If the equatorial plane is defined as the plane

<figure><img src="image_5.png" /><figcaption>Figure 1. ORTEP diagram of (dfepe)(CO)₂Fe(η²-CH₂=CH₂).</figcaption></figure>

containing P(2), Fe, and C(5), then the Fe-C(4)-C(3) plane subtends an angle of only 8° with this plane. C(3) and C(4) lie 0.23 and 0.06 Å above the plane, respectively. The axial Fe-P(1) distance [2.2078 (15) Å] is in agreement with the Fe-P<sub>ax</sub> distances found in the literature.⁹,¹⁰ The equatorial Fe-P(2) [2.1398 (13) Å] distance is similar to that of [(C-F₃)₃P₃Fe(CO)₂ [2.151 (4) Å]¹⁶ but shorter than the Fe-P<sub>eq</sub> distance in (dpepe)Fe(CO)₃ [2.227 (2) Å],⁹ as expected with a better π-acid ligand. The bond distances observed for 5 are consistent with those found for similar complexes. Table V compares key distances for 5, (CO)₄Fe(C₂H₄), and (CO)₃[P(C₆H₅)₃]Fe(C₂H₄). The most notable structural feature of 5 is the remarkably short carbon-carbon length of 1.362 (7) Å for the coordinated ethylene ligand. This distance is one of the shortest known for a coordinated ethylene and is comparable to that in Zeise's salt [KPt-(C₂H₄)Cl₃]·H₂O [1.354 (15) Å].¹⁷ This short length in 5 reflects poor back-bonding to the π* orbitals of ethylene and is a consequence of the strong π-acid character of the fluorinated bidentate phosphine ligand.¹⁸ The C-C bond length in (Ph₃P)(CO)₃Fe(C₂H₄)¹⁵ is somewhat longer (1.398 (8) Å), no doubt due to increased back-bonding as a consequence of the better donor, poorer acceptor properties of P(C₆H₅)₃ relative to the (perfluoroalkyl)phosphine. A comparison with the C-C bond length of (CO)₄Fe(C₂H₄) determined by electron diffraction is not meaningful due to the high degree of uncertainty in this length [1.46 (6) Å].¹⁹

Synthesis of other η²-olefin complexes using similar experimental conditions has been attempted. The propene complex (dfepe)Fe(CO)₂(η²-C₃H₆) (6) could be isolated as a yellow oil which was too thermally unstable at 25 °C to provide an analytically pure sample. Likewise, photolysis of 2 in cyclohexene at -45 °C led to IR evidence for formation of (dfepe)Fe(CO)₂(η²-C₆H₁₀) (7) (ν<sub>CO</sub> = 2004, 1966 cm⁻¹), but isolation of a complex stable at 25 °C was not possible.

**Photoreaction of 2 with Acetylene.** Photolysis of 2 in an ether solution saturated with acetylene led not to the formation of an η²-adduct but rather to the η⁴-cyclopentadienone complex 8, (dfepe)Fe(CO)(η⁴-C₄H₄CO) (30% yield, yellow crystals). The ¹³C NMR shift for the vinylic ring carbons [δ 67 and 81 ppm] indicate that the two double bonds are η²-coordinated while the keto group (ν<sub>CO</sub>)

<footnote>(16) Burg, A. B. *Inorg. Chem.* **1986**, *25*, 4751.</footnote>

<footnote>(17) Hamilton, W. C.; Klanderman, K. A.; Spratley, R. *Acta Crystallogr., Sect. A* **1969**, *25*, S172.</footnote>

<footnote>(18) Mingos, D. M. P. In *Comprehensive Organometallic Chemistry*; Wilkinson, G.; Ed.; Pergamon Press: Oxford, U. K., 1982; Vol. 3, Chapter 19.</footnote>

<footnote>(19) Davis, M. I.; Speed, C. S. *J. Organomet. Chem.* **1970**, *21*, 401.</footnote>
<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /></figure>

= 1646 cm⁻¹ and δCO = 172.5 ppm] is not bound to iron. This reaction is well-known for simple iron carbonyl derivatives,²⁰ but yields of dienone products are generally quite low.

**Photosubstitution of Butadiene.** Generally, Fe(η⁴-diene)L₃ [L = PF₃, P(OCH₃)₃] complexes have been prepared by photolysis of FeL₅,²¹ by reduction of FeCl₂ in presence in dienes and L,²² or by metal atom evaporation techniques.²³ Complexes of the type (η⁴-diene)Fe(CO)L₂ [L = PF₃,²⁴ P(OCH₃)₃,²⁵ L₂ = dppe²⁶] have been described. These systems have been prepared from (η⁴-C₄H₆)₂Fe(CO) by thermal or photochemical displacement of butadiene but not from carbonyl substitution starting from L₂Fe(CO)₃ derivatives. In view of the ready substitution of CO in 2 by monodentate ligands, we sought to prepare bidentate derivatives by similar techniques.

Photolysis of 2 was carried out in an ether solution of butadiene at 0 °C and monitored by IR spectroscopy. The bands for 2 disappear as a strong single band grows at 1996 cm⁻¹, supporting the formation of the η⁴-diene monocarbonyl complex (dfepe)Fe(CO)(η⁴-C₄H₆) (9). Complex

<figure><img src="image_8.png" /></figure>

9 was isolated as an analytically pure yellow crystalline material (40%). The ¹H and ¹³C NMR data clearly support η⁴-coordination of the diene ligand (δH(1)(inner) = 0.05 ppm, δH(1)(outer) = 2.1 ppm δC(2) = 84.8 ppm δC(1) = 38.7 ppm). The similarities of the νCO bands of 9 and (η⁴-C₄H₆)Fe(CO(PF₃)₂,²⁴ 1996 and 2001 cm⁻¹, respectively, again support the suggestion that the dfepe ligand has π-acidic character nearly equivalent to two PF₃ ligands. No reaction of 9 is observed under carbon monoxide (1 atm in THF) or in the presence of trimethyl phosphite either under photolytic or thermal conditions.

**Photoreactions with Methyl Vinyl Ketone and Styrene: Synthesis of (dfepe)Fe(CO)-Transfer Reagents.** α,β-Unsaturated ketone complexes of Fe(CO)₃ have been successfully used as Fe(CO)₃-transfer reagents under very mild conditions.²⁷ The utility of such reagents

led us to investigate the synthesis of (dfepe)(CO)Fe(enone) complexes which could serve as convenient transfer reagents for the (dfepe)(CO)Fe moiety.

Photolysis of 2 at 0 °C in methyl vinyl ketone led to formation of a blood red solution in which the νCO bands for 2 had been replaced by two new bands at 2020 and 1976 cm⁻¹. From this solution the (dfepe)(CO)Fe(η⁴-methyl vinyl ketone) complex 10 (νCO = 1976 cm⁻¹) could be isolated in 20% yield as a dark red analytically pure crystalline material. Owing to the low yield of this material and the extreme sensitivity of 10, it was not practical to develop the transfer chemistry of this complex.

<figure><img src="image_9.png" /></figure>

While styrene is often a quite labile ligand, styrene complexes have seldom been employed synthetically as transfer reagents. One exception is the use of Fe(CO)₄-(styrene) to prepare other alkene complexes.²⁸ On the basis of the stability of iron carbonyl complexes 11 and 12,²⁹ we anticipated that an η⁴-styrene complex of (dfepe)Fe(CO) may be accessible and serve as a convenient thermal source of “(dfepe)Fe(CO)”.

<figure><img src="image_10.png" /></figure>

When 2 is photolyzed in the presence of styrenes, three complexes 13–15 corresponding to η⁴-, η²-, and η⁶-styrene coordination could be isolated in 55%, 20%, and 5%

<figure><img src="image_11.png" /></figure>

yields, respectively. Following the progress of the reaction of IR spectroscopy suggested that the η²-complex 14 is a precursor to the η⁴-complex 13. However, photolysis of a pure sample of the η⁴-complex 13 did not lead to the formation of the η⁶-complex 15.

The minor products η²-14 and η⁶-15 were characterized by IR and NMR spectroscopy; complete details are given in the Experimental Section. The structure of the η²-dicarbonyl complex 14 was confirmed by the presence of two νCO bands at 2021 and 1982 cm⁻¹ similar to the ethylene complex 5(νCO = 2020 and 1979 cm⁻¹) and the observation of ¹³C resonances at δ 32.1 and 57.1 ppm attributable to Cβ and Cα of the coordinated vinyl group. These resonances are each shifted upfield ca. 80 ppm relative to the shifts in free styrene (δ 112 and 136 ppm, respectively) and are in accord with the 92 ppm upfield shift of the olefin

<footnote>(20) *Organic Synthesis via Metal Carbonyls*; Wender, I., Pino, P., Eds.; John Wiley and Sons: New York, 1977; Vol. 2, p 420.</footnote>

<footnote>(21) Kruck, T.; Knoll, L.; Laufenberg, J. *Chem. Ber.* **1973**, *106*, 697.</footnote>

<footnote>(22) (a) Rathke, J. W.; Muetterties, E. L. *J. Am. Chem. Soc.* **1975**, *97*, 3272. (b) Hoberg, H.; Jenni, K.; Raabe, E.; Kruger, C.; Schroth, G. *J. Organomet. Chem.* **1987**, *320*, 325. (c) Harris, T. V.; Rathke, J. W.; Muetterties, E. L. *J. Am. Chem. Soc.* **1978**, *100*, 6966.</footnote>

<footnote>(23) Ittel, S. D.; Van-Catledge, F. A.; Jesson, J. P. *J. Am. Chem. Soc.* **1979**, *101*, 3874.</footnote>

<footnote>(24) Warren, J. D.; Clark, R. *J. Inorganic Chem.* **1970**, *9*, 373.</footnote>

<footnote>(25) Al-Ohaly, A.-R.; Nixon, J. F. *J. Organomet. Chem.* **1980**, *202*, 297.</footnote>

<footnote>(26) Ungermann, C. B.; Caulton, K. G. *J. Organomet. Chem.* **1975**, *94*, C9.</footnote>

<footnote>(27) (a) Vichi, E. J. S.; Fujiwara, F. Y.; Stein, Edison. *Inorg. Chem.* **1985**, *24*, 286. (b) Johnson, B. F. G.; Lewis, J.; Stephenson, G. R.; Vichi, E. J. S. *J. Chem. Soc., Dalton Trans.* **1978**, 369. (c) Brookhart, M.; Graham, C. R.; Nelson, G. O.; Scholes, G. *Ann. N.Y. Acad. Sci.* **1977**, *295*, 254. (d) Howell, J. A. S.; Johnson, B. F. G.; Josty, P. L.; Lewis, J. *J. Organomet. Chem.* **1972**, *39*, 329. (e) Domingos, A. J. P.; Howell, J. A. S.; Johnson, B. F. G.; Lewis, J. *In Inorganic Syntheses*; Holtzlaw, F. H., Ed.; McGraw-Hill: New York, 1976, Vol. 16, p 103.</footnote>

<footnote>(28) Burkinshaw, P. M.; Dixon, D. T.; Howell, J. A. S. *J. Chem. Soc., Dalton Trans.* **1980**, 999.</footnote>

<footnote>(29) Davis, R. E.; Pettit, R. *J. Am. Chem. Soc.* **1970**, *92*, 716.</footnote>
<table><caption>Table V. Relevant Bond Lengths (Å) of Fe(0) η²-Ethylene Complexes</caption><thead><tr><th></th><th>CO</th><th>P(Ph)<sub>3</sub></th><th>(F<sub>5</sub>C<sub>2</sub>)<sub>2</sub>P</th><th>(C<sub>2</sub>F<sub>5</sub>)<sub>2</sub></th><th>(this work)</th></tr></thead><tbody><tr><td>Fe-P<sub>ax</sub></td><td></td><td>2.265 (1)</td><td></td><td>2.2078 (15)</td><td></td></tr><tr><td>Fe-P<sub>eq</sub></td><td></td><td></td><td></td><td>2.1398 (13)</td><td></td></tr><tr><td>Fe-CO<sub>ax</sub></td><td>1.796 (35)</td><td>1.759 (3)</td><td></td><td>1.771 (4)</td><td></td></tr><tr><td>Fe-CO<sub>eq</sub></td><td>1.836 (35)</td><td>1.759 (6), 1.763 (6)</td><td></td><td>1.773 (5)</td><td></td></tr><tr><td>Fe-C<sub>ethylene</sub></td><td>2.117 (3)</td><td>2.095 (7), 2.102 (7)</td><td></td><td>2.125 (4), 2.113 (4)</td><td></td></tr><tr><td>ethylene C-C</td><td>1.46 (6)</td><td>1.398 (8)</td><td></td><td>1.362 (7)</td><td></td></tr></tbody></table>

<sup>a</sup>Davis, M. I.; Speed, C. S. *J. Organomet. Chem.* **1970**, *21*, 401.

<sup>b</sup>Lindner, E.; Schauss, E.; Hiller, W.; Fawzi, R. *Angew. Chem., Int. Ed.* **1984**, *23*, 711. Lindner, E.; Schauss, E.; Hiller, W.; Fawzi, R. *Chem. Ber.* **1985**, *118*, 3915.

<table><caption>Table VI. Fractional Atomic Coordinates and Isotropic Thermal Parameters for (CO)(dfepe)Fe(η⁴-C₆H₅CH=CH₂) (13)</caption><thead><tr><th>atom</th><th>x</th><th>y</th><th>z</th><th>B<sub>iso</sub>, Å<sup>2</sup></th></tr></thead><tbody><tr><td>Fe(1)</td><td>0.53799 (6)</td><td>0.79746 (5)</td><td>0.29625 (3)</td><td>3.025 (22)</td></tr><tr><td>P(1)</td><td>0.72546 (12)</td><td>0.76618 (10)</td><td>0.17117 (7)</td><td>3.59 (5)</td></tr><tr><td>P(2)</td><td>0.58658 (11)</td><td>0.99889 (9)</td><td>0.24936 (6)</td><td>3.16 (4)</td></tr><tr><td>C(1)</td><td>0.3635 (4)</td><td>0.8572 (3)</td><td>0.25003 (24)</td><td>3.55 (18)</td></tr><tr><td>O(1)</td><td>0.2486 (3)</td><td>0.8897 (3)</td><td>0.22229 (21)</td><td>5.43 (17)</td></tr><tr><td>C(2)</td><td>0.8203 (5)</td><td>0.9208 (4)</td><td>0.0985 (3)</td><td>5.43 (24)</td></tr><tr><td>C(3)</td><td>0.6945 (5)</td><td>1.0464 (4)</td><td>0.1189 (3)</td><td>4.75 (22)</td></tr><tr><td>C(11)</td><td>0.6827 (5)</td><td>0.7057 (4)</td><td>0.3941 (3)</td><td>4.65 (21)</td></tr><tr><td>C(12)</td><td>0.5171 (5)</td><td>0.7439 (4)</td><td>0.43930 (24)</td><td>4.33 (21)</td></tr><tr><td>C(13)</td><td>0.3961 (5)</td><td>0.6842 (4)</td><td>0.43379 (23)</td><td>4.03 (19)</td></tr><tr><td>C(14)</td><td>0.4524 (5)</td><td>0.5932 (4)</td><td>0.3778 (3)</td><td>4.10 (20)</td></tr><tr><td>C(15)</td><td>0.3308 (5)</td><td>0.5449 (4)</td><td>0.3630 (3)</td><td>4.73 (21)</td></tr><tr><td>C(16)</td><td>0.1686 (6)</td><td>0.5838 (5)</td><td>0.4001 (3)</td><td>5.61 (25)</td></tr><tr><td>C(17)</td><td>0.1117 (5)</td><td>0.6705 (5)</td><td>0.4571 (3)</td><td>5.59 (24)</td></tr><tr><td>C(18)</td><td>0.2211 (5)</td><td>0.7197 (4)</td><td>0.4735 (3)</td><td>5.16 (24)</td></tr><tr><td>C(21)</td><td>0.6758 (5)</td><td>0.7464 (4)</td><td>0.0680 (3)</td><td>4.35 (22)</td></tr><tr><td>C(22)</td><td>0.5831 (6)</td><td>0.6345 (4)</td><td>0.0888 (3)</td><td>5.45 (25)</td></tr><tr><td>F(23)</td><td>0.5736 (3)</td><td>0.86467 (22)</td><td>0.03609 (16)</td><td>5.64 (14)</td></tr><tr><td>F(24)</td><td>0.8100 (3)</td><td>0.73715 (25)</td><td>-0.00671 (16)</td><td>5.67 (14)</td></tr><tr><td>F(25)</td><td>0.4284 (3)</td><td>0.6577 (3)</td><td>0.14151 (21)</td><td>7.91 (20)</td></tr><tr><td>F(26)</td><td>0.5747 (4)</td><td>0.6226 (3)</td><td>0.01037 (20)</td><td>9.02 (22)</td></tr><tr><td>F(27)</td><td>0.6540 (4)</td><td>0.5185 (3)</td><td>0.1381 (3)</td><td>9.51 (24)</td></tr><tr><td>C(31)</td><td>0.9199 (6)</td><td>0.6298 (6)</td><td>0.1864 (4)</td><td>7.6 (4)</td></tr><tr><td>C(32)</td><td>1.0497 (7)</td><td>0.5805 (6)</td><td>0.1188 (5)</td><td>10.0 (5)</td></tr><tr><td>F(33)</td><td>0.9923 (3)</td><td>0.6782 (3)</td><td>0.23082 (22)</td><td>8.19 (21)</td></tr><tr><td>F(34)</td><td>0.8670 (3)</td><td>0.55065 (24)</td><td>0.25494 (18)</td><td>6.75 (15)</td></tr><tr><td>F(35)</td><td>1.1734 (3)</td><td>0.5001 (3)</td><td>0.15394 (23)</td><td>8.25 (19)</td></tr><tr><td>F(36)</td><td>1.0998 (4)</td><td>0.7025 (3)</td><td>0.04786 (20)</td><td>9.16 (21)</td></tr><tr><td>F(37)</td><td>1.0021 (4)</td><td>0.5112 (3)</td><td>0.08163 (25)</td><td>9.29 (24)</td></tr><tr><td>C(41)</td><td>0.4138 (5)</td><td>1.1524 (3)</td><td>0.2476 (3)</td><td>3.88 (19)</td></tr><tr><td>C(42)</td><td>0.2890 (5)</td><td>1.1596 (4)</td><td>0.3423 (3)</td><td>4.90 (22)</td></tr><tr><td>C(43)</td><td>0.4727 (3)</td><td>1.27026 (20)</td><td>0.20522 (16)</td><td>5.21 (13)</td></tr><tr><td>F(44)</td><td>0.3267 (3)</td><td>1.15024 (22)</td><td>0.19176 (16)</td><td>5.29 (13)</td></tr><tr><td>F(45)</td><td>0.3639 (3)</td><td>1.1573 (3)</td><td>0.40278 (18)</td><td>7.58 (19)</td></tr><tr><td>F(46)</td><td>0.1774 (3)</td><td>1.2704 (3)</td><td>0.33250 (21)</td><td>7.71 (18)</td></tr><tr><td>F(47)</td><td>0.2086 (3)</td><td>1.0559 (3)</td><td>0.38284 (20)</td><td>7.12 (16)</td></tr><tr><td>C(51)</td><td>0.7336 (5)</td><td>1.0375 (4)</td><td>0.3000 (3)</td><td>4.47 (22)</td></tr><tr><td>C(52)</td><td>0.7823 (6)</td><td>1.1775 (5)</td><td>0.2723 (3)</td><td>5.8 (3)</td></tr><tr><td>F(53)</td><td>0.8793 (3)</td><td>0.95493 (24)</td><td>0.27361 (19)</td><td>5.95 (16)</td></tr><tr><td>F(54)</td><td>0.6791 (3)</td><td>0.99613 (23)</td><td>0.39707 (16)</td><td>5.47 (13)</td></tr><tr><td>F(55)</td><td>0.8313 (4)</td><td>1.2318 (3)</td><td>0.17850 (19)</td><td>7.57 (18)</td></tr><tr><td>F(56)</td><td>0.9083 (3)</td><td>1.1593 (3)</td><td>0.30717 (23)</td><td>8.16 (21)</td></tr><tr><td>F(57)</td><td>0.6594 (3)</td><td>1.26185 (24)</td><td>0.30969 (19)</td><td>6.61 (17)</td></tr></tbody></table>

<sup>a</sup>B<sub>iso</sub> is the mean of the principal axes of the thermal ellipsoid.

resonance in 5 (δ 36 ppm) relative to that in free ethylene (δ 128 ppm). A calculated angle of 83° between the two carbonyl groups suggests that the geometry of 14 is probably similar to that of 5.

The structure of the η⁶ complex 15 is based on the absence of CO ligands, the relatively unperturbed ¹H and ¹³C shifts of the vinyl group (e.g., δ₁₃C₈ = 135 ppm, δ₁₃C₉ = 115 ppm) and the substantial upfield shift of all of the arene ¹³C resonances (δ 82.3, 83.6, 86.6 and 101<sub>ipso</sub> ppm). Arene complexes of iron(0) are relatively rare and are generally synthesized by metal atom vapor techniques and from iron(II) chlorides. Examples include (η⁶-C₆H₆)FeL₂ (L =

<figure><img src="image_12.png" /><figcaption>Figure 2. ORTEP diagram of (dfepe)(CO)Fe(η⁴-CH₆H₅CH=CH₂).</figcaption></figure>

<table><caption>Table VII. Selected Bond Distances (Å) and Angles (deg) for (dfepe)(CO)Fe(η⁴-C₆H₅CH=CH₂) (13)</caption><tbody><tr><td>Fe-P(1)</td><td>2.1364 (13)</td><td>C(14)-C(13)</td><td>1.431 (5)</td></tr><tr><td>Fe-P(2)</td><td>2.1313 (11)</td><td>C(11)-C(12)</td><td>1.389 (6)</td></tr><tr><td>Fe-C(11)</td><td>2.111 (4)</td><td>C(12)-C(13)</td><td>1.423 (5)</td></tr><tr><td>Fe-C(12)</td><td>2.045 (3)</td><td>C(14)-C(15)</td><td>1.423 (5)</td></tr><tr><td>Fe-C(13)</td><td>2.045 (3)</td><td>C(15)-C(16)</td><td>1.340 (6)</td></tr><tr><td>Fe-C(14)</td><td>2.278 (4)</td><td>C(16)C(17)</td><td>1.405 (7)</td></tr><tr><td>Fe-C(1)</td><td>1.773 (4)</td><td>C(17)-C(18)</td><td>1.348 (6)</td></tr><tr><td></td><td></td><td>C(18)-C(13)</td><td>1.434 (6)</td></tr><tr><td>P(1)-Fe-P(2)</td><td>88.30 (5)</td><td>C(1)-Fe-C(11)</td><td>160.73 (16)</td></tr><tr><td>P(2)-Fe-C(1)</td><td>90.60 (11)</td><td>P(1)-Fe-C(11)</td><td>97.90 (12)</td></tr><tr><td>P(1)-Fe-C(1)</td><td>99.28 (12)</td><td>P(1)-Fe-C(12)</td><td>136.67 (12)</td></tr><tr><td>Fe-C(13)-C(14)</td><td>75.73 (20)</td><td>P(2)-Fe-C(14)</td><td>167.02 (10)</td></tr><tr><td>C(1)-Fe-C(12)</td><td>136.67 (12)</td><td>Fe-C(14)-C(13)</td><td>66.76 (19)</td></tr><tr><td>Fe-C(11)-C(12)</td><td>67.95 (21)</td><td>P(2)-Fe-C(12)</td><td>100.52 (11)</td></tr><tr><td>P(2)-Fe-C(13)</td><td>129.52 (11)</td><td>Fe-C(12)-C(11)</td><td>73.03 (21)</td></tr><tr><td>C(13)-C(14)-C(15)</td><td>117.9 (3)</td><td>C(17)-C(18)-C(13)</td><td>120.8 (4)</td></tr><tr><td>P(2)-Fe-C(11)</td><td>98.58 (11)</td><td>C(14)-C(13)-C(18)</td><td>118.5 (3)</td></tr><tr><td>C(14)-C(15)-C(16)</td><td>121.1 (4)</td><td>C(16)-C(17)-C(18)</td><td>120.0 (4)</td></tr><tr><td>C(15)-C(16)-C(17)</td><td>121.6 (4)</td><td></td><td></td></tr></tbody></table>

PF₃),³⁰ P(OCH₃)₃,³¹ and P(CH₃)₃,³² Recently, (η⁶-C₆H₅R)Ru(dfepe) (R = H, CH₃, C₆H₁₁) have been prepared by the thermolysis of Ru(dfepe)₂(H)₂ in the corresponding aromatic solvent.⁷

**X-ray Structure Determination of 13.** The structure of the major product, the η⁴-styrene complex 13, was confirmed by single-crystal X-ray crystallography. Single crystals of 13 were grown from methylene chloride at -30 °C. Complex 13 crystallizes in the triclinic system with space group P\I. The unit cell contains two molecules separated by the normal van der Waals distances. Crystallographic data are summarized in Table II, while atomic coordinates appear in Table VI. An ORTEP drawing of the

<footnote>(30) Middleton, R.; Hull, J. R.; Simpson, S. R.; Tomlinson, C. H.; Timms, P. L. *J. Chem. Soc., Dalton Trans.* **1973**, 120.</footnote>

<footnote>(31) Ittel, S. D.; Tolman, C. A. *J. Organomet. Chem.* **1979**, *172*, C47.</footnote>

<footnote>(32) Green, M. L. H.; Wong, L.-L. *J. Chem. Soc., Chem. Commun.* **1984**, 1442.</footnote>
<figure><img src="image_13.png" /></figure>

13

molecule is shown in Figure 2, while Table VII lists selected metrical parameters. The geometry around the iron atom can be described as intermediate between trigonal bipyramidal in which the axial ligands are P(2) and the η²-C₆H₅ moiety and square pyramidal in which the carbonyl ligand occupies the apical position. No significant differences are observed between the two Fe-P bond lengths (2.1364 (13) and 2.1313 (11) Å), and the P(1)-Fe-P(2) angle of 88.3° is similar to that observed in complex 5. The most notable feature of the structure is the very long Fe-C(14) distance of 2.278 (4) Å, which suggests a relatively weak interaction between the C(13)-C(14) double bond and the iron atom. Note also the Fe-C(13) distance (2.159 (3) Å) is considerably longer than the Fe-C(12) length (2.045 (3) Å). (The η⁴-styrene derivative, Fe(CO)₃(η⁴-CH(CH₃)=C(C₆H₅)₂), also possesses a long bond between iron and the ortho-carbon of the η²-bound arene ring (2.230 (8) Å).)³³

**Complex 13 as a Fe(CO)(dfepe)-Transfer Reagent.**
The weak interaction between iron and the η²-bound arene in 13 as supported by the X-ray structural results, suggested that the styrene ligand may be quite labile and that 13 may serve as a convenient Fe(CO)(dfepe)-transfer agent. Indeed, this is the case. Treatment of 13 with CO (THF, 25 °C) resulted in the displacement of the η⁴-bound styrene and formation of 2. Treatment of 13 with 2 equiv of trimethyl phosphite (THF, 25 °C) results in rapid displacement of styrene and formation of the bis(phosphite) adduct 16. Complex 16 shows only one strong vCO band at 1944

<figure><img src="image_14.png" /></figure>

16

cm⁻¹, indicating only one isomer is present rather than two as in the case of (F₃P)₄Fe(CO) which shows two vCO bands (2037 cm⁻¹ CO(axial), 2008 cm⁻¹ CO(equatorial)). The ³¹P NMR spectrum of 16 exhibits a singlet at δ 173 ppm attributed to the phosphite ligands and a septet at δ 132 ppm due to the dfepe ligand. The ¹³C resonances of the carbonyl ligand appear as a triplet at δ 218.1 ppm (²J<sub>PC</sub> = 26 Hz) indicating coupling to only two ³¹P nuclei. An X-ray diffraction study has confirmed the structure of 16 and indicates a slightly distorted trigonal-bipyramidal structure in which CO and one of the dfepe phosphorus atoms occupy the axial positions.³⁴

**Oxidative Addition Reactions of Photogenerated (dfepe)Fe(CO)₂.**
In view of the fact that 16-electron L₄Fe0 complexes undergo a variety of oxidative addition reactions, including oxidative addition of alkanes, we have investigated oxidative addition reactions of photogenerated (dfepe)Fe(CO)₂.

**Reaction with H₂.** Photolysis of complex 2 under a dihydrogen purge in diethyl ether produced the dihydride 17 in a 68% yield. Similar to Fe(CO)₄(H)₂, spectroscopic

<figure><img src="image_15.png" /></figure>

17

data reveal that the dihydride 17 is the classical cis-dihydride 17 and not one in which dihydrogen is bound in an η² fashion.³⁵,³⁶ In contrast to Fe(CO)₄(H)₂ which is thermally unstable at temperatures above -10 °C, 17 is stable at 45 °C for several hours in solution. Pure 17 decomposes slowly under N₂ at -30 °C and must be stored under either Ar or H₂.

The ¹H NMR spectrum at 25 °C exhibits a 1:2:1 triplet pattern at δ -11.1 ppm with J<sub>PH</sub> = 31.4 Hz. Furthermore, the variable-temperature ¹H and ³¹P spectra show that 17 is stereochemically nonrigid. The 400-MHz ¹H NMR spectrum at -84 °C in the high-field region shows two distinct hydride signals at δ -9.5 and -13 ppm. The spectrum observed is consistent only with structure 17 in which the hydride ligands are cis. The observation of two distinct hydride signals clearly suggests a classical dihydride structure for 17 rather than an η²-H₂ structure, since in all previously observed η²-H₂ systems the barriers to H₂ rotation are extremely low.³⁵,³⁷,³⁸ The large J<sub>PH</sub> values are most consistent with terminal hydrides; however, a substantial J<sub>PH</sub> coupling in an η²-H₂ system has been observed in at least one case.³⁷c The signal at δ -9.5 ppm is assigned to Hₐ on the basis of a large trans coupling J<sub>P(2)-Hₐ</sub> = 63 Hz and a smaller cis coupling, J<sub>P(1)-Hₐ</sub> = 41 Hz. The δ -13 ppm signal is assigned to Hₐ on the basis of two nearly equal cis J<sub>PH</sub> couplings of 35 and 37 Hz. As the temperature is raised, the two high-field bands broaden and coalesce at -38 °C. From the coalescence equation, k = π(Δν)/(√2), the rate constant for site exchange of the two hydrogens is 3.2 × 10³ s⁻¹, corresponding to ΔG* = 9.9 ± 0.2 kcal/mol. The ²H spectrum of (dfepe)Fe(CO)₂(D)₂ (18) (prepared by photolysis under D₂) shows a broad signal at δ -11.1 ppm at 25 °C. The variable-temperature

<footnote>(33) Herrmann, W. A.; Weichmann, J. *J. Organomet. Chem.* **1982**, *231*, C69.</footnote>

<footnote>(34) Brookhart, M. S.; Chandler, W. A.; White, P. S.; Santini, C. C. Unpublished results.</footnote>

<footnote>(35) For examples of Fe(H₂)L₄ see: (a) Guggenberger, L. J.; Tutis, D. D.; Flood, M. T.; Marsh, R. E.; Orio, A. A.; Gray, H. B. *J. Am. Chem. Soc.* **1972**, *94*, 1135. (b) Meakin, P.; Muetterties, E. L.; Tebbe, F. N.; Jesson, J. P. *J. Am. Chem. Soc.* **1971**, *93*, 4701. (c) Meakin, P.; Muetterties, E. L.; Jesson, J. P. *J. Am. Chem. Soc.* **1973**, *95*, 75. For examples of Fe(H₂)(CO)₂L₂ see: (d) Berke, H.; Bankhardt, W.; Huttner, G.; Seyerl, J. V.; Zsolnai, L. *Chem. Ber.* **1981**, *114*, 2754. (e) Schubert, U.; Knorr, M. *Inorg. Chem.* **1989**, *28*, 1765. For a discussion on Fe(H₂)(CO)₄ see ref 36 and: (f) Shriver, D. F.; Witmire, K. H. In *Comprehensive Organometallic Chemistry*; Wilkinson, G., Ed.; Pergamon Press: Oxford, U.K., 1982; Vol. 4, Chapter 31.1.7. (g) Ginsberg, A. P. *Transition Met. Chem.* **1965**, *1*, 111. (H) Green, M. L. H.; Jones, D. J. *Adv. Inorg. Chem. Radiochem.* **1965**, *7*, 115. For a discussion of spectroscopic methods for differentiation between H₂ ↔ (H₂) complexes see: (i) Earl, K. A.; Jia, G.; Maltby, P. A.; Morris, R. H. *J. Am. Chem. Soc.* **1991**, *113*, 3027. (j) Crabtree, R. H. *Acc. Chem. Res.* **1990**, *23*, 95. (k) Kubas, G. J.; Unkefer, C. J.; Swanson, B. I.; Fukushima, E. *J. Am. Chem. Soc.* **1986**, *108*, 7000.</footnote>

<footnote>(36) (a) Hieber, W.; Leutert, F. *Nuturwissenschaften* **1931**, *19*, 360. (b) Hieber, W.; Leutert, F. *Z. Anorg. Allg. Chem.* **1937**, *232*, 29. (c) Hieber, W.; Leutert, F. *Z. Anorg. Allg. Chem.* **1932**, *204*, 145. (d) Hieber, W.; Vetter, H. *Z. Anorg. Allg. Chem.* **1933**, *212*, 145. (e) Brauer, G. In *Handbook of Preparative Inorganic Chemistry*, 2nd ed.; Academic Press: New York, 1965; Vol. 2, p 1752. (f) McNeill, E. A.; Scholer, F. R. *J. Am. Chem. Soc.* **1977**, *99*, 6243. (g) Hamilton, D. G.; Crabtree, R. H. *J. Am. Chem. Soc.* **1988**, *110*, 4126.</footnote>

<footnote>(37) (a) Chinn, M. S.; Heinekey, D. M. *J. Am. Chem. Soc.* **1987**, *109*, 5865. (b) Kubas, G. J. *Acc. Chem. Res.* **1988**, *21*, 120. (c) Bianchini, C.; Mealli, C.; Meli, A.; Peruzzi, M.; Zanobini, F. *J. Am. Chem. Soc.* **1988**, *110*, 8725.</footnote>
Scheme I

<figure><img src="image_16.png" /></figure>

behavior is similar to that of the dihydride but shows a lower $T_c$ (-50 °C) due to the smaller $v_{D_2} - v_{D_3}$. The rate of site exchange at $T_c$ is calculated as $4.3 \times 10^2$ s⁻¹, $ΔG^* = 10.2 \pm 0.2$ kcal/mol. The data are not sufficiently accurate to estimate $k_H/k_D$.

The stereochemical nonrigidity of complex 17 is also exhibited in the variable-temperature ³¹P NMR spectrum. At -35 °C two signals are observed at δ 143 and 133 ppm, which broaden and coalesce at -16 °C. The rate of site exchange at $T_c$ (-16 °C) is calculated as $4 \times 10^3$ s⁻¹, $ΔG^* = 10.7 \pm 0.3$ kcal/mol. In contrast to the tricarbonyl complex 2, the dfepe ligand is quite labile in complex 17. Treatment of 17 with excess trimethyl phosphite in hexanes results in the rapid substitution of the chelating dfepe ligand by 2 molar equiv of trimethyl phosphite to give the known complex [(CH₃O)₃P]₂Fe(CO)₂(H)₂³⁸ (19) in 78% yield. NMR and IR data are consistent with a cis stereochemistry for the H and CO ligands and a trans arrangement for the phosphite ligands. The rate of the substitution reaction under pseudo-first-order conditions (8-fold excess of P(OMe)₃) was found to be $7 \times 10^{-3}$ s⁻¹ at -21 °C ($ΔG^* = 17.2$ kcal/mol).

**Reaction with (C₂H₅)₃SiH.** In an attempt to carry out oxidative addition reactions between (dfepe)Fe(CO)₂ and silanes, complex 2 was photolyzed in the presence of triethylsilane in a variety of solvents (see Experimental Section). Two major products were isolated, the dihydride complex, 17, and hexaethyldisiloxane, (Et₃Si)₂O. Even when solvents and glassware were carefully dried, disiloxane was still observed. When this reaction was monitored by GC, triethylsilanol, Et₃SiOH, could be detected as an intermediate. We believe formation of 17 and (Et₃Si)₂O likely involves the reaction sequence shown in Scheme I, in which the silyl hydride, 20, is initially formed but rapidly undergoes the sequence of hydrolysis steps shown. The fact that traces of adventitious water are sufficient to promote formation of 17 suggests that the silyl hydride is extremely reactive toward water. This is likely due to the high stability of the anion, [(dfepe)(CO)₂FeH]⁻,³⁹ which would be the initial product of attack of either water or Et₃SiOH on 20. In an attempt to observe the proposed (dfepe)(CO)₂Fe(H)(SiR₃) intermediate, the reaction was monitored by ¹H NMR and IR spectroscopy. No intermediate was observed in the case of triethylsilane, but when diphenylsilane was used, several new features were observed. The ¹H NMR spectrum exhibited new resonances at δ 2.74 (PCH₂CH₂P) and -11.0 (Fe-H) ppm, while the IR spectrum showed new CO-stretching frequencies at 2041 and 2004 cm⁻¹. These new signals are consistent with an iron silyl hydride,⁴⁰ (dfepe)(CO)₂Fe(H)(SiHPh₂),

but attempts to isolate this species gave only 17.

There is substantial precedent for nucleophilic attack at Si and cleavage of Fe-Si bonds in related complexes. For example, Parish⁴¹ has demonstrated that Fe(CH₂=CH₂)(dppe)₂ catalyzes the coupling of primary alcohols, ROH (R = Me, Et, nPr, nBu), with SiH(OEt)₃ or SiH₂Ph₂, to produce ultimately Fe(H)₂(dppe)₂, Si(OEt)ₙ(OR)₄-n, and SiPh₂(OR)₂, respectively. A mechanism for the catalytic process involving the formation of a (dppe)₂Fe(H)(SiR₃) intermediate followed by nucleophilic attack by alcohol was proposed. It is noteworthy that while Fe(CH₂=CH₂)(dppe)₂ is inactive toward triorganosilanes such as SiH(Et)₃ or SiH(Ph)₃, 2 readily reacts under photolytic conditions with either substrate. Similarly, Crabtree⁴² has proposed a nucleophilic attack by alcohol on an [IrH₂-(MeOH)(η²-HSiEt₃)(PPh₃)₂]⁺ intermediate in the catalytic alcoholysis of hydrosilanes utilizing [IrH₂S₂(PPh₃)₂]SbF₆ (S = solvent) as the catalyst. Closely related to the present work is the observation by Schubert⁴³ that treatment of cis-Fe(CO)₄(SiMe₃)₂ with PPh₃ yields (Me₃Si)₂O, Fe(CO)₃(PPh₃)(H)(SiMe₃), and other products.

**Attempted Oxidative Addition of Alkanes to (dfepe)Fe(CO)₂.** Highly electron-rich Fe(0) complexes of the type L₄Fe (L = phosphine) have previously been observed to undergo oxidative addition reactions with various types of C-H bonds. For example, Ittel and Tolman⁴⁴ have demonstrated that upon thermal elimination of naphthalene from HFeNp(dmpe)₂ (Np = 2-naphthyl) the coordinatively unsaturated Fe(dmpe)₂ intermediate undergoes oxidative addition reactions with aryl C-H bonds and compounds containing activated sp³ C-H bonds, such as CH₃COCH₃, CH₃CH₂CN, and CH₃SOCH₃. Field⁴⁵ further showed that Fe(dmpe)₂, generated by photolysis of Fe(H)₂(dmpe)₂ at -80 °C, activates the sp³ C-H bonds of pentane and cyclopentane. (The corresponding iron alkyl hydrides, Fe(H)(R)(dmpe)₂, are unstable above -30 °C and β-eliminate to form Fe(H)₂-(dmpe)₂ and olefins.)

Photolysis of 2 in alkane solvents was carried out in an effort to detect either alkane complexes⁴⁶ of (dfepe)Fe(CO)₂

<footnote>(40) (a) Hill, R. H.; Wrighton, M. S. *Organometallics* **1987**, *6*, 632. (b) Bellachioma, G.; Cardaci, G. *Inorg. Chem.* **1982**, *21*, 3232. (c) Knorr, M.; Muller, J.; Schubert, U. *Chem. Ber.* **1987**, *120*, 879. (d) Knorr, M.; Schubert, U. *J. Organomet. Chem.* **1989**, *365*, 151.</footnote>

<footnote>(41) Haszeldine, R. N.; Parish, R. V.; Riley, B. F. *J. Chem. Soc., Dalton Trans.* **1980**, 705.</footnote>

<footnote>(42) Luo, X.-L.; Crabtree, R. H. *J. Am. Chem. Soc.* **1989**, *111*, 2527.</footnote>

<footnote>(43) Schubert, U.; Knorr, M.; Strasser, C. *J. Organomet. Chem.* **1991**, *411*, 75.</footnote>

<footnote>(44) (a) Ittel, S. D.; Tolman, C. A.; English, A. E.; Jesson, J. P. *J. Am. Chem. Soc.* **1978**, *100*, 7577. (b) Tolman, C. A.; Ittel, S. D.; English, A. D.; Jesson, J. P. *J. Am. Chem. Soc.* **1979**, *101*, 1742. (c) Tolman, C. A.; Ittel, S. D.; English, A. D.; Jesson, J. P. *J. Am. Chem. Soc.* **1978**, *100*, 4080.</footnote>

<footnote>(45) (a) Baker, M. V.; Field, L. D. *J. Am. Chem. Soc.* **1987**, *109*, 2825. (b) Baker, M. V.; Field, L. D. *J. Am. Chem. Soc.* **1986**, *108*, 7436. (c) Baker, M. V.; Field, L. D. *J. Am. Chem. Soc.* **1986**, *108*, 7433. (d) Baker, M. V.; Field, L. D. *Organometallics* **1986**, *5*, 821.</footnote>

<footnote>(46) Dobson, G. R.; Hodge, P. M.; Healy, M. A.; Poliakoff, M.; Turner, J. J.; Firth, S.; Asali, K. *J. Am. Chem. Soc.* **1987**, *109*, 4218.</footnote>

<footnote>(38) Berke, H.; Huttner, G.; Zsolnai, L. *Z Naturforsch.* **1981**, *36B*, 938.
(39) The [(dfepe)(CO)₂Fe(H)][PPN] complex has been independently synthesized by reacting (dfepe)(CO)₂Fe(H)₂ with KH in Et₂O. IR (Et₂O): ν 1947, 1885 cm⁻¹. ¹H NMR (THF, 21 °C): δ 7.6 (m, PPN cation, 30 H), 2.1 (m, PCH₂CH₂P, 4 H), -8.9 (t, Fe-H, ²JₚH = 20 Hz, 1 H). ¹³C NMR: δ 219.6 (s, CO), 120 (m, CF₂CF₃), 23.75 (m, PCH₂CH₂P).</footnote>
<figure><img src="image_17.png" /><figcaption>Scheme II</figcaption></figure>

or oxidative addition adducts (dfepe)(CO)₂Fe(H)(R) and to contrast the behavior of the highly electrophilic unsaturated species (CO)₂Fe(dfepe) with the electron-rich (dmpe)₂Fe system. Irradiation in hexane at 25 and -30 °C resulted in no IR-observable products and only a very slow decrease in intensity of the νCO bands due to 2. Additionally, a solution of complex 2 in liquid propane at -50 °C was photolyzed in an NMR tube while being purged with argon. After photolysis, propane was removed under vacuum at -50 °C and CD₂Cl₂ was condensed into the tube at -50 °C. ¹H NMR analysis of the solution at -78 °C showed no new resonances and indicated only the presence of 2. The efficient photosubstitution of CO in 2 by other ligands such as P(OMe)₃ (see above) clearly indicates that 2 must readily lose CO under these conditions to form (dfepe)(CO)₂Fe. In alkane solutions, rapid recapture of CO must occur before CO can be removed from solution by the argon purge. Similar results have been observed for (dfepe)Cr(CO)₄.⁴⁷ The nature of the intermediate (or intermediates) formed in alkane solution is unknown. The possibilities include unsolvated (dfepe)Fe(CO)₂, an alkane complex (dfepe)(CO)₂Fe(RH) (21), or the oxidative addition adduct (dfepe)(CO)₂Fe(H)(R) (22) (Scheme II). This reaction is currently being examined by laser transient absorption spectroscopy in an effort to address this question.

One feature of significance is the comparison of the photochemistry of 2 with that of the parent carbonyl complex Fe(CO)₅. Whereas photolysis of Fe(CO)₅ in inert solvents leads to rapid cluster formation to yield Fe₂(CO)₉,⁴⁸ the bulky dfepe ligand in 2 must prevent such clustering reactions and thus only CO capture occurs and the labile dicarbonyl intermediate(s) returns to 2.

## Experimental Section

**General Methods.** All reactions were conducted under a dry and oxygen-free argon atmosphere (unless otherwise noted) using Schlenk techniques with a double-manifold vacuum line. Argon and nitrogen gases were purified by passage through a column of BASF catalyst (R3-11) heated to 130 °C followed by activated molecular sieves. Methylene chloride was distilled in an N₂ atmosphere from phosphorus pentoxide prior to use. THF, benzene, toluene, and diethyl ether were distilled in an N₂ atmosphere from sodium and benzophenone prior to use. Solvents for column chromatography were degassed by purging with argon gas for 30 min. Deuterated solvents were used as received. Hexaethyldisilane,⁴⁹ (benzylideneacetone)iron tricarbonyl,²⁷,⁶, and 1,2-bis(dichlorophosphino)ethane⁵⁰ were prepared as previously described. Glassclad 6C was purchased from Petrarch.

¹H and ¹³C NMR spectra were recorded on Varian XL400 and Bruker AC200 spectrometers in deuterated solvents. When ¹³C data are reported, the following conventions were used: {} indicate proton-decoupled data, () indicate proton-coupled data. ³¹P, ¹⁹F, and ²H NMR spectra were recorded on a Varian XL400 spectrometer.

Chemical shifts were reported by reference to residues of monoprotio solvents (CHDCl₂, δ 5.32 ppm; C₆H₅D₅, δ 7.15 ppm; THF-d₇, δ 3.58 ppm). ³¹P and ¹⁹F were referenced externally to 85% H₃PO₄ (δ 0.00 ppm) and C₆H₅CF₃ (δ 0.00 ppm with upfield chemical shifts taken to be negative). Infrared spectroscopy was conducted in solution on a Mattson Polaris FTIR spectrometer using ICON software. GC experiments were conducted on Hewlett-Packard 5890. The columns used were either a 20-ft × 1/4-in. column packed with 20% QF-1 on Chromasorb W in conjunction with a thermal detector or an HP ultra 1 (cross-linked methylsilicone gum) capillary column 25-m × 0.2-mm × 0.11-mm film thickness in conjunction with a flame ionization detector. Preparative photolyses were carried out using a reflector flood lamp GE No. HR100PFL44. GC/MS was performed by the UNC-Mass Spectrometry Facility using a VG 70-250 SEQ tandem-hybrid MS/MS system operated in the MS1 (first mass spectrometer only) mode, with data acquisition utilizing a 70-250J data system (DEC-Micro-PDP-11 computer). Elemental analyses were performed by either Galbraith, Inc., Knoxville, TN, or ORS, Inc., Whitesboro, NY.

**Synthesis of (C₂F₅)₂PCH₂CH₂P(C₂F₅)₂ (1).** Bis(bis(pentafluoroethyl)phosphino)ethane was prepared using a modified literature procedure.⁵ A 50-mL aliquot of 2.5 M nBuLi (125 mmol) in hexanes was added to a 500-mL flame-dried Schlenk flask equipped with a mechanical stirrer and a closed-system vacuum adapter (Fischer). The hexanes were removed in vacuo, and 150 mL of freshly distilled anhydrous diethyl ether was added under nitrogen to the butyllithium. Approximately 35.0 g of C₂F₅Cl (bp -34 °C, density ca. 1.88 g/mL, 195 mmol) was condensed into a 50-mL oven-dried Schlenk flask at -78 °C. The butyllithium solution in ether was cooled to -95 °C and the C₂F₅Cl was added very slowly by cannula under nitrogen. (Caution: The addition of C₂F₅Cl must be performed slowly. Rapid addition of C₂F₅Cl can result in a rapid pressure buildup.) The reaction mixture was maintained at ca. -95 °C for 1 h. A room-temperature solution of Cl₂PCH₂CH₂PCl₂ (5.75 g, 3.7 mL, 24.8 mmol) in 25 mL of diethyl ether was added slowly by cannula while the reaction mixture was maintained below -90 °C. The mixture was allowed to warm slowly to room temperature (approximately 4-5 h). As the solution temperature increased, the brown solution became dark brown at -70 °C and an extremely viscous white mass formed at 0 °C. The solution was stirred at room temperature for 1 h, after which slightly acidic degassed water (5-10 mL) was added by cannula to the reaction vessel. The mixture was shaken, and the aqueous layer was separated from the ether layer. The organic layer was dried over magnesium sulfate for 1 h and then filtered thru Celite. The diethyl ether was distilled at atmospheric pressure followed by butyl chloride at 67-70 °C. The yellow oil that remained yielded 5-6 g (36-43%) of bis(bis(pentafluoroethyl)phosphino)ethane that was isolated as a clear, colorless liquid upon distillation (55-60 °C/0.14 mmHg, density ca. 1.64 g/mL). ¹H, ¹³C, ³¹P, and ¹⁹F NMR data are similar to those previously reported.⁵

**Synthesis of (CO)₃Fe[(C₂F₅)₂PCH₂CH₂P(C₂F₅)₂] (2).** To an oven-dried 200-mL flask equipped with a nitrogen inlet tube, a magnetic stirring bar, and a condenser was added 0.87 g (3.1 mmol) of (benzylideneacetone)iron tricarbonyl, 0.9 mL (3 mmol) of bis(bis(pentafluoroethyl)phosphino)ethane, and 100 mL of toluene. The reaction mixture was heated at reflux for approximately 2 h and monitored by IR spectroscopy. At the completion of the reaction, as indicated by the disappearance of the 2068, 2008, and 1989 cm⁻¹ bands and the appearance of bands at 2055, 2004, and 1991 cm⁻¹, the toluene was evaporated under reduced pressure. The product was purified by chromatography on a silica column using hexanes. Following the removal of hexanes under reduced pressure overnight, 1.26 g (59%) of [bis(bis(pentafluoroethyl)phosphino)ethane]iron tricarbonyl was obtained as

<footnote>(47) Brookhart, M. S.; Chandler, W. A.; Kessler, R. J.; Liu, Y.; Pienta, N. J.; Santini, C. C.; Hall, C.; Perutz, R. N.; Timney, J. A. Submitted for publication in *J. Am. Chem. Soc.*</footnote>

<footnote>(48) Braye, E. H.; Hubel, W. In *Inorganic Syntheses*; Holtzlaw, F. H., Ed.; McGraw-Hill: New York, 1966; Vol. 8, p. 178.</footnote>

<footnote>(49) Boudjouk, P.; Han, B. H. *Tetrahedron Lett.* **1981**, *22*, 3814.</footnote>

<footnote>(50) Henderson, R. A.; Hussain, W.; Leigh, G. J.; Normanton, F. B. In *Inorganic Syntheses*; Kirschner, S., Ed.; McGraw-Hill: New York, 1985; Vol. 23, p. 141.</footnote>
a yellow oil. IR (pentane): ν 2055, 2004, 1919 cm⁻¹. UV (hexane): λ<sub>max</sub> = 236 and 294 nm. Anal. For C₁₅H₄F₂O₂P₂O₃Fe [found (calcd)]: C, 22.23 (22.19); H, 0.63 (0.57); Fe, 7.37 (7.91). ¹H NMR (CD₂Cl₂, 21 °C): δ 2.6 (m, PCH₂CH₂P). ¹³C NMR: δ 22.6 [[t, ¹JPC ~ ²JPC ~ 21 Hz], PCH₂CH₂P], 120 [[m], CF₂, CF₃], 211.2 [[br s], CO]. ³¹P NMR: δ 140 (quintet, PCF₂CF₃). ¹⁹F NMR: δ -13.3 (s, CF₃), -46.2 (center of the AB part of an ABX system, ²J<sub>F<sub>a</sub>F<sub>b</sub></sub> = 314 Hz, ²J<sub>PF<sub>a</sub></sub> = 65 Hz, ²J<sub>PF<sub>b</sub></sub> = 61 Hz, CF₂).

**Synthesis of (CO)₂Fe[(C₂F₅)₂PCH₂CH₂P(C₂F₅)₂](P(OC-H₃)₃) (4).** A solution of 0.4 g (0.6 mmol) of [bis(bis(pentafluoroethyl)phosphino)ethane]iron tricarbonyl, diethyl ether (30 mL), and a 0.07 mL (0.6 mmol) of trimethyl phosphite was photolyzed at room temperature under argon. The reaction was monitored by IR spectroscopy and judged to be complete as indicated by the disappearance of the bands at 2052, 1999, and 1988 cm⁻¹ and the growth of the 1988- and 1937-cm⁻¹ bands. The resulting yellow solution was evaporated under reduced pressure and the residue purified by silica column chromatography using a 1:9 dichloromethane:pentane eluent. The solvent was evaporated under reduced pressure to yield 354 mg (79%) of [bis(bis(pentafluoroethyl)phosphino)ethane](trimethyl phosphite)iron dicarbonyl as a yellow crystalline material (mp 70-71 °C). IR (pentane): ν 1990, 1940 cm⁻¹. Anal. For C₁₅H₁₃F₂O₃P₃O₅Fe [found (calcd)]: C, 22.89 (22.46); H, 1.62 (1.63); Fe, 6.82 (6.96). ¹H NMR (CD₂Cl₂, 21 °C): δ 2.4 (m, PCH₂CH₂P, 4 H), 3.8 (d, ³JₚH = 12 Hz, P(OCH₃)₃, 9 H). ¹³C NMR: δ 22.7 [[t, ¹JPC ~ ²JPC ~ 21 Hz], PCH₂CH₂P], 54.1 [[s], OCH₃], 120 [[m], CF₂, CF₃], 212.1 [[d, ²JₚC = 50 Hz], CO]. ³¹P NMR: δ 133 (q, ²JₚP = 59 Hz, C₂F₅P), 173 (t, ²JₚP = 8 Hz, P(OMe₃)). ¹⁹F NMR: δ -13.6 (s, CF₃), -45.2 (center of the AB part of an ABX system, ²JₚFₐF₃ = 315 Hz, ²JₚFₐ = 55 Hz, ²JₚF₃ = 66 Hz, CF₂).

**Synthesis of (CO)₂Fe[(C₂F₅)₂PCH₂CH₂P(C₂F₅)₂](η²-CH₂CH₂) (5).** [Bis(bis(pentafluoroethyl)phosphino)ethane]iron tricarbonyl (0.2 g, 0.3 mmol) was dissolved in 25 mL of diethyl ether, and the solution was cooled to 0 °C. The solution was purged with ethylene prior to the beginning of photolysis, and a continuous purge was maintained during photolysis. After ca. 2 h, the solution had become darker yellow and the CO bands of the starting material at 2052, 1999, and 1986 cm⁻¹ had disappeared and two new CO bands at 2020 and 1979 cm⁻¹ had appeared. The solvent was evaporated, and the residue was purified by chromatography on a silica column using hexanes. The first yellow band was collected to yield 0.12 g (60%) of [bis(bis(pentafluoroethyl)phosphino)ethane](η²-ethylene)iron dicarbonyl as a yellow oil after solvent removal. The oil was recrystallized from 2-methylbutane at -30 °C to obtain X-ray-quality single crystals. IR (Et₂O): ν 2020, 1979 cm⁻¹. IR (CH₂Cl₂): ν 2018, 1974 cm⁻¹. Anal. For C₁₄H₈F₂O₂P₂O₄Fe [found (calcd)]: C, 23.78 (23.81); H, 1.42 (1.13). ¹H NMR (C₆D₆, 21 °C): δ 1.7 (m, PCH₂CH₂P, 4 H), 2.5 (t, CH₂=CH₂, ³JₚH = 3 Hz, 4 H). ¹³C NMR: δ 21.3 [[t, ¹JPC ~ ²JₚC ~ 22 Hz], (t, ¹JₚH = 140 Hz), PCH₂CH₂P], 35.6 [[s], (t, JₚH = 160 Hz), CH₂=CH₂], 120 [[m], CF₂, CF₃], 213 [[s], CO]. ³¹P NMR: δ 124.3 (m, C₂F₅P). ¹⁹F NMR: δ -18.4 (s, CF₃), -49.2 (d, JₚF₃ = 67 Hz, CF₂). ¹H NMR (CD₂Cl₂, 21 °C): δ 2.45 (m, PCH₂CH₂P, 4 H), 2.6 (t, CH₂=CH₂, ³JₚH = 4 Hz, 4 H). ¹³C NMR: δ 21.7 [[t, ¹JₚC ~ ²JₚC ~ 22 Hz], PCH₂CH₂P], 35.6 [[s], CH₂=CH₂], 120 [[m], CF₂, CF₃], 214 [[s], CO].

**Synthesis of (CO)₂Fe[(C₂F₅)₂PCH₂CH₂P(C₂F₅)₂](η²-CH₂CHCH₃) (6).** The same procedure used in the synthesis of [bis(bis(pentafluoroethyl)phosphino)ethane](η²-ethylene)iron dicarbonyl was employed for the synthesis of the propylene complex. A 0.2-g (0.3 mmol) sample of [bis(bis(pentafluoroethyl)phosphino)ethane]iron tricarbonyl was dissolved in 25 mL of diethyl ether, and the solution was cooled to 0 °C. Propylene was bubbled through the solution prior to the beginning of photolysis. The solution was photolyzed maintaining a vigorous propylene purge, and the reaction was monitored by IR spectroscopy. After ca. 2 h, the solution had become darker yellow and the CO bands of the starting material at 2052, 1999, and 1986 cm⁻¹ had disappeared and two new CO bands at 2013 and 1974 cm⁻¹ had appeared. The solvent was evaporated, and the residue was purified by chromatography on a silica column using hexanes. The first yellow band was collected to yield [bis(bis(pentafluoroethyl)phosphino)ethane](η²-propylene)iron dicarbonyl as a yellow oil after solvent removal. The propylene complex is thermally unstable and decomposed in CD₂Cl₂ after ca. 7 h at 25

°C or after ca. 12 h as the neat oil. IR (Et₂O): ν 2013, 1974 cm⁻¹. IR (hexane) ν 2017, 1978 cm⁻¹. ¹H NMR (CD₂Cl₂, 21 °C): δ 1.7 (s, CH₃, 3 H), 1.9-3.2 (overlapping bands due to CH₂ propene and PCH₂CH₂P, 6 H), 4.0 (br, CH, 1 H). ¹³C NMR: δ 21.5 [[t, ¹JPC ~ ²JₚC ~ 22 Hz], (t, ¹JₚH = 136 Hz), PCH₂CH₂P], 23.4 [[s], (q, ¹JₚH = 129 Hz), CH₃], 41.9 [[s], (t, ¹JₚH = 154 Hz), CH₂], 55.9 [[s], (d, ¹JₚH = 154 Hz), CH], 120 [[m], CF₂, CF₃], 214 [[s], CO]. ³¹P NMR: δ 124.3 (m, C₂F₅P).

**Synthesis of (CO)Fe[(C₂F₅)₂PCH₂CH₂P(C₂F₅)₂](η⁴-cyclopentadienone) (8).** Complex 2 (0.2 g, 0.3 mmol) was dissolved in 25 mL of diethyl ether, and the solution was cooled to 0 °C. Acetylene was bubbled through the solution prior to the beginning of photolysis. The solution was photolyzed maintaining a vigorous acetylene purge, and the reaction was monitored by IR spectroscopy. After ca. 2 h, the solution had become darker brown, and the CO bands of the starting material at 2052, 1999, 1986 cm⁻¹ had disappeared. The solvent was evaporated and the residue dissolved in methylene chloride. The methylene chloride solution was filtered thru Celite, hexanes were added, and the resulting solution was cooled to -30 °C to yield 0.1 g (20%) of [bis(bis(pentafluoroethyl)phosphino)ethane](η⁴-2,4-cyclopentadienone)iron monocarbonyl as pale yellow crystals. IR (CH₂Cl₂): ν 2016, 1646 cm⁻¹. Anal. For C₁₆H₈F₂O₂P₂O₄Fe [found (calcd)]: C, 26.59 (26.31); H, 1.14 (1.09). ¹H NMR (CD₂Cl₂, 21 °C): δ 2.5 (m, PCH₂CH₂P, 4 H), 4.1 (br, CH, 2 H), 5.33 (br, small coupling of 4 Hz observed, 2 H). ¹³C NMR: δ 22.9 [[t, ¹JₚC ~ ²JₚC ~ 19 Hz], PCH₂CH₂P], 67.2 [[s], CH], 81.1 [[s], CH], 119 [[m], CF₂, CF₃], 172.6 [[s], CO(organic)], 212.7 [[s], CO(metal)].

**Synthesis of (CO)Fe[(C₂F₅)₂PCH₂CH₂P(C₂F₅)₂](η⁴-CH₂CHCHCH₂) (9).** Complex 2 (0.2 g, 0.3 mmol) was dissolved in 25 mL of diethyl ether, and the solution was cooled to -30 °C. 1,3-Butadiene (ca. 5 mL) was condensed into the flask and the resulting solution purged with a flow of argon. The solution was photolyzed, adding more 1,3-butadiene as needed, and the reaction was monitored by IR spectroscopy. After ca. 1.5 h, the solution had become darker yellow and the CO bands of the starting material at 2052, 1999, and 1986 cm⁻¹ had disappeared and one new CO band at 1996 cm⁻¹ had appeared. The solvent was evaporated, and the residue was purified by chromatography on a silica column using hexanes. The first yellow band was collected and recrystallized using hexanes at -30 °C to yield 0.07 g (30%) of [bis(bis(pentafluoroethyl)phosphino)ethane](η⁴-butadiene)iron monocarbonyl as yellow crystals. IR (Et₂O): ν 1996 cm⁻¹. Anal. For C₁₅H₁₀F₂O₄P₂O₄Fe [found (calcd)]: C, 25.79 (25.59); H, 1.47 (1.43). ¹H NMR (C₆D₆, 21 °C): δ 5.1 (br, (H₂, H₂), 2 H), 2.1 (br, (Houter)), 2 H), 1.7 (m, PCH₂CH₂P, 4 H), 0.05 (br, (Hinner)), 2 H). ¹³C NMR: δ 22.2 [[t, ¹JₚC ~ ²JₚC ~ 20 Hz], (t, ¹JₚH = 137 Hz), PCH₂CH₂P], 38.7 [[s], (t, ¹JₚH = 158 Hz), CH₂], 84.8 [[s], (d, ¹JₚH = 166 Hz), CH], 119 [[m], CF₂, CF₃], 217 [[s], CO]. ³¹P NMR: δ 121.9 (m, C₂F₅P).

**Synthesis of (CO)Fe[(C₂F₅)₂PCH₂CH₂P(C₂F₅)₂](η⁴-CH₃COCHCH₂) (10).** Complex 2 (0.25 g, 0.35 mmol) was dissolved in 20 mL of methyl vinyl ketone, and the solution was cooled to 0 °C. The solution was purged with argon prior to the beginning of photolysis. The solution was photolyzed maintaining an argon purge, and the reaction was monitored by IR spectroscopy. After ca. 5 h, the solution had become blood red and the CO bands of the starting material at 2052, 1999, and 1986 cm⁻¹ had disappeared and two new CO bands at 2020 and 1976 cm⁻¹ had appeared. The solvent was evaporated, and the residue was purified by chromatography on a silica column using 2-methylbutane. The first orange band was collected to yield 0.05 g (20%) of [bis(bis(pentafluoroethyl)phosphino)ethane](η⁴-methyl vinyl ketone)iron carbonyl as an orange red oil after solvent removal. The oil was recrystallized using 2-methylbutane at -30 °C. IR (Et₂O): ν 1976 cm⁻¹. IR (CH₂Cl₂): ν 1975 cm⁻¹. Anal. For C₁₅H₁₀F₂O₂P₂O₄Fe [found (calcd)]: C, 24.71 (25.02); H, 1.38 (1.39). ¹H NMR (CD₂Cl₂, 21 °C): δ 5.72 (d of d, JₚH(2)-H(3) = 5.4 Hz, JₚH(1)-H(3) = 9 Hz, H₃), 2.2 (m, PCH₂CH₂P, 1 H), 2.65 (m, PCH₂CH₂P, 1 H), 2.95 (m, PCH₂CH₂P, 2 H), 2.45 (s, CH₃, 3 H), 1.98 (d, JₚH(2)-H(3) = 5.4 Hz, H₂), 1.38 (t, JₚH(1)-H(3) = 9 Hz, JₚH = 7.8 Hz, H₁). ¹³C NMR: δ 21.0 [[s], (q, ¹JₚH = 126 Hz), CH₃], 22.2 [[s], (t, ¹JₚH = 138 Hz), PCH₂CH₂P], 23.4 [[s], (t, ¹JₚH = 127 Hz), PCH₂CH₂P], 36.7 [[s], (t, ¹JₚH = 161), CH₂], 78.5 [[s], (d, ¹JₚH = 168 Hz), CH], 119 [[m], CF₂, CF₃], 147 [[s], CO(organic)], 210 [[br s], CO(metal)].
**Photolysis of (CO)₃Fe[(C₂F₅)₂PCH₂CH₂P(C₂F₅)₂] with Styrene.** Complex 2 (0.3 g, 0.4 mmol) and 10 mL of styrene were dissolved in 150 mL of diethyl ether, and the solution was cooled to 0 °C. The solution was purged with argon prior to the beginning of photolysis. The solution was photolyzed maintaining a vigorous argon purge and the reaction was monitored by IR spectroscopy. After ca. 3 h, the solution had become blood red and the CO bands of the starting material at 2052, 1999, and 1986 cm⁻¹ had disappeared and two new CO bands at 2020 and 1982 cm⁻¹ had appeared. The solvent was evaporated and the residue was purified by chromatography on a silica column using 2-methylbutane. Three complexes were isolated as described below.

**Isolation of (CO)₂Fe[(C₂F₅)₂PCH₂CH₂P(C₂F₅)₂](η⁴-CH₂CHC₆H₅) (13).** The first red band was collected and was a mixture of 13 and 15. The residue was recrystallized at -30 °C in methylene chloride to yield 0.15 g (52%) of [bis(bis(pentafluoroethyl)phosphino)ethane](η⁴-styrene)iron carbonyl as blood red crystals. See the supplementary section for X-ray data. IR (Et₂O): ν 1978 cm⁻¹. Anal. For C₁₉H₁₂F₂O₄P₂OFe [found (calcd)]: C, 30.48 (30.23); H, 1.67 (1.61). ¹H NMR (CD₂Cl₂, 21 °C) (proton assignments were aided by the use of HETCOR): δ 7.5 (d, CH-aromatic), *J*<sub>PH</sub> = 16 Hz, 1 H), 7.1 (br, CH(aromatic), 2 H), 6.9 (br, CH(aromatic), 1 H), 6.3 (br, H₁₄, 1 H), 3.65 (br, H₁₂, 1 H), 3.1 (m, PCH₂CH₂P, 1 H), 2.8 (m, PCH₂CH₂P, 1 H), 2.2 (m, PCH₂CH₂P, 1 H), 1.6 (m, PCH₂CH₂P, 1 H), 1.35 (br, H₁₁(outer), 1 H), -0.3 (m, H₁₁(inner), 1 H). ¹³C NMR: δ 22.4 [[t, ¹J<sub>PC</sub> = 17 Hz], (t, ¹J<sub>CH</sub> = 123 Hz), PCH₂CH₂P], 23.7 [[t, ¹J<sub>PC</sub> = 22 Hz], (t, ¹J<sub>CH</sub> = 138 Hz), PCH₂CH₂P], 35.1 ([s], (t, ¹J<sub>CH</sub> = 154 Hz), CH₂(vinyl)], 67.5 ([s], (d, ¹J<sub>CH</sub> = 162 Hz), CH(vinyl)], 70.8 ([s], (d, ¹J<sub>CH</sub> = 169 Hz), CH(aromatic)), 105.8 [[s], C ipso)], 118 ([m], CF₂, CF₃], 126.8 [[s], (d, ¹J<sub>CH</sub> = 161 Hz), CH(aromatic)], 127.3 ([s], (d, ¹J<sub>CH</sub> = 162 Hz), CH(aromatic)], 129.8 ([s], (d, ¹J<sub>CH</sub> = 158 Hz), CH-aromatic)], 137.1 ([s], (d, ¹J<sub>CH</sub> = 162 Hz), CH(aromatic)], 210.4 [[d, ¹J<sub>PC</sub> = 10 Hz], CO).

**Isolation of (CO)₂Fe[(C₂F₅)₂PCH₂CH₂P(C₂F₅)₂](η²-CH₂CHC₆H₅) (14).** The second band to elute (yellow) was collected and the residue recrystallized at -30 °C in hexane to yield [bis(bis(pentafluoroethyl)phosphino)ethane](η²-styrene)iron dicarbonyl as yellow crystals. IR (hexane): ν 2021, 1983 cm⁻¹. Anal. For C₂₀H₁₂F₂O₄P₂OFe [found (calcd)]: C, 30.32 (30.69); H, 1.56 (1.55). ¹H NMR (CD₂Cl₂, 21 °C): δ 7.2 (br s, CH(aromatic), 5 H), 5.2 (br s, CH(vinyl), 1 H) 1.6–3.3 (m, 5 H), 0.2 (br s, 1 H). ¹³C NMR: δ 21.6 [[br], (t, ¹J<sub>CH</sub> = 108 Hz), PCH₂CH₂P], 32.1 ([s], (t, ¹J<sub>CH</sub> = 153 Hz), CH₂(vinyl)], 57.1 ([s], (d, ¹J<sub>CH</sub> = 159 Hz), CH(vinyl)], 118 ([m], CF₂, CF₃], 126.5 ([s], (d, ¹J<sub>CH</sub> = 158 Hz), CH(aromatic)), 128.7 ([s], (d, ¹J<sub>CH</sub> = 160), CH(aromatic)), 143 ([s], C<sub>ipso</sub>), 213 ([m], CO). ³¹P NMR: δ 125.4 (m, C₂F₅P).

**Isolation of Fe[(C₂F₅)₂PCH₂CH₂P(C₂F₅)₂](η⁶-CH₂CHC₆H₅) (15).** Complexes 13 and 15 coelute off the column. Complex 15 can be conveniently isolated by reacting a diethyl ether solution of 13 and 15 with excess trimethyl phosphite. The ether solution was stirred under Ar and monitored via IR spectroscopy. At the completion of the reaction, as judged by the disappearance of the 1978-cm⁻¹ band and the appearance of a 1934-cm⁻¹ band, the solvent was removed under vacuum and the residue was purified by chromatography on a silica column using 2-methylbutane. The first red fraction was collected to yield a red solid after solvent removal under vacuum. The red solid was recrystallized from hexane at -30 °C to yield ca. 5% [bis(bis(pentafluoroethyl)-phosphino)ethane](η⁶-styrene)iron as red needles. Anal. For C₁₈H₁₂F₂O₄P₂Fe [found (calcd)]: C, 30.24 (29.75); H, 1.86 (1.67). ¹H NMR (CD₂Cl₂, 21 °C): δ 6.4 (m, CH(vinyl), 1 H), 5.8–5.3 (m, CH(aromatic), 5 H, CH₂(vinyl), 1 H) 5.2 (m, CH₂(vinyl), 1 H), 2.1 (m, PCH₂CH₂P, 4 H). ¹³C NMR: δ 21.6 [[t, ¹J<sub>PC</sub> ~ ²J<sub>PC</sub> ~ 24 Hz], (t, ¹J<sub>CH</sub> = 137 Hz), PCH₂CH₂P], 82.3 ([s], (d, ¹J<sub>CH</sub> = 169 Hz), CH(aromatic)), 83.6 ([s], (d, ¹J<sub>CH</sub> = 172 Hz), CH(aromatic)), 86.6 ([s], (d, ¹J<sub>CH</sub> = 170 Hz), CH(aromatic)), 101 ([s], C quaternary aromatic)), 115 ([s], (t, ¹J<sub>CH</sub> = 155), CH₂(vinyl)), 116 ([m], CF₂ CF₃], 135 ([s], (d, ¹J<sub>CH</sub> = 159 Hz), CH(vinyl)). ³¹P NMR: δ 112.4 (m, C₂F₅P).

**Synthesis of Fe[(C₂F₅)₂PCH₂CH₂P(C₂F₅)₂](CO)(P(OC-H₃)₃)₂ (16).** Complex 13 (0.1 g, 0.2 mmol) and excess trimethyl phosphite were stirred under Ar for 1 h. The reaction was monitored by IR spectroscopy and judged to be complete by the disappearance of the 1978-cm⁻¹ band and the appearance of a 1934-cm⁻¹ band. The solvent was removed under vacuum, and

the resulting residue was purified by chromatography on silica using two solvent combinations. Hexanes were first eluted down the column in yield two small colored fractions. The solvent was changed to 40% methylene chloride in hexanes, which eluted a bright yellow band. The yellow solution was evaporated and the residue recrystallized from hexanes at -30 °C to yield 70% (0.08 g, 0.09 mmol) of [bis(bis(pentafluoroethyl)phosphino)ethane]-bis(trimethyl phosphite)iron carbonyl as yellow crystals. IR (Et₂O): ν 1932 cm⁻¹. Anal. For C₁₇H₂₂F₂O₄P₂OFe [found (calcd)]: C, 22.39 (22.74); H, 2.59 (2.47). ¹H NMR (CD₂Cl₂, 21 °C): δ 2.4 (m, PCH₂CH₂P, 4 H), 3.65 (s, POCH₃, 18 H). ¹³C NMR: δ 23.3 [[t, ¹JₚC ~ ²JₚC ~ 19 Hz], PCH₂CH₂P], 52.8 ([s], POCH₃], 117 [[q of q, ²JₚC = 36.6 Hz, ¹JₚC = 313 Hz], CF₃), 121 [[t of t, ¹JₚC = 34 Hz, ¹JₚC = 287 Hz], CF₂], 218 [[t, ²JₚC = 24 Hz], CO]. ³¹P NMR: δ 132.5 (m, C₂F₅P), 174.5 (br, (CH₃O)₃P).

**Synthesis of (CO)₂Fe[(C₂F₅)₂PCH₂CH₂P(C₂F₅)₂](H₂) (17).** A solution of 0.53 g (0.94 mmol) [bis(bis(pentafluoroethyl)-phosphino)ethane]iron tricarbonyl dissolved in 100 mL of diethyl ether was photolyzed at 0 °C during which dihydrogen was purged through the solution. The reaction was monitored via IR spectroscopy and judged to be complete by the disappearance of the 2052-, 1999-, and 1987-cm⁻¹ bands and the growth of the 2058- and 2023-cm⁻¹ bands. The solvent was evaporated under reduced pressure, and the brownish yellow residue was chromatographed on silica under argon using 2-methylbutane. The first yellow band was collected and the solvent removed in vacuo to yield 0.4 g (68%) of [bis(bis(pentafluoroethyl)phosphino)ethane]dihydriodiron dicarbonyl as a pale yellow oil. The product decomposes very slowly under argon at -30 °C. IR (Et₂O): ν 2058 and 2023 cm⁻¹. ¹H NMR (CD₂Cl₂, 21 °C): δ 2.5 (m, PCH₂CH₂P), -11.1 (t, ²JₚH = 31 Hz, 2 H), ¹H NMR (CD₂Cl₂, -84 °C): δ -9.5 (dd, ²Jₚ(1H) = 41 Hz, ²Jₚ(2H) = 63 Hz, H₃), -13 (dd, ²Jₚ(1H) = 35 Hz, ²Jₚ(2H) = 37 Hz, H₄). Dynamic NMR: T_c = -38 °C at 400 MHz. T₁ (-80 °C, CD₂Cl₂) = 275 ms. ¹³C NMR (21 °C): δ 23.0 [[t, ¹JₚC ~ ²JₚC ~ 18 Hz], (t, ¹JₚC = 141 Hz), PCH₂CH₂P], 120 ([m], CF₂, CF₃], 208.2 [[t, ²JₚC ~ 4 Hz], (t, ²JₚC = 9 Hz), 2 CO]. ³¹P NMR (CD₂Cl₂, 21 °C): δ 137.9 (m). ³¹P NMR (CD₂Cl₂, -68 °C): δ 143 (br), 131 (br). Dynamic NMR: T_c = -16 °C at 161 MHz. ¹⁹F NMR: δ -13.3 (s, CF₃), -47 (center of the AB part of an ABX system, ²JₚFₚ = 300 Hz, ²JₚFₐ ~ 45 Hz, ²JₚF₉ = 65 Hz, CF₂).

**Synthesis of (CO)₂Fe[(C₂F₅)₂PCH₂CH₂P(C₂F₅)₂](D₂) (18).** [Bis(bis(pentafluoroethyl)phosphino)ethane]dideuterioiron dicarbonyl was synthesized by photolysis of 2 in THF under a D₂ purge. Workup was the same as for the dihydride complex 17. After solvent evaporation and chromatography, ¹H NMR spectroscopy indicates the presence of the dihydride with an average integration of (methylene:hydride) 10:1. IR (Et₂O): ν 2058 and 2020 cm⁻¹. ²H NMR (CH₂Cl₂, 21 °C): δ -11.14 (m). ²H NMR (CH₂Cl₂, -79 °C): δ -9.6 (m), -12.9 (m). Dynamic NMR: T_c = -54 °C at 61 MHz.

**Photolysis of (CO)₃Fe[(C₂F₅)₂PCH₂CH₂P(C₂F₅)₂] with Et₃SiH.** The following reaction has been carried out using varying amounts of triethylsilane in diethyl ether as well as neat triethylsilane. All conditions gave the same products and results. Complex 2 (0.2 g, 0.3 mmol) was dissolved in 3 mL of triethylsilane and photolyzed under an Ar atmosphere at 0 °C. The reaction was monitored using IR spectroscopy and GC. IR spectroscopy shows the disappearance of the 2052-, 1998-, and 1986-cm⁻¹ bands and appearance of bands at 2057 and 2023 cm⁻¹. The GC conditions were as follows: oven temperature 150 °C, injector temperature 175 °C detector temperature 180 °C, and a flow rate of 50 mL/min (30-ft, packed column). GC shows the growth of a 28.1-min peak, no peaks at higher times. At the completion of the reaction, the crude reaction mixture was separated into two portions. The solvent was evaporated from the first fraction, and the residue was spectroscopically examined by ¹H and ¹³C NMR spectroscopy. The carbonyl-containing species was identified as complex 17 on the basis of comparison to an authentic sample via ¹H and ¹³C NMR and IR spectroscopy. A second product was identified as hexaethyldisiloxane on the basis of comparison to an authentic sample via ¹H and ¹³C NMR analysis. The 28.1-min peak was collected from the second fraction and examined using CI⁺ and EI⁺ GC/MS techniques. The mass spectral data were consistent with hexaethyldisiloxane having a 246 isotopic pattern corresponding to [M]⁺ accompanied by losses of 29 (CH₂CH₃). Hexaethyldisiloxane was singly injected and also coinjected with
a sample of the crude reaction mixture to confirm its identity as the 28.1-min peak. Hexaethyldisilane appears at 34 min and triethylsilane appears at 2.5 min via GC.

Attempts to rigorously exclude water from the photolysis of complex 2 and triethylsilane included treatment of glassware with Glassclad 6C, flame-drying under vacuum, and distillation of triethylsilane from potassium hydride. Under all conditions formation of hexaethyldisiloxane was noted. Triethylsilanol was identified and found to be present during the photolysis of complex 2, and triethylsilane, by capillary GC, retention time 2.31 min, as compared to the retention times of an authentic sample.

**Thermal Reaction of Complex 17 with Trimethyl Phosphite.** Synthesis of Dicarbonylbis(trimethyl phosphite)iron Dihydride (19). Complex 17 (0.2 g, 0.3 mmol) and trimethyl phosphite (0.1 g, 0.9 mmol) were dissolved in hexanes. The reaction was monitored by IR spectroscopy and is essentially complete in less than 10 min (25 °C), as indicated by the disappearance of the carbonyl-stretching frequencies at 17 at 2061 and 2028 cm⁻¹ and the appearance of two new bands at 2016 and 1974 cm⁻¹. The solvent was evaporated under reduced pressure and the brownish yellow residue was chromatographed on silica using two solvent combinations. Hexanes elute a pale yellow band, complex 2. The solvent was changed to 10% ethyl acetate in hexanes, which eluted a bright yellow band. The yellow solution was evaporated and the residue recrystallized from hexanes at -30 °C in yield 78% (0.1 g, 0.2 mmol) of dicarbonylbis(trimethyl phosphite)iron dihydride (19) as yellow crystals. IR, ¹H, and ¹³C NMR data are similar to those previously reported.³⁸

**Kinetics for the Reaction of (CO)₂Fe[(C₂F₅)₂PCH₂CH₂-P(C₂F₅)₂](H₂) (17) and Trimethyl Phosphite.** Concentrations of [bis(bis(pentafluoroethyl)phosphino)ethane]dihydrioiron dicarbonyl at any time were determined from absorbance values at 2056 cm⁻¹ and a Beer's-Lambert plot derived from five standard [bis(bis(pentafluoroethyl)phosphino)ethane]dihydridoiron dicarbonyl solutions in toluene ranging from 0.367 × 10⁻³ to 5.88 × 10⁻³ M (absorbance = -0.007 + 0.034(concentration) with r = 0.99). Pseudo-first-order rate constants were obtained by plotting ln (concentration) vs time.

Kinetic runs were preformed using a Harrick TFC-M25 temperature-controlled flow-through cell in conjunction with an Omega CN310JC solid-state temperature controller and a Neslab ULT-80DD refrigerated circulating bath. A 3-mL B-D multifit glass syringe equipped with a cooling jacket and a small diagonal hole drilled approximately 0.5 in. from the backend of the syringe was connected to the flow-through cell via a B-D luer lock valve. A [bis(bis(pentafluoroethyl)phosphino)ethane]dihydridoiron dicarbonyl in toluene solution was added to the syringe via the

small hole under an Ar atmosphere, and the resulting solution cooled to -21 °C. Excess neat trimethyl phosphite was added to the syringe via the small hole; the solution was quickly shaken to mix, and injected into the IR cell. Absorbances were taken every 20 s for 10 min (t~r~). Two runs were conducted at -21 °C, using 8 equiv of trimethyl phosphite, to give a rate constant of 7 × 10⁻³ s⁻¹ (r = 0.96 and r = 0.94) and ΔG* = 17.2 kcal/mol at -21 °C.

**X-ray Structure Determinations.** Data were collected on an Enraf-Nonius CAD4 diffractometer with graphite-mono-chromated Mo Kα radiation (λ = 0.710 73 Å) using an ω-2θ scan; reflections with I > 2.5σ(I) were considered observed and included in subsequent calculations. The structures were solved by direct methods. Refinement was by full-matrix least squares with weights based on counter statistics. Hydrogen atoms were included in the final cycles of refinement in calculated positions with thermal parameters derived from the atom to which they were bonded. Crystal data and experimental conditions are given in Table II. All computations were performed using the NRCVAX suite of programs.⁵¹,⁵²

**Acknowledgment.** We thank the Gas Research Institute (Contract No. 5087-206-1596) for financial support of this research. C.C.S. acknowledges CNRS for financial support.

**Registry No.** 1, 120263-08-3; 2, 138783-32-1; 3, 138783-33-2; 4, 138783-34-3; 5, 138783-35-4; 6, 138783-36-5; 7, 138783-37-6; 8, 138783-38-7; 9, 138783-39-8; 10, 138783-40-1; 13, 138813-19-1; 14, 138783-41-2; 15, 138783-42-3; 16, 138783-43-4; 17, 138783-44-5; 18, 138813-20-4; 19, 77482-07-6; C₂F₅Cl, 76-15-3; Cl₂PCH₂CH₂PCl₂, 28240-69-9; Et₃SiH, 617-86-7; (dibenzylideneacetone)iron tricarbonyl, 138783-45-6; propylene, 115-07-1; acetylene, 74-86-2; methyl vinyl ketone, 78-94-4; styrene, 100-42-5.

<footnote>(51) Gabe, E. J.; Le Page, Y.; Charland, J.-P.; Lee, F.; White, P. S. J. Appl. Crystallogr. 1989, 22, 384.</footnote>

<footnote>(52) Johnson, C. K. Technical Report No. ORNL-5138; Oak Ridge National Laboratory: Oak Ridge, TN, 1976.</footnote>