First Observation of Alkali Metal Ion Induced <i>Trans</i>–<i>Cis</i> Isomerization of Palladium(<scp>II</scp>) Phosphane Complexes Containing Crown Ether Moieties (2003)

Ion switches: The addition of small metal ions (such as Li+ and Na+) to trans-palladium(II) complexes of PPh2-substituted benzo[15]crown-5 and benzo[18]crown-6 results in the formation of 1:1 trans adducts. However, the addition of large alkali ions (such as K+, Rb+, and Cs+) enforces the complexes to undergo a trans–cis isomerization to enable formation of intramolecular sandwich adducts (see schematic representation). There has been growing interest in the design of allosteric host molecules that can efficiently transmit the information associated with the binding of an ion or molecule to another binding site through structural changes induced on binding.1, 2 Such allosteric binding behavior not only serves as a means to control the binding or release of ions or molecules, but also provides a new strategy for the design and construction of molecular-switching devices. Although trans–cis thermal and photoisomerization reactions are well-known3, 4 and numerous works on photoinduced ion binding have appeared in the literature,3 there have been no reports of metal-ion-induced or -assisted isomerization reactions brought about by allosteric interactions. In this regard, we were interested in investigating the possibility of using metal-ion complexation and allosteric interactions to induce isomerization processes. Herein, we report the unprecedented trans–cis isomerization of dichloropalladium(II) phosphanylcrown complexes that is induced by the binding of alkali-metal ions, as evidenced by UV/Vis, 1H NMR, and 31P NMR spectroscopy as well as ESI-MS, and represents a novel type of allosteric host that is the first of its kind. The reaction of [PdX2(PhCN)2] with two equivalents of PPh2-benzo[15]crown-5 (PPh2(B15C5)) or PPh2-benzo[18]crown-6 (PPh2(B18C6)) in benzene, by using a modification of literature procedures,5 produced [PdX2{PPh2(B15C5)}2] (X=Cl, 1;6 Br, 2; I, 3) and [PdCl2{PPh2(B18C6)}2] (4).5 The identities of 1–4 have been confirmed by 1H, 13C{1H}, and 31P{1H} NMR spectroscopic analysis, positive-ion FAB mass spectrometry, and satisfactory elemental analyses. The crystal structures of 1 and 2 have also been established as having a trans configuration by X-ray crystallographic analysis.7 Although the complexes exist as trans isomers in CD2Cl2, dissolution of these complexes in mixed solution of CD3CN and CD2Cl2 (9:1 v/v) results in an equilibrium between the cis and trans isomers, with the trans form being the majority species. For example, a solution of 1 in CD3CN/CD2Cl2 (9:1 v/v) shows two singlets in the 31P NMR spectra (one at δ=24.02 ppm and one at δ=33.26 ppm) with an integral ratio of 3.57:1 which are ascribed to the trans and cis isomers of 1, respectively. This assignment is consistent with the similar chemical shifts commonly observed in other related [PdX2L2] systems (L=monodentate phosphane ligand).8 The assignment is further supported by an increase in the intensity of the signal corresponding to the cis isomer at δ=33.26 upon irradiation at λ=256 nm of a solution of 1 at −10 °C. This increase is commonly observed in the photoisomerization of trans-[PdX2L2] systems.9 The electronic absorption spectra of solutions of 1–4 in CH3CN showed high-energy bands at about 270–300 nm which are assigned as intraligand (IL) and metal-perturbed IL transitions of the phosphane ligand. The low-energy absorption bands at about 350–430 nm (with extinction coefficients of the order of 104 dm3 mol−1 cm−1), which are strongly dependent on the nature of the halides (λabs: 1(354 nm)<2(374 nm)<3(426 nm)), were tentatively assigned as ligand-to-metal charge-transfer (pπ(X)→4d(Pd); X=Cl, Br, I) transitions. The addition of alkali-metal ions to a solution of 1 in CH3CN/CH2Cl2 (9:1 v/v, 0.1 mol dm−3 nBu4NPF6) resulted in changes in the UV/Vis spectrum with well-defined isosbestic points (Figure 1). Similar spectral changes were not observed in a control experiment using a crown-free analogue [PdCl2{PPh2{3,4-(OMe)2C6H3}}2] (5), which indicates that these changes can be ascribed to the binding of the cations to the polyether cavity. It is interesting to note that addition of NaClO4 to a solution of 1 (6.4×10−5 mol dm−3) in CH3CN/CH2Cl2 (9:1 v/v, 0.1 mol dm−3 nBu4NPF6) resulted in an increase in the absorbance and a small blue shift in the absorption energy of the low-energy band, which reached saturation at about 2.2×10−3 mol dm−3 (Figure 1 a). However, a decrease in the absorbance and a small red shift of the band were observed upon addition of KPF6, which reached saturation at about 6.6×10−5 mol dm−3 (Figure 1 b). The UV/Vis spectral changes upon addition of other alkali-metal salts such as LiClO4 to a solution of 1 are similar to that of NaClO4, whereas RbClO4 and CsClO4 showed similar changes as KPF6. The inserts in Figure 1 show the titration curves of 1 with NaClO4 and KPF6 together with their theoretical fits to the equation for the formation of a 1:1 adduct.10 Stoichiometry studies indicate that 1 forms 1:1 complexes with both Na+ and K+ ions under the conditions studied. Addition of K+ ions (up to a concentration of 3×10−4 mol dm−3) to 4 gave a similar UV/Vis spectral change as that for 1 upon the addition of Li+ and Na+ ions, whereas addition of Cs+ ions to 4 gave a similar spectral change as that observed for 1 upon addition of K+ ions, that is, a decrease in the absorbance and a small red shift in the low-energy absorption band, which reached saturation at about 7.9×10−5 mol dm−3. UV/Vis spectral changes of 1 upon addition of various concentrations of: a) NaClO4 and b) KPF6 in CH3CN/CH2Cl2 (9:1 v/v, 0.1 mol dm−3 nBu4NPF6). Insert: absorbance at 355 nm (▪) as a function of the concentration of Na+ or K+ ions with theoretical fits. Both 1H and 31P NMR studies were undertaken to rationalize these findings. Coordination of alkali-metal ions to both 1 and 4 was signified by downfield shifts of the crown ether protons. Interestingly, signals corresponding to the protons of cis-1 initially shifted downfield upon titration with NaClO4 at low concentrations, which gradually disappeared altogether upon addition of a large excess of NaClO4 (ca. 500-fold), while addition of even a small amount of KPF6 (<1-fold) resulted in the growth of proton signals corresponding to the cis isomer. A more noticeable change could be observed in the 31P NMR titration experiments of 1 with NaClO4 and KPF6. Figure 2 shows the changes in the 31P NMR spectra of a solution of 1 in CD3CN/CD2Cl2 (9:1 v/v) upon complexation of Na+ (Figure 2 a) and K+ ions (Figure 2 b). Addition of Na+ ions initially gave rise to a slight downfield shift of the 31P NMR signal, which is typical for the binding of a metal ion to the crown cavity. However, the addition of an excess of Na+ ions (ca. 500-fold) resulted in the 31P signal corresponding to the cis isomer at δ=33.26 disappearing, and leaving behind only the signal corresponding to the trans isomer. The addition of K+ ions to 1, in contrast, immediately gave rise to an increase in the population of the cis isomer, with the cis form becoming the predominant species as the K+ concentration increased until saturation was reached at a [K+]:[1] ratio of approximately unity. Similar findings were not observed in the control experiment with the crown-free analogue 5 under the same conditions, which supports the importance of the crown moieties in the specific association of the metal ions. We believe that the addition of K+ ions, which are too large to fit into the cavity size of the benzo[15]crown-5 moiety in 1 resulted in the trans to cis isomerization of the palladium(II) complex, while addition of Na+ ions in large excess would favor the formation of the trans isomer (Scheme 1). In general, benzo[15]crown-5 derivatives tend to form 2:1 (crown ether ring/metal ions) sandwich-type complexes with metal ions that are slightly greater in size than the crown ether cavity, that is, K+, Rb+, and Cs+, with K+ ions giving the best fit. Such intramolecular sandwich or tweezer binding modes in 1 would favor the conversion into the cis configuration, in which the two crown ether rings within the same molecule of 1 would bind to one metal ion. On the other hand, the binding of Li+ and Na+ ions into the cavity of benzo[15]crown-5 from solutions at high ion concentrations would favor the formation of the trans isomer as a consequence of electrostatic interactions arising from the binding of two metal ions to a molecule of 1. Similarly, addition of K+ ions, which could fit into the cavity of benzo[18]crown-6, to a solution of 4 would give rise to the formation of the trans isomer at high concentrations of K+ ions, while Cs+ ions should favor the formation of the cis form for a sandwich-type of binding, even at low concentrations. 31P NMR (162 MHz) spectral changes of 1 (1.0×10−3 mol dm−3) upon addition of a) NaClO4 and b) KPF6 in CD3CN/CD2Cl2 (9:1 v/v) at 298 K, with signals corresponding to the cis (•) and trans (▵) isomers indicated. Schematic representation of the trans–cis isomerization in the presence of different metal ions. The log K values obtained from the UV/Vis spectrophotometric methods at low to moderate concentrations of Na+ and K+ ions, that is, under the conditions for formation of a 1:1 adduct, are 3.88±0.02 for 1 with Na+ and 4.36±0.03 for 4 with K+ ions. The close agreement of the experimental data with the theoretical fits confirms the formation of a 1:1 adduct. However, a conversion of the trans-1 isomer into the cis-1 form occurred in the case of the binding of K+ ions to 1, as indicated by 31P NMR spectroscopic studies (Figure 2 b). The presence of both the trans-1 and cis-1 isomers would require a description involving at least four species in equilibrium11 (Scheme 2), assuming that only 1:1 1⋅M adducts are formed under the conditions studied.12 Detailed treatment of the 31P NMR data according to Scheme 2 gave K1, K2, and K3 values of 0.38±0.01, 1.01×106±25 000 and 2.55×105±6300 M−1, respectively. The close resemblance of the experimental data to the theoretical fits confirmed the complexation stoichiometry of the K+ ions to 1 to be 1:1 and, thus, confirmed the validity of Scheme 2. An overall equilibrium constant log K of 5.59±0.02 was obtained for the process M+trans-1⇄cis-1⋅M, where K=K1K2. This result is in close agreement with a log K value of 5.26±0.1 determined by UV/Vis spectrophotometry. Proposed binding equilibria between 1 and K+ ions. Results from the ESI mass spectrometric measurements provided direct evidence for supporting this hypothesis. In the case of 1, only the {1⋅K}+ adduct was observed, irrespective of the concentration of K+ ions used, while for 4, only {4⋅K}+ was observed at low to moderate concentrations of K+ ions, but both the {4⋅K}+ and {4⋅K2PF6}+ ions were observed in the positive ESI-mass spectra at high concentrations of K+ ions. Other alkali-metal ions were also examined: Only {1⋅M}+ and {4⋅M}+ adducts were observed at low to moderate concentrations of Li+ and Na+ ions, however, at high metal-ion concentrations, a small amount of the 1:2 bound adducts {1⋅M2ClO4}+ and {4⋅M2ClO4}+ adducts were also observed. In the case of Cs+, however, only {1⋅Cs}+ and {4⋅Cs}+ appeared irrespective of the concentration of Cs+ ions used. Similarly, for Rb+ ions, only the {1⋅Rb}+ adduct was observed with 1 while both {4⋅Rb}+ and {4⋅Rb2ClO4}+ ions were found with 4 at high concentrations of Rb+ ions. In addition, 1 was found to be the most selective towards the binding of K+ ions, while a selective preference for the binding of Cs+ ions was observed for 4. A likely explanation for this unprecedented ion-induced or -assisted isomerization reaction is that metal ions which are too large to fit into the size of the crown ether cavity tend to form intramolecular sandwich/tweezer complexes with the bis(crown) unit and this provides the driving force for the isomerization of the trans to the cis isomer. In summary, the present bis(crown)-containing [PdX2L2] system represents the first demonstration of a metal-ion-induced switching of the trans–cis isomerization, which may find interesting applications in the design of ion-controlled switching devices. Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2002/2003/z51178_s.pdf or from the author. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.