1P105
F-CN Bond Cleavage by Silyl Iron Complex. Electronic Effect of the Group or Atom
Attached to Cyano Group. A DFT Theoretical Study(Grad. Sch. Inf. Sci., Nagoya Univ.1, Fac. Sci., Assiut Univ.2)
AbdelRahman A. Dahy1,2, Nobuaki Koga1
1.Introduction:
Cleavage of strong bonds in some ligands will give new species which are important in organic
syntheses. There are a number of experimental and theoretical works focusing on cleavage of these
unreactive bonds, especially E−CN (E = C, N, O) bonds in the presence of iron or molybdenum silyl
complexes in cases of E = N and O by Nakazawa et al.1-4 The idea behind using transition metals is that themetal attaching to the ligand through its special part will affect the electronic properties of the strong bondand that the silyl group will facilitate this cleavage. As a matter of fact, in the case of CH3−CN, a newspecies which contains CN and CH3 ligands are formed1 while in the cases of cyanamides and cyanates,
Et3SiCN is formed, to demonstrate the E–CN bond cleavage.2,3 The N−CN bond is difficult to be broken;because, for example, in the case of H2NCN, the lone pair of electrons on amino N atom strengthens thisbond (because of its tautomerization to HN═C═NH), and it is even stronger than CH3−CN bond.2,4 Nsilylated η2-amidino iron complex was isolated and identified by x-ray crystallography in the case ofcyanamides and its heating lead also to the formation of Et3SiCN.2 Accordingly, Nakazawa el al. suggested
the mechanism shown in Scheme in the cases of cyanamides as well as cyanates. It was found that the low
activation energy for the silyl group migration from Fe to NCN is attributed to the hypervalent character of
Si atom and the charges on the Si and N atoms.
In this theoretical study, continuing to our study for the cleavage of E−CN bond in the presence ofsilyl iron complex; (i) the cleavage of F−CN will be studied according to Scheme to show if they are
similar to those in the cases of E = C, N, and O; and (ii) the effect of the group attached to CN on thereaction mechanism will be studied in detail.
2. Computational methods:
Density functional theory (DFT) calculations were performed to verify the feasibility of the reaction
mechanism suggested in Scheme. The optimized structures of intermediates and transition states and theprofiles for potential energy corrected with zero-point energy (ZPE) and the Gibbs free energy for thestudied mechanism are shown in Figure. Unless explicitly stated otherwise, the ZPE-corrected potentialenergies are used in the discussion.
3. Results and Discussion:
As shown in Figure, searching for a minimum for the complex of FCN with complex 1, leads to theend-on (η1-) complex, 2d. 2d is more stable than 1 and FCN by 16.9 kcal/mol. This is because the strong
coordination of FCN to 1 via the lone pair of electrons on the NCN atom. The end-on complex, 2d,isomerizes to the side-on (η2-) complex, 3d, via the TS2d-3d. The activation energy for this step is 13.2
kcal/mol. The vibrational mode with an imaginary frequency at TS2d-3d is the Fe····CCN bond distancestretching. Accordingly, this bond distance shortens during the reaction from 3.054 Å in 2d to 2.745 Å inTS2d-3d, and the Fe−CCN bond forms in 3d with a bond length of 1.882 Å. The next step is the migration ofthe silyl group from Fe to NCN which transforms 3d to 4d via TS3d-4d. The activation energy of 0.6 kcal/mol
for this step is relatively small. The vibrational mode with an imaginary frequency at TS3d-4d is the N····Sibond distance stretching. Accordingly, during the reaction this bond distance shortens from 2.695 Å in 3dto 2.329 Å in TS3d-4d and then the N−Si bond forms in 4d with a bond distance of 1.746 Å. This small
activation energy is because of a large exothermicity of −32.6 kcal/mol, the hypervalent character of Siatom, and the strong electrostatic interaction between the positively charged Si and negatively charged Natoms as stated previously. The Mülliken charges on the Si and N atoms in 3d are +0.923 and −0.296,respectively. Therefore, the Si atom may be strongly attracted by the N atom to form a strong bond. In thenext step, it is expected that 5d having the Fe−C−F three-membered ring, similar to 5's in the cases ofH2NCN, Me2NCN and MeOCN, is formed. In fact, 5d does not form from 4d but instead 4d is directlytransformed to 6d via TS4d-6d. The activation energy for this step is 15.9 kcal/mol and it is the ratedetermining step. The vibrational mode with an imaginary frequency is the combination of cleavage ofFe−N bond and formation of Fe−F bond. Accordingly, the Fe−N bond is elongated from 2.123 Å in 4d to2.890 Å in TS4d-6d and then to 3.006 Å in 6d. On the other hand, the Fe−F bond shortens from 3.082 Å in

4d to 2.773 Å in TS4d-6d and eventually to 1.854 Å in 6d. This reaction coordinate at TS4d-6d is similar to
those at the TSs for 4→5 with E=C, N, and O, to suggest that neither 5d nor TS5d-6d exists. This is ascribedto the large stability of 6d; it is 23.5 kcal/mol more stable than 4d in contrast to the reactions with E=C, N,and O in which 6a and 6c are more stable than 4a and 4c by only 5.9 and 7.9 kcal/mol, respectively, and 6band 6b’ are less stable than 4b and 4b’ by 5.9 and 10.1 kcal/mol, respectively.
In conclusion, the reaction mechanism with E = F is similar to those in cases of E = C, N, and O. The
activation energies for the rate-determining steps are 28.9, 29.9, 28.0, 19.1, and 15.9 kcal/mol in the casesof ERn = CH3, H2N, Me2N, MeO, and F, respectively. Those for MeO and F are much lower than theothers; while in the rate-determining step with ERn = CH3, H2N, and Me2N the structures of ERn groups
must change drastically, the lone pair of electrons on the O and F atoms can easily interact with the Featom.4 The large exothermicity in the case of ERn = F makes a contribution to the lower activation energy.
Scheme. Proposed Scheme for E-CN bond cleavage in the presence of iron silyl complex.

[Fe]
ERn

ERn

3x2xN
C

[Fe]

CC
ERn

SiMe3

C

[Fe]

[Fe]

N C ERn

NN

N
[Fe]

SiMe3

SiMe3

SiMe3

SiMe3

ERn

4x6x5x[Fe] = CpFe(Co)ERn = CH3 (x=a), NH2 (x=b), NMe2 (x=b'), OMe (x=c), and F (x=d)

830

21.

2d63

TS2d-3d(b)TS2d-3d3.7
3.4]

1.179

2.
441.2312529

TS3d-4dTS3d-4d24.6
15.4]3d25.2
16.7]TS4d-6d41.9
35.9]

4d9
1.76

1.771

1 .9

22. 7

73
356

1.
34

3
1.

2774

1.
22 246
1.48
1.

1.

90
2.8

23
2.1

1.8
430.0
[0.0] 2d
16.9
10.0]

1.260

8
1.85

3d1+ FCN

54
1.8

1.219

2.
3

29
1.3

6

2.4
1266

95

45
2.7
1.1

2. 6

1.894
1.273 1.159

2
1.
90

1.274

1.162
1.10912.0

1 + FCN

1.
93

06
2 .1

1.9
35

227
2.1612.

1.882

H

2.3
55

F

2.

O

34C NFe Si

9(a)

57.8
52.5]6d
81.3
76.2]4dTS4d-6d

6dFigure. The (a) optimized structures and (b) the energy profile for the cleavage of F−CN bond in thepresence of iron silyl complex. All bonds are in Ǻ. All energies are ZPE-corrected and relative toCpFe(CO)SiMe3), 1, and F−CN. Energies in parenthesis are free energies relative to 1 and F−CN.
References:(1) Nakazawa, H.; Kawasaki, T.; Miyoshi, K.; Suresh, C. H.; Koga, N. Organometallics, 2004, 23, 117.
(2) Fukumoto, K.; Oya, T.; Itazaki, M.; Nakazawa, H. J. Am. Chem. Soc., 2009, 131, 38.
(3) Fukumoto, K.; Dahy, A. A.; Oya, T.; Hayasaka, K.; Itazaki, M.; Koga, N.; Nakazawa, H.
Organometallics, 2012, 31, 787.
(4) Dahy, A. A.; Koga, N.; Nakazawa, H. Organometallics, 2012, 31, 3995.