Density Functional Theory Study of NâCN and OâCN Bond Cleavage

Article pubs.acs.org/Organometallics

Density Functional Theory Study of N−CN and O−CN Bond Cleavage by an Iron Silyl Complex AbdelRahman A. Dahy,†,‡ Nobuaki Koga,*,‡ and Hiroshi Nakazawa§ †

Department of Chemistry, Faculty of Science, Assiut University, Assiut 71516, Egypt Graduate School of Information Science, Nagoya University, Nagoya 464-8601, Japan § Department of Chemistry, Graduate School of Science, Osaka City University, Sumiyoshi-ku, Osaka 558-8585, Japan ‡

S Supporting Information *

ABSTRACT: Using hybrid density functional theory calculations with the B3LYP functional, the reaction mechanisms for cleavage of R2N−CN (R  H, Me) and MeO−CN bonds in the presence of an unsaturated iron(II) silyl complex, CpFe(CO)SiMe3, were studied. The following sequence of reactions was shown to be favorable: (i) coordination of a nitrile through the lone pair of electrons on the nitrile N atom (NCN) to form an end-on complex, (ii) isomerization of the endon complex to a side-on complex, (iii) migration of the silyl group to NCN facilitated by the hypervalent character of the Si atom and its electrostatic attraction with NCN to form a stable Fe−C−NCN three-membered-ring intermediate with an Fe−NCN dative bond, (iv) dissociation of the NCN atom from Fe and coordination of an amino N atom (NNR2) or methoxy O atom to Fe leading to an Fe−C−NNR2 or Fe−C−O three-membered-ring intermediate, and (v) cleavage of the R2N−C or MeO−C bond to form a silyl isocyanide ligand. Step iv possesses the largest activation energy in the sequence of reactions. The activation energies for the reactions of H2NCN, Me2NCN, and MeOCN were calculated to be 29.9, 28.0, and 19.1 kcal/mol, respectively, on the basis of potential energies with zero-point energy correction. This accounts for the experimental observation that the intermediates formed by silyl group migration can be isolated. The effects of the amino and methoxy groups are discussed by comparing their reaction profiles with that for the reaction of acetonitrile. Localized orbital analysis showed that in the three-membered-ring intermediates formed in step iv, the R2N−C and MeO−C bonds are activated by ring strain, whereas the Me−CN bond is activated by interaction of the Me−C bond with the vacant coordination site that is produced in the dissociation of NCN.

1. INTRODUCTION C−H, C−C, C−N, and C−O bonds are inert, but cleavage of these bonds can provide new species that are useful in organic synthesis. In the last 30 years, a number of experimental and theoretical studies focusing on cleaving C−H,1−3 C−C,4 C− N,5 and C−O6 bonds in the presence of transition-metal complexes have been published. Within C−C bond cleavage, C−CN bonds in organonitriles are more difficult to cleave than C−C bonds in alkanes. Several examples of C−CN bond cleavage have been reported.7−13 We recently demonstrated the reaction of acetonitrile with an iron complex having a silyl ligand affording new complexes that contain CN and Me ligands,7 showing that the C−CN bond was broken during the reactions. We also showed that a silyl ligand is required and iron complexes with germyl or stannyl ligands do not cleave the C−CN bond. Theoretical calculations were performed to clarify the reaction mechanism shown in Scheme 1.7 The reaction was assumed to start with unsaturated complex 1, CpFe(CO)(SiMe3), which is a model of an active intermediate produced photochemically. MeCN coordinates to the Fe atom in this model complex via a lone pair of electrons on the N atom of its nitrile group to form η1-MeCN complex A, which isomerizes to η2-coordinated MeCN complex B. The silyl group then migrates to the nitrile N atom to form η2© 2012 American Chemical Society

Scheme 1. Reaction Pathway for the Proposed C−CN Bond Activation7

iminoacyl intermediate C. In the next step, the C−CN bond is broken to give complex D with methyl and silyl isocyanide ligands. The activation energies with zero-point energy (ZPE) correction (Gibbs free energies of activation are given in parentheses) for the migration and the C−CN bond cleavage were calculated to be 4.0 (4.8) and 15.0 (14.9) kcal/mol, respectively. The low activation energy for CN migratory insertion was attributed to the hypervalent character of the Si atom and the attraction between the positively charged Si atom and negatively charged N atom. The orbital interaction that Received: March 21, 2012 Published: May 8, 2012 3995

dx.doi.org/10.1021/om300232h | Organometallics 2012, 31, 3995−4005

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1 reacts with RnX−CN (RnX = H2N, Me2N, MeO). A Me3Si group was used as a ligand instead of Et3Si in 1 to reduce computation time, which should not affect the results of the calculations. R2N−CN (R = H, Me) and MeO−CN were used as models of cyanamides and a cyanate, respectively. In the first step of the pathway (Scheme 2a), RnX−CN coordinates to the Fe atom in 1 via the lone pair of electrons on the nitrile N atom (NCN) to form the end-on σ complex 2, which isomerizes to the side-on π complex 3. In the next step, the silyl group migrates from Fe to NCN to form complex 4, with an Fe−C−NCN threemembered ring. Cleavage of the Fe−NCN bond and formation of an Fe−X bond follow to give complex 5 with an Fe−C−X three-membered ring. Finally, the RnX−CN bond is cleaved to form complex 6. The silyl isocyanide Me3SiNC in 6 then isomerizes to the silyl cyanide Me3SiCN. This isomerization has been reported in a previous study of the reaction of MeCN to take place after silyl isocyanide dissociates from a silyl isocyanide complex.7 Because RnX−CN has a lone pair of electrons on the X atom, other pathways are conceivable. For cyanamides and cyanates, we compared the pathway shown in Scheme 2b with that in Scheme 2a. In Scheme 2b, RnX−CN coordinates to the Fe atom in 1 via the lone pair of electrons on the X atom (amino N atom (NR2N) in cyanamides and methoxy O atom (OMeO) in cyanates), which is followed by oxidative addition of RnX−CN. Two reaction pathways are possible for this oxidative addition, depending on the positions of the RnX and CN ligands, as shown in Scheme 2b. The aims of the present study are to determine if a reaction mechanism similar to that for C−CN bond cleavage is followed in the cases of cyanamides and cyanate and how substituents (Me, NH2, NMe2, and MeO) affect the reaction profiles. The stability of complex intermediates 4 and the large activation energy for the rate-determining step will also be discussed.

facilitates methyl group migration to the Fe atom was also elucidated. The N−CN bond in cyanamides (R2N-CN) is also known to be difficult to cleave, because the lone pair of electrons on the amino N atom strengthens this bond through conjugation: R2N−CN ↔ R2N+CN−. The bond dissociation energy, D0, for cyanamide (R = H) was calculated to be 116.9 kcal/mol at the B3LYP/6-31G(d) level.14 Tautomerization between cyanamide and diimide (H 2 N−CN ↔ HNCNH) contributes to its stability. The only known experimental study investigating the cleavage of this bond was performed by Von Braun using harsh reaction conditions.15 In a recent study of the reaction of cyanamides with triethylsilane in the presence of an iron complex, Nakazawa et al. found that Et3SiCN is formed, revealing the cleavage of the R2N−CN bond.16 The Nsilylated η2-amidino iron complex CpFe(CO)(C(NMe2) NSiEt3) was isolated, and heating this complex also led to the formation of Et3SiCN,16 indicating that the N-silylated η2amidino iron complex is an intermediate of R2N−CN bond cleavage and that a mechanism similar to that shown in Scheme 1 is followed. The structure of another N-silylated η2-amidino iron complex, Cp*Fe(CO)(C(NMe2)NSiMe2Ph), was analyzed by X-ray crystallography. Nakazawa et al. envisioned a catalytic cycle using Fe and Mo complexes and found that the catalytic reactions were only realized under photochemical and thermal conditions for the Mo complexes, allowing them to propose a mechanism similar to that in Scheme 1.16 In a more recent study by Nakazawa et al., the O−CN bond in cyanates was cleaved in a similar manner in the presence of Fe or Mo silyl complexes, and a catalytic reaction was realized using Mo complexes.17 A mechanism similar to that in Scheme 1 was suggested, which was supported by theoretical calculations. This type of reaction is called a silyl-migrationinduced reaction (SiMI reaction).17 In this theoretical study, the reaction pathway mediated by the Fe silyl complex shown in Scheme 2a was studied using the following proposed reactions. This pathway starts from 1, the same as in our previous study of C−CN bond activation. First,

2. COMPUTATIONAL METHODS Molecular geometries were optimized at the density functional theory level using the B3LYP hybrid functional with the Gaussian 03 suite of programs.18 The B3LYP method takes electron correlation effects into account because it consists of Becke’s three-parameter hybrid exchange functional and the nonlocal correlation functional of Lee, Yang, and Parr.19,20 For Fe, the basis set LANL2DZ was used.21 In this basis set, the 10 innermost electrons of Fe are replaced by the effective core potential determined by Hay and Wadt.21 For other atoms, 6-31G(d) basis functions were selected.22 This method gave reasonable results in the study of C−CN bond cleavage7 with which we compare the results obtained here; therefore, we used the same method of calculation. Normal coordinate analysis was performed for all stationary points to characterize TSs and equilibrium structures. Therefore, the equilibrium structures reported in this paper show positive eigenvalues of the Hessian matrix, while TSs have one negative eigenvalue.23 Intrinsic reaction coordinate calculations near the TS region followed by geometry optimization of both reactants and products were performed for all of the reactions to confirm the connectivity of the TS.24 Unscaled vibrational frequencies were used to calculate ZPE corrections to total energies. The Gibbs free energies were also calculated at a temperature of 298.15 K and a pressure of 1.00 atm. Unless otherwise noted, we discuss the energy profile using ZPEcorrected potential energies. The MOLEKEL program was used to draw optimized structures.25 In order to check the computational method, we also performed the calculations for the reaction of H2N−CN with the 6-311G(d,p) basis set for all atoms.26 Because the reactions of cyanamides were conducted in toluene in the experiments, in these calculations we also took into account the solvation effect of toluene utilizing the

Scheme 2. Reaction Pathways Studied in This Work

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integral equation formalism for the polarizable continuum model (IEFPCM).27 The energy profile shown in Figure S6 in the Supporting Information, especially that from 4a to 6a, is quite similar to those obtained with the small basis set (LANL2DZ+6-31G(d)). The rootmean-square deviation of the activation energies of each reaction step in the favorable reaction pathway between the two calculations is 1.4 kcal/mol (the same value was obtained on the basis of the Gibbs free energies). The corresponding value for the energies of each reaction step is 1.5 kcal/mol (1.6 kcal/mol on the basis of the Gibbs free energies). The largest difference in the activation energies is observed in the isomerization of 2a1 to 3a and that in the energies of reaction is in the reaction of 3a to 4a. Because the differences between the two calculations are qualitatively small, we used the small basis set and the solvation effect was not taken into account in the calculations of the other reactions.

3. RESULTS AND DISCUSSION 3.1. Reinvestigation of CH3−CN Bond Cleavage. Before the results of N−CN and O−CN bond cleavage are presented, we revisit the CH3−CN bond cleavage, because we recently found an intermediate missed in our previous study7 of this reaction shown in the Introduction. This intermediate is the η1iminoacyl complex E, which forms between C and D in Scheme 1. The structures of 1, C, E, D, and the transition state (TS) between C and E (TSC‑E) as well as the TS between E and D (TSE‑D) are shown in Figure 1. During the formation of E from C, the Fe−N dative bond is broken by dissociation of the imino N atom. In E, one of the methyl CH bonds has an agostic interaction with the vacant coordination site of the Fe atom formed as a result of dissociation of the imino N atom. Then, in the reaction of E to form D, the methyl group migrates to this vacant coordination site through TSE‑D. TSE‑D was reported in ref 7 as the TS between C and D. Although the newly found TSC‑E possesses the largest activation energy of 28.9 kcal/mol in the reaction pathway, it is still a more favorable pathway than the alternatives. As a result, the reaction mechanism involves four separate reactions: rearrangement in the MeCN complex, migration of the silyl group to the nitrile N atom, cleavage of the Fe−N dative bond, and migration of the methyl group. Dissociation of the imino N atom creates a vacant coordination site to which a methyl group readily migrates, resulting in C− CN bond cleavage. 3.2. RnX−CN Bond Cleavage and Formation of an Isocyanide Complex. 3.2.1. H2N−CN. The optimized structures of the intermediates and TSs and the associated energy profile for the reaction pathway of H2N−CN bond cleavage are shown in Figures 2 and 3a, respectively. As stated before, we started our study with the model active catalyst 1, which is an unsaturated iron(II) 16-electron complex and has a vacant site that is occupied by an agostic CH bond of one of the methyl groups in the SiMe3 ligand (Figure 1).7 A free H2N− CN molecule has a C−NH2N bond distance of 1.348 Å. This is shorter than the typical C−N single-bond distance (for instance, 1.455 Å in NMe3 at the present level of calculation) because of conjugation of the lone pair electrons with the C N π* orbital; in addition, the nitrile C atom (CCN) is sp hybridized. However, this conjugation is not strong enough to make the H2N plane and C−N bond coplanar: the angle between them is 138.6°. Optimizing the equilibrium structure of end-on σ coordination via the lone pair of electrons on NCN resulted in complex 2a1, as shown in Figure 2. In 2a1, the NCN atom coordinates strongly to the Fe atom with an Fe−NCN bond length of 1.889 Å. 2a1 is more stable than the separated reactants 1 and H2N−

Figure 1. Selected optimized structures involved in the formation of isocyanide complex CpFe(CO)(Me)(CNSiMe 3 ) from 1 and CH3CN.7 All bond lengths are in Å. All energies (in kcal/mol) were calculated at the B3LYP level with ZPE correction relative to 1 + CH3CN. The energies in parentheses are the free energies in kcal/mol relative to 1 + CH3CN.

CN by 24.6 kcal/mol. This high stability, i.e., large binding energy, can be ascribed to saturation of the vacant site on Fe. The CN triple-bond length in free H2N−CN is 1.166 Å, and this distance does not change upon coordination. 2a1 isomerizes to 3a, showing side-on π coordination of H2N− CN via TS2a1‑3a (Figure 2). The CN bond in side-on 3a is 0.056 Å longer than that in end-on 2a1 and free H2N−CN because of back-donation to the π* orbital. The activation energy of 12.5 kcal/mol for the formation of 3a from 2a1 is ascribed to its endothermicity caused by the transformation of σ coordination to weaker π coordination. In the next step, migration of the silyl group to the NCN atom transforms 3a to the η2-amidino complex 4a via TS3a‑4a. Because this reaction formally transforms the CN triple bond into a double bond, the bond length increases from 1.222 Å in 3a to 1.237 Å in TS3a‑4a and 1.283 Å in 4a, and the NH2N−C− N−Si and NH2N−C−N−Fe dihedral angles in 4a are 10.4 and −177.4°, respectively. The conjugation of the amino lone pair of electrons with the CN bond strengthens so that the H2N−C fragment almost becomes coplanar, with a H−NH2N3997


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Figure 2. Optimized structures involved in the formation of isocyanide complex 6a from 1 and H2NCN. All bond lengths are in Å.

Figure 3. Energy profiles for the cleavage of (a) H2N−CN and (b) Me2N−CN bonds with CpFe(CO)(SiMe3) (1). All energies (in kcal/mol) were determined at the B3LYP level with ZPE correction and are relative to 1 + R2NCN (R = H in (a) and R = Me in (b)). The energies in parentheses are the free energies in kcal/mol relative to 1 + R2NCN.

CCN-H dihedral angle of 168.4°. This N-silylated η2-amidino iron complex 4a with a coordinated NCN lone pair of electrons can be considered an Fe(II) 18-electron species. A similar species has been isolated and structurally characterized by X-ray crystallography,16 in good agreement with the theoretical structure of 4a (Table 1). The activation energy for this step is just 0.6 kcal/mol. This is ascribed to the large exothermicity of this step (−29.3 kcal/ mol), the hypervalent character of the Si atom, and significant charges on the NCN and Si atoms, as in the case of the corresponding step in C−CN bond activation.7 The Mulliken (natural) charges on the Si and NCN atoms of 3a are +0.932 (+1.758) and −0.314 (−0.393), respectively. Therefore, the

positively charged Si atom is strongly attracted to the negatively charged NCN atom, inducing formation of a strong bond. The Si−NCN bond in 4a is much stronger than the Fe−Si bond in 3a that is broken during the reaction (D0(Fe−Si) = 36.3 kcal/ mol in 3a and D0(Si−NCN) = 87.6 kcal/mol in H2NCH NSiMe3). This bond alteration results in the large exothermicity of this step. It should be noted that the C−NH2N bond distance in 4a is 1.342 Å, which is 0.006 Å shorter than that in free H2NCN. The C−NH2N bond is not activated in η2-amidino complex 4a. In the next step, 4a is transformed into 5a1, another η2amidino complex, via TS4a‑5a1 (Figure 2). The reaction 3998


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The other possibility for H2N−CN bond cleavage begins with coordination of H2NCN to 1 via the lone pair of electrons on the NH2N atom, as shown in Scheme 2b. Structure optimization of such a complex resulted in complexes 2a2 and 2a3, shown in Figure 4. Although these complexes differ in

Table 1. Comparison of the Bond Lengths (in Å) in the Theoretical Structures of CpFe(CO)(C(NH2)NSiMe3) (4a) and CpFe(CO)(C(NMe2)NSiMe3) (4b) and the Xray Structure of Cp*Fe(CO)(C(NMe2)NSiMe2Ph) from Ref 16 theor (this study) bond

4a

4b

exptl

Fe−CCO Fe−N1 Fe−C C−N1 Si−N1

1.750 2.036 1.867 1.283 1.741

1.746 2.041 1.863 1.287 1.738

1.732(2) 2.043(2) 1.859(2) 1.303(2) 1.7181(18)

coordinate at TS4a‑5a1 is mainly out-of-phase mixing of the Fe···NCN and Fe···NH2N bond-stretching modes. The Fe−NCN bond is broken before reaching TS4a‑5a1, whereas after passing TS4a‑5a1, rotation around the C−NH2N bond occurs so that the lone pair of electrons on the NH2N atom coordinates to the Fe atom. The activation energy of this step (29.9 kcal/mol) is large because of the strong Fe−NCN bond with a distance of 2.036 Å. This large activation energy is consistent with the experimental observations that a species similar to 4a has been isolated and that heating this intermediate produced silyl cyanide.16 The C− NH2N bond in 5a1 (1.529 Å) is 0.187 Å longer than that in 4a because the conjugation between the lone pair of electrons and the CN double bond is disrupted. That this bond is even longer than a typical C−N single bond suggests that the C− NH2N bond is activated by coordination of the lone pair of electrons to the Fe atom, as discussed later. The last step of C−N bond cleavage is the reaction of 5a1 to 6a via TS5a1‑6a. In this step, the C−NH2N bond activated in 5a1 is broken, and it lengthens from 1.529 Å in 5a1 to 1.868 Å in TS5a1‑6a and eventually to 2.565 Å in 6a. The activation energy is just 1.8 kcal/mol, which supports activation of the C−NH2N bond in 5a1. There is a trans isomer of 5a1, called 5a2 (Figure S4c in the Supporting Information), which has a trans NH2N−CN−Si structure and is more stable than 5a1 by 3.0 kcal/mol. However, no reaction pathway for NH2N−C bond cleavage from 5a2 was found. The NH2N−C bond length in 5a1 (1.529 Å) is longer than that in 5a2 (1.498 Å) because of the steric repulsion between SiMe3 and NH2 groups. This suggests that the NH2N−C bond is more activated in 5a1 than in 5a2, which is why bond cleavage occurs through 5a1 and not the more stable 5a2. Variation in the C−NCN bond length reflects the change in orbital hybridization on the CCN atom during the course of the reaction as follows. The C−NCN bond is the shortest in 2a1 and remains short in TS2a1‑3a (1.166 and 1.182 Å, respectively) because the NCN and C atoms adopt sp hybridization, as reflected by the large NCN−C−NH2N angles in 2a1 and TS2a1‑3a of 177.0 and 167.1°, respectively. However, this changes to sp2 hybridization in 4a, which possesses the longest NCN−C bond (1.283 Å). The silyl isocyanide ligand Me3SiNC is then formed as a result of C−NH2N bond cleavage. The C−NCN bond shortens again because of the change in orbital hybridization on the C atom from sp2 in 5a1 as well as 4a to sp in 6a, as seen by the Fe−C−NCN bond angle of 173.0° in 6a.

Figure 4. Optimized structures in the pathways for the formation of (a) CpFe(CO)(NH2)(CN)(SiMe3) (7a) and (b) CpFe(CO)(CN)(NH2SiMe3) (8a) from 1 and H2NCN. All bond lengths are in Å. All energies (in kcal/mol) were calculated at the B3LYP level with ZPE correction and are relative to 1 + H2NCN. The energies in parentheses are the free energies in kcal/mol relative to 1 + H2NCN.

the orientation of the CN group, their stabilities are similar, and both 2a2 and 2a3 are less stable than 2a1 by 11.2 kcal/mol. The coordination of H2NCN to 1 via the lone pair of electrons on the NH2N atom is weaker than that through the lone pair of electrons on the NCN atom. The cleavage of the H2N−CN bond in 2a2 takes place via TS2a2‑7a to give the intermediate 7a, as shown in Figure 4a. This reaction is an oxidative addition of the C−NH2N bond. H2N−CN bond cleavage through 2a2 is less favorable than that through 2a1, not only because 2a2 is less stable than 2a1 but also because cleavage of the H2N−CN bond requires a large activation energy of 31.3 kcal/mol, and TS2a2‑7a is less stable than TS2a1‑3a by 30.0 kcal/mol. Even if the next step involving reductive elimination of the silyl group and CN ligand occurs readily, this pathway can be reasonably excluded. Cleavage of the H2N−CN bond in 2a3 takes place via TS2a3−8a to give CpFe(CO)(CN)(H2NSiMe3) (8a) (Figure 4b). This reaction is not oxidative addition but rather σ metathesis to directly form Me3SiNH2. Because 2a3 is less stable than 2a1 by 11.2 kcal/mol and the σ metathesis of 2a3 requires a large activation energy of 31.5 kcal/mol, this pathway can also be excluded. 3999


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Figure 5. Optimized structures for isomerization of 4b to 5b2 in the pathway for the formation of isocyanide complex 6b from 1 and Me2NCN. All bond lengths are in Å.

activation energy of 28.0 kcal/mol for cleavage of the strong Fe−NCN bond. The second step is coordination of NMe2N, keeping the trans NMe2N−CN−Si structure (4b′ to 5b1 via TS4b′‑5b1). During the course of this step, the agostic interaction is cleaved by rotation of the Me2N group around the NMe2N−C bond. Because the agostic interaction is weak, the activation energy for this step is relatively small (6.1 kcal/mol). The structure of TS4b′‑5b1 suggests that, once the agostic interaction is broken, the lone pair of electrons on Me2N readily coordinates to the Fe atom. The third step is trans to cis isomerization of the NMe2N−CN−Si structure (5b1 to 5b2 via TS5b1−5b2). At TS5b1‑5b2, the reaction coordinate is mainly the Si−NCN−C bending vibration mode. This step is necessary because C−NMe2N bond cleavage occurs from intermediate 5b2 with a cis NMe2N−CN−Si structure, similar to the reaction of H2NCN. The C−NMe2N bond length of 1.525 Å in 5b2 is longer than that of 1.491 Å in 5b1, suggesting activation of the C−NMe2N bond in 5b2. In fact, as shown in Figure S1 in the Supporting Information, cleavage of the C−NMe2N bond from 5b2 requires an activation energy of just 2.4 kcal/mol (Figure 3b), a small activation energy similar to that from 5a1 with a C−N bond distance of 1.529 Å. Because no agostic intermediate exists in the case of H2NCN, the complex with a trans NH2N−CN−Si structure like that of 4b′ is not an intermediate in the reaction pathway. Accordingly, there is a single TS rather than the three required for Me2NCN. Figure 3b shows that 4b′, without an Fe−NCN or Fe−NMe2N dative bond, is an unstable intermediate and thus the activation energy for the transformation of 4b′ to 5b1 is small (6.1 kcal/ mol). The activation energy for trans to cis isomerization is also small (4.8 kcal/mol). Accordingly, it is the first step of 4b to 4b′, which involves dissociation of NCN from Fe, that requires the largest activation energy of these three steps. 4b is the most stable intermediate during the course of the reaction, similar to the case for 4a. Comparison of selected bonds in the X-ray

The present calculations show that the reaction pathway shown in Scheme 2a is favorable and the η2-amidino complex intermediate 4a is a stable intermediate that possesses the largest activation energy in the reaction pathway. These findings account for the experimental results. The free energy profile in Figure 3a shows that the entropy contribution affects cyanamide association to a greater degree than the other steps, as expected. 3.2.2. Me2NCN. In the study of the methyl-substituted cyanamide, we focused on the effect of substituents on the reaction mechanism. Accordingly, here we mainly discuss the large differences observed between the reactions of H2NCN and Me2NCN. The energy profile for the entire reaction pathway is shown in Figure 3b. As for the cleavage of the Me2N−CN bond, the optimized structures for isomerization of the σ complex to the π complex and the migration of a silyl group from the Fe atom to NCN are similar to those in the case of H2N−CN (Figure 2), as shown in Figure S1 in the Supporting Information. The activation energies of these steps with Me2NCN are also similar to those with H2NCN (12.0 and 0.1 kcal/mol versus 12.5 and 0.6 kcal/ mol, respectively), as shown in Figure 3b. The low activation energy for the formation of 4b is ascribed to the same reasons stated for 4a in the case of H2NCN. In comparison with the transformation of 4a to 5a1 via just one transition state (TS4a‑5a1) (Figure 2), the corresponding transformation in the case of Me2NCN (4b to 5b2) requires three steps (Figure 5). In the first step (4b to 4b′ via TS4b‑4b′), the NCN atom dissociates from the Fe atom. In 4b′, the short distance of 1.836 Å between the Fe atom and one of the methyl H atoms and the long C−H bond distance of 1.129 Å are indicators of an agostic interaction between the amino Me group and the Fe atom. This interaction can occur because the Fe atom in this complex is unsaturated. In this step, the NMe2N− CN−Si structure also changes from cis to trans, induced by the steric repulsion between the methyl groups on Si and the amino N atom (NMe2N). This first step requires a large 4000


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Figure 6. Optimized structures in the pathways for the formation of isocyanide complex 6c from 1 and MeOCN. All bond lengths are in Å.

Figure 7. Energy profile for the cleavage of the MeO−CN bond with CpFe(CO)(SiMe3) (1). All energies (in kcal/mol) were calculated at the B3LYP level with ZPE correction and are relative to 1 + MeOCN. The energies in parentheses are the free energies in kcal/mol relative to 1 + MeOCN.

structure16 with their analogues in 4b shows good agreement (Table 1), perhaps better than that with 4a. We also studied the reaction pathways that begin with coordination via the lone pair of electrons on the NMe2N atom, as shown in Scheme 2b. Calculations showed that, as in the case of H2N−CN, such reaction pathways are unfavorable. The coordination of Me2NCN via the lone pair of electrons on the NMe2N atom is weaker than that via the lone pair of electrons on the NCN atom, and the activation energies are higher (Figure 3b and Figure S2 (Supporting Information)). Not only are 2b2 and 2b3 less stable than 2b1 by 21.6 and 20.3 kcal/mol, respectively, but also TS2b2‑7b and TS2b3‑8b with activation energies of 26.3 and 31.7 kcal/mol, respectively, are less stable than TS2b1‑3b by 35.9 and 40.0 kcal/mol, respectively. In summary, the reaction of Me2NCN shown in Figure 3b is very similar to that of H2NCN except for isomerization of the three-membered-ring intermediate 4 to 5. The difference in

isomerization is because the presence of an agostic interaction between the Fe atom and one of the methyl H atoms in Me2 NCN means that the reaction passes through an intermediate with an agostic interaction. 3.2.3. MeOCN. The optimized structures and energy profile for O−CN bond cleavage are shown in Figures 6 and 7, respectively. The profile of the reaction, especially in the first several steps, is similar to those for the reactions of cyanamides. First, the reaction pathway involves formation of end-on complex 2c1 with a binding energy of 22.7 kcal/mol. Next, the end-on complex 2c1 isomerizes to the side-on complex 3c via TS2c1‑3c (Figure 6) with an activation energy of 10.9 kcal/mol. In the next step, migration of the silyl group to the NCN atom takes place so that 3c is transformed to 4c1 via TS3c‑4c1. The activation energy for this step is very small (0.7 kcal/mol), for the same reasons as those in the equivalent reactions of cyanamides. The Mulliken (natural) charges on the Si and NCN 4001


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atoms of 3c are +0.956 (+1.767) and −0.330 (−0.393), respectively. In 4c1, the bond between the methyl C (CMe) and O atoms is located syn to the bond connecting the Fe and CCN atoms. This structure prevents the O atom from coordinating to the Fe atom. Accordingly, 4c1 is transformed to 4c2 via TS4c1‑4c2, so that the CMe−O bond is anti to the Fe−CCN bond, a structure that is favorable for the coordination of the O atom. During this isomerization, the CMe−O bond rotates around the O−CCN bond, and the CMe−O−CCN−NCN dihedral angle changes from −178.2° in 4c1 to −0.8° in 4c2 via −84.6° in TS4c1‑4c2. 4c2 is less stable than 4c1 by 2.9 kcal/mol, and the activation energy for this step is 10.6 kcal/mol. The next step is the transformation of 4c2 to 5c via TS4c2‑5c, in which the Fe−O bond is formed while the Fe−NCN bond is broken (in 4c2, TS4c2‑5c, and 5c the Fe−O distances are 3.105, 2.686, and 2.021 Å, whereas the Fe−NCN distances are 2.092, 2.981, and 3.077 Å, respectively). This transformation corresponds to the reaction of 4a to 5a1 and 4b to 5b1 and requires a substantial activation energy of 19.1 kcal/mol, although this step is endothermic by only 4.8 kcal/mol. A large activation energy is required for this step, because the strong Fe−N bond is broken. Also, the O−CN−Si structure changes from being cis to trans in this step, similar to the reaction of Me2NCN. The O−CCN bond length of 1.491 Å in 5c is longer than that in 4c1 and 4c2, indicating that this bond is activated by the coordination of the oxygen lone pair to the Fe atom. The last step is the cleavage of the O−CCN bond, in which 5c is transformed to 6c via TS5c‑6c. Unlike the reactions of cyanamides, reactant 5c and TS5c‑6c possess a trans O−CN− Si structure. The activation energy of O−CCN bond cleavage is only 0.2 kcal/mol, and the thermal contribution to the free energy makes TS5c‑6c even more stable than 5c. Complex 5 with a cis O−CN−Si configuration does not exist as an equilibrium structure, and attempting to determine the structure of such a complex instead gave 6c, suggesting that O−CCN bond cleavage occurs readily. We also studied the reaction pathways that start with coordination via the lone pair of electrons on the OMeO atom, as shown in Scheme 2b. Calculations showed that, as in the cases of H2N−CN and Me2N−CN, such reaction pathways are less favorable (Figure S3 in the Supporting Information) than that beginning with coordination of NCN. Coordination of MeOCN via the lone pair of electrons on the OMeO atom occurs more weakly than that via the lone pair of electrons on the NCN atom; the TSs from 2c2 and 2c3 (TS2c2‑7c and TS2c3‑8c) are less stable than TS2c2‑3c and require larger activation energies. Therefore, the pathways starting with coordination of the OMeO atom can be excluded. Dissociation of the NCN atom in 4c2 to form 5c requires the largest activation energy during the reaction pathway of 19.1 kcal/mol. Once the NCN atom dissociates from the Fe atom, the O−CCN bond is easily broken, similar to the reactions of cyanamides. The free energy profile in Figure 7 is similar to the ZPE-corrected potential energy profile except for the step involving cyanate association, as was found for the cyanamides. 3.3. Isomerization of Silyl Isocyanide to Silyl Cyanide in Free and Coordinated Environments. Silyl cyanides were observed as products in experiments. Thus, CNSiMe3 formed in the above reactions must isomerize to the more stable NCSiMe3. There are two possible ways for this isomerization to occur. The first is that CNSiMe3 dissociates

from isocyanide complexes 6a−c and then isomerizes to NCSiMe3. The other possibility is that CNSiMe3 isomerizes to NCSiMe3 in the coordination sphere of the Fe complex. The isomerization of free CNSiMe3 has been studied previously, and the activation energy was calculated to be 23.8 kcal/mol.7 The dissociation of CNSiMe3 in 6a−c requires energies of 15.8, 13.7, and 22.4 kcal/mol, respectively, with corresponding changes in Gibbs free energy of 5.9, 1.7, and 10.9 kcal/mol, respectively. Our trial to find the TS for isomerization in the coordination sphere of Fe gave TS structures with Fe−NCN distances longer than 4.5 Å, to indicate that upon isomerization CNSiMe3 dissociates. As a matter of fact, the activation energies obtained were almost the same as those for isomerization of the free ligand, as shown in Figures S4b and S5 in the Supporting Information. We furthermore managed to determine the TS structure for this isomerization in the coordination sphere of Fe in the case of H2NCN. The TS had Fe−N and Fe−C distances of 2.229 and 2.295 Å, respectively, and a large activation energy of 56.9 kcal/mol (Figure S4a). These results show that isomerization of free CNSiMe3 to NCSiMe3 is more favorable than its isomerization in the coordination sphere of Fe. 3.4. Effects of RnX Group. 3.4.1. Rearrangement of Endon Nitrile Complex 2 to Side-on Nitrile Complex 3. The rearrangement of the end-on nitrile complex to the side-on one is sensitive to the electronic effect of the substituents. The reactions of Me−CN, H2N−CN, and Me2N−CN complexes are endothermic by 9.3, 4.7, and 4.9 kcal/mol, respectively, whereas that of MeO−CN is endothermic by only 1.5 kcal/ mol. Investigation of these structures revealed that the closer the Fe atom and CN group in the side-on complexes, the less endothermic the rearrangement is. A more inductively electronwithdrawing methoxy group enhances back-donation from the Fe atom to the CN π* orbital, which stabilizes end-on complex 3c. This effect is more important in the side-on complexes because the π* orbital has a larger lobe on CCN than on NCN, which stabilizes the side-on complex more than the end-on one. 3.4.2. Silyl Group Migration in Side-on Nitrile Complex 3. As mentioned above, silyl group migration is facilitated by the charges on the Si and NCN atoms. The charge on NCN is influenced by the electronic effects of the RnX group; the Mulliken (natural) charges on NCN in 3a,c are −0.314 (−0.393) and −0.330 (−0.393), whereas that in the side-on MeCN complex is −0.268 (−0.299).7 Conjugation of the lone pair of electrons with the CN π* orbital causes the electrons to accumulate on the terminal NCN atom. The larger exothermicity for silyl group migration in 3a,c than in the side-on MeCN complex is ascribed to the increased negative charge. 3.4.3. Dissociation of the NCN Atom in the Fe−C−NCN Three-Membered-Ring Complex 4. The dissociation of the NCN atom of 4c2 to form 5c is the rate-determining step of the reaction of MeOCN. The activation energy for this step of 19.1 kcal/mol is smaller than those in the cases of H2NCN and Me2CN (29.9 and 28.0 kcal/mol, respectively). It should be noted that the Fe−O distance in TS4c2‑5c is shorter than that in 4c2 by 0.349 Å, which is larger than the changes in distance in the cyanamide reactions; the Fe−NNR2 distance in TS4a‑5a and TS4b‑4b′ is shorter than that in 4a,b by 0.211 and 0.225 Å, respectively. Although in the cyanamide reactions these rearrangements are accompanied by rotation of the amino groups, this rotation has not started in TS4a‑5a and TS4b‑4b′; therefore, the amino N atom cannot approach the Fe atom. At 4002


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34.2, and 38.6 kcal/mol for Fe−Me, Fe−NH2, and Fe−OMe bonds. On the basis of these values, it is clear that the reactions in which a weaker bond is broken and a stronger bond is formed are more exothermic. Unlike the reactions of cyanamides, 4c1 and 4c2 are less stable than isocyanide complex 6c. This is because the strong Fe−OMe bond has not been formed in 4c.

TS4c2‑5c for cyanate, the lone pair of electrons on O can readily interact with the Fe atom, resulting in a shorter Fe−O distance and lower activation energy for this step. 3.4.4. C−X Bond Cleavage in the Fe−C−X ThreeMembered-Ring Complex 5. In this section, we analyze the activation of the C−N and C−O bonds in 5 and compare them with Me−CN bond activation. The methyl group in intermediate E in Figure 1 does not have a lone pair of electrons. Therefore, only an agostic interaction can occur between the Me group and Fe atom. The C−C bond is 0.076 Å longer in E than that in C, showing that the C−C bond is activated in E. In addition to the agostic CH bond, the C−C bond interacts with the vacant coordination site on the Fe atom, called an αCC agostic interaction.28 This is clarified by the localized orbital analysis shown in Figure 8a.

4. CONCLUSIONS We have studied the pathways for the reactions of the active complex CpFe(CO)(SiMe3) with R2N−CN (R = H, Me) and MeO−CN, in which the N−CN and O−CN bonds are cleaved. The most favorable pathway starts with the formation of complexes containing nitriles in the end-on coordination mode, which then isomerizes to side-on coordination to facilitate silyl group migration to NCN. This is followed by dissociation of NCN and coordination of NNH2 or OMeO to the Fe atom. Dissociation of NCN has the largest activation energy in the sequence of reactions, and thus it is the rate-determining step. This is in good agreement with the experimental isolation of similar intermediates that have been identified by X-ray crystallography. Finally, the cleavage of R2N−CN and MeO− CN bonds takes place with small activation energies. There is a difference in the rate-determining step between H2N−CN and Me2N−CN. In the case of H2N−CN, there is a single TS, whereas for Me2N−CN there is an intermediate in which the C−H bond of the amino methyl group agostically interacts with the Fe atom. On the other hand, the activation energy of this step is smaller for the reaction of MeO−CN because the O atom can easily interact with the Fe atom in the TS for this step. In contrast, in the TSs for R2N−CN, the amino groups do not rotate and hinder the approach of the lone pair of electrons on NNR2 to the Fe atom. Comparing these reaction profiles with that of Me−CN, we have found that the Me−CN bond is activated by the αCC agostic interaction with Fe, whereas the N−CN and O−CN bonds are activated by ring strain in the three-membered ring that is formed by coordination of the amino N atom or OMeO to Fe.

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Figure 8. Localized orbital for (a) H3C−C, (b) H2N−C, and (c) H3CO−C bonds in E, 5a1, and 5c, respectively.

ASSOCIATED CONTENT

S Supporting Information *

Tables and figures giving the structures of intermediates and transition states for the reactions of Me2NCN, the oxidative addition and σ metathesis for Me2NCN and MeOCN, and isomerization of Me3SiNC in and out of the coordination sphere, the energy profile for the reaction of H2NCN in solution calculated with the 6-311G(d,p) basis set, and Cartesian coordinates of all the optimized structures. This material is available free of charge via the Internet at http:// pubs.acs.org.

On the other hand, in the bond cleavage processes in the reactions of cyanamides and cyanate, the coordination site on the Fe atom is occupied by the lone pair of electrons from the amino or methoxy group. Accordingly, the N−C or O−C bond cannot interact with the Fe atom, unlike the reaction of acetonitrile. The localized orbitals in Figure 8b,c show that, because of the strain in the three-membered structure, the orbitals of these bonds are distorted and thus are activated like a banana bond in cyclopropane.29 3.4.5. Energy of Overall Reactions. While the energy of the reaction of MeCN forming CpFe(CO)(Me)(CNSiMe3) was calculated to be −37.9 kcal/mol,7 those of H2NCN and MeOCN are −43.3 and −57.9 kcal/mol, respectively. To analyze the difference in the energies of these reactions, we calculated the bond dissociation energies of Me−CN, H2N− CN, and MeO−CN bonds in the reactants and those of the Fe−Me, Fe−H2N, and Fe−OMe bonds in the products. The values obtained are 121.9, 116.9, and 106.7 kcal/mol for Me− CN, H2N−CN, and MeO−CN bonds, respectively, and 33.8,

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS Some of the calculations were performed at the Research Center for Computational Science of Okazaki National Research Institutes of Japan. This research was supported by the Core Research for Evolutional Science and Technology (CREST) from the Japan Science and Technology Agency. 4003


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