Theoretical Studies of Inorganic and Organometallic Reaction Mechanisms. 10. Reversal in Stability of Rhodium and Iridium η²-Ethene and Hydridovinyl Complexes

Abstract

Ab initio quantum mechanical calculations were used to examine models for the reaction [η 3 -HB(X) 3 ](CO)M(η 2 -CH 2 CH 2 ) f [η 2 -HB(X) 3 ](CO)M(η 2 -CH 2 CH 2 ) f [η 3 -HB(X) 3 ](CO)M(H)-(CHCH 2 ), for which it is known that the equilibrium lies toward the hydridovinyl product for iridium, with X ) 3-trifluoromethyl-5-methylpyrazol-1-yl, and lies toward the η 2 -ethene reactant for rhodium, with X ) 3,5-dimethylpyrazol-1-yl, and most other related systems. The ligand models tested correspond to X ) NHNH 2 , NHNCHF, N 2 C 3 H 3 (pyrazol-1-yl), N 2 C 3 H 2 F (3-fluoropyrazol-1-yl), N 2 C 3 H 2 F (5-fluoropyrazol-1-yl), N 2 C 3 H 2 CH 3 (3-methylpyrazol-1-yl), and N 2 C 3 H 2 CF 3 (3-(trifluoromethyl)pyrazol-1-yl). For the iridium complexes, the restricted Hartree-Fock (RHF) optimized geometries are similar to the Møller-Plesset second-order (MP2) ones and the energy calculations at the MP2//RHF, MP2//MP2, and MP4SDQ//MP2 levels give reasonable results. For the rhodium complexes, although the RHF energies appear to be in qualitative accord with the calculated results at the MP2// MP2 and MP4SDQ//MP2 levels, the MP2//RHF energies are not in agreement with these results since the RHF geometries of (η 3 -HBX 3 )(η 2 -ethene)rhodium complexes are very sensitive to the electronic environment. In the d 8 η 2 -ethene complexes, Rh favors η 2 -pyrazolylborate over η 3 -pyrazolylborate complexes by -0.7 to -9.9 kcal/mol and the barrier from the hydridovinyl complex is only 12 kcal/mol. Thus, nearly all Rh complexes will exist as (η 2 -ethene)(η 2 -pyrazolylborate) complexes. In contrast, Ir favors the η 3 -pyrazolylborate over η 2 -pyrazolylborate in all complexes and endothermicty to the hydridovinyl complex is reduced by about 20 kcal/mol. The ethene complexes are the most sensitive to the steric properties of the pyrazolylborate ligands. Thus, in the most sterically hindered Ir complexes, the hydridovinyl complex finally becomes more stable than its η 2 -ethene isomer.

Prediction of the Reactive Intermediates in Alkane Activation by Tris(pyrazolyl borate)rhodium Carbonyl

“…Recently, published theoretical work on the rearrangement of (η 3 -Tp)M(CO)(η 2 -CH 2 CH 2 ) to (η 2 -Tp)M(CO)(η 2 -CH 2 CH 2 ) (M = Rh, Ir) shows that both the η 3 -Tp and the η 2 -Tp ethane complexes are stable species connected by a small barrier. The subsequent reaction of these species to the oxidative-addition product (η 3 -Tp)M(CO)(H)(CHCH 2 ) proceeds through a single transition state from both of the ethane isomers.…”

C−H Activation Reactions on Rh(I)−Ethylene Complexes of the Hydrotris(3,5-dimethylpyrazolyl)borate Ligand, TpMe2

“…This dissimilarity may be a reflection of the differences in C−H bond activation mechanistic pathways as discussed below, and this divergence may rely on the thermodynamic stabilities of oxidation states I and III for Rh and Ir. As mentioned earlier in this paper, for the former element the monovalent state is relatively favored …”

“…Whereas the Ir(I)(η-C 2 H 4 ) formulation may be in some cases (e.g., in complexes of the Tp‘ ligands) thermodynamically less favorable than the corresponding Ir(III)H(CHCH 2 ) isomers, for Rh, Rh(III)H(CHCH 2 ) compounds are thermally unstable with respect to Rh(I)(η-C 2 H 4 ), irrespective of the nature of the auxiliary ligand (Cp‘ or Tp‘). The reasons for this are mostly (or exclusively) electronic and reflect the higher tendency of the 5d vs the 4d transition metals to undergo oxidation−addition reactions …”

Direct Dynamics Trajectories Reveal Nonstatistical Coordination Intermediates and Demonstrate that σ and π-Coordination Are Not Required for Rhenium(I)-Mediated Ethylene C–H Activation

“…Despite this general viewpoint, Bergman showed that thermolysis of Cp­(PMe 3 ) 2 Re­(H)­( n -hexyl) induced reductive elimination of hexane followed by subsequent reaction of Cp­(PMe 3 ) 2 Re with ethylene that resulted in a 10:1 mixture of the Re-vinyl hydride ( I ) to the Cp­(PMe 3 ) 2 Re­(η 2 -ethylene) π-complex ( II , Scheme b) . A similar observation was also reported for Fe­(depe) 2 , and Ir/Rh complexes with pyrazolylborate or Cp* ligands. ,− Critically important for the Re reaction, the π-complex is thermodynamically more stable than the Re-vinyl hydride and therefore there is no conversion from the π-complex II to the Re-vinyl hydride I . This result indicates that the π-complex is not required in the mechanism to achieve the Re-vinyl hydride.…”