Theoretical studies on intermolecular versus intramolecular carbon-hydrogen bond activation in rhodium, iridium, and zirconium complexes, and on the ethene to hydridovinyl interconversion in rhodium and iridium complexe

Ab initio quantum mechanical calculations (double-zeta effective core potentials, Hartree-Fock geometries, second order perturbation energies) were performed on model reactions to analyze the behavior of intermolecular versus intramolecular C-H bond activation in rhodium, iridium, and zirconium complexes. Intermolecular reactions (inter) were modeled by (Cp)(X)M + CH$sb4$ $to$ (Cp)(X)M(CH$sb3$)(H), with (M,X) = (Rh,PH$sb3$), (Ir,PH$sb3$), (Zr,Cl), and Cp = C5H5. Intramolecular reactions involving the Cp ring (intra-Cp) were modeled by $rm (X)M(CpRspprime H) to (X)overline{rm M(CpR}spprime$)(H), while those involving the phosphine (intra-P) were modeled by $rm (Cp)M(PHsb2Rspprime H) to (Cp)overline{rm M(PHsb2R}spprime$)(H), with M = Rh, Ir, and R$spprime$ = CH$sb2$, CH$sb2$CH$sb2$. It is found that the thermodynamic exothermicity follows the sequence inter $>$ intra-P $>$ intra-Cp with decreasing differences as the ring size increases. The inter and intra-P reactions beginning with reactants in their singlet states have no kinetic barriers at the MP2 level. The barrier for the intra-Cp reaction decreases as the R$spprime$ fragment increases, particularly for zirconium. For iridium the inter reaction occurs through a $etasp1$-agostic intermediate which has a very small ($<$1 kcal/mol) barrier towards products.;A previously reported mechanism for the elimination of isobutane from (C$sb5$Me$sb5$)$sb2$Zr(H)(CH$sb2$CHMe$sb2$) is modeled, and a lower energy concerted elimination of the isobutyl and a hydride in the Cp$sp{*}$ ring is proposed instead.;Similar calculations were also used to examine models for the reaction (HB(X)$sb3$) (CO)M($etasp2$-CH$sb2$CH$sb2$) $to$ (HB(X)$sb3$) (CO)M(H)(CHCH$sb2$), for which it is known that the equilibrium lies to the hydridovinyl product for iridium with X = 3-trifluoromethyl-5-methylpyrazol-1-yl, and lies to the $etasp2$-ethene reactant for rhodium with X = 3,5-dimethylpyrazol-1-yl. The ligand models tested correspond to X = NH$sb2$, NHNH$sb2$, NCH$sb2$, NCHF, NHNCHF, and N$sb2$C$sb3$H$sb2$F (3-fluoropyrazol-1-yl). All of the models gave qualitatively similar results with X = NH$sb2$ and NCH$sb2$ being closest to the largest model ligand. However, electron correlation, at least at the MP2 level, is necessary for an adequate modeling.