Computational study of the spin-forbidden H₂oxidative addition to 16-electron Fe(0) complexes

Abstract

The spin-forbidden oxidative addition of H 2 to Fe(CO) 4 , Fe(PH 3 ) 4 , Fe(dpe) 2 and Fe(dmpe) 2 [dpe = H 2 PCH 2 CH 2 PH 2 , dmpe = (CH 3 ) 2 PCH 2 CH 2 P(CH 3 ) 2 ] has been investigated by density functional theory using a modified B3PW91 functional. All 16-electron fragments are found to adopt a spin triplet ground state. The H 2 addition involves a spin crossover in the reagents region of configurational space, at a significantly higher energy relative to the triplet dissociation asymptote and, for the case of Fe(CO) 4 ؒH 2 , even higher than the singlet dissociation asymptote. After crossing to the singlet surface, the addition proceeds directly to the classical cis-dihydride product. Only for the Fe(CO) 4 was it possible to locate a stable energy minimum for the non-classical tautomer (dihydrogen complex), but the energy difference between this minimum and the tautomerisation transition state inverts when taking into account the zero-point energy correction. The geometries at the crossing points indicate a "side-on" approach of the H 2 molecule to the metal for the Fe(CO) 4 , Fe(CO) 2 (PH 3 ) 2 and Fe(PH 3 ) 4 systems. These geometries are more reactants-like for the Fe(CO) 4 system and more product-like for the Fe(PH 3 ) 4 system. The crossing point geometry for the Fe(dpe) 2 system, on the other hand, is nearly C 2 -symmetric. The presence of an energy barrier on going from 3 FeL 4 ϩ H 2 to the crossing point is in agreement with the slow observed rates for addition of H 2 to these unsaturated organometallic fragments.

A Two‐State Computational Investigation of Methane CH and Ethane CC Oxidative Addition to [CpM(PH3)]n+ (M=Co, Rh, Ir; n=0, 1)

“…This is quite expected, since the CÀH bond of methane is a very weak donor and does not effectively compete with the ligands that are already present in the coordination sphere in terms of providing electronic saturation to the metal center. [4,5] A similar situation was demonstrated for the repulsive addition of N 2 to triplet [CpMoCl(PH 3 ) 2 ] (whereas CO addition is attractive), [37,38] for the repulsive addition of methane to the triplet metallocenes of Mo and W, [6] for the repulsive addition of H 2 to spin triplet FeL 4 substrates (L = CO, phosphane), [39] and more recently for the repulsive addition of H 2 to [W{N(CH 2 CH 2 NSiMe 3 ) 3 }H]. [40] Methane addition, on the other hand, is attractive to singlet [CpM(PH 3 )] for all metals.…”

Radicals and molecular products from the gas-phase pyrolysis of lignin model compounds. Cinnamyl alcohol

“…The hybrid B3PW91 functional [29,30] is employed in conjunction with the moderate Pople-type basis set 6–31 + G(2d,p) augmented with diffuse and polarization functions [31]. This method is well tested in literature, and has also been evaluated in our previous studies on open-shell systems [32,33]. All quantum mechanical calculations are performed using Gaussian 03 software [34] to obtain optimal equilibrium geometries and transition states verified by vibration mode analysis.…”

Dihydrogen Catalysis: A Remarkable Avenue in the Reactivity of Molecular Hydrogen

“…The participation of spin inversion in the rate-determining steps, known as the multi-state reactivity (MSR), is believed to be a key feature in a number of chemical (mainly TM containing) processes where reactants, intermediates and products have different multiplicities (136)(137)(138)(139)(140)(141)(142)(143)(144)(145)(146)(147).…”