Ab initio quantum chemical calculations for unusual d1 niobium complexes, bond stretch isomers, and molybdenum oxo-transfer enzymes

Abstract

Previous interpretation of the EPR spectra of the dl pseudo- D4h NbCI4(PR3)2 complexes assumed that the unpaired electron resides in the b2g(dxy) orbital. In contrast, simple molecular orbital considerations suggest that the unpaired electron resides in the Jahn-Teller unstable eg(dx,,dy,) orbital. Ab initio self-consistent field calculations predict the ground state to be a Jahn-TeRer-distorted 2Eg, Although the predicted geometry of this state is in agreement with the X-ray structure, the calculated g values for this and other possible states are incompatible with the experimental g values. We conclude that the observed spectra are due to some other species. Previous work has shown that limited basis set self-consistent field calculations on 1, 1, I -triamino-2,2,2-tficyanoethane and 1, 1, I -trianiino- 2,2,2-trinitroethane yield structures with two stable minima. One minimum corresponds to a covalent isomer while the second minimum corresponds to an ionic one. In this work, it is shown that even with larger basis sets and electron correlation two minima are still apparent. The minima corresponding to the ionic isomers are much more stable than the ones corresponding to the covalent isomers, which are barely bound with respect to the barrier separating them. The structure of several other possible isomers and dimer were also investigated. These calculations suggest that the solid material will be mainly ionic with a network of hydrogen bonds. Patterned after Enemark's synthetic model system for dioxomolybdenum enzymes., our theoretical model system produces an energy profile and structures for the various species in the catalytic cycle. Here, our substrate PMe3 approaches [MO(V')02 ] 21 at an 0-Mo-O-P dihedral angle of 90', crosses over a barrier of 14 kcal/mol and rotates to an 0MO-O-P dihedral angle of O' to form an intermediate, [Mo(IV)OOPMe3 ]2+, which is 69 kcal/mol more stable than the reactants. The direction of the substrate's attack leaves the two d electrons in an orbital which is 6 with respect to the remaining spectator Mo-O bond. The displacement of OPR3 by H20 proceeds via an associative mechanism with a 19 kcal/mol barrier. [Mo(17V)OOH2 ]21 then reacts with IMO(VI)0212+ to form [Mo(V)OOH]2+. The addition Of 02 then oxidizes [MO(V)Ooffl2+ to IMO(V')0212+ to complete our model catalytic cycle.