Evaluation of CO Coordination Energies from Spectroscopic Data: On the Use of Vibrational Isotopic Effects

Ligand Field Inversion as a Mechanism to Gate Bioorganometallic Reactivity: Investigating a Biochemical Model of Acetyl CoA Synthase Using Spectroscopy and Computation

The biological global carbon cycle is largely regulated through microbial nickel enzymes, including carbon monoxide dehydrogenase (CODH), acetyl coenzyme A synthase (ACS), and methyl coenzyme M reductase (MCR). These systems are suggested to utilize organometallic intermediates during catalysis, though characterization of these species has remained challenging. We have established a mutant of nickel-substituted azurin as a scaffold upon which to develop protein-based models of enzymatic intermediates, including the organometallic states of ACS. In this work, we report the comprehensive investigation of the S = 1/2 Ni-CO and Ni-CH₃ states using pulsed EPR spectroscopy and computational techniques. While the Ni-CO state shows conventional metal-ligand interactions and a classical ligand field, the Ni-CH₃ hyperfine interactions between the methyl protons and the nickel indicate a closer distance than would be expected for an anionic methyl ligand. Structural analysis instead suggests a near-planar methyl ligand that can be best described as cationic. Consistent with this conclusion, the frontier molecular orbitals of the Ni-CH₃ species indicate a ligand-centered LUMO, with a d⁹ population on the metal center, rather than the d⁷ population expected for a typical metal-alkyl species generated by oxidative addition. Collectively, these data support the presence of an inverted ligand field configuration for the Ni-CH₃ Az species, in which the lowest unoccupied orbital is centered on the ligands rather than the more electropositive metal. These analyses provide the first evidence for an inverted ligand field within a biological system. The functional relevance of the electronic structures of both the Ni-CO and Ni-CH₃ species are discussed in the context of native ACS, and an inverted ligand field is proposed as a mechanism by which to gate reactivity both within ACS and in other thiolate-containing metalloenzymes.

Electronic structure calculations permit identification of the driving forces behind frequency shifts in transition metal monocarbonyls

Our direct DFT decomposition of CO frequency shifts updates the paradigm for metal carbonyl binding.

Neon-matrix spectroscopic and theoretical studies of the reactivity of titanium dimer with diatomic ligands: comparison of reactions with nitrogen and carbon monoxide

The reactivity of diatomic titanium with molecular carbon monoxide has been investigated in solid neon at very low temperature. In contrast to the spontaneous reaction observed between Ti(2) and N(2), our results show that the formation of dititanium oxycarbide (OTi(2)C) from the condensation of effusive beams of Ti and CO in neon matrices involves several intermediate steps including one metastable intermediate. In the absence of electronic excitation, only formation of a Ti(2)(CO) complex occurs spontaneously during the reaction at 9 K of ground state Ti(2) and CO, as reported in solid argon by Xu, Jiang and Tsumori (Angew. Chem. Int. Ed., 2005, 44, 4338). However, during deposition or following electronic excitation, this species rearranges into a new species: the more stable, OTi(2)C oxycarbide form. Several low-lying excited states of OTi(2)C are also observed between 0.77 and 0.89 eV above the ground state, leading to a complex sequence of interacting vibronic transitions, merging into a broad continuum above 1 eV. Observations of Ti(2)(12)C(16)O, Ti(2)(13)C(16)O and Ti(2)(12)C(18)O and natural titanium isotopic data enable the identification of four fundamental vibrations in the ground electronic state and two others in the first two excited states. Quantum chemical calculations predict an open-shell (1)A(g) ground state with Ti-C and Ti-O distances close to 184 pm, and 91 degrees for the TiCTi and TiOTi bond angles, and give fundamental frequencies in good agreement with observation. The reaction paths of the Ti(2) + N(2) --> Ti(2)N(2) and Ti(2) + CO --> Ti(2)(CO) --> OTi(2)C have been investigated and a reaction scheme is proposed accounting for the similarities in nature and properties of the final products, as well as explaining the observation of a coordination complex with Ti(2) only in the case of the carbonyl ligand.