DFT Study of the Structure and Reactivity of the Terminal Pt(IV)-Oxo Complex Bearing No Electron-Withdrawing Ligands

摘要

The recently published [(PCN)Pt=O]~+ complex is interesting as a unique example of a stable d~6 terminal transition metal oxo complex not stabilized by electron withdrawing ligands and as a model of oxo complexes frequently implicated as key intermediates in various processes of oxygen transfer. In the present work, we report an extensive DFT study of its geometric and electronic structure, composition in solution, and reactivity. The thermodynamic data and calculated ~(195)Pt NMR chemical shifts reveal that one solvent molecule is weakly coordinated to the complex in acetone solution. This ancillary ligand is responsible for the diamagnetic state of the complex, retards intramolecular oxygen transfer, and facilitates CO oxidation. Chemical transformations of the coordinated acetone molecule, coordination of other ancillary ligands present in the reaction mixture, and protonation of the Pt-oxo group in nonacidic media are excluded based on thermodynamic or kinetic considerations. Bonding of the terminal oxo ligand with strong electrophiles presents the key interaction in the mechanisms of intramolecular oxygen insertion into the Pt-P bond, in CO oxidation and in water activation mediated by microsolvation. Low affinity of the terminal oxo ligand toward "soft" covalent interactions brings about intermediate formation of agostic hydrido and hydroxo complexes along the reaction pathway of dihydrogen oxidation. Stabilization of the Pt-oxo bonding is attributed to bending of the terminal oxo ligand out of the plane of the complex and to significant transfer of electron density from compact low lying Pt 5d orbitals to more diffuse 6s and 6p orbitals.