Homoleptic seven-coordinate Ti(0) and Zr(0) through a new stabilization mode

Abstract

The stabilization of zero oxidation state group (IV) metal centers in readily accessible compounds on a preparative scale remains a significant challenge. Substituting hydrocarbon ligands in the precursor complexes [Ti(η⁶-toluene)₂] and [Zr(η⁶-cycloheptatriene)₂] with monovalent GaTMP (TMP = 2,2,6,6-tetramethylpiperidinyl) yields the first homoleptic seven-coordinate Ti⁰ and Zr⁰ complexes, [Ti(GaTMP)₇] (1) and [Zr(GaTMP)₇] (2), which are exclusively coordinated by metalloligands. The bonding situation of 1 and 2 was rationalized through DFT calculations, revealing the critical importance of tangential Ga····Ga covalent interactions for stabilizing the compounds. Oxidation of 2 leads to the formation of [Zr(GaTMP)₈]²⁺ (5), showcasing a similar bonding situation. These Ga····Ga interactions arise from significant π-backbonding from the Ti⁰ and Zr⁰ centers into the constructive combinations of the diffuse 4p orbitals of the Ga(I) centers. This unique cooperative feature of the all-Ga metalloligand sphere marks a clear distinction from the bonding properties of formally isolobal carbonyls, phosphines, or N-heterocyclic carbenes.