Measurement of Diphosphine σ-Donor and π-Acceptor Properties in d⁰ Titanium Complexes Using Ligand K-Edge XAS and TDDFT

Abstract

Diphosphines are highly versatile ancillary ligands in coordination chemistry and catalysis because their structures and donor-acceptor properties can vary widely depending on the substituents attached to phosphorus. Experimental and theoretical methods have been developed to quantify differences in phosphine and diphosphine ligand field strength, but experimentally measuring individual σ-donor and π-acceptor contributions to metal-phosphorus bonding remains a formidable challenge. Here we report P and Cl K-edge X-ray absorption spectroscopy (XAS), density functional theory (DFT), and time-dependent density functional theory (TDDFT) studies of a series of [Ph₂P(CH₂) _nPPh₂]TiCl₄ complexes, where n = 1, 2, or 3. The d⁰ metal complexes (Ti⁴⁺) revealed both P 1s → Ti-P π and P 1s → Ti-P σ* transitions in the P K-edge XAS spectra, which allowed spectral changes associated with Ti-P σ-bonding and π-backbonding to be evaluated as a function of diphosphine alkane length. DFT and TDDFT calculations were used to assign and quantify changes in Ti-P σ-bonding and π-backbonding. The calculated results for [Ph₂P(CH₂)₂PPh₂]TiCl₄ were subsequently compared to electronic structure calculations and simulated spectra for [R₂P(CH₂)₂PR₂]TiCl₄, where R = cyclohexyl or CF₃, to evaluate spectral changes as a function of diphosphine ligand field strength. Collectively, our results demonstrate how P K-edge XAS can be used to experimentally measure M-P π-backbonding with a d⁰ metal and corroborate earlier studies showing that relative changes in covalent M-P σ bonding do not depend solely on changes in diphosphine bite angle.