Identification of non-classical C–H⋯M interactions in early and late transition metal complexes containing the CH(ArO)3 ligand

Abstract

The fully optimised DFT structure of the d⁰ complex [{CH(ArO)₃}Ti(NEt₂)] (2) at the B3LYP level compares well with the distorted tetrahedral geometry shown by the X-ray crystal structure. QTAIM analysis of the electron density associated with the C–H⋯Ti interaction shows a well defined bond critical point, a bond path between the hydrogen and titanium centres and a negative value for the energy density indicative of covalency. A natural bond orbital (NBO) picture of the interaction shows that the C–H σ bond electron density donates to a d hybrid orbital on the metal in a linear fashion. Calculated IR and NMR data for the components of the interaction are consistent with experiment. The computed structures for [{CH(ArO)₃}Ti(OPh)] (3), [{CH(ArO)₃}Zr(NEt₂)] (4), [{CH(ArO)₃}Hf(NEt₂)] (5), show tetrahedral geometries and QTAIM and NBO properties similar to (2). [{CH(ArO)₃}Mo(NEt₂)] (6) shows distortion of the tripodal ligand and a reduced C–H⋯M bond angle with properties more consistent with a C–H⋯M side-on donor interaction. In [{CH(ArO)₃}Fe(NEt₂)] (7) the C–H⋯M bond angle is linear and involves a donor interaction. An energy minimised structure maintaining the three fold coordination to the tripodal ligand was not obtained for [{CH(ArO)₃}Ni(NEt₂)]²⁻ but changing from a diethyl amide ligand to phenolato gave energy minimised [{CH(ArO)₃}Ni(OPh)]²⁻ (8). This structure shows a distorted square planar geometry with a substantially bent phenoxo ligand and a near linear C–H⋯M covalent interaction with donor and back bonding properties. The work shows that linear C–H⋯M interactions can have both agostic and weak hydrogen bond-like covalency.