The computational modelling of the structure and reactivity of Rhodium σ-Alkane complexes in the solid-state

Abstract

A series of solid-state [Rh(Cy2PCH2CH2PCy2)(alkane)][BArF 4] (alkane = C3-C6 linear, isobutane, 2-Methylbutane, 3-Methylpentane, Cyclohexane and NBA) σ-alkane complexes were optimised with periodic DFT and investigated with a range of electronic structure analysis techniques including QTAIM, NBO and NCI plots. Intramolecular interactions of the alkane with the Rh centre show a range of hapticities from η1 through to η2 and some η2 interactions supported by varying degrees of interactions with geminal C–H bonds. Intermolecular interactions of the Rh cation with its environment were also explored where the importance of the solid-state environment in alkane binding is shown. However, there is a lack of correlation with the experimental lifetimes of the complexes. Both dehydrogenation and H/D exchange of [Rh(Cy2PCH2CH2PCy2)(Isobutane)][BArF 4] was investigated using an isolated cation model and a solid-state model using periodic DFT. For dehydrogenation two pathways were identified differing by initial 1º or 3º C– H activation. The isolated cation model showed both pathways to have comparable barriers however, in the solid-state, the two pathways become energetically distinct. Hirshfeld surfaces showed destabilising contacts between isobutene and an aryl ring of a BArF 4 - anion at the rate determining H2 reductive coupling transition state in the disfavoured 3º C–H activation pathway. Another case where the solid-state environment controlled reactivity was the H/D exchange of both the 1º and 3º C–H bond participating in a σ-interaction. Here different pathways were favoured in the two models. Finally, the electronic structure of anagostic interactions in Rh((DPEphos-o-R)(NBD)+ (R = H, Me, OMe, and iPr) were studied showing the effect of changing the ortho R group. The largest substituent, iPr, gives the shortest and strongest anagostic interactions by QTAIM and NBO analysis and in general a correlation between H...Rh distance and interaction strength was seen. The anagostic interactions were also found to be predominantly covalent in nature. Computed 1H NMR chemical shifts show a downfield shift experienced by the anagostic proton, as seen experimentally, however the extent of which does not correlate with the strength of the anagostic interaction. Plots of the Laplacian visually rationalised the NMR data, and the occurrence of the downfield shift, with H...Rh bond paths lying over an area of charge accumulation in all cases.