Highly distorted d8-rhodium(+I) and d10-rhodium(−I) complexes: synthesis, reactivity, and structures in solution and solid state

Abstract

A synthesis of the new ligand 5-diphenylphosphanyl-10-methyl-5H-dibenzo[a,d]cycloheptene (^metropp^ph) was developed in order to prepare highly distorted tetra-co-ordinated rhodium(+I) complexes. The ligand ^metropp^ph contains a cycloheptatriene ring in a rigid boat conformation such that a Ph₂P and an olefinic binding site are perfectly arranged for transition metal complexation. Four equivalents of ^metropp^ph react with [Rh₂(μ-Cl)₂(cod)₂] in the presence of KPF₆ to yield almost quantitatively [Rh(+I)(^metropp^ph)₂]PF₆, which was isolated in the form of dark red-violet crystals. A crystal structure analysis reveals that the co-ordination sphere of the rhodium centre in [Rh(+I)(^metropp^ph)₂]⁺ deviates strongly from a square planar arrangement (φ  =  42°). One methyl group in [Rh(+I)(^metropp^ph)₂]⁺ can be deprotonated by KOBu^t to give the allyl complex [Rh(^allyltropp^ph)(^metropp^ph)]. This complex has a structure that may be best described as a distorted trigonal bipyramid. The boron hydride [BEt₃H]⁻ and organolithium reagents, LiR, react with [Rh(^allyltropp^ph)(^metropp^ph)] in allylic alkylation reactions to yield anionic rhodate complexes [Rh(^RCH2tropp^ph)(^metropp^ph)]⁻ (R  =  H, Me, n-Bu, Ph) that formally have a d¹⁰ valence electron configuration. The rhodate [Rh(^metropp^ph)₂]⁻ can be obtained directly by reduction of cation [Rh(^metropp^ph)₂]⁺ with alkali metals. In a sym-proportionation reaction [Rh(^metropp^ph)₂]⁺ and [Rh(^metropp^ph)₂]⁻ give the neutral d⁹-[Rh(^metropp^ph)₂] radical (K  =  1.1 × 10⁷), which is not stable but decomposes with loss of H₂ to give the allyl complex [Rh(^allyltropp^ph)(^metropp^ph)]. The structures in solution of [Rh(^metropp^ph)₂]⁺, [Rh(^allyltropp^ph)(^metropp^ph)], and [Rh(^nBuCH2tropp^ph)(^metropp^ph)]⁻ were determined by NMR techniques, which reveal that (i) they match the solid state structures and (ii) are rather similar to each other. This fact may explain the remarkable electronic flexibility of the rhodium centre, which changes reversibly its formal oxidation state from + I to 0 to − I at rather low negative potentials (ΔE⁰¹  =  − 0.882 V; ΔE⁰²  =  − 1.298 V).