The characterisation and reactivity of a Rhiii η1-σ-alkane complex and the role of a structurally responsive phosphine ligand in solid-state molecular organometallic chemistry

Abstract

A combined experimental and computational study on the single-crystal to single-crystal addition of H₂ to [Rh(κ²-dtbpb)(NBD)][BAr^F₄] [dtbpb = ^tBu₂P(CH₂)₄P^tBu₂, NBD = norbornadiene, Ar^F = 3,5-C₆H₃(CF₃)₂] to give the Rh(III) η¹-σ-alkane complex [Rh(dtbpb’)(H)(η¹-C₇H₁₂)][BAr^F₄] (dtbpb’ = κ³-PCP-^tBu₂PCH₂CHCH₂CH₂P^tBu₂) is reported, in which the supporting phosphine ligand has also undergone C–H activation of one of the methylene groups in the chelate backbone to form a trans-spanning κ³-pincer-type ligand. Characterisation by variable temperature single-crystal X-ray diffraction, solid-state NMR spectroscopy, periodic DFT, QTAIM, NBO and IGMH calculations support this assignment, and also that the norbornane (NBA) alkane ligand can access low energy conformational isomers in the solid-state. Dissolving in CD₂Cl₂ displaces the alkane and the corresponding solvent adduct is formed, [Rh(dtbpb’)(H)(κ¹-ClCD₂Cl)][BAr^F₄]. DFT calculations, supported by experiment, indicate the C–H activation of the backbone occurs after full hydrogenation of the NBD to NBA, and not at a norbornene intermediate. Addition of propene to the crystalline σ-alkane complex displaces the NBA, and the phosphine ligand responds by reforming the κ²-motif, consistent with C–H activation being thermodyamically favoured when trans to a weakly coordinating σ-alkane. These reversible bond activations of the chelating ligand in response to changes in the co-ligands demonstrate that significant structural reorganisation is possible in the crystalline environment.