The Characterisation and Reactivity of a Rh III 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)][BArF4] [dtbpb = tBu2P(CH2)4PtBu2, NBD = norbornadiene, ArF = 3,5-C6H3(CF3)2] to give the Rh(III) ƞ1-σ-alkane complex [Rh(dtbpb’)(H)(ƞ1-C7H12)][BArF4] (dtbpb’ =Κ3-PCP-tBu2PCH2CHCH2CH2PtBu2) 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 and IGHM/QTAIM 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)][BArF4]. 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