Dynamic NMR investigations of fluxionality of 2-(dimethoxymethyl)pyridine and 2,6-bis(dimethoxymethyl)pyridine in rhenium(I) and platinum(IV) complexes

Abstract

The ligands 2-(dimethoxymethyl)pyridine (L¹) and 2,6-bis(dimethoxymethyl)pyridine (L²) formed bidentate chelate complexes with the isoelectronic transition metal moieties Re^IX(CO)₃ and Pt^IVXMe₃ (X = halide). The complexes [ReX(CO)₃L¹] (X = Cl or Br), [PtXMe₃L¹] (X = Br or I) and [ReBr(CO)₃L²] in organic solvents were shown by NMR to undergo fluxional processes which interconvert co-ordinated and pendant OMe groups. Rates and activation energies of these fluxions were measured by NMR methods (1-dimensional bandshape analysis or 2-dimensional exchange spectroscopy). Magnitudes of ΔG^‡ (298.15 K) for the fluxions were in the range 59–85 kJ mol⁻¹ with the order being Re^I–L¹ > Re^I–L² > Pt^IV–L¹. A carbon–carbon bond rotation mechanism is proposed for the fluxions in the L¹ complexes and concerted C–C bond rotation/metallotropic shift processes for the L² complexes. Crystal structures of [ReCl(CO)₃L¹] and [PtIMe₃L¹] revealed distorted octahedral metal centres with N–M–O ‘bite’ angles of 73–75° and a trans relationship of the axial halide and pendant OMe group.