2,2′:6′,2″-Terpyridine (terpy) acting as a fluxional bidentate ligand. Part 1. Trimethylplatinum(IV) halide complexes [PtXMe3(terpy)](X = Cl, Br or I): nuclear magnetic resonance studies of their solution dynamics and crystal structure of [PtIMe3(terpy)]

Abstract

2,2′:6′,2″-Terpyridine (terpy) reacts with trimethylplatinum halides [(PtXMe₃)₄](X = Cl, Br or I) to form stable octahedral complexes fac-[PtXMe₃(terpy)](X = Cl, Br or I) in which the terpy molecule is acting as a bidentate chelate ligand. In solution the complexes are fluxional with the ligand oscillating between equivalent bidentate bonding modes by a mechanism consisting of ‘tick-tock’ twists of the metal moiety through an angle equal to the N–Pt–N angle of the octahedral centre. At below-ambient temperatures rotation of the unco-ordinated pyridine ring is severely restricted with the most favoured rotamers having the plane of the pendant pyridine ring at an angle of ca. 52° with respect to the adjacent co-ordinated pyridine ring plane. The X-ray crystal structure of [PtIMe₃(terpy)] depicts the pendant pyridine N atom cis to iodine and this is the predominant species in solution at low temperatures. At above-ambient temperatures the complexes exhibit intramolecular Pt–Me exchange of axial and equatorial environments. Energy data based on accurate dynamic NMR fittings are reported for the three dynamic processes, namely pendant pyridine rotation, 1,4-Pt–N metallotopic shifts and Pt–Me scramblings.