Square-planar rhodium(I) complexes with aromatic bent terdentate nitrogen ligands as candidates for rod-like extended materials

The crystal structure of 2,6-bis(1-methylbenzimidazol-2-yl)pyridine (L-4) evidenced a quasi trans-trans conformation of the unsubstituted nitrogen atoms of the aromatic rings which is maintained in solution on the NMR timescale. Reactions of L-4, or of its lipophilic analogue 2,6-bis(1-octylbenzimidazol-2-yl)pyridine (L-5), with [Rh(NBD)Cl](2) (NBD=bicyclo[2.2.1]hepta-2,5-diene) produced poorly soluble dimetallic complexes [{Rh(NBD)Cl}(2)L-i] (i=4 or 5). The crystal structure of [{Rh(NBD)Cl}(2)L-5] revealed that the terdentate aromatic unit adopts a distorted trans-trans conformation and acts as a bridging ligand connecting two [Rh(NBD)Cl] units through the nitrogen atoms of the benzimidazole side arms. Solution studies (ES-MS, NMR) indicate that (i) the dimetallic structure is maintained in solution, (ii) the complicated fluxional behaviour observed at room temperature is blocked at 210 K on the NMR timescale and (iii) the bent aromatic core cannot act as a meridional terdentate chelate toward Rh-I. The consequences of this co-ordination behaviour on the molecular anisometry of extended complexes is discussed.