Luminescent complexes of iridium(III) with 6′-phenyl-2,2′-bipyridine and 4′-aryl derivatives:: NAC versus NAN coordination

The reaction of iridium terpyridyl trichloride, [Ir-(tpy)Cl-3], with the following ligands has been investigated: 6'-phenyl-2,2'-bipyridine (HL1), its 4'-tolyl-derivative (HL2), and an annelated version of the latter incorporating an ethylene bridge between C5' and the pendent phenyl ring (HL3). Although all three are potentially C<^>N<^>N-coordinating ligands, none are found to bind in this way in the products isolated. Ligand HL1 binds through its two nitrogen atoms only, leading to a dicationic complex [Ir(tpy)(HL1-N,N)Cl](2+), with an 'N5Cl' coordination sphere. In contrast, the presence of the 4'-tolyl group in HL2 and HL3 favours cyclometallation of the central pyridine ring through position C3', leading to N<^>C-binding and to the monocationic complexes [Ir(tpy)(L-2-C-3',N)Cl](+), 2, and [Ir(tpy)(L-3-C-3',N)Cl](+), 3. The cyclometallated complexes 2 and 3 are intensely luminescent in solution at room temperature (lambda(max) = 541 and 562 nm; tau = 3.2 and 2.4 mu s, respectively: phi = 0.16 for both, in degassed CH3CN at 295 K), probably arising from a state of primarily (MLCT)-M-3 character. In contrast, the N-5-coordinated complex 1 is only weakly emissive under the same conditions. (c) 2005 Academie des sciences. Published by Elsevier SAS. All rights reserved.