Carbene and isocyanide ligation at luminescent cyclometalated 6-phenyl-2,2′-bipyridyl platinum(II) complexes:: Structural and spectroscopic studies

A series of luminescent cyclometalated platinum(II) diamino-carbene complexes, namely, [(CNN)Pt{C(NHR1)(NHR2)}](+) (HCNN = 6-phenyl-2,2'-bipyridine; for R-1 = Bu-t, R-2 = Me (2), NH2 (3), CH2Ph (4); for R-1 = 2,6-Me2C6H3 (Ar'), R-2 = Me (6), NH2 (7), CH2Ph (8)), are synthesized by nucleophilic attack of amines at the coordinated isocyanide ligands of [(CNN)PtC=NR1]+ (R-1 = Bu-t (1), Ar' (5)). The binuclear bridging bis(carbene) derivative [{(CNN)Pt)(2)mu-(C((NHBu)-Bu-t)(NH(CH2)(3))}(2)NMe](2+) (9) is prepared by treatment of 1 with excess 3,3'-diamino-N-methyldipropylarnine. The molecular structures of 2(ClO4). 0.5H(2)O, 3(ClO4). 0.5H(2)O, 4(ClO4), and 6(ClO4) reveal Pt-C(carbene) distances of 1.997(7), 1.992(4), 1.989(6), and 1.996(8) Angstrom, respectively. Short N-C(carbene) bond lengths (mean 1.332 Angstrom) imply substantial p(pi)-p(pi) interactions within the N-C(carbene)-N fragment, and minimal pi bonding in the Pt-C(carbene) moiety is inferred. Weak pi-pi stacking interactions between the CNN ligands are observed in the crystal lattice of 2-4 and 6 (range 3.5-3.6 Angstrom). Complexes 1-9 display structureless emissions, with lambda(max) ranging from 528 to 558 nm in acetonitrile at 298 K, and they are assigned to (MLCT)-M-3 excited states. The emissions of 1 and 5 exhibit noticeably higher quantum yields, which reflect the strong ligand field strength of the isocyanide auxiliaries. The 77 K emissions of 1-9 in frozen acetonitrile are blue-shifted and display well-resolved vibronic structures. At high concentrations (greater than or equal to 10(-5) M), 1 and 5 show an additional low-energy band at 627 (in butyronitrile) and 730 nm which are ascribed to pi-pi excimeric and MMLCT emissions, respectively. The solid-state emissions of complexes 1(ClO4)-9(ClO4)(2) (except 5(ClO4)) at 298 K are red-shifted from solution to lambda(max) 553-579 nm with a shoulder at 595-612 nm. They are tentatively assigned to (MLCT)-M-3 excited states with excimeric character due to weak CNN pi-pi interactions, as evident in the crystal structures. The remarkable low-energy luminescence of 5 (ClO4) in the solid state (lambda(max) 704 nm at 298 K; 781 nm at 77 K) is attributed to MMLCT transitions arising from solid-state intermolecular stacking interactions. Upon photolysis with UV-visible light, reaction between complex 2 and iodomethane in acetonitrile leads to decomposition of the diamino-carbene ligand and generation of the platinum(IV) diiodide cyanide derivative trans-[(CNN)Pt-(C=N)I-2] (10).