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(NHR 1 )(NHR 2 )}] + (HCNN = 6-phenyl-2,2‘-bipyridine; for R 1 = t Bu, R 2 = Me ( 2 ), NH 2 ( 3 ), CH 2 Ph ( 4 ); for R 1 = 2,6-Me 2 C 6 H 3 (Ar‘), R 2 = Me ( 6 ), NH 2 ( 7 ), CH 2 Ph ( 8 )), are synthesized by nucleophilic attack of amines at the coordinated isocyanide ligands of [(CNN)PtC⋮NR 1 ] + (R 1 = t Bu ( 1 ), Ar‘ ( 5 )). The binuclear bridging bis(carbene) derivative [{(CNN)Pt} 2 μ-{C(NH t Bu)(NH(CH 2 ) 3 )} 2 NMe] 2+ ( 9 ) is prepared by treatment of 1 with excess 3,3‘-diamino- N -methyldipropylamine. The molecular structures of 2 (ClO 4 )·0.5H 2 O, 3 (ClO 4 )·0.5H 2 O, 4 (ClO 4 ), and 6 (ClO 4 ) reveal Pt−C(carbene) distances of 1.997(7), 1.992(4), 1.989(6), and 1.996(8) Å, respectively. Short N−C(carbene) bond lengths (mean 1.332 Å) imply substantial p π −p π interactions within the N−C(carbene)−N fragment, and minimal π bonding in the Pt−C(carbene) moiety is inferred. Weak π−π stacking interactions between the CNN ligands are observed in the crystal lattice of 2 − 4 and 6 (range 3.5−3.6 Å). Complexes 1 − 9 display structureless emissions, with λ max ranging from 528 to 558 nm in acetonitrile at 298 K, and they are assigned to 3 MLCT 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 (≥10 - 5 M), 1 and 5 show an additional low-energy band at 627 (in butyronitrile) and 730 nm which are ascribed to π−π excimeric and MMLCT emissions, respectively. The solid-state emissions of complexes 1 (ClO 4 )− 9 (ClO 4 ) 2 (except 5 (ClO 4 )) at 298 K are red-shifted from solution to λ max 553−579 nm with a shoulder at 595−612 nm. They are tentatively assigned to 3 MLCT excited states with excimeric character due to weak CNN π−π interactions, as evident in the crystal structures. The remarkable low-energy luminescence of 5 (ClO 4 ) in the solid state (λ 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 ).