Carbene and Isocyanide Ligation at Luminescent Cyclometalated 6-Phenyl-2,2′-bipyridyl Platinum(II) Complexes: Structural and Spectroscopic Studies

Abstract

A series of luminescent cyclometalated platinum(II) diamino-carbene complexes, namely, [(CNN)Pt{C(NHR1)(NHR2)}]+ (HCNN = 6-phenyl-2,2′-bipyridine; for R1 = tBu, R2 = Me (2), NH2 (3), CH2Ph (4); for R1 = 2,6-Me2C6H3 (Ar′), R2 = Me (6), NH2 (7), CH2Ph (8)), are synthesized by nucleophilic attack of amines at the coordinated isocyanide ligands of [(CNN)-PtC≡NR1]+ (R1 = tBu (1), Ar′ (5)). The binuclear bridging bis(carbene) derivative [{(CNN)Pt}2μ-{C(NHtBu)(NH(CH2) 3)}2NMe]2+ (9) is prepared by treatment of 1 with excess 3,3′-diamino-N-methyldipropylamine. The molecular structures of 2(ClO4)•0.5H2O, 3(ClO4)• 0.5H2O, 4(ClO4), and 6(ClO4) 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 pπ-pπ 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 λmax ranging from 528 to 558 nm in acetonitrile at 298 K, and they are assigned to 3MLCT 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(ClO4)-9(ClO4)2 (except 5(ClO4)) at 298 K are red-shifted from solution to λmax 553-579 nm with a shoulder at 595-612 nm. They are tentatively assigned to 3MLCT 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(ClO4) in the solid state (λmax 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)I2] (10).