cis-dicyanoosmium(II) diimine complexes bearing phosphine or sulfoxide ligands: Spectroscopic and luminescent studies

Abstract

A series of cis-dicyanoosmium(II) complexes [Os(PPh 3) 2(CN) 2(N∩N)] (N∩N = Ph 2phen (2a), bpy (2b), phen (2c), Ph 2bpy (2d), tBu 2bpy (2e)) and [Os(DMSO) 2(CN) 2(N∩N)] (3a-3e, N∩N = Br 2phen (31), Clphen (3g)), were synthesized and their spectroscopic and photophysical properties were examined, and [Os(PMe 3) 2(CN) 2(phen)] (4) with axial PMe 3 ligands was similarly prepared. The molecular structures of 2a, 2c, [2c·Zn(NO 3) 2] ∞, 2d, 2e, 3b, 3d, 3e, and 4 were determined by X-ray crystallographic analyses. The two CN ligands are cis to each other with mean Os-C bond distance of 2.0 Å. The two PR 3 (R = Ph, Me) or DMSO ligands are trans to each other with P/S-Os-P/S angles of ∼77°. The UV-vis absorption spectra of 2a-2e display an intense absorption band at 268-315 nm (ε = ∼(1.54-4.82) × 10 4 M -1 cm -1) that are attributed to π → π*(N∩N) and/or π → π*(PPH 3) transitions. The moderately intense absorption bands with λ max at 387-460 nm (ε = ∼ (2.4-11.3) × 10 3 M -1 cm -1) are attributed to a 1MLCT transition. A weak, broad absorption at 487-600 nm (ε = ∼390-1900 M -1 cm -1) is assigned to a 3MLCT transition. Excitation of 2a-2e in dichloromethane at 420 nm gives an emission with peak maximum at 654-703 nm and lifetime of 0.16-0.67 μs. The emission energies, lifetimes, and quantum yields show solvatochromic responses, and plots of ν max, τ, and Φ, respectively, versus E T (solvent polarity parameter) show linear correlations, indicating that the emission is sensitive to the local environment. The broad structureless solid-state emission of 2a-2e at 298 (λ max 622-707 nm) and 77 (λ max 602-675 nm) K are assigned to 3MLCT excited states. The 77 K MeOH/EtOH (1:4) glassy solutions of 2a-2e also exhibit 3MLCT emissions with λ max = 560-585 nm. The 1MLCT absorption and 3MLCT emission of 3a-3g occur at λ max = 332-390 nm and 553-644 nm, respectively. In the presence of Zn(NO 3) 2, both the 1MLCT absorption and 3MLCT emission of 2c in acetonitrile blue-shift from 397 to 341 nm and 651 to 531 nm, respectively. The enhancement of emission intensity (I/I o) of 2e at 531 nm reached a maximum of ∼810 upon the addition of two equivs of Zn(NO 3) 2. The crystallographic and spectroscopic evidence suggests that 2c undergoes binding of Zn 2+ ions via the cyano moieties. © 2007 American Chemical Society.