Luminescent nitridoosmium(VI) complexes with aryl- and alkylacetylide ligands: Spectroscopic properties and crystal structures

A series of air- and moisture-stable nitridoosmium(VI) complexes with arylacetylide ligands, [ⁿBu₄N][OsN(C≡CC₆H₄R-p)₄ ] (R = H (1), Et (2), OEt (3), Ph (4)), and the alkyl analogue [ⁿBu₄N] [OsN(C≡C^tBu)₄] (5) were synthesized. The structures of 1, 2, and 4 were determined by X-ray crystal analyses. The Os≡N (1.65(1) Å in 1, 1.620(8) Å in 2, and 1.619(6) Å in 4) and Os-C (mean 2.03 Å) distances are similar to that observed for [OsN(CH₂SiMe₃)₄]^-, whereas the C≡C distances (mean 1.20 Å) correspond to terminal arylacetylide groups. The UV-vis absorption spectra of 1-4 display intense absorptions at 248-324 nm (ε ≈ 5 × 10⁴ dm³ mol^-1 cm^-1) that are attributed to overlapping of the [p_π*(N^3-) → d_π*(d_xz, d_yz)] and [π → π*(arylacetylides)] transitions. The moderately intense absorptions at λ_max 403-418 nm (ε ≈ 10³ dm³ mol^-1 cm^-1) are assigned to admixture of [(d_xy)² → (d_xy)¹(d_π*)¹] with intraligand charge-transfer transition of the arylacetylides. The broad structureless solid-state luminescence of 1-5 at 298 (λ_max 600-632 nm) and 77 K (λ_max 599-635 nm) are assigned to the triplet ³[(d_xy)¹(d_π*)¹] excited state. The 77 K glassy solutions of 1-5 also exhibit ³[(d_xy)¹(d_π*)¹] emissions at λ_max 597-628 nm. Like [OsNX₄]^- (X = Cl, Br), complexes 1-5 are nonemissive in dichloromethane at room temperature. Notably, 1 emits weakly at λ_max 643 nm in benzene at room temperature, whereas 5 exhibits a more intense and long-lived luminescence at λ_max 640 nm in diethyl ether. Correspondingly, a well-defined absorption at λ_max 428 nm in the excitation spectrum of 5 (λ_em 640 nm, concentration 8 × 10^-4 mol dm^-3) is assigned to the ¹[(d_xy)²] → ¹[(d_xy)¹(d_π*)¹] transition.