[Os(bipy)(CN)₄]^2- and Its Relatives as Components of Polynuclear Assemblies:  Structural and Photophysical Properties

A series of diimine-tetracyanoosmate anions [Os(diimine)(CN)₄]^2- [diimine = 2,2‘-bipyridine (bipy), 2,2‘-bipyrimidine (bpym), 1,10-phenanthroline (phen), and 4,4‘-^tBu₂-2,2‘-bipyridine (^tBu₂bpy)] were prepared and isolated as their Na⁺ salts (water soluble) or PPN⁺ salts (soluble in organic solvents). Several examples were crystallographically characterized; the Na⁺ salts form a range of 1D, 2D, or 3D infinite coordination polymers via coordination of the cyanide groups to Na⁺ cations in either an end-on or a side-on manner. The [Os(diimine)(CN)₄]^2- anions are solvatochromic, showing three MLCT absorptions, which are considerably blue-shifted in water compared to organic solvents, in the same way as is well-known for the analogous [Ru(diimine)(CN)₄]^2- anions. Luminescence in the red region of the spectrum is very weak but (following the expected solvatochromic behavior) is higher energy and more intense in water. However, by exploiting the effect of metallochromism (ref ), the emission from [Os(^tBu₂bpy)(CN)₄]^2- in MeCN can be very substantially boosted in energy, intensity, and lifetime in the presence of Lewis-acidic metal cations (Na⁺, Ba²⁺, Zn²⁺), which, in a relatively noncompetitive solvent, coordinate to the cyanide groups of [Os(^tBu₂bpy)(CN)₄]^2-. This has an effect similar in principle to hydrogen bonding of the cyanides to δ+ protons of water, but very much stronger, such that in the presence of Zn²⁺ ions in MeCN the ¹MLCT and ³MLCT absorptions are blue-shifted by ca. 7000 cm^-1, and the luminescence moves from 970 nm (vanishingly weak) to 610 nm with a lifetime of 120 ns (dominant component). Thus, the binding of metal cations to the cyanides provides a mechanism to incorporate [Os(diimine)(CN)₄]^2- complexes into polynuclear assemblies and simultaneously increases their ³MLCT energy and lifetime to an extent that makes them comparable to much-stronger luminophores such as Ru(II)-polypyridines.