Synthesis, Spectroscopy, and Structure of [FeRu(μ-dithiolate)(CN)2(CO)4]2-

The salt [K(18-crown-6)](2)[Ru(CN)(2)(CO)(3)] ([K(18-crown-6)](2)[<bold>1</bold>]) was generated by the reaction of Ru(C2H4)(CO)(4) with [K(18-crown-6)]CN. An initial thermal reaction gives [Ru(CN)(CO)(4)](-), which, upon ultraviolet (UV) irradiation, reacts with a second equiv of CN-. Protonation of [<bold>1</bold>](2-) gave [HRu(CN)(2)(CO)(3)](-) ([H<bold>1</bold>](-)), which was isolated as a single isomer with mutually trans cyanide ligands. The complex cis,cis,cis-[Ru(pdt)(CN)(2)(CO)(2)](2-) ([<bold>2</bold>](2-)) was prepared by the UV-induced reaction of [<bold>1</bold>](2-) with propanedithiol (pdtH(2)). The corresponding iron complex cis,cis,cis-[Fe(pdt)(CN)(2)(CO)(2)](2-) ([<bold>3</bold>](2-)) was prepared similarly. The pdt complexes [<bold>2</bold>](2-) and [<bold>3</bold>](2-) were treated with Fe(benzylideneacetone)(CO)(3) to give, respectively, [RuFe (mu-pdt)(CN)(2)(CO)(4)](2-) ([<bold>5</bold>](2-)) and [Fe-2(mu-pdt)(CN)(2)(CO)(4)](2-) ([<bold>4</bold>](2-)). The pathway from [<bold>3</bold>](2-) to Fe-2 complex [<bold>4</bold>](2-) implicates intermetallic migration of CN-. In contrast, the formation of [<bold>5</bold>](2-) leaves the Ru(CN)(2)(CO) center intact, as confirmed by X-ray crystallography. The structure of [<bold>5</bold>](2-) features a "rotated" square-pyramidal Fe(CO)(2)(mu-CO) site. NMR measurements indicate that the octahedral Ru site is stereochemically rigid, whereas the Fe site dynamically undergoes turnstile rotation. Fe-57 M & ouml;ssbauer spectral parameters are very similar for rotated [<bold>5</bold>](2-) and unrotated Fe-2 complex [<bold>4</bold>](2-), indicating the insensitivity of that technique to both the geometry and the oxidation state of the Fe site. According to cyclic voltammetry, [<bold>5</bold>](2-) oxidizes at E-1/2 similar to -0.8 V vs Fc(+/0). Electron paramagnetic resonance (EPR) measurements show that 1e(-) oxidation of [<bold>5</bold>](2-) gives an S = 1/2 rhombic species, consistent with the formulation Ru(II)Fe(I), related to the H-ox state of the [FeFe] hydrogenases. Density functional theory (DFT) studies reproduce the structure, H-1 NMR shifts, and infrared (IR) spectra observed for [<bold>5</bold>](2-). Related homometallic complexes with both cyanides on a single metal are predicted to not adopt rotated structures. These data suggest that [<bold>5</bold>](2-) is best described as Ru(II)Fe(0). This conclusion raises the possibility that for some reduced states of the [FeFe]-hydrogenases, the [2Fe](H) site may be better described as Fe(II)Fe(0) than Fe(I)Fe(I).