Organo ruthenium-nickel dithiolates with redox-responsive nickel sites

Described is a new family of RuNi dithiolates featuring geometrically flexible Ni centers, which enable both acid-base and redox chemistry, behavior that is characteristic of the hydrogenases. Treatment of Ni(pdt)(dxpe) with (cymene)₂Ru₂Cl₄ affords the salt [(cymene)Ru(Cl)(pdt)Ni(diphos)]Cl (pdt^2- = 1,3-propanedithiolate, diphos = dppe = 1,2-bis(diphenylphosphino)ethane ([1Cl]Cl) and diphos = dcpe = 1,2-bis(dicyclohexylphosphino)ethane ([2Cl]Cl)). Cyclic voltammetry revealed that in CH₂Cl₂ solution [1Cl]Cl reduces irreversibly near -1.6 V vs Fc^0/+ followed by the appearance of a reversible 1e ^- couple assigned to the [1]^0/+ couple. Reduction of [1Cl]Cl and [2Cl]Cl with cobaltocene produced the neutral derivatives (cymene)Ru(pdt)Ni(diphos) ([1]⁰, [2]⁰). Crystallographic characterization of these compounds revealed short Ru-Ni distance of 2.5539(5) ([1]⁰) and 2.600(3) Å ([2]⁰). The 2e^- reduction of these chlorides converts the Ni site from square planar to tetrahedral, highlighting the flexibility of the Ni center in these complexes. Variable-temperature NMR studies show that [1]⁰ and [2]⁰ are dynamic by virtue of ring flipping in the RuNi(pdt) core. Complexes [1] ⁰ and [2]⁰ are basic: their conjugate acids, [H1] ⁺ and [H2]⁺, exhibit pK_a^PhCN values of 18.94 and 21.65, respectively. Crystallographic characterization of [H1] ⁺ as its aryl borate salt revealed an unsymmetrical Ru-H···Ni interaction and confirmed that the Ni center converted from tetrahedral to square planar, again demonstrating the flexibility of this site.