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 (pdt2– = 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 Fc0/+ 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 pKaPhCN 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.