Modulation of the electronic structure and the Ni-Fe distance in heterobimetallic models for the active site in [NiFe]hydrogenase

Reaction of the mononuclear Ni(II) thiolate complexes [Ni(L)] [L, L-1, H2L1, bis(2-mercaptoethyl)-1,2-dimercaptoethane; L-2, H2L2, N,N'-dimethyl-N,N'-bis(2-mercaptoethyl)-bis(aminoethyl)sulfide] with [FeCP(CO)(2)I] gives the dithiolate-bridged heterobimetallic species, [Ni(L-1)FeCp(CO)]PF6, 1, and [Ni(L-2)FeCp]1, 2, respectively. Binding of a Fe(CO)3 fragment via reaction of square-planar [Ni(pdt)(dppe)] (dppe, 1,2-diphenylphosphinoethane; pdt(2-), 1,3-propanedithiolate) with Fe-3(CO)(12) or [Fe(CO)(3)(BDA)] (BDA, benzylidene acetone) affords diamagnetic [(dppe)Ni(mu-pdt)Fe(CO)(3)], 3, in which the Ni(II) center is bound tetrahedrally to two thiolate S-donors and to two P-donors. The complex [(dppe)Ni(mu-pdt)Fe(CO)(3)], 3, reacts in solution via rearrangement to afford [(OC)Ni(mu-dppe)(mu-pdt)Fe(CO)(2)], 4, in which one P-donor of dppe is bound to Ni and the other to Fe, and a CO ligand has transferred from Fe to Ni. Additionally, the syntheses of 3 and 4 afford the side products [(dppe)Ni(CO)(2)] and [(OC)(3)Fe(pdt)Fe(CO)(3)] together with the trinuclear species [(dppe)(CO)Fe(mu-CO)(mu-pdt)Fe(mu-pdt)Fe(CO)(3)], 5. Reaction of [Ni(pdt)(dppe)] with [FeCP(CO)211 in CH2Cl2 affords two products [(dppe)Ni(mu-pdt)FeCp(CO)]PF6, 6, and [(dppe)Ni(pdt)(mu-l)Ni(dppe)]-PF6, 7. The complexes 2, 3, and 4 show Ni-Fe distances of 2.539(4), 2.4666(6), and 2.4777(7) angstrom, respectively, with relatively acute dihedral angles of 79.5-81.8 degrees for the Ni-S-2-Fe bridge, thus mimicking the shortened Ni...Fe distance (2.5 angstrom) and the acute dihedral angle of the Ni-S-2-Fe moiety observed in certain active forms of [NiFe]hydrogenase. The role of direct Ni-Fe bonding in these complexes is discussed and linked to electronic structure calculations on [(dppe)Ni(pdt)Fe(CO)(3)], 3, which confirm the presence of a bent Ni(d(z2))-Fe(d(z2)) sigma-bond in a singlet ground state.