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

Abstract

Reaction of the mononuclear Ni(II) thiolate complexes [Ni(L)] [L, L 1 , H 2 L 1 , bis(2-mercaptoethyl)-1,2-dimercaptoethane; L 2 , H 2 L 2 , 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)]PF 6 , 1, and [Ni(L 2 )FeCp]I, 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(μ-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(μ-pdt)Fe(CO) 3 ], 3, reacts in solution via rearrangement to afford [(OC)Ni(μ-dppe)(μ-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(μ-CO)(μ-pdt)Fe(μ-pdt)Fe(CO) 3 ], 5. Reaction of [Ni(pdt)(dppe)] with [FeCp(CO) 2 I] in CH 2 Cl 2 affords two products [(dppe)Ni(μ-pdt)FeCp(CO)]PF 6 , 6, and [(dppe)Ni(pdt)(μ-I)Ni(dppe)]PF 6 , 7. The complexes 2, 3, and 4 show Ni–Fe distances of 2.539(4), 2.4666(6), and 2.4777(7) Å, respectively, with relatively acute dihedral angles of 79.5–81.8° for the Ni–S 2 -Fe bridge, thus mimicking the shortened Ni...Fe distance (2.5 Å) 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 z 2 )-Fe(d z 2 ) σ-bond in a singlet ground state.