Modulation of the Electronic Structure and the Ni-Fe Distance in Heterobimetallic Models for the Active Site in [NiFe]hydrogenase

• Published in: Proceedings of the National Academy of Sciences - PNAS 2005-12, Vol.102 (51), p.18280-18285
• Main Authors: Wenfeng Zhu, Andrew C. Marr, Wang, Qiang, Neese, Frank, Douglas J. E. Spencer, Blake, Alexander J., Paul A. Cooke, Wilson, Claire, Schröder, Martin, Halpern, Jack
• Format: Article
• Language: English
• Subjects: Active sites; Binding Sites; Chemistry; Crystal structure; Diamagnetism; Dihedral angle; Electronic structure; Electronics; Electrons; Enzymes; Hydrogenase - chemistry; Hydrogenase - metabolism; Iron; Iron - metabolism; Ligands; Models, Molecular; Nickel; Nickel - metabolism; Orbitals; Physical Sciences; Protein Structure, Tertiary; Room temperature
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.0505779102

Reaction of the mononuclear Ni(II) thiolate complexes $[Ni(L)] [L, L^1, H_2L^1$, bis(2-mercaptoethyl)-1,2-dimercaptoethane; $L^2, H_2L^2$, N,N'-dimethyl-N,N'-bis(2-mercaptoethyl)-bis(aminoGthyl)sulfide] with $[FeCp(CO)_2I]$ 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 $[(dppe)Ni(\mu -pdt)Fe(CO)_3]$, 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 $[(OC)_3Fe(pdt)Fe(CO)_3]$ 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 $[FeCp(CO)_2I]$ in CH2Cl2 affords two products $[(dppe)Ni(\mu -pdt)FeCp(CO)]PF_6$, 6, and $[(dppe)Ni(pdt)(\mu -I)Ni(dppe)]-PF_6$, 7. The complexes 2, 3, and 4 show Ni-Fe distances of 2.539(4), 2.4666(6), and $2.4777(7) \AA$, respectively, with relatively acute dihedral angles of $79.5-81.8\textdegree$ 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 $Ni(d_{z^2})-Fe(d_{z^2})$ $\sigma-bond$ in a singlet ground state.