Chien-Ming Lee

Mononuclear NiIII−Alkyl Complexes (Alkyl = Me and Et): Relevance to the Acetyl-CoA Synthase and Methyl-CoM Reductase

Mononuclear, distorted trigonal bipyramidal [PPN][Ni(III)(R)(P(C(6)H(3)-3-SiMe(3)-2-S)(3))] (R = Me (1); R = Et (2)) were prepared by reaction of [PPN][Ni(III)Cl(P(C(6)H(3)-3-SiMe(3)-2-S)(3))] and CH(3)MgCl/C(2)H(5)MgCl, individually. EPR, SQUID studies as well as DFT computations reveal that the Ni(III) in 1 has a low-spin d(7) electronic configuration in a distorted trigonal bipyramidal ligand field. The Ni-C bond lengths of 1.994(3) A in 1 and 2.015(3) A in 2 are comparable to that in the Ni(III)-methyl state of MCR (approximately 2.04 A) (Sarangi, R.; Dey, M.; Ragsdale, S. W. Biochemistry 2009, 48, 3146). Under a CO atmosphere, CO triggers homolytic cleavage of the Ni(III)-CH(3) bond in 1 to produce Ni(II)-thiolate carbonyl [PPN][Ni(II)(CO)(P(C(6)H(3)-3-SiMe(3)-2-S)(3))] (3). Additionally, protonation of 1 with phenylthiol generates Ni(III)-thiolate [PPN][Ni(III)(SPh)(P(C(6)H(3)-3-SiMe(3)-2-S)(3))] (4).

Preparative and structural studies on iron(II)-thiolate cyanocarbonyls: relevance to the [NiFe]/[Fe]-hydrogenases.

A number of thermally stable iron(II)-thiolate cyanocarbonyl complexes, cis,cis-[Fe(CN)2(CO)2(CS3-S,S)]2-(1), mer-[Fe(CO)2(CN)3(NCCH3)]-(2)mer-[Fe(CO)3(CN)(CS3-S,S)]-(3), cis-[Fe(CO)2(CN)(S(CH2)2S(CH2)2S-S,S,S)]-(4), [Fe(CO)2(CN)3Br]2-(5), mer-[Fe(CO)2(CN)3(m-SC6H4Br)]2-(6) and mer-[Fe(CO)2(CN)3(SPh)]2-(7) were isolated and characterized by IR and X-ray diffraction analysis. The extrusion of one strong sigma-donor CN- ligand instead of CO from the iron(II) center of the thermally stable complexes [FeII(CO)2(CN)3Br]2-(5) containing less electron-donating bromide reflects the electron-rich character of the mononuclear [FeII(CN)2(CO)2(CS3-S,S)]2-(1) when ligated by by the bidentate thiolate, and the combination of one cyanide, two carbonyls and a tridentate thiolate provides the stable complex 4 as a result of the reaction of complex 5 and chelating ligand [S(CH2)2S(CH2)2S]2-. The preference of the sixth ligand coordinated to the unsaturated [FeII(CO)(CN)2(CS3-S,S)]2- Fe(II) center, the iron-site architecture of the bimetallic Ni-Fe active-site of [NiFe] hydrogenases, is a strong pi-acceptor CO group. Scrutiny of the coordination chemistry of iron(II)-thiolate cyanocarbonyl species [FeII(CO)x(CN)y(SR)z]n- reveals that certain combinations of thiolate, cyanide and carbonyl ligands (3 < or = y+z > or = 4) bound to Fe(II) are stable and this could point the way to understand the reasons for Natures choice of combinations of these ligands in hydrogenases.

Heterotrinuclear complexes containing a hexaselenolato-zinc(II), -cadmium(II), -nickel(II) or -iron(II) core: crystal structures of [(OC)3Fe(µ-SePh)3M(µ-SePh)3Fe(CO)3] (M = Zn, Cd, Ni or Fe) and [Fe(CO)2(phen)(SePh)2] (phen = 1,10-phenanthroline)

A series of linear trinuclear selenolate complexes of the general type [(OC)3Fe(µ-SePh)3M(µ-SePh)3Fe(CO)3] (M = ZnII 1, CdII 2, NiII 3 or FeII 4) has been synthesized by reaction of the appropriate [M(H2O)n]2+ with the chelating metalloligand fac-[N(PPh3)2][Fe(CO)3(SePh)3] in acetonitrile. Their structures were elucidated on the basis of IR, UV/VIS, NMR spectra and X-ray crystallography. Compounds 1–4 are structurally related, each containing two tridentate metalloligands resulting in a MSe6 co-ordination environment of virtual Oh symmetry. Further reaction of 4 with 3 equivalents of 1,10-phenanthroline (phen) initially afforded the intermediate [Fe(phen)3][Fe(CO)3(SePh)3]2 but eventually neutral [Fe(CO)2(phen)(SePh)2] 5 was formed as the sole product. Its crystal structure has been determined. This investigation shows that fac-[Fe(CO)3(SePh)3]– serves as a tridentate chelating metalloligand and selenolate ligand-transfer reagent.