Metal-Thilate Complexes as Models for Metal-Cysteine Centers in Metalloproteins: the Role of the Second Coordination Sphere Interactions
MetadataShow full item record
As a clean energy source, hydrogen is an attractive alternative. Among all the challenges that hydrogen economy has, it is particularly important to develop new ways to generate hydrogen without hydrocarbons. In search of cheap and robust material, we are learning from nature.Hydrogenase enzymes, which occur in a wide variety of microorganisms, catalyze the oxidization or production of molecular hydrogen (2H+ + 2e = H2,). In the case of [NiFe]-hydrogenase enzymes, the Ni is coordinated as a [Ni-(S-Cys)4] center which undergoes a series of redox changes during the catalytic processes. This work is focusing on the synthesis of model compounds that can provide some insights into the mechanism by which H2 is consumed or produced. To mimic the cysteine coordination in metalloezymes, we have used a variety of chelating type ligands with thiolate and phosphine donor atoms: [bis-phenyl-(5-methyl-2-thio-phenyl)phosphine] (abbreviated PS1'), [bis(5-methyl-2-thio-phenyl)phenylphosphine] (abbreviated PS2') and [tris(5-methyl-2-thio-phenyl) phosphine] (abbreviated PS3'), to create new compounds. Herein, we report the synthesis and properties of series of Ni, Pd and Pt complexes of PS2' and/or PS1'. Significantly, a biologically relevant three membered redox series of Ni(II) ↔ Ni(III) ↔ Ni(IV) have been synthesized and the oxidation state of metal center was confirmed by Ni K-edge XAS (X-ray Absorption Spectroscopy). [MII(PS2'H)2] and [MII(PS2')2]2- (M = Ni, Pd and Pt) are square planar complexes with [MS2P2] coordination and two non-coordinated pendant thiols and thiolates respectively. These M(II) complexes can be oxidized to form octahedral M(IV) complexes, [MIV(PS2')2], in which the pendant thiolates have coordinated to the metal. Under certain conditions the M(II) complexes are oxidized without the addition of any external oxidizing agent. The pendant thiol SH's in the [MII(PS2'H)2] complexes are engaged in hydrogen bonding interactions with the metal. This interaction has been demonstrated by X-ray crystallography and by NMR studies which indicated coupling between the thiol SH and 195Pt. 1H and 31P NMR spectroscopic studies have been used to study the thermal and photochemical induced isomerization of [PtII(PS2'H)2] and its oxidation to [MIV(PS2')2]. All four possible isomers of [PtII(PS2'H)2] have been detected in the NMR (Nuclear Magnetic Resonance) spectra, and the structures of three of the isomers have been characterized by X-ray crystallography. The participation of the pendant SH protons in the oxidation of the M(II) complexes has been demonstrated. The pendant thiolate groups of the [MII(PS2')2]2- (M = Pd, Pt) complexes react with CH2Cl2 under very mild condition to give a neutral compound in which the pendant thiolates have become linked by a CH2 group. A characteristic feature of all these studies is the reactivity of the pendant ligands. In metalloproteins, the reactivity of the metal center is determined by the residues in the second coordination sphere as well as the groups which bind directly to the metal.In a second study, the reactivity of [FeII FeII(PS3)2]2- species with molecular oxygen has been investigated. Using different solvent and temperature conditions, two different binuclear products have been isolated and characterized. One in which the phosphine has been oxidized to a phosphine oxide and a second product in which two atoms of oxygen have been added to a thiolate to generate a RSO2- ligand.