Nickel thiolate complexes

Structural models, which are synthesized to imitate features of the proposed structure of the active site. These may be used to demonstrate the chemical conditions, which allow such structures to exist, to investigate their chemical properties and to give a better understanding of the spectroscopic characteristics of the native proteins. Examples of these include the mixed carbonyl/cyano complexes of iron, used to verify the infrared spectra to the hydrogenases (Fig 7.4) (Lai et al. 1998) and the nickel-thiolate complexes which have low redox potentials like the hydrogenases (Franolic et al. 1992). 

Kockerling, M. and Henkel, G. (1993) Mononuclear nickel thiolate complexes containing nickel sites in different oxidation states - molecular definition of [Ni(SCgH40)2] and [Ni(SCgH40)2]-, Chem. Ber., 126, 951-3. 

Muller, A. and Henkel, G. (1995) [Ni2(SC4H9)g] , a novel binuclear nickel-thiolate complex with NiS4 tetrahedra sharing edges and [Ni(SC( H4SiMe3)4] , a structurally related mononuclear complex ion. Z. Naturforsch., B50, 1464-8. 

Sellmann, D., Geipel, F. and Moll, M. (2000) [Ni(NHPnPr3)(S3)], the first nickel thiolate complex modeling the nickel cysteinate site and reactivity of [NiFe] hydrogenase. Angew. Chem. Int. Ed. Engl., 39, 561-3. 

Zhang W, Hong J, Zheng J, Huang Z, Zhou J., Xu R. Nickel-thiolate complex catalyst assembled in one step in water for solar H2 production. J Am Chem Soc. 2011 133 (51) 20680-3. 

Studies on simple nickel-thiolate complexes are pertinent to discussions on the mechanisms of action of hydrogenases. Consider the pathway for reduction of protons. Initial protonation of metal-thiolate species will always occur at the lone pair of electrons on sulfur (most basic site). Protonation at the metal is thermodynamically less favourable, and usually kinetically slower than protonation of a stereochemical lone pair of electrons [48-50]. 

By oxidative addition of aryl sulphides to low-valent nickel complexes, a C—S bond cleavage occurs to form Ni11 thiolate complexes. For example, exposure of diaryl sulphides to [(But3P)3Ni0] leads to oxidative addition, with nickel inserting into the C—S bond (280).814... 

According to this model the iron-hydride bond is cleaved upon illumination and the hydride, together with the proton, leaves the complex. This would lead to re-establishment of the nickel thiolate bond which was weakened in the former state, explaining the changes in the EPR spectrum observed after illumination (Pig. 7.16-III). After a flip of the electronic z-axis the selenium could in this state interact with the unpaired electron in an orbital with d -y2 character. To get EPR-active, CO-treated... 

The use of six equivalents of dihydrogen peroxide leads to a clean conversion of the dithiolate complex to the disulfonate compound. Earlier studies on oxidation of nickel thiolates showed that oxidations with dioxygen stop at monosulfinates. Our observation and the characterization of the first chelating bis-sulfonato nickel complex formed from the direct oxidation of a mononuclear nickel dithiolate, may also provide new insight into the chemistry of sulfur-rich nickel-containing enzymes in the presence of oxygen. 

Davies, S. C., Evans, D. J., Hughes, D. L., Longhurst, S. and Sanders, J. R. (1999) Synthesis and structure of a thiolate-bridged nickel-iron complex Towards a mimic of the active site of NiFe-hydrogenase. Chem. Commun., 1999, 1935-6. 

Kruger, H.-J. and Holm, R. H. (1989) Chemical and electrochemical reactivity of nickel(II,I) thiolate complexes - examples of ligand-based oxidation and metal-centered oxidative addition. Inorg. Chem., 28, 1148-55. 

Lai, C.-H., Reibenspies, J. H. and Darensbourg, M. Y. (1996) Thiolate bridged nickel-iron complexes containing both iron(O) and iron(II) carbonyls. Angew. Chem. Int. Ed. Engl., 35, 2390-3. 

Figure 5-75. The alkylation of a nickel(n) thiolate complex gives a thioether complex. There is no competitive alkylation of the pyridine nitrogen atom.

As mentioned above, reactions of this type have been widely used in the synthesis of macrocyclic ligands. Indeed, some of the earliest examples of templated ligand synthesis involve thiolate alkylations. Many of the most important uses of metal thiolate complexes in these syntheses utilise the reduced nucleophilicity of a co-ordinated thiolate ligand. The lower reactivity results in increased selectivity and more controllable reactions. This is exemplified in the formation of an A -donor ligand by the condensation of biacetyl with the nickel(n) complex of 2-aminoethanethiol (Fig. 5-78). The electrophilic carbonyl reacts specifically with the co-ordinated amine, to give a complex of a new diimine ligand. The beauty of this reaction is that the free ligand cannot be prepared in a metal-free reac-... 

Many exotic electrophiles have been shown to react with co-ordinated thiolate for example new disulfide bonds may be formed by reaction with S2C12. The nickel(n) complex of a very unusual tetrasulfide macrocyclic ligand may be prepared by this method (Fig. 5-83). Notice that this reaction utilises the nickel complex of the N2S2 ligand prepared by a metal-directed reaction in Fig. 5-78. 

Even an olefin may be sufficiently electrophilic to react with co-ordinated thiolate, and some nickel dithiolene complexes have been shown to react smoothly with norbornadiene (Fig. 5-84). Naturally, the dithiolene complexes also react with more conventional electrophiles, such as methyl iodide (Fig. 5-85). 

The chemistry of carbonyls of higher-valent nickel and iron has not been extensively studied, but a model thiolate complex of Ni(II) with carbon monoxide (Scheme 7) and its Fe(II) analog has been synthesized [160], The methyl deriva-... 

The binuclear nickel-thiolate macrocyclic complex (101) displays noteworthy redox behavior, in which one-electron oxidation yields a Ni. Ni product with significant delocalization of the unpaired electron density onto the bridging thiolate ligands and not onto the second nickel ion. The charge delocalization consequently lies between the two redox extremes of nickel(III)-thiolate and nickel(II)-thiyl radical, thus mimicking the Ni-C state in [NiFeJhydrogenase (see 8ection 8). 

Palladium(ll) and nickel(II) complexes containing the bridging pyrazolate ion as well as the thiolate based dinucleating ligand H,L12, [M2(Ll2)(pz )] (Hpz = Hpz or Hdmpz), were obtained (148). The complexes were easily prepared by treating the [M2(Ll2)(CH2COO)] species with Hpz. ... 

The two thiolate-bridged nickel-copper complexes in Fig. 6.6 have been prepared by reaction of [N2S2]Ni-type complexes with [Cu(CO)(PhTtBu)], the latter available via the carbonylation of [Cu(MeCN)(PhTt Bu)]. These complexes can be employed as model for the catalytic site of the acetyl coenzyme A synthase.14... 

Fig. 6.6. Thiolate bridged nickel-copper complexes containing PhTt Bu.

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