Developments in the biomimetic chemistry of cubane-type and higher nuclearity iron-sulfur clusters.

Abstract

A prior thematic issue of Chemical Reviews in 20041 provides broad coverage of the field of biomimetic inorganic chemistry. One principal objective of this field, which is a component of the continually burgeoning multidisciplinary enterprise that is bioinorganic chemistry, is the synthesis of analogues of mononuclear and polynuclear sites in proteins which convey information of significance in interpreting the physical and chemical properties of such sites. This article focuses on polynuclear analogues, specifically higher nuclearity, biomimetic metal-sulfur clusters. Many synthetic and biological clusters can be designated as either strong-field or weak-field. Strong-field clusters are formed by first transition series elements with π-acceptor ligands and by second and third series elements regardless of ligation, and manifest properties arising from large splittings of the d-orbital manifold at individual metal sites. Such species are subsumed under a restrictive definition of clusters as containing two or more metal atoms where direct and substantial metal-metal bonding is present.2 A broader definition now normally employed leads to recognition of weak-field clusters. These species contain σ/π-donor ligands that induce smaller d-orbital splittings favoring individual metal sites with high-spin configurations, magnetic interactions among these individual sites, paramagnetic molecular ground states, and labile ligand binding. These clusters contain first transition series elements. Prominent examples of metals in native weak-field cluster sites include (but are not limited to) Mn3–6 (catalases, photosystem II), non-heme Fe7–10 (O2 carriers, oxygenases, reductases, hydrogenase), Fe-S11–14 (electron transfer, nitrogenase, numerous nonredox functions), Ni15,16 (urease), and Ni-Fe17–19 (hydrogenase). The only exceptions to the weak-field designation are Fe sites in [FeFe]- and [NiFe]-hydrogenases, which occur as the fragments Fe(CO)(CN)(μ2-CO) and Fe(CO)(CN)2(μ2-H), respectively. We also note that the weak-field/strong field distinction does not strictly apply to zinc and copper complexes because ZnII and CuI are necessarily diamagnetic, CuII has a spin-doublet ground state, and CuIII is uniformly diamagnetic. However, all but CuIII manifest the substitutional lability associated with the weak-field case. As examples of weak-field clusters, structures of selected protein-bound sites 1–8 containing Mn, non-heme Fe, Fe-S, Ni, and Ni-Fe (Ni site) are provided in Figure 1. Because of the restricted scope of this article, a selected bibliography of summary accounts of the biomimetic chemistry of mono- and polynuclear sites is presented in Table 1. Citations are in the period 2004–2013 and do not include the contents of the previous thematic issue.1 We note in particular a book devoted to bioinorganic synthesis,20 a treatment of biosynthetic inorganic chemistry involving manipulation of protein-bound sites,21 and biomimetic research on sulfur-ligated sites.22 Other examples of weak-field metal sites in proteins are found in this issue. Figure 1 Schematic structures of illustrative weak-field protein-bound clusters: O2-evolving center in photosystem II (1), [FeFe]-hydrogenase (2), [NiFe]-hydrogenase (3); dinuclear (4), trinuclear cuboidal (5), and tetranuclear cubane-type (6) iron-sulfur clusters; ... Table 1 Selected Bibliography of Biomimetic Inorganic Chemisty: 2004–2013 The purview of this article is the biomimetic chemistry of metal-sulfur clusters as confined to post-2004 advances of homo- and heterometallic iron-sulfur clusters of nuclearity four and higher. The emphasis is on the synthetic approaches to such clusters, some of which in their native condition are implicated directly in numerous enzymological processes.