Biological systems employ polynuclear clusters for the biological activation of small molecule substrates in a way to promote multi-electron redox reactions. For instance, biological N₂ fixation is a process that involves a six-electron reduction and it is catalyzed by a family of metalloenzymes called nitrogenases. The most abundant nitrogenase is the molybdenum-dependent nitrogenase found in diazotrophs that contains the iron-molybdenum cofactor (FeMoco) in its active site – a cluster composed of seven Fe and one Mo atoms bound through bridging sulfides. There are evidences for the presence of hydride bridges in the FeMoco during the N₂ reduction and such ligands are believed to act as sites for storage of electron and proton equivalents, having an essential role in the N₂-binding and reduction. For these reasons, we are investigating the synthesis and reactivity of bromide- and sulfide-bridged triiron clusters and specifically, that of mixed sulfide-hydride systems. In this work, we will discuss our ongoing efforts to synthesize and examine the reactivity of such clusters.

Fe(II) complexes with ligands of an intermediate ligand field strength show magnetic bistability, i.e. switching between the high-spin and low-spin electronic configurations driven by changes in temperature, pressure, or photoexcitation. We are interested in designing multi functional materials that exhibit both spin crossover (SCO) and conductivity by combining para-magnetic Fe(II) cationic complexes and TCNQ^·– anion radicals. It is expected that the latter should provide conducting pathways via the formation of p-p interacting stacks in the solid state structure. The synthetic strategy toward such hybrid materials is two-pronged: first, the TCNQ^·– anions can coordinate to Fe(II) ions that are partially protected by blocking ligands, in order to limit fast precipitation of extended structures; second, these anions can be co-crystallized with completely blocked Fe(II) centers. In both approaches, the goal is to obtain a hybrid crystal structure that features both conducting stacks of fractionally charged organic radicals and SCO Fe(II) complexes. We report the synthesis of several such complexes, which demonstrate significant conductivity values and, in some cases, SCO behavior.

The carborane anion CB₁₁H₁₂ and its derivatives have received widespread attention in the recent years due to its weakly coordinating nature. This allows it to support highly reactive counterions such as H⁺, CH₃⁺, R₃Si⁺, and also make it able to mask empty coordination sites in the coordination sphere of transition metals. With this work, we report synthetic routes for the preparation of alkoxy derivatives, RO- with R = Me, Et, Pr, Bu, of the carborane anion and we explore the possibility for their application in non-coordination chemistry

A recent experimental investigation of CuFe₂Ge₂ revealed that this compound exhibits weak itinerant antiferromagnetism,¹ in agreement with the earlier theoretical prediction.² Based on the similarity of the electronic band structure of CuFe₂Ge₂  and recently discovered superconductor, YFe₂Ge₂,³ it has been suggested that proper doping into the former structure might suppress the antiferromagnetic ordering and lead to the superconducting state.² Guided by the electronic structure calculations performed at the DFT level of theory, we pursued electron doping by exploring the entire range of isostructural solid solutions, CuFe_2–xCo_xGe₂, with an incremental (0.2) increase in the Co content across the series. Magnetic measurements do reveal a gradual suppression of the AFM behavior with the increase in the Co content (x). The low-temperature investigation of the materials that appear at the borderline of AFM ordering might result in the discovery of a superconducting phase transition.

References

1. May, A. F.; Calder, S.; Parker, D. S.; Sales, B. C.; McGuire, M. A. Sci. Rep. 2016, 6, 35325.
2. Shanavas, K. V.; Singh, D. J. PLoS One 2015, 10, 1.
3. Zou, Y.; Feng, Z.; Logg, P. W.; Chen, J.; Lampronti, G.; Grosche, F. M. Phys. Status Solidi - Rapid Res. Lett. 2014, 8, 928.

Developing new synthetic routes to high nuclearity Mn^III,IV clusters has been an integral part of the research in our group. Several years ago we reported a new synthetic method involving ‘reductive aggregation’ of MnO₄^- ions in the presence of excess RCO₂H in MeOH. This approach gave several new Mn₁₂ and Mn₁₆ clusters. However, to date reductive aggregation has not been employed for the synthesis of heterometallic Mn clusters, and this has stimulated our present interest in extending this method to Mn-Ce chemistry. Using reductive aggregation, we have now obtained a family of related Mn-Ce clusters. This presentation will describe their structures and magnetic properties, including the identification of unusual long-range coupling between Mn ions separated by diamagnetic Ce(IV).

As a means to interrogating the fundamental chemical transformations effected by the iron-molybdenum cofactor (FeMoco), we have explored the development of novel ligands designed to generate multiiron compounds and the reactivity profiles of the resultant complexes. In particular, our reported triiron(II) tri(μ-hydride) species provides a platform to understand the reactivity of hydrides bound to high spin iron clusters. To that end, recent results on reductive elimination and oxidative addition, hydride transfer reactivity, and bond activation will be discussed. Notably, these reactivity manifolds are commonly invoked in catalytic cycles for biological metal cluster cofactors.