A family of heterometallic cubane-type clusters with an exo-Fe(CO)3 fragment anchored to the cubane.

Metal-rich boron clusters are members of a rapidly growing family of metallaborane “hybrid” systems which bridges the gap between metal clusters and polyhedral boranes. Clusters such as [Cp4Ni4B4H4] [2] and [Cp4Co4B4H4] [3] (Cp = h-C5H5) can be linked with the well-studied class of tetranuclear metal complexes known as cubanes. Recently, a metal-rich metallaborane with the same geometry, namely, [(Cp*Ru)3(m3CO)Co(CO)2B3H3] (Cp* = h -C5Me5), was reported by Fehlner et al. Cubane is quite a popular shape, especially for combinations of heterometal and main group elements. These compounds are of interest not only because of their contribution to the development of organometallic chemistry, but also for their potential use as models for various industrial and biological catalytic processes. As a part of our ongoing studies on metallaboranes and their derivatives, we recently reported the synthesis of [(Cp*Mo)2B4H4E2] [11] (1: E = S; 2 : E = Se) and arachno[(Cp*RuCO)2B2H6] [12] (3) in good yields. Until now we have focused on the chemistry of metallaboranes with boranes, main group elements, and small organic molecules. We have now extended our studies to transition metal carbonyl compounds such as [Fe2(CO)9], [Mn2(CO)10], and [Co2(CO)8], since earlier work suggested their potential as versatile reagents in cluster-building reactions. Reaction with [Co2(CO)8] led to decomposition, whereas mild pyrolysis of [Fe2(CO)9] with 1–3 in hexane led to hybrid clusters [(Cp*M)2(m3-E)2B2H(m-H){Fe(CO)2}2Fe(CO)3] (4 : M = Mo, E = S; 5 : M = Mo, E = Se; 6 : M = Ru, E = CO). The identities of 5 and 6 were established by a single-crystal X-ray diffraction study, which together with spectroscopic studies demonstrated the existence of novel capped-cubane cluster cores. Although X-ray quality crystals of 4 have not been obtained yet, its identity is inferred by comparison to selenium analogue 5. The overall structure of 5 is intriguing and its geometry can be viewed in a few different ways. The more obvious approach is to recognize the cubane shape made of two Mo, two Fe, two Se, and two B atoms, capped by a third Fe atom attached to one of the B-Fe-Fe faces of the cube (Figure 1). An alternative description of 5 is as a Mo2Fe2 tetrahedron face-capped by two selenium and two boron atoms. Capping one of the resulting Fe2B faces with another Fe(CO)3 group generates capped cubane 5.