Synthesis and property of BrCCH- and BrCCBr-coordinated tetrairon clusters

The reaction of (η5-C5H4Me)4Fe4(HCCH)2 (1) with 1 equiv. of N-bromosuccinimide (NBS) gives the one-electron oxidized form in 83% yield. Further treatment of [1]+ with NBS results in the stepwise bromination of four acetylenic protons to give [(η5-C5H4Me)4Fe4(HCCH)(HCCBr)]+ ([2]+), [(η5-C5H4Me)4Fe4(HCCBr)2]+ ([3a]+), [(η5-C5H4Me)4Fe4(HCCBr)(BrCCBr)]+ ([4]+), and [(η5-C5H4Me)4Fe4(BrCCBr)2]+ ([5]+) in moderate yields, with the isomer of [3a]+, [(η5-C5H4Me)4Fe4(HCCH)(BrCCBr)]+ ([3b]+), formed as a minor product. These compounds are characterized by analytical and spectroscopic techniques, and the molecular structures of [2](PF6), [4](TFPB), and [5](TFPB) are established by X-ray diffraction analysis [TFPB = tetrakis{bis(3,5-trifluoromethyl)phenyl}borate]. The compounds are confirmed to retain the butterfly core of four iron atoms as in [1](TFPB). The bromoacetylene part in [2]+ exhibits high reactivity toward various nucleophiles: Cluster[2]+ is moisture-sensitive and is converted to a mixture of [(η5-C5H4Me)4Fe4(HCCH)(μ3-CH)(μ3-CO)]+ ([6]+) and [1]+. Reactions of [2]+ with ZnR2 (R = Me, Et) give [(η5-C5H4Me)4Fe4(HCCH)(HCC–R)]+ in good yields (R = Me ([9]+, 88%), Et ([10]+, 91%)). Accordingly, treatment of [2]+ with HC triple bond; length of mdashCMgBr and LiSpTol leads to the introduction of the ethynyl and thiolate groups to give [(η5-C5H4Me)4Fe4(HCCH)(HCC–Ctriple bond; length of mdashCH)]+ ([11]+, 95%) and [(η5-C5H4Me)4Fe4(HCCH)(HCC–SpTol)]+ ([12]+, 78%), respectively. Substitution of the bromo group in [2]+ with pyridine affords [(η5-C5H4Me)4Fe4(HCCH)(HCC–Py)]2+ ([13]2+) in 90% yield. The reaction with 4,4′-bipyridyl (bpy) requires the severer conditions (70 °C, 2 days), probably due to the relative low basicity of bpy, giving [(η5-C5H4Me)4Fe4(HCCH)(HCC–bpy)]2+ ([14]2+) in 54% yield. The substitution reaction with 4,4′-bipyridyl is strongly accelerated by treatment with silver salt to give [14]2+ in 90% yield. The products derived from [2]+ and nucleophiles are unequivocally determined by elemental, spectroscopic, and X-ray diffraction analyses.