Synthesis and characterization of μ,μ′-M(B5H8)2 (M=Cd, Hg and Zn): a reassignment of the NMR spectra for 2,3-μ-metalloderivatives of pentaborane(9)

The series of compounds μ,μ′-M(B5H8)2, where M=Cd (I), Zn (II), or Hg (III), has been prepared from the reaction between K(B5H8) and the metal chloride in THF at low temperatures. The species were characterized by multinuclear NMR spectroscopy, elemental analysis and mass spectrometry. The NMR spectral assignments were confirmed using 11B{1H} selective decoupling experiments and heteronuclear 11B–1H chemical shift correlation spectroscopy. The results appeared to be in conflict with the earlier assignment of the 11B-NMR spectra for 2,3-μ-metalloderivatives of pentaborane(9) which appear in the literature. For systems with an electrophilic group replacing a bridging H atom in B5H9, the B atoms most shifted in the 11B-NMR spectrum, which appear at the lower field, were assigned to the ones closest to the metal group. Our results for I–III suggested that the assignment was reversed, that is the higher field resonance is the one closest to the metal group. Thus, we reexamined the spectra of a series of related compounds including 2,3-μ-SnPPh3(B5H8), μ,μ′-SnPh2(B5H8), μ,1′-SnPh2(B5H8), other Sn species and 2,3-μ-Cu(dppe)B5H8. In all cases our assignments were in accord with those for I–III and we suggest that this is general for all such B5H9 derivatives.