Steps on the way to the first dirhodium tetracarboxylate with no axial ligation: synthetic lessons and a plethora of Rh2(O2CR)4L2−n compounds, n=0, 1, 2

We report the preparation and structural features of thirteen new dirhodium tetracarboxylato compounds. Twelve of these were synthesized in early attempts to create Rh2(TiPB)4, where TiPB=2,4,6-triisopropylbenzoate, the first dirhodium tetracarboxylate without axial ligation, the structure of which is also reported here. Several challenges in the synthesis of such a molecule are identified and discussed. Although axial interactions are of paramount importance for Rh2 4+ units, RhRh bond lengths are quite insensitive to the nature, presence or absence of axial ligation. However, the effect of axial ligation is clearly observed using electronic spectroscopy; Rh2(TiPB)4 shows an extremely low energy π*(Rh2)→σ*(Rh2) transition. The energy of this transition is put into the context of previously unreported spectroscopy of Rh2(O2CCPh3)4 bis-adducts.