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 Rh-2(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 Rh-2(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; Rh-2(TiPB)(4) shows an extremely low energy pi*(Rh-2) --> sigma*(Rh-2) transition. The energy of this transition is put into the context of previously unreported spectroscopy of Rh-2(O2CCPh3)(4) bis-adducts. (C) 2002 Published by Elsevier Science B.V.