The first dirhodium tetracarboxylate molecule without axial ligation: new insight into the electronic structures of molecules with importance in catalysis and other reactions.

The first crystalline tetracarboxylato dirhodium paddlewheel complex with no axial ligation has been synthesized and structurally characterized. The compound, tetrakis(2,4,6-triisopropylbenzoato)dirhodium(II,II), Rh(2)(TiPB)(4), exhibits a Rh-Rh bond length of 2.3498(4) A. Interestingly, this is only ca. 0.02 A shorter than that in the corresponding bis-acetone adduct, 2.3700(4) A. The electronic spectrum exhibits two bands in the visible region; the low-energy band A has been previously attributed to a pi(Rh(2))-->sigma(Rh(2)) transition in various bis-adducts. The spectrum of Rh(2)(TiPB)(4)(acetone)(2) shows this band at 610 nm, while that of Rh(2)(TiPB)(4) is greatly displaced to 760 nm. This is very persuasive evidence that the assignment for this transition is correct.