Synthesis and reactivity of porphyrinatorhodium(II)-triethylphosphine adducts:: The role of PEt3 in stabilizing a formal Rh(II) state

Rh(por)H, where por is an octaethyl- or meso-tetraphenylporphyrin dianion,reacts with triethylphosphine to form stable mononuclear paramagnetic formally-Rh-II complexes, Rh(OEP)(PEt3) and Rh(TPP)-(PEt3)(2). The former adduct is also obtained as the sole product of the reaction between Rh-2(OEP)(2) and FEt3. The EPR spectroscopy at 77 K shows both complexes to have mainly porphyrin-based HOMOs. The composition and the reactivity of Rh(TPP.-)(PEt3)(2) support its formulation as Rh-III(TPP'-)(PEt3)(2). In contrast, Rh(OEP)(PEt3)(2) demonstrates the reactivity of both a Rh-II d(7) center and a porphyrin pi -anion radical. The adduct reacts with O-2 as a Rh-II(por) species, originally forming a Rh-III-superoxido derivative. In contrast, with water Rh(OEP)(PEt3) reacts as a porphyrin sz-anion radical, yielding a Rh-III-octaethylphlorin complex. The latter is the first characterized phlorin complex of a heavy transition metal. The dual reactivity of Rh(OEP)(PEt3) is proposed to arise from thermal excitation of the unpaired electron from the porphyrin-based HOMO onto the metal-based LUMO (d sigma*(Rh-P)) Unlike the other reported 1:1 adducts of Rh-II(por) species with sigma -basic ligands, Rh(OEP)(PEt3) is remarkably stable toward disproportionation to Rh-I and Rh-III. To understand the origin of this stability, the affinity of Rh-III(OEP)(+) toward PEt3 and pyridine was measured spectrophotometrically. The high binding affinity of PEt3 to Rh(OEP) is proposed as the underlying cause of the increased stability of Rh(OEP)(PEt3) toward disproportionation.