Synthesis and Reactivity of Porphyrinatorhodium(II)−Triethylphosphine Adducts: The Role of PEt3in 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)(PEt 3 ) and Rh(TPP)(PEt 3 ) 2 . The former adduct is also obtained as the sole product of the reaction between Rh 2 (OEP) 2 and PEt 3 . The EPR spectroscopy at 77 K shows both complexes to have mainly porphyrin-based HOMOs. The composition and the reactivity of Rh(TPP)(PEt 3 ) 2 support its formulation as Rh III (TPP • - )(PEt 3 ) 2 . In contrast, Rh(OEP)(PEt 3 ) demonstrates the reactivity of both a Rh II d 7 center and a porphyrin π-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)(PEt 3 ) reacts as a porphyrin π-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)(PEt 3 ) is proposed to arise from thermal excitation of the unpaired electron from the porphyrin-based HOMO onto the metal-based LUMO (dσ* Rh - P ). Unlike the other reported 1:1 adducts of Rh II (por) species with σ-basic ligands, Rh(OEP)(PEt 3 ) 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 PEt 3 and pyridine was measured spectrophotometrically. The high binding affinity of PEt 3 to Rh(OEP) is proposed as the underlying cause of the increased stability of Rh(OEP)(PEt 3 ) toward disproportionation.