Redox-Induced Coordination Isomerization of a Phosphoniobenzophospholide

摘要

1-Triphenylphosphoniobenzo[c]phospholide 1 reacts with [M(CO)_5Br] (M = Mn, Re) and [Mn(CO)_3(naphthalene)][BF_4] to give complexes cis-[M(CO)_4](1)Br] (5a, b) and [Mn(CO)_3(1)][BF_4] (6a[BF_4]), respectively, featuring eta~1(P)- and eta~5(pi)-coordination of the phosphole ring. The corresponding reactions with [M_2(CO)_10] proceed with conservation of the metal-metal bond and yield, depending on the reaction temperature, dinuclear complexes [M_2(CO)_8(1)] (M = Mn, 7a) or [M_2(CO)_6(1)_2] (M = Mn, Re, 8a, b) with mu_2-bridging eta~1(P):eta~2(P=C) coordination of the phosphole moiety. All complexes formed were characterized by spectroscopic data; 5b, 6a[BF_4], and 8a, b were characterized by X-ray diffraction studies as well. The structural and ~31P NMR data of the dinuclear manganese complex 8a suggest that the interaction between the metal atoms and the eta~2-bound P=C double bond moieties is dominated by the L->M charge-transfer contributions; this hints at a very low back-donation ability of the central M_2(CO)_6 fragment. Investigation of the reactions of the Mn complexes 6a and 8a with Mg or ferrocenium hexafluorophosphate ([Fc][PF_6]), respectively, revealed that the chemically reversible mutual interconversion between both species was feasible. Likewise, oxidation of the rhenium complex 8b with [Fc][PF_6] gave spectroscopic evidence for the formation of a Re analogue of 6a. Electrochemical studies suggested that the oxidation of 8a ->26a invovles two consecutive single-electron-transfer steps, the first of which is electrochemically reversible and produces a metastable radical cation that is detectable by ESR spectroscopy. The mutual interconveresion between 6a and 8a represents the first case of a revresible coordination isomerization of a phosphaarene that is triggerred by a redox process and might stimulate further studies directed at the use of dinuclear phosphaaarene complexes in redox-catalysis.