Paramagnetic Rh(III) complexes from reactions of phosphines with hydride-bridged and nonbridged rhodium phthalocyanine dimers: Metal-to-ligand charge transfer induced by phosphines

摘要

The paramagnetic rhodium phthalocyanine complex (RPc)(PMe3)(2)Rh (4) (RPc = dianion of 1,4,8,11,15,18,22,25-octa-n-pentylphthalocyanine) has been prepared by the reaction of trimethylphosphine (PMe3) with hydride-bridged and nonbridged rhodium phthalocyanine dimers. The reaction of the mu-hydrido complex (RPcH)Rh(mu-H)Rh(RPc) (5) (RPcH = RPc with one of its meso nitrogens protonated) with PMe3 at -10 degrees C produces (RPc)(PMe3)RhH (6), which further reacts with PMe3 to produce 4 and H-2 at higher temperatures. If the solution of 6 is allowed to warm in the absence of free PMe3, H-2 is also liberated and 6 is converted to an equal number of moles of 4 and [(RPc)Rh](2)(PMe3) (7). In contrast, the reaction of 5 with pyridine (py) produces (RPc)(py)RhH (11), which does not react with additional pyridine. Alternatively, 4 is generated by the reaction of the Rh-Rh bonded dimer [(RPc)Rh](2) (1) with PMe3. In this reaction, the formation of 7 as an intermediate may be shown by the titration of 1 with PMe3. Complex 1 reacts with 4 to produce (RPc)Rh- (10) and (RPc)(PMe3)(2-)Rh+ (9). A redox exchange between 4 and 9 ensues. The cobalt analogue of 4, i.e., (RPc)(PMe3)(2)Co (12), was generated reversibly by the reaction of (RPc)Co (13) with PMe3 at low temperatures. The LH NMR and the ESR spectra of 4, 12, and 13 indicate that while 13 is a metal-centered radical, 4 and 12 are best described as ligand-centered radicals. Therefore, 4 and 12 are formulated as Rh(III) and Co(III) complexes of the singly reduced pi-radical anion RPc.-, generated by metal-to-ligand charge-transfer reactions induced by phosphines. Consistent with formulation of 4 as a Rh(III) complex, it is only slowly oxidized in air and its PMe3 ligand is slow to exchange with the free phosphine. [References: 19]