Paramagnetic Rh(III) Complexes from Reactions of Phosphines with Hydride-Bridged and Nonbridged Rhodium Phthalocyanine Dimers:  Metal-to-Ligand Charge Transfer Induced by Phosphines

Abstract

The paramagnetic rhodium phthalocyanine complex (RPc)(PMe3)2Rh (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 μ-hydrido complex (RPcH)Rh(μ-H)Rh(RPc) (5) (RPcH = RPc with one of its meso nitrogens protonated) with PMe3 at −10 °C produces (RPc)(PMe3)RhH (6), which further reacts with PMe3 to produce 4 and H2 at higher temperatures. If the solution of 6 is allowed to warm in the absence of free PMe3, H2 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)2Rh+ (9). A redox exchange between 4 and 9 ensues. The cobalt analogue of 4, i.e., (RPc)(PMe3)2Co (12), was generated reversibly by the reaction of (RPc)Co (13) with PMe3 at low temperatures. The 1H 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 π-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.

Methanol as Hydrogen Source: Transfer Hydrogenation of Aldehydes near Room Temperature

“…This may indicate either deprotonation of the hydroxy group of the imino ligand or formation of a hydride complex occurs. In support of the latter speculation, a triplet at −13.2 ppm ( J Rh‐H =30.3 Hz) appeared in the 1 H NMR spectrum, indicative of the formation of a hydride‐bridged dimeric rhodium species . As the reaction progressed, a monomeric hydride peak, characterized by a doublet at −12.02 ppm, grew in and then disappeared, and eventually a new dominant triplet arose, indicating the formation of a more stable dimeric rhodium species.…”

Dimeric aggregates of five-coordinated methyl(phthalocyaninato)Rh(III): 1H NMR evidence for staggered and slipped cofacial dimers

“…4(g) and their similarity to the corresponding resonances of [(RPc)Rh] 2 in inset B of Fig. 2 suggest that 2 is a dimer with a conformation similar to [(RPc)Rh] 2 , which most likely assumes the 45°-staggered cofacial conformation [12]. Table 1 lists the concentrations of the monomer and the …”

“…We have noted previously that the resonances for the two a-H's are separated by less than 0.2 ppm in monomeric (RPc)Rh complexes, but the separation is on the order of 1.0 ppm for dimeric (RPc)Rh complexes [11,12]. Figure 3 shows the changes in the two resonances for H a1 and H a2 of 1 as the concentration was increased from 0.27 mM to 10.2 mM, suggesting that aggregation occurs at higher concentrations.…”

Rh(III) complexes of singly and doubly reduced phthalocyanine ligands generated by chemical and electrochemical reduction

“…3À [9]. Therefore, (FPc)(Me)Rh À is best described as a Rh(III) complex of the radical anion ligand FPc .…”