Hydrido(1,4,8,11,15,18,22,25-octa-n-pentylphthalocyaninato)- rhodium Dimers:  Single-Crystal X-ray Structure and the Isomerization of the Four Isomers

The hydrido(phthalocyaninato)rhodium dimer [(R8Pc)RhH]2 (1; R8Pc2- = dianion of 1,4,8,11,15,18,22,25-octa-n-pentylphthalocyanine) was synthesized from the reaction of [(R8Pc)(MeOH)2Rh]Cl with H2 at 110 °C in 1-pentanol. A single-crystal X-ray diffraction study shows that 1 is a dimer. The two Pc ligands are essentially planar and staggered by 35(2)°. The two Rh atoms are not significantly displaced from the planes of the ligands; they are separated by 3.347(3) Å. This long Rh−Rh separation suggests that 1 has a bridging hydride ligand. The dimeric structure of 1 is maintained in benzene solution. The 1H NMR spectra show that 1 contains one Rh−H and one N−H moiety for every two R8Pc2- ligands. The νNH band at 3373 cm-1 (the νND band at 2478 cm-1) supports the presence of an N−H moiety in 1. Our belief that protonation occurs on a meso (or peripheral) nitrogen, rather than a pyrrolic nitrogen, is derived from X-ray structural data and 1H NMR results. (1) No uniquely long Rh−N bond is present, and all four pyrrolic nitrogens remain on the same plane, along with the Rh atom. (2) The chemical shift of δ 11.6 ppm for the N−H proton indicates that it is highly deshielded and, therefore, lies outside the extended ring currents of the Pc ligand. Therefore, the hydride-bridged isomer (R8PcH)Rh(μ-H)Rh(R8Pc) (1a), in which the proton of R8PcH- resides on a meso nitrogen, is proposed for 1 in the solid state and in benzene solution. Variable-temperature 1H NMR studies of 1 in toluene-d 8 indicate that when T ≥ 115 °C, 1 dissociates into the monomeric hydride complex (R8Pc)RhH (2); when T ≤ −25 °C, four isomers of 1, all of which have one Rh−H and one N−H moiety, are reversibly formed. Two isomers (1b and 1c), which exhibit fine structure in the hydride resonance due to their coupling to two inequivalent Rh nuclei, are assigned to two rotamers of the terminal hydride dimer (R8PcH)RhRh(R8Pc)H. The fourth isomer (1d), which is the only isomer present at −85 °C, is assumed to be another hydride-bridged dimer, i.e., a rotamer of 1a.