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

The hydrido(phthalocyaninato)rhodium dimer [(R₈Pc)RhH]₂ (<b>1</b>; R₈Pc^2- = dianion of 1,4,8,11,15,18,22,25-octa-<i>n</i>-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 <b>1</b> 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 <b>1</b> has a bridging hydride ligand. The dimeric structure of <b>1</b> is maintained in benzene solution. The ¹H NMR spectra show that <b>1</b> contains one Rh−H and one N−H moiety for every two R₈Pc^2- ligands. The ν_NH band at 3373 cm^-1 (the ν_ND band at 2478 cm^-1) supports the presence of an N−H moiety in <b>1</b>. Our belief that protonation occurs on a <i>meso</i> (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) (<b>1a</b>), in which the proton of R₈PcH^- resides on a <i>meso</i> nitrogen, is proposed for <b>1</b> in the solid state and in benzene solution. Variable-temperature ¹H NMR studies of <b>1</b> in toluene-<i>d</i>₈ indicate that when <i>T</i> ≥ 115 °C, <b>1</b> dissociates into the monomeric hydride complex (R₈Pc)RhH (<b>2</b>); when <i>T</i> ≤ −25 °C, four isomers of <b>1</b>, all of which have one Rh−H and one N−H moiety, are reversibly formed. Two isomers (<b>1b</b> and <b>1c</b>), 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 (<b>1d</b>), which is the only isomer present at −85 °C, is assumed to be another hydride-bridged dimer, i.e., a rotamer of <b>1a</b>.