15,18,22,25-OCTA-N-PENTYLPHTHALOCYANINATO)-RHODIUM DIMERS - SINGLE-CRYSTAL X-RAY STRUCTURE AND THE ISOMERIZATION OF THE 4 ISOMERS

The hydrido(phthalocyaninato)rhodium dimer [(R(8)Pc)RhH](2) (1; R(8)Pc (2-) = dianion of 1,4,8,-11,15,18,22,25-octa-n-pentylphthalocyanine) w as synthesized from the reaction of [(R(8)Pc)(MeOH)(2)Rh]Cl with H-2 a t 110 degrees C in 1-pentanol. A single-crystal X-ray diffraction stud y shows that 1 is a dimer. The two Pc ligands are essentially planar a nd staggered by 35(2)degrees. The two Rh atoms are not significantly d isplaced from the planes of the ligands; they are separated by 3.347(3 ) Angstrom. This long Rh-Ph separation suggests that 1 has a bridging hydride ligand. The dimeric structure of 1 is maintained in benzene so lution. The H-1 NMR spectra show that 1 contains one Rh-H and one N-H moiety for every two R(8)Pc(2-) ligands. The nu(NH) band at 3373 cm(-1 ) (the nu(ND) band at 2478 cm(-1)) supports the presence of an N-H moi ety in 1. Our belief that protonation occurs on a meso (or peripheral) nitrogen, rather than a pyrrolic nitrogen, is derived from X-ray stru ctural data and H-1 NMR results. (1) No uniquely long Rh-N bond is pre sent, and all four pyrrolic nitrogens remain on the same plane, along with the Rh atom. (2) The chemical shift of delta 11.6 ppm for the N-H proton indicates that it is highly deshielded and, therefore, lies ou tside the extended ring currents of the Pc ligand. Therefore, the hydr ide-bridged isomer (R(8)PcH)Rh(mu u-H)Rh(R(8)Pc) (1a), in which the pr oton of R(8)PcH(-) resides on a meso nitrogen, is proposed for 1 in th e solid state and in benzene solution. Variable-temperature H-1 NMR st udies of 1 in toluene-d(8) indicate that when T greater than or equal to 115 degrees C, 1 dissociates into the monomeric hydride complex (R( 8)Pc)RhH (2); when T less than or equal to -25 degrees C, four isomers of 1, all of which have one Rh-H and one N-H moiety, are reversibly f ormed. Two isomers (1b and 1c), which exhibit fine structure in the hy dride resonance due to their coupling to two inequivalent Rh nuclei, a re assigned to two rotamers of the terminal hydride dimer (R(8)PcH)RhR h(R(8)Pc)H. The fourth isomer (1d), which is the only isomer present a t -85 degrees C, is assumed to be another hydride-bridged dimer, i.e., a rotamer of 1a.