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.
