Thiol, Disulfide, and Trisulfide Complexes of Ru Porphyrins: Potential Models for Iron–Sulfur Bonds in Heme Proteins

Thirty-two Ru­(porp)­L2 complexes have been
synthesized, where porp = the dianion of meso-tetramesitylporphyrin
(TMP) or meso-tetrakis­(4-methylphenyl)­porphyrin (H2T-pMe-PP), and L = a thiol,
a sulfide, a disulfide, or a trisulfide. Species studied were with
RSH [R = Me, Et, nPr, iPr, tBu, Bn (benzyl), and Ph],
RSR (R = Me, Bn), RSSR (R = Me, Et, nPr,
Bn) and MeSStBu, and RSSSR (R = Me, Bn).
All the species except two, which were the isolated Ru­(T-pMe-PP)­(tBuSH)2 and Ru­(TMP)­(MeSSMe)2, were characterized in situ. The
disulfide complex was characterized by X-ray analysis. 1H NMR data for the coordinated thiols are the first reported within
metalloporphyrin systems, and are especially informative because of
the upfield shifts of the axial sulfur-containing ligands due to the
porphyrin π-ring current effect, which is also present in the
di- and trisulfide species. The disulfide in the solid state structure
of Ru­(TMP)­(MeSSMe)2 is η1(end-on) coordinated,
the first example of such bonding in a nontethered, acyclic dialkyl
disulfide; 1H–1H EXSY NMR data in solution
show that the species undergoes 1,2-S-metallotropic shifts. Stepwise
formation of the bis­(disulfide) complex from Ru­(TMP)­(MeCN)2 in solution occurs with a cooperativity effect, resembling behavior
of FeII–porphyrin systems where crystal field effects
dominate, but ligand trans-effects are more likely in the Ru system.
The η1(end-on) coordination mode is also favored
for the trisulfide ligand. Discussed also are the remarkable linear
correlations that exist between the ring-current shielding shifts
for the axial ligand C1 protons of Ru­(porp)­(RSxR)2 and x (the number
of S atoms). The Introduction briefly reviews literature on Ru- and
Fe porphyrins (including heme proteins) with sulfur-containing ligands
or substrates, and relationships between our findings and this literature
are discussed throughout the paper.