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

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journal contribution
posted on 22.02.2012, 00:00 by Júlio S. Rebouças, Brian O. Patrick, Brian R. James
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.