Spectroscopic comparisons of MoW(porphyrin)2 heterodimers with homologous Mo2 and W2 quadruple bonds:: A dynamic NMR and resonance Raman study

The rotational barrier for MoW(meso-monotolyl octaethylporphyrin)(2) ([(TOEP)MoW(TOEP)]) has been determined (Delta G(rot)double dagger = 10.6 +/- 0.1 kcal/mol) by variable-temperature NMR and complete band shape analysis and is compared with values previously obtained for the analogous homodimers. The overall quadruple bond strengths of these isostructural dimolybdenum, ditungsten, and molybdenum-tungsten porphyrin dimers have also been compared by calculation of the force constants corresponding to each metal-metal bond stretching frequency as observed by resonance Raman spectroscopy. The Raman results are as follows: [Mo(OEP)](2), nu(MoMo) = 310 cm(-1), k = 2.72 mdyn/Angstrom; [(OEP)MoW(OEP)], nu(MoW) = 279 cm(-1), k = 2.89 mdyn/Angstrom; [Mo(TOEP)](2), nu(MoMo) = 310 cm(-1), k = 2.72 nu(MoMo) [W(TOEP)](2), nu(WW) = 275 cm(-1), k = 4.08 mdyn/Angstrom; and [(TOEP)MoW(TOEP)], nu(MoW) = 278 cm-1, k = 2.87 mdyn/Angstrom. Both the H-1 NMR and Raman specta are consistent with a [(Por)MoW(Por)] structure wherein the Mo(Por) congener experiences a more drastic "bending-back" distortion of the porphyrin macrocycle.