SPECTROSCOPIC COMPARISONS OF MOW(PORPHYRIN)(2) HETERODIMERS WITH HOMOLOGOUS MO-2 AND W-2 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 sha pe analysis and is compared with values previously obtained for the an alogous homodimers. The overall quadruple bond strengths of these isos tructural dimolybdenum, ditungsten, and molybdenum-tungsten porphyrin dimers have also been compared by calculation of the force constants c orresponding to each metal-metal bond stretching frequency as observed by resonance Raman spectroscopy. The Raman results are as follows: [M o(OEP)](2), nu(MoMo) = 310 cm(-1), k = 2.72 mdyn/Angstrom; [(OEP)MoW(O EP)], 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 consiste nt with a [(Por)MoW(Por)] structure wherein the Mo(Por) congener exper iences a more drastic ''bending-back'' distortion of the porphyrin mac rocycle.