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 (Δ G ⧧ rot = 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, ν MoMo = 310 cm - 1, k = 2.72 mdyn/Å; [(OEP)MoW(OEP)], ν MoW = 279 cm - 1, k = 2.89 mdyn/Å; [Mo(TOEP)] 2, ν MoMo = 310 cm - 1, k = 2.72 ν MoMo [W(TOEP)] 2, ν WW = 275 cm - 1, k = 4.08 mdyn/Å; and [(TOEP)MoW(TOEP)], ν MoW = 278 cm - 1, k = 2.87 mdyn/Å. Both the 1 H 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.