A contribution to the coordination chemistry of 2,3,5,6-tetrakis(α-pyridyl)pyrazine (TPP): Synthesis, spectroscopy, electrochemistry, and density-functional study of {[Mo(CO)4]2(μ-TPP)}

The reaction between cis-[Mo(CO)4(piperidine)2] and 2,3,5,6-tetrakis(α-pyridyl)pyrazine (TPP) in the 3:1 (Mo:TPP) stoichiometric ratio afforded the organobimetallic compound {[Mo(CO)4]2(μ-TPP)} (1) in 89% yield. This complex has been characterized by elemental and thermogravimetric analyses, vibrational (FT-IR and FT-Raman) and electronic spectroscopy, and 1H and 13C-CP/MAS NMR spectroscopy. NMR data have revealed that TPP coordinates to each metal at two adjacent pyridine nitrogen atoms, according to a rare bis-bidentate chelating mode. The cyclic voltammogram of 1 in DMF has shown an irreversible metal-based oxidation at +0.77 V versus SCE. A DFT/B3LYP study of two plausible molecular structures for 1 has been carried out in vacuum using the LACV3P(d,p) and LACV3P++(d,p) basis sets. This theoretical study has pointed to the structure with the Mo(CO)4 moieties on opposite sides with respect to the pyrazine ring (trans isomer – C2h symmetry) as the most stable one. The vibrational frequencies of the trans structure have been calculated at the B3LYP/LACV3P(d,p) level and compared with the FT-IR and FT-Raman spectra of 1. An excellent agreement between experimental and calculated frequencies was obtained.