Reactions of cyanooxo-molybdates and -tungstates with picolinic acid, 2-pyridinecarboxaldehyde and 2,2′-pyridil: X-ray crystal structure of (PPh4)2[Mo(CN)4O(pic)] · 2.5H2O

The syntheses of (PPh₄)₂M(CN)₃O(pic)] · nH₂O (M = Mo, W; n = 2.5, 4; pic = picolinate anion) as well as (PPh₄)₂Mo(CN)₄O(apic)] · 3H₂O (apic = 2-pyridinecarboxaldehyde) are presented. The salts have been characterised by elemental analysis, IR and UV-Vis spectroscopy, cyclic voltammetry and X-ray crystal structure determination of (PPh₄)₂Mo(CN)₃O(pic)] · 2.5H₂O. The anion in the latter salt is approximately octahedral with O and N donor atoms of pic situated in the trans and cis position to the MoO bond, respectively. The picolinate complexes have the characteristic MLCT bands in the visible spectra with the absorption maxima linearly dependent on the Reichardt’s E_T solvent parameter. These complexes are shown to exhibit the strongest solvatochromic effect of all studied tricyanooxo and hexacyano complexes of Mo(IV) and W(IV). The obtained values of E_1/2 in these salts 0.333 V (Mo) and 0.018 V (W) in 1,2-C₂H₄Cl₂] were found to be the highest of all M(CN)₃O(LL)]²⁻ type complexes making these salts show reversibility in a remarkable variety of solvents. The oxidation of coordinated apic to pic has been demonstrated by means of electronic spectroscopy. The isolated salts were used to monitor the reaction of M(CN)₄O(H₂O)]²⁻ with 2,2′-pyridil in water-ethanol solution. It was found that 2,2′-pyridil disproportionate to Hpic and apic in the first step and the other decomposition products of 2,2′-pyridil postulated in the literature were not observed.