Crystal structures of mixed-ligand oxocyano complexes of molybdenum(IV) and tungsten(IV) and their reactivity towards molecular oxygen studied by IR spectroscopy

The series of compounds, (PPh4)2[M(CN)4O(L)]·xH2O [M = Mo, W; L = pyrazine (pz), pyridine (py); Ph = phenyl group; x = 0, 2, 3], (PPh4)3[Mo(CN)5O]·7H2O and K(PPh4)2[Mo(CN)5O]·5H2O, which are able to bind molecular oxygen giving peroxo complexes (PPh4)2[M(CN)4O(O2)], have been synthesised. The substrates were characterised by thermogravimetric analysis, vibrational and UV-VIS spectroscopy and the crystal structure determination of (PPh4)2[M(CN)4O(pz)]·3H2O. The latter salts are isomorphous and consist of anions of approximately octahedral geometry, forming a three-dimensional hydrogen-bonded network with channels that enable the penetration of dioxygen. The solid state reactions of all the salts with O2 have been studied by measuring their infrared spectra. The integrated intensities of the O–O, pz or py, MO and CN bands change with time according to pseudo-first-order kinetics. The rate constants kobs range from 0.37 × 10−4 to 1.04 × 10−4 s−1 at 323 K. A detailed study of the function of temperature has been undertaken for (PPh4)2[W(CN)4O(pz)]·3H2O. The activation parameters, ΔH# = 68 ± 13 kJ mol−1 and ΔS# = −117 ± 39 J K−1 mol−1, were determined and an associative mechanism of dioxygen uptake was assumed.