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

Abstract

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 k_obs 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.