Involvement of hydrogen-bonding protons in delocalization of the paramagnetic electron in a single crystal of photoreduced decatungstate

Abstract

Diaquahydrogen tetrakis(di-isopropylammonium) decatungstate hexahydrate, [H₅O₂][NH₂Prⁱ ₂]₄-[W₁₀O₃₂]·6H₂O, has been studied in a single crystal, diamagnetically diluted by [H₅O₂]₂-[NH₂Prⁱ ₂]₄[W₁₀O₃₂]·4H₂O in a ca. 1 : 1 molar ratio, by means of X-band e.s.r. spectroscopy at 77 K. The e.s.r. spectra show the superhyperfine interaction due to eight magnetically equivalent ¹H atoms which are hydrogen-bonding water protons bonded to terminal oxygen atoms at eight equatorial WO₆ sites. There is no observable ¹⁸³W hyperfine interaction. The e.s.r. tensors are g₁= 1.840 ± 0.001, g₂= 1.838 ± 0.001, g₃= 1.831 ± 0.001, A H₁ =(6.7 ± 0.1)× 10^–4, A_H2=(6.2 + 0.1)× 10^–4, and A_H3=(4.6 ± 0.1)× 10^–4 cm^–1, where each value is displaced from the tungsten–oxygen bond direction. The paramagnetic electron orbital involves the direct spin polarization of H (1s) orbitals and is delocalized over eight equatorial octahedra sites, through four nearly linear W–O–W bridges which link the two halves of the anion. The molecular g and A_H values are discussed in terms of the molecular structure. It is concluded that the semioccupied molecular orbital consists of an orbital mixing among four sets of OW–O–WO multiple bonds which arrange the terminal WO groups cis to the nearly linear bridged moieties.