Single-crystal and multi-frequency EPR studies on chemical analogues of Amavadin: V(IV)-doped Ca[Ti(hida)2]·6H2O, and Mo(V)-doped [PPh4][Nb(hida)2] and [NEt4][Ta(R,R-hidpa)2] [H3hida=2,2′-(hydroxyimino)diacetic acid, H3hidpa=2,2′-(hydroxyimino)dipropionic acid]

Q-band single-crystal and powder EPR spectra at multiple frequencies (X, Q and 180 GHz) of V(IV)-doped Ca[Ti(hida)2]·6H2O and Mo(V)-doped [PPh4][Nb(hida)2] and [NEt4][Ta(R,R-hidpa)2] (H3hida=2,2′-(hydroxyimino)diacetic acid; H3hidpa 2,2′-(hydroxyimino)dipropionic acid) are reported. The Ti{V} system has the spin-Hamiltonian parameters g11=1.910, g22=1.987, g33=1.990, A11=176.4 G, A22=53.5 G and A33=46.3 G with non-coincidence between the principal axes of the g- and A-matrices. The Euler angles for the non-concidence are α=11.5°, χ=4.63° and γ=346.0°. The triclinic EPR symmetry is consistent with the low (C1) point symmetry of the [Ti{V}(hida)2]2- anion in the solid state. The small angles of non-coincidence between the principal axes of the g and A matrices are also evident from the Q-band powder spectrum, but not the X-band spectrum. High-frequency (34–180 GHz) EPR measurements on powders of both the Nb{Mo} and Ta{Mo} systems reveal the presence of two magnetically distinct Mo centers in each case. The Nb{Mo} system has g-values of g11(a)=1.9765, g11(b)=1.9755, g22(a)=1.9675, g22(b)=1.9665, g33(a)=1.8870 and g33(b)=1.8840, while the Ta{Mo} system has g11(a)=1.976, g11(b)=1.974, g22(a)=1.970, g22(b)=1.967, g33(a)=1.894 and g33(b)=1.892.