EPR AND NMR-STUDY OF THE SPIN-CROSSOVER TRANSITION IN FE(4,4'-BI-1,2,4-TRIAZOLE)2(SCN)2.H2O AND FE(4,4'-BI-1,2,4-TRIAZOLE)2(SECN)2.H2O - X-RAY STRUCTURE DETERMINATION OF FE(4,4'-BI-1,2,4-TRIAZOLE)2(SECN)2.H2O

EPR and proton N MR studies have been done on the spin-crossover transition in Fe(4,4'-bi-1,2,4-triazole)2(SCN)2.H2O and the related selenocyanate compound. The new selenocyanate compound was found to have the transition temperatures of T(up) = 217 K and T(down) = 211 K, and the room-temperature X-ray structure was obtained. The compound, which crystallizes as monoclinic in the space group C2/c (No. 15), has the cell parameters a = 11.342 (2) angstrom, b = 13.169 (3) angstrom, c = 13.118 (4) angstrom, beta-91.79 (2)-degrees, nu = 1959 (1) angstrom, Z = 4, R = 0.0467, and R(W) = 0.0477. EPR spectra of Cu2+ and Mn2+ doped into single crystals of Fe(4,4'-bi-1,2,4-triazole)2(SCN)2.H2O show only one magnetic site in the diamagnetic phase, indicating a change in structure from the paramagnetic phase. All single crystals pulverized spontaneously when warmed from the diamagnetic to the paramagnetic region of temperature. In the diamagnetic phase of Fe(4,4'-bi-1,2,4-triazole)2(SCN)2.H2O, the spin Hamiltonian parameters for Cu2+ were found to be g parallel-to = 2.280, g perpendicular-to = 2.057, A parallel-to = 177 X 10(-4) cm-1, and A perpendicular-to almost-equal-to 0 and for Mn2+ were g = 2.0, D 0.12 cm-1, and a = -0.0018 cm-1. Similar parameters were obtained from powder spectra for the same ions doped into Fe(4,4'-bi-1,2,4-triazole) 2(SeCN)2.H2O. Proton NMR spectra of Fe(4,4'-bi-1,2,4-triazole)2(SCN)2.H2O, Fe(4,4'-bi-1,2,4-triazole)2(SeCN)2.H2O, and Fe(4,4'-bi-1,2,4-triazole)2(SCN)2 (dehydrated) were obtained over a large range of temperatures, and the room-temperature spectrum of the related compound Co(4,4'-bi-1,2,4-triazole)2(SCN)2.H2O was also obtained. In the paramagnetic samples, the simulations of the NMR powder spectra yielded iron-proton distances for the hydrogen atom closest to the iron atom and the water hydrogen. Simulations for the diamagnetic samples showed the water molecule to exhibit a Pake doublet from which the H-H distance was deduced. The existence of this Pake doublet in the diamagnetic phase indicated lack of mobility for the water protons, which was contrary to what was found from the simulations in the paramagnetic phase. Simulations of the experimental NMR spectra in the spin-crossover region demonstrated unequivocally that the transition must occur in domains rather than in a statistically random fashion.