Magnetic local structures and dynamic behavior of water molecules in a moisture-sensitive spin-crossover system [Fe(II)(R-triazole) 3](X)· nD 2O studied by solid-state high-resolution deuterium NMR

Abstract

The trapped state of crystal water molecules in a moisture-sensitive spin-crossover system of [Fe(II)(R-triazole) 3](ClO 4) 2· nD 2O was investigated by solid-state high-resolution deuterium NMR spectrum. Plus and minus sign and magnitude of hyperfine coupling constant of deuterium of crystal water molecules were determined. The hyperfine coupling constant +87 kHz of D 2O of [Fe(II)(4-NH 2-1,2,4-triazole) 3](ClO 4) 2· nD 2O indicates that the crystal water molecules are not directly bound to Fe(II) ion but sit around Fe(II) ion or triazole ligand. We demonstrated that a high-speed magic angle spinning deuterium NMR spectrum is useful for studying local magnetic structures of the high-spin phase of Fe(II) system. It was also found that most of the D 2O molecules in [Fe(II)(1-H-1,2,4-triazole) 3](ClO 4) 2· nD 2O rotate isotropically near room temperature, whereas D 2O molecules in a similar complex [Fe(II)(4-NH 2-1,2,4-triazole) 3](ClO 4) 2· nD 2O are hindered much more strongly. The complex with mobile crystal water undergoes an abrupt spin-crossover transition, while the complex with strongly trapped water molecules shows relatively gradual transition.