Crystal Structure and Magnetic Behavior of the Decamethylferrocenium and Decamethylchromocenium Salts of Bis(ethylenedithiolato)nickel, [M(Cp*)2][Ni(edt)2] − Magnetic Anisotropy and Metamagnetic Behavior of [Fe(Cp*)2][Ni(edt)2

Abstract

New electron-transfer salts, [M(Cp*)2][Ni(edt)2], with M = Fe and Cr, were prepared and shown to be isostructural by X-ray crystallographic studies. The molecular structure of [Ni(edt)2]− is reported here for the first time. The solid-state structure consists of an array of parallel alternating donors, [Fe(Cp*)2]•+, and acceptors, [Ni(edt)2]•−, ··DADADA·· stacks along [101]. At high temperatures (T > 50−100 K), the magnetic susceptibility obeys the Curie−Weiss expression, with the θ values of −5 and −6.7 K for the Ni and Cr compounds respectively, revealing dominant AFM interactions. At low temperatures metamagnetic behavior was observed in case of [Fe(Cp*)2][Ni(edt)2], with TN = 4.2 K and HC = 14 kG at 2 K, resulting from high magnetic anisotropy, due to the coexistence of strong FM DA intrachain interactions and strong AFM (DA and AA) interchain interactions. Single-crystal magnetization measurements with [Fe(Cp*)2][Ni(edt)2] showed that the transition from the AFM state to the FM high-field state was induced by the application of a magnetic field parallel to the stacking axis. In these compounds the AFM interchain (DA and AA) interactions play a dominant role, due to the large spin density on the periphery of the acceptor and the close AA and DA interchain contacts. Above 4.2 K the 57Fe Mössbauer spectra of [Fe(Cp*][Ni(edt)2] exhibit the classical low spin FeIII ferrocenium singlet. Below this temperature a poorly resolved hyperfine pattern is observed (estimated hyperfine field ca. 350 kG at 3.5 K). The analysis of the spectra obtained in applied fields of 20 kG and 50 kG is consistent with a strongly anisotropic g tensor. A pronounced temperature dependence of the spectra in a field of 50 kG suggests the presence of spin-lattice relaxation effects.