Effect of Metal Dilution on the Thermal Spin Transition of [Fe xZn1- x(bapbpy)(NCS)2].

This study reports on the effects of zinc dilution on the structure and magnetic properties of the mononuclear two-step spin-crossover compound [Fe(bapbpy)(NCS)₂] ( 1; bapbpy = N6, N6′-di(pyridin-2-yl)-2,2′-bipyridine-6,6′-diamine). The zinc analogue of 1, [Zn(bapbpy)(NCS)₂] ( 3), was synthesized and characterized by X-ray powder diffraction, which suggests different structural features from 1. The crystal structure of the related compound [Fe(bapbpy)(NCS)]₂[Zn(NCS)₄] ·3DMF ( 4) was determined by single-crystal X-ray diffraction. Unlike the hexacoordinate Fe^II in 1, the Zn^II ions in 4 are pentacoordinate. Nine diluted powder samples [Fe _xZn_{1- x}(bapbpy)(NCS)₂] were prepared with iron fractions of x = 0.89, 0.81, 0.76, 0.65, 0.60, 0.53, 0.44, 0.38, and 0.24. According to powder X-ray diffraction and infrared spectroscopy, the phase of compound 1 is retained in the zinc-diluted samples when x > 0.53. At higher dilutions (i.e., when x < 0.53), the phase of compound 3 gradually takes over, but the SCO of the iron complexes in the zinc phase remains observable at an iron fraction as low as x = 0.24. Powder X-ray diffraction and infrared spectroscopy data indicate the presence of the zinc phase only (i.e., compound 3) when x ≤ 0.24. Magnetic susceptibility measurements on the diluted samples show that upon decreasing the iron fraction x, both hysteresis cycles become initially narrower, and then vanish to lead to a single-step SCO material at x = 0.76. Upon additional increase of the zinc contents, the cooperativity of the SCO gradually vanishes to lead to a noncooperative SCO material at the lowest iron fraction studied ( x = 0.24). Despite the different coordination properties of the bapbpy ligand towards Fe^II and Zn^II, the spin crossover of the hexacoordinate Fe^II complex is robust enough to withstand dilution into a magnetically silent Zn^II phase that is structurally different from that of the iron compound. [ABSTRACT FROM AUTHOR]