(TETRAKIS(2-PYRIDYLMETHYL)ETHYLENEDIAMINE)IRON(II) PERCHLORATE - PHASE-TRANSITION STUDY ABOUT THE 1ST RAPIDLY INTERCONVERTING FERROUS SPIN-CROSSOVER COMPLEX

A detailed variable-temperature magnetic, thermal and single-crystal a nd powder diffractometry study has been made on [Fe(tpen)](ClO4)2.1/2H 2O. (tpen = tetrakis(2-pyridylmethyl)ethylenediamine.) Solid-state mag netic susceptibilities measurement shows that this complex is a contin uous/complete type Fe(II) spin. crossover system as defined by Gutlich , with an effective critical temperature, T(c), where there are equal amounts of high- and low-spin complexes, equal to 365 K. The Fe-57 Mos sbauer spectrum measured by Hendrickson shows that the interconversion rate between high- and low-spin states is faster than the Mossbauer t ime scale. Single-crystal diffractometry shows that the [Fe(tpen)](ClO 2)2.1/2H2O crystallizes in the monoclinic space group C2/c, which at 2 93 K has a unit cell of a = 40.775(8) angstrom, b = 9.467(2) angstrom, c = 23.851(5) angstrom, and beta = 108.32(6)-degrees with Z = 12, and at 182 K has a unit cell of a = 40.430(3) angstrom, b = 9.460(1) angs trom, c = 23.834(2), angstrom, and beta = 108.59(2)-degrees with Z = 1 2. The refinements were carried out with 3228 and 3639 (2.5sigma) obse rved reflections at 293 and 182 K, respectively, to give R = 0.058 (wR = 0.061) and R = 0.032 (wR = 0.036). At both temperatures there are t wo crystallographically different [Fe(tpen)]+2 cations, showing an oct ahedral coordination strongly distorted to trigonal geometry by the st eric constraints produced by the hexadentate ligand. This distortion i s different in the two nonequivalent cations, which is due to differen t intermolecular interaction produced by the perchlorate anions. An an alysis of the changes in the structure of the two cations as a functio n of temperature from our results and those obtained by Hendrickson sh ows that the two cations act independently in the interconversion proc edure. DSC thermal analysis shows low and broadening maxima in the coo ling and warming process, and a thermal hysteresis which is due to the fast rate of spin-state interconversion. Variable-time cycles studied by powder X-ray diffraction show a variation of preferred orientation during the cooling and warming process, and a variation of the result s according to the elapsed-time, slow procedure (period of 140 min) sh ows a reversible equilibrium transition without thermal hysteresis, wh ile a shorter period (30 min) shows a delay in the spin interconversio n. (C) 1994 Academic Press, Inc.