Novel hybrid spin systems of 7,7',8,8'-tetracyanoquinodimethane (TCNQ) radical anions and 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole (abpt). Crystal structure of [Fe(abpt)(2)(TCNQ)(2)] at 298 and 100 K, Mossbauer spectroscopy, magnetic properties, and infrared spectroscopy of the series [M(II)(abpt)(2)(TCNQ)(2)] (M=Mn, Fe, Co, Ni, Cu, Zn)

The compound [Fe(abpt)(2)(TCNQ)(2)], where TCNQ is the radical anion 7,7',8,8'-tetracyanoquinodimethane and abpt = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole, is an Fe(II) complex containing coordinated radical anions which undergoes a thermally induced spin-crossover with T-c = 280 K. Variable-temperature magnetic susceptibility (7-460 K) and Fe-57 Mossbauer spectroscopy data give evidence for a complete S = 2 (high-spin) <-> S = 0 (low-spin) transition, taking place gradually, without hysteresis. The X-ray structure has been determined at 298 K (1) and 100 K (2). The compound crystallizes in the triclinic space group P (1) over bar with one molecule in the unit cell of dimensions a = 9.277(2) Angstrom, b = 9.876(3) Angstrom, c = 12.272(2) Angstrom, alpha = 69.52(2)degrees, beta = 86.92(2)degrees, and gamma = 81.73(2)degrees for 1 and a = 9.236(2) Angstrom, b = 9.684(1) Angstrom, c = 12.137(2) Angstrom, alpha = 69.26(1)degrees, beta = 87.53(2)degrees, and gamma = 82.38(1)degrees for 2. Two abpt ligands coordinating via pyridyl-N1A and triazole-N1 are in the equatorial positions. Fe-N1 and Fe-N1A distances are 2.08(1) and 2.12(1) Angstrom for 1 and 2.00(2) and 2.02(1) Angstrom for 2, respectively. TCNQ molecules coordinate axially at remarkably short distances i.e., Fe-N1T = 2.16(1) Angstrom for 1 and 1.93(1) Angstrom for 2. The TCNQ molecules are stacked in pairs yielding diamagnetic entities. The FT-IR spectra (100-300 K) show that the TCNQ nu(CN) vibrations are a fingerprint for the different spin states. In the series of the isostructural [M(II)(abpt)(2)(TCNQ)(2)] (M = Mn, Fe, Co, Ni, Cu, Zn) compounds, the nu(CN) absorptions show a shift to higher frequencies as a function of the crystal field stabilization energy, Above T-c, the Cu(II)-doped Fe(II) species shows a broad signal with g(perpendicular to) = 2.09 and g(parallel to) = 2.25 and hyperfine structure (A(parallel to) = 180 G). At T-c and below, the spectrum becomes better resolved and now shows superhyperfine structure (A(N parallel to) = 16 G; nine lines). Above T-c, the Mn(II)-doped Fe(II) compound shows a very broad signal at g = 2.00. The spectrum sharpens at T-c to give a clearly resolved spectrum corresponding to a magnetically isolated Mn(II) ion in a tetragonal environment. The signal is split by the zero-field splitting, yielding major signals at g = 1.6 and g = 5.5 and six hyperfine lines (A(parallel to) = 80 G) that are clearly visible on both signals.