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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, Mössbauer Spectroscopy, Magnetic Properties, and Infrared Spectroscopy of the Series [MII(abpt)2(TCNQ)2] (M = Mn, Fe, Co, Ni, Cu, Zn)

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posted on 06.03.1996, 00:00 authored by Paul J. Kunkeler, Petra J. van Koningsbruggen, Joost P. Cornelissen, André N. van der Horst, Adri M. van der Kraan, Anthony L. Spek, Jaap G. Haasnoot, Jan Reedijk
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 Tc = 280 K. Variable-temperature magnetic susceptibility (7−460 K) and 57Fe Mössbauer 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 P1̄ with one molecule in the unit cell of dimensions a = 9.277(2) Å, b = 9.876(3) Å, c = 12.272(2) Å, α = 69.52(2)°, β = 86.92(2)°, and γ = 81.73(2)° for 1 and a = 9.236(2) Å, b = 9.684(1) Å, c = 12.137(2) Å, α = 69.26(1)°, β = 87.53(2)°, and γ = 82.38(1)° 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) Å for 1 and 2.00(2) and 2.02(1) Å for 2, respectively. TCNQ molecules coordinate axially at remarkably short distances i.e., Fe−N1T = 2.16(1) Å for 1 and 1.93(1) Å for 2. The TCNQ molecules are stacked in pairs yielding diamagnetic entities. The FT-IR spectra (100−300 K) show that the TCNQ νCN vibrations are a fingerprint for the different spin states. In the series of the isostructural [MII(abpt)2(TCNQ)2] (M = Mn, Fe, Co, Ni, Cu, Zn) compounds, the νCN absorptions show a shift to higher frequencies as a function of the crystal field stabilization energy. Above Tc, the Cu(II)-doped Fe(II) species shows a broad signal with g = 2.09 and g = 2.25 and hyperfine structure (A = 180 G). At Tc and below, the spectrum becomes better resolved and now shows superhyperfine structure (AN∥ = 16 G; nine lines). Above Tc, the Mn(II)-doped Fe(II) compound shows a very broad signal at g = 2.00. The spectrum sharpens at Tc 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 = 80 G) that are clearly visible on both signals.