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 [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 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 ν_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 ν_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_⊥ = 2.09 and g_∥ = 2.25 and hyperfine structure (A_∥ = 180 G). At T_c and below, the spectrum becomes better resolved and now shows superhyperfine structure (A_N∥ = 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_∥ = 80 G) that are clearly visible on both signals.