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)
dataset
posted on 1996-03-06, 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 ReedijkThe 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.
