Novel hybrid spin systems of7,7′,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, m?ssbauer spectroscopy, magnetic properties, and infrared spectroscopy of the series [MII(abpt)2(TCNQ)2] (M=Mn, Fe, Co, Ni,Cu,Zn)

Article abstract of DOI:10.1021/ja943960s

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 M?ssbauer spectroscopy data give evidence for a complete S = 2 (high-spin) ? S = O (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 P1 with one molecule in the unit cell of dimensions a = 9.277(2) ?, b = 9.876(3) ?, α = 12.272(2) ?, α = 69.52(2)°, γ = 86.92(2)°, and γ = 81.73(2)° for 1 and a = 9.236(2) ?, b = 9.684(1) ?, c = 12.137(2) ?, 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) ? for 1 and 2.00(2) and 2.02(1) ? for 2, respectively. TCNQ molecules coordinate axially at remarkably short distances i.e., Fe-N1T = 2.16(1) A for 1 and 1.93(1) ? for 2. The TCNQ molecules are stacked in pairs yielding diamagnetic entities. The FT-IR spectra (100-300 K) show that the TCNQ νV_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 Mb(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.

Full text of DOI:10.1021/ja943960s

2190
J. Am. Chem. Soc. 1996, 118, 2190-2197
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,
Mo¨ssbauer Spectroscopy, Magnetic Properties, and
Infrared Spectroscopy of the Series [MII(abpt)2(TCNQ)2]
(M ) Mn, Fe, Co, Ni, Cu, Zn)
Paul J. Kunkeler,1a Petra J. van Koningsbruggen,1a Joost P. Cornelissen,1a
Andre´ N. van der Horst,1b Adri M. van der Kraan,1b Anthony L. Spek,1c
Jaap G. Haasnoot,*,1a and Jan Reedijk1a
Contribution from the Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden UniVersity,
P.O. Box 9502, 2300 RA Leiden, The Netherlands, InteruniVersity Reactor Institute,
Delft UniVersity of Technology, Delft, The Netherlands, and BijVoet Center for
Biomolecular Research, Vakgroep Kristal- en Structuurchemie, Utrecht UniVersity,
Padualaan 8, 3584 CH Utrecht, The Netherlands
ReceiVed December 7, 1994. ReVised Manuscript ReceiVed September 13, 1995X
Abstract: 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 Mo¨ssbauer spectroscopy data give evidence for a complete S ) 2 (high-spin) T 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 P1h with one molecule in the unit cell of
dimensions a ) 9.277(2) Å, b ) 9.876(3) Å, c ) 12.272(2) Å, R ) 69.52(2)°, â ) 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, R ) 69.26(1)°, â ) 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.
Introduction
radicals.3 A variety of iron(II) compounds are known to show
5
a transition from the high-spin state (HS, S ) 2, T2g) to the
In the study of the magnetic properties of coordination
compounds containing transition-metal ions, two important fields
of research have attracted much attention over the past years:
the study of the spin-crossover phenomenon2 and the research
on compounds containing transition-metal ions and organic
1
low-spin state (LS, S ) 0, A1g) on cooling,2 upon increasing
pressure,2b,4 or by light irradiation.5 The interest in these bistable
spin-crossover compounds finds its origin in their possible use
in molecular electronics, for which it would be favorable when
(3) (a) Kahn, O.; Prins, R.; Reedijk, J.; Thompson, J. S. Inorg. Chem.
1987, 26, 3557. (b) Eaton, S. S.; Eaton, G. R. Coord. Chem. ReV. 1978,
26, 207. (c) Pierpont, C. G.; Buchanan, R. M. Coord. Chem. ReV. 1981,
38, 45. (d) Laugier, J.; Rey, P.; Benelli, C.; Gatteschi, D.; Zanchini, C. J.
Am. Chem. Soc. 1986, 108, 6931. (e) Caneschi, A.; Ferraro, F.; Gatteschi,
D.; Rey, P.; Sessoli, R. Inorg. Chem. 1991, 30, 3162. (f) Caneschi, A.;
Gatteschi, D.; Laugier, J.; Rey, P. J. Am. Chem. Soc. 1987, 109, 2191. (g)
Caneschi, A.; Gatteschi, D.; Laugier, J.; Rey, P.; Sessoli, R.; Zanchini, C.
J. Am. Chem. Soc. 1988, 110, 2795. (h) Sessoli, R.; Tsai, H. L.; Schake,
A. R.; Wang, S.; Vincent, J. B.; Folting, K.; Gatteschi, D.; Christou, G.;
Hendrickson, D. N. J. Am. Chem. Soc. 1993, 115, 1804. (i) Caneschi, A.;
Gatteschi, D.; Rey, P. Prog. Inorg. Chem. 1991, 39, 331.
X Abstract published in AdVance ACS Abstracts, February 15, 1996.
(1) (a) Leiden University. (b) Delft University of Technology. (c)
Utrecht University.
(2) (a) Ko¨nig, E.; Ritter, G.; Kulshreshtha, S. K. Chem. ReV. 1985, 85,
219. (b) Gu¨tlich, P. Struct. Bonding 1981, 44, 83. (c) Zarembowitch, J.;
Kahn, O. New. J. Chem. 1991, 15, 181-190. (d) Goodwin, H. A. Coord.
Chem. ReV. 1976, 18, 293. (e) Gu¨tlich, P. In Mo¨ssbauer Spectroscopy
Applied to Inorganic Chemistry; Long, G. J., Ed.; Modern Inorganic
Chemistry Series, Vol. 1; Plenum Press: New York, 1984. (f) Bacci, M.
Coord. Chem. ReV. 1988, 86, 245. (g) Ko¨nig, E. Prog. Inorg. Chem. 1987,
35, 527.
0002-7863/96/1518-2190$12.00/0 © 1996 American Chemical Society

Unique New Type of Fe(II) Spin-CrossoVer Compound
J. Am. Chem. Soc., Vol. 118, No. 9, 1996 2191
Table 1. Crystallographic Data for [Fe(abpt)2(TCNQ)2] at 298 K
(1) and 100 K (2)
the compound exhibits an abrupt transition with hysteresis at a
critical temperature close to ambient temperature.2c Further-
more, spin transitions are believed to play an important role in
biological systems; spin equilibria in certain hemeproteins have
been reported to be coupled to electron transport.6 In particular,
iron(II) compounds containing substituted 1,2,4-triazole ligands
have been found to show spin transitions, in several cases with
hysteresis, and considerably high transition temperatures appear
to be possible.7 In general, spin-crossover behavior has
frequently been observed for Fe(II) ions coordinated by two
didentate chelating nitrogen-donating ligands, together with two
monodentately coordinating nitrogen-donating ligands.2,5c In
these cases, the monodentate ligands are generally in a cis
configuration; a trans geometry has so far only been found for
the two-dimensional compounds [FeX2(btr)2](H2O) (X ) NCS,7c,d
NCSe;7g btr ) 4,4′-bis(1,2,4-triazole)). Organic radicals are
widely used as spin probes in biological systems.3b Compounds
which contain transition-metal ions in combination with coor-
dinating organic radicals are now attracting considerable atten-
tion because of their interesting magnetic properties, arising from
the spin-spin interaction between unpaired electron spins
situated on the metal ion and on the radical,3a,c-i and discrete
molecular units having a large total electron spin in the ground
state have been obtained.3g,h On the other hand, this strategy
also opens a way to prepare molecular ferromagnets.3e,f,8 The
type of organic radical we have focused our attention on is
7,7′,8,8′-tetracyanoquinodimethane (abbreviated as TCNQ).
Recently, compounds of general formula [MII(abpt)2(TCNQ)2]
(M ) Cu, Ni, Co, Fe; abpt ) 3,5-bis(pyridin-2-yl)-4-amino-
1,2,4-triazole) have been reported.9 These compounds represent
one of the rare cases of the TCNQ radical anion being involved
in coordination. Here we report a detailed study on the unusual
spin transition in [FeII(abpt)2(TCNQ)2], with a trans-FeN4N′2
chromophore. In addition, the physical properties of the
isostructural Mn(II), Co(II), Ni(II), Cu(II), and Zn(II) compounds
are described.
compd
1
2
formula
C48H28FeN20
940.73
triclinic
P1h
C48H28FeN20
940.73
triclinic
P1h
molar mass, g mol-1
crystal system
space group
T, K
298
100
a, Å
b, Å
c, Å
9.277(2)
9.876(3)
12.272(2)
69.52(2)
86.92(2)
81.73(2)
1042.3(4)
1
9.236(2)
9.684(1)
12.137(2)
69.26(1)
87.53(2)
82.38(1)
1006.3(3)
1
R, deg
â, deg
γ, deg
V, Å3
Z
dx, Mg/m3
crystal size, mm
radiation (Mo KR)a
F000
1.499
1.552
0.02 × 0.15 × 0.18 0.02 × 0.15 × 0.18
0.71073
482
0.71073
482
4.4
abs coeff, cm-1
linear decay
scan width, deg
4.2
4%
3%
0.90 + 0.35 tan θ 0.90 + 0.35 tan θ
24
3491
3275
θ
max, deg
no. of total data
24
3356
3145
no. of unique data
no. of obsd data, I > 2.5σ(I) 988
1019
difabs correction range
0.74;1.19
0.80;1.17
0.095
0.055
144
2.73
-0.79, 0.70
Rb
Rw
0.088
0.071
c
no. of params
goodness of fit
∆ρ range
144
2.30
-0.60, 0.48
a Graphite monochromator. b R ) [∑(|Fo| - |Fc|)/∑|Fo|]. c Rw
)
2
[∑w(|Fo| - |Fc|)2/∑w|Fo| ]1/2; w-1 ) σ2(F).
according to the method of Geldard and Lions10 from 2-cyanopyridine
(FLUKA A.G.) and hydrazine hydrate (Janssen Chemicals). TCNQ
was obtained from Janssen Chemicals. LiTCNQ was prepared by
adding a boiling solution of 0.03 mol of LiI in 20 mL of acetonitrile
to a boiling solution of 0.01 mol of TCNQ in 200 mL of acetonitrile.
Dark purple microcrystals precipitated.11
Experimental Section
Synthesis of [MII(abpt)2(TCNQ)2] (M(II) ) Mn(II), Fe(II),
Co(II), Ni(II), Cu(II), Zn(II)). The complexes were prepared accord-
ing to a modification of the method described by Cornelissen.9
M(NO3)2‚xH2O (M(II) ) Mn, Co, Ni, Cu, Zn)) or Fe(TsO)2‚6H2O (TsO
) p-tolylsulfonate), abpt, and LiTCNQ in a molar ratio of 1:2:2 were
dissolved separately in 30 mL of methanol. To dissolve abpt
completely, the solution was heated for a few minutes. The metal salt
and abpt solutions were mixed, resulting in a clear solution, after which
the LiTCNQ solution was added quickly. At room temperature black
microcrystals precipitated immediately. These were collected by
filtration, washed with methanol, and analyzed after drying under
vacuum. Single crystals of [Fe(abpt)2(TCNQ)2] were obtained similarly,
but by starting from boiling methanolic solutions. Small black shiny
cubic crystals formed immediately. Of many experiments only few
afforded crystals large enough for single-crystal X-ray measurements.
In some cases small amounts of Cu(NO3)2‚xH2O (Cu/Fe ) 0.02) or
Mn(NO3)2‚xH2O (Mn/Fe ) 0.02) were added to the reaction mixture,
to obtain Cu(II)- or Mn(II)-doped [Fe(abpt)2(TCNQ)2] samples. All
compounds are isostructural as determined by X-ray powder diffraction.
Anal. Calcd for [Fe(abpt)2(TCNQ)2], Viz. C48H28FeN20: C, 61.29; H,
3.00; N, 29.78; Fe, 5.94. Found: C, 60.73; H, 3.29; N, 29.75; Fe, 5.76.
Anal. Calcd for [Zn(abpt)2(TCNQ)2], Viz. C48H28N20Zn: C, 60.67; H,
2.95; N, 29.49; Zn, 6.89. Found: C, 59.58; H, 3.19; N, 28.68; Zn,
6.90.
Materials. Commercially available solvents and metal(II) salts were
used without further purification. The ligand abpt was synthesized
(4) (a) Meissner, E.; Ko¨ppen, H.; Spiering, H.; Gu¨tlich, P. Chem. Phys.
Lett. 1983, 95, 163. (b) Slichter, C. P.; Drickamer, H. G. J. Chem. Phys.
1972, 56, 2142.
(5) (a) Decurtins, S.; Gu¨tlich, P.; Ko¨hler, C. P.; Spiering, H.; Hauser, A.
Chem. Phys. Lett. 1984, 105, 1. (b) Hauser, A.; Gu¨tlich, P.; Spiering, H.
Inorg. Chem. 1986, 25, 4245. (c) Figg, D. C.; Herber, R. H.; Potenza, J.
A. Inorg. Chem. 1992, 31, 2111. (d) Hauser, A. Chem. Phys. Lett. 1986,
124, 543. (e) Decurtins, S.; Gu¨tlich, P.; Hasselbach, K. M.; Hauser, A.;
Spiering, H. Inorg. Chem. 1985, 24, 2174.
(6) Dose, E. V.; Tweedle, M. F.; Wilson, L. J.; Sutin, N. J. Am. Chem.
Soc. 1977, 99, 3886.
(7) (a) Vos, G.; le Feˆbre, R. A.; de Graaff, R. A. G.; Haasnoot, J. G.;
Reedijk, J. J. Am. Chem. Soc. 1983, 105, 1682. (b) Vos, G.; de Graaff, R.
A. G.; Haasnoot, J. G.; van der Kraan, A. M.; de Vaal, P.; Reedijk, J. Inorg.
Chem. 1984, 23, 2905. (c) Vreugdenhil, W.; Haasnoot, J. G.; Kahn, O.;
Theury, P.; Reedijk, J. J. Am. Chem. Soc. 1987, 109, 5272. (d) Vreugdenhil,
W.; van Diemen, J. H.; de Graaff, R. A. G.; Haasnoot, J. G.; Reedijk, J.;
van der Kraan, A. M.; Kahn, O.; Zarembowitch, J. Polyhedron 1990, 9,
2971. (e) Stupik, P.; Zhang, J. H.; Kwiecien, M.; Reiff, W. M.; Haasnoot,
J. G.; Hage, R.; Reedijk, J. Hyperfine Interact. 1986, 28, 725. (f) Stupik,
P.; Reiff, W. M.; Hage, R.; Jacobs, J.; Haasnoot, J. G.; Reedijk, J. Hyperfine
Interact. 1988, 40, 343. (g) Ozarowski, A.; Shunzhong, Y.; McGarvey, B.
R.; Mislankar, A.; Drake, J. E. Inorg. Chem. 1991, 30, 3167. (h)
Vreugdenhil, W.; Gorter, S.; Haasnoot, J. G.; Reedijk, J. Polyhedron 1985,
4, 1769. (i) Kahn, O.; Kro¨ber, J.; Jay, C. AdV. Mater. 1992, 4, 718. (j)
Jay, C.; Grolie`re, F.; Kahn, O.; Kro¨ber, J. Mol. Cryst. Liq. Cryst. 1993,
234, 255. (k) Kro¨ber, J.; Codjovi, E.; Kahn, O.; Grolie`rer, F.; Jay, C. J.
Am. Chem. Soc. 1993, 115, 9810.
Crystallographic Data Collection and Refinement of the Struc-
ture. The structure of [Fe(abpt)2(TCNQ)2] has been determined at 298
K (1) and 100 K (2). Parameters of data collection and refinement are
given in Table 1. X-ray data were collected for a black, tiny plate-
(8) Kahn, O.; Pei, Y.; Journaux, Y. In Inorganic Materials; Bruce, D.
W., O’Hare, D., Eds.; Wiley: New York, 1992; p 59.
(9) Cornelissen, J. P.; van Diemen, J. H.; Groeneveld, L. R.; Haasnoot,
J. G.; Spek, A. L.; Reedijk, J. Inorg. Chem. 1992, 31, 198.
(10) Geldard, J. F.; Lions, F. J. Org. Chem. 1965, 30, 318.
(11) Bozio, R.; Girlando, A.; Pecile, C. J. Chem. Soc., Faraday Trans.
2 1975, 71, 1237.

2192 J. Am. Chem. Soc., Vol. 118, No. 9, 1996
Kunkeler et al.
shaped crystal of approximate dimensions 0.02 × 0.15 × 0.18 mm on
an Enraf-Nonius CAD4T/rotating anode system at 10 kW using graphite
monochromated Mo KR radiation (λ ) 0.710 73 Å). Unit cell
parameters were derived from the SET4 setting angles of 25 reflections
in the range 5° < θ < 15°. The data were corrected for Lp, a small
linear decay, and absorption (DIFABS12) and averaged into a unique
data set. The 298 K structure was solved by Patterson techniques
(DIRDIF-9213) and refined on F by full-matrix least-squares (SHELX-
7614). The iron atom was refined with anisotropic displacement
parameters; all other non-hydrogen atoms were refined with individual
isotropic displacement parameters in view of the limited number of
observed data. Hydrogen atoms were taken into account at calculated
positions and their positions refined riding on their carrier atom with
one common displacement parameter. Scattering factors were taken
from Cromer and Mann.15 Geometrical calculations were done with
PLATON.16 The 100 K structure was refined starting from the 298 K
coordinates.
Figure 1. ORTEP19 projection of [Fe(abpt)2(TCNQ)2] at 298 K. Addi-
tional atoms are generated by the symmetry operation -x, -y, -z.
Table 2. Selecteda Interatomic Distances (Å) for
Fe(abpt)2(TCNQ)2 at 298 K( 1) and 100 K (2)
Elemental Analyses. C, H, N and metal determinations were
performed by the Microanalytical Laboratory of University College,
Dublin, Ireland.
X-ray Powder Diffraction. X-ray powder diagrams of the pow-
dered coordination compounds were obtained with a Guinier-de Wolf
camera using Cu KR radiation and a nickel filter.
1
2
Fe-N1
2.08(1)
2.12(1)
2.16(1)
1.36(2)
1.32(2)
1.30(2)
1.35(2)
1.37(2)
1.39(2)
1.51(3)
1.49(2)
1.34(2)
1.36(2)
1.31(2)
1.39(2)
1.36(2)
1.40(2)
1.39(2)
1.40(3)
1.37(3)
1.39(3)
1.37(2)
1.37(2)
1.13(2)
1.12(2)
1.14(2)
1.15(2)
1.42(2)
1.42(2)
1.43(2)
1.41(2)
1.42(2)
1.43(2)
1.41(2)
1.44(2)
1.38(2)
1.44(2)
1.40(2)
1.36(2)
2.00(2)
2.02(1)
1.93(1)
1.35(2)
1.31(2)
1.29(3)
1.32(2)
1.43(3)
1.41(2)
1.54(3)
1.48(3)
1.37(3)
1.32(2)
1.34(3)
1.36(2)
1.33(2)
1.37(2)
1.38(2)
1.39(3)
1.39(3)
1.41(3)
1.36(2)
1.38(2)
1.14(2)
1.09(2)
1.11(2)
1.16(2)
1.44(2)
1.44(3)
1.47(2)
1.38(2)
1.39(3)
1.44(3)
1.42(2)
1.46(3)
1.39(3)
1.44(2)
1.34(3)
1.35(3)
Fe-N1A
Fe-N1T
N1-N2
IR Spectra. Fourier transform infrared spectra (100-300 K) have
been recorded as KBr pellets in the range 4000-250 cm-1 on a Bruker
IFS 113V FT-IR spectrophotometer (resolution 0.5 cm-1).
EPR Spectra. X-band powder EPR spectra at 9.04 GHz, 298-20
K, and microwave power of 4 mW have been obtained on a Jeol RE2x
electron spin resonance spectrometer using an ESR900 continuous-
flow cryostat. dpph was used as a reference.
Magnetic Measurements. Magnetic susceptibilities were measured
in the temperature range 7-300 K with a fully automated Manics
DSM-8 susceptometer equipped with a TBT continuous-flow cryostat
and a Drusch EAF 16 NC electromagnet, operating at ca. 1.4 T. Data
were corrected for magnetization of the sample holder and for
diamagnetic contributions, which were estimated from the Pascal
constants. The susceptometer was calibrated using Gd2O3. Magnetic
susceptibility measurements between 80 and 460 K were carried out
on an automated Faraday balance, described by Arbouw.17 HgCo-
(NCS)4 was used as a calibrant for the Faraday balance.18
Mo1ssbauer Spectra. Mo¨ssbauer spectra were obtained with a
constant-acceleration spectrometer, which uses a 57Co in Rh source.
Isomer shifts (IS) are reported relative to the NBS standard, disodium
pentacyanonitrosylferrate. The measured spectra were fitted by com-
puter, with calculated subspectra consisting of Lorentzian-shaped lines,
by varying the Mo¨ssbauer parameters in a nonlinear, iterative minimiza-
tion routine.
N1-C5
N2-C3
N4-C3
N4-C5
N4-N6
C3-C2B
C5-C2A
N1A-C2A
N1A-C6A
N1B-C2B
N1B-C6B
C2A-C3A
C2B-C3B
C3A-C4A
C3B-C4B
C4A-C5A
C4B-C5B
C5A-C6A
C5B-C6B
N1T-C9T
N2T-C10T
N3T-C11T
N4T-C12T
C7T-C1T
C8T-C4T
C9T-C7T
C10T-C7T
C11T-C8T
C12T-C8T
C1T-C2T
C3T-C4T
C4T-C5T
C6T-C1T
C2T-C3T
C5T-C6T
Results and Discussion
Description of the Crystal Structure of [Fe(abpt)2-
(TCNQ)2] at 298 K (1) and 100 K (2). The molecular structure
and the atomic numbering of the mononuclear Fe(II) cluster is
depicted in Figure 1, whereas relevant bond length and bond
angle information is given in Tables 2 and 3. The structure
has been determined at 298 K (1) and 100 K (2). It should be
noted that in 1 the compound is primarily in the high-spin state;
only a fraction of low-spin iron(II) has been found to be present
(Vide infra). In the present discussion of the structure, 1 will
a A full listing is available in the supporting information.
be referred to as the high-spin state. In 2 all iron(II) atoms are
in the low-spin state, although a residual amount of high-spin
Fe(II) species may be present (Vide infra). At both temperatures
the space group is P1h. Both structures differ mainly in iron-
donor atom distances and angles. The structure is isostructural
with [Cu(abpt)2(TCNQ)2].9 The triclinic unit cell contains one
mononuclear [Fe(abpt)2(TCNQ)2] unit with the Fe(II) atom at
the inversion center and a pair of centrosymmetrically related
TCNQ anions. The metal ion is in a distorted octahedral
environment, of which the equatorial coordination sphere is
formed by the triazole-N1 and pyridyl-N1A of two abpt
molecules. The 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.
(12) Walker, N.; Stuart, D. Acta Crystallogr. 1983, A39, 158.
(13) Beurskens, P. T.; Admiraal, G.; Beurskens, G.; Bosman, W. P.;
Garc´ıa-Granda, S.; Gould, R. O.; Smits, J. M. M.; Smykalla, C. The DIRDIF
program system, Technical report of the Crystallography Laboratory,
University of Nijmegen, The Netherlands, 1992.
(14) Sheldrick, G. M. SHELX76 Program for crystal structure determi-
nation; University of Cambridge: Cambridge, U.K., 1976.
(15) Cromer, D. T.; Mann, J. B. Acta Crystallogr. 1968, A24, 321.
(16) Spek, A. L. Acta Crystallogr. 1990, A46, C34.
(17) Arbouw, J. W. Ph.D. Thesis, Leiden University, 1974.
(18) O’Connor, C. J.; Sinn, E.; Cukauskas, E. J.; Deaver, B. S., Jr. Inorg.
Chim. Acta 1979, 32, 29.
(19) Johnson, C. K. ORTEP. Report ORNL-3794; Oak Ridge National
Laboratory: Tennessee, 1965.

Unique New Type of Fe(II) Spin-CrossoVer Compound
J. Am. Chem. Soc., Vol. 118, No. 9, 1996 2193
Table 3. Selecteda Angles (deg) for [Fe(abpt)2(TCNQ)2] at 298 K
(1) and 100 K (2)
Table 4. Relevant Interatomic Distances (Å) and Angles (deg) for
the Hydrogen Bonding Interactions in [Fe(abpt)2(TCNQ)2] at 298 K
(1) and 100 K (2)
1
2
D-H· · ·A
D-H
H· · ·A
D· · ·A
D-H· · ·A
N1-Fe-N1A
77.2(5)
91.5(5)
180.0
102.8(5)
88.5(5)
88.3(5)
180.0
80.3(6)
87.3(6)
180.0
99.7(6)
92.7(6)
92.2(6)
180.0
compound 1
N1-Fe-N1T
N6-H1· · ·N1B
N6-H2· · ·H4Ta
1.03(2)
0.87(2)
1.96(2)
2.49(2)
2.83(2)
3.25(2)
141(1)
147(2)
N1-Fe-N1′
N1-Fe-N1A′
N1-Fe-N1T′
compound 2
N1A-Fe-N1T
N1A-Fe-N1A′
N1A-Fe-N1T′
N1T-Fe-N1T′
Fe-N1-N2
N6-H1· · ·N1B
N6-H2· · ·N4Ta
a Positions generated by the symmetry operation: x, y, z-1.
1.03(2)
0.86(2)
1.96(2)
2.39(2)
2.83(2)
3.14(2)
141(2)
146(2)
91.7(5)
180.0
87.8(6)
180.0
135(1)
113(1)
111(1)
105(1)
105(1)
132(2)
123(1)
118(1)
126(1)
116(1)
117(1)
151(1)
113(2)
123(1)
124(2)
107(1)
121(2)
132(2)
121(2)
121(1)
118(1)
109(1)
125(2)
126(2)
116(1)
117(2)
127(2)
121(2)
116(2)
117(2)
119(2)
120(2)
118(2)
119(2)
116(2)
125(2)
120(2)
123(2)
124(2)
122(2)
120(2)
119(2)
119(1)
117(1)
123(2)
125(2)
120(2)
115(2)
175(2)
179(2)
179(2)
178(2)
135(1)
113(1)
112(2)
105(2)
104(2)
135(2)
121(1)
118(1)
127(1)
115(2)
114(2)
162(1)
114(2)
123(2)
123(2)
106(2)
122(2)
133(2)
122(2)
122(2)
115(2)
107(2)
127(2)
126(2)
114(2)
118(2)
128(2)
124(2)
118(2)
117(2)
119(2)
118(2)
119(2)
118(2)
118(2)
124(2)
120(2)
119(2)
122(2)
123(2)
124(2)
122(2)
117(2)
119(2)
124(2)
124(2)
118(2)
117(2)
171(2)
179(2)
178(2)
177(2)
Fe-N1-C5
and 1.3(9)° for 2, whereas these are 3.1(8)° for 1 and 3.8(9)°
for 2, respectively, for the noncoordinated pyridyl ring. The
noncoordinating pyridyl group is oriented in such a way as to
be able to form an intramolecular hydrogen bond with the amino
group of the abpt ligand (See Table 4). This contributes to the
approximate planarity of the abpt ligand and has been observed
in other mononuclear abpt compounds.9,21 The axial positions
are occupied by two monodentate coordinating TCNQ radical
anions at extremely short coordination distances, i.e. Fe-N1T
) 2.16(1) Å for 1 and 1.93(1) Å for 2. This is significantly
shorter than the Cu-N1T distance of 2.442(5) Å found in
[Cu(abpt)2(TCNQ)2].9 The coordinated TCNQ nitrile group
shows the largest deviation from linearity, i.e. N1T-C9T-C7T
is 175(2)° in 1 and 171(2)° in 2, respectively. The average
change in Fe(II)-donor atom bond length is 0.14 Å, which is
at the lower side of the values normally encountered.2c,g
Remarkable is the very large change in Fe-N1T distance of
0.23 Å, which is probably related to the extended π system of
TCNQ, favoring increased back-bonding in the low-spin state.
So far, the largest change observed in Fe-N donor atom distance
on going from high-spin (HS) to low-spin (LS) state is only
0.21 Å.20c The [Fe(abpt)2(TCNQ)2] units are packed in such a
way that the TCNQ molecules form stacked dimers in the
usually found eclipsed conformation, like the one existing in
the (TCNQ)22- dimeric anion.9,22 A view of the crystal packing
is not shown here because it is completely identical to that of
[Cu(abpt)2(TCNQ)2].9 The distance between the least-squares
planes through these stacked TCNQ anion radicals, with
symmetry operations (x, y, z) and (1-x, -y, 1-z) shortens from
3.22 Å in the HS state to 3.15 Å in the LS state. These
interplanar TCNQ-TCNQ spacings in discrete dimers fall in
the range normally observed.9,22 Clearly, the contraction of the
Fe(II) coordination sphere when going from HS to LS state is
accompanied by a more compact packing of the TCNQ units.
It is remarkable that the tremendous shortening of the Fe-N1T
distances is not completely reflected on this TCNQ-TCNQ
stacking distance. This can be related to the increase in the
Fe-N1T-C9T angle from 151(1)° in 1 to 162(1)° in 2. The
N2-N1-C5
N1-N2-C3
C3-N4-C5
C3-N4-N6
C5-N4-N6
Fe-N1A-C2A
Fe-N1A-C6A
C2A-N1A-C6A
C2B-N1B-C6B
Fe-N1T-C9T
N2-C3-N4
N2-C3-C2B
N4-C3-C2B
N1-C5-N4
N1-C5-C2A
N4-C5-C2A
C7T-C1T-C2T
C7T-C1T-C6T
C2T-C1T-C6T
C5-C2A-N1A
C5-C2A-C3A
N1A-C2A-C3A
C3-C2B-N1B
C3-C2B-C3B
N1B-C2B-C3B
C1T-C2T-C3T
C2A-C3A-C4A
C2B-C3B-C4B
C2T-C3T-C4T
C3A-C4A-C5A
C3B-C4B-C5B
C3T-C4T-C5T
C3T-C4T-C8T
C5T-C4T-C8T
C4A-C5A-C6A
C4B-C5B-C6B
C4T-C5T-C6T
N1A-C6A-C5A
N1B-C6B-C5B
C1T-C6T-C5T
C1T-C7T-C9T
C9T-C7T-C10T
C1T-C7T-C10T
C4T-C8T-C11T
C4T-C8T-C12T
C11T-C8T-C12T
N1T-C9T-C7T
N2T-C10T-C7T
N3T-C11T-C8T
N4T-C12T-C8T
(20) (a) Ko¨nig, E.; Watson, K. J. Chem. Phys. Lett. 1970, 6, 457. (b)
Katz, B. A.; Strouse, C. E. J. Am. Chem. Soc. 1979, 101, 6214. (c) Gallois,
B.; Real, J. A.; Hauw, C.; Zarembowitch, J. Inorg. Chem. 1990, 29, 1152.
(d) Baker, A. T.; Goodwin, H. A.; Rae, A. D. Inorg. Chem. 1987, 26, 3513.
(21) (a) Faulmann, C.; van Koningsbruggen, P. J.; de Graaff, R. A. G.;
Haasnoot, J. G.; Reedijk, J. Acta Crystallogr. 1990, C46, 2357. (b) Garc´ıa,
M. P.; Manero, J. A.; Oro, L. A.; Apreda, M. C.; Cano, F. H.; Foces-Foces,
C.; Haasnoot, J. G.; Prins, R.; Reedijk, J. Inorg. Chim. Acta 1986, 122,
235. (c) van Koningsbruggen, P. J.; Goubitz, K.; Haasnoot, J. G.; Reedijk,
J. Manuscript in preparation.
(22) (a) Lacroix, P.; Kahn, O.; Gleizes, A.; Valade, L.; Cassoux, P. New
J. Chem. 1984, 8, 643. (b) Miller, J. S.; Zhang, J. H.; Reiff, W. M.; Dixon,
D. A.; Preston, L. D.; Reis, A. H., Jr.; Gebert, E.; Extine, M.; Troup, J.;
Epstein, A. J.; Ward, M. D. J. Phys. Chem. 1987, 91, 4344. (c) Humphrey,
D. G.; Fallon, G. D.; Murray, K. S. J. Chem. Soc., Chem. Commun. 1988,
1356. (d) Lau, C.-P.; Singh, P.; Cline, S. J.; Seiders, R.; Brookhart, M.;
Marsh, W. E.; Hodgson, D. J.; Hatfield, W. E. Inorg. Chem. 1982, 21,
208.
a Primed atoms are generated by the symmetry operations -x, -y,
-z. A full listing is available in the supporting information.
The N1-Fe-N1A bite angle enlarges, while passing from the
high-spin (77.2(5)°) to the low-spin (80.3(6)°) form. The larger
bite angle in the low-spin complex is a result of the shorter
Fe-N bond lengths. This effect of enlarging bite angles, while
going from high-spin to low-spin state has been reported be-
fore for mononuclear iron(II) compounds with dichelating
N-donating ligands.2g,5c,20 The dihedral angle between the
coordinated pyridyl group and the triazole ring is 4.2(8)° for 1

2194 J. Am. Chem. Soc., Vol. 118, No. 9, 1996
Kunkeler et al.
Table 5. Least-Squares Fit Mo¨ssbauer Data for [Fe(abpt)2(TCNQ)2]a
low-spin state
high-spin state
1-2
1-5
1-6
IS
∆EQ
relative
area (%)
IS
∆EQ
relative
area (%)
IS
∆EQ
relative
area (%)
T (K)
(mm s-1
)
(mm s-1
)
(mm s-1
)
(mm s-1
)
(mm s-1
)
(mm s-1
)
120
150
180
210
225
240
255
270
285
300
315
330
360
0.76
0.75
0.74
0.73
0.72
0.70
0.69
0.66
0.54
0.69
0.69
0.69
0.69
0.69
0.70
0.73
0.73
0.54
100
100
100
97
94
91
85
83
67
1.41
1.37
1.35
1.29
1.25
1.15
1.14
1.15
1.16
1.17
2.06
2.00
1.99
1.93
1.92
1.76
1.80
1.85
1.90
1.95
3
6
9
15
17
33
84
92
92
93
0.44
0.44
0.43
0.41
0.39
0.43
0.44
0.43
16
8
8
7
a IS ) isomer shift, ∆EQ ) quadrupole splitting relative to the numbered singlets.
Figure 2. øT vs T curve (0) (T ) 7-460 K) for [Fe(abpt)2(TCNQ)2].
Points at 470 K indicate sample decomposition.
structure is stabilized further by one weak and one strong
NsH‚‚‚N hydrogen bond, data of which are summarized in
Table 5.
Magnetic Susceptibility Measurements. The magnetic
Figure 3. Selected 57Fe Mo¨ssbauer spectra of [Fe(abpt)2(TCNQ)2].
susceptibility data recorded for [Fe(abpt)2(TCNQ)2] in the
temperature range 7-460 K provide evidence for an S ) 2 (HS)
T S ) 0 (LS) spin-crossover behavior (See Figure 2). The
product of the molar magnetic susceptibility (ø) and the
temperature (T) is 3.70 cm3 mol-1 K at 450 K, which is
consistent with the effective magnetic moment µeff ) 5.44µB,
corresponding to a quintet spin state. Upon lowering the
the dimerized TCNQ units is very strong, resulting in a
diamagnetic behavior over the whole temperature range studied.
Indeed, it has frequently been observed that the antiferromag-
netic exchange coupling within a stacked TCNQ pair is generally
very large due to the strong π interaction.22a,23 The magnetic
data recorded for the X-ray isomorphous [Zn(abpt)2(TCNQ)2]
show that the øT value remains relatively constant at a value of
0 cm3 mol-1 K over the whole temperature range. For the
compounds containing the paramagnetic metal(II) ions Mn, Co,
Ni, and Cu, the øT Vs T curves follow the Curie Law,
corresponding to the respective high-spin metal(II) energy states
(data not shown).
temperature, øT attains a value of 0.25 cm3 mol-1 K (µeff
)
1.41µB) at 7 K. This is very close to the value expected for a
singlet spin state, indicating that the spin-crossover is almost
complete. Although, this residual paramagnetism may be
interpreted as indicative of about 7% of high-spin Fe(II) species,
its presence has not been detected by Mo¨ssbauer spectroscopy
(Vide infra). On the other hand, in the EPR spectra (Vide infra),
signals due to small TCNQ•- radical anion impurities have been
observed, which may also result in paramagnetism at low
temperatures. Further, the µeff value of 5.44µB at 450 K is at
the upper limit of the values normally encountered for high-
spin Fe(II); this excludes the presence of LS Fe(II) at this
temperature. Yet the Mo¨ssbauer spectrum (Vide supra) at 330
K shows an asymmetry of the Fe(II) HS lines. This asymmetry
may be due to high-spin Fe(III) impurity, which could at the
same time explain part of the enhanced µeff value of 1.41µB at
7 K.
Mo1ssbauer Spectroscopy. 57Fe Mo¨ssbauer spectroscopy
measurements have been performed in the temperature range
360-120 K. Representative spectra are presented in Figure 3.
From the spectrum at T ) 120 K, it is seen that besides the
central electric quadrupole doublet small additional contributions
to the spectrum are present at Doppler velocities of about -0.7
and 3.3 mm s-1
. These very small spectral contributions
(absorptions 3 and 4) have been ignored in the analyses of the
measured spectra. As shown in Figure 3, the central doublet
with a small quadrupole splitting (at T ) 120 K) is gradually
The transition temperature of this gradually proceeding spin
transition is about 280 K. The transition does not show any
hysteresis, since the øT Vs T curves recorded at decreasing and
increasing temperatures are identical. Magnetic coupling within
(23) (a) Miller, J. S.; Calabrese, J. C.; Harlow, R. L.; Dixon, D. A.; Zhang,
J. H.; Reiff, W. M.; Chittipeddi, S.; Selover, M. A.; Epstein, A. J. J. Am.
Chem. Soc. 1990, 112, 5496. (b) Schwartz, M.; Hatfield, W. E. Inorg.
Chem. 1987, 26, 2823.

Unique New Type of Fe(II) Spin-CrossoVer Compound
J. Am. Chem. Soc., Vol. 118, No. 9, 1996 2195
Table 6. FT-Infrared Spectroscopy Data for the νCN Absorption
(cm-1) for [M(abpt)2(TCNQ)2] (M(II) ) Mn, Fe, Co, Ni, Cu)
changing into a doublet with a much larger splitting (at T )
360 K) with increasing temperature. These doublets can be
ascribed to the LS and the HS state of Fe(II), respectively. In
that case the spin-transition temperature is determined to occur
at about 280 K, in agreement with the results of the magnetic
susceptibility measurements, presented in Figure 2. In the spin-
transition region, the LS absorption line and to some extent the
HS lines show a significant broadening and an increasing
asymmetry of the HS lines. Although distinct spectra for both
spin states are observed, the behavior is characteristic for
relaxation effects, where the rate constant for the spin-conversion
process (denoted as kLH) is about equal to the nuclear Larmor
precession frequency for the 57Fe nucleus (denoted as ωn).24
The spectra presented in Figure 3 have been analyzed using
single Lorentzian-shaped lines, and the isomer shift (IS) values
and quadrupole splitting (∆EQ) values are deduced from the
individual line positions and listed in Table 5 together with the
spectral contributions (in percent). It should be noted that the
fitting procedure applied does not account for the relaxation
effects, from which result part of the line broadening and
asymmetry of the absorptions. A more correct and detailed
interpretation of the Mo¨ssbauer data may be obtained using the
stochastic theory of line shape of Blume.25 Therefore, it would
be worthwhile to subject the present Mo¨ssbauer data to a line
shape analysis that will eventually produce values of rate
constants for the dynamic spin state interconversion process.
Consistent with the magnetic susceptibility measurements, no
hysteresis in the low-spin/high-spin transition has been observed
by Mo¨ssbauer spectroscopy.
However, the present analyses of the spectra measured at 315,
330, and 360 K indicate that a residual spectral contribution
had to be included in the fitting procedure; this contribution is
listed in the last three columns of Table 5. This impurity
(absorption 6) may be due to HS Fe(III), which is expected to
have a small ∆EQ. This is further supported by the observed
asymmetry in the Fe(II) LS lines at 120 K. As the Debye-
Waller factors for Fe(II) and Fe(III) are far different, a good
estimate of the amount of Fe(III) is not possible. It is expected
that an impurity of about 3% already could give rise to such
“perturbed” Mo¨ssbauer spectra. This also contributes to the
residual magnetism at low temperatures and the high µeff at
450 K.
Infrared Spectroscopy. The FT-IR spectra of the X-ray
isomorphous series of compounds [M(abpt)2(TCNQ)2] (M(II)
) Mn, Fe, Co, Ni, Cu, Zn) are all very similar. The absorption
at 828 cm-1 clearly indicates the presence of solely the
uninegative TCNQ•- radical ion.26 The characteristic strong
absorption around 2190 cm-1 (νCN) is split; this splitting can
be explained by the fact that the nitrile groups are inequivalent,
i.e. the presence of coordinating, noncoordinating, and CN
groups involved in hydrogen bonding. The positions of the νCN
absorptions are listed in Table 6. Absorptions B and C do not
show a large change in frequency upon changing the metal ion
or the spin state. These absorptions might be assigned to the
asymmetric (B) and symmetric (C) vibrations of the noncoor-
dinating CN groups of TCNQ. Absorptions A and D could be
due to the two cyano groups in the closest vicinity of the metal-
(II) ion coordination sphere (one of these being the coordinated
TCNQ nitrile group), since these nitriles will mostly be in-
fluenced by a change in metal ion or spin state. Figure 4 shows
the position of the νCN absorptions as function of the crystal
field stabilization energy (CFSE) in the ideal case of an assumed
M(II)
Mn
dn CFSE (Dq)
ν
CN (A)
ν
CN (B)
ν
CN (C)
ν
CN (D)
d5
0
-4
-6
2185
2187
2185
2188
2189
2198
2159
2159
2158
2149
2150
2154
2156
2160
2169
Fe (HS) d6
2183
2180
2184
2184
2186
Cu
Co
Ni
d9
d7
d8
-8
-12
-24
Fe (LS) d6
2164
a A, B, C, and D represent the various absorptions.
Figure 4. Plot of the νCN absorptions as a function of the crystal field
stabilization energy (CFSE) for assumed Oh symmetry. The lines
drawn are meant as a guide for the eye. The metal ions are as listed
in Table 6.
Oh symmetry around the metal(II) ion. All absorptions show a
shift to higher frequencies as a function of the CFSE, which is
a known behavior.27 It should be noted that the agreement for
Cu(II) is less good, apparently due to its Jahn-Teller distorted
geometry. The FT-IR spectra recorded for [Fe(abpt)2-
(TCNQ)2] in the temperature range 100-300 K monitor the
effect of the spin-crossover on the frequency and intensity of
the νCN absorptions. Clearly, the series of νCN vibrations are a
fingerprint for Fe(II) HS (in Figure 5 denoted as absorptions
A1 and D1) and LS states (absorptions A2 and D2). Moreover,
the spectra recorded at various temperatures represented in
Figure 5 confirm the coexistence of HS and LS states.
EPR Spectroscopy. The thermally induced HS T LS transi-
tion of [Fe(abpt)2(TCNQ)2] has been followed by variable-temp-
erature X-band powder EPR spectroscopy by using the Cu(II)-
and Mn(II)-doped Fe(II) species. The spectra recorded at vari-
ous temperatures for Cu(II)- and Mn(II)-doped Fe(II) com-
pounds are represented in Figures 6 and 7, respectively. Al-
though the TCNQ•- radical anions are dimerized, yielding
diamagnetic entities, a narrow signal at g ) 2.00 remains visible
in all spectra; this signal is attributed to small nondimerized
TCNQ•- impurities.28 Because of the completeness of the tran-
sition, it is likely that the host lattice becomes essentially dia-
magnetic below Tc, whereas it is paramagnetic above Tc. In
the paramagnetic phase the EPR spectra are poorly resolved
due to exchange broadening, but this changes dramatically after
the spin transition to spectra with sharp and distinct features.
However it cannot be excluded that relaxation effects play an
important role. Further investigations on this could be very
useful.
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), in agreement with a copper(II) ion in an elongated
tetragonal environment.29 In contrast, at Tc and below, the
spectrum becomes better resolved and now even shows super-
(24) (a) Adler, P.; Spiering, H.; Gu¨tlich, P. Inorg. Chem. 1987, 26, 3840.
(b) Ko¨nig, E. Struct. Bonding 1991, 76, 51.
(25) Blume, M. Phys. ReV. 1968, 174, 351.
(26) Lunelli, B.; Pecile, C. J. Chem. Phys. 1970, 52, 2375.
(27) Gans, P. In Vibrating molecules; Chapman and Hall LTD: London,
1971; p 179.
(28) Inoue, M.; Inoue, M. B. Inorg. Chem. 1986, 25, 37.
(29) Hathaway, B. J.; Billing, D. E. Coord. Chem. ReV. 1970, 5, 143.

2196 J. Am. Chem. Soc., Vol. 118, No. 9, 1996
Kunkeler et al.
Figure 6. Selected X-band powder EPR spectra of Cu(II)-doped
[Fe(abpt)2(TCNQ)2].
Figure 5. Selected FT-IR spectra in the CN stretch region of
[Fe(abpt)2(TCNQ)2].
lent metal ions has been found.22c,31 However, coordination to
divalent metal ions is rarely observed. Coordination of TCNQ
to Cu(II) and Ni(II) has been found in ML(TCNQ)2 (LH2 )
the tetra-Schiff base macrocycle resulting from the 2/2 con-
densation of 1,3-diaminopropane and 2,6-diformyl-4-meth-
ylphenol).22a Very recently, [MnII(tpa)(TCNQ)(CH3OH)]-
(TCNQ)2‚CH3CN (tpa ) tris(2-pyridylmethyl)amine) has been
reported.32 The present series of six isomorphous compounds
[MII(abpt)2(TCNQ)2], where M(II) ) Mn, Fe, Co, Ni, Cu, and
Zn,9 reported here is unique in this respect. Besides its strong
electron-accepting properties, the unfavorability of TCNQ
toward coordination is also related to crystal packing efficiency
considerations. Usually the TCNQ moieties have a tendency
to form segregated stacks of cations and anions, in which the
TCNQ-TCNQ stacking distances are frequently not equidistant,
giving rise to the formation of di-, tri-, tetra-, and pentamers.22d
Stacking interactions in TCNQ compounds definitely lead to
more stable solid compounds, which are more easily formed.
Indeed, in all compounds known to contain N-coordinating
TCNQ radical anions, the TCNQ units are present in the crystal
lattice in such a way as to be able to form at least stacked
dimers.9,22a,c,31,32 Apparently, the TCNQ molecules only coor-
dinate in cases in which the formation of stacks is possible. A
review article describing the structural and electronic features
hyperfine structure (AN| ) 16 G). The superhyperfine structure
splits each line into nine components, due to the coupling of
the unpaired electron of copper(II) with the four abpt nitrogen
atoms in the equatorial coordination sphere. 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.30 The signal is split by the zero-
field splitting, yielding additional signals at g ) 1.6 and g )
5.5 and six hyperfine lines (A| ) 80 G) are clearly visible on
both signals. As might be expected, the EPR spectra of the
Mn(II)- and Cu(II)-doped diamagnetic isostructural Zn(II)
compound (data not shown) are similar to those of the doped
Fe(II) species at temperatures below Tc.
These results show a striking similarity to those obtained for
the also tetragonally coordinated [Fe(btr)2(NCS)2]H2O com-
pound and its Cu and Mn dopes described by Vreugdenhil et
al.7c and extensively investigated by Ozarowski et al.7g
Concluding Remarks
A unique new type of Fe(II) spin-crossover compound has
been reported. The compound represents the first example of
TCNQ radical anions coordinating monodentately to Fe(II). In
fact, coordination of TCNQ rarely occurs; the molecule has
strong electron affinity, which is caused by the electron-
withdrawing capacity of the four cyano groups. Therefore,
TCNQ easily absorbs an electron and forms the radical anion
TCNQ•-. In several cases coordination of TCNQ•- to monova-
(31) (a) Ballester, L.; Barral, M. C.; Gutie´rrez, A.; Jime´nez-Aparicio,
R.; Martinez-Muyo, J. M.; Perpin˜an, M. F.; Monge, M. A.; Ru´ız-Valero,
C. J. Chem. Soc., Chem. Commun. 1991, 1396. (b) Shields, L. J. Chem.
Soc., Faraday Trans. 2 1985, 81, 1. (c) Grossel, M. C.; Evans, F. A.;
Hriljac, J. A.; Morton, J. R.; LePage, Y.; Preston, K. F.; Sutcliffe, L. H.;
Williams, A. J. J. Chem. Soc., Chem. Commun. 1990, 439.
(30) Dowsing, R. D.; Nieuwenhuijse, B.; Reedijk, J. Inorg. Chim. Acta
1971, 5, 301.
(32) Oshio, H.; Ino, E.; Mogi, I.; Ito, T. Inorg. Chem. 1993, 32, 5697.

Unique New Type of Fe(II) Spin-CrossoVer Compound
J. Am. Chem. Soc., Vol. 118, No. 9, 1996 2197
stood. The absence of hysteresis for the present compound can
therefore at this stage not be rationalized, and further research
will be required.
Clearly, [Fe(abpt)2(TCNQ)2] is the first example of an up to
now unknown family of spin-transition compounds. Because
of its interesting physical properties and possible applications
in molecular electronics, the research on this type of spin-
crossover compound will be explored further, i.e. by changing
the 1,2,4-triazole ligand or the TCNQ derivative. A change in
the coordination environment of the Fe(II) ion is expected to
influence directly the spin-crossover behavior. Such studies may
yield Fe(II) compounds showing a more abrupt spin transition,
perhaps also taking place with hysteresis.
In the present series of isostructural compounds of general
formula [MII(abpt)2(TCNQ)2] (M(II) ) Mn, Fe, Co, Ni, Cu,
Zn),9 in spite of the short M-N(TCNQ) distances, no magnetic
coupling could be observed between the radical anion and the
paramagnetic metal(II) ion. From X-ray structure determina-
tions for the Cu(II)9 and Fe(II) analogues, it became evident
that dimerization of the radical anions occurs. Due to this
dimerization, a very strong antiferromagnetic coupling occurs
within the stacked TCNQ pair. This large diamagnetic unit
apparently does not allow significant superexchange between
the parmagnetic metal(II) ions. Consequently, the magnetic
behavior of the compounds follows the Curie law for the
corresponding metal(II) ions.
It would be highly interesting to “decouple” or to diminish
the magnetic coupling within dimerized TCNQ units. In that
case, stacking interactions between the radical anions should
be prevented. This may be achieved upon using substituted
TCNQ derivatives, such as 2,5-dimethyl-7,7′,8,8′-tetracyano-
quinodimethane (DMTCNQ). This appears particularly inter-
esting in the case of Fe(II). In that way, compounds with an
up to now not observed combination of magnetic phenomena
((direct) exchange interactions and HS T LS spin crossover)
might be obtained. Further studies along this line are in progress
and are expected to yield compounds with a new magnetic
behavior.
Figure 7. Selected X-band powder EPR spectra of Mn(II)-doped
[Fe(abpt)2(TCNQ)2].
of metal compounds containing TCNQ and related polynitrile
π acceptors has very recently become available.33
Spin-crossover behavior has frequently been observed for
Fe(II) ions coordinated by two didentate chelating nitrogen-
donating ligands, together with two monodentately coordinating
nitrogen-donating ligands.2,5c,20 In these cases, the monodentate
ligands are generally in a cis configuration; a trans geometry
has so far only been found for the two-dimensional compounds
[FeX2(btr)2](H2O) (X ) NCS,7c,d NCSe;7g btr ) 4,4′-bis(1,2,4-
triazole)). The trans orientation of the TCNQ radical anions
in the present compound is directly related to the crystal packing
efficiency considerations for the TCNQ radical anions, as
mentioned above.
The compound [Fe(abpt)2(TCNQ)2] is particularly interesting,
since it exhibits a reversible spin-crossover very close to room
temperature. No hysteresis could be detected. It is known that,
in solid systems, the spin-crossover phenomenon is not uniquely
of molecular origin; the existence of interactions between the
molecular units results in a more or less pronounced cooperative
effect. This cooperative effect generally accepted to be closely
related to the occurrence of hysteresis.2c Several attempts have
been undertaken to describe the mechanism accounting for this
cooperativity.2c However, an adequate model has not yet been
found. Several authors even suggested the possibility of
hysteresis at a purely molecular scale, i.e. for molecules having
no interaction.34 Clearly, the factors accounting for hysteresis
occurring in the spin transition are not yet completely under-
Acknowledgment. The authors would like to thank the
“Werkgroep Fundamenteel Materialen Onderzoek” (WFMO) for
financial support. This work was supported in part (A.L.S.)
by the Netherlands Foundation of Chemical Research (SON)
with financial aid from the Netherlands Foundation for Scientific
Research (NWO). Financial support by the European Union,
allowing regular exchange of preliminary results with several
European colleagues (D. Gatteschi, P. Gu¨tlich, and O. Kahn),
under Contract ERBCHRXCT920080, is thankfully acknowl-
edged. Mr. G. A. van Albada is thanked for assistance with
the FT-IR measurements.
Supporting Information Available: Complete listings of
bond lengths and angles and positional parameters and figure
of the crystal packing (15 pages); listings of structure factors
(7 pages). This material is contained in many libraries on
microfiche, immediately follows this article in the microfilm
version of the journal, can be ordered from the ACS, and can
be downloaded from the Internet; see any current masthead page
for ordering information and Internet access instructions.
JA943960S
(33) Kaim, W.; Moscherosch, M. Coord. Chem. ReV. 1994, 129, 157.
(34) Bolvin, H.; Kahn, O.; Vekhter, B. New J. Chem. 1991, 15, 889.