B(C6F5)3 adducts of TCNE- and TCNQ-vanadium complexes as new building blocks for molecule-based magnets

Article abstract of DOI:10.1021/om060433w

The TCNX ligands TCNE and TCNQ reacted in toluene with Cp₂V in the presence of B(C₆F₅)₃ to yield, depending on the stoichiometry, the divanadium(III) complexes [(Cp₂V) ₂{(C₆F₅)₃B·(μ₄- TCNX)·B(C₆F₅)₃}] (X = Q (3), E (4)) or the vanadium(IV) dimers [Cp₂V{(C₆F₅) ₃B·(μ₄-TCNX)· B(C₆F ₅)₃}]₂ (X = E (5), Q (6)), all containing [TCNX]^2- ligands σ-bonded to vanadium through a nitrile nitrogen, as revealed by crystallographic (for 3-5) and magnetic studies (for 3-6).

Full text of DOI:10.1021/om060433w

Organometallics 2006, 25, 4243-4246
4243
B(C6F5)3 Adducts of TCNE- and TCNQ-Vanadium Complexes as
New Building Blocks for Molecule-Based Magnets
Robert Choukroun,* Christian Lorber,† Dominique de Caro, and Laure Vendier
Laboratoire de Chimie de Coordination, CNRS UPR 8241, lie´ par conVention a` l’UniVersite´ Paul
Sabatier, 205 route de Narbonne, 31077 Toulouse Cedex 04, France
ReceiVed May 18, 2006
Summary: The TCNX ligands TCNE and TCNQ reacted in
toluene with Cp2V in the presence of B(C6F5)3 to yield,
depending on the stoichiometry, the diVanadium(III) complexes
[(Cp2V)2{(C6F5)3B‚(µ4-TCNX)‚B(C6F5)3}] (X ) Q (3), E (4))
or the Vanadium(IV) dimers [Cp2V{(C6F5)3B‚(µ4-TCNX)‚
B(C6F5)3}]2 (X ) E (5), Q (6)), all containing [TCNX]2- ligands
σ-bonded to Vanadium through a nitrile nitrogen, as reVealed
by crystallographic (for 3-5) and magnetic studies (for 3-6).
reactivity of Cp2V with TCNX (X ) E, Q). Previously, the
vanadium(III) complexes Cp2VX(η1-TCNE) (X ) Cl, Br, I)
have been prepared from the reaction of Cp2VX and TCNE.5
To our knowledge, and although the structure has been only
partially solved (X ) Br), this represents the only example of
a structurally characterized vanadium-TCNE complex sup-
ported by Cp ligands. TCNE was also reported to substitute
carbon monoxide in Cp2V(CO) to afford a complex formulated
as “Cp2V(TCNE)” on the basis of analytical and spectroscopic
(IR) evidence.6 In a somewhat related vein, we have recently
demonstrated that Cp2V reacts with simple nitriles (RCN) when
they are activated with a Lewis acid L (L ) BCl3, B(C6F5)3,
AlCl3) to give Lewis acid adducts of vanada(IV)azirine com-
plexes, [Cp2V(η2-RCdN‚L)].7 To extend the scope of this
reaction, the reactivity of Cp2V with TCNE and TCNQ in the
presence of the Lewis acid B(C6F5)3 was studied. Here, we
present crystallographic evidence for the coordination mode of
TCNX ligands to the [Cp2V] fragment with formation of VIII
and VIV complexes. The magnetic behavior of these compounds
was also determined.
The discovery in the early 1990s by the group of Miller of
the first room-temperature molecule-based magnet, V[TCNE]2‚
yCH2Cl2 (y ≈ 0.5),1 from the reaction of V(CO)6 or V(C6H6)2
with TCNE (TCNE ) tetracyanoethylene) in dichloromethane,
has stimulated the coordination chemistry of TCNE and related
TCNQ (TCNQ ) 7,7,8,8-tetracyano-p-quinodimethane) deriva-
tives with various transition metals and coligands.2 On the basis
of elemental analysis and IR data, the local structure in the
V[TCNE]2 coordination polymers is assumed to consist of 3D
networks bridging cis-[TCNE]•- ligands between V2+ ions;3
however, unambiguous structural information is not available,
due to the low solubility of this amorphous material. To obtain
insight into the local V coordination geometry of the polymers,
we synthesized discrete molecular vanadium species incorporat-
ing TCNE ligands that could serve as structural and magnetic
model compounds of V[TCNE]2 magnets.
We first investigated the reaction between the nitrile groups
of TCNX molecules with the Lewis acid B(C6F5)3, to verify
that an adduct similar to that found in simple nitriles could be
generated.7,8 The addition of 2 equiv of B(C6F5)3 to a toluene
solution containing TCNE led to a red solution.9 Layering this
solution with pentane gave an orange solid identified as the bis-
(borane) adduct [TCNE‚{B(C6F5)3}2] (1). A similar reaction of
As part of our long-term interest in the organometallic
chemistry of vanadocene (Cp2V),4 we have investigated the
* To whom correspondence should be addressed. E-mail: choukroun@
lcc-toulouse.fr.
† E-mail: lorber@lcc-toulouse.fr.
(1) (a) Manriquez, J. M.; Yee, G. T.; McLean, R. S.; Epstein, A. J.;
Miller, J. S. Science 1991, 252, 1415-1417. (b) Miller, J. S. Inorg. Chem.
2000, 39, 4392-4408. (c) Pokhodnya, K. I.; Epstein, A. J.; Miller, J. S.
AdV. Mater. 2000, 12, 410-413. (d) Miller, J. S.; Epstein, A. J. Chem.
Commun. 1998, 1319-1325.
(2) Selected transition-metal complexes with TCNX ligands: (a) Olbrich-
Deussner, B.; Gross, R.; Kaim, W. J. Organomet. Chem. 1989, 366, 155-
174. (b) Siedle, A. R.; Candela, G. A.; Finnegan, T. F. Inorg. Chim. Acta
1979, 35, 125-130. (c) Moscherosch, M.; Waldho¨r, E.; Binder, H.; Kaim,
W.; Fiedler, J. Inorg. Chem. 1995, 34, 4326-4335. (d) Clerac, R.; O’Khane,
S.; Cowen, J.; Ouyang, X.; Heintz, R.; Zhao, H.; Bazile, M. J.; Dunbar, K.
R. Chem. Mater. 2003, 15, 1840-1850. (e) Vickers, E. B.; Selby, T. D.;
Miller, J. S. J. Am. Chem. Soc. 2004, 126, 3716-3717. (f) Vickers, E. B.;
Selby, T. D.; Thorum, M. S.; Taliaferro, M. L.; Miller, J. S. Inorg. Chim.
2004, 43, 6414-6420. (g) Raebiger, J. W.; Miller, J. S. Inorg. Chem. 2002,
41, 3308-3312. (h) Vickers, E. B.; Senesi, A.; Miller, J. S. Inorg. Chim.
Acta 2004, 357, 3889-3894. (i) Wang, G.; Zhu, H.; Slebodnick, C.; Yee,
G. T. Inorg. Chem. 2006, 45, 1406-1408. (j) Yee, G. T.; Calabrese, J. C.;
Vazquez, C.; Miller, J. S. Inorg. Chem. 1993, 32, 377-378. (k) Campana,
C.; Dunbar, K. R.; Ouyang, X. Chem. Commun. 1996, 2427-2428. (l)
Miyasaka, H.; Campos-Fernandez, C. S.; Cle´rac, R.; Dunbar, K. R. Angew.
Chem., Int. Ed. 2000, 39, 3831-3835.
B(C6F5)3 with TCNQ in CH2Cl2 afforded a red solution from
which the orange crystalline product 2 was separated. An X-ray
structure determination (see the Supporting Information) on a
single crystal clearly confirmed that 2 is the bis(borane) adduct
[µ2-TCNQ‚{B(C6F5)3}2]. The solid-state structure also revealed
(3) Gordon, D. C.; Deakin, L.; Arif, A. M.; Miller, J. S. J. Am. Chem.
Soc. 2000, 122, 290-299.
(4) (a) Choukroun, R.; Lorber, C. Eur. J. Inorg. Chem. 2005, 4683-
4692. (b) Choukroun, R.; Lorber, C.; Vendier, L.; Lepetit, C. Organome-
tallics 2006, 25, 1551-1553.
10.1021/om060433w CCC: $33.50 © 2006 American Chemical Society
Publication on Web 08/04/2006

4244 Organometallics, Vol. 25, No. 18, 2006
Communications
having a V-N single σ bond (1.7-1.8 Å)11 but are in the range
of those found in the pseudohalide vanadocene series of
complexes Cp2VX2 (X ) NCO, N3, NtCNCtN)12 (for
comparison, in the cationic [Cp2VMe(NtCCH3)]+ the V-N
distance is 2.096(4) Å).13 All atoms of the TCNQ framework
in 3, as well as the centroids (Ct) of the cyclopentadienyl rings
and the boron atoms and the vanadium centers, are roughly
contained in the same plane. The V-NtC-C skeleton is
essentially linear, and the CtN(V) bond distances are in the same
order as the CtN(B) bond distances. The C-C distances in the
TCNQ ring are nearly the same (1.376-1.391 Å), leading to a
more benzene-like structure, as observed in [TCNQ]2- 14
. Both
C-C distances attached to the ring are elongated and are best
described as C-C single bonds (C2-C4 ) 1.459(7) Å, C1-
C7 ) 1.477(7) Å) by comparison to the equivalent distances
observed in free TCNQ15 and in the adduct 1 (1.374(2) and
1.384(9) Å, respectively). The most intriguing feature is the
NC-C4-CN and NC-C10-CN carbon chains, in which the
formation of a partial double bond (1.356(8)-1.413(8) Å) could
be suggested by comparison to the same distances observed in
the neutral TCNQ molecule (1.440(4), 1.441(4) Å) and in 2
(1.418(10), 1.435(11) Å). These structural data are consistent
with a fully delocalized [TCNQ]2- system, as suggested in the
ruthenium complex {(µ4-TCNQ)[Ru(NH3)5]4}8+, for which a
crystal structure is not available.16 The IR spectrum of 2 presents
Figure 1. ORTEP drawing of the molecular structure of 3 showing
50% probability ellipsoids and partial atom-labeling schemes. C6F5
groups are depicted as spheres, and H atoms are omitted for clarity.
Ct denotes the centroid of the cyclopentadienyl ring. Selected
interatomic bond distances (Å) and angles (deg): C2-C4 ) 1.465-
(6), C7-C10 ) 1.475(6), V1-Ct1 ) 1.993, V1-Ct2 ) 1.982,
V1-N1 ) 2.041(5), V2-Ct3 ) 1.920, V2-Ct4 ) 1.918, V2-N3
) 2.020(5), B1-N2 ) 1.564(8), B2-N4 ) 1.579(8), N1-C1 )
1.157(7), N3-C11 ) 1.150(7), N2-C3 ) 1.162(7), N4-C12 )
1.159(7); Ct1-V1-Ct2 ) 144.20 Ct3-V2-Ct4 ) 144.10.
characteristic νCN bands in the region expected for [TCNQ]2- 17
,
the attachment of the two B(C6F5)3 groups to the nitrogen atom
of two CN groups in anti-1,2-positions.
but one must keep in mind that the presence of the two
coordinated B(C6F5)3 groups does not allow a direct comparison
with other TCNQ compounds.18 A magnetic study revealed that
the dinuclear complex 3 exhibits an essentially temperature-
independent øT vs. T plot. These data are consistent with two
independent VIII centers (øTcalcd ) 2.00 emu K mol-1, assuming
g ) 2; øT300 K ) 1.98 emu K mol-1). Therefore, 3 is best
described as a [TCNQ]2--containing divanadium(III) complex
arising from a redox process between two Cp2VII donors and
the TCNQ acceptor.
For solubility reasons, the reactivity of Cp2V was studied with
the nonisolated adducts 1 and 2. The addition of 2 equiv of
Cp2V in toluene to a toluene solution of the adduct 2 (generated
in situ from 2 equiv of B(C6F5)3 on TCNQ) afforded after
suitable workup compound 3 as a crystalline dark precipitate.
In a similar way, reacting 2 equiv of Cp2V with TCNE in
the presence of 2 equiv of B(C6F5)3 in toluene afforded the
crystalline dark red complex [(Cp2V)2{(C6F5)3B‚TCNE‚B(C6F5)3}]
(4) (Figure 2).17 Compound 4 crystallized as a toluene solvate,
Dark red single crystals were obtained from a THF/pentane
solution of 3, which according to single-crystal X-ray analysis
were the homobimetallic complex [(Cp2V)2{(C6F5)3B‚TCNQ‚
B(C6F5)3}] (3) (Figure 1). The main feature of the solid-state
structure of 3 is the presence of two [Cp2V] units σ-bonded in
a syn-1,2-positions to the free nitrogen atom of two CN groups
of the Lewis acid adduct of the µ4-TCNQ group, whereas two
B(C6F5)3 groups are attached to the two remaining nitrogen
atoms of the TCNQ ligand.10 The V-N distances of 2.041(5)
and 2.020(5) Å are longer than those found for compounds
and its molecular structure was determined by single-crystal
(11) Selected examples of V-N amido bonds: (a) Choukroun, R.;
Moumboko, P.; Chevalier, S.; Etienne, M.; Donnadieu, B. Angew. Chem.,
Int. Ed. 1998, 37, 3169-3172. (b) Lorber, C.; Choukroun, R.; Donnadieu,
B. Inorg. Chem. 2002, 41, 4217-4226.
(5) Rettig, M. F.; Wing, R. M. Inorg. Chem. 1969, 8, 2685-2689.
(6) Calderazzo, F.; Guelfi, M.; Pampaloni, G. Organometallics 2004,
23, 1519-1524.
(7) (a) Choukroun, R.; Lorber, C.; Donnadieu, B. Chem. Eur. J. 2002,
8, 2700-2704. (b) Choukroun, R.; Lorber, C.; Vendier, L.; Donnadieu, B.
Organometallics 2004, 23, 5488-5492.
(8) Jacobsen, H.; Berke, H.; Dor¨ıng, S.; Kehr, G.; Erker, G.; Fro¨lich,
R.; Meyer, O. Organometallics 1999, 18, 1724-1735.
(9) Note that using 4 equiv of B(C6F5)3 led to the formation of the same
bis-adduct compound.
(10) The µ4 mode of coordination refers to the 4-fold-coordinated TCNX
ligand (see the figures); it is clear in this case that coordination occurs in
syn-1,2-positions with two vanadium centers and in syn-1,2-positions with
two boron atoms.
(12) (a) Honzicek, J.; Erben, M.; Cisarova, I.; Vinklarek, J. Appl.
Organomet. Chem. 2005, 19, 100-101. (b) Honzicek, J.; Erben, M.;
Cisarova, I.; Vinklarek, J. Appl. Organomet. Chem. 2005, 19, 102-103.
Honzicek, J.; Erben, M.; Cisarova, I.; Vinklarek, J. Inorg. Chim. Acta 2005,
358, 814-819.
(13) Choukroun, R.; Lorber, C.; Donnadieu, B. Organometallics 2002,
21, 1124-1126.
(14) 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, 4344-4360.
(15) Long, R. E.; Sparks, R. A.; Trueblood, K. Acta Crystallogr. 1965,
18, 932-939.

Communications
Organometallics, Vol. 25, No. 18, 2006 4245
Figure 2. (left) ORTEP drawing of the molecular structure of 4 showing 50% probability ellipsoids and partial atom-labeling schemes.
C6F5 groups are depicted as spheres, and solvent (1/2 THF) and H atoms are omitted for clarity. (right) A different view to show the twist
of the TCNE moiety (C6F5 groups are omitted). Ct denotes the centroid of the cyclopentadienyl ring. Selected interatomic bond distances
(Å) and angles (deg): C2-C3 ) 1.459(5), V1-Ct1 ) 1.914, V1-Ct2 ) 1.928, V1-N1 ) 2.041(5), V2-Ct3 ) 2.010, V2-Ct4 ) 1.994,
V2-N3 ) 2.020(5), B1-N2 ) 1.564(8), B2-N4 ) 1.579(8), N1-C1 ) 1.157(7), N3-C11 ) 1.150(7), N2-C3 ) 1.162(7), N4-C12 )
1.159(7); Ct1-V1-Ct2 ) 142.30, Ct3-V2-Ct4 ) 146.14.
Figure 3. (left) Drawing of the molecular structure of 5 and partial atom-labeling schemes, with all atoms are depicted as spheres. Solvent
(2 toluene) and H atoms are omitted for clarity. (right) A different view to show the twist of the TCNE moiety and the 14-atom cyclic
arrangement (C6F5 groups are omitted for clarity).
X-ray diffraction. Both vanadium centers are σ-bonded to the
nitrogen atom of the Lewis acid adduct of µ2-TCNE with V-N
distances similar to those found in 3 (2.056(3), 2.049(3) Å).
The TCNE framework is twisted along the central C-C bond
with a dihedral angle of 78.9°. However, the general features
(geometrical parameters) of the C-N bonds are comparable to
those of 3. The central C2-C3 bond of the TCNE in 4 (1.459-
(5) Å) is markedly longer than the analogous CdC in free TCNE
(1.344(3) Å)19 and can be considered as a single bond. The C-C
distances in the NCCCN chain (1.380(5)-1.411(5) Å) show
the formation of a partial double-bond chain between the
nitrogen atom bonded to the vanadium atom and the nitrogen
atom bonded to the boron atom. As for 3, the magnetic
properties of complex 4 are characteristic of two independent
spin-only VIII centers (øT300 K ) 1.98 emu K mol-1).
Next, we have studied the effect of a variation of the relative
stoichiometry of the reactants by performing the reaction using
the ratio TCNX:B(C6F5)3:Cp2V ) 1:2:1 (instead of 1:2:2 for
the above synthesis of 3 and 4). For example, the reaction
between TCNE, 2 equiv of B(C6F5)3, and 1 equiv of Cp2V was
performed in toluene, affording the dark orange complex 5.17
(16) Moscherosch, M.; Kaim, W. Inorg. Chim. Acta 1993, 206, 229-
230.
(17) IR data for all compounds (KBr, cm-1): 1, νCN‚B 2321 (s), νCN
2234 (w); 2, νCN‚B 2307 (s), νCN 2224 (w); 3, νCN‚B 2250 (m), νCN 2201
(w), νCN 2150 (s); 4: νCN‚B 2254 (m), νCN 2198 (w), νCN 2154 (s); 5: νCN‚
B 2250 (m), νCN 2201 (w), νCN 2150 (s); 6: νCN‚B 2260 (m), νCN 2196 (w),
νCN 2134 (s). For comparison purposes: TCNE, νCN 2255 (vs), 2216 (s);
TCNQ, νCN 2228 (s).
(18) For reviews on the assignment of the structure and charge of TCNX-
containing compounds on the basis of their νCN bands, see: (a) Kaim, W.;
Moscherosch, M. Coord. Chem. ReV. 1994, 129, 157-193. (b) Miller, J.
S. Angew. Chem., Int. Ed. 2006, 45, 2508-2525.
(19) (a) Little, R. G.; Pauther, D.; Coppens, P. Acta Crystallogr. 1971,
B27, 1493-1499. (b) Chaplot, S. L.; Mierzejewski, A.; Pawley, G. S. Acta
Crystallogr. 1984, C40, 663-666.
Attempts to obtain high-quality crystals of 5 suitable for an
X-ray structure determination failed. Nevertheless, the quality
of the crystals obtained in THF/pentane was sufficient to
establish an overall structure of the atomic connectivity but not
to report detailed bond distances and angles. The formation of
the dimeric species [Cp2V{(C6F5)3B‚TCNE‚B(C6F5)3}]2 (5) is
observed (Figure 3). Each vanadium atom is linked to two

4246 Organometallics, Vol. 25, No. 18, 2006
Communications
nitrogen atoms of CN groups connected to two twisted
[TCNE‚{B(C6F5)3}2]2- molecules, and a 14-atom chairlike
cyclic structure is observed. The magnetic susceptibility values
determined for 5 are in rather good agreement with two
magnetically uncoupled VIV centers (øTcalcd ) 0.75 emu K
mol-1, assuming g ) 2; øT300 K ) 0.70 emu K mol-1).
Similarly, the analogous reaction conducted with TCNQ
(instead of TCNE), 2 equiv of B(C6F5)3, and 1 equiv of Cp2V
yielded a microcrystalline dark precipitate. We suggest this new
compound to be the dimer [Cp2V{(C6F5)3B‚TCNQ‚B(C6F5)3}]2
(6) on the basis of its elemental analysis, strong analogy to
Figure 4. Magnetic behavior for 6 recorded at 2 K (magnetization
vs applied magnetic field). The inset represents an enlargement
between -600 and +600 G and -15 and +15 emu mol-1
.
as shown in the inset in Figure 4. We conclude that magnetic
ordering in the sample exists at this temperature. The ferro-
magnetism of 6 may arise from the planarity of the TCNQ
molecule, which may serve as an adequate superexchange
mediator between the orbitals of the two metallic ions.7b,22
In conclusion, the reactivity of Cp2V with [TCNX‚
{B(C6F5)3}2] is very different from that previously observed
7
with the activated simple nitrile adducts RCN‚B(C6F5)3 (in
which the vanadaazirine complexes [Cp2V(η2(C,N)-RCN‚
B(C6F5)3)] were obtained). We showed here that B(C6F5)3 can
be used to block two sites of coordination on the TCNX ligand
and to enhance the solubility of the new building block; the
magnetic and structural studies suggest a net two-electron
reaction with formation of [TCNX]2- as a consequence of the
d2 (in 3 and 4) and d1 (in 5 and 6) configurations at the metal
center. Further efforts will be directed toward studying the redox
chemistry of these systems and in the synthesis of new
compounds relevant to [V(TCNE)2] magnets.
spectroscopic data (IR,17 19F and 11B NMR) for 5, and magnetic
studies (vide infra). Indeed, the magnetic behavior of 6 is typical
of a VIV species, but in contrast to 5, the magnetic susceptibilites
measured for 6 are strongly temperature-dependent (øT300 K
)
0.90 emu K mol-1). A Bleany-Bowers treatment20 for 6 renders
the following values: exchange constant J ) -55 cm-1 and
zero-field splitting D ) -150 cm-1. The previously reported
[V(C5Me5)2Br]2(µ2-TCNE),21 in which a VIV center and a
[TCNE]2- ligand are present, was also shown to present a
Acknowledgment. We are grateful to the CNRS for financial
support.
Supporting Information Available: Text giving a full experi-
mental section for the synthesis of all compounds, a figure detailing
a Bleany-Bowers treatment for 6, and CIF files and tables of
atomic coordinates and bond distances and angles for the X-ray
crystal structures. This material is available free of charge via the
Internet at http://pubs.acs.org.
similar temperature-dependent magnetic behavior (øT300 K
≈
0.97 emu K mol-1), except with a very distinct coupling constant
(J ) -0.6 cm-1, D ) 1.5 cm-1). Figure 4 shows the hysteresis
loop for 6 recorded at 2 K (magnetization vs applied magnetic
field). We note the absence of saturation for fields up to 50
kOe. However, a coercive field of Hc ≈ 40 Oe at 2 K is evident,
OM060433W
(20) Kahn, O. In Molecular Magnetism; VCH: Weinheim, Germany,
1993; Chapter 7, pp 135-143.
(21) Baumann, F.; Heilmann, M.; Matheis, W.; Schulz, A.; Kaim, W.;
Jordanov, J. Inorg. Chim. Acta 1996, 251, 239-248.
(22) (a) Choukroun, R.; Donnadieu, B.; Malfant, I.; Haubrich, S.; Frantz,
R.; Guerin, C.; Henner, B. Chem. Commun. 1997, 2315-2316. (b)
Choukroun, R.; Donnadieu, B.; Lorber, C.; Pellny, P.-M.; Baumann, W.;
Rosenthal, U. Chem. Eur. J. 2000, 6, 4505-4509.