Structural, electronic and magnetic properties of metal-metal bonded dinuclear rhenium complexes bridged by organocyanide acceptor ligands

Article abstract of DOI:10.1039/b304509a

The syntheses, spectroscopic properties, redox chemistry, and solid-state structures of products obtained from the reaction of Re₂Cl₄(dppm)₂ (dppm = bis(diphenylphosphino) methane) with the polycyano acceptors TCNQ (7,7,8,

Full text of DOI:10.1039/b304509a

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Structural, electronic and magnetic properties of metal–metal
bonded dinuclear rhenium complexes bridged by organocyanide
acceptor ligands†
Stuart L. Bartley, Mervin J. Bazile, Jr., Rodolphe Clérac,‡ Hanhua Zhao, Xiang Ouyang and
Kim R. Dunbar*
Department of Chemistry, Texas A&M University College Station, Texas 77842-3012, USA.
E-mail: dunbar@mail.chem.tamu.edu
Received 24th April 2003, Accepted 22nd May 2003
First published as an Advance Article on the web 12th June 2003
The syntheses, spectroscopic properties, redox chemistry, and solid-state structures of products obtained from the
reaction of Re Cl (dppm) (dppm = bis(diphenylphosphino) methane) with the polycyano acceptors TCNQ (7,7,8,8-
2
4
2
tetracyanoquinodimethanido) and DM-DCNQI (2,5-dimethyl-N,NЈ-dicyanoquinonediimine) are described. The
compounds [Re Cl (dppm) ] (µ-TCNQ), 1, and [Re Cl (dppm) ] (µ-DM-DCNQI), 2, have been prepared by reaction
2
4
2 2
2
4
2 2
of two equivalents of Re Cl (dppm) with TCNQ and DMDCNQI, respectively, in THF or CH Cl . A single-crystal
2
4
2
2
2
X-ray crystallographic study of [Re Cl (dppm) ] (µ-TCNQ)ؒ10THF revealed the presence of a trans-µ bidentate
2
4
2 2
2
mode for the bridging TCNQ ligand that joins two Re Cl (dppm) molecules through equatorial positions. In a
2
4
2
similar fashion, the compound [Re Cl (dppm) ] (µ-DM-DCNQI)ؒ10THF consists of two Re units coordinated to
2
4
2 2
2
the two nitrile positions of the DM-DCNQI ligand. The electronic properties of both compounds are unusual in
that they exhibit intense, broad absorptions that span the near-IR region and extend into the mid-IR. The electro-
chemistry of the compounds consists of numerous oxidation and reduction processes in the range of ϩ2.0 to Ϫ2.0 V
as determined by cyclic voltammetry. Both 1 and 2 exhibit temperature independent paramagnetism (TIP) with large
Ϫ3
Ϫ3
Ϫ1
χTIP values of 7.29 × 10 and 6.23 × 10 emu mol , respectively.
Introduction
reversible oxidations at E1/2(ox) = ϩ0.35 V and E1/2(ox) = ϩ0.87 V
vs. Ag/AgCl, which renders it well-suited for charge-transfer
Research in materials chemistry has witnessed an explosion of
activity in recent years, due, in part, to the quest for novel
materials that can be used in developing technologies. Although
high-temperature solid-state chemistry continues to figure
prominently in the materials arena, there is heightened interest
in molecule-based materials prepared from low-temperature
solution routes. Frontrunners in this field include the organic
charge-transfer salt TCNQ-TTF (TCNQ = 7,7,8,8-tetracyano-
quinodimethane; TTF = tetrathiafulvalene), the first molecular
crystal to exhibit metallic behavior, and the Bechgaard salts
reactions with TCNQ whose first reduction potential is located
5a,6
at E1/2(red) = ϩ0.28 V (Fig. 1).
In addition to being ideally
suited to undergo spontaneous redox chemistry with each
other, the Re2 donor and the organocyanide acceptor also
possess accessible π symmetry HOMOs which will enhance
electronic delocalization between dimetal units that are con-
nected through the bridging organic group. Based on this
rationale, we set out to study the charge-transfer reactions of
the metal–metal bonded donor complex Re Cl (dppm) with
2
4
2
TCNQ and DM-DCNQI. Herein we report the X-ray struc-
tures of two “dimer-of-dimers” complexes in which two Re Cl -
(
TMTSF) X (TMTSF = tetramethyltetraselenafulvalene) which
2
1
2
4
are among the first organic superconductors. In the area of
(
dppm) molecules are linked by TCNQ or DM-DCNQI. The
2
2
molecular-based magnetic materials, notable early examples
are soluble ferromagnets prepared from Cp *M (M = Fe, Mn)
and polycyano acceptor molecules such as TCNQ and TCNE
spectroscopic and physical properties of these unusual products
are described as well.
2
3
(
tetracyanoethylene). In the course of this research, Manriquez
and Miller et al. discovered that the reaction between V(C H )
6
6 2
and TCNE yields a room-temperature magnet formulated as
V(TCNE) ؒ0.5CH Cl which is believed to contain σ-bound
2
2
2
4
TCNE. This finding is part of a new direction for transition
metal polycyano acceptor chemistry, namely the use of organic
acceptors such as TCNQ and TCNE as nitrile ligands in their
radical anion forms.
In our research laboratories, we are exploring the chemistry
of donor molecules of the type M X (PR ) and M X (P–P) (X
2
4
3
4
n
2
4
2
=
Cl, Br; M = Mo, Re, W; R = Me, Et, Pr ; P–P = dppm, dppe)
with TCNQ and related organic acceptors such as DM-DCNQI
2,5-dimethyl-N,NЈ-dicyanoquinonediimine). In addition to
(
being coordinatively unsaturated, many of these molecules
5
undergo accessible oxidation processes. In particular, the
triply-bonded dirhenium complex Re Cl (dppm) exhibits two
2
4
2
†
Electronic supplementary information (ESI) available: Fig. S1: Cyclic
voltammogram of 1; Fig. S2: Differential pulse voltammogram of 1;
Fig. S3: Cyclic voltammogram of 2. See http://www.rsc.org/suppdata/
dt/b3/b304509a/
‡
Present address: Centre de Recherche Paul Pascal, Avenue du Dr.
Schweitzer, 33600 Pessac, France.
Fig. 1 Selected precursors used for charge-transfer reactions.
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 3
D a l t o n T r a n s . , 2 0 0 3 , 2 9 3 7 – 2 9 4 4
2937

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Table 1 A comparison of the ν(C᎐N) and ν(C᎐C) stretching frequencies for various TCNQ containing compounds
Ϫ1
Ϫ1
Compound
ν(C᎐N)/cm
ν(C᎐C)/cm
Nitrile coordination
TCNQ
2222 (vs)
1542 (s)
1594 (s)
1581 (s)
1b573 (m)
None
None
None
[
Bu N][TCNQ]
2181 (s), 2156 (m)
2200 (s), 2114 (s, br)
2187 (m), 2109 (s, br)
2179 (s), 2153 (m)
2178 (s), 2153 (m)
2200 (m), 2110 (w)
2170 (w), 2105 (s)
4
LiTCNQ
a
2
1
σ-µ
[
[
[
[
FeCp*][TCNQ]
CrCp*][TCNQ]
MnCp*(CO) ][TCNQ]
MnCp*(CO) ] [TCNQ]
None
None
σ
b
1b585 (m)
2
4
σ-µ
2
4
a
b
1
= [Re Cl (dppm) ] (µ-TCNQ). Not reported.
2 4 2 2
Results and discussion
The electron-rich complex Re Cl (dppm) reacts spontaneously
2
4
2
with the acceptor molecules TCNQ and DM-DCNQI to form
highly colored solutions and dark brown–black THF-insoluble
products. Crystals of the products [Re Cl (dppm) ] (µ-TCNQ)ؒ
2
4
2 2
1
1
0THF, 1ؒ10THF, and [Re Cl (dppm) ] (µ-DM-DCNQI)ؒ
0THF, 2ؒ10THF, with interstitial THF molecules of crystal-
2 4 2 2
lization were isolated from a careful layering of separate THF
solutions of the two reactants. The structures of both 1ؒ10THF
and 2ؒ10THF, determined by single-crystal X-ray diffraction
methods, revealed the presence of a bridging bidentate mode
for both TCNQ and DM-DCNQI.
Fig. 2 Electronic absorption spectrum of [Re
Cl
2
(dppm)
]
2
(µ-TCNQ),
4
2
1
, in CH Cl .
2 2
A. [Re Cl (dppm) ] (ꢀ-TCNQ), 1
2
4
2
2
(ii) X-Ray crystal structure of [Re Cl (dppm) ] (ꢀ-TCNQ)ؒ
2 4 2 2
1
0THF, 1ؒ10THF. The preliminary structure of 1 was reported
(
i) Synthesis and spectroscopic properties. One equivalent of
9
earlier in a brief communication by our group. A single-crystal
X-ray study of 1ؒ10THF confirms its 2 : 1 formulation, and
reveals the presence of a µ -bridging mode for TCNQ. The
molecular structure of 1ؒ10THF, depicted in Fig. 3, consists of
TCNQ reacts with two equivalents of Re Cl (dppm) in THF to
2
4
2
yield black microcrystals of 1. The infrared spectral features of
are quite unusual in comparison to typical charge-transfer
products of TCNQ. Selected infrared data for 1 are listed along
with corresponding data for TCNQ and TCNQ in Table 1.
From these data, one can conclude that the compound is not an
outer-sphere charge-transfer product of TCNQ, but rather a
molecule that contains σ-coordinated TCNQ. The determin-
ation of the X-ray crystal structure of 1ؒ10THF provided
1
2
0
Ϫ
two Re units that are linked through the trans cyanide groups
2
of a bridging TCNQ molecule. The molecule is centro-
symmetric, with the midpoint of the TCNQ moiety being
situated on an inversion center. A packing diagram looking
down the a-axis is provided in Fig. 4.
confirmation for this hypothesis. The infrared spectrum of
Ϫ1
1
exhibits stretching modes at 2187 and 2109 cm attributed
to the ν(C᎐N) of the unbound and bound CN groups of
᎐
Ϫ1
TCNQ respectively. The intense ν(C᎐N) band at 2109 cm is
᎐
assigned to the bound nitrile in 1 and is shifted to a lower
frequency as a result of significant d –π* back-bonding. As
π
the C᎐N bond lengthens, the π* orbitals are stabilized, and as
᎐
the C᎐N bond becomes longer, the M–N distance decreases,
᎐
7
thus increasing the overlap of the d –π* orbitals. Moreover, it
π
has been shown that metal ions with strong back-bonding
interactions to nitriles (a net metal-to-ligand charge transfer)
7
exhibit an intensity enhancement of the C᎐N stretch. It is also
᎐
interesting to note that the degree of charge transfer in various
TCNQ complexes can be estimated from the nitrile stretch-
Fig.
3
Thermal ellipsoid representation of [Re Cl (dppm) ] -
2 4 2 2
(µ-TCNQ)ؒ10THF, 1ؒ10THF.
8
ing frequencies. A comparison of the C᎐N and C᎐C stretching
᎐
᎐
0
Ϫ
frequencies of 1 with TCNQ and TCNQ suggests the
assignment of a 1Ϫ charge to the bridging TCNQ in 1, which
requires each dirhenium moiety to bear a partial formal charge
of ϩ0.5.
During the refinement procedure it became necessary to fix
the position of C(1). When allowed to refine unconstrained, the
N(1)–C(1) distance became unreasonably short at 0.92 Å,
which is considerably less than the distance of 1.03 Å that first
appeared in the difference map. Furthermore, the C(1)–C(2)
distance lengthens to an unreasonably long distance of 1.52 Å
when unconstrained. Models that included fixing N(1) and C(1)
were examined. When only N(1) was fixed, the N(1)–C(5) dis-
tance became 0.90 Å, and when C(1) was fixed, the N(1)–C(1)
distance refined to 1.023(8) Å. The latter refinement was chosen
because it is the most chemically reasonable model. In addition
to the aforementioned refinements, the identities of the N and
C atoms were switched such that the group was treated as an
isonitrile, but this led to poorer refinements. The artificially
In addition to the infrared spectra, the electronic absorption
spectra (UV-visible, near and mid IR regions) reveal interesting
behavior for the product [Re Cl (dppm) ] (µ-TCNQ). The spec-
2
4
2 2
trum of 1 performed in CH Cl2 (Fig. 2) contains intense
2
4
Ϫ1
Ϫ1
absorptions at λ = 1080 (ε = 2.56 × 10 M cm ) and 1980 (ε
2.20 × 10 M cm ) nm. The low energies as well as inten-
max
Ϫ1
4
Ϫ1
=
sities of these absorptions are quite unusual and are unlike any
transitions exhibited by the starting materials. In the case of
Re Cl (dppm) , a weak d–d transition occurs at λ = 510 nm (ε
2
4
2
max
2
Ϫ1
Ϫ1
0
=
4.70 × 10 M cm ) and TCNQ displays a sharp π π*
transition at 400 nm.
2
938
D a l t o n T r a n s . , 2 0 0 3 , 2 9 3 7 – 2 9 4 4

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Table 2 Selected bond distances (Å) and angles (Њ) for 1ؒ10THF
Re(1)–Re(2)
Re(1)–Cl(1)
Re(1)–Cl(2)
Re(1)–P(1)
Re(1)–P(3)
Re(1)–N(1)
Re(2)–Cl(3)
Re(2)–Cl(4)
Re(2)–P(2)
Re(2)–P(4)
P(1)–C(7)
2.2895(4)
2.5927(17)
2.4024(16)
2.5066(18)
2.5080(17)
2.054(6)
2.3708(17)
2.3709(16)
2.4542(17)
2.4440(17)
1.828(6)
P(1)–C(15)
P(2)–C(21)
P(2)–C(27)
P(2)–C(7)
P(3)–C(39)
P(3)–C(8)
P(3)–C(33)
P(4)–C(8)
P(4)–C(45)
P(4)–C(51)
N(1)–C(1)
N(2)–C(3)
1.833(7)
1.837(7)
1.837(6)
1.845(7)
1.830(7)
1.831(7)
1.840(7)
1.838(7)
1.843(7)
1.841(8)
1.023(8)
1.148(10)
P(1)–C(9)
1.830(8)
N(1)–Re(1)–Re(2)
N(1)–Re(1)–Cl(2)
Re(2)–Re(1)–Cl(2)
N(1)–Re(1)–P(1)
Cl(2)–Re(1)–P(1)
N(1)–Re(1)–P(3)
Cl(2)–Re(1)–P(3)
P(1)–Re(1)–P(3)
N(1)–Re(1)–Cl(1)
Re(2)–Re(1)–Cl(1)
Cl(2)–Re(1)–Cl(1)
P(1)–Re(1)–Cl(1)
P(3)–Re(1)–Cl(1)
Re(1)–Re(2)–Cl(4)
Re(1)–Re(2)–Cl(3)
Cl(4)–Re(2)–Cl(3)
Re(1)–Re(2)–P(4)
95.83(14)
166.07(15)
97.71(4)
97.35(4)
81.68(6)
86.79(15)
95.28(6)
167.76(6)
81.52(15)
176.41(4)
85.08(5)
85.26(6)
82.66(6)
106.60(4)
112.87(5)
140.47(6)
98.99(4)
Cl(4)–Re(2)–P(4)
Cl(3)–Re(2)–P(4)
Re(2)–Re(1)–P(2)
Cl(4)–Re(2)–P(2)
Cl(3)–Re(2)–P(2)
P(4)–Re(2)–P(2)
C(7)–P(1)–Re(1)
C(9)–P(1)–Re(1)
C(15)–P(1)–Re(1)
C(39)–P(3)–Re(1)
C(8)–P(3)–Re(1)
C(21)–P(2)–Re(2)
C(27)–P(2)–Re(2)
C(7)–P(2)–Re(2)
C(8)–P(4)–Re(2)
C(51)–P(4)–Re(2)
C(45)–P(4)–Re(2)
84.66(6)
87.11(6)
97.34(4)
93.60(6)
83.75(6)
163.39(6)
107.1(2)
118.4(2)
117.5(2)
117.2(2)
110.5(2)
126.8(2)
112.4(2)
103.9(2)
104.5(2)
111.5(2)
124.6(2)
Fig.
4 Packing diagram of [Re Cl (dppm) ] (µ-TCNQ)ؒ10THF,
2 4 2 2
viewed down the a axis.
short C–N distance that refines without constraint is attributed
to librational disorder of the CN groups which serves to distort
the thermal ellipsoids and thus prevents an accurate assignment
of the atom locations for bond distance calculations. It should
be noted that the unligated C(3)–N(2) moiety refines to a
reasonable C–N distance of 1.148(10) Å.
The coordination geometries of the two Re atoms within
each M L unit are different (see Fig. 5). The nitrile-substituted
2
9
metal center, Re(1), adopts an octahedral arrangement consist-
ing of an axial chloride, Cl(1), as well as an equatorial chloride
Cl(2), and an equatorial nitrogen ligand, N(1) in addition to the
trans dppm ligands. The geometry around Re(2) is trigonal bi-
pyramidal which is similar to the geometries of the Re atoms in
the parent complex Re Cl (dppm) . Distances and angles are
Ϫ
Ϫ
(
NCR) ]X (X = Cl , PF ) (R = Me, Et, Ph, 4-PhC H , 1,2-
2 6 6 4
13
C H CN) reported by Walton and co-workers. These com-
6
4
plexes contain equatorially bound nitriles and chlorides, and
one axially coordinated chloride ligand. Comparisons of the
Re–Re, Re–N, and Re–Cl axial distances, and the P–Re–Re–P
torsion angles for this structural type are provided in Table 3.
The values observed for 1ؒ10THF are quite similar to those of
the [Re Cl (dppm) (NCR) ] complexes. The most notable dif-
ference appears in the torsion angles, i.e., the P–Re–Re–P twist
angle (15.7Њ) is considerably smaller than the corresponding
2
4
2
10
within the usual ranges for derivatives of Re Cl (dppm) .
A
2
4
2
list of selected bond distances and angles is provided in Table 2.
Of special note is the Re–Re separation of 2.2895(4) Å, which is
longer than the distance of 2.234(3) Å found in the parent
triply-bonded complex. In the absence of other considerations,
one would predict a shorter metal–metal bond resulting from
depopulation of an antibonding orbital upon oxidation (i.e.
ϩ
2
3
2
2
ϩ
values in the [Re Cl (dppm) (NCR) ] complexes. With a
2
3
2
2
smaller P–Re–Re–P torsion angle and a longer Re–Cl axial
distance, one might anticipate a shorter Re–Re distance for
ؒ10THF than what is found for the [Re Cl (dppm) (NCR) ]
complexes, but the converse is true. It is interesting to note that
in the structure of the mixed-valence Re Re
Re Cl (dppm) , the Re–Re separation is 2.263(1) and the P–Re–
Re–P torsion angle is 3.99Њ. The torsion angle is lower, as
expected, but the Re–Re distance is similar to other Re Re
2
4
2
2
2
4
2
1
11
σ π δ δ*
σ π δ δ* ), but in the present system the appli-
ϩ
1
2 3 2 2
cation of an M L bonding scheme is an oversimplification. The
2
8
combined affect of structural changes and π-delocalization on
the extent of Re–Re bonding must be taken into consideration.
It is also reasonable to argue that an axial chloride interaction
would serve to weaken the σ-component of the metal–metal
bond, thus contributing to an overall lengthening of the Re–Re
II
III
complex
2
5
2
14
II
II
11b
complexes listed in Table 3. This example underscores the
danger in equating the Re–Re separation with formal bond
order or oxidation state.
distance.
Other pertinent structural features of 1ؒ10THF are related to
the fact that the two rhenium atoms are electronically different
which leads to different Re–Cl and Re–P distances. The Re(1)–
Cl distances are ∼0.04 Å shorter than the Re(2)–Cleq distance,
and the Re(1)–P distances are ∼0.07 Å shorter than the Re(2)–P
distances. This same trend is also observed in the structure of
14
Re Cl (dppm) . Once again, these structural differences are
2
5
2
attributed to the different coordination environments around
each rhenium atom, and assignments of formal charges on
either rhenium atom cannot be rationally deduced from these
data.
Fig. 5 Crystal Maker representation without dppm ligands of
Re Cl (dppm) ] (µ-TCNQ)ؒ10THF.
Among the vast reports of TCNQ compounds, there are still
relatively few reports of compounds in which TCNQ acts as a
[
2
4
2 2
12
4
Other M L complexes, such as Re Cl (dppm) (CO), have
ligand. The existence of both terminal σ-TCNQ and σ-µ -
2
9
2
4
2
been noted in earlier studies, but these possess A-frame struc-
tures, whereas the structural type found in 1ؒ10THF, in which
the geometry is trigonal bipyramidal around one of the Re
atoms and octahedral around the other Re atom, has been
noted only in nitrile complexes of the type [Re Cl (dppm) -
TCNQ binding modes for complexes of the unstable [CpMn-
(CO) ] fragment have been proposed on the basis of spectro-
2
scopic data, but the formulae have not been verified by crystal-
15–17
Ϫ1
lography.
The crystal structures of various TCNQ
materials in which the TCNQ behaves as a ligand have been
2
3
2
D a l t o n T r a n s . , 2 0 0 3 , 2 9 3 7 – 2 9 4 4
2939

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Table 3 Comparison of selected bond distances (Å) and angles (Њ) of dirhenium complexes exhibiting an unsymmetrical M L geometry
2
9
Complex
Re–Re
Re–N
Re–Cl(axial)
P–Re–Re–P(av.)
Ref
[
[
[
[
Re Cl (dppm) ] (µ-TCNQ), 1
2.2895(4)
2.270(1)
2.2661(9)
2.265(1)
2.263(1)
2.054(6)
2.066(15)
2.01(1)
2.5927(17)
2.575(7)
2.556(4)
2.492(6)
2.575(6)
14.9
22.2
30.6
32.4
3.99
This work
13a
13a
13a
14
2
4
2 2
Re Cl (dppm) (NCC H ) ][PF ]
2
3
2
6
5
2
6
Re Cl (dppm) (NCC H ) ][PF ]
2
3
2
2
5
2
6
Re Cl (dppm) (1,2-C H (CN) ) ][PF ]
2.05(3)
2
3
2
6
4
2
2
6
Re Cl (dppm)
2
5
2
Table 4 A comparison of bond distances (Å) of several TCNQ containing products
a
b
Bond
TCNQ
1ؒ10THF
N-MePZT-TCNQ
[Fe(C Et ) ][TCNQ]
5
5 2
a
b
c
d
e
1.346
1.446, 1.450
1.374
1.441, 1.440
1.141, 1.139
1.356
1.405, 1.432
1.453
1.484, 1.440
1.023, 1.148
1.349
1.446
1.379
1.422
1.151
1.370
1.418, 1.413
1.423
1.413, 1.419
1.148, 1.143
a
b
N-Methylphenothaizine-TCNQ, ref. 28a. Ref. 28b.
18
performed by a few research groups, including our own. In
our laboratories, we have performed structures on homoleptic
18f
metal TCNQ solids of the type M(TCNQ) (M = Cu, Ag) as
3g
well as solvated Mn(TCNQ) ؒxS polymers. The binding
2
modes vary from cis-µ and syn-µ to µ -TCNQ in these struc-
2
2
4
tures. In the crystal structure of Ag(σ-µ -TCNQ), the Ag
4
Scheme 1
atoms are coordinated to four nitrile positions of independent
TCNQ radicals in a distorted tetrahedral arrangement at an
average Ag–N distance of 2.322 Å, which is clearly indicative of
χT vs. T plot for 1ؒ10THF clearly indicates temperature
independent paramagnetic (TIP) behavior (Fig. 6). A fitting of
χm = χ ϩ C/T leads to a value of χTIP of 7.29 × 10 emu
mol and a paramagnetic “impurity” contribution of C =
0.0541 emu K mol . The magnitude of the TIP is quite large
but this is not unusual based on the fact that Re has a large
orbital contribution to its magnetic moment. The EPR spec-
trum, run at a frequency of 9.52 GHz, of [Re Cl (dppm) ] -
µ-TCNQ), 1, on a crystalline sample exhibits a broad signal
19
a M–N interaction. Additionally, a σ-µ -TCNQ ligand has
4
Ϫ3
TIP
been reported for the complex [{Ru (O CCF ) } (µ -TCNQ)ؒ
2
2
3
4
2
4
Ϫ1
3
(C H )] in which one TCNQ is surrounded by four Ru units
7 8 ∞ 2
20
Ϫ1
and creates a 2D hexagonal network. In this case, the TCNQ
ligand occupies the axial sites of each of the Ru units. The Ru–
2
23
Naxial distance is 2.277(3) Å, which is longer than the Re–N
distance in 1. This is expected since the axial interactions are
weaker than equatorial interactions in metal–metal bonded
2
4
2 2
(
centered at g = 2.14 (Fig. 7). The breadth of the signal is indi-
cative of delocalization of spin density onto the Re2 units,
i.e., the system is not that of a localized organic radical.
complexes. In the organometallic complex [(µ -TCNQ){fac-
4
4ϩ
Re(CO) (bpy)} ] reported by Kaim et. al., the Re–N bond
3
4
21
distances are 2.098(7) and 2.121(8). Both of these values are
longer then the Re–N bond distance in 1, which is reasonable
since the TCNQ in the aforementioned complex is neutral, and
it is known that the reduced form of TCNQ is a better donor
than the neutral form. This further supports our conclusion
that charge transfer has occurred between the Re unit and the
2
TCNQ. A perusal of the literature reveals several other various
22
metal–metal complexes that contain σ-TCNQ.
The bond distances and angles within the TCNQ unit in
1
ؒ10THF are considerably different from those found in struc-
0
tures of TCNQ (Table 4). A comparison of the bond distances
in 1ؒ10THF and other TCNQ containing species with those in
TCNQ , reveals that bonds a and c are longer while bonds
Ϫ
0
b and d are shorter. These bond distances changes are expected
and can be explained by a contribution from the resonance
Ϫ
structure (II in Scheme 1) for TCNQ . As reported in the paper
by Hoekstra et al., the charges on the reduced forms of TCNQ
reside predominately on the N atoms which explains the nucleo-
Ϫ1
Fig. 6 Plot of both the magnetic susceptibility (emu mol ) and χT
Ϫ 18b
Ϫ1
philicity of TCNQ .
(emu K mol ) vs. temperature (K) of [Re Cl (dppm) ] (µ-TCNQ).
2
4
2 2
(
iii) Magnetic properties. The compound [Re Cl (dppm) ] -
(iv) Electronic properties. The electronic properties of
[Re Cl (dppm) ] (µ-TCNQ), 1, are unusual. The parent di-
2
4
2 2
(
µ-TCNQ)ؒ10THF, 1ؒ10THF, is paramagnetic as indicated by
2
4
2 2
the magnetic susceptibility and EPR spectroscopic data. The
rhenium complex Re Cl (dppm) exhibits two reversible
2 4 2
2
940
D a l t o n T r a n s . , 2 0 0 3 , 2 9 3 7 – 2 9 4 4

View Article Online
serving to link two Re Cl (dppm) molecules through σ-bonds
2
4
2
to the two cyano groups. The molecule is centrosymmetric, with
the midpoint of the DM-DCNQI moiety being situated on an
inversion center. The coordination environments around Re(1)
and Re(2) are octahedral and trigonal bipyramidal respectively.
The Re–Re distance of 2.2986(5) Å is slightly longer than the
corresponding distance found in the structure of 1ؒ10THF
(
2.2895(4) Å). The Re(1)–Cl(1) distance of 2.624(2) Å is longer
than the other Re–Cl distances, which is not unexpected, given
that it occupies an axial site (see Fig. 10). The Re(1)–N(1) dis-
tance of 2.033(8) Å is slightly shorter than the Re–N distance
found in the structure of 1ؒ10THF (2.054(6) Å). Other
distances and angles in the dirhenium unit are within the range
12
for derivatives of Re Cl (dppm) . The three-dimensional
2
4
2
structure consists of molecules that stack along the a-axis at a
distance of ∼14 Å between adjacent DM-DCNQI centers.
Closer contacts exist between phenyl rings of the phosphine
substituent and among the solvent molecules, but these are not
expected to be important pathways for communication between
molecules.
Fig.
7
EPR spectrum of [Re Cl (dppm) ] (µ-TCNQ)ؒ10THF at
2 4 2 2
Ϫ160 ЊC.
oxidations located at E1/2 = ϩ0.35 and ϩ0.87 V whereas TCNQ
exhibits two reversible reductions at E1/2 = ϩ0.28 and Ϫ0.30 V
(
vs. Ag/AgCl). The cyclic voltammogram (Fig. S1, ESI†) of 1
reveals numerous accessible redox processes, namely one
irreversible reduction at Ep,c = Ϫ0.19 V and two reversible
oxidations at E1/2 = ϩ0.64 and ϩ0.25 V along with multiple
features at potentials greater than ϩ0.8 V as observed in the
differential pulse voltammagram (Fig. S2, ESI†). These
processes are irreversible and occur very close to one another,
which renders it difficult to assign precise potentials. From the
electronic studies of 1, it seems reasonable to propose that
significant charge transfer has occurred between the dirhenium
unit and the TCNQ ligand which leads to delocalization of
charge over the entire compound. This delocalization is giving
rise to the complex electrochemical properties of 1.
Fig. 9 Thermal ellipsoid representation of [Re Cl (dppm) ] (µ-DM-
2
4
2 2
DCNQI)ؒ10THF, 2ؒ10THF.
B. [Re Cl (dppm) ] (ꢀ-DM-DCNQI), 2
2
4
2 2
(
i) Preparation and spectroscopic properties. The reaction of
two equivalents of Re Cl (dppm) with DM-DCNQI in THF
2
4
2
or benzene leads to microcrystalline samples of [Re Cl -
2
4
(
dppm) ] (µ-DMDCNQI), 2, directly from the reaction solu-
2 2
tion. Single crystals were grown using slow diffusion techniques
similar to those used to obtain crystals of 1. The infrared spec-
Ϫ1
trum of 2 exhibits a strong ν(C᎐N) feature at 2056 cm , and
᎐
the electronic absorption spectrum exhibits intense absorptions
at λmax = 1035 and 1800 nm (Fig. 8). Dicholormethane solutions
of 2 are not stable at room temperature as confirmed by the
slow decay in intensity of the absorptions in the electronic
absorption spectrum.
Fig. 10 Crystal Maker representation without the dppm ligands of
[
Re Cl (dppm) ] (µ-DM-DCNQI)ؒ10THF.
2 4 2 2
(
iii) Magnetic and electronic properties. As in the case of
[
Re Cl (dppm) ] (µ-TCNQ), the molecule [Re Cl (dppm) ] -
2
4
2 2
2
4
2 2
(
µ-DM-DCNQI) is paramagnetic as determined by magnetic
susceptibility (Fig. 11) and EPR (signal centered at g = 2.50,
Fig. 12) measurements. The plot of χ T vs. T (Fig. 11) is
m
typical of temperature independent paramagnetism and is
nearly identical to that of compound 1. An application of the
Ϫ3
same fitting used for 1 leads to a χ value of 6.23 × 10 emu
TIP
Ϫ1
mol and a paramagnetic contribution of C = 0.108 emu K
mol for 2. The electronic properties of 2 are similar to those
Ϫ1
of 1 (see Fig. S3, ESI†). There are three major redox processes,
viz., an irreversible reduction at Ep,c = Ϫ0.35 V and two revers-
ible oxidations at E1/2 = ϩ0.22 and ϩ0.69 V (vs. Ag/AgCl). As
was observed for 1, there are several overlapping irreversible
oxidation features. As concluded earlier, such rich electro-
chemistry is indicative of communication between the redox
units through the TCNQ ligand, but a quantitative analysis of
the degree of coupling is precluded by the complexity of the
voltammograms.
Fig. 8 Electronic absorption spectrum of [Re Cl (dppm) ] (µ-DM-
2
4
2 2
DCNQI), 2, in CH Cl .
2
2
(
ii) X-Ray crystal structure of [Re Cl (dppm) ] (DM-DCN-
2 4 2 2
QI)ؒ10THF, 2ؒ10THF. The molecular structure of 2ؒ10THF,
depicted in Fig. 9, closely resembles the structure of [Re Cl -
2
4
(
dppm) ] (µ-TCNQ)ؒ10THF but with the DM-DCNQI moiety
2 2
D a l t o n T r a n s . , 2 0 0 3 , 2 9 3 7 – 2 9 4 4
2941

View Article Online
Physical measurements
IR spectra were recorded as Nujol mulls on KBr plates with
the use of a Perkin-Elmer 599 or Perkin-Elmer 1800 FTIR
spectrometers. Electronic absorption spectra were measured
with Hitachi U-2000 or IBM 9420 UV-visible spectrophoto-
meters, the latter capable of Near IR measurements. The
31
1
P{ H} spectra were obtained with the use of a Varian XL-
2
00A spectrometer operated at 80.98 MHz or a Varian 300
instrument operated at 121.5 MHz. Electrochemical measure-
ments were carried out on dichloromethane solutions that con-
tained 0.1 M tetra-n-butylammonium hexaflourophosphate or
tetra-n-butylammonium tetrafluoroborate as the supporting
electrolyte. Voltammetric experiments were performed with a
CH Instruments Electrochemical workstation using a Pt disk as
the working electrode. All E1/2 values were referenced to the
silver/silver chloride (Ag/AgCl) electrode at room temperature.
Under our experimental conditions the ferrocenium/ferrocene
couple occurs at E1/2 = ϩ0.47 V vs. Ag/AgCl. Variable-temper-
ature magnetic susceptibility measurements were carried out on
solid microcrystalline samples on a Quantum Design MPMS-2
SQUID magnetometer in the range of 2–300 K.
Ϫ1
Fig. 11 Plot of both the magnetic susceptibility (emu mol ) and χT
Ϫ1
(
emu
K mol ) vs. temperature (K) of [Re Cl (dppm) ] (µ-DM-
2 4 2 2
DCNQI).
Preparation of [Re Cl (dppm) ] (ꢀ-TCNQ), 1
2
4
2 2
(
i) Bulk reaction. A solution containing TCNQ (0.0120 g,
.059 mmol) in THF (15 mL) was added to a 15 mL THF
solution containing Re Cl (dppm) (0.150 g, 0.117 mmol). The
0
2
4
2
mixture was stirred for several minutes and left to stand
undisturbed overnight at r. t. to yield a crop of black micro-
crystals. The product was collected by filtration, washed
with THF (3 × 5 mL), and dried in vacuo; yield 0.140 g (86%).
Anal. Calc. for C120H108N Cl O P Re : C, 49.45; H, 3.74; Cl,
4
8
2
8
4
9
.73. Found: C, 49.38; H, 3.48; Cl, 9.62%. IR (CsI, Nujol,
Fig. 12 Solid-state EPR spectrum of [Re Cl (dppm) ] (µ-DM-
Ϫ1
2
4
2
2
᎐
cm ): ν(C᎐N) 2187 (m), 2109 (s, br), ν(C᎐C) 1573 (m), 1586
᎐
DCNQI) 2 at Ϫ160 ЊC.
31
1
(
w, sh). P{ H} NMR (CDCl ): δ Ϫ14 ppm. Electronic spectro-
3
Ϫ1 Ϫ1 4
scopy (CH Cl ): λmax/nm (ε/M cm ), 1080 (2.56 × 10 ), 1980
2
4
2
Conclusions
(
2.20 × 10 ).
Reactions of two nitrile containing organic acceptors with
(
ii) Slow diffusion reaction. Separate stock solutions of
Re Cl (dppm) resulted in the isolation of discrete “dimer-of-
2
4
2
Re Cl (dppm) (0.150 g, 0.117 mmol, 15 mL of THF) and
dimers” molecules. These compounds which are of the
type [Re Cl (dppm) ] (µ-L), where L = TCNQ or DM-TCNQI,
2
4
2
TCNQ (0.0120 g, 0.059 mmol, 15 mL of THF) were prepared.
A 2 mL quantity of the Re Cl (dppm) solution was syringed
2
4
2 2
exhibit unusual electronic properties and magnetic behavior.
2
4
2
into an 8 mm O.D. Pyrex tube and carefully layered with 2 mL
of THF followed by 2 mL of the TCNQ stock solution. The
tubes were flame sealed under vacuum. After ten days, a crop of
black crystals had formed at the interface of the layers in each
tube (7 tubes total); these were harvested and washed with
The large shift in the ν(C᎐N) bands in the infrared spectra and
intense charge-transfer bands in the electronic spectra indicate
᎐
that interactions between the π-HOMOs of Re Cl (dppm) and
2
4
2
Ϫ1
the π* orbitals of the TCNQ radical are occuring that lead to
communication through this network of π orbitals. The inten-
sities of the electronic transitions are remarkably high at ε ∼10 ,
Ϫ1
4
THF; combined yield 10 mg (6%). IR (CsI, Nujol, cm ):
ν(C᎐N) 2186 (m), 2104 (s, br), ν(C᎐C) 1572 (m), 1586 (w, sh).
which is an indication of mixing of the d-orbitals with the
p-orbitals on the ligand that leads to delocalization in the mole-
cule. Attempts to characterize corresponding polymeric struc-
tures based on this type of alternating donor/acceptor building
blocks are in progress.
᎐
᎐
Preparation of [Re
Cl (dppm) ] (ꢀ-DM-DCNQI), 2
4 2 2
2
(
i) Bulk reaction. A solution containing 0.0073 g (0.039
mmol) of DM-DCNQI in 20 mL of benzene was slowly added
to a stirring solution containing 0.100 g (0.078 mmol) of
Experimental section
Re Cl (dppm) in 10 mL of benzene. Initially the solution
turned dark blue, and after 10 min a black solid was collected
2 4 2
Materials
by filtration, washed with benzene and vacuum dried; yield
Ϫ1
᎐
0
.084 g (78%). IR (CsI, Nujol, cm ): ν(C᎐N) 2056 (s,br),
The TCNQ starting material was purchased from TCI and sub-
limed prior to use. The DM-DCNQI ligand was synthesized
according to literature procedures. The compound Re Cl -
ν(C᎐C) 1586 (w). Anal. Calc. for C H N Cl P Re : C, 48.04;
᎐
110 96
4
8
8
4
3
1
1
24
H, 3.52; N, 2.04. Found C, 47.98; H, 3.81; N, 2.01%. A P{ H}
NMR signal was not observed. Electronic spectroscopy
2
4
(
dppm) was prepared by an adaptation of the literature pro-
2
25
Ϫ1
Ϫ1
4
(
(
CH Cl ): λmax/nm (ε/M cm ), 696 (sh), 1035 (2.7 × 10 ), 1800
2 2
4
cedure. In a typical reaction, 25 mL of reagent grade
n
26
3.6 × 10 ).
methanol was added to a flask containing Re Cl (PBu )
2
6
3 2
Similar results were obtained when THF was used as the
reaction solvent.
(
0.750 g, 0.758 mmol) and dppm (1.55 g, 4.03 mmol). The mix-
ture was refluxed for 3 h to yield a purple solid. After the
solid had settled out of solution, the supernatant was carefully
decanted and the solid was washed with fresh methanol (2 ×
(ii) Slow diffusion reaction. A quantity of DM-DCNQI
(0.0073 g, 0.039 mmol) was dissolved in 5 mL of THF and
slowly added to a Schlenk tube containing 0.100 g (0.078 mmol)
1
0 mL) followed by diethyl ether. Yield after vacuum drying
was 0.88 g (90%).
2
942
D a l t o n T r a n s . , 2 0 0 3 , 2 9 3 7 – 2 9 4 4

View Article Online
Table 5 Crystallographic information for both compounds 1ؒ10THF
and 2ؒ10THF
355; (d ) Proceedings of the International Conference on Science and
Technology of Synthetic Metals (ICSM88) Santa Fe, NM, USA,
Synth. Met., 1988, B1–B656.
1
ؒ10THF
2ؒ10THF
2 See, for example: (a) C. Kollmar and O. Kahn, Acc. Chem. Res.,
1
993, 26, 259; (b) R. M. White, Science, 1985, 229, 11; (c) O. Kahn,
Molecular Magnetism, VCH, New York, 1993; (d ) J. S. Miller, Adv.
Mater., 1992, 4, 298; (e) H. O. Stumpf, Y. Pei, C. Michaut, O. Kahn,
J. R. Renard and L. Ouahab, Chem. Mater., 1994, 6, 257; ( f ) P. Rey,
Acc. Chem. Res., 1989, 22, 392; (g) A. Caneschi, D. Gatteschi,
J. P. Renard, P. Rey and R. Sessoli, Inorg. Chem., 1989, 28, 3314;
Formula
Mw
Space group
T /K
λ/Å
a/Å
b/Å
c/Å
α/Њ
β/Њ
C152H172Cl N O P Re
C150H176Cl N O P Re
4
8
4
10
8
4
8
4
10
8
3491.10
3445.25
¯
¯
P1
P1
173(2)
0.71073
173(2)
0.71073
(
h) J. R. Galán-Mascarós, C. Giménez-Saiz, S. Triki, C. J. Gómez-
García, E. Coronado and L. Ouahab, Agnew. Chem., Int. Ed. Engl.,
995, 34, 1460.
12.1888(3)
14.2908(3)
23.3215(6)
72.8220(10)
81.9380(10)
75.7760(10)
3752.06(16)
1
1.545
3.501
0.0430
0.0670
12.2278(2)
14.3701(2)
23.2604(3)
74.6360(10)
83.2270(10)
75.4460(10)
3808.88
1
1.513
3.448
0.0571
0.0920
1
3
(a) J. Zhang, J. Ensling, V. Ksenofontov, P. Gütlich, A. J. Epstein and
J. S. Miller, Angew. Chem., Int. Ed. Engl., 1998, 37, 657; (b) J. S.
Miller and A. J. Epstein, Angew. Chem. Int. Ed., 1994, 33, 385;
(c) J. S. Miller, A. J. Epstein and W. M. Reiff, Chem. Rev., 1988, 88,
γ/Њ
V/Å
3
2
01; (d ) J. S. Miller, J. C. Calabrese, H. Rommelmann, S. R.
Z
Ϫ3
Chittipeddi, H. H. Zhang, W. M. Reiff and A. J. Epstein, J. Am.
Chem. Soc., 1987, 109, 769; (e) M. S. Ward and D. C. Johnson,
Inorg. Chem., 1987, 26, 4213; ( f ) W. E. Broderick, J. A. Thompson,
E. P. Day and B. M. Hoffman, Science, 1990, 249, 401; (g) H. Zhao,
R. A. Heintz, X. Ouyang, K. R. Dunbar, C. F. Campana and
R. D. Rogers, Chem. Mater., 1999, 11, 736; (h) R. Clérac, S. O’Kane,
J. Cowen, X. Ouyang, R. Heintz, H. Zhao, M. J. Bazile, Jr. and
K. R. Dunbar, Chem. Mater., 2003, 15, 1840.
D /g cm
c
Ϫ1
µ(Mo-Kα)/mm
R (I > 2.00σ(I ))
R (all data)
R (I > 2.00σ(I ))
0.0972
0.1062
0.1353
0.1478 (all data)
w
R (all data)
w
4
5
J. M. Manriquez, G. T. Yee, R. S. McLean, A. J. Epstein and
J. S. Miller, Science, 1991, 252, 1415.
of Re Cl (dppm) dissolved in 5 mL of THF. The Schlenk tube
2
4
2
was placed in the refrigerator for 2 days after which time black
crystals were observed to have formed; these were collected by
filtration, washed with THF, and dried in vacuo; yield 0.076 g
(a) T. C. Zietlow, D. D. Klendworth, T. Nimry, D. J. Salmon and
R. A. Walton, Inorg. Chem., 1981, 20, 947; (b) T. J. Barder,
F. A. Cotton, D. Lewis, W. Schwotzer, S. M. Tetrick and R. A.
Walton, J. Am. Chem. Soc., 1984, 106, 2882.
(
71%).
Ϫ1
0
ϩ
6
7
8
Scan rate 200 mV s , 0.1 M TBAH, in CH Cl . The Cp Fe /Cp Fe
2 2 2 2
couple was referenced at ϩ0.47 V.
H. Taube and A. Johnson, J. Indian Chem. Soc., 1989, 66,
03.
(a) B. Lunelli and C. Pecile, J. Chem. Phys., 1979, 52, 2375;
b) W. Pukacki, M. Pawlak, A. Graja, M. Lequan and R. M.
X-Ray data collection, reduction, and structure determination
5
Single crystals for the crystallographic analyses were mounted
on glass fibers and secured with Dow-Corning grease. The crys-
(
3
tal dimensions are 0.39 × 0.26 × 0.09 mm for 1 and 0.50 × 0.35
Lequan, Inorg. Chem., 1987, 26, 1328; (c) W. Kaim and
M. Moscherosh, Coord. Chem. Rev., 1994, 129, 157; (d ) J. G. Robles-
Martinez, A. Salmeron-Valverde, E. Alonso and C. Soriano, Inorg.
Chem. Acta, 1991, 179, 149.
S. L. Bartley and K. R. Dunbar, Angew. Chem., Int. Ed., 1991, 30,
4
3
×
0.20 mm for 2. Data were collected on a Siemens SMART
1
2
K CCD platform diffractometer in the range of 1.53 < 2θ <
4.71Њ for 1 and 1.51 < 2θ < 24.71Њ for 2 at 173(2) K with graph-
9
ite-monochromated Mo-Kα radiation (λ = 0.71073 Å). Of the
total 23479 reflections for 1 and 24165 reflections for 2 that
were collected, 12635 and 12830 are unique, respectively. The
48.
1
0 (a) P. E. Fanwick, A. C. Price and R. A. Walton, Inorg. Chem., 1988,
7, 2601; (b) A. C. Price and R. A. Walton, Polyhedron, 1987, 6, 729.
11 (a) For a study on the effect of bond length on bond order of
complexes, see: F. A. Cotton, K. R. Dunbar, L. R. Falvello,
M. Tomas and R. A. Walton, J. Am. Chem. Soc., 1983, 105, 4950;
b) for a description the elongation of metal–metal bonds due to
2
2
7a
structures were solved by direct methods (SHEXTL v5.04)
2
and refined by full-matrix least-squares calculations on F
SHELXL-97). The non-hydrogen atoms were refined aniso-
M L
2 8
27b
(
(
tropically, whereas hydrogen atoms were calculated and fixed
during refinement. Full-matrix least-squares refinement based
on 9396 observed reflections for 1 and 8559 observed reflections
for 2 (I > 2.00σ(I )) were employed. The unweighted and
weighted agreement factors of R = Σ||F | Ϫ |F ||/Σ|F | and R =
axial ligation, see: F. A. Cotton and R. A. Walton, in Multiple Bonds
between Metal Atoms, Clarendon Press, Oxford, 2nd edn., 1987,
ch. 2.
12 F. A. Cotton, K. R. Dunbar, A. C. Price, W. Schwotzer and
R. A. Walton, J. Am. Chem. Soc., 1986, 108, 4843.
3 (a) T. J. Barder, F. A. Cotton, L. R. Falvello and R. A. Walton,
Inorg. Chem., 1985, 24, 1258; (b) D. R. Derringer, K.-Y. Shih,
P. E. Fanwick and R. A. Walton, Polyhedron, 1991, 10, 79; (c) P. E.
Fanwick, J.-S. Qi, K.-Y. Shih and R. A. Walton, Inorg. Chim. Acta,
o
c
o
w
2
2 1/2
1
[
Σw(|F | Ϫ |F |) /Σw|F | ] were used. The crystal data and
o c o
details of the structure determination are summarized in
Table 5.
CCDC reference numbers 209028 and 209029.
See http://www.rsc.org/suppdata/dt/b3/b304509a/ for crystal-
lographic data in CIF or other electronic format.
1
990, 172, 65.
14 F. A. Cotton, L. W. Shive and B. R. Stults, Inorg. Chem., 1976, 15,
2239.
1
5 R. Gross and W. Kaim, Angew. Chem., Int. Ed., 1987, 26,
51.
6 B. Olbrich-Deussner, R. Gross and W. Kaim, J. Organomet. Chem.,
989, 366, 155.
17 R. Gross-Lannert, W. Kaim and B. Olbrich-Deussner, Inorg. Chem.,
990, 29, 5046.
8 (a) C. J. Fritchie and P. Arthur, Acta Crystallogr., 1966, 21, 139;
b) A. Hoekstra, T. Spoelder and A. Vos, Acta Crystallogr., Sect. B,
2
1
Acknowledgements
1
K. R. D. gratefully acknowledges the National Science
Foundation for a PI Grant (CHE-9906583) and for equipment
grants to purchase the CCD X-ray equipment (CHE-9807975)
and the SQUID magnetometer (NSF-9974899). We also wish
to thank both Texas A&M University and Michigan State
University for additional financial support.
1
1
(
1
1
972, 28, 14; (c) M. Konno and Y. Saito, Acta Crystallogr., Sect. B,
974, 30, 1294; (d ) S. A. O’Kane, R. Clérac, H. Zhao, X. Ouyang,
J. R. Galán-Mascarós, R. Heintz and K. R. Dunbar, J. Solid
State Chem., 2000, 152, 159; (e) C. Campana, K. R. Dunbar and X.
Ouyang, Chem. Commun., 1996, 2427; ( f ) R. A. Heintz, H. Zhao,
X. Ouyang, G. Grandinetti, J. Cowen and K. R. Dunbar,
Inorg. Chem., 1999, 38, 144; (g) H. Zhao, M. J. Bazile, Jr,
J. R. Galán-Mascarós and K. R. Dunbar, Angew. Chem., Int. Ed.,
2003, 42, 1015.
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D a l t o n T r a n s . , 2 0 0 3 , 2 9 3 7 – 2 9 4 4
2943

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