Chromium(II) complexes bearing 2,6-substituted N,N′-diarylformamidinate ligands

Article abstract of DOI:10.1039/b000326n

The synthesis, isolation and characterisation of several new chromium(II) complexes of the N,N′-bis(2,6-xylyl)-formamidinate ligand (DXylF) has been undertaken. The structure of three such complexes, namely Cr(DXylF)₂, 1, Cr₂(μ-Cl)₂(DXylF)₂(THF)₂, 2, and Cr₂(Ac)₂(DXylF)₂(THF)₂,3, have been determined by X-ray crystallography. It has been shown that the steric demands of the 2,6-xylyl group effectively discourage the formation of the homoleptic paddlewheel structure Cr₂(DXylF)₄: instead, the mononuclear (but stoichiometrically equivalent) complex Cr(DXylF)₂, 1 was obtained. It is likely that the unusual dichromium dichloride-bridged intermediate 2 also owes its stability to the presence of such bulky substituents. Although this complex does not possess a significant metal-metal bonding interaction [Cr-Cr = 2.612(1) A], it has however, proven to be a useful starting material for the preparation of the novel quadruply bonded dichromium mixed acetate-formamidinate complex 3 [Cr-Cr (avg) = 2.339(7) A]. In both 2 and 3, the 2,6-xylyl groups force the formamidinate ligands to oppose one another contrary to the trans effect. In addition, it was found that compound 3 can be accessed from the chromium acetate starting material and, for comparison, an attempt was also made to prepare the N,N′-bis(o-anisyl)formamidinate (DPh^OMeF) analogue. With the formamidinates occupying positions cis to one another and with the absence of axially coordinated THF ligands, the molecular structure of the resultant Cr₂(Ac)₂(DPh^OMeF)₂,4 [Cr-Cr = 2.037(1) A], is different from that of 3. The Royal Society of Chemistry 2000.

Full text of DOI:10.1039/b000326n

View Article Online / Journal Homepage / Table of Contents for this issue
Chromium(II) complexes bearing 2,6-substituted N,NЈ-
diarylformamidinate ligands
F. Albert Cotton,*a Lee M. Daniels,a Carlos A. Murillo*a,b and Paul Schoolera
a Laboratory for Molecular Structure and Bonding, Department of Chemistry,
Texas A&M University, PO Box 30012, College Station, Texas 77842-3012, USA
b Escuela de Química, Universidad de Costa Rica, Ciudad Universitaria, Costa Rica
Received 13th January 2000, Accepted 24th April 2000
Published on the Web 13th June 2000
The synthesis, isolation and characterisation of several new chromium() complexes of the N,NЈ-bis(2,6-xylyl)-
formamidinate ligand (DXylF) has been undertaken. The structure of three such complexes, namely Cr(DXylF)2,
1, Cr2(µ-Cl)2(DXylF)2(THF)2, 2, and Cr2(Ac)2(DXylF)2(THF)2, 3, have been determined by X-ray crystallography.
It has been shown that the steric demands of the 2,6-xylyl group effectively discourage the formation of the
homoleptic paddlewheel structure Cr2(DXylF)4: instead, the mononuclear (but stoichiometrically equivalent)
complex Cr(DXylF)2, 1 was obtained. It is likely that the unusual dichromium dichloride-bridged intermediate 2
also owes its stability to the presence of such bulky substituents. Although this complex does not possess a significant
metal–metal bonding interaction [Cr–Cr = 2.612(1) Å], it has however, proven to be a useful starting material for the
preparation of the novel quadruply bonded dichromium mixed acetate–formamidinate complex 3 [Cr–Cr (avg) =
2.339(7) Å]. In both 2 and 3, the 2,6-xylyl groups force the formamidinate ligands to oppose one another contrary
to the trans effect. In addition, it was found that compound 3 can be accessed from the chromium acetate starting
material and, for comparison, an attempt was also made to prepare the N,NЈ-bis(o-anisyl)formamidinate (DPhOMeF)
analogue. With the formamidinates occupying positions cis to one another and with the absence of axially
coordinated THF ligands, the molecular structure of the resultant Cr2(Ac)2(DPhOMeF)2, 4 [Cr–Cr = 2.037(1) Å],
is different from that of 3.
Recently in this laboratory, we have found that certain DArF
Introduction
ligands (Ar = o-chlorophenyl6,7 and o-bromophenyl7) hinder
The complexation of chromium() by amidinates typically
but do not prevent the formation of the complex Cr2(DArF)4.
In these cases, the intermediate complex Cr2(µ-Cl)(DArF)3
results in the assembly of four bridging ligands about a
quadruply bonded Cr24ϩ core.1 Such a construction brings these
could be isolated en route to the tetra-formamidinate. This type
ligands and the substituents that they bear into very close prox-
imity. One might anticipate that it should be possible to take
advantage of steric interactions to hinder or even prohibit the
of compound was unprecedented;6 these Cr2(µ-Cl)(DArF)3
compounds are the first examples of A-frame complexes poss-
4ϩ
essing quadruply bonded M2 moieties. Their isolation can
formation of such paddlewheel species. Indeed, several studies
possibly be attributed to the unique combination of both steric
exploring similar ideas can already be found in the literature.2–5
In 1977, it was shown that despite the utility of the reaction
between Mo(CO)6 and HDArF (N,NЈ-diarylformamidine, Ar =
phenyl, p-tolyl, m-tolyl, o-tolyl, p-chlorophenyl and 3,5-xylyl)
bulk and axial coordination provided by the formamidinate’s
o-chlorophenyl or o-bromophenyl substituents.6,7
Since there are six substituents for only two axial positions
within the Cr2Cl(DArF)3 motif, four of the aromatic groups
to form Mo2(DArF)4 compounds, no identifiable product could
must locate a proton in the vicinity of the axial positions rather
be obtained in the analogous reaction with the 2,6-xylyl deriv-
than a halide atom.6,7 Thus, we wondered about the effects of
ative.2 Predictably, a later study in 1998 using the ‘super-
giving the phenyl groups no choice about their orientation; i.e.
to see how the substitution of all four ortho protons of the
N,NЈ-diarylformamidinate ligand modifies its coordination
chemistry. We report here some of our results obtained using
the N,NЈ-bis(2,6-xylyl)formamidinate ligand (DXylF). The
significant compounds described in this paper are summarised
amidine’ ligand N,NЈ-bis(2,6-diisopropylphenyl)acetamidine
(HDIPhA) also failed to produce the Mo2(DIPhA)4 compound,
with (N-HDIPhA)Mo(CO)5 and (η6-HDIPhA)Mo(CO)3 being
isolated instead.3
In 1993, it was also shown that whereas the N,NЈ-bis-
(cyclohexyl)formamidinate ligand (DCyF) reacts with CrCl2 to
in Fig. 1.
form the familiar dinuclear complex Cr2(DCyF)4; the N,NЈ-
bis(cyclohexyl)acetamidinate ligand (DCyA) reacts to form
only the mononuclear rectangular-planar complex Cr(D-
Results and discussion
CyA)2.4 This result shows clearly how even a subtle change in
the balance of intra-molecular steric interactions (by the intro-
duction of a methyl group at the seemingly innocent amidinate
carbon atom) can dramatically change the complexation
behaviour of the ligand. Furthermore, the steric bulk of the
amidinate can be increased to such an extent that even the
rectangular planar complex is destabilised, as in the reaction of
N,NЈ-bis(trimethylsilyl)benzamidinate ligand (DTMSB) with
CrCl2 for example, where a mononuclear flattened tetrahedral
complex Cr(DTMSB)2 is obtained instead.5
The reaction of two equivalents of LiDXylF with one equiva-
lent of CrCl2 in THF or toluene affords the red complex
Cr(DXylF)2, 1. The molecular structure of compound 1 was
determined by single-crystal X-ray diffraction and is illustrated
in Fig. 2. Selected bond lengths and angles are listed in Table 1
while crystallographic data are shown in Table 2. The molecule
resides on a centre of inversion. Rather than a dichromium
tetra-formamidinate paddlewheel complex, a mononuclear
planar complex of the same stoichiometry is the product of the
DOI: 10.1039/b000326n
J. Chem. Soc., Dalton Trans., 2000, 2001–2005
This journal is © The Royal Society of Chemistry 2000
2001

View Article Online
reaction. Fig. 2 clearly indicates that the complex is heavily
distorted from the ideal square geometry. For example, the
N(1)–N(2a) inter-ligand distance is much longer than the N(1)–
N(2) intra-ligand (cf. 3.472(3) and 2.219(3) Å, respectively).
Accordingly, the N(1)–Cr–N(2) angle is relatively acute at
65.2(1)Њ. Thus, the metal geometry is best described as rec-
tangular rather than square. It is of note that the planes of the
2,6-xylyl groups do not lie perpendicular to the rectangular
plane, but are inclined at 50.8(1)Њ [calculated from the inter-
section of the C(2)–C(6)–C(7) and the N(1)–Cr–N(2) planes].
Table 1 Selected bond lengths (Å) and angles (Њ) for complexes 1–4
1
Cr–N(1)
Cr–N(2)
2.075(2)
2.045(2)
N(1)–Cr–N(2)
65.17(7)
113.9(2)
125.2(2)
124.4(2)
N(1)–C(9)–N(2)
C(8)–N(1)–C(9)
C(9)–N(2)–C(10)
2
Cr(1)–Cr(1Ј)
Cr(1)–N(1)
Cr(1)–N(2)
Cr(1)–Cl(1)
Cr(1)–Cl(1Ј)
Cr(1)–O(1)
2.612(1)
2.078(2)
2.072(2)
2.370(1)
2.596(1)
2.124(2)
N(1)–Cr–N(2)
172.4(1)
62.59(2)
116.73(2)
149.36(6)
93.91(6)
116.1(2)
116.0(2)
Cr(1)–Cl(1)–Cr(1Ј)
Cl(1)–Cr(1)–Cl(1Ј)
Cl(1)–Cr(1)–O(1)
Cl(1Ј)–Cr(1)–O(1)
C(8)–N(1)–C(9Ј)
C(9)–N(2)–C(10)
3
Cr(A)–Cr(AA)
Cr(A)–Cr(BA)
Cr(C)–Cr(CA)
Cr(D)–Cr(DA)
2.342(1)
2.342(1)
2.329(1)
2.344(1)
N–Cr–O (avg)
Cr–Cr–Oaxial (avg)
O–Cr–O (avg)
N–Cr–N (avg)
90(3)
178.9(9)
176.8(2)
179.3(3)
Fig. 1 Schematic drawings of the compounds described in this paper.
Cr–Cr (avg)
Cr–O (avg)
Cr–N (avg)
2.339(7)
2.016(5)
2.091(3)
2.31(1)
Cr ؒ ؒ ؒ Oaxial (avg)
4
Cr(1)–Cr(2)
Cr(1)–N(1)
Cr(1)–N(3)
Cr(2)–N(2)
Cr(2)–N(4)
Cr(1)–O(1)
Cr(1)–O(5)
Cr(1)–O(7)
Cr(2)–O(4)
Cr(2)–O(6)
Cr(2)–O(8)
2.037(1)
2.023(4)
2.027(4)
2.035(3)
2.020(3)
2.353(3)
2.003(3)
2.017(3)
2.337(3)
2.016(3)
2.010(3)
N(1)–Cr(1)–O(5)
N(2)–Cr(2)–O(6)
N(3)–Cr(1)–O(7)
N(4)–Cr(2)–O(8)
Cr(1)–Cr(2) ؒ ؒ ؒ O(4)
O(1) ؒ ؒ ؒ Cr(1)–Cr(2)
C(7)–N(1)–C(8)
C(8)–N(2)–C(9)
C(22)–N(3)–C(23)
C(23)–N(4)–C(24)
N(1)–C(8)–N(2)
N(3)–C(23)–N(4)
O(5)–C(31)–O(6)
O(7)–C(33)–O(8)
172.2(1)
176.4(1)
176.6(1)
171.6(1)
167.1(1)
166.4(1)
120.0(4)
116.8(4)
117.1(4)
121.6(4)
118.2(4)
118.6(4)
122.1(4)
122.6(4)
Fig. 2 The molecular structure of Cr(DXylF)2, 1. Displacement
ellipsoids are drawn at the 30% probability level. Hydrogen atoms
omitted for clarity.
Table 2 Crystallographic data for complexes 1–4
1
2
3ؒ0.5C6H5Me
4ؒ2C6H5Me
Formula
M
Space group
a/Å
b/Å
c/Å
C34H38CrN4
C42H54Cl2Cr2N4O2
821.79
P1
C49.5H64Cr2N4O6
915.05
P1
C48H52Cr2N4O8
916.94
P21/c
18.424(4)
13.020(3)
20.308(4)
90
112.19(3)
90
4511(2)
1.350
0.539
0.71073
213(2)
554.68
¯
¯
¯
P1
8.529(1)
8.890(1)
10.375(1)
78.187(2)
81.956(2)
82.329(2)
757.9(2)
1.215
8.1949(2)
11.191(1)
11.708(1)
88.543(4)
79.794(5)
71.665(6)
1002.6(2)
1.361
16.074(1)
17.903(2)
18.791(2)
112.427(1)
98.266(2)
93.240(2)
4909.8(7)
1.238
α/Њ
β/Њ
γ/Њ
V/Å3
ρcalc/g cmϪ3
µ/mmϪ1
λ(Mo-Kα)/Å
T/K
0.405
0.71073
213(2)
0.716
0.71073
213(2)
0.492
0.71073
213(2)
Z
1
1
4
4
R11, R12 a
wR21, wR22 a
GOF
0.053, 0.144
0.068, 0.158
1.036
0.037, 0.043
0.091, 0.096
1.087
0.062, 0.171
0.098, 0.198
1.091
0.067, 0.133
0.157, 0.160
1.005
a Superscript 1 denotes the value of the residual considering only the reflections for which I > 2σ(I). Superscript 2 denotes the value of the residual
for all reflections.
2002
J. Chem. Soc., Dalton Trans., 2000, 2001–2005

View Article Online
Fig. 4 The molecular structure of Cr2(Ac)2(DXylF)2(THF)2, 3. Dis-
placement ellipsoids are drawn at the 30% probability level. Hydrogen
atoms omitted for clarity.
Fig. 3 The molecular structure of Cr2(µ-Cl)2(DXylF)2(THF)2, 2. Dis-
placement ellipsoids are drawn at the 30% probability level. Hydrogen
atoms omitted for clarity.
[2.379(1) Å] are cis. It is unclear why two apparently similar
compounds, Cr2(µ-Cl)2(DXylF)2(THF)2, 2, and Cr2(µ-Cl)2-
(DMPM)2Cl2 are actually so dissimilar. This is perhaps a con-
sequence of the metal–metal separation [2.612(1) and 3.237(1)
Å, respectively] and Cl–Cr–Cl angle [116.73(2) and 98.92(4)Њ].
Both of these parameters are a function of the donor atom
separation within the bidentate ligands [cf. N–N = 2.345(3) and
P–P = 3.132(3) Å].
Presumably this twisting is attributable to crystal packing
effects. Although the 2,6-xylyl groups are located at the per-
iphery of the complex, it is apparent they have a profound effect
upon the nature of the chromium environment. Clearly, the
presence of the 2,6-xylyl groups prevents the construction of a
paddlewheel structure since sixteen methyl groups cannot be
accommodated over the two axial positions. Furthermore, the
steric bulk provided by the 2,6-xylyl groups prevents the associ-
ation of molecules of 1 into a non-bridged quadruply bonded
species analogous to Li4[Cr2(C4H8)4], for example.8 Compound
1 is obviously analogous to the N,NЈ-bis(cyclohexyl)acet-
amidinate complex Cr(DCyA)2 discussed in the introductory
remarks.4
In contrast, the reaction of equimolar amounts of LiDXylF
and CrCl2 in THF affords the blue complex Cr2(µ-Cl)2-
(DXylF)2(THF)2, 2. The analogous reaction in toluene does
not result in an unsolvated version of compound 2, rather
the complex Cr(DXylF)2, 1, is obtained with half of the CrCl2
remaining unreacted. The molecular structure of compound 2
was determined by single-crystal X-ray diffraction and is illus-
trated in Fig. 3. Selected bond lengths and angles are listed in
Table 1 while crystallographic data are shown in Table 2. The
unit cell contains a single molecule of 2 that resides on a centre
of inversion. Compound 2 contains two chromium ions which
are separated by 2.612(1) Å. This distance precludes the pres-
ence of significant metal–metal bonding interactions and
From a structural viewpoint the isolation of compound 2 was
not what we sought. Our objective was to obtain a quadruply-
bonded dichromium compound with just two bridging biden-
tate ligands. Although such a quadruply bridged compound
might be obtained by the removal of the THF ligands from
Cr2(µ-Cl)2(DXylF)2(THF)2, 2, by heating or by applying a
vacuum, our efforts to date have not met with success: when
compound 2 was heated under nitrogen, under reduced pres-
sure, or in refluxing toluene, a green intractable material was
obtained consistently. This material has not yet been identified.
Compound 2 has, however, already proven to be a useful
starting material for another novel quadruply bonded species.
The reaction of Cr2(µ-Cl)2(DXylF)2(THF)2, 2, with two
equivalents of sodium acetate in THF affords the orange com-
plex Cr2(Ac)2(DXylF)2(THF)2, 3. The molecular structure of
compound 3 was determined by single-crystal X-ray diffraction
and is shown in Fig. 4. Selected bond lengths and angles are
listed in Table 1 while crystallographic data are shown in Table
2. The structure is comprised of four independent but chem-
ically equivalent molecules of 3 in addition to two toluene sol-
vent molecules (one of which is disordered). Each of the four
molecules of 3 resides on an inversion centre. The molecular
structure of 3 consists of two chromium atoms [avg. Cr–Cr =
2.339(7) Å] which are spanned by two trans bridging form-
amidinate ligands and two trans bridging acetate ligands. The
metal–metal bond is elongated owing to the presence of two
axially coordinated THF ligands. With an average Cr ؒ ؒ ؒ Oaxial
distance of 2.31(1) Å, the THF ligands in 3 are much less tightly
coordinated than in Cr2(µ-Cl)2(DXylF)2(THF)2, 2 [cf. 2.124(2)
Å]. Two carbon atoms in each of the THF ligands were found
to be disordered. Each disorder was modelled over two sites
with equal occupancy.
accordingly compound 2 is found to be paramagnetic (µeff
=
3.25 µB per metal atom perhaps indicating two unpaired
electrons; cf. µeff = 4.77 µB per high-spin d4 metal atom in 1).
Presumably, the proximity of the metal ions is enforced by the
two bridging formamidinate and two bridging chloride ligands.
The formamidinate ligands are found to be trans to one another
presumably for steric reasons. As in compound 1, the 2,6-xylyl
groups in compound 2 are twisted, making an angle of 73.3(3)Њ
to the Cr2N4 plane. The remainder of the coordination sphere is
occupied by two THF molecules. These solvate ligands do not
merely act as weak ‘axial’ donors: with a Cr–O separation of
2.124(2) Å, they are in fact tightly bound. Although it might
appear that each chromium atom occupies a trigonal bipyram-
idal environment (as illustrated in Fig. 3), compound 2 is actu-
ally far less symmetrical than it seems. In particular, the Cr–Cl
distances fall into two distinct long and short pairs at 2.370(1)
and 2.596(1) Å, with each chromium atom possessing a contact
of each type. The long Cr–Cl distances are found to be in the
positions cis to the Cr–O bond while the short Cr–Cl distances
are trans. It is interesting to compare these relationships with
those observed in the paramagnetic and seemingly analogous
dimethylphosphinomethane (DMPM) compound Cr2(µ-Cl)2-
(DMPM)2Cl2.9 In this latter compound, the trend is reversed
with the long Cr–Cl distances [2.595(1) Å] at the positions trans
to the terminal Cr–Cl bond while the short Cr–Cl distances
Compound 3 is of significance because it is the first mixed
formamidinate–acetate dichromium paddlewheel species.
Moreover, all of the dimolybdenum analogues possess the cis
conformation, with the exception of the acetamidinate
compound Mo2(Ac)2(DXylA)2(THF)2 which appeared in the
literature almost twenty years ago.10
Unfortunately a similar outcome could not be observed for
the reactions of Cr2(µ-Cl)2(DXylF)2(THF)2, 2, and two equiva-
lents of LiDXylF, lithium N,NЈ-bis(o-anisyl)formamidinate
(LiDPhOMeF) or lithium N,NЈ-bis(o-tolyl)formamidinate
(LiDPhMeF). The reaction with LiDXylF lead to mononuclear
Cr(DXylF)2, 1, while the latter anisyl and tolyl derivatives gave
J. Chem. Soc., Dalton Trans., 2000, 2001–2005
2003

View Article Online
4ϩ
containing a quadruply-bonded Cr2 moiety presumably for
steric reasons. However, this inhibition does not prevent the
formation of other dinuclear species since the non-metal–metal
bonded complex Cr2(µ-Cl)2(DXylF)2(THF)2, 2, is isolable,
although it is still unclear why this molecule does not adopt a
structure with a quadruple bond: Only compounds with three
or four bridging bidentate ligands (with the exception of the
non-bridged alkyls) have been observed with a dichromium
quadruple bond. Compound 2, however, does react with
sodium acetate to form the mixed acetate–formamidinate
complex trans-Cr2(Ac)2(DXylF)2(THF)2, 3, which possesses a
quadruple bond, but the related mixed formamidinate–
formamidinate compounds do not appear to be accessible.
Compound 3 can also be prepared from the chromium acetate
starting material. With the preparation of cis-Cr2(Ac)2-
(DPhOMeF)2(THF)2, 4, this reaction has been established as a
more general synthesis to novel mixed acetate–formamidinate
species.
Fig. 5 The molecular structure of Cr2(Ac)2(DPhOMeF)2, 4. Displace-
ment ellipsoids are drawn at the 30% probability level. Hydrogen atoms
omitted for clarity.
the homoleptic Cr2(DPhOMeF)4 and Cr2(DPhMeF)4 paddle-
wheel compounds along with Cr(DXylF)2 in a 1:2 ratio. In
the latter cases a disproportionation of the ligands occurred
and the anticipated trans mixed formamidinate paddlewheel
compounds were not obtained.
Experimental
Methods and materials
All manipulations were carried out under a nitrogen atmosphere
using standard Schlenk and drybox techniques unless otherwise
stated. Solvents were purified by conventional methods from
Na/K. The formamidines were prepared by the thermolysis at
130 ЊC of triethyl orthoformate in the presence of two equiv-
alents of the appropriate aniline over 4 h. The white solids
obtained were washed with large amounts of pentane before
use. Other chemicals were purchased from Aldrich and used as
received. Infrared spectra were recorded in the range 4000–1000
cmϪ1 on a Perkin-Elmer 16PC FTIR spectrometer using KBr
pellets; NMR spectra were recorded on a Varian XL-200,
spectrometer; while magnetic measurements were made using
a Johnson Matthey magnetic susceptibility balance. Elemental
analyses were satisfactory for all compounds.
Since Mo2(Ac)2(DXylA)2(THF)2 was prepared from the
reaction of LiDXylA and Mo2(Ac)4,10 we decided to see
whether the analogous reaction between LiDXylF and Cr2(Ac)4
in THF would also take place to afford the complex Cr2(Ac)2-
(DXylF)2(THF)2, 3. Indeed it did and in higher yield (cf. 77%
with 41% starting from 2). For comparison, we also attempted
the reaction of LiDPhOMeF and Cr2(Ac)4 in THF and obtained
the compound Cr2(Ac)2(DPhOMeF)2, 4. The molecular structure
of 4 was determined by single-crystal X-ray diffraction and is
shown in Fig. 5. Selected bond lengths and angles are listed in
Table 1 while crystallographic data are shown in Table 2. The
structure is comprised of one molecule of 4 along with two
toluene solvent molecules. The complex consists of two quad-
ruply bonded chromium atoms at a separation of 2.037(1) Å.
The molecular structure of compound 4 differs from that of
compound 3 in two major ways: firstly, the bridging form-
amidinate and acetate ligands have a cisoid relationship in 4.
Secondly, the THF ligands which considerably elongated the
metal–metal bond in 3 are displaced by two of the four
o-methoxyphenyl groups of the formamidinate [Cr ؒ ؒ ؒ O =
2.337(3) and 2.353(3) Å; cf. 2.31(1) Å for 3]. It is of note that
the methoxyphenyl groups do not elongate the metal–metal
bond as strongly as axial THF molecules. This is because the
methoxyphenyl group is not as strong a σ donor as a THF
molecule and also because the oxygen atom sits directly over the
metal–metal axis in compound 3 whereas it is pulled off to one
side by the formamidinate ligand in 4 [cf. Cr–Cr ؒ ؒ ؒ O (avg) =
178.9(9) for 3 and 166.7(5)Њ for 4]. The 1H-NMR spectrum of 4
in C6D6 shows only a sharp singlet for the MeO protons
(δ = 3.04) illustrating that this conformation is not retained
in C6D6 solution at room temperature. It is also of note that
in the tetra-N,NЈ-bis(o-anisyl)formamidinate complex Cr2-
(DPhOMeF)4 the Cr–Cr distance is 2.140(2) Å, almost 0.1 Å
longer than in 4.8 In Cr2(DPhOMeF)4, the metal–metal bond is
elongated by two axial interactions between two trans form-
amidinate ligands. At 2.402(2) and 2.635(2) Å, the Cr ؒ ؒ ؒ O
contact distances in Cr2(DPhOMeF)4 are surprisingly longer
than in compound 4. Since it is unlikely that the metal–metal
bond length is dependent on whether the cis or the trans ligands
provide the oxygen donors it may then be concluded that the
acetate ligand actually has a slight shortening effect over the
formamidinate.
Syntheses
Cr(DXylF)2, 1. In a typical reaction, MeLi (1.02 cm3, 1.6 M
in diethyl ether, 1.63 mmol) was added dropwise to a suspension
of anhydrous CrCl2 (100 mg, 813 µmol) and HDXylF (410 mg,
1.63 mmol) in THF (20 cm3). The light blue suspension
changed gradually to a turbid red solution. After 6 h, the THF
was then removed under vacuum and the solid red-purple resi-
due washed with ice-cold hexanes (4 × 5 cm3) before extraction
into benzene (30 cm3). After the benzene solution was filtered
through Celite to ensure the removal of LiCl, the solvent was
removed under vacuum to reveal a red powdery solid of
Cr(DXylF)2, 1. Yield: 177 mg, 319 µmol, 39%. The analogous
reaction in toluene required 18 h for completion.
Cr2(ꢀ-Cl)2(DXylF)2(THF)2, 2. MeLi (0.51 cm3, 1.6 M in
diethyl ether, 813 µmol) was added dropwise to a suspension of
anhydrous CrCl2 (100 mg, 813 µmol) and HDXylF (205 mg,
813 µmol) in THF (20 cm3). The light blue suspension changed
gradually to a dark blue solution. After 3 h, the THF was then
removed under vacuum and the solid dark blue residue washed
with ice-cold hexanes (4 × 5 cm3) before extraction into benzene
(30 cm3). After the benzene solution was filtered through Celite
to ensure the removal of LiCl, the solvent was removed under
vacuum to reveal a dark blue powdery solid of Cr2(µ-Cl)2-
(DXylF)2(THF)2, 2. Yield: 139 mg, 169 µmol, 42%.
Cr2(Ac)2(DXylF)2(THF)2 3. From 2. Anhydrous sodium acet-
ate (67 mg, 813 µmol) in THF (20 cm3) was added dropwise to
Cr2(µ-Cl)2(DXylF)2(THF)2 2 (334 mg, 407 µmol) in THF (20
cm3) by cannula transfer. The dark blue solution changed grad-
ually to orange. After 4 h, the THF was removed under vacuum
and the solid orange residue redissolved in benzene (30 cm3).
Conclusions
It has been shown that the DXylF ligand forms a mononuclear
complex Cr(DXylF)2, 1, with CrCl2 rather than a dinuclear one
2004
J. Chem. Soc., Dalton Trans., 2000, 2001–2005

View Article Online
After the benzene solution was filtered through Celite to ensure
the removal of both LiCl and any unreacted sodium acetate, the
solvent was removed under vacuum to reveal an orange poly-
crystalline solid of Cr2(Ac)2(DXylF)2(THF)2, 3. Yield: 146 mg,
168 µmol, 41%.
1043 (m), 1022 (s). 1H-NMR (C6D6): δ 9.01 (s, 2H, form),
6.98–6.87 (m, 12H, Ar), 6.69 (t, 4H, Ar, J = 7.7 Hz), 3.04
(s, 12H, OMe) and 2.39 (s, 6H, Ac). Calc. for C34H36Cr2N4O8ؒ
2C6H5Me: C, 62.9; H, 5.7; N, 6.1%. Found: C, 62.3; H, 5.4; N,
6.4%.
From chromium acetate. In a typical reaction, MeLi (0.51
cm3, 1.6 M in diethyl ether, 813 µmol) was added dropwise to a
suspension of HDXylF (208 mg, 813 µmol) in THF (20 cm3).
This yellow solution was added in turn to a suspension of
anhydrous chromium acetate (138 mg, 407 µmol) in THF (20
cm3) by cannula transfer. The brown suspension turned grad-
ually to orange. After 4 h, the product was isolated as described
in the reaction of Cr2(µ-Cl)2(DXylF)2(THF)2, 2, and sodium
acetate to reveal Cr2(Ac)2(DXylF)2(THF)2, 3, an orange poly-
crytalline solid, yield: 272 mg, 313 µmol, 77%.
Crystallographic studies
Single crystals suitable for X-ray diffraction measurements
were obtained from concentrated toluene solutions at Ϫ20 ЊC.
Crystallographic data for complexes 1, 3ؒ0.5C6H5Me and 4ؒ
2C6H5Me were collected on a Bruker SMART 1000 diffrac-
tometer, and for 2 on a Nonius FAST diffractometer. Both
machines are equipped with a low temperature device. Crystal-
lographic details are given in Table 2.
An analogous procedure using DPhOMeF (205 mg, 813 µmol)
gave the complex Cr2(Ac)2(DPhOMeF)2, 4 (orange, yield: 247
mg, 337 µmol, 83%).
Structure solution and refinement. The positions of the metal
atoms and their first coordination spheres were determined by
direct methods and refined against F2 using SHELXL-93.11
All non-hydrogen atoms were found by successive iterations of
least-squares refinement. Hydrogen atoms were added in calcu-
lated positions and allowed to ride on their parent atoms unless
they appeared in difference maps. For crystalline 1, 2 and
4ؒ2C6H5Me, all of the non-hydrogen atoms were refined aniso-
tropically. For crystalline 3ؒ0.5C6H5Me toluene, however, this
was with the exception of several carbon atoms belonging to
the disordered THF ligands and the disordered toluene solvate;
each disorder was modelled over two sites and the atoms refined
at half occupancy.
Spectroscopic data
1. ϩFABMS: m/z = 554 (Mϩ calc. 554.9) with 304 (Mϩ Ϫ
DXylF) and 253 (DXylFϩ) observed. IR (KBr disk): ν/cmϪ1
=
1640 (m), 1605 (s), 1509 (s), 1373 (sh), 1362 (sh), 1290 (w), 1256
(w), 1245 (m) 1207 (s), 1185 (s), 1139 (w) and 1020 (s). Magnetic
measurement: χmol (corrected molar susceptibility)/103 cgs =
9.726 and µeff/µB = 4.77. Calc. for C34H38CrN4: C, 73.6; H, 6.9;
N, 10.1%. Found: C, 73.8; H, 7.0; N, 10.0%.
2. ϩFABMS: m/z = 412 (M2ϩ calc. 410.9 ) with 253 (DXylFϩ)
observed. IR (KBr disk): ν/cmϪ1 = 1650 (s), 1605 (s), 1570
(s), 1536 (s), 1405 (w), 1354 (m), 1264 (w), 1249 (w), 1236 (m)
1200 (w), 1177 (m) and 1034 (s). Magnetic measurement:
χmol (corrected molar susceptibility)/103 cgs = 8.945 and
µeff/µB = 4.59 (3.25 per chromium atom). Calc. for C42H54Cl2-
Cr2N4O2: C, 61.4; H, 6.6; N, 6.8%. Found: C, 60.9; H, 6.2; N,
7.2%.
CCDC reference number 186/1962.
Acknowledgements
We are grateful to the National Science Foundation and the
Robert A. Welch Foundation for financial support.
References
3. ϩFABMS: m/z = 868 (Mϩ calc. 869.0) with 726 (Mϩ Ϫ
2THF, largest intensity ion peak) and 363 (M2ϩ Ϫ 2THF)
observed. IR (KBr disk): ν/cmϪ1 = 1629 (s), 1607 (vs), 1560 (s),
1545 (s), 1486 (s), 1475 (s), 1441 (s), 1347 (vs), 1340 (sh), 1327
(s), 1319 (s), 1282 (m), 1267 (s), 1170 (s), 1158 (s), 1104
(m), 1060 (m), 1043 (sh), 1032 (s) and 1017 (s). 1H-NMR
(C6D6): δ 8.65 (s, 2H, form), 6.96 (d, 8H, Ar, J = 7.6 Hz), 6.19 (t,
4H, Ar, J = 7.6 Hz), 3.41 (br s, 8H, THF), 2.46 (s, 6H, Ac), 2.29
(s, 24H, Me) and 1.13 (br s, 8H, THF). Calc. for C46H60Cr2N4O6ؒ
0.5C6H5Me: C, 65.0; H, 7.1; N, 6.1%. Found: C, 64.4; H, 6.6; N,
6.5%.
1 F. A. Cotton and R. A. Walton, Multiple Bonds between Metal
Atoms, Oxford University Press, Oxford, 2nd edn., 1993.
2 W. H. De Roode, K. Vrieze, E. A. Koerner von Gustorf and
A. Ritter, J. Organomet. Chem., 1977, 135, 183.
3 R. T. Boeré, V. Klassen and G. Wolmershäuser, J. Chem. Soc.,
Dalton Trans., 1998, 4147.
4 S. Hao, S. Gambarotta, C. Bensimon and J. J. H. Edema, Inorg.
Chim. Acta, 1993, 213, 65.
5 J. K. Buijink, M. Noltemeyer and F. T. Edelmann, Z. Naturforsch.,
Teil B, 1991, 46, 1328.
6 F. A. Cotton, C. A. Murillo and I. Pascual, Inorg. Chem. Commun.,
1999, 2, 101.
7 F. A. Cotton, L. M. Daniels, C. A. Murillo and P. Schooler, J. Chem.
Soc., Dalton Trans., 2000, DOI: 10.1039/b000329h.
8 J. Krausse and G. Schödl, J. Organomet. Chem., 1971, 27, 59.
9 F. A. Cotton, R. L. Luck and K.-A. Son, Inorg. Chim. Acta, 1990,
168, 3.
4. ϩFABMS: m/z = 732 (Mϩ calc. 732.9) with 673 (Mϩ Ϫ Ac)
and 477 (Mϩ Ϫ DXylF) observed. IR (KBr disk): ν/cmϪ1
=
1634 (m), 1605 (s), 1558 (s), 1539 (s), 1486 (s), 1440 (s), 1347
(vs), 1322 (vs), 1280 (m), 1242 (s), 1199 (s), 1176 (s), 1111 (m),
10 F. A. Cotton, W. H. Ilsley and W. Kaim, Inorg. Chem., 1981, 20, 930.
11 G. M. Sheldrick, SHELXL-93, University of Göttingen, 1993.
J. Chem. Soc., Dalton Trans., 2000, 2001–2005
2005