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

Article abstract of DOI:10.1039/b000329h

A study regarding the interaction of several ortlio ring-substituted N,N′-diarylformamidinate ligands (DPh^XF, X = Me, OMe, Cl and Br) with the Cr₂⁴⁺ moiety has been undertaken. X-Ray diffraction and spectroscopic data have shown that while all of these ligands form the well-known paddlewheel type complex Cr₂(DPh^XF)₄,1 [X = Me, OMe, Cl and Br; Cr-Cr = 1.925(1), 2.140(2), 2.208(2) and 2.272(2) A, respectively], two of them form highly unusual A-frame type complexes with short Cr-Cr bonds, and the smallest M-X-M angle ever reported in an A-frame structure. These are Cr₂(μ-Cl)(DPh^XF)₃, 2 [X = Cl and Br; Cr-Cr = 1.940(1) and 1.940(2) A, respectively]. For some of the paddlewheel complexes, the elongation of the metal-metal bond distance out of the 'super-short' range (Cr-Cr > 2.00 A) has been attributed to the presence of axial interactions between two of the ortho substituents and the Cr₂⁴⁺ moiety. The Royal Society of Chemistry 2000.

Full text of DOI:10.1039/b000329h

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Chromium(II) complexes bearing 2-substituted N,NЈ-diarylform-
amidinate 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 14th June 2000
A study regarding the interaction of several ortho ring-substituted N,NЈ-diarylformamidinate ligands (DPhXF,
X = Me, OMe, Cl and Br) with the Cr24ϩ moiety has been undertaken. X-Ray diffraction and spectroscopic data
have shown that while all of these ligands form the well-known paddlewheel type complex Cr2(DPhXF)4, 1 [X = Me,
OMe, Cl and Br; Cr–Cr = 1.925(1), 2.140(2), 2.208(2) and 2.272(2) Å, respectively], two of them form highly unusual
A-frame type complexes with short Cr–Cr bonds, and the smallest M–X–M angle ever reported in an A-frame
structure. These are Cr2(µ-Cl)(DPhXF)3, 2 [X = Cl and Br; Cr–Cr = 1.940(1) and 1.940(2) Å, respectively]. For
some of the paddlewheel complexes, the elongation of the metal–metal bond distance out of the ‘super-short’
range (Cr–Cr > 2.00 Å) has been attributed to the presence of axial interactions between two of the ortho
substituents and the Cr24ϩ moiety.
Introduction
Amidinate complexes of Cr() are now well known.1–8 Most
prevalent are those compounds that possess a quadruply
4ϩ
bonded Cr2 unit surrounded by four bridging amidinate
ligands.1–7 This class of complex, shown schematically in Fig. 1,
is known as the ‘paddlewheel’ because of its structural char-
acteristics.1 Although the earliest dichromium tetra-amidinate
complex to be structurally characterised was that of the N,NЈ-
diphenylacetamidinate ligand (RЉNCRЈNRЉ; RЈ = Me and
RЉ = Ph),2 the formamidinate ligands (RЈ = H) have found
much greater application. This is because the synthetic methods
available allow a wider variation in the substituent RЉ of the
formamidinate ligand. For example, formamidinate paddle-
wheel complexes bearing p-tolyl,3 cyclohexyl,4 m-MeOC6H4,5
5
6
p-ClC6H4,5 3,5-Cl2C6H3 and 3,5-Me2C6H3 groups have all
been structurally characterised (to name but a few). It is of note
that in all of these compounds the Cr–Cr distances do not
exceed 1.93 Å. The implication is that the basicity of the form-
amidinate ligand has little influence upon the metal–metal
separation.
More recently, however, we have shown that whenever ortho
fluoro substituents are present in the aromatic groups of an
N,NЈ-diarylformamidinate ligand (DPhXF), there is a signifi-
cant lengthening of the metal–metal bond.7 For example, in
the tetra-formamidinate paddlewheel complexes Cr2(DPhFF)4
(RЈ = H and RЉ = o-fluorophenyl) and Cr2(DPh5FF)4 (RЈ = H
and RЉ = pentafluorophenyl) the Cr–Cr distances were found to
be 1.968(2) and 2.012(1) Å, respectively.7 The elongation of the
metal–metal bond did not occur because of a reduction in
ligand basicity, but rather because of the presence of Cr ؒ ؒ ؒ F
axial interactions. For these fluorinated derivatives, it was
unclear whether the axial interactions were of σ* or π* char-
acter or both. Our parallel work with the 2,6-bis(phenyl-
imino)piperidinate ligand (DPhIP), however, showed that
intramolecular π* coordination alone can account for a
dramatic elongation of a metal–metal bond. For example, in
the complex Cr2(DPhIP)4, four off-axis Cr ؒ ؒ ؒ N interactions
elongate the Cr–Cr bond to 2.265(1) Å.9
Fig. 1 Schematic representations of the paddlewheel 1 and the
A-frame 2 structures discussed in this paper. The chromium atoms are
quadruply bonded.
When we extended our study to N,NЈ-diarylformamidinate
ligands bearing ortho chloro substituents instead, our (initial)
findings were quite unexpected.8 We observed that three
equivalents of LiDPhClF reacted with two equivalents of CrCl2
to afford the unprecedented A-frame complex Cr2(µ-Cl)-
(DPhClF)3, 2c. In this compound, one chloride and three
formamidinate ligands bridge a dichromium unit (see Fig. 1,
RЈ = H and RЉ = o-C6H4Cl). This was unusually interesting
DOI: 10.1039/b000329h
J. Chem. Soc., Dalton Trans., 2000, 2007–2012
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2007

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Table 1 Selected bond lengths (Å) for complexes 1a–1d and 2d. X is the substituent of the aryl group
1a
1b
1c
1d
2d
Compound
Cr–Cr
Cr–N
Cr2(DPhMeF)4
1.925(1)
2.045(4)–2.094(4)
—
Cr2(DPhOMeF)4
2.140(2)
2.053(7)–2.104(7)
2.402(2), 2.635(2)
Cr2(DPhClF)4
2.208(2)
2.065(8)–2.091(8)
2.766(2)
Cr2(DPhBrF)4
2.272(2)
2.048(8)–2.099(8)
2.890(3), 2.943(2)
Cr2Cl(DPhBrF)3
1.940(2)
2.064(7)–2.019(7)
2.865(2), 2.842(2)
Cr–X
Table 2 Crystallographic data for complexes 1a–1d and 2d
1a
1b
1c
1dؒ2C6H5Me
2d
Formula
M
Space group
a/Å
b/Å
c/Å
C60H60Cr2N8
997.16
P1
C60H60Cr2N8O8
1125.16
P21
10.900(2)
20.524(4)
12.330(3)
90
93.52(3)
90
2753(1)
1.357
C52H36Cl8Cr2N8
1160.49
C2/c
19.313(2)
11.971(3)
22.833(6)
90
110.48(2)
90
4945(2)
1.559
C66H52Br8Cr2N8
1700.44
P1
C39H27Br6ClCr2N6
1198.58
C2/c
19.60(1)
19.157(5)
22.02(1)
90
100.67(3)
90
8125(6)
1.96
¯
¯
10.743(2)
14.242(3)
18.073(4)
76.39(3)
74.84(3)
81.95(3)
2585(1)
1.281
13.051(1)
13.716(1)
20.714(1)
105.937(1)
96.117(1)
112.520(1)
3198.3(3)
1.765
α/Њ
β/Њ
γ/Њ
V/Å3
ρcalc/g cmϪ3
µ/mmϪ1
λ(Mo-Kα)/Å
T/K
0.648
0.71073
158(2)
0.458
0.71073
166(2)
0.919
0.71073
213(2)
5.384
0.71073
213(2)
6.54
0.71073
138(2)
Z
2
2
4
2
8
R11, R12 a
wR21, wR22 a
GOF
0.050, 0.092
0.123, 0.139
1.071
0.047, 0.082
0.100, 0.113
1.091
0.120, 0.146
0.230, 0.253
1.354
0.078, 0.252
0.161, 0.191
1.042
0.057, 0.144
0.094, 0.163
1.179
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.
because it was the only example of an A-frame complex poss-
4ϩ
essing a quadruply bonded M2 moiety;8 all other known
A-frame complexes have long metal–metal separations.10 At
first, we believed that the stability of this compound may have
been a consequence of the bulky nature of the o-chlorophenyl
rings and that the paddlewheel complex might be inaccessible
because there was insufficient space to add a fourth DPhClF
ligand across the dichromium unit. Our further work involving
the DPhClF ligand, however, has shown that the tetra-
formamidinate Cr2(DPhClF)4 is indeed accessible. In this paper,
we wish to report this work and also some other significant
results obtained during the course of our studies into the
complexation behaviour of ortho ring substituted N,NЈ-
di(phenyl)formamidinate ligands (DPhXF; X = Me, OMe, Cl
and Br) toward Cr(). In particular, we have employed ligands
that have substituents of differing sizes and electronegativities
to explore the phenomenon of intra-molecular axial ligation.
These results are summarised in Fig. 1.
Results and discussion
In analogy to the reaction involving LiDPhClF,8 the reaction of
three equivalents of LiDPhBrF with two equivalents of CrCl2 in
Fig. 2 A drawing of the molecular structure of Cr2(µ-Cl)(DPhBrF)3,
THF affords the green complex Cr2(µ-Cl)(DPhBrF)3, 2d. The
2d. Displacement ellipsoids are drawn at the 30% probability level.
Hydrogen atoms omitted for clarity.
molecular structure of 2d was determined by single-crystal
X-ray diffraction and is illustrated in Fig. 2. Selected bond
lengths are listed in Table 1 while crystallographic data are
shown in Table 2. The molecular structure of compound 2d
was found to be isostructural with the ortho chloro derivative
Cr2(µ-Cl)(DPhClF)3, 2c.9 The Cr–Cr distance of 1.940(2) Å [cf.
1.940(1) Å for 2c] is within the normal range for a ‘super-short’
quadruple bond despite the presence of two weak axial
Cr ؒ ؒ ؒ Br interactions. The two mutually trans formamidinate
ligands provide one bromine donor atom each at 2.865(2) and
2.842(2) Å for Cr(1) ؒ ؒ ؒ Br(1) and Cr(2) ؒ ؒ ؒ Br(6), respectively
[cf. 2.741(2) and 2.778(2) Å for the analogous Cr ؒ ؒ ؒ Cl
distances in 2c]. The very short intermetallic separation in 2d
makes the Cr–Cl–Cr angle very acute at 46.53(8)Њ [cf. 46.67(2)Њ
for 2c] and the Cr–Cl distances slightly elongated at 2.454(3)
and 2.458(3) Å [cf. 2.441(1) and 2.440(1) Å for 2c]. The
chromium–nitrogen distances can be divided into two sets since
the formamidinate ligand trans to the bridging chlorine atom
is more tightly bound than the formamidinate groups that
are cis [cf. Cr–Nt ≈ 2.02(1) and Cr–Nc ≈ 2.06(1) Å]. It is of note
that 2c and 2d are exceptional compounds in that they are the
only examples in which a chloride ligand bridges a quadruply
bonded dimetallic unit. Because of the shortness of the M–M
distance, these complexes display the most acute M–X–M angle
ever seen in an A-frame structure.
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Fig. 3 A drawing of the molecular structure of Cr2(DPhMeF)4, 1a.
Displacement ellipsoids are drawn at the 30% probability level. Hydro-
gen atoms omitted for clarity.
Fig. 4 A space-filling model of the molecular structure of Cr2-
(DPhMeF)4, 1a as viewed down the metal–metal axis.
There is a clear distinction in the 1H-NMR spectrum of
compound 2d (in C6D6 at room temperature) between the
signals arising from protons belonging to the formamidinate
ligands cis to the bridging chlorine atom and those belonging to
the formamidinate that is trans. The intensity ratio of the cis
signal to the corresponding trans peak is 2:1 as in the spectrum
of compound 2c under identical conditions. However, while all
of the signals attributable to the cis ligand appear at higher field
than their trans counterparts for 2c, this is not true for com-
pound 2d. For example, the methyne resonances of the cis and
trans ligands in 2c appear at δ 8.55 and 8.49, respectively,
whereas in 2d they are observed at δ 8.45 and 8.82, respectively.
The reaction of three equivalents of LiDPhMeF with two
of CrCl2 in THF or benzene does not afford the analogous
chloride-bridged species; rather the orange complex Cr2(DPhMe
F)4, 1a, is obtained even at Ϫ78 ЊC. This compound is, however,
best prepared by the reaction of two equivalents of LiDPhMe
-
F
with one of CrCl2. The molecular structure of compound 1a
was determined by single-crystal X-ray diffraction. Selected
bond lengths and angles are listed in Table 1 while crystallo-
graphic data are shown in Table 2. From Fig. 3, it can be seen
that each molecule has D4 symmetry with all of the methyl
groups oriented away from the C4 axis (the Cr–Cr bond).
Although compound 1a, with its Cr–Cr distance of 1.925(1) Å,
is a rather unremarkable example of a dichromium tetra-
formamidinate complex, its very existence raised some interest-
ing questions. Firstly, we had already observed that the reaction
of three equivalents of LiDPhClF with two of CrCl2 in THF
gave Cr2(µ-Cl)(DPhClF)3, 2c, but since the chlorine atom is of
comparable size to a methyl group we wondered if the reaction
of four LiDPhClF molecules with two of CrCl2 could proceed
to form Cr2(DPhClF)4, 1c.9 Secondly, an inspection of the
space-filling representation of Cr2(DPhMeF)4, 1a (see Fig. 4),
shows that it is unlikely that a paddlewheel structure can be
formed using an N,NЈ-diarylformamidinate ligand which has
methyl groups in all four ortho positions. The former question
will be answered here while the latter forms the subject of the
following paper.11
Fig. 5 A drawing of the molecular structure of Cr2(DPhOMeF)4, 1b.
Displacement ellipsoids are drawn at the 30% probability level. Hydro-
gen atoms omitted for clarity.
of 1b pack in the non-centrosymmetric space group P21. At
2.140(2) Å, the Cr–Cr bond length in Cr2(DPhOMeF)4, 1b, is
almost 0.2 Å longer than that in Cr2(DPhMeF)4, 1a. This
elongation can be attributed to the presence of axial inter-
actions between the oxygen donor atoms of the methoxy sub-
stituents and the dichromium unit. Only two of the eight
methoxy groups (one each from a pair of mutually trans
formamidinate ligands) are turned in over the axial sites at
distances of 2.635(2) and 2.402(2) Å [Cr(1) ؒ ؒ ؒ O(1) and
Cr(2) ؒ ؒ ؒ O(6), respectively]. It is of note that the oxygen donor
atoms in compound 1b do not sit directly over the Cr–Cr axis
12
It was found that the reaction of two equivalents of the
lithiated cis ring-substituted formamidinates LiDPhOMeF,
LiDPhClF and LiDPhBrF with one equivalent of CrCl2 in THF
or benzene affords the corresponding paddlewheel complex
Cr2(DPhXF)4, 1x, in each case.
as in, for example, the acetate complex Cr2Ac4(H2O)2 [Cr–
Cr = 2.288(2) Å]. They are pulled off to one side by the form-
amidinate ligands such that Cr(2)–Cr(1) ؒ ؒ ؒ O(1) = 156.6(1)Њ
1
and Cr(1)–Cr(2) ؒ ؒ ؒ O(6) = 160.4(2)Њ. The H-NMR spectrum
in C6D6 shows only a sharp singlet at δ 3.05 for all of the
methoxy groups. This is an indication that the solid state con-
formation is not retained in solution at room temperature.
Presumably, the time-averaged equivalency of the methoxy
groups arises from a rotation among the o-anisyl rings.
The molecular structure of Cr2(DPhOMeF)4, 1b, was deter-
mined by single-crystal X-ray diffraction and is illustrated in
Fig. 5. Selected bond lengths and angles are listed in Table 1
while crystallographic data are shown in Table 2. The molecules
J. Chem. Soc., Dalton Trans., 2000, 2007–2012
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mium paddlewheel compounds that are supported only by
bidentate nitrogen donor ligands and no direct axial coordin-
ation, the Cr2(DPhBrF)4, 1d, molecule possesses the longest
metal–metal bond observed to date. Accordingly crystals of 1d
actually appear green rather than orange. It is of importance to
repeat here that the bond lengths in the related A-frame com-
pounds 2c and 2d are 1.940(1) and 1.940(2) Å, while for 1c and
1d they are 2.208(2) and 2.272(2) Å, respectively. The inference
here is that the bridging chloride groups in Cr2(µ-Cl)(DPhClF)3,
2c, and Cr2(µ-Cl)(DPhBrF)3, 2d, counteract the Cr–Cr lengthen-
ing effect of axial ligation observed in Cr2(DPhClF)4, 1c, and
Cr2(DPhBrF)4, 1d.
In analogy to LiDPhMeF, it was found that the reaction
of three equivalents of LiDPhFF or LiDPhOMeF with two of
CrCl2 in THF or benzene does not afford the appropriate
chloride-bridged species even at Ϫ78 ЊC. Again only a tetra-
formamidinate species could be isolated. It is unclear why
A-frame type complexes for the ligands DPhXF (X = Me, OMe
and F) have not yet been isolated.
Conclusions
In this paper we have shown that the presence of donor groups
at the ortho position on the phenyl rings of an N,NЈ-diarylform-
amidinate can significantly lengthen the chromium–chromium
bond in the paddlewheel type complexes Cr2(DPhXF)4, 1x. This
is not the case for the A-frame compounds Cr2(µ-Cl)(DPhClF)3,
2c, and Cr2(µ-Cl)(DPhBrF)3, 2d, in which the chromium–
chromium bond remains in the ‘super-short’ range despite the
presence of comparable axial interactions. These latter A-frame
compounds are intermediates en route to the former paddle-
wheel compounds. It appears that the stabilisation of these
intermediates is possible only for the Cl and Br derivatives
among those N,NЈ-diarylformamidinates studied but there is
no clear reason why. It is perhaps the consequence of a delicate
interplay of axial ligation and steric interactions. We have
also shown that three isomers are possible for the paddlewheel
compounds in the solid state. These are the m-exo isomer (Me)
where all of the substituents are turned away from the metal–
metal axis; the trans-m-endo (OMe) where two of the substitu-
ents from a trans pair of ligands are turned in; and the cis-m-
endo isomer (Cl and Br) where two of the substituents from
a cis pair of ligands are turned in. The methoxy proton equiv-
Fig. 6 A drawing of the molecular structure of Cr2(DPhClF)4, 1c.
Only one orientation of each of the disordered o-chlorophenyl groups
is depicted. Hydrogen atoms omitted for clarity.
1
alency observed in the H-NMR spectrum of Cr2(DPhOMeF)4,
1b, however, indicates that these conformations are probably
not retained in solution at room temperature. Our research into
similarly ligated dimetallic species is ongoing in an effort to
rationalise our observations further.
Fig. 7 A drawing of the molecular structure of Cr2(DPhBrF)4, 1d,
illustrating the disorder of the o-bromophenyl groups. Hydrogen atoms
omitted for clarity.
Experimental
Methods and materials
The structures of Cr2(DPhClF)4, 1c, and Cr2(DPhBrF)4, 1d,
were determined by single-crystal X-ray diffraction. Although
they do not form isomorphous crystals, the molecular struc-
All manipulations were carried out under a nitrogen atmos-
phere 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 equivalents 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 while NMR spectra were
recorded on a Varian XL-200 spectrometer.
Cl
Br
¯
tures of Cr2(DPh F)4, 1c, and Cr2(DPh F)4, 1d, (C2/c and P1,
respectively) are very similar. Selected bond lengths and angles
are listed in Table 1 while crystallographic data are shown
in Table 2. Molecules of 1c reside on inversion centres, while
molecules of 1d occupy general positions. Co-crystallised along
with 1d are two equivalents of disordered toluene solvate
molecules. All of the o-halogenophenyl groups in 1c were found
to be disordered over two orientations (see Fig. 6). In 1d, only
the o-halogenophenyl groups involved in axial ligation were
disordered (see Fig. 7). However, it can be discerned that two of
the halogen groups from a cis pair of formamidinate ligands
interact with the dichromium moiety in both compounds
[Cr(A) ؒ ؒ ؒ Cl(2A) = 2.766(2) Å; and Cr(1) ؒ ؒ ؒ Br(1A) = 2.890(3)
and Cr(2) ؒ ؒ ؒ Br(8A) = 2.943(2) Å]. As a consequence, the
Cr–Cr bonds are considerably elongated to 2.208(2) and
2.272(2) Å, respectively. This means that among all the dichro-
Syntheses
Type 1: paddlewheel complexes. In a typical reaction, MeLi
(1.02 cm3, 1.6 M in diethyl ether, 1.63 mmol) was added drop-
wise to a suspension of anhydrous CrCl2 (100 mg, 813 µmol)
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and HDPhMeF (365 mg, 1.63 mmol) in THF (20 cm3). The gray
suspension changed gradually to a turbid orange solution.
After 3 h, the THF was removed under vacuum and the solid
yellow residue washed with warm hexanes (3 × 20 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 leave an orange poly-
crystalline solid, Cr2(DPhMeF)4, 1a. Yield: 219 mg, 220 µmol,
54%. Analogous procedures using HDPhOMeF (417 mg, 1.63
mmol), HDPhClF (432 mg, 1.63 mmol) or HDPhBrF (577 mg,
1.63 mmol) gave the complexes Cr2(DPhOMeF)4, 1b (orange,
yield: 271 mg, 241 µmol, 59%), Cr2(DPhClF)4, 1c (orange, yield:
187 mg, 161 µmol, 40%) or Cr2(DPhBrF)4, 1d (green, yield: 305
mg, 201 µmol, 49%).
[Cr2Cl(DPhClF)3] 2c. ϩFABMS: m/z = 930 (Mϩ calc. 931.5),
895 (Mϩ Ϫ Cl), 664 (Mϩ Ϫ DPhClF) and 397 (Mϩ Ϫ 2DPhClF)
1
observed. H-NMR (C6D6): δ 8.55 (s, 2Hc, form), 8.49 (s, 2Ht,
form), 7.44 (d, 4Hc, Ar, J = 7.9 Hz), 7.02 (t, 4Hc, Ar, J = 7.9 Hz),
6.89 (d, 4Hc, Ar, J = 8.0 Hz), 6.79 (t, 4Hc, Ar, J = 7.9 Hz), 6.61
(d, 2Ht, Ar, J = 8.0 Hz), 6.54 (t, 2Ht, Ar, J = 7.9 Hz), 6.49 (t,
2Ht, Ar, J = 8.0 Hz) and 6.20 (t, 2Ht, Ar, J = 8.0 Hz). Calc. for
C39H27Cl7Cr2N6: C, 50.3; H, 2.9; N, 9.0. Found: C, 50.3; H, 2.6;
N, 8.9%.
[Cr2Cl(DPhBrF)3] 2d. ϩFABMS: m/z = 1197 (Mϩ calc.
1198.6), 1163 (Mϩ Ϫ Cl) and 846 (Mϩ Ϫ DPhBrF) observed.
ν/cmϪ1 = 1618(m), 1613 (sh), 1603(s), 1593(m), 1584(w),
1567(w), 1505(m), 1491(s), 1420(m), 1370(m), 1349(m),
1322(m), 1260(w), 1204(m), 1192 (sh), 1165(w), 1098(m) and
1
1018(m). H-NMR (C6D6): δ 8.82 (s, 1Ht, form), 8.45 (s, 2Hc,
Type 2: A-frame complexes. In a typical reaction, MeLi (0.76
cm3, 1.6 M in diethyl ether, 1.22 mmol) was added dropwise to
a suspension of anhydrous CrCl2 (100 mg, 813 µmol) and
HDPhClF (323 mg, 1.22 mmol) in THF (20 cm3). The gray
suspension changed gradually to a turbid green solution. After
3 h, the THF was removed under vacuum and the solid green
residue washed with warm hexanes (320 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 leave a green polycrystalline solid,
Cr2(µ-Cl)(DPhClF)3, 2c. This is a modification of the published
procedure. Yield: 143 mg, 154 µmol, 38%. An analogous pro-
cedure using HDPhBrF (431 mg, 1.22 mmol) gave the complex
[Cr2(µ-Cl)(HDPhBrF)3] 2d (green, yield: 234 mg, 195 µmol,
48%).
form), 7.63 (d, 4Hc, Ar, J = 7.9 Hz), 7.10 (d, 4Hc, Ar, J = 7.9
Hz), 6.85 (d, 4Ht, Ar, J = 8.1 Hz), 6.81 (t, 4Hc, Ar, J = 7.9 Hz),
6.56 (d, 2Ht, Ar, J = 8.1 Hz), 6.47 (t, 4Hc, Ar, J = 7.9 Hz) and
6.15 (t, 2Ht, Ar, J = 8.1 Hz). Calc. for C39H27Br6ClCr2N6: C,
39.1; H, 2.3; N, 7.0. Found: C, 38.7; H, 2.0; N, 7.2%.
Crystallographic studies
Single crystals of 1a, 1b, 1c and 2d suitable for X-ray diffraction
measurements were obtained by layering hexanes over a benz-
ene solution at room temperature. Single crystals of 1dؒ
2C6H5Me were obtained from a concentrated toluene solution
at Ϫ20 ЊC. Crystallographic data for complexes 1a, 1b and 2d
were collected on a Nonius CAD4 diffractometer equipped
with a low temperature device. Crystallographic data for com-
plex 1c were collected on a Nonius FAST diffractometer
equipped with a low temperature device. Crystallographic data
for 1dؒ2C6H5Me were collected on a Bruker SMART 1000
diffractometer equipped with a low temperature device.
Spectroscopic data
[Cr2(DPhMeF)4] 1a. ϩFABMS: m/z = 996 (Mϩ calc. 997.1)
and 498 (M2ϩ) observed. IR (KBr disk): ν/cmϪ1 = 1631(s),
1612(s), 1590(m), 1507(s), 1424(m), 1339(m), 1263(w), 1203(w),
1
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-93.13 For
crystalline 1a, 1b and 2d all of the non-hydrogen atoms were
refined anisotropically. For crystalline 1c, this was also done
with the exception of the carbon atoms of the disordered o-
chlorophenyl rings; each disorder was modelled over two sites
and the atoms refined at either 7:3 or 9:1 occupancy. For crys-
talline 1dؒ2C6H5Me, all of the non-hydrogen atoms were refined
anisotropically with the exception of the carbon atoms of the
two disordered toluene molecules; each disorder was modelled
over two sites and the atoms refined to almost equal occupancy.
Half of the o-bromophenyl rings were also disordered. The half
that was not disordered was refined anisotropically. Each
disorder was modelled over two sites and the atoms refined at
3:2 occupancy.
1139(m) and 1039(m). H-NMR (C6D6): δ 8.57 (s, 4H, form),
6.98 (d, 8H, Ar, J = 7.7 Hz), 6.82 (t, 8H, Ar, J = 7.7 Hz),
6.65 (t, 8H, Ar, J = 7.7 Hz), 6.07 (d, 8H, Ar, J = 7.7 Hz) and
2.19 (s, 24H, Me). Calc. for C60H60Cr2N8: C, 72.3; H, 6.1; N,
11.2. Found: C, 72.1; H, 6.0; N, 11.0%.
[Cr2(DPhOMeF)4] 1b. ϩFABMS: m/z = 1124 (Mϩ calc.
1125.2), 869 (Mϩ Ϫ DPhOMeF) and 562 (M2ϩ) observed. IR
(KBr disk): ν/cmϪ1 = 1624(s), 1606(s), 1584(s), 1509(s), 1444(m),
1428(w), 1376(m), 1331(m), 1263(w), 1243(w), 1199(w),
1
1110(m) and 1019(w). H-NMR (C6D6): δ 8.91 (s, 4H, form),
6.84–6.72 (m, 16H, Ar), 6.53 (t, 8H, Ar, J = 8.0 Hz), 6.18 (d, 8H,
Ar, J = 8.0 Hz) and 3.05 (s, 24H, OMe). Calc. for C60H60Cr2-
N8O8: C, 64.0; H, 5.4; N, 10.0. Found: C, 64.1; H, 5.3; N, 10.2%.
[Cr2(DPhClF)4] 1c. ϩFABMS: m/z = 1160 (Mϩ calc. 1160.5),
896 (Mϩ Ϫ DPhClF), 580 (M2ϩ) and 387 (M3ϩ) observed. IR
(KBr disk): ν/cmϪ1 = 1617(m), 1613(s), 1590(m), 1571(s),
1498(s), 1457(w), 1340(m), 1260(m), 1227(m), 1194(w), 1125(w)
CCDC reference number 186/1963.
Acknowledgements
We are grateful to the National Science Foundation and the
Robert A. Welch Foundation for financial support, and also to
Dr Xiaoping Wang for advice regarding X-ray crystallography.
1
and 1034(m). H-NMR (C6D6): δ 8.53 (s, 4H, form), 7.50 (d,
8H, Ar, J = 8.1 Hz), 7.04 (m, 8H, Ar), 6.93 (m, 8H, Ar) and
6.50 (d, 8H, Ar, J = 7.9 Hz). Calc. for C52H36Cl8Cr2N8: C, 53.8;
H, 3.1; N, 9.7. Found: C, 53.5; H, 2.9; N, 9.9%.
[Cr2(DPhBrF)4] 1d. ϩFABMS: m/z = 1518 (Mϩ calc. 1516.4),
1163 (Mϩ Ϫ DPhBrF) and 759 (M2ϩ) observed. IR (KBr disk):
ν/cmϪ1 = 1614(m), 1604(s), 1593(m), 1570(m), 1493(s), 1422(w),
1367(w), 1327(m), 1264(w), 1211(m), 1200(sh), 1192(sh) and
1022(m). 1H-NMR (C6D6): δ 8.76 (s, 4H, form), 7.66 (d, 8H, Ar,
J = 7.6 Hz), 7.02 (t, 8H, Ar, J = 7.6 Hz), 6.97 (t, 8H, Ar, J = 7.6
Hz) and 6.45 (d, 8H, Ar, J = 7.6 Hz). Calc. for C52H36Br8Cr2N8ؒ
2C6H5Me: C, 46.7; H, 3.1; N, 6.6. Found: C, 46.2; H, 2.6; N,
6.9%.
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