Synthesis of chromium(0) and molybdenum(0) bis (η6-arene) derivatives and their monoelectronic oxidation to [M(η6-arene) 2]+ cations

Article abstract of DOI:10.1039/b602783k

M(η⁶-arene)₂ species (M = Cr, arene = 1,3,5-Me₃C₆H₃; M = Mo, arene = 1,3,5-Me ₃C₆H₃, 1,3,5-ⁱPr₃C ₆H₃), have been prepared by a modified Fischer-Hafner synthesis or by metal vapour techniques. The reaction of Cr(η⁶-1, 3,5-Me₃C₆H₃)₂ with the fulvene derivatives pentacarbomethoxycyclopentadiene (pcmcpH), 1-benzoyl-6-hydroxy-6- phenylfulvene (dbcpH), or 1-benzoyl-3-nitro-6-hydroxy-6-phenylfulvene (dbncpH) proceeds with evolution of dihydrogen and formation of the ionic derivatives [Cr(η⁶-1,3,5-Me₃C₆H₃) ₂][E], where E = pcmcp, dbcp, or dbncp. Mo(η⁶-arene) ₂ derivatives (arene = toluene, 1,3,5-Me₃C ₆H₃, 1,3,5-ⁱPr₃C₆H ₃) are oxidized to [Mo(η⁶-arene)₂] ⁺ by pcmcpH. The crystal and molecular structures of [M(η⁶-1,3,5-R₃C₆H₃) ₂][pcmcp] (M = Cr, R = Me; M = Mo, R = Me, ⁱPr) have been solved by X-ray single crystal diffraction. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b602783k

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Synthesis of chromium(0) and molybdenum(0) bis (g6-arene) derivatives and
their monoelectronic oxidation to [M(g6-arene)2]+ cations
Lucia Calucci,a F. Geoffrey N. Cloke,b Ulli Englert,c Peter B. Hitchcock,b Guido Pampaloni,*d
Calogero Pinzino,a Filippo Puccinid and Manuel Volpe†b
Received 23rd February 2006, Accepted 1st June 2006
First published as an Advance Article on the web 22nd June 2006
DOI: 10.1039/b602783k
6
M(g -arene)2 species (M = Cr, arene = 1,3,5-Me3C6H3; M = Mo, arene = 1,3,5-Me3C6H3,
1,3,5-iPr3C6H3), have been prepared by a modified Fischer–Hafner synthesis or by metal vapour
6
techniques. The reaction of Cr(g -1,3,5-Me3C6H3)2 with the fulvene derivatives
pentacarbomethoxycyclopentadiene (pcmcpH), 1-benzoyl-6-hydroxy-6-phenylfulvene (dbcpH), or
1-benzoyl-3-nitro-6-hydroxy-6-phenylfulvene (dbncpH) proceeds with evolution of dihydrogen and
6
formation of the ionic derivatives [Cr(g -1,3,5-Me3C6H3)2][E], where E = pcmcp, dbcp, or dbncp.
6
Mo(g -arene)2 derivatives (arene = toluene, 1,3,5-Me3C6H3, 1,3,5-iPr3C6H3) are oxidized to
6
6
[Mo(g -arene)2]+ by pcmcpH. The crystal and molecular structures of [M(g -1,3,5-R3C6H3)2][pcmcp]
(M = Cr, R = Me; M = Mo, R = Me, iPr) have been solved by X-ray single crystal diffraction.
was almost the rule until the discovery of Cr(I) salts con-
Introduction
taining sterically demanding anions such as 1,2-dibenzoyl-
4-nitro-cyclopentadienyl ([dbncp]−), pentacarbomethoxycyclo-
pentadienyl ([pcmcp]−),7 1,2-dibenzoylcyclopentadienyl ([dbcp]−),
Fifty years have passed since Fischer and Hafner were successful1
in isolating a compound in which two neutral benzene molecules
6
6
and [B(C6F5)4]−, where the [Cr(g -toluene)2]+ cation shows a
were g -bound to a chromium metal centre. Since then, a large
6
number of transition metal bis(g -arene) derivatives have been
substantially cis-eclipsed conformation (i.e., eclipsed ring carbon
atoms with dihedral angles between the methyl groups ranging
from 0 to 6.5◦) of the aromatic ligands.8
obtained; they are compounds in which the metal is usually present
in a reduced oxidation state, the aromatic ligands are coplanar and
the alkyl ring-substituents can be eclipsed or staggered. The exact
conformation of the aromatic groups is determined by a number
of factors, such as the particular metal, the oxidation state and the
coordination environment. The literature shows several examples
of solid state structures of bis(g -arene) complexes encompassing
cationic and/or anionic derivatives and containing arenes with
different degrees of substitution.2
A similar dependence of the relative conformation of the arenes
on the anion occurs with polymethyl-substituted arene cations.9
Examples of molybdenum(I) cations are, however, very limited
6
in number, being restricted to [Mo(g -C6H6)2][FeBr4], (staggered)10
6
6
and [Mo(g -C6H5SiMe2H)2][BF4] (trans-eclipsed).11
During the course of our research on bis-arene derivatives
of early transition elements, we became interested in the solid
6
6
As far as chromium(I) bis(g -arene) derivatives are concerned,
state characterization of Cr(I) and Mo(I) bis(g -arene) species
6
6
the bis(g -benzene) cation, [M(g -C6H6)2]+, generally shows an
eclipsed conformation of the aromatic rings. Only a few cases
are known to deviate from this conformation, examples being
containing sterically demanding anions with electron-withdrawing
substituents, such as fulvene derivatives.8a,b This paper reports on
the synthesis and the structural characterization of novel Cr(I) and
6
6
6
[Cr(g -C6H6)2]PF6,3 [Cr(g -C6H6)2][MeOCrO3],4 and bis[bis(g -
6
Mo(I) derivatives containing the [M(g -arene)2]+ cations, with M =
2
benzene)-chromium](g -26,27-dicyano-26,27-dihydro-fullerene)-
Cr and arene = 1,3,5-Me3C6H3 (Mes), or M = Mo and arene =
Mes or 1,3,5-iPr3C6H3. DFT calculations are presented as a tool
to rationalize the observed solid state structures.
(7,8-dicyano-7,8-dihydro-40H-fulleren-40-yl)-(5,10,15,20-tetrap-
henylporphyrinato)-cobalt(II) (REFCODE5: XOFHIJ01).6
In the case of the bis-toluene cationic derivatives, the
trans-eclipsed conformation (i.e., eclipsed ring carbon atoms
with dihedral angles of ca. 180◦ between the methyl groups)
Results and discussion
Synthesis and spectral properties
aIstituto per i Processi Chimico-Fisici, CNR, Area della Ricerca, Via
G. Moruzzi 1, I-56124 Pisa, Italy
=
The bis-arene derivatives of Cr(0) and Mo(0) M(Mes)2 (M Cr,
Mo)12 have been prepared by the modified Fischer–Hafner synthe-
sis already successfully employed for the preparation of M(Mes)2
bDepartment of Chemistry, University of Sussex, Falmer, Brighton, UK,
BN1 9QJ
(M V, Nb14) or M(g -CH3C6H5)2 (M Cr, Mo). Cr(III) or
Mo(V) chlorides (CrCl3 and MoCl5) react with the Al–AlCl3–
mesitylene reducing system to afford a biphasic system. In the
presence of THF/DME, the crude primary product of the reaction
is further reduced by the excess of aluminium, affording medium
13
6
13
cInstitut fu¨r Anorganische Chemie der RWTH Aachen, Landoltweg 1,
D-52074 Aachen, Germany
=
=
dUniversita` di Pisa, Dipartimento di Chimica e Chimica Industriale,
Via Risorgimento 35, I-56126 Pisa, Italy. E-mail: pampa@dcci.unipi.it;
Fax: +39 50 2219246; Tel: +39 50 2219219
† Present Address: School of Chemistry, University of Nottingham,
Nottingham NG7 2RD, UK
=
to high yields of M(Mes)2 (M Cr, 62%; M = Mo, 29%).
4228 | Dalton Trans., 2006, 4228–4234
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Due to the well known reactivity of alkyl-substituted arenes
under the Fischer–Hafner reaction conditions,15 the isopropyl
substituted derivative was prepared in ca. 30% yield via MVS
techniques.
electron spin and the proton spins of the methyl group, being equal
to −0.28 G from DFT calculation.
The giso and AH values are in agreement with those reported for
the same cation in benzonitrile by Domrachev et al.16
Following the use of fulvene derivatives as oxidizing agents,8a,b
The EPR spectrum of 5 in isotropic solution of different solvents
(i.e. EtOH, THF, CH3CN) consists of a broad, unresolved signal
centered at giso = 1.9857; no resolution of the hyperfine structure
has been obtained by changing the temperature to optimize the
solvent viscosity with respect to the effective cation radius. In
fact, a progressive loss of spectral resolution can be expected
by increasing the methyl substitution on the aromatic rings due
to differences in the cation dynamics influenced by both solvent
6
Cr(Mes)2 and Mo(g -arene)2 (arene = toluene, mesitylene, 1,3,5-
iPr3C6H3) were reacted with pcmcpH; in the case of Cr(Mes)2,
the reaction was also extended to other fulvene derivatives:
1-benzoyl-6-hydroxy-6-phenylfulvene (dbcpH) and 1-benzoyl-3-
nitro-6-hydroxy-6-phenylfulvene (dbncpH), see Scheme 1.
6
and temperature, in analogy to the behaviour observed for [Cr(g -
arene)2]+ cations.17,18
X-Ray crystallography
Scheme 1 Synthesis of the chromium(I) and molybdenum(I) derivatives,
1–6.
Compound 1 crystallizes in the space group P212121 with four
independent cations and anions in the unit cell (Fig. 2). Fig. 3
reports a view of the asymmetric unit along the crystallographic
axis b. Three [Cr(Mes)2]+ cations (from now on defined as Cr1,
Cr3 and Cr4) are assembled in infinite columns, with the fourth
cation (Cr2) lying almost perpendicular to the axis of the column.
The average Cr–Mes geometry is characterized by a Cr–centroid
The Cr(I) and Mo(I) cations, 1–6, are thermally stable crystalline
compounds which slowly decompose in air. The IR spectra of 1–6
6
are similar to those of the corresponding g -toluene derivatives8a,b
and are characterized by strong bands at 1715–1681, 1556 and
1644 cm−1 attributed to the C O stretching vibrations of the
=
˚
distance of 1.629(4) A, with Cr–C bond lengths ranging from
[pcmcp]−, [dbcp]− and [dbncp]− anions.
˚
˚
2.121(5) to 2.180(5) A [average. 2.154(4) A]. The C–C bond
distances within the aromatic rings range from 1.388(7) to 1.538(7)
Compounds 4 and 5 have been studied by EPR spectroscopy.
6
The EPR spectrum of [Mo(g -toluene)2][pcmcp] in THF solution
˚
[average: 1.409(6) A]. Similar values have been observed in
6
(Fig. 1) shows a signal at giso = 1.9859 ascribable to the [Mo(g -
9b
˚
[Cr(Mes)2][E], E = TCNE (2.16 and 1.40 A), TCNQ (2.154
toluene)2]+ cation on the basis of the hyperfine structure. The signal
is characterized by the isotropic hyperfine coupling of the unpaired
electron spin with a set of ten equivalent protons (AH = 4.84 G) and
with the nuclear spin of the 95Mo species (I = 5/2, 15.90% natural
abundance, AMo = 14.99 G). The experimental value of AH, found
by fitting the spectral lineshape, is in good agreement (≤2% error)
with the theoretical mean value (4.75 G) determined by DFT
calculations. In fact, these calculations give isotropic hyperfine
coupling constants of 4.55, 4.94 and 4.65 G for protons in meta-
, ortho-, and para- positions with respect to the methyl group,
respectively. Unfortunately, the quite broad spectral linewidth (ca.
3 G) does not allow the different contributions to be resolved
in the experimental spectrum. Moreover, no experimental data
can be acquired on the hyperfine coupling between the unpaired
9c
6
˚
and 1.41 A), and in [Cr(g -toluene)2][pcmcp] [2.145(5) and
8b
˚
1.414(2) A].
The four independent cations, two of which are shown in Fig. 4,
differ with respect to the relative disposition of the two Mes rings,
the dihedral angles between methyl groups of different rings being
from 2.6◦, 9.9◦, 19.3◦, and 22.3◦ in the case of the Cr3, Cr2, Cr4
and Cr1 cations, respectively. It is interesting to observe that the
conformation that is expected to minimize the repulsions between
6
Fig. 1 EPR spectrum of [Mo(g -CH3C6H3)2][pcmcp] (4) (THF, 300 K).
A, experimental; B, simulated.
Fig. 2 View along a of the four independent [Cr(Mes)2]+ cations in the
structure of [Cr(Mes)2][pcmcp], (1).
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Fig. 5 ORTEP20 view of one of the four independent [pcmcp]− anions in
the structure of [Cr(Mes)2][pcmcp], (1). Hydrogen atoms have been omitted
for clarity. Ellipsoids are drawn at 50% probability.
Fig. 3 Packing diagram (PLUTON19) along the crystallographic b axis;
the stacked independent cations Cr1, Cr3 and Cr4 (cf . Fig. 2) are shown
in projection.
Fig. 4 ORTEP20 view of two of the four independent [Cr(Mes)2]+ cations
in the asymmetric unit of [Cr(Mes)2][pcmcp] (1). Hydrogen atoms have
been omitted for clarity. Ellipsoids are drawn at 50% probability.
the arene rings is that with a 30◦ dihedral angle between methyl
groups (corresponding to a staggered conformation of both methyl
and ring carbon atoms). Considering that the energy differences
between most stable staggered and the eclipsed conformation
are remarkably low (vide infra), the observed conformations will
necessarily be those that minimize the lattice energy of the system.
Fig. 6 PLATON19 view of the [Mo(Mes)2]+ cation in [Mo(Mes)2][pcmcp]
(5). Hydrogen atoms have been omitted for clarity.
◦
˚
Table 1 Selected bond distances (A) and angles ( ) of [Mo(Mes)2][pcmcp]
(5), with estimated standard uncertainties in parentheses referring to the
least significant digit
6
The recently reported [Cr(g -toluene)2][E] system8a,b offers an-
other example of this influence: two substantially cis- and trans-
eclipsed conformers are observed for E = pcmcp, while only the
cis-eclipsed conformer is present for E = dbcp and dbncp.
The four independent ani◦ons exhibit planar rings (the inner
Mo1–C2
Mo1–C1
Mo1–C5
C1–C2
C2–C3
C4–C5
2.255(4)
2.270(4)
2.284(4)
1.405(6)
1.422(6)
1.405(6)
Mo1–C6
Mo1–C4
Mo1–C3
C1–C6
C3–C4
C5–C6
2.262(4)
2.273(4)
2.291(4)
1.419(6)
1.424(5)
1.425(5)
=
C–C–C angle is close to 108 ) and average O–CH3 and C( O)–
◦
˚
O–CH3 bond distances and angles of 1.458(6) A and 114.6(4) ,
respectively. Fig. 5 shows one of the [pcmcp]− anions.
C1–Mo1–C2
C2–Mo1–C3
C4–Mo1–C5
36.17(16)
36.46(15)
35.91(14)
C1–Mo1–C6
C3–Mo1–C4
C5–Mo1–C6
36.49(15)
36.36(14)
36.53(14)
Compound 5 crystallizes in the monoclinic P21/c space group
with one molecule in the asymmetric unit. Fig. 6 shows a view
of the [Mo(Mes)2]+ cation, and a list of bond distances and
angles is reported in Table 1. The cation presents a staggered
conformation of both the methyl groups and the carbon atoms
of the aromatic rings (the dihedral angle between corresponding
methyl groups is 1.6◦) strictly resembling that of the [V(Mes)2]+
cation in [V(Mes)2][AlCl4].13
˚
Mo–centroid and Mo–C distances of 1.7842(15)–1.7772(15) A
and 2.275(3) (av.) A, respectively. Similar distances have been
reported for [Mo(g -C6H6)2][FeBr4] (1.78(1) and 2.26(3) A,
respectively)10 and in [Mo(g -C6H5SiMe2H)2][BF4] (1.789 and
˚
6
˚
6
11
˚
The aromatic rings are almost parallel, subtending a dihedral
2.273 A, respectively), which represent the only other examples
angle of 2.51(17)◦; the centroid–Mo–centroid is 178.3◦ with
of Mo bis(arene) cations reported in the literature.
4230 | Dalton Trans., 2006, 4228–4234
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The [pcmcp]− anion shows disorder in four of the five peripheral
methoxy groups.
is to be ascribed to the three bulky isopropyl groups, which adopt
the staggered conformation to minimize steric interactions; the
6
In order to point out some influence of the ring substituents
on the relative conformations of the arenes, the sterically hin-
same behaviour is observed on going from [Cr(g -C6H6)2]+ to
6
[Cr(g -1,3,5-iPr3C6H3)2]+.9a It is worth noting that the C–C–C
6
dered Mo(g -1,3,5-iPr3C6H3)2, as obtained by co-condensation
angle at substituted carbons on the arene rings are all smaller
than 120◦, consistent with the general rule found by Domenicano
et al.,21 stating that the angles of the quaternary carbon atoms on
the coordinated arene rings are bigger or smaller than the expected
120◦ depending on the electron withdrawing or donating nature
of the carbon atom substituents.
CCDC reference numbers 607082–607084.
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b602783k
of Mo and 1,3,5-iPr3C6H3 vapours, was reacted with pcmcpH
6
(Scheme 1) and the crystal and molecular structure of [M(g -1,3,5-
R3C6H3)2][pcmcp] has been studied.
Compound 6 crystallizes as a bis-THF solvate with two
independent cations in the unit cell; the Mo atoms being located
on inversion centres and the anions in general positions. Due to
the two cations showing similar characteristics, in the following
discussion reference will be made to one of the two independent
molecules only. Fig. 7 shows the cation structure, with a selection
of bond distances and angles given in Table 2.
DFT calculations
Some of us have recently found that the energy gap between the
6
cis- and the trans-eclipsed form of the [M(g -toluene)2]+ (M = Cr,
Mo) species in vacuo is low (M = Cr,8b 0.09 kcal mol-1; M = Mo,22
0.13 kcal mol-1), thus suggesting that the conformations of the
cation can be dictated by cation–anion interactions in the solid
state.
DFT calculations performed in the present work on [M(Mes)2]+
derivatives show that, in vacuo, the energy gap between the eclipsed
and the staggered conformation (alkyl groups at 60◦ with eclipsed
ring carbon atoms) is larger than in the case of the toluene
derivatives (M = Cr, 0.30 kcal mol-1; M = Mo, 0.32 kcal mol-1) but
still small enough to be overcome by intermolecular interactions
in the solid state.
The staggered conformation is the only one allowed even
6
in vacuo for the [Mo(g -1,3,5-iPr3C6H3)2]+ cation according to
DFT calculations, the rotation of the rings toward an eclipsed
conformation causing an increase of the Mo–arene bond distance.
6
It should be noted that dynamic NMR data on [Cr(g -1,3,5-
6
Fig.
7
ORTEP20 view of the [Mo(g -1,3,5-iPr3C6H3)2]+ cation in
iPr3C6H3)2]I, indicate that restricted rotation around the Cr–
arene bond occurs even in solution, causing the existence of two
unequally populated sites between the staggered and the eclipsed
conformer, with an approximate barrier to rotation of 10 kcal
mol-1.9a
[Mo(g -1,3,5-iPr3C6H3)2][pcmcp] (6). Hydrogen atoms have been omitted
for clarity. Ellipsoids are drawn at 50% probability.
6
The arene rings are essentially coplanar and adopt a staggered
conformation of the iso-propyl substituents and an eclipsed
configuration with regard to the ring carbon atoms. A similar
arrangement of aromatic rings has been observed in the lighter
Conclusions
congener [Cr(g -1,3,5-iPr3C6H3)2]I,9a the dihedral angle being 53◦
6
(av.) in the Mo derivative and 52◦ (av.) in the Cr compound.
In this paper, we have presented an improved synthesis of previ-
ously known M(Mes)2 derivatives (M = Cr, Mo) and demonstrated
their ability to undergo one-electron oxidation reactions with
fulvenes containing electron-withdrawing substituents. The new
˚
The Mo–centroid distance of 1.805(5) A is slightly longer than
those found in other cationic Mo(I) complexes.10,11 This elongation
6
Mo(g -1,3,5-iPr3C6H3)2 derivative has been synthesized via MVS
◦
6
˚
Table 2 Selected bond distances (A) and angles ( ) of [Mo(g -1,3,5-
iPr3C6H3)2][pcmcp] (6), with estimated standard uncertainties in paren-
techniques and its monoelectronic oxidation product character-
ized via X-ray diffraction experiment. DFT calculations have been
performed on these cations to assess the energy barrier involved
in ring rotation in vacuo and it has been found that, as this barrier
is very low, the difference between the eclipsed and staggered
conformationis probablybeingimposedbypackingeffectsthatare
able to overcome the electronic requirements for the ring carbons
to be staggered or eclipsed. Striking evidence of this has been
shown by the crystal structures of [M(Mes)2][pcmcp], in which four
different conformations of the cation have been observed for M =
Cr, whereas a rather surprising full eclipsed conformation has been
found in the case of the Mo derivative. It should be pointed out
theses referring to the least significant digit
Mo1–C6
Mo1–C2
Mo1–C5
C(1)–C(2)
C(2)–C(3)
C(4)–C(5)
2.277(4)
2.290(5)
2.305(4)
1.426(6)
1.413(7)
1.422(6)
Mo1–C4
Mo1–C1
Mo1–C3
C(1)–C(6)
C(3)–C(4)
C(5)–C(6)
2.278(4)
2.304(5)
2.313(4)
1.427(6)
1.424(6)
1.404(7)
C1–Mo1–C2
C2–Mo1–C3
C4–Mo1–C5
36.18(16)
35.75(16)
36.14(16)
C1–Mo1–C6
C3–Mo1–C4
C5–Mo1–C6
36.29(16)
36.12(16)
36.68(17)
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6
that in the case of [Cr(g -toluene)2][E] (E = pcmcp, dbcp, dbncp),
of the resulting deep green solution, heptane was stripped off
and the residual solid sublimed at 105 C/0.01 mmHg to afford
Cr(Mes)2 as an air-sensitive, dark brown crystalline solid (6.962 g,
62% yield), which was identified on the basis of IR and NMR
spectroscopy.
◦
cis-eclipsed conformations of the cations have been observed for
E = dbcp, dbncp, and both cis- and trans-eclipsed conformers are
present in the crystals of the pcmcp derivative.
Experimental
Preparation of [Cr(Mes)2][E], E = pcmcp (1), dbcp (2), dbncp (3)
General methods
Compound 1. A 100 ml round-bottom flask was charged with
a solution of Cr(Mes)2 (0.443 g, 1.5 mmol) in 50 ml THF. Solid
pcmcpH (0.538 g, 1.5 mmol) was added to the brown solution,
which turned quickly into an orange suspension with gas (H2)
evolution. The reaction mixture was stirred at room temperature
for 1 h, the solid product being collected via filtration, washed
with THF (5 ml) and dried in vacuo to afford 1 as a red–orange
microcrystalline solid (0.574 g, 58.6% yield). Crystals suitable for
an X-ray single crystal diffraction experiment were obtained by
slowly cooling (243 K) a saturated THF solution. Found: C, 61.1;
H, 5.7. C33H39CrO10 requires: C, 61.2; H, 6.0. IR (Nujol, cm−1):
3064m, 1715s, 1681s, 1273m, 1164s, 1062m, 1033mw, 1013m,
937m, 833m, 786m, 751w, 436s.
All manipulations of air and/or moisture sensitive compounds
were performed under an atmosphere of pre-purified Ar using
standard Schlenk techniques. The reaction vessels were oven dried
◦
at 150 C prior to use, evacuated (10−2 mmHg) and then filled
with Ar. Some manipulations of air-sensitive compounds were
performed inside a Miller–Howe glovebox under a dinitrogen
atmosphere (≤1 ppm O2, ≤5 ppm H2O).
IR spectra were recorded with a Perkin Elmer model FT
1725X instrument on Nujol mulls prepared under exclusion of
moisture and dioxygen, using KBr windows. NMR spectra (1H,
200 MHz; 13C, 50.31 MHz, referenced to TMS) were recorded
with a Varian Gemini 200BB spectrometer. EPR spectra were
recorded using a Varian (Palo Alto, ca, USA) E112 spectrometer
operating at X band, equipped with a Varian E257 temperature
control unit and interfaced to an IPC 610/P566C industrial grade
Advantech computer by means of an acquisition board23 and a
software package specifically designed for EPR experiments.24
Solution experiments were run by placing the sample (typically
15–20 lL) into quartz tubes fitted with a quartz–Pyrex joint
and a Bibby Quickfit Rotaflo PTFE tap (DISA, Milan). EPR
spectra were simulated using the WinSim 32 program.25 DFT
geometry optimization and calculation of the electron spin density
distribution were performed using the Linux version of the
Spartan’02 software.26 We adopted the BP86 scheme, which uses
the functional proposed by Becke27 and Perdew,28 and the 6-
31G** basis set appropriate for calculations of split-valence-
plus-polarization quality. When comparing the theoretical and
experimental isotropic hyperfine coupling constants, we should
take into account the limited basis set used, the influence of the
solvent, and the vibrational and environmental effects to produce
accurate results. Since it is difficult to assess the importance of
such effects, a semi-empirical extrapolation procedure29 has been
adopted to correct the theoretical values and compare them with
the experimentally determined values.
Compound 2. Cr(Mes)2 (0.557 g, 1.9 mmol) was added to a
stirred suspension of dbcpH (0.525 g, 1.9 mmol) in THF (100
ml), causing the formation of an orange–brown reaction mixture,
which was refluxed for 7 h. After reduction of the volume to ca.
15 ml and addition of heptane (25 ml), the microcrystalline solid
was recovered by filtration, washed with heptane (2 × 5 ml) and
dried in vacuo, affording 2 as a yellow–orange solid (0.891 g, 78%
yield). Found: C, 77.5; H, 6.7. C37H37CrO2 requires: C, 78.0; H,
6.5. IR (Nujol, cm−1): 3053m, 1618s, 1600w, 1566w, 1489w, 1423m,
1349s, 1307w, 1285m-w, 1123w, 1055w, 1034w, 1016m, 849s, 740m,
712s, 667m, 574w, 437m-s.
Compound 3. Cr(Mes)2 (0.437 g, 1.5 mmol) was added to a
stirred suspension of dbncpH (0.467 g, 1.5 mmol) in THF (75
ml), causing the immediate formation of a brilliant red reaction
mixture. After overnight stirring, the mixture changed to a pale
yellow suspension, which was filtered and the solid washed with
THF (2 × 5 ml) and dried in vacuo, affording 3 as a pale
yellow solid (0.595 g, 65% yield). Found: C, 72.0; H, 5.8; N, 1.6.
C37H36CrNO4 requires: C, 72.8; H, 5.9; N, 2.3. IR (Nujol, cm−1):
3047m, 1644s, 1624m, 1595m, 1575w, 1398s, 1308m, 1282m, 1260s,
1151m, 1081w, 1028w, 865s, 783w, 758w, 438s.
Chromium(III)-, molybdenum(V)- and aluminium chloride,
finely divided aluminium and 1,3,5-iPr3C6H3, were commercial
products used as received.
X-Ray structure determination of [Cr(Mes)2][pcmcp] (1). Crys-
tals were obtained from THF at low temperature (ca. 243 K).
Data were collected with a Bruker SMART APEX CCD area
6
[Mo(g -toluene)2,13 pcmcpH,30 dbcpH,31 and dbncpH32 were
˚
detector on a D8 goniometer (Mo-Ka radiation, k = 0.71073 A,
prepared according to published procedures.
graphite monochromator) at 110(2) K. On a crystal of approximate
dimensions 0.38 × 0.15 × 0.12 mm, 159 047 reflections with indices
−24 ≤ h ≤ 24, −26 ≤ k ≤ 26, −39 ≤ l ≤ 40 were collected with
the x-scan method in the scan range 1.17 ≤ H ≤ 27.08◦. An
empirical absorption correction (min. trans. 0.86, max. trans 0.95)
Preparation of Cr(Mes)2. A 500 ml round-bottom flask was
charged with finely divided Al (1.53 g, 56.7 mmol) and AlCl3
(6.17 g, 46.4 mmol); Mes (100 ml) was added and the resultant
mixture was stirred at room temperature for 1 h. CrCl3 (6.089 g,
38.4 mmol) was then added and the resulting brown suspension
heated at 120 ◦C for 5 h. After cooling, two layers were observed
and a mixture of THF–DME (100 ml, 75/25 v/v) was added
dropwise. The dark mixture was refluxed for 8 h, then cooled
down to room temperature, filtered and the solid washed with
heptane (5 × 25 ml). The solvent was then removed in vacuo
and the residual solid was extracted in heptane. After filtration
was applied before averaging symmetry equivalent data (Rint
=
0.1216). After merging, 27 282 independent reflections remained
for structure solution by direct methods.33 The structure model
was completed by Fourier difference syntheses and refined with
blocked least-squares on F2.34 Convergence was reached for 27 282
reflections and 1630 variables at agreement factors of wR2 = 0.151
(all data), R1 = 0.074 [observations with I > 2r(I)]. Max. and min.
4232 | Dalton Trans., 2006, 4228–4234
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Table 3 Crystal data and structure refinement for [Cr(Mes)2][pcmcp] (1),
orange suspension with gas evolution by the addition of pcmcpH
(0.627 g, 1.76 mmol). The orange precipitate was collected by
filtration and dried in vacuo affording 4 as a yellow–orange
microcrystalline solid (0.535 g). The mother liquor was slowly
cooled to 278 K to give another crop of solid (0.100 g, 57% total
yield) in the form of long, yellow, air- and moisture-sensitive
needles. Found: C, 54.0; H, 5.3. C29H31MoO10 requires: C, 54.8;
H, 4.9. IR (Nujol, cm−1): 3069w, 1719 vs, 1684vs, 1272m, 1197vs,
1172vs, 1063m, 1011m, 801w, 789w.
6
and [Mo(g -1,3,5-R3C6H3)2][pcmcp], R = Me (5), iPr (6)
Compound
1
5
6·2THF
Empirical formula
Formula weight
T/K
C33H39CrO10
647.6
110(2)
Orthorhombic
19.0585(12)
20.8529(12)
31.3141(18)
—
C33H39MoO10
691.6
120(2)
Monoclinic
12.0547(19)
15.6785(12)
20.1419(16)
—
122.744(2)
—
3201.9(4)
P21/c
4
C53H79MoO12
1004.1
173(2)
Crystal system
Triclinic
˚
a/A
11.6201(5)
12.7525(5)
18.5897(7)
75.820(2)
84.673(2)
76.999(2)
2600.8(2)
˚
b/A
˚
c/A
Preparation of [Mo(g6-1,3,5-R3C6H3)2][pcmcp], R = Me (5),
iPr, (6). The preparation of 5 is described in detail, and 6 is
performed in a similar way. A solution of pcmcpH (0.692 g,
1.94 mmol) in THF (50 ml) was added to Mo(Mes)2 (0.635 g,
1.89 mmol). The solution turned red–orange with H2 evolution (by
GC) and separation of a red–orange solid, which was recovered
by filtration and dried in vacuo affording 5 (0.56 g, 43% yield)
as an orange microcrystalline solid. Found: C, 56.3; H, 5.3.
C33H39MoO10 requires: C, 57.3; H, 5.6. IR (Nujol, cm−1): 3055m,
1715s, 1687s, 1536w, 1521w, 1300w, 1262w, 1191w, 1165w, 1063w,
1034w, 1018m, 995w, 947mw, 836mw, 799w, 786w, 777w, 754w,
722w, 510w, 490w.
a/◦
b/◦
c /◦
—
—
3
˚
V/A
12445.0(13)
P212121
16
0.425
159047
¯
Space group
Z
P1
2
l(Mo-Ka)/mm−1
Reflections
collected
0.466
41121
0.31
33339
Independent
reflections
Rint
27282
7306
9150
0.1216
0.0706
0.054
Final R indices
[I > 2r(I)]
R1 = 0.074
wR2 = 0.141
R1 = 0.096
wR2 = 0.151
0.055
0.116
0.076
0.126
0.064
0.159
0.094
0.177
[all data]
Compound 6. It was isolated as the bis-THF adduct, 6·2THF,
yellow–green, 52% yield. Found: C, 63.0; H, 8.2. C53H79MoO12
requires: C, 63.3; H, 7.9. IR (Nujol + hexachlorobutadiene, cm−1):
3058w, 2962s, 2872s, 1693vs, 1460s, 1366w, 1269m, 1198s, 1167s,
1062mw, 1012mw, 863w, 833w.
−3
˚
from a final Fourier map were 0.702 and −0.652 e A . A selection
of crystal parameters are given in Table 3.
X-Ray structure determination of 5. Crystals were obtained
by cooling the mother liquor at ca. 4 ◦C for some days. Data
were collected with Mo-Ka radiation on a CCD area detector
diffractometer equipped with a graphite monochromator, k =
Preparation of Mo(Mes)2. A 500 ml round-bottom flask was
charged with finely divided aluminium (3.49 g, 129.3 mmol) and
AlCl3 (5.17 g, 38.8 mmol), then Mes (100 ml) was added and the
resultant mixture was stirred at room temperature for 1 h. MoCl5
(8.837 g, 32.3 mmol) was then added and the brown suspension
heated at 135 ◦C for 30 h. After cooling, two layers were observed
and a mixture of THF–DME (100 ml, 80/20 v/v) was added
dropwise. The dark mixture was refluxed for 6 h, then cooled down
to room temperature and filtered. The solvent was then removed
in vacuo and the residual dissolved in heptane. After filtration of
the resulting deep green solution, heptane was stripped off and the
residual solid sublimed at 130 ◦C/0.01 mmHg to afford Mo(Mes)2
as an air-sensitive, dark green crystalline solid (3.148 g, 29% yield),
which was identified on the basis of IR and NMR spectra.
˚
0.71073 A at 120(2) K on a crystal of approximate dimensions
0.62 × 0.23 × 0.07 mm. 41 121 reflections with indices −15 ≤ h ≤
15, −20 ≤ k ≤ 20, −26 ≤ l ≤ 26, were collected with the x-scans
method in the range 2.13 ≤ H ≤ 27.45◦. An empirical absorption
correction (min. trans. 0.761, max. trans. 0.968) was applied
before averaging symmetry equivalent data (Rint = 0.0706). After
merging, 7306 independent reflections remained for structure
solution by direct methods.33 The structure model was completed
by Fourier difference syntheses and refined with full-matrix least-
squares on F2.34 Convergence was reached for 7306 reflections and
404 variables at agreement factors of wR2 = 0.126 (all data), R1 =
0.055 [observations with I > 2r(I)]. Max. and min. from a final
Preparation of Mo(g6-1,3,5-iPr3C6H3)2. In a typical experi-
ment, pre-melted and degassed Mo metal (ca.0.5 g) was electron-
beam evaporated at a power input of ca. 800 W and co-condensed
with excess 1,3,5-iPr3C6H3 (30 g) at 77 K for 3 h. After this time,
the reaction vessel was allowed to reach room temperature under
a dinitrogen atmosphere and the black product was washed from
the reactor walls with warm hexane. The resultant black–green
suspension was then filtered from excess bulk metal through pre-
dried Celite. The solvent was removed from the green solution
and the crude product was subjected to a double, high-vacuum
sublimation to remove unreacted iPr3C6H3. The product was then
−3
˚
Fourier map were 0.837 and −0.687 e A .
X-Ray structure determination of 6. Crystals suitable for the
X-ray study were obtained from THF at room temperature. Data
were collected with Mo-Ka radiation on a KappaCCD area
detector diffractometer equipped with a graphite monochromator,
˚
k = 0.71073 A at 173(2) K on a crystal of approximate dimensions
0.40 × 0.30 × 0.05 mm. 33 339 reflections with indices −13 ≤ h ≤
13, −15 ≤ k ≤ 15, −22 ≤ l ≤ 22, were collected with the x-scans
method in the range 3.73 ≤ H ≤ 25.24◦. An empirical absorption
correction (min. trans. 0.890, max. trans 0.985) was applied before
averaging symmetry equivalent data (Rint = 0.054). After merging,
9150 independent reflections remained for structure solution by
direct methods.33 The structure model was completed by Fourier
difference syntheses and refined with full-matrix least-squares
on F2.34 The solvate molecules were refined with restrained
6
recrystallized from pentane at 193 K to afford pure Mo(g -1,3,5-
iPr3C6H3)2 (ca. 0.8 g, 30% yield based on Mo).
6
Preparation of [Mo(g6-CH3C6H3)2][pcmcp] (4). Mo(g -
CH3C6H5)2 (0.488 g, 1.74 mmol) was dissolved in THF (25 ml)
giving an emerald green solution, which rapidly turned to a deep
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1,2- and 1,3-C · · · C distances. Convergence was reached for 9150
reflections and 598 variables at agreement factors of wR2 = 0.177
(all data), R1 = 0.064 [observations with I > 2r(I)]. Max. and min.
P. Skurski, J. Simons, B. C. Dunn, E. M. Eyring and J. S. Miller, J. Org.
Chem., 2003, 68, 3367.
10 D. O’Hare, M. Kurmoo, R. Lewis and H. Powell, J. Chem. Soc., Dalton
Trans., 1992, 1351.
11 M. L. H. Green, I. Treurnicht, J. A. Bandy, A. Gourdon and K. Prout,
J. Organomet. Chem., 1986, 306, 145.
−3
˚
from a final Fourier map were 1.71 and −1.13 e A near the
solvate molecules.
12 Cr(Mes)2:G. Huttner, E. O. Fischer and C. Elschenbroich,
J. Organomet. Chem., 1965, 3, 330; W. M. Lamanna, J. Am. Chem.
Soc., 1986, 108, 2096; V. Graves and J. J. Lagowski, Inorg. Chem.,
1976, 15, 577; M. L. H. Green and W. H. Silverthorn, J. Chem. Soc.,
Dalton Trans., 1973, 301; Mo(Mes)2:; F. W. S. Benfield, M. L. H. Green,
J. S. Ogden and D. Young, J. Chem. Soc., Chem. Commun., 1973,
866.
Acknowledgements
The authors wish to thank the Ministero dell’Istruzione,
dell’Universita` e della Ricerca (MIUR, Roma), Programmi di
Ricerca Scientifica di Notevole Interesse Nazionale, Cofinanzi-
amento 2004–2005, the Consiglio Nazionale delle Ricerche
(C.N.R.) Progetto CNR-MIUR “Sviluppo di microcelle a com-
bustibile”, and the EPSRC (UK) for financial support. M.V.
wishes to acknowledge the European Community for a Marie
Curie grant.
13 F. Calderazzo, R. Invernizzi, F. Marchetti, F. Masi, A. Moalli, G.
Pampaloni and L. Rocchi, Gazz. Chim. Ital., 1993, 123, 53.
14 F. Calderazzo, G. Pampaloni, L. Rocchi, J. Stra¨hle and K. Wurst,
J. Organomet. Chem., 1991, 413, 91.
15 R. Davis and L. A. P. Kane-Maguire, Chromium Compounds with
2
8
g –g Carbon Ligands in Comprehensive Organometallic Chemistry, ed.
G. Wilkinson, F. G. A. Stone and E. W. Abel, Pergamon Press, New
York, 1982, vol. 3, p. 953.
16 G. A. Domrachev, Yu. A. Shevelev, V. K. Cherkasov, G. V. Markin,
G. K. Fukin, S. Ya. Khorshev, B. S. Kaverin and V. L. Karnatchevich,
Russ. Chem. Bull. Int. Ed., 2004, 53, 2056.
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4234 | Dalton Trans., 2006, 4228–4234
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The Royal Society of Chemistry 2006
©