One-electron redox reactions involving chromium(0) and molybdenum(0) bis-1,3,5-trimethylbenzene derivatives

Article abstract of DOI:10.1016/j.jorganchem.2007.01.044

The reaction of M(η⁶-1,3,5-Me₃C₆H₃)₂, M = Cr, Mo, with the tetrahalides of Groups 4 and 5 elements proceeds with the monoelectronic oxidation of the metal bis-arene to the [M(η⁶-Me₃

Full text of DOI:10.1016/j.jorganchem.2007.01.044

Journal of Organometallic Chemistry 692 (2007) 3144–3150
www.elsevier.com/locate/jorganchem
One-electron redox reactions involving chromium(0)
and molybdenum(0) bis-1,3,5-trimethylbenzene derivatives
a
b,
b
b,1
Ulli Englert , Guido Pampaloni *, Filippo Puccini , Manuel Volpe
a
Institut fu¨r Anorganische Chemie der RWTH Aachen, Landoltweg 1, D-52074 Aachen, Germany
`
Universita di Pisa, Dipartimento di Chimica e Chimica Industriale, Via Risorgimento 35, I-56126 Pisa, Italy
b
Received 22 November 2006; received in revised form 23 January 2007; accepted 24 January 2007
Available online 30 January 2007
Abstract
The reaction of M(g6-1,3,5-Me3C6H3)2, M = Cr, Mo, with the tetrahalides of Groups 4 and 5 elements proceeds with the monoelec-
tronic oxidation of the metal bis-arene to the [M(g6-Me3C6H3)2]+ cation. In the case of MX4, M = Ti, X = Cl, Br, M = V, X = Cl, and
of Nb2Cl10 the reduction products are the titanium(III), vanadium(III) halides and the niobium(IV) chloride, isolated as the solvate
anions [MCl4(THF)2]ꢀ and [NbCl4(CH3CN)]ꢀ. The reaction of the tetrachloro complexes MCl4(THF)2, M = Zr, Hf, with Cr(g6-
1,3,5-Me3C6H3)2 in THF produces the ionic [Cr(g6-1,3,5-Me3C6H3)2][MCl5(THF)], which has been characterized by single-crystal
X-ray diffraction in the case of hafnium.
ꢀ 2007 Elsevier B.V. All rights reserved.
Keywords: Redox reactions; One-electron oxidation; Chromium; Molybdenum; Structure
1. Introduction
In view of the increased reducing power [4] and stability
[5] of the bis-arenes derivatives on increasing the methyl
substitution on the aromatic rings, and aiming to find
out new synthetic procedures to bimetallic compounds
which may act as olefin polymerization catalytic precursors
[1,2], we decided to study the reactivity of the trimethylben-
zene derivatives of Cr(0) and Mo(0), M(g6-1,3,5-
Me3C6H3)2, with Groups 4 and 5 metal halides.
It is known that metal carbonyls such as V(CO)6,
Cr(CO)6 and Mo(CO)6 react with TiCl4 to afford binary
chlorides of general formula MCln Æ nTiCl3 [1]. Some of
us have extended this reaction to M(g6-1,3,5-Me3C6H3)2,
M = V, Nb, showing the formation of solid compounds
of general formula MTinClm, with n and m depending on
the M(g6-1,3,5-Me3C6H3)2/TiCl4 molar ratio [2a]. As far
as the arene derivatives of Group 6 are concerned, it has
been recently reported that M(g6-MeC6H5)2, M = Cr,
Mo, reacts with TiCl4 to give [Cr(g6-MeC6H5)2][-
TiCl4(THF)2] after treatment of the primary product with
THF [3]. On the other hand, the reactivity of the cited tol-
uene derivatives with the tetrachlorides of zirconium(IV)
and hafnium(IV) did not allow the isolation of well-defined
compounds due to partial loss of the arene rings.
2. Results and discussion
The study of the reactivity of M(g6-1,3,5-Me3C6H3)2,
M = Cr, Mo, with metal halides was preceded by experi-
ments aimed to elucidate some difficulties recently encoun-
tered in the synthesis of the bis-trimethylbenzene
compounds of chromium [6]. As a matter of fact, M
(g6-1,3,5-Me3C6H3)2, M = Cr, Mo, could be obtained in
60% (Cr) and 30% (Mo) yields by a modification of the ori-
ginal Fischer–Hafner procedure used to prepare Cr(g6-
C6H6)2 [7]. All attempts to increase the yields of the chro-
mium(0) compound were unsuccessful. On the other hand,
the same procedure applied to the vanadium/1,3,5-trimeth-
*
Corresponding author. Tel.: +39 050 2219 219; fax: +39 050 2219 246.
E-mail address: pampa@dcci.unipi.it (G. Pampaloni).
Present address: School of Chemistry, University of Nottingham,
1
Nottingham NG7 2RD, UK.
0022-328X/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2007.01.044

U. Englert et al. / Journal of Organometallic Chemistry 692 (2007) 3144–3150
3145
ylbenzene- and to the chromium/toluene systems gave
almost quantitative yield of M(g6-arene)2 [8].
with Cr(g6-1,3,5-Me3C6H3)2 led to the slow formation of
slightly soluble yellow solids containing zirconium(IV) [or
hafnium(IV)] and chromium(I) that analyzed as [Cr(g6-
1,3,5-Me3C6H3)2][MCl5(THF)]. The reaction was usually
carried out with a 1/1 metal/Cr(g6-1,3,5-Me3C6H3)2 molar
ratio and the yields, based on the tetrachloro-complex
introduced, never exceeded 50%, the remaining unac-
counted metal probably being in a reduced form in solu-
tion. Thus, the reaction can be represented as in the
following equation:
Although the lower yields in the case of molybdenum
might be due to the instability of the bis(g6-arene) deriva-
tives of molybdenum(0) (which may react with loss of one
or two aromatic rings [3]), the question remained open in
the case of chromium bis(1,3,5-trimethylbenzene).
A reason for this different behaviour could be the higher
stability of the [Cr(g6-1,3,5-Me3C6H3)2]+ cation with
respect to [V(g6-1,3,5-Me3C6H3)2]+. Therefore, we decided
to compare the redox properties of the chromium bis-arene
species with those of the vanadium derivatives under reac-
tion conditions as similar as possible to those typical of the
second step of the Fischer–Hafner synthesis [8]. We used
cobaltocene, a species which has widely been used in orga-
nometallic chemistry as a monoelectronic reducing reagent
in aprotic media [8,9], to test the redox properties of chro-
mium(I) compounds, and we found that only the reactions
reported in Eqs. (1) and (2) (and not the reverse ones) take
place.
Cr(g6-1,3,5-Me3C6H3)2 + 2MCl4(THF)2 + nTHF
! [Cr(g6-1,3,5-Me3C6H3)2][MCl5(THF)] + “MCl3(THF)nþ3
”
where M = Zr, Hf
ð5Þ
The different behaviour of MCl4(THF)2 with respect to
[Cr(g6-1,3,5-Me3C6H3)2], see Eqs. (3) and (5), can be
related to the higher tendency of TiX4 to undergo reduc-
tion with respect to the heavier congeners [11]. Data from
the literature suggest that the reduction of zirconium(IV)
and hafnium(IV) complexes is generally a less favourable
process than that of the corresponding titanium species.
For example, the bis(cyclopentadienyl) derivatives
MCp2Cl2 (M = Zr, Hf) undergo reduction at a more nega-
tive potential than the corresponding titanium analogue
[12]. Besides, the M–halogen bond strengths increase from
titanium to zirconium and hafnium [11].
[Cr(g6-MeC6H5)2]þ + CoCp2 ! Cr(g6-MeC6H5)2 + [CoCp2]þ
ð1Þ
Cr(g6-1,3,5-Me3C6H3)2 + [CoCp2]þ
! [Cr(g6-1,3,5-Me3C6H3)2]þ + CoCp2
ð2Þ
These results are in agreement with the redox potentials
measured for the couples [CoCp2]+/CoCp2 (Eꢁ = ꢀ0.93 V
vs. SCE in THF) [9c], [Cr(g6-MeC6H5)2]+/Cr(g6-
MeC6H5)2 (Eꢁ = ꢀ0.86 V vs. SCE in DMSO) [4d], and
[Cr(g6-1,3,5-Me3C6H3)2]+/Cr(g6-1,3, 5-Me3C6H3)2 (Eꢁ =
ꢀ0.98 V vs. SCE in DMSO) [4d]. Taking into account that
both [V(g6-arene)2]+, arene = toluene, 1,3,5-trimethylben-
zene, are reduced by cobaltocene [8] and considering that
the M(I) ! M(0) reduction is a pivotal step in the
Fischer–Hafner synthesis, the higher tendency to reduction
of vanadium with respect to chromium, and of [Cr(g6-
MeC6H5)2]+ with respect to [Cr(g6-1,3,5-Me3C6H3)2]+,
may explain the lower yields generally observed for the lat-
ter compound.
Thus, if the halogen abstraction process is slow for zir-
conium and hafnium, formation of [Cr(g6-1,3,5-
Me3C6H3)2]Cl will occur in the presence of unreacted
MCl4 thus leading to the occurrence of reaction (6), with
formation of the observed product.
We did several attempts to isolate a Zr(III) or Hf(III)
species but we did not succeed. In order to explain this
result, we can admit that a transient formation of
‘‘MCl3(THF)n’’ is followed by the homolytic cleavage of
the C–O bond of THF and formation of a metal-coordi-
nated butoxy radical; this species may evolve with radical
coupling, the metal regaining the +IV oxidation state. For-
mation of an uranium(IV) butoxy derivative has been
observed in the reactions of UCl3 with NaCp in THF
[13]. Attempts have been made in order to verify the pres-
ence in solution of compounds deriving from the THF acti-
vation, but the only product we observed was 1,3,5-
Me3C6H3 clearly deriving from the bis(arene) cation.
The products arising from the Cr(g6-1,3,5-Me3C6H3)2/
MCl4 system could also be prepared from [Cr(g6-1,3,5-
Me3C6H3)2]Cl and the MCl4(THF)2, see the following
equation:
The tetrachloro derivatives of titanium(IV) and vana-
dium(IV) readily react with M(g6-Me3C6H3)2 in THF to
give the products shown in Eq. (3), two of which can also
be obtained by chloride addition reaction to MX3(THF)3
[10], according to Eq. (4):
M(g6-1,3,5-Me3C6H3)2 + M0Cl4 + 2THF
! [M(g6-1,3,5-Me3C6H3)2][M0Cl4(THF)2]
where M = Cr, Mo; M0 = Ti, V
ð3Þ
ð4Þ
[Cr(g6-1,3,5-Me3C6H3)2]Cl + MCl4(THF)2
[Cr(g6-1,3,5-Me3C6H3)2]X + MX3(THF)3
! [Cr(g6-1,3,5-Me3C6H3)2][MX4(THF)2] + THF
where M = Ti, X = Cl, Br; M = V, X = Cl
! [Cr(g6-1,3,5-Me3C6H3)2][MCl5(THF)] + THF
where M = Zr, Hf
ð6Þ
This is a quite general reaction for the tetrahalo com-
plexes of Group 4 metals: for example, the tetrachlorides
of the Group 4 give the [MCl5(THF)]ꢀ anion by reaction
with [CoCp2]Cl, see the following equation [9a]:
The reactions of ZrCl4 or HfCl4 with Cr(g6-1,3,5-
Me3C6H3)2 have a different outcome. Treatment of ZrCl4
or HfCl4, in the form of their tetrahydrofuran adducts,

3146
U. Englert et al. / Journal of Organometallic Chemistry 692 (2007) 3144–3150
Table 1
[CoCp2]Cl + MCl4(THF)2 ! [CoCp2][MCl5(THF)] + THF
ð7Þ
Selected bond distances (A) and angles (ꢁ) for [Cr(g6-1,3,5-
˚
Me3C6H3)2][HfCl5(THF)]
The compound [Cr(g6-1,3,5-Me3C6H3)2][HfCl5(THF)]
was characterized by X-ray single-crystal diffraction and
its structure is shown in Fig. 1; a selection of bond dis-
tances and angles is collected in Table 1. The precision of
the structural model is limited due to non-merohedral twin-
ning and the presence of two crystallographically indepen-
dent, discrete [Cr(g6-1,3,5-Me3C6H3)2]+ cations and
[HfCl5(THF)]ꢀ anions. The conformations in these inde-
pendent species are quite similar; Fig. 2 provides an overlay
of the cations.
Cr(1)–C(9)
Cr(1)–C(12)
Cr(1)–C(23)
Cr(1)–C(11)
Cr(1)–C(21)
Cr(1)–C(20)
Cr(1)–C(18)
Cr(1)–C(10)
Cr(1)–C(13)
Cr(1)–C(14)
Cr(1)–C(22)
Cr(1)–C(19)
Hf(1)–O(1)
Hf(1)–Cl(3)
Hf(1)–Cl(4)
Hf(1)–Cl(2)
Hf(1)–Cl(1)
Hf(1)–Cl(5)
2.124(15)
2.127(14)
2.128(13)
2.130(13)
2.134(15)
2.135(15)
2.136(14)
2.147(13)
2.147(14)
2.148(13)
2.157(15)
2.166(14)
2.196(9)
2.400(4)
2.408(3)
2.409(3)
2.438(4)
2.444(4)
Cr(2)–C(27)
Cr(2)–C(38)
Cr(2)–C(36)
Cr(2)–C(40)
Cr(2)–C(31)
Cr(2)–C(41)
Cr(2)–C(28)
Cr(2)–C(29)
Cr(2)–C(32)
Cr(2)–C(37)
Cr(2)–C(30)
Cr(2)–C(39)
Hf(2)–O(2)
Hf(2)–Cl(9)
Hf(2)–Cl(7)
Hf(2)–Cl(8)
Hf(2)–Cl(6)
Hf(2)–Cl(10)
2.119(14)
2.120(13)
2.133(14)
2.139(14)
2.142(13)
2.147(13)
2.149(14)
2.159(14)
2.162(14)
2.163(13)
2.164(13)
2.167(14)
2.202(8)
2.391(4)
2.414(4)
2.418(3)
2.434(4)
2.437(3)
The average chromium–mesitylene geometry is charac-
˚
terized by a Cr(1)–centroid distance of 1.610(6) A, with
˚
Cr–C bond lengths ranging from 2.124(15) to 2.166(14) A
˚
[average 2.140 A]. Similar values have been observed in
[Cr(g6-1,3,5-Me3C6H3)2][E], E = TCNE (2.16 A) [15],
˚
˚
TCNQ (2.154 A) [16], pcmcp (pentacarbometoxycyclopen-
O(1)–Hf(1)–Cl(3)
O(1)–Hf(1)–Cl(4)
Cl(3)–Hf(1)–Cl(4)
O(1)–Hf(1)–Cl(2)
Cl(3)–Hf(1)–Cl(2)
Cl(4)–Hf(1)–Cl(2)
O(1)–Hf(1)–Cl(1)
Cl(3)–Hf(1)–Cl(1)
Cl(4)–Hf(1)–Cl(1)
Cl(2)–Hf(1)–Cl(1)
O(1)–Hf(1)–Cl(5)
Cl(3)–Hf(1)–Cl(5)
Cl(4)–Hf(1)–Cl(5)
Cl(2)–Hf(1)–Cl(5)
Cl(1)–Hf(1)–Cl(5)
87.7(3)
178.5(3)
92.3(1)
86.0(2)
173.6(1)
94.0(1)
85.5(3)
89.6(2)
95.9(1)
88.7(1)
84.6(3)
90.4(2)
94.0(1)
90.2(1)
170.1(1)
O(2)–Hf(2)–Cl(9)
O(2)–Hf(2)–Cl(7)
Cl(9)–Hf(2)–Cl(7)
O(2)–Hf(2)–Cl(8)
O(2)–Hf(2)–Cl(8)
Cl(9)–Hf(2)–Cl(8)
Cl(7)–Hf(2)–Cl(8)
O(2)–Hf(2)–Cl(6)
Cl(9)–Hf(2)–Cl(6)
Cl(7)–Hf(2)–Cl(6)
Cl(8)–Hf(2)–Cl(6)
O(2)–Hf(2)–Cl(10)
Cl(9)–Hf(2)–Cl(10)
Cl(7)–Hf(2)–Cl(10)
Cl(8)–Hf(2)–Cl(10)
178.7(3)
tadienyl anion) [2.154(4) A] [6] and in [Cr(g6-
˚
˚
86.1(3)
93.7(1)
87.0(3)
87.0(3)
93.2(1)
172.8(1)
83.8(3)
94.9(1)
92.1(1)
89.4(1)
85.9(3)
95.4(1)
89.1(1)
88.2(1)
MeC6H5)2][pcmcp] [2.145(5) A] [17].
The chromium atom is sandwiched between two stag-
gered mesitylene rings, see Fig. 2. The methyl groups of the
two rings form a dihedral angle of 59.1(6)ꢁ (average) in cat-
ion 1 and of 57.3(5)ꢁ (average) in cation 2 (theoretical value
should be 60ꢁ). It is interesting to observe that the recently
reported [Cr(g6-1,3,5-Me3C6H3)2][pcmcp] [6] crystallizes
with four independent cations and anions in the unit cell dif-
fering with respect to the relative disposition of the two
mesitylene rings (the dihedral angles between methyl groups
of different rings are 2.6ꢁ, 9.9ꢁ, 19.3ꢁ, and 22.3ꢁ).
We have recently observed that the solid state orienta-
tion of the two methyl groups on the two toluene rings
of [M(g6-MeC6H5)2]+ cations, M = V [18], Cr [17,19],
depends on the steric hindrance of the anion. For example,
sterically demanding anions, such as [pcmcp]ꢀ, [1,2-dib-
enzoylcyclopentadienyl]ꢀ and [1,2-dibenzoyl-4-nitrocyclo-
pentadienyl]ꢀ, seem to induce stabilization of the more
compact cis-eclipsed conformation of the methyl groups,
i.e., dihedral angles close to 0ꢁ, in crystallographically quite
different solids.
As far as the [HfCl5(THF)]ꢀ anion is concerned, the
˚
average Hf–Cl (2.419(2) A) bond distance is comparable
to that reported for the same anion in the [Co(C5Me5)2]+
˚
derivative [2.425(4) A] [9a].
Fig. 1. View of one of the two independent [Cr(g6-1,3,5-Me3C6H3)2]+
cations and of one of the two [HfCl5(THF)]ꢀ anions in the structure of
[Cr(g6-1,3,5-Me3C6H3)2][HfCl5(THF)]. Atoms are represented by their
30% probability ellipsoids.
Fig. 2. Overlay [14] of the symmetrically independent but conformation-
ally similar cations in [Cr(g6-1,3,5-Me3C6H3)2][HfCl5(THF)].

U. Englert et al. / Journal of Organometallic Chemistry 692 (2007) 3144–3150
3147
The reaction between Cr(g6-1,3,5-Me3C6H3)2 and
Nb2Cl10 is dependent on the reaction medium: when the
reaction was performed in hydrocarbon solvent at room
temperature or in THF even at low temperature, impure
compounds were obtained. On the other hand, the reaction
in acetonitrile afforded a microcrystalline red solid which
vents were dried by conventional methods prior to use.
TiCl4 was distilled prior to use. Commercial VCl4 was used
as heptane solution (stored at ꢀ30 ꢁC) and analyzed for its
chloride content prior to use. Triphenylmethyl halides were
used as received. The compounds Cr(g6-MeC6H5)2 [8],
M(g6-1,3,5-Me3C6H3)2, M = Cr [6], Mo [6], MCl4(THF)2,
M = Zr, Hf, Nb [21], MCl3(THF)3, M = Ti, V [21],
TiBr3(THF)3 [22], CoCp2 [23], were prepared according
to the literature. IR spectra were recorded on a FT-
1725X instrument on Nujol or polychlorotrifluoroethylene
(PCTFE) mulls or on a FTIR-spectrometer equipped with
a Perkin–Elmer UATR sampling accessory.
was
identified
as
[Cr(g6-1,3,5-Me3C6H3)2][NbCl5-
(CH3CN)], Eq. (8). This compound, as the others of this
series, was also obtained by chloride addition to
NbCl4(THF)2 in CH3CN, Eq. (9):
2Cr(g6-1,3,5-Me3C6H3)2 + Nb2Cl10 + 2CH3CN
! 2[Cr(g6-1,3,5-Me3C6H3)2][NbCl5(CH3CN)]
ð8Þ
[Cr(g6-1,3,5-Me3C6H3)2]Cl+NbCl4(THF)2+CH3CN
4.2. Treatment of Cr(g6-arene)2 with [CoCp2]Br
![Cr(g6-1,3,5-Me3C6H3)2][NbCl5(CH3CN)] +2THF ð9Þ
4.2.1. Arene = toluene
The IR spectrum in the solid state shows the absorptions
typical of the cation at 3053, 1039 and 1004 cmꢀ1: the band
at 2279 cmꢀ1 has been assigned to the CN stretching vibra-
tion of the coordinated acetonitrile. The higher wavenum-
ber shift with respect to free acetonitrile (2245 cmꢀ1) is
typical of coordinated nitrile species [20].
A well stirred solution of Cr(g6-MeC6H5)2 (0.18 g,
0.77 mmol) in THF (30 mL) was treated with solid
[CoCp2]Br (0.20 g, 0.76 mmol). After 15 h stirring at room
temperature. The yellow solid was recovered by filtration,
washed with THF (2 · 5 mL) and identified as [CoCp2]Br
(0.16 g, 78% yield, IR spectroscopy). The volume of the
mother liquor was reduced to 2 mL, added of heptane
(5 mL) and the solid was recovered by filtration, dried in
vacuo and identified as Cr(g6-MeC6H5)2 (0.15 g, 82%
yield).
3. Conclusions
In this paper, the reactivity of M(g6-1,3,5-Me3C6H3)2,
M = Cr, Mo, towards metal halides of Groups 4 and 5 has
been examined. It has been shown that both chromium
and molybdenum derivatives are monoelectronically oxi-
dized to the respective [M(g6-1,3,5-Me3C6H3)2]+ cations
with formation of reduced, solvent-stabilized, anionic metal
halides of general formula [MXnLm]ꢀ. The reactivity of the
bis-1,3,5-Me3C6H3 derivatives parallels that of the reported
bis-toluene compounds M(g6-MeC6H5)2, M = Cr, Mo.
On the other hand, the bis-mesitylene derivative Cr(g6-
1,3,5-Me3C6H3)2 reacts with MCl4(THF)2, M = Zr, Hf, in
THF with formation of [Cr(g6-1,3,5-Me3C6H3)2] [MCl5
(THF)], which have been isolated and characterized in the
solid state via X-ray structural analysis for M = Hf. The iso-
lation of a Cr(I) bis-arene cation suggests that reduction of
zirconium(IV) or hafnium(IV) may occur in the system,
although we have not been able as yet to isolate the product
of the reduction, probably due to its instability in THF.
The crystal structure of [Cr(g6-1,3,5-Me3C6H3)2]
[HfCl5(THF)] has confirmed the conclusions drawn in previ-
ous papers of this series [6,17–19]: the conformation of the
arene rings in the chromium(I) cations in the solid state
depends on the steric demand of the anion and it tends
toward the staggered conformation as the dimensions of
the anion get smaller.
4.2.2. Arene = 1,3,5-Me3C6H3
well stirred solution of Cr(g6-1,3,5-Me3C6H3)2
A
(0.39 g, 1.3 mmol) in THF (25 mL) was treated with solid
[CoCp2]Br (0.35 g, 1.3 mmol). After 12 h stirring at room
temperature some yellow solid was present which was
removed by filtration. The yellow solid was recovered by
filtration, washed with THF (2 · 5 mL) and dried in vacuo
at room temperature affording 0.45 g (93% yield) of [Cr(g6-
1,3,5-Me3C6H3)2]Br identified by IR spectroscopy. The
volume of the mother liquor was reduced to 2 mL, added
of heptane (5 mL) and the solid was recovered by filtration
and dried in vacuo. It was identified as CoCp2 (0.21 g, 85%
yield).
4.3. Treatment of [Cr(g6-arene)2]Br with CoCp2
4.3.1. Arene = toluene
A well stirred solution of [Cr(g6-MeC6H5)2]Br (0.15 g,
0.48 mmol) in THF (30 mL) was treated with solid
CoCp2 (0.090 g, 0.48 mmol). After 15 h stirring at room
temperature the yellow solid was recovered by filtration,
washed with THF (2 · 5 mL) and dried in vacuo at room
temperature affording 0.11 g (91% yield) of [CoCp2]Br
identified by IR spectroscopy. The volume of the mother
liquor was reduced to 2 mL, added of heptane (5 mL)
and the solid was recovered by filtration and dried in
vacuo. It was identified as Cr(g6-MeC6H5)2 (0.089 g,
78% yield).
4. Experimental
4.1. General procedures
Unless otherwise stated, all of the operations were car-
ried out under an atmosphere of prepurified argon. Sol-

3148
U. Englert et al. / Journal of Organometallic Chemistry 692 (2007) 3144–3150
4.3.2. Arene = 1,3,5-Me3C6H3
[Cr(g6-1,3,5-Me3C6H3)2][VCl4(THF)2] (air sensitive, yel-
low microcrystalline solid, 63% yield). Anal. Calc. for
C26H40Cl4CrO2V: C, 49.6; H, 5.1; Cl, 22.5. Found: C,
A well stirred yellow suspension of [Cr(g6-1,3,5-
Me3C6H3)2]Br (0.37 g, 1.0 mmol) in THF (25 mL) was trea-
ted with solid CoCp2 (0.18 g, 0.96 mmol). After 12 h stirring
at room temperature some yellow solid was present which
was removed by filtration. The yellow solid was recovered
by filtration, washed with THF (2 · 5 mL) and dried in
vacuo at room temperature affording 0.27 g (74% yield) of
[Cr(g6-1,3,5-Me3C6H3)2]Br identified by IR spectroscopy.
The volume of the mother liquor was dried in vacuo at room
temperature and the residue was sublimed at 60 ꢁC/
0.05 mmHg, obtaining 0.12 g (67% yield) of CoCp2.
49.9; H, 5.2; Cl, 22.3%. IR (Nujol), m=cmꢀ1: 3052 m, 1608
~
m, 1330 w, 1060 m, 1034 s (THF), 1021 m, 917 w, 868 s
(THF), 721 w, 442 m.
[Mo(g6-1,3,5-Me3C6H3)2][TiCl4(THF)2] (air sensitive,
green microcrystalline solid, 38% yield). Anal. Calc. for
C26H40Cl4MoO2Ti: C, 46.6; H, 6.0; Cl, 21.1. Found: C,
46.7; H, 6.2; Cl, 21.5%. IR (Nujol), m=cmꢀ1: 3047 m, 1516
~
m, 1339 w, 1060 m, 1037 s (THF), 865 s (THF), 722 w,
492 w, 430 w.
[Mo(g6-1,3,5-Me3C6H3)2][VCl4(THF)2] (air sensitive,
green microcrystalline solid, 39% yield). Anal. Calc. for
C26H40Cl4MoO2V: C, 46.4; H, 6.0; Cl, 21.1. Found: C,
4.4. Reactions of Cr(g6-1,3,5-Me3C6H3)2 with
triphenylmethyl halides
46.2; H, 6.2; Cl, 21.3%. IR (Nujol), m=cmꢀ1: 3045 m, 1514
~
Only the reaction of Cr(g6-1,3,5-Me3C6H3)2 with
CPh3Cl is described in detail, the reaction with CPh3Br
being performed in a similar way. A solution of Cr(g6-
1,3,5-Me3C6H3)2 (0.91 g, 3.1 mmol) in THF (50 mL) was
treated dropwise with a solution of CPh3Cl (0.87 g,
3.1 mmol) in THF (50 mL). An orange bright suspension
was obtained. The suspension was stirred for 6 h and the
solid was recovered by filtration, washed with THF and
dried in vacuo at room temperature affording 0.85 g (84%
yield) of [Cr(g6-1,3,5-Me3C6H3)2]Cl as an orange micro-
m, 1341 w, 1064 m, 1035 s (THF), 869 s (THF), 721 w,
490 w, 427 w.
4.6. Preparation of [Cr(g6-1,3,5-Me3C6H3)2]
[MCl4(THF)2], M = Ti, V, from [Cr(g6-1,3,5-Me3-
C6H3)2]Cl and MCl3(THF)3
Only the reaction of [Cr(g6-1,3,5-Me3C6H3)2]Cl with
TiCl3(THF)3 is described in detail, the reaction with
VCl3(THF)3 being performed in a similar way. A well stirred
suspension of [Cr(g6-1,3,5-Me3C6H3)2]Cl (0.26 g, 0.80
mmol) in THF (50 mL) was treated with solid TiCl3(THF)3
(0.29 g, 0.80 mmol). The pale green suspension was stirred
for 2 h at room temperature obtaining a orange suspension.
The solid was recovered by filtration, washed with THF
(2 · 2 mL) and dried in vacuo affording [Cr(g6-1,3,5-
Me3C6H3)2][TiCl4(THF)2] (0.45 g, 90% yield) identified on
the basis of the analytical data and the IR spectrum.
crystalline solid. Anal. Calc. for C18H24ClCr: C, 66.0; H,
ꢀ1
~
7.3. Found: C, 65.9; H, 7.5%. IR (solid state): m=cm
:
3045 m, 1607 w, 1533 m, 1510 w, 1306 w, 1159 w, 1039 s,
1019 m, 887 m, 834 w, 722 w.
[Cr(g6-1,3,5-Me3C6H3)2]Br (bright orange, 78% yield).
Anal. Calc. for C18H24BrCr: C, 58.1; H, 6.5. Found: C,
57.9; H, 6.0%. IR (solid state): m=cmꢀ1: 3046 m, 1533 m,
~
1510 w, 1305 w, 1163 w, 1038 s, 1015 m, 1006 w, 995 w,
886 m, 721 mw, 668 m.
4.7. Preparation of [Cr(g6-1,3,5-Me3C6H3)2]
[TiBr4(THF)2] from [Cr(g6-1,3,5-Me3C6H3)2]Br and
TiBr3(THF)3
4.5. Preparation of [M(g6-1,3,5-Me3C6H3)2]
[M0Cl4(THF)2], M = Cr, Mo; M0 = Ti, V
Only the reaction of Cr(g6-1,3,5-Me3C6H3)2 with TiCl4 is
described in detail, the reaction with VCl4 being performed
in a similar way. A well stirred solution of Cr(g6-1,3,5-
Me3C6H3)2 (0.63 g, 2.2 mmol) in heptane (50 mL) was trea-
ted dropwise with a solution of TiCl4 (0.23 mL, 2.1 mmol) in
heptane (5 mL). The solution became immediately cloudy
and changed to a bulky suspension within 5 min. The solid
was filtered, washed with heptane and dried in vacuo. The
heptane solution did not contain appreciable amounts of
chloride ions (test with silver nitrate). The extremely air sen-
sitive solid was treated with THF (50 mL) and an orange
solid was obtained which was recovered by filtration. After
drying in vacuo, [Cr(g6-1,3,5-Me3C6H3)2][TiCl4(THF)2]
(0.68 g, 49% yield) was isolated as a air sensitive, orange
microcrystalline solid. Anal. Calc. for C26H40Cl4CrO2Ti:
C, 49.9; H, 6.4; Cl, 22.6. Found: C, 49.5; H, 6.8; Cl, 22.1%.
A well stirred suspension of [Cr(g6-1,3,5-Me3C6H3)2]Br
(0.42 g, 1.1 mmol) in THF (70 mL) was treated with solid
TiBr3(THF)3 (0.57 g, 1.1 mmol). The suspension was stir-
red for 4 h at room temperature obtaining a orange suspen-
sion. The solid was recovered by filtration, washed with
THF (2 · 2 mL) and dried in vacuo affording [Cr(g6-
1,3,5-Me3C6H3)2][TiBr4(THF)2] (0.75 g, 85% yield) as a
dark orange microcrystalline solid. Anal. Calc. for
C26H40Br4CrO2Ti: C, 38.8; H, 5.0; Br, 39.7. Found: C,
39.4; H, 4.7; Br, 39.5%. IR (Nujol), m=cmꢀ1: 3060 m,
~
1592 m, 1328 w, 1063 m, 1030 s (THF), 1025 m, 921 w,
859 s (THF), 720 w, 665 w, 442 m.
4.8. Reaction of [Cr(g6-1,3,5-Me3C6H3)2] with
MCl4(THF)2, M = Zr, Hf
IR (Nujol), m=cmꢀ1: 3058 m, 1610 m, 1334 w, 1061 m, 1034
Only the reaction of Cr(g6-1,3,5-Me3C6H3)2 with HfCl4
~
s (THF), 1023 m, 920 w, 868 s (THF), 721 w, 668 w, 445 m.
is described in detail, the reaction with ZrCl4 being per-

U. Englert et al. / Journal of Organometallic Chemistry 692 (2007) 3144–3150
3149
formed in a similar way. A well stirred solution of Cr(g6-
Table 2
Crystallographic data for [Cr(g6-1,3,5-Me3C6H3)2][HfCl5(THF)]
1,3,5-Me3C6H3)2 (0.73 g, 2.5 mmol) in THF (25 mL) was
treated dropwise with a solution of HfCl4(THF)2 (1.1 g,
2.4 mmol) in THF (25 mL). A small amount of orange yel-
low solid was present after 2 h from mixing the reagents.
The suspension was stirred for 48 h at room temperature
and refluxed for 4 h. The orange solid was recovered by fil-
tration, washed with THF (2 · 3 mL) and dried in vacuo at
room temperature affording 0.48 g of an exceedingly air
sensitive solid. Another crop of product (0.29 g) was
obtained from the mother liquor after 48 h at room tem-
perature. The two solid were combined and identified as
[Cr(g6-1,3,5-Me3C6H3)2][HfCl5(THF)] (0.77 g, 43% yield).
Anal. Calc. for C22H32Cl5CrHfO: C, 36.7; H, 4.5; Cl,
24.6. Found: C, 36.9; H, 4.3; Cl, 24.1%. IR (Nujol),
Empirical formula
Formula weight
Crystal system
Space group
C22H32Cl5CrHfO
720.22
Monoclinic
P21/n
20.892(5)
13.601(3)
21.053(5)
118.140(7)
5275(2)
8
˚
a (A)
˚
b (A)
˚
c (A)
b (ꢁ)
3
˚
V (A )
Z
Dcalc (g cmꢀ3
F(000)
)
1.814
2824
4.864
68982
l (cmꢀ1
)
Reflections collected
h Range (ꢁ)
1.13–27.63
Limiting indices
ꢀ27 6 h 6 23, ꢀ17 6 k 6 16, ꢀ9 6 l 6 27
m=cmꢀ1: 3058 m, 1608 m, 1507 w, 1305 w, 1035 m (THF),
~
Independent reflections (Rint
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2r(I)]
Final R indices (all data)
)
12024 (0.1054)
1003 m, 858 s (THF), 722 m-w, 688 w, 520 m-w, 446 m-s.
The mother liquor was refluxed for 3 h but no additional
solid was obtained. The solvent was partially removed in
vacuo and toluene was added which caused the formation
of a dark oily solid which was not investigated further.
A GC–MS analysis of the mother liquor revealed only
detectable amounts of 1,3,5-Me3C6H3.
12024/94/554
1.082
R1 = 0.079, wR2 = 0.201
R1 = 0.104, wR2 = 0.216
P
P
P
P
2
2 1=2
R1 = iFoj ꢀ jFci/j (Fo), wR2 ¼ ½ ½wðF 2o ꢀ F c2Þ ꢂ= ½wðF o2Þ ꢂꢂ
.
The reaction was repeated by using a Cr/Hf molar ratio
of 1/2, but unreacted HfCl4(THF)2 was identified in the
mother liquor by IR spectroscopy.
HfCl4(THF)2 (0.56 g, 1.2 mmol). The pale yellow suspen-
sion was stirred for 2 h at room temperature obtaining a
orange suspension. The solid was recovered by filtration,
washed with THF (3 · 2 mL) and dried in vacuo affording
[Cr(g6-1,3,5-Me3C6H3)2][HfCl5(THF)] (0.88 g, 93% yield)
identified on the basis of the analytical data and the IR
spectrum.
[Cr(g6-1,3,5-Me3C6H3)2][ZrCl5(THF)] (yellow, 44%
yield). Anal. Calc. for C22H32Cl5CrOZr: C, 41.7; H, 5.1;
Cl, 41.7. Found: C, 41.1; H, 5.0; Cl, 42.1%. IR (Nujol),
m=cmꢀ1: 3056 m, 1605 m, 1510 w, 1309 w, 1033 m (THF),
~
1008 m, 862 s (THF), 722 m-w, 685 w, 520 m-w, 446 m-s.
4.11. Reaction of [Cr(g6-1,3,5-Me3C6H3)2]Cl with Nb2Cl10
4.9. X-ray structure determination of [Cr(g6-1,3,5-
Me3C6H3)2][HfCl5(THF)]
A well stirred solution of Cr(g6-1,3,5-Me3C6H3)2 (0.36 g,
1.2 mmol) in CH3CN (25 mL) was treated with solid
Nb2Cl10 (0.33 g, 0.60 mmol). The brown solution turned
reddish. After 1 h stirring at room temperature, a bright
red microcrystalline solid was present which was recovered
by filtration and dried in vacuo at room temperature afford-
ing 0.080 g of red solid. The volume of the mother liquor
was reduced to ca. 5 mL and toluene (15 mL) was added.
The red solid which formed was recovered by filtration,
washed with toluene and dried in vacuo affording 0.50 g
of [Cr(g6-1,3,5-Me3C6H3)2][NbCl5(CH3CN)] (0.58 g, 80%
total yield). Anal. Calc. for C20H27Cl5CrNNb: C, 39.8; H,
4.5; N, 2.3; Cl, 29.4. Found: C, 39.3; H, 4.3; N, 2.1; Cl,
Crystals of [Cr(g6-1,3,5-Me3C6H3)2][HfCl5(THF)] were
grown from THF at ca. 4 ꢁC and the data were collected at
˚
110(2) K with Mo Ka radiation (k = 0.71073 A) on a CCD
area detector diffractometer equipped with a graphite mono-
chromator, on a crystal of approximate dimensions 0.34 ·
0.31 · 0.19 mm. Details for the X-ray data collections are
listed in Table 2. An empirical absorption correction (min.
trans. 0.29, max. trans 0.45) was applied before averaging
symmetry equivalent data (Rint = 0.1054). After merging
12024independentreflectionsremainedforstructuresolution
by direct methods [24]. The structure model was completed by
Fourier difference synthesis and refined with full-matrix least-
squares on F2 [25]. Convergence was reached for 12024 reflec-
tions and 554 variables at agreement factors of R1 = 0.079
[observations with I > 2r(I)] and wR2 = 0.216 (all data).
29.8%. IR (Nujol), m=cmꢀ1: 3053 m, 2277 m, 1506 w, 1408
~
m, 1303 w, 1039 m, 1004 w, 896 w, 879 m, 824 w, 721 m-
w, 521 m-w, 447 m-s.
4.12. Reaction of [Cr(g6-1,3,5-Me3C6H3)2]Cl with
NbCl4(THF)2
4.10. Preparation of [Cr(g6-1,3,5-Me3C6H3)2]
[HfCl5(THF)] from [Cr(g6-1,3,5-Me3C6H3)2]Cl and
HfCl4(THF)2
A well stirred suspension of [Cr(g6-1,3,5-Me3C6H3)2]Cl
(0.37 g, 1.1 mmol) in CH3CN (30 mL) was treated drop-
wise with a solution of NbCl4(THF)2 (0.42 g, 1.1 mmol)
in CH3CN (25 mL). The colour of the solution changed
A well stirred suspension of [Cr(g6-1,3,5-Me3C6H3)2]Cl
(0.42 g, 1.3 mmol) in THF (50 mL) was treated with solid

3150
U. Englert et al. / Journal of Organometallic Chemistry 692 (2007) 3144–3150
(b) N. Ito, T. Saji, K. Suga, S. Aoyagui, J. Organomet. Chem. 229
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Chem. 219 (1981) 43.
to dark brown. After 12 h stirring at room temperature
some yellow solid was present which was removed by filtra-
tion. The volume of the solution was reduced to ca. 10 mL
and toluene (20 mL) was added. The red solid which
formed was recovered by filtration and dried in vacuo at
room temperature affording 0.50 g (75% yield) of [Cr(g6-
1,3,5-Me3C6H3)2][NbCl5(CH3CN)] identified on the basis
of the analytical data and the IR spectrum.
[5] J.A. Connor, Top. Curr. Chem. 71 (1971) 71.
[6] L. Calucci, F.G.N. Cloke, U. Englert, P.B. Hitchcock, G. Pampaloni,
C. Pinzino, F. Puccini, M. Volpe, Dalton Trans. (2006) 4228.
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Acknowledgments
[8] F. Calderazzo, R. Invernizzi, F. Marchetti, F. Masi, A. Moalli, G.
Pampaloni, L. Rocchi, Gazz. Chim. Ital. 123 (1993) 53.
[9] (a) P. Biagini, F. Calderazzo, G. Pampaloni, P.F. Zanazzi, Gazz.
Chim. Ital. 117 (1987) 27;
The authors thank the Ministero dell’Istruzione,
`
dell’Universita e della Ricerca (MIUR, Roma), Programmi
di Ricerca Scientifica di Notevole Interesse Nazionale, Cof-
inanziamento 2004–2005, and the Consiglio Nazionale
delle Ricerche (C.N.R.) Progetto CNR-MIUR ‘‘Sviluppo
di microcelle a combustibile’’, for financial support.
(b) F. Calderazzo, G. Pampaloni, U. Englert, J. Stra¨hle, J. Organo-
met. Chem. 383 (1990) 45;
(c) G. Pampaloni, U. Ko¨lle, J. Organomet. Chem. 481 (1994) 1;
(d) F. Calderazzo, G. Pampaloni, G. Tripepi, Organometallics 16
(1997) 4943.
[10] The chloride and the bromide of the [Cr(g6-1,3,5-Me3C6H3)2]+ cation
have been prepared by the stoicheiometric reaction of Cr(g6-1,3,5-
Me3C6H3)2with CPh3X in THF, see Experimental section.
[11] F.A. Cotton, G. Wilkinson, C.A. Murillo, M. Bochmann, Advanced
Inorganic Chemistry, sixth ed., Wiley, New York, 1999.
[12] (a) M.F. Lappert, C.J. Pickett, P.I. Riley, P.I.W. Yarrow, J. Chem.
Soc., Dalton Trans. (1981) 805;
(b) A. Fakhr, Y. Mugnier, B. Gautheron, E. Laviron, J. Organomet.
Chem. 302 (1986) C7.
[13] G.L. Ter Haar, M. Dubeck, Inorg. Chem. 3 (1964) 1648.
[14] A.L. Spek, J. Appl. Crystallogr. 36 (2003) 7.
Appendix A. Supplementary material
CCDC 627010 contains the supplementary crystallo-
graphic data for [Cr(g6-1,3,5-Me3C6H3)2][HfCl5(THF)].
These data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html, or from the
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or
e-mail: deposit@ccdc.cam.ac.uk. Supplementary data asso-
ciated with this article can be found, in the online version,
at doi:10.1016/j.jorganchem.2007.01.044.
[15] J.S. Miller, D.M. O’Hare, A. Chakraborty, A.J. Epstein, J. Am.
Chem. Soc. 111 (1989) 7853.
[16] D. O’Hare, M.D. Ward, J.S. Miller, Chem. Mater. 2 (1990) 758.
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Pinzino, M. Volpe, P. Zanello, J. Organomet. Chem. 691 (2006) 829.
[18] F. Marchetti, G. Pampaloni, C. Pinzino, J. Organomet. Chem. 691
(2006) 3458.
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