Synthesis and crystal structures of chromium and molybdenum complexes containing the 2,4,6-tris(trifluoromethyl)phenyl ligand

Article abstract of DOI:10.1016/S0022-328X(01)01063-4

Complexes [Cr(NR)₂Cl₂] react with Li(fmes) to yield [Cr(NR)₂(fmes)₂], where fmes=2,4,6-(CF₃)₃C₆H₂. X-ray diffraction studies of the newly synthesised complex with R=adamantyl, and the previously known one with R=tert-butyl, have revealed tetrahedral coordination of the metal, complemented by two secondary CrF interactions with ortho-CF₃ groups of the fmes ligands. The reaction of [Mo(NC₆H₃ⁱPr₂-2,6) ₂Cl₂(dme)] with Li(fmes) gave a dimeric complex [Mo(NC₆H₃ⁱPr₂-2,6) ₂(fmes)Cl·LiCl(dme)]₂. The X-ray structure of the latter (as its pentane solvate) shows a double-ribbon Mo₂Li₂Cl₄ arrangement, with two Cl atoms μ₂-bridging between Mo and Li, and two others μ₃-bridging between one Mo and two Li atoms.

Full text of DOI:10.1016/S0022-328X(01)01063-4

Journal of Organometallic Chemistry 631 (2001) 181–187
www.elsevier.com/locate/jorganchem
Synthesis and crystal structures of chromium and molybdenum
complexes containing the 2,4,6-tris(trifluoromethyl)phenyl ligand
Andrei S. Batsanov a, Keith B. Dillon a,*, Vernon C. Gibson b, Judith A.K. Howard a,
Leela J. Sequeira a, Jing Wen Yao a,1
a Chemistry Department, Uni6ersity of Durham, South Road, Durham DH1 3LE, UK
b Chemistry Department, Imperial College, South Kensington, London SW 7 2AY, UK
Received 11 May 2001; accepted 10 June 2001
Abstract
Complexes [Cr(NR)2Cl2] react with Li(fmes) to yield [Cr(NR)2(fmes)2], where fmes=2,4,6-(CF3)3C6H2. X-ray diffraction
studies of the newly synthesised complex with R=adamantyl, and the previously known one with R=tert-butyl, have revealed
tetrahedral coordination of the metal, complemented by two secondary Cr···F interactions with ortho-CF3 groups of the fmes
i
i
ligands. The reaction of [Mo(NC6H3 Pr2-2,6)2Cl2(dme)] with Li(fmes) gave a dimeric complex [Mo(NC6H3 Pr2-2,6)2(fmes)Cl·
LiCl(dme)]2. The X-ray structure of the latter (as its pentane solvate) shows a double-ribbon Mo2Li2Cl4 arrangement, with two
Cl atoms m2-bridging between Mo and Li, and two others m3-bridging between one Mo and two Li atoms. © 2001 Elsevier Science
B.V. All rights reserved.
Keywords: Crystal structures; Group VI complexes; Imido ligands; Secondary M–F interactions
ortho CF3-groups of fmes. The question arises whether
these distances are merely an incidental consequence of
steric crowding, or whether they correspond to weak
bonding interactions (‘secondary coordination’) [8],
which may play an important role in stabilising the
structure.
Until recently, a carbon-bonded fluorine atom was
regarded as incapable of coordination with a metal.
The polarity of the CꢀF bond is surprisingly low in
view of the high formal electronegativity of fluorine [9],
and ‘organic’ fluorine has proved to be a weak nucle-
ophile, e.g. a very poor acceptor of hydrogen bonds
[10]. However, the possibility of CꢀF“M coordina-
tion, envisaged by Murray-Rust et al. [11], has now
been established experimentally for a number of non-
and post-transition metals, although few cases have
been reported for transition metals [12]. Such interac-
tions were suggested as being essentially non-bonding
in trans-[Pd(fmes)2(tht)2] (tht=tetrahydrothiophene)
and [Pd(fmes)2(bipy)], where Pd···F distances range
1. Introduction
The 2,4,6-tris(trifluoromethyl)phenyl (fmes) ligand
first attracted attention for its ability to stabilise,
through the combination of steric bulkiness and strong
electron-withdrawing ability, compounds of main group
elements with low valence and low coordination num-
ber [1]. Subsequently, fmes was found also to stabilise
complexes, both homoleptic and mixed-ligand, of d-ele-
ments, including Co, Ni [2], Re [3], Pd [4] and Au [5].
Until recently, this field was mostly restricted to late-
and post-transition metals. However, fmes complexes of
earlier transition metals can also be stable. Thus, re-
cently we prepared a number of complexes of V [6], Cr
and Mo [7], some of which were characterised by X-ray
crystallography. A remarkable feature of some of these
complexes is the relatively short distances between the
transition metal atom and the fluorine atoms of the
* Corresponding author. Tel.: +44-191-374-3132; fax: +44-191-
384-4737.
E-mail address: k.b.dillon@durham.ac.uk (K.B. Dillon).
1 Present address: Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK.
,
from 2.766 to 2.982 A [4], whereas rather shorter
distances were observed in [VCl(fmes)2(thf)] and
,
[V(fmes)3OLi(thf)3] (V···F 2.306(2)–2.668(4) A) [6] and
t
,
in [Mo(N Bu)2(fmes)2] (Mo···F 2.467(3)–2.476(3) A) [7].
0022-328X/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(01)01063-4

182
A.S. Batsano6 et al. / Journal of Organometallic Chemistry 631 (2001) 181–187
Scheme 1. Conditions (i): −78 °C, in Et2O, −2 LiCl.
In order to investigate these interactions further, in
six electrons between them to the metal centre, rather
than the usual four each for neutral 1s,2p ligands. This
is a result of the competition between the imido groups
the present work we synthesised two new complexes of
Group 6 metals with fmes ligands, [Cr(NAd)2(fmes)2]
(1) and [Mo(NC6H3 Pr2-2,6)2(fmes)Cl·LiCl(dme)] (3),
i
and determined their X-ray crystal structures, as well as
the structure of the complex [Cr(NtBu)2(fmes)2] (2),
synthesised earlier [7].
2. Results and discussion
Bright red crystalline complex 1 was obtained in
good yield from treatment of [Cr(NAd)2Cl2] [13] with
two equivalents of Li(fmes), see Scheme 1. The
analogous synthesis of complex 2 from [Cr(NtBu)2Cl2]
has been described previously [7]. However, the reac-
tion of [Mo(NC6Hi3Pr2-2,6)2Cl2(dme)] [14] with Li(fmes)
did not result in complete substitution of chloride lig-
ands with fmes, producing instead polynuclear complex
3 (Scheme 1).
Fig. 1. Molecular structure of [Cr(NAd)2(fmes)2] (1), showing the
disorder of one CF3 group. H atoms are omitted for clarity.
The structures of complexes 1, 2 and 3 were deter-
mined by single-crystal X-ray diffraction at 150 K. The
molecular structures of 1 and 2 are shown in Figs. 1
and 2, while relevant bond distances and angles are
listed in Table 1. The same mode of metal coordination
is observed for both complexes as in [Mo(NtBu)2-
(fmes)2] (4) [7], although the crystal packing modes of
all three complexes are quite different. It is instructive
to compare the structures of 1, 2 and 4 with those of
the non-fluorinated analogues [Cr(NtBu)2(mes)2] (5)
and [Mo(NtBu)2(2,6-Me2C6H3)2] (6) [15], as shown in
Table 2. In each case, two s-bonded aryl and two
imido ligands comprise a distorted tetrahedral environ-
ment of the metal atom. The imido ligands are essen-
tially linear, and may be viewed as donating a total of
Fig. 2. Molecular structure of [Cr(NtBu)2(fmes)2] (2). H atoms are
omitted for clarity.

A.S. Batsano6 et al. / Journal of Organometallic Chemistry 631 (2001) 181–187
183
for metal dp orbitals in a tetrahedral coordination
environment which allows only three p-bonds (out of
the possible four) to be formed. A structural conse-
quence of this is seen in rather long MꢀN distances
which lie at the upper limit of the range usually ob-
served. Although according to VSEPR theory [16], the
electron pairs of a multiple bond occupy more space in
the coordination sphere than the electron pair of a
single bond, in 5 and 6 the CꢀMꢀC angle is consider-
ably wider than the NꢀMꢀN angle, due to the steric
requirements of the mesityl groups. A consequence of
the large CꢀMꢀC angle is the partial opening up of two
additional coordination sites in positions trans to the
imido ligands. These sites are occupied by one o-methyl
group of each aryl ligand, one CꢀH bond of each
methyl pointing towards the metal atom. The resulting
M···H contacts are far shorter than the sum of the van
i
Fig. 3. Molecular structure of [Mo(NC6H3 Pr2-2,6)2(fmes)Cl·LiCl-
(dme)]2 (3), showing the disorder of one CF3 group. H atoms are
omitted for clarity.
Table 1
,
Bond distances (A) and bond angles (°) in 1 and 2
1
2
1
2
Bond distances
CrꢀN(1)
CrꢀN(2)
CrꢀC(31)
N(1)ꢀC(11)
1.644(2)
1.631(2)
2.133(3)
1.454(3)
1.609(8)
1.612(8)
2.158(9)
1.48(1)
Cr···F(1)
2.443(2)
2.462(2)
2.133(3)
1.464(4)
2.421(5)
2.468(5)
2.129(9)
1.47(1)
Cr···F(10)
CrꢀC(41)
N(2)ꢀC(21)
Bond angles
N(1)ꢀCrꢀN(2)
N(1)ꢀCrꢀC(31)
N(1)ꢀCrꢀC(41)
N(1)ꢀCrꢀF(1)
N(1)ꢀCrꢀF(10)
C(31)ꢀCrꢀF(1)
C(31)ꢀCrꢀF(10)
F(1)ꢀCrꢀF(10)
CrꢀN(1)ꢀC(11)
CrꢀF(1)ꢀC(37)
CrꢀC(31)ꢀC(32)
CrꢀC(31)ꢀC(36)
112.3(1)
111.4(1)
92.2(1)
112.1(4)
110.0(4)
90.1(4)
156.1(3)
89.2(3)
74.5(3)
72.5(3)
69.4(2)
164.0(7)
111.1(6)
113.1(7)
129.7(7)
C(31)ꢀCrꢀC(41)
N(2)ꢀCrꢀC(31)
N(2)ꢀCrꢀC(41)
N(2)ꢀCrꢀF(1)
N(2)ꢀCrꢀF(10)
C(41)ꢀCrꢀF(1)
C(41)ꢀCrꢀF(10)
138.5(1)
91.0(1)
139.9(3)
92.8(3)
111.8(4)
90.7(3)
157.6(3)
74.1(3)
73.6(3)
111.7(1)
89.37(9)
157.00(9)
72.25(8)
73.81(9)
157.16(9)
89.25(9)
73.82(9)
72.95(8)
70.66(6)
155.2(2)
113.6(2)
116.6(2)
129.7(2)
CrꢀN(2)ꢀC(21)
CrꢀF(10)ꢀC(47)
CrꢀC(41)ꢀC(42)
CrꢀC(41)ꢀC(46)
163.2(2)
109.4(2)
116.3(2)
130.0(2)
155.0(7)
113.9(6)
117.4(7)
128.2(6)
Table 2
Average bond distances and angles in [M(NR)2(fmes)2] complexes (1, 2, 4) and non-fluorinated analogues [M(NR)2R%2] (5,6)*
1
2
4
5
6
M
R
Cr
Ad
Cr
Mo
tBu
Cr
Mo
tBu
tBu
tBu
Bond distances
MꢀN
MꢀC
M···F
1.638(7)
2.133(3)
2.453(9)
1.611(8)
2.144(15)
2.45(2)
1.727(5)
2.247(5)
2.471(4)
1.626(5)
2.034(6)
1.721(13)
2.162(7)
M···H
2.85
2.60, 2.76
Bond angles
NꢀMꢀN
CꢀMꢀC
MꢀCꢀC(R)
MꢀCꢀC(H)
112.3(1)
112.1(4)
139.9(3)
115(2)
110.6(2)
114.7(3)
112.5(3)
138.5(1)
116.5(2)
129.9(2)
138.3(2)
116.5(2)
129.9(2)
121.2(2)
115.1(7)
127.3(4)
122.6(3)
114.3(5)
128.1(5)
129.0(8)
*R%=2,4,6-Me3C6H2 (5), 2,6-Me2C6H3 (6).

184
A.S. Batsano6 et al. / Journal of Organometallic Chemistry 631 (2001) 181–187
,
der Waals radius of H (1.01 A) [17] and theoretically
The Cr···F interactions in 1 and 2 do not affect the
CꢀF bond lengths significantly. This is not surprising,
since the lone electron pairs of fluorine, which interact
with the vacant metal d-orbitals, are of essentially
p-character and hybridise little with the bonding
electrons.
,
calculated [18] radii of isolated atoms of Cr (2.42 A)
,
and Mo (2.56 A).
This effect could be explained in terms of the sheer
bulkiness of the aryl ligands, which push away the
imido ligands and ‘wedge’ the methyl groups into the
coordination sphere. However, the MꢀCꢀC angles of
each arene ligand are asymmetric, the smaller angle
being on the side of the M···H contact, implying attrac-
tive M···H interactions (adding to the 16-electron va-
lence shell of the metal atom) rather than forced
repulsive contacts. Simple steric repulsion energy calcu-
lations failed to reproduce the experimental molecular
geometry of 5 and 6 [15], which also supports the
existence of specific bonding interactions.
Molecules 1, 2 and 4 show even stronger distortions
of the same type. The CꢀMꢀC angles (138–140°) are
far wider than in 5 or 6 (121–123°), or in any transition
metal complex of this type, available in the Cambridge
Crystallographic Database (95–125°, average 111°)
[19]. The additional coordination sites are occupied by
two fluorine atoms from different fmes ligands, F(1)
and F(10) in 1 and 2. The M···F contacts are even
shorter than the M···H contacts in 5 and 6, even though
the van der Waals radius of F exceeds that of H by ca.
Further evidence of metal–fluorine interaction is pro-
vided by NMR spectra. Earlier we have found [7] that
both 1H and 19F spectra of complex 4 in solution
indicate a restricted rotation (‘locked’ conformation) of
ortho-CF3 groups of the fmes ligands below 323 K.
Although variable-temperature NMR experiments have
not been carried out in the present case, similar effects
were observed for complexes 1 and 2 even at room
temperature, indicating the existence of Cr···F interac-
tions in solution, probably of the same kind as were
found in the solid state. In contrast, the para-CF3
groups of fmes must have a very low rotation barrier.
Both such groups in molecule 1 are rotationally disor-
dered. For one of them the disorder was approximated
by two sets of fluorine atom positions with equal
occupancy (50%), for the other we could not find a
satisfactory model, while anisotropic refinement of only
one orientation left substantial diffuse residual electron
−3
,
density between the fluorine atoms (up to 0.7 e A
,
−3
,
,
0.3 A [17]. The Cr···F distances of 2.443(2) and 2.462(2)
A in 1, and 2.421(5) and 2.468(5) A in 2, are intermedi-
ate between the standard single bond in coordination
compounds of 1.870 A [20] and the sum of the van der
Waals radii of ca. 3.7 A [17,18]. Indeed they are com-
parable with the axial CrꢀF distance of 2.43 A in CrF2
[21], and cation–anion contacts of 2.449(5) A in
compared to 0.2 e A
elsewhere in the structure). In
,
,
structure 2, both para-CF3 groups also displayed very
large and elongated displacement ellipsoids of the
fluorine atoms, notwithstanding the low temperature at
which both structures were studied.
,
,
,
It is interesting to compare the coordination of the
Cr and Mo atoms in 1, 2 and 4. The molybdenum atom
is larger than chromium, as shown by the covalent radii
,
,
[Cr(py)4](PF6)2 [22] and 2.407(2) A in [Cr(NCMe)4]-
,
(BF4)2 [23]. It is noteworthy that the latter compound
has the cation and anion strongly associated even in
solution, as indicated by its low conductivity [23]. These
interactions can be regarded as hypervalent bonds (or,
in valence bond terms, as a covalent bondlion pair
resonance) [24]. The order 6 for the bond length d can
be roughly estimated by the formula 6=exp[(R−d)/
0.34], where R is the bond-valence parameter (for
of 1.27 and 1.10 A, respectively [27]. In agreement with
this, both the MꢀN and MꢀC bond lengths in 4 exceed
,
those in 1 and 2 by ca. 0.1 A. Sterically, there is nothing
to prevent the M···F distances elongating proportion-
ally to the covalent bonds, i.e. the whole coordination
sphere expanding isotropically. However, the Mo···F
,
distances are only 0.02 A longer than those for Cr···F,
suggesting relatively stronger attractive interactions
,
,
Cr(VI)ꢀF equal to 1.74 A) [25], yielding 6=0.12 for 1
with Mo (6=0.14, assuming R=1.81 A [25]). It is
noteworthy that Mo···H distances in 6 are shorter than
and 2. The metal coordination in complexes 1 and 2
can thus be described as 4+2, or intermediate between
tetrahedral and octahedral, including secondary MꢀF
bonding. The coordination chemistry of the CF unit in
fluorocarbons towards metal centres has been exten-
sively reviewed by Plenio [12]. In the first transition
series, however, there were considered to be only two
convincing reports of such interaction, one being a very
,
Cr···H in 5, by an average of 0.17 A.
MꢀC distances in complexes 1, 2 and 4 are longer
than in 5 and 6 by ca. 0.1 A. These s-bonds are
,
augmented by dp(M)“pp(C) back-bonding, which
weakens as the CꢀMꢀC angle increases, since the two
ligands are increasingly competing for the same (occu-
pied) metal d-orbital. All imido-ligands in the com-
plexes under consideration can be described as ‘linear’,
although, as indicated earlier, there is competition be-
tween p-donor ligands in a tetrahedral environment,
since only three of the possible four p-bonds can be
formed between the metal and the cis-imido ligands.
Their MꢀNꢀC angles vary considerably without a cor-
,
short F···Ti distance of 2.151(2) A in [Cp*Ti(FPh)]-
[BPh4] [26] and the other in the complex [VCl(fmes)2-
(thf)], discussed above [6]. More examples were de-
scribed for metals in the second transition metal series
[12], but none for molybdenum, apart from compound
4 [7] (see above).
2

A.S. Batsano6 et al. / Journal of Organometallic Chemistry 631 (2001) 181–187
185
Table 3
skeleton has a puckered conformation; the central te-
tragon of Li(1), Li(2), Cl(3) and Cl(4) is planar, the
deviations of other atoms from its plane being as
follows: Mo(1) 0.07, Mo(2) 1.32, Cl(1) −0.26, Cl(2)
,
Bond distances (A) and bond angles (°) for 3
Bond distances
Mo(1)ꢀN(1)
Mo(1)ꢀN(2)
Mo(1)ꢀC(51)
Mo(1)ꢀCl(1)
Mo(1)ꢀCl(3)
Mo(1)···F(9)
Li(1)ꢀO(1)
1.744(7)
1.754(7)
2.226(8)
2.434(3)
2.614(3)
2.528(5)
1.982(15)
2.008(17)
2.629(15)
2.362(15)
2.730(16)
1.376(11)
1.402(10)
Mo(2)ꢀN(3)
Mo(2)ꢀN(4)
Mo(2)ꢀC(61)
Mo(2)ꢀCl(2)
Mo(2)ꢀCl(4)
Mo(2)···F(16)
Li(2)ꢀO(2)
1.730(7)
1.766(7)
2.221(8)
2.433(3)
2.598(3)
2.551(5)
1.954(15)
2.069(16)
2.623(15)
2.567(14)
2.392(14)
1.405(10)
1.407(11)
,
0.56 A. The Cl(3) atom is co-planar with the surround-
,
ing molybdenum and two lithium atoms within 0.04 A,
while Cl(4) has a pyramidal environment, deviating
,
from the plane of Mo(2), Li(1) and Li(2) by 0.64 A.
The coordination of each Li atom is complemented to
distorted trigonal–bipyramidal by a chelating 1,2-
(dimethoxy)ethane (dme) ligand. The Mo(1) and Mo(2)
atoms have distorted tetragonal-pyramidal coordina-
tion (with N(1) and N(3) in apical positions), comple-
mented to distorted octahedral by one weakly
coordinated F atom of the fmes ligand, viz. F(9) trans
to N(1), and F(16) trans to N(3).
Li(1)ꢀO(2)
Li(2)ꢀO(4)
Li(1)ꢀCl(1)
Li(1)ꢀCl(3)
Li(1)ꢀCl(4)
N(1)ꢀC(11)
N(2)ꢀC(21)
Li(2)ꢀCl(2)
Li(2)ꢀCl(3)
Li(2)ꢀCl(4)
N(3)ꢀC(31)
N(4)ꢀC(41)
Bond angles
N(1)ꢀMo(1)ꢀN(2)
N(1)ꢀMo(1)ꢀCl(1)
N(1)ꢀMo(1)ꢀCl(3)
109.1(3)
95.4(1)
98.4(2)
N(3)ꢀMo(2)ꢀN(4)
N(3)ꢀMo(2)ꢀCl(2)
N(3)ꢀMo(2)ꢀCl(4)
N(3)ꢀMo(2)ꢀC(61)
N(3)ꢀMo(2)ꢀF(16)
N(4)ꢀMo(2)ꢀCl(2)
N(4)ꢀMo(2)ꢀCl(4)
N(4)ꢀMo(2)ꢀC(61)
N(4)ꢀMo(2)ꢀF(16)
Cl(2)ꢀMo(2)ꢀCl(4)
Cl(2)ꢀMo(2)ꢀC(61)
Cl(2)ꢀMo(2)ꢀF(16)
Cl(4)ꢀMo(2)ꢀC(61)
Cl(4)ꢀMo(2)ꢀF(16)
C(61)ꢀMo(2)ꢀF(16)
O(3)ꢀLi(2)ꢀO(4)
O(3)ꢀLi(2)ꢀCl(2)
O(4)ꢀLi(2)ꢀCl(4)
O(4)ꢀLi(2)ꢀCl(2)
O(4)ꢀLi(2)ꢀCl(3)
O(3)ꢀLi(2)ꢀCl(4)
O(3)ꢀLi(2)ꢀCl(3)
Cl(2)ꢀLi(2)ꢀCl(4)
Cl(2)ꢀLi(2)ꢀCl(3)
Cl(3)ꢀLi(2)ꢀCl(4)
Mo(2)ꢀN(3)ꢀC(31)
Mo(2)ꢀN(4)ꢀC(41)
108.5(3)
98.0(2)
98.5(2)
105.7(3)
170.9(2)
98.8(2)
152.8(2)
91.4(3)
80.4(2)
80.18(9)
149.7(2)
81.6(1)
77.9(2)
72.5(1)
72.1(2)
85.9(6)
99.3(6)
146.1(7)
90.3(5)
92.9(6)
127.7(7)
97.3(6)
80.4(4)
163.3(6)
87.8(4)
166.2(6)
166.9(6)
The Mo···F distances are slightly longer than in 4, in
accordance with the higher coordination number of the
metal atom. One para-CF3 group of one of the fmes
ligands shows a rotational disorder, approximated by
two orientations with occupancies of 90 and 10%.
N(1)ꢀMo(1)ꢀC(51) 105.4(3)
N(1)ꢀMo(1)ꢀF(9)
N(2)ꢀMo(1)ꢀCl(1)
N(2)ꢀMo(1)ꢀCl(3)
N(2)ꢀMo(1)ꢀC(51)
N(2)ꢀMo(1)ꢀF(9)
Cl(1)ꢀMo(1)ꢀCl(3)
170.8(2)
98.2(2)
152.5(2)
94.8(3)
80.1(3)
78.99(8)
3. Experimental
Cl(1)ꢀMo(1)ꢀC(51) 150.4(2)
Cl(1)ꢀMo(1)ꢀF(9)
Cl(3)ꢀMo(1)ꢀC(51)
Cl(3)ꢀMo(1)ꢀF(9)
C(51)ꢀMo(1)ꢀF(9)
O(1)ꢀLi(1)ꢀO(2)
O(1)ꢀLi(1)ꢀCl(1)
O(1)ꢀLi(1)ꢀCl(3)
O(1)ꢀLi(1)ꢀCl(4)
O(2)ꢀLi(1)ꢀCl(1)
O(2)ꢀLi(1)ꢀCl(3)
O(2)ꢀLi(1)ꢀCl(4)
Cl(1)ꢀLi(1)ꢀCl(3)
Cl(1)ꢀLi(1)ꢀCl(4)
Cl(3)ꢀLi(1)ꢀCl(4)
83.3(1)
77.3(2)
72.4(2)
72.9(2)
85.3(6)
99.5(6)
142.7(8)
88.8(6)
92.1(7)
132.0(7)
102.9(5)
80.0(4)
163.5(7)
84.7(5)
3.1. General
All manipulations were carried out under an atmo-
sphere of dry nitrogen, using standard Schlenk and
cannula techniques, or in a conventional nitrogen-filled
glove-box. Solvents were refluxed over an appropriate
drying agent, and distilled and degassed prior to use.
Elemental analyses were performed by the microanalyt-
ical service of the Chemistry Department at Durham.
NMR spectra were recorded at 25 °C, on a Varian
VXR 400 S spectrometer at 400.0 MHz (1H), 376.32
MHz (19F) or 100.582 MHz (13C); chemical shifts are
referenced to the residual protio impurity of the deuter-
ated solvent (CDCl3). 1,3,5-(CF3)3C6H3 (Hfmes) and
Li(fmes) were prepared by previously published proce-
dures [7].
Mo(1)ꢀN(1)ꢀC(11) 166.7(6)
Mo(1)ꢀN(2)ꢀC(21) 168.7(6)
Mo(1)ꢀF(9)ꢀC(59)
113.2(5)
Mo(2)ꢀF(16)ꢀC(69) 110.7(4)
responding change in the MꢀN distance, in agreement
with the earlier observation [13] that this angle is rela-
tively ‘soft’. The MꢀN bond lengths approach the up-
3.2. Synthesis of [Cr(NAd)2( fmes)2] (1)
,
per limit of the usual range (1.58–1.64 A) for
To [Cr(NAd)2Cl2] [13] (250 mg, 0.59 mmol) in Et2O
(20 ml) were added two equivalents of Li(fmes) (1.20
mmol) in Et2O (20 ml) at −78 °C. The solution was
allowed to warm up to room temperature (r.t.), and
stirred overnight. After removal of solvent the residue
was extracted with pentane (3×40 ml); the extracts
were reduced in volume to ca. 25 ml and cooled to
−78 °C to give deep red crystals of 1; yield 400 mg
(74%), m.p. 158–160 °C. Anal. Found: C, 50.25; H,
3.87; N, 3.49. Calc. for C38H34CrF18N2: C, 50.01; H,
3.75; N, 3.06%. MS: m/e 913 [M+]. 1H-NMR: l 8.05 (s,
1H, m-ArH), 7.82 (s, 1H, m-ArH), 2.02 (s, 3H, AdꢀHg),
metal–imido bonds [28]. In each of the complexes 1, 2
and 4 the two MꢀNꢀC angles differ by 8–9°, while in 5
and 6 they are virtually equal (159–161°) with the same
average value.
The asymmetric unit of structure 3 comprises one
dimeric molecule of the complex (Fig. 3, Table 3) and
one pentane molecule of crystallisation, which is
severely disordered. Molecule 3 contains a double-rib-
bon Mo2Li2Cl4 skeleton, where Cl(1) and Cl(2) are
m2-bridging between Mo and Li, while Cl(3) and Cl(4)
are m3-bridging between Mo and two Li atoms. This

186
A.S. Batsano6 et al. / Journal of Organometallic Chemistry 631 (2001) 181–187
1.88 (s, 6H, AdꢀHd), 1.54 (s, 6H, AdꢀHb). 19F-NMR: l
−56.8 (s, 6F, o-CF3), −59.1 (s, 6F, o-CF3), −62.9 (s,
6F, p-CF3). 13C{1H}-NMR: l 175.8 (br, ipso-C), 138.5
(q, 2JCF=30 Hz, o-CCF3), 137.2 (q, 2JCF=30 Hz,
o-CCF3), 128.7 (q, 2JCF=34 Hz, p-CCF3), 126.7 (q,
1JCF=275 Hz, p-CF3), 123.9 (q, 1JCF=273 Hz, o-
(d, 12H, CHMe2), 1.03 (d, 12H, CHMe2). 19F-NMR: l
−63.6 (p-CF3). 13C{1H}-NMR: l 155–118 (accurate
assignment of aryl resonances in this region was ham-
pered by poor resolution and overlap of signals), 71.2
(s, MeOCH2CH2OMe), 60 (s, MeOCH2CH2OMe), 28.4
(br s, CHMe2), 24.6 (s, CHMe2).
1
CF3), 123.2 (q, JCF=272 Hz, o-CF3), 125.0, 124.8 (br
s, m-C), 82.8 (s, AdꢀCa), 43.7 (s, AdꢀCb), 35.6 (s,
AdꢀCd), 29.5 (s, AdꢀCg).
3.4. X-ray crystallography
Crystals of 1 and 2 (deep-red) and 3 (orange-red) of
X-ray quality, were grown by slow evaporation of
pentane solutions at −20 °C (1) and −78 °C (2 and
3). The X-ray diffraction data for 2 were collected on a
three-circle SMART diffractometer with a 1 K CCD area
detector and processed, using SMART and SAINT soft-
ware [29]. More than a hemisphere of reciprocal space
was covered by three sets of ꢀ-scans, each set at
different „ and/or 2q angles. The data for 1 and 3 were
collected on a four-circle Rigaku AFC6S diffractometer
with (2q/ꢀ scan mode), using graphite-monochromated
3.3. Synthesis of
[Mo(NC6H3iPr2-2,6)2( fmes)Cl·LiCl(dme)] (3)
To [Mo(NC6H3iPr2-2,6)2Cl2·dme] (1.0 g, 1.66 mmol),
prepared as described earlier [14b], in Et2O (20 ml) were
added two equivalents (3.32 mmol) of Li(fmes) in Et2O
(20 ml) at −78 °C. The solution was allowed to warm
up to r.t., and stirred overnight. After removal of
solvent the residue was extracted with pentane (3×40
ml); the extracts were reduced in volume to ca. 25 ml
and cooled to −78 °C to give orange–red crystals of
3; yield 950 mg (63%). Anal. Found: C, 49.51; H, 5.19;
N, 3.56%. Calc. for C37H46Cl2F9LiMoN2O2: C, 49.62;
H, 5.17; N, 3.13%. MS: m/e 763 [Mo(NC6H3iPr2-
2,6)2(fmes)Cl]+. 1H-NMR: l 8.09 (s, 2H, fmes-H),
7.09–7.02 (m, 6H, m,p-C6H3), 3.9 (sept, 2H, CHMe2),
3.7 (s, 6H, MeOCH2CH2OMe), 3.5 (s, 4H, MeOCH2-
(
,
Mo–Ka radiation (u=0.71073 A) and a Cryostream
(Oxford Cryosystems) open-flow N2 gas cryostat, and
integrated using TEXSAN programs [30]. The structures
were solved by direct methods and refined by full-ma-
trix least-squares against F2 of all data, using SHELXTL
software [31]. Crystal data and experimental details are
summarized in Table 4.
3
CH2OMe), 2.9 (sept, 2H, JCH=6.8 Hz, CHMe2), 1.26
Table 4
4. Conclusions
Crystallographic data and experimental details
In conclusion, we consider that M···F secondary
interactions play a significant role in stabilising the
structures of the fmes complexes of Group VI transi-
tion metals.
Compound
1
2
3
Empirical
formula
Formula weight 912.7
C38H34CrF18N2
C
26H22CrF18N2 C74H92Cl4F18Li2-
Mo2N4O4·C5H12
1863.2
756.5
T (K)
150
150
150
Symmetry
Space group
Unit cell dimensions
Triclinic
Monoclinic
P21/n (c 14) P1 (c 2)
Triclinic
(
5. Supplementary material
(
P1 (c 2)
,
a (A)
12.099(1)
12.818(1)
14.466(1)
97.57(1)
113.49(1)
106.36(1)
1896.8(3)
2
12.760(3)
9.451(2)
25.330(5)
90
99.10(3)
90
3016(1)
4
4302
16.127(11)
18.186(10)
18.347(8)
118.80(5)
104.26(5)
92.78(4)
4481(4)
2
Crystallographic data for the structural analysis have
been deposited with the Cambridge Crystallographic
Data Centre, CCDC nos. 161590, 161591 and 161592
for compounds 1, 2 and 3, respectively. Copies of this
information may be obtained free of charge from The
Director, CCDC, 12 Union Road, Cambridge CB2
1EZ, UK (Fax: +44-1223-336033; e-mail: deposit@
ccdc.cam.ac.uk or www: http://www.ccdc.cam.ac.uk).
,
b (A)
,
c (A)
h (°)
i (°)
k (°)
V (A )
3
,
Z
Reflections
collected
Unique
reflections
Rint
10 605
14 123
6167
2810
13 676
0.043
5185
0.116
1795
0.047
7062
Reflections
Acknowledgements
F2\2|(F2)
R[F2\2|(F2)] 0.044
0.077
0.225
0.061
0.183
The authors thank EPSRC for a Senior Research
Fellowship (J.A.K.H.), EPSRC and Durham University
for financial support.
wR(F2), all
0.116
data

A.S. Batsano6 et al. / Journal of Organometallic Chemistry 631 (2001) 181–187
187
[13] M.P. Coles, V.C. Gibson, W. Clegg, M.R.J. Elsegood, Polyhe-
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