Bis(cyclopentadienyl) diimido complexes of molybdenum and tungsten [Cp2M(NR)2] at the limit of pi-bond saturation

Article abstract of DOI:10.1002/1099-0682(200106)2001:6<1617::AID-EJIC1617>3.0.CO;2-B

The new imido compounds [(η⁵-C₅H₅)M(NR)₂(η ¹-C₅H₅)] (R = tBu: M = Mo 2a, W 2b; R = Mes: M = W 2c) have been prepared and the crystal structure of 2c has been determined. Three fl

Full text of DOI:10.1002/1099-0682(200106)2001:6<1617::AID-EJIC1617>3.0.CO;2-B

FULL PAPER
Bis(cyclopentadienyl) Diimido Complexes of Molybdenum and Tungsten
[Cp2M(NR)2] at the Limit of Pi-Bond Saturation
Udo Radius,[a] Jörg Sundermeyer,*[b] Karl Peters,[c] and Hans-Georg von Schnering[c]
Dedicated to Professor Max Schmidt on the occasion of his 75th birthday
Keywords: Imides / Molybdenum / Tungsten / Cyclopentadienyl ligands / Rearrangements
The new imido compounds [(η5-C5H5)M(NR)2(η1-C5H5)] (R =
tBu: M = Mo 2a, W 2b; R = Mes: M = W 2c) have been pre- metal complex fragment between the carbon atoms of the
pared and the crystal structure of 2c has been determined. σ-bonded C5H5 ligand (metallotropic migration, sigmatropic
gand about the M−Cipso-bonding axis, (ii) migration of the
Three fluxional processes can be detected by variable tem- rearrangement), and (iii) the interconversion of hapticity of
perature proton NMR spectroscopic studies of these cyclo- σ- and π-bonded C5H5 ligands.
pentadienyl complexes, namely: (i) rotation of the σ-C5H5 li-
Introduction
highly π-bond saturated bis(cyclopentadienyl) complexes
[(η5-C5H5)M(NR)2(η1-C5H5)].
Molybdenum(VI) and tungsten(VI) bis(organyl) imido
complexes of the type [M(NR)2R1R2] (M ϭ Mo, W) have
been known for twenty years.[1,2] In the early eighties, the
imido-bridged compounds [M(NtBu)(µ-NtBu)(CH3)2]2
were synthesized and characterized by Nugent and co-
workers as the first examples of unsymmetrically bridged
dinuclear imido compounds. Mononuclear complexes can
be obtained by increasing the steric demand of the organyl
ligand of the imido compound. Wilkinson et al.,[3] for ex-
ample, reported the syntheses of symmetrically substituted
bis(aryl) derivatives (R ϭ tBu, R1 ϭ R2 ϭ mesityl, o-tolyl),
whereas bis(neopentyl) compounds, like those prepared by
the groups of Schrock and Osborn,[4] are attractive starting
materials for the synthesis of high valent alkylidene com-
plexes.
Important intermediates for unsymmetrically substituted
bis(organyls) (R1 ϶ R2) are monoorganyl complexes of the
type [M(NR)2R1Cl]. During our studies on reactive, N-ba-
sic [MoϭNR] and [WϭNR] functionalities, we intended to
reduce the Lewis acidity of the metal centers of imido com-
plexes [M(NR)2Cl2] by introducing excellent σ,π donor li-
gands. This can be achieved by a formal exchange of a
[MϪCl] functionality by [M(η5-C5RЈ5)] (RЈ ϭ H, CH3).[5]
Complexes of the type [(η5-C5H5)M(NtBu)2Cl] or mixed
substituted organyl complexes of the type [(η5-
C5H5)M(NtBu)2R2] (R2 ϭ alkyl, aryl)[5] are no longer typ-
ical d-metal Lewis acids.[6] Here we wish to report the syn-
theses, molecular structure and dynamic behavior of the
Results and Discussion
Complex Synthesis
The
cyclopentadienyl
imido
complexes
[(η5-
C5H5)M(NtBu)2Cl] (M ϭ Mo 1a; M ϭ W 1b) were reacted
with typical Lewis bases in order to probe the Lewis acidity
of these compounds. Reaction of trimethylphosphane with
[(η5-C5H5)W(NtBu)2Cl] (1b) led to an ocher colored solid
and
a
brownish oil. One of the products,
[W(NtBu)2Cl2(PMe3)] (3),[7] was identified by 31P NMR
spectroscopy, but no phosphorus resonance was obtained
for the second product. The proton NMR spectrum of the
oily residue shows one sharp singlet in the tBu imido region
at approximately δ ϭ 1.2, and a very broad resonance in
the cyclopentadienyl region. The integration of both signals
shows a ratio of 9:5. These experimental results are in ac-
cordance with
a
ligand dismutation of [(η5-
C5H5)W(NtBu)2Cl] (1b) to give [(C5H5)2W(NtBu)2] (2b)
and [W(NtBu)2Cl2] by a phosphane-induced exchange of a
chloride and a cyclopentadienide ligand. The latter Lewis
acid is trapped by the phosphane to yield
[W(NtBu)2Cl2(PMe3)] (3).
[a]
Institut für Anorganische Chemie,
Engesserstr., Geb. 30.45, 76128 Karlsruhe, Germany
E-mail: uradius@achibm6.chemie.uni-karlsruhe.de
Green and co-workers observed a similar unexpected
substitution reaction of a cyclopentadienyl ligand, with sta-
bilization of the resulting MoIV species, whilst studying the
reduction reactions of [(η5-C5H4Me)Mo(NtBu)Cl2] in the
presence of phosphanes.[8]
[b]
Fachbereich Chemie der Universität Marburg,
Hans-Meerweinstraße, 35032 Marburg, Germany
E-mail: jsu@chemie.uni-marburg.de
[c]
Max-Planck-Institut für Festkörperforschung,
Heisenbergstr. 1, 70506 Stuttgart, Germany
Eur. J. Inorg. Chem. 2001, 1617Ϫ1623
WILEY-VCH Verlag GmbH, 69451 Weinheim, 2001
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U. Radius, J. Sundermeyer, K. Peters, H.-G. von Schnering
FULL PAPER
To confirm the results of this rather unusual displace-
ment of an anionic C5H5 ligand by a phosphane, we pre-
pared 2b independently. All experiments to obtain 2b or its
molybdenum analogue [(C5H5)2Mo(NtBu)2] 2a, by reaction
of the chloro complexes [(η5-C5H5)M(NtBu)2Cl] 1a,b with
(C5H5)Li were unsuccessful. However, refluxing 1a,b with
(C5H5)Na in dimethoxyethane for several hours led to the
formation of the bis(cyclopentadienyl) compounds
[(C5H5)2M(NtBu)2] 2a,b isolated as yellow-brown oils.
These highly covalent organo imides 2a,b can be purified
by high-vacuum distillation or condensation.
Figure 1. Molecular structure of 2c in the solid state;
SCHAKAL[27] plot; hydrogen atoms are omitted for clarity; se-
lected bond lengths (pm) and angles (°): WϪN(1) 176.6(4),
WϪN(2) 175.9(4), WϪC(1) 233.4(6), WϪC(2) 244.0(6), WϪC(3)
250.2(6), WϪC(4) 247.4(6), WϪC(5) 241.6(6), WϪC(6) 220.0(6),
N(1)ϪC(11) 138.7(7), N(2)ϪC(12) 139.2(6), C(1)ϪC(5) 139.9(8),
C(1)ϪC(2) 140.4(10), C(2)ϪC(3) 139.6(9), C(3)ϪC(4) 139.9(9),
As we were not able to obtain crystalline material of the
tert-butyl imido complexes 2a,b by low temperature crystal-
lization techniques, we synthesized the corresponding mesi-
tyl imido tungsten complex [(C5H5)2W(NMes)2] (2c) This
complex can also be obtained from the reaction of C(4)ϪC(5) 137.2(11), C(6)ϪC(7) 149.1(9), C(7)ϪC(8) 133.5(10),
[(C5H5)W(NMes)2Cl] (1c) with sodium cyclopentadienide
C(8)ϪC(9) 142.6(11), C(9)ϪC(10) 136.5(12), C(6)ϪC(10) 146.5(8);
N(1)ϪWϪN(2) 107.7(2), N(1)ϪWϪC(6) 97.3(2), WϪN(1)ϪC(11)
in dimethoxyethane. Single crystals of 2c were grown by the
slow cooling of saturated hexane solutions of 2c to Ϫ60 °C.
164.4(3), WϪN(2)ϪC(12) 166.8(4), N(2)ϪWϪC(6) 99.7(2)
lengths,[9c] whereas the distances WϪC(5) [241.6(6) pm],
WϪC(2) [244.0(0) pm], WϪC(3) [250.2(6) pm], and
WϪC(4) [247.4(6) pm], are significantly elongated. Com-
parable elongations of the WϪC(ring) bonds trans to
strong π-donor ligands have been observed by Bercaw et al.
for
[(η5-C5Me5)W(O)2(O-C5Me5)]
and
[(η5-
C5Me5)W(O)2(η1-C5Me5)], and were attributed to the trans
influence of the oxo ligands.[10] We have observed similar
effects in the structurally characterized half-sandwich imido
complex [(η5-C5Me5)Mo(NtBu)Cl3],[5c] but they are also
characteristic for the nitrosyl compounds of the type [(η5-
C5H5)2Mo(NO)R] (R ϭ alkyl)[11] and for the metallocene
imido derivatives [(η5-C5H5)2V(NoTol)][12] and [(η5-
C5H4Me)2Mo(NtBu)].[13] Calculations on the latter 19 and
20 valence electron (VE) complexes show that the HOMO
of these compounds is a ligand-centered orbital with signi-
ficant contributions to the nitrogen atoms of the imido
group and to the π-system of the cyclopentadienyl units. In
a MO bond description, one of the p orbitals located on
the imido nitrogen atoms competes with the two group or-
bitals of π-symmetry (e1) of the η5-coordinated ring for sta-
bilization with one empty d orbital located on the metal
center. As a result, a high degree of metalϪnitrogen bond-
ing (e.g. MϪN triple bond contributions) leads to a de-
crease in metalϪcyclopentadienyl π-bonding, and thus to a
ring slippage of the η5-C5R5 ring.
Molecular Structure of 2c
[(C5H5)2W(NMes)2] (2c) crystallizes in the triclinic space
¯
group P1. The molecular structure is shown in Figure 1.
Selected bond lengths and angles are given in the caption.
Further details of the crystallographic characterization are
given in Table 1.
[(C5H5)2W(NMes)2] (2c) is a monomer in the solid state.
The tungsten atom is pseudotetrahedrally coordinated by
the two imido nitrogen atoms, by the coordinating carbon
atom of a σ-bonded cyclopentadienyl ligand, and by the
centroid of a π-coordinated cyclopentadienyl unit. The
bonding angles N(1)ϪWϪN(2) 107.7(2)°, N(1)ϪWϪC(6)
97.3(2)°, and N(2)ϪWϪC(6) 99.7(2)° reflect some distor-
tion of the coordination polyhedron. The WϪN bond
lengths of 176.6(4) pm and 175.9(4) pm are consistent, to
some degree, with tungsten imido-nitrogen triple bonding,
corresponding to a bond order of 2.5[9a] and comparable
with
that of
the
π-bond saturated complex
[W(NPh)(NMe2)4] [WϪN(imido)
ϭ
175.8(5)].[9b] The
WϪC(6) distance of 220.0(6) pm is in the range of
tungsten(VI)Ϫcarbon σ-bonds. The tungstenϪcarbon bond
lengths of the π-coordinated cyclopentadienyl ring show
some interesting irregularities: WϪC(1) [233.4(6) pm] falls
1
Variable Temperature H NMR Spectra
The proton NMR spectra of the compounds
into the range of expected tungsten ring carbon bond [(C5H5)2M(NR)2] 2aϪc are temperature dependant. Three
1618
Eur. J. Inorg. Chem. 2001, 1617Ϫ1623

Bis(cyclopentadienyl) Diimido Complexes of Molybdenum and Tungsten
FULL PAPER
ment [(η5-C5H5)M(NR)2] between the carbon atoms of the
monohapto-coordinated cyclopentadienyl ring, and (iii) ro-
tation of the σ-cyclopentadienyl ligand around the
MϪCipso bonding axis.
Table 1. Crystal Structure Analysis of 2c
Compound
Empirical formula
Molecular mass
a [pm]
b [pm]
c [pm]
[Cp2W(NMes)2]
C28H32N2W
580.43
1005.7(1)
1606.5(3)
806.7(1)
The cyclopentadienyl region between δ ϭ 4.0 and 8.5 in
the proton NMR spectra of [(C5H5)2Mo(NtBu)2] (2a), re-
corded at different temperatures, is shown in Figure 2. At
1
α [deg]
ß [deg]
98.55(1)
94.43(1)
363 K, the H NMR spectrum of 2a shows one averaged
signal at δ ϭ 5.78 assigned to the cyclopentadienyl protons
and a resonance at δ ϭ 1.13 assigned to the protons of
the imido group (not shown in Figure 2), with a ratio of
integration of 5:9. On cooling the signal of the tert-butyl
imido group remains constant within experimental error,
while the cyclopentadienyl resonance broadens and splits
into two signals below the coalescence temperature of
308 K. At 283 K, two broad signals of similar width are
detectable [∆G϶(308 K) ϭ 61.0 kJ/mol]. The signal at δ ϭ
6.26 is assigned to the protons of the η1-coordinated cyclo-
pentadienyl ring due to its chemical shift[15] and the charac-
teristic splitting at lower temperatures. The resonance at
higher field (δ ϭ 5.33) can be assigned to the protons of
the η5-coordinated cyclopentadienyl ligand. A further de-
crease in the temperature from 308 K (the point of coales-
cence for the σ/π exchange) to 196 K (the point of coales-
cence for the metallotropic rearrangement) leads to a
broadening of the resonance at δ ϭ 6.26 and to a
γ [deg]
105.54(1)
1232.3(3) ϫ 106
2
V [pm3]
Z
d(calcd) [g·cmϪ3
Crystal system
Space group
Diffractometer
Radiation
]
1.564
Triclinic
¯
P1
Siemens R3m/V
Mo-Kα
graphite
0.2 ϫ 0.45 ϫ 0.75
ω-scan
1.75Ϫ27.5
h ϭ 0 Ǟ 13
k ϭ Ϫ20 Ǟ 19
l ϭ Ϫ10 Ǟ 10
5663
5663
4977
4.70
ψ-scan
Monochromator
Crystal size [mm]
Data collection mode
Theta range [deg]
Recip. latt. segment
No. refl. Measd.
No. unique refl.
No. refl. F Ͼ 3σ(F)
Lin. abs. coeff. [mmϪ1
Abs. Correction
Solution by
Method of refinement
]
Direct phase determination
Full-matrix LSQ; hydrogen positions
of riding model with fixed isotropic U
17.71
Data-to-parameter ratio
R, Rw
Weighting scheme
Largest difference peak
Largest difference hole
Program used
0.031, 0.030
w ϭ 1/σ2(F)
Ϫ3
˚
˚
1.80 eA
Ϫ3
1.15 eA
Siemens SHELXTL PLUS:
G. M. Sheldrick, Program package
SHELXTL-PLUS, Release 4.1,
Siemens Analytical X-ray Instruments
Inc., Madison, WI 53719, USA, 1990
dynamic processes, which have been observed previously in
solutions of other η1/η5 bis(cyclopentadienyl) complexes,
are responsible for this dynamic behavior (see Scheme 1),
namely: (i) hapticity change of σ- and π-coordinated cyclo-
pentadienyl rings, (ii) metallotropic migration of the frag-
Scheme 1. Hapticity change and mechanistic proposal of a metallo-
Figure 2. Variable temperature proton NMR spectra of [(η5-
tropic rearrangement via allyl intermediates in complexes of the C5H5)Mo(NtBu)2(η1-C5H5)] (2a) in [D8]toluene showing the C5H5
type [(η5-C5H5)M(NR)2(η1-C5H5)] (M ϭ Mo, W)
region from δ ϭ 4.00 to 8.50
Eur. J. Inorg. Chem. 2001, 1617Ϫ1623
1619

U. Radius, J. Sundermeyer, K. Peters, H.-G. von Schnering
FULL PAPER
sharpening of the η5-cyclopentadienyl resonance. Below bonded cyclopentadienyl ligand to η3-coordination more
this second coalescence temperature of 196 K, the signal of easy, thus it might initiate the exchange from an η1- via η3-
the η1-C5H5 group splits into two resonances for the ol- to an η5-coordination. A similar flexibility is characteristic
efinic protons at δ ϭ 6.82 and 6.71 (at 188 K), and one for the NO ligand. A mechanism as proposed in Scheme 1
signal at δ ϭ 5.05 for the proton of the CH group bound via intermediates of the type [(η5-C5H5)(η3-C5H5)M(NR)2]
to the metal atom, as expected for a not-yet-resolved has been observed for [(C5H5)2W(CO)2], in which one
AAЈBBЈX spin-spin system.
cyclopentadienyl ligand adopts an η3-, and the other one
Similar spectra can be recorded for the tungsten com- an η5-coordination mode.[22]
pound [(C5H5)2W(NtBu)2] (2b). The coalescence temper-
ature for σ,π exchange is slightly higher for the tungsten Metallotropic Migration
complex 2b [∆G϶(310 K) ϭ 64.4 kJ/mol] than for the mo-
lybdenum analogue 2a.
Over the last 40 years a large number of η1-C5H5 com-
pounds of main group elements and transition metals have
been described.[23] Whilst the migration of the metal sub-
stituent between the ring carbon atoms has recently been
recognized as an intramolecular process,[24] the exact mech-
anism of the metallotropic rearrangement has remained an
unresolved problem for some time. Conclusions were finally
drawn from statistical considerations of the different pro-
cesses under consideration (1,2 shift vs. 1,3 shift vs. random
walk) and the initial broadening of the resonances of the
vinyl protons below coalescence temperature. Investigations
with prochiral complex fragments finally established the
[1,2]-metallotropic migration (see Scheme 2) as the energet-
ically most favorable mechanism.[25] The latter may be re-
garded as a symmetry-allowed [1,5]-sigmatropic rearrange-
ment.
The temperature-dependent 1H NMR spectra of the
tungsten compound [(C5H5)2W(NMes)2] (2c) offer some
peculiarities. At a relatively high coalescence temperature of
328 K [compared with the tert-butyl derivatives 2a (308 K)
and 2b (310 K)], the resonance for the cyclopentadienyl
protons of 2c splits into two signals at δ ϭ 6.25 and 5.47.
By lowering the temperature, the signal at δ ϭ 6.25 further
splits into a set of five resonances. They are assigned to the
vinylic protons of 2c HA/AЈ (δ ϭ 7.01 and 6.90) and HB/BЈ
(δ ϭ 6.57 and 6.55) and to the proton HX at the ipso carbon
(δ ϭ 6.57). At 210 K coalescence of the vinylic pairs of pro-
tons is observed. In the low temperature limiting spectrum
at 193 K, every proton of the η1-coordinated cyclo-
pentadienyl ligand is chemically inequivalent indicating that
at very low temperatures the rotation about the metal ipso-
carbon bond as a third dynamic process, besides the
hapticity change and the metallotropic migration, is slowly
resolved on the NMR time scale. The significantly higher
coalescence temperature for the mesityl complex 2c than for
2a,b may be explained by the higher mass and by the in-
creased steric demand of the migrating group [(η5-
C5H5)W(NMes)2] compared with [(η5-C5H5)2M(NtBu)2]
(M ϭ Mo, W). In a similar manner it has been reported by
Werner et al.[16] that the rate of the σ/π exchange in [(η5-
C5H5)Pd(η1-C5H5)(PR3)] is dependant on the cone angle[17]
of the phosphane ligand PR3.
Scheme 2. [1,2]-metallotropic migration as [1,5]-sigmatropic re-
arrangement
According to Cotton and co-workers,[25] [1,2]-migration
can be identified by a preferred broadening of the signals
of HA/AЈ below the coalescence temperature of this process,
while a [1,3]-shift can be detected by a preferred broadening
Mechanistic Considerations
Hapticity Change
The exchange reaction between σ- and π-bonded cyclo- of the resonances of HB/BЈ. The proton NMR spectra of 2c
pentadienyl ligands has been well investigated, for example, in the relevant temperature range between 220 K and 260 K
for
[(η5-C5H5)3Ti(η1-C5H5)],[18]
[(η5-C5H5)2Mo(η1- show a larger broadening for the HA/AЈ and HX signals than
C5H5)(NO)],[11] [(η5-C5H5)Mo(η1-C5H5)(S2CNR2)2] (R ϭ for HB/BЈ. This is in agreement with a nonrandom mechan-
Me, Bu),[19] [(η5-C5H5)Pd(η1-C5H5)(PiPr3)],[16] [(η5- ism of the metallotropic rearrangement, and the qualitative
C5H5)V(NtBu)(η1-C5H5)(OtBu)],[20]
and
[(η5- consideration indicates a [1,2] shift of the molybdenum or
C5H5)2Nb(NtBu)(η1-C5H5)].[21] Scheme 1 shows a mechan- tungsten center between the carbon atoms of the σ-(C5H5)
istic proposal for the hapticity change in the imido com- ligand. It seems that a concerted [1,5]-sigmatropic re-
pounds [(C5H5)2M(NR)2] (M ϭ Mo, W).
arrangement has a lower energy barrier than the formation
As observed in other molecular compounds with η1- and of an allylic intermediate. However, due to the flexibility in
η5-coordinating cyclopentadienyl ligands, for example in the electron count of the imido ligand it cannot be excluded
[(η5-C5H5)2Mo(η1-C5H5)(NO)],[11] a stepwise or concerted that, at higher temperatures, both the [1,2]- and the energet-
hapticity change is promoted by additional, electronically ically less favorable [1,3]-migration might contribute to the
flexible ligands. The reduction of the imido ligand bond metallotropic migration. We examined the possibility of de-
order (BO) from a six VE donor [NR]2Ϫ (BO ϭ 3) to a four termining rate constants of the low temperature rearrange-
VE donor [NR]2Ϫ (BO ϭ 2) makes the slippage of an η1- ments by recording the spectra at higher frequency; how-
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Bis(cyclopentadienyl) Diimido Complexes of Molybdenum and Tungsten
FULL PAPER
Chloro(cyclopentadienyl)bis(mesitylimido)tungsten(VI)
(1c):
[W(NMes)2Cl2(dme)] (600 mg, 0.96 mmol) was added at Ϫ78 °C to
a suspension of C5H5Li (72.0 mg, 1.00 mmol) in 50 mL diethyl
ether. The yellow solution was brought to room temperature within
30 minutes. After 2 h at room temperature, all volatiles were re-
moved and the residue was dried at 50 °C in vacuo and finally
extracted into 15 mL toluene. The dark red extract was concen-
trated to 3 mL. On addition of 15 mL hexane, a red solid precipit-
ated, which was collected and dried in vacuo. Ϫ Yield: 333 mg
(75%); dec.: 101 °C Ϫ IR (nujol): ν˜ ϭ 3104 w ν(CϪHar), 1600 m
ν (CϭC), 1348 s, 1320 vs and 1288 vs ν (WϭNϪC), 1036 w, 1006
w, 988 m, 936 m, 856 m, 812 s ν (CϪHar)oop, 724 m, 504 w, 460 m,
ever, the rather complex exchange system turned out to be
intractable by the methods available to us.
Conclusions
The new imido complexes [(η5-C5H5)M(NR)2(η1-C5H5)]
(M ϭ Mo, W, R ϭ tBu; M ϭ W, R ϭ Mes) 2aϪc have
been prepared. Although the metal atoms of the bis(or-
ganyl) compounds 2aϪc are in their highest formal oxida-
tion state, the Lewis acidity of the d0 metal centers is well
compensated by both types of the excellent σ-,2π- donor
ligands [NR]2Ϫ and [C5H5]Ϫ. The compounds 2aϪc are π-
bond saturated. That means two potential π-bonds of each
C5H5 ligand (e1 group orbitals of C5H5) and two potential
π-bonds of each imido ligand (p orbitals of N atoms) add
1
424 m cmϪ1. Ϫ H NMR (200.1 MHz, CDCl3): δ ϭ 2.32 (s, 18 H,
Mes-CH3), 6.56 (s, 5 H, C5H5), 6.84 (s, 4 H, Mes-H). Ϫ 13C NMR
(50.3 MHz, CDCl3): δ ϭ 18.70 (s, o-CH3), 20.69 (s, p-CH3), 109.64
(s, C5H5), 127.74 (s, Mes-C3), 131.16 (s, Mes-C2), 133.46 (s, Mes-
C4), 152.73 (s, 2JWC ϭ 33.3 Hz, Mes-C1). Ϫ EI-MS: m/z (%) ϭ 550
(100) [Mϩ], 485 (67) [Mϩ Ϫ C5H5]. Ϫ C23H27ClN2W (550.8): calcd.
C 50.16, H 4.94, N 5.09; found C 49.69, H 5.06, N 5.03.
to a number exceeding the maximum of five π-bonds theor-
etically possible from orbital symmetry considerations in
tetrahedral transition metal complexes.[26] It has been
proven that in their competition for π-bonding with the d0
metal acid, the stronger π-donors [NR]2Ϫ are driving the
weaker π-donors [C5H5]Ϫ out of the π-coordination. There-
fore, these complexes may be considered as 18 VE com-
plexes of a d0 metal center stabilized by four σ-bonds to
two C5H5 and to two imido ligands, as well as by five π-
bonds, two to the η5-C5H5 ligand and three π-bonds to
both imido ligands, leading to an average bond order of
2.5. This consideration implies that a certain amount of π-
electron density remains in ligand-centered nonbonding or-
bitals. Structural data and the σ,π dynamic behavior of the
compounds 2aϪc in solution are consistent with this con-
clusion. The stronger π-donor ability of imido vs. cyclo-
pentadienyl ligands at a d0 metal center has also been dem-
onstrated by Green et al. for the isolobally related niobium
compound [(η5-C5H5)2Nb(NtBu)(η1-C5H5)].[21] Due to the
electronic flexibility of the four vs. six electron imido ligand
[NR]2Ϫ, an allylic species [(η5-C5H5)(η3-C5H5)M(NR)2]
(M ϭ Mo, W) might be considered as an intermediate of
the η5/η1 exchange process and for the metallotropic [1,3]
migration; however, as in the related niobium imido com-
plex, the [1,2]-metallotropic rearrangement is the dominant
mechanism in the dynamic behavior at the low temper-
ature limit.
Bis(tert-butylimido)bis(cyclopentadienyl)molybdenum(VI)
(2a):
Finely powdered 1a (339 mg, 1.00 mmol) was added to a suspen-
sion of (C5H5)Na (106 mg, 1.20 mmol) in 20 mL 1,2 dimethoxy-
ethane at Ϫ50 °C. The orange colored mixture was brought to room
temperature within 30 min. and refluxed for 6 h. All volatiles of
the reddish brown suspension were removed in vacuo and the
brown, oily residue was extracted with 20 mL hexane. The evapor-
ated hexane extract was distilled at 6 ϫ 10Ϫ3 mbar onto a cooled
finger (Ϫ30 °C). The product separated as an orange-brown solid
at the finger, which melted at room temperature to give a brown
oil. Ϫ Yield: 229 mg (63%). Ϫ IR (film): ν˜ ϭ 3084 m ν(CϪHar),
2968 s, 2920 s, 2892 s, 2860 m, 1696 w, 1660 w, 1588 m ν(CϭC),
1440 m, 1356 s, 1248 vs and 1216 vs ν(MoϭNϪC), 1116 m, 1080
m, 1020 m, 964 m, 888 w, 840 m, 800 vs and 736 s ν (CϪHar)oop
,
632 w, 584 w, 548 w, 464 w cmϪ1. Ϫ1H NMR (89.6 MHz, C7D8,
188 K): δ ϭ 1.15 (s, 18 H, NCCH3), 5.05 (br, 1 H, η1-HX), 5.30 (s,
5 H, η5-C5H5), 6.71 (br, 2 H, η1-HB/BЈ), 6.82 (br, 2 H, η1-HA/AЈ);
(89.6 MHz, C7D8, 283 K): δ ϭ 1.14 (s, 18 H, NCCH3), 5.33 (s, 5
H, η5-C5H5), 6.29 (br, 5 H, η1-C5H5); (89.6 MHz, C7D8, 363 K):
δ ϭ 1.13 (s, 18 H, NCCH3), 5.78 (s, 10 H, C5H5); Ϫ 13C NMR
(100.6 MHz, C6D6): δ ϭ 30.98 (s, NCCH3), 68.84 (s, NCCH3),
107.95 (br, C5H5). Ϫ EI-MS: m/z (%) ϭ 370 (1) [Mϩ]. Ϫ
C18H28MoN (368.4): calcd. C 58.69, H 7.66, N 7.61; found C 59.01,
H 7.98, N 7.74.
Bis(tert-butylimido)bis(cyclopentadienyl)tungsten(VI) (2b): Finely
powdered 1b (384 mg, 0.90 mmol) was added to a suspension of
(C5H5)Na (87.4 mg, 0.99 mmol) in 20 mL 1,2 dimethoxyethane at
Ϫ50 °C. The temperature of the orange colored mixture was raised
to room temperature within 30 min. and refluxed for 6 h. All vola-
tile materials of the brown suspension were removed in vacuo and
the brown, oily residue was extracted with 20 mL hexane. The evap-
orated hexane extract was distilled at 6 ϫ 10Ϫ3 mbar onto a
cooled finger (Ϫ30 °C). The product separated as an orange-brown
solid at the finger, which melted at room temperature to give a
brown oil. Ϫ Yield: 252 mg (61%). Ϫ IR (film): ν˜ ϭ 3084 m
ν(CϪHar), 2986 s, 2920 s, 2896 s, 2860 s, 2804 m, 1660 sh, 1612 w
ν(CϭC), 1448 m, 1356 m, 1288 vs and 1248 vs ν(WϭNϪC), 1212
s, 1140 w, 1080 m, 1020 m, 960 w, 868 s and 808 vs and 740 vs
ν(CϪHar)oop, 632 w, 576 w, 544 w, 472 w cmϪ1. Ϫ 1H NMR
(89.6 MHz, C7D8, 360 K): δ ϭ 1.15 (s, 18 H, NCCH3), 5.77 (s, 10
Experimental Section
General: All reactions and subsequent manipulations involving or-
ganometallic reagents were performed under argon (4.8) using
standard Schlenk techniques. Solvents were dried according to
˚
standard procedures, stored over activated 4 A molecular sieves
and degassed prior to use. Ϫ Melting and decomposition points
were determined in closed capillaries on a Büchi SMP 20 or as
DTAs on a duPont 9000 apparatus. Ϫ Infrared spectra were re-
corded as Nujol mulls on KBr windows on a Bruker IFS 25 Ϫ
NMR spectra were recorded on a Jeol FX90Q, a Bruker AC 200
and a Bruker AMX400 spectrometer. Ϫ [W(NMes)2Cl2(dme)] H, C5H5); (89.6 MHz, C7D8, 273 K): δ ϭ 1.16 (s, 18 H, NCCH3),
(M ϭ Mo, W)[6] and [(η5-C5H5)M(NR)2Cl][5a] were prepared as 5.36 (s, 5 H, η5-C5H5), 6.56 (br, 5 H, η1-C5H5); (89.6 MHz, C7D8,
described in the literature.
193 K): δ ϭ 1.19 (s, 18 H, NCCH3), 4.69 (br, 1 H, η1-HX), 5.30 (s,
Eur. J. Inorg. Chem. 2001, 1617Ϫ1623
1621

U. Radius, J. Sundermeyer, K. Peters, H.-G. von Schnering
FULL PAPER
[2] [2a]
5 H, η5-C5H5), 6.65 (br, 2 H, η1-HB/BЈ), 6.80 (br, 2 H, η1-HA/AЈ). Ϫ
13C NMR (100.7 MHz, C7D8): δ ϭ 31.88 (s, NCCH3), 66.69 (s,
NCCH3), 106.90 (br, C5H5). Ϫ EI-MS: m/z (%) ϭ 456 (63) [Mϩ],
441 (59) [Mϩ Ϫ CH3]. Ϫ C18H18N2W (456.3): calcd. C 47.38, H
6.19, N 6.14; found C 47.09, H 6.02, N 6.11.
D. L. Thorn, W. A. Nugent, R. L. Harlow, J. Am. Chem.
[2b]
Soc. 1981, 103, 357Ϫ363. Ϫ
W. A. Nugent, Inorg. Chem.
1983, 22, 965Ϫ969.
[3a] M. B. Hursthouse, M. Motevalli, A. C. Sullivan, G. Wilkin-
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A. C. Sullivan, G. Wilkinson, M. Motevalli, M. B. Hursthouse,
[3]
[3b]
J. Chem. Soc., Dalton Trans. 1988, 53Ϫ60.
R. R. Schrock, R. T. DePue, J. Feldman, K. B. Yap, D. C.
Yang, W. M. Davis, L. Park, M. DiMare, M. Schofield, J. An-
Bis(cyclopentadienyl)bis(mesitylimido)tungsten(VI) (2c): Finely
powdered 1b (551 mg, 1.00 mmol) was added to a suspension of
(C5H5)Na (96.1 mg, 1.10 mmol) in 20 mL 1,2 dimethoxyethane at
Ϫ50 °C. The temperature of the red mixture was raised to room
temperature within 30 min. and refluxed for 12 h. All volatile mat-
erials of the orange red suspension were removed in vacuo and
the oily residue was extracted with 30 mL hexane. The extract was
concentrated to approximately 8 mL. Any precipitating product
was redissolved by warming the solution. Orange red crystals were
isolated after one day at room temperature, and washed with 2 mL
cold pentane (Ϫ70 °C), and dried in vacuo. Ϫ Yield: 429 mg (74%).
m.p.: 176 °C. Ϫ IR (nujol): ν˜ ϭ 2986 s, 2920 s, 2896 s, 2860 s, 2804
m, 1604 w ν(CϭC), 1448 m, 1356 m, 1311 s and 1281 vs ν(Wϭ
NϪC), 1153 s, 1056 w, 1024 w, 1010 w, 980 w, 866 s and 810 s and
[4] [4a]
haus, E. Walborsky, E. Evitt, C. Krüger, P. Betz, Organometal-
[4b]
lics 1990, 9, 2262Ϫ2275. Ϫ
R. R. Schrock, J. S. Murdzek,
G. C. Bazan, J. Robbins, M. DiMare, M. OЈRegan, J. Am.
Chem. Soc. 1990, 112, 3875Ϫ3886. Ϫ [4c] G. Schoettel, J. Kress,
J. A. Osborn, J. Chem. Soc., Chem. Commun. 1989,
[4d]
1062Ϫ1063. Ϫ
N. Bryson, M.-T. Youinou, J. A. Osborn,
Organometallics 1991, 10, 3389Ϫ3392.
[5] [5a]
[5b]
J. Sundermeyer, Chem. Ber. 1991, 124, 1977Ϫ1979. Ϫ
U. Radius, J. Sundermeyer, Chem. Ber. 1992, 125, 2183Ϫ2186.
Ϫ
[5c]
J. Sundermeyer, U. Radius, C. Burschka, Chem. Ber.
1992, 125, 2379Ϫ2384.
[6]
[7]
U. Radius, J. Sundermeyer, H. Pritzkow, Chem. Ber. 1994,
127, 1827Ϫ1835.
1
740 s ν(CϪHar)oop, 595 w, 582 w, 544 w, 505 w, 440 w cmϪ1. Ϫ H
[7a] B. R. Ashcroft, D. C. Bradley, G. R. Clark, R. J. Errington,
NMR (200.1 MHz, C7D8, 375 K): δ ϭ 2.07 (s, 6 H, Mes-p-CH3),
2.19 (s, 12 H, Mes-o-CH3), 5.88 (s, 10 H, C5H5), 6.63 (s, 4 H, Mes-
H); (200.1 MHz, C7D8, 310 K): δ ϭ 2.09 (s, 6 H, Mes-p-CH3), 2.19
(s, 12 H, Mes-o-CH3), 5.47 (br, 5 H, C5H5), 6.25 (br, 5 H, C5H5),
6.60 (s, 4 H, Mes-H); (200.1 MHz, C7D8, 193 K): δ ϭ 2.15 (s, 6 H,
Mes-p-CH3), 2.24 (s, 12 H, Mes-o-CH3), 5.15 (s, 1 H, η1-HX), 5.26
(s, 5 H, η5-C5H5), 6.55 (s, 2 H, η1-HB), 6.57 (s, 1 H, η1-HBЈ), 6.71
(s, 4 H, Mes-H) 6.90 (s, 1 H, η1-HA), 7.01 (s, 1 H, η1-HAЈ). Ϫ 13C
NMR (50.3 MHz, C6D6): δ ϭ 19.47 (s, Mes-o-CH3), 20.92 (s, Mes-
p-CH3), 108.40 (s, C5H5), 128.58 (s, Mes-C3), 131.01 (s, Mes-C2),
132.76 (s, Mes-C4), 153.71 (s, 2JWC ϭ 32.7 Hz, Mes-C1). Ϫ EI-MS:
m/z (%) ϭ 580 (70) [Mϩ], 515 (100) [Mϩ Ϫ C5H5]. Ϫ C28H32N2W
(580.4): calcd. C 57.94, H 5.56, N 4.83; found C 57.06, H 5.66,
N 4.83.
[7b]
[8]
Bis(tert-butylimido)dichloro(trimethylphosphane)tungsten(VI) (3):
PMe3 (100 mg, 1.31 mmol) was added to a solution of 1b (430 mg,
1.01 mmol) in 15 mL toluene. After 2 h at room temperature, all
volatile materials were removed in vacuo, and 15 mL pentane was
added to the brown oily residue. Insoluble, ocher colored 3 was
filtered off, washed with 5 mL pentane and was dried in vacuo. The
mother liquor contained 230 mg (0.50 mmol) of slightly impure 2b.
Ϫ Yield: 201 mg (85%). Ϫ 1H NMR (400.1 MHz, CDCl3): δ ϭ
[12]
[13]
2
1.35 (s, 18 H, NCCH3), 1.73 (d, JPH ϭ 10.4 Hz, 9 H, PCH3); Ϫ
1
31P{1H} NMR (162.0 MHz, CDCl3): δ ϭ 7.55 (s, JWP ϭ 363 Hz,
1
PCH3); Ϫ 13C NMR (100.6 MHz, C6D6): δ ϭ 16.63 (d, JPC
ϭ
4
34.5 Hz, P-CH3), 31.82 (d, JPC ϭ 2.0 Hz, NCCH3), 69.61 (d,
2
3JPC ϭ 2.0 Hz, JWC ϭ 26.5 Hz, NCCH3).
(Eds:. G. Wilkinson, F. G. A. Stone, E. W. Abel), Vol. 3, Perga-
Crystallographic data (excluding structure factors) for the struc-
tures reported in this paper have been deposited with the Cam-
bridge Crystallographic Data Centre as supplementary publication
no. CCDC-133165. Copies of the data can be obtained free of
charge on application to CCDC, 12 Union Road, Cambridge,
CB2 1EZ, UK (Fax: (internat.) ϩ44-1223/336-033; E-mail:
deposit@ccdc.cam.ac.uk).
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Acknowledgments
Support by the Deutsche Forschungsgemeinschaft and the Fonds der
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Bis(cyclopentadienyl) Diimido Complexes of Molybdenum and Tungsten
FULL PAPER
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Two strong π-bonds are formed by the e set orbitals, three
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