Alkylidyne complexes of molybdenum and Tungsten that contain the [(3,4,5-C6H2F3NCH2 CH2)2NMe]2- ligand

Article abstract of DOI:10.1021/om0100176

Reduction of (C₆F₅NHCH₂CH₂)₂NMe with LiAlH₄ produces (3,4,5-C₆H₂F₃NHCH₂ CH₂)₂NMe (H₂[F₃NMe]) in good

Full text of DOI:10.1021/om0100176

Organometallics 2001, 20, 2127-2129
2127
Alk ylid yn e Com p lexes of Molybd en u m a n d Tu n gsten
Th a t Con ta in th e [(3,4,5-C6H2F 3NCH2CH2)2NMe]2- Liga n d
Frank V. Cochran and Richard R. Schrock*
Department of Chemistry, Massachusetts Institute of Technology,
Cambridge, Massachusetts 02139
Received J anuary 5, 2001
Summary: Reduction of (C6F5NHCH2CH2)2NMe with
LiAlH4 produces (3,4,5-C6H2F3NHCH2CH2)2NMe (H2[F3-
NMe]) in good yield. Reactions between H2[F3NMe]
and MCl4 (M ) Mo, W) in the presence of NEt3 yield
pseudooctahedral paramagnetic compounds of the type
[Et3NH]{[F3NMe]MCl3}. Treatment of [Et3NH]{[F3NMe]-
MoCl3} with 3 equiv of Me3CCH2MgCl produced five-
coordinate [F3NMe]Mo(CH2CMe3)(CCMe3), while treat-
ment with 3 equiv of Me3SiCH2MgCl produced five-
coordinate paramagnetic [F3NMe]Mo(CH2SiMe3)2. Upon
heating, [F3NMe]Mo(CH2SiMe3)2 is converted into dia-
magnetic {[F3NMe]Mo(CSiMe3)}2. Reactions between
[Et3NH]{[F3NMe]WCl3} and Me3CCH2MgCl or Me3-
SiCH2MgCl lead to five-coordinate diamagnetic com-
plexes of the type [F3NMe]W(CH2R)(CR), where R )
CMe3, SiMe3 respectively. X-ray studies confirmed the
structures of [F3NMe]Mo(CH2CMe3)(CCMe3) and {[F3-
NMe]Mo(CSiMe3)}2.
ceptible to new modes of functionalization. Recent evi-
dence that this is the case for V and Ta consists of
reports that describe reductive cleavage of dinitrogen
between two vanadium centers12 and the reaction
between dinitrogen and a tantalum hydride to give a
bridging dinitrogen unit that is bound both side-on and
end-on.13 The fact that the reaction between (C6F5-
NHCH2CH2)2NMe and Mo(NMe2)4 led to seven-coordi-
nate [(C6F4(NMe2)NCH2CH2)2NMe]MoF2 rather than
five-coordinate [(C6F5NCH2CH2)2NMe]Mo(NMe2)2 sug-
gested to us that the chemistry of complexes that
contain the [(C6F5NCH2CH2)2NMe]2- ligand ultimately
might be limited by undesirable nucleophilic attack on
the pentafluorophenyl ring.11 Here we report the syn-
thesis and characterization of Mo and W complexes that
contain the apparently more robust [(3,4,5-C6H2F3-
NCH2CH2)2NMe]2- ligand and some reactions that
suggest that R,R-dehydrogenation reactions can be
found in environments outside of the triamidoamine
platform.
The chemistry of complexes that contain a triami-
doamine ligand has been under investigation in our
laboratories for the past few years.1-6 Although the
focus has been on dinitrogen reduction and functional-
ization by Mo complexes,7-10 we also had the oppor-
tunity to prepare and explore Mo(IV) or W(IV) alkyl
complexes.2,3,6 In the process we discovered an unusual
R,R-dehydrogenation reaction that converts a Mo(IV) or
W(IV) alkyl complex into an alkylidyne complex and
molecular hydrogen. This reaction is much more facile
for W than for Mo and in many cases takes place even
when â-hydride elimination or abstraction processes
would be plausible alternatives. We have turned to the
synthesis and study of diamidoamine complexes of
Mo and W, initially those that contain the [(C6F5NCH2-
CH2)2NMe]2- ligand,11 primarily in an effort to prepare
dinitrogen complexes in which the dinitrogen is more
sterically accessible and therefore could be more sus-
In a recent publication14 the transformation of pen-
tafluorophenyl rings bound to nitrogen into what were
proposed to be 2,4,6-C6H2F3 rings with LiAlH4 in
refluxing THF was reported.15 In the belief that 2,4,6-
C6H2F3 rings might be less susceptible to nucleophilic
attack, we attempted a similar reaction between (C6F5-
NHCH2CH2)2NMe and LiAlH4.16 We were gratified to
find that (C6H2F3NHCH2CH2)2NMe was indeed formed
in good yield but surprised to learn through X-ray
studies of compounds described below that the product
was (3,4,5-C6H2F3NHCH2CH2)2NMe, not (2,4,6-C6H2F3-
NHCH2CH2)2NMe (eq 1). This outcome may be the more
4.5LiAlH4
(C6F5NHCH2CH2)2NMe THF, reflux, 4 h8
(3,4,5-C6H2F3NHCH2CH2)2NMe (H2[F3NMe]) (1)
(1) Schrock, R. R. Acc. Chem. Res. 1997, 30, 9.
(2) Schrock, R. R.; Seidel, S. W.; Mosch-Zanetti, N. C.; Shih, K.-Y.;
O’Donoghue, M. B.; Davis, W. M.; Reiff, W. M. J . Am. Chem. Soc. 1997,
119, 11876.
(3) Seidel, S. W.; Schrock, R. R.; Davis, W. M. Organometallics 1998,
17, 1058.
desirable of the two, as the reaction between H2[F3NMe]
and Mo(NMe2)4 was found to yield stable five-coordinate
diamagnetic [F3NMe]Mo(NMe2)2 as a red-brown powder,
(4) Greco, G. E.; O’Donoghue, M. B.; Seidel, S. W.; Davis, W. M.;
Schrock, R. R. Organometallics 2000, 19, 1132.
(5) Reid, S. M.; Neuner, B.; Schrock, R. R.; Davis, W. M. Organo-
metallics 1998, 17, 4077.
(6) Schrock, R. R.; Seidel, S. W.; Mo¨sch-Zanetti, N. C.; Dobbs, D.
A.; Shih, K.-Y.; Davis, W. M. Organometallics 1997, 16, 5195.
(7) Kol, M.; Schrock, R. R.; Kempe, R.; Davis, W. M. J . Am. Chem.
Soc. 1994, 116, 4382.
(12) Clentsmith, G. K. B.; Bates, V. M.; Hitchcock, P. B.; Cloke, F.
G. N. J . Am. Chem. Soc. 1999, 121, 10444.
(13) Fryzuk, M. D.; J ohnson, S. A.; Rettig, S. J . J . Am. Chem. Soc.
1998, 120, 11024.
(14) Buchler, S.; Meyer, F.; J acobi, A.; Kircher, P.; Zsolnai, L. Z.
Naturforsch. 1999, 54b, 1295.
(15) We thank S. Buchler for bringing this publication to our
attention.
(8) O’Donoghue, M. B.; Davis, W. M.; Schrock, R. R. Inorg. Chem.
1998, 37, 5149.
(9) O’Donoghue, M. B.; Davis, W. M.; Schrock, R. R.; Reiff, W. M.
Inorg. Chem. 1999, 38, 243.
(10) Greco, G. E.; Popa, A. I.; Schrock, R. R. Organometallics 1998,
17, 5591.
(16) A solution of (C6F5NHCH2CH2)2NMe (3.37 g, 7.49 mmol) in THF
(20 mL) was added via syringe to LiAlH4 (1.28 g, 33.74 mmol) in THF
(30 mL) at room temperature. The reaction mixture was heated to
reflux for 4 h. The reaction was quenched with water, and all volatile
components were removed in vacuo. The residue was washed with 0.5
M
NaOH (200 mL) and extracted twice into 200 mL of dichlo-
(11) Cochran, F. V.; Bonitatebus, P. J .; Schrock, R. R. Organome-
tallics 2000, 19, 2414.
romethane. The product was isolated as a thick yellow oil from the
organic layers; yield 2.46 g (87%).
10.1021/om0100176 CCC: $20.00 © 2001 American Chemical Society
Publication on Web 04/27/2001

2128 Organometallics, Vol. 20, No. 11, 2001
Communications
in contrast to activation of the ortho fluorides in (C6F5-
NHCH2CH2)2NMe upon reaction with Mo(NMe2)4.11
The reaction between MoCl4(THF)2, H2[F3NMe], and
NEt3 in THF led to a paramagnetic purple product
formulated as [Et3NH]{[F3NMe]MoCl3} on the basis of
1
19F and H NMR spectra and elemental analysis (eq 2).
H2[F3NMe] + MoCl4(THF)2 2.2 NEt ,THF8
3
22 °C, 1 h
[Et3NH]{[F3NMe]MoCl3} (2)
This reaction is analogous to the synthesis of [Et3NH]-
{[(C6F5NCH2CH2)2NMe]MoCl3};11 it proceeds in good
yield despite the fact that there are only three fluorides
on each of the ligand aryl rings. Cation exchange with
Bu4NCl produced a more soluble tetrabutylammonium
salt. The magnetic moment of [Bu4N]{[F3NMe]MoCl3}
was determined by the Evans method17 as modified by
Sur18 to be 3.1 µB, slightly higher than the spin-only
value for two unpaired electrons (2.83 µB).
A reaction similar to that shown in eq 2 between
WCl4(dme), H2[F3NMe], and NEt3 in diethyl ether
produced [Et3NH]{[F3NMe]WCl3}. A tetrabutylammo-
nium salt was prepared in the same manner as de-
scribed above for [Bu4N]{[F3NMe]MoCl3}. A measure-
ment of the magnetic moment of [Bu4N]{[F3NMe]WCl3}
by the Evans method gave µeff ) 2.7 µB.
Addition of 3 equiv of Me3CCH2MgCl to [Et3NH]{[F3-
NMe]MoCl3} in THF yielded [F3NMe]Mo(CH2CMe3)-
(CCMe3) as an amber crystalline solid in 68% yield.19
The alkylidyne carbon resonance was located at 308
ppm in the 13C NMR spectrum. An X-ray diffraction
study revealed the complex to be approximately a tri-
gonal bipyramid with the neopentylidyne ligand occupy-
ing an axial position trans to the amine donor (Figure
1). All bond lengths and angles are in the expected
range. Note that the aryl rings contain fluorides in the
3-, 4-, and 5-positions. We hypothesize that this species
arises via R,R-dehydrogenation of transient [F3NMe]Mo-
(CH2CMe3)2 (eq 3). The ease of R,R-dehydrogenation in
F igu r e 1. Thermal ellipsoid plot (30% probability level)
of the structure of [F3NMe]Mo(CH2CMe3)(CCMe3). Selected
bond distances (Å) and angles (deg): Mo-C(1) ) 1.776(3),
Mo-C(6) ) 2.143(3), Mo-N(1) ) 2.042(2), Mo-N(2) )
2.392(2); N(1)-Mo-N(3) ) 126.43(9), C(6)-Mo-N(1) )
112.67(11), C(1)-Mo-N(2) ) 170.66(10), C(2)-C(1)-Mo )
173.0(2), C(7)-C(6)-Mo ) 129.7(2). Crystal data: C27H35F6-
N3Mo, monoclinic, P21/n, Z ) 4, a ) 12.5977(9) Å, b )
14.7520(11) Å, c ) 15.0992(11) Å, â ) 98.1530(10)°, R1 )
0.0294, wR2 ) 0.0812 (all data).
rather than neopentane is formed, although the im-
mediate product upon loss of neopentane would be what
is perhaps a relatively disfavored Mo(IV) neopentylidene
complex. A minor byproduct isolated from this alkyla-
tion reaction (<5% yield) was {[F3NMe]Mo(CH2CMe3)}2-
(µ-N2), as revealed in an X-ray crystallographic study.
We believe that this product most likely arises via
reduction of the intermediate [F3NMe]Mo(CH2CMe3)-
Cl and capture of dinitrogen by “[F3NMe]Mo(CH2-
CMe3).” Synthesis and characterization of {[F3NMe]Mo-
(CH2CMe3)}2(µ-N2) will be reported fully elsewhere
when a more reliable and high-yield route to it is found.
In contrast to the result shown in eq 3, addition of 3
equiv of Me3SiCH2MgCl to a THF solution of [Et3NH]-
{[F3NMe]MoCl3} led to paramagnetic purple [F3NMe]-
Mo(CH2SiMe3)2. Resonances corresponding to two dif-
ferent trimethylsilyl proton environments were observed
1
in the H NMR spectrum at 3.6 and 1.2 ppm, despite
the paramagnetism of the complex (µeff ) 3.1 µB). A
bridging dinitrogen complex was isolated in low yield
in this case also, as will be reported elsewhere. Heating
a benzene solution of [F3NMe]Mo(CH2SiMe3)2 at 62 °C
for 24 h caused conversion to a bright red diamagnetic
product, an X-ray diffraction study of which revealed it
to be {[F3NMe]Mo(CSiMe3)}2 (Figure 2). This compound
has only slightly different Mo-C bond lengths of
2.203(10) and 2.141(8) Å but an essentially linear alkyl-
idyne ligand (Mo(1A)-C(1)-Si(1) ) 177.9(6)°). An alkyl-
idyne carbon resonance was located at 409 ppm in the
13C NMR spectrum. Tungsten compounds that contain
the W2(µ-CSiMe3)2 core were discovered in 1976,20,21 and
a variety of studies have been published since then.22-29
Other examples of compounds that contain a W2(µ-CR)2
[F3NMe]Mo(CH2CMe3)2 stands in contrast to that pro-
cess in isolable Mo(IV) neopentyl complexes containing
the [(Me3SiNCH2CH2)3N]3- and [(C6F5NCH2CH2)3N]3-
ligands, which are converted to the corresponding
neopentylidyne complexes at appreciable rates only
when heated.2,3 An unusual feature of R,R-dehydroge-
nation in [F3NMe]Mo(CH2CMe3)2 is that dihydrogen
(17) Evans, D. F. J . Chem. Soc. 1959, 2003.
(18) Sur, S. K. J . Magn. Reson. 1989, 82, 169.
(19) A solution of NpMgCl (0.43 mL, 2.27 M in Et2O) was added to
a solution of [Et3NH]{[F3NMe]MoCl3} (0.2 g, 0.29 mmol) in THF (10
mL) at -30 °C. After the reaction mixture was warmed to room
temperature and stirred for 1 h, all volatile components were removed
in vacuo. The residue was extracted into toluene (20 mL) and filtered
through Celite to remove magnesium salts. The filtrate was added
dropwise to a rapidly stirred volume of cold pentane (100 mL) and
filtered a second time. A brown crystalline solid was isolated upon
concentrating the solution to dryness; yield 0.122 g (68%). Analytically
pure, amber crystals suitable for X-ray diffraction were grown from a
mixture of benzene and pentane at -30 °C.
(20) Chisholm, M. H.; Cotton, F. A.; Extine, M. W.; Stults, B. R.
Inorg. Chem. 1976, 15, 2252.
(21) Andersen, R. A.; Gayler, A. L.; Wilkinson, G. Angew. Chem.,
Int. Ed. Engl. 1976, 15, 609.
(22) Chisholm, M. H.; Cotton, F. A.; Extine, M. W.; Murillo, C. A.
Inorg. Chem. 1978, 17, 696.
(23) Chisholm, M. H.; Heppert, J . A.; Huffman, J . C. J . Am. Chem.
Soc. 1984, 106, 1151.

Communications
Organometallics, Vol. 20, No. 11, 2001 2129
alkylidyne carbon atom resonances were located at 296
ppm in the neopentylidyne complex and 337 ppm in the
(trimethylsilyl)methylidyne complex. Apparently the
R,R-dehydrogenation reaction, which is known to be
much faster for W than Mo in triamidoamine alkyl com-
plexes,2,6 becomes the dominant reaction pathway in
both [F3NMe]W(CH2CMe3)2 and [F3NMe]W(CH2SiMe3)2.
We conclude that the [(3,4,5-C6H2F3NCH2CH2)2-
NMe]2- ligand has significant potential in terms of
developing the chemistry of middle oxidation states of
Mo and W, because starting materials can be prepared
readily and because the [(3,4,5-C6H2F3NCH2CH2)2-
NMe]2- ligand appears to be less susceptible to nucleo-
philic attack than the pentafluorophenyl ring in the
[(C6F5NCH2CH2)2NMe]2- ligand. We are interested in
elucidating the mechanisms of the organometallic reac-
tions reported here and determining to what extent
chemistry at the R-carbon can be distinguished from
chemistry at the â-carbon in alkyls that contain â-pro-
tons. We also are interested in developing high-yield
routes to species that contain bound dinitrogen in order
to further explore the chemistry of [(3,4,5-C6H2F3NCH2-
CH2)2NMe]2- complexes with respect to dinitrogen
fixation. Although a variety of complexes that contain
“diamido/donor” ligands have been reported by other
laboratories,12,31-47 those reported in this work and in
a previous communication11 appear to be the only ones
so far that contain Mo or W.
F igu r e 2. Thermal ellipsoid plot (30% probability level)
of the structure of {[F3NMe]Mo(CSiMe3)}2 (ligand backbone
and aryl rings omitted for clarity). Selected bond distances
(Å) and angles (deg): Mo(1)-C(1) ) 2.203(10), Mo(1)-
C(1A) ) 2.141(8), Mo(1)-Mo(1A) ) 2.4152(15), Mo(1)-N(1)
) 2.055(7), Mo(1)-N(2) ) 2.306(7), Mo(1)-N(3) ) 2.093(7);
N(1)-Mo(1)-N(3) ) 127.3(3), C(1)-Mo-N(1) ) 93.1(3),
C(1A)-Mo(1)-N(1) ) 109.9(3), C(1)-Mo(1)-C(1A) )
112.5(3), C(1)-Mo(1)-N(2) ) 161.5(3), C(1A)-Mo(1)-N(2)
) 86.0(3), Si(1)-C(1)-Mo(1) 114.1(4), Si(1)-C(1)-Mo(1A)
) 177.9(6). Crystal data: C42H48N6F12Si2Mo2, monoclinic,
P21/c, Z ) 2, a ) 10.818(2) Å, b ) 16.850(3) Å, c ) 14.216(2)
Å, â ) 104.610(3)°, R1 ) 0.0650, wR2 ) 0.1425 (all data).
Ack n ow led gm en t. R.R.S. thanks the National Sci-
ence Foundation (Grant No. CHE 9988766) and the
National Institutes of Health (Grant No. GM 31978) for
research support. F.V.C. thanks Peter J . Bonitatebus,
J r., for assistance in X-ray crystallography.
Su p p or tin g In for m a tion Ava ila ble: Text giving experi-
mental details, labeled ORTEP diagrams, and tables of crystal
data and structure refinement details, atomic coordinates,
bond lengths and angles, and anisotropic displacement pa-
rameters for [F3NMe]Mo(CH2CMe3)(CCMe3) and {[F3NMe]-
Mo(CSiMe3)}2. This material is available free of charge via the
Internet at http://pubs.acs.org.
core are rare.30 To our knowledge there is no published
example of a compound that contains the Mo2(µ-CR)2
core or any compound that contains a W2(µ-CSiMe3)2
core in which the W(1)-C-Si and W(2)-C-Si angles
are not approximately equal. Therefore, at this stage
we do not know whether the linear form of the µ-CSiMe3
ligands found in {[F3NMe]Mo(CSiMe3)}2 is a character-
istic of Mo2(µ-CR)2 compounds or a feature of M2(µ-CR)2
compounds (M ) Mo, W) that contain a diamido/donor
ligand. The different modes of decomposition of [F3NMe]-
Mo(CH2SiMe3)2 and the proposed intermediate [F3NMe]-
Mo(CH2CMe3)2 are striking.
OM0100176
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