Tungsten(II) alkylimido complexes from insertion of nitriles into tungsten hydride: Alkylideneamido intermediate stage and nitrene group transfer to isocyanide

Article abstract of DOI:10.1021/om3004338

The tetrahydrido complex [WH₄(κ⁴-P4)] (P4 = meso-o-C₆H₄(PPhCH₂CH₂PPh ₂)₂) reacted with aliphatic nitriles AkCN at 80 °C in toluene to afford a series of the imido complexes, [W(NCH₂Ak) (κ⁴-P4)] (2). Similar reactions with aromatic nitriles ArCN proceeded stepwise, leading to formation of the hydrido-alkylideneamido complexes [WH(N=CHAr)(κ⁴-P4)] (3) at the first stage and a subsequent isomerization into the imido complexes [W(NCH₂Ar) (κ⁴-P4)] (4). Conversion of 3 into 4 was accelerated by electron-rich Ar groups such as p-CH₃OC₆H₄, while it was completely inhibited when Ar was the considerably electron-deficient p-CF₃C₆H₄. An X-ray crystallographic study on 2 (Ak = p-ClC₆H₄CH₂) and 4 (Ar = p-tolyl) has disclosed a distorted-square-pyramidal coordination geometry, in which the apical position is occupied by the imido ligand with a W^II-N triple bond. An almost linear W^II=N=C linkage of 3 (Ar = p-tolyl) has also been determined. Complex 2 was capable of nitrene group transfer to isocyanide to produce carbodiimide.

Full text of DOI:10.1021/om3004338

Communication
pubs.acs.org/Organometallics
Tungsten(II) Alkylimido Complexes from Insertion of Nitriles into
Tungsten Hydride: Alkylideneamido Intermediate Stage and Nitrene
Group Transfer to Isocyanide
Qi Xiu Dai, Hidetake Seino,* and Yasushi Mizobe*,†
Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan
S
* Supporting Information
ABSTRACT: The tetrahydrido complex [WH4(κ4-P4)] (P4 =
meso-o-C6H4(PPhCH2CH2PPh2)2) reacted with aliphatic nitriles
AkCN at 80 °C in toluene to afford a series of the imido
complexes, [W(NCH2Ak)(κ4-P4)] (2). Similar reactions with
aromatic nitriles ArCN proceeded stepwise, leading to
formation of the hydrido−alkylideneamido complexes
[WH(NCHAr)(κ4-P4)] (3) at the first stage and a
subsequent isomerization into the imido complexes [W-
(NCH2Ar)(κ4-P4)] (4). Conversion of 3 into 4 was accelerated
by electron-rich Ar groups such as p-CH3OC6H4, while it was
completely inhibited when Ar was the considerably electron-deficient p-CF3C6H4. An X-ray crystallographic study on 2 (Ak = p-
ClC6H4CH2) and 4 (Ar = p-tolyl) has disclosed a distorted-square-pyramidal coordination geometry, in which the apical position
is occupied by the imido ligand with a WII−N triple bond. An almost linear WIINC linkage of 3 (Ar = p-tolyl) has also been
determined. Complex 2 was capable of nitrene group transfer to isocyanide to produce carbodiimide.
ydrogenation of nitrile is a vital method to prepare
primary amines, and the heterogeneous processes under
unsaturated bonds prompted us to investigate reactions with
organonitriles, leading to the discovery of not only conven-
tional alkylideneamido formation but also further conversion
into the imido stage. Here we report this new synthetic route to
imido ligands as well as the structures and reactions of the
resulting imido complexes of WII.
Various aliphatic nitriles AkCN (Ak = 1°−3° alkyls) reacted
with 1 on heating in toluene to form the imido complexes
[W(NCH2Ak)(κ4-P4)] (2) accompanied by formation of 1
equiv of H2 (eq 1). Reactions using 3-fold molar AkCN were
H
hydrogen pressure using transition-metal catalysts have great
industrial importance.1 In comparison, homogeneous hydro-
genation based on transition-metal catalysts has attracted little
interest because of side reactions producing secondary and
tertiary amines etc.,2 whereas a number of complex catalysts
that selectively give primary amines have been developed.3 The
first step in hydrogenation is insertion of a nitrile molecule into
a metal−hydrogen bond. Stoichiometric reactions of this type
have been observed for some hydride complexes and result in
either an alkylideneamido (−NCHR) or iminoacyl (−C(
NH)R) ligand, the former of which is common for
mononuclear complexes.4 Also in heterogeneous systems,
these species are hypothesized to be present on the catalyst
surface, and imido/nitrene (N−CH2R) and carbene (
C(NH2)R) intermediate structures are proposed in addition.5
We have recently developed the synthesis of the tungsten
tetrahydride complex [WH4(κ4-P4)] (1; P4 = meso-o-
C6H4(PPhCH2CH2PPh2)2) and its use in the conversion of
unsaturated organic molecules.6,7 By virtue of the linear
tetraphosphine coligand P4, 1 is much more reactive than the
closely related [WH4(dppe)2] (dppe = Ph2PCH2CH2PPh2).
This is presumed to be mainly due to the particular
coordination geometry brought by P4 as well as labile bonding
between terminal PPh2 groups and the metal center. Previous
studies have shown the remarkable hydride-donating ability of
1, which facilitates conversion of CS2 into a methanedithiolato
ligand (CH2S22−) and hydrodesulfurization of aryl isothiocya-
nates to N-arylmethylamines. Such reactivity toward polar
complete within 3 h at 80 °C without giving any other
complexes, as determined by NMR, while those with n-
C7H15CN proceeded even at 50 °C. Among the series of
compounds 2, that with Ak = p-ClC6H4CH2 (2e) was isolated
as air-sensitive brownish-purple crystals in 89% yield, while
other derivatives could not be crystallized due to their high
solubility in common organic solvents.
The molecular structure of 2e was established by X-ray
crystallography (Figure 1A). The metal center has a distorted-
Received: May 18, 2012
© XXXX American Chemical Society
A
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Communication
Figure 1. Molecular structures of 2e (A), 3a (B), and 4a (C) showing thermal ellipsoids at the 50% probability level. Hydrogen atoms are omitted
for clarity, except for those attached to the α-carbon atoms of organonitrogen ligands and the hydrido ligand. Some important bond lengths (Å) and
angles (deg) not appearing in the text: N−C = 1.436(4), N−C−C = 112.7(2) for 2e; W−H = 1.88(4), N−W−H = 174(1), P(1)−W−P(3) =
133.38(4), P(2)−W−P(4) = 154.45(4) for 3a; W−N = 1.794(2), N−C = 1.449(4), W−N−C = 169.9(2), N−C−C = 114.4(2), P(1)−W−P(3) =
113.13(2), P(2)−W−P(4) = 140.25(2) for 4a.
square-pyramidal geometry with the imido ligand at the apical
position. The transoid P−W−P angles are 122.82(2)° for
P(1)−W−P(3) and 135.31(2)° for P(2)−W−P(4), which are
indicative of slight distortion toward a trigonal bipyramid. The
closely related (N-piperidinyl)imido complex [W(NNC5H10)-
(dppe)2] has been reported to have a distorted-trigonal-
bipyramidal geometry, in which the axial sites are occupied by
two P atoms with a P−W−P angle of 157.1(1)° (equatorial P−
W−P = 107.7(1)°).8 This difference presumably arises from
steric regulation due to three consecutive chelate rings of
tetradentate P4. The W−N distance of 1.785(2) Å in 2e is
typical for a triple bond, which is consistent with the linear W−
N−C linkage (168.4(2)°) due to the sp-hybridized N atom as
well as the 18-electron count around the five-coordinate WII
center. In comparison with the six-coordinate WIV complexes
bearing one P49 or two dppe ligands,10 the W−N bond of the
imido ligand is slightly long (vs 1.721.76 Å) and the W−P
distances are short for 2e. These features are similarly seen in
[W(NNC5H10)(dppe)2] (W−N = 1.781(5) Å), while the six-
coordinate WII complex fac-[W(NNMe2)(dppe)(CO)3] has a
considerably long W−N bond (2.12(1) Å) and bent W−N−N
moiety (139(1)°), reflecting a WN(sp2) character.11 The
31P{1H} NMR spectra of 2 displays two signals at about δ 85
and 117, corresponding to the outer and inner P atoms of P4.
The proton signal due to the NCH2 moiety is unambiguously
assigned according to the vicinal coupling with the Ak group for
each derivative of 2.
and the imido complex [W(NCH2Tol)(κ4-P4)] (4a) became
predominant after 20 h, which was isolated as brown cubic
crystals in 29% yield. As for NMR spectra, 4a closely resembled
its analogues 2. For 3a, the existence of the hydride ligand was
1
confirmed by the H NMR signal, which was observed at δ
−3.42 as a broad quintet (JHP = 38 Hz). The azomethine (N
CH) proton appeared at δ 4.43, weakly coupling with P nuclei
(∼2 Hz). The 31P{1H} NMR spectrum showed two signals at δ
62.0 and 83.4 assignable to the outer and inner P atoms of P4,
respectively. The experiment to heat a toluene solution of
isolated 3a at 80 °C unambiguously confirmed that 4a was
produced via isomerization of 3a. This conversion was
remarkably facilitated by a catalytic amount of [Et3NH][BPh4]
to progress even at 28 °C (see the Supporting Information).
A crystallographic study on 3a and 4a has discriminately
revealed the structures of their organonitrogen ligands (Figure
1B,C). The imido complex 4a was ascertained to be essentially
identical with that of 2e, except for the organic moiety of the
imido ligand. The coordination geometry of 3a is a highly
distorted octahedron, where four P atoms constitute an
equatorial plane. With respect to this plane, the hydrido ligand
is located at the side that the o-phenylene group of P4 is
directed, while the alkylideneamido ligand occupies the wider
side of its opposite. The N−C distance at 1.305(6) Å and the
N−C−C angle of 126.7(4)° in 3a are diagnostic of an N
C(sp2) system. The W−N bond length of 1.863(3) Å, which is
longer by ∼0.07 Å than those of the imido complexes 2e and
4a, suggests a double-bond character, expected from the 18e
rule. The sp hybridization of the N atom is supported by the
linear W−N−C linkage (169.7(3)°). These bond distances and
angles of the alkylideneamido ligand are comparable to those of
other W complexes.14,15
Insertion of nitrile into an M−H bond has been known for a
number of mononuclear complexes.13−16 However, to the best
of our knowledge, the formation of imido ligands via stepwise
transfer of two hydrides to a nitrile carbon on a single metal
center is unprecedented.17 In the case of the dinuclear hydride
[Cp′TaCl2H]2 (Cp′ = η5-C5Me4Et), [(Cp′TaCl2)2(μ-H)(μ-
NCHMe)] was given by reaction with MeCN, which degrades
by addition of ethylene into the mononuclear
[Cp′TaCl2(NCH2Me)] and [Cp′TaCl2{(CH2)4}].18 However,
the mononuclear dihydride complex [Cp*2ZrH2] (Cp* = η5-
C5Me5) undergoes insertions of two TolCN molecules to result
in the bis(alkylideneamido) complex [Cp*2Zr(N
CHTol)2].19 Photolysis of [Tp′W(CO)3H] (Tp′ = hydrotris-
Treatment of 1 with TolCN (Tol = p-tolyl) under the same
conditions as for aliphatic nitriles (3 equiv of nitrile, 80 °C, 3 h)
resulted in release of H2 (0.88 equiv to 1) and deposition of
brown crystals from the reaction mixture, which were identified
as the hydrido−alkylideneamido complex [WH(NCHTol)-
(κ4-P4)] (3a, 37% yield) (Scheme 1). Prolonged heating of the
above reaction mixture gradually decreased the amount of 3a,
Scheme 1. Reaction of the Tetrahydrido Complex 1 with p-
Tolunitrile
B
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Communication
(3,5-dimethylpyrazolyl)borate) with RCN (R = Me, Et,
CH2Ph) provides the insertion products [Tp′W(CO)2(N
CHR)], subsequent protonation of which by HBF4 gives the
WIV imido complexes [Tp′W(CO)2(NCH2R)][BF4].12 Other
transformations of a nitrile into an imido ligand demonstrated
with mononuclear complexes include double protonation at the
carbon atom of nitrile coordinated to Mo0, W0, or ReI 10,20 and
those presumed to proceed via hydrogen transfer from a
methanol to a nitrile ligand in MoII and CrII complexes.21
The alkylideneamido ligand is probably formed via
generation of an open site by dissociation of H2 or one of
the terminal P atoms in P4, followed by insertion of nitrile into
a W−H bond. Possible pathways for the isomerization of 3a to
4a are roughly classified into intra- and intermolecular
hydrogen shifts, although the former seems less probable
because of the mutually trans location of the alkylideneamido
and the hydride ligands. From the reaction of tetradeuterido
complex 1-d4 with TolCN was prepared [WD(NCDTol)(κ4-
P4)] (3a-d,d), whose D atom enrichments were estimated at 93
and 87 atom % for the hydrido ligand and the azomethine
group, respectively. Complex 4a formed by heating this sample
at 80 °C for 20 h in toluene bore predominantly the NCD2Tol
ligand (4a-d2, 59% isotopic purity) containing totally 77 atom
% D at the imido ligand. When a 1:1 mixture of 3a-d,d and
nondeuterated 3a was treated under the same conditions, the
mixture of isotopomers produced was in the ratio 4a-d2:4a-
d:4a-d0 = 14:46:40. This low content of 4a-d2 unequivocally
excluded intramolecular mechanisms.
speculated not to occur bimolecularly for steric reasons but
rather to be mediated by proton carriers such as a trace of H2O.
This may be responsible for the observed D atom contents in
the above labeling experiments being lower than the theoretical
level.
Although imido complexes of W are ubiquitous, most of their
oxidation states are from +4 to +6.22 Imido ligands bound to
low-valent centers such as WII are very rare and are in general
highly reactive,8,11,23,24 while the reactivity of 2 and 4 seems
dissimilar because these are relatively stabilized by the strong
W−N triple bond. Complex 2e reacted with 2,6-dimethyl-
phenyl isocyanide (XyNC) at 80 °C to produce the asymmetric
carbodiimide p-ClC6H4CH2CH2NCNXy (5) in 75% yield
via the coupling of the nitrene functionality and isocyanide
(Scheme 3). The complex part was retrieved as a mixture of W0
Scheme 3. Reaction of the Imido Complex 2e with XyNC
It was spectroscopically confirmed that various aromatic
nitriles ArCN reacted with complex 1 to give the corresponding
species 3, which then converted into 4 in most cases (Scheme
2). While the reactivity of these nitriles toward 1 was not
isocyanide complexes [W(CNXy)n(κ6−n-P4)] (6n, n = 24) in
high gross yield. Nitrene group transfer to isocyanide has been
known for group 8−10 metals25 but is exceptional for early
transition metals.26 It is noticeable that this reaction proceeds
in a reverse direction in comparison with those of
Scheme 2. Reactions of the Tetrahydrido Complex 1 with
Various Aromatic Nitriles under the Same Conditions
t
[MCl2(PMeR2)4] (M = Mo, W; R = Me, Ph) with BuN
CNtBu to give imido−isocyanide complexes of the type
[WCl2(NtBu)(CNtBu)(PMeR2)2].27 The ratio of three W
isocyanide complexes 6n was almost constant during the
reaction course, and transient species were not detected
spectroscopically. Because it has been confirmed that bis-
(isocyanide) complex 62 does not react with excess XyNC
under these conditions,6 63 and 64 presumably result from the
pathway that several numbers of XyNC displace the terminal P
atom(s) of P4 prior to liberation of carbodiimide. The dangling
PPh2 groups in these complexes remain unaffected, although
these P atoms are susceptible to O and S atom transfer from
heterocumulenes under certain conditions.28 Some W and Mo
complexes have been reported to transfer the nitrene group to
tertiary phosphines.24b,29
significantly different, a tendency was observed that isomer-
ization of 3 into 4 was accelerated by an electron-rich Ar group.
Among the examined nitriles, the formation of 4 was fastest for
Ar = p-MeOC6H4CN, while 3e with Ar = p-CF3C6H4 was never
converted into 4 at 80 °C. In contrast, the amido-type complex
was not detected at all along the reactions with aliphatic nitriles.
Conversion to the imido complex is presumably very fast by
virtue of highly electron-donating alkyl groups.
Taking into consideration of these results, conversion of 3
into 4 is proposed to proceed via the following mechanisms.
The alkylideneamido ligand is reversibly protonated at the
azomethine carbon atom to give the imido ligand. If the
resulting cationic WIV complex releases the hydrido ligand as a
proton, the neutral imido complex 4 will be generated. The
accelerating effect of an added proton source supports this
hypothesis, and some imido complexes have been prepared via
protonation pathways (vide supra).10,12,20 The initial proto-
nation step is probably rate determining and depends on the
electron density at the azomethine carbon, which is influenced
by the adjacent organic group. Proton-transfer steps are
In summary, a tetrahydrido complex of W bearing a
tetradentate phosphine coligand undergoes insertion of nitrile,
leading to an imido complex via the alkylideneamido
intermediate. The resulting five-coordinated WII system is
unique in copious imido complexes and performs nitrene group
transfer. Not only the strong hydridic character but also the
ability to create a stable W−N multiple bond may be
responsible for imido ligand formation from nitrile, and 1 is
therefore promising for converting other nitrogen-containing
molecules.
C
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Organometallics
Communication
(11) Arndtsen, B. A.; Schoch, T. K.; McElwee-White, L. J. Am. Chem.
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(12) Feng, S. G.; White, P. S.; Templeton, J. L. J. Am. Chem. Soc.
ASSOCIATED CONTENT
* Supporting Information
Text, a table, a figure, and CIF files giving experimental
procedures, characterization data, and results of the crystallo-
graphic analysis. This material is available free of charge via the
Internet at http://pubs.acs.org.
■
S
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̈
AUTHOR INFORMATION
Corresponding Author
*Fax: +81 3 5452 6311. Tel +81 3 5452 6310. E-mail: seino@
iis.u-tokyo.ac.jp (H.S.).
J. Organomet. Chem. 1985, 280, 343. (c) Jordan, R. F.; Bajgur, C. S.;
Dasher, W. E.; Rheingold, A. L. Organometallics 1987, 6, 1041.
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3152. (b) Figueroa, J. S.; Cummins, C. C. J. Am. Chem. Soc. 2003, 125,
4020.
■
Notes
(16) Temprado, M.; McDonough, J. E.; Mendiratta, A.; Tsai, Y.-C.;
Fortman, G. C.; Cummins, C. C.; Rybak-Akimova, E. V.; Hoff, C. D.
Inorg. Chem. 2008, 47, 9380.
The authors declare no competing financial interest.
†Deceased March 11, 2010.
(17) Some reactions of polynuclear complexes forming μ3-imido
ligands are known: (a) Andrews, M. A.; Kaesz, H. D. J. Am. Chem. Soc.
1979, 101, 7255. (b) Banford, J.; Dawoodi, Z.; Henrick, K.; Mays, M.
J. J. Chem. Soc., Chem. Commun. 1982, 554. (c) Bernhardt, W.;
Vahrenkamp, H. Angew. Chem., Int. Ed. Engl. 1984, 23, 381. (d) Cui,
D.; Tardif, O.; Hou, Z. J. Am. Chem. Soc. 2004, 126, 1312.
(18) Churchill, M. R.; Wasserman, H. J.; Belmonte, P. A.; Schrock, R.
R. Organometallics 1982, 1, 559.
(19) Bercaw, J. E.; Davies, D. L.; Wolczanski, P. T. Organometallics
1986, 5, 443.
(20) (a) Bakir, M.; Fanwick, P. E.; Walton, R. A. Inorg. Chem. 1988,
27, 2016. (b) Esjornson, D.; Bakir, M.; Fanwick, P. E.; Jones, K. S.;
Walton, R. A. Inorg. Chem. 1990, 29, 2055.
(21) (a) Barron, A. R.; Salt, J. E.; Wilkinson, G.; Motevalli, M.;
Hursthouse, M. B. J. Chem. Soc., Dalton Trans. 1987, 2947.
(b) Anderson, S. J.; Wells, F. J.; Wilkinson, G.; Hussain, B.;
Hursthouse, M. B. Polyhedron 1988, 7, 2615.
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ACKNOWLEDGMENTS
■
This work was supported by a Grant-in-Aid for Scientific
Research on Priority Areas (No. 18065005, “Chemistry of
Concerto Catalysis” to Y.M.) and a Grant-in-Aid for Scientific
Research (B) (No. 21350033 to H.S.). H.S. gratefully
acknowledges the special fund of IIS, UT. Q.X.D. is grateful
for a grant from the GCOE program (Chemistry Innovation
through Cooperation of Science and Engineering).
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dx.doi.org/10.1021/om3004338 | Organometallics XXXX, XXX, XXX−XXX