Facile synthesis of (Imido)vanadium(V)-Alkyl, alkylidene complexes containing an n-heterocyclic carbene ligand from their trialkyl analogues

Article abstract of DOI:10.1021/om8007822

Summary: (Imido)vanadium(V)-alkyl, alkylidene complexes containing an N-heterocyclic carbene (NHC) ligand, V(CHSiMe₃)(NR)(CH ₂SiMe₃)(NHC) [R = 1-adamantyl (Ad) or 2,6-Me ₂C₆H₃ (Ar), NHC = 1, -bis(2,6- diisopropylphenyl)imi-dazol-2-ylidene], have been prepared from the trialkyl analogues, V(NR)(CH₂SiMe₃)₃, by αhydogen elimination in C₆D₆in the presence of NHC. The V-C(NHC) bond distance [2.172(2) A] in V(CHSiMe₃)(NAd)(CH ₂SiMe₃)(NHC) is much longer than the V-C(alkylidene) [1.829 A] and the V-C(alkyl) [2.069 A] distances, indicating that NHC plays an important role in the stabilization of the high-oxidation-state vanadium(V)-alkylidene as a neutral carbene ligand.

Full text of DOI:10.1021/om8007822

6400
Organometallics 2008, 27, 6400–6402
Facile Synthesis of (Imido)vanadium(V)-Alkyl, Alkylidene
Complexes Containing an N-Heterocyclic Carbene Ligand from
Their Trialkyl Analogues
Wenjuan Zhang and Kotohiro Nomura*
Graduate School of Material Science, Nara Institute of Science and Technology (NAIST),
8916-5 Takayama, Ikoma, Nara 630-0101, Japan
ReceiVed August 14, 2008
hibited high catalytic activities for ring-opening metathesis
polymerization of norbornene. Although titanium(IV)-alkyl,
alkylidene complexes display unique reactivities with various
organic compounds via a “Tit CiBu” intermediate,8 synthesis
and the reaction chemistry of the vanadium(V)-alkyl, alkylidene
complexes have not been reported so far.9 We thus herein
present that syntheses of (imido)vanadium(V)-alkyl, alkylidene
complexes, V(CHSiMe3)(NR)(CH2SiMe3)(NHC) [R ) 1-ada-
mantyl (Ad), 2,6-Me2C6H3 (Ar); NHC ) 1,3-bis(2,6-diisopro-
pylphenyl)imidazol-2-ylidene], have been achieved from the
trialkyl analogues by R-hydrogen abstraction in the presence
of NHC.10
Summary: (Imido)Vanadium(V)-alkyl, alkylidene comple-
xes containing an N-heterocyclic carbene (NHC) ligand,
V(CHSiMe3)(NR)(CH2SiMe3)(NHC) [R ) 1-adamantyl (Ad) or
2,6-Me2C6H3 (Ar), NHC ) 1,3-bis(2,6-diisopropylphenyl)imi-
dazol-2-ylidene], haVe been prepared from the trialkyl ana-
logues, V(NR)(CH2SiMe3)3, by R-hydrogen elimination in C6D6
in the presence of NHC. The V-C(NHC) bond distance
[2.172(2) Å] in V(CHSiMe3)(NAd)(CH2SiMe3)(NHC) is much
longer than the V-C(alkylidene) [1.829 Å] and the V-C(alkyl)
[2.069 Å] distances, indicating that NHC plays an important
role in the stabilization of the high-oxidation-state Vanadium(V)-
alkylidene as a neutral carbene ligand.
Reaction of V(NAd)Cl3 prepared by the reported method11
with 3.0 equiv of LiCH2SiMe3 in n-hexane gave the trialkyl
analogue V(NAd)(CH2SiMe3)3 (1) in moderate yield (64%), and
the procedure is analogous to that for synthesis of the arylimido
analogue V(NAr)(CH2SiMe3)3 (2).6d Complex 1 was identified
on the basis of NMR spectra and elemental analysis, and the
structure was determined by X-ray crystallography (Figure 1).10
Complex 1 has a rather distorted tetrahedral geometry around
V, and the V-N-C bond angle is 180.00(17)° and the V-N
distance is 1.6317(14) Å; the results would be a unique contrast
to those in V(N-2,6-iPr2C6H3)(CH2Ph)3 [V-N-C 169.0(5)° and
1.641(6) Å].12 The three V-C bond distances and angles are
High-oxidation-state early transition metal alkylidene com-
plexes attract considerable attention1 because they play essential
roles as catalysts in olefin metathesis and Wittig-type group
transfer reactions,1,2 as demonstrated by molybdenum.1a-d,2
Although classical Ziegler-type vanadium catalysts displayed
unique characteristics in olefin coordination insertion polym-
erization (notable reactivity toward olefins),3,4 examples with
vanadium(V)-alkylidenes still have been limited.1e,f,5-7 We
recently introduced that (arylimido)vanadium(V)-alkylidene
complexes containing ketimide6b,e and aryloxo6d ligands ex-
* Corresponding author. Tel: +81-743-72-6041. Fax: +81-743-72-6049.
E-mail: nomurak@ms.naist.jp.
(1) For examples, see: (a) Schrock, R. R. Acc. Chem. Res. 1990, 23,
158. (b) Schrock, R. R. In Alkene Metathesis in Organic Synthesis; Fu¨rstner,
A., Ed.; Springer: Berlin, Germany, 1998; p 1. (c) Schrock, R. R. Chem.
ReV. 2002, 102, 145. (d) Schrock, R. R. In Handbook of Metathesis; Grubbs,
R. H., Ed.; Wiley-VCH: Weinheim, Germany, 2003; Vol. 1, p 8. (e)
Mindiola, D. Acc. Chem. Res. 2006, 39, 813. (f) Mindiola, D.; Bailey, B.;
Basuli, F. Eur. J. Inorg. Chem. 2006, 16, 3135.
(2) For examples, see: (a) Buchmeiser, M. R. Chem. ReV. 2000, 100,
1565. (b) Grubbs, R. H., Ed. Handbook of Metathesis; Wiley-VCH:
Weinheim, Germany, 2003; Vols. 1-3. (c) Khosravi, E., Szymanska-Buzar,
T., Eds. Ring-Opening Metathesis Polymerisation and Related Chemistry;
Kluwer: Dordrecht, The Netherlands, 2002. (d) Imamoglu, Y., Dragutan,
V., Eds. Metathesis Chemistry; Springer: Dordrecht, The Netherlands, 2007.
(3) Examples: (a) Carrick, W. L. J. Am. Chem. Soc. 1958, 80, 6455. (b)
Carrick, W. L.; Kluiber, R. W.; Bonner, E. F.; Wartman, L. H.; Rugg, F. M.;
Smith, J. J. J. Am. Chem. Soc. 1960, 82, 3883. (c) Carrick, W. L.; Reichle,
W. T.; Pennella, F.; Smith, J. J. J. Am. Chem. Soc. 1960, 82, 3887. (d)
Lehr, M. H. Macromolecules 1968, 1, 178.
(6) Examples for terminal vanadium(V)-alkylidenes: (a) Buijink
J.-K. F.; Teuben, J. H.; Kooijman, H.; Spek, A. L. Organometallics 1994,
13, 2922. No reaction between CpV(CHPh)(NAr)(PMe3) and norbornene
or acetone was observed. (b) Yamada, J.; Fujiki, M.; Nomura, K.
Organometallics 2005, 24, 2248. (c) Kilgore, U. J.; Sengelaub, C. A.; Pink,
M.; Fout, A. R.; Mindiola, D. J. Angew. Chem., Int. Ed. 2008, 47, 3769.
No descriptions concerning application as the catalysts. (d) Nomura, K.;
Onishi, Y.; Fujiki, M.; Yamada, J. Organometallics 2008, 27, 3818. (e)
Zhang, W.; Yamada, J.; Nomura, K. Organometallics, 2008, 27, 5353.
(7) Examples for related complexes containing vanadium(V)-carbon
double bonds: (a) Hessen, B.; Meetsma, A.; van Bolhuis, F.; Teuben, J. H.
Organometallics 1990, 9, 1925. (b) Moore, M.; Gambarotta, S.; Yap, G.;
Liable-Sands, L. M.; Rheingold, A. L. Chem. Commun. 1997, 643.
(8) (a) Bailey, B. C.; Fan, H.; Baum, E. W.; Huffman, J. C.; Baik
M.-H.; Mindiola, D. J. J. Am. Chem. Soc. 2005, 127, 16016. (b) Bailey,
B. C.; Fan, H.; Huffman, J. C.; Baik, M.-H.; Mindiola, D. J. J. Am. Chem.
Soc. 2006, 128, 6798. (c) Bailey, B. C.; Fout, A. R.; Fan, H.; Tomaszweski,
J.; Huffman, J. C.; Gary, J. B.; Johnson, M. J. A.; Mindiola, D. J. J. Am.
Chem. Soc. 2007, 129, 2234. (d) Bailey, B. C.; Huffman, J. C.; Mindiola,
D. J. J. Am. Chem. Soc. 2007, 129, 5302. (e) Bailey, B. C.; Fan, H.;
Huffman, J. C.; Baik, M.-H.; Mindiola, D. J. J. Am. Chem. Soc. 2007, 129,
8781. (f) Fout, A. R.; Bailey, B. C.; Tomaszweski, J.; Mindiola, D. J. J. Am.
Chem. Soc. 2007, 129, 12640.
(4) For reviews, see: (a) Gibson, V. C.; Spitzmesser, S. K. Chem. ReV.
2003, 103, 283. (b) Gambarotta, S. Coord. Chem. ReV. 2003, 237, 229. (c)
Hagen, H.; Boersma, J.; van Koten, G. Chem. Soc. ReV. 2002, 31, 357. (d)
Nomura, K. In New DeVelopments in Catalysis Research; Bevy, L. P., Ed.;
NOVA Science Publishers: New York, 2005; p 199.
(5) Examples for isolation of terminal vanadium(III),(IV) alkylidenes:1e,f
(a) Hessen, B.; Meetsma, A.; Teuben, J. H. J. Am. Chem. Soc. 1989, 111,
5977. (b) Hessen, B.; Meetsma, A.; Van Bolhuis, F.; Teuben, J. H.;
Helgesson, G.; Jagner, S. Organometallics 1990, 9, 1925. (c) Basuli, F.;
Kilgore, U. J.; Hu, X.; Meyer, K.; Pink, M.; Huffman, J. C.; Mindiola,
D. J. Angew. Chem., Int. Ed. 2004, 43, 3156. (d) Basuli, F.; Bailey, B. C.;
Brown, D.; Tomaszewski, J.; Huffman, J. C.; Baik, M.-H.; Mindiola, D. J.
J. Am. Chem. Soc. 2004, 126, 10506.
(9) Synthesis of vanadium(IV)-alkyl, alkylidene yielding terminal
vanadium(V)-neopentylidyne upon addition of AgOTf (Tf ) CF3SO2).
Basuli, F.; Bailey, B. C.; Brown, D.; Tomaszewski, J.; Huffman, J. C.;
Baik, M.-H.; Mindiola, D. J. J. Am. Chem. Soc. 2004, 126, 10506.
(10) Experimental details including syntheses, identification, and struc-
tural analysis reports are shown in the Supporting Information.
(11) Zhang, W.; Nomura, K. Inorg. Chem. 2008, 47, 6482.
(12) Murphy, V. J.; Turner, H. Organometallics 1997, 16, 2495.
10.1021/om8007822 CCC: $40.75
2008 American Chemical Society
Publication on Web 11/12/2008

Communications
Organometallics, Vol. 27, No. 24, 2008 6401
Scheme 1
Figure 1. ORTEP drawing for 1. Thermal ellipsoids are drawn
at the 50% probability level, and H atoms are omitted for
clarity.10 Selected bond angles (deg) and distances (Å):
V-N-C(5) 180.00(17), C(1)-V-C(1)* 112.55(7); V-N
1.6317(14), V(1)-C(1) 2.0267(18).
as organocatalysts,16 and NHC in VOCl3(IMes) [IMes ) 1,3-
bis(2,4,6-trimethylphenyl)imidazol-2-ylidene] previously pro-
posed its utility to stabilize high-oxidation-state vanadium(V)
complexes.17 We thus explored the reaction of 1 in the presence
of 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (NHC). Co-
ordination of NHC was not seen if 1 was added with 1.0 equiv
of NHC in C6D6 at 25 °C, but a new resonance ascribed to the
alkylidene proton appeared in the 1H NMR spectrum (13.4 ppm)
when the solution was heated at 80 °C for 18 h.10 Only one
resonance in addition to 1 was observed in the 51V NMR
spectrum.10 However, isolation of the targeted complex failed
due to the difficulty in separating the product from the NHC
remaining in the mixture. Eventually, the targeted complex could
be isolated (22% based on NHC, 15% based on V) when 1 was
treated with 2/3 equiv of NHC at 80 °C in C6D6.10 The complex
could be identified on the basis of NMR spectra and elemental
analysis as V(CHSiMe3)(NAd)(CH2SiMe3)(NHC) (6), having
alkyl, alkylidene, and N-heterocyclic carbene ligands, and the
structure could be determined by X-ray crystallography (Figure
2). Similarly, the reaction of the arlyimido analogue (2) with
NHC afforded V(CHSiMe3)(NAr)(CH2SiMe3)(NHC) (7) in
better yield (75% based on NHC, 60% based on V) than the
adamantylimido analogue (6), and 7 was identified on the basis
of NMR spectra and elemental analysis. The alkylidene protons
(VdCHSiMe3) in 6 and 7 were observed at 13.4 ppm as a broad
resonance coupled with vanadium, and the resonance is
somewhat close to that in V(CHSiMe3)(NAr)(NdCtBu2)(PMe3)
(14.52 ppm)6b but shifted to low field compared to V(CHSiMe3)-
(NAr)(O-2,6-iPr2C6H3)(PMe3) (16.09 ppm).6d However, the
the same [112.55(7)°, 2.0267(18) Å], and the facts are also a
unique contrast to those in the arylimido-tribenzyl complex.12
Reaction of 1 with 1 equiv of C6F5OH in n-hexane afforded
a mixture of V(NAd)(CH2SiMe3)2(OC6F5) (3, 62% yield) and
[V(NAd)(CH2SiMe3)(OC6F5)(µ2-OC6F5)]2 (4, 4% yield), as
shown in Scheme 1, and these two complexes were isolated by
separation and were identified on the basis of NMR spectra and
elemental analysis;10 the structure for 4 could be determined
by X-ray crystallography as a dimeric structure bridged by
oxygen.10,13 Although 1 is a low-coordinate unsaturated trialkyl
complex, coordination of a donor ligand [such as PMe3 (14
equiv) and CH3CN (1 equiv)] was not observed in the 51V NMR
spectra (in C6D6) at room temperature, as seen in the arylimido
analogue (2),6d probably due to the steric bulk of the three
CH2SiMe3 ligands. One of the methods commonly employed
generating metal-alkylidene is the R-hydrogen abstraction
promoted by heat or upon irradiation,1 but thermolysis of 1 in
C6D6 at 80 °C afforded a dimeric species, [V(µ2-NAd)-
(CH2SiMe3)2]2 (5), identified by NMR spectra and elemental
analysis. The structure of 5 was also confirmed by X-ray
crystallography,14 whichwasanalogousto[V(µ2-4-MeC6H4)(2,4,6-
Me3C6H2)2]2 reported previously.15
N-Heterocyclic carbenes (NHC) have been known to play
an essential role as ligands in metal complex catalysts as well
(13) The reaction of 1 with 1 equiv of 2,6-Me2C6H3SH in n-hexane did
not take place, as seen in the attempted reaction of 2.6d The reaction of 1
with 1 equiv of (CF3)2CHOH in n-hexane at 25 °C recovered 1 in high
yield even if the mixture was stirred overnight (>12 h),10 and the results
would be an interesting contrast to the fact that the reaction of 2 with
(CF3)2C(Me)OH cleanly afforded the corresponding alkoxide.6d Detailed
results are shown in the Supporting Information.
(14) We attempted the crystallographic analysis several times, and the
structure could be solved, but the R value may not be satisfactory; these
results are shown in the Supporting Information.
(16) For example: (a) N-Heterocyclic Carbenes in Synthesis; Nolan, S. P.,
Ed.; Wiley-VCH: Weinheim, Germany, 2006. (b) N-Heterocyclic Carbenes
in Transition Metal Catalysis; Glorius, F., Ed.; Springer: Berlin, Germany,
2007. (c) Bourissou, D.; Guerret, O.; Gabbai, F. P.; Bertrand, G. Chem.
ReV. 2000, 100, 39. (d) Enders, D.; Niemeier, O.; Henseler, A. Chem. ReV.
2007, 107, 5605.
(15) Synthesis and structural analysis for [V(µ2-4-MeC6H4)(2,4,6-
Me3C6H2)2]2. Solan, G. A.; Cozzi, P. G.; Floriani, C.; Chiesi-Villa, A.;
Rizzoli, C. Organometallics 1994, 13, 2572.
(17) Abernethy, C. D.; Codd, G. M.; Spicer, M. D.; Taylor, M. K. J. Am.
Chem. Soc. 2003, 125, 1128.

6402 Organometallics, Vol. 27, No. 24, 2008
geometry around vanadium (Figure 2), and the V-C bond
distances for the alkyl, the alkylidene, and the NHC are 2.069(3),
1.829(3), and 2.172(2) Å, respectively; these values (alkyl,
alkylidene) are similar to those in previous reports.6b,10,17 The
V-C(NHC) distance is close to that in VOCl3(IMes) (2.137
Å); the coordination seems like that of neutral carbene, and a
similar explanation would be thus possible. Ring-opening
metathesis polymerization of norbornene by 6 (and 7) took
place,20 but the activities (at 80 °C in benzene) were lower than
that by V(CHSiMe3)(NAr)(NdCtBu2)(PMe3)6b and the activities
at 25 °C were negligible.
We have presented the utility of NHC for stabilization of
high-oxidation-state vanadium-alkylidenes (6 and 7), and an
appropriate donor ligand is necessary to promote R-hydrogen
elimination from two adjacent alkyl species. The present
approaches should be widely applied for preparation of various
early transition metal-alkylidene complexes. Moreover, the
present alkyl-alkylidenes would also be promising intermedi-
ates for the syntheses of not only alkylidynes but also cationic
alkylidenes or alkylidenes containing a series of anionic donor
ligands. Further studies concerning their application as catalysts
including the effect of adamantyl imido ligands are now under
way.
Acknowledgment. The authors express their sincere
thanks to Mr. Shohei Katao (NAIST) for help in the
crystallographic analyses. This work was partly supported
by a Grant-in-Aid for Scientific Research on Priority Areas
(No. 19028047, “Chemistry of Concerto Catalysis”) from
Ministry of Education, Culture, Sports, Science and Technol-
ogy, Japan. W.Z. expresses her thanks to JSPS for a
postdoctoral fellowship (P06349).
Figure 2. ORTEP drawing for 6. Thermal ellipsoids are drawn at
the 50% probability level, and H atoms are omitted for clarity.10
Selected bond angles (deg) and distances (Å): N-V-C(11)
107.00(12),C(11)-V-C(15)111.10(12),C(11)-V-C(19)115.26(11),
C(15)-V-C(19) 101.96(11); V-N(1) 1.637(2), V-C(11) 2.069(3),
V-C(15) 1.829(3), V-C(19) 2.172(2).
reaction of 1 with (more bulky and stable18) 1,3-di-tert-
butylimidazole-2-ylidene did not show any changes in the 51V
NMR spectra even at 80 °C for 3 days (Scheme 1).10 This thus
suggests that this NHC stabilizes 1 probably by weak coordina-
tion but did not promote R-hydrogen elimination under these
conditions.19
Supporting Information Available: Text giving experimental
procedures, CIF files and structure reports for V(NAd)(CH2SiMe3)3
(1), [V(NAd)(CH2SiMe3)(OC6F5)(µ2-OC6F5)]2 (4), [V(µ2-NAd)-
(CH2SiMe3)2]2 (5), and V(NAd)(CHSiMe)(CH2SiMe3)(NHC) [6,
NHC ) 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene], and their
reactions (described in the text and notes). These materials are
available free of charge via the Internet at http://pubs.acs.org.
The (adamantylimido)vanadium(V)-alkyl, alkylidene com-
plex containing NHC (6) has a rather distorted tetrahedral
OM8007822
(18) Poater, P.; Ragone, F.; Giudice, S.; Costabile, C.; Dorta, R.; Nolan,
S. P.; Cavallo, L. Organometallics 2008, 27, 2679.
(19) Two new resonances (may be ascribed to the alkylidene) were seen
in the 51V (and 1H) NMR spectrum when 1 was treated with 14 equiv of
PMe3 in C6D6 at 80 °C,10 but attempted isolation of any complexes from
the mixture was unsuccessful.
(20) Conditions: 6 1.0 µmol, norbornene 200 mg, toluene 5.0 mL, 80
°C for 30 min. Activity 740 turnovers (TON), Mw ) 3.74 × 105, Mw/Mn )
2.0. TON after 1 h was 390 if the reaction with 7 was conducted under the
same conditions.