A vanadium(V) alkylidene complex exhibiting remarkable catalytic activity for ring-opening metathesis polymerization (ROMP)

Article abstract of DOI:10.1021/om0501834

The vanadium(V) alkylidene complex (ArN)V-(=CHSiMe₃)(N=C ^tBu₂)(PMe₃) (6; Ar = 2,6-Me₂C ₆H₃) has been isolated from (ArN)V(CH₂SiMe ₃)₂(N=C^t

Full text of DOI:10.1021/om0501834

2248
Organometallics 2005, 24, 2248-2250
A Vanadium(V) Alkylidene Complex Exhibiting
Remarkable Catalytic Activity for Ring-Opening
Metathesis Polymerization (ROMP)
Junji Yamada, Michiya Fujiki, and Kotohiro Nomura*
Graduate School of Materials Science, Nara Institute of Science and Technology (NAIST),
8916-5 Takayama, Ikoma, Nara 630-0101, Japan
Received March 13, 2005
Scheme 1
Summary: The vanadium(V) alkylidene complex (ArN)V-
(dCHSiMe3)(NdCtBu2)(PMe3) (6; Ar ) 2,6-Me2C6H3) has
been isolated from (ArN)V(CH2SiMe3)2(NdCtBu2) (5) by
R-hydrogen elimination in benzene-d6 at 80 °C. The
complex 6 initiated ring-opening metathesis polymeri-
zation (ROMP) of norbornene, yielding a high-molecular-
weight polymer with unimodal molecular weight distri-
bution (Mw ) 1.15 × 106, Mw/Mn ) 1.6) in high yield
(98%), and the activity increased at higher temperatures
(80 °C).
High-oxidation-state transition-metal alkylidene com-
plexes have attracted considerable attention1 because
of their promise as catalysts (initiators) for olefin
metathesis such as ring-opening metathesis polymeri-
zation (ROMP), ring-closing metathesis (RCM), and
cross-metathesis (CM), etc.,1-3 and successful examples
have been demonstrated by molybdenum1b-d,2 and
ruthenium.2,3 However, examples of olefin metathesis
with vanadium alkylidene complexes4-7 have been
* To whom correspondence should be addressed. Tel: +81-743-72-
6041. Fax: +81-743-72-6049. E-mail: nomurak@ms.naist.jp.
(1) For recent examples, see: (a) Schrock, R. R. Chem. Rev. 2002,
102, 145-179. (b) Schrock, R. R. In Handbook of Metathesis; Grubbs,
R. H., Ed.; Wiley-VCH: Weinheim, Germany, 2003; Vol. 1, pp 8-32.
(c) Schrock, R. R. In Alkene Metathesis in Organic Synthesis; Fu¨rstner,
A., Ed.; Springer: Berlin, 1998; pp 1-36.
(2) For recent examples (books and review), see: (a) Buchmeiser,
M. R. Chem. Rev. 2000, 100, 1565-1604. (b) Grubbs, R. H., Ed.
Handbook of Metathesis; Wiley-VCH: Weinheim, Germany, 2003; Vol.
1-3. (c) Khosravi, E., Szymanska-Buzar, T., Eds. Ring-Opening
Metathesis Polymerisation and Related Chemistry; Kluwer: Dordrecht,
The Netherlands, 2002. (d) Imamoglu, Y., Bencze, L., Eds. Novel
Metathesis Chemistry: Well-Defined Initiator Systems for Specialty
Chemical Synthesis, Tailored Polymer and Advanced Material Ap-
plications; Kluwer: Dordrecht, The Netherlands, 2003.
(3) For example, see: (a) Trnka, T. M.; Grubbs, R. H. Acc. Chem.
Res. 2001, 34, 18-29. (b) Nguyen, S. T.; Trnka, T. M. In Handbook of
Metathesis; Grubbs, R. H., Ed.; Wiley-VCH: Weinheim, Germany,
2003; Vol 3, pp 61-85.
(4) Examples of isolated vanadium(III) and -(IV) alkylidene com-
plexes: (a) Hessen, B.; Meetsma, A.; Teuben, J. H. J. Am. Chem. Soc.
1989, 111, 5977-5978. (b) Hessen, B.; Buijink, J.-K. F.; Meetsma, A.;
Teuben, J. H.; Helgesson, G.; Hakansson, M.; Jagner, S. Spek, A. L.
Organometallics 1993, 12, 2268-2276. (c) Basuli, F.; Kilgore, U. J.;
Hu, X.; Meyer, K.; Pink, M.; Huffman, J. C.; Mindiola, D. J. Angew.
Chem., Int. Ed. 2004, 43, 3156-3159. (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, 10506-10507.
(5) Related isolated examples for vanadium carbene complexes: (a)
Erker, G.; Lecht, R.; Schlund, R.; Angermund, K.; Kru¨ger, C. Angew.
Chem., Int. Ed. Engl. 1987, 26, 666-668. (b) Milczarek, R.; Ru¨sseler,
W.; Binger, P.; Jonas, K.; Angermund, K.; Kruger, C.; Regitz, M.
Angew. Chem., Int. Ed. Engl. 1987, 26, 908-909. (c) Hessen, B.;
Meetsma, A.; van Bolhuis, F.; Teuben, J. H. Organometallics 1990, 9,
1925-1936.
(6) (a) Buijink, J.-K. F.; Teuben, J. H.; Kooijman, H.; Spek, A. L.
Organometallics 1994, 13, 2922-2924. No reaction between CpV-
(CHPh)(NAr)(PMe3) and NBE or acetone was observed. (b) Moore, M.;
Gambarotta, S.; Yap, G.; Liable-Sands, L. M.; Rheingold, A. L. Chem.
Commun. 1997, 643-644.
limited,7 although classical Ziegler type vanadium
catalysts displayed unique characteristics such as syn-
thesis of high-molecular-weight polymer with rapid
propagation in olefin coordination insertion polymeri-
zation8 and syntheses of some vanadium(III) and -(IV)
alkylidenes4,5 as well as vanadium(V) alkylidene com-
plexes6 are known. Since no examples for catalytic olefin
metathesis by vanadium alkylidene complexes are
known,7 we explored the possibility of isolating an olefin
metathesis active vanadium(V) alkylidene complex. In
this paper, we wish to present the first example of
isolation and structural determination of a thermally
stable alkylidene complex that exhibits remarkable
catalytic activity for the ROMP of norbornene (NBE) at
high temperature.9
We focused on (arylimido)vanadium(V) complexes
containing anionic donor ligands, because (ArN)V-
(7) Intramolecular metathesis4d and reaction of CpV(CHCMe3)-
(dmpe) with NBE4b were reported previously. However, the latter case
showed extremely low catalytic activity (TON 0.92 after 96 h at 20
°C) and no NMR or GPC results for the resultant polymers were given
in the text.
(8) Examples: (a) Carrick, W. L. J. Am. Chem. Soc. 1958, 80, 6455-
6456. (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, 3883-3887.
(c) Carrick, W. L.; Reichle, W. T.; Pennella, F.; Smith. J. J. J. Am.
Chem. Soc. 1960, 82, 3887-3892. (d) Lehr, M. H. Macromolecules 1968,
1, 178-184.
(9) Part of these results were introduced at the XXIst International
Conference on Organometallic Chemistry, P151, July 2004, Vancouver,
Canada.
10.1021/om0501834 CCC: $30.25 © 2005 American Chemical Society
Publication on Web 04/12/2005

Communications
Organometallics, Vol. 24, No. 10, 2005 2249
Figure 1. ORTEP drawings for 1a (left) and 3 (right). Thermal ellipsoids are drawn at the 50% probability level, and H
atoms are omitted for clarity.11
Scheme 2
(CH2Ph)2(O-2,6-iPr2C6H3) (Ar ) 2,6-Me2C6H3) initiated
the ROMP of NBE without cocatalyst, affording high-
molecular-weight ring-opened poly(NBE).10 The arylim-
ido-ketimide complexes were thus chosen, because
(ArN)V(CH2SiMe3)2(NdCtBu2) (5) showed better ther-
mal stability than (ArN)V(CH2Ph)2(O-2,6-iPr2C6H3).11
Various vanadium(V) dichloride complexes (1 and 2)
could be prepared in high yields by treating (ArN)VCl3
with LiNdCtBu2 in Et2O (Scheme 1; 84-85%)12 and
were identified by 1H, 13C, and 51V NMR spectra as well
as by elemental analyses.11,12 The crystal structure for
1a (Figure 1, left) showed that 1a has a distorted-
tetrahedral geometry around V, and the bond distances
for V-N(Ar) and V-N(CtBu2) are 1.660 and 1.787 Å,
respectively, suggesting that the complex 1a is a 14-
electron species. Complex 1a reacted with 1 equiv of
PMe3 to afford (ArN)VCl2(L)(PMe3) (3; L ) NdCtBu2),
1
which was exclusively confirmed by H, 13C, and 51V
NMR spectra.13 The crystal structure for 3 (Figure 1,
right) showed that 3 has a distorted-trigonal-bipyrami-
dal structure around V; the P atom is positioned trans
to the N atom in the ketimide ligand, and the V-N
distance (1.839 Å) is thus influenced by electron dona-
tion from PMe3. As suggested by both the rather long
V-P distance (2.527 Å) and the fact that only broad
resonances were seen in the 31P NMR spectrum, the
PMe3 was dissociated exclusively, affording analytically
pure 1a quantitatively if 3 was placed in vacuo. The
reaction of 1a with bis(dimethylphosphino)ethane (dmpe)
afforded (ArN)VCl2(L)(dmpe) (4),11 and the crystal
structure showed that 4 has a distorted-octahedral
geometry around V; V-N(Ar) and V-N(CtBu2) distances
(1.677 and 1.843 Å) are longer than those in 1a, due to
the trans influence of dmpe.
The reaction of 1a with LiCH2SiMe3 in n-hexane
afforded (ArN)V(CH2SiMe3)2(L) (5) in high yield (95%;
(10) Nomura, K.; Sagara, A.; Imanishi, Y. Macromolecules 2002, 35,
1583-1590. Examples of ROMP by vanadium catalysts were cited
therein.
1
Scheme 2), and the complex was identified by H, 13C,
and 51V NMR spectra and elemental analysis.11 The
coordination of PMe3 to 5 was not observed, even by
addition of an excess amount (7.0 equiv) at 25 °C, and
this observation might be due to the steric hindrance
of the rather bulky CH2SiMe3 group around the V metal
center.
(11) Detailed results (including the crystal structures) are given in
the Supporting Information.
(12) Complex 1a: yield 84%; 1H NMR (CDCl3) δ 1.35 (s, 18H,
(CH3)3C-), 2.69 (s, 6H, CH3), 6.94 (m, 3H); 13C NMR (CDCl3) δ 18.9,
30.2, 47.1, 127.4, 128.2, 138.4, 169.3, 210.1; 51V NMR (CDCl3) δ 72.4
(∆ν1/2 ) 237 Hz). Anal. Calcd for C17H27N2Cl2V: C, 53.55; H, 7.14; N,
7.35. Found: C, 53.71; H, 7.14; N, 7.26.
(13) Complex 3: 1H NMR (CDCl3) δ 1.27 (d, 9H, P(CH3)3), 1.39 (s,
18H, (CH3)3C-), 2.51 (s, 6H, (CH3)2), 6.81 (t, 1H), 6.89 (d, 2H); 13C
NMR (CDCl3) δ 13.9, 18.9, 30.9, 47.4, 126.6, 127.4, 138.6, 164.2, 190.0;
51V NMR (CDCl3) δ -76.7 (∆ν1/2 ) 900 Hz).
The complex 5 initiated the ROMP of NBE in toluene,
and the activity increased especially by the addition of
PMe3 at higher temperature (Table 1; run 8 vs runs 1

2250 Organometallics, Vol. 24, No. 10, 2005
Communications
Table 1. Ring-Opening Metathesis Polymerization
(ROMP) of Norbornene (NBE) with 5, 6, or
Ru(CHPh)(Cl)2(PCy3)2 (Ru, Cy ) Cyclohexyl)a
complex
e
(amt/
NBE P/ temp/ time/
Mw
× Mwe/
run µmol) solvent concnb Vc °C
h
TONd 10-4 Mn
1
2
3
4
5
6
7
8
9
5 (10.0) toluene
6 (1.0) benzene 0.22
Ru (1.0) toluene 0.22
benzene 0.22
0.89
3
25
25
25
50
50
80
80
80
80
80
80
80
80
80
6
1.6
3
267
1306
1275
166
46 2.3
54 1.7
49 1.6
1
6 (1.0)
6 (1.0)
3
toluene
0.22
0.89
0.22
0.44
0.44
0.22
3
5 (20.0) toluene
5 (10.0) toluene
5 (10.0) toluene
5 (10.0) toluene
6
6.8
3
3
5
3
101
212
40
1244
967
1583
2071f
350
97 3.0
87 3.2
47 3.0
32 2.8
140 1.3
133 1.4
115 1.6
3
3
10 6 (1.0)
11 6 (1.0)
12 6 (1.0)
13 6 (1.0)
toluene
1
benzene 0.22
benzene 0.22
benzene 0.22
0.5
1
2
1
14 Ru (1.0) toluene
0.22
a Conditions: NBE, 2.12 mmol (run 6, 4.24 mmol; run 7, 1.06
mmol); benzene or toluene, 9.6 mL (run 1, 2.4 mL; runs 6-9, 4.8
mL). b Initial NBE concentration in mmol/mL. c Molar ratio of
PMe3/V. d TON ) NBE consumed (mmol)/V (mmol). e GPC data
vs polystyrene standard. f Yield 98%.
Figure 2. ORTEP drawing for 6. Thermal ellipsoids are
drawn at the 50% probability level, and H atoms are
omitted for clarity.11
and 6). The activity depended upon the NBE concentra-
tion (runs 7 and 8), and the activity decreased on further
addition of PMe3 (run 9).11 The resultant polymer
possessed a ring-opened structure containing a mixture
of cis- and trans-olefinic double bonds.11 Note that a
significant increase in the activity on PMe3 addition at
higher temperature was observed, suggesting the for-
mation of the vanadium alkylidene.10
by the known Ru(CHPh)(Cl)2(PCy)2 under the same
conditions due to the improved thermal stability at 80
°C (runs 10 and 12 vs run 14). This is the first example
that a vanadium alkylidene species catalytically ini-
tiates ROMP, and this is also a rare example of a
“thermally stable” olefin metathesis active transition-
metal alkylidene. Various applications such as ROMP,
acyclic diene metathesis (ADMET) polymerization, CM,
and RCM with 6 can be thus expected, and we are now
exploring these possibilities in more detail.
Encouraged by the above findings, reaction of 5 with
1
PMe3 (7.0 equiv) was thus monitored by H NMR in
benzene-d6 (at 80 °C),14 because R-hydrogen elimination
should be initiated by coordination of PMe3 to the V.
Although no reaction occurred at 25 °C, the reaction did
take place at 80 °C;14 the alkylidene complex (ArN)V-
(dCHSiMe3)(L)(PMe3) (6) could be thus isolated from
the chilled n-hexane solution15 and was identified by 1H,
13C, and 31P NMR spectra and elemental analysis. The
crystal structure (Figure 2) indicates that 6 has a
distorted-tetrahedral geometry around the vanadium
metal center, and the V-CHSiMe3 bond distance (1.860
Å) is close to that in the bicyclic carbene-amide com-
plex (1.876 Å)6b and is shorter than that in the ben-
zylidene complex (1.922 Å);6a the distance is longer than
those in neutral VIII-CHtBu (1.809 Å)4a,b and cat-
ionic VIV-CHtBu complexes (1.787-1.795 Å).4c,d The
V-C-Si bond angle is 121.4°, and the V-P bond
distance (2.433 Å) is shorter than those in 3 and 4
(2.515-2.646 Å). The 1H and 13C NMR spectra showed
resonances corresponding to VdCHSiMe3 at 14.52 and
302.0 ppm, respectively. These results clearly indicate
that 6 is a 16-electron vanadium(V) alkylidene complex.
It should be noted that 6 initiated the ROMP of NBE
with remarkable catalytic activity, affording high-mo-
lecular-weight ring-opened polymer with unimodal mo-
lecular weight distributions (run 13, 98% yield, Mn )
1.15 × 106, Mw/Mn ) 1.6).16 Note that the activity
markedly increased in benzene at higher temperature
(80 °C), and the observed activity was higher than that
Acknowledgment. J.Y. expresses his thanks to the
JSPS for a predoctoral fellowship (No. 5042). J.Y. and
K.N. thank Mr. Shohei Katao (NAIST) for help in
crystallographic analysis. K.N. thanks Prof. Masafumi
Hirano (Tokyo Univ. A&T) for helpful discussions
regarding crystallographic analysis.
Supporting Information Available: Text and tables
giving experimental details, additional polymerization results,
and details of the crystal structure determinations for 1a, 3,
4, and 6; crystallographic data are also given as CIF files. This
material is available free of charge via the Internet at
http://pubs.acs.org.
OM0501834
(15) Compound 6: yield 34%; 1H NMR (C6D6) δ 0.38 (s, 9H, CHSi-
(CH3)3), 0.93 (d, 9H, PMe3), 1.25 (s, 18H, (CH3)3C-), 2.78 (s, 6H, (CH3)2),
6.82 (t, 1H), 7.09 (d, 2H), 14.52 (br, 1H, CHSi(CH3)3); 13C NMR (CDCl3)
δ 2.8, 17.4, 17.7, 20.2, 31.0, 42.4, 121.9, 127.6, 134.5, 161.7, 193.8, 302.0.
Anal. Calcd for C24H46N2PSiV: C, 60.99 (58.68 + VC); H, 9.81; N, 5.93.
Found: C, 58.68; H, 9.72; N, 5.73.
(16) Since the molecular weights for resultant poly(NBE)s were
higher than those estimated on the basis of NBE/V molar ratios, the
catalyst (initiation) efficiency of 6 was not perfect. This is probably
because the dissociation of PMe3 is necessary for the formation of
catalytically active species, because the observed catalytic activity
decreased at higher PMe3/V molar ratio in the ROMP by 5. A study
concerning the actual catalytically active species for the ROMP is now
being carried out.
(14) For more details, see the Supporting Information.