Highly efficient dimerization of ethylene by (Imido)vanadium complexes containing (2-anilidomethyl)pyridine ligands: Notable ligand effect toward activity and selectivity

Article abstract of DOI:10.1021/ja100573d

(Imido)vanadium(V) complexes containing the (2-anilidomethyl)pyridine ligand, V(NR)Cl₂[2-ArNCH₂(C₅H₄N)] (R = 1-adamantyl (Ad), cyclohexyl (Cy), phenyl), exhibit remarkably high catalytic activities (e.g. TOF = 2-730-000 h^-1 (758 s^-1) by V(NAd)Cl₂[2-(2,6-Me₂C₆H₃)NCH ₂(C₅H₄N)) for ethylene dimerization in the presence of MAO, affording 1-butene exclusively (selectivity 90.4 to >99%). The steric bulk of the imido ligand plays an important role in the selectivity (polymerization vs dimerization), and the electronic nature directly affects the catalytic activity (activity: R = Ad > Cy > Ph).

Full text of DOI:10.1021/ja100573d

Published on Web 03/10/2010
Highly Efficient Dimerization of Ethylene by (Imido)vanadium
Complexes Containing (2-Anilidomethyl)pyridine Ligands:
Notable Ligand Effect toward Activity and Selectivity
Shu Zhang and Kotohiro Nomura*
Graduate School of Materials Science, Nara Institute of Science and Technology, 8916-5
Takayama, Ikoma, Nara 630-0101, Japan
Received January 21, 2010; E-mail: nomurak@ms.naist.jp
Abstract: (Imido)vanadium(V) complexes containing the (2-anilidomethyl)pyridine ligand, V(NR)Cl2[2-
ArNCH2(C5H4N)] (R ) 1-adamantyl (Ad), cyclohexyl (Cy), phenyl), exhibit remarkably high catalytic activities
(e.g. TOF ) 2 730 000 h-1 (758 s-1) by V(NAd)Cl2[2-(2,6-Me2C6H3)NCH2(C5H4N)) for ethylene dimerization
in the presence of MAO, affording 1-butene exclusively (selectivity 90.4 to >99%). The steric bulk of the
imido ligand plays an important role in the selectivity (polymerization vs dimerization), and the electronic
nature directly affects the catalytic activity (activity: R ) Ad > Cy > Ph).
insertion polymerization.11 We previously reported that (arylim-
ido)vanadium(V) complexes containing anionic ancillary donor
Introduction
ligands of the type V(N-2,6-Me2C6H3)Cl2(X) (X ) aryloxo,12a-c
Liner R-olefins are important intermediates for a variety of
products (such as detergents, polymers, lubricants, surfactants,
etc.)1 and are largely produced by ethylene oligomerization,2,3
as exemplified by the Shell Higher Olefin Process (SHOP) using
a Ni complex catalyst containing a chelate P-O ligand.2
Although recent examples of ethylene oligomerization, espe-
cially using Ni4 and Fe complexes,5 or ethylene trimerization,
especially using Cr complex catalysts,6,7 have been known, the
examples using group 4 (Ti, Zr)8 and group 5 (V, Ta)9,10
catalysts have been limited. In particular, reports of efficient
complex catalysts with notable catalytic activity have not been
reported so far.
ketimide,12d phenoxyimine,12e (2-anilidomethyl)pyridine12f
)
(7) For reviews and selected more recent examples, see: (a) Dixon, J. T.;
Green, M. J.; Hess, F. M.; Morgan, D. H. J. Organomet. Chem. 2004,
689, 3641–3668. (b) Wass, D. F. Dalton Trans. 2007, 816–819. (c)
Tomov, A. K.; Chirinos, J. J.; Long, R. J.; Gibson, V. C.; Elsegood,
M. R. J. J. Am. Chem. Soc. 2006, 128, 7704–7705. (d) Bru¨ckner, A.;
Jabor, J. K.; McConnell, A. E. C.; Webb, P. B. Organometallics 2008,
27, 3849–3856. (e) McGuinness, D. S.; Suttil, J. A.; Gardiner, M. G.;
Davies, N. W. Organometallics 2008, 27, 4238–4247. (f) Zhang, J.;
Braunstein, P.; Hor, T. S. A. Organometallics 2008, 27, 4277–4279.
(g) Hanton, M. J.; Tenza, K. Organometallics 2008, 27, 5712–5716.
(h) Kirillov, E.; Roisnel, T.; Razavi, A.; Carpentier, J.-F. Organome-
tallics 2009, 28, 2401–2409. (i) Zhang, J.; Li, A.; Hor, T. S. A.
Organometallics 2009, 28, 2935–2937. (j) Dulai, A.; de Bod, H.;
Hanton, M. J.; Smith, D. M.; Downing, S.; Mansell, S. M.; Wass,
D. F. Organometallics 2009, 28, 4613–4616. (k) Tenza, K.; Hanton,
M. J.; Slawin, A. M. Z. Organometallics 2009, 28, 4852–4867. (l)
Vidyaratne, I.; Nikiforov, G. B.; Gorelsky, S. I.; Gambarotta, S.;
Duchateau, R.; Korobkov, I. Angew. Chem., Int. Ed. 2009, 48, 6552–
6556.
Designing vanadium complex catalysts should be promising
for the target, because the classical Ziegler-type catalyst systems
(V(acac)3, VOCl3, etc. and Et2AlCl, EtAlCl2, nBuLi, etc.) display
uniquely high reactivity toward olefins in olefin coordination/
(1) (a) Vogt, D. In Applied Homogeneous Catalysis with Organometallic
Compounds; Cornils, B., Herrmann, W. A., Eds.; Wiley-VCH:
Weinheim, Germany, 2000; pp 245-258. (b) van Leeuwen, P. W. N. N.
Homogeneous Catalysis; Kluwer Academic: Dordrecht, The Nether-
lands, 2004; pp 175-190.
(8) For Ti and Zr complexes, see: (a) Wielstra, Y.; Gambarotta, S.; Chiang,
Y. Organometallics 1988, 7, 1866–1867. (b) Deckers, P. J. W.; Hessen,
B.; Teuben, J. H. Angew. Chem., Int. Ed. 2001, 40, 2516–2519. (c)
Deckers, P. J. W.; Hessen, B.; Teuben, J. H. Organometallics 2002,
21, 5122–5135. (d) You, Y.; Girolami, G. S. Organometallics 2008,
27, 3172–3180. (e) Otten, E.; Batinas, A. A.; Meetsma, A.; Hessen,
B. J. Am. Chem. Soc. 2009, 131, 5298–5312.
(2) (a) Peuckert, M.; Keim, W. Organometallics 1983, 2, 594–597. (b)
Peuckert, M.; Keim, W.; Storp, S.; Weber, R. S. J. Mol. Catal. 1983,
20, 115–127. (c) Keim, W. Angew. Chem., Int. Ed. Engl. 1990, 29,
235–244.
(9) For Ta catalysts, see: (a) Fellmann, J. D.; Rupprecht, G. A.; Schrock,
R. R. J. Am. Chem. Soc. 1979, 101, 5099–5101. (b) Andes, C.; Harkins,
S. B.; Murtuza, S.; Oyler, K.; Sen, A. J. Am. Chem. Soc. 2001, 123,
7423–7424. (c) Arteaga-Mu¨ller, R.; Tsurugi, H.; Saito, T.; Yanagawa,
M.; Oda, S.; Mashima, K. J. Am. Chem. Soc. 2009, 131, 5370–5371.
Ligand-free TaCl5-cocatalyst systems were used in refs 11b and c.
(10) For V complex catalysts, see ref 7g and: (a) Brussee, E. A. C.;
Meetsma, A.; Hessen, B.; Teuben, J. H. Chem. Commun. 2000, 497–
498. (b) Schmidt, R.; Welch, M. B.; Knudsen, R. D.; Gottfried, S.;
Alt, H. G. J. Mol. Catal. A 2004, 222, 17–25. Vanadium complexes
containing chelate bis(imino)pyridine ligands yielded polymers/
(3) For reviews of the metal-catalyzed dimerization of ethylene and
propylene, see: (a) Pillai, S. M.; Ravindranathan, M.; Sivaram, S.
Chem. ReV. 1986, 86, 353–399. (b) Skupinska, J. Chem. ReV. 1991,
91, 613–648.
(4) For selected examples, see: (a) Killian, C. M.; Johnson, L. K.;
Brookhart, M. Organometallics 1997, 16, 2005–2007. (b) Svejda,
S. A.; Brookhart, M. Organometallics 1999, 18, 65–74. (c) Komon,
Z. J. A.; Bu, X.; Bazan, G. C. J. Am. Chem. Soc. 2000, 122, 12379–
12380. (d) Speiser, F.; Braunstein, P.; Saussine, L. Acc. Chem. Res.
2005, 38, 784–793.
oligomers with Schultz-Flory distribution7g,10b
.
(5) (a) Small, B. L.; Brookhart, M. J. Am. Chem. Soc. 1998, 120, 7143–
7144. (b) Britovsek, G. J. P.; Mastroianni, S.; Solan, G. A.; Baugh,
S. P. D.; Redshaw, C.; Gibson, V. C.; White, A. J. P.; Williams, D. J.;
Elsegood, M. R. J. Chem. Eur. J. 2000, 6, 2221–2231.
(6) Reagan, W. K. (Phillips Petroleum Company) EP 0417477, 1991.
(11) For recent reviews (vanadium catalysts), see: (a) Hagen, H.; Boersma,
J.; van Koten, G. Chem. Soc. ReV. 2002, 31, 357–364. (b) Gambarotta,
S. Coord. Chem. ReV. 2003, 237, 229–243. (c) Nomura, K. In New
DeVelopments in Catalysis Research; Bevy, L. P., Ed.; NOVA Science
Publishers: New York, 2005; pp 199-217.
9
4960 J. AM. CHEM. SOC. 2010, 132, 4960–4965
10.1021/ja100573d  2010 American Chemical Society

Ethylene Dimerization by (Imido)vanadium Catalysts
A R T I C L E S
Scheme 1
Scheme 2
showed remarkable catalytic activities for ethylene polymeri-
zation in the presence of Al cocatalysts (methylaluminoxane
(MAO), Et2AlCl, etc.) and the aryloxo analogues showed notable
catalytic activity and comonomer incorporation in ethylene/
norbornene copolymerization.12b,c We also reported that (1-
adamantylimido)vanadium(V) complexes containing aryloxo and
ketimide ligands, V(NAd)Cl2(X) (Ad ) 1-adamantyl), showed
moderate catalytic activities for ethylene polymerization in the
presence of MAO.13 In this paper, we wish to report that
(imido)vanadium(V) complexes containing (2-anilidomethyl)py-
ridine of the type V(NR)Cl2[2-ArNCH2(C5H4N)] (R ) Ad,
cyclohexyl (Cy), phenyl; Ar ) 2,6-Me2C6H3 (a), 2,6-iPr2C6H3
(b)) efficiently dimerize ethylene with both remarkably high
catalytic activities and high selectivity (Scheme 1).
Table 1. Selected Bond Lengths and Angles for
i
V(NAd)Cl2[2-(2,6-R2C6H3)NCH2(C5H4N)] (R ) Me (1a), Pr (1b))
and V(NCy)Cl2[2-(2,6-iPr2C6H3)NCH2(C5H4N)] (3b)a
1a
1b
3b
Bond Distances (Å)
V1-N1
V1-N2
V1-N3
V1-Cl1
V1-Cl2
2.2241(11)
1.8580(12)
1.6517(12)
2.2677(3)
2.2709(4)
2.225(2)
1.8607(18)
1.654(2)
2.2743(6)
2.2680(6)
2.221(2)
1.854(2)
1.648(2)
2.2652(12)
2.2705(7)
Results and Discussion
Bond Angles (deg)
N1-V1-N2
N1-V1-N3
N1-V1-Cl1
N1-V1-Cl2
N2-V1-N3
N2-V1-Cl1
N2-V1-Cl2
N3-V1-Cl1
N3-V1-Cl2
Cl1-V1-Cl2
78.20(4)
174.90(4)
85.49(3)
83.93(3)
97.41(5)
116.92(4)
118.18(3)
98.90(3)
96.02(4)
119.953(16)
77.96(8)
176.95(8)
83.85(5)
85.53(5)
99.05(9)
119.71(6)
114.68(6)
97.19(7)
96.36(7)
120.53(2)
170.20(16)
78.14(9)
175.63(13)
84.08(8)
1. Synthesis of (Imido)vanadium(V) Complexes Containing
(2-Anilidomethyl)pyridine Ligands. A series of (imido)vana-
dium(V) complexes containing (2-anilidomethyl)pyridine
ligands, V(NR)Cl2[2-ArNCH2(C5H4N)] (R ) Ad (1), Cy (3);
Ar ) 2,6-Me2C6H3 (a), 2,6-iPr2C6H3 (b)), V(NAd)Cl2[2-(2,6-
iPr2C6H3)NCH2-6-MeC5H3N] (2b), and V(NPh)Cl2[2-(2,6-
iPr2C6H3)NCH2(C5H4N)] (4b), were prepared by reacting the
corresponding (imido)vanadium(V) trichloride analogues
V(NR)Cl3 with Li[2-ArNCH2(C5H4N)] (prepared by treating
84.37(5)
97.53(12)
114.38(8)
118.36(9)
98.33(13)
97.35(8)
121.90(3),
165.8(4), 158.6(5)
V1-N3-C19(C25) 170.94(10)
n
2-Ar′N(H)CH2(C5H4N) with 1 equiv of BuLi in n-hexane
a Detailed structural analysis data, including CIF files for complexes
1a,b and 3b, are given in the Supporting Information.
at -30 °C)12f in Et2O or toluene (Scheme 2). These pro-
cedures are analogous to those for the synthesis of V(N-2,6-
Me2C6H3)Cl2[2-(2,6-iPr2C6H3)NCH2(C5H4N)] (5b), reported
previously.12f The resultant complexes were identified by
NMR spectra (1H, 13C, 51V) and elemental analyses, and
structures of some complexes (1a,b, 3b) were determined
by X-ray crystallography (Figure 1).14 The reaction of
1b with 2.0 equiv of LiCH2SiMe3 in toluene afforded
the dialkyl complex V(NAd)(CH2SiMe3)2[2-(2,6-iPr2C6H3)-
NCH2(C5H4N)] (6b) in moderate yield (87%). Complex 6b
was identified on the basis of NMR spectra (1H, 13C, 51V)
and elemental analysis.
with two nitrogen atoms from the pyridine and the imido ligands
at the axial positions and an equatorial plane consisting of two
chlorine atoms and the nitrogen in the anilide ligand. Selected
bond distances and angles are summarized in Table 1. The
nitrogen atom in the pyridine is located trans to the imido ligand.
The VdN(3) bond lengths in the alkylimido complexes (1a,b,
3b, 1.648-1.654 Å) are relatively shorter than that in the
arylimido complex (5b, 1.679 Å), probably due to the electronic
donation from the alkyl substituents.
2. Ethylene Dimerization Using V(NR)Cl2[2-ArNCH2(C5H4N)]-
MAO Catalysts. It should be noted that the adamantylimido
analogues (1a,b) exhibit remarkable catalytic activities for
ethylene dimerization in the presence of MAO, affording
1-butene with high selectivities (Table 2). The activity was
affected by the Al/V molar ratios, ethylene pressure without
significant changes in the selectivity of 1-butene. The activity
was also affected by the reaction temperature, and the activity
decreased at 0 °C as well as at 50 °C (runs 4, 9, and 10). Since
the selectivity of 1-butene seems to decreasing (or that of
1-hexene increases) for prolonged times as well as upon an
increase of 1-butene accumulated in the reaction mixture, it is
thus assumed that 1-hexene would be produced by the reaction
The structures of 1a,b and 3b indicate that these complexes
fold a distorted-trigonal-bipyramidal geometry around vanadium
(12) (a) Nomura, K.; Sagara, A.; Imanishi, Y. Macromolecules 2002, 35,
1583–1590. (b) Wang, W.; Nomura, K. Macromolecules 2005, 38,
5905–5913. (c) Wang, W.; Nomura, K. AdV. Synth. Catal. 2006, 348,
743–750. (d) Nomura, K.; Wang, W.; Yamada, J. Stud. Surf. Sci. Catal.
2006, 161, 123. (e) Onishi, Y.; Katao, S.; Fujiki, M.; Nomura, K.
Organometallics 2008, 27, 2590–2596. (f) Zhang, S.; Katao, S.; Sun,
W.-H.; Nomura, K. Organometallics 2009, 28, 5925–5933.
(13) Zhang, W.; Nomura, K. Inorg. Chem. 2008, 47, 6482–6492.
(14) Detailed structural analysis data, including CIF files, for complexes
1a,b and 3b are given in the Supporting Information. Carbon atoms
in the cyclohexyl group in 3b could not be defined, probably due to
their flexible structure (observed as a mixture of two structures).
9
J. AM. CHEM. SOC. VOL. 132, NO. 13, 2010 4961

A R T I C L E S
Zhang and Nomura
Figure 1. ORTEP drawings of 1a (left) and 1b (right). Thermal ellipsoids are drawn at the 50% probability level, and hydrogen atoms are omitted for
clarity. Details are given in the Supporting Information.14
Table 2. Ethylene Dimerization by V(NAd)X2[2-ArNCH2(C5H4N)]
(1a,b, 6b)- and V(NAd)Cl2[2-(2,6-iPr2C6H3)NCH2-6-MeC5H3N]
(2b)-MAO Catalyst Systems (X ) Cl and Ar ) 2,6-Me2C6H3 (1a),
2,6-iPr2C6H3 (1b); X ) CH2SiMe3 and Ar ) 2,6-iPr2C6H3 (6b))a
Table 3. Ethylene Dimerization by V(NR)Cl2[2-ArNCH2(C5H4N)]
(1a,b, 3a,b, 4b, 5b)-MAO Catalyst Systemsa
complex
(ant/µmol)
run
R
Al/Vb
activityc
TOFd
C4′/%e C6′/%e
complex
run (amt/µmol) Al/Vb
ethylene/ temp/ time/
atm
C4′/ C6′/
e e
4
1a (0.5) Ad
500 51 100 1 830 000 92.5
1 500 76 500 2 730 000 97.0
1 000 35 700 1 280 000 92.1
7.5
3.0
7.9
2.9
6.8
1.6
°C
min activityc
TOFd
%
%
13 1a (0.1) Ad
17 1b (0.5) Ad
25 3a (0.5) Cy
26 3a (0.5) Cy
27 3a (0.5) Cy
28 3a (0.5) Cy
29 3a (0.5) Cy
30 3b (0.5) Cy
31 3b (0.5) Cy
32 3b (0.5) Cy
33 3b (0.5) Cy
34 4b (0.5) Ph
35 4b (0.5) Ph
36 4b (0.5) Ph
37 4b (0.5) Ph
1
2
3
4
5
6
7
8
9
1a (0.5)
1a (0.5)
1a (0.5)
1a (0.5)
1a (0.5)
1a (0.5)
1a (0.5) 1 000
1a (0.5) 2 000
1a (0.5)
100
200
500
500
500
500
8
8
8
8
4
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
25 10 1 350
25 10 50 600 1 810 000 94.9 5.1
25 35 400 1 260 000 96.7 3.3
25 10 51 100 1 830 000 92.5 7.5
25 10 25 500 911 000 92.2 7.8
25 20 50 300 1 800 000 90.4 9.6
25 10 40 400 1 440 000 95.5 4.5
25 10 6 780 242 000 95.0 5.0
50 10 8 160 291 000 95.2 4.8
48 200 94.0 6.0
100 15 300
546 000 97.1
5
200 28 200 1 010 000 93.2
500 6 400
229 000 98.4
34 000 >99
18 000 >99
39 000 >99
471 000 97.8
1 000
2 000
940
500
50 1 100
100 13 200
2.2
4.1
3.5
500
500
500
200 32 300 1 150 000 95.9
10 1a (0.5)
11 1a (0.2)
0
10 17 500 625 000 97.9 2.1
500 13 100
200 trace
468 000 96.5
25 10 57 800 2 060 000 96.8 3.2
25 10 45 800 1 640 000 98.0 2.0
25 10 76 500 2 730 000 97.0 3.0
25 60 74 800 2 670 000 92.1 7.9
25 10 42 900 1 530 000 98.2 1.8
25 10 20 200 721 000 95.0 5.0
25 10 35 700 1 280 000 92.1 7.9
12 1a (0.1) 1 000
13 1a (0.1) 1 500
14 1a (0.1) 1 500
15 1a (0.1) 2 000
500 18 300
750 9 600
1 000 5 500
654 000 95.1
343 000 97.2
196 000 >99
1 390 PE
4.9
2.8
16 1b (0.5)
500
38 5b (2.0) 2,6-Me2C6H3
39 5b (2.0) 2,6-Me2C6H3
40 5b (2.0) 2,6-Me2C6H3 1 000
41 5b (2.0) 2,6-Me2C6H3 3 000
200
500
39
42
17 1b (0.5) 1 000
18 1b (0.5) 1 000
19 1b (0.5) 2 000
20 1b (0.5) 3 000
21 1b (0.5) 1000f
22 6b (0.5) 1 000
23 2b (2.0) 1 000
24 2b (2.0) 2 000
1 500 PE
66
189
2 350 PEf
0
10 23 800 850 000 95.2 4.8
25 10 32 300 1 150 000 92.7 7.3
25 10 15 600 557 000 94.1 5.9
25 10 17 900 639 000 94.0 6.0
25 10 16 600 593 000 95.6 4.4
25 10
25 10 2 100
6 740 PEf
a Conditions: toluene 30 mL, ethylene 8 atm, 25 °C, 10 min, d-MAO
white solid. b Al/V molar ratio. c Activity in (kg of ethylene)/((mol of
510
18 000 >99g
V) h). d TOF (turnover frequency)
) (molar amount of ethylene
75 000 95.9g 4.1
reacted)/((mol of V) h). e By GC analysis vs internal standard.
f Polyethylene (PE) was obtained12f (Mw ) 2.55 × 106, Mw/Mn ) 1.9
(run 40); Mw ) 2.93 × 106, Mw/Mn ) 2.6 (run 41)).
a Conditions: toluene 30 mL, ethylene 4 or 8 atm, d-MAO white
solid (prepared by removing AlMe3, toluene from PMAO-S (commercial
sample)). b Al/V molar ratio. c Activity in (kg of ethylene)/((mol of V)
h). d TOF (turnover frequency) ) (molar amount of ethylene reacted)/
((mol of V) h). e By GC analysis vs internal standard. f d-MMAO
(prepared by removing AlMe3, AliBu3, and hexane from a modified
methylaluminoxane (methylisobutylaluminoxane), MMAO-BH (molar
ratio Me/iBu 3.54)) was used in place of d-MAO. g A small amount of
polyethylene (PE) was also collected (ca. 70 mg).
of preformed 1-butene12a with ethylene (runs 3, 4, 6, and run 4
vs runs 12-15). A TOF of 2 730 000 h-1 (758 s-1) with 97.0%
selectivity of 1-butene has been achieved under the optimized
conditions (run 13), and the activity (TOF values) did not change
even after 60 min (run 14).
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4962 J. AM. CHEM. SOC. VOL. 132, NO. 13, 2010

Ethylene Dimerization by (Imido)vanadium Catalysts
A R T I C L E S
tion detector. Elemental analyses were performed by using a
The activities by 1a (Ar ) 2,6-Me2C6H3) were slightly higher
than those by 1b (Ar ) 2,6-iPr2C6H3), but no apparent
differences were seen under the same conditions. Use of MMAO
(modified methylaluminoxane (methylisobutylaluminoxane)) in
place of MAO was also effective in enhancing both the high
catalytic activity and the selectivity (run 21, by 1b). The dialkyl
complex 6b also showed catalytic activity, affording 1-butene
with high selectivity (run 22). The fact thus suggests that similar
catalytically active species are present in this catalysis.
In contrast, attempts using the complex containing a methyl
group in the ortho position (2b) in place of 1b afforded products
in trace amounts under the same conditions (2b, 0.5 µmol of
ethylene, 8 atm, 10 min at 25 °C, Al/V ) 100, 500, 1000, 2000,
same conditions as in runs 1, 3, 5, 7, and 8). A mixture of
1-butene and polyethylene was collected when the reactions by
2b were conducted at higher catalyst concentration (runs 23
and 24). This fact thus clearly suggests that the steric bulk of
the (2-anilidomethyl)pyridine ligand plays a key role in the
selectivity as well as the activity.
Reactions with ethylene using the other (imido)vanadium(V)
complexes V(NR)Cl2[2-ArNCH2(C5H4N)] (R ) Cy (3a,b), Ph
(4b)) were conducted under the same conditions (Table 3). Note
that the cyclohexylimido analogues (3a,b) also showed high
activities, affording 1-butene with high selectivities under the
optimized conditions (Al/V molar ratios, e.g. runs 26 and 32).
Moreover, the phenylimido analogue 4b also afforded 1-butene
as the major product under the same conditions (runs 35-37),
whereas, as reported recently,12f the 2,6-dimethylphenylimido
analogue 5b afforded polyethylene (runs 38-41). These results
thus clearly indicate that the steric bulk of the imido substituent
plays an essential role in the selectivity in this catalysis. These
results also indicate that the activity was affected by the
electronic nature of the imido ligand, because the activities
(under the optimized Al/V molar ratios) by 1b and 3b were
higher than those by 4b.
1
PE2400II Series instrument (Perkin-Elmer Co.). All H, 13C, and
51V NMR spectra were recorded on a JEOL JNM-LA400 spec-
1
trometer (399.65 MHz for H, 100.40 MHz for 13C, and 105.31
MHz for 51V). All spectra were obtained in the solvent indicated
at 25 °C unless otherwise noted. Chemical shifts are given in ppm
and are referenced to SiMe4 (δ 0.00; 1H, 13C), and VOCl3 (δ 0.00;
51V). Coupling constants and half-width values, ∆ν1/2, are given in
Hz.
Synthesis of V(NAd)Cl2[2-(2,6-Me2C6H3)NCH2(C5H4N)] (1a). To
a Et2O solution containing V(NAd)Cl3 (1160 mg, 3.78 mmol) was
added Li[2-(2,6-Me2C6H3)NCH2(C5H4N)] (824 mg, 3.78 mmol) at
-30 °C. The reaction mixture was warmed slowly to room
temperature, and the mixture was then stirred overnight. The solvent
was then removed in vacuo, and the resultant deep red-tan residue
was extracted with toluene. The solvent was removed in vacuo,
and the solid was dissolved in a minimum amount of CH2Cl2. The
CH2Cl2 solution was layered with n-hexane, and orange crystals
were obtained at room temperature upon standing. Yield: 536 mg
(29%). 1H NMR (CDCl3): δ 8.87 (d, 1H, J ) 4.40, Py-H), 7.89 (t,
1H, J ) 7.60, Py-H), 7.51 (t, 1H, J ) 6.40, Py-H), 7.40 (d, 1H, J
) 7.60, Py-H), 7.14-7.07 (m, 3H, Ar-H), 5.12 (s, 2H, NCH2), 2.21
(s, 6H, ArCH3), 1.87 (s, 3H, Ad-H), 1.72 (s, 6H, Ad-H), 1.43 (s,
6H, Ad-H). 13C NMR (CDCl3): δ 163.6, 157.0, 148.9, 137.8, 128.7,
128.2, 127.7, 127.5, 127.3, 126.6, 123.4, 119.8, 81.7, 70.4, 40.8,
35.3, 28.5, 18.1. 51V NMR (CDCl3): δ -97.3 (∆ν1/2 ) 2498 Hz).
Anal. Calcd for C24H30Cl2N3V·0.1(toluene): C, 60.35; H, 6.32; N,
8.55. Found: C, 60.67; H, 6.31; N, 8.50.
Synthesis of V(NAd)Cl2[2-(2,6-iPr2C6H3)NCH2(C5H4N)] (1b).
The synthetic procedure of 1b was similar to that for 1a, except
that Li[2-(2,6-iPr2C6H3)NCH2(C5H4N)] (738 mg, 2.69 mmol) was
used in place of Li[2-(2,6-Me2C6H3)NCH2(C5H4N)]. Orange mi-
1
crocrystals of 1b were collected in a yield of 50% (824 mg). H
NMR (CDCl3): δ 8.94 (d, 1H, J ) 4.40, Py-H), 7.96 (t, 1H, J )
7.40, Py-H), 7.58 (t, 1H, J ) 7.20, Py-H), 7.48 (d, 1H, J ) 7.20,
Py-H), 7.32-7.22 (m, 4H, Ar-H), 5.23 (s, 2H, NCH2), 2.95 (m,
2H, CH(CH3)2), 2.41 (s, toluene), 1.97 (s, 3H, Ad-H), 1.90 (s, 6H,
Ad-H), 1.51 (s, 6H, Ad-H), 1.46 (d, 6H, J ) 6.80, CH(CH3)2),
1.20 (d, 6H, J ) 6.80, CH(CH3)2). 13C NMR (CDCl3): δ 159.6,
156.9, 149.3, 139.2, 137.9, 137.8, 128.9, 128.1, 127.7, 125.2, 124.0,
123.6, 119.8, 82.4, 72.9, 40.9, 35.6, 28.8, 27.6, 25.7, 24.2, 21.4.
51V NMR (CDCl3): δ -78.4 (∆ν1/2 ) 3044 Hz). Anal. Calcd for
C28H38Cl2N3V·0.8(toluene): C, 65.92; H, 7.31; N, 6.86. Found: C,
65.87; H, 7.52; N, 6.91.
Synthesis of V(NAd)Cl2[2-(2,6-iPr2C6H3)NCH2-6-MeC5H3N)]
(2b). To a toluene solution containing V(NAd)Cl3 (404 mg, 1.32
mmol) was added Li[2-(2,6-iPr2C6H3)NCH2(2-MeC5H3N)] (380 mg,
1.32 mmol) at -30 °C. The reaction mixture was warmed slowly
to room temperature, and the mixture was then stirred overnight.
The resultant solution was filtrated through a Celite pad, and the
filter cake was washed with toluene. The solvent was then removed
in vacuo, and the solid was dissolved in a minimum amount of
CH2Cl2 (ca. 1 mL). The CH2Cl2 solution was then layered with
n-hexane, and orange crystals were obtained at room temperature
upon standing. Yield: 310 mg (41%). 1H NMR (CDCl3): δ 7.76 (t,
1H, J ) 7.80, Py-H), 7.30-7.16 (m, 5H, Py-H and Ar-H), 5.16 (s,
2H, NCH2), 2.97 (m, 2H, CH(CH3)2), 2.88 (s, 3H, PyCH3), 1.89
(s, 3H, Ad-H), 1.83 (s, 6H, Ad-H), 1.44 (s, 6H, Ad-H), 1.41 (d,
6H, J ) 6.40, CH(CH3)2), 1.12 (d, 6H, J ) 7.20, CH(CH3)2). 13C
NMR (CDCl3): δ158.4, 158.0, 155.3, 138.2, 137.1, 136.8, 127.9,
127.1, 126.8, 124.2, 124.0, 123.0, 122.4, 116.9, 81.7, 71.4, 39.9,
34.6, 27.8, 26.5, 24.8, 23.1, 22.8. 51V NMR (CDCl3): δ -97.9 (∆ν1/2
) 2423 Hz). Anal. Calcd for C29H40Cl2N3V·0.2(toluene): C, 63.95;
H, 7.34; N, 7.36. Found: C, 63.94; H, 7.62; N, 7.17.
In summary, we have shown that the (imido)vanadium(V)
complexes showed significant catalytic activities (TOF of
2 730 000 h-1 (758 s-1), run 13) for ethylene dimerization,
affording 1-butene exclusively (selectivity >97.0-98.2%, runs
11-13 and 15). We are now exploring more details, including
the effects of ligand substituents, cocatalysts, or codimerization
with R-olefins in this catalysis.
Experimental Section
General Procedure. All experiments were carried out under a
nitrogen atmosphere in a Vacuum Atmospheres drybox. Anhydrous
grade toluene, n-hexane, diethyl ether, and dichloromethane (Kanto
Kagaku Co., Ltd.) were transferred into a bottle containing
molecular sieves (a mixture of 3A 1/16, 4A 1/8, and 13X 1/16) in
the drybox under N2, and were passed through an alumina short
column under an N2 stream prior to use. Polymerization grade
ethylene (purity >99.9%, Sumitomo Seika Co. Ltd.) was used as
received. Toluene and AlMe3 in the commercially available
methylaluminoxane (PMAO-S, 9.5 wt % (Al) toluene solution,
Tosoh Finechem Co.) were removed under reduced pressure (at
ca. 50 °C for removing toluene and AlMe3 and then heated to >100
°C for 1 h for completion) in the drybox to give a white solid.
MMAO-3BH (Me/iBu ) 3.54) was also supplied from Tosoh
Finechem Co. and was used as a white solid after removing
n-hexane, AlMe3, and AliBu3 in vacuo according to a procedure
analogous to that for PMAO-S, except that the resultant solid was
redissolved in n-hexane and the solvent then removed in vacuo to
remove AliBu3 completely.
Synthesis of V(NCy)Cl3. The synthetic procedure for V(NCy)Cl3
is analoguos to that for V(2,6-Me2C6H3)Cl3 reported previously.15
GC analysis was performed with a Shimadzu GC-17A gas
chromatograph (Shimadzu Co. Ltd.) equipped with a flame ioniza-
(15) Buijink, J.-K. F.; Teubin, J. H.; Kooijman, H.; Spek, A. L. Organo-
metallics 1994, 13, 2922.
9
J. AM. CHEM. SOC. VOL. 132, NO. 13, 2010 4963

A R T I C L E S
Zhang and Nomura
Into a sealed glass Schlenk tube, n-octane (20 mL) and VOCl3 (6.92
g, 40 mmol) were added sequentially in the drybox, and cyclo-
hexanyl isocyanate (3.75 g, 30 mmol) was then added to the
mixture. The wall of the reactor was washed with n-octane (30
mL), and the mixture was placed into an oil bath that had been
preheated at 125 °C. The solution was then stirred overnight (>12
h). The tube was connected to a nitrogen line, and the CO2 that
evolved was released, removed from the mixture by opening the
cap during the reaction. After the reaction, the cooled mixture was
filtered through a Celite pad, and the filter cake was washed with
hexane several times to extract V(NCy)Cl3. The combined wash
and the filtrate were brought to dryness under reduced pressure to
remove solvent (octane and hexane). The resultant tan residue was
dissolved in a minimum amount of hot n-hexane. Green micro-
crystals were collected from the chilled solution (placed in the
freezer at -30 °C). Yield: 37% (2.81 g). 1H NMR (CDCl3): δ 4.74
(br, 1H, Cy-H), 2.15-2.10 (m, 4H, Cy-H), 1.98-1.92 (m, 2H, Cy-
H), 1.50-1.38 (m, 4H, Cy-H). 13C NMR (CDCl3): δ 97.2 (br), 33.2,
25.1, 23.8. 51V NMR (CDCl3): δ 37.1 (∆ν1/2 ) 373 Hz). Anal.
Calcd for C6H11Cl3V: C, 28.32; H, 4.36; N, 5.50. Found: C, 28.50;
H, 4.46; N, 5.46.
Synthesis of V(N-Ph)Cl2[2-(2,6-iPr2C6H3)NCH2(C5H4N)]
(4b). To a Et2O solution containing V(NPh)Cl3 (82 mg, 0.3 mmol)
was added Li[2-(2,6-iPr2C6H3)NCH2(C5H4N)] (75 mg, 0.3 mmol)
at -30 °C. The reaction mixture was warmed slowly to room
temperature, and the mixture was then stirred overnight. The solvent
was removed in vacuo, and the resultant deep red-tan residue was
extracted with toluene. The solution was concentrated and layered
with n-hexane. Red microcrystals were obtained from the chilled
solution (-30 °C). Yield: 27 mg (19%). 1H NMR (CDCl3): δ 8.99
(d, 1H, J ) 5.60, Py-H), 7.98 (t, 1H, J ) 7.20, Py-H), 7.60 (t, 1H,
J ) 6.20, Py-H), 7.53 (d, 1H, J ) 8.00, Py-H), 7.32 (t, 1H, J )
7.90, Ar-H), 7.22-7.15 (m, 2H, Ar-H), 7.06-6.98 (m, 3H, Ar-H),
6.70 (d, 2H, J ) 7.80, Ar-H), 5.30 (s, 2H, NCH2), 2.85 (m, 2H,
CH(CH3)2), 1.14 (d, 6H, J ) 6.80, CH(CH3)2), 1.09 (d, 6H, J )
6.40, CH(CH3)2). 13C NMR (CDCl3): δ 160.3, 157.4, 149.6, 139.0,
138.5, 128.2, 128.1, 128.0, 127.9, 124.4, 123.9, 120.3, 74.0, 28.0,
25.3, 24.2. 51V NMR (CDCl3): δ 40.2 (∆ν1/2 ) 2087 Hz). Anal.
Calcd for C24H28Cl2N3V: C, 60.01; H, 5.88; N, 8.75. Found: C,
60.18; H, 5.97; N, 8.63.
Synthesis of V(NAd)(CH2SiMe3)2[2-(2,6-iPr2C6H3)NCH2-
(C5H4N)] (6b). (i) Method 1 from V(NAd)Cl3. To a Et2O solution
(20 mL) containing the lithium salt Li[2-(2,6-iPr2C6H3)-
NCH2(C5H4N)] (445 mg, 1.62 mmol) was added VCl3(NAd) (495
mg, 1.62 mmol) at -30 °C. The reaction mixture was warmed
slowly to room temperature and was stirred for 12 h. The solvent
was then removed in vacuo, and the orange solid was washed with
hexane. Toluene was added to the mixture, and the solution was
cooled to -30 °C. LiCH2SiMe3 (304 mg, 3.24 mmol) was then
added to the solution, and the reaction mixture was warmed slowly
to room temperature and was stirred overnight. The solvent was
then removed in vacuo, and the resulting residue was extracted with
n-hexane. The n-hexane solution was then concentrated in vacuo,
and the concentrated n-hexane solution was placed in the freezer
(-30 °C). Brown microcrystals (310 g, 0.48 mmol) were obtained
from the chilled solution in the yield of 30%.
Synthesis of V(NCy)Cl2[2-(2,6-Me2C6H3)NCH2(C5H4N)]
(3a). To a toluene solution containing V(NCy)Cl3 (255 mg, 1.0
mmol) was added Li[2-(2,6-Me2C6H3)NCH2(C5H4N)] (218 mg, 1.0
mmol) at -30 °C. The reaction mixture was warmed slowly to
room temperature, and the mixture was then stirred for 6 h. Then
the solution was filtered through a Celite pad and the filter cake
was washed with toluene. The combined filtrate and wash was
concentrated in vacuo and layered with n-hexane. Yellow micro-
crystals were obtained from the chilled solution (-30 °C). Yield:
151 mg (35%). 1H NMR (CDCl3): δ 8.91 (d, 1H, J ) 5.20, Py-H),
7.91 (t, 1H, J ) 7.40, Py-H), 7.53 (t, 1H, J ) 6.40, Py-H), 7.44 (d,
1H, J ) 8.40, Py-H), 7.16-7.07 (m, 3H, Ar-H), 5.18 (s, 2H, NCH2),
3.41 (m, Cy-H), 2.19 (s, 6H, Ar-CH3), 1.80-1.76 (m, 2H, Cy-H),
1.51-1.48 (m, 4H, Cy-H), 1.30-1.25 (m, 2H, Cy-H), 1.11-1.05
(m, 2H, Cy-H). 13C NMR (CDCl3): δ 162.0, 157.4, 149.5, 138.3,
128.7, 128.3, 127.0, 123.8, 120.2, 80.9, 71.2, 32.0, 25.5, 23.9, 18.3.
51V NMR (CDCl3): δ -115 (∆ν1/2 ) 1876 Hz). Anal. Calcd for
C20H26Cl2N3V·0.3(toluene): C, 57.96; H, 6.25; N, 9.18. Found: C,
57.99; H, 6.06; N, 9.34.
(ii) Method 2 from 1b. To a toluene solution (20 mL) containing
1b (291 mg, 0.54 mmol) was added LiCH2SiMe3 (102 mg, 1.08
mmol) at -30 °C. The reaction mixture was warmed slowly to
room temperature and was stirred for 6 h. The solvent was then
removed in vacuo, and the resultant residue was extracted with
n-hexane. After the n-hexane was removed, the resultant tan residue
was dissolved in a minimum amount of n-hexane. The chilled
solution (-30 °C) gave brown microcrystals (303 mg, 0.47 mmol)
Synthesis of V(NCy)Cl2[2-(2,6-iPr2C6H3)NCH2(C5H4N)]
(3b). To a Et2O solution containing V(NCy)Cl3 (255 mg, 1.0 mmol)
was added Li[2-(2,6-iPr2C6H3)NCH2(C5H4N)] (274 mg, 1.0 mmol)
at -30 °C. The reaction mixture was warmed slowly to room
temperature, and the mixture was then stirred overnight. The solvent
was removed in vacuo, and the resultant deep red tan residue was
extracted with toluene. This solvent was then removed in vacuo,
and the resultant solid was dissolved in a minimum amount of
CH2Cl2 (ca. 1 mL). The CH2Cl2 solution was layered with n-hexane,
and orange microcrystals were obtained from the chilled solution
1
in a yield of 87%. H NMR (C6D6): δ 9.05 (d, 1H, J ) 5.60, Py-
H), 7.19-7.14 (m, 3H, Ar-H), 6.98 (t, 1H, J ) 8.00, Py-H), 6.80
(t, 1H, J ) 6.80, Py-H), 6.61 (d, 1H, J ) 8.00, Py-H), 4.85 (s, 2H,
NCH2), 3.19 (m, 2H, CH(CH3)2), 2.65 (d, 2H, J ) 8.00, CH2SiMe3),
2.16 (s, 6H, Ad-H), 1.96 (s, 3H, Ad-H), 1.59-1.45 (m, 12H,
CH(CH3)2 and Ad-H), 1.16 (d, 6H, J ) 6.80, CH(CH3)2), 0.71 (d,
2H, J ) 9.60, CH2SiMe3), 0.12 (s, 18H, CH2SiMe3). 13C NMR
(C6D6): δ 162.1, 158.7, 148.1, 142.3, 136.8, 125.6, 124.0, 122.5,
120.6, 69.4, 43.6, 36.6, 31.9, 29.8, 27.1, 27.0, 24.6, 23.0, 14.3, 3.4.
51V NMR (C6D6): δ 237.9 (∆ν1/2 ) 1303 Hz). Anal. Calcd for
C36H60N3Si2V: C, 67.35; H, 9.42; N, 6.55. Found: C, 67.38; H,
9.63; N,6.62.
1
(-30 °C). Yield: 98 mg (20%). H NMR (CDCl3): δ 8.91 (d, 1H,
J ) 5.20, Py-H), 7.91 (t, 1H, J ) 6.80, Py-H), 7.53 (t, 1H, J )
6.20, Py-H), 7.44 (d, 1H, J ) 7.60, Py-H), 7.28-7.18 (m, 3H, Ar-
H), 5.20 (s, 2H, NCH2), 3.59 (m, Cy-H), 2.83 (m, 2H, CH(CH3)2),
1.80-1.75 (m, 2H, Cy-H), 1.59-1.55 (m, 4H, Cy-H), 1.31 (d, 6H,
J ) 6.80, CH(CH3)2), 1.21-1.17 (m, 2H, Cy-H), 1.12 (d, 6H, J )
6.80, CH(CH3)2), 1.06-1.03 (m, 2H, Cy-H). 13C NMR (CDCl3): δ
158.8, 157.0, 149.4, 138.9, 138.1, 127.6, 123.7, 123.6, 120.0, 81.4,
73.2, 31.9, 27.6, 25.8, 25.3, 24.0, 23.9. 51V NMR (CDCl3): δ -97.8
(∆ν1/2 ) 2427 Hz). Anal. Calcd for C24H34Cl2N3V: C, 59.26; H,
7.05; N, 8.64. Found: C, 59.16; H, 7.32; N, 8.53.
Oligomerization of Ethylene. Ethylene oligomerizations were
conducted in a 100 mL stainless steel autoclave. The typical reaction
procedure is as follows. Toluene (29 mL) and a prescribed amount
of MAO solid (prepared from ordinary MAO by removing toluene
and AlMe3) were added into the autoclave in the drybox. The
reaction apparatus was then filled with ethylene (1 atm), and the
complex (0.5 µmol) in toluene (1.0 mL) was then added into the
autoclave; the reaction apparatus was then immediately pressurized
to 7 atm (total 8 atm), and the mixture was magnetically stirred
for 10 min. After the above procedure, the remaining ethylene was
purged at -78 °C, and 0.5 g of heptane was added as an internal
standard. The solution was then analyzed by GC to determinate
the activity and the product distribution.
Synthesis of V(NPh)Cl3. The synthesis of V(NPh)Cl3 was similar
to that for V(NCy)Cl3, except that phenyl isocyanate (3.57 g, 30
mmol) was used in place of cyclohexanyl isocyanate. Red micro-
crystals of V(NPh)Cl3 were collected. Yield: 6.28 g (84%). 1H NMR
(CDCl3): δ 7.56 (d, 2H, J ) 8.0, Ph-H), 7.42 (t, 2H, J ) 7.8, Ph-
H), 7.33 (d, 1H, J ) 7.4, Ph-H). 13C NMR (CDCl3): δ 132.7, 129.0,
126.8. 51V NMR (CDCl3): δ 251.7 (∆ν1/2 ) 322 Hz).
9
4964 J. AM. CHEM. SOC. VOL. 132, NO. 13, 2010

Ethylene Dimerization by (Imido)vanadium Catalysts
A R T I C L E S
Table 4. Crystal Data and Collection Parameters of
refined anisotropically. Hydrogen atoms were included but not
refined. All calculations for complexes 1a,b were performed using
the Crystal Structure17,18 crystallographic software package. All
calculations for analysis of 3b were performed using the Crystal
Structure17,18 crystallographic software package except for refine-
ment, which was performed using SHELXL-97.19
i
V(NAd)Cl2[2-(2,6-R2C6H3)NCH2(C5H4N)] (R ) Me (1a), Pr (1b))
and V(NCy)Cl2[2-(2,6-iPr2C6H3)NCH2(C5H4N)] (3b)
1a
1b
3b
formula
formula wt
C24H30Cl2N3V
482.37
C28H38N3Cl2V C24H23N3Cl2V
538.48 475.31
cryst color, habit
cryst size (mm)
orange, block
0.40 × 0.24 ×
0.20
orthorhombic
P212121 (No. 19) P1 (No. 2)
12.0709(4)
12.5390(5)
15.2621(5)
orange, block red, block
Acknowledgment. This project was partly supported by a Grant-
in-Aid for Scientific Research on Priority Areas (No. 19028047,
“Chemistry of Concerto Catalysis”) from the Ministry of Education,
Culture, Sports, Science and Technology of Japan. S.Z. expresses
his sincere thanks to the JSPS for a postdoctoral fellowship (No.
P08361). We are grateful to Prof. Wen-Hua Sun (Institute of
Chemistry, Chinese Academy of Sciences) for discussions, to Mr.
Shohei Katao (NAIST) for his assistance in the crystallographic
analysis, and thank to Tosoh Finechem Co. for donating MAO and
MMAO.
0.60 × 0.56× 0.26 × 0.20×
0.30
0.15
cryst syst
space group
a (Å)
b (Å)
c (Å)
triclinic
monoclinic
P21/c (No. 14)
17.3177(5)
19.4500(5)
20.2626(6)
j
10.0663(18)
10.537(2)
13.172(3)
96.887(5)
96.829(6)
91.582(5)
1375.9(5)
2
R (deg)
ꢀ (deg)
132.7575(8)
γ (deg)
V (Å3)
2310.02(14)
4
1.387
1008.00
113
5011.2(2)
8
Z
Dcalcd (g/cm3)
F(000)
1.300
568.00
103
1.260
1960.00
153
Supporting Information Available: Text, tables, figures, and
CIF files giving structural analysis data for complexes 1a,b and
3b. This material is available free of charge via the Internet at
http://pubs.acs.org.
temp (K)
µ(Mo KR) (cm-1
)
6.764
5.752
6.229
no. of rflns measd
total
18 728
4198
4080
11 086
4996
40 757
9113
unique
JA100573D
no. of observns
(I > 2.00σ(I))
no. of variables
R1 (I > 2.00σ(I))
wR2 (I > 2.00σ(I)) 0.0519
goodness of fit 1.032
4577
9113
302
0.0169
345
600
(16) DIRDIF94: Beurskens, P. T.; Admiraal, G.; Beurskens, G.; Bosman,
W. P.; de Delder, R.; Israel, R.; Smits, J. M. M. The DIRDIF94
Program System, Technical Report of the Crystallography Laboratory;
University of Nijmegen, Nijmegen, The Netherlands, 1994.
(17) Crystal Structure 3.6.0: Crystal Structure Analysis Package; Rigaku
and Rigaku/MSC, 9009 New Trails Dr., The Woodlands, TX 77381,
2000-2004.
(18) Watkin, D. J., Prout, C. K. Carruthers, J. R.; Betteridge, P. W.
CRYSTALS Issue 10; Chemical Crystallography Laboratory, Oxford,
U.K., 1996.
0.0343
0.0925
1.000
0.0448
0.1323
1.072
Crystallographic Analysis. All measurements were made on a
Rigaku RAXIS-RAPID imaging plate diffractometer with graphite-
monochromated Mo KR radiation. The selected bond lengths and
bond angles as well as crystal collection parameters are given in
Table 4. All structures were solved by direct methods and expanded
using Fourier techniques,16 and the non-hydrogen atoms were
(19) Sheldrick, G. M. SHELX97; University of Go¨ttingen, Go¨ttingen,
Germany, 1997.
9
J. AM. CHEM. SOC. VOL. 132, NO. 13, 2010 4965