C-H and Si-H activation on palladium(II) and platinum(II) complexes with a new methoxyalkyl-substituted diimine ligand

Article abstract of DOI:10.1021/om000720e

The first cationic Pd(II) and Pt(II) complexes of a new methoxyalkyl substituted diimine ligand were prepared. The Pt(II) complex activated an alkyl C-H bond of the methylene proximal to the coordinated OMe group. Pt(II) complex also activated a Si-H bond to form a diimine Pt(IV) silyl complex.

Full text of DOI:10.1021/om000720e

Organometallics 2000, 19, 4193-4195
4193
C-H a n d Si-H Activa tion on P a lla d iu m (II) a n d
P la tin u m (II) Com p lexes w ith a New
Meth oxya lk yl-Su bstitu ted Diim in e Liga n d
Xinggao Fang, Brian L. Scott, J ohn G. Watkin, and Gregory J . Kubas*
Chemistry Division, Los Alamos National Laboratory, MS J 514,
Los Alamos, New Mexico 87545
Received August 17, 2000
Summary: Electrophilic 1,4-bis(methoxypropyl)-2,3-di-
methyl-1,4-diazabutadiene Pd(II) and Pt(II) cations 2a ,b
with a BArF counteranion have been prepared. Complex
2a undergoes intramolecular C-H activation to form
tricyclic 5, and complex 2b adds to Et3SiH to form an
octahedral diimine Pt(IV) silyl hydride complex, 7.
Since the first report from Brookhart’s group on aryl
diimine cationic Pd(II) complexes with bulky substitu-
ents, 1, as highly active catalysts for copolymerization
of R-olefins with functional vinyl monomers, this area
has attracted great attention.1 A significant amount of
research has aimed at tuning the electronic and steric
environments by changing aryl substituents or replacing
one aryl with an alkyl group, which leads to complexes
of diverse olefin polymerization properties.2 Bulky aryls
are used to disfavor associative displacement and chain
transfer to afford high-molecular-weight polymers. An
alkyl-substituted diimine ligand with side arms con-
taining labile functional groups might hinder the â-
elimination process by temporarily occupying a vacant
site and serve the same purpose. We are not aware of
any report on such alkyl-substituted diimine Pd(II)
complexes and report here the first examples of such
Pd(II) and Pt(II) complexes (2a ,b) with a new methoxy-
alkyl-substituted diimine ligand. 2a activates the C-H
bond of the CH2 unit proximal to a pendant methoxy,
and we also find that 2b oxidatively adds HSiEt3 to form
an octahedral Pt(IV) silyl hydride complex.
denoted as NN) with (COD)PdCl(Me)4 affords the neu-
tral complex (NN)PdCl(Me) (4) as yellow crystals, for
which a single-crystal X-ray structure has been ob-
tained.5 The side arms orient away from Pd as shown
6
in eq 1. Metathesis of 4 with NaBArF leads to the
desired complex [(NN)Pd(Me)][BArF] (2a ) as an orange
solid. The 1H and 13C NMR spectra of 2a show two
distinguishable 3-methoxypropyl groups, where pre-
sumably one methoxy coordinates to the vacant site to
form a six-membered ring and the other is unbound.
The bound OMe displays NMR signals at δ 3.57 (OCH3)
and 77.8 (OCH3), both shifted downfield from the δ 3.30
(OCH3) and 65.4 (OCH3) of the free OMe. Solutions of
2a are stable to air/moisture and are more thermally
stable than the aryl diimine complexes 1 (L ) diethyl
1
ether).1a The H NMR spectrum of 2a in CDCl3 exhibits
no noticeable change at room temperature over 30 min
or 1 week if stored at -30 °C. This increased stability
is likely due to the intramolecular coordination of OMe.
However, prolonged standing of a solution of 2a at room
temperature leads to the formation of the tricyclic
complex 5, which is possibly formed by addition of a
C-H of the CH2 group proximal to the coordinated OMe,
followed by CH4 elimination. While Pd insertion into
vinyl or aryl C-H is not unusual,7 this is the first
example of alkyl C-H activation with a cationic diimine
Pd(II) complex under such mild conditions. One other
closely related C-H activation occurs for cationic
Cationic 2a with the weakly coordinating BArF anion
(B[C6H3(3,5-CF3)2]4) is readily synthesized (eq 1).3 Reac-
tion of 3-(methoxypropyl)-substituted diazabutadiene (3,
(1) J ohnson, L. K.; Killian, C. M.; Brookhart, M. J . Am. Chem. Soc.
1995, 117, 6414. (b) J ohnson, L. K.; Mecking, S.; Brookhart, M. J . Am.
Chem. Soc. 1996, 118, 267. (c) Ittel, S. D.; J ohnson, L. K.; Brookhart,
M. Chem. Rev. 2000, 100, 1169.
(2) Selected recent examples: (a) Schleis, T.; Heinemann, J .; Spaniol,
T. P.; Mulhaupt, R.; Okuda, J . Inorg. Chem. Comm. 1998, 1, 431. (b)
Meneghetti, S. P.; Lutz, P. J .; Kress, J . Organometallics 1999, 18, 2734.
(c) Lim, N. K.; Yaccato, K. J .; Dghaym, R. D.; Arndtsen, B. A.
Organometallics 1999, 18, 3953. (d) Albirtz, P. J .; Yang, K.-Y.;
Eisenberg, R. Organometallics 1999, 18, 2747. (e) Mecking, S.; J ohnson,
L. K.; Wang, L.; Brookhart, M. J . Am. Chem. Soc. 1998, 120, 888.
10.1021/om000720e CCC: $19.00 © 2000 American Chemical Society
Publication on Web 09/22/2000

4194 Organometallics, Vol. 19, No. 21, 2000
Communications
[(TMEDA)PtII(Me)(OEt2)]+, reported recently by Ber-
(3) Experimental procedures and characterization of new compounds
are as follows. (a) 3 (NN): NH2(CH2)3OMe (2.25 mL, 22.0 mmol) was
added to a suspension of 4 Å molecular sieves (9.60 g) in toluene (35
mL) at room temperature, followed by 2,3-butanedione (0.88 mL, 10.0
mmol). The resulting mixture was stirred at room temperature for 2
days. The mixture was then filtered through Celite and washed with
CH2Cl2. Volatiles were removed to give
a red oil. The oil was
crystallized from hexane at -78 °C to give the product (1.80 g, 79%)
as a red oil. 1H NMR (CDCl3): δ 1.93 (quintet, 4H, J ) 7.6 Hz), 2.02
(s, 6H), 3.31 (s, 6H), 3.45 (m, 8H). 13C NMR (CDCl3): δ 12.7, 30.9,
49.1, 58.7, 70.9, 168.4. MS (EI): 228 (M+), 197, 169, 114, 73. (b) 4:
(COD)PdCl(Me) (0.226 g, 0.853 mmol) was added to a solution of 3
(0.214 g, 0.938 mmol) in Et2O (10 mL) at -30 °C. The mixture was
stirred at room temperature for 30 min. Volatiles were removed and
the residue was washed with hexane (3×) to give yellow product (0.315
g, 96%). 1H NMR (CDCl3): δ 0.71 (s, 3H), 1.83 (quintet, 2H, J ) 6.4
Hz), 1.94 (quintet, 2H, J ) 6.5 Hz), 2.15 (s, 3H), 2.20 (s, 3H), 3.26 (s,
3H), 3.27 (s, 3H), 3.34 (t, 4H, J ) 5.8 Hz), 3.73 (t, 2H, J ) 7.6 Hz),
3.88 (t, 2H, J ) 7.2 Hz). 13C NMR (CDCl3): δ -1.2, 17.0, 18.1, 29.6,
49.4, 50.8, 58.6, 58.8, 69.5, 70.0, 169.0, 174.6. Anal. Calcd for
C13H27N2O2Pd: C, 40.58; H, 7.02; N, 7.28. Found: C, 40.30; H, 7.32;
N, 7.09. (c) 2a : CH2Cl2 (4 mL) was added to a mixture of 4 (0.096 g,
0.25 mmol) and NaBArF (0.222 g, 0.25 mmol) at room temperature.
The resulting mixture was stirred for 20 min at room temperature
and then filtered through Celite. Volatiles were removed to give a red
syrup, which was washed with hexane (3×) to give a yellow solid (0.260
g, 86%). 1H NMR (CDCl3): δ 0.79 (s, 3H), 1.81 (quintet, 2H, J ) 6.1
Hz), 1.98 (s, br, 5H), 2.21 (s, 3H), 3.30 (s, 3H), 3.35 (t, 2H, J ) 5.4 Hz),
3.43 (br, 2H), 3.57 (s, 3H), 3.69 (t, 2H, J ) 7.4 Hz), 3.76 (t, 2H, J ) 4.5
Hz), 7.55 (s, 4H), 7.70 (s, 8H). 13C NMR (CDCl3): δ 6.4, 17.6, 18.2,
27.8, 29.4, 51.7, 52.7, 58.7, 65.4, 68.8, 77.8, 177.8, 181.7. Anal. Calcd
for C45H39BF24N2O2Pd: C, 44.55; H, 3.22; N, 2.31. Found: C, 44.44;
H, 3.60; N, 2.23. (d) 5: A solution of 2a (0.140 g, 0.116 mmol) in CH2Cl2
(3 mL) was allowed to stand at room temperature in the glovebox for
2 days. A sample was removed, and a 1H NMR spectrum was recorded
after solvent removal. The data showed the coexistence of 2a and 5 in
a ratio of ca. 1/2. Volatiles were removed. Residue was crystallized from
Et2O/hexane at -30 °C to give product (0.015 g, 11%) as red crystals.
1H NMR (CD2Cl2): δ 1.80 (m, 1H), 2.09 (m, 3H), 2.12 (s, 3H), 2.16 (s,
3H), 3.44 (s, 3H), 3.45 (m, 2H), 3.71 (m, 2H), 3.72 (s, 3H), 3.89 (m,
2H), 4.78 (dd, 1H, J ) 7.7, 3.8 Hz). 13C NMR (CD2Cl2): δ 17.0, 18.5,
F igu r e 1. Drawing of 5 showing thermal ellipsoids at the
50% level. Selected bond distances (Å) and angles (deg):
Pd(1)-N(1), 2.02(1); Pd(1)-N(2), 1.960(8); Pd(1)-C(11),
2.109(8); Pd(1)-O(1), 2.085(7). N(1)-Pd(1)-N(2), 78.3(3);
N(1)-Pd(1)-O(1), 96.2(3); N(2)-Pd(1)-C(11), 84.9(4).
caw, which eventually leads to a carbene hydride,
[(TMEDA)PtII(H)(C(Me)(OEt))]+.8 5 is thermally stable,
and its solution does not decompose at room tempera-
ture over days. The structure of 5 has been confirmed
by X-ray analysis (Figure 1)9 and shows distorted-
square-planar cooordination around Pd. The Pd-N(1)
distance, 2.025(12) Å, is longer than Pd-N(2), 1.960(8)
Å, reflecting the stronger trans effects of covalent bound
CH than coordinated OMe. The Pd-O(1) distance,
2.085(7) Å, is typical of Pd-O distances,10a but Pd-
C(11), 2.109(8) Å, is longer than normal Pd-C dis-
tances,10 possibly because of ring strain.
29.7, 40.9, 51.8, 53.8, 57.5, 65.8, 78.6, 89.3. Anal. Calcd for C44H35
B
F24N2O2Pd: C, 44.15; H, 2.93; N, 2.34. Found: C, 44.28; H, 3.19; N,
2.30. (e) Reaction of 2a with HSiEt3: To a solution of 2a (8.6 mg) in
CD2Cl2 (∼0.5 mL) at -78 °C was added Et3SiH (4 µL) to give a yellow
solution. 1H NMR spectra were then recorded at -78 to 20 °C. 1H NMR
(-78 °C): δ -9.87 (s, 1H, Pd-H), 0.90 (m, 15H, SiCH2CH3 and PdCH3),
1.90 (br, 4H), 2.26 (s, 6H), 3.22 (s, 6H), 3.29 (m, 4H), 3.78 (br, 4H).
The signal at -9.87 ppm started to decrease when the temperature
was raised to -20 °C and disappeared completely at 20 °C. Meanwhile,
the rest of the spectrum at 20 °C became complicated, indicating
formation of several products. Volatiles of the reaction mixture were
then analyzed by GC-MS analysis, which showed Et3SiCl and (Et3-
Si)2O as two major components. (f) 6: [Pt(SMe2)Me2]2 (0.178 g, 0.31
mmol) was added to a solution of 3 (0.205 g, 0.90 mmol) in CH2Cl2 (4
mL) at -30 °C. The resulting red solution was stirred at room
temperature for 2 h. Hexane (∼12 mL) was then added, and the
mixture was cooled to -30 °C to give the product (0.170 g, 60%) as
Reaction of 2a in CD2Cl2 with HSiEt3 at -78 °C
cleanly gives a product consistent with σ-silane coordi-
1
nation, [(NN)PdII(Me)(η2-HSiEt3)]+.11 The H NMR spec-
trum contains a high-field signal at δ -9.87 correspond-
ing to the Si-H proton and one single peak at δ 3.22
consistent with equivalent, noncoordinating OCH3
groups. Apparently the η2-HSiEt3 group displaces the
(4) Rulke, R. E.; Ernsting, J . M.; Spek, A. L.; Elsevier: C. J .; van
Leeuwen, P. W. N. M.; Vrieze, K. Inorg. Chem. 1993, 32, 5769.
(5) Crystal data for 4: monoclinic, C2/c, a ) 7.6081(7) Å, b )
9.2632(9) Å, c ) 23.878(2) Å, â ) 92.126(2)°, V ) 1681.7(3) Å3, Z ) 4,
R1(I > 2σ) ) 0.0312 and wR2 ) 0.0744.
(6) Brookhart, M.; Grant, R. G.; Volpe, A. R., J r. Organometallics
1992, 11, 3920.
(7) For example, see: Dyker, G. Angew. Chem., Int. Ed. 1999, 38,
1698 and references therein.
(8) Holtcamp, M. W.; Labinger, J . A.; Bercaw, J . E. J . Am. Chem.
Soc. 1997, 119, 848. Very recently Tempel et al. reported C-H
activation of the methyl groups of both tert-butyl and isopropyl groups
of the aryl diimine complexes 1; see: Tempel, D. J .; J ohnson, L. K.;
Huff, R. L.; White, P. S.; Brookhart, M. J . Am. Chem. Soc. 2000, 122,
6686.
(9) Crystal data for 5: monoclinic, P21/c, a ) 12.7833(9) Å, b )
20.4625(12) Å, c ) 18.2923(13) Å, â ) 95.962(1)°, V ) 4759.0(6) Å3, Z
) 4, R1(I > 2σ) ) 0.1003 and wR2 ) 0.1948.
(10) (a) Rix, R. C.; Brookhart, M.; White, P. S. J . Am. Chem. Soc.
1996, 118, 2436. (b) Kayser, B.; Missling, C.; Knizek, J .; Noth, H.; Beck,
W. Eur. J . Inorg. Chem. 1998, 375. (c) Alsters, P. L.; Boersma, J .;
Smeets, W. J . J .; Spek, A. L.; van Koten, G. Organometallics 1993,
12, 1639.
(11) For examples of similar Fe and Mo silane σ complexes, see:
(a) Scharrer, E.; Change, S.; Brookhart, M. Organometallics 1995, 14,
5686. (b) Luo, X.-L.; Kubas, G. J .; Bryan, J . C.; Burns, C. J .; Unkefer,
C. J . J . Am. Chem. Soc. 1994, 116, 10312. The complex could also be
a diimine Pd(IV) silyl hydride; see: Brookhart, M.; Grant, B. E.;
Lenges, C. P.; Prosenc, M. H.; White, P. S. Angew. Chem., Int. Ed.
2000, 39, 1676.
black crystals. 1H NMR (CDCl3): δ 1.13 (s, 6H, J
) 83.5 Hz), 1.70
(s, 6H), 1.99(m, 4H), 3.30 (s, 6H), 3.38 (t, 4H, J )P5t-.4H Hz), 4.10 (t, 4H,
J ) 7.4 Hz). 13C NMR (CDCl3): δ -14.7 (s, J Pt-C ) 777.2 Hz), 18.3,
30.2, 50.5 (s, J Pt-C ) 30.8 Hz), 69.9, 76.8, 170.6. Anal. Calcd for
C14H30N2O2Pt: C, 37.09; H, 6.62; N, 6.18. Found: C, 37.19; H, 6.93;
N, 6.08. (g) 2b: H(OEt2)2BArF (0.0996 g, 0.098 mmol) was added to a
deep red solution of 6 (0.0446 g, 0.098 mmol), and the resulting solution
was stirred at room temperature for 10 min. Volatiles were removed,
and the residue was triturated with hexane to give yellow product
(0.1050 g, 82%). 1H NMR (CD2Cl2): δ 1.14 (s, 3H, J Pt-H ) 69.6 Hz),
1.95 (s, 3H), 2.05 (s, 3H), 1.90-2.05 (br, 2H), 2.22 (br, 2H), 3.29 (s,
3H), 3.37 (t, 2H, J ) 5.4 Hz), 3.72 (m, 2H), 3.76 (s, 3H), 3.94 (t, 2H, J
) 7.3 Hz), 4.09 (m, 2H), 7.58 (s, 4H), 7.74 (s, 8H). 13C NMR (CD2Cl2):
δ -7.0 (J Pt-C ) 783 Hz), 18.1, 18.7, 28.2, 30.2, 52.4, 55.0, 58.9, 67.8,
69.0, 80.2, 178.5, 178.6. Anal. Calcd for C45H39BF24N2O2Pt: C, 41.51;
H, 3.00; N, 2.15. Found: C, 41.67; H, 3.27; N, 2.13. (h) 7: Et3SiH (0.10
mL) was added to a solution of 2b (0.030 g, 0.023 mmol) in CH2Cl2 (2
mL) at -30 °C. After the mixture was kept at -30 °C for 30 min,
hexane (10 mL, -30 °C) was added and the mixture was kept at -30
°C overnight to give the product (0.027 g, 83%) as light yellow crystals.
1H NMR (-10 °C, CD2Cl2): δ -17.31 (s, 1H, J Pt-H ) 1297.6 Hz), 0.73
(q, 6H, J ) 7.4 Hz), 0.88 (t, 9H, J ) 7.5 Hz), 1.01 (s, 3H, J Pt-H ) 60.0
Hz), 1.73 (br, 1H), 1.87 (br, 2H), 2.14 (br, 1H), 2.22 (s, 3H), 2.37 (s,
3H), 3.28 (s, 3H), 3.30 (s, 3H), 3.41 (t, 2H, J ) 5.4 Hz), 3.55 (br, 1H),
3.85 (t, 1H, J ) 9.8 Hz), 3.99-4.09 (m, 3H), 4.44 (m, 1H). 13C NMR
(-40 °C, CD2Cl2): δ -12.8 (J Pt-C ) 810 Hz), 7.7 (SiCH2, J Pt-C ) 52.8
Hz), 7.9, 18.6, 19.0, 30.0, 31.2, 50.0, 56.5, 59.2, 60.6, 69.1, 73.6, 175.3,
179.4.

Communications
Organometallics, Vol. 19, No. 21, 2000 4195
geometry around Pt, and the Pt-N(2) distance, 2.0558(9)
Å, is longer than the respective Pt-N(1) distance,
1.9555(97) Å, presumably due to the stronger trans
effects of the CH3. The Pt-C(1) distance of 2.0337(123)
Å and Pt-O(2) distance of 2.0696(89) Å are within
normal ranges.10b,14
Unlike those of 2a and the Bercaw complex, solutions
of complex 2b are stable and the NMR spectra in CDCl3
do not change for weeks at room temperature in air.
This high stability is in drastic contrast to the analogous
aryl diimine analogue [(NN)PtII(Me)(OEt2)]+, where NN
) ArNCMeCMeNAr (Ar ) p-MeC6H4), which is not even
observable by NMR spectroscopy because of its ex-
tremely high instability.15 It is clear that coordination
of the intramolecular OMe confers stability, and accord-
ingly, 2b does not add to the C-H bonds of alkanes,
benzene, or toluene. However, the bound OMe is readily
displaced by a Si-H group when the complex is treated
with HSiEt3 at -30 °C, eventually affording the octa-
hedral Pt(IV) hydride 7 as colorless crystals (eq 2). While
oxidative addition of Si-H bonds to cationic diphosphine
Pt(II) complexes to form Pt(IV) silyl hydrides is known,16
this is the first case of addition to a cationic diimine
Pt(II) complex. Previously, diimine Pt(IV) silyl hydrides
were suggested to be too unstable to be observable;17
therefore, it is likely that a pendant OMe stabilizes the
complex by coordinating to the vacant sixth site.18 Still,
solutions of 7 are unstable at room temperature and
decompose to a unidentifiable mixture within ∼20 min.
In summary, we have prepared the first cationic
Pd(II) and Pt(II) complexes of a new methoxyalkyl-
substituted diimine ligand. The Pt(II) complex 2a
activates an alkyl C-H bond of the methylene proximal
to the coordinated OMe group, and the Pt(II) complex
activates a Si-H bond to form a diimine Pt(IV) silyl
complex. Determination of other chemical properties of
these and related complexes with other functional
groups is in progress.19
F igu r e 2. Drawing of 2b showing thermal ellipsoids at
the 50% level. Selected bond distances (Å) and angles
(deg): Pt(1)-N(1), 1.956(1); Pt(1)-N(2), 2.056(9); Pt(1)-
C(1), 2.03(1); Pt(1)-O(2), 2.070(1); N(1)-C(2), 1.31(1);
N(2)-Pt(1)-O(2), 96.6(4); N(2)-Pt(1)-N(1), 78.7(4); O(2)-
Pt(1)-C(1), 86.1(6).
bound OMe in 2a . However, the complex is thermally
unstable and rapidly decomposes as its solution is
warmed to room temperature. As measured by GC-MS,
the volatile products of the reaction contain Et3SiCl and
(Et3Si)2O as two major components, characteristic of
heterolytic cleavage of the η2-Si-H bond.12
The Pt(II) complex 2b is prepared in a related manner
(eq 2). On analogy to 2a , one OMe arm in 2b is bound
Ack n ow led gm en t. This work was supported by the
Department of Energy, Office of Basic Energy Sciences,
Chemical Sciences Division.
Su p p or tin g In for m a tion Ava ila ble: Detailed X-ray crys-
tallographic data of the structures of compounds 4, 5, and 2b.
This material is available free of charge via the Internet at
http://pubs.acs.org.
OM000720E
(14) (a) Kapteijin, G. M.; Meijer, M. D.; Grove, D. M.; Veldman, N.;
Spek, A. L.; van Koten, G. Inorg. Chim. Acta 1997, 264, 211. (b)
Holtcamp, M. W.; Henling, L. M.; Day, M. W.; Labinger, J . A.; Bercaw,
J . E. Inorg. Chim. Acta 1998, 270, 467.
(15) J ohansson, L.; Ryan, O. B.; Tilset, M. J . Am. Chem. Soc. 1999,
121, 1974.
(16) For example, see: Pfeiffer, J .; Kickelbick, G.; Schubert, U.
Organometallics 2000, 19, 62 and references therein.
(17) Hill, G. S.; Rendina, L. M.; Puddephatt, R. J . J . Chem. Soc.,
Dalton Trans. 1996, 1809.
(18) For similar nitrogen-stabilized Pt(IV) alkyl hydrides, see: (a)
O’Reilly, S. A.; White, P. S.; Templeton, J . L. J . Am. Chem. Soc. 1996,
118, 5684. (b) Wick, D. D.; Goldberg, K. J . Am. Chem. Soc. 1997, 119,
10235. (c) Canty, A. J .; Dedieu, A.; J in, H.; Milet, A.; Richmond, M. K.
Organometallics 1996, 15, 2845. (d) Prokopchuk, E. M.; J enkins, H.
A.; Puddephatt, R. J . Organometallics 1999, 18, 2861. (e) Haskel, A.;
Keinan, E. Organometallics 1999, 18, 4677.
1
to the vacant site. The bound OMe has a H NMR signal
at δ 3.76 vs the signal at δ 3.29 of unbound OMe. The
structure is confirmed by X-ray analysis (Figure 2).13
On analogy to 5, 2b exhibits a distorted-square-planar
(12) (a) Huhmann-Vincent, J .; Scott, B. L.; Kubas, G. J . Inorg. Chim.
Acta 1999, 294, 240. (b) Fang, X.-G.; Huhmann-Vincent, J .; Scott, B.
L.; Kubas, G. J . J . Organomet. Chem., in press.
(13) Crystal data for 2b: triclinic, P1h, a ) 13.0244(9) Å, b )
13.099(1) Å, c ) 16.628(1) Å, R ) 87.756(1)°, â ) 82.167(1)°. γ )
61.316(1)°, V ) 2464.5(3) Å3, Z ) 2, R1(I > 2σ) ) 0.0585 and wR2 )
0.1282.
(19) For example, 2a is found to catalyze polymerization of tert-
butylacetylene to give stereoregular polymers.