Nonchelated ZrIV-alkoxide-alkyne complexes

Article abstract of DOI:10.1021/om050947f

Addition of alkynes to [Cp′₂Zr(O^tBu)][B(C ₆F₅)₄] (Cp′ = C₅H ₄Me, 2) at low temperature in CD₂Cl₂ solution yields equilibrium mixtures of Cp′₂Zr(O

Full text of DOI:10.1021/om050947f

Organometallics 2006, 25, 3379-3387
3379
Nonchelated ZrIV-Alkoxide-Alkyne Complexes
Edward J. Stoebenau, III and Richard F. Jordan*
Department of Chemistry, The UniVersity of Chicago, 5735 South Ellis AVenue, Chicago, Illinois 60637
ReceiVed NoVember 3, 2005
Addition of alkynes to [Cp′2Zr(OtBu)][B(C6F5)4] (Cp′ ) C5H4Me, 2) at low temperature in CD2Cl2
solution yields equilibrium mixtures of Cp′2Zr(OtBu)(RCtCR′)+ (3a-g), 2, and free alkyne. The 13C
NMR Cint resonance of terminal alkynes shifts far downfield upon coordination, while the Cterm resonance
shifts upfield, consistent with polarization of the CtC bond with partial positive charge on Cint and
partial negative charge on Cterm, probably due to unsymmetrical coordination. Alkynes bind more strongly
to 2 than do similar alkenes, but are readily displaced by Lewis bases such as THF. Increasing the steric
bulk of the alkyne decreases the coordination strength. Propargyltrimethylsilane coordinates particularly
strongly, due to â-Si stabilization of the positive charge on Cint of the coordinated alkyne. Compounds
3a-g undergo two dynamic processes: reversible alkyne decomplexation and rotation around the metal-
(alkyne centroid) axis.
Scheme 1
Introduction
Alkyne complexes of d0 metals are proposed intermediates
in alkyne dimerization and oligomerization reactions (Scheme
1)1 and are potential models for the d0 metal-alkene complexes
that are intermediates in alkene polymerization.2 d0 metal-
alkyne complexes are challenging to study because alkyne
coordination is weak due to the absence of d-π* back-bonding,
and facile alkyne insertion can occur if M-H, M-R, or other
reactive groups are present. These properties limit the range of
ancillary ligands, solvents, and counterions that may be used
with these species.
Alkyne complexes of d0 metals are very rare. Erker prepared
chelated Cp2M(CMedC{B(C6F5)3}CtCMe) (M ) Zr, Hf)
alkenyl-alkyne complexes (A, Chart 1) by the reaction
of Cp2M(CtCMe)2 with B(C6F5)3.3 Horton prepared
Cp*2Zr(CHdCRCtCR)+ species (B, Cp* ) C5Me5), which
are intermediates in Zr-catalyzed alkyne oligomerization, by
insertion of alkynes into Zr-alkynyl compounds.1a Jordan
reported similar (η5-C2B9H11)(Cp*)Hf(CHdCRCtCR) com-
pounds (C), which were generated by analogous reactions.1e
Chart 1
* To whom correspondence should be addressed. E-mail: rfjordan@
uchicago.edu.
(1) (a) Horton, A. D. Chem. Commun. 1992, 185. (b) Akita, M.; Yasuda,
H.; Nakamura, A. Bull. Chem. Soc. Jpn. 1984, 57, 480. (c) den Haan, K.
H.; Wielstra, Y.; Teuben, J. H. Organometallics 1987, 6, 2053. (d) Heeres,
H. J.; Teuben, J. H. Organometallics 1991, 10, 1980. (e) Yoshida, M.;
Jordan, R. F. Organometallics 1997, 16, 4508. (f) Nishiura, M.; Hou, Z. J.
Mol. Catal. A: Chem. 2004, 213, 101. (g) Mach, K.; Gyepes, R.; Hora´cˇek,
M.; Petrusova´, L.; Kubisˇta, J. Collect. Czech. Chem. Commun. 2003, 68,
1877. (h) Varga, V.; Petrusova´, L.; Cˇ ejka, J.; Hanusˇ, V.; Mach, K. J.
Organomet. Chem. 1996, 509, 235. (i) Ohff, A.; Burlakov, V. V.; Rosenthal,
U. J. Mol. Catal. A: Chem. 1996, 108, 119.
(2) For recent review articles concerning metal-catalyzed alkene polym-
erization see: (a) Mu¨lhaupt, R. Macromol. Chem. Phys. 2003, 204, 289.
(b) Resconi, L.; Cavallo, L.; Fait, A.; Piemontesi, F. Chem. ReV. 2000,
100, 1253. (c) Brintzinger, H. H.; Fischer, D.; Mu¨lhaupt, R.; Rieger, B.;
Waymouth, R. M. Angew. Chem., Int. Ed. Engl. 1995, 34, 1143.
(3) (a) Temme, B.; Erker, G.; Fro¨hlich, R.; Grehl, M. Angew. Chem.,
Int. Ed. Engl. 1994, 33, 1480. (b) Venne-Dunker, S.; Ahlers, W.; Erker,
G.; Fro¨hlich, R. Eur. J. Inorg. Chem. 2000, 1671. (c) Ahlers, W.; Erker,
G.; Fro¨hich, R.; Peuchert, U. J. Organomet. Chem. 1999, 578, 115. (d)
Ahlers, W.; Temme, B.; Erker, G.; Fro¨hlich, R.; Fox, T. J. Organomet.
Chem. 1997, 527, 191. (e) Erker, G.; Venne-Dunker, S.; Kehr, G.;
Kleigrewe, N.; Fro¨hlich, R.; Mu¨ck-Lichtenfeld, C.; Grimme, S. Organo-
metallics 2004, 23, 4391. (f) Burlakov, V. V.; Arndt, P.; Baumann, W.;
Spannenberg, A.; Rosenthal, U. Organometallics 2004, 23, 5188.
The CtC NMR resonances of B and C are 5-10 ppm
downfield from the resonances in typical enynes.1b Finn and
Grimes reported that the reaction of {(η5-Et2C2B4H4)CpTaH}2-
(µ-Cl)2 with diphenylacetylene or 1-phenylpropyne yields
nonchelated alkyne complexes (η5-Et2C2B4H4)CpTa(H)(Cl)-
(PhCtCR) (D; R ) Ph, Me).4 13C NMR chemical shifts for
the CtC carbons in these species were not reported. Theoretical
(4) Curtis, M. A.; Finn, M. G.; Grimes, R. N. J. Organomet. Chem. 1998,
550, 469.
(5) (a) Hyla-Kryspin, I.; Gleiter, R. J. Mol. Catal. A: Chem. 2000, 160,
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964.
10.1021/om050947f CCC: $33.50 © 2006 American Chemical Society
Publication on Web 06/06/2006

3380 Organometallics, Vol. 25, No. 14, 2006
Stoebenau and Jordan
Chart 2
This paper describes the synthesis and properties of nonch-
elated Cp′2Zr(OtBu)(alkyne)+ (Cp′ ) C5H4Me) complexes,
which are models for the putative (C5R5)2Zr(R′)(alkyne)+
intermediates in Zr-catalyzed alkyne oligomerization reactions.14
Results and Discussion
Targets. The objective of this work was to prepare [Cp′2Zr-
(OtBu)(RCtCR′)][B(C6F5)4] alkyne complexes in order to probe
ZrIV-alkyne bonding. The tert-butoxide ligand was used to
generate an electrophilic three-coordinate Zr species that cannot
undergo alkyne insertion due to the strong Zr-O bond.15 The
Cp′ ligand was chosen because it provides a useful symmetry
probe and neutral precursors to Cp′2Zr(OR)+ are readily
available. The B(C6F5)4- anion was chosen for its high stability
and weak coordinating ability.16 Halocarbon solvents were used
to solubilize the ionic species, albeit at the cost of competitive
solvent coordination.17
studies of Cp2ZrMe(HCtCH)+, Cl2ZrH(HCtCH)+, and
Cl2ZrMe(HCtCH)+ predict an unsymmetrical bonding mode
for ZrIV-alkyne complexes.5 For example, the Zr-Calkyne
distances in Cp2ZrMe(HCtCH)+ were calculated to be 2.640
and 2.787 Å (∆d(Zr-C) ) 0.147 Å).
Alkyne complexes of s- and p-block elements have also been
characterized. Gokel reported that crown ethers with tethered
alkynes exhibit Na+- or K+-alkyne interactions in the solid
state (E, Chart 2),6 and Hanusa described the diyne complex
Cp*2Ca(Me3SiCtCCtCSiMe3) (F).7 The NMR resonances of
the alkyne units in these compounds are only slightly shifted
from the free alkyne positions.6b,7 Jones reported that the reaction
Synthesis of [Cp′2Zr(OtBu)][B(C6F5)4]. The reaction of Cp′2-
ZrMe2 with tBuOH yields Cp′2Zr(OtBu)Me (1).18 The reaction
of 1 with [Ph3C][B(C6F5)4] affords [Cp′2Zr(OtBu)][B(C6F5)4]
(2).14a
Generation of Cp′2Zr(OtBu)(alkyne)+ Complexes. Addition
of excess alkyne to a CD2Cl2 solution of 2 at low temperature
results in the formation of an equilibrium mixture of 2, free
alkyne, and [Cp′2Zr(OtBu)(alkyne)][B(C6F5)4] (3a-g), as shown
in eq 1. In a typical experiment, the desired alkyne was added
by vacuum transfer to a frozen solution of 2 in CD2Cl2 in an
NMR tube. The tube was thawed, stored at -78 °C, and
transferred to a precooled NMR probe with minimal transfer
time (<1 min) to prevent side reactions, and the equilibrium in
eq 1 was studied by NMR spectroscopy. The Cp′2Zr(OtBu)-
(alkyne)+ species are thermally sensitive, which precluded
isolation or ESI-MS analysis.
of “GaI” with Me3SiCtCCtCSiMe3 yields a novel GaIII
4
cluster with two Ga-alkyne units (G).8 Wrackmeyer has studied
SnIV-η2-alkynylborate adducts that are intermediates in stannole
formation (e.g., H)9 and has reported similar PbIV-η2-alky-
nylborate compounds.10
Group 11 and 12 metal-alkyne complexes in which d-π*
back-bonding is very weak have also been studied. For example,
d10 AgI compounds normally exhibit only minimal d-π* back-
bonding, as exemplified by (HB{(3,5-CF3)2pz}3)Ag(CO), for
which a very high νCO value (2162 cm-1) was observed.11 Dias
prepared AgI-alkyne adducts such as (HB{(3,5-CF3)2pz}3)Ag-
(HCtCPh) (I; pz ) pyrazolyl).11 Compound I exhibits unsym-
metrical Ag-alkyne bonding in the solid state (d(Ag-Cterm) )
2.263(5) Å; d(Ag-Cint) ) 2.407(5) Å; ∆d(Ag-C) ) d(Ag-
Cint) - d(Ag-Cterm) ) 0.144(7) Å). The coordination shifts (∆δ
) δcoord - δfree) of the alkyne CtC NMR resonances of I are
opposite in sign (∆δ Cterm ) -11.1; ∆δ Cint ) 2.8), which may
be a consequence of the unsymmetrical coordination. Other AgI
alkyne adducts have been reported.12 Starowieyski has studied
ω-alkynyl zinc compounds that have chelated structures with
alkyne-zinc interactions.13
NMR Properties and Solution Structures of Cp′2Zr(OtBu)-
1
(alkyne)+ Complexes. The H and 13C{1H} NMR spectra of
(6) (a) Hu, J.; Barbour, L. J.; Gokel, G. W. J. Am. Chem. Soc. 2001,
123, 9486. (b) Hu, J.; Gokel, G. W. Chem. Commun. 2003, 2536.
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Organometallics 1996, 15, 2660. (d) Gleiter, R.; Karcher, M.; Kratz, D.;
Ziegler, M. L.; Nuber, B. Chem. Ber. 1990, 123, 1461.
(13) Oknin´ska, E.; Starowieyski, K. B. J. Organomet. Chem. 1989, 376,
7.
(14) Preliminary communications: (a) Stoebenau, E. J., III; Jordan, R.
F. J. Am. Chem. Soc. 2003, 125, 3222. (b) Stoebenau, E. J., III; Jordan, R.
F. J. Am. Chem. Soc. 2004, 126, 11170.
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(b) Strauss, S. H. Chem. ReV. 1993, 93, 927. (c) Chen, E. Y.-X.; Marks, T.
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Am. Chem. Soc. 1991, 113, 8570. (f) Turner, H. W. Eur. Patent EP0277004,
1988.
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(9) (a) Wrackmeyer, B.; Kehr, G.; Willbold, S. J. Organomet. Chem.
1999, 590, 93. (b) Wrackmeyer, B.; Vollrath, H.; Ali, S. Inorg. Chim. Acta
1999, 296, 26. (c) Wrackmeyer, B.; Horchler von Locquenghien, K.;
Kundler, S. J. Organomet. Chem. 1995, 503, 289. (d) Wrackmeyer, B.;
Kehr, G.; Wettinger, D. Inorg. Chim. Acta 1994, 220, 161. (e) Wrackmeyer,
B.; Kehr, G.; Boese, R. Angew. Chem., Int. Ed. Engl. 1991, 30, 1370. (f)
Wrackmeyer, B.; Kundler, S.; Milius, W.; Boese, R. Chem. Ber. 1994, 127,
333. (g) Wrackmeyer, B.; Kundler, S.; Boese, R. Chem. Ber. 1993, 126,
1361. (h) Wrackmeyer, B.; Kehr, G.; Sebald, A.; Ku¨mmerlen, J. Chem.
Ber. 1992, 125, 1597.
(10) (a) Wrackmeyer, B.; Horchler, K. J. Organomet. Chem. 1990, 399,
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Nonchelated ZrIV-Alkoxide-Alkyne Complexes
Organometallics, Vol. 25, No. 14, 2006 3381
Table 1. NMR Coordination Shifts (∆δ) for
Cp′2Zr(OtBu)(HCtCR)+ Complexes (3a-d)a
b
b
cpd
alkyne
Cint
Cterm
Cprop
4.5
Hterm
Hprop
3a
3b
3c
3d
HCtCMe
HCtCPh
HCtCSiMe3
HCtCCH2SiMe3
8.9
13.3
4.8
-2.8
-2.1
-2.9
-0.9
1.16
1.23
0.98
1.58
0.46
Figure 2. Polarization of d0 metal-alkyne complexes.
21.7
7.0
0.48
a In CD2Cl2 solution, -89 °C; ∆δ ) δcoord - δfree
resonance.
.
b HCtCCHnR3-n
3a-g each contain four CH resonances and one set of CMe
resonances for the Cp′ ligands, and one set of OtBu resonances,
characteristic of Cs symmetry. In most cases, the Cp′ resonances
are only slightly shifted from those of 2. The Zr-OtBu 1H NMR
resonance appears at δ 1.22 for 2 and shifts less than 0.05 ppm
from this position for 3a,c-g. However, the Zr-OtBu 1H NMR
resonance of the phenylacetylene adduct 3b is shifted upfield
to δ 0.80, which is ascribed to anisotropic shielding of the Ot-
Bu hydrogens by the phenyl ring of the coordinated alkyne.
The NMR spectra of terminal alkyne complexes 3a-d each
contain one set of alkyne resonances, which are shifted from
the free alkyne positions. The Cs symmetry and the presence
of only one set of alkyne signals are consistent with the presence
of a single rotamer of 3a-d in which the alkyne lies in the
plane between the Cp′ ligands or with fast rotation around the
Zr-(alkyne centroid) axis.
Figure 3. Stabilization of propargyltrimethylsilane adduct 3d by
the â-Si effect.
ppm upon coordination. For 3a,d, the propargylic tCCHn 1H
and 13C resonances are shifted downfield by ca. 0.5 and 5-7
ppm, respectively.
The 13C ∆δ values are reminiscent of the divergent coordina-
tion shifts in chelated (C5R5)2Zr(OCMe2CH2CH2CHdCH2)+
complexes19 and the AgI(HCtCPh) adduct I,11 for which X-ray
crystallographic analyses establish that the alkene and alkyne
ligands are unsymmetrically bound. Therefore, the alkyne
ligands in 3a-d are also probably unsymmetrically coordinated
with d(Zr-Cint) > d(Zr-Cterm). Unsymmetrical alkyne coordi-
nation will polarize the CtC triple bond, resulting in positive
charge buildup on Cint and negative charge buildup on Cterm
(Figure 2), which explains the NMR data in Table 1. Consistent
with this picture, propargyltrimethylsilane complex 3d exhibits
the largest ∆δ value for Cint, which is a consequence of â-Si
stabilization of the positive charge on Cint, as shown in Figure
3.20,21 In contrast, the trimethylsilylacetylene adduct 3c exhibits
the smallest ∆δ value for Cint, because the R-Si is better at
stabilizing an anionic charge than a positive charge at Cint,
resulting in reduced alkyne polarization.
The NMR spectra of symmetrical-alkyne adducts 3e,g each
contain one set of alkyne tCR resonances. The Cs symmetry
of 3e,g and the presence of only one set of tCR signals are
consistent with the presence of a single rotamer of 3e,g in which
the alkyne lies perpendicular to the plane between the Cp′
ligands or with fast rotation around the Zr-(alkyne centroid)
1
axis. The 2-butyne H and 13C NMR resonances of 3e have
approximately the same line widths as the other signals of 3e
at -59 °C, where intermolecular alkyne exchange is slow on
the NMR time scale, but broaden as the temperature is lowered
to -89 °C, while the other resonances of 3e remain sharp.
Similar NMR line-broadening effects are observed for the
2-pentyne (3f) and 3-hexyne (3g) adducts. These results indicate
that rotation around the metal-(alkyne centroid) axis is fast on
the NMR time scale at higher temperatures, but decreases in
rate at lower temperatures.
The 1JCH and 2JCtCH coupling constants for the alkyne units
in 3a-d are only slightly perturbed from the free alkyne values,
as summarized in Table 2. These results suggest that the alkyne
structure and the hybridization at the alkyne carbons are not
strongly perturbed by coordination.
The alkyne NMR resonances of the terminal alkyne com-
plexes 3a-d exhibit characteristic coordination shifts (∆δ )
The coordination shifts of internal alkyne complexes 3e-g
are listed in Table 3. The Calkyne NMR resonances of 3e-g shift
δcoord - δfree), which are listed in Table 1. The HCt (Cterm
)
13C resonance shifts upfield by 1-3 ppm, while the tCR (Cint)
resonance shifts significantly downfield (∆δ ) 5-22) upon
coordination. The alkyne region of the 13C{1H} NMR spectrum
of 3a is shown in Figure 1 and illustrates the divergent
coordination shifts of the MeCtCH resonances. The tCH
(17) (a) Wu, F.; Dash, A. K.; Jordan, R. F. J. Am. Chem. Soc. 2004,
126, 15360. (b) Bouwkamp, M. W.; Budzelaar, P. H. M.; Gercama, J.; Del
Hierro Morales, I.; de Wolf, J.; Meetsma, A.; Troyanov, S. I.; Teuben, J.
H.; Hessen, B. J. Am. Chem. Soc. 2005, 127, 14310. (c) Korolev, A. V.;
Delpech, F.; Dagorne, S.; Guzei, I. A.; Jordan, R. F. Organometallics 2001,
20, 3367.
1
(Hterm) H NMR resonance of 3a-d shifts downfield by 1-2
(18) (a) Brandow, C. G. Zirconocenes as Models for Homogeneous
Ziegler-Natta Olefin Polymerization Catalysts. Ph.D. Thesis, California
Institute of Technology, Pasadena, CA, 2001. See also: (b) Wailes, P. C.;
Weigold, H.; Bell, A. P. J. Organomet. Chem. 1972, 34, 155. (c) Takahashi,
T.; Aoyagi, K.; Hara, R.; Suzuki, N. Chem. Lett. 1992, 1693.
(19) Carpentier, J. F.; Wu, Z.; Lee, C. W.; Stro¨mberg, S.; Christopher,
J. N.; Jordan, R. F. J. Am. Chem. Soc. 2000, 122, 7750.
(20) (a) Lambert, J. B. Tetrahedron 1990, 46, 2677. (b) Mu¨ller, T.;
Juhasz, M.; Reed, C. A. Angew. Chem, Int. Ed. 2004, 43, 1543. (c) Mu¨ller,
T.; Meyer, R.; Lennartz, D.; Siehl, H.-U. Angew. Chem., Int. Ed. 2000, 39,
3074. (d) Mu¨ller, T.; Margraf, D.; Syha, Y. J. Am. Chem. Soc. 2005, 127,
10852.
(21) (a) Boag, N. M.; Green, M.; Grove, D. M.; Howard, J. A. K.;
Spencer, J. L.; Stone, F. G. A. J. Chem. Soc., Dalton. Trans. 1980, 2170.
(b) Bartik, T.; Happ, B.; Iglewsky, M.; Bandmann, H.; Boese, R.;
Heimbacht, P.; Hoffmann, T.; Wenschuh, E. Organometallics 1992, 11,
1235. (c) Rosenthal, U.; Burlakov, V. V.; Arndt, P.; Baumann, W.;
Spannenberg, A. Organometallics 2003, 22, 884, and references therein.
Figure 1. Partial 13C{1H} NMR spectrum of 3a (CD2Cl2, -89
°C). Assignments: δ 89 (Cint of 3a), 84 (OCMe3 of 2 and 3a), 80
(Cint of free propyne), 67 (Cterm of free propyne), 64 (Cterm of 3a).
The coordination shifts (free to coordinated) for the alkyne
resonances are shown by arrows.

3382 Organometallics, Vol. 25, No. 14, 2006
Stoebenau and Jordan
Table 2. Coupling Constants for Free Alkynes and Alkynes
Coordinated to 2a
1JCH (Hz)
2JCtCH (Hz)
cpd
alkyne
HCtCMe
HCtCPh
HCtCSiMe3
HCtCCH2SiMe3
coord
free
coord
free
3a
3b
3c
3d
251
246
244
244
249
252
237
247
47
41
xb
50
50
41
50
43
a In CD2Cl2 solution, -89 °C. b Determination of this coupling constant
was precluded by spectral overlaps.
Table 3. NMR Coordination Shifts (∆δ) for
Cp′2Zr(OtBu)(RCtCR′)+ Complexes (3e-g)a
b
b
cpd
alkyne
Calkyne
Cprop
Hprop
0.35
Figure 4. van’t Hoff plot for propyne (circles, solid line) and
2-butyne (triangles, dashed line) coordination in 3a,e in CD2Cl2
solution (eq 1).
3e
3f
MeCtCMe
MeCtCEt
5.4
6.6
7.3c
3.8d
6.1
6.8c
7.2d
7.0
0.36c
xd,e
xe
3g
EtCtCEt
Thermodynamics of Alkyne Binding to 2. Equilibrium
constants for alkyne binding to 2 in eq 1, Keq ) [3][2]-1[L]-1
,
a In CD2Cl2 solution, -89 °C; ∆δ ) δcoord - δfree
.
b tCCHnR3-n
were determined by H NMR and are listed in Table 4.23 Keq
values for 3a-c,e-g were found by studies of eq 1. The Keq
value for propargyltrimethylsilane binding was too large to be
determined directly and, therefore, was determined by a
competition experiment with propyne. In certain cases, Keq
values were determined over a sufficiently wide temperature
range to enable accurate determination of binding enthalpies
and entropies. These values are listed in Table 5.
1
resonance. c MeCt resonances. d EtCt resonances. e Resonance not de-
tected due to spectral overlaps.
Table 4. Equilibrium Constants (Keq) for Alkyne
Coordination to Cp′2Zr(OtBu)(ClCD2Cl)+ in Eq 1a
b
cpd
alkyne
K )
eq (M-1
3d
3a
3e
3f
3g
3b
3c
HCtCCH2SiMe3
HCtCMe
MeCtCMe
MeCtCEt
EtCtCEt
1.0(2) × 105
360(70)
52(3)
49(9)
47(4)
The binding of 2-butyne to 2 to form 3e in eq 1 will be
described as a representative case. The equilibrium constant for
HCtCPh
22(1)
2-butyne coordination in CD2Cl2 solution at -89 °C is Keq
)
HCtCSiMe3
8.1(5)
52(3) M-1. At [2]initial ) 0.08 M and [2-butyne]initial ) 0.10 M,
ca. 75% of the total Cp′2Zr(OtBu)+ exists as 3e. The Keq value
is constant over the concentration ranges [2]initial ) 0.035-0.078
M and [2-butyne]initial ) 0.043-0.10 M. Addition of [Ph3C]-
[B(C6F5)4] (ca. 4 equiv per Zr) as an anion source does not
affect the equilibrium constant at -91 °C in CD2Cl2, indicating
that the anion is not explicitly involved in eq 1. Under these
conditions, Ph3C+ does not interact with 2-butyne or 2. At -38
HCtCtBu
xc
a In CD2Cl2 solution, -89 °C. Keq ) [3][2]-1[L]-1
.
c No complex
b
observed; Keq e 0.1 M-1
.
downfield by 4-7 ppm and the Cprop resonances shift downfield
by ca. 7 ppm upon coordination. The Hprop resonances also
display moderate downfield coordination shifts (∆δ ) ca. 0.4
ppm).
°C, Keq for 2-butyne binding is larger in CD2Cl2 (8.8(4) M-1
)
than in C6D5Cl (2.9(1) M-1). While solvent polarity may
influence the Keq value,24 it is likely that PhCl binds to 2 more
strongly than CD2Cl2 does,17a which would result in a reduced
Keq value in the former solvent. As the temperature is raised,
the 2-butyne complex 3e becomes less favored, and the
equilibrium shifts to free alkyne and 2. A van’t Hoff analysis
(Figure 4) gives ∆H° ) -3.6(3) kcal/mol and ∆S° ) -11(1)
eu (Table 5).23
The data in Table 4 show how the steric and electronic
properties of the alkyne influence Keq. Steric crowding close to
the alkyne triple bond inhibits alkyne coordination. Propyne
binds more strongly than 2-butyne or other internal alkynes, and
tert-butylacetylene does not bind to 2 in CD2Cl2 solution.
Propyne coordination is strong enough to observe NMR line
broadening in a 2/propyne mixture at 22 °C. Internal alkynes
must have one alkyl group in the vicinity of the Cp′ rings in
the Cp′2Zr(OtBu)(RCtCR)+ adduct, while propyne does not.
Stabilization of the partial positive charge on Cint of the
coordinated alkyne enhances alkyne binding. Thus, due to the
The NMR properties of 3e-g can be rationalized by the
unsymmetrical coordination/alkyne polarization model proposed
above for terminal alkyne complexes, coupled with fast
exchange of the ends of the alkyne by rotation around the
metal-(alkyne centroid) axis. Under slow alkyne rotation
conditions, one alkyne carbon resonance of an unsymmetrically
bound symmetrical alkyne would shift slightly upfield, while
the other would shift significantly downfield, as observed for
terminal alkyne adducts 3a-d. Alkyne rotation would average
these resonances, resulting in a net downfield shift, as observed
for 3e-g. The proposed unsymmetrical alkyne coordination in
3e-g is consistent with the computational results for Cp2ZrMe-
(HCtCH)+ noted above.5
Oxymetallocyclobutene-type structures derived from alkyne
insertion, i.e., Cp′2Zr(κ2-C,O-CRdCROtBu)+, are ruled out for
3a-g because such structures would display much larger ∆δ
values for the alkyne fragment than are observed, analogous to
those for azametallocyclobutene species, i.e., Cp2Zr(κ2-C,N-
CRdCRNR).22
a
Table 5. Thermodynamic Data for Equilibria in Eq 1 and Activation Parameters for Decomplexation (k-1
)
∆H°
(kcal/mol)
∆S°
(eu)
∆Hq
(kcal/mol)
∆Sq
(eu)
∆Gq
cpd
alkyne
(kcal/mol)b
3a
3e
HCtCMe
MeCtCMe
-4.1(3)
-3.6(3)
-11(1)
-11(1)
8.5(3)
13.8(5)
-14(1)
11.8(1)
12.9(1)
4(2)
a In CD2Cl2 solution. b At -39 °C.

Nonchelated ZrIV-Alkoxide-Alkyne Complexes
Organometallics, Vol. 25, No. 14, 2006 3383
Figure 6. Eyring plot for propyne (circles, solid line) and 2-butyne
(triangles, dashed line) decomplexation (k-1) from 3a,e in CD2Cl2
solution.
hence the propyne decomplexation rate constant is independent
of the free propyne concentration over the range [propyne] )
0.017-0.048 M. Therefore, direct associative displacement of
coordinated propyne by free propyne (eq 2) does not occur to
a significant extent. Instead, the data imply that propyne is lost
from 3a to give 2, and propyne coordinates to 2 to give 3a (eq
1).
Figure 5. Partial variable-temperature 1H NMR spectra (500 MHz)
of an equilibrium mixture of 3a, 2, and propyne in CD2Cl2 solution
illustrating the broadening of the Hterm resonance of 3a (δ 3.1) due
to reversible propyne decomplexation. The k-1 values are first-
order rate constants for propyne decomplexation. The peak at δ
2.85 is due to an unidentified impurity.
operation of the â-Si effect, propargyltrimethylsilane coordinates
very strongly to 2 to give 3d (Table 4), and the presence of just
1 equiv of propargyltrimethylsilane results in quantitative
formation of 3d up to -8 °C.20
The binding enthalpies in Table 5 show that the Zr-alkyne
bonds in 3a,e are only ca. 4 kcal/mol stronger than the Zr-
ClCD2Cl bond in 2, which underscores the point that the absence
of d-π* back-bonding results in weak alkyne coordination.
Alkyne binding is substantially weaker than coordination of
typical Lewis bases. For example, 1 equiv of THF (per Zr)
completely displaces propyne from 3a to give [Cp′2Zr(OtBu)-
(THF)][B(C6F5)4] (4). Compound 4 can be independently
generated by addition of THF to a solution of 2. The properties
of 4 are similar to those of Cp2Zr(OtBu)(THF)+ and rac-
(EBTHI)Zr(OtBu)(THF)+ (EBTHI ) 1,2-ethylenebis(tetrahy-
droindenyl)).25
Intermolecular Alkyne Exchange. The NMR resonances for
3a, 2, and free propyne in an equilibrium mixture of these
species broaden as the temperature is raised above -69 °C. The
Cp′ CH 1H NMR signals of 3a coalesce with those of 2 between
-38 and -18 °C, and the bound propyne signals of 3a coalesce
with the free propyne signals between -8 and +22 °C. These
dynamic effects are consistent with the exchange of 3a and 2
and of free and coordinated propyne. The broadening of the
Hterm resonance of 3a is shown in Figure 5.
An Eyring analysis (Figure 6) for propyne decomplexation
from 3a gives the activation parameters listed in Table 5. The
negative ∆Sq value suggests that a CD2Cl2 molecule displaces
the coordinated propyne by an associative mechanism.
Similarly, when a solution of 3e is warmed above -48 °C,
the NMR signals of 3e broaden and coalesce with the signals
for 2 and free 2-butyne. First-order rate constants for 2-butyne
decomplexation (k-1) were determined from the excess line
broadening of the 2-butyne 1H NMR resonance of 3e. The first-
order rate constants are independent of the free 2-butyne
concentration over the range [MeCtCMe] ) 0.037-0.079 M.
Therefore, free 2-butyne does not directly displace coordinated
2-butyne in the exchange process.
An Eyring analysis for 2-butyne decomplexation from 3e
(Figure 6) gives the activation parameters listed in Table 5. The
small, positive ∆Sq value and larger ∆Hq value compared to
that for propyne decomplexation from 3a are consistent with a
greater degree of dissociative character for 2-butyne decom-
plexation versus propyne decomplexation. Significant steric
crowding is expected in the transition state for associative CD2-
Cl2 displacement of 2-butyne from 3e.
First-order rate constants for propyne decomplexation from
3a (k-1 in eq 1) were determined from the excess line
broadening of the Hterm resonance of 3a under slow NMR
exchange conditions. The line width of the Hterm resonance and
A free energy diagram comparing propyne and 2-butyne
coordination to 2 is shown in Figure 7. 2-Butyne binds more
weakly, but exchanges more slowly than propyne. These
differences reflect the greater steric crowing in the 2-butyne
adduct.
(22) (a) Walsh, P. J.; Hollander, F. J.; Bergman, R. G. Organometallics
1993, 12, 3705. (b) Harlan, C. J.; Tunge, J. A.; Bridgewater, B. M.; Norton,
J. R. Organometallics 2000, 19, 2365.
(23) If the solvent term is included, the equilibrium constant for eq 1 is
K′eq ) Keq[CD2Cl2], where Keq is defined as in the text. If the solvent
concentration is assumed to be independent of temperature, the value of
∆H° is not affected, but the entropy term becomes ∆S°′ ) ∆S° + R(ln-
[CD2Cl2]), where R(ln[CD2Cl2]) ≈ 5.5 eu.
Comparison of Properties of d0 and d2 Group 4 Alkyne
Complexes. Due to the absence of d-π* back-bonding in
alkyne adducts 3a-g, the properties of these compounds are
quite different from those of “classical” dn metal-alkyne
complexes, in which back-bonding is present. These differences
can be illustrated most clearly by comparing d0 ZrIV-alkyne
adducts to d2 ZrII-alkyne adducts. Buchwald studied the hexyne
adducts Cp2Zr(HCtCnBu)(PMe3) (J) and Cp2Zr(EtCtCEt)-
(24) The dielectric constants (20 °C) are CH2Cl2 9.08, C6H5Cl 5.71. CRC
Handbook of Chemistry and Physics, 67th ed.; Weast, R. C., Astle, M. J.,
Beyer, W. H., Eds.; CRC Press: Boca Raton, FL, 1986; pp E-50-51.
(25) (a) Collins, S.; Koene, B. E.; Ramachandran, R.; Taylor, N. J.
Organometallics 1991, 10, 2092. (b) Hong, Y.; Kuntz, B. A.; Collins, S.
Organometallics 1993, 12, 964.

3384 Organometallics, Vol. 25, No. 14, 2006
Stoebenau and Jordan
Figure 8. Free energy diagram (-39 °C) comparing propyne and
propylene coordination to 2.
Figure 7. Free energy diagram (-39 °C) comparing propyne and
2-butyne coordination to 2.
propyne decomplexation is higher than that for propylene
decomplexation by ca. 0.9 kcal/mol, while the propyne com-
plexation barrier is lower than that for propylene by ca. 0.8
kcal/mol. Thus, the stronger ligand propyne comes off more
slowly, but binds more rapidly than the weaker ligand propylene.
Implications for Alkyne Oligomerization. Catalytic dimer-
ization of terminal alkynes by ZrIV compounds normally occurs
via 1,2-alkyne insertion to give “head-to-tail” H2CdCHRCt
CR dimers as the major or exclusive product (Scheme 1). This
selectivity implies that the Zr-CtCR group migrates to Cint
of the coordinated alkyne.1 Polarization of the CtC bond of
the alkyne ligand in the key Cp2Zr(CtCR)(HCtCR)+ inter-
mediate in the same manner as implied by the NMR data for
the Cp′2Zr(OtBu)(HCtCR)+ models 3a-d provides a reason-
able explanation for this regiochemistry, as the partial positive
charge on Cint of the alkyne renders this carbon susceptible to
nucleophilic attack and thus insertion. It is also possible that
steric crowding between the alkyl group of a terminal alkyne
and the Cp ligands in the Cp2Zr(CtCR)(HCtCR)+ intermedi-
ate would favor the alkyne rotamer in which Cint is oriented
toward the M-R bond, which would increase the preference
for 1,2-alkyne insertion. The shielding of the OtBu hydrogens
of 3b by the phenyl ring of the HCtCPh ligand suggests that
such a rotamer is the favored one.
(PMe3) (K).26 An X-ray crystallographic analysis of 1-hexyne
adduct J revealed the presence of a symmetrically bound alkyne
ligand (d(Zr-Cint) ) 2.211(3) Å; d(Zr-Cterm) ) 2.244(3) Å;
∆d(Zr-C) ) 0.033(4) Å), with a CtC distance of 1.286(5) Å
and a CtC-C angle of 135.8(3)°. The CtC NMR resonances
of J and K are shifted far downfield from the free alkyne
positions (∆δ ) 60-120), and the Hterm resonance of J is also
shifted significantly downfield upon coordination (∆δ ≈ 5).
These results show that the alkyne ligands in J and K are
symmetrically coordinated, the CtC bonds are not polarized,
but the alkyne is strongly structurally distorted such that the
“alkyne” carbons are sp2 hybridized. The d2 compounds Cp*2-
Ti(MeCtCMe),27 Cp*2Ti(HCtCMe),28 Cp2Nb(Et)(HCtCMe),
and Cp2Nb(H)(HCtCSiMe3)29 display similar properties. These
effects are all characteristic of strong d-π* back-bonding, and
these compounds are best described as d0 metallocyclopropenes.
In contrast, the absence of back-bonding in the d0 metal-alkyne
complexes 3a-g results in weak unsymmetrical coordination,
substantial polarization of the CtC bond, but little perturbation
of the alkyne structure and hybridization of the alkyne carbons.30
Other metal-alkyne complexes with weak or nonexistent d-π*
back-bonding (e.g., B, C, E, F, H, and I; Charts 1 and 2) also
show only minor coordination shifts and thus show a similar
contrast to metal-alkyne complexes with back-bonding.
Comparison of d0 Metal-Alkyne and d0 Metal-Alkene
Complexes. Alkynes bind more strongly to 2 than structurally
similar alkenes do.14 For example, propyne binds to 2 almost
70 times stronger than propylene does, and propargyltrimeth-
ylsilane binds about 60 times stronger than allyltrimethylsilane
does, based on the measured Keq values. Steric effects probably
contribute significantly to this difference, as the linear structure
of alkynes creates less steric crowding with the Cp′2Zr backbone
compared to alkenes. The ligand exchange behavior of alkyne
and alkene complexes of 2 is quite similar. In both cases
substrate decomplexation occurs by displacement by solvent in
CD2Cl2 solution.
Experimental Section
General Procedures. All reactions were performed using
glovebox or Schlenk techniques under a purified N2 atmosphere
or on a high-vacuum line. N2 was purified by passage through
columns of activated molecular sieves and Q-5 oxygen scavenger.
CD2Cl2 and C6D5Cl were distilled from P2O5, while THF was
distilled from Na/benzophenone. The synthesis of 2 was described
previously.14a Other reagents were obtained from standard com-
mercial sources. [Ph3C][B(C6F5)4] and propyne were used as
received. Trimethylsilylacetylene and phenylacetylene were dried
over CaH2. Propargyltrimethylsilane, 2-butyne, and 2-pentyne were
dried over 3 Å molecular sieves. 3-Hexyne was dried over Na and
stored over 3 Å molecular sieves prior to use.
NMR spectra were recorded on Bruker DRX 500 or 400
spectrometers in Teflon-valved NMR tubes at ambient probe
temperature unless otherwise noted. 1H and 13C chemical shifts are
reported relative to SiMe4 and were referenced to the residual
solvent signals. 19F NMR spectra are reported and referenced
relative to external neat CFCl3. NMR probe temperatures were
calibrated by a MeOH thermometer.31 Coupling constants are
reported in Hz. Where 13C-gated-{1H} NMR spectra are reported,
standard 13C{1H} NMR spectra were also recorded to assist in
interpretation.
A free energy diagram comparing propylene and propyne
coordination to 2 is shown in Figure 8. Propyne binds more
strongly than propylene by ca. 1.7 kcal/mol. The barrier for
(26) Buchwald, S. L.; Watson, B. T.; Huffman, J. C. J. Am. Chem. Soc.
1987, 109, 2544.
(27) Cohen, S. A.; Bercaw, J. E. Organometallics 1985, 4, 1006.
(28) Brinkmann, P. H. P.; Luinstra, G. A.; Saenz, A. J. Am. Chem. Soc.
1998, 120, 2854.
(29) Yasuda, H.; Yamamoto, H.; Arai, T.; Nakamura, A.; Chen, J.; Kai,
Y.; Kasai, N. Organometallics 1991, 10, 4058.
(30) Structural and vibrational spectroscopic data for TiII and TiIII alkyne
adducts substantiate the weakening of alkyne coordination upon loss of d
electrons. Arndt, P.; Baumann, W.; Spannenberg, A.; Rosenthal, U.;
Burlakov, V. V.; Shur, V. B. Angew. Chem., Int. Ed. 2003, 42, 1414.
(31) Van Geet, A. L. Anal. Chem. 1970, 42, 679.

Nonchelated ZrIV-Alkoxide-Alkyne Complexes
Organometallics, Vol. 25, No. 14, 2006 3385
-
NMR spectra of ionic compounds contain B(C6F5)4 anion
resonances at the free anion positions, as listed in the Supporting
Information. 19F NMR spectra were obtained for all compounds
that contain this anion. NMR data for free alkynes are given in the
Supporting Information.
1.19 (s, 9H, OtBu), 0.21 (s, 9H, SiMe3). 13C-gated-{1H} NMR
1
(CD2Cl2, -89 °C): δ 127.5 (s, ipso Cp′), 117.4 (d, JCH ) 169,
1
1
Cp′ CH), 115.6 (d, JCH ) 171, Cp′ CH), 114.5 (d, JCH ) 174,
1
2
Cp′ CH), 110.1 (d, JCH ) 173, Cp′ CH), 104.3 (d, JCtCH ) 43,
Cint), 83.4 (s, OCMe3), 65.5 (d, 1JCH ) 244, Cterm), 30.8 (q, 1JCH
)
1
1
General Procedure for Generation of [Cp′2Zr(OtBu)(alkyne)]-
[B(C6F5)4] (3a-g). An NMR tube was charged with 2 (25-50 mg),
and CD2Cl2 (0.5-0.7 mL) was added by vacuum transfer at -78
°C. The tube was shaken at -78 °C, giving a yellow solution. The
tube was cooled to -196 °C, and alkyne (excess) was added by
vacuum transfer. The tube was warmed to -78 °C and shaken,
yielding a yellow solution. The tube was placed in an NMR probe
that had been precooled to -89 °C. NMR spectra were obtained
and showed that a mixture of 3a-g, 2, and free alkene was present.
The detailed procedure for propyne adduct 3a and NMR data for
all compounds are given below. The procedure for generation of
3b-g is similar to that for 3a.
126, OCMe3), 14.8 (q, JCH ) 129, Cp′Me), 12.7 (t, JCH ) 131,
CH2SiMe3), -2.6 (q, JCH ) 121, SiMe3).
1
[Cp′2Zr(OtBu)(MeCtCMe)][B(C6F5)4] (3e). 1H NMR (C6D5-
Cl, -35 °C): δ 5.95 (br s, 2H, Cp′ CH), 5.61 (br s, 2H, Cp′ CH),
1.92 (s, 6H, Cp′Me), 1.55 (s, 6H, MeCtCMe), 0.97 (s, 9H, OtBu).
1
The other Cp′ CH resonances are obscured by signals for 2. H
NMR (CD2Cl2, -89 °C): δ 6.33 (br s, 2H, Cp′ CH, overlaps with
signal for 2), 6.30 (br s, 2H, Cp′ CH), 6.23 (br s, 2H, Cp′ CH),
6.00 (br s, 2H, Cp′ CH), 2.23 (s, 6H, Cp′Me), 2.03 (br s, 6H, MeCt
CMe), 1.23 (s, 9H, OtBu, overlaps with signal for 2). 13C{1H} NMR
(C6D5Cl, -35 °C): δ 118.1 (Cp′ CH), 116.4 (Cp′ CH), 114.4 (Cp′
CH), 111.5 (Cp′ CH), 84.4 (OCMe3), 80.0 (CtC), 31.2 (OCMe3),
14.3 (Cp′Me), 8.8 (MeCtCMe). The ipso Cp′ resonance is obscured
by a solvent resonance. 13C{1H} NMR (CD2Cl2, -89 °C): δ 128.5
(ipso Cp′), 118.2 (Cp′ CH), 116.1 (Cp′ CH), 113.7 (Cp′ CH), 110.7
(Cp′ CH), 84.3 (OCMe3), 79.6 (CtC), 31.1 (OCMe3), 14.5 (Cp′Me),
9.7 (br, MeCtCMe).
Generation of [Cp′2Zr(OtBu)(HCtCMe)][B(C6F5)4] (3a). An
NMR tube was charged with 2 (25.5 mg, 0.0255 mmol), and CD2Cl2
(0.70 mL) was added by vacuum transfer at -78 °C. The tube was
shaken at -78 °C, giving a yellow solution. The tube was cooled
to -196 °C, and propyne (0.0265 mmol) was added by vacuum
transfer. The tube was warmed to -78 °C, shaken at -78 °C, and
placed in an NMR probe that had been precooled to -89 °C. NMR
spectra showed the presence of 3a (0.026 M), 2 (0.0073 M), and
free propyne (0.011 M). Raising the temperature from -89 °C has
the effect of decreasing the concentration of 3a, increasing the
concentrations of 2 and free propyne, and broadening the signals
for 3a and to a lesser extent those of 2 and free propyne. As the
temperature is raised, the signals for 3a eventually coalesce with
those of 2 and free propyne. Data for 3a: 1H NMR (CD2Cl2, -89
°C): δ 6.42 (m, 2H, Cp′ CH), 6.37 (m, 2H, Cp′ CH, overlaps with
signal for 2), 6.19 (m, 2H, Cp′ CH), 6.06 (m, 2H, Cp′ CH), 3.08
(q, J ) 1.8, 1H, tCH), 2.23 (d, J ) 1.5, 3H, tCMe), 2.17 (s, 6H,
Cp′Me), 1.23 (s, 9H, OtBu, overlaps with signal for 2). 13C{1H}
NMR (CD2Cl2, -89 °C): δ 128.2 (ipso Cp′), 118.0 (Cp′ CH), 116.1
(Cp′ CH), 115.1 (Cp′ CH), 110.5 (Cp′ CH), 89.1 (d, 2JCtCH ) 47,
[Cp′2Zr(OtBu)(MeCtCEt)][B(C6F5)4] (3f). 1H NMR (CD2Cl2,
-89 °C): δ 6.35 (br s, 2H, Cp′ CH), 6.29 (br s, 2H, Cp′ CH), 6.23
(br m, 2H, Cp′ CH), 6.00 (br s, 2H, Cp′ CH), 2.23 (s, 6H, Cp′Me),
2.06 (unresolved m, 3H, tCMe, partially overlaps with a resonance
for free 2-pentyne), 1.27 (br t, J ) 8, 3H, CH2Me, partially obscured
by OtBu resonances of 3f and 2), 1.23 (s, 9H, OtBu, overlaps with
resonance for 2). The tCCH2 signal is obscured by the Cp′Me
signal for 2. 13C{1H-gated} NMR (CD2Cl2, -89 °C): δ 128.3 (s,
1
1
ipso Cp′), 118.2 (d, JCH ) 177, Cp′ CH), 116.0 (d, JCH ) 171,
1
1
Cp′ CH), 113.8 (d, JCH ) 174, Cp′ CH), 110.7 (d, JCH ) 174,
Cp′ CH), 84.3 (s, OCMe3), 84.0 (slightly br s, CtC), 81.6 (slightly
br s, CtC), 31.1 (q, 1JCH ) 126, OCMe3), 19.0 (br t, 1JCH ) 135,
tCCH2), 14.6 (q, 1JCH ) 128, Cp′Me), 14.5 (partially obscured br
1
q, CH2Me), 9.9 (br q, JCH ) 134, MeCt).
[Cp′2Zr(OtBu)(EtCtCEt)][B(C6F5)4] (3g). 1H NMR (CD2Cl2,
-89 °C): δ 6.33 (br m, 2H, Cp′ CH), 6.28 (br m, 2H, Cp′ CH),
6.20 (br m, 2H, Cp′ CH), 5.98 (br m, 2H, Cp′ CH), 2.22 (s, 6H,
Cp′Me), 1.29 (br t, J ) 7, 6H, MeCH2Ct), 1.22 (s, 9H, OtBu,
overlaps with OtBu signal of 2). The MeCH2Ct signal is obscured
by the Cp′Me signal of 3g and 2. 13C{1H-gated} NMR (CD2Cl2,
1
Cint), 84.2 (OCMe3), 64.2 (d, JCH ) 251, Cterm), 30.8 (OCMe3),
1
14.8 (Cp′Me), 7.6 (q, JCH ) 135, tCMe).
[Cp′2Zr(OtBu)(HCtCPh)][B(C6F5)4] (3b). 1H NMR (CD2Cl2,
-89 °C): δ 7.83 (d, J ) 7.3, 2H, o-Ph), 7.78 (t, J ) 7.3, 1H,
p-Ph), 7.56 (t, J ) 7.4, 2H, m-Ph), 6.34 (br s, 2H, Cp′ CH, overlaps
with signal for 2), 6.30 (br s, 2H, Cp′ CH), 6.09 (br s, 2H, Cp′
CH), 6.06 (br s, 2H, Cp′ CH), 4.46 (s, 1H, tCH), 2.22 (s, 6H,
Cp′Me), 0.80 (s, 9H, OtBu). 13C{1H} NMR (CD2Cl2, -89 °C): δ
136.3 (m-Ph), 135.8 (p-Ph), 129.6 (o-Ph), 127.6 (ipso Cp′), 117.8
(Cp′ CH), 115.7 (Cp′ CH), 114.4 (Cp′ CH), 110.9 (Cp′ CH), 96.1
1
-89 °C): δ 128.2 (s, ipso Cp′), 118.1 (d, JCH ) 172, Cp′ CH),
1
1
115.8 (d, JCH ) 171, Cp′ CH), 113.8 (d, JCH ) 172.4, Cp′ CH),
1
110.6 (d, JCH ) 181, Cp′ CH), 86.5 (s, CtC), 84.3 (s, OCMe3),
1
1
31.1 (q, JCH ) 127, OCMe3), 19.0 (br t, JCH ) 135, CH2Ct),
14.6 (MeCH2Ct),33 14.6 (q, JCH ) 128, Cp′Me).
1
2
1
(d, JCtCH ) 41, Cint), 83.5 (OCMe3), 75.0 (d, JCH ) 246, Cterm),
30.2 (OCMe3), 15.0 (Cp′Me). The ipso-Ph resonance was not
detected.
Competitive Coordination of Propyne and THF to 2. A
solution of 2 (0.0270 mmol) in CD2Cl2 (0.71 mL) in an NMR tube
was generated as described above. The tube was cooled to -196
°C, and THF (0.0292 mmol, 1.08 equiv) was added by vacuum
transfer. The tube was warmed to -78 °C and shaken to give a
yellow solution. The tube was cooled to -196 °C, and propyne
(0.41 mmol, 15 equiv) was added by vacuum transfer. The tube
was warmed to -78 °C, shaken to give a yellow solution, and then
placed in a precooled NMR probe. NMR spectra recorded at -89
and -38 °C revealed the presence of [Cp′2Zr(OtBu)(THF)]-
[B(C6F5)4] (4; 100%), along with free THF (0.097 equiv) and free
propyne (15 equiv). [Cp′2Zr(OtBu)(HCtCMe)][B(C6F5)4] (3a) was
not observed.
Independent Generation of [Cp′2Zr(OtBu)(THF)][B(C6F5)4]
(4). A solution of 2 (0.0701 mmol) in C6D5Cl (0.6 mL) was cooled
to -196 °C, and THF (0.0717 mmol, 1.02 equiv) was added by
vacuum transfer. The tube was warmed to 22 °C and shaken to
give a yellow-orange solution. NMR spectra showed that 4 had
formed quantitatively. Data for 4: 1H NMR (C6D5Cl): δ 5.94-
5.91 (m, 4H, Cp′ CH), 5.79 (q, J ) 2.5, 2H, Cp′ CH), 5.73 (q, J )
[Cp′2Zr(OtBu)(HCtCSiMe3)][B(C6F5)4] (3c). 1H NMR (CD2-
Cl2, -89 °C): δ 6.36 (br m, 2H, Cp′ CH), 6.33 (br m, 2H, Cp′
CH), 6.24 (br m, 2H, Cp′ CH), 6.02 (br m, 2H, Cp′ CH), 3.47 (s,
1H, CH), 2.14 (s, 6H, Cp′Me), 1.24 (s, 9H, OtBu), 0.44 (s, 9H,
SiMe3). 13C{1H-gated} NMR (CD2Cl2, -89 °C): δ 126.9 (s, ipso
1
1
Cp′), 118.3 (d, JCH ) 177, Cp′ CH), 117.1 (d, JCH ) 170, Cp′
1
1
CH), 114.7 (d, JCH ) 177, Cp′ CH), 109.5 (d, JCH ) 179, Cp′
CH), 94.3 (Cint),32 90.1 (d, JCH ) 244, Cterm), 84.8 (s, OCMe3),
1
1
1
30.9 (q, JCH ) 125, OCMe3), 14.8 (q, JCH ) 129, Cp′Me), -0.5
1
(q, JCH ) 122, SiMe3).
[Cp′2Zr(OtBu)(HCtCCH2SiMe3)][B(C6F5)4] (3d). H NMR
(CD2Cl2, -89 °C): δ 6.33 (br m, 2H, Cp′ CH), 6.31 (br s, 2H, Cp′
CH), 6.13 (br m, 2H, Cp′ CH), 5.99 (br m, 2H, Cp′ CH), 3.46 (br
s, 1H, tCH), 2.13 (s, 6H, Cp′Me), 1.91 (br s, 2H, CH2SiMe3),
1
(32) While the resonance is clear in the 13C{1H} NMR spectrum, the
coupling pattern in the 13C{gated-1H} spectrum is obscured by a free alkyne
resonance.

3386 Organometallics, Vol. 25, No. 14, 2006
Stoebenau and Jordan
2.9, 2H, Cp′ CH), 3.51 (m, 4H, THF), 1.85 (s, 6H, Cp′Me), 1.65
(m, 4H, THF), 1.01 (s, 9H, OtBu). 1H NMR (CD2Cl2, -89 °C): δ
6.36 (br s, 2H, Cp′ CH), 6.31 (br s, 2H, Cp′ CH), 6.15 (br s, 2H,
Cp′ CH), 6.07 (br s, 2H, Cp′ CH), 3.96 (br m, 4H, THF), 2.13 (s,
6H, Cp′Me), 2.10 (br m, 4H, THF), 1.21 (s, 9H, OtBu). 13C{1H}
NMR (C6D5Cl): δ 127.6 (ipso Cp′), 117.6 (Cp′ CH), 116.2 (Cp′
CH), 115.3 (Cp′ CH), 111.2 (Cp′ CH), 78.0 (THF), 31.6 (OCMe3),
25.7 (THF), 14.4 (Cp′Me); the OCMe3 resonance was not detected.
13C{1H} NMR (CD2Cl2, -89 °C): δ 126.9 (ipso Cp′), 117.0 (Cp′
CH), 115.0 (Cp′ CH), 114.9 (Cp′ CH), 109.9 (Cp′ CH), 82.3
(OCMe3), 78.0 (THF), 31.0 (OCMe3), 25.7 (THF), 14.3 (Cp′Me).
and [alkyne]initial were varied. For 3a-c,e-g, Keq values were
determined by direct study of the equilibria in eq 1.
For the equilibrium involving 3a, the relative amounts of 3a, 2,
and free propyne were determined from the integrals of the Hterm
resonance of 3a, the OtBu resonance of 2 after the contribution
from 3a was subtracted, and both free propyne resonances. The
number of moles of 3a and 2 was determined by finding their
respective mole fractions and multiplying by the original amount
of 2 used. The number of moles of free propyne was determined
from the ratio of the integrals of the free and coordinated propyne
resonances. Concentrations were determined by dividing the number
of moles by the solution volume. Keq values for 3a were measured
between -89 and -8 °C.
The methodology for determining the Keq values for 3b-c,e-g
was similar. Calculations of molar amounts and Keq values for 3b-g
were determined as described for 3a. Keq values for 3b-d,f,g were
measured at -89 °C. Keq values for 3e were measured between
-89 and -3 °C. The choices for integrals for the relative amounts
involving 3b,c,e-g are given below.
For the equilibrium involving 3b, relative amounts of 3b, 2, and
free phenylacetylene were determined from the integrations of the
Hterm signal of 3b, the Cp′Me and OtBu resonance after the
contribution from 3b was subtracted for 2, and the Hterm resonance
for free phenylacetylene.
For the equilibrium involving 3c, relative amounts of 3c, 2, and
free trimethylsilylacetylene were determined from the integrations
of the SiMe3 resonance of 3c, the OtBu resonance after the
contribution from 3c was subtracted for 2, and the Hterm resonance
of free trimethylsilylacetylene.
For the equilibrium involving 3e, relative amounts of 3e, 2, and
free 2-butyne were determined from the integrations for coordinated
2-butyne, the OtBu resonance after the contribution from 3e was
subtracted for 2, and the free 2-butyne resonance. Keq values were
found between -89 and -3 °C.
For the equilibrium involving 3f, relative amounts of 3f, 2, and
free 2-pentyne were determined from the most upfield Cp′ CH
resonance of 3f, the OtBu resonance after the contribution from 3f
was subtracted for 2, and both Me resonances of free 2-pentyne.
For the equilibrium involving 3g relative amounts of 3g, 2, and
free 2-hexyne were determined from the two most upfield Cp′ CH
resonances of 3g, the Cp′Me resonance after the contribution from
3g was subtracted for 2, and the Me resonance for free 3-hexyne.
The Keq value for formation of propargyltrimethylsilane adduct
3d in eq 1, Keq,3d ) [3d][2]-1[HCtCCH2SiMe3]-1, was determined
by competition experiments with propyne in CD2Cl2 solution at
-89 °C. The equilibrium constant for the reaction
Influence of [Ph3C][B(C6F5)4] on 2-Butyne Coordination to
2. An NMR tube was charged with 2 (19.3 mg, 0.0193 mmol) and
[Ph3C][B(C6F5)4] (48.9 mg, 0.0530 mmol, 2.75 equiv), and CD2-
Cl2 (0.71 mL) was added by vacuum transfer at -78 °C. The tube
was shaken at -78 °C to give a deep yellow solution and was then
cooled to -196 °C. 2-Butyne (0.0241 mmol, 1.25 equiv) was added
by vacuum transfer. The tube was warmed to -78 °C, shaken, and
placed in an NMR probe that had been precooled to -91 °C. NMR
spectra showed that 3e (0.015 M), 2 (0.013 M), free 2-butyne (0.028
M), and Ph3C+ (0.084 M) were present. The B(C6F5)4- concentra-
tion was 0.11 M (sum of [3e] + [2] + [Ph3C+]). The equilibrium
constant, Keq ) [3e][2]-1[MeCtCMe]-1 ) 41(9) M-1, agrees with
1
the value found without added [Ph3C][B(C6F5)4] (52(3) M-1). H,
19F, and 13C NMR spectra of a CD2Cl2 solution containing [Ph3C]-
[B(C6F5)4] (0.042 M) and 2-butyne (0.037 M) showed that there is
no detectable interaction between these species at -86 °C.
Competitive Coordination of Propargyltrimethylsilane and
Propyne to 2. A solution of 2 (0.0206 mmol) in CD2Cl2 (0.63 mL)
was generated as described above. The tube was cooled to -196
°C, and propargyltrimethylsilane (0.026 mmol) was added by
vacuum transfer. The tube was warmed to -78 °C and shaken,
yielding a yellow solution. The tube was cooled to -196 °C, and
propyne (0.085 mmol) was added by vacuum transfer. The tube
was warmed to -78 °C and shaken, yielding a yellow solution.
The tube was placed in an NMR probe that had been precooled to
-89 °C. NMR spectra showed that a mixture of [Cp′2Zr(OtBu)-
(HCtCCH2SiMe3)][B(C6F5)4] (3d, 0.032 M), [Cp′2Zr(OtBu)(HCt
CMe)][B(C6F5)4] (3a; 0.0017 M), free propargyltrimethylsilane
(0.0089 M), and free propyne (0.13 M) was present. 2 was not
present. This equilibrium was studied in three experiments, in which
the initial concentrations were [2]initial ) 0.033(1) M, [HCt
CMe]initial ) 0.14-0.26 M, and [HCtCCH2SiMe3]initial ) 0.040-
0.089 M, and the order of alkyne addition was reversed in one
experiment.
Reaction of 2 with tert-Butylacetylene. An NMR tube was
charged with 2, and CD2Cl2 (ca. 0.60 mL) was added by vacuum
transfer at -78 °C. The tube was shaken at this temperature, giving
a yellow solution. The tube was cooled to -196 °C, and excess
tert-butylacetylene was added by vacuum transfer. The tube was
warmed to -78 °C, shaken, and placed in an NMR probe cooled
to -89 °C. NMR spectra showed that mixtures of 2 and free tert-
butylacetylene were present. No resonances or line broadening
effects indicative of ligand coordination to 2 were observed.
3a + HCtCCH2SiMe3 / 3d + HCtCMe
was defined to be K1 ) [3d][propyne][3a]-1[HCtCCH2SiMe3]-1
1
and was determined by H NMR studies of this equilibrium. The
relative concentrations of 3d, 3a, free propargyltrimethylsilane, and
free propyne were found from the integrals of the Hterm resonance
of 3d, the Hterm resonance of 3a, the CH2 and SiMe3 resonances of
free propargyltrimethylsilane, and the Me resonance of free propyne.
The equilibrium constant for propyne coordination to 2 to form 3a
is Keq,3a ) [3a][2]-1[HCtCMe]-1 and was determined as described
Determination of Thermodynamic Parameters and Exchange
Rates. (A) General Variable-Temperature NMR Procedure.
NMR samples were prepared as described above. The NMR tube
was stored at -78 °C and placed in a precooled NMR probe. The
probe was maintained at a given temperature for 20-30 min to
allow the sample to reach thermal equilibrium, and then NMR
experiments were conducted. The temperature was raised 5 or 10
°C and the procedure repeated until the desired temperature range
had been studied.
above. Keq,3d is given by Keq,3d ) K1Keq,3a
.
Thermodynamic parameters (∆H° and ∆S°) were determined
from van’t Hoff plots of ln(Keq) versus 1/T for each variable-
temperature NMR run. The reported values are the weighted average
of results from at least three runs in which the initial concentrations
of 2 and alkyne were varied.34
(B) Equilibrium Constant Measurements. Equilibrium con-
stants for eq 1, Keq ) [3][2]-1[alkyne]-1, were determined by
multiple (typically 2-4) H NMR experiments in which [2]initial
(33) While the resonance is clear in the 13C{1H} NMR spectrum, the
coupling pattern in the 13C{gated-1H} spectrum is obscured by the Cp′Me
resonance of 3g.
1

Nonchelated ZrIV-Alkoxide-Alkyne Complexes
Organometallics, Vol. 25, No. 14, 2006 3387
(C) Exchange Rate Measurements. Rate constants for alkyne
decomplexation (k-1) for 3a and 3e were determined by variable-
temperature H NMR spectroscopy. For 3a, the rate of propyne
for 2-butyne decomplexation (k-1) were determined by the formula
k-1 ) π(ω - ω0), where ω is the line width at half-height of the
coordinated 2-butyne resonance. All measurements were made in
the slow exchange region between -48 and -8 °C.
For 3a,e, k-1 values were determined in at least three experiments
with different initial concentrations of 2 and alkyne. The k-1 values
were found to be independent of the concentrations of 2, free alkyne,
and 3a or 3e, consistent with the rate law rate ) k-1[3]. Activation
parameters for decomplexation were determined from Eyring plots
of ln(k-1/T) versus 1/T. The reported values are the weighted
averages of results from at least three runs.34
1
decomplexation was determined from the line shape of the tCH
resonance of coordinated propyne by fitting simulated spectra to
experimental spectra using gNMR to perform an iterative full line
shape analysis.35 The line width at half-height of the benzene
resonance (ω0) was used as the line width in the absence of
4
exchange. The JHH value for propyne was set at 1.8 Hz and
assumed to be temperature-independent. The rate values were
converted to first-order rate constants k-1 by the formula rate )
k-1[3a]. Measurements were made over a temperature range of -69
to +22 °C.
For 3e, rate constants for 2-butyne decomplexation were
determined from the line widths of the coordinated 2-butyne
resonance. The line width at half-height of the benzene resonance
was used as the line width in the absence of exchange (ω0) for
coordinated 2-butyne at each temperature. First-order rate constants
Acknowledgment. We thank the NSF (CHE-0212210) for
financial support and the University of Chicago for a William
Rainey Harper fellowship (E.J.S.).
Supporting Information Available: NMR data for free alkynes
(pdf). This material is available free of charge via the Internet at
http://pubs.acs.org.
(34) The weighted average xj is defined as (∑wixi)/(∑wi) and the weighted
standard deviation (σ) is defined by σ2 ) {∑wi(xi - xj)2}/{∑wi}, where the
weighing factors are wi ) 1/σi2.
(35) gNMR, v. 4.1.2; Adept Scientific: Letchworth, UK, 2000.
OM050947F