Mechanistic aspects of the reactions of bis(pentafluorophenyl)borane with the dialkyl zirconocenes Cp2ZrR2 (R = CH3, CH2SiMe3, and CH2C6H5)

Article abstract of DOI:10.1021/om9802313

The reactions of bis(pentafluorophenyl)borane with simple dialkyl zirconocenes Cp₂ZrR₂ (R = CH₃, CH₂SiMe₃, CH₂Ph) proceed via initial alkyl/hydride exchange to yield Cp₂Zr-(H)R and RB(C₆F₅)₂. Two reaction paths are then followed depending on whether further equivalents of HB(C₆F₅)₂ are present or not. If present, HB(C₆F₅)₂ reacts with the newly formed Zr-H moiety to form dihydridoborate compounds, ultimately yielding Cp₂Zr[(μ-H)₂B-(C₆F₅) ₂]₂, 1, and 2 equiv of RB(C₆F₅)₂. Compound 1 was characterized by X-ray crystallography. In the absence of more HB(C₆F₅)₂, the products of alkyl/hydride exchange react to eliminate RH and produce the borane-stabilized alkylidene compounds Cp₂Zr(μ-CH₂)[(μ-H)B(C₆F₅) ₂], 2, and Cp₂Zr{η³-CH(C₆H ₅)[μ-H)B(C₆F₅)₂]}, 4. The latter compound is formed cleanly in 92percent yield and was characterized by X-ray crystallography. Mechanistic studies on these reactions involving partially deuterated compounds reveal that the alkyl/hyride exchange process is reversible and takes place via a stepwise alkide-abstraction-hydridereplacement sequence rather than a concerted, four-centered σ-bond metathesis type mechanism. This is most convincingly demonstrated by the observed inversion of stereochemistry observed when erythro-Cp₂Zr[CH(D)CH(D)-t-C₄H₉](Cl) (³J_HH = 12.82 ± 0.05 Hz) is treated with excess HB(C₆F₅)₂, producing threo-(C₆F₅)₂B-CH(D)CH(D)-t-C₄H ₉ (³J_HH = 5.00 ± 0.05 Hz). Further experiments reveal a H/D scrambling process involving the borane proton and the C_α-H positions of the zirconium alkyl groups (R = CH₃, CH₂Ph). For example, treatment of Cp₂Zr(CD₂C₆D₅)₂ with 1 equiv of HB(C₆F₅)₂ leads to a mixture of isotopomers of 4 and toluene, including C₆D₅CH₃ and C₆D₅CH₂D, suggesting a scrambling process in which the borane engages in multiple contacts with the metallocene reagent prior to alkane elimination. The HTD scrambling event is proposed to involve hydridoborate attack of the remaining alkyl group on the forming metallocene cation as HB(C₆F₅)₂ abstracts the other alkide ligand. The implications of these mechanistic studies within the realms of metallocene activation and metallocene-catalyzed hydroborations are discussed.

Full text of DOI:10.1021/om9802313

Organometallics 1998, 17, 2459-2469
2459
Mech a n istic Asp ects of th e Rea ction s of
Bis(p en ta flu or op h en yl)bor a n e w ith th e Dia lk yl
Zir con ocen es Cp 2Zr R2 (R ) CH3, CH2SiMe3, a n d CH2C6H5)
Rupert E. v. H. Spence,† Warren E. Piers,*,‡ Yimin Sun,† Masood Parvez,†
Leonard R. MacGillivray,§ and Michael J . Zaworotko§
Department of Chemistry, University of Calgary, 2500 University Drive Northwest,
Calgary, Alberta T2N 1N4, Canada, NOVA Research & Technology Corpoation,
2928 16th Street Northeast, Calgary, Alberta T2E 7K7, Canada, and Department of
Chemistry, Saint Mary’s University, Halifax, Nova Scotia B3H 3C3, Canada
Received March 27, 1998
The reactions of bis(pentafluorophenyl)borane with simple dialkyl zirconocenes Cp2ZrR2
(R ) CH3, CH2SiMe3, CH2Ph) proceed via initial alkyl/hydride exchange to yield “Cp2Zr-
(H)R” and RB(C6F5)2. Two reaction paths are then followed depending on whether further
equivalents of HB(C6F5)2 are present or not. If present, HB(C6F5)2 reacts with the newly
formed Zr-H moiety to form dihydridoborate compounds, ultimately yielding Cp2Zr[(µ-H)2B-
(C6F5)2]2, 1, and 2 equiv of RB(C6F5)2. Compound 1 was characterized by X-ray crystal-
lography. In the absence of more HB(C6F5)2, the products of alkyl/hydride exchange react
to eliminate RH and produce the borane-stabilized alkylidene compounds Cp2Zr(µ-CH2)[(µ-
H)B(C6F5)2], 2, and Cp2Zr{η3-CH(C6H5)[(µ-H)B(C6F5)2]}, 4. The latter compound is formed
cleanly in 92% yield and was characterized by X-ray crystallography. Mechanistic studies
on these reactions involving partially deuterated compounds reveal that the alkyl/hyride
exchange process is reversible and takes place via a stepwise alkide-abstraction-hydride-
replacement sequence rather than a concerted, four-centered σ-bond metathesis type
mechanism. This is most convincingly demonstrated by the observed inversion of stereo-
chemistry observed when erythro-Cp2Zr[CH(D)CH(D)-t-C4H9](Cl) (3J HH ) 12.82 ( 0.05 Hz)
is treated with excess HB(C6F5)2, producing threo-(C6F5)2B-CH(D)CH(D)-t-C4H9 (3J HH ) 5.00
( 0.05 Hz). Further experiments reveal a H/D scrambling process involving the borane
proton and the CR-H positions of the zirconium alkyl groups (R ) CH3, CH2Ph). For example,
treatment of Cp2Zr(CD2C6D5)2 with 1 equiv of HB(C6F5)2 leads to a mixture of isotopomers
of 4 and toluene, including C6D5CH3 and C6D5CH2D, suggesting a scrambling process in
which the borane engages in multiple contacts with the metallocene reagent prior to alkane
elimination. The H/D scrambling event is proposed to involve hydridoborate attack of the
remaining alkyl group on the forming metallocene cation as HB(C6F5)2 abstracts the other
alkide ligand. The implications of these mechanistic studies within the realms of metallocene
activation and metallocene-catalyzed hydroborations are discussed.
In tr od u ction
oxidative addition reaction available to late-transition-
metal catalysts,5 these early-transition-metal systems
utilize exclusively nonoxidative pathways that effect
rapid alkyl/hydride exchange6 between boron and the
metal center for B-C bond formation (Scheme 1). This
exchange process has been categorized as an example
of σ-bond metathesis (σBM) and as such has been as-
The reactions of transition-metal complexes with sec-
ondary boranes R2B-H have received increased atten-
tion in recent years. This interest stems primarily from
the fact that a wide range of organometallic compounds
are effective mediators of the addition of B-H bonds
across unsaturated organic functions, i.e., hydrobora-
tion.1 While the earliest work in this relatively young
area of research focused on the use of late-transition-
metal-based catalysts,2 it has recently been shown that
early transition metals3 and lanthanides4 can also cata-
lyze hydroboration reactions. In the absence of the
(2) For selected examples, see: (a) Mannig, D.; No¨th, H. Angew.
Chem., Int. Ed. Engl. 1985, 24, 878. (b) Evans, D. A.; Fu, G. C.;
Hoveyda, A. H. J . Am. Chem. Soc. 1988, 110, 6917. (c) Burgess, K.;
Ohlmeyer, M. J . Tetrahedron Lett. 1989, 30, 395. (d) Westcott, S. A.;
Blom, H. P.; Marder, T. B.; Baker, R. T. J . Am. Chem. Soc. 1992, 114,
8863.
(3) (a) He, X.; Hartwig, J . F. J . Am. Chem. Soc. 1996, 118, 1696. (b)
Erker, G.; Noe, R.; Wingbermu¨hle, D.; Petersen, J . L. Angew. Chem.,
Int. Ed. Engl. 1993, 32, 1213. (c) Pereira, S.; Srebnik, M. J . Am. Chem.
Soc. 1996, 118, 909. (d) Pereira, S.; Srebnik, M. Organometallics 1995,
14, 3127. (e) Bijpost, E. A.; Duchateau, R.; Teuben, J . H. J . Mol. Catal.
A: Chem. 1995, 95, 121.
* To whom correspondence should be addressed. E-mail:
wpiers@chem.ucalgary.ca.
† NOVA Research and Technology Corp.
‡ University of Calgary.
§ Saint Mary’s University.
(1) Burgess, K.; Ohlmeyer, M. J . Chem. Rev. 1991, 91, 1179.
(4) Harrison, K. M.; Marks, T. J . J . Am. Chem. Soc. 1992, 114, 9220.
S0276-7333(98)00231-3 CCC: $15.00 © 1998 American Chemical Society
Publication on Web 05/13/1998

2460 Organometallics, Vol. 17, No. 12, 1998
Spence et al.
Sch em e 1
sumed to occur via the four-centered transition state
commonly invoked for σBM reactions. The most favored
transition state for σBM involving B-H and early
metal-transition-carbon-bonds leads to formation of
B-C and M-H; generation of alkane and metal boryl
products via an alternative σBM transition state has
not, to our knowledge, been observed in these systems.
Olefin insertion into the resultant M-H bond rapidly
regenerates the metal alkyl species; in the absence of
olefin, excess borane R2B-H can complex M-H, forming
a metal borate complex.5a-b,7 Stoichiometric examples
of alkyl/hydride exchange reactions were observed some
time ago in the pioneering work of Marsella and Caulton
in their study of the reactions of Cp2ZrMe2 with di-
borane8 and have also been documented more recently
in other systems.7b,9 Despite these studies, however, a
detailed picture of the mechanism for alkyl/hydride
exchange between (ETM)-R and B-H has not been
developed.
Many of the more recent studies examining these
types of reactions have focused on relatively unreactive
boronic esters such as catechol borane, not surprising
in light of the reluctance of these boranes to add across
CdC or CtC bonds of their own volition. We, on the
other hand, have been exploring the chemistry of the
highly electrophilic borane bis(pentafluorophenyl)-
borane, HB(C6F5)2,10 in this regard. While this borane
is an extremely effective hydroboration reagent which
requires no aid from a transition-metal catalyst to
perform, it nonetheless is also highly reactive toward
early-transition-metal organometallic compounds. This
stoichiometric reactivity is germane to the realm of
group 4 metal-catalyzed hydroborations. In addition to
relevance to metal-catalyzed hydroboration chemistry,
F igu r e 1. ORTEP drawing of the molecular structure
of 1.
the interactions of highly Lewis acidic pentafluorophen-
yl-substituted boranes with group 4 metal metallocenes
are of interest in the field of homogeneous olefin po-
lymerization.11 We have, therefore, examined the reac-
tions of HB(C6F5)2 with simple dialkyl zirconocenes in
some detail. A preliminary account of the reactions of
Cp2ZrMe2 with HB(C6F5)2 has appearred.12
Resu lts a n d Discu ssion .
A. Syn th etic Ch em istr y. In benzene or toluene,
the reaction of 1 equiv of HB(C6F5)2 with Cp2ZrMe2 gives
an intractable mixture of products. In contrast, addition
of 4 equiv of borane to the zirconocene results in one
major zirconium-containing product as well as 2 equiv
of MeB(C6F5)2 (eq 1). The zirconocene product is the
bis-dihydridoborate complex Cp2Zr[(µ-H)2B(C6F5)2]2, 1,
a colorless white solid with poor solubility in aromatic
hydrocarbons. Compound 1 is characterized by a rela-
tively sharp triplet in the 11B NMR spectrum (1J HB
)
64 Hz). In the 1H NMR spectrum, a resonance centered
around 0.38 ppm consisting of four broad humps sepa-
rated by 64 ( 2 Hz (11B, ∼80% abundant, I ) 3/2)
corroborates the 11B data. After our initial report,12 we
have obtained the solid-state structure of this molecule,
confirming its formulation.
(5) Alkyl and dialkyl boranes can react with late-transition-metal
compounds via nonoxidative pathways: (a) Baker, R. T.; Ovenall, E.
W.; Harlow, R. L.; Westcott, S. A.; Taylor, N. J .; Marder, T. B.
Organometallics 1990, 9, 3028. (b) Baker, R. T.; Calabrese, J . C.;
Westcott, S. A.; Marder, T. B. J . Am. Chem. Soc. 1995, 117, 8777. (c)
Westcott, S. A.; Blom, H. P.; Marder, T. B.; Baker, R. T.; Calabrese, J .
C. Inorg. Chem. 1993, 32, 2175. (d) Hartwig, J . F.; Bhandari, S.; Rablen,
P. R. J . Am. Chem. Soc. 1994, 116, 1839.
(6) In this paper, “alkyl/hydride” exchange refers to an alkyl transfer
from the metal to boron with concomitant transfer of hydride from
boron to the metal. The term “hydride/alkyl” exchange, on the other
hand, refers to the reverse process, i.e., alkyl from boron to metal and
hydride from metal to boron.
(7) (a) Hartwig, J . F.; De Gala, S. R. J . Am. Chem. Soc. 1994, 116,
3661. (b) Lantero, D. R.; Ward, D. L.; Smith, M. R. J . Am. Chem. Soc.
1997, 119, 9699.
(8) Marsella, J . A.; Caulton, K. G. J . Am. Chem. Soc. 1982, 104,
2361.
(9) (a) Motry, D. H.; Smith, M. R., III J . Am. Chem. Soc. 1995, 117,
6615. (b) Sun, Y.; Piers, W. E.; Rettig, S. J . Organometallics 1996, 15,
4110. (c) Hartwig, J . F.; Muhoro, C. N.; He, X. J . Am. Chem. Soc. 1996,
118, 10936. (d) Motry, D. H.; Brazil, A. G.; Smith, M. R., III J . Am.
Chem. Soc. 1997, 119, 2743. (e) Sun, Y.; Spence R. E. v. H.; Piers, W.
E.; Parvez, M.; Yap, G. P. A. J . Am. Chem. Soc. 1997, 119, 5132.
(10) Parks, D. J .; Spence, R. E. v. H.; Piers, W. E. Angew. Chem.,
Int. Ed. Engl. 1995, 34, 809.
Figure 1 shows an ORTEP diagram of the molecule,
while Table 1 gives selected metrical data. The hydro-
gen atoms bridging boron and zirconium were located
on the Fourier difference map and refined in those
positions. The molecule crystallizes with 1 equiv of
benzene in the lattice. Parameters associated with the
Cp ligands and C6F5 rings are unremarkable. The
dihydridoborate ligands are bidentate, with Zr-H dis-
tances of ∼2.0 Å and B-H lengths of ∼1.25 Å. The
Zr-B separations of 2.696(10) and 2.679(10) Å are
somewhat longer than the 2.558(4) Å observed for the
(11) Piers, W. E.; Chivers, T. Chem. Soc. Rev. 1997, 345.
(12) Spence, R. E. v. H.; Parks, D. J .; Piers, W. E.; MacDonald, M.;
Zaworotko, M. J .; Rettig, S. J . Angew. Chem., Int. Ed. Engl. 1995, 34,
1230.

Reactions of Bis(pentafluorophenyl)borane
Organometallics, Vol. 17, No. 12, 1998 2461
Ta ble 1. Selected Bon d Dista n ce a n d An gle Da ta
for Com p lex 1
Sch em e 2
bond lengths (Å)
bond angles (deg)
Zr(1)-C(25)
2.503(9)
2.506(10)
2.501(8)
2.478(8)
2.450(8)
2.454(9)
2.502(9)
2.496(10)
2.497(9)
2.479(9)
H(1)-Zr(1)-H(2)
50.30
159.38
105.24
109.43
55.44
Zr(1)-C(26)
Zr(1)-C(27)
Zr(1)-C(28)
Zr(1)-C(29)
Zr(1)-C(30)
Zr(1)-C(31)
Zr(1)-C(32)
Zr(1)-C(33)
Zr(1)-C(34)
H(1)-Zr(1)-H(3)
H(1)-Zr(1)-H(4)
H(2)-Zr(1)-H(3)
H(2)-Zr(1)-H(4)
H(3)-Zr(1)-H(4)
54.19
H(1)-B(1)-H(2)
H(3)-B(2)-H(4)
C(1)-B(1)-C(7)
C(13)-B(2)-C(19)
91.65
97.32
113.5(7)
113.2(7)
C(1)-B(1)
C(7)-B(1)
C(13)-B(2)
C(19)-B(2)
1.63(1)
1.60(1)
1.61(1)
1.62(1)
C(2)-C(1)-C(6)
113.9(9)
119.6(8)
126.5(8)
113.7(9)
121.9(9)
124.4(9)
113.9(9)
121.0(8)
125.0(8)
114.3(8)
122.8(8)
122.9(8)
C(2)-C(1)-B(1)
C(6)-C(1)-B(1)
C(8)-C(7)-C(12)
C(8)-C(7)-B(1)
C(12)-C(7)-B(1)
C(14)-C(13)-C(18)
C(14)-C(13)-B(2)
C(18)-C(13)-B(2)
C(20)-C(19)-C(24)
C(20)-C(19)-B(2)
C(24)-C(19)-B(2)
trajectory a , Scheme 2) and the intriguing complex
Cp2Zr(µ-H)2(CH2(B(C6F5)2)2), 3, if the borane utilizes
approach b. The bonding interactions in 3 have been
described in detail elsewhere.12,16
Zr(1)-H(1)
Zr(1)-H(2)
Zr(1)-H(3)
Zr(1)-H(4)
2.043
2.054
2.177
1.991
Since formation of 2 is accompanied by loss of
methane, it likely forms from the products of alkyl/
hydride exchange in a way similar to that proposed for
the formation of Tebbe’s reagent.17 According to this
model, Cp2TiCl2 reacts with AlMe3 to effect chloro/alkyl
exchange and then methane elimination takes place
from a six-membered transition state akin to that
depicted in I. The observation that separately synthe-
B(1)-H(1)
B(1)-H(2)
B(2)-H(3)
B(2)-H(4)
Zr(1)-B(1)
Zr(1)-B(2)
1.234
1.193
1.295
1.243
2.696(10)
2.679(10)
same parameter in the bidentate13 borate complex
Cp2Zr(H)(η2-H3BCH3),14 a reflection of the greater steric
congestion in 1. This crowding is apparent in the close
nonbonded approaches between adjacent ortho fluorines
(F(1)-F(16) ) 2.64 Å; F(10)-F(11) ) 2.73 Å) and
partially accounts for the compound’s moderate toler-
ance of air and moisture.
Production of 1 as in eq 1 involves initial alkyl/hydride
exchange between B-H and Zr-CH3 to produce a
zirconium hydride complex. In the presence of excess
borane, the Zr-H moiety is immediately complexed,
forming a borate ligand, a process with precedence in
transition-metal hydride chemistry.5a-b,7 The facility of
this latter process is demonstrated by the fact that 1
can also be prepared conveniently from the oligomeric
dihydride [Cp2ZrH2]n and 2 equiv of HB(C6F5)2.
A clean reaction between Cp2ZrMe2 and 1 equiv of
HB(C6F5)2 occurs under slightly different conditions.
When carried out in hexanes as opposed to toluene, a
nearly totally insoluble brick-red solid precipitates from
the reaction medium while methane gas evolves (identi-
sized [Cp2Zr(H)Me]n and MeB(C6F5)2 react immediately
in C6D6 to produce 218 along with vigorous evolution of
CH4 (eq 2) supports the notion that alkane elimination
takes place after alkyl/hydride exchange in these sys-
tems. This process must be extremely facile since the
1
fied by H NMR, 0.16 ppm). As described previously,12
the product was identified as the HB(C6F5)2-complexed
zirconocene methylidene compound Cp2Zr(µ-CH2)[(µ-
H)B(C6F5)2], 2 (an analogue of Cp2Ti(µ-CH2)(µ-Cl)AlMe2,
Tebbe’s reagent15), by completely characterizing its
PMe3 adduct, 2‚PMe3. Uncomplexed 2 is moderately
stable as a solid but decomposes (t1/2 ≈ 20 min) to a
variety of products in benzene solution. In addition to
reacting with Lewis bases, 2 reacts further with HB-
(C6F5)2 to form mixtures of bis(borate) 1 (via attack from
methyl hydrido zirconocene is an almost completely
insoluble oligomer in benzene.
As the discussion above demonstrates, the effect of
the solvent medium on the result of the reaction
between Cp2ZrMe2 and HB(C6F5)2 is dramatic. Al-
though both outcomes likely involve alkyl/hydride ex-
change as a first step, subsequent reactivity is quite
different, depending on the solvent environment. It is
possible that in the aliphatic hydrocarbon medium the
two products of exchange (i.e., “Cp2Zr(H)Me” and MeB-
(C6F5)2) are not effectively solvated and do not fully
(13) Zr-B distances in tridentate borohydride ligands are substan-
tially shorter, e.g. 2.341(3) Å in Zr(η3-BH4)4: Bird, P. H.; Churchill,
M. R. Chem. Commun. 1967, 403.
(14) Kot, W. K.; Edelstein, N. M.; Zalkin, A. Inorg. Chem. 1987, 26,
1339.
(15) (a) Tebbe, F. N.; Parshall, G. W.; Reddy, G. S. J . Am. Chem.
Soc. 1978, 100, 3611. (b) Klabunde, U.; Tebbe, F. N.; Parshall, G. W.;
Harlow, R. L. J . Mol. Catal. 1980, 8, 37.
(16) Radius, U.; Silverio, S. J .; Hoffmann, R.; Gleiter, R. Organo-
metallics 1996, 15, 3737.
(17) Ott, K. C.; de Boer, E. J . M.; Grubbs, R. H. Organometallics
1984, 3, 223.

2462 Organometallics, Vol. 17, No. 12, 1998
Spence et al.
Sch em e 3
characterized by a broad signal at 2.01 ppm for the
methylene protons, was also generated by transmeta-
lation20 from Cp2Zr(CH2SiMe3)2 and ClB(C6F5)2 to con-
firm its identity.
While the trimethylsilylmethyl group favors the path-
way leading to formation of 1, dibenzyl zirconocene
reacts smoothly and cleanly with 1 equiv of HB(C6F5)2
in the alkane elimination manifold, yielding the ben-
zylidene analogue of
2
Cp2Zr{η3-CH(C6H5)[(µ-H)-
B(C6F5)2]}, 4, and 1 equiv of toluene (eq 4). Sonication
escape from each other after alkyl/hydride exchange.
Perhaps the borane reagent HB(C6F5)2, which is very
poorly hexane soluble, cannot effectively compete with
MeB(C6F5)2, which is hexane soluble, for the methyl
hydrido zirconocene product. Furthermore, the ex-
tremely low solubility of product 2 in hexanes essentially
precludes its further reactions with HB(C6F5)2 (Scheme
3) in this solvent. Likely, each of these factors conspire
to allow for the selective formation of 2 over 1 (and/or
3) in hexane solvent as opposed to aromatic media.
It is worthwhile comparing these results to the
recently disclosed chemistry of Hartwig and co-workers
who have examined the reactions of dimethyltitanocene
with the less electrophilic boranes HB(cat) (cat )
catecholate and substituted catecholates). While much
of this work was done within the context of titanium-
catalyzed alkene hydroboration chemistry,3a the boranes
react with Cp2TiMe2 in the absence of olefins9c with loss
of methane and MeB(cat), producing remarkable borane
σ-complexes of titanocene (eq 3).19 In the absence of
of the reaction mixture was employed to induce the
reaction. Unlike 2, complex 4 is stable in the absence
of Lewis-base donors, due to the internal electronic
stabilization provided by the phenyl group on CR.
Multihapto bonding in the benzyl moiety is suggested
1
by the separate resonances observed in the H and 13C-
{1H} NMR spectra for each of the inequivalent phenyl
hydrogen and carbon atoms of the C6H5 unit. One of
1
the ortho C-H units is shifted upfield both in the H
(4.25 ppm) and 13C{1H} (98.0 ppm) NMR spectra,
suggestive of interaction with the metal center.21 Fur-
1
ther, an increased J CH coupling constant of 133 ( 2
Hz22 for the benzylic C-H bond in comparison to the
1
value of 120 Hz measured for the methylene J CH
coupling constant in Cp2Zr(CH2C6H5)2 and those for
normal sp3-hybridized carbon centers23 is indicative of
a distorted geometry about this carbon atom. A broad
1
resonance at -2.2 ppm in the H NMR spectrum con-
firmed the presence of a B-(µ-H)-M group, while the
chemical shift of the boron atom in the 11B NMR spec-
trum (-16.6 ppm, whh ) 190 Hz) is diagnostic of a four-
coordinate boron center with anionic character.24
The solid-state structure of 4 was determined by
X-ray analysis and confirmed many of the features
deduced from the spectroscopic data; Figure 2 shows an
ORTEP diagram of the compound, while Table 2 gives
pertinent bond distance and angle data. In general
terms, 4 may be formulated as a zwitterionic zircono-
cene25 in which the borate counterion is attached co-
valently to the R-carbon of the alkyl ligand. The
detailed mechanistic studies, it thus appears that in the
reactions of these very different boranes with Cp2TiMe2,
alkyl/hydride exchange is again the initial reaction.
However, formation of the σ-complex suggests that in
the titanium chemistry, reductive elimination of CH4
from in-situ-generated “Cp2Ti(H)Me” (either spontane-
ous or borane-induced) competes effectively with the
other possible reaction channels, i.e., borate formation
or alkane loss involving MeB(cat). Clearly, subtle
effects are at play in the reactions of HBR2 with Cp2M-
(CH3)2 derivatives.
The reactions of bulkier bis(trimethylsilylmethyl)
zirconocene with HB(C6F5)2 provide further support for
an alkyl/hydride-exchange-borane-complexation se-
quence leading to 1 (Scheme 3). In this instance, the
alkyl groups can be selectively removed one at a time
by treatment of the zirconocene with 2 equiv of borane
in sequence. The alkyl borate complex Cp2Zr(CH2-
SiMe3)[(µ-H2)B(C6F5)2] could be isolated or treated with
a further dual portion of borane to yield 1. The alkyl
borane byproduct of this reaction, Me3SiCH2B(C6F5)2,
(20) (a) Fagan, P. J .; Burns, E. G.; Calabrese, J . C. J . Am. Chem.
Soc. 1988, 110, 2979. (b) Fryzuk, M. D.; Bates, G. S.; Stone, C. J . Org.
Chem. 1988, 53, 4425. (c) Cole, T. E.; Quintanilla, R.; Rodewald, S.
Organometallics 1991, 10, 3777. (d) Fagan, P. J .; Nugent, W. A.;
Calabrese, J . C. J . Am. Chem. Soc. 1994, 116, 1880.
(21) Addition of a small amount of THF to benzene solutions of 4
results in formation of an adduct in which the phenyl resonances for
the benzylidene moiety return to a “normal” pattern. Although we have
not fully characterized 4‚THF, it is apparent that the THF donor is
able to displace the multihapto benzylidene ligand.
(22) (a) Because C1 is bonded to a boron atom, its resonance in the
13C
NMR spectrum was broad; measurement of the C1-H coupling
constant was accomplished using the HMQC pulse sequence with gated
decoupling. (b) Robinson V. J .; Bain, A. D. Magn. Reson. Chem. 1993,
31, 865. (c) Bax, A.; Subramanian, S. J . Magn. Reson. 1986, 67, 565.
1
(23) This J CH value is similar to that found in cyclobutane: Gordon,
A. J .; Ford, R. A. The Chemist’s Companion; Wiley: New York, 1972.
1
Cf. also the J CH value of 120 Hz measured for the methylene C-H
(18) As identified by its characteristic 1H NMR spectrum.12 (1H
NMR: 5.70 ppm, 10H; 5.30 ppm, 2H).
(19) See also: Muhoro, C. N.; Hartwig, J . F. Angew. Chem., Int. Ed.
Engl. 1997, 36, 1510.
bonds in Cp2Zr(CH2C6H5)2.
(24) Kidd, R. G. In NMR of Newly Accessible Nuclei; Laszlo, P., Ed.;
Academic Press: New York, 1983; Vol. 2.
(25) Piers, W. E. Chem. Eur. J . 1998, 4, 13.

Reactions of Bis(pentafluorophenyl)borane
Organometallics, Vol. 17, No. 12, 1998 2463
plane of the metallocene which bisects the two Cp rings.
Indeed, the centroid of C1-C2-C3 essentially lies in
this plane, a further point in favor of an η3-benzyl
bonding description.
The multihapto benzylidene unit affects the reactivity
of 4 with further equivalents of HB(C6F5)2. By blocking
the other face of the alkylidene unit, the coordinated
phenyl group effectively precludes attack of the borane
via approach b (eq 5) and formation of the benzylidene
analogue of 3. Consequently, the only zirconium prod-
F igu r e 2. ORTEP drawing of the molecular structure of
4.
uct observed in this reaction is 1. The diboryl toluene
compound shown in eq 5 is presumed to be the other
major product, on the basis of its NMR spectra (see
Experimental Section), particularly the very broad28
(whh ≈ 850 Hz) 11B chemical shift of 74.0 ppm diag-
nostic for neutral borane centers.24
Ta ble 2. Selected Bon d Dista n ce a n d An gle Da ta
for Com p lex 4
bond lengths (Å)
bond angles (deg)
Zr-Cp1
2.509(8)
2.516(8)
2.492(9)
2.526(10)
2.506(9)
2.518(12)
2.477(13)
2.413(17)
2.492(22)
2.532(15)
Zr-C1-C2
82.2(6)
87.6(6)
Zr-Cp2
Zr-Cp3
Zr-Cp4
Zr-Cp5
Zr-Cp6
Zr-Cp7
Zr-Cp8
Zr-Cp9
Zr-Cp10
Zr-C1-B
C2-C1-B
133.5(8)
117.3(9)
126.3(9)
116.2(8)
122.1(9)
119.0(10)
118.5(10)
122.9(10)
121.2(9)
113.8(8)
124.0(9)
107.7(8)
107(4)
B. Mech a n istic Stu d ies. The poor solubility of HB-
(C6F5)2 in solvents it does not react with tends to
preclude detailed kinetic studies of these reactions. The
stoichiometric synthetic studies described above, par-
ticularly those involving Cp2ZrMe2, point to an initial
exchange reaction followed by competing processes
leading either to borate 1 or Tebbe-type compounds such
as 2 and 4. While formation of borate ligands from
metal hydrides and HB(C6F5)2 seems a straightforward
process, the nature of the alkyl/hydride exchange and
the alkane elimination step leading to 2 and 4 are
unclear. Experiments making use of deuterium labeling
provide fundamental insights into these processes.
I. Alk yl/Hyd r id e Exch a n ge. 1,2-Dideuterated neo-
hexyl groups have been used extensively as probes for
determining the character of a reaction by observing the
effect of the transformation on the stereochemistry of
this group.29 Through hydrozirconation,30 organozirco-
nium derivatives of such probes are readily prepared
with >95% diastereospecificity.31 We, thus, prepared
erythro-Cp2Zr[CH(D)CH(D)-t-C4H9](Cl) as a model sub-
strate for the alkyl/hydride exchange reaction with HB-
(C6F5)2. This compound reacts rapidly with an excess
(2.5-3 equiv) of HB(C6F5)2 to give monoborate species
Cp2Zr[(µ-H)2B(C6F5)2]Cl, 5, as the zirconium-containing
product. Compound 5’s identity was confirmed via its
independent synthesis from [Cp2Zr(Cl)H]n (Schwartz’s
reagent) and HB(C6F5)2 (see Experimental Section). As
C1-C2-C3
C1-C2-C7
C3-C2-C7
C2-C3-C4
C3-C4-C5
C4-C5-C6
C5-C6-C7
C2-C7-C6
C1-B-C20
C1-B-C30
C20-B-C30
Zr-HB-B
Zr-C1
C1-C2
C1-B
2.341(10)
1.456(12)
1.545(15)
1.661(15)
1.612(15)
2.02(7)
C20-B
C30-B
Zr-HB
B-HB
C2-C3
C2-C7
C3-C4
C4-C5
C5-C6
C6-C7
Cpcent-Zr-Cpcent
127.10(5)
1.35(8)
1.408(14)
1.421(15)
1.416(15)
1.411(18)
1.372(18)
1.365(16)
electron-deficient zirconium center gains electron den-
sity from a µ-hydride ligand shared with the boron
center and the multihapto benzyl unit. The Zr-C1
distance of 2.341(10) Å is typical of a normal Zr-Calkyl
linkage,26 and the Zr-C2 length of 2.583 Å, while
longer, is within the range (2.627-2.648 Å) of M-Cipso
lengths observed in known η2-benzyl complexes of
zirconium.27 Although the Zr-C3 distance is longer still
at 2.737 Å, the perturbations in the chemical shifts of
the C3-H unit discussed above argue for an η3-bonding
description. The geometery about C1 is indeed dis-
torted, as evidenced by the acute Zr-C1-C2 and Zr-
C1-B angles of 82.2(6)° and 87.6(6)°, respectively. The
B-C1-C2 angle is 133.5(8)° because C2 dips below the
1
the H{2H} NMR spectra in Figure 3 demonstrate, the
stereochemistry associated with the 1,2-d2-neohexyl
probe in the zirconium starting material (A) undergoes
clean inversion (>95%) to yield threo-(C6F5)2B-CH(D)-
(28) Due to the quadrupolar nature of the boron nucleus, extremely
broad 11B resonances are characteristic of 1,1-diboryl compounds.
(29) (a) Bock, P. L.; Boschetto, D. J .; Rasmussen, J . R.; Demers, J .
P.; Whitesides, G. M. J . Am. Chem. Soc. 1974, 96, 2814. (b) Igau, A.;
Gladysz, J . A. Organometallics 1991, 10, 2327. (c) Piers, W. E. J . Chem.
Soc. Chem. Commun. 1994, 309.
(30) Labinger, J . A. In Comprehensive Organic Synthesis; Trost, B.
M., Ed.; Pergamon: New York, 1991; Vol. 8, p 667.
(31) Labinger, J . A.; Hart, D. W.; Seibert, W. E.; Schwartz, J . J .
Am. Chem. Soc. 1975, 97, 3851.
(26) (a) Hunter, W. E.; Hrncir, D. C.; Vann Bynum, R.; Penttila, R.
A.; Atwood, J . L. Organometallics 1983, 2, 750. (b) J ordan, R. F.;
Bajgur, C. S.; Willett, R.; Scott, B. J . Am. Chem. Soc. 1986, 108, 7410.
(27) (a) J ordan, R. F.; LaPointe, R. E.; Bajgur, C. S.; Echols, S. F.;
Willett, R. J . Am. Chem. Soc. 1987, 109, 4111. (b) J ordan, R. F.;
LaPointe, R. E.; Baenziger, N.; Hinch, G. D. Organometallics 1990, 9,
1539. (c) Bochmann, M.; Lancaster, S. J . Organometallics 1993, 12,
633. (d) Bochmann, M.; Lancaster, S. J .; Hursthouse, M. B.; Abdul
Malik, K. M. Organometallics 1994, 13, 2235.

2464 Organometallics, Vol. 17, No. 12, 1998
Spence et al.
Sch em e 4
Sch em e 5
1
F igu r e 3. (A) Partial H NMR spectrum of erythro-Cp2-
Zr[CH(D)CH(D)-t-C4H9](Cl). (B) Partial 1H NMR spectrum
of the product mixture obtained from reaction of erythro-
Cp2Zr[CH(D)CH(D)-t-C4H9](Cl) and excess HB(C6F5)2. (C)
1
Partial H NMR spectrum of erythro-(C6F5)2B-CH(D)CH-
(D)-t-C4H9 formed from reaction of HB(C6F5)2 and E-1-d1-
3,3-dimethyl-1-butene. All spectra were recorded at 300
MHz.
this orbital, two possibilities which have been suggested
for this process.32 If this mechanism for alkyl/hydride
exchange is general, there are also ramifications of this
result vis-a`-vis early-transition-metal catalyzed hy-
droborations, particularly asymmetric reductions where
the stereochemistry at CR is at issue. It should be noted
that for less electrophilic boranes the alkide abstraction
pathway may be disfavored and the concerted σBM
pathway dominant; this is a question that remains to
be addressed.
CH(D)-t-C4H9 (3J HH ) 5.00 ( 0.05 Hz) as the boron
exchange product (B). For comparison, the top trace
in Figure 3 (C) is that of erythro-(C6F5)2B-CH(D)CH-
(D)-t-C4H9 (3J HH ) 11.30 ( 0.05 Hz), independently
generated from E-1-d1-3,3-dimethyl-1-butene and DB-
(C6F5)2.
This result has significant mechanistic implications
for the alkyl/hydride exchange step in these systems. A
concerted, four-centered σBM-type mechanism would be
expected to result in retention of configuration in the
probe (Scheme 4). The observation of inversion is
suggestive of a stepwise process in which the borane
abstracts via backside attack of the alkyl group and
replaces it with a hydride ligand from the resulting
borate counterion, as depicted on the left-hand side of
Scheme 4. Abstraction of alkide groups from alkyl
zirconocenes by B(C6F5)3 is a well-known method for
generating metallocene cations.27d,32 The fact that
abstraction using HB(C6F5)2 results in an inversion of
stereochemistry at the abstracted carbon atom supports
electrophilic attack on the back lobe of the Zr-C σ
bonding orbital instead of in the internuclear region of
Alkide abstraction from Cp2ZrMe2 and derivatives
using B(C6F5)3 has been shown to be rapidly revers-
ible;33 not surprisingly, therefore, the alkyl/hydride
exchange reactions involving HB(C6F5)2 are also revers-
ible. When [Cp2Zr(Cl)H]n is treated with H3CB(C6F5)2,
a product mixture is observed which can only form after
hydride/alkyl exchange, which is the reverse of alkyl/
hydride interchange6 (Scheme 5). For example, the
primary zirconium-containing product in this reaction
is 5, which forms from Schwartz’s reagent and the HB-
(C6F5)2 generated from hydride/alkyl exchange. A sec-
ond product, whose appearance coincides with the
elimination of CH4, we assign as the chloride-bridged
analogue of 2, i.e., [Cp2Zr(µ-CH2)(µ-Cl)B(C6F5)2], on the
1
basis of its H (5.75 ppm, 10H; 2.97 ppm 2H) and 11B
(-4.0 ppm, cf. -1.1 ppm for 2) NMR spectra. We have
(32) Yang, X.; Stern, C. L.; Marks, T. J . J . Am. Chem. Soc. 1994,
116, 10015.
(33) Deck, P. A.; Marks, T. J . J . Am. Chem. Soc. 1995, 117, 6128.

Reactions of Bis(pentafluorophenyl)borane
Organometallics, Vol. 17, No. 12, 1998 2465
not been able to synthesize this compound indepen-
dently, and thus, this assignment is tentative. The
presence of 5 and CH4 in the product mixture of this
reaction, however, is strong evidence for an antipodal
hydride/alkyl exchange and shows that these processes
are reversible under mild conditions. The rapid revers-
ibility of this exchange is crucial for explaining the
observed H/D scrambling phenomenon described in the
next section.
II. H/D Scr a m blin g. Crossover experiments aimed
at probing the nature of the alkane elimination step
were performed utilizing specifically deuterated re-
agents. The results of these experiments failed to
address this point conclusively (vide infra) but revealed
instead a H/D scrambling process which implies another
level of complexity in the reactions of Cp2ZrR2 with HB-
(C6F5)2. Reaction of HB(C6F5)2 with Cp2Zr(CD2C6D5)2
gives a complex mixture of isotopomers (eq 6), with the
proton label distributed randomly among all possible
locations except the phenyl group.34 Significantly, all
F igu r e 4. Partial 400 MHz 1H NMR spectrum (toluene
methyl region) of the product mixture formed in the
reaction of Cp2Zr(CD2C6D5)2 and HB(C6F5)2.
three possible proton isotopomers of toluene are ob-
served in the product mixture,35 as demonstrated by the
1
partial 400 MHz H NMR spectrum shown in Figure 4
(toluene methyl region).36 This observation leads to the
conclusion that the borane engages in multiple contacts
with the metallocene prior to alkane elimination and
generation of products. Consistent with this is the
observation that when Cp2Zr(CD2C6D5)2 is treated with
only 0.5 equiv of HB(C6F5)2, a detectable amount of the
proton label is incorporated into the benzylic positions
(δ 1.82 ppm) of unreacted Cp2Zr(CD2C6D5)2. In a
separate experiment, it was determined that the ben-
zylic protons of free toluene do not exchange (at room
temperature or at 50 °C) with any of the protons of 4
by treating d7-4 with perproteo toluene. Thus, the
protons are not scrambled in some process involving free
toluene. Finally, the inverse experiment, i.e., the reac-
tion of Cp2Zr(CH2C6H5)2 with DB(C6F5)2, yields similar
results in that all benzylic positions plus the bridging
F igu r e 5. Partial 400 MHz 1H NMR spectrum of the
reaction between [Cp2Zr(H)Me]n and CD3B(C6F5)2 showing
the mixture of methane isotopomers produced. Portions of
the mulitplets due to the CD2H2 (5 lines) and CD3H (7 lines)
isotopomers are obscured due to overlapping.
Scrambling of the deuterium label is also observed
when one starts with the products of alkyl/hydride
exchange; thus, in the reaction of [Cp2Zr(H)Me]n with
CD3B(C6F5)2, the full range of proton isotopomers of
methane is produced (Figure 5). While the CDH3 and
CD3H products may be accounted for by “simple”
methane elimination, the presence of CH4 and CD2H2
can only be explained by invoking H/D scrambling and
multiple contacts between the two reagents. On the
surface, this experiment suggests that the scrambling
event occurs in the alkane elimination step after alkyl/
hydride exchange; however, the facile reversibility of the
alkyl/hydride and alkyl/alkyl exchanges must be con-
sidered before reaching this conclusion.
2
hydride contain deuterium, as determined by H{1H}
NMR spectroscopy.
(34) (a) H/D exchange between the alkyl groups and the Cp ligands
of Cp2ZrR2 compounds occurs at higher temperatures (≈165 °C).34b In
none of the experiments described do we observe involvement of the
Cp ligands. (b) Razuvaev, G. A.; Mar’in, V. P.; Andrianov, Y. A. J .
Organomet. Chem. 1979, 174, 67.
(35) Toluene-d8 may also have been present, but no attempt was
made to detect and quantify this isotopomer.
(36) The identity of CHD2C6D5 was confirmed by its separate
synthesis from KCD2C6D5 and H2O; quenching KCH2C6H5 with D2O
gave an essentially identical pattern as that observed for CH2DC6D5.

2466 Organometallics, Vol. 17, No. 12, 1998
Spence et al.
Sch em e 6
Consideration was also given to a H/D scrambling
mechanism involving hydride transfer from the newly
formed hydridoborate to the abstracted carbon with
concomitant loss of a deuteron to the metal center, i.e.
II. This process resembles internal hydride and alkyl40
transfers observed in R-halo boronic esters. Two ob-
servations mitigate against such a mechanism for the
H/D scrambling observed in the reactions described
here. First, no deuterium incorporation into the MeB-
(C6F5)2 product is observed when Cp2Zr(CH3)Cl is
treated with an excess of DB(C6F5)2. Second, if a
mechanism involving II was operative, the experiments
described above involving erythro-Cp2Zr[CH(D)CH(D)-
t-C4H9](Cl) would have led to at least some (C6F5)2B-
CH2CH(D)-t-C4H9, which was not observed (see Figure
3). Thus, when the other metallocene girdle ligand is
not an alkyl, as in these experiments where L ) Cl,
scrambling cannot occur and the alkyl/hydride exchange
is completed by direct transfer of the borate hydride to
the zirconium.41
Referring back to Scheme 6, after alkyl/hydride ex-
change and one H/D scrambling event, the initial prod-
uct mixture of isotopomers can undergo irreversible
elimination of toluene (in this case the PhCD3 or
PhCD2H isotopomers) or one of two exchanges: hydride/
alkyl exchange (path b) or alkyl/alkyl exchange (path
c). The latter event leads to the same mixture of toluene
isotopomers upon elimination, but the former leads to
dibenzyl zirconocene with a proton in the benzylic
position which was formerly fully deuterated. This
Cp2Zr(CDHPh)(CD2Ph) isotopomer is now able to re-
enter the process (top of the Scheme 6) to interact with
another equivalent of HB(C6F5)2, accounting for the
observation of PhCDH2 and PhCH3 in the mixture of
toluenes produced. Note that entry into the process can
also occur at the bottom of the Scheme 6, accommodat-
A plausible explanation for these observations is de-
picted in Scheme 6, using the experiment of eq 6 as an
example. As the benzyl group is abstracted from the
zirconium center, the developing cationic center is
stabilized by an R-agostic interaction. The hydrido-
borate counterion does not completely dissociate in this
model but, instead, nucleophilically attacks the R-carbon
atom; in so doing, the hydride displaces the agostic
deuteron at CR and alkyl/hydride exchange (a) is com-
plete along with one H/D scrambling event. Although
nucleophilic attack of an alkyl ligand at CR is rare,
halides have been shown to do this in some systems.29a,37
In this case, the alkyl group is rendered susceptible to
attack by the developing R-agostic interaction, and thus,
another way to view this process is as nucleophilic
attack on an incipient alkylidene ligand, which has
ample precedent.38 Of course, complete R elimination
to yield a cationic alkylidene hydride is precluded in this
system since the zirconium center is of d0 electron
configuration. As a fully concerted process, however,
the H/D exchange event is not unlike the modified
Green-Rooney mechanism for the insertion of olefins
into d0 metal-carbon bonds,39 in which an R-agostic
interaction stablizes the transition state of olefin inser-
tion without undergoing complete elimination.
(40) (a) Matteson, D. S.; Kandil, A. A.; Soundararajan, R. J . Am.
Chem. Soc. 1990, 112, 3964. (b) Matteson, D. S.; Majumdar, D. J . Am.
Chem. Soc. 1980, 102, 7588. (c) Matteson, D. S.; Ray, R.; Rocks, R. R.;
Tsai, D. J . Organometallics 1983, 2, 1536.
(41) A third possibility involving proton scrambling into the R
position of the alkyl ligand in the alkyl hydride product of exchange
(i.e., Cp2Zr(CD2Ph)H) via a σ-complex was proposed as the most likely
mode of scrambling by a referee. We also considered this but rejected
the possibility on the basis of the following considerations. While
precedent for this mode of scrambling certainly exists, in the systems
in which it has been observed,40a,b reductive elimination of RH is highly
endothermic and scrambling occurs only at or near the high temper-
atures required for loss of RH. In the absence of Lewis bases,40c this
scenario would also be the case for Cp2Zr(R)H and none of the
experiments in which we observe facile scrambling were performed
anywhere near the temperatures which would be required to induce
reductive elimination of alkane from Cp2Zr(R)H. Furthermore, in cases
where selectively deuterated Cp2Zr(R)X (X ) H or D) have been
prepared,40c,d,e deuterium scrambling of the type that would require
the intermediacy of a σ-complex would appear to be very slow at room
temperature. (a) Buchanan, J . M.; Stryker, J . M.; Bergman, R. G. J .
Am. Chem. Soc. 1986, 108, 1537. (b) Parkin, G.; Bercaw, J . E.
Organometallics 1989, 8, 1172. (c) Gell, K. I.; Schwartz, J . J . Am. Chem.
Soc. 1981, 103, 2687. (d) Gell, K. I.; Schwartz, J . J . Am. Chem. Soc.
1978, 100, 3246. (e) Gell, K. I.; Postin, B.; Schwartz, J .; Williams, G.
M. J . Am. Chem. Soc. 1982, 104, 1846.
(37) (a) Halpern, J .; Chan, M. S.; Hanson, J .; Roche, T. S.; Topich,
J . A. J . Am. Chem. Soc. 1975, 97, 1606. (b) Magnuson, R. H.; Halpern,
J .; Levitin, I. Y.; Vol’pin, M. E. J . Chem. Soc. Chem. Commun. 1978,
44.
(38) Casey, C. P.; Miles, W. M. J . Organomet. Chem. 1983, 254, 333.
(39) (a) Brookhart, M.; Green, M. L. H.; Wong, L. Prog. Inorg. Chem.
1988, 36, 1. (b) Piers, W. E.; Bercaw, J . E. J . Am. Chem. Soc. 1990,
112, 9406. (c) Krauledat, H.; Brintzinger, H. H. Angew. Chem., Int.
Ed. Engl. 1990, 29, 1412. (c) Leclerc, M. K.; Brintzinger, H. H. J . Am.
Chem. Soc. 1995, 117, 1651. (e) Grubbs, R. H.; Coates, G. W. Acc. Chem.
Res. 1996, 29, 85.

Reactions of Bis(pentafluorophenyl)borane
Organometallics, Vol. 17, No. 12, 1998 2467
ing the scrambling observed in the reaction of [Cp2Zr-
(H)Me]n with CD3B(C6F5)2. In this case, only one H/D
scrambling event is necessary to allow for production
of CH4 and CH2D2 in the product mixture.
III. Alk a n e Elim in a tion . The loss of alkane from
Cp2Zr(H)R and RB(C6F5)2 can occur via a six-membered
transition state (III) similar to what has been proposed
for the alkane elimination step in the formation of
Tebbe’s reagent (I above).17 An alternative view of this
the metal or via a H/D scrambling nucleophilic attack
at the R-carbon of the second alkyl group on the metal,
if present. When excess borane is present, Zr-H func-
tions are trapped to form very stable hydridoborate com-
plexes. In the absence of borane, the products of alkyl/
hydride exchange, namely “Cp2Zr(H)R)” and RB(C6F5)2,
react together to eliminate alkane and produce com-
pounds that are essentially HB(C6F5)2-complexed al-
kylidenes of zirconocene.
The alkide abstraction pathway represents a distinct
nonoxidative mechanism for B-C bond formation in the
reaction of metal carbon bonds with a secondary bo-
ranes. The other previously observed mechanism,
namely σ-bond metathesis,4,5d is fundamentally different
in terms of the effect on the stereochemistry of the
R-carbon. It is, therefore, crucial to determine which
mechanism is operative in this step for catalyzed
hydroborations where the stereochemistry at CR is at
issue; this study helps to delineate the factors which
cause one mechanism to dominate over the other. On
the basis of this work, it appears that less electrophilic
boranes engage in concerted, four-centered σ-bond me-
tathesis and more electrophilic boranes, such as HB-
(C6F5)2, utilize alkide abstraction as the primary mode
of B-C bond formation. Caution must be taken in
applying this generalization, since the Lewis acidity of
the conjugate acid of LnM-R (i.e., LnM+) will likely also
influence the favorability of each pathway. Further
studies are necessary to address these questions in
detail.
elimination would invoke deprotonation of the boron
methyl group as in IV, not unreasonable given the
ability of boron to stabilize R-carbanions.42 To distin-
guish between these two possibilities, the reaction of in-
situ-generated PhCH2B(C6F5)2 with [Cp2Zr(H)Me]n was
carried out; elimination through III would be expected
to produce toluene, while elimination via IV would
result in the expulsion of methane. In fact, as shown
in eq 7, this experiment does not provide a definitive
Exp er im en ta l Section
Gen er a l. General procedures have been described in detail
elsewhere.43 Routinely employed solvents (hexanes, toluene,
and THF) were purified using the Grubbs purification sys-
tem.44 For the NMR data reported for new compounds,
19F-19F couplings are not given nor are the 13C data for the
pentafluorophenyl groups; the former were in all cases unre-
markable, while the latter were not always detectable. The
reagents Cp2ZrCl2, PMe3, and [Cp2Zr(H,D)Cl]n were purchased
from Aldrich and used as received. Literature methods were
used to prepare [Cp2ZrH2]n,45 [Cp2Zr(H)Me]n,46 Cp2ZrMe2,26a
result since both methane and toluene are observed as
products, along with corresponding amounts of 2 and
4. This could mean that both transition states are
accessible, but in light of the facile alkyl/alkyl exchange
in these systems (see Scheme 6), it seems likely that
equilibration occurs before elimination from either III
or IV. Thus, these experiments cannot distinguish
between the two possibilities, although a significant
preponderance of 4 over 2 in the product mixture (∼3:
1) points toward IV as the preferred transition state for
elimination.
49
Cp2Zr(CH2SiMe3)2,47 Cp2Zr(CH2C6H5)2,48 KCX2C6X5 (X ) H,
D), XB(C6F5)2 (X ) H/D,10 Cl50), and erythro-Cp2Zr[CH(D)CH-
(D)-t-C4H9](Cl)31 and compounds 1, 2, 2‚PMe3, and 3.12
P r ep a r a tion of Cp 2Zr {η3-CH(C6H5)[(µ-H)B(C6F 5)2]}, 4.
To a flask containing Cp2Zr(CH2C6H5)2 (403 mg, 1 mmol) and
HB(C6F5)2 (346 mg, 1 mmol) was added dry hexane (30 mL).
The reaction suspension was sonicated for 20 min and then
stirred for a further 1 h at room temperature. Over this time
the yellow zirconocene starting material was replaced by an
orange precipitate. This solid was isolated by filtration,
washed with hexane, and pumped dry under vacuum. The
Su m m a r y a n d Con clu d in g Rem a r k s
The reactions of the highly electrophilic borane HB-
(C6F5)2 with even the simplest of the bent metallocenes,
Cp2ZrR2, are striking for their complexity and vari-
ability with respect to changes in reaction conditions
and the nature of R. We have shown that reversible
alkyl/hydride exchange dominates in the initial stages
of these reactions and that the intimate mechanism of
this process involves alkide abstraction followed by
replacement with a hydride from the resulting hydri-
doborate counterion. The latter can occur directly to
(43) Spence, R. E. v. H.; Piers, W. E. Organometallics 1995, 14, 4617.
(44) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J . Organometallics 1996, 15, 1518.
(45) Wailes, P. C.; Weigold, H. J . Organomet. Chem. 1970, 24, 405.
(46) J ordan, R. F.; Bajgur, C. S.; Dasher, W. E.; Rheingold, A. L.
Organometallics 1987, 6, 1041.
(47) J effery, J .; Lappert, M. F.; Luong-Thi, N. T.; Webb, M.; Atwood,
J . L.; Hunter, W. E. J . Chem. Soc., Dalton Trans. 1981, 1593.
(48) (a) Fachinetti, G.; Fochi, G.; Floriani, C. J . Chem. Soc., Dalton
Trans. 1977, 1946. (b) Bradley, P. B.; Scotton, M. J . J . Chem. Soc.,
Perkin Trans. 2 1981, 419.
(49) Schlosser, M.; Hartmann, J . Angew. Chem., Int. Ed. Engl. 1973,
12, 508.
(50) (a) Chambers, R. D.; Chivers, T. J . Chem. Soc. 1965, 3933. (b)
Spence, R. E. v. H.; Piers, W. E.; MacGillivray, L. R.; Zaworotko, M. J .
Acta Crystallogr., Sect. C 1995, C51, 1688.
(42) Krishnamurthy, S.; Brown, H. C. J . Org. Chem. 1980, 45, 849.
(b) Yoshida, T.; Negishi, E. J . Chem. Soc., Chem. Commun. 1974, 763.
(c) Pelter, A.; Smith, K.; Brown, H. C. Borane Reagents; Academic
Press: New York, 1988; pp 22-23.

2468 Organometallics, Vol. 17, No. 12, 1998
Spence et al.
yield of compound 4 was 602 mg, 0.92 mmol, 92%. 1H NMR
(C6D6): 7.14 (m, 1H, C6H5), 6.86 (m, 1H, C6H5), 6.68 (m, 2H,
C6H5), 5.03 (s, 5H, C5H5), 4.43 (s, 5H, C5H5), 4.25 (m, 1H, C6H5),
2.43 (s, 1H, PhCH), -2.2 (br, 1H, µ-H). 13C{1H} NMR (C6D6):
139.2 (ipso), 138.5, 133.3, 129.2, 123.8, 98.0 (C6H5), 109.4, 106.9
(C5H5), 69 (br, PhCH). 11B{1H} NMR (C6D6): -16.6. 19F{1H}
NMR (C6D6) -128.7 (1F), -130.8 (3F), -159.0 (1F), -159.9
(1F), -164.4 (1F), -164.6 (1F), -164.9 (2F). Anal. Calcd for
Ta ble 3. Su m m a r y of Da ta Collection a n d
Str u ctu r e Refin em en t Deta ils for a n d 1 a n d 4
1
4
formula
fw
cryst syst
a, Å
b, Å
c, Å
C
41H22B2F20Zr
C29H17BF10Zr
657.46
monoclinic
13.9296(15)
8.5508(9)
21.5196(18)
94.13(3)
2556.5(4)
P21/c
1007.43
monoclinic
14.042(5)
18.776(5)
15.567(4)
105.46(2)
3995(1)
P21/n
C
29H17F10ZrB: C, 52.98; H, 2.61. Found: C, 53.33; H, 2.65.
IR (KBr, Nujol, cm-1): 1638 m, 1592 w, 1514 s, 1339 w, 1283
w, 1111 w, 1091 s, 1016 w, 972 s, 895 w, 824 s, 812 s, 766 m.
Rea ction of 4 w ith HB(C6F 5)2. Compound 4 (10 mg, 0.015
mmol) was loaded into a 5 mm NMR tube along with
HB(C6F5)2 (16 mg, 0.045 mmol) and C6D6 (0.7 mL). The tube
was capped, shaken, and briefly heated (∼50 °C) until the
contents of the tube were dissolved. The tube was gently
heated for a further 10 min, until production of 1 was complete.
The C6D6 was removed in vacuo, and the solid was extracted
with 2 × 2 mL of hexanes. Removal of the hexanes and
redissolution of the resulting solid in C6D6 gave a sample
enriched in the borane coproduct, C6H5CH[B(C6F5)2]2. 1H
NMR (C6D6): 6.96 (m, 2H), 6.84 (m, 2H), 6.77 (m, 1H), 4.72
(br s, 1H). 13C{1H} NMR (partial, C6F5 resonances not
reported): 137.5, 130.0, 129.3, 127.6 (C6H5), 59.8 (CHB2, a
DEPT experiment confirms the presence of one hydrogen atom
bonded to this carbon). 11B{1H} NMR: 74.0 (whh ≈ 850 Hz).
19F{1H} NMR (C6D6): -131.2, -147.7, -162.3.
R ea ct ion of Cp 2Zr (CD2C6D5)2 w it h n E q u iva len t s of
HB(C6F 5)2. Cp2Zr(CD2C6D5)2 (12 mg, 0.03 mmol) and HB-
(C6F5)2 (10 mg, 0.03 mmol or 5 mg, 0.015 mmol) were loaded
into a 5 mm NMR tube. C6D6 was added, and the tube was
immediately capped and shaken. A rapid color change ensued,
and the sample was assayed by 1H NMR spectroscopy. An
analogous procedure using Cp2Zr(CH2C6H5)2, DB(C6F5)2, and
C6H6 was also performed and analyzed via 2H{1H} NMR
spectroscopy.
â, deg
V, Å3
space group
Z
4
4
F(000)
1992.00
1.691
0.404
1304
1.71
0.51
0.051
d
calc, mg m-3
µ, mm-1
R1
0.046
Rw
0.045
0.047
gof
2.18
2.44
Cp2Zr(CH2SiMe3)2 gave a solution of Me3SiCH2B(C6F5)2. 1H
NMR: 2.01 (s, CH2, 2H), -0.08 (s, SiCH3, 9H).
Rea ction of [Cp 2Zr (H)Me]n w ith MeB(C6F 5)2. [Cp2Zr-
(H)Me]n (15 mg, 0.06 mmol) was loaded into a 5 mm NMR
tube and covered with ∼0.3 mL of C6D6. A solution of MeB-
(C6F5)2 (23 mg, 0.06 mmol) in C6D6 (0.3 mL) was layered onto
this suspension; the tube was quickly capped and shaken. An
immediate color change to that characteristic of 2 was observed
1
along with gas evolution. Immediate H NMR analysis (prior
to decomposition of 2) revealed the presence of methane along
with 2 as the major metallocene product (∼80%). Reaction
using CD3B(C6F5)2 was carried out in an analogous fashion.
Rea ction of Cp 2Zr (er yth r o-CH(D)CH(D)CMe3)Cl w ith
HB(C6F 5)2. Cp2Zr(erythro-CH(D)CH(D)CMe3)Cl (11 mg, 0.03
mmol) was loaded into a 5 mm NMR tube and dissolved in
C6D6. Solid HB(C6F5)2 (33 mg, 0.09 mmol) was added to the
tube, which was then capped and shaken. After 5 min,
undissolved material was centrifuged to the top of the tube
and the 1H{2H} NMR spectrum was recorded.
P r ep a r a tion of Cp 2Zr (CH 2SiMe3)[(µ-H2)B(C6F 5)2]. To
an evacuated flask at -78 °C containing Cp2Zr(CH2SiMe3)2
(380 mg, 0.96 mmol) and HB(C6F5)2 (666 mg, 1.92 mmol) was
condensed hexane (20 mL). The reaction was warmed to room
temperature and then sonicated for 1 h. During this time, a
white precipitate formed, which was isolated by filtration,
washed with a little hexane, and pumped dry under vacuum.
The yield was 509 mg, 0.78 mmol, 81%. 1H NMR (C6D6): 5.52
(s, 10H, C5H5), 0.93 (s, 2H, CH2Si), 0.01 (s, 9H, SiCH3), -0.9
(br, 2H, µ-H). 13C{1H} NMR (C6D6): 110.8 (Cp), 44.9 (CH2),
P r ep a r a tion of er yth r o-(C6F 5)2BCH(D)CH(D)-t-C4H9: To
a suspension of DB(C6F5)2 (18 mg, 0.05 mmol) in C6D6 (0.7 mL)
was added E-1-d1-3,3-dimethyl-1-butene (7 µL, 0.055 mmol)
via syringe. The tube was capped and shaken, the suspension
cleared. The sample was assayed by 1H{2H} NMR spectros-
copy and found to be >95% pure. 1H NMR (C6D6): 1.80 (CR-
3
H, d, 1H, J HH ) 11.3 ( 0.3 Hz), 1.36 (Câ-H, d, 1H), 0.88
(C(CH3)3, s, 9H). 13C{1H} NMR (C6D6): 148.4, 146.0, 144.9,
142.3, 138.9, 136.4, (C6F5), 38.2 (t, Câ), 28.8 (CCH3), 26.7(br,
CR). 19F{1H} NMR (C6D6) -132.4 (d, J ) 18.4 Hz), -149.0 (t,
J ) 21.4 Hz), -162.8 (m). 11B NMR (C6D6): 73.3.
1
3.3 (SiCH3). 11B NMR (C6D6): -8.7 (t, J BH ) 67 Hz).19F{1H}
NMR (C6D6): -133.7, -158.3, 164.0. Anal. Calcd for
C
26H23F10ZrBSi: C, 47.64; H, 3.54. Found: C, 49.61; H, 3.54.
IR (KBr, Nujol, cm-1): 2190 w (br), 2115 w (br), 2038 w (br),
1644 m, 1515 s, 1334 w, 1286 w, 1112 m, 1098 m, 1015 w, 960
s, 852 m, 816 s.
Gen er a tion of Cp 2Zr [µ-H2B(C6F 5)2](Cl). Schwartz’s re-
agent (10 mg, 0.04 mmol) and HB(C6F5)2 (15 mg, 0.042 mmol)
were loaded into a 5 mm NMR tube, and C6D6 was added. The
suspension was shaken and sonicated for 15 min and analyzed
by multinuclear NMR spectroscopy. 1H NMR (C6D6): 5.57 (s,
P r ep a r a tion of MeB(C6F 5)2. To a solution of Cp2ZrMe2
(252 mg, 1 mmol) in toluene (10 mL) was added a toluene
solution of ClB(C6F5)2 (760 mg, 2 mmol). The reaction was
stirred for 1 h at room temperature and filtered to remove
precipitated Cp2ZrCl2. The toluene was removed in vacuo, and
the residue was transferred to a sublimator equipped with a
water-cooled probe. Sublimation at ∼30-40 °C under high
vacuum yielded 552 mg (77%) of pure MeB(C6F5)2. This
material had NMR spectroscopic data identical to that reported
by Marks et al.14c
Gen er a tion of C6H5CH2B(C6F 5)2 a n d Me3SiCH2B(C6F 5)2.
Cp2Zr(CH2Ph)2 (9 mg, 0.02 mmol) was loaded into a 5 mm
NMR tube along with ClB(C6F5)2 (18 mg, 0.04 mmol). Benzene-
d6 was added, and the tube was capped and shaken. Undis-
solved material was centrifuged to the top of the tube, and
the 1H NMR spectrum was recorded. The presence of Cp2-
ZrCl2 was noted. PhCH2B(C6F5)2. 1H NMR: 6.90-7.05 (m,
C6H5, 5H), 3.27 (br m, BCH2, 2H). 13C{1H} NMR: 137.4, 129.3,
129.0, 126.4 (C6H5), 39.1 (BCH2). An identical procedure using
1
10H, C5H5), 0.42 (br q, 2H, µ-H, J HB ) 68 ( 3 Hz). 11B NMR
1
(C6D6): -10.3 (t, J BH ) 68 Hz).19F{1H} NMR (C6D6): -131.4,
-155.7, 162.0.
Rea ction of [Cp 2Zr (H)Me]n w ith P h CH2B(C6F 5)2.
A
solution of ClB(C6F5)2 (21 mg, 0.054 mmol) in C6D6 (0.4 mL)
was rapidly added to a solution of Cp2Zr(CH2Ph)2 (11 mg, 0.027
mmol) in C6D6 (0.3 mL) with vigorous stirring. The resulting
solution was transferred to a 5 mm NMR tube and checked
by 1H NMR, revealing only signals for Cp2ZrCl2 and PhCH2B-
(C6F5)2. To this tube was added [Cp2Zr(H)Me]n (13 mg, 0.055
mmol) without allowing it to contact the solution prior to
capping. The tube was capped and shaken vigorously, and
the sample was immediately analyzed by 1H NMR spectros-
copy.
X-r a y Cr ysta llogr a p h y for 1. A summary of crystal data
and refinement details for all structures is given in Table 3.
Crystals of 1 were grown from a toluene solution at room

Reactions of Bis(pentafluorophenyl)borane
Organometallics, Vol. 17, No. 12, 1998 2469
temperature. Suitable crystals were placed in glass capillaries,
taking precautions to prevent loss of solvent of crystallization,
sealed, and mounted onto a Rigaku AFC6S diffractometer.
Measurements were made using graphite-monochromated Mo
KR radiation (λ ) 0.710 69 Å) at -73 °C with the ω-2θ scan
technique to a maximum 2θ value of 50.1°. The structure was
solved by direct methods and refined by full-matrix least-
squares calculations. The non-hydrogen atoms were refined
anisotropically; hydrogen atoms bridging the B and Zr atoms
were included in the refinement at postions located from a
difference map, while the rest of the hydrogen atoms were
included at geometrically idealized positions with C-H ) 0.95
Å and were not refined. At convergence, R and Rw were 0.046
and 0.045. The maximum and minimum peaks on the final
difference map were 0.72 and -0.79 e-/Å3, respectively. All
calculations were performed using the TEXAN51 crystal-
lographic software package of Molecular Structure Corp.
X-r a y Cr ysta llogr a p h y for 4. Single crystals suitable for
X-ray crystallography were mounted in thin-walled glass
capillaries and optically centered in the X-ray beam of an
Enraf-Nonius CAD-4 diffractometer. Measurements were
made using graphite-monochromated Mo KR radiation (λ )
0.709 30 Å). Unit cell dimensions were determined via least-
squares refinement of the setting angles of 24 reflections, and
intensity data were collected using the ω-2θ scan mode in
the range 29.00-34.00°. Data were corrected for Lorentz,
polarization, and absorption effects. The structure was solved
using direct methods. Aryl and Cp hydrogen atoms were
placed in calculated positions (DC-H ) 1.00 Å), and H1 was
located via inspection of a difference Fourier map and fixed;
the temperature factors for these hydrogens were based upon
the carbon atom to which they are bonded. HB was located
via inspection of a difference Fourier map and refined.
A
weighting scheme based upon counting statistics was used
2
with the weight modifier k in kFo (0.000 050) being deter-
mined via evaluation of the variation in the standard reflec-
tions that were collected during the course of data collection.
Neutral atom scattering factors were taken from International
Tables for X-ray Crystallography.52 Values of R and Rw at
convergence were 0.051 and 0.047, respectively. All crystal-
lographic calculations were conducted with the PC version of
the NRCVAX program package53 locally implemented on an
IBM-compatible 80486 computer.
Ack n ow led gm en t. This work was generously sup-
ported by the NOVA Research and Technology Corp. of
Calgary, Alberta, NSERC of Canada’s Research Part-
nerships office (CRD program), and the University of
Calgary. W.E.P. thanks Mr. J ames Blackwell for valu-
able discussions and the Alfred P. Sloan Foundation for
a Research Fellowship (1996-98).
Su p p or tin g In for m a tion Ava ila ble: Tables of atomic
coordinates and isotropic thermal parameters, bond lengths
and angles, H atom coordinates, and anisotropic thermal
parameters for compounds 1 and 4 (25 pages). Ordering
information is given on any current masthead page.
OM9802313
(52) International Tables for X-ray Crystallography; Kynoch Press:
Birmingham., England, 1974; Vol. IV.
(51) Crystal Structure Analysis Package; Molecular Structure
Corp: 1985 and 1992.
(53) Gabe, E. J .; Le Page, Y.; Charland, J .-P.; Lee, F. L.; White, P.
S. J . Appl. Crystallogr. 1989, 22, 384.