Reaction of bis(pentafluorophenyl)borane with methylidyne complexes: Synthesis and characterization of a cationic tungsten(VI) borylalkylidyne hydride

Article abstract of DOI:10.1021/om034318m

Reaction of the tungsten methylidyne complexes (dmpe)₂W(X)≡CH (X = Cl, OTf) with HB(C₆F₅)₂ yields zwitterionic compounds upon electrophilic attack of the methylidyne ligand, in which the methylidyne hydrogen is strongly α-agostic (X = Cl) or is transferred to the tungsten center (X = OTf). Removal of hydride from the coordinated HB(C₆F₅)₂ with [Ph₃C]⁺[B(C₆F₅)₄] ^- gives a cationic borylalkylidyne hydride species.

Full text of DOI:10.1021/om034318m

314
Organometallics 2004, 23, 314-316
Rea ction of Bis(p en ta flu or op h en yl)bor a n e w ith
Meth ylid yn e Com p lexes: Syn th esis a n d Ch a r a cter iza tion
of a Ca tion ic Tu n gsten (VI) Bor yla lk ylid yn e Hyd r id e
Edwin F. van der Eide,1a Warren E. Piers,*,1a Patricio E. Romero,1a
Masood Parvez,1a and Robert McDonald1b
Department of Chemistry, University of Calgary, 2500 University Drive NW,
Calgary, Alberta T2N 1N4, Canada, and X-ray Structure Laboratory, Department of
Chemistry, University of Alberta, Edmonton, Alberta T6G 2G2, Canada
Received November 24, 2003
Summary: Reaction of the tungsten methylidyne com-
plexes (dmpe)2W(X)tCH (X ) Cl, OTf) with HB(C6F5)2
yields zwitterionic compounds upon electrophilic attack
of the methylidyne ligand, in which the methylidyne
hydrogen is strongly R-agostic (X ) Cl) or is transferred
to the tungsten center (X ) OTf). Removal of hydride
from the coordinated HB(C6F5)2 with [Ph3C]+[B(C6F5)4]-
gives a cationic borylalkylidyne hydride species.
gating its reactivity with a variety of organometallic
compounds, including compounds with M-C single
bonds (Cp2MR2, M ) Zr,9 Ti10), coordinated olefins, and
MdC double bonds (Cp2Ta(CH3)dCH2).11 A hallmark
of several of these reactions is the production of orga-
nometallic compounds with unprecedented boron-con-
taining ligands. To extend these studies, we have
investigated the reactions of HB(C6F5)2 with the MtC
triple bond in the tungsten methylidyne complex
(dmpe)2W(Cl)tCH,12 first reported by Schrock in 1981.13
This chemistry has led to the formation of a novel
cationic borylalkylidyne complex, reported herein.
The fundamental reactivity of boranes with transi-
tion-metal organometallic compounds is germane to
important catalytic reactions such as olefin polymeri-
zation,2 hydroboration,3 and hydrocarbon borylation.4
Perfluoroarylboranes are a critical subclass of boranes,
due to their high stability and Lewis acidity.5
Some years ago, we first synthesized bis(pentafluo-
rophenyl)borane, HB(C6F5)2,6 a highly active hydrobo-
ration reagent and synthon for the incorporation of
Lewis acidic -B(C6F5)2 groups into molecular frame-
works.7 A large part of the reason for this high activity,
apart from the strongly Lewis acidic nature of this
borane, is the relative ease with which it dissociates into
the reactive monomer from the dimeric form it assumes
as a solid. Thus, in solution, significant concentrations
of monomeric HB(C6F5)2 are present at room tempera-
ture.
The dmpe-stabilized methylidyne complex (dmpe)2W-
(Cl)tCH was prepared as described by Schrock et al.
from the closely related PMe3 ligated species (PMe3)4W-
(Cl)tCH by simple substitution chemistry.12b To aid in
the acquisition of NMR and IR spectroscopic data, the
2
13C- and H-labeled isotopomers were similarly gener-
ated from labeled (PMe3)4W(Cl)t13CH and (PMe3)4W-
(Cl)tCD, prepared from (PMe3)4WCl2 and Al(13CH3)3 or
Al(CD3)3. The chloride ligand in these compounds can
be exchanged for triflate by treatment with Me3SiOTf.
The use of the chelating phosphine precluded side
reactions leading to formation of the PMe3 adduct of HB-
(C6F5)2, which became problematic in the reactions of
(PMe3)4W(Cl)tCH with the borane.
In addition to exploring the hydroboration chemistry
of this borane,6,8 we have been systematically investi-
As expected on the basis of the earlier Schrock
studies,12 reaction of the bulky electrophile HB(C6F5)2
with (dmpe)2W(X)tCH (X ) Cl, OTf) leads to formation
of zwitterionic adducts, 1a (X ) Cl) and 1b (X ) OTf),
in which the borane attacks the methylidyne carbon
(Scheme 1). In 1a , the spectroscopic data indicate that
the methylidyne hydrogen remains associated with the
methylidyne carbon, although it engages in a strong
R-agostic interaction with the tungsten center. Its
resonance appears in the region typical of such d2
* To whom correspondence should be addressed. Tel: 403-220-5746.
E-mail: wpiers@ucalgary.ca. S. Robert Blair Professor of Chemistry
(2000-2005).
(1) (a) University of Calgary. (b) University of Alberta.
(2) Chen, E. Y.-X.; Marks, T. J . Chem. Rev. 2000, 100, 1391.
(3) (a) Irvine, G. J .; Lesley, M. J . G.; Marder, T. B.; Norman, N. C.;
Rice, C. R.; Robins, E. G.; Roper, W. R.; Whittell, G. R.; Wright, L. J .
Chem. Rev. 1998, 98, 2685. (b) Brauschweig, H.; Angew. Chem., Int.
Ed. 1998, 37, 1786. (c) Burgess, K.; Ohlmeyer, M. J . Chem. Rev. 1991,
91, 1179. (d) Beletskaya, I.; Pelter, A. Tetrahedron 1997, 53, 4957.
(4) (a) Ishiyama, T.; Takagi, J .; Ishida, K.; Miyaura, N.; Anastasi,
N. R.; Hartwig, J . F. J . Am. Chem. Soc. 2002, 124, 390. (b) Tse, M. K.;
Cho, J .-Y.; Smith, M. R. Org. Lett. 2001, 3, 2831. (c) Shimada, S.;
Batsanov, A. S.; Howard, J . A. K.; Marder, T. B. Angew. Chem., Int.
Ed. 2001, 40, 2168. (d) Cho, J .-Y.; Tse, M. K.; Holmes, D.; Maleczka,
R. E., J r.; Smith, M. R. Science 2002, 295, 305. (e) Ishiyama, T.;
Miyaura, N. J . Organomet. Chem. 2003, 680, 3.
(9) Spence R. E. v H.; Piers, W. E.; Sun, Y.; Parvez, M.; MacGillivray,
L. R.; Zaworotko, M. J . Organometallics 1998, 17, 2459.
(10) Chase, P. A.; Piers, W. E.; Parvez, M. Organometallics 2000,
19, 2040.
(11) Cook, K. S.; Piers, W. E.; Woo, T. K.; Rettig, S. J .; McDonald,
R. Organometallics 2001, 20, 3927.
(12) (a) Sharp, P. R.; Holmes, S. J .; Schrock, R. R.; Churchill, M.
R.; Wasserman, H. J . J . Am. Chem. Soc. 1981, 103, 965. (b) Holmes,
S. J .; Clark, D. N.; Turner, H. W.; Schrock, R. R. J . Am. Chem. Soc.
1982, 104, 6322.
(13) Other studies involving alkylidynes and boranes: (a) Wadepohl,
H.; Arnold, U.; Prizkow, H. Angew. Chem., Int. Ed. Engl. 1997, 36,
974. (b) Braunschweig, H.; Klinkhammer, K. W.; Koster, M.; Radaki,
K. Chem. Eur. J . 2003, 9, 1303.
(5) (a) Piers, W. E.; Chivers, T. Chem. Soc. Rev. 1997, 345. (b) Piers,
W. E. Adv. Organomet. Chem., in press.
(6) Parks, D. J .; Piers, W. E., Yap, G. P. A. Organometallics 1998,
17, 5492.
(7) (a) Spence, R. E. v H.; Piers, W. E. Organometallics 1995, 14,
4617. (b) Kunz, D.; Erker, G.; Fro¨hlich, R.; Kehr, G. Eur. J . Inorg.
Chem. 2000, 409. (c) Carpenter, B. E.; Piers, W. E.; Parvez, M.; Yap,
G. P. A.; Rettig, S. J . Can. J . Chem. 2001, 78, 857.
(8) Parks, D. J .; Piers, W. E. Tetrahedron 1998, 54, 15469.
10.1021/om034318m CCC: $27.50 © 2004 American Chemical Society
Publication on Web 01/07/2004

Communications
Organometallics, Vol. 23, No. 3, 2004 315
Sch em e 1
F igu r e 1. Crystalmaker diagram of the molecular struc-
ture of 1b. Selected bond distances (Å): W-P(11), 2.506-
(2); W-P(12), 2.4833(16); W-P(21), 2.5351(18); W-P(22),
2.4763); W-O(1), 2.400(5); W-C(1), 1.792(6); C(1)-B,
1.613(9); C(31)-B, 1.651(9); C(41)-B, 1.628(9). Selected
bond angles (deg): P(11)-W-P(21), 87.00(8); P(12)-W-
P(22), 117.83(6); W-C(1)-B, 168.5(5); C(31)-B-C(41),
109.5(5).
agostic alkylidenes14 at -7.58 ppm as a quintet (3J PH
2
) 9.9(2) Hz, J WH ) 41(1) Hz), indicating that the
position of agostic coordination is dynamic on the NMR
time scale; the room-temperature 31P NMR spectrum
exhibits a single resonance at 20.7 ppm with tungsten
satellites (1J PW ) 253 Hz). Low-temperature NMR
studies failed to freeze out a slow exchange regime,
indicating a low barrier for migration of the agostic
replacement of Cl for OTf yields an equilibrium mixture
of (dmpe)2Ta(OTf)dC(H)tBu and (dmpe)2Ta(H)(OTf)t
CtBu.15
1
hydrogen between CP2 faces. A very low J CH value of
1
50(1) Hz, obtained from the H NMR spectrum of 13C-
1a , is consistent with the agostic nature of this bond.
The hydridoborate moiety is indicated by an 11B NMR
chemical shift of -18.1 ppm and a broad quartet
This assignment for 1b was supported by X-ray
crystallographic analysis of colorless needles of the
compound; a Crystalmaker diagram of the molecular
structure is shown in Figure 1 along with selected
metrical parameters. While the hydride bonded to
tungsten was not located and refined, its presence in
the P4W plane between P(12) and P(22) is clearly
indicated by the very different interligand P-W-P
angles: P(11)-W-P(21) ) 87.00(8)° and P(12)-W-
P(22) ) 117.83(6)°. The W-C(1) bond distance of 1.792-
(6) Å is at the low end of those found for WtC,17 and
the W-C(1)-B angle deviates from linearity (168.5(5)°)
probably to alleviate adverse steric interactions. The
C(1)-B distance of 1.613(9) Å is typical of a boron-
carbon single bond.
Removal of H2 from compounds 1 would lead to
complexes with a novel boron-containing ligand. Heat-
ing samples of 1a leads to ill-defined decomposition
processes in which no definitive evidence for H2 loss was
observed. We thus sought to remove the elements of H2
via a hydride abstraction/deprotonation protocol. While
we have been unsuccessful in completing the protocol,
the distorted alkylidene 1a reacts with [Ph3C]+[B(C6F5)4]-
in benzene to yield Ph3CH and an orange oil, 2, which
precipitates out of this medium. The oil can be solubi-
lized with bromobenzene, but to acquire NMR data, the
resulting solid that crystallizes from C6D6/C6D5Br must
be redissolved in CD2Cl2/C6D5Br mixtures, in which it
is sparingly soluble. NMR data are consistent with
formation of a compound in which the B-H hydride has
been removed,18 and the agostic alkylidene hydrogen
has transferred to the W center (Scheme 2).
1
resonance in the H NMR spectrum at 3.23 ppm (1J BH
) 88(2) Hz). Zwitterionic complex 1a is isoelectronic
with the tantalum alkylidene complex (dmpe)2Ta(Cl)d
C(H)tBu,15 which also exhibits a strong R-agostic inter-
action with NMR properties similar to those described
for 1a .16
The triflate product 1b exhibits NMR properties
distinctly different from those observed for 1a . While
the parameters associated with the hydridoborate moi-
ety are similar, those pertaining to the methylidyne
C-H unit indicate that the hydrogen has transferred
completely to the metal. No resonance in the upfield
region around -7.5 ppm associated with R-agostic
protons, as seen for 1a , was observed. Rather, a quintet
for the W-D moiety in d-1b was detected at 1.90 ppm
2
(2J PD ) 6.1 Hz) in the H{1H} NMR spectrum, which is
obscured by dmpe proton resonances in the 1H NMR
spectrum. Furthermore, the two compounds differ in
their color; the formally d2 1a is orange, while the d0
configuration of 1b arrived at upon oxidative addition
of the methylidyne C-H bond is consistent with its
observed lack of color. In the isoelectronic Ta manifold,
(14) Brookhart, M.; Green, M. L. H.; Wong, L.-L. Prog. Inorg. Chem.
1988, 36, 1.
(15) Churchill, M. R.; Wasserman, H. J .; Turner, H. W.; Schrock,
R. R. J . Am. Chem. Soc. 1982, 104, 1710.
(16) IR spectroscopy gave inconclusive data concerning the R-agostic
interaction for 1a . A very low stretch of 2200 cm-1 was reported for
the agostic C-H moiety of (dmpe)2Ta(Cl)dC(H)tBu,14 but we do not
observe a band in this region of the spectrum. Rather, a weak band in
the spectrum of 1a at 2801 cm-1 is observed, and this band disappears
in the spectrum of d-1a . While this falls within the range of 2250-
2800 cm-1 generally observed for agostic C-H functions,13 the expected
(17) Churchill, M. R.; Rheingold, A. L.; Wasserman, H. J . Inorg.
Chem. 1981, 20, 3392.
(18) Courtenay, S.; Mutus, J . Y.; Schurko, R. W.; Stephan, D. W.
Angew. Chem., Int. Ed. 2002, 41, 498.
ν
C-D band at ∼2000 cm-1 was not observed, so we cannot with complete
confidence assign this to the agostic C-H stretch.

316 Organometallics, Vol. 23, No. 3, 2004
Communications
Sch em e 2
F igu r e 2. Crystalmaker diagram of the molecular struc-
ture of 2. Selected bond distances (Å): W(1)-C(13), 1.821-
(5); W(1)-P(2), 2.4861(12); W(1)-P(3), 2.5041(12);
W(1)-Cl(1), 2.5524(12); W(1)-P(4), 2.5545(12); W(1)-P(1),
2.5710(13); B(1)-C(13), 1.512(7); B(1)-C(14), 1.578(7);
B(1)-C(20), 1.591(7). Selected bond angles (deg): P(2)-
W(1)-P(3), 118.22(4); P(4)-W(1)-P(1), 85.51(4); C(13)-
B(1)-C(14), 124.7(4); C(13)-B(1)-C(20), 120.9(4); C(14)-
B(1)-C(20), 114.0(4); B(1)-C(13)-W(1), 176.0(4).
The 19F NMR spectrum of 2 exhibits two sets of three
resonances in a 2:1 ratio. The more intense set is
attributed to the [B(C6F5)4]- anion by virtue of the small
separation (3.4 ppm) between the resonances for the
para and meta fluorines.19 The smaller set, however,
has a more substantial ∆δm,p gap of 10.4 ppm, which is
suggestive of the three-coordinate boron center expected
upon H- abstraction. The 11B NMR spectrum shows two
signals: one at -17 ppm for the borate boron and one
sum of the C-B-C angles is 359.6(4)°), and the W(1)-
C(13)-B(1) angle is essentially linear at 176.0(4)°. The
W(1)-C(13) distance of 1.821(2) Å is indicative of a
tungsten-carbon triple bond, but the C(13)-B(1) length
of 1.521(6) Å is ∼0.1 Å shorter than a typical boron-
carbon single bond. Together with the spectroscopic
data, it appears that an apt description of the bonding
in 2 falls somewhere between the “borylalkylidyne” and
“boravinylidene” resonance structures shown in Scheme
2. The importance of the latter resonance structure is
perhaps indicated by the fact that hydride abstraction
from the hydridoborate moiety in 1a triggers transfer
of the agostic alkylidene hydrogen to the metal, allowing
for π donation to the newly vacant Lewis acid orbital
on boron.
Attempts to deprotonate 2 have thus far met with
failure, probably due to preferential nucleophilic attack
by the base at the electrophilic boron center of the
borylalkylidyne/boravinylidene ligand. Future research
will continue in this vein using weakly nucleophilic
bases, in addition to exploring the reactivity and bond-
ing of these novel new boron-containing ligands.
1
at 40 ppm for the neutral boron atom. In the H NMR
spectrum, no signal for the hydridoborate is present, nor
is there a resonance in the region associated with an
R-agostic C-H. Rather, a symmetrical pentet appears
2
at 4.5 ppm, with a large J PH value of 39(1) Hz (cf. the
value of 9.9(2) Hz seen in 1a ). Upon cooling to 220 K,
this resonance transforms into a triplet of triplets with
2J PH values of 70(1) Hz and ∼8(2) Hz (the small coupling
was barely resolvable in the low-temperature limit).
These values compare well to the couplings of 90 and
10 Hz observed in (dmpe)2Ta(H)(ClAlMe3)tCtBu be-
tween the Ta-H and the trans and cis dmpe phos-
phines, respectively.15 Finally, the 13C resonance for the
alkylidyne carbon was located (using 13C-labeled 2) as
2
a broad signal at 265.0 ppm, and the J CH value of 5.5-
(2) Hz unequivocally establishes that cleavage of the
C-H bond has occurred to produce compound 2.
Scheme 2 indicates that 2 may be viewed as being a
borylalkylidyne, a boravinylidene, or even a tungsten
carbide stabilized borenium ion. The 19F and 11B NMR
data discussed above are consistent with a measure of
π-donation from WtC to the vacant p orbital on the
boron, since the ∆δm,p and 11B chemical shift values fall
between the normal values for purely three-coordinate
neutral (∆δm,p ≈ 15-18 ppm, 11B > 50 ppm) and four-
coordinate neutral adducts (∆δm,p ≈ 7-8 ppm, 11B ≈
10-20 ppm). To probe this issue further, the molecular
structure of 2 (the cationic portion is shown in Figure
2) confirms the gross features of the structure of 2 as
determined spectroscopically. The pattern of P-W-P
angles indicates the presence of a W-H moiety. The
boron center is clearly trigonal planar in geometry (the
Ack n ow led gm en t. Funding for this work came
from the Natural Sciences and Engineering Research
Council of Canada in the form of a Discovery Grant (to
W.E.P.). E.F.v.d.E. wishes to thank the Marco Polo
Fonds of the University of Groningen for financial
support.
Su p p or tin g In for m a tion Ava ila ble: Text giving experi-
mental details and tables of atomic coordinates, anisotropic
displacement parameters, and all bond distances, bond angles,
and torsion angles for the structurally characterized molecules
presented here; crystallographic data are also given as CIF
files. This material is available free of charge via the Internet
at http://pubs.acs.org.
(19) Horton, A. D.; de With, J . Organometallics 1997, 16, 5424.
OM034318M