Synthesis and reactivity of tantalocene zwitterions stabilized by ground-stat α-agostic interactions via reaction of B(C6F5)3 with Cp′2Ta (double bond CH2)(CH3) (Cp′ = C5H5, C5H4Me)

Article abstract of DOI:10.1021/om000176+

Zwitterionic tantalocene derivatives are formed when the perfluorophenyl-substituted borane B(C₆F₅)₃ is reacted with the tantalocene methyl methylidene complexes Cp′₂Ta(double bond CH₂)(CH₃) (Cp′ = C₅H₅, C₅H₄Me). The products arise from borane attack of the methylidene ligand and are characterized by a strong, ground-state α-agostic interaction, and insertion of tert-butyl isocyanide occurs exclusively into the agostic Ta-C bond, producing iminoacyl zwitterions.

Full text of DOI:10.1021/om000176+

Organometallics 2000, 19, 2243-2245
2243
Syn th esis a n d Rea ctivity of Ta n ta locen e Zw itter ion s
Sta bilized by Gr ou n d -Sta te r-Agostic In ter a ction s via
Rea ction of B(C6F 5)3 w ith Cp ′2Ta (dCH2)(CH3) (Cp ′ ) C5H5,
C5H4Me)
Kevin S. Cook,† Warren E. Piers,*,† Steven J . Rettig,‡,1 and Robert McDonald§
Departments of Chemistry, University of Calgary, 2500 University Drive NW, Calgary,
Alberta, Canada T2N 1N4, University of British Columbia, 2036 Main Mall, Vancouver,
British Columbia, Canada V6T 1Z1, and University of Alberta, Edmonton, Alberta, Canada
Received February 24, 2000
Summary: Zwitterionic tantalocene derivatives are formed
when the perfluorophenyl-substituted borane B(C6F5)3 is
reacted with the tantalocene methyl methylidene com-
plexes Cp′2Ta(dCH2)(CH3) (Cp′ ) C5H5, C5H4Me). The
products arise from borane attack of the methylidene
ligand and are characterized by a strong, ground-state
R-agostic interaction, and insertion of tert-butyl isocya-
nide occurs exclusively into the agostic Ta-C bond,
producing iminoacyl zwitterions.
tantalocene10 derivatives exhibiting rare ground-state
R-agostic interactions.
As a classic example of a nucleophilic carbene, Cp2-
Ta(dCH2)CH3 reacts rapidly with electrophiles at the
methylene ligand.8 Predictably, therefore, reaction of
Cp′2Ta(dCH2)CH3 (Cp′ ) C5H5, C5H4Me) with B(C6F5)3
proceeds rapidly in toluene to afford the zwitterionic
tantalocene complexes Cp′2Ta[CH2B(C6F5)3]CH3 (1a ,b)
in excellent yield (eq 1). Borane attack at the meth-
We have been studying the fundamental chemistry
involved in the reactions of early-transition-metal or-
ganometallic compounds with highly electrophilic, per-
fluoroaryl-substituted boranes.2 While the main impetus
for these studies stems from the role these boranes play
in the activation of olefin polymerization catalysts,3 a
detailed understanding of the interaction of boranes
with simple hydrocarbyl ligands has additional rel-
evance to catalytic hydroboration4 and diboration5 pro-
cesses. We recently reported6 preliminary results on the
ylidene carbon is signaled by the upfield shift of the now
boron-broadened resonances for the methylene protons
(2.10 ppm) and carbon atom (150.3 ppm) of 1a in
comparison to Cp2Ta(dCH2)CH3 (10.14 and 224.0 ppm,
respectively). A resonance at -9.4 ppm in the 11B NMR
spectrum and a meta-para chemical shift difference of
4.7 ppm in the 19F NMR spectrum11 are indicative of
borate formation and a significant degree of charge
separation in the zwitterions 1.
7
reactions of HB(C6F5)2 with Schrock’s methyl meth-
ylidene compound Cp2Ta(dCH2)CH3;8 here we disclose
the reactions of this organometallic compound with the
tertiary borane B(C6F5)3.9 The products are zwitterionic
* To whom correspondence should be addressed. Phone: 403-220-
5746. FAX: 403-289-9488. E-mail: wpiers@ucalgary.ca.
† University of Calgary.
‡ University of British Columbia.
§ University of Alberta.
The tantalum centers in compounds 1 are stabilized
by a strong R-agostic interaction involving one of the
C-H bonds of the electron-rich methylene unit, as
evidenced by established criteria.12 In solution, a low
average 1J CH value of 98.7(5) Hz is observed for the CH2
unit in 1a , compared with a value of 129.1(5) Hz for
the methyl C-H bonds. In the solid, a low-frequency
stretch of 2742 cm-1 (νCD ) 2026 cm-1) augments the
findings of an X-ray structural analysis,13 in which both
CH2 hydrogens were located and refined, clearly estab-
lishing the presence of a ground-state R-agostic linkage
(Figure 1). The agostic C(12)-H(14) bond is elongated
by 0.27(4) Å compared to the conventional C(12)-H(15)
bond, while the Ta-C(12) distance is notably shorter
(1) Deceased October 27, 1998.
(2) (a) 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. (b) Sun, Y.; Spence, R. E. v. H.; Piers, W. E.; Parvez, M.;
Yap, G. P. A. J . Am. Chem. Soc. 1997, 119, 5132. (c) Sun, Y.; Piers, W.
E.; Yap, G. P. A. Organometallics 1997, 16, 2509. (d) Spence, R. E. v.
H.; Piers, W. E.; Sun, Y.; Parvez, M.; MacGillivray, L. R.; Zaworotko,
M. J . Organometallics 1998, 17, 2459. (e) Lee, L. W. M.; Piers, W. E.;
Parvez, M.; Rettig, S. J .; Young, V. G., J r. Organometallics 1999, 18,
3904. (f) Piers, W. E.; Sun, Y.; Lee, L. W. M. Top. Catal. 1999, 7, 133.
(3) (a) Piers, W. E.; Chivers, T. Chem. Soc. Rev. 1997, 345. (b) Yang,
X.; Stern, C. L.; Marks, T. J . J . Am. Chem. Soc. 1994, 116, 10015.
(4) (a) Harrison, K. N.; Marks, T. J . J . Am. Chem. Soc. 1992, 114,
9220. (b) Pereira, S.; Srebnik, M. Organometallics 1995, 14, 3127. (c)
He, X.; Hartwig, J . F. J . Am. Chem. Soc. 1996, 118, 1696. (d) Hartwig,
J . F.; Muhoro, C. N.; He, X. J . Am. Chem. Soc. 1996, 118, 10936. (e)
Lantero, D. R.; Ward, D. L.; Smith, M. R. J . Am. Chem. Soc. 1997,
119, 9699. (f) Lantero, D. R; Miller, S. L.; Cho, J .-Y.; Ward, D. L.; Smith,
M. R., III. Organometallics 1999, 18, 235. (g) Muhoro, C. N.; He, X.;
Hartwig, J . F. J . Am. Chem. Soc. 1999, 121, 5033. (h) Hartwig, J . F.;
Muhoro, C. N. Organometallics 2000, 19, 30.
(9) Massey, A. G.; Park, A. J . J . Organomet. Chem. 1964, 2, 245.
(10) (a) Piers, W. E. Chem. Eur. J . 1998, 4, 13. (b) Bochmann, M.
Top. Catal. 1999, 7, 9.
(11) Horton, A. D.; de With, J . Organometallics 1997, 16, 5424-
5436.
(12) Brookhart, M.; Green, M. L. H.; Wong, L.-L. Prog. Inorg. Chem.
1988, 36, 1.
(5) Braunschweig, H. Angew. Chem., Int. Ed. 1998, 37, 1786.
(6) Cook, K. S.; Piers, W. E.; Rettig, S. J . Organometallics 1999, 18,
8, 1575.
(7) Parks, D. J .; Piers, W. E.; Yap, G. P. A. Organometallics 1998,
17, 5492.
(8) Schrock, R. R.; Sharp, P. R. J . Am. Chem. Soc. 1978, 100, 2389.
10.1021/om000176+ CCC: $19.00 © 2000 American Chemical Society
Publication on Web 05/09/2000

2244 Organometallics, Vol. 19, No. 12, 2000
Communications
Sch em e 1
engendering unfavorable steric interactions between the
-B(C6F5)3 moiety and a Cp ring as the agostic C-H
bond strives to align with the plane bisecting the rings.
In fact, the agostic C-H vector is about 30° out of this
plane, illustrating the effect a bulky substituent can
have on the geometry of such an interaction. It is
nonetheless strong enough to stabilize 1a against
potential decomposition to neutral products by -C6F5
transfer to tantalum.17
F igu r e 1. Molecular structure of 1a . Selected bond
distances (Å): Ta(1)-Cp(1), 2.107; Ta(1)-Cp(2), 2.113; Ta-
(1)-C(11), 2.218(5); Ta(1)-C(12), 2.141(5); Ta(1)-H(14),
2.26(4); C(12)-H(14), 1.26(4); C(12)-H(15), 0.99(5); B(1)-
C(12), 1.661(7); B(1)-C(13), 1.675(7); B(10-C(19), 1.643(8);
B(1)-C(25), 1.649(6). Selected bond angles (deg): Cp(1)-
Ta(1)-Cp(2), 131.9; C(11)-Ta(1)-C(12), 97.0(2); C(11)-Ta-
(1)-H(14), 68.6(11); Ta(1)-C(12)-H(14), 78.7(19); Ta(1)-
C(12)-B(1); 145.6(3); Ta(1)-C(12)-H(15), 95.1(29); B(1)-
C(12)-H(14), 123.6(20).
Compounds 1 are zwitterionic analogues of neutral
group 4 bis(alkyl)metallocenes, and we have begun to
examine their chemistry for comparative purposes.
Thus, insertion of tert-butyl isocyanide proceeds smoothly
in tetrahydrofuran with excess isocyanide to give the
N-out iminoacyl zwitterions 2a ,b (Scheme 1) as meta-
stable kinetic isomers (vide infra). Interestingly, the
insertion occurs regioselectively into the Ta-CH2 bond,
as indicated by the more pronounced downfield shift of
the boron-broadened methylene resonance (to 3.75 ppm
from 2.10 ppm) in comparison to the slight upfield
change in the methyl resonance (to 0.37 ppm from 0.55
ppm) upon insertion. Selective CO and RNC insertions
into the metallocene M-C bond of the more sterically
demanding R group are generally the norm.18 11B (δ
-10.9 ppm) and 19F (∆m,p ) 4.3 ppm) NMR spectroscopy
shows that the -CH2-B(C6F5)3 borate fragment re-
mains intact during insertion under these conditions.19
A resonance at 219.8 ppm for the iminoacyl carbon atom
is characteristic of η2-binding for this ligand, a common
bonding mode in high-valent early-d-block metals.20 We
assign the kinetically formed isomer the “N-out” con-
figuration on the basis of a 2D ROESY experiment,
which shows cross-peaks between the tert-butyl and
methylene protons and between the methylene and
methyl resonances, with no cross-peak between the tert-
(2.141(5) Å) than the Ta-C(11) length of 2.218(5) Å. The
Ta-C(12)-H(14) angle is only 78.7(19)°.
Since no evidence exists for an R-agostic stabilization
in the related compounds Cp2Ta[CH2Al(CH3)3]CH3 (1J CH
) 128 Hz8) and [Cp2Ta(CH3)2]+[B(C6F5)4]- 14 (1J CH ) 126
Hz), its presence in 1a is a consequence of more
extensive charge separation and subsequent delocaliza-
tion of borate charge onto the methylene carbon, making
its C-H bonds better donors. Structural observation of
R-agostic interactions are of interest due to the role
these structures play in the olefin chemistry of metal-
locenes.15 As they are usually associated with transition-
state structures, well-characterized ground-state R-ag-
ostic interactions are rare and usually imposed by steric
factors which preclude â-agostic structures.16 In 1a , the
agostic interaction occurs for electronic reasons, despite
(13) Crystals of 1a were grown from dichloroethane and form as a
solvate. Crystal data for 1a : C34H23F15BCl4Ta, 0.12 × 0.25 × 0.30 mm,
triclinic, P1h (No. 2), a ) 10.6312(10) Å, b ) 11.7797(9) Å, c ) 15.4744-
(15) Å, R ) 100.8110(12)°, â ) 90.9989(9)°, γ ) 108.215(2)°, V ) 1802.1-
(3) Å3, Z ) 2, fw ) 1050.10, Dcalcd ) 1.935 g cm-3, 2θmax ) 61.0°, Mo
KR radiation, λ ) 0.710 69 Å, T ) -93 °C, 16 290 measured reflections,
(17) (a) Scollard, J . D.; McConville, D. H.; Rettig, S. J . Organome-
tallics 1997, 16, 1810. (b) Thorn, M. G.; Vilardo, J . S.; Fanwick, P. E.;
Rothwell, I. P. Chem. Commun. 1998, 2427. (c) Zhang, S.; Piers, W.
E.; Gao, X.; Parvez, M. Submitted for publication in J . Am. Chem. Soc.
(18) Approach of the isocyanide from the lateral position of the
metallocene wedge associated with the sterically more bulky alkyl
group pushes this group into the central position of the wedge, the
sterically less demanding site: (a) Lappert, M. F.; Luong-Thi, N. T.;
Milne, C. R. C. J . Organomet. Chem. 1979, 174, C35. (b) J effrey, J .;
Lappert, M. F.; Luong-Thi, N. T.; Webb, M.; Atwood, J . L.; Hunter, W.
E. J . Chem. Soc., Dalton Trans. 1981, 1593. (c) Erker, G. Acc. Chem.
Res. 1984, 17, 103 and references therein.
8283 unique, 6426 reflections with Inet > 3.0σ(Inet), µ ) 34.46 cm-1
,
minimum/maximum transmission 0.7040 and 1.0000, residuals on F2
(all data) R ) 0.069, Rw ) 0.072, GOF ) 1.39, 530 parameters.
(14) See the Supporting Information for details on the synthesis
and characterization of this compound and its tBuNC insertion
product.
(15) Grubbs, R. H.; Coates, G. W. Acc. Chem. Res. 1996, 29, 85.
(16) (a) Etienne, M.; Mathieu, R.; Donnadeau, B. J . Am. Chem. Soc.
1997, 119, 3218. (b) Guo, Z.; Swenson, D. C.; J ordan, R. F. Organo-
metallics 1994, 13, 1424. (c) Poole, A. D.; Williams, D. N.; Kenwright,
A. M.; Gibson, V. C.; Clegg, W.; Hockless, C. R.; O’Neil, P. A.
Organometallics 1993, 12, 2549. (d) J affart, J .; Mathieu, R.; Etienne,
M.; McGrady, J . E.; Eisenstein, O.; Maseras, F. Chem. Commun. 1998,
2011.
t
(19) (a) In other solvents, such as benzene, formation of the BuNC
19b
adduct of B(C6F5)3
is competitive with insertion. Presumably,
dissociation of B(C6F5)3 from 1 is more facile in this solvent. (b)
J acobsen, H.; Berke, H.; Do¨ring, S.; Kehr, G.; Erker, G.; Fro¨hlich, R.;
Meyer, O. Organometallics 1999, 18, 1724.
(20) Durfee, L. D.; Rothwell, I. P. Chem. Rev. 1988, 88, 1059.

Communications
Organometallics, Vol. 19, No. 12, 2000 2245
butyl and methyl protons present. This assignment was
confirmed by an X-ray structural analysis of the 1,1′-
dimethyl analogue 2b, which crystallizes as two inde-
pendent molecules in the unit cell, one of which is
disordered over two conformations with 65:35 occupan-
cies.21 An ORTEP diagram of the nondisordered mol-
ecule (A) is shown in Figure 2, clearly showing the N-out
orientation of the iminoacyl ligand. Metrical parameters
are comparable to those of related compounds,22,23
although 2b has the lowest value of ∆24 (-0.061 Å) of
any tantalum iminoacyl compound reported to date,
consistent with the fact that it is a more electron
deficient tantalum center.
Typically, N-in isomers are more thermodynamically
stable than N-out structures in metallocene and related
iminoacyl complexes.21 Indeed, the N-out isomers of 2
can be converted quantitatively to the N-in derivatives
by heating at 65 °C for several days (eq 2). These
F igu r e 2. Molecular structure of 2b. Selected bond
distances (Å): Ta-C(1), 2.259(7); Ta-C(2), 2.182(6); Ta-
N(1), 2.121(5); C(2)-N(1), 1.264(8); C(4)-N(1), 1.501(9);
C(2)-C(3), 1.500(9); B(1)-C(3), 1.698(9). Selected bond
angles (deg): C(1)-Ta-C(2), 80.6(3); C(1)-Ta-N(1), 114.7-
(2); C(2)-Ta-N(1) 34.1(2); Ta-C(2)-C(3), 156.3(5); C(3)-
C(2)-N(1), 131.2(6); C(2)-C(3)-B(1), 124.1(6); C(3)-B(1)-
C(31), 110.8(5); C(3)-B(1)-C(41), 110.1(5); C(3)-B(1)-
C(51), 107.6(5).
istry, the presence of a ground-state R-agostic interac-
tion hampers insertion chemistry, in contrast to the
facilitating role such interactions play in stabilizing
transition states for olefin insertion reactions.15 Disen-
gagement of the R-agostic interaction in zwitterions 1
frees up a metallocene orbital to accommodate incoming
tBuNC, which presumably approaches via the lateral
site next to the -CH2B(C6F5)3 group, forming interme-
diate I.18 This intermediate was not detected even at
low temperature (-80 °C), indicating that insertion to
give the kinetic N-out isomer of 2a is rapid. That
isocyanide binding is rate-limiting is supported by the
following observations: (1) the reaction rate increases
when a larger concentration of tBuNC is employed (still
pseudo-first-order conditions); (2) kobs at 17 °C for the
relatively harsh conditions are presumably required due
to the stronger Ta-N interaction present in zwitterions
2 (manifested in the low value of ∆, vide supra). The
opportunity to observe and characterize both insertion
isomers, and study their interconversion, is rare.23
Mechanistically, the formation of the kinetic insertion
products N-out-2 appears to proceed as shown in
Scheme 1. Dissociation of the R-agostic C-H bond is
required, as indicated by a substantial inverse isotope
effect of 0.84(4) at 30 °C on the rate of the reaction for
1a vs 1a -d 5.25 Inverse isotope effects have been observed
in other reactions where dissociation of an agostic
interaction is the initial step,26 reflecting the inverse
equilibrium isotope effect (EIE) for the agostic/nona-
gostic preequilibrium (i.e., KD > KH). Furthermore, the
rate of insertion of tBuNC into [Cp2Ta(CH3)2]+[B(C6F5)4]-,
which does not feature an R-agostic interaction, is much
faster than that observed for 1a .14 Thus, in this chem-
t
more basic isocyanide BuNC (7.34 × 10-4 M-1 s-1) is
larger than for the less basic C6H5CH2NC (3.61 × 10-4
t
M-1 s-1); (3) kobs for insertion of BuNC in 1a is larger
than for the (slightly) more electron rich 1,1′-dimethyl
derivative 1b (1.47 × 10-4 M-1 s-1); (4) the activation
entropy is -31.4(5) eu, suggesting an associative rate-
limiting step.
(21) Crystals of the 1,1′-dimethyltantalocene derivative 2b suitable
for X-ray analysis were grown from CH2Cl2. Crystal data for 2b:
C38H30F15BCl2NTa, 0.28 × 0.26 × 0.11 mm, triclinic, P1h (No. 2), a )
14.223(4) Å, b ) 16.875(5) Å, c ) 18.026(5) Å, R ) 107.887(5)°, â )
109.932(5)°, γ ) 96.671(5)°, V ) 3751.1(18) Å3, Z ) 4, fw ) 1048.29,
Dcalcd ) 1.856 g cm-3, 2θmax ) 51.5°, Mo KR radiation, λ ) 0.710 73 Å,
T ) -80 °C, 27 577 measured reflections, 14 270 unique, 11 320
reflections with Inet > 2.0σ(Inet), µ ) 3.179 mm-1, minimum/maximum
transmission 0.3255 and 0.6783, final R indices R1 ) 0.0521, wR2 )
0.1410, GOF ) 1.028, 1087 parameters.
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 Research Grant to
W.E.P. and Scholarship support (PGSA and PGSB) to
K.S.C. W.E.P. also thanks the Alfred P. Sloan Founda-
tion for a Research Fellowship (1996-2000).
(22) Bazan, G. C.; Donnelley, S. J .; Rodriguez, G. J . Am. Chem. Soc.
1995, 117, 2671.
(23) Boring, E.; Sabat, M.; Finn, M. G.; Grimes, R. N. Organome-
tallics 1997, 16, 3993.
Su p p or tin g In for m a tion Ava ila ble: Text giving experi-
mental details, figures giving representative kinetic plots, and
full listings of crystallographic data, atomic parameters,
hydrogen parameters, atomic coordinates, and complete bond
distances and angles for 1a and 2b. This material is available
free of charge via the Internet at http://pubs.acs.org.
(24) ∆ ) d(Ta-N) - d(Ta-C). The lower this parameter, the more
electron deficient the metal center.20
(25) For details on the kinetic studies and representative kinetic
plots, see the Supporting Information.
(26) Fryzuk, M. F.; Haddad, T. S.; Rettig, S. J . Organometallics
1991, 10, 2026.
OM000176+