Solution and solid-state characteristics of imine adducts with tris(pentafluorophenyl)borane

Article abstract of DOI:10.1021/om011086n

Adducts of the N-benzyl imines Ph(R)C=NBn (R = Ph, CH₃, H) and ^tBu(CH₃)C=NBn and the N-phenyl imine Ph(H)C=NPh with the Lewis acid tris(pentafluorophenyl)borane, B(C₆F₅)₃ have been prepared

Full text of DOI:10.1021/om011086n

1400
Organometallics 2002, 21, 1400-1407
Solu tion a n d Solid -Sta te Ch a r a cter istics of Im in e
Ad d u cts w ith Tr is(p en ta flu or op h en yl)bor a n e
J ames M. Blackwell,† Warren E. Piers,*,†,1 Masood Parvez,† and
Robert McDonald‡
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 December 21, 2001
t
Adducts of the N-benzyl imines Ph(R)CdNBn (R ) Ph, CH3, H) and Bu(CH3)CdNBn and
the N-phenyl imine Ph(H)CdNPh with the Lewis acid tris(pentafluorophenyl)borane,
B(C6F5)3, have been prepared and characterized in solution and in the solid state. For each
t
imine, the Lewis acid is N-bound, with the exception of the sterically demanding base Bu-
(CH3)CdNBn, which reacts with B(C6F5)3 through its enamine tautomer to form an R-C
bound adduct, 5. In the N-bound imine-borane adducts 1-4, steric crowding and π-stacking
between C6F5 and C6H5 rings results in restricted rotation about the B-N and B-C bonds;
1
the dynamic behavior which results can be studied using variable-temperature 19F and H
NMR spectroscopy. For the unsymmetrical ketimines and aldimines, kinetic adducts of the
thermodynamically dominant free imine are observed to form initially upon treatment with
B(C6F5)3; these adducts slowly convert thermally to the thermodynamic adducts of the less
stable imine. For the ketimine Ph(CH3)CdNBn, both the kinetic adduct 2-k and the
thermodynamic adduct 2-t were fully characterized. In the solid-state structures of all the
adducts except 5, one or two C6F5/C6H5 stacking interactions are present; in one case, the
adduct 3-t between B(C6F5)3 and Ph(H)CdNBn, intermolecular stacking interactions are
apparent in the crystal-packing diagram. The implications of the structures of these adducts
for Lewis acid mediated addition of nucleophiles to imines is discussed.
In tr od u ction
It is widely acknowledged that the possibility exists
for geometrical isomers (syn vs anti) of imine adducts
of Lewis acids (eq 1). Although it has often been
The addition of nucleophiles to imines is an important
transformation in organic synthesis.2 Although addition
of strong nucleophiles can occur without external acti-
vation, weaker addends are often assisted by a Lewis
acid, either in stoichiometric amounts or in catalytic
quantities.3 Even with strong nucleophiles, use of a
Lewis acid can be advantageous in order to modulate
the reactivity or control the stereoselectivity. The role
of the Lewis acid in these reactions is mostly thought
to involve further polarization of the CdN bond via
coordination of the Lewis acid to the imine nitrogen lone
pair, although few detailed mechanistic studies probing
this question have been reported.4
hypothesized that competitive reaction via these isomers
can affect the stereochemical outcome of addition to the
imine, there are few studies which experimentally
demonstrate that such syn-anti isomerism is an im-
portant factor to consider.5 The most striking recent
example is the enantioselective reduction of imines, as
mediated by titanocene catalysts reported by Buchwald
et al.6 In this chemistry, the enantioselectivity can be
modulated by reducing the H2 pressure and allowing
for syn-anti isomerization to become kinetically com-
petitive with hydrogenation.
* To whom correspondence should be addressed. Phone: 403-220-
5746. Fax: 403-289-9488. E-mail: wpiers@ucalgary.ca.
† University of Calgary.
‡ University of Alberta.
(1) S. Robert Blair Professor of Chemistry 2000-2005. NSERC E.
W. R. Steacie Fellow 2001-2003.
(2) Larock, R. C. Comprehensive Organic Transformations, 2nd ed.;
Wiley-VCH: New York, 1999; pp 835-866.
Despite the importance of Lewis acid/imine adducts
in this arena, few studies concerning the solution and
(3) (a) Denmark, S. E.; Nicaise, O. J .-C. In Comprehensive Asym-
metric Catalysis; J acobsen, E. N., Pfaltz, A., Yamamoto, H., Eds.;
Springer-Verlag: New York, 1999; Vol. 2, pp 923-964. (b) Bloch, R.
Chem. Rev. 1998, 98, 1407. (c) Enders, D.; Reinhold, U. Tetrahedron:
Asymmetry 1997, 8, 1895.
(5) (a) Keck, G. E.; Enholdm, E. J . J . Org. Chem. 1985, 50, 146. (b)
Shimizu, M.; Kume, K.; Fujisawa, T. Tetrahedron Lett. 1982, 23, 3739.
(c) Chen, C.-M.; Brown, H. C. J . Am. Chem. Soc. 2000, 122, 4217.
(6) Willoughby, C. A.; Buchwald, S. L. J . Am. Chem. Soc. 1994, 116,
8952.
(4) Aubrecht, K. B.; Winemillar, M. D.; Collum, D. B. J . Am. Chem.
Soc. 2000, 122, 11084.
10.1021/om011086n CCC: $22.00 © 2002 American Chemical Society
Publication on Web 02/28/2002

Imine Adducts with Tris(pentafluorophenyl)borane
Organometallics, Vol. 21, No. 7, 2002 1401
Sch em e 1
solid-state structures of the adducts between monoden-
tate imines and main-group Lewis acids have been
undertaken.7 Our interest in addition reactions to
which exhibits a rigid structure in solution, as evidenced
by the observation of 15 separate signals for the aryl
fluoro groups in the 19F NMR spectrum at room tem-
perature. The absence of exchange both between and
carbonyl8 and imine9 functions as catalyzed by the
10-12
strong organometallic Lewis acid B(C6F5)3
has led
us to explore the stoichiometric reactivity of this borane
with simple aldimines and ketimines. Due to the high
stability and strength of this Lewis acid, adducts with
a variety of weak Lewis bases have been readily
generated and studied both in solution and in the solid
state.13 Herein we describe stoichiometric reactions
between B(C6F5)3 and four N-benzyl imines and one
N-phenyl imine and the solution and solid-state char-
acteristics of the resulting adducts. The nature of the
thermodynamic products of these complexations is de-
pendent on the features of the imine, and the results
have implications for the role of the Lewis acid in the
promoted reactions of imines with nucleophiles.
within the C6F5 rings suggests that rotation about the
B-N vector and the B-C bonds is restricted in this
congested adduct. Heating the sample to 60 °C results
in severe broadening in the spectrum, indicating that
some freedom to rotate exists. In the 1H NMR spectrum,
the benzylic protons are diastereotopic in the static
structure but are observed to coalesce at about 40 °C,
corresponding to a barrier of ∆Gq ) 59.9(5) kJ mol-1
for the exchange of these two protons.
Resu lts a n d Discu ssion
Solu tion Str u ctu r e a n d Dyn a m ics of B(C6F 5)3-
Im in e Ad d u cts. The symmetrical N-benzyl ketimine
Ph2CdNBn forms an adduct with B(C6F5)3, 1, (eq 2),
For unsymmetrical ketimines or aldimines, the situ-
ation is somewhat more complex. In these systems, a
kinetic adduct is observed initially, followed by slow
emergence of a thermodynamic adduct (Scheme 1) over
the course of about 1 h. For example, for the reaction
between B(C6F5)3 and the ketimine Ph(CH3)CdNBn
(which exists in solution as a 17:1 equilibrium mixture
of the E and Z isomers6) at -40 °C, the initial adduct
(7) Of course, many examples of transition-metal and main-group
complexes that incorporate ligands in which an imine function is part
of a chelate array exist; we do not include this class of compounds in
this statement.
(8) (a) Parks, D. J .; Piers, W. E. J . Am. Chem. Soc. 1996, 118, 9440.
(b) Parks, D. J .; Blackwell, J . M.; Piers, W. E. J . Org. Chem. 2000, 65,
3090. (c) Blackwell, J . M.; Piers, W. E. Org. Lett. 2000, 2, 695. (d)
Blackwell, J . M.; Piers, W. E.; McDonald, R. J . Am. Chem. Soc. 2002,
124, 1295.
1
2-k is observed by both H and 19F NMR spectroscopy.
When the temperature is raised, a new adduct, 2-t,
appears, eventually supplanting the kinetic adduct such
that the final ratio is approximately 10:1 in toluene. In
C6D6, where the final ratio of 2-t to 2-k is 5:1, 1-D NOE
experiments support the assignment of the kinetic
adduct as the isomer with the borane syn to the imine
phenyl group. For example, irradiation of the N-benzylic
protons in 2-k results in NOE enhancement of the
methyl protons; such an enhancement is not observed
in 2-t. Furthermore, in a 19F/1H NOE experiment,
irradiation of the o-fluorine resonances in 2-t results in
a positive enhancement of the ketimine methyl protons
which is absent in the kinetic structure. As indicated
in Scheme 1, similar results are obtained for the
aldimine Ph(H)CdNBn, where the products 3-k and 3-t
are observed to form.
(9) Blackwell, J . M.; Sonmor, E.; Scoccitti, T.; Piers, W. E. Org. Lett.
2000, 2, 3921.
(10) (a) Massey, A. G.; Park, A. J .; Stone, F. G. A. Proc. Chem. Soc.
1963, 212. (b) Massey, A. G.; Park, A. J . J . Organomet. Chem. 1964,
2, 245.
(11) For reviews on the use of this Lewis acid in organic synthesis
see: (a) Piers, W. E.; Chivers, T. Chem. Soc. Rev. 1997, 345. (b)
Ishihara, K.; Yamamoto, Eur. J . Org. Chem. 1999, 527, 7. For a review
on its use in organometallic chemistry see: Chen, E. Y.-X.; Marks, T.
J . Chem. Rev. 2000, 100, 1391.
(12) Recent examples of the use of B(C6F5)3 as a Lewis acid in
organic transformations: (a) Blackwell, J . M.; Foster, K. L.; Beck, V.
H.; Piers, W. E. J . Org. Chem. 1999, 64, 4887. (b) Hassfield, J .;
Christmann, M.; Kalesse, M. Org. Lett. 2001, 3, 3561. (c) Gevorgyan,
V.; Liu, J .-X.; Rubin, M.; Benson, S.; Yamamoto, Y. Tetrahedron Lett.
1999, 40, 8919. (d) Gevorgyan, V.; Rubin, M.; Benson, S.; Liu, J .-X.;
Yamamoto, Y. J . Org. Chem. 2000, 65, 6179. (e) Rubin, M.; Gevorgyan,
V. Org. Lett. 2001, 3, 2705. (f) Imamura, K.; Yoshikawa, E.; Gevorgyan,
V.; Sudo, T.; Asao, N.; Yamamoto, Y. Can. J . Chem. 2001, 79, 1624.
(13) (a) Ro¨ttger, D.; Erker, G.; Fro¨hlich, R.; Kotila, S. J . Organomet.
Chem. 1996, 518, 17. (b) Bradley, D. C.; Hursthouse, M. B.; Motevalli,
M.; Zheng, D. H. J . Chem. Soc., Chem. Commun. 1991, 7. (c) Bradley,
D. C.; Harding, I. S.; Keefe, A. D.; Motevalli, M.; Zheng, D. H. J . Chem.
Soc., Dalton Trans. 1996, 3931. (d) Parks, D. J .; Piers, W. E.; Parvez,
M.; Atencio, R.; Zaworotko, M. J . Organometallics 1998, 17, 1369.
The process by which this isomerization of adducts
occurs is likely via the free imines, rather than a Lewis
acid activated structure akin to I depicted in Scheme

1402 Organometallics, Vol. 21, No. 7, 2002
Blackwell et al.
t
1.14 This is supported by the observation that, in the
presence of an excess of borane, the isomerization of 2-k
to 2-t or of 3k to 3t is significantly inhibited. The rate-
limiting step in the adduct isomerization is likely the
thermal isomerization of the free imine rather than the
dissociative step necessary to produce free imine. At
room temperature, the adducts are kinetically labile, as
demonstrated by the facile dissociative substitution of
less basic imines (e.g. Ph2CdNBn) with more basic
bonding partners (e.g. Ph(CH3)CdNBn). These equilib-
ria are established essentially upon mixing. On the
other hand, first-order rate constants for the thermal
E/ Z isomerization of N-alkyl-substituted imines (which
occurs via inversion at the imine N) yield half-lives on
the order of a few hours for this process,15 the same time
scale as the observed equilibration of 2-k /3-k to 2-t/3-t.
Thus, upon mixing, the borane reacts rapidly with the
thermodynamically prevalent free imine to give the
more sterically hindered adduct but gradually converts
to the thermodynamically more favored adduct by
trapping the less stable Z isomer of the imine.
tion of 1 equiv of borane to Bu(CH3)CdNBn, approxi-
mately 40% of the imine is converted to a new compound
for which the NMR spectral data is in disaccord with
those expected for either kinetic or thermodynamic
N-bound adducts analogous to those described above.
This new product predominates in solution when 3 equiv
more of B(C6F5)3 is added. In the 19F NMR spectrum of
this mixture, in addition to signals for free borane, three
new signals are observed, suggesting that free rotation
of the C6F5 moieties is possible in this species. The 11B
NMR spectrum shows a resonance at -12.2 ppm, about
10 ppm upfield of the resonances typically observed for
adducts 1-4 and indicative of a borate-type boron rather
than a neutral, four-coordinate center. The 1H NMR
spectrum shows three nonaromatic signals in a 2:2:9
ratio rather than the 2:3:9 ratio observed for the free
ketimine. Finally, the 13C NMR spectrum shows four
nonaromatic signals, including one at 212.8 ppm for the
imine CdN carbon. This resonance is shifted signifi-
cantly downfield relative to that observed for the free
imine (175.1 ppm) in C6D6. These data are consistent
with the formation of the iminium zwitterion 5, as
shown in eq 3. This species presumably forms because
Spectroscopically, the kinetic adducts 2-k and 3-k ,
where B(C6F5)3 is found syn to the imine phenyl group,
behave very similarly to adduct 1, where the borane has
no choice but to be syn to a phenyl group. In the 19F
NMR spectra of these adducts, 15 separate signals are
again observed, and coalescence behavior in the benzylic
1
protons is found in the H NMR spectra, with a barrier
for this exchange of ∆Gq ) 58.5(5) kJ mol-1 at 20 °C
estimated for kinetic adduct 3-k . A measurement of the
analogous barrier for 2-k could not be made, since the
diastereotopic benzylic protons in 2-k have nearly
coincident chemical shifts. The barrier in 3-t, where
borane is syn to the aldimine proton, is somewhat lower
(∆Gq ) 52.1(5) kJ mol-1 at -10 °C). Nonetheless, in both
2-t (∆Gq ) 66.6(5) kJ mol-1 at 56 °C) and 3-t, restricted
rotation in the B-N and B-C bonds is observed,
resulting in diastereotopic benzylic protons and C6F5
rings and attesting to the steric crowding inherent even
in the thermodynamic structures of these adducts.
To probe the nature and strength of C6F5/C6H5
π-stacking interactions in these adducts (see below), the
reactions of two further imines with B(C6F5)3 were
studied. For N-benzylideneaniline (Ph(H)CdNPh), in
which the N-Bn group is replaces with an N-Ph
substitutent, solution chemistry very similar to that
described above was observed. However, in this in-
stance, only the thermodynamic isomer of the adduct,
4-t, was fully characterized. The last imine studied was
steric effects destabilize borane binding to the imine
nitrogen. The sterically demanding Bu group cannot
accommodate either the borane or the benzyl group in
a syn orientation. Thus, the borane traps the imine as
its enamine tautomer, forming 5.
t
This assignment was confirmed by X-ray crystal-
lography; an ORTEP diagram of 5 is given in Figure 1,
along with selected bond distances and angles. The
metrical data are supportive of the zwitterionic reso-
nance form, although the C(1)-N(1) bond length of
1.3024(18) Å is slightly longer than the typical CdN
distance of 1.28 Å and the C(1)-C(2) separation of
1.4750(19) Å is somewhat shorter than that expected
for an sp2-sp3 C-C bond (∼1.50 Å). Also, the B-C(2)
distance of 1.715(2) Å is rather long in comparison to
the other B-C bonds in the structure and also to the
B-C bond length of 1.675(2) Å in the related adduct
B(C6F5)3‚CH2PPh3.16 These data are thus consistent
with the observation that, in solution, this is a weak
adduct whose formation must be driven by the presence
of excess Lewis acid. To our knowledge, this type of
reactivity between an imine and a Lewis acid has not
been observed or given much consideration previously.
Solid Sta te Str u ctu r es of N-Bou n d Im in e Ad -
d u cts. The adducts 1-4 are all white crystalline solids
which can be isolated analytically pure in 50-90% yield
after one recrystallization. Thus, it was possible to
analyze the adducts 1, 2-k , 2-t, 3-t, and 4-t by X-ray
crystallography. Comparative metrical data are given
in Table 1; for 2-k , two independent molecules with
similar parameters were refined. In general, the in-
t
the aliphatic ketimine Bu(CH3)CdNBn, which under-
goes an unexpected reaction with B(C6F5)3. Upon addi-
(14) Acid-catalyzed isomerizations of imines generally occur only in
the presence of a nucleophile (usually the counteranion of the acid
moiety): J ohnson, J . E.; Morales, N. M.; Gorczyca, A. M.; Dolliver, D.
D.; McAllister, M. A. J . Org. Chem. 2001, 66, 7979 and references
therein.
(15) McCarty, C. G. In The Chemistry of the Carbon-Nitrogen Double
Bond; Patai, S., Ed.; Wiley-Interscience: Toronto, 1970; p 363.
(16) Do¨ring, S.; Erker, G.; Fro¨hlich, R.; Meyer, O.; Bergander, K.
Organometallics 1998, 17, 2183.

Imine Adducts with Tris(pentafluorophenyl)borane
Organometallics, Vol. 21, No. 7, 2002 1403
Ta ble 2. Dista n ces (Å) betw een Sta ck ed C6F 5 a n d
N-Ben zyl C6H5 Rin gs
position
1
2-k (a)
2-k (b)
2-t
3-t
a/a′
b/b′
c/c′
d/d′
e/e′
f/f′
3.052
3.338
3.829
3.992
3.691
3.212
3.123
3.503
3.905
3.997
3.652
3.219
3.092
3.476
3.945
4.018
3.621
3.178
3.121
3.392
3.769
3.935
3.686
3.237
3.062
3.119
3.454
3.710
3.655
3.336
av
3.519
3.567
3.555
3.523
3.389
C6H6 pair have been measured,18 while other studies
suggest that each fluorine contributes about 2.5 kJ
mol-1 to the interaction strength.19 Nonetheless, these
weakly energetic interactions can have remarkable
effects on subsequent chemistry,20 and in the present
study, the stereochemical rigidity observed in solution
for several of these adducts can be at least partially
rationalized in terms of the intramolecular stacking
interactions.
F igu r e 1. ORTEP diagram of 5. Selected bond distances
(Å): N-C(1), 1.305(2); C(1)-C(2), 1.475(2); B-C(2), 1.715-
(2); B-C(21), 1.665(2); B-C(31), 1.657(2); B-C(41), 1.667-
(2); N-C(10), 1.489(2). Selected bond angles (deg): C(10)-
N-C(1), 126.7(1); N-C(1)-C(2), 119.5(1); N-C(1)-C(3),
117.2(1); C(3)-C(1)-C(2), 112.7(1); C(1)-C(2)-B, 121.6-
(1).
Two different intramolecular π-stacking interactions
are found in this family of compounds. One exists
between the phenyl group of the benzyl moiety and a
borane C6F5 ring, while the other is between the syn
iminyl phenyl group and is similar in character to those
found in the B(C6F5)3‚PhC(O)R adducts mentioned
above.13d Table 2 collects the intrastack distances for
the N-benzyl/C6F5 interaction in these imine adducts,
while Table 3 lists those for the iminyl phenyl/C6F5
stacks. The average separation between the ring atoms
found in all cases are well within observed distances
for other π-stacked complexes.13d,20
Ta ble 1. Metr ica l P a r a m eter s for N-Bou n d
B(C6F 5)3-Im in e Ad d u cts
CdN
(Å)
N-B
(Å)
N-CBn CdN- CdN-B CBn-N-
(Å) CBn (deg) (deg) B (deg)
adduct
1
2-k
a
1.297(6) 1.642(8) 1.502(6) 117.0(4) 133.2(5) 109.6(4)
1.300(5) 1.630(6) 1.504(5) 116.8(4) 132.2(4) 110.6(3)
1.310(5) 1.658(6) 1.498(5) 118.0(4) 132.2(4) 109.5(4)
1.293(2) 1.640(2) 1.502(2) 118.1(1) 131.2(2) 110.5(1)
1.285(2) 1.627(3) 1.481(2) 121.1(2) 124.6(2) 114.3(1)
1.296(4) 1.649(4) 1.454(4) 118.4(6) 119.6(3) 121.8(2)
b
2-t
3-t
4-ta
a
CBn in this compound is Cipso of the N-phenyl group.
Adducts 1 and 2-k , where the borane is coordinated
syn to an iminyl phenyl group, feature both of these
stacking interactions. Figures 2 and 3 show views of 1
and 2-k , respectively, which emphasize the two interac-
tions. In general, the average values for the intrastack
distances of the aromatic carbons are slightly smaller
for the benzyl/C6F5 interaction than the iminyl phenyl/
C6F5 stack but both interactions are geometrically very
similar, being separated by the same number of atoms.
In the adducts 2-t and 3-t , where the borane is now
coordinated anti to the iminyl phenyl group, this stack-
ing interaction is lost and only that with the N-benzyl
group is possible. Figure 4 shows a view of 2-t which
illustrates the presence of only one intramolecular
stacking interaction with the N-Bn group. Interest-
tramolecular metrical data are similar in these adducts.
The CdN distances are only slightly elongated from the
expected value of ∼1.28 Å for a “normal” CdN bond;
B-N distances are somewhat longer than those found
in B(C6F5)3-nitrile adducts (1.595(3)-1.616(3) Å17), as
expected on the basis of the higher steric requirements
of imine Lewis bases. The angles about N can be
rationalized by consideration of the steric properties of
the group syn to the borane.
Perhaps more interesting are the nonbonding interac-
tions present in these molecules, in the form of π-stack-
ing interactions between -C6F5 and -C6H5 moieties. We
have previously noted such an interaction in the solid-
state structures of the adducts between B(C6F5)3 and
PhC(O)R (R ) OEt, NiPr2) and, through low-tempera-
ture NMR spectroscopy, demonstrated their importance
in solution structures as well.13d The ability of these two
groups to stack relates to their oppositely charged
quadrupoles; perfluorinated rings are electron poor
above the aromatic plane, whereas protioaryl rings are
electron rich. The strength of the interaction is not
great; estimates of about 15.5 kJ mol-1 for the C6F6/
(18) Williams, J . H. Acc. Chem. Res. 1993, 26, 593.
(19) Cozzi, F.; Ponzini, F.; Annunziata, R.; Cinquinei, M.; Siegel, J .
S. Angew. Chem., Int. Ed. Engl. 1995, 34, 1019.
(20) (a) Coates, G. W.; Waymouth, R. M. Science 1995, 267, 217. (b)
Pietsch, M. A.; Rappe, A. K. J . Am. Chem. Soc. 1996, 118, 10908. (c)
Coates, G. W.; Dunn, A. R.; Henling, L. M.; Dougherty, D. A.; Grubbs,
R. H. Angew. Chem., Int. Ed. Engl. 1997, 36, 248. (d) Coates, G. W.;
Dunn, A. R.; Henling, L. M.; Ziller, J . W.; Lobkovsky, E. B.; Grubbs,
R. H. J . Am. Chem. Soc. 1998, 120, 3641. (e) Weck, M.; Dunn, A. R.;
Matsumoto, K.; Coates, G. W.; Lobkovsky, E. B.; Grubbs, R. H. Angew.
Chem., Int. Ed. 1999, 38, 2741. (f) Dai, C.; Nguyen, P.; Marder, T. B.;
Scott, A. J .; Clegg, W.; Viney, C. Chem. Commun. 1999, 2493.
(17) J acobsen, H.; Berke, H.; Do¨ring, S.; Kehr, G.; Erker, G.;
Fro¨hlich, R.; Meyer, O. Organometallics 1999, 18, 1724,

1404 Organometallics, Vol. 21, No. 7, 2002
Blackwell et al.
F igu r e 3. ORTEP diagram of 2-k (molecule A). See Tables
1-3 for selected metrical parameters.
F igu r e 2. ORTEP diagram of 1. See Tables 1-3 for
selected metrical parameters.
Ta ble 3. Dista n ces (Å) betw een Sta ck ed C6F 5 a n d
Im in yl C6H5 Rin gs
position
1
2-k (a)
2-k (b)
a/a′
b/b′
c/c′
d/d′
e/e′
f/f′
3.084
3.579
4.176
4.243
3.795
3.313
3.062
3.534
4.121
4.224
3.756
3.258
3.017
3.445
3.887
3.901
3.543
3.173
av
3.698
3.659
3.494
F igu r e 4. ORTEP diagram of 2-t. See Tables 1-3 for
selected metrical parameters.
ingly, however, for aldimine adduct 3-t, which crystal-
lizes in a different space group than ketimine adduct
2-t, intermolecular stacking interactions are observed.
While only one intramolecular stack is possible (Figure
5a), the molecules line up in the crystal such that an
alternating arrangement of benzylic C6H5 and B-C6F5
rings are made possible (Figure 5b). The intermolecular
distances between the rings range from 3.34 to 3.967
Å, which are again well within the distances expected
for stacked rings. This motif is absent in the packing
structures of all the other adducts described herein,
including the closely related adduct 2-t, which differs
only in the presence of a methyl group on the imine
carbon rather than hydrogen. Finally, the structure of
the adduct with the N-phenyl-substituted imine, 4-t ,
reveals that a stacking interaction with this group is
geometrically more difficult (Figure 6) due to the dif-
ficulty in attaining an eclipsed geometry. As a result,
the average inter-ring distance of 4.23 Å is somewhat
larger for this species compared to those discussed
above.
-C6F5 rings and the benzylic protons in these adducts
in that rotation about the N-B and N-CBn bonds may
be to some extent “geared” by π-stacking interactions,
as shown in Scheme 2. For example, this effect may
partially rationalize why the barrier of 58.5(5) kJ mol-1
(20 °C) for exchange of the benzylic protons in 3-k ,
which has two stacking interactions available, is higher
than that found for the thermodynamic isomer 3-t (52.1-
(5) kJ mol-1, -10 °C) in which only one stacking
interaction is possible. However, since adduct 4-t, where
the π-stacking interaction with the N-phenyl group is
weaker, also exhibits a rigid structure (as evidenced by
the 15 inequivalent 19F nuclei), more standard steric
factors must also be at play in restricting the rotation
about the N-B and N-CBn bonds. Still, these stacking
interactions undoubtedly play a role in the solution
structure and dynamic behavior of adducts 1-3, in
particular at low temperatures, and these effects may
be important in directing the subsequent reactivity of
the adducts with nucleophiles.
It is possible that these π-stacking interactions may
contribute to the barriers observed for exchange of the
In summary, we have prepared a number of imine
adducts of B(C6F5)3 and found that, while kinetic

Imine Adducts with Tris(pentafluorophenyl)borane
Organometallics, Vol. 21, No. 7, 2002 1405
F igu r e 6. ORTEP diagram of 4-t. See Tables 1-3 for
selected metrical parameters.
ketimines were synthesized by mixing the ketone and amine
in toluene and heating to reflux using a Dean-Stark appara-
tus in the presence of catalytic ZnCl2, followed by purification.
Syn th esis of P h 2CdN(Bn )‚B(C6F 5)3 (1). Crystals of ad-
duct 1 suitable for X-ray crystallography and elemental
analysis were obtained by mixing B(C6F5)3 (51 mg, 0.10 mmol)
and Ph2CdNPh (30 mg, 0.11 mmol) in toluene (approximately
0.5 mL) in a 1 dram vial. Hexane was added until the mixture
became cloudy, and then the mixture was heated until all of
the precipitate dissolved. The mixture was cooled to room
temperature, over which time colorless crystals formed (yields
ranging from 50 to 90%) from the first crystallization. NMR
spectral characterization was carried out by mixing 1:1 ratio
1
(0.1 mmol) of 1 and Ph2CdNPh in C7D8. H NMR (C7D8, -40
°C): 7.89 (t, 1H, J ) 9.2 Hz), 7.75 (br s, 1H), 6.95-6.24 (m,
13H), 5.79 (d, 1H, J ) 15.6 Hz), 4.87 (d, 1H, J ) 15.6 Hz); 1H
NMR (C7D8, 300 MHz, 25 °C; δ): 8.20-6.24 (m, 15H), 5.79 (br
s, 1H), 4.98 (br s, 1H). 13C NMR (C6D6, 100 MHz, -40 °C; δ):
190.2 (CdN), 139.4, 138.2, 136.3, 136.2, 133.8, 132.0, 131.7,
131.1, 129.3, 128.4, 128.1, 128.0, 127.4, 127.2, 126.9, 126.0,
125.4, 124.4, 61.1 (d), 21.8. 19F NMR (C7D8, 282 MHz, -40 °C;
δ): -119.5 (app t, 1F, o-F), -123.6 (app t, 1F, o-F), -129.0 (d,
1F, o-F), -131.4 (d, 1F, o-F), -132.5 (app t, 1F, o-F), -137.0
(app t, 1F, o-F), -155.6 (app t, 1F, p-F), -156.4 (app t, 1F,
p-F), -156.8 (app. t, 1F, p-F), -162 to -163 (m, 4F, m-F’s),
-163.8 (app t, 1F, m-F), -164.2 (app t, 1F, m-F). 11B NMR
(C7D8, 64.18 MHz, 25 °C; δ): -3.7. IR (KBr, cm-1): 1646 (Cd
N), 1573, 1513, 1470, 1282, 1088, 972, 796, 698. Anal. Calcd
for C38H17NBF15: C, 58.26; H, 2.19; N, 1.79. Found: C, 58.17;
H, 2.28; N, 1.75.
F igu r e 5. (a) ORTEP diagram of 3-t. See Tables 1-3 for
selected metrical parameters. (b) Partial crystal packing
diagram for 3-t, showing the intermolecular stacking
arrangement.
adducts with the thermodynamically most stable geo-
metric isomer of the free imine can be formed at low
temperature, under conditions where the imines are
labile the thermodynamic adduct (with the less stable
geometric isomer of the imine) is readily formed. In
cases where neither isomer of the free imine coordinates
to the borane strongly, an adduct can be formed through
the enamine tautomer (5) if R-protons are present.
While these results suggest that the geometry of the
adducts can be controlled in stoichiometric regimes,
under conditions where borane is present in catalytic
quantities, the speciation and reactivity of the imine-
borane adducts will be governed by the complex inter-
play of the relevant rate constants.
Ch a r a cter iza tion of (E)-P h (CH3)CdN(Bn )‚B(C6F 5)3 (2-
k ). Crystals of adduct 2-k were obtained while attempting to
isolate the thermodynamic isomer 2-t. tThe imine Ph(CH3)Cd
NBn and B(C6F5)3 were mixed in a 1:1 ratio (0.10 mmol) in
toluene. A white precipitate formed that was dissolved by
heating gently; the solution was cooled slowly to room tem-
perature. X-ray-quality crystals deposited from this mixture,
which contained both 2-k and 2-t, although visually different
crystals were not distinguished in this sample. The crystal
structure obtained was of the kinetic isomer 2-k . To character-
ize 2-k by NMR spectroscopy, the adduct was generated at
low temperature by adding a solution of B(C6F5)3 in C7D8 to
Ph(CH3)CdNBn dissolved in C7D8 in an NMR tube cooled to
-78 °C. 1H NMR (C7D8, 400 MHz, -20 °C; δ): 7.38 (t, 1H),
6.89 (t, 1H), 6.82-6.76 (m, 3H), 6.71 (t, 1H), 6.52 (br s, 1H),
Exp er im en ta l Section
Gen er a l Con sid er a tion s. General procedures have been
described in detail elsewhere.8d The imines used in this study
were either purchased from Aldrich-Sigma and used as
received or prepared via standard methodology. Aldimines
were prepared by condensing the aldehyde and amine in CH2-
Cl2 followed by purification by recrystallization or distillation;
6.34-6.22 (m, 3H), 4.69 (br s, 2H, CH2), 1.51 (s, 3H, CH3). 19
F
NMR (C7D8, 282 MHz, -40 °C; δ): -118.1 (br t, 1F, o-F),
-124.4 (t, 1F, o-F), -129.3 (d, 1F, o-F), -133.0 (m, 2F, o-F’s),

1406 Organometallics, Vol. 21, No. 7, 2002
Blackwell et al.
Sch em e 2
-139.6 (s, 1F, o-F), -155.9 (t, 1F, p-F), -156.1 (t, 1F, p-F),
-156.9 (t, 1F, p-F), -162.4 to -163.2 (m, 4F, m-F’s), -164.1
to -164.6 (m, 2F, m-F’s). 13C NMR analysis was precluded by
the propensity of 2-k to precipitate over longer periods of time
in C7D8, CD2Cl2, and other solvents. The infrared spectrum
and elemental analysis were obtained on the solid that
immediately precipitates when B(C6F5)3 and Ph(CH3)CdNBn
are mixed at low temperature. IR (KBr, cm-1): 1639 (CdN),
1601, 1515, 1459, 1278, 1088, 979, 760, 698, 504. Anal. Calcd
for C33H15NBF15: C, 54.95; H, 2.10; N, 1.94. Found: C, 54.54;
H 2.05; N, 1.94.
Hz), 4.31 (d, 1H, J ) 14.9 Hz). 19F NMR (C7D8, 282 MHz, -40
°C; δ): -121.1 (br s, 1F, o-F), -122.6 (app t, 1F, o-F), -124.3
(d, 1F, o-F), -127.0 (br s, 1F, o-F), -127.9 (d, 1F, o-F), -132.0
(br s, 1F, o-F), -150.7 (app t, 2F, p-F), -150.9 (app t, 1F, p-F),
-157.0 (app td, 1F, m-F), -157.5 (app td, 1F, m-F), -158.0
(app td, 1F, m-F), -158.3 (app td, 1F, m-F), -159.2 (app td,
1F, m-F), -159.5 (app td, 1F, m-F). 11B NMR (C6D6, 64.18
MHz, 25 °C; δ): -6.6. Infrared spectral and analytical data
were obtained on precipitate formed at low temperature. IR
(KBr, cm-1): 1651 (CdN), 1531, 1383, 1290, 1115 (br), 963
(br), 793. Anal. Calcd for C33H13NBF15: C, 54.34; H, 1.85; N,
1.98. Found: C, 54.84; H, 2.00; N, 1.89.
Syn t h esis of (Z)-P h (CH 3)CdN(Bn )‚B(C6F 5)3 (2-t ). A
thermodynamic mixture of 2-t and 2-k (10:1) was prepared
by mixing Ph(CH3)CdNBn and B(C6F5)3 in C7D8 followed by
heating to redissolve the precipiate that initially forms.
Syn th esis of (Z)-P h (H)CdN(Bn )‚B(C6F 5)3 (3-t). Colorless
crystals of 3-t were obtained by dissolving a 1:1 mixture of
B(C6F5)3 and Ph(H)CdNBn (0.1 mmol) in toluene, adding
hexane until cloudy, heating to dissolution, and then standing
at room temperature (77% recovery). 1H NMR (300 MHz, -40
°C; δ): 8.68 (d, 1H, J ) 7.2 Hz, CHdN), 7.05 (brs, 2H), 6.79
(t, 1H, J ) 7.4 Hz), 6.70-6.63 (m, 3H), 6.57 (t, 2H, J ) 7.4
Hz), 6.35 (m, 2H), 5.25 (d, 1H, J ) 15.6 Hz), 4.87 (d, 1H, J )
15.6 Hz). 19F NMR (282 MHz, -40 °C; δ): -129.0 (m, 2F, o-F’s),
-129.2 (br s, 1F, o-F), -131.2 (d, 1F, o-F), -134.2 (d, 1F, o-F),
-136.3 (app t, 1F, o-F), -153.5 (app t, 1F, p-F), -155.9 (app
t, 1F, p-F), -156.0 (app t, 1F, p-F), -159.8 (app td, 1F, m-F),
-161.8 - -162.3 (m, 2F, m-F’s), -162.5 - -162.8 (m, 2F,
m-F’s), -163.9 (m, 1F, m-F). 13C NMR (CDCl3, 100 MHz, -40
°C; δ): 171.0 (CdN), 135.6, 132.8, 132.4, 130.2, 128.6, 128.4,
127.8, 125.3, 56.2 (d, CH2) (broad resonances from B(C6F5)3
not included). 11B NMR (C7D8, 64.18 MHz, 25 °C; δ): -3.3.
1
Complete NMR spectral characterization was carried out. H
NMR of 2-t (300 MHz, -40 °C; δ): 7.25-6.35 (m, 8H), 6.08
(m, 2H), 5.27 (d, 1H, J ) 16.9 Hz), 5.02 (d, 1H, J ) 16.9 Hz),
1.94 (s, 3H). 1H NMR (300 MHz, 25 °C; δ): 6.83-6.52 (m, 8H),
6.16 (m, 2H), 5.23 (d, 1H, J ) 16.4 Hz), 5.03 (d, 1H, J ) 16.4
Hz), 2.09 (br s, 3H). 19F NMR (282 MHz, -40 °C; δ): -129.8
(br s, 2F, o-F’s), -130.2 (d, 1F, o-F), -130.5 (d, 1F, o-F), -132.7
(br s, 2F, o-F’s), -155.0 (app t, 1F, p-F), -155.2 (app t, 1F,
p-F), -155.4 (app t, 1F, p-F), -160.8 (app t, 1F, m-F), -161.4
(app t, 1F, m-F), -162.2 (app t, 1F, m-F), -162.6 (m, 1F, m-F),
-162.9 (m, 1F, m-F), -163.4 (app t, 1F, m-F) (plus minor
resonances for t-k ). 13C NMR (C6D6, 100 MHz, 25 °C; δ):
191.82 (CdN), 138.31, 135.14, 130.30, 128.76, 128.05, 127.15,
125.48, 124.82, 60.55 (m, CH2), 28.94 (m, CH3). 11B NMR
(128.34 MHz, 25 °C; δ): -4.7. Crystals of 2-t were obtained
from the 10:1 mixture of 2-t and 2-k (0.10 mmol) dissolved in
C7D8 in an NMR tube (62% recovery). IR (KBr, cm-1): 1651
(CdN), 1596, 1519, 1454 (br), 1372, 1279, 1105 (br), 968. Anal.
Calcd for C33H15NBF15: C, 54.95; H, 2.10; N, 1.94. Found: C,
54.51; H, 1.93; N, 1.99%. These IR spectral and analytical data
were obtained on crystalline material obtained under thermo-
dynamic conditions.
Anal. Calcd for C32H12NBF15
: C, 54.34; H, 1.85; N, 1.98.
Found: C, 54.19; H, 1.58; N, 1.98. Infrared spectral and
analytical data were obtained on crystalline material obtained
under thermodynamic conditions.
Ch a r a cter iza tion of P h (H)CdN(P h )‚B(C6F 5)3 (4-t). An
NMR tube was charged with Ph(H)CdNPh (18 mg, 0.10
mmol), B(C6F5)3 (51 mg, 0.10 mmol), and C7D8. NMR spectra
of this compound were measured. 1H NMR (C7D8, 300 MHz,
25 °C; δ): 7.89 (s, 1H, C(H)dN), 7.20-6.50 (m, 10H). 13C NMR
(C6D6, 25 °C; δ): 174.76, 151.03, 143.11, 135.28, 134.61, 133.14,
131.47, 130.02, 129.90, 129.79, 129.57, 129.41, 125.81. 19F
NMR (C7D8, 282 MHz, -20 °C; δ): -122.1 (d, 1F, o-F), -126.1
(d, 1F, o-F), -126.9 (d, 1F, o-F), -132.4 (s, 1F, o-F), -133.8
(d, 1F, o-F), -138.6 (d, 1F, o-F), -155.0 (app t, 1F, p-F), -155.7
(app t, 1F, p-F), -155.9 (app t, 1F, p-F), -162.4 (m, 1F, m-F),
-162.9 (m, 1F, m-F), -163.4 to -163.7 (m, 2F, m-F’s), -164.9
(m, 1F, m-F), -165.1 (m, 1F, m-F). Crystals suitable for X-ray
analysis were obtained from the NMR tube. IR spectral and
analytical data were obtained on this crystalline material. IR
Ch a r a cter iza tion of (E)-P h (H)CdN(Bn )‚B(C6F 5)3 (3-k ).
The adduct 3-k was generated at low temperature by adding
a solution of B(C6F5)3 (26 mg, 0.05 mmol) to the aldimine (9
mg, 0.05 mmol) dissolved in C7D8 in an NMR tube at low
1
temperature. H and 19F NMR spectra could be obtained, but
a
13C NMR spectrum was not obtained, since 3-k precipitates
in the NMR tube soon after it is formed. Warming the reaction
mixture to room temperature leads to conversion of adduct 3-k
1
to adduct 3-t. H NMR (C7D8, 300 MHz, -40 °C; δ): 7.69 (s,
1H), 7.04 (d, 2H, J ) 9.8 Hz), 6.86-6.66 (m, 4H), 6.54 (t, 2H,
J ) 8.5 Hz), 6.32 (d, 2H, J ) 6.9 Hz), 4.68 (d, 1H, J ) 14.9

Imine Adducts with Tris(pentafluorophenyl)borane
Organometallics, Vol. 21, No. 7, 2002 1407
Ta ble 4. 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 Im in e Ad d u cts of B(C6F 5)3.
1
2-k
2-t
3-t
4-t
5
formula
C
38H17BNF15
0.5C8H7
‚
C33H15BNF15
0.5C7H8
767.34
‚
C33H15BNF15
C32H13BNF15
C31H11BNF15
C31H19BNF15
fw
829.42
triclinic
12.706(5)
13.574(7)
11.860(8)
113.10(4)
101.03(5)
96.88(5)
1803.3(20)
P1h
721.27
triclinic
9.6120(3)
11.4664(4)
13.4295(4)
101.71(2)
88.101(2)
86.36(2)
1442.73(8)
P1h
707.24
693.22
701.28
cryst syst
a, Å
monoclinic
22.036(2)
15.833(1)
20.376(1)
79.983(2)
115.582(1)
81.833(1)
6412.0(8)
P21/c
monoclinic
14.144(4)
13.679(4)
15.863(5)
triclinic
9.983(3)
17.673(4)
8.184(2)
monoclinic
16.530(2)
9.8869(9)
18.070(2)
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
space group
Z
115.267(6)
104.60(2)
104.153(2)
2775.2(14)
P21/n
4
1368.1(6)
P1h
2
1.683
0.163
5148
4853
434
0.040
0.041
2863.5(4)
P21/n
4
1.627
0.163
14 351
5877
433
2
8
2
d
calcd, mg m-3
1.527
1.590
1.660
1.693
0.169
13 490
5741
µ, mm-1
0.143
0.154
0.17
no. of rflns
6699
32 358
11 944
6465
no. of unique rflns
6387
12 952
no. of variables
541
957
451
442
R
0.040
Rw
0.040
R1
0.0765
0.1457
0.939
0.046
0.118
1.02
0.0371
0.0870
0.904
0.0340
0.0908
1.031
wR2
GOF
1.40
2.79
(KBr, cm-1): 1667 (CdN), 1651, 1596, 1520, 1088, 766. Anal.
Calcd for C31H11NBF15: C, 53.45; H, 1.60; N, 2.02. Found: C,
53.71; H, 1.43; N, 2.06.
structure was solved by direct methods and expanded using
Fourier techniques. The non-hydrogen atoms were refined
anisotropically; hydrogen atoms were included at geometrically
idealized positions with C-H ) 0.95 Å and were not refined.
All calculations were performed using SHELXTL97.22
2-k , 3-t, 5. Colorless crystals were obtained from toluene
(2-k ), toluene/hexanes (3-t), or C6D6 (5). Data were collected
on a Bruker P4/RA/SMART 1000 CCD diffractometer23 using
Mo KR radiation at -80 °C. Unit cell parameters were
obtained from a least-squares refinement of the setting angles
of 4839 reflections from the data collection. An empirical
absorption correction was applied to the data through use of
the SADABS procedure. The structures were solved using the
direct methods program SHELXS-86,24 and full-matrix least-
squares refinement on F2 was completed using the program
SHELXL-93.25 Hydrogen atoms were assigned positions based
on the geometries of their attached carbon atoms and were
given isotropic thermal parameters 20% greater than the
equivalent isotropic displacement parameters of the attached
carbons. For 2-k , along with one molecule of cocrystallized
toluene, the asymmetric unit was found to contain two
crystallographically independent molecules of 2-k , which
showed no significant differences in bond lengths, bond angles,
or conformational angles.
Syn th esis of Zw itter ion 5. Colorless crystals of 5 were
t
obtained in 44% yield by mixing B(C6F5)3 and Bu(CH3)CdNBn
in a 1:1 ratio (0.1 mmol each) in toluene at room temperature
followed by cooling to -30 °C. Characterization of 5 in solution
t
was achieved by mixing B(C6F5)3 and Bu(CH3)CdNBn in a
4:1 ratio, under which conditions 5 predominates. 1H NMR
(400 MHz, C6D6; δ): 7.07 (br s, 1H, NH), 7.00-6.80 (m, 3H),
6.36 (d, 2H, J ) 6.9 Hz), 3.40 (br s, 2H, CH2B), 3.30 (d, 2H, J
) 5.7 Hz, NCH2Ph), 0.36 (s, 9H). 13C NMR (100 MHz, C6D6;
δ): 212.87 (CdN), 132.39, 130.07, 130.00, 127.58, 50.60, 42.99,
42.95 (C(CH3)3), 27.07 (CH3). 19F NMR (282 MHz, C6D6; δ):
-131.41 (br s, 2F, o-F), -158.88 (br s, 1F, p-F), -164.24 (brs,
2F, m-F). 11B NMR (128.4 MHz, C6D6; δ): -12.2. Infrared
spectral and analytical data were obtained on crystalline 5 that
had been crushed into a powder. IR (KBr, cm-1): 3632 (w),
3540 (w), 3356 (s, NH), 2972, 1644 (CdN), 1618, 1516, 1454,
1368, 1270, 1148, 1081, 974, 862, 754, 688. Anal. Calcd for
C
31H19NBF15: C, 53.09; H, 2.73; N, 2.00. Found: C, 53.09; H,
2.20; N, 1.96.
X-r a y Cr ysta llogr a p h y. A summary of crystal data and
refinement details for all structures is given in Table 4.
Suitable crystals were covered in Paratone oil, mounted on a
glass fiber, and immediately placed in a cold stream on the
diffractometer employed.
1 a n d 4-t. Crystals of 1 and 4-t were grown from a toluene
solution. Measurements were made using a Rigaku AFC6S
diffractometer with a graphite-monochromated Mo KR radia-
tion (λ ) 0.710 69 Å) source at -103 °C with the ω-2θ scan
technique to a maximum 2θ value of 50.1°. The structure was
solved by direct methods and expanded using Fourier tech-
niques. The non-hydrogen atoms were refined anisotropically;
hydrogen atoms were included at geometrically idealized
positions with C-H ) 0.95 Å and were not refined. All
calculations were performed using the teXsan crystallographic
software package of Molecular Structure Corp.21
2-t. Crystals of 2-t were grown from a toluene solution.
Measurements were made using a Nonius KappaCCD diffrac-
tometer with graphite-monochromated Mo KR radiation (λ )
0.710 73 Å). Cell constants obtained from the refinement of
6086 reflections in the range 1.0 < θ < 27.5° corresponded to
a primitive triclinic cell. The data were collected at 170(2) K
using the ω-æ scans to a maximum θ value of 27.5°. 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 Research Grant (to
W.E.P.), an E. W. R. Steacie Fellowship (2001-2003;
to W.E.P.) and Scholarship support (PGSB; to J .M.B.).
J .M.B. also thanks the Killam Foundation for a Fellow-
ship and the Alberta Heritage Foundation for a Stein-
hauer Award. Mr. Eric Sonmor is thanked for prelimi-
nary experiments and technical support at an early
stage of the project.
Su p p or tin g In for m a tion Ava ila ble: Full listings of
crystallographic data, atomic parameters, hydrogen param-
eters, atomic coordinates, and complete bond distances and
angles for 1, 2-k , 2-t, 3-t, 4-t, and 5. This material is available
free of charge via the Internet at http://pubs.acs.org.
OM011086N
(22) Sheldrick, G. M., SHELXTL97: Program for Crystal Structure
Determination; University of Go¨ttingen, Go¨ttingen, Germany, 1997.
(23) Programs for diffractometer operation, data collection, data
reduction, and absorption correction were those supplied by Bruker.
(24) Sheldrick, G. M. Acta Crystallogr. 1990, A46, 467-473.
(21) teXan: Crystal Structure Analysis Package; Molecular Struc-
ture Corp., 1985, 1992.
(25) Sheldrick, G. M. SHELXL-93: Program for Crystal Structure
Determination; University of Go¨ttingen, Go¨ttingen, Germany, 1993.