Stepwise boron-to-zinc C6F5 group transfer in a Zn - calixarene system

Article abstract of DOI:10.1021/om800904y

Methylzinc calixarene complex (1) reacts with one equivalent of B(C ₆F₅)₃ to give the ion-pair (calixarene)-Zn ^δ+ - Me - B(C₆F₅)₃ ^δ- complex (2), which readily undergo

Full text of DOI:10.1021/om800904y

Organometallics 2009, 28, 929–932
929
Stepwise Boron-to-Zinc C6F5 Group Transfer in a Zn-Calixarene
System
Natalie Kotzen, Israel Goldberg, and Arkadi Vigalok*
School of Chemistry, The Sackler Faculty of Exact Sciences, Tel AViV UniVersity, Tel AViV 69978, Israel
ReceiVed September 17, 2008
proceed differently, giving products of the alkyl abstraction
rather than C6F5 transfer (eq 2).5,6
Summary: Methylzinc calixarene complex (1) reacts with one
equiValent of B(C6F5)3 to giVe the ion-pair (calixarene)-
Znδ+-Me-B(C6F5)3 complex (2), which readily undergoes
δ-
RnM-R + B(C6F5)3 h RnM-C6F5 + R-B(C6F5)2 (1)
the pentafluorophenyl group transfer to the zinc center. Reaction
between 1 and H2O-B(C6F5)3 results in the formation of the
LnM-R + B(C6F5)3 h LnM+R-B(C6F5)3-
(2)
ion-pair (calixarene)Znδ+-HO-B(C6F5)3 complex (4). In
δ-
contrast to 2, complex 4 requires harsher reaction conditions
to undergo the C6F5 group transfer, with the reaction taking
place under the autocatalytic conditions.
We recently reported the preparation and reactivity of a series
of bimetallic (zinc) calixarene inclusion complexes (Figure 1).7
In these complexes, one of the alkylzinc centers (exocyclic) is
accessible for interactions with other organic and inorganic
reagents, while the second one (endocyclic) is located deep
inside the hydrophobic calixarene cavity that protects it from
reacting with other reagents.7c The X-ray structures of several
zinc calixarene complexes showed weak complexation of the
ether groups of the calixarene lower rim to the exocyclic metal
center. Here we present the results of our studies of reactivity
of these complexes with perfluoroaryl Lewis acids, including
the unexpected pattern in the pentafluorophenyl group transfer
to Zn.
Introduction
Interactions of B(C6F5)3 and its derivatives with transition
metal alkyl complexes received a great deal of attention in the
development of highly active catalysts for alkene polymeriza-
tion.1 Alkyl, usually methyl, groups can be readily abstracted
by this strong Lewis acid generating an ion-pair with
R-B(C6F5)3 as anion.2 The chemistry of such ion-pairs has
-
been extensively studied, particularly with regard to the catalyst
activity and stereoselectivity of the polymerization process.3 The
degree of coordination of the R-B(C6F5)3- anion with the cation
can significantly influence the catalysts’ activity, while the
pentafluorophenyl group transfer from the anion to the transition
metal center often appears as an important contributor to the
catalyst deactivation.4 In contrast to transition metals, such group
transfer represents a very common reactivity pathway in the
reaction of nontransition metal alkyls with B(C6F5)3. For
example, various alkylzinc and alkylaluminum precursors react
with B(C6F5)3 to produce the corresponding pentafluorophenyl
metal complexes (eq 1).5a,b Interestingly, in coordinating
solvents or in the presence of multidentate ligands the reactions
Results and Discussion
Preparation of the Zwitterionic Calixarene Zinc Complexes.
We recently showed that the reaction between 1 and B(C6F5)3
resulted in the methyl group removal from the exocyclic zinc
center and formation of a new calixarene zinc complex, assigned
8
1
the structure of 2 (Scheme 1). The H NMR spectrum of 2
-
showed the signal of the methyl group of the CH3-B(C6F5)3
anion at 0.2 ppm. Complex 2 was relatively unstable in solution
(vide supra), however, when the bipyridine (bipy) ligand was
added, the quantitative formation of the corresponding thermally
stable adduct 3 was observed (Scheme 1).8 The latter was fully
spectroscopically characterized and showed typical NMR data
* Corresponding author. E-mail: avigal@post.tau.ac.il.
(5) (a) Walker, D. A.; Woodman, T. J.; Hughes, D. L.; Bochmann, M.
Organometallics 2001, 20, 3772–3776. (b) Klosin, J.; Roof, G. R.; Chen,
E. Y.-X.; Abboud, K. A. Organometallics 2000, 19, 4684–4686. (c) Amo,
V.; Andres, R.; de Jesus, E.; de la Mata, F. J.; Flores, J. C.; Gomez, R.;
Gomes-Sal, M. P.; Turner, J. F. C. Organometallics 2005, 24, 2331–2338.
(d) Dagorne, S.; Guzei, I. A.; Coles, M. P.; Jordan, R. F. J. Am. Chem.
Soc. 2000, 122, 274–289. (e) Qian, B.; Ward, D. L.; Smith, M. R., III.
Organometallics 1998, 17, 3070–3076. (f) Milione, S.; Grisi, F.; Centore,
R.; Tuzi, A. Organometallics 2006, 25, 266–274. (g) Spitzmesser, S. K.;
Gibson, V. C. J. Organomet. Chem. 2003, 673, 95–101. (h) Hannant, M. D.;
Schormann, M.; Bochmann, M. J. Chem. Soc., Dalton Trans. 2002, 4071–
4073. (i) Robson, D. A.; Rees, L. H.; Mountford, P.; Schro¨der, M. Chem.
Commun. 2000, 1269–1270.
(1) For general references see: (a) Chen, E. Y.-X.; Marks, T. J. Chem.
ReV. 2000, 100, 1391–1434. (b) Luo, L.; Marks, T. J. Top. Catal. 1999, 7,
97–106. (c) Bochmann, M. Top. Catal. 1999, 7, 9–22.
(2) For a review see Piers, W. E. AdV. Organomet. Chem. 2005, 52,
1–76.
(3) (a) Yang, X.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1991,
113, 3623–3625. (b) Bochmann, M.; Lancaster, S. J.; Hursthouse, M. B.;
Abdul Malik, K. M. Organometallics 1994, 13, 2235–2243. (c) Landis,
C. R.; Rosaaen, K. A.; Sillars, D. R. J. Am. Chem. Soc. 2003, 125, 1710–
1711. (d) Zhang, S.; Piers, W. E.; Gao, X.; Parvez, M. J. Am. Chem. Soc.
2000, 122, 5499–5509. (e) Bochmann, M. J. Chem. Soc., Dalton Trans.
1996, 255–270.
(4) (a) Yang, X.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1994,
116, 10015–10031. (b) Thorn, M. G.; Etheridge, Z. C.; Fanwick, P. E.;
Rothwell, I. P. J. Organomet. Chem. 1999, 591, 148–162. (c) Scollard, J. D.;
McConville, D. H.; Rettig, S. J. Organometallics 1997, 16, 1810–1812.
(d) Jime´nez Pindado, G.; Thornton-Pett, M.; Bochmann, M. J. Chem. Soc.,
Dalton Trans. 1997, 3115–3127. (e) Jime´nez Pindado, G.; Thornton-Pett,
M.; Bouwkamp, M.; Meetsma, A.; Hessen, B.; Bochmann, M. Angew.
Chem., Int. Ed. Engl. 1997, 36, 2358–2361. For theoretical studies see: (f)
Wondimagegn, T.; Xu, Z.; Vanka, K.; Ziegler, T. Organometallics 2004,
23, 3847–3852.
(6) For a comparative analysis of B(C6F5)3 reactivity toward transition
and nontransition metal alkyls, see: Bochmann, M.; Lancaster, S. J.;
Hannant, M. D.; Rodriguez, A.; Schormann, M.; Walker, D. A.; Woodman,
T. J. Pure Appl. Chem. 2003, 75, 1183–1195.
(7) (a) Bukhaltsev, E.; Goldberg, I.; Vigalok, A. Organometallics 2004,
23, 4540–4543. (b) Bukhaltsev, E.; Goldberg, I.; Vigalok, A. Organome-
tallics 2005, 24, 5732–5736. (c) Bukhaltsev, E.; Frish, L.; Cohen, Y.;
Vigalok, A. Org. Lett. 2005, 7, 5123–5126.
(8) Kotzen, N.; Goldberg, I.; Vigalok, A. Inorg. Chem. Commun. 2005,
8, 1028–1030.
10.1021/om800904y CCC: $40.75
2009 American Chemical Society
Publication on Web 01/09/2009

930 Organometallics, Vol. 28, No. 3, 2009
Notes
Figure 1
Figure 4. ORTEP view of a molecule of 6 with thermal ellipsoids
shown at 50% probability. Hydrogen atoms are omitted for clarity.
Selected bond distances (Å) and angles (deg): Zn(1)-C(1) 1.932(3),
Zn(2)-C(2)1.968(2),Zn(1)-O(1)1.9529(16),Zn(2)-O(1)1.9552(16),
Zn(2)-O(2) 2.3384(17), Zn(1)-Zn(2) 3.0134, O(3)-Zn(2)-O(1)
80.38(7), O(4)-Zn(2)-O(2) 162.14(6).
bearing etheric ligands, calixarene lower rim providing the
intramolecular ether complexation.
Figure 2. ORTEP view of a molecule of 3 with thermal ellipsoids
shown at 50% probability. Hydrogen atoms are omitted for clarity.
Selected bond distances (Å) and angles (deg): Zn(1)-C(1) 1.925(4),
Zn(1)-O(1) 1.957(3), Zn(2)-O(1) 2.000(3), Zn(2)-O(2) 2.323(3),
Zn(2)-N(1) 2.070(4), B(1)-C(2) 1.627(7), Zn(1)-Zn(2) 3.0329,
O(3)-Zn(2)-O(1) 80.08(10), O(4)-Zn(2)-O(2) 160.44(9), N(1)-
Zn(2)-N(2) 79.09(14).
The composition of 2, obtained initially in the reaction
between 1 and B(C6F5)3, was clearly established by NMR
spectroscopy and its reactivity with bipy or phen to give 3.
However, our numerous attempts to reproduce the experiments
were only “partially successful”, as the obtained complex 4
showed different spectroscopic properties than 2. In particular,
1
we were puzzled by the fact that no H, 13C and 11B NMR
signals could be assigned to the anion of 4. In contrast, all NMR
signals of the cationic part of the molecule were sharp and well-
resolved at room temperature and appeared to be similar to those
in 2, with the exception of the B-Me signal that was clearly
1
missing. For example, the H NMR signal of the internal Me-
Zn group showed a typical high-field signal at -2.50 ppm, while
the CH2Ph group signals of the calixarene ligand gave rise to a
singlet at 5.11 ppm, a slight downfield shift from the same signal
in starting 1. In addition, a new broad signal at 5.15 ppm with
relative integration of 1 appeared in the 1H NMR spectrum. The
19F spectrum of 4 in C6D6 showed the signals at -134.00 (d),
-155.82 (t), and -162.79 (t) ppm, the latter two corresponding
to the m- and p-fluoro substituents. The large difference in the
chemical shifts between these atoms (6.97 ppm in the case of
-
4) is often used as an indication of R-B(C6F5)3 attached to
the metal cation in the ion pair-like complex.9 Unfortunately,
while several batches of good looking crystals of 4 were
prepared, the crystals showed extremely high sensitivity to the
environment change and decomposed upon the attempts to
analyze them by X-ray crystallography. Moreover, the addition
of bipy or phenanthroline (phen) resulted in the formation of
5a,b, which also showed no signals due to the methyl group in
Figure 3. ORTEP view of a molecule of 5b with thermal ellipsoids
shown at 50% probability. Hydrogen atoms are omitted for clarity.
Selected bond distances (Å) and angles (deg): Zn(1)-C(1) 1.936(7),
Zn(1)-O(1) 1.956(4), Zn(2)-O(1) 1.980(4), Zn(2)-O(2) 2.269(5),
Zn(2)-N(1) 2.082(5), B(1)-O(5) 1.451(9), Zn(1)-Zn(2) 3.0565,
O(3)-Zn(2)-O(1) 78.69(16), O(4)-Zn(2)-O(2) 160.61(14),
N(1)-Zn(2)-N(2) 80.1(2).
1
either H or 13C NMR although the NMR signals due to the
calixarene-based cation were nearly identical to those in 3.
Finally, we were able to crystallize complex 5b which unex-
pectedly showed the presence of an OH group at the boron anion
-
for the out-of-sphere R-B(C6F5)3 and bimetallic calixarene
cation. The X-ray structure of 3 is shown in Figure 2. The
formation of 2 in reaction between 1 and B(C6F5)3 was in
agreement with the reactivity pattern of alkylzinc complexes
(9) (a) Horton, A. D.; de With, J.; van der Linden, A. J.; van de Weg,
H. Organometallics 1996, 15, 2672–2674. (b) Horton, A. D.; de With, J.
Chem. Commun. 1996, 1375–1376.

Notes
Organometallics, Vol. 28, No. 3, 2009 931
Scheme 1
Scheme 2
Scheme 3
instead of the methyl group (Figure 3).10 This led us to verify
the purity of the starting B(C6F5)3 purchased from different
suppliers. Although the 19F NMR spectra of all batches of the
commercial compound testified for the presence of a single
material, there were some differences in the chemical shifts in
the compounds obtained from different suppliers. Close exami-
nation of these differences showed that while one of the
suppliers indeed provided B(C6F5)3, batches obtained from two
other suppliers contained H2O-B(C6F5)3 instead.11 Thus, the
reaction between 1 and H2O-B(C6F5)3 resulted in the formation
of the zwitter-ionic 4, which upon the reaction with bipy or
phen gave complexes 5a and 5b, respectively (Scheme 2). The
formation of 4 likely proceeds via the cleavage of the metal-
methyl bond by the acidic hydrogen of the coordinated water
molecule.10a,e Although no X-ray structure of 4 is available,
the large downfield shift of the HO-B(C6F5)3 hydrogen atom
in the 1H NMR spectrum at 5.15 ppm suggests that the anion is
strongly coordinated to the Zn center. For comparison, the same
signal appears at ca. 2.37 ppm in 5a or 5b.12
Scheme 4
Pentafluorophenyl Group Transfer to Zinc. As mentioned
previously, complex 2 was unstable at room temperature. We
found that, after several hours at 25 °C, complex 2 was
completely converted to the pentafluorophenyl zinc complex 6
(Scheme 3), with the reaction following simple first order
kinetics (t1/2 ≈ 60 min). The X-ray structure of 6 is shown in
Figure 4 and represents the first example of the crystallographi-
cally characterized bimetallic calixarene inclusion complexes
with two different organozinc groups inside and outside the
cavity. The bonding distances and angles in 6 are very similar
to those in other alkylzinc calixarene complexes. We note
however, a slightly shorter Zn1-Zn2 distance of 3.013 Å
compared with 3.06 Å typically observed in other similar
complexes.7a,b
Unlike 2, complex 4 showed significantly higher thermal
stability and was stable in solution at room temperature.
However, heating of 4 in benzene resulted in the formation of
the same C6F5 group transfer product 6 (Scheme 4). Surprisingly,
following up the conversion of 4 to 6 by 1H NMR spectroscopy
showed a clear induction period in this group-transfer reaction.
Heating pure samples of 4 at different temperatures showed no
decomposition for long periods of time, followed by rapid
conversion to 6 (Figure 5). In contrast, when 4 was not purified
thoroughly, prior to heating, the conversion to 6 was significantly
faster with no induction period observed. This made us consider
the possibility of a Lewis acid-catalyzed C6F5 transfer in 4, since
free three-coordinate boron byproduct is formed during the
-
(10) For a first example of a metal complex with the HO-B(C6F5)3
anion, see: (a) Hill, G. S.; Manojlovic-Muir, L.; Muir, K. W.; Puddephatt,
R. J. Organometallics 1997, 16, 525–530. See also: (b) Bergquist, C.; Parkin,
G. J. Am. Chem. Soc. 1999, 121, 6322–6323. (c) Doerrer, L. H.; Green,
M. L. H. J. Chem. Soc., Dalton Trans. 1999, 4325–4329. (d) Danopoulos,
A. A.; Galsworthy, J. R.; Green, M. L. H.; Cafferkey, S.; Doerrer, L. H.;
Hursthouse, M. B. Chem. Commun. 1998, 2529–2530. (e) Zhang, F.; Kirby,
C. W.; Hairsine, D. W.; Jennings, M. C.; Puddephatt, R. J. J. Am. Chem.
Soc. 2005, 127, 14196–14197.
(13) The resulting boron species were found to participate in a series of
equilibria in solution, with the monomeric form dominant in dry nonpolar
solvents: (a) Beringhelli, T.; D’Alfonso, G.; Donghi, D.; Maggioni, D.;
Mercandelli, P.; Sironi, A. Organometallics 2003, 22, 1588–1590. (b)
Beringhelli, T.; D’Alfonso, G.; Donghi, D.; Maggioni, D.; Mercandelli, P.;
Sironi, A. Organometallics 2004, 23, 5493–5502. (c) Beringhelli, T.;
D’Alfonso, G.; Donghi, D.; Maggioni, D.; Mercandelli, P.; Sironi, A.
Organometallics 2007, 26, 2088–2095.
(11) For comparative NMR analyses of the two compounds, see:
Beringhelli, T.; Maggioni, D.; D’Alfonso, G. Organometallics 2001, 20,
4927–4938.
(12) There is little literature information available regarding the position
-
of the HO-B(C6F5)3 signal in metal complexes (ref 10). In some cases,
the signal could not be observed. Bergquist, C.; Fillebeen, T.; Morlok,
M. M.; Parkin, G. J. Am. Chem. Soc. 2003, 125, 6189–6199.

932 Organometallics, Vol. 28, No. 3, 2009
Notes
1H NMR: -2.53 (3H, s, ZnMe), 0.96 (18H, s, Bu), 1.24 (18H,
t
s, tBu), 2.87 (4H, d, JHH ) 14.0 Hz, ArCH2Ar), 3.91 (4H, d, JHH
)
14.0 Hz, ArCH2Ar), 5.11 (4H, s, CH2Ph), 5.15 (1H, s, HOB),
6.85-7.21 (18H, m, Ph and Ar). 13C NMR: -18.59, 31.22, 31.75,
34.20, 34.25, 34.32, 78.42, 120.25 (m), 126.16, 127.16, 128.41,
129.34, 129.78, 130.36, 131.46, 133.75, 137.89 (dm), 140.48 (dm),
143.27, 148.44 (dm), 149.26, 149.72, 155.82. 19F NMR: -134.00
(d, JFF ) 19.6 Hz), -155.82 (t, JFF ) 20.7 Hz), -162.79 (t, JFF
)
19.3 Hz). Anal. for C77H70BF15O5Zn2: Found (calcd): C 62.10
(61.72), H 5.00 (4.72).
Synthesis of 5. One equivalent of bipy or phen in C6D6 was
added to a solution of Zn2Me2Bn2cax (20 mg, 0.02 mmol) in C6D6,
followed by addition of 1 equiv of B(C6F5)3 (10.4 mg, 0.02 mmol).
Changing the addition order did not significantly alter the product
yield. The product was purified by crystallization from toluene/
pentane solution (90%).
Figure 5. Kinetic profiles of conversion of 4 to 6 at different
temperatures.
1
reaction.13 Indeed, when small amounts of free (C6F5)3B were
added to a solution of pure 4 in C6D6, very rapid conversion to
6 was observed, the reaction being completed in less than 20
min at 63 °C. To our knowledge, such Lewis acid accelerated
group transfer reaction is unprecedented. We believe that the
strong interaction of the hydroxy group of the anion with the
zinc center hinders the latter from reacting with one of the C6F5
groups. Thus, the Lewis acid, which is likely to coordinate to
the oxygen atom of HO-B(C6F5)3,10c causes the anion dis-
sociation from the zinc coordination sphere, making the C6F5
group transfer significantly more facile.
5a: H NMR: 8.87 (d, J ) 8.2 Hz, 2H, bipy); 7.71 (d, J ) 4.9
Hz, 2H, bipy); 7.52 (t, J ) 7.8 Hz, 2H, bipy); 7.29 (s, 4H, Ar);
7.18 (s, 4H, Ar); 6.88 (t, J ) 7.5 Hz, 2H, Ph); 6.50 (t, J ) 7.7 Hz,
4H, Ph); 6.25 (t, J ) 6.5 Hz, 2H, bipy); 6.02 (d, J ) 7.5 Hz, 4H,
Ph); 3.99 (d, J ) 13.6 Hz, 4H, Ar-CH2-Ar); 4.00 (s, 4H, CH2Ph);
3.25 (d, J ) 13.6 Hz, 4H, Ar-CH2-Ar); 1.35 (s, 18H, t-Bu); 1.09
(s, 18H, t-Bu); -2.35 (s, 3H, ZnMe). 13C NMR: 158.16, 152.03,
149.66, 148.26, 147.10, 142.73, 141.95, 133.02, 132.72, 130.06,
129.62, 128.69, 128.02, 127.81, 127.62, 126.54, 125.37 (aromatic),
78.79 (CH2-Ph); 34.76 (Ar-CH2-Ar); 34.47, 34.35 (C(CH3)3);
31.92, 31.32 (C(CH3)3); -18.39 (ZnMe). 19F NMR: -135.39 (d, J
) 22 Hz), -162.22 (t, J ) 20 Hz), -165.95 (t, J ) 20 Hz).
In summary, we reported the preparation of zwitter-ionic
1
-
5b: H NMR: 8.04 (m, 4H, phen); 7.70 (s, 2H, phen); 7.39 (s,
calixarene zinc complexes with Me-B(C6F5)3 and HO-
B(C6F5)3- as anions. The complexes undergo the transmetalation
reaction to give the neutral pentafluorophenyl Zn complexes.
While this reaction is very rapid with Me-B(C6F5)3 anion and
4H, Ar); 7.24 (s, 4H, Ar); 6.74 (m, 2H, phen); 6.37 (t, J ) 7.5 Hz,
2H, Ph); 6.05 (t, J ) 7.64 Hz, 4H, Ph); 5.73 (d, J ) 7.4 Hz, 4H,
Ph); 4.16 (d, J ) 13.8 Hz, 4H, Ar-CH2-Ar); 3.86 (s, 4H, CH2Ph);
3.38 (d, J ) 13.8 Hz, 4H, Ar-CH2-Ar); 1.40 (s, 18H, t-Bu); 1.11
(s, 18H, t-Bu); -2.26 (s, 3H, ZnMe).13C NMR: 147.92, 132.67,
130.06, 129.10, 126.71, 126.13 (aromatic); 78.80 (CH2-Ph); 34.86,
34.43 (C(CH3)3); 31.96, 31.35 (C(CH3)3), -8.15 (ZnMe).19F NMR:
-135.18 (d, J ) 22 Hz), -163.00 (t, J ) 20 Hz), -166.37 (t, J )
20 Hz). FAB-MS: found (calcd) 1153 (1153).
Preparation of 6. Complex 6 was obtained quantitatively upon
reacting 1 (20 mg, 0.02 mmol) with B(C6F5)3 (10.4 mg, 0.02 mmol)
in benzene (1 mL) at RT for 5 h. It could also be obtained by
heating 4 (20 mg, 0.013 mmol) at 60 °C for 8 h. After the
completion, the solvent was partially evaporated and pentane was
added. The solution was cooled to -30 °C for overnight and solid
6 was filtered and dried in vacuum.
1H NMR: 6.75-7.28 (18H, m, Ph and Ar), 4.56 (4H, s, CH2Ph),
4.41 (4H, d, JHH ) 13.8 Hz, ArCH2Ar), 3.26 (4H, d, JHH ) 13.8
Hz, ArCH2Ar), 1.28 (18H, s, tBu), 1.08 (18H, s, tBu), -2.31 (3H,
s, ZnMe). 13C NMR: 157.50, 152.07, 148.90, 148.59 (dm), 141.83,
140.63 (dm), 136.99 (dm), 134.75, 133.38, 130.58, 130.45, 129.04,
128.43, 127.29, 126.29, 117.31 (t), 78.07, 34.86, 34.46, 34.21,
31.94, 31.44, -17.88 (ZnMe). 19F NMR: -114.61 (m), -156.47
(t, JFF ) 19.7 Hz), -161.2 (m). FAB-MS: measured (calcd) 1140
(1140).
-
obeys the first-order kinetics, the reaction with HO-B(C6F5)3
as the anion requires heating and shows long induction period.
The reaction is autocatalytic as it is accelerated by the presence
of a three-coordinate boron reagent, which is formed as a
byproduct. We believe that these findings will be useful for
studies of the boron-to-metal group transfer transformations in
main group and transition metal complexes.
Experimental Part
General Data. All operations with air- and moisture-sensitive
compounds were performed in a nitrogen-filled Innovative Technol-
ogy glovebox. All solvents were degassed and stored under high-
purity nitrogen over activated 4Å molecular sieves. All deuterated
commercially available reagents were degassed and used as
received. The NMR spectra were recorded in C6D6 on Bruker AC
200 MHz or Bruker AMX 400 MHz spectrometers. Kinetic
measurements were recorded by 1H NMR on Bruker AC 200 MHz
1
after prior heating of the apparatus to the desired temperature. H
and 13C NMR signals are reported in ppm downfield from TMS.
19F chemical shifts are reported in ppm upfield from CFCl3.
Elemental analysis was performed in the laboratory for microanaly-
sis at the Hebrew University of Jerusalem. Compounds 1-3 have
been reported previously.7,8
Acknowledgment. We thank the Israel Science Founda-
tion for supporting this work.
Synthesis of 4. One equivalent of B(C6F5)3 · H2O (10.4 mg, 0.02
mmol) was added to a solution of 1 (20 mg, 0.02 mmol) in C6D6
at room temperature. The reaction was instantaneous as confirmed
Supporting Information Available: Crystallographic data in
CIF format for complexes 3, 5b, and 6. This material is available
free of charge via the Internet at http://pubs.acs.org.
1
by H and 19F NMR spectroscopy. After addition of toluene, the
solution was partially evaporated and the product was precipitated
by addition of nitromethane at room temperature, filtered and dried
in vacuum (88%).
OM800904Y