Alkali-metal sandwich complexes of a 1,2-diaza-3,5-diborolyl ligand featuring η1, η2, η3, and η4 coordination modes

Article abstract of DOI:10.1021/om061091x

The ring closure of 1,2-diisopropylhydrazine and 1,1-bis(phenylchloroboryl) ethane produced a heterocyclic compound with a CB₂N₂ framework. Deprotonation of this precursor yielded 1,2-diisopropyl-4-methyl-3,5- diphenyl-1,2-diaza-3,5-diborolyl, a cyclopentadienyl analogue containing only one carbon atom in the ring skeleton. Lithium, sodium, and potassium salts of the new ligand have been prepared and characterized. The structures of both monomeric and polymeric sodium metallocenes, as well as the structures of polymeric potassium metallocenes incorporating the new heterocycle, have been determined by single-crystal X-ray diffractometry. These structures reveal not only many similarities but also significant differences in the coordination behavior of the new ligand compared with that of the carbon-based analogue.

Full text of DOI:10.1021/om061091x

1750
Organometallics 2007, 26, 1750-1756
Alkali-Metal Sandwich Complexes of a 1,2-Diaza-3,5-diborolyl
Ligand Featuring η1, η2, η3, and η4 Coordination Modes
Hanh V. Ly, Taryn D. Forster, Andrea M. Corrente, Dana J. Eisler, Jari Konu,
Masood Parvez, and Roland Roesler*
Department of Chemistry, UniVersity of Calgary, 2500 UniVersity DriVe NW, Calgary,
Alberta T2N 1N4, Canada
ReceiVed NoVember 30, 2006
The ring closure of 1,2-diisopropylhydrazine and 1,1-bis(phenylchloroboryl)ethane produced a
heterocyclic compound with a CB2N2 framework. Deprotonation of this precursor yielded 1,2-diisopropyl-
4-methyl-3,5-diphenyl-1,2-diaza-3,5-diborolyl, a cyclopentadienyl analogue containing only one carbon
atom in the ring skeleton. Lithium, sodium, and potassium salts of the new ligand have been prepared
and characterized. The structures of both monomeric and polymeric sodium metallocenes, as well as the
structures of polymeric potassium metallocenes incorporating the new heterocycle, have been determined
by single-crystal X-ray diffractometry. These structures reveal not only many similarities but also significant
differences in the coordination behavior of the new ligand compared with that of the carbon-based analogue.
substitution of two carbon fragments, RC, in the cyclopenta-
dienyl framework with the pairs (RB-, R′N+)9 and (RB-, O+)10
in the 1,2-positions, and (RB-, S+)11 in the 1,2- and 1,3-
positions. These rings were shown to have coordinative proper-
ties similar to those of cyclopentadienyl and proved to be
feasible ancillary ligands in transition-metal catalysts.12 Het-
erocyclic anionic ligands with a PnC5-n (n ) 2 - 4) skeleton,
which display a rich coordination chemistry, have been prepared
through the formal substitution of RC fragments with phospho-
rus atoms.13 Carbon-free cyclopentadienyl analogues of the type
E5- have been reported for E ) P, As, Sb,14 and the coordination
chemistry of the lighter derivatives has been thoroughly
investigated.15 With the exception of the group 15 derivatives
described above, few cyclopentadienyl analogues containing
more than two heteroelements in the ring framework have been
described. Recently, Sekiguchi isolated the lithium salt of a C2-
Introduction
On the quest to tune the electronic properties of the cyclo-
pentadienyl ligand, various five-membered heterocyclic ana-
logues have been obtained by the formal replacement of carbon
atoms in the ring skeleton with isoelectronic main-group
fragments. Relatively soon after the first report on sandwich
compounds, metal complexes containing monoanionic ligands
with C4E skeletons were isolated. Complexes with E belonging
to group 15,1 including pyrrolyl,2 phospholyl,3 arsolyl,4 stibolyl,5
and bismolyl,6 have been synthesized. Similar complexes
incorporating the heavier group 14 elements silicon7 and
germanium8 have been known since the mid-1990s. Anionic
C3BN, C3BO, and C3BS rings were obtained through the formal
* To whom correspondence should be addressed. E-mail: roesler@
ucalgary.ca.
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10.1021/om061091x CCC: $37.00 © 2007 American Chemical Society
Publication on Web 03/01/2007

Alkali-Metal Sandwich Complexes
Organometallics, Vol. 26, No. 7, 2007 1751
GeSi2 ligand,16 and we reported on the synthesis of lithium and
zinc complexes containing a heterocycle with a CB2N2 skel-
eton.17
Scheme 1
Alkali-metal salts are among the most common precursors
to complexes containing cyclopentadienyl ligands,18 and several
of these salts of pnictogen-containing cyclopentadienyl ana-
logues have been structurally characterized. For ligands incor-
porating boron, however, only two structures of lithium
derivatives containing ligands with C3BN19 and CB2N2
17
skeletons have been reported. We were interested in developing
new ligands with inorganic skeletons and investigating their
coordination chemistry. Reported herein are the syntheses and
structural characterization of alkali-metal salts containing a new
cyclopentadienyl analogue with a CB2N2 framework, as well
as the complete structural discussion of the recently com-
municated lithium 1,2-diisopropyl-4-methyl-3,5-dimethyl-1,2-
diaza-3,5-diborolyl (2a).17
Results and Discussion
The 1,2-diaza-3,5-diborolyl ligand 1 was prepared in a fashion
similar to that for its analogue featuring methyl groups on
boron17 and was preferred because of the lower solubility and
diethyl ether but insoluble in hydrocarbons. Deprotonation
resulted in a shift in the 11B NMR signal from 46 ppm in 1 to
38-39 ppm in the alkali-metal salts. Additionally, a dramatic
low-field shift was observed for the broad 13C NMR signal
corresponding to the ring carbon, from 26.8 ppm in the
protonated form of the ligand, 1, to 91.5-96.8 ppm in 2b, 3,
and 4. This is typical for metal cyclopentadienyl complexes with
pronounced ionic character. For example, in pentamethylcy-
clopentadiene, deprotonation results in a shift of the analogous
carbon signal from 52.2 ppm (in C6D6) to 105.2 ppm (Cp*Na
better crystallization properties of its complexes. The symmetric
17,20
diphenylation of MeCH(BCl2)2
with tetraphenyltin (eq 1)
1
was achieved by using a variation of the method described for
H2C(BCl2)2, and the product was purified by distillation.21 In
the 11B NMR spectrum, MeCH(BPhCl)2 displayed a broad signal
at 66.9 ppm. For the synthesis of symmetric diisopropylhydra-
zine, the ketazine obtained through condensation of hydrazine
with acetone was hydrogenated with molecular hydrogen in the
presence of platinum deposited on carbon, as shown in eq 2.22
in THF-d8). The H NMR spectra of all salts described here
featured only one doublet for the methyl groups of the isopropyl
substituents on nitrogen, indicating that the coordination envi-
ronments on both faces of the ligand are identical on the NMR
time scale. This would be in agreement with a solvent-separated
ion-pair structure in solution, consisting of free anionic ligands
7
and solvated alkali-metal ions. The Li NMR spectrum of 2b
in THF-d8 further supports this hypothesis, featuring a chemical
shift typical for [Li(thf)4] (-2.8 ppm).17
Structural determinations by single-crystal X-ray diffracto-
metry have been performed for the compounds 3, 3a (3‚3thf),
4a (4‚thf), and 4b (4‚2thf) (Table 1). The metric parameters of
the π ligand (Table 2) display little variation among all of the
structures discussed in this paper. The ring is practically planar
in 2a, with a sum of the intraannular bond angles of 539.9°.
The planarity of the CB2N2 ring (Figure 1) is relatively good
also in the structures of 3a, 4a, and 4b (sum of bond angles of
539.4-539.6°). However, careful examination reveals a slight
folding (6-7°) along one of the transannular B-N axes,
resulting in an envelope conformation. The substituents on the
ring carbon and boron atoms lie in the plane of the ring, with
the extraannular C-C and B-C bonds forming angles of less
than 4° with the ring plane. The isopropyl groups in structures
3a, 4a, and 4b lie noticeably above and below the ligand plane,
with the extraannular N-C bonds forming angles of 26-33°
with the plane of the ring. The ligand geometry in 3 deviates
slightly from this generalization, with the sum of the intraannular
bond angles being 539.2°, a folding angle of the envelope
conformation of 8.5°, and the B(2)-C(15) and N(1)-C(3) bonds
forming angles of 6.1 and 12.5°, respectively, with the best plane
of the CB2N2 ring. In 2a, only one of the isopropyl groups lies
outside the ligand plane, which forms angles of 2.4 and 23.2°
with the two N-C bonds.
The air-sensitive product was dried on amalgamated aluminum
foil. Ring closure between MeCH(BPhCl)2 and diisopropylhy-
drazine occurred easily in hexane in presence of triethylamine
(eq 3), and 1 was obtained as a colorless, viscous liquid that
could not be distilled but had satisfactory purity according to
NMR data.
Deprotonation of 1 occurred quantitatively in THF in the
presence of strong bases, yielding the salts 2b, 3, and 4 (Scheme
1). These salts were soluble in THF and sparingly soluble in
(16) Lee, V. Y.; Kato, R.; Ichinohe, M.; Sekiguchi, A. J. Am. Chem.
Soc. 2005, 127, 13142.
(17) Ly, H. V.; Forster, T. D.; Maley, D.; Parvez, M.; Roesler, R. Chem.
Commun. 2005, 4468.
(18) (a) Harder, S. Coord. Chem. ReV. 1998, 176, 17; 2000, 199, 331
(erratum). (b) Jutzi, P.; Burford, N. Chem. ReV. 1999, 99, 969. (c) Jutzi, P.;
Reumann, G. Dalton Trans. 2000, 2237.
(19) Schmid, G.; Zaika, D.; Lehr, J.; Augart, N.; Boese, R. Chem. Ber.
1988, 121, 1873.
(20) (a) Deforth, T.; Kaschke, M.; Stock, H.; Pritzkow, H.; Siebert, W.
Z. Naturforsch. 1997, 52B, 823. (b) Enders, M.; Gangnus, B.; Hettrich, R.;
Magos-Martin, Z.; Stephan, M.; Pritzkow, H.; Siebert, W.; Zenneck, U.
Chem. Ber. 1993, 126, 2197.
(21) Ederer, B.; Metzler, N.; No¨th, H. Chem. Ber. 1993, 126, 2003.
(22) (a) Lochte, H. L.; Bailey, J. R.; Noyes, W. A. J. Am. Chem. Soc.
1921, 43, 2597. (b) Lochte, H. L.; Noyes, W. A.; Bailey, J. R. J. Am. Chem.
Soc. 1922, 44, 2556.
At 1.50 Å, the intraannular B-C bonds are 0.1 Å shorter
than the corresponding extraannular bonds and comparable to

1752 Organometallics, Vol. 26, No. 7, 2007
Table 1. Selected Data and Structure Refinement Details for 3, 3a, 4a, and 4b
Ly et al.
3
3a
4a
4b
empirical formula
formula wt
cryst syst
space group
a (Å)
b (Å)
c (Å)
C20H27B2N2Na
340.05
monoclinic
P21/n
11.711(2)
10.170(2)
17.149(3)
106.34(3)
1959.8(7)
4
1.152
0.085
2.35-25.03
3418 (Rint ) 0.027)
3431/0/230
1.070
C32H51B2N2NaO3
556.36
monoclinic
P21/c
17.466(4)
9.358(2)
20.269(4)
90.48(3)
3313(1)
C24H35B2KN2O
428.26
monoclinic
Cc
6.055(1)
17.628(5)
22.867(6)
96.50(2)
2425(1)
4
1.173
0.236
3.5-27.5
2759 (Rint ) 0.023)
2759/2/271
1.05
C28H43B2KN2O2
500.36
monoclinic
Pn
11.852(7)
6.165(2)
20.203(1)
96.06(2)
1468(1)
2
1.132
0.207
3.5-27.5
3335 (Rint ) 0.043)
3335/2/316
1.03
â (deg)
V (Å3)
Z
d
4
calcd (g cm-3
)
1.116
0.080
3.19-27.49
7579 (Rint ) 0.061)
7579/0/366
1.016
µ (mm-1
θ range (deg)
)
no. of indep rflns
no. of data/restraints/params
GOF on F2
R1 (I > 2σ(I))
wR2 (I > 2σ(I))
0.0528
0.1274
0.0593
0.138
0.036
0.090
0.053
0.122
Table 2. Selected Bond Lengths (Å) and Angles (deg) for 2a, 3, 3a, 4a, and 4b
2a (M ) Li)
3 (M ) Na)a
3a (M ) Na)
1.494(3)
4a (M ) K)
4b (M ) K)
C(1)-B(1)
C(1)-B(2)
B(1)-N(1)
B(2)-N(2)
N(1)-N(2)
C(1)-C(2)
B(1)-C(9)
B(2)-C(15)
N(1)-C(3)
N(2)-C(6)
M(1)-C(1)
M(1a)a-C(1)
M(1)···C(2)
M(1a)···C(2)
M(1)-B(1)
M(1)-B(2)
M(1a)-B(1)
M(1a)-B(2)
M(1)-N(1)
M(1)-N(2)
M(1a)-N(1)
M(1a)-N(2)
M(1)-O(1)
M(1)···M(1a)
1.513(4)
1.498(4)
1.457(4)
1.479(4)
1.466(3)
1.519(4)
1.589(4)
1.593(4)
1.485(3)
1.481(3)
2.225(6)
2.480(6)
2.786(6)
3.622(6)
2.543(6)
2.750(6)
2.370(6)
2.354(6)
2.075(5)
2.112(5)
3.313(6)
3.179(6)
1.514(3)
1.503(4)
1.506(3)
1.458(4)
1.459(3)
1.444(3)
1.516(3)
1.595(4)
1.589(4)
1.481(3)
1.473(3)
3.058(2)
3.033(2)
3.321(3)
3.331(3)
3.211(3)
3.752(3)
3.477(3)
3.315(3)
3.898(3)
4.256(2)
3.992(3)
3.832(3)
2.733(2)
6.055(3)
1.491(7)
1.486(6)
1.465(6)
1.465(6)
1.456(5)
1.530(6)
1.579(7)
1.584(6)
1.476(5)
1.472(5)
3.127(4)
3.156(4)
3.247(4)
3.203(4)
3.802(4)
3.425(4)
3.364(4)
3.895(4)
4.419(4)
4.143(4)
4.131(4)
4.483(4)
2.726(4), 2.816(4)
6.165(4)
1.496(3)
1.452(3)
1.481(3)
1.449(2)
1.526(3)
1.581(3)
1.590(3)
1.482(3)
1.489(3)
2.801(2)
2.624(5)-2.755(5)
3.744(4)
3.296(5)-3.644(5)
2.884(2)
2.629(2)
2.517(5)-2.654(5)
3.971(5)-3.399(5)
2.872(2)
1.498(3)
1.460(3)
1.453(3)
1.460(3)
1.521(3)
1.589(3)
1.586(3)
1.479(3)
1.471(3)
2.616(2)
3.139(3)
2.874(2)
3.101(3)
3.351(2)
3.540(2)
2.610(2)
2.766(5)-3.144(5)
3.077(5)-3.695(6)
2.333(2)-2.344(2)
4.381(8)
5.01(1)-5.21(1)
B(1)-C(1)-B(2)
C(2)-C(1)-B(1)
C(2)-C(1)-B(2)
C(1)-B(1)-N(1)
C(1)-B(1)-C(9)
C(9)-B(1)-N(1)
C(1)-B(2)-N(2)
C(1)-B(2)-C(15)
C(15)-B(2)-N(2)
B(1)-N(1)-N(2)
B(1)-N(1)-C(3)
C(3)-N(1)-N(2)
B(2)-N(2)-N(1)
B(2)-N(2)-C(6)
C(6)-N(2)-N(1)
106.9(3)
126.4(3)
126.6(3)
107.5(3)
128.3(3)
124.2(3)
107.6(2)
128.8(3)
123.4(3)
109.6(2)
123.4(3)
119.0(2)
108.3(2)
125.1(2)
118.7(2)
539.9
104.9(2)
128.5(2)
126.6(2)
108.3(2)
129.2(2)
122.5(2)
109.2(2)
126.3(2)
123.5(2)
109.8(2)
125.7(2)
120.3(2)
107.1(2)
124.4(2)
114.7(2)
539.2
103.7(2)
129.7(2)
126.5(2)
110.0(2)
128.0(2)
121.9(2)
109.8(2)
126.2(2)
124.0(2)
107.5(2)
121.6(2)
116.3(2)
108.5(2)
126.9(2)
113.9(2)
539.5
103.3(2)
129.6(2)
127.1(2)
109.8(2)
126.7(2)
123.5(2)
110.1(2)
126.0(2)
123.8(2)
108.5(2)
126.0(2)
112.8(2)
107.9(2)
124.6(2)
111.3(2)
539.6
104.2(4)
128.8(4)
126.9(4)
110.1(4)
127.4(4)
122.5(4)
110.0(4)
127.6(4)
122.4(4)
107.4(3)
125.3(3)
112.2(3)
107.7(3)
123.9(3)
112.0(3)
539.4
∑
pentagon angles
a In complex 3, M(1a) is M(2), as shown in Figure 3.
the B-C bonds featured by 1,2-azaborolinyl9 and borataben-
zene23 complexes (1.48-1.53 and 1.50-1.55 Å, respectively).
The B-N bonds (1.46 Å) are intermediate in length between
the B-N bonds observed in borazines (1.42-1.44 Å)24 and 1,2-
azaborolinyl complexes (1.46-1.50 Å),9 indicating partial
multiple-bond character. At 1.45-1.46 Å, the N-N bonds are
much closer in length to a single bond (1.45 Å in hydrazine)25
than to a double bond (1.34 Å in pyridazine).26 For com-
parison, the N-N bonds in η5-pyrazolato ligands measure
1.38-1.40 Å.27 It appears, therefore, that the electron
delocalization on the N-B-C-B-N skeleton does not extend
over the N-N bond. The pyramidal arrangement at the nitrogen
centers is nevertheless puzzling, given the multiple-bond
character of the B-N bonds; one contributing cause to this
geometry could be the steric strain between the bulky isopropyl
substituents.
(23) (a) Fu, G. C. AdV. Organomet. Chem. 2001, 47, 101. (b) Ashe, A.
J., III; Al-Ahmad, S.; Fang, X. J. Organomet. Chem. 1999, 581, 92.

Alkali-Metal Sandwich Complexes
Organometallics, Vol. 26, No. 7, 2007 1753
Figure 1. Fragment from the polymeric structure of 4a, showing
the structure of the ligand and the relative position of the
coordinated metal centers. The thermal ellipsoids are drawn at the
50% probability level, and the hydrogen atoms have been omitted
for clarity.
Figure 3. Structure of a fragment of the polymeric chain of 3,
with thermal ellipsoids drawn at the 50% probability level. For
clarity, only the R-carbon atoms in the organic substituents are
represented and all hydrogen atoms have been omitted.
ranges 2.18-2.42 and 2.05-2.41 Å, respectively.30 In lithium
boratabenzene sandwiches, the Li-B and Li-C bonds are
somewhat longer, usually in the ranges 2.45-2.58 and 2.30-
2.56 Å, respectively.31 Although the coordination mode in 2a
has been described as µ,η3:η4,17 the Li(1)-B(1) and Li(1)-
B(2) distances (2.543(6) and 2.751(6) Å) are situated outside
the typical range observed for Li-B bonds in related com-
pounds, and the binding mode is more accurately described as
µ,η1:η4 (Figure 7). At 2.480(6) Å, the Li(1a)-C(1) separation
is greater than all Li-C distances observed in polymeric
lithiocenes containing carbon-based ligands. The alternating
ligand planes form angles of 21.4° with each other, and the
consecutive rings are staggered at an angle of 180°. The distance
between neighboring lithium ions (4.381(8) Å) is larger than
in polymeric lithiocenes incorporating carbon-based ligands
(3.9-4.0 Å),29 and the lithium ions are closer to the plane of
the η4 face (1.861(5) Å) than to the plane of the η1 face (2.196-
(5) Å).
Figure 2. Structure of the polymeric chain of 2a in the solid state.
The hydrogen atoms have been omitted for clarity.
A polydecker sandwich structure in the solid state is found
also for the solvent-free sodocene 3 (Figure 3). The structure is
slightly disordered, with one of the two independent sodium
ions, Na(2), occupying several different positions in relation to
the anion. This sodium ion was modeled as a 27:23 isotropic
mixture; the remaining two positions are generated by symmetry.
While Na(1) is symmetrically coordinated between the η3 faces
of the perfectly parallel ligands, Na(2) is disordered around an
η2 position with respect to the other face of the ligands, resulting
in an µ,η2:η3 coordination mode (Figure 7). The separation
between neighboring metals is 5.01(1)-5.21(1) Å; for com-
parison, in reported polymeric sodium cyclopentadienyls, the
Na‚‚‚Na distance is between 4.61 and 4.86 Å for solvent-free
In a previous communication we reported the crystallographic
structural determination of 2a (Figure 2),17 which revealed a
polydecker sandwich structure with a bifacially coordinating
ligand, typical for solid, base-free alkali-metal cyclopentadi-
enyls.18 Atypically for cyclopentadienyls, though, the bridging
ligand coordinates to lithium in an η1 fashion with one face
and an η4 fashion with the other face. The η1 coordination of
the lithium ion involves a short bond to carbon (2.225(6) Å),
while the η4 coordination is comprised of two short bonds to
nitrogen (2.075(5) and 2.112(5) Å) and two bonds to boron
(2.354(6) and 2.369(6) Å). The Li-N bond lengths are shorter
than those observed in lithium complexes containing η5-
coordinating 1,2-azaboryl (2.142(7) Å)19 and pyrrolyl type
ligands (2.161(4), 2.185(1) Å).28 In polymeric lithium cyclo-
pentadienyls, the Li-C bond distances range from 2.22 to 2.37
Å,29 while the Li-B and Li-C bond distances in sandwich
complexes containing η5-coordinated five-membered boron
heterocycles are in the
(29) (a) Evans, W. J.; Boyle, T. J.; Ziller, J. W. Organometallics 1992,
11, 3903. (b) Dinnebier, R. E.; Behrens, U.; Olbrich, F. Organometallics
1997, 16, 3855. (c) Dinnebier, R. B.; Schneider, M.; van Smaalen, S.;
Olbrich, F.; Behrens, U. Acta Crystallogr. 1999, B55, 35. (d) Nie, W.; Qian,
C.; Chen, Y.; Sun, J. Organometallics 2001, 20, 5780. (e) Dinnebier, R.
E.; van Smaalen, S.; Olbrich, F.; Carlson, S. Inorg. Chem. 2005, 44, 964.
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Chem., Int. Ed. Engl. 1990, 29, 317. (b) Herberich, G. E.; Eigendorf, U.;
Englert, U. Chem. Ber. 1993, 126, 1397. (c) Grigsby, W. J.; Power, P. P.
J. Am. Chem. Soc. 1996, 118, 7981. (d) Scheschkewitz, D.; Menzel, M.;
Hofmann, M.; Schleyer, P. v. R.; Geiseler, G.; Massa, W.; Harms, K.;
Berndt, A. Angew. Chem., Int. Ed. 1999, 38, 2936.
(24) (a) Boese, R.; Maulitz, A. H.; Stellberg, P. Chem. Ber. 1994, 127,
1887. (b) Jaska, C. A.; Temple, K.; Lough, A. J.; Manners, I. J. Am. Chem.
Soc. 2003, 125, 9424. (c) Ja¨schke, T.; Jansen, M. Z. Anorg. Allg. Chem.
2004, 630, 239.
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220, 193.
(27) (a) Perera, J. R.; Heeg, M. J.; Schlegel, H. B; Winter, C. H. J. Am.
Chem. Soc. 1999, 121, 4536. (b) Hitzbleck, J.; Deacon, G. B.; Ruhlandt-
Senge, K. Angew. Chem., Int. Ed. 2004, 43, 5218.
(28) (a) Jubb, J.; Gambarotta, S.; Duchateau, R.; Teuben, J. H. J. Chem.
Soc., Chem. Commun. 1994, 2641. (b) Bonomo, L.; Solari, E.; Scopelliti,
R.; Latronico, M.; Floriani, C. Chem. Commun. 1999, 2227.
(31) (a) Paetzold, P.; Finke, N.; Wennek, P.; Schmid, G.; Boese, R. Z.
Naturforsch. 1986, 41B, 167. (b) Hoic, D. A.; Davis, W. M.; Fu, G. C. J.
Am. Chem. Soc. 1995, 117, 8480. (c) Herberich, G. E.; Englert, U.; Schmidt,
M. U.; Standt, R. Organometallics 1996, 15, 2707. (d) Herberich, G. E.;
Ganter, B.; Pons, M. Organometallics 1998, 17, 1254. (e) Lee, R. A.;
Lachicotte, R. J.; Bazan, G. C. J. Am. Chem. Soc. 1998, 120, 6037. (f)
Herberich, G. E.; Englert, U.; Ganter, B.; Pons, M.; Wang, R. Organome-
tallics 1999, 18, 3406. (g) Zheng, X.; Herberich, G. E. Organometallics
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Organometallics 2004, 23, 3085.

1754 Organometallics, Vol. 26, No. 7, 2007
Ly et al.
Figure 6. Structure of the polymeric chain of 4b in the solid state,
with thermal ellipsoids drawn at the 50% probability level. For
clarity, only the R-carbon atoms in the organic substituents are
represented and all hydrogen atoms, as well as the methylene groups
in THF, have been omitted.
Figure 4. Molecular structure of 3a, with thermal ellipsoids drawn
at the 50% probability level. For clarity, in one of the phenyl
substituents only the ipso carbon atom is represented, and all
hydrogen atoms have been omitted.
couple of sodium boratabenzene structures are available for
comparison.33a,b In the latter complexes, the Na-B distances
are much longer than in 3, being 2.910(6) and 2.976(2) Å, while
the Na-C distances range between 2.732(2) and 3.000(2) Å.
The Na-B distances are significantly longer (2.896(3) and
3.130(3) Å) also in a dimeric Na salt featuring the Et6C2B4H-
ligand.33c In a complex featuring π-coordinating pyrazolyl
ligands, the Na-N distances are 2.494(2)-2.638(2) Å, and in
a complex containing π coordinating pyrrolyl ligands the Na-N
separation is 2.694(1) Å.34 The Na(1)-N(1) and Na(1)-B(1)
distances (2.872(2) and 2.884(2) Å, respectively) are substan-
tially longer than the bonding interactions described above, and
therefore the bonding mode of the ligand is best described as
η3 (Figure 7).
In contrast to 2a and 3, 3a is monomeric in the solid state
and features a piano-stool structure, with the sodium cation being
coordinated by the π ligand and three tetrahydrofuran molecules
(Figure 4). The Na-O bonds (2.333(2)-2.344(2) Å) and
O-Na-O angles (87-100°) fall in the typical ranges for
solvated sodium cations. At 2.616(2) Å, the Na-C bond is
comparatively short,32 and in fact shorter bonds (2.544(3)-
2.575(6) Å) have been observed only for dicyclopentadienyl-
sodium anions, a solvent-free cyclopentadienylsodium deriva-
tive, and a strained-geometry cyclopentadienylsodium species
featuring a pendant aza macrocycle.35 The sodium atom in 3a
is positioned above the B(1)-C(1) bond (Figure 4), and
therefore the Na(1)-B(2) distance is considerably longer than
the Na(1)-B(1) distance (3.101(3) vs 2.874(2) Å). Both
distances, however, are more than 0.3 Å longer than the Na-
(2)-B(1) bond in 3, and therefore the binding mode of the
heterocyclic ligand in 3a is best described as η1. The shortest
Na-N distance is more than 3.35 Å, and the distance of the
metal to the ligand plane is 2.609(2) Å.
Figure 5. Structure of the polymeric chain of 4a in the solid state,
with thermal ellipsoids drawn at the 50% probability level. For
clarity, only the R-carbon atoms in the organic substituents are
represented and all hydrogen atoms have been omitted.
structures29b,c,32g and between 4.89 and 5.21 Å for structures
containing solvated sodium.32 The disorder of the metal ion
prevents a detailed discussion of the η2 coordination of Na(2)
in 3. The η3 coordination of Na(1) is comprised of one short
bond to nitrogen (2.612(2) Å), one bond to carbon (2.801(2)
Å), and one relatively short bond to boron (2.631(2) Å). The
Na-C distances in sodium cyclopentadienyls typically range
from 2.60 Å to over 3.20 Å, with the shortest Na-C bond in
each complex being between 2.616(9) and 2.795(2) Å.29b,32
Sodocenes featuring five-membered, boron-containing π ligands
have not been structurally characterized to date; however, a
Compounds 4a and 4b are polymeric in the solid state,
featuring polydecker sandwich structures containing potassium
ions intercalated with parallel, π-coordinating diazadiborolidine
(32) (a) Aoyagi, T.; Shearer, H. M. M.; Wade, K.; Whitehead, G. J.
Organomet. Chem. 1979, 175, 21. (b) Rogers, R. D.; Atwood, J. L.; Rausch,
M. D.; Macomber, D. W.; Hart, W. P. J. Organomet. Chem. 1982, 238,
79. (c) Rabe, G.; Roesky, H. W.; Stalke, D.; Pauer, F.; Sheldrick, G. M. J.
Organomet. Chem. 1991, 403, 11. (d) Herberich, G. E.; Fischer, A.
Organometallics 1996, 15, 58. (e) Bock, H.; Hauck, T.; Na¨ther, C.; Havlas,
Z. Z. Naturforsch. 1997, 52B, 524. (f) Ka¨hler, T.; Behrens, U.; Neander,
S.; Olbrich, F. J. Organomet. Chem. 2002, 649, 50. (g) Tedesco, C.;
Dinnebier, R. E.; Olbrich, F.; van Smaalen, S. Acta Crystallogr. 2001, 57B,
673. (h) Cole, M. L; Jones, C.; Junk, P. C. Dalton Trans. 2002, 896. (i)
Nishinaga, T.; Yamazaki, D.; Stahr, H.; Wakamiya, A.; Komatsu, K. J.
Am. Chem. Soc. 2003, 125, 7324. (j) Enders, M.; Kohl, G.; Pritzkow, H.
Organometallics 2004, 23, 3832.
(33) (a) Lee, B. Y.; Wang, S.; Putzer, M.; Bartholomew, G. P.; Bu, X.;
Bazan, G. C. J. Am. Chem. Soc. 2000, 122, 3969. (b) Lee, B. Y.; Bazan, G.
C. J. Am. Chem. Soc. 2000, 122, 8577. (c) Wrackmeyer, B.; Schanz, H.-J.;
Milius, W. Angew. Chem., Int. Ed. Engl. 1997, 36, 75.
(34) (a) Kuhn, N.; Henkel, G.; Kreutzberg, J. Angew. Chem., Int. Ed.
Engl. 1990, 29, 1143. (b) Deacon, G. B.; Delbridge, E. E.; Evans, D. J.;
Harika, R.; Junk, P. C.; Skelton, B. W.; White, A. H. Chem. Eur. J. 2004,
10, 1193.
(35) (a) Harder, S.; Prosenc, M. H.; Rief, U. Organometallics 1996, 15,
118. (b) Leung, W.-P.; Song, F.-Q.; Xue, F.; Mak, T. C. W. J. Chem. Soc.,
Dalton Trans. 1997, 4307. (c) Giesbrecht, G. R.; Gebauer, A.; Shafir, A.;
Arnold, J. Dalton Trans. 2000, 4018.

Alkali-Metal Sandwich Complexes
Organometallics, Vol. 26, No. 7, 2007 1755
Figure 7. Perpendicular projections onto the CB2N2 plane, revealing the hapticities of the ligands (from left to right): 2a (µ,η1:η4), 3
(µ,η2:η3), 3a (η1), 4a (µ,η2:η2), and 4b (µ,η2:η2).
ligands (Figures 5 and 6). This is not typical for polymeric
potassium cyclopentadienyls, which usually display bent ge-
ometries with nonparallel ligands.18 The main difference
between the two structures is the number of tetrahydrofuran
molecules completing the coordination sphere of each of the
potassium ions: one in 4a and two in 4b. With lengths between
2.723(2) and 2.816(4) Å, the K-O bonds fall in the usual range.
The multidecker sandwich structure in 4a, where potassium ions
have a lower coordination number, is more compact, with a
K‚‚‚K separation of 6.055(3) Å versus 6.165(4) Å in 4b. For
comparison, the K‚‚‚K distance in bent, polymeric potassium
cyclopentadienyls is between 5.52 and 5.85 Å.29b,32c,h,35b,36
Uncharacteristic for alkali-metal cyclopentadienyls, the potas-
sium ions in 4a and 4b are not contained between the ligands
but are instead situated outside of the columns formed by the
parallel CB2N2 rings, alternating on both sides in the vicinity
of the two endocyclic C-B bonds (Figure 7). The K-C bond
distances are 3.033(2) and 3.058(2) Å in 4a and 3.127(4) and
3.156(4) Å in 4b. In polymeric potassium cyclopentadienyls,
the K-C distances range from 2.95 to over 3.30 Å, with the
shortest K-C bond in each complex being between 2.952(2)
and 3.074(7) Å.29b,32c,h,35b,36 The potassium ions are closer to
one of the boron atoms of each of the coordinating ligands.
The two shorter K-B distances are 3.211(3) and 3.315(3) Å in
4a and 3.364(4) and 3.425(4) Å in 4b, while the longer K-B
distances are 3.477(3) and 3.752(3) Å in 4a and 3.802(4) and
3.895(4) Å in 4b. Potassium borole salts have not been
structurally characterized, but a few potassium borata- and
diboratabenzene complexes displaying monomeric and poly-
meric sandwich structures have been reported. The K-B
distances in these complexes fall between 3.062(13) and 3.686-
(6) Å, while the K-C distances are situated between 2.952(3)
and 3.572(5) Å.37 Taking into account the geometry and bond
distances, it appears that the best description for the bonding
of the diazadiborolidine ligands in 4a and 4b is µ,η2:η2. The
K-N distances are greater than 3.8 Å.
Conclusion
Two cyclopentadienyl analogues with a CB2N2 framework,
1,2-diisopropyl-4-methyl-3,5-dimethyl-1,2-diaza-3,5-diborolyl
and 1,2-diisopropyl-4-methyl-3,5-diphenyl-1,2-diaza-3,5-diboro-
lyl, have been synthesized, and their coordination chemistry
toward alkali metals was investigated. The results reveal
substantial similarities but also important differences with
respect to the all-carbon analogues. In agreement with the 13C
NMR data, the structural analyses indicate electron delocaliza-
tion over the NBCBN skeleton. The intraannular B-C and B-N
distances are intermediate in length between those corresponding
to single and double bonds; however, the N-N bond lengths
are typical for single bonds. The latter observation, together
with the fact that the substituents on nitrogen feature significant
deviations from the ring plane, implies that the electron
delocalization over the ring framework does not extend over
the N-N bond. Similar to cyclopentadienyl, the new π ligands
coordinate face-on, forming monomeric (3a) and polymeric (2a,
3, 4a, 4b) sandwich complexes. The distances separating the
metal ions are systematically larger in the compounds described
herein than in the polymeric cyclopentadienyl analogues,
indicating the lower coordinating ability of the new ligands.
These ligands nevertheless exhibit a remarkable variability in
their coordination manner, and because the ring atoms have
different radii, the assignment of the coordination mode is not
unequivocal. Taking into account the geometry and bond
distances reported in the literature, η1 (C), η2 (BC), η3 (NBC),
and η4 (BNNB) coordination modes have been assigned.
Relatively short bonding distances between the metal and the
ring carbon are present in all compounds, while only some of
the complexes feature short distances between the metal and
boron or nitrogen. By comparison, the cyclopentadienyl ligand
displays almost exclusively the η5 coordination in alkali-metal
sandwich complexes. No trends regarding the preference of the
ligand for a specific coordination mode can be formulated at
this point, but it appears obvious that the energetic differences
between various coordination modes are small. This is not
unexpected, given the predominantly ionic character of the
bonding in the complexes described herein. The coordination
ability of the new ligands toward main-group and transition
metals is currently under investigation.
The distances between potassium and the methyl substituents
of the ring carbon are quite short as a result of the unusual
position of the potassium ions, on the outside of the columns
formed by the CB2N2 rings (Figure 5 and 6). The K-C(2)
distances range between 3.203(4) and 3.331(3) Å, only 0.05-
0.30 Å longer than the K-C(1) bonds and shorter than some
of the distances between potassium and the ring carbon in
polymeric potassium cyclopentadienyls (2.95 Å to over 3.30
Å).29b,32c,h,35b,36 This geometry is not observed in any of the other
compounds described in this paper or in other reported potas-
sium sandwich compounds.
Experimental Section
General Considerations. All operations were performed under
an argon atmosphere using standard Schlenk and glovebox tech-
niques. Solvents were dried and deoxygenated prior to use. Starting
materials were prepared according to reported procedures or
purchased from commercial suppliers. NMR spectra were collected
on a Bruker Advance DRX-400 spectrometer and calibrated with
respect to C6D5H (1H, 7.15 ppm), THF-d7 (1H, 3.58 ppm), C6D6
(13C, 128.39 ppm), THF-d8 (13C, 67.57 ppm), BF3‚Et2O (11B, 0
ppm), and LiCl (7Li, 0 ppm). Mass spectra were performed by the
Analytical Instrumentation Laboratory, Department of Chemistry,
University of Calgary. Elemental analyses of compounds 1-4 failed
to produce satisfactory results, potentially due to boron carbide
formation under the sample burning conditions.
(36) (a) Jordan, V.; Behrens, U.; Olbrich, F.; Weiss, E. J. Organomet.
Chem. 1996, 517, 81. (b) Karsch, H. H.; Graf, V. W.; Reisky, M. Chem.
Commun. 1999, 1695. (c) Evans, W. J.; Brady, J. C.; Fujimoto, C. H.;
Giarikos, D. G.; Ziller, J. W. J. Organomet. Chem. 2002, 649, 252. (d)
Evans, W. J.; Giarikos, D. G.; Ziller, J. W. J. Organomet. Chem. 2003,
688, 200.
(37) (a) Mu¨ller, P.; Huck, S.; Ko¨ppel, H.; Pritzkow, H.; Siebert, W. Z.
Naturforsch. 1995, 50B, 1476. (b) Hoic, D. A.; DiMare, M.; Fu, G. C. J.
Am. Chem. Soc. 1997, 119, 7155. (c) Zheng, X.; Herberich, G. E. Eur. J.
Inorg. Chem. 2003, 2175.

1756 Organometallics, Vol. 26, No. 7, 2007
Ly et al.
3
Synthesis of 1,1-Bis(chlorophenylboryl)ethane. Bis(chlorobo-
ryl)ethane17,20 (5.31 g, 27.7 mmol) was added to Ph4Sn (7.90 g,
18.5 mmol), and the neat mixture was stirred at ambient temperature
for 2 h. Subsequently, the colorless suspension was heated and
maintained at 80 °C overnight. Vacuum distillation yielded PhSnCl3
at 60-80 °C/0.1 Torr and the desired product at 100-115 °C/0.10
Torr, as a colorless liquid (6.50 g, 85%). 1H NMR (400 MHz, C6D6,
25 °C): δ 1.51 (d, 3H, 3JHH ) 6.6 Hz, HCCH3), 3.22 (q, 1H, 3JHH
) 6.6 Hz, HCCH3), 7.01-7.16 (m, 6H, m-, p-C6H5), 7.98 (d, 4H,
o-C6H5). 11B NMR (128 MHz, C6D6, 25 °C): δ 66.9 (s, br, LW1/2
) 454 Hz).
m-C6H5), 7.45 (d, 4H, JHH ) 7.3 Hz, o-C6H5). 13C NMR (100
MHz, THF-d8, 25 °C): δ 14.7 (s, CCH3), 23.9 (s, CH(CH3)2), 50.7
(s, CH(CH3)2), 94.8 (s, br, CCH3), 124.1 (s, p-C6H5), 126.6 (s,
m-C6H5), 135.1 (s, o-C6H5), 151.2 (s, br, i-C6H5). 11B NMR (128
MHz, THF-d8, 25 °C): δ 37.9 (s, br, LW1/2 ) 600 Hz). 7Li NMR
(155 MHz, THF-d8, 25 °C): δ -2.8 (s, LW1/2 ) 14 Hz).
Synthesis of (1,2-Diisopropyl-1,2-diaza-3,5-diphenyldiboro-
lyl)sodium (3). A solution of 1 (200 mg, 0.629 mmol) in THF (20
mL) was added to a stirred solution of sodium bis(trimethylsilyl)-
amide (NaHMDS; 115 mg, 0.629 mmol) in THF (15 mL), forming
a yellow mixture, which was stirred at ambient temperature for 1
day to ensure the reaction had completed. The volatiles were
removed under vacuum, producing a colorless solid, which was
washed twice with hexane (30 mL) and dried under vacuum. The
Synthesis of Me2CdNNdCMe2. Hydrazine (10 g, 0.312 mol)
was slowly mixed with acetone (100 mL) under external cooling,
in a flask containing molecular sieves (3 Å, 100 g). The mixture
was left sitting at ambient temperature for 24 h, and then the solution
was decanted and distilled at atmospheric pressure. The product
was collected as a clear colorless liquid at 134 °C (26.7 g, 74%).
1H NMR (400 MHz, C6D6, 25 °C): δ 1.80 (d, 12H, CCH3).
Synthesis of 1,2-Diisopropylhydrazine. A Parr hydrogenation
apparatus was loaded with Me2CdNNdCMe2 (26.7 g, 0.238 mol),
ethanol (36 mL), acetic acid (5.35 mL), and the Pt/C catalyst (10%,
100 mg). The apparatus was deoxygenated and stirred under 5 bar
of H2 for 24 h. The resulting mixture was filtered, and the filtrate
was decomposed with HCl solution (12.1 M, 50 mL). Volatiles
were removed in vacuo, and the residue was dissolved in water
(60 mL) and treated with solid KOH (34.0 g, 0.606 mol). The
product was extracted with diethyl ether (3 × 50 mL), and the united
extracts were dried over amalgamated aluminum foil and distilled
at atmospheric pressure, yielding an air-sensitive colorless liquid
1
product was obtained as a colorless powder (159 mg, 75%). H
NMR (400 MHz, THF-d8, 25 °C): δ 1.12 (d, 12H, 3JHH ) 6.8 Hz,
CH(CH3)2), 1.75 (s, 3H, CCH3), 3.90 (sep, 2H, JHH ) 6.8 Hz,
3
CH(CH3)2), 6.98 (t, 2H, 3JHH ) 7.3 Hz, p-C6H5), 7.13 (t, 4H, 3JHH
) 7.3 Hz, m-C6H5), 7.46 (d, 4H, 3JHH ) 7.3 Hz, o-C6H5). 13C NMR
(100 MHz, THF-d8, 25 °C): δ 13.9 (s, CCH3), 23.9 (s, CH(CH3)2),
50.5 (s, CH(CH3)2), 91.5 (s, br, CCH3), 125.0 (s, p-C6H5), 127.1
(s, m-C6H5), 135.0 (s, o-C6H5), 149.2 (s, br, i-C6H5). 11B NMR
(128 MHz, THF-d8, 25 °C): δ 39.1 (s, br, LW1/2 ) 667 Hz).
Colorless block crystals of 3a that were suitable for X-ray
diffraction analysis were obtained by storing a concentrated solution
of 3 in THF/hexane at -35 °C for a few days. Colorless thin plates
of 3 were obtained by slow room-temperature evaporation from a
solution of 3 in C6H6/THF.
1
boiling at 124 °C (16.4 g, 59%). H NMR (400 MHz, C6D6, 25
3
°C): δ 0.97 (d, 12H, JHH ) 6.2 Hz, CH(CH3)2), 2.34 (s, br, 2H,
Synthesis of (1,2-Diisopropyl-1,2-diaza-3,5-diphenyldiboro-
lyl)potassium (4). A solution of 1 (1.35 g, 4.24 mmol) in THF (20
mL) was added to a stirred solution of potassium bis(trimethylsilyl)-
amide (KHMDS; 0.950 g, 4.24 mmol) in THF (15 mL), forming a
yellow mixture. The mixture was stirred at ambient temperature
for 12 h, and the volatiles were removed under vacuum, producing
a colorless solid, which was washed twice with hexane (30 mL)
and dried under vacuum. The product was obtained as a colorless
3
NH), 2.74 (sep, 2H, JHH ) 6.2 Hz, CH(CH3)2). 13C NMR (400
MHz, C6D6, 25 °C): δ 21.6 (s, CH(CH3)2), 50.9 (s, CH(CH3)2).
The NMR data correlate with the reported values.38
Synthesis of 1,2-Diisopropyl-1,2-diaza-3,5-diphenyldiboroli-
dine (1). A solution of 1,1-diisopropylhydrazine (1.27 g, 10.9 mmol)
and triethylamine (2.21 g, 21.8 mmol) in hexane (30 mL) was
slowly added to a stirred solution of 1,1-bis(chlorophenylboryl)-
ethane (3.00 g, 10.9 mmol) in hexane (20 mL). The mixture was
stirred at ambient temperature for 12 h, and the white precipitate
was filtered off. Volatiles were removed under full vacuum, leaving
1
powder (1.41 g, 93%). H NMR (400 MHz, THF-d8, 25 °C): δ
1.12 (d, 12H, 3JHH ) 6.8 Hz, CH(CH3)2), 1.73 (s, 3H, CCH3), 3.89
1
3
3
behind the product as a thick colorless liquid (2.46 g, 71%). H
(sep, 2H, JHH ) 6.8 Hz, CH(CH3)2), 6.97 (t, 2H, JHH ) 7.3 Hz,
3
p-C6H5), 7.13 (t, 4H, 3JHH ) 7.3 Hz, m-C6H5), 7.43 (d, 4H, 3JHH
)
NMR (400 MHz, C6D6, 25 °C): δ 1.12 (d, 3H, JHH ) 6.8 Hz,
3
7.3 Hz, o-C6H5). 13C NMR (100 MHz, THF-d8, 25 °C): δ 14.3 (s,
CCH3), 23.9 (s, CH(CH3)2), 50.3 (s, CH(CH3)2), 96.8 (s, br, CCH3),
124.7 (s, p-C6H5), 127.1 (s, m-C6H5), 134.6 (s, o-C6H5), 149.9 (s,
br, i-C6H5). 11B NMR (128 MHz, THF-d8, 25 °C): δ 38.6 (s, br,
LW1/2 ) 560 Hz). Colorless crystals that were suitable for X-ray
diffraction analysis were obtained by either storing a concentrated
solution of 4 in THF/hexane at -35 °C (4b) or by slow diffusion
of hexane into a THF solution of the salt (4a).
HCCH3), 1.13 (d, 6H, JHH ) 6.9 Hz, CH(CH3)2), 1.14 (d, 6H,
3
3JHH ) 6.9 Hz, CH(CH3)2), 3.91 (sep, 2H, JHH ) 6.9 Hz,
CH(CH3)2), 7.20-7.31 (m, 6H, m-, p-C6H5), 7.46 (d, 4H, o-C6H5).
13C NMR (100 MHz, C6D6, 25 °C): δ 9.32 (s, CCH3), 24.0, 24.2
(s, CH(CH3)2), 26.8 (s, br, CCH3), 49.4 (s, CH(CH3)2), 127.9 (s,
p-C6H5), 128.2 (s, m-C6H5), 133.0 (s, o-C6H5), 141.8 (s, br, i-C6H5).
11B NMR (128 MHz, C6D6, 25 °C): δ 46.0 (s, br, LW1/2 ) 552
Hz). MS (EI+, 70 eV; m/z (%)): 318 (100) [M]+, 303 (88) [M-
Me]+, 190 (48) [M - (iPrN)2Me + H]+.
Synthesis of (1,2-Diisopropyl-1,2-diaza-3,5-diphenyldiboro-
lyl)lithium (2b). A yellow solution of lithium 2,2,6,6-tetrameth-
ylpiperidide (LiTMP), formed in situ from 1.6 M n-butyllithium
in hexane (3.20 mL, 5.12 mmol) and tetramethylpiperidine (TMP;
0.724 g, 5.12 mmol) in THF (3 mL), and a solution of 1 (1.63 g,
5.12 mmol) in THF (20 mL) were precooled to -35 °C for 1 h.
The precooled solutions of 1 and LiTMP were mixed and kept at
-35 °C for 12 h. The solvent was removed under vacuum,
producing a pale yellow residue, which was washed several times
with hexane (30 mL) and dried under vacuum. The product was
Acknowledgment. This work was supported by the Natural
Sciences and Engineering Research Council of Canada, the
Canada Foundation for Innovation, and the Alberta Science and
Research Investments Program.
Supporting Information Available: Figures giving NMR
spectra for all new compounds, as well as complete crystallographic
data in CIF and table formats. This material is available free of
charge via the Internet at http://pubs.acs.org. CIF files are also
available online from the Cambridge Crystallographic Data Centre
(CCDC Nos. 629092 (3), 629093 (3a), 629094 (4a), and 629095
(4b)).
1
obtained as an off-white powder (680 mg, 41%). H NMR (400
MHz, THF-d8, 25 °C): δ 1.07 (d, 12H, 3JHH ) 6.8 Hz, CH(CH3)2),
3
1.72 (s, 3H, CCH3), 3.80 (sep, 2H, JHH ) 6.8 Hz, CH(CH3)2),
3
3
6.90 (t, 2H, JHH ) 7.3 Hz, p-C6H5), 7.06 (t, 4H, JHH ) 7.3 Hz,
(38) Engel, P. S.; Wu, W.-X. J. Am. Chem. Soc. 1989, 111, 1830.
OM061091X