The preparation and crystal structures of η1-derivatives of 2-phenyl-1,2-azaboratabenzene

Article abstract of DOI:10.1021/om701224c

The reaction of potassium 2-phenyl-1,2-azaboratabenzene with Cp ₂ZrCl₂ gave 5 in which the heterocyclic rings are η¹-bound to zirconium.

Full text of DOI:10.1021/om701224c

Organometallics 2008, 27, 1345–1347
1345
The Preparation and Crystal Structures of η1-Derivatives of
2-Phenyl-1,2-azaboratabenzene
Jun Pan, Jeff W. Kampf, and Arthur J. Ashe III*
Department of Chemistry, UniVersity of Michigan, Ann Arbor, Michigan 48109-1055
ReceiVed December 6, 2007
on the preparation and crystal structure of 5, the first η1 metal
complex of a 1,2-azaboratabenzene (Scheme 1).
Summary: The reaction of potassium 2-phenyl-1,2-azaborataben-
zene with Cp2ZrCl2 gaVe 5 in which the heterocyclic rings are
η1-bound to zirconium.
Results and Discussion
1,2-Dihydro-1,2-azaborine (1) is a 6 π-electron aromatic ring
that is isoelectronic with benzene. Monocyclic derivatives of 1
were first prepared in very low yield by Dewar1 and White2 in
the early 1960s. The Dewar group subsequently reported several
fused-ring relatives of 13 and more recently there has been
renewed interest in these boron-nitrogen hetercycles for their
potential application in optical and electronic devices.4 In
2000–2001 we reported on two general syntheses of 1,2-dihydro-
1,2-azaborines which make this heterocycle readily available
for study.5,6 Structural data on several 1,2-dihydro-1,2-azabo-
rines show that the B-N ring is completely planar with intraring
bond distances which are consistent with those of an aromatic
ring.7–9 Furthermore, derivatives of 1 undergo the most char-
acteristic aromatic reaction, electrophilic substitution.8
Potassium 2-phenyl-1,2-azaboratabenzene (3a) was prepared
by the reaction of 1a with potassium bis(trimethylsilylamide)
in toluene. After filtration 3a was dissolved in THF and added
to Cp2ZrCl2. The reaction gave a 56% yield of 5 that could be
1
recrystallized from toluene. The H NMR spectrum of 5 in
CDCl3 is first order and can be assigned by inspection. Aside
from the absence of the NH signal and the presence of the Cp
signal, the spectrum is very similar to that of 1a and indeed
shows only small changes in the chemical shifts of the
corresponding signals. These data suggest that the 1,2-azabo-
ratabenzene ligand is σ- rather than π-coordinated. This mode
of coordination was confirmed by obtaining an X-ray crystal
structure. The molecular structure of 5 is illustrated in Figure 1
and selected bond distances and angles are shown in Table 1.
The molecular structure of 5 is that of a bis(amido)zir-
conocene. The bulky, planar 1,2-azaboratabenzene groups are
related to each other by an approximate C2 axis bisecting the
N-Zr-N angle. These ligands are inclined to the N-Zr-N
plane by 31.22(6)° and 28.37(6)°, while the 2-phenyl groups
are canted out of the 1,2-azaboratabenzene planes by 41.42(6)°
and 58.60(5)°.
Inspection of the molecular structure of 5 shows that the ring
nitrogen atoms are σ-bonded to zirconium. The 1,2-dihydro-
1,2-azaborine rings of 5 are planar within 0.02 Å. The intraring
distances of the C4BN rings are identical (average difference,
0.01 Å) with those calculated for the parent heterocycle 1 at
the MP2/6-31G* level.11 They are also very similar to experi-
mental distances found for several metal-free 1,2-dihydro-
1,2-azaborines.7–9 Thus the σ-bound zirconium does not
significantly effect the electron delocalization of the 1,2-dihydro-
1,2-azaborine heterocycle.
The bonding in 5 appears to closely resemble that of
bis(pyrrolyl)zirconocene 6a12a and bis(2,5-dimethylpyrrolyl)zir-
conocene 6b,12b the structures of which were reported by J. L.
Atwood and co-workers. For all three compounds the Cp rings
are π-bound while the aromatic nitrogen heterocycles are
σ-bonded to Zr(IV). The Zr-N distances of 5 (2.238(1),
2.247(1) Å) and 6b (2.22(2), 2.25(2) Å) are identical. However,
the Zr-N distances of 5 and 6b are somewhat larger than those
of the less hindered 6a (2.171(2), 2.167(2) Å). The N-Zr-N
bond angles decrease with the size of the ligands: 5 (111.9(1)°)
> 6b (106.4(1)°) > 6a (95.7(1)°). On the other hand, the Zr-N
bond angles of 5 are much less symmetrical than those of 6a
and 6b. For 5 the Zr-N-B angles (142.4(1)°, 136.4(1)°)
average 36° greater than the Zr-N-C angles (99.9(1)°,
106.5(1)°). The corresponding differences for 6a and 6b are
only 8° and 13°. The greater difference in the Zr-N bond angles
The coordination chemistry of 1 and its derivatives is of
particular interest to us. 2-Phenyl-1,2-dihydro-1,2-azaborine (1a)
forms arene-like Cr(CO)3 complex 2.7 The removal of the NH
proton from 1a produces the conjugate base 1,2-azaborataben-
zene (3a), which reacts with [Cp*RuCl]4 to form η6 Ru complex
4.10 Since 2-phenyl-1,2-azaborabenzene (3a) can be N-methyl-
ated to form 1b and N-silylated to form 1c, it is nucleophilic at
nitrogen. Therefore it seemed likely that it might form σ-metal
complexes with the more electophilic metals. We report here
* Corresponding author. E-mail: ajashe@umich.edu.
(1) (a) Dewar, M. J. S.; Marr, P. A. J. Am. Chem. Soc. 1962, 84, 3782.
(b) Davies, K. M.; Dewar, M. J. S.; Rona, P. J. Am. Chem. Soc. 1967, 89,
6294.
(2) White, D. G. J. Am. Chem. Soc. 1963, 85, 3634.
(3) (a) For reviews, see: Dewar, M. J. S. Prog. Boron Chem. 1964, 1,
235. (b) Fritsch, A. J. Chem. Heterocycl. Compd. 1977, 30, 381.
(4) (a) Bosdet, M. J. D.; Piers, W. E.; Sorensen, T. S.; Parvez, M. Angew.
Chem., Int. Ed. 2007, 46, 4940. (b) Bosdet, M. J. D.; Jaska, C. A.; Piers,
W. E.; Sorensen, T. S.; Parvez, M. Org. Lett. 2007, 9, 1395. (c) Jaska,
C. A.; Piers, W. E.; McDonald, R.; Parvez, M. J. Org. Chem. 2007, 72,
5234. (d) Jaska, C. A.; Emslie, D. J. H.; Bosdet, M. J. D.; Piers, W. E.;
Sorensen, T. S.; Parvez, M. J. Am. Chem. Soc. 2006, 128, 10885.
(5) Ashe, A. J., III; Fang, X. Org. Lett. 2000, 2, 2089.
(6) Ashe, A. J., III; Fang, X.; Fang, X.; Kampf, J. W. Organometallics
2001, 20, 5413.
(7) Pan, J.; Kampf, J. W.; Ashe, A. J., III Organometallics 2006, 25,
197.
(8) Pan, J.; Kampf, J. W.; Ashe, A. J., III Org. Lett. 2007, 9, 679.
(9) Marwitz, A. J. V.; Abbey, E. R.; Jenkins, J. T.; Zakharov, L. N.;
Liu, S.-Y. Org. Lett. 2007, 9, 4905.
(10) Pan, J.; Kampf, J. W.; Ashe, A. J., III Organometallics 2004, 23,
5626.
10.1021/om701224c CCC: $40.75
2008 American Chemical Society
Publication on Web 02/27/2008

1346 Organometallics, Vol. 27, No. 6, 2008
Notes
Scheme 1
of 5 is likely a consequence of the intrinsic lower symmetry of
the 2-phenyl-1,2-azaboratabenzene ligand and the greater crowd-
ing in the metallocene wedge.
difference between the two structures is that the exocyclic bond
angles at nitrogen are more symmetrical for 1c. The Si-N-B
and the Si-N-C bond angles differ by only 9°.
For comparison we have also obtained a crystal structure for
1c, since it also has an electropositive substituent at nitrogen
but is less crowded. The molecular structure of 1c is illustrated
in Figure 2 and selected bond distances and angles are shown
in Table 1. The bond distances of the 1,2-azaboratabenzene
groups of 1c and 5 are not significantly different. The major
In summary we have prepared and characterized 5, the first
η1-coordinated 1,2-azaboratabenzene. Prior work had shown
that 1,2-azaboratabenzene could form η6-metal complexes.
Thus this anionic heterocycle can form σ- or π-metal
complexes depending upon the metal. In this respect 1,2-
azaboratabenzene has a coordination chemistry similar to that
Figure 1. The solid-state structure of 5 (ORTEP). Thermal ellipsoids are set at the 50% probability level. Hydrogen atoms have been
omitted for clarity.
Table 1. Comparison of Selected Bond Distances (Å) and Bond Angles (deg) for 5 and 1c with the Calculated Bond Distances for 1a
bond, angle
5 (M ) Zr)b
1c (M ) Si)b
1 (M ) H)b
B(1)-N(1); B(2)-N(2)
1.449(2); 1.445(2)
1.519(2); 1.521(2)
1.362(3); 1.360(2)
1.468(3); 1.409(2)
1.362(2); 1.364(2)
1.366(2); 1.372(2)
1.579(2); 1.586(2)
2.238(2); 2.247(2)
111.85(4)
1.448(2)
1.518(2)
1.364(2)
1.413(2)
1.355(2)
1.390(2)
1.579(2)
1.801(1)
1.433
1.510
1.510
1.417
1.370
1.367
B(1)-C(4); B(2)-C(14)
C(4)-C(3); C(14)-C(13)
C(3)-C(2); C(13)-C(12)
C(2)-C(1); C(12)-C(11)
C(1)-N(1); C(11)-N(2)
B(1)-C(5); B(2)-C(15)
N(1)-M; N(2)-M
N(1)-Zr-N(2)
M-N(1)-B(1); M-N(2)-B(2)
M-N(1)-C(1); M-N(2)-C(11)
B(1)-N(1)-C(1); B(2)-N(2)-C(11)
142.4(1); 136.4(1)
99.9(1); 106.5(1)
117.1(1); 116.7(1)
125.4(1)
116.1(1)
118.4(1)
a Reference 11. b M is the exocyclic substituent at N.

Notes
Organometallics, Vol. 27, No. 6, 2008 1347
Experimental Section
General Methods. All reactions were carried out under an
atmosphere of argon or nitrogen. Solvents were dried using standard
procedures. High-resolution mass spectra were recorded on a VG-
250S spectrometer with an electron impact at 70 eV. The NMR
spectra were obtained with a Varian INOVA 400 or 500 spectrom-
eter. The 1H NMR and 13C NMR spectra were calibrated by using
signals from solvents referenced to Me4Si. The 11B NMR spectra
were referenced to external BF3-OEt2.
Bis(η5-cyclopentadienyl)bis(η1-2-phenyl-1,2-azaborataben-
zene)zirconium(IV) (5). A 0.5 M solution of potassium bis(tri-
methylsilyl)amide (23 mL, 11.5mmol) in toluene was added
gradually to a solution of 1a (1.63 g, 10.5 mmol) in 20 mL of
toluene at 0 °C. The mixture was stirred at 0 °C for 1 h and allowed
to warm to room temperature for 4 h. After filtration and removal
of the solvent in vacuo, the residue was washed with 2 × 10 mL
of toluene and 2 × 10 mL of pentane followed by drying in vacuo
to give 3a as a white powder (1.93 g, 95%). The 1H, 11B, and 13
C
NMR spectra were as reported.10 This material was taken up in 20
mL of THF and the resulting solution was added to a solution of
Cp2ZrCl2 (1.30 g, 4.45 mmol) in 50 mL of THF at -78 °C. The
mixture was stirred at -78 °C for 2 h and allowed to warm to
room temperature for 10 h. The solvent was removed in vacuo and
the residue was extracted with 3 × 25 mL of toluene. After filtration
and removal of the solvent the residue was washed with 3 × 15
mL of hexanes. After drying the product (1.32 g, 56%) was an
amber powder, which was recrystallized from toluene to give yellow
needles. 1H NMR (400 MHz, CDCl3): δ 6.06 (s, 10H, CpH), 6.20
(t, J ) 6.2 Hz, 2H, C(5)H), 6.78 (d, J ) 10.6 Hz, 2H, C(3)H),
7.09(d, J ) 6.2 Hz, 2H,C(6)H), 7.38 (t, J ) 7.6 Hz, 2H, ArH),
7.46 (t, J ) 7.6 Hz, 4H, ArH), 7.57 (dd, J ) 10.6, 6.2 Hz, 2H,
C(4)H), 7.61 (d, J ) 7.6 Hz, 4H, ArH). 13C NMR (100.6 MHz,
CDCl3): δ 109.1, 115.4, 127.3, 127.8, 131.8 (br), 132.7, 137.3,
142.4. 11B NMR (160.4 MHz, CDCl3): δ 39.8. HRMS (EI, m/z)
calcd for C30H2811B2N290Zr [M - 2H] 526.1329, found 526.1327.
Satisfactory CHN combustion analyses could not be obtained.
However, the NMR spectra were consistent with a sample of high
purity.
Figure 2. The solid-state structure of 1c (ORTEP). Thermal
ellipsoids are set at the 50% probability level. Hydrogen atoms have
been omitted for clarity.
Table 2. Crystal Data and Data Collection Parameters for 5 and 1c
5
1c
empirical formula
fw
C30H28B2N2Zr
529.38
C13H18BNSi
227.18
temp, K
150(2)
150(2)
wavelength, Å
cryst syst
space group
a, Å
b, Å
c, Å
0.71073
monoclinic
P2(1)/c
14.737(16)
11.604(1)
15.199(2)
108.281(2)
2470.9(5); 4
1.423
0.71073
monoclinic
P2(1)/c
10.024(1)
15.631(2)
8.525(1)
96.184(2)
1327.8(3); 4
1.136
ꢀ, deg
V, Å3; Z
calcd density, Mg/m3
abs coeff, mm-1
F(000)
0.467
1088
0.150
488
cryst size, mm3
limiting indices
0.50 × 0.26 × 0.20 0.32 × 0.26 × 0.21
-19 e h e 18
-15 e k e 15
-20 e l e 20
-13 e h e 13
-20 e k e 20
-11 e l e 11
12621/3304
1,2-Dihydro-2-phenyl-1-(trimethylsilyl)-1,2-azaborine (1c).
One equivalent of trimethylsilyl chloride was added to a THF
1
solution of 3a obtained as above. The H NMR and mass spectra
no. of reflns collected/unique 22435/6139
showed that 1c was identical with previously obtained material.10
Single-Crystal X-ray Crystallography. Crystals of 1c and 5
were obtained from crystallization from the melt and recrystalli-
zation from toluene, respectively. Crystallographic and data col-
lection parameters are collected in Table 2. ORTEP drawings of 5
and 1c showing the atom-numbering scheme used in refinement
are given in Figures 1 and 2, respectively. Selected bond distances
and angles are collected in Table 1. Additional crystallographic
data are available in the Supporting Information.
abs cor
semiempirical from equivalents
refinement method
no. of data/restrains/params
GOF on F2
full-matrix least squares on F2
6139/0/316
3304/0/148
1.050
1.043
final R indices (I > 2σ(I))
R1 ) 0.0244
wR2 ) 0.0643
R1 ) 0.0280
wR2 ) 0.0667
0.402 and -0.321
R1 ) 0.0348
wR2 ) 0.0923
R1 ) 0.0424
wR2 ) 0.0981
0.338 and -0.214
R indices (all data)
largest diff peak and hole,
e/Å
Acknowledgment. Partial support of this research has
been provided by NSF.
of pyrrolyl, which is also know to form both σ-12 and π-metal
complexes.13–15
Supporting Information Available: CIF files giving X-ray
characterization of 5 and 1c and figures giving 1H NMR spectra of
5 and 1c. This material is available free of charge via the Internet
at http://pubs.acs.org.
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OM701224C
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