Synthesis and structural characterization of 9-azido-9-borafluorene: Monomer and cyclotrimer of a borole azide

Article abstract of DOI:10.1021/ic902436s

The reaction of 2,2′-dilithiobiphenyl, generated from 2,2′-dibromobiphenyl and n-BuLi, with BCl₃ using n-hexane as solvent provides a high-yielding, simple preparative route to 9-chloro-9-borafluorene 1a. This in turn can be reacted with trimethylsilyl azide to yield 9-azido-9-borafluorene 2. Compound 2 exists as a cyclic trimer in the solid state, but in dichloromethane solution the monomer can coexist with the trimer. Azide 2 is rather unstable both in the solid state and in solution, and it transforms with trace amounts of water and oxygen to the 1,3,5-tris(2-biphenylyl)cyclotriboroxane that was characterized by X-ray diffraction analysis. Addition of pyridine and t-butyl pyridine to azide 2 afford the corresponding pyridine adducts. Compound 1a as well as the Lewis base adducts of 2 have been characterized by multinuclear NMR spectroscopy, and the structures were confirmed by X-ray diffraction analysis. The structural features of azide 2 and the strong Lewis acidity of its boron center have also been investigated by computational chemistry techniques at the MP2 level of theory.

Full text of DOI:10.1021/ic902436s

Inorg. Chem. 2010, 49, 4499–4506 4499
DOI: 10.1021/ic902436s
Synthesis and Structural Characterization of 9-Azido-9-Borafluorene: Monomer
and Cyclotrimer of a Borole Azide
†
,§
‡
,†,§
Sunanda Biswas, Iris M. Oppel, and Holger F. Bettinger*
†
‡
Lehrstuhl f u€ r Organische Chemie II, and Lehrstuhl f u€ r Analytische Chemie, Ruhr-Universit a€ t Bochum,
§
Universit a€ tsstrasse 150, 44780 Bochum, Germany, and Institut f u€ r Organische Chemie, Universit a€ t T u€ bingen,
Auf der Morgenstelle 18, 72076 T u€ bingen, Germany
Received December 8, 2009
0
0
The reaction of 2,2 -dilithiobiphenyl, generated from 2,2 -dibromobiphenyl and n-BuLi, with BCl using n-hexane as
3
solvent provides a high-yielding, simple preparative route to 9-chloro-9-borafluorene 1a. This in turn can be reacted
with trimethylsilyl azide to yield 9-azido-9-borafluorene 2. Compound 2 exists as a cyclic trimer in the solid state, but in
dichloromethane solution the monomer can coexist with the trimer. Azide 2 is rather unstable both in the solid state and
in solution, and it transforms with trace amounts of water and oxygen to the 1,3,5-tris(2-biphenylyl)cyclotriboroxane
that was characterized by X-ray diffraction analysis. Addition of pyridine and t-butyl pyridine to azide 2 afford the
corresponding pyridine adducts. Compound 1a as well as the Lewis base adducts of 2 have been characterized by
multinuclear NMR spectroscopy, and the structures were confirmed by X-ray diffraction analysis. The structural
features of azide 2 and the strong Lewis acidity of its boron center have also been investigated by computational
chemistry techniques at the MP2 level of theory.
Introduction
After the first report of an azidoborane [B(N ) ] by Wiberg
Chart 1
3
3
1
and Michaud in 1954, a number of acyclic azidoboranes of
2-8
type A (R BN ; R = alkyl, aryl, oxo, amino)
(Chart 1)
2
3
have been successfully synthesized. Whereas a number of
2,8-10
heterocyclic azidoboranes of type B are known,
only
two examples of stable carbocyclic azidoboranes of type C
11
have been described. Members of type D are still unknown.
Most of the known azidoboranes are monomeric in solu-
7,9,12
tion and in the solid state.
But the presence of a Lewis
acidic boron center and a basic nitrogen center allows
structural diversity by intermolecular Lewis acid-base inter-
*
To whom correspondence should be addressed. E-mail: holger.bettinger@
action. The tendency to aggregate strongly depends on the
uni-tuebingen.de.
(
(
(
1) Wiberg, E.; Michaud, H. Z. Naturforsch. 1954, 96, 497.
2) Pieper, W.; Schmitz, D.; Paetzold, P. Chem. Ber. 1981, 114, 3801.
3) Paetzold, P. I.; Maier, G. Angew. Chem. 1964, 76, 343; Angew. Chem.,
Lewis acidity of the boron center that is controlled by
11,13
electronic effects as well as by steric effects.
When R is
small (Type A), the azide forms oligomers. A typical example
Int. Ed. Engl. 1964, 3, 315.
5
(
(
(
(
4) Paetzold, P.; Truppat, R. Chem. Ber. 1983, 116, 1531.
is Me BN3 that was found by Paetzold et al. to show
2
5) Paetzold, P. I.; Hansen, H.-J. Z. Anorg. Allg. Chem. 1966, 345, 79.
6) Meier, H.-U.; Paetzold, P.; Schr o€ der, E. Chem. Ber. 1984, 117, 1954.
temperature dependent oligomer formation in solution.
Likewise, the small and electron withdrawing halogens
of dihaloazidoboranes [BX N (X = F, Cl, Br)] result in
7) Fraenk, W.; Klap
o€ tke, T. M.; Krumm, B.; Mayer, P.; N o€ th, H.;
Piotrowski, H.; Suter, M. J Fluorine Chem. 2001, 112, 73.
2
3
(
(
8) Paetzold, P.; Maier, G. Chem. Ber. 1970, 103, 281.
9) Fraenk, W.; Habereder, T.; Klap o€ tke, T. M.; N o€ th, H.; Polborn, K.
14-17
the formation of trimers (BX N ) .
In 1971, M u€ ller
2
3 3
J. Chem. Soc., Dalton Trans. 1999, 4283.
10) Bettinger, H. F.; Filthaus, M.; Bornemann, H.; Oppel, I. M. Angew.
Chem., Int. Ed. 2008, 47, 4744.
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Mackiewicz, T. v. Z. Anorg. Allg. Chem. 2004, 630, 2641.
12) Steuber, E. v.; Elter, G.; Noltemeyer, M.; Schmidt, H.-G.; Meller, A.
(
(
13) Paetzold, P. I. Fortschr. Chem. Forsch. 1967, 8, 437.
(14) Paetzold, P. I. Z. Anorg. Allg. Chem. 1963, 326, 47.
(
(15) Paetzold, P. I.; Gayoso, M.; Dehnicke, K. Chem. Ber. 1965, 98, 1173.
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(
(17) Wiberg, N.; Joo, W.-C.; Schmid, K. H. Z. Anorg. Allg. Chem. 1972,
Organometallics 2000, 19, 5083.
394, 197.
r 2010 American Chemical Society
Published on Web 04/16/2010
pubs.acs.org/IC

4
500 Inorganic Chemistry, Vol. 49, No. 10, 2010
Biswas et al.
Chart 2
Scheme 1. Synthesis of 1a
1
6
reported the crystal structure of (BCl N ) , wherein the
2
3 3
six-membered (BN) ring has a skew boat conformation
3
2
4
and each boron atom is attached to two chlorine atoms
and two nitrogen atoms in a distorted tetrahedral fashion.
Later on Klap o€ tke et al. reported the NMR data for the
as a long-lived species in strong acid were unsuccessful.
Numerous rate measurements
24-28
display a rate retardation
in forming the fluorenyl carbocation compared to the benz-
hydryl system, and calculations indicate that carbocation III
1
8
known (BF N ) . Since then only three additional aggre-
2
3 3
28-30
gated azidoboranes have been structurally characterized in
is 8-10 kcal/mol less stable.
Excitation energy calcula-
1
8
31
the solid state by Klap o€ tke et al.: the trimer [C F B(N ) ] ,
tions propose antiaromatic character in III, that is, the
longest wavelength absorption of cyclopentadienyl cation
increases upon annulation going to indenyl and fluorenyl
6
5
3 2 3
7
19
and the dimers [(C F ) BN ] and [(C H F ) BN ] . An
6
5 2
3 2
6
3
2 2
3 2
oligomeric structure is also likely for solid [C H F B(N ) ]
6
3
2
3 2
7
32
based on vibrational spectroscopy. These azides are oligo-
meric in the solid state and, with the exception of the
insoluble [(C F ) BN ] , transform completely into mono-
cation. Already in 1966, Armstrong and Perkins have
compared the calculated (using the semiempirical PPP
Hamiltonian) and experimental absorption spectra of bora-
fluorene systems with suitable models. They concluded that
neutral borafluorene (IV) has very small π charge on boron
and that the presence of the bond connecting the two phenyl
rings at ortho positions “effectively cuts off π electron
conjugation via boron.” This suggests that in borafluorenes
the boron center is a strong electron acceptor. Piers et al.
reported that the boron center in the perfluorinated diben-
zannulated borole is more Lewis acidic than the open per-
6
5 2
3 2
mers in solution.
The stability of carbocyclic azidoboranes of Type C
appears to depend on the ring size. 1-Azido-3-methylbora-
cyclopentane (n = 4) undergoes ring expansion at room tem-
perature while carbocyclic azidoboranes with n = 6, 7 are
32
11
found to be stable at room temperature. An attempt to
9
synthesize 9-BBN-N failed as reported by N o€ th et al.
3
A carbocyclic motif that has not been explored in the
context of azide formation is the borole system (Type D).
Borole (II) is isoelectronic to the cyclopentadienyl cation (I)
33
fluorinated diarylborane systems.
Herein, we report the synthesis and structural character-
ization of the azide of dibenzoborole (Type D) and its
cyclotrimerization. The cyclotrimer exists in the solid state
and can coexist with the monomer in dichloromethane
solution. The combination of the strongly Lewis acidic boron
(Chart 2). A quantitative comparison of unsaturated five
membered ring systems based on geometric, energetic, and
magnetic criteria has been convincingly interpreted by
20
Schleyer et al. in terms of the aromatic and antiaromatic
21
nature of these species. Recently Braunschweig et al.
of the borole and the Lewis basic N atom in a single
R
observed that in the solid state structure of pentasubstituted
borole PhBC Ph the bond length alteration (BLA) in the
molecule limits the stability of the azide at room temperature.
But pyridine adducts that are also structurally characterized
enjoy enhanced stability.
4
4
BC ring is significantly smaller than computed for the
4
antiaromatic species in the gas phase. This was ascribed to
result from intermolecular interactions that reduce the Lewis
acidity of the boron center and induce the observed changes
in bond lengths. Piers et al. could not identify the intermole-
cular interactions in the solid state of the highly symmetric
perfluorinated pentaphenylborole and ascribed the apparently
Results and Discussion
9
-Azido-9-borafluorene is obtained from the chloride 1a
Scheme 1) by reaction with trimethylsilyl azide. Chloride 1a
was first prepared in 1963 by K o€ ster and Benedikt from
(
34,35
9-alkyl-9-borafluorene.
Later, this compound was
22
reduced BLA to crystallographic disorder. Yamaguchi and
co-workers also noticed π interaction as judged by intermole-
cular distances in less symmetric boroles [Ph C BAr, Ar =
synthesized either by transmetalation or by salt elimination.
36
Narula and N o€ th reported a high yielding (75%) trans-
4
4
metalation procedure using a mercury containing precursor.
To avoid the highly toxic mercury compound we have
improved the salt elimination procedure reported by Hong
p-MeC H , p-Me SiC H , p-FC H ]. However, still signifi-
6
4
3
6
4
6
4
23
cant bond length alternation was present in the BC unit.
4
Dibenzoborole or 9-borafluorene, IV, is isostructural with
the fluorenyl cation, III, whose antiaromaticity is still a
matter of debate. Efforts to generate this fluorenyl cation
(
(
25) Ledwith, A.; Morris, D. G. J. Chem. Soc. 1964, 508.
26) Cowell, G. W.; George, T. D.; Ledwith, A.; Morris, D. G. J. Chem.
Soc. B 1966, 1169.
(
(
27) Cowell, G. W.; Ledwith, A. J. Chem. Soc. B 1967, 695.
(
18) Fraenk, W.; Klap o€ tke, T. M.; Krumm, B.; N o€ th, H.; Suter, M.;
Warchhold, M. J. Chem. Soc., Dalton Trans. 2000, 4635.
19) Fraenk, W.; Klap o€ tke, T. M.; Krumm, B.; Mayer, P. Chem. Com-
mun. 2000, 667.
20) Schleyer, P. v. R.; Freeman, P. K.; Jiao, H.; Goldfuss, B. Angew.
Chem., Int. Ed. Engl. 1995, 34, 337.
21) Braunschweig, H.; Fernandez, I.; Frenking, G.; Kupfer, T. Angew.
Chem., Int. Ed. 2008, 47, 1951.
22) Fan, C.; Piers, W. E.; Parvez, M. Angew. Chem., Int. Ed. 2009, 48,
955.
23) So, C.-W.; Watanabe, D.; Wakamiya, A.; Yamaguchi, S. Organo-
metallics 2008, 27, 3496.
24) Olah, G. A.; Prakash, G. K. S.; Liang, G.; Westerman, P. W.; Kunde,
K.; Chandrasekhar, J.; Schleyer, P. v. R. J. Am. Chem. Soc. 1980, 102, 4485.
28) Amyes, T. L.; Richard, J. P.; Novak, M. J. Am. Chem. Soc. 1992, 114,
8
032.
29) Rodriquez, C. F.; Vuckovic, D. L.; Hopkinson, A. C. J. Mol. Struct.
THEOCHEM 1996, 363, 131.
30) Jiao, H.; Schleyer, P. v. R.; Mo, Y.; McAllister, M. A.; Tidwell, T. T.
J. Am. Chem. Soc. 1997, 119, 7075.
31) Dietz, F.; Tyutyulkov, N.; Rabinovitz, M. J. Chem. Soc., Perkin
Trans. 2 1993, 157.
(
(
(
(
(
(
(
(
32) Armstrong, D. R.; Perkins, P. G. J. Chem. Soc., A 1966, 1026.
2
(33) Chase, P. A.; Piers, W. E.; Patrick, B. O. J. Am. Chem. Soc. 2000, 122,
(
1
2911.
(34) K o€ ster, R.; Benedikt, G. Angew. Chem. 1963, 75, 419.
(35) K o€ ster, R.; Willemsen, H.-G. Liebigs Ann. Chem. 1974, 1843.
(36) Narula, C. K.; N o€ th, H. J. Organomet. Chem. 1985, 281, 131.
(

Article
Inorganic Chemistry, Vol. 49, No. 10, 2010 4501
˚
Figure 1. (a) Molecular structure of 1a. Selected bond lengths [A] and
angles [deg] in 1a: B(9)-Cl(1) 1.752(3), B(9)-C(9A) 1.547(4), C(8A)-B-
(
9) 1.536(4), C(4A)-C(9A) 1.409(3), C(4A)-C(4B) 1.481(3), C(4B)-
Figure 2. Molecular structure of 1b. Hydrogen atoms are omitted for
˚
clarity. The red dots mark the center of rings. Selected bond lengths [A]
and angles [deg] (theoretical data obtained at the RI-MP2/SVP level of
theory are given in parentheses): B(21)-C(2) 1.558(5) (1.557); C(2)-C(1)
C(8A) 1.416(3); Cl(1)-B(9)-C(8A) 126.3(2), C(8A)-B(9)-C(9A) 105.9(2),
B(9)-C(8A)-C(4B) 106.8(2), C(8A)-C(4B)-C(4A) 110.2(2); (b) Di-
meric subunit in the crystal lattice of 1a. The intermolecular B(9)
3
3 3
˚
C(9A) distance is 3.485 A. Hydrogen atoms are omitted for clarity.
1
.414(4), (1.422); C(1)-C(12) 1.480(5) (1.473); C(12)-C(7) 1.415(5)
(
1.422); and C(7)-B(30) 1.551(5), (1.557); B B 2.948 (2.743); Selected
3 3 3
37
dihedral angle [deg]: C(2)-C(1)-C(12)-C(7) 43.40 (41.9).
and Chung. As initially reported, this procedure suffers from
a low yield (36%). But the simple modification reported here
improves the yield considerably: carrying out the dilithiation
of 2,2 -dibromobiphenyl in n-hexane rather than in ether
gives 1a in 90% yield with respect to 2,2 -dibromobiphenyl.
2
C-C single bond between two sp -hybridized carbon atoms
0
38
˚
1.49 A) and similar to the corresponding length in 1a py.
To achieve the planar geometry, the bond angles inside the
(
3
0
-
3
five membered ring also become smaller by 10-15° than
The chloride 1a is purified by distillation at 90 °C/10
2
the value expected for sp hybridized atoms. This induces
mbar. During distillation the yellow liquid 1a rapidly forms
nice yellow needle shaped crystals upon contact with the cold
trap (0 °C). These are suitable for X-ray analysis.
some strain that is relieved upon pyramidalization. This issue
was previously discussed by Narula and N o€ th in the context
of the X-ray structure of the pyridine adduct of 9-chloro-9-
9-Chloro-9-borafluorene 1a crystallizes in the monoclinic
45
borafluorene, 1a py. The crystal packing of 1a reveals the
space group P2 /c with four formula units in the unit cell.
Figure 1a shows the projection of the molecule. The molecule
is planar having a sum of bond angles around the boron atom
of 359.9(2). The B-Cl bond length [1.752(3) A] is in the usual
range.
and C(8A)-B(9) 1.536(4) A] are shorter than typical B-C
bonds in other three-coordinated boranes [e.g., 1.589(5)
and 1.571(3) in Ph B, 1.579(2) and 1.580(3) in Mes B ]
3
1
intermolecular interaction between the boron atom and the
π system of a neighboring molecule. The shortest intermole-
˚
˚
cular B C distance is 3.485 A (Figure 1b) which is shorter
3 3 3
39-41
˚
than that of pentasubstituted borole compounds reported
The B-C bond lengths [B(9)-C(9A) 1.547(4) A
21,23
earlier.
˚
An even shorter intramolecular distance between
aryl
a boron center of a borafluorene and an aromatic ring was
46
42
43
observed by Hoefelmeyer et al.
3
3
Depending on the purity of 1a, crystallization can either be
and slightly shorter than in other 9-borafluorene compounds
33
retarded or not be observed at all. In one instance we were
[
e.g., 1.549(3) in F C B(C F ), 1.556(7) and 1.573(6) in
8 12 6 5
0
able to obtain a byproduct 1b [2,2 -bis(9-borafluoren-9-
1
-(4-tert-butylphenyl)-7-tert-butyl-9-(bis-2,6-(4-tert-butyl-
44
yl)biphenyl] as deep red crystals. These grew from the yellow
oil after standing for an extended period of time without
interruption (Figure 2). The mechanism of formation of 1b is
not clear yet; further studies will be pursued in the future.
Compound 1b crystallizes monoclinically in the space
phenyl)phenyl)-9-borafluorene. The B-C bond lengths in
a are also shorter than in its pyridine adduct 1a py that was
1
3
prepared and structurally characterized by Narula and N o€ th
previously. Likewise, the unusually long B-Cl bond in
1
45
a py [1.900(2) average] shortens significantly in free 1a to
3
˚
˚
group P2 /c with Z = 4. The planar dibenzoborole units
1
.752(3) A. The lengths of the C(4A)-C(9A) [1.409(3) A]
1
˚
have comparable bond lengths and angles. The syn orienta-
tion of the two dibenzoborole units in 1b is unexpected in
view of their size and results in a dihedral angle C(2)-
C(1)-C(12)-C(7) of 43.4°. This is rather small compared
and C(4B)-C(8A) [1.416(3) A] bonds are also similar to
those of 1a py [1.404(3) and 1.408(3)] and in agreement with
3
˚
an aromatic ring system (1.40 A). The distance between
C(4A)-C(4B) [1.481(3) A] bond is slightly shorter than the
˚
0
to the value of this particular dihedral angle in 1,1 -bi-
0
47
naphthyl-2,2 -diboronic acid (96.7°). In the syn conforma-
(37) Hong, H.; Chung, T. C. J. Organomet. Chem. 2004, 689, 58.
(
38) Buchalski, P.; Grabowska, I.; Kami nꢀ ska, E.; Suwi nꢀ ska, K. Organo-
tion these two 9-borafluorene units have a short distance of
˚
3
.636 A between the centers of six membered rings indicative
metallics 2008, 27, 2346.
39) Macdonald, C. L. B.; Gorden, J. D.; Voigt, A.; Filipponi, S.; Cowley,
A. H. Dalton Trans. 2008, 1161.
40) Wagner, T.; Eigendorf, U.; Herberich, G. E.; Englert, U. Struct.
Chem. 1994, 5, 233.
of π-π interaction between two dibenzoborole units. The
(
˚
boron-boron distance (2.948 A) is similar to that of 1,8-
(
˚
bis(diphenylboryl)naphthalene (3.002 A) indicating a signifi-
cant overlap of p orbitals of the two boron centers as
(
(
41) Ashe, A. J.; Klein, W.; Rousseau, R. Organometallics 1993, 12, 3225.
42) Zettler, F.; Hausen, H. D.; Hess, H. J. Organomet. Chem. 1974, 72,
z
48
described by Hoefelmeyer and Gabbai. The average
1
57.
(
43) Olmstead, M. M.; Power, P. P. J. Am. Chem. Soc. 1986, 108, 4235.
44) Wehmschulte, R. J.; Diaz, A. A.; Khan, M. A. Organometallics 2003,
(
(46) Hoefelmeyer, J. D.; Sol ꢀe , S.; Gabbai, F. P. Dalton Trans. 2004, 1254.
(47) Schilling, B.; Kaiser, V.; Kaufmann, D. E. Chem. Ber. 1997, 130, 923.
(48) Hoefelmeyer, J. D.; Gabbai, F. P. J. Am. Chem. Soc. 2000, 122, 9054.
2
2, 83.
(
45) Narula, C. K.; N o€ th, H. Inorg. Chem. 1985, 24, 2532.

4
502 Inorganic Chemistry, Vol. 49, No. 10, 2010
Biswas et al.
be due to an unknown decomposition product of the azide.
The azide 2 is soluble in benzene, toluene, and dichloro-
methane but decomposes in chloroform. Its stability in
dichloromethane solution is still limited though as it slowly
decomposes to a greenish compound even at -20 °C after a
few days. Because of the instability of this azide, the isolation
and purification is very difficult. Although the nitrogen
content is below the commonly accepted critical value of
5
0
2
5%, sensitivity also depends on other parameters. Paetzold
has stated that the formation of oligomers turn the corres-
11
ponding azides into labile and dangerous compounds. The
strongly Lewis acidic boron center could potentially catalyze
the (spontaneous) decomposition of this azide. Because of this
dangerous nature and the observed instability, we have not
done further investigations of the monomer-trimer equilibra-
tion in solution.
Figure 3. LUMO (a symmetry, E = 0.30 eV) of 1b as computed at the
RI-MP2/SVP level of theory.
˚
distances between B-C
(1.555 A) and in the borole
biphenyl
2
˚
units B-C (1.566 A) are in the range of regular B-C(sp )
distances. The structure observed experimentally is in good
agreement with theory indicating that the syn conformation
The coexistence of monomer and oligomer in solution is in
1
8
19
contrast to the behavior of [(C F B(N ) ] or[(C F ) BN ]
3 2
6
5
3 2 3
6
5 2
7
and [(C H F ) BN ] . These are trimeric or dimeric only
6
3
2 2
3 2
49
is not merely the result of packing forces. The close proxi-
mity of the boron atoms allows for an overlap of the p
orbitals in a σ fashion (Figure 3). This reduces the lowest
unoccupied molecular orbital (LUMO) level and bathochro-
mically shifts the longest wavelength absorption that is
originating from the highest occupied to a lowest unoccupied
molecular orbital (HOMO) f LUMO excitation (562 nm,
B3LYP/TZVPP//RI-MP2/SVP level of theory).
in solid state and monomers in solution. On the other
hand, spectroscopic data for (BX N ) [X = F, Cl, Br] show
2
3 3
evidence only for trimeric azides in the solid state as well as in
14-18
solution.
The crystallization of the 9-azido-9-borafluorene 2 is
difficult. We managed to get a few crystal blocks grown from
dichloromethane at -20 °C but several other trials to
produce high quality crystals in different solvents and under
various conditions have failed. An X-ray analysis confirms
that 9-azido-9-borafluorene is trimeric in nature in the solid
state, but the crystal data is not of sufficiently good quality to
discuss the geometry parameters (Figure 4).
We thus resort to computational chemistry techniques
utilizing second-order perturbation theory (MP2) in con-
junction with the resolution of the identity (RI) approxima-
tion and a polarized split-valence double-ζ basis set (RI-
MP2/SVP). We first consider the monomer 2a that is com-
9-Azido-9-borafluorene 2a was synthesized by the reaction
of 1a with trimethylsilyl azide (TMSN ) in dichloromethane
3
solution. We found that the crude yellow reaction solution
11
shows a B signal at δ 50.2 which is expected for a typical
three coordinated monomeric azidoborane 2a. After removal
of all volatiles from the reaction mixture followed by redis-
11
B
solution in CD Cl , we observe an additional signal in the
2
2
NMR spectrum at δ 5.0 along with δ 50.3. This new signal at
δ 5.0 lies in the region of tetracoordinated boron centers and
is assigned to an oligomer 2b which appears to be formed in
the solid state and that persists in solution (Scheme 2).
For further understanding, we have measured the IR
spectrum of the crude reaction mixture. This shows a strong
˚
puted to be planar. The B-N distance of 1.436 A and C-C
R
˚
˚
(1.422 averaged and 1.486 A) and B-C distances (1.566 A
averaged) in the central borole ring are as expected for
dibenzoannulated derivatives. The LUMO is a π* orbital
-1
0
0
absorption at 2139 cm that is typical for the asymmetric
of a symmetry and has its largest coefficient on the boron
atom. The azide unit is slightly bent (N-N-N angle: 172.5°)
while the B-N-N angle is 125°. This is similar to previous
stretching frequency of monomeric azidoboranes (2100-2200
-
1 4
cm ). After removal of the solvent from the reaction
mixture and redissolution in dichloromethane two absorp-
tions in the asymmetric stretching region of azides are
51,52
computational results for azidoboranes
experimental data.
and available
7,9,12
-
1
observed: one is at 2136 cm and another one is blue-shifted
Two isomers 2b-A and 2b-B (Figure 5) of the trimer can be
located as stationary points on the potential energy surface.
-
1
to 2176 cm in dichloromethane solution. This frequency
shift to higher wavenumber is in agreement with an oligomer
The conformer 2b-A is of C symmetry and has a twist-like
2
2
b of 9-azido-9-borafluorene. IR data obtained for (BF N )
arrangement of the central six-membered ring. As observed
2
3 3
-
1
17,18
-1 14,15
16,18
(2236 cm ),
(BCl N ) (2210 cm
)
and [(C F ) -
in the single crystal structures of trimeric azides,
the azide
2
3 3
6 5 2
-
1 19
BN ] (2202 cm
)
clearly show that the N-N stretching
units are essentially linear. One of the azide groups and a
3
2
vibration of bridging azides are always shifted to higher
wavenumber compared to that of monomeric free azides.
We have tried to explore the aggregation behavior of 2
by slightly increasing the temperature (see Supporting In-
formation, Figures S6 and S7). After keeping the tempera-
ture at 40 °C for 3 h, the intensity of the signal at δ 5.1
decreases compared to the signal at δ 50.3 as expected for a
monomer-oligomer equilibrium. However, an additional
weak signal at δ 40.0 is observed after warming which could
transannular boron center lie on the C axis that bisects the
2
formal C-C single bond between the phenyl rings in the
corresponding borafluorene unit. The N atoms are in an
R
almost trigonal planar coordination environment with sums
of bond angles of 357.2 and 360.0° for theoff-axis andon-axis
˚
atoms, respectively. The B-N bond lengths, 1.591 to 1.606 A,
16,18
are withinthe range expected for a dative interaction.
The
(50) Smith, P. A. S. The Chemistry of Open-Chain Organic Nitrogen
Compounds; W. A. Benjamin, Inc.: New York, 1966; Vol. 2, p 214.
(51) Fraenk, W.; Klap o€ tke, T. M. J. Fluorine Chem. 2001, 111, 45.
(52) Bettinger, H. F. J. Organomet. Chem. 2006, 691, 4480.
(
49) Eliel, E. L.; Wilen, S. H.; Doyle, M. P. Basic Organic Stereochemistry;
Wiley-Interscience: New York, 2001; pp 626-627.

Article
Inorganic Chemistry, Vol. 49, No. 10, 2010 4503
Scheme 2. Synthesis of 9-Azido-9-borafluorene 2 and Its Pyridine Adducts and Key Spectral Features of 2-4
˚
3
58.8°). Noteworthy are two close contacts of 2.235 A
between the hydrogen atoms of the borafluorene units.
Both isomers are essentially isoenergetic as they lie within
.1 kcal mol in energy in favor of 2b-A at the RI-MP2/
-
1
0
SVP level. The trimerization of azide 2a is highly exother-
-
1
mic, ΔE(0 K) = 57 kcal mol . Because of the size of the
system we do not consider enthalpy or entropy corrections.
It is interesting to compare the cyclotrimerization of 2a and
51
F BN that was studied by Fraenk and Klap o€ tke at a
2
3
similar level of theory (MP2(full)/6-31G**). Formation of a
cyclotrimer of F BN3 with a chairlike conformation is
exothermic by 22.5 kcal mol according to these authors.
3 3 3
Figure 4. Molecular structure of 2b (left) and depiction of its B (N )
core(right). Two dichloromethanesolvent moleculesand hydrogenatoms
are omitted for clarity.
2
-
1
51
At our level of theory, we find that besides the chair also a
twist conformation of (F BN ) corresponds to a minimum
2
3 3
-
1
that is 3.2 kcal mol lower in energy than the chair form.
The energy of trimerization, -35.7 kcal mol , is however
significantly larger at our level of theory than the previously
-1
51
reported value. The source for the discrepancy is unclear
at this time: both the RI and the frozen core approximations
can be excluded as identical or very similar reaction energies
are obtained in the case of conventional or full MP2
calculations. On the other hand, the basis sets SVP versus
6-31G(d,p) are of too similar quality to be responsible for
such a large difference.
In any event, the computed exothermicity of the trimeri-
zation of azide 2a to 2b is larger than that for the trimeriza-
tion of F BN , at least in the gas phase without any enthalpy
Figure 5. Structures of the two conformers of cyclotrimer 2b as com-
˚
puted at the RI-MP2/SVP level of theory. Left: selected bond lengths [A]
and angles [deg] of 2b-A: N1-B1 1.591; B1-N2 1.606; N2-B2 1.602;
2
3
0
0
N1-B1-N2 102.1; B1-N2-B2 128.7; N2-B2-N2 98.5; B1-N1-B1
or entropy corrections. This shows that the boron center in
azide 2a is highly Lewis acidic, and is stabilized by forma-
tion of a strong σ bond to the R-nitrogen atom of another
molecule. The coexistence of monomeric and trimeric
forms of 2 in solution indicate that the solvent appears to
have a pronounced effect on the energy of cyclotrimeriza-
tion, more so than for F BN that is only known in its
1
27.8 (symmetry equivalent atoms are labeled with primes). Right:
˚
selected bond lengths [A] and angles [deg] of 2b-B: B1-N1 1.597;
0
N1-B2 1.586; B2-N2 1.619; N1 -B1-N1 102.0; B1-N1-B2 127.6;
0
N1-B2-N2 100.6; B2-N2-B2 132.6 (symmetry equivalent atoms are
labeled with primes).
B-C bonds are shorter for the two symmetry identical
2
3
˚
borafluorene subunits, 1.600 and 1.603 A, but longer for
trimeric form.
˚
the third unit, 1.613 A.
As expected, the azide 2 is extremely sensitive toward
degradation. In the presence of trace amounts of water
and air it converts to 1,3,5-tris(2-biphenylyl)cyclotriboro-
xane 5 as depicted in Figure 6. The white compound 5
crystallizes in a triclinic space group P1 (Z = 4) with
two independent enantiomeric molecules in the asym-
metric unit that have equal lengths of equivalent bonds.
The B O ring is nearly planar with B-O bond distances
The second isomer 2b-B is of C symmetry and features a
s
central six-membered ring in a boat conformation. Again
the azide units are essentially linear. One of the borafluorene
units and a transannular azide lie within the symmetry
˚
plane. The B-N bond lengths, 1.586,1.597, and 1.619 A,
differ more than in 2b-A, while the planarity of the N
R
atoms is slightly reduced (sum of bond angles: 356.7° and
3
3

4
504 Inorganic Chemistry, Vol. 49, No. 10, 2010
Biswas et al.
Figure 7. Molecular structure of 3. Hydrogen atoms are omitted for
˚
clarity. Selectedbondlength lengths[A] andangles [deg] in single molecule:
Figure 6. Molecular structure of 5. Hydrogen atoms are omitted for
˚
clarity. Selected bond length lengths [A] and angles [deg] in one enantio-
N(21)-B(29) 1.559(3), N(21)-N(22) 1.201(2), N(22)-N(23) 1.136(2),
N(24)-B(29) 1.619(2), C(28A)-B(29) 1.610(3), C(29A)-B(29) 1.598(3),
C(24B)-C(28A) 1.406(2), C(24A)-C(24B) 1.485(2), C(24A)-C(29A)
mer: O(101)-B(102) 1.376(15), O(101)-B(106) 1.381(14), O(103)-
B(102) 1.377(15), O(103)-B(104) 1.378(15), O(105)-B(106) 1.364(15),
O(105)-B(104) 1.385(15), B(102)-C(101) 1.562(17), B(104)-C(113)
1.409(2); N(21)-N(22)-N(23) 175.9(2), B(29)-N(21)-N(22) 119.99(17),
1.557(17), B(106)-C(125) 1.563(17); B(102)-O(101)-B(106) 121.4(10),
C(29A)-B(29)-C(28A) 101.12(16), C(24B)-C(28A)-B(29) 108.58(17),
C(24A)-C(29A)-B(29) 108.93(16), C(28A)-C(24B)-C(24A) 110.68(16),
C(29A)-C(24A)-C(24B) 110.54(17).
B(102)-O(103)-B(104) 120.9(9), B(106)-O(105)-B(104) 121.6(9),
O(101)-B(102)-O(103) 118.9(10), O(101)-B(102)-C(101) 121.6(11),
O(103)-B(102)-C(101) 119.4(10), O(103)-B(104)-O(105) 118.7(11),
O(103)-B(104)-C(113) 124.3(11), O(105)-B(104)-C(113) 117.0(10),
O(105)-B(106)-O(101) 118.4(10).
˚
from 1.364(15) to 1.385(15) A, and the boroxine ring
angles are close to 120° with the B-O-B angles slightly
larger [120.9(9)-121.6(9)°] than the O-B-O angles
[
118.4(10)-118.9(10)°]. This is similar to boroxin crystals
53-56
investigated earlier.
Note that Wehmschulte et al. have reported that the steri-
cally encumbered borafluorene compounds 1-(4-tert-butyl-
phenyl)-7-tert-butyl-9-(bis-2,6-(4-tert-butylphenyl)phenyl)-9-
borafluorene and 1-(3,5-dimethylphenyl)-6,8-dimethyl-9-(bis-
2
,6-(3,5-dimethylphenyl)phenyl)-9-boraflorene are air and
44
moisture stable. Exposure of the pyridine adduct of 1-(4-
tert-butylphenyl)-7-tert-butyl-9-(bis-2,6-(4-tert-butylphenyl)-
phenyl)-9-borafluorene to concentrated aqueous HCl for one
week in the presence of air led to cleavage of one of the B-C
bonds of the borafluorene unit. The splitting pattern is similar
Figure 8. Molecular structure of 4. Hydrogen atoms are omitted for
˚
44
for the highly stable compound of Wehmschulte et al. and
our labile 9-azido-9-borafluorene 2.
clarity. Selected bond length lengths [A] and angles [deg]:B(1)-C(2)
1.611(19), B(1)-C(8) 1.617(2), C(2)-C(1) 1.413(17), C(1)-C(7)
1
1
.485(18), C(7)-C(8) 1.410 (17), N(1)-N(2) 1.208(14), N(2)-N(3)
We also have performed the trapping reaction of 2 with
pyridine and t-butyl pyridine as Lewis bases. Addition of
pyridine to the reaction mixture as well as to the redissolved
product (2a and 2b) in dichloromethane, in separate experi-
ments, yields the identical white stable pyridine adduct of
˚
˚
.138(15), B(1)-N(1) 1.548(16) A, B(1)-N(4) 1.608(17) A ; N(1)-N-
(
1
2)-N(3) 174.4(13), B(1)-N(1)-N(2) 121.7(11), B(1)-C(2)-C(1)
09.1(11), C(2)-C(1)-C(7) 110.8(11), C(1)-C(7)-C(8) 110.6(11), C-
(7)-C(8)-B(1) 109.1(11), C(2)-B(1)-C(8) 100.5(10).
1
1
compound crystallizes triclinically in the P1 space group with
eight formula units in the unit cell and four molecules in the
asymmetric unit. The boron atom is tetrahedrally arranged
9
-azido-9-borafluorene 3. This is characterized by an B signal
-
1
at δ 4.1 and a strong absorption at 2116 cm (νasymN ) in
3
the IR vibrational spectra. This red shift of the azide asym-
metric stretching vibration is very well-known for pyridine
adducts of different azidoboranes as reported earlier for
and bonded to pyridine with simple B-N dative bond
˚
(
average length B-Npy 1.61 A). Comparing this distance
-
1
-1 9
with earlier investigatedcrystal structures of pyridine adducts
CatBN (2169 cm ) and CatBN py (2132 cm ); [(C F ) -
3
3
3
5 2
6
5 2
9
-
1
-1 19
˚
of azidoboranes, that is, 9-BBN-N py [1.637(3) A] or (2-
BN3]2 (2202 cm ) and (C F ) BN py (2137 cm );
3
3
6
3
3
7
-
1
˚
FC H ) BN py [1.621(3) A], suggests that the boron center
in dibenzoborole is slightly more acidic and acts as a better
acceptor toward the pyridine donor. The azide group
[
(
C F B(N ) ] (2200/2180/2142 cm ) and C F B(N ) py
6
4 2
3
3
6
5
3 2 3
6
5
3 2
3
-
1 18
2146/2130 cm ).
Slow evaporation of solvent from the CH Cl solution
2
2
attached to boron is slightly bent [N -N -N 174-176°]
of compound 3 produces white needle shaped crystals. The
crystal structure of compound 3 (Figure 7) confirms the
existence of monomeric azide 2a in the reaction mixture. This
R
β
γ
˚
in all molecules 3. The B-N bond length of 1.562 A (avg.) is
R
˚
a little shorter than a typical B-N single bond (1.59 A). The
˚
˚
N -N (avg. 1.136 A) is shorter than N -N (avg. 1.192 A),
β
γ
R
β
and these bond distances have lengths between those of a
(
53) Brock, C. P.; Minton, R. P.; Niedenzu, K. Acta Crystallogr., Sect. C
987, 43, 1775.
54) Beckett, M. A.; Strickland, G. C.; Varma, K. S.; Hibbs, D. E.;
Hursthouse, M. B.; Mailk, K. M. A. J. Organomet. Chem. 1997, 535, 33.
55) Boese, R.; Polk, M.; Bl €a ser, D. Angew. Chem. 1987, 99, 239; Angew.
Chem., Int. Ed. Engl. 1987, 26, 245.
56) Alcaraz, G.; Euzenat, L.; Mongin, O.; Katan, C.; Ledoux, I.; Zyss, J.;
Blanchard-Desce, M.; Vaultier, M. Chem. Commun. 2003, 2766.
9
˚
˚
N-N double (1.24 A) and a triple bond (1.10 A).
The trapping reaction of 2 also has been carried out with
1
(
t-butyl pyridine producing 9-azido-9-borafluorene t-butyl
3
(
py 4. After washing with pentane, the white, crystalline, moi-
sture sensitive clean product is obtained. Addition of t-butyl
(
11
pyridine shifts the B NMR signal upfield to δ 3.7 and the IR

Article
Inorganic Chemistry, Vol. 49, No. 10, 2010 4505
-
1
stretching frequency to lower frequency (νasymN 2125 cm
)
12.1 mL of n-BuLi (19.2 mmol) was added dropwise. After
complete addition, the ice bath was removed and the white
solution stirred for 3 days at ambient temperature. After
removal of the solvent in vacuo, the remaining white solid was
washed with n-hexane (3 ꢀ 100 mL). Then a white slurry was
made in 150 mL of hexane by vigorous stirring, and 9.6 mL of
3
compared to free azide 2. The single crystal X-ray analysis
confirms the structure of 4 as depicted in Figure 8.
Compound 4 crystallizes in the monoclinic space group
P2 /c with Z = 4. It shows tetrahedral coordination geometry
around the boron atom. Again, the azide group is slightly bent
1
BCl (1 M in n-hexane, 9.6 mmol) was added slowly to it at 0 °C.
3
at N(2) [N(1)-N(2)-N(3) 174.4°] and the N(2)-N(3) bond
After stirring overnight, the bright yellow solution was filtered
from the white solid and concentrated to ∼5 mL. This remaining
˚
˚
[
1.138(15) A] is shorter than N(1)-N(2) bond [1.208(14) A].
˚
˚
The B(1)-N(1) [1.548(16) A] and B(1)-N(4) [1.608(17) A]
distances are comparable to those of pyridine adduct 3. The
-3
oily liquid was distilled at 90 °C at 10 mbar to give (1.72 g,
0
8
9-borafluorene as bright yellow needle shape crystals that were
.6 mmol, 90% relative to 2,2 -dibromobiphenyl) pure 9-chloro-
˚
bond lengths in the BC4 unit, B(1)-C(2) [1.611(19) A],
˚
13
B(1)-C(8) [1.617(2) A] are in the range that is typical for
good enough for X-ray measurement. mp 52 °C, C NMR
3
B-C(sp ) single bonds for tetrahedral boron center
2 2
(CD Cl ): 153.2, 135.4, 132.6, 128.7 and 119.9, the carbon atom
57
11
directly attached to boron can not be detected. B NMR
˚
˚
1
.60-1.64 A. The bond lengths C(2)-C(1) [1.413(17) A],
1
(CD Cl ): 63.6. H NMR (CD Cl ): 7.53 (d, J = 7.0 Hz, 2 H),
˚
˚
C(1)-C(7) [1.485(15) A], and C(7)-C(8) [1.410(17) A] re-
semble the structural parameters found in 9-chloro-9-
borafluorene. Thus, it can be concluded that these bond
lengths have little dependence on the coordination number
of the boron center.
2
2
2
2
7
.39-7.35 (m, 4 H), 7.17-7.11 (m, 2 H).
-Azido-9-borafluorene (2). A solution of 0.1 g (0.5 mmol)
-chloro-9-borafluorene 1 in 10 mL of CH Cl was treated with
9
9
0
2
2
.067 mL of (0.5 mmol) trimethylsilyl azide at -78 °C. The
reaction mixture was gradually warmed to room temperature
after stirring overnight. The resulting crude bright yellow solu-
tion was monitored by boron NMR showing complete conver-
Conclusions
1
1
2 2
sion into monomeric boron species 2a. B NMR (CD Cl ):
The target compound, 9-azido-9-borafluorene 2, is the first
example of an azidoborane of the borole system (Type D).
The azide can be prepared from 9-chloro-9-borafluorene 1a.
The boron center appears to be very Lewis acidic: Compound
-1
5
0.2; IR (nujol þ CH
After complete removal of all volatile products, the remaining
pale yellow solid product shows two signals for 2a and 2b in
2 2 3
Cl ): 2139 vs [νasym(N )] cm .
11
boron NMR. B NMR (CD
CH
2
Cl
): 2176 s and 2136 vs [νasym(N
-Azido-9-borafluorene pyridine (3). Trimethylsilyl azide
2
): 50.3 and 5.0. IR (nujol þ
-1
3
2
exists as a monomer 2a in solution just after its synthesis,
2
Cl
2
)] cm .
but as a cyclotrimer 2b in the solid state. Because of the
pronounced instability of 2b the structural details could not
obtained from experiment. Computations indicate that two
conformational isomers lie very close in energy. The com-
puted energy of cyclotrimerzation of 2a in the gas phase at
9
3
(0.067 mL, 0.5 mmol) was added to a solution of 0.1 g (0.5 mmol)
of 9-chloro-9-borafluorene dissolved in 10 mL of CH Cl
2
2
at -78 °C. The reaction mixture is allowed to warm slowly and
stirred overnight. Then 0.04 mL (0.5 mmol) of pyridine was
added to the reaction mixture subsequently. After that the solvent
was removed in vacuo leaving a white solid that was purified by
trituration with n-pentane from dichloromethane solution and
dried at ambient temperature under reduced pressure to produce
pure product 3. Slow evaporation of the CH Cl solvent from the
0
K is larger than for F BN that only exists in trimeric form
2 3
in solution. Dissolution of 2b in dichloromethane, however,
produces both 2a and 2b. This indicates that solvent effects
are more important for 2 than for F BN . Azide 2 is easily
decomposed into a boroxin that could be structurally char-
acterized. It also can be stabilized by forming adducts with
pyridines. The latter include only one azide unit as shown by
X-ray crystallography.
2
3
2
2
solution of the product produced colorless crystals which were
found to be suitable for X-ray crystallography. Yield 0.131 g
1
1
13
(91%), mp 176-178 °C (decomp.). B NMR (CD Cl ): 4.1.
C
NMR (CD Cl ): 149.0, 144.4, 141.9, 129.6, 128.4, 127.1, 126.2,
2
2
2
2
1
19.6. H NMR (CD
1
2 2
Cl ): 8.68 (d, J=6.8 Hz, 2 H), 8.07 (t, J=8.1
Hz, 1 H), 7.68(d, J = 7.4Hz, 2 H), 7.63-7.57(m, 2 H), 7.33-7.27
Experimental and Computational Details
1
4
(
(
(
1
7
m, 4 H), 7.18-7.12 (m, 2 H); N NMR (CD Cl ): -75.6
2
2
General Procedures. All manipulations were carried out
under an inert atmosphere of argon gas by standard Schlenk
techniques or in a glovebox. The solvent n-hexane was dried
N-py), -138.4 (N-2), -210.9 (N-3), N-1 not detected. IR
Cl ): ν = 2929 w, 2854 m, 2116 vs [νasym(N )],
620 vs, 1457 vs, 1377 m, 1158 sw, 1058 w, 919 m, 874 s, 830 sw,
43 vs, 690 m, 690 s, 653 s, 619 s.
-Azido-9-borafluorene t-butyl Pyridine (4). Compound 4
nujol þ CH
2
2
3
over CaH just before use. The NMR spectra were recorded on a
2
Bruker DPX 250 MHz spectrometer. Spectra were referenced to
9
1
residual solvent ( H, C) and externally ( B: BF
13
11
14
3
3
3
OEt
2
,
N:
CH NO ). The NMR spectra were measured in CD Cl . It was
was prepared similarly as described for compound 3 using
.07 mL (1 equivalent) of t-butyl pyridine. After removal of all
volatile residues by vacuum evaporation, colorless compound
3
2
2
2
0
˚
purchased from Deutero GmbH, dried over 4 A molecular sieve.
and was Argon saturated before use. Dry solvent (CH Cl ),
,2-dibromobenzene, n-BuLi (1.6 M in n-hexane), BCl (1 M in
2
2
was obtained by a trituration process as described above for 3.
This compound is crystallized from CH Cl . Then the white
2 2
1
3
n-hexane), TMSN (Aldrich), pyridine, t-butyl pyridine pur-
3
needle shaped crystals were washed with n-pentane to give a
pure product. Yield 0.152 g (88%), mp 204-206 °C (decomp.).
chased from Acros or Aldrich were used as received without any
0
8,59
modification. 2,2 -Dibromobiphenyl was prepared as described
in the literature.
11
13
5
2 2 2 2
B NMR (CD Cl ): 3.7. C NMR (CD Cl ): 167.3, 149.1,
1
1
(
7
43.9, 129.6, 128.4, 127.0, 123.3, 119.6, 35.7, 29.9. H NMR
CD Cl ): 8.53 (d, J = 6.7 Hz, 2 H), 7.66 (d, J = 7.4 Hz, 2 H),
.54 (d, J = 6.7 Hz, 2 H), 7.33-7.25 (m, 4 H), 7.16-7.11 (m, 2
Caution! Azides described here may be explosive; appropriate
safety precautions need to be taken.
2
2
3
7
9
-Chloro-9-borafluorene (1a). To a cooled solution (0 °C) of
14
H), 1.31 (s, 9 H). N (CD
207.6 (N-3), N-1 not detected. IR (nujol þ CH
ν = 3050 s, 2956 vs, 2925 vs, 2854 vs, 2125 vs [ν (N )],
2
Cl
2
): -73.0 (N-py), -135.8 (N-2),
0
3
g (9.6 mmol) of 2,2 -dibromobiphenyl in 150 mL of n-hexane,
-
2
Cl ):
2
asym
3
1
9
632 vs, 1596 m, 1504 m, 1432 vs, 1352 s, 1221 s, 1151 sw, 1083 s,
29 s, 873 s, 851 s, 829 s, 742 vs, 707 m, 681 s.
(
(
(
57) Hoffman, K.; Weiss, E. J. Organomet. Chem. 1974, 67, 221.
58) Gilman, H.; Gaj, B. J. J. Org. Chem. 1957, 22, 447.
59) Leroux, F.; Schlosser, M. Angew. Chem. 2002, 114, 4447; Angew.
X-ray Crystallography. The intenisity data were collected
on an Oxford Diffraction Xcalibur2 diffractometer with a
Chem., Int. Ed. 2002, 41, 4274.

4
506 Inorganic Chemistry, Vol. 49, No. 10, 2010
Biswas et al.
Table 1. Crystal Data and Structure Refinements
1
a
1b
3
4
5
empirical formula
formula weight
T/K
C
198.44
103
0.71073 A
monoclinic
P2(1)/c
3.8570(5)
4.0680(17)
17.495(3)
90°
95.237(15)°
90°
945.3(2)
12
H
8
BCl
C
478.17
108
0.71073 A
monoclinic
P2(1)/c
17.6344(17)
10.2537(9)
14.1172(13)
90°
103.205(9)°
90°
2485.1(4)
4
1.278
36
H
24
B
2
C
284.12
113
0.71073 A
triclinic
P1
9.7657
17.5892
18.4303
68.734(4)°
89.620(3)°
81.217(3)°
2911.5(2)
8
17
H
13BN
4
C
21
H
21BN
4
C
540.01
108
0.71073 A
triclinic
P1
13.0852(5)
13.3035(6)
19.6082(7)
70.466(4)°
74.187(3)°
61.449(4)°
2799.41(22)
4
36
H
27
B
3
O
3
340.23
110
0.71073 A
monoclinic
P2(1)/c
10.7761(5)
13.7559(7)
12.5533(5)
90°
96.300(4) °
90°
1849.60(15)
4
1.222
˚
˚
˚
˚
˚
wavelength
crystal system
space group
˚
a (A)
˚
b (A)
˚
c (A)
R (deg)
β (deg)
γ (deg)
3
˚
V (A )
Z
4
1.394
0.350
408
3
d
calcd (mg/m )
1.296
0.079
1184
1.281
0.078
1128
-
1
μ (mm
F(000)
)
0.071
1000
0.073
720
crystal size (mm)
θ range (deg)
index range
0.435 ꢀ 0.063 ꢀ 0.054
2.90 to 27.65
-2 e h e 5
0.27 ꢀ 0.24 ꢀ 0.18
2.96 to 25.25
-21 e h e 20
-12 e k e 12
-16 e l e 16
27172/4498
0.36 ꢀ 0.32 ꢀ 0.21
2.64 to 25.25
-8 e h e 11
-20 e k e 21
-22 e l e 22
24852/10505
0.52 ꢀ 0.43 ꢀ 0.41
2.96 to 25.24
-12 e h e 12
-15 e k e 16
-14 e l e 15
9125/3300
0.58 ꢀ 0.55 ꢀ 0.23
3.06 to 27.50.
-16 e h e 16
-15 e k e 17
-24 e l e 25
25207/12695
2
-
-
11 e k e 18
22 e l e 21
no. of reflections
collected/unique
4325/2139
[R(int) = 0.0768]
98.0%
[R(int) = 0.1124]
99.8%
[R(int) = 0.0427]
99.5%
[R(int) = 0.0218]
98.7%
[R(int) = 0.0199]
98.6%
completeness to θ =
27.50
absorption correction
refinement method
none
full-matrix
least-squares on F
2139/0/127
0.959
none
full-matrix
least-squares on F
4498/0/343
1.133
none
full-matrix
least-squares on F
10505/0/793
0.819
none
full-matrix
least-squares on F
3300/0/238
0.945
none
full-matrix
least-squares on F
12695/0/757
0.882
2
2
2
2
2
data/restraints/params
goodness-of-fit, F
2
R
R
1
, wR
, wR
2
[I > 2σ(I)]
(all data)
0.0397, 0.0472
0.1368, 0.0573
0.221 and -0.261
0.0797, 0.1691
0.1014, 0.1749
0.227 and -0.229
0.0380, 0.0640
0.0906, 0.0692
0.256 and -0.184
0.0351, 0.0855
0.0504, 0.0890
0.215 and -0.176
0.0360, 0.0789
0.0591, 0.0821
0.305 and -0.220
1
2
largest diff. peak and
)
-3
˚
hole (e A
Sapphire2 CCD. The crystal structures were solved by direct
methods using SHELXS-97 and refined with SHELXL-97.
For refinement details see Table 1 and for further details please
see Supporting Information.
Crystal structure of 2b: the crystals were extremely air- and
moisture sensitive and even covered with oil at 100 K they
decomposed quite rapidly on the diffractometer. Nevertheless
we were able to get a data set which was good enough to con-
the resolution of the identity (RI) approximation for the fast
computation of two-electron integrals (RI-MP2). The com-
putations used the SVP basis set (C, B, N, F: (7s4p1d)/[3s2p1d];
6
0
60
62,63
6
1,63
H: (4s1p)/[2s1p]) and the corresponding fitting bases.
excitation energy of 1b was computed at the B3LYP
The
func-
tional as implemented in Turbomole in conjunction with the
6
4,65
6
6
TZVP basis set (C, B: (11s6p1d)/[5s3p1d]; H: (5s1p)/[3s1p]) at
the RI-MP2/SVP geometry using the time-dependent DFT
formalism.
6
7
firm the connectivity of 2b: pale yellow crystal, 0.22 ꢀ 0.20 ꢀ
3
.10 mm , triclinic, P1, a =11.069(4) A, b=11.233(3) A, c =
˚
˚
0
˚
1
6.894(5) A, R = 90.44(2)°, β = 91.11(3)°, γ = 118.21(3)°,
Acknowledgment. Financial support by the Deutsche
Forschungsgemeinschaft is gratefully acknowledged. We also
thank Manuela Winter for recording the crystal data, and
Professor Dr. Peter Paetzold for helpful discussions.
3
3
˚
V=1850(1) A , Z=2, Fcalc=1.240 g/cm , θmax=23.7°, λ (Mo
˚
K
2
R
) = 0.71073 A, T = 110(2) K, 5621 independent reflections,
223 [I > 2σ(I)], 237 parameter, R = 0.216, wR = 0.537 (for
all data).
Calculations. The computations were performed with the
Supporting Information Available: Experimental and spectro-
6
1
11
1
13
14
Turbomole program. Geometries were fully optimized within
the respective point group constraints at the second order
Møller-Plesset perturbation theory level in conjunction with
scopic details ( B, H, C, N NMR data) for the new
compounds and CIF files of the X-ray crystal structures includ-
ing tables of crystallographic data. This material is available free
of charge via the Internet at http://pubs.acs.org.
(
(
60) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112.
61) Ahlrichs, R.; B €a r, M.; H €a ser, M.; Horn, H.; K o€ lmel, C. Chem. Phys.
(64) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
Lett. 1989, 162, 165.
(65) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785.
(
(
62) Weigend, F.; H €a ser, M. Theor. Chem. Acc. 1997, 97, 331.
(66) Sch €a fer, A.; Huber, C.; Ahlrichs, R. J. J. Chem. Phys. 1994, 100,
63) Weigend, F.; H €a ser, M.; Patzelt, H.; Ahlrichs, R. Chem. Phys. Lett.
5829.
1
998, 294, 143.
(67) Bauernschmitt, R.; Ahlrichs, R. Chem. Phys. Lett. 1996, 256, 454.