Synthesis and structure of [Cr{η6-C6H 5)2B{NtBu(sime3)}}]and [Cr{(η6- C6H5)2(BNMe2)2}], the first boron-bridged metalloarenophanes

Article abstract of DOI:10.1021/om049872q

Reaction of [Cr(η⁶-C₆H₅Li) ₂]·tmen with (RR′)-NBCl₂ (R=R′ = SiMe₃; R=R′ = iPr; R = SiMe₃, R′ = tBu) and B₂Br₂(nmE₂)₂, respectively, in

Full text of DOI:10.1021/om049872q

1
968
Organometallics 2004, 23, 1968-1970
Syn th esis a n d Str u ctu r e of
6
[
Cr {(η -C H ) B{NtBu (SiMe )}}] a n d
6
5 2
3
6
[
Cr {(η -C H ) (BNMe ) }], th e F ir st Bor on -Br id ged
6
5 2
2 2
Meta lloa r en op h a n es
,
†
‡
‡
‡
Holger Braunschweig,* Melanie Homberger, Chunhuas Hu, Xiaolai Zheng,
†
†
†
Emanuel Gullo, Guy Clentsmith, and Matthias Lutz
Institut f u¨ r Anorganische Chemie, Universit a¨ t W u¨ rzburg, Am Hubland,
D-97074 W u¨ rzburg, Germany, and Institut f u¨ r Anorganische Chemie der
Technischen Hochschule, D-52056 Aachen, Germany
Received February 23, 2004
6
Summary: Reaction of [Cr(η -C6H5Li)2]‚tmen with (RR′)-
NBCl2 (R ) R′ ) SiMe3; R ) R′ ) iPr; R ) SiMe3, R′ )
tBu) and B2Br2(NMe2)2, respectively, in hexane yielded
BNRR′}] (4a -c: R ) R′ ) SiMe3; R ) R′ ) iPr; R )
6
SiMe , R′ ) tBu) and [Cr{(η -C H ) (BNMe ) }] (5) as
3
6
5 2
2 2
the first examples of [1]- and [2]metalloarenophanes
with a bridging first-row element. Compounds 4a -c
were obtained from reactions of the corresponding
6
the strained [1]borachromoarenophanes [Cr{(η -C6H5)2-
BNRR′}] (4a -c; R ) R′ ) SiMe3; R ) R′ ) iPr; R )
SiMe3, R′ ) tBu) and [2]borachromoarenophanes [Cr-
6
aminodihaloboranes with a suspension of [Cr(η -C6H5-
6
{
(η -C6H5)2(BNMe2)2}] (5), respectively, in moderate
Li)2]‚tmen according to eq 1 and isolated as dark red
yields as dark red solids; compounds 4c and 5 were
structurally characterized. 4c exhibits the largest arene-
arene tilt angle (26.6(3)°) reported for any [n]metal-
loarenophane.
1
Our recent investigations on both strained and
unstrained2
a-e
[1]borametallocenophanes led to com-
5
pounds of the type [Fe{(η -C5H4)2BNR2}] (1; R ) various
ligands), which represent the first examples of [1]-
ferrocenophanes with a bridging first-row element. Such
strained ansa-ferrocenes are well-known with various
bridging main-group elements as well as transition
metals and have attracted considerable interest, due to
their possible use as precursor molecules for the syn-
thesis of novel organometallic polymers via ring-opening
polymerization (ROP).3 In contrast to the well-docu-
mented chemistry of [1]ferrocenophanes, related [1]-
metalloarenophanes have been scarcely investigated
and are restricted to a very few silicon- and germanium-
crystalline solids in yields ranging from 42% to 48%.5
All complexes are extremely sensitive toward air and
moisture and decompose readily in polar solvents such
as MeOH, CH2Cl2, DMF, and DME at ambient temper-
6
ature with formation of [Cr{(η -C6H6)2] and various
phenylborane species. This enhanced sensitivity in
solvents commonly used as electrochemical standard
mediums prevented collection of CV data: similar
bridged chromium and vanadium complexes of the type
behavior was reported for the silicon analogues 2a ,b.4
6
[
3
M{(η -C6H5)2ER2}] (2a ,b, M ) Cr, E ) Si, R ) Me, Ph;
The NMR spectroscopic data, which closely resemble
those of 2a ,b, indicate a significantly tilted structure
, M ) V, E ) Si, R ) various ligands).4
The present paper reports on the syntheses and full
characterizations of the complexes [Cr{(η -C6H5)2-
6
(3) (a) Stoeckli-Evans, H.; Osborne, A. G.; Whiteley, R. H. J .
Organomet. Chem. 1980, 194, 91. (b) Seyferth, D.; Withers, H. P.
Organometallics 1982, 1, 1275. (c) Broussier, R.; Da Rold, A.; Gauth-
eron, B.; Dromzee, Y.; J eannin, Y. Inorg. Chem. 1990, 29, 1817. (d)
Broussier, R.; Da Rold, A.; Gautheron, B.; Dromzee, Y.; J eannin, Y. J .
Organomet. Chem. 1992, 427, 231. (e) Pudelski, J . K.; Gates, D. P.;
Rulkens, R.; Lough, A. J .; Manners, I. Angew. Chem. 1995, 107, 1633;
Angew. Chem., Int. Ed. Engl. 1995, 34, 1506. (f) Herberhold, M. Angew.
Chem. 1995, 107, 1985; Angew. Chem., Int. Ed. Engl. 1995, 34, 1837.
(g) Rulkens, R.; Lough, A. J .; Manners, I. Angew. Chem. 1996, 108,
1929; Angew. Chem., Int. Ed. Engl. 1996, 35, 1805. (h) Manners, I.
Angew. Chem. 1996, 108, 1712; Angew. Chem., Int. Ed. Engl. 1996,
35, 1602 and references therein.
(4) For chromium species see: (a) Elschenbroich, C.; Hurley, J .;
Metz, B.; Massa, W.; Baum, G. Organometallics 1990, 9, 889. (b)
Hultzsch, K. C.; Nelson, J . M.; Lough, A. J .; Manners, I. Organome-
tallics 1995, 14, 5496. (c) Elschenbroich, C.; Schmidt, E.; Gondrum,
R.; Metz, B.; Burghaus, O.; Massa, W.; Wocadlo, S. Organometallics
1997, 16, 4589. For vanadium species see: (d) Elschenbroich, C.;
Bretschneider-Hurley, A.; Hurley, J .; Massa, W.; Wocadlo, S.; Pebler,
J .; Reijerse, E. Inorg. Chem. 1993, 32, 5421. (e) Elschenbroich, C.;
Bretschneider-Hurley, A.; Hurley, J .; Behrendt, A.; Massa, W.; Wocad-
lo, S.; Reijerse, E. Inorg. Chem. 1995, 34, 743.
*
To whom correspondence should be addressed. Tel: +49 (0)931
8
88 5260. Fax: +49 (0)931 888 4623. E-mail: holger.braunschweig@
mail.uni-wuerzburg.de.
†
Universit a¨ t W u¨ rzburg.
Technische Hochschule Aachen.
‡
(1) (a) Braunschweig, H.; Dirk, R.; M u¨ ller, M.; Nguyen, P.; Gates,
D. P.; Manners, I. Angew. Chem. 1997, 109, 2433; Angew. Chem., Int.
Ed. 1997, 36, 2338. (b) Braunschweig, H.; Dirk, R.; Englert, U.;
Berenbaum, A.; J a¨ kle, F.; Lough, A. J .; Manners, I. J . Am. Chem. Soc.
2
000, 122, 5765.
2) (a) Braunschweig, H.; von Koblinski, C.; Wang, R. Eur. J . Inorg.
(
Chem. 1999, 69. (b) Braunschweig, H.; von Koblinski, C.; Mamuti, M.;
Englert, U.; Wang, R. Eur. J . Inorg. Chem. 1999, 1899. (c) Braunsch-
weig, H.; von Koblinski, C.; Englert, U. Chem. Commun. 2000, 1049.
(
d) For other structurally authentic [1]borazirconocenophanes with
three-coordinate boron see: Ashe, A. J .; Fang, X.; Kampf, J . W.
Organometallics 1999, 18, 2288. (e) For a recent review on boron-
bridged group 4 ansa-metallocenes see: Shapiro, P. J . Eur. J . Inorg.
Chem. 2001, 321. (f) Braunschweig, H.; Breitling, F. M.; Gullo, E.;
Kraft, M. J . Organomet. Chem. 2003, 680, 31.
1
0.1021/om049872q CCC: $27.50 © 2004 American Chemical Society
Publication on Web 04/01/2004

Communications
Organometallics, Vol. 23, No. 9, 2004 1969
9
F igu r e 1. Molecular structures of 4c (left) and 5 (right). Selected distances (Å) and angles (deg) for 4c: B-N ) 1.398(6),
B-C21 ) 1.601(7), B-C11 ) 1.617(7), C11-C12 ) 1.410(6), C14-C15 ) 1.385(7), Cr-C11 ) 2.084(5), Cr-C14 ) 2.182-
(
)
5); C11-B-N1 ) 129.3(5). Selected distances (Å) and angles (deg) for 5: B1-N1 ) 1.390(9), B1-B2 ) 1.718(12), B2-N2
1.376(9), B2-C21 ) 1.590(10), C21-C22 ) 1.432(8), C14-C15 ) 1.396(8), Cr-C11 ) 2.142(6), Cr-C14 ) 2.159(6),;
C11-B1-N1 ) 120.3(5), N1-B1-B2 ) 129.4(6), N2-B2-B1 ) 127.3(6).
for 4a -c in solution.4a-c In particular, the high-field-
meta and para hydrogen atoms. Further spectroscopic
data such as the B NMR signal at δ 46.3 ppm meet
1
11
shifted and separated H NMR signals for the ortho,
meta, and para hydrogen atoms of the aryl ligands
ranging from δ 3.63 to 4.77 ppm corroborate the pres-
ence of strained molecules.4b The B NMR resonances
between δ 43.2 and 50.4 ppm match those of 1, thus
being in the expected range for aminodiarylboranes.
the expectations.
The different geometries for the monoboron- and
diboron-bridged complexes in solution, as already indi-
cated by the NMR spectroscopic data, were proven by
X-ray structure determinations (Figure 1).7 Suitable
single crystals of 4c and 5 were obtained from hexane
11
According to the aforementioned synthesis the related
6
6
[
2]borachromoarenophane [Cr{(η -C6H5)2(BNMe2)2}] (5)
at -30 °C. [Cr{(η -C6H5)2B{NtBu(SiMe3)}}] (4c) crystal-
was obtained in 39% yield as a crystalline solid from
reaction of the diborane(4) B2Br2(NMe2)2 (eq 2). Al-
lizes in the monoclinic space group P21/c, and the
molecule adopts approximate Cs symmetry in the crys-
tal. The much greater tilt angle of 26.6(3)° with respect
6
6
to 14.4° for the silylene-bridged analogue [Cr{(η -C6H5)2-
4
b
SiMe2}] (2b) proves the expected effect of the small
bridging boron atom, which affords a molecule with a
significantly greater intrinsic strain. As a consequence,
the Cr-Cipso distances are smaller while the Cr-Cpara
distances are larger in 4c than in 2b. The two benzene
rings adopt an almost eclipsed position, indicated by the
torsion angle cg1-C11-C21-cg2 ) 2.0(5)°. The diboron-
though showing a similar appearance with respect to
color and sensitivity, the diboron-bridged species 5
represents a significantly less strained molecule. For
example, this becomes evident from the less shielded
resonances (δ 4.46-4.69 ppm) of the aryl protons in the
6
bridged complex [Cr{(η -C6H5)2(BNMe2)2}] (5) crystal-
lizes in the orthorhombic space group Pna21, and the
molecule adopts C2 symmetry in the crystal. The
significantly decreased strain of 5 in comparison with
4
c is reflected in a smaller tilt angle of only 10.1(3)°.
1
H NMR spectrum. Those signals are also less sepa-
The benzene rings adopt now a staggered geometry, as
rated, thus showing only one broad resonance for the
(
6) Spectroscopic data for 5: 1H NMR δ 2.80 (s, 6H; Me
6H; Me N), 4.46 (br, 6 H; m-/p-C ), 4.69 (br s, 4 H, o-C
NMR δ 46.3; C{ H} NMR δ 0.59 (Me N), 43.68 (Me N), 77.22, 78.22,
80.36 (C ); MS (70 eV) m/z (%) 316 (100) [M ], 273 (22) [M - NMe
263 (16) [M - Cr], 211 (2) [M - (Me
Me NBC ], 51 (30) [Cr]. Anal. Calcd for C16
H, 7.02; N, 8.87. Found: C, 60.95; H, 6.82; N, 8.54.
(7) Crystal data for 4c: C19 28BCrNSi, M ) 361.33, monoclinic,
space group P2 /c (No. 14), a ) 6.391(4) Å, b ) 11.612(2) Å, c ) 25.490-
2
N), 2.84 (s,
11 1
(
5) Spectroscopic data for 4a : 1H NMR δ 0.43 (s, 18 H; SiMe
m, 4 H; o-C ), 4.53 (m, 4 H; m-C ), 4.73 (m, 2 H; p-C
H} NMR: δ 50.4; C{ H} NMR δ 5.39 (SiMe
3
), 3.63
2
6
H
5
6 5
H ); B{ H}
1
1
13
1
(
{
6
H
5
6
H
5
6
H
5
); B-
2
2
1
13
1
+
+
3
), 74.84, 79.52, 82.32
6
H
5
2
],
], 132 (14) [M+ - Cr -
CrN : C, 60.80;
+
+
+
+
(C
6
H
5
); MS (70 eV) m/z (%) 377 (12) [M ], 325 (15) [M - Cr], 310 (53)
2 2 2
N) B
+
], 299 (19) [M+ - C
Si) N], 78 (6) [C ], 73 (38) [SiMe
28BCrNSi : C, 57.27; H, 7.48; N, 3.71. Found: C, 57.00;
], 248 (19) [M+ - C
[
M
1
- Cr - CH
3
6
H
5
6
H
5
- Cr],
2
6
H
5
H
22
B
2
2
60 (14) [(Me
Calcd for C18
3
H
2
6
H
5
3
], 52 (2) [Cr]. Anal.
2
H
r
1
3
H, 7.28; N, 3.83. Spectroscopic data for 4b: H NMR δ 1.25 (d, J (H,H)
1
3
3
-3
)
7.02 Hz, 12 H; Me), 3.63 (sept, J (H,H) ) 7.02 Hz, 2 H; CHMe
.82 (m, 4 H; o-C ), 4.59 (m, 4 H; m-C ), 4.77 (m, 2 H; p-C
B{ H} NMR δ 43.2; C{ H} NMR δ 24.44 (Me), 48.68 (CHMe
2
),
);
(3) Å, â ) 97.13(2)°, V ) 1877(1) Å , Z ) 4, Fcalcd ) 1.28 cm , µ(Mo
-1
3
6
H
5
6
H
5
6
H
5
KR) ) 0.66 mm , T ) 223(2) K. 8109 reflections measured, of which
1
1
1
13
1
8a
2
), 74.69,
); MS (70 eV) m/z (%) 265 (18) [M - Cr], 250 (100)
], 165 (15) [M - iPr
], 58 (6) [iPrN], 52 (4) [Cr]. Anal. Calcd for C18
4075 were unique (Rint ) 0.070), solution by direct methods,
+
8b
2
7
M
C
9.83, 82.31 (C
6
H
3
5
refinement on F to R1 ) 0.069 for observed data and wR2 ) 0.169
+
+
[
[
- Cr - CH
5
2
N - Cr], 100 (2) [iPr
2
N], 78 (6)
24BCrN: C, 68.14;
for all data. Crystal data for 5: C16
1
hombic, space group Pna2 (No. 33), a ) 18.989(4) Å, b ) 8.7368(8) Å,
3 -3
22 2 2 r
H B CrN , M ) 315.98, orthor-
6
H
H
H, 7.63; N, 4.42. Found: C, 67.98; H, 7.51; N, 4.39. Spectroscopic data
for 4c: 1H NMR δ 0.49 (s, 9 H; SiMe
), 1.54 (s, 9 H; tBu), 3.66 (m, 4 H;
o-C ), 4.55 (m, 4 H; m-C ), 4.72 (m, 2 H; p-C
δ 47.41; C{ H} NMR δ 6.47 (SiMe ), 34.70 (tBu), 73.12, 73.93, 79.18,
9.23, 82.18 (C ); MS (70 eV) m/z (%) 361 (3) [M ], 309 (2) [M -
c ) 9.422(3) Å, V ) 1563.2(5) Å , Z ) 4, Fcalcd ) 1.34 Mg m , µ(Mo
-1
3
KR) ) 0.72 mm , T ) 203(2) K, 3367 reflections measured, of which
1
1
1
8a
6
H
5
6
H
5
6
H
5
); B{ H} NMR
3054 were unique (Rint ) 0.045), solution by direct methods,
1
3
1
8b
2
3
refinement on F to R1 ) 0.062 for observed data and wR2 ) 0.161
for all data.
+
+
7
6
H
5
+
+
Cr], 294 (100) [M - Cr - CH
3
], 238 (10) [M - Cr - SiMe
3
], 222 (8)
], 144 (79)
], 57 (56) [tBu], 52 (12) [Cr]. Anal. Calcd
H24BCrNSi: C, 63.14; H, 7.81; N, 3.88. Found: C, 62.84; H, 7.24;
(8) (a) Sheldrick, G. M. SHELXS-97, Program for Crystal Structure
Solution; University of G o¨ ttingen, G o¨ ttingen, Germany, 1997. (b)
Sheldrick, G. M. SHELXL-97, Program for Crystal Structure Refine-
ment; University of G o¨ ttingen, G o¨ ttingen, Germany, 1997.
(9) Spek, A. L. Acta Crystallogr. 1990, A46, C34.
+
+
[
[
M
- Cr - SiMe
3 3 3
- CH ], 165 (9) [M - Cr - tBuNSiMe
tBuNSiMe ], 73 (72) [SiMe
for C19
N, 3.61.
3
3

1
970 Organometallics, Vol. 23, No. 9, 2004
Communications
indicated by the torsion angle cg1-C11-C21-cg2 )
Ack n ow led gm en t. This work was supported by the
Deutsche Forschungsgemeinschaft and the Fonds der
Chemischen Industrie.
1
1
1.4(5)°. Besides, the boron-nitrogen (1.390(9) and
.376(9) Å) and boron-boron distances (1.718(12) Å) are
in the expected range for B-N double and B-B single
bonds, respectively. Currently, investigations on utiliz-
ing the aforementioned borachromoarenophanes as
precursors for the buildup of polymeric materials are
in progress. Preliminary polymerization studies show
that 4a undergoes ring opening upon treatment with
BuLi with formation of dimers, as indicated by mass
spectrometry. Thermally induced ROP, however, ap-
pears to be less suitable, since heating of the [1]-
borachromoarenophanes to temperatures above 200 °C
is accompanied by decomposition and deposition of
chromium metal.
Su p p or tin g In for m a tion Ava ila ble: Text detailing the
synthetic procedures and the structure determination details
for 4c and 5. This material is available free of charge via the
Internet at http://pubs.acs.org. Crystallographic data (exclud-
ing structure factors) for the structures reported in this paper
have been deposited with the Cambridge Crystallographic
Data Centre as Supplementary Publication Nos. CCDC-157467
(4c) and CCDC-157467 (5). Copies of the data can be obtained
free of charge on application to the CCDC, 12 Union Road,
Cambridge CB21EZ, U.K. (fax, (+44)1223-336-033; E-mail,
deposit@ccdc.cam.ac.uk).
OM049872Q