Isocyanide and ylidene complexes of boron: Synthesis and crystal structures of (2-(trimethylsiloxy)phenyl isocyanide)-triphenylborane and (1,2-dihydrobenzoxazol-2-ylidene)-triphenylborane

Article abstract of DOI:10.1021/om950773+

2-(Trimethylsiloxy)phenyl isocyanide (1) reacts at -60 °C with triphenylborane to form the isocyanide adduct (2-(trimethylsiloxy)phenyl isocyanide)-triphenylborane (2). Upon desilylation in MeOH with a catalytic amount of KF at -30 °C 2 is converted into

Full text of DOI:10.1021/om950773+

Organometallics 1996, 15, 1251-1256
1251
Isocya n id e a n d Ylid en e Com p lexes of Bor on : Syn th esis
a n d Cr ysta l Str u ctu r es of (2-(Tr im eth ylsiloxy)p h en yl
isocya n id e)-Tr ip h en ylbor a n e a n d
(1,2-Dih yd r oben zoxa zol-2-ylid en e)-Tr ip h en ylbor a n e
Matthias Tamm, Thomas Lu¨gger, and F. Ekkehardt Hahn*
Institut fu¨r Anorganische und Analytische Chemie, Freie Universita¨t Berlin,
Fabeckstrasse 34-36, D-14195 Berlin, Germany
Received September 29, 1995X
2-(Trimethylsiloxy)phenyl isocyanide (1) reacts at -60 °C with triphenylborane to form
the isocyanide adduct (2-(trimethylsiloxy)phenyl isocyanide)-triphenylborane (2). Upon
desilylation in MeOH with a catalytic amount of KF at -30 °C 2 is converted into the ylidene
adduct (1,2-dihydrobenzoxazol-2-ylidene)-triphenylborane (3). The X-ray crystal structures
of 2 and 3 are reported. When it is heated, 2 dimerizes to give 1,4-bis(2-(trimethylsiloxy)-
phenyl)-2,2,3,5,5,6-hexaphenyl-2,5-dibora-2,5-dihydropyrazine (4). Si-O bond cleavage leads
to 1,4-bis(2-hydroxyphenyl)-2,2,3,5,5,6-hexaphenyl-2,5-dibora-2,5-dihydropyrazine (5). At-
tempts to induce formation of a boron-bridged diylidene complex via attack of the hydroxyl
oxygens in 5 at the pyrazine carbon atoms failed. Instead, complex 6, containing one bridging
N,O-ylidene ligand, was obtained by partial hydrolysis of 5. The molecular structure of 6,
which crystallizes with one molecule of methanol and acetone each per formula unit, is
reported.
In tr od u ction
dinated to a Lewis acid exclusively as a σ-donor with
no back-bonding from the metal center. The lack of
back-bonding should facilitate the intramolecular car-
bene formation after Si-O bond cleavage. Recently we
described preliminary results of the reaction of 1 with
TiCl4.4
We have demonstrated the use of 2-(trimethylsiloxy)-
phenyl isocyanide (1) for the synthesis of transition-
metal carbene complexes. Complexes with the 1,2-
dihydrobenzoxazol-2-ylidene ligand are obtained from
complexes of 1 after Si-O bond hydrolysis via an
intramolecular cycloaddition of the hydroxyl oxygen
atom to the isocyanide carbon atom.1 Even complexes
of 1 with low-valent transition metals, where the
isocyanide is normally deactivated toward nucleophilic
attack by strong (dfp)π back-bonding from the metal
center, will yield carbene complexes.1 For example, the
complex [W(CNC6H4-2-OSiMe3)(CO)5] gives after Si-O
bond cleavage the corresponding carbene complex,1b
while the complex [W(CNCH2CH2-2-OSiMe3)(CO)5] after
Si-O bond cleavage remains the isocyanide complex
[W(CNCH2CH2-2-OH)(CO)5].2a However, late or high-
valent transition metals also give complexes with ox-
azolidin-2-ylidene ligands from coordinated 2-hydroxy-
lalkyl isocyanides.2 Recently we have shown that the
intramolecular ylidene formation can be suppressed by
the generation of 2-hydroxyphenyl isocyanide at metal
fragments where enhanced (dfp)π back-bonding oc-
curs.3
In this contribution we wish to report on the reaction
of 1 with triphenylborane to yield the isocyanide adduct
[(C6H5)3B(1)] (2) and the conversion of 2 to the ylidene
complex {(C6H5)3B[CN(H)(C6H4-2-O)]} (3) and on our
attempts to prepare diborane heterocycles containing
two bridging 1,2-dihydrobenzoxazol-2-ylidene ligands.
Although Casanova and Hesse have already studied
isocyanide5 and carbene6 adducts of alkylboranes in the
1960s, the availability of stable nucleophilic carbenes7
has recently stimulated some interest in the synthesis
of novel carbene adducts of main-group elements.8
(4) Hahn, F. E.; Lu¨gger, T. J . Organomet. Chem. 1995, 501, 341.
(5) (a) Casanova, J ., J r.; Schuster, R. E. Tetrahedron Lett. 1964, 405.
(b) Casanova, J ., J r.; Kiefer, H. R.; Kuwada, D.; Boulton, A. H.
Tetrahedron Lett. 1965, 703. (c) Hesse, G.; Witte, H. J ustus Liebigs
Ann. Chem. 1965, 687, 1. (d) Hesse, G.; Witte, H.; Bittner, G. J ustes
Liebigs Ann. Chem. 1965, 687, 9.
(6) Bittner, G.; Witte, H.; Hesse, G. J ustus Liebigs Ann. Chem. 1968,
713, 1.
(7) (a) Arduengo, A. J ., III; Harlow, R. L.; Kline, M. J . Am. Chem.
Soc. 1991, 113, 361. (b) Arduengo, A. J ., III; Dias, H. V. R.; Harlow,
R. L.; Kline, M. J . Am. Chem. Soc. 1992, 114, 5530. (c) Igau, A.;
Baceiredo, A.; Trinquier, G.; Bertrand, G. Angew. Chem., Int. Ed. Engl.
1989, 28, 621. (d) Gillette, G. R.; Baceiredo, A.; Bertrand, G. Angew.
Chem., Int. Ed. Engl. 1990, 29, 1429.
After having studied the cyclization of 2-hydroxy-
phenyl isocyanide at relatively electron-rich metal frag-
ments, we became interested in examining the carbene
formation in a system where the isocyanide 1 is coor-
(8) Selected examples from group 13: (a) Arduengo, A. J ., III; Dias,
H. V. R.; Calabrese, J . C.; Davidson, F. J . Am. Chem. Soc. 1992, 114,
9724. (b) Cowley, A. H.; Gabbai, F. P.; Carrano, C. J .; Mokry, L. M.;
Bond, M. R.; Bertrand, G. Angew. Chem., Int. Ed. Engl. 1994, 33, 578.
(c) Kuhn, N.; Henkel, G.; Kratz, T.; Kreutzberg, J .; Boese, R.; Maulitz,
A. H. Chem. Ber. 1993, 126, 2041. Group 14: (d) Arduengo, A. J ., III;
Dias, H. V. R.; Calabrese, J . C.; Davidson, F. Inorg. Chem. 1993, 32,
1541. (e) Kuhn, N.; Kratz, T.; Bla¨ser, D.; Boese, R. Chem. Ber. 1995,
128, 245.
X Abstract published in Advance ACS Abstracts, J anuary 1, 1996.
(1) (a) Hahn, F. E.; Tamm, M. J . Chem. Soc., Chem. Commun. 1993,
842. (b) Hahn, F. E.; Tamm, M. J . Organomet. Chem. 1993, 456, C11.
(2) (a) Fehlhammer, W. P.; Bartel, K.; Weinberger, B.; Plaia, U.
Chem. Ber. 1985, 118, 2220. (b) Fehlhammer, W. P.; Bartel, K.; Plaia,
U.; Vo¨lkl, A.; Liu, A. T. Chem. Ber. 1985, 118, 2235. (c) Plaia, U.;
Stolzenberg, H.; Fehlhammer, W. P. J . Am. Chem. Soc. 1985, 107,
2171.
(3) (a) Hahn, F. E.; Tamm, M. J . Chem. Soc., Chem. Commun. 1995,
569. (b) Hahn, F. E.; Tamm, M. Organometallics 1995, 14, 2597.
(9) Fehlhammer, W. P.; Hoffmeister, H.; Stolzenberg, H.; Boyadjiev,
B. Z. Naturforsch. 1989, 44B, 419.
0276-7333/96/2315-1251$12.00/0 © 1996 American Chemical Society

1252 Organometallics, Vol. 15, No. 4, 1996
Tamm et al.
Sch em e 1
F igu r e 1. ORTEP plot of the molecular structure of 2,
showing 50% probability thermal ellipsoids.
4 (Scheme 1). The tendency toward dimerization upon
warming is also observed if samples of 2 which have
been kept cool right until the measurement are sub-
jected to mass spectroscopy. The spectrum exhibits
signals at m/ z 433 for monomeric 2 and at m/ z 866 for
dimeric 4.
To establish 2 unequivocally as a monomeric isocya-
nide adduct of triphenylborane, we carried out an X-ray
structure determination. Structure investigations are
rare for triorganoborane Lewis base adducts, and only
a few molecular structures have been reported for Lewis
bases with carbon donor atoms.8c,12,13 To our knowledge
2 is so far the only crystallographically characterized
isocyanide adduct of a triorganoborane Lewis acid.
A plot of the molecular structure of 2 is presented in
Figure 1. It confirms that Ph3B and 1 form a monomeric
1:1 adduct at low temperature. The boron atom in 2
resides in the center of a slightly distorted tetrahedron
(smaller C1-B-C(Ph) angles, larger C(Ph)-B-C(Ph)
angles; Table 1). Thus, the distortion is determined by
the sterically less demanding isocyanide ligand. The
distance B-C1 is shorter than the B-C(Ph) distances,
which we attribute to the different hybridization (sp for
C1, sp2 for C(Ph)) of the carbon ligands at the boron
atom.
Resu lts a n d Discu ssion
Syn th esis of Com p lexes 2 a n d 3. 1:1 adducts of
isocyanides with triorganoboranes are normally un-
stable compounds at ambient temperature which tend
to dimerize and rearrange to 2,5-dibora-2,5-dihydro-
pyrazines.5b,c On the other hand, aliphatic isocyanides
form moderately stable 1:1 adducts with Ph3B.5d,9
However, triphenylborane adducts with aromatic iso-
cyanides are still unstable and tend to dimerize.5d To
prevent phenyl migration from boron to the isocyanide
carbon and subsequent dimerization, we synthesized the
borane complex 2 from 1 and Ph3B in CH2Cl2 at low
temperature (below -20 °C, Scheme 1).
Within the 3σ error range, the C1-N bond distance
(1.148(2) Å) is comparable to the corresponding C-N
distances found in the free 2-alkoxyphenyl isocyanide
CH2(O-C6H4-2-NC)214 (1.153(3) and 1.150(3) Å).14 The
isocyanide in 2 is coordinated exclusively as a σ-donor
to the borane fragment. However, the lack of back-
bonding in 2 does not cause a shortening of the C-N
distance when compared to tungsten(0) complexes of
2-alkoxyaryl isocyanides. In the tungsten complexes the
effect of back-bonding is clearly visible by a reduction
If kept below -20 °C, complex 2 is stable in solution
and in the solid state for longer periods. It can be
crystallized from diethyl ether at -26 °C. In the IR
spectrum of 2 the absorption for the CN stretching
frequency (2244 cm-1) is shifted to significantly higher
wavenumbers compared to the free isocyanide 1 (2120
cm-1),10 showing the isocyanide to act exclusively as a
σ-donor. A substantial increase of ν(CN) upon coordina-
tion of isocyanides to Lewis acidic centers is normal and
was previously observed in boron5,9 and lanthanide11
adducts of various isocyanides.
Warming to room temperature causes the almost
colorless crystalline 2 to darken. IR spectra taken from
samples left at room temperature for 24 h show the
absence of 2 (no ν(CN)) and are almost identical with
those reported for 2,5-dibora-2,5-dihydropyrazines (strong
absorption around 1550-1560 cm-1 for the B(R)CdN-
(R)B moiety),5 indicating complete dimerization of 2 to
of ν(CN) up to 100 cm-1 15
.
Coordinated 1 reacts upon cleavage of the Si-O bond
under carbene formation,1,3 if the isocyanide carbon is
not deactivated for nucleophilic attack by back-bonding.
The force constant for the CN bond of coordinated 1
(12) Schmidbaur, H.; Mu¨ller, G.; Milewski-Mahrla, B.; Schubert, U.
Chem. Ber. 1980, 113, 2575.
(13) Fehlhammer, W. P.; Hoffmeister, H.; Boyadjiev, B.; Kolrep, T.
Z. Naturforsch. 1989, 44B, 917.
(14) Hahn, F. E.; Imhof, L.; Lu¨gger, T.; Tamm, M. Manuscript in
preparation.
(15) (a) Hahn, F. E.; Tamm, M. Organometallics 1992, 11, 84. (b)
Hahn, F. E.; Tamm, M. Angew. Chem., Int. Ed. Engl. 1991, 30, 203.
(10) J utzi, P.; Gilge, U. J . Organomet. Chem. 1983, 246, 159.
(11) (a) Fischer, E. O.; Fischer, H. J . Organomet. Chem. 1966, 6,
141. (b) Andersen, R. A. Inorg. Chem. 1979, 18, 1507.

Isocyanide and Ylidene Complexes of Boron
Organometallics, Vol. 15, No. 4, 1996 1253
Ta ble 1. Selected Bon d Dista n ces (Å) a n d An gles
(d eg) for 2 a n d 3a
2
3
B-C1
1.616(2)
1.148(2)
1.400(2)
1.387(2)
1.395(2)
1.373(3)
1.385(2)
1.382(3)
1.387(2)
1.633(2)
1.321(2)
1.401(2)
1.391(2)
1.375(2)
1.387(2)
1.401(2)
1.393(2)
1.378(2)
1.3431(13)
1.3889(14)
N-C1
N-C2
C2-C3
C2-C7
C3-C4
C4-C5
C5-C6
C6-C7
O-C1
O-C7
O-Si
B-C11
B-C17
B-C23
1.353(2)
1.6796(12)
1.639(2)
1.635(3)
1.634(2)
B-C8
B-C14
B-C20
1.631(2)
1.638(2)
1.639(2)
F igu r e 2. ORTEP plot of the molecular structure of 3,
showing 50% probability thermal ellipsoids.
O-C1-N
108.11(10)
121.40(10)
130.47(10)
111.01(10)
108.27(9)
O-C1-B
N-C1-B
176.5(2)
172.5(2)
C1-N-C2
C1-O-C7
Si-O-C7
131.25(10)
118.66(14)
119.19(14)
122.2(2)
119.3(2)
119.4(2)
121.2(2)
120.4(2)
117.72(14)
124.8(2)
117.5(2)
103.40(13)
106.32(13)
107.33(13)
113.64(14)
112.67(13)
112.62(14)
N-C2-C3
N-C2-C7
C3-C2-C7
C2-C3-C4
C3-C4-C5
C4-C5-C6
C5-C6-C7
O-C7-C2
O-C7-C6
C2-C7-C6
C1-B-C11
C1-B-C17
C1-B-C23
C11-B-C17
C11-B-C23
C17-B-C23
134.23(11)
104.21(10)
121.57(11)
115.96(12)
121.80(12)
121.80(12)
115.24(12)
108.40(10)
127.97(11)
123.63(11)
107.36(9)
106.80(9)
107.22(9)
111.76(9)
114.18(10)
109.14(9)
F igu r e 3. Resonance stabilization of 3 and schematic
representation of complex 8.
to a B-C(aryl) bond. The hybridizations of all four
boron-bound carbon atoms are identical (sp2).
The structural characterization of 3 allows a direct
comparison of the molecular parameters for a triphen-
ylborane-bound ylidene resulting from cyclization of an
C1-B-C8
C1-B-C14
C1-B-C20
C8-B-C14
C8-B-C20
C14-B-C20
aromatic isocyanide with those of {Ph3B[CN(H)CH2-
a
Estimated standard deviations are given in parentheses.
CH2O]} (8) (Figure 3), formed after cyclization of the
aliphatic isocyanide in [Ph3B-CNCH2CH2-OSi(CH3)3].13
The boron-C(ylidene) distances in both complexes are
almost identical (1.633(2) Å in 3 and 1.623(4) Å in 8).
In 3 one finds longer C(ylidene)-heteroatom distances
and shorter distances between the heteroatoms and the
other attached carbon atoms than in 8. This indicates
less stabilization of the carbene center by (pfp)π
bonding from the heteroatoms (Figure 3, B) and a
contribution of aromatic stabilization in 3 as shown by
A in Figure 3. The former stabilization is not possible
in 8, containing, apart from the carbene carbon, only
saturated carbon atoms in the five-membered chelate
ring. Comparison of the structural parameters of the
ylidene ligand in 3 to those found in other low-valent
transition-metal complexes with the 1,2-dihydrobenzox-
azol-2-ylidene ligand1,3 reveals little sensitivity toward
the attached metal fragment, showing the ylidene to act
exclusively as a σ-donor no matter whether back-
bonding is feasible or not.
In the course of studying the preparation of 3 from 2
at room temperature, we obtained on one occasion, after
multiple recrystallizations from methanol/acetone, col-
orless crystals of a new compound. The X-ray crystal
structure analysis (Figure 4) revealed that this com-
pound was the dimeric 2,5-dibora heterocycle 6, depicted
in Scheme 1.
The structure analysis shows a six-membered ring in
which two Ph2B fragments are bridged by a C- and
N-diborated benzoxazol-2-yl group and by a C- and
allows us to predict if the intramolecular cyclization can
occur, and we have calculated a limiting value of
approximately 1730 N m-1 for immediate carbene
formation.3b The force constant for the CN bond in 2
was calculated to be 1918 N m-1, which illustrates that
coordination of 1 to Ph3B actually activates the isocya-
nide toward cyclization. Consequently, 2 reacts in
methanol at -30 °C in the presence of a catalytic
amount of KF to give the ylidene complex 3 as colorless
crystals in 73% yield (Scheme 1).
In the 1H NMR spectrum of 3 (in CDCl3) a broad
singlet is found at 10.76 ppm for the resonance of the
NH proton. No absorption in the isocyanide region is
found in the IR spectrum. Once formed, 3 is stable
toward dimerization and no indication for the formation
of a dimer is obtained from the mass spectrum, which
shows a signal at m/z 361 (M+) with 98% intensity.
The molecular structure of 3 was determined by X-ray
diffraction (Figure 2). The boron atom in 3 is tetrahe-
drally coordinated. All four B-C distances are identical
within statistical limits. The B-C(Ph) distances are
identical in 2 and 3, but the B-C(ylidene) distance in 3
is significantly longer than the B-C(isocyanide) dis-
tance in 2 (Table 1). Again the different hybridization
states (sp for the isocyanide, sp2 for the ylidene) are
responsible for the observed elongation of the B-C-
(ylidene) bond. The similarity in the four B-C distances
is not surprising, since the B-C(ylidene) bond is not
really dative but instead can be considered to be similar

1254 Organometallics, Vol. 15, No. 4, 1996
Tamm et al.
out at room temperature. We suspect that the forma-
tion of 6 results from the partial hydrolysis of the
intermediate bis(benzoxazol-2-yl)-bridged 2,5-diborapy-
razine 7, which could have been formed in the reaction
sequence shown in Scheme 1.
In order to gain insight into the formation of 6, we
wanted to evaluate the possibility of synthesizing 7
intentionally. Although we could obtain and character-
ize the dimeric compounds 4 and 5 unambiguously by
thermal dimerization of 2 and consecutive desilylation,
we have not yet been able to convert 5 into the cyclic
bis(ylidene) complex 7 and to find any indication of
benzene evolution at this stage. The attempted dimer-
ization of the ylidene complex 3 under thermal condi-
tions failed as well. Further investigations toward a
controlled synthesis of 7 and related compounds are
currently going on.
F igu r e 4. ORTEP plot of the molecular structure of 6,
showing 50% probability thermal ellipsoids.
Con clu sion s
2-(Trimethylsiloxy)phenyl isocyanide (1) reacts at low
temperature with Ph3B to give the first monomeric,
crystallographically characterized Ph3B isocyanide (1:
1) adduct 2. Desilylation of 2 leads to the ylidene adduct
of Ph3B 3, which is thermally stable. The isocyanide
complex 2 dimerizes at room temperature to the 2,5-
dibora-2,5-dihydropyrazine 4. Desilylation of 4 is pos-
sible. In the course of this reaction an ylidene-bridged,
six-membered 2,5-dibora heterocycle was obtained by
a presently unknown mechanism.
Ta ble 2. Selected Bon d Dista n ces (Å) a n d An gles
(d eg) for 6a
O1-C1
O1-C7
O2-C20
O2-B2
O3-C26
N1-C1
N1-C2
N1-B2
N2-C20
N2-C21
C1-B1
C2-C3
1.353(2)
1.384(2)
1.278(2)
1.536(3)
1.355(3)
1.307(3)
1.407(3)
1.590(3)
1.322(3)
1.420(3)
1.597(3)
1.382(3)
C2-C7
1.373(3)
1.385(3)
1.386(4)
1.380(3)
1.379(3)
1.622(3)
1.375(3)
1.394(3)
1.381(3)
1.370(3)
1.371(3)
1.383(3)
C3-C4
C4-C5
C5-C6
C6-C7
C20-B1
C21-C22
C21-C26
C22-C23
C23-C24
C24-C25
C25-C26
Exp er im en ta l Section
All operations were performed under an atmosphere of dry
argon by using Schlenk and vacuum techniques. Solvents
were dried by standard methods and distilled prior to use.
NMR spectra were recorded on Bruker AM 250 (250 MHz) or
AM 270 (270 MHz) instruments. Infrared spectra were taken
on a Perkin-Elmer 983 instrument in KBr. Elemental analy-
ses (C, H, N) were performed at the Freie Universita¨t Berlin
on a Heraeus CHN-Rapid elemental analyzer. Mass spectra
(EI, 70 eV) were recorded on a Varian MAT 711 instrument.
2-(Trimethylsiloxy)phenyl isocyanide (1) was synthesized ac-
cording to the method of J utzi.10 Triphenylborane was pur-
chased from Aldrich and was used as received.
C1-O1-C7
C20-O2-B2
C1-N1-C2
C1-N1-B2
C2-N1-B2
C20-N2-C21
O1-C1-N1
O1-C1-B1
N1-C1-B1
N1-C2-C3
N1-C2-C7
C3-C2-C7
C2-C3-C4
C3-C4-C5
C4-C5-C6
C5-C6-C7
106.3(2)
128.7(2)
108.3(2)
125.6(2)
126.1(2)
132.6(2)
111.1(2)
121.5(2)
127.4(2)
133.5(2)
105.7(2)
120.8(2)
116.4(2)
121.7(2)
122.4(2)
114.8(2)
O1-C7-C2
108.5(2)
127.5(2)
124.0(2)
116.9(2)
126.1(2)
117.0(2)
125.1(2)
114.7(2)
120.2(2)
119.8(2)
120.2(2)
120.4(2)
120.3(2)
116.6(2)
124.2(2)
119.2(2)
O1-C7-C6
C2-C7-C6
O2-C20-N2
O2-C20-B1
N2-C20-B1
N2-C21-C22
N2-C21-C26
C22-C21-C26
C21-C22-C23
C22-C23-C24
C23-C24-C25
C24-C25-C26
O3-C26-C21
O3-C26-C25
C21-C26-C25
(2-(Tr im eth ylsiloxy)p h en yl isocya n id e)-Tr ip h en ylbo-
r a n e (2). A solution of triphenylborane (3.44 g, 14 mmol) in
50 mL of dichloromethane was cooled to -60 °C, causing BPh3
to precipitate. 1 (2.72 g, 14 mmol) was slowly added to the
stirred suspension. A clear yellowish solution was formed, and
stirring was continued for 1 h at -60 °C. The solvent was
removed at -30 °C, and 2 was obtained as a white solid in
quantitative yield. Dissolving a 900 mg (2 mmol) sample of 2
in 20 mL of diethyl ether and cooling to -26 °C gave colorless
crystals of 2 suitable for X-ray diffraction. The major fraction
(5.1 g, 12 mmol) was directly converted to 3. IR: ν 2244 (s,
a
Estimated standard deviations are given in parentheses.
O-diborated (2-hydroxyphenyl)formamide. Complex 6
crystallizes with one molecule each of methanol and
acetone per formula unit, which are hydrogen-bonded
to the phenolic hydrogen and to each other (the posi-
tional parameters for H(N2), H(O3) and H(O5) were
identified and refined in the least-squares refinement).
Bond distances and angles of the benzoxazol-2-ylidene
fragment in 6 (C1-C7, N1, O1; Table 2) are within
statistical limits identical with those found for the same
fragment in 3. However, the C1-B1 distance (1.597(3)
Å) is now even shorter than in 3. Formation of the N1-
B2 bond is not surprising, since we have shown that
coordinated 1,2-dihydrobenzoxazol-2-ylidene ligands are
easily alkylated at the carbene nitrogen atom.1,3
+
CN). MS (LR, 70 eV, m/ z): 866 (M2+, 7.1), 789 (M2 - Ph,
9.5), 624 [M2+ - Ph - (C6H4-2-OSiMe3), 24.1], 551 [M2+ - Ph
- (C6H4-2-OSiMe3) - SiMe3, 33.0], 433 (M+, 12.0), 360 (M+
-
SiMe3, 26.1), 165 (BPh2+, 82.9), 73 (SiMe3, 100). Elemental
analyses were impossible to obtain due to the rapid dimeriza-
tion of 2 at room temperature, giving 4 with identical values
for C, H, and N.
(1,2-D ih y d r o b e n zo x a zo l-2-y lid e n e )-T r ip h e n y lb o -
r a n e (3). A 50 mL portion of methanol was condensed onto a
cooled sample of 2 (5.10 g, 12 mmol) obtained from the reaction
described above. A catalytic amount of KF (10 mg, 0.2 mmol)
was added, and the suspension was stirred at -30 °C for 2 h.
The reaction mixture was warmed to 0 °C, giving a clear
Because 3 is stable against dimerization once it has
formed, we assume that formation of 6 can only have
occurred after dimerization of 2. This is conceivable,
since all reactions leading to isolation of 6 were carried

Isocyanide and Ylidene Complexes of Boron
Organometallics, Vol. 15, No. 4, 1996 1255
Ta ble 3. Su m m a r y of Cr ysta l Str u ctu r e Da ta a
2
3
6
cryst size, mm
formula
fw
0.41 × 0.35 × 0.17
C28H28BNOSi
433.44
0.36 × 0.29 × 0.12
0.55 × 0.35 × 0.15
C42H40B2N2O5
674.42
C25H20BNO
361.26
space group
a, Å
b, Å
P1h (No. 2)
11.007(4)
11.374(3)
11.346(4)
108.03(2)
98.82(3)
108.72(2)
1228(2)
P1h (No. 2)
P1h (No. 2)
9.201(5)
20.108(8)
10.379(2)
80.33(2)
108.01(3)
90.71(4)
1799(2)
2
9.535(3)
9.945(2)
10.759(5)
80.53(3)
79.91(3)
73.81(2)
957.4(6)
2
c, Å
R, deg
â, deg
γ, deg
V, Å3
Z
2
dc, g/cm3
do, g/cm3
µc, cm-1
radiation (λ, Å)
data collecn temp, °C
2θ range, deg
hkl range
1.172
1.16
1.10
1.253
1.26
0.70
1.245
1.23
0.75
Mo KR, 0.710 73
-120(2)
4.5 e 2θ e 46
-100(2)
4.5 e 2θ e 50
-11 e h e 12, -12 e k e 12,
0 e l e 13
variable: min 2.35, max 16.5
4109
3598
3.89
5.99
0.08
1.333
334
<0.01
-0.09, 0.37
-120(2)
2 e 2θ e 50
-11 e h e 12, -11 e k e 11,
0 e h e 10, -23 e k e 23,
-11 e l e 0
-11 e l e 11
scan speed (θ), deg/min
variable: min 1.65, max 16.5
variable: min 1.65, max 5.49
no. of unique data
3544
2979
3.35
5.05
0.08
1.178
402
<0.01
-0.10, 0.31
6331
4283
4.71
6.34
0.08
1.351
478
0.02
2
2
no. of obsd data, Fo g 3σ(Fo
)
R, %b
Rw, %b
p factorb
GOFb
no. of variables
max shift/error
residual electr dens, e/Å3
-0.09, 0.28
a
b
Estimated standard deviations are given in parentheses. See ref 19.
solution, from which a colorless solid started to precipitate
after 5 min. Stirring was continued at 0 °C for 2 h and at
room temperature overnight. The solvent was removed in
vacuo, and the white solid was recrystallized from acetone/
was obtained as a white powder (1.03 g, 62%). Anal. Calcd
for C50H40B2N2O2 (Mr ) 722.50): C, 83.12; H, 5.58; N, 3.88.
Found: C, 82.22; H, 5.64; N, 3.62. MS (LR, 70 eV, m/ z): 722
(M+, 3.2), 644 (M+ - H - Ph, 13.7), 566 (M+ - 2H - 2Ph,
3.0), 527 (M+ - Ph - O-2-C6H4-NC, 11.0), 449 (M+ - 2Ph -
HO-2-C6H4-NC, 16.5), 360 (M+ - H - HO-2-C6H4NC-BPh3,
100), 284 (M+ - H - Ph - HO-2-C6H4NC-BPh3, 41.9), 165
(BPh2+, 55.1).
P r ep a r a tion of 6. Complex 6 was obtained in approxi-
mately 20% yield (relative to 2) by stirring 2 g of 2 in methanol
for 3 days at room temperature followed by multiple recrys-
tallization of the product from wet methanol/acetone.
Cr ysta l Str u ctu r e An a lyses. Crystals of 2 and 6 are
temperature-sensitive (dimerization or loss of solvent), while
3 is stable at room temperature. Suitable specimens of 2 and
6 were selected at -120 °C using a device similar to that
described by Veith16 and mounted in the cold stream
methanol at -26 °C (3.11 g, 73%). Anal. Calcd for C25H20
-
BNO (Mr ) 361.25): C, 83.12; H, 5.58; N, 3.88. Found: C,
82.81; H, 5.64; N, 3.61. 1H NMR (250 MHz, acetone-d6): δ
7.05 (tt, 3 H, BPh3 4-CH), 7.15 (td, 6 H, BPh3 3,5-CH), 7.34
(dd, 6 H, BPh4 2,6-CH), 7.46 (m, 2 H, Ar H), 7.68 (m, 1 H, Ar
H), 7.78 (m, 1 H, Ar H). The NH resonance at δ 10.76 was
only observed in CDCl3. 11B NMR (28.69 MHz, acetone-d6):
δ -9.2 (s). MS (LR, 70 eV, m/ z): 361 (M+, 98.0), 284 (M+
-
Ph, 21.9), 165 (BPh2+, 100).
1,4-Bis(2-(t r im e t h ylsiloxy)p h e n yl)-2,2,3,5,5,6-h e xa -
p h en yl-2,5-d ibor a -2,5-d ih yd r op yr a zin e (4). A solution of
triphenylborane (5.49 g, 23 mmol) in 70 mL of dichloromethane
was cooled to -60 °C, causing the BPh3 to precipitate. 1 (4.34
(-120(2) °C) of an Enraf-Nonius CAD-4 diffractometer.
A
g, 23 mmol) was slowly added to the stirred suspension.
A
crystal of 3 was selected in air and mounted at -100(2) °C on
an Enraf-Nonius CAD-4 diffractometer. Important crystal and
data collection details are listed in Table 3. Data for all three
compounds were collected using ω-2θ scans. Raw data were
reduced to structure factors17 (and their esd’s) by correcting
for scan speed, Lorentz, and polarization effects. No crystal
decay was detected, and no absorption corrections were
applied. The space group was found to be P1h for all three
compounds. All three structures were solved by direct meth-
clear yellowish solution was formed, and stirring was contin-
ued for 1 h at -60 °C. The solvent was removed at 0 °C. The
residue was kept at 80 °C for 30 min by the use of a water
bath, causing the reaction mixture to turn dark green. The
residue was treated with hot pentane (80 mL) and filtered.
Washing with pentane and drying under vacuum gave 4 as a
white powder (4.5 g, 46%). Anal. Calcd for C56H56B2N2O2Si2
(Mr ) 866.86): C, 77.59; H, 6.51; N, 3.23. Found: C, 77.07;
H, 6.53; N, 3.18. MS (LR, 70 eV, m/ z): 866 (M+, 24.5), 789
(M+ - Ph, 27.9), 712 (M+ - 2Ph, 24.1), 598 [M+ - Ph - Me3-
SiO-2-C6H4-NC, 20.4], 551 [M+ - Ph - (C6H4-2-OSiMe3) -
SiMe3, 19.6], 521 (M+ - 2Ph - Me3SiO-2-C6H4-NC, 13.6), 433
[M+ - Me3SiO-2-C6H4-NC-BPh3, 17.8], 360 (M+ - Me3SiO-2-
C6H4-NC-BPh3 - SiMe3, 30.7), 268 (Ph3B-CN+, 28.7), 165
(BPh2+, 37.9), 73 (SiMe3, 100).
1,4-Bis(2-h yd r oxyp h en yl)-2,2,3,5,5,6-h exa p h en yl-2,5-
d ibor a -2,5-d ih yd r op yr a zin e (5). A suspension of 4 (2.0 g,
2.3 mmol) in 60 mL of CHCl3/MeOH (1:1) was treated with a
catalytic amount of KF (10 mg, 0.2 mmol) and heated under
reflux for 2 days. The solvent was removed in vacuo, and the
residue was extracted with 100 mL of methanol. After
filtration the clear solution was evaporated to dryness, and 5
(16) Veith, M.; Ba¨rnighausen, H. Acta Crystallogr. 1974, B30, 1806.
(17) Neutral scattering factors were used: International Tables for
X-Ray Crystallography; Kynoch Press: Birmingham, England, 1974;
Vol. IV, Table 2.2B. Terms of anomalous dispersion from: Interna-
tional Tables for X-Ray Crystallography; Kynoch Press: Birmingham,
England, 1974; Vol. IV, Table 2.3.1.
(18) Churchill, M. R. Inorg. Chem. 1972, 12, 1213.
(19) MolEN: Molecular Structure Solution Procedures, Program
Descriptions; Enraf-Nonius, Delft, The Netherlands, 1990. Definition
2
2
of residuals: R ) ∑||Fo| - |Fc||/∑|Fo|, Rw ) [∑w||Fo| - |Fc|| /∑w|Fo| ]1/2
,
and GOF ) [∑w||Fo| - |Fc|| /(no - np)]1/2 with no ) number of structure
2
factors, np ) number of parameters, w ) 1/[σF]2, σF ) σF /2F, and σF
2
2
) {[σI]2 + [pF2]2}1/2
.
(20) J ohnson, C. K. ORTEP II; Report ORNL-5138; Oak Ridge
National Laboratory: Oak Ridge, TN, 1971.

1256 Organometallics, Vol. 15, No. 4, 1996
Tamm et al.
ods. The positional parameters for all non-hydrogen atoms
were refined by using first isotropic and later anisotropic
thermal parameters. Difference Fourier maps calculated at
this stage showed for all three molecules the positional
parameters of the hydrogen atoms. All hydrogen positions
were refined for 2 and 3. For 6 the positional parameters for
all N- and O-bonded hydrogens as well as those for the
methanol molecule were refined; the others were added to the
structure model at calculated positions (d(C-H) ) 0.95 Å)18
and are unrefined. The isotropic temperature factors for
hydrogens were fixed to be 1.3 times the Beq value of the parent
atom. All calculations were carried out with the MolEN
package.19 ORTEP20 was used for all molecular drawings.
Ack n ow led gm en t. This work was supported by the
Deutsche Forschungsgemeinschaft and the Fonds der
Chemischen Industrie. We are indebted to the BASF
AG for a predoctoral grant to M.T. (1991-1993).
Su p p or tin g In for m a tion Ava ila ble: For 2, 3, and 6,
tables of atomic coordinates, all bond distances and angles,
and anisotropic thermal parameters and figures giving ad-
ditional atom labeling (15 pages). Ordering information is
given on any current masthead page.
OM950773+