A stable N-heterocyclic carbene with a diboron backbone

Article abstract of DOI:10.1021/ja050129e

The synthesis of a novel five-membered inorganic ring, a stable N-heterocyclic carbene with a diboron backbone, is reported. A pentacarbonyltungsten complex containing the new carbene is also described. Spectroscopic evidence indicates that the sterically

Full text of DOI:10.1021/ja050129e

Published on Web 03/04/2005
A Stable N-Heterocyclic Carbene with a Diboron Backbone
Kelly E. Krahulic, Gary D. Enright,† Masood Parvez, and Roland Roesler*
Department of Chemistry, UniVersity of Calgary, 2500 UniVersity DriVe NW, Calgary, Alberta, T2N 1N4 Canada,
and Steacie Institute for Molecular Sciences, 100 Sussex DriVe, Ottawa, Ontario, K1A 0R6, Canada
Received January 9, 2005; E-mail: roesler@ucalgary.ca
Stable N-heterocyclic carbenes1 display a rich coordination
Scheme 1. Synthesis and Coordination of B2-Backbone Carbene
chemistry and have found a multitude of applications as catalyst
components due to their remarkable ligand properties.2 Derivatives
with various substitution patterns and ring sizes from four to six
have been characterized, with the five-membered rings being by
far the most common. A variety of inorganic analogues have been
prepared as well, where the N2C: moiety was replaced by isolobal
fragments N2E:, E ) Si,3a-c Ge,3c-e Sn,3c,f N+,3g P+,3h,i As+,3i or
Ga-.3j,k
Scheme 1. The carbene was isolated as a pale yellow powder with
good solubility in benzene, toluene, and THF. Rapid decomposition
was observed in CH2Cl2. Neither the solid nor the solutions of 3b
showed any signs of decomposition when stored for a few weeks
at room temperature in an inert atmosphere; however, the B-N
bonds hydrolyzed rapidly in air, generating the corresponding
amidine. The NMR spectra displayed the expected signals, most
notably the singlet in the 11B NMR corresponding to the backbone
boron atoms at 33.2 ppm and the 13C NMR signal for the carbene
carbon at 304 ppm. This value corresponds to the highest downfield
shift reported for a diaminocarbene. Large, pale yellow crystals
were obtained by cooling a solution of the carbene in a 1:1 hexane/
toluene mixture.
The growing significance of N-heterocyclic carbenes as spectator
ligands in catalysis and our interest in five-membered inorganic
heterocycles prompted us to investigate the stability of carbenes
with inorganic backbones, as well as the influence of the inorganic
backbone on the ligand properties of the carbon ligand. To our
knowledge, only one N-heterocyclic carbene with an inorganic
backbone (:C(NR)2PR′) has been reported so far.4
Attempts to close a five-membered ring starting from known
precursors of type 1, with R ) tBu, 2,6-Me2C6H5,5 and carbon or
even silicon fragments upon elimination of HCl or LiCl were not
successful. However, the method used by Grubbs et al. for the
synthesis of the first four-membered N-heterocyclic carbene4a easily
generated, in high yield, the imidazolium precursor 2a, which
displayed 1H and 13C NMR signals corresponding to the ring proton
and carbon at 7.93 and 175 ppm, respectively, as well as a 11B
NMR signal at 31.3 ppm. Single-crystal X-ray diffraction revealed
the expected structure featuring separated ions. The short endocyclic
C-N bonds (1.335(3) and 1.316(3) Å) are consistent with electron
delocalization over the N-C-N fragment, which is observed in
all iminium analogues. The significant difference between the
lengths of the endo- and exocyclic B-N bonds (1.511(3) and
1.521(3) Å vs 1.383(4) and 1.374(3) Å) indicates electron donation
primarily from the exocyclic nitrogen to boron. As compared to
imidazolium cations with a carbon framework, the B-B backbone
is significantly longer (1.71 vs ca. 1.34 Å for CdC and ca. 1.51 Å
for C-C backbone). The B2N2C framework is a novel ring system.
The only B2N2C ring reported to date contains a coordinative bond
between a borate and an ammonium moiety.6
Compound 3b crystallizes as a modulated structure that presented
significant difficulties in the structural refinement. In the primary
cell the structure was modeled as consisting of fully disordered
molecules across a 2-fold axis. The carbene ring (Figure 1) is nearly
planar. As compared to diaminocarbenes with a carbon backbone,
the relatively long B-B bond does not result in a very significant
opening of the N1-C1-N2 angle (108.45(8)° vs 101-106° and
106-108°, respectively). However, it has a notable effect on the
phenyl groups, which form an unusually small dihedral angle,
thereby protecting the carbene carbon. The N2-C12 and N5-C24
bonds form an angle of only 120.5° (phenyl-phenyl dihedral angle
121.3°), as compared to 145-146° in analogues with a CdC
backbone.7 Similarly to 2a, the endocyclic B-N bonds in 3b are
ca. 0.1 Å longer than their exocyclic counterparts. This suggests
little electron donation from nitrogen to boron within the ring
skeleton and indicates the similarity with the isolobal organic
framework A rather than B. The hindered rotation around the
exocyclic B-N bond is confirmed by the nonequivalence of the
Deprotonation of the precursor 2a with potassium hexamethyl-
disilazide failed to generate the free carbene and led instead to a
complex mixture of products that was not further investigated. The
imidazole analogue 2b containing the bulkier 2,6-diisopropylphenyl
group was synthesized using the same method and subsequently
yielded the free carbene 3b in 65% isolated yield according to
1
methyl groups on nitrogen in the room temperature H and 13C
NMR spectra.
To assess the coordinative properties of the novel carbene, its
behavior toward selected transition metal complexes was investi-
gated. The reaction of 3b with (PhCN)2PdCl2 led to a complex
† Steacie Institute for Molecular Sciences.
9
4142
J. AM. CHEM. SOC. 2005, 127, 4142-4143
10.1021/ja050129e CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
Figure 1. ORTEP-like diagram of 3b at 30% probability. Hydrogen atoms
have been omitted for clarity. Selected bond lengths (Å) and angles (deg):
N5-C1 1.352(1), N2-C1 1.404(1), N2-B3 1.460(2), N5-B4 1.470(2),
B3-B4 1.731(2), B3-N6 1.380(2), B4-N9 1.374(2); N2-C1-N5 108.45(8),
C1-N2-B3 116.70(9), C1-N5-B4 114.84(9), N2-B3-B4 98.01(9),
N5-B4-B3 101.79(9).
Figure 2. ORTEP-like diagram of 4b at 50% probability. Hydrogen atoms
have been omitted for clarity. Selected bond lengths (Å) and angles (deg):
W1-C6 2.283(4), N1-C6 1.384(5), N2-C6 1.382(5), N1-B1 1.482(5),
N2-B2 1.499(5), B1-B2 1.700(7), B1-N3 1.398(6), B2-N4 1.394(6),
C3-W1 1.991(5), Ceq-W1 2.036(5)-2.040(5); N1-C6-N2 109.2(3),
C6-N1-B1 114.5(3), C6-N2-B2 114.3(3), N1-B1-B2 101.0(3),
N2-B2-B1 100.0(3).
mixture of products, whereas no reaction was observed with
[Cp*RuCl]4 in THF. The latter reagent is known to react quickly
with N-heterocyclic carbenes, yielding deep blue complexes.8
The UV-promoted reaction of 3b with tungsten hexacarbonyl
produced the expected pentacarbonyl derivative 4b, which was
readily crystallized from benzene. The reaction proceeded quanti-
tatively in benzene, whereas in tetrahydrofuran only formation of
(CO)5W(thf) was observed. The signal for the carbene carbon in
the 13C NMR spectrum shifted upfield upon coordination to 259.1
ppm. The observation of 183W satellites (1JWC 110.5 Hz) confirms
Foundation for Innovation, and the Alberta Science and Research
Investments Program.
Supporting Information Available: Experimental details and
complete spectroscopic data for 2-4; crystallographic details including
CIF files for 2a, 3b, and 4b; and the complete ref 9c. This material is
available free of charge via the Internet at http://pubs.acs.org.
References
(2)
the W-C bond formation. The A1 stretching vibration of the
(1) Arduengo, A. J., III; Harlow, R. L.; Kline, M. J. Am. Chem. Soc. 1991,
113, 361-363.
W(CO)5 moiety was observed at 2054 cm-1, at a lower wavenumber
compared to those of the related N-heterocyclic carbene complexes
(2060-2065 cm-1).9 This observation, along with the high value
of the 1JWC coupling constant for the carbene carbon in comparison
to similar complexes containing five-membered N-heterocyclic
carbenes with a carbon backbone (95-96 Hz),9c indicates that 3b
is a better σ-donor than classical carbenes. Its low reactivity is
probably influenced by the very efficient protection of the carbene
carbon by the diisopropylphenyl groups.
(2) (a) Bourissou, D.; Guerret, O.; Gabba¨ı, F. P.; Bertrand, G. Chem. ReV.
2000, 100, 39-91. (b) Jafarpour, L.; Nolan, S. P. J. Organomet. Chem.
2001, 617-618, 17-27. (c) Hillier, A. C.; Grasa, G. A.; Viciu, M. S.;
Lee, H. M.; Yang, C.; Nolan, S. P. J. Organomet. Chem. 2002, 653, 69-
82. (d) Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41, 1290-1309.
(e) Perry, M. C.; Burgess, K. Tetrahedron: Asymmetry 2003, 14, 951-
961.
(3) (a) Denk, M.; Lennon, R.; Hayashi, R.; West, R.; Belyakov, A. V.; Verne,
H. P.; Haaland, A.; Wagner, M.; Metzler, N. J. Am. Chem. Soc. 1994,
116, 2691-2692. (b) Haaf, M., Schmedake, T. A.; West, R. Acc. Chem.
Res. 2000, 33, 704-714. (c) Tokitoh, N.; Ando, W. In ReactiVe
Intermediate Chemistry; Moss, R. A., Platz, M. S., Jones, M., Jr., Eds.;
John Wiley & Sons: Hoboken, 2004; pp 651-715. (d) Herrmann, W.
A.; Denk, M.; Behm, J.; Scherer, W.; Klingan, F.-R.; Bock, H.; Solouki,
B.; Wagner, M. Angew. Chem. 1992, 104, 1489-1492; Angew. Chem.,
Int. Ed. Engl. 1992, 31, 1485-1488. (e) Ku¨hl, O. Coord. Chem. ReV.
2004, 248, 411-427. (f) Braunschweig, H.; Gehrhus, B.; Hitchcock, P.
B.; Lappert, M. F. Z. Anorg. Allg. Chem. 1995, 621, 1922-1928. (g)
Boche, G.; Andrews, P.; Harms, K.; Marsch, M.; Rangappa, K. S.;
Schimeczek, M.; Willeke, C. J. Am. Chem. Soc. 1996, 118, 4925-4930.
(h) Denk, M. K.; Gupta, S.; Ramachandran, R. Tetrahedron Lett. 1996,
37, 9025-9028. (i) Denk, M. K.; Gupta, S.; Lough, A. J. Eur. J. Inorg.
Chem. 1999, 41-49 and references therein. (j) Schmidt, E. S.; Jockisch,
A.; Schmidbaur, H. J. Am. Chem. Soc. 1999, 121, 9758-9759. (k) Baker,
R. J.; Jones, C.; Platts, J. A. J. Am. Chem. Soc. 2003, 125, 10534-10535.
(4) (a) Despagnet-Ayoub, E.; Grubbs, R. H. J. Am. Chem Soc. 2004, 126,
10198-10199. (b) Despagnet-Ayoub, E.; Grubbs, R. H. Organometallics
2005, 24, 338.
The crystal structure of 4b (Figure 2) reveals the expected
octahedral geometry at tungsten, with one coordination site occupied
by the carbene. The metal as well as the axial and two equatorial
CO groups lie in the plane of the carbene ring. Except for a dramatic
opening of the dihedral angle between the planes of the phenyl
rings to 157.9°, no significant changes in the metric parameters of
the carbene occur upon coordination. The steric repulsion between
the carbonyl and the diisopropylphenyl groups also causes the
equatorial carbonyl groups lying in the carbene plane to be nota-
bly bent toward the axial position (C1-W1-C4 163.3(2)° vs
C2-W1-C5 179.4(2)°). The C6-W1 bond length of 2.283(4) Å
is similar to the values found for other tungsten diaminocarbene
complexes.
The fact that 3b is a better σ-donor than classical carbenes
demonstrates the potential of carbenes with an inorganic backbone
as ligands in transition metal chemistry, as well as the major
influence of the inorganic backbone on the coordination ability of
the carbon ligand. The synthesis of analogues with less bulky
substituents on the nitrogen atoms adjacent to the carbene carbon
is currently being investigated.
(5) Patton, J. T.; Feng, S. G.; Abboud, K. A. Organometallics 2001, 20, 3399-
3405.
(6) Haubold, W.; Hrebicek, J.; Sawitzki, G. Z. Naturforsch. 1984, 39B, 1027-
1031.
(7) Arduengo, A. J., III; Krafczyk, R.; Schmutzler, R.; Craig, H. A.; Goerlich,
J. R.; Marshall, W. J.; Unverzagt, M. Tetrahedron 1999, 55, 14523-
14534.
(8) (a) Huang, J.; Stevens, E. D.; Nolan, S. P.; Petersen, J. L. J. Am. Chem.
Soc. 1999, 121, 2674-2678. (b) Hillier, A. C.; Sommer, W. J.; Yong, B.
S.; Petersen, J. L.; Cavallo, L.; Nolan, S. P. Organometallics 2003, 22,
4322-4326.
(9) (a) Herrmann, W. A.; Goossen, L. J.; Ko¨cher, C.; Artus, G. R. J. Angew.
Chem., Int. Ed. Engl. 1996, 35, 2805-2807. (b) Kernbach, U.; Mu¨hl, M.,
Polborn, K.; Fehlhammer, W. P.; Jaouen, G. Inorg. Chim. Acta 2002,
334, 45-53. (c) Liu, S.-T. et al., Organometallics 1999, 18, 2145-2154.
Acknowledgment. This work was supported by the Natural
Sciences and Engineering Research Council of Canada, the Canada
JA050129E
9
J. AM. CHEM. SOC. VOL. 127, NO. 12, 2005 4143