Angewandte
Chemie
Carbenes
The Synthesis of B2(SIDip)2 and its Reactivity Between the
Diboracumulenic and Diborynic Extremes
Julian Bçhnke, Holger Braunschweig,* Theresa Dellermann, William C. Ewing, Kai Hammond,
J. Oscar C. Jimenez-Halla, Thomas Kramer, and Jan Mies
Dedicated to Professor Todd B. Marder on the occasion of his 60th birthday
Abstract: A new compound with the formula L-B2-L wherein
the stabilizing ligand (L) is 1,3-bis[diisopropylphenyl]-4,5-
dihydroimidazol-2-ylidene (SIDip) has been synthesized, iso-
lated, and characterized. The p-acidity of the SIDip ligand,
intermediate between the relatively non-acidic IDip (1,3-
bis[diisopropylphenyl]imidazol-2-ylidene) ligand and the
much more highly acidic CAAC (1-[2,6-diisopropylphenyl]-
3,3,5,5-tetramethylpyrrolidin-2-ylidene) ligand, gives rise to
a compound with spectroscopic, electrochemical, and struc-
tural properties between those of L-B2-L compounds stabilized
by CAAC and IDip. Reactions of all three L-B2-L compounds
with CO demonstrate the differences caused by their respective
ligands, as the p-acidities of the CAAC and SIDip carbenes
enabled the isolation of bis(boraketene) compounds
(L(OC)B-B(CO)L), which could not be isolated from reac-
tions with B2(IDip)2. However, only B2(IDip)2 and B2(SIDip)2
could be converted into bicyclic bis(boralactone) compounds.
the electronic properties of stable carbenes,[3] a number of
different experimental methods have been employed in hopes
of quantifying their respective donor/acceptor capabilities.[4]
Strategies involving NMR spectroscopy, notably comparing
the 31P, 77Se, and 13C NMR signals of carbene phosphinidine,[5]
carbene selenide,[6] and carbene platinum[7] complexes have
shown the method to be a sensitive tool for the evaluation of
carbene character. Measurements of the structural, vibra-
tional, and redox characteristics of organometallic complexes
bearing a wide range of carbenes have likewise provided
a wealth of information.[4,8]
In recent years, our group has reported the syntheses of
two similar compounds utilizing carbenes with distinct and
divergent electronic characters. One of these was the first
example of a diboryne (B2(IDip)2, 1, IDip = 1,3-bis[diisopro-
pylphenyl]imidazol-2-ylidene),[9]
a compound containing
a triple bond between two boron atoms. The second was
a molecule best described as a diboracumulene (B2(CAAC)2,
2, CAAC = cyclic (alkyl)(amino) carbene, in this case 1-[2,6-
diisopropylphenyl]-3,3,5,5-tetramethylpyrrolidin-2-yli-
S
ince Arduengoꢀs initial isolation of a stable N-heterocyclic
carbene (NHC),[1] these extraordinarily useful ligands have
become widely available and have truly transformed many
aspects of organometallic and main-group chemistry. Work
since Arduengo, particularly by Bertrand and co-workers, has
shown the diamino-NHC, where the carbene carbon is part of
a ring and flanked by two nitrogen atoms, is just one example
of a broad array of stable, singlet carbenes.[2] Alterations in
the identity of the elements adjacent to the carbene carbon
atom, as well as in the size, degree of unsaturation, and
substitution pattern on the backbone of the carbene, lead to
significant electronic differences in both the s-donating
capacity of the lone-pair of electrons on the carbene carbon
atom and in the p-acidity of the formally vacant p orbital on
the carbene carbon atom. On top of the numerous computa-
tional studies seeking to theoretically evaluate and compare
dene),[10] an analogue of dicationic butatriene. The differences
À
À
in the B B and C B bond lengths in these compounds are the
direct result of differences in the p-acidities of the IDip and
CAAC ligands. The formally empty p orbital on the carbene
carbon atom of the CAAC ligand, stabilized by only one
nitrogen lone-pair, is the recipient of a larger degree of p-
backdonation from the electron rich B2 unit than the
comparatively electronically saturated p orbital on the car-
bene carbon atom of IDip. This results in more extensive
À
spread of p-electron density across the B C bonds of 2 than in
À
À
1, in turn resulting in a longer B B bond and shorter B C
bonds in 2.
In hopes of further probing the sensitivity of the B2 unit to
changes in carbene electronic character, we turned to
a carbene with p-acidity between those of CAAC and IDip,
synthesizing B2(SIDip)2 (3, SIDip = 1,3-bis[diisopropyl-
phenyl]-4,5-dihydroimidazol-2-ylidene) in a method similar
to the syntheses of 1 and 2. The aforementioned comparative
studies of carbenes have indicated that SIDip, identical to
IDip with the exception that the C2 backbone of the five-
membered ring is saturated, is more p-acidic than IDip, but
much less acidic than CAAC (Scheme 1).[5–7]
[*] J. Bçhnke, Prof. Dr. H. Braunschweig, T. Dellermann, Dr. W. C. Ewing,
K. Hammond, Dr. T. Kramer, Dr. J. Mies
Institut für Anorganische Chemie
Julius-Maximilians-Universität Würzburg
Am Hubland, 97074 Würzburg (Germany)
E-mail: h.braunschweig@uni-wuerzburg.de
Dr. J. O. C. Jimenez-Halla
Department of Chemistry, Division of Natural and Exact Sciences
University of Guanajuato, campus Gto
The sodium napthalenide reduction of 1,2-(SIDip)2-B2Br4
furnished 3 in moderate yields. In agreement with previous
NMR studies,[5–7] the 11B NMR shift in 3 (d = 58 ppm) falls
between that of 1 (d = 39 ppm) and 2 (d = 80 ppm). Single
Noria Alta s/n 36050 Guanajuato (Mexico)
Supporting information for this article is available on the WWW
Angew. Chem. Int. Ed. 2015, 54, 13801 –13805
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
13801