The Synthesis of B2(SIDip)2 and its Reactivity between the Diboracumulenic and Diborynic Extremes

Article abstract of DOI:10.1002/anie.201506368

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, isolated, and characterized. The π-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 structural 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 π-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 reactions with B₂(IDip)₂. However, only B₂(IDip)₂ and B₂(SIDip)₂ could be converted into bicyclic bis(boralactone) compounds.

Full text of DOI:10.1002/anie.201506368

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201506368
German Edition: DOI: 10.1002/ange.201506368
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
under http://dx.doi.org/10.1002/anie.201506368.
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unit and the empty p orbital on the carbene carbon atom. In 2,
where the conjugation between B and C is large, the nearly
orthogonal orientation of these ligands allows the alignment
of the carbene p orbitals with the two pBB orbitals. In 1, where
À
the double-bond character of the B C bond is substantially
lower, this angle is less important. Computational analysis has
À
hinted that rotation about the B C bond in 1 is not
substantially hindered by p-overlap, and that the 568 angle
between the planes of the NHCs is a result of optimal packing
of the bulky ligand architecture.[11] The intermediate angle in
3 is a compromise between these two factors, as the double
À
bond character of the B C bond is slightly greater, while the
bulk of the ligands is the same as in 1. Interestingly, there is
a slight but noticeable degree of pyramidalization at each of
the nitrogen atoms of the NHCs in 3 (Æangles = 352.98 avg.).
This pyramidalization, owing to the presence of pBB!carbene
backdonation, is notably absent from the nitrogen atoms of
1[9] and un-complexed SIDip.[12] All of these factors, in
conjunction with cyclic voltammetry data (Figure S1 in the
Supporting Information) indicating two one-electron oxida-
tion peaks at À1.05 V and + 0.44 V (vs. Fc/Fc+; Fc =
[(C5H5)2Fe]), falling between those of 1 (À1.28 V and
Scheme 1. The syntheses of compounds 1–3.
crystals suitable for X-ray diffractometry were grown by
slowly evaporating a benzene solution of 3 (Figure 1a).
As in both 1 and 2, the central C-B-B-C unit of 3 is linear,
with B-B-C angles measuring 179.6(2)8 and 178.6(2)8. The
À
À
length of the B B bond in 3 lies between those of the B B
À
bonds in 1 and 2, as do the B C bonds in 3 that are, on
average, shorter than in 1 and longer than in 2 (Figure 1b).
The N1-C1-C2-N3 and N2-C1-C2-N4 dihedral angles, roughly
describing the angle formed by the planes of the five-
membered NHC rings, measure approximately 738 and 728,
respectively. This can be compared to the angle between the
NHC planes in 1 (ca. 568) and 2 (ca. 808), which speak to the
comparative importance of overlap of the p orbitals of the B2
+ 0.11 V)[9] and
2
(À0.55 V, no second oxidation was
observed),[10] agree with the electronic description of 3 as
intermediate between diborynic 1 and diboracumulenic 2 as
a result of the intermediate p-acidity of the SIDip ligand.
Beyond the structural and spectroscopic comparisons of
À
1–3, the existence of a reactive B B multiple bond in each
compound presents a unique opportunity to compare the
influence of the carbene through reactivity studies. The
reaction of 1 with CO has been previously shown to result in
the reductive insertion of four CO molecules in the formation
of a bicyclic bis(boralactone) (4, Scheme 2).[13] Bertrand and
Stephan also reported an example of CO complexed to
a CAAC-stabilized boron atom.[14] When 2 was treated with
an excess of CO at room temperature, the solution turned
from deep purple to orange, concomitant with the emergence
of a new peak in the 11B NMR spectrum at d = À22 ppm. Slow
evaporation of the reaction mixture yielded crystals suitable
for X-ray study, which proved to be the bis(boraketene) 5
(Figure 2a). When 3 was stirred under an atmosphere of CO,
the immediate emergence of a new 11B NMR resonance at d =
À29 ppm was observed and identified as the bis(boraketene)
6 on account of its highfield 11B NMR shift, in reasonable
agreement with the DFT predicted chemical shift (d =
À33 ppm), and the observation of a new lowfield 13C NMR
resonance. A second very small 11B NMR resonance was
observed at d = À2 ppm that grew over the course of three
days as the signal at d = À29 ppm gradually disappeared.
Isolation and crystallization of this new product indicated the
formation of the SIDip-stabilized bis(boralactone) (7, Fig-
ure 2b). Stopping the reaction after 15 min allowed the
isolation of 6, and conversion into 7 could be halted by
holding the reaction at À308C. In the absence of a CO
atmosphere 6 slowly decomposed, even at low temperatures.
The conversion of 6 into 7 led us to believe that 5 might
likewise be pushed into forming a bis(boralactone) if heated
under a CO atmosphere; however, even at 1508C and under
50 bar of CO, no formation of the hypothetical compound II
Figure 1. a) Crystallographically determined solid-state structure of 3.
Thermal ellipsoids are set at 50% probability and have been omitted
for atoms on the organic periphery. Hydrogen atoms have likewise
been omitted, save those on the backbone of the NHCs. Selected
bond lengths [Š] and angles [8]: B1–B2 1.465(2), C1–B1 1.480(2), C2–
B2 1.482(2); Ccarbene-N 1.383 avg., B1-B2-C2 178.6(2), C1-B1-B2 179.6-
À
À
(2), N1-C1-N2 106.4(1), N4-C2-N3 106.4(1). b) The B C and B B
bond lengths [Š] in 1–3.
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Scheme 2. Syntheses of 4–7 and thermochemistry of the reaction
sequences. The DG values (kcalmolÀ1) have been calculated at
298.15 K at the M05-2x/6-311G(d) level of theory for the reaction of
the substrate with two molecules of CO to the subsequent products.
Figure 2. Crystallographically determined solid-state structures of a) 5
and b) 7. Thermal ellipsoids are set at 50% probability and have been
omitted for the ligand periphery. For 5: B1–B2 1.744(2), C3–B1
1.514(2), C2–B7 1.512(2), B1–C1 1.477(2), B2–C2 1.477(2), C1–O1
1.173(2), C2–O2 1.173(2); B1-C1-O1 170.0(2), B2-C2-O2 170.6(2), Ci-
B1-B2-C2 93.3(1). For 7: C3–B1 1.548(2), B1–C1 1.450(2), C1–O1
1.394(2), O1–C2 1.432(2), C2–O2 1.213(2); N1-C3-N2 109.1(1).
was observed. The unexpected stability of 5 is highlighted by
the fact that 5 was air and moisture stable, showing no change
when stored open on the benchtop for more than one day.
The solid-state structure of 5 (Figure 2a) is reminiscent of
the previously reported bis(isocyanide) adducts of 2,[15] while
= =
the B C O unit is similar to those found in Stephan and
bonding into the CO fragment as found in 5, as observed in
HOMO and HOMOÀ1 of both compounds (Figure S14a).
The p-systems depicted in these orbitals show delocalization
across both the CO and carbene moieties, but the degree to
which the orbital encompasses the carbene is greater in 5 than
in 6 (Figure S14a).
Bertrandꢀs CAAC-B(CO) compound[14] and
a recently
reported bis(carbonyl) arylborylene.[16] Each of these com-
pounds, with neutral boron bound to two Lewis bases, may be
drawn with formal lone pairs of electrons on boron, but their
planarity indicates delocalization of these electrons into
À
extended p systems. The B CCAAC bonds in 5 (1.514(2) Š,
The thermochemistry of the reaction sequence from
compounds 1–3 to their bis(boraketene) compounds and on
to their respective bis(boralactone) forms is indicative of the
electronic properties of the stabilizing carbenes. The initial
bis(boraketene) formation is the most favorable for 2
(À34.1 kcalmolÀ1), which is able to stabilize the extra pair
of electrons (formally residing at boron) through backdona-
tion into both CO and CAAC. NPA analysis of the three
bis(boraketene) complexes (I, 5, and 6) shows 5 to have the
least negative charge on boron (5 À0.27, 6 À0.31, I À0.33,
Figure S15). The formation of 6 from 3 is slightly less
favorable (À29.6 kcalmolÀ1) as the lone pair of electrons
cannot be distributed as freely into the carbene ligand, and
formation of the hypothetical I is the least favorable, since
IDip is the least acidic of the carbenes studied. The relative
stabilities of the bis(boraketene) compounds are further
reflected in the conversion into their respective bis(boralac-
tone) forms, where the opposite trend is observed. In this case,
the formation of 4 from I is highly exergonic (À30.5 kcal
1.512(2) Š) are short relative to CAAC bound to tetrahedral
boron (1,2-(CAAC)2-B2Br4, 1.754(5) Š),[10] indicating
a degree of multiple bond character, but less than in 2
where these bonds measure 1.459(2) and 1.458(2) Š.[10]
À
Multiple bond character is also observed in the B CCO
bonds, which at 1.477(2) Š are shorter that the bonds between
CO and tetrahedral boron (e.g., (C3F7)3B(CO) 1.660(3) Š,
(C2F5)3B(CO) 1.618(2) Š, 1.623(2) Š).[17,18] The slightly
greater length of the C ꢀ O bond (1.173(2) Š, 1.173(2) Š) in
comparison to CO bound to tetrahedrally coordinated boron
(1.105–1.124 Š)[17,18] is likewise indicative of pB!p*CꢀO back-
ꢀ
donation. This idea is borne out in the C O stretching
frequency (n˜ = 1928 cmÀ1), which is significantly decreased
from that of free CO (n˜ = 2143 cmÀ1), and is in line with the
stretching frequencies of CO ligands on electron-rich, late
transition metals, where M!COp* backbonding is substan-
tial.[19] The CO stretching frequency in 6 (n˜ = 1929 cmÀ1) is in
the same range, indicating an equivalent amount of back-
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molÀ1), while the formation of 7 from 6 is slightly less
favorable (À26.1 kcalmolÀ1), and the conversion of 5 to the
undetected compound II is essentially thermoneutral.
Keywords: boron · carbenes · main-group chemistry ·
multiple bonds · thermochemistry
How to cite: Angew. Chem. Int. Ed. 2015, 54, 13801–13805
Angew. Chem. 2015, 127, 14006–14010
Simply based on these numbers, it is perhaps surprising
that no formation of II is observed, as the energetic
equivalence of the two forms suggests an equilibrium between
the two and the possibility of pushing the reaction to II
forward under high CO pressures. The fact that II was not
observed led us to examine potential energetic barriers
blocking the formation of II from 5 by computationally
determining the mechanistic pathways from B2L2 to bis(bor-
alactone) (Figure S16–S18).[20] According to these pathways
the intermediate immediately following the bis(boraketene)
complex is a bis(m-carbonyl) arrangement wherein each CO
ligand bridges the two boron atoms (Scheme 3). Calculations
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indicated that this conversion is highly unfavorable for 5
(+ 23.8 kcalmolÀ1), though energetically favorable for 6
(À14.7 kcalmolÀ1) and I (À17.3 kcalmolÀ1). The trend in
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representing the most unfavorable by far, best explains why
the reaction halts at this point, even under large over-
pressures of CO. The large differences in these energies are
a direct result of the stabilization of 5 through delocalization
of electrons into both the CO and CAAC ligand.
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acidity in the construction of a complex with properties
between those of compounds stabilized by highly acidic
carbenes and relatively non-acidic carbenes. Beyond the
conventional spectroscopic comparisons of these three
related species, we have herein demonstrated how the
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a given reaction pathway.
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Acknowledgements
We acknowledge the Deutsche Forschungsgemeinschaft for
the generous financial support of this work. J.O.C.J.-H.
acknowledges Supercomputer Center of University of Gua-
najuato and CONACyT through project #123732.
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[20] For computational efficiency, the ONIOM method was used to
determine the mechanism of reaction from 1 to 4. The calculated
data was checked against pure DFT data to ensure viability of
the method (see Supporting Information). Unless otherwise
noted, all other calculated thermochemical and spectroscopic
data reported in the text have been determined by pure DFT
calculations as described in the Supporting Information and in
the text.
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À
[18] The B C length can also be compared to the CO adduct of
pentaperfluorophenylborole (1.609(3) Š) and (CF3)3B(CO)
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Received: July 10, 2015
Published online: October 1, 2015
Angew. Chem. Int. Ed. 2015, 54, 13801 –13805
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 13805