Access to B=S and B=Se double bonds via sulfur and selenium insertion into a B-H bond and hydrogen migration

Article abstract of DOI:10.1021/ja105021d

Stable compounds with a boron-chalcogen (S or Se) valence double bond have been prepared via sequences involving insertion of the chalcogen into a B-H bond and subsequent hydrogen migration. X-ray diffraction studies and density functional theory calculat

Full text of DOI:10.1021/ja105021d

Published on Web 07/22/2010
Access to BdS and BdSe Double Bonds via Sulfur and Selenium Insertion
into a B-H Bond and Hydrogen Migration
Hao Wang, Jianying Zhang, Hongfan Hu, and Chunming Cui*
State Key Laboratory of Elemento-organic Chemistry, Nankai UniVersity, Tianjin 300071, People’s Republic of China
Received June 9, 2010; E-mail: cmcui@nankai.edu.cn
that 2, by insertion of an electronegative fragment into the B-H bond,
may be synthetically useful as an LB equivalent since the resulting
Abstract: Stable compounds with a boron-chalcogen (S or Se)
valence double bond have been prepared via sequences involving
acidic hydrogen atom may migrate to the exocyclic methylene group.
insertion of the chalcogen into a B-H bond and subsequent
To prove the hypothesis, the reactions of 2 with sulfur and selenium
hydrogen migration. X-ray diffraction studies and density func-
tional theory calculations on the resulting compounds provide
convincing evidence for the boron-chalcogen multiple bonding.
were investigated, as sulfur and selenium are known to undergo
insertion reactions with a B-H bond. Reaction of 2 with 1 equiv of
elemental sulfur in n-hexane at room temperature proceeded slowly
to give an orange-yellow solution after stirring at room temperature
overnight, from which yellow crystals of 3 were obtained after workup.
The synthesis and isolation of main-group compounds contain-
ing multiple bonding to group 16 elements have attracted much
attention.1 Monomeric oxoboranes and their heavy analogues
RBE (E ) S, Se, Te) have long been known to be extremely
reactive and unstable under normal conditions. Recent isolation
and structural characterization of the oxoboryl complex trans-
[(Cy3P)2BrPt(BO)] (Cy ) cyclohexyl) and the AlCl3-stabilized
oxoborane [HC(CMe)2(NC6F5)2]BOfAlCl3 represent break-
throughs in oxoborane chemistry.2,3 However, even though
reports on boron-chalcogen double-bond species RBE as
reactive intermediates4 and high-temperature gas-phase species5
have appeared, such room-temperature-stable compounds are still
elusive, probably because of their intrinsic high reactivity and
the lack of suitable synthetic approaches. Herein we report on
the synthesis and isolation of the first isolable monomeric thioxo-
and selenoxoboranes featuring well-defined BdE (E ) S, Se)
double bonds from an N-heterocyclic borane via sequences
involving sulfur and selenium insertion into the B-H bond and
subsequent hydrogen migration, as shown in Scheme 1.
Bulky ꢀ-diketiminato ligands have proved to be effective for the
stabilization of heavy group 13 element carbene analogues,6 but the
analogous boron compounds have not been obtained to date. Theoreti-
cal calculations indicate that such boron analogues should have a very
small singlet-triplet energy gap and be extremely reactive, unlike their
heavy analogues.7 In the course of our studies on the boron analogues,
we attempted to generate the species by dehydrohalogenation of the
ꢀ-diketiminato chloroborane LBHCl (1, L ) [HC(CMe)2(NAr)2]-, Ar
) 2,6-Me2C6H3) with N-heterocyclic carbenes following the protocol
for the synthesis of N-heterocyclic silylenes developed by us.8 Thus,
the desired chloroborane 1 was synthesized by the reaction of LLi9
with Me2S·BHCl2 in diethyl ether. Treatment of 1 with 1 equiv of
1,4-di(tert-butyl)imidazol-2-ylidene10 in THF indeed yielded a dehy-
drohalogenated product, as indicated by the formation of the insoluble
imidazolium salt. Analysis of the resulting soluble product by multi-
nuclear NMR spectroscopy, however, indicated the formation of borane
2 (Scheme 1) rather than the expected boron carbene analogue LB.
Apparently, one of the ꢀ-methyl groups on the ꢀ-diketiminato ligand
backbone is deprotonated during the reaction. The similar dehydro-
halogenation of ꢀ-diketiminato silicon and germanium halides has been
reported to yield novel N-heterocyclic silylene and germylene species,
respectively.11a,b 2 can be considered as the intramolecular C-H
activation product of the isomeric monovalent species LB. We reasoned
The reaction with selenium was carried out at 70 °C in toluene, and
the selenoxoborane 4 was obtained as orange crystals after crystal-
lization from THF. Compounds 3 and 4 are extremely air- and
moisture-sensitive. They were fully characterized by 1H, 13C, and 11
B
NMR spectroscopies, elemental analysis, and single-crystal X-ray
analysis.
Scheme 1
The 1H and 13C NMR spectra of 3 and 4 showed the characteristic
patterns for the ꢀ-diketiminato ligand L, indicating that the exocyclic
CdCH2 group in 2 had been hydrogenated. The 11B NMR spectra
of 3 and 4 exhibited broad resonances at 36.79 and 40.88 ppm,
respectively, which are downfield-shifted relative to the value of
29.3 ppm found in hydroborane 2 but fall in the range for three-
coordinate diamino-substituted boron species.12 The BdS and
BdSe stretching vibration modes of 3 and 4 (1161 and 1136 cm-1
)
Figure 1. ORTEP drawing of 4 with 30% probability ellipsoids. Hydrogen
atoms have been omitted for clarity. Selected bond lengths (Å) and angles
(deg) for 3: B1-S1, 1.741(2); B1-N1, 1.483(3); B1-N2, 1.484(3);
N1-B1-S1, 124.35(18); N2-B1-S1, 124.13(18); N1-B1-N2, 111.52(18).
For 4: B1-Se1, 1.896(4); B1-N1, 1.489(6); B1-N2, 1.476(6); N1-C2,
1.358(5);C2-C3,1.406(7);C3-C4,1.365(7);N2-C4,1.343(5);N1-B1-Se1,
123.8(4); N2-B1-Se1, 123.6(4); N1-B1-N2, 112.6(3).
9
10998 J. AM. CHEM. SOC. 2010, 132, 10998–10999
10.1021/ja105021d  2010 American Chemical Society

C O M M U N I C A T I O N S
observed in their IR spectra are well-correlated with the theoretical
predictions (see Figures S3 and S4 in the Supporting Information).
These spectroscopic data are consistent with the structures of 3
and 4. The molecular structures of 3 and 4 were unambiguously
verified by single-crystal X-ray analysis.
and π orbitals (1.97e) for the B-S double bond, with the π orbital
being strongly polarized toward the sulfur atom (77.31%). The two B-N
σ bonds are formed by the high-p-character hybrid orbitals (sp2.52) of the
boron atom with the sp1.61 hybrid orbitals of the nitrogen atoms. Inspection
of the frontier Kohn-Sham orbitals (Figure 2) shows that the HOMO
corresponds mainly to the sulfur lone pair and the HOMO-1 to the B-S
π bond with the ligand π* components. The calculated 11B and 77Se NMR
chemical shifts are in good agreement with the experimental values (see
Table S2 in the Supporting Information).
In summary, the first stable molecular compounds with a
boron-chalcogen valence double bond have been prepared via
insertion of chalcogen into a B-H bond followed by hydrogen
migration through the use of a bulky ꢀ-diketiminato ligand. 3 and
4 feature short B-E distances and a trigonal-planar geometry
around the boron atom, thereby providing convincing evidence for
the boron-chalcogen multiple bonding. Reactivity studies of these
compounds and extension of the synthetic route to other systems
are currently in progress.
Crystals of 3 and 4 suitable for X-ray diffraction studies were
obtained from THF at -40 °C. Compounds 3 and 4 are isomorphous
and have similar molecular geometries. The structure of 4 is
representatively shown Figure 1, where selected bond parameters for
3 and 4 are given. Both 3 and 4 are monomeric, with terminal S and
Se atoms, respectively. The B-S bond length of 1.741(2) Å is
significantly shorter than the sum of the covalent radii of boron and
sulfur (∼1.87 Å) and shorter than those in the three-coordinate boron
compounds containing a B-SR bond (1.77-1.805 Å),13 in which lone-
pair donation from the sulfur atom to the boron atom may exist. The
B1-Se1 bond length [1.896(4) Å] in 4 is shorter than the B-Se bond
lengths (1.960-2.13 Å) found in a small number of structurally
characterized organic compounds containing a B-Se bond.4c-e,14 The
geometry around the boron atom in both 3 and 4 was found to be
approximately trigonal-planar [the sum of the angles ) 360.00(18)°
for 3 and 360.0(4)°for 4]. These structural features are consistent with
the multiply bonded character of the B-S(Se) bond.
The four B-N bond lengths [1.483(3) and 1.484(3) Å in 3;
1.489(6) and 1.476(6) Å in 4] are almost equal to each other, and
the C-C and C-N bonds in the planar B-N-C-C-C-N ring
tend to equalize in each of the two molecules; this indicates the
electron delocalization in the N-C-C-C-N backbone. The B-N
bond lengths in 3 and 4 are within the reported range for
ꢀ-diketiminato boron compounds.3,15
Although π bonding between boron and sulfur has been proposed
in the three-coordinate boron compounds having the formula R2BSR
(R ) alkyl, aryl),1a compounds 3 and 4 represent the first well-defined
stable compounds containing a valence double bond between boron
and heavy chalcogens. Because of the electron delocalization in the
central six-membered heterocycle, there is no structural distinction
between the dative and covalent B-N bonds that can be observed.
Nevertheless, 3 and 4 can be formally described as having an intramo-
lecular donor-stabilized boron-chalcogen double bond. The syntheses of
several N-heterocyclic carbene-stabilized silanechalcogenones supported
by a closely related N-heterocyclic ligand via chalcogenation of the
corresponding silylenes have recently been reported.16
Acknowledgment. We are grateful to the National Natural Science
Foundation of China (Grant 20725205) for the support of this work.
Supporting Information Available: Experimental details for the
synthesis and characterization of the compounds in this paper, CIFs
for 3 and 4, and calculation details. This material is available free of
charge via the Internet at http://pubs.acs.org.
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