A stable neutral diborene containing a B=B double bond

Article abstract of DOI:10.1021/ja075932i

The syntheses and molecular structures of four compounds are reported: 1 (RBBr₃), 2 (R(H)₂B-B(H)₂R), 3 (R(H)B=B(H)R), and 4 (RBH₃) (R = :C{N(2,6-Prⁱ₂C₆H₃)CH}₂

Full text of DOI:10.1021/ja075932i

Published on Web 09/21/2007
A Stable Neutral Diborene Containing a BdB Double Bond
Yuzhong Wang, Brandon Quillian, Pingrong Wei, Chaitanya S. Wannere, Yaoming Xie,
R. Bruce King, Henry F. Schaefer, III, Paul v. R. Schleyer, and Gregory H. Robinson*
Department of Chemistry and the Center for Computational Chemistry, The UniVersity of Georgia,
Athens, Georgia 30602-2556
Received August 7, 2007; E-mail: robinson@chem.uga.edu
While carbon-carbon double bonds (I) are ubiquitous the
corresponding multiple-bond chemistry of boron, the group 13
neighbor of carbon, is undeveloped.1,2 Although the isoelectronic
diboron dianions, [R2BBR2]2- (II), and their alkali metal salts were
predicted two decades ago to be viable BdB double-bond candi-
dates,3 subsequent corroborating synthetic and structural efforts have
been sparse.4-6 Neutral diborenes(2), based on the parent (III), are
less attractive boron-boron double-bond alternatives as they have
been predicted to be highly reactive, with triplet ground states and
two one-electron π-bonds.7 Nevertheless, the synthesis and structural
characterization of stabilized derivatives of (III) is a fascinating
challenge.1,8 The electron deficiency of the boron atoms in (III)
invites complexation by Lewis base ligands (IV).
Figure 1. Molecular structure of 2 (thermal ellipsoids represent 30%
probability; hydrogen atoms on carbon omitted for clarity). Selected bond
distances (Å) and angles (deg): B(1)-B(1A) 1.828(4), B(1)-C(1) 1.577(2),
B(1)-H(1) 1.155(18), B(1)-H(2) 1.147(19); B(1A)-B(1)-C(1) 107.45(16),
B(1A)-B(1)-H(1) 110.7(9), B(1A)-B(1)-H(2) 110.3(9), C(1)-B(1)-
H(1) 108.9(9), C(1)-B(1)-H(2) 108.1(10), H(1)-B(1)-H(2) 111.3(13).
documented hydrogen abstraction from ethereal solvents13 in the
presence of alkali metals.14-16
The analogy between neutral ligated BdB double bonded
compounds, (IV), and ethene derivatives (I) is compelling. Indeed,
the isolobal relationship between CH and BCO groups has led to
the extensive computational development of BCO chemistry.9 Thus,
OC(H)BdB(H)CO ((IV), L: ) CO) and ethene are isolobal.9c The
ethyne analogue, OCBBCO, has been characterized by FTIR in
matrix isolation.2 Other Lewis bases, L: in (IV), particularly bulky,
sterically demanding N-heterocyclic carbene (NHC) ligands, are
quite intriguing owing to their high stability and strong electron-
donor capabilities.10 We now report the experimental realization
and molecular structure11 of R(H)BdB(H)R (R ) :C{N(2,6-
Pri2C6H3)CH}2), 3. Significantly, compound 3 is the first structurally
characterized neutral diborene containing a BdB double bond. The
nature of this BdB double bond is further delineated by density
functional theory (DFT) computations.
We employed carbenes as stabilizing ligands in organo-group
13 chemistry over a decade ago with the synthesis and structural
determination of R′:M(CH3)3 (R′ ) :C{N(Pri)C(CH3)}2; M ) Al,
Ga).12 Extending this work, we allowed RBBr3, 1,11 to react with
KC8 in diethyl ether and isolated two products: 2, R(H)2B-B(H)2R,
as air-stable, colorless block crystals, and 3 as air-sensitive, orange-
red sheet-like crystals (eq 1).
We also prepared the carbene:borane adduct, R:BH3, 4. The 11
B
NMR resonances of 4 (RBH3), 2 (R(H)2B-B(H)2R), and 3
(R(H)BdB(H)R) are -35.38, -31.62, and +25.30 ppm, respec-
tively. The 11B signal of 4 is a quartet (JBH ) 83.38 Hz), while 2
displays a singlet with shoulders (w1/2 ) 188 Hz) and 3 displays a
broad singlet (w1/2 ) 946 Hz). The 1H NMR imidazole resonances
of 4, 2, and 3 are 6.31, 6.21, and 6.14 ppm, respectively.
X-ray structural analysis reveals that 2 has a center of symmetry
about the (H)2B-B(H)2 core (Figure 1). The hydrides (B-H) in 2,
3, and 4 were located in the difference Fourier map. The B-B
bond distance in 2 (1.828(4) Å) compares well to that computed
for the CO-ligated analogue OC(H)2BB(H)2CO (1.819 Å)9c and to
those in an activated m-terphenyl based diborate (1.83(2) Å)18 as
well as a 2,3-diboratabutadiene dianion (1.859(8) Å).19 However,
the bond distance in 2 is longer than those in three-coordinate
diboron compounds (1.682(16) to 1.762(11) Å).17 The boron atoms
in 2 reside in tetrahedral geometries. The N2C3 ring of the NHC
ligand is almost perpendicular to the B-B-C plane with a N(1)-
C(1)-B(1)-B(1A) torsion angle of -89.3°. The B-C bond
distance in 2 (1.577(2) Å) is somewhat shorter than that in 1
(1.623(7) Å), but is similar to that in 4 (1.585(4) Å).
KC8
3 crystallizes in the orthorhombic space group P212121 (No. 19).
Each asymmetric unit contains two independent, and nearly
identical, molecules of 3 (Figure 2; only one molecule of 3 is
shown). The B-C bond distances, 1.547(15) Å (av), are marginally
shorter than those of 1, 2, and 4. Moreover, in contrast to 2, one
C3N2 carbene ring of 3 is nearly coplanar with the B2H2 core (N(1)-
C(1)-B(1)-B(2) torsion angle, -13.8°), while the other is stag-
gered more (N(4)-C(28)-B(2)-B(1) torsion angle, -30.0°). The
R(H) B-B(H) R + R(H)BdB(H)R
RBBr3
Et O8
(1)
2
2
2
1
2
3
The stoichiometric ratio of 1 to KC8 has been observed to affect
the yield of 3. A higher yield of 3 (12%) was obtained with a
stoichiometric 1:KC8 ratio of 1:5.4. Greater amounts of KC8
decrease the yield of 3. At a 1:KC8 ratio of 1:9, only 2 was isolated.
The unexpected formation of 2 and 3 appears to involve the well-
9
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J. AM. CHEM. SOC. 2007, 129, 12412-12413
10.1021/ja075932i CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
orbital (LMO)20 representation of the B-B σ bond is shown in the
Supporting Information). Natural bond orbital (NBO) electron
occupancies of the B-B σ- and π-bonding orbitals in 3a are 1.943
and 1.382, respectively. The Wiberg and NLMO/NPA B-B bond
indices, 1.408 and 1.656, respectively, also document the BdB
double bond character in 3a.
The computed boron-boron Wiberg bond indexes along the
OC(H)2B-B(H)2CO (ethane-like), OC(H)BdB(H)CO ((IV), L: )
CO) (ethene-like), and OCBBCO (ethyne-like) series, 0.870, 1.308,
and 1.953, respectively, are instructive. The 1.0, 2.0, 3.0 unit bond-
order values of the hydrocarbon series are not to be expected for
the corresponding boron-boron analogues owing to the resonance
contributions of Lewis structures. Nevertheless, the single-, double-,
and triple-bond descriptions of boron-boron bonds discussed here
are appropriate.
In summary, we have synthesized and characterized the first
stable neutral diborene and computationally probed the nature of
the novel boron-boron double bond. Related studies on the
chemistry of boron-boron multiple bonds are ongoing.
Figure 2. Molecular structure of 3 (thermal ellipsoids represent 30%
probability; hydrogen atoms on carbon omitted for clarity). Selected bond
distances (Å) and angles (deg): B(1)-B(2) 1.561(18), B(1)-C(1) 1.543(15),
B(1)-H(1) 1.14(2), B(2)-C(28) 1.532(15), B(2)-H(2) 1.13(2); B(2)-
B(1)-C(1) 128.3(12), B(2)-B(1)-H(1) 124(4), C(1)-B(1)-H(1) 107(4),
B(1)-B(2)-C(28) 126.1(12), B(1)-B(2)-H(2) 128(4), C(28)-B(2)-H(2)
105(4).
Acknowledgment. We are grateful to the National Science
Foundation (Grants CHE-0608142 and CHE-0209857) for support.
Note Added after ASAP Publication. After this paper was
published ASAP September 21, 2007, production errors were fixed
in the graphics showing structures (I)-(IV) and eq 1. The corrected
version was published ASAP September 25, 2007.
Figure 3. Representation of the HOMO and HOMO-1 orbitals of 3a.
three-coordinate boron atoms in 3 adopt trigonal planar geometries.
The most notable feature of 3, however, is the BdB bond. The
BdB bond distance of 1.560(18) Å (av) in 3 is not only
considerably shorter than the B-B distance in 2 (1.828(4) Å), but
also shorter than those reported for [Mes2BB(Mes)Ph]2- (1.636-
(11) Å)4 and for [{Ph(Me2N)BB(NMe2)Ph}]2- (1.627 Å (av)),5
which purportedly contained a “strong B-B π-bond”. Furthermore,
the BdB bond distance in 3 compares well to those in dianionic
tetra(amino)diborates (1.566(9) to 1.59(1) Å)6 and to the computed
BdB bond lengths for the OC(H)BdB(H)CO ((IV), L: ) CO)
analogue (1.590 Å)9c and for diborene(2), (III)3 (1.498-1.515 Å).
The computed B-B distance of 1.45 Å reported for OCBBCO, a
compound “with some triple bond character”, is shorter.2,9c Notably,
the B-B bond distance difference of 0.27 Å between 2 and 3 is
comparable to the corresponding difference (about 0.2 Å) between
ethane and ethene. Likewise, the C-C bond distance difference of
0.1 Å between ethene and ethyne corresponds to the difference
between 3 and OCBBCO (0.11 Å).2,9c Thus, the structural details
of 3 are consistent with a BdB double bond.
The nature of 3 was investigated by performing B3LYP/6-
311+G** DFT computations20 on the simplified R(H)BdB(H)R
(R ) :C(NHCH)2) model, 3a (Figure 3). Both 3a and the OC(H)Bd
B(H)CO ((IV), L: ) CO) analogue9c are planar and have C2h
symmetry, whereas the corresponding R moieties in 3 are twisted
because of the greater steric demands of the very bulky N(aryl)
ligands. The computed B-B bond lengths in 3a (1.591 Å) and (IV)
(L: ) CO) (1.590 Å)9c are virtually identical and are close to the
error bound of the corresponding experimental distance of 3
(1.561 (18) Å). The B-C length (1.547 (15) Å (av)) of 3 also agrees
with the computed value (1.531 Å) for 3a. Perhaps due to reduced
steric repulsion between the ligands, the B-B-C bond angle in
3a (120°) is less than the average value in 3, 126.7(12)°.
Supporting Information Available: Complete ref 20, full details
of the syntheses, computations, and X-ray crystal determination,
including the cif files. This material is available free of charge via the
Internet at http://pubs.acs.org.
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