Planar, twisted, and trans-bent: Conformational flexibility of neutral diborenes

Article abstract of DOI:10.1021/ja800257j

The potassium graphite reduction of R-BBr₃ (R- = :C{N(2,4,6-Me₃C₆H₂)CH}₂) in Et₂O led to the isolation of 3 (R-(H)B=B(H)R-) and 4 (R-(H)₂B-B(H)₂R-), with B=B double an

Full text of DOI:10.1021/ja800257j

Published on Web 02/21/2008
Planar, Twisted, and Trans-Bent:
Conformational Flexibility of Neutral Diborenes
Yuzhong Wang, Brandon Quillian, Pingrong Wei, Yaoming Xie, Chaitanya S. Wannere,
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 January 11, 2008; E-mail: robinson@chem.uga.edu
Realization of the fascinating potential of boron homonuclear
multiple bond chemistry has long frustrated chemists.1-3 Boron-
boron double bonds are represented by two olefin-like classes of
Et2O resulted in isolation of red-colored 3 (15.8%), together with
colorless 4, R′(H)2B-B(H)2R′. Reduction using a R′BBr3:KC8 ratio
of 1:6.2 only resulted in 4. Similar to the formation of 1 and 2,17
the preparation of 3 and 4 involves the well-documented hydrogen
abstraction from ethereal solvents in the presence of alkali metals.
Both 3a, as black red crystals, and 3b, as ruby-colored crystals,
were isolated from the 1:5 Et2O/hexane solvent mixture, while 3c
was crystallized from the parent Et2O solution. Despite their three
different conformations in the solid state, 3a-c, exhibit identical
1H and 11B NMR spectra in C6D6 solution. Furthermore, the broad
singlet 11B NMR resonance of 3 (+23.45, w1/2 ) 587 Hz)
corresponds to that of diborene 1 (+25.30, w1/2 ) 946 Hz). The
1H NMR imidazole resonances of 3 and 4 are 5.96 and 5.91,
respectively. There is no evidence for different isomers or states
of 3 in solution. We conclude that 3a-c are polymorphssthe same
compound crystallizing in different forms.22 The space groups for
3a-c are P21/c, P-1, and P21/c, respectively, and their packing
patterns are completely different.18
The 11B signal of 4 (-31.20) is a triplet (JBH ) 83.38 Hz) like
that of diborane 2 (-31.62).17 The core of 4 consists of two
tetrahedral C(H)2B units connected by a boron-boron single bond
(1.795(5) Å).18 Evidently due to the smaller steric repulsion between
the carbene ligands, the B-B distance in 4 is shorter than that in
2 (1.828(4) Å).
The trans-bent C(H)BdB(H)C boron-boron double bond is the
most remarkable structural feature of 3c (Figure 1). Its trans-bending
angle, θ ) 36°, is the same as that in the heavier Group 14 ethylene
congener, [R(Mes)GedGe(Mes)R] (R ) 2,6-Pri2C6H3).23 The
central BdB bond distance in 3c (1.679(9) Å) is 0.116 Å shorter
compounds: (1) the isoelectronic diboron dianions, [R2BBR2]2-
,
and (2) the Lewis base-stabilized neutral diborene complexes,
L(H)BdB(H)L (L ) Lewis base). Although diboron dianions and
their alkali metal salts were proposed as promising BdB double
bond candidates two decades ago,4 corroborating synthetic and
structural evidence has been rare.5-7 In contrast, neutral Lewis base-
stabilized diborenes are attractive alternatives. While the highly
reactive parent neutral diborene(2), HBdBH,8 has only been
characterized in matrices,9 complexation with appropriate Lewis
base ligands is a promising approach to viable L(H)BdB(H)L
derivatives. Although the theoretical development of BCO
chemistry10-14 included the computational prediction of the car-
bonyl-stabilized diborene, OC(H)BdB(H)CO,12 such complexes
have not been experimentally realized. In this regard, bulky
N-heterocyclic carbene (NHC) ligands are attractive due to their
strong electron-donating properties coupled with their ability to
provide effective protection to the HBdBH core.15,16
Our recent potassium graphite reduction of RBBr3 (R )
:C{N(2,6-Pri2C6H3)CH}2) afforded R(H)BdB(H)R, 1, the first
structurally characterized neutral diborene as well as a diborane
complex, R(H)2B-B(H)2R, 2.17 We now utilize a less bulky NHC
ligand (R′ ) :C{N(2,4,6-Me3C6H2)CH}2) to prepare the second
neutral diborene, R′(H)BdB(H)R′, 3, as well as the corresponding
R′(H)2B-B(H)2R′ diborane, 4. In contrast to planar diborene 1,
the new diborene, 3, exhibits remarkable conformational variations
in the solid state. X-ray determinations of three different crystals
reveal not only planar (3a) but also twisted (3b) and trans-bent
(3c) molecular structures! Herein we report these results and the
computational examination of 3 and 4.18,19
While trans-bent geometries of the heavier group 13 dianionic
alkene analogues, [H2EdEH2]2- (E ) Al, Ga, In), are predicted to
be favored over planar alternatives,20 both diboron dianions (E )
B)4-7 and the Lewis base-stabilized neutral diborenes (1 and
OC(H)BdB(H)CO)12,17 prefer planar geometries. Hence, the twisted
(3b) and trans-bent (3c) structures of 3 are unexpected. The
pyramidal tricoordinate boron atoms in 3c contrast with the
predominant trigonal planar geometries. Indeed, pyramidal boron
environments have only been reported in cyclic systems.21
Figure 1. Molecular structures of 3b and 3c (thermal ellipsoids represent
30% probability; hydrogen atoms on carbon are omitted for clarity). Selected
bond distances (Å) and angles (deg): For 3b, B(1)-B(2) 1.582(4), B(1)-
C(1) 1.541(4), B(2)-C(22) 1.541(4), B(1)-H(1) 1.117(17), B(2)-H(2)
1.12(3); C(1)-B(1)-B(2) 125.0(2), C(1)-B(1)-H(1) 109.9(16), B(2)-
B(1)-H(1) 124.9(16), C(22)-B(2)-B(1) 125.1(2), C(22)-B(2)-H(2)
107.0(15), B(1)-B(2)-H(2) 127.2(15). For 3c, B(1)-B(1A) 1.679(9),
B(1)-C(1) 1.565(5), B(1)-H(1) 1.109(18); C(1)-B(1)-B(1A) 118.6(5),
C(1)-B(1)-H(1) 107.7(19), H(1)-B(1)-B(1A) 118(2).
Our earlier study showed that the RBBr3:KC8 ratio affects the
diborene yield.17 The reaction of R′BBr3 with KC8 in a 1:5 ratio in
9
3298
J. AM. CHEM. SOC. 2008, 130, 3298-3299
10.1021/ja800257j CCC: $40.75 © 2008 American Chemical Society

C O M M U N I C A T I O N S
reported for 1 (1.408 and 1.656, respectively), support the presence
of a BdB double bond in 3c despite its ca. 0.1 Å boron-boron
elongation and trans-bent geometry. The distortion exhibited by
3c does not decrease the boron-boron bond order substantially and
supports the dictum “the electronic structure, rather than bond
distances, determines the nature of multiple bonds”.30
The experimental realization of three distinct polymorphic
structures of diborene 3 may be attributed to a combination of,
inter alia, packing effects in the crystal, crystallizing-solvent
polarity, and intramolecular ligand steric effects.
Figure 2. Representation of the frontier orbitals of trans-bent 3c.
than that of the corresponding B-B single bond of 4 (1.795(5) Å),
but it is about 0.1 Å longer than those in 1 (1.560(18) Å, av), in
dianionic (tetraamino)diborates7 (1.566(9) to 1.59(1) Å), and in
OC(H)BdB(H)CO (1.590 Å, computed).12 Notably, the BdB bond
distance of 3c is only about 0.05 Å longer than in [Mes2BB-
(Mes)Ph]2- (1.636(11) Å)5 and [{Ph(Me2N)BB(NMe2)Ph}]2- (1.627
Å, av).6 Each boron atom in 3c is pyramidal with a 344.3° bond
angle sum. As far as we are aware, 3c is the first example of
pyramidal tricoordinate boron in an acyclic environment. The cyclic
silaborirane, CH2SiH2BH,24 and its analogs have been computed
to have pyramidal geometries due to heteroatom-boron p orbital
interactions. Constrained systems like 1-boraadamantane25 neces-
sarily have nonplanar boron geometries.
In contrast to the trans-bent structure of 3c, isomer 3a possesses
the same planar C(H)BdB(H)C core as observed in 1.18 Each boron
atom in 3b (Figure 1) also has a planar tricoordinate environment.
However, 3b adopts a twisted geometry with a 18.1° dihedral angle
between the two CBH planes. The BdB double bond distance of
3b (1.582(4) Å) is similar to those of 1 (1.560(18) Å (av)) and 3a
(1.602(5) Å). Remarkably, the BdB bond distance in the crystal
structure of 3c (1.679(9) Å) is about 0.1 Å longer. The boron-
boron double bond character of 3 is further supported by the
πBdB-π*BdB absorption (λmax ) 574 nm).
Acknowledgment. We are grateful to the National Science
Foundation (Grants CHE-0209857, CHE-0451445, and CHE-
0608142) for support.
Supporting Information Available: 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.
References
(1) Cowley, A. H. J. Organomet. Chem. 2004, 689, 3866-3872.
(2) Zhou, M.; Tsumori, N.; Li, Z.; Fan, K.; Andrews, L.; Xu, Q. J. Am. Chem.
Soc. 2002, 124, 12936-12937.
(3) Maier, C.-J.; Pritzkow, H.; Siebert, W. Angew. Chem., Int. Ed. 1999, 38,
1666-1668.
(4) Kaufmann, E.; Schleyer, P. v. R. Inorg. Chem. 1988, 27, 3987-3992.
(5) Moezzi, A.; Olmstead, M. M.; Power, P. P. J. Am. Chem. Soc. 1992,
114, 2715-2717.
(6) Moezzi, A.; Bartlett, R. A.; Power, P. P. Angew. Chem., Int. Ed. 1992,
104, 1082-1083.
(7) Noth, H.; Knizek, J.; Ponikwar, W. Eur. J. Inorg. Chem. 1999, 1931-
1937.
(8) Dill, J. D.; Schleyer, P. v. R.; Pople, J. A. J. Am. Chem. Soc. 1975, 97,
3402-3409.
(9) Knight, L. B., Jr.; Kerr, K.; Miller, P. K.; Arrington, C. A. J. Phys. Chem.
1995, 99, 16842-16848.
The B3LYP/6-311+G** DFT optimization of 3c,19 starting with
the X-ray coordinates, led to a planar geometry and a BdB bond
distance of 1.605 Å, essentially identical with the experimental value
(1.602(5) Å) of planar 3a. The polymorphism exhibited by 3 may
be attributed to the combination of a number of factors including
(1) the flat potential energy surface; (2) the packing effects in
crystals;26,27 (3) the polarity of the solvent used for crystallization;
and (4) the intramolecular steric repulsion of the carbene ligands.
The different packing patterns of 3a-c suggest that the associated
distinct packing effects may contribute to the stabilization of these
polymorphs.18 Differences in solvent polarity are known to
significantly affect conformational isomerism of molecular28 and
supramolecular29 systems. Indeed, 3a and 3b were isolated from
1:5 Et2O/hexane solvent mixtures, whereas 3c was crystallized from
pure Et2O. Compared to the more sterically demanding ligands in
1, the smaller ligands in 3 can adjust their orientations more easily
to packing forces.
Our numerous computations19 employing simplified ligand
models R′′(H)BdB(H)R′′ (R′′ ) :C(NRCH)2, with R ) H or CH3)
confirmed the flatness of the potential energy surface. The planar
3a models had C2h symmetry. The Ci models for trans-bent 3c
optimized to the same C2h geometries. The only minimum (in C2
symmetry) corresponding to 3b (R ) CH3) had a small planarization
barrier. Consequently, the X-ray coordinates of 3c were used for
the MO model shown in Figure 2. Boron-boron π-bonding
dominates the HOMO, while the HOMO-1 has mixed B-B and
B-H σ bonding character (Figure 2). The Wiberg (1.445) and
NLMO/NPA (1.515) B-B bond indices, comparable to those
(10) Wu, H.-S.; Jiao, H.; Wang, Z.-X.; Schleyer, P. v. R. J. Am. Chem. Soc.
2003, 125, 4428-4429.
(11) Wu, H.-S.; Qin, X.-F.; Xu, X.-H.; Jiao, H.; Schleyer, P. v. R. J. Am. Chem.
Soc. 2005, 127, 2334-2338.
(12) Wang, Z.-X.; Chen, Z.; Jiao, H.; Schleyer, P. v. R. J. Theor. Comput.
Chem. 2005, 4, 669-688.
(13) Qin, X.-F.; Wu, H.-S.; Jiao, H. J. Mol. Struct. 2007, 810, 135-141.
(14) Qin, X.-F.; Wu, H.-S.; Jiao, H. J. Mol. Model. 2007, 13, 927-935.
(15) Scott, N. M.; Nolan, S. P. Eur. J. Inorg. Chem. 2005, 1815-1828.
(16) Arduengo, A. J., III Acc. Chem. Res. 1999, 32, 913-921.
(17) Wang, Y.; Quillian, B.; Wei, P.; Wannere, C. S.; Xie, Y.; King, R. B.;
Schaefer, H. F., III; Schleyer, P. v. R.; Robinson, G. H. J. Am. Chem.
Soc. 2007, 129, 12412-12413.
(18) See the Supporting Information for synthetic and crystallographic details.
(19) Computations: The DFT calculations of 3 and 3c were performed at the
B3LYP/6-311+G** level with the Gaussian 03 program (computational
details are in the Supporting Information): Frisch, M. J.; et al. Gaussian
03, reVision C.02; Gaussian, Inc.: Wallingford, CT, 2004.
(20) Bridgeman, A. J.; Nielsen, N. A. Inorg. Chim. Acta 2000, 303, 107-115.
(21) Bouhadir, G.; Bourissou, D. Chem. Soc. ReV. 2004, 33, 210-217.
(22) Bernstein, J. Polymorphism in Molecular Crystals; Oxford University
Press: Oxford, U.K., 2002.
(23) Power, P. P. J. Chem. Soc., Dalton Trans. 1998, 2939-2951.
(24) Giju, K. T.; Phukan, A. K.; Jemmis, E. D. Angew. Chem., Int. Ed. 2003,
42, 539-542.
(25) Wrackmeyer, B.; Milius, W.; Tok, O. L.; Bubnov, Y. N. Chem. Eur. J.
2002, 8, 1537-1543.
(26) Steed, J. W.; Atwood, J. L. Supramolecular Chemistry, 1st ed.; John Wiley
& Sons, Ltd.: Chichester, U.K., 2000.
(27) Steed, J. W. In Frontiers in Crystal Engineering, 1st ed.; Tiekink, E. R.
T., Vittal, J., Eds.; John Wiley & Sons, Ltd.: Chichester, U.K., 2006; pp
67-90.
(28) Kitamura, M.; Gao, G.; Nakajima, K.; Takahashi, T. Chem. Lett. 2002,
1076-1077.
(29) Huang, X.-C.; Zhang, J.-P.; Lin, Y.-Y.; Chen, X.-M. Chem. Commun.
2005, 2232-2234.
(30) Xie, Y.; Grev, R. S.; Gu, J.; Schaefer, H. F.; Schleyer, P. v. R.; Su, J.; Li,
X.-W.; Robinson, G. H. J. Am. Chem. Soc. 1998, 120, 3773-3780.
JA800257J
9
J. AM. CHEM. SOC. VOL. 130, NO. 11, 2008 3299