Boron-pnictogen multiple bonds: Donor-stabilized P=B and As=B bonds and a hindered iminoborane with a B-N triple bond

Article abstract of DOI:10.1021/ic062076n

Reaction of the hindered phosphino- and arsinoboranes, Ar*Pn(H)-B(Br) Tmp (Ar* = -C₆H₃-2,6-(C₆H ₂-2,4,6-ⁱPr₃)₂; Tmp = 2,2,6,6-tetramethylpiperidino; Pn = P and As, 1 and 3, respectively) with 4-dimethylaminopyridine, DMAP, afforded the boranylidenephosphane and arsane, Ar*Pn=B(DMAP)Tmp (Pn = P and As, 2 and 4) as deep red-purple solids. The analogous aminoboranes Ar′N(H)-B(X)Tmp (Ar′ = -C₆H ₃-2,6-(C₆H₂-2,4,6-Me₃)₂; X = Cl and Br; 5 and 6) did not display any reactivity with DMAP, but in the presence of the amide base, Na[N(SiMe₃)₂], the clean formation of the uncomplexed iminoborane Ar′N≡BTmP (7) was observed. Attempts to generate an Sb=B bond were unsuccessful, as the required stibinoborane precursor, Ar*Sb(H)-B(Br)Tmp, could not be prepared; in place of clean Sb-B bond formation, the reduced product Ar*Sb=SbAr* was obtained. All compounds were characterized spectroscopically, and the X-ray crystal structures of 1, 2, 4, 6, and 7 were determined.

Full text of DOI:10.1021/ic062076n

Inorg. Chem. 2007, 46, 2971−2978
Boron
−Pnictogen Multiple Bonds: Donor-Stabilized P
dB and AsdB
Bonds and a Hindered Iminoborane with a B
−N Triple Bond
Eric Rivard, W. Alexander Merrill, James C. Fettinger, Robert Wolf, Geoffrey H. Spikes, and
Philip P. Power*
Department of Chemistry, UniVersity of California, DaVis, California 95616
Received October 30, 2006
Reaction of the hindered phosphino- and arsinoboranes, Ar*Pn(H)
Tmp 2,2,6,6-tetramethylpiperidino; Pn P and As, 1 and 3, respectively) with 4-dimethylaminopyridine, DMAP,
afforded the boranylidenephosphane and arsane, Ar*Pn B(DMAP)Tmp (Pn P and As, 2 and 4) as deep red-
purple solids. The analogous aminoboranes Ar N(H) B(X)Tmp (Ar′ ) −C6H3-2,6-(C6H2-2,4,6-Me3)2; X Cl and
Br; 5 and 6) did not display any reactivity with DMAP, but in the presence of the amide base, Na[N(SiMe3)2], the
clean formation of the uncomplexed iminoborane Ar BTmp (7) was observed. Attempts to generate an Sb
bond were unsuccessful, as the required stibinoborane precursor, Ar*Sb(H) B(Br)Tmp, could not be prepared; in
place of clean Sb B bond formation, the reduced product Ar*Sb SbAr* was obtained. All compounds were
characterized spectroscopically, and the X-ray crystal structures of 1, 2, 4, 6, and 7 were determined.
−
B(Br)Tmp (Ar* ) −C6H3-2,6-(C6H2-2,4,6-iPr3)2;
)
)
d
)
′
−
)
′
Nt
dB
−
−
d
Introduction
at boron; for example, the dimerization of the parent species
HBdPH has been estimated to be exothermic by over 54
kcal/mol.5a The bent geometries and concomitant ease of
dimerization calculated for heavy boranylidenepnictanes
RBdPnR′ are a result of the increased inversion barrier at
the pnictogen center upon descending the group. This effect
makes it more difficult to achieve a linear RBPnR′ arrange-
ment when heavy Group 15 elements are present.
Since the landmark preparation of a kinetically stabilized
PdP double bond by Yoshifuji in 1981,1 the synthesis of
related species which exhibit multiple bonding to a heavy
pnictogen (Group 15) atom has been an active field of study.2
So far, the successful isolation of these systems was made
possible mainly with the use of bulky ligands. One system
which has resisted isolation using hindered ligands alone has
been the boranylidenepnictane series RBdPnR′ (Pn ) P, As,
Sb, and Bi), as well as their heavier triel (Al-Tl) analogues.3
The lightest member of the boranylidenepnictane series,
iminoboranes RBtNR′, display essentially B-N triple
bonding and are readily accessible as monomeric species,4
while the heavier Group 15 analogues are predicted to
possess PndB double bonds with bent geometries at the
pnictogen center in the gas phase.5 Furthermore, the enthal-
pies of dimerization of the latter species are anticipated to
be highly exothermic due to the presence of an active lone
pair at the pnictogen and an accompanying empty p-orbital
To deal with the above-mentioned issues, we questioned
if boron-pnictogen multiple bonding could be preserved by
competitively binding a Lewis basic donor molecule to the
empty p-orbital at boron within the RBdPnR′ unit. The
reverse of this principle has been successfully used by Paine,
No¨th, and co-workers to isolate the unique species, (CO)5Cr-
(Et3C)PdBTmp (Tmp ) 2,2,6,6-tetramethylpiperidino). In
this system, a Lewis acidic Cr(CO)5 unit was used to bind
to the active lone pair at phosphorus in order to mitigate the
dimerization tendency of the boranylidenephosphane moiety.6
Recently we reported the implementation of a simple donor-
stabilization protocol in the isolation of rare examples of
PdB and AsdB double bonds7 and now describe in full
detail this chemistry and our attempts to prepare the
analogous BdN and BdSb bonds.
* To whom correspondence should be addressed. E-mail: pppower@
ucdavis.edu.
(1) Yoshifuji, M.; Shima, I.; Inamoto, N.; Hirotsu, K.; Higuchi, T. J. Am.
Chem. Soc. 1981, 103, 4587.
(2) Power, P. P. Chem. ReV. 1999, 99, 3463 and references therein.
(3) Paine, R. T.; No¨th, H. Chem. ReV. 1995, 95, 343.
(4) (a) Paetzold, P. AdV. Inorg. Chem. 1987, 31, 123. (b) No¨th, H. Angew.
Chem., Int. Ed. Engl. 1988, 27, 1603.
(5) (a) Watts, J. D.; Van Zant, L. C. Chem. Phys. Lett. 1996, 251, 119.
(b) Kerins, M. C.; Fitzpatrick, N. J.; Nguyen, M. T. Polyhedron 1989,
8, 969.
(6) Linti, G.; No¨th, H.; Polborn, K.; Paine, R. T. Angew. Chem., Int. Ed.
Engl. 1990, 29, 682. For the recent isolation of an unstable bora-
nylidenephosphane using AlBr3 as a Lewis acid, see: Knable, K.;
Klapo¨tke, T. M.; No¨th, H.; Paine, R. T.; Schwab, I. Eur. J. Inorg.
Chem. 2005, 1099.
10.1021/ic062076n CCC: $37.00
Published on Web 03/06/2007
© 2007 American Chemical Society
Inorganic Chemistry, Vol. 46, No. 8, 2007 2971

Rivard et al.
Table 1. Crystallographic Data for Compounds 1, 2, 4, 6, and 7
1
2
4
6
7
empirical formula
fw
C48H74.5BBrNP
787.28
C55H84BN3P
829.03
C58H91AsBN3
916.07
C33H44BBrN2
559.42
C33H43BN2
478.50
color
colorless
0.57 × 0.17 × 0.16
needle
triclinic
P1h
red
red
colorless
0.25 × 0.19 × 0.09
block
triclinic
P1h
colorless
0.23 × 0.15 × 0.12
prism
monoclinic
P21/n
cryst dimens (mm3)
cryst habit
cryst syst
space group
unit cell dimensions
a (Å)
0.68 × 0.57 × 0.26
0.18 × 0.08 × 0.07
plate
plate
monoclinic
P21/n
monoclinic
P21/n
16.0804(7)
16.9371(8)
17.5600(8)
79.3360(10)
84.3330(10)
83.0610(10)
4651.2(4)
4
12.8832(8)
19.9163(12)
20.7647(13)
90
95.8420(10)
90
5300.3(6)
4
1.039
12.7587(13)
21.142(2)
20.465(2)
90
96.813(2)
90
5481.3(9)
4
1.110
10.9159(5)
11.5290(6)
11.7840(6)
89.2140(10)
89.2680(10)
83.5650(10)
1473.43(13)
2
14.2149(11)
12.1924(9)
16.3375(12)
90
90.0310(10)
90
2831.5(4)
4
1.122
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
D (g cm-3
)
1.124
0.949
1698
1.261
1.419
592
abs coeff (mm-1
F(000)
)
0.088
1820
0.657
1992
0.064
1040
T (K)
90(2)
90(2)
90(2)
90(2)
90(2)
θ range (deg)
total data
1.18-25.25
44 092
1.78-27.50
46 847
1.79-25.25
37 429
1.78-30.00
16 728
1.67-30.51
39 179
observed data I > 2σI
Rint
16 823
0.0382
12 155
0.0325
9934
0.0904
8567
0.0273
8520
0.0458
params
1002
560
588
388
326
R1/wR2 (all data)
R1/wR2 (I > 2σI)
difference map (e‚Å-3
0.0720/0.1136
0.0409/0.1046
0.479/-0.438
0.0678/0.1658
0.0556/0.1564
1.040/-0.794
0.1071/0.2001
0.0775/0.1875
1.018/-0.684
0.0579/0.1411
0.0461/0.1358
0.638/-1.002
0.0729/0.1744
0.0576/0.1675
0.520/-0.471
)
to a glass fiber, and quickly placed in a low-temperature stream of
nitrogen.13 Data for compound 1 were recorded on a Bruker
SMART 1000 instrument using Mo KR radiation (λ ) 0.71073 Å)
in conjunction with a CCD detector; data for compounds 2, 4, 6,
and 7 were similarly obtained except using a Bruker APEX II
instrument. The collected reflections were corrected for Lorentz
and polarization effects by using Blessing’s method as incorporated
into the program SADABS.14,15 The structures were solved by direct
methods and refined with the SHELXTL v.6.1 software package.16
Refinement was by full-matrix least-squares procedures with all
carbon-bound hydrogen atoms included in calculated positions and
treated as riding atoms. Compound 1 crystallized with two
molecules of hexane, of which one molecule of hexane was
disordered over two positions and modeled with a 67:33 occupancy
ratio. Compound 7 crystallized as a pseudomerohedral twin and
was refined using the TWIN instruction in SHELXTL (twin matrix
1 0 0 0 -1 0 0 0 -1). Some details of the data collection and
refinement are given in Table 1. Full crystallographic details for 2
and 4 are found in ref 7.
Preparation of Ar*P(H)-B(Br)Tmp (1). To a solution of
Ar*PH2 (0.964 g, 1.88 mmol) in 12 mL of Et2O was added nBuLi
(0.78 mL, 2.5 M solution in hexanes, 1.95 mmol), which resulted
in the formation of a bright yellow solution containing Ar*PH(Li).
After 1 h, this solution was added dropwise to a solution of
TmpBBr2 (0.588 g, 1.89 mmol) in 10 mL of Et2O. The resulting
pale yellow slurry was stirred for 24 h, and the solvent was removed
to give a white solid. The product was extracted with 40 mL of
hexanes and filtered through Celite to give a colorless solution.
Concentration of the filtrate to ca. 6 mL, followed by cooling to
ca. -20 °C for 24 h, resulted in the precipitation of a white
Experimental Section
General. All reactions were performed with the use of modified
Schlenk techniques under an atmosphere of nitrogen or in a Vacuum
Atmospheres Nexus drybox. Solvents were dried using a Grubbs-
type8 solvent purification system manufactured by Glass Contour
and degassed twice (freeze-pump-thaw method) prior to use.
Ar*PnH2 (Ar* ) -C6H3-2,6-(C6H2-2,4,6-iPr3)2; Pn ) P, As, and
Sb),9 Ar′NH2 (Ar′ ) -C6H3-2,6-(C6H2-2,4,6-Me3)2),10 TmpBCl2,11
12
and TmpBBr2 were prepared according to literature procedures.
nBuLi (2.5 M solution in hexanes), 4-dimethylaminopyridine
(DMAP), and Na[N(SiMe3)2] were purchased from commercial
sources and used as received. H, 13C{1H}, and 31P NMR spectra
1
were obtained on a Varian Mercury 300 MHz spectrometer (300.1,
75.5, and 121.5 MHz, respectively) and referenced either internally
to residual protio benzene in the C6D6 solvent or externally to 85%
H3PO4. 11B{1H} NMR spectra were acquired with an INOVA 400
MHz spectrometer (96.6 MHz) and referenced externally to BF3‚
OEt2. Infrared spectra were recorded as Nujol mulls between CsI
plates using a Perkin-Elmer 1430 instrument, while UV-vis data
were recorded on a Hitachi-1200 instrument. Melting points were
obtained in sealed glass capillaries under nitrogen using a Mel-
Temp II apparatus and are uncorrected.
X-ray Crystallography. Crystals of appropriate quality for X-ray
diffraction studies were removed from a Schlenk tube under a
stream of nitrogen and immediately covered with a thin layer of
hydrocarbon oil (Paratone). A suitable crystal was selected, attached
(7) For a brief communication on some of this work, see: Rivard, E.;
Merrill, W. A.; Fettinger, J. C.; Power, P. P. Chem. Commun. 2006,
3800.
(8) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J. Organometallics 1996, 15, 1518.
(9) Twamley, B.; Hwang, C.-S.; Hardman, N. J.; Power, P. P. J.
Organomet. Chem. 2000, 609, 152.
(10) Wright, R. J.; Steiner, J.; Beaini, S.; Power, P. P. Inorg. Chim. Acta
2006, 359, 1939.
(13) Hope, H. Prog. Inorg. Chem. 1995, 41, 1.
(14) Blessing, R. H. Acta Crystallogr. 1995, 51A, 33.
(15) Sheldrick, G. M. SADABS, Version 2.10, Siemens Area Detector
Absorption Correction; Universita¨t Go¨ttingen: Go¨ttingen, Germany,
2003.
(11) No¨th, H.; Weber, S. Z. Naturforsch. 1983, 38B, 1460.
(12) Glaser, B.; No¨th, H. Chem. Ber. 1986, 119, 3253.
(16) Sheldrick, G. M. SHELXTL, Verison 6.1; Bruker AXS, Inc.: Madison,
WI, 2002.
2972 Inorganic Chemistry, Vol. 46, No. 8, 2007

Boron-Pnictogen Multiple Bonds
microcrystalline solid that was separated from the mother liquor
and dried under vacuum (0.950 g, 68%). Crystals of 1 suitable for
X-ray diffraction were subsequently grown from a cold (-40 °C)
4 as red-purple dichroic crystals after 2 days (0.095 g, 16%; based
upon the quantitative formation of 3). 1H NMR (C6D6): δ 0.77 (d,
J ) 6.6 Hz, 6H, CH(CH3)2), 0.87 (br, 4H, CH2 Tmp), 1.22 (d, J )
6.6 Hz, 6H, CH(CH3)2), 1.35 (d, J ) 6.9 Hz, 18H, CH(CH3)2),
1.48 (s, 12H, CH3 Tmp), 1.55 (m, 2H, CH2 Tmp), 1.81 (s, 6H,
N(CH3)2 DMAP), 1.85 (d, J ) 6.9 Hz, 6H, CH(CH3)2), 2.64 (septet,
J ) 6.9 Hz, 2H, CH(CH3)2), 2.92 (septet, J ) 6.6 Hz, 2H,
CH(CH3)2), 3.05 (septet, J ) 6.9 Hz, 2H, CH(CH3)2), 5.61 (br d,
2H, ArH DMAP), 7.14 (d, J ) 1.8 Hz, 2H, ArH), 7.24 (t, 1H,
ArH), 7.33 (br, 2H, ArH), 7.48 (d, J ) 7.5 Hz, 2H, ArH), and 9.42
(br d, 2H, ArH DMAP). 11B NMR (C6D6): δ 51.2 (br, ∆ν1/2 ) ca.
750 Hz). 13C{1H} NMR (C6D6): δ 14.4, 19.3, 23.0, 23.8, 24.7,
26.1, 27.2, 27.6, 31.47, 31.53, 32.0, 33.3, 35.0, 38.5, 42.5, 52.1,
120.2, 121.2, 124.6, 129.6, 141.3, 145.0, 147.0, 148.0, 148.7, 154.3,
and 154.6. mp (°C): 147 (chars); 169-173 (melts). UV-vis
(hexane, nm [ꢀ, cm-1 M-1]): 328 (shoulder) and 584 [2530].
1
solution in hexanes. H NMR (C6D6): δ 0.88 (t, J ) 6.6 Hz, 4H,
CH2 Tmp), 1.16 (s, 12H, CH3, Tmp), 1.23 (br d, 12H, CH(CH3)2),
1.25 (d, J ) 6.9 Hz, 12H CH(CH3)2), 1.36 (m, 2H, CH2 Tmp),
1.54 (d, J ) 6.9 Hz, 12H, CH(CH3)2, 2.84 (septet, J ) 6.9 Hz, 2H,
CH(CH3)2), 3.38 (very broad, 4H, CH(CH3)2), 4.04 (d, 1JHP ) 214.2
Hz, 1H, PH), 7.18 (m, 1H, ArH), 7.20 (s, 4H, ArH), and 7.24-
7.26 (m, 2H, ArH). 11B NMR (C6D6): δ 46.3 (br, ∆ν1/2 ) 1140
Hz). 13C{1H} NMR (C6D6): δ 14.3, 24.4, 26.2, 31.2, 32.4 (d, J )
9.1 Hz), 34.8, 35.3, 57.7, 138.5, 139.1 (d, J ) 12.5 Hz), 145.1 (d,
J ) 10.3 Hz), 147.5, and 148.6. 31P NMR (C6D6): δ -80.1 (d,
1JPH ) 214 Hz). IR (Nujol mull, CsI plates): 2265 (m), V(P-H).
mp (°C): 214-216.
Preparation of Ar*PdB(DMAP)Tmp (2). Precooled toluene
(-78 °C, 12 mL) was added to a Schlenk flask containing a mixture
of 1 (0.269 g, 0.36 mmol) and DMAP (0.114 g, 0.90 mmol). Upon
the addition of toluene, a pale yellow solution was observed along
with undissolved DMAP. The reaction was slowly warmed to room
temperature and stirred for 2 days to give a purple solution along
with a white precipitate (DMAP‚HBr). The reaction was filtered,
the solvent was then removed from the filtrate, and the product
was crystallized from hexanes (4 mL, ca. -20 °C, 2 weeks) to
give large well-formed rods of 2 that were dark red in color (0.045
g, 16%). 1H NMR (C6D6): δ 0.72 (d, J ) 7.2 Hz, 6H, CH(CH3)2),
0.81 (br, 4H, CH2 Tmp), 1.24 (d, J ) 6.9 Hz, 6H, CH(CH3)2),
1.36 (d, J ) 6.9 Hz, 12H, CH(CH3)2), 1.46 (d, J ) 6.6 Hz, 6H,
CH(CH3)2), 1.51 (s, 12H, CH3 Tmp), 1.58 (m, 2H, CH2 Tmp), 1.82
(s, 6H, N(CH3)2 DMAP), 1.90 (d, J ) 6.6 Hz, 6H, CH(CH3)2),
2.93 (septet, 2H, CH(CH3)2), 3.18 (septet, 2H, CH(CH3)2), 3.36
(septet, 2H, CH(CH3)2), 5.55 (br d, 2H, CH DMAP), 7.13 (s, 2H,
ArH), 7.24 (m, 1H, ArH), 7.35 (m, 1H, ArH), 7.36 (s, 2H, ArH),
7.48 (d, J ) 7.5 Hz, 2H, ArH), and 8.95 (br d, 2H, CH DMAP).
11B NMR (C6D6): δ 41.2 (br, ∆ν1/2 ) ca. 800 Hz). 13C{1H} NMR
(C6D6): δ 14.4, 19.4, 23.1, 23.70, 23.75, 24.6, 24.7, 26.1, 27.7,
31.4, 32.0, 35.0, 38.4, 42.7, 52.7, 120.2, 121.2, 124.0, 125.9, 129.6,
140.6, 145.0, 146.5 (d, J ) 5.2 Hz), 147.0, 148.6, 150.6, and 154.5.
31P NMR (C6D6): δ 57.3 (s). mp (°C): 169-172 (dec). UV-vis
(hexane, nm [ꢀ, cm-1 M-1]): 370 (shoulder), 534 [2280].
Synthesis of Ar*AsdB(DMAP)Tmp (4). To a solution of
Ar*AsH2 (0.602 g, 1.08 mmol) in 15 mL of Et2O (0.570 g, 1.11
mmol) was added nBuLi (0.45 mL, 2.5 M solution in hexanes, 1.13
mmol), which resulted in the formation of an orange-yellow solution
containing Ar*AsH(Li). After 45 min, this solution was added
dropwise to a solution of TmpBBr2 (0.393 g, 1.26 mmol) in 8 mL
of Et2O. The resulting pale yellow solution was stirred for 24 h,
and the solvent was removed to give a white solid. The product
was extracted with 25 mL of hexanes and filtered through Celite
to give a colorless solution of Ar*As(H)-B(Br)Tmp (3).17 The
solution of in situ generated 3 was then added to excess DMAP
(0.345 g, 2.76 mmol) to give a deep blue-violet solution over a
white precipitate. This mixture was stirred for 24 h and filtered
through a pad of Celite. Cooling of the filtrate to ca. -20 °C yielded
Attempted Preparation of Ar*Sb(H)-B(Br)Tmp. Following
a literature procedure,9 a solution of Ar*SbH(Li) in diethyl ether
n
was prepared from Ar*SbH2 and BuLi. The red-orange solution
of Ar*SbH(Li) was then added to either TmpBCl2 or TmpBBr2 in
cold (-78 °C) diethyl ether. In both instances, deep orange solutions
were obtained after warming the reaction mixture to room tem-
perature. Filtration and cooling of the reaction mixtures to ca. -20
°C afforded clean samples of olive green Ar*SbdSbAr* as judged
by 1H and 13C{1H} NMR spectroscopy (20-35% isolated yield).18
The remaining mother liquors were analyzed by NMR and were
shown to contain complicated mixtures from which pure stibinobo-
rane Ar*Sb(H)-B(X)Tmp (X ) Cl or Br) could not be isolated.
Preparation of Ar′N(H)-B(Cl)Tmp (5). To a solution of
Ar′NH2 (0.496 g, 1.66 mmol) in 12 mL of diethyl ether was added
dropwise a solution of nBuLi (0.69 mL, 2.5 M solution in hexanes,
1.72 mmol). The resulting yellow solution was stirred for 45 min
and added to a solution of TmpBCl2 (0.373 g, 1.67 mmol) in 5 mL
of Et2O. The reaction mixture was then stirred for 16 h to give a
yellow solution over a white precipitate. Removal of the solvent
gave a pale yellow residue to which was added 20 mL of hexanes.
The mixture was then filtered through Celite and concentration of
the filtrate to ca. 15 mL resulted in the spontaneous crystallization
of a colorless product. Crystallization was continued at ca. -20
°C (24 h) to give a crop of well-formed colorless crystals (365
mg, 45%). 1H NMR (C6D6): δ 0.85 (s, 12H, CH3 Tmp), 1.20 (t, J
) 6.6 Hz, 4H, CH2 Tmp), 1.41 (m, 2H, CH2 Tmp), 2.19 (s, 6H,
p-CH3 Mes), 2.20 (s, 12H, o-CH3 Mes), 4.99 (s, N-H, 1H), 6.85
(s, 4H, Ar-H Mes), and 7.0-7.1 (m, 3H, Ar-H). 11B NMR (C6D6):
δ 31.0 (br, ∆ν1/2 ) ca. 530 Hz). 13C{1H} NMR (C6D6): δ 17.3,
20.98, 21.06, 30.7, 39.4, 51.6, 126.5, 128.9, 130.2, 136.3, 136.6,
137.5, 138.7, and 140.4. IR (Nujol mull, CsI plates): 3358 cm-1
(w, sharp), V(N-H). mp (°C): 193-198 (dec).
Preparation of Ar′N(H)-B(Br)Tmp (6). To a solution of
Ar′NH2 (0.677 g, 2.27 mmol) in 15 mL of diethyl ether was added
dropwise a solution of nBuLi (0.90 mL, 2.5 M solution in hexanes,
2.25 mmol). The resulting yellow solution was stirred for 45 min
and added to a solution of TmpBBr2 (0.698 g, 2.24 mmol) in 5 mL
of benzene. The reaction mixture was then stirred for 16 h to give
a yellow solution over a white precipitate. Removal of the solvent
gave a pale yellow residue to which was added 30 mL of hexanes.
The mixture was then filtered through Celite and concentration of
the filtrate to ca. 20 mL and cooling to ca. -20 °C to give a crop
(17) Despite numerous attempts, we were not able to obtain a clean sample
of the arsinoborane 3. Analysis of crude product by 1H and 13C NMR
revealed the presence of unreacted Ar*AsH2 along with unknown Tmp-
containing products. IR spectroscopy contained two identifiable As-H
bands at 2105 and 2080 cm-1, with the former band assignable to
Ar*AsH2.9 We assume that 3 is generated by the reaction of Ar*AsH-
(Li) and TmpBBr2 due the isolation of clean boranylidenearsane 4 in
the subsequent reaction with DMAP (albeit in low yield). However,
at the moment we cannot rule out the presence of a reaction surrogate
to 3 in solution, which is also transformed to 4 upon reaction with
DMAP.
1
of well-formed colorless crystals after 2 days (0.250 g, 21%). H
NMR (C6D6): δ 0.84 (s, 12H, CH3 Tmp), 1.16 (t, J ) 6.6 Hz, 4H,
(18) Twamley, B.; Sofield, C. D.; Olmstead, M. M.; Power, P. P. J. Am.
Chem. Soc. 1999, 121, 3357.
Inorganic Chemistry, Vol. 46, No. 8, 2007 2973

Rivard et al.
Scheme 1. Possible Routes to Stable Pnictogen-Boron Double
Bonds
CH2 Tmp), 1.42 (quintet, J ) 6.6 Hz, 2H, CH2 Tmp), 2.19 (s, 6H,
p-CH3 Mes), 2.22 (s, 12H, o-CH3 Mes), 5.36 (s, N-H, 1H), 6.84
(s, 4H, Ar-H Mes), and 7.01-7.13 (m, 3H, Ar-H). 11B NMR
(C6D6): δ 30.2 (br, ∆ν1/2 ) ca. 630 Hz). 13C{1H} NMR (C6D6):
δ 17.8, 21.0, 21.2, 30.4, 40.3, 51.7, 126.7, 129.0, 130.5, 136.4,
136.7, 137.4, 138.8, and 140.7. IR (Nujol mull, CsI plates): 3362
cm-1 (w, sharp), V(N-H). mp (°C): 201-204 (dec).
Attempted Preparation of Ar′NdB(DMAP)Tmp. Reaction of
5 and 6 with DMAP did not give any noticeable reaction (even
upon prolonged heating of 5 in refluxing toluene, 5 days). Attempts
to generate Ar′NdB(DMAP)Tmp by reacting either 5 or 6 with
DMAP in the presence of the silylamide salt Na[N(SiMe3)2] gave
exclusively the iminoborane product Ar′NtBTmp 7 as determined
by NMR, IR, and X-ray diffraction studies; see below for an
independent preparation of 7.
Figure 1. Molecular structure of 1 with thermal ellipsoids presented at
the 30% probability level. All carbon-bound hydrogen atoms have been
omitted for clarity. Selected bond lengths (Å) and angles (deg) with
bracketed values belonging to a second molecule of 1 in the asymmetric
unit: P(1)-B(1) 1.958(3) [1.950(3)], P(1)-C(1) 1.839(2) [1.839(2)], P(1)-
H(1) 1.29(2) [1.30(2)], B(1)-N(1) 1.390(3) [1.389(3)]; C(1)-P(1)-B(1)
106.93(11) [107.52(11)], C(1)-P(1)-H(1) 98.2(9) [98.6(11)], P(1)-B(1)-
N(1) 122.31(18) [122.78(18)], P(1)-B(1)-Br(1) 115.17(14) [114.79(14)],
Br(1)-B(1)-N(1) 122.23(18) [122.26(19)], B(1)-N(1)-C(37) 119.6(2)
[119.80(19)], B(1)-N(1)-C(41) 122.8(2) [122.7(2)].
Preparation of Ar′NtBTmp (7). To a mixture of 6 (0.216 g,
0.41 mmol) and Na[N(SiMe3)2] (81 mg, 0.44 mmol) was added 12
mL of THF. The initially clear, pale yellow mixture clouded upon
stirring for 24 h. The volatiles were then removed, and the product
was extracted with 20 mL of hexanes. After the mixture was filtered
through Celite, the colorless filtrate was concentrated to 8 mL and
cooling to ca. -20 °C for 3 days yielded 7 as small colorless prisms
(0.080 g, 44%). 1H NMR (C6D6): δ 0.71 (s, 12H, CH3 Tmp), 0.96
(t, J ) 6.6 Hz, 4H, CH2 Tmp), 1.18 (quintet, J ) 6.6 Hz, 2H, CH2
Tmp), 2.24 (s, 6H, p-CH3 Mes), 2.26 (s, 12H, o-CH3 Mes), 6.90
(s, 4H, Ar-H Mes), and 6.98-7.09 (m, 3H, Ar-H). 11B NMR
(C6D6): δ 10.6 (br, ∆ν1/2 ) ca. 500 Hz). 13C{1H} NMR (C6D6):
δ 17.5, 20.6, 21.2, 31.1, 38.1, 51.6, 120.9, 128.3, 128.7, 135.7,
136.0, 136.7, and 139.4. IR (Nujol mull, CsI plates): 2037 [w,
V(10BtN)] and 1982 cm-1 [m, V(11BtN)]. mp (°C): 193-196.
we constructed the requisite phosphinoborane precursor 1
from which HBr elimination could be induced to yield a
PdB bond.
Compound 1 was prepared by the reaction of the diha-
loborane, TmpBBr2, with 1 equiv of Ar*P(H)Li in diethyl
ether and isolated as a white microcrystalline solid in
moderate yield (68%). The 31P NMR spectrum of 1 consisted
of a doublet resonance at -80.1 ppm (1JPH ) 214 Hz)
attributable to a PH group, while a broad signal at +46.3
ppm was detected by 11B NMR spectroscopy; the latter signal
lies within the typical range observed for three coordinate
boron environments.20 Due to the broad nature of both the
31P and 11B resonances, no P-B coupling was observed.
Crystals suitable for study by X-ray diffraction were
subsequently grown from cold hexane, and the structure of
1 is shown in Figure 1.
Phosphinoborane 1 exists as a monomer in the solid state
with two independent molecules in the asymmetric unit of
the crystal lattice. The average P-B length in 1 is 1.954(4)
Å, while the average B-N distance is 1.390(4) Å. The
pyramidal phosphorus geometry, in conjunction with the long
P-B bond length, strongly suggests a lack of any appreciable
PfB dative π bonding. For comparison, the P-B bond
lengths in the related Tmp-substituted phosphinoboranes
MesP(H)-B(Cl)Tmp and [(iPr2N)2B]P(H)-B(Cl)Tmp are
1.948(3) and 1.925(5) Å, respectively.19a,21 The average B-N
distance in 1 was similar to that in the above-mentioned Tmp
derivatives [1.380(3) and 1.397(5) Å] and considerably
Results and Discussion
Preparation of the Phosphinoborane 1 and Its Conver-
sion into the Boranylidenephosphane 2. As mentioned in
the Introduction, we initially set out to prepare species with
multiple bonding between boron and a heavy pnictogen atom,
e.g., P and As, with the aid of donor molecules (illustrated
conceptually in the lower portion of Scheme 1). In line with
prior work in this field,19 we viewed the unsaturated species
Ar*PdB(L)Tmp (Ar* ) -C6H3-2,6-(C6H2-2,4,6-iPr3)2; L )
donor) as a suitable synthetic target. It was hoped that the
steric bulk imposed by the terphenyl ligand at phosphorus,
coupled with the facile introduction of the Tmp group at
boron, would encourage the synthesis of a stable PdB double
bond when a suitable donor molecule was present. Therefore,
(19) (a) Arif, A. M.; Boggs, J. E.; Cowley, A. H.; Lee, J. G.; Pakulski, M.;
Power, J. M. J. Am. Chem. Soc. 1986, 108, 6083. (b) Ko¨lle, P.; Linti,
G.; No¨th, H.; Wood, G. L.; Narula, C. K.; Paine, R. T. Chem. Ber.
1988, 121, 871.
(20) No¨th, H.; Wrackmeyer, B. Nuclear Magnetic Resonance Spectroscopy
of Boron Compounds; Springer: Berlin, 1978.
(21) Dou, D.; Wood, G. L.; Duesler, E. N.; Paine, R. T.; No¨th, H. Inorg.
Chem. 1992, 31, 1695.
2974 Inorganic Chemistry, Vol. 46, No. 8, 2007

Boron-Pnictogen Multiple Bonds
to an allowed π-to-π* transition associated with the PdB
chromophore.
As depicted in Figure 2, monomeric 2 displays bonding
parameters fully consistent with the presence of a discrete
PdB double bond. Most relevant is the short P-B bond
distance of 1.8092(17) Å, which is considerably shorter than
typical P-B single bonds (e.g., the average P-B single bond
length in 1 is 1.954(4) Å). Furthermore, the phosphorus
center in 2 is 2-coordinate with a C(ipso)-P-B bending
angle of 114.36(7)°, consistent with approximate sp2 hy-
bridization at phosphorus. As anticipated, the neighboring
boron center adopts a trigonal planar geometry [angle sum
) 360.0(2)°] with a dative B-N interaction at 1.571(6) Å
(av) involving DMAP,24 along with a shorter B-N single
bond distance [1.4837(19) Å] incorporating the Tmp ligand.
The long B-N length to the Tmp group, coupled with the
relative twisted orientation of the nitrogen (Tmp) and boron
planes, suggests that minimal π-bonding exists between N
and B; thus, the π-bonding is concentrated between phos-
phorus and boron.
Compound 2 represents the first example of a neutral base-
stabilized boranylidenephosphane, though multiple bonding
between phosphorus and boron has been observed previously.
For example, comparably short P-B bond lengths exist in
the anionic borylphosphides [RPdBR′2]- [1.810(4)-1.833-
(6) Å],25 and short distances were occasionally observed in
a series of hindered monomeric phosphinoboranes R2PBR′2
[1.839(8)-1.871(3) Å].26 In addition, a very short P-B
interaction of 1.7453(5) Å was found in the unique acid-
stabilized species [(CO)5Cr(Et3C)PdBTmp]; this compound
also exhibited a nearly linear P-B-N(Tmp) angle of 176.1-
(3)° and a short B-N length of 1.339(5) Å, consistent with
the allenic bonding form >PdBdN<.6 The presence of an
sp-hybrized boron center in [(CO)5Cr(Et3C)PdBTmp], coupled
with an electron-withdrawing Cr(CO)5 moiety at phosphorus,
could help explain the ca. 0.06 Å contraction in the P-B
bond length when compared to our base-stabilized analogue
2. However, it is also plausible that the longer P-B distance
in 2 could arise from the greater steric congestion at the P
and B centers in this compound.
Figure 2. Molecular structure of 2 with thermal ellipsoids presented at
the 30% probability level. The hydrogen atoms and disordered DMAP have
been omitted for clarity. Selected bond lengths (Å) and angles (deg) with
values corresponding to the disordered DMAP in parentheses: P(1)-B(1)
1.8092(17), P(1)-C(1) 1.8509(14), B(1)-N(1) 1.4837(19), B(1)-N(2)
1.593(2) [1.548(2)]; C(1)-P(1)-B(1) 114.36(7), P(1)-B(1)-N(1) 119.48-
(11), P(1)-B(1)-N(2) 127.41(13) [132.7(4)], N(1)-B(1)-N(2) 113.09-
(14) [107.8(4)], B(1)-N(1)-C(38) 117.98(12), B(1)-N(3)-C(42) 117.84-
(12), C(38)-N(1)-C(42) 119.16(11).
shorter than the B-N(Tmp) bond length of 1.465(3) Å within
the dimeric diphosphadiboretane [tBuPBTmp]2.22 Further-
more, planar geometries exist at both the boron and nitrogen
centers in 1, suggesting that some multiple bonding exists
between these two atoms.
Upon mixing 1 with an excess of DMAP in toluene,23
a
deep purple mixture was observed. After separation of the
DMAP‚HBr byproduct, deep-red crystals were obtained and
identified as the novel base-stabilized boranylidenephos-
phane, 2, with the help of X-ray crystallography (Figure 2,
Scheme 2) in combination with NMR (1H, 11B, 13C, and 31P)
and UV-vis spectroscopy. This product was exceedingly
air and moisture sensitive but was found to be quite thermally
robust (mp ) 169-172 °C).
Consistent with the assigned structure, broad singlet
resonances were detected in the 31P (+57.3 ppm) and 11B
(+41.2 ppm) NMR spectra of 2. The 31P NMR resonance
was considerably deshielded in comparison to that of the
phosphinoborane precursor 1 (-80.1 ppm); the 11B NMR
signal for 2 is similar to that for 1, indicating the retention
of a 3-coordinate boron environment. The UV-vis spectrum
of 2 was dominated by a broad absorption centered at 534
nm (ꢀ ) 2280 M-1 cm-1) which has been tentatively assigned
Preparation of the Boranylidenearsane 4. Given the
successful preparation of 2, we decided to investigate the
preparation of heavier pnictogen congeners. Following the
protocol described above, the lithiated arsenide Ar*AsH-
(Li) was reacted with 1 equiv of TmpBBr2 to give the
arsinoborane 3.17 Subsequent treatment of 3 with an excess
of DMAP in hexanes rapidly afforded a dark blue-violet
solution, which yielded dark purple-red dichroic crystals upon
workup. These crystals were identified as the novel bora-
nylidenearsane 4 by X-ray crystallography (Figure 3) and
further analyzed by NMR and UV-vis spectroscopy. As in
(22) No¨th, H.; Schwartz, M.; Weber, S. Chem. Ber. 1985, 118, 4716.
(23) For reports concerning the use of DMAP to stabilize reactive main
group species, see: (a) Thomas, F.; Schulz, S.; Nieger, M. Eur. J.
Inorg. Chem. 2001, 161. (b). Schulz, S. Struct. Bond. 2002, 103, 117.
(c) Rivard, E.; Huynh, K.; Lough, A. J.; Manners, I. J. Am. Chem.
Soc. 2004, 126, 2286. (d) Bla¨ttner, M.; Nieger, M.; Ruban, A.;
Schoeller, W. W.; Niecke, E. Angew. Chem., Int. Ed. 2000, 39, 2768.
(e) Schulz, S.; Thomas, F.; Priesmann, W. M.; Nieger, M. Organo-
metallics 2006, 25, 1392. (f) An interesting DMAP-mediated route to
labile Group 13-15 bonds has been reported: Thomas, F.; Schulz,
S.; Mansikkama¨ki, H.; Nieger, M. Angew. Chem., Int. Ed. 2003, 42,
5641.
(24) For comparison, the B-N distances within the DMAP‚BX3 adducts
(X ) halogen or C6F5) range from 1.589(5) to 1.602(6) Å; Gerald
Lesley, M. J.; Woodward, A.; Taylor, N. J.; Marder, T. B.; Cazenobe,
I.; Ledoux, I.; Zyss, J.; Thornton, A.; Bruce, D. W.; Kakkar, A. K.;
Chem. Mater. 1998, 10, 1355.
(25) Bartlett, R. A.; Rasika Dias, H. V.; Feng, X.; Power, P. P. J. Am.
Chem. Soc. 1989, 111, 1306.
(26) Pestana, D. C.; Power, P. P. J. Am. Chem. Soc. 1991, 113, 8426.
Inorganic Chemistry, Vol. 46, No. 8, 2007 2975

Rivard et al.
Scheme 2. Preparation of the PdB and AsdB Double-Bonded Species 2 and 4
the case of 2, crystals of the arsenic derivative 4 were
exceedingly air- and moisture-sensitive, yet possessed con-
siderable thermal stability (decomposes at 147 °C).
The 11B NMR spectrum of 4 consisted of a broad
resonance at +51 ppm, while a transition at 584 nm (ꢀ )
2530 M-1 cm-1) was found in the UV-vis spectrum. This
absorption is significantly red-shifted from that of 2 and is
in line with the anticipated weaker AsdB bond in 4 (i.e.,
smaller π-to-π* energy gap).
The molecular structure of 4 closely resembles that of its
phosphorus counterpart 2. A short As-B bond [1.914(6) Å]
was found which parallels the AsdB distances observed
[1.926(6) and 1.936(11) Å] within the borylarsenide anion
[PhAsdBMes2]-, which is, to our knowledge, the only other
example of a structurally authenticated AsdB double bond
in a molecular species.27,28 For comparison, the As-B
distances within the 3-coordinate arsinoboranes R2As-BR2
(which do not have appreciable As-B multiple bonding)
range from 2.06 to 2.20 Å.27b The B-N distances in 4 were
similar to those observed in 2 with distances of 1.567(7) and
1.469(7) Å to the DMAP and Tmp ligands, respectively. The
C(ipso)-As-B bending angle of 113.5(2)° (i.e., sp2-hydrid-
ized arsenic), in conjunction with the planar geometry at
boron, reinforces the assignment of an AsdB double bond
in 4.
Attempted Preparation of the Stibinoboranes of Ar*Sb-
(H)-B(X)Tmp (X ) Cl and Br). Attempts to form a
hitherto unknown SbdB bond were not successful, as our
efforts to prepare the required stibinoborane Ar*Sb(H)-B(X)-
Tmp (X ) Cl or Br) from TmpBX2 and the known stibinide,
Ar*SbH(Li), resulted in the formation of significant quanti-
ties of the olive green distibene Ar*SbdSbAr*18 in place of
clean Sb-B bond formation. Analogous synthetic routes
were not explored for bismuth, as the required bismuthane
precursor, Ar*BiH2, is unstable and spontaneously eliminates
H2 at low temperatures to yield the dibismuthene,
Ar*BidBiAr*.9,29 It seems a possiblity that the increased
stability of the lone pair in the heavier elements decreases
the nucleophilic character of the pnictogen center, thus
reducing the probability of Sb-B bond formation by
nucleophilic attack on boron. This effect could also increase
the possiblity of Li-X (X ) halogen) exchange with
TmpBX2, and in the case above, subsequent HX elimination
from an unstable Ar*Sb(H)X intermediate might have
occurred to give the distibene Ar*SbdSbAr*.30
Synthesis of the Aminoboranes 5 and 6 and Their
Conversion into the Iminoborane 7. The chemistry of
iminoboranes RBtNR′ has been well explored, and a
number of interesting reaction sequences including their
cycloaddition and coordination chemistries have been dis-
covered.4,31,32 Interestingly, to our knowledge, there have not
been any examples of iminoboranes acting as Lewis acids
in the presence of neutral donors, while the role of imino-
boranes as Lewis bases in the presence of electron-deficient
substrates has been well documented.33 Therefore, we were
motivated to extend our donor-stabilization protocol to
include the BdN double-bonded species ArNdB(DMAP)-
Tmp, which would contain the hitherto unknown iminobo-
rane ArNtBTmp unit acting as a Lewis acid toward the
strong donor DMAP (Ar ) terphenyl ligand).
Recently we developed a high-yield, large-scale synthesis
of the very bulky aniline Ar′NH2 via the reduction of the
readily available azido precursor Ar′N3 with Li[AlH4] (Ar′
) -C6H3-2,6-(C6H2-2,4,6-Me3)).10,34 Hence we decided to
use Ar′NH2 as a reaction synthon to access the aminoborane
(29) Hardman, N. J.; Twamley, B.; Power, P. P. Angew. Chem., Int. Ed.
2000, 39, 2771.
(30) For, example halophosphines of the general form RPH(X) tend to
eliminate HX to yield phosphinidene oligomers (RP)x: Cowley, A.
H.; Kilduff, J. E.; Norman, N. C.; Pakulski, M.; Atwood, J. L.; Hunter,
W. E. J. Am. Chem. Soc. 1983, 105, 4845.
(31) (a) Paetzold, P.; von Plotho, C.; Schmidt, G.; Boese, R.; Schrader,
B.; Bougeard, D.; Pfeiffer, U.; Gleiter, R.; Scha¨fer, W. Chem. Ber.
1984, 117, 1089. (b) Paetzold, P.; von Plotho, C.; Niecke, E.; Ru¨ger,
R. Chem. Ber. 1983, 116, 1678. (c) Ma¨nnig, D.; Narula, C. K.; No¨th,
H.; Wietelmann, U. Chem. Ber. 1985, 118, 3748. (d) Klo¨fkorn, C.;
Schmidt, M.; Spaniol, T.; Wagner, T.; Costisor, O.; Paetzold, P. Chem.
Ber. 1995, 128, 1037.
(27) (a) Petrie, M. A.; Shoner, S. C.; Rasika Dias, H. V.; Power, P. P.
Angew. Chem., Int. Ed. Engl. 1990, 29, 1033. (b) Petrie, M. A.;
Olmstead, M. M.; Hope, H.; Bartlett, R. A.; Power, P. P. J. Am. Chem.
Soc. 1993, 115, 3221.
(28) Short P-B and As-B distances have been reported in the Zintl anions
[PBP]3- and [AsBAs]3-, see: (a) von Schnering, H. G.; Somer, M.;
Hartweg, M.; Peters, K. Angew. Chem., Int. Ed. Engl. 1990, 29, 65.
(b) Somer, M.; Carrillo-Cabrera, W.; Peters, K.; von Schnering, H.-
G. Z. Anorg. Allg. Chem. 2000, 626, 897.
(32) (a) Paetzold, P.; Delpy, K. Chem. Ber. 1985, 118, 2552. (b) Bulak,
E.; Herberich, G. E.; Manners, I.; Mayer, H.; Paetzold, P. Angew.
Chem., Int. Ed. Engl. 1988, 27, 958. (c) Manners, I.; Paetzold, P. J.
Chem. Soc. Chem. Commun. 1988, 183. (d) Braunschweig, H.;
Paetzold, P.; Boese, R. Chem. Ber. 1990, 123, 485.
(33) (a) No¨th, H.; Weber, S. Chem. Ber. 1985, 118, 2554. (b) Bo¨ck, B.;
Braun, U.; Habereder, T.; Mayer, P.; No¨th, H. Z. Naturforsch. 2004,
59B, 681.
(34) Gavenonis, J.; Tilley, T. D. Organometallics 2004, 23, 31.
2976 Inorganic Chemistry, Vol. 46, No. 8, 2007

Boron-Pnictogen Multiple Bonds
Figure 5. Molecular structure of 7 with thermal ellipsoids presented at
the 30% probability level. Hydrogen atoms have been omitted for clarity.
Selected bond lengths (Å) and angles (deg): N(1)-B(1) 1.254(3). C(1)-
N(1) 1.366(2), B(1)-N(2) 1.374(3), N(2)-C(26) 1.491(2), N(2)-C(29)
1.496(2); C(1)-N(1)-B(1) 171.7(2), N(1)-B(1)-N(2) 179.4(2), B(1)-
N(2)-C(26) 119.99(16), B(1)-N(2)-C(29) 119.42(17), C(26)-N(2)-
C(29) 120.59(14).
Figure 3. Molecular structure of 4 with thermal ellipsoids presented at
the 30% probability level. The hydrogen atoms have been omitted for clarity.
Selected bond lengths (Å) and angles (deg): As(1)-B(1) 1.914(6), As-
(1)-C(1) 1.987(5), B(1)-N(1) 1.469(7), B(1)-N(2) 1.567(7); C(1)-As-
(1)-B(1) 113.5(2), As(1)-B(1)-N(1) 119.3(4), As(1)-B(1)-N(2) 128.6-
(4), N(1)-B(1)-N(2) 112.1(4), B(1)-N(1)-C(38) 120.3(4), B(1)-N(1)-
C(42) 118.1(4), C(38)-N(1)-C(42) 118.3(4).
phosphinoborane derivative 1 (11B, 46.3 ppm), yet still appear
in the spectral region associated with 3-coordinate boron
environments.20 The N-H groups in 5 and 6 were identified
by 1H NMR spectroscopy and appeared as singlet resonances
at 4.99 and 5.36 ppm, respectively, while weak N-H
vibrations were located at 3358 and 3362 cm-1 in the IR
spectra of 5 and 6, respectively.
The molecular structure of 6 was determined by X-ray
crystallography, and the resulting thermal ellipsoid plot is
depicted in Figure 4. Of note, a considerably shorter B-N
distance was found between the boron center and the aniline
nitrogen N(1) [1.390(3) Å] when compared to the remaining
B-N interaction involving the amino nitrogen of the Tmp
ligand [B(1)-N(2) ) 1.446(3) Å]. As in 1, there appears to
be some B-N π-bonding present (i.e., between B(1) and
N(1)); however, in this case it does not involve the Tmp
ligand. The slightly pyramidal nature of the amide nitrogen
atom of the Tmp group in 6 [angle sum at N(2) ) 355.0-
(5)°], coupled with the longer B-N distance than in
phosphinoborane 1 support the lack of appreciable
B-N(Tmp) π-bonding in 6.35 Furthermore, the Tmp group
is rotated by ca. 90° from the N-B-N plane, which should
dramatically reduce the chance of any π-bonding between
the N atom in the Tmp group and boron.
Figure 4. Molecular structure of 6 with thermal ellipsoids presented at
the 30% probability level. All carbon-bound hydrogen atoms have been
omitted for clarity. Selected bond lengths (Å) and angles (deg): N(1)-
B(1) 1.390(3), C(1)-N(1) 1.432(2), B(1)-N(2) 1.446(3), C(1)-N(1)-B(1)
132.63(18), N(1)-B(1)-N(2) 124.4(2), N(1)-B(1)-Br(1) 113.39(16), Br-
(1)-B(1)-N(2) 122.25(17), B(1)-N(2)-C(25) 117.32(16), B(1)-N(2)-
C(29) 117.83(16), C(25)-N(2)-C(29) 119.86(15).
precursors Ar′N(H)-B(X)Tmp (X ) Cl and Br; 5 and 6).
The aminoboranes 5 and 6 were synthesized by the coupling
of in situ generated Ar′NH(Li) (derived from Ar′NH2 and
nBuLi) with TmpBCl2 and TmpBBr2, respectively, in diethyl
ether. Compounds 5 and 6 were isolated as colorless crystals
in low to moderate yield and were characterized by 1H, 11B,
and 13C{1H} NMR and IR spectroscopy and, in the case of
6, X-ray diffraction (Figure 4).
The NMR spectra of 5 and 6 were consistent with the
assigned monomeric structure, as broad resonances were
observed in the 11B NMR spectra at 31.0 and 30.2 ppm,
respectively. These resonances were shielded relative the
In an attempt to generate the iminoborane adduct
Ar′NdB(DMAP)Tmp, both 5 and 6 were treated with
DMAP in either toluene or hexanes. No reaction was
observed at room temperature in hexanes, and upon refluxing
5 with DMAP in toluene, no change was detected after 5
1
days (by H NMR). From these results, we reasoned that a
stronger base was needed in order to induce HX elimination
from the aminoboranes 5 and 6. Upon mixing either 5 or 6
with the silylamide salt, Na[N(SiMe3)2], and an excess of
DMAP in THF, a new colorless crystalline product was
(35) No¨th, H.; Stolpmann, H.; Thomann, M. Chem. Ber. 1994, 127, 81.
Inorganic Chemistry, Vol. 46, No. 8, 2007 2977

Rivard et al.
Scheme 3. Preparation of the Iminoborane 7
[B(1)-N(2)] was also observed. This bond is considerably
shorter than the B-N(Tmp) distance in the precursor 6
[1.446(3) Å], and the contraction of this bond in 7 can be
attributed, in part, to the decrease in the covalent radius of
boron (sp2 to sp hydridization) from 6 to 7, although a
contribution to the structure of the canonical form
Ar*NdBdTmp cannot be ruled out.
The isolation of 7 highlights the ability of terphenyl ligands
to stabilize low coordination environments, as the related
species, (tBuNBTmp)2, is a dimer despite the presence of a
bulky tert-butyl group at nitrogen.22,39 Mixing 7 with an
excess of DMAP did not result in any appreciable coordina-
tion; thus, the strong B-N intramolecular triple bonding
interaction in 7 is preferred over a dative interaction with
DMAP.
obtained upon workup. This product no longer contained
N-H functionality; however, the anticipated DMAP adduct
was not formed, as resonances belonging to DMAP were
not found in the NMR spectrum of the isolated product.
Notably, a single 11B NMR resonance was detected at 10.6
ppm which closely matched the 11B NMR chemical shifts
(11.8-12.2 ppm) reported for the N-substituted iminoboranes
Mes*NtB-N(SiMe3)R (Mes* ) C6H2-2,4,6-tBu3; R ) alkyl
or silyl group).36 More convincing evidence that the product
was the hindered iminoborane Ar′NtBTmp (7) was obtained
from the infrared spectrum, as two closely spaced bands were
found at 2037 and 1982 cm-1 in an approximate ratio of
1:4. Similar vibrations are found in various iminoboranes
with the higher frequency band belonging to the ν(10BtN)
mode, while the more abundant 11B isotope (ca. 80%
abundance) has a ν(11BtN) vibrational mode that is lower
in frequency by 50-60 cm-1.37 Fortunately, crystals suitable
for an X-ray diffraction study could be grown from hexanes
that conclusively identified the product as the iminoborane
7 (Figure 5, Scheme 3).
Upon inspecting the structure of 7, the most prominent
feature of the molecule is the nearly linear geometry of the
C(ipso)-N-B-N core [N(1)-B(1)-N(2) angle ) 179.4-
(2)°; C(1)-B(1)-N(1) angle ) 171.7(2)°]. Moreover, there
is a very short B(1)-N(1) distance of 1.254(3) Å which
signals the presence of a formal B-N triple bond in 7; of
note, typical B-N lengths in structurally characterized
iminoboranes range from ca. 1.23 to 1.26 Å.36,38 In addition,
a relatively short B-N(Tmp) bond length of 1.374(3) Å
Conclusions
A new class of stable main group species featuring PdB
and AsdB bonds was prepared using a simple donor-
stabilization strategy. Attempts to generate the SbdB
analogue have not been successful to date due to undesired
redox chemistry, while the formation of a stable B-N triple
bond occurred in preference to a donor-stablilized BdN
double bond. Future work will involve the preparation of
new unsaturated bonding environments using this method
and exploring the reaction chemistry of the boranylidene-
pnictanes 2 and 4.
Acknowledgment. The authors are grateful to the Na-
tional Science Foundation and the U.S. Department of Energy
for financial support. E.R. also thanks the Natural Sciences
and Engineering Research Council (NSERC) of Canada for
a Postdoctoral Fellowship. R.W. thanks the Alexander von
Humboldt Foundation for a Feodor Lynen Research Fellow-
ship. We wish to acknowledge Kirstin Campbell and
Michelle Powell for their help in preparing the cover artwork.
We also thank Dr. Roland Fischer for acquiring 11B NMR
spectral data and Drs. Håkon Hope and Marilyn M. Olmstead
for crystallographic assistance.
Supporting Information Available: X-ray crystallographic
structures in CIF format. This material is available free of charge
via the Internet at http://pubs.acs.org.
(36) Luthin, W.; Elter, G.; Heine, A.; Stalke, D.; Sheldrick, G. M.; Meller,
A. Z. Anorg. Allg. Chem. 1992, 608, 147.
(37) Kla¨boe, P.; Bougeard, D.; Schrader, B.; Paetzold, P. Spectrochim. Acta
1986, 41A, 53.
IC062076N
(38) Elter, G.; Neuhaus, M.; Meller, A.; Schmidt-Ba¨se, D. J. Organomet.
Chem. 1990, 381, 299.
(39) Power, P. P. J. Organomet. Chem. 2004, 689, 3904 and references
therein.
2978 Inorganic Chemistry, Vol. 46, No. 8, 2007