A donor-stabilization strategy for the preparation of compounds featuring P=B and As=B double bonds

Article abstract of DOI:10.1039/b609748k

A new class of heavier group 15 compounds demonstrating multiple bonding with boron has been synthesized using a simple donor-stabilization protocol. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b609748k

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A donor-stabilization strategy for the preparation of compounds
featuring PLB and AsLB double bonds
Eric Rivard, W. Alexander Merrill, James C. Fettinger and Philip P. Power*
Received (in Berkeley, CA, USA) 10th July 2006, Accepted 2nd August 2006
First published as an Advance Article on the web 17th August 2006
DOI: 10.1039/b609748k
(Tmp = 2,2,6,6-tetramethylpiperidino).7 Against the backdrop of
A new class of heavier group 15 compounds demonstrating
this work, we speculated that suitable donor molecules could be
used to facilitate access to a boron–pnictogen multiple bond.
In line with prior work in this field,8 we viewed the unsaturated
species Ar*PLB(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 boranylidenephosphane
multiple bonding with boron has been synthesized using a
simple donor-stabilization protocol.
The preparation of stable compounds with multiple bonds between
heavier main group elements is an extremely active field of study.1
In many instances the presence of suitably bulky ligands enables
the isolation of kinetically stable monomeric species with unusual
bonding. In this context, our group has profited by the use of
hindered terphenyl ligands (Ar) in order to prepare a series of main
group species of formula ArEEAr (E = group 13–15 element).2
These compounds display novel reactivity and, in the case of the
digermyne derivative, ArGeGeAr, clean activation of H2 was
observed under ambient conditions.3 As part of efforts to obtain
new unsaturated inorganic systems, we are now exploring the use
of donor molecules to isolate bonding motifs previously unattain-
able with bulky ligands alone. We now report our preliminary
investigations, which involve the successful preparation of rare
species featuring well-defined PLB and AsLB bonds.
derivative when
a suitable donor molecule was present.
Therefore, we constructed the requisite phosphinoborane pre-
cursor Ar*P(H)–B(Br)Tmp (2) from which HBr elimination could
be induced to yield a PLB bond. Compound 2 was prepared from
the reaction of the known dihaloborane9 TmpBBr2 with one equiv.
of Ar*P(H)Li10 in diethyl ether.11
Upon mixing 2 with an excess of the strong donor DMAP
(4-dimethylaminopyridine) in toluene,12 a deep purple mixture
was observed immediately.{ After separation of the
DMAP?HBr byproduct, deep-red crystals were obtained and
identified as the novel base-stabilized boranylidenephosphane,
Ar*PLB(DMAP)Tmp 3, with the help of X-ray crystallography{
(Fig. 1, Scheme 2) in combination with NMR (1H, 11B, 13C and
31P) and UV-vis spectroscopy. This product was found to be
exceedingly air- and moisture-sensitive, though quite thermally
robust (Mp = 169–172 uC).
Stable compounds with triple bonds between boron and
nitrogen were first reported in the 1980s through the seminal
preparation of monomeric iminoboranes RNMBR by Paetzold and
coworkers.4 These species are characterized by their extremely
˚
short B–N linkages (1.23 to 1.26 A) and linear geometry, both
consistent with the presence of B–N triple bonds. However, the
phosphorus congeners of iminoboranes have not been isolated to
date.5 Calculations performed on the simplest boranylidenepho-
sphane, HPBH, highlight some possible reasons behind the
previously observed synthetic difficulties. Unlike linear iminobor-
anes, HPBH has been predicted to be a bent molecule (H–P–B =
Consistent with the assigned structure, broad singlet resonances
were detected in the 31P (+57.3 ppm) and 11B (+41.2 ppm) NMR
spectra of 3. The 31P NMR resonance was considerably downfield
1
of that of the phosphinoborane precursor 2 (280.1 ppm; JPH
=
211 Hz); the 11B NMR signal for 3 lies within the range typically
observed for three-coordinate boron environments.13 The UV-vis
spectrum of 3 consists of a broad absorption centered at 534 nm
(e = 2280 M21 cm21) which has been tentatively assigned to an
allowed p to p* transition associated with the PLB chromophore.
As depicted in Fig. 1, monomeric 3 displays bonding parameters
fully consistent with the presence of a discrete PLB double bond.
Most relevant is the presence of a very short P–B bond distance of
˚
94.5u) with a B–P bond length of 1.756 A, indicative of a PLB
double bond rather than a triple bond. Furthermore, due to the
presence of an unoccupied p-orbital at boron and an active lone
pair at phosphorus, the dimerization energy of HPBH has been
calculated to be exothermic by over 54 kcal/mol.6 Therefore in
order to suppress dimerization of RPLBR9 units and thus preserve
PLB multiple bonding: (a) large substituents need to be present at
P and B, (b) the lone pair at P has to be sequestered by a Lewis
acceptor, and/or (c) the empty p-orbital at B has to be occupied by
either an intra- or intermolecular donor (Scheme 1).
Currently, only one example of a stable boranylidenephosphane
analogue is known, [(CO)5Cr(Et3C)PLBTmp] (1), and its
synthesis requires the binding of a Lewis acidic Cr(CO)5 unit to
phosphorus in order to suppress dimerization of the PLB core
Department of Chemistry, University of California, Davis, California,
95616, USA. E-mail: pppower@ucdavis.edu; Fax: +1 5307528995;
Tel: +1 5307526913
Scheme 1 Possible routes to stable pnictogen–boron double bonds.
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3800 | Chem. Commun., 2006, 3800–3802

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˚
176.1(3)u and a short B–N length of 1.339(5) A, consistent with the
allenic bonding form .PLBLN,. The presence of an sp-hybrized
boron center in 1, coupled with an electron withdrawing Cr(CO)5
˚
moiety at phosphorus, helps explain the ca. 0.06 A contraction in
the P–B bond length in 1 when compared to our base-stabilized
analogue 3.
Given the successful preparation of 3, we decided to investigate
the preparation of heavier congeners. Following the protocol
described above, the lithium arsinide Ar*AsH(Li)10 was reacted
with one equiv. of TmpBBr2 to give the arsinoborane Ar*As(H)–
B(Br)Tmp 4.11 Subsequent treatment of 4 with an excess of
DMAP in hexanes rapidly afforded an intense dark-blue solution
which yielded dark-purple/red dichroic crystals upon work-up.
These crystals were identified as the novel boranylidenearsane
Ar*AsLB(DMAP)Tmp 5 by X-ray crystallography (Fig. 2) and
further analyzed by NMR and UV-vis spectroscopy. As in the case
of 3, crystals of the arsenic derivative 5 were exceedingly air- and
moisture-sensitive, yet had considerable thermal stability (decom-
position at 147 uC). Furthermore, samples of 5 remained
unchanged in benzene solution for ca. one month.
Fig. 1 Molecular structure of 3 with thermal ellipsoids presented at a
30% probability level. All hydrogen atoms and disordered DMAP have
˚
been omitted for clarity. Selected bond lengths (A) and angles (deg) with
The 11B NMR spectrum of 5 consisted of a broad resonance at
+51 ppm, while a transition at 584 nm (e = 2530 M21 cm21) was
the dominant feature of the UV-vis spectrum. This absorption is
significantly red-shifted from that of 3, consistent with the weaker
AsLB bond in 5 [i.e. smaller HOMO(p) to LUMO(p*) gap].
The molecular structure of 5 closely matches that of its
values involving 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(5)]; 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(1)–C(42) 117.84(12),
C(38)–N(1)–C(42) 119.16(11).
˚
phosphorus counterpart 3. A short As–B bond [1.914(6) A] was
˚
1.8092(17) A, which is considerably shorter than typical P–B single
˚
bonds (e.g. P–B single bond length in 2 is 1.954(4) A).
found which parallels the AsLB distances observed [1.926(6) and
2
˚
1.936(11) A] within the borylarsinide anion [PhAsLBMes2] ,
Furthermore, the phosphorus center in 3 is 2-coordinate with a
C(ipso)–P–B bending angle of 114.36(7)u, consistent with sp2-
hybridization at phosphorus. As anticipated, the neighboring
boron center adopts a trigonal planar geometry [angle sum =
which is, to our knowledge, the only other example of a
structurally authenticated AsLB double bond.17 For comparison,
the As–B distances within the 3-coordinate arsinoboranes R2As–
BR2 (which do not have appreciable As–B multiple bonding)
˚
360.0(2)u] with a dative B–N interaction at 1.571(6) A (avg.)
range from 2.06 to 2.20 A.17b The B–N distances in 5 were similar
˚
involving DMAP,14 along with a B–N single bond of 1.4837(19) A
˚
with the Tmp ligand. The observed B–N bond length to the Tmp
group, coupled with the almost perpendicular orientation of Tmp,
suggests that minimal p-bonding exists between N and B; thus the
p-bonding is concentrated between the P and B centers.
Although compound 3 represents the first example of a neutral
base-stabilized boranylidenephosphane, multiple-bonding between
phosphorus and boron has been observed previously. For
example, comparably short P–B bond lengths exist in the anionic
borylphosphides [RPLBR92]2 [1.810(4) to 1.833(6) A],15 and longer
˚
distances were observed in a series of hindered monomeric
phosphinoboranes R2PBR92 [1.839(8) to 1.871(3) A].16 In addition,
˚
˚
a very short P–B interaction of 1.7453(5) A was observed in the
novel acid-stabilized species [(CO)5Cr(Et3C)PLBTmp] 1;7a this
compound also exhibited a nearly linear P–B–N(Tmp) angle of
Fig. 2 Molecular structure of 5 with thermal ellipsoids presented at a
30% probability level. All hydrogen atoms have been omitted for clarity.
˚
Selected bond lengths (A) 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).
Scheme 2 Preparation of the PLB and AsLB bonded species 3 and 5.
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Chem. Commun., 2006, 3800–3802 | 3801

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(Rint = 3.25%). Goodness-of-fit on F2 = 1.102, final R indices [I . 2s(I)],
R1 = 5.35%, wR2 = 15.75% (all data), DMAP was disordered over two
positions (76/24 ratio); 5?(hexane), C58H91AsBN3: monoclinic, space group
˚
to those observed in 3 with distances of 1.567(7) and 1.469(7) A to
the DMAP and Tmp ligands, respectively. The C(ipso)–As–B
bending angle of 113.5(2)u (i.e. sp2-hydridized arsenic), in
conjunction with the planar geometry at boron, reinforces the
assignment of an AsLB double bond in 5.
˚
˚
˚
P21/n, a = 12.7587(13) A, b = 21.142(2) A, c = 20.465(2) A, a = c = 90u, b =
96.813(2)u, V = 5481.3(9) A , Z = 4, m(Mo-Ka) = 0.657 mm21. 37429 total
3
˚
reflections, 9934 independent (Rint = 9.04%). Goodness-of-fit on F2
=
1.089, final R indices [I . 2s(I)], R1 = 7.75%, wR2 = 16.58% (all data).
CCDC 614372–614373. For crystallographic data in CIF or other
electronic format see DOI: 10.1039/b609748k
Attempts to form a hitherto unknown SbLB bond were not
successful, because our efforts to prepare the required stibinobor-
ane Ar*Sb(H)–B(Br or Cl)Tmp from TmpBBr2 (or TmpBCl2)18
and the known antimonide, Ar*SbH(Li)10 resulted in the
formation of significant quantities of the olive green distibene11
Ar*SbLSbAr* in place of clean Sb–B bond formation.
In conclusion, a new class of stable main group species featuring
PLB and AsLB bonds was prepared by using a simple donor-
stabilization strategy. Future work will involve the preparation of
new unsaturated bonding environments using this method, as well
as exploring the reaction chemistry of the boranylidene pnictanes 3
and 5.
1 P. P. Power, Chem. Rev., 1999, 99, 3463.
2 For stable group 13 dimetallenes, see: (a) R. J. Wright, A. D. Phillips,
S. Hino and P. P. Power, J. Am. Chem. Soc., 2005, 127, 4794; (b)
H. Schumann, C. Janiak, J. Pickardt and U. Bo¨rner, Angew. Chem., Int.
Ed. Engl., 1987, 26, 789; (c) M. S. Hill, P. B. Hitchcock and
P. B. Pongtavornpinyo, Angew. Chem., Int. Ed., 2005, 44, 4231; (d)
R. J. Wright, A. D. Phillips, N. J. Hardman and P. P. Power, J. Am.
Chem. Soc., 2002, 124, 8538; (e) N. J. Hardman, R. J. Wright,
A. D. Phillips and P. P. Power, J. Am. Chem. Soc., 2003, 125, 2667; (f)
R. J. Wright, A. D. Phillips and P. P. Power, J. Am. Chem. Soc., 2003,
125, 10784. For group 14 dimetallynes, see: (g) P. P. Power, Chem.
Commun., 2003, 2091; (h) A. Sekiguchi, R. Kinjo and M. Ichinohe,
Science, 2004, 305, 1755; (i) N. Wiberg, W. Niedermayer, G. Fischer,
H. No¨th and M. Suter, Eur. J. Inorg. Chem., 2002, 1066. For group 15
analogues, see: (j) M. Yoshifuji, I. Shima, N. Inamoto, K. Hirotsu and
T. Higuchi, J. Am. Chem. Soc., 1981, 103, 4587; (k) N. Tokitoh, Y. Arai,
R. Okazaki and S. Nagase, Science, 1997, 277, 78; (l) B. Twamley,
C. D. Sofield, M. M. Olmstead and P. P. Power, J. Am. Chem. Soc.,
1999, 121, 3357.
We wish to acknowledge the National Science Foundation and
the Department of Energy for financial support of this work. ER is
grateful to NSERC of Canada for a Postdoctoral Fellowship. The
authors also wish to thank Dr Roland Fischer for acquiring 11B
NMR data, and Drs Geoffrey Spikes, Ha˚kon Hope and Marilyn
Olmstead for crystallographic assistance.
3 G. H. Spikes, J. C. Fettinger and P. P. Power, J. Am. Chem. Soc., 2005,
127, 12232.
4 P. Paetzold, Adv. Inorg. Chem., 1987, 31, 123.
Notes and references
{ All manipulations were carried out under strictly anhydrous and
anaerobic conditions. 3: Pre-cooled toluene (278 uC, 12 mL) was added
to a Schlenk flask containing a mixture of 2 (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
mixture was slowly warmed to room temperature and stirred for 2 d to give
a purple solution along with a white precipitate (DMAP?HBr). The
reaction mixture was filtered and the solvent was then removed and the
product was crystallized from hexane (4 mL, ca. 220 uC, 2 weeks) to give
large well-formed rods of 3 that were dark-red in color (0.045 g, 16%). 1H
NMR (C6D6): d 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, ArH), 8.95 (br d, 2H, CH DMAP). 11B NMR (C6D6): d 41.2
(br, Dn1/2 = ca. 800 Hz). 13C{1H} NMR (C6D6): d 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): d 57.3 (s). Mp (uC): 169–172 (dec). UV-vis
(hexane, nm [e, cm21 M21]): 370 (shoulder), 534 [2280].
5 R. T. Paine and H. No¨th, Chem. Rev., 1995, 95, 343.
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7 (a) G. Linti, H. No¨th, K. Polborn and R. T. Paine, Angew. Chem., Int.
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boranylidenephosphane using AlBr3 as a Lewis acid, see: (b) K. Knabel,
T. M. Klapo¨tke, H. No¨th, R. T. Paine and I. Schwab, Eur. J. Inorg.
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8 (a) A. M. Arif, J. E. Boggs, A. H. Cowley, J. G. Lee, M. Pakulski and
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11 Full details concerning the preparation and structural characterization
of precursors 2 and 4 will appear in a forthcoming publication.
12 DMAP has been used to stabilize monomeric heavy group 13/15
tetraorganyls: (a) F. Thomas, S. Schulz and M. Nieger, Eur. J. Inorg.
Chem., 2001, 161; (b) S. Schulz, Struct. Bonding, 2002, 103, 117. An
interesting DMAP-mediated route to labile heavy group 13–15 bonds
has been reported: ; (c) F. Thomas, S. Schulz, H. Mansikkama¨ki and
M. Nieger, Angew. Chem., Int. Ed., 2003, 42, 5641.
Compound 5 was prepared in a similar manner to 3 except that hexane
was used as the reaction solvent. Data for 5: Red-purple dichroic crystals;
15% yield. H NMR (C6D6): d 0.77 (d, J = 6.6 Hz, 6H, CH(CH3)2), 0.87
1
13 H. No¨th and B. Wrackmeyer, Nuclear Magnetic Resonance
Spectroscopy of Boron Compounds; Springer, Berlin, 1978.
14 For comparison, the B–N distances with the DMAP?BX3 adducts (X =
(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 (m, 1H, ArH), 7.33 (br, 2H,
ArH), 7.48 (d, J = 7.5 Hz, 2H, ArH), 9.42 (br d, 2H, ArH DMAP). 11B
NMR (C6D6): d 51.2 (br, Dn1/2 = ca. 750 Hz). 13C{1H} NMR (C6D6): d
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 (uC): 147 (chars); 169–173 (melts). UV-vis (hexane, nm
[e, cm21 M21]): 328 (shoulder), 584 [2530].
˚
halogen or C6F5) range from 1.589(5) to 1.602(6) A: M. J. Gerald Lesley,
A. Woodward, N. J. Taylor, T. B. Marder, I. Cazenobe, I. Ledoux,
J. Zyss, A. Thornton, D. W. Bruce and A. K. Kakkar, Chem. Mater.,
1998, 10, 1355.
15 R. A. Bartlett, H. V. Rasika Dias, X. Feng and P. P. Power, J. Am.
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16 D. C. Pestana and P. P. Power, J. Am. Chem. Soc., 1991, 113, 8426.
17 (a) M. A. Petrie, S. C. Shoner, H. V. Rasika Dias and P. P. Power,
Angew. Chem., Int. Ed. Engl., 1990, 29, 1033; (b) M. A. Petrie,
M. M. Olmstead, H. Hope, R. A. Bartlett and P. P. Power, J. Am.
Chem. Soc., 1993, 115, 3221.
{ Details of the X-ray diffraction studies: 3?(hexane)0.5, C55H84BN3P:
˚
monoclinic, space group P21/n, a = 12.8832(8) A, b = 19.9163(12) A, c =
˚
3
˚
˚
18 H. No¨th and S. Weber, Z. Naturforsch., B: Anorg. Chem. Org. Chem.,
1983, 38, 1460.
20.7647(13) A, a = c = 90u, b = 95.8420(10)u, V = 5300.3(6) A , Z = 4,
m(Mo-Ka) = 0.088 mm21. 46847 total reflections, 12155 independent
3802 | Chem. Commun., 2006, 3800–3802
This journal is ß The Royal Society of Chemistry 2006