Quasi-thermoneutral P → B interactions within di- and tri-phosphine boranes

Article abstract of DOI:10.1021/ic7006556

Spectroscopic, structural, and theoretical evidence is provided for both open (no intramolecular P → B interaction) and closed (with intramolecular P → B interaction) forms of the di- and tri-phosphine boranes [o-(iPr ₂P)C₆H₄

Full text of DOI:10.1021/ic7006556

Inorg. Chem. 2007, 46, 5149−5151
Quasi-Thermoneutral P
Boranes
f B Interactions within Di- and Tri-Phosphine
Se´bastien Bontemps,† Ghenwa Bouhadir,† Philip W. Dyer,‡ Karinne Miqueu,*,§ and Didier Bourissou*,†
Laboratoire He´te´rochimie Fondamentale et Applique´e (UMR CNRS 5069),
UniVersite´ Paul Sabatier, 118, route de Narbonne, 31062 Toulouse Cedex 09, France,
Department of Chemistry, Durham UniVersity, South Road, Durham, DH1 3LE, U.K.,
Equipe de Chimie-Physique (UMR 5254-IPREM), and UniVersite´ de Pau et des Pays de l’Adour,
AVenue de l’UniVersite´, BP 1155, 64013 Pau Cedex, France
Received April 5, 2007
Spectroscopic, structural, and theoretical evidence is provided for
both open (no intramolecular P B interaction) and closed (with
intramolecular P B interaction) forms of the di- and tri-phosphine
Depending on the disposition of the donor and acceptor
moieties about an organic backbone, ambiphilic compounds
may adopt closed or open forms in which there is, or is not,
an intramolecular donor f acceptor interaction, respectively.9
Chemical interconversion between these two forms has been
quite extensively studied for amine-borane (NB) systems,
with its pivotal role in chemosensing having been recognized,
for example.10 In contrast, P f B interactions in PB
derivatives have been scarcely investigated; only the limiting
cases that either have or do not have an interaction have
been described to date.11
f
f
boranes [o-(iPr2P)C6H4]2BPh (2) and [o-(iPr2P)C6H4]3B (3).
Ambiphilic compounds combining both donor and accep-
tor functionalities have attracted increasing interest over the
past 20 years as multi-center catalysts,1 molecular probes,2
and nonlinear optical materials.3 Notably, recent investiga-
tions on phosphine-borane (PB) derivatives have further
extended the synthetic interest of ambiphilic compounds, with
PB-containing species having been demonstrated to be (i)
versatile ligands for transition metals4 affording unusual M
f B interactions;5 (ii) metal-free systems capable of revers-
ible dihydrogen activation under mild conditions;6 (iii) direct
precursors for photoisomerizable heterodienes;7 and (iv)
readily tuneable fluorescent systems.8
Here we report on the unusual behavior encountered in
o-(di- and tri-phosphine)-substituted triarylboranes 2 and 3.
Spectroscopic analyses reveal the coexistence in solution of
the open and closed forms for both compounds 2 and 3, with
the open form 2o and closed form 3c having been structurally
authenticated. These observations are supported by DFT
calculations, which substantiate the propensity of the o-
phenyl spacer to induce quasi-thermoneutral intramolecular
P f B interactions in these systems.
* To whom correspondence should be addressed. Email: dbouriss@
chimie.ups-tlse.fr. Fax: +33 5 6155 8204.
† University Paul Sabatier (Toulouse).
‡ Durham University.
(5) Transition metal f borane interactions were first structurally authen-
ticated within metallaboratranes: Hill, A. F.; Owen, G. R.; White, A.
J. P.; Williams, D. J. Angew. Chem., Int. Ed. 1999, 38, 2759. For
recent discussions on the precise nature of such interactions, see: Hill,
A. F. Organometallics 2006, 25, 4741 and Parkin, G. Organometallics
2006, 25, 4744.
§ Universite´ de Pau et des Pays de l’Adour.
(1) (a) Rowlands, G. J. Tetrahedron 2001, 57, 1865. (b) Gro¨ger, H. Chem.
Eur. J. 2001, 7, 5247. (c) Shibasaki, M.; Kanai, M.; Funabashi, K.
Chem. Commun. 2002, 1989. (d) Ma, J.-A.; Cahard, D. Angew. Chem.,
Int. Ed. 2004, 43, 4566.
(2) Boronic acids featuring a pendant amino group have been used for
chemosensing, see (a) James, T. D.; Sandanayake, K. R. A. S.; Shinkai,
S. Angew. Chem., Int. Ed. 1996, 35, 1910. (b) Zhu, L.; Zhong, Z.;
Anslyn, E. V. J. Am. Chem. Soc. 2005, 127, 4260. (c) Bresner, C.;
Aldridge, S.; Fallis, I. A.; Jones, C.; Ooi, L.-L. Angew. Chem., Int.
Ed. 2005, 44, 3606. In addition, o-(Ph2N)C6H4[B(C6F5)2] was proved
to act as a Lewis acid/Lewis base trap toward H2O and HCl: (d)
Roesler, R.; Piers, W. E.; Parvez, M. J. Organomet. Chem. 2003, 680,
218.
(6) (a) Welch, G. C.; San Juan, R. R.; Masuda, J. D.; Stephan, D. W.
Science 2006, 314, 1124. (b) Kubas, G. J. Science 2006, 314,
1096.
(7) Bebbington, M.; Bontemps, S.; Bouhadir, G.; Bourissou, D. Angew.
Chem., Int. Ed. 2007, 46, 3333.
(8) Dibenzophosphaborins were recently found to exhibit tuneable
fluorescent properties: (a) Agou, T.; Kobayashi, J.; Kawashima, T.
Org. Lett. 2005, 7, 4373. (b) Agou, T.; Kobayashi, J.; Kawashima, T.
Inorg. Chem. 2006, 45, 9137.
(9) For the NB system (2-picolyl)BCy2, an equilibrium between the
monomeric and dimeric forms has been observed: Vergnaud, J.; Ayed,
T.; Hussein, K.; Vendier, L.; Grellier, M.; Bouhadir, G.; Barthelat,
J.-C.; Sabo-Etienne, S.; Bourissou, D. Dalton Trans. 2007, DOI:
10.1039/b704993p.
(3) Entwistle, C. D.; Marder, T. B. Angew. Chem., Int. Ed. 2002, 41, 2927
and references therein.
(4) (a) Bontemps, S.; Gornitzka, H.; Bouhadir, G.; Miqueu, K.; Bourissou,
D. Angew. Chem., Int. Ed. 2006, 45, 1611. (b) Bontemps, S.; Bouhadir,
G.; Miqueu, K.; Bourissou, D. J. Am. Chem. Soc. 2006, 128, 12056.
(c) Emslie, D. J. H.; Blackwell, J. M.; Britten, J. F.; Harrington, L. E.
Organometallics 2006, 25, 2412. (d) Oakley, S. R.; Parker, K. D.;
Emslie, D. J. H.; Vargas-Baca, I.; Robertson, C. M.; Harrington, L.
E.; Britten, J. F. Organometallics 2006, 25, 5835.
(10) Zhu, L.; Shabbir, S. H.; Gray, M.; Lynch, V. M.; Sorey, S.; Anslyn,
E. V. J. Am. Chem. Soc. 2006, 128, 1222 and references therein.
(11) Braunschweig, H.; Dirk, R.; Ganter, B. J. Organomet. Chem. 1997,
545-546, 257 and references therein.
10.1021/ic7006556 CCC: $37.00
Published on Web 05/25/2007
© 2007 American Chemical Society
Inorganic Chemistry, Vol. 46, No. 13, 2007 5149

COMMUNICATION
Scheme 1. Synthesis and Open/Closed Structures of the Di- and Tri-
Phosphine-Boranes 2 and 3
Figure 1. Thermal ellipsoid diagrams (50% probability) for the open form
2o. Thermal ellipsoid for iPr groups and hydrogen atoms are omitted for
clarity.
These calculations support the hypothesis that there exists
an equilibrium between the open and closed forms in solution
at room temperature, with negligible ring strain being induced
by the o-phenyl spacer.17
Table 1. Experimentally Determined and Calculated 31P and 11B NMR
Chemical Shifts (in ppm) at 298 K unless Otherwise Stated
To explore these P f B interactions further, NMR studies
of the open/closed equilibrium were carried out in solution
at low temperature and in the solid state.13 Interestingly, in
THF-d8 at -100 °C, the 31P NMR signal for 3 splits into
two signals at δ 31P ) +26.1 (P f B) and -2.9 (free P)
ppm, in a 1:2 relative ratio by integration, respectively. A
rather similar 31P NMR pattern was obtained for a solid-
state sample with three signals of near-identical relative
integration being observed at +28.5, +1.0, and -1.1 ppm.
Combined with the 11B NMR signal observed at +13.0 ppm
in the solid state,18 these data are most likely attributable to
a frozen closed form of 3. This has been further corroborated
by computing the 11B and 31P NMR chemical shifts for the
optimized open and closed forms of 3 via the GIAO method
(Table 1). Notably, the experimental data are in excellent
agreement with those predicted for the closed form 3c, but
differ significantly from those of the open form 3o.
For the related diphosphine-borane 2, the two phosphorus
atoms remain equivalent upon cooling a THF solution to
-100 °C, and only a slight broadening of the 31P NMR signal
(δ31P ) +11 ppm) was noticed. In the solid state, a single
resonance was observed at δ31P ) -2 ppm. More signifi-
cantly, the solid-state 11B NMR resonance at +71.0 ppm
strongly suggests that unlike 3, the diphosphine-borane 2
adopts the open form in the solid state. This difference in
behavior is supported by the computed NMR data, 11B NMR
chemical shifts of +64.4 and +12.1 ppm being predicted
for the open and closed forms 2o and 2c, respectively.
Intrigued by the different spectroscopic data obtained for
2 and 3 in the solid state, single crystals suitable for X-ray
diffraction analyses were grown for both compounds. In
agreement with the conclusions drawn from the NMR
investigations, the di- and tri-phosphine-boranes were found
to adopt the open (2o) and closed (3c) forms, respectively
(Figures 1 and 2, Table 2). As would be expected, rather
exptl/theor
conditions
11B
31P
2
exptl
exptl
theor
theor
exptl
exptl
exptl
theor
theor
THF
solid
-
+43.1
+71.0
+64.4
+12.1
+50.1
-
+13.0
+14.5
+65.4
+11.0
-2.0
2o
2o
2c
3
3c
3c
3c
3o
+13.4, +12.3
+25.5 (P f B), +9.8
+4.9
+26.1 (P f B), -2.9
+28.5 (P f B), +1.0, -1.1
+25.8 (P f B), +8.0, +4.0
+15.1, +10.5, +8.6
-
THF
THFa
solid
-
-
a At 173 K.
The diphosphine borane 2 was prepared as previously
described4a by bromine-lithium-boron exchange starting
from the readily available (o-bromophenyl)phosphine 112 and
PhBCl2 (Scheme 1). Following the same strategy with BCl3,
the related triphosphine-borane 3 was obtained in 47%
isolated yield.13 The solution-state (THF-d8) multinuclear
NMR data (Table 1) for 2 and 3 are very similar and merit
little comment, except for the 11B NMR chemical shifts (2,
+43.1 ppm; 3, +50.1 ppm), which are in between those of
free triarylboranes (δ 11B ≈ 70 ppm)14 and of the few
phosphine adducts thereof (δ 11B ≈ 0 ppm).15 Comparison
of all these 11B NMR data suggests that compounds 2 and 3
exist in solution either in rapid equilibrium between the open
and closed forms or as intermediate structures. In order to
probe this unusual behavior, DFT calculations were per-
formed on the complete systems 2 and 3 at the BP86/6-31G*
level of theory. In both cases, two minima of similar energy
(∆G < 3 kcal/mol at 25 °C), associated with the open and
closed forms, were located on the potential hypersurface.16
(12) Tamm, M.; Dressel, B.; Baum, K.; Lu¨gger, T.; Pape, T. J. Organomet.
Chem. 2003, 677, 1.
(13) See Supporting Information for details.
(14) (a) Brown, N. M. D.; Davidson, F.; Wilson, J. W. J. Organomet. Chem.
1981, 209, 1. (b) Brown H. C.; Racherla, U. S. J. Org. Chem. 1986,
51, 427.
(15) (a) Burford, N.; Losier, P.; MacDonald, C.; Kyrimis, V.; Bakshi, P.
K.; Cameron, T. S. Inorg. Chem. 1994, 33, 1434. (b) Jacobsen, H.;
Berke, H.; Do¨ring, S.; Kehr, G.; Erker, G.; Fro¨hlich, R.; Meyer, O.
Organometallics 1999, 18, 1724.
(16) For both compounds, the absence of P f B interaction in the open
forms 2o and 3o is supported by the long PB distances (>3 Å), while
the corresponding closed forms 2c and 3c feature short PB distances
(∼2.2 Å).
(17) A very similar P-B distance (2.22 Å) and a near identical P-B
bonding energy (∆E with ZPE correction ) 2.8 kcal/mol) were
predicted for the related unconstrained intermolecular adduct iPr2-
PPh f BPh3.13
(18) No 11B NMR signals could be detected for low-temperature solutions
of 2 and 3.
5150 Inorganic Chemistry, Vol. 46, No. 13, 2007

COMMUNICATION
Table 2. Computed and Experimentally Determined Selected
Geometric Data for 2 and 3 (Bond Lengths in Å, Bond Angles in deg)
BP1 BP2 BP3 ΣBR ΣP1R ΣP2R ΣP3R
2o (exptl) 2.993 2.993
2o (theor) 3.081 3.053
2c (theor) 2.154 3.318
3c (exptl) 2.154 3.307
3c (theor) 2.221 3.428
3o (theor) 3.376 3.232
-
359.9 309.8 309.8
360.0 310.5 311.1
350.7 328.3 303.5
-
-
-
-
-
3.267 348.9 328.4 301.0 302.7
3.317 349.6 327.5 307.4 303.9
3.259 359.8 306.8 317.1 308.4
the noticeable pyramidalization of the boron environment
(ΣBR ) 348.9°).20
Figure 2. Thermal ellipsoid diagrams (50% probability) for the closed
form 3c. Thermal ellipsoid for iPr groups and hydrogen atoms are omitted
for clarity.
In conclusion, the characterization of both the open and
closed forms of the di- and tri-phosphine boranes 2 and 3
provides evidence for quasi-thermoneutral P f B interac-
tions. Future development of PB systems as molecular probes
and activators21 should greatly benefit from such investiga-
tions on P f B interactions. Through modulation of the
organic backbone and substitution pattern, we are currently
trying to identify new PB systems that would adopt the
closed, protected, form in the ground state, but whose related
open, reactive, form would remain accessible energetically.
long B-P distances (∼3.0 Å) and a planar environment
around boron (ΣBR ) 359.9°) were observed for 2o. The
two phosphino-substituted phenyl rings are orientated almost
perpendicularly to the boron coordination plane, while the
unsubstituted ring is rotated out of the plane by 31.5°. This
latter observation, combined with the slightly shortenened
B-Cipso bond length (1.56/1.59 Å), may indicate some
stabilization of the Lewis acid via π-donation. The presence
of an intramolecular P f B interaction in the related
triphosphine-borane 3c is apparent from the short PB distance
(2.1519 vs 3.27 and 3.31 Å for the two other P atoms) and
Acknowledgment. The CNRS, UPS, ANR, and Royal
Society are warmly acknowledged for financial support of
this work. We also thank IDRIS (CNRS, Orsay, France) for
calculation facilities and Dr. H. Gornitzka (X-ray Diffraction
Study Service, Universite´ Paul Sabatier, France) and
Dr. D. Apperley (EPSRC Solid-state NMR Service, Durham
University, UK) for their assistance.
(19) This value is in the upper range of those observed in phosphine-
borane adducts [2.05 Å in (-PhNCH2CH2NPhPPh-)BPh315a and 2.18
Å in Ph3P f B(C6F5)315b], including in four-membered rings: Breen,
T. L.; Stephan, D. W. Organometallics 1997, 16, 365.
(20) The related triphosphine-alane [o-(Ph2PCH2)C6H4]3Al exhibits a
pentacoordinate aluminium (C3P2) center and trigonal-bipyramidal
geometry: Mu¨ller, G.; Lachmann, J.; Rufin´ska, A. Organometallics
1992, 11, 2970.
(21) The critical role of intermolecular P f B interactions in the heterolytic
cleavage of dihydrogen by phosphine/borane combinations has been
recently demonstrated: Welch, G. C.; Stephan, D. W. J. Am. Chem.
Soc. 2007, 129, 1880.
Supporting Information Available: Experimental and com-
putational details, crystallographic data. This material is available
free of charge via the Internet at http://pubs.acs.org.
IC7006556
Inorganic Chemistry, Vol. 46, No. 13, 2007 5151