On the versatile and unusual coordination behavior of ambiphilic ligands o-R2P(Ph)BR′2

Article abstract of DOI:10.1021/ja0637494

Monophosphine-boranes were shown to behave as bidentate ambiphilic ligands, whose structural versatility has been illustrated by the preparation of Pd^II and Au^I complexes featuring unusual P → M-Cl → B and P → M → B interactions, res

Full text of DOI:10.1021/ja0637494

Published on Web 08/23/2006
On the Versatile and Unusual Coordination Behavior of Ambiphilic Ligands
o-R2P(Ph)BR′2
Se´bastien Bontemps,† Ghenwa Bouhadir,† Karinne Miqueu,*,§ and Didier Bourissou*,†
Laboratoire He´te´rochimie Fondamentale et Applique´e (UMR CNRS 5069), UniVersite´ Paul Sabatier, 118, route de
Narbonne, F-31062 Toulouse Cedex 09, France, and Laboratoire de Chimie The´orique et Physico-Chimie
Mole´culaire (UMR 5624-FR 2606), AVenue de l’UniVersite´, BP 1155, 64013 Pau Cedex, France
Received May 29, 2006; E-mail: dbouriss@chimie.ups-tlse.fr.
Scheme 1
Transition metal complexes featuring Lewis acid moieties directly
linked to the metal center or at the near periphery have attracted
increasing interest over the past decade. In particular, the structural
characterization of metallaboratranes A and B have allowed for a
better understanding of the propensity of group 13 Lewis acids to
behave as σ-acceptor ligands.1-3 Ligands featuring pendant borane
and alane moieties have also been investigated as promising
candidates for organometallic catalysis, notably via intramolecular
activation of M-X bonds.4,5 Accordingly, Zargarian recently
induced spectacular rate enhancement in PhSiH3 dehydrogenative
oligomerization with the bifunctional Me2PCH2AlMe2 cocatalyst,
which was proposed to form bridged NiII complexes C.6
In this context, we have previously described the coordination
of a preformed tridentate PBP ligand, leading to D-type complexes
featuring RhI f B interactions.7 Here we report the synthesis and
structural characterization of E-type complexes, providing thereby
the first evidence for M f B interactions that are supported by a
single donor buttress. The structural versatility of such ambiphilic
monophosphine-borane ligands is also substantiated by the prepa-
ration of a related E′-type complex featuring a rare example of P
f M-X f B coordination.8
center with the metal fragment, whose nature was deduced from
an X-ray diffraction study (Figure 1).9 Complex 2a adopts a six-
membered ring structure, the bidentate PB ligand bridging the Pd-
Cl bond via P f Pd and Cl f B interactions. The latter contact is
clearly supported by the short ClB distance (2.16 Å) and by
noticeable pyramidalization of the boron environment (ΣBR
)
349.1°). Complex 2a affords a rare example of M-Cl f B
interaction.11
In addition, the phosphine-borane 1a readily displaced the labile
dimethyl sulfide ligand from (Me2S)AuCl in dichloromethane at
1
room temperature, as deduced from the mass, 31P, and H NMR
data of the resulting complex 3a (45% isolated yield). The 11B NMR
chemical shift remains almost identical upon coordination (δ11B
80 ppm for 3a versus 76 ppm in 1a), suggesting a weak, if any,
interaction of the boron center with the metal fragment. The
coordination mode of 3a was unambiguously established by an
X-ray diffraction study (Figure 1).9,12 As a result of the nearly linear
PAuCl arrangement (170.18°), the ClB distance (4.08 Å) is much
higher than that observed in 2a, ruling out the presence of a ClfB
interaction. The gold atom deviates from the mean plane of the
ligand (torsion angle AuPCC of 30°), but the AuB distance remains
rather short (2.90 Å).13 This value is much smaller than the sum of
the van der Waals radii (∼3.7 Å) and suggests the presence of a
gold-boron interaction, despite the marginally pyramidalized
environment around boron (ΣBR ) 358.6°).14
To confirm this unusual bonding situation, the related complex
3b featuring a highly electrophilic borafluorene moiety15 was
prepared from the corresponding phosphine-borane 1b. In marked
contrast with that observed for 3a, the presence of a gold-boron
interaction in solution was indicated by the 11B NMR resonance
signal (3b, 55 ppm), which is shifted to high-field by about 10
ppm compared to that of free PhBFlu (64.5 ppm)16a and very similar
to that recently reported for a borafluorene/π-complex (57 ppm).16b
In the solid-state structure of 3b,9,12 the torsion AuPCC angle was
found to be only 13°, resulting in a significantly shortened boron-
gold distance (2.66 Å) and in a slightly pyramidalized environment
around boron (ΣBR ) 355.8°).
The monophosphine-borane ligand 1a was readily prepared in
77% yield by coupling the related (o-bromophenyl)phosphine with
chlorodicyclohexylborane.9,10 The propensity of 1a to act as an
ambiphilic bidentate ligand via P f M-Cl f B or P f M f B
interactions was then investigated. With this in mind, PdCl(allyl)
and AuCl were chosen as representative metal fragments of various
electronic and geometric properties (d8/d10 configuration and bent/
linear P f M-Cl skeleton).
The dimeric precursor [Pd(µ-Cl)(allyl)]2 was readily cleaved with
1a (Scheme 1). The corresponding complex 2a was isolated as a
yellow powder in 56% yield and fully characterized by multinuclear
NMR and mass spectrometry. The 11B NMR resonance signal for
2a (47 ppm) is shielded by 29 ppm compared to that of the free
ligand 1a. This supports a rather strong interaction of the boron
† Universite´ Paul Sabatier.
§ Laboratoire de Chimie The´orique et Physico-Chimie Mole´culaire.
9
12056
J. AM. CHEM. SOC. 2006, 128, 12056-12057
10.1021/ja0637494 CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
References
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(2) Related M f B interactions have been pointed out in boryl-bridged
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Figure 1. Thermal ellipsoid diagram (50% probability) of 2a (left) and 3b
(right). Selected bond lengths (Å) and angles (deg): (2a) P1-Pd1 2.296(5),
Pd1-Cl1 2.352(1), Cl1-B1 2.165(2), Pd1-C1 2.136(2), Pd1-C3 2.205(2),
C5-B1-C16113.14(17),C5-B1-C22118.37(17),C16-B1-C22117.58(17);
(3b) P1-Au1 2.242(2), Au1-Cl1 2.302(2), Au1-B1 2.663(8), P1-Au1-
Cl1 170.18(6), C1-B1-C13 125.6(6), C1-B1-C24 127.8(7), C13-B1-
C24 102.4(6).
Table 1. Experimental and Theoretical (*) Data for Complexes
3a,b: Selected Bond Length (Å), Boron Pyramidalization and
Torsion Angle (deg), and Total Atomic and Fragment Charges
Derived from NPA Analyses
geometric data
BR
NPA charges
complex
AuB
Σ
AuPCC
P
Au
Cl
BR′
2
3a
3b
3a*
3b*
2.90
2.66
2.99
2.63
358.6
355.8
358.5
353.8
30.0
13.1
21.1
5.7
(5) For a cooperative activation of dba between Pd and a phosphine-
thioether-borane ligand, see Emslie, D. J. H.; Blackwell, J. M.; Britten,
J. F.; Harrington, L. E. Organometallics 2006, 25, 2412-2414.
1.00
1.02
0.31
0.41
-0.58
-0.57
0.29
0.14
(6) Fontaine, F.-G.; Zargarian, D. J. Am. Chem. Soc. 2004, 126, 8786-8794.
(7) Bontemps, S.; Gornitzka, H.; Bouhadir, G.; Miqueu, K.; Bourissou, D.
Angew. Chem., Int. Ed. 2006, 45, 1611-1614.
To gain further insight into the unusual AuB interactions
encountered in complexes 3a,b, DFT calculations were carried out
at the BP86/[LanL2DZ(Au),6-31G*(C,P,B,Cl,H)] level of theory.9
The optimized geometries well reproduced those obtained experi-
mentally (Table 1). Noteworthy, the shortening of the AuB distance
from 3a to 3b is accompanied by a significant decrease of the total
natural charge for the BR′2 fragment (from 0.29 to 0.14), as derived
from Natural Population Analyses (NPA).17 The noticeable influ-
ence of the boron electrophilicity supports some transfer of electron
density from the metal to the σ-acceptor ligand. This is further
substantiated by the increase predicted for the charge of the gold
atom from (i-Pr2PPh)AuCl (0.25) to 3a (0.31) and 3b (0.41). The
contribution of such donor (Au) f acceptor (B) interactions in
complexes 3a,b was also evidenced by second-order perturbation
theory analyses (NBO calculations).9 So far, M f B interactions
have only been characterized in 16e or 18e complexes using tri- or
tetra-dentate ligands.1,7 Complexes 3 provide evidence for such
interactions that occur in 14e complexes and that are supported by
a single donor buttress.
(8) A related complex featuring a P f Ru-H f B interaction has been
obtained by borane insertion into a Ru-C bond: Baker, R. T.; Calabrese,
J. C.; Westcott, S. A.; Marder, T. B. J. Am. Chem. Soc. 1995, 117, 8777-
8784.
(9) See Supporting Information for details.
(10) According to 31P and 11B NMR spectroscopy, 1a adopts a monomeric
structure in solution, the steric hindrance around boron apparently
preventing dimerization as well as solvent coordination.
(11) According to the Cambridge Structural Database, the structural charac-
terization of M-Cl f B interactions has so far been limited to a few
titanium complexes featuring the CpB(C6F5)2 ligand4b and to an osmium
complex featuring an amido-borane ligand: Crevier, T. J.; Bennett, B.
K.; Soper, J. D.; Bowman, J. A.; Dehestani, A.; Hrovat, D. A.; Lovell,
S.; Kaminsky, W.; Mayer, J. M. J. Am. Chem. Soc. 2001, 123, 1059-
1071.
(12) Large Au‚‚‚Au distances (> 5.5 Å) were observed in 3a,b, precluding
aurophilic interactions (Schmidbaur, H. Chem. Soc. ReV. 1995, 24, 391-
400).
(13) Braunstein, Herberich et al. proposed that weak AuB interactions might
be responsible for the sterically congested conformation adopted by a
borole FeAu2 cluster (AuB distances of 3.06 and 3.30 Å): Braunstein,
P.; Herberich, G. E.; Neuschu¨tz, M.; Schmidt, M. U.; Englert, U.; Lecante,
P.; Mosset, A. Organometallics 1998, 17, 2177-2182.
(14) M f B interactions without boron pyramidalization have already been
observed in boryl bridged complexes.2a,b
In conclusion, monophosphine-boranes were shown to behave
as bidentate ambiphilic ligands via P f M-X f B or P f M f
B interactions. Further investigations are currently in progress (i)
to evaluate the scope of such unusual bonding situations with
regards to the metal and co-ligands involved, (ii) to precise the
relative stability of the various coordination modes for a given
complex, and (iii) to determine the influence of the boron
coordination on the geometry and reactivity of the resulting
complexes.18,19
(15) (a) Eisch, J. J.; Galle, J. E.; Kozima, S. J. Am. Chem. Soc. 1986, 108,
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(16) (a) Romero, P. E.; Piers, W. E.; Decker, S. A.; Chau, D.; Woo, T. K.;
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D.; Sole´, S.; Gabba¨ı, F. P. J. Chem. Soc., Dalton Trans. 2004, 1254-
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(17) The same trend, AuB shortening and decrease of the BR′2 charge, was
predicted from 3b to its perfluorinated analogue [i-Pr2PPhBFlu(F8)]AuCl
featuring a borafluorene moiety of higher electrophilicity but similar steric
demand.
(18) For theoretical investigations of the deviation of ML4 complexes from
planarity upon interaction with Lewis acids, see Aullo´n, G.; Alvarez, S.
Inorg. Chem. 1996, 35, 3137-3144. For theoretical investigations of
coordination number and deformation energy in d10 complexes of group
11 metals, see Carvajal, M. A.; Novoa, J. J.; Alvarez, S. J. Am. Chem.
Soc. 2004, 126, 1465-1477.
(19) The critical role of acid cocatalysts in gold(I)-mediated transformations
has been recently highlighted: (a) Dyker, G. Angew. Chem., Int. Ed. 2000,
39, 4237-4239. (b) Hoffmann-Ro¨der, A.; Krause, N. Org. Biomol. Chem.
2005, 3, 387-391. (c) Hashmi, A. S. K. Angew. Chem., Int. Ed. 2005,
44, 6990-6993.
Acknowledgment. We are grateful to the CNRS, UPS, and
French Ministry of Research and New Technologies (Grant ACI
JC4091) for financial support of this work. IDRIS (CNRS, Orsay,
France) is acknowledged for calculation facilities.
Supporting Information Available: Experimental and computa-
tional details; spectroscopic and X-ray crystallographic data for 1-3
(PDF, CIF). This material is available free of charge via the Internet at
http://pubs.acs.org.
JA0637494
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