Gold(I) complexes of phosphanyl gallanes: From interconverting to separable coordination isomers

Full text of DOI:10.1002/anie.200900737

Communications
DOI: 10.1002/anie.200900737
Ambiphilic Ligands
Gold(I) Complexes of Phosphanyl Gallanes: From Interconverting to
Separable Coordination Isomers**
Marie Sircoglou, Maxime Mercy, Nathalie Saffon, Yannick Coppel, Ghenwa Bouhadir,
Laurent Maron,* and Didier Bourissou*
Over the last few years, several groups have been investigat-
ing the coordination chemistry of so-called ambiphilic donor–
acceptor ligands.[1] The combination of phosphane and borane
or alane moieties has allowed more insight to be gained into
unusual metal–Lewis acid interactions[2–4] and the activation
ꢀ
of M X bonds through bridging coordination, leading
ultimately to heterolytic cleavage.[4b,c,g,5] Representative
examples are the neutral and zwitterionic gold(I) complexes
A
[4d] and B,[5e] derived from diphosphanyl borane (DPB) and
diphosphanyl alane (DPA) ligands, respectively (Scheme 1).
The spectacular results obtained in recent years in the
coordination chemistry of gallium[6–10] prompted us to study
ambiphilic phosphanyl gallane ligands,[11] and herein we
report on their unusual coordination properties. Upon
coordination to the AuCl fragment, both neutral complexes
of type A featuring an unprecedented gold–gallane interac-
tion, and zwitterionic complexes of type B, have been
characterized. Remarkably, the coexistence and interconver-
sion of these coordination isomers can be directly monitored
by NMR spectroscopy with diphosphanyl gallane (DPG),
whereas the neutral and zwitterionic complexes derived from
Scheme 1. Neutral and zwitterionic gold(I) complexes derived from
ambiphilic diphosphanyl borane and diphosphanyl alane ligands.
the corresponding triphosphanyl gallane (TPG) proved read-
ily separable and were both structurally characterized.
The DPG 1 was prepared by adding o-lithiated diisopro-
pylphenylphosphine to GaCl3 in toluene at room temper-
ature. Treatment of 1 with [AuCl(SMe2)] in dichloromethane
(DCM) afforded the desired complex 2 in 33% yield of
isolated product (Scheme 2).[12] Colorless crystals suitable for
[*] M. Mercy, Dr. L. Maron
University of Toulouse, INSA, UPS, LPCNO
135 avenue de Rangueil, 31077 Toulouse (France)
and
CNRS, LPCNO UMR 5215
31077 Toulouse (France)
E-mail: laurent.maron@irsamc.ups-tlse.fr
M. Sircoglou, Dr. G. Bouhadir, Dr. D. Bourissou
University of Toulouse, UPS, LHFA
118 route de Narbonne, 31062 Toulouse (France)
and
CNRS, LHFA UMR 5069, 31062 Toulouse (France)
Fax: (+33)5-6155-8204
E-mail: dbouriss@chimie.ups-tlse.fr
Dr. Y. Coppel
Laboratoire de Chimie de Coordination, UPR8241 CNRS
205 route de Narbonne, 31077 Toulouse (France)
Scheme 2. Coordination of the diphosphanyl gallane ligand 1 to AuCl.
Dr. N. Saffon
Structure Fꢀdꢀrative Toulousaine en Chimie Molꢀculaire, FR 2599
118 route de Narbonne, 31062 Toulouse cedex 9 (France)
X-ray diffraction analysis were obtained from a DCM/
pentane solution at ꢀ308C.[13] Complex 2 adopts the zwitter-
ionic form 2ZI in the solid state (Figure 1). The gallium center
[**] The CNRS, UPS and ANR (ANR-06-BLAN-0034) are warmly
acknowledged for financial support of this work. Fruitful discus-
sions with Dr. K. Miqueu (Pau, France) have been particularly
appreciated. L.M. thanks CalMip (CNRS, Toulouse, France) and
Cines (CNRS, Toulouse, France) for calculation facilities and the
Institut Universitaire de France.
ꢀ
is bound to the two chlorine atoms (Ga Cl = 2.266 and
2.278 ꢀ) and adopts a slightly distorted tetrahedral environ-
ment. As a result, the gold center is only two-coordinate (P-
Au-P = 1658). The Au···Ga and shortest Au···Cl distances are
3.02 and 3.04 ꢀ, respectively, indicating a complete transfer of
the chloride from gold to gallium.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200900737.
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To gain some insight into the structure of 2N and to further
confirm its possible interconversion with 2ZI, DFT calcula-
tions were carried out at the B3PW91/SDD(Au,P,Ga,Cl),6-
31G**(C,H) level of theory, both in the gas phase and in
CDCl3 solution (C-PCM model).[12] The optimized structure
of the zwitterionic form 2ZI is in excellent agreement with that
determined crystallographically. Another minimum was
located for the corresponding neutral square planar complex
2N. The short Au···Ga distance (2.59 ꢀ) and noticeable
pyramidalization of the gallium environment (SGaa = 3458)
in 2N are diagnostic of a Au!Ga interaction. This result is
further corroborated by NBO analysis, in which a dative Au!
Ga interaction (DEint = 33 kcalmolꢀ1) is found at the second-
order perturbation level. In marked contrast with that
observed for the related DPB and DPA complexes A and
B,[14] the zwitterionic and neutral forms of complex 2 are
essentially isoenergetic (DG = 0.3 kcalmolꢀ1 in the gas phase,
and 1.0 kcalmolꢀ1 in CDCl3 solution), supporting the hypoth-
esis of their coexistence. Furthermore, the dipole moment
predicted for 2ZI is about twice that computed for 2N (m = 10.7
vs. 5.2 D in the gas phase), which is in agreement with the
decrease of the 2ZI/2N ratio when the polarity of the solvent
decreases.
The interconversion of the neutral and zwitterionic forms
is associated with the transfer of the chloride between the
metal and the Lewis acid site of the ambiphilic ligand. To slow
down or even prevent this process, we then envisioned the
preparation of cage complexes with reduced flexibility by
introducing a third phosphanyl buttress at gallium. Addition
of GaCl3 to three equivalents of the o-lithiated diisopropyl-
phenylphosphine afforded the TPG 3, which was subse-
quently reacted with [AuCl(SMe2)] in DCM (Scheme 3).[12]
The ensuing complex 4 once again coexists in solution
Figure 1. View of complex 2ZI with ellipsoids set at 50% probability.
Hydrogen atoms are omitted and isopropyl and phenyl groups are
simplified for clarity.
Although the solid-state structure of the DPG complex 2
is very similar to that of the DPA complex B,[5e] spectroscopic
analyses revealed a very different situation in solution.
Indeed, on dissolution of crystals of 2 in chloroform, two
species were observed by 31P NMR spectroscopy, with d =
70 ppm (major) and 86 ppm (minor). The major compound
was assigned to the zwitterionic form 2ZI, which has a single
signal at 71 ppm in the solid state. Dynamic interconversion of
the two species in CDCl3 was supported by the presence of
cross peaks between the two resonance signals in the 2D 31P/
31P EXSY NMR spectrum.[12] The activation barrier for the
exchange of the two forms was estimated to be circa (15 ꢁ
1) kcalmolꢀ1 by variable-temperature and spin saturation
1
transfer H NMR experiments.[12] Furthermore, the propor-
tion of the zwitterionic form, as estimated from 31P and
1H NMR, was found to vary noticeably with the solvent
polarity. From 68% in pure CDCl3, it increases to 88% in the
more polar CD2Cl2, and decreases to 52% in CDCl3/CCl4 1:3
(Figure 2). All these data are consistent with the exchange in
solution between the zwitterionic form 2ZI and the corre-
sponding neutral form 2N in the slow regime of the NMR
timescale.
Scheme 3. Coordination of the triphosphanyl gallane ligand 3 to AuCl.
between the zwitterionic and neutral forms. But in the case
of compound 4, the two coordination isomers proved
separable, as complex 4N (4% yield of isolated product), but
not 4ZI (80% yield of isolated product), is soluble in pentane.
The two coordination isomers of complex 4 lie close in energy
(DG = 4.1 kcalmolꢀ1 in the gas phase).[12] However, in marked
contrast with the related DPG complex 2, no sign of
interconversion between the zwitterionic and neutral forms
of the TPG complex 4 was detected over days when solutions
of pure 4ZI or 4N were monitored by NMR spectroscopy at
room temperature, even at 508C.
Figure 2. 31P NMR spectra of complex 2 in different solvent combina-
tions at 258C.
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Communications
The structures of both complexes were unambiguously
corresponding triphosphanyl borane,[4f,h] the two complexes
4ZI and 4N display approximately C3 symmetry in the solid
state, with average P-Au-Ga-Cipso torsion angles q[20] of 278
and 198, respectively.
confirmed by X-ray analyses (Figure 3).[13] In 4ZI, the gold
center is surrounded by the three phosphorus atoms organ-
Over the last few years, the four conceivable coordination
modes of ambiphilic ligands have been exemplified exper-
imentally and authenticated crystallographically. To further
increase the versatility of such bifunctional ligands, dynamic
systems susceptible to interconvert between different forms
are particularly desirable. But to date, only indirect evidence
for such an exchange process has been reported.[5b,c,f] Herein
we reported the direct observation by NMR spectroscopy of
the neutral and zwitterionic forms of the (DPG)AuCl
complex 2, including their mutual interconversion in solution.
The presence of a third phosphanyl buttress at gallium was
found to inhibit the transfer of the chloride between the metal
and Lewis acid centers, so that the two coordination isomers
are then separable and could be both structurally character-
ized. Dynamic systems derived from ambiphilic ligands offer
new possibilities to finely tune the reactivity of complexes,
and thereby open interesting perspectives in catalysis.
Received: February 6, 2009
Published online: April 9, 2009
Keywords: ambiphilic ligands · coordination modes ·
.
dynamic exchange · gallium · zwitterions
[1] a) F.-G. Fontaine, J. Boudreau, M.-H. Thibault, Eur. J. Inorg.
Chem. 2008, 5439 – 5454; b) I. Kuzu, I. Krummenacher, J. Meyer,
F. Armbruster, F. Breher, Dalton Trans. 2008, 5836 – 5865.
[2] The ability of Lewis acids to act as s-acceptor ligands was
recognized early on: a) R. B. King, Adv. Chem. Ser. 1967, 62,
203 – 220; b) M. L. H. Green, J. Organomet. Chem. 1995, 500,
127 – 148.
[3] Transition metal–borane interactions were first structurally
authenticated in metal boratranes derived from hydridotris-
(imazolyl)borate ligands: A. F. Hill, G. R. Owen, A. J. P. White,
D. J. Williams, Angew. Chem. 1999, 111, 2920 – 2923; Angew.
Chem. Int. Ed. 1999, 38, 2759 – 2761.
Figure 3. Views of the two coordination isomers 4ZI (a) and 4N (b) with
ellipsoids set at 50% probability. Hydrogen atoms and solvate
molecules are omitted and isopropyl and phenyl groups are simplified
for clarity.
ized in a trigonal planar environment (sum of P-Au-P bond
angles = 3598). In the neutral complex 4N, the gold center is
pentacoordinate and adopts a quasi ideal trigonal bipyramidal
geometry.[15] The AuGa distance in 4ZI (2.969 ꢀ) is very
similar to that observed in 2ZI, and is much shorter in 4N
(2.578 ꢀ), and fits well with the sum of covalent radii (2.48–
2.58 ꢀ).[16] The presence of a Au!Ga interaction in 4N is
further supported by the inward pyramidalization of the
gallium environment (sum of C-Ga-C bond angles = 3478). In
4ZI, the gallium center also adopts a pseudo-tetrahedral
environment, but points opposite to the gold center (outward
pyramidalization, SC-Ga-C = 3568). According to NBO anal-
[4] a) S. Bontemps, H. Gornitzka, G. Bouhadir, K. Miqueu, D.
Bourissou, Angew. Chem. 2006, 118, 1641 – 1644; Angew. Chem.
Int. Ed. 2006, 45, 1611 – 1614; b) S. Bontemps, G. Bouhadir, K.
Miqueu, D. Bourissou, J. Am. Chem. Soc. 2006, 128, 12056 –
12057; c) S. R. Oakley, K. D. Parker, D. J. H. Emslie, I. Vargas-
Baca, C. M. Robertson, L. E. Harrington, J. F. Britten, Organo-
metallics 2006, 25, 5835 – 5838; d) M. Sircoglou, S. Bontemps, M.
Mercy, N. Saffon, M. Takahashi, G. Bouhadir, L. Maron, D.
Bourissou, Angew. Chem. 2007, 119, 8737 – 8740; Angew. Chem.
Int. Ed. 2007, 46, 8583 – 8586; e) S. Bontemps, M. Sircoglou, G.
Bouhadir, H. Puschmann, J. A. K. Howard, P. W. Dyer, K.
Miqueu, D. Bourissou, Chem. Eur. J. 2008, 14, 731 – 740; f) S.
Bontemps, G. Bouhadir, W. Gu, M. Mercy, C.-H. Chen, B. M.
Foxman, L. Maron, O. V. Ozerov, D. Bourissou, Angew. Chem.
2008, 120, 1503 – 1506; Angew. Chem. Int. Ed. 2008, 47, 1481 –
1484; g) D. J. H. Emslie, L. E. Harrington, H. A. Jenkins, C. M.
Robertson, J. F. Britten, Organometallics 2008, 27, 5317 – 5325;
h) M. Sircoglou, S. Bontemps, G. Bouhadir, N. Saffon, K.
Miqueu, W. Gu, M. Mercy, C.-H. Chen, B. M. Foxman, L.
Maron, O. V. Ozerov, D. Bourissou, J. Am. Chem. Soc. 2008, 130,
16729 – 16738.
ysis, the magnitude of the Au!Ga interaction in 4N (DEint
=
25 kcalmolꢀ1) is comparable to that predicted for 2N. A few
adducts between transition metals (Mo, Fe, Co, Ru, Rh, Pt)
and trihalogallanes have been structurally characterized,[17]
but to the best of our knowledge, complex 4N is the first
example of such a M!GaR3 interaction with a Group 11
element.[18,19] Similar to metal boratranes derived from the
3456
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[5] a) R. T. Baker, J. C. Calabrese, S. A. Westcott, T. B. Marder, J.
Am. Chem. Soc. 1995, 117, 8777 – 8784; b) F.-G. Fontaine, D.
Zargarian, J. Am. Chem. Soc. 2004, 126, 8786 – 8794; c) M.-H.
Thibault, J. Boudreau, S. Mathiotte, F. Drouin, O. Sigouin, A.
Michaud, F.-G. Fontaine, Organometallics 2007, 26, 3807 – 3815;
d) A. Fischbach, P. R. Bazinet, R. Waterman, T. D. Tilley,
Organometallics 2008, 27, 1135 – 1139; e) M. Sircoglou, G.
Bouhadir, N. Saffon, K. Miqueu, D. Bourissou, Organometallics
2008, 27, 1675 – 1678; f) S. Bontemps, G. Bouhadir, D. C.
Apperley, P. W. Dyer, K. Miqueu, D. Bourissou, Chem. Asian
J. 2009, 4, 428 – 435.
[6] R. A. Fischer, J. Weiss, Angew. Chem. 1999, 111, 3002 – 3022;
Angew. Chem. Int. Ed. 1999, 38, 2830 – 2850.
[7] For a dinuclear complex bridged by a bare gallium atom, see: K.
Ueno, T. Watanabe, H. Tobita, H. Ogino, Organometallics 2003,
22, 4375 – 4377.
[8] For complexes featuring terminally bound gallium(I), see: a) B.
Buchin, C. Gemel, T. Cadenbach, I. Fernꢁndez, G. Frenking,
R. A. Fischer, Angew. Chem. 2006, 118, 5331 – 5334; Angew.
Chem. Int. Ed. 2006, 45, 5207 – 5210; b) S. Aldridge, Angew.
Chem. 2006, 118, 8275 – 8277; Angew. Chem. Int. Ed. 2006, 45,
8097 – 8099; c) T. Cadenbach, C. Gemel, T. Bollermann, I.
Fernꢁndez, G. Frenking, R. A. Fischer, Chem. Eur. J. 2008, 14,
10789 – 10796.
[9] For terminal gallylene complexes, see: a) T. Cadenbach, C.
Gemel, D. Zacher, R. A. Fischer, Angew. Chem. 2008, 120,
3487 – 3490; Angew. Chem. Int. Ed. 2008, 47, 3438 – 3441;
b) N. D. Coombs, W. Clegg, A. L. Thompson, D. J. Willock, S.
Aldridge, J. Am. Chem. Soc. 2008, 130, 5449 – 5451; c) N. D.
Coombs, D. Vidovic, J. K. Day, A. L. Thompson, D. D. Le P-
evelen, A. Stasch, W. Clegg, L. Russo, L. Male, M. B. Hurst-
house, D. J. Willock, S. Aldridge, J. Am. Chem. Soc. 2008, 130,
16111 – 16124; d) H.-J. Himmel, G. Linti, Angew. Chem. 2008,
120, 6425 – 6427; Angew. Chem. Int. Ed. 2008, 47, 6326 – 6328.
[10] For d- and f-block gallyl complexes derived from anionic
gallium(I) heterocycles, see: a) R. J. Baker, C. Jones, Coord.
Chem. Rev. 2005, 249, 1857 – 1869; b) P. L. Arnold, S. T. Liddle, J.
McMaster, C. Jones, D. P. Mills, J. Am. Chem. Soc. 2007, 129,
5360 – 5361; c) C. Jones, A. Stasch, W. D. Woodul, Chem.
Commun. 2009, 113 – 115; d) S. T. Liddle, J. McMaster, D. P.
Mills, A. J. Blake, C. Jones, W. D. Woodul, Angew. Chem. 2009,
121, 1097 – 1100; Angew. Chem. Int. Ed. 2009, 48, 1077 – 1080.
[11] a) O. T. Beachley, Jr., M. A. Banks, M. R. Chruchill, W. G.
Feighery, J. C. Fettinger, Organometallics 1991, 10, 3036 – 3040;
b) G. Mꢂller, J. Lachmann, Z. Naturforsch. B 1993, 48, 1544 –
1554; c) H. Schumann, F. Girgsdies, B. Heymer, J. Kaufmann, C.
Marschall, W. Wassermann, Z. Anorg. Allg. Chem. 2007, 633,
2268 – 2273; d) A. M. V¼lean, S. Gꢃmez-Ruiz, P. Lꢄnnecke, I.
Silaghi-Dumitrescu, L. Silaghi-Dumitrescu, E. Hey-Hawkins,
Inorg. Chem. 2008, 47, 11284 – 11293.
[12] See the Supporting Information for details.
[13] CCDC 719580 (2ZI), CCDC 719581 (4ZI), and CCDC 719582
(4N) contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.
uk/data_request/cif.
[14] The difference in energy was estimated to be circa 10 kcalmolꢀ1
in favor of the neutral form for DPB, but in favor of the
zwitterionic form for DPA.[5e]
[15] The Ga-Au-Cl fragment is almost linear (1798) and the sum of P-
Au-P bond angles is 3548.
[16] a) B. Cordero, V. Gꢃmez, A. E. Pletro-Prats, M. Revꢅs, J.
Echeverrꢆa, E. Cremades, F. Barragꢁn, S. Alvarez, Dalton Trans.
2008, 2832 – 2838; b) P. Pyykkꢄ, M. Atsumi, Chem. Eur. J. 2009,
15, 186 – 197.
[17] a) R. A. Fischer, A. Miehr, H. Hoffmann, W. Rogge, C. Boehme,
G. Frenking, E. Herdtweck, Z. Anorg. Allg. Chem. 1999, 625,
1466 – 1474; b) G. Linti, G. Li, H. Pritzkow, J. Organomet. Chem.
2001, 626, 82 – 91; c) E. Leiner, O. Hampe, M. Scheer, Eur. J.
Inorg. Chem. 2002, 584 – 590; d) U. Vogel, M. Scheer, Z. Anorg.
Allg. Chem. 2003, 629, 1491 – 1495; e) M. Cokoja, C. Gemel, T.
Steinke, F. Schrꢄder, R. A. Fischer, Dalton Trans. 2005, 44 – 54;
f) T. Steinke, C. Gemel, M. Cokoja, M. Winter, R. A. Fischer,
Dalton Trans. 2005, 55 – 62; g) N. R. Bunn, S. Aldridge, D. L.
Kays (nꢅe Coombs), N. D. Coombs, J. K. Day, L.-L. Ooi, S. J.
Coles, M. B. Hursthouse, Organometallics 2005, 24, 5879 – 5890;
h) H. Braunschweig, K. Gruss, K. Radacki, Inorg. Chem. 2008,
47, 8595 – 8597.
[18] Transition metal complexes of organyl gallanes have only been
spectroscopically characterized to date: J. M. Burlitch, M. E.
Leonowicz, R. B. Petersen, R. E. Hughes, Inorg. Chem. 1979, 18,
1097 – 1105.
[19] The coordination of low-valent Ga(I) ligands has recently
allowed the structural characterization of a few Au–Ga bonds:
a) U. Anandhi, P. R. Sharp, Angew. Chem. 2004, 116, 6254 – 6257;
Angew. Chem. Int. Ed. 2004, 43, 6128 – 6131; b) A. Kempter, C.
Gemel, R. A. Fischer, Inorg. Chem. 2005, 44, 163 – 165; c) A.
Kempter, C. Gemel, N. J. Hardman, R. A. Fischer, Inorg. Chem.
2006, 45, 3133 – 3138; d) S. P. Green, C. Jones, D. P. Mills, A.
Stasch, Organometallics 2007, 26, 3424 – 3430.
[20] This parameter has been introduced to quantify the chiral twist
of hydridotris(methimazolyl)borate ligands in the ensuing C3-
symmetric complexes: M. R. St.-J. Foreman, A. F. Hill, A. J. P.
White, D. J. Williams, Organometallics 2003, 22, 3831 – 3840.
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