Syntheses and X-ray crystal structures of cationic, two-coordinate gold(i) π-alkene complexes that contain a sterically hindered o-biphenylphosphine ligand

Article abstract of DOI:10.1039/b914632f

A number of cationic gold π-alkene complexes of the form {[P(t-Bu) ₂o-biphenyl]Au(π-alkene)}⁺SbF₆^- were isolated and three were analyzed by X-ray crystallography.

Full text of DOI:10.1039/b914632f

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Syntheses and X-ray crystal structures of cationic, two-coordinate
gold(I) p-alkene complexes that contain a sterically hindered
o-biphenylphosphine ligandw
Timothy J. Brown, Marina G. Dickens and Ross A. Widenhoefer*
Received (in Berkeley, CA, USA) 20th July 2009, Accepted 3rd September 2009
First published as an Advance Article on the web 24th September 2009
DOI: 10.1039/b914632f
isolation of {[P(t-Bu)2o-biphenyl]Au[Z2-H2CQC(CH2)5]}+SbF6
ꢀ
A number of cationic gold p-alkene complexes of the form
ꢀ
{[P(t-Bu)2o-biphenyl]Au(p-alkene)}+SbF6 were isolated and
three were analyzed by X-ray crystallography.
(1a) in 99% yield (Chart 1) as an air and thermally stable white
solid that was characterized by NMR, combustion analysis,
and X-ray crystallography (see below). The presence of a gold
p-alkene bond in solution was established by the large
difference in the 13C NMR resonances of the olefinic carbon
atoms of bound [d 169.4 (s), 91.8 (t)] and free [d 150.0 (s), 106.4 (t)]
methylene cyclohexane. The downfield shift of the CMe2
carbon and upfield shift of the CH2 carbon point to
unsymmetric binding of the alkene to gold with enhanced
gold s-donation to the unsubstituted alkene terminus. In
addition to 1a, gold pꢀ-alkene complexes {[P(t-Bu)2o-biphenyl]-
Au(Z2-alkene)}+SbF6 [alkene = cis-2-butene (1b), isobutylene
(1c), 1-hexene (1d), 2,3-dimethyl-2-butene (1e), trans-2-butene
(1f), 2-methyl-2-butene (1g), styrene (1h), 4-methylstyrene (1i),
4-bromostyrene (1j), and 4-trifluoromethylstyrene (1k) were
isolated in Z90% yield and were fully characterized (Chart 1).
Slow diffusion of hexanes into a CH2Cl2 solution of
methylene cyclohexane complex 1a at 4 1C gave colorless
crystals of 1a suitable for X-ray analysis (Fig. 1, Table 1).z
Complex 1a adopts a distorted linear conformation with a
P–Au–alkene(centroid) angle of 1651 with the coordinated CQC
bond displaced away from the protruding phenyl group of the
o-biphenyl moiety. The CQC bond of methylene cyclohexane
is bound unsymmetrically to gold with a short Au–CH2 and a
long Au–CR2 interaction (Dd = 0.155 A) (Fig. 1). Crystals of
the 2,3-dimethyl-2-butene solvate complex 1eꢁCH2Cl2 and
Cationic gold(I) complexes have recently emerged as effective
p-activation catalysts for the functionalization of C–C multiple
bonds.1 Although outer-sphere mechanisms have been widely
evoked for these transformations, only recently have
well-defined examples of cationic two-coordinate gold(I)
p-complexes appeared.2–10 Macchioni et al.2 and Nolan
et al.3 have reported cationic, two-coordinate p-alkene
complexes that contain an N-heterocyclic carbene (NHC)
ligand while Sadighi et al.4 and Bertrand et al.5 have reported
analogous [(NHC)Au(p-alkyne)]+ complexes. Echavarren
et al.6 and Bertrand et al.5 have reported the X-ray crystal
structures of cationic, two-coordinate gold p-arene complexes.
Shapiro and Toste have reported multimeric, cationic,
two-coordinate gold(I) p-alkyne and p-alkene complexes in
which the p-ligand is tethered to a triaryl phosphine ligand.7
Russell et al. have very recently described monomeric,
cationic gold(I) p-alkene complexes that contain a tri-tert-
butylphosphine ligand.8
As part of our ongoing interest in the gold(I)-catalyzed
hydroamination of alkenes,11 we recently reported the
syntheses and X-ray crystal structures of
a family of
monomeric, cationic gold p-alkene complexes that contained
the NHC ligand IPr [IPr = 1,3-bis(2,6-diisopropylphenyl)-
imidazol-2-ylidine].12 Along with NHCs, sterically hindered
o-biphenylphosphines have shown considerable utility as
supporting ligands for gold(I)-catalyzed alkene hydroamination11
and related gold(I)-catalyzed transformations.13,14 However,
no information regarding the structure and reactivity of
cationic p-alkene complexes that contain these ligands has
been disclosed. Herein we report the syntheses, X-ray crystal
structures, and equilibrium analyses of gold(I) p-alkene
complexes that contain a sterically hindered o-biphenyl-
phosphine ligand.
Treatment of
a
methylene chloride suspension of
[P(t-Bu)2o-biphenyl]AuCl and AgSbF6 (1 : 1) with methylene
cyclohexane (4 equiv.) at room temperature for 6 h led to
Duke University, French Family Science Center, Durham,
NC 27708-0346, USA. E-mail: rwidenho@chem.duke.edu
w Electronic supplementary information (ESI) available: Experimental
procedures, spectroscopic data, and crystallographic data (65 pages).
CCDC 740535–740537. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/b914632f
Chart 1 Synthesis of gold p-alkene complexes 1a–1k.
Chem. Commun., 2009, 6451–6453 | 6451
ꢂc
This journal is The Royal Society of Chemistry 2009

View Article Online
Table 2 Equilibrium constants for reaction of aliphatic alkenes and
vinyl arenes with 2 in CD2Cl2 at ꢀ60 1C
Alkene
Keq
Vinyl arene
Keq
2-Methyl-2-butene 156 ꢃ 8
H2CQCH-4-C6H4OMe 32 ꢃ 2
H2CQCH-4-C6H4Me 9.4 ꢃ 1.8
Isobutylene
1-Hexene
136 ꢃ 7
133 ꢃ 10
126 ꢃ 9
32 ꢃ 2
H2CQCH-4-C6H4H
H2CQCH-4-C6H4Br
2.3 ꢃ 0.2
cis-2-Butene
Propene
0.47 ꢃ 0.05
H2CQCH-4-C6H4CF3 0.05 ꢃ 0.01
trans-2-Butene
2,3-Dimethyl-2-
butene
14.1 ꢃ 0.4
—
—
—
0.44 ꢃ 0.22 —
coordinated alkene and the protruding aryl group of the
o-biphenylphosphine moiety, which has no counterpart in
the case of the IPr ligand, can be effectively minimized through
rotation about the Au–alkene bond in the case of trisubstituted
alkenes but not in the case of tetrasubstituted alkenes.
To evaluate the effect of alkene electron density on binding
affinity to {[P(t-Bu)2o-biphenyl]Au}+, we determined Keq for
displacement of NCArF from 2 with 4-substituted styrene
derivatives H2CQCH-4-C6H4X (X = OMe, Me, H, Br, CF3).
Keq decreased with decreasing electron density by a factor
of B640 (Table 2) and a plot of log(KX/KH) vs. the
Hammett s-parameter was linear with r = ꢀ3.4 ꢃ 0.2
(Fig. 2). This value is considerably more negative than was
the value obtained for binding of 4-substituted styrenes to the
cationic [(IPr)Au]+ fragment (r = ꢀ2.4),12 which in turn was
considerably more negative than the values obtained for
related cationic, electrophilic late transition metal complexes.15
We studied the kinetics of isobutylene exchange with 1c as a
function of [isobutylene] in CD2Cl2 at 25 1C employing 1H
NMR line broadening techniques. A plot of kobs vs. [isobutylene]
over the concentration range 3–71 mM was linear with a
second-order rate constant for isobutylene exchange of
Fig. 1 ORTEP diagrams of 1a (top), 1eꢁCH2Cl2 (middle), and 1i
(bottom). Ellipsoids are shown at 50%. Counterion, solvent, and
hydrogen atoms are omitted for clarity.
Table 1 Selected bond lengths (A) and bond angles (1) for 1a,
1eꢁCH2Cl2, and 1i
Complex
C1–C2 Au–C1 Au–C2 Au–P P–Au–CQC(centroid)
1a
1.369
2.210
2.292
2.199
2.365
2.293
2.308
2.299 165.1
2.287 162.5
2.286 161.9
1eꢁCH2Cl2 1.325
1i 1.319
4-methylstyrene complex 1i were also analyzed by X-ray
diffraction (Fig. 1, Table 1). These complexes also displayed
a distorted linear geometry with P–Au–alkene(centroid) angles
of B1621. The 2,3-dimethyl-2-butene ligand of complex
1eꢁCH2Cl2 is bonded symmetrically to gold whereas the CQC
bond of vinyl arene complex 1i is bonded unsymmetrically
to gold with a shorter Au–CH2 and a longer Au–C(H)Ar
interaction (Dd = 0.11 A) (Fig. 1).
k
ex = 75 ꢃ 3 Mꢀ1 sꢀ1 (DGz = 15.0 ꢃ 0.1 kcal molꢀ1). Eyring
analysis of the second-order rate constants for isobutylene
exchange from 11 to 40 1C provided the activation parameters:
DHz = 5 ꢃ 1 kcal molꢀ1 and DSz = ꢀ33 ꢃ 4 eu. These data
support an associative pathway for isobutylene exchange,
presumably via the cationic bis(alkene) intermediate
{[P(t-Bu)2o-biphenyl]Au(Z2-H2CQCMe2)2}+SbF6ꢀ. Isobutylene
exchange with 1c occurs with a lower enthalpy of activation
To evaluate the effect of alkene substitution on the binding
affinity of alkenes to the 12 electron cationic gold fragment
{[P(t-Bu)2o-biphenyl]Au}+, equilibrium constants for displace-
ment of NCArF [NCArF = NRC-3,ꢀ5-C6H3(CF3)2] from
{[P(t-Bu)2o-biphenyl]Au(NCArF)}+SbF6 (2) with alkenes
1
were determined in CD2Cl2 at ꢀ60 1C employing H NMR
analysis. Keq decreased by a factor of B350 in the order
2-methyl-2-butene 4 isobutylene E 1-hexene 4 cis-2-butene 4
propene 4 trans-2-butene c 2,3-dimethyl-2-butene. The
equilibrium constants for reaction of alkenes with 2 were
generally larger than those observed for the correspondinꢀg
reactions with NHC derivative [(IPr)Au(NCArF)]+SbF6
with the notable exception of 2,3-dimethyl-2-butene, which
was among the strongest binding alkenes in the case of
[(IPr)Au]+ (Keq = 50 ꢃ 3),12 but was the weakest binding
aliphatic alkene in the case of {[P(t-Bu)2o-biphenyl]Au}+.
Presumably, the destablizing interactions between the
Fig. 2 Plot of log KX/KH vs. the Hammett s-parameter (X = OMe,
Me, Br, and CF3) for the reaction of vinyl arenes with 2 in CD2Cl2
at ꢀ60 1C (r = ꢀ3.4 ꢃ 0.2).
ꢂc
This journal is The Royal Society of Chemistry 2009
6452 | Chem. Commun., 2009, 6451–6453

View Article Online
and a more negative entropy of activation than did isobutylene
2 D. Zuccaccia, L. Belpassi, F. Tarantelli and A. Macchioni, J. Am.
Chem. Soc., 2009, 131, 3170.
´
3 P. de Fremont, N. Marion and S. P. Nolan, J. Organomet. Chem.,
2009, 694, 551.
4 J. A. Akana, K. X. Bhattacharyya, P. Muller and J. P. Sadighi,
¨
exchange with {(IPr)Au(Z2-H2CQC(CH3)2}+SbF6 (DHz =
ꢀ
8 ꢃ 1 kcal molꢀ1, DSz = ꢀ27 ꢃ 4 eu).12 It is not clear whether
the lower DHz in the case of 1c reflects diminished alkene–
ligand steric interaction in the corresponding three-coordinate
J. Am. Chem. Soc., 2007, 129, 7736.
5 (a) V. Lavallo, G. D. Frey, S. Kousar, B. Donnadieu and
G. Bertrand, Proc. Natl. Acad. Sci. U. S. A., 2007, 104, 13569;
(b) V. Lavallo, G. D. Frey, B. Donnadieu, M. Soleilhavoup and
G. Bertrand, Angew. Chem., Int. Ed., 2008, 47, 5224.
intermediates or the greater electrophilicity of the gold center
in 1c relative to {(IPr)Au(Z2-H2CQC(CH3)2}+SbF6
In summary, we have synthesized a family of cationic, linear
gold p-alkene complexes that contain a sterically hindered
P(t-Bu)2(o-biphenyl) ligand, and in three cases we have
characterized these complexes by X-ray crystallography.
ꢀ
.
6 E. Herrero-Gomez, C. Nieto-Oberhuber, S. Lopez, J. Benet-Buchholz
´ ´
and A. M. Echavarren, Angew. Chem., Int. Ed., 2006, 45, 5455.
7 N. D. Shapiro and F. D. Toste, Proc. Natl. Acad. Sci. U. S. A.,
2008, 105, 2779.
8 T. N. Hooper, M. Green, J. E. McGrady, J. R. Patel and
C. A. Russell, Chem. Commun., 2009, 3877.
`
Two points regarding the nature of these complexes vis-a-vis
the corresponding [(IPr)Au(p-alkene)]+ complexes are worth
noting. Firstly, the steric effect of the protruding phenyl ring
of the o-biphenylphosphine ligand is apparent in each of
the complexes analyzed by X-ray crystallography. These
complexes displayed significantly more deviation from linearity
[P–Au–alkene(centroid) = 162–1651] than did the analogous
9 For recent examples of neutral two-coordinate and cationic
three-coordinate gold p-alkene complexes see: (a) J. A. Flores
and H. V. R. Dias, Inorg. Chem., 2008, 47, 4448; (b) H. V. R.
Dias, M. Fianchina, T. R. Cundari and C. F. Campana, Angew.
Chem., Int. Ed., 2008, 47, 556; (c) H. V. R. Dias and J. Wu, Angew.
Chem., Int. Ed., 2007, 46, 7814; (d) J. Wu, P. Kroll and H. V. R.
Dias, Inorg. Chem., 2009, 48, 423.
10 For recent examples of two-coordinate gold s-hydrocarbyl
complexes see: (a) L.-P. Liu, B. Xu, M. S. Mashuta and
G. B. Hammond, J. Am. Chem. Soc., 2008, 130, 17642;
[(IPr)Au(p-alkene)]+ complexes [C(carbene)–Au–alkene(centroid)
=
172–1781] with the alkene ligand displaced away from the
proximal phenyl ring. Secondly, the pronounced electronic depen-
dence of the alkene binding affinity to {[P(t-Bu)2o-biphenyl]Au}+
points to an Au–alkene interaction that is dominated by
alkene-Au s-donation and/or electrostatic interactions at
the expense of Au-alkene p-donation.16 Thus the bias toward
s-donation in these o-biphenylphosphine complexes exceeds
that of the analogous [(IPr)Au(Z2-alkene)]+ complexes,
consistent with the greater s-donating properties of NHC
ligands relative to dialkyl aryl phosphine ligands.17
(b) A. Furstner, M. Alcarazo, R. Goddard and C. W. Lehmann,
¨
Angew. Chem., Int. Ed., 2008, 47, 3210; (c) D. Weber,
´
M. A. Tarselli and M. R. Gagne, Angew. Chem., Int. Ed., 2009,
48, 5733; (d) M. Fananas-Mastral and F. Aznar, Organometallics,
´
2009, 28, 666; (e) A. S. K. Hashmi, T. D. Ramamurthi and
F. Rominger, J. Organomet. Chem., 2009, 694, 592.
11 (a) Z. Zhang, S. D. Lee and R. A. Widenhoefer, J. Am. Chem. Soc.,
2009, 131, 5372; (b) C. F. Bender and R. A. Widenhoefer, Chem.
Commun., 2008, 2741; (c) C. F. Bender and R. A. Widenhoefer,
Org. Lett., 2006, 8, 5303; (d) C. F. Bender and R. A. Widenhoefer,
Chem. Commun., 2006, 4143; (e) X. Han and R. A. Widenhoefer,
Angew. Chem., Int. Ed., 2006, 45, 1747; (f) Y. Shi, S. D. Ramgren
and S. A. Blum, Organometallics, 2009, 28, 1275.
12 T. J. Brown, M. G. Dickens and R. A. Widenhoefer, J. Am. Chem.
Soc., 2009, 131, 6350.
13 (a) H. Li and R. A. Widenhoefer, Org. Lett., 2009, 11, 2671;
Acknowledgement is made to the NSF (CHE-0555425) for
support of this research and to the NCBC (2008-IDG-1010)
for support of the Duke University NMR facility. TJB thanks
Duke University for a Burroughs Wellcome Fellowship.
(b) P. Mauleon, R. M. Zeldin, A. Z. Gonzalez and F. D. Toste,
´ ´
J. Am. Chem. Soc., 2009, 131, 6348; (c) C. Ferrer, C. H. M. Amijs
and A. M. Echavarren, Chem.–Eur. J., 2007, 13, 1358;
(d) Z. Zhang, C. Liu, R. E. Kinder, X. Han, H. Qian and
R. A. Widenhoefer, J. Am. Chem. Soc., 2006, 128, 9066;
(e) C. Nieto-Oberhuber, S. Lopez and A. M. Echavarren, J. Am.
Chem. Soc., 2005, 127, 6178.
Notes and references
z Crystal data for 1a: C27H39AuF6PSb, M = 827.27, monoclinic, a =
8.1045(9), b = 17.2929(19), c = 20.611(2) A, b = 95.833(6)1, V =
2873.7(6) A3, T = 100 K, space group P2(1)/c, Z = 4, reflections
measured 73 553, 11 601 unique (Rint = 0.0587) which were used in all
calculations. The final R1 was 0.0436 and the final wR(F2) was 0.0650
(all data). CCDC 740535. Crystal data for 1eꢁCH2Cl2:
C27H41AuCl2F6PSb, M = 900.18, monoclinic, a = 9.4018(9), b =
16.0494(16), c = 21.377(2) A, b = 93.985(5)1, V = 3217.9(5) A3, T =
100 K, space group P2(1)/c, Z = 4, reflections measured 61 645, 8078
unique (Rint = 0.0406) which were used in all calculations. The final
R1 was 0.0310 and the final wR(F2) was 0.0670 (all data). CCDC
740536. Crystal data for 1i: C29H37AuF6PSb, M = 849.27, ortho-
rhombic, a = 12.702(4), b = 18.693(6), c = 25.481(7) A, V =
6050(3) A3, T = 100 K, space group Pbca, Z = 8, reflections
measured 104 499, 6695 unique (Rint = 0.0826) which were used in
all calculations. The final R1 was 0.0415 and the final wR(F2) was
0.0683 (all data). CCDC 740537.
14 For recent application of the P(t-Bu)2(o-biphenyl) ligand with
other transition metals see: (a) T. Zou, S. Pi and J. Li, Org. Lett.,
2009, 11, 453; (b) S. J. Hwang, S. H. Cho and S. Chang, J. Am.
Chem. Soc., 2008, 130, 16158; (c) M. A. Dureen, A. Lough,
T. M. Gilbert and D. W. Stephan, Chem. Commun., 2008, 4303;
(d) L. J. Goossen, B. Zimmermann and T. Knauber, Angew.
Chem., Int. Ed., 2008, 47, 7103.
15 (a) T. Fueno, T. Okuyama, T. Deguchi and J. Furukawa, J. Am.
Chem. Soc., 1965, 87, 170; (b) H. Kurosawa and N. Asada,
J. Organomet. Chem., 1981, 217, 259; (c) H. Kurosawa,
T. Majima and N. Asada, J. Am. Chem. Soc., 1980, 102, 6996;
(d) F. R. Hartley, Chem. Rev., 1973, 73, 163.
16 For computational analysis of cationic gold(I) p-complexes see:
(a) T. Ziegler and A. Rauk, Inorg. Chem., 1979, 18, 1558;
1 (a) Z. Li, C. Brouwer and C. He, Chem. Rev., 2008, 108, 3239;
(b) R. A. Widenhoefer, Chem.–Eur. J., 2008, 14, 5382;
(c) D. J. Gorin, B. D. Sherry and F. D. Toste, Chem. Rev., 2008,
108, 3351; (d) A. Arcadi, Chem. Rev., 2008, 108, 3266;
(b) R. H. Hertwig, W. Koch, D. Schroder, H. Schwarz, J. Hruak
´
¨
and P. Schwerdtfeger, J. Phys. Chem., 1996, 100, 12253;
(c) M. S. Nechaev, V. M. Rayon and G. Frenking, J. Phys.
Chem. A, 2004, 108, 3134.
´
(e) E. Jime
´
nez-Nu
´
nez and A. M. Echavarren, Chem. Rev., 2008,
17 (a) W. A. Herrmann, Angew. Chem., Int. Ed., 2002, 41, 1290;
(b) I. Huang, H.-J. Schanz, E. D. Stevens and S. P. Nolan,
Organometallics, 1999, 18, 2370.
108, 3326; (f) N. Bongers and N. Krause, Angew. Chem., Int. Ed.,
2008, 47, 2178.
ꢂc
This journal is The Royal Society of Chemistry 2009
Chem. Commun., 2009, 6451–6453 | 6453