A phosphine gold(i) π-alkyne complex: Tuning the metal alkyne bond character and counterion position by the choice of the ancillary ligand

Article abstract of DOI:10.1021/ic100093n

The intra- and interionic structures of a mononuclear phosphine gold(I) alkyne complex [(PAr^F₃)Au(2-hexyne)]BF₄ [1BF₄; Ar^F = 3,5-bis(trifluoromethyl)phenyl] and its analogous complex [(NHC)Au(2-hexyne)]BF₄ [2BF₄; NHC = 1,3-bis(diisopropylphenyl)imidazol-2ylidene] have been investigated by combining 1D and 2D multinuclear NMR spectroscopy and density functional theory calculations. It has been found that alkyne in 1BF₄ is depleted of its electron density to a greater extent than that h 2BF₄. This correlates with the Δδ(¹³C) NMR of the carbon-carbon triple bond. Instead, 2BF₄ is much more ''Kinetically stable than 1BF₄. ¹⁹F^-1H HOKY NMR experiments indicate that the counterion locates close to the gold atom in 1BF₄ (differently from that previously observed in the few other gold(I) ion pairs studied), exactly where the computed Coulomb potential indicates that partial positive charge accumulates. 2010 American Chemical Society.

Full text of DOI:10.1021/ic100093n

3080 Inorg. Chem. 2010, 49, 3080–3082
DOI: 10.1021/ic100093n
A Phosphine Gold(I) π-Alkyne Complex: Tuning the Metal-Alkyne Bond Character
and Counterion Position by the Choice of the Ancillary Ligand
Daniele Zuccaccia, Leonardo Belpassi, Luca Rocchigiani, Francesco Tarantelli,* and Alceo Macchioni*
ꢀ
Dipartimento di Chimica, Universita degli Studi di Perugia, Via Elce di Sotto, 8, 06123 Perugia, Italy
Received January 16, 2010
The intra- and interionic structures of a mononuclear phosphine
mechanism is generally thought to proceed,4 have been isolated
gold(I) alkyne complex [(PArF )Au(2-hexyne)]BF4 [1BF4; ArF =
in only a handful of cases5 and never with phosphine ligands,6
which are among the most widely used. At the same time, ion-
pairing interactions in [LAu(alkyne)]X have never been inves-
tigated, although it is known that the counterion7 may have a
significant role in affecting the activity8,9 and regio-10 and
stereoselectivity.11
3
3,5-bis(trifluoromethyl)phenyl] and its analogous complex [(NHC)Au-
(2-hexyne)]BF4 [2BF4; NHC = 1,3-bis(diisopropylphenyl)imidazol-2-
ylidene] have been investigated by combining 1D and 2D multinuclear
NMR spectroscopy and density functional theory calculations. It has
been found that alkyne in 1BF4 is depleted of its electron density to a
greater extent than that in 2BF4. This correlates with the Δδ(13C) NMR
of the carbon-carbon triple bond. Instead, 2BF4 is much more
“kinetically stable” than 1BF4. 19F-1H HOESY NMR experiments
indicate that the counterion locates close to the gold atom in 1BF4
(differently from that previously observed in the few other gold(I) ion
pairs studied), exactly where the computed Coulomb potential indicates
that partial positive charge accumulates.
Here we report on the synthesis and characterization of
[(PArF3)Au(2-hexyne)]BF4 [1BF4; ArF = 3,5-bis(trifluoro-
methyl)phenyl; Scheme 1] and [(NHC)Au(2-hexyne)]BF4
[2BF4; NHC = 1,3-bis(diisopropylphenyl)imidazol-2-ylidene].
1BF4 represents the first example6b of a mononuclear gold
alkyne complex with a phosphine ancillary ligand.
An integrated experimental (NMR) and theoretical
[density functional theory (DFT)]12 investigation on the
intra- and interionic structures of 1BF4 and 2BF4 has
provided very useful information on the effect of the ancillary
ligand on the Au-alkyne chemical bond and unprecedented
indications on the relative anion-cation orientation.
1BF4 was generated in situ within an NMR tube at 204 K
and completely characterized by 1D and 2D multinuclear
NMR experiments in rigorously anhydrous and deoxygenated
CD2Cl2 (Scheme 1 and the Supporting Information, SI).
Under such experimental conditions, 1BF4 is stable for a
few hours, but it quickly decomposes at higher temperature
Gold(I) cationic complexes [LAuþ X-] [L = phosphine,1
3 3 3
N-heterocyclic carbene (NHC), and cyclic (alkyl)(amino)car-
benes (CAAC);2 X- =a weakly coordinating anion] are increa-
singly used as catalysts in a great variety of reactions involving
the activation of carbon-carbon triple bonds.3 Despite this,
π complexes [LAu(alkyne)]X, through which the reaction
*To whom correspondence should be addressed. E-mail: alceo@unipg.it
(A.M.), franc@thch.unipg.it (F.T.).
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3378.
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(3) Hashmi, A. S. K. Chem. Rev. 2007, 107, 3180–3211. Li, Z.; Brouwer, C.;
(>223 K) (SI) yielding [(PArF ) Au]BF4 and colloidal gold.13
3 2
(6) (a) To the best of our knowledge, only bimetallic encumbered alkyne-
tethered phosphinegold(I) complexes have been synthesized: Shapiro, N. D.;
Toste, F. D. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 2779–2782. (b) During
the review procedure of this work, a paper appeared reporting the synthesis of [P(t-
Bu)3Au(alkyne)]SbF6 complexes: Hooper, T. N.; Green, M.; Russell, C. A. Chem.
Commun. 2010, DOI:10.1039/b923900f.
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He, C. Chem. Rev. 2008, 108, 3239–3265. Jimenez-Nnez, E.; Echavarren, A. M.
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€
(12) For a recent review on the theoretical chemistry of gold, see: Pyykko,
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(13) The stability of analogous complexes with terminal alkynes is much
reduced.
r
pubs.acs.org/IC
Published on Web 03/11/2010
2010 American Chemical Society

Communication
Inorganic Chemistry, Vol. 49, No. 7, 2010 3081
Scheme 1
The coordination of 2-hexyne causes a deshielding of the
PArF3 phosphorus resonance from 36.81 ppm in PArF AuCl
3
to 41.65 ppm. The chemical shifts of carbons 2 and 3
(Scheme 1) move from 78.75 and 80.81 ppm in the uncoordi-
nated alkyne to 90.26 (doublet, J2
= 8.06 Hz, Δδ =
C2-P
11.51 ppm) and 91.79 ppm (doublet, J2C1-P = 8.18 Hz, Δδ =
10.98 ppm), respectively, in 1BF4.6b The very similar Δδ and
J2C-P values of C2 and C3 suggest that 2-hexyne coordinates
symmetrically at gold. Indeed, the DFT-computed bond
lengths are almost identical:6b Au-C2 = 2.31 A and
˚
˚
Au-C3 = 2.29 A (SI).
Deshielding values of the C2 and C3 quaternary carbons in
2BF4 are 6.8 and 6.6 ppm, respectively, thus much smaller
than those in 1BF4 and in line with those found for similar
complexes.5b,f The DFT calculations (SI) show that 2-hexyne
is coordinated symmetrically also in 2BF4, with Au-C
Figure 1. Two sections of the low-temperature 19F-1H HOESY NMR
spectrum (376.65 MHz, 204 K, CD2Cl2) of 1BF4. Asterisks denote the
˚
resonances of free 2-hexyne. Average interionic distances (A) are shown
in blue.
˚
˚
distances (Au-C2 = 2.27 A and Au-C3 = 2.29 A) very
similar to those in 1BF4.
The computed [LAuþ alkyne] dissociation energies in
3 3 3
1BF4 (37.4 kcal/mol) and 2BF4 (38.1 kcal/mol) are also very
close and are typical of other alkyne complexes.5f Quite
remarkably, by contrast, the alkyne ligand undergoes a
significantly larger electron density depletion in 1þ than in
2þ. In particular, charge transfer14 from the alkyne to LAuþ
is 0.25 electrons in 1þ versus 0.15 electrons in 2þ, and this
correlates very well with the carbon-carbon triple bond
Δδ (SI). Consistently, Δδ in Cl-Au-3-hexyne is only
5.3 ppm,5d and our computed charge transfer is reduced to
0.07 electrons (SI). It appears evident that LAuþ in 1þ is
globally more acidic than in 2þ. The very similar interaction
energies and structured of 1þ and 2þ further suggest the
presence, in the NHC complex, of a smaller alkynef AuLþ σ
donation and a larger AuLþ f alkyne π back-donation,
producing the observed smaller net charge transfer. Decom-
position of the computed electron density flux in terms of σ
donation and π back-donation is of key interest,5d,f,6a and we
are currently investigating it in depth. The relative anion-ca-
tion position in 1BF4 and 2BF4 was investigated by perform-
ing 19F-1H HOESY NMR experiments15 in CD2Cl2. 1BF4 is
particularly suitable for such studies because the intramole-
cular distances CF3 H8 or CF3 H10 can be used as refe-
Figure 2. Left: Ball-and-stick representation of the most stable ion
pair of 1BF4. Right: Coulomb potential of 1þ mapped on an elec-
tronic isodensity surface (F = 0.007 e/A ; Coulomb potential in atomic
units).
3
˚
preferentially located in the space around the gold atom
between the phosphine and alkyne ligands, closer to the
phosphine, as depicted in Figures 1 and 2 (left). This relative
anion-cation orientation is similar to that found, very
recently, in the solid-state structure of the [P(t-Bu)3Au-
(4,4-dimethylpent-2-yne)]SbF6 complex.6b The computed
Coulomb potential of 1þ, mapped on an electronic
isodensity surface (Figure 2, right), neatly confirms these
conclusions, showing that the most attractive regions
in 1þ are the phosphorus atom and H8 of the aryl
substituent close to the gold site (blue color in
Figure 2). The average H F4B- distances for H1, H4,
3 3 3
3 3 3
3 3 3
rences to quantify the average H F interionic distances (SI).16
and H8 resulting from NOE measurements (4.9, 5.3, and
3 3 3
˚
3.7 A, respectively; Figure 1) are in good agreement with
A strong contact is present between the fluorine atoms of
the counterion and H8 of the phosphine ligand (Figure 1).
those calculated,17 considering the energy of the most
-
stable relative anion-cation configurations of the 1BF4
Medium-strength contacts are observed between BF4
˚
ion pair (4.5, 5.6, and 4.1 A; SI).
and H1, H4, and H10, whereas the anion does not show
any interaction with H5 and H6. This nuclear Overha-
The relative anion-cation orientation in 1BF4 is signifi-
cantlydifferent from thosefoundin[LAu(4-Me-styrene)]BF4
complexes, where the anion preferentially locates on the side
-
user enhancement (NOE) pattern indicates that BF4 is
(14) Belpassi, L.; Infante, I.; Tarantelli, F.; Visscher, L. J. Am. Chem. Soc.
2008, 130, 1048–1060.
(15) Macchioni, A. Eur. J. Inorg. Chem. 2003, 195-205 and references
cited therein.
(16) Zuccaccia, C.; Bellachioma, G.; Cardaci, G.; Macchioni, A. J. Am.
Chem. Soc. 2001, 123, 11020–11028. Macchioni, A.; Magistrato, A.; Orabona, I.;
Ruffo, F.; Rothlisberger, U.; Zuccaccia, C. New J. Chem. 2003, 27, 455–458.
(17) The Æræ distances are computed considering that free rotation of the
methyl and methylene groups is faster than the overall molecular correlation
time (r-3 average), while the restricted rotation of the phenyl ring is slower
than the overall ion-pair correlation time (r-6 average): Neuhaus, D.;
Williamson, M. The Nuclear Overhauser Effect in Structural and Conforma-
tional Analysis; VCH Publishers: New York, 1989.

3082 Inorganic Chemistry, Vol. 49, No. 7, 2010
Zuccaccia et al.
of olefin (L = PPh3) or the L ligand (L = NHC).18 19F-1H
HOESY NMR spectroscopy and DFT calculations show that
the latter orientation is indeed predominant also in 2BF4 (SI).
Two distinct sets of resonances were visible in the 1H and
13C NMR spectra for free and bound 2-hexyne when a slight
excess of 2-hexyne was used for generating 1BF4; this
indicates that the exchange process is slow in the Δδ time
scale. 1H EXSY NMR (exchange spectroscopy) experi-
ments19 were performed in order to derive kinetic para-
meters of the exchange process. An exchange rate constant
kobs = 7.9 ( 0.8 s-1 and a second-order constant k2 =
kobs/[2-hexyne] = 158 ( 20 M-1 s-1 at 204 K were obtained.
For 2BF4, line-shape analysis and 1H EXSY NMR (SI)
indicate a kobs that increases with the 2-hexyne concentration
(SI), thus suggesting an associativeexchange mechanism with
k2 = 26 ( 2 M-1 s-1 at 298 K. If an associative mechanism20
is also operative in 1BF4, its alkyne exchange is clearly much
faster than that of 2BF4 (by 2-3 orders of magnitude,
assuming an activation energy comparable to that of alkene
exchange20).21
allowed us to shed some light on the role played by the L
ancillary ligand in modulating Au-alkyne and ion-pair
interaction patterns. More pronunced electron density deple-
tion occurs at the alkyne in 1BF4 than in 2BF4 because of the
higher acidity of LAuþ in 1þ compared to that in 2þ.22 This
makes alkyne coordinated at the PArF3Auþ moiety more
susceptible to nucleophilic attack. Instead, 2BF4 is much
more “kinetically stable” than 1BF4, probably because of the
steric protection exerted by the isopropyl groups of NHC
that inhibit associative decomposition pathways. All of these
findings support the view that catalysts with phosphine
ancillary ligands are more effective in activating alkyne
substrates (higher TOF), while NHC catalysts are more
robust (higher TON), as found, for example, in the hydration
of alkynes to ketones.23,24 Finally, the presence of CF3
electron-withdrawing groups in 1BF4 causes a shift of the
counterion, which tends to locate close to the gold atom in a
position different from those previously observed in the few
other gold(I) ion pairs studied.18 This illustrates how the
relative anion-cation position, an important factor in de-
termining the catalytic activity and selectivity, can be finely
modulated by the proper selection of the ancillary ligand.
In conclusion, the synthesis and characterization of a
mononuclear phosphine gold(I) alkyne complex (1BF4) and
a comparison with its analogous NHC complex (2BF4) have
Acknowledgment. This work was supported by grants
ꢀ
from the Ministero dell’Istruzione, dell’Universita e della
Ricerca through PRIN and FIRB programmes.
(18) Zuccaccia, D.; Belpassi, L.; Tarantelli, F.; Macchioni, A. J. Am.
Chem. Soc. 2009, 131, 3170–3171.
(19) Perrin, C. L.; Dwyer, T. J. Chem. Rev. 1990, 90, 935–967.
(20) For a detailed study on olefin exchange in NHC and phosphinegold
alkene complexes, see: (a) Brown, T. J.; Dickens, M. G.; Widenhoefer, R. A.
J. Am. Chem. Soc. 2009, 131, 6350–6351. (b) Brown, T. J.; Dickens, M. G.;
Widenhoefer, R. A. Chem. Commun. 2009, 6451–6453. (c) Hooper, T. N.; Green,
M.; McGrady, J. E.; Patela, J. R.; Russell, C. A. Chem. Commun. 2009, 3877–
3879.
Supporting Information Available: Details of the syntheses,
NMR characterization, and computational studies. This
material is available free of charge via the Internet at http://
pubs.acs.org.
(21) Note that, if the exchange mechanisms were dissociative, different
rates would likely imply different interaction energies, in contrast with our
calculations.
(23) PPh3: Mizushima, E.; Sato, K.; Hayashi, T.; Tanaka, M. Angew.
Chem., Int. Ed. 2002, 41, 4563–4565.
(22) The computed alkyne f LAuþ charge transfer in (PPh3)Au(2-hexyne)þ
is very similar (0.21 electrons; SI) to that in 1BF4.
ꢀ
(24) NHC: Marion, N.; Ramon, R. S.; Nolan, S. P. J. Am. Chem. Soc.
2009, 131, 448–449.