Geminally diaurated gold(I) aryls from boronic acids

Article abstract of DOI:10.1002/anie.201201744

The electrophilic [Ph₃PAu]NTf₂ quickly diaurates aryl boronic acids providing stable geminally diaurated complexes (see example; yellow, Au; orange, P; blue, C) that have been spectroscopically, and in some cases crystallographically

Full text of DOI:10.1002/anie.201201744

Angewandte
Chemie
DOI: 10.1002/anie.201201744
Gold Aryl Complexes
Geminally Diaurated Gold(I) Aryls from Boronic Acids**
James E. Heckler, Matthias Zeller, Allen D. Hunter, and Thomas G. Gray*
Gold catalysts and catalyst precursors have commanded
attention for more than a decade. Several reviews[1–8] highlight
the utility of gold in organic transformations. Recent efforts
have succeeded in isolating putative intermediates of alkene
and alkyne transformations. In 2008, Hammond and co-
workers generated aurated g-lactone complexes from
Scheme 1.
allenes.[9] Later, Bertrand, Hashmi, and their respective co-
workers independently reported vinylgold(I) species derived
from alkynes.[10] Gagnꢀ and co-workers first implicated
geminally diaurated species as resting states in catalytic
cycles of gold by reacting vinylgold(I) species with one
equivalent of gold(I) cation.[11] Fꢁrstner and co-workers later
showed that transmetalation from boron using the Gagosz
complex[12] ([Ph3PAu]NTf2, Tf = trifluoromethanesulfonyl)
gave only geminally diaurated species regardless of the
number of equivalents used.[13]
Such species are not recent discoveries. Work by Nes-
meyanov and co-workers in the 1970s produced the first
geminally diaurated aromatic ring systems, bis(triphenylphos-
phine)gold(I) complexes of ferrocene.[14] The work was soon
expanded to include adducts of heteroarenes.[15] Schmidbaur
and co-workers have reported geminally diaurated diketones
and disulfones,[16] as well as arenes, although with lithiated
reagents.[17] Most recently, Hashizume et al. generated a gem-
diaurated 2-naphthyl species by protonation with perchloric
acid of mono-gold complexes, with subsequent capture of the
released cationic gold fragment.[18] In contrast to the recent
development of diaurated vinyl complexes, the frontier of
arene gem-diauration remains relatively quiet. A very recent
report by Gagnꢀ and co-workers disclosed a preparation of
several diaurated species from parent monoaurated arenes.[19]
Such species deserve further investigation.
or they entail the loss of one half equivalent of aryl ligand—
a costly issue in the case of value-added organic ligands. Here,
we disclose syntheses of geminally diaurated arenes from
arylboronic acids[22] without the need for activating silver
salts, Brønsted acids, or Grignard reagents. The reactions are
mild, quick, and high-yielding. In most cases the isolated
solids are pure. No air- or moisture-free techniques were
employed. All products are fully characterized by 1H and
31P NMR spectroscopy and elemental analysis, unless other-
wise noted; 13C NMR data are not reported because attempts
to detect the bridging, ipso-carbon were unsuccessful.[23] For
several of the products, crystals suitable for X-ray diffraction
analysis were grown.
Reaction of one-half equivalent of arylboronic acid with
[Ph3PAu]NTf2 in diethyl ether led to the precipitation of the
expected diaurated complex [(m-aryl)(Ph3PAu)2]NTf2
Due to the isolobality of the proton and the LAu+
fragment (L = phosphine, carbene, and others),[20,21] diaurated
arenes are analogues of Wheland intermediates in electro-
philic aromatic substitutions (Scheme 1). However, future
studies would benefit from mild methods of generating these
complexes. All established routes require hazardous reagents,
Scheme 2.
(Scheme 2). Washing the solid with diethyl ether removed
the boronimidate side product and small amounts of
unreacted boronic acid. The as-isolated solids persist at
room temperature, but decompose in solution within days.
Monitoring the reaction by 31P{1H} NMR spectroscopy in
tetrahydrofuran at room temperature showed the concom-
itant disappearance of starting material (d = 26.0 ppm) and
appearance of two resonances: one major peak (ꢀ 98%,
approximately d = 37.0 ppm) and one minor (d = 45.3 ppm).
The latter, minor peak corresponds to an in-solution decom-
position product [(Ph3P)2Au]+, in agreement with previous
reports.[11,13] This peak gradually becomes dominant over
a course of days in both THFand chlorinated solvents. Table 1
collects new gold compounds and yields of isolated products.
[*] J. E. Heckler, Prof. Dr. T. G. Gray
Department of Chemistry, Case Western Reserve University
Cleveland, OH 44106 (USA)
E-mail: tgray@case.edu
Homepage: http://gray.case.edu/
Dr. M. Zeller, Prof. Dr. A. D. Hunter
Department of Chemistry, Youngstown State University
Youngstown, OH 44555 (USA)
[**] We the U.S. National Science Foundation (CHE- 1057659 to T.G.G.)
for support. The diffractometer was funded by NSF Grant 0087210,
by the Ohio Board of Regents Grant CAP-491, and by Youngstown
State University.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201201744.
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Table 1: Geminally diaurated arenes and yields of isolated products.
Arylboronic acid
[Product]NTf2
Yield
[%]
1
2
95
45
3
4
5
94
61
99
Figure 1. Thermal ellipsoid projection of indolyl derivative 6 (50%,
100 K). Hydrogen atoms are omitted for clarity. Unlabeled atoms are
carbon.
6
83
an approximately linear geometry about the P-Au-C bonds;
the bond angles are 176.46(19)8 and 173.2(2)8. The molecule
features a tight intramolecular aurophilic interaction; the
Au···Au distance is 2.7699(7) ꢂ. Examination of key bond
lengths shows diminished aromaticity that is presumably
compensated for by the stabilizing aurophilic interaction. The
7
96
98
89
91
ꢁ
ꢁ
8
bond lengths C1 C2 and C6 C1 are 1.401(10) and 1.455(9) ꢂ,
respectively; those of the parent indole are 1.399 and
1.414 ꢂ.[24] The longer bonds suggest that positive charge
delocalizes across the benzopyrrole skeleton and diminishes
the aromaticity.
9
The photophysical properties of 11 are representative.
Optical spectra of this compound were collected in CH2Cl2.
Absorption of 11 obeys Beerꢃs law, and extends to 330 nm
(see Supporting Information). Room-temperature emission
(294 nm excitation) extends from 350–700 nm with a Stokes
shift of 6700 cmꢁ1. Emission spectra of 11 and of its mono-
gold analogue [Ph3PAu(1-naphthyl)][22] appear in Figure 2.
The emission profile of a solution of di-gold 11 is red-shifted
10
11
12
90
98
13
79
X-ray crystallographic studies confirmed the geminal
nature of the dinuclear complexes. Compounds 1, 6, 7, 8,
and 12 grew suitable crystals by diffusion of diethyl ether into
either saturated chloroform or THF solutions. Figure 1 shows
the solid-state structure of 6. Both gold atoms bind to C1 of
the indole scaffold in a distorted tetrahedral geometry; the
Au1-C1-Au2 angle is 81.3(2)8 and lies perpendicular to the
C2-C1-C6 angle, which is 117.8(6)8. Both gold atoms maintain
Figure 2. Room-temperature emission spectra of mono-gold and gemi-
nal di-gold 1-naphthyl complexes in methylene chloride.
2
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from that of the mono-gold compound. Luminescence spectra
of both compounds show vibronic structure, with peak-to-
peak spacings near 1200 cmꢁ1. Structured emission is typical
for gold(I) aryls; the peak separations suggest that ring
deformation modes are activated in the emitting state. The
vibronic emission and the large Stokes shift suggest phos-
phorescence from a triplet excited state, as observed in other
gold(I) organometallics.[25–29]
geminally diaurated species. These results suggest some gold
participation in the excited states of geminally diaurated aryls,
whereas phosphorescence of normal gold(I) aryls and alkyn-
yls is ligand-centered.[25–29,31–33]
The available crystal structures suggest disrupted aroma-
ticity, as do the optimized geometries. Perturbations of
carbon–carbon bond lengths decrease with distance from
the dimetallation site. The same trend prevails for natural
ꢁ
Density functional theory calculations have been applied
to models of 1, 6, and 11, where PMe3 ligands replace PPh3 for
computational simplicity. These model complexes are 1’, 6’
and 11’, respectively. Geometries were optimized with gold–
gold distances constrained to those in the crystal structures.
These constraints are imposed because of the dispersive
nature of aurophilic interactions,[30] which DFT does not
entirely capture. Converged geometries of 1’ and 6’ agree well
with the crystal structures. Harmonic vibrational frequency
calculations find all three to be energy minima. Full computa-
tional details appear in the Supporting Information.
For the three model compounds, the highest-occupied
Kohn–Sham orbital (HOMO) resides on the aryl ligand. The
lowest unoccupied Kohn–Sham orbital (LUMO) has mixed
aryl–gold(I) character. Figure 3 depicts a frontier orbital
energy diagram of 6’; that for 11’ appears in the Supporting
Information. Plots of selected orbitals appear at right.
Methylene chloride solvation is included using a polarizable
continuum model. In these compounds, the HOMO lies
mainly on the aryl ligand, whereas the LUMO, the
HOMOꢁ1, and other near-frontier orbitals have sizable
(> 10%) contributions from the (phosphine)gold(I) frag-
ments. Percentages are of electron density, from Mulliken
population analysis. A de facto separability of gold- and aryl-
based orbitals in normal gold(I) aryls does not extend to
bond orders calculated for aromatic C C bonds. The mean
bond length between the metalated and adjacent carbons is
1.413 ꢂ for 1’, 6’, and 11’ (range: 1.391–1.437 ꢂ); the
corresponding value calculated for [Me3PAuPh] (Cs symme-
try) is 1.403 ꢂ. For the three model compounds, the mean
gold–carbon bond length is 2.169 ꢂ (range: 2.162–2.174 ꢂ);
that computed for [Me3PAuPh] is 2.065 ꢂ. The average,
calculated C-Cmetalated-C bond angle is 118.08; for mono-gold
[Me3PAuPh], it is 116.48. A single metalation at carbon does
not much perturb sp2 hybridization, and the aurated carbon
does not approach a tetrahedral geometry. The mean
calculated Au-C-Au angle of 1’, 6’ and 11’ is 78.08; the
aurophilic interaction precludes sp3 hybridization at C.
Wiberg bond orders in the Lowdin basis[34] also suggest
diminished aromaticity. Table 2 collects bond orders calcu-
lated for 1’, 6’, and 11’; for their mono-gold analogues, and for
the corresponding Wheland complexes. Both calculated geo-
metries and bond orders suggest that geminal diauration
degrades aromaticity, but less so than outright protonation
(Wheland complex formation). In agreement with Reed and
co-workers,[35] we find that positive charge does not collect at
carbon. Natural atomic charges are positive and roughly equal
(ca. 0.25) for gold and hydrogen, and largest (> 0.95) for
phosphorus. Carbon atoms have negative charges, be they
protonated, geminally diaurated, or aromatic. The mean
natural charge of dimetalated car-
bons across 1’, 6’, and 11’ is ꢁ0.50.
In diaurated compounds, positive
charge buildup at ortho and para
sites is muted compared to that in
Wheland complexes.
Time-dependent DFT calcula-
tions indicate that the aryl ligand
dominates the emission and low-
energy absorption features. Naph-
thyl 11’ is typical. The lowest
excited singlet and triplet states
!
are mainly LUMO HOMO
transitions; the triplet has small
contributions from other triplet
states with which it undergoes
configuration interaction. The
frontier orbitals of geminally dia-
urated aryls have greater gold
character than in mono-gold ana-
logues. Hence, emission from 11’
is not cleanly ligand-centered, but
the naphthyl moiety dominates.
Calculations on 1’, 6’, and other di-
Figure 3. Partial Kohn–Sham orbital energy diagram of 6’. Plots of selected orbitals and percentage
compositions in terms of fragments appear at right. Implicit methylene chloride solvation is included;
see Supporting Information for details.
gold aryls herein suggest that their
excited states are similar.
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Table 2: Bond lengths and calculated bond orders.
Keywords: aurophilicity · boronic acids · gold · luminescence ·
Wheland cations
.
Experimental
bond length [ꢀ]
(1)
Calculated
bond length [ꢀ]
(1’)
Bond order
(1’)
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ꢁ
Au C1
ꢁ
C1 C2
2.153(4)
1.396(6)
1.382(6)
1.371(7)
2.170, 2.172
1.409
1.387
0.384
1.413
1.396
1.361
ꢁ
C2 C3
ꢁ
C3 C4
1.390
Experimental
bond length [ꢀ]
(6)
Calculated
bond length [ꢀ]
(6’)
Bond order
(6’)
ꢁ
Au C5
2.126(6)
1.401(10)
1.455(9)
1.366(9)
1.352(14)
2.162, 2.163
1.406
1.426
1.379
1.363
0.426, 0.425
1.337
1.337
1.455
1.615
ꢁ
C4 C5
ꢁ
C5 C6
ꢁ
C6 C7
ꢁ
C2 C3
Calculated
bond length [ꢀ]
Bond order
(11’)
(11’)
ꢁ
Au C1
2.174, 2.174
1.393
1.406
1.369
1.369
0.448, 0.448
1.592
1.277
1.423
1.474
ꢁ
C1 C2
ꢁ
C2 C3
ꢁ
C3 C4
ꢁ
C5 C6
ꢁ
C6 C7
1.408
1.370
1.236
1.490
ꢁ
C7 C8
In summary, a method of generating geminally diaurated
arenes is disclosed. Arylboronic acids were reacted with an
electrophilic (phosphine)gold(I) cation source in ether,
producing a series of air- and moisture-stable solids. The
reaction tolerates a range of boronated substrates. Isolation of
these species demonstrates the gem-diaurated arene to be
stable, and a plausible resting state in gold-catalyzed trans-
formations of olefins and alkynes. Because of the isolobality
of the gold(I) cation with the proton, these species also
represent analogues of Wheland intermediates of electro-
philic aromatic substitution. It is hoped that the addition of
these organometallics to the menagerie of gold-based (pre)-
catalysts will stimulate future investigations.
[15] A. N. Nesmeyanov, E. G. Perevalova, O. B. Afanasova, M. V.
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2248 – 2252.
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Int. Ed. Engl. 1982, 21, 711 – 724.
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Chem. 2006, 118, 8368 – 8371; Angew. Chem. Int. Ed. 2006, 45,
8188 – 8191.
[23] The insensitivity to 13C NMR probing of some bridging/ipso
carbons in gem-diaurated species has been reported. See
Refs. [11], [13], and [17].
[24] S. Park, A. Gahlmann, Y. He, J. Feenstra, A. H. Zewail, Angew.
Chem. 2008, 120, 9638 – 9641; Angew. Chem. Int. Ed. 2008, 47,
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[25] L. Gao, M. A. Peay, D. V. Partyka, J. B. Updegraff III, T. S. Teets,
A. J. Esswein, M. Zeller, A. D. Hunter, T. G. Gray, Organo-
metallics 2009, 28, 5669 – 5681.
[26] D. V. Partyka, L. Gao, T. S. Teets, J. B. Updegraff III, N.
Deligonul, T. G. Gray, Organometallics 2009, 28, 6171 – 6182.
[27] R. A. Vogt, M. A. Peay, T. G. Gray, C. E. Crespo-Hernµndez, J.
Phys. Chem. Lett. 2010, 1, 1205 – 1211.
Experimental Section
11: A suspension of [Ph3PAu]NTf2 (0.026 mmol) in diethyl ether
(2 mL) was added to
a solution of 1-naphthylboronic acid
(0.015 mmol) in diethyl ether (2 mL). The white mixture was stirred
in the dark for 6 h. The mixture was filtered, washed with 3 ꢄ 2 mL
diethyl ether and pentane, and dried in vacuo to yield a light gray
powder. Crystals suitable for X-ray diffraction were obtained by
diffusion of diethyl ether into a saturated chloroform solution. Yield:
1
16 mg (90%). H NMR (CDCl3): d = 8.46–8.42 (td, 2H, ArH4,7, J =
6.9 Hz), 8.21–8.19 (d, 1H, ArH2, J = 8.3 Hz), 8.04–8.02 (d, 1H, ArH8,
J = 7.6 Hz), 7.79–7.66 ppm (m, 3H, ArH3,5,6). 31P{1H} NMR (CDCl3):
d = 37.4 ppm (s). Elemental analysis calcd for C48H37Au2F6NO4P2S2
[%]: C 43.48, H 2.81, N 1.065; found: C 43.25, H 2.76, N 0.93.
[28] L. Gao, D. V. Partyka, J. B. Updegraff III, N. Deligonul, T. G.
Gray, Eur. J. Inorg. Chem. 2009, 2711 – 2719.
Received: March 4, 2012
Published online: && &&, &&&&
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Hunter, T. G. Gray, Chem. Eur. J. 2012, 18, 2100 – 2112.
4
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[30] H. Schmidbaur, A. Schier, Chem. Soc. Rev. 2012, 41, 370 – 412.
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2429.
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F. Mohr, Organometallics 2009, 28, 2931 – 2934.
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Cheung, N. Zhu, J. Am. Chem. Soc. 2002, 124, 14696 – 14706.
[34] K. B. Wiberg, Tetrahedron 1968, 24, 1083 – 1096.
[35] C. A. Reed, K.-C. Kim, E. S. Stoyanov, D. Stasko, F. S. Tham,
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1804.
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Gold Aryl Complexes
The electrophilic [Ph3PAu]NTf2 quickly
diaurates aryl boronic acids providing
stable geminally diaurated complexes
(see example; yellow, Au; orange, P; blue,
C) that have been spectroscopically, and
in some cases crystallographically, char-
acterized. Geminally diaurated arenes are
putative intermediates in gold(I) catalysis
and are analogues of Wheland inter-
mediates of electrophilic aromatic sub-
stitutions.
J. E. Heckler, M. Zeller, A. D. Hunter,
T. G. Gray*
&&&& — &&&&
Geminally Diaurated Gold(I) Aryls from
Boronic Acids
Gold-Aryl-Komplexe
Das elektrophile [Ph3PAu]NTf2 reagiert
mit Boronsꢁuren unter Bildung stabiler
geminal diaurierter Komplexe (siehe Bei-
spiel; gelb Au, orange P, blau C), die
spektroskopisch und in einigen Fꢁllen
kristallographisch charakterisiert wurden.
Geminal diaurierte Arene sind mutmaß-
liche Zwischenstufen in Gold(I)-Katalysen
und Analoga der Wheland-Intermediate
in elektrophilen aromatischen Sub-
stitutionen.
J. E. Heckler, M. Zeller, A. D. Hunter,
T. G. Gray*
&&&& — &&&&
Geminally Diaurated Gold(I) Aryls from
Boronic Acids
6
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Gold Aryl Complexes
The electrophilic [Ph3PAu]NTf2 quickly
diaurates aryl boronic acids providing
stable geminally diaurated complexes
(see example; yellow, Au; orange, P; blue,
C) that have been spectroscopically, and
in some cases crystallographically, char-
acterized. Geminally diaurated arenes are
putative intermediates in gold(I) catalysis
and are analogues of Wheland inter-
mediates of electrophilic aromatic sub-
stitutions.
J. E. Heckler, M. Zeller, A. D. Hunter,
T. G. Gray*
&&&& — &&&&
Geminally Diaurated Gold(I) Aryls from
Boronic Acids
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