Synthesis and solution- and solid-state characterization of gold(I) rings with short Au...Au interactions. Spontaneous resolution of a gold(I) complex

Article abstract of DOI:10.1021/ja064609x

We report the synthesis and solution- and solid-state characterization of gold(I) rings with short 1,9-transannular Au...Au interactions. The 9- and 16-membered gold(I) rings were prepared by reacting 9,9-dimethyl-4,5-bis(diphenylphosphino)-xanthene and (Me₂S)AuCl in the presence of AgNO3 in the molar ratio of 1:0.5:1 and 1:1:1, respectively. X-ray crystallographic measurements in conjunction with solution X-ray diffraction and NMR methods have been used to determine the structure of gold(I) rings, and we also gained insight into the dynamics. The nine-membered gold(I) ring is chiral, and the crystal contains only one of the two enantiomers, either right- or left-handed. To the best of our knowledge this represents the first example of crystallization-induced spontaneous resolution of a binuclear gold(I) cycle. The 16-membered ring with 1,9-transannular Au...Au interaction is in a figure-eight conformation. Copyright

Full text of DOI:10.1021/ja064609x

Published on Web 09/13/2006
Synthesis and Solution- and Solid-State Characterization of Gold(I) Rings
with Short Au‚‚‚Au Interactions. Spontaneous Resolution of a Gold(I)
Complex
Andrea Dea´k,* Tu¨nde Megyes, Ga´bor Ta´rka´nyi, Pe´ter Kira´ly, La´szlo´ Biczo´k, Ga´bor Pa´linka´s,* and
Peter J. Stang*,#
Institute of Structural Chemistry, Chemical Research Center, Hungarian Academy of Sciences, P.O. Box 17,
H-1525, Budapest, Hungary
Received June 29, 2006; E-mail: palg@chemres.hu; deak@chemres.hu; stang@chem.utah.edu
Scheme 1. Synthesis of Gold(I) Rings 1 and 2
Within the field of supramolecular inorganic chemistry, the
metal-assisted self-assembly of metallacycles, helicates, racks,
ladders, grids, and cages represents an impressive achievement of
molecular design and assembly.1 The structure, conformation, and
topology of these supramolecules should depend not only on the
nature of the organic backbone, but also on the nature of the metal-
ligand interaction; moreover, unique properties may result when
metal-metal bonds are introduced.2 The propensity of gold(I) for
linear coordination geometry featuring Au‚‚‚Au attractions in
binuclear precursor molecules has been exploited in constructing
macrocycles,3a-f catenanes,3e-h and polymers.3i-n The Au‚‚‚Au
interaction generally occurs perpendicular to the principal axis of
the linearly two-coordinated Au(I) centers, and its typical length
ranges from 2.75 to 3.40 Å.4 Experimental and theoretical studies
indicate that the strength of the aurophilic Au‚‚‚Au attractions is
comparable to that of hydrogen bonding.5 The short aurophilic
contacts observed in gold(I) supramolecules frequently induce
intriguing spectroscopic and optoelectronic properties. Therefore,
such complexes became ideal candidates for use in the development
of molecular sensors and switches or energy storage devices.3
Eight- to eleven-membered rings in which two or more consecu-
tive atoms are gold are relatively few, and macrocycles are even
more rare.6 Short Au‚‚‚Au interactions have been observed in eight-
and nine-membered gold(I) cyclic systems with Ph2P-(CH2)n-
PPh2 diphosphine ligands; moreover, nine-membered gold(I) rings
have been prepared using diphosphine ligands derived from rigid
heteroatomic backbones.7 Hence, it was of interest to see whether
the self-assembly of diphosphine ligands derived from heteroatomic
backbones with Au(I) ions would afford macrocycles showing short
Au‚‚‚Au interactions.
Herein, we report the synthesis and structural characterization
of nine-membered and 16-membered gold(I) rings showing short
1,9-transannular Au‚‚‚Au aurophilic interactions. They were pre-
pared by reacting 9,9-dimethyl-4,5-bis(diphenylphosphino)-xanthene
(xantphos) and (Me2S)AuCl in the presence of AgNO3 (Scheme
1). The 2:1 stoichiometric combination of a dichloromethane
solution of (Me2S)AuCl and xantphos produced the [(AuCl)2(µ-
xantphos)] complex7 which was further reacted with AgNO3, and
the resulting [(AuNO3)2(µ-xantphos)] (1) was isolated. Likewise,
the reaction of (Me2S)AuCl with xantphos and AgNO3 in a molar
ratio of 1:1:1 in dichloromethane yields [Au2(µ-xantphos)2](NO3)2
(2). Moreover, the reaction of 1 with xantphos also gave the
complex 2, indicating that 1 is a likely intermediate in the formation
of 2.
report on platinum metallacycles.8 The understanding of fluxionality
of gold(I) metallacycles and their structure in solution is also
limited.9 Therefore, by use of solution X-ray diffraction and NMR
methods we provide insight into the structure as well as the
dynamics of gold(I) macrocycle 2. Moreover, the photophysical
properties of 1 and 2 were studied by UV-visible and luminescent
spectroscopy.
Diffraction-quality single crystals of both 1 and 2 were grown
by diffusion of diethyl ether into a dichloromethane solution of
each individual complex. The crystals 1 and 2 were isolated in
Paratone-N cryoprotectant and successfully subjected to low-
temperature X-ray diffraction study. The X-ray structure determi-
nation revealed the existence of disordered solvent molecules in
each case. Figure 1 shows the skeleton of 1, where the nitrate anions
are coordinated to the gold centers bridged by the xantphos ligand,
each having a linear coordination geometry and a short 1,9-
transannular Au‚‚‚Au distance to form a nine-membered ring. The
ligand backbone is folded and twisted by 51.4(1)° about the
Au‚‚‚Au axis, most likely to accommodate the two Au(I) ions
at a close distance of 2.950(1) Å. The nitrate groups are rotated by
59.1(1) and 50.7(1)° with respect to the least-squares plane of the
nine-membered cycle, and they display a propeller-like arrangement
with respect to one another. The crystal structure of the related
[(AuCl)2(µ-xantphos)] has been reported.7
Complex 1 crystallized in the noncentrosymmetric, monoclinic
space group P21; thus the molecule is chiral, and the crystal contains
only one of the two enantiomers, either right- or left-handed.
Refinement of the Flack parameter led to a value of ∼0, which
confirms the enantiomeric purity of the crystal. Although many
double- and triple-helical metal complexes have been reported,10
the simpler monohelical complexes have received much less
attention.11 This is perhaps because spontaneous resolution more
likely occurs when strong, selective, and directional interactions,
such as coordination and/or hydrogen bonding, extend the neigh-
boring chiral molecules into multidimensional arrays.12
The structure determination of metallacycles in solution using
X-ray diffraction is a very challenging problem with only a single
# Department of Chemistry, University of Utah, 315 S 1400 E Room 2020, Salt
Lake City, UT 84112.
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10.1021/ja064609x CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
Figure 3. Ortep view of the cationic skeleton of 2, illustrating the 16-
membered gold(I) ring showing a figure-eight conformation [equivalent
atoms generated by i ) (-x + 1, y, -z + /2)] with thermal ellipsoids at
the 30% probability level. Selected distances (Å) and angles (deg):
Au(1)‚‚‚Au(1)i 2.858(1); Au(1)-P(1) 2.326(1); Au(1)-P(2) 2.331(1); P(1)-
Au(1)-P(2)i 160.8(1); P(1)-Au(1)-Au(1)i 102.1(1); P(2)-Au(1)i-Au(1)
93.1(1).
Figure 1. Ortep view of the skeleton of 1, illustrating the nine-membered
gold(I) ring and the coordinated nitrate anions with thermal ellipsoids at
the 30% probability level. Selected distances (Å) and angles (deg):
Au(1)-Au(2) 2.950(1); Au(1)-P(1) 2.224(3); Au(2)-P(2) 2.226(3);
Au(1)-O(2) 2.199(9); Au(2)-O(5) 2.091(8); P(1)-Au(1)-O(2) 168.3(3);
P(2)-Au(2)-O(5) 173.3(3).
3
into account, the molecule of 2 exists in a figure-eight conformation,
where the 16-membered ring contains one short Au‚‚‚Au linkage
(2.858(1) Å), as shown in Figure 3. This represents an example of
a 16-membered ring with a 1,9-transannular Au‚‚‚Au interaction.
This structure crystallized in centrosymmetric space group C2/c,
thus both right- and left-handed molecules are present in the crystal.
To check whether the nine- and 16-membered gold(I) rings retain
their structure in solution, wide-angle X-ray scattering and NMR
experiments were performed. The solubility of 1 is extremely low
in the usual protic and aprotic organic solvents, thus solution X-ray
diffraction was not possible. Despite its limited solubility (<4 mM),
1
the H NMR of 1 exhibited sharp resonances in CD2Cl2 at room
temperature. The phosphorus nuclei of 1 at δ 18.3 ppm are more
shielded when compared to the [(AuCl)2(µ-xantphos)] complex.7
1
The H and 31P NMR spectra for 2 were measured using the
Figure 2. View illustrating the C-H‚‚‚O bonded homochiral helices of 1,
which are also interconnected by C-H‚‚‚O interactions to form an
interhelical network.
same sample (179 mM) prepared for solution wide-angle X-ray
diffraction experiments. Noteworthy in the 1H and 31P NMR spectra
are the broad resonances found at room temperature (Supporting
Information Figures S4 and S5). Temperature-dependent 31P NMR
experiments on 2 were performed, and it was observed that by
raising the temperature to +70 °C, the corresponding NMR signals
became much sharper, while after recooling (to +25 °C) the
complex remained intact. Given that a more dilute (2 mM) solution
of 2 behaved similarly, the most plausible explanation for the room-
temperature line-broadening is the conformational motion of 2,
rather than self-association or oligomerization processes. This has
been further proven by low-temperature 31P NMR, where a pair of
AB phosphorus doublets appeared with a large, two-bond 31P-31P
coupling constant (2JP-P ) 318.3 Hz, -30 °C), indicating that each
gold atom is bound to two chemically nonequivalent phosphorus
sites. The large 2JP-P scalar coupling mediated by a gold atom fits
As shown in Figure 2, the molecules of 1 are linked throughout
via C-H‚‚‚O and Au‚‚‚π interactions into a 2D array. This is
achieved in such a way that one of the nitrate groups with its O(2)
and O(3) atoms is involved as an acceptor in the C-H‚‚‚O hydrogen
bonding. These C-H‚‚‚O and additional Au‚‚‚π interactions link
the molecules of 1 into homochiral helices, which are also
interconnected with each other by C-H‚‚‚O contacts (Supporting
Information, Table S1). Thus, all oxygen atoms of the second nitrate
function are involved in C-H‚‚‚O interactions, which connect the
C-H‚‚‚O and Au‚‚‚π bonded helices into an interhelical meander-
shaped network. The role of the nitrate anions played in the C-
H‚‚‚O linking of the molecules is chirally discriminative and
required that complex 1 exhibits the same absolute configuration.
Therefore, the chirality is preserved and extended into the crystal.
To the best of our knowledge this represents the first example of
crystallization-induced spontaneous resolution of a binuclear gold-
(I) metallacycle. This chiral gold(I) complex has potential in
absolute asymmetric (stereoselective) syntheses where optically
active materials are prepared from achiral (or racemic) starting
materials in the absence of optically active catalyst or reagents.
The X-ray structural data of 2 indicate that it consists of
[Au2(xantphos)2]2+ cations in which two strands of xantphos are
folded relative to each other and held together by two Au(I) ions
(Figure 3). In addition, the structure contains uncoordinated nitrate
anions, as well as disordered solvent molecules. In this structure,
the ligand backbone is folded and tilted by 72.2(1)° with respect
to the Au‚‚‚Au axis, and the two xantphos ligands bridge the Au(I)
ions to form a short aurophilic attraction. With this interaction taken
1
well within the range of reported values.13 Coalescence of the H
and 31P resonances near room temperature is attributed to the
interconversion of the mirror image conformers of 2. The activation
enthalpy (∆Hq) for the process was determined by 1H and 31P
temperature-dependent NMR line shape analyses and found to be
∆Hq(1H) ) +48.9 kJ/mol and ∆Hq(31P) ) +48.4 kJ/mol (Sup-
porting Information Figures S6-S9). The good agreement between
the 1H and 31P activation parameters suggests that the same
exchange phenomenon is followed on both NMR time-scales.
Although difficult to separate from steric factors, the experimental
∆Hq is in agreement with literature data 4,13,14 and is probably only
a slight overestimation of the aurophilic interaction energy.
To assess the aurophilicity of 2 after dissolution, the solution
X-ray measurement was performed in high-purity nitromethane
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C O M M U N I C A T I O N S
2005 and the Hungarian Scientific Research Funds (OTKA)
K68498 projects and the NSF at Utah (Grant CHE-0306720). A
diffractometer purchase grant from the National Office for Research
and Technology (Grant MU-00338/2003) is gratefully acknowl-
edged.
Supporting Information Available: Synthetic details, X-ray
1
crystallographic data for of 1 and 2 in CIF format; H and 31P NMR
line shape analysis; experimental and evaluation method for solution
X-ray diffraction measurements; absorption and luminescence spec-
troscopy. This material is available free of charge via the Internet at
http://pubs.acs.org.
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Figure 4. Difference radial distribution function (green line) g(r) for
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Au‚‚‚Au aurophilic interaction; (blue line) Au‚‚‚P interaction.
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Acknowledgment. This work was supported by the Hungarian
GVOP-3.2.1.-2004-04-0210/3.0, NAP VENEUS05 OMFB-00650/
JA064609X
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