Synthesis and structure of a dinuclear gold(II) complex with terminal fluoride ligands

Article abstract of DOI:10.1021/ic200394q

The potential for reductive elimination of fluorine from dinuclear gold(II) for catalysis has prompted our efforts to synthesize a dinuclear gold(II) fluoride complex. This has been achieved with bis(2,6-dimethylphenyl) formamidinate bridging ligands. In order to obtain this product, it was necessary first to synthesize the corresponding dinuclear gold(II) nitrate, which reacts readily with KF in a metathesis reaction. The nitrate complex and fluoride complexes have been structurally characterized. The Au-Au distance in the dinuclear fluoride, 2.595 A, is longer than the distance found in the analogous chloride complex, 2.567 A. This result is consistent with the presence of a fluoride "π electron effect" on the filled Au 5d orbitals. The Raman spectrum shows an Au-Au stretch at 206 cm^-1, which agrees with Woodruffs rules and the density functional theory computational model used for modeling the complex.

Full text of DOI:10.1021/ic200394q

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Synthesis and Structure of a Dinuclear Gold(II) Complex with Terminal
Fluoride Ligands
Doris Y. Melgarejo,† Gina M. Chiarella,† John P. Fackler, Jr.,*,† Lisa M. Perez,§ Alexandre Rodrigue-Witchel,‡
and Christian Reber‡
†Department of Chemistry, Texas A&M University, College Station, Texas 77843, United States
‡Department of Chemistry, University of Montreal, Montreal, Quebec H3C 3J7, Canada
§Laboratory for Molecular Simulation, Texas A&M University, College Station, Texas 77843, United States
S Supporting Information
b
complex containing cyclometalated tertiary arsine ligands is
produced by metathesis with AgF.9 Two others with Re2 or
6þ
ABSTRACT: The potential for reductive elimination of
fluorine from dinuclear gold(II) for catalysis has prompted
Re27þ cores10 and hpp (hpp = 1,3,4,6,7,8-hexahydro-2H-pyrimido
our efforts to synthesize a dinuclear gold(II) fluoride com-
[1,2a]pyrimidine) ligands contain terminal fluorines, formeꢀd
plex. This has been achieved with bis(2,6-dimethylphe-
nyl)formamidinate bridging ligands. In order to obtain
this product, it was necessary first to synthesize the corre-
sponding dinuclear gold(II) nitrate, which reacts readily
with KF in a metathesis reaction. The nitrate complex and
fluoride complexes have been structurally characterized. The
AuꢀAu distance in the dinuclear fluoride, 2.595 Å, is longer
than the distance found in the analogous chloride complex,
2.567 Å. This result is consistent with the presence of a fluoride
“π electron effect” on the filled Au 5d orbitals. The Raman
spectrum shows an AuꢀAu stretch at 206 cmꢀ1, which agrees
with Woodruff’s rules and the density functional theory
computational model used for modeling the complex.
using a fluorinating agent or from decomposition of the PF6
anion or from an oxidizing silver reagent.
Our approach to the elusive FꢀAuII bond was through
substitution of a labile gold(II) nitrate, 1. The nitrate as a ligand
attached to the dinuclear gold(II) is more labile than halides. By
disproportionation under nitrogen of the dinuclear gold(I)
formamidinate2 in the presence of nitrate, the dinuclear gold(II)
nitrate was formed. Metathesis of this complex with KF leads to
the dinuclear fluoride, 2, which was crystallized (see the Support-
ing Information). The nitrate complex also readily reacts with
other halides to form previously characterized products.2
Both 1 and the dinuclear gold(II) fluoride complex 2 have
been characterized structurally. The AuIIꢀF distance in 2 of
2.287(11) Å is shorter (ca. 0.07) than that found in the related2
chloride [2.366(3) Å] but longer than the AuIIꢀF distances in
discrete AuF42- units [2.076(9)ꢀ2.190(9) Å].11ꢀ13 This reflects
the strong trans influence of the AuIIꢀAuII bond, a result similar
to that found in the lantern-type dinuclear complex of platinum-
(III).9 The AuIꢀF bond distance in the carbene monomer8 is
notably shorter [2.0281(17) Å]. The carbene ligand efficiently
removes the electronic density from the AuIꢀF bond into its π
system. In the dicationic complexes1 of N-heterocyclic carbene
(NHC) gold(III) fluorides, the bridging AuIIIꢀF bonds
[2.034(3) Å] are in the expected range of other bridging bonds
[2.03(4)ꢀ2.10(5) Å] and longer than terminal AuIIIꢀF bonds
[1.77(5)ꢀ1.84(5) Å].
s Mankad and Toste recently described,1 the catalytic
A
reactions of gold have exploited the gold(I)/gold(III) redox
cycle. Dinuclear gold(II) complexes also are well-known to under-
go redox reactions but, to date, have not been involved in
discussions of gold catalysis.2 A dinuclear gold complex in
combination with fluoride offers a potential system for delivering
fluorine to carbon moieties, although this has yet to be found.
Given theimportanceoforganofluorineinmedicineand industry,3
we have explored the synthesis of a dinuclear gold(II) fluoride.
As a result of the special bonding characteristics of fluorine and
its small size, electronegativity, and polarizing ability, AuꢀF
bonding has been a rarity. There are numerous anionic
[AuFx]nꢀ moieties4ꢀ7 but only one example of a gold(I) fluoride
complex.8 Dinuclear gold(II) complexes with AuꢀF bonds were
not known prior to this work. Bennett et al.9 treated the dinuclear
complex [Au2Cl2(μ-C6H4PPh2)2] with AgF and isolated a ma-
terial with singlet 31P and 19F NMR resonances. However, this
material was not characterized. The use of XeF by Leary and
Bartlett6 to induce the oxidative addition of [Au2(μ-2-
MeC6H3PPh2)2] did not give consistent spectroscopic results.
Here we report the first example of a well-characterized AuꢀAu-
bonded complex with terminal fluoride ligands, 2, and its pre-
cursor, 1, with labile, terminal nitrate ligands (Figures 1 and 2).
With heavy-metal ions, there are a few reported dinuclear
The FꢀAuII bond produces a lengthening of the AuꢀAu
distance, 2.595(2) Å, in 2 compared to the nitrate 1 and also to
the analogous chloride, 2.567(2) Å.2 This is good evidence for
the fluoride π-electron effect (filled 2p orbitals on F at a short
AuꢀF distance) upon filled 5d orbitals in the AuIIꢀAuII bond.
A similar observation has been noted with Re26þ and hpp ligands,10
yet the metalꢀmetal distance in the dinuclear lantern complex of
6þ
Pt2 is shorter [2.6530(4) Å] with terminal fluorides than with
chlorides or bromides.9 The marked deviation from linearity of the
AuꢀAuꢀF angle, 167.39(4)° [which in the chloride is 177.8°(3)],
also was noted in the Pt26þ(d7) lantern complex [166.78°(5)] but
Received: February 28, 2011
Published: April 14, 2011
6þ
complexes with terminal fluoride. A neutral lantern type of Pt2
r
2011 American Chemical Society
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dx.doi.org/10.1021/ic200394q Inorg. Chem. 2011, 50, 4238–4240
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bond. The short AuꢀAu distance of 2.4857(6) Å, similar to the
distance found [2.489(10) Å] with terminal benzoates, is a result
of the weak electron donation of oxo ligands. As is expected from
this short distance, the associated Raman vibration ν is found to
be the one with the highest frequency for a AuIIꢀAuII bond. This
observation is based on a computational model, 200.2 cmꢀ1
,
Woodruff’s rules, 198.7 cmꢀ1, and the actual Raman measure-
ment, 206 cmꢀ1 (see the SI).
In 2, the AuIIꢀF distance of 2.287(11) Å is shorter (ca. 0.07)
than that in the related chloride2 [2.366(3) Å] but longer3 than
AuIIꢀF bonds in discrete AuF42ꢀ units [2.076(9)ꢀ2.190(9) Å],
reflecting the trans influence (vida infra) of the opposite
AuIIꢀAuII bond. The 19F NMR spectrum shows a single peak
at 2.314 ppm in C6D6. The analogous lantern-type complex9 of
platinum(III) also shows an enlargement of the terminal PtꢀF
bond compared to [PtF6]2ꢀ units. In comparison, the AuIꢀF
bond in the Sadighi8 monomer is notably shorter [2.0281(17)
Å]. The carbene (NHC) ligand efficiently withdraws the electro-
nic density into its π system. In the dicationic complex1 of a NHC
gold(III) fluoride, the bridging AuIIIꢀF bonds [2.034(3) Å] are
in the expected range of other bridging AuIIIꢀF bonds
[2.03(4)ꢀ2.10(5) Å] and longer than nonbridging ones
[1.77(5)ꢀ1.84(5) Å].14
Figure 1. Representation of [Au2(2,6-dimethylformyl)2(NO3)2] (1) with
50% probability ellipsoids. Hydrogen atoms and the solvent are omitted.
’ ASSOCIATED CONTENT
S
Supporting Information. Details of the syntheses, ana-
b
lytical results, Raman (experimental and computational), NMR,
and UVꢀvis spectra, structural data, and X-ray crystallographic
data in CIF format. This material is available free of charge via the
Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: fackler@mail.chem.tamu.edu.
Figure 2. Thermal ellipsoid representation (50% probability) of 2.
Hydrogen atoms are omitted.
’ ACKNOWLEDGMENT
The Robert A. Welch Foundation of Houston, TX (Grant
A-0960), is acknowledged for financial support. The Laboratory
for Molecular Simulation and the Supercomputing Facility at
Texas A&M University are thanked for providing software,
support, and computer time.
not in the Re26þ(d4) analogue. Although this bent angle brings the
fluoride close to two methyl hydrogen atoms on the ligand, the
distance of 3.22(5) Å is larger than the sum of the CꢀH F van
3 3 3
der Waals radii (2.673 Å).
The negatively charged anions (nitrate or fluoride) connect
laterally to the gold atoms. Crystallization of 1 in three solvents
(toluene, THF, and acetone) renders triclinic crystals in the P1
space group with each solvent molecule; even so, the a, b, and c
parameters and the R, β, and γ angles are nearly the same from
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