Well-Defined Dinuclear Gold Complexes for Preorganization-Induced Selective Dual Gold Catalysis

Article abstract of DOI:10.1002/anie.201603938

The synthesis, reactivity, and potential of well-defined dinuclear gold complexes as precursors for dual gold catalysis are explored. Using the preorganizing abilities of the ditopic PN^HP^iPr(L^H) ligand, dinuclear Au^I–Au^Icomplex 1 and mixed-valent Au^I–Au^IIIcomplex 2 provide access to structurally characterized chlorido-bridged cationic species 3 and 4 upon halide abstraction. For 2, this transformation involves unprecedented two-electron oxidation of the redox-active ligand, generating a highly rigidified environment for the Au₂core. Facile reaction with phenylacetylene affords the σ,π-activated phenylacetylide complex 5. When applied in the dual gold heterocycloaddition of a urea-functionalized alkyne, well-defined precatalyst 3 provides high regioselectivities for the anti-Markovnikov product, even at low catalyst loadings, and outperforms common mononuclear Au^Isystems. This proof-of-concept demonstrates the benefit of preorganization of two gold centers to enforce selective non-classical σ,π-activation with bifunctional substrates.

Full text of DOI:10.1002/anie.201603938

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201603938
German Edition: DOI: 10.1002/ange.201603938
Gold Catalysis
Well-Defined Dinuclear Gold Complexes for Preorganization-Induced
Selective Dual Gold Catalysis
Vincent Vreeken, Daniꢀl L. J. Broere, Anne C. H. Jans, Marianne Lankelma, Joost N. H. Reek,
Maxime A. Siegler, and Jarl Ivar van der Vlugt*
Abstract: The synthesis, reactivity, and potential of well-
defined dinuclear gold complexes as precursors for dual gold
catalysis are explored. Using the preorganizing abilities of the
ditopic PNHPiPr (LH) ligand, dinuclear AuI–AuI complex 1 and
mixed-valent AuI–AuIII complex 2 provide access to
structurally characterized chlorido-bridged cationic species 3
and 4 upon halide abstraction. For 2, this transformation
involves unprecedented two-electron oxidation of the redox-
active ligand, generating a highly rigidified environment for the
Au2 core. Facile reaction with phenylacetylene affords the
s,p-activated phenylacetylide complex 5. When applied in the
dual gold heterocycloaddition of a urea-functionalized alkyne,
well-defined precatalyst 3 provides high regioselectivities for
I
ꢀ
Scheme 1. Top: coordination of C C bonds on mononuclear Au
species and a strategy to induce selective s,p-activation using well-
the anti-Markovnikov product, even at low catalyst loadings,
defined diphosphine-supported dinuclear AuI–AuI. Bottom: known
and outperforms common mononuclear AuI systems. This
(left) and novel (right) coinage metal coordination chemistry of
proof-of-concept demonstrates the benefit of preorganization
of two gold centers to enforce selective non-classical
s,p-activation with bifunctional substrates.
PNHPiPr ligand LH, including unprecedented ligand-based reactivity.
The proximity of both gold centers has occasionally been
credited to enhance reactivity.[6] However, to the best of our
knowledge the competence of well-defined dinuclear
s-,p-alkynide complexes in dual gold catalysis has never
been reported, despite the potential benefits of two pre-
organized gold centers with respect to chemoselectivity and
activity for this type of reaction. Additionally, facile access to
dinuclear mixed-valent gold complexes might be an interest-
ing target in the context of cascade and AuI/AuIII cooperative
catalysis.[7]
G
old catalysis has flourished over the past 15 years,
enabling a wide range of transformations.[1] More recently,
dual gold catalysis has been developed successfully.[2]
Whereas “traditional” mono-gold catalysis relies on
p-activation of a substrate by a cationic AuI center, dual
gold catalysis typically involves both s- and p-activation (of
ꢀ
one or two functionalities, such as C C bonds) by two gold
centers. The prevailing strategy utilizes mononuclear AuI
complexes to induce this mode of activation (Scheme 1).[3]
However, this strategy offers no handles to induce pre-
organization of both gold centers to specifically target well-
defined s,p-activation of, for example, unsaturated hydro-
The ditopic tridentate ligand PNHPiPr (LH; Scheme 1)
displays versatile coordination chemistry with respect to
a wide range of transition metals,[8] including Cu[9] and Ag.[10]
Strikingly, no single complex of gold with this type of ditopic
framework is known to date. Furthermore, chemistry related
to the redox-active nature[11] of LH has been well-established
in nickel, manganese, and rhenium complexes,[12] but ligand
redox-activity with Group 11 metals is limited to one example
with CuI, leading to dimerization on one of the para-positions
of the L backbone.[9a] New avenues for gold coordination
chemistry and catalysis may become accessible by developing
strategies to preorganize and stabilize multiple gold centers
on suitable ligand platforms and to selectively bind and
activate bifunctional substrates. To address these challenges,
we herein 1) uncover the versatile coordination chemistry of
gold using the redox-active ditopic PNHPiPr ligand, 2) disclose
the interesting ligand and metal-based reactivity of (mixed-
valent) dinuclear species, and 3) establish well-defined pre-
catalyst systems ideally suited for s,p-activation of alkynes
and demonstrate their use in highly selective dual gold
catalysis.
À
carbon C C multiple bonds whilst avoiding p- or
s + p-coordination. Furthermore, no control over the selec-
tive binding of bifunctional substrates (such as heterocycliza-
tions) can be achieved in this manner.
The synthesis, coordination chemistry and catalytic appli-
cations of dinuclear AuI complexes are well-developed.[4,5]
[*] V. Vreeken, Dr. D. L. J. Broere, A. C. H. Jans, M. Lankelma,
Prof. Dr. J. N. H. Reek, Dr. J. I. van der Vlugt
Homogeneous, Bioinspired and Supramolecular Catalysis, van ’t
Hoff Institute for Molecular Sciences, University of Amsterdam
Science Park 904, 1098 XH Amsterdam (The Netherlands)
E-mail: j.i.vandervlugt@uva.nl
Dr. M. A. Siegler
Small Molecule X-ray Crystallography Facility, Department of
Chemistry, John Hopkins University
3400 N. Charles St., Baltimore, MD 21218 (USA)
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201603938.
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The straightforward reaction of PNHPiPr with AuCl(SMe2)
in a 1:2 ratio provides white solid 1 (31P NMR: d 40.9 ppm)
with a 1H NMR spectrum suggestive of a C2 symmetric
species (Scheme 2). Single crystal X-ray diffraction analysis
determination confirms the sole formation of species 2, with
the central deprotonated amido nitrogen coordinating to the
square-planar AuIII center (Figure 1).
The PNP-ligand backbone in 2 is severely twisted, with
a C6-C1-C7-C8 torsion angle of 76.4(8)8. This distortion
1
relates to the upfield shifted aromatic signal in the H NMR
spectrum, which corresponds to the shielded C(6)-H
hydrogen that is positioned on top of the second phenyl
ring. The intramolecular Au–Au distance of 4.641 ꢀ excludes
an aurophilic d8–d10 interaction. Cyclic voltammetry does not
indicate reversible oxidation of the potentially redox-active
ligand scaffold in 2.
A prerequisite for catalysis with a AuX(L) precursor (X is
a halide) is the generation of a vacant coordination site by
halide abstraction with, for example, Ag+ salts or a suitable
Lewis acid. Addition of one equivalent of AgNTf2 (or related
silver salts or GaCl3; see the Supporting Information) to 2,
which bears multiple chlorido fragments, leads to a mixture of
species according to 31P NMR spectroscopy. However,
addition of two equivalents results in rapid decoloration of
the reaction solution and generation of a single symmetric
product (31P NMR: d 40.4 ppm), suggestive of the presence of
only AuI-phosphine fragments. The 1H NMR spectrum shows
only one methyl signal for the ditolylamine backbone and
chemically identical isopropyl groups at phosphorus. Most
notably, only two instead of the anticipated three aromatic
hydrogen signals are observed, together with a downfield
signal at d 10.53 ppm. This signal integrates for one hydrogen
and is attributed to an -NH fragment. ESI-MS data suggest
that the dinuclear complex, bearing only one chlorido ligand,
remains intact during this transformation. The structure of
complex 3 could be elucidated by X-ray structure determi-
nation of single crystals grown from CH2Cl2–pentane
(Figure 2).
Scheme 2. Synthesis of well-defined Au2-complexes 2–5 from AuI–AuI
precursor 1, generated by reaction of ligand LH with AuCl(SMe2)
(2 equiv).
Clearly, halide abstraction by AgNTf2, related silver salts,
or GaCl3, induces a highly unusual metal-based reduction of
mixed-valent species
2
to yield AuI–AuI species
3
Figure 1. Displacement ellipsoid plots (50% probability level) of
1 (left) and 2 (right). For clarity, the counterion and hydrogen atoms
À
are not shown. Selected bond lengths (ꢀ) and angles (8), for 1: Au1
À
À
P1 2.2495(8), Au1 Cl1 2.3010(7), Au1 Au2 3.23791(17), P1-Au1-Cl1
À
À
173.45(3), C6-N1-C12 126.3(3). For 2: Au1 P1 2.2655(21), Au1 N1
À
À
À
2.041(7), Au1 Cl1 2.354(2), Au1 Cl2 2.3039(22), Au2 P2 2.2401(23),
Au1 Au2 4.641, P1-Au1-Cl1 178.39(8), P1-Au1-N1 84.51(19), N1-Au1-
À
Cl2 172.66(20), C1-N1-C7 116.4(7), torsion ] C6-C1-C7-C8 76.4(8).
reveals an intramolecular Au1–Au2 distance of
3.23791(17) ꢀ, which suggests an aurophilic d10–d10 interac-
tion[13] in the solid state (Figure 1). Reaction of 1 with one
equivalent of dichloro-l3-(iodanyl)benzene (PhICl2) in
CH2Cl2 instantaneously generates a dark purple solution
that shows two signals in the 31P NMR spectrum at d 44.3 (P2)
and 105.3 ppm (P1). The strongly downfield shifted signal for
P1 supports coordination to a AuIII center by site-selective
two-electron AuI oxidation with formation of AuI–AuIII
derivative 2. Mass spectrometry supports an overall
Au2Cl3(L) configuration. The 1H NMR spectrum contains
one strongly upfield shifted aromatic signal at d 5.90 ppm but
no -NH signal could be identified. X-ray structure
Figure 2. Displacement ellipsoid plot (50% probability level) for the
cationic part of 3. The NTf2 counterion, lattice solvent molecules, and
hydrogen atoms are omitted for clarity. Selected bond lengths (ꢀ) and
À
À
À
angles (8): Au1 P1 2.2500(12), Au2 P2 2.2500(12), Au1 Cl1
À
À
2.3484(12), Au1 Au2 3.0758(3), C2 C8 1.449(7), P1-Au1-Cl1
178.53(4), Au1-Cl1-Au2 81.74(4).
2
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À
À
(concomitant with oxidative C C coupling of the two C H
groups ortho to the nitrogen in the ligand backbone), to
generate a carbazole framework wherein the nitrogen is
reprotonated. The gold centers are bridged by a single
chlorido ligand, leading to an acute ]Au1-Cl1-Au2 of
81.74(4)8. Relative to “open” complex 1, the intramolecular
Au–Au distance decreased by approximately 0.16 ꢀ in this
“closed” derivative, as a result of increased directional
positioning of the phosphine lone pairs, despite the larger
natural bite angle.[14] To the best of our knowledge, this is the
to Au(2) (Figure 3). This is the first crystallographically
characterized intramolecular Au2(s,p-acetylide) complex
with a diphosphine ligand, and serves to illustrate that
preorganization of two gold centers may lead to selective
substrate coordination. Complex 4 reacts in a similar fashion,
leading to the well-defined species 5’ that was characterized
by NMR spectroscopy and mass spectrometry (see the
Supporting Information).
Having established that dinuclear complex 3 engages in
À
well-defined s,p-activation of C C triple bond systems, the
À
first report of redox-chemistry occurring at the ortho C H
heterocyclization (intramolecular hydroamination) of urea-
functionalized alkyne 6 was selected as a proof-of-concept
reaction to probe the catalytic competence of this well-
defined Au2 species. Markovnikov addition to generate a
six-membered ring (8) involves p-activation by a single gold
species, whereas dinuclear s,p-activation results in
anti-Markovnikov addition to give a five-membered ring (7;
Scheme 3).[17] Using 2.5 mol% of 3 in DMF at 608C for five
positions of the PNP framework within the coordination
sphere of any transition metal. The existence of a single
chlorido bridgehead between two AuI centers is relatively
rare.[15] Notably, no single example exists of an intramolecular
Au-Cl-Au bridge stabilized by a single dinucleating ligand.
Reaction of 1 (featuring the “open” ligand) with one
equivalent of AgNTf2 generates tetranuclear bis(chlorido)-
bridged complex 4 as a crystalline solid (]Au1-Cl1-Au2
96.78(4)8). This species is likely in equilibrium with the
dinuclear monomer in solution (see the Supporting Informa-
tion for details about complex 4).
Substitution of the chlorido bridge in these dinuclear gold
species might release a masked AuI-cation. Reaction of 3 with
excess phenylacetylene leads to broadening of the 31P NMR
signal and appearance of an additional singlet at d 43.3 ppm,
which converts to a single product 5 (with complete
conversion) upon addition of one equivalent of AgNTf2 or
K2CO3. No signal corresponding to the terminal CH of the
Scheme 3. Heterocyclization of 1-(ethynylphenyl)urea (6) into either
anti-Markovnikov product 7 or Markovnikov addition product 8.
hours leads to full conversion and high regioselectivity for the
five-membered indole 7 (85%), in accordance with a selective
s,p-acetylide mechanism. External base (such as K2CO3)
inhibits the reactivity, while addition of one equivalent of
AgNTf2 with respect to the catalyst provides slightly higher
regioselectivity for 7 (90%); presumably this is due to faster
generation of the s,p-acetylide species. In the absence of
additives (Ag+ or base), bis(chlorido)-bridged tetranuclear
complex 4 displays a similar preference for formation of 7.
The high regioselectivity achieved with dinuclear catalysts 3
and 4 is attributed to the ligand-enforced proximity of both
AuI centers. Stoichiometric reaction of 3 and 6 shows sole
formation of the s,p-activated substrate by ESI-MS (Support-
ing Information, Figure S27).
1
alkyne was observed for this species by H NMR spectro-
scopy, suggesting formation of a gold(acetylide) fragment.
The overall symmetry of the complex appears to be retained
during this transformation, which points to rapid exchange of
the phenylacetylide between the two gold centers.[16] Single
crystal X-ray structure determination corroborates the dual
I
I
ꢀ
interaction of the -C CPh ligand with the Au –Au complex,
that is, s-coordination of the terminal phenylacetylide carbon
C(15) to Au(1) and p-coordination of the triple bond system
Dilution studies were performed to investigate the effect
of decreased catalyst loading on the level of regiocontrol for
the conversion of 6 to 7 and 8. The findings clearly validate
our hypothesis, as the high regioselectivity for 7 obtained with
catalyst 3 is independent of the catalyst concentration
(Figure 4). In contrast, dilution experiments with mono-
nuclear AuCl(PtBu3)—reported as the best catalyst to form
7 by s,p-activation[17]—result in a sharp drop in selectivity.[18]
These results demonstrate the benefits of well-defined pre-
organization of two gold centers to enforce selective
s,p-activation and to mediate regioselective dual gold catal-
ysis with functionalized alkynes, even at low catalyst loadings.
In summary, we have demonstrated that the redox-active
PNHPiPr ligand is a suitable framework to preorganize two
gold centers for selective s,p-activation of functionalized
alkynes. Halide abstraction from mixed-valent AuI–AuIII
complex 2 using AgNTf2 results in highly unusual reactivity
Figure 3. Displacement ellipsoid plot (50% probability level) for the
cationic part of 5. The NTf2 counterion, lattice solvent molecules, and
hydrogen atoms are omitted for clarity. Selected bond lengths (ꢀ) and
À
À
À
angles (8): Au1 P1 2.2899(10), Au2 P2 2.2676(11), Au1 C15 2.019(5),
À
À
À
Au2 Ct(C15-C16) 2.201, C15 C16 1.227(6), Au1 Au2 3.1110(2), P1-
Au1-C15 176.81(13), Au1-C15-C16 174.2(4), P2-Au2-Ct(C15-C16)
174.99, C15-C16-C17 170.0(5).
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dilution conditions.
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Acknowledgements
This research was funded by the European Research Council
(ERC) through Starting Grant 279097 (EuReCat) to J.I.v.d.V.;
A.C.H.J. and J.N.H.R. thank the ERC for Advanced Grant
339786. We thank Dr. Marc Devillard for valuable tips and
tricks and Ed Zuidinga for ESI-MS measurements.
Keywords: s,p-activation · bridging acetylide ·
dinuclear gold complexes · dual gold catalysis ·
redox-active ligand
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[17] A. Gimeno, A. B. Cuenca, S. Suꢄrez-Pantiga, C. Ramꢇrez de Ar-
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[18] See also: D. Ye, J. Wang, X. Zhang, Y. Zhou, X. Ding, E. Feng, H.
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[19] Because of the anticipated complexity associated with this multi-
step “cascade” process, which converts 2 into 3, a detailed
mechanistic investigation of this transformation is deemed
beyond the scope of this study and will be reported elsewhere.
Received: April 26, 2016
Revised: May 27, 2016
Published online: && &&, &&&&
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Gold Catalysis
Worth its weight in gold: Coordination of
gold(I) to a rigid bis(o-tolylamino)di-
phosphine ligand allows for versatile
chemistry, including the isolation of
a mixed-valent AuI–AuIII species. Halide
abstraction induces ligand-to-gold two-
electron transfer, producing a carbazolyl-
diphosphine analogue with a bridging
chlorido ligand. This preorganized AuI–
AuI species enables s,p-activation of
alkynes and is an isolable precatalyst for
highly selective heterocyclizations.
V. Vreeken, D. L. J. Broere, A. C. H. Jans,
M. Lankelma, J. N. H. Reek, M. A. Siegler,
J. I. van der Vlugt*
&&&& — &&&&
Well-Defined Dinuclear Gold Complexes
for Preorganization-Induced Selective
Dual Gold Catalysis
6
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!