Coordination chemistry of gold with: N -phosphine oxide-substituted imidazolylidenes (POxIms)

Article abstract of DOI:10.1039/c9nj04911h

N-Phosphine oxide-substituted imidazolylidenes (POxIms) have been employed as heteroditopic ligands towards gold centres. Both bis-carbene and mono-carbene gold(i) complexes have been obtained, depending on the steric bulk of the employed POxIm ligand. Ox

Full text of DOI:10.1039/c9nj04911h

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Coordination chemistry of gold with N-phosphine
oxide-substituted imidazolylidenes (POxIms)†
Cite this: New J. Chem., 2019,
43, 17275
a
ab
b
Lorenzo Branzi,a Marco Baron,
Paolo Sgarbossa,
Lidia Armelao,
Marzio Rancan,
c
d
e
a
Claudia Graiff,
Alexander Pothig
and Andrea Biffis
*
¨
N-Phosphine oxide-substituted imidazolylidenes (POxIms) have been employed as heteroditopic ligands
towards gold centres. Both bis-carbene and mono-carbene gold(I) complexes have been obtained,
depending on the steric bulk of the employed POxIm ligand. Oxidation of the gold(I) complexes with
halogens or halogen synthons allows access to both bis-carbene and mono-carbene gold(III) complexes,
depending on the nature of the starting gold(I) complex and on the oxidation conditions. In all these
complexes, the phosphanyl oxide moiety of the ligand is found to not be coordinated by the gold centre,
although the crystallographic and spectroscopic characterization of the compounds suggests in most cases
the existence of weak electrostatic interactions. A preliminary screening of the complexes as precatalysts in
the intermolecular hydroamination of phenylacetylene with mesitylamine has also been carried out.
Received 28th September 2019,
Accepted 8th October 2019
DOI: 10.1039/c9nj04911h
rsc.li/njc
gold–gold closed shell interactions (aurophilic interactions).7–9
Finally, NHC ligands have been recently highlighted as promising
Introduction
Gold complexes with stable carbene, most notably N-heterocyclic for the derivatization and stabilization of metallic gold surfaces and
carbene (NHC), ligands represent a class of coordination com- nanoparticles, including atomically precise gold nanoclusters.10,11
pounds that has been enjoying steadily growing interest among
On the basis of the above, it is not surprising that a great
the scientific community over the last two decades. The main variety of carbene ligands have been developed, with the scope
reason for this interest is the stability of the gold–carbene of producing gold complexes with novel properties. Among this
bond,1 which is the key feature enabling the extensive application variety, NHC ligands with a tethered additional functionality,
of these complexes in several different fields of chemistry. Thus, potentially able to coordinate metal centres, appear particularly
gold–NHC complexes have been highlighted several times as interesting. For example, heteroditopic ligands of this kind
robust catalysts for diverse chemical reactions.2–4 They are suffi- have been employed for the production of gold(I) complexes
ciently stable to survive for long times in biological media, hence with a strongly distorted coordination geometry, which exhibit
they enable the transport of gold centres inside living cells, where peculiar luminescence properties.12 On the other hand, NHC
gold can display its bioactivity (e.g. as an antitumoral agent).5,6 ligands with an additional pendant functional group can be
Functional gold–NHC complexes as well as polynuclear gold employed to facilitate unusual oxidative addition reactions to a
adducts with NHC ligands can be involved in the construction gold(I) centre forming the corresponding gold(III) complex (as
of supramolecular architectures featuring molecular recognition recently highlighted by Bourissou with functional phosphane
or photophysical properties, thanks, e.g., to the presence of ligands13), or to coordinate additional metal centres, providing
access to polymetallic complexes. Finally, these functionalities
a
may also assist the NHC-bound gold centre in performing a
catalytic reaction (cooperative catalysis).
`
Dipartimento di Scienze Chimiche, Universita degli Studi di Padova,
Via F. Marzolo 1, 35131 Padova, Italy. E-mail: andrea.biffis@unipd.it
b ICMATE-CNR, c/o Dipartimento di Scienze Chimiche,
We have an ongoing interest in gold–NHC complexes for
diverse applications.14–18 In this contribution, we investigate
the coordination chemistry of gold centres with a novel kind of
functional NHC ligand, namely N-phosphine oxide-substituted
imidazolylidenes, alternatively termed POxIms. These carbenes
have been originally developed in recent years and employed as
organocatalysts by the group of Hoshimoto,19–21 and we are
currently performing a comprehensive investigation on their
use as ligands for transition metals. After a first report on the
coordination chemistry of palladium(II) with these ligands,22
`
Universita degli Studi di Padova, Via F. Marzolo 1, 35131 Padova, Italy
c
`
Dipartimento di Ingegneria Industriale, Universita degli Studi di Padova,
Via F. Marzolo 9, 35131 Padova, Italy
d
`
Dipartimento di Scienze Chimiche, della Vita e della Sostenibilita Ambientale,
`
Universita degli Studi di Parma, Parco Area delle Scienze 17/A, 43124 Parma,
Italy
e
¨
Catalysis Research Centre & Department of Chemistry, Technische Universitat
¨
Munchen, Ernst-Otto-Fischer-Strasse 1, 85747, Garching, Germany
† Electronic supplementary information (ESI) available. CCDC 1911218 and
1942489–1942492. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c9nj04911h
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in this contribution, we now turn our attention to complexes of
POxIm ligands with gold, and we show that a great variety
of complexes with different gold : ligand ratios and different
oxidation states for gold are accessible, which in some cases
show a remarkable activity as precatalysts in model intermolecular
alkyne hydroamination reactions.
Results and discussion
The synthesis of the cationic precursors of POxIm ligands is
straightforward (Scheme 1) and follows the route previously
developed by Hoshimoto.19 The resulting N-phosphanyl oxide
imidazolium precursors 1 and 2 are remarkably stable to air
and moisture.
From these precursors, the free POxIms 3 and 4 can be
generated by treatment with a base, liberating the carbenes as
air-sensitive compounds that can be isolated19,20 or directly
reacted with suitable metal precursors to yield the corresponding
complexes.22 In the context of the preparation of POxIm–Au(I)
compounds, though, we have employed another, simpler
approach that starts directly from the protonated precursors 1
and 2 and makes use of an inorganic base such as potassium
carbonate to deprotonate the imidazolium cation.18,23,24 Key to
success is the formation of ion pairs between the imidazolium
cation and an anionic gold(I) polyhalide complex, which facilitates
deprotonation with concomitant carbene transfer to gold; conse-
quently, since the imidazolium precursors 1 and 2 do not contain
halides as counteranions, we purposely added one equivalent of
KCl to the reaction mixture. The aim in our case was to prepare
complexes of general stoichiometry POxImAuCl, hence we per-
formed the reaction with a 1 : 1 stoichiometric ratio between the
carbene precursor and [AuCl(SMe2)]. Quite surprisingly, this
reaction was successful in the case of carbene precursor 1,
yielding complex 5, whereas it furnished the corresponding
cationic dicarbene gold(I) complex cation 6 in the case of
precursor 2 (Scheme 2). In the latter case, excess gold undergoes
dismutation to gold(0) and gold(III) as AuCl4ꢀ, which produces
the complex counteranion in the isolated product (complex
6-AuCl4); on the other hand, when 0.5 equivalents of [AuCl(SMe2)]
was employed, the reaction yielded cleanly the cationic complex
with triflate as counteranion (complex 6-OTf).
Scheme 2 Synthesis of POxIm–gold(I) complexes.
carbene precursor with 0.5 equivalents of [AuCl(SMe2)] or reaction
of preformed complex 5 with 1 equivalent of the free carbene 3
invariably resulted in the isolation of complex 5 from the reaction
mixture. Conversely, reaction between preformed free carbene 4
and [AuCl(SMe2)] in a 1 : 1 ratio invariably led to the formation of
the cationic biscarbene complex 6-AuCl4. Thus, it seems that the
different steric bulk of the two carbene ligands determines
the different reaction outcome irrespective of the employed
synthetic pathway.
Crystals of complex 5 and of the cation 6 could be grown
and their structures could be determined. In the case of 6, slow
recrystallization of compound 6-AuCl4 resulted in crystals
containing the cation 6 with chloride as counteranion. This is
indicative of a certain degree of ligand exchange between gold(I)
and gold(III) in solution. Additional evidence of such an exchange
stems from the ESI-MS characterization of 6-AuCl4, in which
beside the expected mass peak of 6 at m/z 889, a second peak at
m/z 959 is also detected, whose mass and isotopic pattern are
consistent with [(4)2AuCl2]+, i.e. a POxIm–gold(III) complex.
The two structures are reported in Fig. 1 and 2. The most
interesting aspect of the two structures is the different orienta-
tion of the phosphanyl oxide moiety in the two cases; in the
case of complex 5, the P–O bond is turned anti to the metal and
towards one of the hydrogens of the imidazole backbone, which
clearly indicates that there is no interaction between gold and
the oxide. On the other hand, complex 6-Cl is symmetric with a
twofold axis passing through the metal atom and with both
phosphanyl oxide moieties oriented syn to the metal; although
there seems to be no proper coordination of the oxide to the
metal, the recorded Au–O distance (2.96 Å) is nevertheless
significantly shorter than the sum of the van der Waals radii
of gold and oxygen (3.18 Å). Thus, there could be a weak
electrostatic interaction between the positively charged gold
centre and the oxygen of the phosphanyl oxide, which carries a
We followed by 1H NMR the latter reaction at room tempera-
ture: we could monitor the slow conversion of precursor 4 to the
cationic biscarbene complex 6, but no intermediate formation
of a monocarbene complex was recorded at any point. Remark-
ably, attempts to prepare the cationic biscarbene complex with
ligand 3 or the neutral monocarbene complex with ligand 4 were
unsuccessful. In the former case, either reaction between the
Scheme 1 Synthesis of the POxIm ligands.
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complexes with the same ligand, in which the P–O bond is
invariably oriented towards the metal (Dd = ca. 0.3 ppm).22,25
A
similar observation was previously made by the group of
Hoshimoto upon comparing the 1H NMR spectra of cationic
POxIm precursors 1 and 2 (Dd = ca. 0.7 ppm) with that of free
carbenes 3 and 4 (Dd = ca. 1.5 ppm).19 Again, the chemical shift
difference correlated with the recorded orientation of the P–O
bond in the crystal structure of the compounds, which pointed
towards H4 in the free carbenes and away from it in the
precursors. We tend to attribute the large chemical shift
difference observed with compounds 3, 4 and 5 to the existence
of an intramolecular attractive interaction between H4 and the
phosphanyl oxide group (which is an excellent hydrogen bond
acceptor),26 and to take this chemical shift difference as proof
of the syn/anti orientation of the phosphanyl oxide group with
respect to the metal in POxIm complexes in solution.
An indirect confirmation of our assumption comes from the
NMR characterization of cation 6, which presents more
complex features than those of 5. Indeed, two sets of signals
in a 1 : 1 ratio are observed in the 1H, 13C and 31P NMR spectra
of 6 in CD3CN, differing in particular in the chemical shifts of
the H4 and H5 resonances; this is consistent with the two
POxIm ligands in 6 possessing a different conformation in
solution, one with the P–O bond syn to gold (Dd = 0.25 ppm)
and the other one anti to it (Dd = 0.56 ppm); a complete
attribution of the signals to each set was possible through
2D-NMR experiments (see Fig. S11 in the ESI†). Interestingly,
the chemical shift values of the various signals depend signifi-
cantly on the nature of the counteranion of 6 and of the solvent,
to the point that in the case of 6-AuCl4 in CDCl3, they coalesce to
a single set of signals with large chemical shift differences
between H4 and H5 (Dd = 0.93 ppm); this is indicative of the
fact that under these conditions, the two P–O bonds in 6 present
an all-anti conformation.
Fig. 1 ORTEP drawing of complex 5. Ellipsoids are drawn at the 50%
probability level. Hydrogen atoms have been omitted for clarity. Selected
bond distances (Å) and angles (1): C1–Au 1.982(5), Au–Cl1 2.275(1),
C1–Au–Cl1 178.01(11).
We subsequently turned to the preparation of gold(III) complexes
with the same ligands, using the well established gold(I) oxidation
Fig. 2 ORTEP drawing of complex 6-Cl. Ellipsoids are drawn at the 50% strategy with elemental halogens or halogen synthons, which
previously proved useful with several NHC–gold compounds.14
Indeed, oxidation of compound 5 with PhICl2 cleanly led to
the corresponding POxIm–gold(III) trichloride complex 7, as
outlined in Scheme 3.
probability level. Hydrogen atoms and the chloride anion have been
omitted for clarity. Selected bond distances (Å) and angles (1): C1–Au
27
2.008(6), O–Au 2.957(6), C1–Au–C10 178.9(3). Symmetry operation: ‘ =
1
2
1 ꢀ x, +y, ꢀ z.
The crystal structure of complex 7 (Fig. 3) was determined
partial negative charge, although the steric bulk of the mesityl and exhibited the expected features, namely a slightly longer
group of the second POxIm ligand also favours an orientation Au–C bond (2.012(3) Å) compared to the corresponding
of the phosphanyl oxide group with the less sterically demanding gold(I) complex 5 (1.982(5) Å),28 and a longer Au–Cl bond for
PQO bond instead of the t-butyl groups pointing towards gold. the chloride ligand trans to the carbene (Cl3–Au1 2.311(1) Å),
The chlorine atom is disordered in two positions around a
twofold axis and is not involved in interactions.
The difference between the two compounds is much more
apparent in solution. The 1H NMR spectrum of 5 is simple and
shows the expected features but a strong difference in the
chemical shift of the backbone hydrogens of the imidazole
ring: the hydrogen in 5-position (H5) resonates at 7.15 ppm
whereas the one in 4-position (H4) is considerably deshielded
and is found at 8.02 ppm (Dd = 0.87 ppm). This difference is
much greater than that previously recorded for palladium(II)
Scheme 3 Preparation of POxIm–gold trichloride 7 by oxidation of the
gold(I) precursor 5.
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1
H4 and H5 signals in the H NMR spectrum (Dd = 0.19 ppm),
which suggests a syn conformation in solution.
The spectral features of complexes 8–9 were instead notably
simpler than those of the parent gold(I) cationic complex 6. In
particular, in the 1H NMR spectrum, only one set of signals was
invariably present, and the separation of the H4 and H5 signals
(Dd = 0.34 ppm for 8, 0.17 ppm for 9) was in all cases compatible
with an all-syn conformation of the phosphanyl oxide group
with respect to gold.
We attempted to grow crystals of all these compounds and
were successful in the case of complexes 9-OTf and 9-AuCl4. The
latter crystals, though, did not contain thꢀe expected cationic
biscarbene gold(III) dibromide with AuCl4 as counteranion,
but instead they contained a neutral gold(III) complex (pseudo-10),
similar in structure to complexes 7 and 10 but with scrambled
halide ligands coordinated to the gold centre, which are in part
chloride and in part bromide (Fig. 4, atomic occupancies of Cl1/
Br1 0.156(4)/0.844(4), Cl2/Br2 0.711(4)/0.289(4), Cl3/Br3 0.187(4)/
0.813(4)). Again, formation of such a complex is a further
indication of ligand exchange ꢀin solution between the cation
in complex 9 and the AuCl4 counteranion. The structure
confirms the syn orientation of the P–O bond that was assumed
for complex 10, with a Au–O distance of 2.910(3) Å, slightly
shorter than in the case of the complexes investigated before.
Finally, the crystal grown from a solution of complex 9-OTf
contained indeed the expected biscarbene gold(III) dibromide cation
(Fig. 5). The syn conformation of the two phosphanyl oxide moieties
was again confirmed, and the Au–O distance was found to be
slightly shorter than the previous cases (2.889(3) Å), in line with the
higher electrophilicity expected for a monocationic gold(III) centre.
The application of the synthesized complexes as catalysts
has been preliminarily evaluated using as a test reaction the
hydroamination of phenylacetylene with mesitylamine, which
proceeds in a fully Markovnikov fashion under gold catalysis to
produce the corresponding imine (Scheme 5).29,30
Fig. 3 ORTEP drawing of complex 7. Ellipsoids are drawn at the 50%
probability level. Hydrogen atoms have been omitted for clarity. Selected
bond distances (Å) and angles (1): C1–Au 2.012(3), Cl3–Au 2.311(1), Cl2–Au
2.281(1), Cl1–Au 2.283(1); Cl1–Au–Cl2 179.02(4), C1–Au–Cl3 175.86(9).
due to the strong trans influence of the carbene ligand. On the
other hand, in contrast to 5, the P–O bond in 7 takes up a syn
conformation to gold; the oxygen distance to gold is found to be
2.949(2) Å in length, which is very close to the Au–O distance in
the related gold(I) cation 6 and is again shorter than the sum of
the van der Waals radii of Au and O. The 1H NMR characterization
of the complex indicates that this conformation is maintained
also in solution, as the signals of H4 and H5 are very close
(Dd = 0.01 ppm).
Oxidation of the complex 6-OTf or 6-AuCl4 led instead to
different results depending on the amount and nature of the
employed oxidant (Scheme 4). Use of a stoichiometric quantity
of PhICl2 or Br2 produced cleanly the cationic biscarbene
complexes 8–9, whereas use of excess PhICl2 led to the neutral
monocarbene complex 10, a POxIm gold(III) trichloride complex
analogous to 7. Complex 10 was not structurally characterized
(see below), but its spectral features closely resemble those of
the analogous complex 7, including the small separation of the
Fig. 4 ORTEP drawing of compound pseudo-10 that crystallizes out of
solutions of complex 9-AuCl4. Ellipsoids are drawn at the 50% probability
level. Solvent molecule and hydrogen atoms have been omitted for clarity.
Selected bond distances (Å) and angles (1): Au1–C1 2.021(4), Au1–Cl1 2.38(5),
Au1–Cl2 2.330(8), Au1–Cl3 2.38(3), Au1–Br1 2.408(3), Au1–Br2 2.422(8),
Au1–Br3 2.414(2), O1–Au1 2.910(3), C1–Au1–Br1 89.55(14), C1–Au1–Br2
174.2(2), C1–Au1–Br3 92.16(12), C1–Au1–Cl1 89.2(11), C1–Au1–Cl2
178.2(2), C1–Au1–Cl3 91.6(6).
Scheme 4 Preparation of gold(III) complexes with POxIm ligand 4.
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Table 1 Catalytic activity of the complexes in hydroamination
Hydroamination Hydration
Entry Catalyst
Time (h) yield (%)
yield (%)
1
2
3
4
5
6
7
8
5-AgSbF6
4
24
4
24
4
24
4
24
4
24
4
24
4
24
4
24
66
85
52
71
57
73
2
9
15
6
6
6
6
0
1
1
7-AgSbF6
7-AgSbF6 (3 eq.)
9-OTf
3
9-OTf–AgSbF6
10-AgSbF6
12
13
69
85
61
65
65
86
1
11
11
9
9
8
10-AgSbF6 (3 eq.)
IPrAuCl–AgSbF6
14
Fig. 5 ORTEP drawing of complex 9-OTf. Ellipsoids are drawn at the 50%
probability level. Hydrogen atoms have been omitted for clarity. Selected
bond distances (Å) and angles (1): Au–C1 2.061(3), Au–Br1 2.4099(8),
Au1–Br2 2.4252(8), Br1–Au–Br2 180.0, C1–Au–C1’ 175.67(16), C1–Au–Br1
Reaction conditions: 0.983 mmol of phenylacetilene, 0.951 mmol of
mesitylamine, 1 mol% gold precatalyst, 1 mol% AgSbF6, 40 1C. Yields
determined by 1H-NMR.
1
87.84(8), C1–Au–Br2 92.16(8). Symmetry operation: ‘ = ꢀ x, +y, 1 ꢀ z.
2
to evaluate whether its cationic nature was per se sufficient to
activate the reagents upon an addition/substitution reaction.
However, complex 9-OTf turned out to be almost completely
inactive as a catalyst (entry 4), and even after addition of one
equivalent of AgSbF6, its catalytic activity increased only marginally
(entry 5). We were quite surprised by this result, but we later found
out that the substitution of the bromido ligands in this complex is
all but trivial: we checked the reaction of 9-OTf with one equivalent
AgOTf and we were unable to isolate a formally dicationic
Scheme 5 The hydroamination reaction employed in this study.
The corresponding ketone, stemming from the hydrolysis of product stemming from the removal of a bromide ligand by
the initially formed imine by traces of water, is often present as silver, whereas we observed by NMR a mixture of species
a reaction byproduct. Gold(I) and also gold(III) centres are in possibly associated with the coordination of silver to the pen-
general quite efficient in catalysing this reaction, which has dant phosphanyl oxide groups of the complex. In contrast,
often been employed as a standard tool to quantify the reactivity of neutral mono-NHC gold(I) and gold(III) complexes 5, 7 and 10
these complexes.2,18,30 In this connection, it is important to remark were active catalysts for the reaction once activated with one
that the pendant phosphanyl oxide moiety could exert a positive equivalent of AgSbF6 (entries 1, 2 and 6). Addition of excess
role in alkyne hydrofunctionalisation reactions of this kind, as it silver salt (3 equiv.) in the case of gold(III) complexes did not
could interact with the protonated nucleophile activating it and promote further the reaction (entries 3 and 7), which is not
directing its attack to the alkyne p-coordinated to the metal, as surprising since it has been reported that with NHC–Au(III)
suggested for example by C. Hahn et al. in the case of a complexes of this kind, only one halido ligand can be removed
diphosphine monoxide gold(I) complex closely related to some by the action of Ag+.32 The gold(III) complexes 7 and 10 appear
of the complexes reported herein.31
much more active than the related gold(III)–NHC complexes
Tests were run using neat conditions that we already employed reported by Messerle et al. and featuring ligands with pendant
for the evaluation of the catalytic performance of dinuclear gold(I) pyrazole groups, which provide 61% conversion after 16 hours
complexes with di-NHC ligands.18 One equivalent of AgSbF6 with in the same reaction under much more drastic reaction conditions
respect to gold was generally added as a reaction promoter, in (100 1C, 2 mol% Au).33 The catalytic performance of the new gold(I)
order to remove a halide ligand liberating a coordination site on and gold(III) complexes appears similar and their initial activity is
the gold centre for interaction with the reagents; we previously comparable with that of the benchmark commercial gold(I) NHC
showed that silver salts such as AgSbF6 exhibit per se negligible complex IPrAuCl, which allows a 65% yield to be reached in 4 hours
activity in the hydroamination reaction under the employed under identical reaction conditions (entry 8). However, the catalyti-
reaction conditions.18 We did not test the gold(I) dicarbene cally competent species formed in situ upon halide removal do not
cation 6 as a catalyst, since we expected it to be catalytically appear to be exceedingly stable, as the reaction considerably slows
inactive, given the difficulty in removing an NHC ligand from down with time and does not reach full conversion of the alkyne
gold(I). The results are reported in Table 1.
(with the only exception of entry 1), even after prolonged
We initially tested the cationic dicarbene gold(III) complex reaction times. Consequently, the recorded catalytic efficiency
9-OTf in the absence of the AgSbF6 promoter, since we wanted of these complexes does not seem to reach the values recorded
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with the most productive gold catalysts reported to date in the Synthesis of complex 6-AuCl4
literature for this reaction, such as the zwitterionic complex
0.2435 g (0.4904 mmol) of precursor 2, 0.0398 g (0.5338 mmol)
of KCl, 0.8272 g (5.6553 mmol) of K2CO3 and 0.13976 g
(0.5012 mmol) of [(Me2S)AuCl] were placed in a round-bottomed
flask under Ar. 10.0 ml of dry acetonitrile were then added; the
flask was placed in an oil bath thermostated at 65 1C and the
reaction mixture was stirred for 24 h. The mixture was filtered over
Celite and the solids were washed with acetonitrile (3 ꢁ 3.0 ml).
The solution was evaporated to dryness and the residue was taken
up in 10.0 ml of dichloromethane. Undissolved solids were
removed by filtration and the solution was finally evaporated to
dryness and treated with 10.0 ml of Et2O to isolate the product as a
white solid. Yield 50.6% (0.1524 g). 1H-NMR (CD3CN): d = 7.94 (m,
1H, Him), 7.64 (m, 1H, Him), 7.38 (m, 1H, Him), 7.36 (m, 1H, Him),
7.10–7.08 (m, 4H, Haryl), 2.35 (6H, CH3para), 2.19 (s, 6H, CHo3rtho),
2.00 (s, 6H, CH3ortho), 1.71 (s, 6H, CHo3rtho), 1.52 (d, 3JHP = 15.96 Hz,
reported by the group of Lavallo, which features a phosphane ligand
with a carboranyl substituent and allows full conversions to be
reached with as low as 0.001 mol% Au (TONs up to 95 000).34
Experimental
Materials and methods
All manipulations of air and moisture sensitive compounds
were carried out in a glove box or using standard Schlenk
techniques under an atmosphere of argon or dinitrogen. The
reagents were purchased from Aldrich as high-purity products
and generally used as received. All solvents were purified and
dried by standard methods. Ligand precursors 1 and 219 and
reagent PhICl235 were prepared according to literature procedures.
NMR spectra were recorded using a Bruker Avance spectrometer
3
18H, CH3tBu), 1.37 (d, JHP = 15.46 Hz, 18H, CH3tBu). 1H-NMR
(CDCl3): d = 8.01 (m, 2H, Him), 7.08 (m, 2H, Him), 6.99 (s, 4H, Haryl),
2.34 (s, 6H, CH3para), 2.01 (s, 12H, CHo3rtho), 1.58 (d, 3JHP = 16.08 Hz,
36H, CH3tBu). 31P-NMR (CDCl3): d = 72.70. 13C-NMR (CDCl3): d =
173.40 (d, 2JCP = 8.16 Hz, Ccarb), 140.21 (s, Caryl), 134.94 (s, Caryl),
134.38 (s, Caryl), 129.71 (s, CHaryl), 125.58 (d, J = 3.46 Hz, CHim),
121.3 (d, J = 4.13 Hz, CHim), 38.70 (d, 1JCP = 62.01 Hz, CtBu), 27.53
(s, CH3tBu), 21.33 (s, CH3para), 17.87 (s, CHo3rtho). ESI-MS (m/z):
889.38 [100%, [(M-AuCl4)+], 959.30 [45% (M-AuCl2)+]]. Elemental
analysis: calcd (%) for C40H62N4P2O2Au2Cl4 (MM 1228.18):
C 39.10, H 5.09; N 4.56; found: C 39.32, H 5.40, N 4.28.
1
working at 300 MHz (300.1 MHz for H, 75.5 MHz for 13C and
121.5 MHz for 31P); chemical shift (d) values are reported in units
of ppm relative to the residual solvent signals and to external 85%
H3PO4 (for 31P). ESI-MS analysis was performed using a LCQ-Duo
(Thermo-Finnigan) operating in positive ion mode. Instrumental
parameters: capillary voltage 10 V; spray voltage 4.5 kV; capillary
temperature 200 1C; mass scan range from 150 to 2000 amu; N2
was used as sheath gas; the He pressure inside the trap was kept
constant. The pressure directly read by an ion gauge (in the absence
of the N2 stream) was 1.33 ꢁ 10ꢀ5 Torr. Sample solutions, prepared
by dissolving the compounds in acetonitrile, were directly infused
into the ESI source by a syringe pump at 8 ml minꢀ1 flow rate.
Elemental analysis was carried out using a Fisons EA 1108 CHNS-O
instrument or with a Carlo Erba analyzer.
Synthesis of complex 6-OTf
0.2004 g (0.4360 mmol) of precursor 2, 0.01780 g (0.2388 mmol)
of KCl, 0.2758 g (0.1996 mmol) of K2CO3 and 0.0573 g
(0.2050 mmol) of [(Me2S)AuCl] were placed in a round-bottomed
flask under Ar. 10.0 ml of dry acetonitrile was then added; the flask
Synthesis of complex 5
0.1004 g (0.1862 mmol) of precursor 1, 0.0166 g (0.2227 mmol) was placed in an oil bath thermostated at 40 1C and the reaction
of KCl, 0.2555 g (1.849 mmol) of K2CO3 and 0.0522 g (0.1868 mmol) mixture was stirred for 72 h. The mixture was filtered over Celite
of [(Me2S)AuCl] were placed in a round-bottomed flask under Ar. and the solids were washed with acetonitrile (3 ꢁ 3.0 ml). The
30.0 ml of dry acetonitrile was then added; the flask was placed in solution was evaporated to dryness and the residue was taken up
an oil bath thermostated at 60 1C and the reaction mixture was in 10.0 ml of dichloromethane. Undissolved solids were removed
stirred for 3.0 h. The mixture was filtered over Celite and the solids by filtration and the solution was finally evaporated to dryness and
were washed with acetonitrile (3 ꢁ 3.0 ml). The solution was treated with 10.0 ml of Et2O to isolate the product as a white solid.
1
evaporated to dryness and the resulting solid was treated with Yield 75.0% (0.1598 g). H-NMR (CD3CN): d = 7.87 (m, 1H, Him),
5.0 ml of chloroform. Undissolved solids were removed by filtra- 7.60 (m, 1H, Him), 7.35 (m, 1H, Him), 7.31 (m, 1H, Him), 7.00 (m,
tion and the solution was finally evaporated to dryness to yield the 4H, Haryl), 2.25 (6H, CH3para), 1.90 (s, 6H, CH3ortho), 1.71 (s, 6H,
product as a white solid. Yield 89.6% (0.1036 g). 1H-NMR (CDCl3): CH3ortho) 1.32 ppm (d, 3JHP = 15.91 Hz, 18H, CH3tBu), 1.24 (d, 3JHP
=
d = 8.01 (m, 1H, Him), 7.50 (t, 3JHH = 7.78 Hz, 1H, Haryl), 7.27 (d, 2H), 15.39 Hz, 18H, CH3tBu). 31P-NMR (CD3CN): d = 71.05 (1P), 64.07
3
7.14 (m, 1H, Him), 2.31 (sept, JHH = 7.01 Hz, 2H, CHiPr), 1.57 (d, (1P). 13C-NMR (CD3CN): d = 189.76 (d, 2JCP = 8.29 Hz, Ccarb), 184.74
3JHP = 15.60 Hz, 18H, CH3tBu), 1.32 (d, 3JHH = 6.91 Hz, 6H, CH3iPr), (d, JCP = 8.86 Hz, Ccarb), 140.84 (s, Caryl), 140.60 (s, Caryl), 136.48
2
1.13 (d, JHH = 6.90 Hz, 6H, CH3iPr). 13C-NMR (CDCl3): d 174.5 (s, Caryl), 136.17 (s, Caryl), 135.98 (s, Caryl), 135.50 (s, Caryl), 130.13
3
(d, J = 8.1 Hz, Ccarb), 145.4 (s, Caryl), 134.3 (s, Caryl), 131.1 (s, CHaryl), (s, CHaryl), 130.0 (s, CHaryl), 126.49 (d, J = 3.07 Hz, CHim), 125.31
125.3 (d, J = 3.4 Hz, CHim), 124.5 (s, CHaryl), 122.3 (d, J = 3.9 Hz, (d, J = 2.67 Hz, CHim), 123.62 (d, J = 3.40 Hz, CHim), 122.59 (d,
CHim), 38.8 (d, J = 62.0 Hz, CtBu), 28.8 (s, CHiPr), 27.5 (s, CH3tBu), J = 3.63 Hz, CHim), 39.37 (d, JCP = 11.70 Hz, CtBu), 35.02 (d, 1JCP
=
1
24.3 (s, CH3iPr). 31P-NMR (CDCl3): d 72.8 (s). ESI-MS (m/z): 643.35 13.00 Hz, CtBu), 27.34 (s, CH3tBu), 26.48 (s, CH3tBu), 21.07 (s, CHp3ara),
[75%, (M + Na)+], 1263.05 [100%, (M2 + Na)+]. Elemental analysis: 17.89 (s, CH3ortho). ESI-MS (m/z): 889.38 [100% (M-OTf)]+. Elemental
calcd (%) for C23H37AuClN2OP (MM 620.68): C 44.49, H 6.01, analysis: calcd (%) for C41H62N4P2O5SF3Au (MM 1038.35): C 47.40,
N 4.51; found: C 44.55, H 6.37, N 4.73.
H 6.01, N 5.39, S 3.09; found: C 47.74, H 6.46, N 5.30, S 2.86.
17280 | New J. Chem., 2019, 43, 17275--17283 This journal is ©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2019

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Paper
Synthesis of complex 7
135.35 (s, Caryl), 128.87 (s, CHaryl), 128.34 (m, CHim), 124.66
1
(m, CHim), 37.79 (d, JCP = 62.60 Hz, CtBu), 26.41 (s, CH3tBu),
0.1016 g (0.1636 mmol) of complex 5 and 0.0579 g (0.1963 mmol)
of PhICl2 were placed in a round-bottomed flask under Ar.
10.0 ml of dry acetonitrile was then added and the resulting
solution was stirred at room temperature for 5.0 h. All volatiles
were subsequently evaporated under vacuum and the resulting
yellowish-white solid was extensively washed with diethylether
(3 ꢁ 10.0 ml). Yield 57.2% (0.0648 g). 1H-NMR (CD3CN):
20.74 (s, CHp3ara), 19.17 (s, CHo3rtho). ESI-MS (m/z): 889.35 [60%
(M-OTf-2Br)+], 1049.16 [100% (M-OTf)+]. Elemental analysis:
calcd (%) for C41H62N4P2O5SF3Br2Au (MM 1198.16): C 41.08,
H 5.21, N 4.67, S 2.67; found: C 40.91, H 5.58, N 4.67, S 2.42.
Synthesis of complex 9-AuCl4
d = 7.81–7.80 (m, 2H, Him), 7.63 (t, 1H, JHH = 7.81 Hz, Haryl),
3
0.1467 g (0.1194 mmol) of complex 6-AuCl4 was dissolved in a
round-bottomed flask under Ar into 5.0 ml of dry CH3CN.
0.390 ml (0.220 mmol) of a 0.564 M solution of Br2 in dry
CH3CN was then added, and the resulting solution was stirred
at room temperature for 3.0 h, during which time a yellow solid
precipitated out of the solution. The solution was concentrated
under vacuum to about one fifth of its original volume.
The solid product was filtered off and washed with Et2O (2 ꢁ
2.0 ml). Yield 43.4% (0.0956 g). 1H-NMR (CD3CN): d = 7.80
(m, 2H, Him), 7.63 (s, 2H, Him), 7.09 (s, 4H, Haryl), 2.36 (s, 6H,
7.43 (d, 2H, 3JHH = 7.81 Hz, Haryl), 2.63 (sept, 2H, 3JHH = 6.54 Hz,
3
3
CHiPr), 1.45 (d, 18H, JPH = 16.02 Hz, CH3tBu), 1.30 (d, JHH
=
3
6.54 Hz, 6H, CH3iPr), 1.04 (d, JHH = 6.54 Hz, 6H, CH3iPr).
31P-NMR (CD3CN): d = 73.33. 13C-NMR (CD3CN): d = 147.49
(s, CHaryl), 133.91 (s, Caryl), 133.57 (s, CHaryl), 130.26 (d, J =
3.24 Hz, CHim), 125.49 (s, CHaryl), 125.14 (d, J = 3.20 Hz, CHim),
39.41 (d, 1JCP = 61.48 Hz, CtBu), 29.76 (s, CHiPr), 27.15 (s, CH3tBu),
26.49 (s, CH3iPr), 22.67 (s, CH3iPr). ESI-MS (m/z): 728.93 [60%
(M + K)+], 1420.76 [100% (M2 + K)+]. Elemental analysis: calcd
(%) for C23H37N2POCl3Au (MM 691.59): C 39.93, H 5.39, N 4.05;
found: C 39.42, H 5.27, N, 3.97.
3
CHp3ara), 2.21 (s, 12H, CH3ortho), 1.45 (d, JCP = 16.01 Hz, 36H,
CH3tBu). 31P-NMR (CD3CN): d = 73.38. 13C-NMR (DMSO-d6): d =
140.21 (s, Caryl), 135.17 (s, Caryl), 135.18 (s, Caryl), 129.39 (s, Caryl),
128.93 (d, J = 3.00 Hz, Cim), 124.68 (d, J = 3.38 Hz, Cim), 38.21
Synthesis of complex 8
1
(d, JCP = 61.33 Hz, CtBu), 26.63 (s, CH3tBu), 20.60 (s, CHp3ara),
0.0234 g (0.0225 mmol) of complex 6-OTf and 0.0076 g
(0.0270 mmol) of PhICl2 were placed in a round-bottomed flask
under Ar. 5.0 ml of dry acetonitrile was then added and the
resulting solution was stirred at room temperature for 5.0 h. All
volatiles were subsequently evaporated under vacuum and the
resulting yellowish-white solid was extensively washed with
diethylether (2 ꢁ 2.0 ml). Yield 92.1% (0.0230 g). 1H-NMR
(CD3CN): d = 7.68 (s, 2H, Him), 7.34 (s, 2H, Him), 6.93
(s, 4H, Haryl), 2.49 (s, 6H, CH3para), 1.76 (s, 12H, CHo3rtho), 1.41
19.18 (s, CHo3rtho). ESI-MS (m/z): 889.35 [60% (M-OTf-2Br)+],
1049.16 [100% (M-OTf)+]. Elemental analysis: calcd (%) for
C
40H62N4P2O2Br2Cl4Au2 (MM 1387.99): C 34.60, H 4.50, N
4.04; found: C, 34.54; H, 4.54; N, 3.97.
Synthesis of complex 10
0.0659 g (0.0536 mmol) of complex 6-AuCl4 and 0.0619 g
(0.2251 mmol) of PhICl2 were placed in a round-bottomed flask
under Ar. 5.0 ml of dry acetonitrile was then added and the
resulting solution was stirred at room temperature overnight.
All volatiles were subsequently evaporated under vacuum and
the resulting yellowish-white solid was extensively washed with
diethylether (3 ꢁ 3.0 ml). Yield 52.6% (0.0183 g). 1H-NMR
(d, JCP = 15.49 Hz, 36H, CH3tBu). 31P-NMR (CD3CN): d = 67.70.
3
13C-NMR (CD3CN): d = 158.38 (m, Ccarb), 140.63 (s, Caryl),
136.13 (s, Caryl), 134.56 (s, Caryl), 130.56 (s, CHaryl), 128.30
(d, J = 2.78 Hz, CHim), 124.75 (d, J = 3.54 Hz, CHim), 39.34
1
(d, JCP = 62.68 Hz, CtBu), 27.12 (s, CH3tBu), 21.33 (s, CHp3ara),
(CD3CN): d = 7.83 (s, 1H, Him), 7.64 (s, 1H, Him), 7.11 (s, 2H,
18.53 (s, CHo3rtho). ESI-MS (m/z): 889.33 [15% (M-OTf-2Cl)+],
3
Haryl), 2.37 (s, 3H, CH3para), 2.15 (s, 6H, CHo3rtho), 1.43 (d, JHP
=
959.28 [100% (M-OTf)+]. Elemental analysis: calcd (%) for
15.87 Hz, 18H, CH3tBu). 31P-NMR (CD3CN): d = 73.00. 13C-NMR
(CD3CN): d = 142.11 (s, Caryl), 136.82 (s, Caryl), 130.46 (s, Caryl),
128.77 (m, Cim), 125.85 (m, Cim), 39.33 (d, 1JCP = 61.52 Hz, CtBu),
27.11 (s, CH3tBu), 21.16 (s, CHp3ara), 18.88 (s, CH3ortho). 13C-NMR
(DMSO-d6): d = 140.37 (s, Caryl), 135.32 (s, Caryl), 135.07 (s, Caryl),
C
41H62N4P2O5SF3Cl2Au (MM 1109.26): C 44.37, H 5.63, N 5.05,
S 2.89; found: C 44.21, H 5.83, N 4.98, S 2.69.
Synthesis of complex 9-OTf
0.1029 g (0.0990 mmol) of complex 6-OTf was dissolved in 129.40 (s, Caryl), 128.49 (m, Cim), 125.49 (m, Cim), 37.63 (d, CtBu),
a round-bottomed flask under Ar into 3.0 ml of dry CH3CN. 26.28 (s, CH3tBu), 20.64 (s, CH3para), 18.13 (s, CH3ortho). ESI-MS
0.980 ml (0.099 mmol) of a 0.101 M solution of Br2 in dry (m/z): 670.89 [60% (M + Na)+], 1322.73 [100% (M2 + Na)+].
CH3CN was then added, and the resulting solution was stirred Elemental analysis: calcd (%) for C20H31N2POCl3Au (MM
at room temperature for 3.0 h, during which time a white solid 649.54): C 36.97, H 4.81, N 4.31; found: C 36.88, H, 4.70, N 4.48.
precipitated out of the solution. The solution was concentrated
Crystallography
under vacuum to about one fifth of its original volume. The
solid product was filtered off and washed with Et2O (2 ꢁ The crystallographic data for complexes 5, 6-Cl and 7 were
2.0 ml). Yield 70.1% (0.0832 g). 1H-NMR (CD3CN): d = 7.63 obtained by mounting a single crystal on a glass fiber and
(m, 2H, Him), 7.45 (s, 2H, Him), 6.96 (s, 4H, Haryl), 2.29 (s, 6H, transferring it to an APEX II Bruker CCD diffractometer. The
3
CHp3ara), 2.16 (s, 12H, CHo3rtho), 1.26 (d, JCP = 15.66 Hz, 36H, APEX 3 program package36 was used to obtain the unit-cell
CH3tBu). 31P-NMR (CD3CN): d = 68.56. 13C-NMR (DMSO-d6): d = geometrical parameters and for the data collection (30 s/frame
154.18 (d, 2JCP = 6.32 Hz, Ccarb), 138.60 (s, Caryl), 135.29 (s, Caryl), scan time for a sphere of diffraction data). The raw frame data
This journal is ©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2019 New J. Chem., 2019, 43, 17275--17283 | 17281

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were processed using SAINT37 and SADABS37 to obtain the Conclusions
data file of the reflections. The structures were solved using
SHELXT38 (Intrinsic Phasing method in the APEX 3 program).
The refinement of the structures (based on F2 by full-
matrix least-squares techniques) was carried out using the
SHELXTL-2014/7 program38 in the WinGX suite v.2014.1.39
The hydrogen atoms were introduced in the refinement in
defined geometry and refined ‘‘riding’’ on the corresponding
carbon atoms.
In this contribution, we have examined the coordination chemistry
of N-phosphine oxide-substituted imidazolylidenes (POxIms) as
heteroditopic ligands for gold centres. We have shown that it is
possible to obtain both bis-carbene and mono-carbene gold(I)
complexes, depending on the steric bulk of the employed POxIm
ligand. From these complexes, gold(III) complexes can be prepared
upon gold(I) oxidation with halogens or halogen synthons. Again,
both bis-carbene and mono-carbene gold(III) complexes are acces-
sible, depending on the nature of the starting gold(I) compound
and on the oxidation conditions. The complexes were found to
undergo ligand exchange relatively easily and to generally present
the phosphanyl oxide group uncoordinated to the gold centre,
though weak electrostatic interactions between the metal and
the phosphanyl oxygen seem to be present in most cases. A
preliminary screening of the complexes as precatalysts in the
intermolecular hydroamination of phenylacetylene with mesityl-
amine has showcased the good catalytic performance of the
mono-carbene gold(I) and gold(III) complexes, which is compar-
able to the benchmark IPrAuCl. However, the stability of these
catalytic systems seems to be an issue, so that steps will be taken
to increase the robustness of these complexes and to exploit the
opportunities provided by the pendant phosphanyl oxide group
in a cooperative catalytic event with the gold centre. Efforts in
these directions are currently underway.
Data for compound 9-OTf were collected using an Oxford
Diffraction Gemini E diffractometer, equipped with a 2 K ꢁ 2 K
EOS CCD area detector and sealed-tube Enhance (Mo) and (Cu)
X-ray sources. Single crystals of the compounds were fastened
on the top of a Lindemann glass capillary. Data were collected by
means of the o-scans technique using graphite-monochromated
radiation. Detector distance was set at 45 mm. The diffraction
intensities were corrected for Lorentz/polarization effects as well
as with respect to absorption. Empirical multi-scan absorption
corrections using equivalent reflections were performed with the
scaling algorithm SCALE3 ABSPACK. Data reduction, finalization
and cell refinement were carried out through the CrysAlisPro
software. Accurate unit cell parameters were obtained by
least squares refinement of the angular settings of strongest
reflections, chosen from the whole experiment.
Data for the crystals derived from complex 9-AuCl4 (Fig. 4)
were collected using a single crystal X-ray diffractometer equipped
with a CMOS detector (APEX III, kCMOS), a TXS rotating anode
with Mo Ka radiation (l = 0.71073 Å), and a Helios optic using the
APEX III software package.36 The crystals were fixed on the top of a
kapton microsampler with perfluorinated ether, transferred to the
diffractometer, and frozen under a stream of cold nitrogen.
A matrix scan was used to determine the initial lattice
parameters. Reflections were merged and corrected for Lorentz
and polarization effects, scan speed, and background using
SAINT.37 Absorption corrections, including odd and even order
spherical harmonics, were performed using SADABS.37 The
structures were solved with Olex240 by using ShelXT38 structure
solution program by Intrinsic Phasing and refined with the
ShelXL41 refinement package using least-squares minimization.
In the last cycles of refinement, non-hydrogen atoms were
refined anisotropically. Hydrogen atoms were included in
calculated positions, and a riding model was used for their
refinement.†
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
The University of Padova is gratefully acknowledged for the
financial support (P-DiSC BIRD 2017). MB thanks the DAAD
(Short-Term Grants, 2018 (57378443)) for a scholarship.
Notes and references
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