Chemistry of gold(III) with pyridine-carboxamide ligands

Article abstract of DOI:10.1016/j.poly.2013.11.022

The pyridine-carboxamide ligand PhNHC(O)2-C₅H₄N reacts with Na⁺[AuCl₄]^- either by cation exchange, to give [PhNHC(O)2-C₅H₄NH][AuCl₄], or by ligand substitution to give

Full text of DOI:10.1016/j.poly.2013.11.022

Polyhedron 69 (2014) 61–67
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Chemistry of gold(III) with pyridine-carboxamide ligands
⇑
Nasser Nasser, Richard J. Puddephatt
Department of Chemistry, University of Western Ontario, London N6A 5B7, Canada
a r t i c l e i n f o
a b s t r a c t
+
ꢀ
Article history:
5 4 4
The pyridine-carboxamide ligand PhNHC(@O)A2-C H N reacts with Na [AuCl ] either by cation
Received 2 October 2013
Accepted 22 November 2013
Available online 1 December 2013
exchange, to give [PhNHC(@O)A2-C
5
H
4
NH][AuCl
4 2
], or by ligand substitution to give [AuCl
N)]. Similar reactions with bis(pyridine-carboxamide) ligands gave
2
0
(
j
-N,N -PhNC(@O)A2-C
5
H
4
2
0
complexes [AuCl
@O)A2-C
be isolated. The complex [AuCl
lyst for oxidative cyanation of PhNMe
gold(I), gold(II) and gold(III) intermediates.
2
(
j
-N,N -RNC(@O)A2-C
5
4
H N)], with R = (CH
2
)
3
NHC(@O)A2-C
5
H
4
N
or 2-C
6
H
4
NHC
(
5 4
H N, in which the ligands are bidentate, but no complexes with tetradentate ligands could
Keywords:
Gold
Pyridine-carboxamide
Amide
2
0
2
(j
-N,N -PhNC(@O)A2-C
5 4
H N)] in methanol solution is an efficient cata-
CN, and it is proposed that the catalysis involves
2
to give PhMeNCH
2
Ó 2013 Elsevier Ltd. All rights reserved.
Catalysis
1
. Introduction
[4], it was of interest to study their chemistry with gold(I) and
gold(III) and to test the catalytic activity of the complexes formed.
The results are reported below.
Transition metal complexes with pyridine-carboxamide ligands
have been shown to have useful properties in catalysis or molecu-
lar materials, and could be useful as pharmaceuticals or photonic
materials [1–3]. In catalysis, the ligands are particularly useful in
oxidation reactions since they are not themselves easily oxidized,
and also in biological systems since the carboxamide groups are
compatible with proteins [4–6]. Gold complexes are also useful
in catalysis, biology and molecular materials [7–10], yet there are
relatively few reports of complexes of gold with pyridine-carbox-
amide or related ligands [3,11–16]. Some of these are shown in
Chart 1. Coordination occurs with deprotonation of the amide
group and it has been suggested that this leads to enhanced stabil-
ity of the chelate complex [17,18]. The effect has been used to
selectively adsorb gold(III) on protein materials [19]. Simple 2-pyr-
idyl-carboxamide ligands give neutral complexes such as A or B
2
. Experimental
2.1. Materials and methods
All reactions were carried out in inert atmosphere of dry nitro-
gen using standard Schlenk techniques, unless otherwise specified.
All solvents used for air and moisture sensitive materials were
purified using an innovative Technology Inc. PURE SOLV solvent
purification system (SPS). NMR spectra were recorded at ambient
temperature, unless otherwise noted (ca. 25 °C), on Varian Mercury
1
4
00 or Varian Inova 400 or 600 spectrometers. H chemical shifts
1
3 4
are reported relative to TMS ( H), 85% H PO . Mass spectrometric
analysis was carried out using an electrospray PE-Sciex Mass Spec-
trometer (ESI-MS) coupled with TOF detector. For X-ray structure
determination, a suitable crystal of each compound was coated
in Paratone oil and mounted on a glass fiber loop. X-ray data were
collected at 150 K with
Apex II diffractometer with graphite-monochromated Mo K
(Chart 1), while bridged bis(2-pyridyl-carboxamide) ligands can
act as tetradentate ligands, as in C, or as bidentate ligands, as in
D [11–15].
In earlier papers, we have been interested in developing the
chemistry of gold with bis(phosphine-carboxamide) ligands [20–
x
and
u
scans by using a Bruker Smart
radi-
a
2
2]. These ligands favor the oxidation state gold(I) and the amide
ation (k = 0.71073 Å) and Bruker SMART software [27]. Unit cell
parameters were calculated and refined from the full data set. Cell
refinement and data reduction were performed using the Bruker
APEX2 and SAINT programs respectively [28]. Reflections were scaled
and corrected for absorption effects using SADABS [29]. All structures
were solved by either Patterson or direct methods with SHELXS and
refined by full-matrix least-square techniques against F2 using
SHELXL [30]. All non-hydrogen atoms were refined anistropically.
The hydrogen atoms were placed in calculated positions and
refined using the riding model. Crystal data are summarized in
group usually does not coordinate but may take part in hydrogen
bonding. In attempts to use the gold(I) complexes in oxidation
catalysis, the phosphine donors were often oxidized to phosphine
oxides, which are not good ligands for gold. Pyridine-carboxamide
ligands have been used in supramolecular chemistry with late
transition metals [2,23–26] and, since they are not easily oxidized
⇑
Corresponding author.
E-mail address: pudd@uwo.ca (R.J. Puddephatt).
0
277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2013.11.022

6
2
N. Nasser, R.J. Puddephatt / Polyhedron 69 (2014) 61–67
0
.16 mmol) and isolated as a red solid. Yield: 0.082 g (89%). NMR
Cl
Cl
N
1
in DMSO-d
6
at 400 MHz: d( H) = 10.48 [s, 1H, NH], 9.46 [d, 1H,
R
N
N
3
6
3
3
3
J
HH = 7 Hz, H ], 8.62 [d, 1H, JHH = 7 Hz, H ], 8.41 [d, 1H, JHH = 7 -
Cl
Cl
O
6
0
3
3
0
O
Hz, H ], 8.31 [d, 1H, JHH = 7 Hz, H ], 8.1–8.2 [m, 4H], 7.1–7.6 [m,
4H]. Anal. Calc. for C18 13AuCl : C, 36.94; H, 2.24; N, 9.57.
Found: C, 36.46; H, 1.80; N, 9.23%. Single crystals were grown by
slow diffusion of n-pentane into a methanol solution of the
compound.
NH
Au
N
O
H
2 4 2
N O
N
O
Au+
N
N
A, R = H
B, R = CH Ph
N
N
Cl
Cl
O
2
C
Au
2.2.3. [AuCl
This was prepared in a similar way from CH
N) (0.050 g, 0.18 mmol) and NaAuCl
ꢁ2H
2
{CH
2
(CH
2
NCO–2-C
5
H
4
N)(CH
2
NHCO–2-C
5
H
4
N)]
(CH
2
NHCO–2-
(0.070 g,
D
2
C
H
5 4
2
4
2
O
Chart 1. 2-Pyridine-carboxamide complexes of gold(III).
0.18 mmol) and isolated as a yellow solid. Yield: 0.090 g (93%).
1
6
NMR in DMSO-d
1
H , H ], 7.56–7.60 [m, 2H, H , H ], 3.55 [m, 2H, CH
2
C, 32.69; H, 2.74; N, 10.16. Found: C, 32.27; H, 2.71; N, 9.75%. Sin-
gle crystals were grown by slow diffusion of n-pentane into a
dichloromethane solution of the compound.
6
at 400 MHz: d( H) = 9.28 [d, 1H, H ], 8.82 [t,
0
6
5
3
H, NH], 8.62 [d, 1H, H ], 8.50 [t, 1H, H ], 7.95–8.04 [m, 3H, H ,
0
0
0
5
Table 1 and in the CIF files (CCDC 964344–964346). DFT calcula-
tions were carried out by using the Amsterdam Density Functional
program based on the BLYP functional, with double-zeta basis set
and first-order scalar relativistic corrections [31]. The ligands 1,2-
H N) and CH (CH NHCO–2-C H N) were pre-
5 4 2 2 2 5 4 2
pared by the literature method [32,33].
4
3
4
2
N], 3.36 [m,
H, CH N], 1.84 [quin, 2H, CH ]. Anal. Calc. for C15H15AuCl N O :
2
2
2 4 2
C H
6 4
(NHCO–2-C
2
.3. Catalysis
2
2
.2. Synthesis of new complexes
To a stirred solution of N,N-dimethylaniline (0.96 g, 7.9 mmol)
.2.1. [AuCl
To a solution of PhNHC(@O)A2-C
THF (5 mL) was added solution of NaAuCl
.25 mmol) in THF (5 mL). Excess Na CO (0.3 g) was added to
2 5 4
(2-C H NAC(@O)NPh]
in methanol (15 mL) was added [AuCl
2
(2-C
5 4
H NAC(@O)NPh]
5
H
4
N (0.05 g, 0.25 mmol) in
ꢁ2H (0.10 g,
(
0.18 g, 0.39 mmol), followed by t-BuOOH (9.5 mmol, as a 5 M
a
4
2
O
3
solution in decane) and Me SiCN (1.97 mL, 15.8 mmol). The reac-
tion was allowed to stir until all N,N-dimethylaniline was con-
sumed (5 h, monitored by TLC), then quenched by addition of
0
2
3
the reaction mixture, which was allowed to stir for 12 h, then fil-
tered to remove insoluble material. The solvent was evaporated
under vacuum, and the red solid product was washed with n-pen-
tane and diethyl ether and dried under vacuum. Yield: 0.09 g (78%).
excess aqueous NaHCO
acetate, the combined organic layer was washed with brine, dried
over anhydrous MgSO , and the solvent was removed under re-
duced pressure to give the product PhMeNCH CN (1.1 g, 95%).
: d( H) = 7.35 [m, 2H, Ph-o]; 6.98 [t, 1H, Ph-p]; 6.91
m, 2H, Ph-m]; 4.14 [s, 2H, CH CN]; 2.99 [s, 3H, CH ]. ESI-MS: m/
z = 146.
In attempts to identify intermediates, the reactions in CDCl
3
. The mixture was extracted with ethyl
1
3
6
4
6
NMR in DMSO-d at 400 MHz: d( H) = 9.37 [d, 1H, JHH = 7 Hz, H ],
3
5
3
4
2
8
[
6
.56 [t, 1H,
m, 5H, Ph]. Anal. Calc. for C12
.02. Found: C, 30.54; H, 1.73; N, 5.68%. Single crystals were grown
J
HH = 7 Hz, H ], 8.05–8.13 [m, 2H, H , H ], 7.29–7.39
1
NMR in CDCl
[
3
H
9
AuCl O: C, 30.99; H, 1.95; N,
2 2
N
2
3
by the slow diffusion of pentane into a solution of the sample dis-
solved in a mixture of methanol, dichloromethane, chloroform and
acetone.
3
1
were monitored by H NMR spectroscopy. Reagents were added
in different sequences and NMR spectra were recorded at each
stage. Yields of volatile products in the final solutions were deter-
mined by GC–MS and less volatile products were identified by ESI-
In a similar synthesis, but with stoichiometric amount of Na2-
3
, recrystallization gave a mixture of red crystals of the above
CO
complex with pale yellow crystals, identified as [PhNHC(@O)A2-
C H NH] [AuCl ] by X-ray structure determination.
MS. In a typical experiment, to a solution of PhNMe
.2 mmol) in CDCl (0.2 mL) was added a solution of [AuCl
N–C(@O)NPh] (4.5 mg, 9.7 mol) in CDCl (0.1 mL). The solu-
2
(24 mg,
+
ꢀ
5 4 4
0
C
3
2
(2-
H
5 4
l
3
1
2
.2.2. [AuCl
This was prepared in a similar way from 1,2-C
N) (0.050 g, 0.16 mmol) and NaAuCl
ꢁ2H
2 6
{1,2-C H
4
(NCO–2-C
5
H
4
N)(NHCO–2-C
5 4
H N)}]
tion immediately became violet-blue in color. In the H NMR, the m
and p-phenyl resonances which overlap at d 6.77–6.81 in free
PhNMe gave broad resonances shifted to 6.85 [1H, m] and 6.94
2
6
H
4
(NHCO–2-
(0.063 g,
C
H
5 4
2
4
2
O
Table 1
Crystal and refinement data.
Complex
4
5
6
Formula
C
12
H
4
11AuCl N
2
O
C
12
H
9 2
AuCl N
2
O
15 2 4 2
C H15AuCl N O
Fw
537.99
monoclinic
465.08
monoclinic
P2 /c
551.18
monoclinic
C2/c
Crystal system
Space group
P2
1
/n
1
a
b
c
b
Z
V
8.8311(3)
7.5187(3)
23.3384(8)
100.880(2)
4
1521.78(10)
2.348
10.364
16.1441(17)
16.8323(17)
9.6879(9)
92.914(3)
8
2629.2(5)
2.350
11.584
29.0237(13)
7.9773(4)
18.0225(9)
126.3750(10)
8
3359.7(3)
2.179
D
l
calc
(mm 1
ꢀ
)
9.092
Reflection/restraint/parameter
, I > 2 (I)
wR (all data)
3358/0/181
0.0186
0.0382
6037/0/325
0.0423
0.0859
4167/0/217
0.0165
0.0397
R
1
r
2

N. Nasser, R.J. Puddephatt / Polyhedron 69 (2014) 61–67
63
[
2H, p]. Some free ligand PhNHC(@O)–2-C
After 1 h, a solution of Me SiCN (50
was added. Reaction occurred to give Me
5
H
4
N was also detected.
+
3
l
L, 0.4 mmol) in CDCl
3
(0.1 mL)
1
3
SiCl [d( H) = 0.4 ppm]
[AuCl4]-
1
O
NH
O
N
N
and (Me
The PhNMe
Si)
3 2
O [d( H) = 0.023] and more free ligand was formed.
[AuCl ]-
4
H
resonances returned to their normal positions. After
H+
2
H
1
h, t-BuOOH (0.24 mmol) in CDCl
changes were decay of resonances for Me
with an increase in resonances for Me
Me Si) O.
3
(0.1 mL) was added. The chief
SiCN and Me SiCl, along
SiOH [d( H) = 0.14] and
N
[AuCl
]-
3
3
4
1
B
3
1
+
-
4
-
-
BH
2Cl
(
3
2
O
N
N
Cl
3
. Results and discussion
5
Au
Cl
3.1. Synthesis and structures of gold(III) complexes
The reactions of the ligands 1–3, shown in Chart 2, with
Scheme 1. Synthesis of complexes 4 and 5 (B = base).
4
Na[AuCl ] were studied. In their deprotonated forms, ligand 1 is
expected to act as a bidentate anionic ligand (compare A, B in
Chart 1), while ligands 2 and 3 could act as tetradentate dianionic
ligands (compare C, Chart 1) or as bidentate anionic ligands (com-
pare D, Chart 1).
ꢀ
4
The ligand 1 did not substitute the chloride ligands of [AuCl ]
in neutral or acidic solution, but could react to give the pyridinium
salt [PhNHC(@O)–2-C NH][AuCl ], 4 (Scheme 1). However, a li-
gand substitution reaction did occur in the presence of base to give
H
5 4
4
2
0
the complex [AuCl
2 5 4
(j -N,N -PhNC(@O)–2-C H N)], 5, according to
Scheme 1. Several attempts to prepare the potential complex cat-
2
0
+
ion [Au(
5 4 2
j -N,N -PhNC(@O)–2-C H N) ] , using excess of the ligand
1
to replace the remaining chloride ligands in 5, were unsuccessful.
ꢀ
Complex 4 retained the yellow color of the [AuCl
4
]
ion, while 5
was red in color, so the successful substitution reaction to give 5
is readily observed.
The structure of complex 4 is shown in Fig. 1. There is hydrogen
bonding NHꢁ ꢁ ꢁO@C between pyridinium NH protons and carbonyl
groups of neighboring cations, and also hydrogen bonding between
ꢀ
the amide NH proton and a chloride ligand of the [AuCl
4
]
anion.
The heavy atoms in the cation are roughly coplanar, with the angle
between the amide group atoms [N(1)O(1)C(7)C(8)] and the pyri-
dyl and phenyl groups being 6.9° and 3.1° respectively, allowing
good p-conjugation.
The structure of complex 5 is shown in Fig. 2. There are two
independent, but similar, molecules in the unit cell. As in complex
Fig. 1. The structure of complex 4. Selected bond distances: Au(1)–Cl(1) 2.2810(8),
Au(1)–Cl(2) 2.2893(8), Au(1)–Cl(3) 2.2857(8), Au(1)–Cl(4) 2.2778(9), O(1)–C(7)
1.230(4), N(1)–C(7) 1.341(4) Å. H-bond distances: O(1)ꢁ ꢁ ꢁN(2A) 2.85, N(1)ꢁ ꢁ ꢁCl(3)
3.53 Å. Symmetry equivalent: A, ꢀx,ꢀy,ꢀz.
4, the amide group is close to planar [e.g. sums of the angles at C(1)
and N(1) are each 359.9°], and the gold(III) centers have square
planar stereochemistry. The 2-pyridyl group is roughly coplanar
with the amide group [twist is 5.7° and 11.9° for the Au(1) and
Au(2) molecules respectively], but the phenyl group is twisted
out of the amide plane [ twist is 65.4° and 71.6° for the Au(1)
and Au(2) molecules respectively]. This phenyl twist is necessary
to avoid steric interactions with the cis chloride ligand, but will re-
4
The ligands 2 and 3 reacted with Na[AuCl ] according to
Scheme 2. In each case, the product, 6 or 7, contained the ligand
in its bidentate form, with one pyridine-carboxamide arm of the li-
gand not coordinated to gold(III). Reactions were also carried out
4
using excess Na[AuCl ], in attempts to form a digold(III) complex
duce p-conjugation with the amide group.
with the ligand acting as a bis(bidentate), but these were not suc-
cessful. In addition, attempts to induce displacement of the
remaining chloride ligands in 6 or 7 to give a tetradentate complex
analous to C (Chart 1), by addition of silver trifluoroacetate, failed
to give pure products.
The Au(1) and Au(2) molecules of 5 stack independently
through weak intermolecular Auꢁ ꢁ ꢁCl and, for the Au(2) molecules,
Auꢁ ꢁ ꢁO secondary interactions (Fig. 3).
The structure of complex 6 is shown in Fig. 4. The coordination
geometry is similar to that in complex 5, but the supramolecular
structure is different. There are hydrogen bonds between NH
groups of the free carboxamides and carbonyl groups of coordi-
nated carboxamides to give dimers [R(2,2)(16) in graph theory,
N(4)ꢁ ꢁ ꢁO(1A) 2.90 Å].
O
NH HN
O
O
NH
O
NH HN
O
N
N
Above and below the gold(III) centers, the shortest intermolec-
ular contacts are Au(1)ꢁ ꢁ ꢁCl(1A) 3.63 Å and Au(1)ꢁ ꢁ ꢁAu(1B) 3.88 Å,
as shown in Fig. 5. In combination with the hydrogen bonding
N
N
N
1
2
3
(Fig. 4), these secondary interactions give rise to a supramolecular
Chart 2. Pyridine-carboxamide ligands.
network structure.

6
4
N. Nasser, R.J. Puddephatt / Polyhedron 69 (2014) 61–67
N
N
-
[
AuCl4]
B
O
O
NH
HN
O
+
-
-
BH
2Cl
-
NH
O
N
N
Cl
Cl
Au
N
2
6
O
N
O
N
H
]-
H
N
[AuCl
B
4
N
NH
O
N
Cl
Cl
O
Au
+
-
-
BH
2Cl
N
-
N
3
7
Scheme 2. Synthesis of complexes 6 and 7.
Fig. 2. The structures of the two independent molecules of complex 5. Selected
bond parameters: Au(1)–N(1) 2.018(7), Au(1)–N(2) 2.013(7), Au(1)–Cl(1) 2.288(3),
Au(1)–Cl(2) 2.272(2), Au(2)–N(3) 2.006(8), Au(2)–N(4) 2.022(8), Au(2)–Cl(3)
2
.265(3), Au(2)–Cl(4) 2.286(3), O(1)–C(1) 1.223(11), O(2)–C(19) 1.242(11), N(1)–
C(1) 1.351(11), N(3)–C(19) 1.322(12) Å; N(2)–Au(1)–N(1) 82.2(3)°, Cl(2)–Au(1)–
Cl(1) 89.79(10)°, N(3)–Au(2)–N(4) 81.4(3)°, Cl(3)–Au(2)–Cl(4) 89.65(10)°.
Fig. 4. The structure of complex 6, showing a pair of hydrogen bonded molecules.
Selected bond parameters: Au(1)–N(1) 2.003(2), Au(1)–N(3) 2.036(2), Au(1)–Cl(1)
2.2714(7), Au(1)–Cl(2) 2.2891(7), O(1)–C(1) 1.226(3), N(1)–C(1) 1.347(3), O(2)–
C(10) 1.232(3), N(4)–C(10) 1.334(3) Å. Symmetry equivalent: A, 1 ꢀ x,y,½ ꢀ z.
Fig. 3. Secondary bonding interactions in the packing of complex 5. Selected
distances: Au(1)ꢁ ꢁ ꢁCl(2B) 3.80, Au(2)ꢁ ꢁ ꢁCl(4A) 3.75, Au(2)ꢁ ꢁ ꢁO(2B) 3.49 Å. Symmetry
equivalents: A, x,½ ꢀ y,ꢀ½ + z; B, x,½ ꢀ y,½ + z.
t
PhNMe
PhMeNCH
The mechanism of the catalytic reaction using [AuCl
2
þ Me
3
SiCN þ BuOOH
t
!
2
CN þ Me
2
SiOH þ BuOH
ð1Þ
The crystals obtained for complex 7 were always twinned in
such a way that detwinning was only partially successful. The
structure shown in Fig. 7 is therefore poorly defined. The molecular
structure is similar to that for complex 6 (Fig. 4), but there is no
intermolecular hydrogen bonding in 7. As in complex 5 (Fig. 2),
2
(bipy)]Cl
was suggested to be as shown in Scheme 3 [34], and involves a
gold(III)–gold(V) cycle. The gold cation E is oxidized by BuOOH
t
to give an oxogold(V) intermediate F. This oxidizes PhNMe
2
to
] , which isomerizes and forms the
iminium complex G, which then reacts with cyanide to give the
product PhMeNCH CN.
A previous claim of an oxogold complex was made, but was
subsequently withdrawn [44], and theory suggests that M = O
bonding is likely to be weak for late transition metals [45]. Gold(III)
tends to form Au -O) linkages rather than terminal Au@O
+
the cation radical [PhNMe
2
6 4
the C H group is twisted out of the amide plane, by 78° in 7, to re-
lieve steric hindrance.
2
3.2. Catalysis
p-
The oxidative
a-cyanation of tertiary amines is a useful meth-
2
(l
2
odology for C–C coupling steps in organic synthesis [34–43]. A typ-
ical procedure is illustrated in Eq. (1) [34], and gives a 98% yield of
groups [46], while gold(V) oxides have not been characterized. In
addition, gold(V) is a difficult oxidation state to access under mild
conditions, so this proposed mechanism of Scheme 3 should be
reconsidered. DFT calculations, using the BLYP functional with
double zeta basis set and scalar relativistic correction [31], were
carried out on the proposed intermediate complex F. The axial
PhMeNCH
plex [AuCl
2
CN from PhNMe
2
when catalyzed by the gold(III) com-
2
(bipy)]Cl in methanol solvent. With other catalytic
systems, different sources of cyanide and different oxidants have
been used [35–43] (see Fig. 6).

N. Nasser, R.J. Puddephatt / Polyhedron 69 (2014) 61–67
65
Fig. 5. Part of the network structure of complex 6, formed by a combination of NHꢁ ꢁ ꢁO@C, Auꢁ ꢁ ꢁCl and Auꢁ ꢁ ꢁAu secondary interactions. Distances: N(4)ꢁ ꢁ ꢁO(1) 2.90,
Au(1)ꢁ ꢁ ꢁCl(1A) 3.63, Au(1)ꢁ ꢁ ꢁAu(1B) 3.88 Å.
Me SiOH +
3
N
N
Cl
Au+
Cl
PhMeN-CH CN
2
tBuOOH
tBuOH
Me SiCN
3
E
+
PhMe N CH2
Cl
N
N
Cl
Au+
Cl
N
Au
N
Cl
OH
O
G
F
PhNMe2
Scheme 3. Proposed mechanism of catalysis of
34].
2
a-cyanation of PhNMe from Ref.
[
trigonal bipyramidal stereochemistry (Fig. 7). The calculated Au–
O distances do not indicate multiple bonding (Fig. 7). However,
the calculation indicates that the formation of either structure of
Fig. 6. The structure of complex 7. Selected bond parameters: Au(1)–N(1) 2.00(3),
Au(1)–N(2) 2.00(3), Au(1)–Cl(1) 2.292(8), Au(1)–Cl(2) 2.255(9) Å.
t
Fig. 7 by oxidation with BuOOH (giving t-butanol as co-product)
ꢀ
1
is strongly unfavorable, by at least 90 kcal mol , so the mecha-
nism of Scheme 3 is very unlikely under the mild conditions of
the catalysis.
The cyanation catalysis was studied, using the conditions re-
ported previously [34], using the neutral complex 5 in place of
the reported [AuCl
reactivity was observed, giving PhMeNCH
on conversion of PhNMe , as analysed by GC–MS. The minor
product was PhMeNCH CH@O, formed in 2% yield. Under these
2
(bipy)]Cl catalyst in methanol solution. Similar
2
CN in 98% yield, based
2
2
conditions, no free ligand 1 was detected, indicating that the unit
Au(1-H) stays intact during the catalysis. The solution became
violet-blue in color on addition of PhNMe
UV–Vis spectrum contained a broad band centered at 590 nm. Oxi-
dation of PhNMe with copper(II) in acetonitrile is reported to give
the cation radical PhNMe
tion band at 470 nm [47,48], while other researchers have sug-
2
to complex 5, and the
2
+
2
, which is characterized by an absorp-
Fig. 7. The calculated lowest energy singlet and triplet state structures for the
+
+
hypothetical complex [Au(@O)Cl
2
(bipy)] .
2
gested that PhNMe is only formed as a transient intermediate
[
49]. The species that gives rise to the 590 nm band is probably a
+
radical derived from the transient PhNMe
2
but its identity is not
oxo stereochemistry
F is not favored and the more stable
clear.
structures are a triplet state with equatorial oxo group (better
considered as a gold(IV)-oxyl diradical) and a singlet state with
The catalysis is not efficient in chloroform solution [34], the
maximum yield of PhMeNCH CN being 20%, but it was possible
2

6
6
N. Nasser, R.J. Puddephatt / Polyhedron 69 (2014) 61–67
tBuOH
Me SiOH
but the evidence does not support a mechanism involving gold(V).
Gold(III)–gold(I) cycles are commonly invoked for other C–C cou-
pling reactions catalyzed by gold complexes [57,58].
Another possible mechanism might involve the intermediacy of
digold complexes. In such a mechanism, it could be possible to
avoid a paramagnetic gold intermediate J and to provide an easier
route to reductive elimination of the product, avoiding the strained
gold(I) intermediate L. Binuclear mechanisms are now well estab-
lished for palladium(II) and platinum(II) catalysts, for example
Me SiCN
3
3
HCl
N
N
Cl
Cl
H O
2
Au
H O
2
tBuOOH
Me SiCl
3
2
HCl
5
N
Cl
H
N
Au
Au
N
CN
N
L
PhNMe2
PhMeN-CH CN
2
.
+
PhNMe2
involving binuclear, diamagnetic Pd(III)
can involve amidate bridges [59–63]. The gold(III) catalyst 5 does
form -stacked complexes (Fig. 3), but we have seen no evidence
2
intermediates, and they
.-
N
CH
2
NMePh
N
N
Cl
p
Au
Au
J
for ligand bridged dimers analogous to the platinum blues and re-
lated compounds [63]. Nevertheless, it should be emphasized that
Scheme 4 is not the only possible mechanism of catalysis and that
a binuclear mechanism is not eliminated.
N
CN
CN
K
.
+
.
+
PhNMe2
HCl
PhMeHN CH2
Scheme 4. A possible mechanism for oxidative cyanation.
4
. Conclusions
to obtain insight into the mechanism by monitoring reactions in
CDCl by H NMR spectroscopy, by UV–Vis spectroscopy and by
3
ESI-MS. Scheme 4 shows a potential reaction mechanism. In indi-
The pyridine-carboxamide ligands, in deprotonated form, bind
1
to gold(III) as bidentate ligands, but the bis(pyridine-carboxamide)
ligands do not easily act as tetradentate ligands. In the typical com-
vidual reactions, complex 5 did not react with the peroxide
2
0
2 5 4
plex [AuCl (j -N,N -PhNC(@O)A2-C H N)], 5, the gold(III) and the
t
BuOOH but it did react with Me
3
SiCN and with PhNMe
2
.
amidopyridine groups are planar, but the phenyl substituent is
twisted out of the plane. The neutral complex 5 is an effective cat-
Complex 5 reacted with Me
change to give Me SiCl [identified by its H NMR spectrum, with
d(MeSi) = 0.4 ppm] and the complexes [Au(1-H)Cl(CN)], H, and
Au(1-H)(CN) ], identified by their ESI-MS with m/z = 455 and
46, respectively. Using excess Me SiCN, free ligand 1 was also
3
SiCN in CDCl
3
solution by anion ex-
1
3
alyst for oxidative cyanation of PhNMe
to the known cationic catalyst [AuCl
, with activity comparable
(bipy)] [34]. The detailed
2
+
2
[
2
mechanism of catalysis is not established, but a cycle involving
gold(III), gold(II) and gold(I) complexes is tentatively suggested.
The work adds significantly to the known structural and catalytic
properties of gold(III) pyridine and amido complexes [1–3,7,8,63–
4
3
formed, suggesting that cyanide is able to displace 1 from gold(III)
t
under these conditions. Addition of BuOOH to this solution led to
solvolysis of both Me
ment of the ligand 1 from gold(III), as monitored by H NMR.
Reaction of complex 5 in CDCl solution with excess PhNMe led
3 3
SiCN and Me SiCl, with complete displace-
6
7].
1
3
2
Acknowledgments
immediately to a violet-blue coloration, characterized in the
UV–Vis spectrum by a broad band centered at 620 nm. This is
tentatively attributed to the same cation radical as observed in
We thank the NSERC (Canada) for financial support.
methanol, with kmax at 590 nm. The most obvious feature of the
H NMR spectrum was that the phenyl resonances of PhNMe were
2
Appendix A. Supplementary data
1
shifted and broadened, attributed to easy electron transfer
between the cation radical and free PhNMe . When Me SiCN was
CCDC 964344–964346 contains the supplementary crystallo-
graphic data for complexes 4–6. These data can be obtained free
of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or
from the Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336 033; or e-mail:
deposit@ccdc.cam.ac.uk.
2
3
added to this solution, the blue color was quenched and the phenyl
1
resonances returned to their normal positions. The H NMR spectra
also indicated the formation of Me
3 3 2
SiOH and (Me Si) O as hydroly-
sis products. The amine PhNMe did not react with t-BuOOH so a
2
key role of the gold(III) complex is to oxidize the amine to its cation
radical, which appears to be a necessary intermediate.
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imental data. The gold(III) complex 5 is shown to react with Me3-
SiCN to give cyanogold(III) complexes, including complex H. It is
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