Dinuclear gold(i) dithio- and diselenophosph(in)ate complexes forming mononuclear gold(iii) oxidative addition complexes and reversible chemical reductive elimination products

Article abstract of DOI:10.1039/c3dt52557k

Dinuclear gold(i) dithio- and diselenophosph(in)ate complexes were prepared to serve as precursors for subsequent oxidative addition (OA) chemistry following reaction with mild oxidant iodine, I₂. The new OA products circumvented the formation

Full text of DOI:10.1039/c3dt52557k

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Dinuclear gold(I) dithio- and diselenophosph(in)ate
complexes forming mononuclear gold(III) oxidative
addition complexes and reversible chemical
reductive elimination products†
Cite this: Dalton Trans., 2014, 43,
663
Yu-Chen Lee,a Yan-Ru Lin,a Bo-Yuan Liou,a Jian-Hong Liao,a Nina K. Gusarova,b
Boris A. Trofimov,b Werner E. van Zyl*c and C. W. Liu*a
Dinuclear gold(I) dithio- and diselenophosph(in)ate complexes were prepared to serve as precursors for
subsequent oxidative addition (OA) chemistry following reaction with mild oxidant iodine, I2. The new OA
products circumvented the formation of the expected dinuclear Au(II) complexes, but instead formed
novel chelating mononuclear square-planar gold(III) products of the type [AuI2{E2PR2}] (R = (CH2)2Ph; E =
S, 2; E = Se, 3) and [AuI2{Se2P(OR)2}] (R = Et, 4; iPr, 5) directly. We further demonstrate that this process is
chemically reversible as all the Au(III) complexes undergo chemical reductive elimination to the starting
dinuclear Au(I) complexes in the presence of SnI2 as determined by 119Sn and 31P NMR. The complexes
(2–5) were all prepared from the reaction between I2 and the corresponding dinuclear gold(I) precursor
Received 16th September 2013,
Accepted 25th September 2013
material. All new complexes were characterized by H, 31P, 77Se (for E = Se) NMR, infrared spectroscopy,
1
DOI: 10.1039/c3dt52557k
www.rsc.org/dalton
elemental analysis and single crystal X-ray diffraction.
Introduction
Oxidative addition (OA) is an important reaction class as it
allows the formation of new metal–ligand bonds by cleavage of
a bond in an organic or main-group reagent. The reagents can
typically vary from low to high polarity. Furthermore, OA reac-
tions lead to an increase of both the oxidation state and
coordination number of the metal centre(s) involved.1 As such,
homogeneous catalytic cycles frequently contain an OA (and
its reverse, reductive elimination) reaction, currently applied to
a number of large industrial processes.2 Homogeneous gold
catalysts in particular have witnessed tremendous develop-
ment during the past decade.3 In gold chemistry, OA reactions
of dinuclear gold(I) ylide complexes of the type [Au(CH2)2-
PPh2]2 (A) (Schmidbaur ylide) are the best known and by far
the most intensely studied, see Scheme 1.4
Scheme 1 Classic dinuclear gold(II) OA products with a formal Au(II)–
Au(II) bond obtained from small molecules adding across a variety of
gold(I) metallacycles. A = P, B = B’ = CH2 (A); A = P, B = S, B’ = CH2 (B); A
= CH, B = B’ = NR (C); X = Cl, Br, I, and other small molecules.
The OA event can be achieved if the gold centers are in
close proximity; hence dinuclearity is an essential consider-
ation to stabilize the d9 system with the formation of a formal
Au(II)–Au(II) bond upon oxidation.5 Hence, factors related to
the extent of the Au–Au bonding interaction, the electronic
structure of each metal atom, and the nature of the ligand
must be considered.6
Numerous reports have shown that small molecules such as
X2 (X = Cl, Br, I), CH3I, CH3NO2, SO2, SOCl2, H2S, (PhCOO)2
and S8 can successfully add across the two Au centres of the
metallacycle, forming dinuclear Au(II) complexes.4,5,7
aDepartment of Chemistry, National Dong Hwa University, Hualien, Taiwan 97401,
Republic of China. E-mail: chenwei@mail.ndhu.edu.tw; Fax: +886-3-8633570
bA. E. Favorsky Irkutsk Institute of Chemistry, Siberian Branch of the Russian
Academy of Sciences, Irkutsk 664033, Russian Federation
cSchool of Chemistry and Physics, University of KwaZulu-Natal, Westville Campus,
Private Bag X54001, Durban, 4000, South Africa. E-mail: vanzylw@ukzn.ac.za
†CCDC 876097–876099 for (3–5), 960263 for (1), and 960264 (2). For crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/c3dt52557k
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The development of the methylenethiophosphane ligand in
complexes of the type [Au(CH2)(S)PPh2]2 (B) shows the same
reactivity trends as (A).8 Fackler and co-workers also made
Experimental
Materials and measurements
inroads with the amidinate ligand in complexes of the type All chemicals were purchased from commercial sources and
[Au(NRCHNR)]2 (R = bulky) (C), now well established.9 These used as received. Solvents were purified following standard
systems all contain C–P–C (A), C–P–S (B) and N–C–N (C) brid- protocols.20 All of the reactions were performed in oven-dried
ging monoanionic ligands.
Schlenk glassware by using standard inert-atmosphere tech-
In continuing the studies of (A), M. Laguna and co-workers niques. The starting [AuCl(THT)]18a and dinuclear [AuSe2PR2]2
examined the reactions of [Au(Cl)(CH2)2PPh2]2 with various (R = OEt, OiPr, CH2CH2Ph) complexes were prepared according
alkynes in an attempt to form digold(II) acetylide complexes, to the reported methods.18b,c The ligand [NH4][S2P-
but instead formed the first dinuclear gold(III) alkyne (CH2CH2Ph)2] was prepared according to the literature meth-
complex.4e The study further demonstrated that although ods.18d The elemental analyses were done using a Perkin-
dinuclear gold(II) complexes can afford heterovalent gold(I)/ Elmer 2400 CHN analyzer. NMR spectra were recorded on a
gold(III) complexes by disproportionation,4f the inverse Bruker Advance DPX300 FT-NMR spectrometer, which operates
reaction, that is, formation of digold(II) complexes by at 300 MHz while recording 1H, 121.49 MHz while recording
comproportionation of gold(I)/gold(III) complexes, can also 31P, and 57.24 MHz while recording 77Se nuclei. The 31P{1H}
be achieved.
and 77Se{1H} NMR are referenced externally against 85%
The less studied bridging S–C–S case involves the [Au2{S2CC- H3PO4 (δ = 0 ppm) and PhSeSePh (δ = 463 ppm), respectively.
(CN)2}]2− complex which does add X2 (halide or other small The chemical shift (δ) and coupling constant (J) are reported
molecule) but importantly, the negative charge of the resulting in parts per million and hertz, respectively. The NMR spectra
Au(II) complex appears to greatly improve stability10 because were recorded at ambient temperature unless noted otherwise.
for the related S–C–S bridging neutral dithiocarbamate com- IR spectra were recorded on a JASCO FT-IR 401 spectrometer at
plexes of the type [AuS2CNR2]2, an OA reaction had been 25 °C using CsI plates. Melting points were measured using a
suggested, but was found to be too unstable to isolate at room Fargo MP-2D melting point apparatus.
temperature.11 Recently, the addition of the elusive F2 across
Safety note
type (C) had been achieved.12 In all the standard cases for A, B
and C outlined above, the important observation is that the Selenium and its derivatives are toxic! These materials should be
dinuclear unit remains intact and is wholly isolable and stable handled with caution.
after the OA event. Even in cases where isolated dinuclear
Synthesis
Au(II) complexes are subjected to further oxidation to Au(III)
species, the dinuclear unit still remains intact.5
[Au{S2P(CH2CH2Ph)2}]2, 1. A solution of NH4[S2P-
The present study focused on the E–P–E (E = S, Se) bridging (CH2CH2Ph)2] (0.201 g, 0.619 mmol) and [AuCl(tht)] (0.198 g,
ligands as a seemingly innocent extension of the above, but its 0.619 mmol) were mixed in 20 mL of THF. The solution was
OA reactions consistently revealed a complete departure from stirred at −50 °C for 1 h under nitrogen. The solvent was evap-
the above reported observations. Gold(I) analogues with brid- orated under vacuum to obtain [Au2{S2P(CH2CH2Ph)2}2] as a
ging S–P–S ligands have been reported extensively,13 and the yellow powder. The NH4Cl byproduct was removed by filtration,
reversible addition of Cl2 to the heteroleptic complex the product extracted into CH2Cl2 and the solvent subsequently
[Au2{(CH2)2PMe2}{S2PPh2}] has been demonstrated.14 The removed under reduced pressure. Yield: 0.284 g (91%). Anal.
complex [Au(S2P(OH)Ph)2]Cl, reported in 1988, was the first Calcd for C32H36Au2P2S4: C, 38.25; H, 3.61. Found: C, 38.11; H,
structurally characterized mononuclear gold(III) complex with a 3.34. mp: 155 °C (dec). 1H NMR (300 MHz, CDCl3, δ, ppm): 2.59
S–P–S chelating ligand;15 to date it has no Se counterpart, and (m, 8H, CH2Ph), 3.12 (m, 8H, PCH2), 7.28 (m, 20H, Ph). 31P
its mechanism of formation remains unclear. Changing to the NMR (121.49 MHz, CDCl3, δ, ppm): 77.7. Anal. Calcd for
heavier Se analogue, dinuclear gold(I) complexes with Se- C32H36Au2P2S4: C, 38.25; H, 3.61. Found: C, 38.11; H, 3.34.
donor ligands are studied considerably less.16 Although the
[AuI2{S2P(CH2CH2Ph)2}], 2. A mixture of [Au2{S2P-
Liu group has studied diselenophosphate complexes and clus- (CH2CH2Ph)}2] 1 (0.100 g, 0.099 mmol) and I2 (0.050 g,
ters for a number of years,17 the first dinuclear gold(I) com- 0.198 mmol) was stirred in 20 mL of THF at −50 °C under a
plexes with Se–P–Se bridging ligands, [Au{Se2P(OR)2}]2 (R = Et, nitrogen atmosphere for 1 h. The purple solution obtained
iPr), was described.18 Recently, one of us communicated an was filtered and dried under vacuum. The residue was washed
example of a structurally characterized gold(III) complex with a with de-ionized water and again dried under vacuum to obtain
S–P–S chelate as well as an oxidative bromination product [AuI2{S2P(CH2CH2Ph)2}] as a purple powder. Yield: 0.132 g
related to complexes 2–5 reported here, but for the corres- (88%). Anal. Calcd for C16H18AuI2PS2: C, 25.41; H, 2.40.
ponding dithiophosphonate ligand.19 The present study Found: C, 25.30; H, 2.23. mp: 133 °C (dec). 1H NMR (300 MHz,
cements the view with several examples that OA reactions for CDCl3, δ, ppm): 2.30 (m, 8H, CH2Ph), 3.07 (m, 8H, PCH2), 7.20
E–P–E bridging ligands across gold(I) centers do not lead to (m, 20H, Ph). 31P NMR (121.49 MHz, CDCl3, δ, ppm): 97.4.
readily isolable dinuclear gold(II) complexes, but instead show Anal. Calcd for C16H18AuI2PS2: C, 25.41; H, 2.40. Found: C,
a different and unexpected reactivity pathway.
25.30; H, 2.23.
664 | Dalton Trans., 2014, 43, 663–670
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Reductive elimination of [AuI2{S2P(CH2CH2Ph)2}],
2
by C4H10AuI2O2PSe2·4H2O: C, 5.99; H, 2.26. Found: C, 5.59; H,
1
SnI2. The reaction of [AuI2{S2P(CH2CH2Ph)2}] 2 (0.147 g, 2.04. mp: 185 °C. H NMR (300 MHz, CDCl3, δ, ppm): 1.51 (t,
0.194 mmol) and SnI2 (0.038 g, 0.101 mmol) was stirred in 3JHH = 7.1 Hz, 6H, OCH2CH3), 4.36 (m, 4H, OCH2CH3), 31P{1H}
20 mL of CHCl3 at −50 °C under a nitrogen atmosphere. The NMR (121.49 MHz, CDCl3, δ, ppm): 53.55 (JP–Se = 519.5 Hz);
purple solution rapidly turned red, and stirring of the resulting 77Se NMR (57.24 MHz, CDCl3, δ, ppm): 345.9 (d, JP–Se = 519.8
red solution was continued for 1 h. The reaction mixture was Hz). IR [CsI, ν(Au–Se)]: 222 cm−1
.
filtered and a dark-red precipitate was collected by filtration,
[AuI2{Se2P(OiPr)2}], 5. Compound 5 was synthesized in a
washed with water and dried under vacuum to obtain SnI4 similar fashion as 4 above by utilizing NH4[Se2P(OiPr)2]. Yield:
as a dark-red powder. Yield: 0.058 g (95%). 119Sn NMR 0.048 g (81.4%). Anal. Calcd for C6H14AuI2O2PSe2·0.3C4H8O: C,
1
(111.92 MHz, THF, δ, ppm): −1781.5. The filtrate, on the other 11.09; H, 2.12. Found: C, 11.21; H, 2.49. mp: 172 °C. H NMR
3
hand, was evaporated to dryness under a vacuum to obtain a (300 MHz, CDCl3, δ, ppm): 1.47 (d, JHH = 6.3 Hz, 12H, OCH-
dark-yellow solid. The solid was washed with deionized water, (CH3)2), 4.95 (m, 2H, OCH(CH3)2), 31P{1H} NMR (121.49 MHz,
then subjected to column chromatography (Al2O3) by using CDCl3, δ, ppm): 45.95 (JP–Se
=
511.4 Hz); 77Se NMR
THF as an eluent to afford yellow solution. This was dried (57.24 MHz, CDCl3, δ, ppm): 376.32 (d, JP–Se = 511.6 Hz) IR
under vacuum to obtain 1 as a yellow powder. Yield: 0.074 g [CsI, ν(Au–Se)]: 222 cm−1
(76%). 31P NMR (121.49 MHz, CDCl3, δ, ppm): 77.7.
.
X-ray structure determination
[AuI2{Se2P(CH2CH2Ph)2}], 3. Compound 3 was synthesized
in a fashion similar to 4 by utilizing K[Se2P{(CH2)2Ph}2]
(0.041 g, 0.09 3 mmol). Yield: 0.061 g (76.7%). Anal. Calcd for
C16H18AuI2PSe2·0.9C4H8O: C, 25.73; H, 2.78. Found: C, 26.04;
Crystals were mounted on glass fibers with epoxy resin, and all
geometric and intensity data were collected on a Bruker APEXII
CCD diffractometer using graphite monochromated MoKα radi-
ation (λ = 0.71073 Å). Data reduction was carried out using the
SAINT-Plus software.21 An empirical absorption correction was
applied using the SADABS program.22 Structures were solved by
direct methods and refined by full-matrix least-squares on F2
using the SHELXTL software package,23 incorporated in the
SHELXTL/PC version 5.10.24 Crystal data and selected bond
lengths and angles for 1–5 are summarized in Tables 1–4.
1
H, 2.36. mp: 138 °C. H NMR (300 MHz, CDCl3, δ, ppm): 2.62
(m, 4H, CH2P), 3.12 (m, 4H, CH2Ph), 7.28 (m, 10H, Ph),
31P{1H} NMR (121.49 MHz, CDCl3, δ, ppm): 42.9 (JP–Se
377 Hz). 77Se NMR (57.24 MHz, CDCl3, δ, ppm): 25.28 (d, JP–Se
=
=
377.3 Hz). IR [CsI, ν(Au–Se)]: 222 cm−1
.
[AuI2{Se2P(OEt)2}], 4. Stirring of NH4[Se2P(OEt)2] (0.028 g,
0.094 mmol) and [AuCl(tht)] (0.030 g, 0.094 mmol) in THF
(30 mL) at −50 °C under a nitrogen atmosphere for 3 h yielded
a yellow solution of [Au{Se2P(OEt)2}]2 and NH4Cl. Iodine, I2
(0.025 g, 0.098 mmol) was added and stirring continued for
30 min. After evaporation, the residue was extracted by CH2Cl2
(20 mL) and dried under vacuum to obtain a black purple
powder. The resulting black purple powder was recrystallized
from CHCl3 at −20 °C. Yield: 0.059 g (86.7%). Anal. Calcd for
Results and discussion
Synthesis
In this study, we focused on I2 because as an oxidant it seems
to reveal quite different reactivity trends in gold chemistry
Table 1 Crystallographic data for complexes 1–5
1
2
3
4
5
Formula
Mr
Space group
C
32H36Au2P2S4
C16H18AuI2PS2
756.16
C2/c
C16H18Au I2PSe2
849.96
P21/n
C4H10AuI2O2PSe2
729.78
P21/n
C12H28Au2I4O4P2Se4
1515.66
P1
1004.72
ˉ
ˉ
P1
a (Å)
b (Å)
c (Å)
α (°)
β (°)
γ (°)
9.6496(2)
12.8257(2)
14.1389(3)
79.90°
85.08°
74.07°
1655.28(6)
2
2.016
9.224
296(2)
13.013(3)
9.3266(19)
18.341(4)
90°
107.03(3)°
90°
2128.4(9)
4
2.360
9.4588(8)
11.8888(10)
18.7731(18)
90.00
102.103(4)
90.00
2064.2(3)
4
2.735
8.827(2)
8.851(3)
18.150(5)
90.00
101.497(7)
90.00
1389.4(7)
4
3.489
20.36
296(2)
8.7465(11)
8.7967(11)
21.903(3)
90.353(3)
100.611(3)
90.214(3)
1656.4(4)
2
V
Z
ρcalcd (g cm−3
)
3.039
17.08
296(2)
μ (mm−1
T (K)
)
10.079
296(2)
13.72
296(2)
Reflections collected
Independent reflections [Rint
Final R indices [I > 2σ(I)]a,b
R Indices (all data)
GooF
8034
7198
4143
2442
6381
]
6807 [Rint = 0.0336]
0.0243, 0.0514
0.0334, 0.0545
1.033
1879 [Rint = 0.0344]
0.0241. 0.0530
0.0316, 0.0574
1.141
3652 [Rint = 0.070]
0.0424, 0.1168
0.0489, 0.1207
1.150
1343 [Rint = 0.075]
0.0447 , 0.1033
0.1149, 0.1393
1.013
4654 [Rint = 0.039]
0.0519 , 0.1383
0.0768 , 0.1559
1.041
Largest diff. peak and hole (e Å−1
)
1.118 and −0.677
0.752 and −1.580
2.809 and −3.148
1.430 and −1.749
1.765 and −1.875
a R1 = ∑||Fo| − |Fc||/∑|Fo|. b wR2 = {∑[w(Fo2 − Fc2)2]/∑[w(Fo2)2]}1/2
.
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be isolated as a yellow powder. At room temperature, the Au(III)
complexes slowly decompose in solution: small amounts of
3–5 yield Au(0) metal and oxidized ligand (see below) before
OA takes place; this problem is circumvented by performing
the reaction at −50 °C.
Complexes 2–5 were all prepared in situ, once the dinuclear
gold(I) species formed in solution (tested by TLC), solid iodine
was added (molar ratio 1 : 1 with respect to ammonium salt)
and stirred for an additional 30 minutes. Complexes 1–5 were
all isolated in high >75% yield.
Table 2 Selected bond distances (Å) and angles (°) for 1 and 2 with
e.s.d.s. in parentheses
Complex 1
Au(1)–Au(1A)
Au(1)–S(1)
3.2668(2)
2.292(1)
174.41
P(1)–S(1)
Au(1)–Au(2)
2.023(1)
3.4558(2)
S(3)–Au(2)–S(4)
Complex 2
Au(1)–I(1)
Au(1)–S(1)
2.5997(7)
2.356(2)
100.8(1)
S(1)–P(1)
I(1)–Au(1)–S(1A)
2.051(2)
175.44(4)
S(1)–P(1)–S(1A)
NMR and IR spectroscopy
1
The complexes 1–5 were characterized by solution H and 31P
Table 3 Selected bond distances (Å) and angles (°) for complexes 3–4
with e.s.d.s. in parentheses
NMR. A key question is whether any sign of an OA Au(II)–Au(II)
complex could be observed. The OA reaction was monitored by
31P NMR at −50 °C where only the starting dinuclear gold(I)
precursor material and a Au(III) product was detected, see the
next section. Warming the solution up to room temperature,
another product was identified as the oxidized form of the
ligand which is a thermosensitive compound, see Scheme 2.
We propose that the oxidation is due to either the temperature
effect or the slight excess of I2 used. In our hands, no trace of
Au(II) species was ever detected by NMR, let alone isolated. It
should further be pointed out that the number of non-
stoichiometric Se linkages between the P atoms of the oxidized
ligand is dependent on the nature of the R group which was
independently identified, as follows: (RO)2P(Se)–Sen–(Se)P(OR)
Complex 3
Au–I
Au–Se
Se–Au–Se
2.6288(6)–2.6412(6)
2.4483(8)–2.4523(8)
84.99(3)
Se–P
I–Au–I
Se–P–Se
2.192(2)–2.201(2)
94.65(2)
97.82(8)
Complex 4
Au–I
Au–Se
2.6063(18)–2.6069(16)
2.449(2)–2.455(2)
86.04(7)
Se–P
I–Au–I
Se–P–Se
2.165(5)–2.169(6)
94.92(5)
101.0(2)
Se–Au–Se
Table 4 Selected bond distances (Å) and angles (°) for complexes 5 with
e.s.d.s. in parentheses
Au–I
Au–Se
Se–Au–Se
2.6017(13)–2.6097(14)
2.4326(18)–2.4604(18)
85.85(6)–85.91(5)
Se–P
I–Au–I
Se–P–Se
2.171(5)–2.181(4)
94.28(4)–94.75(5)
99.96(17)–100.06(16)
i
[n = 1 for R = CH2CH2Ph; n = 2 for R = Pr; n = 3 for R = Et].27
The dinuclear Au(I) complex 1 showed a singlet peak at
77.7 ppm, the remaining mononuclear Au(III) complexes
showed singlet peaks resonating at 97.4 (2), 42.9 (3), 53.55 (4)
and 45.95 (5) ppm, respectively. Complex 2 was clearly shifted
more downfield, presumably due to the dithio moiety vs. the
diseleno moieties on 3–5. The 77Se NMR for 3–5 showed a far
larger variation. The 77Se NMR showed doublet peaks due to
JP–Se coupling resonating at 25.28 ppm for the diseleno-
phosphinate 3, but significantly more downfield at 345.9 and
376.32 ppm for the diselenophosphates 4 and 5, respectively.
IR spectra of compounds 3–5 were recorded and displayed a
ν(Au–Se) stretching vibration peak at 222 cm−1 for both 4 and
5, respectively, a slight bathochromic shift, ∼20 cm−1, to their
precursors.18
when compared to Cl2 and Br2. This is demonstrated by the
following three examples: (i) the addition of I2 to iodogold(I)
compounds is more strongly ligand-dependent when com-
pared to Cl2 and Br2, suggesting that a crucial threshold in oxi-
dation potential is operative and that both electronic and
steric effects control the outcome of the reaction,25 (ii) elemen-
tal gold powder undergoes oxidative dissolution with thio-
amide–I2 adducts, where the S-donor to I2 interaction is
believed to play an important charge-separation role in
explaining the dissolution mechanism,26 (iii) the complex [Au-
(CH2)(S)PPh2]2 (B), mentioned above, shows interesting OA
properties with I2, forming both a homovalent Au(II)–Au(II) and
a heterovalent Au(I)–Au(III) dinuclear complex, isolated separ-
ately through a simple change in solvent polarity.8 These pro-
cesses are unique to the Au–I system.
A summary of the synthesis methodology used is outlined
in Scheme 2. Complex 1 serves as an example for the prepa-
ration of other precursors to obtain complexes 2–5 and was
obtained from the reaction between a solution of NH4[S2P-
(CH2CH2Ph)2] and [AuCl(tht)] (molar ratio 1 : 1) in 20 mL of
THF. The mixture was stirred at −50 °C for 1 h under nitrogen.
The solvent was evaporated under vacuum. The material was
extracted into dichloromethane and filtered over anhydrous
magnesium sulfate to remove NH4Cl. The solvent of the filtrate
was removed under reduced pressure, allowing complex 1 to
Proof of reversible chemical reductive elimination
The present study makes the substantial claim that the OA
Au(III) products (2–5) are chemically reversible through a reduc-
tive elimination reaction with SnI2 and we demonstrate that
the original Au(I) starting material not only reforms, but the
fate of the reductant has also been established. The reductive
elimination of halogens from a metal centre is a process of
interest and usually examined in the context of catalysis and
sustainable energy. RE has unfavourable thermodynamics,
and is typically driven by a trapping mechanism due to the fast
kinetics of the OA reaction of halogens. A number of Au(III)
thiolate complexes have been shown to undergo reversible
reductive elimination–oxidative addition processes in solution
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Scheme 2 Summary of synthesis methodology used and products formed in the present study.
via thermal and photochemical pathways.28 A thermally trig- yield of 76%, and 31P NMR at 77.7 ppm confirmed complex 1.
gered reductive elimination of bromine from Au(III) as a path Cases of RE reactions from Au–Se systems are rare. More than
to Au(I)-based coordination polymers was also achieved.29 40 years ago, van der Linden and Nijssen reported on a series
Thus, we reacted complex 2 with SnI2 (molar ratio 2 : 1) in of gold(III) diselenocarbamates of the type [Au{Se2CNEt2}2]Br
CH3Cl at −50 °C under a nitrogen atmosphere and monitored and [AuBr2{Se2CNEt2}].31 The former was demonstrated to
the progress of the reaction over one hour by 31P NMR, see undergo reversible one-electron reduction while the latter did
Fig. 1.
not isolate from the OA reaction of Au(I), unlike the case in the
The purple coloured solution turned red rapidly and present study.
yielded a precipitate. The reaction mixture was filtered and the
deep-red precipitate was removed by filtration and confirmed
X-ray crystal structures
to be SnI4 whose 119Sn NMR shift is at −1781.5 ppm,30 in a Complexes 1–5 were all characterized by single-crystal X-ray
yield of 95%.
crystallography. Complex 1 is the only dinuclear complex,
The SnI4 species confirms the fate of the SnI2 reductant as shown in Fig. 2. Complex 1 crystallized in the triclinic space
ˉ
the sole oxidized product. The filtrate was subsequently evap- group P1. Selected bond lengths and angles are listed in
orated to dryness under a vacuum to obtain a dark-yellow Table 2 and a perspective view of 1 is shown in Fig. 2. X-ray
solid. Following work-up, the yellow powder was isolated in a analysis reveals that complex 1 consists of the commonly
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Fig. 1 The reaction between 2 and SnI2 monitored by 31P NMR spec-
troscopy at 0, 5, 10, 15, 30 and 60 min increments. At T = 0 min
(bottom), only the Au(III) complex 2 is observed, whilst after 60 min (top),
only the precursor complex 1 is observed.
Fig. 3 Thermal ellipsoid drawing (30% probability) of 2. Hydrogen
atoms are omitted for clarity.
shown in Fig. 4. Complex 3 showed the Au–I and Se–P bond
lengths at 2.62 and 2.19 Å, respectively, while the Au–Se bond
length is at 2.43 Å. The bond angles for I–Au–I, Se–Au–Se, and
Se–P–Se are at 94.6, 84.9 and 97.8°, respectively. Complex 3
also revealed some intermolecular I⋯(Se and Se⋯(Se inter-
actions at 3.779 and 3.511 Å, respectively, to form a 1D chain
((Fig. 5). Surprisingly these weak interactions do not exist in
compounds 4 and 5.
Complex 4 crystallizes in the monoclinic system in the
space group P21/c, shown in Fig. 6. Complex 4 is clearly mono-
nuclear with a slightly distorted square-planar geometry about
the Au atom and has Au–I, Au–Se and Se–P bond lengths of
2.60, 2.44 and 2.16 Å, respectively. Significant bond angles
include I–Au–I, Se–Au–Se, and Se–P–Se at 94.9, 86.0 and
101.0°, respectively, in all respects slightly larger than for 3.
Complex 5 crystallizes in the triclinic system in space group
ˉ
P1, as shown in Fig. 7. Complex 5 is virtually identical to 4 in
terms of bond angles and lengths.
Fig. 2 Thermal ellipsoid drawing (30% probability) of 1. Hydrogen
atoms are omitted for clarity.
observed 8-membered gold(I) metallacycle in an elongated
chair conformation. The complex shows an intramolecular
Au⋯Au interaction at a distance of 3.2668(2) Å, as well as an
intermolecular Au⋯Au interaction at a distance of 3.4558(2) Å.
The gold centres are slightly distorted from linearity as shown
by the S–Au–S bond angle of 174.4°. Complex 2 crystallized in
the monoclinic space group C2/c. Selected bond lengths and
angles are listed in Table 2 and a perspective view of 2 is
shown in Fig. 3. X-ray analysis reveals that complex 2, similar
to 3–5, consists of a 4-coordinate square planar geometry
around the gold(III) center.
Complex 2 contains a chelating S–P–S ligand and two cis
iodo ligands. Complexes 3–5 are the seleno analogues of the
study and are structurally very similar. Complex 3 crystallized
in the monoclinic space group P21/n. Selected bond lengths
and angles are listed in Table 3 and a perspective view of 3 is
Fig. 4 Thermal ellipsoid drawing (30% probability) of 3. Hydrogen
atoms are omitted for clarity.
668 | Dalton Trans., 2014, 43, 663–670
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product. A novel series of complexes containing for the first
time both a Se–P–Se chelating and iodo ligands across a Au(III)
center are reported. The importance of these OA reactions is
that they departed entirely from any related dinuclear gold(I)
chemistry that has gone before through a simple change to
E–P–E bridging ligands.
This result strongly favours the idea that the OA event is
driven by the electronic properties of the ligand since there is no
Au–Au bond or interaction to support stability in the final
Au(III) product. The dinuclear units ruptured under mild con-
ditions into stable and well defined Au(III) complexes in high
yield. We further demonstrate that the process is reversible,
and chemical (as opposed to thermal or photochemical) reduc-
tive elimination could be achieved using the reductant SnI2.
We anticipate that more surprising results will continue in
this system until such a time that OA pathways across dinuc-
lear gold(I) centers can be given a sound and predictable
theoretical footing.
Fig. 5 Portion of crystal packing structure of complex 3 showing Se⋯
(Se and Se⋯(I interactions.
Acknowledgements
Financial supports from the National Science Council of
Taiwan (NSC 100-2923-M-259-001-MY3) are gratefully acknowl-
edged. We thank Dr I.-Y. Wu of Rezwave Technology Inc. for
the help in 119Sn NMR experiments.
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