One-step formation of cyclometallated Au(III) N,S-heterocyclic carbene: Crystallographic analysis

Article abstract of DOI:10.1039/b909661b

Gold(I) N,S-heterocyclic carbene AuBr(NSHC) (NSHC = N-allylbenzothiazolin- 2-ylidene) (1) with an allyl pendant is oxidised by iodine to give [AuI ₂(NSHC)₂]⁺[I₃]^- (2) and a cyclometallated byproduct A

Full text of DOI:10.1039/b909661b

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www.rsc.org/dalton | Dalton Transactions
One-step formation of cyclometallated Au(III) N,S-heterocyclic carbene:
crystallographic analysis†
Xiaoyan Han, Lip Lin Koh, Zhiqiang Weng* and T. S. Andy Hor*
Received 15th May 2009, Accepted 13th July 2009
First published as an Advance Article on the web 30th July 2009
DOI: 10.1039/b909661b
Gold(I) N,S-heterocyclic carbene AuBr(NSHC) (NSHC = N-allylbenzothiazolin-2-ylidene) (1) with an
allyl pendant is oxidised by iodine to give [AuI2(NSHC)2]+[I3]- (2) and a cyclometallated byproduct
AuBr2(h-C6H4SCNCH2CHCH2Br-C,C¢) (3). The latter can be prepared directly from bromination of 1.
Similar reaction with the crotyl (but-2-en-1-yl) derivative AuBr(NSHC) (NSHC = N-crotyl-
=
benzothiazolin-2-ylidene) (4) gives an oxidative addition product AuBr3(C6H4SCNCH2CH CHCH3)
(5). X-Ray single-crystal crystallographic analysis of 3 reveals a 5-membered cyclometallated ring in a
sq planar metal that gives two types of AuIII–C bonds. Similar structural analysis has also been carried
out in 1, 2 and 5.
reacts with N-3-propenylbenzothiazolium bromide to yield
Introduction
AuBr(NSHC) (NSHC = N-allylbenzothiazolin-2-ylidene) (1).
The recent emergence of gold carbenes1 is one of the key highlights
of N-heterocyclic carbene (NHC) chemistry.2 This is traced to the
Oxidation of 1 by excess iodine in CH2Cl2 gives the ex-
pected Au(III) but complicated by ligand migration, bromide
structure and bonding curiosities of such s-dominant carbene
moiety3 on the electron-rich and soft late-metals that usually
require a balance of s and p ligands. Much of the interest is also
driven by the catalytic interest of gold.4 Recent developments also
suggested some unique catalytic5 and biomedical6 features of gold
carbenes. To date, Au(I) NHC complexes7 continue to emerge but
their Au(III) analogues are still uncommon. Oxidative addition of
Au(I) (d10) to Au(III) (d8) by halogen remains the preferred method
for the likes of AuCl3(NSHC)8 (N,S-heterocyclic carbene), which
was among the earliest Au(III) carbenes, AuBr3(NHC) complexes,
which were crystallographically established9, and, more recently,
cationic [AuI2(NHC)2]+10 and [Au2Br2(bis-NHC)2]2+.11 We have
been studying the use of NSHC as an alternative to the more
common NHC ligands in d8 complexes.12 In this paper, we present
an alternative methodology for Au(III) carbene preparation by
using an olefin-in-proximity provided by an allyl N-substituent
of the carbene to capture the halogen for concomitant olefin
bromination and gold oxidation. This generates in a single step
a cyclometallated Au(III) NSHC complex and exemplifies the use
of an allyl functionalised NSHC carbene ligand.13
dissociation and iodination to give the bis-carbene complex
[AuI2(NSHC)2]+I3 (2), the NHC form of the cation has been
-
reported earlier10 (Scheme 1). The formation of a triiodide salt
from addition of iodine to iodide has also been observed in
similar cationic Pd-NSHC carbene complexes.12a The byproduct
AuBr2(C6H4SCNCH2CHCH2Br) (3) was unexpected as there was
no external or additional source of bromine or bromide. It proba-
bly arises from oxidation of 1 by adventitious Br2 (generated from
bromide oxidation in 1). Complex 3 can be prepared in higher yield
directly from the oxidation of 1 with Br2 (Scheme 1). It probably
occurs via a pathway similar to the ionic mechanism known for
halogen addition to olefin (Scheme 2). Similar cyclometalated
reactions have been reported in the bromination of Au(I), but
not Pd(II), with olefinic tertiary phosphines.14 To the best of our
knowledge, this is the first cyclometalated product of Au(III)
carbene. The ESI-MS mass spectrometric data of 2 support a
dibiscarbene moiety ([AuI2(NSHC)2]+ m/z = 800.7).
Results and discussion
Attempts to prepare a stable Au(I) NSHC complex from
the reaction between AuCl(tht) (tht = tetrahydrothiophene)
and N-3-propenylbenzothiazolium bromide in the presence of
NaOtBu or KOtBu could not give any isolatable and iden-
tifiable product. However, metathesis reaction of AuCl(tht)
with LiN(SiMe3)2 gives [Au{N(SiMe3)2}(tht)], which further
Scheme 1 Synthesis of Au(III) NSHC complexes from halogenation
of 1.
Department of Chemistry, National University of Singapore, 3 Sci-
ence Drive 3, Kent Ridge, Singapore 117543. E-mail: chmwz@nus.edu.
sg, andyhor@nus.edu.sg; Fax: +65 68731324
† CCDC reference numbers 732077–732079 and 739702. For crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/b909661b
The crotyl (but-2-en-1-yl) derivative of 1, viz. AuBr(NSHC)
(NSHC = N-crotylbenzothiazolin-2-ylidene) (4) has been pre-
pared for comparison. Bromination of 4 does not lead to
7248 | Dalton Trans., 2009, 7248–7252
This journal is
The Royal Society of Chemistry 2009
©

Scheme 2 Proposal mechanism of cyclometallation in 3.
cyclometallation under similar conditions. An X-ray analysis of
the product reveals a straightforward oxidative addition at the
=
metal to give AuBr3(C6H4SCNCH2CH CHCH3) (5). Experi-
ments on related complexes with other alkenyl substituents are
ongoing.
Fig.
2
ORTEP diagram of complex 2 with 30% thermal ellip-
Complexes 1–3 and 5 have been isolated and characterised by
X-ray single-crystal diffraction analysis. Complex 1 (Fig. 1) is a
soids and labeling scheme; hydrogen atoms and solvent are omit-
◦
˚
ted for clarity. Selected bond lengths (A) and angles ( ): Au(1)–C(1)
2.04(1), Au(1)–C(11) 2.05(1), Au(1)–I(1) 2.614(1), Au(1)–I(2) 2.614(1),
C(9)–C(10) 1.33(3), C(19A)–C(20A) 1.32(1), C(1)–Au(1)–C(11) 176.4(5),
C(1)–Au(1)–I(2) 92.2(4), C(1)–Au(1)–I(1) 88.1(4), C(11)–Au(1)–I(2)
91.3(3), C(11)–Au(1)–I(1) 88.4(3), I(2)–Au(1)–I(1) 174.76(4), I(5)–I(4)–I(3)
177.90(5).
d10 complex with linear Au(I). The Au(1)–C(1) bond (1.989(7) A)
˚
is comparable to those in other AuBr(NHC) complexes such
9
˚
as (IPr)–AuBr (1.975(5) A). The allyl olefin group (C(9)–C(10)
˚
1.32(1) A), which maintains its p character, is twisted away from
the metal to avoid non-bonding interactions. Complexes 2, 3 and
5 (Fig. 2–4) are d8 Au(III) with a typical sq planar geometry.
Complex 2 has two mutually-trans carbenes with relatively long
˚
Au-bonds [Au(1)–C(1) 2.04(1) A] presumably weakened by the
high trans influence of the carbene ligand. Complex 3 is most
intriguing with two types of Au–C bonds, namely, the shorter
˚
˚
Au–C(carbene) (1.97(1) A) and longer Au–C(alkyl) (2.11(1) A),
within a cyclometallated 5-membered ring. Their difference in
trans influence leads to two trans Au–Br bonds of significantly
˚
different lengths (2.469(1) and 2.547(1) A). The shorter Au–Br
bond is trans to the carbene, suggesting that the alkyl CH exerts
a stronger trans influence than the NSHC carbene. The crotyl
substituent of 5 is twisted away from the metal such that there
is negligible interaction between the pendant olefin and the gold
center; this could offer a possible explanation on the absence of
proximity effect that could drive cyclometallation. Its Au–carbene
˚
bond (Au(1)–C(1) 2.02(1) A) is longer than that in 3, indicating
the strength inherent of a chelate in the latter. This weaker Au–C
bond in 5, together with the absence of cyclometallation, leads to
Fig.
3
ORTEP diagram of complex 3 with 30% thermal ellip-
soids and labeling scheme; hydrogen atoms and solvent are omit-
◦
˚
ted for clarity. Selected bond lengths (A) and angles ( ): Au(1)–C(1)
1.97(1), Au(1)–C(9) 2.11(1), Au(1)–Br(1) 2.547(1), Au(1)–Br(2) 2.469(1),
C(9)–C(10) 1.49(2), C(1)–Au(1)–C(9) 82.1(5), C(1)–Au(1)–Br(2) 174.7(3),
C(9)–Au(1)–Br(2) 92.6(4), C(1)–Au(1)–Br(1) 91.5(3), C(9)–Au(1)–Br(1)
173.4(4), Br(2)–Au(1)–Br(1) 93.79(4), C(10)–C(9)–C(8) 113.5(1).
an overall shortening, and presumably strengthening, of all Au–Br
bonds (Au(1)–Br(1) 2.420(1), Au(1)–Br(2) 2.431(1), Au(1)–Br(3)
˚
2.409(1) A) compared to those of 3 (Au(1)–Br(1) 2.547(1), Au(1)–
˚
Br(2) 2.469(1) A).
All these Au(III) complexes (2–3 and 5) are significantly more
stable than the corresponding Au(I) complexes (1 and 4) which
decompose slowly to colloidal gold even in a protected atmo-
sphere. There is no evidence of decomposition of 3 through the
known reductive elimination pathway. A preliminary study of the
catalytic activities of 1 and 3 towards hydration of phenylacetylene
suggested that the Au(III) complex is about two-fold more active
Fig. 1 ORTEP diagram of complex 1 with 30% thermal ellipsoids and
labeling scheme; hydrogen atoms are omitted for clarity. Selected bond
◦
˚
lengths (A) and angles ( ): Au(1)–C(1) 1.989(7), Au(1)–Br(1) 2.3790(8),
C(9)–C(10) 1.32(1), C(1)–Au(1)–Br(1) 177.1(2), C(10)–C(9)–C(8) 124.8(7).
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 7248–7252 | 7249
©

5.36–5.32 (m, 2H, CH2CHCH2). 13C NMR (125.77 MHz, THF-
d8), 142.4, 133.2, 131.0, 127.9, 126.2, 123.0, 118.9, 116.0, 57.4.
C(NCS) cannot be detected under this condition. Anal. Calcd for
1, C10H9AuBrNS: C, 26.57; H, 2.01; N, 3.10. Found: C, 26.78; H,
2.01; N, 3.11.
Preparation of [AuI2(NSHC)2]+I3 (2)
-
Iodine (50 mg, 0.2 mmol) was added to a suspension of
AuBr(NSHC) 1 (30 mg, 0.066 mmol) in CH2Cl2 (2 mL) and the
mixture stirred for 1 h at r.t. The solvent was removed by vacuum
and the residual iodine removed by washing with hexane (2 mL).
The residue was redissolved in CH2Cl2 (1 mL), the solution filtered
through Celite, and allowed to stand at low temperature (-30 ◦C)
to obtain crystals of 2 (red) and 3 (colorless). Yield of 2: 10.2 mg
(0.0086 mmol, 39%). 1H NMR (500 MHz, CD2Cl2): 8.02 (d, 2H,
J = 8.2 Hz, Ar-H), 7.90 (d, 2H, J = 8.8 Hz, Ar-H), 7.72–7.67 (m,
4H, Ar-H), 6.19 (br, 2H, CH), 5.54 (m, 4H, CH2), 5.42 (d, 4H,
Fig. 4 ORTEP diagram of complex 5 with 30% thermal ellipsoids and
labeling scheme; hydrogen atoms and solvent are omitted for clarity.
◦
˚
Selected bond lengths (A) and angles ( ): Au(1)–C(1) 2.02(1), Au(1)–Br(1)
2.420(1), Au(1)–Br(2) 2.431(1), Au(1)–Br(3) 2.409(1), C(9)–C(10) 1.33(2),
C(1)–Au(1)–Br(1) 87.9(3), C(1)–Au(1)–Br(2) 176.5(3), Br(1)–Au(1)–Br(2)
91.36(4), Br(1)–Au(1)–Br(3) 176.31(4), Br(2)–Au(1)–Br(3) 91.89(5),
N(1)–C(1)–S(1) 113.0(7).
J = 6.3 Hz, CH CH2). 13C NMR (125.77 MHz, CD2Cl2): 142.4,
=
135.1, 128.7, 127.8, 127.4, 123.0, 116.0, 58.0. C(NCS) cannot be
detected under this condition. Yield of 3: 2.1 mg (0.003 mmol,
15%). MS (ESI, positive mode) m/z (%): 800.7 (80) [M - I3]+.
Anal. Calcd for 2, C20H18AuI5N2S2: C, 20.32; H, 1.53; N, 2.37.
Found: C, 17.32; H, 1.36; N, 2.00. The elemental analysis remained
unsatisfactory despite repeated purification and analysis, possibly
than Au(I), perhaps reflecting the instability of the latter. Details
of these catalytic reactions will be reported in due course.
Conclusions
This study suggested that cyclometalated Au(III) heterocyclic car-
benes can be conveniently prepared, isolated and are significantly
more stable than their Au(I) heterocyclic carbene precursor. The
conversion is achieved by a single-step allyl-assisted oxidation by
halogen. The presence of two robust and trans directing Au–C
bonds could provide a ready source of cyclometallated [Au(NHC)]
moieties. Facile dissociation of the bromide(s) could also help to
generate the vacant site needed for catalytic initiation.
-
due to contamination by I5 .
Preparation of AuBr2(C6H4SCNCH2CHCH2Br) (3)
Bromine (20 mg, 0.12 mmol) was added to the suspension of
AuBr(NSHC) 1 (30 mg, 0.066 mmol) in THF (2 mL) and the
mixture stirred for 1 h at r.t. The residual bromine and THF
was removed under vacuum. The residual solid was redissolved in
THF (1 mL) and the solution filtered through Celite and kept at
low temperature (-30 ◦C) to obtain 3. Yield: 33 mg (0.054 mmol,
81%). 1H NMR (500 MHz, DMSO-d6): d 8.39 (d, 1H, J = 7.6 Hz,
Ar-H), 8.30 (d, 1H, J = 8.1 Hz, Ar-H), 7.84–7.76 (m, 2H, Ar-
H), 5.03 (m, 1H, CH2), 4.75 (m, 1H, CH2), 4.42–4.28 (m, 3H),
13C NMR (125.77 MHz, DMSO-d6): d 186.8 (NCS), 141.3, 133.4,
129.4, 127.9, 124.9, 118.3, 59.0, 57.9. Anal. Calcd for 3·0.5THF,
C12H13AuBr3O0.5NS: C, 22.24; H, 2.02; N, 2.16. Found: C, 22.79;
H, 2.15; N, 2.12.
Experimental
General procedures
All experiments were performed under a dry argon atmosphere
using a Labmate glove box or by Schlenk techniques. THF and
CH2Cl2 were distilled over benzophenone sodium-ketyl and CaH2
1
respectively and kept under Ar. H and 13C NMR spectra were
recorded on Bruker AMX 500 spectrometers. ESI mass spectra
were obtained using a Finnigan LCQ. Elemental analyses were
performed on a Perkin-Elmer PE 2400 elemental analyzer at the
Department of Chemistry, National University of Singapore.
Preparation of AuBr(NSHC) (NSHC =
N-crotylbenzothiazolin-2-ylidene) (4)
Compound 4 was prepared and purified using a method similar
to that of 1. Yield: 62%. 1H NMR (500 MHz, DMSO-d6): d 8.26
(d, 1H, J = 8.2 Hz, Ar-H), 8.17 (d, 1H, J = 8.2 Hz, Ar-H), 7.76–
7.68 (m, 2H, Ar-H), 5.88 (m, 1H, CH), 5.79 (m, 1H, CH), 5.40 (d,
2H, J = 5.6 Hz, CH2), 1.66 (d, 3H, J = 6.3 Hz, CH3). 13C NMR
(125.77 MHz, DMSO-d6), 202.1 (NCS), 142.4, 133.2, 132.0, 128.7,
127.0, 124.5, 124.0, 116.8, 57.1, 17.89. Anal. Calcd for 4·0.2THF,
C11.8H12.6AuBrO0.2NS: C, 29.49; H, 2.64; N, 2.91. Found: C, 29.66;
H, 2.41; N, 3.15.
Preparation of AuBr(NSHC) (NSHC =
N-allylbenzothiazolin-2-ylidene) (1)
A solution of LiN(SiMe3)2 (34 mg, 0.2 mmol) in THF (2 mL) was
added to a suspension of [AuCl(tht)] (64 mg, 0.2 mmol) in THF
(2 mL). The mixture was stirred for 5 min, after which a suspension
of N-allylbenzothiazolium bromide (45 mg, 0.18 mmol) in THF
(3 mL) was added and stirred for 2 h. The resultant mixture was
filtered through Celite. The solvent and HN(SiMe3)2 were removed
under vacuum. The residual solid was dissolved in THF (1 mL)
and left at low temperature (-30 ◦C) to afford white precipitate of
1. Yield: 45 mg (0.1 mmol, 50%). 1H NMR (500 MHz, THF-d8):
d 8.17 (m, 1H, Ar-H), 8.08 (d, 1H, J = 8.2 Hz, Ar-H), 7.75–7.67
(m, 2H, Ar-H), 6.22–6.16 (m, 1H, CH), 5.60–5.58 (m, 2H, CH2),
=
Preparation of AuBr3(C6H4SCNCH2CH CHCH3) (5)
Compound 5 was prepared from Br2 and 4 using a procedure
similar to that of 3. Yield: 65%. 1H NMR (500 MHz, DMSO-d6):
7250 | Dalton Trans., 2009, 7248–7252
This journal is
The Royal Society of Chemistry 2009
©

Table 1 Selected crystallographic data of complexes 1-3 and 5
1
2·CH2Cl2
3·CH2Cl2
5·0.25THF
Formula
fw
C10H9AuBrNS
452.12
Triclinic
P-1
7.9989(6)
8.0038(6)
9.5214(7)
92.4370(10)
94.8290(10)
112.4620(10)
559.48(7)
2
2.684
16.863
412
7173
2573
C21H20AuCl2I5N2S2
1266.88
Triclinic
P-1
11.1192(9)
11.3298(9)
13.0049(10)
86.981(2)
74.880(2)
82.729(2)
1568.6(2)
2
2.682
9.928
1140
11094
7120
5277
1.62–27.48
0.0349
317
C11H11AuBr3Cl2NS
696.86
Triclinic
P-1
7.8579(11)
10.8044(15)
11.3771(15)
64.654(2)
78.777(3)
87.187(3)
855.6(2)
2
2.705
16.022
636
5904
3901
3235
2.02–27.50
0.0373
191
C12H13AuBr3NO0.25
647.99
Triclinic
P-1
8.9850(16)
9.9425(18)
11.506(2)
109.135(3)
99.547(4)
101.778(4)
920.0(3)
2
2.339
14.612
592
6324
4184
S
crystal system
space group
˚
a/A
˚
b/A
˚
c/A
a/deg
b/deg
g /deg
3
˚
V/A
Z
Dc/g cm-3
m/mm-1
F(000)
No of reflections collected
No of unique reflections
No. of observed reflections
q range [deg]
R(int)
2374
3072
2.15–27.49
0.0391
127
2.26–27.48
0.0220
200
Parameters
T/K
223(2)
293(2)
0.0678
0.1788
0.0899
0.1934
1.043
4.543
-2.879
293(2)
0.0674
0.1667
0.0794
0.1739
1.074
5.358
-3.762
295(2)
a
R1 [I > 2s(I)]
0.0331
0.0846
0.0363
0.0860
1.066
2.143
-1.192
0.0468
0.1319
0.0705
0.1566
1.038
1.805
-0.946
b
wR2 [I > 2s(I)]
a
R1 (all data)
b
wR2 (all data)
GOFc
-3
˚
Drmax/e A
Drmin/e A
-3
˚
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
1/2
a R1 = ꢀF0| - |Fcꢀ/ |F0|. b wR2 = {w (| F0| - |Fc|)2/ w|F0|2}
and p is the total number of parameters refined.
.
c GOF = { w(|F0| - |Fc|)2/(n - p)}1/2, where n is the number of reflections
d 8.40–8.34 (m, 2H, Ar-H), 7.85–7.78 (m, 2H, Ar-H), 5.19 (m, 1H,
CH), 5.06–4.92 (m, 2H, CH2), 4.48 (m, 1H, CHCH3), 1.87 (d, 3H,
J = 6.9 Hz, CH3), 13C NMR (125.77 MHz, DMSO-d6): d 187.9
(NCS), 141.0, 133.4, 129.5, 128.0, 125.1, 118.3, 64.8, 26.4. Anal.
Calcd for 5·0.6THF, C13.4H15.8AuBr3O0.6NS: C, 24.05; H, 2.38; N,
2.09. Found: C, 24.21; H, 2.24; N, 2.29.
Notes and references
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Chem., 2006, 691, 4350; (b) R. Tonner, G. Heydenrych and G. Frenking,
Chem.–Asian J., 2007, 2, 1555.
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N. Marion, L. Fensterbank, M. Malacria, S. P. Nolan and L. Cavallo,
Angew. Chem., Int. Ed., 2008, 47, 718.
The crystals were mounted on quartz fibers and X-ray data
collected on a Bruker AXS APEX diffractometer, equipped with a
CCD detector, using graphite-monochromated MoKa radiation
˚
(l = 0.71073 A). The data were corrected for Lorentz and polar-
5 (a) G. Seidel, R. Mynott and A. Fu¨rstner, Angew. Chem., Int. Ed., 2009,
48, 2510; (b) N. Marion and S. P. Nolan, Chem. Soc. Rev., 2008, 37,
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M. M. Shaikh and P. Ghosh, J. Organomet. Chem., 2007, 692, 4259;
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effects with SADABS. Structure solution and refinement were
carried out with the SHELXTL suite of programs.15 The structures
were solved by direct methods to locate the heavy atoms, followed
by difference maps for the light non-hydrogen atoms. Select crystal
data for complexes 1-3 and 5 are summarized in Table 1.
Acknowledgements
We are grateful to the Agency of Science, Technology & Research
(R143-000-364-305), Ministry of Education (R143-000-361-112)
and the National University of Singapore for financial support
and the reviewers for some useful comments. We thank G. K.
Tan and S. W. Chien for their assistantance in X-ray analysis and
monograph preparation respectively.
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7252 | Dalton Trans., 2009, 7248–7252
This journal is
The Royal Society of Chemistry 2009
©