Selective Au-C cleavage in (C∧N∧C)Au(III) aryl and alkyl pincer complexes

Article abstract of DOI:10.1021/om300666j

Treatment of gold(III) pincer complexes (C^∧N ^∧C)*AuX with trifluoroacetic acid (X = C₆F ₅, thiophenyl, Me, Et) or of (C^∧N^∧C) *AuOAc^F with AgOAc^F/arylboronic acids leads to the selective cleavage of a C-Au bond under mild conditions to give the bidentate complexes (HC-C^∧N)*Au(X)(OAc^F) [(C ^∧N^∧C)* = 2,6-(C₆H ₃Bu^t)₂pyridine]. Alkylation of (C ^∧N^∧C)*Au(OAc^F) with AlR ₃ (R = Me, Et) proved to be a high-yielding route to gold(III) alkyls. Au-C cleavage significantly influences reactivity, e.g., with boronic acids. The photoemission of the cleavage product (HC-C^∧N) *Au(C₆H₄F)(OAc^F) is about an order of magnitude more intense than that of its tridentate parent compound.

Full text of DOI:10.1021/om300666j

Communication
pubs.acs.org/Organometallics
Selective Au−C Cleavage in (C∧N∧C)Au(III) Aryl and Alkyl Pincer
Complexes
Dan A. Smith, Dragos-̧ Adrian Rosç a, and Manfred Bochmann*
Wolfson Materials and Catalysis Centre, School of Chemistry, University of East Anglia, Norwich, NR4 7TJ, U.K.
S
* Supporting Information
ABSTRACT: Treatment of gold(III) pincer complexes (C∧N∧C)*AuX with trifluoroacetic acid (X = C6F5, thiophenyl, Me, Et)
or of (C∧N∧C)*AuOAcF with AgOAcF/arylboronic acids leads to the selective cleavage of a C−Au bond under mild conditions
to give the bidentate complexes (HC-C∧N)*Au(X)(OAcF) [(C∧N∧C)* = 2,6-(C6H3But)2pyridine]. Alkylation of (C∧N∧C)*Au-
(OAcF) with AlR3 (R = Me, Et) proved to be a high-yielding route to gold(III) alkyls. Au−C cleavage significantly influences
reactivity, e.g., with boronic acids. The photoemission of the cleavage product (HC-C∧N)*Au(C6H4F)(OAcF) is about an order
of magnitude more intense than that of its tridentate parent compound.
old(III) complexes with bidentate and tridentate cyclo-
metalated phenylpyridine ligands of the types (C∧N)-
C∧N)*Au(C6H4F)(OAcF) (2), a product of Au−C bond
protolysis, which was obtained in 76% yield (Scheme 1). In
order to test whether Ag+ was necessary for this cleavage, the
aryls (C∧N∧C)*AuAr (3a, Ar = 2-thiophenyl; 3b, Ar = C6F5)
were treated with HOAcF in dichloromethane, which also gave
cleanly the Au−C cleaved products 4 and 5, respectively.7
Electrophilic attack occurs exclusively on the phenyl-C of the
pincer ligand, while the aryl or alkyl ligands trans to the
pyridine donor are not affected. There is no reaction with
weaker acids, such as CH3COOH.
As the crystal structure of 2 confirms, the reaction is
regioselective, leading exclusively to the isomer where the
C6H4F ligand is located trans to the pyridine donor. The crystal
structure of 1 was determined for comparison (Figure 1). In the
bidentate complex, both the Au−OAcF and the Au−N
distances are significantly elongated compared to 1, due to
the trans influence of the strong aryl σ-donor, suggesting a
loosening of the pyridine coordination. The Au−O distance of
2.130(5) Å in complex 2 suggests a weak bond to the anion
similar to that found in the triflate complex (tpy)Au(Me)(OTf)
(2.1382(17) Å) (tpy = p-tolylpyridine anion).8
G
AuX2 and (C∧N∧C)AuX have attracted much attention in
recent years, mainly because of their interesting photophysical
properties and antitumor activities.1−4 The pincer complexes
(C∧N∧C)AuX are very attractive since the rigid ligand
framework with trans-oriented Au−C bonds is particularly
structurally and chemically robust and provides excellent
stability toward reductive elimination, which is the prevalent
reaction path of Au(III) complexes with more labile ligands.5 It
now appears that these C∧N∧C pincer complexes may be more
reactive that previously thought, particularly in the presence of
silver salts, which are often components in gold catalyst
mixtures.
We have recently reported the synthesis of (C∧N∧C)*AuX
complexes with labile and/or basic ligands X = OAcF, OMe,
and OH (OAcF = CF3CO2), as well as the facile reaction of
(C∧N∧C)*AuOH with arylboronic acids to give gold aryls
(C∧N∧C)*AuAr. The latter show aryl-modulated photo-
emissions in solution [(C∧N∧C)* = 2,6-(C6H3But)2pyridine].6
We show here that one of the gold−phenyl bonds of the pincer
framework can undergo unexpectedly facile and selective
cleavage with AgOAcF or HOAcF to give bidentate (C∧N)-
gold(III) aryls or alkyls.
In contrast to the reaction of boronic acids with
(C∧N∧C)*AuOH, there was no reaction with p-fluorophenyl-
boronic acid when (C∧N∧C)*AuOAcF (1) was used as the
starting material. However, we found that in the presence of
AgOAcF 1 was readily converted to a new compound, (HC-
In an effort to attempt the synthesis of related gold(III) alkyl
complexes, the reaction of 1 with trialkylaluminum reagents
was explored, which generated the corresponding complexes
(C∧N∧C)*AuR (6a, R = Me; 6b, R = Et) in yields of 60% and
Received: July 16, 2012
Published: August 8, 2012
© 2012 American Chemical Society
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dx.doi.org/10.1021/om300666j | Organometallics 2012, 31, 5998−6000

Organometallics
Communication
Scheme 1
distance in 6b is elongated compared to the corresponding
Au(III) hydroxide6 and trifluoroacetate complexes. Treatment
of 6a,b with HOAcF afforded the expected complexes (HC-
C∧N)*AuR(OAcF) 7a,b in essentially quantitative yields
(Scheme 2). Heyn, Tilset, and co-workers recently synthesized
the related gold methyl compound (tpy)AuMe(Cl) by
alkylation of (tpy)AuCl2 with SnMe4; however, this reaction
was accompanied by extensive reduction to gold metal and gave
the Au(III) methyl complex in only 8% yield.8 By contrast,
reduction to gold metal was not in evidence during the
formation of 6a,b using aluminum alkyls.
Given that the gold(III) aryls 3 showed interesting
photoluminescence,6 the photoemission behavior of 2 and 4
was investigated for comparison. Surprisingly, bidentate 2
showed a strongly increased solution photoemission intensity
and quantum yield (ΦP = 1.0 × 10−1) compared to tridentate
(C∧N∧C)*AuC6H4F (ΦP = 1.3 × 10−2), while the emission
wavelengths of both compounds remain essentially identical
(Figure 3).9 By contrast, compound 1, which has the same
number of C, N, and O donors as 2 but in a different
configuration, shows no photoluminescence. Photoemission in
(C∧N∧C)*AuC6H4F is from a ligand-based triplet state and
shows vibrational fine structure. This is also observed for 2. By
contrast, the thiophenyl compound 4 luminesces much less
strongly than its (C∧N∧C)-ligated precursor 3a. The origin of
these variations is as yet unclear.
Figure 1. Molecular structures of 1 (left) and 2 (right). Ellipsoids are
drawn at 50% probability. Selected bond distances (Å) and angles
(deg): 1: Au−C(1) 2.060(4), Au−C(17) 2.069(4), Au−N 1.959(3),
Au−O(1) 2.019(3); N(1)−Au−O(1) 175.46(7). 2: Au−C(17)
2.017(7), Au−C(28) 2.029(7), Au−O(1) 2.130(5), Au−N(1)
2.164(6); N(1)−Au−C(28) 172.4(2).
40%, respectively, with some losses in the case 6b due to its
high solubility. The structure of 6b is shown in Figure 2. In line
with the trans influence of the alkyl σ-donor, the Au−N
In conclusion, we have shown that tridentate C∧N∧C gold
pincer complexes are readily converted into bidentate
analogues by Au−C bond cleavage with AgOAcF or simply
HOAcF. Such cleavage has a significant impact on reactivity, as
shown by the failure of (C∧N∧C)*AuOAcF to react with
boronic acids unless AgOAcF is present. Tridentate and
bidentate gold alkyl complexes are similarly accessible through
high-yielding alkylations with AlR3. In all cases the electrophilic
Figure 2. Molecular structure of (C∧N∧C)*AuEt (6b). Ellipsoids are
drawn at 50% probability. Selected bond distances (Å) and angles
(deg): Au−C(1) 2.042(8), Au−N(1) 2.051(7), Au−C(3) 2.092(8),
Au−C(19) 2.075(9); C(1)−Au−N(1) 179.2(3).
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Organometallics
Communication
Scheme 2
REFERENCES
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(1) For examples of (C∧N) complexes and cytotoxic activity see:
Marcon, G.; Carottu, S.; Coronnello, M.; Messori, L.; Mini, E.; Orioli,
P.; Mazzei, T.; Cinellu, M. A.; Minghetti, G. J. Med. Chem. 2002, 45,
1672. (b) Kilpin, K. J.; Henderson, W.; Nicholson, B. K. Polyhedron
2007, 26, 204. (c) Cocco, F.; Cinellu, M. A.; Minghetti, G.; Zucca, A.;
Stoccoro, S.; Maiore, L.; Manassero, M. Organometallics 2010, 29,
1064. (d) Shaw, A. P.; Tilset, M.; Heyn, R. H.; Jacobsen, S. J. Coord.
Chem. 2011, 64, 38. (e) Zhang, J. J.; Sun, R. W. Y.; Che, C. M. Chem.
Commun. 2012, 48, 3388.
(2) For examples of photoemitting (C∧N) complexes see: (a) Garg,
J. A.; Blaque, O.; Fox, T.; Venkatesan, K. Inorg. Chem. 2010, 49, 11463.
(b) Garg, J. A.; Blaque, O.; Venkatesan, K. Inorg. Chem. 2011, 50,
5430. (c) Wong, K. M.-C.; Zhu, N.; Yam, Y. W.-W. Chem.Eur. J.
2011, 17, 130.
(3) For examples of (C∧N∧C) complexes and cytotoxic activity see:
(a) Li, C. K. L.; Sun, R. W. Y.; Kui, S. C. F.; Zhu, N.; Che, C. M.
Chem.Eur. J. 2006, 12, 5253. (b) Meng, X.; Moriuchi, T.; Tohnai,
N.; Miyata, M.; Kawahata, M.; Yamaguchi, K.; Toshikazu, H. Org.
Biomol. Chem. 2011, 9, 5633. (c) Yan, J. J.; Chow, A. L. F.; Leung, C.
H.; Sun, R. W. Y.; Ma, D. L.; Che, C. M. Chem. Commun. 2010, 46,
3893. (d) Che, C. M.; Sun, R. W. Y.; Chow, L. F.; Yan, J. U.S. Patent
Application 2011/0098264, 2011.
Figure 3. Normalized photoemission of (C∧N∧C)*Au(C6H4F) (red)
and complex 2 (blue).
(4) For examples of photoemitting (C∧N∧C) complexes see:
(a) Yam, V. W.-W.; Wong, K. M.-C. Chem. Commun. 2011, 47,
11579. (b) Wong, K. H.; Cheung, K.-K.; Chan, M. C.-W.; Che, C. M.
Organometallics 1998, 17, 3505. (c) Shirasawa, N.; Akino, N.;
Nakatani, T. U.S. Patent Application 2008/0114151, 2008, to
Sumitomo. (d) Wong, K.M. C.; Hung, L. L.; Lam, W. H.; Zhu, N.;
Yam, V. W.-W. J. Am. Chem. Soc. 2007, 129, 4350. (e) Yang, B. Z.;
Zhou, X.; Liu, T.; Bai, F. Q.; Zhang, H. X. J. Phys. Chem. A 2009, 113,
9396. (f) Au, V. K. M.; Wong, K. M.-C.; Tsang, D. P. K.; Chan, M. Y.;
Zhu, N.; Yam, V. W.-W. J. Am. Chem. Soc. 2010, 132, 14273. (g) To,
W. P.; Tong, G. S. M.; Lu, W.; Ma, C.; Liu, J.; Chow, A. L. F.; Che, C.
M. Angew. Chem., Int. Ed. 2012, 51, 2654.
cleavage is selective for the Au−phenyl bond of the pincer
ligand, while unconstrained alkyl, aryl, or heteroaryl bonds are
not attacked.
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental details and full characterization data (EA, 1H, 13
C
NMR data for 2−7; crystallographic experimental details and
structural data for 1, 2, and 6b). This information is available
free of charge via the Internet at http://pubs.acs.org.
(5) Schmidbaur, H.; Schier, A. Organometallics 2010, 29, 2.
(6) Rosc
48, 7247.
̧ a, D.-A.; Smith, D. A.; Bochmann, M. Chem. Commun. 2012,
AUTHOR INFORMATION
Corresponding Author
*Tel: +44 1603 592044. E-mail: m.bochmann@uea.ac.uk. Fax:
+44 016035 92044.
(7) A reviewer raised the point that the cleavage may be the result of
an Au(III)−Ag+ metallophilic interaction. We believe that the cleavage
in the presence of AgOAcF is most simply explained by the generation
of HOAcF by adventitious water.
■
(8) Venugopal, A.; Shaw, A. P.; Tornroos, K. W.; Heyn, R. H.; Tilset,
̈
Notes
M. Organometallics 2011, 30, 3250.
(9) Similarly, Yam found higher quantum yields for (C∧N)Au bis-
alkynyl complexes than for related tridentate compounds, ref 2c.
(10) Coles, S. J.; Gale, P. Chem. Sci. 2012, 3, 683.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Leverhulme Trust. We are
grateful for Johnson Matthey PLC for a loan of gold salts.
D.A.R. thanks the University of East Anglia for a studentship.
We are grateful to Dr. Yimin Chao (UEA) for access to
spectrophotometric facilities and to the National Crystallo-
graphic Service, University of Southampton,10 for data
collection of complex 1.
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dx.doi.org/10.1021/om300666j | Organometallics 2012, 31, 5998−6000