"Click" chemistry in metal coordination spheres: Copper(I)-catalyzed 3+2 cycloadditions of benzyl azide and platinum polyynyl complexes trans-(C6F5)(p-tol3P) 2Pt(C≡C)nH (n = 2-6)

Article abstract of DOI:10.1021/om900680q

Summary; Benzyl azide and the terminal polyynyl complexes trans-(C ₆F₅)(p-tol₃P)₂Pt(C≡C) _nH(n = 2-6) undergo 3+2 "click" cycloadditions in the presence of CuSO₄/ascorbic acid to give the 1,

Full text of DOI:10.1021/om900680q

Organometallics 2009, 28, 5597–5599 5597
DOI: 10.1021/om900680q
“Click” Chemistry in Metal Coordination Spheres: Copper(I)-Catalyzed
3þ2 Cycloadditions of Benzyl Azide and Platinum Polyynyl Complexes
trans-(C6F5)(p-tol3P)2Pt(CtC)nH (n = 2-6)
ꢀ
Sebastien Gauthier, Nancy Weisbach, Nattamai Bhuvanesh, and
John A. Gladysz*
Department of Chemistry, Texas A&M University, P.O. Box 30012, College Station, Texas 77842-3012
Received July 31, 2009
Summary: Benzyl azide and the terminal polyynyl complexes
trans-(C6F5)(p-tol3P)2Pt(CtC)nH (n = 2-6) undergo 3þ2
“click” cycloadditions in the presence of CuSO4/ascorbic acid
P)2Pt(CtC)2nPt(Pp-tol3)2(Ar) (PtC4nPt).11 The latter exhi-
bit surprisingly high stabilities, with decomposition tempera-
tures in excess of 150 °C even at chain lengths of 20 to 28
carbon atoms (Ar = p-tol). However, the educts become
exceedingly labile at longer chain lengths, with hexatriynyl
complexes (PtC6H) representing the limit for isolability in
spectroscopically pure form. This also generally applies to
purely organic systems.12 The trick to synthesizing PtC24Pt
and PtC28Pt (Ar = p-tol) is to generate the dodecahexaynyl
and tetradecaheptaynyl complexes PtC12H and PtC14H
under conditions that maximize the chances for subsequent
bimolecular coupling as opposed to unimolecular or solvent-
mediated decomposition.11b
As such, we were interested in investigating the potential
utility of copper(I)-promoted azide cycloadditions as diag-
nostic tests for the generation of labile PtC2nH species.
Importantly, Tykwinski has established the applicability
of such trapping reactions to organic conjugated terminal
diynes, triynes, and tetraynes.13 In this communication, we
report that analogous reactions can be realized with plati-
num derivatives, including the first click cycloadditions
involving any type of terminal pentayne or hexayne. The
resulting organometallic triazoles are exceedingly stable and
readily crystallize.
to give the 1,2,3-triazoles trans-(C6F5)(p-tol3P)2Pt(CtC)n-1
-
CdCHN(CH2C6H5)NdN, four of which are structurally
characterized.
Over the course of the last seven years, the copper(I)-
catalyzed “click” 3þ2 cycloaddition of azides and termi-
nal alkynes1,2 has been extended to virtually every class of
molecules. Furthermore, numerous in vivo applications3
and means of functionalizing a variety of types of solid
phases4 have been developed. However, the inorganic
and organometallic communities have come relatively late
to this game. Outside of robust ferrocenyl systems,5 there
have only been a handful of click reactions conducted in
metal coordination spheres. These include the derivatiza-
tion of bridging formamidinate, porphyrin, carbene, and
pincer ligands that contain aryl CtCH moieties as descri-
bed by Ren,6 Collman,7 Casarrubios and Sierra,8 and van
Koten.9,10
We have had an ongoing interest in platinum terminal
polyynyl complexes of the formula trans-(Ar)(p-tol3P)2Pt-
(CtC)nH (PtC2nH) and their oxidative homocoupling to
diplatinum polyynediyl complexes trans,trans-(Ar)(p-tol3-
As shown in Scheme 1 (top), equimolar quantities of the
pentafluorophenyl-substituted complex trans-(C6F5)(p-tol3-
P)2Pt(CtC)2H (Pt0C4H)11a and benzyl azide were combined
in DMF. Then aqueous CuSO4 and ascorbic acid, a standard
recipe for generating copper(I),2 were added. After ca. 16 h
at room temperature, a chromatographic workup gave a
new complex, 1 (65% based upon the structure established
below), which was characterized by microanalysis and NMR
(1H, 13C, 31P, 19F) and IR spectroscopy, as summarized in the
Supporting Information.
*Corresponding author. E-mail: gladysz@mail.chem.tamu.edu.
(1) (a) Tornøe, C. W.; Christensen, C.; Meldal, M. J. Org. Chem.
2002, 67, 3057. (b) Rostovtsev, V. V.; Green, L. G.; Fokin, V. V.; Sharpless,
K. B. Angew. Chem., Int. Ed. 2002, 41, 2596; Angew. Chem. 2002, 114,
2708.
(2) Review: Meldal, M.; Tornøe, C. W. Chem. Rev. 2008, 108, 2952.
(3) (a) Beatty, K. E.; Xie, F.; Wang, Q.; Tirrell, D. A. J. Am. Chem.
Soc. 2005, 127, 14150. (b) Deiters, A.; Schultz, P. G. Bioorg. Med. Chem.
Lett. 2005, 15, 1521.
(4) Lutz, J.-F. Angew. Chem., Int. Ed. 2007, 46, 1018; Angew. Chem.
2007, 119, 1036.
The 13C NMR spectrum of 1 (CDCl3) showed only a single
CtC linkage (δ 109.0, 101.9 ppm), and the tCH 1H NMR
signal of Pt0C4H (δ 1.46 ppm) was replaced by one with a
chemical shift plausible for a triazole dCH moiety (s, 6.15
ppm). Crystals were obtained from CH2Cl2/pentane, and
the X-ray structure was determined as described in the
Supporting Information. The resulting thermal ellipsoid
diagram, depicted in Figure 1, confirmed 1 to be the expected
(5) (a) Siemeling, U.; Rother, D. J. Organomet. Chem. 2009, 694,
1055, and references therein. (b) Das, M. R.; Wang, M.; Szunerits, S.;
Gengembre, L.; Boukherroub, R. Chem. Commun. 2009, 2753.
(6) Chen, W.-Z.; Fanwick, P. E.; Ren, T. Inorg. Chem. 2007, 46,
3429.
ꢀ
(7) Decreau, R. A.; Collman, J. P.; Yang, Y.; Yan, Y.; Davaraj, N. K.
J. Org. Chem. 2007, 72, 2794.
(8) Baeza, B.; Casarrubios, L.; Ramırez-Lopez, P.; Gomez-Gallego,
ꢀ
ꢀ
M.; Sierra, M. A. Organometallics 2009, 28, 956.
(9) McDonald, A. R.; Dijkstra, H. P.; Suijkerbuijk, B. M. J. M.; van
Klink, G. P. M.; van Koten, G. Organometallics 2009, 28, 4689.
(10) Closely related work: Sawoo, S.; Dutta, P.; Chakraborty, A.;
Mukhopadhyay, R.; Bouloussa, O.; Sarkar, A. Chem. Commun. 2008,
5957.
(12) Wang, C.; Batsanov, A. S.; West, K.; Bryce, M. R. Org. Lett.
2008, 10, 3069.
(11) (a) Mohr, W.; Stahl, J.; Hampel, F.; Gladysz, J. A. Chem.;Eur.
J. 2003, 9, 3324. (b) Zheng, Q.; Bohling, J. C.; Peters, T. B.; Frisch, A. C.;
Hampel, F.; Gladysz, J. A. Chem.;Eur. J. 2006, 12, 6486.
(13) (a) Luu, T.; McDonald, R.; Tykwinski, R. R. Org. Lett. 2006, 8,
6035. (b) See also: West, K.; Hayward, L. N.; Batsanov, A. S.; Bryce, M. R.
Eur. J. Org. Chem. 2008, 5093.
r
2009 American Chemical Society
Published on Web 09/09/2009
pubs.acs.org/Organometallics

5598 Organometallics, Vol. 28, No. 19, 2009
Scheme 1. Syntheses of “Click” Adducts 1-5a
Gauthier et al.
a Conditions A1: THF, wet n-Bu4NþF- in THF and then ClSiMe3. Conditions A2: DMF, benzyl azide, cat. CuSO4 and ascorbic acid, H2O.
Conditions B: THF, then benzyl azide in DMF, then cat. aqueous CuSO4 and ascorbic acid, then wet n-Bu4NþF- in THF.
1,4-disubstituted 1,2,3-triazole. Metrical parameters are sup-
plied in the Supporting Information.
subsequent 3þ2 cycloadditions (conditions B, Scheme 1).
Slight excesses of benzyl azide were now employed (2.0-1.5
equiv), and wet n-Bu4NþF- was the last species added.
A model reaction with Pt0C6SiEt3 gave 2 in 62% yield
after workup, establishing the compatibility of the various
reactants. Analogous procedures were then applied to
Pt0C10SiEt3 and Pt0C12SiEt3. As shown in Scheme 1, the
target 3þ2 cycloadducts 4 and 5 were isolated in 74% and
78% yields.
Next, Pt0C6H and Pt0C8H were generated by the proto-
desilylation of the corresponding triethylsilyl derivatives
Pt0C6SiEt3 and Pt0C8SiEt3 with wet n-Bu4NþF- in THF
(“conditions A1” in Scheme 1).11a This was followed by the
addition of ClSiMe3, an additive that has proved beneficial
in subsequent homocoupling reactions.11 A DMF solution
of benzyl azide and aqueous solutions of CuSO4 and ascorbic
acid were then introduced (“conditions A2”). Workups gave
the 3þ2 cycloadducts 2 and 3 in 70% and 60% yields. These
were characterized analogously to 1.
Crystals of 2-4 or solvates thereof could also be obtained.
Those of 3 and 4 were yellow, whereas 1 and 2 were colorless.
The X-ray structures were determined, and the resulting
thermal ellipsoid plots are given in Figure 1. Although the
bond lengths and angles about platinum (Supporting
Information) are similar, there are minor conforma-
tional differences. For example, 3 exhibits the most linear
Pt-CtC-Ct segment (sum of bond angles 355.3° vs
When a similar reaction was attempted with Pt0C10SiEt3,14
no tractable products were obtained. This was seen as
consistent with a rapid independent decomposition of
Pt0C10H, in accord with results obtained in oxidative homo-
couplings.11 Thus, protodesilylations of Pt0C2nSiEt3 were
attempted in the presence of all reactants required for
337.7-345.5°).15 Also,
1 and 4 exhibit pronounced
(14) (a) This complex is available from the cross-coupling of Pt0C6H
and HC4SiEt3, analogously to closely related procedures described
(15) For analyses of bond angle (and bond length) trends in sp carbon
chains, see: Szafert, S.; Gladysz, J. A. Chem. Rev. 2003, 103, 4175; 2006,
106, PR1-PR33.
earlier.11 (b) Kuhn, H. Doctoral Dissertation, Universitat Erlangen-
€
€
Nurnberg, 2009.

Communication
Organometallics, Vol. 28, No. 19, 2009 5599
-5.75(15)° for 1, 10.21(15)° and 14.27(15)° for 4, and higher
average values for 2 and 3).
A few spectroscopic trends merit note. For example, the
CtCCdCHN 1H NMR signals of 1-5 show a pronounced
monotonic downfield shift (δ 6.15, ca. 7.09 (overlapping
signals), 7.39, 7.51, 7.56 ppm). The Brønsted acidities of
conjugated terminal polyynes R(CtC)nH are known to
increase with chain length.17 Hence, the Pt(CtC)n- moieties
become increasingly more electron withdrawing. The 31P
NMR chemical shifts exhibit a weaker monotonic trend
1
(18.0 to 17.2 ppm), paralleled by an increase in the JPPt
values (2613, 2624, 2631, 2651, 2687 Hz).
Finally, Pt0C4H and the rhenium azide complex (η5-
C5H5)Re(NO)(PPh3)(N3)18 were combined under condi-
tions analogous to those in Scheme 1. However, no reaction
was observed. Nonetheless, other types of cycloaddi-
tions of metal azide complexes and alkynes have been
reported.19
In summary, we have shown that platinum polyynyl
complexes with terminal CH linkages readily undergo
copper(I)-promoted “click” 3þ2 cycloadditions with orga-
nic azides. These reactions can furthermore be effected
by conducting protodesilylations of triethylsilyl polyynyl
complexes in the presence of the azide and copper(I) cata-
lyst. This enables the trapping of labile terminal poly-
ynes that would undergo rapid independent decomposition
of the time scale of sequential reactions. There are a num-
ber of obvious extensions of this work, such as to reac-
tants with still longer sp carbon chains and ruthenium
catalysts that afford complementary cycloaddition regio-
chemistries,20 and further efforts will be reported in due
course.
Acknowledgment. We thank the U.S. National Science
Foundation (CHE-0719267) and Johnson Matthey
(platinum loans) for support.
Supporting Information Available: Experimental procedures
and characterization for all new compounds, and CIF files with
crystallographic data for 1-4. This material is available free of
charge via the Internet at http://pubs.acs.org.
(16) (a) Stahl, J.; Mohr, W.; de Quadras, L.; Peters, T. B.; Bohling, J. C.;
Martın-Alvarez, J. M.; Owen, G. R.; Hampel, F.; Gladysz, J. A. J. Am.
Chem. Soc. 2007, 129, 8282. (b) de Quadras, L.; Bauer, E. B.; Mohr, W.; Bohling,
J. C.; Peters, T. B.; Martín-Alvarez, J. M.; Hampel, F.; Gladysz, J. A. J. Am. Chem.
Soc. 2007, 129, 8296. (c) de Quadras, L.; Bauer, E. B.; Stahl, J.; Zhuravlev, F.;
Hampel, F.; Gladysz, J. A. New J. Chem. 2007, 31, 1594. (d) Owen, G. R.; Stahl,
J.; Hampel, F.; Gladysz, J. A. Chem.;Eur. J. 2008, 14, 73.
(17) Eastmond, R.; Johnson, T. R.; Walton, D. R. M. J. Organomet.
Chem. 1973, 50, 87.
Figure 1. Thermal ellipsoid plots (50% probability level) of the
crystal structures of (top to bottom) 1, 2, 3, and 4.
(18) Dewey, M. A.; Knight, D. A.; Klein, D. P.; Arif, A. M.; Gladysz,
J. A. Inorg. Chem. 1991, 30, 4995.
C6H4CH3/C6F5/C6H4CH3 stacking interactions, as found in
many other crystal structures of trans-(C6F5)(p-tol3P)2Pt
species.11a,16 This can be quantified by the average distances
between the centroids of the C6F5 and C6H4CH3 rings
(3.497-3.554 A for 1, 4 vs 3.955-3.975 A for 2, 3), and the
near-zero Cipso-P-Pt-Cipso torsion angles (4.74(15)° and
(19) Chen, C.-K.; Tong, H.-C.; Hsu, C.-Y. C.; Lee, C.-Y.; Fong, Y.
H.; Chuang, Y.-S.; Lo, Y.-H.; Lin, Y.-C.; Wang, Y. Organometallics
2009, 28, 3358 (see Scheme 2 therein).
(20) Boren, B. C.; Narayan, S.; Rasmussen, L. K.; Zhang, L.;
Zhao, H.; Lin, Z.; Jia, G.; Fokin, V. V. J. Am. Chem. Soc. 2008,
130, 8923.