Expanding iClick to group 9 metals
Graphical abstract
Gold-azides undergo cycloaddition (iClick) to group 9 azido complexes to give triazolate bridged heterobimetallic complexes.
Introduction
Metallopolymers are on the cusp of revolutionizing the energy sector, information storage, and materials synthesis [1]. Transition metal chemistry marries polymer science in these hybrid materials wherein the metal ion imparts new properties unimaginable for organic polymers alone [2]. The challenge for synthetic chemists is to find new methods to covalently incorporate metal ions into polymer chains. Preparing metallopolymers requires either appending metal ions to an already existing polymer backbone or incorporating them into the repeating unit directly [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32]. Conjugated platinum(II) acetylide oligomers/polymers, studied extensively by Schanze and co-workers [33], [34], [35], [36], [37], [38], [39], [40], are hallmark examples of incorporating metal ions into the propagating repeat unit. The platinum ions engender photophysical properties that are challenging or impossible to replicate by organic polymers alone. New methods are necessary though to extend the synthetic versatility and control over metallopolymer composition, molecular weight, optical, and conductive properties.
In 2010 [41], we published the cycloaddition reaction between a gold(I)-acetylide and a gold(I)-azide to from a triazolate linked digold complex (Fig. 1a); and termed the reaction inorganic click (iClick) to acknowledge the two metal ions that substitute either protons or organic substituents in typical copper-catalyzed azide-alkyne cycloadditions (CuAAC) [42], [43]. Subsequently, we extended the reaction to include Pt(II) ions, and proposed methods to exploit the reaction to prepare metallopolymers (Fig. 1b). Recently, by combining iClick with multiple aurophilic interactions, we prepared solution stable organogold oligomers [44]. Interrogating the mechanism [45] of reaction (a) reveals that the Au(I)-acetylide is a necessary partner to a successful iClick reaction. Au(I)-acetylide is necessary because it plays the role of the in situ generated Cu(I)-acetylide in prototypical CuAAC reactions [46]. Ongoing studies focus on alleviating the requirement for a Au(I)-acetylide partner. In the meantime, to be a useful synthetic methodology and to ultimately apply iClick to the synthesis of metallopolymers, extension across the periodic table to new metal ions is necessary. In this manuscript we extend the iClick reaction to now include group 9 azido complexes (see Fig. 2, Fig. 3).
Section snippets
Synthesis and characterization of [Rh(CO)(PPh3)2][PPh3Au](μ-N3C2C6H4NO2) (3), {[Rh(CO)(PPh3)][PPh3Au](μ-N3C2C6H4NO2)}2 (4)
To expand iClick to group 9 transition metals, the first reaction involved treating the gold(I)-acetylide, PPh3Au(CCC6H4NO2) (2) [47], with the rhodium-azide, Rh(CO)(PPh3)2N3 (1) [48], [49], [50] at ambient temperature in chloroform. Upon combining the reagents an instantaneous color change from orange to bright yellow occurs. Three products, the hetero-bimetallic complex, [Rh(CO)(PPh3)2][PPh3Au](μ-N3C2C6H4NO2) (3), the hetero-tetranuclear complex, {[Rh(CO)(PPh3)][PPh3Au](μ-N3C2C6H4NO2)}2 (4),
Conclusions
The reaction between Au(I)-acetylide and group 9 azide complexes results in the [3+2] azide/acetylide cycloaddition to give (initially for Rh) heterobimetallic complexes bridged by a triazolate. In the case of Rh(I) the reaction proceeds to eventually form a tetranuclear complex via dimerization, whereas for Ir(I) the reaction stops at the monomer. Even heating complex 6 at 50 °C does not promote dimerization. An explanation is that the Ir–PPh3 bond must be favoured over the Ir–N bond that would
Materials and methods
Unless specified otherwise, all manipulations were performed under an inert atmosphere using standard Schlenk or glove-box techniques. Hexanes, methylene chloride, tetrahydrofuran and toluene were degassed by sparging with high purity argon, and were dried using a GlassContour drying column. Methanol was dried over anhydrous copper(II)sulfate, distilled and stored over 4 Å molecular sieves; chloroform-d (Cambridge Isotopes) was dried over copper(II) sulfate/calcium chloride, distilled, and
Acknowledgements
Research supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award DE-SC0010510. KAA thanks UF and the NSF for funds to purchase X-ray equipment (CHE-0821346).
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