10.1002/anie.201712602
Angewandte Chemie International Edition
COMMUNICATION
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complex (Figure 3a). This was done by adding excess PPh3,
followed by further 30 minutes milling with ethanol. The PPh3
acted as both a reducing and a complexation agent, yielding the
complex Au(PPh3)Cl (2a) and triphenylphosphine oxide.
Reduction of Au(III) to Au(I) was evident by change of the reaction
mixture color to white, and formation of 2a was verified by HR-MS
of the crude reaction mixture. The complexation of PPh3 and
formation of triphenylphosphine oxide were evident by 31P-NMR
spectroscopy in CDCl3 (Figure 3). Compound 2a was separated
by washing with diethylether and recrystallization from CHCl3
(Figure 3), or by washing with water to remove Oxone®
byproducts.
Due to the cost of palladium and gold, all reactions were
done at 0.2 mmol scale. We have also demonstrated scale-up to
at least 2 mmol for palladium metal: purified 1d-f were obtained
in 87% (1.42 g of (PPh3)PdCl2·CHCl3), 81% (1.47 g of
(PPh3)PdBr2·CHCl3) and 69% (1.38 g of (PPh3)PdI2·CHCl3) yields,
respectively (see ESI).
Preliminary results also reveal 56% conversion of platinum
into (NH4)2PtCl6 with Oxone® and NH4Cl (see ESI), as well as
potential for metal separation: milling three-component mixtures
of Pd, Pt and Au in approximate 1:2:1 molar ratios with limited
Oxone and NH4Cl led to selective dissolution of only Pd and Au.
Depending on the milling conditions, activation of Au is preferred
by 2 or 3 times to Pd (see ESI).
In summary, we demonstrated mechanochemistry for
simple, rapid and safe conversion of palladium and gold into
water-soluble, well-defined salts. A second mechanochemical
step enables one-pot transformation of noble metals into well-
defined, catalytically active complexes. To the best of our
knowledge, this is the first strategy for noble metal activation and
recycling that is solvent-free, avoids harsh oxidative or
complexation reagents, and proceeds at room temperature. The
halides used for this mechanochemical process are readily
available (NH4Cl, KCl) and inexpensive, particularly compared to
the end product,[32] and the oxidant is stable on storage,
chemically similar to those already used in solution.[33} Although
the solid-state procedure generates sulfate byproducts of
Oxone reduction, it offers promise for replacement of
aggressive HNO3, aqua regia, molten salts or cyanides. Further
work on generality, scalability and selectivity is ongoing.
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Experimental Section
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Details of all experiments, diffraction, and spectroscopic data are given in
the Electronic Supplementary Information (ESI). In a general procedure,
mechanochemical reactions were done on a Retsch MM400 mill operating
at 30 Hz, using a ZrO2 grinding assembly (10 mL jar, one 10 mm diameter
ball of 3.5 g). Palladium or gold metal, a halide salt, an oxidant, and an
optional liquid additive were milled for 30 minutes. For two-step, one-pot
reactions, the ligand and a liquid additive were then added, and the
reaction milled for 30 minutes more. The products were purified by
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standard
recrystallization
and
chromatographic
techniques.
Crystallographic data for trigonal 1d were deposited with the Cambridge
Crystallographic Data Centre, deposition code CCDC 1564633.
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Keywords: mechanochemistry • noble metals • green chemistry
• sustainability • coordination chemistry
[18] (a) P. Baláž, M. Achimovičová, M. Baláž, P. Billik, Z. Cherkezova-
Zheleva, J. M. Criado, F. Delogu, E. Dutková, E. Gaffet, F. J. Gotor, R.
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