Org Prep Daily

April 5, 2019

Cesium trans-[tetrachloro Bis-(1H-Indazole)-Ruthenate(III)] hydrate

Filed under: procedures — milkshake @ 3:26 am



RuCl3.xH2O 100.0g (x~3, 382mmol) was combined with conc. HCl 0.6L and non-denatured ethanol 0.6 L. The mixture was stirred and distilled under air at normal pressure until the total volume of the mixture was below 400 mL. The distillates were discarded. The obtained dark brown solution remaining in the distillation flask was cooled, filtered through a medium porosity glass Buchner funnel and the filtrates were adjusted with conc. HCl to total volume about 0.5L.

In the meantime, indazole 300g (2.54 mol; 6.64eq.) was dissolved in a mixture of water 800 mL and conc. HCl 4.0 L (with 20 min stirring), the solution was filtered through a medium porosity glass Buchner funnel. This indazole solution was charged into a 15L glass-and-teflon jacketed reactor equipped with an efficient paddle-shaped stirrer, 0.5L addition funnel, a thermoprobe and air-cooled reflux condenser topped with a gas outlet tube for HCl gas release. An additional volume of conc. HCl 4.0L was then charged to the  reactor, the circulator-heating was set to 90C. The temperature in the reactor was let to stabilize for at least 30 min and then carefully maintained at 90 C throughout. The solution of ruthenium trichloride in HCl was added dropwise, at about 250rpm stirring, over a period of 5 hours, using an addition funnel with a stem extended with a piece of polyethylene tubing (to limit splashing). The addition funnel was washed down with a small volume of HCl (2x50mL). The obtained brownish slurry was then stirred at 90C for additional 10 hours. The reaction mixture was cooled down to 25C, with stirring, the slurry of the precipitated product was drained from the reactor through bottom valve into a 15L polyethylene bucket. The solids were collected by filtration on a large (3L) medium porosity glass Buchner funnel, the reactor was washed down with 2M aqueous HCl and the washings were added to the Buchner funnel. The product was rinsed with additional 2M HCl, about 2L, and partially dried by suction overnight. This provided a wet cake (moist with the residual 2M HCl) of the indazolium salt, 598g,  as a brown sticky solid. [Note 1]

The moist indazolium salt was transferred into a 10L wide-mouth flask equipped with an efficient mechanical stirrer with a teflon paddle. CsCl 180g (1.07mol, 2.8 eq., powdered briefly with a spatula to break any lumps) was added to the flask, followed by methyl ethyl ketone 2.0L and non-denatured ethanol (99%) 1.8L The mixture was stirred at 200 rpm for 5 min, the stirring was then turned to high speed and continued for 2 hours at 700 rpm at ambient temperature (22 C). The resulting bright orange slurry was collected by filtration (3 L medium porosity Buchner funnel), the solids were rinsed thoroughly with 99% non-denatured ethanol and partially dried by suction, for about 1 hour. The obtained bright orange cesium salt in the form of MEK-solvate intermixed with residual CsCl was transferred into a large 4 L beaker. 1 L of a 2:1 (v/v) ethanol-water mixture was added and the slurry was stirred in open beaker for 15 min at about 350 rpm. During this time the bright orange color of the MEK-solvated cesium salt slurry faded into cinnamon red-brown color of the hydrate. The solids were collected by filtration (using the same Buchner funnel),  washed thoroughly with 99% ethanol, about 1L. The material was dried by suction overnight  (14 h). The yield was 226.9 g (91.5% theory) of a red-brown heavy solid. The product is approximately monohydrate (it forms initially as dihydrate but loses a part of the solvated water upon drying). The material is bench stable.

HPLC purity 98.5-99% by HPLC (SB Zorbax C18, 3 micron, 4.6x150mm,  a 12 min 10% to 90% linear gradient of MeCN(+0.1%TFA) in water(+0.1%TFA) at 1.0mL/min), the product composition was confirmed by elemental analysis and X-ray crystallography

Note 1: The indazolium salt is a potent contact irritant. Indazole and ruthenium trichloride are caustic to skin. HCl is very corrosive. A full face shield and a protective apron are recommended when loading the reactor with large volumes of conc. HCl. The reactor needs to be completely disassembled and cleaned after the preparation, to prevent damage to seals and metal parts, and decontaminated from Ru residues (rinse with acetone, followed by methanol with added conc. ammonia, about 20:1 by volume, followed by water and acetone rinse)

Note 2: Since Ru(III) salts are paramagnetic, NMR is not helpful for purity determination. HPLC is useful but the resolution of the impurities is very specific to the particular type of reverse-phase HPLC column (SB-C18 Zorbax 3 micron). It is best to use single injections and prepare the individual HPLC samples just before the analysis because the material gradually decomposes in solution.



  1. Hi milkshake,
    Why do you do first step, distillation with Ethanol?

    Comment by pikolinian — April 6, 2019 @ 11:26 am

    • To purify commercial RuCl3. Also, one needs to cook RuCl3 with conc. HCl for awhile to form a RuCl6(3-) solution, (RuCl3 has limited solubility in cold HCl, and the hexachloro complex formation is not instant, Ru(III) complexes being kinetically lazy). Ethanol is added as a reducing agent, it gets oxidized to acetaldehyde and acetaldehyde-derived byproducts, that are fortunately volatile and distill out. The purpose of refluxing RuCl3 in the presence of ethanol is to reduce any Ru(IV) impurity that might be present in the commercial-grade RuCl3 from the manufacture. The resulting reduced Ru(II) in concentrated HCl is not very stable and gets quickly reoxidized by air back to Ru(III). But if you were to do this ethanol reflux under inert atmosphere, you would get so-called ruthenium blue, which is a mixture of mixed valency polynuclear ruthenium(II+III) chloro complexes bridged by Cl ligands.

      Comment by milkshake — April 6, 2019 @ 11:38 am

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