Halide metathesis in overdrive: mechanochemical synthesis of a heterometallic group 1 allyl complex

As a synthesis technique, halide metathesis (n RM + M'Xn → RnM' + n MX) normally relies for its effectiveness on the favorable formation of a metal halide byproduct (MX), often aided by solubility equilibria in solution. Owing to the lack of significant thermodynamic driving forces, intra-alkali metal exchange is one of the most challenging metathetical exchanges to attempt, especially when conducted without solvent. Nevertheless, grinding together the bulky potassium allyl [KA']∞ (A' = [1,3-(SiMe3)2C3H3]–) and CsI produces the heterometallic complex [CsKA'2]∞ in low yield, which was crystallographically characterized as a coordination polymer that displays site disorder of the K+ and Cs+ ions. The entropic benefits of mixed Cs/K metal centers, but more importantly, the generation of multiple intermolecular K…CH3 and Cs…CH3 interactions in [CsKA'2]∞, enable an otherwise unfavorable halide metathesis to proceed with mechanochemical assistance. From this result, we demonstrate that ball milling and unexpected solid-state effects can permit seemingly unfavored reactions to occur.


S1
with potassium tert-butoxide in hexanes solution. Toluene was degassed with argon and dried over activated alumina using a solvent purification system, then stored over 4 Å molecular sieves in a glovebox. Hexanes were distilled under nitrogen over NaK/benzophenone radical [2[, then stored over 4 Å molecular sieves in a glovebox. Benzene-d6 was obtained from Cambridge Isotopes and stored over 4 Å molecular sieves.

Mechanochemical protocol.
Planetary milling was performed with a Retsch PM100 mill, 50 mL stainless steel grinding jar type C, and a safety clamp for air-sensitive grinding. Mixer milling was performed with a Retsch model MM200 mill. Ball milling reactions used 50 stainless steel (440 grade) ball bearings ( 3 /16 in (5 mm), 0.44 g) or 3 stainless steel (440 grade) ball bearings ( 1 /2 in (12.7 mm), 8.4 g) that were thoroughly cleaned with detergent and water, then washed with acetone, and dried in a 125 °C oven prior to use. A typical reaction was sealed under an inert atmosphere prior to grinding. The ground mixture was extracted with minimal hexanes (<100 mL) and filtered through a medium porosity ground glass frit. The extraction is designed to dissolve the complex, and the filtration removes residual KI. The filtrate was then dried under vacuum prior to NMR analysis.

Synthesis of [CsKA′2]
. Solid CsI (0.157 g, 0.604 mmol) and K[A′] (0.445 g, 1.98 mmol) were added to a 50 mL stainless steel grinding jar (type C). The jar was charged with stainless steel ball bearings (½ in dia, 3 count) and closed tightly with the appropriate safety closer device under an N2 atmosphere. The reagents were milled for 15 min at 600 rpm, resulting in a pale yelloworange solid. The solid was extracted with hexanes and filtered through a medium porosity ground glass frit, providing a yellow-tinted filtrate. Removal of hexanes under vacuum resulted in a turbid S3 yellow solution that deposited a precipitate as the product was concentrated. Removal of hexanes under vacuum yielded a yellow solid in low yield (12 mg, 4% yield). The solid was dissolved in hexanes, and the solution slowly evaporated over the course of a week to promote crystal growth.
As the concentration of the solution increased, it became more orange. X-ray analysis of the crystals revealed them to be the bimetallic [CsKA´2] complex. The crystals were highly soluble in C6D6, giving a bright red solution. 1

Procedures for X-ray crystallography [K1.5Cs1.5(1,3-(SiMe3)2C3H3)3]
A crystal was placed onto the tip of a thin glass optical fiber and mounted on a Bruker SMART APEX II CCD platform diffractometer at the X-ray Crystallographic Facility, Department of Chemistry, University of Rochester (Rochester, NY). Data collection was conducted at 100 K using MoK radiation (graphite monochromator) [3]. The structure was solved using SHELXT-2014/5 [4] and refined using SHELXL-2014/7 [5]. The space group P1 was determined based on intensity statistics. A direct-methods solution was calculated which provided most non-hydrogen atoms from the E-map. Full-matrix least squares/difference Fourier cycles were performed which located the remaining non-hydrogen atoms. All non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms on the allylic portions of the anions were found from the difference Fourier map and their positions were refined independently from those of their respective bonded carbon atoms. However, their isotropic displacement parameters were refined relative to the (equivalent) anisotropic displacement parameters of their respective bonded carbon atoms. All other hydrogen atoms were placed in ideal positions and refined as riding atoms with relative isotropic displacement parameters.

[(Benzene)K(1,3-(SiMe3)2C3H3)], [(toluene)K(1,3-(SiMe3)2C3H3)]
X-ray crystallographic data were collected on a Rigaku Oxford Diffraction Supernova diffractometer. Crystal samples were handled under immersion oil and quickly transferred to a cold nitrogen stream. The crystals were kept at 100 K during data collection. Under Olex2 [6], the structure was solved with the SHELXT 7 structure solution program using direct methods and refined with the SHELXL [5] refinement package using least squares minimization. All non-hydrogen atoms were refined with anisotropic displacement parameters.

General procedures for calculations
All calculations were performed with the Gaussian 09W [8] or Gaussian 16 (Linux) suite of programs [9]. The B3PW91 functional, which incorporates Becke's three-parameter exchange functional [10] with the 1991 gradient-corrected correlation functional of Perdew and Wang [11], was used. For dispersion-corrected calculations, Grimme's D3 correction [12] with additional Becke-Johnson damping was used [13] (Gaussian keyword: empiricaldispersion=GD3BJ). For the energy of metal allyl complex formation (M + + [A´] -→ [MA´]), the def2-TZVPD basis set was used on all atoms, with the accompanying ECP used for Cs [14]. For all other calculations, the def2-TZVP basis set was used on all atoms, with an ECP on Cs. An ultrafine grid was used for all calculations (Gaussian keyword: int=ultrafine).