General methods
THF and toluene were passed through columns containing molecular sieves, then stored over a potassium mirror (toluene), or over 4 Å molecular sieves (THF) and thoroughly degassed prior to use. Hexane and diethyl ether were purchased anhydrous from Tokyo Chemical Industry, dried over activated molecular sieves for 7 days, then stored over a potassium mirror. For NMR spectroscopy, C6D6 and C4D8O were dried by refluxing over potassium, and then vacuum transferred and degassed by three freeze-pump-thaw cycles before use. NMR spectra were recorded on either a Bruker Avance III HD 400 or Bruker Avance III 500 spectrometer operating at 400.07/500.13 (1H), 61.4/76.8 (2H), 100.60/125.78 (13C{1H}) or 79.48/99.36 (29Si{1H}) MHz. To achieve a greater signal to noise ratio, the 29Si{1H} NMR spectra were acquired with a DEPT24 pulse sequence. NMR spectra were recorded at 298 K unless otherwise stated and were referenced to residual solvent signals in the case of 1H and 13C{1H} experiments, or externally referenced to SiMe4 (29Si{1H}). FTIR spectra were recorded on a Bruker Alpha II spectrometer with Platinum-ATR module. Magnetic measurements were performed using a Quantum Design MPMS3 superconducting quantum interference device (SQUID) magnetometer operating at 1.8–400 K under applied fields up to 7 T; experimental details are in S7.1. The electron paramagnetic resonance (EPR) spectra were recorded in continuous-wave (C.W.) mode, using a Bruker EMX Plus Spectrometer operating at X-band (9.36 GHz) and variable temperatures (5 to 293 K); experimental details are in S7.2. Mass spectrometry were carried out by Dr Sharad Mistry at the University of Leicester, using a Waters Acquity XEVO Q ToF spectrometer (electrospray). Elemental microanalyses were carried out by the London Metropolitan University Elemental Analysis service. Anhydrous CaI2 was purchased from Alfa Aesar and baked at 200 ºC for 4 hours prior to use. HN(Mes)(SiMe3),54 K[N(Mes)(SiMe3)]55 and 117 were prepared according to literature procedures. All glassware used to store or to investigate the reactivity of 2 and 2ʹ was first treated with trimethylsilyl chloride to remove trace hydroxide residues, rinsed with deionized water and acetone, and dried at 150°C for 16 hours. Solid state reactions were performed using either: i) an IKA ULTRA-TURRAX Tube Drive Disperser; 20 mL polypropylene milling jar, 4 × 3 mm stainless steel ball-bearings, or ii) a Retch MM440 mixer mill; 25 mL stainless steel milling jar, 1 × 10 mL stainless steel ball-bearing.
Synthesis of K + [{Ca{N(Mes)(SiMe 3 )} 3 K]e – (2):
Method 1: 1
(0.350 g, 0.5 mmol, 1 eq.), freshly-cut potassium metal (19.6 mg, 0.5 mmol, 1 eq.) and 4 stainless steel ball-bearings (3 mm diameter) were added to a 25 mL polypropylene milling jar inside an argon-filled glovebox before being milled at 4000 rpm for 3 hours. Upon completion, the resultant blue powder (0.360 g, quantitative yield) was decanted and stored in silylated glassware.
Method 2: 1
(2.457 g, 3.524 mmol, 1 eq.), freshly-cut potassium metal (0.138 g, 3.524 mmol 1 eq.) and 1 stainless steel ball-bearing (10 mm diameter) were added to a 25 mL stainless steel milling jar inside an argon-filled glovebox before being milled at 30 Hz rpm for 30 minutes. Upon completion, the resultant blue powder (0.360 g, quantitative yield) was decanted and stored in silylated glassware.
2ʹ: 1
(0.500 g, 0.717 mmol, 1 eq.), freshly-cut potassium metal (58.6 mg, 1.5 mmol, 2.1 eq.) and 1 stainless steel ball-bearing (10 mm diameter) were added to a 25 mL stainless steel milling jar inside an argon-filled glovebox before being milled at 30 Hz rpm for 30 minutes. Upon completion, the resultant blue powder (0.550 g, quantitative yield) was decanted and stored in silylated glassware.
Synthesis of [K(η6-C6H6)4][{Ca[N(Mes)(SiMe3)](H)}2K3] (3A) and [K(η6-C7H8)4][{Ca[N(Mes)(SiMe3)](H)}2K3] (3B):
Method 1
Benzene (2 mL) was added to a freshly prepared sample of 2 (0.201 g, 0.27 mmol) and stirred at room temperature, yielding an instantaneous formation of a black mixture which lightened to form an orange solution. Slow evaporation of this solution yielded 3A as large, yellow crystals overnight (0.060 g, 0.03 mmol, 24.9%).
Method 2
1,4-Cyclohexadiene (10 mL) was added to a freshly prepared sample of 2 (0.500 g, 0.68 mmol) and stirred at room temperature for 16 hours. The resulting yellow suspension was filtered yielding 3A as a yellow solid (0.166 g, 0.11 mmol, 16.5%). Crystallization from toluene afforded 3B, where benzene molecules binding the potassium counterion are replaced by toluene.
1H NMR (500 MHz, 298 K, C6D6): δ/ppm = 0.55 (s, 54H, Si(CH3)3), 2.07 (s, 18H, para-CH3), 2.29 (s, 36H, ortho-CH3), 2.66 (s, 2H, Ca-H), 6.66 (s, 12H, Ar-H).
13C{1H} (125 MHz, 298 K, C6D6): δ/ppm = 4.88 (Si(CH3)3), 20.39 (para-CH3), 22.30 (ortho-CH3), 123.48 (Ar-C), 128.89 (Ar-C), 134.78 (Ar-C), 159.20 (Ar-C).
29Si{H} (80 MHz, 298 K, C6D6) δ/ppm = ‒13.4.
Synthesis of [K(η6-C6D6)4][{Ca[N(Mes)(SiMe3)](D)}2K3] (3C):
Benzene-d6 (0.5 mL) was added to a freshly prepared sample of 2 (0.017 g, 0.023 mmol) in a J. Young’s NMR tube yielding an instantaneous formation of a dark brown mixture. The reaction mixture was frozen using liquid nitrogen and the solvent removed by sublimation in vacuo yielding a light brown powder, which was fully dried under reduced pressure. Benzene-H6 (0.5 mL) was added and the 2H NMR spectrum obtained. 2H{1H} NMR (61 MHz, C6H6, 298K): δ/ppm = 2.73 (s, Ca-H).