Preparation and ESR studies of salts of crown ether complex cations and nitridotechnetium(VI) anions: structures of the twinned ‘infinite sandwich’ [Cs(18-crown-6)][TcNCl 4 ] complex and of [Rb(15-crown-5) 2 ] [TcNCl 4 (OH 2 )]

We report the preparations and electron spin resonance (ESR) studies of a series of Cs + and Rb + crown ether (15-crown-5 and 18-crown-6) salts of [TcNCl 4 ] - and [MoOCl 4 ] - , and a new refinement of the X-ray structure of [Cs(18-crown-6)] [TcNCl 4 ] involving a twinned P4/n scattering model. This refinement addresses some unsatisfactory aspects of the original disordered model. The structure consists of an ‘infinite sandwich’ Cs + cation/crown ether configuration and polymeric [TcNCL 4 ] - anions, with Tc≡N distances of 1.561(36) Å, and anti-parallel chains of –––Tc≡N–––Tc≡N –––with Tc···Tc separations of 4.275(4) Å. The structure determination of [Rb(15-crown-5) 2 ][TcNCl 4 (OH 2 )] is also described and shows a lattice containing 2:1 ‘sandwich’-type crown ether/alkali-metal cations and isolated anions in the disordered lattice. The presence of Tc≡N infrared stretching frequencies at ca . 1074 cm -1 indicates the absence of strong electron donors coordinated to Tc in the sixth position (i. e. isolated anions), whereas the polymeric chain structure of [Cs(18-crown-6)] [TcNCL 4 ] is characterized by a Tc≡N stretching frequency of 1041 cm -1 . Three product compounds, the ‘infinite sandwich’ [Cs(18-crown6)][TcNCl 4 ], [Cs(18-crown-6)] [TcNCL 4 (OH 2 )] with two Tc≡N stretching frequencies at 1055 and 1046.5 cm -1 , and [Cs(18-crown-6) 2 ][TcNCl 4 containing 2:1 crown ether/Cs + ‘sand­wich’ cations and isolated [TcNCl 4 ] - anions, could be obtained from reaction of Cs + /18-crown-6 and [TcNCL 4 ] - depending on reaction conditions and solvent. The ESR spectra of [Cs(18-crown-6)] [TcNCl 4 ] over the temperature range 130-290 K indicate the presence of exchange interactions along the linear ––– Tc≡N––– Tc≡N––– chains. Spectra for other crown ether salts are consistent with isolated [TcNCL 4 ] - or [TcNCL 4 (OH 2 )] - anions, with dipolar broadening effects apparent.