Hydrogen Bonds Stabilize Chloroselenite Anions: Crystal Structure of a New Salt and Donor-Acceptor Bonding to SeO₂

The single-crystal X-ray diffraction structure characterizing a new 4-methylbenzamidinium salt of chloroselenite [C₈H₁₁N₂][ClSeO₂] is reported. This is only the second crystal structure report on a ClSeO₂^- salt. The structure contains an extended planar hydrogen bond net, including a double interaction with both O atoms of the anion (an R228 ring in Etter notation). The anion has the shortest Se-Cl distances on record for any chloroselenite ion, 2.3202(9) Å. However, the two Se-O distances are distinct at 1.629(2) and 1.645(2) Å, attributed to weak anion-anion bridging involving the oxygen with the longer bond. DFT computations at the RB3PW91-D3/aug-CC-pVTZ level of theory reproduce the short Se-Cl distance in a gas-phase optimized ion pair, but free optimization of ClSeO₂^- leads to an elongation of this bond. A good match to a known value for [Me₄N][ClSeO₂] is found, which fits to the Raman spectroscopic evidence for this long-known salt and to data measured on solutions of the anion in CH₃CN. The assignment of the experimental Raman spectrum was corrected by means of the DFT-computed vibrational spectrum, confirming the strong mixing of the symmetry coordinate of the Se-Cl stretch with both ν₂ and ν₄ modes.