Structure of the Lithium Thiocyanate-Tetramethylpropylenediamine Complex Dimer, [LiNCS·Me₂N(CH₂)₃NMe₂]₂, with Asymmetric NCS-Bridge Bonding: A New Bonding Mode for the Thiocyanate Ligand

The new complex [LiNCS·Me₂N(CH₂)₃NMe₂]₂ has been prepared in high yield by reacting BuⁿLi with a suspension of solid NH₄NCS in toluene containing the ligand Me₂N(CH₂)₃NMe₂[TMPDA]. Its crystal structure has been determined by X-ray crystallography at 153 K. Crystal data: monoclinic, space group P2₁/c, a = 10.616(2) Å, b = 13.309(3) Å, c = 16.694(3) Å, β = 101.09(3)°, Z = 4. In the structure, two complexed (TMPDA)Li⁺ units are bridged through N-bonded NCS⁻ anions. However, an unprecedented feature is that these μ₂-NCS⁻ bridges are asymmetric; each leans over toward one Li⁺ and away from the other, but each does so to different extents, so leading to four distinct LiNC angles [range 108.8(1)–167.0(2)°] and to four distinct Li-N ring distances [range 1.978(3)–2.095(3) Å]. These structural features are interpreted in terms of the supposed dimer being actually two loosely-associated monomers, each slightly distorted from the ideal linearity of a complexed LiNCS monomer. Such an interpretation is supported by the results of 6-31G basis set ab initio MO calculations on LiNCS and (LiNCS)₂ and on their complexes with NH₃; in particular, the lowest energy optimized structure of (LiNCS)₂·4NH₃, a model for the title complex, has also an asymmetric ε₂-NCS bonding mode. Solution studies (IR spectroscopic and cryoscopic) on the complex indicate extensive dissociation of the solid-state structure into separate monomers but cast doubts on the proposed correlation between v(NCS) stretching frequencies and NCS⁻ bonding modes.