Topotactic Transformation of the Transition-Metal Cyanamide Li2MnSn2(NCN)6

The reversible (de)intercalation of Li from Li₂MnSn₂(NCN)₆ has been demonstrated using NO₂BF₄ and n-BuLi as chemical oxidation and reduction agents, respectively. These reactions are topotactic with deintercalated Li_2–xMnSn₂(NCN)₆ (x = 0–1.7) retaining the $P3̅1m$ structure as the NCN^2– scaffold tilts such as to accommodate the introduction of Li vacancies, together with a concomitant contraction of the Mn–N bond length while Mn²⁺ is oxidized toward Mn⁴⁺. This oxidation is also confirmed from SQUID magnetometry which evidences a decrease in the paramagnetic moment and, also, from XANES measurements showing a clear shift of the Mn K-edge position to higher energies upon Li deintercalation. The tentative application of Li₂MnSn₂(NCN)₆ in Li-ion battery materials was also investigated using electrochemical methods in an organic electrolyte. Cyclic voltammetry measurements reveal two pairs of anodic and cathodic peaks which are assigned to Mn²⁺/Mn³⁺ and the Mn³⁺/Mn⁴⁺ redox-pairs. These results provide the first experimental indication toward the as yet untapped intercalation chemistry of transition-metal carbodiimides.