Multifunctional Magnetic Molecular {[Mn^II(urea)₂(H₂O)]₂[Nb^IV(CN)₈]}_n System: Magnetization-Induced SHG in the Chiral Polymorph

Incorporation of urea molecules into the magnetic system Mn II -[Nb IV (CN) 8 ] results in formation of two magnetic polymorphs {[Mn II (H 2 O)(urea) 2 ] 2 [Nb IV (CN) 8 ]} n chiral α and centric β crystallizing in space groups P 4 1 and P 1̅, respectively. Both polymorphs exhibit extremely similar structures and are soft ferrimagnets with critical temperatures of 43 and 42 K, respectively. The presence of coordinated urea in the noncentrosymmetric α produces NLO functionality, second harmonic generation (SHG), whereas centric β is just another molecular magnet. As a result of interaction of both functionalities below magnetic ordering temperature, the magnetization-induced SHG (MSHG) is observed for polymorph α. The observed MSHG/SHG signal ratio was the largest among molecule-based noncentrosymmetric magnets. The presence of urea molecules in both polymorphs gives also rise to their interesting topotactic reactivity. Both polymorphs can be easily transformed into a new entity {[Mn II (H 2 O)] 2 [Nb IV (CN) 8 ]·2.5MeOH} n 2b with significant shift of critical temperature T c to 70 K. Subsequent hydration of 2b leads to {[Mn II (H 2 O) 2 ] 2 [Nb IV (CN) 8 ]·4H 2 O} n 3 with T c of 47 K.