Structurally Characterised New Twisted Conformer for Cyclen, Controlled by Metal ion Complexation as Seen in NiII and CuII Complexes with halides and pseudohalides

Serendipitous self-assembly is a potential approach in achieving structurally versatile and unexpected solids with chosen metal-ligand combinations. Cyclen (1,4,7,10-tetraazacyclododecane) is an interesting flexible molecule, well explored for biological and material applications of its metal complexes. The dihedral angle (θ) between four nitrogen atoms of free cyclen is close to 0°. Still, it may vary upon complexation, allowing flexibility to adopt any two conformers, viz., (i) planar and (ii) twisted. The present work establishes the crucial role of metal ion in controlling the conformation adopted by cyclen through metal complexation. SCXRD studies of complexes of cyclen with CuII and NiII with halides and pseudohalides as counter ions reveal the existence of a planar conformation (θ = ~0-3°) for cyclen in CuII systems and a highly twisted conformation (hitherto unreported) in NiII systems (θ = ~52-55°). Further, SQUID magnetic studies for the dimeric [Ni1] complex reveal antiferromagnetic interactions between the two Ni centres.