A New Kinetically Preferable Polymorph of 1-(4′-Cyanobenzyl)pyridinium bis(maleonitriledithiolato)nickelate with Spin-Peierls-type Transition

A new kinetically preferable polymorph of 1-(4′-cyanobenzyl)pyridinium bis(maleonitriledithiolato)nickelate (β-[CN-Py][Ni(mnt)2]) was achieved and characterized by element microanalysis for C, H, and N elements, powder X-ray diffraction (PXRD), and differential scanning calorimetry (DSC) techniques. β-[CN-Py][Ni(mnt)2] shows a magnetostructural phase transition together with a thermal anomaly at ∼192 K. In the high-temperature phase (HTP), the β-[CN-Py][Ni(mnt)2] crystal belongs to monoclinic space group P21/c, and its asymmetric unit is comprised of one pair of CN-Py+ and [Ni(mnt)2]- ions. Both CN-Py+ and [Ni(mnt)2]- ions form uniform columnar stacks, respectively. In the low-temperature phase (LTP), the space group of the crystal is lowered to triclinic P1̄, its asymmetric unit changes into two pairs of anions and cations, and moreover, both stacks of CN-Py+ and [Ni(mnt)2]- ions are dimerized. β-[CN-Py][Ni(mnt)2] features an S = 1/2 Heisenberg regular antiferromagnetic (AFM) linear chain system with |J| = 86(9) K in HTP, while a spin gap is opened with Δ= 568 K in LTP. The magnetic phase transition in β-[CN-Py][Ni(mnt)2] shows the spin-Peierls transition characters from the changes of both crystal structure and magnetic nature.