# Solid-State Transformation (Stotal = 0, 1, and 2) in a Ni2+ Chelate with Two tert-Butyl 5-(p-Biphenylyl)-2-pyridyl Nitroxides

## TL;DR

This paper reports the synthesis and characterization of a new nickel(II) complex with unique spin-state transformation properties observed at low temperatures.

## Contribution

The study introduces a novel nickel(II) complex exhibiting a three-state spin transition mechanism supported by experimental and computational evidence.

## Key findings

- The complex shows a large torsion angle change of 36° and 37° upon cooling, the largest reported for similar compounds.
- Magnetic measurements reveal gradual spin transition-like behavior around 143 K with a non-zero χmT plateau.
- DFT calculations confirm the proposed three-state model involving S_total = 0, 1, and 2.

## Abstract

A novel S = 1/2 paramagnetic chelating ligand tert-butyl 5-(p-biphenylyl)-2-pyridyl nitroxide (bppyNO) and its S = 1 nickel(II) ion complex [Ni(bppyNO)2Br2] were synthesized. X-ray crystallography revealed a 2p–3d–2p heterospin triad, with half of the molecule being crystallographically independent. A relatively planar chelate geometry with the torsion angle ϕ(Ni-O-N-C2py) = −10.6(5)° at 300 K becomes significantly out-of-plane distorted with ϕ = −46.9(8) and 26.1(11)° at 90 K accompanied by disorder at the oxygen site. The torsion angle changes, Δϕ = 36° and 37°, are among the largest reported for related compounds. Magnetic measurements indicate gradual and incomplete spin transition-like behavior around 143(2) K. A three-state model involving an intermediate-spin (Stotal = 1) state is proposed to explain non-zero χmT plateau in a low-temperature region. Density functional theory calculations using the determined structures support the proposed mechanism. Furthermore, geometry optimizations assuming Stotal = 0, 1, and 2 are in good agreement with the present model.