Bis-meridional Fe²⁺ spincrossover complexes of phenyl and pyridyl substituted 2-(pyridin-2-yl)-1,10-phenanthrolines

A series of Fe²⁺ SCO complexes of substituted 2-(pyridin-2-yl)-1,10-phenanthrolines 2 was prepared and the SCO (spincrossover) properties were characterized in the solid state (X-ray crystallography, SQUID magnetometry) and in solution (VT-¹H NMR spectroscopy), augmented by theoretical modelling. Bis-meridional coordination of the tridendate 2a-c and tetradentate 2d ligands gives octahedral and distorted trigonal-dodecahedral complexes [Fe(2)₂]²⁺, respectively, which were identified as SCO complexes with the transition temperature T_1/2 below room temperature. SCO in the solid state is limited to bromo-substituted [Fe(2a)₂]²⁺ (Dalton Trans., 2017, 46, 6218-6229) and [Fe(2b)₂]²⁺ with a pyridine-appended phenyl group, whereas solution state NMR studies reveal SCO behaviour for all complexes, which is in agreement with DFT derived results. As anticipated from its N6(+2) coordination in the HS state, DFT structure modelling of [Fe(2d)₂]²⁺ identified deviation from a structure-conserving SCO reaction coordinate; that is, Fe-N breathing is accompanied by a change in the coordination number. Accordingly, a remarkably slow SCO is observed in [Fe(2d)₂]²⁺, owing to an extended coordinate. De-novo defined characteristic temperatures T(τ_HSLS) are introduced as structure-dependent parameters deemed to define the onset of phenomenological "slow" SCO. The rich phenomenology of the NMR spectra of [Fe(2)₂]²⁺ is identified to be largely controlled by the dynamics of spin-state exchange and a qualitative illustration of the NMR-reporters of SCO is suggested.