Bis-meridional Fe2+ spincrossover complexes of phenyl and pyridyl substituted 2-(pyridin-2-yl)-1,10-phenanthrolinesElectronic supplementary information (ESI) available. CCDC 1549553-1549556. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c7dt02320k

A series of Fe 2+ 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- 1 H 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. Fe 2+ spincrossover complexes [Fe( L ) 2 ] 2+ (L = substituted (pyridin-2-yl)-1,10-phenanthroline) were prepared and SCO with changing coordination numbers was identified by 1 H NMR spectroscopy and in silico modeling.