The influence of ligands on the spin state of a metal ion is of central importance for bioinorganic chemistry, and the production of base‐metal catalysts for synthesis applications. Complexes derived from [Fe(bpp)2]2+ (bpp=2,6‐di{pyrazol‐1‐yl}pyridine) can be high‐spin, low‐spin, or spin‐crossover (SCO) active depending on the ligand substituents. Plots of the SCO midpoint temperature (T 1/2 ) in solution vs. the relevant Hammett parameter show that the low‐spin state of the complex is stabilized by electron‐withdrawing pyridyl (“X”) substituents, but also by electron‐donating pyrazolyl (“Y”) substituents. Moreover, when a subset of complexes with halogeno X or Y substituents is considered, the two sets of compounds instead show identical trends of a small reduction in T 1/2 for increasing substituent electronegativity. DFT calculations reproduce these disparate trends, which arise from competing influences of pyridyl and pyrazolyl ligand substituents on Fe‐L σ and π bonding.

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Chlorination of 2,6‐bis(pyrazol‐1‐yl)pyrazine (bppz) with NaClO in acetic acid afforded 2,6‐bis(4‐chloropyrazol‐1‐yl)pyrazine (L2Cl). 2,6‐Bis(4‐bromopyrazol‐1‐yl)pyrazine (L2Br), 2,6‐bis(4‐iodopyrazol‐1‐yl)pyrazine (L2I), 2,6‐bis(4‐methylpyrazol‐1‐yl)pyrazine (L2Me), and 2,6‐bis(4‐nitropyrazol‐1‐yl)pyrazine (L2NO2) were also prepared by reactions of the preformed 4‐substituted pyrazoles with 2,6‐dichloropyrazine. The reduction of L2NO2 with iron powder gave 2,6‐bis(4‐aminopyrazol‐1‐yl)pyrazine (L2NH2) and L2I was converted into 2,6‐bis[4‐(phenylethynyl)pyrazol‐1‐yl]pyrazine (L2CCPh) by a Sonogashira coupling reaction. The salts [Fe(L2Me)2]X2 (X– = BF4– and ClO4–) underwent thermal spin‐crossover abruptly at around 200 K in one and two steps, respectively. The [Fe(L2Me)2]X2 salts exhibited different light‐induced excited spin‐state trapping (LIESST) behavior; the BF4– salt behaves classically [T(LIESST) = 93 K], but the ClO4– salt undergoes a multistep LIESST relaxation. In contrast, solid [Fe(L2Cl)2][BF4]2 adopts a fixed 2:1 high/low‐spin‐state population that does not change with temperature below 300 K, whereas [Fe(L2Br)2][BF4]2 and [Fe(L2I)2][BF4]2 form low‐spin solvated crystals that are transformed into high‐spin powders on drying. The pyrazinyl group in the L2R ligands slightly stabilizes the low‐spin state of the complexes, as determined by solution‐phase magnetic measurements. The crystal structure of [Fe(L2CCPh)(OH2)z][BF4]2 contains a disordered mixture of six‐ (z = 3) and seven‐coordinate (z = 4) iron centers.

The influence of ligands on the spin state of ametal ion is of central importance for bioinorganic chemistry,and the production of base-metal catalysts for synthesis applications. Complexes derived from [Fe(bpp) 2 ] 2+ (bpp = 2,6-di{pyrazol-1-yl}pyridine) can be high-spin, low-spin, or spin-crossover (SCO) active depending on the ligand substituents.Plots of the SCO midpoint temperature (T1 = 2 )i ns olution vs.t he relevant Hammett parameter showt hat the low-spin state of the complex is stabilized by electron-withdrawing pyridyl ("X") substituents,b ut also by electron-donating pyrazolyl ("Y") substituents.M oreover,w hen as ubset of complexes with halogeno Xo rYs ubstituents is considered, the two sets of compounds instead show identical trends of asmall reduction in T1 = 2 for increasing substituent electronegativity.D FT calculations reproduce these disparate trends,w hich arise from competing influences of pyridyl and pyrazolyl ligand substituents on Fe-L s and p bonding.The ability of first-row transition ions to adopt different spin states in strong or weak ligand fields is of great importance to their catalysis and reactivity. [1][2][3] Fore xample,f undamental mechanistic steps in biological and synthetic oxidation catalysis involve ac hange in spin state at an iron catalyst center, described as two-state reactivity.[3] Catalysts with different resting spin states follow different pathways through these two-state processes,l eading to altered reactivity and product distributions.[4] Similar considerations also apply for "base-metal" catalysts for organometallic reactions, [5] which give access to high-spin active species with different reactivity patterns compared to conventional precious-metal catalysts. [6,7] Another consequence of spin-state dichotomy is the phenomenon of spin crossover (SCO), where am olecular or framework compound exhibits atransition between high-and low-spin states under aphysical stimulus. [8,9] SCO compounds have been developed into versatile molecular switches for molecular materials chemistry and nanoscience. [9,10] Ther elationship between chemical structure and spin state is central to these phenomena. [2,11] Asterically crowded ligand sphere generally leads to high-spin complexes. [12] However,t he effect of ligand electronic character on metalion spin state is less clear-cut, with electron-withdrawing substituents being reported to stabilize either the lowspin [13][14][15][16] or the high-spin state [17,18] in different series of compounds.W hile the literature includes data from solution and the solid-state,t hese effects are best quantified by solution measurements which determine ac omplexs spin state in the absence of crystal-packing effects or any other influences from ar igid solid lattice. [19] We report herein ac omprehensive study to resolve this contradiction, through as urvey of twenty-five complexes from the [Fe(bppfamily (bpp X,Y = a2 ,6-di(pyrazol-1-yl)pyridine derivative; Scheme 1).[20] Our results show that substituents at the X and Ysites ...