Challenges in Engineering Spin Crossover: Structures and Magnetic Properties of Six Alcohol Solvates of Iron(II) Tris(2‐picolylamine) Dichloride

Abstract

"Structure determines properties", this assertion is a central paradigm in materials research as well as in molecular biology. It is a prime reason for the popularity of the notion of crystal engineering, whose goal it is to develop novel materials with predefined and tuned properties.[1] Such engineering necessarily proceeds in two steps: 1) establishing empirical or theoretical relationships between solid-state structure and desired properties, 2) tailoring of the structure by a synthesis process.[2] The property of main interest in spincrossover compounds is the thermal evolution of magnetic behavior. Herein we report on a serendipitous observation derived from magnetic and structural studies on six different alcohol solvates of iron(ii) tris(2-picolylamine) dichloride (1), in which the size of the solvating molecules ranges from methanol to tert-butyl alcohol. The six crystal structures are either isostructural or polytypic. In spite of evident structural similarity, the spin-transition curves are very different.

Structural Determination of a Short-Lived Excited Iron(II) Complex by Picosecond X-Ray Absorption Spectroscopy

“…The R Fe-N values of 0:2 A extracted from both the XANES and the EXAFS are in very good agreement with the LS-to-HS bond elongation from DFT calculations on Fe II bpy 3 2 [35], and with the values derived for other Fe(II)-based SCO complexes using time resolved [22] and static structural methods [9][10][11][12][13][14][15][16][17][18]20,21]. That the bond elongation is nearly the same for all complexes, despite their largely different HS lifetimes, confirms the fact that the driving force for the HS-LS relaxation is determined by the energetics of the HS state, rather than its structure [36].…”

“…Schematic potential energy curves of Fe(II)-SCO complexes as a function of the Fe-N bond distance, and pathways of nonradiative relaxation leading to the short-lived quintet state (0.6 ns for Fe II bpy 3 2 ), upon excitation of the singlet MLCT state in the UV. [9][10][11][12][13][14][15][16][17][18][19][20][21]. Recently, Fe K-edge steady-state XAS of the LS Fe tren py 3 PF 6 2 and its HS analogue Fe tren 6-Me-py 3 PF 6 2 were combined to interpret the picosecond transient spectra of the photoactivated Fe tren py 3 PF 6 2 complex, showing good agreement with the anticipated result for the HS complex [22].…”

Improved Cooperativity of Spin-Labile Iron(III) Centers by Self-Assembly in Solution

“…This is predominantly because spin transitions are crucially affected by intermolecular interactions beyond crystal packing effects, such as elastic strain. 2 Such intermolecular interactions are the classical domain to be engineered by supramolecular chemistry. 3 Specifically, the promotion of close metal-metal interactions may stimulate cooperative behavior, which is expected to result in more abrupt spin transitions.…”

Spin-Crossover Behavior of Hofmann-Type-Like Complex Fe(4,4’-bipyridine)Ni(CN)4·nH2O Depending on Guest Species

“…Spin-crossover phenomena depending on guest inclusion are already known in several Fe II complexes [54][55][56][57][58][59][60][61][62][63][64]. Among them, this case, Fe II (bpy)Ni II (CN) 4¨n H 2 O(n = 2.5), is unique in the simultaneity of vapochromism and spin transition.…”