The Effect of Ligand Design on Metal Ion Spin State—Lessons from Spin Crossover Complexes

Abstract

Abstract:The relationship between chemical structure and spin state in a transition metal complex has an important bearing on mechanistic bioinorganic chemistry, catalysis by base metals, and the design of spin crossover materials. The latter provide an ideal testbed for this question, since small changes in spin state energetics can be easily detected from shifts in the spin crossover equilibrium temperature. Published structure-function relationships relating ligand design and spin state from the spin crossover literature give varied results. A sterically crowded ligand sphere favors the expanded metal-ligand bonds associated with the high-spin state. However, steric clashes at the molecular periphery can stabilize either the high-spin or the low-spin state in a predictable way, depending on their effect on ligand conformation. In the absence of steric influences, the picture is less clear since electron-withdrawing ligand substituents are reported to favor the low-spin or the high-spin state in different series of compounds. A recent study has shed light on this conundrum, showing that the electronic influence of a substituent on a coordinated metal ion depends on its position on the ligand framework. Finally, hydrogen bonding to complexes containing peripheral N-H groups consistently stabilizes the low-spin state, where this has been quantified.

Towards the Molecular Design of Spin‐Crossover Complexes of 2,6‐Bis(pyrazol‐3‐yl)pyridines

“…This spin‐crossover (SCO) phenomenon is often observed for the iron(II) ion in octahedral N 6 coordination environments, where the switching occurs between differently coloured diamagnetic low‐spin (LS, S =0) and paramagnetic high‐spin (HS, S =2) states . Tailoring it for the above applications relies on proper chemical modifications of the ligands identified mostly through systematic screening of solutions (by NMR spectroscopy that is behind the popular Evans method) of closely related iron(II) complexes with bi‐ and tridentate or higher denticity ligands. The resulting structure‐function relationships, which are behind a successful ‘truly molecular’ design of SCO compounds, established the stabilization of the HS state by bulky groups close to the donor nitrogen atoms, with a less clear‐cut role of subtle electronic or other effects of more remote substituents .…”

“…Tailoring it for the above applications relies on proper chemical modifications of the ligands identified mostly through systematic screening of solutions (by NMR spectroscopy that is behind the popular Evans method) of closely related iron(II) complexes with bi‐ and tridentate or higher denticity ligands. The resulting structure‐function relationships, which are behind a successful ‘truly molecular’ design of SCO compounds, established the stabilization of the HS state by bulky groups close to the donor nitrogen atoms, with a less clear‐cut role of subtle electronic or other effects of more remote substituents . The contradictory electronic effects have been recently reconciled in a solution‐state study of 2,6‐bis(pyrazol‐1‐yl)pyridines (1‐bpp; Scheme ), which are among the most popular ligands to induce an SCO behaviour in iron(II) complexes, and to control it with a judicious choice of substituents .…”

Switching Magnetic Properties by a Mechanical Motion

“…Apart from temperature and light, chemical [25], electrical [26], or pressure [27] stimuli have also been used to induce a spin crossover. Several stimuli have been used to trigger magnetic properties changes [3] in three main areas: (i) changing the spin state of a transition metal in spin crossover (SCO) systems [4][5][6][7][8][9][10][11]; (ii) switching the exchange interaction between different spin carriers [12,13]; and (iii) switching single-molecule magnet (SMM) properties [14,15]. Spin-crossover complexes [16][17][18][19][20] are particularly interesting for such purpose, as they can exist in two stable spin states and have been largely studied.…”

Interaction Channels Between Perfluorinated Iron Phthalocyanine and Cu(111)

“…Different interaction channels are known for phthalocyanines, involving both the central metal atom and the macrocycle – in many cases the charge transfer is expected to be bidirectional . Due to its flexibility in the spin ground state, iron complexes might be of particular interest in the view of electronic properties . Compared to other metal substrates, less attention was paid to iron phtalocyanines on copper substrates .…”