Spin State Chemistry : Modulation of Ligand pKa by Spin State Switching in a [2´2] Iron(II) Grid-Type Complex Article - Juin 2018

Sébastien Dhers, Abhishake Mondal, David Aguilà, Juan Ramirez, Sergi Vela, Pierre Dechambenoit, Mathieu Rouzieres, Jonathan Nitschke, Rodolphe Clérac, Jean-Marie Lehn

Sébastien Dhers, Abhishake Mondal, David Aguilà, Juan Ramirez, Sergi Vela, Pierre Dechambenoit, Mathieu Rouzieres, Jonathan Nitschke, Rodolphe Clérac, Jean-Marie Lehn, « Spin State Chemistry : Modulation of Ligand pKa by Spin State Switching in a [2´2] Iron(II) Grid-Type Complex  », Journal of the American Chemical Society, juin 2018, pp. 8218 - 8227. ISSN 0002-7863

Abstract

The iron(II) [2´2] grid complex Fe-8H has been synthesized and characterized. It undergoes spin-crossover (SCO) upon deprotonation of the hydrazine-based terpyridine-like ligand. The deprotonation patterns have been determined by X-ray crystallography and 1H NMR spectroscopy and discussed in relation to the spin state of the iron(II) centers which influences greatly the pKa of the ligand. The synthesis of the magnetically-silent zinc(II) analogue is also reported and its (de)protonation behavior has been characterized to serve as reference for the study of the Fe II grid complexes. DFT computations have also been performed in order to investigate how the successive deproto-nation of the bridging ligands affects the SCO behavior within the grid. n INTRODUCTION The search for materials having tunable properties is a very active research area. 1 In this context, special attention has been given to the modulation of magnetic properties by physical means such as temperature, 2 pressure and light, 2,3 or through chemical processes affecting the ligand field in complexes. 4 Spin crossover (SCO) complexes are interesting candidates, particularly iron(II) complexes, which exhibit a switching process between a paramagnetic high spin state (HS, S = 2) and a dia-magnetic low spin state (LS, S = 0). 5 Polymetallic [2´2] grid-like complexes 6 with modulable properties can be seen as molecular precursors of metallosupramolecular architectures or materials due to their ability to generate ordered arrangements of multiple grid entities by self-assembly at the air-water interface, 7 through hydrogen-bond formation in the solid array 8 and on adsorption onto a graphite surface. 9 It has been shown that tetranuclear iron(II) [2´2] grids undergo multiple spin state switching under the action of three physical triggers, temperature, pressure and light. 2,10 The influence of hydrogen-bond donors, 11 as well as that of counterions and solvent, 12 have also been shown to allow the modulation of the magnetism of these architectures. On the other hand, [2´2] grids presenting ionizable N-H sites undergo reversible protonic modulation of optical and redox properties. 13 One thus expects that the magnetic properties of such entities may also be modulated by reversible ligand deprotonation without destruction of the initial complex. 2,13 The hydrazine-based ditopic isomeric ligand H2L (Scheme 1) offers the opportunity to both generate [2´2] grid architectures by self-assembly and to study the protonic modulation of their physico-chemical properties, due to their ionizable N-H sites. The final step of the synthesis of such a ditopic ligand consists in the condensation reaction of one equivalent of 4,6-bis(hydrazino)-2-phenyl-pyrimidine with two equivalents of 2-pyri-dine-carboxaldehyde to yield ligand H2L. This ligand contains two com-plexation subunits of terpyridine (terpy) type, where the hydrazone function acts as an ionizable group whose acidity is greatly enhanced on complexation, as compared to the free ligands. Scheme 1. Synthesis of the ditopic ionizable hydrazine-based ligand H2L and its self-assembly into the corresponding tetranu-clear [Fe4(H2L)4] 8+ [2´2] grid complex by coordination with Fe II cations. Red spheres represent Fe metal ions. Hydrogen atoms that are colored in blue emphasize the deprotonation sites on the [2´2] grid. ABSTRACT : When immersed in solutions containing Cu(II) cations, the microporous metal−organic material P11 ([Cd 4 (BPT) 4 ]·[Cd(C 44 H 36 N 8)(S)]·[S], BPT = bi-phenyl-3,4′,5-tricarboxylate) undergoes a transformation of its [Cd 2 (COO) 6 ] 2− molecular building blocks (MBBs) into novel tetranuclear [Cu 4 X 2 (COO) 6 (S) 2 ] MBBs to form P11-Cu. The transformation occurs in single-crystal to single-crystal fashion, and its stepwise mechanism was studied by varying the Cd 2+ /Cu 2+ ratio of the solution in which crystals of P11 were immersed. P11-16/1 (Cd in framework retained, Cd in encapsulated porphyrins exchanged) and other intermediate phases were thereby isolated and structurally characterized. P11-16/1 and P11-Cu retain the microporosity of P11, and the relatively larger MBBs in P11-Cu permit a 20% unit cell expansion and afford a higher surface area and a larger pore size. P orous metal−organic materials (MOMs) that incorporate

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