Nanoscale Coordination Polymer of Dimanganese(II) as Infinite, Flexible Nanosheets with Photo‐Switchable Morphology

Abstract

An original co‐ligand association, consisting of 4,4'‐azopyridine and 1,3‐bis(carboxypropyl)tetramethyldisiloxane, is used for complexing MnII, to obtain a 2D dinuclear coordination array having infinite horizontal dimensions and nanometric thickness. Due to the shielding effect of the tetramethyldisiloxane units facing outwards on both sides, the sheets are free of any chemical or physical intermolecular interactions, as crystallographic analysis reveals. The compound shows two glass transitions, at –51 and 33 °C, and begins to decompose at 292 °C. The co‐existence in the structure of the highly hydrophobic tetramethyldisiloxane spacers and the polar metal complexed units confers amphiphilic character to the compound, making it capable of reducing the surface tension of the DMF, from 35.8 to 29.4 mN/m, at a critical micellar concentration around 1 wt.‐% complex. The presence of aggregates in solution is evidenced by dynamic light scattering. The light sensitive azopyridine units make the compound photo‐responsive. This behavior occurs both in solution as well as in the film, where the compound self‐assembles in micellar morphology when UV‐irradiated. Both conductivity and dielectric permittivity of the film increase as a result of trans–cis configuration. Magnetic measurement data indicate two antiferromagnetic coupled spins, S = 5/2, that are able to interact with small external magnetic field, i.e., during MFM scanning.

From Amorphous Silicones to Si-Containing Highly Ordered Polymers: Some Romanian Contributions in the Field

“…However, highly ordered, crystalline polymeric covalent structures, where siloxane motifs alternate with coordinated metal blocks, were obtained with carboxylic ligands, 1,3-bis(carboxypropyl)tetramethyldisiloxane (H 2 CX) prepared for the first time in 1999 [ 15 ], bis(p-carboxyphenyl)diphenylsilane (H 2 L 1 ), and bis(3,4-dicarboxyphenyl)dimethylsilane (H 4 L 2 ) were prepared later [ 126 ] ( Figure 9 a). They were used alone to directly coordinate the metal ions [ 127 , 128 , 129 ] or complexes of the latter with vacancies [ 55 , 99 , 130 , 131 ] or in combination with ancillary organic ligands (imidazole [ 132 , 133 ], 4,4′-bipyridyl [ 134 ], 4,4′-azopyridine [ 56 , 132 , 135 ], 1,2-di(4-pyridyl)ethylene [ 134 ], 1,10-phenanthroline [ 126 ], etc.) ( Figure 9 b) and metal ions or clusters, which allow highly directional intermolecular interactions (metal-ligand, hydrogen bonds and π-π stacking interactions).…”

“…The flexibility of this segment allows the adoption of both cis - [ 133 , 135 ], and trans -oide [ 55 , 133 , 135 ] conformations, in dependence on the coordination geometry of the secondary building units (SBUs) and H-bonding configuration. It was observed that while the complexation of metals with silicone ligands almost always leads to 1D structures [ 55 , 99 , 128 , 131 , 132 , 134 ], in the presence of an organic co-ligand 2D structures are often formed [ 56 , 127 , 132 , 133 , 134 , 135 ]. The 2D structures built with the H 2 Cx ligand are of interest from the perspective of 2D materials, presenting as hydrophobic nano-sheets with very weak interactions between them [ 56 ].…”

“…It was observed that while the complexation of metals with silicone ligands almost always leads to 1D structures [ 55 , 99 , 128 , 131 , 132 , 134 ], in the presence of an organic co-ligand 2D structures are often formed [ 56 , 127 , 132 , 133 , 134 , 135 ]. The 2D structures built with the H 2 Cx ligand are of interest from the perspective of 2D materials, presenting as hydrophobic nano-sheets with very weak interactions between them [ 56 ]. The carboxylate groups of the silicon-based ligands in these complexes also evidenced a versatile coordination behavior, in other words, ionic, monodentate, bidentate bridging, bidentate chelating or some combination of them.…”

“…Besides the highly ordered structure, in the obtained coordination polymers, the presence of the metal and of its coordination block induces specific properties such as optical [ 56 , 128 ], magnetic [ 55 , 56 , 126 , 127 ], electrical [ 99 ], catalytic [ 134 ], self-assembly ability in film and solution [ 55 , 99 ] or some porosity [ 126 , 127 , 135 ], not found in classical amorphous silicones. In addition, the co-existence in the structure of highly hydrophobic methylsilicone units and somewhat polar coordinated blocks gives the polymer an amphiphile character enabling it, when it is soluble, to reduce the surface tension of the solvent and to self-assemble in solution beyond its specific critical aggregation concentration.…”

“…Carboxylic ligands containing silicon in the substrate (a) and organic co-ligands (b) used; 1D coordination polymer based on: diphenylsilane-spaced dicarboxylate (H2L1 ligand) coordinated to the NiL [131] (c), tetramethyl-Figure 9. Carboxylic ligands containing silicon in the substrate (a) and organic co-ligands (b) used; 1D coordination polymer based on: diphenylsilane-spaced dicarboxylate (H 2 L 1 ligand) coordinated to the NiL[131] (c), tetramethyldisiloxane-spaced dicarboxylate (H 2 CX ligand) coordinated to the NiL preformed complex[130] (d) and to oxime-bridged Mn 6+ cluster[55] (e); Covalent 2D coordination polymers looking like nano-sheets consisting of: dimanganese coordinated by H 2 CX and 4,4'-azopyridine[56] (f); trinuclear (g) and pentanuclear (h) manganese coordinated with H 2 L 1[127].…”