Abstract
Polyiodide networks are currently of great practical interest for the preparation of new electronic materials. The participation of metals in the formation of these networks is believed to improve their mechanical performance and thermal stability. Here we report the results on the construction of polyiodide networks obtained using Cu(II) complexes of a series of pyridinol-based tetraazacyclophanes as countercations. The assembly of these crystalline polyiodides takes place from aqueous solutions on the basis of similar structural elements, the [CuL]2+ and [Cu(H–1L)]+ (L = L2, L2-Me, L2-Me3) complex cations, so that the peculiarities induced by the increase of N-methylation of ligands, the structural variable of ligands, can be highlighted. First, solution equilibria involving ligands and complexes were analyzed (potentiometry, NMR, UV–vis, ITC). Then, the appropriate conditions could be selected to prepare polyiodides based on the above complex cations. Single-crystal XRD analysis showed that the coordination of pyridinol units to two metal ions is a prime feature of these ligands, leading to polymeric coordination chains of general formula {[Cu(H–1L)]}nn+ (L = L2-Me, L2-Me3). In the presence of the I–/I2 couple, the polymerization tendency stops with the formation of [(CuL)(CuH–1L)]3+ (L = L2-Me, L2-Me3) dimers which are surrounded by polyiodide networks. Moreover, coordination of the pyridinol group to two metal ions transforms the surface charge of the ring from negative to markedly positive, generating a suitable environment for the assembly of polyiodide anions, while N-methylation shifts the directional control of the assembly from H-bonds to I···I interactions. In fact, an extended concatenation of iodine atoms occurs around the complex dimeric cations, the supramolecular I···I interactions become shorter and shorter, fading into stronger forces dominated by the orbital overlap, which is promising for effective electronic materials.
Highlights
Small polyazacycloalkanes have aroused a great interest since the earliest years of macrocyclic chemistry, as the convergent arrangement of their donor atoms allows for strong complexation of many transition-metal ions
The presence of the ionizable hydroxyl group on the pyridine ring led to the involvement of the pyridinol group in the coordination to two metal ions
This does not occur in solution, where only Cu(II) complexes with 1:1 metal:ligand stoichiometry are formed, while, in the solid state, bridging coordination of the hydroxyl group gives rise to polymeric coordination chains of L2-Me, L2-Me3)
Summary
Small polyazacycloalkanes have aroused a great interest since the earliest years of macrocyclic chemistry, as the convergent arrangement of their donor atoms allows for strong complexation of many transition-metal ions. The significant stability of the protonated center B can be related to its intramolecular hydrogen-bond interaction with the nitrogen atom of the pyridinol moiety in its ketone form (Figure 4). Assisted OH···O− H-bonds involving their protonated and deprotonated pyridinol oxygen atoms (2.59(1) Å for O1···O2− Oin4[−(CinuL[2(-CMueL)2(C-MuHe)−(1CLu2H-M−1eL)2I3-]M2+e)aIn]d2+)2..6I0n(1[)(CÅufLo2r -MO3e·)·-· (CuH−1L2-Me)I]2+, adjacent cations form additional NH···I− H-bonds with the coordinated I28 anions (Figure 9a, N16··· I28, 3.53(1) Å; N14···I28, 3.61(1) Å) By this way, chains of equivalent binuclear complexes develop along the b axis, which are completely surrounded by an intricate network of polyiodide anions and iodine molecules. Despite the presence of an iodine molecule occupying the sixth position of a distorted octahedron (Figure 8c), the [(CuL2-Me3)(CuH−1L2-Me3)I]2+ dinuclear cations form chains developing along the b axis, where protonated and deprotonated pyridinol oxygen atoms from neighboring units give strong OH···O− H-bonds (Figure 11a). Tails of the 400/411 nm bands extend toward longer wavelengths in accordance with the presence of lower energy absorption components expected for polyiodides higher than I3−, which are more clearly evidenced by the shoulder emerging at about 600 nm in the spectrum of 4
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