Spin Interactions and Magnetic Anisotropy in a Triangular Nickel(II) Complex with Triaminoguanidine Ligand Framework

The trinuclear nickel(II) complex [Ni 3 (saltag t Bu )(bpy) 3 (H 2 O) 3 ]Cl (H 5 saltag t Bu = 1,2,3‐tris[(5‐ tert ‐butylsalicylidene)amino]guanidine) was synthesized and characterized by experimental as well as theoretical methods. The complex salt crystallizes with three molecules of dimethylformamide (dmf) and water as [Ni 3 (saltag t Bu )(bpy) 3 (H 2 O) 3 ]Cl · 3dmf · 3H 2 O ( 1 ) in the trigonal space group P 3 , with the complex located on a threefold rotation axis, which is consistent with the molecular C 3 symmetry of the complex cation. Magnetic measurements reveal an antiferromagnetic coupling ( J = –35.9 cm –1 ) between the nickel(II) ions leading to a diamagnetic ground state for the trinuclear complex cation. Theoretical investigations based on broken‐symmetry DFT confirm the antiferromagnetic exchange within the complex cation of 1 . Additional single‐ion CASSCF ab initio studies reveal that magnetic anisotropy is present in the system. The experimental and theoretical results for 1 are compared with those of a structurally similar nickel(II) complex that is based on the bromo‐substituted derivative of the triaminoguanidine ligand. The differences in their magnetic properties can be attributed to the stronger elongation of the pseudo‐octahedral coordination sphere at the nickel(II) centers in case of 1 . The analysis of the magnetic properties of 1 clearly shows that for such exchange coupled systems reliable parameters for the magnetic anisotropy cannot be extracted from experimental data alone.image John Wiley & Sons, Ltd

Photonic Effects on the Radiative Decay Rate and Luminescence Quantum Yield of Doped Nanocrystals

Nanocrystals (NCs) doped with luminescent ions form an emerging class of materials. In contrast to excitonic transitions in semiconductor NCs, the optical transitions are localized and not affected by quantum confinement. The radiative decay rates of the dopant emission in NCs are nevertheless different from their bulk analogues due to photonic effects, and also the luminescence quantum yield (QY, important for applications) is affected. In the past, different theoretical models have been proposed to describe the photonic effects for dopant emission in NCs, with little experimental validation. In this work we investigate the photonic effects on the radiative decay rate of luminescent doped NCs using 4 nm LaPO4 NCs doped with Ce3þ or Tb3þ ions in different refractive index solvents and bulk crystals. We demonstrate that the measured influence of the refractive index on the radiative decay rate of the Ce3þ emission, having near unity QY, is in excellent agreement with the theoretical nanocrystal-cavity model. Furthermore, we show how the nanocrystal-cavity model can be used to quantify the nonunity QY of Tb3þ-doped LaPO4 NCs and demonstrate that, as a general rule, the QY is higher in media with higher refractive index

(Di)-aminoguanidine Functionalization through Transamination: An Avenue to an Auspicious Class of Supramolecular Synthons

N-4H-1,2,4-Triazol-4-yl-guanidine hydrochloride (L1) and a heterocyclized 3,4-diamino-4H-1,2,4-triazole hydrochloride (L2) were synthesized in good yields thanks to a transamination reaction of aminoguanidine and diaminoguanidine hydrochlorides, whose mechanism is presented. L1 and L2 crystallize in monoclinic (P21/a) and triclinic (P1̅) space groups, respectively. Hydrogen bonding interactions dictated by H-donor, acceptor rich ligand framework together with lattice chloride ions in L1 and L2 organize the crystal packing into a three-dimensional (3D) supramolecular network. The crystal structure of L1 is the first one for a 1,2,4-triazole ligand constructed from aminoguanidine. Both L1 and L2 that exist in monocationic form were introduced as supramolecular synthons to prepare Zn(II) complexes. Scanning electron microscopy (SEM) images on a bulk sample of [Zn(L1)2Cl2]Cl2·H2O (1) reveals a spontaneous aggregation of porous balls of coccolith morphology with surface decorated sickle-shaped particles of 100 nm thickness, whereas a powder sample of [Zn(L2)2Cl2]·H2O (2) shows neat rectangular blocks of 470 nm thickness. Colorless crystals of 2 crystallize in a monoclinic space group (C2/c). Zn(II) ions bound chloride with NH6B···Cl(1) = 2.495(2) Å and N···H type bonding (N6···H3C3 = 2.743(2) Å; N2···H7N7 = 1.989(2) Å) affording a 3D supramolecular network. The degree of pyramidality, ΣN = 38°, observed at one of the terminal amines in L2 is still retained in 2, but “flipping” of orientation of attached protons due to directional H-bonding is observed too. Porosity partitioning by mercury porosimetry measurements on 1 and 2 reveals interparticular porosities between 7 and 15 μm. N2(g) and H2(g) gas adsorption capacities of 1 and 2 were measured by BET which shows no preference for N2(g) but a low irreversible H2(g) uptake of 3 cm3/g