Catecholase activity has been explored by combined experimental and theoretical approaches using di-copper(ii) complexes of Mannich-base ligands.

The development of contrast agents specifically designed for high‐field magnetic resonance imaging (MRI) is required because the relaxation efficiency of classic Gd(III) contrast agents significantly decreases with increasing magnetic field strengths. With an idea of exploring the unique structure of lanthanide (Ln) 15‐MC‐5 metallacrowns, we developed a series of water‐soluble Gd(III) aqua‐complexes, bearing aminohydroxamate (glycine, α‐alanine, α‐phenylalanine and α‐tyrosine) ligands, with increasing number of water molecules directly coordinated to the Gd(III) ion: Gd(H2O)4[15‐MCCu(II)Glyha‐5](Cl)3 (1(Gd)), Gd(H2O)4[15‐MCCu(II)Alaha‐5](Cl)3 (2(Gd)), Gd(H2O)3[15‐MCCu(II)Phalaha‐5](Cl)3 (3(Gd)) and Gd(H2O)3[15‐MCCu(II)Tyrha‐5](Cl)3 (4(Gd)). In these systems, the Ln(III) central ion is coordinated by five oxygen donor atoms of the ligands and three or four inner‐sphere water molecules. The X‐ray crystal structure of metallacrown Ln(H2O)3,4[15‐MCCu(II)Rha‐5]3+ agrees with density functional theory predictions. The calculations demonstrate that the exchange of coordinated water molecules can proceed easily, resulting in increased relaxivity parameters. The longitudinal relaxivities (r1) of 1(Gd)–4(Gd) in water at ultrahigh magnetic field of 9.4 T were determined to be 11.5, 14.8, 13.9 and 12.2 mM−1 s−1, respectively. The ability to increase the number of Ln(III) inner‐sphere water molecules up to four, the planar metallacrown structure and the rich hydration shell due to strong hydrogen bonds between the [15‐MC‐5] moiety and bulk water molecules provide new opportunities for potential MRI applications.

No abstract

A boraguanidinato-stabilized germylene, [(i-Pr)NB(N-2,6-MeCH)]Ge, reacts with alkynes RC[triple bond, length as m-dash]CR selectively in a 2 : 1 molar ratio to afford 3,4-R,R'-1,2-digermacyclobut-3-enes 1a-e as the products of formal [2 + 2 + 2] cyclization [R/R' = Me/Me (1a), Ph/Ph (1b), Ph/H (1c), t-Bu/H (1d) and Cy/H (1e)]. Ferrocenyl-substituted alkynes react similarly, yielding the corresponding ferrocenylated 3,4-R,R'-1,2-digermacyclobut-3-enes 2a-d [where R/R' = Fc/H (2a), Fc/Me (2b), Fc/Ph (2c), and Fc/Fc (2d); Fc = ferrocenyl]. By contrast, only one of the triple bonds available in conjugated diynes RC[triple bond, length as m-dash]CC[triple bond, length as m-dash]CR is activated with the germylene, while the second one remains intact even in the presence of an excess of the germylene. The exclusive formation of 3,4-R,(C[triple bond, length as m-dash]CR)-1,2-digermacyclobut-3-enes 3a-c [R = Ph (3a), t-Bu(3b), and Fc (3c)] was ascribed to a steric repulsion around the second triple bond. On the other hand, the reaction of the germylene with more flexible dialkyne fc(C[triple bond, length as m-dash]CPh) (fc = ferrocene-1,1'-diyl) proceeded in the expected manner, producing compound 4, where both triple bonds are transformed into 1,2-digermacyclobut-3-ene rings by reaction with four equivalents of the germylene. All compounds were characterized by multinuclear NMR spectroscopy, Raman and IR spectroscopy, and in the case of 1a-c, 2a, 2c, 3a, 3b and 4, also by single-crystal X-ray diffraction analysis. The ferrocenyl substituted compounds were studied by cyclic voltammetry (CV). Finally, the plausible reaction pathway was studied for a model reaction of [(i-Pr)NB(N-2,6-MeCH)]Ge with MeC[triple bond, length as m-dash]CMe using DFT computations.

Experimental and theoretical studies of a new strontium metallacrown reveal the specific central ion influence on molecular and electronic structures of metallamacrocycles.

The water-soluble copper(II)-lanthanum(III) complex based on phenylalaninehydroximate ligands La(H 2 O) 4 -[15-MC CuPhalaha -5](Cl) 3 (1) has been synthesized and characterized structurally and spectroscopically. The X-ray structure of 1 demonstrated that the ring copper ions are placed in distorted square pyramidal geometries with various axial coordination ligands. UV/Vis absorption spectra reveal halochromic behavior of 1 in the aqueous solution which represents reversible color changes from blue to colorless in response to pH variations. This effect has been described as pH-induced "on-off-on" switch through protonation and deprotonation of the phenylalanine-[a] G. A. Razuvaev

Polynuclear metallamacrocyclic complexes named as metallacrowns are considered as a promising exceptional class of macroheterocyclic compounds that structurally resembles crown ethers. In this communication, we report a general synthetic approach and characterization of the first metallamacrocyclic Bi(III)-Cu(II) 15-MC-5 complex derived from pyrazinohydroxamic acid. The molecular and crystal structures of complex were determined from single-crystal X-ray diffraction studies. The single-crystal structure reveals the classic metallamacrocyclic 15-MC-5 configuration. The Bi(III) ion is located at the center of the 15-MC-5 ring consisting of five [Cu(II)-N-O] repeat units. Detailed NBO and QTAIM studies based on DFT calculations were carried out for {Bi[15-MC Cu(II)Pyzha-5]} 3+ species.

The reactivity of the N,N-chelated germylene with pseudohalides was investigated producing various types of novel heterocycles.