Enantiomeric Complexes Based on Ruthenium(III) and 2,2'-Biimidazole: X-ray Structure and Magnetic Properties

We have prepared and characterized two Ru(III) compounds based on the 2,2'-biimidazole (H2biim) ligand, namely, a single complex of formula cis-[RuCl2(H2biim)2]Cl·4H2O (1) and a racemic mixture of formula {cis-[RuCl2(H2biim)2]Cl}2·4H2O (2), which contains 50% of Ru(III) complex 1. Both compounds crystallize in the monoclinic system with space groups C2 and P21 for 1 and 2, respectively. These complexes exhibit the metal ion bonded to four nitrogen atoms from two H2biim molecules and two chloride ions, which balance part of the positive charges in a distorted octahedral geometry. Significant differences are observed in their crystal packing, which leads to the observation of differences in their respective magnetic behaviors. Despite having imidazole rings in both compounds, π-π stacking interactions occur only in the crystal structure of 2, and the shortest intermolecular Ru···Ru separation in 2 is consequently shorter than that in 1. Variable-temperature dc magnetic susceptibility measurements performed on polycrystalline samples of 1 and 2 reveal different magnetic behaviors at low temperatures: while 1 behaves pretty much as a magnetically isolated mononuclear Ru(III) complex with S = 1/2, 2 exhibits the behavior of an antiferromagnetically coupled system with S = 0 and a maximum in the magnetic susceptibility curve at approximately 3.0 K.

Hexahalorhenate(iv) salts of protonated ciprofloxacin: antibiotic-based single-ion magnets

In the crystal lattice of two novel ReIV compounds, the paramagnetic [ReCl6]2− and [ReBr6]2− anions are well separated from each other through two protonated forms of the antibiotic ciprofloxacin. These compounds behave as single-ion magnets (SIMs).

Field-Induced Single-Ion Magnet Phenomenon in Hexabromo- and Hexaiodorhenate(IV) Complexes

Two mononuclear ReIV complexes of general formula (PPh4)2[ReX6] [PPh4+ = tetraphenylphosphonium cation, X = Br (1) and I (2)] have been prepared and structurally and magnetically characterised. Both compounds crystallise in the triclinic system with space group Pī. Their structures are made up of hexahalorhenate(IV), [ReX6]2−, anions, and bulky PPh4+ cations. Each ReIV ion in 1 and 2 is six-coordinate and bonded to six halide ions in a quasi regular octahedral geometry. In their crystal packing, the [ReX6]2− anions are well separated from each other through the organic cations, generating alternated anionic and cationic layers, and no intermolecular Re−X···X−Re interactions are present. Variable-temperature dc magnetic susceptibility measurements performed on microcrystalline samples of 1 and 2 show a very similar magnetic behaviour, which is typical of noninteracting mononuclear ReIV complexes with S = 3/2. Ac magnetic susceptibility measurements reveal the slow relaxation of the magnetisation in the presence of external dc fields for 1 and 2, hence indicating the occurrence of the field-induced single-ion magnet (SIM) phenomenon in these hexabromo- and hexaiodorhenate(IV) complexes.

Halogen⋯halogen interactions in the self-assembly of one-dimensional 2,2′-bipyrimidine-based CuIIReIV systems

Two one-dimensional Cu^IIRe^IV coordination polymers of the general formula {[Re^IVCl₄(μ-bpym)Cu^IIX₂]·solvent}_n [where bpym = 2,2′-bipyrimidine, X = Cl (1) and Br (2), and solvent = H₂O (1) and CHCl₃ (2)] have been prepared and characterised structurally and magnetically.

Magneto-structural correlations in a family of ReIVCuII chains based on the hexachlororhenate(iv) metalloligand

Six novel one-dimensional chloro-bridged Re^IVCu^II complexes of formula {[Cu(L)₄][ReCl₆]}_n, where L = imidazole (Imi, 1), 1-methylimidazole (Meim, 2), 1-vinylimidazole (Vim, 3), 1-butylimidazole (Buim, 4), 1-vinyl-1,2,4-triazole (Vtri, 5) and N,N'-dimethylformamide (DMF, 6) are characterised structurally, magnetically and theoretically. The structures exhibit significant differences in Cu-Cl bond lengths and Re-Cl-Cu bridging angles, resulting in large differences in the nature and magnitude of magnetic exchange interactions between the Re^IV and Cu^II ions. Theoretical calculations reveal the coupling to be primarily ferromagnetic, increasing in magnitude as the bridging angle becomes smaller and the bond lengths shorten.

Hexahalorhenate(iv) salts of metal oxazolidine nitroxides

Eight coordination compounds of formulae [Fe^II(L˙)₂][Re^IVCl₆] (1a), [Fe^II(L˙)₂][Re^IVBr₆] (1b), [Co^II(L˙)₂][Re^IVCl₆]·CH₃CN (2a), [Co^II(L˙)₂][Re^IVBr₆] (2b), [Ni^II(L˙)(CH₃CN)₃][Re^IVCl₆]·CH₃CN (3a), [Ni^II(L˙)(CH₃CN)₃][Re^IVBr₆]·3CH₃CN (3b), [Cu^II(L˙)₂][Re^IVCl₆] (4a) and [Cu^II(L˙)₂][Re^IVBr₆] (4b), where L˙ is the aminoxyl radical chelating ligand, 4,4'-dimethyl-2,2'-di(2-pyridyl)oxazolidine-N-oxide, have been synthesised. Structural and magnetic studies reveal metal-radical intramolecular antiferromagnetic interactions in the [M^II(L˙)₂]²⁺ cations in the iron, cobalt and copper based compounds (1a, 1b, 2a, 2b, 4a and 4b) with the central metal ion low-spin in the case of iron (1a and 1b) and a gradual, cobalt based, spin-crossover transition present in 2a and 2b. The nickel based compounds, 3a and 3b, were analysed in the dried form (3a(dried) and 3b(dried)) and directly in acetonitrile (3a(solvated) and 3b(solvated)). Microanalysis and IR spectroscopy on 3a(dried) and 3b(dried) suggest that the dried samples are best formulated as [Ni^II(L˙)(H₂O)₃][Re^IVX₆], where X = Cl (3a(dried)) and Br (3b(dried)). All forms of 3a and 3b exhibit cationic metal-radical ferromagnetic interactions resulting in S = 3/2 ground states. In addition, 3a(dried) exhibits spin-canting behaviour with an ordering temperature of 2.7 K, an open hysteresis loop with a coercive field H_c = 580 Oe, and a remanent magnetisation M_r = 0.21μ_B, resulting in a canting angle of ∼1.8°. In contrast, 3b(dried) shows no spin-canting behaviour; a maximum in χ_Mvs. T at T = 3 K suggesting long-range antiferromagnetic ordering. 3a(solvated) and 3b(solvated) show no indication of long-range magnetic ordering, unlike 4a and 4b where anomalies are evident in the low-temperature magnetic susceptibility measurements.

Bisthienylethene Th2im and its complex (Th2imH)2[ReCl6]: crystalline-phase photochromism, and photochemical regulation of luminescence and magnetic properties

Molecular assembly of bisthienylethene Th2im (1) and [ReCl6]2− anions leads to the complex (Th2imH)2[ReCl6] (2), in which a [ReCl6]2(-) anion connects two equivalent Th2imH+ cations through Cl⋯N/C hydrogen bonds. Crystal structures of 1 and 2 indicate that two thiophene groups of each Th2im/Th2imH+ molecule adopt a photoactive antiparallel conformation. Thus, two compounds show crystalline-phase photochromism (CPP), i.e. reversible structural transformation between the open form and the closed form upon alternately irradiating the sample with UV light (365 nm) and visible light (574 nm for 1, 624 nm for 2). It was found that the CPP behaviors of 1 and 2 could regulate their luminescence and/or magnetic properties. Their solid-state emissions (433, 448, 482, 531 and 570 nm for 1, and 460, 489, 535 and 593 nm for 2) exhibited weaker intensities after UV irradiation with 365 nm light. Besides CPP and luminescence, compound 2 shows field-induced slow magnetic relaxation. Before and after UV irradiation, this compound revealed different magnetic behaviors, including the differences in the shape of the χMT vs. T plot, D parameter, and the values of the relaxation barrier Ueff and the preexponential factor τ0.

Synthesis, crystal structure and magnetic properties of H2tppz[ReCl6] and [Cu(bpzm)2(μ-Cl)ReCl3(μ-ox)Cu(bpzm)2(μ-ox)ReCl3(μ-Cl)]n

Two new Re(iv) compounds of formulae H2tppz[ReCl6] (1) and [Cu(bpzm)2(μ-Cl)ReCl3(μ-ox)Cu(bpzm)2(μ-ox)ReCl3(μ-Cl)]n (2) [tppz = 2,3,5,6-tetrakis(2-pyridyl)pyrazine and bpzm = bis(pyrazolyl-1-yl)methane] have been prepared and their crystal structures determined by X-ray diffraction on single crystals. Compound 1 is a mononuclear species whose structure consists of octahedral hexachlororhenate(iv) anions and diprotonated H2tppz(2+) cations which are arranged in the unit cell as alternating anionic and cationic layers, held together by electrostatic forces. The structure of 2 is made up of alternating [Cu(1)(bpzm)2](2+) and [(ox)ReCl3(μ-Cl)Cu(2)(bpzm)2(μ-Cl)ReCl3(ox)](2-) entities interlinked by oxalate bridges to afford a neutral heterobimetallic chain. The oxalate group adopts the didentate (at Re)/monodentate (at Cu) bridging mode. The magnetic behavior of 1 and 2 has been investigated over the temperature range 1.9-295 K. 1 is a magnetically diluted Re(iv) complex, the relatively large value of the zero-field splitting of the ground level [D = -15.8(2) cm(-1)] accounting for the variation of χMT in the low temperature range. Weak intrachain ferromagnetic interactions between Re(iv) and Cu(ii) through oxalate (J1 = +0.15 cm(-1)) and single chloro (J2 = +4.9 cm(-1)) bridges occur in 2 which are obscured by the large zero-field splitting of the Re(iv) ion (DRe = 42 cm(-1)). In addition, interchain antiferromagnetic interactions are also involved in 2 which are responsible for the metamagnetic behavior observed, the value of the critical dc magnetic field (Hc) being 20 kOe.

The effect of crystal packing and Re(IV) ions on the magnetisation relaxation of [Mn6]-based molecular magnets

The energy barrier to magnetisation relaxation in single-molecule magnets (SMMs) proffers potential technological applications in high-density information storage and quantum computation. Leading candidates amongst complexes of 3d metals ions are the hexametallic family of complexes of formula [Mn₆O₂(R-sao)₆(X)₂(solvent)_y] (saoH₂=salicylaldoxime; X=mono-anion; y=4–6; R=H, Me, Et, and Ph). The recent synthesis of cationic [Mn₆][ClO₄]₂ family members, in which the coordinating X ions were replaced with non-coordinating anions, opened the gateway to constructing families of novel [Mn₆] salts in which the identity and nature of the charge balancing anions could be employed to alter the physical properties of the complex. Herein we demonstrate initial experiments to show that this is indeed possible. By replacing the diamagnetic ClO₄⁻ anions with the highly anisotropic Re^IV ion in the form of [Re^IVCl₆]²⁻, the energy barrier to magnetisation relaxation is increased by up to 30 %.

Metamagnetic behaviour in a new Cu(II)Re(IV) chain based on the hexachlororhenate(IV) anion

A new chloro-bridged heterobimetallic Cu(II)Re(IV) chain of formula {Cu(pyim)(Him)2ReCl6}n·MeCN (·MeCN) has been prepared and magnetostructurally characterised. Compound is the first example of the [Re(IV)Cl6](2-) anion acting as a metalloligand towards a paramagnetic metal ion.

Formation of tetrameric water clusters driven by a cavitand template

We describe an innovative approach to the generation of tetrameric water clusters in the solid state. The specific H-bond pattern induced by the tetraphosphonate cavitand template via its rigidly preorganized P=O acceptor groups leads to the exclusive formation of the unique cyclic homodromic water tetramer of C(4) symmetry.