Passing two strings through the same ring using an octahedral metal center as template: a new synthesis of [3]rotaxanes.

Octahedral transition metal centers such as Fe(II), Co(II), and Co(III) have been used as templates in the construction of [3]pseudorotaxanes and [3]rotaxanes from various acyclic and macrocyclic fragments. The species obtained consist of a ring threaded by two string-like compounds. Such systems are relatively uncommon in the [3]rotaxane family, the most usual form being made up of a single axis threaded through two rings. The key structural feature of the present systems is the coordinating unit incorporated in the various organic fragments and used in conjunction with the metal to gather and thread the two filaments through the ring. This bidentate chelate is derived from 8,8'-diphenyl-3,3'-bi-isoquinoline, a very rare example of an endotopic but nonsterically hindering ligand. The stoppered [3]rotaxanes were obtained by using an open-chain fragment bearing azide groups as end functions, followed by click chemistry using a propargyl ether attached to a very bulky group. A particularly attractive X-ray structure was obtained for a cobalt(III)-complexed [3]pseudorotaxane consisting of a 41-membered ring and two crescent-shaped threaded components. The Fe(II) and Co(III) complexes were characterized by (1)H NMR and ES-MS. By taking advantage of the markedly different kinetic properties of the two oxidation states, Co(II) and Co(III), it was possible to proceed to fast coordination or decoordination reactions (for the divalent state) or, when needed, to "freeze" the complexes due to the kinetic inertness of the trivalent state and to study them by (1)H NMR. Finally, demetalation of the two stoppered compounds prepared was performed. This demetalation reaction was fast for the Co(II)-complexed [3]rotaxane, whereas decomplexation of the Fe(II) equivalent required harsh conditions which were not compatible with the stability of the metal-free rotaxane. Interestingly, the thermal stability of the free [3]rotaxane toward unthreading and formation of its constitutive elements was only limited. (1)H NMR measurements showed that the half-life of the rotaxane is about one week at room temperature in dichloromethane. A variable-temperature study revealed that the unthreading reaction leading to dissociation of the [3]rotaxane has a remarkably high entropy of activation, in agreement with the intuitive view that the unthreading process involves a highly ordered transition state.

Selective benzylic C-C coupling catalyzed by a bioinspired dicopper complex.

A highly preorganized bioinspired dicopper complex with imidazole ligation catalyzes the selective benzylic para-C-H activation of 2,4,6-trimethylphenol under aerobic conditions, yielding either the stilbenequinone or 4-methoxymethyl-2,6-dimethylphenol depending on the solvent used.

Pyrazolate-based copper(II) and nickel(II) [2 x 2] grid complexes: protonation-dependent self-assembly, structures and properties.

The pyrazole-based diamide ligand N,N'-bis(2-pyridylmethyl)pyrazole-3,5-dicarboxamide (H(3)L) has been structurally characterised and successfully employed in the preparation of [2 x 2] grid-type complexes. Thus, the reaction of H(3)L with Cu(ClO(4))2.6H(2)O or Ni(ClO(4))2.6H(2)O in the presence of added base (NaOH) affords the tetranuclear complexes [M(4)(HL(4))].8H(2)O (1: M = Cu, 2: M = Ni). Employment of a mixture of the two metal salts under otherwise identical reaction conditions leads to the formation of the mixed-metal species [Cu(x)Ni(4-x)(HL)(4)].8H(2)O (x

Oligonuclear copper complexes of a bioinspired pyrazolate-bridging ligand: synthesis, structures, and equilibria in solution.

The synthesis of a new bioinspired dinucleating ligand scaffold based on a bridging pyrazolate with appended bis[2-(1-methylimidazolyl)methyl]aminomethyl chelate arms is reported. This ligand forms very stable copper complexes, and a series of different species is present in solution depending on the pH. Interconversions between these solution species are tracked and characterized spectroscopically, and X-ray crystallographic structures of three distinct complexes that correspond to the species present in solution from acidic to basic pH have been determined. Overall, this provides a comprehensive picture of the copper coordination chemistry of the new ligand system. Alterations in the protonation state are accompanied by changes in nuclearity and pyrazolate binding, which cause pronounced changes in color and magnetic properties. Antiferromagnetic coupling between the copper(II) ions is switched on or off depending on the pyrazole binding mode.

Tuning of inter- versus intrachain magnetic interactions in cyano-bridged Ni(II)/M(III) (M = Cr, Fe, Co) chain complexes.

The reaction of [M(CN)6]3- (M = Cr3+, Fe3+, Co3+) with the nickel(II) complex of 2,4-diamino-1,3,5-triazin-6-yl-{3-(1,3,5,8,12-pentaazacyclotetradecane)} ([NiL]2+) in excess of ANO3 or ACl (A = Li+, Na+, K+, Rb+, Cs+, NH4+) leads to the cyano-bridged dinuclear assemblies A{[NiL][M(CN)6]}.xH2O (x = 2-5). X-ray structures of Li{[NiL][Cr(CN)6]}.5H2O, NH4{[NiL][Cr(CN)6]}.3.5H2O, K{[NiL][Cr(CN)6]}.4H2O, K{[NiL][Fe(CN)6]}.4H2O, Rb{[NiL][Fe(CN)6]}.3.5H2O, and Cs{[NiL][Fe(CN)6]}.3.5H2O, as well as the powder diffractometry of the entire Fe(III) series, are reported. The magnetic properties of the assemblies are dependent on the monocation A and discussed in detail. New efficient pathways for ferromagnetic exchange between Ni(II) and Fe(III) or Cr(III) are demonstrated. Field dependencies of the magnetization for the Fe(III) samples at low temperature and low magnetic field indicate a weak interchain antiferromagnetic coupling, which is switched to ferromagnetic coupling at increasing magnetic field (metamagnetic behavior). The interchain magnetic coupling can be tuned by the size of the A cations.

Determination of the solution structures of melamine-based bis- and tris-macrocyclic ligand copper(II) complexes.

A combination of molecular mechanics (MM), electron paramagnetic resonance spectroscopy (EPR), and spectra simulation (MM-EPR) has been used to determine the solution structures of di- and trinuclear copper(II) complexes of melamine-based oligomacrocyclic ligands. The spin Hamiltonian parameters of the mononuclear, melamine-appended macrocyclic ligand copper(II) complex have been determined by EPR spectroscopy and were also studied with DFT methods. These spin Hamiltonian parameters, together with the structural parameters obtained from models optimized with MM, have been used for the simulation of the EPR spectra of the di- and trinuclear complexes. For the dinuclear complex, the syn isomer is preferred over the anti, for which an X-ray structure exists; for the trinuclear complex, the syn,syn isomer is preferred over the syn,anti form. Additional support for these assignments comes from DFT calculations, and this demonstrates that the MM-DFT-EPR method is a reliable approach for the determination of solution structures and for the analysis of spin Hamiltonian parameters of dipolar, coupled transition metal complexes (g and A tensors and J values).

Encapsulation of cyanometalates by a tris-macrocyclic ligand tricopper(II) complex: syntheses, structural variation, and magnetic exchange coupling pathways.

The reaction of [M(CN)(6)](3-) (M = Cr(3+), Mn(3+), Fe(3+), Co(3+)) and [M(CN)(8)](4-/3-) (M = Mo(4+/5+), W(4+/5+)) with the trinuclear copper(II) complex of 1,3,5-triazine-2,4,6-triyltris[3-(1,3,5,8,12-pentaazacyclotetradecane)] ([Cu(3)(L)](6+)) leads to partially encapsulated cyanometalates. With hexacyanometalate(III) complexes, [Cu(3)(L)](6+) forms the isostructural host-guest complexes [[[Cu(3)(L)(OH(2))(2)][M(CN)(6)](2)][M(CN)(6)]][M(CN)(6)]30 H(2)O with one bridging, two partially encapsulated, and one isolated [M(CN)(6)](3-) unit. The octacyanometalates of Mo(4+/5+) and W(4+/5+) are encapsulated by two tris-macrocyclic host units. Due to the stability of the +IV oxidation state of Mo and W, only assemblies with [M(CN)(8)](4-) were obtained. The Mo(4+) and W(4+) complexes were crystallized in two different structural forms: [[Cu(3)(L)(OH(2))](2)[Mo(CN)(8)]](NO(3))(8)15 H(2)O with a structural motif that involves isolated spherical [[Cu(3)(L)(OH(2))](2)[M(CN)(8)]](8+) ions and a "string-of-pearls" type of structure [[[Cu(3)(L)](2)[M(CN)(8)]][M(CN)(8)]](NO(3))(4) 20 H(2)O, with [M(CN)(8)](4-) ions that bridge the encapsulated octacyanometalates in a two-dimensional network. The magnetic exchange coupling between the various paramagnetic centers is characterized by temperature-dependent magnetic susceptibility and field-dependent magnetization data. Exchange between the CuCu pairs in the [Cu(3)(L)](6+) "ligand" is weakly antiferromagnetic. Ferromagnetic interactions are observed in the cyanometalate assemblies with Cr(3+), exchange coupling of Mn(3+) and Fe(3+) is very small, and the octacoordinate Mo(4+) and W(4+) systems have a closed-shell ground state.