No abstract

The azido ligand is one of the most investigated ligands in magnetochemistry. Despite its importance, not much is known about the ligand field of the azido ligand and its influence on magnetic anisotropy. Here we present the electronic structure of a novel five-coordinate Co(II)-azido complex (1), which has been characterized experimentally (magnetically and by electronic d-d absorption spectroscopy) and theoretically (by means of multireference electronic structure methods). Static and dynamic magnetic data on 1 have been collected, and the latter demonstrate slow relaxation of the magnetization in an applied external magnetic field of H = 3000 Oe. The zero-field splitting parameters deduced from static susceptibility and magnetizations (D = -10.7 cm(-1), E/D = 0.22) are in excellent agreement with the value of D inferred from an Arrhenius plot of the magnetic relaxation time versus the temperature. Application of the so-called N-electron valence second-order perturbation theory (NEVPT2) resulted in excellent agreement between experimental and computed energies of low-lying d-d transitions. Calculations were performed on 1 and a related four-coordinate Co(II)-azido complex lacking a fifth axial ligand (2). On the basis of these results and contrary to previous suggestions, the N3(-) ligand is shown to behave as a strong σ and π donor. Magnetostructural correlations show a strong increase in the negative D with increasing Lewis basicity (shortening of the Co-N bond distances) of the axial ligand on the N3(-) site. The effect on the change in sign of D in going from four-coordinate Co(II) (positive D) to five-coordinate Co(II) (negative D) is discussed in the light of the bonding scheme derived from ligand field analysis of the ab initio results.

Abstract. Even though the existence of 1,2,3-triazoles has been known for more than a century, the recent discovery of a copper(I) catalyzed version of this reaction has attributed unprecedented importance to these compounds. Coordination and organometallic chemists have benefited from this modular synthetic route, and have accessed ligands based on both the triazoles as well as the triazolylidenes. The wide variation of steric and electronic properties that can be achieved for this ligand class has made them useful for generating metal complexes

The new ligands, 1-(4-isopropyl phenyl)-4-(2-pyridyl)-1,2,3-triazole, 1 and 1-(mesityl)-4-(2-pyridyl)-1,2,3-triazole, 2 were prepared by the reactions of the respective azides with 2-ethynylpyridine following the "click method". These ligands together with the reported ligands 1-(phenyl)-4-(2-pyridyl)-1,2,3-triazole, 3 and 1-(benzyl)-4-(2-pyridyl)-1,2,3-triazole, 4 were reacted with palladium and platinum precursors to give mononuclear cis-dichloropalladium and platinum complexes containing the triazole ligands. Structural characterisation of the free ligand 3 shows that the central N-N bond in the triazole ring has double bond character and hence is best described as an "azo-like" N-N double bond. The pyridine ring in 3 has an almost "anti" conformation with respect to the central triazole ring. The metal centers bind to the ligands through the pyridine N and a triazole N atom. The metal-N(triazole) distances are shorter than the metal-N(pyridine) distances. Cyclic voltammograms of the ligands show reduction processes that appear at extreme negative potentials. Coordination of metal centers induces huge anodic shifts of the reduction potentials due to sigma-polarisation by the metal centers. UV/Vis spectra of the ligands and complexes are also discussed. The properties of such chelating triazole ligands towards palladium and platinum centers is being compared and contrasted to the widely used 2,2'-bipyridine ligand.

We present the synthesis and characterization of enantiomerically pure [6]helicene o-quinones (P)-(+)-1 and (M)-(-)-1 and their application to chiroptical switching and chiral recognition. (P)-(+)-1 and (M)-(-)-1 each show a reversible one-electron reduction process in their cyclic voltammogram, which leads to the formation of the semiquinone radical anions (P)-(+)-1(•-) and (M)-(-)-1(•-), respectively. Spectroelectrochemical ECD measurements give evidence of the reversible switching between the two redox states, which is associated with large differences of the Cotton effects [Δ(Δε)] in the UV and visible regions. The reduction of (±)-1 by lithium metal provides [Li(+){(±)-1(•-)}], which was studied by EPR and ENDOR spectroscopy to reveal substantial delocalization of the spin density over the helicene backbone. DFT calculations demonstrate that the lithium hyperfine coupling A((7)Li) in [Li(+){(±)-1(•-)}] is very sensitive to the position of the lithium cation. On the basis of this observation, chiral recognition by ENDOR spectroscopy was achieved by complexation of [Li(+){(P)-(+)-1(•-)}] and [Li(+){(M)-(-)-1(•-)}] with an enantiomerically pure phosphine oxide ligand.