A novel mixed-valence trinickel complex containing nickel(I), nickel(II) and iodonium moieties

Ligand effects on the structure, mixed-valence sites and magnetic properties of novel pentanickel string complexes

We report the syntheses, characterization, electronic structures and magnetic properties of four redox pairs of novel nickel-extended metal-atom chain (EMAC) complexes containing pyridine-, naphthyridine- and sulfonyl-containing ligands (H₂Tspnda and H₂Mspnda) (1-2 and 5-6). We further study the corresponding phenyl-substituted ligands (H₂Tsphpnda and H₂Msphpnda) (3-4 and 7-8) to examine the details of ligand effects. The X-ray structure of one-electron-reduced [Ni₅]⁹⁺ complexes shows shorter Ni-Ni bond distances (2.2646(6) for 1, 2.2943(7) for 3, 2.2436(11) for 5 and 2.2322(8) Å for 7) in comparison with an average Ni-Ni distance of 2.3187(8) Å for these complexes, indicative of a partial metal-metal bond interaction in the mixed-valence [Ni₂]³⁺ (S = 3/2) unit. The most striking result is that the [Ni₂]³⁺ site migrates from Ni(1)-Ni(2) to Ni(2)-Ni(3) when we replace the p-tolyl-sulfonyl group with methyl-sulfonyl group. These complexes present a rare example of the effect of crystal packing on the symmetric molecular structure yielding unsymmetric electronic distribution. Cyclic voltammetry measurements show four reversible redox waves and display the lower potentials of the [Ni₅]⁹⁺ complexes. These unusual lower potentials facilitate one-electron oxidation of these four complexes to [Ni₅]¹⁰⁺-core forms. We applied the magnetic susceptibility and EPR measurements to examine the magnetic properties of these four [Ni₅]⁹⁺-core pentanickel complexes and study the bonding nature of these mixed-valence [Ni₂]³⁺ units. Indeed, the results of EPR measurements reflect the migration of the mixed-valence site and the change of symmetry. Surprisingly, the oxidized [Ni₅]¹⁰⁺ counterparts behave differently: complex 2 exhibits an antiferromagnetic interaction with J = -13.59 cm^-1 between the two terminal Ni ions, while the others (4, 6 and 8) display diamagnetic properties as all of the Ni²⁺ ions are in low-spin (S = 0) states. These three complexes, to the best of our knowledge, are the first examples of all Ni²⁺ ions in a null spin configuration for pentanickel chains. Even though the structures of these complexes are similar to each other, their corresponding electronic structure and oxidized products show drastic changes in their magnetic properties and bonding nature. These differences of the properties and bonding nature of these pentanickel complexes are attributed to the ligand effects.

Shuttling of nickel oxidation states in N4S2 coordination geometry versus donor strength of tridentate N2S donor ligands

Seven bis-Ni(II) complexes of a N(2)S donor set ligand have been synthesized and examined for their ability to stabilize Ni(0), Ni(I), Ni(II) and Ni(III) oxidation states. Compounds 1-5 consist of modifications of the pyridine ring of the tridentate Schiff base ligand, 2-pyridyl-N-(2'-methylthiophenyl)methyleneimine ((X)L1), where X = 6-H, 6-Me, 6-p-ClPh, 6-Br, 5-Br; compound 6 is the reduced amine form (L2); compound 7 is the amide analog (L3). The compounds are perchlorate salts except for 7, which is neutral. Complexes 1 and 3-7 have been structurally characterized. Their coordination geometry is distorted octahedral. In the case of 6, the tridentate ligand coordinates in a facial manner, whereas the remaining complexes display meridional coordination. Due to substitution of the pyridine ring of (X)L1, the Ni-N(py) distances for 1~5 < 3 < 4 increase and UV-vis λ(max) values corresponding to the (3)A(2g)(F)→(3)T(2g)(F) transition show an increasing trend 1~5 < 2 < 3 < 4. Cyclic voltammetry of 1-5 reveals two quasi-reversible reduction waves that correspond to Ni(II)→Ni(I) and Ni(I)→Ni(0) reduction. The E(1/2) for the Ni(II)/Ni(I) couple decreases as 1 > 2 > 3 > 4. Replacement of the central imine N donor in 1 by amine 6 or amide 7 N donors reveals that complex 6 in CH(3)CN exhibits an irreversible reductive response at E(pc) = -1.28 V, E(pa) = +0.25 V vs saturated calomel electrode (SCE). In contrast, complex 7 shows a reversible oxidation wave at E(1/2) = +0.84 V (ΔE(p) = 60 mV) that corresponds to Ni(II)→Ni(III). The electrochemically generated Ni(III) species, [(L3)(2)Ni(III)](+) is stable, showing a new UV-vis band at 470 nm. EPR measurements have also been carried out.

The nature of the chemical bond in linear three-body systems: from i3- to mixed chalcogen/halogen and trichalcogen moieties

The 3 centre-4 electrons (3c-4e) and the donor/acceptor or charge-transfer models for the description of the chemical bond in linear three-body systems, such as I(3) (-) and related electron-rich (22 shell electrons) systems, are comparatively discussed on the grounds of structural data from a search of the Cambridge Structural Database (CSD). Both models account for a total bond order of 1 in these systems, and while the former fits better symmetric systems, the latter describes better strongly asymmetric situations. The 3c-4e MO scheme shows that any linear system formed by three aligned closed-shell species (24 shell electrons overall) has reason to exist provided that two electrons are removed from it to afford a 22 shell electrons three-body system: all combinations of three closed-shell halides and/or chalcogenides are considered here. A survey of the literature shows that most of these three-body systems exist. With some exceptions, their structural features vary continuously from the symmetric situation showing two equal bonds to very asymmetric situations in which one bond approaches to the value corresponding to a single bond and the second one to the sum of the van der Waals radii of the involved atoms. This indicates that the potential energy surface of these three-body systems is fairly flat, and that the chemical surrounding of the chalcogen/halogen atoms can play an important role in freezing different structural situations; this is well documented for the I(3) (-) anion. The existence of correlations between the two bond distances and more importantly the linearity observed for all these systems, independently on the degree of their asymmetry, support the state of hypervalency of the central atom.