Stabilizing a NiII-aqua complex via intramolecular hydrogen bonds: Synthesis, structure, and redox properties

Abstract

Hydrogen bonds within the secondary coordination sphere are effective in controlling the chemistry of synthetic metal complexes. Coupling the capacity of hydrogen bonds with those of redox-active ligands offers a promising approach to enhance the functional properties of transition metal complexes. These qualities were successfully illustrated with the [NNN]3−pincer ligand N,N′ -(azanediylbis(2,l-phenylene))bis(2,4,6-triisopropyl-benzene-sulfonamido ([ibaps]3−) through the preparation of the NiII-OH2 complex, [NiII(ibaps)(OH2)]−. The [ibaps]3− ligand contains two appended sulfonamido groups that support the formation of intramolecular hydrogen bonds. The bulky 2,4,6-triisopropylphenyl rings are necessary to ensure that only one ligand binds to a single metal ion. The molecular structure of the complex shows a square planar N3O primary coordination sphere and two intramolecular hydrogen bonds involving the aqua ligand. Electrochemical measurements in acetonitrile revealed two oxidation events at potentials below that of the ferrocenium/ferrocene couple. Oxidation with 1 equiv of ferrocenium produced the one-electron oxidized species, [Ni(ibaps)(OH2)]. Experimental and computational studies support this assignment.

δ-C(sp3)H Activation of Free Alcohols Enabled by Rationally Designed H-Bond-Acceptor Ligands

“…Taken together, the crystallographic studies, extensive catalyst and substrate SAR data, computational modelling, evidence from solvent screening data (see Figs. S4 and S12), and literature precedent for stabilizing intramolecular H-bonding to sulfonamides in Ni(II)-aquo complexes 40 strongly support a key role for the proposed H-bonding interaction in enabling alcohol-directed C(sp 3 )-H activation.…”

Modulation of the Bonding between Copper and a Redox-Active Ligand by Hydrogen Bonds and Its Effect on Electronic Coupling and Spin States

“…This structural constraint positions the 2,4,6-triisopropylphenyl groups in a syn conformation, and within the crystal lattice, they are nearly coplanar (the angle between the planes of the rings is 172.7°). We note that the molecular structure of a related Ni complex, [Ni II (ibaps)(OH 2 )] − , also has analogous intramolecular H-bonds, but the sulfonamido groups are in an anticonformation . It thus appears that either conformation can support H-bonds to the aqua ligand.…”

“…We note that the molecular structure of a related Ni complex, [Ni II (ibaps)(OH 2 )] − , also has analogous intramolecular H-bonds, but the sulfonamido groups are in an anticonformation. 36 It thus appears that either conformation can support H-bonds to the aqua ligand.…”

“…For instance, the absorption spectra of the 2-L and 3-L complexes resemble those reported for the oxidized forms of complexes with [NNN] redox-active ligands. 9,10,21,35,36,41 The isolation of 1-L, 2-L, and 3-L provided an opportunity to examine the effects of the ancillary ligands on both the molecular and electronic structures and the impact of intramolecular H-bonds in regulating magnetic coupling between a metal center and a multidentate, redox-active ligand.…”

“…To explore whether exchange coupling can similarly be controlled within synthetic nonheme complexes, we have investigated the magnetic properties of Cu II complexes with the redox-active ligand N,N’ -(azanediyly bis(2,1-phenylene))bis(2,4,6-triisopropyl-benzene-sulfonamido) ([ibaps] 3– , Figure E) having a series of ancillary ligands (Figure F and G). We previously demonstrated that Fe, Ga, and Ni complexes of [ibaps] 3– have rich electrochemical properties and that the sulfonamido groups can be H-bond acceptors to promote intramolecular H-bond formation. , Examination of the redox and structural properties of the Cu complexes reported here allowed us to monitor differences in the electronic structure with four different ancillary ligands that have 0, 1, or 2 H-bond donors over three oxidation levels. Owing to the similar energies of the d x2–y2 orbital of the Cu II center and the redox-active π orbital of [ibaps] 3– , a nascent bond forms between a tilted d x2–y2 orbital and the p z orbital of the central N atom of [ibaps] 3– , which we discovered promotes molecular distortions away from the customary square planarity of four-coordinate Cu II complexes.…”