Tricoordinate Nontrigonal Pnictogen-Centered Radical Anions: Isolation, Characterization, and Reactivity.

The search for main-group element-based radicals is one of the main research topics in contemporary chemistry because of their fascinating chemical and physical properties. The Group 15 element-centered radicals mainly feature a V-shaped two coordinate structure, with a couple of radical cations featuring trigonal tricoordinated geometry. Now, nontrigonal compounds R3 E (E=P, As, Sb) were successfully synthesized by introducing a new rigid tris-amide ligand. The selective one-electron reduction of R3 E afforded the first stable tricoordinate pnictogen-centered radical anion salts; the pnictogen atoms retain planar T-shaped structures. EPR spectroscopy and calculations reveal that the spin density mainly resides at the p orbitals of the pnictogen atoms, which is perpendicular to the N3 E planes.

An Iminosemiquinone-Coordinated Oxidovanadium(V) Complex: A Combined Experimental and Computational Study.

Ligand H4Sar(AP/AP) contained two terminal amidophenolate units that were connected by a disulfane bridge. The ligand reacted with VOSO4·5H2O in the presence of Et3N under air and provided a mononuclear octahedral oxidovanadium complex (1). X-ray crystal structure analysis of complex 1 revealed that the oxidation state of the V ion was V and the VO3+ unit was coordinated to an iminosemiquinone radical anion. An isotopic signal at g = 1.998 in the X-band electron paramagnetic resonance (EPR) spectrum and the solution magnetic moment µeff = 1.98 µB at 298 K also supported the composition. The formation of complex 1 preceded through the initial generation of a diamagnetic VO2+-iminoisemiquinone species, as established by time-dependent UV-vis-near-IR (NIR), X-band EPR, and density functional theory studies. The UV-vis-NIR spectrum of complex 1 consisted of four ligand-to-metal charge-transfer transitions in the visible region, while an intervalence ligand-to-ligand charge transfer appeared at 1162 nm. The cyclic voltammogram of the complex showed four oxidation waves and one reduction wave. Spectroelectrochemical studies at fixed potentials revealed that the oxidation and reduction processes were ligand-based.

Solid-state valence tautomeric octahedral {CoII[(BQ-N-Cat)]2}0 complex formation via ligand-centered phenolic C-O bond breaking and Co-O bond making.

Ligand H4LO(AP/AP) underwent ligand-centered C-O bond cleavage during a complexation reaction with Co(ii)-salt. The thus formed octahedral {CoII[(BQ-N-Cat)]2}0 complex showed valence tautomerization in the solid state. While the process was triggered by the presence of lattice solvent, the nature of the solvent molecule has less effect on the process.

An unprecedented one-step synthesis of octahedral Cu(ii)-bis(iminoquinone) complexes and their reactivity with NaBH4.

A new non-innocent ligand, H2L(AP(o-NO2-OPh)), was synthesized. The ligand H2L(AP(o-NO2-OPh)) reacted with 0.5 equivalents of CuCl2·2H2O and provided the corresponding mononuclear four-coordinate [Cu(ii)-bis(iminosemiquinone)] complex (1). Interestingly, the ligand upon reacting with 2 equivalents of CuCl2·2H2O in the presence of Et3N and air provided the corresponding trans-dichloride-bound six-coordinate mononuclear Cu(ii)-complex (2) in one step. To extend the validity of the newly developed one-step synthesis of the trans-dichloride-bound Cu(ii)-bis(iminoquinone) complex using a non-innocent ligand and 2 equivalents of CuCl2·2H2O, the previously reported non-innocent ligand H2L(AP) was further examined. Thus the formed trans-dichloride-bound Cu(ii)-bis(iminoquinone) complex was designated as complex 3. The complexes were characterized by IR, mass, UV-Vis/NIR, X-band EPR, and X-ray single crystal diffraction techniques. Molecular structure analysis confirmed that in 1 the oxidation state of the coordinating ligands was [L(ISQ(o-NO2-OPh))](1-), i.e., one-electron oxidized iminosemiquinone. In 2 and 3·CH2Cl2, both the coordinated-ligands were present in their two-electron oxidized iminoquinone form. The iminoquinone-complex formation was found to proceed with the generation of CuCl salt. X-band EPR spectrum measurement confirmed that both the iminoquinone-complexes were paramagnetic and the unpaired electron was located at the 3dx(2)-y(2) orbital of Cu(ii) ions. When the Cu(ii)-bis(iminoquinone) complex 3 was subjected to react with NaBH4 in dry CH3CN, H2 gas was formed along with the generation of the corresponding Cu(ii)-bis(iminosemiquinone) complex. GC analyses were performed for the identification of H2 gas.

Conformational selection underpins recognition of multiple DNA sequences by proteins and consequent functional actions.

Recognition of multiple functional DNA sequences by a DNA-binding protein occurs widely in nature. The physico-chemical basis of this phenomenon is not well-understood. The E. coli gal repressor, a gene regulatory protein, binds two homologous but non-identical sixteen basepair sequences in the gal operon and interacts by protein-protein interaction to regulate gene expression. The two sites have nearly equal affinities for the Gal repressor. Spectroscopic studies of the Gal repressor bound to these two different DNA sequences detected significant conformational differences between them. Comprehensive single base-substitution and binding measurements were carried out on the two sequences to understand the nature of the two protein-DNA interfaces. Magnitudes of basepair-protein interaction energy show significant variation between homologous positions of the two DNA sequences. Magnitudes of variation are such that when summed over the whole sequence they largely cancel each other out, thus producing nearly equal net affinity. Modeling suggests significant alterations in the protein-DNA interface in the two complexes, which are consistent with conformational adaptation of the protein to different DNA sequences. The functional role of the two sequences was studied by substitution of one site by the other and vice versa. In both cases, substitution reduces repression in vivo. This suggests that naturally occurring DNA sequence variations play functional roles beyond merely acting as high-affinity anchoring points. We propose that two different pre-existing conformations in the conformational ensemble of the free protein are selected by two different DNA sequences for efficient sequence read-out and the conformational difference of the bound proteins leads to different functional roles.

Unprecedented iminobenzosemiquinone and iminobenzoquinone coordinated mononuclear Cu(ii) complex formation under air.

Two non-innocent 2-anilino-4,6-di-tert-butylphenol units were connected through an -ortho bridging S atom and provided the ligand . The ligand reacted with CuCl2·2H2O in the presence of Et3N under air and generated a mononuclear Cu(ii) complex (1, [CuL(S(IBQ/ISQ))Cl](0)). A single crystal X-ray diffraction analysis confirmed that in the complex one of the two 2-anilido-4,6-di-tert-butylphenol arms was present in its one-electron oxidized iminobenzosemiquinone form, while the other was in its two-electron oxidized iminobenzoquinone form. The monoradical-containing Cu(ii) complex was diamagnetic in nature owing to a strong antiferromagnetic coupling between the single electron containing dx(2)-y(2) magnetic orbital of Cu(ii) and the radical-centered unpaired electron.