Brendan J. Liddle

Chemical switching behaviour of tricarbonylrhenium(I) complexes of a new redox active 'pincer' ligand.

The structures and optoelectronic properties of tricarbonylrhenium(I) complexes of di(2-pyrazolyl-p-tolyl)amine in its neutral and deprotonated (uninegative amido) form were investigated. Reactions of the complexes with Brønsted acids or bases result in distinctive changes of colour and electrochemical activity owing to the non-innocent nature of the ligand.

Using sterics to promote reactivity in fac-Re(CO)3 complexes of some ‘non-innocent’ NNN-pincer ligands

Two new redox active ligands based on di(2-(3-organopyrazolyl)-p-tolyl)amine have been prepared in order to investigate potential effects of steric bulk on the structures, electronic properties, or reactivity of tricarbonylrhenium(I) complexes. Replacing the hydrogens at the 3-pyrazolyl positions with alkyl groups causes significant distortion to the ligand framework due to potential interactions between these groups when bound to a fac-Re(CO)(3) moiety. The distortions effectively increase the nucleophilic character of the central amino nitrogen and ligand-centered reactivity of the metal complexes.

Manipulating self-assembly in silver(I) complexes of 1,3-di-N-pyrazolylorganyls.

A series of ligands with two pyrazolyls (pz) linked by either a propyl (pz(2)prop), a benzyl (pz(2)Bn or pzBnpz*, where Bn = benzyl and pz* = 3,5-dimethylpyrazolyl), or a 1,8-naphthyl (pz(2)naphth) spacer and their silver(I) tetrafluoroborate complexes have been prepared with the intent of evaluating how the conformational flexibility of the ligands would affect the supramolecular assembly of the 1:1 [Ag(ligand)](BF(4)) complexes and their capacity for promoting short Ag...Ag interactions. The noncoordinating nature of the tetrafluoroborate anion ensured low coordination numbers to the silver(I) centers, thereby allowing the metal ion to participate in multiple noncovalent interactions that dictate the ligand conformations and supramolecular isomerism observed in the solid state. In the solid state, the complex [Ag(CH(3)CN)(pz(2)prop)](BF(4)) forms a cyclic bimetallic cation that assembles into one-dimensional chains as a result of Ag-pi and CH...F noncovalent interactions, in a manner distinct from the known nitrate derivative. With [Ag(pz(2)Bn)](BF(4)), either cyclic bimetallic cations or coordination polymers can be formed depending on the solvents used for crystallization, where acetone promotes the formation of the former while acetonitrile gives the latter. The complex [Ag(pzBnpz*)](BF(4)) forms two different one-dimensional coordination polymers in the same flask during crystallization from acetone/Et(2)O, where the presence or absence of the included solvent dictates the differences in the secondary coordination sphere of (and noncovalent interactions involving) silver(I). In all the above cases, neighboring silver atoms are separated beyond van der Waals contact. In contrast, the complex [Ag(pz(2)naphth)](BF(4)) x 2 CH(3)CN forms discrete cyclic bimetallic cations where the rigid ligand enforces a short (3.19 A) Ag...Ag contact. All complexes are extensively dissociated in a CH(3)CN solution, as indicated from a combination of (1)H NMR and positive-ion electrospray ionization mass spectral data.

Homoleptic nickel(II) complexes of redox-tunable pincer-type ligands

Different synthetic methods have been developed to prepare eight new redox-active pincer-type ligands, H(X,Y), that have pyrazol-1-yl flanking donors attached to an ortho-position of each ring of a diarylamine anchor and that have different groups, X and Y, at the para-aryl positions. Together with four previously known H(X,Y) ligands, a series of 12 Ni(X,Y)2 complexes were prepared in high yields by a simple one-pot reaction. Six of the 12 derivatives were characterized by single-crystal X-ray diffraction, which showed tetragonally distorted hexacoordinate nickel(II) centers. The nickel(II) complexes exhibit two quasi-reversible one-electron oxidation waves in their cyclic voltammograms, with half-wave potentials that varied over a remarkable 700 mV range with the average of the Hammett σ(p) parameters of the para-aryl X, Y groups. The one- and two-electron oxidized derivatives [Ni(Me,Me)2](BF4)n (n = 1, 2) were prepared synthetically, were characterized by X-band EPR, electronic spectroscopy, and single-crystal X-ray diffraction (for n = 2), and were studied computationally by DFT methods. The dioxidized complex, [Ni(Me,Me)2](BF4)2, is an S = 2 species, with nickel(II) bound to two ligand radicals. The mono-oxidized complex [Ni(Me,Me)2](BF4), prepared by comproportionation, is best described as nickel(II) with one ligand centered radical. Neither the mono- nor the dioxidized derivative shows any substantial electronic coupling between the metal and their bound ligand radicals because of the orthogonal nature of their magnetic orbitals. On the other hand, weak electronic communication occurs between ligands in the mono-oxidized complex as evident from the intervalence charge transfer (IVCT) transition found in the near-IR absorption spectrum. Band shape analysis of the IVCT transition allowed comparisons of the strength of the electronic interaction with that in the related, previously known, Robin-Day class II mixed valence complex, [Ga(Me,Me)2](2+).

Electronic Communication Across Diamagnetic Metal Bridges: A Homoleptic Gallium(III) Complex of a Redox-Active Diarylamido-Based Ligand and Its Oxidized Derivatives

Complexes with cations of the type [Ga(L)(2)](n+) where L = bis(4-methyl-2-(1H-pyrazol-1-yl)phenyl)amido and n = 1, 2, 3 have been prepared and structurally characterized. The electronic properties of each were probed by electrochemical and spectroscopic means and were interpreted with the aid of density functional theory (DFT) calculations. The dication, best described as [Ga(L(-))(L(0))](2+), is a Robin-Day class II mixed-valence species. As such, a broad, weak, solvent-dependent intervalence charge transfer (IVCT) band was found in the NIR spectrum in the range 6390-6925 cm(-1), depending on the solvent. Band shape analyses and the use of Hush and Marcus relations revealed a modest electronic coupling, H(ab) of about 200 cm(-1), and a large rate constant for electron transfer, k(et), on the order of 10(10) s(-1) between redox active ligands. The dioxidized complex [Ga(L(0))(2)](3+) shows a half-field ΔM(s) = 2 transition in its solid-state X-band electron paramagnetic resonance (EPR) spectrum at 5 K, which indicates that the triplet state is thermally populated. DFT calculations (M06/Def2-SV(P)) suggest that the singlet state is 21.7 cm(-1) lower in energy than the triplet state.

CCDC 1047359: Experimental Crystal Structure Determination

Related Article: Jeewantha S. Hewage, Sarath Wanniarachchi, Tyler J. Morin, Brendan J. Liddle, Megan Banaszynski, Sergey V. Lindeman, Brian Bennett, and James R. Gardinier|2014|Inorg.Chem.|53|10070|doi:10.1021/ic500657e