A new pincer ligand is synthesized to be dianionic, with the potential to be redox active. It has pyrrrole rings attached to both ortho sites of a pyridine, as the linking element. This H2L can be doubly deprotonated and then used to replace two chloride ligands in MCl2(NCPh)2, to form LM(NCPh) for M = Pd, Pt. The acid form H2L reacts with ZnEt2 with elimination of only 1 mol of ethane to yield (HL)ZnEt, a three-coordinate species with one pendant pyrrole NH functionality. This molecule binds the Lewis base p-dimethylaminopyridine (DMAP) to give first a simple 1:1 adduct that eliminates ethane on heating to form four-coordinate LZn(DMAP), which has an unusual structure due to the strong preference of the pincer ligand to bind in a mer (planar) geometry. A molecule with two HL(-) ligands each bonded in a bidentate manner to FeCl2 is synthesized and shown to contain four-coordinate iron with a flattened-tetrahedral structure. The electrochemistry of LM(NCPh) and (L)Zn(DMAP) shows three oxidation processes, which is interpreted to involve at least two oxidations of the pyrrolide arms.
An unexpected doubling in redox storage emerging from a new pincer ligand upon bis-ligation of iron(ii) is described. When tetrazine arms are present at the two ortho positions of pyridine, the resulting bis-tetrazinyl pyridine (btzp) pincer ligand displays a single one-electron reduction at ca. -0.85 V vs. Ag/AgCl. Complexation to iron, giving the cation Fe(btzp)2(2+), shows no oxidation but four reduction waves in cyclic voltammetry instead of the two expected for the two constituent ligands. Mossbauer, X-ray diffraction and NMR studies show the iron species to contain low spin Fe(ii), but with evidence of back donation from iron to the pincer ligands. CV and UV-Vis spectroelectrochemistry, as well as titration studies as monitored by CV, electronic spectra and EPR reveal the chemical reversibility of forming the reduced species. DFT and EPR studies show varying degrees of delocalization of unpaired spin in different species, including that of a btzp(-1) radical anion, partnered with various cations.
Binding an electron deficient pincer ligand which strongly dictates planar, mer stereochemistry, to a metal which prefers tetrahedral structure, e.g., d(10) CuCl, is explored for possible intramolecular redox chemistry. Experiment shows that the pincer ligand 2,2'-bis-tetrazinyl pyridine, btzp, forms a complex (btzp)CuCl which is a chloride-bridged polymer in the solid state, hence with 20 valence electrons around copper. DFT calculations show that even the monomer has nonplanar copper with the tetrazinyl nitrogen lone pairs somewhat misdirected away from copper, with long Cu/N bonds, in a singlet ground state; 13.9 kcal/mol less stable is a triplet, whose electronic structure shows one electron from the ground state Cu(I) has been transferred to a pincer π* orbital. Outer sphere electron transfer to (btzp)CuCl yields (btzp)Cu where the added electron has gone into the pincer, to leave a ligand-centered radical, characterized by EPR, chemical reactivity, and X-ray photoelectron spectroscopy.
The synthesis and characterization of a "super-saturated" six-coordinate complex of monovalent copper (PF6(-) salt) and the potentially oxidizing pincer ligand 2,6, bis-(methyl tetrazinyl)pyridine, btzp, are described. This cation has a structure with the two pincers symmetry related, but each btzp having one short and one long Cu-N(tetrazine) bond; facile exchange is observed between short and long tetrazine donors. The structure shows no evidence of full electron transfer from Cu(+) to tetrazine, and DFT calculations not only confirm this conclusion, but also show that the lowest energy triplet state, an excited state relative to the singlet ground state, has the MLCT character where the electron lost by copper now resides in both tetrazine rings of only one pincer ligand; the two pincers in this excited state are inequivalent, having charged btzp(0) and btzp(-1). The unusual orientation of the distinct tetrazines in the ground state structure of Cu(btzp)2(+) is attributed to charge/dipole attraction unhindered by significant entropy cost.
Two different neutral tridentate imine-donor pincer ligands interact with excess MCl2 (M = Co or Cu) to form compounds of the same stoichiometry, (LMCl2)2·MCl2, where the assembling force is the electron richness of the terminal chlorides on the LMCl2 unit. Finite aggregation occurs for M = Co, but for M = Cu, an infinite polymeric structure is adopted, all because MCl2 is bifunctional, which thus bridges multiple MCl units. The bis-pyrazolylpyridine ligand has two acidic NH protons, and both of these are involved in intramolecular hydrogen bonds. The generality of this Lewis acid aggregation is discussed.
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