David McMorran

Five new complexes containing AgI salts and the ligand 3,4‐dipyridyl ketone (L34) were synthesized and characterized, namely {[AgL34]ClO4}∞ (1), {[AgL34]BF4}∞ (2), {AgL34NO3}∞ (3), {AgL34CF3CO2}∞ (4) and [AgL34CF3SO3]4 (5). Both the anion and the ligand conformation were found to affect the structure packing. This set of 1:1 M/L structures demonstrated the role of anion binding and ligand conformation in determining the molecular architecture; the increasing alignment of cations in neighboring M‐L chains in structures 1–4 can be attributed to the anions' ability to coordinate to AgI cations. The conformation of the ligand was shown to determine whether a discrete or polymeric architecture formed. In complexes 1–4 the N3py atom was in a cis orientation, with 1 and 2 forming offset chains held together in 2‐D sheets. The anion in 3 helped link 1‐D chains together to form double chains with the double chains offset in the 2‐D sheet. Complex 4 was present as a 2‐D sheet, with the bifurcating CF3CO2– anion causing the alignment of the AgI cations within the sheet. Complex 5 was the only discrete species, with L34 adopting a trans arrangement and acting as corner pieces of a molecular square. DFT calculations showed that the two conformations of the free ligand had a difference in energy of only 2.1 kJ mol–1. A set of 1:1 Ag:L34 structures demonstrates how anion control can tailor the structural outcome, with more coordinating anions leading to increasing alignment of Ag ions within the coordination polymers. The conformation of L34 determines whether the outcome is polymeric or discrete.

A series of [Co-III(N4Py)(X)](ClO4)n (X = Cl-, Br-, OH-, N-3(-), NCS- -kappa N, n = 2: X = OH2, NCMe, DMSO-kappa O, n = 3) complexes containing the tetrapyridyl N-5 ligand N4Py (N4Py = 1,1-di(pyridin-2-yl)-N,N-bis(pyridin-2-ylmethyl)methanamine) has been prepared and fully characterized by infrared (IR), UVvisible, and NMR spectroscopies, high-resolution electrospray ionization mass spectrometry (HRESI-MS), elemental analysis, X-ray crystallography, and electrochemistry. The reduced Co(II) and Co(I) species of these complexes have been also generated by bulk electrolyses in MeCN and characterized by UVvisible and EPR spectroscopies. All tested complexes are catalysts for the photocatalytic production of H-2 from water at pH 4.0 in the presence of ascorbic acid/ascorbate, using [Ru(bpy)3](2+) as a photosensitizer, and all display similar H-2-evolving activities. Detailed mechanistic studies show that while the complexes retain the monodentate X ligand upon electrochemical reduction to Co(II) species in MeCN solution, in aqueous solution, upon reduction by ascorbate (photocatalytic conditions), [Co-II(N4Py)(HA)](+) is formed in all cases and is the precursor to the Co(I) species which presumably reacts with a proton. These results are in accordance with the fact that the H-2-evolving activity does not depend on the chemical nature of the monodentate ligand and differ from those previously reported for similar complexes. The catalytic activity of this series of complexes in terms of turnover number versus catalyst (TONCat) was also found to be dependent on the catalyst concentration, with the highest value of 230 TONCat at 5 x 10(-6) M. As revealed by nanosecond transient absorption spectroscopy measurements, the first electron-transfer steps of the photocatalytic mechanism involve a reductive quenching of the excited state of [Ru(bpy)(3)](2+) by ascorbate followed by an electron transfer from [Ru-II(bpy)(2)(bpy(center dot-))](+) to the [Co-II(N4Py)(HA)]+ catalyst. The reduced catalyst then enters into the H-2-evolution cycle.

[PdCl2(L)] and [PtCl2(L)] complexes of ((2-pyridyl)pyrazol-1-ylmethyl)benzoic acids have been prepared and characterised. In the solid state molecules assemble via π(chelate)-stacking and a variety of hydrogen bonding motifs, the natures of which was also explored using Hirshfeld surface analysis and DFT calculations. Cytotoxicity studies on the [PtCl2(L)] complexes are also reported. [Display omitted] •Pd(II) and Pt(II) complexes containing distal hydrogen-bonding sites are reported.•Hydrogen bonding and π-stacking interactions mediate complex assembly.•Many of the π-stacking interactions involve the chelate rings.•Hirshfeld surface analysis provides insight into complex assembly. The new ligands 3-(3-(2-pyridyl)pyrazol-1-ylmethyl)benzoic acid (L2) and 5-(3-(2-pyridyl)pyrazol-1-ylmethyl)benzene 1,3-dicarboxylic acid (L3) are reported and the synthesis and characterisation of [PdCl2(L)] and [PtCl2(L)] complexes of these and the previously reported 4-(3-(2-pyridyl)pyrazol-1-ylmethyl)benzoic acid (L1) are described. In the solid state, the square planar complexes assemble via hydrogen bonding interactions involving COOH and M–Cl groups as well as by various π-stacking interactions involving the aromatic rings on the ligands and, notably, the chelate rings. Hirshfeld surface analysis has been used to gain insight into the assembly of the molecules. Preliminary studies of the biological cytotoxicity of the [PtCl2(L)] complexes against A549 and MDA-MB-231 cancer cell lines are reported.

The synthesis and characterization of the new ligand N,N-bis(2-quinolylmethyl)-N-bis(2-pyridyl)methylamine (2PyN2Q), a derivative of N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl) methylamine (N4Py) is reported. Purification of both N4Py and 2PyN2Q on Dowex cation exchange columns as their hydrochloride salts allowed the isolation of the byproducts N3Py and 2PyNQ, respectively. The X-ray crystal structure of [H(2)N4Py](PF6)(2) shows that the two more basic picolyl nitrogens are protonated. X-ray structural analyses of the copper(II) complexes [Cu(N4Py)(NO3)](NO3) (1) and [Cu(2PyN2Q)(NO3)](NO3) (2) show that the binding of the nitrate ligand is influenced by the steric bulk of the quinoline rings while electrochemical studies show that the poorer basicity of the quinolinyl donors makes 2 more easily reduced than 1. X-ray structural analyses of the zinc(II) complexes [Zn(N4Py)(OH2)](CF3SO3)(2)center dot H2O (3a center dot H2O), [Zn(N4Py)(CH3OH)](ClO4)(2)center dot CH3OH (4 center dot CH3OH) and [Zn(2PyN2Q)(NCCH3)](ClO4)(2)center dot CH3CN (6 center dot CH3CN) again show that the quinoline rings have a significant influence on the way in which the monodentate ligand can bind to the zinc center. The X-ray crystal structure of [Zn(2PyNQ)(2)](ClO4)(2) (7) is also reported. In 7 the zinc(II) ion is bonded to two pyridyl donors and one amine donor from each of two ligands and the quinolinyl donors are not bound. (C) 2014 Elsevier B.V. All rights reserved.

Two inverse 2-pyridyl-1,2,3-triazole "click" ligands, 2-(4-phenyl-1H-1,2,3-triazol-1-yl)pyridine and 2-(4-benzyl-1H-1,2,3-triazol-1-yl)pyridine, and their palladium(II), platinum(II), rhenium(I), and ruthenium(II) complexes have been synthesized in good to excellent yields. The properties of these inverse "click" complexes have been compared to the isomeric regular compounds using a variety of techniques. X-ray crystallographic analysis shows that the regular and inverse complexes are structurally very similar. However, the chemical and physical properties of the isomers are quite different. Ligand exchange studies and density functional theory (DFT) calculations indicate that metal complexes of the regular 2-(1-R-1H-1,2,3-triazol-4-yl)pyridine (R = phenyl, benzyl) ligands are more stable than those formed with the inverse 2-(4-R-1H-1,2,3-triazol-1-yl)pyridine (R = phenyl, benzyl) "click" chelators. Additionally, the bis-2,2'-bipyridine (bpy) ruthenium(II) complexes of the "click" chelators have been shown to have short excited state lifetimes, which in the inverse triazole case, resulted in ejection of the 2-pyridyl-1,2,3-triazole ligand from the complex. Under identical conditions, the isomeric regular 2-pyridyl-1,2,3-triazole ruthenium(II) bpy complexes are photochemically inert. The absorption spectra of the inverse rhenium(I) and platinum(II) complexes are red-shifted compared to the regular compounds. It is shown that conjugation between the substituent group R and triazolyl unit has a negligible effect on the photophysical properties of the complexes. The inverse rhenium(I) complexes have large Stokes shifts, long metal-to-ligand charge transfer (MLCT) excited state lifetimes, and respectable quantum yields which are relatively solvent insensitive.

A family of tripodal tetraamine ligands incorporating two pyrazolyl and one 1,2,3-triazolyl donor arm have been synthesized in modest-to-excellent yields (42-90 %) using the copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) reaction. Mono-, bis-, and tris-tripodal ligand scaffolds were readily generated using this method. The coordination chemistry of the ligands with cobalt(III) ions has been studied, and cobalt(III) carbonato complexes of the ligands have been isolated and characterized spectroscopically and crystallographically. X-ray crystallography and NMR spectroscopy of the mono-metallic complexes showed that racemic mixtures of the cis-isomer are formed selectively. The di- and tri-metallic systems could not be crystallized, but NMR spectroscopy indicates that these compounds were isolated as mixtures of stereoisomers.

Access to readily functionalized ligand architectures is of crucial importance in a range of different areas including catalysis, metallopharmaceuticals, bioimaging, metallosupramolecular chemistry, mechanically interlocked architectures, and molecular machines. The mild and modular Cu(I)-catalyzed 1,3-cycloaddition of terminal alkynes with organic azides (the CuAAC “click” reaction) allows the ready formation of functionalized 1,4-disubstituted-1,2,3-triazole scaffolds, and this has led to an explosion of interest in the coordination chemistry of these heterocycles. The parent 1,4-disubstituted-1,2,3-triazole units can potentially act as monodentate or bridging ligands. Examples of both the monodentate (through either the N3 nitrogen or C5 carbon positions of the 1,2,3-triazole) and bridging (through the N2 and N3 nitrogen atoms) coordination modes have been structurally characterized. A diverse array of bi-, tri-, and polydentate ligands incorporating 1,4-disubstituted-1,2,3-triazole units have also been synthesized and characterized. When the chelate pocket involves coordination through the N3 nitrogen atom of the 1,2,3-triazole, these are called “regular” click ligands. While these are the most common type of “click” chelate, “inverse” ligands in which the 1,2,3-triazole unit coordinates through the less electron-rich N2 nitrogen atom have also been synthesized and characterized. The resulting “click” complexes are beginning to find applications in catalysis, metallosupramolecular chemistry, photophysics, and as metallopharmaceuticals and bioimaging agents.