This tutorial review provides an overview of the theoretical and experimental investigations that resulted in the recognition of anion-pi interactions, i.e., non-covalent forces between electron deficient aromatic systems and anions. Several pioneering theoretical studies revealed that these interactions are energetically favorable (approximately 20-50 kJ mol(-1)). Anion-pi interactions are gaining significant recognition, and their pivotal role in many key chemical and biological processes is being increasingly appreciated. The design of highly selective anion receptors and channels represent important advances in this nascent field of supramolecular chemistry.

Syntheses and characterization of two polymorphs of Cu(TCNQ) have been carried out and the results correlated to films of the materials. Reactions of CuI with TCNQ or [Cu(CH3CN)4][BF4] with TCNQ- lead to blue-purple needles of Cu(TCNQ) phase I (1). A slurry of this kinetic product in CH3CN yields a second crystalline phase of Cu(TCNQ), phase II (2), which exhibits a platelet morphology. Powder X-ray diffraction and scanning electron microscopy data revealed that both phases are present in films of Cu(TCNQ) formed by oxidation of Cu foil by TCNQ in CH3CN. X-ray photoelectron spectra of the two phases are indistinguishable from each other and are indicative of the presence of Cu(I). Single-crystal X-ray studies were undertaken on very small crystals of the two samples, the results of which reveal that subtle geometrical changes for the nitrile arrangements around the four-coordinate Cu(I) centers lead to major changes in the architectural framework of the polymers. Phase I was indexed in the tetragonal crystal system, but due to disorder and twinning, the crystal diffracted to only ∼40° in 2θ. The data were solved and refined in the monoclinic Pn space group. The polymeric motif consists of Cu atoms surrounded by four nitrile lone pairs of independent TCNQ- molecules arranged in a distorted tetrahedral geometry. A quadruply twinned crystal of Cu(TCNQ) phase II was indexed in the monoclinic system and resolved by deconvolution methods. The Cu(I) ions in phase II occupy the tetrahedral positions of a Cooperite structure (PtS), and the TCNQ- radicals occupy the square planar sites. In both phases there are two interpenetrating lattices present. In phase I the TCNQ- units are involved in close π-stacking interactions at ∼3.24 Å whereas in phase II the closest approach of the rings is ∼6.8 Å. In qualitative agreement with these observations are the magnetic properties; 1 is essentially diamagnetic and 2 displays Curie−Weiss behavior down to very low temperatures. The charge-transport properties of the samples revealed that, while they are both semiconductors, 1 is a good semiconductor with a room-temperature conductivity of 0.25 S cm-1 and a band gap of 0.137 eV whereas 2 is a very poor semiconductor with σ(rt) = 1.3 × 10-5 S cm-1 and a band gap of 0.332 eV. Cu(TCNQ) film devices have been found to switch between two states that exhibit very similar conducting properties.

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This Account summarizes the DNA binding properties of anticancer active dinuclear Rh, Re, and Ru compounds. The combined results of NMR spectroscopy, X-ray crystallography, and various biochemical tools provide incontrovertible evidence that dinuclear complexes are favorably poised to bind to purine nucleobases, DNA fragments, and double-stranded DNA. Moreover, direct DNA photocleavage in vitro is effected by dirhodium compounds in the presence of electron acceptors in solution or directly attached to the dirhodium core. This research has provided valuable insight in the interactions of dinuclear compounds with DNA, knowledge that is an excellent backdrop for rational design of promising dinuclear drugs.

The study of the noncovalent force between π-acidic aromatic systems and anions, referred to as the anion-π interaction, has recently emerged as a new branch of supramolecular chemistry. The anion-π contact is complementary to the cation-π interaction, a prominent noncovalent force involved in protein structure and enzyme function. Until recently, the scientific community had overlooked the anion-π interaction due to its ostensibly counterintuitive nature. Pioneering theoretical studies in 2002, however, established that anion-π interactions are energetically favorable (~20-70 kJ/mol) and prompted a flurry of reports in support of their existence. The interest in anion-π contacts was further fueled by the importance of anions in key chemical and biological processes and the involvement of π-rings in anion recognition and transport. Anion-π interactions hold great promise for the design of selective anion receptors, hosts or scaffolds, colorimetric sensors, and catalysts and may also affect biological functions. Currently, the area of anion-π research is highly topical in the scientific community and on a meteoric rise in the chemical literature. This Account highlights our leading findings in this burgeoning area. Our work has focused on comprehensive investigations of several unprecedented supramolecular systems, in which the anions and their close anion-π contacts are the driving elements of the final architectures. We surveyed several heterocyclic π-acidic aromatic systems amenable to anion-π contacts and explored the subtle interplay between ligand π-acidity, anion identity, and metal ions in mediating the ensuing self-assembled architectures. The reactions we performed between solvated first-row transition metal ions and the π-acidic ligands bptz (3,6-bis(2-pyridyl)-1,2,4,5-tetrazine) or bmtz (3,6-bis(2-pyrimidyl)-1,2,4,5-tetrazine) resulted in unprecedented metallacycles. Our investigations revealed that the identity of the encapsulated ion dictates the metallacycle nuclearity and close anion-π contacts are critical for the metallacycle stability. Our X-ray crystallographic, NMR spectroscopic, and mass spectrometric (MS) studies demonstrated that the tetrahedral ([BF4](-), [ClO4](-)) and octahedral ([SbF6](-), [AsF6](-), [PF6](-)) anions template discrete molecular squares and pentagons, respectively. The metal ions occupy the vertices, and bptz or bmtz moieties span the edges of the metallacycles. The encapsulated anions occupy the π-acidic cavities of the metallacycles and establish multiple close directional F/O···C(tetrazine) contacts with the edges. The observation of notable (19)F solid-state NMR chemical shifts reflects the short contacts of the encapsulated anions, findings that we corroborated by DFT calculations. The solution NMR data support the conclusion that bona fide metallacycle templation and interconversion between the metallacycles in solution occurs only in the presence of the appropriate anions. The NMR, MS, and CV data underscore the remarkable metallapentacycle stability despite the a...

The study of paramagnetic compounds based on 4d and 5d transition metals is an emerging research topic in the field of molecular magnetism. An essential driving force for the interest in this area is the fact that heavier metal ions introduce important attributes to the physical properties of paramagnetic compounds. Among the attractive characteristics of heavier elements vis-à-vis magnetism are the diffuse nature of their d orbitals, their strong magnetic anisotropy owing to enhanced spin-orbit coupling, and their diverse structural and redox properties. This critical review is intended to introduce readers to the topic and to report recent progress in this area. It is not fully comprehensive in scope although we strived to include all relevant topics and a large subset of references in the area. Herein we provide a survey of the history and current status of research that has been conducted on the topic of second and third row transition metal molecular magnetism. The article is organized according to the nature of the precursor building blocks with special topics being highlighted as illustrations of the special role of heavier transition metal ions in the field. This paper is addressed to readers who are interested in molecular magnetism and the application of coordination chemistry principles to materials synthesis (231 references).

Reactions of 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine (bptz) with solvated first-row transition metals M(II) (M(II) = Ni, Zn, Mn, Fe, Cu) have been explored with emphasis on the factors that influence the identity of the resulting cyclic products for Ni(II) and Zn(II). The relatively small anions, namely [ClO4]- and [BF4]-, lead to the formation of molecular squares [{M4(bptz)4(CH3CN)8} subsetX][X]7, (M = Zn(II), Ni(II); X = [BF4]-, [ClO4]-), whereas the larger anion [SbF6]- favors the molecular pentagon [{Ni5(bptz)5-(CH3CN)10} subsetSbF6][SbF6]9. The molecular pentagon easily converts to the square in the presence of excess [BF4]-, [ClO4]-, and [I]- anions, whereas the Ni(II) square can be partially converted to the less stable pentagon under more forcing conditions in the presence of excess [SbF6]- ions. No evidence for the molecular square being in equilibrium with the pentagon was observed in the ESI-MS spectra of the individual square and pentagon samples. Anion-exchange reactions of the encapsulated ion in [{Ni4(bptz)4(CH3CN)8} subsetClO4][ClO4]7 reveal that a larger anion such as [IO4]- cannot replace [ClO4]- inside the cavity, but that the linear [Br3]- anion is capable of doing so. ESI-MS studies of the reaction between [Ni(CH3CN)6][NO3]2 and bptz indicate that the product is trinuclear. Mass spectral studies of the bptz reactions with Mn(II), Fe(II), and Cu(II), in the presence of [ClO4]- anions, support the presence of molecular squares. The formation of the various metallacyclophanes is discussed in light of the factors that influence these self-assembly reactions, such as choice of metal ion, anion, and solvent.

Pentanuclear, cyanide-bridged clusters [M(tmphen)2]3[M'(CN)6]2 (M/M' = Zn/Cr (1), Zn/Fe (2), Fe/Fe (3), Fe/Co (4), and Fe/Cr (5); tmphen = 3,4,7,8-tetramethyl-1,10-phenanthroline) were prepared by combining [M'III(CN)6]3- anions with mononuclear complexes of MII ions with two capping tmphen ligands. The clusters consist of a trigonal bipyramidal (TBP) core with three MII ions in the equatorial positions and two M'III ions in the axial positions. Compounds 1-4 are isostructural and crystallize in the monoclinic space group P21/c. Complex 5 crystallizes in the enantiomorphic space group P3221. The magnetic properties of compounds 1 and 2 reflect the contributions of the individual [CrIII(CN)6]3- and [FeIII(CN)6]3- ions. The FeII ions in compounds 3 and 4 exhibit a gradual, temperature-induced spin transition between high spin (HS) and low spin (LS), as determined by the combination of Mössbauer spectroscopy, magnetic measurements, and single-crystal X-ray studies. The investigation of compound 5 by these methods and by IR spectroscopy indicates that cyanide linkage isomerism occurs during cluster formation. The magnetic behavior of 5 is determined by weak ferromagnetic coupling between the axial CrIII centers mediated by the equatorial diamagnetic FeII ions. Mössbauer spectra collected in the presence of a high applied field have allowed, for the first time, the direct experimental observation of uncompensated spin density at diamagnetic metal ions that bridge paramagnetic metal ions.