Chun Y. Liu

Bio: Chun Y. Liu is an academic researcher from Texas A&M University. The author has contributed to research in topics: Oxidation state & Crystal structure. The author has an hindex of 12, co-authored 13 publications receiving 438 citations. Previous affiliations of Chun Y. Liu include University of Costa Rica & Tongji University.

Modifying electronic communication in dimolybdenum units by linkage isomers of bridged oxamidate dianions.

Abstract: Reactions of Mo2(O2CCH3)(DAniF)3, DAniF = N,N‘-di-p-anisylformamidinate, with oxamidate dianions [ArNC(O)C(O)NAr]2-, Ar = C6H5 and p-anisyl, give pairs of isomeric compounds where the [Mo2] units are bridged by the oxamidate anions. For the α isomers, the C−C unit of the dianion is nearly perpendicular to the Mo−Mo bonds, and these are essentially perpendicular to each other. For the β isomers, the corresponding C−C unit and the Mo−Mo bonds are essentially parallel to each other. Each type of isomer is stable in solution. The electronic communication as measured by the ΔE1/2 for the oxidation of each of the Mo2 units is significantly better for the β isomers. This is supported also by the appearance of what is conventionally called an intervalence charge-transfer band in the near infrared region upon oxidation of the β isomers but not the α isomers. Molecular mechanics and DFT calculations help explain the relative conformations in the α isomers and the relative energy differences between the α and β isomers.

Strong Electronic Coupling between Dimolybdenum Units Linked by the N,N‘-Dimethyloxamidate Anion in a Molecule Having a Heteronaphthalene-like Structure

Abstract: Very strong electronic communication, manifested in a KC value of ca. 109, has been found in a system of three compounds separated by one electron oxidation where each compound contains two metal−metal bonded Mo2n+ units linked by a dimethyloxamidate anion, CH3N(O)C−C(O)NCH32-. The Mo−Mo distances increase as the oxidation state increases and the bond order decreases, while the diamagnetic, doubly oxidized species becomes essentially planar and resembles a naphthalene molecule. Calculations at the DFT level indicate that a strong transition in the near-IR region, shown by the singly oxidized and paramagnetic species, is best described as a HOMO−1 → SOMO transition.

A Fractional Bond Order of 1/2 in Pd25+−Formamidinate Species; The Value of Very High-Field EPR Spectra

Abstract: Reaction of Pd2(DAniF)4, 1, (DAniF = di-p-anisylformamidinate) with 1 equiv of AgPF6 in CH2Cl2 at or below −10 °C produces the paramagnetic species [Pd2(DAniF)4]PF6, 1-PF6, that has been studied by X-ray crystallography, UV−vis spectroscopy, electrochemistry, and multifrequency (9.5, 34.5, 110, and 220 GHz) EPR spectroscopy. Upon oxidation of the precursor, the Pd−Pd distance decreases by 0.052 A from 2.6486(8) to 2.597(1) A. The EPR spectra show broad signals with line widths of about 1000 G. The spectra collected at high field show a large spread of g tensor components (∼0.03), but these are masked at lower frequencies (9.5 and 34.5 GHz). A reinvestigation using high-field EPR of the p-tolyl analogue, which is the only other structurally characterized Pd25+ species (Cotton, F. A.; Matusz, M.; Poli, R.; Feng, X. J. Am. Chem. Soc. 1988, 110, 1144), shows that this species, which had been reported to give an isotropic 9.5 GHz EPR spectrum, also gives anisotropic 110 and 220 GHz EPR spectra with a similarly...

The First Designed Syntheses of Bis-dimetal Molecules in Which the Bridges Are Diamidate Ligands

Abstract: The first deliberate syntheses of molecules in which pairs of quadruply bonded Mo2 units are bridged by N,N'-diarylterephthaloyldiamidate (aryl = Ph, m-CF3Ph) ligands are described. The addition of neutral N,N'-diarylterephthaloyldiamide to 2 equiv of [Mo2(DAniF)3(MeCN)2]+ (DAniF = N,N'-di-p-anisylformamidinate) followed by the introduction of excess H3CO- in MeCN results in the formation of (DAniF)3Mo2[(C6H5)NC(O)C6H4(O)CN(C6H5)]Mo2(DAniF)3 (1) and (DAniF)3Mo2[[(m-CF3)C6H5]NC(O)C6H4(O)CN[(m-CF3)C6H5]]Mo2(DAniF)3 (2). The DeltaE1/2 for the oxidation of each Mo2 unit is greater for these terephthaloyldiamidate-bridged molecules (approximately 100 mV) than for the analogous terephthalate-bridged compound (approximately 60 mV). Variation in the nature of the substituents on the diamidate nitrogen atoms offers a means to fine-tune the oxidation potentials of the Mo2 units.

Systematic preparation of Mo 2 4+ building blocks for supramolecular assemblies.

Abstract: The preparation of additional and useful building blocks for the construction of supramolecular entities with quadruply bonded Mo(2)(4+) units has been explored, and five new mixed-ligand complexes with three types of ligands and various basicities are reported. The ligands used were the DAniF (N,N'-di-p-anisylformamidinate) anion, the acetate anion, and neutral acetonitrile molecules. The formamidinate ligands are the least labile, and the acetonitrile molecules are the most labile. This difference as well as a relatively strong trans directing influence by the formamidinate anions in ligand substitution reactions allows designed synthesis of various mixed-ligand building blocks, including rare pairs of cis and trans isomers. The new compounds are cis-Mo(2)(DAniF)(2)(O(2)CCH(3))(2) (1), trans-Mo(2)(DAniF)(2)(O(2)CCH(3))(2) (2), trans-[Mo(2)(DAniF)(2)(O(2)CCH(3))(CH(3)CN(eq)())(2)]BF(4) (3), trans-[Mo(2)(DAniF)(2)(CH(3)CN(eq)())(4)](BF(4))(2) (4), and [Mo(2)(O(2)CH(3))(CH(3)CN(eq)())(6)(CH(3)CN(ax)())](BF(4))(3) (5), where eq and ax designate equatorial and axial ligands, respectively. A comparison with some previously synthesized complexes is given along with a discussion of the overall reactivity of all compounds.

Supramolecular coordination: self-assembly of finite two- and three-dimensional ensembles.

Abstract: Fascination with supramolecular chemistry over the last few decades has led to the synthesis of an ever-increasing number of elegant and intricate functional structures with sizes that approach nanoscopic dimensions Today, it has grown into a mature field of modern science whose interfaces with many disciplines have provided invaluable opportunities for crossing boundaries both inside and between the fields of chemistry, physics, and biology This chemistry is of continuing interest for synthetic chemists; partly because of the fascinating physical and chemical properties and the complex and varied aesthetically pleasing structures that supramolecules possess For scientists seeking to design novel molecular materials exhibiting unusual sensing, magnetic, optical, and catalytic properties, and for researchers investigating the structure and function of biomolecules, supramolecular chemistry provides limitless possibilities Thus, it transcends the traditional divisional boundaries of science and represents a highly interdisciplinary field In the early 1960s, the discovery of ‘crown ethers’, ‘cryptands’ and ‘spherands’ by Pedersen,1 Lehn,2 and Cram3 respectively, led to the realization that small, complementary molecules can be made to recognize each other through non-covalent interactions such as hydrogen-bonding, charge-charge, donor-acceptor, π-π, van der Waals, etc Such ‘programmed’ molecules can thus be self-assembled by utilizing these interactions in a definite algorithm to form large supramolecules that have different physicochemical properties than those of the precursor building blocks Typical systems are designed such that the self-assembly process is kinetically reversible; the individual building blocks gradually funnel towards an ensemble that represents the thermodynamic minimum of the system via numerous association and dissociation steps By tuning various reaction parameters, the reaction equilibrium can be shifted towards the desired product As such, self-assembly has a distinct advantage over traditional, stepwise synthetic approaches when accessing large molecules It is well known that nature has the ability to assemble relatively simple molecular precursors into extremely complex biomolecules, which are vital for life processes Nature’s building blocks possess specific functionalities in configurations that allow them to interact with one another in a deliberate manner Protein folding, nucleic acid assembly and tertiary structure, phospholipid membranes, ribosomes, microtubules, etc are but a selective, representative example of self-assembly in nature that is of critical importance for living organisms Nature makes use of a variety of weak, non-covalent interactions such as hydrogen–bonding, charge–charge, donor–acceptor, π-π, van der Waals, hydrophilic and hydrophobic, etc interactions to achieve these highly complex and often symmetrical architectures In fact, the existence of life is heavily dependent on these phenomena The aforementioned structures provide inspiration for chemists seeking to exploit the ‘weak interactions’ described above to make scaffolds rivaling the complexity of natural systems The breadth of supramolecular chemistry has progressively increased with the synthesis of numerous unique supramolecules each year Based on the interactions used in the assembly process, supramolecular chemistry can be broadly classified in to three main branches: i) those that utilize H-bonding motifs in the supramolecular architectures, ii) processes that primarily use other non-covalent interactions such as ion-ion, ion-dipole, π–π stacking, cation-π, van der Waals and hydrophobic interactions, and iii) those that employ strong and directional metal-ligand bonds for the assembly process However, as the scale and degree of complexity of desired molecules increases, the assembly of small molecular units into large, discrete supramolecules becomes an increasingly daunting task This has been due in large part to the inability to completely control the directionality of the weak forces employed in the first two classifications above Coordination-driven self-assembly, which defines the third approach, affords a greater control over the rational design of 2D and 3D architectures by capitalizing on the predictable nature of the metal-ligand coordination sphere and ligand lability to encode directionality Thus, this third strategy represents an alternative route to better execute the “bottom-up” synthetic strategy for designing molecules of desired dimensions, ranging from a few cubic angstroms to over a cubic nanometer For instance, a wide array of 2D systems: rhomboids, squares, rectangles, triangles, etc, and 3D systems: trigonal pyramids, trigonal prisms, cubes, cuboctahedra, double squares, adamantanoids, dodecahedra and a variety of other cages have been reported As in nature, inherent preferences for particular geometries and binding motifs are ‘encoded’ in certain molecules depending on the metals and functional groups present; these moieties help to control the way in which the building blocks assemble into well-defined, discrete supramolecules4 Since the early pioneering work by Lehn5 and Sauvage6 on the feasibility and usefulness of coordination-driven self-assembly in the formation of infinite helicates, grids, ladders, racks, knots, rings, catenanes, rotaxanes and related species,7 several groups - Stang,8 Raymond,9 Fujita,10 Mirkin,11 Cotton12 and others13,14 have independently developed and exploited novel coordination-based paradigms for the self-assembly of discrete metallacycles and metallacages with well-defined shapes and sizes In the last decade, the concepts and perspectives of coordination-driven self-assembly have been delineated and summarized in several insightful reviews covering various aspects of coordinationdriven self-assembly15 In the last decade, the use of this synthetic strategy has led to metallacages dubbed as “molecular flasks” by Fujita,16 and Raymond and Bergman,17 which due to their ability to encapsulate guest molecules, allowed for the observation of unique chemical phenomena and unusual reactions which cannot be achieved in the conventional gas, liquid or solid phases Furthermore, these assemblies found applications in supramolecular catalysis18,19 and as nanomaterials as developed by Hupp20 and others21,22 This review focuses on the journey of early coordination-driven self-assembly paradigms to more complex and discrete 2D and 3D supramolecular ensembles over the last decade We begin with a discussion of various approaches that have been developed by different groups to assemble finite supramolecular architectures The subsequent sections contain detailed discussions on the synthesis of discrete 2D and 3D systems, their functionalizations and applications

Bimetallic Pd(III) complexes in palladium-catalysed carbon–heteroatom bond formation.

Abstract: Palladium is a common transition metal for catalysis, and the fundamental organometallic reactivity of palladium in its 0, I, II and IV oxidation states is well established. The potential role of Pd(III) in catalysis has not been investigated because organometallic reactions that involve Pd(III) have not been reported previously. In this article we present the formation of carbon–heteroatom bonds from discrete bimetallic Pd(III) complexes and show the synergistic involvement of two palladium atoms of the bimetallic core during both oxidation and reductive elimination. Our results challenge the currently accepted mechanism for oxidative palladium catalysis via Pd(II)–Pd(IV) redox cycles and implicate bimetallic palladium complexes in redox catalysis. The new mechanistic insight provides an opportunity to explore rationally the potential of bimetallic palladium catalysis for synthesis.

Modern Transition-Metal-Catalyzed Carbon–Halogen Bond Formation

Abstract: The high utility of halogenated organic compounds has prompted the development of a vast number of transformations which install the carbon–halogen motif. Traditional routes to these building blocks have commonly involved multiple steps, harsh reaction conditions, and the use of stoichiometric and/or toxic reagents. In this regard, using transition metals to catalyze the synthesis of organohalides has become a mature field in itself, and applying these technologies has allowed for a decrease in the production of waste, higher levels of regio- and stereoselectivity, and the ability to produce enantioenriched target compounds. Furthermore, transition metals offer the distinct advantage of possessing a diverse spectrum of mechanistic possibilities which translate to the capability to apply new substrate classes and afford novel and difficult-to-access structures. This Review provides comprehensive coverage of modern transition metal-catalyzed syntheses of organohalides via a diverse array of mechanisms. Atte...

Recent developments in palladium catalysed carbonylation reactions

Abstract: Recently, carbonylation reactions have gained considerable interest as they are becoming a versatile tool in the synthesis of pharmaceuticals, agrochemicals and their intermediates. Nowadays, a plethora of transition metal catalysts are available for the synthesis of various functional groups like ureas, carbamates, oxamates, oxamides, α-keto amides, ketones, esters, etc. using carbonylation methodology. Several carbonylation reactions such as aminocarbonylation, alkoxycarbonylation, double carbonylation and oxidative carbonylation, provide efficient and attractive alternatives to the conventional synthetic routes on a laboratory or industrial scale. Oxidative carbonylation is an important reaction as it allows direct carbonylative C–H bond activation. A double carbonylation reaction provides a one step alternative route for the synthesis of α-keto amides, oxamides, and oxamates. It also eliminates the use of conventional thermally unstable and toxic reagents like oxalyl chloride. Several recent studies have focused on the various aspects of these reactions, including catalyst–product separation, and catalyst recoverability and reusability. In view of this, developments in anchoring homogeneous catalysts using various techniques like biphasic catalysis and supported liquid phase catalysis are gaining importance. Carbonylation routes using these techniques are simple, efficient, economical, avoid the use of ligands, and give the desired products in excellent yields. The use of phosphine ligands is disadvantageous as it leads to air/moisture sensitivity, tedious work-up procedures and high work-up costs. Several phosphine-free carbonylation routes eliminate the use of phosphine ligands, and provide economical and simple methods for these transformations. In this review we have summarized the recent trends in carbonylative transformations, which have undergone a rapid development.