Xiaoguang Bao

Water-Free Rare Earth-Prussian Blue Type Analogues: Synthesis, Structure, Computational Analysis, and Magnetic Data of {LnIII(DMF)6FeIII(CN)6}∞(Ln = Rare Earths Excluding Pm)

Water-free rare earth(III) hexacyanoferrate(III) complexes, {Ln(DMF)(6)(mu-CN)(2)Fe(CN)(4)}(infinity) (DMF = N,N-dimethylformamide; Ln = Sm, 1; Eu, 2; Gd, 3; Tb, 4; Dy, 5; Ho, 6; Er, 7; Tm, 8; Yb, 9; Lu, 10; Y, 11; La, 12; Ce, 13; Pr, 14; Nd, 15), were synthesized in dry DMF through the metathesis reactions of [(18-crown-6)K](3)Fe(CN)(6) with LnX(3)(DMF)(n) (X = Cl or NO(3)). Anhydrous DMF solutions of LnX(3)(DMF)(n) were prepared at room temperature from LnCl(3) or LnX(3).nH(2)O under a dynamic vacuum. All compounds were characterized by IR, X-ray powder diffraction (except for 10), and single crystal X-ray diffraction (except for 2, 7, 10). Infrared spectra reveal that a monotonic, linear relationship exists between the ionic radius of the lanthanide and the nu(mu-CN) stretching frequency of 1-10, 12-15 while 11 deviates slightly from the ionic radius relationship. X-ray powder diffraction data are in agreement with powder patterns calculated from single crystal X-ray diffraction results, a useful alternative for bulk sample confirmation when elemental analysis data are difficult to obtain. Eight-coordinate Ln(III) metal centers are observed for all structures. trans-cyanide units of [Fe(CN)(6)](3-) formed isocyanide linkages to Ln(III) resulting in one-dimensional polymeric chains. Structures of compounds 1-9 and 11 are isomorphous, crystallizing in the space group C2/c. Structures of compounds 12-15 are also isomorphous, crystallizing in the space group P2/n. One unique polymeric chain exists in the structures of 1-9 and 11 while two unique polymeric chains exist in structures of 12-15. One of the polymeric chains of 12-15 is similar to that observed for 1-9, 11 while the other is more distorted and has a shorter Ln-Fe distance. Magnetic susceptibility measurements for compounds 3-6, 8, 11 were performed on polycrystalline samples of the compounds.

Encapsulation of Guests within a Gated Molecular Basket: Thermodynamics and Selectivity

Molecular basket 1 has been designed to contain a set of aromatic gates, each with rotational mobility restricted via intramolecular hydrogen bonding. This structural, yet dynamic, feature of the host has been revealed to permit the formation of a transient enclosed space capable of containing haloalkanes, whose size/shape, electronic and entropic attributes contributed to the thermodynamics of binding. Markedly, the basket is capable of mediating the trafficking of a broad range of molecules.

Molecular Recognition of a Transition State

Self-assembled or covalent hosts capable of enclosing space offer a confined environment for accommodating small to medium-sized molecules. When the isotropic solvent shell around a molecule is substituted by the framework of the host, unique properties can arise, including the stabilization of transient intermediates and catalysis of chemical reactions. The research groups of Cram and Sherman were among the first to observe and characterize the limited rotational mobility of smaller molecules residing in carceplexes, and these seminal studies invoked steric interactions as a source of the retardation. Subsequently, Rebek and co-workers characterized encapsulation complexes with guests having limited translational mobility, thereby establishing the phenomenon of social isomerism and revealing new types of supramolecular chirality. The conformational interconversion of encapsulated guest(s) has also been studied in artificial hosts, 7] and complexation was almost uniformly identified to retard or to have no effect, relative to a proper reference system. The deceleration has been speculated to arise from steric and electronic characteristics of the hosts affecting the reactant as well as the transition state(s) of the interconverting guest. The activation barrier for the ring flipping of 1,4-thioxane and 1,4-dioxane required an additional 1.6–1.8 kcalmol 1 (DDG ) within the restrictive interior of carcerands. The chair–chair interconversion process for cyclohexane was noted to occur slower in “jelly doughnut” (DDG 0.3 kcal mol ) and resorcin[4]arene (DDG = 0.25 0.10 kcal mol ) based cavitands. In the first case, this was rationalized by invoking favorable C H···p interactions to stabilize the chair ground state. Analogous studies on the rotation of the amide bond in encapsulated environments showed such an interconversion occurred at a faster/slower rate in hydrophobic, supramolecular assemblies than in polar or nonpolar (DDG 1–3 kcalmol ) solvents, respectively. In light of such discoveries, we report a rather unusual case of accelerated ring flipping of cyclohexane inside newly developed hosts—gated molecular baskets. We measured the kinetics of the conformational interconversion of [D11]cyclohexane (C6D11H) by quantitative NMR spectroscopy and used electronic structure methods to identify the origin of the observed acceleration. Gated molecular baskets (Figure 1) were designed as models for examining the kinetics of molecular encapsulation. These dynamic hosts comprise a bowl-shaped platform with three pyridine-based gates at its rim. The gates are transiently connected through a seam of intramolecular hydrogen bonds for controlling the in/out trafficking of guests (Figure 1). As a prelude to studies on the relationship

Four-state switching characteristics of a gated molecular basket.

The development of working molecular devices relies on the ability to extrinsically modulate function via structure. We have found that gated molecular basket 1 can be reversibly interconverted among four unique structural states (see above). Controlling the relative population of these states, the recognition characteristics of the basket can be finely tuned.