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Highly selective rare-earth separation has become increasingly important due to the indispensable role of these elements in various cutting-edge technologies including clean energy. However, the similar physicochemical properties of rare-earth elements (REEs) render their separation very challenging, and the development of new selective receptors for these elements is potentially of very considerable economic and environmental importance. Herein, we report the development of a series of 4-phosphoryl pyrazolone receptors for the selective separation of trivalent lanthanum, europium, and ytterbium as the representatives of light, middle, and heavy REEs, respectively. X-ray crystallography studies were employed to obtain solid-state structures across 11 of the resulting complexes, allowing comparative structure-function relationships to be probed, including the effect of lanthanide contraction that occurs along the series from lanthanum to europium to ytterbium and which potentially provides a basis for REE ion separation. In addition, the influence of ligand structure and lipophilicity on lanthanide binding and selectivity was systematically investigated via n -octanol/water distribution and liquid-liquid extraction (LLE) studies. Corresponding stoichiometry relationships between solid and solution states were well established using slope analyses. The results provide new insights into some fundamental lanthanide coordination chemistry from a separation perspective and establish 4-phosphoryl pyrazolone derivatives as potential practical extraction reagents for the selective separation of REEs in the future.

The development of molecule-based switchable materials remains an important challenge in the field of molecular science. Achievement of a structural phase transition induced by adsorption/desorption of guest molecules in spin crossover (SCO) Co(II) compounds is of significant interest because of the possibility that the spin state of the magnetic anisotropic high-spin (HS, S = 3/2) and low-spin (LS, S = 1/2) states can be switched via the induced changes in associated intermolecular interactions. In this study, we demonstrated a reversible magnetic switching associated with spin state conversion, along with a single-crystal to single-crystal (SCSC) phase transition induced by dehydration/rehydration. [Co(terpy) 2 ](BF 4 ) 2 ·H 2 O ( 1 ·H 2 O; terpy = 2,2':6',2''-terpyridine) assembles in the solid state via π-π and CH-π interactions involving adjacent terpyridine cores along the ab direction to form two-dimensional (2D) layered domains. 1 ·H 2 O exhibits gradual and incomplete SCO, from fully HS to ca. 0.5 HS, and the field-induced single-molecule magnet (SMM) behavior attributed to the presence of the anisotropic partial high-spin Co(II) species. 1 ·H 2 O undergoes a SCSC transformation accompanied by a change from the tetragonal space group I 4 1 / a to P 4 2 / n via a dehydration process. Dehydrated 1 exhibits a reverse thermal hysteresis behavior ( T 1/2↑ = 287 K; T 1/2↓ = 270 K) in the gradual SCO region from fully HS to ca. 0.5 HS, followed by an ordinary thermal hysteresis ( T ' 1/2↑ = 195 K; T ' 1/2↓ = 155 K) to fully LS Co(II). A temperature-dependent single-crystal X-ray structural analysis revealed that the reverse hysteresis can be attributed to an order/disorder structural phase transition of the BF 4 - anions involving a symmetry breaking to yield the monoclinic space group P 2 1 / n and orbital (angular momentum) transition ( L T). Both the SCSC phase transition and magnetic behavior are switchable by dehydration/rehydration processes; thus 1 again adsorbs water at room temperature to give both the original structure and its magnetic behavior.

Preferential formation of cis - or trans -palladium(II) complexes controlled via the exocyclic binding sites embedded in dithiamacrocycles ( L 1 = -S(CH 2 ) 2 S-; L 2 = -S(CH 2 ) 2 O(CH 2 ) 2 S-) is reported. From the reaction with K 2 PdCl 4 , the shorter sulfur-to-sulfur separation in L 1 preferentially leads to the formation of cis -[Pd( L 1 )Cl 2 ] ( 1 ), while L 2 , incorporating a larger sulfur-to-sulfur separation, coordinates in a trans fashion to form a cyclic dimer, trans -[Pd 2 ( L 2 ) 2 Cl 4 ] ( 2 ). The observed results illustrate the possibility for the controlled formation of cis/trans square-planar complexes through binding-site design. When palladium(II) acetate was substituted for K 2 PdCl 4 in the above reaction, L 1 gave no product, while L 2 resulted in the formation of a unique tripalladium(II) complex, [Pd 3 ( L 2 )(CH 3 C(═N)OC(═N)CH 3 )(CH 3 COO) 4 ] ( 3 ), in which three Pd II atoms are linked by acetimidic anhydride, CH 3 C(═N)OC(═N)CH 3 , derived from the acetonitrile solvent employed. In the 1 H NMR spectrum for 3 , specific methylene signals for methylene protons adjacent to S donors exhibit large complexation-induced splitting of the geminal proton signals into axial and equatorial proton peaks, thus indicating magnetically nonequivalent geminal protons that reflect the restricted conformation of the metallabicycle.

Two-dimensional Hofmann-type coordination polymers of type Mn(H 2 O) 2 [Pd(CN) 4 ]· x H 2 O ( 1· x H 2 O ; x = 0, 1, and 4), Mn(H 2 O)(MeOH)[Pd(CN) 4 ]·2MeOH ( 2·2MeOH ), and Mn(MeOH) 2 [Pd(CN) 4 ]·MeOH ( 3·MeOH ) have been synthesized. The homosolvent-bound 1·4H 2 O , 1·H 2 O , and 3·MeOH polymers consist of undulating layer structures, whereas the structure of heterosolvent-bound 2·2MeOH consists of "Janus-like" flat layers in which water-bound and MeOH-bound-sides are present. 1·4H 2 O and 1·H 2 O exhibited anisotropic two-dimensional thermal expansions involving structural transformations of the undulating layers; one layer axis expands while the other contracts. 2·2MeOH exhibits anisotropic thermal expansion in which the flat layers shift sideways as the temperature is increased, with colossal interlayer expansion occurring (α c = +200 MK -1 over 140-180 K, α c = +165 MK -1 over 200-280 K). 3·MeOH also showed colossal interlayer expansion (α c = +216 MK -1 ) together with expansion of the undulating layers.

The behavior of single-ion magnets (SIMs) that reflects large distortions of their coordination environments caused by the packing of long alkyl chains for two Co(II) complexes of the type [Co(C n -terpy) 2 ](BF 4 ) 2 (C n -terpy = 4'-alkoxy-2,2':6',2″-terpyridine; n = 10 (1), 16 (2)) is reported. 1·2MeOH, which features a highly distorted octahedral high-spin Co(II) center, exhibits field-induced slow magnetic relaxation under an applied dc field of 1000 Oe. Further detailed analysis of the relaxation process indicated the prevalence of the Raman process at low temperature. Surprisingly, 2 shows a reverse spin transition (rST) and also exhibits remarkable field-induced SIM behavior, revealing the presence of magnetic anisotropy for this high-spin Co(II) species that is triggered by a structural phase transition. We present here the first examples of the coexistence of field-induced slow magnetic relaxation and rST associated with structural phase transitions involving long-alkyl-chain conformational changes from gauche to anti. These results indicate the prospect of inducing SIM properties in other distorted high-spin Co(II) species bearing long alkyl chains.

Metal dilution effects on reverse spin transition (rST) in mixed crystals of type [Co(1-x)Zn(x)(C16-terpy)2](BF4)2 (x = 0.1-0.7) were investigated by comparison with behavior of [Co(1-x)Fe(x)(C16-terpy)2](BF4)2 (x = 0.1-0.4). In the mixed crystals, the Zn complexes increased rST temperatures linearly with increasing values of x, without changing the hysteresis width, while the Fe complexes decreased rST temperatures. Moreover, the strength of the metal dilution effects in the CoZn mixed crystals is weaker than what occurs in the CoFe mixed crystals.

The use of a highly efficient reductive amination procedure for the postsynthetic end-capping of metal-templated helicate and tetrahedral supramolecular structures bearing terminal aldehyde groups is reported. Metal template formation of a [Fe2L3](4+) dinuclear helicate and two [Fe4L6](8+) tetrahedra (where L is a linear ligand incorporating two bipyridine domains separated by one or two 1,4-(2,5-dimethoxyaryl) linkers and terminated by salicylaldehyde functions is described. Postassembly reaction of each of these "open" di- and tetranuclear species with excess ammonium acetate (as a source of ammonia) and sodium cyanoborohydride results in a remarkable reaction sequence whereby the three aldehyde groups terminating each end of the helicate, or each of the four vertices of the respective tetrahedra, react with ammonia then undergo successive reductive amination to yield corresponding fully tertiary-amine capped cryptate and tetrahedral covalent cages.

A rare, discrete, mixed-valent, heterometallic Fe(III)/Cu(II) cage, [Cu6Fe8L8](ClO4)12·χsolvent (H3L = tris{[2-{(imidazole-4-yl)methylidene}amino]ethyl}amine), was designed and synthesized via metal-ion-directed self-assembly with neutral tripodal metalloligands. The formation of this coordination cage was demonstrated by X-ray crystallography, ESI mass spectrometry, FT-IR, and UV-vis-NIR spectroscopy.

The metal complexation properties of the naturally occurring Maillard reaction product isomaltol HL(2) are investigated by measurement of its stability constants with copper(II), zinc(II), and iron(III) using potentiometric pH titrations in water, by structural and magnetic characterization of its crystalline complex, [Cu(L(2))(2)]·8H(2)O, and by density functional theory calculations. Strong complexation is observed to form the bis(isomaltolato)copper(II) complex incorporating copper in a typical (pseudo-)square-planar geometry. In the solid state, extensive intra- and intermolecular hydrogen bonding involving all three oxygen functions per ligand assembles the complexes into ribbons that interact to form two-dimensional arrays; further hydrogen bonds and π interactions between the furan moiety of the anionic ligands and adjacent copper(II) centers connect the complexes in the third dimension, leading to a compact polymeric three-dimensional (3D) arrangement. The latter interactions involving copper(II), which represent an underappreciated aspect of copper(II) chemistry, are compared to similar interactions present in other copper(II) 3D structures showing interactions with benzene molecules; the results indicate that dispersion forces dominate in the π system to chelated copper(II) ion interactions.

Reaction of the O(2)S(2)X-macrocycles (L(1), X = S; L(2), X = NH; and L(3), X = O) with Cu(ClO(4))(2) x 6 H(2)O affords 1:1 (M/L) square-pyramidal Cu(II) complexes when X = S and NH but yields a rare 1:2 sandwich-type tetrahedral Cu(I) complex when X = O; the X-ray structures of all three complexes are reported. Substitution of O for S or NH in the ligand structure thus results in a donor-set-induced II/I oxidation state change of the copper, and this is accompanied by a square-pyramidal to tetrahedral topological change in the solid state. Spectrophotometric titration data (including Job plots) indicate that similar behavior occurs in acetonitrile. In further experiments aimed at investigating the generality of the above redox behavior, it was shown that the 16- and 18-membered analogs of the 17-membered L(3) also induce a similar II/I redox change in acetonitrile. It was demonstrated for L(3) that the above-induced Cu(II/I) change is also maintained when the reaction solvent is changed from acetonitrile to methanol or ethanol.

New N-phenyl (L(1)), azo-coupled (L(2)), and tri-linked (L(3)) substituted derivatives of a parent dibenzo-N(3)O(2) macrocycle, 1,12,15-triaza-3,4:9,10-dibenzo-5,8-dioxacycloheptadecane (L(4)), have been synthesized. Competitive seven-metal transport studies across a bulk chloroform membrane employing an aqueous source phase containing equal molar concentrations of Co(II), Ni(II), Cu(II), Zn(II), Cd(II), Ag(I), and Pb(II) as their nitrate salts have been performed using both L(1) and L(3) as the ionophore, with the results discussed in terms of those obtained previously for related mono-ring (L(5)) and di-linked (L(6)) macrocyclic systems. Sole transport selectivity for Cu(II) was observed in each case. On a per macrocyclic cavity basis the tri-linked analogue L(3) gave less efficient Cu(II) transport than its monomeric or di-linked analogues. At least in part, this may reflect the tendency of tri-linked derivative L(3) to form a 2:1 complex (metal/ligand) with Cu(II) under the conditions employed; such a stoichiometry was demonstrated to occur in acetonitrile using both spectrophotometric titration and Job plot procedures. The azo-coupled derivative (L(2)) yields a red solution (lambda(max) = 495 nm, epsilon(max) = 23000 M(-1) cm(-1)) and undergoes significant hypochromic metal-induced shifts (Delta lambda(max) = 54-174 nm) on metal addition. Cu(II) induces the largest shift (Delta lambda(max) = 174 nm), corresponding to a color change from red to pale-yellow. The X-ray structures of red L(2) x HNO(3) together with its Cu(II) complexes, [Cu(L(2))NO(3)]NO(3) x CH(2)Cl(2) (6) (pale-yellow) and [{Cu(L(2))}(2)(mu-OH)(2)](ClO(4))(2) x 2 CH(2)Cl(2) x 2 H(2)O (7) (dark-red), are reported. The structural determinations have allowed insight into the structure-function relationships governing the observed color variation between these species.

The interaction of the diastereopure N,N'-bis(acetylacetone)cyclohexanediimine ligands, L(1)(1R,2R), L(2)(1S,2S), and their 1:1 mixture, with Zn(II) chloride has been investigated. Four new coordination polymers, [ZnL(1)Cl2 x H2O]n (1), [ZnL(2)Cl2 x H2O]n (2), [ZnL(2)Cl2]n (3), and [Zn6(L(2))6Cl12 x 2 H2O]n (4), each consisting of an infinite single helical chain displaying different pitches and/or handedness have been isolated. The complexed Schiff base ligands are present in their deprotonated enol forms, and the nitrogen atoms, which do not coordinate, are protonated because of proton transfer from the adjacent enol oxygen (coupled with concomitant N-H...O bond formation); each bound ligand is thus pseudo-zwitterionic. The respective zinc centers are bound to two chloro ligands and two oxygen donors from acacH-imine units belonging to different N,N'-bis(acetylacetone)cyclohexanediimine ligands such that the coordination at each zinc is distorted tetrahedral. Compounds 1 and 2, prepared from enantiopure L(1) and L(2), respectively, are enantiomers with similar structures, with the helical pitch in each being 17.0 A. Overall, the structure of 3 may be described as a one-dimensional helical chain with a pitch of 17.3 A, with each period corresponding to two L(2) ligands and two metal centers. The structure of [(Zn6L(2)6Cl12) x 2 H2O]n (4) contains six Zn(II) centers connected via six L(2) ligands to form a "bowed" helical repeat unit, with the pitch of the helix corresponding to 43.5 A. Supramolecular (intra- and intermolecular) aspects of all these unusual polymeric structures are discussed. Finally, the synthesis and characterization of an unprecedented six zinc-node discrete supramolecular assembly, [Zn6(L(1))3(L(2))3Cl12] (5), incorporating a 78-membered metallacycle, is also reported.