Metallocyclopentadienyl derivatives of bivalent samarium,europium and ytterbium

It has been established that the reaction of metallocyclopentadienyl mercurials with an excess of elemental lanthanides Ln(Ln = Sm, Eu, Yb) in tetrahydrofuran yields substituted cyclopentadienyl derivatives of the metals.     

Syntheses,structures, and electronic interactions of dicyanamide/tricyanomethanide-bridged binuclear organometallic complexes

Dicyanamide-bound mononuclear compounds Cp(dppe)FeN(CN)2 (3) and Cp(PPh3)2RuN(CN)2 (4) were isolated in high yields by the reactions of Cp(dppe)FeCl (1) and Cp(PPh3)2RuCl (2), respectively, with excess sodium dicyanamide. Compounds 3 and 4 are excellent precursors for the design of dicyanamide-bridged binuclear complexes [[Cp(dppe)Fe]2N(CN)2](SbF6) (5) and [[Cp(PPh3)2Ru]2N(CN)2](SbF6) (6) by the incorporation with 1 and 2, respectively. Controlling oxidation of 5 with ferrocenium hexafluorophosphate afforded the mixed-valence compound [[Cp(dppe)Fe]2N(CN)2](PF6)2 (5a) which exhibits a broad absorption band in the near-infrared region (centered at 1500 nm, epsilon = 750 cm-1 M-1) due to the intervalence charge transfer of Robin and Day class II mixed-valence system. Tricyanomethanide-bound mononuclear compounds Cp(dppe)FeC(CN)3 (7) and Cp(PPh3)2RuC(CN)3 (8) were prepared by the same methods as 3 and 4 using potassium tricyanomethanide as the starting material instead. The tricyanomethanide-bridged binuclear complexes [[Cp(dppe)Fe]2C(CN)3](CF3SO3) (9) and [[Cp(PPh3)2- Ru]2C(CN)3](SbF6) (10) were prepared by the reactions between 7 and 1 and between 8 and 2, respectively. Cyclic voltammograms of the dicyanamide/tricyanomethanide-bridged binuclear complexes showed stepwise reversible one-electron oxidation waves with the potential separation of the two redox couples in the range 0.14-0.25 V, indicating the demonstrably electronic communication is operative between the organometallic components through a dicyanamide/tricyanomethanide spacer with metal...metal distances more than 7.8 A. Furthermore, the electronic coupling transmitted by the tricyanomethanide is appreciably greater than that by the dicyanamide. The complexes 3-10 were characterized by elemental analysis, IR, UV-vis, 1H and 31P NMR, and ES-MS. The crystal structures of 3 and 5-9 were determined by X-ray crystallography.     

The electronic structures and bonding of some transition-metal monoborides

The band structures and densities of states for the isostructural monoborides of Ti, Mn, Fe, and Co have been calculated by a LCAO-TB method. These results are related to the physical and spectroscopic properties of the materials. A satisfactory account of the ESCA spectra and a qualitative correlation with the electrical and magnetic properties is afforded by the calculations. A second series of calculations by the periodic cluster method is also carried out in order to reveal more details of the bonding and the effects of diffusion of electrons into the unoccupied levels above the Fermi edge.     

Crystal structures of dicarboxy-2,2'-bipyridyl complexes: the role of hydrogen bonding and stacking interactions

The crystal structures of three complexes of dicarboxy-2,2'-bipyridyl ligands, 5,5'-dicarboxy-2,2'-bipyridyl (1) and 4,4'-dicarboxy-2,2'-bipyridyl (2) are reported. [Rh(1H)3] shows two interpenetrating, homochiral rhombohedral networks linked by short carboxylate-carboxylic acid hydrogen bonds, in which each complex acts as a node for six hydrogen bonds. [Ru(1H2)(1H)2] forms only four such hydrogen bonds, leading to the formation of heterochiral chains held together by stacking between bipyridyls. [Co(2H)3] can in principle form six hydrogen bonds, but in practice forms only four in a layer structure where stacking interactions are important. This is attributed to differences in molecular shape.     

Secondary bonding interactions observed in two arsenic thiolate complexes

Treatment of N-(2-mercaptoethyl)-1,8-naphthalimide (HL) with stoichiometric amounts of AsCl(3) and base affords AsL(2)Cl and AsL(3) complexes stabilized in part by secondary As...O bonding interactions.     

New mono- and bis-carbene samarium complexes: synthesis, X-ray crystal structures and reactivity

New samarium carbene complexes have been synthesized and characterized by X-ray diffraction analysis; the carbenic nature was assessed by reactivity studies.     

Intermolecular interactions and geometry of non-rigid molecules in weak crystalline complexes: Vibrational spectra of 4,4′-dinitrobiphenyl complexes with biphenyl,biphenyl derivatives and p-terphenyl

Raman spectroscopy is used to study the complexes of 4,4′-dinitrobiphenyl with biphenyl, 4-hydroxybiphenyl, 4-bromobiphenyl and p-terphenyl, which crystallize in a highly unusual geometry. Their phonon spectra at 125 K and 18 K are compared and the effect of isotopic substitution of biphenyl on the phonon spectra of its complex is examined. Internal vibrations of the components in the crystalline complex are compared with those observed in the pure crystals of the components. The results from both phonon and intramolecular vibration studies show that these complexes form in fixed stoichiometries, are governed by geometrical factors, and are stabilized primarily by van der Waals interaction, although other kinds of interactions may provide additional stabilization. The 4,4′-dinitrobiphenyl molecule as well as biphenyl and p-terphenyl are centrosymmetric and remain so when the complexes are cooled from room temperature to 18 K. For biphenyl complex, this conclusion is supported by the observed IR spectra which show mutual exclusion between IR-active and Raman active vibrations. Crystal splitting is observed on the 410 cm^?1 vibration of 4,4′-dinitrobiphenyl. This splitting is attributed to the presence of more than one 4,4′-dinitrobiphenyl molecules in the complex unit.     

Hydrogen–hydrogen bonding in biphenyl revisited

The evidence for the stabilizing nature of the H–H bonding in planar biphenyl is succinctly reviewed. The stabilizing nature of the H–H bonding is revealed through a comparison of the atomic energy of every atom in planar biphenyl with the same atom in the twisted equilibrium structure. It is shown that the barrier to rotation via the planar transition state is the net resultant of a stabilisation of the four ortho-hydrogen atoms (by 8 kcal/mol each), a stabilisation of the two para-carbon atoms (by 3 kcal/mol each) and by the dominant destabilisation of the two carbon atoms joining the two rings—the two junction carbon atoms—(by 22 kcal/mol each). The energetic stabilisation of the four ortho-hydrogen atoms is further shown to be in large proportion due to the formation of the hydrogen–hydrogen interatomic surface. Furthermore, neither the “bond order” between the two junction carbon atoms nor the total electron delocalisation between the two rings exhibit a significant change in going from the planar to the twisted equilibrium geometry. These findings are in contrast with the classical view of a balance between “steric non-bonded repulsion” and better electron delocalisation as a function of the twist dihedral angle. Similar conclusions have been recently reached by Pacios and Gómez through a study of the electrostatic potential at the position of the hydrogen nuclei. We dedicate this article to Professor TM Krygowski on the occasion of his 70th birthday wishing him a long and productive life.     

Synthesis and structural characterization of novel mixed-valent samarium and divalent ytterbium and europium complexes supported by amine bis(phenolate) ligands

The amine-elimination reactions of Ln[N(SiMe3)2]2(THF)2(Ln=Sm, Yb and Eu) with amine bis(phenol)s (L1H2=[BunN(CH(2)-2-OC6H(2)-3,5-But2)2]H2; L2H2=[Me2NCH2CH2N(CH(2)-2-OC6H(2)-3,5-But2)2]H2) were investigated. It was found that the number of heteroatom(s) in the ligands has a profound effect on the reaction outcome for the samarium systems. Reaction of the tetradentate diamino-bis(phenol)s L2H2 with Sm[N(SiMe3)2]2(THF)2 afforded a yellow solution, which indicated the complete oxidation of the SmII species, yellow being the characteristic color of SmIII species, while the same reaction with Eu[N(SiMe3)2]2(THF)2 gave a divalent complex with a dimeric structure (EuL2)2. Using the tridentate amine bis(phenol)s L1H2 as the reagent, the novel mixed-valent samarium complex SmIII2SmIIL1(4) was prepared by the same reaction. Both reactions of L1H2 with Yb[N(SiMe3)2]2(THF)2 and Eu[N(SiMe3)2]2(THF)2 yielded the normal divalent lanthanide complexes: monomeric complex for YbII, YbL1(THF)3 and dimeric complex for EuII, (EuL1)2. All of the complexes are well characterized with elemental analyses, IR and 1H NMR spectra for , and , as well as X-ray crystal structure determination in the cases of complexes , , and .     

Modeling the electronic structures of the ground and excited states of the ytterbium atom and the ytterbium dimer: A modern quantum chemistry perspective

We present a comprehensive theoretical study of the electronic structures of the Yb atom and the Yb₂ molecule, respectively, focusing on their ground and lowest-lying electronically excited states. Our study includes various state-of-the-art quantum chemistry methods such as CCSD, CCSD(T), CASPT2 (including spin-orbit coupling), and EOM-CCSD as well as some recently developed pCCD-based approaches and their extensions to target excited states. Specifically, we scan the lowest-lying potential energy surfaces of the Yb₂ dimer and provide a reliable benchmark set of spectroscopic parameters including optimal bond lengths, vibrational frequencies, potential energy depths, and adiabatic excitation energies. Our in-depth analysis unravels the complex nature of the electronic spectrum of Yb₂, which is difficult to model accurately by any conventional quantum chemistry method. Finally, we scrutinize the bi-excited character of the first excited state and its evolution along the potential energy surface.     

Strength and nature of hydrogen bonding interactions in mono- and di-hydrated formamide complexes

In this work, mono- and di-hydrated complexes of the formamide were studied. The calculations were performed at the MP2/6-311++G(d,p) level of approximation. The atoms in molecules theory (AIM), based on the topological properties of the electronic density distribution, was used to characterize the different types of bonds. The analysis of the hydrogen bonds (H-bonds) in the most stable mono- and di-hydrated formamide complexes shows a mutual reinforcement of the interactions, and some of these complexes can be considered as "bifunctional hydrogen bonding hydration complexes". In addition, we analyzed how the strength and the nature of the interactions, in mono-hydrated complexes, are modified by the presence of a second water molecule in di-hydrated formamide complexes. Structural changes, cooperativity, and electron density redistributions demonstrate that the H-bonds are stronger in the di-hydrated complexes than in the corresponding mono-hydrated complexes, wherein the σ- and π-electron delocalization were found. To explain the nature of such interactions, we carried out the atoms in molecules theory in conjunction with reduced variational space self-consistent field (RVS) decomposition analysis. On the basis of the local Virial theorem, the characteristics of the local electron energy density components at the bond critical points (BCPs) (the 1/4? (2)ρ(b) component of electron energy density and the kinetic energy density) were analyzed. These parameters were used in conjunction with the electron density and the Laplacian of the electron density to analyze the characteristics of the interactions. The analysis of the interaction energy components for the systems considered indicates that the strengthening of the hydrogen bonds is manifested by an increased contribution of the electrostatic energy component represented by the kinetic energy density at the BCP.     

Surface segregation in polymer blends and interpolymer complexes with increasing hydrogen bonding interactions

The evolution of surface composition in polymer blends and interpolymer complexes was studied using X‐ray photoelectron spectroscopy (XPS) and Time‐of‐Flight secondary ion mass spectroscopy (ToF‐SIMS). For immiscible and miscible poly(styrene‐co‐4‐vinyl phenol)/poly(styrene‐co‐4‐vinyl pyridine) (STVPh/STVPy) blends, surface enrichment by the lower surface energy component STVPh was always observed. Increasing VPh contents in STVPh from 0 to 16 mol % spans the transition from immiscible to miscible blends; the differences in surface free energies between STVPh and STVPy decreased, but surface enrichment of STVPh continued to increase. This is due to the strong hydrogen bonded self‐association of STVPh, which dominates over the immiscibility to miscibility transition in controlling the surface composition. In the immiscible and miscible blends, decreasing the molecular weights of STVPy, which decreased the surface free energy of STVPy, systematically reduced surface enrichment by STVPh. For STVPh/STVPy complexes formed at VPh contents higher than 21 mol %, surface enrichment of STVPh is barely detectable. STVPh and STVPy form a new supramolecular species. Interpolymer complexation is now the decisive factor controlling the surface composition, dominating over the surface free energy differences; the effect of STVPy molecular weight variation on the surface composition is also negligible for the interpolymer complexes. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1924–1930, 2005     

Distinct electronic effects on reductive eliminations of symmetrical and unsymmetrical bis-aryl platinum complexes

Symmetrical bis-aryl platinum complexes (DPPF)Pt(C(6)H(4)-4-R)(2) (R = NMe(2), OMe, CH(3), H, Cl, CF(3)) and electronically unsymmetrical bis-aryl platinum complexes (DPPF)Pt(C(6)H(4)-4-R)(C(6)H(4)-4-X) (R = CH(3), X = NMe(2), OMe, H, Cl, F, CF(3); R = OMe, X = NMe(2), H, Cl, F, CF(3); R = CF(3), X = H, Cl, NMe(2); and R = NMe(2), X = H, Cl) were prepared, and the rates of reductive elimination of these complexes in the presence of excess PPh(3) are reported. The platinum complexes reductively eliminated biaryl compounds in quantitative yields with first-order rate constants that were independent of the concentration of PPh(3). Plots of Log(k(obs)/k(obs(H))) vs Hammett substituent constants (sigma) of the para substituents R and X showed that the rates of reductive elimination reactions depended on two different electronic properties. The reductive elimination from symmetrical bis-aryl platinum complexes occurred faster from complexes with more electron-donating para substituents R. However, reductive elimination from a series of electronically unsymmetrical bis-aryl complexes was not faster from complexes with the more electron-donating substituents. Instead, reductive elimination was faster from complexes with a larger difference in the electronic properties of the substituents on the two platinum-bound aryl groups. The two electronic effects can complement or cancel each other. Thus, this combination of electronic effects gives rise to complex, but now more interpretable, free energy relationships for reductive elimination.     

Thermal stabilities,electronic properties and structures of metformin-metal complexes

The atomic superposition and electron delocalization molecular orbital (ASED-MO) theory was used to calculate structures and relative stabilities of metformin-metal complexes. The relative stabilities and decomposition pathways were discussed in terms of bond order, binding energy and the nature of charge on the central metal atom. The electronic transitions and their energy gaps were also studied. The optimization of the structures shows that the most stable state is distorted from planarity for Co^II and Ni^II complexes.     

Pentadienyls vs cyclopentadienyls and reversal of metal-ligand bonding affinity with metal oxidation state: synthesis, molecular structures, and electronic structures of high-valent zirconium pentadienyl complexes

Molecules of the form Cp(6,6-dmch)ZrX(2) (Cp = eta(5)-cyclopentadienyl, X = Cl, Br, I; 6,6-dmch = eta(5)-6,6-dimethylcyclohexadienyl) have been synthesized, and the molecular and electronic structures have been investigated. These molecules allow direct comparison of the bonding and properties of pentadienyl and cyclopentadienyl ligands in the same high-oxidation-state metal complexes. Unlike the well-known Cp(2)ZrX(2) analogues, these Cp(6,6-dmch)ZrX(2) molecules are intensely colored, indicating significantly different relative energies of the frontier orbitals. Also unusual, the average Zr-C distances to the 6,6-dmch pentadienyl ligand are about 0.1 A longer than the average Zr-C distances to the cyclopentadienyl ligand for these Zr(IV) complexes, opposite of what is observed for the Zr(II) complex Cp(2,6,6-tmch)Zr(PMe(3))(2) (tmch = eta(5)-2,6,6-trimethylcyclohexadienyl), reflecting a dramatic reversal in the favorability of the bonding depending on the metal oxidation state. The experimental and computational results indicate that the color of the Cp(6,6-dmch)ZrX(2) complexes is due to a 6,6-dmch ligand-to-metal charge-transfer band. Compared to the Cp(2)ZrX(2) analogues, the Cp(6,6-dmch)ZrX(2) molecules have a considerably less stable HOMO that is pentadienyl-based and an essentially unchanged metal-based LUMO. Also, the lowest unoccupied orbital of pentadienyl is stabilized relative to cyclopentadienyl and becomes a better potential delta electron acceptor, thus contributing to the differences in structure and reactivity of the low-valent and high-valent metal complexes.     

Theoretical study of rhenium dinuclear complexes: Re-Re bonding nature and electronic structure

Four dinuclear rhenium complexes, [Re2Cl8](2-) (1), [Re2(mu-Cl)3Cl6](2-) (2a), [Re2(mu-Cl)3Cl6](-) (2b), and [Re2(mu-Cl)2Cl8](2-) (3), were theoretically investigated by the CASSCF, MRMP2, SA-CASSCF, and MCQDPT methods. Interesting differences in electronic structure and Re-Re bonding nature among these complexes are clearly reported here, as follows: In 1, the ground state is the 1A1g state. The approximate stabilization energies by the sigma, pi, and delta bonding interactions are evaluated to be 4.36, 2.89, and 0.52 eV, respectively, by the MRMP2 method. In 2a, the ground state is the 2E" state. The approximate stabilization energy by two degenerate delta bonding interactions is estimated to be 0.36 eV by the MCQDPT method. One delta bonding interaction of 2a is much weaker than that of 1, which is discussed in terms of the Re-Re distance and the Re oxidation state. In 2b, the ground state is the 1A1' state, of which multiconfigurational nature is extremely large unlike that of the 2E" ground state of 2a despite similarities between 2a and 2b. In 3, the sigma, pi, and delta bonding interactions are not effectively formed between two Re centers. As a result, the 1Ag, 3B1u, 5Ag, and 7B1u states are in almost the same energy within 0.03 eV. This result is consistent with the paramagnetism of 3 experimentally reported.     

Theoretical studies on relation among structures,electric structures and magnetic interactions in MMX complexes

The electronic structures of Ni₂(dta)₄I and Pt₂(dta)₂I complexes were examined by using UB3LYP calculations in terms of relation between magnetic interactions and structures of those complexes. The calculated effective exchange integrals (J_ab) values by using experimental structures were −488 and −1274 cm⁻¹ for Ni₂(dta)₄I and Pt₂(dta)₄I complexes, respectively. Natural orbital (NO) analyses revealed the origin of the difference in their antiferromagnetic interactions. In order to explain a variety of structural phases in those complexes, such as averaged valence (AV), spin density wave (SDW), charge polarization (CP) and alternate charge polarization (ACP), potential and J_ab surfaces were also investigated. The calculated potential surfaces explained experimental results that Ni₂(dta)₄I complex only took AVSDW phase, while Pt₂(dta)₄I complex underwent AVSDW, CP and ACP phases by temperature.     

Distribution of samarium and ytterbium in rats measured by enriched stable isotope tracer technique and INAA

In the present study a method using enriched stable isotope tracer and instrumental neutron activation analysis (INAA) was developed to study the dynamic distribution of rare earth elements (REEs) in a variety of organs and tissues of Wistar rats. Stable isotopes ¹⁵²Sm and ¹⁶⁸Yb were selected as tracers for the experiment. Intravenously injected ¹⁵²Sm and ¹⁶⁸Yb in chloride form could be quickly absorbed and distributed in almost all the organs and tissues of interest, including liver, skeleton, kidney, spleen, heart, lung, testicle, and blood serum. Liver and skeleton had high ability to take up ¹⁵²Sm and ¹⁶⁸Yb under the experimental conditions, whereas the contents of the elements in other organs were generally lower than 2% of the given dose during the whole experimental period. The difference in distribution of ¹⁵²Sm and ¹⁶⁸Yb in the body was also discussed.