Free-Radical Approach to 4-Substituted Dipicolinates

Dipolar and Contact Paramagnetic NMR Chemical Shifts in AnIV Complexes with Dipicolinic Acid Derivatives

Actinide +IV complexes (An^IV = Th^IV, U^IV, Np^IV, and Pu^IV) with two dipicolinic acid derivatives (DPA and Et-DPA) have been studied by ¹H and ¹³C NMR spectroscopies and first-principles calculations. The Fermi contact and dipolar contributions to the actinide-induced shifts (AIS) are evaluated from a temperature dependence analysis, combined with ab initio results. It allows an experimental estimation of the axial anisotropy of the magnetic susceptibility Δχ_ax and of the hyperfine coupling constants of the NMR-active nuclei. Due to the compactness of the coordination sphere, the magnetic anisotropy of the paramagnetic center is small, and this makes the contact contribution to be the dominant one, even on the remote atoms. The sign of the hyperfine coupling constants and related spin densities is alternating on the nuclei of the ligand cycle, denoting a preponderant spin polarization mechanism. This is well reproduced by unrestricted density functional theory (DFT) calculations. Those values are furthermore slightly decreasing in the actinide series, which indicates a small decrease of the covalency from U^IV to Pu^IV.

Temperature Dependence of 1 H Paramagnetic Chemical Shifts in Actinide Complexes, Beyond Bleaney's Theory: The AnVI O2 2+ -Dipicolinic Acid Complexes (An=Np, Pu) as an Example

Actinide +VI complexes ( A n V I = U V I , N p V I and P u V I ) with dipicolinic acid derivatives were synthesized and characterized by powder XRD, SQUID magnetometry and NMR spectroscopy. In addition, N p V I and P u V I complexes were described by first principles CAS based and two-component spin-restricted DFT methods. The analysis of the ¹ H paramagnetic NMR chemical shifts for all protons of the ligands according to the X-rays structures shows that the Fermi contact contribution is negligible in agreement with spin density determined by unrestricted DFT. The magnetic susceptibility tensor is determined by combining SQUID, pNMR shifts and Evans' method. The SO-RASPT2 results fit well the experimental magnetic susceptibility and pNMR chemical shifts. The role of the counterions in the solid phase is pointed out; their presence impacts the magnetic properties of the N p V I complex. The temperature dependence of the pNMR chemical shifts has a strong 1 / T contribution, contrarily to Bleaney's theory for lanthanide complexes. The fitting of the temperature dependence of the pNMR chemical shifts and SQUID magnetic susceptibility by a two-Kramers-doublet model for the N p V I complex and a non-Kramers-doublet model for the P u V I complex allows for the experimental evaluation of energy gaps and magnetic moments of the paramagnetic center.

Syntheses and anti-HIV and human cluster of differentiation 4 (CD4) down-modulating potencies of pyridine-fused cyclotriazadisulfonamide (CADA) compounds

CADA compounds selectively down-modulate human cell-surface CD4 protein and are of interest as HIV entry inhibitors and as drugs for asthma, rheumatoid arthritis, diabetes and some cancers. Postulating that fusing a pyridine ring bearing hydrophobic substituents into the macrocyclic scaffold of CADA compounds may lead to potent compounds with improved properties, 17 macrocycles were synthesized, 14 with 12-membered rings having an isobutylene head group, two arenesulfonyl side arms, and fused pyridine rings bearing a para substituent. The analogs display a wide range of CD4 down-modulating and anti-HIV potencies, including some with greater potency than CADA, proving that a highly basic nitrogen atom in the 12-membered ring is not required for potency and that hydrophobic substituents enhance potency of pyridine-fused CADA compounds. Cytotoxicities of the new compounds compared favorably with those of CADA, showing that incorporation of a pyridine ring into the macrocyclic scaffold can produce selective compounds for potently down-modulating proteins of medicinal interest.

Insight of the Metal-Ligand Interaction in f-Element Complexes by Paramagnetic NMR Spectroscopy

The magnetic properties of Ln^III and An^III complexes formed with dipicolinate ligands have been studied by NMR spectroscopy. To know precisely the geometries of these complexes, a crystallographic study by single-crystal X-ray diffraction (XRD) and extended X-ray absorption fine structure (EXAFS) in solution was performed. Several methods to separate the paramagnetic shifts observed in the NMR spectra were applied to these complexes. Methods using a number of nuclei of the dipicolinate ligands revealed an abrupt change in the geometries of the complexes and a metal-ligand interaction in the middle of the lanthanide series. A study of the variation of the paramagnetic shifts with temperature demonstrated that higher-order terms of the dipolar and contact contributions are required, especially for the lightest Ln^III and almost all the studied An^III . Bleaney's parameters <S_z >_a and C a D relating to the contact and dipolar terms, respectively, were deduced from experimental data and compared with the results of ab initio calculations. Quite a good agreement was found for the temperature dependencies of <S_z >_a and C a D . However, the C a D values obtained from cation magnetic anisotropy calculations showed some discrepancies with the values derived from Bleaney's equation defined for Ln^III . Other parameters, such as the crystal field parameter and the hyperfine constants F_i obtained from the experimental data of the [An(ethyl-dpa)₃ ]^3- complexes (ethyl-dpa=4-ethyl-2,6-dipicolinic acid), are at odds with the assumptions underlying Bleaney's theory.

Sub-10-nm patterning via directed self-assembly of block copolymer films with a vapour-phase deposited topcoat

Directed self-assembly (DSA) of the domain structure in block copolymer (BCP) thin films is a promising approach for sub-10-nm surface patterning. DSA requires the control of interfacial properties on both interfaces of a BCP film to induce the formation of domains that traverse the entire film with a perpendicular orientation. Here we show a methodology to control the interfacial properties of BCP films that uses a polymer topcoat deposited by initiated chemical vapour deposition (iCVD). The iCVD topcoat forms a crosslinked network that grafts to and immobilizes BCP chains to create an interface that is equally attractive to both blocks of the underlying copolymer. The topcoat, in conjunction with a chemically patterned substrate, directs the assembly of the grating structures in BCP films with a half-pitch dimension of 9.3 nm. As the iCVD topcoat can be as thin as 7 nm, it is amenable to pattern transfer without removal. The ease of vapour-phase deposition, applicability to high-resolution BCP systems and integration with pattern-transfer schemes are attractive properties of iCVD topcoats for industrial applications.

C-H Functionalization of Azines

Azines, which are six-membered aromatic compounds containing one or more nitrogen atoms, serve as ubiquitous structural cores of aromatic species with important applications in biological and materials sciences. Among a variety of synthetic approaches toward azines, C-H functionalization represents the most rapid and atom-economical transformation, and it is advantageous for the late-stage functionalization of azine-containing functional molecules. Since azines have several C-H bonds with different reactivities, the development of new reactions that allow for the functionalization of azines in a regioselective fashion has comprised a central issue. This review describes recent advances in the C-H functionalization of azines categorized as follows: (1) S_NAr reactions, (2) radical reactions, (3) deprotonation/functionalization, and (4) metal-catalyzed reactions.

Synthesis of Mono- and Dinuclear Vanadium Complexes and Their Reactivity toward Dehydroperoxidation of Alkyl Hydroperoxides

Several vanadium(V) complexes with either dipic-based or Schiff base ligands were synthesized. The complexes were fully characterized by elemental analysis, IR, ¹H, ¹³C, and ⁵¹V NMR spectroscopy, as well as mass spectrometry and X-ray diffraction. Furthermore, they were tested toward their catalytic deperoxidation behavior and a significant difference between 4-heptyl hydroperoxide and cyclohexyl hydroperoxide was observed. In the case of 4-heptyl hydroperoxide, the selectivity toward the corresponding ketone was higher than with cyclohexyl hydroperoxide. DFT calculations performed on the vanadium complex showed that selective decomposition of secondary hydroperoxides with vanadium(V) to yield the corresponding ketone and water is indeed energetically feasible. The computed catalytic path, involving cleavage of the O-O bond, hydrogen transfer, release of ketone/water, and finally addition of hydroperoxide, can proceed without the generation of radical species.

Heptacoordinate Heteroleptic Salan (ONNO) and Thiosalan (OSSO) Titanium(IV) Complexes: Investigation of Stability and Cytotoxicity

Seven heptacoordinate titanium(IV) complexes were synthesized based on the concept of hetero-bis-chelate stabilization of salan (ONNO) and thiosalan (OSSO) titanium(IV)alkoxides with 2,6-pyridinedicarboxylic acid (dipic) and derivatives thereof. The resulting compounds were investigated in a solid by X-ray diffraction and in solution by NMR spectroscopy. A thiosalan (OSSO) titanium(IV) complex could be isolated and its conformational stabilization by dipic was shown by (1)H NMR spectroscopy to lead to nonfluxional behavior even at room temperature. The stability of selected complexes was assessed at pH 1.9, 6.8, and 12.1 by an UV-vis monitored hydrolysis study with >5 Mio. equivalents of water. Even at pH 12.1 [L(1)Ti(dipic)(1)] showed t1/2 of more than 2 days. The cytotoxicity of all compounds was investigated in two human carcinoma cell lines. IC50-values in the range of cisplatin were achieved by all tested compounds except for [L(4)Ti(dipic)(1)], which was proven to be nontoxic. The functionalization of dipic was thus well tolerated and did neither interfere with the stability nor the cytotoxicity of the heteroleptic complexes.

An exploration of homo- and heterometallic UO22+ hybrid materials containing chelidamic acid: synthesis, structure, and luminescence studies

Eight U(vi)–chelidamate compounds highlight the structural significance of modified ligands as sources for direct metal–ligand coordination or halogen-based interaction platforms.