¹H-MAS-NMR chemical shifts in hydrogen-bonded complexes of chlorophenols (pentachlorophenol, 2,4,6-trichlorophenol, 2,6-dichlorophenol, 3,5-dichlorophenol, and p-chlorophenol) and amine, and H/D isotope effects on ¹H-MAS-NMR spectra

Pd(II) Complexes with Pyridine Ligands: Substituent Effects on the NMR Data, Crystal Structures, and Catalytic Activity

A wide range of functionalized pyridine ligands have been employed to synthesize a variety of Pd(II) complexes of the general formulas [PdL₄](NO₃)₂ and [PdL₂Y₂], where L = 4-X-py and Y = Cl^– or NO₃^–. Their structures have been unambiguously established via analytical and spectroscopic methods in solution (NMR spectroscopy and mass spectrometry) as well as in the solid state (X-ray diffraction). This in-depth characterization has shown that the functionalization of ligand molecules with groups of either electron-withdrawing or -donating nature (EWG and EDG) results in significant changes in the physicochemical properties of the desired coordination compounds. Downfield shifts of signals in the ¹H NMR spectra were observed upon coordination within and across the complex families, clearly indicating the relationship between NMR chemical shifts and the ligand basicity as estimated from pK_a values. A detailed crystallographic study has revealed the operation of a variety of weak interactions, which may be factors explaining aspects of the solution chemistry of the complexes. The Pd(II) complexes have been found to be efficient and versatile precatalysts in Suzuki–Miyaura and Heck cross-coupling reactions within a scope of structurally distinct substrates, and factors have been identified that have contributed to efficiency improvement in both processes.

A wide range of pyridine derivatives have been employed to synthesize a variety of di- and tetrasubstituted Pd(II) complexes of square-planar geometry. This in-depth characterization has shown that the functionalization of ligand molecules with groups of either electron-withdrawing or -donating nature results in significant changes in the physicochemical properties of the coordination compounds. Moreover, the complexes have been found to be of practical utility as efficient precatalysts for both Suzuki−Miyaura and Heck cross-coupling reactions.

Synthesis and Characterization of Pt(II) Complexes with Pyridyl Ligands: Elongated Octahedral Ion Pairs and Other Factors Influencing 1H NMR Spectra

Our goal is to develop convenient methods for obtaining trans-[Pt^II(4-Xpy)₂Cl₂] complexes applicable to 4-substituted pyridines (4-Xpy) with limited volatility and water solubility, properties typical of 4-Xpy, with X being a moiety targeting drug delivery. Treatment of cis-[Pt^II(DMSO)₂Cl₂] (DMSO = dimethyl sulfoxide) with 4-Xpy in acetonitrile allowed isolation of a new series of simple trans-[Pt^II(4-Xpy)₂Cl₂] complexes. A side product with very downfield H2/6 signals led to our synthesis of a series of new [Pt^II(4-Xpy)₄]Cl₂ salts. For both series in CDCl₃, the size of the H2/6 Δδ [coordinated minus "free" 4-Xpy H2/6 shift] decreased as 4-Xpy donor ability increased from 4-CNpy to 4-Me₂Npy. This finding can be attributed to the greater synergistic reduction in the inductive effect of the Pt(II) center with increased 4-Xpy donor ability. The high solubility of [Pt^II(4-Xpy)₄]Cl₂ salts in CDCl₃ (a solvent with low polarity) and the very downfield shift of the [Pt^II(4-Xpy)₄]Cl₂ H2/6 signals for the solutions provide evidence for the presence of strong {[Pt^II(4-Xpy)₄]²⁺,2Cl^-} ion pairs that are stabilized by multiple CH···Cl contacts. This conclusion gains considerable support from [Pt^II(4-Xpy)₄]Cl₂ crystal structures revealing that a chloride anion occupies a pseudoaxial position with nonbonding (py)C-H···Cl contacts (2.4-3.0 Å). Evidence for (py)C-H···Y contacts was obtained in NMR studies of [Pt^II(4-Xpy)₄]Y₂ salts with Y counterions less capable of forming H-bonds than chloride ion. Our synthetic approaches and spectroscopic analysis are clearly applicable to other nonvolatile ligands.

Models for B12-conjugated radiopharmaceuticals. Cobaloxime binding to new fac-[Re(CO)3(Me2bipyridine)(amidine)]BF4 complexes having an exposed pyridyl nitrogen

New mononuclear amidine complexes, fac-[Re(CO)3(Me2bipy)(HNC(CH3)(pyppz))]BF4 [(4,4'-Me2bipy (1), 5,5'-Me2bipy (2), and 6,6'-Me2bipy (3)] (bipy = 2,2'-bipyridine), were synthesized by treating the parent fac-[Re(I)(CO)3(Me2bipy)(CH3CN)]BF4 complex with the C2-symmetrical amine 1-(4-pyridyl)piperazine (pyppzH). The axial amidine ligand has an exposed, highly basic pyridyl nitrogen. The reaction of complexes 1-3 with a B12 model, (py)Co(DH)2Cl (DH = monoanion of dimethylglyoxime), in CH2Cl2 yielded the respective dinuclear complexes, namely, fac-[Re(CO)3(Me2bipy)(μ-(HNC(CH3)(pyppz)))Co(DH)2Cl]BF4 [(4,4'-Me2bipy (4), 5,5'-Me2bipy (5), and 6,6'-Me2bipy (6)]. (1)H NMR spectroscopic analysis of all compounds and single-crystal X-ray crystallographic data for 2, 3, 5, and 6 established that the amidine had only the E configuration in both the solid and solution states and that the pyridyl group is bound to Co in 4-6. Comparison of the NMR spectra of 1-3 with spectra of 4-6 reveals an unusually large "wrong-way" upfield shift for the pyridyl H2/6 signal for 4-6. The wrong-way H2/6 shift of (4-Xpy)Co(DH)2Cl (4-Xpy = 4-substituted pyridine) complexes increased with increasing basicity of the 4-Xpy derivative, a finding attributed to the influence of the magnetic anisotropy of the cobalt center on the shifts of the (1)H NMR signals of the pyridyl protons closest to Co. Our method of employing a coordinate bond for conjugating the fac-[Re(I)(CO)3] core to a vitamin B12 model could be extended to natural B12 derivatives. Because B12 compounds are known to accumulate in cancer cells, such an approach is a very attractive method for the development of (99m)Tc and (186/188)Re radiopharmaceuticals for targeted tumor imaging and therapy.

Molecular dynamics of neutral polymer bonding agent (NPBA) as revealed by solid-state NMR spectroscopy

Neutral polymer bonding agent (NPBA) is one of the most promising polymeric materials, widely used in nitrate ester plasticized polyether (NEPE) propellant as bonding agent. The structure and dynamics of NPBA under different conditions of temperatures and sample processing are comprehensively investigated by solid state NMR (SSNMR). The results indicate that both the main chain and side chain of NPBA are quite rigid below its glass transition temperature (Tg). In contrast, above the Tg, the main chain remains relatively immobilized, while the side chains become highly flexible, which presumably weakens the interaction between bonding agent and the binder or oxidant fillers and in turn destabilizes the high modulus layer formed around the oxidant fillers. In addition, no obvious variation is found for the microstructure of NPBA upon aging treatment or soaking with acetone. These experimental results provide useful insights for understanding the structural properties of NPBA and its interaction with other constituents of solid composite propellants under different processing and working conditions.