Bernard Omondi

Tandem ethylene oligomerisation and Friedel–Crafts alkylation of toluene catalysed by bis-(3,5-dimethylpyrazol-1-ylmethyl)benzene nickel(II) complexes and ethylaluminium dichloride

Three ligands, 1,2-bis(3,5-dimethylpyrazol-1-ylmethyl)benzene (L1), 1,3-bis(3,5-dimethylpyrazol-1-ylmethyl)benzene (L2) and 1,4-bis(3,5-dimethylpyrazol-1-ylmethyl)benzene (L3), were reacted with either nickel(II) chloride or nickel(II) bromide to produce four nickel complexes, Ni(L1)Br2 (1), Ni(L1)Cl2 (2), Ni(L2)Br2 (3), and Ni(L1)Br2 (4). The complexes were either mononuclear, 1 and 2, or polymeric, 3 and 4, depending on the positions of the pyrazolyl units on the benzene linker in the ligand. This was established from the crystal structures of 1, 2 and 3. All four complexes upon activation with ethylaluminium dichloride produced a tandem catalyst system that oligomerised ethylene to mainly 1-butene and 1-hexene and subsequently used the olefins present in the reaction medium to alkylate toluene that was used as solvent in the reactions. This led to mono-, di- and tri-alkyltoluenes with ethylene, butene and hexene.

Review: Multimetallic silver(I)–pyridinyl complexes: coordination of silver(I) and luminescence

This review highlights some structural features and luminescent properties of homo- and hetero-multinuclear silver(I)–pyridinyl complexes. It focuses on the coordination and geometry of the silver(I) ions to the pyridinyl-nitrogen. For this reason, we have considered only pyridinyl-N–Ag(I) complexes whose crystal data are available. In addition, this review does not consider mononuclear silver(I)–pyridinyl complexes as these have been reviewed elsewhere. This is motivated by the fact that multinuclear silver(I)–pyridinyl complexes have been shown to be more stable in solution, possess enhanced properties, and have fascinating structures compared to their mononuclear counterparts. The introduction highlights pyridinyl ligands used in complexation of silver(I) ions. The main body highlights complexation of silver(I) through pyridinyl nitrogen and the interactions found in the multinuclear silver(I)–pyridinyl complexes as well as the coordination number and geometry of silver(I) centers. Though silver(I) has been flaunted to prefer linear twofold coordination geometry, from this review, it is clear that higher coordination numbers in varied geometries are possible. These include distorted trigonal planar, T-shaped, distorted tetrahedral, trigonal bipyramidal, and octahedral geometries. Coordination of silver(I) to pyridinyl ligands and their metalloligands has been observed to impart or enhance luminescent properties in the ensuing complexes. Graphical abstract Possible coordination modes of silver(I) by pyridinyl ligands and Ag⋯Ag interactions.

Polymorphism and phase transformations in 2,6-disubstituted N-phenylformamides: the influence of hydrogen bonding, chloro-methyl exchange, intermolecular interactions and disorder

The crystal structures, thermal behaviour and phase transformations of a series of 2,6-disubstituted-N-phenylformamides have been investigated. A phase transformation was only observed when chlorine was one of the substituents. Crystals of the room-temperature form of 2,6-dichloro-N-phenylformamide (1) and 2-chloro-6-methyl-N-phenylformamide (2) are isomorphous. Both compounds are orthorhombic at room temperature and transform to a monoclinic high-temperature form at 155 and 106 °C, respectively. The room-temperature structures of 1 and 2 consist of chains of N–H⋯O hydrogen-bonded molecules stacked in an alternating arrangement along the crystallographic a direction. The high-temperature forms of compounds 1 and 2 (grown by sublimation) are both monoclinic but not isomorphous, with one short axis of about 4.3 A, and consist of chains of N–H⋯O hydrogen-bonded molecules stacked along the short axis, related by translation. When both of the chlorine substituents are replaced by methyl groups, as in 2,6-dimethyl-N-phenylformamide (3), the crystals do not undergo any phase transition on heating and only an orthorhombic form, space group P212121, has been isolated. Examination of the molecular geometry and structural properties of 3 indicates that it is a hybrid structure of the low- and high-temperature forms of compounds 1 and 2. This contribution analyzes the effect of chloro-methyl exchange, the steering ability of chlorine, and the role of weak interactions on the structural and thermal properties of the compounds studied. In addition, a mechanism for the phase change in 1 is proposed and rationalized through the examination of the structures themselves together with lattice energy calculations.

Advances in carbon nanotubes as efficacious supports for palladium-catalysed carbon-carbon cross-coupling reactions

Since the 1970s, palladium-catalysed carbon–carbon (C–C) bond formation has made a critical impact in organic synthesis. In early studies, homogeneous palladium catalysts were extensively used for this reaction with limitations such as difficulty in separation and recycling ability. Lately, heterogeneous palladium-based catalysts have shown promise as surrogates for conventional homogeneous catalysts in C–C coupling reactions, since the product is easy to isolate, while the catalyst is reusable and hence sustainable. Recently, a better part of these heterogeneous palladium catalysts are supported on carbon nanotubes (Pd/CNTs), that have shown superior catalytic performance and better recyclability since the CNT support imparts stability to the palladium catalyst. This review discusses the wide variety of surface functionalization techniques for CNTs that improve their properties as catalyst supports, as well as the methods available for loading the catalyst nanoparticles onto the CNTs. It will survey the literature where Pd/CNTs catalysts have been utilized for C–C coupling reactions, with particular emphasis on Suzuki–Miyaura and Mizoroki–Heck coupling reactions. It will also highlight some of the important parameters that affect these reactions.

Zn(II) and Cu(II) formamidine complexes: structural, kinetics and polymer tacticity studies in the ring-opening polymerization of ε-caprolactone and lactides

Treatment of N,N′-bis(2,6-dimethylphenyl)formamidine (L1), N,N′-bis(2,6-diisopropylphenyl)formamidine (L2), and N,N′-dimesitylformamidine (L3) with Zn(OAc)2·2H2O or Cu(OAc)2·H2O produced the corresponding Zn(II) and Cu(II) N,N′-diarylformamidine complexes [Zn3(L1)2(OAc)6] (1), [Zn2(L2)2(OAc)4] (2), [Zn2(L3)2(OAc)4] (3) and [Cu2(L2)2(OAc)4] (4), respectively. While complex 1 is trinuclear, compounds 2–4 are dimeric in the solid state. The X-band EPR spectra of complex 4 in solid and solution states are consistent with perfect axial symmetry and confirm retention of the dinuclear paddle-wheel core in the solution state. Complexes 1–4 formed active catalysts in the ring opening polymerization (ROP) of e-caprolactone (e-CL) and lactides (LA). Complexes 1 and 3 exhibited higher rate constants of 0.1009 h−1 and 0.0963 h−1 compared to the rate constants of 0.0479 h−1 and 0.0477 h−1 observed for 2 and 4, respectively, in the ROP of e-CL at 110 °C. Higher rate constants of 0.5963 h−1 and 1.2962 h−1 were obtained for complexes 1 and 3 in the ROP of LAs compared to those reported in the ROP of e-CL at 110 °C. Activation parameters were determined as ΔH‡ = 25.08 kJ mol−1 and ΔS‡ = −201.7 J K−1 mol−1 for the ROP of e-CL using 3. Investigation of the kinetics of polymerization of e-CL and LAs revealed first order dependence of the polymerization reactions on monomer concentration. Moderate molecular weight polymers of up to 21 286 g mol−1 exhibiting relatively moderate molecular weight distributions and moderately heterotactic PLAs with Pr up to 0.65 were obtained.

Synthesis, characterization, and cytotoxic and antimicrobial activities of ruthenium(II) arene complexes with N,N-bidentate ligands

Abstract Three new complexes, [(η6-C6H6)RuCl(C5H4N-2-CH=N-Ar)]PF6 (Ar = phenylmethylene (1), (4-methoxyphenyl)methylene (2), and phenylhydrazone (3)), were prepared by reacting [(η6-C6H6)Ru(μ-Cl)Cl]2 with N,N′-bidentate ligands in a 1 : 2 ratio. Full characterization of the complexes was accomplished using 1H and 13C NMR, elemental and thermal analyses, UV–vis and IR spectroscopy and single crystal X-ray structures. Single crystal structures confirmed a pseudo-octahedral three-legged, piano-stool geometry around Ru(II), with the ligand coordinated to the ruthenium(II) through two N atoms. The cytotoxicity of the mononuclear complexes was established against three human cancer cell lines and selectivity was also tested against non-cancerous human epithelial kidney (HEK 293) cells. The compounds were selective toward the tumor cells in contrast to the known anti-cancer drug 5-fluoro uracil which was not selective between the tumor cells and non-tumor cells. All the compounds showed moderate activity against MCF7 (human breast adenocarcinoma), but showed low antiproliferative activity against Caco-2 and HepG2. Also, antimicrobial activities of the complexes were tested against a panel of antimicrobial-susceptible and -resistant Gram-negative and Gram-positive bacteria. Of special interest is the anti-mycobacterial activity of all three synthesized complexes against Mycobacterium smegmatis, and bactericidal activity against resistant Enterococcus faecalis and methicillin-resistant Staphylococcus aureus ATCC 43300.

Group 12 dithiocarbamate complexes: Synthesis, characterization, and X-ray crystal structures of Zn(II) and Hg(II) complexes and their use as precursors for metal sulfide nanoparticles

ABSTRACT Zn(II), Cd(II), and Hg(II) dithiocarbamate complexes were synthesize and characterized by elemental analysis, thermogravimetric analysis, UV-Vis, FTIR, and 1H- and 13C-NMR spectroscopy. Single-crystal X-ray crystallography revealed that the Zn complex has a centrosymmetric dimeric structure while the Hg complex crystallizes with two monomeric molecules of the mercury complex and two molecules of toluene solvent in the asymmetric unit. The compounds were used as single molecule precursors to synthesize HDA capped metal sulfides nanoparticles with average crystallite size ranging from 7 to 22 nm. The optical properties of the nanoparticles showed evidence of quantum confinement.

Concomitant polymorphic behavior of di-μ-thiocyanato-κ2N:S;κ2S:N-bis[bis(tri-p-fluorophenylphosphine-κP)silver(I)]

The structures of two polymorphs, both monoclinic P2(1)/n [polymorph (I)] and P2(1)/c [polymorph (II)], of di-mu-thiocyanato-kappa(2)N:S;kappa(2)S:N-bis[bis(tri-p-fluorophenylphosphine-kappaP)silver(I)] complexes have been determined at 100 K. In both polymorphs the complex has a dinuclear structure where the silver(I) coordinates to two phosphine ligands and two bridging thiocyanate anions to form complexes with distorted tetrahedral geometry. Polymorph (I) has just one half of the [Ag(2)(SCN)(2){P(4-FC(6)H(4))(3)}(4)] molecule at (0, 1/2, 0) from the origin in the asymmetric unit. Polymorph (II) has one and a half molecules of [Ag(2)(SCN)(2){P(4-FC(6)H(4))(3)}(4)] in the asymmetric unit; the half molecule is situated at (0, 1, 1/2), while the full molecule is located at (1/3, 1/2, 1/3) from the origin. The Ag-P bond distances range from 2.4437 (4) to 2.4956 (7) A in both polymorphs. The Ag-S distances are 2.5773 (7) A in (I) and 2.5457 (5), 2.5576 (5) and 2.5576 (5) A in (II). The full molecule in polymorph (II) has slightly shorter Ag-N bond distances [2.375 (1) and 2.367 (2) A] compared with the half molecules in both polymorphs [2.409 (2) A in (II) and 2.395 (2) A in (I)]. The two polymorphs are compared using r.m.s. overlay calculations as well as half-normal probability plot analysis.

Isomorphism in monomeric 1:3 complexes of silver(I) salts with tri-p-tolylphosphine.

Reaction of silver(I) salts with three equivalents of tri-p-tolylphosphine in CH(3)CN resulted in a series of isomorphous complexes [AgX{P(4-MeC(6)H(4))(3)}(3)] (X = Br, SCN, ClO(4)). These complexes all crystallize in the orthorhombic space group Pna2(1). The complexes with X = Br, SCN are distorted tetrahedral around the silver(I) atom, whereas the ClO(4)(-) complex is distorted trigonal planar around the silver. The new complexes are compared with each other using r.m.s. overlay calculations as well as half-normal probability plot analysis and with the previously reported isomorphous chloride, bromide as well as the non-isomorphous iodide complexes.

Cocrystal of cis- and trans-N-phenyl- formamide

N-Phenylformamide, C(7)H(7)NO, crystallizes with two molecules in the asymmetric unit which have different conformations of the formylamino group, one being cis and the other trans. This is the first example of an arylformamide crystal containing both conformational isomers and it may thus be considered a cocrystal of the two conformers. The two molecules in the unit cell are linked through N-H...O hydrogen bonding to two other molecules, thereby forming hydrogen-bonded tetramers within the crystal structure.