Stephen O. Ojwach

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.

Multidentate bis(pyrazolylmethyl)pyridine ligands: coordination chemistry and binding properties with zinc(II) and cadmium(II) cations

The coordination chemistry and cationic binding properties of 2,6-bis(pyrazol-1-ylmethyl)pyridine (L1), 2,6-bis(3,5-dimethylpyrazol-1-ylmethyl)pyridine (L2), and 2,6-bis(3,5-ditertbutylpyrazol-1-ylmethyl)pyridine (L3) with zinc(II) and cadmium(II) have been investigated. Reactions of L2 with zinc(II) and cadmium(II) nitrate or chloride salts produced monometallic complexes [Zn(NO3)2(L2)] (1), [ZnCl2(L2)] (2), [Cd(NO3)2(L2)] (3), and [CdCl2(L2)] (4). Solid state structures of 1 and 3 confirmed that L2 binds in a tridentate mode. While the nitrates in the zinc complex (1) adopt monodentate binding fashion, in cadmium complex (3), they exhibit bidentate mode. L1–L3 show binding efficiencies of 99% for zinc(II), 60% for lead(II), and 30% for cadmium(II) cations from aqueous solutions of the metal ions. Theoretical studies using Density Functional Theory were consistent with the observed extraction results.

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.

Pyridyl)benzoazole palladium(II) complexes as homogeneous catalysts in hydrogenation of alkenes and alkynes

Reactions of 2-(2-pyridyl)benzimidazole (L1), 2-(2-pyridyl)benzothiazole (L2) and 2-(2-pyridyl)benzoxazole (L3) with either [PdCl2(NCMe)2] or [PdClMe(COD)] produced complexes [(PdCl2(L1)] (1), [(PdCl2(L2)] (2), [(PdCl2(L3)] (3) and [(PdClMe(L1)] (4) in good yields. Treatment of 1 with PPh3 gave the cationic complex [(Pd(L1)ClPPh3]Cl (5), while reactions of 4 with one equiv. of PPh3 and NaBAr4 (Ar = 3,5-(CF3)2C6H3) produced the cationic complex [(Pd(L1)MePPh3]BAr4 (6). Single-crystal X-ray analysis has been used to elucidate the structure of complex 6. The complexes formed active catalysts in hydrogenation reactions of alkenes and alkynes. Hydrogenation of terminal alkenes was accompanied by isomerization to the internal isomers, while alkyne reactions involved a two-step process producing alkenes and the respective alkanes. While the kinetics data and formation of inactive palladium nanoparticles support the homogeneous nature of the active species, mercury drop experiments indicated a possible role of the nanoparticles in the re-generation of the active species.

Ring-opening polymerization of ε-caprolactone catalysed by (pyridyl)benzoazole Zn(II) and Cu(II) complexes

This paper describes the synthesis of (pyridyl)benzoazole Zn(II) and Cu(II) complexes and their applications as catalysts in ring-opening polymerization (ROP) of ε-caprolactone (ε-CL). Reactions of 2-(3-pyridyl)-1H-benzimidazole (L1), 2-(2-pyridyl)-1H-benzothiazole (L2) and 2-(2-pyridyl)-1H-benzimidazole (L3) with Zn(II) and Cu(II) acetates produced the corresponding complexes; [Zn2(L1)2(OAc)4)] (1), [Cu2(L1)2(OAc)4] (2), [Zn(L2)(OAc)2)] (3), [Zn(L3)(OAc)2)] (4) and [Cu(L3), (OAc)2)] (5). Molecular structures of complexes 2 and 5a revealed that while L1 adopts a monodentate binding mode, through the pyridyl nitrogen atom, L3 exhibits a bidentate coordination mode. All the complexes formed active catalysts in the ROP of ε-CL to afford moderate molecular weight polymers. The kinetics of the ROP reactions of ε-CL were pseudo-first-order with respect to monomer and catalysts.

Structural, kinetic, and DFT studies of the transfer hydrogenation of ketones mediated by (pyrazole)pyridine iron(II) and nickel(II) complexes

A series of iron(II) and nickel(II) complexes chelated by 2-pyrazolyl(methyl)pyridine (L1), 2,6-bis(pyrazolylmethyl)pyridine (L2), and 2,6-bis(pyrazolyl)pyridine (L3) ligands have been investigated as transfer hydrogenation (TH) catalysts for a range of ketones. Nine chelates in total were studied: [Ni(L1)Br2] (1), [Ni(L1)Cl2] (2), [Fe(L1)Br2] (3), [Ni(L2)Br2] (4), [Ni(L2)Br2] (5), [Fe(L2)Cl2] (6), [Ni(L3)Br2] (7), [Ni(L3)Br2] (8), and [Fe(L3)Cl2] (9). Attempted crystallization of complexes 4 and 6 afforded stable six-coordinate cationic species 4a and 6a with a 2 : 1 ligand : metal (L : M) stoichiometry, as opposed to the monochelates that function as precursors to catalytic species for TH reactions. Crystallization of 7·4H2O and 8·2H2O, in contrast, afforded tri- and bis(aqua) salts of L3 chelated to Ni(II) in a 1 : 1 L : M stoichiometry, respectively. Complexes 1–9 formed active catalysts for the TH of a range of ketones in 2-propanol at 82 °C. Both the nature of the metal ion and ligand moiety had a discernible impact on the catalytic activities of the complexes, with nickel(II) chelate 5 affording the most active catalyst (kobs, 4.3 × 10−5 s−1) when the inductive phase lag was appropriately modelled in the kinetics. Iron(II) complex 3 formed the most active TH catalyst without a significant inductive phase lag in the kinetics. DFT and solid angle calculations were used to rationalize the kinetic data: both steric shielding of the metal ion and electronic effects correlating with the metal–ligand distances appear to be significant factors underpinning the reactivity of 1–9. Catalysts derived from 1 and 9 exhibit a distinct preference for aryl ketone substrates, suggesting the possible involvement of π-type catalyst⋯substrate adducts in their catalytic cycles. A catalytic cycle involving only 4 steps (after induction) with stable DFT-simulated structures is proposed which accounts for the experimental data for the system.

Neutral and cationic (pyrazolylmethyl)pyridine palladium(II) complexes: kinetics and chemoselectivity studies in hydrogenation of alkenes and alkynes

Reactions of (3,5-dimethylpyrazolylmethyl)pyridine (L1) and (3,5-diphenylpyrazolylmethyl)pyridine (L2) with either [PdCl2(NCMe)2] or [PdClMe(COD)] afforded the respective neutral palladium complexes, [PdCl2(L1)] (1), [PdCl2(L2)] (2) and [PdClMe(L1)] (3). Treatment of complex 1 with equimolar amounts of PPh3 or PPh3/NaBAr4 produced the corresponding cationic complexes [Pd(L1)ClPPh3]Cl (4) and [Pd(L1)ClPPh3]BAr4 (5), respectively. Complexes 1–5 formed active catalysts in hydrogenation of alkenes and alkynes. Isomerization reactions were predominant in the hydrogenation reactions of terminal alkenes, while hydrogenation of alkynes involved a two-step process via alkene intermediates prior to the formation of the respective alkenes. The lack of induction periods in the hydrogenation reactions in addition to pseudo-first-order kinetics with respect to the substrates established the homogeneous nature of the active species.

Packing forces in dichloridobis(3,5-diphenyl-1H-pyrazole-κN2)cobalt(II) dichloromethane hemisolvate

The title compound, [CoCl2(C15H12N2)2]·0.5CH2Cl2, was crystallized from a binary mixture of dichloromethane and hexane and a dimeric supramolecular structure was isolated. The Co(II) centre exhibits a distorted tetrahedral geometry, with two independent pyrazole-based ligands occupying two coordination sites and two chloride ligands occupying the third and fourth coordination sites. The supramolecular structure is supported by complementary hydrogen bonding between the pyrazole NH group and the chloride ligand of an adjacent molecule. This hydrogen-bonding motif yields a ten-membered hydrogen-bonded ring. Density functional theory (DFT) simulations at the PBE/6-311G level of theory were used to probe the solid-state structure. These simulations suggest that the chelate undergoes a degree of conformational distortion from the lowest-energy geometry to allow for optimal hydrogen bonding in the solid state.

Coordination behavior and binding properties of (3,5-dimethyl-1H-pyrazol-1-yl)ethanol with Cu(II), Zn(II), Cd(II), and Pb(II) metals

The coordination chemistry and binding properties of (3,5-dimethyl-1H-pyrazol-1-yl)ethanol (L1) with Cu(II), Zn(II), Cd(II), and Pb(II) have been investigated. Reactions of L1 with Zn(II) and Cd(II) nitrate or chloride salts produced the corresponding monometallic and bimetallic complexes [Zn(NO3)2(L1)] (1), [ZnCl2(L1)] (2), [Cd(NO3)2(L1)2] (3) and [Cd2Cl2(μ-Cl2)(L1)2] (4). Solid-state structures of 3 and 4 confirmed the bidentate coordination mode of L1. The extraction efficiency of the metals by L1 is dependent upon the size and electronic properties of the metal. Binding affinities of 92% for Cu(II), 90% for Zn(II), 70% for Pb(II), and 61% for Cd(II) were observed.

trans-Bis[3,5-bis­(trifluoro­meth­yl)-1H-pyrazole-κN2]dichloro­palladium(II) monohydrate

The palladium(II) center in the title compound, trans-[PdCl2(C5H2F6N2)2]·H2O, possesses a distorted square-planar geometry. The NH groups are positioned on the same side of the PdN2Cl2 coordination plane. Four symmetry-independent strong hydrogen bonds of three types (N—H⋯Cl, N—H⋯Cl and O—H⋯Cl) hold the structure together.