Temperature Dependence of the Ring Opening of Cyclopropene Imines on Thorium Metallocenes.

The reactions of two highly strained cyclopropenimine ligands L1H and L2H (L1H = N1,N1,N2,N2-tetraisopropyl-3-iminocycloprop-1-ene-1,2-diamine, L2H = N1,N1,N2,N2-tetracyclohexyl-3-iminocycloprop-1-ene-1,2-diamine) with three thorium precursors Cp*2ThCl2, Cp*2Th(Cl)(CH3), and Cp*2Th(CH3)2 were studied. At -20 °C, L1H and L2H react with Cp*2ThCl2 to form Th1 (Th1 = Cp*2ThCl2(L1H)) and Th2 (Th2 = Cp*2ThCl2(L2H)), respectively, where the neutral ligand coordinates to the thorium metal center. Coordination of the ligand to the thorium metal center introduces aromaticity at the cyclopropene ring of the ligand. Reaction at room temperature results in the ring opening of the ligand to form Th3 (Th3 = Cp*2ThCl2((Z)-2,3-bis(diisopropylamino)acrylonitrile) and Th4 (Th4 = Cp*2ThCl2((Z)-2,3-bis(dicyclohexylamino)acrylonitrile), where the cyclopropenimine converts into a nitrile and coordinates to the thorium metal center. Reaction of L1H and L2H with Cp*2Th(Cl)(CH3) and/or Cp*2Th(CH3)2 at -20 °C results in a rapid methanolysis reaction and forms Cp*2Th(L1/L2)(CH3/Cl)-type complexes Th5 (Th5 = Cp*2Th(L1)(CH3)), Th6 (Th6 = Cp*2Th(L2)(CH3), Th7 (Th7 = Cp*2Th(L1)(Cl), and Th8 (Th8 = Cp*2Th(L2)(Cl). On the other hand, at room temperature, these reactions result in a ring opening of the ligand. Room-temperature reaction of L1H and L2H with Cp*2Th(CH3)2 results in Th9 (Th9 = Cp*2Th(CH3)((Z)-3-imino-N1,N1,N2,N2-tetraisopropylbut-1-ene-1,2-diamine) and Th10 (Th10 = Cp*2Th(CH3)((Z)-3-imino-N1,N1,N2,N2-tetracyclohexylbut-1-ene-1,2-diamine). Similarly, at room temperature, L1H and L2H react with Cp*2Th(Cl)(CH3) to form Th11 (Th11 = Cp*2Th(Cl)((Z)-3-imino- N1,N1,N2,N2-tetraisopropylbut-1-ene-1,2-diamine) and Th12 (Th12 = Cp*2Th(Cl)((Z)-3-imino-N1,N1,N2,N2-tetracyclohexylbut-1-ene-1,2-diamine). The ring-opening reaction is assisted by the nucleophilic attack of the thorium-coordinated methyl group to the highly strained cyclopropene imine carbon.

Catalytic regeneration of metal-hydrides from their corresponding metal-alkoxides via the hydroboration of carbonates to obtain methanol and diols.

Thorium complexes decorated with 5-, 6-, and 7-membered N-heterocyclic iminato ligands containing mesityl wingtip substitutions have been synthesized and fully characterized. These complexes were found to be efficient in the hydroboration of cyclic and linear organic carbonates with HBpin or 9-BBN promoting their decarbonylation and producing the corresponding boronated diols and methanol. In addition, the hydroboration of CO2 breaks the molecule into "CO" and "O" forming boronated methanol and pinBOBpin. Moreover, the demanding depolymerization of polycarbonates to the corresponding boronated diols and methanol opens the possibility of recycling polymers for energy sources. Increasing the core ring size of the ligands allows a better performance of the complexes. The reaction proceeds with high yields under mild reaction conditions, with low catalyst loading, and short reaction times, and shows a broad applicability scope. The reaction is achieved via the recycling of a high-energy Th-H moiety from a stable Th-OR motif. Experimental evidence and DFT calculations corroborate the formation of the thorium hydride species and the reduction of the carbonate with HBpin to the corresponding Bpin-protected alcohols and H3COBpin through the formate and acetal intermediates.

A Sacrificial Iminato Ligand in the Catalytic Cyanosilylation of Ketones Promoted by Organoactinide Complexes.

Four new complexes containing the bis(pentamethylcyclopentadienyl)thorium(IV) moiety, Cp*2Th(L1)(Me) (Th2), Cp*2Th(L2)(Me) (Th3), Cp*2Th(L1)Cl (Th5), and Cp*2Th(L2)Cl (Th6), were synthesized in quantitative yields via the protonolysis reaction of the metallocene precursor complexes Cp*2Th(Me)2 (Th1) and Cp*2Th(Me)Cl (Th4) and the respective six- and seven-membered N-heterocyclic neutral imine ligands L1H and L2H. The molecular structures of all the complexes were established by single-crystal X-ray structure analyses. The synthesized complexes along with the precursor complexes were employed as catalysts for the cyanosilylation reaction of ketones with trimethylsilyl cyanide (Me3SiCN). The removal of the iminato ligand is necessary to trigger the reaction, allowing the formation of the active catalyst.

Bis-Imidazole Methane Ligated Ruthenium(II) Complexes: Synthesis, Characterization, and Catalytic Activity for Hydrogen Production from Formic Acid in Water.

A series of half sandwich arene-ruthenium complexes [(η6-arene)RuCl(κ2-L)]+ ([Ru]-1-[Ru]-10) containing bis-imidazole methane-based ligands {4,4'-(phenylmethylene)bis(2-ethyl-5-methyl-1H-imidazole)} (L1), {4,4'-((4-methoxyphenyl)methylene)bis(2-ethyl-5-methyl-1H-imidazole)} (L2), {4,4'-((2-methoxyphenyl)methylene)bis(2-ethyl-5-methyl-1H-imidazole)} (L3), {4,4'-((4-chlorophenyl)methylene)bis(2-ethyl-5-methyl-1H-imidazole)} (L4), and {4,4'-((2-chlorophenyl)methylene)bis(2-ethyl-5-methyl-1H-imidazole)} (L5) are synthesized. The synthesized and purified complexes ([Ru]-1-[Ru]-10) are further employed for hydrogen production from formic acid in aqueous medium. Among the investigated complexes, [(η6-p-cymene)RuCl(κ2-L2)]+ [Ru]-2, having Ru(II) coordinated 4-methoxy phenyl substituted bis-imidazole methane ligand (L2), outperformed over others, displaying a higher catalytic turnover of 8830 and high efficiency (TOF = 1545 h-1) with appreciably high long-term stability for formic acid dehydrogenation in water.

Synthesis, structure and catalytic activity of manganese(ii) complexes derived from bis(imidazole)methane-based ligands.

New mononuclear manganese(ii) complexes [Mn(κ2-L1)(OAc)2] ([Mn]-1), [Mn(κ2-L2)(OAc)2] ([Mn]-2) and [Mn(κ2-L3)(OAc)2] ([Mn]-3) with imidazole based ligands {4,4'-(phenylmethylene)bis(2-ethyl-5-methyl-1H-imidazole)} (L1), {(4,4'-((2-methoxy phenyl)methylene)bis(2-ethyl-5-methyl-1H-imidazole)} (L2) and {4,4'-((2-chlorophenyl) methylene)bis(2-ethyl-5-methyl-1H-imidazole)} (L3) are synthesized and fully characterized by a variety of techniques. Furthermore, the molecular structures of complexes [Mn]-1 and [Mn]-2 are established by single crystal X-ray structure analysis. The synthesized manganese(ii) complexes exhibited efficient catalytic oxidative coupling of primary amines in air under solvent-free conditions to the corresponding imines in moderate to good yields.

Dioxygenation Reaction of a Cobalt-Nitrosyl: Putative Formation of a Cobalt-Peroxynitrite via a {CoIII(NO)(O2-)} Intermediate.

A cobalt-nitrosyl complex, [(BPI)Co(NO)(OAc)], 1 {BPI = 1,3-bis(2'-pyridylimino)isoindol} was prepared and characterized. Structural characterization revealed that the cobalt center has a distorted square pyramidal geometry with the NO group coordinated from the apical position in a bent fashion. The addition of dioxygen (O2) to the dichloromethane solution of complex 1 resulted in the formation of nitro complex, [(BPI)Co(NO2)(OAc)], 2. It was characterized structurally. Kinetic studies suggested the involvement of an associative mechanism. FT-IR spectroscopic studies suggested the formation of the intermediate 1a [(BPI)CoIII(NO)(O2-)(OAc)] in the reaction. The intermediate 1a decomposed to complex 2 via a presumed peroxynitrite intermediate which was implicated by its characteristic phenol ring nitration reaction.

Reaction of a Co(III)-Peroxo Complex and NO: Formation of a Putative Peroxynitrite Intermediate.

A Co(II) complex, [Co(L)2]Cl2, 1 of the ligand L (L = bis(2-ethyl-4-methylimidazol-5-yl)methane) upon reaction with H2O2 in methanol solution at -40 °C resulted in the formation of the corresponding Co(III)-peroxo complex [Co(L)2(O2)]+ (2). The addition of NO gas to the freshly generated solution of the complex 2 led to the formation of the Co(II)-nitrato complex 3 through the putative formation of a Co(II)-peroxynitrite intermediate, 2a. The intermediate 2a was found to mediate the nitration of the externally added phenol resembling the nitration of tyrosine in biological systems.

Reaction of a Nitrosyl Complex of Cobalt Porphyrin with Hydrogen Peroxide: Putative Formation of Peroxynitrite Intermediate.

The cobalt porphyrin complex [(Cl4TPP)Co], 1, {Cl4TPP = 5,10,15,20-tetrakis(4'-chlorophenyl)porphyrinate dianion} in dichloromethane solution was subjected to react with nitric oxide (NO) gas and resulted in the formation of the corresponding nitrosyl complex [(Cl4TPP)Co(NO)], 2, having {CoNO}8 description. It was characterized by spectroscopic studies and single-crystal X-ray structure determination. It did not react with dioxygen. However, in CH2Cl2/CH3CN solution, it reacted with H2O2 to result in the Co-nitrito complex [(Cl4TPP)Co(NO2)], 3, with the simultaneous release of O2. It induced ring nitration to the added phenol in an appreciable yield. The reaction presumably proceeds through the formation of corresponding Co-peroxynitrite intermediate.

Nitric Oxide Reactivity of a Cu(II) Complex of an Imidazole-Based Ligand: Aromatic C-Nitrosation Followed by the Formation of N-Nitrosohydroxylaminato Complex.

A binuclear Cu(II) complex, 1, [Cu2(L-)2(OAc)](OAc) of imidazole-based ligand LH {LH = 2-(bis(2-ethyl-5-methyl-1H-imidazol-4-yl)methyl)phenol} was synthesized and characterized spectroscopically and structurally. Addition of an equivalent amount of nitric oxide (NO) by a gastight syringe to the acetonitrile:methanol (5:1, v/v) solution of complex 1 at room temperature resulted in the reduction of Cu(II) center to Cu(I) with concomitant C-nitrosation of the ligand. Spectroscopic characterization of the resulting Cu(I) complex (1a) of the C-nitrosylated ligand, L' {L' = 2-(bis(2-ethyl-5-methyl-1H-imidazol-4-yl)methyl)-4-nitroso-phenol} has been done. The Cu(I) complex, 1a, further reacted with NO to result in the corresponding N-nitrosohydroxylaminato complex, 2, [Cu2(L-ONNO)2](OAc)2 through the formation of a Cu(I)-nitrosyl intermediate. A small fraction of the nitrosyl intermediate decomposed to the corresponding Cu(II) complex 3, [Cu(L')2], and N2O in a parallel reaction.

Effect of ligand denticity on the nitric oxide reactivity of cobalt(ii) complexes.

The activation of nitric oxide (NO) by transition metal complexes has attracted a wide range of research activity. To study the role of ligand denticity on the NO reactivity of Co(ii) complexes, three complexes (, and ) were prepared with ligands , and [ = N(1),N(2)-bis(2,4,6-trimethylbenzyl)ethane-1,2-diamine; = N(1)-(2,4,6-trimethylbenzyl)-N(2)-(2-((2,4,6-trimethylbenzyl)amino)ethyl)ethane-1,2-diamine] and = N(1)-(2,4,6-trimethylbenzyl)-N(2),N(2)-bis(2-((2,4,6-trimethylbenzyl)amino)ethyl)ethane-1,2-diamine], respectively. The complexes differ from each other in terms of denticity and flexibility of the ligand frameworks. In degassed methanol solution, they were exposed to NO gas and their reactivity was studied using various spectroscopic techniques. In the case of complex with a bidentate ligand, reductive nitrosylation of the metal ion with concomitant dinitrosation of the ligand framework was observed. Complex with a tridentate ligand did not undergo reductive nitrosylation; rather, the formation of [Co(III)(NO(-))] was observed. The nitrosyl complexes were isolated and structurally characterized. On the other hand, complex with a tetradentate tripodal ligand did not react with NO. This can be attributed to the geometry of the complex as well as due to the accessibility of the corresponding redox potential.

Reductive nitrosylation of nickel(ii) complex by nitric oxide followed by nitrous oxide release.

Ni(ii) complex of ligand ( = bis(2-ethyl-4-methylimidazol-5-yl)methane) in methanol solution reacts with an equivalent amount of NO resulting in a corresponding Ni(i) complex. Adding further NO equivalent affords a Ni(i)-nitrosyl intermediate with the {NiNO}(10) configuration. This nitrosyl intermediate upon subsequent reaction with additional NO results in the release of N2O and formation of a Ni(ii)-nitrito complex. Crystallographic characterization of the nitrito complex revealed a symmetric η(2)-O,O-nitrito bonding to the metal ion. This study demonstrates the reductive nitrosylation of a Ni(ii) center followed by N2O release in the presence of excess NO.

Oxo Transfer from Nitrogen Dioxide to Nitrito Group in a Copper(II) Complex.

Reaction of Cu(II) complex [Cu(II)(LH)(O2CCH3)2] (1) [LH = 4,6-ditert-butyl-2-((2-picolyl(isopropyl)amino)methyl)phenol] with equivalent amount of NO2 leads to the reduction of Cu(II) to Cu(I) with concomitant nitration at the phenol ring of the ligand. This resulted in the in situ formation of intermediate Cu(I) complex of the nitrated ligand (L'H). Additional equivalent of NO2 coordinates to the Cu(I) complex to form corresponding O-nitrito Cu(II) complex [Cu(II)(L'(η(1)-ONO)] (2). Subsequent addition of NO2 led to the corresponding O-nitrato complex, [Cu(II)(L')(η(1)-ONO2)] (3) with concomitant formation of NO. Complexes 2 and 3 were isolated and structurally characterized. The formation of NO in the reaction was established by spin-trapping experiment. Isotopic labeling experiment revealed that the oxo transfer takes place from NO2 to the coordinated η(1)-ONO group.