Ligand-metal cooperativity in quinonoid based nickel(II) and cobalt(II) complexes for catalytic hydrosilylative reduction of nitriles to amines: electron transfer and mechanistic insight.

The sustainable production of privileged amines by the catalytic reduction of nitriles with an inexpensive silane polymethylhydrosiloxane (PMHS) holds great promise to replace conventional synthetic routes that have limited applicability and involve the use of expensive metal catalysts. The use of late 3d-metal complexes provides an excellent platform for the rational design of inexpensive catalysts with exquisite control over their electronic and structural features through metal-ligand cooperativity. In this context, we have realistically designed two complexes based on nickel(II) and cobalt(II) with a redox-active imino-o-benzoquinonato ligand. The compounds were characterized by a suite of spectroscopic methods, cyclic voltammetry and single-crystal X-ray diffraction. Both complexes showed excellent catalytic activity in transforming various organonitriles into the corresponding primary amines selectively using the inexpensive PMHS. The catalytic performance of the complexes was evaluated by various control experiments and spectroscopic studies with detailed computational calculations revealing the crucial role of the non-innocent imino-o-benzoquinonato ligand and metal(II) ion cooperativity in controlling the reactivity and selectivity of the key metal-hydride intermediates in the course of catalytic reduction.

Catalytic Reduction of Nitriles by Polymethylhydrosiloxane Using a Phenalenyl-Based Iron(III) Complex.

The reduction of nitriles to primary amines using an inexpensive silane such as polymethylhydrosiloxane (PMHS) is an industrially important reaction. Herein we report the synthesis of an earth-abundant Fe(III) complex bearing a phenalenyl-based ligand that was characterized by mass spectroscopy, elemental analysis, cyclic voltammetry, and single-crystal X-ray diffraction. The complex showed excellent catalytic activity toward reduction of aromatic, heteroaromatic, aliphatic, and sterically crowded nitriles to produce primary amines using polymethylhydrosiloxane (PMHS).

A safe and efficacious Pt(ii) anticancer prodrug: design, synthesis, in vitro efficacy, the role of carrier ligands and in vivo tumour growth inhibition.

Diaminocyclohexane-Pt(ii)-phenalenyl complexes (1 and 2) showed an appropriate balance between efficacy and toxicity. Compound 2 showed nearly two-fold higher tumour growth inhibition than oxaliplatin in a murine NSCLC tumour model, when a combined drug development approach was used. The fluorescent properties of phenalenone were utilized to understand the mechanistic details of the drug.

Tuning the electronic properties in ruthenium-quinone complexes through metal coordination and substitution at the bridge.

A rare example of a mononuclear complex [(bpy)2 Ru(L(1) -H )](ClO4 ), 1(ClO4 ) and dinuclear complexes [(bpy)2 Ru(µ-L(1) -2H )Ru(bpy)2 ](ClO4 )2 , 2(ClO4 )2 , [(bpy)2 Ru(µ-L(2) -2H )Ru(bpy)2 ](ClO4 )2 , 3(ClO4 )2 , and [(bpy)2 Ru(µ-L(3) -2H )Ru(bpy)2 ](ClO4 )2 , 4(ClO4 )2 (bpy=2,2'-bipyridine, L(1) =2,5-di-(isopropyl-amino)-1,4-benzoquinone, L(2) =2,5-di-(benzyl-amino)-1,4-benzoquinone, and L(3) =2,5-di-[2,4,6-(trimethyl)-anilino]-1,4-benzoquinone) with the symmetrically substituted p-quinone ligands, L, are reported. Bond-length analysis within the potentially bridging ligands in both the mono- and dinuclear complexes shows a localization of bonds, and binding to the metal centers through a phenolate-type "O(-) " and an immine/imminium-type neutral "N" donor. For the mononuclear complex 1(ClO4 ), this facilitates strong intermolecular hydrogen bonding and leads to the imminium-type character of the noncoordinated nitrogen atom. The dinuclear complexes display two oxidation and several reduction steps in acetonitrile solutions. In contrast, the mononuclear complex 1(+) exhibits just one oxidation and several reduction steps. The redox processes of 1(1+) are strongly dependent on the solvent. The one-electron oxidized forms 2(3+) , 3(3+) , and 4(3+) of the dinuclear complexes exhibit strong absorptions in the NIR region. Weak NIR absorption bands are observed for the one-electron reduced forms of all complexes. A combination of structural data, electrochemistry, UV/Vis/NIR/EPR spectroelectrochemistry, and DFT calculations is used to elucidate the electronic structures of the complexes. Our DFT results indicate that the electronic natures of the various redox states of the complexes in vacuum differ greatly from those in a solvent continuum. We show here the tuning possibilities that arise upon substituting [O] for the isoelectronic [NR] groups in such quinone ligands.

One-pot synthesis of symmetric and asymmetric p-quinone ligands and unprecedented substituent induced reactivity in their dinuclear ruthenium complexes.

The compounds 2-[2-(trifluoromethyl)-anilino]-5-hydroxy-1,4-benzoquinone (L(1)), 2,5-di-[2-(trifluoromethyl)-anilino]-1,4-benzoquinone (L(2)), 2-[2-(methylthio)-anilino]-5-hydroxy-1,4-benzoquinone (L(3)), and 2,5-di-[2-(methylthio)-anilino]-1,4-benzoquinone (L(4)) were prepared in high yields by reacting 2,5-dihydroxy-1,4-benzoquinone with the corresponding amines in a one-pot synthesis in refluxing acetic acid. This straightforward and "green" synthesis delivers biologically relevant asymmetric p-quinones such as L(1) and L(3) in a rare, simple, one-step process. The proposed synthetic route is general and can be applied to generate a variety of such molecules with different substituents on the nitrogen atoms. Structural characterization of L(2) and L(4) shows electron delocalization across the "upper" and "lower" parts of the molecule, thus showing the importance of charge separated species in the proper description of such molecules. Reactions of these ligands with [Cl(η(6)-Cym)Ru(µ-Cl)(2)Ru(η(6)-Cym)Cl] (Cym = p-Cymene = 1-isopropyl-4-methyl-benzene) in the presence of a base result in the formation of complexes [{Cl(η(6)-Cym)Ru}(2)(µ-L(-2H)(1))] (1), [{Cl(η(6)-Cym)Ru}(2)(µ-L(-2H)(2))] (2), [{Cl(η(6)-Cym)Ru}(2)(µ-L(-2H)(3))] (3), and [{Cl(η(6)-Cym)Ru}(2)(µ-L(-2H)(4))] (4). Structural characterization of 2 and 4 shows a rare syn-coordination of the chloride atoms. The SMe groups in 3 and 4 are not coordinated to the ruthenium center, and the bridging ligands thus function in a bis-bidentate form. Abstraction of the chloride atoms in these complexes with AgClO(4) in CH(3)CN results in the expected formation of solvent substituted complexes [{(CH(3)CN)(η(6)-Cym)Ru}(2)(µ-L(-2H)(1))][ClO(4)](2) (5[ClO(4)](2)) and [{(CH(3)CN)(η(6)-Cym)Ru}(2)(µ-L(-2H)(2))][ClO(4)](2) (6[ClO(4)](2)) with the ligands where there are no additional donor atoms on the nitrogen substituents. The same chloride abstraction reaction in the cases of 3 and 4 leads to an unprecedented substituent induced release of the Cym ligand, resulting in complexes of the form [(CH(3)CN)(η(6)-Cym)Ru(µ-L(-2H)(3))Ru(CH(3)CN)(3)][ClO(4)](2) (7[ClO(4)](2)) and [{(CH(3)CN)(3)Ru}(2)(µ-L(-2H)(4))][ClO(4)](2) (8[ClO(4)](2)), where the SMe groups are now coordinated to the metal center. In the case of complex 3, which contains an asymmetric bridging ligand, Cym release is observed only at the side that contains an additional SMe donor, thus proving the necessity of such donor substituents for the observed reactivity. The increase in Lewis acidity at the ruthenium center on chloride abstraction is made responsible for SMe coordination and the rigidity of the ligand systems, and their concomitant failure to coordinate in a "fac" manner as is required for a piano stool configuration results in the eventual Cym release. The bridging ligand which then coordinates in a bis-meridional fashion in 8[ClO(4)](2) results in a bis-pincer type of coordination. These observations were validated by a structural analysis of 8[ClO(4)](2). The results show the potential hemilabile character of ligands such as L(3) and L(4). Electrochemical and spectroscopic investigations are reported on 8[ClO(4)](2), and substitution reactions of the CH(3)CN molecules are presented to show the use of 8[ClO(4)](2) as a versatile precursor for other reactions.

First structurally characterized mono- and dinuclear ruthenium complexes derived from zwitterionic quinonoid ligands.

Electron transfer and delocalization of the pi systems were investigated in mono- and dinuclear Ru complexes with the zwitterionic ligand L = N,N'-diisopropyl-2-amino-5-alcoholate-1,4-benzoquinonemonoiminium.