From nitrobenzenes to substituted tetrahydroquinolines in a single step by a domino reduction/imine formation/aza-diels-alder reaction.

The three-component reaction between a nitrobenzene, an aldehyde, and a dienophile in the presence of iron powder as a reductant and montmorillonite K10 as a catalyst in aqueous citric acid delivers the products of an aza-Diels-Alder (Povarov) reaction with high endo-selectivity and yields up to 99%.

Formation of novel T-shaped NNN ligands via rare-earth metal-mediated Si-H activation.

Reactions of silylamides [Ln{N(SiHMe2)2}3(thf)2] with sterically crowded terphenylamine DmpNH2 (Dmp = 2,6-Mes2C6H3 with Mes = 2,4,6-Me3C6H2) afforded via a template reaction the formation of a new tridentate ligand, and derived complexes of composition [LnN{SiMe2N(Dmp)}2] (Ln = Ce, Pr) were obtained. Usage of the even more bulky amine Ar*NH2 (Ar* = 2,6-Trip2C6H3 with Trip = 2,4,6-iPr3C6H2) yielded the free protonated ligand NH{SiMe2NH(Ar*)}2.

Tuning spin-spin coupling in quinonoid-bridged dicopper(II) complexes through rational bridge variation.

Bridged metal complexes [{Cu(tmpa)}2(µ-L(1)-2H)](ClO4)2 (1), [{Cu(tmpa)}2(µ-L(2)-2H)](ClO4)2 (2), [{Cu(tmpa)}2(µ-L(3)-2H)](BPh4)2 (3), and [{Cu(tmpa)}2(µ-L(4)-2H)](ClO4)2 (4) (tmpa = tris(2-pyridylmethyl)amine, L(1) = chloranilic acid, L(2) = 2,5-dihydroxy-1,4-benzoquinone, L(3) = (2,5-di-[2-(methoxy)-anilino]-1,4-benzoquinone, L(4) = azophenine) were synthesized from copper(II) salts, tmpa, and the bridging quinonoid ligands in the presence of a base. X-ray structural characterization of the complexes showed a distorted octahedral environment around the copper(II) centers for the complexes 1-3, the donors being the nitrogen atoms of tmpa, and the nitrogen or oxygen donors of the bridging quinones. In contrast, the copper(II) centers in 4 display a distorted square-pyramidal coordination, where one of the pyridine arms of each tmpa remains uncoordinated. Bond-length analyses within the bridging ligand exhibit localization of the double bonds inside the bridge for 1-3. In contrast, complete delocalization of double bonds within the bridging ligand is observed for 4. Temperature-dependent magnetic susceptibility measurements on the complexes reveal an antiferromagnetic coupling between the copper(II) ions. The strength of antiferromagnetic coupling was observed to depend on the energy of the HOMO of the bridging quinone ligands, with exchange coupling constants J in the range between -23.2 and -0.6 cm(-1) and the strength of antiferromagnetic coupling of 4 > 3 > 2 > 1. Broken-symmetry density functional theory calculations (DFT) revealed that the orientation of magnetic orbitals in 1 and 2 is different than that in 3 and 4, and this results in two different exchange pathways. These results demonstrate how bridge-mediated spin-spin coupling in quinone-bridged metal complexes can be strongly tuned by a rational design of the bridging ligand employing the [O] for [NR] isoelectronic analogy.

Cobalt complexes with "Click"-derived functional tripodal ligands: spin crossover and coordination ambivalence.

We demonstrate the use of a Cu(I) catalyzed "Click" reaction in the synthesis of novel ligands for spin crossover complexes. The reaction between azides and alkynes was used to synthesize the reported tripodal ligand tris[(1-benzyl-1H-1,2,3-triazol-4-yl)methyl]amine, TBTA, and the new ligands tris[(1-cyclohexyl-1H-1,2,3-triazol-4-yl)methyl]amine, TCTA, and tris[(1-n-butyl-1H-1,2,3-triazol-4-yl)methyl]amine, TBuTA. Reactions of TBTA with Co(ClO(4))(2) lead to complexes of the form [Co(TBTA)(CH(3)CN)(3)](ClO(4))(2), 1, and [Co(TBTA)(2)](ClO(4))(2), 2, where complex formation can be controlled by the metal/ligand ratio and the complexes 1 and 2 can be chemically and reversibly switched from one form to another in solution resulting in coordination ambivalence. The benzyl substituents of TBTA in 2 show intramolecular C-H-π T-stacking that generates a chemical pressure to stabilize the low spin (LS) state at lower temperatures. The structural parameters of 2 are consistent with a Jahn-Teller active LS Co(II) (elongation) ion showing four short and two long bonds. 2 shows spin-crossover (SCO) behavior in the solid state and in solution with a high T(0) close to room temperature which is driven by the T-stacking. 1 remains high spin (HS) between 2 and 400 K. Reversible chemical switching is observed between 1 and 2 at room temperature, with an accompanying change in the spin state from HS to LS. The importance of the intramolecular T-stacking in driving the SCO behavior is proven by comparison with two analogous compounds that lack an aromatic substituent and remain HS down to very low temperatures.

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.

Benzo-1,3,2-diazaphospholide and benzo-1,3,2-diazaphospholium: an isoelectronic aromatic anion-cation pair.

Isoelectronic benzo-1,3,2-diazaphospholium cations and benzo-1,3,2-diazaphospholide anions were prepared from the same phosphazane precursor; both species display according to computational studies similar aromaticity as the neutral benzo-1,3,2-diazaphosphole but are chemically more stable due to their ionic nature.

Synthesis of bis(phosphinoferrocenyl) copper complexes from zwitterionic quinonoid ligands and their structural and redox properties.

Reactions of N,N'-di-n-butyl-2-amino-5-alcoholate-1,4-benzoquinonemonoiminium L(1), or N,N'-diisopropyl-2-amino-5-alcoholate-1,4-benzoquinonemonoiminium L(2) with [{(dppf)Cu}(2)(mu-Cl)(2)] (dppf = 1,1'-bis(diphenylphosphino)ferrocene) or [{(dispf)Cu}(2)(mu-Cl)(2)] (dispf = 1,1'-bis(diisopropylphosphino)ferrocene) led to the formation of the heterodinuclear complexes [(dppf)(CuL(1)(-H))] (2), [(dppf)(CuL(2)(-H))] (3), [(dispf)(CuL(1)(-H))] (4), and [(dispf)(CuL(2)(-H))] (5). The crystal structure of L(2) was determined by X-ray diffraction and shows that the molecule exists in a 6pi + 6pi zwitterionic form, with two chemically connected but electronically nonconjugated pi-subunits. The crystal structures of complexes 2-4 show a distorted tetrahedral coordination environment for the Cu(I) center and a more localized pi-system for the ligands. Cyclic voltammetry on the ligands and complexes indicates various redox processes. The first oxidation of the complexes leads to an electron paramagnetic resonance supported formulation where the ligand radical is bound to Cu(I). UV-visible spectroscopy of the ligands and the complexes is also reported and discussed.

Stabilizing the elusive ortho-quinone/copper(I) oxidation state combination through pi/pi interaction in an isolated complex.

The heterodinuclear compound [(PhenQ)Cu(dppf)](BF4), PhenQ = 9,10-phenanthrenequinone and dppf = 1,1'-bis(diphenylphosphino)ferrocene, was identified structurally and spectroscopically (NMR, IR, UV-vis) as a copper(I) complex of a completely unreduced ortho-quinone. Crystallographic and DFT calculation results suggest that this stabilization of a hitherto elusive arrangement is partially owed to intramolecular pi/pi interactions phenyl/PhenQ. Intermolecular PhenQ/PhenQ pi stacking is also observed in the crystal. According to DFT calculations, the pi interactions are responsible for the considerably distorted coordination geometry at CuI with one short and one longer Cu-O and Cu-P bond, respectively, and with bond angles at copper ranging from 99 degrees to 133 degrees. Electrochemical reduction proceeds reversibly at low temperatures to yield an EPR spectroscopically characterized semiquinone-copper(I) species.