Circular Heterochiral Titanium-Based Self-Assembled Architectures.

Circular trinuclear helicates have been synthesized from a bis-biphenol strand (LH4), titanium isopropoxide, and various diimine ligands. These self-assembled architectures constructed around three TiO4N2 nodes have a heterochiral structure (C1 symmetry) when 2,2'-bipyridine (A), 4,4'-dimethyl-2,2'-bipyridine (B), 4,4'-bromo-2,2'-bipyridine (C), or 4,4'-dimethyl-2,2'-bipyrimidine (D) is employed. Within these complexes, one nitrogen ligand is endo-positioned inside the metallo-macrocycle, whereas the other two diimine ligands point outside the helicate framework. This investigation highlights that the nitrogen ligand which does not participate in the helicate framework of the complex controls the overall symmetry of the helicate since the 2,2'-bipyrimidine chelate (F) ends in the formation of a homochiral aggregate (C3 symmetry). The lack of symmetry found in the solid state for the trinuclear species ([Ti3L3(B)3], [Ti3L3(C)3], and [Ti3L3(D)3]) is observed for these complexes in solution (dichloromethane or chloroform). Remarkably, the 2,2'-bipyrazine ligand (ligand E) ends in the formation of a hexameric aggregate formulated as [Ti6L6(E)6], whereas the use of 4,4'-dimethyl-2,2'-bipyrimidine (ligand D) permits to generate the dinuclear complexes ([Ti2L(D)2(OiPr)4] and [Ti2L2(D)2]) in addition to the trimeric structure [Ti3L3(D)3]. The behavior of [Ti3L3(A)3] in solution, on the other hand, is unique since an equilibrium between the homochiral and the heterochiral form is reached within 17 days after the complex has been dissolved in dichloromethane (C3-[Ti3L3(A)3]/C1-[Ti3L3(A)3] ratio = 0.3). In chloroform, the heterochiral form of [Ti3L3(A)3] is stable for the same period of time, evidencing the dependence of this stereochemical transformation toward the solvent medium. The thermodynamic and kinetic parameters linked to this stereochemical equilibrium have been obtained and point to the fact that the transformation is intramolecular and not induced by the presence of external ligands. The thermodynamic constant of the C1-[Ti3L3(A)3]/C3-[Ti3L3(A)3] equilibrium is found to be K = 0.34 ± 10%. Further evidence to rationalize this solvent-induced symmetry switch is obtained via a DFT calculation and classical molecular dynamics. In particular, this computational investigation elucidates the reason why the stereochemical transformation of a heterochiral architecture into a homochiral structure is possible only for a trinuclear assembly containing ligand A.

A Chiral [2+3] Covalent Organic Cage Based on 1,1'-Bi-2-naphthol (BINOL) Units.

A [2+3] chiral covalent organic cage is produced through a dynamic covalent chemistry approach by mixing two readily available building units, viz. an enantiopure 3,3'-diformyl 2,2'-BINOL compound (A) with a triamino spacer (B). The two enantiomeric (R,R,R) and (S,S,S) forms of the cage C are formed nearly quantitatively thanks to the reversibility of the imine linkage. The X-ray diffraction analysis of cage (S,S,S)-C highlights that the six OH functions of the BINOL fragments are positioned inside the cage cavity. Upon reduction of the imine bonds of cage C, the amine cage D is obtained. The ability of the cage D to host the 1-phenylethylammonium cation (EH+) as a guest is evaluated through UV, CD and DOSY NMR studies. A higher binding constant for (R)-EH+ cation (Ka=1.7 106±10 % M-1) related to (S)-EH+ (Ka=0.9 106±10 % M-1) is determined in the presence of the (R,R,R)-D cage. This enantiopreference is in close agreement with molecular dynamics simulation.

From monomeric complexes to double-stranded helicates constructed around trans-TiO4N2 motifs with intramolecular inter-ligand hydrogen-bonding interactions.

A coordination chemistry involving trans-TiO4N2 motifs is described, where the oxygen ligands are 2,2'-biphenolato derivatives (L1 or L2) and the nitrogen ligands are pyridine (Pyr), 2,3-dihydro-7-azaindole (DHA) or 2-(methylamino)pyridine (MePyr). Monomeric complexes and double-stranded helicates incorporating this set of ligands are characterized. The monomeric species are obtained from the precursor [Ti(L1)2(HOiPr)2] whereas the helical dinuclear architectures are synthesized following a multicomponent self-assembly approach starting from L2H4, Ti(OiPr)4 and two equivalents of the nitrogen ligand. It is proposed that the trans isomerism in TiO4N2 observed in these structures results from the destabilisation of the cis isomer by the trans influence of the Ti-N bonds. The crystal structures and infra-red analysis demonstrate hydrogen bonding interactions occurring between the NH group of DHA or MePyr and the oxygens belonging to the titanium coordination sphere. The strength of these interactions is estimated using the PACHA (Partial Atomic Charges Analysis) software.

Monomeric Ti(iv)-based complexes incorporating luminescent nitrogen ligands: synthesis, structural characterization, emission spectroscopy and cytotoxic activities.

This manuscript describes the synthesis of a series of neutral titanium(iv) monomeric complexes constructed around a TiO4N2 core. The two nitrogen atoms that compose the coordination sphere of the metallic center belong to 2,2'-bipyrimidine ligands homo-disubstituted in the 4 and 4' positions by methyl (2a), phenylvinyl (2b), naphthylvinyl (2c) or anthrylvinyl (2d) groups. The crystal structures of these complexes named [Ti(1)2(2a)], [Ti(1)2(2b)], [Ti(1)2(2c)] and [Ti(1)2(2d)] (where 1 is a 2,2'-biphenolato ligand substituted in the 6 and 6' positions by phenyl groups) are reported. The hydrolytic stability of the four complexes is evaluated by monitoring the evolution of the free 2a-d signals by 1H NMR spectroscopy. For the conditions tested (6 mM, DMSO-d6/D2O: 8/1), a rather good stability with t1/2 ranging from 180 to 300 min is determined for the complexes. In the presence of an acid (DCl), the hydrolysis of [Ti(1)2(2a)] is faster than without an acid. The cytotoxic activity against gastric cancer cells of the titanium-based compounds and the free disubstituted 2,2'-bipyrimidine ligands is tested, showing IC50 ranging from 6.2 ± 1.2 µM to 274 ± 56 µM. The fluorescence studies of the ligands 2a-d, and the complexes [Ti(1)2(2a-d)] reveal an important fluorescence loss of the ligands 2c and 2d upon coordination with the Ti(1)2 fragment. Frontier orbitals obtained by DFT calculations permit us to explain this fluorescence quenching.

Cyclopalladated complexes of 4-aryl-2,1,3-benzothiadiazoles: new emitters in solution at room temperature.

The cyclopalladation of the 4-aryl-2,1,3-benzothiadiazoles 1a-c with palladium acetate in acetic acid afforded the novel dimeric complexes 2a-c in good yields. These were then converted into the monomeric pyridine-, chloro-coordinated cyclometallated complexes 3a-c through reaction with lithium chloride in acetone and then pyridine in dichloromethane. All complexes were fully characterized by means of NMR, IR and elemental analysis. The X-ray structure of complex 2c revealed that it presents transoid geometry, whereas the X-ray structure of 3c shows that the pyridine ligand and the thiazole ring are mutually trans. Photophysical properties were investigated by means of UV-Vis absorption and fluorescence emission in solution. Solid-state diffuse reflectance UV-Vis spectra (DRUV) were also applied in order to better characterize the complexes photophysics in the solid state. All complexes present intense absorption at around 300 nm (λ(1)) via(1)LC transitions located in BTD ligands, and additional low energy absorption bands, higher than 450 nm (λ(2)) of (1)MLCT character. The complexes are fluorescent in solution at room temperature, where two emission bands could be observed, a high energy band (excitation @ λ(1)) ascribed to the ligand emission and an additional red shifted low intense band (excitation @ λ(2)) due to the complex emission.

µ(3)-Carbonato-κO:O':O''-tris-{(η-ben-zene)[(R)-1-(1-amino-ethyl)naphthyl-κC,N]ruthenium(II)} hexa-fluorido-phosphate dichloro-methane solvate.

The title compound, [Ru(3)(C(12)H(12)N)(3)(CO(3))(C(6)H(6))(3)]PF(6)·CH(2)Cl(2), was obtained unintentionally as the product of an attempted deprotonation of the monomeric parent ruthenium complex [Ru(C(12)H(12)N)(C(6)H(6))(C(2)H(3)N)]PF(6). The carbonate ligand bridges three half-sandwich cyclo-ruthenated fragments, each of them exhibiting a pseudo-tetra-hedral geometry. The configuration of the Ru atoms is S. The naphthyl groups of the enanti-opure cyclo-ruthenated benzylic amine ligands point in the same direction, adopting a propeller shape.

Cycloruthenated primary and secondary amines as efficient catalyst precursors for asymmetric transfer hydrogenation.

[reaction: see text] Ruthenacycles obtained by cyclometalation of enantiopure aromatic primary or secondary amines with [(eta6-benzene)RuCl2]2 or with [(eta6-p-cymene)RuCl2]2 are efficient catalysts for asymmetric transfer hydrogenation (TOF up to 190 h(-1) at room temperature). Enantioselectivities in the transfer hydrogenation of acetophenone ranged from 38% to 89%. It is possible to prepare the catalysts in situ, which allows the use of high throughput experimentation.

Synthesis and structural studies of binuclear platinum(II) complexes with a novel phosphorus-nitrogen-phosphorus ligand.

The new pyridinediphosphinite ligand PONOP (1) was synthesized in one step from 2,6-pyridinedimethanol and diphenylchlorophosphane. Reaction of 1 with PtCl(2)(PhCN)(2) led to the neutral homobimetallic complex [Pt(2)Cl(4)(PONOP)(2)] (2), where the benzonitriles have been substituted by the phosphorus atoms of 1. The X-ray structure of 2 revealed a metallamacrocycle where the pyridines remained free. Addition of 2 equiv of [Cu(MeCN)(4)](BF(4)) to 2 led to CuCl and to the binuclear dicationic complex [Pt(2)Cl(2)(PONOP)(2)](BF(4))(2) (3), where chloro ligands have been substituted by pyridine groups. Conversely, reaction of 3 with chloride anions gave back complex 2. Solid-state (X-ray) and solution (NMR) studies indicated that after the transformation of 2 into 3, the platinum centers were brought much closer and the pyridinediphosphinite ligand was stiffened. The methylene NMR protons of 3 were strongly deshielded, and the corresponding proton-phosphorus coupling constants followed a Karplus-type relationship.

Versatile Ligands for Palladium-Catalyzed Asymmetric Allylic Alkylation.

Palladium complexes of the chiral diphosphanes 1 and 2 which possess a rigid backbone and a large bite angle catalyze the alkylation of allyl compounds with both high enantioselectivities and reaction rates, particularly with less sterically demanding substrates. 1: R=Me, X=S; 2: R=H, X=C(CH3 )2 .