A remarkable solvent effect on the nuclearity of neutral titanium(IV)-based helicate assemblies.

The spontaneous self-assembly of a neutral circular trinuclear Ti(IV) -based helicate is described through the reaction of titanium(IV) isopropoxide with a rationally designed tetraphenolic ligand. The trimeric ring helicate was obtained after diffusion of n-pentane into a solution with dichloromethane. The circular helicate has been characterized by using single-crystal X-ray diffraction study, (13) C CP-MAS NMR and (1) H NMR DOSY solution spectroscopic, and positive electrospray ionization mass-spectrometric analysis. These analytical data were compared with those obtained from a previously reported double-stranded helicate that crystallizes in toluene. The trimeric ring was unstable in a pure solution with dichloromethane and transformed into the double-stranded helicate. Thermodynamic analysis by means of the PACHA software revealed that formation of the double-stranded helicates was characterized by ΔH(toluene)=-30 kJ mol(-1) and ΔS(toluene)=+357 J K(-1) mol(-1) , whereas these values were ΔH(CH2 Cl2 )=-75 kJ mol(-1) and ΔS(CH2 Cl2 )=-37 J K(-1) mol(-1) for the ring helicate. The transformation of the ring helicate into the double-stranded helicate was a strongly endothermic process characterized by ΔH(CH2 Cl2 )=+127 kJ mol(-1) and ΔH(n-pentane)=+644 kJ mol(-1) associated with a large positive entropy change ΔS=+1115 J K(-1) ⋅mol(-1) . Consequently, the instability of the ring helicate in pure dichloromethane was attributed to the rather high dielectric constant and dipole moment of dichloromethane relative to n-pentane. Suggestions for increasing the stability of the ring helicate are given.

Reusable polystyrene-supported Pd catalyst for Mizoroki-Heck reactions with extremely low amounts of supported Pd.

In this communication, we report that by running Mizoroki-Heck reactions of various aryl iodides in toluene-DMF, only extremely low amounts of supported Pd (0.04 mequiv.) are required to obtain almost quantitative yields. The catalyst was reused successfully up to three times.

Spontaneous symmetry breaking during self-assembly of a double stranded biphenolate-based Ti(IV)-helicate.

A binuclear Ti(IV)-based helicate synthesized from a symmetric tetrahydroxyheptaphenylene strand was self-assembled in solution and shown to undergo a spontaneous head-to-tail differentiation according to single-crystal X-ray diffraction.

Synthesis and characterization of a monomeric octahedral C2-symmetric titanium complex bearing two 3,3'-diphenyl-2,2'-biphenol ligands.

The 3,3'-diphenyl-2,2'-biphenol (dpbpolH(2)) ligand is shown to react with Ti(OPr(i))(4) in a 2:1 ratio to yield a novel C(2)-symmetric mononuclear octahedral Ti(IV) bis-biphenolate complex, cis-Ti(dpbpol)(2)(HOPr(i))(2).