Ernesto Rufino-Felipe

Calix[4]arenes of aluminum and gallium with benzimidazolyl ligands: steric control of the conformation via substitution on the ligand.

Complexes [bzimAlR(2)](4) [bzim = benzimidazolate; R = Et (2), (i)Bu (3)], [mbzimAlR(2)](4) [mbzim = 2-methylbenzimidazolate; R = Et (6), (i)Bu (7)], [dmbzimAlR(2)](4) [dmbzim = 5,6-dimethylbenzimidazolate; R = Me (9), Et (10), (i)Bu (11)], and [tmbzimAlR(2)](4) [tmbzim = 2,5,6-trimethylbenzimidazolate; R = Me (12), Et (13), (i)Bu (14)] have been prepared via alkane elimination and coordinative self-assembly upon the reaction of benzimidazole ligands with aluminum alkyls in benzene, toluene, or xylene. Characterization of the complexes was achieved by spectroscopic methods, microanalysis, and X-ray crystallography of 2, 7, 10, 11, 13, and 14. The complexes reported herein and the aluminum and gallium analogues 1, 4, 5, and 8 reported in a previous paper (1) are predominantly tetranuclear aggregates related to calix[4]arenes in which the benzimidazolyl ligands bind two metal atoms in a η(1):η(1) fashion. X-ray crystallography demonstrates that modulation of the conformation adopted by these metallacalix[4]arenes is achieved by proper substitution on the C atom at the 2 position of the benzimidazolyl ligand. An H substituent for 1, 2, 4, 10, and 11 favors a chair conformation with a small cavity and approximate C(2h) symmetry, while a CH(3) substituent for 5, 7, 8, 13, and 14 introduces enough repulsion to switch the conformation to a 1,3-alternate or double cone with a concomitant larger cavity and approximate C(2v) symmetry.

Synthesis and characterization of aluminum complexes incorporating Schiff base ligands derived from pyrrole-2-carboxaldehyde

The reaction of 1,2-Ethylendiamine-N,N′-bis(1H-pyrrol-2-yl)methylene (L1H), 1,2-benzenediamine-N,N′-bis(1Hpyrrol-2-yl)methylene (L2H) and 3,4-Dimethyl-1,2-benzenediamine-N,N′-bis(1 H-pyrrol-2-yl)methylene (L3H) with one equivalent of AlR2R′ (R = alkyl, R′ = alkyl, Cl) in toluene afforded the monometallic complexes [LAlR′] (1–12). All the complexes were characterized by analytical and spectroscopic methods. Compounds 1, 3, 4, 6, 7, and 11 were characterized bya singlecrystal X-ray structural analyses. Due to the rigidity imposed by the ligand, 1, 3 and 4, with an ethylene “backbone” connecting the two nitrogens, adopt a severely distorted tbp geometry (av. = 0.68). On the other hand 6, 7 and 11, with a phenylene backbone, adopt an intermediate tbp-sqp geometry (av. = 0.43). The study of the X-ray crystal structures suggests that, like group 13 Salen complexes, flexible bridges between the two imine groups favor a tbp geometry while more rigid backbones, like phenylene rings, lead to an intermediate sqp and tbp geometry. Based on the geometrical parameter for previously reported aluminum complexes with Schiff base ligands and those presented herein, it is possible to propose that upon the coordination geometries of five coordinated aluminum complexes are less tbp distorted ( ≈ 1) the catalytic activity on ROP increases.

Lithium Complexes Derived of Benzylphosphines: Synthesis, Characterization and Evaluation in the ROP of rac-Lactide and ε-Caprolactone

A series of lithium complexes ([Ph2P(o-C6H4-CH2Li·TMEDA)] (1-Li), [PhP(o-C6H4-CH3)(o-C6H4-CH2Li·TMEDA)] (2-Li), [PhP(o-C6H4-CH2Li·TMEDA)2] (2-Li2) and [P(o-C6H4-CH2Li·TMEDA)3] (3-Li3)) was prepared from mono-, di- and tri-benzylphosphines and varying amounts of nBuLi and was characterized extensively by IR and 1H, 7Li, 13C and 31P NMR spectroscopy. The molecular structures of complexes 1-Li and 2-Li were determined by single-crystal X-ray diffraction studies. The two complexes have monomeric structures in the solid state comprising seesaw lithium atoms. In each case, the ligand exhibits an asymmetric C-C η2-coordination mode and an intramolecular P-Li bond interaction. Theoretical calculations at Density functional theory (DFT) level M06/6111+G(2d,p) show that indeed a P-Li bond is established which can be explained as the P lone pair (sp1.26) being partially delocalized on an available sp2 orbital on Li (sp2.04) and additional bonding contribution of the phosphorous atom to Li stems from further delocalization of a σ P-C orbital into the sp2 orbital on Li. The observed short contact distances between an aromatic ipso carbon and Li in the crystal structures of 1-Li and 2-Li are explained as due to the interaction of a σ C-Li orbital into the π* orbital of a C-C aromatic bond. Preliminary tests show compounds 1-Li, 2-Li, 2-Li2 and 3-Li3 are active catalysts in the solvent free ring-opening polymerization (ROP) of ε-caprolactone (ε-CL) and rac-lactide (rac-LA). High conversions to polycaprolactones were obtained in short periods of time: 1–6 min at 25 °C. Additionally, all four lithium complexes behave as moderately good initiators for the ROP of rac-LA showing high conversions to polylactides at 140 °C in one hour.