ABSTRACT
New sterically encumbered tripodal aminetris(aryloxide) ligands N(CH(2)C(6)H(2)-3-(t)Bu-5-X-2-OH)(3) ((tBu,X)LH(3)) with relatively electron-rich phenols are prepared by Mannich condensation (X = OCH(3)) or by a reductive amination/Hartwig-Buchwald amination sequence on the benzyl-protected bromosalicylaldehyde (X = N[C(6)H(4)-p-OCH(3)](2)), followed by debenzylation using Pearlman's catalyst (Pd(OH)(2)/C). The analogous dianisylamino-substituted compound lacking the tert-butyl group ortho to the phenol ((H,An(2)N)LH(3)) is also readily prepared. The ligands are metalated by titanium(IV) tert-butoxide to form the five-coordinate alkoxides LTi(O(t)Bu). Treatment of the tert-butoxides with aqueous HCl yields the five-coordinate chlorides LTiCl, and with acetylacetone gives the six-coordinate diketonates LTi(acac). The diketonate complexes (tBu,X)LTi(acac) show reversible ligand-based oxidations with first oxidation potentials of +0.57, +0.33, and -0.09 V (vs ferrocene/ferrocenium) for X = (t)Bu, MeO, and An(2)N, respectively. Both dianisylamine-substituted complexes (R,An(2)N)LTi(acac) (R = (t)Bu, H) show similar electrochemistry, with three one-electron oxidations closely spaced at approximately 0 V and three oxidations due to removal of a second electron from each diarylaminoaryloxide arm at approximately + 0.75 V. The new electron-rich tripodal ligands therefore have the capacity to release multiple electrons at unusually low potentials for aryloxide groups.
ABSTRACT
Novel bis(beta-diketones) linked by 2,2'-biphenyldiyl, 2,2'-tolandiyl, and 2,2'-bis(methylene)biphenyl moieties have been prepared. All are metalated readily by titanium(IV) isopropoxide, but the nature of the complexes formed depends on the linker structure. The biphenyl-bridged ligand gives only traces of a mononuclear complex, which is thermodynamically unstable with respect to oligomerization. The tolan-bridged ligand does form mononuclear complexes, but only as a mixture of geometric isomers. In contrast, the substituted 2,2'-bis-(2,4-dioxobutyl)biphenyl ligands, R2BobH2 (R = tBu, p-Tol), react with Ti(OiPr)4 to give, initially, a mixture of monomer and oligomers, which is converted quantitatively to monomer upon heating in the presence of excess Ti(OiPr)4. Only a single relative configuration of the biphenyl and bis(chelate) titanium moieties, established by crystallography of (tBu2Bob)Ti(O-2,6-iPr2C6H3)2 to be the (R)-/(S)- diastereomer, is observed. The kinetic and thermodynamic robustness of the (R2Bob)Ti framework is confirmed by reactions with Lewis acids. For example, (Tol2Bob)Ti(OiPr)2 reacts with trimethylsilyl triflate or triflic acid to substitute one or both of the isopropoxide groups with triflates without any redistribution or loss of the diketonate ligands. Cationic complexes can be prepared by abstraction of triflate from (Tol2Bob)Ti(OiPr)(OTf) with Na[B(C6H3(CF3)2)4]. For example, in the presence of diethyl ether, the crystallographically characterized [(Tol2Bob)Ti(OiPr)(OEt2)][B(C6H3(CF3)2)4], containing a rapidly dissociating ether ligand, is formed.
ABSTRACT
Dibenzoylmethane derivatives with one (L1H2) or both (L2H3, L3H3) benzenes linked at their ortho positions to 4,6-di-tert-butylphenol moieties by two-carbon linkers have been synthesized. The mono-beta-diketone-monophenol ligand L1H2 is metalated by titanium alkoxides to form the homoleptic complex (L1)2Ti and heteroleptic complexes (L1)Ti([OCH2CH2]2NR) (R = H, CH3), and reacts with Cp3Sc to form CpSc(L1). These are the first examples of complexes of a beta-diketonate ligand which is further chelating to a single metal center. Crystallographic analysis of (L1)2Ti indicates that the 10-membered ring allows chelation of the phenoxide with little strain, and both fac and mer geometries are accessible in solution. Protonolysis of the second cyclopentadienyl ring of Cp3Sc appears to take place by an indirect, Cp3Sc-catalyzed pathway.
ABSTRACT
The monomeric titanium(IV) hydroxide complex, LTi(OH)(LH(3)= tris(2-hydroxy-3,5-di-tert-butylbenzyl)amine), which is sterically inhibited from condensation to a mu-oxo dimer, cannot be prepared by hydrolysis of the alkoxide, with K(eq)= 0.012 for hydrolysis of the titanium methoxide in THF.
ABSTRACT
Sulfonamide and amide derivatives of tris(aminoethyl)amine (TREN) are known to facilitate phospholipid translocation across vesicle and erythrocyte membranes; that is, they act as synthetic translocases. In this report, a number of new TREN-based translocases are evaluated for their abilities to bind phosphatidylcholine and translocate a fluorescent phosphatidylcholine probe. Association constants were determined from (1)H NMR titration experiments, and translocation half-lives were determined via 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD)/dithionite quenching assays. A rough correlation exists between translocase/phosphatidylcholine association constants and translocation half-lives. The tris-sulfonamide translocases are superior to the tris-amide versions because they associate more strongly with the phospholipid headgroup. The stronger association is due to the increased acidity of the sulfonamide NHs as well as a molecular geometry (as shown by X-ray crystallography) that is able to form tridentate complexes with one of the phosphate oxygens. Two fluorescent translocase analogues were synthesized and used to characterize membrane partitioning properties. The results indicate that the facilitated translocation of phospholipids by TREN-derived translocases is due to the formation of hydrogen-bonded complexes with the phospholipid headgroups. In the case of zwitterionic phosphatidylcholine, it is the neutral form of the translocases that rapidly associates with the phosphate portion of the phosphocholine headgroup. Complexation masks the headgroup polarity and promotes diffusion of the phospholipid-translocase complex across the lipophilic interior of the membrane.
