Mononucleating bis(beta-diketonate) ligands and their titanium(IV) complexes.
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.
Ugrinova
Vesela
V
Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556-5670, USA.
Noll
Bruce C
BC
Brown
Seth N
SN