Mononucleating Bis(β-diketonate) Ligands and Their Titanium(IV) Complexes

Novel bis(β-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)-Λ/(S)-Δ 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.