Exploration of the Suitability of Bicyclic Guanidinates as Ligands in Catalytic Chemistry Mediated by Titanium

Abstract

The synthesis, structure, and reactivity of titanium complexes supported by the bicyclic guanidinate ligand derived from 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine (hppH) are described. In situ reaction between a THF solution of the lithium salt, (hppLi)n, and titanium chloride starting materials afforded the series of compounds Ti(hpp)nCl4-n(THF)x [n = 1, x = 1 (1); n = 2, x = 0 (3); n = 3, x = 0 (4)]. The dimeric, base-free complex [Ti(hpp)Cl2(-Cl)]2 (2) was synthesized from the reaction of the silylated ligand precursor, hppSiMe3, with TiCl4 in CH2Cl2. Preliminary olefin polymerization studies of 1-3 using MAO as activator indicated low activities. The reaction between 3 and 2 equiv of AlMe3 resulted in formation of a novel trimetallic Ti(III) species, Ti{AlMe2(hpp)2}Cl(-Cl)AlMe3 (5), highlighting the noninnocent behavior of the ligands. The bis(benzyl) complex Ti(hpp)2(CH2Ph)2 (6) was synthesized from the alkane elimination reaction between the neutral ligand precursor, hppH, and Ti(CH2Ph)4. Attempted generation of the corresponding mono-benzyl cation through reaction with the neutral borane B(C6F5)3 again afforded reduced species. The synthesis of titanium tert-butylimido compounds [Ti(-NtBu)(hpp)Cl]2 (7) and [Ti(NtBu)(hpp)(-hpp)]2 (8) from the salt metathesis reaction of 1 and 2 equiv of (hppLi)n, respectively, with Ti(NtBu)Cl2(py)3 is reported. The molecular structures of compounds 2 and 3 and 5-8 are reported, and the distribution of -electron density throughout the guanidinate ligands is discussed in detail.