DACH-bridged (DACH = trans-1,2-diaminocyclohexane) Bis(iminophosphonamide) derivatives of groups 3 and 13 and their use in the enantiomorphic polymerization of methyl methacrylate

A straightforward and flexible one-pot synthesis of a series of racemic frans-1,2-diaminocyclohexyl (DACH)-linked N-aryl bis(iminophosphonamines) is reported. These compounds were readily metalated with either nBuLi or Et ₃Al. The five-coordinate asymmetric environments provided by the ligands in the monometallic species were readily apparent from the useful and diagnostic ³¹P-{¹H} NMR spectra and correlated with the observed solid-state configurations. Similar reactions with the sterically encumbered yttrium amide [Y{N(SiMe₃)₂}₃] were only observed to produce the analogous N₅-coordinated amide under forcing conditions, while reactions with the less sterically encumbered [M{N(SiHMe₂)₂}₃(THF)_n] (M = Se (n = 2), Y (n = 3)) gave smooth conversion to the desired monometallic chelate complexes. All of the monometallic and bimetallic group 3 and group 13 compounds were assessed for the stereoselective polymerization of methyl methacrylate. Although the aluminum compounds were inactive as single-component initiators, activation with Gibson's three-component (L₂AlR/MAD/Ni(acac) ₂) system yielded active catalysts. The isolated polymers revealed a definite syndiotactic (>80%) bias, albeit with broad polydispersity indices (ca. 2) at ambient temperatures. In contrast, the yttrium complexes were found to function as efficient single-component polymerization initiators and yielded PMMA, which was predominantly isotactic (>80%) and, in some cases, close to monodisperse. Comparison to control experiments in which [Y{N(SiMe3)2}3] was employed as initiator provided evidence that it was the asymmetric environment of the bis(iminophosphonamide) ligand, which was exerting a pronounced (i.e. enantiomorphic) effect upon polymer propagation.