The replacement of the imine functionality in the ubiquitous phenoxy-imine (FI) ligands by a more robust and donor N,N,N′-trisubstituted amidine function was examined and gave rise to the synthesis of five new phenoxy-amidine (FA) ligands (L1− L5). The solid-state structure of four proligands has been determined by X-ray diffraction analysis and showed that the amidine moiety is in a trans-configuration. The reaction of the phenol-amidine proligands with AlMe 3 afforded mononuclear (L1−L5)AlMe 2 (1a−5a). A similar alkane elimination route was used from ZnEt 2 and led to dinuclear [(L1−L5)ZnEt] 2 complexes (1b−5b) or to homoleptic (L2/L4) 2 Zn complexes (2b′, 4b′) depending on the metal/ligand ratio used. The structure of these complexes has been determined by NMR spectroscopy ( 1 H, 13 C, HMBC, HSQC, DOSY, and NOESY experiments) and X-ray diffraction study for seven of them. The crystal structure of the Al complexes showed FA ligands coordinated in a chelate fashion via the O atom of the aryloxy group and the imino-N atom, indicating that the amidine function has undergone trans−cis isomerization upon coordination. A similar chelating coordination mode was observed for the FA ligands with Zn metal ions. These complexes were used as initiators for the ring-opening polymerization of rac-lactide. FA−Zn complexes gave the best performance, affording polylactic acid with a narrow molecular weight distribution and heterotactic bias (Pr up to 0.75). Remarkably, some of these complexes were able to tolerate the presence of a large amount of lactic acid to the point of using it as a co-initiator during the polymerization reaction.

Ti-imido complex [TiCl­(N t Bu)­(BIPP)] [1; BIPP = bis­(iminophosphoranyl)­phosphide ligand] reacts with terminal alkynes R–CCH (R = phenyl, isopropenyl, cyclopropyl, and 2-pyridyl) via P–P bond cleavage of the BIPP ligand. The resulting complexes [TiCl­(NPN′)­(NPhPPh2)] (2a–d) contain a pincer-type NPN′ phosphide ligand that incorporates the terminal alkyne and the imido ligand from complex 1. Complexes 2a–d feature two chiral centers (Ti and P) with interdependent absolute configurations; thus, they are formed stereoselectively. Complex 2a (R = phenyl) undergoes chloride abstraction with [Et3SiHSiEt3]­[B­(C6F5)4], yielding [Ti­(NPN′)­(NPhPPh2)]­[B­(C6F5)4] (3). Complex 3 is a moderately active and stereoselective initiator for the ring-opening polymerization of rac-lactide. Complex 3 activates the CO bond of 4-iodobenzaldehyde to give complex 4 as a single diastereomer despite the presence of three chiral centers. Complex 3 undergoes transmetallation with SbCl3, yielding [Sb­(NPN′)]­[B­(C6F5)4] (5) and [TiCl3(NPhPPh2)] (6) selectively. The bonding situation in 3 and 5 was analyzed using Bader’s atoms in molecules and the electron localization function, showing that the nitrogen atoms of the NPN′ ligand are electronically similar, and that the metal–phosphide interaction is more polar in the case of titanium.

Four cationic amidotitanocene complexes [Cp2Ti­(NRR′)]­[B­(C6F5)4] (Cp = η5-C5H5; 1a: R = R′ = p-anisyl; 1b: R = p-fluorophenyl, R′ = p-anisyl; 1c: R = p-fluorophenyl, R′ = phenyl; 1d: R = phenyl, R′ = 2-pyridyl) were synthesized. Complexes 1a–d undergo Ti–N bond homolysis under visible light irradiation. Complexes 1a–c catalyze the polymerization of phenylsilane to yield branched polysilane polymers with molecular weights (Mw) up to approximately 3000 and dispersity indexes (Đ) of 1.4–1.6. Previously reported Group 4 cationic amidometallocene complexes [Cp2Ti­(NPh2)]­[B­(C6F5)4] (Ia) and Cp2Zr­(NPh2)]­[MeB­(C6F5)3] (IIa) were also tested in the hydrosilylation of carbonyl compounds with triethylsilane (Et3SiH). In some cases, complex Ia afforded completely reduced products (e.g., ethylbenzene from acetophenone), while IIa was generally more selective (e.g., (1-phenylethoxy)­triethylsilane from acetophenone) but also more active. Complex IIa could also convert anisole derivatives to phenoxysilanes with high efficiency (TON = 2000).