Several new group 4 metal amide complexes with recently developed constrained-geometry carborane ligands, [η 5 :σ-Me 2 A(C 9 H 6 )(C 2 B 10 H 10 )]M(NR 2 ) 2 and [η 5 :σ-Me 2 A(C 5 H 4 )(C 2 B 10 H 10 )]M-(NR 2 ) 2 (A ) C, Si; M ) Ti, Zr; R ) Me, Et), were prepared in good yield via amine elimination reactions of M(NR 2 ) 4 and the corresponding neutral ligands in toluene. Treatment of the amide complexes with 2.5 equiv of Me 2 NH‚HCl or Me 3 N‚HCl led to high yields of the corresponding chloride complexes, [{η 5 :σ-Me 2 A(C 9 H 6 )(C 2 B 10 H 10 )}ZrCl(µ-Cl) 1.5 ] 2 -(A ) C, Si), which could also be prepared from an equimolar reaction between ZrCl 4 (THF) 2 and [Me 2 C-(C 9 H 6 )(C 2 B 10 H 10 )]Li 2 . In contrast, excess Me 3 SiCl could only convert [η 5 :σ-Me 2 C(C 5 H 4 )-(C 2 B 10 H 10 )]Ti(NMe 2 ) 2 to [η 5 :σ-Me 2 C(C 5 H 4 )(C 2 B 10 H 10 )]TiCl(NMe 2 ), due to steric effects. All these new complexes have been fully characterized by 1 H, 13 C, and 11 B NMR and IR spectroscopic data and elemental analyses. Thirteen out of fourteen newly prepared complexes have been structurally characterized by single-crystal X-ray diffraction studies. The chloride complexes exhibit a very high ethylene polymerization activity when they are activated with MMAO.

A series of rare-earth (RE) metal complexes (Y, Sm, Nd, and La) stabilized by polydentate N-methylethylenediamine-bridged tris­(phenolato) ligands was synthesized and characterized. Lanthanum complexes showed good activity in catalyzing the cycloaddition reaction of terminal epoxides with CO2 under ambient conditions (i.e., room temperature, 1 bar CO2), giving rise to cyclic carbonates in 49–99% yields. More importantly, generally challenging internal epoxides were also transformed into cyclic carbonates in 70–99% yields in the presence of 1 bar CO2 at 60 °C. This is the first RE-based catalyst for efficient cycloaddition of CO2 and epoxides under ambient conditions and is among the most active catalysts for this important transformation. The lanthanum complex was also recycled six times. Kinetic study of the cycloaddition of cyclohexene oxide and CO2 was conducted, and the kinetic equation was determined as follows: rate = k[epoxide]1[CO2]0[cat.]1.26[TBAI]0.4. The Gibbs activation energy (333 K) was determined to be 29.8 kcal/mol.

A series of neutral lanthanide alkoxides supported by an amine-bridged bis(phenolate) ligand were synthesized, and their catalytic behaviors for the polymerization of rac-lactide (LA) and rac-β-butyrolactone (BBL) were explored. The reactions of (C(5)H(5))(3)Ln(THF) with amine-bridged bis(phenol) LH(2) [L = Me(2)NCH(2)CH(2)N{CH(2)-(2-OC(6)H(2)Bu(t)(2)-3,5)}(2)] in a 1:1 molar ratio in THF for 1 h and then with 1 equiv each of 2,2,2-trifluoroethanol, benzyl alcohol, and 2-propanol gave the neutral lanthanide alkoxides LLn(OCH(2)CF(3))(THF) [Ln = Y (1), Yb (2), Er (3), Sm (4)], LY(OCH(2)Ph)(THF) (5), and LY(OPr(i))(THF) (6), respectively. These lanthanide alkoxides are sensitive to moisture, and the yttrium complex [(LY)(2)(μ-OPr(i))(μ-OH)] (7) was also isolated as a byproduct during the synthesis of complex 6. Complexes 1-6 were well characterized by elemental analyses and IR and NMR spectroscopy in the cases of complexes 1 and 4-6. The definitive molecular structures of all of these complexes were determined by single-crystal X-ray analysis. It was found that complexes 1-6 can initiate efficiently the ring-opening polymerization of rac-LA and rac-BBL in a controlled manner. For rac-LA, polymerization gave polymers with very narrow molecular weight distributions (PDI ≤ 1.12) and very high heterotacticity (P(r) up to 0.99). The observed activity-increasing order is in agreement with the order of the ionic radii, whereas the order for stereoselectivity is in the reverse order. For rac-BBL polymerization, the resultant polymers have narrow molecular distributions (PDI ≤ 1.26) and high syndiotacticity (P(r) up to 0.83). It is worth noting that the activity-decreasing order Yb > Er > Y >> Sm is observed for rac-BBL polymerization, which is opposite to the order of ionic radii and to the order of activity for rac-LA polymerization. The ionic radii of lanthanide metals have no obvious effect on the stereoselectivity for rac-BBL polymerization, which is quite different from that for rac-LA polymerization. End-group analysis of the oligomer of rac-BBL suggested that elimination side reactions occurred slowly in these systems, which led to chain cleavage and the formation of crotonate (and carboxy) end groups.

Two dinuclear aluminum alkyl complexes supported by a piperazidine-bridged bis(phenolato) group were prepared, and both complexes exhibited extremely high activity for the ring-opening polymerization of ε-caprolactone. In the presence of benzyl alcohol (BnOH), the polymerization accelerated dramatically.

A series of novel salan-ligated rare-earth metal amide complexes were prepared and employed as initiators for the ROP of rac-β-butyrolactone (rac-BBL). Tuning the substituents on the N atoms of the ligand frameworks from aromatic groups to aliphatic groups results in switching tacticity from iso-tactic to syndio-tactic in rac-BBL polymerization.