Aluminum and gallium amidinate complexes, {RC(NR‘)2}MMe2 (R, R‘ = alkyl; M = Al, Ga), react with the “cationic activators” [Ph3C][B(C6F5)4] and B(C6F5)3 to yield cationic Al and Ga alkyl species whose structures are strongly influenced by the steric properties of the amidinate ligand. The reaction of acetamidinate Al complexes {MeC(NR‘)2}AlMe2 (R‘ = i Pr, 1a; R‘ = Cy, 3a) with 0.5 equiv of [Ph3C][B(C6F5)4] or B(C6F5)3 yields {MeC(NR‘)2}2Al2Me3 + (R‘ = i Pr, 2a +; R‘ = Cy, 4a +) as the B(C6F5)4 - or MeB(C6F5)3 - salts. X-ray crystallographic analyses establish that 2a + and 4a + are double-amidinate-bridged dinuclear cations, in which the two metal centers are linked by μ-η1,η1 and μ-η1,η2 amidinate bridges. NMR studies show that 2a + undergoes two dynamic processes in solution:  (i) a μ-η1,η1/μ-η1,η2 amidinate exchange and (ii) Me exchange between the two metal centers. The reaction of {MeC(N i Pr)2}GaMe2 (1b) with 0.5 equiv of B(C6F5)3 yields {MeC(N i Pr)2}2Ga2Me3 + (2b +), whose structure and dynamic properties are similar to those of 2a +. The reaction of the bulkier t Bu-substituted amidinate complexes { t BuC(N i Pr)2}MMe2 (M = Al, 6a; M = Ga, 6b) with 0.5 equiv of [Ph3C][B(C6F5)4] yields { t BuC(N i Pr)2}MMe2·{ t BuC(N i Pr)2}MMe+ (M = Al, 7a +; M = Ga, 7b +) as the B(C6F5)4 - salts, the former of which is thermally unstable. An X-ray crystallographic analysis establishes that 7b + is a single-amidinate-bridged dinuclear cation, in which the two metal centers are linked by a μ-η1,η2 amidinate bridge. NMR data establish that the structures of 7a + and 7b + are similar and both species are rigid in solution. 6a and 6b also react with B(C6F5)3 to yield [7a][MeB(C6F5)3] and [7b][MeB(C6F5)3], respectively, which decompose by C6F5 - transfer to yield { t BuC(N i Pr)2}M(Me)(C6F5) (M = Al, 9a; M = Ga, 9b) and boron species. The “super-bulky” amidinate complexes { t BuC(N t Bu)2}MMe2 (M = Al, 12a; M = Ga, 12b) react with 1 equiv of [Ph3C][B(C6F5)4] to yield { t BuC(N t Bu)2}MMe+ (M = Al, 13a +; M = Ga, 13b +) as the B(C6F5)4 - salts. The salts [13a][B(C6F5)4] and [13b][B(C6F5)4] are thermally unstable and could not be isolated. However, the NMR data for 13a + and 13b + in C6D5Cl are consistent with base-free, three-coordinate structures or labile, four-coordinate solvated cations. These results provide a starting point for understanding the mechanism and reactivity trends in ethylene polymerization catalyzed by cationic Al amidinate species.

Polylactide (PLA) is an attractive polymeric material due to its origin from annually renewable resources and its biodegradability. The ring-opening polymerization (ROP) of lactide initiated by Lewis acidic and oxophilic metal-based catalysts constitutes the method of choice to access PLA in a controlled and stereoselective manner. The design and synthesis of ligand-supported metal complexes to act as effective ROP initiators of lactide monomers have been the subject of numerous investigations over the past decades. In view of their oxophilic nature, well-defined group 4 metal complexes supported by polydentate supporting ligands have appeared as active initiators for lactide ROP. This perspective summarizes various classes of structurally well-defined group 4 metal initiators developed for lactide ROP. It also provides observed trends regarding their catalytic performance. Whenever appropriate and possible, catalyst structure-ROP performance (i.e. activity, control and stereoselectivity) relationships are rationalized.

No abstract

No abstract

International audienceOver the past five years, Ga(III) and most notably In(III) precursors have attracted a growing interest for application in ROP catalysis of cyclic esters, primarily lactide, and may now be considered as potentially efficient ROP initiators of cyclic esters/carbonates. Despite their higher cost (vs. Al), Ga and In derivatives exhibit key attractive features including: (i) Ga(III) and In(III) are biocompatible metal centers and (ii) their precursors are typically more stable than organoaluminum species in polar media. The present contribution reviews discrete Ga(III) and In(III) compounds thus far developed as ROP initiators of cyclic esters/carbonates. The very few reports on Ga(III)-mediated ROPs of cyclic ethers are also included. In addition to the ROP performances of such species, the synthesis and structural characterization of these initiators are also provided and thoroughly discussed with, whenever appropriate, the establishment of structure/reactivity relationships and mechanistic pathways

The present contribution describes the synthesis and structural characterization of a novel family of robust titanium complexes, supported by a tridentate pincer ligand of the type bis-phenolate-N-heterocyclic carbene [ tBu (OCO) 2-]. For the most part, these complexes were found to be accessible in high yields via an alcohol elimination route involving the reaction of the imidazolinium salt [ tBu (OCO)H 3 ]Cl (1) with ClTi(O i Pr) 3 or Ti(O i Pr) 4 to afford the corresponding NHC-Ti complexes [ tBu (OCO)]TiCl 2 (2) and [ tBu (OCO)]TiCl(O i Pr) (3), respectively, when the reaction is carried out in noncoordinative solvents such CH 2 Cl 2 and toluene. When these reactions are performed in THF, the corresponding Ti-THF adducts [ tBu (OCO)]TiCl 2 (THF) (2-THF) and [ tBu (OCO)]TiCl(O i Pr)(THF) (3-THF) are isolated in quantitative yields. The molecular structures of complexes 2, 2-THF, and 3-THF were determined by X-ray crystallographic studies, establishing the effective coordination of the tBu (OCO) 2pincer to Ti. While the alcohol elimination pathway appears to be most suited to access Ti complexes of the type [ tBu (OCO)]TiX 2 (X = halide, alkoxide), the amine elimination was also found to be effective, albeit in lower yield. Thus, the reaction of salt species 1 with Ti(NMe 2 ) 4 in THF afforded the corresponding NHC-Ti amido complex [ tBu (OCO)]TiCl(NMe 2 )(THF) (5) in a modest yield. The direct reaction of the salt species 1 with 0.5 or 1 equiv of TiCl 4 (THF) 2 in the presence of NEt 3 afforded the homoleptic bis-adduct Ti complex [ tBu (OCO)] 2 Ti (6), whose molecular structure was confirmed by X-ray crystallographic analysis. As for the potential of such complexes in catalysis, the Ti isopropoxide chloro complex 3-THF was found to readily initiate the ring-opening polymerization of rac-lactide in a controlled manner and, interestingly, without apparent involvement of the NHC moiety in the catalytic process. The tridentate nature of the tBu (OCO) 2ligand as well as some level of steric protection provided by the t Bu groups may rationalize the excellent stability of the Ti-NHC bond in the present systems.

We describe the synthesis, structure, and reactivity of low-coordinate Al-alkyl and -alkoxide cationic complexes incorporating the sterically bulky aminophenolate bidentate ligand 6-(CH(2)NMe(2))-2-CPh(3)-4-Me-C(6)H(2)O- (N,O). These complexes are derived from the ionization of neutral dialkyl Al complexes (N,O)Al2) (1 a, R=Me; 1 b, R=iBu), readily obtained by alkane elimination between AlR3 and the corresponding aminophenol ligand, with the alkyl abstracting reagents B(C(6)F(5))3 and [Ph(3)C][B(C(6)F(5))4]. The reactions of 1 a,b with B(C(6)F(5))3 yield complicated mixtures or decomposition products, however the ionization of the Al-diisobutyl derivative 1 b with [Ph(3)C][B(C(6)F(5))4] affords a stable four-coordinate Al-PhBr cationic adduct [(N,O)Al(iBu)(PhBr)]+ (3+), as deduced from elemental analysis data. Complex 3+ readily coordinates Lewis bases such as THF to form the corresponding adduct [(N,O)Al(iBu)(thf)]+ (4+), and also rapidly chain-transfers with 1-hexene to yield the three-coordinate Al-hexyl cation [(N,O)Al-hexyl]+ (5+). Both cations 3+ and 5+ slowly dimerize to form unprecedented organoaluminum dications [(N,O)AlR+]2 (3'++, R=iBu; 5'++, R=hexyl) as deduced from X-ray crystallographic analysis. Cation 3+ reacts quickly with iPrOH to form a stable Lewis acid/base adduct [(N,O)Al(iBu)(HOiPr)]+ (6+), which constitutes the first X-ray characterized adduct between an Al-alkyl complex and a simple ROH. The Al-ROH proton in 6+ is readily abstracted by NMe(2)Ph to form the neutral isopropoxide Al complex [(N,O)Al(iBu)(OiPr)] (7). Upon reaction with THF, cation 6+ undergoes an intramolecular proton transfer to yield the ammonium Al-THF complex [(eta1-HN,O)Al(iBu)(OiPr)(thf)] (8 b+), in which the aminophenolate is eta1-coordinated to the Al center. Cation 8 b+ can then be converted to the desired Al-alkoxide derivative [(N,O)Al(OiPr)(thf)](+) (10+), by an intramolecular protonolysis reaction, as confirmed by X-ray crystallography. The synthesized Al-alkyl cations form robust four-coordinate adducts in the presence of cyclic esters such as epsilon-caprolactone and (D,L)-lactide, but no insertion chemistry occurs, illustrating the poor ability of the Al-R+ moiety to ring-open. In contrast, the Al-alkoxide cation 10+ polymerizes epsilon-caprolactone in a controlled manner with excellent activity, but is inactive in the polymerization of (D,L)-lactide and L-lactide. Control experiments with L-lactide show that cation 10+ ring-opens L-lactide to yield a robust five-coordinated Al--lactate cation [(N,O)Al(eta2-L-lactate-OiPr)(thf)]+ (13+), derived from a monoinsertion of L-lactide into the Al--OiPr bond of 10+, that does not further react. Cation 13+ may be regarded as a structurally characterized close mimic of the initial intermediate in the ring opening polymerization (ROP), of lactides by [{LX}M(OR)(L)] (where LX-=bidentate monoanionic ligand and L=labile ligand) metal complex initiators.

We describe the synthesis of the new Zn–N‐heterocyclic carbene (NHC) alkoxide complexes [(S,CNHC)ZnCl(OBn)]2 (5) and [(O,CNHC)ZnCl(OBn)]2 (6) for use as ring‐opening polymerization (ROP) initiators for lactide polymerization. Complexes 5 and 6 are readily available through an alcoholysis reaction between BnOH and the corresponding Zn–NHC ethyl species [(S,CNHC)ZnCl(Et)] (3) and [(O,CNHC)ZnCl(Et)] (4), and species 3 and 4 were obtained from the reaction of ZnEt2 with the N‐methyl‐N'‐ethylphenylsulfide (1⋅HCl) and N‐methyl‐N'‐ethylmethylether (2⋅HCl) imidazolium salts, respectively. Both solution and solid‐state structural data for Zn benzyloxide species 5 and 6 agree with dimeric structures under the studied conditions (reaction conditions: CH2Cl2 or THF, room temperature). A computational analysis of species 3 and 4 supports a dimeric structure in solution. The ZnII alkoxide species 5 and 6 were found to mediate either the ROP of lactide (in an effective and controlled manner) to produce chain length‐controlled polylactide (PLA) or, in the presence of an alcohol source such as MeOH, the controlled degradation of PLA through extensive transesterification reactions to afford methyl lactate as the major product. A thorough DFT computational analysis of the ROP of lactide initiated by complex 5 was performed, which revealed that the operating coordination–insertion mechanism was assisted by the second Zn center, leading to a lower‐energy ROP process; this result may be of interest for the future design of well‐defined and high‐performance metal‐based catalysts.