Well‐Defined Cationic Alkyl– and Alkoxide–Aluminum Complexes and Their Reactivity with ε‐Caprolactone and Lactides

Abstract

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.

Electronically Unsaturated Three‐Coordinate Aluminum Hydride and Organoaluminum Cations

“…A number of four‐, five‐ and six‐coordinate cationic aluminum species are known with their applications ranging from polymerization of cyclic ethers (propylene oxide), esters (ϵ‐capralactone), isobutene, and β‐hydrogen‐transfer reactions to olefins or ketones . It is not surprising that two and three‐coordinate cationic aluminum species, due to the coordinative unsaturation at the metal, are very rare, and the well‐characterized ones are shown in Figure …”

An Unusual N‐Bridged (Amido)(hydrido)(phenoxido)aluminium Dinuclear Compound – The Role of Nitrogen Substituents in Determining Nuclearity: A Combined Experimental and Theoretical Study

“…For ligands containing both oxygen and nitrogen as potential bridging atoms, it is found that compounds having oxygen bridges between aluminium atoms [23,24,39,[47][48][49][50] predominate over those with nitrogen bridges. [37,51] Examples are known with both O and N atoms bridging in hydroxypyridine ligands (in an isolobal disposition to carboxylatebridged ligands) [25] or with an amidate group [52][53][54][55] between two aluminium atoms.…”

“…The values observed in 5 are in the range observed for other complexes with a bridging N atom [37,57,58] and comparable to Al-N dative bond lengths. [50,59] The six-membered ring chelate Al metallacycle is significantly twisted, angles between the planes containing the Al 2 N 2 and C 6 H 2 rings ranging from 32.88°to 33.48°. [32,38] In view of these experimental results, we were interested in understanding the factors that determine the tendency to stabilize mononuclear or dinuclear structural dispositions in (amido)(hydrido)(phenoxido) aluminium compounds.…”

Deoxygenierende Reduktion von Kohlendioxid zu Methan, Toluol und Diphenylmethan mit [Et2Al]+ als Katalysator

“…Die Reaktionsprodukte sind C 6 D 5 CH 3 und (Et 3 Si) 2 O. Zwischenzeitlich wurde Methanol gebildet, das aber weiterreagierte. [25] Zum Vergleich haben wir auch die katalytische Aktivität von AlBr 3 in der CO 2 [26] und wurden unter anderem für die Herstellung von Aluminiumoxiden bei hçheren Temperaturen eingesetzt. [22] In vielen der obigen Reaktionen, insbesondere denen mit langen Reaktionszeiten und unter Erhitzen, war Et 4 Si das Endprodukt und nicht das erwartete Siloxan (Et 3 Si) 2 O. Da es bekannt ist, dass Siloxane und Silylether unter Substituentenaustausch in Gegenwart einer Lewis-Säure reagieren, [23] wurde (Et 3 Si) 2 Es sollte auch nicht unerwähnt bleiben, dass CO 2 nicht mit 1 reagierte, auch nicht nach 30 h bei 80 8C.…”