CHEMISTRY OF MAGNESIUM ALKYLS COMPLEXES SUPPORTED BY A ß–DIIMINATO LIGAND

The use of biocompatible magnesium complexes as catalysts or initiators for the generation of biodegradable and biocompatible polymers derived from renewable resources is an attractive alternative for replacing the petroleum-based polymers. Herein, the magnesium alkyls complexes in the form of (BDI)MgR(THF) where BDI = CH(CMeNC₆H₃-2,6-Prⁱ₂), R = Me, Et, Prⁿ, Pr ⁱ, Buⁿ, n-hexyl, and CH₂CH₂Ph are synthesized, characterized and used as pre-catalysts for the ring-opening polymerization, ROP, of cyclic esters. Those where R = CH₂CH₂X (X = alkyl or phenyl) have the characteristic a-CH₂ proton resonances which are the part of an AA¿XX¿ pattern. Single crystal X-ray crystallographic studies on R= Bu ⁿ reveal that the molecule has a local N₂MgC(O) coordination which can be viewed as a distorted tetrahedral geometry. The Mg-Buⁿ bond in (BDI)MgBuⁿ(THF) readily reacts with alcohols, ROH, and amines, HNR₂, yielding the kinetic products (BDI)Mg(OR)(THF) and (BDI)Mg(NR₂)(THF), respectively. This Mg-Buⁿ group also shows greater reactivity relative to the BDI ligand in reactions involving carbon dioxide. With PhCHO, Ph₂CO and cyclohexanone, (BDI)MgBuⁿ(THF) reacts via ß-hydride transfer to generate the appropriate alkoxide with the elimination of 1-butene. Similarly L-LA reacts by ß-H transfer to give poly- L-lactide (P-L-LA) and 1-butene. On the other hand, rac-LA yields atactic polylactide, PLA, in toluene/dichloromethane solutions but in the presence of = 10 equiv. of THF, heterotactic PLA is formed. These indicate the involvement of THF at the Mg²⁺ centre during the polymerization.

In solution, (BDI)MgX(L) complexes, where X = Buⁿ, NEt₂, and Bu^t_, L = THF, 2-MeTHF, py and DMAP, are shown to undergo exchange of the free and coordinate ligands L. The dynamic line-shape ¹H and ¹³C {¹H} NMR analyses have been determined for the three exchange processes: (i) the apparent rotation of the aryl groups of the BDI, (ii) the exchange between free and coordinated ligands L, and (iii) bound THF rotation. With low concentrations of added L (THF, 2-MeTHF) in toluene-d₈, the L exchange occurs via a dissociative process and follows the order Buⁿ > NEt₂ > OBu^t for the X series and 2-MeTHF > THF > py > DMAP for the L series. At higher concentrations of THF for X = Buⁿ and OBu^t, but not for X = NEt ₂, an associative interchange process is observed for THF adducts involving a five-coordinate Mg²⁺ activated complex.

In polymerization events where X = Buⁿ, the ligands L have an effect on the homopolymerization of rac-LA and e-CL and the polymerization rates are in the same order as the L exchange rate. The propagation rate of e-CL is about 10, 6.8, and 5.5 times faster than rac-LA for THF, py, and DMAP adducts, respectively. However, the presence of LA completely suppresses the polymerization of e-CL. Model compounds (BDI)M(OCMe₂COOEt)(L) where M = Mg or Zn, L = py or DMAP representing the propagation of LA are synthesized. Single crystal X-ray crystallography, FTIR and variable temperature NMR studies reveal the chelation of CO group in such complex in both solid and solution. All model complexes are active for ROP of rac-LA (Mg > Zn) but are not for e-CL. In the presence of the equal mixture of rac-LA and e -CL, only ROP of rac-LA is observed. These clearly indicate the suppression of ROP of e-CL resulting from the chelation effect of the carbonyl group in such complex.