Coordination Chemistry and Reactivity of Monomeric Alkoxides and Amides of Magnesium and Zinc Supported by the Diiminato Ligand CH(CMeNC6H3-2,6-iPr2)2. A Comparative Study

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Abstract
The preparation and characterization of a series of closely related magnesium and zinc compounds are reported: LMg(NiPr2)(THF), 1; LZn(NiPr2), 2; LMg(OtBu)(THF), 3; LZn(OtBu), 4; and LZn(OSiPh3)(THF), 6; where L = CH(CMeNC6H3-2,6-iPr2)2. Their dynamic solution behavior has been examined by variable-temperature NMR studies and reveals that THF reversibly dissociates in toluene-d8 or CD2Cl2 and that exchange with free THF occurs by a dissociative process. Compounds 1 − 4 and 6 all initiate and subsequently sustain ring-opening polymerization (ROP) of lactides. For a related series of compounds LMX(THF)n, where n = 1 or 0, the rate of initial ring-opening follows the order M = Mg > Zn and X = OtBu > NiPr2 > NSi2Me6 > OSiPh3. In THF at 25 °C, compounds 3 and 4 polymerize 100 equiv of rac-lactide to >95% conversion in 5 and 80 min for M = Mg and Zn, respectively, and yield ca. 90% heterotactic PLA, (isi + sis tetrads). The reactions proceed faster in methylene chloride, but for M = Mg, a Bernoulian distribution of tetrads is formed from rac-lactide (3iii:2isi:sii:sis:iis) prior to trans-esterification. Polymerization of L-LA in toluene-d8 and THF-d8 by 3 and 4 have been studied by VT 1H NMR spectroscopy: the resting state for zinc is proposed to be a monomeric species akin to LZn(η2-OCHMeC(O)OMe), whereas the magnesium complex appears to be dimeric LMg(μ-OP)2MgL. None of the compounds is capable of initiating homopolymerization of propylene oxide (PO) or cyclohexene oxide (CHO), although the magnesium amide 1 effects ring-opening by allylic proton abstraction and the dimeric compound [LMg(μ-OC6H9)]2, 7, is formed. Reactions with carbon dioxide are also described, along with the characterization of LZnO2CNiPr2, 8, which is shown to be inert with respect to CHO and PO at room temperature. All the compounds are hydrolytically sensitive, and LZn(μ-OH)2ZnL, 5, has been isolated from hydrolysis of compound 4. The crystal and molecular structures are reported for compounds 1−5, 7, and 8. These results are compared with those recently reported by Coates et al. (J. Am. Chem. Soc.2001, 123, 3229−3238).