Hybrid Scorpionate/Cyclopentadienyl Magnesium and Zinc Complexes: Synthesis, Coordination Chemistry, and Ring-Opening Polymerization Studies on Cyclic Esters

The reaction of the hybrid scorpionate/cyclopentadienyl lithium salt [Li(bpzcp)(THF)] [bpzcp = 2,2-bis(3,5-dimethylpyrazol-1-yl)-1,1-diphenylethylcyclopentadienyl] with 1 equiv of RMgCl proceeds cleanly to give very high yields of the corresponding monoalkyl κ2-NN-η5-C5H4 magnesium complexes [Mg(R)(κ2-η5-bpzcp)] (R = Me 1, Et 2, nBu 3, tBu 4, CH2SiMe3 5, CH2Ph 6). Hydrolysis of the hybrid lithium salt [Li(bpzcp)(THF)] with NH4Cl/H2O in ether cleanly affords the two previously described regioisomers: (bpzcpH) 1-[2,2-bis(3,5-dimethylpyrazol-1-yl)-1,1-diphenylethyl]-1,3-cyclopentadiene (a) and 2-[2,2-bis(3,5-dimethylpyrazol-1-yl)-1,1-diphenylethyl]-1,3-cyclopentadiene (b). Subsequent reaction of the bpzcpH hybrid ligand with ZnR2 quantitatively yields the monoalkyl κ2-NN-η1(π)-C5H4 zinc complexes [Zn(R){κ2-η1(π)-bpzcp}] (R = Me 7, Et 8, tBu 9, CH2SiMe3 10). Additionally, magnesium alkyls 1, 2, 4, and 5 can act as excellent cyclopentadienyl and alkyl transfers to the zinc metal center and yield zinc alkyls 7−10 in good yields. The single-crystal X-ray structures of the derivatives 4, 5, 7, and 10 confirm a 4-coordinative structure with the metal center in a distorted tetrahedral geometry. Interestingly, whereas alkyl magnesium derivatives 4 and 5 present a η5 coordination mode for the cyclopentadienyl fragment, zinc derivatives 7 and 10 feature a peripheral η1(π) arrangement in the solid state. Furthermore, the reaction of the hybrid lithium salt [Li(bpzcp)(THF)] with 1 equiv of ZnCl2 in tetrahydrofuran (THF) affords very high yields of the chloride complex [ZnCl{κ2-η1(π)-bpzcp}] (11). Compound 11 was used as a convenient starting material for the synthesis of the aromatic amide zinc compound [Zn(NH-4-MeC6H4){κ2-η1(π)-bpzcp}] (12), by reaction with the corresponding aromatic primary amide lithium salt. Alternatively, aliphatic amide and alkoxide derivatives were only accessible by protonolysis of the bis(amide) complexes [M{N(SiMe3)2}2] (M = Mg, Zn) and the mixed ligand complex [EtZnOAr)] with the hybrid ligand bpzcpH to afford [Zn(R){κ2-η1(π)-bpzcp}] (R = N(SiMe3)2 13, R = 2,4,6-Me3C6H2O 14) and [Mg{N(SiMe3)2}(κ2-η5-bpzcp)] (15). Finally, alkyl and alkoxide-containing complexes 1−10 and 14 can act as highly effective single-component living initiators for the ring-opening polymerization of ε-caprolactone and lactides over a wide range of temperatures. ε-Caprolactone is polymerized within minutes to give high molecular weight polymers with medium-broad polydispersities (Mn > 105, Mw/Mn = 1.45). Lactide afforded poly(lactide) materials with medium molecular weights and polydispersities as narrow as Mw/Mn = 1.02. Additionally, polymerization of l-lactide occurred without racemization in the propagation process and offered highly crystalline, isotactic poly(l-lactides) with very high melting temperatures (Tm = 165 °C). Microstructural analysis of poly(rac-lactide) by 1H NMR spectroscopy revealed that propagations occur without appreciable levels of stereoselectivity. Polymer end group analysis showed that the polymerization process is initiated by alkyl transfer to the monomer.