Well-Defined Alkyl Heteroscorpionate Magnesium Complexes as Excellent Initiators for the ROP of Cyclic Esters

Abstract

The reaction of the heteroscorpionate lithium salts [Li(tbpamd)(THF)] [tbpamd = N-ethyl-N‘-tert-butylbis(3,5-dimethylpyrazol-1-yl)acetamidinate] and [Li(pbpamd)(THF)] [pbpamd = N,N‘-diisopropylbis(3,5-dimethylpyrazol-1-yl)acetamidinate] with 1 equiv of RMgCl proceeds to give very high yields of the neutral heteroscorpionate alkyl magnesium complexes [Mg(R)(NNN)] (NNN = tbpamd, R = C3H5 1, t Bu 2, CH2SiMe3 3; NNN = pbpamd, R = C3H5 4, t Bu 5, CH2SiMe3 6). On heating toluene solutions of complexes 1 − 3, 5, and 6, a ligand redistribution process occurs to give the corresponding 6-coordinated sandwich complexes [Mg(tbpamd)2] (7) and [Mg(pbpamd)2] (8). Interestingly, the allyl derivative 4 can be easily transformed to 8 at room temperature. In addition, the cationic sandwich complex [Mg(tbpamdH)2]Cl2 (9) [tbpamdH = N-ethyl-N‘-tert-butylbis(3,5-dimethylpyrazol-1-yl)acetamidine] was obtained from 7 by means of a protonation process. Finally, alkyl-containing complexes 1 − 6 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 seconds to give high molecular weight polymers with narrow polydispersities. Lactide afforded PLA materials with medium molecular weights and polydispersities as narrow as M w/M n = 1.05. Additionally, polymerization of l-lactide occurred without racemization in the propagation process and offered highly crystalline, isotactic poly(l-lactides) with high melting temperatures (T m = 160 °C). Polymer end-group analysis shows that the polymerization process is initiated by alkyl transfer to the monomer.

Unprecedented Formation of the First Alkaline‐Earth‐Metal Complex Bearing an Asymmetrical gem‐Dithiolato Heteroscorpionato Ligand

“…However, the S-C(sp 3 )-S angles, which have an average value of 107.8(4)°, are consistent with a non-distorted tetrahedral geometry for the new C(sp 3 ) atom. As expected, the Mg-N bond lengths are not similar, and the axial distances [Mg1-N1 2.14(1) Å, Mg2-N5 2.18(1) Å, Mg3-N11 2.18(1) Å] are slightly longer than their equatorial counterparts, with the exception of Mg2-N7 [Mg1-N4 2.12(1) Å, Mg2-N7 2.22(1) Å, Mg3-N9 2.11(1) Å], but the latter values compare well with those previously observed in related heteroscorpionato alkylmagnesium complexes [12] to the new sp 3 carbon atom and present two different bond lengths, d(C α -C β ) = 1.51(1) Å and d(C β -C γ ) = 1.30(1) Å, on average, for the single and the double bond, respectively, which is in good agreement with literature precedents. [12,18] Single crystals of 2 suitable for X-ray diffraction crystallography were also readily grown from a dichloromethane solution at -26°C.…”

“…This type of behavior was previously observed in similar homoscorpionato alkylmagnesium systems by Parkin [17] and, more recently, in less sterically hindered alkylmagnesium heteroscorpionates reported by our group. [12] The difference in reactivity with the lithium salt, which was observed within this series of Grignard reagents, may be due to the potential η 3 coordination mode of the allyl ligand in a proposed monoalkyl complex, which could initially form before to the allyl insertion into the thiocarbonyl bond. In addition, the occurrence of this ligand-exchange process might be also favored by a polar solvent such as tetrahydrofuran and by the low steric demand of the methyl groups in the bis(3,5-dimethylpyrazol-1-yl)methane fragment, which are not sufficiently bulky to act as tetrahedral enforcers.…”

“…In a similar manner to the synthesis of analogous heteroscorpionato magnesium alkyls, [12][13][14] thf suspensions of the lithium dithioacetate [Li(bdmpzdta)(THF)] [15] [bdmpzdta = bis(3,5-dimethylpyrazol-1-yl)dithioacetate], which acts as a very attractive scaffold for the synthesis of discrete chlorido and alkoxido early-transition-metal complexes, [15] were treated with a series of commercially available Grignard reagents RMgCl (R = C 3 H 5 , Me, and Et) in a 1:1 molar ratio. Quite unexpectedly, the reaction with C 3 H 5 MgCl led to the complex [Mg(κ 3 -NNS-μ-S-bppbte)] 3 (1) [bppbte = 1,1-bis(3,5-dimethyl-1H-pyrazol-1-yl)pent-4-ene-2,2-bis(thiolate)], which bears an asymmetrical gem-dithiolato heteroscorpionato ligand in a κ 3 -NNS-μ-S coordination mode.…”

Magnesium Pyrazolyl-Indolyl Complexes as Catalysts for Ring-Opening Polymerization of L-Lactide

“…The Mg-Nindolyl bond lengths (2.1067(9) Å) are close to those found in magnesium bis-indolyl complexes (2.0907(16) Å for trans-form; 2.126(2) and 2.138(2) Å for cis-form) [28]. The Mg-Npyrazole bond lengths (2.2156(9) Å) are within those found in magnesium complexes bearing pyrazole functionality (2.100(3)-2.21(1) Å) [35][36][37][38]. The ligand precursor Pz H IndH was prepared via copper-catalyzed N-arylation reaction from 7-bromoindole with pyrazole [44].…”

“…They demonstrated efficient catalytic activities towards the ROP of cyclic esters. Considering the catalytic activities demonstrated by some pyrazolyl-containing magnesium complexes [35][36][37][38], we intended to introduce the pyrazolyl substituents as pendant functionalities into indole. We expected the combination of pyrazole and indole groups could be the candidates for ligand precursors.…”