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.

Scorpionates represent one of the most versatile types of tridentate ligand that can coordinate to a wide variety of elements, e.g. from early to late transition metals, and the coordination chemistry of these systems has developed greatly in recent years. This Perspective gives an account of studies on the following aspects: (1) the preparative methods for a new class of heteroscorpionate [RR'C(pz)2] ligand derived from bis(pyrazol-1-yl)methane and (2) the description of metal complexes containing these ligands, examples of which incorporate a range of different metals from the Periodic Table.

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The reaction between NaNH2 and B(C6F5)3 affords the amidodiborate anion [H2N{B(C6F5)3}2]-, the structure of which shows multiple intramolecular NH···F hydrogen bonding. Reaction with HCl affords [H(OEt2)2][H2N{B(C6F5)3}2], while treatment of zirconocene dimethyls with [CPh3][H2N{B(C6F5)3}2] gives highly active alkene polymerization catalysts.

The reaction of bis(3,5-di-tert-butylpyrazol-1-yl)methane (bdtbpzm) with BunLi and carbodiimide derivatives, namely, N,N′-diisopropyl and 1-tert-butyl-3-ethyl carbodiimides, gives rise to the new sterically hindered lithium acetamidinate [Li(tbptamd)(THF)] (1) [tbptamd = N-ethyl-N′-tert-butylbis(3,5-di-tert-butylpyrazol-1-yl)acetamidinate] and [Li(pbptamd)(THF)] (2) [pbptamd = N,N′-diisopropylbis(3,5-di-tert-butylpyrazol-1-yl)acetamidinate]. Subsequent hydrolysis of 1 and 2, and the recently reported 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 NH4Cl/H2O in ether cleanly affords the corresponding amidine ligands Htbpamd (3), Hpbpamd (4), Htbptamd (5), and Hpbptamd (6) in very good yields. The X-ray diffraction molecular structure of 3 was obtained. Reaction of the amidine-heteroscorpionate ligands 3–6 with 1 equiv of ZnR′2 proceeds in very high yields to give the neutral heteroscorpionate alkyl zinc complexes [Zn(R′)(NNN)] (NNN = tbpamd, R′ = Me 7, Et 8, CH2SiMe3 9; NNN = pbpamd, R′ = Me 10, Et 11, CH2SiMe3 12; NNN = tbptamd, R′ = Me 13, Et 14; NNN = pbptamd, R′ = Me 15, Et 16). The single-crystal X-ray structures of the derivatives 8, 12, 15, and 16 confirm a four-coordinative structure with the zinc metal center in a distorted tetrahedral geometry and the heteroscorpionate ligands arranged in κ3-coordination mode. The new lithium salts 1 and 2 and the alkyls 7–9, 13, and 14 can act as efficient single-component 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-medium molecular weight polymers with medium broad values of 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 = 165 °C). rac-Lactide polymerization also produces enriched levels of heterotactic poly(lactide). Polymer end group analysis shows that the polymerization mediated by alkyl zinc complexes is initiated by alkyl transfer to monomer.

New neutral and zwitterionic chiral NNO‐donor scorpionate ligands 1 and 2 were designed to obtain new mononuclear and dinuclear NNO‐heteroscorpionate aluminum complexes. Reaction of 1 with [AlR3] (R=Me, Et) in a 1:1 or 1:2 molar ratio afforded the neutral mononuclear alkyl complexes [AlR2(κ2‐bpzappe)] {R=Me (3), Et (4); bpzappeH=2,2‐bis(3,5‐dimethylpyrazol‐1‐yl)‐1‐[4‐(dimethylamino)phenyl]‐1‐phenylethanol} and bimetallic complexes [{AlR2(κ2‐bpzappe)}(μ‐O){AlR3}] [R=Me (5), Et (6)]. By reaction of complexes 3–6 with PhCH2Br, mononuclear and dinuclear cationic aluminum complexes [AlR2(κ2‐bbpzappe)]Br {R=Me (7), Et (8); bbpzappeH=N‐benzyl‐4‐[2,2‐bis(3,5‐dimethyl‐1H‐pyrazol‐1‐yl)‐1‐hydroxy‐1‐phenylethyl]‐N,N‐dimethylbenzenaminium bromide} and [{AlR2(κ2‐bbpzappe)}(μ‐O){AlR3}]Br [R=Me (9), Et (10)] were synthesized. Both neutral aluminum complexes in the presence of Bu4NBr and cationic aluminum complexes were investigated as catalysts for cyclic carbonate formation from epoxides and carbon dioxide. Amongst them, complex 10 was found to be an efficient one‐component catalyst for the synthesis of cyclic carbonates from both monosubstituted and internal epoxides and was shown to have broad substrate scope.

The chiral and the enantiopure bis(pyrazol-1-yl)methane-based NNO-donor scorpionate ligands in the form of the alcohol compounds (bpzbeH) [bpzbe = 1,1-bis(3,5-dimethylpyrazol-1-yl)-3,3-dimethyl-2-butoxide], (bpzteH) [bpzte = 2,2-bis(3,5-dimethylpyrazol-1-yl)-1-para-tolylethoxide], and (R,R)-bpzmmH {(R,R)-bpzmm = (1R)-1-[(1R)-6,6-dimethylbicyclo[3.1.1]-2-hepten-2-yl]-2,2-bis(3,5-dimethylpyrazol-1-yl)ethoxide} have been utilized to obtain new NNO-scorpionate zinc alkyl complexes. The reaction of bpzbeH, bpzteH (racemic mixture), or (R,R)-bpzmmH (enantiopure) with [ZnMe2] in a 1:1 molar ratio in toluene afforded the mononuclear and monoalkyl zinc complexes [Zn(Me)(κ3-NNO)] (1–3), respectively. However, when the same reaction was carried out with [ZnEt2], [Zn t Bu2], or [Zn(CH2SiMe3)2], new dinuclear complexes of the type [Zn(R)(κ-NNμ-O)]2 (κ-NNμ-O = bpzbe, R = Et 4, t Bu 5, CH2SiMe3 6; bpzte, R = Et 7, t Bu 8, CH2SiMe3 9; (R,R)-bpzmm, R = Et 10, t Bu 11, CH2SiMe3 12) were obtained. The single-crystal X-ray structure of derivative 1 confirms a monomeric 4-coordinative structure in which the metal center is in a distorted tetrahedral geometry with the heteroscorpionate ligands in a κ3 coordination mode, whereas 4·2C 7 H 8 , 6, and 7 reveal an asymmetric κ-NNμ-O arrangement in a dimeric molecular disposition. Interestingly, alkyl-containing zinc complexes 1, 4, 5, 6, 9, and 12 can act as single-component initiators for the ring-opening polymerization of lactides at 50 °C. Thus, lactides were polymerized to afford PLA materials with low molecular weights in a few hours. The polymerizations are living, as evidenced by the narrow polydispersities (M w/M n = 1.03) of the isolated polymers in conjunction with the linear nature of the number-average molecular weight versus conversion plot. Inspection of the kinetic parameters for l-LA showed that propagations present the usual pseudo-first-order dependence on monomer and catalyst concentration. 1H NMR and MALDI-TOF mass spectra confirmed that the initiation occurs through nucleophilic attack of alkyl on the lactide monomer. Microstructural analysis of poly(rac-lactide) by 1H NMR spectroscopy revealed that the myrtenal substituent on the alkoxide fragment has a moderate influence on the degree of stereoselectivity, producing at mild temperatures enriched-heterotactic PLAs with a P r value of up to 0.77.