The assembly of architecturally sophisticated, high-nuclearity coordination complexes from the reactions of relatively simple ligands with suitable metal ions remains an area of intense activity driven not only by the aesthetic appeal of the structures, but also by the potentially wide applications of the resulting supramolecular entities as functional materials.[1] In particular, stereochemically well-defined building blocks are needed for the assembly of metallo-supramolecular structures, the dimensions of which are greater than those found in traditional covalent-based chemistry.[2] Helicates represent one of the paradigmatic branches of supramolecular architecture and these metallo-supramolecular arrays that contain one or more strand ligands have attracted much attention.[3] The understanding of the interplay between the metals and the ligands in helicates and their applications in the selective preparation of complicated organised chemical architectures remains the subject of systematic investigations, with great potential applications in asymmetric synthesis [4] and in the development of double-helical molecular architectures because life itself is encoded within double-helical DNA arrays. [5] Furthermore, heteroscorpionates are amongst the most versatile types of tridentate ligand and they can coordinate to a wide variety of elements. [6,7] In the last decade our research group has contributed widely to this field, designing new heteroscorpionate ligands related to the bis(pyrazol-1-yl)methane system and incorporating several pendant donor arms.[8] Acetamide and thioacetamide heteroscorpionate ligands exhibit high versatility in terms of coordination modes due to the existence of three possible tautomers (Scheme 1).[9] Inspired by this versatility and taking into consideration tautomer c, which could be capable of forming a helical surface by attachment to metal atoms, we attempted to generate a series of single-helical dinuclear complexes that could be appropriate building blocks to self-assemble into supramolecular helical structures. Herein, we report the [a] Prof.
The reactions of the hybrid scorpionate/cyclopentadiene compounds, as a mixture of regioisomers1-[2,2-bis(3,5-dimethylpyrazol-1-yl)-1,1-diphenylethyl]-1,3-cyclopentadiene and 2-[2,2-bis(3,5-dimethylpyrazol-1-yl)-1,1-diphenylethyl]-1,3-cyclopentadiene (bpzcpH) and 1-[2,2-bis(3,5-dimethylpyrazol-1-yl)-1-tert-butylethyl]-1,3-cyclopentadiene and 2-[2,2-bis(3,5-dimethylpyrazol-1-yl)-1-tert-butylethyl]-1,3-cyclopentadiene (bpztcpH)with [Lu(CH2SiMe3)3(thf)2] proceed in very high yields to give the free solvent neutral heteroscorpionate dialkyl lutetium complexes [Lu(CH2SiMe3)2(bpzcp)] (1) and chiral [Lu(CH2SiMe3)2(bpztcp)] (2). The structures in solution of 1 and 2 were investigated by VT NMR spectroscopy, and a fluxional behavior corresponding to an exchange between the alkyl groups was observed. The lutetium complex [Lu(CH2SiMe3)2(bpztcp)(thf)] (3) was isolated as an enantiomerically enriched complex. Supramolecular CH−π interactions between molecules in crystals of 3 have been identified in its X-ray molecular analysis, and they explain the formation of a conglomerate among molecules of 3. Complexes 1–3 are efficient catalysts for the intramolecular hydroamination of aminoalkenes, giving TOF values of up to 475 h–1 at 90 °C for 2,2-diphenyl-pent-4-enylamine (4) by using complex 3 as catalyst. Enantioselectivities up tp 70% ee were achieved in the cyclization of the 1,2-disubstituted olefin 6 with the high enantiopurity complex 3. The hydroamination reactions show apparently zero-order rate dependence on substrate concentration and first-order rate dependence on catalyst concentration. Additionally, bicyclization of 2-allyl-2-methylpent-4-enylamine (10) was achieved at 60 and 100 °C, giving exo,exo-2,4,6-trimethyl-1-azabicyclo[2.2.1]heptane (12). The protonolysis reaction of complex [Lu(CH2SiMe3)2(bpztcp)] (2) with 2 equiv of 2,2-diphenyl-pent-4-enylamine (4) yielded a dipyrrolidinide lutetium complex [Lu(NC4H5-2-Me-4,4-Ph2)2(bpztcp)] (13) as a mixture of two diastereoisomers. The structures of the complexes were determined by spectroscopic methods, and the X-ray crystal structures of 3 and 13 were also established.
With a sting in its tail: An enantiopure neodymium complex (see scheme) acts as an efficient single-site initiator for the controlled ring-opening polymerization of rac-lactide, forming isotactic polyester. The heteroscorpionate complex was characterized spectroscopically and by X-ray diffraction.
Five-coordinate alkyl aluminium complexes [AlR(κ(2)-pbpam)2] (R = Me 1, Et 2), [AlR(κ(2)-sbpam)2] (R = Me 3, Et 4) and [AlR{κ(2)-(S)-mbpam}2] (R = Me 5, Et 6), -pbpam [pbpam = N-phenyl-2,2-bis(3,5-dimethylpyrazol-1-yl)acetamidate], sbpam [sbpam = N-sec-butyl-2,2-bis(3,5-dimethylpyrazol-1-yl)acetamidate] and (S)-mbpam [(S)-mbpam = (S)-(-)-N-α-methylbenzyl-2,2-bis(3,5-dimethylpyrazol-1-yl)acetamidate] were obtained by an alkane elimination route involving the reaction of the previously reported acetamide heteroscorpionate precursors with 0.5 equiv. of the corresponding AlR3. In the solid state, complexes 1-6 adopt a distorted trigonal bipyramidal structure with the heteroscorpionate ligands arranged in a κ(2)-NO coordination mode. The molecular structures of 1-6 in solution were studied by VT NMR spectroscopy and a fluxional exchange between coordinated and noncoordinated pyrazole rings was observed. This process led to interconversion between the different isomers. Compounds 4 and 6 were used as precursors for the synthesis of the aryloxide aluminium compounds [Al(OR)(κ(2)-sbpam)2] (7) and [Al(OR){κ(2)-(S)-mbpam}2] (8) (R = 2,6-Me2C6H3O) by reaction with the corresponding 2,6-dimethylphenol. The structures of the complexes were determined by spectroscopic methods, and the X-ray crystal structures of 2, 6 and 7 were also established. Five-coordinate compounds 1-8 were evaluated as initiators in the ring-opening polymerization of rac-lactide. Compound 5 was also evaluated in the presence of a co-initiator. Finally, a study of the block and random copolymerization of ε-caprolactone and L-lactide was carried out.
The synthesis, structures, and ring-opening polymerization (ROP) activity of heteroscorpionate aluminum alkyl and aryloxide complexes are reported. The reactions of the acetamide and thioacetamide heteroscorpionate protio ligands pbptamH (pbptamH = N-phenyl-2,2-bis(3,5-dimethylpyrazol-1-yl)thioacetamide), pbpamH (pbpamH = N-phenyl-2,2-bis(3,5-dimethylpyrazol-1-yl)acetamide), sbpamH (sbpamH = N-sec-butyl-2,2-bis(3,5-dimethylpyrazol-1-yl)acetamide), and (S)-mbpamH ((S)-mbpamH = (S)-(-)-N-R-methylbenzyl-2,2-bis(3,5-dimethylpyrazol-1-yl)acetamide) with 1 equiv of AlR 3 (R = Me, Et, i Bu) proceed in very high yields to give the neutral heteroscorpionate dialkyl aluminum complexes [AlR 2 {κ 2 -pbptam}] (R = Me (1), Et (2)), [AlR 2 {κ 2 -pbptam}] (R = Me (3), Et (4), i Bu (5)), [AlR 2 {κ 2 -pbptam}] (R = Me (6), Et (7), i Bu (8)), and [AlR 2 {κ 2 -(S)-mbpam}] (R = Me (9), Et (10)). In the solid state, complexes 1-10 adopt a tetrahedral structure with the heteroscorpionate ligands arranged in a κ 2 coordination mode; in the case of the thioacetamidate derivatives 1 and 2 a κ 2 NN coordination mode is observed, whereas the acetamidate derivatives 3-10 present a κ 2 NO coordination mode. The structures in solution of 1-10 were investigated by VT NMR spectroscopy, and fluxional exchange between coordinated and noncoordinated pyrazole rings was observed, producing interconversion between the different isomers. Compounds 7 and 10 were used as convenient starting materials for the synthesis of the aryloxide aluminum compounds [Al(OR) 2 {κ 2 -sbpam}] (11) and [Al(OR) 2 {κ 2 -(S)-mbpam}] (12) (R = 2,6-Me 2 C 6 H 3 O) by reaction with the corresponding 2,6-dimethylphenol. The complexes [AlMe{κ 3 -pbptam}][MeB(C 6 F 5 ) 3 ] (13), [AlMe{κ 3 -pbptam}][B(C 6 F 5 ) 4 ] (14), [AlEt{κ 3 -pbptam}][B(C 6 F 5 ) 4 ] (15), [AlEt{κ 3 -sbpam}][B(C 6 F 5 ) 4 ] (16), and [AlEt{κ 3 -(S)-mbpam}]-[B(C 6 F 5 ) 4 ] (17) are derived from the ionization of the neutral dialkyl aluminum complexes 1, 2, 7, and 10 with the alkyl abstracting reagent B(C 6 F 5 ) 3 or [Ph 3 C][B(C 6 F 5 ) 4 ].The NMR data were consistent with an overall C s -symmetric structure for 13-15 and C 1symmetric structure for 16 and 17, which indicates an effective κ 3 coordination of the corresponding heteroscorpionate ligand to the cationic aluminum center. The structures of the complexes were determined by spectroscopic methods, and the X-ray crystal structures of 1, 2, and 7 were also established. Finally, exhaustive comparative catalytic studies of the aluminum complexes 1-10, 13, and 14 in the ring-opening polymerization of rac-lactide, L-lactide, and ε-caprolactone are also described.
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