Search

The use of aminoacidato Cp*Ir(III)-complexes in catalytic alcohol amination reactions of primary and secondary alcohols with amines permits to carry out these transformations at very mild reaction ...

[Cp*Ir(Pro)Cl] (Pro = prolinato) was identified among a series of Cp*-iridium half-sandwich complexes as a highly reactive and selective catalyst for the alkylation of amines with alcohols. It is active under mild conditions in either toluene or water without the need for base or other additives, tolerates a wide range of alcohols and amines, and gives secondary amines in good to excellent isolated yields.

The zinc(II) coordination chemistry of the bioinspired bis(tetradentate) ligand 3,5-bis{bis[2-(1-methylimidazolyl)methyl]aminomethyl}pyrazole (L) was studied in detail both in solution and in the solid state. Six different species were determined in aqueous solution by means of potentiometry within the pH range 211, namely mononuclear [ZnH2L]4+, dinuclear [Zn2L]4+, [Zn2H1L]3+ and [Zn2H2L]2+, as well as tetranuclear [Zn4H5L2]3+ and [Zn4H6L2]2+. Complexes representing the protonation state of four of these species could be isolated and structurally characterized by X-ray diffraction, namely [ZnH2L(MeCN)](ClO4)4 (1), [Zn2H1L](SO3CF3)3 (4), [Zn2H1L(MeOHOMe)](ClO4)2 (5a), [Zn2H1L(OMe)](ClO4)2 (5b), and [(Zn2H1L)2(mu-O)(mu-OH)](ClO4)3 (6). In addition, a bis(pyrazolato)-bridged complex, [(ZnH1L)2](ClO4)2 (2), was obtained and structurally characterized, and it was found to exhibit ligand dynamics in solution. Both 5a and 5b are likely present in solution, which requires significant plasticity of the Zn...Zn distance (4.21 angstrom in 5a vs. 3.38 angstrom in 5b). Interconversion of 5a and 5b is proposed to be a facile process, supporting the idea that the ZnO2H3Zn moiety is a functionally relevant hydrolytic zincaqua structure in oligozinc metalloenzymes. The tetranuclear complex 6 that features two {Zn2H1L} subunits connected by a central mu-oxido-mu-hydroxido moiety is unprecedented in zinc(II) chemistry. Combining X-ray crystallography with potentiometry, ESI mass spectrometry, as well as NMR and EXAFS spectroscopy has provided a comprehensive picture of the multitude of species present in this rather complex system.

Metallo-β-lactamases (mβls) cause bacterial resistance toward a broad spectrum of β-lactam antibiotics by catalyzing the hydrolytic cleavage of the four-membered β-lactam ring, thus inactivating the drug. Minutiae of the mechanism of these enzymes are still not well understood, and reports about binding studies of the substrates to the enzymes as well as to synthetic model systems are rare. Here we report a new pyrazolate-based bioinspired dizinc complex (1) reminiscent of the active site of binuclear mβls. Since 1 does not mediate hydrolytic degradation of β-lactams, the binding of a series of common β-lactam antibiotics (benzylpenicillin, cephalotin, 6-aminopenicillanic acid, ampicillin) as well as the inhibitor sulbactam and the simplest β-lactam, 2-azetidinone, to the dizinc core of 1 could now be studied in detail by NMR and IR spectroscopy as well as mass spectrometry. X-ray crystallographic information was obtained for 1 and its complexes with 2-azetidinone (2) and sulbactam (3); the latter represents the first structurally characterized dizinc complex with a bound β-lactam drug. While 2-azetidinone was found deprotonated and bridging in the clamp of the two zinc ions in 2, in 3 and all other cases the substrates preferentially bind via their carboxylate group within the bimetallic pocket. The relevance of this binding mode for mβls and consequences for the design of functional model systems are discussed.

The unsymmetrical dinucleating ligand 2-(N-isopropyl-N-((2-pyridyl)methyl)aminomethyl)-6-(N-(carboxylmethyl)-N-((2-pyridyl)methyl)aminomethyl)-4-methylphenol (IPCPMP or L) has been synthesized to model the active site environment of dinuclear metallohydrolases. It has been isolated as the hexafluorophosphate salt H(4)IPCPMP(PF(6))(2) x 2 H(2)O (H(4)L), which has been structurally characterized, and has been used to form two different Zn(II) complexes, [{Zn(2)(IPCPMP)(OAc)}(2)][PF(6)](2) (2) and [{Zn(2)(IPCPMP)(Piv)}(2)][PF(6)](2) (3) (OAc = acetate; Piv = pivalate). The crystal structures of and show that they consist of tetranuclear complexes with very similar structures. Infrared spectroscopy and mass spectrometry indicate that the tetranuclear complexes dissociate into dinuclear complexes in solution. Potentiometric studies of the Zn(II):IPCPMP system in aqueous solution reveal that a mononuclear complex is surprisingly stable at low pH, even at a 2:1 Zn(II):L ratio, but a dinuclear complex dominates at high pH and transforms into a dihydroxido complex by a cooperative deprotonation of two, probably terminally coordinated, water molecules. A kinetic investigation indicates that one of these hydroxides is the active nucleophile in the hydrolysis of bis(2,4-dinitrophenyl)phosphate (BDNPP) enhanced by complex 2, and mechanistic proposals are presented for this reaction as well as the previously reported transesterification of 2-hydroxypropyl p-nitrophenyl phosphate (HPNP) promoted by Zn(II) complexes of IPCPMP.

A set of pyrazole-bridged bis(imidazolium) compounds [H3L1]X2 - [H3 L4]X2 (L1 = 3,5-bis[1-(tert-butyl)imidazolium-1-ylmethyl]-1H-pyrazole; L2 = 3,5-bis[1-(tert-butyl)imidazolium- 1-ylmethyl]-4-phenyl-1H-pyrazole; L3 = 3,5-bis[1-(1-adamantyl)imidazolium-1-ylmethyl]-1Hpyrazole; L4 = 3,5-bis[1-(1-adamantyl)imidazolium-1-ylmethyl]-4-phenyl-1H-pyrazole; X = Cl−, BF4 − or PF6 −) has been prepared, and three compounds have been characterized by X-ray crystallography. The unique [H3L4][H2L4](PF6)3 features a dimeric face-to-face arrangement of two molecules due to the involvement of both the pyrazole-NH and the imidazolium C2H in hydrogen bonding. [H3L1]X2 - [H3L4]X2 serve as precursors for silver(I) complexes with compartmental pyrazolate-bridged bis(NHC) ligands. The complexes have been readily prepared by the Ag2O route and feature either the known [(L1−4)2Ag4]2+ or the new [(H2L1)4Ag4]8+ motif, depending on the solvent for the reaction (MeCN or acetone). [(H2L1)4Ag4](PF6)8 contains a central (pzAg)4 ring with pendant imidazolium side arms. Upon further reaction with Ag2O in MeCN it was found to undergo transformation to the corresponding [(L1)2Ag4](PF6)2. All complexes have been thoroughly studied by NMR spectroscopy in solution, and preliminary luminescence data of [(H2L1)4Ag4](PF6)8 have been recorded