Search

We unravel the mechanisms of chelation-bond formation in nickel dicarbonyl complexes with bidentate phosphines and demonstrate that σ donation drives the carbonyl stretching response to the metal–ligand bond formation., The relation between spectroscopic observables and the detailed metal–ligand bonding features in chelation complexes is addressed using both experimental and state-of-the-art theoretical and computational methods. We synthesized and characterized a set of six nickel dicarbonyl complexes of general formula [Ni(CO)2(PP)], where PP is an atropoisomeric chelating diphosphine ligand. The analysis of the obtained experimental data and the basicity and oxidative potentials of the free ligands suggests a close relation between the donor ability of the chelating ligand and the carbonyl stretching frequencies observed in the complexes. We then use theory to unravel the detailed mechanisms of chelation-bond formation in terms of partial charge flows between the molecular orbitals of the fragments. By extending the promising, recently published natural orbitals for chemical valence/charge displacement (NOCV/CD) analysis scheme we provide a thorough, quantitative description of the several charge fluxes following the metal–ligand bond formation and demonstrate that the carbonyl stretching frequencies in the considered complexes selectively respond to the σ-donation charge flow from the phosphorus lone pairs of the ligands, with the frequency shift being in quantitative correlation with the extent of the ligand-to-metal charge transfer.

The luminescent square-planar Pt(II) complexes of the type [Pt(C^N)(N^N)] + X - , where (C^N) is the cyclometallated 2-phenylpyridine ligand, (N^N) a chiral diamine and BF 4 − or CH 3 COO − as counterion (X − ) have been synthesized. The chiral diamine Campy has been used, resulting in a mixture of regioisomers. The complexes were characterized and their structures in solution were determined by a combination of different spectroscopic and chiroptical techniques (NMR, UV–Vis, ECD, IR, VCD and CPL) associated with electrochemical, DFT and TDDFT studies. Remarkably, the acetate derivatives proved to be soluble and stable in water for at least a month. The electrochemical behaviour of the [Pt(C^N)(N^N)]BF 4 was investigated and the LUMO energies estimated. The photophysical properties of the new compounds were deeply investigated in water or dimethyl sulfoxide solution by UV–Vis, steady state emission and time-correlated single-photon counting spectroscopy.

The octahedral ionic Ir(iii) complex with a dual stereogenic centre of general formula Δ,Λ-(R,S)-[(ppy)2Ir(Me-Campy)]X, where ppy = 2-phenylpyridine and Me-Campy = 2-methyl-5,6,7,8-tetrahydroquinolin-8-amine, and the complex Λ-(R,S)-[(ppy)2Ir(H-Campy)]X, where ppy = 2-phenylpyridine, H-Campy = 8-amino-5,6,7,8-tetrahydroquinolines and X(-) = Cl(-) as a counterion in both cases, have been characterized by vibrational circular dichroism (VCD), which turns out to be efficacious in diastereomeric discrimination. Moreover, the single crystal X-ray structure of the complex Δ-(R)-[(ppy)2Ir(Me-Campy)]Cl is reported here. Density functional theory (DFT) calculations allow us to conclude that the most important doublet feature in the VCD spectra is associated with a clear vibrational exciton structure located on the two dissymmetrically disposed phenylpyridine ligands. The features in the VCD spectra associated with the (R) or (S)-central chirality configuration are identified and commented on. DFT calculations provide also the interpretation of electronic circular dichroism (ECD) spectra. Finally, circularly polarized luminescence (CPL) spectra are presented as an additional chiroptical characterization of these luminescent iridium complexes.

Vibrational circular dichroism, VCD, gives evidence that the carbon monoxide chromophore in a heteroleptic cyclopentadienyl Ru(II)-carbonyl complex is very sensitive to the chirality of the metal centre and becomes an excellent probe to define the configuration of chiral metal complexes.

The relation between spectroscopic observables and the detailed metal-ligand bonding features in chelation complexes is addressed using both experimental and state-of-the-art theoretical and computational methods. We synthesized and characterized a set of six nickel dicarbonyl complexes of general formula [Ni(CO)2(PP)], where PP is an atropoisomeric chelating diphosphine ligand. The analysis of the obtained experimental data and the basicity and oxidative potentials of the free ligands suggests a close relation between the donor ability of the chelating ligand and the carbonyl stretching frequencies observed in the complexes. We then use theory to unravel the detailed mechanisms of chelation-bond formation in terms of partial charge flows between the molecular orbitals of the fragments. By extending the promising, recently published natural orbitals for chemical valence/charge displacement (NOCV/CD) analysis scheme we provide a thorough, quantitative description of the several charge fluxes following the metal-ligand bond formation and demonstrate that the carbonyl stretching frequencies in the considered complexes selectively respond to the σ-donation charge flow from the phosphorus lone pairs of the ligands, with the frequency shift being in quantitative correlation with the extent of the ligand-to-metal charge transfer.

Chiral (R)-(-) and (S)-(+)-8-amino-5,6,7,8-tetrahydroquinoline, hereafter defined (R)-CAMPY and (S)-CAMPY, have proved to be valuable chiral ligands in Ru(II) precatalysts for the reduction of prochiral ketones by asymmetric hydrogen transfer AHT. These ligands bear chiral information on an alkyl ring fused to a pyridine and therefore have a flat nature, characterized by a high conformational rigidity. It represent the first application in AHT of a member of a family of ancillary ligands already known as starting material for medicinal compouds. The reaction with [RuCl2PPh3] furnished the (OC-6-14)-[RuCl2(PPh3)2(CAMPY)] which evolved to (OC-6-42-C)- [RuCl2(PPh3)2(CAMPY)] in refluxing toluene. The absence of a suitable crystal for X-ray structure was easily overcame by a combination of routines 2D-NMR experiments based on an unprecedent 4-bond P-H coupling. The formation of (OC-6-42-C)- [RuCl2(PPh3)2(CAMPY)] is totally stereoselective at the chiral metal center; the same behavior is followed when triphenyl phosphines are displaced by chelating diphosphines like Ph2P(CH2)4PPh2 (DPPB) and Ph2P(CH2)3PPh2 (DPPP). The flat and rigid nature of CAMPY alone gave catalyst able to reduce acetophenone with moderate e.e.; substitution of triphenylphosphines in the complex with an achiral chelating diphosphines produce Ru-catalysts witch gave enantioselectivities up to 96%. © 2012 Bentham Science Publishers.

Three homologous series of luminescent octahedral ionic Ir(III) complexes (1–12) with a dual stereogenic center of general formula Δ,Λ (R,S)[(ppy)2Ir(R-campy)]X, where ppy=2-phenylpyridine, R-campy=2-methyl-5,6,7,8-tetrahydroquinolin-8-amine (Me-campy) or 8-amino-5,6,7,8-tetrahydroquinolines (H-campy) and as counterions X−=Cl− or CH3COO− have been synthesized and characterized. The NMR characterization of each complex highlighted the diastereoisomeric purity and the absolute configuration has been confirmed by Electronic Circular Dichroism spectroscopy. The absorption and the luminescence properties of the compounds in solution and in solid state have been investigated by UV–vis, steady-state emission and time-correlated single-photon counting spectroscopy. The obtained results from the 12 compounds highlight the difficult to correlate photophysical properties in solution to the stereochemistry, while excited states decay studies of the solid state samples indicate a correlation between photophysics and packing mode which is affected by the different stereochemistry.

In this research work simple unsymmetrical 1,3-diamines were studied. The synthesis of the diamines started from non-commercial available compounds. S-5a and S,S-5c were obtained by biocatalysis with non conventional yeast, Rhodotorula rubra MIM 147, with excellent 99% e.e. and d.e. up to 90%. Different approaches of synthesis were applied to the same backbone to study both the steric and electronic effects of the ligands. The reactivity of the corresponding ruthenium complexes was evaluated in the asymmetric hydrogen transfer reduction of acetophenone as a standard substrate and of other different aryl ketones, highlighting the flexibility of the six membered chelating ring. A screening of the reaction conditions indicated aqueous media in the presence of HCOONa as a hydrogen donor to be the best system for overcoming the lack of stereocontrol thus allowing us to obtain 56% e.e. in the reduction of acetophenone with the complex in which the ligand was diamine 1, revealed as the best in terms of reactivity and stereoselectivity also in the reduction of the other different aryl ketones, in particular for α-tetralone, i (63% e.e.).

Aqua iridium(III) complexes with 8-amino-5,6,7,8-tetrahydroquinolines CAMPY L1 and its derivatives as chiral ligands proved to be very efficient catalysts for the reduction of a wide range of prochiral aryl ketones, revealing a variety of behaviours in terms of reaction rate and stereoselectivity. As standard substrates, differently substituted acetophenones were studied and good enantioselectivity (86% ee) was achieved in the reduction of 1-(o-tolyl)ethan-1-one 6. Particularly interesting was the ATH reaction in the case of β-amino keto esters, precursors of β-lactams and azetidinones. The best results were obtained with [Cp*Ir(H2O)(L1)]SO4affording the corresponding diastereomeric alcohols in an (R,S)-configuration with an excellent 99% ee in the reduction of 2-(benzamido methyl)-3-oxo-3-(4-(trifluoromethyl)phenyl)propanoate 12. © 2014 Elsevier Ltd. All rights reserved.

Cis -RuCl 2 (diphosphine)(CAMPY) complexes, chiral at the metal centre with matching or mismatching chiralities between diphosphine and CAMPY were prepared and the configuration at the metal was determined in solution by a complete set of NMR investigations; CAMPY is ( R )-(−) or ( S )-(+)-8-amino-5,6,7,8-tetrahydroquinoline. The complexes were used in ATH reactions of different aryl ketones with 2-propanol as hydrogen source. The effects of the chirality at the metal were studied and enantiomeric excesses up to 99% were obtained.