RECENT DEVELOPMENTS IN THE METALLOSUPRAMOLECULAR AND MOLECULAR STRUCTURES OF THE COBALT,IRON AND VANADIUM COMPLEXES OF THE DIANIONIC TETRADENTATE SCHIFF BASE LIGANDS OF SALICYLIDENEIMINE AND ACETYLACETONEIMINE

Abstract

Four different types of metallosupramolecular structures are distinguishable for the title complexes. These types are described as: (a) The metal complex could react with other metal ions as a ligand, (b) Dimerization occurs through Lewis acid and Lewis base interactions of the metal ion and the coordinated oxygen atom of the ligand with those of an adjacent molecule, (c) Dimerization and chain formation occur using the donor-acceptor behaviour of the oxovanadium (IV) ion, and (d) Molecular association occurs through the bridged fluoride. Types (b), (c) and (d) are self-assembly. Chemical understanding of those types could lead to designing, and establishing procedures for, the preparation of new metallosupramolecular structures of homo- and heterobinuclear (as well as polynuclear) metal complexes with similar or mixed ligands.     

Catalytic Zinc Complexes for Phosphate Diester Hydrolysis

Creating efficient artificial catalysts that can compete with biocatalysis has been an enduring challenge which has yet to be met. Reported herein is the synthesis and characterization of a series of zinc complexes designed to catalyze the hydrolysis of phosphate diesters. By introducing a hydrated aldehyde into the ligand we achieve turnover for DNA‐like substrates which, combined with ligand methylation, increases reactivity by two orders of magnitude. In contrast to current orthodoxy and mechanistic explanations, we propose a mechanism where the nucleophile is not coordinated to the metal ion, but involves a tautomer with a more effective Lewis acid and more reactive nucleophile. This data suggests a new strategy for creating more efficient metal ion based catalysts, and highlights a possible mode of action for metalloenzymes.     

Quantitative evaluation of Lewis acidity of metal ions with different ligands and counterions in relation to the promoting effects of Lewis acids on electron transfer reduction of oxygen

The g(zz) values of ESR spectra of superoxide (O(2)(.-) complexes of metal ion salts acting as Lewis acids with different ligands and counterions were determined in acetonitrile at 143 K. The binding energies (DeltaE) of (O(2)(.-)/Lewis acid complexes have been evaluated from deviation of the g(zz) values from the free spin value. The DeltaE value is quite sensitive to the difference in the counterions and ligands of metal ion salts acting as Lewis acids. On the other hand, the fluorescence maxima of the singlet excited states of 10-methylacridone/Lewis acid complexes are red-shifted as compared with that of 10-methylacridone, and the relative emission energies (Deltahnu(f)) vary significantly depending on the Lewis acidity of metal ion salts with different counterions and ligands. The promoting effects of Lewis acids were also examined on electron transfer from cobalt(II) tetraphenylporphyrin to oxygen in acetonitrile at 298 K, which does not occur in the absence of Lewis acids under otherwise the same experimental conditions. Both DeltaE and Deltahnu(f) values are well correlated with the promoting effects of Lewis acids on the electron transfer reduction of oxygen. Such correlations indicate that DeltaE and Deltahnu(f) values can be used as quantitative measures of Lewis acidity of metal ion salts with different ligands and counterions. The Lewis acidity thus determined can also be applied to predict the promoting effects of Lewis acids on organic synthesis.     

Unimolecular reactions of dihydrated alkaline earth metal dications M2+(H2O)2, M = Be, Mg, Ca, Sr, and Ba: salt-bridge mechanism in the proton-transfer reaction M2+(H2O)2 --> MOH+ + H3O

The unimolecular reactivity of M(2+)(H(2)O)(2), M = Be, Mg, Ca, Sr, and Ba, is investigated by density functional theory. Dissociation of the complex occurs either by proton transfer to form singly charged metal hydroxide, MOH(+), and protonated water, H(3)O(+), or by loss of water to form M(2+)(H(2)O) and H(2)O. Charge transfer from water to the metal forming H(2)O(+) and M(+)(H(2)O) is not favorable for any of the metal complexes. The relative energetics of these processes are dominated by the metal dication size. Formation of MOH(+) proceeds first by one water ligand moving to the second solvation shell followed by proton transfer to this second-shell water molecule and subsequent Coulomb explosion. These hydroxide formation reactions are exothermic with activation energies that are comparable to the water binding energy for the larger metals. This results in a competition between proton transfer and loss of a water molecule. The arrangement with one water ligand in the second solvation shell is a local minimum on the potential energy surface for all metals except Be. The two transition states separating this intermediate from the reactant and the products are identified. The second transition state determines the height of the activation barrier and corresponds to a M(2+)-OH(-)-H(3)O(+) "salt-bridge" structure. The computed B3LYP energy of this structure can be quantitatively reproduced by a simple ionic model in which Lewis charges are localized on individual atoms. This salt-bridge arrangement lowers the activation energy of the proton-transfer reaction by providing a loophole on the potential energy surface for the escape of H(3)O(+). Similar salt-bridge mechanisms may be involved in a number of proton-transfer reactions in small solvated metal ion complexes, as well as in other ionic reactions.     

MIL-53系列金属有机骨架化合物的合成及在Strecker反应中的催化性能

通过改变金属中心和配体官能团，合成了4个MIL-53系列金属有机骨架化合物NH₂-MIL-53（Sc），NH₂-MIL-53（Al），MIL-53（Sc）和NO₂-MIL-53（Sc）；研究了它们在合成α-氨基腈的Strecker反应中的催化性能.结果表明，NH₂-MIL-53（Sc）对于Strecker反应具有优异的催化性能，并具有可重复使用性.通过对比NH₂-MIL-53（Sc）和NH₂-MIL-53（Al）的催化性能，分析了金属离子半径和金属中心不饱和配位点对Strecker反应的影响.通过对比NH₂-MIL-53（Sc）与MIL-53（Sc）的催化性能，证明了作为路易斯碱中心的氨基可通过与路易斯酸中心的协同作用，有效促进Strecker反应的进行.通过对比NO₂-MIL-53（Sc）与MIL-53（Sc）的催化性能，认为催化剂结构上的硝基对于Strecker反应的进行具有重要影响.最后，通过对比NH₂-MIL-53（Sc）、硝酸钪和2-氨基对苯二甲酸（ATA）配体对催化性能的影响，总结出催化剂在纳米尺度的孔道结构是提高Strecker反应选择性的关键因素.     

Syn- and anti-selective Prins cyclizations of delta,epsilon-unsaturated ketones to 1,3-halohydrins with Lewis acids

Ten acyclic and monocyclic delta,epsilon-unsaturated ketones, with and without methyl substituents on the double bond, underwent halide-terminated Prins (halo-Prins) cyclizations under anhydrous conditions in the presence of Lewis acids. TiCl4, TiBr4, BCl3, and BBr3 promoted syn-selective cyclizations to sterically congested chloro- and bromohydrins, while SnCl4, SnBr4, InCl3, ZrCl4, and several other Lewis acids effected highly anti-selective reactions to furnish the corresponding trans halohydrins. The stronger Lewis acids (TiX4 and BX3) favor the syn process that involves axial delivery of a halide ligand. Competition experiments showed that substitution at the delta carbon (methallyl enones) led to increased rates (40-50-fold), while substitution at the epsilon position (cis and trans crotyl enones) retarded the rate and eroded the selectivity of the cyclizations. The trends in syn vs anti selectivity, reactivity, and effects of different Lewis acidic metal halides are rationalized by competitive reaction pathways proceeding through syn carbocation-halide ion pairs and a higher order transition state that leads to inversion of configuration and formation of trans halohydrins, along with cyclic olefins arising from proton elimination.     

Metal ion complexation by acyclic polyethers with lipophilic amide, thioamide, and amine end groups

A series of acyclic polyethers with lipophilic amide, thioamide, and amine end groups was synthesized. Metal ion transport across bulk liquid membranes and measurement of thermodynamic parameters for ligand-metal ion complexation by titration calorimetry show strong selectivity for complexation of lead ion over other metal ion species for the diamide ligand. Lead ion complexation by the acyclic polyether diamide involves the amide oxygens and silver ion coordination by a dithioamide analog involves the thioamide sulfurs. With a proper length of the ethereal linkage, the ligand wraps around the metal ion in a pseudocyclic fashion.     

Chelation-controlled molecular morphology: aminal to imine rearrangements

Reactions between the tripodal hydroxytriamine, 2,2-bis(aminomethyl)-3-aminopropan-1-ol, "hytame", and the isomeric pyridine aldehydes generate in all cases the tris(aminal) species based on a 1,3,5-triaza-adamantane skeleton. In all cases also, the product from water under basic conditions consists of an approximately 1:9 mixture of the triequatorial and monoaxial-diequatorial isomers. While all these tripyridyltriaza-adamantanes appear capable of acting as Lewis bases, in particular cases metal ion binding leads to a radical structural rearrangement. These cases involve the pyridine-2-aldehyde derivatives only and certain transition metal ions (notably Fe(II)), and result in the conversion of the tris(aminal) into its isomeric tris(imine) form. This is apparently favoured because it can act as a hexadentate ligand towards a single metal ion, although kinetic influences are clearly important in this chemistry because template reactions of the triamine, pyridine-2-aldehyde and several metal ions give much better yields of the tris(imine) complex than do analogous rearrangement reactions. For the low-spin, kinetically inert Fe(II) complex of the tris(imine), its formation is apparently so favourable that it is generated via aldehyde unit exchange when the aza-adamantanes derived from pyridine-3- and -4-aldehyde are heated with a mixture of Fe(II) and pyridine-2-aldehyde. When the kinetically labile Zn(II) complex is treated with EDTA, the metal ion is extracted but the released ligand does not undergo valence tautomerisation to what would be expected to be the triaxial isomer of the tripyridyltriaza-adamantane but instead rapidly undergoes partial hydrolysis before slowly forming the mixture of triequatorial and monoaxial-diequatorial isomers.     

Bis(oxazolinyl)phenyl transition metal complexes: synthesis, asymmetric catalysis, and coordination chemistry

Molecular catalysts for organic synthesis should be constructed to be tailored to target reactions and their desirable conditions. In our search for them, we have studied new types of transition metal molecular catalysts dressed with a tridentate N,C,N modular ligand, which consists of a C2-symmetric side-by-side phenyl group with chiral bis(oxazolinyl) substituents. The ligand, 2,6-bis(oxazolinyl)phenyl abbreviated as Phebox, can connect covalently to transition metals by the central carbon atom. Here, we review our recent work on the chemistry of Phebox and its metal complexes, including preparation, structural analysis, asymmetric Lewis acid catalysis, asymmetric hydrosilylation, asymmetric conjugate reduction, asymmetric reductive aldol reaction, and organometallic reactions.     

Lewis Acid Binding and Transfer as a Versatile Experimental Gauge of the Lewis Basicity of Fe0, Ru0, and Pt0 Complexes

A number of zerovalent ruthenium tri‐ and tetracarbonyl complexes of the form [Ru(CO)_5?nL_n] (n=1, 2) with neutral phosphine or N‐heterocyclic carbene donor ligands have been treated with the Lewis acids GaCl₃ and Ag⁺ to form a range of metal‐only Lewis pairs (MOLPs). The spectroscopic and structural parameters of the adducts are compared to each other and to related iron carbonyl based MOLPs. The Lewis basicity of the original Ru⁰ complexes is gauged by transfer experiments, as well as through the degree of pyramidization of the bound GaCl₃ units and the Ru?M bond lengths. The work shows the benefits of the MOLP concept as one of the few direct experimental gauges of metal basicity, and one that can allow comparisons between metal complexes with different metal centers and ligand sets.     

A Quantitative Approach of the Interaction between Metal Triflates and Organic Ligands Using Electrospray Mass Spectrometry

The interaction between two Lewis ??superacid?? catalysts Zn(OTf)₂ and In(OTf)₃ and series of amide and phosphate ligands is quantitatively characterized by electrospray ionization mass spectrometry (ESI-MS). A specific feature of the ESI-MS spectra of the mixture of metal triflates and Lewis bases is the formation of ionic adducts resulting from the displacement of one triflate anion by two neutral ligands. A ligand competition model is developed, which describes the relative intensities of the ionic adducts as a function of relative ligand concentrations. The relative affinities deduced from the ligand competition method are combined in an affinity scale for the metal triflate.     

A study of the nature of coordination in metal monothiocarbamates based on infrared spectroscopy and the HSAB concept

The nature of coordination in metal monothiocarbamates is shown to depend on the hardness or softness of the metal ton. Thus, the monothiocarbamate ion acts as a monodentate ligand with metal-sulphur bending when the metal ion is a soft acid while it acts as a bidentate ligand when the metal ion is a hard acid; it can exhibit either behaviour when the metal ion is a borderline acid. In dialkyltin and dialkylmonocholorotin complexes, the monothiocarbamate ion acts as a bidentate ligand with strong Sn-S bonding while in trialkyl-or triaryl-tin complexes it acts essentially as a monodentate ligand. Thus, R₃Sn(I) seems to be a soft or borderline acid while R₂Sn(II) is a hard acid.     

Enhancing Phosphate Diester Cleavage by a Zinc Complex through Controlling Nucleophile Coordination

Metal‐ion complexes are the most effective artificial catalysts capable of cleaving phosphate diesters under mild aqueous conditions. A central strategy for making these complexes highly reactive has been to use ligand‐based alcohols that are coordinated to the ion, providing an ionised nucleophile under neutral conditions but at the expense of deactivating it. We have created a highly reactive Zn complex that is 350‐fold more reactive than an alcohol analogue by preventing the nucleophile binding to the metal ion. This strategy successfully delivers the benefits of efficient nucleophile delivery without strongly deactivating the metal ion Lewis acidity nor the oxyanion nucleophilicity. Varying the leaving group reveals that the transition state of the reaction is much further advanced than the reaction with hydroxide.     

Development of an unusually highly enantioselective hetero-Diels-Alder reaction of benzaldehyde with activated dienes catalyzed by hypercoordinating chiral aluminum complexes

The effect of Lewis acid catalysis of the hetero-Diels-Alder reaction between benzaldehyde and activated dienes (e.g. the Danishefsky's diene) has been investigated. In the present work we decided to study a series of chiral aluminum complexes as potential catalysts for the hetero-Diels-Alder reaction in order to gain a better understanding of the effect on the chiral induction of varying the steric and electronic environment of the metal ion. The results of this study prompted us to conclude that steric effects in the ligand coordination sphere and hypercoordination are strongly contributing factors to the optical yield of the reaction. Optimization of the reaction culminated in the synthesis of the hetero-Diels-Alder product in 99.4% ee and 97% yield of the isolated product. Based on the experimental results the mechanism for the hetero-Diels-Alder reaction is discussed and it is postulated that hypercoordination to the chiral aluminum Lewis acid center is of importance for the reaction.     

Adsorption and selectivity characteristics of several human serum proteins with immobilised hard Lewis metal ion-chelate adsorbents

In this investigation, human serum has been used as an example of a crude protein mixture to define the protein binding characteristics and selectivity of several immobilised hard Lewis metal ion affinity chromatographic (IMAC) adsorbents. Specifically, the binding properties of immobilised O-phosphoserine (im-OPS) and 8-hydroxyquinoline (im-8-HQ), with immobilised iminodiacetic acid as a control system, have been investigated in combination with the hard Lewis metal ions, Al3+, Ca2+, Fe3+, Yb3+, and the borderline metal ion, Cu2+, over the pH range pH 5.5 to pH 8.0 with buffers of 0.5 M ionic strength. The same IMAC adsorbents were also investigated for their protein binding capabilities with buffers of an ionic strength of 0.06 M at pH 5.5 and pH 8.0. The binding behaviour of four "marker" proteins, namely transferrin, alpha2-macroglobulin, gammaglobulin and human serum albumin have furthermore been employed to monitor the differences in protein selectivity exhibited by these IMAC systems. The experimental findings confirm that these hard Lewis metal ion IMAC adsorbents function in a "mixed" binding mode with both coordination and electrostatic characteristics evident, depending on the ionic strength and pH of the equilibration or elution buffers. Based on a screening protocol, several members of the im-Mn+-8-HQ and im-Mn+-OPS adsorbent series have been identified with high selectivity for transferrin and alpha2-macroglobulin. These hard Lewis metal ion IMAC adsorbents thus provide attractive alternatives for selective fractionation of human serum proteins.     

Enantioseparation of α‐hydroxy acids by chiral ligand exchange CE with a dual central metal ion system

Using two kinds of central metal ions in a background electrolyte, ligand exchange CE was investigated for the simultaneous enantioseparation of dl ‐malic, dl ‐tartaric, and dl ‐isocitric acids. Ligand exchange CE with 100 mM d ‐quinic acid as a chiral selector ligand and 10 mM Cu(II) ion as a central metal ion could enantioseparate dl ‐tartaric acid but not dl ‐malic acid or dl ‐isocitric acid. A dual central metal ion system containing 0.5 mM Al(III) ion in addition to 10 mM Cu(II) ion in the background electrolyte enabled the simultaneous enantioseparation of the three α‐hydroxy acids. These results suggest that the use of a dual central metal ion system can be useful for enantioseparation by ligand exchange CE.     

Perspective on dirhodium carboxamidates as catalysts

Dirhodium compounds are emerging as highly efficient catalysts for diverse reactions, and those with carboxamidate ligands have the broadest applications. The unique features of these compounds are their structural rigidity, ease of ligand exchange, open diaxial sites for coordination with Lewis bases, and their low oxidation potential. As consequences of this, dirhodium carboxamidates are efficient and effective catalysts for metal carbene reactions, Lewis acid-catalyzed processes, and chemical oxidations. With chiral carboxamidate ligands these dirhodium compounds show exceptional enantiocontrol for intramolecular cyclopropanation and carbon-hydrogen insertion reactions of diazoacetates, and they are also highly efficient and selective for hetero-Diels-Alder reactions. Their limitations lie in their moderate reactivities for metal carbene generation and Lewis acid catalysis and in the cost of the precious metal rhodium.     

Mass spectrometric studies on small open-chain piperazine-containing ligands and their transition metal complexes

Electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry was used to characterize the complexes formed between open-chain piperazine-containing ligands and transition metal salts (Cobalt, Copper, Zinc, and Cadmium as chlorides, nitrates, and acetates). Only single-charged complexes were observed, formed of one ligand (L) and mainly one metal ion (M). Since the net charge of the complexes was one, a counterion (X) was attached to some of the complexes, with formation of [L + M + X]+ complexes, and a proton was lost from others, as in [L - H + M]+ complexes. In most cases the composition of the complexes was more dependent on the ligand than the metal salt. Collision-induced dissociation measurements showed that complexes with related composition often differed in structure, or that interactions between the ligand and the metal ion were not alike. The metal ion influenced considerably the fragmentation pathways of the ligands, so that the fragmentation products could be used to deduce the binding sites of the metal. The variations observed in fragmentation behavior of complexes possessing the same ligand but different metal ions can mostly be explained by the ionic radius and electronic configuration of the metal ion. The results indicated a preference of the piperazine ring of the coordinated ligand for the boat conformation.     

Synthesis of the first chiral bidendate bis(trifluoromethyl)phosphane ligand through stabilization of the bis(trifluoromethyl)phosphanide anion in the presence of acetone

Lewis acid/Lewis base adduct formation of the P(CF3)2- ion and acetone leads to a reduced negative hyperconjugation and, therefore, limits the C--F bond activation. The resulting increased thermal stability of the P(CF3)2- ion in the presence of acetone allows selective substitutions and enables the synthesis of the first example of a chiral, bidentate bis(trifluoromethyl)phosphane ligand: a DIOP derivative, [(2,2-dimethyl-1,3-dioxolane-4,5-diyl)bis(methylene)]bis(diphenylphosphane), in which the phenyl groups at the phosphorus atoms are replaced by strong electron-withdrawing trifluoromethyl groups. The resulting high electron-acceptor strength of the synthesized bidentate (CF3)2P ligand is demonstrated by a structural and vibrational study of the corresponding tetracarbonyl-molybdenum complex. The stabilization of the P(CF3)2- ion in the presence of acetone is based on the formation of a dynamic Lewis acid/Lewis base couple, (CF3)2PC(CH3)2O-. Although there is no spectroscopic evidence for the formation of the formulated alcoholate ion, the intermediate formation of (CF3)2PC(CH3)2O- could be proved through the reaction with (CF3)2PP(CF3)2, which yields the novel phosphane-phosphinite ligand (CF3)2PC(CH3)2OP(CF3)2. This ligand readily forms square-planar Pt(II) complexes upon treatment with solid PtCl2.     

Binding of the two-valence-electron metal ions Sc(+), Ti(2+), and V(3+) to the second-row hydrides BeH(2), BH(3), NH(3), OH(2), and FH: a computational study

We report results from a computational study of the binding in complexes formed from one of the transition-metal ions Sc(+), Ti(2+), or V(3+), each of which has two valence electrons outside an argon core, and one of the second-row hydrides FH, OH(2), NH(3), BH(3), or BeH(2). The complexes that involve the electron-rich ligands FH, OH(2), and NH(3) have strong ion-dipole components to their binding. There are large stabilization energies for sigma-interactions that transfer charge from occupied lone-pair natural bond orbitals on the F, O, or N atom of the (idealized) Lewis structure into empty non-Lewis orbitals on the metal ions; these interactions effectively increase electron density in the bonding region between the metal ion and liganded atom, and the metal ions in these complexes act in the capacity of Lewis acids. The complexes formed from the electron-poor hydrides BH(3) and BeH(2) consistently incorporate bridging hydrogen atoms to support binding, and there are large stabilization energies for interactions that transfer charge from the Be-H or B-H bonds into the region between the metal ion and liganded atom. The metal ions in Sc(+)-BeH(2), Ti(2+)-BeH(2), Ti(2+)-BH(3), and V(3+)-BH(3) act in the capacity of Lewis acids, whereas the scandium ion in Sc(+)-BH(3) acts as a Lewis base.