Interplay and Competition Between Two Different Types of Redox-Active Ligands in Cobalt Complexes: How to Allocate the Electrons?

The field of molecular transition metal complexes with redox-active ligands is dominated by compounds with one or two units of the same redox-active ligand; complexes in which different redox-active ligands are bound to the same metal are uncommon. This work reports the first molecular coordination compounds in which redox-active bisguanidine or urea azine (biguanidine) ligands as well as oxolene ligands are bound to the same cobalt atom. The combination of two different redox-active ligands leads to mono- as well as unprecedented dinuclear cobalt complexes, being multiple (four or six) center redox systems with intriguing electronic structures, all exhibiting radical ligands. By changing the redox potential of the ligands through derivatisation, the electronic structure of the complexes could be altered in a rational way.     

Synthesis and Structures of Bis(alkene) Complexes of Tungsten and Molybdenum, [M(CO)2(dpphen)(dbf)2] (M = W or Mo; dpphen = 4,7‐diphenyl‐1,10‐phenanthroline; dbf = dibutylfumarate)

Tungsten and molybdenum complexes [M(CO)₂(dpphen)(dbf)₂] (M = W 1 or Mo 2 ; dpphen = 4,7‐diphenyl‐1,10‐phenanthroline; dbf = dibutylfumarate) have been synthesized and structurally characterized by X‐ray diffraction analysis. In both complexes which have similar structure, the metal atom co‐ordination is distorted octahedral with dpphen and two CO groups in the equatorial plane and the metal atom binds in an η²‐fashion to the C–C bonds of two dbf ligands. The two C–C bonds are almost mutually orthogonal. The two complexes are different in conformation which result from face selection of the two dbf ligands for coordination to the metal atom.     

Metal complexes of some thiocarbohydrazone ligands: synthesis and structure

The synthesis, structure, physico-chemical investigation and biological studies of some metal complexes of thiocarbohydrazone ligands are described. The ligands were obtained by condensation of N,N′-thiocarbohydrazide with carbonyl compounds such as 2-hydroxyacetophenone or 5-chlorosalicylaldehyde. A variety of binuclear or mononuclear complexes were obtained with the ligands in mono-, bi- and/or tri-deprotonated forms. The bonding sites are the azomethine nitrogen atom, phenolic oxygen atom and thiol sulfur atom. The metal complexes exhibit either tetrahedral or octahedral structures. Preliminary antimicrobial screening showed that the ligands and their metal complexes possess antimicrobial activity towards bacteria and fungi.     

Metal Complexes Bearing Terminal Borylene Ligands

More and more metal complexes with terminal borylene ligands will be synthesized. Although these ligands in metal complexes must be stabilized either by integration of the boron atom into a polyhedral skeleton ( 1 ) or by B–N π interactions with a bulky amino group ( 2 ), the route to new complexes with terminal RB ligands (R=alkyl, aryl) is clearly indicated.     

2-亚胺-1,10-邻二氮杂菲铁(Ⅱ)类催化剂活性与中心金属原子净电荷的关系

应用金属原子净电荷相关性(MANCC)方法研究了铁(Ⅱ)类催化剂活性. 取代基的电子效应占主导作用的催化剂, 活性与中心金属原子净电荷有较好的相关性. 当取代基呈供电子效应时, 催化剂活性随着电荷的增大而升高, 当取代基呈吸电子效应时, 催化剂活性随着电荷的增大而降低. 在此基础上推测烯烃聚合反应催化过程中可能存在两种不同的活性中心, 一种是[LFe-R]⁺, 另一种是[LFe-R]²⁺或[Fe(Cl)RL]⁺. 当取代基的电子效应和空间效应均对活性有影响时, 发现催化剂的两卤素净电荷差值越小, 催化活性越强.     

Design of highly active iron-based catalysts for atom transfer radical polymerization: tridentate salicylaldiminato ligands affording near ideal Nernstian behavior

Iron(II) complexes bearing monoanionic tridentate salicyladiminato ligands are shown to be highly efficient catalysts for atom transfer radical polymerization (ATRP). Polymerization rates for styrene are among the highest reported for iron-mediated ATRP in nonpolar media, correlating well with E1/2 potentials and DeltaEp values for the complexes. The rigidity of the tridentate ligands, combined with ample space around the metal center to accommodate a halogen atom, we believe to be an important factor in the efficient ATRP behavior of these systems.     

Chiral boron-bridged bisoxazoline (borabox) ligands: structures and reactivities of Pd and Cu complexes

Anionic boron-bridged bisoxazolines (borabox ligands) have been synthesized and characterized in their protonated forms. The ligands are tuneable over a wide range, allowing either alkyl or aryl substituents at the oxazoline rings and the central bridging boron atom. The structural parameters of this new ligand type have been investigated by X-ray analyses of palladium and copper complexes. Electronic properties have been studied by (13)C NMR spectroscopy and by DFT calculations on palladium allyl complexes and compared to those of analogous bisoxazoline (box) complexes. Borabox complexes are more electron-rich at the metal center than their neutral box congeners, and as a consequence of the longer bonds between the bridging atom and the oxazoline rings, their bite angles are larger. Palladium(II) complexes bearing an unsubstituted allyl ligand and homoleptic copper(II) complexes each possess an almost flat chelate ring. NMR analysis of a (1,3-diphenylallyl)(borabox)palladium complex showed a 92:8 mixture of (syn,syn) and (anti,syn) allyl isomers, in contrast with a previously reported box analogue that existed exclusively in the (syn,syn) form. Comparison of the corresponding crystal structures revealed that the distance between the bisoxazoline and the allyl ligand in the borabox complex is shorter. In the copper-catalyzed allylic oxidation of cyclohexene and cyclopentene with tert-butyl perbenzoate, borabox ligands gave results similar-and in some cases superior-to those obtained with analogous box ligands.     

Foliacenes: ab initio modeling of metallocomplexes exhibiting a unique form of 16-electron, metal-induced aromaticity

Ab initio calculations predict that the cyclic trefoilenes 2 can be stabilized by formation of a complex 4 with early transition metals. The metal atom within the complex is nested within the carbon ring and is considerably closer to the ring centroid than in traditional metallocene complexes. Stabilization is explained by a unique form of 16-electron delocalization involving the metal atom, for which we suggest the name "foliate aromaticity". The aromaticity of various polyfoliate systems such as 9 suggests this 16-electron motif is more robust than Clar-like aromatic 6pi-sextets. The open hemisphere of the metal in such foliacene complexes is predicted to coordinate a variety of ligands.     

Synthesis,Coordination Chemistry and Bonding of Strong N‐Donor Ligands Incorporating the 1H‐Pyridin‐(2E)‐Ylidene (PYE) Motif

A range of N‐donor ligands based on the 1H‐pyridin‐(2E)‐ylidene (PYE) motif have been prepared, including achiral and chiral examples. The ligands incorporate one to three PYE groups that coordinate to a metal through the exocyclic nitrogen atom of each PYE moiety, and the resulting metal complexes have been characterised by methods including single‐crystal X‐ray diffraction and NMR spectroscopy to examine metal–ligand bonding and ligand dynamics. Upon coordination of a PYE ligand to a proton or metal‐complex fragment, the solid‐state structures, NMR spectroscopy and DFT studies indicate that charge redistribution occurs within the PYE heterocyclic ring to give a contribution from a pyridinium–amido‐type resonance structure. Additional IR spectroscopy and computational studies suggest that PYE ligands are strong donor ligands. NMR spectroscopy shows that for metal complexes there is restricted motion about the exocyclic C? N bond, which projects the heterocyclic N‐substituent in the vicinity of the metal atom causing restricted motion in chelating‐ligand derivatives. Solid‐state structures and DFT calculations also show significant steric congestion and secondary metal–ligand interactions between the metal and ligand C? H bonds.     

Synthesis and structure of metal complexes containing zwitterionic N-hydroxyimidazole ligands

Cu(II) and Zn(II) complexes of N-hydroxyimidazoles were synthesised by reacting simple metal perchlorate salts with the imidazole ligand in alcohol and formulated with a metal:ligand ratio of 1:2. The X-ray crystal structures of five complexes (four Cu(II) and one Zn(II)) were obtained and each showed the two trans, N-hydroxyimidazole ligands forming six-membered, chelate rings with the metal. Both of the NO chelating, neutral N-hydroxyimidazole ligands are in the zwitterion form, with the uncoordinated imidazole imine N atom being protonated and the oxime O atom deprotonated. In the solid state the complexes form hydrogen-bonded supramolecular structures.     

P?N ligands in lanthanide chemistry

The coordination chemistry of inorganic amides in Group 3 and lanthanide chemistry is discussed. Three different ligand systems (phosphino‐amides, bis(phosphino)amides, and bis(phosphinimino)methanides) that consist of one or more P N units were used. In this series the steric demand of the ligands is increased in a stepwise fashion and the negative charge is delocalized over more atoms. These properties were used in the design of new lanthanide complexes. For all three compounds the synthesis of the alkali metal derivatives is reported first, followed by the reaction of the alkali metal salts with various lanthanide trichlorides. Further reactions of the obtained lanthanide complexes as well as their application as catalysts are discussed. Most of the reported complexes show a dynamic behavior in solution. In phosphinoamide and bis(phosphino)amide complexes, in which the phosphorus atom is in oxidation state +3, there is always a weak coordination of the phosphorus atom to the lanthanide atom observed. In bis(phosphinimino)methanide complexes, in which the phosphorus atom is in oxidation state +5, no such interaction is noticeable. Instead a weak coordination of the methine atom to the center metal can be seen in the solid state. © 2002 Wiley Periodicals, Inc. Heteroatom Chem 13:514–520, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.10096     

Green synthesis of new Ge(IV) complexes with bio-potent ligands and their antimicrobial,DNA cleavage,and antioxidant activities

Green microwave supported synthesis, spectral, antimicrobial, DNA cleavage, and antioxidant studies of Ge(IV) complexes with bio-potent ligands, 1-acetylferrocenehydrazinecarboxamide (L¹H) and 1-acetylferrocenehydrazinecarbothioamide (L²H) have been carried out. The ligands and their respective complexes have been characterized on the basis of elemental analysis, IR, ¹H and ¹³C NMR spectra, and X-ray powder diffraction studies. The ligands are coordinated to the Ge(IV) via azomethine nitrogen and thiolic sulfur atom/ enolic oxygen atom. Both ligands and their complexes demonstrated appreciable fungicidal and bactericidal properties. The metal complexes demonstrated stronger antimicrobial than the respective free ligands. DNA cleavage activity of the complexes study revealed higher activity of the complexes than the ligands. Antioxidant activity of the complexes was tested for their hydrogen peroxide scavenging.     

Complexes of phosphorus halides with two or more coordination bonds

The review considers complexes of phosphorus as central atom with the variously dentate ligands of different topology and type of donor groups. The most stable complexes form the N-donor ligands, which are strong nucleophiles. The P-donor ligands stabilize the lower oxidation state of phosphorus. Only a few examples of complexes is known of the neutral O- and S-donor ligands. The most stable are the chelates with the ligands forming with the phosphorus atom alongside a donor-acceptor bond also a common covalent bond.     

Heterocyclic carbenes: synthesis and coordination chemistry

The chemistry of heterocyclic carbenes has experienced a rapid development over the last years. In addition to the imidazolin-2-ylidenes, a large number of cyclic diaminocarbenes with different ring sizes have been described. Aside from diaminocarbenes, P-heterocyclic carbenes, and derivatives with only one, or even no heteroatom within the carbene ring are known. New methods for the synthesis of complexes with N-heterocyclic carbene ligands such as the oxidative addition or the metal atom template controlled cyclization of beta-functionalized isocyanides have been developed recently. This review summarizes the new developments regarding the synthesis of N-heterocyclic carbenes and their metal complexes.     

Low-temperature complexes of europium atoms and dimers with cyanophenylpyridines and alkylcyanophenylpyridines

It is shown by IR-Fourier and UV-visible spectroscopy that two biligand complexes of different stoichiometric composition Ln·2L and 2Ln·2L (Ln = Eu; L = 4-pentyl-4′-cyanophenylpyridine) are formed in the course of low-temperature co-condensation of metal and ligand vapors on the surfaces cooled with liquid nitrogen. Quantum-chemical modeling of the equilibrium structures of the mono-and binuclear complexes of europium with unsubstituted cyanophenylpyridyl and para-ethylcyanophenylpyridyl ligands was carried out. The main geometrical parameters of these compounds were determined. For mononuclear complexes, there is a competition between two mechanisms of coordination of the metal atom: formation of sandwich π complexes by interactions with the π orbitals of the aromatic ligand system and σ coordination at the nitrogen atoms of the pyridine ring. The sandwich structures of the binuclear complexes with 4-ethylcyanophenylpyridine are stabilized by N…H intermolecular contacts between the N atom of the cyano group and the terminal H atom of the ethyl group. The spectral shifts and the relative thermal stability of complexes with varying nuclearity are discussed.     

Paramagnetic NMR Shifts for Saddle-Shaped Five-Coordinate Iron(III) Porphyrin Complexes with Intermediate-Spin Structure

Calculable results: Complex density functional calculations and spin distribution analyses have been performed for planar and saddled iron(III) porphyrin complexes. The spin populations and the extent of the interactions between the metal and the porphyrin orbitals were determined, which can explain the large change of meso-carbon atom chemical shifts observed for different porphyrin ligands.     

Novel chiral dimesogenic bidentate ligands and their Cu(II) and Pd(II) metal complexes

The synthesis and characterization of cholesterol-based dimesogenic bidentate ligands and their Cu(II) and Pd(II) metallomesogens are reported in detail. To understand structure-property relationships in these materials the terminal alkoxy chains and the central metal atom have been varied. Our studies reveal that chiral dimesogenic bidentate ligands with n-butyloxy chains exhibit smectic A (SmA), twist grain boundary and chiral nematic (N*) mesophases while substitution with either n -decyloxy or 3,7-dimethyloctyloxy chains also show a ferroelectrically switchable chiral smectic C (SmC*) mesophase. The metal complexes with n-butyloxy chains show only the SmA phase whereas higher chain length derivatives exhibit N* phase irrespective of the metal atom present. The ligands are thermally stable whereas their metal complexes, especially Pd(II) systems, seem to be heat sensitive. Spontaneous polarization, response time and tilt angle measurements have been carried out in the smectic C* phase of the two ligands.     

含硅金属化合物的研究进展

近年来,含硅金属化合物由于在催化工业等领域的广泛应用受到了化学工作者的重视。本文综述了以硅原子或硅杂链为配体的桥形成的金属络合物、金属硅氧烷、硅-金属化合物、金属不饱和硅化物、含硅多金属络合物五种含硅金属化合物的研究进展。最后,并对该领域的研究方向提出了展望。     

Synthetic, structural, and antimicrobial studies of organotin(IV) complexes of semicarbazone, thiosemicarbazone derived from 4-hydroxy-3-methoxybenzaldehyde

Reaction of dibutyltin dichloride, dimethyltin dichloride, and tributyltin chloride with ligands derived from thiosemicarbazone and semicarbazone leads to the formation of a new series of organotin(IV) complexes of general formula R₂SnCl₂·L and R₃SnCl·L (where L ligands derived from the condensation of thiosemicarbazide and semicarbazide with 4-hydroxy-3-methoxybenzaldehyde). The authenticity of these ligands and their metal complexes have been established on the basis of elemental analysis, conductance measurements, molecular weight determinations, infrared, ¹H NMR, ¹³C NMR, ¹¹⁹Sn NMR, and UV spectral studies. These studies showed that the ligands coordinate to the metal atom in a bidentate. An octahedral structure is proposed for the organotin(IV) complexes. The ligands and its metal complexes are screened for their antimicrobial activities against some Gram-positive and Gram-negative bacteria, and fungus. The studies demonstrated that metalation can increase the antimicrobial activity rather than the free ligands.     

Novel chiral dimesogenic bidentate ligands and their Cu(II) and Pd(II) metal complexes

The synthesis and characterization of cholesterol-based dimesogenic bidentate ligands and their Cu(II) and Pd(II) metallomesogens are reported in detail. To understand structure-property relationships in these materials the terminal alkoxy chains and the central metal atom have been varied. Our studies reveal that chiral dimesogenic bidentate ligands with n -butyloxy chains exhibit smectic A (SmA), twist grain boundary and chiral nematic (N * ) mesophases while substitution with either n -decyloxy or 3,7-dimethyloctyloxy chains also show a ferroelectrically switchable chiral smectic C (SmC * ) mesophase. The metal complexes with n -butyloxy chains show only the SmA phase whereas higher chain length derivatives exhibit N * phase irrespective of the metal atom present. The ligands are thermally stable whereas their metal complexes, especially Pd(II) systems, seem to be heat sensitive. Spontaneous polarization, response time and tilt angle measurements have been carried out in the smectic C * phase of the two ligands.