Metal–Ligand Cooperativity of the Calix[4]pyrrolato Aluminate: Triggerable C−C Bond Formation and Rate Control in Catalysis

Metal-ligand cooperativity (MLC) had a remarkable impact on transition metal chemistry and catalysis. By use of the calix[4]pyrrolato aluminate, [ 1 ]⁻, which features a square-planar Al^III, we transfer this concept into the p-block and fully elucidate its mechanisms by experiment and theory. Complementary to transition metal-based MLC (aromatization upon substrate binding), substrate binding in [ 1 ]⁻ occurs by dearomatization of the ligand. The aluminate trapps carbonyls by the formation of C−C and Al−O bonds, but the products maintain full reversibility and outstanding dynamic exchange rates. Remarkably, the C−C bonds can be formed or cleaved by the addition or removal of lithium cations, permitting unprecedented control over the system's constitutional state. Moreover, the metal-ligand cooperative substrate interaction allows to twist the kinetics of catalytic hydroboration reactions in a unique sense. Ultimately, this work describes the evolution of an anti-van't Hoff/Le Bel species from their being as a structural curiosity to their application as a reagent and catalyst.     

Metal–Ligand Cooperativity of the Calix[4]pyrrolato Aluminate: Triggerable C−C Bond Formation and Rate Control in Catalysis

Metal‐ligand cooperativity (MLC) had a remarkable impact on transition metal chemistry and catalysis. By use of the calix[4]pyrrolato aluminate, [ 1 ]^?, which features a square‐planar Al^III, we transfer this concept into the p‐block and fully elucidate its mechanisms by experiment and theory. Complementary to transition metal‐based MLC (aromatization upon substrate binding), substrate binding in [ 1 ]^? occurs by dearomatization of the ligand. The aluminate trapps carbonyls by the formation of C?C and Al?O bonds, but the products maintain full reversibility and outstanding dynamic exchange rates. Remarkably, the C?C bonds can be formed or cleaved by the addition or removal of lithium cations, permitting unprecedented control over the system's constitutional state. Moreover, the metal‐ligand cooperative substrate interaction allows to twist the kinetics of catalytic hydroboration reactions in a unique sense. Ultimately, this work describes the evolution of an anti‐van't Hoff/Le Bel species from their being as a structural curiosity to their application as a reagent and catalyst.     

Reversible OH-bond activation and amphoterism by metal–ligand cooperativity of calix[4]pyrrolato aluminate

Most p-block metal amides irreversibly react with metal alkoxides when subjected to alcohols, making reversible transformations with OH-substrates a challenging task. Herein, we describe how the combination of a Lewis acidic square-planar-coordinated aluminum(iii) center with metal–ligand cooperativity leverages unconventional reactivity toward protic substrates. Calix[4]pyrrolato aluminate performs OH-bond activation of primary, secondary, and tertiary aliphatic and aromatic alcohols, which can be fully reversed under reduced pressure. The products exhibit a new form of metal–ligand cooperative amphoterism and undergo counterintuitive substitution reactions of a polar covalent Al–O bond by a dative Al–N bond. A comprehensive mechanistic picture of all processes is buttressed by isolation of intermediates, spectroscopy, and computation. This study delineates how structural constraints can invert thermodynamics for seemingly simple addition reactions and invert common trends in bond energies.

The combination of structural constraint and metal–ligand cooperativity in calix[4]pyrrolato aluminate inverts common trends of bond energies and enables reversible OH-bond activation.     

The Preparation, Characterization and Biological Activity of Palladium(II) and Platinum(II) Complexes of Tridentate NNS Ligands Derived from S-methyl- and S-benzyldithiocarbazates and the X-ray Crystal Structure of the [Pd(mpasme)Cl] Complex

New complexes of general formula, [M(NNS)Cl] (M = Pd^II, Pt^II; NNS = anionic forms of the 6-methyl-2-formylpyridine Schiff bases of S-methyl- and S-benzyldithiocarbazates) have been prepared and characterized by a variety of physico-chemical techniques. Based on conductance and spectral evidence, a square-planar structure is assigned to these complexes. The crystal and molecular structure of the [Pd(mpasme)Cl] complex (mpasme=anionic form of the 6-methyl-2-formylpyridine Schiff base of S-methyldithiocarbazate) has been determined by X-ray diffraction. The complex has a distorted square-planar geometry with the ligand coordinated to the palladium(II) ion via the pyridine nitrogen atom, the azomethine nitrogen atom and the thiolate sulfur atom; the fourth coordination position around the palladium(II) ion is occupied by the chloride ligand. The distortion from a regular square-planar geometry is ascribed to the restricted bite angle of the ligand. Both the Schiff bases exhibit strong cytotoxicity against the human ovarian cancer (Caov-3) cell lines, the S-methyl derivative being two times more active than the S-benzyl derivative. The [Pt(mpasme)Cl] complex is moderately active but the palladium(II) complex is weakly active against this cancer. None of the complexes of Hmpsbz are active against Caov-3. The Schiff base, Hmpasme exhibits moderate activity against the bacteria, MRSA, P. aeruginosa and S. typhimurium but is inactive against B. subtilis. Coordination of the ligand with palladium(II) substantially reduces its activity. The Schiff base, Hmpasbz and its palladium(II) and platinum(II) complexes are inactive against these bacteria. The Schiff bases and their palladium(II) and platinum (II) complexes are inactive against the pathogenic fungi, C. albican, Aspergillus ochraceous and Saccharomyces cerevisiae.     

Polycyclic Azoniahetarenes: Assessing the Binding Parameters of Complexes between Unsubstituted Ligands and G‐Quadruplex DNA

Polycyclic azoniahetarenes were employed to determine the effect of the structure of unsubstituted polyaromatic ligands on their quadruplex‐DNA binding properties. The interactions of three isomeric diazoniadibenzo[b,k]chrysenes ( 4 a – c ), diazoniapentaphene ( 5 ), diazoniaanthra[1,2‐a]anthracene ( 6 ), and tetraazoniapentapheno[6,7‐h]pentaphene ( 3 ) with quadruplex DNA were examined by DNA melting studies (FRET melting) and fluorimetric titrations. In general, penta‐ and hexacyclic azoniahetarenes bind to quadruplex DNA (K_b≈10⁶ M ^?1) even in the absence of additional functional side chains. The binding modes of 4 a – c and 3 were studied in more detail by ligand displacement experiments, isothermal titration calorimetry, and CD and NMR spectroscopy. All experimental data indicate that terminal π stacking of the diazoniachrysenes to the quadruplex is the major binding mode; however, because of different electron distributions of the π systems of each isomer, these ligands align differently in the binding site to achieve ideal binding interactions. It is proposed that tetraazonia ligand 3 binds to the quadruplex by terminal stacking with a small portion of its π system, whereas a significant part of the bulky ligand most likely points outside the quadruplex structure, and is thus partially placed in the grooves. Notably, 3 and the known tetracationic porphyrin TMPyP4 exhibit almost the same binding properties towards quadruplex DNA, with 3 being more selective for quadruplex than for duplex DNA. Overall, studies on azonia‐type hetarenes enable understanding of some parameters that govern the quadruplex‐binding properties of parent ligand systems. Since unsubstituted ligands were employed in this study, complementary and cooperative effects of additional substituents, which may interfere with the ligand properties, were eliminated.     

Structure of the Complex of [Ru(tpm)(dppz)py]2+ with a B‐DNA Oligonucleotide—A Single‐Substituent Binding Switch for a Metallo‐Intercalator

We report the synthesis of three new complexes related to the achiral [Ru(tpm)(dppz)py]²⁺ cation (tpm=tripyridazole methane, dppz=dipyrido[3,2‐a:2′,3′‐c]phenazine, py=pyridine) that contain an additional single functional group on the monodentate ancillary pyridyl ligand. Computational calculations indicate that the coordinated pyridyl rings are in a fixed orientation parallel to the dppz axis, and that the electrostatic properties of the complexes are very similar. DNA binding studies on the new complexes reveal that the nature and positioning of the functional group has a profound effect on the binding mode and affinity of these complexes. To explore the molecular and structural basis of these effects, circular dichroism and NMR studies on [Ru(tpm)(dppz)py]Cl₂ with the octanucleotides d(AGAGCTCT)₂ and d(CGAGCTCG)₂, were carried out. These studies demonstrate that the dppz ligand intercalates into the G²–A³ step, with {Ru(tpm)py} in the minor groove. They also reveal that the complex intercalates into the binding site in two possible orientations with the pyridyl ligand of the major conformer making close contact with terminal base pairs. We conclude that substitution at the 2‐ or 3‐position of the pyridine ring has little effect on binding, but that substitution at the 4‐position drastically disrupts intercalative binding, particularly with a 4‐amino substituent, because of steric and electronic interactions with the DNA. These results indicate that complexes derived from these systems have the potential to function as sequence‐specific light‐switch systems.     

Evaluation of alkaloids binding to the parallel quadruplex structure [d(TGGGGT)]4 by electrospray ionization mass spectrometry

In this study, electrospray ionization mass spectrometry (ESI‐MS) was used to investigate the binding interaction of six alkaloids with parallel intermolecular G‐quadruplex [d(TGGGGT)]₄, and five alkaloids including berberine, jatrorrhizine, palmatine, tetrandrine, and fangchinoline showed complexation with the target DNA. Relative binding affinities were estimated on the basis of mass spectrometric data. The slight differences in chemical structures of berberine, jatrorrhizine, and palmatine had little influence on their binding affinities to [d(TGGGGT)]₄. Tetrandrine and fangchinoline selectively bound to [d(TGGGGT)]₄ versus duplex DNA. Collision‐induced dissociation (CID) experiments showed that the complexes with berberine, jatrorrhizine, and palmatine dissociated via strand separation and ligand retaining in the strand while the complexes with tetrandrine and fangchinoline were dissociated via ligand elimination. A comparison of dissociation patterns in CID experiments of complexes with the alkaloids to those with the traditional G‐quadruplex DNA binders suggested an end‐stacking binding mode for tetrandrine and fangchinoline and an intercalation binding mode for berberine, jatrorrhizine, and palmatine to the target DNA. The current work not only provides deep insight into alkaloid/[d(TGGGGT)]₄ complexes and useful guidelines for design of efficient anticancer agents but also demonstrates the utility of ESI‐MS as a powerful tool for evaluating interaction between ligand and quadruplex DNA. Copyright © 2012 John Wiley & Sons, Ltd.     

Metal-mediated self-assembly of pyridylcalixarenes: prevention of intramolecular metal chelation is essential in constructing molecular capsules.

To construct calixarene-based molecular capsules utilizing the pyridyl-Pd(II) interaction, reactions of cone-pyridylcalix[4]arene 3, cone-pyridylcalix[5]arene 13, and cone-pyridylcalix[4]arene bis-crown 16 with square-planar Pd(II) complex 7 were investigated. Because of the coexistence of intermolecular binding and chelate-forming intramolecular binding, the reactions of cone-pyridylcalix[4]arene 3 or cone-pyridylcalix[5]arene 13 with cis-Pd(II) complex 7 yield complicated, structure-unknown oligomers. The short dioxyethylene bridges on the lower rim of pyridylcalix[4]arene bis-crown 16 rigidify the cone conformation and thus prohibit 16 from the intramolecular binding with a metal component. Thus, two cone-tetrapyridylcalix[4]arene bis-crown 16 and four cis-Pd(II) complex molecules self-assemble into molecular capsules that exist as a parallel/antiparallel conformer mixture in a nearly 1:1 ratio. The results demonstrated that to prevent entropically favorable intramolecular binding is essential is constructing higher capsule-like structures with calixarene building blocks by self-assembling.     

Carbon monoxide induced decomposition of the active site [Ni-4Fe-5S] cluster of CO dehydrogenase

During the past two years, crystal structures of Cu- and Mo-containing carbon monoxide dehydrogenases (CODHs) and Ni- and Fe-containing CODHs have been reported. The active site of CODHs from anaerobic bacteria (cluster C) is composed of Ni, Fe, and S for which crystallographic studies of the enzymes from Carboxydothermus hydrogenoformans, Rhodospirillum rubrum, and Moorella thermoaceticarevealed structural similarities in the overall protein fold but showed substantial differences in the essential Ni coordination environment. The [Ni-4Fe-5S] cluster C in the fully catalytically competent dithionite-reduced CODH II from C. hydrogenoformans (CODHII(Ch)) at 1.6 A resolution contains a characteristic mu(2)-sulfido ligand between Ni and Fe1, resulting in a square-planar ligand arrangement with four S-ligands at the Ni ion. In contrast, the [Ni-4Fe-4S] clusters C in CO-treated CODH from R. rubrum resolved at 2.8 A and in CO-treated acetyl-CoA synthase/CODH complex from M. thermoacetica at 2.2 and 1.9 A resolution, respectively, do not contain the mu(2)-sulfido ligand between Ni and Fe1 and display dissimilar geometries at the Ni ion. The [Ni-4Fe-4S] cluster is composed of a cubane [Ni-3Fe-4S] cluster linked to a mononuclear Fe site. The described coordination geometries of the Ni ion in the [Ni-4Fe-4S] cluster of R. rubrum and M. thermoacetica deviate from the square-planar ligand geometry in the [Ni-4Fe-5S] cluster C of CODHII(Ch). In addition, the latter was converted into a [Ni-4Fe-4S] cluster under specific conditions. The objective of this study was to elucidate the relationship between the structure of cluster C in CODHII(Ch) and the functionality of the protein. We have determined the CO oxidation activity of CODHII(Ch) under different conditions of crystallization, prepared crystals of the enzyme in the presence of dithiothreitol or dithionite as reducing agents under an atmosphere of N(2) or CO, and solved the corresponding structures at 1.1 to 1.6 A resolutions. Fully active CODHII(Ch) obtained after incubation of the enzyme with dithionite under N(2) revealed the [Ni-4Fe-5S] cluster. Short treatment of the enzyme with CO in the presence of dithiothreitol resulted in a catalytically competent CODHII(Ch) with a CO-reduced [Ni-4Fe-5S] cluster, but a prolonged treatment with CO caused the loss of CO-oxidizing activity and revealed a [Ni-4Fe-4S] cluster, which did not contain a mu(2)-S. These data suggest that the [Ni-4Fe-4S] cluster of CODHII(Ch) is an inactivated decomposition product originating from the [Ni-4Fe-5S] cluster.     

平面四方金属苯配合物的二阶超极化率的量子化学计算

反位平面四方型过渡金属有机配合物[XM(PEt₃)₂-C₆H₄-A] (M＝Pd, Pt; X＝Br, I; A＝NO₂, CHO), 具有较高的二阶超极化率. 采用从头算方法对该配合物的二阶超极化率进行了研究. 构型在MP2/Lanl2DZ水平优化. 基组采用赝势价分裂基Lanl2DZ添加弥散函数和极化函数. HF水平计算(个别情况用MP2计算验证)表明, 苯的对位取代基A的吸电子能力越强, 金属对位配体X的供电子能力越强, 则配合物的二阶超极化率越大.     

Transition Metal Complexes with the Thiosemicarbazide-based ligands. XII. Complexes of nickel(II) with benzoylacetone S-methylisothiosemicarbazone and N(1)-2-butylidene-4-oxo-4-phenyl-N(4)-salicylidene-S-methylisothiosemicarbazide

A template synthesis procedure yielded [Ni(HL¹)NH₃]I, where HL¹ is the monoanion of the terdentate ONN benzoylacetone S-methylisothiosemicarbazone ligand. The reaction of this complex with an excess of NH₄NCS, pyridine, or hydrazine resulted in the complexes [Ni(HL¹)(NH₃)NCS] and [Ni(L¹)A] (A = Py, N₂H₄). The monoanionic form of the ligand is obtained by deprotonation of the enolic form of the benzoylacetone moiety, whereas the dianion is formed by additional deprotonation of the terminal NH₂ group. Finally, the reaction of [Ni(HL¹)NH₃]I with salicyladehyde produced the NiL² complex in which L² stands for the dianion of the ONNO ligand N(1)-2-butylidene-4-oxo-4-phenyl-N(4)-salicylidene-S-methylisothiosemicarbazide. All complexes are diamagnetic and have a square-planar configuration, except for [Ni(HL¹)(NH₃)NCS], for which te data of i.r. spectra suggest a square-pyramidal structure. The electronic absorption spectra of the ethanolic solutions of all complexes are characteristic of typical square-planar coordination of nickel(II).     

Synthesis,spectral, and biological studies of transition metal chelates of N-[1-(3-aminopropyl)imidazole]salicylaldimine

Tridentate chelate complexes M[LX?·?2H₂O], where M?=?Co(II), Ni(II), Cu(II), Zn(II), and Cd(II) have been synthesized from the Schiff base L?=?N-[1-(3-aminopropyl)imidazole]salicylaldimine and X?=?Cl. Microanalytical data, UV-Vis, magnetic susceptibility, IR, ¹H-NMR, mass, and EPR techniques were used to confirm the structures. Electronic absorption spectra and magnetic susceptibility measurements suggest square-planar geometry for copper complex and octahedral for other metal complexes. EPR spectra of copper(II) complex recorded at 300?K confirm the distorted square-planar geometry of the copper(II) complex. Biological activities of the ligand and metal complexes have been studied on Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa, and Candida albicans by the well diffusion method. The activity data show the metal complexes to be more potent than the parent ligand against two bacterial species and one fungus. The electrochemical behavior of the copper complex was studied by cyclic voltammetry.     

Spectroscopic and spectrometric studies of anion recognition with calix[4]pyrroles in different reaction conditions

The binding studies of calix[4]pyrroles (1–6) with fluoro, chloro, bromo, iodo and sulphato anions generated from normal-tetrabutylammoniumfluoride, normal-tetrabutylammoniumchloride, normal-tetrabutylammoniumbromide, normal-tetrabutylammoniumiodide, and normal-tetrabutylammoniumsulphate respectively were investigated by electrospray ionization mass spectrometry (ESI-MS) in dichloromethane–acetonitrile in negative ion mode. The efficacy of a particular calix[4]pyrrole to bind with anions was found maximum at low cone voltage of the instrument, at high cone voltage the binding was suppressed due to removal of anion from the cavity of the macrocycles. The binding strength was found inversely proportional to the size of anion for a particular calix[4]pyrrole. The fragmentation pattern of calix[4]pyrrole was observed at higher cone voltage of ESI-MS and was interpreted. The association constants of calix[4]pyrroles and anions obtained from electronic transition studies were in good agreement with that observed from ¹H NMR titration studies.     

Vesicle formed by amphiphilc cucurbit[6]uril: versatile, noncovalent modification of the vesicle surface, and multivalent binding of sugar-decorated vesicles to lectin

We report a novel vesicle formed by an amphiphilic CB[6] derivative, the surface of which can be easily modified via host-guest interactions by taking advantage of molecular cavities, readily accessible at the vesicle surface, and their strong affinity toward polyamines. Amphiphilic CB[6] derivative 1 synthesized by reaction between (allyloxy)12CB[6] and 2-[2-(2-methoxyethoxy)ethoxy]ethanethiol affords a vesicle that has been characterized by TEM, light scattering, and fluorescent dye entrapment experiments. Treatment of vesicle 1 with FITC (fluorescein isothiocyanate)-spermine conjugate ligand 2, in which spermine serves as a binding motif to CB[6] and FITC as a fluorescent tag, produced a surface-modified vesicle, which can be easily visualized by a confocal microscope. This result provides us with a new noncovalent, modular approach to the modification of vesicle surfaces. By treating the vesicle derived from the amphiphilic CB[6] with a tag-attached polyamine, we can easily decorate the surface of the vesicle with the tag. Sugar-decorated vesicles were prepared by this noncovalent method, and their interactions with concanavalin A (ConA) were studied. The binding constant of the vesicle decorated with mannose-spermidine conjugate 3 to ConA was measured to be approximately 3 x 104 M-1, which is almost 3 orders of magnitude higher than that of free ligand 3 to ConA (K = approximately 50 M-1). On the other hand, the binding constant of the vesicle coated with galactose-spermidine conjugate 4 to ConA was too small to be measured. These results illustrate the specific and multivalent interactions between the mannose-decorated vesicle and ConA. The ability for facile surface modification suggests many practical applications, including its use in targeted drug delivery and immunization.     

Phosphido pincer complexes of platinum: synthesis, structure and reactivity

A series of platinum(II) complexes supported by the tridentate bis(phosphine)phosphido ligand bis(2-diisopropylphosphinophenyl)phosphide) [(i)Pr-PPP] have been synthesized and characterized (1-4). X-Ray structural studies of [(i)Pr-PPP]PtCl (1) and [(i)Pr-PPP]PtCH(3) (3) complexes show meridional [(i)Pr-PPP] ligands around approximately square-planar platinum centers. Structural data and NMR analysis highlight a strong trans influence for the phosphido phosphorous donor, comparable to that of the anionic aryl carbon of the classic PCP pincer complexes. A series of thermally stable [PPP]Pt(IV) compounds, including [PPP]Pt(CH(3))(2)X [X = I (5) and SbF(6) (6)], were also synthesized. The study of the binding affinity of SO(2) and NO to complex 1 has also been addressed.     

Synthesis,characterization and ascorbic acid oxidase mimetic catalytic activity of copper(II) picolyl hydrazone complexes

Four new picolyl hydrazones were prepared via Schiff-base condensation of picolonic acid hydrazide with α-formyl-(L¹), α-acetyl-(L²), α-benzoyl-(L³) pyridine and α-formyl-(L⁴) thiophene. Copper(II) complexes of these hydrazones and a series of copper(II) complexes containing (L²) and various anions (Cl, Br, NO₃, SCN, SO₄, ClO₄, AcO, PF₆ and BF₄) have been synthesized. Elemental, thermal analysis, molar conductivity, magnetic moment measurements and spectral (i.r., electronic and e.s.r.) studies have been used to characterize the prepared compounds. The overall structure and reactivity of the reported copper(II) chelates critically depend on the ligand structure and the nature of counter anion incorporated in the complex molecule. Octahedral [complex (7)], square-pyramidal [complex (8)] and square-planar monomeric species [complexes (1–6), (9) and (10)] and a dimeric structure with oxygen bridge in square-planar geometry [complexes (11) and (12)] were suggested. The reported copper(II) complexes exhibit promising oxidase catalytic activity towards the aerobic oxidation of vitamin C. A linear correlation exists between the oxidase catalytic activity and the Lewis-acidity of the central copper(II) ion created by the donating properties of the parent ligand, as well the irregularity of the coordination environment. The probable mechanistic implications of the catalytic oxidation reactions are discussed.     

Electrochemical synthesis of metal complexes of schiff bases: The crystal structure of bis{2-[(4-methylphenyl)iminomethyl]pyrrolato}copper(II)

The electrochemical oxidation of anodic nickel, copper, zinc or cadmium in acetonitrile solutions of Schiff bases (HL) derived from H-pyrrole-2-carbaldehyde and substituted anilines gives compounds of general formula ML₂. The crystal structure of bis{2-[(4-methylphenyl)iminomethyl]pyrrolato}copper(II) has been determined by X-ray diffraction. The compound crystallizes in the monoclinic space group P2₁/n with a = 9.356(2), b = 16.697(2), c = 14.145(2) Å and β = 108.47(2)°. The crystal structure consists of monomeric molecules in which the central CuN₄ unit has distorted square-planar geometry with a dihedral angle of 25.8(3)° between the coordination planes. The IR, ¹H NMR and UV-visible spectra of the complexes are discussed and related to the structure.     

(3,8,9,14‐Tetraethyl‐2,4,13,15‐tetramethyltripyrrinato)(trifluoroacetato)­palladium(II)

In the crystal structure, the title compound {systematic name: [2,5‐bis­(4‐ethyl‐3,5‐dimethyl‐2H‐pyrrol‐2‐ylidene­meth­yl)‐1H‐pyrrolato](trifluoroacetato)palladium(II)}, [Pd(C₂F₃O₂)(C₂₇H₃₄N₃)], forms chiral mol­ecules with a helical distortion of the tripyrrinate ligand backbone and an essentially planar PdN₃O core, with Pd—N distances ranging from 1.977 (3) to 2.045 (3) Å and a Pd—O distance of 2.051 (2) Å. This distortion of the organic ligand is considered as the conformational answer to the steric inter­action of the terminal methyl groups of the tripyrrinate ligand with the donor O atom of the trifluoro­acetate group.     

An Aluminum Telluride with a Terminal Al=Te Bond and its Conversion to an Aluminum Tellurocarbonate by CO2 Reduction

Facile access to dimeric heavier aluminum chalcogenides [(NHC)Al(Tipp)-μ-Ch]₂ (NHC=IiPr (1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene, IMe₄ (1,3,4,5-tetramethylimidazol-2-ylidene); Tipp=2,4,6-iPr₃C₆H₂; Ch=Se, Te) by treatment of NHC-stabilized aluminum dihydrides with elemental Se and Te is reported. The higher affinity of IMe₄ in comparison with IiPr toward the Al center in [(NHC)Al(Tipp)-μ-Ch]₂ can be used for ligand exchange. Additionally, the presence of excess IMe₄ allows for cleavage of the dimers to form a rare example of a neutral multiply bonded heavier aluminum chalcogenide in the form of a tetracoordinate aluminum complex, (IMe₄)₂(Tipp)Al=Te. This species reacts with three equivalents of CO₂ across two Al−C^NHC and the Al=Te bond affording a pentacoordinate aluminum complex containing a dianionic tellurocarbonate ligand [CO₂Te]²⁻, which is the first example of tellurium analogue of a carbonate [CO₃]²⁻.     

Square-planar copper(II) complexes with a novel tetradentate amido-carboxylate ligand. Crystal structure of [Co(H2O)6][Cu(mda)] · 2H2O

A novel O—N—N—O-type tetradentate ligand H₄mda (H₄mda = malamido-N,N-diacetic acid) and the corresponding square-planar copper(II) complexes have been prepared and characterized. The mda^4– ligand coordinates to the copper(II) ion via two pairs of deprotonated ligating atoms (two carboxylate oxygens and two deprotonated amide nitrogens) with in-plane square chelation. A four-coordinate, square-planar geometry has been established crystallographically for the [Co(H₂O)₆][Cu(mda)] · 2H₂O complex. Structural data correlating the square-planar geometry of the [Cu(mda)]^2– unit are discussed in relation to information obtained for similar complexes. The i.r., electronic, absorption and reflectance spectra of the complexes are analysed in comparison with related complexes of known geometries.