Modification of a Common ß-diketiminate NacNac Framework via Sequential Lithiation and Small Molecule Insertion.

A widely utilised class of ligands in synthesis and catalysis, ß-diketiminate (BDI) or NacNac compounds were initially considered innocent in the sense that they remained intact in all their applications. That changed when the γ-C-H unit of their NCCCN backbone was found to engage in reactions with electrophiles. Here, we show that this special reactivity can be used advantageously to prepare tripodal modifications of the common NacNac ligand derived from 2,6-diisopropylphenyl-ß-methyldiketimine [NacNacH (Me, Dipp)]. Lithiation to give NacNacLi, followed by reactions with isocyanates, isothiocyanates and a carbodiimide, have afforded a series of tripodal NacNac variants having N,N,N,O; N,N,N,S; or N,N,N,N potential dentation sites, many of which have been crystallographically characterised. Distinct ligating modes of these new ligands have been elucidated through the crystal structures of their lithiated derivatives.

Chiral-at-Metal Racemization Unraveled for MX2 (a-chel)2 by means of a Computational Analysis of MoO2 (acnac)2.

Octahedral chiral-at-metal complexes MX2 (a-chel)2 (a-chel=asymmetric chelate) can rearrange their ligands by four mechanisms known as the Bailar (B), Ray-Dutt (RD), Conte-Hippler (CH), and Dhimba-Muller-Lammertsma (DML) twists. Racemization involves their interconnections, which were computed for MoO2 (acnac)2 (acnac=ß-ketoiminate) using density functional theory at ωB97x-D with the 6-31G(d,p) and 6-311G(2d,p) basis sets and LANL2DZ for molybdenum. Racemizing the cis(NN) isomer, being the global energy minimum with trans oriented imine groups, is a three step process (DML-CH-DML) that requires 17.4 kcal/mol, while all three cis isomers (cis(NN), cis(NO), and cis(OO)) interconvert at ≤17.9 kcal/mol. The B and RD twists are energetically not competitive and neither are the trans isomers. The interconnection of all enantiomeric minima and transition structures is summarized in a graph that also visualizes valley ridge inflection points for two of the three CH twists. Geometrical features of the minima and twists are given. Lastly, the influence of N-substitution on the favored racemization pathway is evaluated. The present comprehensive study serves as a template for designing chiral-at-metal MX2 (a-chel)2 catalysts that may retain their chiral integrity.

Hexaphenyltriphenylene-Based Multicomponent Metallacages: Host-Guest Complexation for White-Light Emission.

A hexaphenyltriphenylene-based hexatopic pyridyl ligand is designed and used to prepare three hexagonal prismatic metallacages via metal-coordination-driven self-assembly. Owing to the planar conjugated structures of the hexaphenyltriphenylene skeleton, such metallacages show good host-guest complexation with a series of emissive dyes, which have been further used to tune their emission in solution. Interestingly, based on their complementary emission colors, white light emission is achieved in a mixture of the host metallacages and the guests.

Insight into the Structural Ambiguity of Actinide(IV) Oxalate Sheet Structures: A Case for Alternate Coordination Geometries.

Plutonium(IV) oxalate hexahydrate (Pu(C2 O4 )2 ⋅ 6 H2 O; PuOx) is an important intermediate in the recovery of plutonium from used nuclear fuel. Its formation by precipitation is well studied, yet its crystal structure remains unknown. Instead, the crystal structure of PuOx is assumed to be isostructural with neptunium(IV) oxalate hexahydrate (Np(C2 O4 )2 ⋅ 6 H2 O; NpOx) and uranium(IV) oxalate hexahydrate (U(C2 O4 )2 ⋅ 6 H2 O; UOx) despite the high degree of unresolved disorder that exists when determining water positions in the crystal structures of the latter two compounds. Such assumptions regarding the isostructural behavior of the actinide elements have been used to predict the structure of PuOx for use in a wide range of studies. Herein, we report the first crystal structures for PuOx and Th(C2 O4 )2 ⋅ 6 H2 O (ThOx). These data, along with new characterization of UOx and NpOx, have resulted in the full determination of the structures and resolution of the disorder around the water molecules. Specifically, we have identified the coordination of two water molecules with each metal center, which necessitates a change in oxalate coordination mode from axial to equatorial that has not been reported in the literature. The results of this work exemplify the need to revisit previous assumptions regarding fundamental actinide chemistry, which are heavily relied upon within the current nuclear field.

Bivalent Metal Chelates with Pentadentate Azo-Schiff Base Derived from Nicotinic Hydrazide: Preparation, Structural Elucidation, and Pharmacological Activity.

Azo-Schiff base ligand (N'-((E)-2-hydroxy-5-((E)-(2-hydroxyphenyl)diazenyl)benzylidene)nicotinohydrazide) and its Mn(II), Co(II), Ni(II), Cu(II), Zn(II) and Pd(II) chelates were prepared and elucidated. The geometrical structures of the prepared chelates were characterized by several spectroanalytical techniques and thermogravimetric analysis. The obtained data revealed that the chelates have (1M:1L), (1M:2L), (1M:3L), and (1M:4L) molar ratios. The infrared spectra displayed that the H2 L ligand behaves in a pentacoordinate fashion in chelates of Mn(II), Ni(II), and Cu(II) ions. However, in Zn(II) and Pd(II) chelates, the ligand is coordinated as a tetradentate species (NONO) through nitrogen atoms of azomethine and azo groups as well as oxygen atoms of phenolic hydroxy, and carbonyl groups. Besides, it was concluded that the oxygen atoms of carbonyl and hydroxy groups along with the azomethine nitrogen atom of the ligand are bounded with Co(II) ion in metal chelate (2). According to the measured molar conductance values, the chelates of Cu(II), Zn(II), and Pd(II) are weak electrolytes, but Mn(II), Co(II), and Ni(II) chelates are ionic. The azo-Schiff base ligand and its prepared metal chelates were tested for their antioxidant and antibacterial properties. The Ni(II) chelate was found to be considered an effective antioxidant agent. In addition, the available antibacterial data suggest that the Ni(II) and Co(II) chelates may be employed as inhibitor agents against Proteus vulgaris, Escherichia coli, and Bacillus subtilis bacteria. Furthermore, the data showed that, in comparison to the ligand and other metal chelates, copper(II) chelate (4) exhibited higher action against Bacillus subtilis bacteria.

A Series of Novel 3D Coordination Polymers Based on the Quinoline-2,4-dicarboxylate Building Block and Lanthanide(III) Ions-Temperature Dependence Investigations.

A series of novel 3D coordination polymers [Ln2(Qdca)3(H2O)x]·yH2O (x = 3 or 4, y = 0-4) assembled from selected lanthanide ions (Ln(III) = Nd, Eu, Tb, and Er) and a non-explored quinoline-2,4-dicarboxylate building block (Qdca2- = C11H5NO42-) were prepared under hydrothermal conditions at temperatures of 100, 120, and 150 °C. Generally, an increase in synthesis temperature resulted in structural transformations and the formation of more hydrated compounds. The metal complexes were characterized by elemental analysis, single-crystal and powder X-ray diffraction methods, thermal analysis (TG-DSC), ATR/FTIR, UV/Vis, and luminescence spectroscopy. The structural variety of three-dimensional coordination polymers can be ascribed to the temperature effect, which enforces the diversity of quinoline-2,4-dicarboxylate ligand denticity and conformation. The Qdca2- ligand only behaves as a bridging or bridging-chelating building block binding two to five metal centers with seven different coordination modes arising mainly from different carboxylate group coordination types. The presence of water molecules in the structures of complexes is crucial for their stability. The removal of both coordinated and non-coordinated water molecules leads to the disintegration and combustion of metal-organic frameworks to the appropriate lanthanide oxides. The luminescence features of complexes, quantum yield, and luminescent lifetimes were measured and analyzed. Only the Eu complexes show emission in the VIS region, whereas Nd and Er complexes emit in the NIR range. The luminescence properties of complexes were correlated with the crystal structures of the investigated complexes.

Multistate Structural Switching of [3]Catenanes with Cyclic Porphyrin Dimers by Complexation with Amine Ligands.

Catenanes with multistate switchable properties are promising components for next-generation molecular machines and supramolecular materials. Herein, we report a ligand-controlled switching method, a novel method for the multistate switching of catenanes controlled by complexation with added amine ligands. To verify this method, a [3]catenane comprising cyclic porphyrin dimers with a rigid π-system has been synthesized. Owing to the rigidity, the relative positions among the cyclic components of the [3]catenane can be precisely controlled by complexation with various amine ligands. Moreover, ligand-controlled multistate switching affects the optical properties of the [3]catenanes: the emission intensity can be tuned by modulating the sizes and coordination numbers of integrated amine ligands. This work shows the utility of using organic ligands for the structural switching of catenanes, and will contribute to the further development of multistate switchable mechanically interlocked molecules.

Binding, Release and Functionalization of Intact Pnictogen Tetrahedra Coordinated to Dicopper Complexes.

The bridging MeCN ligand in the dicopper(I) complexes [(DPFN)Cu2 (µ,η1 : η1 -MeCN)][X]2 (X=weakly coordinating anion, NTf2 (1 a), FAl[OC6 F10 (C6 F5 )]3 (1 b), Al[OC(CF3 )3 ]4 (1 c)) was replaced by white phosphorus (P4 ) or yellow arsenic (As4 ) to yield [(DPFN)Cu2 (µ,η2 : η2 -E4 )][X]2 (E=P (2 a-c), As (3 a-c)). The molecular structures in the solid state reveal novel coordination modes for E4 tetrahedra bonded to coinage metal ions. Experimental data and quantum chemical computations provide information concerning perturbations to the bonding in coordinated E4 tetrahedra. Reactions with N-heterocyclic carbenes (NHCs) led to replacement of the E4 tetrahedra with release of P4 or As4 and formation of [(DPFN)Cu2 (µ,η1 : η1 -Me NHC)][X]2 (4 a,b) or to an opening of one E-E bond leading to an unusual E4 butterfly structural motif in [(DPFN)Cu2 (µ,η1 : η1 -E4 Dipp NHC)][X]2 (E=P (5 a,b), E=As (6)). With a cyclic alkyl amino carbene (Et CAAC), cleavage of two As-As bonds was observed to give two isomers of [(DPFN)Cu2 (µ,η2 : η2 -As4 Et CAAC)][X]2 (7 a,b) with an unusual As4 -triangle+1 unit.

Electrochemical Behaviour and Chemical Species of Sm(II) in AlCl3 -NaCl with Different Lewis Acidity.

AlCl3 -NaCl was utilized as an electrolyte in this work due to its low melting point and Lewis acidity, in which samarium exists in two oxidation states, Sm(III) and Sm(II), resulting in unique electrochemical behaviours. Sm metal dissolves in AlCl3 -NaCl melt to form SmCl2 , which is verified by electrochemical and spectroscopic techniques. As the Lewis acidity of the melt increases, the diffusion coefficient of Sm(II) gradually increases, and the activation energy of diffusion decreases. Moreover, an additional co-reduction peak of Sm3+ and AlCl4 - is observed to be more positive than that of Al(0)/Al(III) in Lewis basic melt, which may be tightly correlated with the variation of Sm(II) coordination in AlCl3 -NaCl melt and ligand variation from Cl- to AlCl4 - and Al2 Cl7 - as the Lewis acidity of the AlCl3 -NaCl melt increases, according to the in situ electronic absorption spectra of Sm in this melt.

Single Atom Catalysts: An Overview of the Coordination and Interactions with Metallic Supports.

Catalyst utilization is a key economic factor in heterogeneous catalysis, particularly, when noble metals are used as the active phase. A huge saving on catalyst cost can be achieved with developing a single atomic layer of the active catalyst on a given cheap support. Besides the economic benefit, single atom catalysts (SACs) have also shown superior activity and selectivity relative to catalytic particles or nanoparticles; yet they are prone to aggregation and deactivation. The development of effective, stable, and commercially viable SACs is still a huge challenge. One of the remaining key obstacles is the ability to easily and effectively tune SACs-support interactions and coordination in a way that enables the production of robust, stable, and versatile SACs. Accordingly, the coordination and interactions between metallic supports and SACs and their impacts on SACs stability and activity are reviewed in this article.

Coordination Sites for Sodium and Potassium Ions in Nucleophilic Adeninate Contact ion-Pairs: A Molecular-Wide and Electron Density-Based (MOWED) Perspective.

The adeninate anion (Ade-) is a useful nucleophile used in the synthesis of many prodrugs (including those for HIV AIDS treatment). It exists as a contact ion-pair (CIP) with Na+ and K+ (M+) but the site of coordination is not obvious from spectroscopic data. Herein, a molecular-wide and electron density-based (MOWED) computational approach implemented in the implicit solvation model showed a strong preference for bidentate ion coordination at the N3 and N9 atoms. The N3N9-CIP has (i) the strongest inter-ionic interaction, by -30 kcal mol-1, with a significant (10-15%) covalent contribution, (ii) the most stabilized bonding framework for Ade-, and (iii) displays the largest ion-induced polarization of Ade-, rendering the N3 and N9 the most negative and, hence, most nucleophilic atoms. Alkylation of the adeninate anion at these two positions can therefore be readily explained when the metal coordinated complex is considered as the nucleophile. The addition of explicit DMSO solvent molecules did not change the trend in most nucleophilic N-atoms of Ade- for the in-plane M-Ade complexes in M-Ade-(DMSO)4 molecular systems. MOWED-based studies of the strength and nature of interactions between DMSO solvent molecules and counter ions and Ade- revealed an interesting and unexpected chemistry of intermolecular chemical bonding.

Coordination Chemistry of Nucleotides and Antivirally Active Acyclic Nucleoside Phosphonates, including Mechanistic Considerations.

Considering that practically all reactions that involve nucleotides also involve metal ions, it is evident that the coordination chemistry of nucleotides and their derivatives is an essential corner stone of biological inorganic chemistry. Nucleotides are either directly or indirectly involved in all processes occurring in Nature. It is therefore no surprise that the constituents of nucleotides have been chemically altered-that is, at the nucleobase residue, the sugar moiety, and also at the phosphate group, often with the aim of discovering medically useful compounds. Among such derivatives are acyclic nucleoside phosphonates (ANPs), where the sugar moiety has been replaced by an aliphatic chain (often also containing an ether oxygen atom) and the phosphate group has been replaced by a phosphonate carrying a carbon-phosphorus bond to make the compounds less hydrolysis-sensitive. Several of these ANPs show antiviral activity, and some of them are nowadays used as drugs. The antiviral activity results from the incorporation of the ANPs into the growing nucleic acid chain-i.e., polymerases accept the ANPs as substrates, leading to chain termination because of the missing 3'-hydroxyl group. We have tried in this review to describe the coordination chemistry (mainly) of the adenine nucleotides AMP and ATP and whenever possible to compare it with that of the dianion of 9-[2-(phosphonomethoxy)ethyl]adenine (PMEA2- = adenine(N9)-CH2-CH2-O-CH2-PO32) [or its diphosphate (PMEApp4-)] as a representative of the ANPs. Why is PMEApp4- a better substrate for polymerases than ATP4-? There are three reasons: (i) PMEA2- with its anti-like conformation (like AMP2-) fits well into the active site of the enzyme. (ii) The phosphonate group has an enhanced metal ion affinity because of its increased basicity. (iii) The ether oxygen forms a 5-membered chelate with the neighboring phosphonate and favors thus coordination at the Pα group. Research on ANPs containing a purine residue revealed that the kind and position of the substituent at C2 or C6 has a significant influence on the biological activity. For example, the shift of the (C6)NH2 group in PMEA to the C2 position leads to 9-[2-(phosphonomethoxy)ethyl]-2-aminopurine (PME2AP), an isomer with only a moderate antiviral activity. Removal of (C6)NH2 favors N7 coordination, e.g., of Cu2+, whereas the ether O atom binding of Cu2+ in PMEA facilitates N3 coordination via adjacent 5- and 7-membered chelates, giving rise to a Cu(PMEA)cl/O/N3 isomer. If the metal ions (M2+) are M(α,ß)-M(γ)-coordinated at a triphosphate chain, transphosphorylation occurs (kinases, etc.), whereas metal ion binding in a M(α)-M(ß,γ)-type fashion is relevant for polymerases. It may be noted that with diphosphorylated PMEA, (PMEApp4-), the M(α)-M(ß,γ) binding is favored because of the formation of the 5-membered chelate involving the ether O atom (see above). The self-association tendency of purines leads to the formation of dimeric [M2(ATP)]2(OH)- stacks, which occur in low concentration and where one half of the molecule undergoes the dephosphorylation reaction and the other half stabilizes the structure-i.e., acts as the "enzyme" by bridging the two ATPs. In accord herewith, one may enhance the reaction rate by adding AMP2- to the [Cu2(ATP)]2(OH)- solution, as this leads to the formation of mixed stacked Cu3(ATP)(AMP)(OH)- species, in which AMP2- takes over the structuring role, while the other "half" of the molecule undergoes dephosphorylation. It may be added that Cu3(ATP)(PMEA) or better Cu3(ATP)(PMEA)(OH)- is even a more reactive species than Cu3(ATP)(AMP)(OH)-. - The matrix-assisted self-association and its significance for cell organelles with high ATP concentrations is summarized and discussed, as is, e.g., the effect of tryptophanate (Trp-), which leads to the formation of intramolecular stacks in M(ATP)(Trp)3- complexes (formation degree about 75%). Furthermore, it is well-known that in the active-site cavities of enzymes the dielectric constant, compared with bulk water, is reduced; therefore, we have summarized and discussed the effect of a change in solvent polarity on the stability and structure of binary and ternary complexes: Opposite effects on charged O sites and neutral N sites are observed, and this leads to interesting insights.

Structural Origin of Enhanced Circularly Polarized Luminescence in Hybrid Manganese Bromides.

Chiral hybrid metal halides with a high dissymmetry factor (glum ) and a superior photoluminescence quantum yield (PLQY) are promising candidates for circularly polarized luminescence (CPL) light sources. Here, we report eight new chiral hybrid manganese halides, crystallizing in the non-centrosymmetric space group P21 21 21 and showing intense CPL emissions. Oppositely-signed circular dichroism (CD) and CPL signals are detected according to the R- and S-configurations of the chiral alkanolammonium cations. Time-resolved PL spectra show long averaged decay lifetimes up to 1 ms for (R-3-quinuclidinol)MnBr3 (R-1). The glum of polycrystalline samples for coordinated structures (23×10-3 ) is more than doubled compared with the non-coordinated ones (8.5×10-3 ), due to the structural variations. R-1 exhibit both a high glum and a high PLQY (50.2 %). The effective chirality transfer mechanism through coordination bonds, with strongly emissive MnII centers, enables a new class of high-performance CPL materials.

Pas de Deux of an NO Couple: Synchronous Photoswitching from a Double-Linear to a Double-Bent Ru(NO)2 Core under Nitrosyl Charge Conservation.

The {Ru(NO)2 }10 dinitrosylruthenium complex [Ru(NO)2 (PPh3 )2 ] (1) shows photo-induced linkage isomerism (PLI) of a special kind: the two NO ligands switch, on photo-excitation, synchronously from the ground state (GS) with two almost linear RuNO functions to a metastable state (MS) which persists up to 230 K and can be populated to ≈50 %. The MS was experimentally characterised by photo-crystallography, IR spectroscopy and DS-calorimetry as a double-bent variant of the double-linear GS. The experimental results are confirmed by computation which unravels the GS/MS transition as a disrotatory synchronous 50° turn of the two nitrosyl ligands. Although 1 shows the usual redshift of the N-O stretch on bending the MNO unit, there is no increased charge transfer from Ru to NO along the GS-to-MS path. In terms of the effective-oxidation-state (EOS) method, both isomers of 1 and the transition state are Ru-II (NO+ )2 species.

Magnesium(II)-ATP Complexes in 1-Ethyl-3-Methylimidazolium Acetate Solutions Characterized by 31 Mg ß-Radiation-Detected NMR Spectroscopy.

The complexation of MgII with adenosine 5'-triphosphate (ATP) is omnipresent in biochemical energy conversion, but is difficult to interrogate directly. Here we use the spin- 1/2 ß-emitter 31 Mg to study MgII -ATP complexation in 1-ethyl-3-methylimidazolium acetate (EMIM-Ac) solutions using ß-radiation-detected nuclear magnetic resonance (ß-NMR). We demonstrate that (nuclear) spin-polarized 31 Mg, following ion-implantation from an accelerator beamline into EMIM-Ac, binds to ATP within its radioactive lifetime before depolarizing. The evolution of the spectra with solute concentration indicates that the implanted 31 Mg initially bind to the solvent acetate anions, whereafter they undergo dynamic exchange and form either a mono- (31 Mg-ATP) or di-nuclear (31 MgMg-ATP) complex. The chemical shift of 31 Mg-ATP is observed up-field of 31 MgMg-ATP, in accord with quantum chemical calculations. These observations constitute a crucial advance towards using ß-NMR to probe chemistry and biochemistry in solution.

Metalloradical Cations and Dications Based on Divinyldiphosphene and Divinyldiarsene Ligands.

Metalloradicals are key species in synthesis, catalysis, and bioinorganic chemistry. Herein, two iron radical cation complexes (3-E)GaCl4 [(3-E).+ = [{(IPr)C(Ph)E}2 Fe(CO)3 ].+ , E = P or As; IPr = C{(NDipp)CH}2 , Dipp = 2,6-iPr2 C6 H3 ] are reported as crystalline solids. Treatment of the divinyldipnictenes {(IPr)C(Ph)E}2 (1-E) with Fe2 (CO)9 affords [{(IPr)C(Ph)E}2 Fe(CO)3 ] (2-E), in which 1-E binds to the Fe atom in an allylic (η3 -EECvinyl ) fashion and functions as a 4e donor ligand. Complexes 2-E undergo 1e oxidation with GaCl3 to yield (3-E)GaCl4 . Spin density analysis revealed that the unpaired electron in (3-E).+ is mainly located on the Fe (52-64 %) and vinylic C (30-36 %) atoms. Further 1e oxidation of (3-E)GaCl4 leads to unprecedented η3 -EECvinyl to η3 -ECvinyl CPh coordination shuttling to form the dications (4-E)(GaCl4 )2 .

Atypical and Asymmetric 1,3-P,N Ligands: Synthesis, Coordination and Catalytic Performance of Cycloiminophosphanes.

Novel seven-membered cyclic imine-based 1,3-P,N ligands were obtained by capturing a Beckmann nitrilium ion intermediate generated in situ from cyclohexanone with benzotriazole, and then displacing it by a secondary phosphane under triflic acid promotion. These "cycloiminophosphanes" possess flexible non-isomerizable tetrahydroazepine rings with a high basicity; this sets them apart from previously reported iminophophanes. The donor strength of the ligands was investigated by using their P-κ1 - and P,N-κ2 -tungsten(0) carbonyl complexes, by determining the IR frequency of the trans-CO ligands. Complexes with [RhCp*Cl2 ]2 demonstrated the hemilability of the ligands, giving a dynamic equilibrium of κ1 and κ2 species; treatment with AgOTf gives full conversion to the κ2 complex. The potential for catalysis was shown in the RuII -catalyzed, solvent-free hydration of benzonitrile and the RuII - and IrI -catalyzed transfer hydrogenation of cyclohexanone in isopropanol. Finally, to enable access to asymmetric catalysts, chiral cycloiminophosphanes were prepared from l-menthone, as well as their P,N-κ2 -RhIII and a P-κ1 -RuII complexes.

Seven-Membered Cyclic Potassium Diamidoalumanyls.

The seven-membered cyclic potassium alumanyl species, [{SiNMes }AlK]2 [{SiNMes }={CH2 SiMe2 N(Mes)}2 ; Mes=2,4,6-Me3 C6 H2 ], which adopts a dimeric structure supported by flanking K-aryl interactions, has been isolated either by direct reduction of the iodide precursor, [{SiNMes }AlI], or in a stepwise manner via the intermediate dialumane, [{SiNMes }Al]2 . Although the intermediate dialumane has not been observed by reduction of a Dipp-substituted analogue (Dipp=2,6-i-Pr2 C6 H3 ), partial oxidation of the potassium alumanyl species, [{SiNDipp }AlK]2 , where {SiNDipp }={CH2 SiMe2 N(Dipp)}2 , provided the extremely encumbered dialumane [{SiNDipp }Al]2 . [{SiNDipp }AlK]2 reacts with toluene by reductive activation of a methyl C(sp3 )-H bond to provide the benzyl hydridoaluminate, [{SiNDipp }AlH(CH2 Ph)]K, and as a nucleophile with BPh3 and RN=C=NR (R=i-Pr, Cy) to yield the respective Al-B- and Al-C-bonded potassium aluminaborate and alumina-amidinate products. The dimeric structure of [{SiNDipp }AlK]2 can be disrupted by partial or complete sequestration of potassium. Equimolar reactions with 18-crown-6 result in the corresponding monomeric potassium alumanyl, [{SiNDipp }Al-K(18-cr-6)], which provides a rare example of a direct Al-K contact. In contrast, complete encapsulation of the potassium cation of [{SiNDipp }AlK]2 , either by an excess of 18-cr-6 or 2,2,2-cryptand, allows the respective isolation of bright orange charge-separated species comprising the 'free' [{SiNDipp }Al]- alumanyl anion. Density functional theory (DFT) calculations performed on this moiety indicate HOMO-LUMO energy gaps in the of order 200-250 kJ mol-1 .

Coordination-Driven Selective Formation of D2 Symmetric Octanuclear Organometallic Cages.

The coordination-driven self-assembly of organometallic half-sandwich iridium(III)- and rhodium(III)-based building blocks with asymmetric ambidentate pyridyl-carboxylate ligands is described. Despite the potential for obtaining a statistical mixture of multiple products, D2 symmetric octanuclear cages were formed selectively by taking advantage of the electronic effects emanating from the two types of chelating sites - (O,O') and (N,N') - on the tetranuclear building blocks. The metal sources and the lengths of bridging ligands influence the selectivity of the self-assembly. Experimental observations, supported by computational studies, suggest that the D2 symmetric cages are the thermodynamically favored products. Overall, the results underline the importance of electronic effects on the selectivity of coordination-driven self-assembly, and demonstrate that asymmetric ambidentate ligands can be used to control the design of discrete supramolecular coordination complexes.

Titanium-Oxo Clusters with Bi- and Tridentate Organic Ligands: Gradual Evolution of the Structures from Small to Big.

Homometallic titanium oxo clusters are one of the most important groups of metal oxo clusters, with more than 300 examples characterized by X-ray structure analyses. Most of them are uncharged and are obtained by partial hydrolysis and condensation of titanium alkoxo derivatives. The cluster cores, ranging from 3 to >50 titanium atoms, are stabilized by organic ligands. Apart from residual OR groups, carboxylato and phosphonato ligands are most frequent. The article critically reviews and categorizes the known structures and works out basic construction principles by comparing the different cluster types.