Superhalogens as Building Blocks of Super Lewis Acids

Lewis acids play an important role in synthetic chemistry. Using first-principle calculations on some newly designed molecules containing boron and organic heterocyclic superhalogen ligands, we show that the acid strength depends on the charge of the central atom as well as on the ligands attached to it. In particular, the strength of the Lewis acid increases with increasing electron withdrawing power of the ligand. With this insight, we highlight the importance of superhalogen-based ligands in the design of strong Lewis acids. Calculated fluoride ion affinity (FIA) values of B[C₂BNO(CN)₃]₃ and B[C₂BNS(CN)₃]₃ show that these are super Lewis acids.     

Carbodicarbenes and Related Divalent Carbon(0) Compounds

Quantum‐chemical calculations using DFT and ab initio methods have been carried out for fourteen divalent carbon(0) compounds (carbones), in which the bonding situation at the two‐coordinate carbon atom can be described in terms of donor–acceptor interactions L→C←L. The charge‐ and energy‐decomposition analysis of the electronic structure of compounds 1 – 10 reveals divalent carbon(0) character in different degrees for all molecules. Carbone‐type bonding L→C←L is particularly strong for the carbodicarbenes 1 and 2 , for the “bent allenes” 3 a , 3 b , 4 a , and 4 b , and for the carbocarbenephosphoranes 7 a , 7 b , and 7 c . The last‐named molecules have very large first and large second proton affinities. They also bind two BH₃ ligands with very high bond energies, which are large enough that the bis‐adducts should be isolable in a condensed phase. The second proton affinities of the complexes 5 , 6 , and 8 – 10 bearing CO or N₂ as ligand are significantly lower than those of the other molecules. However, they give stable complexes with two BH₃ ligands and thus are twofold Lewis bases. The calculated data thus identify 1 – 10 as carbones L→C←L in which the carbon atom has two electron pairs. The chemistry of carbones is different from that of carbenes because divalent carbon(0) compounds CL₂ are π donors and thus may serve as double Lewis bases, while divalent carbon(II) compounds are π acceptors. The theoretical results point toward new directions for experimental research in the field of low‐coordinate carbon compounds.     

Dative bonds versus electron solvation in tri‐coordinated beryllium complexes: Be(CX)3 [X = O,S, Se,Te, Po] and Be(PH3)3 versus Be(NH3)3

The formation mechanism and bonding scheme for the titled molecules is proposed based on high‐level theoretical calculations. All species are formed via three dative bonds from the ligands to Be. For Be(CX)₃ [X = O, S, Se, Te, Po] species and Be(PH₃)₃ the two electrons of beryllium are promoted from 2s to 2p facilitating these bonds. At the same time, electronic density is donated toward the ligands via the π‐frame. In contrast, ammonia partly solvates the valence 2s electrons of Be, which are delocalized in the periphery of the molecule. Based on the proton affinity of all of these complexes and their derivatives, we found that they are strong Lewis bases. For example, Be(PMe₃)₃ is stronger than ammonia and its ethyl substituted analogue and could serve as the basis of more efficient frustrated Lewis pairs.     

A New Glimpse into the CO2‐Philicity of Carbonyl Compounds

We report a theoretical study on non‐conventional structures of 1:1 complexes between carbon dioxide and carbonyl compounds. These structures have never been reported before but are relevant for understanding the solubility of carbonyl compounds in supercritical CO₂. The work is based on the results of ab initio calculations at the MP2 and CCSD(T) levels using aug‐cc‐pVDZ and aug‐cc‐pVTZ basis sets. Investigated systems include aldehydes, ketones and esters, together with some fluorinated derivatives. The results are interpreted in terms of natural bond orbital analyses. Harmonic vibrational frequency calculations have also been done in order to compare them with available experimental data. We show for the first time that complexes where CO₂ behaves globally as a Lewis base are stable in the case of ketones and esters, but not in the case of aldehydes, and their stability is similar to that of traditional complexes in which CO₂ behaves as a Lewis acid. This finding considerably modifies the concept of CO₂‐philicity and may have important ramifications in the development of green reactions in supercritical CO₂.     

On the Electronic Impact of Abnormal C4‐Bonding in N‐Heterocyclic Carbene Complexes

Sterically similar palladium dicarbene complexes have been synthesized that comprise permethylated dicarbene ligands which bind the metal center either in a normal coordination mode via C2 or abnormally via C4. Due to the strong structural analogy of the complexes, differences in reactivity patterns may be attributed to the distinct electronic impact of normal versus abnormal carbene bonding, while stereoelectronic effects are negligible. Unique reactivity patterns have been identified for the abnormal carbene complexes, specifically upon reaction with Lewis acids and in oxidative addition‐reductive elimination sequences. These reactivities as well as analytical investigations using X‐ray diffraction and X‐ray photoelectron spectroscopy indicate that the C4 bonding mode increases the electron density at the metal center substantially, classifying such C4‐bound carbene ligands amongst the most basic neutral donors known thus far. A direct application of this enhanced electron density at the metal center is demonstrated by the catalytic H₂ activation with abnormal carbene complexes under mild conditions, leading to a catalytic process for the hydrogenation of olefins.     

Quantum-chemical study of interaction of dipyrrolylmethenes with boron trifluoride and other lewis acids

The geometric parameters and energies of the products of donor-acceptor interaction of dipyrrolylmethenes with BF₃ and other inorganic Lewis acids were calculated by quantum-chemical methods. The bond nature and the energies of formation of the donor-acceptor complexes under consideration were analyzed. It was shown that the complexes with p-element fluorides are noticeably stabilized by hydrogen bonds involving the hydrogen atom of the NH group of dipyrrolylmethene and the nearest fluorine atom of a Lewis acid. Hydrogen bonding promotes further elimination of HF in the synthesis of boron fluoride complexes of dipyrrolylmethenes. The energy profile was calculated for the reaction of formation of the boron fluoride complex with dipyrrolylmethene through the intermediate donor-acceptor complex.     

Tertiary Amine and N‐Heterocyclic Carbene Coordinated Haloalanes—Synthesis,Structure, and Application

The preparation of several tertiary amine and N‐heterocyclic carbene coordinated chloro‐ and bromoalanes has been studied and routes to their gram‐scale synthesis optimized. This provides a catalogue of well‐characterized, thermally stable haloalanes for future application. All complexes have been investigated by spectroscopy (IR, NMR) and, where possible, single‐crystal X‐ray diffraction structure determination. A particular focus of this article is the relative thermal stabilities of the complexes, which provides a useful handle for the aerobic stability of Group 13 hydride complexes. These thermal data have been elucidated in full and rationalized relative to one another on the basis of Lewis base donation, steric shielding, and relative inductive halide strengthening of the aluminum hydride bonds by halides. All of the four‐coordinate complexes reported exist as distorted tetrahedra in the solid state with aluminum to N/C‐donor bonds that shorten with the increasing Lewis acidity of the aluminum Lewis acid. The five‐coordinate complexes [AlBrH₂(Quin)₂] and [AlBr₂H(Quin)₂] (Quin=quinuclidine) exist in a trigonal‐bipyramidal geometry in the solid state with the amine donors situated in the apical positions. Five chloroalanes; [AlClH₂(Quin)], [AlClH₂(Quin)₂], [AlCl₂H(Quin)₂], [AlClH₂(IMes)], and [AlCl₂H(IMes)] (IMes=1,3‐bis(2,4,6‐trimethylphenyl)imidazol‐2‐ylidene), the latter two of which are aerobically stable, have been applied to the hydroalumination of carbonyl and heterocycle substrates and their chemo‐, regio‐, and stereoselectivities compared to those of Group 13 hydride reagents cited in the literature. Overall, the reactivities of these species are comparable to non‐halogenated alane complexes with the additional benefit of aerobic stability, non‐pyrophoricity, and enhanced regioselectivity borne out of greater Lewis acidity.     

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.     

Alkali Metal Covalent Bonding in Nickel Carbonyl Complexes ENi(CO)3−

The alkali metal‐nickel carbonyl anions ENi(CO)₃^? with E=Li, Na, K, Rb, Cs have been produced and characterized by mass‐selected infrared photodissociation spectroscopy in the gas phase. The molecules are the first examples of 18‐electron transition metal complexes with alkali atoms as covalently bonded ligands. The calculated equilibrium structures possess C_3v geometry, where the alkali atom is located above a nearly planar Ni(CO)₃^? fragment. The analysis of the electronic structure reveals a peculiar bonding situation where the alkali atom is covalently bonded not only to Ni but also to the carbon atoms.     

Covalent Hypercoordination: Can Carbon Bind Five Methyl Ligands?

C(CH₃)₅⁺ is the first reported example of a five‐coordinate carbon atom bound only to separate (that is, monodentate) carbon ligands. This species illustrate the limits of carbon bonding, exhibiting Lewis‐violating “electron‐deficient bonds” between the hypercoordinate carbon and its methyl groups. Though not kinetically persistent under standard laboratory conditions, its dissociation activation barriers may permit C(CH3)₅⁺ fleeting existence near 0 K.     

Covalent Hypercoordination: Can Carbon Bind Five Methyl Ligands?

C(CH₃)₅⁺ is the first reported example of a five‐coordinate carbon atom bound only to separate (that is, monodentate) carbon ligands. This species illustrate the limits of carbon bonding, exhibiting Lewis‐violating “electron‐deficient bonds” between the hypercoordinate carbon and its methyl groups. Though not kinetically persistent under standard laboratory conditions, its dissociation activation barriers may permit C(CH3)₅⁺ fleeting existence near 0 K.     

Observation of Main‐Group Tricarbonyls [B(CO)3] and [C(CO)3]+ Featuring a Tilted One‐Electron Donor Carbonyl Ligand

A combined experimental and theoretical study on the main‐group tricarbonyls [B(CO)₃] in solid noble‐gas matrices and [C(CO)₃]⁺ in the gas phase is presented. The molecules are identified by comparing the experimental and theoretical IR spectra and the vibrational shifts of nuclear isotopes. Quantum chemical ab initio studies suggest that the two isoelectronic species possess a tilted η¹(μ₁‐CO)‐bonded carbonyl ligand, which serves as an unprecedented one‐electron donor ligand. Thus, the central atoms in both complexes still retain an 8‐electron configuration. A thorough analysis of the bonding situation gives quantitative information about the donor and acceptor properties of the different carbonyl ligands. The linearly bonded CO ligands are classical two‐electron donors that display classical σ‐donation and π‐back‐donation following the Dewar–Chatt–Duncanson model. The tilted CO ligand is a formal one‐electron donor that is bonded by σ‐donation and π‐back‐donation that involves the singly occupied orbital of the radical fragments [B(CO)₂] and [C(CO)₂]⁺.     

Stimuli‐Responsive Frustrated Lewis‐Pair‐Type Reactivity of a Tungsten Iminoazaphosphiridine Complex

Reactions of 3‐imino‐azaphosphiridine complexes 1 a,b with carbodiimides 2 a,b , isocyanates 3 a,b , and carbon dioxide are described. Whereas exchange of the carbodiimide unit occurs in the first case, an overall ring expansion takes place with phenyl isocyanate ( 3 a ) and carbon dioxide to yield complexes 4 and 5 bearing novel 1,3,5‐oxazaphospholane ligands; the isopropyl derivative 3 b did not react under these conditions. DFT calculations provide insight into the pathway of the reaction with carbon dioxide with model complex 1 c , revealing effects of initial non‐covalent interactions with the substrate onto the ring bonding, thus triggering an initially masked frustrated Lewis‐pair‐type behavior.     

Multiple Bonding Between Group 3 Metals and Fe(CO)3−

Heteronuclear Group 3 metal/iron carbonyl anion complexes ScFe(CO)₃^?, YFe(CO)₃^?, and LaFe(CO)₃^? are prepared in the gas phase and studied by mass‐selective infrared (IR) photodissociation spectroscopy as well as quantum‐chemical calculations. All three anion complexes are characterized to have a metal–metal‐bonded C_3v equilibrium geometry with all three carbonyl ligands bonded to the iron center and a closed‐shell singlet electronic ground state. Bonding analyses reveal that there are multiple bonding interactions between the bare group‐3 elements and the Fe(CO)₃^? fragment. Besides one covalent electron‐sharing metal–metal σ bond and two dative π bonds from Fe to the Group 3 metal, there is additional multicenter covalent bonding with the Group 3 atom bonded to Fe and the carbon atoms.     

Anti‐neurotoxic evaluation of synthetic and characterized metal complexes of thiosemicarbazone derivatives

Quinoline‐2‐caboxyaldehyde thiosemicarbazone (HL¹) and quinoline ‐2‐caboxyaldehyde N‐dimethyl thiosemicarbazone (HL²) metal complexes were prepared and characterized using analytical and spectroscopic techniques. The measurements showed that ligands behave as monovalent or neutral tridentate ligands bonding via azomethine, quinoline ring nitrogen atoms and sulfur atoms in thiol or thion forms. The anti‐neurotoxic effect of ligands and their complexes showed that, exposure to aluminum increase oxidative stress in the brain, an effect that could be offset by concomitant thiosemicarbazone complexes. Complexes could be having an effect on absorption or excretion of aluminum, due to their chelating activity. These findings may shed light on the potential clinical importance of thiosemicarbazone complexes in Alzheimer's disease.     

Interpnictogen Cations: Exploring New Vistas in Coordination Chemistry

Pnictine derivatives can behave as both 2e^? donors (Lewis bases) and 2e^? acceptors (Lewis acids). As prototypical ligands in the coordination chemistry of transition metals, amines and phosphines also form complexes with p‐block Lewis acids, including a variety of pnictogen‐centered acceptors. The inherent Lewis acidity of pnictogen centers can be enhanced by the introduction of a cationic charge, and this feature has been exploited in recent years in the development of compounds resulting from coordinate Pn–Pn and Pn–Pn′ interactions. These compounds offer the unusual opportunity for homoatomic coordinate bonding and the development of complexes that possess a lone pair of electrons at the acceptor center. This Review presents new directions in the systematic extension of coordination chemistry from the transition series into the p‐block.     

Tuning the Halogen/Hydrogen Bond Competition: A Spectroscopic and Conceptual DFT Study of Some Model Complexes Involving CHF2I

Insight into the key factors driving the competition of halogen and hydrogen bonds is obtained by studying the affinity of the Lewis bases trimethylamine (TMA), dimethyl ether (DME), and methyl fluoride (MF) towards difluoroiodomethane (CHF₂I). Analysis of the infrared and Raman spectra of solutions in liquid krypton containing mixtures of TMA and CHF₂I and of DME and CHF₂I reveals that for these Lewis bases hydrogen and halogen‐bonded complexes appear simultaneously. In contrast, only a hydrogen‐bonded complex is formed for the mixtures of CHF₂I and MF. The complexation enthalpies for the C?H ??? Y hydrogen‐bonded complexes with TMA, DME, and MF are determined to be ?14.7(2), ?10.5(5) and ?5.1(6) kJ mol^?1, respectively. The values for the C?I ??? Y halogen‐bonded isomers are ?19.0(3) kJ mol^?1 for TMA and ?9.9(8) kJ mol^?1 for DME. Generalization of the observed trends suggests that, at least for the bases studied here, softer Lewis bases such as TMA favor halogen bonding, whereas harder bases such as MF show a substantial preference for hydrogen bonding.     

Boron and other Triel Lewis Acid Centers: From Hypovalency to Hypervalency

MP2/aug‐cc‐pVTZ calculations were performed on triel trifluorides, ZF₃ (Z=B, Al, Ga, In, Tl), and their complexes with N₂ and HCN species, which are acting as Lewis bases. Interaction of the ZF₃ trigonal structure with a single HCN or N₂ ligand may lead to a tetrahedral structure with the tetravalent triel atom, whereas interaction of ZF₃ with two ligands results in a trigonal bipyramidal structure with a pentacoordinate Z atom. Consequently, the Z atom in ZF₃ compounds suffers from electron deficiency (hypovalency), it obeys the octet rule in ZF₃–NCH and ZF₃–N₂ moieties, and it may be considered as a hypervalent center in complexes of ZF₃ with two ligands. Much weaker interactions are observed for the boron complexes than for the other triel systems. This is because of the lower acidity of the B center relative to that of the other triel centers, and it may be the result of a stronger backbonding effect for BF₃ than for the other triel trifluorides. Interactions in aluminum complexes are characterized by meaningful electrostatic contributions, whereas for gallium complexes, the most important electron charge shift is observed as a result of complexation. Analysis of the geometry of the triel complexes and of the Z???N interactions (triel bonds) in these complexes is based on ab initio calculations as well as on the quantum theory of atoms in molecules and the natural bond orbitals method.     

Peroxo Complexes of Molybdenum(VI) Containing Mannich Base Ligands

The molybdenum(VI)-peroxo complexes containing Mannich base ligands having a formula as [MoO(O2)2(L-L)] [where L-L＝morpholinobenzyl benzamide (MBB), piperidinobenzyl benzamide (PBB), morpholinobenzyl urea (MBU), piperidinobenzyl urea (PBU), morpholinobenzyl thiourea (MBTU), piperdinobenzyl thiourea (PBTU)] have been synthesized and characterized by physico-chemical, electrochemical techniques and TGA/DTA studies. The complexes have been prepared by stirring ammonium molybdate and excess of 30% aqueous-H2O2 and then treatment with ethanolic solution of the ligand. Studies revealed that these complexes were non-electrolytes and diamagnetic in nature. The ligands are bound to metal in a bidentate mode through carbonyl oxygen/thiocarbonyl sulphur and the ring nitrogen. The cyclic voltammograms of the complexes show two quasi-reversible steps involving complexes. The complexes have also been tested for antibacterial activity against Salmonella and Kleibsella. The antibacterial study of the ligands and complexes indicate that the complexes exhibit higher activity than the free ligands.