Neutral Lewis Bases Research Articles

Yttrium Complexes with Group 13 Heterobenzene-Type Ligands.

The yttrium gallabenzene complex [(1-Me-3,5-tBu2 -C5 H3 Ga)(μ-Me)Y(2,4-dtbp)] is accessible from Y(GaMe4 )3 and K(2,4-dtbp) via a tandem salt metathesis/methane elimination (2,4-dtbp=2,4-di-tert-butyl-pentadienyl). The pentadienyl ligand in [(1-Me-3,5-tBu2 -C5 H3 E)(μ-Me)Y(2,4-dtbp)] (E=Al, Ga) is easily displaced by salt metathesis with KC5 Me5 and KTpMe,Me (TpMe,Me =tris(pyrazolyl-Me2 -3,5)borato) affording [(1-Me-3,5-tBu2 -C5 H3 E)(μ-Me)Y(TpMe,Me )] and [(1-Me-3,5-tBu2 -C5 H3 E)(μ-Me)Y(C5 Me5 )]. The yttrium center in [(1-Me-3,5-tBu2 -C5 H3 E)(μ-Me)Y(2,4-dtbp)] readily forms adducts with neutral Lewis bases like 4-DMAP (4-dimethylaminopyridine), PMe3 , DMPE (1,2-bis(dimethylphosphino)ethane), and DME (1,2-dimethoxyethane). In stark contrast, addition of TMEDA (N,N,N',N'-tetramethylethylenediamine) results in methyl/pentadienyl exchange between aluminum and yttrium resulting in [(1-(2,4-dtbp)-1-Me-3,5-tBu2 -C5 H3 Al)Y(Me)(tmeda)]. The bonding features of the newly synthesized complexes are analyzed by single-crystal X-ray diffraction (SCXRD) and heteronuclear (89 Y, 31 P) NMR spectroscopy.

Dynamic Metalloporphyrin-Based [2]Rotaxane Molecular Shuttles Stimulated by Neutral Lewis Base and Anion Coordination.

A series of dynamic metalloporphyrin [2]rotaxane molecular shuttles comprising of bis-functionalised Zn(II) porphyrin axle and pyridyl functionalised macrocycle components are prepared in high yield via active metal template synthetic methodology. Extensive variable temperature 1 H NMR and quantitative UV-Vis spectroscopic titration studies demonstrate dynamic macrocycle translocation is governed by an inter-component co-ordination interaction between the macrocycle pyridyl and axle Zn(II) metalloporphyrin, which serves to bias a 'resting state' co-conformation. The dynamic shuttling behaviour of the interlocked structures is dramatically inhibited by the addition of a neutral Lewis base such as pyridine, but can also be tuned via post-synthetic rotaxane demetallation of the porphyrin axle core to give free-base, or upon subsequent metallation, Ni(II) [2]rotaxane analogues. Importantly, the Lewis acidic Zn(II) porphyrin axle component is also capable of coordinating anions which induces mechanical bond shuttling behaviour resulting in a novel optical sensing response.

Bismuth Cations: Fluoride Ion Abstraction, Isocyanide Coordination, and Impact of Steric Bulk on Lewis Acidity.

The molecular compound [BiDipp2 (SbF6 )], containing the bulky, donor-free bismuth cation [BiDipp2 ]+ has been synthesized and fully characterized (Dipp=2,6-iPr2 -C6 H3 ). Using its methyl analog [BiMe2 (SbF6 )] as a second reference point, the impact of steric bulk on bismuth-based Lewis acidity was investigated in a combined experimental (Gutmann-Beckett and modified Gutmann-Beckett methods) and theoretical approach (DFT calculations). Reactivity studies of the bismuth cations towards [PF6 ]- and neutral Lewis bases such as isocyanides C≡NR' revealed facile fluoride ion abstraction and straightforward Lewis pair formation, respectively. The first examples of compounds featuring bismuth-bound isocyanides have been isolated and fully characterized.

Cationic Symmetrically and Unsymmetrically Substituted Diboranes and Bis(diboranes) with Direct Boron‐Boron Bond: Synthesis by Substitution, Stability and Properties

AbstractStarting with diboranes with two electron‐rich bridging bicyclic guanidinate substituents, we report in this work the rational synthesis of new dicationic symmetrically‐ and unsymmetrically‐substituted diboranes in SN1‐type substitution reactions in which triflato or bromo substituents are replaced by neutral Lewis bases. The scope of such substitution reactions and their rate are analyzed with different pyridine derivatives of variable Lewis basicity. The first substitution step, leading to a monocationic diborane with one anionic substituent (triflate or bromide) and one neutral Lewis base, proceeds much faster than the second substitution step leading to a dicationic diborane with two neutral Lewis bases. The different time scales for the substitution steps could be used to conveniently synthesize in one‐pot reactions several dicationic, unsymmetrically‐substituted diboranes with two different neutral Lewis bases.

Probing the Potential of Hitherto Unexplored Base-Stabilized Borylenes in Dinitrogen Binding.

Computational investigations were carried out to probe the potential of several dicoordinate, singly base-stabilized borylenes of the form [L→BR] (L=neutral Lewis base) in dinitrogen binding. The calculated reaction free energies and activation barriers associated with the formation of mono- and diborylene-N2 adducts suggest the presence of thermally surmountable kinetic barriers towards their possible isolation. Our results show that the exergonicity of dinitrogen activation and fixation is linearly dependent on the natural charge at the boron center, which can be tuned to design novel boron-based compounds with potential applications to small-molecule activation. EDA-NOCV analysis reveals strong binding of dinitrogen to these base-stabilized borylenes.

Cyclic (Amino)(Aryl)Nitrenium Cations with Lewis Acidity Controlled by Remote Substituents

Comprehensive SummaryN‐Heterocyclic nitrogen Lewis acids are important in synthetic chemistry. Stable cyclic (amino)(aryl)nitrenium cations, cyc‐1+—cyc‐3+, were synthesized by chemical oxidation of aryl azo compounds with different substituents, iPr, H and I, at the para positions of the phenyl group. The excited triplet states of cyc‐1+—cyc‐3+ abstract H‐atoms step by step to generate radical intermediates cyc‐1H•+—cyc‐3H•+ traced by EPR spectroscopy and products cyc‐1H2+—cyc‐3H2+ characterized by single crystal X‐ray diffraction. The Lewis acidity of species cyc‐1+—cyc‐3+ are remote substituent‐dependent. Cyc‐2+—cyc‐3+ have much higher acidity than those previously reported congeners based on energies of LUMOs. The electrophilicity enables them to form Lewis adducts with neutral Lewis base Me3P, and to gain one‐electron to produce neutral radicals cyc‐1•—cyc‐3•.

Halogen bonding in the co-crystallization of potentially ditopic diiodotetrafluorobenzene: a powerful tool for constructing multicomponent supramolecular assemblies.

Halogen bonding is emerging as a significant driving force for supramolecular self-assembly and has aroused great interest during the last two decades. Among the various halogen-bonding donors, we take notice of the ability of 1,4-diiodotetrafluorobenzene (1,4-DITFB) to co-crystallize with diverse halogen-bonding acceptors in the range from neutral Lewis bases (nitrogen-containing compounds, N-oxides, chalcogenides, aromatic hydrocarbons and organometallic complexes) to anions (halide ions, thio/selenocyanate ions and tetrahedral oxyanions), leading to a great variety of supramolecular architectures such as discrete assemblies, 1D infinite chains and 2D/3D networks. Some of them act as promising functional materials (e.g. fluorescence, phosphorescence, optical waveguide, laser, non-linear optics, dielectric and magnetism) and soft materials (e.g. liquid crystal and supramolecular gel). Here we focus on the supramolecular structures of multicomponent complexes and their related physicochemical properties, highlight representative examples and show clearly the main directions that remain to be developed and improved in this area. From the point of view of crystal engineering and supramolecular chemistry, the complexes summarized here should give helpful information for further design and investigation of the elusive category of halogen-bonding supramolecular functional materials.

Alkali Metal Cation Affinities of Neutral Maingroup-Element Hydrides across the Periodic Table.

We have carried out an extensive quantum chemical exploration of gas-phase alkali metal cation affinities (AMCAs) of archetypal neutral bases across the periodic system using relativistic density functional theory. One objective of this work is to provide an intrinsically consistent set of values of the 298 K AMCAs of all neutral maingroup-element hydrides XHn of groups 15–18 along the periods 1–6. Our main purpose is to understand these trends in terms of the underlying bonding mechanism using Kohn–Sham molecular orbital theory together with a canonical energy decomposition analysis (EDA). We compare the trends in XHn AMCAs with the trends in XHn proton affinities (PAs). We also examine the differences between the trends in AMCAs of the neutral XHn bases with those in the corresponding anionic XHn–1– bases. Furthermore, we analyze how the cation affinity of our neutral Lewis bases changes along the group-1 cations H+, Li+, Na+, K+, Rb+, and Cs+.

On the Aggregation and Sensing Properties of Zinc(II) Schiff-Base Complexes of Salen-Type Ligands.

The zinc(II) ion forms stable complexes with a wide variety of ligands, but those related to Schiff-bases are among the most largely investigated. This review deals with the peculiar aggregation characteristics of Zn(II) Schiff-base complexes from tetradentate N2O2 salen-type ligands, L, derivatives from salicylaldehydes and 1,2-diamines, and is mostly focused on their spectroscopic properties in solution. Thanks to their Lewis acidic character, ZnL complexes show interesting structural, nanostructural, and aggregation/deaggregation properties in relation to the absence/presence of a Lewis base. Deaggregation of these complexes is accompanied by relevant changes of their spectroscopic properties that can appropriately be exploited for sensing Lewis bases. Thus, ZnL complexes have been investigated as chromogenic and fluorogenic chemosensors of charged and neutral Lewis bases, including cell imaging, and have shown to be selective and sensitive to the Lewis basicity of the involved species. From these studies emerges that these popular, Lewis acidic bis(salicylaldiminato)Zn(II) Schiff-base complexes represent classical coordination compounds for modern applications.

Nitrogen-Based Lewis Acids: Synthesis and Reactivity of a Cyclic (Alkyl)(Amino)Nitrenium Cation.

A room-temperature-stable crystalline cyclic (alkyl)(amino)nitrenium cation 2 features cationic nitrogen atom with a smaller HOMO-LUMO gap compared to that of a 1,2,3-triazolium 5 (an N-heterocyclic nitrenium cation). The low-lying LUMO of 2 results in an enhanced electrophilicity, which allowed for the formation of Lewis adducts with neutral Lewis bases, such as Me3 P, nBu3 P, and IiPr. The N-based Lewis acid 2 also forms an FLP with tBu3 P but subsequently reacts with (PrS)2 to cleave the S-S bond. Both experimental and theoretical results suggest that the Lewis acidity of 2 is stronger than its N3 analogues.

Nitrogen‐Based Lewis Acids: Synthesis and Reactivity of a Cyclic (Alkyl)(Amino)Nitrenium Cation

AbstractA room‐temperature‐stable crystalline cyclic (alkyl)(amino)nitrenium cation 2 features cationic nitrogen atom with a smaller HOMO–LUMO gap compared to that of a 1,2,3‐triazolium 5 (an N‐heterocyclic nitrenium cation). The low‐lying LUMO of 2 results in an enhanced electrophilicity, which allowed for the formation of Lewis adducts with neutral Lewis bases, such as Me3P, nBu3P, and IiPr. The N‐based Lewis acid 2 also forms an FLP with tBu3P but subsequently reacts with (PrS)2 to cleave the S−S bond. Both experimental and theoretical results suggest that the Lewis acidity of 2 is stronger than its N3 analogues.

Characterization of chalcogen bonding interactions via an in-depth conceptual quantum chemical analysis.

The chalcogen bond has been acknowledged as an influential noncovalent interaction (NCI) between an electron-deficient chalcogen (donor) and a Lewis base (acceptor). This work explores the main features of chalcogen bonding through a large-scale computational study on a series of donors and acceptors spanning a wide range in strength and character of this type of bond: (benzo)chalcogenadiazoles (with Ch = Te/Se/S) versus halides and neutral Lewis bases with O, N, and C as donor atoms. We start from Pearson's hard and soft acids and bases (HSAB) principle, where the hard nature of the chalcogen bond is quantified through the molecular electrostatic potential and the soft nature through the Fukui function. The σ-holes are more pronounced when going down in the periodic table and their directionality matches the structural orientation of donors and acceptors in the complexes. The Fukui functions point toward an n→σ*-type interaction. The initial conjectures are further scrutinized using quantum mechanical methods, mostly relating to the systems' electron density. A Ziegler-Rauk energy decomposition analysis shows that electrostatics plays a distinctly larger role for the soft halides than for the hard, uncharged acceptors, associated with the softness matching within the HSAB principle. The natural orbital for chemical valence analysis confirms the n→σ* electron donation mechanism. Finally, the electron density and local density energy at the bond critical point in the quantum theory of atoms in molecules study and the position of the spikes in the reduced density gradient versus density plot in the NCI theory situate the chalcogen bond in the same range as strong hydrogen bonds. © 2017 Wiley Periodicals, Inc.

Nucleophilic addition and substitution at coordinatively saturated boron by facile 1,2-hydrogen shuttling onto a carbene donor.

The reaction of [(cAACMe)BH3] (cAACMe = 1-(2,6-iPr2C6H3)-3,3,5,5-tetramethylpyrrolidin-2-ylidene) with a range of organolithium compounds led to the exclusive formation of the corresponding (dihydro)organoborates, Li+[(cAACMeH)BH2R]- (R = sp3-, sp2-, or sp-hybridised organic substituent), by migration of one boron-bound hydrogen atom to the adjacent carbene carbon of the cAAC ligand. A subsequent deprotonation/salt metathesis reaction with Me3SiCl or spontaneous LiH elimination yielded the neutral cAAC-supported mono(organo)boranes, [(cAACMe)BH2R]. Similarly the reaction of [(cAACMe)BH3] with a neutral donor base L resulted in adduct formation by shuttling one boron-bound hydrogen to the cAAC ligand, to generate [(cAACMeH)BH2L], either irreversibly (L = cAACMe) or reversibly (L = pyridine). Variable-temperature NMR data and DFT calculations on [(cAACMeH)BH2(cAACMe)] show that the hydrogen on the former carbene carbon atom exchanges rapidly with the boron-bound hydrides.

N‐Heterocyclic Carbene (NHC) Adducts of [(C5H5)2Mn] Revisited: Nothing More, Nothing Less than High‐Spin (S = 5/2) Molecules!

Addition of N‐heterocyclic carbenes [L = 1,3‐di‐tert‐butylimidazolin‐2‐ylidene (ItBu), 1,3‐diisopropyl‐4,5‐dimethylimidazolin‐2‐yildene (IiPrMe2), 1,3‐di‐mesitylimidazolin‐2‐yildene (IMes), and 1,3‐bis(2,6‐diisopropylphenyl)imidazolin‐2‐yildene (IPr)] to manganocene [(C5H5)2Mn] forms the neutral Lewis base adducts [(C5H5)2Mn(L)] (2–5) in moderate yields. Complexes 2–4 were structurally characterized and their solid‐state magnetism was investigated between T = 2.6–300 K. In contrast to literature reports, these materials exclusively adopt a high‐spin (S = 5/2) ground state.

Rationalizing and Disrupting Fluxional Processes in Agostically Stabilized 14‐Electron Alkyliridium Hydride Complexes

AbstractWe have investigated the fundamental patterns of reactivity for the cationic IrIII complexes [(6‐Mes)(6‐Mes′)Ir(H)]+ and [(7‐Mes)(7‐Mes′)Ir(H)]+ [6‐Mes, 7‐Mes = ring‐expanded N‐heterocyclic carbene (NHC) ligands, ′ indicates that NHC is tethered through an iridium‐bound CH2 group], each of which undergoes degenerate fluxional exchange between an iridium‐bound agostic C–H bond and the discrete alkyl/hydride ligands resulting from C–H activation. A comparison between the activation parameters ΔH‡ and ΔS‡ determined by variable‐temperature NMR (VT‐NMR) spectroscopy line‐shape analysis and those obtained from DFT calculations on model systems offers a basis to discriminate between different possible exchange mechanisms, and an oxidative addition pathway gives the lowest barrier and also the best agreement between experiment and theory. The addition of neutral Lewis base donors to [(6‐Mes)(6‐Mes′)Ir(H)]+ resulted in the displacement of the weak agostic C–H···Ir interaction and the generation of static (alkyl)iridium(III) hydride complexes such as [(6‐Mes)(6‐Mes′)Ir(H)(CNtBu)2]+. The analogous carbonyl complex is more labile, with the enhanced π‐acceptor properties of the ancillary ligands leading to C–H reductive elimination and to the formation of the IrI complex [(6‐Mes)2Ir(CO)2]+, which incorporates unactivated NHC donors and has the ability to reversibly take up H2.

‘Counting ions’ in Alfred Werner’s coordination chemistry using electrical conductivity measurements

From 1888 till 1892 Alfred Werner, the founder of coordination chemistry, developed together with his friend Arturo Miolati electrical conductivity of ionic complexes as an auxilliary analytical tool for structural elucidations of complexes. The used electrical conductivity device was based on a design of Wilhelm Ostwald consisting of a generator for alternating current (induction apparatus with Wagner hammer), a measuring cell with platinated electrodes and a rheostatic part including a buzzer for balancing resistivity conditions. Electrical conductivities were examined in the ion-isomeric series of Pt(II)(NH3)nCl2 (n = 0 - 4), Pt(IV)(NH3)nCl4 (n = 0 - 6) and Co(III)(NH3)nCl3 (n = 0 - 6) complexes producing approximately V-shaped curves in dependence of the stoichiometry factor n. Replacing in the coordination spheres neutral Lewis base type ligands by ‘anionic residues’ generated charged complexes, which by conductivity measurements laid grounds for Alfred Werner’s coordination theory [primary (from ‘anionic residues’) and secondary valencies (from Lewis bases)] and the Nobel Prize in 1913. From 1893 on seven PhD theses were prepared in Alfred Werner’s group, which dealt with conductivity measurements establishing identification processes of complexes by ‘ion counting’. In 1902 Alfred Werner ceased to apply electrical conductivity in his group switching to meanwhile more timely coordination chemistry fields.

Insights into the Formation of Borabenzene Adducts via Ligand Exchange Reactions and TMSCl Elimination from Boracyclohexadiene Precursors

The bonding properties of borabenzene with various neutral Lewis bases have been investigated. 1-Chloro-4-isopropyl-2-(trimethylsilyl)-2,4-boracyclohexadiene reacts with a number of Lewis bases—notably pyridine, PMe3, PCy3, and PPh3—to afford 4-isopropylborabenzene–base adducts. These adducts can undergo ligand exchange to afford new borabenzene complexes. The scope and mechanism of the reaction, as well as the steric and electronic properties of different adducts, were studied experimentally and computationally.

A cationic aqua complex of an orthoplatinated primary amine – A versatile intermediate for derivatization

In the presence of silver salts of weakly or non-coordinating anions, the iodo ligand in a cycloplatinated complex of phenylethylamine may be replaced by a coordinated water molecule. The resulting cationic aqua complex may either be isolated in the form of its tetrafluoroborate or perchlorate salt or prepared in situ. In subsequent derivatization reactions, the aqua ligand may be substituted under mild conditions. Two classes of cycloplatinated primary amines, interesting in view of their potential cytostatic properties, are accessible via this useful intermediate: Its aqua ligand may be displaced either by (pseudo)halides or by neutral donor ligands. With the anionic (pseudo)halides, neutral water-insoluble coordination compounds are obtained: The corresponding isocyanato complex has been structurally characterized, and we have converted the original iodo complex into a radiolabeled derivative. With neutral Lewis bases, cationic complexes of moderate solubility are formed; we report the crystal structure of the 3,5-lutidine derivative.

Vinylogous Aldol Reaction of a Silyloxydiene Promoted by Sulfoxides, as Lewis Bases: 1,4–asymmetric Induction Mediated by a Remote Methylsulfinyl Group

A versatile procedure for the γ –vinylogous aldol reaction of the dioxinone–derived silyl enolether, via enolate activation with an appropriate neutral Lewis base, was formulated. High levels of diastereo– and enantioselectivity using chiral 2–methylsulfinyl benzaldehyde were obtained, pointing out the dual role of the methylsulfinyl group as incorporated chiral inductor and activator of the silyloxydiene in the stereoselective vinylogous aldol reaction.

Synthesis and Lewis acid properties of a ferrocene-based planar-chiral borenium cation

The first planar chiral ferrocenylborenium species (pR)-3(+) is obtained in the enantiopure form by halide abstraction from the corresponding chloroborane adduct (pR)-2 using Krossing's salt. Competition experiments suggest that the Lewis acidity of (pR)-3(+) is higher than that of B(C6F5)3 towards anions and slightly lower towards neutral Lewis bases. The ferrocenylborenium species (pR)-3(+) is examined as a catalyst for the stereoselective hydrosilylation of ketones.