Nonbonding Electron Pairs Research Articles

Synthesis, Complexation Behaviors, and Crystal Structures of Azamacrocycles Having Tropon-2-yl Groups

Abstract Azamacrocycles 1–3, having one to three tropon-2-yl groups, were found to display unique absorption spectral changes in the presence of guest cations. Aza-15-crown-5 1, 1,7-diaza-12-crown-4 2a, 1,7-diaza-15-crown-5 2b, 1,10-diaza-18-crown-6 2c, and 1,5,9-triazacyclododecane 3 derivatives showed Ba2+, Zn2+, Ca2+, Ba2+, and Zn2+ selectivities, respectively. The crystal structures of free hosts 2a and 2c, the calcium thiocyanate complexes of 1 and 2c, the barium thiocyanate complex of 1, and the zinc thiocyanate complex of 2a were elucidated by X-ray analysis. The 2a·zinc and 2c·calcium thiocyanate complexes consist of an ion pair with a zinc tetrathiocyanate anion and a 2a·zinc cation and with a calcium hexathiocyanate anion and a 2c·H2O·calcium cation, respectively. The aminotropone moiety of 2a and 2c forms a conjugated 10-electron system. Complexation gave rise to deconjugation between the tropone ring and the nonbonding electron pair of the nitrogen atom to cause absorption spectral changes.

The cyanoimido ligand as an oxo analogue. Novel approaches to the preparations of cyano(imino)-aza-phosphorus(V) and N-cyanoaziridine.

The known Os(IV)-cyanoimido complexes, mer-Et4N[OsIV(bpy)(Cl)3(NalphaCNbeta)] (mer-[OsIV=N-CN]-) (bpy = 2,2'-bipyridine) and trans-[OsIV(tpy)(Cl)2(NalphaCNbeta)] (trans-[OsIV=N-CN]) (2,2':6',2' '-terpyridine), have formal electronic relationships with high oxidation state Ru and Os-oxo and -dioxo complexes. These include multiple bonding to the metal, the ability to undergo multiple electron transfer, and the availability of nonbonding electron pairs for donation. Thermodynamic, oxo-like behavior is observed for mer-[OsIV=N-CN]- in the pH-dependence of its Os(VI/V) to Os(III/II) redox couples in 1:1 (v/v) CH3CN:H2O. Oxo-like behavior is also observed in the reaction between mer-[OsVI(bpy)(Cl)3(NalphaCNbeta)]PF6 and benzyl alcohol to give mer-[OsIV(bpy)(Cl)3(NalphaCNbetaH2)]PF6 and benzaldehyde. The reaction is first order in each reactant with kbenzyl(CH3CN, 25.0 +/- 0.1 degrees C) = (8.6 +/- 0.2) x 102 M-1 s-1. Formal NCN degrees transfer, analogous to O-atom transfer, occurs in reactions with tertiary phosphine and hexenes. In CH3CN under N2, a rapid reaction occurs between trans-[OsIV=N-CN] and PPh3 (kPPh3(DMF, 25.0 +/- 0.1 degrees C) = 4.06 +/- 0.02 M-1 s-1) to form the nitrilic N-bound Os(II)-(N-cyano)iminophosphorano product, trans-[OsII(tpy)(Cl)2(NalphaCNbetaPPh3)] (trans-[OsII-NalphaC-Nbeta=PPh3]). It undergoes solvolysis at 45 degrees C after 24 h to give trans-[OsII(tpy)(Cl)2(NCCH3)] and (N-cyano)iminophosphorane (NalphaC-Nbeta=PPh3). The analogue to epoxidation, N-cyanoaziridination of cyclohexene and 1-hexene by mer-[OsIV=N-CN]- and trans-[OsIV=N-CN], occurs at Nbeta to give the Os(IV)-N-cyanoaziridino complexes, mer-Et4N[OsII(bpy)(Cl)3(NalphaCNbetaC6H10)] and trans-[OsII(tpy)(Cl)2(NalphaCNbetaC6H11)], respectively. Oxidation to mer-[OsV(bpy)(Cl)3(NalphaCNbeta)]- greatly accelerates N-cyanoaziridination of cyclohexene, which is followed by slow solvolysis to give mer-[OsIII(bpy)(Cl)3(NCCH3)] and N-cyanoaziridine (NC-NC6H10). The Os-(N-cyano)aziridino complexes are the first well-characterized examples of coordinated cyanoaziridines.

Nonoctahedral structures.

There is always a special reason when six-coordinate molecules or complexes are nonoctahedral. The Jahn-Teller distortion is the longest known example. Also well understood now is the steric activity or nonactivity of a nonbonding electron pair, for example XeF(6). In the past few years, it became obvious that six-coordinate d(0) and d(1) complexes with sigma only ligands such as Mo(CH3)(6) are far from octahedral; rather, they are trigonal prismatic or even C(3v) distorted trigonal prismatic. This phenomenon can be explained with simple molecular orbital or valence bond models.

The role of non-bonding electron pairs in intermetallic compounds.

The Electron Localisation Function, ELF pictorially visualises chemists' intuitive ideas of single and multiple bonds as well as non-bonding electron pairs in molecules. The power of the representation of chemical bonds via ELF is that on the one hand covalent, polar, and ionic bonds are distinguishable, and that on the other hand ELF can be calculated for molecules and solids. This enables us to transfer the ideas of chemical bonding from molecular to intermetallic compounds. Localised two-electron-two-centre bonds and lone pairs are present in solid-state valence compounds (Zintl phases) as expected by the 8-N rule. In solids, lone pairs are generally more contracted than in molecules due to 'lone-pair repulsion'. In intermetallic compounds localised electrons predominantly occur in the form of lone pairs. Lattice vibrations influence the strength of lone pair interactions and non-bonded interactions lead to an exchange of delocalised and localised electrons. Such a mechanism of local electron pair formation gives rise to ideas of a chemical view of the phenomenon of superconductivity in intermetallic compounds.

The nature of topological parameters. II. The composition of topological parameters.

The composition of thirty topological parameters of various types including the chi, kappa and tau parameters and the Wiener indices was determined. Those derived from alkyl and substituted alkyl groups are a function of branching and steric effects; those derived from haloalkanes and haloalkenes are a function of some combination of polarizability, dipole moment, the number of halogen atoms of each type, and the ration of the number of branches to the number of atoms in the longest chain. It follows that topological parameters are actually counts of the number of atoms of each type, bonds of each type, valence, nonbonding and core electrons, branches at each position, OH and NH bonds, and nonbonding electron pairs on O and/or N atoms. These counts can be related to polarizability, dipole moment, hydrogen bonding, and steric effects, which are the true determinants of properties and biological activities. Topological parameters themselves do not have a cause and effect relationship with the quantities they are used to model. They function because: 1. They are counts of quantities that are related to the fundamental quantities that determine properties. 2. They are composite parameters which for the purpose of obtaining a predictive empirical relationship are able to represent a data set as well as pure parameters.

Axial and equatorial hydrogen bonds: jet-cooled rotational spectrum of the pentamethylene sulfide...hydrogen fluoride complex.

Two different axial and equatorial hydrogen-bonded conformers of the complex formed by pentamethylene sulfide and hydrogen fluoride have been generated in a pulsed supersonic expansion and characterised by means of Fourier transform microwave spectroscopy. The ground-state rotational spectra of six isotopomers (C(5)H(10)S...HF, C(5)H(10)S ...DF, C(5)H(10)(34)S ...HF, (13)C(alpha)C(4)H(10)S ...HF, (13)C(beta)C(4)H(10)S...HF and (13)C(gamma)C(4)H(10)S ...HF) have been analysed for both conformers in the frequency range 5.5-18.5 GHz. The rotational parameters were used to derive C(s) structures for the conformers, with hydrogen fluoride pointing to the domain of the nonbonding electron pairs at either the axial or equatorial position of the sulfur atom. The axial form was found to be the more stable, in contrast with the observation for the pentamethylene sulfide...HCl complex. No equatorial-to-axial relaxation was observed when He or Ar were used as the carrier gas. The conformational behaviour is compared with that of related six-membered rings and discussed in terms of the existence of secondary hydrogen bonding between the halogen atom and the nearest H atoms of the methylene groups of the ring. No significant structural distortion of pentamethylene sulfide upon complexation was detected from a comparison with the structure of the isolated monomer. Finally, an ab initio study was carried out to complement the experimental results.

Natural J coupling (NJC) analysis of the electron lone pair effect on NMR couplings: 2. The anomeric effects on 1 J (C, H) couplings and its dependence on solvent

The electronic origin of the influence of the anomeric effect (negative hyperconjugative interaction, NHI) on the Fermi contact (FC) term of 1 J(C, H) couplings has been studied from a theoretical point of view at the DFT-B3LYP level. The HN=CH2, molecule was chosen as the primary model compound, in which both FC 1 J(C, H) couplings were decomposed into bond contributions with the natural J coupling dissection approach (NJC). Differences between the 1 J (C, H)FC couplings for C——H bonds in synperiplanar and antiperiplanar orientations with respect to the nitrogen non-bonding electron pair closely follow the experimental trend. They are made up chiefly of three NJC contributions: ‘bond’, ‘direct lone pair’ and the ‘carbon-core orbitals’. The NHI influence on these terms was studied by applying the natural bond orbital (NBO) deletion procedure to the charge transfer interaction into the antiperiplanar (C——H) antibond (n(N)→(C——H)*) prior to the NJC dissection calculation. The dielectric solvation effect on both the total FC terms and the respective NJC contributions was estimated by carrying out the calculations using the polarization continuum model. Inhibition of the anomeric effect is evident when the solvent polarity is increased. NHI saturates rapidly with increasing solvent dielectric. Specific solute-solvent interaction effects on 1 J(C, H) couplings were estimated by evaluating molecular complex models of the form CH2=HN…S (S = H2O and DMSO).

Constraining of small-ring cyclic ether triads by stereodefined spiroannulation to an inositol orthoformate platform. Solution- and gas-phase alkali metal binding affinities for three- to five-membered ring structural combinations.

The structural features most conducive to complexation of the alkali metal ions Li(+), Na(+), and K(+) in a series of constrained inositol orthoformate derivatives have been probed in solution, in the solid state, and in the gas phase by electrospray ionization mass spectrometry. The eight spirotricyclic polyethers differ in the size of the rings containing the potentially ligating oxygen atoms. Although the ring sizes have been limited to three to five atoms inclusively, the combinations of oxirane, oxetane, and tetrahydrofuran are rather extensive and consist of many options. The overall trend for lithium ion affinity is [5.5.5] > [ 5.5.4] > [4.4.4] > [5.5.3] > [5.4.3] > [4.4.3] > [1.1.1] > [3.3.1], an ordering that correlates with the differing polarizabilities of the oxygen atoms, ease of alignment of the nonbonded electron pairs, and the overall size of the ligand as gauged by nonbonded O......O distances.

Hydrogen desorption from polycrystalline platinum chemically modified by sulfur pre-coverage

Hydrogen desorption energetics from clean polycrystalline platinum (Pt(Poly)) and (Pt(Poly)) with sulfur (S) pre-coverage (S/Pt(Poly)) have been studied using temperature programmed desorption (TPD) over a wide range of hydrogen and S coverages. Hydrogen desorption energies have been calculated and tabulated. At saturation, hydrogen desorbs from clean Pt(Poly) in three states (384, 320, and 282 K). The 384 K state is attributed to hydrogen from defect sites. The 320 and 282 K states are both attributed to hydrogen from (1 1 1) sites, the latter state induced by repulsive lateral interaction between hydrogen in neighboring sites. On clean Pt(Poly), the desorption energy for hydrogen from defect sites is essentially fixed over the entire coverage range, dilute to saturation. In contrast, the desorption energy for hydrogen from (1 1 1) sites begins to decrease when about 50% of the surface is saturated. The decrease onset is attributed to significant accumulation of hydrogen in fcc three-fold hollows relative to hcp hollows. With increasing hydrogen and S coverages, the energies of desorption converge to 4.5 kcal/mol, the barrier height to hydrogen diffusion on (1 1 1) terraces of d-band transition metal surfaces via classical over-the-barrier hopping. Therefore, increasing S coverage has the same effect on the binding strength of Pt( Poly) for hydrogen as increasing hydrogen coverage (site blocking). But more interestingly, at high surface coverage, atomic hydrogen recombination immediately after hopping out of the adsorption well without surface diffusion is indicated. Though, S site blocking has been found as the primary cause of decreased hydrogen desorption temperatures, several trends in the data cannot be rationalized using a pure site blocking argument alone. Sulfur induced additional repulsive lateral interaction at high coverage is strongly suggested, through-space, short range electronic effects. Further, long range S effects (through space) are also clearly supported. The uniform percent decrease in hydrogen adsorption independent of exposure is not consistent with pure site blocking but can be readily explained by a decreased sticking coefficient on S/Pt(Poly) surfaces due to long range S effects. In particular, the strong inhibition of hydrogen adsorption on S/Pt(Poly) surfaces (the long range effect) is attributed mostly to electrostatic repulsion of approaching hydrogen caused by non-bonding electron pairs in hybrid valence sulfur orbitals normal to the surface. Insight has also been gained into the dynamics of hydrogen adsorption. The study of fixed hydrogen exposures as a function of S coverage shows zero order kinetics for the initial rate of desorption of the 320 K state, i.e. the leading edges are `pinned'. Thus, hydrogen adsorption on Pt(Poly) in patches nucleated at defects that attain similar density independent of coverage is implied. Key steps in the adsorption and desorption trajectories as indicated by this TPD work are illustrated in a schematic.

Calculational study of the protonation of BXH(2) and BX(2)H (X = F and Cl). structures of BXH(3)(+) and BX(2)H(2)(+) and their dihydrogen complexes BXH(5)(+) and BX(2)H(4)(+).

Ab initio calculations on monoprotonated BFH2 indicate that the B−H protonated 1a is 9.7 kcal/mol more stable than the F-protonated 1b. Thus the bonded electron pair of the B−H bond is a better donor than the nonbonded fluorine electron pair in BFH2.

Synthesis and characterization of non-centrosymmetric TeSeO4.

The synthesis, crystal structure, and characterization of a non-centrosymmetric oxide, TeSeO4, are reported. The material was synthesized by combining TeO2 and SeO2 in a quartz tube and heating at 370 degrees C. TeSeO4 has a three-dimensional structure containing [SeO(3/2)](+) cations linked to [TeO(5/2)](-) anions. Both the Se(4+) and the Te(4+) atoms are in asymmetric environments owing to their nonbonded electron pair. The material is SHG active, with a SHG intensity of 400 times SiO(2). Crystal data: monoclinic, space group Ia (No. 9, cell choice 3), with a = 4.3568(8) A, b = 12.465(3) A, c = 6.7176(15) A, beta = 90.825(4) degrees, and Z = 4.

Electrocatalytic response of amino acids at CuMn alloy electrodes

Abstract The anodic voltammetric responses obtained for ethylenediamine and representative amino acids in 0.10 M NaOH (>0.5 V vs. SCE) are significantly larger at preanodized CuMn (95:5) and CuMn (90:10) rotated disk electrodes as compared to the responses at preanodized pure Cu electrodes. The larger response at the CuMn electrodes is demonstrated to result from increased values of the apparent number of electrons transferred (napp) and/or the apparent heterogeneous rate constants (kapp) for the corresponding anodic reactions at these alloy electrodes. It is speculated that these effects are the result of enhanced adsorption of the reactant species at Mn-sites within the response mechanisms at the preanodized CuMn electrode surfaces. We speculate further, that the Mn-sites, whose 3d-orbital occupancy is lower than that of Cu-sites, can adsorb the reactant species via the nonbonded electron pairs of their amine moieties in the alkaline media. The electrocatalytic benefit of adsorption is anticipated to come as a result of an increased lifetime of the reactant species within the applied interfacial potential at the electrode ∣ solution interface.

Solvatochromism of Heteroaromatic Compounds: VI. Comparison of the Empirical and Theoretical Approaches to Description of Solvatochromism in Nonspecific Solvation of Nitropyrazoles

The effect of nonspecific solvation on the long-wave absorption band in the UV spectra of isomeric nitropyrazoles was studied. The aprotic inert and aprotic protophilic solvents exert different spectroscopic effects. In the former solvents, the solvatochromism is mainly governed by orientation interactions, and in the latter solvents, along with the orientation interactions, the UV spectrum is appreciably influenced by the induction, polarization, and dispersion interactions, which is due to the presence of readily polarizable nonbonding electron pairs and/or a pi-electron system in molecules of such solvents.

Synthesis and Structure of a Monomeric Aluminum(I) Compound [{HC(CMeNAr)2 }Al] (Ar=2,6-iPr2 C6 H3 ): A Stable Aluminum Analogue of a Carbene.

A monovalent aluminum monomer [{HC(CMeNAr)2}Al] was obtained by reduction of the corresponding aluminum(III) diiodide. The molecular structure shows a unique two-coordinate aluminum center and a planar heterocyclic Al-N-C-C-C-N six-membered ring system (see picture). A nonbonded lone pair of electrons on the Al atom indicates the Lewis base character of the aluminum center. Ar=2,6-iPr2C6H3.

Diprotonated Hydrogen Halides (H3X2+) and Gitonic Protio Methyl- and Dimethylhalonium Dications (CH3XH22+ and (CH3)2XH2+): Theoretical and Hydrogen−Deuterium Exchange Studies1

Density functional theory (DFT) calculations at the B3LYP/6-31G** level were performed to investigate geometries and energies of a series of diprotonated hydrogen halides (H3X2+) and gitonic protio methyl- and dimethylhalonium dications (CH3XH22+ and (CH3)2XH2+). On the basis of computed energies, proton affinities and related thermodynamic parameters were also calculated. A hydrogen/deuterium exchange experiment of dimethylbromonium ion, (CH3)2Br+, with an excess of 4:1 DF/SbF5 superacid indicated lack of exchange at the methyl group supporting protonation of the nonbonded electron pair on the bromine leading to the involvement of gitonic protodimethylbromonium dication. A similar hydrogen/deuterium exchange experiment was also carried out on dimethylchloronium ion, (CH3)2Cl+.

Structure−Reactivity Relationships and Intrinsic Reaction Barriers for Nucleophile Additions to a Quinone Methide: A Strongly Resonance-Stabilized Carbocation

Second-order rate constants kNu (M-1 s-1) were determined for addition of a wide range of nucleophiles to the simple quinone methide 4-[bis(trifluoromethyl)methylene]cyclohexa-2,5-dienone (1) to give the nucleophile adduct 1-Nu in water. Equilibrium constants were determined for the overall addition of HBr and HI to 1 to give H-1-Nu, and the data were used to calculate equilibrium constants for the addition of Br- and I- to 1, and to estimate equilibrium constants for the addition of Cl- and AcO-. The values of log kNu show a linear correlation with the Ritchie nucleophilicity parameter N+ with a slope s = 0.92 ± 0.10 that is essentially the same as the electrophile-independent value of 1.0 for highly resonance-stabilized carbocations. Marcus intrinsic barriers Λ of 12.4, 13.9, 15.4, and 19.8 kcal/mol are reported for the addition of I-, Br-, Cl-, and AcO- to 1, respectively. The thermodynamic barriers ΔG° and intrinsic barriers Λ for addition of Br-, Cl-, and AcO- to 1 are 8.4 ± 1.0 and 5.2 ± 0.2 kcal/mol larger, respectively, than the corresponding barriers for addition of these nucleophiles to the triphenylmethyl carbocation. It is concluded that, by the criterion of its chemical reactivity, 1 behaves as a highly resonance-stabilized carbocation. Values of N+ = 4.0, 2.2, 1.2 and 0.60, respectively, are reported for I-, Br-, Cl-, and AcO-, which do not form stable adducts to Ritchie electrophiles. The slope of 2.0 (r = 0.98) for the linear correlation between Ritchie (N+) and Swain−Scott (n) nucleophilicity parameters shows that there is substantially greater bonding between the nucleophile and carbon at the transition state for nucleophile addition to sp2-hybridized carbon than for addition to sp3-hybridized carbon. Azide ion and nucleophiles with a nonbonding electron pair(s) at atoms adjacent to the nucleophilic site (α-effect nucleophiles) exhibit significant positive deviations from this correlation.

Inert Pair Effects in Tin and Lead Dihalides: Crystal Structure of Tin(II) Bromide

The crystal structure of stannous bromide, SnBr2, is compared with those of its analogs: SnCl2, PbBr2, and PbCl2. The structure of SnBr2 has been refined by Rietveld analysis of X-ray powder diffraction data in the orthorhombic space group Pbnm, with a=10.5251(7) Å, b=8.3936(5) Å, c=4.2369(2) Å, and Z=4. Refinement terminated with Rwp=11.90% and Rex=19.11%. Sn is in a trigonal-pyramidal coordination with three Br atoms, two of which are bridging and one terminal. The bridging units form polymeric chains, which run parallel to the c-axis. SnBr2 is related to, but is not isostructural with, analogs, PbCl2, PbBr2, and SnCl2. All the structures possess (MX2)n chains (M=Sn or Pb, X=Cl or Br), but those in SnBr2 are shifted with respect to each other, resulting in a difference in the secondary M–X coordination shell. Stereochemical distortions caused by the nonbonding valence electron pairs on the metal atoms are compared using unit cell parameters normalized to take into account differences in covalent radii between the structures.

Gauging the Donor Ability of the C-Si Bond. Results from Low-Temperature Structural Studies of gauche and Antiperiplanar β-Trimethylsilyl-cyclohexyl Esters and Ethers by Use of the Variable Oxygen Probe.

A plot of C–OR bond distance against pKa(ROH) for esters and ether derivatives of the antiperiplanar β-silyl alcohol (4) has a slope of –5.310–3. This represents a stronger response of the C–OR distance than was observed for the corresponding gauche β-silyl alcohol (5) and its derivatives. This result is consistent with the greater donor ability of a C–Si bond compared with a C–C bond. Comparison of these plots with those previously reported for derivatives (3) of tetrahydropyran-2-ol reveals that a C–Si bond is a weaker donor than an oxygen non-bonded pair of electrons. An estimate of the pKa value for 2,4-dinitrobenzenesulfenic acid (14) was derived from these plots.

Living Radical Polymerization with Reversible Addition−Fragmentation Chain Transfer (RAFT Polymerization) Using Dithiocarbamates as Chain Transfer Agents

Living radical polymerization with reversible addition−fragmentation chain transfer (RAFT polymerization) can be achieved with the use of dithiocarbamate derivatives that have the nonbonded electron pair of the nitrogen included as part of an aromatic system. These compounds have been shown to be highly effective in RAFT polymerization of styrene and (meth)acrylate esters to produce polymers of predetermined molecular weight and narrow polydispersity (usually <1.2). By contrast, simple N,N-dialkyl dithiocarbamates (those compounds previously described as “photoiniferters”) are ineffective as RAFT agents. The reasons for the different behavior are discussed.

Comments on the Treatment of Aromaticity and Acid-Base Character of Pyridine and Pyrrole in Contemporary Organic Chemistry Textbooks

Presentations of aromaticity and acid-base character of pyridine and pyrrole in 18 contemporary organic chemistry textbooks were surveyed. Superficially, each of these two monoaza aromatic heterocycles retains an electron pair on its sp2-hybridized ring nitrogen, ostensibly available for neutralization with a proton to form a salt. The two nonbonded electrons in pyridine, in an sp2-hybridized orbital outside the ring, are not interacting with the 6π-electron aromatic sextet and thus are fully available for neutralization to form an N+-H pyridinium cation. However, in pyrrole, the nonbonded electron pair on nitrogen is part of the aromatic 6π- electron sextet and is not available for NH salt formation, since this would generate an aliphatic (highly reactive) diene imminium cation, which would destroy pyrrole's aromaticity. However, electrophilic attack of a proton in an irreversible manner attacks C-2 of pyrrole to form a resonance-stabilized cationic intermediate capable of further transformations.