Substituent effects in second row molecules : Silicon-containing compounds

Substituent interaction energies are calculated by ab initio molecular orbital methods for the two series SiH₂X^- and SiH₃X for the directly bound substituents X = BH₂, CH₃, NH₂, OH, F and the results compared with those for the corresponding first row species. Interactions with the groups NH₂, OH, F are as large in the neutral as in the anionic series and this is attributed to the presence of important π-bonding interactions, supplementing the effects of inductive withdrawal of σ-electrons. The restoration of charge neutrality by π-donation to silicon is more important in the neutral molecules, σ-electron transfer from silicon in the anions. π-Bonding with the π-acceptor substitutent BH₂ is favourable, as it is in the CH₃X and CH₂X^- systems, but with π-donor substituents the interactions are always destabilizing.     

Substituent effects in second row molecules: Molecular orbital studies of phosphorus(III) compounds

Substituent effects in directly bonded P(III) compounds are investigated by ab initio MO calculations of relative energies and the results compared with those for the corresponding nitrogen species. The investigation covers substitution by X = BH₂, CH₃, NH₂, OH, F in PHX^-, PH₂X, and PH₃X⁺ series molecules with some attention also to PX₃ and PX₃H⁺ species. Except for compounds containing the π-acceptor substituent BH₂, σ-interactions dominate substitution behaviour but the second row species tolerate electron withdrawal better than their first row analogues, the severe destabilization of NH₂X and NH₃X⁺ by σ-electron withdrawal being absent from PH₂X and PH₃X⁺. In contrast to the σ-withdrawing NH₂ group, the PH₂ group is characterized as a mild σ-donor. PH^- is a σ-donor and PH⁺₃ a σ-acceptor. π-Bonding to the second row atom is an important means of maintaining electroneutrality in the PH₃X⁺ series, where dπ functions have a bigger role than pπ functions.     

Substituent effects in second-row molecules: Basis set performance in calculations of normal valency phosphorus and sulfur compounds

Ab inito molecular orbital calculations of the phosphorus- and sulfur-containing series PH₂X, PH₃X⁺, SHX, and SH₂X⁺ (X = H, CH₃, NH₂, OH, F) have been carried out over a range of Gaussian basis sets and the results (optimized geometrical structures, relative energies, and electron distributions) critically compared. As in first-row molecules there are large discrepancies between substituent interaction energies at different basis set levels, particularly in electron-rich molecules; use of basis sets lower than the supplemented 6-31G basis incurs the risk of obtaining substituent stabilizations with large errors, including the wrong sign. Only a small part of the discrepancies is accounted for by structural differences between the optimized geometries. Supplementation of low level basis sets by d functions frequently leads to exaggerated stabilization energies for π-donor substituents. Poor performance also results from the use of split valence basis sets in which the valence shell electron density is too heavily concentrated in diffuse component of the valence shell functions, again likely to occur in electron-rich molecules. Isodesmic reaction energies are much less sensitive to basis set variation, but d function supplementation is necessary to achieve reliable results, suggesting a marginal valence role for d functions, not merely polarization of the bonding density. Optimized molecular geometries are relatively insensitive to basis set and electron population analysis data, for better-than-minimal bases, are uniform to an unexpected degree.     

Substituent effects in formally quintuple-bonded ArCrCrAr compounds (Ar = terphenyl) and related species

The effects of different terphenyl ligand substituents on the quintuple Cr-Cr bonding in arylchromium(I) dimers stabilized by bulky terphenyl ligands (Ar) were investigated. A series of complexes, ArCrCrAr (1-4; Ar = C6H2-2,6-(C6H3-2,6-iPr2)2-4-X, where X = H, SiMe3, OMe, and F), was synthesized and structurally characterized. Their X-ray crystal structures display similar trans-bent C(ipso)CrCrC(ipso) cores with short Cr-Cr distances that range from 1.8077(7) to 1.8351(4) A. There also weaker Cr-C interactions [2.294(1)-2.322(2) A] involving an C(ipso) of one of the flanking aryl rings. The data show that the changes induced in the Cr-Cr bond length by the different substituents X in the para positions of the central aryl ring of the terphenyl ligand are probably a result of packing rather than electronic effects. This is in agreement with density functional theory (DFT) calculations, which predict that the model compounds (4-XC6H4)CrCr(C6H4-4-X) (X = H, SiMe3, OMe, and F) have similar geometries in the gas phase. Magnetic measurements in the temperature range of 2-300 K revealed temperature-independent paramagnetism in 1-4. UV-visible and NMR spectroscopic data indicated that the metal-metal-bonded solid-state structures of 1-4 are retained in solution. Reduction of (4-F3CAr')CrCl (4-F3CAr' = C6H2-2,6-(C6H3-2,6-iPr2)2-4-CF3) with KC8 gave non-Cr-Cr-bonded fluorine-bridged dimer {(4-F3CAr')Cr(mu-F)(THF)}2 (5) as a result of activation of the CF3 moiety. The monomeric, two-coordinate complexes [(3,5-iPr2Ar*)Cr(L)] (6, L = THF; 7, L = PMe3; 3,5-iPr2Ar* = C6H1-2,6-(C6H-2,4,6-iPr3)2-3,5-iPr2) were obtained with use of the larger 3,5-Pri2-Ar* ligand, which prevents Cr-Cr bond formation. Their structures contain almost linearly coordinated CrI atoms, with high-spin 3d5 configurations. The addition of toluene to a mixture of (3,5-iPr2Ar*)CrCl and KC8 gave the unusual dinuclear benzyl complex [(3,5-iPr2Ar*)Cr(eta3:eta6-CH2Ph)Cr(Ar*-1-H-3,5-iPr2)] (8), in which a C-H bond from a toluene methyl group was activated. The electronic structures of 5-8 have been analyzed with the aid of DFT calculations.     

Substituent effects of nitro group in cyclic compounds

Structural Chemistry - Numerous studies on nitro group properties are associated with its high electron-withdrawing ability, by means of both resonance and inductive effect. The substituent effect...     

Substituent effects in the zinc(II) coordination chemistry of isomeric pyridylpyrazole ligands

Abstract

Pyrazoles with unsymmetric substitution are useful ligands in coordination chemistry, but are under-developed due to synthetic challenges in accessing the pure isomers. We have prepared four new structurally related N-(2-pyridyl)-3,5-dialkylpyrazole ligands, L1–L4, and probed their coordination chemistry in the crystalline phase and in solution to elucidate a relationship between steric influence of the alkyl substituents, the stability of the subsequent metal complexes, and their crystal packing influences. We find that L1 and L2, bearing linear or branched alkyl substituents, show similar stabilities and crystal packing motifs featuring π···π and C-H···Cl interactions in the crystalline complexes 1 and 2, respectively. The cyclohexyl-fused species L3 and L4 vary both in the solution stability of complexes 3 and 4, respectively, and in their crystal packing. Complex [ZnCl₂(L3)] (3) is a mononuclear complex similar to 1 and 2, albeit with π···π interactions disallowed by the bulk of the cyclohexyl ring. Reaction of isomeric L4 with ZnCl₂ gives two polymorphic complexes, 4α and 4β, of the form [Zn₂Cl₂(μ₂-Cl)₂(L4)₂], varying only in their long-range packing modes. These results show the importance of understanding the steric influences in substituted pyridylpyrazoles, which determine both stability in solution and speciation in the crystalline phase.     

pi bonding in second and third row molecules: testing the strength of Linus's blanket

The flexibility of valence bond (VB) theory provides a new method of calculating pi-bond energies in the double-bonded species H(m)A=BH(n), where A, B = C, N, O, Si, P, S. This new method circumvents the problems usually associated with obtaining pi-bond strengths by targeting only the pi bond, while all other factors remain constant. In this manner, a clean separation between sigma- and pi effects can be achieved which highlights some expected trends in bond strength upon moving from left to right and up and down the Periodic Table. Intra-row pi bonds conform to the classic statement by Pauling [L. Pauling, The Natiure of the Chemical Bond, Cornell University Press, Ithaca, 1960, 3rd edition] regarding the relationship of heteronuclear bond strengths to their homonuclear constituents whereas inter-row pi bonds do not. This variance with Pauling's statement is shown to be due to the constraining effect of the underlying sigma bonds which prevents optimal p(pi)-p(pi) overlap. While Pauling's statement was based on the assumption that the resonance energy (RE) would be large for heteronuclear and small for homonuclear bonds, we have found large REs for all bonds studied herein; this leads to the conclusion that REs are dependent not only on the electronegativity difference but also the electronegativity sum of the constituent atoms. This situation where the bond is neither covalent nor ionic but originates in the covalent-ionic mixing has been termed charge shift (CS) bonding [S. Shaik, P. Maitre, G. Sini, P. C. Hiberty, J. Am. Chem. Soc. 1992, 114, 7861]. We have shown that CS bonding extends beyond single sigma bonds in first row molecules, thus supporting the idea that CS-bonding is a ubiquitous bonding form.     

Bispidine copper(II) compounds: effects of the rigid ligand backbone

Approximative density-functional theory calculations indicate that the tetradentate ligand L (L = 2,4-bis-(2-pyridyl)-3,7-diaza-[3.3.1]-bicyclononane) enforces an unusual and strong binding of a co-ligand (substrate) to a copper(II) center. The co-ligand in [Cu(L)(Cl)](+) completes a square-pyramidal coordination around copper(II) and binds in the equatorial plane rather than on the apical position. This configuration is a stable geometric isomer for the model complex [Cu(NH3)2(imine)2(Cl)](+), but it is disfavored by approximately 10 kJ mol(-1) and not commonly observed for CuN4 chromophores with a monodentate co-ligand. The equatorial coordination increases the bond energy of the copper(II)-chloride bond by approximately 80 kJ mol(-1), and similar results are expected for other copper(II)-L-substrate complexes, some of which show strong catalytic activity or unusual stability. Despite the enforced configuration, L does not impose significant steric strain on the copper(II) center but is well preorganized for the Jahn-Teller labile ion in this unusual geometry. The preorganization extends to the orientation of the pyridine donors (torsion angle around the copper-pyridine bond), and this seems to be of importance in the reactivity of the copper-L complexes and their derivatives.     

Substituent effects in homoleptic iron(II) and ruthenium(II) complexes of 4′-hydrazone derivatives of 2,2′:6′,2″-terpyridine

The synthesis and characterization of eight homoleptic iron(II) and ruthenium(II) complexes containing 4′-hydrazone-substituted 2,2′:6′,2″-terpyridine ligands are described. ¹H NMR spectroscopic data illustrate that the coordinated ligands undergo facile rotation about the C_pyridine–N_hydrazone bond when the N atom is methylated, and hindered bond rotation when the hydrazone NH unit is available for hydrogen bonding to solvent molecules. Detailed structural studies illustrate how the flexibility of the backbone of the complexes leads to significant variation in packing. Throughout the series of solid structures, the packing is dictated by a combination of face-to-face aromatic π-stacking, edge-to-face aromatic interactions and classical and non-classical hydrogen bonding.     

Inclusion compounds formed between α,ω-(long-carbon-chain)-diaminecademium(II) tetracyanonickelate(II) host and aromatic guest molecules

Inclusion compounds with the general formula Cd(NH₂(CH₂) _n NH₂)Ni(CN)₄·xG were prepared forn=4 to 8, and for G of such an aromatic guest molecule as pyrrole, benzene, aniline, toluene, toluidine, xylene, xylidine, dichlorobenzene, trimethylbenzene, tetramethylbenzene, ethylbenzene, styrene, or isopropylbenzene, with varyingx. Generally, longer chain lengths of ,-diamine in the host permit the inclusion of bulkier guest molecules. However, the presence of an amino group on the phenyl ring of the guest appears to impart a special affinity with the hosts.     

Substituent effects in five oxo-centered trinuclear rhodium(III) clusters

We here report the rates of water substitution by methanol-d(4) for four new oxo-centered trinuclear rhodium(III) clusters with different carboxylate-bridging ligands, [Rh3(mu3-O)(mu-O2CR)6(OH2)3]+ (R = CH2CH3, CH2CH2Cl, CH2Cl, and CHCl2), and [Rh3(mu3-O)(mu-O2CCH3)6(OH2)3]+. By varying the R group alkyl chain, water substitution rates were found to span almost 3 orders of magnitude (k(298K) = 1.2 x 10(-2)-2.3 x 10(-5) s(-1)) and reflect the following trend R = CH2CH3 > CH3 > CH2CH2Cl > CH(2)Cl > CHCl2. Activation parameters for substitution point toward a dissociative activation pathway (DeltaH = 99-115 kJ mol(-1); DeltaS = 48-52 J mol(-1) K(-1)), indicating that there is little association with the incoming methanol molecule during the formation of the transition-state complex. Because the mechanism for substitution in all five trimers has a considerable dissociative character, substitution rates are likely very similar to water exchange rates. These data suggest that the kinetic reactivity of the ligated waters is heavily influenced by the inductive ability of the aliphatic substituents, but yet the mechanism of substitution remains virtually unchanged. Structural data are also reported for the four new rhodium(III) trimer salts as well as 103Rh NMR spectra. We find that 103Rh NMR chemical shifts span more than 200 ppm and mirror the same reactivity trend found for the rates of water substitution (103Rh delta (9406-9620 ppm): R = CH2CH3 < CH3 < CH2CH2Cl < CH2Cl < CHCl2). Taken together, these data suggest a means for estimating water exchange rates for other oxo-centered rhodium(III) trimers from chemical shift data alone.     

Adamantylcyclopentadienyldicarbonyliron(II) compounds

Synthesis of 1- and 2-adamantyl derivatives of η⁵-cyclopentadienyldicarbonyliron is reported. The 2-adamantyl is prepared from the metal carbonyl anion but the 1-adamantyl derivative is prepared by decarbonylation of the acyl derivative. 1-adCOFe(CO)₂(η⁵-C₅H₅) is in turn prepared from 1-AdCOCl and NaFe-(CO)₂(η⁵-C₅H₅). Phosphine-substituted acyl derivatives AdCOFe(CO)(PR₃)-(η⁵-C₅H₅) are also reported.     

Substituent control of hydrogen bonding in palladium(II)-pyrazole complexes

Inter- and intramolecular hydrogen bonding of an N-H group in pyrazole complexes was studied using ligands with two different groups at pyrazole C-3 and C-5. At C-5, groups such as methyl, i-propyl, phenyl, or tert-butyl were present. At C-3, side chains L-CH(2)- and L-CH(2)CH(2)- (L = thioether or phosphine) ensured formation of chelates to a cis-dichloropalladium(II) fragment through side-chain atom L and the pyrazole nitrogen closest to the side chain. The significance of the ligands is that by placing a ligating side chain on a ring carbon (C-3), rather than on a ring nitrogen, the ring nitrogen not bound to the metal and its attached proton are available for hydrogen bonding. As desired, seven chelate complexes examined by X-ray diffraction all showed intramolecular hydrogen bonding between the pyrazole N-H and a chloride ligand in the cis position. In addition, however, intermolecular hydrogen bonding could be controlled by the substituent at C-5: complexes with either a methyl at C-5 or no substituent there showed significant intermolecular hydrogen bonding interactions, which were completely avoided by placing a tert-butyl group at C-5. The acidity of two complexes in acetonitrile solutions was estimated to be closer to that of pyridinium ion than those of imidazolium or triethylammonium ions.     

Substituent interaction effects in aromatic molecules in RP-HPLC with acetonitrile-water and tetrahydrofuran-water eluents

Summary Retention volumes of monosubstituted benzenes, benzoic acids, phenols and anilines have been measured in reversed-phase liquid chromatography. Buffered acetonitrile-water and tetrahydrofuran-water eluents were used with an octadecylsilylsilica adsorbent. From the net retention volumes a substituent interaction effect was calculated and described with the linear free energy relationship developed by Taft. The positive sign of the values of the -parameters, figuring in this relationship, was interpreted in terms of hydrogen bonding between the solutes and the eluent.     

Substituent effects of phosphorus and arsenic containing groups in aromatic substitution—II : Nitration of benzylphosphonic acid and related compounds

The reactivities of benzylphosphonic acid (protonated and unprotonated) and β-phenylethylphosphonic acid have been determined for nitration. The highly selective para/ortho orientation has been confirmed. The relative reactivities of systems PhX and PhCH₂X for some substituents are discussed.     

Substituent effects of beta-diketiminate ligands on the structure and physicochemical properties of copper(II) complexes

Substituent effects of beta-diketiminate ligands on the structure and physicochemical properties of the copper(II) complexes have been systematically investigated by using 3-iminopropenylamine derivatives R1LR3H, R3-N=CH-C(R1)=CH-NH-R3, where R1 is Me, H, CN, or NO2, and R3 is Ph, Mes (mesityl), Dep (2,6-diethylphenyl), Dipp (2,6-diisopropylphenyl), or Dtbp (3,5-di-tert-butylphenyl). When the ligands with R3=Ph or Dtbp were treated with CuII(OAc)2, bis(beta-diketiminate) copper(II) complexes exhibiting distorted tetrahedral geometries were obtained, the crystal structures of which were nearly the same as each other regardless of the alpha-substituent (R1); dihedral angles between the two beta-diketiminate coordination planes are 62.5 +/- 1.2 degrees, and the Cu-N bond lengths are 1.959 +/- 0.008 A. The distorted tetrahedral structures are maintained in solution, but the spectroscopic features, especially gII values of the ESR spectra and the d-d bands of the absorption spectra, as well as the electrochemical behaviors of the complexes, are significantly affected by the electronic nature of R1. The ligands with R3=Mes and Dep, on the other hand, gave di(mu-hydroxo)dicopper(II) complexes, and their crystal structures as well as spectroscopic and electrochemical features have also been explored. Furthermore, the ligand with the more sterically encumbered aromatic substituent (Dipp) provided a mononuclear four-coordinate square planar copper(II) complex supported by one beta-diketiminate ligand and one didentate acetate ion. Thus, the beta-diketiminate ligands with a variety of substituents (R1 and R3) have been explored to provide coordinatively unsaturated (four-coordinate) mononuclear and dinuclear copper(II) complexes with significantly different coordination geometry and properties.     

Geometry optimization calculations in molecules containing second row atoms with various polarization function basis sets

Ab initio MO geometry optimization studies on a number of molecules containing second-row atoms using various polarization basis sets are presented. In particular the use of gaussian bond functions is shown to be a good substitute for the more conventional (and expensive) atom-centered d functions. Molecules examined to a double zeta plus polarization basis set level are HCP, P₂, SO, SO₂, H₂S, SF₂, CIF, HCl and Cl₂.     

pi-Indenyl tin(II) and lead(II) compounds

The syntheses and structures of the first indenyl-substituted tin(II) complexes, [Sn{1,3-(SiMe3)2C9H5}2] and [Sn(C5Me5)-{1,3-(SiMe3)2C9H5}], are described; the lead(II) analogue of the latter compound has also been prepared and structurally characterized.