Neutral Lewis Bases Research Articles

Adducts of titanium tetrahalides with neutral lewis bases. Part II. Kinetics and reaction mechanism: a variable temperature and variable pressure proton NMR study

The ligand exchange reaction cis-TiX 4·2L + *L ai cis-TiX 4·L*L + L has been studied for a series of cis-TiX 4·2L adducts (X=Cl, Br; L=Lewis base) in CH 2Cl 2 and CHCl 3 by variable temperature and variable pressure proton NMR. The exchange reactions are characterized by large, positive values of Δ H* (50.2-61.5 kJ/mol), Δ S* (9.2-103 J/mol K) and Δ V* (17.5-26.1 cm 3/mol) and by first order rate laws. For this series of adducts, Δ G* (213 K) for ligand exchange decreases as the stability of the adduct decreases. It was concluded that this ligand exchange process occurs via a dissociative, D, mechanism. The cis/ trans isomerization of TiCl 4·2TMPA in CHCl 3 was also investigated. In view of the small but positive ΔV* iso (6.2 cm 3/mol), it was concluded that the isomerization process occurs via an intramolecular twist mechanism with an expanded six-coordinate transition state.

Adducts of titanium tetrahalides with neutral Lewis bases. Part I. Structure and stability: a vibrational and multinuclear NMR study

Raman, infra-red and multinuclear NMR spectroscopy were used to establish the structure of several TiX 4·2L adducts (X=F, Cl, Br; L=Lewis base) in inert solvents. In contrast to the analogous SnX 4·2L adducts where a cis- trans equilibrium prevails, most of the TiX 4·2L adducts studied were found to have only the cis configuration. Trans isomers were observed but their formation was dependent on the donor ability of the ligand. In dichloromethane solution, the adducts with L=Me 2O, Me 2S, (MeOCH 2-) 2, Et 2S, THT, Me 2Se, MeCN, Me 2CO, Cl(MeO) 2PO, Cl 2(MeO)PO, Cl 3PO and Cl 2(Me 2N)PO were found to have the cis configuration only. For the adducts with L=THF, Cl(Me 2N) 2PO and TMPA, a cis- trans equilibrium was observed. The thermodynamic parameters were measured for cis- trans isomerization for TiCl 4·2TMPA in CHCl 3; these parameters are: K iso 277=[ trans] / [ cis]=0.36, Δ H° iso=− 1.3 ± 1.3 kJ/mol, Δ S° iso=−13.1 + 7.5 J/mol K, and Δ V° iso= − 1.3+0.8 cm 3/mol. A complex equilibrium involving cis and trans isomers and the ionic complex [TiCl 3·3HMPA]Cl was found to occur for the TiCl 4 adduct with L=HMPA. 1H NMR was used to establish the relative stabilities of the cis adducts and the following sequence was obtained: Me 2O ∼ MeCN < Me 2CO < Me 2S < Me 2Se < Cl(MeO) 2PO < TMPA < CI(Me 2N) 2PO.

ChemInform Abstract: High‐Pressure NMR Kinetics. Part 26. Adducts of Antimony(V) Pentachloride with Neutral Lewis Bases.

ChemInform Abstract: High‐Pressure NMR Kinetics. Part 26. Adducts of Antimony(V) Pentachloride with Neutral Lewis Bases.

ChemInform Abstract: Adducts of Antimony(V) Pentachloride with Neutral Lewis Bases.

AbstractThe adducts (I) are synthesized by adding the appropriate quantities of SbCl5 to the Lewis bases.

Adducts of antimony(V) pentachloride with neutral lewis bases. Part I. Structure and stability: a vibrational and NMR study

A large number of SbCl 5·L adducts were synthesized (L = RCN, R 3− n R′ n PO, ethers and various other oxygen donors). The adducts can be assigned to the C 4 v point group according to the infrared and Raman spectra of both the solids and the solutions in dibromethane. Antimony-121 NMR was used to monitor the extent of auto-ionisation due to reaction: 2SbCl 5·L ⇌ SbCl 4·2L + + SbCl 6 −. This increases with the polarity of the solvent and the donor strength of the ligand, but was always less than 3%. 1H and 13C{ 1H} NMR were used to establish the relative stability constants of the series of adducts in dichloromethane. A linear correlation of slope near unity is observed between the relative free energy of formation and Gutmann's donor number. A correlation is found between the adduct stabilities and the various metal—chloride stretching frequencies.

Adducts of antimony(V) pentachloride with neutral lewis bases. Part II. Kinetics and reaction mechanism: a variable temperature and variable pressure proton NMR study

The ligand exchange reaction SbCl 5·L + *L⇌ SbCl 5· *L + L has been studied in CH 2Cl 2 and (CHCl 2) 2 as a function of temperature and of pressure by using 1H NMR. First order rate laws, positive activation entropies ranging from +9.8 to +37.3 cal K −1 mol −1, positive activation volumes from +18.2 to +30.0 cm 3 mol −1 and a linear free energy relationship of slope -1.09 ± 0.06 have been observed for a series of L including nitriles, ethers, amides and -PO donor ligands. It is concluded that a limiting dissociative, D, mechanism exists for the whole series of ligand exchange reactions studied.

Kinetic and stereochemical factors in the hydralumination of alkenes and the configurational stability of the resulting carbonaluminum bond

A series of cyclic olefins was subjected to hydralumination by diisobutyl-aluminum hydride, in order to determine the stereochemistry, the regiochemistry and the influence of structure on reactivity. The reactivity gradation observed was: acenaphthylene > 1,1-dimethyl-3-trimethylsilylindene > 1-methylacenaphthylene > 1,1-dimethylindene > 1-phenylacenaphthylene ⪢ 1,1-dimethyl-3-phenylindene. By working in donor solvents it was demonstrated that cis-hydralumination is the kinetically controlled mode of addition. The reactivity for 3-substituted-1,1-dimethylindenes reflected polar effects, whereas that for 1-substituted acenaphthylenes responded to steric effects. The regiochemistry of hydralumination was generally determined by steric factors, except for the case of the silylindene. The isomerization of the hydralumination adduct could occur either by carbonaluminum bond inversion or by dehydralumination. The former process took place readily in the absence of Lewis bases, but the latter process required elevated temperatures and protracted times. The stable cis-hydralumination adducts could be protodealuminated with retention of configuration at the CAl bond, although acenaphthenyl systems gave significant amounts of 1,3-dihydroacenaphthylenes. The nature of these unusual hydrolysis products was elucidated by deuterium labeling, 1H NMR and chemical degradation studies. A complete series of C 2- and C 3-deuterated-1,1-dimethylindenes was synthesized unambiguously in order to permit these foregoing studies. The configurational stability of these organoaluminum compounds toward heat, neutral Lewis bases and carbanions was evaluated. These observations, together with the temperature-dependent 1H NMR spectrum of 1-acenaphthenyl(diisobutyl)aluminium, lend some insight into the pathways of carbonaluminum bond inversion.

Complexes of stannous fluoride and other group IVB dihalides with mammalian hemoproteins.

Stannous fluoride, the widely used anticaries toothpaste additive, and other tin and germanium dihalides form complexes with hemoproteins such as hepatic cytochrome P-450, hemoglobin, and peroxidase. These complexes are characterized by visible spectra closely similar in shape, molar absorptivity, and absorbance maxima to those obtained with analogous complexes of carbon monoxide. Evidence is presented that the observed complexes are formed with uncharged MX2 or LMX2, moieties (where L is a neutral Lewis base, M is a metal, and X is a halogen) acting as pi-acceptor ligands.

Adducts of tin(IV) tetrahalides with neutral Lewis bases. II. Quantitative study of the Cis- trans equilibria by NMR

The cis- trans of SnX 4·2L and SnX 4·LL adducts in solution were studied quantitatively by 19F, 31P and 1H NMR spectroscopy ( X = F, Cl, Br; L = Me 2O, Et 2O, Me 2S, Et 2S, Me 2Se, MeCN, Me 3CCN, Me 2CO, TMPA, Me 2N(MeO) 2PO, HMPA; LL = (MeOCH 2−) 2). The relative stabilities for cis-and trans-SnX 4·2L adducts were estimated, and the following sequence obtained: HMPA > Me 2N(MeO) 2PO > TMPA Me 2Se > Me 2S > Me 2O. The existence of SnCl 4·L a·L b mixed species when tin tetrachloride is present with two different ligands has been shown, and their concentrations at equilibrium obey a statistical distribution. The cis- trans isomerizationconstant is relatively independent of temperature, but depend strongly on solvent polarity, the trans isomer being favoured in weakly polar media The trends in the isomerization constant as a function of substitutions of the ligand and the halogen is discussed on the basis of tin(IV) orbital overlap.

Adducts of tin(IV) tetrahalides with neutral lewis bases. I. Vibrational study of the cis- trans isomerism in solution

A large number of SnX 4·2L and SnX 4·L-L tin(IV) tetrahalide adducts were synthesized and analyzed (X = Cl, Br; L = Me 2O, Et 2O, THF, Me 2S, Et 2S, THT, Me 2Se, MeCN, Me 3CCN, Me 3CO, HMPA, TMPA; L−L = (MeOCH 2−) 2). Their cis or trans geometry was determined by infrared and Raman spectroscopy. For the SnCl 4·2THT adduct, both isomers could be isolated by changing the solvent polarity. The existence of a cis-trans equilibrium in solution has been demonstrated for the majority of the adducts by Raman measurements. This equilibrium strongly depends upon the solvent polarity, the cis isomer being favoured in a highly polar medium. Using a simplified model for Wilson's FG matrices, the Sn-X force constants were determined and the following sequences deduced for the strength of the tin-ligand bond in SnX 4·2L: Me 2Se > Me 2S ∼ THT ∼ Et 2S > Me 2O ∼ THF ∼ Et 2O and HMPA > TMPA.

Fluorine-19 nuclear magnetic resonance study of some pentafluorotitanate complexes with neutral ligands

A 19F n.m.r. study of the reactions of [TiF5]– with neutral Lewis bases is reported. The factors which contribute to the formation of [TiF5]–·D complexes are considered. Fluorine-19 n.m.r. data for over 30 complexes are presented and the effects of donor basicity, pπ–dπ bonding, and solvent interactions on the chemical shifts are discussed.