D.N. Kravtsov

A 19F NMR study of electronic effects in organomercury, -tin and -lead derivatives of some fluorophenols and fluorothiophenols

Abstract A number of organomercury, -tin and -lead derivatives of m- and p-fluoro-substituted phenols and thiophenols of the type RnMOAr and RnMSAr have been prepared. The 19F chemical shifts relative to fluorobenzene have been measured in various solvents for all these compounds. On the basis of the data obtained, the electronic features of the substituents RnMO and RnMS have been studied and compared with those of OH, CH3O, SH and CH3S groups. The influence of various factors on the electronic effect of the organometallic substituent and on its solvent susceptibility is discussed. The coordinating abilities of the substituents (C2H5)3SnO and (C2H5)3-SnS in solution have been compared with those of the OH and SH groups.

A 19F NMR study of steric hindrance and intramolecular coordination in organo-mercury, -tin and -lead derivatives of 2,6-dimethyl-4-fluorothiophenol and 2,6-dibromo-4-fluorothiophenol

Abstract Some organomercury, -tin and -lead derivatives of 2,6-dimethyl-4-fluorothiophenol and 2,6-dibromo-4-fluorothiophenol of the general type RpMSAr have been prepared. The fluorine chemical shifts of the compounds realtive to internal fluorobenzene and the 19F SCS values for the corresponding substituents have been determined and discussed in terms of steric hindrance and intramolecular coordination, the behaviour of the organometallic substituents RnMS in these respects being compared with that of the SH and CH3S groups. The anomalously low steric requirements of RnMS groups, as well as the differences in the strength of intramolecular coordination and its influence on the acceptor power of the metal atom, in the compouds of mercury, tin and lead have been demonstrated.

A Comparative PMR study of Exchange reactions in organo- Mercury, -TIN and -LEAD derivatives of some thiols

Abstract Metal—proton and metal—metal exchange reactions have been studied by PMR for thiophenol, 2-methylthiophenol, 2,6-dimethylthiophenol, benzyl mercaptan and their C 6 H 5 HG, (C 6 H 5 ) 3 Sn and (C 6 H 5 ) 3 Pb derivatives in chlorobenzene and pyridine solutions. In chlorobenzene the metal—metal exchange has been found to proceed in many cases at a greater rate than the metal—proton type, the exchange mobility of hydrogen and organometallic groups in chlorobenzene increasing in the order (C 6 H 5 ) 3 Sn 6 H 5 ) 3 Pb 6 H 5 Hg. In the case of the (C 6 H 5 ) 3 Sn and (C 6 H 5 ) 3 Pb groups, pyridine accelerates the metal—proton exchange to a greater extent than the metal—metal exchange. The influence of various factors on the exchange reactions has been studied Analysis of the experimental findings and literature data has led to the conclusion that most probably the mechanism of the exchange reactions involves an associative pathway, the ease of exchange being mainly determined by the ability of the migrating group to form a cyclic transition state with delocalized bonds. The data on the exchange equilibria of the organometallic derivatives of 2-methylthiophenol and 2,6-dimethylthiophenol with thiophenol and its derivatives demonstrate that the C 6 H 5 HgS, (C 6 H 5 ) 3 SnS and (C 6 H 5 ) 3 PbS groups have equal steric requirements when involved in non-bonded interactions with o -methyl substituents.

A 19F NMR study of electronic effects in some organo-antimony and organobismuth compounds containing metal—heteroatom bonds

Abstract The 19 F NMR technique has been used to study the ground-state electronic effects of univalent organo-antimony and -bismuth substituents in compounds of the type (C 6 H 5 ) n MSC 6 H 4 F-3, (C 6 H 5 ) n MSC 6 H 4 F-4, (C 6 H 5 ) 4 SbOC 6 H 4 F-3, (C 6 H 5 ) 4 SbOC 6 H 4 F-4 and (4-FC 6 H 4 ) 2 SbX, where (C 6 H 5 ) n M = (C 6 H 5 ) 2 Sb, (C 6 H 5 ) 2 Bi and (C 6 H 5 ) 4 Sb, X = C 6 H 5 S, CH 3 COO, Cl, Br. It has been found that the electron-donating effect of the sulphur-containing groups increases in the order: (C 6 H 5 ) 2 SbS 6 H 5 ) 2 BiS 6 H 5 ) 4 SbS, the substituents (C 6 H 5 ) 4 SbS and (C 6 H 5 ) 4 SbO being the most electron-releasing among the (C 6 H 5 ) n MS and (C 6 H 5 ) n MO groups containing heavy non-transition metals. From the viewpoint of solvent susceptibility of their electronic effects the (C 6 H 5 ) 2 SbS, (C 6 H 5 ) 2 BiS and (C 6 H 5 ) 4 SbS groups resemble the (C 6 H 5 ) 3 SnS and (C 6 H 5 ) 3 PbS substituents, and differ from the C 6 H 5 HgS group, whereas the (C 6 H 5 ) 4 SbO substituent differs in this respect from all other (C 6 H 5 ) n MO groups. The low conjugating ability of the (C 6 H 5 ) 2 SbS substituent and slight influence of steric hindrance upon its electronic effect have been explained by the operation of conformational factors. It has been established that the electronic interactions across the antimony—heteroatom bonds are mainly of inductive character and that the order of electron withdrawal for the ArSbX substituents can be reversed on transfer from an inert to coordinating solvent.

The 19F NMR spectra of some fluoroaryl mercurials and the electronic effect of mercury substituents

A number of new fluoroaryl mercurials have been prepared, and for the compounds indicated the 19F NMR spectra have been recorded in chloroform, tetrahydrofuran and dimethyl sulphoxide. The values of σ1 and σOR for mercury substituents have been derived from the fluorine shielding parameters. The dependence of the electronic effect of mercury substituents upon the solvent, the nature of the groups on mercury, steric hindrance and possible intramolecular coordination is discussed.

A Comparative 19F NMR study of electronic effects in some fluoroaryl compounds of antimony and bismuth and in their carbon and nitrogen analogues

Abstract A number of m - and p -fluorophenyl compounds of type Ar 2 QC 6 H 4 F (Q = Sb, Bi, N and CH; Ar = C 6 H 5 , C 6 H 4 F) have been prepared. The l0 F chemical shifts relative to fluorobenzene have been measured in cyclohexane, chloroform and pyridine for all these compounds. On the basis of the data obtained, the electronic nature of the Ar 2 Sb and Ar 2 Bi substituents has been studied and compared with that of the corresponding (C 6 H 5 ) 2 N and (C 6 H 5 ) 2 CH groups. The electronic effect of the Ar 2 Sb and Ar 2 Bi substituents has been shown to be mainly inductive, its solvent susceptibility being in most cases close to zero relative to both electron- and proton- donating solvents.

A 19F NMR investigation of the transmission of electronic effects through tin-aromatic carbon bonds

A number of triaryl(4-fluorophenyl)tin compounds have been prepared. The fluorine chemical shifts relative to fluorobenzene as an internal standard have been measured for all these compounds in chloroform, and a linear relationship between these values and the Σσo values for the substituted phenyl groups attached to the tin atom has been demonstrated. The results obtained suggest that the transmission of electronic effects from one aromatic ring to another in compounds of the type Ar4Sn occurs mainly by an inductive mechanism. In conjunction with the data on similar systems containing other bridging atoms, the results obtained provide a quantitative measure of the relative ability of tin-aromatic carbon bonds to transmit electronic effects.

Influence of substituent effects, steric hindrance and intramolecular coordination on the exchange rates and equilibria in substituted benzenesulphonamides and their N-arylmercury derivatives

The kinetics of exchange reactions of the metalmetal and metalproton types and exchange equilibria of the metalproton type have been studied by PMR for arylmercury derivatives of N-methyl-p-chlorobenzenesulphonamide, some substituted benzenesulphonanilides and their arylmercury derivatives. The metalmetal exchange has been shown to be accelerated by electron-accepting substituents in the arylmercury group and retarded by electron-donating substituents. The metalproton exchange is accelerated by electron-withdrawing substituents in the aryl radical on nitrogen and inhibited by electron-releasing substituents, steric hindrance and intramolecular coordination. Both electron-accepting and electron-donating substituents in the arylmercury group can accelerate the metalproton exchange. It has been established that purely polar effects of substituents in the aryl group on nitrogen affect the exchange equilibrium of the metalproton type only slightly, whereas steric hindrance and intramolecular coordination exert a considerable influence upon it. Analysis of the data obtained has demonstrated that with five-membered chelate rings the intramolecular coordinate bond involving the C6H5Hg group is stronger than the corresponding internal hydrogen bond, whereas with six-membered chelate rings the contrary is the case.

A PMR study of exchange equilibria between thiophenols and their organo-antimony and -mercury derivatives

The exchange reactions between substituted thiophenols and their (C6H5)2Sb and C6H5Hg derivatives are slow on the PMR time scale in chloroform. The corresponding equilibrium constants indicate that the strength of the virtually chelate rings involving the (C6H5)2Sb group is lower than those of the chelate rings involving the C6H5Hg group or hydrogen.

A 19F NMR study of the transmission of electronic effects in triarylantimony and triarylbismuth compounds. A comparison with the nitrogen- and carbon-containing analogues

Abstract A 19F NMR study of the transmission of electronic effects has been made for the systems Ar2EC6H4F-4 (E = Sb, Bi, CH, N). The fluorine chemical shifts obtained are correlated with the polar constants (Σσo and Σσ) of the substituents, suggesting that electronic effects are transmitted through the SbCar, BiCar and CCar bonds predominantly by an inductive mechanism, whereas the transmission through the NCar bonds is contributed to significantly by classical resonance effects due to competitive conjugation of the lone pair with the aromatic rings, and the substituents therein. A dual parameter correlation of the fluorine chemical shifts with the inductive (σI) and resonance (σoR and σR) parameters of the substituents in the aromatic rings has led to similar conclusions. The inductive transmission through the bridging Sb and Bi atoms has been assigned to the absence of conjugation of lone pair and vacant d-orbitals of the metals with π-electron systems of the aromatic rings. On the basis of the values of the ϱ coefficients for the correlation equations obtained it has been established that the transmitting ability of the BiCar bonds is close to that of the CalCar bonds and considerably lower than the transmitting ability of the NCar bonds.