(Cyclopentadienyl)dihalogenochromium-Donor Compounds. Synthesis and NMR Investigation

A series of paramagnetic (S = 3/2) chromium half-sandwiches of the type CpCrX2D (D = donor) were synthesized by starting from [CpCrX2]2, Cp2Cr, CpCrCl2(THF), and Cp*Cr(CH3)2[P(CH3)3]. Besides the parent cyclopentadienyl (Cp) the alkylated derivatives CH3C5H4, (CH3)5C5 (Cp*), and C2H5(CH3)4C5 were bound to chromium. The electronegative substituent was X = F, Cl, Br, I, and triflate, while the donors were three ethers, four ketones, dimethylsulfoxide, acetonitrile, methylisocyanide, pyridine, and seven molecules ER3 where E = N, P, As, Sb. The half-sandwiches were partly isolated and partly established in solution. The 13C and 1H NMR spectra showed strongly shifted signals which allowed to quantitatively investigate the equilibrium between CpCrX2D and the anti ferromagnetic species [CpCrX2]2. The NMR signals of CpCrX2D and its substituted derivatives appeared in characteristic ranges thus providing a means of rapid identification. Considerable spin density was found to be induced on the ligands; it is negative in the Cp π system. As for the donors, inter- and intramolecular dynamic behavior as well as selective spin transfer to the γ protons of acetonitrile, methylisocyanide, and ketones was detected.

29Si and 13C NMR spectra of some alkyldiphenylchlorosilanes, alkyldiphenylsilanoles and bis(alkyldiphenylsilyl)chromates

29Si and 13C NMR spectra of alkyldiphenylchlorosilanes, alkyldiphenylsilanoles, and bis(alkyldiphenylsilyl)chromates (C6H5)2Si(R)X have been studied. The chemical shifts δ(29Si) are most affected by nature of the electronegative substituent X, i.e. Cl, OH or CrO3OSi(R). (C6H5)2. The δ(29Si) shifts due to effects of the substituents R can be correlated successfully with values of the Taft constants σ. Slopes of the correlation dependences have negative signs for all the groups of the studied compounds (increasing electron-acceptor ability of substituent R causes upfield shifts of δ(29Si), and their values decrease with increasing electronegativity of the substituent X (sensitivity of the δ(29Si) shifts due to sustituent R increases with increasing electronegativity of the substituent X). Methyl groups at γ-position of the chain of substituent R cause a diamagnetic shift of δ(29Si) by 0.6 to 3.3 ppm. The δ(13C) shifts depend little on nature of the substituent X. Chemical shifts δ(13C) of carbon atoms at α-, β-, and γ-positions of the substituent R with respect to silicon show a good linear correlation with the δ(13C) shifts of the corresponding carbon atoms in aliphatic hydrocarbons CH3R. Slopes of the correlation dependences of all the groups of studied compounds are close to 1, the intercepts for α- and β-positions are negative because of induction effect. Positive value of the intercepts for γ-positions reflects the influence of sterical γ-effect.