Steric effects

About: Steric effects is a research topic. Over the lifetime, 16112 publications have been published within this topic receiving 319615 citations. The topic is also known as: steric hindrance.

Complexity with simplicity: a steric continuum of chelating diamines with copper(I) and dioxygen

Abstract: Copper(I) complexed by simple peralkylated diamines reacts with O2 at low temperature in aprotic solvents to create a diverse set of thermally sensitive, polynuclear copper products. Structural and spectroscopic characterization of several complexes provides keen insights into the complexity of this ostensibly simple, yet important, biological and industrial reaction. A correlation of a series of closely related copper(I) complexes with their oxygenated products highlights the preeminent, though not absolute, role of the ligand in these multi-step formation reactions. A steric continuum of diamine ligands provides a sensitive probe into this copper(I)/O2 reactivity. The diverse product distribution is interpreted through a single unifying mechanism.

Separation of polar and steric effects on absolute rate constants and arrhenius parameters for the reaction of tert‐butyl radicals with alkenes

Abstract: Absolute rate constants and their temperature dependencies were measured for the reaction of tert-butyl radicals with 24 substituted ethenes and several other compounds in 2-propanol solution by time-resolved electron spin resonance. At 300 K the rate constants cover the range from 60 M−1 s−1 (1,2-dimethylene) over 16,500 M−1 s−1 (vinyl-chloride) to 460,000 M−1 s−1 (2-vinylpyridine). For the mono- and 1,1-disubstituted ethenes log k300 increases and the activation energy decreases with increasing electron affinity of the olefins. The frequency factors are in the range log A/M−1 s−1 = 7.5 ± 1.0 as typical for addition reactions, with minor exceptions. Electron affinity (polar) and steric effects on reactivity are separated for the addition of tert-butyl to chloro- and methyl-substituted ethylenes. A comparison with rate data for methyl, ethyl, 2-propyl, and other radicals indicates both polar and steric effects on radical substitution.

Elucidation of the Solution Structures of Transition Metal Complex Ion Pairs by NOE NMR Experiments

Abstract: Our results from structural investigations of transition metal complex ion pairs in solution by homonuclear and heteronuclear 1D- and 2D-NOE NMR spectroscopy are reviewed. Both model (A) and catalytically active (B) complexes have been taken into account; their general formulas are: trans-[ML2(N,X)(Y)(Z)]+A− (A) {M = FeII, RuII and OsII, L = PMe3, Y = CO, Z = COMe, A− = BPh3R−, CF3SO3−, BF4−, PF6− and B[3,5-(CF3)2C6H3]4−; M = IrIII, L = PR3, Y = Z = H and A− = BPh4−, CF3SO3−, BF4−, PF6−} and [M′(N,X)(R,olefin)]+A− (B) {M′ = Pt, R = Me, olefin = ethene, propene and CH2=CHCOOMe, A− = BF4−; M′ = Pd, (R,olefin) = η1,η2-5-methoxycyclooctenyl, A− = BPh4−, CF3SO3−, BF4−, PF6−, SbF6− and B[3,5-(CF3)2C6H3]4−}, where N,X = bis(pyrazolyl)methane, bipyridine, α-diimines, α-diamines or α-oxo imines. The detection of dipolar interionic interactions in 1H-NOESY and 19F,1H-HOESY NMR spectra allowed relative anion-cation orientations to be determined, these being well defined in most cases, with one orientation predominating in solution. We often found that the anion approaches the complexes from the side of the N,X-ligands, due to an accumulation of positive charge at the junction of the moieties containing the N- and X-donor atoms. In square-planar complexes B, the apparently favoured apical positions were often protected either by electronic factors [N,X = bis(2-pyridyl)amine or bis(pyrazolyl)methane] or by a combination of electronic and steric factors (N,X = α-diimines). For M′ complexes, bearing Ar−N=C(Me)−C(Me)=N−Ar ligands, the position of the counter-anion is finely modulated by the steric hindrance of the ortho-aryl substituents. The accessibility of the metal centre, which is directly associated with the catalytic performances of the complexes, was determined by using the counteranion as a probe. Finally, average interionic distances were estimated for model complexes A, bearing unsymmetrical BPh3R− counter-anions, based on the quantification of interionic NOEs. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2003)