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.

Sterically Congested Phosphite Ligands: Synthesis, crystallographic characterization, and observation of unprecedented eight-Bond 31P,31P coupling in the 31P-NMR spectra

Abstract: The synthesis and characterization of 2-{1-{3,5-bis(1,1-dimethylethyl)-2-{[2,4,8,10-tetrakis(1,1-dimethylethyl)dibenzo[d,f][1,3,2]dioxaphosphepin-6-yl]oxy}phenyl}ethyl}-4,6-bis(1,1-dimethylethyl)phenyl diphenyl phosphite (6) is described. In the 31P-NMR spectrum (1H-decoupled) of 6, an unprecedented eight-bond P,P coupling of J = 72.8 Hz is observed. In the X-ray crystal structure of 6, an intramolecular P–P distance of 3.67 A is found, which is within the sum of the van-der-Waals radii of the P-atoms. The observed intramolecular P–P distance suggests that a through-space coupling mechanism is operative. The solid-state conformation of 6 is compared to the conformation obtained by semi-empirical MO geometry optimizations (PM3 method). The calculated geometry suggests that the solid-state structure is near a true energy minimum, but that crystal-packing forces decrease the intramolecular P–P distance in the solid state. In the absence of crystal-packing forces, however, the collisional and vibrational energy available in solution may lead to the population of states with a shortened intramolecular P–P distance in 6. The proximity of the P-atoms in 6 is due to restricted conformational freedom resulting from steric congestion within the molecule. The free energy of activation (ΔG* = 10.2 and 10.8 kcal/mol for unequal populations of exchanging conformers) for ring inversion of the dibenzo[d,f][1,3,2]dioxaphosphepin ring in 6 is determined by variable-temperature 31P-NMR spectroscopy. Semi-empirical MO calculation on model compounds suggest that the structure of the transition state for ring inversion has the two aryl rings and O-atoms in a common plane, with the P-atom lying above this plane.

Strong Trans Influence Methoxymethyl Ligand in B(12) Cobaloxime and Imine/Oxime Model Complexes: Structural, Spectroscopic, and Molecular Mechanics Investigations

Abstract: The CH(2)OCH(3) ligand has a large trans influence comparable to that of bulky alkyl groups but lacks the complication of marked steric effects. However, crystals of model complexes with this ligand have proved difficult to obtain. The crystal structures of the cobaloximes reported here, Me(3)BzmCo(DH)(2)CH(2)OCH(3).0.6CH(3)OH (1) and 4-MepyCo(DH)(2)CH(2)OCH(3) (2) [DH = monoanion of dimethylglyoxime, Me(3)Bzm = 1,5,6-trimethylbenzimidazole, and 4-Mepy = 4-methylpyridine], triple the number of cobaloxime structures with CH(2)OCH(3). Also, these are the first structures in this class of models with an N-donor planar heterocyclic axial donor ligand, L. The planes of the Me(3)Bzm and 4-Mepy ligands are almost perpendicular to the respective planes of the four equatorial DH nitrogen donors and bisect the (O(-))N-Co-N(OH) angles. The Co-N(L) bond distances average 2.096(2) A, confirming the strong trans influence of CH(2)OCH(3). Geometry optimization via molecular mechanics using MacroModel 5.0 and an AMBER-type force field was applied to both cobaloxime-type and imine/oxime-type B(12) models. In our initial work with some imine/oxime models, the calculated structures did not compare well to the solid-state structures. Therefore, adjustments to the force field were evaluated. The major adjustment that improved the fit of the computed and experimental structures was an approximately 10% reduction of the van der Waals (vdw) parameters for both the N donors and the C(sp(2)) atoms linked to the N donors; this adjustment may reflect the electron-withdrawing effect of the metal center. Analysis of (1)H-(13)C coupling constants of Me(3)Bzm lends support to the concept that the atoms in the ligands were modified, but only slightly, by the metal center. To reproduce the dependence of geometric features on the trans influence, different force field parameters for L-Co bonds were used for compounds with weak (Cl) and those with strong (CH(2)OCH(3)) trans influence ligands. These small modifications allowed us to model the structural features of both classes of models well.

Formation and reactivity of (tetraarylporphyrinato)rhodium(II) monocarbonyls: bent RhIICO complexes that react like acyl radicals

Abstract: Reactions for a series of rhodium(II) porphyrins with CO are used in illustrating the use of ligand steric effects in both promoting and inhibiting CO coupling to form α-diketone complexes ((por)RhC(O)C(O)Rh(por))