Structural Studies Of Steric Effects In Phosphine Complexes

Author : Brian David Rekken
Publisher :
ISBN 13 : 9781267969538
Total Pages : pages
Book Rating : 4.9/5 (695 download)

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Download or read book Steric Effects and Dispersion Forces Within Dichalcogenolate (O-Se) Metallylenes (Si-Pb) and Other Encumbered Systems written by Brian David Rekken and published by . This book was released on 2012 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: The understanding of how steric effects can manipulate a compound's geometry, physical properties, and reactivity is discussed. The aim of this research is to expand the approach by which chemists use steric hindrance to control reactivity and demonstrate that classic or intuitive understandings of steric effects are not always straightforward in predicting the structure of a compound or outcome of a reaction. This is established principally by the synthesis and characterization of new periodic trends for the heavier, divalent group 14 elements (Si-Pb) and several iron(II) and boron(III) compounds. Three series of functionalized terphenyl compounds, which include primary amines, secondary amines, and mercaptans; a secondary phosphine; and the boron derivatives of the amides are presented. The data clearly reveal the presence of steric effects that result in bond lengthening and wide angles. Furthermore, trends within the X-ray crystallographic measurements as well as IR and 1H NMR spectroscopy reveal that as the ligands become bulkier, there is a downfield shift of the E-H (E = N, S) signal in the 1H NMR spectra, except in cases where the bulkiest ligand, Ar(Pri8) ((ArPri8) = C6H-2,6(C6H2- 2,4,6-Pr(i)3)2-3,5-Pr(i)2), is used which have a distinct upfield shift. Derivatives of the amines were used to produce several novel examples of aminoboranes which can be sterically adjusted to control C-H bond activation and a potassium amide with unusual packing. The synthesis and characterization of the first stable, acyclic two-coordinate silylenes, Si(SAr(Me6))2, (Ar(Me6) = C6H3-2,6(C6H2-2,4,6-Me3)2) by reduction of SiBr2(SAr(Me6))2 with a magnesium(I) reductant, as well as Si(SAr(Pri4))2 (Ar(Pri4) = C6H3-2,6(C6H3- 2,6-Pr(i)2)2) and Si(SAr(Pri6))2 (Ar(Pri6) = C6H3-2,6(C6H3- 2,4,6-Pr(i)3)2) by Rieke's magnesium, with a catalytic quantity of anthracene, are described. They feature V-shaped silicon coordination with a S-Si-S angles ranging between 90.52(2) and 84.8(1)° and an average Si-S distances spanning 2.158(3)-2.089(9) [angstrom]. Although they react readily with an alkyl halide, none of the silylenes react with hydrogen under ambient conditions probably as a result of the large energy difference between the frontier silicon lone pair and 3p orbitals (ca. 4.3 eV for Si(SAr(Me6))2). The complexes, Pb(ChAr(Pri4))2 (Ch = O (1), S (2); Ar(Pri4) = C6H3-2,6-(C6H3-2,6-Pr(i)2)2) were synthesized by alcoholysis and salt metathesis routes and represent the first fully characterized monomeric, two-coordinate, lead dichalcogenolates in the solid state. Structural studies showed that the S-Pb-S angle (77.21(4)°) is ca. 22° narrower than the corresponding O-Pb-O angle. 207Pb NMR and electronic spectroscopy show that the HOMO-LUMO separation decreases from the bisaryloxo plumbylene to the bisthiolato derivative. Furthermore, the monothiolato lead(II) complex, {Pb( Br)([mu]-SAr(Me6))}2 (3) and the lithium tristhiolato plumbate, LiPb(SAr(Me6))3 (4), were isolated from the same reaction. Taken with 2, a step-wise mechanism processing from the monothiolato- to trithiolato Pb(II) complexes is discussed. The synthesis, spectroscopic and structural characterization of an extensive series of acyclic, monomeric tetrylene dichalcogenolates of formula M(ChAr)2 (M = Si, Ge, Sn, Pb; Ch = O, S, or Se; Ar = bulky m-terphenyl ligand) are described. They were found to possess several unusual features--the most notable of which is their strong tendency to display acute interligand, Ch-M-Ch, bond angles that are often well below 90°. Furthermore, and contrary to normal steric expectations, the interligand angles were found to become narrower as the size of the ligand was increased. Experimental and structural data in conjunction with high-level DFT calculations, including corrections for dispersion effects, led to the conclusion that dispersion forces played a key role in stabilizing their acute interligand angles. Two Fe(II) complexes of formula Fe{N(H)Ar(Pri)(n)}2 (n = 6 (1), 4 (2)) were prepared which feature rigidly linear N-Fe-N bond angles that allow for the Fe(II) complexes to afford an orbital component to their overall magnetic moment which is typically not observed for first-row transition metals. The magnetic moment of 1 (5.4 [mu](B)) and 2 (5.6 [mu](B)) is then compared with those of other linear Fe(II) complexes which all have magnetic moments greater than the expected spin-only value (4.9 [mu](B)). In addition, the synthesis and characterization of the mono-primary amido Fe(II) and Sn(II) complexes with bridging halides (Fe{Br}{N(H)Ar(Pri)(8)})2 (3) and (Sn{Cl}{N(H)Ar(Pri)(4)})2 (4) are presented.