Advisory Committee Chair
Gary M Gray
Advisory Committee Members
Houston Byrd
Tracy P Hamilton
Christopher M Lawson
Aaron L Lucius
Document Type
Dissertation
Date of Award
2014
Degree Name by School
Doctor of Philosophy (PhD) College of Arts and Sciences
Abstract
The factors influencing the regioselectivities and activities of phosphorus-donor substituted transition metal catalysts in styrene hydroformylation have been studied. These factors are: (1) steric and electronic properties of the phosphorus-donor ligand (2) reaction conditions, including solvent, pressure, and temperature and (3) addition of co-catalysts, such as Lewis acids. Chapter 1 focused on analysis of the combinatorial effects of pressure, solvent, and alkali metal salt on catalysts containing a well-characterized metallacrown ether ligand, (2,2'-(C12H8O2)P(CH2CH2O)4P(2,2'-O2H8C12)). Previously, the regioselectivities of Rh(I) complexes containing (2,2'-(C12H8O2)P(CH2CH2O)4P(2,2'-O2H8C12)) in styrene hydroformylation reactions were determined in two different solvent systems, with or without LiBPh44 or NaBPh4, and at a variety of pressures (5, 10 and 20 atm), however only a qualitative analysis of the data was carried out. In this dissertation, a quantitative analysis of the styrene hydroformylation data was carried out using four 32 factorial designs to determine the contributions of pressure (P), alkali metal salt (S:Rh) and any interactions thereof (P*S:Rh) on regioselectivity (%iso). The Rh(I) catalyst containing (2,2'-(C12H8O2)P(CH2CH2O)4P(2,2'-O2H8C12)) is quite sensitive the reaction conditions surveyed, giving % iso values ranging from 38 to 86% depending on reaction conditions. Pressure has the largest effect on the regioselectivity of this catalyst system, regardless of solvent or alkali metal salt concentration, and the magnitude of the effect in dichloromethane is nearly twice that of tetrahydrofuran. Alkali metal salt, (S:Rh), is the second largest contributor to regioselectivity, but the nature of the salt (LiBPh4 or NaBPh4) is not important whereas the interaction terms between pressure and salt are significantly different (P*S:Rh). The effects of alkali metal salts and those of pressure are synergistic, which may indicate that the mechanism of action is similar. In the second chapter the structure/activity relationships of Rh(I) complexes containing three different novel ligands are determined in styrene hydroformylation. Phosphite or phosphordiamidite ligands derived from 2-(pyridine)methanol or 2,6-pyridinedimethanol were synthesized and characterized by multinuclear NMR spectroscopy. Evaluations of the σ-donating abilities of the ligands via 31P-77Se coupling constants indicate that the phosphite ligand is more basic than the phosphordiamidites. Coordination preferences of the ligands in model metal complexes of cis-tetracarbonylmolybdenum(0) or cis-dichloropalladium(II) centers were elucidated via multinuclear 1D and 2D NMR spectroscopy and X-ray crystallography. Both ligands derived from 2-(pyridine)methanol coordinated in a cis-κ2-P,N fashion in both the molybdenum and the palladium metal complexes, however the phosphordiamidite ligand derived from 2,6-pyridinedimethanol coordinated predominantly in a cis-κ2-P,P fashion. The molybdenum complex of the ligand derived from 2,6-pyridinedimethanol initially bound in a cis-κ2-P,P fashion but then underwent spontaneous carbonyl elimination to give a fac-Mo(CO)4(κ3-P,N,P) complex. X-ray crystallographic studies also determined that the steric parameters of these ligands are not very different. Styrene hydroformylation reactions catalyzed by Rh(I) complexes of these new ligands were carried out at multiple temperatures and pressures, with and without alkali metal salts, and were compared to two simple phosphites. All of the ligands studied have a positive order with respect to syngas, and catalysts containing P-donor/pyridine hybrid ligands have nearly double the activities of catalysts containing phosphites functionalized by ether groups. The regioselectivities of the catalysts are surprisingly similar when compared at the same temperature and pressure (80 °C, 20 atm, no salt: 23 ± 1.4 % n-aldehyde) given the wide array of structural features (i.e. P,N chelating, bidentate phosphite metallacrown ethers, monodentate phosphites, phosphoramidites). The regioselectivities of catalysts containing ligands derived from 2-(pyridine)methanol or 2,6-pyridinedimethanol were not sensitive to the addition of alkali metal salt, whereas the catalyst containing phosphite ligands did have significant changes in regioselectivity. A 22 factorial design experiment and analysis determined that the effects of both pressure and temperature on regioselectivities of catalyst containing 2(P(pyrr)2(oxymethyl)pyridine are statistically significant, and that by varying these parameters over the range studied one can vary the % n-aldehyde from 15 to 50%. The data were reduced to one equation, where the percent n-selectivity is defined as a function of the constant (a0), the main affects, (aP and aT), individual linear contributions of pressure and temperature, and the interaction term (aPT) (% n-aldehyde = 30.98 - 10.63P + 7.08T - 2.63PT). From this, a response surface was generated, and the equation was used to predict regioselectivity outcomes for various temperature and pressure conditions, and to predict conditions necessary to optimize either n or iso-selectivity. Chapter 3 describes the synthesis and an in-depth analysis of the steric and electronic descriptors for four complimentary complexes of biaryl phosphite ligands that may be considered precursors of many ligands used in transition metal catalysis. The systematic variation (biphenoxy vs. binaphthoxy biaryl groups, chloro vs. oxo groups as the third substituents) allowed for the effects on the steric and electronic properties of the ligands to be independently evaluated. NMR spectroscopy measurements of W-P coupling and X-ray crystallographic data have shown that changing the biaryl groups from biphenyl to binaphthyl does not affect the electron donor ability of the ligand or the steric descriptors. In contrast, changing the third substituent from chloro to oxo has a significant effect on the electronic nature of the phosphorus, but the effect on the steric properties of the ligands is limited. These finding are quite interesting because catalysts derived from biphenoxy phosphites are reported to give different activities and selectivities compared to those derived from binaphthoxy.
Recommended Citation
Hastings, Samantha Delmont, "Phosphorus-Donor Substituted Transition Metal Complexes: Coordination and Alkene Hydroformylation" (2014). All ETDs from UAB. 1889.
https://digitalcommons.library.uab.edu/etd-collection/1889