Amidinates and guanidinates are promising supporting ligands in organometallic and coordination chemistry, highly valued for their accessibility, tunability, and comparability with other popular anionic N-chelating hard donor ligands like β-diketiminates. By far the most powerful way to access these ligands involves direct metal-nucleophile insertion into N,N’- substituted carbodiimides. However, the majority of reported examples require the use of commercially accessible carbodiimide peptide coupling reagents with simple alkyl substituents leading to low variation in potential substituents. Presented here is the design, synthesis, and isolation of a novel N,N’-bis[3-(diphenylphosphino)propyl]carbodiimide via an Aza-Wittig reaction between two previously described air stable substrates. At room temperature, 3-(diphenylphosphanyl-borane)-propylisocyanate was added to N-(3-(diphenylphospino)propyl)-triphenylphosphinimine, leading to product formation in minutes. One-pot phosphine-borane deprotection, followed by simple filtration of the crude mixture through a small, basic silica plug using pentane and diethyl ether granted the corresponding carbodiimide in high purity and yield (over 70%), confirmed by 1H, 13C, and 31P NMR spectroscopy. In addition to accessing different central carbon substituents, modification of phosphine substituents should be easily accessible through minor variations in the synthesis. With these precursors, anionic amidinates and guanidinates capable of κ4 -N,N,P,P-coordination may be accessed. The ability of the labile phosphine arms to associate and dissociate may facilitate catalysis. Thus, this carbodiimide provides a tunable, reliable one step precursor to novel substituted amidinates and guanidinates for homogeneous transition metal catalysis.
Transition metals have been extensively employed to address various challenges
related to catalytic organic transformations, small molecule activation, and energy storage
over the last few decades. Inspired by recent catalytic advances mediated by redox noninnocent
pyridine diimine (PDI) and α-diimine (DI) ligand supported transition metals,
our group has designed new PDI and DI ligands by modifying the imine substituents to
feature donor atoms. My doctoral research is focused on the development of PDI and DI
ligand supported low valent first row metal complexes (Mn, Fe, Co) and their application
in bond activation reactions and the hydrofunctionalization of unsaturated bonds.
First two chapters of this dissertation are centered on the synthesis and
application of redox non-innocent ligand supported low valent iron complexes. Notably,
reduction of a DI-based iron dibromide led to the formation of a low valent iron
dinitrogen compound. This compound was found to undergo a sequential C-H and C-P
bond activation processes upon heating to form a dimeric compound. The plausible
mechanism for dimer formation is also described here.
Inspired by the excellent carbonyl hydrosilylation activity of our previously
reported Mn catalyst, (Ph2PPrPDI)Mn, attempts were made to synthesize second generation
Mn catalyst, which is described in the third chapter. Reduction of (PyEtPDI)MnCl2
furnished a deprotonated backbone methyl group containing Mn compound
[(PyEtPDEA)Mn] whereas reduction of (Ph2PEtPDI)MnCl2 produced a dimeric compound,
[(Ph2PEtPDI)Mn]2. Both compounds were characterized by NMR spectroscopy and XRD
analysis. Hydrosilylation of aldehydes and ketones have been studied using
[(PyEtPDEA)Mn] as a pre-catalyst. Similarly, 14 different aldehydes and 6 different
ii
formates were successfully hydrosilylated using [(Ph2PEtPDI)Mn]2 as a pre-catalyst.
Encouraged by the limited number of cobalt catalysts for nitrile hydroboration, we
sought to develop a cobalt catalyst that is active for hydroboration under mild conditions,
which is discussed in the last chapter. Treatment of (PyEtPDI)CoCl2 with excess NaEt3BH
furnished a diamagnetic Co(I) complex [(PyEtPDIH)Co], which exhibits a reduced imine
functionality. Having this compound characterized, a broad substrate scope for both
nitriles and imines have been investigated. The operative mechanism for nitrile
dihydroboration has been investigated based on the outcomes of a series of stoichiometric
reactions using NMR spectroscopy.
In this study, an innovative composite membrane that integrates water-selective molecular sieve particles into a liquid-barrier chemically resistant polymer film is synthetized. This plan manipulates distinctive aspects of the two materials used to create the membranes: (1) the innate permeation and selectivity of the molecular sieves, and (2) the decay-resistant, versatile, and mechanical strength of the liquid-barrier polymer support matrix.
To synthesize the membrane, Linde Type A (LTA) zeolite particles are anchored to the porous substrate, producing a single layer of zeolite particles capable of transporting water through the membrane. Thereafter, coating the chemically resistant latex polymer filled the space between zeolites. Finally, excess polymer was etched from the surface to expose the zeolites to the feed solution. The completed membranes were tested in reverse osmosis mode with deionized water, sodium chloride, and rhodamine solutions to determine the suitability for water recovery.
The main distinguishing characteristics of the new membrane design compared with current composite membrane include: (1) the use of an impermeable polymer broadens the range of chemical resistant polymers that can be used as the polymer matrix; (2) the use of zeolite particles with specific pore size insures the high rejection of the neutral molecules since water is transported through the zeolite rather than the polymer; (3) the use of latex dispersions, environmentally friendly water based-solutions, as the polymer matrix shares the qualities of low volatile organic compound, low cost, and non- toxicity.
Using the phosphine containing PDI chelate, Ph2PPrPDI, several low-valent molybdenum complexes have been synthesized and characterized. While the zerovalent monocarbonyl complex, (Ph2PPrPDI)MoCO, catalyzes the reduction of aldehyde C=O bonds, the C-H activated Mo(II) complex, (6-P,N,N,N,C,P-Ph2PPrPDI)MoH was found to be the first well-defined molybdenum catalyst for reducing carbon dioxide to methanol. Along with low- oxidation state compounds, a Mo(IV) complex, [(Ph2PPrPDI)MoO][PF6]2 was also synthesized and utilized in electrocatalytic hydrogen production from neutral water. Moreover, with the proper choice of reductant, an uncommon Mo(I) oxidation state was stabilized and characterized by electron paramagnetic resonance spectroscopy and single crystal X-ray diffraction.
While the synthesized (PDI)Mo complexes unveiled versatile reduction chemistry, varying the ligand backbone to DI uncovered completely different reactivity when bound to molybdenum. Unlike PDI, no chelate-arm C-H activation was observed with the propyl phosphine DI, Ph2PPrDI; instead, a bis(dinitrogen) Mo(0) complex, (Ph2PPrDI)Mo(N2)2 was isolated. Surprisingly, this complex was found to convert carbon dioxide into dioxygen and carbon monoxide under ambient conditions through a novel tail-to-tail CO2 reductive coupling pathway. Detailed experimental and theoretical studies are underway to gain further information about the possible mechanism of Mo mediated direct conversion of CO2 to O2.