Access Type

Open Access Dissertation

Date of Award

January 2023

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Chemistry

First Advisor

Stanislav Groysman

Abstract

Methylene diphenyl diisocyanate (4,4’-MDI) is an aromatic diisocyanate, which is widely used as a monomer in the polyurethane industry. One of the main challenges with using 4,4’-MDI is its dimerization into uretdione. The polyurethane industry seeks solutions that could suppress dimerization of 4,4’-MDI and extend its shelf-life. The focus of this dissertation is on the design of chemical solutions to this problem including: (1) reversible coordination of MDI (heteroallene) to low-valent mono- or dimolybdenum complexes in redox-active ligand environment, and (2) reversible coordination of MDI with two equivalents of an appropriate N-heterocyclic carbene.

MDI (isocyanate) is a heteroallene, and heteroallenes can undergo coordination to the reactive low-valent transition metal centers, including molybdenum. Towards coordination of MDI, design of an appropriate low-valent molybdenum system supported by redox-active ligand is required. Despite significant interest in low-valent molybdenum chemistry, complexes of formally molybdenum(0) with redox-active bidentate iminopyridine and tridentate bis(imino)pyridine ligands are relatively rare. In Chapter 2, we focus on the synthesis and electronic structures of mononuclear and dinuclear Mo complexes in redox active bidentate iminopyridine ligand environments, and ytheir reactivity with heteroallenes including MDI. Reaction of mononucleating ligand L1 (N-(2,6-diisopropylphenyl)-1-( yridine-2-yl)methanimine) with Mo(CO)3(NCMe)3 precursor resulted in Mo(L1)(CO)3(NCMe). Treatment of Mo(L1)(CO)3(NCMe) with CS2 in THF or prolonged stirring in CH2Cl2 transformed it into Mo(L1)(CO)4, instead the coordination of CS2. The investigation of the electronic structure of the Mo(L1)(CO)3(NCMe) and Mo(L1)(CO)4 systems demonstrated that the iminopyridine ligand in both cases is fully oxidized, and the molybdenum center is Mo(0). The reaction of open-chain dinucleating bis(iminopyridine) ligand L2 (N,N'-(2,7-di-tert-butyl-9,9-dimethyl-9H-xanthene-4,5-diyl)bis(1-(pyridin-2-yl)methanimine)) resulted in Mo2(L2)(CO)6(NCMe)2, which was then similarly transformed into the octacarbonyl Mo2(L2)(CO)8 upon treatment with CS2. Mo(L1)(CO)3(NCMe) failed to produce an isolable product upon reaction with pyridine N-oxide. Oxidation of Mo0(L1)(CO)3(NCMe) with I2 produced heptacoordinate MoII(L1)(CO)3(I)2. In either case we did not observe the activation of the heteroallenes (CS2, phenyl isocyanate, or MDI) by coordinating to the Mo metal centers, including monometallic iodine complexes.

In Chapter 3, we focused on the chemistry of low-valent potentially tridentate bis(imino)pyridine ligands with low-valent molybdenum, and the reactivity of the resulting complexes with heteroallenes. Both mononucleating and homodinucleating bis(imino)pyridine ligands were investigated. The reaction of potentially tridentate bis(imino)pyridine N-mesityl-1-(6-((E)-(mesitylimino)methyl)pyridin-2-yl)methanimine ligand (L3) with Mo(CO)3(NCMe)3 precursor produced Mo(L3)(CO)3(NCMe), in which L3 coordinated Mo(0) in a bidentate fashion, with one of the imine arms unbound. Mo(L3)(CO)4 was produced upon treatment of Mo(L3)(CO)3(NCMe) with CS2 in THF or stirring in CH2Cl2. Treatment of these monometallic Mo complexes with I2 resulted in the tridentate coordination of L3 in Mo(L3)(CO)(I)2. The reaction of macrocyclic di(bis(imino)pyridine ligand L4 with Mo(CO)3(NCMe)3 resulted in the mixture of Mo2(L4)(CO)6(NCMe)2 and Mo2(L4)(CO)8. Treatment of the iodide complexes with heteroallene (MDI) in the presence of AgPF6 did not produce any isolable products. Computational studies of the electronic structures of bis(imino)pyridine molybdenum complexes revealed that the metal is molybdenum(0), similarly to the iminopyridine case.

In Chapter 4, we have explored the application of N-heterocyclic carbenes (NHCs) towards stabilization of MDI. The reaction of MDI with two different bulky commercially available N-heterocyclic carbenes, IPr (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) and IMes (IMes = 1,3-dihydro-1,3-bis (2,4,6-trimethylphenyl)imidazol-2-ylidene), produced respective zwitterionic bis(amidates) in nearly quantitative yields. The resulted adducts, MDI-(IPr)2 and MDI-(IMes)2, were characterized using NMR and IR spectroscopy, mass spectrometry, and X-ray crystallography. The stability of these zwitterionic bis(amidates) and the release of MDI from the adduct were also studied. Adducts were treated with Cu(I) to form the respective bimetallic metal complex MDI-(NHC)2-(CuCl)2, and the effect of temperature for decomposition rate of the complex was explored to evaluate the efficiency of MDI release. The steric/electronic nature (IPr vs IMes) of N-substituents in NHCs has a significant effect on MDI release. Furthermore, insertion of sulfur into Cu-NHC bonds (to form thiourea) improved the efficiency of MDI release while minimizing the MDI loss due to dimer/trimer formation which was observed in the previous case. The separation of the released MDI from the final reaction mixture can be achieved by extracting MDI into cyclohexane.

Download

Share

COinS