Explorations in Oxazoline Chemistry: Transition metal complexes of 2-acylmethyl-2-oxazoline ligands and novel thionated and selenated derivatives

Candidate: Matthew Hill

Supervisor: Dr. Robert Gossage

Abstract:

A series of six oxazolines (the Tohda oxazolines) have been used to synthesize a
series of novel acyl metal complexes of early transition metals vanadium (vanadyl) and manganese, as well as group 12 metals zinc, cadmium, and mercury. Five vanadyl complexes were characterized by solid-state X-ray crystallography, with the phenyl-derivative used as a benchmark for the epoxidation of styrene into styrene oxide (phenyloxirane) but this was only marginally better than the blank test performed in absence of catalyst under the same conditions. A manganese complex was synthesized but was not isolated due to instability in solution. A test of the catalytic potential of this complex for the cross-coupling of iodobenzene and thiophenol yielded an indeterminate result, with more work needed in this regard. The group 12 metal centers yielded interesting structural features: the six structurally characterized zinc derivatives were all monomeric tetrahedrons, while two cadmium analogues (Cd-1a,c) were dimeric, featuring the first known examples of monoleptic monometallic cadmium complexes. Cadmium complex Cd-1e in contrast coordinates an additional excess nitrogen base from the mother liquor, thus yielding a series of cadmium-N-base adducts (Cd-1e-(a-f). Subjecting
Cd-1e-a to heat and vacuum and recrystallization in air removes the N-base to yield a dimeric form of Cd-1e analogous to Cd-1a and Cd-1c, except featuring an additional
bridging oxygen in the form of water. The mercuric complexes Hg-1a-c provide a good overview of group 12 metal chemistry through their monomeric seesaw geometry which is believed to be indicative of relativistic effects at play. This differentiates them from the tetrahedral monomeric zinc and dimeric cadmium counterparts.

In the second part of the research project, the Tohda oxazoline proligands were
subject to chalcogen displacement Lawesson’s reagent, swapping the chelating acyl
oxygen with sulfur to yield six stable thioketones characterized by NMR spectroscopy and HRMS, with one derivative characterized by solid-state X-ray crystallography. Use of by pentaphosphorous sulfide (P4S10) as thionating agent instead chelate a S=P=S unit yielded the first known examples of dithioxophospharane oxazolines (Ditphoxes). Early work in establishing their coordination ability has provided some spectroscopic evidence (31P-NMR) of coordination to platinum. The analogous selenation by Woollins’ reagent gave a selenazolidine in a low yield.

Lastly, thioketone derivative 2a that was synthesized in the previous part was to
synthesize thiolate metal complexes with a variety of metal centers, including Group 9, 10, 11, and 12 metal centers and heavy p-block elements Pb and Tl. The novel cobalt thiolate Co-2a was tested for the polymerization of methylmethacrylate, and early results failed to provide evidence of conversion to polymethylmethacrylate. Early work synthesizing a rhodium cyclooctadiene oxazoline thiolate yielded some spectroscopic evidence of coordination. Nickel complex Ni-2a was tested against the previously synthesized acyl analogues Ni-1a for the polymerization of styrene into polystyrene, and results were indicated that Ni-1a outperformed Ni-2a on this process, synthesizing a higher molecular weight polymer with a PDI nearer to unity and a higher conversion. The novel palladium thiolate Pd-2a was compared against previously synthesized Pd-1a for the Suzuki cross-coupling of iodobenzene and phenylborate but failed to yield and product, likely due to sulfur poisoning of the catalytic activity. Novel Platinum thiolate Pt-
2a was tested for the silylation of styrene, and results of the 29Si-NMR spectrum indicate that the silylation may have occurred, albeit at elevated temperatures. Copper thiolate Cu-2a was synthesized to yield a deep green powder, but due to instability in solution has not been structurally characterized. The group 12 thiolates (Zn-2a, Cd-2a, Hg-2a) were also synthesized and compared structurally to their acyl analogues, with mercury thiolate Hg-2a analyzed by computational methods against Hg-1a and Zn-1a to determine if relativistic effects are involved on the structural differences. Lastly, thallium and lead thiolates Tl-2a and Pb-2a were synthesized in an effort to test the reactivity of the thioketone 2a with toxic heavy p-block metals. Tl-2a has been spectroscopically characterized and is awaiting solid-state characterization, while there is some evidence spectroscopically for Pb-2a, but due to solubility issues full characterization has remained elusive.