Donor functionalized N-heterocyclic carbenes and olefins in complexation with early transition and main group metals

Goswami, Santu (2025) Donor functionalized N-heterocyclic carbenes and olefins in complexation with early transition and main group metals. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Santu Goswami (20RS023)) 20RS023.pdf - Submitted Version Restricted to Repository staff only Download (11MB)

Abstract

N-heterocyclic carbene (NHC) coordination to electropositive main group and d⁰ early transition metals is typically weak due to the lack of back bonding. In such scenarios, NHCs with sidearms consisting of hard anionic donors have shown merits by providing extra kinetic stability through chelation and also by offering more stereoelectronic versatility. The NHC-aryloxide hybrid with a methylene linker [ArOCH₂NHC]⁻ (L) is one such ligand class that has displayed a limited but promising range of coordination chemistries so far. We have first chosen Ti(IV) metal which has no d electron and making its monomeric aryloxide tethered NHC complex gives it stability to remain in monomeric form and later when we have tried to explore Al(III) chemistry on the same ligand system by reacting with AlMe₃ and KHMDS then also we get a stable Al(III) NHC complex. Both the complexes are robust catalysts for ROP of ɛ-caprolactone. In our next work, we have explored Bi(III) chemistry in the same ligand system. By stoichiometric reaction of the LH₂Br with Bi(HMDS)₃ we got a monomeric Bi(III) NHC complex. Then we thought we could incorporate another ligand on the same metal complex by reacting with another equivalent ligand and one equivalent of KHMDS. Unexpectedly, we get a new type of Bi(III) metal complex named as Bi(III) NHO metal complex with only 40 % yield, which has an unsymmetrical tethered aryloxide side arm. This is the first example of an NHO bismuth metal complex. Later, we successfully synthesized the aryloxide side-arm NHO ligand precursor and were also able to make the corresponding Bi (III) complex with high yield (75 %). When we reacted NHOBi(III)Br complex with one equivalent AgSbF₆ in THF we got its corresponding Bi(III) cation complex, which is successfully characterized by NMR and SCXRD. This NHOBi(III) complex is a good catalyst for isocyanate dimerization, and it gives urea in toluene at room temp. In the last, we have made a pyridine-based phenolic side arm pre-NHO ligand. But when we reacted with this ligand system with AlMe3 and KHMDS stoichiometrically in toluene at room temperature for 12 h, we did not get the corresponding NHO metal complex; rather, we got a zwitterionic Al (III) metal complex. Due to its positive and negative charge lying in the vicinal position, this complex shows luminescence in the presence of UV light in solution as well as in the crystalline solid state. We have studied its photophysical properties. Unexpectedly, this complex is highly stable in triplet state ( τav = 69 μs in toluene).

Actions (login required)

View Item