Diiminopyridine complexes of Ti, Zr and Hf
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Date
2018
Authors
Rahimi, Naser
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Abstract
A series of formally low-valent titanium complexes of a diiminopyridine (DIP) ligand were synthesized and the oxidation states of metal and ligand were characterized by a combination of techniques: Nuclear Magnetic Resonance, X-ray Diffraction, X-ray Photoelectron Spectroscopy as well as Density Functional Theory calculations. It was elucidated that the unpaired electron in (DIP)TiCl3 mostly resides in a metal 3d orbital, implying a TiIII oxidation state. Interestingly, one electron reduction of (DIP)TiCl3 to (DIP)TiCl2 led into oxidation of TiIII to TiIV, subsequently reduction of DIP0 to DIP2-.
As an extension to the heavier group IV metals, complexes of (DIP)MCl4 (M = Zr, Hf) were synthesized and reduced. In contrast to the Ti case, attempts to prepare (DIP)MCl3 failed, reduction proceeding immediately to the formally divalent complexes (DIP)MCl2, which were shown to contain dianionic chelates like their Ti counterparts. (DIP)ZrCl2 crystallizes as a dimer while its hafnium counterpart is monomeric in the solid state.
Attempts to use the reducing power of the two electrons which reside in the pi system of the ligand in (DIP)TiCl2 failed. However, its zirconium and hafnium counterparts broke a C-Cl bond of CH2Cl2 instantly. (DIP)TiMe2 reacted with hydrazobenzene (PhNH-NHPh), furnishing a titanium amido-imido complex (DIP+Me)Ti(NHPh)(=NPh).
A unique H-shift isomerization of the ligand was observed in several cases. Alkylation of (DIP)MCl2 (M: Ti, Hf) using LiR or RMgX in excess resulted in formation of (DIP+H/-H)MR2, in which one H atom has been transferred from one imine methyl group to the other imine carbon. (DIP)ZrCl2 underwent the same type of isomerization on standing whereas it was not observed in either (DIP)TiCl2 or (DIP)HfCl2.
When DIP ligands bearing various sizes of N-aryl fragments were reacted with MBn4 (M: Zr, Hf), two benzyl groups migrated to the imine carbons, forming a mixture of Cs and C2 isomers. In contrast to previously reported mono-alkylated DIP complexes, heating either of the pure diastereomers led into re-establishing of the Cs/C2 isomerization equilibrium rather than transfer of benzyl groups to the pyridine ring.
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Keywords
Redox-active ligands, diiminopyridine, titanium, zirconium, chemical noninnocence, H-shift isomerization