Amidinates, guanidinates and iminopyrrolidinates: Understanding precursor thermolysis to design a better ligand

The thermolysis of metal compounds incorporating amidinate-type ligands (RN(H)C(X)NR, where R is any alkyl and X is an alkyl or an amido) show a great complexity and diversity. Amidinates and guanidinates of copper(I), aluminium(III) and gallium(III) can undergo thermal decomposition by two main routes: elimination of carbodiimide (here called “deinsertion”) and by abstraction of a β-hydrogen. In general terms carbodiimide deinsertion tends to be a low temperature and solution-based thermolysis route, and β-hydrogen elimination tends to be a high temperature and gas phase thermolysis path. There is little difference between amidinates and guanidiantes in this respect: both ligands show these decomposition pathways on a variety of metal centres. The iminopyrrolidinate (ip) ligand can be designed without β-hydrogens, and so the ligand tert-butyl-imino-2,2-dimethylpyrrolidinate (tBu-ip) was used on copper(I) to successfully show that this compound does not undergo these two thermolyses, but undergoes a much higher thermolysis through loss of the iminotertbutyl group and subsequent decomposition of the five-membered ring. This “redesigned” ligand boosts the thermal stability of the copper(I) dimer from 225 °C for [CuI{(iPrN)2CNMe2}]2 to 350 °C for [CuI(Me2-tBu-ip)]2.