Magnesium hydrides bearing sterically demanding amidinate ligands: synthesis, reactivity and catalytic application

Clare Bakewell

doi:10.1039/d0dt02523b

Abstract

The synthesis and characterisation of a series of magnesium complexes bearing sterically demanding amidinate ligands is reported; this includes magneisum amides (1a and 1b), hydrides (3a and 3b) and alkyl complexes (2b). The solid and solution state behaviour of the complexes has been investigated using single crystal X-ray diffraction and NMR spectroscopy, revealing the magnesium hydrides to exist as dimers in the solid state, dispite the sterically demanding ligand systems and showing a degree of monomeric character in solution. The stoichiometric and catalytic activity of the amidinate complexes were investigated, with the complexes found to efficiently mediate both the hydroamination of N,N'-diisopropylcarbodiimide and the Tishchenko reaction. The metal hydrides are highly reactive towards coordinating substrates, showing a significant increase in catalytic rate compared with more ubiquitous β-diketiminate magnesium hydrides.

Highlights

There is a growing interest in replacing expensive transitions metal based catalysts with cheap, Earth abundant and nontoxic main-group metals
Two ligand systems were chosen to be the focus of this study, LH1 and LH2, both of which were synthesised by modifying literature procedures (Fig. S1†)
The catalytic activity of these complexes has been investigated, with compounds 1a–b and 3a–b shown to efficiently mediate the hydroboration of N,N′-diisopropylcarbodiimide. 1a and 3b are extremely proficient at catalysing the Tishchenko reaction (100 equivalents < 10 min) and 1a–b/3a–b all react stoichiometrically with phenylacetylene. Comparing this reactivity with other magnesium hydrides draws mixed conclusions. 1a and 3b catalyse the hydroboration of N,N′-diisopropylcarbodiimide significantly faster than the Mg–H dimer A, but unlike A they show no reactivity towards alkenes

Summary

Introduction

There is a growing interest in replacing expensive transitions metal based catalysts with cheap, Earth abundant and nontoxic main-group metals. Paper hydrosilylation and the hydroboration of carbodiimides.[7] Sterics have been used to favour the formation of monomeric, low-coordinate compounds leading to the isolation of a ‘super bulky’ β-diketiminate magnesium hydride complex, D.

Results

Conclusion