Tetra-Amido Macrocyclic Ligand (TAML) at Silicon(IV): A Structurally Constrained, Water-Soluble Silicon Lewis Superacid.

Tetracoordinate silicon species are typically tetrahedral, weak Lewis acids, and often sensitive to moisture. In this study, we present a tetra-amido macrocyclic ligand (TAML)-substituted Si(IV), isolated as its bis(pyridine) adduct. Due to structural constraint toward anti van't-Hof/Le Bel geometry, this compound exhibits Lewis superacidity and effectively catalyzes the hydroboration of pyridine. Kinetic and computational analyses of the catalytic cycle reveal that TAML-Si(IV) acts as a hydride transfer agent, and the hydrido silicate key intermediate is isolated. Notably, the Lewis acid is highly soluble (5 g/L) and long-term stable in water. Unlike previously described silicon-H2O adducts, the bound water becomes substantially acidified, reaching the BroÌ·nsted superacidity range. A comparison of water affinity versus pKa lowering confirms our previous theory of the strength and the effect of Lewis acids. Overall, the compound's unlimited water compatibility and its mechanistically understood catalytic efficiency mark significant progress in applying structural constraint strategies for p-block element-based catalysis, while the acidification touches critical aspects of zeolite and silica surface chemistry.

Hyperpolarisation of weakly binding N-heterocycles using signal amplification by reversible exchange.

Signal Amplification by Reversible Exchange (SABRE) is a catalytic method for improving the detection of molecules by magnetic resonance spectroscopy. It achieves this by simultaneously binding the target substrate (sub) and para-hydrogen to a metal centre. To date, sterically large substrates are relatively inaccessible to SABRE due to their weak binding leading to catalyst destabilisation. We overcome this problem here through a simple co-ligand strategy that allows the hyperpolarisation of a range of weakly binding and sterically encumbered N-heterocycles. The resulting 1H NMR signal size is increased by up to 1400 times relative to their more usual Boltzmann controlled levels at 400 MHz. Hence, a significant reduction in scan time is achieved. The SABRE catalyst in these systems takes the form [IrX(H)2(NHC)(sulfoxide)(sub)] where X = Cl, Br or I. These complexes are shown to undergo very rapid ligand exchange and lower temperatures dramatically improve the efficiency of these SABRE catalysts.