Stabilization of titanium(IV) and indium(III) complexes by coordination of [MoO3(1,4,7-triazacyclononane)] metalloligand in aqueous solution

Abstract

A neutral weakly-donating [MoO3(tacn)] (tacn = 1,4,7-triazacyclononane) metalloligand has proved useful to stabilize Ti4+ or In3+ cation in aqueous solution. Thus, the reaction of [MoO3(tacn)] with TiOSO4 or In(NO3)3 in water affords a complex cation [Ti{MoO3(tacn)}6]4+ (1) or [In{MoO3(tacn)}6]3+ (2), respectively. The Ti4+ or In3+ cation is homoleptically coordinated by six [MoO3(tacn)] metalloligands. Each [MoO3(tacn)] coordinates Ti4+ or In3+ via one of its three oxygen atoms; the other two oxygens are hydrogen-bonded to NH groups of the neighboring [MoO3(tacn)]. These hydrogen bonding networks (i) strengthen [MoO3(tacn)] coordination to the Ti4+ or In3+ center and (ii) orient the six [MoO3(tacn)] units into two tripod-like arms that can catch a cation. One of the arms binds a tetramethylammonium cation through non-covalent CH···O bonds in 1 or a fac-{In(OH2)3}3+ cation through In–O coordination bonds in 2. Complex 1 dissolves intact in dry acetonitrile, as confirmed by 1H NMR spectroscopy and ESI-mass spectrometry. Addition of water or formamide to the acetonitrile solution disrupts the hydrogen bond networks and triggers dissociation of [MoO3(tacn)] from Ti4+, and importantly, the number of dissociated ligands can be managed by changing the amount of water or formamide. Therefore 1 is a prominent precursor for bottom-up synthesis of titanium–oxo clusters.