Macrocyclic Ligands for Molecular Hydrides of s-Block Metals

Abstract

Hydrides of s-, p-, dand f-block metals are insoluble solid state materials. Currently, hydrides of highly electropositive metals (electronegativity < ≈ 1.6) such as MgH2, Ca(BH4)2, and LaNi5H6 are being considered as hydrogen storage materials to allow reversible hydrogen uptake and release. Metal hydrides could also become useful as inexpensive and non-toxic precursors for homogeneous catalyst, if they can be dissolved. By introducing ligands such as CO, phosphines, and N-heterocyclic carbenes (NHCs) homogeneous hydride catalyst precursors based on late transition metals such as Wilkinsonʼs catalyst became widely available. When early transition metals were considered, their more electropositive (oxophilic) character and large size required new types of more electron-rich ligands to saturate the low valence electron counts of these metals. Bis(h-cyclopentadienyl) ligand systems tremendously contributed to the rapid development of early transition metal compounds, including Brintzingertype ansa-zirconocene catalysts for stereoselective olefin hydrogenation and poly merization. Generally, when dealing with highly electropositive, large metal centers, the use of chelating ligands is preferred, to suppress intermolecular ligand exchange reactions during catalysis. TACD ligands, derived from the NNNN macrocycle cyclen (1,4,7,10-tetraazacyclododecane or [12]aneN4) were initially developed for group 3 metals as alternatives to the ubiquitous cyclopentadienyl ligand class (Fig. 1). Commercial success of gadolinium(III) complexes containing DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate) ligands as MRI contrast agents inspired us to study simpler derivatives such as (Me3TACD)H, the N,N′,N′′ methylated ligand [12]aneN4. 7 Me3TACD acts as an amido triamine supporting ligand (7-electron L3X or 9-electron L4X-type ligand, depending on the presence of p-donation of the amido function). Originally, (Me3TACD)H was conceived as a surrogate for the 5-electron L2X-type cyclopentadienyl ligand to support large metal centers such as lanthanides. The neutral L4-type ligand N,N′,N′′,N′′′ methylated cyclen Me4TACD (Me4[12]aneN4) is similar to the crown ether 12-crown-4 but acts as a more kinetically inert ancillary ligand for metal ions with different size of the s-, p-, d-, and f-block.