Cationic Zirconium Hydrides Supported by an NNNN-Type Macrocyclic Ligand: Synthesis, Structure, and Reactivity

An air- and light-sensitive, but thermally stable tris­[(trimethylsilyl)­methyl]­zirconium complex containing an NNNN-type macrocyclic ligand [Zr­(Me3TACD)­(CH2SiMe3)3] (1; Me3TACD = Me3[12]­aneN4: 1,4,7-trimethyl-1,4,7,10-tetraazacyclododecane) was prepared by reacting [Zr­(CH2SiMe3)4] with (Me3TACD)­H. Reaction of the zirconium tris­(alkyl) 1 with a Lewis or Brønsted acid gave a dialkyl cation with a weakly coordinating anion [Zr­(Me3TACD)­(CH2SiMe3)2]­[A] [A = Al­{OC­(CF3)3}4 (2a), B­{3,5-C6H3(CF3)2}4 (2b), B­(3,5-C6H3Cl2)4 (2c), and BPh4) (2d)]. Hydrogenolysis of 2a–2c resulted in the formation of the dinuclear tetrahydride dication [{Zr­(Me3TACD)­(μ-H)2}2]­[A]2 (3a–3c). Compounds 1–3 were characterized by multinuclear NMR spectroscopy, and the solid-state structures of 1, 2c, and 3b were established by single-crystal X-ray diffraction studies. The dinuclear hydride complex 3b exhibits a quadruply bridged {Zr2(μ-H)4} core in solution and in the solid state with a relatively short Zr···Zr distance of 2.8752(11) Å. Density functional theory computations at the B3PW91 level reproduced this structure (Zr···Zr distance of 2.900 Å). The cationic hydride complex 3b reacted with excess carbon monoxide in tetrahydrofuran at room temperature to give ethylene in 25% yield based on 3b. Upon analysis of 13C NMR spectra of the reaction mixture using 13CO, oxymethylene and enolate complexes were detected as intermediates among other complexes.