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.