New Side-On Bound Dinitrogen Complexes of Zirconium Supported by an Arene-Bridged Diamidophosphine Ligand and Their Reactivity with Dihydrogen

A new dinitrogen complex, deep blue-green {[NPN]*Zr(THF)}2(μ-η2:η2-N2) ([NPN]* = {[N-(2,4,6-Me3C6H2)(2-N-5-MeC6H3)]2PPh}), was prepared in high yield by the reduction of [NPN]*ZrCl2 with 2.2 equiv
of KC8 in THF under N2. The solid-state molecular structure shows that N2 is strongly activated (N−N bond
length:  1.503(6) Å) and bound side-on to two Zr atoms. Coordinated THF can be readily replaced by
adding pyridine (py) or PMe2R (R = Me, Ph) to the complex to obtain {[NPN]*Zr(py)}2(μ-η2:η2-N2) or {[NPN]*Zr(PMe2R)}(μ-η2:η2-N2){Zr[NPN]*} in high yield. X-ray diffraction experiments show that the N2 moiety is
strongly activated and remains side-on bound to Zr for the py and PMe2Ph adducts; interestingly, only one
PMe2Ph coordinates to the Zr2N2 unit. {[NPN]*Zr(PMe2R)}(μ-η2:η2-N2){Zr[NPN]*} reacts slowly with H2 to
provide {[NPN]*Zr(PMe2R)}(μ-H)(μ-η2:η2-N2H){Zr[NPN]*}, as determined by isotopic labeling, and multinuclear NMR spectroscopy. The THF adduct does not react with H2 even after an extended period, whereas
the pyridine adduct does undergo a reaction with H2, but to a mixture of products.