Molecular Phosphide Complexes of Zirconium

Molecular Zr^IV phosphides are extremely rare, with no examples containing a one-coordinated and terminal triple-bonded phosphorus atom. We report here an isolable and relatively stable Zr phosphide complex, [(PN)₂Zr≡P{μ₂-Na(OEt₂)}]₂ (4), stemming from a cyclometalated Zr-hydride, [(PN)(PN')Zr(H)] (2), and NaPH₂. Complex 2 is prepared from two- or one-electron reductions of precursors [(PN)₂ZrCl₂] (1) or metastable Zr^III[(PN)₂ZrCl], respectively. Oxidation of 2 with ClCPh₃ (2 equiv) or I₂ (1 equiv) resulted in re-formation of the PN ligand and isolation of the Zr^IV complexes [(PN)₂ZrX₂] (X = Cl, 1; X = I, 3), whereas addition of a weak acid to 2 allowed us to intercept the hydrido-halide intermediate [(PN)₂Zr(I)(H)] (X = Cl, I) spectroscopically before conversion to 1 or 3. Complex 2 exchanged with D₂ (1 atm) to fully deuterate the methylene all ortho-methyl groups of the PN and PN' ligands and the hydride. Discrete salts of 4 can be readily prepared from Na⁺ encapsulation with the crown ether 1,4,7,10,13,16-hexaoxacyclooctadecane (18-C-6) or cryptand 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane (222-Kryptofix) to form [Na(18-C-6)(THF)₂][(PN)₂Zr≡P] (5) and [Na(222-Kryptofix)][(PN)₂Zr≡P] (6), respectively, which were structurally and spectroscopically characterized. Compounds 4-6 demonstrate exceptionally short Zr≡P bonds (4, 2.3270(18) Å; 5, 2.291(3) Å; 6, 2.2989(17) Å) and highly downfield ³¹P NMR spectral resonances (4, 819 ppm; 5 and 6, 927 and 955 ppm) in accord with a terminal phosphide ligand. The Zr≡P motif in 4-6 can be stabilized via coordination to a softer Tl⁺ ion to form a nonsolvated phosphide [(PN)₂Zr≡P{μ₂-Tl}]₂ (7), exhibiting a phosphide resonance at ∼711 ppm.