From dinuclear zirconium(+3)-phosphido complexes {[Cp2ZrP(H)R]2} to the first neutral zirconocene-phosphinidene dimers [(Cp2ZrPR)2] by P-H dehydrogenation

The syntheses and structures of the new zirconium(+3) silylphosphanido complexes {[CP2ZrP(H)R](2)} 1a {Cp = eta(5)-C5H5; R = SiMe2C(iPr)Me-2}, 1b (R = SiiPr(3)), and 1c (R = SiF(tBu) (2,4,6-iPr(3)C(6)H(2)) are reported. The latter are easily accessible by the reaction of the lithium salts of the corresponding primary silylphosphanes with the Schwartz reagent [Cp2Zr(H)Cl] in THF at -70 degrees C. They can be isolated in the form of diamagnetic red-black crystals in 68-85% yield. The central structural motif of the complexes consists of planar (1a, 1b) and slightly puckered Zr2P2 rings (1c), and contains two d(1)-Zr(+3) centers, which are antiferromagnetically coupled (super exchange, through-bond interaction) although the transannular Zr-Zr separation is around 360 pm. Thus, the compounds have a biradicaloid electronic structure. Heating of solutions of 1a-c in toluene in the presence of Pd/C or [(Ph3P) Pt-2(C2H4)] results in catalytic dehydrogenation of the P-H bonds, affording the first neutral zirconocene-phosphinidene dimers [Cp2ZrPR](2)3a-c in the form of deep red crystals in 72-88% yield. The composition of the products is proven by multinuclear NMR spectroscopy, mass spectrometry and correct C,H-combustion analyses. The molecular structures of 3a and 3c were additionally confirmed by X-ray crystal structure analyses, showing that they consist of Zr2P2 rings with significantly shorter Zr-P distances than those observed in 1a and 1c. Interestingly, the dehydrogenation of the mostly sterically crowded derivative 1c occurs also in boiling toluene solutions without using a catalyst, but the process is prevented in a H-2 atmosphere. This suggests that the Zr(+3) centers in 1c can serve as active sites for the P-H bonding activation., which, however, are deactivated in the presence of H-2. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451. Weinheim, Germany, 2005).