Synthesis and Reactivity of Cyclopentadienylhafnium Phosphido Complexes. Hydrogenolysis and Carbon Monoxide Insertion for Hf-PR2 Bonds

Article abstract of DOI:10.1021/ja00302a013

The syntheses of mono(pentamethylcyclopentadienyl)hafnium derivatives with the bulky, soft donor ligand di-(tert-butyl)phosphide are described.Cp^*HfCl2 (1) (Cp^* = ?⁵-C5Me5) and Cp^*HfCl2 are obtained from Cp^*HfCl3 and LiP(CMe3)2.Compound 1 reacts with neopentyllithium or benzylpotassium to afford Cp^*HfCl(CH2CMe3) or Cp^*HfCl(CH2Ph), whereas Cp^*HfMe2 and Cp^*HfCl(Ph) are best prepared by treatment of Cp^*HfMe2Cl or Cp^*HfPhCl2 with LiP(CMe3)2.Cp^*HfMe2 reacts smoothly with H2 to yield the surprisingly unreactive *HfMe(μ-H)<μ-P(CMe3)2>>2, whose X-ray determined structure (a=11.484(2) Angstroem, b=16.447(5) Angstroem, c=11.826(2) Angstroem, β=118.92(1) deg, P2₁/c) shows a quadruply bridged dimer (Hf-CH3 =2.258(9) Angstroem; Hf-P =2.805(2), 2.807(2) Angstroem; Hf-H =2.12(13), 2.33(13) Angstroem; Hf-ring centroid :2.264 Angstroem).The hafnium-phosphide bonds are also susceptible to hydrogenolysis.Thus, Cp^*HfCl2 reacts rapidly with 2-3 atm of H2 with the release of free HP(CMe3)2.The relative rates of hydrogenolysis of the Hf-R bond of Cp^*HfCl(R) (R = P(CMe3)2, CH2CMe3, and CH2Ph) suggest that the Hf-P(CMe3)2 bond energy is the weakest in the series.The similarity of Hf-P(CMe3)2 bond energies to those of hafnium alkyls is also evidenced by the rapid insertion of CO with 1 to generate the fierst carboxyphosphide derivative of a transition element, Cp^*HfCl22-C(O)P(CMe3)2>, whose structure was determined (a=14.919(2) Angstroem, b=9.952(1) Angstroem, c=15.779(2) Angstroem, P2₁2₁2₁).The carboxyphosphide fragment shows some lengthening of the C-O bond, 1.284(6) Angstroem, and shortening of the P-C bond, 1.782(5) Angstroem, to suggest substantial contribution from P=C-O and P-C=O resonance forms (other important bond lenghts : Hf-O 2.117(4) Angstroem; Hf-C(acyl) = 2.03(5) Angstroem; Hf-Cl = 2.383(2), 2.393(2) Angstroem; Hf-ring centroid = 2.157 Angstroem).