Synthetic, structural, and bonding studies of phosphido-bridged early-late transition-metal heterobimetallic complexes

Article abstract of DOI:10.1021/om00158a037

The mononuclear terminal phosphide complexes Cp₂M(PR₂)₂ (M = Zr, Hf; R = Et, Ph, Cy) are shown to act as chelating bis(phosphine) ligands to Ni, Pd, and Pt (M′) in compounds of the form Cp₂M(μ-PR₂)₂M′L_n (n = 1, 2) and {Cp₂M(μ-PR₂)₂}₂M′. Reaction of Cp₂M(PEt₂)₂ with Ni(1,5-COD)₂ yields either Cp₂M(μ-PEt₂)₂Ni(1,5-COD) or {Cp₂(μ-PEt₂)₂}₂Ni depending on the stoichiometry employed. Analogous reactions with Pd(PPh₃)₄ and Pt(PPh₃)₃ yield mixtures of Cp₂M(μ-PEt₂)₂M′(PPh₃) and {Cp₂M(μ-PEt₂)₂}₂M′ (M′ = Pd, Pt) when the M/M′ ratio is <2. With bulkier substituents, Cp₂M(PR₂)₂ (R = Ph, Cy) react with Pt(1,5-COD)₂ and CpPd(η-2-Me-allyl) in the presence of phosphines and phosphites yielding only Cp₂M(μ-PR₂)₂M′L (L = PR′₃, P(OR′)₃). The M′L₂ complexes Cp₂M(μ-PPh₂)₂M′(DMPE) were also prepared. The three classes of compounds were characterized by X-ray crystallography. The structure of Cp₂Zr(μ-PEt₂)₂Ni(μ-PEt₂) ₂HfCp₂ (C2/c, Z = 8, a = 34.823 (4), b = 10.991 (2), c = 21.606 (3) A?, β = 105.65 (1)°) consists of two 16-e^- d⁰ metal centers coupled to a pseudotetrahedral 18-e^- d¹⁰ Ni center by PEt₂ bridges. The MP₂Ni rings are nearly planar with an average M?Ni separation of 3.027 (1) A?. The structure of Cp₂Hf(μ-PPh₂)₂Pd(PPh₃) (C2/c, Z = 8, a = 27.701 (8), b = 10.654 (5), c = 30.330 (11) A?, β = 103.16 (5)°) consists of edge-shared, 16-e^- pseudotetrahedral Hf(IV) and trigonal planar Pd(0) centers with a puckered HfP2Pd ring and a Hf?Pd separation of 2.896 (1) A?. In the structure of Cp₂Hf(μ-PPh₂)₂Pd(DMPE) (C2/c, Z = 8, a = 25.361 (12), b = 19.584 (5), c = 18.814 (13) A?, β = 126.15 (5)°) the 16-e^- Hf(IV) and 18-e^- Pd(0) centers are pseudotetrahedral, the HfP₂Pd ring is puckered and the Hf?Pd separation is 2.983 (1) A?. Extended Hu?ckel molecular orbital calculations performed on the model complexes Cp₂Zr(μ-PH₂)₂M′L_n (M′L_n = Pt(PH₃), Pt(DMPE), Rh(η⁵-indenyl), Ni(μ-PH₂)₂ZrCp₂, and Mo(CO)₄) indicate the presence of M′ → Zr donor-acceptor metal-metal bonds that become weaker along the series. The calculations also predict low barriers to MP₂M′ ring inversion for the Pt complexes, consistent with low-temperature ¹H and ³¹P NMR spectroscopic observations.