Generation of AuPd22/Au2Pd21 analogues of the high-nuclearity Pd23(CO)20(PEt3)10 cluster containing 19-atom centered hexacapped-cuboctahedral (ν2-octahedral) metal fragment: structural-to-synthesis approach concerning formation of Au2Pd21(CO)20(PEt3)10

Abstract

Reactions of Pd(PEt₃)₂Cl₂ and Au(PPh₃)Cl in DMF with NaOH under CO atmosphere gave rise to the unique capped three-shell homopalladium Pd₁₄₅(CO)_x(PEt₃)₃₀ (x ∼ 60) and two neutral Au–Pd clusters: Au₂Pd₂₁(CO)₂₀(PEt₃)₁₀ (1) and Au₂Pd₄₁(CO)₂₇(PEt₃)₁₅ (following article). Similar reactions with Pd(PMe₃)₂Cl₂ being used in place of Pd(PEt₃)₂Cl₂ afforded Au₂Pd₂₁(CO)₂₀(PMe₃)₁₀ (2), the trimethylphosphine analogue of 1, and the electronically equivalent [AuPd₂₂(CO)₂₀(PPh₃)₄(PMe₃)₆]⁻ monoanion (3) as the [PPh₄]⁺ salt. Each of these three air-sensitive 23-atom heterometallic Au–Pd clusters was obtained in low yields (7–25%); however, their geometrical similarities with the known cuboctahedral-based homopalladium Pd₂₃(CO)₂₀(PEt₃)₁₀ (4), recently obtained in good yields from Pd₁₀(CO)₁₂(PEt₃)₆, suggested an alternative preparative route for obtaining 1. This “structure-to-synthesis” approach afforded 1 in 60–70% yields from reactions of Pd₁₀(CO)₁₂(PEt₃)₆ and Au(PPh₃)Cl in DMF with NaOH under N₂ atmosphere. Both the compositions and atomic arrangements for 1, 2 and 3 were unambiguously established from low-temperature single-crystal CCD X-ray crystallographic determinations in accordance with their nearly identical IR carbonyl frequencies. Cluster 1 was also characterized by ³¹P{¹H} NMR, cyclic voltammetry (CV) and elemental analysis. The virtually identical Au₂Pd₂₁ core-architectures of 1 and 2 closely resemble that of 4, which consists of a centered hexa(square capped)-cuboctahedral Pd₁₉ fragment of pseudo-O_h symmetry that alternatively may be viewed as a centered Pd₁₉ ν₂-octahedron (where ν_n designates (n + 1) equally spaced atoms along each edge). [AuPd₂₂(CO)₂₀(PPh₃)₄(PMe₃)₆]⁻ (3) in the crystalline state ([PPh₄]⁺ salt) consists of two crystallographically independent monoanions 3A and 3B; a superposition analysis ascertained that their geometries are essentially equivalent. A CV indicates that 1 reversibly undergoes two one-electron reductions and two one-electron oxidations; these reversible redox processes form the basis for an integrated structural/electronic picture that is compatible with the existence of the electronically-equivalent 1–3 along with the electronically-nonequivalent 4 (with two fewer CVEs) and other closely related species.