High-nuclearity [Pd13Ni13(CO)34]4− containing a 26-atom Pd13Ni13 core with an unprecedented five-layer close-packed triangular stacking geometry: possible substitutional Pd/Ni crystal disorder at specific intralayer nickel sites

Abstract

[Pd₁₃Ni₁₀(Ni_3 − xPd_x)(CO)₃₄]⁴⁻1 has been prepared in essentially quantitative yields (>90%) from room-temperature reactions of [Ni₆(CO)₁₂]²⁻ with either Pd(OAc)₂ in DMF or Pd(MeCN)₄(BF₄)₂ in DMSO. The crystal structure of 1 as the [PPh₄]⁺ salt was determined from low-temperature CCD X-ray crystallographic analyses of three crystals. 1 was also characterized by elemental analysis, IR, ¹H NMR, and electrochemical (CV) measurements. Its structurally unprecedented 26-atom metal polyhedron consists of a 5-layer close-packed triangular stacking arrangement of pseudo-C_3v symmetry with an [a(Ni)b(Ni₃)c(Pd₆)a(Pd₇Ni₃)c(Ni₃M₃)] sequence. The overall geometry of the 20-atom ν₃ tetrahedral Pd₁₃Ni₇ framework in the top four ccp layers is analogous to the pseudo-T_d cubic geometry of the metal-core previously found in [Pd₁₆Ni₄(CO)₂₂(PPh₃)₄]²⁻ and [Os₂₀(CO)₄₀]²⁻. Each M in the bottom fifth Ni₃M₃ layer denotes an inner triangular atomic site where a substitutional Pd/Ni crystal disorder may occur; the composite composition of the three M sites corresponds to Ni_3 − xPd_x with x = 0.45 for crystal A and 0 for B and C. We thereby formulate 1 to possess a stoichiometric Pd₁₃Ni₁₃ core even though crystallographic evidence from crystal A indicates the existence of a crystal disorder involving a small substitution of Pd for Ni at two of the three inner atomic M sites within the Ni₃M₃ layer. The entire geometry of 1 (including the 34 carbonyl ligands) ideally conforms to C_3v trigonal symmetry. A qualitative structure/bonding analysis was performed in order to correlate the geometry of 1 with its electron count. The maximum metal-core diameter is ca. 0.87 nm parallel and 0.56 nm perpendicular to the principal 3-fold axis.