Ligand Substitution in Cubic Clusters:  Surprising Isolation of the Cocrystallization Products of Cu₈(μ₈-Se)[S₂P(OEt)₂]₆ and Cu₆[S₂P(OEt)₂]₆

The cluster {Cu₈(μ₈-Se)[S₂P(OEt)₂]₆}_0.54{Cu₆[S₂P(OEt)₂]₆}_0.46 (2) was prepared in 78% yield from the reaction of Cu₈(Se)[Se₂P(OPr)₂]₆ (1) and NH₄S₂P(OEt)₂ in toluene. The central selenide ion in 2 was characterized by ⁷⁷Se NMR at δ −976 ppm. The simulated solid-state ³¹P NMR spectrum shows two components with an intensity ratio close to 55:45. The peak centered at 100.7 ppm is assigned to the ³¹P nuclei in the hexanuclear copper cluster, and that at 101.1 ppm is due to the octanuclear copper cluster. The single-crystal X-ray diffraction analysis confirms the cocrystallization structures of Cu₈(Se)[S₂P(OEt)₂]₆ (54%) and Cu₆[S₂P(OEt)₂]₆ (46%) (2:  trigonal, space group R3̄, a = 21.0139(13) Å, c = 11.404(3) Å, γ = 120°, Z = 3). While the octanuclear copper cluster possesses a 3-fold crystallographic axis which pass through the Cu(2), Se, and Cu(2A) atoms, the six copper atoms having the S₆ point group symmetry in Cu₆[S₂P(OEt)₂]₆ form a compressed octahedron. The Cu₈(μ₈-Se) cubic core in Cu₈(μ₈-Se)[S₂P(OEt)₂]₆ is larger in size than the metal core in Cu₈(μ₈-Se)[Se₂P(OPr)₂]₆ (1) although the bite distance of the Se-containing bridging ligand is larger than that of the S ligand. To understand the nature of the structure contraction of the metal core and metal−μ₈-Se interaction, molecular orbital calculations have been carried out at the B3LYP level of density functional theory. MO calculations suggest that Cu−μ₈-Se interactions are not very strong and a half bond can be formally assigned to each Cu−μ₈-Se bond. Moderate Cu···Cu repulsion exists, and it is the bridging ligands that are responsible for the observed Cu···Cu contacts. Hence, the S-ligating copper clusters have greater Cu···Cu separations because each Cu carries more positive charge in the presence of the more electronegative S-containing ligands.