Rare, Hexatomic, Boat-Shaped, Cross-Linked Bis(iminodiphenylphosphorano)methanediide Pincer Carbon Bridged Photoluminescent Copper Clusters Capped with Methyl or Halide Bridges

The dimeric dilithium methanediide salt [Li₂C(Ph₂PNSiMe₃)₂]₂ ([Li₂L]₂, L = [C(Ph₂P=NSiMe₃)₂]²⁻) reacted with 6 equiv of [NEt₄CuCl₂] and 2 equiv of LiMe in THF or ether to give exclusively the six-copper cluster complex [Cu₆L₂(CH₃)₂] (1). Similarly, [Li₂L]₂ reacted with 3 equiv of the bimetallic copper halide complexes [(cod)₂Cu₂X₂] (X = Cl, Br, I) in the same solvent to give good yields of three halide-capped six-copper clusters [Cu₆L₂(X)₂] (X = Cl (2), Br (3), I (4)) with structures very similar to that of 1. These four hexacopper clusters (1−4) as a family show similar ³¹P NMR spectra with an AB pattern with slightly different chemical shifts but identical coupling constants in solution. The reactivity of 2 was explored, and it was demonstrated that 1 can be quantitatively generated by addition of 2 equiv of LiMe to 2. Moreover, the cluster [Cu₆L₂(O^tBu)₂] (5) with a similar cage structure is quantitatively generated by reaction of 2 with 2 equiv of NaO^tBu in solution according to ³¹P NMR spectroscopy. An additional copper complex with a bicopper formulation, [Cu₂L(PPh₃)₂] (6), was synthesized by reaction of [Li₂L]₂ with 2 equiv of [(NEt₄)Cu(PPh₃)Cl₂]. We also observed a monocopper iodide complex with the doubly protonated parent methylene bridged ligand H₂L, [CuI(H₂L)] (7), as a minor component (about 10%) during the preparation of complex 4. Complexes 1−4, 6, and 7 were characterized by X-ray crystal diffraction. All four clusters (1−4) show a rare boat-shaped conformation of hexacopper clusters assembled by cross-linking of two copper atoms via geminal substitution on one bis(iminophosphorano)methanediide ligand. Two of these units combine along with two additional copper atoms to form the boat cluster. There is extensive direct Cu−Cu bonding. The cluster is doubly capped, top and bottom, by methyl groups (1) or halides (2−4, X = Cl, Br, I). All of these six-copper clusters absorb and emit in the UV−vis region. The absorption bands and photoluminescent wavelengths are substituent dependent; thus, optical properties of the complex are tunable by means of substitution of the capping ligands. The spectral and bonding characteristics were explored by means of Gaussian DFT calculations and NBO analysis.