Lewis-base adducts of Group 1B metal(I) compounds. Part 16. Synthesis, structure, and solid-state phosphorus-31 nuclear magnetic resonance spectra of some novel [Cu₄X₄L₄] (X = Halogen, L = N, P base) 'cubane' clusters

Recrystallization of [Cu₄l₄(PPh₃)₄] from toluene has yielded a new polymorph of that compound, (1), which has been shown by single-crystal X-ray diffraction analysis to have a tetrametallic 'cubane' structure rather than the expected 'step' structure. Crystals are monoclinic, space group P2₁/n, with a = 19.47(1), b = 26.94(1), c = 13.528(5) Å, β = 98.98(4)°, Z = 4 tetramers; R was 0.06 for N_o = 3 681. Cu-I distances range from 2.653(3) to 2.732(3) Å, with Cu ⋯ Cu 2.874(5)-3.164(4) and I ⋯ I 4.234(2)-4.496(3) Å. All adducts of stoicheiometry [M₄X₄(PPh₃)₄] (M = Cu or Ag; X = Cl, Br, or I) have now been synthesized and structurally characterized in a cubane configuration. Recrystallization of copper(I) chloride and bromide from triethylamine also yields tetrameric cubane 1: 1 adducts [X = Cl (2) or Br (3)], as does the reaction of copper(I) chloride with the very bulky ligand 2-[bis(trimethylsilyl)methyl]pyridine, to give [Cu₄Cl₄(tmspy)₄] (4). These three complexes have also been crystallographically characterized, (2) and (4) being the first reported cubane type tetramers for the copper(I) chloride-nitrogen base system. Complexes (2) and (3) are isostructural with their triethylarsine and -phosphine counterparts, being cubic, space group I4̄3m, with a = 12.162(5) Å in (2) and 12.368(3) Å in (3); Z = 2 tetramers. Cu-Cl,Br distances are 2.441(4) and 2.537(3) Å respectively. For (4), the crystals are tetragonal, space group I4₁/a, with a = 18.620(4), c = 20.079(5) Å, Z = 4 tetramers. Although the Cu₄Cl₄ cubane core of the molecule has crystallographically imposed 4̄ symmetry, the geometry is very unsymmetrical as a consequence of the ligand bulk, with Cu-Cl 2.225(2)-2.636(2), Cu ⋯ Cu 2.960(2)-3.194(2), and Cl ⋯ Cl 3.838(3)-3.866(3) Å. Residuals R for (2), (3), (4) were 0.040, 0.038, and 0.040 respectively for N_o = 136, 136, and 1 008 'observed' reflections. The solid-state ³¹P n.m.r. spectra of the triphenylphosphine cubane clusters show significant differences to those with a 'step' geometry; these differences are related to the crystallographic environment of the phosphorus nuclei.