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

Abstract

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 I3m, 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 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.