Conformational analysis of a helically distorted gold(I) macrocycle derived from xantphos: evidence for the aurophilic Au--Au interaction from NMR.

Variable-temperature solution (1)H- and (31)P-NMR experiments are reported for [Au(2)(xantphos)(2)](NO(3))(2) gold(I) complex incorporating a 16-membered Au(2)P(4)C(8)O(2) macrocycle derived from the 9,9-dimethyl-4,5-bis(diphenylphosphino)xanthene (xantphos) ligand. These NMR studies provide detailed structural information about its solution structure in comparison to the solid-state structure available from single crystal X-ray diffraction. Two-dimensional (1)H-NOESY spectroscopy at low temperatures allowed the full assignment of proton resonances, which are severely exchange-broadened at room temperature. The (1)H--(1)H spatial connectivities of the large number of non-equivalent protons unravel the complete stereochemistry of the gold(I) macrocycle in its symmetrically twisted conformation. Chemical exchange between particular proton pairs has allowed understanding the molecular dynamics and ruled out formation of coordination oligomers. Spectral analysis of the unique phosphorous AA'BB' spin system has revealed three-bond (31)P--(31)P scalar couplings mediated through the Au--Au bond, and has confirmed the existence of the aurophilic contact in low-temperature solutions. The solution (13)C and (31)P spectra are compared to the solid state (31)P-MAS and (13)C--CP/MAS spectra, where signal doubling effects due to the crystal packing have been discovered.