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Terminal Gold-Oxo Complexes

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posted on 12.09.2007, 00:00 by Rui Cao, Travis M. Anderson, Paula M. B. Piccoli, Arthur J. Schultz, Thomas F. Koetzle, Yurii V. Geletii, Elena Slonkina, Britt Hedman, Keith O. Hodgson, Kenneth I. Hardcastle, Xikui Fang, Martin L. Kirk, Sushilla Knottenbelt, Paul Kögerler, Djamaladdin G. Musaev, Keiji Morokuma, Masashi Takahashi, Craig L. Hill
In contradiction to current bonding paradigms, two terminal Au-oxo molecular complexes have been synthesized by reaction of AuCl3 with metal oxide-cluster ligands that model redox-active metal oxide surfaces. Use of K102-P2W17O61]·20H2O and K2WO4 (forming the [A-PW9O34]9- ligand in situ) produces K15H2[Au(O)(OH2)P2W18O68]·25H2O (1); use of K10[P2W20O70(OH2)2]·22H2O (3) produces K7H2[Au(O)(OH2)P2W20O70(OH2)2]·27H2O (2). Complex 1 crystallizes in orthorhombic Fddd, with a = 28.594(4) Å, b = 31.866(4) Å, c = 38.241(5) Å, V = 34844(7) Å3, Z = 16 (final R = 0.0540), and complex 2 crystallizes in hexagonal P6(3)/mmc, with a = 16.1730(9) Å, b = 16.1730(9) Å, c = 19.7659(15) Å, V = 4477.4(5) Å3, Z = 2 (final R = 0.0634). The polyanion unit in 1 is disorder-free. Very short (∼1.76 Å) Au-oxo distances are established by both X-ray and 30 K neutron diffraction studies, and the latter confirms oxo and trans aqua (H2O) ligands on Au. Seven findings clarify that Au and not W is present in the Au-oxo position in 1 and 2. Five lines of evidence are consistent with the presence of d8 Au(III) centers that are stabilized by the flanking polytungstate ligands in both 1 and 2:  redox titrations, electrochemical measurements, 17 K optical spectra, Au L2 edge X-ray absorption spectroscopy, and Au-oxo bond distances. Variable-temperature magnetic susceptibility data for crystalline 1 and 2 establish that both solids are diamagnetic, and 31P and 17O NMR spectroscopy confirm that both remain diamagnetic in solution. Both complexes have been further characterized by FT-IR, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and other techniques.