A combined neutron and X-ray diffraction study of hydrogenbis(NN-dimethylacetamide) tetrachloroaurate(III) revealing a short hydrogen bond

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Abstract
A combined room-temperature neutron and X-ray diffraction study of the title compound [{(CH3)2NC(CH3)O}2H]-[AuCl4] is reported. The compound crystallizes with two molecules in a monoclinic unit cell of space group P21/a with lattice parameters a= 11.597(3), b= 10.404(2), c= 7.657(2)Å, and β= 108.61(1)°. The structure has been solved by the heavy-atom method and the final values for R(F0 2) were 0.088 for 617 neutron data and 0.082 for 817 X-ray data. It consists of distinct [AuCl4]– anions and [(dma)2H]+ cations (dma = dimethyl-acetamide) with the gold and the bridging hydrogen atoms located at centres of symmetry. The hydrogen bond is ‘symmetrical’ as a result of crystallographic requirements. The O O distance is 2.430(16)Å. Thermalmotion analysis of neutron data indicates that methyl groups attached to nitrogen have higher rotational amplitudes resulting in short apparent C–H bond lengths [average 0.96(4)Å] compared with the methyl group attached to carbonyl carbon which has an average C–H bond length of 1.02(2)Å. Other average, uncorrected, neutron-determined bond distances are: Au–Cl 2.265(3); C–O 1.259(8); N–CH3 1.473(9); N–C 1.299(6); and C–C 1.502(8)Å. The structural analysis clearly indicates that earlier conclusions about the presence of six-co-ordinate gold and nitrogen-co-ordinated dma based upon spectroscopic data are erroneous.

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