Interaction of N-(4-methoxyphenyl)-2,4,6-trinitroaniline with tris(triphenylphosphinegold)oxonium tetrafluoroborate; X-ray structure of salt [O(AuPPh3)3]2 2{(4-MeO)C6H4-N-C6H2-[2,4,6-(NO2)3]}