ASSIGNMENT OF THE LIGATING NITROGEN IN O,O'-DIHYDROXYAZOARENE COMPLEXES OF NICKEL(II), PALLADIUM(II), AND PLATINUM(II) BY H-1 AND C-13 NMR-SPECTROSCOPY

The H-1 and C-13 NMR spectra of the soluble square-planar o,o'-dihydroxyazoarene-platinum(II) complexes [Pt-(L) (tba)] (L = 5,5'-dichloro-2,2'-dihydroxyazobenzenate (dhab), (5-chloro-2-hydroxyphenylazo)-3-oxo-N-phenylbutanamidate (hpab), or (5-chloro-2-hydroxyphenylazo)-2-naphtholate (hpan) and tba = tributylamine) and the protonated ligands have been assigned by use of homo- and heteronuclear 2D correlated NMR spectroscopy. The assignment of spectra of the analogous complexes [M(L)(py)] (M = nickel(II), palladium(TI), and platinum(II) and py = pyridine) has been achieved by comparison of the chemical shifts. C-13 chemical shifts of corresponding carbon signals of the 5-chloro-2-hydroxyphenyl group, common to all the eight complexes with hpab and hpan, are very similar to each other, and in turn these shifts are very similar to the one set of signals from the 5-chloro-2-hydroxyphenyl groups in the four complexes of the symmetrically substituted ligand dhab. These chemical shifts exhibit the same periodic change when the ligand is coordinated to different group 10 metal ions, and the signals are assumed to originate from 5-chloro-2-hydroxyphenyl groups with equal coordination environment in all the reported complexes; thus C-13 NMR spectroscopy is believed to constitute an excellent way of establishing isostructurality. On the basis of the crystal structures of complexes with the unsymmetrical ligands hpab and hpan, all the reported complexes with these ligands are N-alpha-isomers coordinated to the metal at the azo nitrogen attached to the 5-chloro-2-hydroxyphenyl substituent. The changes in C-13 chemical shift with coordination can be used to distinguish N-alpha- from N-beta-isomers. Deuterium isotope effects on C-13 chemical shifts in the hpab complexes with one exchangeable amine proton and in the free unsymmetrically substituted ligands (hpab)H-2 and (hpan)H-2 themselves reveal intramolecular hydrogen bonds and predominant tautomeric forms, these being possible indicators for predicting the identity of the ligating nitrogen. (n)J(195Pt- C)(13) couplings in the [Pt(L)(tba)] complexes have been measured at low field, and the magnitudes of these couplings are analyzed in terms of coupling paths in the two annulated five- and six-membered rings formed by the ligand and the central platinum(II) ion.