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'-dihydro xyazoarene-platinum(II) complexes [Pt-(L) (tba)] (L = 5,5'-dichloro-2, 2'-dihydroxyazobenzenate (dhab), oro-2-hydroxyphenylazo)-3-oxo-N-pheny lbutanamidate (hpab), or (5-chloro-2-hydroxyphenylazo)-2-naphtholate ( hpan) and tba = tributylamine) and the protonated ligands have been as signed by use of homo- and heteronuclear 2D correlated NMR spectroscop y. The assignment of spectra of the analogous complexes [M(L)(py)] (M = nickel(II), palladium(TI), and platinum(II) and py = pyridine) has b een achieved by comparison of the chemical shifts. C-13 chemical shift s of corresponding carbon signals of the 5-chloro-2-hydroxyphenyl grou p, common to all the eight complexes with hpab and hpan, are very simi lar to each other, and in turn these shifts are very similar to the on e 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 coordinate d to different group 10 metal ions, and the signals are assumed to ori ginate from 5-chloro-2-hydroxyphenyl groups with equal coordination en vironment in all the reported complexes; thus C-13 NMR spectroscopy is believed to constitute an excellent way of establishing isostructural ity. On the basis of the crystal structures of complexes with the unsy mmetrical ligands hpab and hpan, all the reported complexes with these ligands are N-alpha-isomers coordinated to the metal at the azo nitro gen 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 i n the free unsymmetrically substituted ligands (hpab)H-2 and (hpan)H-2 themselves reveal intramolecular hydrogen bonds and predominant tauto meric forms, these being possible indicators for predicting the identi ty 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 ce ntral platinum(II) ion.