Cyclometallated Compounds of Palladium(ii), Rhodium(iii), and Iridium(iii).

Abstract

A number of new complex compounds of palladium, gold, rhodium, iridium, and platinum have been synthesized and characterized. The organic ligands used in the preponderance of reactions involve some nitrogen heterocycle such as 2-arylpyridines, 2,6-diarylpyridines, benzo{h}quinoline, and caffeine. In most cases, the organic ligand underwent an intramolecular metallation (designated "orthometallation" or more generally "cyclometallation") with subsequent formation of a chelate ring containing a covalent metal to carbon bond. The effect of varying the substituents on the aryl moiety of the 2-arylpyridines supported the hypothesis that after initial N-complexation, the 2-position of the aryl nucleus experiences an electrophilic attack by the palladium atom. Several different palladium starting materials were used and it was found that: (a) Pd(acetylacetonate)(,2) did not react; (b) PdCl(,2)(C(,6)H(,5)CN)(,2) did not metallate, but did give a 2:1 (ligand-to-metal) adduct; and (c) both PdCl(,4)('2-) and Pd(CH(,3)COO)(,2) gave the desired cyclometallated products. Palladium(II) acetate has proven to be a more useful starting material than the tetrachloropalladata, since the resulting acetato-bridged dimers, unlike the chloro-bridged dimers, are soluble in polar organic solvents. Reactions of gold in such forms as AuCl(,4)('-), (C(,6)H(,5))(,3)PAuCl, and dichloro-(mu)-1,2-bis(diphenylphosphino)ethanedigold(I) failed to produce any cyclometallated products under a broad assortment of conditions. Only compounds containing 1:1 (ligand to metal) adducts were formed. Occasionally, reduction to metallic gold occurred. Both rhodium and iridium formed biscyclometallated products containing two chelated rings per six-coordinated trivalent metal atom. In contrast to the four-coordinate palladium(II) chlorobridged dimers, the chloro-bridged dimers of rhodium and iridium were conveniently soluble in organic solvents. Platinum formed a compound which was analogous to those of the palladium chloro-bridged dimers. Detailed high resolution ('1)H NMR studies (200 MHz) showed that cyclometallation did occur on the aryl nucleus of the 2-arylpyridines. The 6-heteroaryl protons and those 'ortho' to the metal-carbon bond are both shifted from the free ligand position. Depending on the metal, the shifts are either up- or downfield. These shifts can be explained by either through-space interactions of overlying aromatic rings and/or through-bond (metal-to-ligand) effects. The ('1)H NMR spectra of these biscyclometallated rhodium and iridium complexes have established a trans-nitrogen octahedral structure for all of these d('6) compounds. Both acetato- and chloro-bridged dimers react with certain ligands to give mononuclear complexes containing the intact cyclometallated ligand. ('1)H NMR studies of palladium and rhodium complexes containing diethyldithiocarbamate and 2-arylpyridines as chelating ligands strongly suggest that in the d('8) palladium compound there is metal-to-ligand back bonding whereas in the d('6)