Experimental evidence for the noninnocence of o-aminothiophenolates: Coordination chemistry of o-iminothionebenzosemiquinonate(1-) pi-radicals with Ni(II), Pd(II), Pt(II)

The ligand 2-mercapto-3,5-di-tert-butylaniline, H[L-AP], an o-aminothiophen ol, reacts with metal(II) salts of Ni and Pd in CH3CN or C2H5OH in the pres ence of NEt3 under strictly anaerobic conditions with formation of beige to yellow cis-[M-II(L-AP)(2)] (M = Ni (1), Pd (2)) where (L-AP)(1-) represent s the o-aminothiophenolate(1-) form. The crystal structure of cis-[Pd-II(L- AP)(2)] [HN(C2H5)(3)][CH3CO2] has been determined by X-ray crystallography. In the presence of air the same reaction produces dark blue solutions from which mixtures of the neutral complexes trans/cis-[M-II(L-ISQ)(2)] (M = Ni (1a/1b), Pd (2a/2b), and Pt (3a/3b)) have been isolated as dark blue-black solid materials. By using HPLC the mixture of 3a/3b has been separated int o pure samples of 3a and 3b, respectively; (L-ISQ)(1-) represents the o-imi nothionebenzosemiquinonate(1-) pi -radical. The structures of 1a.dmf and 3a .CH2Cl2 have also been determined. All compounds are square-planar and diam agnetic. H-1 NMR spectroscopy established the cis reversible arrow trans eq uilibrium of 1a/1b, 2a/2b, and 3a/3b in CH2Cl2 solution where the isomeriza tion rate is very fast for the Ni, intermediate for the Pd, and very slow f or the Pt species. It is shown that the electronic structures of 1a/1b, 2a/ 2b, 3a, and 3b are best described as diradicals with a singlet ground state . The spectro- and electrochemistries of all complexes display the usual fu ll electron transfer series where the monocation, the neutral species, the mono- and dianions have been spectroscopically characterized. X-band EPR sp ectra of the monocations [1a/1b](+) and [3a](+) support the assignment of a n oxidation-state distribution as predominantly [M-II(L-ISQ)(L-IBQ)](+) whe re (L-IBQ)(0) represents the o-iminothionequinone level. In contrast, the E PR spectra of the monoanions [1a/1b]- and [3a](-) indicate an [M-II(L-ISQ)( L-AP-H)](-) distribution but with a significant contribution of the [M-I(L- ISQ)(2)](-) resonance hybrid; (L-AP-H)(2-) represents the o-imidothiophenol ato(2-) oxidation level. Analysis of the geometric features of 120 publishe d structures of complexes containing ligands of the o-aminothiophenolate ty pe show that high precision X-ray crystallography allows to discern the dif fering protonation and oxidation levels of these ligands. o-Amino-thiopheno lates are unequivocally shown to be noninnocent ligands; the (L-ISQ)(1-) ra dical form is quite prevalent in coordination compounds and the electronic structure of a number of published complexes must be reconsidered.