Development of Metallodithiolates as a New Class of Versatile Ligands to Transition Metals

Abstract

Metallodithiolate ligands play an important role in providing the appropriate electronic environment for the catalytic function of various metalloenzymes; acetyl coA synthase serves as paradigm. Using tetradentate N_(2)S_(2) ligands to bind transition metals, a library of well-characterized synthetic analogues has been established. These metal-bound cis-dithiolates, display a wide range of reactivity, including S-based metallation, oxygenation, and alkylation.

Sulfur’s affinity for gold(I) is well known and typically follows Pearson’s HSAB theory. With this in mind, the NiN_(2)S_(2) moieties were used to synthesize propeller-type nickel-gold complexes within a group of [Ni(N_(2)S_(2))_(x)Au_(y)] ( x = 1 or 2; y = 1, 2, or 4) complexes. The solid-state molecular structures of these square planar cis-dithiolate nickel complexes to which gold(I) is bound contain within them classical aurophilic interactions. Electrochemical studies reveal a positive shift in the Ni^(II/I) couple for the [Ni(N_(2)S_(2))_(x)Au_(y)] complexes as compared to the NiN_(2)S_(2) precursors.

The incorporation of the paramagnetic vanadyl ion, [VO]^(2+), in N_(2)S_(2) motifs has been studied in our laboratory. Previous studies of M(N_(2)S_(2)) complexes where M is Ni^(2+), ZnCl^(+), and Fe(NO) with the W(CO)_(x) reporter unit have been explored to determine the donor ability of these metallodithiolate ligands.

According to their response or binding to a tungsten carbonyl unit, we have found that the sulfurs of the dianionic [(VO)(ema)]^(2-) are activated as nuclephilies. To further investigate the donor ability of the neutral vanadyl-bound thiolate sulfurs, the oxidized {Fe(NO)_(2)}^(9) unit was used.

Cleavage of the (μ-I)_(2)[Fe(NO)_(2)]_(2) dimer (in the {Fe(NO)_(2)}^(9) form) was achieved with two metalloligands, Ni(bme-daco) and (VO)bme-daco, and also N-heterocyclic carbene, IMes. Using infrared spectroscopy, the ν(NO) stretching frequencies of the LFe(NO)_(2)I (L = Ni(bme-daco), (VO)bme-daco, and IMes) complexes were used to report the donor ability of the ligands. Cyclic voltammetry for the LFe(NO)_(2)I complexes show a more accessible {Fe(NO)_(2)}^(9/10) couple for stronger donors. Electron paramagnetic resonance (EPR) measurements of the LFe(NO)_(2)I complexes demonstrate super-hyperfine coupling of the ^(127)I to the unpaired electron on iron, and complex equilibria that indicates dissociation of the metallodithiolate ligand from the dinitrosyl iron unit.