Reactivity studies of pincer metal complexes stabilized by a bis(1,2,3-triazol-5-ylidene)carbazolide ligand scaffold

Abstract

The stability and reactivity of two analogous bis(1,2,3-triazol-5-ylidene)carbazolide ligand scaffolds were exploited during the preparation and characterization of a range of novel metal complexes. The CNC-pincer supported metal complexes demonstrated various unique reactivity patterns, while the ligands stabilized complexes believed to be only transient intermediates in catalysis. A nickel hydride complex mediated the reversible CO2 insertion across the metal hydrogen bond at ambient temperatures, with the reaction ensuing to form a nickel formate complex which, when exposed to negative pressures at room temperature, resulted in the facile regeneration of the hydride product. The in situ generated formate complex was reacted with various chloride containing trapping reagents resulting to the isolation of the corresponding nickel chloride product, but more importantly, evidenced the reversible nature of the reaction. Catalytically active metal complexes of ruthenium and rhodium could also be prepared and it was demonstrated that the atmospherically stable ruthenium complex catalyzed the transfer hydrogenation reaction of acetophenone to the corresponding alcohol with iso-propanol as sacrificial hydrogen atom donor, albeit with unsatisfactory catalytic activity. The air-stable rhodium oxygen adduct prepared, was found to catalyze the alkyne dimerization and hydrothiolation reactions with a high degree of selectivity for one product, out of a possible three isomers. The selective and step-wise double hydrothiolation of a dithiol with two non-equivalent alkynes were also readily catalyzed by the same complex. Moreover, the two different catalytic reactions (dimerization and hydrothiolation) could be performed as a one-pot tandem process. Accordingly, the gem-enyne prepared in situ from the Rh-catalyzed dimerization of a terminal alkyne was subjected to hydrothiolation with an alkyl thiol. This resulted in the syn addition of the thiol across the internal alkyne of the enyne, yielding the 1,3- and 1,4-gem-ene-β-E-vinyl sulfide products with a ratio of 1:1. Coordination of the ligands to (tht)AuCl yielded the corresponding neutral, three coordinate gold(I) complexes featuring an unusual T-shaped geometry. The redox potential of the Au(I)/Au(III) couple was determined through cyclic voltammetry, indicating reversible oxidation even at potentials negative relative to ferrocene. In addition, computational analysis predicted a gold(I) complex with Lewis basic character. The gold(I) products could therefore be reacted with electrophiles, which directly contrasts with classical gold(I) species. The unique reactivity allowed for the preparation of the first example of a cationic gold(III) hydride complex, with protic and not hydridic character. Additionally, treatment of the gold(I) species with alkylating electrophiles resulted in alkylation of the amido nitrogen to yield the cationic gold(I) complex, or alkylation and oxidation of the metal center to form the gold(III) methyl product. The first example of an atmospherically stable cationic gold(III) fluoride complex could also be isolated, after reacting the gold(I) product with Selectfluor. The results, in addition to some very recent literature reports, disprove the long-standing belief that gold cannot mimic the reactivity of the platinum group metals. Finally, the first example of a triazolylidene-coordinated lanthanide complex has also been reported. The exploratory study suggested that specific ligand geometry is required during the isolation of the highly reactive species; the ligand featuring the bulky 2,6-diisopropylphenyl wingtip groups did not yield stable complexes, contrasting intuitive reasoning. However, the use of the smaller 2,4,6-trimethylphenyl functionalized pincer ligand did lead to isolation of the CNC-pincer coordinated yttrium product featuring benzyl bonds due to the C-H activation of two of the 2,4,6-trimethylphenyl wingtip substituents.