Synthesis and reactivity studies of nickel arylchalcogenolate complexes

Abstract

Metal complexes of oxygen, sulfur, selenium, or tellurium comprise the class of compounds referred to collectively as metal chalcogenolates. In this area of chemistry, the majority of attention has been devoted to metal complexes containing alkoxide (M-OR) or thiolate (M-SR) ligands. Complexes of the heavier congeners selenium and tellurium are not as widely studied due to the difficulty of preparation and handling of these complexes. Currently, advances in industrial applications and biomimetic research have generated new motivation for the development of metal chalcogenolates, including metal selenolates (M-SeR) and tellurolates (M-TeR). The aim of this dissertation is to provide a more thorough understanding of chalcogen chemistry as it relates to metal chalcogenolate species: specifically, the chemistry of nickel chalcogenolates. The structures and reactivity of three different isostructural nickel arylchalcogenolate series of the type LnNi-EAr are discussed. The series of coordination complexes are explored with the supporting ligands (Ln) tetramethylcyclam (tmc), hydrotris(3-phenyl-5-methylpyrazolyl)borate (TpPh,Me), and hydrotris(3-tert-butyl-5-methylpyrazolyl)borate (TptBu,Me). ☐ The series of [Ni(tmc)EAr](OTf) (E = O, Se, Te; Ar = C6H5) complexes were generated by metathesis reactions of the nickel(II) precursor and sodium salts of the corresponding chalcogen, NaEAr. X-ray crystallographic characterization and spectroscopic studies have established the geometric and electronic structures of these complexes. The tellurium analog, however, is unstable at room temperature, and thus has only been characterized spectroscopically. The observed spectroscopic and structural characteristics reveal distinct trends in accordance with the variation of the identity of the arylchalcogenolate substituent. An additional series of arylselenolates [Ni(tmc)SeAr](OTf) (Ar = C6H4-4-Cl and C6F5) was synthesized to define the electronic effects of the arylchalcogenolate ligand. The reactivity of [Ni(tmc)SeC6H5](OTf) and [Ni(tmc)SeC6H4-4-Cl] with various alkyl halides was investigated, and the observed reactivity was consistent with SN2 alkylation at the selenolate heteroatom. ☐ Exploitation of hydrotris(pyrazolyl)borate ligands has allowed for even greater expansion of nickel chalcogenolate chemistry. Both TpPh,Me and TptBu,Me ligands support pseudotetahedral [TpR,Me]Ni(EAr) complexes. However, the TptBu,Me scaffold incorporates more sterically encumbering tert-butyl groups in the 3-pyrazole position, which proved to be a hindrance to development of the complete isostructural series. On the other hand, the TpPh,Me derivatives were found to support the complete range of [TpPh,Me]Ni(EAr) complexes, including those for E = O, Se, Te; Ar = C6H5, as well as an arylselenolate series [TpPh,Me]Ni(SeC6H4-4-X) (X = H, Cl, Me, OMe) designed to uncover the electronic effects of the aryl selenolates. Spectroscopic and structural characterization were in agreement with expected trends among the chalcogens. Additionally, the two TpR,Me ligand systems displayed disparate reactivity for reactions of [TpR,Me]Ni(SC6H5) with methyl iodide in THF. Surprisingly, for R = tert-butyl the nickel arylthiolate reacted more quickly than its less sterically hindered R = phenyl counterpart. Full computational and kinetic analyses of the reactivity of [TpPh,Me]Ni(SC6H5) with MeI revealed that the electrophilic alkylation reaction occurs via an associative mechanism, via a classical SN2 transition state. Were the same mechanism operative for the [TptBu,Me]Ni(SC6H5) complex, then the alkylation rate should be much slower due to the presence of the tert-butyl groups. These relative rates likely indicate that the reaction mechanism for alkylation of [TptBu,Me]Ni(SC6H5) is different.