Synthesis and reactivity of low valent chromium (I) precursors supported by hydrotris(pyrazolyl)borate ligands for dioxygen and small molecule activation

Theopold, Klaus H.Dioxygen activation has garnered a lot of interest in both academia and industry. Transition metals such as silver are able to coordinate and split dioxygen to yield reactive transition metal oxo complexes. These complexes are then able to transfer the oxygen atom to exogenous substrates producing valuable products. Chromium complexes have been found to coordinate and activate dioxygen and have been well documented in the literature. In this vein, a series of chromium (I) dinitrogen complexes supported by the hydrotris(pyrazolyl)borate ligands hydrotris(3-tert-butyl-5-methyl)pyrazolylborate and hydrotris(3,5-di-iso-propyl)pyrazolylborate were synthesized. Both [TptBu,MeCr]2(μ-N2) and [Tp iPr2Cr]2(μ-N2) were found to react with dioxygen to yield the corresponding TptBu,MeCrV(O) 2 and [TpiPr2CrV]2(O) 2(μ-O)2 respectively in good yields. The mechanism by which this transformation occurred was studied through the synthesis of potential intermediates such as [TptBu,MeCr]2(μ-O) 2 and [TpiPr2Cr]2(μ-O)2. Additionally, the synthesis of TptBu,MeCr(η-S2) yielded a model for the corresponding TptBu,MeCr(η-O 2), which has yet to be crystallographically characterized in the literature. Both a mononuclear, albeit spin forbidden, mechanistic pathway, as well as, a binuclear mechanistic pathway are possible and supported by the experimentation. Additionally, [TptBu,MeCr]2(μ-N 2) was found to react with organic azides to produce TptBu,Me CrNR (R = adamantyl, trimethylsilyl, and phenyl). TpiPr2CrNAd was also synthesized and was found transfer the nitrene group to CO to produce isocyanates. Interestingly, in an attempt to synthesize the corresponding chromium isocyanate complex using both AdNCO and PhNCO, the products which were isolated were found to be [TpiPr2Cr]2(μ:η 3:η3-AdNC(O)N-NC(O)NAd) and [TpiPr2Cr] 2(μ:η3:η3-PhNC(O)N-NC(O)NPh). These products may arise from the reaction of the activated N2 moiety in [TpiPr2Cr]2(μ-N2) with the corresponding isocyanate making new N – C bonds. Unexpectedly, [TptBu,MeCr]2(μ-N 2) was found to be unreactive towards several small molecules such as alkynes and alkenes, and decomposed when exposed to CO. An alternative chromium (I) precursor, TptBu,MeCr(η2-C2(SiCH 3)2), was synthesized. This complex was found to react with ethylene to yield [TptBu,MeCr]2(μ-η2:η 2-C2H4), with CO to produce [TptBu,Me Cr]2(η1:CO)2(μ-CO)(Et 2O), with other alkynes such as diphenyl acetylene to form Tp tBu,MeCr(η2-C2(C6H5 )2), and finally, with N2 to give [TptBu,Me Cr]2(μ-N2).University of Delaware, Department of Chemistry and BiochemistryPh.D