Synthesis, characterization and reactivity of imido chromium(VI) complexes
Date
2018
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Publisher
University of Delaware
Abstract
This dissertation describes research on imido chromium(VI) complexes. The relative instability of the highest oxidation state and the weaker metal-bonds of the first-row metal generally make for sparse organometallic chemistry of hexavalent chromium. The motivation of my research is to make high valent chromium complexes supported by imido ligands and explore their reactivity in C-H activation, olefin metathesis and the activation of other small molecules (e.g. PhNCO, CO2). ☐ Chapter 1 describes the syntheses of bis(arylimido) chromium(VI) complexes, (ArN)2CrR2 (Ar = 2,6-diisopropylphenyl, R = CH2SiMe3, Ph, 4-tolyl, 2,6-dimethylphenyl, 3,5-dimethylphenyl and 2,4,6-trimethylphenyl, aka mesityl). The corresponding Grignard reagents were employed in alkylations or arylations of the starting material (ArN)2CrCl2 to make each complex. Bis(imido) bis(phenyl) chromium was prepared through arylation by triphenylaluminum, AlPh3. The thermostability of these chromium(VI) dialkyls and diaryls has been studied. ☐ Chapter 2 describes the synthesis and reactivity of chromium(VI) alkylidenes. Three of them, namely (ArN)2Cr(=CHCMe3)(PPh3), (ArN)2Cr(=CHSiMe3)(PPh3) and (Et4N)[(ArN2Cr(=CHSiMe3)(CH2SiMe3)] have been structurally characterized. Transient neopentylidene [(ArN)2Cr(=CHCMe3)], which is generated by α-hydrogen abstraction, is able to do C-H bond activation of alkanes, [2+2] cycloaddition with alkenes, and N-H activation of tert-butylamine. Unusual chromium(IV) complexes were formed via C-H bond activation of cyclohexane. These results are attributed to β-hydrogen elimination of intermediate (ArN)Cr(CH2CMe3)(Cy). [2+2] cycloaddition with alkenes afforded chromacyclobutanes, which are key intermediates in olefin metathesis. Trimethylsilylmethylidene THF adduct (ArN)2Cr(=CHSiMe3)(THF-d8) also shows reactivity in C-H bond activation of alkanes. ☐ Chapter 3 describes the synthesis and reactivity of tris(imido) chromium(VI) complexes, including both neutral and ionic compounds. K[(tBuN)3CrCl] was prepared by deprotonation of (tBuN)2Cr(NHtBu)Cl by KN(SiMe3)2. The tris(imido) anion undergoes nucleophilic substitution by PPh3 and tBuNH2 to form (tBuN)3Cr(PPh3) and (tBuN)2Cr(NHtBu)2, respectively. (tBuN)2Cr(NHtBu)2 loses one amido hydrogen to form K[(tBuN)3Cr(NHtBu)] when it is treated with KN(SiMe3)2. The imido ligands of K[(tBuN)3CrCl] as well as (tBuN)3Cr(PPh3) are attacked by electrophile MeI to produce (tBuN)2Cr(NMetBu)Cl and (tBuN)2Cr(NMetBu)I, respectively. An alternate way to make tris(imido) anion is deprotonation of (tBuN)2Cr(NHtBu)Cl by alkyl lithium reagent, Me3SiCH2Li. The corresponding product Li[(tBuN)3CrCl] can be alkylated by a second equiv. of Me3SiCH2Li to form Li[(tBuN)3Cr(CH2SiMe3)]. Reactivity studies of tris(imido) complexes show cycloaddition with PhNCO or CO2 to form metallacycles.