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Chemistry and Biochemistry


Samuel Johnson



Creative Commons License

Creative Commons Attribution 4.0 International License
This work is licensed under a Creative Commons Attribution 4.0 International License.


Studies into the mechanism of 8-aminoquinoline-directed nickel-catalyzed C(sp3)–H arylation with iodoarenes were performed. Paramagnetic complexes such as a high spin four-coordinate disphenoidal Ni([AQpiv]-κN,N)2 (where [AQpiv] = C9NH6NCOtBu) and {[AQpiv]Ni(O2CtBu)}2 undergo the key C–H activation step. The ubiquitous base Na2CO3 is found to hinder catalysis and was rate-limiting; replacement of Na2CO3 with NaOtBu gave improved catalytic turnovers under milder conditions but also occurred without carboxylic acid and phosphine additives. Products of C–H activation supported by PPh3 or PiBu3 were isolated. The C–H functionalization reaction orders with respect to 7·PiBu3, iodoarenes, and phosphines were determined. Hammett analysis using electronically different aryl iodides suggests a concerted oxidative addition mechanism for the C–H functionalization step; DFT calculations were also performed to support this finding. The facile carbon atom abstraction reaction by [(iPr3P)Ni]5H6 with various terminal alkenes to give [(iPr3P)Ni]5H4(μ5-C) occurs via a common highly reactive intermediate [(iPr3P)Ni]5H4, which was isolated by the reaction of [(iPr3P)Ni]5H6 with norbornene. Temperature-dependent 1H and 31P{1H} NMR chemical shifts of [(iPr3P)Ni]5H4 are consistent with a thermally populated triplet excited state only 2 kcal mol−1 higher energy than the diamagnetic ground state. [(iPr3P)Ni]5H4 catalyzes the dimerization of norbornene to stereoselectively provide exclusively (Z) anti-(bis-2,2'-norbornylidene). To understand these atypical clusters from a theoretical point of view, the structures, bonding and properties of pentanuclear nickel hydride clusters supported by electron rich iPr3P of the type [(iPr3P)Ni]5Hn (n = 4, 6, 8) and their anionic models where iPr3P are substituted by H− are investigated by DFT. All clusters are calculated to adopt the same square pyramidal core geometry and calculations show singlet-triplet gaps of [(iPr3P)Ni]5H4 and [(iPr3P)Ni]5H6 matching experimental values. The positions of hydrides are calculated to match those experimentally for [(iPr3P)Ni]5H4 and [(iPr3P)Ni]5H6. Bonding in this series of clusters is investigated using model complexes and shows there are 3 skeletal electron pairs (SEPs) in [(iPr3P)Ni]5H4 and the addition of additional 2 molecules of H2 to form [(iPr3P)Ni]5H6 and [(iPr3P)Ni]5H8 occupies the two MOs in the e* set that are antibonding to 2 SEPs’ MOs. Orbital composition analysis indicates that MOs of e* set are not entirely antibonding due to the mixing of higher energy px and py orbitals. Further investigations were done to better understand the mechanism by which the precatalyst [(iPr3P)Ni]5H6 stereoselectively dimerizes alkenes such as norbornene. Attempts to generate a polymer by reaction of norbornadiene with [(iPr3P)Ni]5H6 instead lead to the 2+2 cyclodimerized norbornadiene, whereas cyclopentene gave a mixture of 1,1'-bi(cyclopentylidene) and 1-cyclopentylcyclopentene as organic products. Catalysis terminated when [(iPr3P)Ni]5H6 was fully converted to the unusual twisted trapezoidal pentanuclear cluster (iPr3P)4Ni5(C10H13)H5, which gives some insight into the nature of the catalytic intermediates as it can be readily hydrogenated with H2 to give back [(iPr3P)Ni]5H6. Attempts to functionalize the organic fragment in (iPr3P)4Ni5(C10H13)H5 with Ph2SiH2 instead gave (iPr3P)4Ni5(SiPh2)(SiPh2H)H5 which retains the same geometry and is accessible using [(iPr3P)Ni]5H6 and Ph2SiH2. Et3SiH reacts with [(iPr3P)Ni]5H6 to give a new distorted pentagonal cluster [(iPr3P)Ni]5(μ5-SiEt)H7 from Si‒H and multiple Si‒C bond cleavage.

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