UC Berkeley

Chapter 1 IntroductionChapter 2 Isocyanide Activation by Phosphinimide-Ligated Cr(II) CentersThis report describes the synthesis and reactivity of a heteroleptic phosphinimide/phosphinimine Cr(II) complex HL’CrAdHMDS, which was obtained by incomplete protonolysis of Cr(HMDS)2(THF)2 in the presence of the bisphosphinimine pro-ligand H2L’Ad. Addition of common ligands such as N-heterocyclic carbenes and isocyanides displaces the remaining HMDS ligand concomitant with complete deprotonation of the phosphinimine pro-ligand furnishing three- and four-coordinated complexes respectively. The resulting tetrahedral chromium(II) bis(isocyanide) species was found to adopt an unusual S = 1 spin state at room temperature. Under certain reaction conditions, one coordinated isocyanide ligand was found to insert into a chromium-phosphinimide bond. The described compounds are characterized by single-crystal X-ray diffraction and solution-phase spectroscopy and the mechanism for isocyanide insertion was investigated by DFT calculations.Chapter 3 Four-Electron Oxidative Addition of an N=N Double Bond at a Chromium MetallocyclopropeneThis report describes the synthesis of a pseudo-tetrahedral chromium alkyne complex supported by a bidentate phosphinimide ligand and its reactivity with azobenzene. Characterization of the former by structural and computational methods reveals an unprecedented extent of alkyne activation by a formal chromium(II) center, suggesting that this complex is best described as a chromium(IV)-metallocyclopropene. Exposure of this compound to an azobenzene derivative results in the formation of a chromium(VI) diimido complex, which constitutes a rare 4-electron oxidative addition of an N=N double bond. The isolation of a chromium(IV)-hydrazido intermediate enabled mechanistic investigations of this challenging bond cleavage process. This work substantiates the notion that terminal phosphinimide ligands can engender first-row transition metal ions with exceptional reactivity.Chapter 4 Two-electron reactivity between Phthalazines and a Chromium MetallocyclopropeneThis report describes the diverging reactivity of two N=N double bond containing phthalazines, namely 5,8-difuorophthalazine and 1,4-diphenylphthalazine, with a bisphosphinimide chromium metallacyclopropene acting as a masked chromium(II) synthon originally envisioned to demonstrate reversible N=N bond cleavage across a single metal center. Addition of 5,8-difuorophthalazine to the chromium synthon did not result in N=N bond cleavage, instead an imino-pinacol coupling of two equivalents of phthalazine atop the chromium center was observed. The desired cleavage was observed when 1,4-diphenylphthalazine, featuring a different substitution pattern to prevent dimerization, was used. Crystallographic structure determination revealed that both 1,4-diphenylphthalazine reacts in a two-electron process forming a bisketimide. Utilizing benzil as a trapping reagent the imino-pinacol coupling of 5,8-difuorophthalazine could be reversed under thermodynamic control.Chapter 5 Demonstrating Reversible N=N-Bond Cleavage Across a Single Chromium CenterOxidative addition and reductive elimination of single bonds are fundamental organometallic reactions paramount to numerous catalytic applications of transition metal complexes, however an analogous examples involving double of this reactions featuring the formation and cleavage of double bonds are exceedingly rare individually and a reversible example remains unknown. Discovery of such processes would greatly enhance the toolkit available to organometallic chemists for the development of new catalytic processes. This report describes the reactivity of the N=N double bond containing heterocycle, 2,9-difluorobenzo[c]cinnoline with a bisphosphinimide chromium metallacyclopropene acting as a masked chromium(II) synthon, demonstrating reversible four-electron N=N bond cleavage across a single metal center. Addition of 2,9-difluorobenzo[c]cinnoline to the bisphosphinimide chromium metallacyclopropene lead to the formation of an intermediate, which features 2,9-difluorobenzo[c]cinnoline weekly bound to the chromium center. This intermediate can be converted to the corresponding N-N cleaved product under thermodynamic control in first order. Crystallographic structure determination and spectroscopic analysis revealed that 2,9-difluorobenzo[c]cinnoline reacts in a four-electron process forming the chromium(VI) bisimido complex. Utilizing benzil as a trapping reagent, 2,9-diflourobenzo[c]cinnoline can be displaced from the intermediate and in conjunction with green light the 4-electron oxidative addition of 2,9-difluorobenzo[c]cinnoline could be reversed under photochemical conditions respectively, demonstrating a reversible 4-electron cleavage of a N=N double bond across a single transition metal center.