Ligand Dictated Redox Controlled Transformations of Transition Metals

Abstract

Webber, Christopher K. Ligand Dictated Redox Controlled Transformations of Transition Metals (Under the direction of T. Brent Gunnoe)

Understanding the factors that affect redox transformations of transition metals is important for the optimization of catalytic processes. Transition metal mediated processes such as oxidative C–H functionalization and electrocatalytic water oxidation often require control of redox transformations during catalytic turnover. In brief, this dissertation discusses various aspects of transition metal mediated processes that involve formal redox changes at the metal center.
Previously, our group has utilized “capping arene” ligated transition metal complexes to facilitate several chemical transformations. This dissertation discusses Co-capping arene complexes that were investigated for heterogenized electrocatalytic water oxidation and current efforts to expand the synthetic scope of capping arene complexes. The complexes (6-FP)Co(NO3)2 (6-FP = 1,2-di(quinolin-8-yl)benzene), (5-FP)Co(NO3)2 (5-FP = 1,2-bis(1H-pyrrolo[2,3-b]pyridin-1-yl)benzene) and [(A4-FP)M(H2O)3][ClO4]2 (A4-FP = 1,2,4,5-tetrakis(1H-pyrrolo[2,3-b]pyridin-1-yl)benzene) were each investigated as immobilized catalysts {on Ordered Mesoporous Carbon (OMC) and Multi-walled Carbon Nanotubes (MWCNT)}. In addition, two new capping arene ligands, 1,2-di(9H-carbazol-1-yl)benzene (C2-FP) and 1,2-bis(9,9-dimethyl-9,10-dihydroacridin-4-yl)benzene (A2-FP), bearing N–H binding sites to function as dianionic ligands, upon deprotonation of the N–H groups, were synthesized. Further exploration of these ligands demonstrated their ability to form bis-phosphine ligands, 1,2-bis(9-(diisopropylphosphaneyl)-9H-carbazol-1-yl)benzene (C2-IPPFP) and 1,2-bis(9-(diphenylphosphaneyl)-9H-carbazol-1-yl)benzene (C2-PhPFP).
Other groups have reported pyridine-alkoxide (pyalk) supported transition metal complexes for the study of electrocatalytic processes. Herein, we used the pyalk proligands diphenyl(pyridin-2-yl)methanol ([H]PhPyalk), 1-(pyren-1-yl)-1-(pyridin-2-yl)ethan-1-ol ([H]PyrPyalk), 1-(pyridine-2-yl)-1-(thiophen-2-yl)ethan-1-ol ([H]ThioPyalk), and 1-(ferrocenyl)-1-(pyridin-2-yl)ethan-1-ol ([H]FePyalk) to synthesize CuII complexes that vary in nuclearity and secondary coordination sphere. Also, the proligand 1-(ferrocenyl)-1-(5-methoxy-pyridin-2-yl)ethan-1-ol ([H]FeOMePyalk) was synthesized with a methoxy substituted pyridine; however, the isolation of a CuII complex ligated by [H]FeOMePyalk was not possible. Under variable reaction conditions, the pyalk ligands reacted with CuII precursors and formed either mononuclear or dinuclear CuII complexes depending on the amount of ligand added. The resulting complexes were characterized by single crystal X-ray diffraction, elemental analysis, and cyclic voltammetry.
A series of Pt–Sb complexes with two or three L-type quinoline side arms were prepared and studied. Two ligands, tri(8-quinolinyl)stibane (SbQ3, Q = 8-quinolinyl) and 8,8′-(phenylstibanediyl)diquinoline (SbQ2Ph), were used to synthesize the PtII–SbIII complexes (SbQ3)PtCl2 and (SbQ2Ph)PtCl2. Chloride abstraction with AgOAc provided the bis-acetate complexes (SbQ3)Pt(OAc)2 and (SbQ2Ph)Pt(OAc)2. To better understand the electronic effects of the Sb moiety, analogous bis-chloride complexes were oxidized to an overall formal oxidation state of +7 (i.e., Pt + Sb formal oxidation states = 7) using dichloro(phenyl)-λ3-iodane (PhICl2) and 3,4,5,6-tetrachloro-1,2-dibenzoquinone (o-chloranil) as two-electron oxidants. Depending on the oxidant, different conformational changes occur within the coordination sphere of Pt as confirmed by single-crystal X-ray diffraction and NMR spectroscopy. In addition, the nature of Pt–Sb interactions was evaluated via molecular and localized orbital calculations (calculations performed by the group of Professor Dan Ess).
The bis-acetate complexes (SbQ3)Pt(OAc)2 and (SbQ2Ph)Pt(OAc)2 were used to study C–Cl acetoxylation of 1,2-dichloroethane (DCE) to generate 2-chloroethylacetate and the complexes (SbQ3)PtCl2 and (SbQ2Ph)PtCl2, respectively. The first acetoxylation step produced the intermediates (SbQ3)Pt(Cl)(OAc) and (SbQ2Ph)Pt(Cl)(OAc). The reactions were studied using pseudo first order kinetics (excess DCE) in order to compare the rates of reaction of (SbQ3)Pt(OAc)2 and (SbQ2Ph)Pt(OAc)2, which revealed that kobs = 2.34(3)×10−4 s−1 for (SbQ3)Pt(OAc)2 and 0.60(2)×10−4 s−1 for (SbQ2Ph)Pt(OAc)2. The intermediate (SbQ3)Pt(Cl)(OAc) was synthesized independently, and the solid-state structure was determined using single crystal X-ray diffraction. Density functional theory (DFT) calculations (performed by the group of Professor Dan Ess) were used to examine possible Pt-mediated mechanisms for the reactions of (SbQ3)Pt(OAc)2 or (SbQ2Ph)Pt(OAc)2 with DCE. Calculations support either a nucleophilic attack by the Pt center on a C–Cl bond or direct substitution from a coordinated acetate anion on a C–Cl bond of DCE.
The proligand SbQ3 can be oxidized with o-chloranil to furnish a Lewis acidic SbV ligand, Q3Sb(o-chloranil). Preliminary studies show the formation of Q3Sb(o-chloranil)CuOTf, which contains a formal CuSb Z-type interaction, via the reaction of Q3Sb(o-chloranil) with the low valent CuI precursor [CuOTf]2•C6H6. The complex has been characterized by single crystal X-ray diffraction, 1H 13C{1H} and 19F{1H} NMR spectroscopies and elemental analysis. Additionally, RhI precursors, [Rh(µ–Cl)(CO)2]2and ([Rh(µ–TFA)(C2H4)]2), have been demonstrated to react with (o-chloranil)SbQ3 to form the transmetalated complexes (o-chloranil)SbQ3RhCl(CO) and Q2Sb(o-chloranil)(TFA)Rh(ethyl-Q) (ethyl-Q = [2-(8-quinolinyl-κN)ethyl-κC]), respectively. During the synthesis using ([Rh(µ–TFA)(C2H4)]2), ethylene insertion has been observed to form a metallocyclic product, which suggests potential application in alkenylation reactions.