Study of Multielectron Redox Chemistry in Bimetallic Complexes and Metal-Free Macrocycles
Multielectron redox chemistry is fundamental for the activation of small molecules (e.g. H2O, CO2, CO, N2). Although these processes are of great importance, they are not well understood. Unravelling the mechanisms of electron transfer and its principles is essential for the design and development of efficient catalysts. Our approach towards the understanding of multielectron redox chemistry includes theoretical study of mechanistic pathways in the hydrogen evolution reaction (HER) by metal-free porphyrinoids, the study of electronic communication in non-symmetric multimetallic two-state models (D–B–A) and the tuning of the electronic properties of bimetallic systems (Mo, and Au) by ligand control. In Chapter 2, the free-base meso-tetra(pentafluorophenyl)porphyrin was found to be electrocatalytically active for hydrogen gas generation in the presence of p-toluenesulfonic acid. The electrochemical potential of hydrogen evolution (–1.31 vs Fc/Fc+ in THF and –0.69 V vs Ag/Ag+ MeCN) is comparable to metal containing electrocatalysts such as metallated porphyrins or other metallated macrocycles. In combination of spectroscopic and spectroelectrochemical observations along with DFT computations, we proposed the most thermodynamically favorable hydrogen generation mechanism to be a (1) reduction, (2) protonation, (3) reduction, (4) protonation (E-P-E-P) pathway in THF, and a (1) protonation, (2) protonation, (3) reduction, (4) reduction (P-P-E-E) pathway in MeCN. In Chapter 3, the free-base 5, 10, 15-tris(pentafluorophenyl)corrole showed to be active towards hydrogen evolution in acetonitrile under acidic conditions. The electrochemical potential of HER using p-toluenesulfonic acid (–1.22 V vs Fc/Fc+ in acetonitrile) is comparable to other metalated and metal-free macrocycles. In combination of experimental observations and Density Functional Theory calculations a mechanistic pathway was obtained when a strong was used as proton source (p-toluenesulfonic acid). The most favourable hydrogen gas generation mechanism is a (1) protonation, (2) reduction, (3) reduction, (4) protonation (PEEP) pathway when p-toluenesulfonic acid is used. On the other hand, when a weaker acid (benzoic acid) was used, the metal-free corrole cannot catalyse the proton reduction reaction. Chapter 4 depicts the synthesis and characterization of three non-symmetric multimetallic systems of the D–B–A type for the study of Inner-Sphere electron transfer. In the first part of this chapter, two dimers-of-dimers ([Mo2(DAniF)3]2(C12O3NH7) and [Mo2(DAniF)3]2(C18O2N2H12)) were prepared and studied. Cyclic voltammetry on these compounds displayed multielectron redox processes with two one-electron and three one-electron oxidations for [Mo2(DAniF)3]2(C12O3NH7) and [Mo2(DAniF)3]2(C18O2N2H12), respectively. Calculation of their comproportionation constants based on the obtained data (Kc = 6.36 x 103 and Kc = 9.7 x 1010) regarded [Mo2(DAniF)3]2(C12O3NH7) and [Mo2(DAniF)3]2(C18O2N2H12) as Class II and Class III on the Robin–Day classification. The HOMO and HOMO-1 energy gap, which is directly related to the interaction between the Mo2 centers, suggests the electronic coupling of the dimolybdenum units in [Mo2(DAniF)3]2(C18O2N2H12) are stronger than that on [Mo2(DAniF)3]2(C12O3NH7). This result is consistent with the larger separation between the redox potentials waves. In the second part, the effect of a non-metallic redox active unit in the electronics of the D–B–A system, where a C60 fullerene cage was used as the second active site, was studied. We describe the synthesis and characterization of Mo2(DAniF)3(C69O2NH8). The UV-Vis spectra showed a strong band in the UV region corresponding to a MLCT transition from the interaction of the δ orbitals of the Mo2 core with the π* orbitals of the bridging ligand. This electronic transition was assigned based on the nature of the HOMO and LUMO and supported by TDDFT calculations. Electrochemical studies showed a one-electron oxidation at 0.543 V vs Ag/Ag+ related to the Mo24+/5+ process, as well as reduction events associated to reductions of the fullerene cage. The change on the reduction events suggests the presence of communication between the donor and acceptor sites in the form of Mo25+ → C60 charge transfer. The last part of this chapter explores the study of charge transfer in a D–B–A system where both the donor and acceptor are organic units, while the bridging ligand is a dimolybdenum center. We synthesized and characterized Mo2(DAniF)3(C18O2N2H12) and trans-Mo2(DAniF)3[C18O2N2H12]2 by means of 1H NMR, mass spectrometry and single crystal X-Ray diffraction. Cyclic voltammetry showed a one-electron oxidation at –0.03 and –0.017 V vs Fc/Fc+ for Mo2(DAniF)3(C18O2N2H12) and trans-Mo2(DAniF)3[C18O2N2H12]2 corresponding to the Mo24+/5+ process. The presence of a second redox event assigned to oxidation of the organic ligands was observed at 0.465 and 0.148 V vs Fc/Fc+ to the mono and trans complex, respectively. Integration of the redox waves showed a one-electron and two-electron oxidation for Mo2(DAniF)3(C18O2N2H12) and trans-Mo2(DAniF)3[C18O2N2H12]2. The presence of only one wave for the second event in the trans complex suggested the ligands are not considerably coupled. Chapter 5 reports the synthesis and characterization of a series of dimolybdenum paddlewheel complexes of the type Mo2(DAniF)4-n(hpp)n (n = 1 – 3) where DAniF is the anion of N,N’-di-p-anisyl-formamidine and hpp is the anion of 1,3,4,6,7,8-Hexahydro-2H-pyrimido[1,2-a]pyrimidine. The effect on the electronic structure of these tetragonal paddlewheel dimolybdenum was studied upon systematic substitution of formamidinate ligands by the more basic guanidinates. Mo—Mo distances in the paddlewheel structures decreased upon guanidinate ligand substitution, and were found to be 2.0844(6), and 2.0784(6), for Mo2(DAniF)3(hpp) and trans-Mo2(DAniF)2(hpp)2, respectively. Electrochemical studies show that the half wave potential of the Mo25+/Mo24+ couple shifts cathodically upon ancillary ligand substitution ranging from –0.286 V for the tetraformamidinate complex to –1.795 V for the tetraguanidinate analogue, and with redox potentials of –0.75, –1.07 and –1.14 V for Mo2(DAniF)3(hpp), Mo2(DAniF)2(hpp)2, and Mo2(DAniF)(hpp)3, respectively. The presence of a second redox event assigned to the Mo26+/Mo25+ couple was not observed until two guanidinate ligands were introduced. Raman spectroscopy shows that the v(M-M) stretch gets systematically strengthen upon formamidinate ligand substitution by the guanidinate ligand hpp. The destabilization of the delta bond by the basic hpp ligand was measured using DFT calculations by tracking the energy of the frontier orbitals. The decrease in the HOMO-LUMO gap was supported by the red-shift in the UV-Vis spectra of the compounds, 412, 442, and 450 nm for Mo2(DAniF)3(hpp), Mo2(DAniF)2(hpp)2, and Mo2(DAniF)(hpp)3, respectively. In the first part of Chapter 6 we report the synthesis and characterization of a series of digold complexes of the type Au2(DippF)2Cln (n = 0, 2) were DippF is the anion of N,N’-di-isopropyl-formamidine. The formation of the gold (I) and (II) species was studied under different solvents, namely THF and DCM. Au…Au distances in the structures decreased upon oxidative addition of Cl2, and were found to be 2.7385(7) and 2.5303(3) Å, for Au2(DippF)2 and Au2(DippF)2Cl2, respectively. Electrochemical studies show a one-electron reversible oxidation for Au2(DippF)2 at 0.843 V vs Fc/Fc+, and a non-reversible one-electron oxidation at 0.15 V vs Fc/Fc+ for Au2(DippF)2Cl2, assigned as oxidations of the Au2 center and ligand. The decrease in the HOMO-LUMO gap due to oxidative addition was supported by the red-shift in the UV-Vis spectra of the compounds, 350, and 470 nm for Au2(DippF)2 and Au2(DippF)2Cl2.In addition, the high energy bands observed were assigned to MLCT and LMCT electronic transitions as supported by TDDFT calculations. Finally, we ...
Rodriguez Lopez, Nancy, "Study of Multielectron Redox Chemistry in Bimetallic Complexes and Metal-Free Macrocycles" (2019). ETD Collection for University of Texas, El Paso. AAI27666071.