Emily Montgomery

Lily Tepen

Ion Manganese Tricarbonyl Complexes with Asymmetric 2-Iminopyridine Ligands: Toward Decoupling Steric and Electronic Factors in Electrocatalytic CO2 Reduction

The overall reaction for converting carbon dioxide to carbon monoxide is CO2 + 2 H+ + 2 e-CO + H2O. For the reduction of [Mn(CO3)α-diimine)]2, two approaches can be taken.[1],[2],[3] The anionic pathway reduces the dimer at a potential that is more negative than the parent complex. By doing this, we are left with a five-coordinate anion to which CO2 coordinates. This complex is then reduced with a Bronsted acid.[4]-[5] The less common approach is using oxidative addition.[6] This technique requires coordination of CO3 to the Mn dimer, typically in the presence of a Bronsted acid.[7]The purpose of this study was to examine compounds with different R groups to determine reactivity. For our purposes, we focused on MnDIPIMP. This complex has two isopropyl R groups and a hydrogen R group. The different side chains have effects on electrochemistry, steric hindrance, and formation of dimers, among other topics.

The procatalyst used in this experiment was [MnBr(CO)3(DIPIMP)] (P). From this parent material, many different reactions can occur. (Scheme 1) With the addition of 2 electrons, we can make the active catalyst (A). In these reductions, the presence of a Bronsted acid is required for catalysis with the tricarbonyl Mn α-diimine complexes.[8],[9] These catalysts have yielded results that show one electron reduction of the initial complex (MnBr(CO)3(R-bpy) to a Manganese dimer, specifically [Mn(CO)3(R-bpy)]2.8,9Changing the ligands of a series affects the efficiency of the process. This shows that the type of catalysts used is versatile and tunable, while also determining distribution of intermediate species.[10] In these complexes, the pyridine and phenyl rings have little overlap. The exception would be in MnIMP, where the two ring structures are less orthogonal.10The rotation of the phenyl ring is inhibited by two bulky isopropyl (iPr) groupsin the complex MnDIPIMP.10 Although they limits sterics, the two iPr side chains only result in a .20 eV energy difference when comparing MnIMP and MnDIPIMP.10

The reduction outlined in Scheme 1 can be further analyzed by using infrared spectroscopy. When choosing analysis techniques, IR spec is a preferred tool in the cathodic processes because it will show strong carbonyl tracking. It is important to note, though, that the IR bands will be from the dimer and the five-coordinate anion.4By marking different times in the reaction in different colors, Figure 1 can help better understand the conversion of CO2 to CO.

Different types of atmosphere can greatly affect chemistry. As pointed out in scheme 1, there is a separate set of reactions that take place in an inert atmosphere. In the presence of CO2, the complex forms free bicarbonate (F) and subordinate formate (S). (Figure 2) The peaks on the right side of the graph correspond to F and S formation. The formation of a dimer opens the molecule up for the opportunity to form the free bicarbonate.

Emily Montgomery

Lily Tepen

Scheme 1:Main transformation pathways upon reduction under an inert atmosphere and an atmosphere of CO2. The DIPIMP structure can have steric hindrance, which can limit the amount of reactivity. The overall reaction is CO2 + 2 H+ + 2 e-CO + H2O.

Emily Montgomery

Lily Tepen

Figure 1: IR spectra of MnDIPIMP throughout the reduction. For Before Reduction (m=black) (λmax,i,m[1/cm], hi,m, σi,m[1/cm], ni,m): 2030, .2, 5, (i=1); 1945, .11, 7, (i=2); 1920, .1, 10, (i=3). For Early in Reaction (m=red) (λmax,i,m[1/cm], hi,m, σi,m[1/cm], ni,m): 2060, .05, 3, (i=1); 2030, .1, 5 (i=2); 1965, .06, 15, (i=3); 1940, .05, 8, (i=4); 1915, .065, 12, (i=5); 1880, .01, 10, (i=6); 1820, .015, 18, (i=7). For Late in Reaction (m=green) (λmax,i,m[1/cm], hi,m, σi,m[1/cm], ni,m): 2050, .005, .01 (i=1); 2030, .015, 4, (i=2); 2010, .015, 5, (i=3); 1930, .15, 10, (i=4); 1895, .03, 10, (i=5); 1820, .13, 12, (i=6). For After Reduction (m=blue) (λmax,i,m[1/cm], hi,m, σi,m[1/cm], ni,m): 2010, .01, 5 (i=1); 1930, .19, 8, (i=2); 1900, .021, 11, (i=3); 1820, .18, 12, (i=4).

Figure 2:IR spectral changes accompanying in situ reduction of MnDIPIMP in CO2-saturated acetonitrile/0.2M [Bu4N][PF6] within an OTTLE cell. Each color represents a different times throughout the course of reduction. The black line represents before the reduction. It shows IR of the parent compound (P). The red line is early in the reaction. There is still a lot of P, and not much of the product, A, yet. The green line represents later in the reaction. There is only some P left, and a lot of A formed. The two blue lines represent the reaction in the presence of CO2. It shows that there is a lot of A, and the reaction even goes past that point to form free bicarbonate (F) and subordinate formate (S).

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