Complexes as Catalysts. needs to be reduced before it can be further utilized. This reduction can be difficult to carry out

Transcription

1 Reduction of CO 2 to CO Using Asymmetric 2-Iminopyridine Ligands in Mn(CO) 3 Complexes as Catalysts The carbon in carbon dioxide (CO 2 ) exists in its highest possible oxidation state and needs to be reduced before it can be further utilized. This reduction can be difficult to carry out because of the high stability of CO 2 but catalysts can help make this reduction more feasible. One such catalyst that has been studied is (2-R 1-6-R 2 -phenyl)(r-imino)pyridine which allows H, -CH 3, i Pr, and t Bu as substituents for the R-groups (Scheme 1). 1 In this catalytic system a procatalyst is added and then reduced by adding two electrons and losing a bromide ion, forming a five coordinate anion. This five coordinate anion then either forms a dimer in inert atmosphere or reacts with CO 2 and protons to form a formyl substituted Mn complex which can react with protons and lose water and eventually release CO as a product, returning to the original cation state. This catalytic system overcomes previous challenges to CO 2 reduction catalysts by resisting dimer formation and minimizing the overpotential required for CO 2 reduction. The specific complex studied in detail in this paper is denoted as MnDIPIMP and has i Pr substituents for R 2 and R 3. As is shown in Scheme 1 the protocatalyst (P) will first be debrominated to a fivecoordinated cation before any subsequent reactions can occur. The debrominated P cation can then undergo a variety of reactions, the first of which is its two electron reduction to the five coordinate active catalyst anion (A). In the absence of CO 2 and steric hindrance from the R- substituents A will dimerize with P, losing bromide, to form the dimer (D). In a CO 2 atmosphere CO 2 will coordinate to the manganese and form the intermediate I1 which will start the catalytic CO 2 reduction cycle. D can also undergo a two electron reduction and form two molecules of A. The debrominated P can react with acetonitrile and gain an electron to form the complex M or react with water to form the hydrated complex H These reactions are evidenced by the IR spectrum in Fig. 1.

2 The IR spectrum for the molecules, H, P, A, and M is shown (Fig. 1). IR spectroscopy is used in this wavenumber region because the many CO ligands offer convenient IR markers. By the use of cyclic voltammetry the molecules appeared in the order P, A, H, M and could be easily detected by IR spectroscopy, an analytical procedure called IR-SEC (Infrared- Spectroelectrochemistry). In Fig. 1 A shows strong peaks at 1823 cm -1 and 1899 cm -1 as well as a weak peak at 2007 cm -1. The procatalyst P shows strong peaks at 2030 cm -1 and two smaller twin peaks at 1950 cm -1 and 1927 cm -1. These peaks are similar to those of the hydrated complex (H) which, as a result of the coordination of a nucleophile in the same position formerly occupied by bromine in P shows analogous but weaker peaks at 2030 cm -1, 1950 cm -1, and 1927 cm -1 in addition to some unique peaks such as those at 2050 cm -1 and 1960 cm -1. H also shows a weak peak at 1823 cm -1 indicating that a vibrational mode only forbidden in the compound P is now permitted when water or acetonitrile (in the case of M), replaces bromine for coordination in MnDIPIMP. The acetonitrile substituted complex M shows the same peak characteristic of octahedral coordination at 2030 cm -1 but also peaks characteristic of the five-coordinate anion A at 1927 cm -1 and 1823 cm -1. Additionally, M shows a spur at 1899 cm -1 which, although slightly present in A, is really only a fully identifiable vibrational mode in M. Under the experimental conditions used in Fig. 1 no dimer or CO 2 -complex formation was observed indicating that the products observed in Fig. 1 are the only products long lived enough to be observed by IR spectroscopy when the procatalyst P is reduced in argon saturated acetonitrile. The Mn-Mn dimer is absent in Fig. 1 because thesteric hindrance caused by the substituents in MnDIPIMP impede its formation. It will also not be observed if it is reduced at equal or lower potentials as the parent complex. The IR spectrum of the products of the reduction of P by cyclic voltammetry in the presence of both CO 2 and acetonitrile is also shown (Fig. 2) with the molecules appearing in the order P, A, H, C, M, F and S. C is the pentacarbonyl complex, F free bicarbonate, and S subordinate formate. The same peaks seen in Fig. 1 are conserved in Fig. 2 indicating that the

3 species P, A, H, and M are conserved to at least some extent. An additional curve in turquoise representing the molecules C, F, and S is shown in Fig. 2 showing analogous peaks to M, and A at 2007 cm -1, 1927 cm -1, and 1900 cm -1 as well as the A, M, and H peak at 1820 cm -1 shifted to 1805 cm -1. Additional peaks unique to C, F, and S are visible at 1910 cm -1, 1690 cm -1, 1650 cm - 1, and 1610 cm -1. Interestingly, the blue curve that represents A also shows the F and S characteristic peaks between 1700 cm -1 and 1600 cm -1. The bicarbonate complex (B) is not visible in this spectrum because CO 2 reduction to CO is inefficient and A slowly converts to C displacing bicarbonate and hindering formation of the B. Bicarbonate is formed as a result of carbonic acid dissociation which happens normally when CO 2 is dissolved in water. Fig. 2 shows that bicarbonate (F), subordinate formate (S), and a pentacarbonyl complex (C) are the new species formed when P is reduced in the presence of acetonitrile and CO 2. References (1) Spall, S. J. P.; Keane, T.; Tory, J.; Cocker, D. C.; Adams, H.; Fowler, H.; Meijer, A. J. H. M.; Hartl, F.; Weinstein, J. A. Manganese Tricarbonyl Complexes with Asymmetric 2- Iminopyridine Ligands: Toward Decoupling Steric and Electronic Factors in Electrocatalytic CO 2 Reduction. Inorganic Chemistry 2016, 55,

4 Scheme 1. Reactions of the procatalyst (P) and the resulting reactions in inert atmosphere and the catalytic pathway in CO 2 atmosphere. A two electron reduction follows the debromination of P to yield A. A can then react with CO 2 by nucleophilic attack followed by two protonations to yield H 2 O and CO and returning the oxidized form of the original catalyst. Debrominated P can also be hydrated to yield H or react with acetonitrile followed by a one electron reduction to form M.

5 0.2 P A A Absorbance 0.1 P H H M M f v, m = 2000!!!!!! f! v 1900 Wavenumbers, cm -1 where f! v = h n e!(!!!!"#)! 2πσ!!!! 1700 Fig. 1 Simulated IR spectrum resulting from reduction in argon-saturated acetonitrile of procatalyst (P) yielding five-coordinate anion (A), hydrated Mn complex (H), and the acetonitrile complex (M). Color represents time in reaction with black being early in the reaction, blue a little later, red even later, and green the last. Gaussian function parameters (v max,i,m [cm -1 ], h i,m, σ i,m [cm -1 ], n i,m ) where i indicates gaussian function number and m indicates chemical species: v max,1,p = 2030, h 1,P = 2.11, σ 1,P = 4.32, n 1,P = 0.195, v max,2,p = 1947, h 2,P = 1.00, σ 2,P = 4.50, n 2,P = 0.089, v max,3,p = 1930, h 3,P = 2.50, σ 3,P = 16.00, n 3,P = 0.049, v max,4,p = 1919, h 4,P = 0.60, σ 4,P = 4.90, n 4,P = 0.049, v max,1,a = 2005, h 1,A = 0.07, σ 1,A = 3.50, n 1,A = 0.008, v max,2,a = 1927, h 2,A = 2.10, σ 2,A = 4.70, n 2,A = 0.178, v max,3,a = 1915, h 3,A = 0.3, σ 3,A = 8.00, n 3,A = 0.015, v max,4,a = 1900, h 4,A = 0.30, σ 4,A = 16, n 4,A = 0.013, v max,5,a = 1820, h 5,A = 4.75, σ 5,A =

6 11.00, n 5,A = 0.172, v max,1,m = 2030, h 1,M = 0.20, σ 1,M = 4.00, n 1,M = 0.02, v max,2,m = 2005, h 2,M = 0.17, σ 2,M = 3.50, n 2,M = 0.019, v max,3,m = 1940, h 3,M = 0.40, σ 3,M = 9.00, n 3,M = 0.018, v max,4,m = 1927, h 4,M = 1.70, σ 4,M = 4.70, n 4,M = 0.144, v max,5,m = 1910, h 5,M = 0.70, σ 5,M = 15.00, n 5,M = 0.019, v max,6,m = 1895, h 6,M = 0.20, σ 6,M = 8.00, n 5,M = 0.010, v max,6,m = 1820, h 6,M = 3.58, σ 6,M = 11.00, n 6,M = 0.130, v max,1,h = 2050, h 1,H = 0.52, σ 1,H = 4.14, n 1,H = 0.049, v max,2,h = 2030, h 2,H = 1.29, σ 2,H = 4.89, n 2,H = 0.105, v max,3,h = 2005, h 3,H = 0.07, σ 3,H = 3.50, n 3,H = 0.008, v max,4,h = 1965, h 4,H = 0.40, σ 4,H = 9.00, n 4,H = 0.018, v max,5,h = 1963, h 5,H = 0.20, σ 5,H = 3.60, n 5,H = 0.022, v max,6,h = 1948, h 6,H = 1.30, σ 6,H = 7.00, n 6,H = 0.074, v max,7,h = 1930, h 7,H = 0.85, σ 7,H = 7.40, n 7,H = 0.046, v max,8,h = 1920, h 8,H = 0.55, σ 8,H = 5.30, n 8,H = 0.041, v max,9,h = 1910, h 9,H = 0.10, σ 9,H = 6.00, n 9,H = 0.007, v max,10,h = 1820, h 10,H = 0.42, σ 10,H = 11.00, n 10,H =

7 Fig. 2 IR spectrum resulting from reduction in CO 2 -saturated acetonitrile of procatalyst (P) yielding five-coordinate anion (A), hydrated Mn complex (H), pentacarbonyl complex (C), acetonitrile complex (M), free bicarbonate (F), and subordinate formate (S). Color represents time in reaction with black being early in the reaction, blue appearing next, then red, green, and at the end of the reduction cycle turquoise.