Communication pubs.acs.org/IC
Exploring the Role of Carbonate in the Formation of an Organomanganese Tetramer Karthika J. Kadassery, Suman Kr Dey, Alan E. Friedman, and David C. Lacy* Department of Chemistry, University at Buffalo, SUNY, Buffalo, New York 14260-3000, United States S Supporting Information *
conditions are not possible for studies with PSII. We have found a system for probing the possible role of bicarbonate for the cluster assembly by constructing an anhydrous tetranuclear manganese carbonate containing cluster that collapses into a known16 manganese cubane [Mn(CO)3(μ3-OH)]4 (1) upon the addition of H2O (Scheme 1) and herein describe these findings.
ABSTRACT: The formation of metal−oxygen clusters is an important chemical transformation in biology and catalysis. For example, the biosynthesis of the oxygenevolving complex in the enzyme photosystem II is a complicated stepwise process that assembles a catalytically active cluster. Herein we describe the role that carbonato ligands have in the formation of the known tetrameric complex [Mn(CO) 3 (μ 3 -OH)] 4 (1). Complex 1 is synthesized in one step via the treatment of Mn2(CO)10 with excess Me3NO·2H2O. Alternatively, when anhydrous Me3NO is used, an OH-free synthetic intermediate (2) with carbonato ligands is produced. Complex 2 produces carbon dioxide, Me3NO·2H2O, and 1 when treated with water. Labeling studies reveal that the μ3-OH ligands in 1 are derived from the water and possibly the carbonato ligands in 2.
Scheme 1. Synthesis of 1 from 2 or Mn2(CO)10
T
he recognition that bicarbonate has a functional role in photosynthesis dates back to the original discovery of the “bicarbonate effect” that Warburg defined in 1958.1,2 Since then, many hypotheses have been put forward to explain the role of bicarbonate at the donor side in the enzyme photosystem II (PSII) such as (i) bicarbonate is a substrate, (ii) bicarbonate is a functioning structural component, (iii) bicarbonate is necessary for the formation of the cluster, and (iv) bicarbonate is involved in proton shuttling.3 Hypotheses involving bicarbonate as a substrate or a functioning structural component are rejected because of the results from X-ray crystallographic measurements,4 analysis of oxygen-evolving-complex (OEC) decomposition products,5 light-induced-turnover studies,6−8 and computations.9 However, the remaining hypotheses have significant experimental support. For example, there is ample evidence that bicarbonate is a mobile proton acceptor from the substrate water (H2O). The concomitant formation of 4 mol equiv of carbon dioxide (CO2) for every 1 equiv of O2 under illumination of PSII-containing membranes strongly supports the hypothesis that bicarbonate accepts protons from the OEC during turnover.10 Additionally, the importance of bicarbonate in “photoactivation”, a term used to describe the formation of the OEC prior to H2O oxidation turnover, is established.11−13 While synthetic models have successfully aided in the understanding of the ground-state structural components of the OEC,14 there are few photoactivation model reactions.15 Intermediates in the OEC formation that contain bicarbonate but do not have all of the nominally H2O-derived ligands installed are likely difficult to isolate because anhydrous © 2017 American Chemical Society
Zaworotko et al. first reported the synthesis and structure of 1· (C6H6)2 in 1991 and later reported additional studies on its supramolecular chemistry.17−21 We reasoned that 1 could be used as a potential platform to study H2O-splitting chemistry because other similar complexes, namely, organometallic chromophores with H2O-derived ligands, have been shown to have this type of reactivity.22−25 Thus, we began our investigation by first synthesizing 1 and exposing it to broad-band irradiation in toluene and confirmed the presence of both dihydrogen (H2) and hydrogen peroxide (H2O2; Scheme 2). Our intention to elucidate the source of hydrogen and oxygen atoms in the Scheme 2. Summarized Photochemical Decomposition of 1
Received: June 6, 2017 Published: July 19, 2017 8748
DOI: 10.1021/acs.inorgchem.7b01438 Inorg. Chem. 2017, 56, 8748−8751
Communication
Inorganic Chemistry
that the oxygen atom in Me3NO serves as the oxidant for the conversion of Mn0 in Mn2(CO)10 to the MnI ions in 1 and 2. The presence of carbonato ligands in 2 provides evidence for this premise because CO is only a two-electron reductant and therefore requires two more electrons to form carbonate from CO2 and Me3NO. Titration of Mn2(CO)10 with Me3NO and monitoring of the transition using gas chromatography (GC) confirms that CO and CO2 form and reach a maximum of 3 equiv per molecule of Mn2(CO)10 (Table S1). Following the titration with transmission FTIR reveals that a metastable intermediate (3) forms that contains a ν(CO) at 2081 cm−1 (Figure 2) in addition to other ν(CO) peaks consistent with a coordinated Me3N ligand (Figure S3).
photodecomposition of 1 prompted us to investigate the source of the hydroxido ligands in 1 and its formation mechanism. The general synthetic route that we employ to 1 is a modified version of the literature procedure and involves the treatment of Mn2(CO)10 with 6 equiv of Me3NO·2H2O in tetrahydrofuran (THF; Scheme 1).17 We explored the ability of 1 to exchange with free H2O in acetonitrile (MeCN). By monitoring the speciation of 1 with mass spectroscopy (MS), we found that over the course of 1 week the oxygen atoms in 1 do not exchange with free H218O in MeCN; this is to be expected for six-coordinate d6 centers.26 However, the hydrogen atoms of 1 do exchange with D2O/MeCN (Figure S1). Furthermore, replacing the hydrate Me3NO·2H2O with anhydrous Me3NO in the reaction with Mn2(CO)10 affords a different compound (2) that does not have ν(OH) in the ATR-FTIR spectrum (Figure S2), thus demonstrating that H2O is directly involved in the formation of 1. Solutions of 2 produce crystalline material suitable for X-ray diffraction when stored in toluene at −35 °C. The molecular structure of 2 reveals an uncommon set of ligands surrounding two sets of dissimilar MnI(CO)3 units (Figure 1).27 One set of
Figure 2. Titration of Mn2(CO)10 with Me3NO in toluene was monitored with transmission FTIR spectroscopy. The 2111, 2080, and 1648 cm−1 traces were followed as unique signatures for Mn2(CO)10, 3, and 2, respectively.
The ν(CO) peak at 2081 cm−1 and the overall pattern of the remaining ν(CO) features are similar to those of known Mn2(CO)9L complexes (L = 2-methylpicoline, 2084 cm−1; MeCN, 2092 cm−1; pyridine, 2089 cm−1; PPh3, 2091 cm−1), and therefore we tentatively assign 3 as the complex Mn2(CO)9(Me3N) and deduce that its oxidation by Me3NO is part of the overall conversion of Mn2(CO)10 into 2.30 Attempts to isolate 3 and other intermediates resulting from additional Me3NO resulted in powders that contained 1, 2, and Mn2(CO)10. The former is believed to arise from H2O sensitivity because solutions of 2 often convert to 1 over extended periods inside a glovebox and the latter from an unreacted starting material or inherent lability of 3. Nevertheless, the use of excess anhydrous Me3NO allows us to obtain good quantities of crystalline 2 for the subsequent synthesis of 1. The complete conversion of 2 into 1 is possible with a minimum of 4 equiv of H2O. Following the reaction in THF with transmission FTIR spectroscopy confirms this premise as the peak associated with 1 reaches a maximum once 4 equiv of H2O is allowed to react with 2 (Figure 3). Furthermore, a white precipitate forms during the reaction, and we identified it as Me3NO·2H2O with ATR-FTIR spectroscopy. Thus, the remaining two H2O molecules are incorporated into the product 1. When 4 equiv of 97% enriched H218O is used to synthesize 1 from 2, MS analysis reveals that a mixture of isotopologues of 1 are obtained that include fully unlabeled (16O4-1), fully labeled (18O4-1), and each possible combination of 18OH and 16OH ligands (16O318O-1/16O218O2-1/16O18O3-1; Figure 4). It is possible to increase the enrichment of 1 by using 60 equiv of 97% H218O, thus changing the distribution to favor the dilabeled (16O218O2-1), trilabeled (16O18O3-1), and tetralabeled (16O4-1) labeled isotopologues but never achieving complete incorpo-
Figure 1. X-ray molecular structure of 2 with ellipsoids at 50% probability. The hydrogen atoms and solvent molecules are not shown. Selected bond lengths (Å): Mn1−Mn2 = 2.940(1), Mn1−O1 = 2.008(1), Mn1−O3 = 2.026(1), Mn1−O2 = 2.083(1), Mn1−COave = 1.802(1), O2−N1 = 1.420(2), Mn3−N2 = 2.189(1), Mn3−COave = 1.798(2).
MnI(CO)3 ions share two bridging monodentate carbonato {(μη1-CO3)-κ1-O}2 ligands and a μ-(O)Me3N ligand. These two manganese ions are separated by only 2.940(1) Å; this is nearly the same as the Mn−Mn bond length (2.923 Å) in Mn2(CO)10.28 Charge balance requires that either all four manganese centers to be in the 1+ oxidation state or a MnII2Mn02 state, and because the molecule is diamagnetic, the best description is the former. For instance, if a metal−metal bond were present between Mn1 and Mn2, Mn3 and Mn4 would have to be in the 2+ oxidation state, resulting in a paramagnetic molecule.29 The other set of MnI ions in 2 are separately bonded to the two carbonato ligands in a bidentate fashion {(η2-CO3)-κ2-O′O″}, and each contains a terminal Me3N ligand. The room temperature 1H NMR spectra of the dissolved crystals and bulk material in C6D6 contain two sets of resonances for the Me3N groups that integrate as 9 and 18 protons for the bridging Me3NO and terminal Me3N ligands, respectively, indicating that the solution and solid-state structures are similar (Figure S2). Headspace analysis after the formation of 1 from Mn2(CO)10 revealed the presence of carbon monoxide (CO) and CO2 and only trace H2 (
