Elsevier

Inorganica Chimica Acta

Volume 483, 1 November 2018, Pages 510-515
Inorganica Chimica Acta

Research paper
A multimetal-ligand cooperative approach to CO2 activation

https://doi.org/10.1016/j.ica.2018.08.039Get rights and content

Highlights:

  • Ligand substitution at high-spin Fe(II) centers.

  • Cooperative metal-ligand activation of CO2 using basic pyrazolate sites to act as proton acceptors.

  • Hydration of Fe(II) complexes leads to protonation of ligand backbone.

  • Reduction of hydrated Fe(II) complexes is ligand-centered.

Abstract

The octahedral, S = 2, bis-pyrazolatopyridyl complex LFe(DMAP)3 reacts with CO2 in CH3CN to yield (LH)Fe(DMAP)2(O3C)Fe2(L)(HL), which is the product of capture of one CO2 by one doubly deprotonated water molecule, then binding of one iron to each carbonate oxygen. In order to eliminate DMAP as a participant, [LiClFe2L2] is shown to add one water per Fe2 unit to give [LiClFe2(HL)(L)(OH)], with the Fe2L2 unit capturing hydroxide, and this entire formula unit is linked into a dimer by Li/OH interactions, supplemented by hydrogen bonds to OH from nucleophilic pyrazolate nitrogens. This is suggested to be the process where carbonate is formed, promoted by the ability of the LFe unit to increase the nucleophilicity of H2O, and then attack CO2. Related to that conversion, we establish the structure of the conjugate base [Fe2L2(O)]. Collectively, this shows the combined action of multiple Fe(II) reactants, together with pyrazolate nucleophile and Bronsted base in the hydration of CO2, the conversion effected by carbonic anhydrase.

Introduction

Ocean acidification is one of many potentially economic and environmental threats resulting from rising atmospheric levels of CO2 [1], [2], [3], [4]. In this process, nucleophilic attack of H2O on free CO2 produces HCO3 and H+, the latter attacking CO32–, which is needed in the life cycle of numerous aquatic organisms [5], [6], [7], [8], [9]. The microscopic reversal of carbonic acid dehydration, carbonic anhydrase catalyzes the conversion of CO2 and H2O to H2CO3 and its conjugate bases [10], [11], [12], [13], [14], [15], [16], which is slow without catalysis, in spite of the growing recognition that nucleophilic attack can be accomplished on the carbon of CO2 [4], [17], [18], [19], [20], [21]. The challenge is the degree of nucleophilicity, where hydroxide has an advantage over H2O. Carbonic anhydrase employs the earth abundant metal Zn2+ and that metal appears to function as an electrophile to stabilize the emerging electron rich carbonate oxygen during the mechanism. Indeed, it has been found that deprotonation of H2O is in fact the slow step of this reaction, and the nearby protein residues to the Zn2+ assist in deprotonating H2O [22], [23], [24]. Secondary coordination sphere effects are hallmarks of enzyme active sites, and a great deal of research in the past decade in coordination chemistry has focused on development of ligand scaffolds with proton-responsive functionalities to lower the activation barrier on catalytically relevant intermediates [25], [26], [27], [28], [29], [30], [31], [32]. Proximal basic sites to the metal center represent effective catalyst design for reactions that involve proton shuttling, such as in carbonic anhydrase, as well as in other enzymatic systems [27], [33], [34], [35], [36], [37], [38].

Catalytic reduction of CO2 has frequently employed Lewis acids (B or Si) in reduction of CO2 with E-H bonds [4], [18], [39], [40], [41], [42], [43], [44]. A close nonmetal analog of this concept is found in a pyrazole bound by nitrogen to a B(C6F5)2 group which binds CO2 by nucleophilic attack of the second pyrazole nitrogen on carbon and electrophilic attack of boron at one CO2 oxygen (Scheme 1) [41]. Here, a Lewis acid and Lewis base cooperatively bind CO2; nucleophilic attack at carbon pushes electron density to one of the oxygens, making it δ- and thus satiated by proximate electrophile.

Our approach to establishing Lewis acidic iron in proximity to a pyrazolate Lewis base begins with dehydrohalogenation, the removal of a ligand-based proton and a halide from a metal (Scheme 2, Eq. (1)), targeting LFe. Here, the proton-responsive pyrazole can be an effective Bronsted acid in dehydrohalogenation, yielding pyrazolate, which creates an amide (bound to the metal) and unmasks an outwardly directed β-nitrogen lone pair and (unsaturated) metal in LFe (Scheme 2). Pincer ligands containing pyrazole have been shown to be competent for dehydrohalogenation, and this process is reversible on Ru [25], [45], [46], [47].

Once formed, the naked lone pair of the pyrazolate is an interesting potential site for metal-ligand cooperative substrate activation (Scheme 2, bottom). We and others have shown that this pyrazolate β-nitrogen lone pair is not innocent, and indeed can interact with substrate, but also with other electrophiles such as metal cations [48], [49], [50], [51], [52]. In rare cases, metal-free pyrazolates have been shown to activate CO2 when coupled with boron Lewis acids [18], [40], [41], [42], [43], [53] in its catalytic reduction with HBpin. Thus, incorporating β-nitrogen lone pairs is an attractive feature in a potential CO2 reduction catalyst. Weakening of one C-O bond through nucleophilic attack at carbon reduces the energy required for full C-O bond cleavage. Coupled with an oxophilic metal such as Fe, our goal became utilizing the cooperative binding of CO2 using iron electrophile and the β-nitrogen of pyrazolate as the nucleophile. Here we report that multiple electrophiles bind to pyrazolate β-nitrogen including transition metals and alkali metals. In addition, water protons become competing electrophiles for pyrazolate base, releasing hydroxide to interact preferentially with CO2. We thus report here on the hydrolytic capture of CO2 reminiscent of the carbonic anhydrase mechanism.

Section snippets

Results

We have reported elsewhere [48] that LFe(DMAP)3 (2) is an octahedral species with available pyrazolate β-nitrogens as potential Bronsted bases. This molecule exists in a fast thermal equilibrium between S = 0 and S = 2 spin states, and the higher spin state (occupied σFeN* orbitals) makes it subject to rapid DMAP ligand loss. Dissociation of one DMAP ligand opens up a coordination site at Fe for substrate binding, and under pseudo-first order conditions could favor CO2 capture by pyrazolate (N

Reaction of LFe(DMAP)3 with CO2

Reaction of 200 mmHg CO2 (2 mol per mole Fe) with a CD3CN solution of 2 gives a fast color change from deep red to orange. After stirring 1 h, 1H NMR spectroscopy shows complete consumption of reagent complex to primarily one product, which exhibits 2 resolvable tBu signals (3.49, 3.18 ppm) and one signal for DMAP methyl groups (10.9 ppm) (Fig. S5). There are two ways to explain inequivalent arms: either “derivatization” of one arm by CO2 (Scheme 2), or formation of a Fe2L2 species, via

Origin of the polymetallic CO2 activation

Given the presence of an Fe2L2 unit binding CO2-derived oxygens in 3, we next sought a DMAP-free approach to activation of CO2 with a preformed divalent iron unit, i.e. that in LiClFe2L2, (Scheme 3) in which chloride is located in the site occupied by carbonate, above, suggesting that this might be a general locus for substrate activation.

Product of hydration of LiClFe2L2

Consistent with an oxophilic metal such as Fe and a Bronsted basic pyrazolate β-nitrogen, LiFe2L2Cl is highly reactive towards water. The presence of adventitious water during crystallization attempts with LiFe2L2Cl [48], [57] leads to the isolation of the species [LiClFe2(HL)(L)(OH)]2, 4 (Fig. 2), whose structure was established by single crystal X-ray diffraction. This is thus the product of addition of H2O to [LiClFe2L2]. The dimer 4 has rigorous crystallographic C2 symmetry so parameters

Discussion

While the 1:1 L:Fe stoichiometry creates a deficiency of ligands, even a Fe2L2 bimetallic fragment has 4-coordinate Fe(II) in an approximate sawhorse geometry, thus a high energy species, with a strong thermodynamic preference to bind a fifth ligand between the two Fe. This work highlights the capability of this Fe2L2 unit to accept bridges of varied donor strength, charge, and size, such as OH, O2–, and CO32– here. The pyrazolate bridge that links two Fe centers in a [Fe2L2X] unit is

Conclusions

Capture of water and enhancement of oxo nucleophilicity by deprotonation is the major reactivity exhibited here. The close proximity to iron of Bronsted basic sites in the LFe unit mimics enzymatic catalytic centers by accomplishing deprotonation, away from the metal center, such that the corresponding conjugate base can bind to the metal. Here we show that the bis-pyrazolate pincer can accept protons from H2O, and this resulting OH and O2– can be nucleophilic to substrate CO2. The need for

General

All reactions were performed in a MBraun Lab Master 130 glovebox under N2 atmosphere or with a Schlenk line using standard techniques. Commonly used laboratory solvents were obtained from an Innovative Technology SPS-400 PureSolv solvent purification system and stored over activated molecular sieves·THF and benzene were additionally stored over Na metal prior to use. Deuterated and uncommon solvents were dried with CaH2 for ca. 12 h before undergoing vacuum transfer through standard

Reaction of LFe(DMAP)3 with CO2 (3)

A 5 mm J-Young NMR tube was charged with 20 mg (0.029 mmol) LFe(DMAP)3 and 0.6 mL of CD3CN under an N2 atmosphere. This solution was treated three times with the freeze–pumpthaw technique to remove dissolved N2. This evacuated tube was then charged with 200 mmHg CO2 and the solution allowed to thaw. An immediate color change from dark red to orange-red was observed. To enhance head-space gas mixing into solution, the tube was allowed to tumble on a rotary NMR tube rotator for 1 h before NMR

[LiClFe2(HL)(L)(OH)]2 (4)

A 100 mL round bottom flask was charged with 300 mg (H2L)FeCl2, a PTFE-coated magnetic stir bar, and 15 mL THF. To this solution was added 230 mg LiN(SiMe3)2 in 5 mL THF dropwise, which resulted in an immediate color change from pale orange to deep red–orange. The solution was allowed to stir at room temperature for 12 h. Volatiles were removed from the reaction mixture under reduced pressure to yield a tacky red solid. To this solid was added 30 mL toluene, followed by filtration through a

[(18-crown-6)K(H2O)2]2[Fe2L22-O)] (5)

80 mg (0.101 mmol) of LiClL2Fe2 were dissolved in 7 mL of THF forming an orange-red solution. The reaction mixture was cooled to –78 °C and 3 equiv. of (18-crown-6)K(THF)2C10H8 in 7 mL of THF were added dropwise. Reaction mixture turned dark-green and was stirred overnight in a slowly warming cold well. After 12 h, the reaction mixture (now a reddish-violet) was filtered through a short Celite pad, concentrated and layered with pentane. Crystals suitable for X-ray diffraction analysis were

Acknowledgements

This work was supported by the Indiana University Office of Vice President for research and the National Science Foundation, Chemical Synthesis Program (SYN), by grant CHE-1362127. BJC additionally thanks the Raymond Siedle Fellowship for funding.

Conflict of Interest

The authors declare no conflict of interest.

References (62)

  • K. Kishore et al.

    Tetrahedron

    (1999)
  • K. Umehara et al.

    Inorg. Chim. Acta

    (2014)
  • C.B. Turley et al.

    Reviewing the Impact of Increased Atmospheric CO2 on Oceanic pH and the Marine Ecosystem, in Avoiding Dangerous Climate Change

    (2006)
  • C.-T.A. Chen et al.

    Nat. Clim. Change

    (2017)
  • F.F. Perez et al.

    Nature

    (2018)
  • A.M. Appel et al.

    Chem. Rev.

    (2013)
  • B.D. Eyre et al.

    Science

    (2018)
  • X. Zheng et al.

    Environ. Sci. Pollut. Res.

    (2018)
  • J.L. Drake et al.

    Limnol. Oceanogr.

    (2018)
  • D.N. Silverman et al.

    Acc. Chem. Res.

    (2007)
  • D.W. Christianson et al.

    Acc. Chem. Res.

    (1996)
  • D.N. Silverman et al.

    Acc. Chem. Res.

    (1988)
  • I. Bertini et al.

    Acc. Chem. Res.

    (1983)
  • R.H. Prince et al.

    Angew. Chem. Int. Ed. Eng.

    (1972)
  • D.M. Ermert et al.

    Angew. Chem. Int. Ed.

    (2015)
  • K.X. Bhattacharyya et al.

    Organometallics

    (2008)
  • A.J. Morris et al.

    Acc. Chem. Res.

    (2009)
  • Y. Zhang et al.

    Organometallics

    (2013)
  • G. Miscione et al.

    Theoretical Chemistry Accounts: Theory

    Computation, & Modeling

    (2007)
  • A. Bottoni et al.

    J. Am. Chem. Soc.

    (2004)
  • J.M. Stauber et al.

    Inorg. Chem.

    (2017)
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