Access to Metal Centers and Fluxional Hydride Coordination Integral for CO2 Insertion into [Fe3(μ-H)3]3+ Clusters

Abstract

CO₂ insertion into tri(μ-hydrido)triiron(II) clusters ligated by a tris(β-diketiminate) cyclophane is demonstrated to be balanced by sterics for CO₂ approach and hydride accessibility. Time-resolved NMR and UV-vis spectra for this reaction for a complex in which methoxy groups border the pocket of the hydride donor (Fe₃H₃L², 4) result in a decreased activation barrier and increased kinetic isotope effect consistent with the reduced sterics. For the ethyl congener Fe₃H₃L¹ (2), no correlation is found between rate and reaction solvent or added Lewis acids, implying CO₂ coordination to an Fe center in the mechanism. The estimated hydricity (50 kcal/mol) based on observed H/D exchange with BD₃ requires Fe-O bond formation in the product to offset an endergonic CO₂ insertion. μ₃-hydride coordination is noted to lower the activation barrier for the first CO₂ insertion event in DFT calculations.

Figure 1.

a) Structure comparison of the [Fe₃Br₃N₆] core of 1 (red) and 3 (blue). Two-headed arrow highlights the narrowed distance between iron and terminal bromide of 3. b) Mößbauer spectrum of Fe₃Br₃L¹ (1, top) and Fe₃Br₃L² (3, bottom). The black noisy lines represent the experimental data. The colored lines are simulated quadrupole doublets, as described in the text (see Table S1 for simulation parameters), whereas the black, solid line is a composite spectrum obtained by combining individual doublets.

Figure 2.

Changes of ¹H NMR integration values grouped by species 4 (a), 6a (b), 6b (c), and 6c (d) for reaction of 4 with CO₂ (95 mM) in benzene-d₆ at 25 °C. Resonances are specified in the legend for which the integrations were fit using an A → B → C → D as described in the experimental. Fitted rate constants from 4, 6a, and 6b were used to simulate the data for 6c. Each set of points represents the integration for the corresponding chemical shift as described in the legend. Peaks with minor in contributions are omitted for clarity.

Figure 3.

The affected area for the topographic steric maps around the bridging hydride of 2 (left, green circle), and the maps of 2 (middle, %V_Bur=82.8%) and 4 (right, %V_Bur= 80.1%). C, N, H, and Fe are represented as grey, blue, white, and orange spheres with atomic radii, respectively.

Figure 4.

(a) ¹H NMR spectrum of 2 in toluene-d₈. Peaks are marked as follows: -CH₂- of aminomethyl A, -CH- of beta-ketiminate B, CH₃- of β-ketiminate C, CH₃- of ethyl D, -CH₂- of ethyl E, and solvent and solvent impurities S. (b) ¹H NMR spectrum from mixing equimolar amounts of 2 and deuterated borane THF adduct (BD₃·THF) in toluene-d₈ for 20 h. Peaks from the minor species are denoted as *. All four isotopologues (H₃, H₂D, HD₂ and D₃) are visible as two groups of four nearby peaks at A (c) in a statistical 1:3:3:1 ratio, which are located spatial vicinity to the bridging hydrides/deuterides. Relative intensities were determined through the deconvolution of the peak with Lorentzian functions (green: ¹H NMR spectrum, red: fitted Lorentzian functions for each component, black: sum of all component functions). Only the parts of the spectra from δ 160 to −60 ppm are shown for clarity.

Figure 5.

Hydride insertion mechanism for the conversion of 2•CO₂ to 5a showing the rearrangement of the Fe₃H₃ core. Free energies are given in kcal mol⁻¹. Inset figures display ball-and-stick configuration of metal clusters. C, N, O, H, and Fe are represented as grey, blue, red, white, and orange spheres. Bonds formed during the reaction are represented as sky-blue sticks.

Scheme 1.

Reported multi-iron complexes with bridging hydrides coordinated by β-diketiminate-type ligands.

Scheme 2.

Ligand and complex synthesis. ^a Benzyl potassium for H₃L¹, LDA for H₃L², r.t. in THF, 10 min ^b 80 °C for 1, 50 °C for 2 in C₇H₈, 20h ^c r.t. in C₇H₈, 10 min

Scheme 3.

Reaction product of trihydride complexes (2 or 4) with CO₂.

Scheme 4.

Hydride complexes with reported KIE values for CO₂ insertion.

Scheme 5.

Proposed associative pathway for H₂/CO exchange.