Cationic Effects on the Net Hydrogen Atom Bond Dissociation Free Energy of High-Valent Manganese Imido Complexes

Abstract

Local electric fields can alter energy landscapes to impart enhanced reactivity in enzymes and at surfaces. Similar fields can be generated in molecular systems using charged functionalities. Manganese(V) salen nitrido complexes (salen = N,N′-ethylenebis(salicylideneaminato)) appended with a crown ether unit containing Na⁺ (1-Na), K⁺, (1-K), Ba²⁺ (1-Ba), Sr²⁺ (1-Sr), La³⁺ (1-La), or Eu³⁺ (1-Eu) cation were investigated to determine the effect of charge on pK_a, E_1/2, and the net bond dissociation free energy (BDFE) of N–H bonds. The series, which includes the manganese(V) salen nitrido without an appended crown, spans 4 units of charge. Bounds for the pK_a values of the transient imido complexes were used with the Mn(VI/V) reduction potentials to calculate the N–H BDFEs of the imidos in acetonitrile. Despite a span of >700 mV and >9 pK_a units across the series, the hydrogen atom BDFE only spans ~6 kcal/mol (between 73 and 79 kcal/mol). These results suggest that the incorporation of cationic functionalities is an effective strategy for accessing wide ranges of reduction potentials and pK_a values while minimally affecting the BDFE, which is essential to modulating electron, proton, or hydrogen atom transfer pathways.

The activation of strong heteroatom–hydrogen (X–H) bonds using high-valent metal oxidos or nitridos as hydrogen atom acceptors is a robust area of bioinspired reaction chemistry.^1-4 The free energy of these reactions is dependent on the hydrogen atom bond dissociation free energies (BDFEs) for the reactants and products. BDFE values are comprised of both the pK_a and redox potential (E_1/2) according to eq 1 (Chart 1).^5-7 Exergonic reactivity with metal oxidos/nitridos requires the BDFE values for the resultant hydroxido/imido bonds to exceed that of the targeted X–H bond. However, the relative contributions of pK_a and E_1/2 to the BDFE are also critical for steering reactivity.^8,9 The difference in reduction potential (ΔE_1/2) and pK_a (ΔpK_a) between the donor and acceptor governs the most favorable reaction pathway for proton, electron, or concerted hydrogen atom transfer (HAT).^10-12 In some cases, the rate of HAT correlates more strongly with ΔE_1/2 or ΔpK_a than with ΔG(H^•),^13-15 deviating from the Bell–Evans–Polanyi principle that the overall free energy governs kinetic reactivity.^16,17 This occurrence can lead to a kinetic selectivity for the cleavage of stronger X–H bonds in the presence of weaker bonds. Thus, understanding how synthetic variations modify the reduction potential, pK_a, and consequential BDFE is critical for controlling HAT reactions.¹⁸

Chart 1.

(Left) Manganese Nitrido Complexes and (Right) Thermodynamic Properties Investigated in This Study

Most studies have used inductive effects to modulate these quantities, leading to modest changes; an alternative approach is to use the secondary coordination sphere. Borovik and co-workers used hydrogen-bond donation from an amide ligand to demonstrate basicity-controlled HAT to a manganese oxido (Figure 1a, left).¹⁹ Tolman and co-workers found that the incorporation of sulfonate or trimethylammonium substituents expands the range of the Cu(III/II) reduction potential (E_1/2) by 275 mV in Cu(II) hydroxide complexes (Figure 1a, right).²⁰ Despite the change in E_1/2, the BDFE of the Cu(II) aqua species remained relatively constant while the kinetics of hydrogen atom transfer to the Cu(III) hydroxide varied, which is attributed to steric contributions of the counterions in addition to possible electrostatic effects. Thus far, direct correlations between changes in thermochemical parameters and electrostatic effects at molecular complexes have been minimally explored.^21-24

Figure 1.

(a) Secondary coordination sphere effects on E_1/2 or pK_a in metal hydroxides (refs 19 and 20). (b) Activation of salen manganese nitridos ((i) ref 42; (ii) refs 26 and 30; (iii) ref 72 and this work).

In this study, we report the effect of proximal mono-, di-, and trications on high-valent Mn nitrido complexes and the hydrogen atom BDFEs of their associated imidos. Manganese nitrido complexes are used in many catalytic and stoichio-metric reactions that form intermediate imidos (Figure 1b).^27-46 Although their BDFEs are central in catalytic nitrogen cycles^47-52 and C–H activation,^53-58 few values have been measured in comparison to isoelectronic metal oxido analogues. The salen-crown framework provides a unique platform for isolating the effect of the cationic charge on the metal center (Chart 1). Non-redox-active cations in the crown modify the electric field potential around the redox-active metal.²⁵ Our previous investigation with complexes 1-Na, 1-K, 1-Ba, and 1-Sr exhibited anodic shifts of over 400 mV of the Mn(VI/V) reduction potential (Table 1) with increasing cationic charge.²⁶

Table 1.

Summary of Thermodynamic Parameters

complex	E 1/2 c	pKad	N–H BDFEe
A a	0.43	8.0–9.4	73–75
1-Na a	0.59	6.2–8.0	75–77
1-K a	0.62	6.2–8.0	75–78
1-Ba a	0.80	0.2–2.6	71–75
1-Sr a	0.88	0.2–2.6	73–76
1-La b	1.02	<0.2	<76
1-Eu b	1.13	<0.2	<79

^aReference 26.

^bThis work.

^cIn V; Mn^VI/V(N) couple vs Fe(C₅H₅)₂^+/0 in MeCN.

^dProtonation of nitrido complexes in MeCN-d₃ at 20 °C as determined by ¹H NMR.

^eIn kcal/mol using experimentally measured E_1/2 and pK_a (eq 1 in Chart 1).

We have now synthesized two derivatives with +3 cations, 1-La and 1-Eu, so that our series spans four different units of charge (see the Supporting Information). Single crystals suitable for X-ray diffraction of 1-La (Figure 2a) and 1-Eu (Figure S32) were obtained from concentrated acetonitrile solutions.

Figure 2.

(a) Solid-state molecular structure of 1-La with 50% probability ellipsoids. Hydrogen atoms are omitted for clarity. See the Supporting Information for a full list of bond lengths and angles. (b) Scan-rate-dependent cyclic voltammetry of 1-La (2 mM) showing Mn^VI/V oxidation event (scan rates 10–2500 mV/s, 0.2 M TBAPF₆, CH₃CN).

The electrochemical properties of 1-La and 1-Eu were measured in acetonitrile using cyclic voltammetry (Figure 2b and the Supporting Information). We previously described the bimolecular coupling of the oxidized Mn(VI) species to form 2 equiv of the corresponding Mn(III) complex and N₂ or an EC mechanism (electron transfer, chemical step) (Figure 1b-ii).^26,30 Lau and Man recently reported the reactivity of a Mn(VI) nitrido complex that was also isolated and structurally characterized despite undergoing similar bimolecular reactivity.⁵⁹ In our prior study with the mono- and dicationic nitrido complexes, an increase in charge corresponded to a slower rate of bimolecular coupling. Consistent with this trend, there is no evidence of bimolecular coupling upon oxidation of 1-La to Mn(VI) even at slow scan rates (10 mV/s) (Figure 1b and Figures S16 and S17). The redox event is reversible, and there is no reduction event corresponding to the Mn(III/II) couple at more negative potentials, which would be the expected product of bimolecular coupling.

On the basis of the oxidation event observed for 1-Eu, the E_1/2 is about ~1.13 V. However, we note that the cathodic wave for the Mn(VI/V) redox couple is smaller than the anodic wave (Figure S19). There is evidence that, upon oxidation, adventitious Na⁺ ions displace the europium(III) ions due to the latter’s poor fit in the crown (ionic radii of 102 and 94.7 pm, respectively).⁶⁰ Analytically pure 1-Eu was used for cyclic voltammetry and the 1-Na oxidation peak was not observed in the initial oxidative scan. However, after several oxidation cycles, an additional cathodic redox feature appeared at ~0.6 V (vs [Fe(C₅H₅)₂]^+/0) (Figure S21), which matches the oxidation potential of the Mn(V) nitrido with Na⁺ in the crown cavity.

The E_1/2 values for the Mn(VI/V) reduction potential of 1.02 V (1-La) and ~1.13 V (1-Eu) vs [Fe(C₅H₅)₂]^+/0 correspond to anodic shifts of 600 and 730 mV in comparison to the noncrown (salen)MnN (A) or changes of 14 and 16 kcal/mol, respectively, in the E_1/2 contributions to the N–H BDFE (eq 1 in Chart 1 and Figure 3).

Figure 3.

Plot showing the compensatory relationship between the Mn^VI/V E_1/2 and pK_a for complexes with a cationic charge and contribution to BDFE. Axis values were calculated as (a) 23.06(E_1/2) and 0) 1.37(pK_a) as shown in eq 1 in Chart 1.

We investigated the protonation of the manganese(V) nitrido complexes to determine the pK_a values. The direct detection and isolation of the parent electrophilic transition-metal imido complexes is challenging because of accessible coupling, disproportionation, and nitrene transfer pathways.^61-65 Acid titration experiments were conducted at room temperature or −35 °C in acetonitrile and were monitored by UV–vis spectroscopy through changes in two absorption bands associated with the manganese nitrido at ~380 and ~600 nm. No spectral features corresponding to a putative imido were observed.

Lau and co-workers previously discussed the instability of the imido formed following protonation of the nitride, where 2 equiv of the resulting imido complex couples to form a manganese(III) μ-diazene species, which rapidly decomposes to give the final Mn(III), N₂, and NH₃ (Figure 1b-i).⁴² Indeed, in our studies the UV–vis spectrum at the end point of the acid titration corresponded to that of the Mn(III) complexes.

In an effort to observe the imido, we synthesized the ¹⁵N-labeled nitrido complex 1-Ba(¹⁵N). Protonation of 1-Ba(¹⁵N) with 1 equiv of [H(OEt₂)₂][BF₄] in acetonitrile-d₃ was monitored by ¹H and ¹⁵N NMR spectroscopy at −30 °C. Only resonances corresponding to the starting material and Mn(III) product were observed. An analysis of the gas headspace following protonation of 1-Ba(¹⁵N) showed ¹⁴N¹⁵N and ¹⁵N₂, further supporting the coupling pathway proposed by Lau. It is possible that, instead of protonating on the nitrido, nucleophilic attack at the imine of the salen could occur.^66-68 Alternatively, protonation of the crown ether could displace the bound cation.^69-71 However, these possibilities are unlikely, as there is quantitative recovery of the of the manganese(III) complex following protonation, indicating that the reactivity proceeds as shown in Figure 1.b-i.

The instability of the manganese salen imidos precludes establishing an equilibrium for protonation of the nitrido. Bounds for the pK_a values were determined by ¹H NMR in acetonitrile-d₃ at room temperature using acids of known pK_a values (see the Supporting Information). Notably, the pK_a values span ~9 units, with the basicity of the Mn(V) nitrido decreasing with an increasing charge of the bound cation.

The BDFEs for the imido N–H bonds are given in Table 1. Despite the changes in pK_a and reduction potential, the BDFE is relatively constant across the series, indicating that the positive shift in reduction potential is largely compensated by acidification of the imido. Although the determination of N–H bond strengths is challenging due to the reactive nature of the intermediate imido, these values are essential to predicting reactivity. A computational study by Cundari and co-workers on these complexes determined that an increase in charge at the bound cation resulted in an increase in the N–H BDFE and lower free energy barriers for hydrogen atom transfer (HAT) from methane.⁷² Our experimentally determined BDFE values do not indicate an increasing N–H BDFE with higher cation charges. However, this may be due to our use of bracketed pK_a values due to the instability of the imidos. Still, we explored the hydrogen atom abstraction (HAA) reactivity of the Mn(VI) complexes with a hydrogen atom donor, 9,10-dihydroanthracene (DHA, BDFE(DMSO) = 72.9 kcal/mol)¹⁸ (Scheme 1).

Scheme 1. Reaction of Manganese Complexes with DHA following Oxidation.

^aReaction performed by spectroelectrochemical UV–vis in MeCN at 20 °C. ^bReaction performed by chemical oxidation with 1 equiv of tris(2,4-dibromophenyl)aminium hexachloroantimonate in n-PrCN at −40 °C. See the Supporting Information for experimental details.

UV–visible spectroelectrochemistry was used to monitor the reactivity of complexes A and 1-Ba with 100 equiv of DHA. Upon oxidation of A to the Mn(VI) species, only spectral changes that correspond to the formation of Mn(III) were observed (Figure S13). Recovery of the solution following electrolysis and analysis by ¹H NMR spectroscopy showed no evidence for the formation of anthracene, the expected product of hydrogen atom abstraction. For 1-Ba, however, absorption bands corresponding to anthracene (340–380 nm)⁷³ increased in intensity during electrolysis (Figure S14), which was confirmed by ¹H NMR spectroscopy of the recovered solution. Spectroelectrochemical electrolyses of solutions of 1-La with 100 equiv of DHA were unsuccessful due to the positive potential required to oxidize 1-La, which resulted in the direct oxidation of DHA. Therefore, we performed chemical oxidation of A, 1-Ba, and 1-La with tris(2,4-dibromophenyl)-aminium hexachloroantimonate (Magic Green) under an inert atmosphere in n-PrCN at −40 °C. Following the in situ generation of the Mn(VI) nitrido and consumption of Magic Green, 10 equiv of DHA was added and the reaction mixture was stirred at −40 °C until no further reaction was observed. ¹H NMR spectroscopy was used to quantify the yield of anthracene (2) for each manganese complex (Scheme 1, inset table, and Figure S15). Two equivalents of manganese is required to form 1 equiv of anthracene (two HAT events); thus, the maximum possible yield of anthracene is 50%. The results of the chemical oxidation were variable due to competitive bimolecular coupling of the Mn(VI) complex, even at −40 °C (Figure S23). However, we note that chemical oxidation of 1-La at −40 °C exhibited a minimal decay over 5 h.³⁰ The addition of cationic charge has a slightly positive effect on C–H activation. The hydrogen atom abstraction exhibited by 1-Ba and 1-La may be due to the inhibition of bimolecular coupling following oxidation to Mn(VI) due to the charge.²⁶ However, additional electrostatic interactions facilitating HAT cannot be ruled out.

Our results establish that BDFEs for the manganese imido N–H bonds change minimally with charge despite significant changes to E_1/2 and pK_a. We also demonstrate enhanced reactivity for HAT when a cation is bound using the hydrogen atom donor DHA. Our findings demonstrate the utility of electric fields for tuning the reduction potential, pK_a, and BDFE, while also differentially affecting hydrogen atom transfer. Future work will focus on understanding electrostatic effects for controlling different pathways for proton and electron transfer as well as C–H activation.