Rhenium(V)-oxo corrolazines: isolating redox-active ligand reactivity

Abstract

The synthesis of the first example of a third-row metallocorrolazine characterized by single crystal X-ray diffraction is reported. This Re^V(O) porphyrinoid complex shows an exclusively ligand-based reactivity with strong acids and oxidizing agents. The one-electron oxidized π-cation-radical complex is capable of H-atom abstraction.

The synthesis of rhenium complexes is of interest due to their potential use as catalysts for oxygen-transfer,¹ X–H (X = Si, B, P and H) bond activations,² and in CO₂ photoreduction reactions.³ In addition, certain isotopes of rhenium are used in medical imaging, diagnostics, and therapeutics.⁴ Although not found in natural heme systems, a few examples of rhenium porphyrins have been synthesized, including Buchler’s first report of high-valent Re^V(O)(X) porphyrins.⁵ A decade later, the ring-contracted high-valent metal-oxo Re^V(O)(TCF₃C) (TCF₃C = 5,10,15-tris(trifluoromethyl)corrole) was serendipitously prepared in an attempted porphyrin metallation reaction, providing the only example of a rhenium corrole.⁶

High-valent metal-oxo porphyrinoid complexes are of significant interest because of their role in synthetic and biological oxidation catalysis.⁷ Our group recently described the characterization of high-valent Mn^V(O) and Cr^V(O) corrolazine (Cz) complexes by single crystal X-ray diffraction (XRD), and showed that these complexes exhibited dramatically different abilities to abstract hydrogen atoms from X–H (X = O, C) bonds.⁸ In earlier efforts, it was also shown that the Mn^V(O)(Cz) complex could be chemically oxidized to give Mn^V(O)(Cz^+•), the first example of an Mn^V(O) π-radical cation complex. The latter complex showed greatly enhanced O-atom transfer (OAT) reactivity compared to its neutral precursor.⁹ Lewis/Brønsted acids (LA) were also shown to have a profound influence on Mn^V(O)(Cz), stabilizing the valence tautomer Mn^IV(O)(Cz^+•)(LA) and providing a rare example of a chemically driven, reversible valence tautomerization. The reactivity of Mn^IV(O)(Cz^+•)(LA) was significantly different from Mn^V(O)(Cz), with the metal-oxo unit and redox-active Cz ligand functioning together to carry out H-atom transfer (HAT) and OAT reactions.¹⁰ Recently, much attention has been given to redox-active ligands and how they operate in conjunction with a metal ion to mediate various chemical transformations.¹¹

Herein we report the synthesis and characterization by XRD of an Re^V(O)(Cz) complex, the first metallocorrolazine containing a third-row metal ion, and a rare example of a structurally characterized rhenium porphyrinoid complex. This complex was prepared as an isoelectronic analog of Mn^V(O)(Cz). Addition of strong Brønsted acids does not lead to stabilization of a valence tautomer in this case, but rather the reversible protonation of a remote site on the ligand. It is shown that the redox-active Cz ring can participate in electron-transfer and H-atom transfer reactions without the involvement of the metal-oxo unit. The H-atom transfer reactivity for the one-electron-oxidized Re^V(O)(Cz^+•) is also shown to be strongly dependent on the nature of the external one-electron oxidant through a surprising observation of “zero-order” kinetics.

The synthesis of the Re^V(O) corrolazine complex was accomplished by metallation of TBP₈CzH₃ (1) with excess ReCl₅ in refluxing decalin (Scheme 1). The product was purified by chromatography and gave Re^V(O)(TBP₈Cz) (2) as a dark green solid (99% yield). The UV-vis spectrum of 2 exhibits a Soret band at 460 nm, which is the most red-shifted for any metallocorrolazine, and a Q-band at 670 nm. The ¹H NMR spectrum of this complex is diamagnetic, consistent with a low-spin Re^V ion. Eight doublets appear between 8.44 – 7.26 ppm, which can be assigned to the peripheral para-substituted phenyl substituents (Fig. S1^†). An LDI-TOF mass spectrum gives M⁺ = 1557.38 m/z, in good agreement with the Re(O)(TBP₈Cz) formulation.

Scheme 1.

Synthesis of Re^V(O)(TBP₈Cz) (2).

Confirmation of the structure of 2 comes from single crystal X-ray crystallography. Single crystals of 2 were obtained by vapor diffusion of acetonitrile in toluene. The molecular structure is shown in Fig. 1, revealing a 5-coordinate Re center, with a terminal oxo ligand at a Re–O distance of 1.682(5) Å. This distance is consistent with an Re–O triple bond.^1e,12 The Re ion is displaced by ca. 0.74 Å from the plane defined by the four N_pyrrole atoms. For comparison, Re^V(O)(TCF₃C) exhibits an Re–O bond length of 1.662(2) Å and an out-of-plane distance of 0.701 Å.⁶ The Re–O distance in 2 is significantly elongated compared to Mn^V(O) (1.5455(18) Å) and Cr^V(O) (1.553(2) Å) corrolazines,^8a as expected for a third-row metal ion. The Re ion is also much further out of plane, by at least 0.2 Å, compared to either Mn^V or Cr^V complexes.

Fig. 1.

Displacement ellipsoid plot (30% probability level) of Re^V(O)(TBP₈Cz) (2). Selected bond distances (Å): Re–O, 1.682(5), Re–N_ave, 1.976, Re–N_plane, 0.739. a) Top view; b) side view (peripheral aryl groups were omitted). In both cases, the disorder and H-atoms were removed for clarity.

We previously found that Mn^V(O)(TBP₈Cz) reacts with Lewis and Brønsted acids at 23 °C to give Mn^IV(O)(TBP₈Cz^•+)(LA) (LA = H⁺, Zn^II, B(C₆F₅)₃), in which the paramagnetic MnIV valence tautomer is stabilized over the diamagnetic, low-spin Mn^V species through a proposed weakening of the Mn–O π-bonding from coordination of LA to the terminal oxo group.¹⁰ In contrast, the addition of the Brønsted acid [H(OEt₂)₂][B(C₆F₅)₄] (HBArF) to Mn^V(O)(TBP₈Cz) at low temperature (−60 °C) led to protonation of a remote meso-nitrogen atom on the Cz ring and retention of the low-spin Mn^V configuration. Protonation of the meso-N atoms in Mn^III corrolazines was also definitively confirmed by XRD.¹³ To determine the reactivity of the isoelectronic Re^V(O) analog toward Brønsted acids, the reaction of 2 with HBArF was examined. Addition of one equiv of HBArF to 2 in CH₂Cl₂ causes clear shifts in the UV-vis spectrum (Fig. 2a), although these changes are not consistent with formation of a Cz π-radical-cation. The final spectrum is more typical of protonation at the remote site on the Cz ligand.¹³ Spectral titration with HBArF supports a 1:1 binding stoichiometry (inset, Fig. 2a) to form [Re^V(O)(TBP₈Cz)(H)]⁺ (3).

Fig. 2.

a) UV-vis spectral changes upon addition of HBArF (0–2 equiv) to 2 in CH₂Cl₂. Inset: spectral titration at 723 nm showing maximal formation of product at 1 equiv HBArF. b) Comparison of ¹H NMR spectra (400 MHz) near the aryl region of 2 (black) and 3 (red) in CD₂Cl₂ at 23 °C.

Characterization of the monoprotonated complex 3 was performed by 1D (Fig. 2B) and 2D (Fig. S3–4^†) NMR spectroscopy. The ¹H NMR spectrum of 3 is diamagnetic, confirming that the paramagnetic valence tautomer Re^IV(O)(Cz^+•) is not observed. Spectra for 2 and 3 are shown in Fig. 2b (see Fig. S2^†). A new peak is observed at 13.45 ppm for the protonated complex, and integrates to 1H. This peak can be assigned to protonation of a meso-N of the corrolazine ring, shifted downfield by the ring current effect. For comparison, the meso C–H proton of Zn octaethylporphyrin appears at ca. 10 ppm.¹⁴ The aromatic pattern is complex, consistent with addition of H⁺ to one of the meso-N atoms that does not lie on the mirror plane bisecting the pyrrole-pyrrole linkage. The NMR spectrum for 2 at 23 °C is similar to what was observed for protonation of Mn^V(O)(TBP₈Cz) at −60 °C.¹³

The IR spectrum of 2 shows an intense peak at 997 cm⁻¹ (Fig. 3a), in the region expected for the stretching frequency of an Re^V–O triple bond. However, an intense peak at ~997 cm⁻¹ has also been observed in several other metallocorrolazines, and may arise from Cz vibrational modes.¹⁵ To conclusively identify the Re–O stretch, we synthesized the isotopically labeled Re^V(¹⁸O)(TBP₈Cz) (2-¹⁸O) by addition of excess H₂18O (80 equiv) to the Re metallation reaction involving TBP₈CzH₃ (1) and ReCl₅ in decalin. This method yielded the Re^V(O) complex with >99% ¹⁸O isotopic incorporation as seen by LDI-MS (Fig. S5^†), and indicates a mechanism of oxygen incorporation that involves hydrolysis of an Re^V(Cl)₂(TBP₈Cz) precursor.5 The appearance of a new band upon ¹⁸O substitution at 945 cm⁻¹ is accompanied by a slight decrease in intensity of the band at 997 cm⁻¹ (Fig. 3). These spectral changes are consistent with the v(Re¹⁶O) mode overlapping with the intense peak at 997 cm⁻¹. The predicted isotopic shift for ¹⁸O substitution in an isolated Re–O diatomic oscillator is 52 cm⁻¹, in excellent agreement with the predicted value of v(Re¹⁶O) = 997 cm⁻¹. The Re-O stretch is close to that observed for Re^V(O) corrole (994 cm⁻¹),⁶ but higher than most Re^V(O) porphyrins, phthalocyanines^5,12b or N-confused porphyrin.^12a

Fig. 3.

ATR-IR spectra (800 – 1200 cm⁻¹) of a) 2-¹⁶O (black) and 2-¹⁸O (red), b) 2-¹⁸O (red), (3-¹⁸O)(BArF⁻) (blue), and (3-¹⁸O)(OTf⁻) (green). Asterisk (*) = peaks associated with OTf⁻ or BArF⁻ counterions.

The effect of protonation on the Re–O bond can be probed by IR spectroscopy (Fig. 3b). Upon protonation of 2 with HBArF, a sharp peak at 3280 cm⁻¹ appears, consistent with the N–H stretch of a meso-NH group (Fig. S6^†). At the same time, the v(Re¹⁸O) peak at 945 cm⁻¹ disappears, and only a broad band associated with BArF⁻ is present at 975 cm⁻¹ without any clear evidence for a new Re-¹⁸O stretch. However, replacing HBArF with HOTf causes the broad peak at 975 cm⁻¹ to disappear, revealing a new peak for v(Re¹⁸O) = 956 cm⁻¹ (Fig. 3b). These data indicate that an 11 cm⁻¹ blue shift occurs for the metal-oxo stretch, implicating a strengthening of the metal–oxo bond upon protonation of a remote site on the ligand.

Density functional theory (DFT) calculations were performed on complexes 2 and 3 to support structural and spectroscopic assignments. Geometry optimizations were performed at the PBE0/LANL2TZ/6-31G** level of theory, beginning with the crystal structure coordinates for 2. The peripheral aryl substituents were replaced by H atoms to facilitate the calculations. The optimized geometry for 2 matched well with the experimentally determined crystal structure. Geometry optimizations for protonated 2 were performed with the H⁺ attached at either the meso-N or the terminal oxo ligand. These calculations showed that the O–H tautomer is +29 kcal/mol higher in energy than the N–H tautomer 3 (Fig. S7^†, Table S2^†), and indicate that the meso-N is the preferred site of protonation. Frequency calculations give v(Re–O) = 1055 cm⁻¹ for 2 and 1064 cm⁻¹ for 3, which are both higher than the corresponding experimental values. DFT is known to overestimate vibrational frequencies due to systematic errors.¹⁶ However, the difference between the calculated v(Re–O) values for 2 and 3 (Δv(ReO) = 9 cm⁻¹) is in excellent agreement with experiment (11 cm⁻¹).

The Mn^V(O)(TBP₈Cz) complex reacts rapidly with phosphine derivatives through an O-atom transfer mechanism.^10c Attempts to react 2 with the phosphine derivatives PPh₃, PMe₃, or PEt₃ in CH₂Cl₂ at 23°C showed no reaction even over prolonged reaction times (5 d). Similarly, H-atom donors such as TEMPOH or 2,4-di-tert-butylphenol were unreactive toward 2, although the isoelectronic Mn^V(O) complex readily abstracts H-atoms from both of these substrates.^8b Reduction of 2 with strong one-electron donors such as cobaltocene (E_red = −1.33 V vs SCE)¹⁷ was also unsuccessful. We speculated that 3, with an additional full unit of positive charge, might show enhanced oxidative reactivity compared to 2, but reactions with PR₃ or ArOH substrates led only to deprotonation and recovery of 2. Both the neutral and monoprotonated Re^V(O) complexes appear inert to either H-atom or O-atom transfer reactions, suggesting a greatly enhanced stability for Re^V(O) compared to Mn^V(O) in the corrolazine environment.

To gain further insights into the reactivity of 2 and 3, cyclic voltammetric measurements were performed (Fig. 4). Complex 2 exhibits a single reversible wave at 1.04 V vs SCE, which is close to an assigned Cz ring oxidation for Mn^V(O)(TBP₈Cz),⁹ as well as other metallocorrolazines.^15a However, there are no other redox events observed for 2, in contrast to the Mn^V(O)complex, which exhibits an Mn^V/Mn^IV couple near −0.05 V.¹⁵ Upon protonation of 2, the ring oxidation wave disappears, leaving only an irreversible reduction at −0.17 V as seen in the CV for 3. Thus for 2 and 3 there are no clearly accessible metal-based redox couples, consistent with a lack of HAT and OAT reactivity. However, the reversible oxidation seen for 2 at 1.04 V suggested that a one-electron oxidized Re^V(O)(Cz^+•) π-cation-radical complex might be accessible by chemical oxidation.^9,18

Fig. 4.

a) Cyclic voltammograms of 2 (black line) and 3 (red line) in CH₂Cl₂, with 0.1 M TBAPF₆ supporting electrolyte, 150 mV/s. b) UV-vis spectral changes of 2 observed upon addition of [Ar’₃N^+•][SbCl₆⁻] (0–1 equiv) in CH₂Cl_2. Inset: X-band EPR of Re^V(O)(TBP₈Cz)^•+ (4) (3 mM) at 298 K in CH₂Cl₂.

Reaction of Re^V(O)(TBP₈Cz) with [Ar’₃N^+•][SbCl₆⁻] (Ar’ = 4-BrC₆H₄, E_red = 1.16 V vs SCE)¹⁷ resulted in isosbestic conversion to a new species with a broadened Soret peak at 445 nm and a relatively weak, red-shifted band at 760 nm (Figure 4b). These features are characteristic of a corrole,¹⁹ porphyrazine,²⁰ and corrolazine π-radical.^9–10,18 Spectral titration showed one equiv of oxidant was required for complete formation of the π-radical cation complex (Fig. S8^†). EPR spectroscopy revealed a sharp singlet at g = 2.00, similar to Mn^V(O) and Mn^V(imido) Cz-π-radical cation species.^9,18 Quantitation of the EPR signal showed a 94% yield of the oxidized product. Attempts to isolate this product as a solid were unsuccessful, but taken together the data show that 2 can be oxidized in situ by Ar’₃N^+• to give the monocationic complex Re^V(O)(TBP₈Cz^+•) (4).

In earlier work, dramatic enhancements in OAT reactivity were seen for Mn^V(O)(TBP₈Cz^+•) compared to its neutral precursor,⁹ but the Re^V analog 4 remained unreactive to OAT. Addition of phosphine derivatives to 4 in CH₂Cl₂ led only to 1−e⁻ reduction, restoring 2 with no evidence of oxo-transfer. However, addition of the H-atom donor 9,10-dihydroanthracene (DHA) to 4 resulted in quantitative conversion to monoprotonated 3. Analysis by GC-FID showed anthracene was produced in 90% yield, confirming that formal HAT occurs via the net reaction in Scheme 2. Although these observations implied that 4 was abstracting an H-atom from DHA, UV-vis time course experiments under pseudo-first-order conditions (excess DHA) showed a surprising linear dependence for the decay of 4 and production of 3 (Fig. S10^†). These data were indicative of a reaction zero-order in 4. The zero-order kinetics can be explained by a mechanism involving back electron-transfer between Ar’₃N and 4, establishing the equilibrium shown in Eq 1. The redox potentials for 2 and [Ar’₃N^+•][SbCl₆⁻] indicate a relatively small K_ET = 107. The minor Ar’₃N^+• species can then oxidize DHA to give anthracene and Ar’₃N in the rate-determining step (Eq 2), while the released H⁺ binds to 2 in preference to Ar’₃N (pKa ≤ −4).^21a Independent experiments confirm that [Ar’₃N^+•][SbCl₆⁻] oxidizes DHA to anthracene (98%) relatively rapidly in a second-order process (k = 0.126 M⁻¹ s⁻¹) (Fig. S11^†).

Scheme 2.

Reaction of 4 with 9,10-DHA to form 3 and anthracene.

$${Re}^{\mathrm{V}}(\mathrm{O})(\text{Cz})+{\text{Ar}’}_3{\mathrm{N}}^{•+}\rightleftharpoons {Re}^{\mathrm{V}}(\mathrm{O})({\text{Cz}}^{•+})+{\text{Ar}’}_3\mathrm{N}(K_{ET})$$ (1)

$${\text{Ar}’}_3{\mathrm{N}}^{•+}+\mathbf{0}.\mathbf{5}\text{DHA}\to {\text{Ar}’}_3\mathrm{N}+\mathbf{0}.\mathbf{5}\text{anthracene}+{\mathrm{H}}^{+}(k_1)$$ (2)

Replacement of [Ar’₃N^+•][SbCl₆⁻] with the more powerful oxidant Ce^IV(NH₄)₂(NO₃)₆ (CAN, E_red = +1.33 V)¹⁷ led to a distinct change in mechanism. Oxidation of 2 with CAN in CH₂Cl₂/CH₃CN (100:1 v/v) gives a UV-vis spectrum similar to 4, and addition of excess DHA results in an exponential decay of 4 with the concomitant formation of 2 (Eq 3) (Fig. S12^†). These data are consistent with a pseudo-first-order process. The use of the stronger oxidant CAN greatly disfavors back electron-transfer. In this case, the rate-determining step involves the direct reaction of 4 with DHA, and variation of [DHA] leads to a k₂ = 6.3 × 10⁻³ M⁻¹ s⁻¹. This reaction likely occurs through an HAT mechanism between DHA and 4 to give 3, which is then rapidly deprotonated by the NO₃⁻ counterion^21b to give 2. Control experiments show that 3 is rapidly deprotonated by Bu₄N⁺NO₃⁻ (Fig. S14^†).

$${Re}^{\mathrm{V}}(\mathrm{O})({\text{Cz}}^{•+})({{\text{NO}}_3}^{-})+\mathbf{0}.\mathbf{5}\text{DHA}\to {Re}^{\mathrm{V}}(\mathrm{O})(\text{Cz})+{\mathrm{H}}^{+}{{\text{NO}}_3}^{-}+\mathbf{0}.\mathbf{5}\text{anthracene}(k_2)$$ (3)

We have reported the synthesis and XRD characterization of the first third-row metallocorrolazine. This Re^V(O) complex is strikingly inert to both H-atom and O-atom transfer reactions, in contrast to its isoelectronic Mn^V(O) analog. Protonation of 2 gives a cationic Re^V(O) complex and no evidence of valence tautomerism, supporting the conclusion that protonation occurs exclusively on the meso-N atom and not on the oxo ligand. Taking advantage of the inertness of the Re–O group, we have provided the first insights into the reactivity of a porphyrinoid π-radical-cation completely decoupled from its high-valent metal-oxo core. This Cz π-radical-cation, which contains a weakly basic meso-N site, appears to be competent to abstract H-atoms from relatively weak C-H substrates. These observations suggest that porphyrin π-radical cations, including those found in heme enzyme metal-oxo intermediates, may have as yet unidentified roles to play in oxidative reactivity.