Activation of Metal Oxo and Nitrido Complexes by Lewis Acids

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Activation of metal oxo and nitrido complexes by Lewis acids Liu Yingying, and Tai-Chu Lau J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b13100 • Publication Date (Web): 01 Feb 2019 Downloaded from http://pubs.acs.org on February 3, 2019

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Journal of the American Chemical Society

Activation of Metal Oxo and Nitrido Complexes by Lewis Acids Yingying Liu and Tai-Chu Lau* Department of Chemistry and Institute of Molecular Functional Materials, City University of Hong Kong, Tat Chee Avenue, Kowloon Tong, Hong Kong, China. ABSTRACT: Metal oxo species play key roles as oxidants in chemical and biological systems. Although Brønsted acids have long been known to enhance the oxidizing power of metal oxo complexes, the use of Lewis acids (LA), such as metal ions, to activate these complexes has received much less attention until recently. The report of the presence of a Mn4CaO5 cluster active site in the oxygen-evolving center (OEC) of photosystem II (PS II) in 2004 has stimulated intense interest in understanding the interaction of Lewis acids with metal-oxo species. This perspective analyzes the various modes of activation of metal oxos by Lewis acids and the pathways for the oxidation of various substrates by LA/M=O systems. The interaction of Lewis acids with metal nitrides will also be discussed, although it is much less studied than metal oxos.

1. Introduction High-valent metal oxo species play key roles in many biological and chemical oxidation processes.1-5 It is well established that the oxidizing power of oxo species can be enhanced by Brønsted acids. On the other hand, the use of Lewis acids (LA), such as metal ions, to activate metal-oxo species has received much less attention. However, the discovery of the Mn4CaO5 cluster active site in the oxygenevolving center (OEC) of photosystem II (PS II) in 2004 has stimulated intense interest in studying the interaction of metal ions with metal-oxo species.6-10 There are a number of advantages in the use of Lewis acids over Brønsted acids for the activation of metal oxo complexes. For instance, Lewis acids can interact with metal oxos through both σ and π bonding effects. A wide variety of Lewis acids with different acidities and structures can be used to tune both the electronic and steric effects of LA/M=O systems. Over the past 15 years, substantial progress has been made in understanding how Lewis acids activate metal oxo species. In particular, a number of synthetic iron and manganese oxo complexes have been reported recently as models of both heme and non-heme metalloenzymes.11-19 This has made possible detailed studies of the activation of these metal oxo complexes by a variety of Lewis acids. This perspective will first analyze in general the various ways that metal oxo species can be activated by Lewis acids. This is followed by more detailed discussions of the interaction of Lewis acids with various metal oxo complexes, as well as the mechanisms of oxidation of various substrates by these LA/M=O systems.

The interaction of Lewis acids with metal nitrides will also be discussed, although so far it has received much less attention than M=O. 2. Interaction of Lewis acids with metal-oxo species The main pathways for oxidation by a metal oxo species are: 1. Electron transfer (ET): Mn+=O + X → Mn-1=O + X+ 2. H-atom transfer (HAT): Mn+=O + RH → Mn-1OH + R• 3. O-atom transfer (OAT): Mn+=O + X → Mn-2 + XO All these processes may be enhanced by Lewis acids. In some cases, a switch of the mechanism may occur, e.g. from HAT to ET. The Lewis acids that have been used for the activation of metal oxo species include redox-inactive metal ions such as Li+, Mg2+, Ca2+, Sr2+, Al3+, Zn2+ and Ln3+. Non-metallic Lewis acids such as BF3 and B(C6F5)3 have also been studied. Redoxactive metal ions such as Fe3+ are also effective, although in this case the activating effects may be accompanied by redox changes. There are several modes of interaction between LA and M=O (Figure 1): a) Direct binding of LA to M=O. This would usually result in an increase in the redox potential of M=O that correlates with the acidity of the LA. b) Binding of LA to M=O is accompanied by intramolecular ET to give a valence tautomer. This may occur for M=O with redox-active ancillary ligand. c) LA may bind to one or more donor atoms of an ancillary ligand. Electron-withdrawing effects of the LA through the ligand would increase the electrophilicity of M=O. d) LA may abstract a non-oxo ligand from the M=O species. This may create a vacant site for catalysis; the resulting M=O would also become more oxidizing.

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to give a dark green solution which is capable of oxidizing cyclohexane to cyclohexanone in 60% yield over 4 h at 23 oC. The green species generated in solution is proposed to be trans-[Ru(O)2(OAc)4]2. Upon adding various Lewis acids to the green solution, the oxidation of alkanes is greatly accelerated (Figure 3).

Figure 1. Modes of interaction between LA and metal-oxo.

3. Mono-oxo versus poly-oxo species The effects of a Lewis acid on mono-oxo complexes may be different from that on poly-oxo (di-, tri- or tetra-oxo) complexes. Binding of LA would normally cause mono-oxo species to become a stronger oxidant due to electronwithdrawing effects of the LA. This would enhance ET reactions. However, there may be steric effects on HAT and OAT reactions, especially if the Lewis acid is bulky. On the other hand, LA-bound poly-oxo species can make use of a free oxo ligand to carry out HAT and OAT reactions, hence steric effects may not be a problem (Figure 2). Moreover, multiple binding of LA to two or more oxo ligands is possible, which may further activate the metal-oxo species.

XO L M O L X L M O +LA L O

L MO L O

LA

Figure 3. Effects of Lewis acids on the oxidation of alkanes by Ba[Ru(O)3(OH)2] in CH2Cl2/CH3COOH. The yield is based on Ba[Ru(O)3(OH)2] acting as a two-electron oxidant.

Ruthenium mono-oxo complexes. Recently Yin et al. reported that the catalytic epoxidation of alkenes by Ru(bpy)2Cl2 (bpy = 2,2'-bipyridine) using PhI(OAc)2 as oxidant is accelerated by Al3+, and a Al3+/RuIV=O adduct is proposed to be the active intermediate.22 The aerobic dehydrogenation of saturated C–C bonds using the same ruthenium catalyst is also greatly accelerated by Lewis acids such as Sc3+, Al3+ and Zn2+. The Lewis acid is proposed to play a dual role; it accelerates the aerobic oxidation of the Ru(II) catalyst by forming a LA/RuIV=O adduct (Figure 4).23 However, it is likely that for both catalytic reactions the Lewis acid also plays an addition role of abstracting Cl from Ru(bpy)2Cl2 to generate a vacant site for binding to the oxidant.

LA

RH

LA L MO R L OH

Figure 2. HAT and OAT reactions of LA/poly-oxo metal species.

LA may also bind two metal-oxos together to facilitate multi-electron processes, e.g. intermolecular OO coupling. The activation of various metal oxo and nitrido complexes by Lewis acids will be analyzed in the following sections. 4. Activation of oxo complexes by Lewis acids 4.1. Ruthenium oxo complexes Ruthenium poly-oxo complexes. Our group first reported the activation of a metal oxo species by Lewis acids in 1993.20 Barium ruthenate(VI), Ba[Ru(O)3(OH)2], has a trigonal bipyramidal structure with three oxo ligands in the trigonal plane.21 It is a red solid that is insoluble in common organic solvents. However, upon adding acetic acid to a suspension of Ba[Ru(O)3(OH)2] in CH2Cl2, the red solid gradually dissolves

Figure 4. Aerobic dehydrogenation by LA/Ru(bpy)2Cl2. The yield is based on the substrate.

4.2. Chromium oxo complexes [nBu4N]2[Cr2O7] is inactive towards the oxidation of alkanes in CH2Cl2 for more than 1 day at room temperature. However, in the presence of just a few equiv. of a Lewis acid such as BF3 or AlCl3, oxidation of alkanes to the corresponding carbonyl compounds occurs within 5 min at 23 oC (Figure 5).24


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Figure 5. Oxidation of alkane by LA/[nBu4N]2[Cr2O7].

4.3. Manganese oxo complexes Manganese poly-oxo complexes. Our group reported in 2006 that the rates of oxidation of alkanes by KMnO4 in CH3CN are greatly accelerated by the presence of a few equiv. of BF3.25 BF3 forms an adduct with MnO4, which oxidizes cyclohexane 107 faster than MnO4 alone (Figure 6). Density functional theory (DFT) calculations using CH4 as substrate show that BF3 binds to one of the oxo ligands of MnO4, this activates H-atom abstraction from CH4 by an unbound oxo ligand, with activation barrier lowered by 6 Kcal mol-1.

the UV/Vis region are absent in the final reaction solution.27 This is in contrast to oxidation of alkanes by BF3/MnO4, where the system acts as a 3-electron oxidant to give MnO2. A similar accelerating effect was reported in sulfide oxidation by LA/MnO4.28 Watkinson et al. reported that the catalytic epoxidation of styrenes by a tri-μ-oxo dinuclear Mn catalyst using H2O2 as oxidant is activated by Sc3+ (Figure 8).29 Low yields of epoxide and long reaction time (20-60 min) were observed in the absence of LA. However, in the presence of 0.5 mol% Sc3+, epoxide yields of up to 99% were obtained within 3 min.

Figure 8. Sc3+ activated epoxidation by a tri-μ-oxo dinuclear Mn catalyst using H2O2.

Figure 6. Oxidation of alkane and alcohol by LA/KMnO4.

DFT calculations also show that in CH4 oxidation, the 2:1 adduct [2BF3·MnO4] is more reactive, with an activation barrier 12 Kcal mol-1 lower than that of MnO4 alone. Hence addition of high concentrations of BF3 to MnO4 is expected to form the 2:1 adduct, which should be more oxidizing.25 However, this species readily decomposes to give O2.26 From mechanistic studies and DFT calculations, the O2 evolution is proposed to occur via intramolecular O⋯O coupling of the [MnO2(OBF3)2] species (Figure 7). The proposed manganese product [MnF2(OBF2)2] can be detected by ESI/MS.

Recently, Agapie et al. have designed a [Mn3CaO4]6+ cubane as model for the OEC in PS II (Figure 9).30 Agapie’s group also reported the synthesis of a series of high-valent Mn3M(µ4-O)(µ2-O) clusters comprising three manganese centers and a redox-inactive metal M. The reduction potentials (E1/2) of these clusters show a linear correction with the pKa of M(aqua)n+ (which is a measure of the Lewis acidity of Mn+).31 Similar effects were found for the [MMn3(µ3-O)4] (M = Ca2+, Sr2+, Zn2+, Sc3+, Y3+) cluster system.32

OO O Ca OO O O O OMn OON MnN O Mn N O N N O N Figure 9. Structure of [Mn3CaO4] cubane. Curved lines schematically represent 2-pyridyl groups.

Figure 7. Proposed mechanism for O2 evolution from BF3/KMnO4.

BF3/MnO4 also oxidizes alcohols to carbonyl compounds at a rate that is 107 faster than MnO4 alone. Other Lewis acids, such as Sc(OTf)3, Zn(OTf)2, Ca(OTf)2 and Ba(OTf)2 can also enhance the oxidation of alcohols by MnO4.27 Notably, the presence of Ca2+ can increase the rate of oxidation of CH3OH by a factor of 104 (Figure 6). In the presence of Lewis acid, MnO4 functions as a 5-electron oxidant for alcohols, as evidenced by the formation of 2.5 mol of carbonyl product per mol of KMnO4 in the oxidation of cyclohexanol and benzyl alcohol. Moreover, characteristic absorption peaks of MnO2 in

The effects of Ln3+ on the electronic properties of manganese oxo clusters were also investigated by Agapie’s group through the design of [LnMnIV3O4(OAc)3(DMF)n]+ clusters (Ln = La3+, Ce3+, Nd3+, Eu3+, Gd3+, Tb3+, Dy3+, Yb3+ and Lu3+; n = 2 or 3) supported by a trianionic ligand. A linear correlation between E1/2 and ionic radii or pKa of Ln3+ was also found.33 Manganese mono-oxo complexes. Goldberg et al. found that binding of Zn2+ to the MnV oxo complex, (TBP8Cz)MnV(O) (TBP8Cz = octakis(p-tertbutylphenyl)corrolazinato(3−)), induces intramolecular electron transfer to produce the MnIV oxo π-cation radical complex [(TBP8Cz+•)MnIV(O)-Zn2+], which is a valence tautomer of the MnV oxo species (Figure 10).34, 35 This



Figure 11. Valence tautomerization of (tpfc)MnV(O) induced by Lewis acid.

The activation of the MnIV oxo complexes [(N4Py)MnIV(O)]2+ and [(Bn-TPEN)MnIV(O)]2+ (N4Py = N,Nbis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine; Bn-TPEN = N-benzyl-N,N’,N’-tris(2-pyridylmethyl)-1,2-diaminoethane) by Sc3+ have been studied in detail by Fukuzumi and Nam.41, 42 Binding of Sc3+ to the MnIV(O) complexes results in the formation of MnIV(O)Sc3+ and MnIV(O)(Sc3+)2 species, which have been characterized by various spectroscopic methods, including electron paramagnetic resonance (EPR), electrospray ionization mass spectrometry (ESI-MS) and Xray absorption near edge structure/extended X-Ray absorption fine structure (XANES/EXAFS). Combined with DFT calculations, the structure of the MnIV(O)(Sc3+)2 species is proposed as having one Sc3+ directly bound to the oxo motif, while the other Sc3+ is located in the secondary coordination sphere (Figure 12).41, 42

3

3

3

3

3

3

IV

2+ FO C CF O S O S O O O O ScO OScO S CF OSO OOS O FS C O O CF C O FO N MnN N N N IV

2+ OCF S O O O FS C O O ScOS O O O CF N MnN N N N

3

The reactivity of [(TBP8Cz+•)MnIV(O)LA] (LA = TFA, Zn(OTf)2, B(C6F5)3 and HBArF) towards CH bond activation of various alkylaromatics has also been investigated.37 The second-order rate constant (k) for the oxidation of xanthene increases with increasing acidity of the LA. The observed large deuterium isotope effects and linear correlation of logk with bond dissociation energy (BDE) of the substrates indicate a HAT mechanism. In these reactions [(TBP8Cz+•)MnIV(O)LA] again functions as a one-electron oxidant to give a MnIV product, whereas (TBP8Cz)MnV(O) functions as a two-electron oxidant. The one-electron HAT reaction rates of (TBP8Cz)MnV(O) are accelerated by Lewis acids, in accordance with a more positive redox potential upon binding to LA. In contrast, the rate of OAT from MnV(O) to PPh3 is dramatically decreased upon binding to LA; a 104-fold decrease in rate was observed in the case of LA = Zn2+.38 This is attributed to the less electrophilic oxo ligand of the MnIV(O) π-radical-cation species compared to that of the MnV(O) species, which is at a higher oxidation state.38 However, steric effects between the bulky phosphine and the LA-bound MnIV=O may also be a factor.

is faster than that of (tpfc)MnV(O) for LA = B(C6F5)2, Zn2+ and Ca2+; the rate increases by 6-, 21- and 31-fold, respectively. Remarkably, (tpfc+•)MnIV(O)Ca2+ is the most reactive species, despite its rate of formation is the slowest. This observation may provide insight in nature’s choice of Ca in OEC. These results indicate that, similar to (TBP8Cz)MnV(O), (tpfc)MnV(O) exhibits an increase in ET and HAT reaction rates, but a decrease in OAT rates in the presence of LA. Again, as in the case of [(TBP8Cz+•)MnIV(O)LA], steric effects may play a role in inhibiting OAT reactions, whereas HAT reactions are usually less susceptible to steric effects.

3

conversion can be reversed by using 1,10-phenanthroline to sequester the Zn2+ ion. As expected, [(TBP8Cz+•)MnIV(O)Zn2+] is a stronger oxidant than (TBP8Cz)MnV(O). However, it functions only as a oneelectron oxidant in reaction with ferrocene (Fc), whereas (TBP8Cz)MnV(O) functions as a two-electron oxidant in reaction with [Fe(C5HMe4)2]. In HAT reaction of 2,4,6-tri-tertbutylphenol (2,4,6-TTBP) with the MnV(O) complex, a modest rate enhancement of ~3 fold was observed in the presence of Zn2+ ion.34 B(C6F5)3 was also used as Lewis acid and it shows similar valence tautomerization effects as Zn2+ on the MnV(O) species (Figure 10).36 However, in HAT reaction of 2,4,6TTBP with the MnV(O) complex, a rate increment of ~100 fold by B(C6F5)3 was found, indicating that the HAT reactivity of MnIV(O)(π-cation-radical)(LA) increases with increasing Lewis acidity of LA.36

3

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Figure 12. Structure of [(N4py)MnIV(O)Sc3+]2+ [(N4py)MnIV(O)(Sc3+)2]2+ from DFT calculations (ref 41). Figure 10. Reactivity of (TBP8Cz)MnV(O) and its valence tautomer.

Abu-Omar et al. also reported the valence tautomerization of a manganese-oxo corrole complex, (tpfc)MnV(O) (tpfc = 5,10,15-tris(pentafluorophenyl)corrole), induced by CF3COOH (TFA)39 and various other LA40 (Figure 11). This MnV(O) species exhibits an increase in the rates of ET reactions in the presence of TFA. On the other hand, (tpfc+•)MnIV(O)LA does not undergo OAT with PhSMe, whereas this reaction is observed with (tpfc)MnV(O). This indicates that (tpfc)MnV(O) is more reactive than its valence tautomer in OAT reactions. However, HAT reaction of (tpfc+•)MnIV(OLA)n+ with 2,4-di-tert-butyl phenol (2,4-DTBP)

and

The OAT reaction of [(N4Py)MnIV(O)]2+ with PhSMe is enhanced by ~2200-fold upon binding to Sc3+. On the other hand, in the reaction with 1,4-cyclohexadiene (CHD) to give benzene, the rate of the MnIV(O) complex is decreased by a factor of ~5 and ~180 upon binding to one and two Sc3+, respectively. It is proposed that in the OAT reaction with PhSMe, the mechanism with [(N4Py)MnIV(O)]2+ alone is direct OAT. However, in the presence of Sc3+, the mechanism is switched to initial rate-limiting ET followed by rapid O transfer. This is due to much higher redox potentials of [MnIV(O)]2+Sc3+ (1.28 V vs SCE) and [MnIV(O)]2+(Sc3+)2 (1.36 V) than [MnIV(O)]2+ (0.78V).42 The ET mechanism is expected to be little effected by steric effects from Sc(OTf)3. On the other hand, in the reaction of [MnIV(O)]2+Sc3+ and


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[MnIV(O)]2+(Sc3+)2 with CHD, the ET mechanism is not favorable since CHD has a much higher oxidation potential than PhSMe. In this case the mechanism is HAT, which would require close contact between Mn=O and CH bond of CHD, hence there should be significant steric effects from the Sc(OTf)3 bound to the oxo ligand. In contrast, in the Mn(V) oxo (corrolazine) and (corrole) complexes described above, HAT reactions are accelerated by Lewis acids. This could be due to the less bulky LA used in these complexes (Zn(OTf)2 and B(C6F5)3); also the corrolazine and corrole ligands are planar and so there should be less steric effects in HAT reactions. Yin et al. reported that Lewis acids can activate the catalytic epoxidation of alkenes by a MnII catalyst, LMnCl2 (L = N,N’-dimethyl-N,N’-bis(2-pyridylmethyl)-1,2ethanediamine, BPMEN), via a LMnIV=O⋯LA active intermediate.43, 44 In the absence of Lewis acid, catalytic epoxidation of cyclooctene by LMnCl2 using PhI(OAc)2 as oxidant results in only 7.6% conversion and 6.4% yield of epoxide. Upon adding 2 equiv. of Sc3+ the conversion becomes 100% with 94% yield of epoxide. Reaction of LMnCl2 with PhI(OAc)2 is proposed to generate the known di-μ-oxoMn2(III,IV) species, which is very sluggish in olefin epoxidation. Upon adding Lewis acid, it was proposed that cleavage of the dinuclear core occurs, leading to the formation MnIV=O⋯LA (LA = Sc3+, Al3+, etc.) which is highly active. It was also proposed that the presence of the Lewis acid also changes the epoxidation mechanism from radical-type OAT to concerted OAT (Figure 13).43, 44 Sc3+ should also enhance the dissociation of Cl from LMnCl2 to generate a vacant site for OAT with PhI(OAc)2.

Figure 13. Proposed mechanism for Lewis acid promoted epoxidation by a MnII catalyst.

Yin also reported that the catalytic oxidation of PhSMe to PhS(O)Me and PhS(O)2Me by [MnII(Me2Bcyclen)Cl2] (Me2Bcyclen is a cross-bridged cyclen ligand) and cis[MnIV(Me2Bcyclen)(OH)2]2+ using H2O2 as oxidant are enhanced by Ca2+.45 Other group II ions also exhibit similar accelerating effects. 4.4. Iron oxo complexes Iron poly-oxo complexes. The ferrate(VI) ion, FeO42, is the only iron(VI) oxo complex known. K2[FeO4] is a strong oxidant in acidic medium, it will spontaneously liberate O2 in aqueous acidic solution.46, 47 BaFeO4, on the other hand, is a stable, non-hygroscopic solid that is insoluble in common organic solvents. However, it is slightly soluble in CH2Cl2 containing CH3COOH to give a red solution that is stable for a few hours at room temperature. The red solution can be activated by various metal chlorides to oxidize alkanes (Figure 14).48

Figure 14. Oxidation of alkanes by MgCl2/FeO42.

Recently, our group also observed that O2 evolution from FeO42 can be promoted by Ca2+.49 K2FeO4 is stable in water at pH 9-10 for >10 h at 23 oC. However, O2 is detected immediately upon adding a few equiv. of Ca2+ to K2FeO4 in water at pH 9-10. The rate of the reaction is second-order in [FeO42] and first-order in [Ca2+]. Isotope labeling experiments reveal that both O atoms in the evolved O2 come from FeO42. These results suggest a binuclear mechanism for O2 evolution. DFT calculations suggest that the role of Ca2+ is to bind two FeO42 units together to facilitate O–O coupling (Figure 15).49

Figure 15. Proposed mechanism for O2 evolution by Ca2+/FeO42 at pH 9-10.

Agapie’s group also reported the synthesis of a series of Fe3M(µ4-O)(µ2-OH) clusters comprising three iron centers and a redox-inactive metal M (Figure 16). Similar to the Mn clusters discussed in 4.3, these clusters exhibit a linear dependence between E1/2 and Lewis acidity of Mn+.50

Figure 16. Structure of Fe3Sc(µ4-O)(µ2-OH) cluster. Curved lines in structure schematically represent 2-pyridyl groups.

Iron mono-oxo complexes. The X-ray crystal structure of a complex that contains Sc3+ bound to the oxo ligand of a nonheme iron(IV) oxo species was reported by Fukuzumi and Nam in 2010. The complex is initially formulated as the neutral [(TMC)FeIV(O)ScIII(OTf)4(OH)] (TMC = 1,4,8,11tetramethyl-1,4,8,11-tetraazacyclotetradecane) (Figure 17a).51 However, DFT calculations by Swart suggest that the complex should be better formulated as [(TMC)FeIII(O)ScIII(OTf)4(OH2)] (Figure 17b).52 Further characterization of this complex by Münck and Que using Mossbauer, X-ray absorption spectroscopy (XAS) and EPR indicates that this is indeed a high-spin iron(III) complex, as suggested by Swart.53



Figure 17. a) originally proposed and b) actual structure of complex formed between Sc(OTf)3 and [(TMC)FeIV(O)]2+.

Extensive work has been carried out by Fukuzumi and Nam on the effects of redox-inactive metal ions on [(N4Py)FeIV(O)]2+.54-58 Upon binding to Sc3+, the one-electron reduction potential of [(N4Py)FeIV(O)]2+ in CH3CN (Ered = 0.51 V vs SCE)59 is shifted to positive direction according to the Nernst equation (Eq 1): 𝐸red = 𝐸0𝑟𝑒𝑑 +

(

2.3𝑅𝑇 𝐹

)log (1 + 𝐾 [Sc 1

3+

] + 𝐾1𝐾2[Sc3 + ]2) (1)

where 𝐸0𝑟𝑒𝑑 is the one-electron reduction potential without Sc3+, and K1 and K2 are the formation constants of 1:1 and 1:2 complexes of [(N4Py)FeIV(O)]2+ with Sc3+, respectively (structures are similar to the Mn complexes in Figure 12).54 The Ered in the presence of 0.2 M Sc3+ is 1.35 V, which is 0.84 V higher than that without Sc3+. In accordance with this higher Ered, the oxidation of [Fe(bpy)3]2+ by [(N4Py)FeIV(O)]2+ in the presence of 12 mM Sc3+ is ca. 8 orders of magnitude faster than without Sc3+. The electron transfer reaction of [(N4Py)FeIV(O)]2+ is also accelerated by various other metal ions using Fc as reductant. The rate law is shown in Eq 2: kobs = k0 + k1[Mn+] + k2[Mn+]2 (2) Both logk1 and logk2 values for various metal ions exhibit good linear correlations with ΔE values (ΔE is the binding energy of metal ion with O2, which is a quantitative measure of the Lewis acidity of metal ions) (Figure 18).51, 54

On the other hand, the rate of oxidation of benzyl alcohol by [(N4Py)FeIV(O)]2+, which is proposed to occur by HAT mechanism, is not accelerated by Sc3+.57 Presumably HAT reaction of the {[(N4Py)FeIV(O)]2+2Sc3+} species is not favored due to steric effects, similar to the case of [(N4Py)MnIV(O)]2+ described in section 4.3. In contrast, for more electron-rich substrates, such as 2,5-(MeO)2C6H3CH2OH, the oxidation is accelerated by Sc3+. In this case, the reaction occurs by ET, which is much more favored for this substrate due to its lower oxidation potential. Similarly, the oxidation of toluene to benzyl alcohol by [(N4Py)FeIV(O)]2+ occurs by HAT mechanism; but in the presence of Sc(OTf)3, the mechanism is switched to ET. Similar activating effects are also observed using HOTf.58 The oxidative dimerization and N-demethylation of N,Ndimethylanilines are also greatly enhanced by Sc(OTf)3, and the mechanism involves initial ET.56 Yin et al. reported that both the yields and rates are enhanced by Sc3+ in the catalytic epoxidation of alkenes by a Fe(BPMEN) complex using aqueous H2O2.60 It was proposed that in the presence of Sc3+, FeIIIOOH, FeIV=O are more reactive species; while in the absence of Sc3+, FeV=O species is the most reactive species. Presumably, the more basic FeIIIOOH and FeIV=O species are more readily activated by Sc3+. In CeIV-driven water oxidation catalyzed by the non-heme iron complex α-[FeII(CF3SO3)2(mcp)] (mcp = N,N’-dimethylN,N’-bis(2-pyridylmethyl)-1,2-cis-diaminocyclohexane), an intermediate formulated as [(mcp)FeIV(O)(µ-O)CeIV(NO3)3]+ was trapped (Figure 19), which was characterized by cryospray ionization high resolution mass spectrometry (CSIMS) and resonance Raman (rRaman) spectroscopy.61 The CeIV may function as a Lewis acid to activate the iron oxo species towards OO bond formation. This intermediate also resembles the fundamental MnV(O)(µ-O)Ca(OH2) structural motif of the OEC unit in PS II. Later Que et al. reported a facile and reversible formation of iron(III)-oxo-cerium(IV) adduct by reacting [(N4Py)FeIV(O)]2+ with (NH4)2[Ce(NO3)6].62 The X-ray crystal structure of the complex has been determined (Figure 20). CeIV has been widely used as an oxidant for organic substrates63 and in water oxidation64; it is likely that it functions both as an oxidant and as a Lewis acid in many of these reactions.

Figure 18. Plots of log k1 (red circles) and log k2 (blue squares) vs ΔE. Reprinted with permission from ref 54. Copyright 2011 American Chemical Society.

The oxidation of thioanisoles to sulfoxides by [(N4Py)FeIV(O)]2+ is also significantly enhanced by Sc3+.55 In this case a switch in the mechanism of oxidation also occurs. In the absence of Sc3+, there is direct OAT from FeIV=O to ArSR to give ArS(O)R. However, in the presence of Sc3+, the mechanism is switched to initial ET followed by O transfer: FeIV(O) + ArSR → [FeIII(O), ArSR+] → FeII + ArS(O)R. This behavior is similar to that of the corresponding MnIV=O complexes described in section 4.3.

Page 6 of 13

Figure 19. Formation [O=FeIVOCeIV] intermediate.


Page 7 of 13

Lewis acids are known to form adducts with oxo complexes of Re, Mo, W and Ce (Figure 23).68-70 However, the reactivity of these adducts has not been investigated.

4.5. Cobalt oxo complexes Redox-inactive metal ions such as Sc3+ has been reported to stabilize cobalt(IV) oxo complexes.65, 66 Ray et al. reported that [CoII(TMG3tren)(OTf)]+ (TMG3tren = tris[2-(Ntetramethylguanidyl)ethyl]amine) can be oxidized by 2-(tertbutylsulfonyl)iodosylbenzene (sPhIO) in the presence of Sc(OTf)3 to generate a species formulated as [(TMG3tren)CoIV(µ-O)ScIII(OTf)3]2+, which was supported by EPR, Co K-edge XAS/EXAFS and DFT calculations (Figure 21).65 In accordance with its formulation as a [CoIVOSc3+] species, this complex can perform OAT reaction with PPh3 to generate PPh3O, ET reaction with Fc to generate Fc+ and CoIII, and oxidation of 1,10-dihydroanthracene to anthracene and anthraquinone. However, Borovik et al. suggested that an alternative formulation of this species is [(TMG3tren)CoIII(µOH)ScIII(OTf)3]2+ by comparison with related species bearing the CoIIIOHSc3+ motif.67

N N 1)PhIO N N 2) Sc(OTf) N Co N N N N O N ScOTf TfOOTf

2+

3

5

6

5

3

5

6

3

4.7. Activation by binding to a non-oxo ligand. Our group reported in 2004 that the very stable nitridoosmate(VIII) ion, [Os(N)(O)3], is readily activated by FeCl3 to undergo stoichiometric and catalytic oxidation of alkanes in CH2Cl2/CH3COOH at ambient conditions.71 The adduct [Os(N)(O)3FeCl3] can be detected by ESI/MS. It was proposed that FeCl3 binds to the nitrido ligand rather than to an oxo ligand. The resulting adduct is activated towards HAT reactions by an oxo ligand (Figure 24). The osmium product after alkane oxidation is [Os(N)Cl4], with the Cl ligands derived from FeCl3. The alkane oxidation can be made catalytic by using FeCl3/[Os(N)Cl4] as catalyst and 2,6-dichloropyridine Noxide (Cl2pyO), H2O2 or tBuOOH as oxidant, with turnover numbers (TONs) up to 900, 1000 and 4200, respectively. Efficient alkane oxidation can also be carried out using Sc(OTf)3/[Os(N)Cl4] as catalyst. (Table 1).72 The proposed mechanism is shown in Figure 25.

3

+

M = Mo, W NN N OOMO B(CF)

Figure 23. Examples of M=OB(C6F5)3 (M = Re, Mo and W).

Figure 20. Structure of [(N4Py)FeIIIOCeIV(OH2)2(NO3)4]+.

N N N N N Co N N N N OTf N

5

6

Re OO OB(CF)

5

6

6

B(CF) CF O N Cl Re N N CF

s

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 24. Oxidation of alkanes by FeCl3/[Os(N)(O)3]. Adapted with permission from ref 71. Copyright 2004 American Chemical Society.

Figure 21. The formation of [CoIVOSc3+].

Using a similar strategy, Ray and Nam also generated cobalt(IV) oxo species bearing a tetraamido macrocyclic ligand (TAML) that is stabilized by various Lewis acids, [(TAML)CoIV(O)Mn+]n2 (Mn+ = Sc3+, Ce3+, Y3+ and Zn2+) (Figure 22).66 The [(TAML)CoIV(O)Sc3+]+ species is characterized by EPR, XAS/EXAFS and DFT calculations. [(TAML)CoIV(O)Sc3+]+ reacts readily with hydrocarbons with weak CH bonds. The observed large deuterium isotope effects and linear correlation between the log(rate constant) and the CH BDE suggest a HAT mechanism. In OAT reaction of [(TAML)CoIV(O)Sc3+]+ with thioanisoles, the rate increases with increasing Lewis acidity of the metal ion, similar to Fe and Mn systems.41, 55

Figure 25. Catalytic oxidation of alkanes by LA/[Os(N)Cl4]. Adapted with permission from ref 72. Copyright 2008 American Chemical Society.

Table 1. Catalytic oxidation of cyclohexane (CyH) by LA/[Os(N)Cl4]/ ROOH. a ROOH

Figure 22. Formation of [(TAML)CoIV(O)M3+]+.

4.6. Other oxo species.

Lewis acid

Yield / % CyOH

Cy=O

CyCl

1

tBuOOH

FeCl3

67

21

3

2

H2O2

FeCl3

60

26

2

3

Cl2pyO

FeCl3

57

20

5

4

tBuOOH

Sc(OTf)3

57

31

2

5

H2O2

Sc(OTf)3

60

29

2


Journal of the American Chemical Society a Reaction conditions: temperature, 23 oC; cyclohexane, 1.2 M; solvent, CH2Cl2/CH3COOH (5:2, v/v); [Os(N)Cl4], 1.25 mM; FeCl3, 10 mM; Sc(OTf)3, 5 mM; reaction time, 5-10 min. (ref 72)

Collins et al. have designed a MnV(O) complex of a macrocyclic tetraamido ligand that bears secondary cation binding sites (Figure 26a).73 The affinity of this complex for various metal ions, such as Li+, Na+, Zn2+, Ba2+and Sc3+, has been investigated. The LA is bound onto a secondary site incorporated in the tetraamido ligand, rather than on the oxo group. The LA/MnV=O adduct exhibits accelerating effects in the oxidation of PPh3 to PPh3O and in allylic oxidation of 2,3dimethyl-2-butene. Later, Fukuzumi and Nam used a similar strategy to prepare [MnV(O)(TAML)](Sc3+), which was characterized by various spectroscopic techniques.74 The Sc3+ is proposed to bind to an O atom of TAML, rather than to the oxo ligand (Figure 26b). Binding of Sc3+ to [MnV(O)(TAML)] increases the redox potential (by 0.73 V), which accounts for the enhancement of its reactivity towards OAT reaction with PPh3 and ET reaction with ferrocene derivatives.

O N ON O Mn N N N O O

b

O  N ON O NMnN O OTf O Sc TfOOTf V

a

V

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 13

Figure 26. a) Structures of a) [MnV(O)(TAML)] reported by Collins and b) [MnV(O)(TAML)](Sc3+) by Fukuzumi and Nam.

4.8. Activation by abstraction of a non-oxo ligand [VIV(O)Cl(TPA)]+ (TPA = tris(2-pyridylmethyl)amine) and related complexes are activated by Al3+ to catalyze the aerobic oxidation of 1,4-cyclohexadiene to benzene via a HAT mechanism.75 It was proposed that apart from interacting with the oxo ligand and stabilizing the superoxo intermediate, Al3+ also assists in the dissociation of chloride to generate a vacant site for O2 binding.75 A similar role of LA on Ru(bpy)2Cl2 and LMnCl2 has been discussed above. 5. Activation of nitrido complexes by Lewis acids Similar to metal oxo complexes, high-valent metal nitrido (M≡N) complexes can function as oxidants via ET, HAT and N-atom transfer pathways. These pathways may also be enhanced by Lewis acids. However, so far the interaction of Lewis acids with M≡N has received much less attention than M=O.76 Manganese nitrido complexes. Groves et al. first reported that cyclooctene can be converted to a N-trifluoroacetylated aziridine product by a manganese(V) porphyrin nitrido complex in the presence of trifluoroacetic anhydride (TFAA), which converts (porphyrin)MnV≡N to (porphyrin)MnV=NC(O)CF3 as the active aziridination species (Figure 27).77 The acetyl group may be viewed as functioning as a Lewis acid to activate the nitrido species.

Figure 27. Aziridination by TFAA/(porphyrin)MnV≡N. Adapted with permission from ref 77. Copyright 1983 American Chemical Society.

Subsequently Carreira et al. used a similar strategy to activate [MnV(N)(saltmen)] (saltmen = N,N’-1,1,2,2tetramethylethylenebis(salicylideneaminato))78 and [MnV(N)(3-R-sal-R’)2] (H-3-R-sal-R’ = substituted salicylimine) towards amination of alkenes.79 Komatsu et al. reported asymmetric aziridination of alkenes using a chiral manganese nitrido complex in the presence of ptoluenesulfonyl anhydride.80 Our group reported that (salen)MnV≡N can be activated by Lewis acids such as BF3, Al(OTf)3 and Fe(OTf)3 to afford parent aziridines from alkenes (Figure 28).81 By using a chiral salen ligand, asymmetric aziridination of styrenes with enantiometric excess of up 91% yield can be achieved.

Figure 28. Aziridination by LA/(salen)MnV≡N.

Osmium nitrido complexes. The activation of [OsVI(N)(O3)] by FeCl3 and Sc(OTf)3 is described in section 4.7. Leung et al. have also studied the interaction of Au(I) and Pt(II) with [Os(N)(O)3]. The X-ray crystal structures of [Au(PPh3)(µ-N)Os(O)3)] and cis-[Pt(PMe3)3(µ-N)Os(O)3] show the binding of the nitrido ligand to Au and Pt center, respectively (Figure 29).82 Similarly, Meyer et al. have reported the binding of CpCo and PtCl2(SMe2)2 to TpOs(N)Cl2 (Tp = hydrotris(1-pyrazolyl)borate).83 However, the reactivity of these adducts has not been reported.


Page 9 of 13 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 29. Structures of LA/Os≡N complexes.

Molybdenum nitrido complexes. Molydenum nitrides are activated by B(C6F5)3 towards alkyne metathesis (Figure 30).84 The X-ray structures of two BR3/Mo≡N adducts show that the boron atom binds to the nitrido ligand, resulting in significant elongation of the Mo≡N distance. Infrared spectra (IR) also show that the Mo≡N bond is weakened upon binding to the borane. It was proposed that the weakening of the Mo≡N bond lowers the barrier for alkyne metathesis. Similarly, catalytic alkyne metathesis by Mo(N)(OR)3 (OR = OCMe2CF3 and OCMe3) are promoted by the addition of Lewis acids such as MgBr2, MgI2 and BPh3.85

Apart from M=O, investigation on the activation of nonmetallic oxo species such as IO4, ClO3, etc. by LA could also lead to new oxidizing agents. Compared to oxo complexes, the activation of nitrido complexes by Lewis acids has received much less attention. Investigation of this area may lead to novel methods for the nitrogenation of organic substrates. Similarly, imido complexes are believed to be active intermediates in many useful nitrogenation reactions, but their interaction with Lewis acids has not been explored. Corresponding Author * E-mail: [email protected]. ORCID Tai-Chu Lau: 0000-0002-0867-9746

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT Financial support from the Research Grants Council of Hong Kong (CityU 11336816) and the Hong Kong University Grants Committee (AoE/P-03-08) are greatly acknowledged.

Figure 30. Alkyne metathesis by B(C6F5)3/Mo≡N.

6. Conclusions and outlook In the past two decades there has been significant advance in Lewis acid/metal oxo chemistry. The binding of a Lewis acid to an oxo ligand increases the redox potential of the metal oxo species, hence increases the driving force for ET. For mono-oxo species, the presence of a Lewis acid may cause steric effects in OAT and HAT reactions, in this case a change in mechanism from HAT/OAT to ET may occur. On the other hand, for a poly-oxo species, ET, HAT and OAT pathways may all be enhanced by Lewis acid. Ca2+ was found to activate a number of metal-oxo systems. The activation may simply arise from electron-withdrawing effects of the Ca2+ ion. In the case of O2 evolution from FeO42 induced by Ca2+, there is evidence that the main role of Ca2+ is to bring two FeO42 ions together to facilitate intermolecular OO bond formation. These findings should provide valuable insights into the role of Ca2+ in OEC of PS II. It is anticipated that LA/M=O systems will find useful applications in catalysis and organic synthesis in the near future. Mild Lewis acids such as Ca2+ or Zn2+ can have significant activating effects on metal-oxo species, so systems based on these metal ions may be useful for oxidation of substrates with acid-sensitive functional groups. Moreover, by designing suitable chiral Lewis acids, it may be possible to carry out asymmetric oxidation, where the Lewis acid provides both accelerating and chiral induction effects. A number of MOH complexes are also founded to be good HAT reagents,86-97 and the use of Lewis acids to activate this class complexes should be of interest. A number of MIII(µ-OH)LA (M = Fe, Mn) complexes have been isolated by Borovik et al.67, 98 Metal superoxo, peroxo/hydroperoxo species are key intermediates in the activation of dioxygen by metal complexes, and studies of the interaction of Lewis acids with these species would be of fundamental interest. Indeed a number of reports in this area have recently appeared.99-108

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SYNOPSIS TOC

O Substrate

LA

Mn+

+

LA

O Mn+

L

L

LA

Mn+

O

O

O L

LA

Mn+ L


LA

Mn1 L

Activation of Metal Oxo and Nitrido Complexes by Lewis Acids

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