DOI: 10.1002/chem.201304412

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& Water Oxidation

Transformation of a Cp*–Iridium(III) Precatalyst for Water Oxidation when Exposed to Oxidative Stress Cristiano Zuccaccia,[a] Gianfranco Bellachioma,[a] Olga Bortolini,[b] Alberto Bucci,[a] Arianna Savini,[a] and Alceo Macchioni*[a]

Abstract: The reaction of [Cp*Ir(bzpy)NO3] (1; bzpy = 2-benzoylpyridine, Cp* = pentamethylcyclopentadienyl anion), a competent water-oxidation catalyst, with several oxidants (H2O2, NaIO4, cerium ammonium nitrate (CAN)) was studied to intercept and characterize possible intermediates of the oxidative transformation. NMR spectroscopy and ESI-MS techniques provided evidence for the formation of many species that all had the intact Ir–bzpy moiety and a gradually more oxidized Cp* ligand. Initially, an oxygen atom is trapped in between two carbon atoms of Cp* and iridium, which gives an oxygen–Ir coordinated epoxide, whereas the remaining three carbon atoms of Cp* are involved in a h3 interaction with iridium (2 a). Formal addition of H2O to 2 a or H2O2 to 1 leads to 2 b, in which a double MeCOH functionali-

Introduction Iridium complexes have recently come to the attention of the scientific community as effective precatalysts for the oxidation of water to molecular oxygen,[1–15] which is considered the most challenging process for the realization of an artificial photosynthetic apparatus aimed at producing solar fuels.[16–21] Most of iridium precatalysts reported so far have the strongly electron-donating Cp* ligand (Cp* = pentamethylcyclopentadienyl anion), which is thought to be important for achieving the high oxidation states necessary to oxidize water. Interest in these systems began in 2009 when Crabtree showed that [Cp*Ir(ppy)Cl] (ppy = 2-phenylpyridine) was capable of oxidatively splitting water by using cerium ammonium nitrate (CAN) as a sacrificial oxidant (SO) with remarkable TOF (turnover frequency) and TON (turnover number).[2] Following this seminal discovery, several other organometallic complexes of general

[a] Dr. C. Zuccaccia, Prof. G. Bellachioma, A. Bucci, A. Savini, Prof. Dr. A. Macchioni Department of Chemistry, Biology and Biotechnology University of Perugia, Via Elce di Sotto 8, 06123 Perugia ( Italy) Fax: (+ 39) 075-5855598 E-mail: [email protected] [b] Prof. O. Bortolini Department of Chemical and Pharmaceutical Sciences University of Ferrara, Via Borsari 46, 44121 Ferrara (Italy) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201304412. Chem. Eur. J. 2014, 20, 3446 – 3456

zation of Cp* is present with one MeCOH engaged in an interaction with iridium. The structure of 2 b was unambiguously determined in the solid state and in solution by X-ray single-crystal diffractometry and advanced NMR spectroscopic techniques, respectively. Further oxidation led to the opening of Cp* and transformation of the diol into a diketone with one carbonyl coordinated at the metal (2 c). A h3 interaction between the three non-oxygenated carbons of “ex-Cp*” and iridium is also present in both 2 b and 2 c. Isolated 2 b and mixtures of 2 a–c species were tested in wateroxidation catalysis by using CAN as sacrificial oxidant. They showed substantially the same activity than 1 (turnover frequency values ranged from 9 to 14 min 1).

formula [Cp*IrL1L2L3]Xn have been identified as competent precatalysts for water oxidation by using CAN and NaIO4 as chemical SOs and, more recently,[22] [Ru(bpy)3] + 2/S2O8 2 as a photochemical SO. Studies aimed at designing new and better performing catalysts have been flanked by structural and kinetic investigations with the ultimate goal of understanding the reaction mechanism and the nature of the active species.[23] It was soon realized that [Cp*IrL1L2L3]Xn precatalysts undergo an oxidative transformation under the harsh experimental conditions of catalysis.[9, 24] This casts some shade on the real nature of the catalytic active species that may be the starting complex itself, a stillmolecular derivative generated by the partial oxidation of some ligands, or even nanoparticles (IrO2 or Ir(OH)3) derived from the complete oxidation of all organic ligands coordinated at the metal center. A heated dispute is in progress on this topic, and clues supporting both the molecular view and the one based on nanoparticles have been found.[24] Clearly, the selection of L ligands and SO is expected to play a crucial role in directing the oxidative transformation of the precatalyst toward a still-molecular species rather down to the formation of nanoparticles. An astonishing example, in this respect, has been reported by Fukuzumi, who studied [Cp*Ir(4,4’-R2-2,2’bipy)(H2O)]X2 (bipy = bipyridine) with CAN as the SO.[24c] He found clear evidence for the formation of Ir(OH)3 nanoparticles when R = OH, by complete degradation of Cp* and bipy ligands, whereas in other cases the catalysis is likely molecular and the active species has probably lost the Cp* but still has

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Full Paper the intact bipy ancillary ligand. Clues supporting the selective loss of Cp* come from the elegant studies by Lin and co-workers on [Cp*IrLn] catalysts attached to Zr carboxylate metal–organic frameworks (MOFs)[25] and, more recently, by Crabtree et al.[26] The latter showed that the [Cp*Ir(pyalc)OH] (pyalc = 2(2’pyridyl)-2-propanolate) precatalysts, during water oxidation catalysis, driven by NaIO4, rapidly lost Cp* to form the dicationic [IrIV(pyalc)(H2O)2(m-O)]2 complex that is proposed to be the catalytic resting state.[26] Independent of the endpoint of the oxidative aging of [Cp*IrL1L2L3]Xn precatalysts, it is clear that the most sensible point for oxidative attack is the Cp* ligand.[9, 24b, 27] Some lines of evidence, mainly based on NMR spectroscopy, indicate that the C CH3 functionality of Cp* is attacked by oxygen atoms from water, which leads to double functionalization of both the quaternary carbon and the methyl group.[27] The exact nature of the resulting species has been only hypothesized, whereas the final organic products of Cp* degradation have been identified as acetic acid, glycolic acid, formic acid, and CO2. Very little is known about what occurs between the initial oxidation of Cp* and its complete transformation. Herein we report the results of an integrated approach based on NMR spectroscopy, ESI-MS, and X-ray studies, aimed at intercepting the intermediates of the oxidative transformation of [Cp*Ir(bzpy)NO3] complex (1; bzpy = 2-benzoylpyridine, Scheme 1), which is a competent catalyst for water oxida-

Scheme 1. Starting complex and intercepted intermediates of the oxidative transformation.

tion.[4, 9] Oxidation reactions performed with H2O2, CAN, and NaIO4 delineated a reaction pathway that involved initial formation of an unconventional iridium–oxo species in which an oxygen atom is captured by the precatalyst due to a cooperative action of the metal and Cp* ligand leading to the formation of an epoxide Cp*-O-Ir functionality (2 a, Scheme 1). Further oxidation affords a double functionalization of Cp* to form complex 2 b with a HO-Cp*-OH-Ir moiety (Scheme 1). Complex 2 b has been isolated and completely characterized in the solid state by using X-ray diffractometric single-crystal analysis. Pushing the oxidative process still further causes the breaking of a C C bond of Cp* with the formation of a diacetyl complex (2 c, Scheme 1). Isolated 2 b and mixtures of species derived from the oxidative transformation of 1 have been tested in water-oxidation catalysis and shown to be active, with performances comparable to that of 1.

Results and Discussion Reactions of 1 with oxidants were initially performed in an NMR tube to follow the evolution of different species derived Chem. Eur. J. 2014, 20, 3446 – 3456

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from the oxidative transformation with time and find the proper experimental conditions to intercept and characterize reaction intermediates. Isolated species or mixtures enriched in certain species were successively studied by using NMR spectroscopy, ESI-MS, and X-ray diffractometry. NMR measurementsMonitoring the reactions of 1 with oxidants Experiments were conducted in [D6]acetone/D2O (1:1 v/v) with NMe4BF4 as an internal standard. Reaction of 1 with H2O2 The 1H NMR spectrum recorded after the addition of 0.2 equivalents of H2O2 showed new resonances due to the formation of two species (2 a and 2 b, 4.6 and 5.2 %, respectively, see the Supporting Information), in addition to those of residual 1 (87 % with respect to the initial quantity). Species 2 a and 2 b display five singlets with relative intensities of 3:3:3:3:3 at chemical shift values typical of Me groups of Cp*. Two highfrequency multiplets diagnostic of hydrogen atoms in ortho positions with respect to the bzpy nitrogen atom (2 a, dH = 8.90 ppm; 2 b, dH = 8.82 ppm), well separated from the more intense aromatic resonances of 1, are present in the aromatic region. Integration of the two regions from d = 12.0 to 6.0 ppm and from d = 2.9 to 1.0 ppm indicated that signal loss in the aromatic region is below the detectable limit of NMR spectroscopy and only 6 % in the aliphatic one.[28] The relative abundance of 1, 2 a, and 2 b and the total amount of detectable species were successively monitored by adding a total of 2.2 equivalents of H2O2, 0.2 equivalents at a time. The results are shown in Figure 1 (Table S1 in the Supporting Information). From Figure 1, it is clear that the concentration of 1 steadily decreases down to 1 % of the initial value after the addition of 2.2 equivalents of H2O2. At the same time, the amount of both 2 a and 2 b initially increases, reaches a maximum at about 1 and 1.8 equivalents of H2O2 for 2 a and 2 b, respectively, and then starts to decrease. Close inspection of the 1H NMR spectra showed that after the addition of 0.6 equivalents of H2O2 three new sets of resonances become apparent. One of them (2 c) displays five singlets with relative intensities of 3:3:3:3:3 in the aliphatic region and a high-frequency doublet of doublet of doublet at d = 9.23 ppm. The relative concentration of 2 c reaches a maximum of 6.5 % at 2.2 equivalents of H2O2. The relative amount of the other two species (2 d and 2 e) increases almost in parallel to reach a relative value of about 11 to 12 % at 2.2 equivalents of H2O2, as deduced from integration of their high-frequency doublet of doublet of doublets at d = 9.43 and 9.38 ppm. Some acetic acid starts forming as the amount of added H2O2 increases, reaching 1.3 % with respect to the initial amount of Cp*Me groups. A total of 11 % signal is lost according to integration of the aliphatic region, whereas a very small signal decrement is observed in the aromatic region (about 2– 3 %). When the experiment was repeated by adding a larger amount of H2O2 it was found that 2 c, 2 d, and 2 e reach a maxi-

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Figure 2. Distributions of iridium species during the reaction of 1 with CAN.

and 2 c does not differ too much from the H2O2 case if one takes into account the fact that CAN is a one-electron oxidant, whereas H2O2 and NaIO4 usually behave as two-electron oxidants. For example, the maximum relative concentrations of 2 b and 2 c are obtained at about 4 and 9 equivalents of CAN, whereas the disappearance of 1 occurs after the addition of 6– 7 equivalents of CAN. Figure 1. Evolution of 1H NMR spectra during the reaction of 1 with H2O2 (top) and distributions of iridium species (bottom).

Characterization of 2a–c: common features Functionalized Cp*

mum abundance between 3.2 and 4 equivalents of H2O2 (7.9, 24, and 16.7 %, respectively, see the Supporting Information); the quantity of acetic acid steadily increases and reaches 14.9 % at 142.5 equivalents (see the Supporting Information). Under these conditions, a total signal loss of about 30 and 22 % is detected in the aliphatic and aromatic regions, respectively. Reaction of 1 with NaIO4 The results of the transformation of 1 upon adding sub-stoichiometric quantities of NaIO4 are similar to those observed for H2O2, except that the maximum relative concentrations of 2 a and 2 b is a little higher and occurs when about 1 to 1.2 equivalents of NaIO4 are added. At three equivalents of oxidant, the apparent signal loss in the aliphatic and aromatic regions of the 1H NMR spectrum is 18 and 13 %, respectively, whereas only 6 % of the total Cp*Me groups of 1 are transformed into acetic acid. Reaction of 1 with CAN The results of the transformation of 1 upon successive additions of sub-stoichiometric quantities of CAN are shown in Figure 2. The first important difference with respect to the other oxidants is that 2 a formed in a much smaller quantity, whereas 2 d and 2 e were not detected at all. On the contrary, formation of acetic acid and erosion of aliphatic signals are enhanced: after the addition of 36 equivalents, 40.5 % of the total Cp*Me groups of 1 have been transformed into acetic acid and about 34 % of signal is lost in the aliphatic region of the 1H NMR spectrum. In contrast, the distribution of 1, 2 b, Chem. Eur. J. 2014, 20, 3446 – 3456

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As mentioned above, 2 a–c intermediates display five distinct singlets with relative intensities of 3:3:3:3:3 in the aliphatic region assignable at the methyl groups of functionalized Cp*. The proximity relationships between one methyl and its neighbors were easily established by 1H NOESY NMR studies. 1H, 13 C HMQC and 1H, 13C HMBC NMR spectra allowed the carbon of each methyl group (1JC H) and its quaternary carbon (2JC H) to be distinguished, respectively. In addition, long-range correlations between the methyl groups and quaternary carbons of vicinal moieties (3JC H) observed in the 1H, 13C HMBC NMR spectra corroborated the proximity relationships determined by 1H NOESY NMR experiments. The quaternary carbon at the highest frequency was arbitrarily labeled as C1, and all protons and carbons were distinguished (Table 1, see the Experimental Section). It is worth noting that the 13C chemical shifts of C2-C1-C5 carbons in 2 a–c intermediates are very similar and consistent with having an allylic moiety.[29] The allyl central C1 carbon falls at about d = 20–30 ppm higher than the quaternary carbon in 1, which is reasonable due to its higher sp2 nature. Terminal C2 and C5 carbons resonate at much lower frequencies than the quaternary carbon in 1 and there is a difference of about d = 20 ppm between them in all three intermediates. Because the carbon chemical shift is strongly affected by the ligands in trans relative position, this suggests that the allyl has the same trans ligands in 2 a–c. C3 and C4 carbons in 2 a and 2 b fall at chemical shift values not very different from that of 1 (Table 1), whereas they appear at remarkably higher frequency in 2 c, which indicates oxidation into C=O moieties. As far as H6–H10 resonances are concerned, little can be said except that there is a gradual global shift to higher frequency on going from 2 a to 2 c.

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Full Paper Table 1. Chemical shifts (d [ppm]) of functionalized Cp* carbons and protons for 2 a ([D6]acetone/D2O), 2 b ([D2]methylene chloride), and 2 c ([D2]methylene chloride), compared with those of 1 ([D2]methylene chloride): C1–C5 = 84.6 ppm, C6 C10 = 9.0 ppm, H6 H10 = 1.32 ppm. 2a

2b

2c

allylic moiety C1 C2 C5 C6, H6 C7, H7 C10, H10 C2

106.2 65.5 46.8 10.1, 1.09 9.6, 1.69 9.0, 0.32 65.5

115.5 69.7 47.6 11.7, 1.30 10.7, 1.86 9.3, 0.48 69.7

103.4 65.5 46.1 13.3, 1.44 16.1, 1.99 19.3, 0.65 65.5

oxidized moiety C3 C4 C8, H8 C9, H9

90.6 84.4 19.2, 1.05 20.4, 0.96

94.4 85.8 20.8, 1.20 16.9, 0.75

227.1 208.2 31.4, 2.27 28.9, 1.49

Figure 3. A section of the 1H NOESY spectrum ([D2]methylene chloride, 298 K) of 2 b (see Scheme 2 for numbering) showing the selective H9/H11, H7/H21, and H6/21 interactions. An exchange cross peak between H2O and H23 is also shown.

Bzpy ligand Assignment of all protons and carbons of the bzpy ligand was accomplished by starting from H11 and H21, the resonances of which are known to appear as the highest and lowest deshielded doublets, respectively, in such systems. Other resonances were easily assigned by combining information from 1 H COSY, 1H NOESY, 1H,13C HMQC, and 1H,13C HMBC NMR spectra (see the Experimental Section). It is important to note that the bzpy ligand remains intact in all intermediates, as demonstrated by having unequivocally found all proton and carbon resonances, including that due to the bridging carbonyl (C16, see the Experimental Section). Functionalized Cp*/Bzpy ligand relative position 1

H ROESY spectra of the reaction mixtures indicate that there are selective interactions between aromatic and aliphatic resonances for all intermediates; this means that functionalized Cp* and intact bzpy ligands are coordinated at the same iridium center. In more detail, the H21 proton always shows a NOE contact with H6 and H7 (Figure 3, Scheme 2), which suggests that the cyclometalated aryl and allyl stay in cis relative position in all intermediates 2 a–c.

Scheme 2. Numbering of protons and carbons and principal NOE NMR spectroscopy contacts.

much smaller than 3JCCOH ;[30] 3) the high-frequency shift is similar to that observed in related Cp*Ir-O(H)-R complexes.[31] The resonance of H24 was not observed, most likely because it is very broad. This broadening may be due to fast exchange with the residual water in the deuterated solvent or, alternatively, to the fact that H24 may be engaged in intermolecular

Characterization of 2a–c: specific features Let us first consider intermediate 2 b, which was prepared in reasonably good purity, based on its abundance as a function of the amount of added oxidant (Figure 1), and successfully crystallized. The distinctive feature of 2 b in solution in [D2]methylene chloride is the presence of a singlet at dH = 8.17 ppm, which is likely due to an OH proton. The latter was assigned to H23 because 1) it shows strong NOEs with H8 and H11 and weaker NOEs with H9 and H7 (Figure 3); 2) it has a long-range correlation with C4, stronger than that with C3 (Figure 4), which is in agreement with the notion that 2JCOH is Chem. Eur. J. 2014, 20, 3446 – 3456

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Figure 4. A section of the 1H,13C HMBC NMR spectrum ([D2]methylene chloride, 298 K) of 2 b (see Scheme 2 for numbering) showing the long-range scalar correlations of C3 and C4 with H23.

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Full Paper hydrogen bonds, as found in the solid state (see below).[32] To verify this hypothesis, comparative diffusion NMR spectroscopy measurements[33] have been carried out for 1 and 2 b in [D2]methylene chloride. Diffusion NMR allows the hydrodynamic dimension of a species in solution to be derived by properly elaborating the measured translational self-diffusion coefficient through the Stokes–Einstein equation.[33c] It was found that the hydrodynamic volume of 2 b is 800 3, which is about 1.7 times larger than that of 1 (VH = 490 3). This is consistent with the self-aggregation of 2 b in solution, probably driven by intermolecular hydrogen bonding. During attempts to isolate 2 b through fractional crystallization, one of the resulting fractions was substantially enriched in 2 c. This offered us the possibility of studying the molecular structure of 2 c by 1 D and 2 D NMR spectroscopy methods in [D2]methylene chloride. The most important finding was the chemical shifts of C3 and C4 (dC = 227.1 and 208.2 ppm, respectively), which are typical of carbonyl moieties. In contrast to 2 b, there are no H8/C4 and H9/C3 long-range interactions and there is no NOE between H8 and H9 (Figure 5). This indicates that the C3 C4 bond of 2 b has broken to form two C=O moieties at C3 and C4. The higher chemical shift of C3 with respect to that of C4 suggests that the oxygen atom at C3 is coordinated to the metal center. Unfortunately, intermediate 2 a could not be isolated and studied by NMR spectroscopy in nonaqueous solvents. As

a consequence, the structure of 2 a is only proposed. The distinctive feature of 2 a is the presence of H7/H10 and H8/H9 exchange cross peaks in the 1H ROESY NMR spectrum (Figure 6). This indicates that 2 a has the same substituents at the C3 and

Figure 6. A section of 1H ROESY NMR spectrum ([D6]acetone/D2O, 298 K) of 2 a; H7/H10 and H8/H9 exchange peaks are indicated by arrows.

C4 carbons. There are several structures that are compatible with such a situation but the formation of a C3 C4 epoxide, probably with the oxygen atom coordinated at iridium, appears to be the most likely (2 a in Scheme 1), also based on the ESI-MS data reported below. X-ray single-crystal structure of 2 b

Figure 5. Top spectrum: A section of 1H NOESY NMR spectrum ([D2]methylene chloride, 298 K) of 2 c in which H8/H9 NOE is absent. Bottom spectrum: A section of the 1H, 13C HMBC NMR spectrum ([D2]methylene chloride, 298 K) of 2 c in which H8/C4 and H9/C3 long-range interactions are absent. Chem. Eur. J. 2014, 20, 3446 – 3456

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As anticipated, single crystals of 2 b suitable for investigation by X-ray diffractometry were obtained by fractional crystallization (see the Experimental Section). An ORTEP view of 2 b is shown in Figure 7. Complex 2 b exhibits a pseudo-octahedral structure in which two coordination positions are occupied by the allylic structure of the 1,2,5-allyl-3,4-diol-pentamethylpentane ligand, two are occupied by the bzpy ligand, one by an oxygen atom bridging iridium and C3, and the last one is occupied by an oxygen atom of the nitrate anion. The overall structure is similar to that of piano-stool precursor 1,[4] in which the coordinated h5Cp* fragment is substituted by the h3-allyl-OH moiety, both of which are monoanionic L2X ligands. The two rings of bzpy, linked to iridium by the N1 atom and the C22, are perfectly planar and form an angle of about 1208. The Ir O1 and O1 C3 bond lengths and the C3-O1-Ir angle are consistent with those found in the other few complexes in which the C-O-Ir fragment is present.[34] The bond angles of O1-Ir-C22, O1-Ir-N1, and IrO1-O4 are 177.40(16), 94.28(14), and 93.53(12)8, respectively, which confirms the pseudo-octahedral structure. O1 is in trans

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Figure 7. An ORTEP view of 2 b (ellipsoids drawn at the 50 % probability level, hydrogen atoms have been omitted for clarity).

relative position to the cyclometalated C22 carbon of bzpy, whereas it is in cis relative position to both the N1 atom of bzpy and the O4 atom of the nitrate anion. The cyclometalated phenyl of bzpy is located in cis relative position with respect to the C1-C2-C5 allyl moiety, which is in perfect agreement with the above-described NMR spectroscopy findings. Finally, the separation between oxygen O1, which bridges iridium and C3, and O5 of the nitrate is 2.68 , which suggests the establishment of an O1 H23···O5 intramolecular hydrogen bond. Indeed, residual electron density was found between O1 and O5, which allow H23 to be located at the proper separation from both O1 (0.899 ) and O5 (1.836 ) and with a reasonable O1-H23-O5 angle (155.848). The O2 H24 moiety is also involved in intermolecular hydrogen bonding with O6 (O2···O6, 2.922 ; O2 H24···O6, 2.120 ; O2-H24-O6, 165.178). Complete information regarding the resolution of the structure, bond lengths, and bond angles are given in the Supporting Information.

ESI(+)-MS analysis of 10 4 m solutions of this precursor, dissolved in ACN/10 % H2O (ACN = acetonitrile). The spectrum consists of two ionic clusters centered at m/z 510 (193Ir), which corresponds to [1 NO3] + , and m/z 551, which was assigned to [1 NO3 + ACN] + ions. This latter species may release the ACN solvent ligand upon collision-induced dissociation (MS/MS experiments). For exact mass values, simulated/experimental isotopic patterns, and MS/MS spectra see the Supporting Information. Shortly after the addition of H2O2 to this solution in fivefold excess, two new ionic species directly associated to an oxidation process were detected and identified as the result of an oxygen atom insertion, that is, [1 NO3 + O] + (m/z 526) and [1 NO3 + ACN + O] + (m/z 567), both referable to intermediate [2 a NO3] + . When these ions were mass selected and submitted to collision, induced dissociation loss of an oxygen atom, together with other fragmentations, was observed. In addition, if H218O was used, no modification of the isotopic cluster by labeling with 18O was detected.[38] This evidence rules out the formation of an Ir=O moiety in 2 a, but strongly supports the Ir-O-Cp* bridged structure. Gradual modifications of the ESI(+) mass spectrum are observed at longer reaction times (3–5 h) and are probably associated with further oxidative degradation taking place on the Cp* ligand. Of these new clusters, the isotopic envelope at m/z 544 that formally corresponds to the insertion of H2O2 on [1 NO3] + is worth noting. By a careful balance of 1 and H2O2 it was possible to obtain this species as the predominant ion (Figure 8), and to isolate the complex for single-crystal analysis.

ESI-MS measurements Electrospray ionization mass spectrometry (ESI-MS)[35] in conjunction with tandem MS/MS analysis has been widely used for the online monitoring of reactions,[36] due to the ability of ESI to capture reactants, intermediates (including labile species), and products with great efficiency. Molecules, supramolecules of high polarity (i.e., transition-metal complexes), and most ionic species present in the reaction solution can be easily and gently transferred from solution to the gas-phase and rapidly characterized.[36] Working in both positive- and negative-ion modes, ESI()-MS monitoring provides, therefore, continuous snapshots of the changing ionic composition of reaction solutions, and thus facilitates the detection of key intermediates.[37] The investigation of 1 and related intermediates or products formed under oxidizing conditions started with high-resolution Chem. Eur. J. 2014, 20, 3446 – 3456

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Figure 8. ESI(+) mass spectrum of the ion cluster formally corresponding to the insertion of H2O2 on [1 NO3] + .

The collision-induced mass spectrum of [1 NO3 + H2O2] + , that is, [2 b NO3] + , shows loss of water as sole fragmentation channel (Figure 9), which confirms the presence of hydroxyl groups as proposed for structure 2 b. An analogous sequence of experiments was performed by using NaIO4 as the primary oxidant. The overall trend is almost identical to that observed with H2O2, but oxidation and oxo degradation are much slower and less efficient.[39] However, when CAN was used, a fast and progressive modification of the Ir complex signals was observed, as exemplified by the series of mass spectra depicted in Figure 10.

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Figure 9. MS/MS of the ion at m/z 544 that corresponds to [2 b NO3] + .

Note that the group of ions in the range of m/z 560 to 570 are the result of the overlap of two distinct isotopic clusters that correspond to [2 a NO3 + ACN] + (m/z 567) and the same oxidized species deprived of two hydrogen atoms after Cp* methyl-to-aldehyde oxidation (m/z 565).[40] As well as the [2 NO3 + ACN 2H] + ion, a new ionic species forms over time, with a base peak after 15 min, which corresponds to the insertion of two oxygen atoms [2 c NO3 + ACN] + (m/z 583) and impeccably fits with structure 2 c. Catalytic tests with 2 b and mixtures of intermediates The catalytic activity of isolated 2 b and powders recovered after treating 1 with appropriate amounts of oxidant (Table 2) was evaluated by using CAN as the sacrificial oxidant, by means of UV/Vis spectroscopy and manometry. Experiments were carried out with catalyst concentration in the range of 2.5 to 125 mm and with 5 or 10 mm of CAN. Data are summarized in Table 2. Mixture 1 contains 2 b (30 %) and several other species with abundances of less than 11 %, whereas 1 is 1.1 %. In mixtures 2 and 3, species 1 and 2 a–e are not present at all. As can be seen from Table 2, 2 b and mixtures of intermediates are active in water oxidation and show substantially the same TOF than 1. Two extreme scenarios are compatible with such observations. All species derived from the oxidative transformation of 1 carry on their specific catalytic cycle with analogous performance (multisite hypothesis; Scheme 3). Alternatively, 1, 2 a, 2 b, 2 c, etc. are all precursors of the real catalytically active species (single-site hypothesis; Scheme 3) that may not have any functionalized Cp* but only the bzpy ligand, in analogy with the recent proposal by Crabtree and co-workers.[26] Because oxygen is produced pretty quickly compared with the time necessary for the complete disappearance of 1 and 2 a–e and, importantly, in parallel with CO2 evolution,[23] the multisite hypothesis would seem more probable. However, it appears rather unlikely that complexes with considerably different ancillary ligands (compare 1 with 2 b, for example) exhibit almost the same TOF values. This would support the single-site hypothesis with the possible active single site (IrIII(2 x+) in Scheme 3) still containing the intact bzpy-ligand. In Chem. Eur. J. 2014, 20, 3446 – 3456

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Figure 10. Changes with time of the ESI(+) mass spectrum of 1 in ACN/10 % H2O in the presence of CAN as the oxidant.

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Table 2. Comparison of the catalytic activity of 1, 2 b, mixture 1 (1 + H2O2 (2.2 equiv)), mixture 2 (1 + CAN (36 equiv)), and mixture 3 (1 + H2O2 (142 equiv)). TOF [min 1][a] 1 mixture 1 mixture 2 mixture 3 2b

UV/Vis[b]

Manometry[c]

8.4 10.6 12.1 9.3 9.0

9.5 13.4 12.7 11.0 10.7

[a] Mean values derived from individual TOF values of a single run. [b] TOF values were determined on the basis of a zero-order kinetic treatment (At/A0 = 1 4kobst/C0). [c] TOF values were evaluated on the basis of the linear fit of TON vs. t trends.

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Full Paper two ketone functionalities (2 c). One of the latter is likely coordinated at the iridium. Prolonged oxidative stress generates many unidentified species, probably containing more oxygenated and still coordinated “ex-Cp*” ligand until a complete degradation of Cp* is reached. It is worth noting that in all intermediates 1) the functionalized Cp* acts as a L2X ligand due to the presence of an allyl (LX) and neutral-O arm (L) and 2) the bzpy ligand is intact. Isolated 2 b and mixtures of Scheme 3. Multisite and single-site hypotheses for water oxidation with precatalyst 1. The cycles reported in the scheme consist of a double proton-coupled electron transfer (PCET) process that transforms IrIII OH to IrV=O; the intermediates are active in water latter undergoes attack by water to form a hydroperoxo complex (IrIII OOH) from which molecular oxygen is liberoxidation catalysis driven by ated and the initial aqua complex is restored. The catalytic cycle is oversimplified, especially the O2 generation CAN with a TOF comparable step; for a more detailed discussion on the mechanism of water oxidation with iridium catalysts see refs. [9], [23], with that of 1. This suggests [26]. a multisite mechanism in which a single species 2 x, with a molecular nature, brings about most of the catalytic cycles but also fact, we had no indication of oxidative transformation of bzpy species 1, 2 a–c, etc. probably act as homogeneous catalysts in all our experiments. To reconcile all observations, we probefore they oxidatively transform. pose that a multisite mechanism is active with a 2 x species carrying out most of the cycles. As stated above, all intercepted intermediates still have the bzpy ancillary ligand; this induces us to think that all active species are molecular in our case. Experimental Section Nevertheless, the multisite hypothesis is also applicable to sysGeneral procedures tems with ancillary ligands more prone to be oxidatively transformed than bzpy, which would lead to a 2 x nanoparticulate All manipulations were performed in air. All solvents were purified material. by using standard methods. Deuterated solvents were used as re-

Conclusion We succeeded in intercepting and characterizing three intermediates (2 a–c) of the oxidative transformation of 1, which is a competent catalyst for water oxidation. 2 a–c form when oxidants of different nature (H2O2, CAN, and NaIO4) are used, consequently, the oxidative transformation appears to be rather general. Initially an oxygen atom is transferred to 1 to give 2 a (Scheme 4).

ceived. 1D and 2D 1H, 13C, 1H COSY, 1H,13C HMQC, 1H,13C HMBC, 1 H NOESY, and 1H ROESY NMR spectra were measured by using a Bruker DRX 400 equipped with a QNP probe or by using a Bruker Avance III HD 400 spectrometer equipped with a smartprobe. Referencing is relative to external TMS (1H and 13C). The number of transients and data points for 1D and 2D experiments were chosen according to the sample concentration and desired final digital resolution. 1H PGSE NMR measurements were performed by using a GREAT 1/10 gradient unit and a QNP probe with a Z-gradient coil on the Bruker DRX 400 spectrometer at 298 K without spinning. The standard stimulated-echo or double-stimulated-echo pulse sequences were used. Dt data were treated as described in the literature to derive hydrodynamic dimensions.[33c] The standard deviation for the hydrodynamic volumes is approximately 10–15 %.

NMR monitoring of catalyst speciation after addition of oxidants Scheme 4. Oxidative transformation of 1.

The latter may be considered an unconventional iridium(III)– oxo species stabilized by a cooperative action of the metal and Cp* ligand and derived from the elusive iridium(V)–oxo species.[41] Species 2 a successively transforms into 2 b through an apparent water addition at the epoxide moiety. Further oxidation causes the opening of the Cp* ligand and formation of Chem. Eur. J. 2014, 20, 3446 – 3456

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Complex 1 (5 mg, 0.0087 mmol), NMe4BF4, and [D6]acetone (0.3 mL) were placed in an NMR tube and D2O (0.3 mL) was added. A stock solution (0.35 m, 1 mL) of each oxidant (H2O2, NaIO4, or CAN) in D2O was prepared in a separate vial. Predefined aliquots of the oxidant solution were then added to the catalyst solution by using a micrometric syringe (5 mL = 0.2 equiv of oxidant). After each addition, a quantitative 1H NMR spectrum was recorded to monitor the catalyst speciation by using the resonance of NMe4 + as an internal standard.

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Full Paper Isolation of 2 b Isolation of 2 b was attempted several times under slightly different experimental conditions. The most efficient procedure consisted of adding H2O (6 mL) and, successively, H2O2 (2 equiv; 3 mL of a 0.116 m solution in H2O), in 10 portions over a period of about 10 min, to a solution of 1 (100 mg, 0.175 mmol) in [D6]acetone (6 mL). The progress of the reaction was monitored by 1H NMR. The solution was evaporated to dryness under vacuum and the resulting solid was extracted several times with a mixture of diethyl ether/benzene (1:10 v/v). After filtering, the volume of the filtrate was reduced under vacuum. Complex 2 b was purified by a series of fractional crystallizations from ether/benzene/n-hexane. In one case, orange-yellow single crystals suitable for X-ray analysis were obtained.

190.9 (s, C16), 152.9 (s, C11), 152.4 (s, C15), 139.9 (s, C13), 137.8 (s, C21), 135.4 (s, C17), 132.3 (s, C20), 129.8 (s, C22), 129.02 (s, C12), 128.97 (s, C14), 128.9 (s, C18), 124.6 (s, C19), 103.4 (s, C1), 65.5 (s, C2), 46.1 (s, C5), 31.4 (s, C8), 28.9 (s, C9), 19.3 (s, C10), 16.1 (s, C7), 13.3 ppm (s, C6). Data for 2 a: 1H NMR (400 MHz, [D6]acetone/D2O 1:1, 298 K): d = 8.91 (ddd, 3JHH = 5.8, 4JHH = 1.76, 5JHH = 0.76 Hz, H11), 8.21 (m, H14), 8.18 (m, H13), 7.8 (m, H12), 7.43 (dd, 3JHH = 7.6, 4JHH = 1.7 Hz, H18), 7.17 (dd, 3JHH = 7.6, 4JHH = 1.2 Hz, H21), 7.06 (m, H20), 6.96 (m, H19),

Data for 2 b: 1H NMR ([D2]methylene chloride, 298 K): d = 8.87 (ddd, JHH = 5.8, 4JHH = 1.9, 5JHH = 0.8 Hz, H11), 8.33 (dd, 3JHH = 7.9, 4JHH = 1.5 Hz, H14), 8.17 (s, OH), 8.08 (dt, 3JHH = 7.6, 4JHH = 1.5 Hz, H13),

3

1.69 (s, H7), 1.09 (s, H6), 1.05 (s, H8), 0.96 (s, H9), 0.32 ppm (s, H10); C{1H} NMR (100 MHz [D6]acetone/D2O 1:1, 298 K): d = 196.2 (C16), 151.6 (C15), 150.0 (C11), 139.3 (C13), 139.0 (C21), 136.0 (C17), 131.9 (C22), 131.8 (C20), 128.6 (C12), 128.3 (C18), 127.9 (C14), 123.1 (C19), 106.2 (C1), 90.6 (C3), 84.4 (C4), 65.5 (C2), 46.8 (C5), 20.4 (C9), 19.2 (C8), 10.1 (C6), 9.6 (C7), 9.0 ppm (C10).

13

7.70 (ddd, 3JHH = 7.6, 4JHH = 5.8, 5JHH = 1.5 Hz, H12), 7.61 (dd, 3JHH = 7.7, 4JHH = 1.6 Hz, H18), 7.16 (dt, 3JHH = 7.6, 4JHH = 1.5 Hz, H20), 7.13 (dd, 3JHH = 7.8, 4JHH = 1.8 Hz, H21), 7.06 (ddd, 3JHH = 7.9, 4JHH = 6.0, 5 JHH = 1.7 Hz, H19), 1.86 (s, H7), 1.30 (s, H6), 1.20 (s, H8), 0.75 (s, H9), 0.48 ppm (s, H10); 13C{1H} NMR ([D2]methylene chloride, 298 K): d = 191.7 (s, C16), 152.5 (s, C15), 149.5 (s, C11), 138.9 (s, C21), 138.7 (s, C13), 136.1 (s, C17), 132.1 (s, C22), 131.9 (s, C20), 128.6 (s, C18), 128.3 (s, C12), 126.1 (s, C14), 123.8 (s, C19), 115.5 (s, C1), 94.4 (s, C3), 85.8 (s, C4), 69.7 (s, C2), 47.6 (s, C5), 20.8 (s, C8), 16.9 (s, C9), 11.7 (s, C6), 10.7 (s, C7), 9.3 ppm (s, C10); ESI MS m/z calcd for [2 b NO3] + : 542.1440, 544.1444; found: 542.144007, 544.1487 (Figure S20 in the Supporting Information).

Characterization of 2 c During the purification of 2 b by fractional crystallization, a fraction enriched in 2 c was obtained. This offered the possibility of investigating the molecular structure of 2 c by 1D and 2D NMR spectroscopy methods in [D2]methylene chloride. Data for 2 c: 1H NMR ([D2]methylene chloride, 298 K): d = 9.27 (ddd, JHH = 5.7, 4JHH = 1.6, 5JHH = 0.5 Hz, H11), 8.49 (ddd, 3JHH = 8.0, 4JHH = 1.7, 5JHH = 0.5 Hz, H14), 8.20 (dt, 3JHH = 7.7, 4JHH = 1.5 Hz, H13), 7.70 (ddd, 3JHH = 7.5, 4JHH = 5.7, 5JHH = 1.5 Hz, H12), 7.73 (dd, 3JHH = 7.7, 4 JHH = 1.7 Hz, H18), 7.16 (dt, 3JHH = 7.7, 4 JHH = 1.6 Hz, H20), 7.11 (ddd, 3JHH = 7.8, 4JHH = 6.0, 5JHH = 1.4 Hz, H19), 7.03 (dd, 3JHH = 7.7, 4JHH = 1.3 Hz, H21), 2.27 (s, H8), 1.99 (s, H7), 1.46 (s, H9), 1.44 (s, H6), 0.65 ppm (s, H10); 13 1 C{ H} NMR ([D2]methylene chloride, 298 K) d = 227.1 (s, C3), 208.2 (s, C4),

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Partial NMR characterization of 2 d and 2 e The quaternary carbon resonance of functionalized Cp* at the highest frequency was arbitrarily labeled as C1. Data for 2 d: 1H NMR (400 MHz, [D6]acetone/D2O 1:1, 298 K): d = 9.43 (ddd, 3JHH = 5.8, 4JHH = 1.3, 5JHH = 0.7 Hz, H11), 7.86, (m, H12), 8.21 (m, H13), 8.17 (m, H14), 7.83 (dd, 3JHH = 7.6, 4JHH = 1.1 Hz, H21), 7.67 (dd, 3JHH = 7.8, 4JHH = 1.6 Hz, H18), 7.28 (m, H19), 7.16 (m, H20), 1.24 (s, H9), 1.067 (s, H8), 1.062 (s, H7), 0.83 (s, H10), 0.61 ppm (s, H6); 13C{1H} NMR (100 MHz [D6]acetone/D2O 1:1, 298 K): d = 155.3 (s, C11), 136.7 (s, C21), 128.5 (s, C18), 104.1 (s, C1), 96.1 (s, C4), 92.0 (s, C5), 88.2 (s, C3), 84.3 (s, C2), 23.8 (C9), 16.2 (s, C7), 14.0 (s, C8), 12.2 (s, C6), 9.4 ppm (s, C10). Data for 2 e: 1H NMR (400 MHz, [D6]acetone/D2O 1:1, 298 K): d = 9.38 (ddd, 3JHH = 5.9, 4JHH = 1.4, 5JHH = 0.7 Hz, H11), 7.89, (m, H12), 8.14 (m, H13), 8.20 (m, H14), 8.03 (dd, 3JHH = 8.0, 4JHH = 1.0 Hz, H21), 7.64 (dd, 3JHH = 7.8, 4JHH = 1.6 Hz, H18), 7.38 (m, H19), 7.16 (m, H20), 1.30 (s, H6), 0.99 (s, H8), 0.77 (s, H7), 0.6 (s, H10), 0.48 ppm (s, H9); 13 1 C{ H} NMR (100 MHz [D6]acetone/D2O 1:1, 298 K): d = 153.4 (s, C11), 135.9 (s, C21), 129.3 (s, C18), 95.9 (s, C1), 95.0 (s, C3), 86.4 (s, C5), 80.7 (s, C4), 78.7 (s, C2), 20.8 (s, C8), 20.7 (s, C6), 17.2 (s, C7), 11.9 (s, C9), 10.5 ppm (s, C10).

X-ray crystallography A single crystal of 2 b suitable for X-ray diffraction (red block, approximate dimensions 0.25  0.17  0.12 mm) was obtained by crystallization from ether/benzene/n-hexane. Data were collected by using an XCALIBUR (Kuma4CCD) diffractometer and MoKa graphite monochromated radiation (l = 0.71069 ), w scans and the frame data were acquired by using the CRYSALIS (CCD 171) software. The crystal-to-detector distance was 65.77 mm. The structure was solved using direct methods and refined against IF2I. The Laue

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Full Paper symmetry was determined to be 1.870 g cm 3 (Z = 4 and Mr = 605.66) and the investigation of the observed systematic absences are consistent with the monoclinic space group P21/c (no. 14). The data were collected at RT. The lattice parameters found were a = 9.649(5), b = 14.465(7), c = 15.667(6) ; b = 100.418(5)8; V = 2150.6(17) 3. Data were collected to 2qmax of 58.928 in the index ranges 13  h  13, 19  k  17, and 20  l  20 with a total of 21 320 reflections collected, of which 529 were rejected and 5613 were unique reflections with independent R(int) = 0.0875, up to a resolution of 0.72 . The frames were then processed by using the CRYSALIS (RED 171) software to give the hkl file corrected for scan speed, background, and Lorentz and polarization effects. Standard reflections, measured periodically, showed no apparent variation in intensity during data collection and thus no correction for crystal decomposition was necessary. The data were corrected for absorption by using the SADABS program.[42] The structure was solved by the direct method by using the Sir97[43] program and refined by the full-matrix least-squares method on F2 by using SHELXL-97[44] WinGX[45] version. All nonhydrogen atoms were refined anisotropically. Almost all hydrogen atoms were added at calculated positions and refined by using a riding model, only the hydrogen bonded to O1 was determined instrumentally. The final cycle of full-matrix least-squares refinement against IFI2 was based on 3968 observed reflections [F0 > 4s(F0)] and 289 variable parameters and converged with unweighted and weighted agreement factors of R = 0.0373 and Rw = 0.0755, and GOF = 0.922. Data concerning significant bond lengths, angles, and angles between planes are reported in the Supporting Information. CCDC 971168 (2 b) contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

ESI-MS measurements ESI mass spectra were collected in positive-ion mode by using a HRMS Q-star pulsar-i instrument (MDS Sciex Applied Biosystems, Toronto, Canada) at 10 000 resolution and an LCQ Duo (ThermoQuest, San Jose, CA, USA), by introducing solutions of precursor 1 (10 4 m) in ACN/10 % H2O. H2O2, NaIO4, and CAN were added in equimolar amounts or from three- to fivefold excess. Instrumental parameters for the positive-ion mode conditions: capillary voltage 1.30 V, spray voltage 4.50 kV, capillary temperature 170 8C, mass scan range from m/z 100 to 1000; N2 was used as the sheath gas. The samples were injected into the ESI source by a syringe pump at a constant flow rate of 8 mL min 1.

Catalytic experiments UV/Vis experiments were carried out with a catalyst concentration in the range of 2.5–125 mm, whereas the concentration of CAN was kept constant at 5 or 10 mm. Generally, an aqueous solution of catalyst in HNO3 (2 mL; pH 1) was added to a cuvette under stirring at 25 8C. After background correction, the solution of CAN (1 mL, pH 1; 30 or 15 mm) was injected into the catalyst solution. The depletion of Ce4 + was followed at l = 390 nm (5 mm CAN) or at l = 410 nm (10 mm CAN). Differential manometry experiments were performed by first preparing a 0.16 m CAN solution (HNO3, pH 1). Aliquots of this solution (150–300 mL) were injected into HNO3 (4–4.5 mL, pH 1) in the reactor. The same amount of water was added to the reference reactor. Both reactors were thermostatted at 25 8C and the solution was stirred until a stable baseline was reached. Under these conditions, 40–600 mL of water and catalyst solution were injected into the refChem. Eur. J. 2014, 20, 3446 – 3456

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erence reactor and the main reactor, respectively. In all experiments, the CAN concentration was 5 or 10 mm and catalyst concentration was in the range of 2.5–125 mm.

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Received: November 11, 2013 Revised: December 17, 2013 Published online on February 12, 2014

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