Synthesis and Characterization of Cationic Rhodium Peroxo Complexes

Article pubs.acs.org/Organometallics

Synthesis and Characterization of Cationic Rhodium Peroxo Complexes Judy Cipot-Wechsler,† Danielle Covelli,‡ Jeremy M. Praetorius,† Nigel Hearns,† Olena V. Zenkina,† Eric C. Keske,† Ruiyao Wang,† Pierre Kennepohl,*,‡ and Cathleen M. Crudden*,† †

Department of Chemistry, Queen’s University, Chernoff Hall, 90 Bader Lane, Kingston, Ontario, Canada K7L 3N6 Department of Chemistry, The University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z1

‡

S Supporting Information *

ABSTRACT: Mononuclear cationic rhodium complexes of dioxygen have been synthesized and characterized. Crystallographic, spectroscopic, and computational results support the conclusion that these complexes are best described as RhIII{O22−} (rhodium(III) peroxo) complexes, in contrast to recently reported neutral analogues that are best described as RhI{1O2} adducts. The nature of the ligand trans to the O2 ligand is crucial in defining the electronic nature of the RhO2 bonding. It is determined that π-donor ligands such as the halidesin conjunction with sufficient steric bulkcan stabilize the formation of RhI{1O2} adducts, whereas stronger field ligands lead to the stabilization of asymmetric O2 binding that ultimately favors formation of higher coordinate RhIII peroxo species. The factors that control the relative stabilization of RhIII{O22−} versus RhI{1O2} species are related to the well-established Dewar−Chatt−Duncanson model that has been successfully used to describe the bonding in isoelectronic transition-metal alkene complexes. The specific factors that control the stabilization of one electromer (resonance structure) over another are explored and discussed in detail.

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INTRODUCTION

dioxygen ligand, resulting from one-electron (superoxo) and two-electron (peroxo) reduction of O2, respectively.16,35 Early theoretical studies of O2 as a ligand in transition-metal complexes suggested that another side-on bonding mode could also be accessed, particularly in cases where efficient π overlap between the metal and the dioxygen ligand is difficult to achieve.2 In principle, such weak binding situations would enable the formation of an M−O2 bond without net reduction of the ligand and concomitant oxidation of the metal (Figure 1). Recently, we reported crystallographic, spectroscopic, and computational characterization of dioxygen adducts of rhodium N-heterocyclic carbene complexes (Chart 1) that have unusually short O−O bond lengths and square-planar geometries.36 A detailed investigation of [ClRh(IPr)2(O2)] (1) by X-ray crystallography, Raman spectroscopy, L-edge X-ray absorption spectroscopy (XAS), and DFT calculations revealed

The coordination of dioxygen to transition metals and organometallic complexes is of significant interest, given dioxygen’s essential role in aerobic life forms and potential as an environmentally benign terminal oxidant.1−12 For 1:1 metal−dioxygen complexes, one of two possible binding approaches is typically observed: end-on (η1) or side-on (η2).6,13−17 These can be further differentiated as peroxo (O22−) or superoxo (O2−) complexes depending on the O−O bond lengths and stretching frequencies observed by X-ray crystallography and vibrational spectroscopy, respectively.3,16,18 The more common η2-peroxo bonding mode is characterized by longer bond lengths (∼1.4−1.5 Å) and lower stretching frequencies (∼800−930 cm−1),3,7,14,19−28 whereas η2-superoxo complexes possess characteristically shorter bond lengths (∼1.2−1.3 Å) and higher stretching frequencies (1050−1200 cm−1).3,13,14,16,18,29−35 Formally, these two bonding scenarios correspond to differing electronic configurations and charge distributions between the metal fragment and the coordinated © 2012 American Chemical Society

Received: August 8, 2012 Published: October 5, 2012 7306

dx.doi.org/10.1021/om300766x | Organometallics 2012, 31, 7306−7315

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accomplish this goal, we endeavored to synthesize cationic analogues of 1 and 4. Herein, we describe the synthesis of cationic [Rh(NHC)2(O2)(CH3CN)2]X complexes from their neutral [ClRh(NHC)2(O2)] precursors. As a consequence of this study, we document the dramatic effect that the coordination number has on the redox properties of the metal, specifically with regard to binding of dioxygen. Considering the rarity of complexes of types 1−10, understanding under what conditions they can be expected to form, compared with when typical peroxides will be produced, is of significant importance.

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Figure 1. Various bonding modes for side-on metal−O2 complexes.

RESULTS AND DISCUSSION Synthesis and Structure. Initial reactions of 1 with the silver salts AgBF4, AgOTf, and AgSbF6 were performed in THF but resulted in inseparable mixtures of complexes, likely due to the fluxional coordination of THF to the metal in varying stoichiometries. Upon changing the solvent to acetonitrile, the clean formation of a single species was achieved upon reaction of 1 with silver metathesis reagents, generating [Rh(IPr)2(O2)(MeCN)2]+[X]− (11·X, where X = OSO2CF3 (11·OTf), BF4 (11·BF4), SbF6 (11·SbF6)) in 2 h at ambient temperature (Scheme 1, eq 1). Removal of excess solvent, followed by

Chart 1. Rhodium(I) Complexes of Dioxygen Characterized by Square-Planar Geometries and Unusually Short O−O Bond Lengths36,39−43

Scheme 1. Synthesis and Yields of Complexes 11·X and 12·X via Halide Abstraction in Acetonitrile

that it, and by extension its analogues [ClRh(IMes)2(O2)] (3) and [ClRh(IMes)(PPh3)(O2)] (5), are best described as rhodium(I) complexes of singlet oxygen.36 Bonding in these complexes results from a combination of σ donation from a filled O2(π) orbital as well as back-bonding into one of the empty O2(π*) orbitals, in a manner that is completely analogous to that which is observed in weakly bound side-on metal−olefin complexes. In short, bonding in these RhI−O2 complexes can be well described by the Dewar−Chatt− Duncanson model that is commonly used for metal−olefin complexes.37,38 Milstein41 and Caulton40 have also described Rh complexes of O2 that are square planar and are characterized by shorter than normal O−O bond lengths (Chart 1, complexes 9 and 10). In the case of compounds 9 and 10, on the basis of the magnitude of the 1JRh−P coupling constant, Milstein suggested that oxygen binds without changing the oxidation state of Rh.41 Caulton also suggested that 10 is a Rh(I) complex, with the inability of the otherwise highly electron rich PNP-ligated Rh to reduce O2 being ascribed to the coordinative unsaturation of the complex.40 In addition, although ClRh(PCy3)2(O2) (7) and ClRh(PiPr3)2(O2) (8) were incompletely characterized,42,43 available data are consistent with this same type of bonding mode, where dioxygen binds without oxidation of the metal (and reduction of O2). Considering that all of these complexes are functionalized with bulky, electron-rich ligands, the question arises as to what is special about this ligand set that promotes this unusual type of bonding. Since it is difficult to separate electronic effects from the steric effects in the NHC and pincer ligands used to date, we set out to prepare related complexes in which the sterics are kept the same but the overall electron density at the metal is reduced. As a simple way to

taking up the resulting solid in trifluorotoluene, filtration through Celite, and subsequent washings with diethyl ether, resulted in isolation of the pure compound. Similarly, treatment of [ClRh(SIMes)2(O2)]45 (4) with AgOSO2CF3 in acetonitrile resulted in the generation of [Rh(SIMes) 2 (O 2 )(MeCN)2]+[OSO2CF3]− (Scheme 1, eq 2). In all cases, beige powders were isolated, unlike the typical blue or dark green complexes typically observed by our group36,39 and Milstein’s for Rh−singlet O2 complexes,41 providing the first hint that a dramatic difference in bonding was to be observed.44 Characterization of these complexes by 1H and 13C{1H} NMR revealed no observable effect of the counterion on the solution structure, with all three complexes featuring virtually identical spectra. The room-temperature 1H NMR spectra of the complexes gave the expected signals for the IPr ligands, and coordinated acetonitrile was observed as a very broad singlet at 1.92 ppm. Importantly, the methyl group of the acetonitrile ligand integrated for six protons, indicating that two molecules were bound to Rh. Although the corresponding signal was absent in the 13C{1H} spectra, upon cooling to 253 K, a sharp resonance at 5.90 ppm appeared, which correlated to the 1H 7307


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and corresponding oxygen atoms of 159.88 and 159.59°. The dihedral angle between the heterocyclic rings of the trans carbene ligands for 11·BF4 is 64.63°. The heterocyclic rings of the carbene ligands are completely planar, with a negligible deviation from planarity, as represented by the torsion angle (N1C2C3N2) of 0.8°. The heterocyclic rings of the carbene ligands are almost perpendicular to their corresponding DiPP wingtip groups with a dihedral angle of 75.57°. Interestingly, although the O2 ligand remains bound to Rh during abstraction of the chloride ligand, the O−O bond lengthens significantly to 1.428(3) Å. This represents a difference of ca. 0.1 Å in comparison to the parent complex 1. Most importantly, this lengthening of the O−O bond moves it to within what is expected for typical peroxidic O−O bonds.13,19−24,26 A slight shortening of the Rh−O bonds and an expansion of the O−Rh−O dihedral angle is also observed relative to complex 1 (Table 1). As expected, the geometry of

resonance in the HSQC spectra. Assignment of this signal to the two equivalent coordinated acetonitrile molecules was further corroborated by the 1H and 2H NMR spectra of 11·SbF6 synthesized with d3-acetonitrile (d-11·SbF6). In the 1H NMR spectrum of d3-11·SbF6, the broad singlet at 1.92 ppm was absent and the corresponding signal was observed in the 2 H NMR spectrum. Similarly, 1H NMR spectra of the complex 12·OTf gave the expected signals for the SIMes ligands, and coordinated acetonitrile was observed as a broad singlet at 1.98 ppm. 12·OTf is highly unstable in solution and rapidly decomposes, precluding the possibility of obtaining 13C NMR spectra of sufficiently high S/N. However, X-ray-quality brown plate-like crystals of complex 12·OTf could be obtained by slow diffusion of hexane into a concentrated acetonitrile solution of the product, as will be discussed below. Having established the stoichiometry of coordination complexes 11·BF4 and 12·OTf via NMR, we turned to X-ray crystallography for a better understanding of the connectivity, geometry, and O2 bonding. Shown in Figure 2 is an X-ray

Table 1. Comparison of the O−O Binding of 1 versus 11·BF4 and of 4 versus 12·OTf param

1

11·BF4

4

12·OTf

rO−O (Å) rRh−O(av) (Å) O−Rh−O angle (deg) ν(O−O) (cm−1)

1.323(3)a 2.012a 38.40(8)a

1.428(3)b 1.977b 42.35(10)b

1.372(16)b 2.097 38.3(5)

1.439(8)b 1.914 44.1(2)

1010c

910d

1056e

856e

a

b

Determined by X-ray crystallography, from ref 36. Determined by X-ray crystallography, from this study. cDetermined by Raman resonance spectroscopy by comparison to its 18O2 isotopomer. d Determined by IR spectroscopy by comparison to its 18 O 2 isotopomer. eEstimated by IR spectroscopy without examination of 18 O2 isotopomer.

the complex is octahedral, which is also typical of Rh(III) complexes. In fact, all of the crystallographic data obtained for 11·BF4 (coordination-sphere geometry, bond lengths and angles) are consistent with reduction of the O2 ligand and formation of the more typical η2-peroxo bonding mode observed for Rh(O2) complexes. 12·OTf was also subjected to analysis by X-ray crystallography. As shown in Figure 3, this complex is also characterized by octahedral geometry and has an O−O bond length of 1.439(8) Å, which would also lead to the conclusion that this species is a peroxidic Rh(III) compound. In this case, the lengthening of the O−O bond as determined by crystallography was again extreme, since the starting complex (4) has an O−O bond length of 1.372(16) Å. 12·OTf has a distorted-octahedral geometry with an angle between the carbene atoms and rhodium of 167.50° and angles between the nitrogen atoms of the trans acetonitrile ligands, rhodium center, and oxygen atoms of 162.80 and 156.65°. The dihedral angle between the heterocyclic rings of the trans carbene ligands for 12·OTf is 35.65°. The heterocyclic rings of the carbene ligands are nearly planar, with minor deviation from planarity represented by the torsion angle (N1C2C3N2) of 6.66°. The heterocyclic rings of the carbene ligands are almost perpendicular to their corresponding wingtip groups with a dihedral angle of 84.40° (see the Supporting Information for the X-ray structure and details of 4). Spectroscopic Analysis. Since the determination of O−O bond lengths by X-ray crystallographic analysis can be problematic, we also probed the nature of the coordinated O2 by IR spectroscopy.7,14,27,28 A signal at 910 cm−1 in the IR

Figure 2. Crystallographically determined structure of 11·BF4 displaying thermal ellipsoids at the 50% confidence level. Hydrogen atoms have been removed for clarity. Hydrogen atoms and the counterion are omitted for clarity. Selected interatomic distances (Å) and angles (deg): Rh(1)−O(1), 1.974(2); Rh(1)−O(2), 1.980(3); Rh(1)−C(1), 2.085(3); Rh(1)−C(28), 2.097(3); Rh(1)−N(5), 2.091(3); Rh(1)−N(6), 2.105(3); O(1)−O(1A), 1.428(3); N(1)− C(1), 1.379(4); N(2)−C(1), 1.369(4); C(2)−C(3), 1.327(5); O(1)− Rh(1)−C(1), 87.65(12); O(1)−Rh(1)−C(28), 89.15(11); C(1)− Rh−C(28), 176.71(13); O(1)−Rh(1)−N(5), 117.89(11); O(1)− Rh(1)−N(6), 159.59(12); C(1)−Rh(1)−N(5), 85.04(12); C(1)− Rh(1)−N(6), 98.34(12); C(28)−Rh(1)−N(5), 97.16(12), C(28)− Rh(1)−N(6), 84.41(12).

crystallographic analysis of 11·BF4. Complexes 11·BF4 and 12·OTf crystallize in monoclinic P21/c and C2/c space groups, respectively (see Table S1 in the Supporting Information). Unlike previous neutral O2 complexes we have prepared, which are uniformly square planar, 11·BF4 has a distorted-octahedral geometry, with O2 and a pair of acetonitrile molecules occupying the equatorial plane and the IPr ligands positioned trans to one another. The slightly distorted octahedral geometry of 11·BF4 is represented by an angle between the carbene atoms and rhodium of 176.70° and angles between the nitrogen atoms of the trans acetonitrile ligands, rhodium center, 7308


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ν(O−O) values for 11·X are completely consistent with the longer O−O bond length observed for 11·BF4 by X-ray crystallography and are 100 cm−1 lower than the ν(O−O) band observed in the Raman spectra of 1 (Table 1, entry 4). Furthermore, this value is consistent with the assignment of 11·X as rhodium(III) η2-peroxo complexes. These results are well supported by DFT calculated vibrational frequencies for the N-Me analogues of 1 and 11·X, which yield ν(O−O) values of 988 and 915 cm −1, respectively. Rh L3-edge and Rh K-edge X-ray absorption spectroscopy (XAS) was used to determine the electronic structure of 11·BF4 and to gain greater insight into the electronic origin of the differences between these species.46 The Rh L3-edge XAS spectrum is dominated by electric dipole allowed Rh 4d ←Rh 2p transitions; for this reason, metal L-edge spectroscopy has proven to be a useful tool for assigning the electronic structure.47−49 The Rh L3-edge spectra for complexes 1 and 11·BF4 are shown in Figure 5A, and the results from a quantitative fit of the data are given in Table 2. The two features observed in the spectrum of 1 have been previously assigned as resulting from a Rh 4dσ*(x2 − y2) ← Rh 2p transition at higher energy, and a weaker RhO2(π*) ← Rh 2p shoulder ∼2 eV below the main peak.36 In contrast, 11·BF4 exhibits a single intense main feature that corresponds well with previously reported sixcoordinate Rh(III) complexes, as Rh 4dσ* ← Rh 2p. The intensity of the features corresponding to Rh 4d final states is significantly greater in 11·BF4, due to the four available holes (4dx2−y2 and 4dz2 are both empty). These data clearly indicate that 1 and 11·BF4 have very different electronic structures about the metal center, with the cationic complex 11·BF4 behaving like a traditional RhIII−O22− complex. Rhodium K-edge XAS further supports our conclusions that there are significant electronic differences between 1 and 11·BF4, as indicated in Figure 5B. There is a marked difference in the energy of the edge feature, as determined by the first inflection point of the data, which is ∼2 eV higher in 11·BF4 in comparison with 1, suggesting that the cationic complex

Figure 3. Crystallographically determined structure of 12·OTf displaying thermal ellipsoids at the 50% confidence level. Hydrogen atoms have been removed for clarity. Hydrogen atoms (except for the SIMes45 backbone hydrogens) and the counterion are omitted for clarity. Selected interatomic distances (Å) and angles (deg): Rh(1)− O(1), 1.895(5); Rh(1)−O(2), 1.933(5); Rh(1)−C(1), 2.089(7); Rh(1)−C(22), 2.094(6); Rh(1)−N(5), 2.067(5); Rh(1)−N(6), 2.064(6); O(1)−O(2), 1.439(8); N(1)−C(1), 1.339(9); N(2)− C(1), 1.340(9); C(2)−C(3), 1.513(11); O(1)−Rh(1)−C(1), 85.4(2); O(1)−Rh(1)−C(22), 84.2(2); C(1)−Rh(1)−C(22), 167.5(3); O(1)−Rh(1)−N(5), 112.5(2); O(1)−Rh(1)−N(6), 118.8(2); C(1)−Rh(1)−N(5), 95.9(2); C(1)−Rh(1)−N(6), 94.4(2); C(22)−Rh(1)−N(5), 94.5(2), C(22)−Rh(1)−N(6), 93.6(2).

spectra of 11·BF4 was observed, which is in the expected region for the O−O single bond in peroxides. Comparison of the IR spectra of 16O2-11·OTf and its 18O2-11·OTf isotopomer shows them to be identical except for a 50 nm red shift of the band at 910 cm−1 in the spectrum of 16O2-11·OTf, to 860 cm−1 (Figure 4), in good agreement with that predicted by a simple harmonic oscillator calculation of the O−O stretch (857 cm−1). As expected, the band at 910 cm−1 was observed for all three complexes of 11·X synthesized in this study. The observed

Figure 4. IR spectra of 11·OTf (solid line) and its corresponding isotopomer 18O2-11·OTf (dashed line). 7309


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Figure 6. Valence MO diagram of complex 1 as determined from BP86/TZVP DFT calculations on fully optimized geometries of the complex. Kohn−Sham orbitals of particular interest are also shown.

Figure 5. (A) Rh L3-edge and (B) Rh K-edge XAS spectra of complexes 1 and 11·BF4.

Table 2. XAS Transitions and Peak Intensities at the Rh L3 Edge for Complexes 1 and 11·BF4 Obtained from Quantitative Fits of the XAS Dataa RhO2 π* ← Rh 2p3/2 complex

energy (eV)

normalized intensity

1 11·BF4

3006.8 ± 0.1

2.1 ± 0.5

Rh 4dσ*(x2 − y2)← Rh 2p3/2 energy (eV)

normalized intensity

3008.2 ± 0.1 3008.2 ± 0.1

3.0 ± 0.5 5.0 ± 1.0

a

Normalized intensities are referenced to the combined L2/L3 edge jump during the fit procedure.

contains a more oxidized metal center. The observed shift is consistent with the presence of a formal Rh(III) center in 11·BF4. DFT Calculations. To further explore the electronic structure of this class of molecules, density functional theory (DFT) calculations were performed on compounds 1 and 11+.50 Optimized geometries are in good agreement with the crystallographically defined molecular structures. A summary of the most important contributions to M−L bonding are given in the form of valence MO diagrams for both 1 and 11+ (Figures 6 and 7). The valence MO diagram for the complete structural model of 1 reproduces results previously obtained for the simplified N-Me derivative of 1,36 indicating a putative RhI−1O2 binding motif with significant back-bonding into the empty

Figure 7. Valence MO diagram of complex 11+ as determined from BP86/TZVP DFT calculations on a fully optimized geometry of the complex. Kohn−Sham orbitals of particular interest are also indicated.

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behavior that is completely consistent with the formation of symmetric η2-1O2 adducts, as previously observed in 1.52 However, computed geometries of complexes possessing πacceptor ligands trans to the O2 ligand result in asymmetric binding of X, as exemplified in Figure 8 for the complex bound to the weak π-acceptor ligand MeCN, [RhO2(IMe)2MeCN]+. The neutral MeCN ligand preferentially occupies a position trans to one of the oxygen atoms rather than lying directly opposite the center of mass of the O2 ligand as is observed for halides. The same effect is observed for anionic π-acceptor ligands (e.g., CN−), suggesting that the charge of the ligand is not responsible for the structural asymmetry.26 Although the geometries of the complexes differ somewhat, each of these species retains a formal RhI{1O2} electronic configuration, with a large splitting between the LUMO and LUMO+1 orbitals (ΔE2 in Figure 9) and similar computed O− O bond lengths. The geometric changes are associated, however, with equally subtle changes in the orbital interactions that define the bonding in these complexes. The symmetric complex formed with a π donor such as Cl− is best described as resulting from (i) a σ bonding interaction between the filled O2 πip orbital and an empty Rh 4dx2−y2 (with concomitant formation of an empty σ* corresponding to the LUMO+1 orbital, see Figure 10) and (ii) π back-bonding from the Rh 4dxz to the empty O2 π*ip (which correlates with the antibonding LUMO). These interactions reflect classic Dewar− Chatt−Duncanson bonding interactions. Notably, the π-donor contributions from the trans chloro ligand serve to enhance and stabilize the π interaction between the metal and O2 ligands. In contrast, in complexes where the trans ligand is a stronger field ligand, an asymmetric bonding mode is obtained. In these cases, the ligands reorganize themselves to maximize σ type interactions, presumably due to weakening of the π backbonding interaction. As a result, the two bonding interactions in an asymmetric complex (e.g., the MeCN complex as shown in Figure 10) differ from that observed in the chloro complex. Distortion away from the symmetric binding mode thus leads to (i) a pure σ interaction to the oxygen trans to the MeCN ligand (labeled OA in Figure 10) and (ii) a weaker pseudo-σ interaction with the other oxygen atom (OB). This effect is reflected by an increase in ΔE2 and a decrease in the HOMO− LUMO gap (ΔE1). Although the electronic configuration still

O2(π*) orbital. The overall electronic structure description of cationic complex 11+ differs from that of 1, most notably in the energy and relative orbital contributions in the LUMO. In contrast to a mostly ligand (O2)-based LUMO in 1, complex 11+ exhibits a predominantly metal-based LUMO, although the overall nature of the MO remains very similar. This shift in overall Rh 4d character in 11+ results in a concomitant increase in the calculated charge at the metal center as well as a significant increase in the HOMO−LUMO gap (ΔEHL), as demonstrated in Table 3. These indicators are consistent with Table 3. Calculated Xharges at Rh and HOMO/LUMO Gap (ΔEHL) from DFT Calculations51 qRh complex ClRh(O2)(IMe)2 ClRh(O2)(IPr)2 (1) [Rh(O2) (IMe)2(MeCN)2]+ [Rh(O2) (IPr)2(MeCN)2]+ (11+)

ChElPG

Voronoi Loewdin

ΔEHL (eV)

ΔEL/L+1 (eV)

0.21 0.24 0.60

1.15 1.23 1.68

0.31 0.36 0.57

0.81 0.76 1.71

1.68 1.40 0.36

0.62

1.61

0.59

1.51

0.08

11+ having a more oxidized metal center, with concomitant reduction of the dioxygen ligand. Indeed, all of our experimental and computational data are consistent with describing 11+ as a peroxo complex, as opposed to the singlet dioxygen description of 1 and related complexes. Two potential rationales for the observed differences between these complexes are (1) the influence of the ligand trans to the dioxygen, increasing the ability of the metal to back-bond into the O2 ligand such that it can be reduced to a traditional peroxide, and (2) the change in coordination number of the complex by the addition of the second nitrile ligand, increasing the ability of the metal to reduce dioxygen. To explore the impact of the trans ligand (X) in stabilizing the RhI{1O2} configuration, we used in silico models where X = Cl−, Br−, I−, MeCN, CN−. Full geometry optimizations of the [X Rh(O2)(IMe)2]+ complexes indicate that the nature of the ligand has a subtle effect on the O2 binding geometry. The halo complexes exhibit

Figure 8. Selected metric parameters from optimized geometries (BP86/TZVP + COSMO) obtained for ClRhO2(IMe)2 (left), [Rh(IMe)2(O2)MeCN]+ (middle), and [Rh(IMe)2(O2)(MeCN)2] + (right). The IMe ligands (above and below the plane of the image) have been removed for clarity. The O2−Rh−X angle is the average of the two O−Rh−X angles. The arrow indicates the formation of an “open” coordination position for the addition of another ligand (see text). 7311


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Figure 9. MO diagrams derived from DFT calculations on a series of XRh(IMe)2(O2) complexes where X = Cl−, Br−, I−, NH3, MeCN, CN−. On the far right, the valence MO diagram for [Rh(IMe)2(O2)(MeCN)2]+ is also given for comparison. Kohn−Sham orbital energies have been adjusted such that EHOMO is in each of the complexes is arbitrarily set to 0 eV (note that the HOMO corresponds to the O2 π*op nonbonding orbital in each of the complexes). Major differences are observed in both ΔE1 (the HOMO−LUMO gap) and ΔE2 upon shifting to π-acceptor ligands. The cyano ligand was chosen as an example of an anionic π-accepting ligand.

reflects a 1O2 adduct, a change in the bonding has resulted in a redistribution of the bonding interactions and concomitant formation of an “open” coordination position, which is presumably poised to generate a six-coordinate peroxo complex upon addition of an additional ligand. Addition of a second nitrile ligand to form [Rh(IMe)2(O2)(MeCN)2]+ has an substantial impact on bonding at the metal center.53 Most notably, the O−O bond distance increases to 143 pm and the energy of the LUMO increases, as seen from the large increase in the HOMO−LUMO gap (ΔE1 ≈ 1.5 eV) as compared to that of the mononitrile complex. The LUMO is now best described as a Rh 4d orbital involved in a σ* interaction with the two nitriles and a π* interaction with the in-plane O2 π*, formally generating a RhIII{O22−} complex upon addition of the second nitrile. This change to the peroxo species results from the additional σ bonding interaction provided by the second acetonitrile ligand, shifting the metal 4d orbitals to higher energy (via charge donation from MeCN), thus enabling greater charge transfer to the O2 ligand. This result is consistent with DFT studies by Caulton, which showed that adding an extra ligand to the metal (in this case neutral amine ligands) results in increase in the reducing power of the system and longer predicted O−O bond lengths.40 In addition, it has also been demonstrated that oxidative addition into a C− H bond is facilitated by the presence of additional ligands on Ir, including specifically acetonitrile.54

The formal description of these limiting Dewar−Chatt− Duncanson electronic configurations should reflect the relative energies of the Rh 4dxz and O2 π*ip orbitals that are involved in the putative π back-bonding interaction observed in both 1 and 11+. An estimate of the energy splitting between the Rh 4dxy and the nonbonding O2 π*op orbital can be obtained by projecting the Rh 4d orbital from the contributing molecular orbitals (see the Supporting Information for details). We thus estimate that the Rh 4dxy in RhO2(IMe)2Cl is lower in energy than the O2 π*op by ∼1.5 eV, whereas the same Rh 4d orbital in [Rh(IMe)2(O2)(MeCN)2]+ is higher in energy than the nonbonding O2 π* orbital by ∼1.2 eV. These results are in good agreement with the overall limiting MO representations given in Figure 11, where the major difference between 1 and 11+ results from the relative energy of the metal 4d relative to the O2 π* orbitals. Notably, each of these limiting cases involves significant delocalization of the RhO2 π* electron pair. This is reminiscent of behavior observed for side-on-bound CuO2 species, first reported by Fujisawa and co-workers29 and subsequently spectroscopically and computationally characterized by Solomon et al.46 In the cases of CuO2 bonding, the systems have been described as a highly covalent CuII−O2− complex, whose bonding is dominated by a highly delocalized electron pair in a CuO2 in-plane π orbital. Cu K-edge XAS spectroscopy is most consistent with a Cu(II) formal oxidation state, reflecting nearly isoenergetic Cu 3d and O2 π* fragment orbitals. Although the 7312


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coordination number is the key to the increased reducing power of the cationic species. Complexes 5−10 (Chart 1), like 1−4, are characterized by highly bulky, electron-rich ligands yet feature lower lying metal 4d orbitals, due to the low coordination number. In cases where such coordinative unsaturation can be stabilized by steric constraints, and with judicious choice of a trans ligand that enhances π back-bonding with the dioxygen ligand, one may stabilize RhI{1O2} adducts. However, the ability to generate an electron-rich six-coordinate complex allows for the formation of the more typical RhIII peroxo complexes.54 We note that, from a molecular orbital perspective, the key feature in defining the electronic nature of the metal−dioxygen bond is therefore the relative energies of the metal 4d and O2 π* orbitals, as represented in Figure 11. This postulate is in agreement with that made by Caulton describing the chemistry of complex 10, a square-planar RhO2 complex with an O−O bond length of 1.36 Å.40 Computational studies accompanying the description of the synthesis and reactivity of compound 10 implied that the unsaturated nature of the pincer complex was responsible for its inability to reduce O2. Consistent with this, the O−O bond is predicted to be lengthened into the peroxidic region when donors such as NH3 and DMAP are introduced in silico into the coordination sphere of the metal. In addition, increased reducing capacity has been tied to coordination number in the chemistry of C−H activation.54

Figure 10. Pictorial representation of the lowest lying empty Kohn− Sham molecular orbitals for Rh Cl(IMe)2(O2) (left), Rh(IMe)2(O2)(MeCN)+ (center), and Rh(IMe)2(O2)(MeCN)2 (right). The modification to the in silico nitrile complex results in a marked shift in the O2 ligand bonding to the metal centerasymmetric η2-O2 binding results in σ-like interactions between the metal and each of the oxygen atoms, deviating from the DCD description of bonding predicted for the chloro complex. Binding of a second nitrile results in the regeneration of a symmetric structure, but with a large increase in the energy of the LUMO and a change from ligand to metal centered due to the addition of another ligand.

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overall bonding descriptions in 1 and 11+ are quite similar to that observed in the copper system, the Rh K-edge XAS spectra are most consistent with representing these as formal Rh(I) and Rh(III) species, respectively, especially within the context defined by the DCD bonding model. Further investigations are ongoing to explore these details in more depth.

ASSOCIATED CONTENT

S Supporting Information *

Tables, figures, text, and CIF files giving X-ray crystallographic data for 11·BF4, 4, and 12·OTf, experimental procedures, and details of computational analyses. This material is available free of charge via the Internet at http://pubs.acs.org. Crystallographic data are also available through the Cambridge Crystallographic Database for complex 4 (CCDC 901275), 11·BF4 (CCDC 891245), and 12·BF4 (CCDC 891246).

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CONCLUSIONS Crystallographic, spectroscopic, and computational results all support the conclusion that the neutral, tetracoordinate complexes 1 and 4 differ significantly in their electronic structure from their higher coordinate cationic counterparts 11 and 12. The former are reflective of highly covalent RhI{1O2} adducts, whereas the latter are more typical RhIII peroxo complexes. Although it seems counterintuitive that introducing a positive charge on the complex would increase its ability to reduce a coordinated ligand such as dioxygen, it is clear from our results and from literature precedent that the increase in

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (C.M.C.); pierre. [email protected] (P.K.). Notes

The authors declare no competing financial interest.

Figure 11. Simplified MO representations of the two limiting cases for RhO2 bonding. 7313


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ACKNOWLEDGMENTS Support for this research has been provided to P.K. and C.M.C. via the Natural Sciences and Engineering Research Council of Canada (NSERC) through Discovery Research Grants and a discovery accelerator supplement to C.M.C. The NSERC is also acknowledged for a postgraduate fellowship to E.C.K., a Canada postgraduate fellowship to J.M.P., and postdoctoral fellowships to J.C.-W. and N.H. Queen’s University is thanked for graduate awards to E.C.K. and J.M.P. Portions of this research were carried out at the Stanford Synchrotron Radiation Lightsource, a Directorate of SLAC National Accelerator Laboratory and an Office of Science User Facility operated for the U.S. Department of Energy Office of Science by Stanford University. The SSRL Structural Molecular Biology Program is supported by the DOE Office of Biological and Environmental Research and by the National Institutes of Health, National Institute of General Medical Sciences (including P41GM103393), and the National Center for Research Resources (P41RR001209). This research has been enabled by the use of computing resources provided by WestGrid and Compute/Calcul Canada.

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with that obtained for the full [RhO2(IPr)2(MeCN)2]+ complex. The geometries differ in the relative orientation of the two N-heterocyclic carbene ligands due to steric encumbrance in the IPr complex, but this has very little influence on the bonding to the O2 ligand. (54) Martin, M.; Torres, O.; Onate, E.; Sola, E.; Oro, L. A. J. Am. Chem. Soc. 2005, 127, 18074−18084.

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Synthesis and Characterization of Cationic Rhodium Peroxo Complexes

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