ABNO Complexes

Article pubs.acs.org/JACS

Electronic Structural Analysis of Copper(II)−TEMPO/ABNO Complexes Provides Evidence for Copper(I)−Oxoammonium Character Richard C. Walroth,† Kelsey C. Miles,§ James T. Lukens,† Samantha N. MacMillan,† Shannon S. Stahl,*,§ and Kyle M. Lancaster*,† †

Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York 14853, United States Department of Chemistry, University of WisconsinMadison, 1101 University Avenue, Madison, Wisconsin 53706, United States

§

S Supporting Information *

ABSTRACT: Copper/aminoxyl species are proposed as key intermediates in aerobic alcohol oxidation. Several possible electronic structural descriptions of these species are possible, and the present study probes this issue by examining four crystallographically characterized Cu/aminoxyl halide complexes by Cu K-edge, Cu L2,3edge, and Cl K-edge X-ray absorption spectroscopy. The mixing coefficients between Cu, aminoxyl, and halide orbitals are determined via these techniques with support from density functional theory. The emergent electronic structure picture reveals that Cu coordination confers appreciable oxoammonium character to the aminoxyl ligand. The computational methodology is extended to one of the putative intermediates invoked in catalytic Cu/aminoxyl-driven alcohol oxidation reactions, with similar findings. Collectively, the results have important implications for the mechanism of alcohol oxidation and the underlying basis for cooperativity in this cocatalyst system.

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INTRODUCTION The selective oxidations of alcohols to aldehydes and ketones are important, frequently used reactions in organic synthesis.1 There are numerous stoichiometric reagents and catalytic systems available for these transformations, including chromium oxides,2 pyridine·SO33 and NaOCl/TEMPO (TEMPO = 2,2,6,6-tetramethylpiperidine N-oxyl),4 but there is growing demand for “greener” methods, such as those capable of using oxygen as the stoichiometric oxidant.5 Recent advances in Cu/ aminoxyl catalyst systems have led to aerobic alcohol oxidation methods that match or surpass the selectivity, scope and practicality of traditional oxidation methods.6 In recent years, the Stahl lab has reported two complementary catalyst systems: (1) (bpy)CuI/TEMPO/NMI (bpy = 2,2′-bipyridine; NMI = N-methylimidazole), which demonstrates high chemoselectivity for primary alcohols over secondary alcohols,7 and (2) (MeObpy)CuI/ABNO/NMI (MeObpy = 4,4′-dimethoxy-2,2′bipyridine; ABNO = 9-azabicyclo[3.3.1]nonane N-oxyl), which exhibits excellent reactivity with both primary and secondary alcohols8 (Scheme 1). Mechanistic studies of Cu/aminoxyl-catalyzed alcohol oxidation reactions have elucidated the origin of the electronic and steric effects that favor the oxidation of activated alcohols over aliphatic alcohols, and primary alcohols over secondary alcohols, respectively. The proposed mechanism involves oneelectron redox cycling of the two co-catalysts: CuII/CuI and R2NO•/R2NOH.9−11 These experimental and computational studies support the involvement of a Cu/aminoxyl adduct as © 2017 American Chemical Society

Scheme 1

the active species in the two-electron oxidation step. This mechanism contrasts the traditional alcohol oxidation pathway with aminoxyl reagents, which involves two-electron redox cycling between oxoammonium and hydroxylamine species (i.e., [R2NO]+/ R2NOH) (Scheme 2). A recent electrochemical study12 directly compared these pathways and showed that alcohol oxidation by CuII/TEMPO proceeds nearly 5-fold faster than the corresponding TEMPO oxoammonium process under comparable conditions even while operating at 500 mV lower potential. These observations, which highlight the profound redox cooperativity between CuII and aminoxyl Received: July 10, 2017 Published: September 18, 2017 13507

DOI: 10.1021/jacs.7b07186 J. Am. Chem. Soc. 2017, 139, 13507−13517

Article

Journal of the American Chemical Society

produce M(n+1)+−(−ONR2). The intermediate case is a purely covalent interaction best described as Mn+−(•ONR2). Where a metal/aminoxyl species will lie on this spectrum will be influenced by the identity of the transition metal and its ancillary ligands.17 For example, recent characterization of a series of [Ti−ONR2]3+ complexes by metal and ligand K-edge X-ray absorption spectroscopy (XAS) indicated that such species are best described as TiIV−(−ONR2).17 The 3d manifold of Cu lies at a much deeper binding energy relative to that of Ti. The Lancaster group has recently experimentally demonstrated that this periodic property causes an inversion of classical metal and ligand orbital ordering in some Cu coordination complexes.18 In addition to the role of Cu/aminoxyl species in catalytic oxidation reactions, the present study provides an opportunity to explore further the relationship between ligand field inversion and reactivity (or lack thereof). We report herein a computationally supported Xray spectroscopic study of [Cu−ONR2]2+ units using Cu Kedge, Cl K-edge, and Cu L2,3-edge XAS. The analysis includes two new Cu/ABNO complexes, which have been synthesized and characterized by X-ray crystallography, together with two previously reported Cu/TEMPO complexes (Scheme 4).15d,16a

Scheme 2. Comparison of Mechanisms for (a) Cu/ Aminoxyl- and (b) Aminoxyl-Catalyzed Alcohol Oxidation Reactions

Scheme 4. Cu/Aminoxyl Complexes Analyzed in This Study: [Cu(TEMPO)Cl2]2 (1), Cu(TEMPO)Br2 (2), [Cu(ABNO)Cl2]2 (3), and [Cu(ABNO)Br2]2 (4)

Scheme 3. (a) Resonance Forms of a Free Aminoxyl, (b) Possible Binding Modes for Aminoxyl on a Metal Center, and (c) Accessible Valence Tautomers of Cu−ONR2 Complexes

Specifically targeted are the mixing coefficients between Cu 3d and R2NO π* in the occupied and unoccupied molecular orbitals (MOs) involved in Cu−ONR2 bonding, which report on the Cu−NO covalency. Experimental values for the Cu− ONR2 MO mixing coefficients are used to validate electronic structure calculations; these calculations are then extended to the key intermediate proposed in Cu/aminoxyl-catalyzed aerobic oxidation of alcohols, which is not experimentally accessible owing to thermodynamically unfavorable binding of the aminoxyl to bpy-ligated CuII.9b The emergent picture suggests that [Cu−ONR2]2+ units are best formulated with a substantial contribution of CuI−(O+NR2) character. The oxoammonium character that arises from aminoxyl coordination to a CuII has clear implications for the cooperative role of the CuII and aminoxyl species in alcohol oxidation reactions.

species, raise important fundamental questions concerning the electronic structure of Cu/aminoxyl adducts. Aminoxyl radicals are generally described as having electronic character that is intermediate between two limiting resonance structures (Scheme 3a).13 The spin density of more complex aminoxyl radicals has been experimentally determined by NMR, EPR and polarized neutron diffraction, and the results reveal an approximately equal distribution of spin between the N and O atoms.14 As ligands, aminoxyl radicals can bind to a metal in either an end-on15 (O-bound) η1 or side-on16 (N−O bound) η2 mode (Scheme 3b). If ligated end-on, the N−O π* orbital serves as a σ-donor; if bound side-on, R2NO• may act as both a σ-donor and π-acceptor. To understand the electronic structure of a metal-aminoxyl unit, [M−ONR2]n+, consideration must be extended to three valence tautomers (Scheme 3c). In one limit, formal electron transfer from R2NO• to Mn+ produces a configuration best described as a M(n−1)+− oxoammonium: M(n−1)+−(O+NR2). The opposite extreme assumes reduction of the aminoxyl by the metal center to

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EXPERIMENTAL SECTION

X-ray Absorption Spectroscopy: Sample Preparation, Measurements, and Data Analysis. All data were measured at the Stanford Synchrotron Radiation Lightsource (SSRL) under ring conditions of 3.0 GeV and 500 mA. All samples were prepared in 13508



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an inert-atmosphere glovebox and were measured as solids. For Cu Kedge measurements, samples were ground with BN to a final concentration of 10 wt% Cu, pressed into 1 mm aluminum spacers and sealed with 37 μm Kapton tape. For Cl K-edge measurements, samples were ground to a fine powder and evacuated extensively to ensure complete removal of any residual or co-crystallized CH2Cl2, and then spread to a vanishing thickness onto 38 μm low-S Mylar film. For Cu L2,3-edge measurements, samples were ground to a fine powder and spread in a thin layer on carbon tape affixed to an Al paddle. Cu K-Edges. All Cu K-edge measurements were collected using beamline 7−3. Samples were maintained at 10 K in a liquid He cryostat during data collection. Spectra were collected in transmission mode, with X-rays detected by ionization chambers immediately downstream and upstream of the sample. A Cu foil and a third ionization chamber upstream of the sample were used for internal energy calibration, setting the first inflection point of the Cu foil scan to 8980.3 eV. Spectra were processed using Sixpack and Igor Pro.19 The region below 8970 eV was used to fit a linear background, while the region above 9000 eV was flattened with a piecewise spline and set to an average intensity of 1. Cl K-Edges. All Cl K-edge measurements were collected using beamline 4−3 under ring conditions of 3 GeV and 500 mA. All samples were measured in a He atmosphere at room temperature in fluorescence mode using a Lytle detector. Intensity was normalized with respect to the incident beam using a He-filled ion chamber upstream of the sample. The incident beam energy was calibrated by setting the first inflection point in the Cl K-edge spectrum of KCl to 2824.8 eV. The region below 2800 eV was used to fit a linear background, and the region above 2830 eV was flattened with a piecewise spline and set to an average intensity of 1. Raw data was processed using Sixpack and Igor Pro. Peaks were fit using a Python package developed in-house which employed the nonlinear leastsquares fitting algorithm implemented in SciPy and equation S4. Cu L2,3-Edges. All Cu L2,3-edge measurements were collected on the 31-pole wigglerbeamline 10−1 with a 1000 lines/mm spherical grating monochromator and 29 μm entrance and exit slits. Data were measured by monitoring the change in sample current through detection of the total electron yield (TEY). The drain current was normalized to incident photon flux with a gold-grid reference monitor. Incident beam energy was calibrated by collecting (nmph)2[CuCl4] (nmph = N-methylphenethylammonium) standards (EL3 = 931.05 eV) before and after each sample. Samples were maintained at room temperature under an ultra high vacuum (10−9 Torr) during collection. Linear and quadratic fits were used for the pre-edge and postedge regions, respectively. The post-edge region was flattened and set to an average value of 1. The edge jumps were subtracted using a statistics-sensitive nonlinear iterative peak-clipping (SNIP) algorithm as implemented in PyMCA.20 Peaks were fit using a Python package developed in-house which employed the nonlinear least-squares fitting algorithm implemented in SciPy and equation S2. Calculations. Density functional theory (DFT) calculations were performed using version 3.0.3 of the ORCA computational chemistry package.21 Single-point and time-dependent density functional theory (TDDFT) calculations used Head-Gordon’s range-separated ω-B97X functional,22 the zeroth order regular approximation for relativistic effects (ZORA)23 as implemented by van Wüllen,24 and the scalar relativistically contracted def2-TZVP(-f)-ZORA basis set.25 Solvation was modeled with the conductor-like screening model (COSMO) employing an infinite dielectric.26 In addition to the above parameters, for XAS calculations an expanded core potential basis set (CP(PPP)) was used for Cu, and the integration grid accuracy for Cu extended to 7. Spectra were calculated using a previously reported TDDFT protocol.27 [(MeObpy)(ABNO)Cu(OnPr)]+, TEMPO, TEMPO+, and TEMPO− were geometry optimized using the BP86 functional, the ZORA-def2-TZVP(-f) basis set on all atoms, ZORA, and COSMO with a dielectric of 9.08 (CH2Cl2).

Article

RESULTS Synthesis and Characterization of ABNO Complexes. Two CuX2/ABNO complexes were synthesized, and crystals suitable for single-crystal X-ray analysis were grown by slow evaporation of the desired complex in equal parts of dichloromethane and diethyl ether (Figure 1). The X-ray

Figure 1. X-ray crystal structures of 3 and 4 shown with 50% probability ellipsoids. The dimers reside on a crystallographic inversion center. All H atoms are omitted for clarity.

crystal structure of [Cu(ABNO)Cl2]2 (3) reveals a dimer with bridging chlorides and the aminoxyl binding in an η2 mode (Figure 1a). The dimer resides on a crystallographic inversion center. The X-ray crystal structure of [Cu(ABNO)Br2]2 (4) also features a dimer, which also resides on a crystallographic inversion center (Figure 1b). The N−O distance of the ligated aminoxyl is 1.3071(11) Å for complex 3 and 1.3194(4) Å for complex 4. The N−O bond lengths for the ABNO complexes are slightly longer than those found in the previously reported TEMPO complexes15d which have an N−O bond distance of 1.276(2) Å for complex 1 and 1.304(8) Å for complex 2. All complexes exhibit a η2 binding mode with the aminoxyl except for 1. The η1 binding mode of 1 possibly arises from a steric clash between the sterically encumbered TEMPO ligand and the short Cu−Cl bonds (relative to Cu−Br bonds; Table 1). Cu K-Edge XAS. Complexes 1−4, which were chosen to validate electronic structure calculations, comprise species with different aminoxyl and halide ligands, as well as different aminoxyl and halide coordination modes (Scheme 4). Previous studies have described 1 and 2 as Cu2+−(•ONR2) based on the crystallographically determined bond angles of 1 and ab initio calculations on 2.16a However, direct spectroscopic investigations of the electronic structure of these complexes have not 13509


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Journal of the American Chemical Society Table 1. Key Structural Metrics for Compounds 1−4a

dipole forbidden, but gain intensity via a quadrupolar mechanism.30 Depending on the symmetry of the complex in question, additional intensity may be gained from dipole mixing, rendering the intensities of pre-edge peaks primarily dependent on geometry with covalency considerations secondary. For the species studied here, the shifts in geometry will dominate over electronic effects and so we limited our analysis of Cu K-edge XAS to the energies of the observed transitions. Typically, observation of a pre-edge feature in Cu K-edge XAS is thought to rule out the CuI oxidation state because the conventional assignment necessitates vacancies in the Cu 3d manifold. Recently, however, pre-edge features were reported for bis-diimine CuI complexes and assigned to Cu 1s → diimine π* transitions.31 In the present systems, the high degree of overlap between the N−O π* and Cu dx2−y2 orbital could reasonably lead to transitions to acceptor orbitals dominated by ligand character, thus favoring a d10 description. Assignment of these pre-edge features can be facilitated by TDDFT calculations in concert with complementary spectroscopies (vide inf ra). Cu K-edge XAS studies of Cu complexes bearing innersphere superoxide (O2−), a species that is isoelectronic with aminoxyl, have also highlighted the ambiguity in interpreting the pre-edge features of these spectra. While XAS studies of end-on bound superoxide have not been reported, the side-on bound peroxides 5 and 6 have been extensively studied (Scheme 5).32 The Cu K-edge XAS of 5 features a pre-edge

d, Å

a

compound

N−O

C−Xterminal

C−Xbridging

1 2 3 4

1.276(2) 1.304(8) 1.3071(11) 1.3194(4)

2.155(1) 2.270(2) 2.1734(3) 2.2996(8)

2.284(1) N/A 2.1852(3) 2.3095(8)

Full structural metrics are provided in the Supporting Information.

Scheme 5. Cu Superoxide (5) and Peroxide (6) Adducts Previously Characterized Using Cu K- and L2,3-Edge XAS29,32

Figure 2. Cu K-edge XAS spectra of (a) Cl-containing complexes 1 and 3 and (b) Br-containing complexes 2 and 4. The 1s → 3d preedge feature is magnified 20-fold in the inset.

been reported. The two new ABNO species, 3 and 4, enable a comparison between TEMPO and ABNO interactions with Cu. The XANES regions from the Cu K-edge XAS obtained for 1−4 are plotted in Figure 2, with second derivatives given in Figures S1−S4. All four compounds have a weak but resolved pre-edge peak at ca. 8979.8 eV, which is on the high end reported for CuII species, though lower than that of wellcharacterized CuIII species (Table 2).28 However, pre-edge peak position has been shown to be significantly affected by ligand contribution to the LUMO, and it can be problematic to use this metric as a raw determinant of physical oxidation state.29 The aforementioned pre-edge feature is conventionally assigned to a Cu 1s → 3d excitation. Such excitations are

peak at 8978.6 eV, and thus was described as a bona f ide CuII− (O2−) species. Solomon and co-workers formulated 6 as a CuIII−peroxo, [CuIII−(O22−)], based on its 8980.7 eV pre-edge feature. Recently, Wieghardt and co-workers contended that 6 is better described as a CuII−(O2−) species with modest mixing of the CuIII−(O22−) configuration into its ground state.29 That complexes bearing [Cu−O2]+ units can adopt the limiting CuII−(O2−) configurations suggests that [Cu−ONR2]2+ units with an isoelectronic but less electronegative redox-active ligand may favor an even more reduced metal center: CuI− (O+NR2). For 5 and 6, both ligand field effects and the overall charge on Cu contribute to the pre-edge peak position. The pre-edge peak position of square pyramidal-like species such as 5 occur at lower energy than square planar species with similar level of charge on Cu, such as 6.29 We observe no such shift in comparing pseudo-square planar 2 to the more square pyramidal 4. This indicates either that the axial halide is having little effect on the energy level of the HOMO, or that the effects are being mitigated by changes in overall charge on Cu. Higher energy Cu K-edge XAS transitions at ca. 8986.5 eV are conventionally assigned as Cu 1s → Cu 4p transitions, with

Table 2. Cu K-Edge Peak Energies, in eVa

a

compound

Cu 1s → LUMO

Cu 1s → Cu 4p

1 2 3 4

8979.9 8979.9 8979.7 8979.8

8986.8 8986.2 8986.8 8986.3

Values measured from smoothed second derivatives of spectra. 13510


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Journal of the American Chemical Society contributions from ligand-to-metal charge transfer (LMCT) shakedown.33 This dipole-allowed transition leads to a drastic increase in intensity relative to the lower energy feature. Solomon et al. demonstrated a dependence on ligand identity for the position of rising edge peaks, arising either due to nearedge scattering or influence of ligand to metal shakedown transition.33 For compounds 1−4 the energy of this transition correlates with halide identity, with Br-containing species at lower energy relative to Cl-containing species. The region along the rising edge is sensitive to the overall charge at the metal center. This correlation to ligand identity may arise from increased charge donation from the Br to the Cu relative to Cl as the more diffuse orbitals in Br allow for better overlap and interaction with empty Cu 4p orbitals. Alternatively, near-edge scattering effects could be affected by having heavier atoms in the first coordination sphere. Cl K-Edge XAS. The ambiguities that arise in interpreting Cu K-edge XAS necessitate complementary methods to more accurately describe the interaction of Cu and its ligands. To this end, we obtained Cl K-edge XAS data to evaluate the contribution of Cl to the LUMO(s) of 1 and 3. The intensity of pre-edge peaks in ligand K-edge XAS have been shown to be a direct probe of the ligand character in the LUMO of metal complexes.34 Briefly, the oscillator strength D0 is related to the %Cl 3p in the accepting MO, the number of holes being accessed in the transition h, the number of absorbers n, and the radial dipole integral IS (effectively the intensity of a pure Cl 1s → Cl 3p transition) (eq 1).34 (%Cl 3p)Ish D0(Cl 1s → Ψ*) = 3n

Figure 3. Cl K-edge XAS data obtained for (a) 1 and (b) 3. Experimental data are plotted in red. Composite pseudo-Voigt fits are overlaid as dotted black line. Individual fitted pseudo-Voigt peaks are in gray. Residual edge intensity was accounted for by fitting a broadened step function, which is plotted in blue. Second derivatives are plotted above each spectrum in red.

(1)

Figure 3 shows the experimental Cl K-edge XAS obtained of 1 and 3 and Table 3 lists their peak amplitudes and positions. While 1 has a single pre-edge peak, 3 has a pre-edge feature that is split into two nearly overlapping peaks separated by 0.5 eV. We assign the peak at 2819.4 eV as belonging to the terminal Cl and the higher energy peak at 2819.9 eV to the bridging Cl. A bridging Cl is expected to donate more of its charge than a terminal ligand, thereby raising the energy of the final state formed after creation of a core hole on the bridging Cl.35 This small change in charge donation between terminal and bridging ligands is not seen in 1, suggesting that all four Cl atoms in the molecule are donating equally to the Cu centers. Using the well-established35 value for the Cl character in (nmph)2[CuCl4] (9.75% Cl character per bond) as a calibrant, we determined an experimental value of 2.42% Cl character per bond in 1. In 3 we observe 2.46% Cl character per terminal Cl and 2.66% Cl character per bridging Cl. Cu L2,3-Edge XAS. Figure 4 shows the experimental Cu L2,3-edge XAS spectra for compounds 1−4. First-row metal L2,3-edge XAS is an experimental probe of metal 3d contributions to unoccupied and partially occupied valence orbitals.32,36 The summed intensity (2IL2 + IL3) is related to the Cu 3d character (%Cu 3d), the number of holes being accessed in the transition h, and the intensity of the pure Cu 2p → Cu 3d transition, Is (eq 2). (2IL 2 + IL3)(Cu 2p → Ψ*) = (%Cu 3d)Ish

Table 3. Peak Energies and Amplitudes for 1 and 3, as Well as the Calculated %Cl 3p Character in the LUMO, Reported per Cl Bond compound

energy, eV

areaa

%Cl

1

2819.7(5)

1.12(8)

2.26(6)

3

2819.4(4) 2819.9(8)

0.66(1) 0.61(2)

2.66(6) 2.46(5)

2820.6b

0.676b

9.75b

(nmph)2[CuCl4]

Voigt peaks were fit to experimental spectra using an in-house Python script. bTaken from ref 36. a

essentially the same Cu 3d character in their LUMOs: 28 ± 3%. This may be construed as the absence of 56% of a 3d electron from the Cu manifoldin other words, intermediacy between the CuI−(O+NR2) and CuII−(•ONR2) limits. DFT Calculations. TDDFT has been established as a predictive method for pre-edge features in K-edge XAS of transition metals.27 While core potentials are poorly modeled by commonly employed density functionals such as BP86 and B3LYP, the spacings between valence orbitals are wellreproduced. For hybrid functionals, a scalar shift is sufficient to correct calculated XAS energies to experimental values. Errors in relative predicted peak energies depend on the functional used, but in general are less than 0.5 eV. The Lancaster group has previously used the B3LYP functional to reproduce the experimental Cu K-edge XAS features of nine Cu

(2)

The L3 and L2 peak energies are intermediate between 5 and 6, both of which have been assigned as formally CuII species. The fit areas and calculated values for Cu parentage are shown in Table 4. Within error, all four aminoxyl complexes have 13511


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Table 5. Peak Positions for 1−4 from Experimental Spectra in Addition to ω-B97X Predicted Spectra position, eV ΔEc

compound

experimentala

TDDFTb

1

8979.9 8986.8

8979.9 8984.8

0.0 2.0

2

8979.9 8986.2

8980.5 8986.4

−0.6 −0.2

3

8979.7 8986.8

8980.3 8986.7

−0.6 0.1

4

8979.8 8986.3

8980.3 8986.4

−0.5 −0.1

Figure 4. Background- and rising-edge-subtracted Cu L2,3-edge XAS data obtained for compounds 1−4. averagec

0.5

Peak positions were obtained by fitting to the second derivative of the experimental spectra. bTDDFT calculations were initiated following single-point DFT calculations employing the ω-B97X function, the ZORA-def2-TZVP(-f) basis set on all atoms, and COSMO. Peaks were adjusted using a calibrant library published in ref 32. cRelative error as well as the average of the absolute errors are also listed. a

complexes across a wide range of formal oxidation states and coordination environments.31 The range-separated ω-B97X provides superior matches between experimental and calculated covalency in Cu complexes18 and is thus employed here. Examining the resulting predictions for 1−4 (Table 5, Figures S9−S12), we found that ω-B97X replicates the experimental features with good fidelity. While the first pre-edge feature is predicted with high accuracy for 1, it is overestimated by ca. 0.5 eV for the other three compounds. In all four, the pre-edge is predicted to be an XAS-ML feature dominated by Cu1s → NO π* character. The second pre-edge feature is underestimated in 1 by 2 eV, by far the largest error for the four compounds, but accurately predicted within ∼0.1 eV for the other three compounds. This feature is predicted to be primarily a Cu 1s → Cu 4p excitation. Simplified MO diagrams depicting the principal orbital interactions between the CuIIX2 fragment SOMO and R2NO• π* SOMO are given in Figure 5. Table 6 lists the corresponding contribution to the Cu−NO antibonding LUMO for 1−4 based on ω-B97X calculations, along with the experimentally derived values from Cl K-edge data and Cu L2,3-edge data. As the LUMO and LUMO+1 are degenerate in the dimeric species, and both give equal contribution to features in XAS spectra, their values have been averaged for interpretation. Overall, ω-B97X predicts an electronic structure for each complex in good agreement with experimental observables. The LUMO that is predicted for 1−4 is predominantly R2NO based, implying an appreciable amount of electron density transferred from R2NO• to CuII, conferring oxoammonium character to the R2NO ligand. For the side on species, the Cu d manifold lies buried beneath a region of halide

character, while for the end-on 1, the metal and halide orbitals become more mixed. The end-on binding of R2NO in 1 precludes back bonding, lowering the electron density on the NO and leading to higher oxoammonium character in the ground state. Extension to Catalytic Alcohol Oxidation via (bpy)Cu(TEMPO) Adducts. The Cu/TEMPO adducts 1 and 2 have been shown previously to mediate stoichiometric oxidation of benzyl alcohol.15d The mechanism of these reactions is not well understood, but these observations create an important link between the crystallographically characterized complexes 1−4 and the highly effective (bpy)Cu catalyst systems that have been developed for aerobic alcohol oxidation. Previous experimental and computational studies show that TEMPO and ABNO do not form stable adducts with bpy-ligated CuII species. For example, EPR spectroscopic analysis of catalytic reaction mixtures provided no evidence for TEMPO binding to Cu,9a and DFT calculations indicated that coordination of TEMPO to [(bpy)CuII(OnPr)]+ is thermodynamicaly unfavorable by ΔG = 13.6 kcal/mol.9b The aforementioned studies, however, provide the basis for expansion of the DFT treatment to the (bpy)Cu/TEMPO/alkoxide intermediate 7 (Scheme 6), which has been proposed to be the active species in the (bpy)Cu/TEMPO-catalyzed alcohol oxidation.9,11 The reaction

Table 4. Fit Peak Positions and Areas for the Complexes of Interest, along with the Calculated %Cu Character L3

a

L2

compound

position, eV

area

position, eV

area

%Cu character

1 2 3 4 (nmph)2[CuCl4] 5 6

931.4(2) 931.4(2) 931.3(2) 931.3(2) 931.2(1) 930.8b 932.7b

5.18(3) 5.94(3) 5.31(3) 5.01(3) 5.73(3)

951.2(1) 951.1(1) 951.0(1) 951.1(1) 950.9(1) 950.7b 952.7b

1.64(5) 1.82(5) 1.54(5) 1.37(4) 1.83(5)

27(2) 31(2) 27(2) 25(2) 61a 20b 28b

Taken from ref 36. bTaken from ref 33. 13512


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Figure 5. Partial MO diagrams for 1−4 derived from DFT calculations employing the ω-B97X functional and the ZORA-def2-TZVP(-f) basis set on all atoms. Depicted MOs correspond to the interaction between the CuIIX2 3d SOMO and the R2NO π* SOMO. MOs are printed at an isolevel of 0.03 au.

proton/two-electron (i.e., “hydride”) transfer from the alkoxide to the Cu/aminoxyl fragment.9b,11 We used [(bpy)(TEMPO)Cu(OnPr)]+ (7-OnPr) as a model for our DFT studies. Figure 6 shows a partial MO diagram for 7-OnPr, with the corresponding MO coefficients provided in Table S5. Like the aminoxyl complexes 1−4 described above (Figure 5), the LUMO is dominated by R2NO character (41.1%), with additional contributions from Cu (25.7%) and the O and α-C of OnPr (O−C) (15.3%). The orbital plot of the HOMO shows some O−C π bonding character, while the LUMO shows O−C π* character. The d orbital manifold of Cu is highly mixed with the ligand-based orbitals and is fully occupied. These features correlate with appreciable CuI/ oxoammonium character in the ground state of 7-OnPr, as in complexes 1−4. The Stahl lab had previously reported that using 4,4′-2,2′bipyridine (MeObpy) instead of bipyridine along with ABNO as the aminoxyl species resulted in improved reactivity toward secondary alcohols. We investigated an analogous species to 7OnPr, [(MeObpy)(ABNO)Cu(OnPr)]+ (8-OnPr) and found it

Table 6. Contributions to the LUMO of 1−4 Based on Cu LEdge Data for Cu and Cl K-Edge Data for Cla %Cu 3d

%Cl 3p

compound

Cu L2,3-edge

DFT

% error

Cl K-edge

DFT

% error

1 2 3 4

27 31 27 25

18.9 29.3 30.2 28.9

22.2 5.5 11.9 15.6

9.1

7.6

16

10.3

9.4

8.7

Calculations employed the ω-B97X functional, the ZORA-def2TZVP basis set on all atoms, and solvation modeled with COSMO. For dimeric species, the LUMO and LUMO+1 were averaged as both give rise to an equal energy feature in XAS spectra.

a

is proposed to proceed via inner-sphere transfer of a hydrogen atom from the coordinated alkoxide to the aminoxyl, affording the aldehyde or ketone product together with (bpy)CuI and TEMPOH (the latter readily isomerizing to the more stable TEMPO-H species). This step formally consists of a one13513


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have been previously characterized by similar approaches and are best described by the TiIV−(−ONR2) limiting configuration (Scheme 7).17 This comparison is chemically

Scheme 6. Proposed Details of O−H Bond Activation within the Catalytic Cycle of Aerobic Cu/TEMPO Alcohol Oxidationa

Scheme 7. Comparison of the Electronic Description of Ti/ TEMPO and Cu/TEMPO after Reversible TEMPO Binding

a

reasonable, as the greater effective nuclear charge (Zeff) experienced by the 3d manifold of Cu relative to Ti will stabilize these orbitals sufficiently to invert mixing between the metal and R2NO, leading to a LUMO dominated by the R2NO π* orbital. The electronic structural description of the metal/ aminoxyl fragments also aligns with the typical reactivity trends, whereby TiIII is a reductant while CuII is an oxidant. This contrasting electronic structural description belies the similar homolytic dissociation of R2NO• from both metal centers. Our formulation of the electronic structure of 1−4 contrasts a previous description of the electronic structure of 2. Rey and co-workers16a first reported the synthesis of 2 accompanied by X-ray crystallographic analysis and an extended Hückel calculation. The electronic structural formulations derived from these two methods were found to be at odds: while orbital population analysis from the calculation supported a CuI−(O+NR2) formulation, Rey and co-workers advocated for a strict CuII−(•ONR2) formulation due to the 1.304(8) Å N−O distance and pyramidalized R2NO fragment of the TEMPO ligand. Consequently, the Cu−ONR2 bonding was described as a strongly antiferromagnetic interaction. However, this assignment rests on structures of free TEMPO, TEMPO+, and TEMPO−. Wieghardt and co-workers37 have since demonstrated that structural metrics of organic nitrosyl (RNO) units are perturbed from free ligand values upon metal complexation. Aminoxyl R2NO fragments can be expected to follow suit. The aforementioned formulation would also be expected to produce a broken symmetry38 (BS) solution; however, no reasonable solution emerges when carrying out a BS(1,1) calculation using ω-B97X/ZORA-def2TZVP(-f) on 2. BS predicts a triplet ground state arising from strong (J = 600−1200 cm−1) ferromagnetic coupling, contrary to the diamagnetic nature of 2. Moreover, this solution is ca. 1.4 eV higher in energy than the closed-shell restricted Kohn− Sham solution. All told, BS can be excluded from consideration. Singlet diradical association of Cu and TEMPO is inappropriate and must give way to a covalent bonding description. Benard and co-workers39 advanced computational treatment of 2 to the ab initio SCF/CI level. These authors endorsed the CuII−(•ONR2) formulation, owing, in part, to their calculated atomic charges on N and O in 2, which did not differ from those of free TEMPO. This conclusion is weakened, however, by the well-recognized method dependence of calculated atomic charges40 and several authors have also documented

Adapted from ref 9b.

Figure 6. Partial MO diagram of 7-OnPr depicting the interaction between the (bpy)(OnPr)CuII 3d SOMO and R2NO π* SOMO. MOs are printed at an isolevel of 0.03 au.

had modestly higher Cu parentage (28.5%) in the LUMO, with modestly decreased NO π* character (35.4%). Overall, 8-OnPr possesses effectively the same electronic structure as 7-OnPr inasmuch as it exhibits a high degree of CuI/oxoammonium character. The electronic structural similarity of formally CuII/ ABNO and CuII/TEMPO units suggests that the enhanced reactivity toward secondary alcohols is likely a steric consequence of the ABNO vs TEMPO substitution.

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DISCUSSION The [Cu−ONR2]2+ complexes characterized in this work exhibit an intermediate electronic structure that most closely aligns with the CuI−(O+NR2) limiting configuration. These results offer a notable contrast to [Ti−ONR2]3+ complexes that 13514


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adduct having CuI/oxoammonium character that achieves oxoammonium-like reactivity without full oxidation of the aminoxyl fragment. A notable feature of the proposed mechanism (Scheme 6) is that the alcohol is activated for hydride transfer by formation of an alkoxide ligand for CuII. In this manner, the Cu center serves as a template and an activator for both the alcohol and aminoxyl reaction partners, thereby enabling facile hydride transfer from the alkoxide to the “oxoammonium” fragment.

the tenuous relationship between calculated charges and oxidation state.41 This complication is evident in the combined atomic charges for N and O derived from Löwdin42 and Mulliken population analyses of DFT calculations employing ω-B97X/ZORA-def2-TZVP(-f) on 1−4 as well as TEMPO•, TEMPO+, and TEMPO− (Table 7). The values starkly contrast Table 7. DFT-Calculated Atomic Charges Summed for N and O in Free and Coordinated Aminoxyl Unitsa

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charge, e compound

Löwdin

Mulliken

1 2 3 4 TEMPO+ TEMPO• TEMPO−

+0.37 +0.28 +0.23 +0.22 +0.46 −0.07 −0.63

+0.13 −0.01 −0.02 −0.04 +0.27 −0.28 −0.66

CONCLUSION We have elucidated the electronic structure of four Cu−ONR2 complexes using a combination of XAS techniques and DFT calculations. In all the Cu/aminoxyl complexes studied here, the LUMO is predominantly R2NO based, with only ∼28% Cu character. By comparison, well characterized CuII complexes exhibit >50% Cu character when investigated by Cu L2,3-edge XAS. This small amount of hole character on Cu is at odds with the formal CuII assignment, pointing instead to a more reduced metal after aminoxyl bindinga configuration weighted toward CuI. For the Cl containing species the halide character in the LUMO was experimentally found to be ∼9%, leaving ∼63% of the hole character to reside on the aminoxyl NO fragment. Not only would oxidation state formalism obscure this important electronic structural detail, but also conventional interpretation of Cu K-edge XAS would lead to an erroneous “CuII” assignment that would mask the cooperative activation by Cu of R2NO• toward R2NO+. The Cu-coordinated aminoxyl fragments are thus found to adopt significant oxoammonium character. This electronic structural description reveals that coordination of the aminoxyl to CuII provides access to an oxoammonium-like species even though CuII lacks the driving force to promote full one-electron oxidation of TEMPO• to TEMPO+. Identification of ancillary ligands that maximize ground state oxoammonium character in Cu/aminoxyl complexes may afford access to more potent oxidation catalysts.

Calculations employed the ω-B97X functional, the ZORA-def2TZVP basis set on all atoms, and solvation modeled with COSMO.

a

the results from Benard and co-workers: in all complexes, the charges on the NO fragment become more positive relative to those of free TEMPO•, in accord with a more oxidized aminoxyl ligand. The spectroscopic and computational studies elaborated above show that Cu/TEMPO adducts, including the crystallographically characterized complexes 1−4 and the key proposed intermediate 7 in (bpy)Cu/TEMPO-catalyzed aerobic alcohol oxidation, have substantial CuI/TEMPO+ character. This electronic structural description highlights the ability of the CuII and TEMPO co-catalysts to achieve two-electron oxoammonium reactivity via their partnership as two oneelectron oxidants. As noted in the Introduction, this Cu/ TEMPO partnership mediates alcohol oxidation with a nearly 5-folder higher rate than TEMPO+, even though both of the partners have substantially lower reduction potentials than TEMPO+ (by ≥500 mV).12 This observation provides a clear demonstration of co-catalyst “cooperativity” that may be distinguished from other modes of co-catalysis. A sequential, multi-step redox pathway in which the CuII reoxidizes TEMPOH to TEMPO+ (Scheme 8) was proposed in an

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b07186. X-ray crystallographic data for 3 (CIF) X-ray crystallographic data for 4 (CIF) Syntheses and characterization of 1−4, X-ray crystallographic methods and data, Cu K-edge second derivatives, Cu L2,3-edge background subtractions, Orca input files, TDDFT-calculated Cu K-edge XAS, molecular orbital compositions, molecular orbital plots, and input coordinates for DFT calculations, including Figures S1−S22 and Tables S1−S20 (PDF)

Scheme 8. Sequential, Multi-Step Redox Pathway Originally Considered as a Mechanism for Cu/TEMPO-Catalyzed Alcohol Oxidation

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AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected]

early report of Cu/TEMPO-catalyzed alcohol oxidation;43 however, later studies showed that CuII is not a sufficiently strong oxidant to mediate oxidation of TEMPOH to TEMPO+.9a The present electronic structural studies show that Cu/aminoxyl adducts undergo inner-sphere (partial) charge transfer from the aminoxyl to CuII, resulting in the

ORCID

Shannon S. Stahl: 0000-0002-9000-7665 Kyle M. Lancaster: 0000-0001-7296-128X Notes

The authors declare no competing financial interest. 13515


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(15) (a) Dong, T. Y.; Hendrickson, D. N.; Felthouse, T. R.; Shieh, H. S. J. Am. Chem. Soc. 1984, 106, 5373−5375. (b) Felthouse, T. R.; Dong, T. Y.; Hendrickson, D. N.; Shieh, H. S.; Thompson, M. R. J. Am. Chem. Soc. 1986, 108, 8201−8214. (c) Dickman, M. H.; Porter, L. C.; Doedens, R. J. Inorg. Chem. 1986, 25, 2595−2599. (d) Laugier, J.; Latour, J. M.; Caneschi, A.; Rey, P. Inorg. Chem. 1991, 30, 4474−4477. (e) Pervukhina, N.; Romanenko, G.; Podberezskaya, N. J. Struct. Chem. 1994, 35, 367−390. (f) Nguyen, T.-A. D.; Wright, A. M.; Page, J. S.; Wu, G.; Hayton, T. W. Inorg. Chem. 2014, 53, 11377−11387. (16) (a) Caneschi, A.; Grand, A.; Laugier, J.; Rey, P.; Subra, R. J. Am. Chem. Soc. 1988, 110, 2307−2309. (b) Jaitner, P.; Huber, W.; Hunter, G.; Scheidsteger, O. J. Organomet. Chem. 1983, 259, C1−C5. (c) Jaitner, P.; Huber, W.; Gieren, A.; Betz, H. Z. Anorg. Allg. Chem. 1986, 538, 53−60. (d) Dickman, M. H.; Doedens, R. J. Inorg. Chem. 1982, 21, 682−684. (e) Mindiola, D. J.; Waterman, R.; Jenkins, D. M.; Hillhouse, G. L. Inorg. Chim. Acta 2003, 345, 299−308. (f) Ito, M.; Matsumoto, T.; Tatsumi, K. Inorg. Chem. 2009, 48, 2215−2223. (g) Zhu, Z.; Fettinger, J. C.; Olmstead, M. M.; Power, P. P. Organometallics 2009, 28, 2091−2095. (h) Isrow, D.; Captain, B. Inorg. Chem. 2011, 50, 5864−5866. (17) DeBeer George, S.; Huang, K.-W.; Waymouth, R. M.; Solomon, E. I. Inorg. Chem. 2006, 45, 4468−4477. (18) Walroth, R. C.; Lukens, J. T.; MacMillan, S. N.; Finkelstein, K. D.; Lancaster, K. M. J. Am. Chem. Soc. 2016, 138, 1922−1931. (19) Webb, S. M. Phys. Scr. 2005, 2005, 1011. (20) Van Espen, P., Spectrum Evaluation. In Handbook of X-ray Spectrometry, 2nd ed.; Van Grieken, R., Markowicz, R., Eds.; CRC Press: New York, 2001; pp 260−264. (21) Neese, F. Wiley Interdiscip. Rev. Comput. Mol. Sci. 2012, 2, 73− 78. (22) Chai, J.-D.; Head-Gordon, M. J. Chem. Phys. 2008, 128, 084106. (23) van Lenthe, E.; van der Avoird, A.; Wormer, P. E. S. J. Chem. Phys. 1998, 108, 4783−4796. (24) van Wüllen, C. J. Chem. Phys. 1998, 109, 392−399. (25) Weigend, F.; Ahlrichs, R. Phys. Chem. Chem. Phys. 2005, 7, 3297. (26) Klamt, A.; Schüürmann, G. J. Chem. Soc., Perkin Trans. 2 1993, 799−805. (27) DeBeer George, S.; Petrenko, T.; Neese, F. J. Phys. Chem. A 2008, 112, 12936−12943. (28) DuBois, J. L.; Mukherjee, P.; Stack, T. D. P.; Hedman, B.; Solomon, E. I.; Hodgson, K. O. J. Am. Chem. Soc. 2000, 122, 5775− 5787. (29) Tomson, N. C.; Williams, K. D.; Dai, X.; Sproules, S.; DeBeer, S.; Warren, T. H.; Wieghardt, K. Chem. Sci. 2015, 6, 2474−2487. (30) Hahn, J. E.; Scott, R. A.; Hodgson, K. O.; Doniach, S.; Desjardins, S. R.; Solomon, E. I. Chem. Phys. Lett. 1982, 88, 595−598. (31) Walroth, R. C.; Uebler, J. W. H.; Lancaster, K. M. Chem. Commun. 2015, 51, 9864−9867. (32) Sarangi, R.; Aboelella, N.; Fujisawa, K.; Tolman, W. B.; Hedman, B.; Hodgson, K. O.; Solomon, E. I. J. Am. Chem. Soc. 2006, 128, 8286−8296. (33) Kau, L. S.; Spira-Solomon, D. J.; Penner-Hahn, J. E.; Hodgson, K. O.; Solomon, E. I. J. Am. Chem. Soc. 1987, 109, 6433−6442. (34) Sproules, S.; Weyhermüller, T.; Goddard, R.; Wieghardt, K. Inorg. Chem. 2011, 50, 12623−12631. (35) Glaser, T.; Hedman, B.; Hodgson, K. O.; Solomon, E. I. Acc. Chem. Res. 2000, 33, 859−868. (36) George, S. J.; Lowery, M. D.; Solomon, E. I.; Cramer, S. P. J. Am. Chem. Soc. 1993, 115, 2968. (37) Tomson, N. C.; Labios, L. A.; Weyhermüller, T.; Figueroa, J. S.; Wieghardt, K. Inorg. Chem. 2011, 50, 5763−5776. (38) (a) Neese, F. J. Phys. Chem. Solids 2004, 65, 781−785. (b) Noodleman, L. J. Chem. Phys. 1981, 74, 5737−5743. (c) Noodleman, L.; Davidson, E. R. Chem. Phys. 1986, 109, 131−143. (39) Rohmer, M. M.; Grand, A.; Benard, M. J. Am. Chem. Soc. 1990, 112, 2875−2881. (40) (a) Meister, J.; Schwarz, W. J. Phys. Chem. 1994, 98, 8245− 8252. (b) Jansen, M.; Wedig, U. Angew. Chem., Int. Ed. 2008, 47, 10026−10029.

ACKNOWLEDGMENTS K.M.L. gratefully acknowledges NSF (CAREER: CHE1454455) and the ACS Petroleum Research Fund (55181DNI6) for support. K.M.L. is an A.P. Sloan fellow. S.S.S. acknowledges the DOE (DE-FG02-05ER15690) for support. R.C.W. was supported by the NIH, Award Number T32GM008500 from the NIGMS. We thank Roald Hoffmann for valuable discussions and a critical read of this manuscript. XAS data were obtained at SSRL, which is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-76SF00515. The SSRL Structural Molecular Biology Program is supported by the Department of Energy’s Office of Biological and Environmental Research, and by NIH/HIGMS (including P41GM103393).

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