Oxidative Cleavage of CH3 and CF3 Radicals from BOXAM Nickel

Jul 24, 2012 - The corresponding radicals R• can be generated by either anodic (electrochemical) or aerial oxidation and were detected by spin trapp...
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Oxidative Cleavage of CH3 and CF3 Radicals from BOXAM Nickel Complexes Axel Klein,*,† David A. Vicic,*,‡ Christian Biewer,† Iris Kieltsch,‡ Kathrin Stirnat,† and Claudia Hamacher† †

Institut für Anorganische Chemie, Universität zu Köln, Greinstraße 6, D-50939 Köln, Germany Department of Chemistry, University of Hawaii, 2545 McCarthy Mall, Honolulu, Hawaii 96822, United States

‡

S Supporting Information *

ABSTRACT: Oxidation of the nickel complexes [(BOXAM)Ni(R)] (HBOXAM = bis((4-isopropyl-4,5-dihydrooxazol-2-yl)phenyl)amine) with R = CF 3 , CH 3 , Cl leads to the corresponding radical cationic complexes, which rapidly undergo homolytic Ni−R bond splitting. The corresponding radicals R• can be generated by either anodic (electrochemical) or aerial oxidation and were detected by spin trapping experiments using N-tert-butyl-α-phenylnitrone (PBN). The splitting reaction was also monitored by UV−vis−near-IR absorption spectroelectrochemistry, showing the formation of the “byproduct” complex [(BOXAM)Ni(THF)]+ in THF solution. Additionally, the undissociated complex radicals [(BOXAM)Ni(R)]•+ were studied by EPR and UV−vis−near-IR spectroelectrochemistry, revealing inter alia largely ligand-centered oxidation. DFT calculations on ground and oxidized states support this finding.

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INTRODUCTION Organonickel complexes have come into focus in recent years for their application as catalysts in C−C coupling reactions.1−7 From two different points of view the use of the trifluoromethyl group in such C−C coupling reactions is highly interesting and challenging. While the introduction of the trifluoromethyl group, the simplest perfluoroalkyl group, is of growing importance in agrichemical, pharmaceutical, and materials chemistry,8−12 the methods to introduce CF3 are still underdeveloped; today mainly stoichiometric synthetic procedures are applied to introduce the CF3 group (usually based on Cu complexes),13−20 and only recently have the first catalytic protocols been developed.21 In this context we have recently studied the complex [(BOXAM)Ni(CF3)] (BOXAM = bis((4-isopropyl-4,5-dihydrooxazol-2-yl)phenyl)amine; Scheme 1), its CH3 derivative (for comparison of CF3 vs CH3), and the precursor complex [(BOXAM)NiCl].22 This last species might be a suitable precatalyst for a catalytic CF3 alkylation process. In this study

we observed that the redox behavior of the three BOXAM nickel complexes strongly depends on the coligand Cl, CF3, or CH3, and the complexes display the same trends in oxidation potentials observed for the corresponding phosphine complexes [(dippe)Ni(R)2] (dippe = 1,2-bis(diisopropylphosphino)ethane; R = CH3, CF3)22 in the sense that the [(BOXAM)Ni(CF3)] complex was much more difficult to oxidize than the [(BOXAM)Ni(CH3)] complex (−0.17 V for R = CH3 vs +0.42 V for R = CF3). Furthermore, we found that while the one-electron oxidation of the CH3 complex is completely irreversible (also at lower temperature and higher scan rates), the CF3 and Cl derivatives exhibit a certain degree of reversibility. A detailed investigation of the CH3 complex revealed that one-electron oxidation or treatment with oxygen leads to the complex [(BOXAM)Ni(THF)]+ and a homolytic cleavage of CH3 was assumed. Finally, from the similarity of the oxidation potentials for the Cl and CF3 complexes we concluded that the redox chemistry might be centered at the BOXAM ligand, while the redox chemistry of the methyl derivative resembles far more that of the phosphine complex [(dippe)Ni(CH3)2] and might be more nickel-centered. To support these assumptions, detailed spectroelectrochemical (EPR and UV−vis−near-IR absorption) experiments have been carried out (under anaerobic conditions) on the complexes [(BOXAM)Ni(R)] (R = CH3, CF3, Cl) and [(BOXAM)Ni(THF)](OAc) and are reported herein.

Scheme 1. Schematic Drawing of [(BOXAM)Ni(R)] Complexes

Received: April 26, 2012

© XXXX American Chemical Society

A

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RESULTS AND DISCUSSION The synthesis and analytical data of the BOXAM nickel complexes [(BOXAM)Ni(R)] (R = CH3, CF3, Cl) and [(BOXAM)Ni(THF)](OAc) were previously reported.22 In addition, the electrochemistry of the four complexes has been presented previously. To examine the role of THF as ligand in the complex [(BOXAM)Ni(THF)]+, we investigated the electrochemistry of the complex in various solvents. In all solvents the one-electron-oxidation wave was not completely reversible, comparable to the case for complexes carrying Cl or CF3 coligands (for CVs see the Supporting Information). EPR Spectroelectrochemistry. In order to establish the character of the one-electron-oxidized complex species and the character of the Ni−R bond scission (ionic or radical type), we carried out a number of EPR experiments under anaerobic conditions. In the first series we wanted to probe for the products of the Ni−R splitting reaction by carrying out anodic electrolysis on the three complexes (potentials 0 V for R = CH3, 0.5 V for R = CF3, and 0.6 V for R = Cl) in the presence (and absence) of the spin trap N-tert-butyl-α-phenylnitrone (PBN; Scheme 2). Spectra are shown in Figure 1.

signal, a doublet of a triplet signal, simulated with aN = 14.1 G and aH = 3.2 G represents the CH3−PBN adduct (sim I in Figure 1), in line with earlier work reporting values of aN ≈ 14 G (NPBN) and aH ≈ 3.4 G (β-HPBN).23−25 The major signal observed in our experiments was a simple narrow triplet with a relatively small aN = 7.99 G and nonresolved (or missing) aH < 0.50 G (sim II in Figure 1). The underlying species could be PhC(O)N(•O)tBu formed by oxidation of the trap, in line with reported aN values of 7.5−8 G for this species.27,28 Also for the Cl derivative the species PhC(O)N(•O)tBu could be observed (aN = 8.00 G) together with a simple triplet with a larger aN of 15.28 G (see the figures in the Supporting Information), which could not be assigned. It is well established that the Cl−PBN adduct (aCl ≈ 4−6 G; aβ‑H ≈ 1 G)25,26 is highly unstable and easily undergoes oxidation to PhC(O)N(•O)tBu.27,28 We can thus conclude that while •CF3 reacts readily with the spin trap and the spin adduct is quite stable, •CH3 and •Cl also perhaps react rapidly with PBN; however, the spin adducts are markedly less stable. To test if the THF solvent reacts with the spin adducts, we carried out the same experiments for the CF3 and the Cl derivatives in MeCN solution. For R = CF3 we found exclusively the spin-trapped radical CF3−PBN (simulation identical with that above), while for the Cl derivative again the three-line signal for the oxidized adduct (aN = 8 G) was observed, this time without further species. Although the results were slightly different in MeCN, a direct involvement of THF is unlikely, since no THF radical could be observed.29,30 It is more likely that the nature of the solvent has a slight influence on the stability of the spin adducts. When carrying out electrolysis experiments at low potentials (−0.6 to +0.2 V for R = Me; 0 to +0.8 V for R = CF3, Cl) under the same conditions without adding the spin trap, we did not observe any EPR signal at 298 K. At 110 K in glassy frozen solution very broad and unresolved EPR signals were observed (Figure 2 and Supporting Information). When carrying out the same electrolysis experiments at lower temperatures (278 K) and freezing the solutions and recording spectra at an early stage of the electrolysis (a few minutes), we also did not observe signals at 278 K, while at 110 K the broad signals were detectable. Thus, we conclude that the underlying species were EPR silent in solution at ambient temperature (278−298 K). Electrolysis at higher potentials (without PBN) led to the observation of narrow resolved EPR signals centered at around g = 2.006 (Figure 2 and Supporting Information). The narrow

Scheme 2. Reaction of the Spin Trap PBN with a Radicala

a

Nuclei in boldface represent possible coupling partners with the unpaired electron. Note that nuclei of the R group might possibly also couple.

For all three complexes narrow 1:1:1 triplet signals were detected at g = 2.006 97 (CH3), 2.006 41 (CF3), and 2.006 16 (Cl). The general triplet character of the signals is due to hyperfine splitting (hfs) of the unpaired electron to the isotope 14 N (I = 1, 99.64% natural abundance). This and the observed g values at around g = 2.006 indicate the formation of PBNbound radical species.23−26 The signal of the CF3 adduct shows additional hfs to 19F (I = 1/2, 100% natural abundance), giving a triplet of quintets with the simulated coupling constant aN = 13.91 G, aH = 1.17 G (βH of PBN), and aF = 1.77 G (trapped CF3 group), in line with reported values.23−25 For the CH3 derivative essentially two signals were observed. The minor

Figure 1. X-band EPR spectra obtained during anodic electrolysis of [(BOXAM)Ni(CF3)] (left) and [(BOXAM)Ni(CH3)] (right) in the presence of PBN in THF/nBu4NPF6, recorded at 293 K with simulations (lower traces). B

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Figure 2. X-band EPR spectra obtained during anodic electrolysis of [(BOXAM)Ni(CH3)] in THF/nBu4NPF6, electrolyzed at 298 K: (left) spectrum recorded after electrolysis at −0.6 to +0.2 V and measured at 110 K in glassy frozen solution (with simulation below); (center) spectra recorded after electrolysis at +0.2 to 1.0 V and measured at 110 K (broad signal) or at 298 K (narrow signal); (right) hf resolved spectrum recorded at 298 K (narrow signal) with simulation (below) at g = 2.006, aH(3H) = 11.21 G, aH(2H) = 1.585 G, aN(1N) = 1.18 G, Gauss/Lorentzian = 1, and line width 1.25 G.

Table 1. EPR Data of Nickel Complexesa complex •+

[(BOXAM)Ni(CH3)] [(BOXAM)Ni(CF3)]•+ [(BOXAM)NiCl]•+ [Ni(TMPP-O)2]•+ [Ni(L1)]•− [Ni(L2)]•− [Ni(emi)]− [(NCN)NiCl2] [(PPh3)2Ni(Fmes)Br]•+ [Ni(C6Cl5)4]·−

giso or gav

g3

g2

g1

Δg

ref

2.073 2.077 2.073 2.20 2.173 2.221 2.223 2.196 2.435 2.533

2.12 2.11 2.10 2.28 2.255 2.366 2.44 2.369 2.825 2.84

2.08 2.09 2.09 2.28 2.255 2.303 2.27 2.195 2.453 2.84

2.02 2.03 2.03 2.04 2.008 1.994 1.96 2.024 1.947 1.92

0.09 0.08 0.07 0.24 0.247 0.372 0.48 0.345 0.878 0.92

this work this work this work 31 32 33 34 35 36 37

Definitions: gav = averaged g value = (g1 + g2 + g3)/3; Δg = g anisotropy = g1 − g3. Abbreviations: TMPP-O = monodemethylated form of tris(2,4,6trimethoxypheny1)phosphine (TMPP); H4L1 = N-[2-(oxalylamino)phenyl]oxalamic acid; NCN = C6H3(CH2NMe2)2-o,o′: H4L2 = 6,6-diethyl3,9,9,12,12,14,14-octamethyl-1,4,8,11-tetraazacyclotetradecane-2,5,7,10,13-pentaone; H4emi = N,N-ethylenebis(2-mercaptoisobutyramide); Fmes =2,4,6-tris(trifluoromethyl)phenyl. a

CH3, Cl). Such organometallic radicals would also be in line with the relatively large g values, the small g anisotropy, and their EPR silence at ambient temperature. Although in these radicals the nickel atom (and the coligand R) has a marked influence on the EPR spectroscopy, the total contribution of nickel to the singly occupied molecular orbital (SOMO) is probably small (see also DFT Calculations) and the description of the radical complex is more like [(BOXAM•)NiII(R)]+ (A) than [(BOXAM−)NiIII(R)]+ (B). The high similarity of the corresponding EPR spectra (at 110 K) strongly supports the resonance form A with the spin mainly centered on the (neutral) BOXAM• radical ligand. To distinguish between “real” Ni(III) complexes and Ni(II) complexes of noninnocent ligands carrying largely the unpaired electron on the ligand, both the g anisotropy (Δg) and the averaged g values (gav) are indicative. Examples of essential nickel contributions to the unpaired electron in a radical complex in a sense of Ni(III) were the trimethoxyphenylphosphine complex [Ni(TMPP-O)2]•+ (Δg = 0.24),31 the oxamato complex [Ni(L1)]•− (H4L1 = N-[2-(oxalylamino)phenyl]oxalamic acid; Δg = 0.247),32 the tetraamidonickel complex [Ni(L2)]•− (H4L2 = 6,6-diethyl-3,9,9,12,12,14,14-octamethyl1,4,8,11-tetraazacyclotetradecane-2,5,7,10,13-pentaone; Δg = 0.372),33 and [Ni(emi)]•− (H4emi = N,N-ethylenebis(2mercaptoisobutyramide); Δg = 0.486).34 Interestingly, corresponding organonickel(III) complexes (having a NiIII−C bond) such as the pincer complex [(NCN)NiCl2]• (Δg = 0.345),35 the phosphine complex

and resolved character of the signals suggests that they do not represent nickel-containing radical species, and although simulations were satisfactorily carried out, no unequivocal information on the composition of the underlying species could be extracted (for some details, see the Supporting Information). Bulk electrolysis experiments and a detailed analysis of the products will be carried out in the near future to assign these species. The broad signals at 110 K (Figure 2) are rhombic in nature. The spectra have been simulated by assuming three g values, and due to the ill-resolved character of the signals the accuracy of the obtained g values is low and can be only provided at a level ±0.005. Nevertheless, the results confirm that the three species are not identical (Table 1) and the g values lie in the typical range of organometallic radicals containing nickel (thus “organic” radicals coordinated to diamagnetic Ni(II)). The latter assumption is supported by the relatively small g anisotropy Δg of the three species (CH3, 0.09; CF3, 0.08; Cl, 0.07). We thus assign the signals observed at 110 K to the oxidized complexes [(BOXAM)Ni(R)]•+ (R = CF3, CH3, Cl), prior to dissociation. Importantly, when recording EPR spectra on anodically electrolyzed solutions of [(BOXAM)Ni(THF)]+ in THF/nBu4NPF6, we failed at both 298 and 115 K to observe reasonable signals. The missing signal at 115 K for the oxidized THF complex (which should be [(BOXAM)Ni(THF)]•2+) strongly supports our assumption that the broad unresolved spectra recorded for the oxidized complexes at 110 K are due to the nondissociated species [(BOXAM)Ni(R)]•+ (R = CF3, C

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trans-[(PPh 3) 2 Ni(Fmes)Br]•+ (Δg = 0.878),36 and the homoleptic complex [Ni(C6Cl5)4]•− (Δg = 0.92),37 exhibit even higher Δg values. As Table 1 shows, the gav values also show the same trend. An interesting insight was recently provided by Thomas et al.38 They reported that the EPR spectroscopy of the oxidized Ni complex [Ni(L3)]•+ containing the salen-type ligand (H2L3 = 6,6′-(2,2′)-bipyridine-6,6′-diyl)bis(2,4-di-tert-butylphenol) depends on the absence or presence of axial ligands such as pyridine. For the spectrum in the absence of pyridine (distorted-square-planar complex) a rhombic signal with g3 = 2.06, g2 = 2.014, and g1 = 1.986 (gav = 2.02, Δg = 0.074) was observed. In line with earlier observations,39 the spectral features are assigned to the ligand-centered radical complex [NiII(L3•)]+, which is a phenoxyl radical bound to diamagnetic Ni(II). For the broad signal obtained in the presence of pyridine, the g values g3 = 2.217, g2 = 2.179, and g1 = 2.032 were obtained from a simulation (gav = 2.1426, Δg = 0.185). The authors state that “these values are far from those expected for a high-spin Ni(II) ion antiferromagnetically coupled to a phenoxyl radical”40 and conclude that “the unpaired electron is mostly hosted by a metal dz2 orbital. Therefore the nickel ion is at its (+III) oxidation state and resides within an elongated octahedral geometry”.33,39−41 It is noticeable that the high-field component is split into five lines of intensity 1:2:3:2:1, as expected for the interaction of the electronic spin with two equivalent nitrogen nuclei. The “pyridine adduct” of [NiII(L3•)]+ thus represents the octahedral nickel(III) complex [NiIIIL3(Py)2]+ involving two axially bound pyridines. Such “switches” in character of a nickel radical upon changes in the coordination sphere have been reported before.32,34,39 DFT Calculations. In order to help rationalize the electrochemical and EPR data, DFT calculations were performed on the parent complexes [(BOXAM)Ni(R)] and [(BOXAM)Ni(THF)]+ and the oxidized species. Unrestricted geometry optimizations (data are given in the Supporting Information) were performed using the SV(P) basis set, and analyses of the frontier molecular orbitals of the optimized structures (Table 2) show that, in the gas phase, the HOMOs and LUMOs of the three neutral BOXAM complexes are similar in energy in comparison to the cationic complex [(BOXAM)Ni(THF)]+.

contributions coming from the p orbitals of the amide nitrogen. From this point of view it is easy to understand why the electrochemical oxidation potentials are equal in magnitude, because if the oxidation takes place on the pincer ligand, the potentials are likely to be minimally influenced by the nature of the coligands on the nickel (CH3 vs CF3 vs Cl). Recent calculations on the different behavior of CF3 vs CH3 as ligands in transition metal complexes have shown that CF3 is a strong σ-donor ligand, comparable to CH3.42−44 Additionally, CF3 exhibits some degree of back-bonding (to σ*(C−F)), while CH3 exhibits virtually none, which is, for example, the reason M−CH3 bonds are frequently longer than M−CF3 bonds in complexes of electron-rich transition metals such as Mn(I),43 Rh(I),43 Ru(II),44 Os(II),44 Ni(II),42 and Pt(II),43 while for early transition metals in high oxidation states such as Ti(IV) or Hf(IV)44 the sequence of bond lengths is inverted. This is completely in line with our calculated geometries (see the Supporting Information), the calculated LUMOs depicted in Figure 3 and the stabilization of the nickel-centered LUMO energy. In contrast to the nickel-centered LUMO, the HOMOs of the BOXAM complexes have mainly amide-ligand N p orbital character and are thus less influenced by the nature of the R coligand. This is fully in line with recent calculations on the related bis (amino)amide nickel complexes [(L)Ni(R)] (L = bis(2-(N,N-dimethylamino)phenyl)amine; R = CF3, Cl, CH3), also exhibiting mainly amide-centered HOMOs.42 However, while in the latter system CF3 and Cl exhibit similar HOMO energies with the CH3 derivative slightly destabilized, in the BOXAM complexes the HOMO energy of the CF3 complex takes an intermediate position between the CH3 (higher energy) and Cl (lower energy) derivatives, which might be explained by slightly higher nickel contributions to the HOMO. The calculated spin density (Figure 4) of the radical cationic complexes [(BOXAM)Ni(R)]•+ shows large contributions of the diphenylamido group of the BOXAM ligand but also spin density on the nickel atom and, for the Cl derivative (as expected), on the Cl atom. Since we have no detailed EPR data from these species, we can only state that qualitatively the picture from the DFT calculations agrees with the EPR spectra, from which a mainly ligand-centered character of the unpaired electron was concluded. UV−Vis−Near-IR Spectroelectrochemistry. Upon oxidation of [(BOXAM)Ni(CH3)] in THF/nBu4NPF6 the longwavelength absorption band at 530 nm and the UV band at 350 nm bleach out, while a new band arises at 670 nm and the 440 nm band increases in intensity and shifts slightly to the blue (Figure 5, left). Comparison with the spectrum of [(BOXAM)Ni(THF)]+ (Figure 6) reveals no similarities; thus, we can assume that [(BOXAM)Ni(CH3)]•+ is formed, can be detected by this method, and is not rapidly converted to [(BOXAM)Ni(THF)]+ by splitting of •CH3. Further oxidation (sweeping the voltage from 0.2 to 1.3 V, Figure 5, right) leads to drastic changes in the spectra. A new long-wavelength band at 630 nm grows in and quickly dominates the spectrum. The bands with maxima at 440 and 320 nm vanish and leave a broad structured absorption band at around 400 nm. Similar behavior is observed during the anodic oxidation of the CF3 and Cl derivatives. However, here the two processes cannot be completely resolved and the spectra for the oxidized complexes [(BOXAM)Ni(R)]•+ (R = CF3, Cl) are superimposed by the absorptions of the THF complex (spectra in the Supporting Information). Nevertheless, the absorption

Table 2. Calculated Frontier Orbital Energies complex [(BOXAM)Ni(CH3)] [(BOXAM)Ni(CF3)] [(BOXAM)NiCl] [(BOXAM)Ni(THF)]+

orbital 124 123 136 135 128 127 139 138

(LUMO) (HOMO) (LUMO) (HOMO) (LUMO) (HOMO) (LUMO) (HOMO)

orbital energy (eV) −3.671 −2.310 −4.207 −2.626 −4.056 −2.871 −6.001 −7.172

Closer inspection shows that the relative HOMO energies for the three complexes agree qualitatively well with the oxidation potential, putting the CF3 group alongside the Cl ligand with their complexes exhibiting higher potential than the CH3 derivative. Graphical representations of the HOMOs and LUMOs for [(BOXAM)Ni(R)] (R = CF3, CH3, Cl) are shown in Figure 3. Further analysis revealed that the HOMOs of all three complexes were mainly ligand-centered, with the major D

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Figure 3. Calculated HOMO (bottom) and LUMO (top) of [(BOXAM)Ni(R)] (R = CH3 (left), CF3 (center), Cl (right)). All lobes are shown at 0.02 isovalues.

Figure 4. Calculated spin density plot of [(BOXAM)Ni(R)]•+ (R = CH3 (left), CF3 (center), Cl (right)): isovalue 0.02 and density 0.04.

Figure 5. Absorption spectra taken during anodic electrolysis of [(BOXAM)Ni(CH3)] in THF/nBu4NPF6 solutions: (left) voltage range −0.6 to +0.2 V; (right) voltage range +0.2 to +1.3 V.

E

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coligand in comparison to the strong electron donor coligands CH3 and CF3 would explain the observed red shift, and from this point of view the THF complex also fits this series. Importantly, these assignments agree qualitatively very well with the DFT calculated HOMOs and LUMOs of the complexes, showing markedly higher HOMO−LUMO gaps for the CH3 and CF3 complexes, in comparison to the Cl- and THF-containing derivatives (Table 2). If we look at the oxidative spectroelectrochemistry of [(BOXAM)Ni(CH3)], the first oxidation (Figure 5) leads to a spectrum of a species which is characterized also by three characteristic bands (λ3−λ5) in the visible region. For the two other derivatives [(BOXAM)Ni(R)] (R = CF3, Cl) similar changes were observed, with one marked difference for R = Cl. Here, the long-wavelength band λ5 for the oxidized species is blue-shifted in comparison to the band for the parent complex (Table 3). If the assignment of the long-wavelength absorption bands of the parent complexes is correct, the oxidation process [(BOXAM)Ni(R)] → [(BOXAM)Ni(R)]•+ should lead to a stabilization of the ligand-centered HOMO and thus to a blue shift of the corresponding LMCT bands with decreased intensity (half empty). In this sense for the complexes with R = CH3, CF3 the λ4 absorptions might be interpreted as the blueshifted long-wavelength LMCT bands (λ5 for the parent species). The long-wavelength absorptions λ5 might then be interpreted as intraligand (SOMO−LUMO) transitions. For R = Cl the long-wavelength absorption λ5 is blue-shifted in comparison to λ5 of the parent complex, in line with LMCT character. For this complex, the SOMO to LUMO absorption, proposed for the R = CH3, CF3 derivatives, was not observed. This is probably due to the rapid conversion of the oxidized complex [(BOXAM)NiCl]•+ to the THF complex [(BOXAM)Ni(THF)]+ and the oxidation of the latter (same potential). Both the THF complex and its oxidation products (see the Supporting Information) exhibit strong absorption bands covering the range from 500 to 800 nm, thus probably obscuring the weak long-wavelength absorption for the oxidized R = Cl complex. To summarize, the results from UV−vis−nearIR spectroelectrochemistry confirm qualitatively the assumption of a largely ligand-centered oxidation [(BOXAM−)NiII(R)]/[(BOXAM•)NiII(R)]•+. In future work we will seek detailed insight into the absorption spectroscopy of parent and oxidized species by TD-DFT calculations.

Figure 6. Absorption spectra of [(BOXAM)Ni(CH3)] (solid line) and [(BOXAM)Ni(THF)](OAc) (dashed line) (both in THF) and anodically oxidized [(BOXAM)Ni(CH 3 )] (dotted line) (in THF/nBu4NPF6 solution).

data could be extracted (Table 3). In line with the EPR experiments, the absorption spectra observed upon electrolysis at higher potentials are probably due to decomposition products of unclear composition. Table 3. Absorption Data of HBOXAM and BOXAM Nickel Complexesa complex

λ1

λ2

λ3

λ4

λ5

[(BOXAM)Ni(CH3)] [(BOXAM)Ni(CH3)]+ [(BOXAM)Ni(CF3)] [(BOXAM)Ni(CF3)]+ [(BOXAM)NiCl] [(BOXAM)Ni(Cl)]+ [(BOXAM)Ni(THF)]+ (HBOXAM)

224, 243 210, 242 239, 276 223 220, 283 280 226 sh, 283 255

305 323 326 324 333 330 333 304

349 373 379 395 333 394 382 358

443 445 436 440 458 455 450

535 670 520 630 680 527 620

a

Absorption maxima in nm, measured in THF/nBu4PF6 solutions. Oxidized species were generated by in situ electrolysis in an OTTLE cell.

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A closer look at the collected absorption data in Table 3 shows that the absorption spectra of the parent complexes [(BOXAM)Ni(R)] are characterized by several bands of medium intensity in the visible to UV range (300−700 nm; λ2−λ5 in Table 3) and intense absorptions in the UV (λ1). The long-wavelength bands (λ5) for the CH3 and CF3 derivatives lie in the same range (535 and 521 nm, respectively) and explain the red color of the compounds, while the Cl derivative and the cationic complex [(BOXAM)Ni(THF)]+ deviate markedly, absorbing at 620 and 630 nm, respectively, and consequently have a green color. The UV bands (λ1) can be assigned to intraligand (π−π*) transitions in the BOXAM ligand, since they also occur for the protonated ligand HBOXAM. The two further bands λ2 and λ3 for HBOXAM can also be observed in the nickel complexes and were assigned to further BOXAMcentered intraligand transitions. The long-wavelength bands λ4 and λ5 both undergo red shifts along the series CF3 < CH3 < THF < Cl. Both absorption bands are assigned to ligand (BOXAM−) to metal (Ni(II)) charge transfer (LMCT) transitions. Stabilization of the nickel LUMO by the Cl

CONCLUSIONS The homolytical bond splitting and formation of R• radicals (R = CF3, CH3, Cl) following one-electron oxidation of the nickel complexes [(BOXAM)Ni(R)] (BOXAM = bis((4-isopropyl4,5-dihydrooxazol-2-yl)phenyl)amine) could be established by spin trapping experiments using N-tert-butyl-α-phenylnitrone (PBN). The splitting reaction was also monitored by UV−vis− near-IR absorption spectroelectrochemistry, showing the formation of the “byproduct” complex [(BOXAM)Ni(THF)]+ in THF solution from the radical cationic complexes [(BOXAM)Ni(R)]•+. Furthermore, the character of the undissociated complex radicals [(BOXAM)Ni(R)]•+ was studied by EPR and UV−vis−near-IR spectroelectrochemistry, revealing inter alia largely ligand-centered oxidation. DFT calculations on ground and oxidized states confirm the largely ligand-centered character of the radical complexes [(BOXAM•)NiII(R)]•+. Furthermore, the DFT calculations help to understand the conflicting observations that, while the electrochemical oxidation potentials place the CF3 complex F

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alongside with the Cl derivative and the cationic THF complex [(BOXAM)Ni(THF)]+, pointing to an electron-withdrawing character of CF3, the long-wavelength absorption bands (=HOMO−LUMO transition) of CF3 and CH3 are very similar and are both far higher in energy, in comparison to those for the Cl complex and the cationic THF complex. The DFT calculated HOMO and LUMO energies are completely in line with both findings and show that while the HOMO energies (locus of the oxidation) are similar for CF3 and Cl, the HOMO−LUMO gaps are similar for CH3 and CF3.

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EXPERIMENTAL SECTION

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ASSOCIATED CONTENT

General Considerations. All manipulations were performed using standard Schlenk and high-vacuum techniques or in a nitrogen-filled drybox, unless otherwise noted. Solvents were dried using a MBRAUN MB SPS-800 solvent purification system. Electrochemical experiments were carried out in 0.1 M nBu4NPF6 solutions using a three-electrode configuration (glassy-carbon electrode, Pt counter electrode, Ag/AgCl reference) and an Autolab PGSTAT30 potentiostat and function generator. Data were processed using GPES 4.9 (General Electrochemical System Version 4.9). The ferrocene/ferrocenium couple (FeCp2/FeCp2+) served as internal reference. Spectroelectrochemical investigations (UV−vis−near-IR) were performed at ambient temperature using an OTTLE (optical transparent electrochemical) cell designed by J. Fiedler, Prague, Czech Republic.45 UV−vis−near-IR absorption spectra were recorded using a Varian Cary50 Scan photospectrometer. ESR spectra were recorded in the X-band on a Bruker ELEXSYS 500E instrument equipped with a Bruker Variable Temperature Unit ER 4131VT (500−100 K). g values were determined using a dpph sample. Spectral simulation was performed using Bruker SimFonia. Quantum Chemical Calculations. All calculations were performed using density functional theory as implemented in the Turbomole6.346 package using the resolution of identity (RI) approximation.47−49 The unconstrained geometry optimizations were performed at the (RI-)BP86/SV(P) level. All minima were confirmed as such by the absence of imaginary frequencies.

S Supporting Information *

Figures, text, and a table giving cyclic voltammograms of [(BOXAM)Ni(THF)](OAc), further EPR spectra, results from DFT calculations, and UV−vis−near-IR absorption spectra from spectroelectrochemical investigations. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (A.K.); [email protected] (D.A.V.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS C.H. and A.K. are grateful for support by the DFG (DFG KL 1194/4-1 and 5-1). D.A.V. thanks the Office of Basic Energy Sciences of the U.S. Department of Energy (DE-FG0207ER15885) and the National Science Foundation (CHE0822523) for support of this work.

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REFERENCES

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