Synthesis and Characterization of Aluminum ... - ACS Publications

Oct 29, 2015 - Open Access ... Andrew M. Poitras†, Justin A. Bogart‡, Bren E. Cole‡, Patrick J. Carroll‡, Eric J. Schelter‡, and Christopher...
0 downloads 0 Views 3MB Size
Download PDF
Article pubs.acs.org/IC

Synthesis and Characterization of Aluminum Complexes of RedoxActive Pyridyl Nitroxide Ligands Andrew M. Poitras,† Justin A. Bogart,‡ Bren E. Cole,‡ Patrick J. Carroll,‡ Eric J. Schelter,‡ and Christopher R. Graves*,† †

Department of Chemistry & Biochemistry, Albright College, 13th & Bern Streets, Reading, Pennsylvania 19612, United States P. Roy, Diana T. Vagelos Laboratories, Department of Chemistry, University of Pennsylvania, 231 South 34th Street, Philadelphia, Pennsylvania 19104, United States

‡

S Supporting Information *

ABSTRACT: The aluminum complexes (RpyNO−)2AlCl (RpyNO− = N-tert-butyl-N-(2-pyridyl)nitroxyl; R = H (1), CH3 (2), CF3 (3)) were prepared in 80−98% yield through the protonolysis reaction between the pyridyl hydroxylamine ligand precursors RpyNOH and dimethylaluminum chloride. Complex 1 was also prepared using a salt metathesis route in 92% yield. Complexes 1−3 were characterized using 1H and 13C NMR spectroscopies. Single-crystal X-ray diffraction analysis of the complexes revealed that 1−3 are isostructural, with the AlIII cation in all cases being five coordinate with distorted square pyramidal geometries. The geometry of complex 1 was studied using DFT, which showed primarily ligand-based frontier molecular orbitals. Reaction of 1 with NaOt-Bu gave (pyNO−)2AlOt-Bu (4), while reaction of 1 with AgBPh4 gave [(pyNO−)2Al(THF)2][BPh4] (5) in 54% and 87% yields, respectively. Compounds 4 and 5 were both characterized using 1H and 13C NMR spectroscopies and compound 5 by X-ray diffraction. Complexes 1−5 were also characterized by UV−vis electronic absorption spectroscopy and electrochemistry. The cyclic voltammograms of the complexes show two separate oxidation process, the potentials of which are dependent on both the substitution pattern of the RpyNO− ligands and the anion that completes the aluminum coordination sphere. A correlation was determined between the chemical shift of the t-Bu of the RpyNO− ligand in the 1H NMR spectroscopy and the potentials of the redox events for complexes 1−4.

■

INTRODUCTION The development of aluminum coordination complexes implementing redox-active and noninnocent ligands contributes to the expansion of the portfolio of aluminum-based reaction chemistry.1 Seminal work from the Berben group has demonstrated that redox-active ligand aluminum complexes can enable redox and ligand-facilitated reactivity and catalysis.2 We have similarly been interested in the synthesis of aluminum complexes of redox-active ligands and recently reported the synthesis of Al-α-diimine complexes across all three ligand oxidation states.3 Reported complexes in this area of chemistry have typically involved chelating nitrogen donor ligands, including 2,2′-bipyridine,4 α-diimine,3,5 BIAN6 (BIAN = 1,2bisiminoacenaphthylene), iminopyridine,7 and bis(imino)pyridine8 ligands. Conversely, redox-active ligand aluminum complexes supported by non-nitrogen heteroatoms are much less common. The series of aluminum ONO pincer ligand (ONOH3 = bis(3,5-di-tert-butyl-2-phenol)amine) complexes reported by the Heyduk group9 represent one example. Recently, the Hayton group reported the synthesis of the AlCl3(η1-TEMPO) (TEMPO = 2,2,6,6-tetramethylpiperidineN-oxyl) complex and demonstrated its competence as an oxidant toward various organic molecules.10 © XXXX American Chemical Society

We recently expanded our interest to investigate the coordination chemistry of nitroxide-based ligands to AlIII. The nitroxide functional group is well-known to exist over three oxidation states: The fully reduced form of the nitroxide group, the aminoxyl anion (NO−), can be oxidized by one electron to the neutral, radical form (NO0), which in turn can be further oxidized to the give the oxoammonium cation (NO+) (Figure 1). Three of us have recently developed chemistry for a suite of pyridyl-hydroxylamines and demonstrated their redox activities.11 The redox energetics of these compounds are predictable and span a wide potential window dependent on the substitution pattern of the pyridine ring. We hypothesized

Figure 1. Nitroxide functional group across three oxidation states. Received: August 25, 2015

A

DOI: 10.1021/acs.inorgchem.5b01941 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry that the Al-pyridyl nitroxide complexes would span a similar window and offer a set of structurally similar compounds with variable redox properties. Herein, we report the synthesis of a family of aluminum complexes supported by pyridyl nitroxide ligands. The electrochemical characterization of the complexes demonstrates that the redox properties can be tuned through the substitution pattern of the ligand.

■

RESULTS AND DISCUSSION Synthesis of Aluminum Complexes 1−5. Reaction of 2 equiv of a 2-(t-butyl-hydroxylamine)pyridine (RpyNOH) with AlMe 2 Cl in toluene cleanly affords the [κ 2 −N py ,O-2( t ‑ B u NO − )py R ] 2 AlCl complexes (pyNO − ) 2 AlCl (1), (CH3pyNO−)2AlCl (2), and (CF3pyNO−)2AlCl (3) in 80−98% yield (Scheme 1). Complex 1 can also be prepared through salt Scheme 1. Synthesis of the Al-Pyridyl Nitroxide Complexes 1−3

Figure 2. Inverse correlation between 1H NMR chemical shift of the tBu protons and Hammett parameter (σp) of the RpyNO− ligand substituent for complexes 1−3.

The 13C NMR spectra of 1−3 all have characteristic signals for the t-Bu group, with resonances at ∼28 ppm (C(CH3)3) and ∼62 ppm (C(CH3)3). All three complexes have five aromatic signals in their 13C NMR spectra. In the case of 3, several of the aromatic signals are quartets, arising from C−F coupling. This coupling is also observed for the CF3 resonance. Compound 3 was also characterized by 19F NMR spectroscopy; a single resonance was observed at −62.18 ppm, which is slightly upfield from the corresponding signal in the 19F NMR of the free ligand.11 We performed metathesis reactions on compound 1. Reaction of 1 with 1 equiv of NaOt-Bu in THF at −25 °C gives (pyNO−)2AlOt-Bu (4) as a yellow solid in 54% yield after workup and crystallization from pentane at −25 °C (Scheme 2). NMR analysis of the crude reaction products indicated 4 as

metathesis. Reaction of ligand precursor with NaN(SiMe3)2 in tetrahydrofuran (THF) followed by addition of 0.5 equiv of AlCl3 affords 1 in 92% yield. All three compounds were isolated as yellow solids that are indefinitely stable in the solid state when stored under an N2 environment at −25 °C. Complexes 1−3 were readily characterized by 1H and 13C NMR spectroscopies. The 1H NMR spectra of 1−3 in C6D6 show neither the O−H signal of the ligand precursor nor the Al−CH3 signal, indicating that both protonation reactions have occurred fully in all cases. The remaining ligand signatures are readily assignable and in the expected chemical ranges, indicating that the complexes are diamagnetic with an AlIII cation supported by fully reduced nitroxide ligands across all three complexes. In all cases, there is a single set of ligand-based resonances, indicating symmetry equivalent RpyNO− ligands. All three complexes have a single resonance attributable to the t-Bu protons, which appear as singlets. Compound 1 has four unique aromatic resonances that integrate with equal intensities and all appear as multiplets. The 1H NMR spectra of 2 and 3 both have three unique aromatic resonances of equal intensities. While the ligand precursors RpyNOH all have resonances assignable to the t-Bu protons at ∼1.30 ppm, there is a systematic trend in chemical shift for the t-Bu protons in the 1H NMR of 1−3. An upfield change in the t-Bu resonance in the 1H NMR of 3 (δ = 1.18 ppm) is observed relative to 1 (δ = 1.35 ppm). Conversely, there is a downfield change in the signal in the 1H NMR of 2 (δ = 1.44 ppm). An inverse correlation is present between the chemical shift for the t-Bu resonance and the Hammett parameter (σp) 12 of the substituent on the aromatic ring (Figure 2). A similar correlation is seen in the redox potentials observed in the cyclic voltammograms of the complexes (vide infra), suggesting that the t-Bu chemical shift is a reflection of the overall electronic properties of the Al−nitroxide complexes.

Scheme 2. Synthesis of Complexes 4 and 5 by Metathesis Reactions with 1

the predominant product. The low yield obtained after purification is a reflection of the high solubility of the compound in hydrocarbon solvents. The 1H NMR spectrum of 4 has two t-Bu signals integrating in a 2:1 ratio that correspond to the pyridyl nitroxide ligands and the alkoxide ligand, respectively, as well as four unique aromatic resonances of equal integration. The 13C NMR of 4 is similar to that of 1, with the addition of an extra set of resonances corresponding to the carbon atoms of the Ot-Bu group. Our attempts to oxidize 1 with either CuCl or AgCl to form a complex of the type (pyNO − )(pyNO 0 )AlCl 2 were B

DOI: 10.1021/acs.inorgchem.5b01941 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Table 1. Crystallographic Data for Complexes 1−3 and 5 formula a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z FW (g/mol) space group T (K) λ (Å) Dcalc (Mg m−3) μ (mm−1) R1 (I > 2σ(I)) wR2 (all data) Tmax/Tmin GOF (F2)

1

2

(3·toluene0.5)×2

5

C18H26N4O2ClAl 8.5966(10) 23.621(3) 9.6039(11) 90 97.033(6) 90 1935.5(4) 4 392.86 P21/n 100(1) 0.71073 1.348 0.263 0.0320 0.0833 0.7456/0.6993 1.016

C20H30N4O2ClAl 17.6435(6) 8.6012(3) 44.3270(16) 90 93.585(2) 90 6713.7(4) 12 420.91 P21/n 100(1) 0.71073 1.249 0.232 0.0373 0.0916 0.7456/0.6827 1.034

C47H56N8O4F12Cl2Al2 11.8663(9) 8.8006(6) 26.587(2) 90 100.472(4) 90 2730.2(3) 2 1149.86 P21/n 100(1) 0.71073 1.399 0.241 0.1257 0.3443 0.7456/0.4692 1.141

C50H62N4O4BAl 13.214(2) 12.694(2) 27.138(5) 90 102.217(9) 90 4449.0(13) 4 820.83 P21/c 100(1) 0.71073 1.225 0.095 0.0474 0.1266 0.7456/0.6961 1.008

unsuccessful, leaving only unreacted 1. Reaction of 1 with WCl6 produced free ligand and otherwise intractable products. However, reaction of 1 with 1 equiv of AgBPh4 in THF at room temperature gave [(pyNO−)2Al(THF)2][BPh4] (5) in 87% yield. The 1H NMR spectrum of 5 was collected in THFd8 and displayed the expected four aromatic signals and single tBu resonance (δ = 1.60 ppm) assignable to the protons in the pyNO− ligands. In addition, three aromatic signals for the BPh4− anion and resonances for the THF molecules were observed. Solid-State Structures of 1−3 and 5. The structures of the complexes 1−3 assigned by NMR spectroscopy were corroborated by X-ray crystallography. Crystallographic data are provided in Table 1, and selected bonding metrics for the complexes are provided in Table 2. Single crystals of 1 were grown from a hexanes solution at −25 °C (Figure 3). Single crystals of 2 were grown from a THF/pentane mixture at −25

Figure 3. Solid-state structure of (pyNO−)2AlCl (1). Ellipsoids are projected at 30% probability, and H atoms are omitted for clarity.

°C. The molecule crystallizes with three independent halfmolecules in the asymmetric unit. The three molecules are very similar and the bond distances and angles are effectively identical, so the discussion will only focus on the data for the molecule composed of aluminum atom 1 (Figure 4). Single crystals of 3 were grown from a THF/pentane mixture at −25 °C (Figure 5) and crystallize with a half molecule of interstitial

Table 2. Selected Experimental and Calculated Bond Distances (Å) and Angles (deg) for 1 and 2 1exp Al(1)−Cl(1) Al(1)−O(1) Al(1)−O(2) Al(1)−N(1) Al(1)−N(3) N(2)−O(1) N(4)−O(2) O(1)−Al(1)−N(1) O(1)−Al(1)−N(3) O(2)−Al(1)−N(1) O(2)−Al(1)−N(3) O(1)−Al(1)−O(2) N(1)−Al(1)−N(3) O(1)−Al(1)−Cl(1) O(2)−Al(1)−Cl(1) N(1)−Al(1)−Cl(1) N(3)−Al(1)−Cl(1)

2.2053(6) 1.7842(10) 1.7844(10) 1.9488(12) 1.9538(12) 1.4122(14) 1.4073(14) 82.02(5) 91.00(5) 82.02(5) 82.21(5) 141.55(5) 159.47(5) 110.12(4) 108.32(4) 99.21(4) 101.33(4)

1theory 2.1967 1.8198 1.8119 1.9984 1.9998 1.3891 1.3971 81.62 90.05 91.19 81.32 129.69 161.38 115.01 115.29 99.23 99.39

2exp 2.1905(5) 1.7881(11) 1.7835(11) 1.9628(12) 1.9699(12) 1.4008(14) 1.4199(15) 82.35(5) 87.06(5) 91.64(5) 82.32(5) 136.54(5) 157.29(5) 111.07(4) 112.34(4) 100.52(4) 102.03(4)

Figure 4. Solid-state structure of (CH3pyNO−)2AlCl (2). Ellipsoids are projected at 30% probability, and H atoms are omitted for clarity. C

DOI: 10.1021/acs.inorgchem.5b01941 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 5. Solid-state structure of (CF3pyNO−)2AlCl (3). Ellipsoids are projected at 30% probability. H atoms and an interstitial toluene molecule have been omitted for clarity.

Figure 6. Solid-state structure of the [(pyNO−)2Al(THF)2]+ cation of 5. Ellipsoids are projected at 30% probability, and H atoms are omitted for clarity. Selected bond distances (Å): Al(1)−O(1), 1.8127(12); Al(1)−O(2), 1.8138(13); Al(1)−O(3), 1.9726(13); Al(1)−O(4), 1.9736(12); Al(1)−N(3), 1.9727(15); Al(1)−N(1), 1.9806(15); N(2)−O(1), 1.4107(17); N(4)−O(2), 1.3885(18). Selected bond angles (deg): O(1)−Al(1)−O(2), 177.32(6); O(1)−Al(1)−O(3), 86.23(5); O(2)−Al(1)−O(3), 95.74(6); O(1)−Al(1)−O(4), 96.60(6); O(2)−Al(1)−O(4), 85.40(5); O(3)−Al(1)−O(4), 84.75(5); O(1)−Al(1)−N(3), 96.73(6); O(2)−Al(1)−N(3), 81.46(6); O(3)−Al(1)−N(3), 90.69(6); O(4)−Al(1)−N(3), 165.59(6); O(1)−Al(1)−N(1), 81.70(6); O(2)−Al(1)−N(1), 96.58(6); O(3)−Al(1)−N(1), 165.70(6); O(4)−Al(1)−N(1), 89.04(6); N(3)−Al(1)−N(1), 98.28(6).

toluene in the unit cell. The quality of the data for 3 is enough to establish connectivity but not good enough to discuss or make comparisons in bonding metrics. All three complexes 1−3 are five-coordinate at the aluminum cation, with two Npy,O-bound bidentate pyridyl nitroxide ligands and a chloride comprising the coordination sphere. In all three complexes, the two pyridyl nitroxide ligands are arranged in the basal plane, with the O−Al−O and N−Al−N atoms in mutually trans arrangements, giving the complexes near C2 symmetry and supporting the equivalent ligands observed in the 1H and 13C NMR spectra. The AlIII cations in 1 and 2 are best described as distorted square pyramidal according to the τ5 parameter,13 with τ5 values of 0.30 and 0.35, respectively. The average N−O distance in 1 and 2 is 1.41 Å, which is indicative of a N−O single bond14 and supports our ligand oxidation state assignment. All of the N−O distances in 1 and 2 are slightly longer than the N−O bond lengths observed for the pyridine nitroxide cerium complexes {CeIII(μ-RpyNO−)(RpyNO−)2}2 (N−Oave = 1.37 Å) and CeIV(RpyNO−)4 (N−Oave = 1.38 Å).15 The average Al−O distance in 1−3 is 1.79 Å, which compares well to the analogous distance observed for the AlCl3(η1-TEMPOH) compound (TEMPOH = 2,2,6,6-tetramethylpiperdinium-Noxy, Al−O = 1.7745(8) Å) prepared by Hayton and coworkers,10b which also has a N−O−Al nitroxide bond. At 2.12053(16) Å (for 1) and 2.1905(5) (for 2), the Al−Cl bond lengths for the complexes are unremarkable and are in the range of other terminal Al−Cl bonds reported for fivecoordinate aluminum(III) complexes.16 Likewise, the Al−N bond lengths in 1 and 2 are in the range of other structurally characterized compounds with Al−pyridine bonds.17 Single crystals of 5 were grown from slow evaporation of pentane into a THF solution at −25 °C. The aluminum ion in the [(pyNO−)2Al(THF)2]+ cation is six-coordinate with two bidentate pyridyl nitroxide ligands and two THF molecules making up the primary coordination sphere (Figure 6). The ion has a distorted octahedral geometry, with the O(1)−Al(1)− O(2) angle of 177.32° being the largest and the O−Al−N angles formed between a given pyNO− ligand and the aluminum ion (O(1)−Al(1)−N(1) = 81.46(6)°; O(2)− Al(1)−N(3) = 81.70(6)°) being the smallest. The two THF molecules are coordinated in a cis geometry, giving the ion near C2 symmetry. The Al−O and Al−N distances in 5 are slightly longer than the corresponding average distances for 1 and 2, which we attribute to the differences in coordination number

between the complexes. The N−O distances for 5 are within the range of those observed for 1 and 2, supporting that the ligand oxidation state has not changed. Density Functional Theory (DFT) Studies. The full geometry of compound 1 was optimized using density functional theory (Figure S12 and Table S1). The computed bond distances and angles were found to be in good agreement with those obtained in the X-ray data (Table 2). All of the computed metrics centered around the aluminum ion are within 0.05 Å and 10°, with the exception of the O(1)−Al(1)− O(2) angle. The charge distribution of complex 1 was studied by the natural bonding orbital (NBO) method, with selected data given in Table 3. The aluminum(III) cation has a large Table 3. Charge Distribution of Complex 1 Al N(1) N(2) O(1)

1.816 −0.672 −0.092 −0.750

Cl N(3) N(4) O(2)

−0.611 −0.664 −0.108 −0.765

positive charge, while the bonding heteroatoms and chloride have negative charges. The pyridine fragment of both pyNO− ligands is polarized toward the nitrogen atoms as signified by the negative charge on N(1) and N(3) atoms and positive charges throughout the rest of the pyridine ring. We also examined the molecular orbitals of complex 1 (Figure 7). The HOMO is ligand based, with electron density primarily delocalized across the N−O and pyridine groups of one of the pyridyl nitroxide ligands with minimal contribution from the other pyNO and chloride ligands. The HOMO−1 is essentially the symmetry equivalent of the HOMO, with electron density delocalized primarily over the other pyridyl nitroxide ligand. The energy difference between the HOMO D

DOI: 10.1021/acs.inorgchem.5b01941 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 7. Frontier molecular orbitals of the (pyNO−)2AlCl (1) complex.

orbitals, with a small amount of contribution from the N−O group in the virtual orbital for the 251 nm transition of 1. Electrochemistry. The cyclic voltammograms of 1−3 were collected in THF and exhibit two features (Figure 9 and Table

and HOMO−1 is 0.229 eV. The LUMO is localized primarily on one of the pyridine rings, with minimal contribution from the N−O moieties. The LUMO+1, which is 0.071 eV higher in energy, has electron density primarily across the other pyridine ligand. Absorption Spectra. The absorption spectra of compounds 1−5 were collected in dichloromethane (Figure 8).

Figure 9. Cyclic voltammograms of compounds 1−5 recorded in 0.1 M [n-Pr4N][BArF] THF solutions.

4), which we attribute to two sequential one-electron oxidation processes corresponding to the N−O−/N−O• couples of each R pyNO− ligand. We do not observe the other two oxidation processes corresponding to the N−O•/NO+ couples in the electrochemical window. The observed processes are quasireversible (for 1 and 2) or irreversible (for 3) with diminished

Figure 8. UV−vis spectra of 1.5 × 10−4 M solutions of 1−5 in CH2Cl2.

The spectra for compounds 1−3 appear similar, with a dominant feature at ∼280 nm (εM ≈ 14 000−15 000) and a smaller feature at ∼345 nm (εM ≈ 3500−4000). The spectrum of 4 is also similar, although there is a bathochromic shift in the smaller feature by ∼20 nm relative to 1−3. The UV−vis spectrum of compound 5 has the feature at ∼280 nm, but the smaller, lower energy feature is absent. TD-DFT computations were conducted to assist in the assignments of the electronic transitions. The results for compound 1 show four major vertical excitations, two intense features at 251 and 261 nm and two less intense features at 337 and 355 nm (see the Supporting Information). All four bands involve transitions from ligand-based orbitals to primarily pyridine-based π*

Table 4. Electrochemical Potentials (V vs Fc/Fc+) for Complexes 1−5 pyNO−pyNO−/ pyNO0pyNO− 1 2 3 4 5 E

pyNO0pyNO−/ pyNO0pyNO0

Epa

Epc

Epa

Epc

ΔEpa

0.34 0.27 0.60 0.24 0.55

0.30 0.22

0.70 0.56 0.90 0.66 0.88

0.50 0.45

0.36 0.34 0.30 0.42 0.33

0.19 0.54

0.44

DOI: 10.1021/acs.inorgchem.5b01941 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

As the compound becomes easier to oxidize, as signified by a shift in redox potential to more negative values, there is a similar downfield shift for the resonance of the t-Bu pyNO− protons. Similarly, a shift in the redox potentials to more positive values correlates to an upfield shift in t-Bu resonances. The chemical shift also correlated to the Hammett parameter (vide supra), suggesting that the chemical shift of the t-Bu resonance is indicative of the overall electronic properties of the complexes.

return reduction features in all cases. There is an approximately equal spacing between the two redox events across the three complexes, with separations for the Epa values in the range of 0.30−0.36 V. As with the pyridyl nitroxide ligand precursors,11 there is a systematic variation in the redox potentials for both features in the CVs for 1−3 with changing substituent on the pyridine ring. Complex 2 has the most negative potentials, as expected from the electron-donating CH3 group (σp = −0.1712), while the electron-withdrawing CF3 group (σp = 0.5412) shifts the features for 3 to more positive potentials. Similar tuning of redox events through ligand substitution was reported by the Heyduk group for a series of tantalum complexes supporting tris-amido-based ligands.18 In all three complexes 1−3, coordination to the AlIII cation shifts the redox events to more positive potential relative to free ligand,11 indicating that the Lewis acidic metal stabilizes the reduced form of the ligand. For example, the first oxidation in 1 occurs at ∼0.30 V, while the pyNOH/pyNO0 couple for free ligand comes at E1/2 = −0.39 V (vs Fc/Fc+). This is consistent with chemistry reported by the Hayton group who have recently shown that coordination of the TEMPO radical to AlX3 (X = Cl− or Br−) activates the compound and results in a more oxidizing reagent.10 The cyclic voltammograms of 4 and 5 were also collected in THF. Compound 4 has two quasi-reversible redox events separated by 0.42 V, while 5 has one quasi-reversible and one irreversible redox event separated by ∼0.33 V. Relative to 1, the pyNO−pyNO−/pyNO0pyNO− couple for 4 is negatively shifted by ∼0.1 V. The Berben group reported a similar dependence on redox potentials for the series of (IP−) 2 Al−X (IP = iminopyridine; X = monoanionic ligand) complexes with variable X ligands.7b Switching the inner sphere chloride ligand in 1 to the noncoordinating anion BPh4− in complex 5 results in a shift by ∼0.2 V for both events to a more positive potential. A similar dependence on coordination sphere was seen in our Al-α-diimine system3a between the (LMes2−)AlCl(THF) and (LMes−)AlCl2 ((LMes = N,N′-bis[2,4,6-trimethylphenyl]-2,3dimethyl-1,4-diazabutadiene) complexes, as well as in Berben’s iminopyridine system.7c Interestingly, there is a correlation between the chemical shift of the t-Bu group of the pyridyl nitroxide ligand and the potential of the redox events for complexes 1−4 (Figure 10).

■

CONCLUSIONS AND OUTLOOK We have demonstrated that pyridyl nitroxide ligands can be coordinated to an AlIII ion to afford a new class of aluminum complexes supporting redox-active ligands. The electrochemical characterization of the complexes shows two redox events for all complexes, and the oxidation potentials of the redox events can be tuned through the substituent on the pyNO− ligand or through varying the counterion in the AlIII coordination sphere. We are currently investigating the reactivity profiles for the complexes through the development of one- and two-electron oxidation chemistries. Additionally, we are developing aluminum complexes of multidentate nitroxide ligands to provide more robust complexes with reversible electrochemical behaviors.

■

EXPERIMENTAL SECTION

Physical Measurements. 1H and 13C NMR spectra were recorded at ambient temperature in C6D6 using a Varian 400 MHz spectrometer (399.78 MHz for 1H, 100.52 MHz for 13C). Chemical shifts were referenced to residual solvent. s = singlet, d = doublet, t = triplet, q = quartet, m = multiplet, dd = doublet of doublets, b = broad. The 19F NMR spectrum of 3 was collected using a Varian 400 MHz spectrometer (376.17 MHz for 19F). The chemical shift was referenced to an external fluorobenzene in C6D6 standard set to −113.15 ppm. Elemental analyses were performed either at the University of California, Berkeley Microanalytical Facility, on a PerkinElmer Series II 2400 CHNS analyzer (for 1) or at Complete Analysis Laboratories on a CHN analyzer by Thermo Electron (for 2−5). Electrochemical measurements were done in a glovebox under a dinitrogen environment using a BASi Epsilon-EC Potentiostat/Galvanostat. A glassy carbon working electrode, a platinum wire auxiliary electrode, and a silver wire plated with AgCl as a quasi-reference electrode were utilized. Potentials were reported versus ferrocene, which was added as an internal standard for calibration at the end of each run. Solutions employed during these studies were ∼3 mM in analyte and 100 mM in [n-Pr4N][BArF] (BArF− = B(3,5-CF3)2-C6H3)4−) in ∼3 mL of THF. All data were collected in a positive-feedback IR compensation mode. Absorbance spectra were collected using an Agilent 8453 UV−vis spectroscopy system in anhydrous dichloromethane at ambient temperature and pressure. Absorbances are reported in nm. X-ray Structure Determination. X-ray diffraction data were collected on a Brüker APEX II CCD diffractometer employing graphite-monochromated Mo-Kα radiation. Rotation frames were integrated using SAINT,19 producing a listing of unaveraged F2 and σ(F2) values, which were then passed to the SHELXTL20 program package for further processing and structure solution. The intensity data were corrected for Lorentz and polarization effects and for absorption using SADABS.21 The structures were solved by direct methods and refined by full-matrix least-squares based on F2 using SHELXL-97.22 All non-hydrogen atoms were refined anisotropically, and hydrogen atoms were refined using a riding model. Computational Details. The structure optimization of 1 was performed with the Gaussian 09,23 revision D.01 program using the B3LYP hybrid DFT method and the 6-31G basis set on all atoms. Geometry optimization was performed using the crystal structure geometry as the initial starting point. Frequency calculations found no imaginary frequencies, confirming that the optimized structures were

Figure 10. Linear correlation between 1H NMR chemical shift of the tBu protons and both the first (red) and the second (blue) N−O−/N− O• oxidation potentials. F

DOI: 10.1021/acs.inorgchem.5b01941 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry minima. Molecular orbitals were rendered with the program Chemcraft v1.8 at an isovalue of ±0.03 au. NBO analyses were performed using the NBO6 package.24 Preparation of Compounds. All reactions and manipulations were performed under an inert atmosphere (N2) using standard Schlenk techniques or in a Vacuum Atmospheres, Inc. Nexus II drybox equipped with a molecular sieves 13X/Q5 Cu-0226S catalyst purifier system. Glassware was dried overnight at 150 °C before use. C6D6 and THF-d8 were purchased from Sigma-Aldrich and stored over potassium metal prior to use. Tetrahydrofuran, hexanes, pentane, and toluene were purchased from Fisher Scientific. These solvents were sparged for 20 min with dry argon and dried using a commercial two-column solvent purification system comprising columns packed with Q5 reactant and neutral alumina, respectively (for hexanes, toluene, and pentane), or two columns of neutral alumina (for THF). The ligand precursors N-tert-butyl-N-(2-pyridyl)hydroxylamine, Ntert-butyl-N-2-(5-methylpyridyl)-hydroxylamine, and N-tert-butyl-N-2[5-(trifluoromethyl)pyridyl]-hydroxylamine were prepared as previously reported.11 AgBPh425 and [n-Pr4N][BArF]26 were prepared according to literature procedures. All other reagents were purchased from commercial sources and used as received. Synthesis of (pyNO−)2AlCl (1) by Protonolysis. N-tert-Butyl-N(2-pyridyl)hydroxylamine (1.00 g, 6.0 mmol) was added to a flask equipped with a magnetic stirbar and dissolved in toluene (∼50 mL). A 1.0 M solution of dimethylaluminum chloride in hexanes (3.0 mL, 3.0 mmol) was added to the stirring solution, and the resulting mixture was stirred at room temperature. After 12 h, the reaction was filtered over a Celite-padded frit, and volatiles were removed from the filtrate. The resultant oily solid was triterated with pentane (3 × 10 mL) and dried under reduced pressure to give 1 as a yellow solid. Yield: 1.01 g, 2.6 mmol (86%). Synthesis of (pyNO−)2AlCl (1) by Salt Metathesis. N-tert-ButylN-(2-pyridyl)hydroxylamine (1.00 g, 6.0 mmol) was added to a flask equipped with a magnetic stirbar and dissolved in THF (∼25 mL). NaN(SiMe3)2 (1.24 g, 6.6 mmol) was added to the stirring solution in small portions over 0.5 h. The resulting reaction was stirred at room temperature for 4 h after which aluminum trichloride (0.40 g, 3.0 mmol) was added. The reaction was stirred for 12 h at room temperature after which the reaction was filtered over a Celite-padded frit and volatiles were removed from the filtrate. Crude product was dissolved in boiling hexanes (25 mL) and filtered over a Celite-padded frit. Solvents were removed from the filtrate to give 1 as a yellow solid. Yield: 1.09 g, 2.8 mmol (92%). Characterization Data for (pyNO−)2AlCl (1). 1H NMR (C6D6): δ 8.24 (m, 2H), 6.67 (m, 2H), 6.44 (m, 2H), 6.01 (m, 2H), 1.35 (s, 18H). 13C{1H} NMR (C6D6): δ 156.1, 143.4, 137.1, 110.6, 109.6, 61.4 (C(CH3)3), 28.5 (C(CH3)3). Anal. Calcd for C18H26AlClN4O2: C, 55.03; H, 6.67; N, 14.26. Found: C, 54.82; H, 6.80; N, 14.15. UV−vis spectrum (CH2Cl2), λmax (εM): 276 (13 820), 345 (3440). Synthesis and Characterization of (CH3pyNO−)2AlCl (2). N-tertButyl-N-2-(5-methylpyridyl)-hydroxylamine (0.18 g, 1.0 mmol) was added to a flask equipped with a magnetic stirbar and dissolved in toluene (∼25 mL). A 1.0 M solution of dimethylaluminum chloride in hexanes (0.50 mL, 0.50 mmol) was added to the stirring solution, and the resulting mixture was stirred at room temperature. After 12 h, the reaction was filtered over a Celite-padded frit, and volatiles were removed from the filtrate. The resultant oily solid was triterated with pentane (3 × 10 mL) and dried under reduced pressure to give 2 as a yellow solid. Yield: 0.17 g, 0.40 mmol (80%). 1H NMR (C6D6): δ 8.17 (bs, 2H), 6.60 (m, 2H), 6.54 (m, 2H), 1.69 (s, 6H), 1.44 (s, 18H). 13 C{1H} NMR (C6D6): δ 156.4, 142.1, 139.2, 120.7, 110.2, 61.6 (C(CH3)3), 28.6 (C(CH3)3), 16.9 (py-CH3). Anal. Calcd for C20H30AlClN4O2: C, 57.07; H, 7.18; N, 13.31. Found: C, 56.89; H, 7.35; N, 13.44. UV−vis spectrum (CH2Cl2), λmax (εM): 277 (13 630), 347 (4140). Synthesis and Characterization of (CF3pyNO−)2AlCl (3). N-tertButyl-N-2-[5-(trifluoromethyl)pyridyl]-hydroxylamine (0.23 g, 1.0 mmol) was added to a flask equipped with a magnetic stirbar and dissolved in toluene (∼25 mL). A 1.0 M solution of dimethylaluminum chloride in hexanes (0.50 mL, 0.50 mmol) was added to the

stirring solution, and the resulting mixture was stirred at room temperature. After 12 h, the reaction was filtered over a Celite-padded frit, and volatiles were removed from the filtrate. The resultant oily solid was triterated with pentane (3 × 10 mL) and dried under reduced pressure to give 3 as a yellow solid. Yield: 0.26 g, 0.49 mmol (98%). 1H NMR (C6D6): δ 8.53 (bs, 2H), 6.73 (dd, J = 2.4 Hz, J = 9.6 Hz, 2H), 6.07 (d, J = 9.6 Hz, 2H), 1.18 (s, 18H). 13C{1H} NMR (C6D6): δ 153.6, 141.7 (q, JC−F = 4.6 Hz), 133.0, 124.6 (q, JC−F = 269.2 Hz, CF3), 112.8 (q, JC−F = 34.5 Hz), 108.8, 62.0 (C(CH3)3), 28.0 (C(CH3)3). 19F{1H} NMR (C6H6): δ −62.18. Anal. Calcd for C20H24AlClF6N4O2: C, 45.42; H, 4.57; N, 10.59. Found: C, 45.55; H, 4.55; N, 10.82. UV−vis spectrum (CH2Cl2), λmax (εM): 283 (15 370), 346 (3760). Synthesis and Characterization of (pyNO−)2AlOt-Bu (4). Compound 1 (0.25 g, 0.64 mmol) was weighed into a flask with THF (∼25 mL), and the solution was cooled to −25 °C. In a separate flask, a solution of sodium tert-butoxide (0.060 g, 0.64 mmol) in THF (∼10 mL) was cooled to −25 °C. The alkoxide solution was added to the solution of 1, and the resultant reaction was stored at −25 °C. After 12 h, the reaction was filtered over a Celite-padded frit, and volatiles were removed from the filtrate. Crude reaction products were taken up into pentane (∼10 mL), filtered over Celite, and cooled to −25 °C. After 48 h, crystallized solid was collected, washed with cold pentane, and dried under reduced pressure to give 4 as a yellow solid. Yield: 0.15 g, 0.35 mmol (54%). 1H NMR (C6D6): δ 8.30 (m, 2H), 6.72 (m, 2H), 6.43 (m, 2H), 6.07 (m, 2H), 1.38 (s, 9H), 1.37 (s, 18H). 13C NMR (C6D6): δ 155.6, 144.2, 136.4, 109.3, 109.0, 67.0 (O− C(CH3)3), 60.6 (N−C(CH3)3), 34.1 (O−C(CH3)3), 28.4 (N− C(CH3)3). Anal. Calcd for C22H35AlN4O3: C, 61.38; H, 8.19; N, 13.01. Found: C, 61.06; H, 8.38; N, 13.02. UV−vis spectrum (CH2Cl2), λmax (εM): 286 (14 500), 366 (2500). Synthesis and Characterization of [(pyNO−)2Al(THF)2][BPh4] (5). Compound 1 (0.25 g, 0.64 mmol) was weighed into a flask equipped with a magnetic stir bar and was dissolved in THF (∼25 mL). AgBPh4 (0.27 g, 0.64 mmol) was added to the stirring solution, and the reaction was stirred at room temperature. After 12 h, the reaction was filtered over a Celite-padded frit, and volatiles were removed from the filtrate. The resultant solid was triterated with pentane (3 × 10 mL) and dried under reduced pressure to give 5 as a yellow solid. Yield: 0.44 g, 0.54 mmol (87%). 1H NMR (THF-d8): δ 7.71 (d, J = 5.6 Hz, 2H), 7.31 (t, J = 7.6 Hz, 2H), 7.24 (bm, 8H, B− Ph), 6.95 (d, J = 9.6 Hz, 2H), 6.81 (t, J = 6.8 Hz, 8H, B−Ph), 6.66 (t, J = 6.4 Hz, 4H, B−Ph), 6.34 (t, J = 6.8 Hz, 2H), 3.63 (m, 4H), 1.77 (m, 4H), 1.60 (s, 18H). 13C NMR (THF-d8): δ 165.4 (q, JC−B = 49 Hz), 154.7, 143.2, 139.3, 125.8 (q, JC−B = 3 Hz), 121.9, 110.9, 109.5, 61.6, 28.8. Anal. Calcd for C50H62AlBN4O4: C, 73.16; H, 7.61; N, 6.83. Found: C, 73.40; H, 7.61; N, 7.09. UV−vis spectrum (CH2Cl2), λmax (βM): 277 (12 890).

■

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b01941. 1 H and 13C NMR spectra of 1−5; 19F NMR spectrum of 3; tables of coordinates from geometry optimization of 1; visualization of the donor/acceptor orbitals for the TDDFT modeled electronic transitions; and full ref 23 (PDF) X-ray data for compounds 1−3, 5 (CIF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. G

DOI: 10.1021/acs.inorgchem.5b01941 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

■

Barbon, S. M.; Drover, M. W.; Dawe, L. N.; Kerton, F. M. Organometallics 2012, 31, 8145−8158. (Al−Cl = 2.2102(9) Å) (d) Munoz-Hernandez, M.-A.; Keizer, T. S.; Wei, P.; Parkin, S.; Atwood, D. S. Inorg. Chem. 2001, 40, 6782−6787 (Al−Cl = 2.182(4) Å, 2.1753(14) Å). (17) For a selection of Al−Npy distances, see: (a) Timoshkin, A. Y.; Bodensteiner, M.; Sevastianova, T. N.; Lisovenko, A. S.; Davydova, E. I.; Scheer, M.; Graßl, C.; Butlak, A. V. Inorg. Chem. 2012, 51, 11602− 11611. (Al−N = 1.930(2) Å, 1.935(3) Å) (b) Mason, M. R.; Smith, J. M.; Bott, S. G.; Barron, A. R. J. Am. Chem. Soc. 1993, 115, 4971−4984. (Al−N = 2.029(2) Å) (c) van Poppel, L. H.; Bott, S. G.; Barron, A. R. J. Chem. Soc., Dalton Trans. 2002, 3327−3332. (Al−N = 2.008(3) Å) (d) van Poppel, L. H.; Bott, S. G.; Barron, A. R. Aust. J. Chem. 2004, 57, 503−506 (Al−N = 2.089(1) Å). (18) Munha, R. F.; Zarkesh, R. A.; Heyduk, A. F. Inorg. Chem. 2013, 52, 11244−11255. (19) SAINT; Bruker AXS Inc.: Madison, WI, 2009. (20) SHELXTL; Bruker AXS Inc.: Madison, WI, 2009. (21) Sheldrick, G. M. SADABS; University of Göttingen: Göttingen, Germany, 2007. (22) Sheldrick, G. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, 64, 112−122. (23) Frisch, M. J.; et al. Gaussian 09; Gaussian, Inc.: Wallingford, CT, 2009. The full reference is available in the Supporting Information. (24) Glendening, E. D.; Badenhoop, J. K.; Reed, A. E.; Carpenter, J. E.; Bohmann, J. A.; Morales, C. M.; Landis, C. R.; Weinhold, F. NBO 6.0; Theoretical Chemistry Institute, University of Wisconsin: Madison, WI, 2013. (25) Balazs, G.; Cloke, F. G. N.; Green, J. C.; Harker, R. M.; Harrison, A.; Hitchcock, P. B.; Jardine, C. N.; Walton, R. Organometallics 2007, 26, 3111−3119. (26) Thomson, R. K.; Scott, B. L.; Morris, D. E.; Kiplinger, J. L. C. R. Chim. 2010, 13, 790−802.

ACKNOWLEDGMENTS C.R.G. thanks Albright College and the ACS-Petroleum Research Fund (PRF 52181-UNI3) for financial support of this work. E.J.S. acknowledges the Chemical Sciences, Geosciences, and Biosciences Division, Office of Basic Energy Sciences, Early Career Research Program of the U.S. Department of Energy, under Award no. DE-SC0006518 for support of this work. B.E.C. thanks the NSF-GRF program for financial support. We thank the U.S. National Science Foundation for support of the computing cluster (CHE-0131132) used in this work.

■

REFERENCES

(1) Berben, L. A. Chem. - Eur. J. 2015, 21, 2734−2742. (2) (a) Myers, T. W.; Berben, L. A. J. Am. Chem. Soc. 2011, 133, 11865−11867. (b) Myers, T. W.; Berben, L. A. J. Am. Chem. Soc. 2013, 135, 9988−9990. (c) Myers, T. W.; Berben, L. A. Chem. Commun. 2013, 49, 4175−4177. (d) Myers, T. W.; Berben, L. A. Chem. Sci. 2014, 5, 2771−2777. (e) Thompson, E. J.; Berben, L. A. Angew. Chem., Int. Ed. 2015, 54, 11642−11646. (3) (a) Cole, B. E.; Wolbach, J. P.; Dougherty, W. G., Jr.; Piro, N. A.; Kassel, W. S.; Graves, C. R. Inorg. Chem. 2014, 53, 3899−3906. (b) Koellner, C. A.; Piro, N. A.; Kassel, W. S.; Goldsmith, C. R.; Graves, C. R. Inorg. Chem. 2015, 54, 7139−7141. (4) Herzog, S.; Geisler, K.; Praekel, H. Angew. Chem. 1963, 75, 94. (5) (a) Cloke, F. G. N.; Dalby, C. I.; Henderson, M. J.; Hitchcock, P. B.; Kennard, C. H. L.; Lamb, R. L.; Raston, C. L. J. Chem. Soc., Chem. Commun. 1990, 1394−1396. (b) Baker, R. J.; Farley, R. D.; Jones, C.; Kloth, M.; Murphy, D. M. J. Chem. Soc., Dalton Trans. 2002, 3844− 3850. (c) Cloke, F. G. N.; Dalby, C. I.; Daff, P. J.; Green, J. C. J. Chem. Soc., Dalton Trans. 1991, 181−184. (6) Lukoyanov, A. N.; Fedushkin, I. L.; Hummert, M.; Schumann, H. Russ. Chem. Bull. 2006, 55, 422−428. (7) (a) Myers, T. W.; Berben, L. A. Inorg. Chem. 2012, 51, 1480− 1488. (b) Myers, T. W.; Holmes, A. L.; Berben, L. A. Inorg. Chem. 2012, 51, 8997−9004. (c) Myers, T. W.; Kazem, N.; Stoll, S.; Britt, R. D.; Shanmugam, M.; Berben, L. A. J. Am. Chem. Soc. 2011, 133, 8662− 8672. (8) (a) Jurca, T.; Dawson, K.; Mallov, I.; Burchell, T.; Yap, G. P. A.; Richeson, D. S. Dalton Trans. 2010, 39, 1266−1272. (b) Scott, J.; Gambarotta, S.; Korobkov, I.; Knijnenburg, Q.; De Bruin, B.; Budzelaar, P. H. M. J. Am. Chem. Soc. 2005, 127, 17204−17206. (c) Thompson, E. J.; Myers, T. W.; Berben, L. A. Angew. Chem., Int. Ed. 2014, 53, 14132−14134. (9) Szigethy, G.; Heyduk, A. F. Dalton Trans. 2012, 41, 8144−8152. (10) (a) Nguyen, T.-A. D.; Wright, A. M.; Page, J. S.; Wu, G.; Hayton, T. W. Inorg. Chem. 2014, 53, 11377−11387. (b) Scepaniak, J. J.; Wright, A. M.; Lewis, R. A.; Wu, G.; Hayton, T. W. J. Am. Chem. Soc. 2012, 134, 19350−19353. (11) Bogart, J. A.; Lee, H. B.; Boreen, M. A.; Jun, M.; Schelter, E. J. J. Org. Chem. 2013, 78, 6344−6349. (12) Hansch, C.; Leo, A.; Taft, R. W. Chem. Rev. 1991, 91, 165−195. (13) Addison, A. W.; Rao, T. N.; Reedijk, J.; Van Rijn, J.; Verschoor, G. C. J. Chem. Soc., Dalton Trans. 1984, 1349−1356. (14) Miessler, G. L.; Fisher, P. J.; Tarr, D. A. Inorganic Chemistry, 5th ed.; Pearon: Boston, 2014; pp 288−290. (15) (a) Bogart, J. A.; Lewis, A. J.; Boreen, M. A.; Lee, H. B.; Medling, S. A.; Carroll, P. J.; Booth, C. H.; Schelter, E. J. Inorg. Chem. 2015, 54, 2830−2837. (b) Bogart, J. A.; Lewis, A. J.; Medling, S. A.; Piro, N. A.; Carroll, P. J.; Booth, C. H.; Schelter, E. J. Inorg. Chem. 2013, 52, 11600−11607. (16) For representative Al−Cl bond lengths in five-coordinate aluminum complexes with a N,N,O,O coordination environment, see: (a) Johnstone, N. C.; Aazam, E. S.; Hitchcock, P. B.; Fulton, J. R. J. Organomet. Chem. 2010, 695, 170−176. (Al−Cl = 2.1610(16) Å) (b) Yu, R.-C.; Hung, C.-H.; Huang, J.-H.; Lee, H.-Y.; Chen, J.-T. Inorg. Chem. 2002, 41, 6450−6455. (Al−Cl = 2.170(6) Å) (c) Ikpo, N.; H

DOI: 10.1021/acs.inorgchem.5b01941 Inorg. Chem. XXXX, XXX, XXX−XXX