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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Synthesis and Characterization of Neutral Ligand α‑Diimine Complexes of Aluminum with Tunable Redox Energetics Henry H. Wilson,† Connor A. Koellner,‡ Zain M. Hannan,† Caroline B. Endy,‡ Mark W. Bezpalko,§ Nicholas A. Piro,§ W. Scott Kassel,§ Matthew D. Sonntag,‡ and Christopher R. Graves*,† †
Department of Chemistry and Biochemistry, Swarthmore College, 500 College Avenue, Swarthmore, Pennsylvania 19081, United States ‡ Department of Chemistry and Biochemistry, Albright College, 13th and Bern Streets, Reading, Pennsylvania 19612, United States § Department of Chemistry, Villanova University, 800 Lancaster Avenue, Villanova, Pennsylvania 19085, United States S Supporting Information *
ABSTRACT: The synthesis and full characterization of a series of neutral ligand α-diimine complexes of aluminum are reported. The compounds [Al(LAr)2Cl2)][AlCl4] [LAr = N,N′-bis(4-R-C6H4)-2,3-dimethyl-1,4-diazabutadiene] are structurally analogous, as determined by multinuclear NMR spectroscopy and solid-state X-ray diffraction, across a range of electron-donating [R = Me (2), tBu (3), OMe (4), and NMe2 (5)] and electron-withdrawing [R = Cl (6), CF3 (7), and NO2 (8)] substituents in the aryl side arm of the ligand. UV−vis absorption spectroscopy and electrochemistry were used to access the optical and electrochemical properties, respectively, of the complexes. Both sets of properties are shown to be dependent on the R substituent. Density functional theory calculations performed on the [Al(LPh)2Cl2)][AlCl4] complex (1) indicate primarily ligand-based frontier orbitals and were used to help support our discussion of both the spectral and electrochemical data. We also report the reaction of the LPh ligand with both AlBr3 and AlI3 and demonstrate a different reactivity profile for the heavier halide relative to the lighter members of the group.
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INTRODUCTION An important challenge in chemistry is the development of synthetic reactions that are not only effective, efficient, and selective but also practical and environmentally benign.1 Aluminum is an attractive choice for the development of such systems given its high natural abundance2,3 and low toxicity,4 especially compared to its heavy-metal counterparts. Aluminum coordination complexes have historically been defined by the stability of the trivalent oxidation state,2,4 and research in this area has resulted in the development of a multitude of systems applicable for the Lewis acid activation of molecules.5 The lack of readily accessible multielectron redox states for aluminum has limited the application of its complexes in processes dependent on oxidative and reductive transformations. One approach that has been utilized to circumvent this limitation is the preparation of novel aluminum coordination complexes supporting redox-active, noninnocent ligands. The development of metal complexes supported by redox-active ligands has resulted in new classes of compounds for use in multielectron transformations in recent years and has served to expand the reaction profiles of various main-group, transition, and f-block metal complexes.6−9 Several classes of aluminum complexes supporting redoxactive ligands have been prepared. The majority of these systems have involved ligands incorporating nitrogen heter© XXXX American Chemical Society
oatoms (Figure 1) that have included complexes of 2,2′bipyridine,10,11 iminopyridine,12−14 and bis(imino)pyridine ligands.15−17 Our work in this field has focused on investigating the coordination chemistry of N-aryl-substituted α-diimine ligands [ArNC(CH3)C(CH3)NAr] to aluminum. Our interest in this class of ligands is 2-fold. First, α-diimines are well-known to exhibit rich redox behavior and can be singly or doubly reduced to form the radical anionic (LAr−) and dianionic (LAr2−) species, respectively (Figure 2).18 Second, α-diimines are easily prepared and highly modifiable. The N−C−C−N backbone can be derivatized with a wide array of substituents in either the C or N positions that alter both their steric and electronic properties and those of their metal complexes. A detailed study demonstrating how the substitution pattern effects the ligand electronic properties of the 1,2-bis(imino)acenaphthylene (BIAN) ligand has recently been reported.19 The coordination chemistry of the related 1,4-diazabutadiene (DAB) has been reported. Raston, Cloke, and co-workers prepared the Al(tBu-DAB2−)(tBu-DAB−) (tBu-DAB = N,N′-ditert-butyl-1,4-diazabutadiene) complex through the oxidation of Special Issue: Applications of Metal Complexes with LigandCentered Radicals Received: January 5, 2018
A
DOI: 10.1021/acs.inorgchem.8b00045 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Figure 1. Nitrogen-based redox-active ligands that have been coordinated to aluminum.
Figure 2. α-Diimine ligand across three oxidation states.
[Mg2Cl3(THF)6][(dipp-BIAN2−)AlCl2].25 The doubly reduced complex [Na(Et2O)2][(dipp-BIAN2−)Al(CH3)2] was prepared via the in situ reduction of a neutral ligand with 2 equiv of sodium, followed by salt metathesis with AlCl3.27 The neutral ligand complex [(dipp-BIAN)AlX2]AlX4 (X = Cl and Br) could also be accessed through a 1:2 reaction between the ligand and AlX3.28 More recently, Anstey, Tomson, and co-workers reported the neutral ligand complex [(dmp-BIAN)3Al][PF6]3 [ d m p - B I A N = 1 , 2- bi s (3 , 5 -d im e t h y l p h e n y l i m i n o ) acenaphthylene], which was prepared through oxidation of the reduced ligand complex (dmp-BIAN−)3Al with 3 equiv of AgPF6.29 The cyclic voltammogram of the (dmp-BIAN−)3Al complex showed three one-electron oxidations, which the authors assigned to the sequential oxidation of each of the dmpBIAN− ligands. We have prepared and structurally characterized aluminum complexes across all three oxidation states of the α-diimine ligand. The doubly and singly reduced complexes (Lmes2−)AlCl(THF) and (Ldipp−)AlCl2 [Lmes = N,N′-bis(2,4,6-trimethylphenyl)-2,3-dimethyl-1,4-diazabutadiene; Ldipp = N,N′-bis(2,6-diisopropylphenyl)-2,3-dimethyl-1,4-diazabutadiene] were prepared by in situ reduction of the ligands with 2 or 1 equiv, respectively, of sodium metal, followed by salt metathesis with AlCl3.30 The complexes were characterized by electrochemistry, with the cyclic voltamograms of both compounds exhibiting two redox events assignable to the LAr2−/LAr− and LAr−/LAr0 couples. We demonstrated that the redox chemistry of the Lmes ligand could be utilized to prepare the singly reduced complex
aluminum vapor with a neutral tBu-DAB ligand. Physical characterization of the complex revealed the electronic structure to be best described as an Al3+ ion coordinated by a doubly reduced tBu-DAB2− ligand and another tBu-DAB− singly reduced radical anion.20,21 The Cowley group reacted the low-valent [Al(C5Me5)]4 complex with 4 equiv of mes-DAB [mes-DAB = N,N′-bis(2,4,6-trimethylphenyl)-1,4-diazabutadiene] to prepare the (mes-DAB2−)Al(C5Me5) compound.22 Interestingly, the analogous reaction between tBu-DAB and [Al(C5Me5)]4 gives Al(tBu-DAB2−)(tBu-DAB−) as the major product. Neutral ligand DAB complexes have also been prepared: The reaction of tBu-DAB with 2 equiv of AlMeCl2 gives [(tBu-DAB)AlClMe][AlCl3Me]23 and the 1:1 reaction between dipp-DAB [dipp-DAB = N,N′-bis(2,6-diisopropylphenyl)-1,4-diazabutadiene] and AlI3 gives [(dipp-DAB)AlI2][I].24 Jones and co-workers also demonstrated that the singly reduced (dipp-DAB−)AlI2 complex could be prepared by using an Al/AlI3 mixture as the metal source. Aluminum complexes of the BIAN ligand have also been prepared. Fedushkin and co-workers synthesized the singly reduced ligand complex (dipp-BIAN−)AlCl2 [dipp-BIAN = N,N′-bis(2,6-diisopropylphenylimino)acenaphthylene] 25 through the reaction of a Al/AlCl3 mixture with a neutral ligand. The corresponding (dipp-BIAN−)AlR2 (R = Me, Et, or i Bu) could also be prepared through the salt metathesis reaction of AlXR2 (X = Cl or Br) with (dipp-BIAN−)Na(Et2O)2.26 Similar chemistry between the (dipp-BIAN 2−)Mg(THF)3 reagent and AlCl3 yielded the doubly reduced ligand complex B
DOI: 10.1021/acs.inorgchem.8b00045 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry Scheme 1. Synthesis of the Aluminum α-Diimine Neutral Ligand Complexes 1−8
(Lmes−)AlCl2 through the one-electron oxidative functionalization of (Lmes2−)AlCl(THF) with either AgCl or CuCl. Although Cui and co-workers were able to prepare the neutral ligand [(Ldipp)AlCl2][AlCl4] complex through the reaction between (Ldipp−)AlEt2 and BCl3,31 we were unable to isolate neutral ligand complexes of the Lmes or Ldipp ligands through either direct complexation with AlCl3 or oxidation of the singly reduced complexes. However, a 1:1 reaction of the LPh [LPh = N,N′-bis(phenyl)-2,3-dimethyl-1,4-diazabutadiene] ligand, which lacks substituents on the aryl ring, with AlCl3 resulted in formation of the neutral ligand complex [Al(LPh)2Cl2][AlCl4].32 Herein we report the synthesis and characterization of a family of neutral ligand [Al(LAr)2Cl2][AlCl4] complexes that systematic vary in substituents on the aromatic side arm. We have elucidated the electronic structure of the complexes using a combination of multinuclear NMR spectroscopy, absorption spectroscopy, electrochemistry, and theory. Our electrochemical characterization demonstrates that the redox properties can be tuned through the substitution pattern of the ligand. We also report the reaction chemistry of the LPh ligand with the suite of AlX3 (X = halide) reagents.
substituents on the aromatic rings. In all cases, products precipitate from solution during the course of the reactions as analytically pure powders, all of which are stable in the solid state when stored under a N2 environment.33 The reaction between the electron-rich α-diimines is significantly fast that the 12 h reaction time is unnecessary. For example, under reaction conditions analogous to those outlined above, complex 5 could be isolated in 71% yield after only 4 h. In our original report for the synthesis of 1, we used 1,2-dimethoxyethane (DME) as the solvent, and a similar precipitation facilitated isolation of the product.32 The use of DME as the solvent for the wider range of [Al(LAr)2Cl2)][AlCl4] complexes herein still effectively affords products in all cases, but the solubility of the complexes differs dramatically with the R substituent on the aromatic ring and complicates isolation of the more soluble analogues. We found that using diethyl ether as the solvent uniformly results in precipitation of products in higher purity than the corresponding DME reactions across the series of LAr ligands and allows for facile isolation of complexes supported by α-diimine ligands with both electron-donating and -withdrawing substituents. In the case of complex 8 [LAr = LNO2 = N,N′-bis(4-nitrophenyl)-2,3-dimethyl-1,4-diazabutadiene], the ligand starting material is not sufficiently soluble in diethyl ether, and dichloromethane was used as the solvent to prepare the compound under otherwise identical reaction conditions. Complex 8 was isolated as an analytically pure powder in 63% yield. The color of the [Al(LAr)2Cl2)][AlCl4] complexes is dependent on the specific R group and ranges from pale yellow (for 1, 2, and 6) to orange (for 3 and 4) to red (for 7 and 8) to purple (for 5). Complexes 1−8 were readily characterized by 1H and 13C NMR spectroscopies. In all cases, the 1H NMR spectra exhibit two signals attributable to the CH3CN protons of the ligand backbones and are consistent with the C2 symmetry of the [Al(LAr)2Cl2)]+ cation corroborated by the solid-state structures (vide infra). For complexes with R groups containing protons (2−5), two unique resonances for the R groups are also observed. Similarly, the 19F NMR of 7 has two signals assignable to inequivalent CF3 groups. The complexes have
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RESULTS AND DISCUSSION Synthesis of Aluminum Complexes 1−11. The [Al(LAr)2Cl2)][AlCl4] complexes 1−7 [LAr = N,N′-bis(phenyl)2,3-dimethyl-1,4-diazabutadiene (LPh), N,N′-bis(4-methylphenyl)-2,3-dimethyl-1,4-diazabutadiene (LTol), N,N′-bis(4-tert-butylphenyl)-2,3-dimethyl-1,4-diazabutadiene (LtBu), N,N′-bis(4methoxyphenyl)-2,3-dimethyl-1,4-diazabutadiene (L OMe ), N,N′-bis[4-(N,N-dimethylamino)phenyl]-2,3-dimethyl-1,4-diazabutadiene (LNMe2), N,N′-bis(4-chlorophenyl)-2,3-dimethyl1,4-diazabutadiene (LCl), and N,N′-bis(4-trifluorophenyl)-2,3dimethyl-1,4-diazabutadiene (LCF3)] were prepared through the reaction of equimolar amounts of LAr and AlCl3 in diethyl ether and isolated as solids after filtration from the reaction medium in 64−88% yield (Scheme 1). The reaction is equally successful for diimine ligands incorporating both electron-donating (Me, t Bu, OMe, and NMe2) and electron-withdrawing (Cl and CF3) C
DOI: 10.1021/acs.inorgchem.8b00045 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
to a broad singlet (for Ar with electron-donating groups). Complex 5, which has the most electron-donating groups as gauged by its Hammett parameter,34 lacks a signal at ∼5 ppm and has only two broad, unresolved signals in the aromatic region at room temperature.35 Together, this suggests that electron-donating substituents result in more facile rotation about the N−CAr bonds relative to their electron-withdrawing counterparts. We propose that this results from an increased δ+ character of the ortho hydrogen with electron-withdrawing groups, which results in a stronger H---Cl interaction in these cases. We previously reported the variable-temperature (VT-) NMR of complex 1 over the 278−350 K temperature range. At higher temperatures, rotation becomes more rapid, causing the upfield resonance at 5.13 ppm to become less resolved and ultimately coalesce into the normal range of aromatic protons. Full collapse of the resonance occurred by 350 K. The analogous VT-NMR of complex 4 (Figure S33) shows a collapse of the upfield aromatic proton signal by 330 K, which supports a weaker H---Cl interaction in 4 relative to 1. In both cases, the CH3CN signals do not change across the temperature range, suggesting that the ligands remain firmly bound to the metal ion. Our attempts to collect similar VTNMR data for either complex 7 or 8 were unsuccessful because both of these compounds are unstable in CD3CN at elevated temperatures and begin to decompose prior to 350 K. The 13C NMR spectra of 1−8 also corroborate the C2 symmetry of the cations and the hindered rotation about the N−CAr bonds. All of the complexes have two unique resonances for both the CH3CN (at ∼173 ppm) and CH3CN (at ∼20 ppm) nuclei of the α-diimine backbone. Two sets of resonances were also observed for the 13C nuclei of the Me, tBu, OMe, NMe2, and CF3 substituents in 2−5 and 7, respectively. In cases where rotation about the N−CAr bond is slower at room temperature (R = H, Cl, CF3, and NO2), 12 unique aromatic environments are recorded. For complexes with faster rotation, the number of signals decreases. For the alkyl-substituted complexes 2 and 3, there are eight major aromatic environments and four smaller, broader environments. The broader signals are not present in complex 4, which has only eight sharp signals assignable to aromatic 13C nuclei. The
Figure 3. Representation of the [Al(LPh)2Cl2)]+ cation of 1 rendered from the X-ray structure data depicting the hydrogen bonding between the ortho C−H of the aryl rings and chloride ligands.
several aromatic environments in their 1H NMR spectra, all of which are multiplets or broad signals. We attribute these broad signals to hindered rotation about the N−CAr bonds, which is facilitated by hydrogen-bonding interactions between the chloride ligands and an ortho hydrogen of one of the aryl groups in each of the diimine ligands, as displayed for the [Al(LPh)2Cl2)]+ cation in Figure 3. This hydrogen bonding is also observed in the solid-state structures of complexes 1, 2, 4, and 5 (vide infra), which have an average H---Cl distance of 2.96 Å. The protons involved in the hydrogen bonds are shifted upfield to ∼5 ppm and range in resolution from a well-defined doublet-of-doublet (for Ar with electron-withdrawing groups) Scheme 2. Synthesis of Complexes 9−11
D
DOI: 10.1021/acs.inorgchem.8b00045 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry Table 1. Selected Bond Distances (Å) and Angles (deg) for 1, 2, 4, 5, and 9 1 Al−N
Al−X C−Cbackbone C−Nimine
N−Al−N
N−Al−X
X−Al−X
2.0612(12) 2.0334(12) 2.0399(12) 2.0710(12) 2.2342(5) 2.2222(5) 1.5055(19) 1.505(2) 1.2881(18) 1.2863(18) 1.2893(18) 1.2865(18) 166.10(5) 78.04(5) 92.09(5) 91.26(5) 77.93(5) 84.48(5) 92.70(4) 96.88(4) 88.70(4) 171.24(4) 97.27(4) 91.34(4) 171.92(4) 89.08(4) 98.15(2)
1theory 2.1326 2.0924 2.2249 1.5037 1.2821 1.2835 168.16 76.60 94.42 82.83 91.00 96.42 87.88 167.85 102.33
2
4
2.032(4) 2.064(3) 2.047(4) 2.062(4) 2.2314(14) 2.2271(15) 1.512(5) 1.506(7) 1.294(5) 1.283(5) 1.279(5) 1.287(6) 167.76(16) 78.10(14) 92.48(14) 92.79(15) 78.25(15) 83.55(13) 91.28(11) 97.02(11) 89.59(10) 171.34(11) 96.90(11) 90.77(10) 170.71(11) 88.79(10) 98.49(6)
13
2.0485(16) 2.0565(17) 2.2237(7) 1.504(3) 1.288(3) 1.285(3) 170.08(10) 78.24(7) 94.05(7) 79.36(9) 91.28(5) 95.06(5) 90.52(5) 167.33(5) 100.46(4)
5
9
2.049(3) 2.051(3) 2.068(3) 2.037(3) 2.2298(11) 2.2229(12) 1.506(4) 1.503(5) 1.277(4) 1.291(4) 1.285(4) 1.290(4) 168.77(11) 78.28(10) 93.34(10) 93.26(11) 78.06(11) 82.19(10) 90.88(8) 96.49(8) 88.55(8) 170.45(9) 96.32(8) 90.81(8) 168.82(9) 90.23(8) 99.59(4)
2.107(2) 2.065(2) 2.054(2) 2.063(3) 2.3985(9) 2.3742(9) 1.506(4) 1.507(4) 1.281(4) 1.280(4) 1.286(4) 1.286(4) 166.09(10) 76.42(10) 91.69(10) 93.81(10) 77.58(10) 84.05(10) 91.53(7) 95.67(8) 88.28(7) 169.97(8) 95.34(8) 95.30(7) 169.37(8) 90.41(7) 98.20(3)
chloro and bromo analogues. Adjusting the stoichiometry to 2:1 LPh/AlI3 gives 10 as an orange powder in 93% yield. The analogous reaction in DME was unsuccessful, which we attribute to the insoluble nature of AlI3 under the reaction conditions. The 1H NMR spectrum of the [Al(LPh)2I2)]I complex also suggests a C2-symmetric cation, indicating a similar structure for the [Al(LPh)2X2)]+ species across the series of halide ligands. Unlike the [Al(LPh)2Cl2)][AlCl4] and [Al(LPh)2Br2)][AlBr4] complexes, which are structurally stable in various coordinating solvents, the inner-sphere iodo ligands in 10 are readily displaced by acetonitrile. Dissolving 10 in acetonitrile results in an immediate color change from orange to dark purple. After stirring for 12 h and removal of solvents, [Al(LPh)2(NCMe)2)]I3 (11) was isolated in 98% yield as a purple powder. The 1H and 13C NMR spectra of 11 indicate that C2 symmetry is maintained in the [Al(LPh)2(NCMe)2)]3+ cation. The 27Al NMR spectrum displays only one resonance at 12.9 ppm. Solid-State Structures of 1, 2, 4, 5, 9, and 11. The structures of complexes 1, 2, 4, 5, 9, and 11 were corroborated by X-ray crystallography. Crystallographic data for these complexes are provided in Table S1 or in ref 32 (for 1). Details concerning refinement of the crystallographic data for 2, 4, 5, 9, and 11 can be found in the Supporting Information. Complexes 2 and 11 refined with R1 values of ∼10%, primarily because of disorder in their respective anions, and the geometric data provided should be treated as such. Single crystals of 1, 2, 4, and 5 were all grown from the slow diffusion of diethyl ether into concentrated acetonitrile solutions at −25 °C. All of the molecules crystallize as cation−anion pairs, with the anion being an AlCl4− tetrahedron and the cation being made up of an Al3+ ion coordinated by two
C NMR of 5 was collected at 350 K and also shows eight unique 13C environments in the aromatic region. The 27Al NMR spectra of 1−8 all have two unique signals supporting two independent aluminum ions in solution. All of the complexes have a sharp signal at 103.6 ppm (Δν1/2 ∼ 6 Hz) corresponding to the AlCl4− anion. The second signal, corresponding to the [Al(LAr)2Cl2]+ cation, is both broader (Δν1/2 ∼ 100−3000 Hz) and less intense and comes at ∼29 ppm. We also explored complexation of the LPh ligand with other AlX3 (X = F, Br, I, and OTf) reagents (Scheme 2). In either DME or diethyl ether, the reaction with aluminum trifluoride was unsuccessful, with only uncoordinated ligand being recovered from the reaction mixture, which we attribute to the strong Al−F interactions throughout the polymeric structure of the AlF3 reagent. Extended heating of the LPh/ AlF3 mixture in DME at 100 °C similarly did not lead to the formation of a coordination complex. The reaction between LPh and Al(OTf) 3 was also unsuccessful and resulted in decomposition of the ligand in either DME or diethyl ether. The reaction between LPh and AlBr3 in diethyl ether gives the [Al(LPh)2Br2)][AlBr4] (9) complex as a pale-orange powder in 80% yield. The reaction was also successful when DME was used as the solvent, giving 9 in 65% yield. Complex 9 was characterized by 1H, 13C, and 27Al NMR spectroscopies, with the spectra looking very similar to those for the [Al(LPh)2Cl2)][AlCl4] complex. The resonances for the AlBr4− anion (δ = 80.7 ppm) and [Al(LPh)2Br2)]+ cation (δ = 20.0 ppm) in the 27 Al NMR spectra are upfield relative to the analogous signals in 1. The reaction between LPh and AlI3 in diethyl ether gives the [Al(LPh)2I2)]I complex 10, where the second 1 equiv of AlI3 does not coordinate the displaced I− anion, as occurs in the E
DOI: 10.1021/acs.inorgchem.8b00045 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry α-diimine and two chloride ligands. In all cases, the aluminum ions in the [Al(LAr)2Cl2]+ cations are coordinated in slightly distorted octahedral geometries, with the two chloride ligands being in a cis arrangement to give the cations near C2 symmetry, which is in agreement with the structures deduced from the NMR spectral data outlined above. Selected bonding metrics for the [Al(LAr)2Cl2]+ cations for these complexes are provided in Table 1, and representations of 2, 4, and 5 are shown in Figures 4−6. We reported the structural data for complex 1 previously32 but include the bonding metrics herein for completeness and comparison purposes.
Figure 6. Solid-state structure of the [Al(LNMe2)2Cl2]+ cation of complex 5 showing only the dominant orientation of two of one disordered NMe2 group. Ellipsoids are projected at 30% probability. Hydrogen atoms have been omitted for clarity.
(LMes−)AlCl2 has C−C and C−N distances of 1.438(2) and 1.355(2)/1.348(2) Å, respectively, while the doubly reduced (LMes2−)AlCl(THF) has a C−C distance of 1.341(4) Å and C− N distances of 1.438(4) and 1.440(4) Å.30 The Al−N distances in the [Al(LAr)2Cl2]AlCl4 complexes average 2.05 Å, which is slightly longer than the corresponding average bond length in either [(diip-DAB)AlI2][I] (Al−Nave = 1.92 Å)24 or [(tBu-DAB)AlClMe][AlCl3Me] (Al−N = 1.96 Å).23 These distances for the neutral ligand complexes are all longer than the average Al−N distances in previously reported singly (1.90 Å)20,21,24,30,31,37 and doubly (1.82 Å)20−22,24,30 reduced aluminum α-diimine adducts, as is anticipated from the lesser Coulombic attraction between the aluminum(III) ion and neutral ligand. A similar trend was observed for the iminopyridine complexes reported by Berben,12−14 where the Al−Nimido distances get progressively shorter as the oxidation state of the ligand changes from neutral [Al−N = 2.038(4) Å] to singly reduced [Al−N = 1.856(1)−1.989(3) Å] to doubly reduced [Al−N = 1.806(1)−1.879(2) Å] across the complexes. The Al-BIAN complexes show a similar trend in the Al−N bond distance with changing ligand oxidation state.25−27,29 Single crystals of 9 were also grown by the slow evaporation of diethyl ether into a concentrated acetonitrile solution at −25 °C. The structure of 9 is analogous to that of 1, with the chloride ligands being replaced by bromides in both the cation and anion (Figure 7). The average C−C (1.51 Å) and C−N
Figure 4. Solid-state structure of the [Al(LTol)2Cl2]+ cation of complex 2. Ellipsoids are projected at 30% probability. Hydrogen atoms and an interstitial diethyl ether molecule have been omitted for clarity.
Figure 5. Solid-state structure of the [Al(LOMe)2Cl2]+ cation of complex 4, showing only the dominant orientation of two of one disordered OMe group. Ellipsoids are projected at 30% probability. Hydrogen atoms have been omitted for clarity.
The average C−C and C−N bond distances in the α-diimine ligands for the complexes are 1.50 and 1.29 Å, respectively, confirming that the ligands remain fully oxidized across the series.36 There is no systematic trend in these metrics across the complexes. The C−C bond distances in our compounds are slightly longer than those reported for either the [(dippDAB)AlI2][I] [C−C = 1.442(13) Å; C−N = 1.325(12) and 1.289(11) Å]24 or [(tBu-DAB)AlClMe][AlCl3Me] [C−C = 1.459(9) Å; C−N = 1.284(7) and 1.279(8) Å]23 neutral ligand complex, although the C−N distances are in better agreement. Our bonding metrics also compare well to the average C−C (1.51 Å) and C−N (1.28 Å) bond distances reported for the neutral ligand complex [(dmp-BIAN)3Al][PF6]3.29 As expected, the C−C and C−N bonding distances are longer and shorter, respectively, relative to analogous bond distances in aluminum complexes with either singly or doubly reduced α-diimine ligands. For example, the singly reduced ligand complex
Figure 7. Solid-state structure of the [Al(LPh)2Br2]+ cation of complex 9. Ellipsoids are projected at 30% probability. Hydrogen atoms have been omitted for clarity. F
DOI: 10.1021/acs.inorgchem.8b00045 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
excellent agreement between the computed and experimental bond angles around the aluminum ion, with the biggest deviation being ∼4° for the Cl−Al−Cl angle. The charge distribution of the [Al(LPh)2Cl2]+ cation was studied by the natural bonding orbital (NBO) method, with selected data given in Table 2. The aluminum(III) cation has a large positive charge, while the bonding nitrogen and chloride atoms have negative charges. Relative to the singly and doubly reduced ligands in (LMes−)AlCl2 and (LMes2−)AlCl(THF), DFT analyses of which have previously been reported,30 the nitrogen atoms of the neutral diimine ligands in 1 acquire less electron density. There is a systematic trend for the charge density at the nitrogen atoms moving across the series from neutral → singly reduced → doubly reduced, reflective of the change in the oxidation state of the ligands.
(1.28 Å) bond distances in 9 are in the range of those for the [Al(LAr)2Cl2]AlCl4 complexes outlined above. The Al−N distances in 9 are also in the range of those for the [Al(LAr)2Cl2]AlCl4 complexes, albeit the average Al−N distance is slightly longer at 2.07 Å. Complex 11 was crystallized from acetonitrile at room temperature. The molecule crystallizes as a [Al(LPh)2(NCMe)2)]+3 cation with three outer-sphere I− counterions. The [Al(LPh)2(NCMe)2)]3+ cation is structurally similar to its [Al(LPh)2X2)]+ counterparts, with the inner-sphere halide ligands being replaced with acetonitrile molecules (Figure 8).
Table 2. Charge Distribution of the [Al(LPh)2Cl2]+ Cation of Complex 1 Al Cl(1)/Cl(2)
1.4573 −0.5204
N(1)/N(4) N(2)/N(3)
−0.5261 −0.5255
We also examined the molecular orbitals of the [Al(LPh)2Cl2]+ cation. The lowest unoccupied molecular orbital (LUMO) and LUMO+1 are primarily ligand-based and located on the N−C−C−N backbones of the two α-diimines with a bonding interaction between the C−C groups and an antibonding interaction between the C−N groups (Figure 9). The two orbitals are close to degenerate, with an energy difference of 0.1734 eV. The highest occupied molecular orbital (HOMO) and HOMO−1 (Figure S34) are also similar in structure and energy (energy difference of 0.0966 eV), with the electron density primarily spread on one phenyl ring of each αdiimine ligand as well as on the two chloride ligands. Absorption Spectra. The absorption spectra of compounds 1−8 were collected in acetonitrile at 298 K at concentrations of ∼3 × 10−5 M. The data are summarized in Table 3, and the spectra are shown in Figure S35. All of the spectra are similar, with a dominant feature in the range of 220−260 nm (εM ∼ 11000−59000 M−1 cm−1) along with a small shoulder and a less intense feature in the range of 320−530 nm (εM ∼ 2600− 13000 M−1 cm−1). For both complexes 1 and 8, the major feature is shifted out of the solvent window. Time-dependent DFT (TD-DFT) calculations indicate that the transitions involve excitation from orbitals primarily located on the aryl side arms (HOMO, HOMO−1, etc.) to ligands primarily located on the N−C−C−N diimine backbone (LUMO, LUMO+1, etc.). A visualization of the five most intense transitions and their accompanying energies is shown in Figure S36. Relative to 1, there is a consistent red shift of all three features for complexes with electron-donating groups in the LAr ligand. We attribute this to an increase in the energy of the HOMO commensurate with electron-donating substituents, resulting in a smaller HOMO−LUMO gap. A similar destabilization of the HOMO was employed to explain the electronic absorption spectra of a series of indium(III) bis(arylamino)acenaphthene complexes recently reported by ́ Soo, and Garcia.́ 38 Diaz, Electrochemistry. The cyclic voltammograms of 1−7 were collected in CH2Cl2. The solubility of complex 8 in CH2Cl2 is not sufficient, and acetonitrile was used to collect its electrochemical data. Figure 10 shows the cyclic voltammogram
Figure 8. Solid-state structure of the [Al(LPh)2(NCMe)2]+ cation of complex 11. Ellipsoids are projected at 30% probability. Hydrogen atoms have been omitted for clarity. Selected bond distances (Å): Al(1)−N(1), 2.020(8); Al(1)−N(2), 1.993(8); Al(1)−N(3), 2.006(8); Al(1)−N(4), 2.007(8); Al(1)−N(5), 1.94(3); Al(1)− N(6), 1.97(2); C(1)−C(3), 1.507(13); C(5)−C(7), 1.494(13); N(1)−C(1), 1.291(11); N(2)−C(3), 1.270(12); N(3)−C(5), 1.289(12); N(4)−C(7), 1.277(13). Selected bond angles (deg): N(1)−Al(1)−N(2), 79.6(3); N(1)−Al(1)−N(3), 89.1(3); N(1)− Al(1)−N(4), 96.1(3); N(2)−Al(1)−N(3), 97.8(3); N(2)−Al(1)− N(4), 175.2(4); N(3)−Al(1)−N(4), 79.8(3); N(1)−Al(1)−N(5), 95.1(9); N(1)−Al(1)−N(6), 169.1(7); N(2)−Al(1)−N(5), 90.5(11); N(2)−Al(1)−N(6), 90.5(8); N(3)−Al(1)−N(5), 171.3(11); N(3)− Al(1)−N(6), 87.7(6); N(5)−Al(1)−N(4), 92.1(11); N(6)−Al(1)− N(4), 93.5(8); N(5)−Al(1)−N(6), 89.6(9).
The solvent molecules are coordinated in a cis arrangement, maintaining the C2 symmetry of the ion. Although the C−C and C−N bond distances in 11 are comparable to those in the [Al(LPh)2X2)]+ ions, the average Al−N distance (2.01 Å) is shorter, which we propose is due to the increased Lewis acidity of the [Al(NCMe)2)]3+ cation relative to its [AlX2]+ counterparts. Electronic Structure of the [Al(LAr)2Cl2)]AlCl4 Complexes 1−8. Density Functional Theory (DFT) Studies. The full geometry of the [Al(LPh)2Cl2]+ cation of compound 1 was optimized using DFT, with the ion being constrained to C2 symmetry (Table S2). The computed bond distances were found to be in good agreement with those obtained in the X-ray data (Table 1). The deviation of the bond distances for the Al− Cl interactions as well as the N−C−C−N backbone of the ligand is less than 0.01 Å. There is a greater discrepancy (∼0.05−0.07 Å) in all of the Al−N interactions, which are systematically longer in the computed structure. There is G
DOI: 10.1021/acs.inorgchem.8b00045 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 9. LUMO and LUMO+1 of the [Al(LPh)2Cl2]+ cation of complex 1.
ligand. Optimization of the singly reduced ([Al(LAr)2Cl2]) and doubly reduced ([Al(LAr)2Cl2]−) compounds and visualization of their frontier orbitals support these assignments (see the Supporting Information): The singly unoccupied molecular orbital of the [Al(LAr)2Cl2] complex is localized on one diimine ligand, and the HOMO of the [Al(LAr)2Cl2]− complex is delocalized over both diimines. The first redox event is reversible or quasireversible for all compounds except 8, for which the process is nonreversible. The second oxidation feature is uniformly less reversible across the compounds, with diminished and/or shifted return oxidation features in all cases. Isolation of the [Al(LAr0)2Cl2]+/[Al(LAr0)(LAr−)Cl2] feature increases the reversibility across the compounds (Figure 11), although the process
Table 3. Absorption Data for Complexes 1−8 Taken in Acetonitrile 1 2 3 4 5 6 7 8
λ1 (nm)
εM1 (M−1 cm−1)
λ2 (nm)
λ3 (nm)
εM3 (M−1 cm−1)
222 219 228 263 223 237
24000 35000 25000 48000 59000 11000
283 257 257 262 310 283 297 230
333 363 361 394 529 325 365 322
16000 2400 3200 2600 6200 4600 3500 8800
Figure 10. Cyclic voltammogram of compound 1 recorded in 0.1 M [nPr4N][BArF] CH2Cl2 solution recorded at 500 mV s−1. Figure 11. Cyclic voltammograms of the isolated first redox event for compounds 1−8.
for 1 as a representative case, with full scans of 2−8 provided in the Supporting Information. The data for all complexes are summarized in Table 4. All of the compounds exhibit two features that we attribute to sequential one-electron oxidation processes corresponding to LAr/LAr− couples on each diimine
is still only quasireversible for both 7 and 8. There is a systematic variation in the redox potentials for the feature across the complexes, with changing substituent on the aromatic ring spanning ∼0.6 V. Complex 5 has the most negative E1/2 potential, as expected from the electron-donating N(CH3)2 group (σp = −0.8334), while E1/2 for complex 8, which incorporates the electron-withdrawing NO2 group (σp = 0.77834), comes at the most positive potential. Considering the whole range of complexes, there is a linear correlation between the E1/2 values and the Hammett parameter of the substituent on the aromatic ring (Figure 12), indicating that the redox energetics of the [Al(LAr)2Cl2]AlCl4 complexes can be tuned via a judicial choice in the ligand substituent. We observed a similar tuning of redox events through ligand substitution in our pyridyl nitroxide (RpyNO−)2AlCl system39 as did the
Table 4. Electrochemical Potentials (V vs Fc/Fc+) for Complexes 1−8
1 2 3 4 5 6 7 8
E1/2 for [Al(LAr0)2Cl2]+/[Al(LAr0) (LAr−)Cl2]
Epa for [Al(LAr0)(LAr−)Cl2]/ [Al(LAr−)2Cl2]−
−0.98 −1.05 −1.07 −1.05 −1.24 −0.91 −0.67 −0.62
−1.72 −1.58 −1.68 −1.62 −1.59 −1.37 −1.46 −0.99 H
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diffractometer manipulations, including data collection, integration, scaling, and absorption corrections were carried out using the Bruker Apex2 software.41 Unit cell parameters were obtained from 60 data frames, 0.5° ϕ, from three different sections of the Ewald sphere. For compounds 2 and 11, the structures were solved by direct methods and refined by full-matrix least squares based on F2 using SHELXL.42 For compounds 4 and 9, the structures were solved using SuperFlip43 and refined by full-matrix least squares on F2 using the Oxford University Crystals for Windows program.44,45 Compound 5 was solved using SIR-9246 and refined by full-matrix-least-squares on F2 using the Oxford University Crystals for Windows program.44,45 All nonhydrogen atoms were refined anisotropically, and hydrogen atoms were refined using a riding model. Details regarding specific solution refinements for each compound are provided in the Supporting Information. Computational Details. Structure optimizations of the [Al(LPh)2Cl2]+ cation of 1 and the reduced Al(LPh)2Cl2 and [Al(LPh)2Cl2]− species were performed with the Gaussian 09,47 revision B.01, program using the B3LYP hybrid DFT method and the 6311+G(2d,p) basis set on all atoms. Geometry optimization was performed using the crystal structure geometry for 1 as the initial starting point for [Al(LPh)2Cl2]+. Optimization for Al(LPh)2Cl2 and [Al(LPh)2Cl2]− were started from C1 symmetry. Frequency calculations found no imaginary frequencies, confirming that the optimized structures were minima. Molecular orbitals were rendered with the program VMD v1.9.4* at an isovalue of ±0.06 au, and NBO analyses were performed using the NBO6.0 package.48 TD-DFT calculations were performed using the CAM-B3LYP functional, and the first 20 states were calculated. Solvent effects were accounted for using the polarizable continuum model with acetonitrile as the solvent. Materials and Supplies. Preparation of Compounds. All reactions and manipulations were performed under an inert atmosphere (N2) using standard Schlenk techniques or in a Vacuum Atmospheres, Inc., NextGen drybox equipped with both oxygen and moisture purifier systems. Glassware was dried overnight at 150 °C before use. CD3CN and CDCl3 were purchased from Sigma-Aldrich, degassed, and stored over 4 Å molecular sieves prior to use. 1,2Dimethoxyethane, acetonitrile, diethyl ether, and dichloromethane were sparged for 20 min with dry argon and dried using a commercial two-column solvent purification system comprising two columns packed with neutral alumina. With the exception of the nitrated derivative, all of the α-diimine ligands were prepared according to the protocol reported by the Stanford group for the N,N′-bis(phenyl)-2,3dimethyl-1,4-diazabutadiene (LPh) compound.49 The N,N′-bis(4nitrophenyl)-2,3-dimethyl-1,4-diazabutadiene (LNO2) ligand was prepared following the protocol developed for the corresponding BIAN ligand.38 [nPr4N][BArF] was prepared according to a literature procedure.50 All other reagents were purchased from commercial sources and used as received. General Synthesis of [Al(LAr)2Cl2][AlCl4] 1−8. LAr (1.00 mmol) was dissolved in diethyl ether (for 1−7) or CH2Cl2 (for 8) in a 125 mL flask equipped with a magnetic stirbar. The specific amount of solvent was varied across the series to ensure a homogeneous solution of the ligand. AlCl3 (0.133 g, 1.00 mmol) was dissolved in 25 mL of either diethyl ether (for 1−7) or CH2Cl2 (for 8), and the solution was added to the stirring solution of the ligand. The resulting reaction was stirred at room temperature for 12 h, after which the reaction product was collected as a powder by filtration over a medium-porosity frit. The powder was washed with a liberal amount of diethyl ether and dried in vacuo. Characterization Data for [Al(LPh)2Cl2][AlCl4] (1). Yield: 0.314 g, 0.425 mmol (85%) of a pale-yellow powder. The spectral data for complex 1 is identical with that previously reported. 1H NMR (CD3CN): δ 7.73 (m, 6H), 7.43 (m, 4H), 7.35 (t, J = 7.2 Hz, 2H), 7.22 (m, 4H), 6.92 (d, J = 6.8 Hz, 2H), 5.13 (d, J = 7.6 Hz, 2H), 2.38 (s, 6H, CH3CN), 2.14 (s, 6H, CH3CN). Characterization Data for [Al(LTol)2Cl2][AlCl4] (2). Yield: 0.303 g, 0.381 mmol (76%) of a pale-yellow powder. 1H NMR (CD3CN): δ 7.53 (m, 4H), 7.35 (b, 2H), 7.23 (b, 2H), 7.03 (m, 4H), 6.75 (dd, J =
Figure 12. Hammett plot of the constants σ for the para substituents versus redox potential for the first redox of the [Al(LAr)2Cl2][AlCl4] complexes 1−8.
Heyduk group for a series of tantalum complexes supporting triamido-based ligands.40
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CONCLUSIONS AND OUTLOOK We have described a comprehensive investigation on the synthesis and characterization of a series of neutral ligand aluminum α-diimine complexes. The [Al(LAr)2Cl2][AlCl4] structural motif tolerates a wide range of both electronwithdrawing and -donating substituents within the aromatic groups of the ligand, which can be used to systematically tune the electronics of the aluminum complex. Future work will involve the preparation of both singly and doubly reduced variants of the complexes. We are also working toward correlating the redox energietics of the complexes to their chemical reactivity.
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EXPERIMENTAL SECTION
Physical Measurements. 1H and 13C NMR spectra were recorded at ambient temperature (unless otherwise noted) in CD3CN or CDCl3 using a Bruker 400 MHz spectrometer (399.78 MHz for 1H and 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, td = triplet of doublets, and b = broad). 27Al NMR spectra were collected in CD3CN using a Bruker 400 MHz spectrometer (104.25 MHz for 27Al). Chemical shifts were referenced to an external 0.6 M Al(D2O)6Cl3 in D2O standard. The 19F NMR spectrum of 7 was collected in CD3CN using a Bruker 400 MHz spectrometer (376.17 MHz for 19F). CHN analyses were performed on bulk samples at either the Complete Analysis Laboratories (for 2 and 9) or the Midwest Microlab. Electrochemical measurements were done in a glovebox under a dinitrogen environment using a CHI 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 analyte and 100 mM [nPr4N][BArF] (BArF− = B(3,5-CF3)2-C6H3)4−) in ∼5 mL of anhydrous CH2Cl2 (or MeCN for 8). All data were collected in a positive-feedback IR compensation mode at 500 mV s−1. Absorbance spectra were collected using an Agilent Cary series UV−vis spectrophotometer in anhydrous acetonitrile at ambient temperature and pressure. X-ray Structure Determination. X-ray diffraction data were collected on a Bruker APEX II CCD diffractometer employing graphite-monochromated (λ = 0.71073 Å) Mo Kα radiation. All I
DOI: 10.1021/acs.inorgchem.8b00045 Inorg. Chem. XXXX, XXX, XXX−XXX
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NMR (CD3CN): δ 174.9 (CN), 173.8 (CN), 148.3, 147.5, 127.0, 126.6, 125.4, 125.1, 125.0, 124.5, 124.3, 123.8, 123.3, 121.8, 20.2 (H3CCN), 20.0 (H3CCN). 27Al NMR (CD3CN): δ 103.6 (AlCl4−), 30.4 (Al(LNO2)2Cl2+). Anal. Calcd for C32H28Al2Cl6N8O8: C, 41.81; H, 3.07; N, 12.19. Found: C, 42.07; H, 3.35; N, 11.98. Synthesis and Characterization of [Al(LPh)2Br2][AlBr4] (9). AlBr3 (0.266 g, 1.00 mmol) was added to a 125 mL flask equipped with a magnetic stirbar and diethyl ether (25 mL). To the stirring suspension was added LPh (0.236 g, 1.00 mmol) as a solution in ∼5 mL of diethyl ether. The resulting reaction was stirred at room temperature for 12 h, after which the reaction product was collected as a powder by filtration over a medium-porosity frit. The pale-orange powder was washed with a liberal amount of diethyl ether and dried in vacuo. Yield: 0.400 g, 0.398 mmol (80%) of a pale-orange powder. 1H NMR (CD3CN): δ 7.72 (m, 6H), 7.60 (m, 2H), 7.40 (td, J = 7.6 and 1.2 Hz, 2H), 7.32 (m, 4H), 7.20 (td, J = 7.8 and 1.2 Hz, 2H), 6.88 (m, 2H), 5.03 (d, J = 8.7 Hz, 2H), 2.40 (s, 6H, CH3CN), 2.14 (s, 6H, CH3CN). 13 C{1H} NMR (CD3CN): δ 173.8 (CN), 173.5 (CN), 144.8, 143.8, 132.3, 131.6, 130.1, 129.8, 129.1, 129.1, 125.4, 125.3, 123.2, 121.3, 20.9 (H3CCN), 20.7 (H3CCN). 27Al NMR (CD3CN): δ 80.7 (AlBr4−), 20.0 (Al(LPh)2Br2+). Anal. Calcd for C32H32Al2Br6N4: C, 38.21; H, 3.21; N, 5.57. Found: C, 38.13; H, 3.49; N, 5.53. Synthesis and Characterization of [Al(LPh)2I2]I (10). AlI3 (0.204 g, 0.500 mmol) was added to a 125 mL flask equipped with a magnetic stirbar and diethyl ether (25 mL). To the stirring suspension was added LPh (0.236 g, 1.00 mmol) as a solution in ∼5 mL of diethyl ether. The resulting reaction was stirred at room temperature for 12 h, after which the reaction product was collected as an orange powder by filtration over a medium-porosity frit. The powder was washed with a liberal amount of diethyl ether and dried in vacuo. Yield: 0.407 g, 0.462 mmol (93%) of a pale-orange powder. 1H NMR (CDCl3): δ 8.18 (t, J = 7.3 Hz, 2H), 8.04 (d, J = 7.3 Hz, 2H), 7.64 (m, 4H), 7.54 (d, J = 7.4 Hz, 2H), 7.37 (m, 4H), 7.27 (m, 2H), 7.14 (t, J = 7.3 Hz, 2H), 4.79 (d, J = 7.6 Hz, 2H), 2.74 (s, 6H, CH3CN), 2.38 (s, 6H, CH3CN). 13C{1H} NMR (CDCl3): δ 171.8 (CN), 171.5 (C N), 143.3, 142.2, 132.6, 131.1, 129.4 (b, two overlapping signals), 128.9, 128.3, 125.8, 125.7, 122.0, 121.7, 22.8 (H3CCN), 22.1 (H3CCN). Anal. Calcd for C32H32AlI3N4: C, 43.66; H, 3.66; N, 6.36. Found: C, 43.43; H, 3.66; N, 6.24. Synthesis and Characterization of [Al(LPh)2(NCCH3)2]I3 (11). Compound 10 (0.058 g, 0.066 mmol) was added to a 25 mL vial equipped with a magnetic stirbar and dissolved in acetonitrile (∼15 mL). The resulting purple solution was stirred for 1 h, after which the solvents were removed and the solid was dried in vacuo and then collected. Yield: 0.062 g, 0.065 mmol (98%) of a purple powder. 1H NMR (CD3CN): δ 7.80 (m, 4H), 7.75 (m, 4H), 7.56 (td, J = 7.8 and 1.4 Hz, 2H), 7.43 (t, J = 7.5 Hz, 2H), 7.36 (m, 2H), 7.29 (td, J = 7.8 and 1.2 Hz, 2H), 6.87 (d, J = 7.9 Hz, 2H), 4.97 (dd, J = 7.8 and 1.5 Hz, 2H), (s, 6H, CH3CN), 2.52 (s, 6H, CH3CN), 1.96 (NCCH351). 13 C{1H} NMR (CD3CN): δ 178.6 (b, two overlapping signals, CN), 142.6, 142.4, 132.1, 131.8, 130.6, 130.5, 130.1, 129.6, 122.1, 121.9, 121.2, 121.1, 23.6 (H3CCN), 23.5 (H3CCN). 27Al NMR (CD 3 CN): δ 12.9 (Al(L Ph ) 2 (NCMe) 2 + ). Anal. Calcd for C36H38AlI3N6: C, 44.93; H, 3.98; N, 8.73. Found: C, 44.59; H, 4.06; N, 8.58.
2.2 and 8.0 Hz, 2H), 5.08 (b, 2H), 2.54 (s, 6H), 2.35 (s, 6H), 2.34 (s, 6H), 2.13 (s, 6H). 13C{1H} NMR (CD3CN): δ 172.8 (CN), 172.5 (CN), 141.9, 141.1, 139.7, 138.5, 132.1, 131.3, 129.6, 129.0, 124.4, 123.9, 122.6, 120.5, 20.7, 20.6, 19.9, 19.8. 27Al NMR (CD3CN): δ 103.6 (AlCl4−), 29.4 (Al(LTol)2Cl2+). Anal. Calcd for C36H40Al2Cl6N4: C, 54.36; H, 5.07; N, 7.04. Found: C, 54.46; H, 5.14; N, 7.23. Characterization Data for [Al(LtBu)2Cl2][AlCl4] (3). Yield: 0.308 g, 0.320 mmol (64%) of a pale-orange powder. 1H NMR (CD3CN): δ 7.76 (d, J = 8.6 Hz, 4H), 7.44 (b, 2H), 7.36 (b, 2H), 7.19 (b, 2H), 7.06 (dd, J = 2.8 and 8.5 Hz, 2H), 6.78 (dd, J = 1.7 and 8.4 Hz, 2H), 5.05 (b, 2H), 2.35 (s, 6H, CH3CN), 2.07 (s, 6H, CH3CN), 1.46 (s, 18H, C(CH3)3), 1.30 (s, 18H, C(CH3)3). 13C{1H} NMR (CD3CN): δ 172.9 (CN), 172.3 (CN), 152.8, 151.6, 141.9, 141.0, 128.9, 127.6, 126.3, 125.1, 124.2, 123.6, 122.2, 120.3, 35.2 (C(CH3)3), 34.8 (C(CH3)3), 31.1 (C(CH3)3), 31.0 (C(CH3)3), 19.8 (H3CCN), 19.7 (H3CCN). 27Al NMR (CD3CN): δ 103.6 (AlCl4−), 29.9 (Al(LtBu)2Cl2+). Anal. Calcd for C48H64Al2Cl6N4: C, 59.82; H, 6.69; N, 5.81. Found: C, 60.09; H, 6.59; N, 5.70. Characterization Data for [Al(LOMe)2Cl2][AlCl4] (4). Yield: 0.315 g, 0.367 mmol (73%) of an orange powder. 1H NMR (CD3CN): δ 7.41 (b, 2H), 7.21 (t, J = 6.8 Hz), 7.03 (d, J = 8.6 Hz), 6.86 (b, 4H), 6.73 (d, 8.8 Hz), 5.24 (b, 2H), 3.93 (s, 6H, OCH3), 3.79 (s, 6H, OCH3), 2.31 (s, 6H, CH3CN), 2.14 (s, 6H, CH3CN). 13C{1H} NMR (CD3CN): δ 173.0 (CN), 172.6 (CN), 160.3, 159.7, 137.0, 136.3, 125.2, 122.0, 116.4, 116.2, 56.2 (OCH3), 55.8 (OCH3), 19.9 (H3CC N), 19.8 (H3CCN). 27Al NMR (CD3CN): δ 103.6 (AlCl4−), 29.5 (Al(LOMe)2Cl2+). Anal. Calcd for C26H40Al2Cl6N4O4: C, 50.31; H, 4.69; N, 6.52. Found: C, 49.99; H, 4.79; N, 6.48. Characterization Data for [Al(LNMe2)2Cl2][AlCl4] (5). Yield: 0.404 g, 0.443 mmol (88%) of a purple powder. 1H NMR (CD3CN): δ 6.84 (b, 16H), 3.09 (s, 12H, N(CH3)2), 2.99 (s, 12H, N(CH3)2), 2.32 (s, 6H, CH3CN), 2.16 (s, 6H, CH3CN). 13C{1H} NMR (CD3CN): δ 171.4 (CN), 169.8 (CN), 150.9, 150.4, 132.9, 132.0, 125.1, 113.3, 112.4, 110.7, 39.8 (N(CH3)2), 39.5 (N(CH3)2), 19.1 (H3CC N), 18.8 (H3CCN). 27Al NMR (CD3CN): δ 103.6 (AlCl4−), 29.6 (Al(LNMe2)2Cl2+). Anal. Calcd for C40H52Al2Cl6N8·CH2Cl2: C, 49.42; H, 5.46; N, 11.24. Found: C, 49.59; H, 5.62; N, 11.32. Characterization Data for [Al(LCl)2Cl2][AlCl4] (6). Yield: 0.323 g, 0.368 mmol (74%) of a pale-yellow powder. 1H NMR (CD3CN): δ 7.73 (m, 4H), 7.44 (b, 4H), 7.30 (d, J = 8.2 Hz), 7.13 (dd, J = 2.6 and 8.5 Hz, 2H), 6.87 (dd, J = 2.6 and 8.4 Hz, 2H), 5.18 (d, J = 8.0 Hz), 2.36 (s, 6H, CH3CN), 2.19 (s, 6H, CH3CN). 13C{1H} NMR (CD3CN): δ 174.2 (CN), 173.7 (CN), 142.6, 141.9, 135.1, 134.0, 131.9, 131.3, 129.5, 128.9, 126.4, 125.7, 124.0, 122.6, 20.3 (H3CC N), 20.2 (H3CCN). 27Al NMR (CD3CN): δ 103.6 (AlCl4−), 29.8 (Al(LCl)2Cl2+). Anal. Calcd for C32H28Al2Cl10N4: C, 43.82; H, 3.22; N, 6.39. Found: C, 43.66; H, 3.32; N, 6.44. Characterization Data for [Al(LCF3)2Cl2][AlCl4] (7). The best yield for compound 7 was obtained by running the reaction on a smaller 0.69 mmol (ligand and AlCl3) scale. Yield: 0.231 g, 0.228 mmol (66%) of a red powder. 1H NMR (CD3CN): δ 8.11 (m, 4H), 7.80 (d, J = 8.1 Hz, 2H), 7.64 (d, J = 8.2 Hz, 2H), 7.57 (d, J = 8.1 Hz, 2H), 7.42 (d, J = 8.2 Hz, 2H), 7.18 (d, J = 8.0 Hz, 2H), 5.18 (d, J = 8.0 Hz, 2H), 2.43 (s, 6H, CH3CN), 2.22 (s, 6H, CH3CN). 13C{1H} NMR (CD3CN): δ 175.0 (CN), 174.0 (CN), 147.2, 146.6, 131.4 (q, JC−F = 33.1 Hz), 130.3 (q, JC−F = 32.7 Hz), 129.4 (q, JC−F = 3.7 Hz), 128.7 (q, JC−F = 3.8 Hz), 126.9 (b), 125.8 (b), 125.8, 125.7, 124.2 (q, JC−F = 267.3 Hz, CF3), 123.7 (q, JC−F = 330.7 Hz, CF3), 123.1, 123.0, 20.6 (H3CCN), 20.5 (H3CCN). 19F{1H} NMR (CD3CN): δ −63.10, −63.12. 27Al NMR (CD3CN): δ 103.6 (AlCl4−), 30.3 (Al(LCF3)2Cl2+). Anal. Calcd for C36H28Al2Cl6F12N4: C, 42.76; H, 2.79; N, 5.54. Found: C, 42.23; H, 3.03; N, 5.04. Characterization Data for [Al(LNO2)2Cl2][AlCl4] (8). Yield: 0.289 g, 0.314 mmol (63%) of a red powder. 1H NMR (CD3CN): δ 8.62 (m, 4H), 8.30 (dd, J = 8.8 and 2.4 Hz, 2H), 8.14 (dd, J = 8.7 and 2.4 Hz, 2H), 7.67 (dd, J = 8.8 and 2.0 Hz, 2H), 7.47 (dd, J = 9.0 and 2.9 Hz, 2H), 7.20 (dd, J = 8.9 and 2.8 Hz, 2H), 5.44 (dd, J = 8.8 and 2.1 Hz, 2H), 2.44 (s, 6H, CH3CN), 2.24 (s, 6H, CH3CN). 13C{1H}
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00045. Details regarding refinement for complexes 2, 4, 5, 9, and 11, 1H NMR spectrum of 1, 1H, 13C, and 27Al NMR spectra of 2−9 and 11, 1H and 13C NMR spectra of 10, 19 F NMR spectrum of 7, VT-NMR spectra of 4, tables of coordinates from geometry optimizations of the [Al(LPh)2Cl2]+ cation and reduced [Al(LPh)2Cl2] and J
DOI: 10.1021/acs.inorgchem.8b00045 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry [Al(LPh)2Cl2]− species, additional visualizations of frontier orbitals, UV−vis spectra of 1−8, visualization of the donor/acceptor orbitals for the TD-DFT-modeled electronic transitions of 1, full-scan cyclic voltammograms for 2−8, and full ref 47 (PDF)
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Accession Codes
CCDC 1814573−1814577 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
W. Scott Kassel: 0000-0002-6764-9045 Matthew D. Sonntag: 0000-0002-8883-1030 Christopher R. Graves: 0000-0001-5853-2446 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS C.R.G. thanks Swarthmore College and the American Chemical Society Petroleum Research Fund (PRF 52181-UNI3) for financial support of this work. H.H.W. was supported by the Peter and Aleck Fellowship in Environmental Studies. We thank the National Science Foundation (Grant CHE-1337494) for funding toward the NMR spectrometer used in this work. We thank Dr. Jun Gu (University of Pennsylvania) for help in collecting the 27Al NMR spectra.
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