Mono- and Bimetallic Aluminum Alkyl, Aryl, and Hydride Complexes of

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Mono- and Bimetallic Aluminum Alkyl, Aryl, and Hydride Complexes of a Bulky Dipyrromethene Ligand Christopher G. Gianopoulos, Kristin Kirschbaum, and Mark R. Mason* Department of Chemistry, School of Green Chemistry and Engineering, The University of Toledo, 2801 West Bancroft Street, Toledo, Ohio 43606-3390, United States S Supporting Information *

ABSTRACT: Reactions of an appropriate organoaluminum reagent with a sterically demanding dipyrromethene (2) produced the monomeric complexes (N,N)AlR2 (where (N,N) is deprotonated dipyrromethene; R = Me (3), tBu (4), Ph (5), H (6)). The alkyl and aryl derivatives 3−5 are surprisingly stable and tolerant to exposure to air, moisture, and silica gel. Dihydride 6 is less robust and reacts with water to form the oxo-bridged aluminoxane [(N,N)AlH]2O (7). The presence of terminal hydrides in 6 and 7 was confirmed by NMR and IR spectroscopy. Furthermore, observed Al−H stretching frequencies agree well with those predicted by DFT calculations. We also report the addition of tBuLi to the meso position of 2, affording 5-tert-butyl-5-phenyl2,2′-dimesityldipyrromethane (8). In addition to NMR (1H, 13C) and IR spectroscopy, compounds 2−8 were further characterized by X-ray crystallography. Solid-state structures show that the aluminum and dipyrromethene ligand are nearly planar when the coligands at aluminum are slender.

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INTRODUCTION Organoaluminum chemistry has been an active area of research for many years, owing to the numerous industrial applications of aluminum alkyls, including cocatalysts in Ziegler−Natta polymerization, activators for olefin oligomerization (aufbau reaction, dimersol process, and Chevron−Phillips process), catalysts for ring-opening polymerization of epoxides and other polar monomers, precursors for chemical vapor deposition of semiconductors, and reducing and alkylating agents in fine chemical production.1 Methylaluminoxane, generated by controlled hydrolysis of trimethylaluminum, is a prototypical example of the complicated nature of seemingly simple aluminum chemistry in that many species can be present in equilibrium.2 Simple, multivalent ligands capable of bridging metal centers tend to give rise to clusters.1 Accordingly, examples of monomeric aluminum complexes with multiply bonded ligands, such as alkylidenes and imides, are scarce or unknown.3 Roesky, Power, and co-workers reported the only example of a monomeric imidoaluminum complex with a terminal imido moiety stabilized by a bulky, chelating, monoanionic β-diketiminate ligand synthesized via low-valent aluminum(I).4 Furthermore, access to a stable, monomeric Al(I) precusor allowed for the preparation of several unusual species through oxidative addition.5 Sterically protected organoaluminum complexes have also been shown to oligomerize or polymerize alkenes in the absence of a transition metal. Jordan and co-workers reported the polymerization of © XXXX American Chemical Society

ethylene at atmospheric pressure, albeit with low activity, using aluminum complexes stabilized by an N,N-bidentate, monoanionic amidinate ligand.6 Related aluminum complexes stabilized by guanidinate ligands are active catalysts for the hydroamination of carbodiimides, as shown by the Bergman group.7 Recently, Betley and co-workers reported the use of new sterically demanding α,α′-diaryl or α,α′-dialkyl dipyrromethene ligands for stabilizing imido complexes of iron and cobalt.8−10 Variation of substituents at the α or β positions of pyrrole and the dipyrromethene meso position affords the ability to tune both the electronic and steric properties in dipyrromethene ligands.11 In this context, we became interested in the potential for bulky dipyrromethene ligands to stabilize reactive, monomeric aluminum(III) centers with divalent coligands. Despite the popularity of BODIPY luminophores, dipyrromethene complexes of the heavier group 13 elements are virtually unexplored and are largely limited to six-coordinate homoleptic complexes of the form M(N,N)3, where (N,N) represents a deprotonated dipyrromethene ligand.12−19 The groups of Dolphin12,13 and Cohen14,15 have primarily focused on Ga and In chemistry, while Hsieh reported an organometallic thallium complex of the form (N,N)TlMe2.16 Ikeda and coReceived: July 16, 2014

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Figure 1. Compounds presented herein.

Scheme 1. Preparation of Dipyrromethene 2

Scheme 2. Preparation of Aluminum Compounds 3−7 and Dipyrromethane 8

meso position with such specificity and can in fact be utilized to install B−E bonds from the parent BODIPYs.22 Our goal was to synthesize a number of complexes of the type (N,N)AlR2 that are convenient to prepare and provide entry into a novel area of organoaluminum chemistry. Herein, we report the synthesis and characterization of dipyrromethane 1, dipyrromethene 2, monomeric organoaluminum complexes 3−6, and the bimetallic aluminoxane 7 (Figure 1). We also report attack of tBuLi at the meso position of 2 to afford dipyrromethane 8.

workers reported a salen-like luminescent dipyrromethene complex of aluminum.19 Our initial investigations have focused on the synthesis and characterization of a series of dipyrrinatoaluminum complexes with alkyl, aryl, and hydride coligands in order to ensure that the expected complexes were obtainable through alkane, arene, or hydrogen elimination pathways. As the dipyrromethene ligand is susceptible to reduction, the preparation and isolation of the most simple BODIPY derivative, namely that of borane, has proven to be intractable, although a cationic dipyrromethene complex of the form [(N,N)BH] + has been described.20,21 Reaction of borane and dipyrromethene leads to reduction of the ligand by attack of hydride at the meso position and ultimately yields a neutral complex of the form (N,N)BH, where (N,N) is a divalent dipyrromethane ligand.21 Other organometallic nucleophiles do not necessarily attack the

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RESULTS AND DISCUSSION

Synthesis and Characterization of Monomeric Dipyrromethene Complexes. Dipyrromethene 2 was prepared with slight modifications to the procedure employed by Betley for the preparation of the analogous meso-mesityl dipyrromethene.8 Condensation of 2-mesitylpyrrole23 with benzaldehyde B

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Furthermore, observed frequencies agree well with the Al−H stretching frequencies of 1812 and 1822 cm−1 obtained from DFT/B3LYP (6-31G*) calculations (see the Supporting Information). Interestingly, compound 6 fluoresces green under black-light irradiation while 3−5 are nonemissive, although characterization of the photophysical properties of these complexes is beyond the aim of this work. It is interesting to note, although it is unclear why, that preparation of alane complex 6 is so straightforward while borane ((N,N)BH2) analogues of BODIPY have evaded isolation and characterization.21 In contrast to the syntheses for 3, 5, and 6, reaction of AltBu3 with 2 produces only a small amount of 4, isomer 4b, and 8, in the presence of a complicated mixture of products. Conditions for the high-yield preparation of 4 through tBuH elimination were not identified, but recovery of small amounts of 4, 4b, and 8 was accomplished by preparative thin-layer chromatography on silica gel plates and elution with 9/1 hexanes/ethyl acetate. Salt metathesis was found to be a more practical method for the preparation of 4. Reaction of the in situ generated lithium salt of 2 with an equimolar amount of ClAltBu2 afforded 4 in 58% yield. Isomeric 4b, (N,N)AltBuiBu, was present as the major impurity and is the result of β-hydrogen elimination from t Bu 3 Al to transiently afford isobutylene and HAl t Bu 2 . Subsequent insertion of isobutylene ultimately yields i BuAltBu2, as reported by Lehmkuhl.29 As for 3, 5, and 6, complex 4 exhibits C2v symmetry in solution, as evidenced by NMR spectroscopy. The tBu group was observed as a singlet at 0.38 ppm (CDCl3). The 1H NMR spectrum of the isomeric, mixed tBu/iBu complex 4b was consistent with lowering of symmetry from C2v to Cs. Accordingly, two mesityl hydrogen and two o-methyl resonances were present in 4b, as the mesityl groups are unable to rotate. The resonances of the meso-phenyl group were unperturbed in comparison to those of 4, which is consistent with free rotation of the phenyl group. Compound 8, which was afforded in small amounts during attempts to prepare 4 from tBu3Al, was prepared nearly quantitatively (1H NMR) by the reaction of 2 with 2 equiv of t BuLi. The first equivalent of tBuLi presumably deprotonates the dipyrromethene, while the second equivalent attacks the meso carbon to give the dilithium salt of 8. Aqueous workup followed by chromatography and crystallization gave 8 in 33% yield. The tert-butyl group is observed in the room-temperature 1 H NMR spectrum as a broad singlet at 1.29 ppm that sharpens significantly as the temperature is increased (see the Supporting Information, Figure S18). The structure was further confirmed by X-ray crystallography, and the ORTEP diagram is depicted in Figure 2. Reaction of 2 with 2 equiv of MeLi, under conditions identical with those for the formation of 8, led only to recovery of ligand 2. Accordingly, care must be taken when selecting an organometallic reagent for deprotonation of dipyrromethenes, as strong nucleophiles readily add to the meso carbon. It is interesting to note that the addition of alkyllithium reagents to the meso carbon is well established in the porphyrin literature,30 although, to the knowledge of the authors, it has been undocumented in dipyrromethene chemistry. Reactivity of Dipyrromethene Complexes. The alkyl and phenyl complexes 3−5 are remarkably robust and can be handled in air for short periods of time with minimal decomposition. Similarly stable aluminum complexes, supported by sterically demanding β-diketiminate ligands, have

yielded dipyrromethane 1 as a 1:1 methanol adduct in 40% yield upon crystallization from methanol. Subsequent oxidation of 1 with DDQ afforded 2 in 71% yield upon separation of H2DDQ (Scheme 1). The chemical shift of the N−H proton in 2, in the presence of moisture, is observed at 12.72 ppm (CDCl3) as a broad singlet and shifts downfield and sharpens in strictly anhydrous samples. A singlet integrating to four protons is observed for the mesityl hydrogens, and two doublet resonances integrating to two protons each are observed for the pyrrolyl hydrogens. The para and ortho mesityl methyl resonances are observed as two singlets in the alkyl region and integrate to 6 and 12 protons, respectively. The chemical shift of the CN pyrrolyl α-carbon in the 13C NMR spectrum was observed at 154.68 ppm, and its assignment is supported by the observation of cross-peaks with pyrrolyl hydrogens in the 2D HMBC spectrum. Crystals of 2 can be grown from concentrated toluene or chloroform solutions, and X-ray crystallography confirmed the expected connectivity of 2 (see the Supporting Information, Figure S1). Complexes 3, 5, and 6 were prepared in good yield by reactions of ligand 2 with an equimolar amount of an organoaluminum source (AlR3; R = Me, Ph, H) in toluene, as summarized in Scheme 2. The preparation of dimethyl complex 3 proceeded smoothly and was nearly quantitative on the basis of NMR analysis of the crude product. The 1H NMR data for 3 are consistent with a complex of C2v symmetry bearing two methyl groups on aluminum; these groups are observed as a broad singlet at −1.29 ppm in CDCl3. Accordingly, the Al−Me carbon resonance in the 13C NMR spectrum is broad and is observed at −9.00 ppm. All 13C NMR chemical shifts were assigned in accordance with the results of HMQC and HMBC 2D NMR experiments. Reaction of Ph3Al·OEt2 with 2 similarly afforded the diphenyl complex 5 as yellow-orange needles in 78% yield after crystallization from a concentrated toluene solution. C2v symmetry is observed in solution by 1H NMR spectroscopy, suggesting that the Al−Ph groups are free to rotate. The phenyl groups on aluminum give rise to ring current effects, shifting the o-methyl and mesityl hydrogen resonances of 5 upfield of those for 3 or 4. The o-methyl resonance is observed at 1.40 ppm for 5 (cf. 2.06 ppm for 3 and 2.16 ppm for 4), and the mesityl hydrogen chemical shift is at 6.33 ppm for 5 (cf. 6.87 ppm for 3 and 6.88 ppm for 4). The Al−C resonance in the 13C NMR spectrum is a broad singlet observed at 146.07 ppm. Treatment of 2 with H3Al·NMe2Et afforded 6 as red crystals in 40% isolated yield, but the reaction was nearly quantitative, as determined by the 1H NMR spectrum of the crude reaction mixture. The 1H NMR spectrum of 6 indicates C2v symmetry in solution, and the hydrides appear as a broad singlet integrating to two protons at 3.44 ppm in CDCl3 and 4.25 ppm in C6D6. These are comparable with the hydride chemical shifts of 4.58 and 4.80 ppm reported for the β-diketiminatoaluminum complexes HC[CMeN(o-xylyl)]2AlH224 and HC[CMeN(CH3)]2AlH225 in C6D6, as well as chemical shifts reported for other terminal aluminum hydride complexes.26 The presence of terminal hydrides was further confirmed by the observation of aluminum hydride stretches at 1809 and 1828 cm−1 in the infrared spectrum. These frequencies are within the typical range observed for terminal aluminum hydride complexes26,27 and are quite similar to those reported for the related β-diketiminate complexes HC[CMeN(o-xylyl)]2AlH224 and HC[CMeN(2,6-iPrC6H3)]2AlH2,28 which were observed at 1819, 1787 cm−1 and 1832, 1795 cm−1 , respectively. C

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NMR, 3.95 ppm; IR, νAl−H 1804, 1795 cm−1)24 and other complexes of aluminum with terminal hydrides.26,27 Calculated aluminum−hydride stretching frequencies for 7 of 1845 and 1851 cm−1, obtained by DFT at the B3LYP/6-31 G* level (see the Supporting Information), also agree well with observed results. Oxygen-bridged aluminum complexes with terminal hydrides are rare, owing to the reactivity and accessibility of hydride ligands. Furthermore, dimers are even less observed as, in the absence of significant steric bulk, oxygen prefers to bridge three aluminum centers. Compound 7 is one of only a few examples of a bimetallic oxo-bridged aluminum complex with terminal hydrides to be structurally characterized.24,33 X-ray Crystallography. Molecular structures of 2−8 were confirmed by X-ray crystallography (see Figures 2−7, Figure S1, and Table S1 (Supporting Information)). The molecular structure of 3 is depicted in Figure 3. Rigorous C2 symmetry is

Figure 2. ORTEP diagram of 8. Thermal ellipsoids are drawn at the 50% probability level. Non N−H hydrogen atoms are omitted for clarity.

been reported in the literature.31 More striking is the stability of 3−5 to brief exposure to silica gel, which is likely due to steric stabilization of otherwise reactive organoaluminum groups. Unreacted organoaluminum and ligand 2 are thus easily removed by quickly filtering the crude reaction mixtures through a pad of silica gel. Dihydride complex 6 is considerably more reactive than 3−5. The presence of adventitious moisture leads to the formation of oxygen-bridged dimer 7 (Scheme 1). Controlled hydrolysis of 6 through the addition of 1/2 equiv of water generates mixtures of 7 and 2. In order to avoid complete hydrolysis to 2, water was introduced slowly through the use of hydrated aluminum sulfate, Al2(SO4)3·18H2O. Hydrated salts have found industrial and academic utility in the synthesis of alkylaluminoxanes2a,c,32 and function well for our purposes, mitigating the formation of 2. Controlled hydrolysis of 6 with Al2(SO4)3·18H2O afforded the bimetallic oxo-bridged terminal hydride complex 7 in 54% yield. Compound 7 possesses C2 symmetry in solution, as evidenced by 1H NMR spectroscopy. This symmetry relates the two dipyrromethene aluminum moieties in the dimer but does not relate the two mesityl or two pyrrole rings per dipyrromethene aluminum fragment. Thus, the 1H NMR spectrum exhibits four doublet resonances for the pyrrolyl βhydrogens and four singlet resonances for the inequivalent mesityl hydrogens on each ligand. Three of these resonances are clearly resolved, while one pyrrolyl resonance overlaps with a mesityl hydrogen resonance. Accordingly, six methyl resonances are also observed, two of which are clearly resolved while the others occur as two sets of two closely spaced singlets. One of the o-methyl groups is significantly shifted upfield and is observed at 1.09 ppm (cf. 2.12 ppm in 6); the cause of this can be gleaned from the structural data and will be discussed shortly. Variable-temperature NMR spectra suggest that the geometry of 7 is rigid up to at least 55 °C, as resonances begin to broaden upon warming but do not coalesce (see the Supporting Information, Figure S19). It is interesting to note that the analogous β-diketiminate compound (HC[CMeN(oxylyl)]2AlH)2O possesses C2v symmetry in solution, suggesting more flexibility about the Al−O−Al linkage relative to that for 7.24 The presence of terminal hydrides was confirmed by the observation of a broad singlet integrating to two protons at 3.27 ppm in the 1H NMR spectrum in CDCl3, as well as by Al−H stretches at 1833 and 1849 cm−1 in the infrared spectrum. These data are comparable with those observed in the βdiketiminate analogue (HC[CMeN(o-xylyl)]2AlH)2O (1H

Figure 3. ORTEP diagram of 3. Thermal ellipsoids are drawn at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (deg): Al(1)−N(1), 1.936(2); Al(1)− C(19), 1.962(2); N(1)−Al(1)−N(1A), 92.2(1). Symmetry operator (A): −x, −y, z.

observed in the solid state, with Al, Cmeso, Cipso‑Ph and Cpara‑Ph lying on a 2-fold axis in the space group Pnn2. The pyrrolyl rings are planar, and the aluminum atom sits in the plane of the ligand. Al−C and Al−N bond lengths are similar to those reported for analogous β-diketiminate aluminum complexes.24,31 Complex 4, which crystallizes in the monoclinic space group P21/n (see Figure 4), does not lie on a C2 axis, in contrast to the structure of 3. The ligand is slightly puckered, with an angle of 14.80(8)° between planes of the two pyrrole rings. The aluminum atom sits 0.8768(6) Å above the mean plane defined by the five atoms which form the six-membered chelate ring with aluminum. Accordingly, the environments of the tBu groups are different. One of the tBu groups lies between the mesityl groups, and the other is oriented away from the dipyrromethene ligand. Al−C bond lengths are approximately 0.1 Å longer than those in 3 due to the increased steric bulk of the tert-butyl groups. Presumably, C2 symmetry in solution is a result of the AltBu2 fragment rocking such that the tBu groups are interconverted on the NMR time scale. A similar motion in solution is likely for 5 as well, which would explain the equivalence of AlPh2 chemical shifts in the 1H NMR spectrum. Complex 5, which crystallizes in the monoclinic space group P21/c (see Figure 5), adopts a conformation between those of 3 D

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Figure 4. ORTEP diagram of 4. Thermal ellipsoids are drawn at the 50% probability level. Hydrogen atoms are omitted for clarity. One orientation of the disordered tBu group (methyls on C(34)) is shown for clarity. Selected bond distances (Å) and angles (deg): Al(1)−N(1), 1.967(2); Al(1)−N(2), 1.958(2); Al(1)−C(34), 2.037(2); Al(1)− C(38), 2.047(2); N(1)−Al(1)−N(2), 91.87(7).

Figure 6. ORTEP diagram of 6. Thermal ellipsoids are drawn at the 50% probability level. Non Al−H hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (deg): Al(1)−N(1), 1.902(4); Al(1)−N(2), 1.915(3); Al(1)−H(1), 1.51(4); Al(1)−H(2), 1.54(4); N(1)−Al(1)−N(2), 93.3(2).

a closest distance of less than 3.4 Å to two mesityl aryl carbons on the other ligand. The structures presented herein depict a steric environment which is considerably different from that of β-diketiminate ligands, as can be seen in Figure 8. Overlaying the structures of 3 and an analogous β-diketiminate complex, HC(H3CCNAr)2AlMe2 (Ar = o-xylyl),24 shows that there is increased bulk flanking the metal center in 3. However, there appears to be slightly less steric hindrance in 3 directly above the metal center relative to the most bulky β-diketiminate complexes with a group such as 2,6-diisopropylphenyl at nitrogen, which extend closer to the metal center than do the omethyl groups in 3. This difference in steric influence will likely affect the kinetic stabilization of reactive moieties by group 13 dipyrromethene complexes and alter their catalytic activities.

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Figure 5. ORTEP diagram of 5. Thermal ellipsoids are drawn at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (deg): Al(1)−N(1), 1.928(1); Al(1)− N(2), 1.921(1); Al(1)−C(101), 1.972(1); Al(1)−C(201), 1.994(5); N(1)−Al(1)−N(2), 93.90(4).

CONCLUSIONS Sterically demanding ligands have been applied with great success to the preparation of elusive and reactive motifs which are otherwise challenging to handle. The results of this work show there is striking similarity with bulky dipyrromethenes and β-diketiminate ligands, as evidenced by structural similarity and the robust character of the dialkyl aluminum complexes. Additionally, access to a bimetallic oxo-bridged dimer and its apparent rigidity further corroborate the presence of significant steric bulk for this ligand system. It is worth noting that the first example of an emissive dipyrromethene complex of a group 13 element with hydride coligands has been prepared and structurally characterized. Currently we are pursuing aluminum complexes with other coligands such as alkoxides and amides, for application to the ring-opening polymerization of polar monomers, as well as the development of this ligand platform for low-valent group 13 chemistry.

and 4. The planes of the two pyrrolyl rings are twisted by 9.19(4)° with respect to each other, and the aluminum atom sits 0.2448(4) Å above the plane defined in the same way as described above for 4. The structure of terminal hydride complex 6 (see Figure 6), which crystallizes in the orthorhombic space group Pna21, closely resembles that of 3 and is similar to structures of analogous β-diketiminate complexes.24,28 The ligand is nearly planar, and aluminum sits within the ligand plane. The hydrides bound to aluminum were located in the difference Fourier maps and refined isotropically. In the solid-state structure of 7 (see Figure 7), the Al−O−Al angle is 166.742(1)° and is comparable to Al−O−Al bond angles observed in similar βdiketiminate complexes with Al−O−Al linkages.24 The structure also suggests the cause of the upfield shift of one of the o-methyl resonances observed in the 1H NMR spectrum. One of the mesityl groups is nearly sandwiched by the two mesityl groups of the other ligand. This arrangement gives rise to a ring current effect at one of the o-methyl groups, which has

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EXPERIMENTAL SECTION

General Procedures. All reactions were performed under an atmosphere of purified argon using standard inert-atmosphere techniques. Toluene, hexanes, dichloromethane, and chloroform were purchased from Fisher Scientific. Toluene was distilled from sodium, hexanes were distilled from sodium benzophenone ketyl solubilized with tetraglyme, and dichloromethane was distilled from E

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Figure 8. Overlay of crystal structures of 3 (yellow) and HC(H3CCNAr)2AlMe2 (gray; Ar = o-xylyl) displayed as balls and sticks. respectively, and dried over activated molecular sieves and degassed with argon prior to use. Solution NMR spectra were recorded on either a Varian Inova 600 MHz spectrometer or Bruker Avance III 600 MHz spectrometer with a cryoprobe. Chemical shifts are reported relative to tetramethylsilane. All reported 13C NMR chemical shifts are singlets unless otherwise noted. Assignments of 1H and 13C NMR chemical shifts were aided by APT and 2D COSY, HMQC, and HMBC experiments. Melting points were obtained using either a Thomas Scientific Uni-Melt or Laborartory Devices Mel-Temp melting point apparatus and are uncorrected. Infrared spectra were recorded on a PerkinElmer FT-IR Spectrum Two spectrometer. Elemental analyses were performed either by Galbraith Laboratories or in house using a PerkinElmer 2400 Series II CHNS/O Elemental Analyzer. Preparation of 5-Phenyl-2,2′-dimesityldipyrromethane (1). Dichloromethane (100 mL) was added to a flask charged with 2mesitylpyrrole (7.00 g, 38.0 mmol) and benzaldehyde (2.00 g, 19.0 mmol) under argon. Pyridinium tosylate (1.00 g, 4.00 mmol) was then quickly added to the flask, the reaction vessel was flushed with argon, and the mixture was stirred for 12 h. The reaction mixture was filtered through a 30 cm3 pad of silica gel, resulting in a yellow filtrate, which was reduced to an oil in vacuo. The crude product was triturated with hexanes and filtered, resulting in a white solid of sufficient purity for the oxidation to afford 2. The light yellow filtrate was further concentrated to yield a second crop of product. Analytically pure crystals were obtained from methanol, yielding colorless needles of 1· MeOH. Yield: 3.5 g, 40%. Mp: 134−135 °C. 1H NMR (CDCl3, 600 MHz): δ 7.66 (br s, 2H, NH), 7.29 (m, 5H, Ph), 6.89 (s, 4H, Mes-H), 5.98 (m, 2H, pyrr), 5.95 (m, 2H, pyrr), 5.54 (s, 1H, CH), 2.29 (s, 6H, p-CH3), 2.10 (s, 12H, o-CH3). 13C{1H} NMR (CDCl3, 600 MHz): δ 142.57 (ipso-Ph), 138.49 (Mes), 137.62 (Mes), 131.99 (α-pyrr), 130.85 (Mes), 128.97 (α-pyrr), 128.75 (Ph), 128.47 (Ph), 128.17 (Mes-H), 127.07 (Ph), 108.40 (β-pyrr), 107.35 (β-pyrr), 44.18 (methine), 21.17 (p-CH3 ), 20.80 (o-CH 3 ). Anal. Calcd for C33H34N2·CH3OH: C, 83.22; H, 7.81; N, 5.71. Found: C, 83.16; H, 7.95; N, 5.58. Preparation of meso-Phenyl-α,α′-dimesityldipyrromethene (2; (N,N)H). Dichlorodicyanoquinone (7.57 g, 0.033 mol) was slowly added to a stirred solution of dipyrromethane 1 (14.35 g, 0.031 mol) in toluene (100 mL), resulting in an immediate color change to deep purple. The flask was then capped with a septum and flushed with argon, and the mixture was stirred overnight. The mixture was transferred to a separatory funnel and washed with saturated NaHCO3(aq) (3 × 25 mL) followed by 1 M NaOH(aq) (3 × 25 mL). (Note: we have found that extraction of the crude mixture with

Figure 7. ORTEP diagram of 7. Thermal ellipsoids are drawn at the 50% probability level. Non Al−H hydrogen atoms and lattice toluene molecules are omitted for clarity. All atoms in the second ligand, except N atoms, are depicted as sticks, and only one orientation of the disordered phenyl group is depicted for clarity (top). Mesityl groups are omitted and only one orientation of the disordered phenyl group is depicted for clarity (bottom). Selected bond distances (Å) and angles (deg): Al(1)−N(1), 1.925(2); Al(1)−N(2), 1.927(2); Al(2)−N(3), 1.923(2); Al(2)−N(4), 1.929(2); Al(1)−O(1), 1.689(1); Al(2)− O(1), 1.688(1); N(1)−Al(1)−N(2), 93.99(6); N(3)−Al(2)−N(4), 94.01(7); Al(1)−O(1)−Al(2), 166.75(9). calcium hydride prior to use. Anhydrous aluminum chloride, pyridinium tosylate, sodium bicarbonate, and trimethylaluminum (1.6 M in hexane) were purchased from Aldrich and used as received. 2,3-Dichloro-5,6-dicyanoquinone and n-butyllithium (1.6 M in hexane) were purchased from Acros and used without further purification. Benzaldehyde and sodium hydroxide were obtained from Fisher Scientific and used without purification. Silica gel (220− 450 mesh) and Florisil (60−100 mesh) were purchased from Alfa Aesar and Fisher Scientific, respectively, and used as received. Celite was purchased from Fisher and dried overnight in a 130 °C oven prior to use. 2-Mesitylpyrrole was prepared with modifications to the procedure of Sadighi and co-workers.23 Tri-tert-butylaluminum,29,34,35 triphenylaluminum etherate,36 and N,N-dimethylethylaminoalane37 were prepared as previously described. Chloroform-d and benzene-d6 were purchased from Cambridge Isotope Laboratories and Aldrich, F

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storage. Although 4b was only observed by 1H and 13C NMR spectroscopy, compounds 4 and 4b can be separated by serial recrystallization from toluene, which affords a mother liquor enriched in 4b (2:1 4b:4 as determined by 1H NMR spectroscopy). 1H NMR (CDCl3, 600 MHz): δ 7.56 (m, 2H, o-Ph), 7.48 (m, 3H, m,p-Ph, overlaps with 4), 6.91 (s, 2H, Mes-H), 6.88 (s, 2H, Mes-H, overlaps with 4), 6.81 (d, 3JHH = 4.2 Hz, 2H, pyrr), 6.31 (d, 3JHH = 4.2 Hz, 2H, pyrr), 2.29 (s, 6H, CH3), 2.28 (s, 6H, CH3), 2.08 (s, 6H, CH3), 0.76, (m, 1H, AlCH2CH(CH3)2), 0.59 (s, 9H, AltBu), 0.17 (d, 3JHH = 6.6 Hz, 6H, AlCH 2CH(CH3)2), −0.55 (d, 3JHH = 6.6 Hz, 2H, AlCH2CH(CH3)2). 13C{1H} NMR (CDCl3, 600 MHz): δ 161.80, 140.78, 138.56, 138.39, 137.86, 136.70, 133.82, 131.74, 129.20, 128.95, 128.53, 128.35, 120.53, 31.04, 27.82, 24.63, 21.21, 21.20, 21.17. Note: the meso carbon and carbons bound to aluminum, Al-C(CH3)3 and Al-CH 2CH(CH3)2, were not observed. An additional carbon resonance was either not observed or is obscured by resonances of 4 which are of higher intensity. Preparation of (N,N)AlPh2 (5). A toluene solution (15 mL) of Ph3Al·OEt2 (0.82 g, 0.25 mmol) was added to a stirred toluene solution of 2 (0.10 g, 0.23 mmol) at room temperature. The reaction mixture was stirred overnight and then filtered through a small plug of silica gel under reduced pressure. Analytically pure, light orange crystals were obtained by recrystallization from a concentrated toluene solution stored at −20 °C. Yield: 0.112 g, 78%. Mp: 216 °C dec. 1H NMR (CDCl3, 600 MHz): δ 7.74 (m, 2H, Ph), 7.56 (m, 3H, Ph), 7.17 (m, 4H, AlPh2, o-CH), 7.10 (m, 2H, AlPh2, p-CH), 6.97 (m, 4H, AlPh2, m-CH), 6.90 (d, 3JHH = 4.2 Hz, 2H, pyrr), 6.33 (s, 4H, Mes-H), 6.27 (d, 3JHH = 4.2 Hz, 2H, pyrr), 2.07 (s, 6H, p-CH3), 1.40 (s, 12H, oCH3). 13C{1H} NMR (CDCl3, 600 MHz): δ 163.03 (CN), 146.88 (meso), 146.07 (br, ipso-Ph-Al), 139.43 (α-pyrr), 138.10 (Mes), 138.05 (o-Ph-Al), 137.67 (ipso-Ph), 137.43 (Mes), 134.40 (β-pyrr), 131.01 (oPh), 130.07 (Mes), 129.20 (p- or m-Ph), 127.66 (p- or m-Ph), 127.44 (Mes-H), 126.86 (p-Ph-Al), 126.02 (m-Ph-Al), 120.44 (β-pyrr), 21.04 (p-CH3), 20.20 (o-CH3). Anal. Calcd for C45H41AlN2: C, 84.87; H, 6.49; N, 4.40. Found: C, 84.53; H, 6.43; N, 4.41. Preparation of (N,N)AlH2 (6). A toluene solution (25 mL) of H3Al·NMe2Et (0.12 g, 1.2 mmol) was added to a stirred toluene solution of 2 (0.50 g, 1.1 mmol) at −78 °C. The resulting mixture was stirred overnight and then filtered through Celite to remove insolubles. The crude product was purified by recrystallization from a concentrated toluene solution stored at −20 °C, affording analytically pure red crystals. Yield: 0.209 g, 40%. Mp: 209 °C dec. IR (mineral oil, νAl−H, cm−1): 1828, 1808. 1H NMR (CDCl3, 600 MHz): δ 7.64 (m, 2H, o-Ph), 7.55 (m, 1H, p-Ph), 7.51 (m, 2H, m-Ph), 6.90 (d, 3JHH = 4 Hz, 2H, pyrr), 6.87 (s, 4H, Mes-H), 6.36 (d, 3JHH = 4 Hz, 2H, pyrr), 3.44 (br s, 2H, AlH2), 2.26 (s, 6H, p-CH3), 2.12 (s, 12H, o-CH3). 1H NMR (C6D6, 600 MHz): δ 7.17 (m, 2H, o-Ph, partially obscured by residual solvent), 7.12 (m, 1H, p-Ph), 7.03 (m, 2H, m-Ph), 6.78 (d, 3 JHH = 4 Hz, 2H, pyrr), 6.77 (s, 4H, Mes-H), 6.13 (d, 3JHH = 4 Hz, 2H, pyrr), 4.25 (br s, 2H, AlH2), 2.28 (s, 12H, o-CH3), 2.08 (s, 12H, oCH3). 13C{1H} NMR (CDCl3, 600 MHz): δ 161.98 (CN), 146.87 (meso), 139.10 (Mes), 138.73 (α-pyrr), 137.43 (Mes), 137.09 (ipsoPh), 134.65 (β-pyrr), 130.95 (o-Ph), 130.01 (Mes), 129.37 (p-Ph), 128.32 (Mes-H), 127.71 (m-Ph), 119.42 (β-pyrr), 21.37 (p-CH3), 20.22 (o-CH3). Anal. Calcd for C33H33AlN2: C, 81.79; H, 6.86; N, 5.78. Found: C, 81.42; H, 6.73; N, 5.52. Preparation of [(N,N)AlH]2O (7). A toluene solution (25 mL) of H3Al·NMe2Et (0.111 g, 1.08 mmol) was added to a stirred toluene solution of 2 (0.469 g, 1.03 mmol) at −78 °C. The resulting mixture was stirred overnight, at which point Al2(SO4)3·18H2O (0.020 g, 0.540 mmol of H2O) was added to the crude mixture with a solid addition tube. The mixture was filtered after stirring at room temperature for 2 weeks. The crude solution was reduced to an oily residue in vacuo and triturated with hexanes. The mixture was allowed to settle, the hexanes extract was decanted off, and the crude solid was washed with hexanes (2 × 5 mL). Crude 7 was purified by crystallization from a concentrated toluene solution stored at −20 °C, affording X-rayquality red crystals of the toluene solvate 7·1.5C7H8. Repeated recrystallization afforded pure crystals of 7·1.5C7H8. Yield: 0.273 g, 54%. Mp: 262 °C dec. IR (mineral oil, νAl−H, cm−1): 1849, 1833. 1H

NaHCO3(aq) and NaOH(aq) facilitates removal of H2DDQ prior to chromatography. It is worth noting that purple precipitate is obtained following oxidation. The purple material was determined to be an acid−base adduct of the form [(N,N)H2]+[HDDQ]−. Although the adduct is poorly soluble in CDCl3, the color changes from purple to red-orange upon treatment with NEt3 and both 2 and H2DDQ can be observed by NMR spectroscopy.) The organic layer was then concentrated and passed through a pad of Florisil, washing with chloroform until the eluent was pale yellow. Solvent was removed from the collected solution under reduced pressure, yielding dipyrromethene 2 as an orange powder. Samples for elemental analysis were obtained by recrystallization from a concentrated toluene solution stored overnight at −20 °C, affording orange crystals. Yield: 10.15 g, 71%. Mp: 188−190 °C. 1H NMR (CDCl3, 600 MHz): δ 12.72 (br s, 1H, NH), 7.63 (m, 2H, o-Ph), 7.48 (m, 3H, m,p-Ph), 6.91 (s, 4H, Mes-H), 6.64 (d, 3JHH = 4 Hz, 2H, pyrr), 6.30 (d, 3JHH = 4 Hz, 2H, pyrr), 2.30 (s, 6H, p-CH3), 2.23 (s, 12H, o-CH3). 13C{1H} NMR (CDCl3, 600 MHz): δ 154.68 (CN), 140.80 (α-pyrr), 139.91 (ipsoPh), 137.93 (Ph), 137.63 (Mes), 137.24 (Mes), 131.82 (Mes), 131.10 (o-Ph), 128.74 (meso), 128.54 (Mes-H), 128.48 (β-pyrr), 127.66 (Ph), 119.47 (β-pyrr), 21.22 (p-CH3), 20.91 (o-CH3). Anal. Calcd for C33H32N2: C, 86.80; H, 7.06; N, 6.13. Found: C, 86.61; H, 7.35; N, 6.10. Preparation of (N,N)AlMe2 (3). A solution of dipyrromethene 2 (0.40 g, 0.88 mmol) in toluene (25 mL) was added via cannula to a cooled solution (−78 °C) of AlMe3 (0.48 mL, 2.0 M in hexanes, 0.97 mmol), which had been diluted in several milliliters of toluene. After it was stirred at −78 °C for 1 h, the light orange solution was warmed to room temperature, stirred for an additional 2 h, and filtered through a small silica gel pad under reduced pressure. The resulting solution was concentrated in vacuo, stored at −20 °C overnight, and filtered to isolate 220 mg of analytically pure dichroic orange/green crystals as the first crop of product. Further concentration of the filtrate afforded 120 mg of a second crop of crystals. Yield: 0.340 g, 75%. Mp: 215 °C dec. 1H NMR (CDCl3, 600 MHz): δ 7.65 (m, 2H, o-Ph), 7.50 (m, 3H, m,p-Ph), 6.87 (s, 4H, Mes-H), 6.83 (d, 3JHH = 3.9 Hz, 2H, pyrr), 6.32 (d, 3JHH = 3.9 Hz, 2H, pyrr), 2.28 (s, 6H, p-CH3), 2.06 (s, 12H, oCH3), −1.29 (s, 6H, Al(CH3)2). 13C{1H} NMR (CDCl3, 600 MHz): δ 161.78 (CN), 146.68 (meso), 138.90 (α-pyrr), 138.74 (Mes), 137.89 (Mes), 137.70 (ipso-Ph), 134.03 (β-pyrr), 131.20 (Mes), 130.97 (o-Ph), 129.06 (p-Ph), 127.83 (Mes-H), 127.52 (m-Ph), 119.60 (β-pyrr), 21.31 (p-CH3), 20.67 (o-CH3), −9.02 (Al(CH3)2). Anal. Calcd for C35H37AlN2: C, 82.00; H, 7.27; N, 5.46. Found: C, 82.39; H, 7.45; N, 5.36. Preparation of (N,N)AltBu2 (4). A solution of tBu2AlCl was generated in situ by addition of neat tBu3Al (0.315 g, 1.58 mmol) to a toluene (4 mL) slurry of AlCl3 (0.105 g, 0.787 mmol). After the mixture was stirred overnight, the required amount of the resulting solution (0.89 g solution, 0.096 g tBu2AlCl, 0.55 mmol) was withdrawn and added to a toluene solution (15 mL) of the lithium salt of 2, generated by deprotonation of 2 (0.25 g, 0.55 mmol) with nBuLi (1.6 M hexane, 0.34 mL, 0.55 mmol). The reaction mixture was stirred overnight and filtered through a small silica gel plug under reduced pressure. Removal of solvent afforded compound 4 as a red-orange solid, contaminated by a small amount of isomeric (N,N)Al(iBu)(tBu) (4b). Several recrystallizations from a concentrated toluene solution stored at −20 °C afforded pure 4 as dichroic red/green crystals. Yield: 0.192 g, 58%. Mp: 224 °C dec. 1H NMR (CDCl3, 600 MHz): δ 7.60 (m, 2H, o-Ph), 7.48 (m, 3H, m,p-Ph), 6.88 (s, 4H, Mes-H), 6.84 (d, 3 JHH = 4.2 Hz, 2H, pyrr), 6.26 (d, 3JHH = 4.2 Hz, 2H, pyrr), 2.30 (s, 6H, p-CH3), 2.16 (s, 12H, o-CH3), 0.38 (s, 18H, AltBu). 13C{1H} NMR (CDCl3, 600 MHz): δ 163.08 (CN), 146.44 (meso), 141.83 (α-pyrr), 138.82 (Mes), 138.17 (ipso-Ph), 137.57 (Mes), 134.96 (βpyrr), 133.12 (Mes), 131.11 (o-Ph), 128.99 (p-Ph), 128.47 (Mes-H), 127.53 (m-Ph), 121.11 (β-pyrr), 31.17 (AlC(CH3)3), 21.32 (p-CH3), 21.04 (o-CH3), 16.92 (br, AlC(CH3)3). Anal. Calcd for C41H49AlN2: C, 82.51; H, 8.28; N, 4.69. Found: C, 82.01; H, 8.49; N, 4.75. Isolation of (N,N)Al(iBu)(tBu) (4b). Compound 4b was observed by 1H NMR spectroscopy as an impurity during the preparation of 4. It is present due to the isomerization of tBu3Al to iBuAltBu2 during G

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NMR (CDCl3, 600 MHz): δ 7.80 (m, 2H, Ph), 7.60−7.49 (m, 8H, Ph), 6.82 (d, 3JHH = 4 Hz, 2H, pyrr), 6.68 (m, 4H, pyrr and Mes-H overlap), 6.60 (s, 2H, Mes-H), 6.52 (s, 2H, Mes-H), 6.44 (s, 2H, MesH), 6.10 (d, 3JHH = 4 Hz, 2H, pyrr), 6.00 (d, 3JHH = 4 Hz, 2H, pyrr), 3.27 (br s, 2H, AlH), 2.28 (s, 6H, CH3), 2.07 (two overlapped s, 12H, CH3), 1.96 (two overlapped s, 12H, CH3), 1.08 (s, 6H, CH3). 13 C{1H} NMR (CDCl3, 600 MHz): δ 163.79 (CN), 161.53 (C N), 145.51 (meso), 139.14 (α-pyrr), 138.93 (α-pyrr), 138.25, 138.15, 138.04, 137.27, 137.23, 136.17, 133.64 (β-pyrr), 133.20 (β-pyrr), 131.74 (Ph), 131.35, 130.54 (Ph), 130.29, 129.20, 128.98, 128.38 (Ph), 128.00 (Mes-H), 127.64 (Ph), 127.52 (Mes-H), 127.43 (MesH), 126.68 (Mes-H), 120.44 (β-pyrr), 119.00 (β-pyrr), 21.74 (p-CH3), 21.22 (o-CH3), 21.16 (p-CH3), 20.78 (o-CH3), 20.52 (o-CH3), 20.38 (o-CH3). Anal. Calcd for 7·C7H8, C73H72Al2N4O: C, 81.54; H, 6.75; N, 5.21. Found: C, 81.46; H, 7.14; N, 5.12. Preparation of 5-tert-Butyl-5-phenyl-2,2′-dimesityldipyrromethane (8). A solution of tBuLi (1.7 M hexane, 0.65 mL, 1.1 mmol) was added dropwise at room temperature to a stirred toluene solution (5 mL) of 2 (0.240 g, 0.525 mmol). After the mixture was stirred for several hours, methanol (2 mL) was slowly added to the reaction mixture, followed by addition of water (2 mL). After quenching, the color of the reaction mixture lightened considerably. The mixture was then transferred to a separatory funnel and the aqueous layer removed. The organic layer was washed with water (2 × 5 mL), and the combined aqueous layers were extracted with toluene (5 mL). Organic solutions were combined and reduced to an oil in vacuo. The crude oil is fairly pure as determined by 1H NMR spectroscopy, although TLC reveals several colored impurities. The oil was purified by silica gel chromatography (9/1 hexanes/ethyl acetate), resulting in a yellow oil after removal of solvent. A second column was run under the same conditions in order to remove colored impurities that coeluted with 8 on the first column. Only yellow fractions were kept. After removal of solvent, the oil solidified upon storage at −20 °C for about 1 week. Xray-quality crystals were afforded by dissolving 8 in a minimal amount of hot isopropyl alcohol and storing at −20 °C for several days. Yield: 0.087 g, 33%. Mp: 113−115 °C. 1H NMR (CDCl3, 600 MHz): δ 7.56−7.52 (m, 2H, Ph), 7.41 (br s, 2H, NH), 7.28−7.22 (m, 3H, Ph), 6.88 (s, 4H, Mes-H), 6.41 (m, 2H, pyrr), 6.00 (s, 2H, pyrr), 2.30 (s, 6H, p-CH3), 2.08 (s, 12H, o-CH3), 1.29 (br s, 9H, tBu). 13C{1H} NMR (CDCl3, 600 MHz): δ 144.39 (ipso-Ph), 138.37 (Mes), 137.50 (Mes), 134.21 (α-pyrr), 130.85 (Mes), 130.75 (o-Ph), 128.10 (MesH), 127.79 (α-pyrr), 127.12 (Ph), 126.62 (Ph), 110.18 (β-pyrr), 107.58 (β-pyrr), 58.45 (meso), 39.49 (C(CH3)3), 29.34 (br, C(CH3 ) 3 ), 21.14 (p-CH3 ), 20.85 (o-CH 3 ). Anal. Calcd for C37H42N2: C, 86.33; H, 8.22; N, 5.44. Found: C, 86.68; H, 9.25; N, 5.46. X-ray Crystallography. Crystals of 2−7 were grown from concentrated toluene solutions stored at −20 °C. Crystals of 8 were grown from a concentrated isopropyl alcohol solution stored at −20 °C. Diffraction data were collected on either a Siemens three-circle goniometer with SMART 6000 CCD detector or with a Bruker Duo diffractometer with an APEX II CCD detector. Frame data were acquired with SMART 5.625 software38 using Mo Kα radiation (λ = 0.71073 Å) or ApexDuo software using either Mo Kα radiation (λ = 0.71073 Å) or Cu Kα radiation (λ = 1.54178 Å). Cell constants were obtained from the complete data set, and frame data were integrated using SAINT PLUS 6.22 or SAINT V8.34A.39 Absorption correction was made using the program SADABS.40 The structures were solved by direct methods and refined by least-squares methods against F2 using the SHELXTL program suite41 and SHELXLE GUI.42 All ordered non-H atoms were refined with anisotropic displacement parameters. Hydrogen atoms were located in the difference Fourier map and refined with isotropic atomic displacement parameters when possible. Otherwise, hydrogen atoms were calculated on idealized positions and refined with a riding model. One of the tBu groups in 4 was treated with a two-component disorder model (rotation of methyl groups around the quaternary carbon, C(34) in Figure 3) and refined to occupancies of 55% and 45%. One of the phenyl groups bound to aluminum in 5 was also treated as a two-component disorder and refined to occupancies of 70% and 30%. Two toluene molecules were

located in the asymmetric unit of 7, one of which is disordered over an inversion center. Additionally, this toluene was modeled as a twocomponent disorder with occupancies refined to 29% and 21%. The other lattice toluene was modeled as a three-component disorder and refined to occupancies of 48%, 31%, and 21%. The meso-phenyl group on one of the ligands was also modeled as a three-component disorder and refined to occupancies of 40%, 30%, and 30%. Details of data collection, solution, and refinement are given in Table S1 (see the Supporting Information).

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ASSOCIATED CONTENT

* Supporting Information S

Figures, tables, and CIF files giving crystallographic data for 2− 8, selected NMR spectra, and details of DFT calculations on 6 and 7. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for M.R.M.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Xiche Hu for assistance with the DFT calculation of aluminum−hydride stretching frequencies for 6 and 7. Partial funding for this research was provided by a University of Toledo Interdisciplinary Research Initiative Award. The CCD facility of the Ohio Crystallography Consortium located at the University of Toledo was established with grants from the Ohio Board of Regents and ONR.

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REFERENCES

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