Bulky β-Diketones Enabling New Lewis Acidic Ligand Platforms

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Bulky β‑Diketones Enabling New Lewis Acidic Ligand Platforms Eser S. Akturk, Steven J. Scappaticci, Rachel N. Seals, and Michael P. Marshak*,† Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309, United States S Supporting Information *

synthesis of a β-diketonate ligand substituted with the bulky 2,6bis(2,4,6-trimethylphenyl)phenyl group, which enables the isolation of complexes of the type M(Aracac)2Cl(solv) [M = Ti, V, Cr; solv = tetrahydrofuran (THF), acetonitrile (CH3CN)], which are, to the best of our knowledge, the first of this type. The synthesis of β-diketonate ligands typically utilizes the Claisen condensation reaction, wherein an enolizable ketone is acylated by an ester in the presence of base. This synthesis does not provide high yields when bulky substituents are used. Alternatively, the use of an acyl chloride instead of an ester can increase the driving force for the reaction but can result in the formation of an O-acylated species as a kinetically favored product.30 We found that the reaction of excess potassium pinacolone enolate31 with m-dimesitylbenzoyl chloride, prepared from the corresponding benzoic acid,32 gives a 6:1 product ratio of O-acylated to β-diketonate; however, transmetalation of potassium pinacolone enolate with ZnBr2 provides the βdiketone product 1 exclusively (Scheme 1). This procedure, which has been scaled to over 25 g, results in isolated yields of 89% as a colorless crystalline solid.

ABSTRACT: The synthesis of a sterically encumbered βdiketone ligand (Aracac) substituted with 2,6-(2,4,6Me3C6H2)2C6H3 is described. Coordination complexes of the type M(Aracac)2Cl(solv) (M = Ti, V, Cr; solv = THF, CH3CN) were prepared by the reaction of Aracac with MCl 3 (M = V, Cr) or with TiCl 4 to generate Ti(Aracac)2Cl2, followed by reduction. These complexes show a trend of alternating the cis/trans geometric preference with increasing dn electron count (n = 0, 1, 2, 3), which is rationalized in part by the unusual ability of βdiketonates to behave as either a weak π donor or a π acceptor in the cis and trans geometries, respectively. In this way, the bis-β-diketonate platform can accommodate the varying electronic demands of the coordinated metal ion. These results demonstrate the ability to limit the coordination of β-diketonates on metal complexes for the first time, providing a chemically robust and coordinatively versatile platform for mechanistic investigations, metal functionalization, and improved catalyst design.

Scheme 1. Synthesis of the β-Diketone Ligand 1

β-Diketonate ligands hold an important place in the history of coordination chemistry and continue to be among the most ubiquitous ligands for use as catalysts, precatalyts, NMR shift reagents, biochemically active agents, and volatile reagents for metal vapor deposition.1−8 Perhaps the most unique feature distinguishing them from many other organic ligands is their ability to coordinate the majority of all known elements. One challenge with β-diketonate chemistry, particularly in the area of catalysis, is their tendency to form substitutionally inert complexes of the type M(acac)3 or M(acac) 4 (acac = acetylacetonate) for most transition and main-group metals. 9−13 Alternately, metals complexed with two βdiketonates tend to form oligomeric species that can have unfavorable solubility characteristics or lead to bimolecular decomposition pathways during catalysis.14−16 The introduction and control of sterically bulky functional groups on electronically related β-diketiminate ligands has enabled an abundance of new chemistry and catalysis involving the resulting metal complexes.17−25 Translating these successes to β-diketonates, a potentially more Lewis acidic platform, requires the placement of bulky functional groups on one or more of the three carbon atoms because, unlike the nitrogen atoms in β-diketimines, the oxygen atoms in β-diketonates cannot be modified. Any functional group on one of the carbon atoms is directed away from the metal center; therefore, the use of m-terphenyl, which can direct steric bulk back toward the metal center,26−29 was investigated as a way to impose steric control with the β-diketonate platform. Here we report the © XXXX American Chemical Society

The X-ray crystal structure of the ligand (Figure 1) shows an asymmetric β-diketone core with an acidic proton coordinated to the O(1) atom. Differing C−O bond lengths of O(1)−C(1) = 1.318(2) Å and O(2)−C(3) = 1.256(2) Å, as well as C−C bond

Figure 1. Molecular structure of 1, with hydrogen atoms removed, except for O−H, which was found from the difference Fourier map. Received: August 15, 2017

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DOI: 10.1021/acs.inorgchem.7b02077 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

Figure 2. Molecular structures of (a) 2, (b) 3, (c) 4, and (d) 5. Carbon, nitrogen, oxygen, chlorine, and metal atoms are shown in gray, blue, red, green, and dark blue, respectively. Noncoordinated solvent and hydrogen atoms were removed for clarity.

Because complexes 4 and 5 possess identical ligands, the difference in cis and trans coordination observed in the solid state suggests an electronic origin rather than a crystal packing effect. Thus, DFT calculations were performed for the acac anion, Ti(acac)2Cl2, and M(acac)2Cl(THF) (M = Ti, V, Cr). The highest occupied (HOMO) and lowest unoccupied (LUMO) molecular orbitals of the acac anion (Figure 3) represent the π-

lengths of C(2)−C(1) = 1.365(2) Å and C(3)−C(2) = 1.424(2) Å, further suggest that the oxygen atom closer to the more strongly electron-donating tert-butyl group is more basic than the one near the aromatic group. Despite this asymmetry, the IR spectrum shows a single broad CO stretch at 1592 cm−1, suggesting tautomerization in the π system.33 Opposing the bulky aryl group is the tert-butyl group, which was chosen to provide stability by blocking reactivity at the electrophilic C(1) and C(3) atoms, as well as the nucleophilic C(2) atom. In addition, this has been shown to prevent coordination of the adjacent ketone oxygen atom to more than one metal and enable the isolation of molecular rather than oligomeric species.34,35 Finally, it provides favorable solubility characteristics that support 1 and the resulting metal complexes to maintain a wide range of solubility in nonpolar solvents such as hexane and toluene, as well as in polar organic solvents such as dichloromethane and THF. Deprotonation of the β-diketone ligand with potassium hydride, followed by the addition of TiCl4(THF)2, provides Ti(Aracac)2Cl2 (2) in 68% isolated yield (Figure 2a). The X-ray crystal structure of 2 shows that the ligands adopt a cis configuration, which was described by Bradley and Holloway for the related cis-Ti(acac)2Cl2 to result from enhanced π donation in this geometry, relative to the trans isomer. 36 The pseudooctahedral geometry is distorted by the steric repulsion of the chlorine atoms ∠Cl(1)−Ti(1)−Cl(2) = 98.28(2)°, which is compensated for by the angle of the β-diketonate oxygen atoms, ∠O(1)−Ti(1)−O(3) = 82.98(5)°, coordinated trans to the chlorine atoms. The reduction of 2 with KC8 results in the formation of Ti(Aracac)2Cl(NCCH3) (3; Figure 2b). The X-ray crystal structure shows that the β-diketonate ligands adopt a planar configuration in the solid state with the chloride and acetonitrile ligands in a trans geometry. Complex 3 has a magnetic moment of 1.75(2) μB and an electron paramagnetic resonance (EPR) signal consistent with a monomeric d1 electron configuration.10,37 The visible absorption spectrum of 3 features absorptions at 427 nm (ε = 1400 M−1 cm−1) and 687 nm (ε = 2200 M−1 cm−1), which are too intense to be pure d → d transitions but suggest mixing of the π* orbitals of the coplanar bis(β-diketonates). Metalation of 1 with VCl3 and CrCl3 in THF results in the formation of V(Aracac)2Cl(THF) (4) and Cr(Aracac)2Cl(THF) (5), respectively (Figure 2c,d). The X-ray structures of 4 and 5 show cis and trans geometries, and the magnetic moments of 2.81(2) and 3.83(9) μB confirm the assignments of these complexes as monomeric high-spin d2 and d3 electron configurations, respectively.

Figure 3. Resonance structures of the acac anion (top) and the calculated HOMO and LUMO orbitals (bottom).

donating and π-accepting orbitals of the ligand, which can interact to a greater or lesser extent with transition-metal d orbitals depending on their relative energies. In general, a more electropositive or oxidized metal center such as TiIV will interact more strongly with the HOMO, while a less electropositive or oxidized metal will interact more strongly with the LUMO. In the cis arrangement, the π-type HOMO of β-diketonates can interact with each of the three π-type d orbitals: dxz, dyz, and dxy. This allows maximum electron donation to electrophilic early-transition-metal centers. This additional 6-electron π donation allows 2 to achieve a stable 18-electron configuration.36 In contrast, in the trans arrangement, the acac HOMO only provides interactions with one d orbital, dxz. Instead, the coplanar arrangement found in the trans geometry enables mixing of the π-type LUMOs of the two β-diketonates, stabilizing one orbital to act as a π acceptor. The calculated singly occupied molecular orbital (SOMO) of 3 (Figure 4) shows how the dyz orbital of the metal can form a bonding interaction with the π-accepting LUMOs of the coplanar β-diketonates. These interactions are also likely responsible for the trans geometry of the d1 complex V(acac)2Cl2.38 Nevertheless, the energy differences calculated for cis/trans geometries of M(acac)2Cl(THF) (M = Ti, V, Cr) are ≤2 kcal mol−1 for each complex, suggesting that these complexes can isomerize between cis and trans geometries in solution by loss of the coordinated THF ligand followed by a Berry pseudorotation.14,39−41 The ability of β-diketonates to act as both π donors or π acceptors by delocalizing the charge can explain spectroscopic observations by Lintvedt and Fatta, who suggest that strong πback-bonding in various metal tris(β-diketonates) is responsible for a Racah parameter (B) that is significantly attenuated from B

DOI: 10.1021/acs.inorgchem.7b02077 Inorg. Chem. XXXX, XXX, XXX−XXX

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Michael P. Marshak: 0000-0002-8027-2705 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Figure 4. Schematic diagram illustrating the symmetry of the π-bonding interaction of the acac π* LUMO orbitals with the metal 3dyz orbital (left). Calculated SOMO of Ti(acac)2Cl(THF) (right).

Notes †

M.P.M. is an inventor on U.S. Patent 9,676,693, which comprises various bulky β-diketones, other related ligands, and their metal complexes. The authors declare no competing financial interest.

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the free ion form, despite observing a weak ligand field, comparable with H2O.12 This observation also confirms calculations by Pritchard and Autschbach that suggest interactions between the acac π* orbitals with the metal t2g parent orbital set in M(acac)3 (M = Fe, Cr, Ru).42 Complexes 2−5 demonstrate the ability of the β-diketone ligand 1 to sterically control the metal coordination for the first time. β-Diketones are unique ligands in that they provide a combination of weak ligand-field strength from the oxygen-atom ligation and the stability of bidentate coordination. As a result, they can engender particularly Lewis acidic metals with a high density of states that facilitate rapid substrate substitution kinetics during catalytic cycles. Over the years, there have been many reactions developed that utilize metals complexed with βdiketonate ligands; however, in most cases, the mechanism of these catalytic reactions is poorly understood because the added catalyst is in the form of M(acac)3 (M = Ti, V, Cr, Mn, Fe, Co, Ru, Rh, Ir) and is typically coordinated in an inactive, octahedral fashion.43,44 Complexes 3−5 represent the first trivalent βdiketonate complexes that use sterics to prevent ligand rearrangement to form stable M(acac)3 products. These M(Aracac)2Cl complexes should enable a detailed mechanistic investigation of catalysts approaching the weak ligand field limit such as those involved in Ziegler−Natta olefin polymerization and olefin epoxidation mechanisms.4,43 More generally, these ligands can provide a robust acidic ligand platform for elements across the periodic table to investigate cis/trans coordination preferences, observe neutral and anionic substrate binding, block bimolecular decomposition pathways, and facilitate the isolation and investigation of reactive mechanistic intermediates.

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ACKNOWLEDGMENTS We thank Dr. Annette Erbse for assistance with the EPR data, Aaron Crossman for assistance with melting point collection, and Franklin D. R. Maharaj with computational assistance. This work was funded by laboratory startup funds provided by the University of Colorado, Boulder, CO.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02077. Experimental procedures, NMR data, EPR data, UV−vis spectra, IR spectra, mass spectrometry data, computational data, crystallographic details, and tables (PDF) Accession Codes

CCDC 1556398−1556400 and 1562764−1562765 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. C

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DOI: 10.1021/acs.inorgchem.7b02077 Inorg. Chem. XXXX, XXX, XXX−XXX