Communication pubs.acs.org/Organometallics
Oxygen-Bound Trifluoromethoxide Complexes of Copper and Gold Cheng-Pan Zhang and David A. Vicic* Department of Chemistry, University of Hawaii, 2545 McCarthy Mall, Honolulu, Hawaii 96822, United States S Supporting Information *
ABSTRACT: Well-defined copper and gold complexes have been prepared which contain the shortest structurally characterized metal−oxygen bonds between transition metals and a trifluoromethoxide moiety. The trifluoromethoxide ligand is O-bound to both the copper and gold centers, with a copper−oxygen distance of 1.849(4) Å and a gold−oxygen distance of 2.058(4) Å. Density functional theory (DFT) calculations on all new trifluoromethoxy complexes were performed in order to obtain bond lengths and angles that are not influenced from any intermolecular contacts in the solid state and also to provide a first glimpse of the electronic features of this previously unknown ligand. Table 1. Hansch Lipophilicity Parameters (π)11 for Selected Functional Groups
The trifluoromethoxide group has recently been described as “the least well-understood” fluorine-containing substituent.1 The lack of fundamental studies on this group is exacerbated by the fact that the [CF3O] anion readily decomposes into fluorophosgene and fluoride, even at low temperatures (Scheme 1).2−5 This reactivity is akin to that of trifluorScheme 1. Decomposition of the Trifluoromethoxide Anion
omethanol, which readily loses HF at room temperature.6 The crystal structure of the [((CH3)2N)3S][OCF3] salt reveals a short C−O bond length of 1.227(4) Å and elongated C−F bond lengths of 1.390(3) and 1.397(4) Å, which the authors believe is consistent with a negative hyperconjugative effect related to Scheme 1.7 Despite the tendency of trifluoromethoxide to decompose, important methods to form trifluoromethyl ethers based on the trifluoromethoxide anion have started to emerge.3,4,8,9 Interest in the organic trifluoromethyl ether products stems from the fact that fluorination dramatically affects the lipophilicities relative to their nonfluorinated counterparts, much more so than other functional groups (Table 1). The enhanced lipophilicities for their relatively small sizes make the OCF3 and the SCF3 functional groups important for the design of new molecules capable of crossing lipid membranes.10 Further interest in organic trifluoromethoxides stems from the unique structural properties that arise from having a geminal combination of an alkoxy or aryloxy group with fluorine atoms.1 For instance, experimental structures of trifluoromethoxybenzenes without ortho substituents exhibit no conjugation of the oxygen nonbonding electrons with the aromatic systems.4 Furthermore, mesomeric structures involving fluorine detachment offer electrostatic possibilities that are less prevalent in the nonfluorinated forms.1 Despite the growing interest in trifluoromethoxylation reactions, there has been no report of a catalytic method for transferring a trifluoromethoxy group from a metal to an © 2012 American Chemical Society
organic substrate. Crystal structures of alkali-metal-based ionic [M][OCF3] complexes are known for M = K, Rb, Cs, with the shortest M−O distance of 2.74 Å for potassium, yet none of these salts have been successfully used in catalytic methods.12 Moreover, there is not a single report of a structurally characterized transition metal bearing a covalently attached trifluoromethoxy ligand. In order to both better understand the fundamental features of transition-metal trifluoromethoxy complexes and to gain insights into the development of new trifluormethoxylation catalysts, we sought to prepare a trifluoromethoxy complex that was stable enough to isolate and structurally characterize. AgOCF3 and CuOCF3 are both known salts2 which exhibit nearly identical and very broad signals in their 19F NMR spectra (δ (MeCN) −26.8 and −23.4, respectively; see the Supporting Information for examples of actual spectra). These species have never been structurally characterized; therefore, we questioned whether or not they contain bona fide metal−oxygen bonds or if they exist as solvated ion pairs. In this report we now confirm the nature of the bonding in “CuOCF3”. Crystals of CuOCF3 Special Issue: Copper Organometallic Chemistry Received: April 4, 2012 Published: May 7, 2012 7812
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can be grown by layering an acetonitrile solution with THF and cooling to −35 °C. The crystals are extremely oxygen and thermally sensitive, and even Paratone oil covered crystals started turning black within seconds en route to the diffractometer. Despite the poor data set that resulted with these semidecomposed crystals, the nature of the bonding could indeed be confirmed to be a solvent-separated ion pair (Figure 1) with four acetonitrile ligands occupying the
Figure 1. ORTEP diagram of [(MeCN)4Cu][OCF3]. Ellipsoids are shown at the 50% probability level.
Figure 2. ORTEP diagrams of 1 (left) and 2 (right). Ellipsoids are shown at the 50% probability level. Hydrogens are omitted for clarity. Select contact distances (Å): Cu−C1 = 2.680(11), Au−C1 = 2.868(6).
coordination sphere of copper. Caution should be taken in interpreting the details of the [(MeCN)4Cu][OCF3] structure, as symmetrical ionic salts of [OCF3] are well-known to be disordered, providing unrealistic bond lengths and angles for the anionic trifluoromethoxy moiety.12,13 A further search for which ligands and metals could promote more covalent bonding to the trifluoromethoxide ligand was then carried out. The 19F NMR signatures of the new complexes turned out to be a good indicator of whether the compounds exist as a solvent-separated ion pair, for the more covalent structures all had sharp 19F resonances in the NMR spectra relative to the more ionic structures (see below). Nheterocyclic carbenes (NHCs) were tested, as they have a special place in fluoroalkane chemistry; the first well-defined and structurally characterized LCuI−CF3 complexes were prepared using these types of ligands.14 We wanted to establish if NHC ligands could also be used to prepare the related LCuI− OCF3 derivatives. It was found that the reaction of a THFcontaining solution of [(SIMes)Cu−Cl] (SIMes = 1,3dimesitylimidazolin-2-ylidene) with acetonitrile-solvated AgOCF3 at room temperature indeed led to a new product, which we tentatively assign as [(SIMes)Cu−OCF3] (19F NMR (THF/MeCN 20/1) δ −34.1, sharp singlet). However, all attempts to isolate this new copper−trifluoromethoxy species as a solid resulted in decomposition to uncharacterized mixtures. Solid-state instability is known for transition-metal complexes bearing fluoroalkyl-containing ligands.4,15,16 Use of the bulkier starting material [(SIPr)Cu−Cl] ((SIPr = N,N′bis(2,6-diisopropylphenyl)-4,5-dihydroimidazol-2-ylidene) with AgOCF3 led to a similar species according to 19F NMR spectroscopy, and gratifyingly this copper-containing product was also robust enough to survive crystallizations at −30 °C. X-ray analysis revealed the structure to be [(SIPr)Cu-OCF3] (1; eq 1), and the ORTEP diagram of 1 is shown in Figure 2. Compound 1 crystallized in the Pn space group, with two independent molecules differing in both the rotation of the trifluoromethyl group and the intermolecular contacts that are observed with neighboring molecules. Intermolecular contacts were observed for both the oxygen atom and the fluorines of
the trifluoromethoxide ligand. Bond lengths for 1 are provided in Table 2. The copper−oxygen distance was observed to be Table 2. Comparison of Experimental and Calculated Bond Lengths (Å) and Angles (deg) in Various Copper Structuresa
bond length or angle complex 1 exptl Cu−O Cu−C2 O−C1 C1−F1 C1−F2 C1−F3 C2−Cu−O
1.849(5) 1.875(5) 1.23(1) 1.39(1) 1.33(1) 1.35(1) 177.1(2)
complex 1 calcd
complex 3 exptl17
1.848 1.885 1.287 1.358 1.402 1.377 179.0
1.793(7) 1.861(8) 1.31(1)
175.9(3)
a
Experimental bond lengths for 1 are for the rotamer which has a fluorine anti to the copper. The calculated geometry of 1 was also optimized with a fluorine anti to copper.
1.841(3) Å on average, much shorter than those in any previously known metal−OCF3 salt.12 A remarkably short carbon−oxygen bond is also observed and, coupled to the fact that short intramolecular copper-to-carbon contacts (which average 2.688(7) Å) are also present, suggests geometries not unlike those present in η2-like complexes (see below). The copper contacts observed in this structure are much shorter than that observed in the nonfluorinated 3 (Table 2), which measure 2.74(1) Å.17 Moreover, the Cu−O−C1 angle in 1 is 120.7(5)°, while that for complex 3 is longer at 123.3(7)°. 7813
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DFT calculations were performed on both 1 and 2 in order to establish the optimized bond lengths and angles in the gas phase, where no intermolecular contacts influence the bond parameters. Key results for the geometry optimization are summarized in Tables 2 and 3. Of note, the gas-phase calculations predict smaller oxygen−carbon−fluorine bond angles and elongated carbon−oxygen bonds relative to those in [((CH3)2N)3S][OCF3], which might suggest greater interactions of the trifluoromethoxide with the metal centers than are implied in the solid-state structures. The DFT studies also provide a first glimpse of the electronic features of this previously unknown ligand. The calculated HOMOs and LUMOs of 1 and 2 are depicted in Figure 3. The largest
The reactivity of silver and gold analogues was then explored. Silver was found to be even more prone to decomposition than copper, as addition of ligands such as SIPr and 1,10phenanthroline to solvated AgOCF3 led to rapid decomposition, even in the solution state. Gold, however, was amenable to trifluoromethoxide coordination. As in the case of copper, the reaction of [(SIPr)Au−Cl] with AgOCF3 led cleanly to a new product (eq 1) in the solution phase (19F NMR (THF/MeCN 20/1) δ −36.6, sharp singlet), and this new gold-containing product was robust enough to survive crystallizations at −30 °C. Compound 2 also crystallized in the Pn space group, with one independent molecule in the unit cell showing no disorder (shown in Figure 2, right) and another showing a disorder in the orientation of the trifluoromethyl unit. Nevertheless, the disorder could be successfully modeled as rigid groups, and the data were refined to a final R value (I > 2.00σ(I)) of 3.2%. Thus, the first structural data for an O-bound gold trifluoromethoxide was obtained, and bond parameters could be extracted from the nondisordered molecule. Important bond lengths and angles for 2 are shown in Table 3. The carbon− Table 3. Comparison of Experimental and Calculated Bond Lengths (Å) and Angles (deg) in Various Gold Structures
bond length or angle
complex 1 exptl
complex 1 calcd
complex 4 exptl18
Au−O Au−C2 O−C1 C1−F1 C1−F2 C1−F3 C2−Au−O O−C1−F1 O−C1−F2 O−C1−F3
2.058(4) 1.978(5) 1.247(7) 1.386(8) 1.367(7) 1.357(8) 177.27(17) 115.8(6) 114.5(5) 116.3(5)
2.061 1.987 1.295 1.377 1.357 1.388 178.8 113.9 112.2 114.4
1.990(8) 1.971(1) 1.423(14)
Figure 3. (top) Calculated (B3LYP/m6-31g*) LUMO (left, −0.754 eV), HOMO (middle, −5.92 eV), and HOMO-1 (right, −6.293) of copper complex 1. (bottom) Calculated (LANL2DZ basis set and effective core potential on gold/6-31g** on all other atoms) LUMO (left, −0.900 eV), HOMO (middle, −6.643 eV), and HOMO-1 (right, −6.671 eV) of copper complex 1.
complex 5 exptl
1.227(4) 1.390(3) 1.397 (4) 1.390 (3)
molecular orbital coefficients (Mulliken) for the HOMO of 1 were copper-centered, while those for the LUMO were dominated by the NHC ligand. The HOMO-1 contributions came largely from the oxygen pz orbital and linear combinations of the copper d orbitals. The largest molecular orbital coefficients (Mulliken) for the HOMO of 2 were dominated by the oxygen py and linear combinations of gold d and s orbitals (based on its Cartesian coordinate system), while those for the LUMO were dominated by the NHC ligand. The HOMO-1 contributions came largely from the oxygen px orbital and the gold dxy orbital. Orbital interactions suggestive of overlap of the electron densities between the metals and the carbon−oxygen bonds of 1 and 2 are evident from Figure 3. A natural bond orbital (NBO) analysis of 1 was also performed in order to qualitatively describe the nature of bonding in the OCF3 group. The default Lewis reference structure determined by the NBO program was an ionic Lewis structure, with [(SIPr)Cu+−OCF3] as the best fit. In this model, the carbon hybrids to fluorine atoms all have more p character (sp3.31, sp3.61, and sp3.41) leaving more s character to bond with oxygen; the carbon hybrid to oxygen was calculated to be sp2.05. The oxygen hybrid to carbon in this model was calculated to be sp1.7, and the polarization coefficients indicate that about 67.7% of the electron density in the C−O bond is polarized to the
178.0(4) 116.4(3) 115.8(3) 116.4(3)
oxygen bond in 2 was determined to be 1.247(7) Å, only slightly longer than the 1.227(4) Å bond observed in [((CH3)2N)3S][OCF3]. The oxygen−carbon−fluorine bond angles in the gold complex were slightly smaller than those observed in [((CH3)2N)3S][OCF3]. The atoms O1, F2, and F3 all possess intermolecular contacts shorter than the sum of the van der Waals radii with hydrogen atoms of adjacent NHC ligands; therefore, some care must be taken in interpreting the bond distances and angles involving the oxygen and fluorine atoms. Nevertheless, the X-ray data clearly establish the coordination mode of the trifluoromethoxide to gold. A striking comparison in Table 3 is the carbon−oxygen bond distance in the trifluoromethoxy complex 2 relative to that in the tertbutoxide complex 4. When negative hyperconjugation effects can be ignored (as is the case in 4), a much longer C−O bond length is observed (1.423(14) Å vs 1.247(7) Å). 7814
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more electronegative oxygen atom. The fluorine hybrids to carbon were sp2.43, sp2.57, and sp2.77, with all electron densities polarized to fluorine (74.1, 74.3, and 74.9%). NBO analyses of both structures also provided information about atomic charges (Figure 4). The calculated charges of the trifluoromethoxy
(5) Taylor, S. L.; Martin, J. C. J. Org. Chem. 1987, 52, 4147. (6) Christe, K. O.; Hegge, J.; Hoge, B.; Haiges, R. Angew. Chem., Int. Ed. 2007, 46, 6155. (7) Farnham, W. B.; Smart, B. E.; Middleton, W. J.; Calabrese, J. C.; Dixon, D. A. J. Am. Chem. Soc. 1985, 107, 4565. (8) Huang, C.; Liang, T.; Harada, S.; Lee, E.; Ritter, T. J. Am. Chem. Soc. 2011, 133, 13308. (9) Langlois, B. R.; Roques, N. J. Fluorine Chem. 2007, 128, 1318. (10) Zhang, C.-P.; Vicic, D. A. J. Am. Chem. Soc. 2012, 134, 183. (11) Hansch, C.; Leo, A.; Unger, S. H.; Kim, K. H.; Nikaitani, D.; Lien, E. J. J. Med. Chem. 1973, 16, 1207. (12) Arlt, J.; Jansen, M. Chem. Ber. 1991, 124, 321. (13) Zhang, X.; Seppelt, K. Inorg. Chem. 1997, 36, 5689. (14) Dubinina, G. G.; Furutachi, H.; Vicic, D. A. J. Am. Chem. Soc. 2008, 130, 8600. (15) Taw, F. L.; Clark, A. E.; Mueller, A. H.; Janicke, M. T.; Cantat, T.; Scott, B. L.; Hay, P. J.; Hughes, R. P.; Kiplinger, J. L. Organometallics 2012, 31, 1484. (16) Tavener, S. J.; Adams, D. J.; Clark, J. H. J. Fluorine Chem. 1999, 95, 171. (17) Goj, L. A.; Blue, E. D.; Delp, S. A.; Gunnoe, T. B.; Cundari, T. R.; Pierpont, A. W.; Petersen, J. L.; Boyle, P. D. Inorg. Chem. 2006, 45, 9032. (18) Johnson, M. T.; Janse, v. R. J. M.; Axelsson, M.; Ahlquist, M. S. G.; Wendt, O. F. Chem. Sci. 2011, 2, 2373. (19) Yamaguchi, Y.; Ichioka, H.; Klein, A.; Brennessel, W. W.; Vicic, D. A. Organometallics 2012, 31, 1477. (20) Algarra, A. G.; Grushin, V. V.; MacGregor, S. A. Organometallics 2012, 31, 1467.
Figure 4. Calculated natural atomic charge distributions in 1 and 2.
ligand were of interest, as charge distributions on the trifluoromethyl ligand itself have provided much insight into fundamental aspects of metal-mediated trifluoromethylation chemistry.15,19,20 The carbon atoms of the trifluoromethoxide groups, situated between four other more electronegative atoms, were found to bear strikingly large positive charges of +1.32 for both 1 and 2. The oxygen atoms were found to bear the most negative charges in both molecules (−0.87e and −0.85e for 1 and 2, respectively). In conclusion, the first well-defined transition-metal trifluoromethoxide complexes have been prepared, structurally characterized, and evaluated computationally. With these new complexes in hand, it is hoped that explorations of stoichiometric reactions with organic substrates will ultimately lead to development of new methods and a better understanding of how to manipulate the trifluoromethoxide group.
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ASSOCIATED CONTENT
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AUTHOR INFORMATION
S Supporting Information *
Text, tables, figures, and CIF files giving experimental procedures, NMR data for all new compounds, X-ray data for all newly determined structures, and optimized calculated Cartesian coordinates for 1 and 2. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS D.A.V. thanks the Office of Basic Energy Sciences of the U.S. Department of Energy (DE-FG02-07ER15885). We thank Roger Cramer for assistance with handling the disorder in the crystal structure of 2.
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REFERENCES
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