Communication pubs.acs.org/Organometallics
Direct Synthesis of a Copper(II) N‑Heterocyclic Carbene Complex in Air Daniel J. O’Hearn and Robert D. Singer* Atlantic Centre for Green Chemistry, Department of Chemistry, Saint Mary’s University, Halifax, Canada B3H 3C3 S Supporting Information *
ABSTRACT: Copper N-heterocyclic carbene complexes (CuNHCs) are prevalent in the literature, and their stability and catalytic activity have been well characterized and established. In an overwhelming majority of the reported examples, copper is present as Cu(I), owing to the favorable soft−soft interaction. In contrast, there are very few reports of CuNHCs containing Cu(II). This can be explained by the unfavorable intermediate/hard−soft interaction between carbon and Cu(II). The syntheses of examples reported thus far require the exclusion of air and moisture and involve the use of NHC transfer reagents or the use of free NHCs. This results in tedious, multistep procedures, making their preparation more time consuming and costly. Herein we report the synthesis and characterization of two air- and moisture-stable Cu(II)-NHC complexes by direct combination of 1,3-bis(pyridin-2-ylmethyl)-1H-imidazolium chloride with copper(II) hexafluorosilicate. Analysis by single-crystal X-ray diffraction revealed that the species could be isolated in both distorted-octahedral and distortedsquare-pyramidal geometries.
I
report, examples of Cu(II)-NHCs without anionic tethers were characterized by single-crystal X-ray diffraction.3 In this case, the ligands were functionalized with a pyridyl/picolyl group that bonded to the copper center, stabilizing it enough for isolation and characterization by single-crystal X-ray diffraction. Furthermore, when the same pyridyl-functionalized ligand was used in conjunction with CuI to catalyze an Ullmann-type etherification reaction, greater catalytic activity was achieved in comparison to using CuI alone or using NHC ligands lacking a pyridyl functionality (i.e., IMes-HCl).3 This increase in catalytic activity was attributed to the ligand-induced increase in stability of the higher oxidation states of copper found in the catalytic cycle.3 Although examples of Cu(II)-NHCs lacking anionic tethers are present in the literature, multistep syntheses that must be carried out in the absence of air and moisture have been required to afford the final products.3,4,10 In such instances, the synthetic route typically relies on either utilizing an NHC transfer reagent, requiring multiple steps, or reacting the highly reactive, free NHC with a copper(II) salt. In either case, air and moisture must almost invariably be excluded. Furthermore, when an NHC transfer reagent is used, some reaction times can span several days.3,4,10,11 Herein we report, to the best of our knowledge, the first synthesis of a Cu(II)-NHC by direct combination of an imidazolium salt (1,3-bis(pyridin-2-ylmethyl)-1H-imidazolium
n recent years, copper N-heterocyclic carbene complexes (Cu-NHCs) have found applications as catalysts1 and as NHC transfer reagents.2 In the clear majority of these cases, the copper is present in oxidation state Cu(I), where the soft−soft acid−base interaction, Cu(I)−C, results in a highly stable complex. On the other hand, examples in which Cu(II)-NHCs have been isolated are relatively rare, owing to the less favorable intermediate/hard−soft Cu(II)−C interaction. Stable examples typically consist of anionic tethers, making the resulting complexes more sensitive to air and moisture.3 Preparation typically proceeds by utilization of NHC transfer reagents or by a combination of the Cu(II) salt and the free NHC. These methods can be problematic, sometimes giving inconsistent results.4 Furthermore, these preparative routes require the exclusion of moisture, air, and sometimes light, and reaction times can span several days, making the synthetic process tedious and time consuming.3,4 Early examples of isolable Cu(II)-NHCs tend to rely on NHC ligands that have been functionalized on one or both imidazolium nitrogen atoms with anionic tethers. Typically, these take the form of an alkoxide or phenoxide. The tether occupies one of the coordination sites of the copper ion, contributing to its stability and allowing for isolation of the complex.3,5,6,8 Although anionic tethers are predominant, one of the earliest reports of a Cu(II)-NHC was stabilized by a relatively weak electrostatic interaction with an amine-functionalized ligand.9 This complex was not characterized by singlecrystal X-ray diffraction; however, the structure was implied by isolation of the Cu(I) analogue, which could be reversibly oxidized to Cu(II) via cyclic voltammetry.9 In a more recent © 2017 American Chemical Society
Received: June 28, 2017 Published: August 29, 2017 3175
DOI: 10.1021/acs.organomet.7b00489 Organometallics 2017, 36, 3175−3177
Communication
Organometallics
Cu1−O2. It is implicit that these weaker interactions involve labile ligands that can easily exchange. The Cu1−C1 bond length in complex 2 is 1.915(3) Å, which is at the short end of previously reported Cu(II)−C bond lengths, ranging between 1.91 and 2.01 Å.4−8 In comparison with the other reports of pyridyl-/picolyl-functionalized Cu(II)NHCs, which have an average Cu−C bond length of 1.96 Å, it is clear that the Cu−C distance found in 2 is significantly shorter, potentially indicating more π back-bonding and further stabilization.13,14 Another factor contributing to the stability of 2 is the chelating/pincer action of the two picolyl nitrogen atoms. Furthermore, there is extensive hydrogen bonding between the two coordinating methanol molecules and two of the fluorine atoms in the SiF62− moieties, forming a onedimensional (1D) polymeric chain. Single crystals of 3 were obtained from the same blue precipitate that formed 2 by slow crystallization from DMSO-d6 over a 2−3 month period in an NMR tube. The crystal structure of 3 (Figure 2) revealed that a square-pyramidal
(1)) and a Cu(II) salt (copper(II) hexafluorosilicate) under atmospheric conditions. 1,3-Bis(pyridin-2-ylmethyl)-1H-imidazolium chloride (1) was prepared by a modified literature method.11,12 The combination of 1 with CuSiF6 (Scheme 1) in methanol Scheme 1. Synthesis of the Cu(II)-NHCs
resulted in a deep blue solution followed by precipitate formation after stirring overnight in air at room temperature. The precipitate could be recrystallized by first dissolving the slightly soluble solid in methanol followed by vapor diffusion with chloroform. This afforded deep blue block crystals of 2 suitable for singlecrystal analysis. X-ray crystallographic analysis of 2 revealed that a rare, pseudo-octahedral Cu(II)-NHC complex had formed (Figure 1).4 The observation of broad, poorly resolved peaks in
Figure 2. Cocrystal structure of 3, showing the 2:1 ratio between the Cu(II)-NHC and [Cu(SO(CD3)2)6]2+ moieties, respectively. The thermal ellipsoids are drawn at the 50% probability level. Hydrogen atoms have been omitted for clarity. Selected bond lengths (Å): Cu1− C1 1.9319(19), Cu1−Cl1 2.2796(5), Cu1−N3 2.1202(17), Cu1−N4 2.1255(17), Cu1−O1 2.2919(15). Selected bond angles (deg): C1− Cu1−Cl1 174.12(6), C1−Cu1−O1 89.92(7), N3−Cu1−N4 171.04(6).
Figure 1. (a) Asymmetric unit of 2 showing the NHC−Cu bond and (b) hydrogen bonding between the SiF62− anion and the axial and equatorial methanol molecules. The thermal ellipsoids are drawn at the 50% probability level. All hydrogen atoms not bonded to heteroatoms have been omitted for clarity. Selected bond lengths (Å): Cu1−C1 1.915(3), Cu−N3 2.083(3), Cu1−N3′ 2.057(3), Cu−F1 2.5647(19), Cu1−O1 1.993(2), Cu1−O2 2.473(3). Hydrogen bonds (Å): H1- - F5, D−H 0.78(2), H−A = 1.98(3); H2- - -F4, D−H 0.87(2), H−A 1.79(3). Selected bond angles (deg): F1−Cu1−O2 171.02(9), C1− Cu1−O1 170.67(13), N3−Cu1−N3′ 170.91(11).
Cu(II)-NHC had cocrystallized with [Cu(SO(CD3)2)6]2+ in a 2:1 ratio, respectively. The nearest atom neighboring the vacant coordination site of Cu1 is one of the fluorine atoms on SiF62−, having a distance of 3.2142(5) Å between them. This distance is too large to be representative of any meaningful interaction and is likely just a result of the crystal packing. The longer Cu1−C1 bond length of 1.9319(19) Å is possibly indicative of a lesser degree of π back-bonding in comparison to that in complex 2. Despite the observation that the Cu−C bond length increases as the coordination number decreases, this statement does not hold true for a previously reported collection of Cu(II)-NHCs.3 The methanol molecules that were present in 2 have been replaced with a single DMSO molecule and a chloride anion. This ability to exchange monodentate ligands provides reasonable grounds to hypothesize that Cu(II) chelated by the NHC shown in 2 and 3 might facilitate catalysis by providing vacant coordination sites around the copper center. Catalytic systems of interest are those involving a Cu(II) complex as the catalytically active species, such as aziridination
the 1H NMR spectrum, the charge balance of the structure, and the presence of Jahn−Teller distortions about the copper center all support the presence of a paramagnetic Cu(II) complex. Figure 1 shows that the NHC, 1,3-bis(pyridin-2ylmethyl)-1H-imidazole-2-ylidene derived from 1, coordinates to the Cu(II) center as a tridentate NHC pincer ligand, occupying three of the equatorial positions. A methanol molecule coordinating through its oxygen atom occupies the fourth equatorial position. The two axial positions are occupied by a SiF62− anion and another methanol molecule. These equatorial ligands represent electrostatic contacts, having distances of 2.5647(19) Å for Cu1−F1 and 2.473(3) Å for 3176
DOI: 10.1021/acs.organomet.7b00489 Organometallics 2017, 36, 3175−3177
Communication
Organometallics reactions,15 amination of 1,3-dicarbonyls,16 and ring-opening cross-coupling of cyclopropanols with sulfonyl azides.17 Furthermore, a report in which the Cu(I)-NHC IPrCu(DBM) (DBM = dibenzoylmethane) was used as the active catalyst in an aziridination reaction gave moderate to good yields,18 but thus far, no such reaction has been catalyzed by a Cu(II)-NHC. Investigations into the catalytic activity of 2 and 3 are ongoing in our laboratory. In conclusion, the air-stable Cu(II)-NHC complex 2 was successfully synthesized in methanol without the use of NHC transfer reagents and without the exclusion of air and moisture. The unusually stable complex is also labile enough with respect to solvent molecules and other monodentate ligands to easily exchange, allowing the copper(II) center to potentially be open for catalysis as in complex 3.
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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.organomet.7b00489. Single-crystal X-ray crystallographic data and spectral data for compounds 1−3 where appropriate (PDF) Accession Codes
CCDC 1558885 and 1558887 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 for R.D.S.:
[email protected]. ORCID
Daniel J. O’Hearn: 0000-0001-7105-3378 Robert D. Singer: 0000-0002-2708-9485 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors wish to thank the Natural Sciences and Engineering Research Council of Canada (NSERC Discovery to R.D.S.) and Saint Mary’s University for financial support for this research.
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REFERENCES
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DOI: 10.1021/acs.organomet.7b00489 Organometallics 2017, 36, 3175−3177
