Communication pubs.acs.org/IC
Amidine Production by the Addition of NH3 to Nitrile(s) Bound to and Activated by the Lewis Acidic [Re6(μ3‑Se)8]2+ Cluster Core William C. Corbin,† Gary S. Nichol,‡ and Zhiping Zheng*,† †
Department of Chemistry and Biochemistry, The University of Arizona, Tucson, Arizona 85721, United States School of Chemistry, The University of Edinburgh, David Brewster Road, Edinburgh EH9 3FJ, U.K.
‡
S Supporting Information *
supported by the Lewis acidic [Re6(μ3-Se)8]2+ cluster core and with the hope of discovering other synthetically significant chemical transformations, we have recently set out to explore the synthesis of amidines, a family of compounds that are significant in medicinal chemistry,27−29 for CO2/CS2 sequestration,30,31 and as ligands for the synthesis of metallocyclic and heterocyclic complexes,32−39 It is of note that amidines are among the strongest neutral bases, protonation of which leads to the corresponding amidinium salts, which are useful as precursors to stable carbenes40 and as organocatalysts.41 We note that the synthesis of amidines usually involves the multistep Pinner reaction using free nitriles and a strong acid or relies on additions to single-metal-activated nitriles.42 Frequently required are stringent reaction conditions,43 precious metals,44−46 or the use of sensitive starting materials.47 We also note the possible cis and trans additions of a nucleophile to the nitrile CN bond, producing respectively the E and Z isomers of the imino (CN) metal complex; production of both isomers is commonly observed, although the E isomer complex is thermodynamically favored because of less steric hindrance.42 We present herein reactions between NH3 and CH3CN complexes of the [Re6(μ3-Se)8]2+ cluster core,14 [Re6(μ3Se)8(PEt3)5(CH3CN)](BF4)2 (1), trans-[Re6(μ3Se) 8 (PEt 3 ) 4 (CH 3 CN) 2 ](BF 4 ) 2 (2), and cis-[Re 6 (μ 3 Se)8(PEt3)4(CH3CN)2](BF4)2 (3), producing the corresponding acetamidino complexes [Re6(μ3-Se)8(PEt3)5L](BF4)2 [4; L = acetamidine or HNC(NH 2 )(CH 3 )], trans-[Re 6 (μ 3 Se)8(PEt3)4L2](BF4)2 (5), and cis-[Re6(μ3-Se)8(PEt3)4L2](BF4)2 (6; crystallized out as a CH2Cl2 solvate with 1.5 solvent molecules per cluster unit) (Scheme 1 and the Supporting Information). The addition of NH3 to the cluster-bound CH3CN, achieved in an aerobic, room temperature environment, was facile and atom-economical. Briefly, NH3 was bubbled into a CH2Cl2 solution of one of the cluster complexes for 15 min, after which the reaction mixture was sealed and stirred for an additional 24 h. The product was obtained as an orange-yellow crystalline solid in essentially quantitative yield upon removal of the solvent. All three new compounds were characterized by 31P and 1H NMR and elemental analyses (CHN) and structurally determined by single-crystal X-ray diffraction. As a representative, the spectroscopic characterization and crystallographic analysis of 4 are presented here to highlight the salient changes upon ligand transformation. The NMR spectra of
ABSTRACT: Acetonitrile bound to and activated by the Lewis acidic [Re6(μ3-Se)8]2+ cluster core was transformed into acetamidine in quantitative yield using NH3 as the nucleophile at room temperature. The amidine ligand was removed by treating the cluster−acetamidine complexes with trifluoroacetic acid in CH3CN, affording amidinium trifluoroacetate and the starting acetonitrile complexes.
L
ewis acid catalysts are important in many chemical transformations. A commonly encountered challenge in their application is their sensitivity toward air and/or moisture, and maintenance of stringent reaction conditions is thus generally required.1−5 Moreover, separation of the catalyst from the product in a homogeneous reaction mixture can sometimes prove difficult, which increases not only the chance of product contamination but also purification costs. Catalysts of aerobic stability and ease of separation are therefore desired. In this context, we and others have developed a cluster-based Lewis acidic platform for synthetically meaningful transformations. Specifically, the octahedral cluster core of [Re6(μ3Se)8]2+, being Lewis acidic because of the 2+ charge, has been found to support a great variety of aerobically stable complexes.6−11 with some of the ligands being activated for further reactions.12−19 In our work, with triethylphosphino (PEt3) ligands occupying and passivating some of the Re sites, cluster-bound acetonitrile (CH3CN) undergoes stereospecific and atom economical nucleophilic addition with methanol and ethanol, affording the corresponding imino ester complexes.20,21 Szczepura and co-workers, on the other hand, reported the addition of N3− to nitrile(s) bound to the same cluster core to produce tetrazolate complexes in a click-chemistry fashion.22,23 We later showed that the nitrile ligand can also be exchanged for organic azides, and subsequent decomposition of the azido cluster complexes upon ultraviolet irradiation produced the corresponding imino complexes in a stereospecific fashion.24,25 Besides the cluster−nitrile complexes, we have also succeeded in the preparation of the carbonyl complexes of the same cluster core. Such cluster carbonyls are similar to the classical organometallic carbonyls in terms of their back-bonding electronic structure, with the cluster-bound CO ligand(s) being activated for further transformations. For example, upon reaction with CH3Li, the corresponding acyl complex of the cluster was obtained.26 No similar hexametal cluster systems have shown such reactivity. In an effort to assess the scope of the reactions © 2016 American Chemical Society
Received: July 9, 2016 Published: September 16, 2016 9505
DOI: 10.1021/acs.inorgchem.6b01643 Inorg. Chem. 2016, 55, 9505−9508
Communication
Inorganic Chemistry
Scheme 1. Synthesis of Amidine Complexes with Crystal Structures Shown along the C3 Axis of the Cluster Corea
a
Atom labeling is provided for complex 4. Counterions, solvent molecules, and phosphine ethyl groups are omitted for clarity. Anisotropic displacement ellipsoids are shown at the 50% probability level.
unambiguously establishes the planarity of the acetamidino ligand. The neutrality of ligand L is corroborated by the presence of two BF4− anions in the structure of compound 4. The N1−C31 bond distance at 1.288 Å is typical of an NC bond, while the length of C31−N2 at 1.330 Å falls between the typical length of an N−C single bond (1.47 Å) and that of a double bond (1.29 Å), thus suggesting significant double-bond character of the formal amine bond and extensive π conjugation within the N1−C31− N2 connection. This conclusion is also supported by the planar structure of the ligand; because of this planarity, the chemical environments of the two amine H atoms are considerably different, as alluded to above. The positions of the amine H atoms in the crystal structure also provide an inclination to the possible mechanistic pathway that this reaction follows. In our previous work of alcohol addition to cluster-bound CH3CN to generate imino ester complexes of the cluster, product preference for the Z isomer with the alkoxyl group and cluster unit on the same side of the CN bond over the E isomer was established based on NMR data and further supported by crystallographic studies.20 We hypothesized that this preference originates from kinetic control due to the formation of a bifurcated hydrogen bond involving the alcohol OH and the two Se atoms on one corner of the cluster. The structure of complex 4 is consistent with such a mechanistic proposal: Both the N−H2A---Se4 distance at 2.753 Å and that of N−H2A---Se8 at 2.916 Å are perfectly set for hydrogen-bonding interactions (Figure 2). Similar structural parameters are found in the structures of complexes 5 and 6 with two trans- and cis-disposed L ligands, respectively (Scheme 1). With transformation of the organic ligand(s) complete, our endeavors turned to removing the amidino ligand and
the crude reaction product, shown in Figure 1, indicate a very clean reaction. The 31P resonances show upfield shifts with
Figure 1. 31P (left) and 1H (right) NMR spectra of 1 and 4. *: CDHCl2.
respect to those of 1, with the signal of the 4 equiv PEt3 ligands being more shifted than that of the sole PEt3 that is trans to the L ligand. These observations suggest that the newly formed ligand L is more electron-rich than CH3CN, which is consistent with the fact that the cis-PEt3 ligands are in closer proximity to L and are therefore electronically affected more than the trans-PEt3 ligands. In 1H NMR, the resonance at 2.92 ppm of the cluster-bound CH3CN is replaced by a singlet at 2.37 ppm upon reaction. In addition, three new singlets at 7.12, 6.44, and 5.00 ppm appear, corresponding respectively to the imine and amine H atoms; the presence of three separate peaks indicates that the amine H atoms are inequivalent. This conclusion is consistent with the crystal structure of 4 (Scheme 1, top right), which 9506
DOI: 10.1021/acs.inorgchem.6b01643 Inorg. Chem. 2016, 55, 9505−9508
Communication
Inorganic Chemistry Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported, in part, by National Science Foundation Grants CHE-0750530 and CHE-1152609.
Figure 2. Crystal structure of 4 showing ligand planarity and hydrogen bonding between the −NH2 group and two Se atoms on the edge of the cluster core. N−H2A---Se4 = 2.753 Å and N−H2A---Se8 = 2.916 Å. Color code: cyan, Re; orange, Se; purple, N; gray, C; white, H.
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(1) Lu, Z.; Schweighauser, L.; Hausmann, H.; Wegner, H. Metal-Free Ammonia−Borane Dehydrogenation Catalyzed by a Bis(borane) Lewis Acid. Angew. Chem., Int. Ed. 2015, 54, 15556−15559. (2) Aikawa, K.; Kondo, D.; Honda, K.; Mikami, K. Lewis Acid Catalyzed Asymmetric Three-Component Coupling Reaction: Facile Synthesis of α-Fluoromethylated Tertiary Alcohols. Chem. - Eur. J. 2015, 21, 17565−17569. (3) Guo, H.; Herdtweck, E.; Bach, T. Enantioselective Lewis Acid Catalysis in Intramolecular [2 + 2] Photocycloaddition Reactions of Coumarins. Angew. Chem., Int. Ed. 2010, 49, 7782−7785. (4) Zhou, J.; Jiang, B.; Meng, F.; Xu, Y.; Loh, T. B(C6F5)3: A New Class of Strong and Bulky Lewis Acid for Exo-Selective Intermolecular Diels− Alder Reactions of Unreactive Acyclic Dienes with α,β-Enals. Org. Lett. 2015, 17, 4432−4435. (5) Wang, Z.; Chen, S.; Ren, J.; Wang, Z. Cooperative Photo-/Lewis Acid Catalyzed Tandem Intramolecular [3 + 2] Cross-Cycloadditions of Cyclopropane 1,1-Diesters with α,β-Unsaturated Carbonyls for Medium-Sized Carbocycles. Org. Lett. 2015, 17, 4184−4187. (6) Long, J.; McCarty, L.; Holm, R. A Solid-State Route to Molecular Clusters: Access to the Solution Chemistry of [Re6Q8]2+ (Q = S, Se) Core-Containing Clusters via Dimensional Reduction. J. Am. Chem. Soc. 1996, 118, 4603−4616. (7) Zheng, Z.; Holm, R. Cluster Condensation by Thermolysis: Synthesis of a Rhomb-Linked Re12Se16 Dicluster and Factors Relevant to the Formation of the Re24Se32Tetracluster. Inorg. Chem. 1997, 36, 5173−5178. (8) Zheng, Z.; Gray, T.; Holm, R. Synthesis and Structures of Solvated Monoclusters and Bridged Di- and Triclusters Based on the Cubic Building Block [Re6(μ3-Se)8]2+. Inorg. Chem. 1999, 38, 4888−4895. (9) Selby, H.; Roland, B.; Zheng, Z. Ligand-Bridged Oligomeric and Supramolecular Arrays of the Hexanuclear Rhenium Selenide Clusters− Exploratory Synthesis, Structural Characterization, and Property Investigation. Acc. Chem. Res. 2003, 36, 933−944. (10) Ledneva, A.; Brylev, K.; Smolentsev, A.; Mironov, Y.; Molard, Y.; Cordier, S.; Kitamura, N.; Naumov, N. Controlled synthesis and luminescence properties of trans[Re6S8(CN)4(OH)2−n(H2O)n]n−4octahedral rhenium(III) cluster units (n = 0, 1 or 2). Polyhedron 2014, 67, 351−359. (11) El Osta, R.; Demont, A.; Audebrand, N.; Molard, Y.; Nguyen, T.; Gautier, R.; Brylev, K.; Mironov, Y.; Naumov, N.; Kitamura, N.; Cordier, S. Supramolecular Frameworks Built up from Red-Phosphorescent trans-Re6 Cluster Building Blocks: One Pot Synthesis, Crystal Structures, and DFT Investigations. Z. Anorg. Allg. Chem. 2015, 641, 1156−1163. (12) Gabriel, J.; Boubekeur, K.; Uriel, S.; Batail, P. Chemistry of Hexanuclear Rhenium Chalcohalide Clusters. Chem. Rev. 2001, 101, 2037−2066. (13) Shestopalov, M.; Mironov, Y.; Brylev, K.; Fedorov, V. First molecular octahedral rhenium cluster complexes with terminal As- and Sb-donor ligands. Russ. Chem. Bull. 2008, 57, 1644−1649. (14) Zheng, Z.; Long, J.; Holm, R. A Basis Set of Re6Se8 Cluster Building Blocks and Demonstration of Their Linking Capability: Directed Synthesis of an Re12Se16 Dicluster. J. Am. Chem. Soc. 1997, 119, 2163−2171.
regenerating the starting CH3CN−cluster complex with the hope of completing a formal cycle and recycling the cluster-based potential catalyst. We succeeded in our efforts by treating the amidine−cluster complex 4 with excess trifluoroacetic acid in CH3CN. The regenerated CH3CN complex 1 was separated from acetamidinium trifluoroacetate by a straightforward extraction using CH2Cl2 as the organic phase. The identity of these two products was verified by multinuclear NMR (1H and 31 P NMR for 4 and 1H and 19F NMR for acetamidinium trifluoroacetate) in comparison with the respective literature references. Analogous removal of the amidino ligand and reformation of the corresponding CH3CN−cluster complexes were also attempted by using 5 and 6, with both reactions showing complete conversion. These results clearly point to chemical transformations at multiple sites on the same cluster core and the potential of achieving high catalytic efficiency. It is also of note that reacting the recovered CH3CN−cluster complexes with NH 3 yielded the same results as the independently prepared nitrile species. In summary, CH3CN bound and activated by the Lewis acidic cluster core of [Re6(μ3-Se)8]2+ can react with NH3 and be facilely converted to acetamidine in quantitative yield. The reaction produced exclusively the Z isomer of the amidine−cluster complexes. The newly formed ligand can be removed in the form of an amidinium salt by treating the cluster complexes with an organic acid containing a noncoordinating anion such as trifluoroacetic acid in CH3CN, regenerating the starting and potentially catalytic CH3CN−cluster complexes.
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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.6b01643. Experimental details, multinuclear NMR spectra, and crystallographic data with refinement details (PDF) X-ray crystallographic information for CCDC 1034324 (4) (CIF) X-ray crystallographic information for CCDC 1034325 (5) (CIF) X-ray crystallographic information for CCDC 1034326 (6) (CIF)
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. 9507
DOI: 10.1021/acs.inorgchem.6b01643 Inorg. Chem. 2016, 55, 9505−9508
Communication
Inorganic Chemistry
(32) Khalifa, M.; Bodner, M.; Berglund, J.; Haley, M. Synthesis of Nsubstituted aryl amidines by strong base activation of amines. Tetrahedron Lett. 2015, 56, 4109−4111. (33) Edelmann, F. Lanthanide amidinates and guanidinates: from laboratory curiosities to efficient homogeneous catalysts and precursors for rare-earth oxide thin films. Chem. Soc. Rev. 2009, 38, 2253−2268. (34) Edelmann, F. Advances in the Coordination Chemistry of Amidinate and Guanidinate Ligands. Adv. Organomet. Chem. 2008, 57, 183−352. (35) Caron, S.; Wei, L.; Douville, J.; Ghosh, A. Preparation and Utility of Trihaloethyl Imidates: Useful Reagents for the Synthesis of Amidines. J. Org. Chem. 2010, 75, 945−947. (36) Nagasawa, K. Application of Organosuperbases to Total Synthesis. Superbases for Organic Synthesis; John Wiley & Sons Ltd.: Chichester, U.K., 2009; pp 211−250. (37) Harjani, J.; Liang, C.; Jessop, P. A Synthesis of Acetamidines. J. Org. Chem. 2011, 76, 1683−1691. (38) Lobanov, P. S.; Darı ́n, D. V. Acetamidines and acetamidoximes containing an electron-withdrawing group at the α-carbon atom: their use in the synthesis of nitrogen heterocycles (review). Chem. Heterocycl. Compd. 2013, 49, 507−528. (39) Barker, J.; Kilner, M. The coordination chemistry of the amidine ligand. Coord. Chem. Rev. 1994, 133, 219−300. (40) Alder, R.; Blake, M.; Bufali, S.; Butts, C.; Orpen, A.; Schütz, J.; Williams, S. Preparation of tetraalkylformamidinium salts and related species as precursors to stable carbenes. J. Chem. Soc., Perkin Trans. 1 2001, 1, 1586−1593. (41) Sereda, O.; Clemens, N.; Heckel, T.; Wilhelm, R. Imidazolinium and amidinium salts as Lewis acid organocatalysts. Beilstein J. Org. Chem. 2012, 8, 1798−1803. (42) Kukushkin, V.; Pombeiro, A. Additions to Metal-Activated Organonitriles. Chem. Rev. 2002, 102, 1771−1802. (43) Fairlie, D.; Jackson, W. Amination of coordinated nitriles: synthesis of metal complexes of amidines and guanidines. Inorg. Chem. 1990, 29, 140−143. (44) Kukushkin, V.; Ilichev, I.; Wagner, G.; Revenco, M.; Kravtsov, V.; Suwinska, K. Rhodium(III)-Assisted Nucleophilic Addition of Ammonia to Metal-Bound Benzyl Cyanide: Crystal Structures of mer[RhCl3(PhCH2CN)3]·1/2 C6H5CH3 and mer-[RhCl 3{PhCH2C(NH2)NH}3]. Eur. J. Inorg. Chem. 2000, 2000, 1315−1319. (45) Bertani, R.; Catanese, D.; Michelin, R.; Mozzon, M.; Bandoli, G.; Dolmella, A. Reactions of platinum(II)−nitrile complexes with amines. Synthesis, characterization and X-ray structure of the platinum(II)− amidine complex trans-[PtCl2{Z-N(H)C(NHMe)Me}2]. Inorg. Chem. Commun. 2000, 3, 16−18. (46) Michelin, R.; Bertani, R.; Mozzon, M.; Sassi, A.; Benetollo, F.; Bombieri, G.; Pombeiro, A. Cis addition of dimethylamine to the coordinated nitriles of cis- and trans-[PtCl2(NCMe)2]. X-ray structure of the amidine complex cis-[PtCl2{E-N(H)C(NMe2)Me}2]·CH2Cl2. Inorg. Chem. Commun. 2001, 4, 275−280. (47) Garigipati, R. An efficient conversion of nitriles to amidines. Tetrahedron Lett. 1990, 31, 1969−1972.
(15) Shestopalov, M.; Mironov, Y.; Brylev, K.; Kozlova, S.; Fedorov, V.; Spies, H.; Pietzsch, H.; Stephan, H.; Geipel, G.; Bernhard, G. Cluster Core Controlled Reactions of Substitution of Terminal Bromide Ligands by Triphenylphosphine in Octahedral Rhenium Chalcobromide Complexes. J. Am. Chem. Soc. 2007, 129, 3714−3721. (16) Molard, Y.; Dorson, F.; Brylev, K.; Shestopalov, M.; Le Gal, Y.; Cordier, S.; Mironov, Y.; Kitamura, N.; Perrin, C. Red-NIR Luminescent Hybrid Poly(methyl methacrylate) Containing Covalently Linked Octahedral Rhenium Metallic Clusters. Chem. - Eur. J. 2010, 16, 5613−5619. (17) Efremova, O.; Brylev, K.; Kozlova, O.; White, M.; Shestopalov, M.; Kitamura, N.; Mironov, Y.; Bauer, S.; Sutherland, A. Polymerisable octahedral rhenium cluster complexes as precursors for photo/ electroluminescent polymers. J. Mater. Chem. C 2014, 2, 8630−8638. (18) Mironov, Y.; Brylev, K.; Smolentsev, A.; Ermolaev, A.; Kitamura, N.; Fedorov, V. New mixed-ligand cyanohydroxo octahedral cluster complex trans-[Re6S8(CN)2(OH)4]4−, its luminescence properties and chemical reactivity. RSC Adv. 2014, 4, 60808−60815. (19) Cordier, S.; Molard, Y.; Brylev, K.; Mironov, Y.; Grasset, F.; Fabre, B.; Naumov, N. Advances in the Engineering of Near Infrared Emitting Liquid Crystals and Copolymers, Extended Porous Frameworks, Theranostic Tools and Molecular Junctions Using Tailored Re6 Cluster Building Blocks. J. Cluster Sci. 2015, 26, 53−81. (20) Orto, P.; Selby, H.; Ferris, D.; Maeyer, J.; Zheng, Z. Alcohol Addition to Acetonitrile Activated by the [Re6(μ3-Se)8]2+ Cluster Core. Inorg. Chem. 2007, 46, 4377−4379. (21) Zheng, Z. Chemical transformations supported by the [Re6(μ3Se)8]2+ cluster core. Dalton Trans. 2012, 41, 5121−5131. (22) Szczepura, L.; Oh, M.; Knott, S. Synthesis and electrochemical study of the first tetrazolate hexanuclear rhenium cluster complex. Chem. Commun. 2007, 4617−4619. (23) Durham, J.; Tirado, J.; Knott, S.; Oh, M.; McDonald, R.; Szczepura, L. Preparation of a Family of Hexanuclear Rhenium Cluster Complexes Containing 5-(Phenyl)tetrazol-2-yl Ligands and Alkylation of 5-Substituted Tetrazolate Ligands. Inorg. Chem. 2012, 51, 7825− 7836. (24) Tu, X.; Boroson, E.; Truong, H.; Muñoz-Castro, A.; Arratia-Pérez, R.; Nichol, G.; Zheng, Z. Cluster-Bound Nitriles Do Not Click with Organic Azides: Unexpected Formation of Imino Complexes of the [Re6(μ3-Se)8]2+ Core-Containing Clusters. Inorg. Chem. 2010, 49, 380− 382. (25) Tu, X.; Truong, H.; Alster, E.; Muñoz-Castro, A.; Arratia-Pérez, R.; Nichol, G.; Zheng, Z. Geometrically Specific Imino Complexes of the [Re6(μ3-Se)8]2+Core-Containing Clusters. Chem. - Eur. J. 2011, 17, 580−587. (26) Orto, P.; Nichol, G.; Okumura, N.; Evans, D.; Arratia-Pérez, R.; Ramirez-Tagle, R.; Wang, R.; Zheng, Z. Cluster carbonyls of the [Re6(μ3-Se)8]2+ core: synthesis, structural characterization, and computational analysis. Dalton Trans. 2008, 4247−4253. (27) Guile, S.; Alcaraz, L.; Birkinshaw, T.; Bowers, K.; Ebden, M.; Furber, M.; Stocks, M. Antagonists of the P2X7 Receptor. From Lead Identification to Drug Development. J. Med. Chem. 2009, 52, 3123− 3141. (28) Greenhill, J.; Lue, P. Amidines and Guanidines in Medicinal Chemistry. Prog. Med. Chem. 1993, 30, 203−326. (29) Maccallini, C.; Patruno, A.; Bešker, N.; Alí, J.; Ammazzalorso, A.; De Filippis, B.; Franceschelli, S.; Giampietro, L.; Pesce, M.; Reale, M.; Tricca, M.; Re, N.; Felaco, M.; Amoroso, R. Synthesis, Biological Evaluation, and Docking Studies of N-Substituted Acetamidines as Selective Inhibitors of Inducible Nitric Oxide Synthase. J. Med. Chem. 2009, 52, 1481−1485. (30) Ang, M.; Phan, L.; Alshamrani, A. K.; Harjani, J. R.; Wang, R.; Schatte, G.; Mosey, N. J.; Jessop, P. G. Contrasting Reactivity of CS2 with Cyclic vs. Acyclic Amidines. Eur. J. Org. Chem. 2015, 2015, 7334− 7343. (31) Jessop, P. G.; Phan, L.; Carrier, A.; Robinson, S.; Dürr, C. J.; Harjani, J. R. A solvent having switchable hydrophilicity. Green Chem. 2010, 12, 809−814. 9508
DOI: 10.1021/acs.inorgchem.6b01643 Inorg. Chem. 2016, 55, 9505−9508
