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N-heterocyclic carbene copper complexes tethered to iron carbidocarbonyl clusters
Inorganic Chemistry Communications 49 (2014) 27–29
Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
N-heterocyclic carbene copper complexes tethered to iron carbidocarbonyl clusters☆ Roberto Della Pergola a,⁎, Annalisa Sironi a,b, André Rosehr a,1, Valentina Colombo b, Angelo Sironi b a b
Università Milano Bicocca, Dipartimento di Scienze Ambientali e del Territorio e di scienze della Terra, piazza della Scienza 1, 20126 Milano, Italy Università degli Studi di Milano, Dipartimento di Chimica, Via Golgi 19, 20133 Milano, Italy
a r t i c l e
i n f o
Article history: Received 3 June 2014 Received in revised form 2 September 2014 Accepted 7 September 2014 Available online 8 September 2014
a b s t r a c t The reactions of [Fe4C(CO)12(CuNCMe)x]x − 2 (x = 1, 2) with ImiPr·HBF4 (ImiPr = N,N′-bis(isopropyl)imidazol2-ylidene) in the presence of NaH yielded [Fe4C(CO)12(CuImiPr)x]x − 2. The hetero-substituted [Fe4C(CO)12 (CuCl)(CuImiPr)]− was obtained by the deprotonation of the corresponding chloride salt. All products have been characterized by elemental analysis, NMR and single crystal X-ray diffraction analyses. © 2014 Elsevier B.V. All rights reserved.
Keywords: Iron Copper Heterometallic clusters NHC ligands Interstitial carbide Crystal structure
The clusters [Fe5C(CO)14(CuL)]−, [Fe4C(CO)12(CuL)2] and [Fe4C(CO)12 (CuL)]− (L = Cl−, NCMe) [1] contain metallic centers in both positive (Cu) and negative (Fe) oxidation states. For this reason they can be functionalized with ligands that are common in the coordination chemistry of copper, but are quite unusual for carbonyl species; as a consequence, we have recently described the syntheses and the structures of several derivatives, containing a large variety of ligands, with Cl-, N-, S-, P-, O-, and H-donor atoms [1–3]. To further expand this chemistry, we pursued the synthesis of “true” organometallic clusters (i.e. substituted with organic C-donors), and devoted our first efforts to N-heterocyclic carbenes (NHCs), since they are nowadays widely employed in the coordination chemistry of almost all metallic elements [4]. In this context, NHC complexes of copper represent a significant and constant fraction [5], finding applications in many different fields such as catalysis [6], reduction of CO2 [7], construction of supramolecular architectures [8] and MOFs [9], luminescent metallo-complexes [10] and metallodrugs [11]. To the contrary, the substitution of carbonyl clusters with NHCs is relatively less explored, the syntheses are not systematic and the products are much less predictable [12]. Structurally characterized examples are mainly clusters of Ru [13], Os [14,15] and Pd [16]. NHC-substituted heterometallic
☆ Dedicated to the memory of Prof. Mario Manassero. ⁎ Corresponding author. E-mail address: [email protected] (R. Della Pergola). 1 Present address: Institut für Technische und Makromolekulare Chemie, Universität Hamburg, Bundesstr. 45, 20146 Hamburg, Germany.
http://dx.doi.org/10.1016/j.inoche.2014.09.007 1387-7003/© 2014 Elsevier B.V. All rights reserved.
clusters are sparse and are invariably obtained by condensation of preformed M-NHC complexes: representative species include MoNiNiNHC [17], Ru2(Pt-NHC)2, Ru3(Pt-NHC)2 and Ru3Pt-NHC [18], Os3(PdNHC)n (n = 1–3) [19], and Os3Ag-NHC [15]. The three NHC-substituted Fe–Cu heterometallic clusters here described fulfill several aspects of this chemistry which, according to the recent review by Cabeza et al., need to be investigated [12]: they i) make use of metals different from Os and Ru, ii) are high nuclearity heterometallic compounds (n N 4) and iii) adopt a non-planar structure. The most common ways to form carbene complexes are the deprotonation of imidazolium salts with strong bases (such as NaH or tBu-OK) [20] or the “oxide route”, that is the reaction of the imidazolium salt with Ag2O, followed by metal transfer [21]. As a first attempt, we reacted [Fe5C(CO)14(CuNCMe)]− with the carbene transfer agent [(ImiPr)AgCl] (ImiPr = N,N′-bis(isopropyl)imidazol-2-ylidene) and obtained a mixed salt of formula (NEt4)2[Fe4C(CO)12(Cu-ImiPr)] [Fe4C(CO)12(CuCl)(CuImiPr)], in the following denoted as (NEt4)1·(NEt4)3, containing two different monoanionic clusters, both bearing a NHC ligand. The formation of this compound confirmed that the silver complex could indeed transfer the carbenic ligand to copper, but caused unwanted side reactions. Thus, to prepare selectively (NEt4)1 we reacted, in two separated solutions, (NEt4)2[Fe4C(CO)12] with [Cu(NCMe)4]+ and ImiPr·HBF4 with NaH, and we mixed them in a second step. Even if it was not possible to grow crystals suitable for X-ray diffraction, spectroscopy and chemical analyses concurred to confirm that pure (NEt4)[Fe4C(CO)12(Cu-ImiPr)] was obtained. [Fe4C(CO)12(Cu-ImiPr)2] (2) was synthesized similarly from [Fe4C(CO)12(CuNCMe)2], ImiPr·HBF4 and NaH. Moreover, (NEt4)
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R. Della Pergola et al. / Inorganic Chemistry Communications 49 (2014) 27–29
[Fe4C(CO)12(CuCl)(Cu-ImiPr)], (NEt4)3, was obtained selectively by reacting [Fe4C(CO)12(CuNCMe)2] with ImiPr·HCl, after deprotonation of the latter with NaH in THF [22]. (NEt4)1, 2 and (NEt4)3 were characterized by elemental analysis, IR, 1H, and 13C NMR. Sharp and distinct signals have been detected in a narrow range (175–180 ppm) and unambiguously assigned to the quaternary carbon atoms of the NHC directly bound to copper [23]. Only one sharp signal was observed in the region of the carbonyl ligands, being indicative of fluxional behavior at room temperature. The crystal structures of 2 [24] and (NEt4)3 [25] have been determined by single crystal X-ray diffraction. The molecular structures of the two clusters are shown in Figs. 1 and 2. Suitable single crystals for structure solution of 1 have not been obtained. Nevertheless, structural solution of the very small and poorly diffracting single crystals of the mixed salt (NEt4)1·(NEt4)3, has been carried out. Owing to the low crystal quality, all the derived structural details were affected by large experimental uncertainty (see Supporting information) [26]. However, the comparable parameters obtained for 3 – in (NEt4)3 – and (NEt4) 1·(NEt4)3 suggest that the overall picture of 1 is reasonable, as far as stoichiometry, connectivity and shape – not bonding parameters – are concerned. The metallic cores of 2 and 3 are octahedral, with the two copper atoms cis to one another. As previously suggested for related systems, 2 and 3 can be better considered as adducts between the carbide atom of [Fe4C(CO)12]2− [27,28] (the electron donor) and linear [Cu(I)-NHC]+ or [Cu(I)–Cl] fragments (the acceptor). Indeed, the C–Cu–L moieties are remarkably linear. However, both weak bonding interactions and steric repulsions combine to determine the actual structure formed. For instance, we observe that the [Cu(I)–NHC]+ unit accepts some electron density from semibridging carbonyls. This requires a slight reorganization of the parent [Fe4C(CO)12]2− geometry [29], mainly in the butterfly folding (102.1 vs 104.3° in 2 and 3, respectively) and in the Fe(1)–C– Fe(3) angles (178.8(2) vs 176.8(3) ° in 2 and 3, respectively) which are clearly related to the different bulkiness of Cu–X ligands. In this respect, the comparison between the Cu1–Cu2 distances and the Cu1–C (interstitial)–Cu2 angles [105.0(1) vs 94.5(1)°] is very informative, and nicely highlights the distortions induced by two neighbor NHC ligands in 2. The Cu–Cu separation found in 2 (3.060(8) Å) is well within the range observed for cuprophilic Cu(I)–Cu(I) interactions [30], and
Fig. 1. The solid state structure of [Fe4C(CO)12(Cu-ImiPr)2] (2). Selected bond distances: Cu(1)–Cu(2), 3.060(8); Cu(1)–C(1), 1.931(3); Cu(1)–C(3A), 2.479(3); Fe(3)–C(3A)– O(3A), 168.0(3); Cu(2)–C(1A), 2.432(3), Fe(1)–C(1A)–O(1A), 167.1(3); Fe–Fe(ave), 2.675; Fe–C(ave), 1.909; Cu–C(ave), 1.928 Å. Hydrogen atoms are omitted for clarity. Ellipsoids are drawn at 50% probability.
Fig. 2. The solid state structure of [Fe4C(CO)12(CuCl)(Cu-ImiPr)]− (3). Selected bond distances: Cu(1)–Cu(2), 2.811(1); Cu(1)–C(1) 1.933(4); Cu(2)–Cl(1) 2.1528(14); Cu(1)–C(3A), 2.501(5); Fe(3)–C(3A)–O(3A), 169.8(4); Fe–Fe(ave) 2.654; Fe–C(ave) 1.907; Cu–C(ave) 1.914 Å. Hydrogen atoms are omitted for clarity. Ellipsoids are drawn at 50% probability.
should not be considered exceptional. NHC ligands are known to be sterically demanding ligands, owing to the short M–C bond length, and to the presence of bulky substituents on the two N atoms. However they have a two-dimensional shape and several new sterical parameters have been developed for this type of ligand [31]. The cluster framework of 1 consists of a distorted trigonal bipyramid, with two long (N3.4 Å) Cu–Fe and one short (b 2.6 Å) Fe–Fe edge in the equatorial plane (Fig. 3). For other substituted clusters of general formula [Fe4C(CO)12CuL]−, different isomeric structures have been observed, ranging from the C2v trigonal bipyramid (L = NHC, Cl−, BH− 4 ) to the almost regular Cs square-based pyramid (L = pyrazine), and they were related by DFT calculation to the electronic properties of the ligands [2]. The C2v geometry adopted by 1 is consistent with the higher σ-donor properties of the NHC ligand if compared to those of trialkylphosphines [32], and also comparable with those of anionic ligands.
Fig. 3. The molecular geometry of [Fe4C(CO)12(Cu-ImiPr)]− (1). C, dark gray; N, green; O, red; Fe, blue; Cu, orange; H, light gray. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
R. Della Pergola et al. / Inorganic Chemistry Communications 49 (2014) 27–29
Addition of Au-PR + 3 to anionic carbonyl clusters was widely used in the past to locate their basic sites [28,33], to expand their nuclearity, and to reduce molecular symmetry [34]. In the particular case of [Fe4C(CO)12]2−, the behavior toward soft metallic electrophiles can be compared with the addition of the isolobal hard H+ [28]. The latter can add to the carbide [35], but prefers the Fe–Fe hinge [36], whereas AuL+ [37] and CuL+ bind only at the carbide. In the resulting structures the Au–Au interactions or Cu–Cu interactions are strong enough to offset the repulsions between sterically hindered ligands. The clusters 1–3 suggest that M-NHC+ fragments (with M = Cu, Ag, Au) can be the logical extension of this approach, and can be exploited to introduce into molecular clusters many different functionalities. Ditopic N-heterocyclic carbene spacers [38] and [Fe4C(CO)12Cu2] [39] can be used for the assembly of oligomeric structures. Since both NHC ligands [40,41] and Fe4Cu2 clusters are known to allow moderate electronic communication, these species would possibly display peculiar electrochemical behavior. Acknowledgments This work was funded by MIUR (PRIN 2007 2007WMYH23) and University of Milano Bicocca (FAR). VC thanks the Università degli Studi di Milano for partial funding (Piano di Sviluppo 2014, Dipartimento di Chimica, Linea B1n° 17777). Appendix A. Supplementary material Deposited as supplementary material: experimental details, table of crystallographic data and figures with complete labeling. Crystallographic data for the structural analysis have been deposited with the Cambridge Crystallographic Data Centre with CCDC No. CCDC 985062–985064 copies & quoot of this information may be obtained free of charge from: The Director – CCDC − 12 Union Road, Cambridge CB2 1EZ, UK – Fax. (int code) +44(1223)336-033 – or Email: deposit@ ccdc.cam.ac.uk or http://www.ccdc.cam.ac.uk. Supplementary data associated with this article can be found, in the online version, at http://dx. doi.org/10.1016/j.inoche.2014.09.007. These data include MOL files and InChiKeys of the most important compounds described in this article. References [1] R. Della Pergola, A. Sironi, L. Garlaschelli, D. Strumolo, C. Manassero, M. Manassero, S. Fedi, P. Zanello, F. Kaswalder, S. Zacchini, Inorg. Chim. Acta 363 (2010) 586. [2] R. Della Pergola, M. Bruschi, A. Sironi, M. Manassero, C. Manassero, D. Strumolo, S. Fedi, P. Zanello, Dalton Trans. 40 (2011) 5464. [3] R. Della Pergola, A. Sironi, C. Manassero, M. Manassero, Organometallics 29 (2010) 5885. [4] Special issues on metal-carbene complexes a) Dalton Trans. 42 (2013) 7245; a) Dalton Trans. (2009) 6873; b) Coord. Chem. Rev. 251 (2007) 595; c) Chem. Rev. 109 (2009) 3209; d) W.D. Jones, J. Am. Chem. Soc. 131 (2009) 15075; e) J. Organomet. Chem. 690 (2005) 5397. [5] J.C.Y. Lin, R.T.W. Huang, C.S. Lee, A. Bhattacharyya, W.S. Hwang, I.J.B. Lin, Chem. Rev. 109 (2009) 3561. [6] a) M.M. Díaz-Requejo, P.J. Pérez, J. Organomet. Chem. 690 (2005) 5441; b) S. Díez-González, N. Marion, S.P. Nolan, Chem. Rev. 109 (2009) 3612. [7] a) D.S. Laitar, P. Müller, J.P. Sadighi, J. Am. Chem. Soc. 127 (2005) 17196; b) K.X. Bhattacharyya, J.A. Akana, D.S. Laitar, J.M. Berlin, J.P. Sadighi, Organometallics 27 (2008) 2682. [8] J. Chun, I.G. Jung, H.J. Kim, M. Park, M.S. Lah, S.U. Son, Inorg. Chem. 48 (2009) 6353. [9] J. Chun, H.S. Lee, I.G. Jung, S.W. Lee, H.J. Kim, S.U. Son, Organometallics 29 (2010) 1518. [10] a) K. Matsumoto, N. Matsumoto, A. Ishii, T. Tsukuda, M. Hasegawa, T. Tsubomura, Dalton Trans. (2009) 6795; b) V.A. Krylova, P.I. Djurovich, M.T. Whited, M.E. Thompson, Chem. Commun. (2010) 6696. [11] a) M.-L. Teyssot, A.-S. Jarrousse, M. Manin, A. Chevry, S. Roche, F. Norre, C. Beaudoin, L. Morel, D. Boyer, R. Mahiou, Dalton Trans. (2009) 6894; b) S.J. Tan, Y.K. Yan, P.P.F. Lee, K.H. Lim, Future Med. Chem. 2 (2010) 1591. [12] J.A. Cabeza, P. Garcia-Àlvarez, Chem. Soc. Rev. 40 (2011) 5389. [13] a) M.I. Bruce, M.L. Cole, R.S.C. Fung, C.M. Forsyth, M. Hilder, P.C. Junk, K. Konstas, Dalton Trans. (2008) 4118; b) J.A. Cabeza, I. del Río, D. Miguel, E. Pérez-Carreño, M.G. Sánchez-Vega, Organometallics 27 (2008) 211; c) J.A. Cabeza, J.F. van der Maelen, S. García-Granda, Organometallics 28 (2009) 3666;
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[22] Synthesis of (NEt4)1—34 mg (0.14 mmol) of ImiPrHBF4 were suspended in 3 mL of anhydrous THF and solid NaH (60% suspension, 6 mg; 0.15 mmol) was added, stirring at RT. [NEt4][Fe4C(CO)12(CuNCMe)] was prepared in situ from of 115.0 mg (0. 14 mmol) of [NEt4]2[Fe4C(CO)12] and [Cu(NCMe)4]PF6 (58 mg; 0.16 mmol) dissolved in 10 mL of THF. The liquid phase of the carbene suspension was added to the cluster solution. Small amounts of a tan solid were eliminated by filtration, and 30 mL of heptane was added dropwise, causing the formation of the dark microcrystalline product, which gives satisfactory elemental analyses, and whose purity is sufficient for spectroscopic characterization. Yield 81 mg (65%). Synthesis of 2 — 63.6 mg (0. 26 mmol) of ImiPrHBF4 were suspended in 3 mL of anhydrous THF and solid NaH (60 % suspension in vaseline oil, 12 mg; 0.3 mmol) was added, stirring at RT. A solution of 99.0 mg (0.13 mmol) of [Fe4C(CO)12(CuNCMe)2] in 3 mL of THF was prepared and then the liquid phase of the carbene suspension carefully collected and added to the cluster solution. After 12 hours of stirring, an IR analysis indicated the formation of a new compound. The reaction mixture was layered with heptane for stratification. Yield 71 mg (56 %). Synthesis of (NEt4)3 — [Fe4C(CO)12(CuNCMe)2] (151 mg; 0.19 mmol) was dissolved in 20 mL of THF. In a second vessel were placed 5 mL of anhydrous THF, ImiPr·HCl (36 mg; 0.19 mmol) and NaH (60% susp. in vaseline oil, 8 mg; 0.2 mmol). The two solutions were mixed, and the resulting solution was stirred at room temperature for 30 min. NEt4BF4 (50 mg; 0.23 mmol) was added, and allowed to dissolve for 1 h. The solution was concentrated to about 5 mL, filtered, and layered with 15 mL of cyclohexane. Yield 62 mg (32%). Other experimental details, characterization data and detailed X-ray structure determination are deposited in the ESI. [23] a) L.A. Goj, E.D. Blue, S.A. Delp, T.B. Gunnoe, T.R. Cundari, A.W. Pierpont, J.L. Petersen, P.D. Boyle, Inorg. Chem. 45 (2006) 9032; b) D. Tapu, D.A. Dixon, C. Roe, Chem. Rev. 109 (2009) 3385. [24] Cell parameters: triclinic P-1; a = 11.545(3), b = 11.689(3), c = 18.051(5) Å; α = 90.930(10), β = 105.150(10), γ = 115.150(10)°; V = 2105.7(10) Å3; Z = 2. [25] Cell parameters: orthorhombic Pna21; a = 11.356(3), b = 36.248(8), c = 9. 582(3) Å; V = 3944.3(18) Å3; Z = 4. [26] Cell parameters: monoclinic P21; a = 9.791(7), b = 21.234(14), c = 19.3732(13) Å; β = 104.475(1)°; V = 3900(3) Å3; Z = 2. [27] M. Tachikawa, E.L. Muetterties, J. Am. Chem. Soc. 102 (1980) 4541. [28] R. Reina, O. Riba, O. Rossell, M. Seco, P. Gómez-Sal, A. Martín, Organometallics 16 (1997) 5113. [29] R.F. Boehme, P. Coppens, Acta Crystallogr. B37 (1981) 1914. [30] H.L. Hermann, G. Boche, P. 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Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
N-heterocyclic carbene copper complexes tethered to iron carbidocarbonyl clusters☆ Roberto Della Pergola a,⁎, Annalisa Sironi a,b, André Rosehr a,1, Valentina Colombo b, Angelo Sironi b a b
Università Milano Bicocca, Dipartimento di Scienze Ambientali e del Territorio e di scienze della Terra, piazza della Scienza 1, 20126 Milano, Italy Università degli Studi di Milano, Dipartimento di Chimica, Via Golgi 19, 20133 Milano, Italy
a r t i c l e
i n f o
Article history: Received 3 June 2014 Received in revised form 2 September 2014 Accepted 7 September 2014 Available online 8 September 2014
a b s t r a c t The reactions of [Fe4C(CO)12(CuNCMe)x]x − 2 (x = 1, 2) with ImiPr·HBF4 (ImiPr = N,N′-bis(isopropyl)imidazol2-ylidene) in the presence of NaH yielded [Fe4C(CO)12(CuImiPr)x]x − 2. The hetero-substituted [Fe4C(CO)12 (CuCl)(CuImiPr)]− was obtained by the deprotonation of the corresponding chloride salt. All products have been characterized by elemental analysis, NMR and single crystal X-ray diffraction analyses. © 2014 Elsevier B.V. All rights reserved.
Keywords: Iron Copper Heterometallic clusters NHC ligands Interstitial carbide Crystal structure
The clusters [Fe5C(CO)14(CuL)]−, [Fe4C(CO)12(CuL)2] and [Fe4C(CO)12 (CuL)]− (L = Cl−, NCMe) [1] contain metallic centers in both positive (Cu) and negative (Fe) oxidation states. For this reason they can be functionalized with ligands that are common in the coordination chemistry of copper, but are quite unusual for carbonyl species; as a consequence, we have recently described the syntheses and the structures of several derivatives, containing a large variety of ligands, with Cl-, N-, S-, P-, O-, and H-donor atoms [1–3]. To further expand this chemistry, we pursued the synthesis of “true” organometallic clusters (i.e. substituted with organic C-donors), and devoted our first efforts to N-heterocyclic carbenes (NHCs), since they are nowadays widely employed in the coordination chemistry of almost all metallic elements [4]. In this context, NHC complexes of copper represent a significant and constant fraction [5], finding applications in many different fields such as catalysis [6], reduction of CO2 [7], construction of supramolecular architectures [8] and MOFs [9], luminescent metallo-complexes [10] and metallodrugs [11]. To the contrary, the substitution of carbonyl clusters with NHCs is relatively less explored, the syntheses are not systematic and the products are much less predictable [12]. Structurally characterized examples are mainly clusters of Ru [13], Os [14,15] and Pd [16]. NHC-substituted heterometallic
☆ Dedicated to the memory of Prof. Mario Manassero. ⁎ Corresponding author. E-mail address: [email protected] (R. Della Pergola). 1 Present address: Institut für Technische und Makromolekulare Chemie, Universität Hamburg, Bundesstr. 45, 20146 Hamburg, Germany.
http://dx.doi.org/10.1016/j.inoche.2014.09.007 1387-7003/© 2014 Elsevier B.V. All rights reserved.
clusters are sparse and are invariably obtained by condensation of preformed M-NHC complexes: representative species include MoNiNiNHC [17], Ru2(Pt-NHC)2, Ru3(Pt-NHC)2 and Ru3Pt-NHC [18], Os3(PdNHC)n (n = 1–3) [19], and Os3Ag-NHC [15]. The three NHC-substituted Fe–Cu heterometallic clusters here described fulfill several aspects of this chemistry which, according to the recent review by Cabeza et al., need to be investigated [12]: they i) make use of metals different from Os and Ru, ii) are high nuclearity heterometallic compounds (n N 4) and iii) adopt a non-planar structure. The most common ways to form carbene complexes are the deprotonation of imidazolium salts with strong bases (such as NaH or tBu-OK) [20] or the “oxide route”, that is the reaction of the imidazolium salt with Ag2O, followed by metal transfer [21]. As a first attempt, we reacted [Fe5C(CO)14(CuNCMe)]− with the carbene transfer agent [(ImiPr)AgCl] (ImiPr = N,N′-bis(isopropyl)imidazol-2-ylidene) and obtained a mixed salt of formula (NEt4)2[Fe4C(CO)12(Cu-ImiPr)] [Fe4C(CO)12(CuCl)(CuImiPr)], in the following denoted as (NEt4)1·(NEt4)3, containing two different monoanionic clusters, both bearing a NHC ligand. The formation of this compound confirmed that the silver complex could indeed transfer the carbenic ligand to copper, but caused unwanted side reactions. Thus, to prepare selectively (NEt4)1 we reacted, in two separated solutions, (NEt4)2[Fe4C(CO)12] with [Cu(NCMe)4]+ and ImiPr·HBF4 with NaH, and we mixed them in a second step. Even if it was not possible to grow crystals suitable for X-ray diffraction, spectroscopy and chemical analyses concurred to confirm that pure (NEt4)[Fe4C(CO)12(Cu-ImiPr)] was obtained. [Fe4C(CO)12(Cu-ImiPr)2] (2) was synthesized similarly from [Fe4C(CO)12(CuNCMe)2], ImiPr·HBF4 and NaH. Moreover, (NEt4)
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[Fe4C(CO)12(CuCl)(Cu-ImiPr)], (NEt4)3, was obtained selectively by reacting [Fe4C(CO)12(CuNCMe)2] with ImiPr·HCl, after deprotonation of the latter with NaH in THF [22]. (NEt4)1, 2 and (NEt4)3 were characterized by elemental analysis, IR, 1H, and 13C NMR. Sharp and distinct signals have been detected in a narrow range (175–180 ppm) and unambiguously assigned to the quaternary carbon atoms of the NHC directly bound to copper [23]. Only one sharp signal was observed in the region of the carbonyl ligands, being indicative of fluxional behavior at room temperature. The crystal structures of 2 [24] and (NEt4)3 [25] have been determined by single crystal X-ray diffraction. The molecular structures of the two clusters are shown in Figs. 1 and 2. Suitable single crystals for structure solution of 1 have not been obtained. Nevertheless, structural solution of the very small and poorly diffracting single crystals of the mixed salt (NEt4)1·(NEt4)3, has been carried out. Owing to the low crystal quality, all the derived structural details were affected by large experimental uncertainty (see Supporting information) [26]. However, the comparable parameters obtained for 3 – in (NEt4)3 – and (NEt4) 1·(NEt4)3 suggest that the overall picture of 1 is reasonable, as far as stoichiometry, connectivity and shape – not bonding parameters – are concerned. The metallic cores of 2 and 3 are octahedral, with the two copper atoms cis to one another. As previously suggested for related systems, 2 and 3 can be better considered as adducts between the carbide atom of [Fe4C(CO)12]2− [27,28] (the electron donor) and linear [Cu(I)-NHC]+ or [Cu(I)–Cl] fragments (the acceptor). Indeed, the C–Cu–L moieties are remarkably linear. However, both weak bonding interactions and steric repulsions combine to determine the actual structure formed. For instance, we observe that the [Cu(I)–NHC]+ unit accepts some electron density from semibridging carbonyls. This requires a slight reorganization of the parent [Fe4C(CO)12]2− geometry [29], mainly in the butterfly folding (102.1 vs 104.3° in 2 and 3, respectively) and in the Fe(1)–C– Fe(3) angles (178.8(2) vs 176.8(3) ° in 2 and 3, respectively) which are clearly related to the different bulkiness of Cu–X ligands. In this respect, the comparison between the Cu1–Cu2 distances and the Cu1–C (interstitial)–Cu2 angles [105.0(1) vs 94.5(1)°] is very informative, and nicely highlights the distortions induced by two neighbor NHC ligands in 2. The Cu–Cu separation found in 2 (3.060(8) Å) is well within the range observed for cuprophilic Cu(I)–Cu(I) interactions [30], and
Fig. 1. The solid state structure of [Fe4C(CO)12(Cu-ImiPr)2] (2). Selected bond distances: Cu(1)–Cu(2), 3.060(8); Cu(1)–C(1), 1.931(3); Cu(1)–C(3A), 2.479(3); Fe(3)–C(3A)– O(3A), 168.0(3); Cu(2)–C(1A), 2.432(3), Fe(1)–C(1A)–O(1A), 167.1(3); Fe–Fe(ave), 2.675; Fe–C(ave), 1.909; Cu–C(ave), 1.928 Å. Hydrogen atoms are omitted for clarity. Ellipsoids are drawn at 50% probability.
Fig. 2. The solid state structure of [Fe4C(CO)12(CuCl)(Cu-ImiPr)]− (3). Selected bond distances: Cu(1)–Cu(2), 2.811(1); Cu(1)–C(1) 1.933(4); Cu(2)–Cl(1) 2.1528(14); Cu(1)–C(3A), 2.501(5); Fe(3)–C(3A)–O(3A), 169.8(4); Fe–Fe(ave) 2.654; Fe–C(ave) 1.907; Cu–C(ave) 1.914 Å. Hydrogen atoms are omitted for clarity. Ellipsoids are drawn at 50% probability.
should not be considered exceptional. NHC ligands are known to be sterically demanding ligands, owing to the short M–C bond length, and to the presence of bulky substituents on the two N atoms. However they have a two-dimensional shape and several new sterical parameters have been developed for this type of ligand [31]. The cluster framework of 1 consists of a distorted trigonal bipyramid, with two long (N3.4 Å) Cu–Fe and one short (b 2.6 Å) Fe–Fe edge in the equatorial plane (Fig. 3). For other substituted clusters of general formula [Fe4C(CO)12CuL]−, different isomeric structures have been observed, ranging from the C2v trigonal bipyramid (L = NHC, Cl−, BH− 4 ) to the almost regular Cs square-based pyramid (L = pyrazine), and they were related by DFT calculation to the electronic properties of the ligands [2]. The C2v geometry adopted by 1 is consistent with the higher σ-donor properties of the NHC ligand if compared to those of trialkylphosphines [32], and also comparable with those of anionic ligands.
Fig. 3. The molecular geometry of [Fe4C(CO)12(Cu-ImiPr)]− (1). C, dark gray; N, green; O, red; Fe, blue; Cu, orange; H, light gray. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
R. Della Pergola et al. / Inorganic Chemistry Communications 49 (2014) 27–29
Addition of Au-PR + 3 to anionic carbonyl clusters was widely used in the past to locate their basic sites [28,33], to expand their nuclearity, and to reduce molecular symmetry [34]. In the particular case of [Fe4C(CO)12]2−, the behavior toward soft metallic electrophiles can be compared with the addition of the isolobal hard H+ [28]. The latter can add to the carbide [35], but prefers the Fe–Fe hinge [36], whereas AuL+ [37] and CuL+ bind only at the carbide. In the resulting structures the Au–Au interactions or Cu–Cu interactions are strong enough to offset the repulsions between sterically hindered ligands. The clusters 1–3 suggest that M-NHC+ fragments (with M = Cu, Ag, Au) can be the logical extension of this approach, and can be exploited to introduce into molecular clusters many different functionalities. Ditopic N-heterocyclic carbene spacers [38] and [Fe4C(CO)12Cu2] [39] can be used for the assembly of oligomeric structures. Since both NHC ligands [40,41] and Fe4Cu2 clusters are known to allow moderate electronic communication, these species would possibly display peculiar electrochemical behavior. Acknowledgments This work was funded by MIUR (PRIN 2007 2007WMYH23) and University of Milano Bicocca (FAR). VC thanks the Università degli Studi di Milano for partial funding (Piano di Sviluppo 2014, Dipartimento di Chimica, Linea B1n° 17777). Appendix A. Supplementary material Deposited as supplementary material: experimental details, table of crystallographic data and figures with complete labeling. Crystallographic data for the structural analysis have been deposited with the Cambridge Crystallographic Data Centre with CCDC No. CCDC 985062–985064 copies & quoot of this information may be obtained free of charge from: The Director – CCDC − 12 Union Road, Cambridge CB2 1EZ, UK – Fax. (int code) +44(1223)336-033 – or Email: deposit@ ccdc.cam.ac.uk or http://www.ccdc.cam.ac.uk. Supplementary data associated with this article can be found, in the online version, at http://dx. doi.org/10.1016/j.inoche.2014.09.007. These data include MOL files and InChiKeys of the most important compounds described in this article. References [1] R. Della Pergola, A. Sironi, L. Garlaschelli, D. Strumolo, C. Manassero, M. Manassero, S. Fedi, P. Zanello, F. Kaswalder, S. Zacchini, Inorg. Chim. Acta 363 (2010) 586. [2] R. Della Pergola, M. Bruschi, A. Sironi, M. Manassero, C. Manassero, D. Strumolo, S. Fedi, P. Zanello, Dalton Trans. 40 (2011) 5464. [3] R. Della Pergola, A. Sironi, C. Manassero, M. Manassero, Organometallics 29 (2010) 5885. [4] Special issues on metal-carbene complexes a) Dalton Trans. 42 (2013) 7245; a) Dalton Trans. (2009) 6873; b) Coord. Chem. Rev. 251 (2007) 595; c) Chem. Rev. 109 (2009) 3209; d) W.D. Jones, J. Am. Chem. Soc. 131 (2009) 15075; e) J. Organomet. Chem. 690 (2005) 5397. [5] J.C.Y. Lin, R.T.W. Huang, C.S. Lee, A. Bhattacharyya, W.S. Hwang, I.J.B. Lin, Chem. Rev. 109 (2009) 3561. [6] a) M.M. Díaz-Requejo, P.J. Pérez, J. Organomet. Chem. 690 (2005) 5441; b) S. Díez-González, N. Marion, S.P. Nolan, Chem. Rev. 109 (2009) 3612. [7] a) D.S. Laitar, P. Müller, J.P. Sadighi, J. Am. Chem. Soc. 127 (2005) 17196; b) K.X. Bhattacharyya, J.A. Akana, D.S. Laitar, J.M. Berlin, J.P. Sadighi, Organometallics 27 (2008) 2682. [8] J. Chun, I.G. Jung, H.J. Kim, M. Park, M.S. Lah, S.U. Son, Inorg. Chem. 48 (2009) 6353. [9] J. Chun, H.S. Lee, I.G. Jung, S.W. Lee, H.J. Kim, S.U. Son, Organometallics 29 (2010) 1518. [10] a) K. Matsumoto, N. Matsumoto, A. Ishii, T. Tsukuda, M. Hasegawa, T. Tsubomura, Dalton Trans. (2009) 6795; b) V.A. Krylova, P.I. Djurovich, M.T. Whited, M.E. Thompson, Chem. Commun. (2010) 6696. [11] a) M.-L. Teyssot, A.-S. Jarrousse, M. Manin, A. Chevry, S. Roche, F. Norre, C. Beaudoin, L. Morel, D. Boyer, R. Mahiou, Dalton Trans. (2009) 6894; b) S.J. Tan, Y.K. Yan, P.P.F. Lee, K.H. Lim, Future Med. Chem. 2 (2010) 1591. [12] J.A. Cabeza, P. Garcia-Àlvarez, Chem. Soc. Rev. 40 (2011) 5389. [13] a) M.I. Bruce, M.L. Cole, R.S.C. Fung, C.M. Forsyth, M. Hilder, P.C. Junk, K. Konstas, Dalton Trans. (2008) 4118; b) J.A. Cabeza, I. del Río, D. Miguel, E. Pérez-Carreño, M.G. Sánchez-Vega, Organometallics 27 (2008) 211; c) J.A. Cabeza, J.F. van der Maelen, S. García-Granda, Organometallics 28 (2009) 3666;
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[22] Synthesis of (NEt4)1—34 mg (0.14 mmol) of ImiPrHBF4 were suspended in 3 mL of anhydrous THF and solid NaH (60% suspension, 6 mg; 0.15 mmol) was added, stirring at RT. [NEt4][Fe4C(CO)12(CuNCMe)] was prepared in situ from of 115.0 mg (0. 14 mmol) of [NEt4]2[Fe4C(CO)12] and [Cu(NCMe)4]PF6 (58 mg; 0.16 mmol) dissolved in 10 mL of THF. The liquid phase of the carbene suspension was added to the cluster solution. Small amounts of a tan solid were eliminated by filtration, and 30 mL of heptane was added dropwise, causing the formation of the dark microcrystalline product, which gives satisfactory elemental analyses, and whose purity is sufficient for spectroscopic characterization. Yield 81 mg (65%). Synthesis of 2 — 63.6 mg (0. 26 mmol) of ImiPrHBF4 were suspended in 3 mL of anhydrous THF and solid NaH (60 % suspension in vaseline oil, 12 mg; 0.3 mmol) was added, stirring at RT. A solution of 99.0 mg (0.13 mmol) of [Fe4C(CO)12(CuNCMe)2] in 3 mL of THF was prepared and then the liquid phase of the carbene suspension carefully collected and added to the cluster solution. After 12 hours of stirring, an IR analysis indicated the formation of a new compound. The reaction mixture was layered with heptane for stratification. Yield 71 mg (56 %). Synthesis of (NEt4)3 — [Fe4C(CO)12(CuNCMe)2] (151 mg; 0.19 mmol) was dissolved in 20 mL of THF. In a second vessel were placed 5 mL of anhydrous THF, ImiPr·HCl (36 mg; 0.19 mmol) and NaH (60% susp. in vaseline oil, 8 mg; 0.2 mmol). The two solutions were mixed, and the resulting solution was stirred at room temperature for 30 min. NEt4BF4 (50 mg; 0.23 mmol) was added, and allowed to dissolve for 1 h. The solution was concentrated to about 5 mL, filtered, and layered with 15 mL of cyclohexane. Yield 62 mg (32%). 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