Organometallics 2010, 29, 2901–2903 DOI: 10.1021/om100165s
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A [I-Ca-I-Ca-I-Ca-I]2þ Chain Stabilized by Two Chelating β-Diketiminate Ligands Sankaranarayana Pillai Sarish, Anukul Jana, Herbert W. Roesky,* Thomas Schulz, and Dietmar Stalke Institut f€ ur Anorganische Chemie, Universit€ at G€ ottingen, Tammannstrasse 4, 37077 G€ ottingen, Germany Received March 1, 2010
We were able to obtain a [I-Ca-I-Ca-I-Ca-I]2þ chain stabilized by two chelating β-diketiminate ligands from the reaction of L1H (L1 = CH{Et2NCH2CH2N(CMe)}2) with KN(SiMe3)2 and CaI2. [L1CaI(μ-ICaI-μ)ICaL1] (1) was characterized by NMR spectroscopy, mass spectrometry, microanalysis, and X-ray structural analysis. The X-ray structural analysis of 1 reveals a unprecedented Ca3I4 chain structure with the calcium atoms exhibiting different coordination modes within the complex. 1 is anticipated as a possible suitable precursor for low-valent calcium compounds.
Introduction Complexes containing metal-metal bonds are one of the synthetic challenges with implications in other areas of chemistry.1 The interest also stems from several perspectives including synthetic methodology, structure, and theoretical involvement. Generally these unique species are obtained by using bulky protecting groups, and they form various aggregates in the solid state. Several group 13 and group 14 metal-metal complexes have been prepared and structurally characterized.2 Compared to groups 13 and 14 the field of alkaline earth metals is still in its infancy.3 Recently Jones and co-workers were successful in isolating a Mg(I) dimer containing a Mg-Mg single bond.4 The compounds were prepared by the reduction of a β-diketiminate and guanidinate-stabilized Mg(II) halides with potassium. This success initiated the research of other subvalent alkaline earth metal compounds, especially of calcium(I) derivatives. Extrapolating this work to calcium requires a suitable precursor, which provides kinetic and electronic stabilization to the metal center from the spectator ligand. A further impetus came from the recent theoretical calculation that predicted that Cp2Ca2 is a
relatively stable species.5 Moreover, the NBO analysis shows a covalent single Ca-Ca bond and an ionic bonding between the metal and the Cp ring. Following this, Westerhausen et al. theoretically investigated the stabilization of a diaryldicalcium(I) complex.6 Their finding indicates that ortho-substituted diaryl dicalcium(I) is stabilized by steric and electronic factors. More recently the same group was successful in isolating an inverse sandwich complex of calcium with formal oxidation state þ1.7 In this regard there is an increasing interest in the generation of a compound featuring a Ca-Ca bond with a formal oxidation state of þ1. It is notable that most of the subvalent metal complexes are obtained by using halides as precursors. A handful of halide complexes8 of calcium that are preferentially stabilized by β-diketiminate ligands consist of an impressive array of otherwise barely accessible molecules.9-11 β-Diketiminate ligand supported calcium iodide complexes were reported by Winter and co-workers in 2006.9 In recent times, a calcium fluoride and a chloride were prepared as stable species by exploiting the unique electronic and steric effect offered by the β-diketiminate ligand L (L=CH(CMe2,6-iPr2C6H3N)2).10 Following this route another calcium fluoride complex was also reported by Hill and co-workers.11 All these complexes are isolated as THF adducts and are dimers in the
*To whom correspondence should be addressed. E-mail: hroesky@ gwdg.de. (1) Carmona, E.; Galindo, A. Angew. Chem. 2008, 120, 6626–6637; Angew. Chem., Int. Ed. 2008, 47, 6526-6536. (2) (a) Pu, L.; Twamley, B.; Power, P. P. J. Am. Chem. Soc. 2000, 122, 3524–3525. (b) Stender, M.; Phillips, A. D.; Wright, R. J.; Power, P. P. Angew. Chem. 2002, 114, 1863–1865; Angew. Chem., Int. Ed. 2002, 41, 1785-1787. (c) Sekiguchi, A.; Kinjo, R.; Ichinohe, M. Science 2004, 305, 1755–1757. (d) Green, S. P.; Jones, C.; Junk, P. C.; Lippert, K.-A.; Stasch, A. Chem. Commun. 2006, 3978–3980. (e) Nagendran, S.; Sen, S. S.; Roesky, H. W.; Koley, D.; Grubm€ uller, H.; Pal, A.; Herbst-Irmer, R. Organometallics 2008, 27, 5459– 5463. (f) Sen, S. S.; Jana, A.; Roesky, H. W.; Schulzke, C. Angew. Chem. 2009, 121, 8688–8690; Angew. Chem., Int. Ed. 2009, 48, 8536-8538. (3) (a) Stalke, D. Angew. Chem. 1994, 106, 2256–2259; Angew. Chem., Int. Ed. Engl. 1994, 33, 2168-2171. (b) Rieckhoff, M.; Pieper, U.; Stalke, D.; Edelmann, F. T. Angew. Chem. 1993, 105, 1102–1104; Angew. Chem., Int. Ed. Engl. 1993, 32, 1079-1081. (4) (a) Green, S. P.; Jones, C.; Stasch, A. Science 2008, 318, 1754– 1757. (b) Overgaard, J.; Jones, C.; Stasch, A.; Iversen, B. B. J. Am. Chem. Soc. 2009, 131, 4208–4209. (5) Xie, Y.; Schaefer III, H. F.; Jemmis, E. D. Chem. Phys. Lett. 2005, 402, 414–421.
(6) Westerhausen, M.; G€artner, M.; Fischer, R.; Langer, J.; Yu, L.; Reiher, M. Chem.;Eur. J. 2007, 13, 6292–6306. (7) Krieck, S.; G€ orls, H.; Yu, L.; Reiher, M.; Westerhausen, M. J. Am. Chem. Soc. 2009, 131, 2977–2985. (8) (a) Davidson, M. G.; Raithby, P. R.; Snaith, R.; Stalke, D.; Wright, D. S. Angew. Chem. 1991, 103, 1696–1697; Angew. Chem., Int. Ed. Engl. 1991, 30, 1648-1650. (b) Westerhausen, M.; Digeser, M. H.; G€ uckel, C.; N€ oth, H.; Knizek, J.; Ponikwar, W. Organometallics 1999, 18, 2491–2496. (c) Sitzmann, H.; Weber, F.; Walter, M. D.; Wolmersh€auser, G. Organometallics 2003, 22, 1931–1936. (9) El-Kaderi, H. M.; Heeg, M. J.; Winter, C. H. Polyhedron 2006, 25, 224–234. (10) (a) Ruspic, C.; Harder, S. Inorg. Chem. 2007, 46, 10426–10433. (b) Nembenna, S.; Roesky, H. W.; Nagendran, S.; Hofmeister, A.; Magull, J.; Wilbrandt, P.-J.; Hahn, M. Angew. Chem. 2007, 119, 2564–2566; Angew. Chem., Int. Ed. 2007, 46, 2512-2514. (11) Barrett, A. G. M.; Crimmin, M. R.; Hill, M. S.; Hitchcock, P. B.; Procopiou, P. A. Angew. Chem. 2007, 119, 6455–6458; Angew. Chem., Int. Ed. 2007, 46, 6339-6342.
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Published on Web 06/04/2010
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Sarish et al.
Scheme 1. Preparation of β-Diketiminate-Supported Calcium Iodide Complex 1
solid state. Additionally our group reported the first soluble CaF2 complexes [(Cp*TiF2)6(CaF2thf)2] (Cp*=C5Me5)12 and [(Cp*TiF3)4CaF2] or [(C5Me4Et)TiF3]4CaF2. In these examples CaF2 is trapped in a soluble organometallic matrix.13 So the question arises whether is it possible to obtain calcium iodide as a solvent-free complex or even trap CaI2 within an organometallic fragment. Moreover, it is worth mentioning that it is difficult to obtain heavier alkaline earth metal complexes in a pure form due to the high reactivity14 and the tendency toward ligand redistribution,15 even though much success has been achieved in recent times. To reach such a goal, we considered that a ligand that provides a high coordination number around the metal atom might be a suitable precursor. The β-diketiminate ligand is isoelectronic to cyclopentadienyl and β-diketonate ligands, and its steric and electronic effect can be easily tuned at both the nitrogen and carbon atoms.16 In order to synthesize such a complex, the ligand L1H (L1 = CH{Et2NCH2CH2N(CMe)}2) with two pendant donor arms was utilized and proved to be suitable for our purpose.17 The ligand was already found to be a versatile support for the preparation of a number of solvent-free lanthanide complexes including halides.18
Results and Discussion Attempts to obtain a solvent-free dimeric calcium compound with 1:1:1 stoichiometry of reactants give a mixture of products, predominately 1. However, by adjusting the stoichiometry, we are able to obtain exclusively compound 1. The reaction of 2 equiv of L1H and 2 equiv of KN(SiMe3)2 with 3 equiv of CaI2 in THF at room temperature led to the formation of a [I-Ca-I-Ca-I-Ca-I]2þ chain stabilized by two chelating β-diketiminate ligands, [L1CaI(μ-ICaIμ)ICaL1] (1) (Scheme 1). Single crystals of compound 1 3 4C7H8 were obtained by storing a concentrated toluene (12) Liu, F. Q.; Stalke, D.; Roesky, H. W. Angew. Chem. 1995, 107, 2004–2006; Angew. Chem., Int. Ed. Engl. 1995, 34, 1872-1874. (13) Pevec, A.; Demsar, A.; Gramlich, V.; Petricek, S.; Roesky, H. W. J. Chem. Soc., Dalton Trans. 1997, 2215–2216. (14) (a) Fischer, R.; G€ orls, H.; Westerhausen, M. Inorg. Chem. Commun. 2005, 8, 1159–1161. (b) Kriek, S.; G€orls, H.; Westerhausen, M. J. Organomet. Chem. 2009, 694, 2204–2209. (15) (a) Hanusa, T. P. Polyhedron 1990, 9, 1345–1362. (b) Hanusa, T. P. Coord. Chem. Rev. 2000, 210, 329–367. (c) Westerhausen, M. Angew. Chem. 2001, 113, 3063–3065; Angew. Chem., Int. Ed. 2001, 40, 2975-2977. (d) Alexander, J. S.; Ruhlandt-Senge, K. Eur. J. Inorg. Chem. 2002, 2761–2774. (16) (a) Bourget-Merle, L.; Lappert, M. F.; Severn, J. R. Chem. Rev. 2002, 102, 3031–3065. (b) Mindiola, D. J. Angew. Chem. 2009, 121, 6314– 6316; Angew. Chem., Int. Ed. 2009, 48, 6198-6200. (17) Neculai, D.; Roesky, H. W.; Neculai, A. M.; Magull, J.; Schmidt, H.-G.; Noltemeyer, M. J. Organomet. Chem. 2002, 643-644, 47–52. (18) (a) Neculai, D.; Roesky, H. W.; Neculai, A. M.; Magull, J.; Herbst-Irmer, R.; Walfort, B.; Stalke, D. Organometallics 2003, 22, 2279–2283. (b) Neculai, A. M.; Roesky, H. W.; Neculai, D.; Magull, J. Organometallics 2001, 20, 5501–5503. (c) Neculai, A. M.; Neculai, D.; Nikiforov, G. B.; Roesky, H. W.; Schlicker, C.; Herbst-Irmer, R.; Magull, J.; Noltemeyer, M. Eur. J. Inorg. Chem. 2003, 3120–3126.
Figure 1. Crystal structure of 1 3 4C7H8. Selected bond distances (A˚) and angles (deg): Ca(1)-N(1) 2.428(6), Ca(1)-N(2) 2.427(6), Ca(1)-N(3) 2.573(6), Ca(1)-N(4) 2.603(6), Ca(1)-I(1) 3.2109(18), Ca(1)-I(2) 3.1253(17), Ca(2)-N(1) 2.540(6), Ca(2)-N(2) 2.510(6), Ca(2)-C(2) 2.837(7), Ca(2)-C(3) 2.890(6), Ca(2)-C(4) 2.903(7), Ca(2)-I(1) 3.1618(16), Ca1 3 3 3 Ca2 3.6426(17); N(1)-Ca(1)N(2) 73.87(19), N(3)-Ca(1)-N(4) 134.70(19), N(1)-Ca(1)-I(2) 82.41(15), N(2)-Ca(1)-I(2) 81.22(14), N(3)-Ca(1)-I(2) 91.29(14), N(4)-Ca(1)-I(2) 91.86(14), I(1)-Ca(1)-I(2) 173.61(5), Ca(1)I(1)-Ca(2) 69.72(3). All hydrogen atoms have been omitted for clarity.
solution of 1 in a freezer at -32 °C. Crystals were dried under vacuum for 4 h to remove four toluene molecules. Compound 1 is soluble in organic solvents such as benzene, toluene, and THF, and it is characterized by NMR spectroscopy (1H, 13C), mass spectrometry, and elemental and X-ray singlecrystal analysis. The 1H NMR spectrum of 1 shows broad resonances at room temperature. Therefore, the 1H NMR experiment was conducted at higher temperature (343 K) to give a better resolution of the resonances. The 1H NMR spectrum shows a singlet (4.57 ppm) for the methine CH protons, and the 13 C NMR spectrum exhibits a singlet (96.07 ppm) for the methine CH carbon atoms. In the EI mass spectrum no molecular ion peak corresponding to 1 was detected, and only fragment ions were observed. However, ions that are in accordance with the proposed formula of 1 were detected. Furthermore, the structure of 1 was determined by single-crystal X-ray diffraction studies. 1 crystallizes in the monoclinic space group P2/c with four molecules of toluene in the asymmetric unit. The core structure reveals that one CaI2 is connected to two terminal L1CaI units (Figure 1). The coordination geometry around the metal atoms is different for the central calcium unit compared to those of the terminal ones. It is worth mentioning that the Ca1 3 3 3 Ca2 distance (3.6426(17) A˚) through the ligand L1 is much shorter
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Organometallics, Vol. 29, No. 13, 2010
than the Ca 3 3 3 Ca distance (4.279(3) A˚) found by Westerhausen through 1,3,5-triphenylbenzene.7 The terminal calcium atoms are coordinated by four nitrogen atoms of L1 and two iodine atoms exhibiting a pseudo-octahedral coordination geometry around the metal atoms and therefore prove that the ligand forms a tetradentate coordination polyhedron. Ca1 exhibits an η2-coordination mode through the nitrogen atoms of L1, which is similar to that in calcium fluoride and chloride complexes containing the 2,6-diisopropylphenyl-substituted ligand, L. In addition Ca1 is also coordinated to the nitrogen atoms of the two pendant donor side-arms. The Ca1-N bond lengths of the side-arms (2.588av A˚) are longer than those of the backbone (2.4275av A˚), due to the coordinative and ionic nature involved in different bonding modes. Moreover, the central calcium atom Ca2 is coordinated to two iodine atoms and two η5-β-diketiminate rings. The calcium-nitrogen bond lengths are 2.540 and 2.510 A˚, while the calcium-carbon distances are 2.837 A˚ C(2), 2.890 A˚ C(3), and 2.903 A˚ for C(4). For comparison the calcium-nitrogen and calcium-carbon bond lengths range from 2.34 to 2.36 and from 2.84 to 2.90 A˚, respectively, in the homoleptic calcium compound Ca(η5-LtBu)2.19 As expected, the average Ca-I (3.166av A˚) bond distance is comparable with that of another calcium complex, [Ca(η5-LtBu)(μ-I)(thf)]2 [LtBu = CH(CMetBuN)2] (3.144av A˚).9 Interestingly, within the pseudo-octahedral coordination of the Ca atoms the I1-Ca1-I2 bond angle of 173.61° is close to linear.
Experimental Section General Procedures. All manipulations were performed in a dry and oxygen-free atmosphere (N2 or Ar) using Schlenk-line and glovebox techniques. Solvents were purified with the M-Braun solvent drying system. CaI2 and KN(SiMe3)2 (95%) were purchased from Aldrich and used as such. 1H (300 MHz) and 13C (75.47 MHz) NMR spectra were recorded on a Bruker Avance 300 NMR spectrometer. Chemical shifts are reported in ppm with reference to SiMe4 (external). Mass spectra were obtained with a Finnigan MAT 8230 or a Varian MAT CH5 instrument (70 eV) by the EI-MS method. Melting points were measured in sealed glass tubes on a B€ uchi B-540 melting point apparatus. Elemental analyses were performed by the Analytisches Labor des Instituts f€ ur Anorganische Chemie der Universit€at G€ ottingen. Preparation of [L1CaI(μ-ICaI-μ)ICaL1]. L0 H (1.482 g, 5.00 mmol) (L1 = CH{Et2NCH2CH2N(CMe)}2) and KN(SiMe3)2 (1.047 g, 5.25 mmol) were dissolved in THF (60 mL) and stirred for 5 h at room temperature. The clear solution was added to a suspension of (19) El-Kaderi, H. M.; Heeg, M. J.; Winter, C. H. Organometallics 2004, 23, 4995–5002. (20) (a) Kottke, T.; Stalke, D. J. Appl. Crystallogr. 1993, 26, 615–619. (b) Stalke, D. Chem. Soc. Rev. 1998, 27, 171–178.
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CaI2 (2.783 g, 10.00 mmol) in THF (60 mL) at room temperature and stirred for another 12 h. All volatiles were removed under vacuum, and the residue was extracted with toluene (90 mL). The solution was concentrated to ca. 20 mL and stored in a freezer at -32 °C to get single crystals of 1 3 4C7H8. 1 3 4C7H8 was treated under vacuum to remove the four toluene molecules. Yield: 2.18 g, 1.79 mmol, 71.7%. Mp: 269-272 °C (dec). 1H NMR (300 MHz, C7D8, 343 K): δ 4.57 (s, 2H, CH), 3.58, 3.31-3.26, 3.05-3.03, 2.5 (br, 32H, CH2CH2N, CH3CH2N, and CH2CH2N), 1.88 (s, 12H, CH3), 0.82 (t, 24H, CH2CH3) ppm. 13C{1H} NMR (75.47 MHz, C6D6): δ 174.26, 96.07, 54.00, 47.43, 46.98, 23.19, 8.86 ppm. EI-MS: m/z (%) 296.3 (100) [L0 H]þ. Anal. Calcd for C34H70Ca3I4N8 (1218.83): C, 33.50; H, 5.79; N, 9.19. Found: C, 33.83; H, 5.90; N, 9.16. Crystal Structure Determination. Single-crystal structure analysis of 1 3 4C7H8: The data for 1 were collected from a shock-cooled crystal at 100(2) K20 on a Bruker SMART-APEX II diffractometer with a D8 goniometer equipped with a finefocus INCOATEC Mo-microsource.21 The data set of 1 was integrated with SAINT,22 and an empirical absorption correction (SADABS) was applied.23 The structure was solved by direct methods (SHELXS-97)24 and refined by full-matrix leastsquares methods against F2 (SHELXL-97).24 All non-hydrogen atoms were refined with anisotropic displacement parameters. The hydrogen atoms were refined isotropically at calculated positions using a riding model with their Uiso values constrained to 1.5 times the Ueq of their pivot atoms for terminal sp3-carbon atoms and 1.2 times for all other carbon atoms. Disordered moieties were refined using bond length restraints, rigid bond restraints, similarity restraints, and ADP restraints. Crystallographic data (excluding structure factors) for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre. Copies of the data can be obtained free of charge via www.ccdc.cam.ac.uk/data_ request/cif. CCDC number: 766995, Mr=1587.36, 0.18 0.05 0.05 mm, monoclinic, space group P2/c, a = 15.550(6) A˚, b = 16.668(6) A˚, c=14.904(5) A˚, β=116.273(8)°, V=3.46(1) nm3, Z=2, Fcalcd=1.522 Mg/m3, μ(Mo KR)=2.063 mm-1, T=100(2) K, 2θmax = 1.90, 25.08 reflections measured, 6112 independent (Rint=0.1146), R1 = 0.0577 (I > 2σ(I)), wR2=0.1027 (all data), residual density peaks 0.881 to -0.969 e A˚-3.
Acknowledgment. Support of the Deutsche Forschungsgemeinschaft is highly acknowledged. Supporting Information Available: X-ray data for 1 (CIF). This material is available free of charge via the Internet at http:// pubs.acs.org. (21) Schulz, T.; Meindl, K.; Leusser, D.; Stern, D.; Graf, J.; Michaelsen, C.; Ruf, M.; Sheldrick, G. M.; Stalke, D. J. Appl. Crystallogr. 2009, 42, 885–891. (22) SAINT-NT; Bruker AXS Inc.: Madison,WI, 2000. (23) Sheldrick, G. M. SADABS 2.0; Universit€at G€ottingen, G€ottingen, Germany, 2000. (24) Sheldrick, G. M. Acta Crystallogr. Sect. A 2008, A64, 112–122.
