Organometallics 2009, 28, 1579–1581
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An Ether-Free, Internally Coordinated Dialkylcalcium(II) Complex† Martyn P. Coles,*,‡ Sebnem E. So¨zerli,§ J. David Smith,‡ Peter B. Hitchcock,‡ and Iain J. Day‡ Departments of Chemistry, UniVersity of Sussex, Brighton BN1 9QJ, U.K., and Celal Bayar UniVersity, Manisa, Turkey 45030 ReceiVed December 8, 2008 Summary: The alkylpotassium compound KC{SiMe3}2{SiMe2hpp} (hppH ) 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine) reacts with a slurry of CaI2 in toluene to giVe the internally coordinated dialkylcalcium(II) complex Ca(C{SiMe3}2{SiMe2hpp})2. The molecular structure in the solid state shows short Ca-N and long Ca-C bonds and a wide C-Ca-C angle. The 1H and 13C NMR spectra suggest that the metallacycle is transiently opened in toluene at room temperature. Solutions of Grignard reagents have been used in synthesis for over 100 years. In contrast, corresponding compounds of heavier group 2 elements have been used only rarely, as their greater reactivity makes them less easy to make and more difficult to handle. Since sensitivity toward air and moisture is much reduced in compounds containing bulky organic groups, it is possible that, for ligand transfer involving such groups, organometallic compounds of heavier elements could prove to be useful synthetic reagents. In the last 5 years interest in both the heteroleptic compounds RCaX (X ) anionic group) and the homoleptic compounds CaR2 has been considerable and a number of alkyl- (especially benzyl-),1,2 alkynyl-,3 and arylcalcium4,5 compounds have been reported. Some have found applications as catalysts for the polymerization of styrene6-8 or have been implicated in catalytic cycles for lactide polymerization,9 hydroamination,10 and hydrophosphination.11 As far as we are aware, only one solvent-free σ-bonded dialkylcalcium(II), Ca{C(SiMe3)3}2 (1), has been reported.12 Other species that have been structurally characterized contain * To whom correspondence should be addressed. E-mail: m.p.coles@ sussex.ac.uk. Fax: +44 (0)1273-876687. Tel: +44 (0)1273-877339. † Dedicated to Professor Mike Lappert on the occasion of his 80th birthday. ‡ University of Sussex. § Celal Bayar University. (1) Feil, F.; Harder, S. Organometallics 2000, 19, 5010–5015. Harder, S.; Mu¨ller, S.; Hu¨bner, E. Organometallics 2004, 23, 178–183. (2) Feil, F.; Mu¨ller, C.; Harder, S. J. Organomet. Chem. 2003, 683, 56– 63. Hitchcock, P. B.; Khvostov, A. V.; Lappert, M. F. J. Organomet. Chem. 2002, 663, 263–268. Harder, S. Angew. Chem., Int. Ed. 2003, 42, 3430– 3434. (3) Green, D. C.; Englich, U.; Ruhlandt-Senge, K. Angew. Chem., Int. Ed. 1999, 38, 354–357. (4) Ruspic, C.; Harder, S. Organometallics 2005, 24, 5506–5508. Fischer, R.; Go¨rls, H.; Westerhausen, M. Inorg. Chem. Commun. 2005, 8, 1159–1161. Fischer, R.; Ga¨rtner, M.; Go¨rls, H.; Westerhausen, M. Organometallics 2006, 25, 3496–3500. Ga¨rtner, M.; Go¨rls, H.; Westerhausen, M. Organometallics 2007, 26, 1077–1083. Fischer, R.; Go¨rls, H.; Westerhausen, M. Organometallics 2007, 26, 3269–3271. Fischer, R.; Ga¨rtner, M.; Go¨rls, H.; Yu, L.; Reiher, M.; Westerhausen, M. Angew. Chem., Int. Ed. 2007, 46, 1618–1623. (5) Westerhausen, M.; Ga¨rtner, M.; Fischer, R.; Langer, J. Angew. Chem., Int. Ed. 2007, 46, 1950–1956. (6) Harder, S.; Feil, F.; Knoll, K. Angew. Chem., Int. Ed. 2001, 40, 4261– 4264. (7) Harder, S.; Feil, F.; Weeber, A. Organometallics 2001, 20, 1044– 1046. (8) Harder, S.; Feil, F. Organometallics 2002, 21, 2268–2274.
coordinated ether solvent, and most show calcium coordination numbers of 5 and 6.5,13 We have now replaced one methyl group in the C(SiMe3)3 ligand by the bicyclic guanidinate hpp (hppH ) 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine), containing a strongly donating imine nitrogen atom, and obtained an ether-free σ-bonded dialkylcalcium(II) complex in which the metal, unusually,8,14,15 has coordination number 4.
The alkylpotassium compound KC{SiMe3}2{SiMe2hpp}16 reacted with calcium iodide in toluene over a period of days to give the calcium derivative Ca(C{SiMe3}2{SiMe2hpp})2 (2), which was isolated as a white crystalline air-sensitive solid. The composition was established by elemental analysis and NMR spectroscopy, which also showed that the compound was stable in toluene at 25 °C over several months and up to 70 °C for a short period. A clean melting point could not be obtained. At 80-82 °C there was a distinct and irreversible color change to red. On further heating the crystals maintained their form but darkened and by ca. 200 °C were black. The structure in the solid state was determined by an X-ray crystallographic study, which showed that the asymmetric unit consists of two independent but essentially identical molecules. Molecular parameters for one molecule are discussed here; those for the other molecule are given in the caption to Figure 1. A (9) Westerhausen, M.; Schneiderbauer, S.; Kneifel, A. N.; So¨ltl, Y.; Mayer, P.; No¨th, H.; Zhong, Z.; Dijkstra, P. J.; Feijen, J. Eur. J. Inorg. Chem. 2003, 3432–3439. (10) Lachs, J. R.; Barrett, A. G. M.; Crimmin, M. R.; Kociok-Ko¨hn, G.; Hill, M. S.; Mahon, M. F.; Procopiou, P. A. Eur. J. Inorg. Chem. 2008, 4173–4179. Crimmin, M. R.; Casely, I. J.; Hill, M. S. J. Am. Chem. Soc. 2005, 127, 2042–2043. (11) Crimmin, M. R.; Barrett, A. G. M.; Hill, M. S.; Hitchcock, P. B.; Procopiou, P. A. Organometallics 2008, 27, 497–499. Crimmin, M. R.; Barrett, A. G. M.; Hill, M. S.; Hitchcock, P. B.; Procopiou, P. A. Organometallics 2007, 26, 2953–2956. (12) Eaborn, C.; Hawkes, S. A.; Hitchcock, P. B.; Smith, J. D. Chem. Commun. 1997, 1961–1962. (13) Alexander, J. S.; Ruhlandt-Senge, K. Eur. J. Inorg. Chem. 2002, 2761–2774. Westerhausen, M.; Ga¨rtner, M.; Fischer, R.; Langer, J.; Yu, L.; Reiher, M. Chem. Eur. J. 2007, 13, 6292–6306. (14) Cloke, F. G. N.; Hitchcock, P. B.; Lappert, M. F.; Lawless, G. A.; Royo, B. J. Chem. Soc., Chem. Commun 1991, 72, 4–726. Crimmin, M. R.; Barrett, A. G. M.; Hill, M. S.; MacDougall, D. J.; Mahon, M. F.; Procopiou, P. A. Chem. Eur. J. 2008, 14, 11292–11295. (15) Westerhausen, M.; Weinrich, S.; Ossberger, M.; Mitzel, N. W. Z. Anorg. Allg. Chem. 2003, 629, 575–577. (16) Coles, M. P.; So¨zerli, S. E.; Smith, J. D.; Hitchcock, P. B. Organometallics 2007, 26, 6691–6693.
10.1021/om8011618 CCC: $40.75 2009 American Chemical Society Publication on Web 02/04/2009
1580 Organometallics, Vol. 28, No. 5, 2009
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Figure 1. One of the independent molecules of Ca(C{SiMe3}2{SiMe2hpp})2 in the crystal structure of 2, with 30% thermal ellipsoids. Selected bond lengths (Å) and angles (deg) (corresponding values for the second molecule are given in braces): Ca-C8, 2.673(2) {2.664(2)}; Ca-C24, 2.664(2) {2.681(2)}; Ca-N2, 2.364(2) {2.356(2)}; Ca-N5, 2.382(2) {2.371(2)}; C1-N1, 1.372(3) {1.375(3)}; C1-N2, 1.318(3) {1.315(3)}; C1-N3, 1.367(3) {1.366(3)}; C17-N4, 1.371(3) {1.367(3)}; C17-N5, 1.312(3) {1.310(3)}; C17-N6, 1.365(3) {1.372(3)}; C8-Ca-C24, 144.51(7) {146.69(7)}; N2-Ca-N5, 98.69(7) {94.21(6)}.
comparison with the previously studied 1 shows several noteworthy features. First, the Ca-C bonds in 2 (2.673(2), 2.664(2) Å) are significantly longer than those in 1 (2.459(9) Å)12 and are at the upper end of the range (2.483(5)-2.805(1) Å) shown in other alkylcalcium compounds.1,7,14,17 In contrast, the Ca-N bonds (2.364(2)-2.382(2) Å) are short, reflecting the strong basicity of the imino nitrogen of the guanidine fragment (cf. 2.567(2), 2.632(2) Å in organocalcium compounds with internal coordination from the less basic -NMe2 group7). Short Ca-N bonds (2.275(6)-2.467(6) Å) are also found in Ca{N(SiMe3)2}2,18 β-diketiminato,10,11,19 bis(iminophosphorano)methyl,17,20 and triazenido complexes,21 and the azametallacyclopropane complex [Ca(η2-Ph2CNPh)(hmpa)3].22 The coordination around calcium in 2 is that of a distorted tetrahedron, with the short Ca-N bonds forming a narrow angle (98.69(7)°) and the long Ca-C bonds a wide angle (144.51(7)°). Remarkably, the wide X-Ca-X angle is almost the same as those in 1 (149.7(6)°), Ca(C5Me5)2 (147°), and CaI2(g) (148°);12 coordination of two extra nitrogen donors does not change the C-Ca-C geometry. For comparison, the C-Ca-C angles in Ca{CH(SiMe3)2}2(L)2 (L ) 1,4-dioxane, THF) are 134-136°.14 (17) Orzechowski, L.; Jansen, G.; Harder, S. J. Am. Chem. Soc. 2006, 128, 14676–14684. (18) Westerhausen, M.; Schwarz, W. Z. Anorg. Allg. Chem. 1991, 604, 127–140. (19) Barrett, A. G. M.; Boorman, T. C.; Crimmin, M. R.; Hill, M. S.; Kociok-Ko¨hn, G.; Procopiou, P. A. Chem. Commun. 2008, 5206–5208. Spielmann, J.; Jansen, G.; Bandmann, H.; Harder, S. Angew. Chem., Int. Ed. 2008, 47, 6290–6295. Spielmann, J.; Buch, F.; Harder, S. Angew. Chem., Int. Ed. 2008, 47, 9434–9438. Barrett, A. G. M.; Crimmin, M. R.; Hill, M. S.; Hitchcock, P. B.; Procopiou, P. A. Angew. Chem., Int. Ed. 2007, 46, 6339–6342. Barrett, A. G. M.; Crimmin, M. R.; Hill, M. S.; Hitchcock, P. B.; Procopiou, P. A. Organometallics 2007, 26, 4076–4079. Nembenna, S.; Roesky, H. W.; Nagendran, S.; Hofmeister, A.; Magull, J.; Wilbrandt, P.-J.; Hahn, M. Angew. Chem., Int. Ed. 2007, 46, 2512–2514. (20) Ahmed, S. A.; Hill, M. S.; Hitchcock, P. B. Organometallics 2006, 25, 394–402. (21) Barrett, A. G. M.; Crimmin, M. R.; Hill, M. S.; Hitchcock, P. B.; Kociok-Ko¨hn, G. Inorg. Chem. 2008, 47, 7366–7376. (22) Buch, F.; Harder, S. Organometallics 2007, 26, 5132–5135.
Figure 2. Variable-temperature 1H NMR spectra (Si-Me region) of 2 (recorded in C6D5CD3 at 600 MHz). Peaks marked with an asterisk correspond to HC{SiMe3}2{SiMe2hpp} arising from adventitious hydrolysis.
Parameters within the C{SiMe3}2{SiMe2hpp} ligand are similar to those in the lithium, potassium, and zinc derivatives.16,23 Information about the structure of 2 in toluene-d8 may be derived from the SiMen region of 600 MHz 1H NMR spectra (Figure 2). The two very broad peaks observed at room temperature24 (δ 0.30 and 0.42) merge at higher temperatures (up to 70 °C) to give two unresolved peaks, with the more intense signal at lower frequency. At -10 °C there are four sharp peaks, with the two larger peaks having 3 times the intensity of the two smaller peaks. An EXSY experiment shows there is no detectable exchange between the stronger and weaker peaks but slow exchange within the two larger and within the two smaller peaks. These results indicate that at low temperature there are two distinct SiMe3 environments and two distinct methyl environments within SiMe2 for each ligand (in contrast with the related compound NaC(SiMe3)2(SiMe2PPh2)25). The mean SiMe3 resonance is coincident with the mean SiMe2 resonance at approximately 36 °C, and at -40 °C it is at higher frequency than either SiMe2 resonance. Further evidence comes from 13C spectra, which show four sharp peaks at -40 °C and two at 70 °C, and from a two-dimensional 1H-29Si HMBC spectrum at -20 °C. Both SiMe2 1H peaks correlate with the single 29Si peak at δ 6.9 and the SiMe3 1H peaks at δ 0.36 and 0.47 with the 29Si peaks at δ -11.1 and -11.5, respectively. Correlations are not observed at room temperature, due to efficient transverse relaxation caused by the exchange process. The coincident chemical shifts for SiMe2 and SiMe3 protons make it difficult to determine precise coalescence temperatures, (23) Coles, M. P.; El-Hamruni, S. M.; Smith, J. D.; Hitchcock, P. B. Angew. Chem., Int. Ed. 2008, 47, 10147–10150. (24) Due to the high sensitivity of 2 to air and moisture, resonances corresponding to the neutral ligand, HC{SiMe3}2{SiMe2hpp}, are usually observed in the NMR spectra. See ref 16 for assignments. (25) Avent, A. G.; Bonafoux, D.; Eaborn, C.; Hill, M. S.; Hitchcock, P. B.; Smith, J. D. J. Chem. Soc., Dalton Trans. 2000, 2183–2190.
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Organometallics, Vol. 28, No. 5, 2009 1581
Scheme 1. Possible Fluxional Processes for Compound 2 in Solution
Experimental Section
but that for the more intense peaks must be close to 38 °C at 600 MHz, where the 1H signal is a single broad resonance. This corresponds to a free energy of activation ∆Gq ) 63 kJ mol-1 for the exchange process. In 13C spectra, the SiMe3 peaks coalesce at about 45 °C, corresponding to ∆Gq ) 65 kJ mol-1, and the SiMe2 peaks at 10 °C, corresponding to ∆Gq ) 60 kJ mol-1. These activation energies are probably the same within experimental error. A simple ring flip does not make the environments of SiAMe3 and SiBMe3 the same (Scheme 1) or those of methyl groups MeC and MeD. These would, however, become equivalent if the long Ca-C bond were transiently broken to give a zwitterionic species with charge separated between Ca and the carbanionic center. Inversion at carbon together with rotation at the C8-Si1 and Si1-N1 bonds interchanges both the SiMe3 groups and the two SiMe environments (Scheme 1). Similar inversions at carbon have been postulated previously.26 Alternatively, the Ca-C bond could remain intact and the Ca-N bond could be transiently broken. The NMR spectra bear some resemblance to those of the tin metallacycle Sn(κC,κN-C{SiMe3}2{SiMe2-2-C5H4N})Me(OTf)2 (C5H4N ) 2-pyridyl), for which it was postulated that the fivemembered ring was preserved in solution but that the two methyl environments within the SiMe2 group and the two SiMe3 environments became equivalent at room temperature and above.27 The exchange in this compound was attributed to transient breaking of the Sn-N bond (∆Gq ) 62 kJ mol-1); additional inequivalences were attributed to ring flipping, but these were observed only below -60 °C (at 500 MHz), indicating that the activation energy for this process was considerably lower than that for bond cleavage. (26) Eaborn, C.; Clegg, W.; Hitchcock, P. B.; Hopman, M.; Izod, K.; O’Shaughnessy, P. N.; Smith, J. D. Organometallics 1997, 16, 4728–4736. (27) Al-Juaid, S. S.; Avent, A. G.; Eaborn, C.; Hill, M. S.; Hitchcock, P. B.; Patel, D. J.; Smith, J. D. Organometallics 2001, 20, 1223–1229.
Air and moisture were excluded as far as possible by the use of flame-dried glassware and an N2 blanket gas. Samples were prepared in an N2-filled drybox. NMR spectra were recorded on a Varian VNMRS 600 spectrometer with an X{1H} probe equipped with a z gradient. Chemical shifts are given relative to SiMe4. Synthesis of Ca(C{SiMe3}2{SiMe2hpp})2 (2). KC{SiMe3}2{SiMe2hpp} (0.50 g, 1.27 mmol) and CaI2 (0.18 g, 0.63 mmol) were mixed as solids, and toluene (40 mL) was added. The white slurry was stirred at room temperature for 3 days, during which time the CaI2 was consumed. Removal of the volatiles and extraction of the residue with pentane afforded a clear pale yellow solution that was stored at -30 °C. After 1 week, a small number of colorless crystals were isolated. Yield: 0.17 g, 37%. Anal. Calcd for C32H72CaN6Si6: C, 51.28; H, 9.68; N, 11.21. Found: C, 51.15; H, 9.55; N, 11.18. 1H NMR (toluene-d8, 399.5 MHz, 30 °C): δ 3.29, 3.00, 2.63, 2.49, 1.59, 1.35 (br m, 2H, hpp-CH2), 0.42 (v br, 24H, SiMen). 13C NMR (toluene-d8, 150.8 MHz, -40 °C): δ 159.0 (hpp-CN3), 47.7, 47.4, 42.0, 40.8, 23.4, 23.0 (hpp-CH2), 8.4, 7.9 (SiMe3), 7.2, 7.0 (SiMe2). 29Si NMR (toluene-d8, 119.2 MHz, 20 °C): δ 6.9 (SiMe2), -11.1, -11.5 (SiMe3).28 Crystallographic data were collected on a Bruker Smart CCD 1000 diffractometer and corrected for Lorentz and polarization effects. The structure of 2 was determined by direct methods and by full least-squares refinement (SHELX-97)29 with anisotropic thermal parameters for non-hydrogen atoms. Hydrogen atoms were placed in calculated positions and refined in riding mode with fixed thermal parameters. Crystal data: monoclinic, space group P21/c with a ) 22.5814(4) Å, b ) 19.9824(2) Å, c ) 20.5016(3) Å, β ) 109.414(1)°, Z ) 8, Fcalcd ) 1.14 Mg m-3, R1 ) 0.044 (I > 2σ(I)), wR2 (all data) ) 0.122, GOF ) 1.017.
Acknowledgment. We thank Celal Bayar University for granting leave to S.E.S. and Chemetall GmbH for gifts of chemicals. Supporting Information Available: Figures, tables, and a CIF file giving details of the X-ray crystallographic determination for 2 and 13C and 1H-29Si HMBC NMR spectra. This material is available free of charge via the Internet at http://pubs.acs.org. OM8011618 (28) Cf. Ca(C{SiMe3}3)2 at-12.1 ppm; incorrectly reported as +12.1 ppm.12 (29) Sheldrick, G. M. SHELXL-97, Program for the Refinement of Crystal Structures; University of Go¨ttingen, Go¨ttingen, Germany, 1997.
