Communication pubs.acs.org/Organometallics
A Molecular Complex with a Formally Neutral Iron Germanide Motif (Fe2Ge2) Anukul Jana,†,§ Volker Huch,† Henry S. Rzepa,*,‡ and David Scheschkewitz*,† †
Krupp-Chair of General and Inorganic Chemistry, Saarland University, 66125 Saarbrücken, Germany Department of Chemistry, Imperial College London, London SW7 2AZ, United Kingdom
‡
S Supporting Information *
ABSTRACT: We report the synthesis and isolation of a stable complex containing the formally neutral Fe2Ge2 motif, which is stabilized by the coordination of an Nheterocyclic carbene to the germanium and of carbon monoxide to the iron center. [(NHCiPr2Me2)GeFe(CO)4]2 is obtained by reduction of the NHCiPr2Me2-coordinated dichlorogermylene adduct of Fe(CO)4, which in turn is obtained from the reaction of Fe2(CO)9 with GeCl2·NHCiPr2Me2 (NHCiPr2Me2 = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene). The solid-state structure of the title compound reveals two distinct coordination modes for the Fe(CO)4 fragments: bridging (π-type) and terminal (σtype). In solution, the rapid equilibrium between the two modes was resolved by NMR at −35 °C. Reaction with propylene sulfide at room temperature affords the sulfide-bridged digermanium complex with two terminal Fe(CO)4 moieties.
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Scheme 1. NHC-Coordinated Element(0) Motifs I (E = Si− Sn) and II (E = C−Ge) Reported in the Literature9−13
n recent years the chemistry of ligand-coordinated compounds with heavier group 14 elements in the formal 0 oxidation state has attracted considerable interest.1 In particular, the bonding situations in these systems has been subjected to intense scrutiny.2 In light of the revived discussion regarding the use of arrows for the representation of donor− acceptor bonding,3 it has become even more pressing to develop a better understanding of the donor−acceptor properties of main-group elements. As we recently argued, reversibility of coordination (e.g., demonstrated by facile ligand exchange) is an experimental criterion justifying a description as a dative bond.4 The thus implied preparative potential of such compounds as E(0) transfer reagents has been demonstrated by Gudat et al. with bis(imine)-coordinated Sn(0).5 Theoretical predictions by Frenking et al. regarding the use of N-heterocyclic carbenes (NHCs) as stabilizing ligands6 prompted groundbreaking studies on carbon(0) compounds (bent allenes or carbones) by the groups of Bertrand7 and Fürstner.8 Robinson et al. employed an NHC as ligand in their synthesis of a Si2 complex,9 which Bertrand somewhat daringly dubbed “a soluble allotrope of silicon”.10 The series of stable mono- (II) and binuclear (I) E(0) derivatives (E = Si, Ge, Sn) with coordinated NHCs11 was expanded through contributions by the groups of Driess12 and Jones (Scheme 1).13 In addition, Frenking has pointed out the potential of so-called tetrylones of type II as donors toward transition-metal systems,14 which so far has only been experimentally documented in the case of carbon.7 Very recently, however, Robinson et al. reported a copper(I) chloride complex of his prototypical (NHC)Si2(NHC) compound of type I and alluded to its potential for different coordination modes (π versus σ), although only the terminal mode was observed.15 © XXXX American Chemical Society
We have recently introduced NHC-coordinated heavier vinylidenes, silagermenylidenes 116 and 2,17 as versatile synthons to heavier group 14 chemistry (Scheme 2).4b As these compounds can formally be considered as heterocoupled dimers of Si(II) and Ge(0) fragments, we were particularly inspired by the facile formation of the Fe(CO)4 complex 3 from 2 and Fe2(CO)9.17 We thus focused on the synthesis of a FeGe system devoid of the Si(II) fragment.18 Iron germanide in the bulk and Scheme 2. Chemical Structures of 1−3 (R = Tip = 2,4,6iPr3C6H2, NHCiPr2Me2 = 1,3-Diisopropyl-4,5dimethylimidazol-2-ylidene)16,17
Special Issue: Mike Lappert Memorial Issue Received: December 16, 2014
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DOI: 10.1021/om501286g Organometallics XXXX, XXX, XXX−XXX
Communication
Organometallics
We then reduced the germanium(II) complex 5 with 2.3 equiv of KC8 in THF, resulting in the near-quantitative formation of a red compound, which was isolated in 46% yield (Scheme 3).23 The microanalysisalthough slightly high in nitrogen and low in carbonsupports the formation of a chlorine-free species. The 1H and 13C NMR spectra at room temperature display only one set of signals for the NHC ligand, leading to the initial assumption of either the monomeric species [(NHC)GeFe(CO)4] or a symmetrical oligomer [(NHC)GeFe(CO)4]n such as [6]. The longest wavelength absorption at λmax 469 nm (ε = 1080 L mol−1cm−1) is significantly blue-shifted in comparison to NHCDip-stabilized digermanium(0) (λmax 527 nm).13a Compound 7 is stable both in the solid state and in solution under an argon atmosphere but is extremely sensitive to oxygen and moisture. Single-crystal X-ray structural analysis of 7 (Figure 2) revealed a dimeric structure in the solid state. In contrast to
nanoparticular state has received considerable attention as a magnetic semiconductor.19 Despite the availability of numerous donor-stabilized germanium(0) species of type I13a and II,12b,20 coordination compounds either through the lone pairs of electrons or through the multiple bonds have not been reported to date except for the aforementioned copper complex of I (E = Si) reported by Robinson.15 Herein, we describe the synthesis, isolation, and reactivity of a Lewis acid−base (donor−acceptor) stabilized digermanium(0) and so experimentally address the two pertinent features of the electronic structure of the digermanium analogue of I (Scheme 1), namely both the π bond and the nonbonding pair of electrons. We anticipated that the reaction of NHCiPr2Me2-stabilized dichlorogermylene 421 with Fe2(CO)9 would yield a suitable precursor to such a system in analogy to the reported synthesis of a related silicon(II) chloride species.22 The Fe(CO)4 complex 5 (Scheme 3) was prepared by mixing 4 Scheme 3. Synthesis of Donor−Acceptor Coordinated Ge(0) Species 7
Figure 2. Molecular structure of 7 in the solid state (thermal ellipsoids at 30% probability, hydrogen atoms omitted for clarity). Selected bond lengths (Å) and angles (deg): Ge2−Ge1 2.4442(2), Ge2−Fe2 2.4112(3), Ge2−Fe1 2.5970(3), Ge1−Fe1 2.6614(3), Ge2−C12 2.0701(16), Ge1−C1 2.0763(17); Ge2−Fe1−Ge1 55.380(7), Ge2− Ge1−Fe1 60.973(8), Ge1−Ge2−Fe1 63.647(7), C12−Ge2−Fe2 109.42(4), C12−Ge2−Ge1 106.48(4), Fe2−Ge2−Ge1 138.457(10), C12−Ge2−Fe1 106.30(4), Fe2−Ge2−Fe1 122.821(10), C1−Ge1− Ge2 100.90(5), C1−Ge1−Fe1 110.65(5).
with 1.1 equiv of Fe2(CO)9 in THF at room temperature and was isolated in 81% yield by crystallization from hexane.23 The constitution of compound 5 was proven by 1H and 13C NMR spectroscopy, elemental analysis, and single-crystal X-ray diffraction (Figure 1).
expectations, however, only one of the two Fe(CO)4 moieties is terminally bonded to a germanium atom in a σ fashion (Ge2− Fe2 2.4112(3) Å), while the other assumes a bridging position and thus can formally be considered as a digermene complex (Ge2−Fe1 2.5970(3), Ge1−Fe1 2.6614(3) Å). The Ge−Ge bond length is 2.4442(2) Å, which is significantly larger than that of NHCDip-stabilized digermanium(0) (2.3490(8) Å).13a The tetracoordinated germanium center of 7 is somewhat pyramidalized by coordination of the bridging Fe(CO)4 moiety (∑(angles at Ge2) = 354.35°). Previous attempts to generate digermene complexes exclusively resulted in the formation of germylene complexes and thus the complete cleavage of the GeGe bond.24 In order to gather more information about the electronic structure of 7, we optimized the full experimental structure by DFT at the (dispersion corrected, with a continuum solvent field for toluene) ωB97XD/6-311G(d,p) level of theory to a local minimum (no imaginary frequency).23 From the NBO analysis,23 two bonding orbitals can be identified as responsible for the interaction of the digermanium unit with the bridging Fe(CO)4 moiety. The form of these orbitals seems to suggest predominant σ character and thus provides some support for a
Figure 1. Molecular structure of 5 in the solid state (thermal ellipsoids at 30% probability, hydrogen atoms omitted for clarity). Selected bond lengths (Å) and angles (deg): Ge−C1 2.0268(16), Ge−Fe 2.3127(3); C1−Ge−Fe 128.74(5). B
DOI: 10.1021/om501286g Organometallics XXXX, XXX, XXX−XXX
Communication
Organometallics metallacyclopropane-type structure according to the Dewar− Chatt−Duncanson model.25 An ELF analysis,23,26 however, shows that the disynaptic basins for these bonds are strongly depleted to 1.21e and 0.73e in favor of a nonbonding electron pair at germanium (2.60e) as well as the Ge−Ge bond (2.48e). In contrast, the ELF calculations for the parent cyclopropanes (c-E3H6) determine the populations of the E−E disynaptic basins to values close to two electrons (1.75e (E = C), 2.00e (E = Si), 2.22e (E = Ge)). We conclude that the interaction between the digermanium moiety and the bridging Fe(CO)4 is weak and should therefore display a certain π-complex character as well. Accordingly, the isomeric species with two terminal Fe(CO)4 groups [6], which corresponds to a local minimum, turned out to be higher in free energy by only ΔΔG298 = 7.4 kcal mol−1.23 The small energy difference between [6] and 7 implies a facile exchange of coordination modes of the Fe(CO)4 moieties and thus a possible degenerate equilibrium between 7 and 7′ (Scheme 3). Indeed, at 300 K the 1H NMR spectrum of 7 displays only one resonance for the isopropyl CH of the two NHCiPr2Me2 moieties (δ 6.14, hept, 4H, iPr CH), which resolves into three resonances in a 2:1:1 ratio at 233 K (see Figure S7 in the Supporting Information). This observation supports the fluxional behavior of the Fe(CO)4 moieties to be expected on the basis of the theoretical calculations.23 In order to experimentally address the accessibility of the two possible reaction sites, namely the nonbonding electron pair in 7 and the Ge−Ge double bond in [6], we choose propylene sulfide as a source of sulfur, which is known to react with either isolated functionality, as in germylenes27 and digermenes.28 The reaction of propylene sulfide with 7 at room temperature in toluene affords compound 8 with a threemembered Ge2S motif in a 42% isolated yield as yellow blocks (Scheme 4). Accordingly, the longest wavelength absorption of 8 at λmax 434 nm (ε 4820 L mol−1cm−1) is blue-shifted in comparison to 7 (λmax 469 nm).
Figure 3. Molecular structure of 8 in the solid state (thermal ellipsoids at 30% probability, hydrogen atoms, and solvent of crystallization omitted for clarity). Selected bond lengths (Å) and angles (deg): Ge1−Ge2 2.4203(6), Ge1−S 2.2760(12), Ge2−S 2.2992(13), Ge1− C1 2.034(4), Ge2−C16 2.016(4), Ge1−Fe1 2.3583(8), Ge2−Fe2 2.3601(8); Ge1−S−Ge2 63.87(3), S−Ge1−Ge2 58.53(3), S−Ge2− Ge1 57.60(3), C1−Ge1−S 107.37(12).
the coordination environment of Ge1 and Ge2 in 8 is geometrically closer to digermene structures than in the sulfur-free precursor. Although the notion of π complexes of heavier group 14 double bonds to chalcogen atoms had been brought forward in the 1990s by West et al., e.g. in the context of disilathiiranes,30 a cyclic delocalization model of the chalcogen lone pairs proposed by Inagaki and Ma is probably more appropriate.31 The direct comparison of transition-metal vs chalcogen coordination in 7 and 8, respectively, invites some further considerations of the topic. In conclusion, with the synthesis of 7 we have shown that ditetrel(0) moieties can act as donors toward transition-metal fragments with both nonbonding and π-bonding electron density. In contrast, the very recently reported dicationic dimetallodigermenes feature two terminal tungsten centers; the GeGe π bond does not take part in complex formation.32 In our case, the facile degenerate exchange between the two coordination modes of the Fe(CO)4 moieties demonstrates the weak bonding of the formal GeGe double bond to the iron centers, which was rationalized by DFT calculations as being nonetheless of the metallacyclopropane type. The simultaneous presence of formal Ge(0) and Fe(0) centers in a molecular Fe2Ge2 entity suggests its application as a single-source precursor for iron germanide (FeGe), an aspect that is currently being investigated in our laboratories.
Scheme 4. Synthesis of Digermathiirane 8
Single crystals suitable for an X-ray structural analysis of 8 were obtained from either a saturated toluene solution or a warm hexane solution after the solution was kept at 0 °C overnight (Figure 3). Notably, both iron fragments now adopt terminal positions with respect to the two germanium atoms. Curiously, the Ge−Ge bond length of 2.4203(6) Å is slightly contracted in comparison to that of 7 (2.4442(2) Å). The Ge2S motif constitutes an almost isosceles triangle with germanium− sulfur distances of Ge1−S 2.2760(12) and Ge2−S 2.2992(13) Å, very close to the Ge−S bond lengths of the known digermathiirane Mes4Ge2S (2.277 and 2.263 Å, Mes = 2,4,6trimethylphenyl).29 The sums of bond angles about the germanium centers of compound 8 excluding the sulfur atom (at Ge1, 358.37°; at Ge2, 356.31°) show that their coordination spheres do not deviate from planarity by much. On the basis of these findings
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AUTHOR INFORMATION
S Supporting Information *
Text, tables, figures, and CIF files giving experimental procedures and spectral and crystallographic data, including atomic positional and thermal parameters for 5, 7, and 8. This material is available free of charge via the Internet at http:// pubs.acs.org. Computational data and details are available via a digital repository.23 Corresponding Authors
*E-mail for H.S.R.:
[email protected]. *E-mail for D.S.:
[email protected]. Present Address
§ Tata Institute of Fundamental Research Centre for Interdisciplinary Sciences, Hyderabad 500075, India.
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DOI: 10.1021/om501286g Organometallics XXXX, XXX, XXX−XXX
Communication
Organometallics Notes
(19) (a) Lungu, G. A.; Apostol, N. G.; Stoflea, L. E.; Costescu, R. M.; Popescu, D. G.; Teodorescu, C. M. Materials 2013, 6, 612−625. (b) Porter, N. A.; Gartside, J. C.; Marrows, C. H. Phys. Rev. B 2014, 90, 024403. (c) Bernhardt, J.; Lebech, B.; Beckman, O. J. Phys. F: Met. Phys. 1984, 14, 2379−2393. (20) Chu, T.; Belding, L.; van der Est, A.; Dudding, T.; Korobkov, I.; Nikonov, G. I. Angew. Chem., Int. Ed. 2014, 53, 2711−2715. (21) Rupar, P. A.; Staroverov, V. N.; Ragogna, P. J.; Baines, K. M. J. Am. Chem. Soc. 2007, 129, 15138−15139. (22) Ghadwal, R. S.; Azhakar, R.; Pröpper, K.; Holstein, J. J.; Dittrich, B.; Roesky, H. W. Inorg. Chem. 2011, 50, 8502−8508. (23) Experimental details are supplied in the Supporting Information. Computational details are available via the Interactive data table published at DOI: 10.6084/m9.figshare.1206310 (shortdoi: 10/wdd) and the further digital repository links therein. CCDC-986900 (5), -986901 (7), and -986902 (8) contain supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac. uk/data_request/cif. (24) (a) Lappert, M. F.; Miles, S. J.; Power, P. P.; Carty, A. J.; Taylor, N. J. J. Chem. Soc., Chem. Commun. 1977, 458−459. (b) Hitchcock, P. B.; Lappert, M. F.; Miles, S. J.; Thorne, A. J. J. Chem. Soc., Chem. Commun. 1984, 480−482. (25) (a) Dewar, M. J. S. Bull. Soc. Chim. Fr. 1951, 18, C71−C77. (b) Chatt, J.; Duncanson, L. A. J. Chem. Soc. 1953, 2939−2947. (26) Feixas, F.; Matito, E.; Duran, S. M.; Silvi, B. J. Chem. Theory Comput. 2010, 6, 2736−2742. (27) For example: (a) Veith, M.; Becker, S.; Huch, V. Angew. Chem., Int. Ed. Engl. 1989, 28, 1237−1238. (b) Jana, A.; Ghoshal, D.; Roesky, H. W.; Objartel, I.; Schwab, G.; Stalke, D. J. Am. Chem. Soc. 2009, 131, 1288−1293. (28) Sasamori, T.; Miyamoto, H.; Sakai, H.; Furukawa, Y.; Tokitoh, N. Organometallics 2012, 31, 3904−3910. (29) Tsumuraya, T.; Sato, S.; Ando, W. Organometallics 1988, 7, 2015−2019. (30) (a) West, R.; DeYoung, D. J.; Haller, K. J. J. Am. Chem. Soc. 1985, 107, 4942−4946. (b) Grev, R. S.; Schaefer, H. F., III J. Am. Chem. Soc. 1987, 109, 6577−6585. (31) Ma, J.; Inagaki, S. J. Phys. Chem. A 2000, 104, 8989−8994. (32) Inomata, K.; Watanabe, T.; Tobita, H. J. Am. Chem. Soc. 2014, 136, 14341−14344.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Funding by the Deutsche Forschungsgemeinschaft (DFG SCHE906/5-1), Alfried Krupp von Bohlen und HalbachFoundation, and COST Action CM1302 (Smart Inorganic Polymers) is gratefully acknowledged. A.J. thanks the TCIS Hyderabad for providing a knowledge exchange grant.
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DOI: 10.1021/om501286g Organometallics XXXX, XXX, XXX−XXX
