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Organometallics 2010, 29, 5754–5756 DOI: 10.1021/om100838u
The First-Row Transition Metal Interstitial Hydride Anion [{PhP(CH2)3Fe}4(μ4-H)]Cristina Berges Serrano, Robert J. Less, Mary McPartlin, Vesal Naseri,* and Dominic S. Wright* Chemistry Department, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, U.K. Received August 27, 2010 Summary: The metal-exchange reaction of the lithium complex [{PhP(CH2)3}2(Li 3 thf)4] (1) with FeI2 gives the tetranuclear FeII compound [Li(thf)4]þ[{PhP(CH2)3Fe}4(μ4-H)]- (2), a rare example of a first-row transition metal interstitial hydride. In the past decade or so there has been considerable interest in the coordination chemistry of main group element-based anionic ligands containing nitrogen donor functionality.1-4 Most well studied are the series of isoelectronic tripodal anions [RE(NR)3]x- (E=main group element) (Figure 1a,b,c), which exhibit a rich coordination chemistry with a broad range of transition and main group metals. Our interest in these species has been renewed by the realization that isovalent phosphorus-based group 13 anions of this class, [MeE(PPh)3]4- (E=Al, Ga, In, Figure 1d), have the remarkable ability to form supramolecular ionic cages.5 This feature is seen in the series of isostructural anions [{MeE(PPh)3Li4}4(μ4-Cl)]-, composed of a supramolecular, tetrahedral arrangement of four [MeE(PPh)3]4- anions that surround a Li16 core. The inverse coordination of the Clanion by a Li4 unit at the center of the [{MeE(PPh)3Li4}4(μ4Cl)]- anion effectively templates this structure.6 We showed recently that deprotonation of the commercially available phosphonium salt [PhP(CH3)3]þI- with tBuLi results in a new carbon-based representative of this class of ligands, [PhP(CH2)3]2- (Scheme 1).7 This novel phospholide arrangement offers new opportunities in organometallic chemistry, being a strong σ-donor (via the lone pairs of the CH2 groups) and a σ-acceptor (via the σ* orbital of the Ph-P bond).8 We present here the first study of the coordination chemistry of this type of ligand with a transition metal; the tetranuclear anion [{PhP(CH2)3Fe}4(μ4-H)]- (2) reported is a rare example of a first-row transition metal interstitial hydride. *To whom correspondence should be addressed. E-mail: vn226@ cam.ac.uk;
[email protected] (1) Group 13: (a) McDonald, W. S.; McDonald, T. R. R. Acta Crystallogr., B 1972, 28, 1619. (b) Dozzi, G.; Perego, G.; Mazzei, A.; Cucinella, S., J. Orgamomet. Chem. 1977, 137, 257. (2) Group 14: (a) Brauer, D. J.; Burger, H.; Liewald, G. R. J. Organomet. Chem. 1986, 308, 119. (b) Veith, M.; Spaniol, A.; Pohlmann, J.; Gross, F.; Huch, V. Chem. Ber. 1993, 126, 2625. (3) Group 15: Beswick, M. A.; Wright, D. S. Coord. Chem. Rev. 1998, 176, 373. (4) Group 16: (a) Chivers, T.; Gao, X.; Parvez, M. Angew. Chem., Int. Ed. Engl. 1995, 134, 2549. (b) Ilge, D.; Wright, D. S.; Stalke, D. Chem.; Eur. J. 1998, 4, 2275. (5) (a) Duer, M. J.; Garcia, F.; Kowenicki, R. A.; Naseri, V.; McPartlin, M.; Stein, R.; Wright, D. S. Angew. Chem., Int. Ed. 2005, 44, 5729. (b) Duer, M. J.; Garcia, F.; Goodman, J. M.; Hehn, R. A.; Kowenicki, R. A.; Naseri, V.; McPartlin, M.; Stead, M. L.; Stein, R.; Wright, D. S. Chem.;Eur. J. 2007, 1251. (6) Mulvey, R. E. J. Chem. Soc., Chem. Commun. 2001, 1049. (7) Less, R. J.; Naseri, V.; Wright, D. S. Organometallics 2009, 28, 3594. (8) For reviews of other phosphorus ylides as ligands, see: (a) Schmidbaur, H. Acc. Chem. Res. 1974, 8, 62. (b) Kaska, W. C. Coord. Chem. Rev. 1983, 48, 1. pubs.acs.org/Organometallics
Published on Web 10/06/2010
The 1:1 stoichiometric reaction of the lithium precursor 1 with FeI2 in thf produces a dark brown solution, from which dark brown crystals of [Li(thf)4]þ[{PhP(CH2)3Fe}4(μ4-H)](2) were isolated in 44% yield.9 The 1H and 31P{1H} NMR spectra of the complex at room temperature show sharp resonances, strongly suggesting that it is diamagnetic in solution. In addition, 2 is also EPR silent at room temperature in thf solution and in the solid state. This provides key evidence that the FeII centers in the anion of 2 are low-spin (t2g6 eg0). It also provides indirect evidence of the presence of the H- ion within the [{PhP(CH2)3Fe}4(μ4-H)]- anion, which would otherwise have to possess a single, unpaired electron to balance the charge in 2, i.e., paramagnetic (FeII)3(FeI) rather than diamagnetic (FeII)4(μ4-H). In addition to the Ph and CH2 resonances for the [PhP(CH2)3]2- ligands, the room-temperature 1H NMR spectrum shows a broad resonance at δ -11.2, which we assign to the μ4-H atom.10 Although the reaction producing 2 can be reproduced easily, the origin of the hydride in the anion of 2 is not understood at this stage. This hydride ion is most likely to arise from the [PhP(CH2)3]2- ligand rather than from decomposition of thf solvent, since to our knowledge there are no examples of ring-opening or ring-cleavage reactions of thf that generate H-.11 The low-temperature [120(2) K] crystallographic study of 212 shows it to have an ion-separated structure in the solid state, (9) Synthesis of 2: a mixture of solid [{PhP(CH2)3}(Li 3 thf)4] (40.0 mg, 0.065 mmol) and FeI2 (20.0 mg, 0.065 mmol) was cooled to -78 °C, and 6 mL of thf was added. The suspension was then stirred for 2 h at room temperature to give a dark brown solution. Storage at 5 °C for 38 h gave [Li(thf)4]þ[{PhP(CH2)3Fe}4(μ4-H)]- (2) (8.0 mg, 7.2 μmol, 44%) as brown crystals. 1H NMR (400.14 MHz, d8-thf, þ25 °C): δ 7.85 (s, 4H, p-CH Ph), 7.45 (s, 8H, o-C-H Ph), 7.33 (s, 8H, m-C-H Ph), 3.65 (mult, 16H, thf), 1.80 (mult, 16H, thf), 1.28 (s, 24H, CH2), -11.2 [1H (br), μ4-H]. 31P{1H} (161.97 MHz, d8-thf, þ25 °C): δ 42.6 (s). Anal. Found: C 56.0, H 6.7, P 11.5. Calcd for 2: C 55.7, H 6.9, P 11.1. (10) Values of δ = -3 to -25 are typical of terminal, μ2- and μ3-Fe hydrides. See: Karet, G. B.; Stera, C. L.; Norton, D. M.; Shriver, D. F. J. Am. Chem. Soc. 1993, 115, 9979. Ohki, Y.; Suzuki, H. Angew. Chem., Int. Ed. 2000, 39, 3121. Ohki, Y.; Kojima, T.; Oshima, M.; Suzuki, H. Organometallics 2001, 20, 2654. Gloaguen, F.; Lawence, J. D.; Rauchfuss, T. B.; Benard, M.; Rohmer, M.-M. Inorg. Chem. 2002, 41, 6573. Smith, J. M.; Lachicotte, R. J.; Holland, P. L. J. Am. Chem. Soc. 2003, 125, 15752. Brown, S. D.; Mehn, M. P.; Peters, J. C. J. Am. Chem. Soc. 2005, 127, 13147. Femoni, C.; Iapalucci, M. C.; Longoni, G.; Zacchini, S.; Zarra, S. Inorg. Chem. 2009, 48, 1599. (11) Kennedy, A. R.; Klett, J.; Mulvey, R. E.; Wright, D. S. Science 2009, 326, 707, and references therein. (12) Crystallographic study of 2: C52H77Fe4LiO4P4, M = 1120.36, crystal system tetragonal, space group I4, Z = 2, a = 17.3852(3) A˚, b=17.3852(3) A˚, c=8.9337(2) A˚, V=2700.17(9) A˚3, μ(Mo KR)=1.213 mm-1, Fcalc=1.378 Mg m-3, T = 120(2) K. Total reflections 14 139, unique 3347 [R(int) = 0.045]. R1 = 0.028 [I > 2σ(I)] and wR2 = 0.062 (all data). Data were collected on a Nonius KappaCCD diffractometer and solved by direct methods and refined ottingen, by full-matrix least-squares on F2 (G. M. Sheldrick, SHELX-97; G€ Germany, 1997). CCDC XXXX contains the supplementary crystallographic data for 2. Data can be obtained free of charge via www.ccdc.cam.ac.uk/ conts/retrieving.html (or from The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: þ44 1223 336 033;
[email protected]). r 2010 American Chemical Society
Communication
Organometallics, Vol. 29, No. 22, 2010
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Figure 1. Isoelectronic main group anions of groups 13, 14, and 15. Scheme 1
composed of tetranuclear anions [{PhP(CH2)3Fe}4(μ4-H)]- and [Li(thf)4]þ cations. In addition to the previously mentioned spectroscopic studies, the refinement of the X-ray structure of 2 further confirms the existence of a hydride at the center of the core. Refinement of the structure with the central atom as H(He) gives substantially better R-factors than refinement as neutral H or without any atom at the center.13 The Fe4(μ4-H) anion of 2 (Figure 2a) is only one of a few structurally authenticated examples of an interstitial hydride of a first-row transition metal.14 This anion appears to be the only example of a firstrow hydride containing a (metal)4(μ4-H) core. The only other examples of this type of tetranuclear arrangement to be structurally characterized throughout the periodic table are YIII complexes such as [{(L)YH2]4 [L = tris-dimethylpyrazolyl-borate (TpMe2), Me4C5SiMe3],15 the WIII complex [(tBuCH2O)12W4(μ4-H)]-,16 and the RhI complex [Rh2(PNNP)(CO)2}2(μ4-H)] [PNNP=3,5-bis(diphenylphosphinomethyl)pyrazolate].17 In the solid-state structure, the [PhP(CH2)2]2- ligands exhibit a 50:50 disordering over two symmetry-related sites above each of the Fe3 faces (Figure 2b). As a result of this, ranges of bond lengths and angles for both components of disorder are quoted
in the figure caption to Figure 2 and in this discussion. The Fe4(μ4-H) anion of 2 has overall S4 symmetry in which there is a slight tetragonal distortion of the Fe4 unit, with two “short” [Fe(1) 3 3 3 Fe(1A, 1B) 2.7826(7)A˚] and one “long” [Fe(1) 3 3 3 Fe(1C) 2.8235(7)A˚] Fe 3 3 3 Fe contact being made to each of the Fe atoms. These distances are within the range of 2.56-2.86 A˚ found in Fe4 cubanes based on [Fe(μ3-X)]4 (X = S, Se, NR) cores, in which a range of metal oxidation states from Fe0 to FeIV are present.18 In the low oxidation state (FeI)4 and (FeI)3(Fe0) clusters [Fe(NO)(μ3-S)]4n (n = 0, -1), containing strongfield NO ligands, relatively short Fe-Fe bonding interactions have been found [2.641(1)-2.704(1) A˚].19 However, the longer distances in 2 are consistent with only weak Fe 3 3 3 Fe interactions at best. The Fe-H bond lengths to the hydride ion [1.7124(4) A˚] are within the range of values found previously for a broad variety of iron hydrides (mean 1.69 A˚ in structurally characterized compounds of this type).20 Significantly, the C-Fe bond lengths [2.099(5)2.140(5) A˚] are also similar to those observed in [(η5-Cp)Fe(CO)2(CH2PPh3)]þ [2.11(1) A˚], the only previously reported FeII ylide complex.21
(13) Refinement of the structure with the central atom as H- (He) gives R1 = 0.028 (with thermal displacement parameter U = 0.021), whereas refinement as a neutral H atom gives R1 = 0.031 (U = 0.0001) and with nothing at the center gives R1 = 0.034 (peak > 1 e A˚-3). (14) [H2Ni12(CO)21]3-: Dahl, L. F.; Longoni, R. W.; Chini, P.; Schultz, A. J.; Williams, J. M. Adv. Chem. Ser. 1978, 167, 93. [HCo(CO)15]-: Hart, D. W.; Teller, R. G.; Wei, C.-Y.; Bau, R.; Longoni, G.; Campanella, S.; Chini, P.; Koetzle, T. F. Angew. Chem., Int. Ed. Engl. 1979, 18, 80. Hart, D. W.; Teller, R. G.; Wei, C.-Y.; Bau, R.; Longoni, G.; Campanella, S.; Chini, P.; Koetzle, T. F. J. Am. Chem. Soc. 1981, 103, 1456. Vastly different chemical shifts have been observed for interstitial hydrides of transition metals in general (ca. δ þ20 to -30). (15) (a) Cui, D.; Tardif, O.; Hou, Z. J. Am. Chem. Soc. 2004, 126, 1312. (b) Luo, Y.; Baldamus, J.; Tardif, O.; Hou, Z. Organometallics 2005, 24, 4362. (c) Cheng, J.; Saliu, K.; Kiel, G. Y.; Ferguson, M. J.; McDonald, R.; Takats, J. Angew. Chem., Int. Ed. 2008, 47, 4910. (16) (a) Budzichowski, T. A.; Chisolm, M. H.; Huffman, J. C.; Eisenstein, O. Angew. Chem., Int. Ed. Engl. 1994, 33, 191. (b) Budzichowski, T. A.; Chisolm, M. H.; Huffman, J. C.; Kramer, K. S.; Eisenstein, O. J. Chem. Soc., Dalton Trans. 1998, 2563. (17) Tanaka, S.; Akita, M Angew. Chem., Int. Ed. 2001, 40, 2865.
(18) For examples see: (a) (FeI)3(Fe0): Chu, C. T.-W.; Gall, R. S.; Dahl, L. F. J. Am. Chem. Soc. 1982, 104, 737. (b) (FeI)4: Gall, R. S.; Chu, C. T.-W.; Dahl, L. F. J. Am. Chem. Soc. 1974, 96, 4019. (c) (FeII)4: Riese, U.; Harms, K.; Pebler, J.; Dehnicke, K. Z. Anorg. Allg. Chem. 1999, 625, 746. (d) (FeII)3(FeIII): Carney, M. J.; Papaefthymion, G. C.; Whitener, M. A.; Spartalian, K.; Frankel, R. B.; Holm, R. H. Inorg. Chem. 1988, 27, 346. (e) (FeII)2(FeIII)2: Bobrik, M. A.; Laskowski, E. J.; Johnson, R. W.; Gillum, W. O.; Berg, J. M.; Hodgson, K. O.; Holm, R. H. Inorg. Chem. 1978, 17, 1402. (f) (FeIII)4: Verma, A. K.; Lee, S. C. J. Am. Chem. Soc. 1999, 121, 10838. (g) (FeIII)3(FeIV): Verma, A. K.; Nzif, T. N.; Achim, C.; Lee, S. C. J. Am. Chem. Soc. 2000, 122, 11013. (19) Chu, C. T.-W.; Lo, F. Y.-K.; Dahl, L. F. J. Am. Chem. Soc. 1982, 104, 3409. (20) Search of the Cambridge Crystallographic Data Base (March, 2010) using Conquest: Bruno, J.; Cole, J. C.; Edgington, P. R.; Kessler, M.; Macrae, C. F.; McCabe, P.; Pearson, J.; Taylor, R. Acta Crystallogr. 2002, B58, 389, and Vista, A Program for the Analysis and Display of Data, Retrieved from the CSD; Cambridge Crystallographic Data Centre: Cambridge, England, 1994. (21) Guerchais, V.; Astruc, D.; Nunn, C. M.; Cowley, A. H. Organometallics 1997, 27, 667.
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Figure 2. (a) Tetrahedral arrangement of the anion of 2. Only one of the disordered components of the [PhP(CH2)3]2- ligands is shown. H atoms (except the interstitial) have been omitted for clarity. Thermal ellipsoids are drawn at the 30% probability level. Selected bond lengths (A˚) and angles (deg); Fe(1) 3 3 3 Fe(1A) 2.7826(7), Fe(1) 3 3 3 Fe(1B) 2.7826(7), Fe(1)-Fe(1C) 2.8235(7), Fe(1)-C(51) 2.099(5)-2.140(5), Fe(1)-H(1) 1.7124(4), P(2)-C(51,52,61) 1.746(6)-1.775(5), P(2)-C(Ph) 1.845(2), Fe-Fe-Fe 60.98(1)-59.513(7), CH2-P(2)-CH2 108.2(3)-109.7(3). (b) Disordering of the CH2 groups over two symmetry-related sites above each of the Fe3 faces of the Fe4 unit in the anion.
In summary the structure of 2 reveals the potential for [RP(CH2)3]2- ligands to form deltahedral clusters and cages as a result of their inherent ability to cap triangular (metal)3 faces. The “encapsulation” of the H- anion within the tetrahedral Fe4 core of the anion of 2 can be regarded conceptually as related to the inverse coordination of halide ions within more ionically bonded supramolecular arrangements.5 We are continuing our studies of the coordination chemistry of this class of ylides, particularly with respect to
their structure-directing properties in molecular and supramolecular systems.
Acknowledgment. We thank the EPSRC for financial support and Dr. J. E. Davies (Cambridge) for collecting X-ray data on 2. Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org.
