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Tethered Heavy Dicarbene Analogues: Synthesis and Structure of Ditetryldiyl Ethers (Ar0 E)2(μ-O) (E = Ge, Sn; Ar0 = C6H3-2,6-(C6H3-2,6-iPr2)2) Owen T. Summerscales, Marilyn M. Olmstead, and Philip P. Power* Department of Chemistry, University of California, Davis, California 95616, United States
bS Supporting Information ABSTRACT: Ditetryldiyl ethers (Ar0 E)2(μ-O) (E = Ge, Sn; Ar0 = C6H3-2,6-(C6H32,6-iPr2)2) which contain a pair of two-coordinate Sn or Ge atoms were successfully prepared by the reaction of the corresponding dimetallylene (Ar0 E)2 with 1 equiv of pyridine N-oxide. The tin oxide was found to cocrystallize with a hydroxide impurity: {Ar0 Sn(μ-OH)}2. Structural determination revealed a slightly bent oxide EOE core (E = Ge, 154.8(1)°; E = Sn, 154.7(3)°) and a trans orientation of the Ar0 groups in each complex. The compounds were also characterized by NMR and electronic spectroscopy.
T
he amphoteric character of singlet carbenes and their heavier group 14 element analogues has generated a wealth of chemistry for these interesting species1,2 and their transition-metal derivatives.3 For the heavier element derivatives (“metallylenes”) the two-coordinate centers are characterized by the presence of a nonbonded pair and by a vacant p orbital and are typically stabilized by bulky ligands which prevent dimerization to dimetallenes.2,4 Molecules containing two metallylene centers are rare but can be prepared using electronegative bridging ligands: e.g., imido-bridged {(Mes*NH)Ge}2(μ-NMes*)5 and (Ar0 Sn)2(μ-NSiMe3) (Ar0 = C6H3-2,6-(C6H32,6- i Pr 2 )2 ), the latter also being characterized by X-ray crystallography.6 They can also be prepared using longer chains of atoms connecting the metallylenes.79 In addition, basestabilized derivatives in which the heavy atom is three- rather than two-coordinate have been recently reported with monatomic bridging ligands: cf. {(L2)E}2(μ-X) (E = Si, Ge; X = O, S; L2 = amidinate).10,11 The basic character of these compounds was demonstrated by the successful synthesis of a chelated nickel derivative. Bridged dimetallylenes of the general formula {(R)E}2(μ-X) (E = group 14 element; X = bridging atom; R = monodentate ligand) have not been reported, however. They are of interest not only from a fundamental perspective but also as chelating ligands in transition-metal catalysis. Toward this objective, we have successfully prepared the ditetryldiyl ethers (Ar0 E)2(μ-O) (E = Ge (1), Sn (2)), which contain a pair of two-coordinate Sn or Ge centers. Oxidation of the digermyne Ar0 GeGeAr0 12 or distannyne 0 Ar SnSnAr0 13 using an excess of O2 or N2O (bond dissociation energy (BDE) 39.0 kcal/mol)14 led to overoxidation and isolation of the hydroxides {Ar0 Ge(OH)}2(μ-O2)(μ-O)6 and r 2011 American Chemical Society
Scheme 1. Formation of Ditetryldiyl Ethers from Dimetallynes
{Ar0 E(μ-OH)}2,15 as reported previously. Use of TEMPO (BDE 101.0 kcal/mol) gave the spin-trapped molecule Ar0 Ge(TEMPO) or, in the case of tin, the hydroxide {Ar0 Sn(μ-OH)}2.15 We report here the stoichiometric use of an oxygen transfer reagent of intermediate strength, pyridine N-oxide (BDE 63.3 kcal/mol). Reaction with Ar0 GeGeAr0 in toluene successfully gave 1 in 42% yield as a pure compound (Scheme 1).16 Similar reaction with Ar0 SnSnAr0 led to a mixture of the desired bridging oxide 2 and {Ar0 Sn(μOH)}2 (3). Screening against the oxygen transfer reagents Me3NO, Me2SO and Me3PO gave the same mixture in a similar ratio for Me3NO and Me2SO but no reaction for Me3PO. The hydroxide is speculated to form from a putative {Ar0 Sn•(μ-O)}2 singlet diradicaloid the product of overoxidation which presumably CH activates solvent or ligand (Scheme 2). Slowing the rate of addition, changing the concentration of reagents or the temperature of Received: May 17, 2011 Published: June 07, 2011 3468
dx.doi.org/10.1021/om2004018 | Organometallics 2011, 30, 3468–3471
Organometallics
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Scheme 2. Formation of μ-Oxo (2) and μ-Hydroxide Species (3) from Distannyne
reaction did not remove the impurity. The reaction was carried out under strict anhydrous conditions, and it is not likely that the hydroxide arose from an H2O contaminant - no evidence was found for {Ar0 Ge(μOH)}2 in the parallel reaction with 1 using the same reagents. Changing solvent from toluene to an aliphatic solvent (methylcyclohexane) did not change the propensity for hydroxide formation. Furthermore, the use of d8-toluene did not alter the integral value of the hydroxide proton in the 1H NMR spectrum but it is conceivable that CH activation occurred with a sacrificial equivalent of the terphenyl ligand. The CH activation of terphenyl ligands has been previously demonstrated in a digermanium species which also appears to occur via diradicaloid intermediate.6 It is currently unclear why the Sn reaction is more easily overoxidized than that of the Ge compound, although the larger size of Sn in addition to its lower reactivity may allow the coordination of two equivalents of pyridine N-oxide before oxygen transfer occurs. Although 2 could not be isolated in a pure form, reliable data regarding its structure and chemical properties could be obtained nonetheless. The digermyldiyl ether 1 was isolated as a green air-sensitive solid which displays limited solubility in hydrocarbon solvents.16 Structural determination using X-ray methods revealed a pair of two-coordinate Ge centers bridged by a single oxygen atom, which was found to be disordered equally over two positions related by a mirror plane (Figure 1).17 The terphenyl ligands are disposed in a trans-bent geometry through an angle of 110.0(1)° for O1Ge1C1. The central GeOGe moiety is found to have a bending angle of 154.8(2)°, in contrast to that found in the bulky imido analogue (Ar0 Ge)2(μ-NAd) at 111.38(9)°.18 The GeO distance of 1.760(2) Å is similar to that found in divalent {(L2)Ge}2(μ-O) (1.766(5) Å)11 but is much shorter than those found for (Ar0 GeOH)2 (1.976(6) and 1.974(6) Å), consistent with a bridging oxo rather than hydroxo unit. The internuclear
Figure 1. Thermal ellipsoid (50%) plot of 1. H atoms are not shown. Selected bond lengths (Å) and bond angles (deg) for 1: Ge(1)O(1) = 1.760(2), Ge(1)C(1) = 2.0215(13), Ge(1) 3 3 3 Ge(1)0 = 3.868(3); Ge(1)O(1)Ge(1)0 = 154.75(12), O(1)Ge(1)C(1) = 109.98(8).
Ge 3 3 3 Ge separation is nonbonding at 3.868(3) Å and is much longer than that in the more strongly bent hydroxide (Ar0 GeOH)2 (3.127(4) Å). In the crystal chosen for studies, the tin analogue 2 cocrystallized with hydroxide 3.19 Both 2 and 3 have identical ligand spatial coordinates (hence their ready cocrystallization), showing a similar trans-bent geometry but with different mutually perpendicular tinoxygen core moieties (Figure 2). The central unit was refined best in two parts: 63% [Sn(μ-OH)2Sn] in a geometry 3469
dx.doi.org/10.1021/om2004018 |Organometallics 2011, 30, 3468–3471
Organometallics
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Figure 2. Thermal ellipsoid (50%) plots of cocrystallized 2 and 3. H atoms are not shown. Selected bond lengths (Å) and bond angles (deg): for 2, Sn(2)O(2) = 1.957(6), Sn(2)C(1) = 2.2348(15), Sn 3 3 3 Sn = 3.894(1), Sn(2)O(2)Sn(2)0 = 154.7(3); for 3, Sn(1)O(1) = 2.1354(14), Sn(1)O(1)0 = 2.1360(13), Sn(1)C(1) = 2.2200(11), Sn 3 3 3 Sn = 3.433(1), Sn(1)O(1)Sn(1)0 = 107.0(2).
already reported and 37% [Sn(μ-O)Sn] in a geometry isostructural with that found in the Ge congener 1, with an O atom disordered over two positions. As with 1, we find the SnXSn angle to be less bent in (Ar0 Sn)2(μ-X) for X = O (2: 154.7(3)°) than for bulky X = NSiMe3 (105.4(3)°)6 or for hydroxide 3 (107.0(2)°).13 Likewise, the Sn 3 3 3 Sn distance is longer in 2 (3.894(1) Å) than in 3 (3.433(1) Å). The tinoxo distance SnO = 1.957(6) Å in 2 is much shorter than that in the hydroxide 3 (2.135(1) Å), which is also consistent with the ca. 0.2 Å difference between oxo and hydroxo germanium analogues. The 1H NMR spectra of 1 and 2 show the expected signals for 0 Ar and in the case of 2 showed signals consistent with the hydroxide impurity 3 in ca. 20% proportion (although this figure varied slightly between different batches of crystalline samples). The more tightly bonded Ge compound shows diastereotopic isopropyl resonances, whereas the Sn analogue does not display this magnetic inequivalence, presumably for steric reasons. A signal in the 119Sn{1H} NMR spectrum was found for 2 at 903.7 ppm, comparable with that of (Ar0 Sn)2(μ-NSiMe3) at 907.1 ppm,6 and is significantly downfield of the hydroxide impurity 3 at 381.6 ppm.13 A distinctive absorption is observed in the UV visible spectra for the n f p transition at 450 nm (ε = 870) for 1 and at a slightly lower energy of 460 nm (ε = 1150) for 2. The formation of the tin products is proposed to occur via Scheme 2. The first step is very likely the formation of a 1:1 adduct in which the pyridine oxide behaves as a Lewis base donor to the nþ LUMO of the distannyne in the C(Ar0 )SnSnC(Ar0 ) plane. The formation of such a complex is precedented by the synthesis of isonitrile adducts of the digermenes6 and distannynes.20 The formation of related adducts is known for disilynes, digermynes, and distannynes upon treatment with isonitriles.6,21 The initial Ar0 SnSn(pyrO)Ar0 can then form 2 by pyridine elimination with rearrangement of the unsymmetric monoxide species. 2 may then react with another 1 equiv of pyrO to afford the diradicaloid {Ar0 Sn•(μ-O)}2, which may also form via the bis pyridine oxide adduct {Ar0 Sn(pyrO)}2. The diradicaloid {Ar0 Sn•(μ-O)}2 may then abstract H from the solvent to give 3. In principle, the diradical bridging dioxide species {Ar0 Ge•(μ-O)}2 should be accessible from the monoxide 1 by addition of another 1 equiv of oxidizing agent. However, addition of 0.9
equiv of Me3NO or Me2SO to solutions of 1 in methylcyclohexane gave a colorless, intractable mixture of products, presumably due to the expected high reactivity of the anticipated diradical. This is in contrast to the case for the highly colored diimido {Ar0 Ge•(μ-NSiMe3)}2 and oxo/imido (Ar0 Ge•)2(μ-O)(μ-NDipp) (Dipp = C6H3-2,6-iPr2) singlet diradicaloids, which are found to be sufficiently stable to isolate in pure form at room temperature.22,23 DFT calculations performed on simple model compounds have shown that replacement of an imido with an oxo group decreases the HOMOLUMO gap by 8.1 kcal/mol, in turn increasing the triplet diradicaloid character and also the instability of the molecule.23 Attempts to reverse the oxidation process by reacting 2 with phosphines or alkali metals were unsuccessful. It has been generally accepted that isolation of stable metallylenes requires bulky ligands for kinetic stabilization. These results demonstrate that bridging oxo derivatives may be synthesized for dimetallylenes that maintain a two-coordinate group 14 element atom when used in combination with bulky terminal coligands.
’ ASSOCIATED CONTENT
bS
Figures giving the 1H NMR spectra of 1 and the mixture of 2 and 3 and CIF files giving the crystallographic data for 13. This material is available free of charge via the Internet at http://pubs.acs.org. Supporting Information.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT We are grateful to the U.S. Department of Energy (No. DEFG02-07ER4675-03) for support of this work and to Dr. Y. Peng for useful discussions. ’ REFERENCES (1) Martin, D.; Soleilhavoup, M.; Bertrand, G. Chem. Sci. 2011, 2, 389. 3470
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Organometallics (2) Mizuhata, Y.; Sasamori, T.; Tokitoh, N. Chem. Rev. 2009, 109, 3479. (3) (a) Crabtree, R. H.The Organometallic Chemistry of the Transition Metals, 4th ed.; Wiley-Interscience: Hoboken, NJ, 2005. (b) Bourissou, D.; Guerret, O.; Gabbai, F. P.; Bertrand, G. Chem. Rev. 2000, 100, 39. (b) Liddle, S. T.; Edworthy, I. S.; Arnold, P. L. Chem. Soc. Rev. 2007, 36, 1732. (4) Kira, M. J. Organomet. Chem. 2004, 689, 4475. (5) Hitchcock, P. B.; Lappert, M. F.; Thorne, A. J. J. Chem. Soc., Chem. Commun. 1990, 1587. (6) Cui, C. M.; Olmstead, M. M.; Fettinger, J. C.; Spikes, G. H.; Power, P. P. J. Am. Chem. Soc. 2005, 127, 17530. (7) Braunschweig, H.; Hitchcock, P. B.; Lappert, M. F.; Pierssens, L. J. M. Angew. Chem., Int. Ed. Engl. 1994, 33, 1156. (8) Braunschweig, H.; Drost, C.; Hitchcock, P. B.; Lappert, M. F.; Pierssens, L. J. M. Angew. Chem., Int. Ed. Engl. 1997, 36, 261. (9) (a) Zabula, A. V.; Hahn, F. E.; Pape, T.; Hepp, A. Organometallics 2007, 26, 1972. (b) Hahn, F. E.; Zabula, A. V.; Pape, T.; Hepp, A. Eur. J. Inorg. Chem. 2007, 17, 2405. (10) Wang, W.; Inoue, S.; Yao, S.; Driess, M. J. Am. Chem. Soc. 2010, 132, 15890. (11) Zhang, S. H.; So, C. W. Organometallics 2011, 30, 2059. (12) Stender, M.; Phillips, A. D.; Wright, R. J.; Power, P. P. Angew. Chem., Int. Ed. 2002, 41, 1785–1787. (13) Pu, L.; Phillips, A. D.; Richards, A. F.; Stender, M.; Simons, R. S.; Olmstead, M. M.; Power, P. P. J. Am. Chem. Soc. 2003, 125, 11626–11636. (14) Luo, Y. R. Handbook of Bond Dissociation Energies in Organic Compounds, CRC Press: Boca Raton, FL, 2003. (15) Spikes, G. H.; Peng, Y.; Fettinger, J. C.; Steiner, J.; Power, P. P. Chem. Commun. 2005, 6041. Numerous attempts at stoichrometric oxidation of Ar‡SnSnAr‡ or Ar*SnSnAr* (Ar* = C6H3-2,6(C6H2-2,4, 6-Pri3)2) by O2 or N2O with rigorous exclusion of air and moisture afforded {ArSn(μ-OH)}2 (Ar = Ar‡, Ar*) products which displayed hydroxide-bridged structures similar to that of 3 as well as some variation in SnO bond lengths. (16) All reactions were performed under anaerobic and anhydrous conditions. 1: to a solution of (Ar0 Ge)2 (0.303 g, 0.32 mmol) in 30 mL of toluene was added dropwise a solution of pyridine N-oxide (0.026 g, 0.32 mmol) at 78 °C, resulting in a color change from dark red to orange with a green precipitate upon warming. Heating the solution to 100 °C under static vacuum enabled the dissolution of the green precipitate, and upon cooling to room temperature overnight, green crystals of the product 1 were obtained (0.130 g, 0.13 mmol, 42% yield). Mp: 180 °C dec. 1H NMR (300 MHz, C6D6, 298 K): δ 1.03 (d, 24H, o-CH(CH3)2, 3JHH = 6.9 Hz), 1.20 (d, 24H, o-CH(CH3)2, 3JHH = 6.9 Hz), 2.89 (sept, 8H, CH(CH3)2, 3JHH = 6.9 Hz), 7.057.49 ppm (m, 18H, m-C6H3, p-C6H3,m-Dipp, and p-Dipp, Dipp = C6H3-2,6-Pri2). 13 C{1H} NMR (C6D6, 100.6 MHz, 298 K): δ 23.7 (CH(CH3)2), 25.4 (CH(CH3)2), 30.8 (CH(CH3)2), 123.6 (m-Dipp), 129.0 (p-C6H3), 129.2 (o-Dipp), 137.2 (m-C6H3), 142.7 (p-Dipp), 146.9 (i-Dipp), 169.1 (o-C6H3). UV: λmax (ε) 450 nm (870 mol1 L cm1). 2 and 3: to a solution of (Ar0 Sn)2 (0.283 g, 0.27 mmol) in 30 mL of toluene was added dropwise a solution of pyridine N-oxide (0.022 g, 0.27 mmol) at 78 °C, resulting in a color change from dark green to orange with a red precipitate upon warming. Heating the solution to 100 °C under static vacuum enabled the dissolution of the red precipitate, and upon cooling to room temperature overnight, orange crystals of a mixture of 2 and 3 were obtained (0.152 g). 1H NMR (300 MHz, C6D6, 298 K): (2 only) δ 1.04 (d, 48H, o-CH(CH3)2, 3JHH = 6.9 Hz), 3.09 (sept, 8H, CH(CH3)2, 3JHH = 6.9 Hz), 7.057.49 ppm (m, 18H, m-C6H3, p-C6H3, m-Dipp, and p-Dipp). 13C{1H} NMR (C6D6, 100.6 MHz, 298 K): δ 23.8 (CH(CH3)2), 25.8 (CH(CH3)2), 30.6 (CH(CH3)2), 123.8 (m-Dipp), 129.1 (p-C6H3), 130.2 (o-Dipp), 137.8 (m-C6H3), 143.9 (p-Dipp), 146.9 (i-Dipp), 147.4 (o-C6H3). 119Sn{1H} NMR (C6D6, 100.6 MHz, 298 K): δ 903.7 ppm. UV: λmax (ε) 460 nm (1150 mol1 L cm1). (17) Crystal data for 1 at 90(2) K with Cu KR radiation (λ = 0.710 73 Å): triclinic, space group P1, Z = 2, a = 9.1539(3) Å,
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b = 12.7546(5) Å, c = 13.0263(5) Å, R = 63.729(2)°, β = 81.404(2)°, γ = 69.289(2)°, V = 1275.61(8) Å3, R1 = 0.0249 for 5502 observed reflections (I > 2σ(I)), wR2 = 0.0630 (all data). (18) Wang, X.; Ni, C.; Zhu, Z.; Fettinger, J. C.; Power, P. P. Inorg. Chem. 2009, 48, 2464. (19) Crystal data for 2/3 at 90(2) K with Cu KR radiation (λ = 0.710 73 Å): triclinic, space group P1, Z = 1, a = 11.9117(8) Å, b = 13.3946(4) Å, c = 13.7183(5) Å, R = 111.263(2)°, β = 103.040(2)°, γ = 102.296(2)°, V = 1879.52(15) Å3, R1 = 0.0239 for 10 351 observed reflections (I > 2σ(I)), wR2 = 0.0633 (all data). (20) Peng, Y.; Wang, X.; Fettinger, J C.; Power, P. P. Chem. Commun. 2010, 943. (21) (a) Yamaguchi, T.; Sekiguchi, A.; Driess, M. J. Am. Chem. Soc. 2010, 132, 14061. (b) Takeuchi, K.; Ichinohe, M.; Sekiguchi, A. J. Am. Chem. Soc. 2008, 130, 16848. (c) Peng, Y.; Wang, X.; Fettinger, J. C.; Power, P. P. Chem. Commun. 2010, 943. (22) Cui, C.; Brynda, M.; Olmstead, M. M.; Power, P. P. J. Am. Chem. Soc. 2004, 126, 6510. (23) Wang, X.; Peng, Y.; Olmstead, M. M.; Fettinger, J. C.; Power, P. P. J. Am. Chem. Soc. 2009, 131, 14164.
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