Article pubs.acs.org/JACS
Dynamic Behavior and Isomerization Equilibria of Distannenes Synthesized by Tin Hydride/Olefin Insertions: Characterization of the Elusive Monohydrido Bridged Isomer Shuai Wang, Madison L. McCrea-Hendrick, Cory M. Weinstein,§ Christine A. Caputo,‡ Elke Hoppe, James C. Fettinger, Marilyn M. Olmstead, and Philip P. Power* Department of Chemistry, University of California, 1 Shields Avenue, Davis, California 95616, United States S Supporting Information *
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ABSTRACT: The tin(II) hydride [Ar Pr6Sn(μ-H)]2(Ar Pr6 = C6H3-2,6(C6H2-2,4,6-iPr3)2) (1a) reacts with 2 iequiv of ethylene or t-butylethylene at ca. 25 °C to yield Sn2(Ar Pr6)2R2(R = ethyl or i t-butylethyl), which exist either as a symmetric distannene i Pr6 Ar Pr6(R)SnSn(R)Ar (2ai or 5a) or an unsymmetric stannyli stannylene Ar Pr6SnSnR Ar Pr6 (3a). In contrast, the less crowded i 2 i Pr4 Sn(II) hydride [Ar Sn(μ-H)]2 (Ar Pr4 = C6H3-2,6(C 6 H3 i 2,6-iPr2)2) (1b) reacts with excess ethylene toi give Ar Pr4(CH2CH3)2Sn(CH2CH2)Sn(CH2CH3)(CHCH2)Ar Pr4 (4) featuring five ethylene equivalents, one of which is dehydrogenated to an i Pr4 vinyl, −CHCH , group. The Ar isomers of 2ai and 3a, i.e., 2 i i [Ar Pr4Sn(C2H5)]2 (2b) andi Ar Pr4SnSn(C2H5)2Ar Pr4 (3b) are obtained by reaction of [Ar Pr4Sn(μ-Cl)]2 with EtLi or EtMgBr. 1 The isomeric pairs 2a and 3a are separated by crystallization at different temperatures. Variable-temperature H NMR i i Pr6 Pr4 spectroscopy indicates fast i ethyl group i exchange between Ar(C2H5)SnSn(C2H5)Ar (Ar = Ar (2a) or Ar (2b)) and ArSnSn(C2H5)2Ar (Ar = Ar Pr6 (3a) or Ar Pr4 (3b)) with ΔG⧧ = 14.2 ±i 0.65 kcal mol−1 for 2a/3a and 14.8 ± 0.36 kcal mol−1 for i 2b/3b. The bulkier distannenes [ArSn(CH2CH2tBu)]2 (Ar = Ar Pr6 (5a) or Ar Pr4 (5b)), obtained from 1a or 1b and t-butylethylene, dissociate to ArSnCH2CH2tBu monomers in solution. At lower temperature, they interconvert with their stannylstannylene isomers with parameters Keq = 4.09 ± 0.16 for 5a and 6.38 ± 0.41 for 5b and ΔGeq = −1.81 ± 0.19 kcal mol−1 for 5a and −1.0 ± 0.03 kcal mol−1 for 5b at 298 K. Thei 1:1 reaction of 1a or 1b with 5a or 5b yields the unknown monoi hydrido species Sn2RHAr2 which has the structure Ar Pr6Sn−Sn(H)(CH2CH2tBu)Ar Pr6 (6a) or the monohydrido bridged i
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Ar Pr4Sn(μ-H)Sn(CH2CH2tBu)Ar Pr4 (6b). The latter represents the first structural characterization of a monohydrido bridged isomer of a ditetrelene.
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Jones and co-workers used the bulky amido ligand, −N(Ar†)(SiPri3) (Ar† = C6H2-2,6-{C(H)Ph2}2-4-iPr), to stabilize a singly bonded digermyne which reacted with H2 to form monomeric Ar†(SiPri3)NGeH that catalyzed the hydroboration of carbonyl groups to afford alcohols.30 In addition, in situ generated Sn(II) hydrides were shown to catalyze the dehydrocoupling of amines and boranes.31,32 The high reactivity of the group 14 metal hydrides ArEH (E Ge, Sn) is due to their partially open shell structures which have an occupied s and an empty p-orbital which can interact with the frontier orbitals of unsaturated hydrocarbons and related molecules.7,10
INTRODUCTION Since the discovery of stable low-valent group 14 hydrides in 2000,1 several Sn(II) and Ge(II) hydride species supported by a variety of bulky aryl,2−7 amido,8−10 β-diketiminate,11−15 N-heterocyclic carbene,16−19 and phosphine groups20 have been reported. Most were synthesized by reduction of a halogensubstituted precursor, but it was shown i also that ithe direct reactions of H2 with a digermyne, Ar Pr4GeGeAr Pr4,21 and i Pr4 distannyne, ArSnSnAr(Ar = Ar -4-X, X = H, SiMe3, F),22 under ambient conditions gave Ge(II) and Sn(II) hydrides. Their reactions have attracted increasing attention.23,24 For example, they react with alkali metals Li, Na, and K to give reduced products with variety of structures.25 They also undergo several facile reactions that include C−H activation and the hydroelementation of small molecules.7,26−30 Roesky and co-workers reported that the monomeric Sn(II) hydride, [HC(CMeNAr)2]SnH (Ar = 2,6-iPr2C6H3), could hydrostannylate ketones, CO2, and terminal alkynes to give Sn(II) alkoxides, stannylene formates, and alkenyl stannylenes.26−28 © 2017 American Chemical Society
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In 2012, it was shown that the reaction of Ar Pr4GeGeAr Pr4 with cyclopentene, which resulted ini C−H activation to afford i Pr4 the unsymmetrical digermene Ar Pr4Ge(H)Ge(c-C 5H 9)Ar , i i Pr4 Pr4 proceeded via a Ge(II) hydride (i.e., (Ar GeH)2 or Ar GeH) Received: March 6, 2017 Published: April 11, 2017 6586
DOI: 10.1021/jacs.7b02269 J. Am. Chem. Soc. 2017, 139, 6586−6595
Article
Journal of the American Chemical Society
−1 Scheme 1. Calculated Relative Energies (kcal mol ) of Different Isomeric Forms of the Parent Species E2H4 (E = Si − Sn)33,34 with i i Pr4 Pr4 Energies for the Substituted [Ar SnH]2 (Ar = C6H3-2,6-(C6H3-2,6-iPr2)2)35 Given on the Right-Hand Sidea
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The numbers represent the energies of the isomeric forms found as minima on the potential surface
9.1 kcal mol−1 more stable than the trans-pyramidal distannene H2SnSnH2. Planar isomeric forms H2EEH2(E = Sn − Pb) are never minima on the potential surface and in the case of tin lie at a significantly higher energy of 18.5 kcal mol−1.34 Work by this group on the organo-substituted hydrides, in collaboration with Nagase and Guo, showed that the stability of the asymmetric 2 isomers increases with the increasing size of the substituents. i Pr4 Schleyer and co-workers performed calculations on [Ar SnH]2 (Scheme 1).36 Thei energy difference between the monobridging i Pr4 and the doubly bridged hydrido isomer Ar Pr4Sn(μ-H)Sn(H)Ar i hydrido distannene [Ar Pr4Sn(μ-H)]2 of 5.4 kcal mol−1 is lower than that of the parent Sn2H4; however, stable derivatives of a singly hydrido bridged distannene isomer have been unknown up to now. Ini this paper, we report the reactions of the Sn(II) hydrides, i [Ar Pr6Sn(μ-H)]2 (1a) and [Ar Pr4Sn(μ-H)]2 (1b), with unactivated alkenes which afford tetraorgano-ditin products. We i show that the symmetric isomer, [Ar Pr6Sn(C2H5)]2 (2a), which dissociates to monomers in solution,i is in equilibrium iwith the unsymmetric stannylstannylene, Ar Pr6SnSn(C2H5)2Ar Pr6 (3a) with a fast exchange rate for the ethyl group sites. The isomers can be isolated by crystallization at different temperatures. In contrast, 1b reacts with 5 equiv of ethylene to give the unii Pr 4 (CH que product, Ar 2 CH 3 ) 2 Sn(CH 2 CH 2 )Sn(CH 2 CH 3 )i (CHCH2)Ar Pr4 (4), featuring a CH2CH2 moiety bridging two tin atoms one of which carries an ethenyl rather than an ethyl i i group. However, the synthesis of Ar Pr4SnSn(C2H5)2Ar Pr4 (3b) can be effected via salt i metathesis, and this species is also in equilibrium with [Ar Pr4Sn(C2H5)]2 (2b) in solution. In addition, we report the first monohydrido stannylstannylene i i Ar Pr6SnSn(H)(CH2CH2tBu)Ar Pr6 (6a) and the monohydrido
intermediate.7 Jones and co-workers also showed that the divalent amido hydrides, L†-EH (E Sn or Ge) hydroelementate a variety of cyclic and linear unactivated olefins to give monomeric germylenes and stannylenes.10 From the experimental standpoint, stable group 14 derivatives of the parent E2H4 species can be isolated using bulky coligands such as terphenyl groups in place of hydrogen. For tin they have been shown to exist in two isomeric forms based either on the doubly bridged (HE(μ-H)2EH)1,2 or the unsymmetrical HEEH3 structures.2 For germanium the most stable isomer is a digermene based on the multiple bonded trans-pyramidalized H2EEH2 structure.6 Sekiguchi and co-workers have reported the reaction of a dislyne RSiSiR (R = SiiPr[CH(SiMe3)2]2 with R 2 NH (R = Et, Ph) to afford a monohydridosubstituted asymmetric disilenes R(R′2E)SiSiHR (R = SiiPr[CH(SiMe3)2]2 R′ = Et2N or Ph2N).36 Tokitoh and co-workers have synthesized the terminal dihydridodisilene, ArHSiSiHAr (Ar = C6H2-2,6-CH(SiMe3)2, R = H or CH(SiMe3)2).37 Rivard and co-workers have shown that, using a combination of Lewis acids and bases, complexes of the symmetric H2EEH2 isomers can be isolated and characterized.16,17 Nonetheless the reactions of the low-valent/ low-oxidation state heavier group 14 hydrides are complicated by the fact that they can exist in several isomeric forms separated by relatively small energies. In 1990, Trinquier calculated the structures and energies of the parent divalent heavier group 14 hydrides, E2H4 (E = Si − Sn) (Scheme 1),34 and showed that several isomers corresponding to minima on the potential surface are separated by
