Tin(II) Hydrides as Intermediates in Rearrangements of Tin(II) Alkyl

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Tin(II) Hydrides as Intermediates in Rearrangements of Tin(II) Alkyl Derivatives 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, Davis, 1 Shields Avenue, Davis, California 95616, United States S Supporting Information *

ABSTRACT: Reactions of the Sn(II) hydrides [ArSn(μ-H)]2 (1) (Ar = AriPr4 (1a), AriPr6 (1b); AriP4 = C6H3-2,6-(C6H32,6-iPr2)2, AriPr6 = C6H3-2,6-(C6H2-2,4,6-iPr3)2) with norbornene (NB) or norbornadiene (NBD) readily generate the bicyclic alkyl-/alkenyl-substituted stannylenes, ArSn(norbornyl) (2a or 2b) and ArSn(norbornenyl) (3a or 3b), respectively. Heating a toluene solution of 3a or 3b at reflux afforded the rearranged species ArSn(3-tricyclo[2.2.1.02,6]heptane) (4a or 4b), in which the norbornenyl ligand is transformed into a nortricyclyl group. 1H NMR studies of the reactions of 4a or 4b with tert-butylethylene indicated the existence of an apparently unique reversible β-hydride elimination from the bicyclic substituted aryl/alkyl stannylenes 2a or 2b and 3a or 3b. Mechanistic studies indicated that the transformation of 3a or 3b into 4a or 4b occurs via a β-hydride elimination of 1a or 1b to regenerate NBD. Kinetic studies showed that the conversion of 3a or 3b to 4a or 4b is first order. The rate constant k for the conversion of 3a into 3b was determined to be 3.33 × 10−5 min−1, with an activation energy Ea of 16.4 ± 0.7 kcal mol−1.

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INTRODUCTION

In an earlier investigation on the addition of element− hydrogen moieties to olefins, it had been shown by Brown and co-workers that hydroboration of norbornene gave exonorborneol upon hydrolysis.22 Calculations by Nagase and co-workers showed that the hydroboration occurs as a two-step process that includes the formation of a three-membered-ring π complex intermediate featuring one boron atom and two alkenyl carbons and a B−H moiety involved in a fourmembered-ring transition state.23 The exo regioselectivity of hydroboration of norbornene was also demonstrated,24 and calculations by Koga and co-workers showed that exo selectivity is preferred because of the lower energy required to deform the exo configuration relative to that of an endo form during the reaction.25 Sn(IV) species were reported to hydrostannylate norbornadiene (NBD) to afford R3Sn(norbornenyl) (R = nBu, Ph) with transition-metal catalysts.26 It has been shown that the nortricylane isomers of NBD are more thermodynamically stable than the unsaturated bicyclic NBD form.27 An anionic/free-radical pathway has been suggested for the rearrangement of a norbornenyl tin(IV) species to give tricyclo[2.2.1.02,6]heptane via a tin(IV) species (Scheme 1, path I).28 In the formation of [2.2.1.02,6]heptane-3mercuric chloride by the reaction of HgCl2 with 5-norbornene2-boronic acid, an electrophilic displacement mechanism via a transient mercury−alkene complex was proposed (Scheme 1, path II).29 Transition-metal-catalyzed conversion of norbornadiene from its valence energy-rich isomer quadricyclane (Q) has

The synthesis and reactivity of low-valent group 14 element hydrides with small molecules such as CO2, ketones, and alkynes have been the subject of increasing attention in the past 15 years.1−19 The first divalent group 14 element hydride, [AriPr6Sn(μ-H)]2, was reported in 20001 and a related Ge−Gebonded germanium analogue, AriPr4(H)GeGe(H)AriPr4, was reported in 2003.6 It was subsequently shown that the latter species is also produced, amongst other germanium hydride products, by the direct reaction of H2 with the digermyne AriPr4GeGeAriPr4 under ambient conditions. Moreover, the observation of AriPr4GeH3 among these products suggested the existence of AriPr4GeH as a divalent intermediate in the reaction.20 We also reported that the germanium(II) hydride AriPr4GeH is generated during the C−H activation of cyclopentene by digermyne, AriPr4GeGeAriPr4.7 Experimental data indicated also that the low-valent hydrides of germanium and tin were formed as reactive intermediates in the reactions of AriPr4GeGeAriPr4 and AriPr4SnSnAriPr4 with cyclopentadiene, resulting in the elimination of H2 and formation of AriPr4E(η5C5H5) (E = Ge, Sn) products.7 In a series of papers,8−10 Jones and co-workers showed that the bulky amido ligand L† (=− N(Ar†)(SiPri3); Ar† = C6H2{C(H)Ph2}2iPr-2,4,6) stabilized a singly bonded digermyne as well as a monomeric hydrido germylene which, along with its tin analogue, hydrometalated unsaturated alkenes and alkynes.10 In further work, we showed that the reaction of the Sn(II) hydrides 1a and 1b with certain acyclic olefins yielded the corresponding Sn(II) alkyl species.21 The rearrangement of metal alkyls through metal hydrides has not been previously reported. © 2017 American Chemical Society

Received: March 6, 2017 Published: April 11, 2017 6596

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Scheme 1. Previously Proposed Possible Mechanistic Pathways from Norbornenyl to Nortricyclic Group: (I) Anionic/FreeRadical Pathway; (II) Electrophilic Displacement Pathway

Scheme 2. Reaction between Sn(II) Hydrides and Bicyclic Olefins

Figure 1. Thermal ellipsoid (50%) plot of AriPr4Sn(norbornenyl) (3a) (left) and AriPr4Sn(3-tricyclo[2.2.1.02,6]heptane) (4a) (right). H atoms, except these involved in the closest H- - -H approaches, are not shown. Selected bond lengths (Å) and bond angles (deg): 3a, Sn(1)−C(1) 2.192(5), Sn(1)−C(2) 2.211(6), C(2)−C(7) 1.523(3), C(4)−C(5) 1.254(3), C(1)−Sn(1)−C(2) 102.4(14), Sn(1)−C(2)−C(3) 107.4(12), Sn(1)−C(2)− C(7) 120.8(16); 4a, Sn(1)−C(1) 2.202(3), Sn(1)−C(2) 2.214(3), C(5)−C(7) 1.745(10), C(1)−Sn(1)−C(2) 98.26(12), Sn(1)−C(2)−C(3) 114.0(2), Sn(1)−C(2)−C(7) 112.9(3).

been widely studied, as it can be utilized for solar energy storage and controlled energy release.30−34 A few rare examples

of main-group complexes capable of catalyzing this process were reported without mechanistic details, however.35−37 In a 6597

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Journal of the American Chemical Society Table 1. Summary of Key Structural and Spectroscopic Data for Compounds 2a,b−4a,b and 5 Sn−C(aryl) (Å)

Sn−C(alkyl) (Å)

2.193(4) 2.133(6) 2.192(5) 2.188(5) 2.196(19) 2.208(5) 2.207(6) 2.200(6)

2.177(5) 2.221(17) 2.211(6) 2.226(8) 2.198(2) 2.219(6) 2.215(8) 2.209(7)

iPr4

Ar Sn(norbornyl) (2a) AriPr6Sn(norbornyl) (2b) AriPr4Sn(norbornenyl) (3a) AriPr6Sn(norbornenyl) (3b) AriPr4Sn(nortricyclyl) (4a) AriPr6Sn(nortricyclyl) (4b) AriPr4Sn(norbornyl)SnAriPr4 (5)

1.747 (6) 1.759(14)

λmax (nm) 504 494 502 494 505 501 496

119

Sn{1H} NMR (ppm) 1555 1530 1709 1682 1867 1822 1484

= AriPr4 or AriPr6), although there may also be other possibilities21) was confirmed by the disappearance of the tin hydride resonance at 9.15 ppm in the 1H NMR spectrum. The diastereotopic character of methine hydrogens of the isopropyl groups of the terphenyl ligands affords two septets at 3.21 ppm for both compounds. In the 119Sn{1H} NMR spectra, sharp resonances at 1555 ppm for 2a and 1708 ppm for 3a are consistent with the low-valent monomeric formulations which are comparable to the values of the previously reported monomeric stannylenes.20,39,40,42,50,51 The dark red color of both compounds is consistent with the relatively strong absorptions observed at 504 nm for AriPr4Sn(norbornyl), and 502 nm for AriPr4Sn(norbornenyl), which are due to symmetryallowed n to p transitions. Structural data for AriPr6Sn(norbornyl) (2b) and AriPr6Sn(norbornenyl) (3b), including spectroscopic data, are given in the Supporting Information. In the 1H NMR spectrum of 3a, signals from a minor terphenyl-ligated species that had intensities ca. 11% of those of 3a were also observed. Storage of a benzene solution of 3a for 2 weeks at room temperature resulted in an increased intensity of the original minor resonances at 1.48, 1.56, 1.66, and 2.36 ppm and a decreased intensity of the vinyl resonance of the norbornenyl ligand at 5.70 and 6.12 ppm (Figure 2). Heating

computational study Clark and co-workers performed calculations which showed that the conversion of Q to NBD, catalyzed by SnCl2 either in the solid state or in a methanol solution, is possible to have reactivity in the singlet or triplet state, involving an endo attack of SnCl2. In this paper, we investigate the mechanism of the reactions of [ArSn(μ-H)]2 (1a or 1b) species with the bicyclic olefins norbornene and NBD and show that the products are the stannylenes ArSn(norbornyl) (2a or 2b) and ArSn(norbornenyl) (3a or 3b), respectively. The rearranged products, ArSn(3-tricyclo[2.2.1.02,6]heptane) (4a and 4b), featuring a tricyclyl ligand, were first detected as minor products in the reaction of 1a or 1b with NBD. The complexes 4a and 4b were obtained as the major products by refluxing the corresponding toluene solutions of 3a or 3b for 12 h. Several mechanistic pathways were tested, and a mechanism for conversion from 3a or 3b to 4a or 4b is proposed.

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C(5)−C(7) (Å)

RESULTS AND DISCUSSION

Hydrostannylation of Norbornene and Norbornadiene. A 2 equiv amount of the bicyclic molecules norbornene (NB) or norbornadiene (NBD) was added to toluene solutions of the hydrides [ArSn(μ-H)]2 (Ar = AriPr4 (1a), AriPr6 (1b)) at room temperature (Scheme 2). An almost instantaneous color change from blue to dark red was observed. The solutions were stirred for 12 h and concentrated under reduced pressure. Storage of the solution at ca. −20 °C yielded crystals of sufficient quality for X-ray diffraction studies. The molecular structures of AriPr4Sn(norbornyl) (2a) (see the Supporting Information) and AriPr4Sn(norbornenyl) (3a) (Figure 1, left) reveal monomeric tin aryl/alkyl units with bent two-coordinate geometry at the tin atoms (see Figure 1, left). The norbornyl and norbornenyl ligands are bound at the exo positions with a relatively narrow Cipso−Sn−C interligand angles of 100.77(18)° in 2a and of 102.42° in 3a, in comparison to those in other two-coordinate aryl/aryl or aryl/alkyl Sn(II) (range 105.6−117.6°).38−43 The bent geometry at tin is due to the presence of a stereochemically active lone pair. In addition, dispersive interactions44 between the rigid substituents and the terphenyl ligands may also be contributing to the narrower angle. Norbornyl and norbornenyl groups, being more rigid hydrocarbons, are known to show stronger dispersive interactions relative to their acyclic analogues.44−48 This is further indicated by the close intramolecular H- - -H distances ca. 2.1−2.5 Å between the protons of the bicyclic substituents and the isopropyls of the terphenyls (Table 1). These distances are shorter than the sum of van der Waals radii of two hydrogen atoms (2.40 Å).49 The completion of the hydrostannylation reaction (it is assumed that the reactive tin species is monomeric ArSnH (Ar

Figure 2. 1H NMR spectroscopy illustrating the gradual formation of AriPr4Sn(3-tricyclo[2.2.1.02,6]heptane) (4a) and disappearance of AriPr4Sn(norbornenyl) (3a) upon storage of a d6-benzene solution over 2 weeks.

toluene solutions of both 3a and 3b at 110 °C for 12 h led to a 90% conversion to the new species, suggesting that 3a and 3b are the kinetically favored products whereas the new species represent the thermodynamically favored products. New signals appearing at 1867 ppm for 4a and 1821 ppm for 4b in the 119 Sn{1H} NMR spectra further confirm the 1H NMR spectral results and are consistent with a monomeric stannylene.20,39,40,42,50,51 Red crystals were obtained in 68% and 6598

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Journal of the American Chemical Society Scheme 3. Mechanistic Studies of the Formation of ArSn(3-tricyclo[2.2.1.02,6]heptane) (4)

53% yields by recrystallization from toluene at ca. −20 °C. The data showed these to be pure ArSn(3-tricyclo[2.2.1.02,6]heptane) (4a or 4b). In the crystal structure of 4a (Figure 1, right), the tin atom, now substituted by a nortricyclyl ligand, has an interligand C(1)−Sn(1)−C(2) angle (98.26(12)°) slightly narrower than those seen in 2a (100.9(18)°) and 3a (102.4(14)°). The C(5)−C(7) distance in the cyclopropyl ring at 1.745(10) Å is significantly longer than a normal C−C single-bond distance (1.54 Å). However, such lengthening has been reported previously for the nortricyclane group in the transition-metal complex Cp2Ru2(m-CH-nortricyclyl)(CO)52 (1.796(17) Å) and in a nortricyclene acetic acid derivative (1.748(18) Å).53 Mechanistic Studies of the Formation of 4a. A series of experimental investigations were carried out to shed light on the mechanism of the isomerization to the tricyclic derivatives. Thus, 4a was obtained as the only product upon heating a C6D6 solution of 3a to 100 °C for 12 h in the presence of the radical trap 2,6-di-tert-butyl-4-methylpyridine with no indication of radical formation from the 1H NMR spectrum. This suggests a nonradical pathway for the rearrangement (Scheme 3, top). Since our group and Jones and co-workers have shown recently that Ge(II) hydrides were implicated as intermediates in the activation of C−H bonds of cyclopentene or cyclohexadiene,7,54 we also tested the possibility of the reversible βhydride elimination of a Sn(II) hydride upon refluxing 2a. Thus, 3,3-dimethylbut-1-ene (tert-butylethylene), which should react with any 1a produced during the reaction, was added to a d6-benzene solution of 2a. 1H NMR spectroscopy indicated gradual consumption of tert-butylethylene and formation of monomeric AriPr4Sn(CH2CH2tBu), which features a signal at 0.83 ppm, representing the terminal methyl groups on the substituted tert-butylethyl group. The appearance of a triplet signal at 5.95 ppm indicates the regeneration of norbornene, which indicates that 2a or 2b exists in reversible equilibria with the tin hydride species as a consequence of β-hydride elimination (Scheme 3, middle).

To check if one aryl tin hydride species could electrophilically displace another and result in rearrangement, 1/2 equiv of the hydride 1a was added to 3a. No indication of the formation of 4a was observed; instead, the doubly inserted product AriPr4Sn(norbornyl)SnAriPr4 (5) was obtained as red crystals in 73% yield (Scheme 3, bottom). X-ray diffraction studies revealed a structure (Figure 3) in which two AriPr4Sn moieties

Figure 3. Thermal ellipsoid plot (50%) of AriPr4Sn(norbornyl)SnAriPr4 (5). H atoms and some flanking aryl groups and isopropyl groups are not shown. Selected bond lengths (Å) and bond angles (deg): Sn(1)− C(1) 2.224(5), Sn(1)−C(2) 2.209(5), Sn(2)−C(5) 2.243(5), Sn(2)− C(9) 2.216(4), C(1)−Sn(1)−C(2) 99.7(18), Sn(1)−C(2)−C(3) 112.1(17), Sn(1)−C(2)−C(7) 102.9(9), C(5)−Sn(2)−C(9) 101.8(18), Sn(2)−C(5)−C(6) 114.9(17), Sn(2)−C(5)−C(4) 105.3(9).

are bound to a norbornyl moiety in trans-exo positions, with interligand Cipso−Sn−C angles of 99.3(3) and 101.8(3)° which are similar to those in 3a and 4a. The C(2)−C(7) and C(4)− C(5) distances of 1.559(4) and 1.508(4) Å are consistent with single bonds. The absence of olefinic resonances between 5.7 and 6.1 ppm in the 1H NMR spectrum of 5 confirms the reaction of both olefinic moieties of NBD. The identical electronic environments of the two Sn atoms are confirmed by the observation of a single 119Sn{1H} NMR signal at 1484 ppm. 6599

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stereochemistry of hydroboration of bicyclic olefins, which indicates the hydrostannylation of rigid olefins is preferred at the less hindered side of the bicyclic substituents.24,25 Further support for the preference for the exo isomer is available from 1 H NMR spectroscopy. The coupling constants for the hydrogens from carbons bound to tin and the bridgehead protons were all found to be near 0.6 Hz, which is comparable to the 2JHH value of bridgehead protons with endo protons in norbornene.55 VT 1H NMR Studies. To further investigate the mechanistic pathway, kinetic studies and thermodynamic studies were carried out via a series of high-temperature 1H NMR experiments. A d8-toluene solution of 3a was monitored by 1 H NMR spectroscopy at 80 °C for 15 h. The conversion from 3a to 4a was determined to be overall first order with a rate constant of k = 3.33 × 10−5 min−1. In accordance with transition state theory,56 the activation energy for the transition state was determined via a series of conversion reactions of 3a at various temperatures to be Ea = 16.4 ± 0.7 kcal mol−1, ΔS⧧ = 0.0645 ± 0.002 kcal mol−1, and ΔG⧧ = −2.81 ± 0.1 kcal mol−1 at ca. 25 °C (Figure 4 and the

A further experiment involved the reaction of 2 equiv of quadricyclane (Q) with [AriPr4Sn(μ-H)]2 at ambient temperature to check if the formation of 4a is due to β-hydride elimination followed by rearrangement of NBD. 1H NMR spectroscopy revealed that the reaction of 1a with Q gave the same spectrum as the reaction of 1a with NBD, which has 3a as the major product. This shows that Q can be converted to a norbornenyl group in the presence of the Sn(II) hydrides at ambient temperature. A mechanism for this process is proposed, as shown in Scheme 4. In this mechanism, the Scheme 4. Proposed Mechanism of the Formation of ArSn(norbornenyl) from the Reaction of a Tin(II) Hydride and Quadricyclane (Q)

HOMO involving the nonbonded pair of electrons on the tin interacts with the antibonding orbital, σ*C1−C2, to weaken the C1−C2 bond in the three-membered ring, which is followed by a movement of electron density from σC1−C2 to πC2−C3. The Sn−H bond is cleaved simultaneously by electron donation from σC3−C4 to the antibonding orbital σ*Sn1−H. Thus, the possibility of the generation of Q as an intermediate during the formation of 4a was eliminated. However, with the conversion of the norbornenyl group from Q and the confirmation of βhydride elimination of 3a or 3b, this suggests that 1a or 1b could be used as a main-group complex catalyst model for solar energy storage (Scheme 5). It is noteworthy that in the bicyclic substituted aryl/alkyl tin compounds 2, 3, and 5, all of the tin atoms are bound at exo positions. This exo stereoselectivity is analogous to the

Figure 4. Kinetic studies of the conversion from 3a to 4a at different temperatures.

Supporting Information). This indicates that the formation of 4a required relatively high activation energy; however, the energy is still low enough that the transition state is accessible at room temperature and the reaction can occur spontaneously. This is supported by the small negative temperature-dependent ΔG⧧ value and is consistent with the experimental observation. By eliminating the alternative mechanistic pathways, we propose that the formation of the rearranged tricyclic product

Scheme 5. (ArSnH)2 (1a or 1b) Catalyzed Interconversion of NBD and Q

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Journal of the American Chemical Society 4a or 4b is likely a consequence of β-hydride elimination from 3a or 3b, which are in pre-equilibria with 1a or 1b and NBD. The hydride 1a or 1b then reacts with NBD via a unique endo1,3-hydrostannylation as the rate-determining step to yield 4a or 4b, as shown in eqs 1 and 2.

which makes the formation of 4 the driving force for the completion of the reaction.

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CONCLUSION Low valent tin(II) hydrides [ArSn(μ-H)]2 (Ar = AriPr4 (1a), AriPr6 (1b)) react readily with bicyclic olefins, i.e. norbornene and norbornadiene, to afford the hydrostannylation products ArSn(norbornyl) (2a or 2b) or ArSn(norbornenyl) (3a or 3b). Rearrangement of 3a or 3b to ArSn(3-tricyclo[2.2.1.02,6]]heptane) (4a or 4b) occurs upon heating. It was concluded on the basis of experimental data that the reactions involve unique metal hydride transformation pathways, including β-hydride elimination of 1a or 1b from 3a or 3b, and an endo-syn addition of 1a or 1b to NBD.

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In the proposed mechanism shown in Scheme 6, 3a or 3b is the kinetically favored product due to the aforementioned lower steric hindrance of the exo face. In the transition state of the syn addition, similar to hydroboration, a four-membered ring involving the Sn−H moiety and two ethenyl carbon atoms is formed. Donation of the lone pair electrons from Sn to the πC1C6* orbital cleaves the C1−C6 π bond. Back donation from πC1C6 to σSn−H* leads to C6−H bond formation and Sn−H bond breaking. In contrast, the formation of 4 is from Sn−H endo addition at the 1,3-position of NBD. A sixmembered ring, including all four doubly bound carbons and the Sn−H moiety, is formed at the transition state. The C1 C6 double bond is cleaved via a route similar to that of exo addition. In this case, Sn−H is no longer aligned with the C1− C6 bond and the π electrons can be donated from πC1C6 to πC3C4* to form the C4−C6 single bond with cleavage of the C3C4 double bond. Compound 4 forms via C3−H single bond formation and Sn−H bond breaking via π electron donation from πC3C4 to σSn−H*. Even though this species is not energetically favored, the rearranged nortricyclyl group leads to a geometry where β-hydride elimination is prohibited,

EXPERIMENTAL SECTION

General Procedures. All operations were carried out under anaerobic and anhydrous conditions using modified Schlenk techniques. All solvents were dried over alumina columns and degassed prior to use. The 1H, 13C, and 119Sn NMR spectroscopic data were collected on a Varian 600 MHz spectrometer. The 119Sn NMR data were referenced to SnBu4 (−11.7 ppm). Variabletemperature 1H NMR spectroscopy was carried out on a Bruker 500 MHz spectrometer. Infrared spectroscopy was carried out on Nujol mulls using a Bruker Tensor 27 IR spectrometer. UV−visible spectroscopy was carried out on dilute hexane solutions in 3.5 mL quartz cuvettes using an Olis 17 Modernized Cary 14 UV/vis/NIR spectrophotometer. [AriPr4Sn(μ-H)]2 (1a) and [AriPr6Sn(μ-H)]2 (1b) were synthesized according to literature methods.1,3 tert-Butylethylene and norbornadiene were dried over CaH2 and trap-to-trap distilled prior to use. Norbornene was melted at ca. 50 °C, dried over CaH2, and distilled under reduced pressure prior to use. 2,6-Di-tert-butyl-4methylpyridine was distilled under reduced pressure prior to use. AriPr4Sn(norbornyl) (2a). To a rapidly stirred pale green solution of [AriPr4Sn(μ-H)]2 (1a) (0.459 g, 0.443 mmol) in dry degassed toluene (ca. 40 mL) was added a solution of norbornene (0.0918 g, 0.976 mmol) in toluene (ca. 15 mL) over 10 min at ca. 25 °C. A color

Scheme 6. Proposed Mechanism for the Formation of ArSn(norbornenyl) (3a or 3b) and ArSn(3-tricyclo[2.2.1.02,6]heptane) (4a or 4b)

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°C. 1H NMR (600 MHz, C6D6, 298 K): δ 0.79−0.94 (m, 3H), 1.13 (t, 12H, 3J = 12 Hz, CH(CH3)2), 1.21 (dd, 12H, 3J = 6 Hz, CH(CH3)2), 1.33 (s, 1H, CH), 1.36 (d, 6H, 3J = 6 Hz, CH(CH3)2), 1.41 (d, 6H, 3J = 6 Hz, CH(CH3)2), 1.70 (ddd, 1H, 3J = 6 Hz, CH), 1.96 (s, 1H, CH), 2.78 (sept, 2H, 3J = 6 Hz, CH(CH3)2), 2.83 (s, 1H, CH), 3.25 (sept, 2H, 3J = 6 Hz, CH(CH3)2), 3.30 (sept, 2H, 3J = 6 Hz, CH(CH3)2), 5.74 (dd, 1H, 3J = 6 Hz, CHCH), 6.04 (dd, 1H, 3J = 6 Hz, CH CH), 7.20 (s, 4H, ArH), 7.33−7.40 (m, 3H, ArH). 13C{1H} NMR (150 MHz, C6D6, 298 K): δ 22.2, 22.64, 22.72, 23.9, 26.2, 26.3, 30.6, 30.7, 41.3, 43.4, 46.8, 56.4, 121.2, 121.4, 126.4, 127.9, 129.1, 132.9, 134.8, 136.2, 144.2, 146.6, 146.7, 148.7, 180.0. 119Sn{1H} NMR (223.6 MHz, C6D6, 298 K): δ 1682. FT-IR (CsI, Nujol, selected): 795 and 405 (Sn−C stretching and bending). λmax 494 (ε = 1190 in L mol−1 cm−1). AriPr4Sn(3-tricyclo[2.2.1.02,6]heptane) (4a). A red solution of 3a (0.22 g, 0.360 mmol) in toluene (ca. 20 mL) was heated to reflux in an oil bath for 12 h with rapid stirring. The solution was then cooled to 25 °C and concentrated to ca. 1 mL under reduced pressure. Red crystals were obtained from storage of the solution at ca. −18 °C. Yield: 0.15 g (68%). Mp: 150−154 °C. 1H NMR (600 MHz, C6D6, 298 K): δ 0.18 (t, 1H, 3J = 6 Hz, CH), 0.85 (t, 1H, 3J = 5.4 Hz, CH), 0.97 (d, 1H, 3J = 10.2 Hz, CH2), 1.00 (d, 1H, 3J = 10.2 Hz, CH2), 1.07 (d, 6H, 3J = 6.9 Hz, CH(CH3)2), 1.08 (d, 1H, 3J = 5.4 Hz, CH), 1.10 (d, 6H, 3J = 6.9 Hz, CH(CH3)2), 1.30 (m, 1H, CH), 1.32 (d, 6H, 3J = 6.9 Hz, CH(CH3)2), 1.37 (d, 6H, 3J = 6.9 Hz, CH(CH3)2), 1.49 (d, 1H, 3J = 9.6 Hz, CH2), 1.57 (br, 1H, CH), 1.66 (d, 3J = 9.6 Hz, CH2), 2.37 (br, 1H, CH), 3.26 (sept, 4H, 3J = 6.9 Hz, CH(CH3)2), 7.04− 7.38 (m, 9H, m-C6H3, p-C6H3, m-Dipp and p-Dipp; Dipp =2,6-iPr2C6H3). 13C{1H} NMR (C6D6, 125 MHz, 298 K): δ 8.59, 10.5, 10.9, 22.8, 22.9, 26.7, 31.0, 31.1, 32. 5, 37.3, 71.8, 123.6, 123.8, 126.6, 128.9, 129.3, 129.6, 137.2, 144.5, 147.3. 119Sn{1H} NMR (C6D6,223.6 MHz, 298 K): δ 1867. IR (CsI, Nujol; selected, cm−1): 705 and 380. λmax 505 nm (ε = 950 L mol−1 cm−1). AriPr6Sn(3-tricyclo[2.2.1.02,6]heptane) (4b). A red solution of 3b (0.35 g, 0.503 mmol) in toluene (ca. 30 mL) was heated to reflux in an oil bath for 12 h with rapid stirring. The solution was then cooled to ca. 25 °C and concentrated to ca. 3 mL under reduced pressure. Red crystals were obtained from storage of the solution at ca. −18 °C. Yield: 0.27 g (77%). Mp: 159 °C. 1H NMR (600 MHz, C6D6, 298 K): δ 0.17 (t, J = 6 Hz, 1H, nbd), 0.88 (t, J = 6 Hz, 1H, nbd), 1.00 (dd, J = 6 Hz, 2H, nbd), 1.09 (dd, J = 6 Hz, 2H, nbd), 1.13 (d, J = 12 Hz, 6H, CH(CH3)2), 1.15 (d, J = 6 Hz, 6H, CH(CH3)2), 1.17 (d, J = 6 Hz, 6H, CH(CH3)2), 1.19 (d, J = 6 Hz, 6H, CH(CH3)2), 1.39 (d, J = 6 Hz, 6H, CH(CH3)2), 1.43 (d, J = 6 Hz, 6H, CH(CH3)2), 1.49 (s, 1H, CH), 1.58 (s, 1H, CH), 1.66 (d, J = 6 Hz, 1H, CH), 2.76 (sept, J = 6 Hz, 2H), 3.30 (m, 4H), 7.17 (s, 4H, ArH), 7.33−7.40 (m, 3H, ArH). 13 C{1H} NMR (150 MHz, C6D6, 298 K): δ 8.0, 9.9, 10.3, 22.5, 22.6, 23.81, 23.84, 26.3, 26.4, 30.6, 30.7, 30.8, 32.0, 34.4, 36.9, 70.5, 121.1, 121.3, 126.3, 129.1, 134.7, 144.3, 146.82, 146.88, 148.7, 180.2. 119 Sn{1H} NMR (223.6 MHz, C6D6, 298 K): δ 1821. IR (CsI, Nujol; selected, cm−1): 700 and 370. λmax 501 nm (ε = 1050 L mol−1 cm−1). AriPr4Sn(C7H10)SnAriPr4 (5). To a solution of 1a (0.155 g, 0.30 mmol) in dry degassed toluene (ca. 20 mL) was added freshly distilled norbornadiene (0.015 mL, 0.15 mmol) via syringe at ca. 25 °C with rapid stirring. An immediate color change from green to red was observed. The solution was stirred for 12 h, concentrated under reduced pressure to ca. 3 mL, and then stored at ca. −18 °C to afford red crystals of 5. Yield: 0.13 g (77%). Mp: 168−170 °C. 1H NMR (600 MHz, C6D6, 298 K): δ 0.40 (d of t, 2H, 3J = 13.2 Hz, Sn-CH), 0.85 (d, 1H, 3J = 9.6 Hz, SnCH−CH2), 0.88 (d, 1H, 3J = 9.6 Hz, SnCH−CH2), 1.08 (d, 12H, 3J = 6.9 Hz, CH(CH3)2), 1.12 (d, 12H, 3J = 6.9 Hz, CH(CH3)2), 1.25 (br, 2H, SnCH−CH), 1.36 (d, 12H, 3J = 6.9 Hz, CH(CH3)2), 1.37 (d, 6H, 3J = 6.9 Hz, CH(CH3)2), 1.65 (br 2H, SnCH−CH2), 1.80 (d, 1H, 3J = 6.9 Hz, bridgehead), 1.82 (d, 1H, 3 J = 6.9 Hz, bridgehead), 3.16 (sept, 4H, 3J = 6.9 Hz, CH(CH3)2), 3.25 (sept, 4H, 3J = 6.9 Hz, CH(CH3)2), 6.99−7.35 (m, 18H, m-C6H3, pC6H3, m-Dipp and p-Dipp; Dipp = 2,6-iPr2-C6H3). 13C{1H} NMR (C6D6, 125 MHz, 298 K): δ 22.9, 23.2, 24.3, 24.4, 25.6, 26.5, 26.6, 30.9, 31.0, 31.8, 40.0, 123.6, 124.0, 126.5, 128.9, 144.6, 147.0, 147.1, 178.1. 119Sn{1H} NMR (C6D6, 223.6 MHz, 298 K): δ 1484.4. IR (CsI,

change from green to reddish purple was observed within 15 min. The solution was stirred for 16 h, concentrated under reduced pressure to ca. 10 mL, and stored at ca. −18 °C to afford 2a as dark purple crystals. Yield: 0.35 g (63%). Mp: 148−155 °C. 1H NMR (600 MHz, C6D6, 298 K): δ 0.52 (m 1H, nb CH2), 0.94 (d 1H, 3J = 9 Hz, nb CH2), 0.99 (tr 1H 3J = 8.4 Hz, nb CH2), 1.06 (d, 6H, 3J = 6.9 Hz, CH(CH3)2), 1.10 (d, 6H, 3J = 6.9 Hz, CH(CH3)2), 1.22 (tr 1H, 3J = 4.8 Hz, nb CH2), 1.30 (tr, 1H 3J = 12 Hz, nb CH2), 1.35 (m, 1H, nb CH2), 1.37 (d, 12H, 3J = 6.9 Hz, CH(CH3)2), 1.42 (d 1H, 3J = 9 Hz, nb CH2), 1.67 (b, 1H, bridgehead CH), 1.77 (d, 1H, 3J = 4.8 Hz, nb CH), 1.79 (tr, 1H, 3J = 4.8 Hz, nb CH2), 2.17 ((b, 1H, bridgehead CH), 3.19 (sept, 2H, 3J = 6.9 Hz, CH(CH3)2), 3.25 (sept, 2H, 3J = 6.9 Hz, CH(CH3)2), 7.12−7.35 (m, 9H, m-C6H3, p-C6H3, m-Dipp and pDipp; Dipp =2,6-iPr2−C6H3) ppm. 13C{1H} NMR (C6D6, 125 MHz, 298 K): δ 22.7, 26.3, 30.7, 35.2, 37.7, 37.8, 38.0, 61.3, 123.3, 123.7, 128.7, 129.1 137.2, 144.3, 146.7. 119Sn{1H} NMR (C6D6, 223.6 MHz, 298 K): δ 1555. IR (CsI, Nujol; selected, cm−1): 720 and 375 (Sn−C stretching and bending). λmax 504 nm (ε = 950 L mol−1 cm−1). AriPr6Sn(norbornyl) (2b). To a mixture of [AriPr6Sn(μ-H)]2 (1b) (0.250 g, 0.415 mmol) and norbornene (0.40 g, 0.416 mmol) was added dry degassed toluene (ca. 30 mL) at ca. 25 °C with rapid stirring. The blue mixture turned dark red after ca. 5 min. The reaction was stirred for 12 h and the solvent removed under reduced pressure. The red residue was dissolved in pentane (ca. 60 mL) and filtered using a filter-tipped cannula. The solvent was concentrated under reduced pressure to ca. 2 mL. Red crystals of 2b suitable for X-ray diffraction were grown in a 7 °C refrigerator. Yield 0.130 g (45%). Mp: 147 °C. 1H NMR (600 MHz, C6D6, 298 K): δ 0.55 (s, 2H, CH2), 0.96 (d, 2H, 3J = 12 Hz, CH2), 1.11 (d, 6H, 3J = 6 Hz, CH(CH3)2), 1.15 (d, 6H, 3J = 6 Hz, CH(CH3)2), 1.21 (d, 12H, 3J = 6 Hz, CH(CH3)2), 1.43 (d, 12H, 3J = 6 Hz, CH(CH3)2), 1.64 (s, 1H, CH), 1.77 (s, 2H, CH2), 2.19 (s, 2H, CH2), 2.77 (sept, 2H, 3J = 6 Hz, CH(CH3)2), 3.25 (sept, 2H, 3J = 6 Hz, CH(CH3)2), 3.30 (sept, 2H, 3J = 6 Hz, CH(CH3)2), 7.17 (s, 4H, ArH), 7.33−7.37 (m, 3H, ArH). 13C{1H} NMR (150 MHz, C6D6, 298 K): δ 22.6, 22.7, 23.8, 26.2, 26.3, 27.7, 30.5, 30.6, 30.7, 34.4, 35.1, 37.5, 37.6, 37.8, 60.0, 121.1, 121.5, 126.2, 129.0, 135.0, 144.3, 146.6, 148.7. 119Sn{1H} NMR (223.6 MHz, C6D6, 298 K): δ 1530. FT-IR (CsI, Nujol, selected, cm−1): 790 and 395 (Sn−C stretching and bending). λmax 494 nm (ε = 800 L mol−1 cm−1). AriPr4Sn(norbornenyl) (3a). To a rapidly stirred solution of 1a (0.33 g, 0.32 mmol) in dry degassed toluene (ca. 40 mL) was added freshly distilled norbornadiene (0.068 mL, 0.67 mmol) via syringe over 3 min at ca. 25 °C. An immediate color change from pale green to reddish purple was observed. Stirring was continued for 14 h and the solution was concentrated under reduced pressure to ca. 10 mL then stored at ca. −18 °C to afford 3a as dark purple crystals. Yield: 0.21 g, (54%). Mp: 149−153 °C. 1H NMR (600 MHz, C6D6, 298 K): δ 0.74 (tr, 1H, 3J = 9.6 Hz, nbd CH2), 0.82 (m, 1H, nbd CH2), 1.07 (d, 6H, 3J = 6.9 Hz, CH(CH3)2), 1.08 (d, 6H, 3J = 6.9 Hz, CH(CH3)2), 1.10 (tr, 1H, 3J = 7.5 Hz, nbd CH2), 1.29 (d, 6H, 3J = 6.9 Hz, CH(CH3)2), 1.34 (d, 6H, 3J = 6.9 Hz, CH(CH3)2), 1.36 (tr, 1H, 3J = 7.5 Hz, nbd CH2), 1.69 (m, 1H, nbd CH), 2.02 (s, 1H, nbd bridgehead CH), 2.79 (s, 1H, nbd bridgehead CH), 3.18 (sept, 2H, 3J = 6.9 Hz, CH(CH3)2), 3.25 (sept, 2H, 3J = 6.9 Hz, CH(CH3)2), 5.73 (m, 1H, nbd CHCH), 6.02 (m, 1H, nbd CHCH), 7.10−7.38 (m, 9H, m-C6H3, p-C6H3, m-Dipp and p-Dipp; Dipp = 2,6-iPr2-C6H3). 13C{1H} NMR (C6D6, 125 MHz, 298 K): δ 22.1, 23.1, 26.6, 31.0, 42.0, 43.8, 47.3, 57.6, 123.7, 126.7, 129.0, 129.6, 133.4, 136.6, 137.4, 144.5, 147.1, 180.2. 119Sn{1H} NMR (C6D6, 223.6 MHz, 298 K): δ 1709. IR (CsI, Nujol, selected, cm−1): 790 and 380 (Sn−C stretching and bending). λmax 502 nm (ε = 950 L mol−1 cm−1). AriPr6Sn(norbornenyl) (3b). To a rapidly stirred blue solution of 1b (0.280 g, 0.465 mmol) in dry degassed toluene (ca. 30 mL) was added freshly distilled norbornadiene (1.0 mL, 9.8 mmol). The reaction mixture became dark red, and stirring was continued at ca. 25 °C for 12 h. The solvent was removed under reduced pressure, and the red residue was dissolved in pentane (ca. 60 mL) and filtered using a filter-tipped cannula. The solvent was concentrated under reduced pressure to ca. 2 mL. Red crystals of 3b suitable for X-ray diffraction were grown from a 7 °C refrigerator. Yield 0.103 g (32%). Mp: 145 6602

DOI: 10.1021/jacs.7b02271 J. Am. Chem. Soc. 2017, 139, 6596−6604

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Journal of the American Chemical Society Nujol; selected, cm−1): 705, 580 and 380. λmax 496 nm (ε = 950 L mol−1 cm−1).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b02271. Experimental procedures and spectral data not given in the text (PDF) Crystallographic data for compounds 2a,b−4a,b and 5 (CIF)

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AUTHOR INFORMATION

Corresponding Author

*P.P.P.: fax, +1-530-732-8995; tel, +1-530-752-8900; e-mail, [email protected]. ORCID

Philip P. Power: 0000-0002-6262-3209 Present Addresses †

Department of Chemistry and Biochemistry, University of California San Diego, La Jolla, CA 92093, USA. ‡ Department of Chemistry, University of New Hampshire, Durham, NH 03824, USA. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We wish to acknowledge the U.S. Deparment of Energy (DEFG02-07ER4675) Office of Basic Energy Sciences for support of this work. We also thank Dr. David Liptrot for helpful advice and discussions.

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REFERENCES

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DOI: 10.1021/jacs.7b02271 J. Am. Chem. Soc. 2017, 139, 6596−6604