Steric and Electronic Properties of the Bulky Terphenyl Ligand ArtBu6

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Steric and Electronic Properties of the Bulky Terphenyl Ligand ArtBu6 (ArtBu6 = C6H3‑2,6-(C6H2‑2,4,6‑tBu3)2) and Synthesis of Its Tin Derivatives ArtBu6SnCl, ArtBu6SnSn(H)2ArtBu6, and ArtBu6SnSnArtBu6: A New Route to a Distannyne via Thermolysis of the Asymmetric Hydride ArtBu6SnSn(H)2ArtBu6 Luis G. Perla, Jan M. Kulenkampff, James C. Fettinger, and Philip P. Power* Downloaded via UNIV OF NEW ENGLAND on October 29, 2018 at 19:04:55 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

Department of Chemistry, One Shields Avenue, University of California, Davis, California 95616, United States S Supporting Information *

ABSTRACT: A larger scale, modified synthesis of the bulky terphenyl ligand iodo precursor ArtBu6I (1; ArtBu6 = C6H3-2,6-(C6H2-2,4,6-tBu3)2), featuring six tert-butyl groups on the flanking aryl rings, and the synthesis of its tin derivatives ArtBu6SnCl (2), ArtBu6SnSn(H)2ArtBu6 (3), and ArtBu6SnSnArtBu6 (4) are described. For the key reagent ArtBu6I (1), recently reported by Schrock, Hoyveda, and co-workers, modifications to the synthesis using the easily prepared Grignard reagent EtMgBr to obtain ArtBu6MgX, as well as use of their published trituration and iodination protocols in hexane, allowed isolation of ca. 80 g quantities of ArtBu6I (1). The tin(II) halide derivative 2 and the unsymmetrical hydride 3 were synthesized by salt metathesis and reduction with DIBAL (iBu2AlH), respectively. The distannyne 4 was obtained by a new synthetic route by the dehydrogenation of 3 at 100 °C in toluene. It was also synthesized by reduction of 2 with KC8. All compounds were characterized using single-crystal X-ray diffraction, multinuclear NMR, UV−vis, and IR spectroscopy. The 119Sn NMR and UV−vis spectra of 2−4 reveal absorptions considerably shifted from those of other terphenyl-substituted analogues and along with their structural and reactive behavior show that the steric demands of ArtBu6 are the highest among terphenyl ligands and are sufficiently great to prevent formation of the stannylene Sn(ArtBu6)2.

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INTRODUCTION Sterically encumbering, monodentate, uninegative ligands1 are widely used in the stabilization of highly reactive compounds that have low coordination numbers and/or multiple bonding.1,2 Several ligand types2−19 have been used to isolate a large variety of these reactive species and include alkyls,3 aryls,4 amides,6−8 phosphides,9 alkoxides,10−12 and chalcogenolates.13−15 Currently, however, it is probable that mterphenyl groups (a subclass of the aryls) either as ligand substituents or as directly bound ligands, stabilize the widest range of low-coordinate and multiple-bonded species.2,16−23 For example, the use of terphenyl ligands has enabled the isolation of numerous low-coordinate transition-metal compounds17,19 including the quintuple-bonded AriPr4CrCrAriPr4 (AriPr4 = C6H3-2,6-(C6H3-2,6-iPr2)2)).20 In the main-group elements the terphenyl ligands have enabled the isolation of the first heaver group 13 and 14 element alkyne analogues18,21−24 of the elements gallium, germanium, tin, and lead as well as the first stable divalent group 14 element hydride derivatives.25−27 The heavier group 14 dimetallynes and hydrides have shown unusual reactivity with unsaturated species such as olefins and small molecules such as H2 that often result in cycloadditions or hydrogenations.28−32 The © XXXX American Chemical Society

structures and reactivity of the dimetallynes and hydrides can be altered by varying the steric demand and substitution pattern of the terphenyl ligand being used.30−32 We were attracted to the bulky terphenyl ligand ArtBu6 recently reported by Schrock, Hoyveda, and co-workers33 (Scheme 1) due to its likely high steric demand. This terphenyl group, potentially the largest known, has six tert-butyl substituents on the flanking Scheme 1. Overview of the Synthesis of the Hexa-tert-butyl Terphenyl Iodide ArtBu6I (1)

Received: August 29, 2018

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DOI: 10.1021/acs.organomet.8b00624 Organometallics XXXX, XXX, XXX−XXX

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Organometallics aryl rings (ArtBu6 = C6H3-2,6-(C6H2-2,4,6-tBu3)2). Previous attempts by us to synthesize its key iodo derivative synthon ArtBu6I (1) were unsuccessful due to the formation of ArtBu6H.34 However, the recent report of Schrock, Hoyveda, and their co-workers showed that formation of the arene is avoidable by using a hydrocarbon instead of tetrahydrofuran as a solvent on quenching the generated ArtBu6MgX with iodine to yield the iodide, ArtBu6I.33 Herein, we describe a multigram synthesis of ArtBu6I and, in order to assess the steric properties of the ArtBu6 ligand, we describe the characterization of the tin(II) halide ArtBu6SnCl (2), the asymmetric tin(I/III) hydride Ar t B u6 SnSn(H) 2 Ar t B u 6 (3), and distannyne ArtBu6SnSnArtBu6 (4) derivatives (Scheme 2). Scheme 2. General routes to terphenyl-based tin compounds presented hereina

Figure 1. X-ray structure of 2. Ellipsoids are given at 50% probability; hydrogens are not shown. The tert-butyl groups are illustrated as wireframes for clarity. Selected bond lengths and angles: Sn1−Cl1 2.4266(10) Å, Sn1−Cipso 2.218(3) Å, ∠Cipso−Sn1−Cl1 100.19(3)°.

raphy. The X-ray crystal structure showed that 2 is a monomer with two-coordinate tin in the solid state. The only other monomeric species43 of this type with a terphenyl ligand that is cleanly formed without contamination by either its dimer or adventitious iodide42 is AriPr8SnCl (AriPr8 = C6H-2,6-(C6H22,4,6-iPr3)2-3,5-iPr2).28 The related species AriPr6SnCl (AriPr6 = C6H3-2,6-(C6H2-2,4,6-iPr3)2) was reported as a mixture of monomer and dimer in the solid state. Other organo- or amido-tin(II) halides generally have dimeric structures in the solid state, as a consequence of the ease of chloride bridging and their generally lower steric crowding.41−43 The Cl−Sn− Cipso angle is 100.19(3)° in 2, which is similar to the values of 102.31(15)° in AriPr6SnI28 and 98.77(9)° in AriPr8SnClAriPr8 28 and 99.67(6)° in AriPr6SnCl.43 The Sn−Cl and Sn−Cipso distances in 2 are 2.4266(11) and 2.218(3) Å, respectively, which are lengthened in comparison to those in the monomeric AriPr8SnCl, which has Sn−Cl and Sn−Cipso lengths of 2.371(3) and 2.176(6) Å, respectively,28 or in monomeric AriPr6SnCl with bond lengths of 2.4088(8) and 2.180(2) Å, respectively.43 We surmise that the lengthened Sn−C and Sn− Cl bonds in 2 are a consequence of the increased electron density on the central part of the ligand (due to the greater electron donating character of the tert-butyl groups) which, in turn, moves electron density onto the tin atom and decreases its electropositive character and hence the ionic contribution to the strength of the Sn−C and Sn−Cl bonds. The UV−vis spectrum for 2 features an absorption at 385 nm that can be assigned to a transition arising from a tin nonbonding pair to a 5p orbital. This value is at a shorter wavelength (higher energy) than the 416 and 395 nm values observed for AriPr8SnCl and AriPr6SnCl, which are similar to those of other reported terphenyl tin(II) halides.28 It is premature to draw any inference from the shorter wavelength (higher energy) of this absorption owing to its dependence on the relative energies of the ground and excited states. However, the 119Sn NMR spectrum of 2 afforded a single resonance at 922 ppm which is significantly further downfield than the values reported for AriPr6SnCl (793 ppm) and AriPr8SnCl (751 ppm).28 The downfield shift seems counterintuitive, given the greater electron releasing character of the ArtBu6 ligand

Reaction conditions: (i) KC8, Et2O, −78 °C; (ii) DIBAL, Et2O, −78 °C; (iii) toluene, 100 °C, 4 days. a

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RESULTS AND DISCUSSION Synthesis of ArtBu6I (1). Earlier, we had attempted to synthesize the sterically encumbering terphenyl, hypothesizing35 that during the final step of the synthesis the generated aryl-Grignard XMgArtBu6 could decompose, resulting in Mg−C bond dissociation followed by hydrogen atom abstraction from the ethereal solvent. However, Schrock, Hoyveda, and coworkers33 reported the successful synthesis of 1, wherein they hypothesized that the penultimate step is dictated by radical chemistry in the reaction between iodine and the solvent;35 they exchanged the ethereal solvent for hexane, thereby avoiding the formation of ArtBu6H, and succeeded in the isolation of 1 (Scheme 1). We modified this synthetic procedure to use the easily prepared EtMgBr instead of Me3SiCH2MgCl to generate the 1,3-dichloroaryl Grignard reagent. In the final step, following the protocol of Schrock and Hoyveda, we removed the ethereal solvents and triturated with hydrocarbon followed by a final addition of hydrocarbon with subsequent quenching with iodine. We report that 1 can be synthesized in multigram (ca. 80 g) quantities similar to the amounts of 2,6-terphenyls previously reported by this group and by several others.36−40 With a moderate-scale, relatively efficient route to 1, we used this ligand for the synthesis of several derivatives of low-valent tin in order to test its applicability and steric properties (Scheme 2). Synthesis, Structure, and Spectroscopy of ArtBu6SnCl (2). The synthesis of ArtBu6SnCl (2) was carried out in a way similar to those previously reported for terphenyl tin(II) halides. 28 ,41,42 One equivalent of the lithium salt LiArtBu6(Et2O)2,33 was dissolved in Et2O and added to a suspension of SnCl2 in Et2O cooled in an ice bath. After filtration and extraction with hexane, followed by concentration to incipient crystallization, storage at ca. −20 °C resulted in the isolation of 2 (Figure 1) as bright orange crystals that were suitable for single-crystal X-ray crystallogB

DOI: 10.1021/acs.organomet.8b00624 Organometallics XXXX, XXX, XXX−XXX

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Figure 2. (left) Side-on view of the X-ray structure of 3. (right) End-on view of 3 illustrating the C1−Sn1−Sn2−C2 torsion angle. Ellipsoids are given at 50% probability with hydrogens not shown and tert-butyl groups shown as wireframes for clarity. Selected bond lengths and angles: Sn1− Sn2 2.8547(7) Å, Sn1−C1 2.209(4) Å, Sn2−C2 2.197(4) Å, ∠C1−Sn1−Sn2 105.07(10)°, ∠C2−Sn2−Sn1 117.25(11)°, ∠C1−Sn1−Sn2−C2 160.0(2)°.

central aryl rings lie almost perpendicularly to each other, as also observed for AriPr8SnSn(H)2AriPr8.26 The similarity in the structures of AriPr8SnSn(H)2AriPr8 and 3 clearly underline the very large size of the ArtBu6 substituent. The 1H NMR spectrum of 3 in d8-toluene at −10 °C exhibited a broad singlet at 7.06 ppm corresponding to the tin hydride resonance and is similar to that reported in AriPr8SnSn(H)2AriPr8.26 When the temperature was lowered, multiple ArtBu6 resonances were observed due to the restricted rotation of the aryl rings. Interestingly, no 119Sn NMR signal was observed at room temperature; however, upon cooling to −70 °C a single broad resonance was observed upfield at −94 ppm. This was assigned to the tetravalent Sn(III) atom. No downfield signal for the remaining two-coordinate Sn atom was observed. The absence of an observable signal for the divalent Sn in the solution 119Sn NMR spectrum resembles the behavior of AriPr8SnSn(H)2AriPr8 26 and is presumably due to the chemical shift anisotropy at Sn1. However, the Sn(III) signal is observed at a location that is almost 130 ppm upfield of the corresponding signal for AriPr8SnSn(H)2AriPr8. It is likely that the upfield shift is a result of the enhanced electron-releasing character of the ArtBu6 group generated by the tert-butyl substituents (for tetravalent tin the ground and excited states are much farther apart than in divalent tin and changes in paramagnetic shielding are less important). The UV−vis spectrum of 3 exhibited a broad absorption at 541 nm associated with a tin n → p transition consistent with the retention of the asymmetric structure in solution. This is in contrast to AriPr8SnSn(H)2AriPr8, which displays three bands at 422, 480, and 622 nm, suggesting the existence of more than one species in solution. It seems likely that the solution structure of 3 is stabilized by the larger size of the substituent.26 The IR spectrum of 3 contained two different bands at 1850 and 1740 cm−1 corresponding to asymmetric and symmetric stretches of the Sn−H moieties. These bands may be compared to those of AriPr8SnSn(H)2AriPr8 (1810 and 1783 cm−1)26 and AriPr6SnSn(H)2AriPr6 (1828 and 1771 cm−1).25 Synthesis and Spectroscopy of ArtBu6SnSnArtBu6 (4). The synthesis of the distannyne ArtBu6SnSnArtBu6 (4) was achieved by two different routes from either 2 or 3. Initially, 4 was isolated by heating a toluene solution of 3 at 100 °C for 4 days. The intention was to generate a tin cluster species in the same manner as for Sn7(AriPr4)2, which was obtained by

which should increase electron density at tin, thereby affording greater shielding and hence an upfield shift of the 119Sn NMR signal. However, the greater electron releasing character of the ArtBu6 ligand also increases the energy of the nonbonded pair, which may increase the mixing of the ground and excited states of 2. This increased mixing increases the paramagnetic shielding, and since paramagnetic effects augment the applied field, this yields a downfield movement of the signal.44 The synthesis of the bis(terphenyl) stannylene Sn(ArtBu6)2 was attempted by the reaction of 2 equiv of (Et2O)2LiArtBu6 with SnCl2 in diethyl ether. However, the only products isolated from the reaction, even after 2 days of reflux, were 2 and unreacted (Et2O)2LiArtBu6. It seems probable that the very high steric demand of the ArtBu6 ligand does not permit coordination of a second such ligand to the tin atom. Currently, Sn(AriPr6)2 remains the most sterically encumbered terphenyl stannylene to have been reported.45 Synthesis, Structure, and Spectroscopy of ArtBu6SnSn(H)2ArtBu6 (3). The treatment of 2 with 1 equiv of DIBAL at −78 °C in diethyl ether resulted in the isolation of ArtBu6SnSn(H)2ArtBu6 (3) as dark blue plates from a violet hexane solution in 25% yield. The X-ray crystal structure of 3 (Figure 2) revealed an unsymmetrical Sn−Sn bonded structure in which one of the tins (Sn2) carries two hydrogens. This represents the second example of an X-ray crystal structure of an unsymmetrical Sn(II) hydride, the first being AriPr8SnSn(H)2AriPr8.26 In common with the latter, the tin hydrogen atoms in 3 were not located directly in the X-ray crystal structure, although their presence was apparent from the structural configuration at each tin. Their presence was also established by multinuclear NMR and IR spectroscopy (the tin hydrogens in AriPr8SnSn(H)2AriPr8 were later located by X-ray crystallography in a redetermination of the structure).46 The observation of the unsymmetrical structure is consistent with calculations which show that increasing the steric demand of the organic substituent favors the unsymmetrical over the symmetrical, doubly hydrogen bridged isomer26 and also underlines the very large size of the ArtBu6 ligand. The Sn−Sn distance in 3 (2.8547(7) Å) is shorter than that in AriPr8SnSn(H)2AriPr8 (2.9157(10) Å),26 and the Sn−Cipso distances of 2.209(4) Å (Sn1) and 2.197(4) Å (Sn2) are marginally shorter than those in Ar iPr8 SnSn(H) 2 Ar iPr8 (2.247(6) and 2.228(5) Å).26 The torsion angle for the C1− Sn1−Sn2−C2 array is 160.0(2)° (Figure 2, right), and the C

DOI: 10.1021/acs.organomet.8b00624 Organometallics XXXX, XXX, XXX−XXX

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(2.193(1)−2.200(8) Å).28 However, the Sn−Sn−Cipso bending angles 119.10(13) and 119.81(14)° are narrower than in AriPr4SnSnAriPr4 (125.24(7)°) and AriPr8SnSnAriPr8 (125.1(2)− 127.6(2)°). 28 The C1−Sn1−Sn2−C2 torsion angle is −160.8(2)°, with the two aryl groups facing each other in a near-perpendicular fashion. The bending angles between the central aryl ring and the flanking rings, labeled as C(A)−C(B) and C(B)−C(E) (Figure 3), are 177.3(4), 176.3(4), 170.9(4), and 167.1(4)° for all four flanking rings, indicating that some of the flanking rings are tilted toward each other, a possible consequence of London dispersion force interactions between the tBu groups. The smallest of these angles are associated with a short H−H contact between the flanking aryl rings of ca. 2.319 Å. Two absorbances are observed in the UV−vis spectrum at 392 and 536 nm; these are similar to the patterns seen in the distannynes Ar i P r 4 SnSnAr i P r 4 (410 and 597 nm), AriPr6SnSnAriPr6 (409 and 593 nm), and AriPr8SnSnAriPr8 (424 and 612 nm)28 and are indicative of multiple Sn−Sn bonding in solution.23,28 However, these absorptions appear at shorter wavelengths than for other multiple-bonded distannynes. As is the case for other distannynes, no solution 119Sn NMR resonances were observed for 4, most likely due to the anisotropy of the shift tensor.

heating a solution of the symmetric tin(II) hydride [AriPr4Sn(H)]2.47 Heating a solution of 3 led to a color change from deep blue to red. Setting the solution aside for 2 days at room temperature resulted in the isolation of amber crystals of 4 as well as some tin metal precipitate. Further heating of the solution produced more tin metal but not the cluster formation seen for the decomposition of [AriPr4Sn(H)]2.47 1H NMR spectroscopy of the crude reaction mixture did indicate multiple ArtBu6 resonances; thus, it is likely that other tin species are generated by the thermolysis. Further investigations are currently underway to establish their identity. This synthetic route represents the first isolation of a distannyne from a low-oxidation-state tin(II) hydride. We recently showed that the hydride [AriPr4Sn(H)]2 can exist in a reversible equilibrium with the distannyne AriPr4SnSnAriPr4 and H2 in toluene at 80 °C, but when the solution is cooled, the hydride [AriPr4Sn(H)]2 is isolated and not the distannyne because the equilibrium greatly favors the hydride.48 Distannyne 4 can also be prepared by the reduction of 2 in an ethereal solvent with an equimolar amount of freshly prepared KC8, which led to a color change from orange to deep green. Concentration of the solution and storage at −20 °C led to the isolation of 4 as amber crystals. The X-ray crystal structure of 4 (Figure 3)

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CONCLUSIONS The described ArtBu6 derivatives of lower oxidation state tin confirm that the ArtBu6 ligand is probably the most sterically encumbering terphenyl ligand described to date.34 The data indicate that it is at least as crowding as the AriPr8 terphenyl ligand. The latter, in addition to its two flanking C6H2-i-Pr3 rings, carries two i-Pr substituents on the central aryl ring at the meta positions which greatly augment its steric encumbrance by “pushing” the flanking rings toward the reactive (e.g., tin) center. The ligand ArtBu6 features no such substituents on the central ring, and the large steric demand of the flanking −C6H2-2,4,6-tBu3 rings is sufficient to produce a clean monomeric structure for the terphenyl tin(II) halide 2 and an asymmetric structure for tin hydride 3. Furthermore, the distannnyne 4 features the longest multiple Sn−Sn bond among multiple-bonded distannynes. In addition, the steric clash does not allow the coordination of two ArtBu6 groups to the same tin atom in a putative Sn(ArtBu6)2 molecule. The isolation of both 3 and 4 provide a useful starting point to compare the reactivity of larger sterically encumbered terphenyl tin compounds with small molecules.

Figure 3. X-ray structure of 4. Ellipsoids are given at 50% probability with hydrogens not shown and tert-butyl groups shown as wireframes for clarity. Selected bond lengths and angles: Sn1−Sn2 2.7621(6) Å, Sn1−C1 2.184(5) Å, Sn2−C2 2.183(5) Å, ∠C1−Sn1−Sn2 119.10(13)°, ∠C2−Sn2−Sn1 119.81(14)°, ∠C1−Sn1−Sn2−C2 −160.8(2)°.

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EXPERIMENTAL SECTION

General Procedures. All manipulations were carried out under air- and moisture-free conditions using modified Schlenk line or glovebox techniques under a nitrogen atmosphere. All solvents were dried over alumina columns and then stored under an alkali-metal mirror in sealed ampules and freeze−pump−thawed three times prior to use. ArtBu6I (ArtBu6 = C6H3-2,6-(C6H2-2,4,6-tBu3)2) was synthesized using a modification of an originally reported procedure.33 2,4,6-Tritert-butylbenzene (Mes*H),52 1-bromo-2,4,6-tritert-butylbenzene (Mes*Br),53 2,6-dichloro-1-iodobenzene,54 and LiArtBu6 33 were synthesized as previously described. 1H and 13C NMR spectra were collected on a Bruker 400 MHz instrument and referenced internally to solvent residual peaks of CDCl3, C6D6 ,or C7D8. 119Sn NMR spectra were collected on a Bruker 400 MHz instrument and referenced to SnMe4 in C6D6. Infrared spectroscopy was carried out on Nujol mulls using a PerkinElmer 1430 Ratio Recording infrared spectrophotometer. UV−visible spectroscopy was conducted using

revealed a trans-bent planar C(ipso)SnSnC(ipso) core with a Sn−Sn bond length of 2.7621(6) Å, which is the longest Sn− Sn distance observed in a multiple-bonded distannyne. Longer Sn−Sn bonds (ca. 2.97−3.08 Å)28,49,50 have been observed in single-bonded distannyne isomers which have strongly bent structures with Sn−Sn−C bending and angles that are usually