The Reactions of Aryl Tin(II) Hydrides {AriPr6Sn(μ-H)}2 (AriPr6

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The Reactions of Aryl Tin(II) Hydrides {AriPr6Sn(μ-H)}2 (AriPr6 = C6H3‑2,6-(C6H2‑2,4,6‑iPr3)2) and {AriPr4Sn(μ-H)}2 (AriPr4 = C6H3‑2,6(C6H3‑2,6‑iPr2)2) with Aryl Alkynes: Substituent Dependent Structural Isomers Madison L. McCrea-Hendrick, Shuai Wang, Kelly L. Gullett, James C. Fettinger, and Philip P. Power* Department of Chemistry, The University of California Davis, 1 Shields Avenue, Davis, California United States S Supporting Information *

ABSTRACT: The reactions of the aryl tin(II) hydrides {AriPr6Sn(μ-H)}2 (AriPr6 = C6H3-2,6-(C6H2-2,4,6-iPr3)2) and {AriPr4Sn(μ-H)}2 (AriPr4 = C6H3-2,6-(C6H3-2,6-iPr2)2) with aryl alkynes were investigated. Reaction of {AriPr6Sn(μ-H)}2 and {AriPr4Sn(μ-H)}2 with 2 equiv of diphenyl acetylene, PhCCPh, afforded the aryl alkenyl stannylenes AriPr6SnC(Ph)C(H)Ph (1) and AriPr4SnC(Ph)C(H)Ph (2). In contrast, the analogous reactions of {AriPr6Sn(μ-H)}2 with 2 equiv of phenyl acetylene, HCCPh, afforded a high yield of the cis-1,2 addition product AriPr6(H)SnC(H)C(Ph)Sn(H)AriPr6 (3), which has a four-membered Sn2C2 core structure comprised of two Sn−Sn bonded Sn(H)AriPr6 units bridged by a −C(H)C(Ph)− moiety. The corresponding reaction of the less bulky hydride {AriPr4Sn(μ-H)}2 with 2 equiv of phenyl acetylene leads to AriPr4SnC(H)C(Ph)Sn(H)2AriPr4 (4) which unlike 3 has no Sn−Sn bonding. Instead, the tin atoms are connected solely by a −C(H)C(Ph)− moiety. Each tin atom carries a AriPr4 substituent but one is also substituted by two hydrogens. The difference in behavior between PhCCPh and HCCPh is attributed mainly to the difference in steric bulk of the two substrates. The different products 3 and 4 are probably a consequence of the difference in size and dispersion force interactions of the AriPr6 and AriPr4 substituents. Compounds 1−4 were characterized by 1H, 13C, and 119Sn NMR, UV−vis, and IR spectroscopy and structurally by X-ray crystallography.

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INTRODUCTION Organotin hydrides are widely used as reducing agents in organic syntheses.1−4 These hydrides generally involve tetravalent tin having a formally saturated valence shell with the result that activation of the organometallic species often requires elevated temperatures and radical initiators such as azoisobutyrlnitrile (AIBN).5 A transition metal catalyst5,6 can mediate the hydrostannylation of unactivated substrates, as can main group Lewis acids such as triethylborane7 or tris(pentafluorophenyl)borane8,9 The use of homogeneous sonochemical hydrostannylation of unsaturated alkenes and alkynes allows reaction temperatures as low as −60 °C to be used.10 Scheme 1 summarizes some of the known reactions of tin(IV) hydrides. In contrast, the use of divalent tin(II) hydrides in hydrostannylation reactions is less widespread but is attracting increasing attention since they were reported in 2000.11 Several groups have studied the reactions of ligated tin(II) hydrides with a variety of unactivated substrates.12−16 Among these are the alkynes, and Jones and co-workers have shown that the stable amido tin(II) hydride, (Ar)(SiPh3)NSnH (Ar = C6H2{C(H)Ph2}2Pri-2,6,4), hydrostannylates PhCCMe to yield a cis-hydrostannylated olefin product of formula Sn(L†)C(Ph)C(Me) (L† = −N(Sii-Pri3)C6H2{C(H)Ph2}2-iPr2,6,4).12 Roesky and co-workers reported that the tin(II) © XXXX American Chemical Society

hydride [{HC(CMeNAr)2}SnH], (Ar = C6H3-iPr2), reacted with a series of unsaturated molecules that included olefins, ketones, aldehydes and alkynes (Scheme 2).13−16 In all cases, cis-hydroelementation of the alkynes occurred under mild conditions without use of a transition metal or radical catalyst. We reported the reactivity of the original organotin(II) hydrides {AriPr6Sn(μ-H)}211 and {AriPr4Sn(μ-H)}218 toward acyclic and bicyclic olefins as well as their solution dynamics.19,20 These reactions revealed an exclusive cis-addition of the Sn−H bond across olefins during hydrostannylation. However, variable-temperature (VT) NMR studies indicated that the solution structures of the products of hydrostannylation of acyclic olefins are temperature dependent. The hydrostannylation of ethylene by {AriPr6 Sn(μ-H)}2 afforded two different isomers based on the temperature at which the product was crystallized.19 At 25 °C, the symmetric, formally double-bonded distannene {Sn(AriPr6)Et}2 insertion product was crystallized as a red solid and is the major isomer in solution. However, when the product was crystallized at ca. −20 °C the asymmetric Sn(I)/Sn(III) stannylstannylene, AriPr6SnSn(Et)2AriPr6, was isolated as green crystals, and this Received: July 26, 2017

A

DOI: 10.1021/acs.organomet.7b00570 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Scheme 1. Summary of Some Reactions of Tin(IV) Hydrides with Alkynes5,7−10

Scheme 2. Hydrostannylation Using Jones’12 Amido Tin(II) Hydride, Roesky’s17 Tin(II) Hydride, and [{HC(CMeNAr)2}SnH]

yielded monomeric cis-addition aryl/alkenyl stannylene products exclusively (Scheme 3). In sharp contrast reactions of the two tin hydrides with phenyl acetylene yielded either the cisaddition product AriPr6(H)SnC(H)C(Ph)Sn(H)AriPr6 (3) or the unsymmetric Sn(II)/Sn(IV) species AriPr4SnC(H)CPhSn(H)2AriPr4, 4 (Scheme 4). Scheme 4. Addition of Phenyl Acetylene to Aryl Tin(II) Hydrides

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species exists as the major isomer in solution at low temperatures. The hydrostannylation of norbornadiene afforded a product in which the norbornenyl moiety had rearranged to a nortricyclene unit.20 Moreover, experiments indicated that the rearrangement proceeds through a metal hydride intermediate. We now report the reactions of {AriPr6Sn(μ-H)}2 and {AriPr4Sn(μ-H)}2 with diphenyl acetylene and phenyl acetylene. The reactions with diphenyl acetylene

RESULTS Tin(II) hydrides {AriPr6Sn(μ-H)}2 and {AriPr4Sn(μ-H)}2 reacted with 2 equiv of diphenyl acetylene via a cishydrostannylation route to form monomeric tin(II) aryl/ alkenyls ArSn{C(Ph)C(H)Ph} (Ar = AriPr6, AriPr4) as red solids in 64% (1) and 79% (2) yields (Scheme 3). The X-ray crystal

Scheme 3. cis-Addition of Aryl Tin(II) Hydrides to Diphenyl Acetylene

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DOI: 10.1021/acs.organomet.7b00570 Organometallics XXXX, XXX, XXX−XXX

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Figure 1. Thermal ellipsoid (30%) plots for AriPr6Sn{C(Ph)C(H)Ph} (1, left) and AriPr4Sn{C(Ph)C(H)Ph} (2, right). Hydrogen atoms are not shown. Orange: Sn, gray: C, and green: olefinic H. Selected bond lengths (Å) and angles (deg): 1: C1−Sn1 2.2061(18); Sn1−C37 2.2020(17); C37−C38 1.353(3); C1−Sn−C37 98.15(7); 2: C1−Sn1 2.1994(17); Sn1−C37 2.2021(18); C31−C32 1.351(3); C1−Sn1−C31 98.63(7).

structures of compounds 1 (Ar = AriPr6) and 2 (Ar = AriPr4) are shown in Figure 1. The data for 1 show that the Cipso−Sn−C angle is 98.15(6)°, with a very similar Cipso−Sn−C angle of 98.63(7)° being observed for 2. The Cipso−Sn bond lengths in 1 and 2 span the narrow range of 2.1994(17)−2.2054(16) Å and are similar to those previously reported by this group and by others (2.168(8)−2.204(8) Å)11,18−31 as well as being close to the sum (2.17 Å) of the single bond radii of carbon (0.77 Å) and tin (1.40 Å).32 The Cethenyl−Sn bond lengths are 2.2020(17) Å in 1 and 2.2061(18) Å in 2. These values are slightly shorter than the Cethenyl−Sn range, 2.241−2.330 Å, reported by Jones and Roesky.12−16 The Cethenyl−Cethene distances in (1) and (2) are 1.353(3) Å and 1.351(3) Å, consistent with double bonding and lie in the range of other reported alkenes bound to tin(II).33−36 The 119Sn{1H} NMR spectra display single downfield resonances at 1573.9 ppm (1) and 1601.2 ppm (2), indicating tin atoms that are low coordinate. The 119Sn{1H} NMR chemical shifts are in agreement with those of other two-coordinate diorgano tin and related species that range between ca. 1200 and 2600 ppm.21−23,37−39 The UV−vis spectra feature one absorption at 507 nm (300 M−1 cm−1) for 1 and 509 nm (1086 M−1 cm−1) for 2, consistent with an n → p transition of a divalent organotin species.19,20 The reaction of 1 equiv of diphenyl acetylene with {AriPr6Sn(μ-H)}2 yielded only 1 and unreacted {AriPr6Sn(μ-H)}2 as determined by spectroscopy, and the reaction with an excess of diphenyl acetylene also yielded 1 as the only tin containing ethenyl product. {AriPr6Sn(μ-H)}2 and {AriPr4Sn(μ-H)}2 reacted with only 1 equiv of phenyl acetylene (i.e., one alkyne for two tins) even in the presence of a large excess of phenyl acetylene (Scheme 4). This reaction produced two different products dependent on the type of terphenyl substituent (AriPr6 versus AriPr4) used. For the slightly larger tin substituent, AriPr6, a pale red solution was obtained and crystallization from pentane in a ca. −18 °C freezer afforded an almost colorless solution that yielded colorless crystals suitable for X-ray diffraction studies. Attempts to isolate a product from the red solution at room temperature as well as by using different solvents (Et2O, PhMe, hexanes, and C6H6) proved unsuccessful. Heating a C6D6 solution of 3 to 110 °C for 6 h showed no change in the 1H NMR spectrum, indicating a thermodynamically stable product. The X-ray crystal structure of 3 is shown in Figure 2 and establishes the

Figure 2. Structure of 3. Thermal ellipsoids are at 30% probability. AriPr6(H)Sn(CHCPh)Sn(H)AriPr6 (3) Hydrogen atoms, except those attached to the tins or C(73), are not shown. Selected bond lengths (Å) and angles (deg): Sn1−Sn2 2.7791(3), C1−Sn1 2.172(2), C37− Sn2 2.170(2), C73−Sn1 2.158(2), C74−Sn2 2.185(2), C73−C74 1.341(3), Sn1−H1 1.64(3), Sn2−H2 1.64(3), C1−Sn1−H1 104.7(12), Sn2−Sn1−H1 115.2(14), Sn1−Sn2−H2 110.7(10), C1− Sn1−C73 116.06(9), C37−Sn2−C74 116.87(8).

formula AriPr6(H)Sn(CHCPh)Sn(H)AriPr6 (3). The core of the molecule is comprised of a quadrilateral of two carbon and two tin atoms that form an almost planar array with a torsion angle of 4.00(12)° for the Sn1−C74−C73−Sn2 units. The tin−tin bond length is 2.7791(3) Å, is consistent with single bonding,32 and is in the range observed for the few known distannacyclobut-3-enes, 2.7444−2.9004 Å.33,34,36 The Sn− C(aryl) bond lengths are 2.172(2) and 2.170(2) Å. The tin− carbon distances within the Sn2C2 ring are 2.158(2) and 2.185(2) Å. The C−C bond length in the core is 1.341(3) Å and is consistent with double bonding.32 The tin atoms are each four-coordinate and are bound to an AriPr6 group, an ethenyl carbon, a partner tin, and to a hydrogen atom. The proton-decoupled 119Sn{1H} NMR spectrum of 3 displays two upfield resonances at −148.6 (Sn−C(H)) and −218.0 ppm (Sn−C(Ph)), both of which lie within the range for four-coordinate organotin complexes.37 The different shifts are due to the different chemical environments of the tin atoms wherein the more crowded Sn(2) lies further upfield. The C

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stannylenes.19,20 The four-coordinate tin atom has Sn−C bond lengths of 2.166(3) Å (terphenyl) and 2.172(3) Å (ethenyl) that are similar to those observed in 3. The 119Sn{1H} NMR spectrum displays two resonances; one at −324.0 ppm associated with four-coordinate tin, and a second resonance at 1349.2 ppm assignable to the twocoordinate tin.37 In the proton-coupled 119Sn NMR spectrum the resonance at 1349.2 ppm appears as a singlet, indicating an absence of coupling to the olefinic C−H. The resonance at −324.0 ppm appears as a doublet of triplets and is coupled to the olefinic C−H with 3JSn−H value of 255 Hz and 1JSn−H of 1870 Hz. Wesemann and co-workers also observed coupling of the 119Sn nuclei to the olefinic C−H with JSn−CH(ethenyl) values ranging from 126−240 Hz.36 The 1H NMR spectrum of 4 displays a singlet resonance at 8.6 ppm assigned to the olefinic C−H proton with two tin satellites with couplings of 132 (Sn(2)) and 606 Hz (Sn(1)). The UV−vis spectrum of 4 displays one absorption at 501 nm (540 M−1 cm−1) consistent with an n → p transition for the divalent tin. Similar λmax values have been observed for aryl tin(II) alkyl complexes, including 1 and 2, and ranging between 490 and 510 nm.19,20,40 Mechanism. In benzene solution {AriPr4Sn(μ-H)2}2 exists mainly as the symmetric bridged hydride form (90%) and ca. 10% as the asymmetric Sn(I)/Sn(III) dimer by 1H NMR spectroscopy.25 In contrast, NMR studies of {AriPr6Sn(μ-H)2}2, in solution indicates that it exists mainly (ca. 90%) as the Sn(I)/Sn(III) asymmetric species and 10% as the symmetrically bridged hydride dimer.25 Thus, to varying extents three forms (including a monohydrido bridged species also identified recently as a structure,19 Scheme 5, center) probably exist simultaneously in solution equilibrium.

proton-coupled 119Sn NMR spectrum reveals two separate resonance patterns as a double doublet of doublets centered at −148.6 ppm as well as a doublet of triplets centered at −218.0 ppm. The resonance at −148.6 ppm is assigned to Sn(1) (cf. Figure 2) which displays a 1JSn−H of 1594 Hz and further hydrogen couplings, 2JSn−H(C) and 2JSn−H(Sn), of 97 and 369 Hz, respectively. The resonance at −218.0 ppm is assigned to Sn(2) and features a 1JSn−H coupling of 1788 Hz, and 2JSn−H and 3 JSn−C(H) values of 134 Hz. The resonance centered at −148.6 ppm in the proton-decoupled 119Sn{1H} NMR spectrum displays satellites that arise from the coupling of the two different tin isotopomers, 119Sn and 117Sn, to their partner tin to which they are bonded and feature couplings of 4763 and 4112 Hz, respectively. The resonance centered at −218.0 ppm displays very similar coupling with values of 4763 Hz J119Sn−119Sn and 4110 Hz J119Sn−117Sn. The values of these coupling constants are in the same range as those reported by Wesemann for distanna-cyclo-but-3-enes.36 The 1H NMR spectrum displays two sets of Sn−H resonances. One, at 6.15 ppm. has a doublet of doublet pattern and features JSn−H coupling of 129 Hz. The second Sn−H resonance appears at 6.52 ppm as a doublet with JSn−H coupling of 135 Hz. It is noteworthy that the ethenyl hydrogen atom (C73, 5.77 ppm) also displays coupling to the tin of 96 Hz. The FT-IR spectrum features one absorption for the Sn−H stretching band at 1845 cm−1. The reaction of {AriPr4Sn(μ-H)}2 with phenyl acetylene yielded red crystals of formula AriPr4Sn(CHCPh)Sn(H)2AriPr4 (4, Figure 3). In this structure, there are two tin atoms

Scheme 5. Schematic Drawing of the Structures of Aryl Tin(II) Hydrides in Solution

The doubly bridged dimeric structure is likely to be the least reactive species because of the lack of an open coordination site at tin where the bridging hydrides occupy and hinder access to the tin atom’s p-orbitals. Reactivity based on the monohydrido bridged tin isomer19 (Scheme 6, upper) might be similar to that reported by Organ8 and co-workers wherein the alkyne coordinates to a low-coordinate tin atom to form an alkenyl cation whereupon hydride atom transfer occurs from the second tin atom to carbon. Subsequent Sn−Sn bond cleavage forms products 1 and 2 as well 0.5 equiv of (ArSnH)2. Products 3 and 4 could also be formed by a slight modification of this route. Instead of hydride transfer from tin to carbon, the tin−tin bond is cleaved and a new Sn−C bond is formed (Scheme 6, lower). Product 3 may form from an initial species with a structure analogous to 4 as shown in Scheme 7. In this case, a hydrogen is transferred from the tin(IV) atom to the unoccupied p-orbital on tin(II), to form a zwitterionic species (Scheme 7). The nonbonding electrons of the negatively charged tin atom coordinate to the positively charged tin atom forming a Sn−Sn bond in which both tin atoms are in an oxidation state of +3 (Scheme 7). The AriPr6 ligand would be in a trans configuration with respect to each tin atom in order to minimize the steric

Figure 3. Thermal ellipsoid plot of 4 at 30% probability. Only the hydrogen atoms attached to Sn(2) and the olefinic carbon C(61) are shown (both in green). Selected bond lengths (Å) and angles (deg): C1−Sn1 2.243(3), C61−Sn1 2.198(3), C31−Sn2 2.166(3), C62−Sn2 2.172(3), C61−C62 1.342(3), C1−Sn1−C61 95.76(11), C31−Sn2− C62 119.22(10), Sn2−H1 1.59(3), Sn2−H2 1.59(2).

connected by an ethenyl group. One of the tin atoms is twocoordinate with an oxidation state of +2 and is bound to the terphenyl AriPr4 substituent and a carbon from the ethenyl group. The other tin atom is four-coordinate with an oxidation state +4 and is bound to two hydrogen atoms in addition to the ethenyl and AriPr4 groups. The separation between the two tin atoms is 3.7945(4) Å which is ca. 1 Å greater than the sum of the covalent radii of tin (2.80 Å), indicating the absence of Sn− Sn bonding.32 The Sn−C bond lengths involving the twocoordinate tin are 2.243(3) Å (terphenyl) and 2.198(3) Å (ethenyl) with a relatively narrow C−Sn−C interligand angle of 95.76(11)°, resemble those observed in similarly substituted D

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Scheme 6. Possible Mechanisms for the Reaction of Alkynes with Monohydrido Bridging Aryl (Ar = AriPr6 or AriPr4) Tin(II) Hydride Dimers

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Scheme 7. Possible Formation Routes for Product 3

EXPERIMENTAL SECTION

General Considerations. All manipulations were performed under anaerobic and anhydrous conditions using Schlenk techniques and a vacuum atmospheres drybox. {AriPr6Sn(μ-H)}211 and {AriPr4Sn(μ-H)}218 were prepared according to literature procedures. Solvents were dried and stored over potassium. Physical measurements were carried out under anaerobic and anhydrous conditions. 1H, 13C{1H}, 119 Sn, and 119Sn{1H} NMR spectra were collected on a Varian spectrometer and referenced to known standards. IR spectra were recorded as Nujol mulls between CsI plates on a PerkinElmer 1430 spectrometer. UV−visible spectra were recorded as dilute hexane solutions in 3.5 mL quartz cuvettes using an Olis 17 Modernized Cary 14 UV/vis/NIR spectrophotometer. Melting points were determined on a Meltemp II apparatus using glass capillaries sealed with vacuum grease and are uncorrected. AriPr6Sn{C(Ph)C(H)Ph} (1). To a mixture of {AriPr6Sn(μ-H)}2 (0.510 g, 0.424 mmol) and diphenyl acetylene (0.151 g, 0.847 mmol) was added toluene (ca. 30 mL). The solution, initially turquoise, gradually became red and was stirred for 12 h and then filtered using a filter tipped cannula. The solvent was concentrated under reduced pressure to ca. 5 mL. Storage of the solution in a ca. 7 °C refrigerator afforded red crystals of 1 suitable for X-ray diffraction. Yield: 0.211 g (64%), mp: 147 °C 1H NMR (600 MHz, C6D6, 298 K, δ) 1.12 (d, J = 6 Hz, 12H, CH(CH3)2), 1.25 (d, J = 6 Hz, 12H, CH(CH3)2), 1.34 (d, J = 6 Hz, 12H, CH(CH3)2), 2.74 (sept, J = 6 Hz, 2H, CH(CH3)2), 3.42 (sept, J = 6 Hz, 4H, CH(CH3)2), 6.68 (d, J = 6 Hz, 1H, ArH), 6.88 (m, 1H, ArH), 6.89 (t, J = 6 Hz, ArH), 7.04 (t, J = 6 Hz, 2H, ArH), 7.08 (d, J = 6 Hz, 2H, ArH), 7.18 (s, 4H, ArH), 7.36 (t, J = 6 Hz, 1H, ArH), 7.45 (d, J = 6 Hz, 2H, ArH), 7.62 (s, 1H, ArSn{PhCC(H)Ph}). 13C{1H} NMR (150 MHz, C6D6, 298 K, δ) 23.01, 23.76, 26.72, 31.13, 34.31, 121.47, 124.74, 125.63, 126.84, 127.16, 127.87, 128.53, 130.29, 130.40, 134.45, 135.11, 138.91, 145.09, 146.89, 147.0, 149.02, 181.47, 192.76. 119Sn{1H} (223.6 MHz, C6D6 298 K, δ) 1573.9 ppm. λmax (ε in mol−1 L cm−1): 507 (300) FT-IR (CsI, Nujol: Selected): 735 and 375 (Sn−C) AriPr4Sn{C(Ph)C(H)Ph} (2). To a green solution of {AriPr4Sn(μH)}2 (0.215 g, 0.207 mmol) in toluene (30 mL) was added 2 equiv of diphenyl acetylene (0.074 g, 0.416 mmol) dropwise. A gradual color change from green to dark red was observed. The solution was stirred for 12 h and concentrated to ca. 3 mL. Storage at −20 °C afforded red crystals of 2 suitable for X-ray diffraction. Yield: 0.23 g (79%), mp: 148−152 °C. 1H NMR (600 MHz, C6D6, 298 K, δ) 1.12 (d, 12H, 3J = 6.9 Hz CH(CH3)2), 1.25 (d, 12H, 3J = 6 Hz CH(CH3)2), 3.34 (sept, 4H, 3J = 6 Hz, CH(CH3)2), 6.75 (dd, 6 Hz, 2H), 6.83 (d, 6 Hz, 1H), 6.88 (t, 6 Hz, 2H), 7.06 (d, 6 Hz, 4H), 7.11 (s, 1H), 7.16 (d, 6H, 1H), 7.38 (dd, 6 Hz, 3H), 7.43 (s, 1H), 7.44 (s, 1H), 7.51 (dd, 6 Hz, 1H), 7.59 (s, 1H) ppm. 13C{1H} NMR (C6D6, 150 MHz, 298 K, δ) 23.2, 27.0, 31.4, 123.9, 125.3, 126.0, 127.2, 127.7, 128.3, 128.4, 128.5, 128.6, 129.1, 129.3, 130.6, 136.6, 137.0, 145.3, 147.2 ppm. 119Sn{1H} NMR (C6H6, 233.6 MHz, 198 K, δ) 1601.2 ppm. λmax (ε in mol−1 L cm−1): 509 nm (1086). IR (CsI, Nujol; Selected): 730 and 380 (Sn−C stretching and bending)

environment around the metal atom. An alternative mechanism for the formation of 3 from a 4-type isomer could occur via an Sn−H activation via the divalent tin.41 The difference in reactivity between the AriPr6 and AriPr4 substituted species appears to be due to the para-isopropyl group on the flanking aryl ring. This may induce an interaction between the tin atoms, possibly due to increased dispersion force attraction between the para-iPr substituents. A further possible mechanistic route comes from the unsymmetric stannylstannylene isomer. In this mechanism a concerted [2 + 2] sigmatropic rearrangement occurs with cleavage of the Sn−Sn bond and formation of two new Sn−C bonds (Scheme 8). Scheme 8. Formation of 4 via a Concerted [2 + 2] Cycloaddition

Conclusion. The reaction of {AriPr6 Sn(μ-H)} 2 and {AriPr4Sn(μ-H)}2 with diphenyl acetylene afforded the Sn−H cis-inserted alkenyl stannylenes (1 and 2) as red solids. The reaction of {AriPr6Sn(μ-H)}2 with phenyl acetylene yielded distannacyclobut-3-ene 3 in which there is a hydrogen and an AriPr6 group bound to each tin. In contrast, the reaction of {AriPr4Sn(μ-H)}2 with phenyl acetylene yielded 4, which has an unsymmetric structure in which one tin atom is two-coordinate and is bound to an AriPr4 group and an ethenyl group that bridges to a second tetravalent tin atom that also carries two hydrogens and an AriPr4 group. A mechanism for the insertion of alkynes into the tin−hydrogen bonds is proposed to occur via a monohydrido bridged tin(II) dimer. E

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Organometallics

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AriPr6(H)Sn(C(H)C(Ph))Sn(H)AriPr6 (3). To a turquoise solution of {AriPr6Sn(μ-H)}2 (0.220 g, 0.188 mmol) in toluene, ca. 30 mL, was added phenyl acetylene (1.0 mL, 9.10 mmol). The color of the solution faded, the solution was stirred for 12 h, and then the solvent was removed under reduced pressure. The residue was dissolved in pentane ca. 70 mL and filtered using a filter-tipped cannula. The solution was concentrated under reduced pressure to ca. 5 mL. Storage of the solution in a −18 °C freezer afforded colorless crystals of 3 suitable for X-ray diffraction. Yield 0.201 g (82%), mp: 138 °C 1H NMR (600 MHz, C6D6, 298 K, δ) 0.90 (d, J = 6 Hz, 6H, CH(CH3)2), 0.95 (d, J = 6 Hz, 6H, CH(CH3)2), 1.06 (dd, J = 6 Hz, 12H, CH(CH3)2), 1.11 (d, J = 6 Hz, 6H, CH(CH3)2), 1.15 (d, J = 6 Hz, 6H, CH(CH3)2), 1.26 (d, J = 6 Hz, 6H, CH(CH3)2), 1.30 (d, J = 6 Hz, 6H, CH(CH3)2), 1.36 (dd, J = 6 Hz, 24H, CH(CH3)2), 2.70 (sept, J = 6H, 2H, CH(CH3)2), 2.85−3.00 (m, 8H, CH(CH3)2), 3.13 (sept, J = 6 Hz, 2H, CH(CH3)2), 5.77 (d, JHH = 6 Hz, 2JSn−H = 96 Hz, 1H) 6.15 (dd, JHH = 6 Hz, JSnH = 132 Hz, 1H, Sn−H), 6.52 (d, JHH = 6 Hz, JSnH = 138 Hz, 1H, Sn−H), 6.88 (d, J = 6 Hz, 1H, ArH), 6.98 (d, J = 6 Hz, 4H, ArH), 7.04 (t, J = 6 Hz, 2H, ArH), 7.10 (t, J = 6 Hz, 1H, ArH), 7.14 (d, J = 6 Hz, 4H, ArH), 7.18 (d, J = 6 Hz, 4H, ArH), 7.25 (d, J = 6 Hz, 2H, ArH). 13C{1H} NMR (125 MHz, C6D6, 298 K, δ) 22.83, 22.96, 23.51, 23.84, 24.0, 24.40, 24.58, 25.20, 25.22, 25.72, 26.38, 30.56, 30.66, 30.80, 30.95, 34.14, 34.58, 120.81, 121.04, 121.18, 121.37, 126.46, 126.90, 127.28, 127.38, 127.65, 127.81, 128.02, 139.29, 139.40, 143.88, 145.79, 146.20, 146.22, 146.36, 146.44, 147.17, 147.84, 148.07, 148.17, 148.65, 157.09, 172.71. 119Sn{1H} NMR (223.6 MHz, C6D6 298 K, δ) −218.0 (1JSnH = 1788 Hz, 2JSnH = 134 Hz, J119Sn−119Sn = 4763 Hz, J119Sn−117Sn = 4112 Hz), −148.6 (1JSnH = 1594 Hz, 2JSnH = 369 Hz, 2 JSnH = 97 Hz, J119Sn−119Sn = 4763 Hz, J119Sn−117Sn = 4110 Hz) ppm. FTIR (CsI, Nujol: Selected): 1845 (Sn−H), 700 (Sn−C), 395 (Sn−C). AriPr4Sn(CHCPh)Sn(H)2AriPr4 (4). To a green solution of [AriPr4Sn(μ-H)]2 (0.195 g, 0.377 mmol) in toluene (30 mL) was added 1 equiv of phenyl acetylene (41.3 μL, 0.377 mmol) dropwise. An instant color change from green to dark red was observed. The solution was stirred for 20 min and concentrated to ca. 2 mL. Storage at −20 °C afforded red crystals suitable for X-ray diffraction. Yield: 0.18 g (84%), mp: 150−155 °C. 1H NMR (600 MHz, C6D6, 298 K, δ) 1.04 (d, 24H, 3J = 6 Hz CH(CH3)2), 1.12 (d, 12H, 3J = 6 Hz CH(CH3)2), 1.26 (d, 12H, 3 J = 7.5 Hz CH(CH3)2), 2.85 (sept, 4H, 3J = 6 Hz, CH(CH3)2), 3.12 (sept, 4H, 3J = 6 Hz, CH(CH3)2), 5.05 (s, 2H, Sn−H), 6.95−7.35 (m 23H) 8.60 (s, 1H, JSn−C(H) = 126 Hz, 288 Hz, Sn-CH) ppm. 13C{1H} NMR (C6D6, 125 MHz, 298 K, δ) 22.9, 23.0, 26.3, 26.6, 31.0, 77.9, 123.5, 123.7, 126.8, 126.9, 127.2, 128.5, 128.7, 129.1, 129.6, 129.7, 132.4, 137.3, 142.0, 144.8, 147.0, 147.2, 148.9, 161.2, 179.4, 195.1 ppm. 119Sn{1H} NMR (C6H6, 233.6 MHz, 298 K, δ) −324.0, 1349.2 ppm. λmax (ε in mol−1 L cm−1): 501 nm. (540) IR (CsI, Nujol; Selected): 1860 (Sn−H stretching) 710 and 380 (Sn−C stretching and bending). X-ray Crystallography. Crystals of 1−4 suitable for single crystal X-ray diffraction studies were covered in silicone oil and attached to a glass fiber on the mounting pin at the goniometer. Crystallographic measurements were collected at 90 K with a Bruker APEX II DUO diffractometer using Mo Kα (λ = 0.71073 Å) radiation. The crystal structures were corrected for Lorentz and polarization effects with SAINT42 and absorption using Blessing’s method as incorporated into the program SADABS.43,44 The SHELXTL program was used to determine the space groups and set up the initial files.45 The structures were determined by direct methods using the program SHELXS and refined with the program SHELXL.46 All non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms were placed in idealized positions throughout the refinement process and refined as riding atoms with individual isotropic refinement parameters. A summary of data collection and refinement parameters is given in the Supporting Information.

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00570. Spectroscopic data and crystallographic information for 1−4 (PDF) Accession Codes

CCDC 1563866−1563869 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: 1-530-752-8900. Fax: 1-530-732-8995. E-mail: [email protected]. ORCID

James C. Fettinger: 0000-0002-6428-4909 Philip P. Power: 0000-0002-6262-3209 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the U.S. Department of Energy (DE-FG0207ER4675) office of Basic Energy Sciences for support of this work. We thank the National Science Foundation (grant nos. 0840444 and 1531193) for the funding for the purchase of a dual-source and dual-microsource X-ray diffractometers.

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REFERENCES

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DOI: 10.1021/acs.organomet.7b00570 Organometallics XXXX, XXX, XXX−XXX