Complete Hydrogen Transfer: Tin Hydride Reactivity toward

2 hours ago - Benzonitrile shows an insertion reaction with the low-valent organotin hydride to yield a dimeric insertion product, whereas the isonitr...
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Article Cite This: Organometallics XXXX, XXX, XXX−XXX

Complete Hydrogen Transfer: Tin Hydride Reactivity toward Adamantylisonitrile and Benzonitrile Frederik S. W. Aicher, Klaus Eichele, Hartmut Schubert, and Lars Wesemann* Institut für Anorganische Chemie, Universität Tübingen, Auf der Morgenstelle 18, 72076 Tübingen, Germany S Supporting Information *

ABSTRACT: Adamantylisonitrile and benzonitrile were reacted with bulky substituted organotin trihydride [Ar*SnH3] [Ar* = (C6H3-2,6-Trip2), Trip = 2,4,6-triisopropylphenyl]. They do not show any reaction at room temperature as well as at 80 °C. After activation of the organotin trihydride with diethylmethylamine in the isonitrile case three hydrogen atoms were transferred from the tin atom to the isonitrile unit and a carbon tin bond was formed to give an intramolecular adduct between a diorganostannylene and a dialkylamine. Benzonitrile as well as adamantylisonitrile react both with low-valent organotin hydride [Ar*SnH]2. Benzonitrile shows an insertion reaction with the low-valent organotin hydride to yield a dimeric insertion product, whereas the isonitrile carbon atom of adamantylisonitrile abstracts three hydrogen atoms from the low-valent organotin hydride to give an equimolar mixture between (adamantylmethylamido)organostannylene and a bis(isonitrile)distannyne adduct.

INTRODUCTION In contrast to the less investigated thermolabile stannane SnH4, which decomposes at 0 °C, the organotin(IV) hydride tributyltin hydride (Bu3SnH) is known as a versatile reducing and hydrostannylation reagent in organic transformation reactions.1−3 The chemistry of diorganotin dihydrides, e.g., Ph2SnH2 and Et2SnH2, has already been investigated more than 50 years ago by Kuivila and Neumann.4−7 A reductive elimination of hydrogen induced by Lewis bases like DMF or pyridine to yield oligomeric stannanes was reported by both groups. Organotin trihydrides like aryltin trihydrides are wellknown compounds which can be synthesized by hydride substitution using LiAlH4 starting from the respective aryltin trichloride.8 Tilley et al. have studied the reaction of TripSnH3 (Trip = 2,4,6-triisopropylphenyl) with reactive organometallic coordination compounds to give hydride transfer from the tin atom to the transition metal and formation of a metal−tin bond.9−11 Tin(II)dihydride was isolated and stabilized by an ylidic Wittig ligand donor and an electrophilic transition metal fragment like [W(CO) 5 ]. 12,13 Highly reactive dimeric organotin(II) hydrides [(Ar*SnH)2] [Ar* = (C6H3-2,6Trip2)] were isolated for the first time by Power et al. by using very bulky terphenyl groups as substituents at the tin atom.14 Roesky and co-workers reported low-valent hydrides of tin stabilized by chelating nitrogen ligands whereas Jones presented a low-valent tinhydride substituted by a bulky amido substituent.15,16 Without any additional catalyst these hydrides show hydrostannylation and addition reactions toward alkenes, alkynes, diazo derivatives, ketones, and carbon dioxide.17−20 Furthermore, low-valent tin hydrides also act as catalysts in hydroboration or dehydrocoupling reactions.21−23 © XXXX American Chemical Society

We have studied the reductive elimination of hydrogen from organotin trihydrides substituted by bulky aryl substituents or substituted by the less bulky Lappert substituent [−CH(SiMe3)2] to give highly reactive low-valent tetrameric organotin hydrides like [(Me3Si)2CH-SnH]4 or Lewis base stabilized adducts [Ar*SnH(L)] [L = MeNHC, DMAP] [MeNHC: 1,3,4,5-tetramethylimidazol-2-ylidene, DMAP: 4(N,N-dimethylamino)pyridine].24−27 The hydrogen elimination was induced by NHC, DMAP, or by an excess of diethylmethylamine.25,27 Furthermore, complete hydrogen elimination of the three hydrogen atoms of the organotin trihydride was also observed in reaction with NHC to yield low-valent metalloid tin clusters.24,27 In this publication we would like to present the reaction of organotin hydrides with the Lewis bases adamantylisonitrile and benzonitrile.

RESULTS AND DISCUSSION A toluene solution of a 1:2 mixture between bulky substituted tin trihydride Ar*SnH3 [Ar* = (C6H3-2,6-Trip2), Trip = C6H22,4,6-iPr3] and adamantylisonitrile was investigated by NMR spectroscopy. After 1 day at room temperature and even after 8 h at 80 °C no new signals could be detected in the 119Sn{1H} and 1H NMR spectra. Obviously, organotin(IV) trihydride 1 does not show any reaction with adamantylisonitrile. Reductive elimination of hydrogen from organotin trihydrides was found to be activated in reaction with an excess of diethylmethylamine. This reaction turned out to be a high yield procedure for the formation of low-valent organotin(II) hydrides [(RSnH)n] Received: April 9, 2018


DOI: 10.1021/acs.organomet.8b00207 Organometallics XXXX, XXX, XXX−XXX


Organometallics in high purity (R: Ar*, n = 2; Ar′, n = 4, Ar′ = (C6H3-2,6-Mes2), Mes = C6H2-2,4,6-Me3; −CH(SiMe3)2, n = 4].14,25,26 To activate trihydride 1 Ar*SnH3 we added 18 equiv of diethylmethylamine to the mixture between organotin trihydride 1 and adamantylisonitrile (Scheme 1). Immediately, the colorless mixture turned yellow to become red after 4−5 h. Scheme 1. Reaction of Trihydride 1 with Adamantylisonitrile and Excess of Diethylmethylamine at Room Temperature

Figure 2. Phosphastannirane 3.36

Comparing the structural data of two ring molecules 2 and 3, the interatomic distances in the nitrogen ring are smaller. The two angles at tin [C1−Sn−N = 37.9(1)°] and carbon [Sn− C1−N = 75.1(1)°] are slightly smaller in comparison to the values in 3 [angle at Sn: 42.1(1)°, C: 79.3(1)°] and the angle at nitrogen [Sn−N−C1 = 67.0(1)°] is larger than the angle at phosphorus [Sn−P−C = 58.6(1)°] inside the ring molecule 3. The interatomic Sn−N distance in 2, 2.326(2) Å, is longer than distances between these atoms found in stannylene amides [2.055(6)−2.256(3) Å].22,38 In the product molecule 2, we found formation of a Sn−C bond and a donor−acceptor interaction between a dialkylamine nitrogen and a stannylene tin atom. Three hydrogen atoms were transferred from the tin to the isonitrile CN moiety upon reduction of the organotin trihydride into a diorgano stannylene. This hydrogen transfer can be compared with the transfer of two hydrogen atoms to the carben carbon atom in the reaction of N-heterocyclic carbenes (NHCs) with organotin trihydrides.24,26,27 In the room-temperature 1H NMR spectrum of 2, we detect for the CH2 group of the three-membered ring only one doublet at 1.94 ppm showing a coupling constant of 7.8 Hz. The NH moiety exhibits a triplet at 1.12 ppm also with a 3J coupling constant of 7.8 Hz. Due to too short relaxation times of the 119Sn nucleus, the 2JSn−H coupling between the CH2 unit of the ring and the tin atom was only observed in the 250 MHz 1H NMR spectrum of 2. Because for the geometry of the ring moiety of 2 two signals in the 1H NMR spectrum of the CH2 group are expected we also measured the NMR spectra at −80 °C. Besides broadening of all signals, we do not detect the expected separation of the resonance for the CH2 group at 1.94 ppm into two signals at this temperature. Fortuitous isochronicity aside, an intramolecular rearrangement of the molecule converting the two protons of the CH2 group into each other should be responsible for the high symmetry in solution. A possible pathway could be a Sn−N bond breaking reaction followed by rotation along the N−CH2 or CH2−Sn bond, inversion at the nitrogen atom and Sn−N adduct formation. The 119Sn NMR resonance for the cyclic stannylene adduct 2 was found at −465 ppm. Diarylstannylene adducts with Lewis bases like NHCs were found to exhibit 119Sn NMR signals at −150.7 ppm.24 A cyclic disilyl stannylene forms an adduct with PEt3 and shows a resonance in the 119Sn NMR spectrum at −224.4 ppm.39 In the case of arylstannylenes stabilized by nitrogen donors the signals for the 119Sn NMR resonance were found at slightly lower field: [2-{(C6H3-2,6Pri2)NCH}C6H4]2Sn 69.2 ppm; [C6H3(CH2NMe2)2-2,6][4tol]Sn 209.6 ppm.40,41 In comparison, the 119Sn NMR resonance for phosphastannirane 3 (Figure 2), which was found at 716 ppm, lies at low field in comparison to other adducts making a structural change of 3 in solution feasible. Because the 119Sn NMR resonance of 3 could not be observed in the solid state, we were not able to compare the solution and

After evaporation of the solvent, a minimal amount of nhexane was used to recrystallize the product. Red crystals of compound 2 were obtained overnight in a yield of 48%. The reaction product 2 was characterized by single-crystal X-ray diffraction, NMR spectroscopy, and elemental analysis. The molecular structure of reaction product 2 is presented in Figure 1 together with selected interatomic distances and angles. To

Figure 1. Mercury37 plot of the molecular structure of 2. Thermal ellipsoids are shown at 50% probability level. iPr groups of the aryl substituents are omitted for clarity. The adamantyl and CNSn ring moieties show a disorder in the crystal in two positions of ratio 90/10. In the figure only the major component is shown. Selected bond lengths [Å] and angles are given: Sn−N 2.326(2), Sn−C1 2.214(2), C1−N 1.479(2), Sn−C2 2.2391(15), N−C3 1.480(2), N−Sn−C1 37.91(6), Sn−C1−N 75.13(10), C1−N−Sn 66.96(9), C1−Sn−C2 92.43(7), N−Sn−C2 95.27(5), C3−N−Sn 126.06(12), C1−N−C3 120.02(14).

our surprise, the three hydrogen atoms in the main product (90% by 1H NMR spectroscopy) were transferred to the isonitrile unit, and no molecular hydrogen was liberated. At this stage of the study we do not want to speculate about the role of the amine.25 A three-membered ring molecule, an azastanniridine, was isolated. This Sn−N−C ring molecule is to the best of our knowledge not known. However, examples of the cyclic Si−N−C molecule were presented in the literature.28−35 The homologous phosphastannirane (3 shown in Figure 2) was synthesized in reaction between lithiated benzyldiphenylphosphine and terphenyltinchloride [(Ar*SnCl)2].36 B

DOI: 10.1021/acs.organomet.8b00207 Organometallics XXXX, XXX, XXX−XXX



adduct 6 and methylamidostannylene 5 was formed. Isonitrile adducts of distannynes were already published in the literature and prepared in reaction of distannynes with an isonitrile donor.46,47 The 119Sn NMR resonance of 6 (−212 ppm) lies at higher field in comparison to the signal found for [Ar′Sn(CNtBu)]2 (181 ppm) [Ar′ = C6H3-2,6-(C6H3-2,6-iPr2)2].46 (Adamantylmethylamido)organostannylene 5 and the bis(isonitrile)distannyne adduct 6 are the products of a 3-fold hydrogen transfer from the tin to the isonitrile carbon atom. Power et al. and Jones et al. published the oxidative addition of hydrogen at distannynes to give low-valent organotin hydrides.16,47−49 Recently Power’s group presented with the reductive elimination of hydrogen at 80 °C from low-valent organotin hydrides proof for the reversibility of this hydrogen addition.50 With the room temperature hydrogen transfer presented here we found another example for distannyne formation starting from low-valent organotin hydrides. Unfortunately, we were not able to separate compounds 5 and 6 by crystallization from various solvents. Therefore, reaction products 5 and 6 were characterized by NMR spectroscopy and elemental analysis as a 1:1 mixture. Compound 5 was also synthesized and characterized separately in reaction between deprotonated adamantylmethylamide and aryltin chloride [(Ar*SnCl)2] in a yield of 72% (see the Supporting Information for details). However, single crystals of each compound suitable for crystal structure determination were isolated from the reaction mixture. The molecular structure of bis(isonitrile)distannyne adduct 6, which was placed in the Supporting Information, is comparable with the structure of the bis(t-butylisonitrile) distannyne adduct [Ar′Sn(CNtBu)]2 [Ar′ = C6H3-2,6(C6H3-2,6-iPr2)2].47 The Sn−Sn distance in 6 of 2.9168(4) Å is only slightly shorter than the Sn−Sn bond characterized in [Ar′ Sn(CNtBu)]2 [2.9282(6) Å].46 In Figure 3, the molecular structure in the solid state of the stannylene 5 is shown. Due to the steric bulkiness of both substituents, the terphenyl and adamantyl groups, compound 5 is a monomeric dialkylamidoarylstannylene. In comparison, the diethylamido terphenyltin compound (ArMe6SnNEt2)2 [ArMe6 = C6H3-2,6-(C6H3-2,4,6-Me3)2] is in solution a dimeric compound exhibiting a Sn2N2 four-membered ring structure.22 The monomeric structure of adamantylamide 5 corroborates with a resonance in the 119Sn NMR at 890 ppm, whereas the diethylamide dimer exhibits the tin resonance at −155.2 ppm.22 Compound 5 is another example for a monomeric organoamidostannylene.51,52 Details of the structure solution have been placed in the Supporting Information. In comparison to azastanniridine 2, the Sn−N distance in 5 is substantially shorter [5: 2.044(2) Å; 2: 2.326(2) Å]. The Sn−N distance of 2.044(2) Å in amidostannylene 5 is only slightly shorter than the Sn−N distances found in bisamidostannylenes [2.102(6)− 2.063(2) Å].22,53−59 The sum of angles around the nitrogen

solid state NMR data with the solid state structure of 3 (Figure 2). In order to discuss the solution 119Sn NMR chemical shift values of 2 (−465 ppm) and 3 (716 ppm) with respect to their molecular structures in the solid state, we used the ADF program to calculate the chemical shift of the 119Sn NMR signals.42 In a previous study, ADF calculations of 119Sn NMR chemical shifts for a series of compounds resulted in good agreement with experimentally determined 119Sn NMR chemical shifts.43 In the case of phosphine adduct 3, the calculated resonance lies at significantly higher field in comparison to the solution data (3 calcd −380 ppm, exp 716 ppm), whereas the calculated chemical shift of 2 was found in comparable range to the experimentally determined value (2 calcd −639 ppm, exp −465 ppm). We interpret these findings as an indicator for a feasible ring opening−closure equilibrium (Scheme 2), which was already discussed for the interpretation Scheme 2. Equilibrium between Closed and Open Stannylene Lewis Pair (3: E = P, R = Ph, R1, R2 = Ph; 2: E = N, R = H, R1 = Ad, R2 = H)

of the 1H NMR data of 2 (vide supra). The solution 119Sn NMR data are in the case of stannylene 3 an indicator for the equilibrium lying in solution more on the side of the stannylene structure without Sn−P adduct formation, and for compound 2, the equilibrium lies on the side of the cyclic adduct structure. Benzyl-terphenyl stannylene is in solution a distannene exhibiting a 119Sn NMR resonance at 1206 ppm.44 Like phosphastannirane 3, azastanniridine 2 is also a chiral molecule with the tin and nitrogen atoms as centers of chirality. Because we detect only one set of signals in the 1H NMR spectrum and one 119Sn NMR resonance for compound 2, the formation of the cyclic molecule 2 is a diastereoselective reaction. The reaction of trihydride 1 with adamantylisonitrile only started after addition of Et2MeN, which is a reagent to form the low-valent hydride [(Ar*SnH)2] (4) and dihydrogen in reaction with Ar*SnH3.25,45 Therefore, we were speculating about the role of the Sn(II)hydride 4 in the formation of azastanniridine 2 (Scheme 1). Monitoring the formation of 2 by NMR spectroscopy, we found signals indicating also a small amount (