Cyclic Distannene or Bis(stannylene) with a Ferrocenyl Backbone

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Cyclic Distannene or Bis(stannylene) with a Ferrocenyl Backbone: Synthesis, Structure, and Coordination Chemistry Jessica Henoch,† Armin Auch,† Fatima Diab,† Klaus Eichele,† Hartmut Schubert,† Peter Sirsch,† Theresa Block,‡ Rainer Pöttgen,‡ and Lars Wesemann*,† †

Institut für Anorganische Chemie, Eberhard-Karls-Universität Tübingen, Auf der Morgenstelle 18, 72076 Tübingen, Germany Institut für Anorganische und Analytische Chemie, Universität Münster, Corrensstrasse 30, D-48149 Münster, Germany

‡

S Supporting Information *

ABSTRACT: 1,1′-Dilithioferrocene was reacted with 2 equiv of isopropyl (Ar*) or methyl (Ar′) substituted terphenyl tin(II) chloride. Reaction product 1, carrying the bulkier terphenyl substituent Ar*, displays a bis(stannylene) structure in the solid state without formation of a tin−tin bond. Temperature-dependent solution 119Sn NMR spectroscopy, however, revealed a dynamic interplay between bis(stannylene) (100 °C) and cyclic distannene (−80 °C). In contrast to 1, the less bulky Ar′ substituent results in a cyclic distannene 2. On the basis of temperature-dependent 119Sn NMR spectroscopy the Sn−Sn bond of compound 2 was preserved up to 100 °C. Both compounds were further characterized by solidstate 119Sn NMR spectroscopy as well as 119Sn and 57Fe Mössbauer spectroscopy. 1 reacted as a chelating ligand with nickel and palladium complexes [Ni(cod)2] and [Pd(nbe)3] (nbe = norbornene). In the resulting coordination compounds the nonstabilized stannylene acts as a donor as well as an acceptor ligand and shows a dynamic switch from donor to acceptor behavior in the monopalladium complex.

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On the basis of low-temperature 119Sn NMR spectroscopy data Masamune proposed Lappert’s stannylene [Sn(CHSiMe3)2]2 to be an equilibrium mixture in methylcyclohexane between dimeric distannene and monomeric stannylene.27 Power presented a benzyl-substituted distannene, which shows a dimeric structure in the solid state and on the basis of the 119Sn NMR chemical shift retains this structure also in solution.17 We are interested in intramolecular Lewis pairs based on electrophilic stannylenes. Our synthetic approach uses as Lewis base either substituted phosphines or the stannylenes themselves.28−33 To build molecules with two stannylene units within one molecule we previously employed xanthene (A), naphthalene (B), and acenaphthene (C) fragments as a backbone (Chart 1).31−33 Depending on the steric requirements of the other substituents the stannylene units form either a cyclic distannene, which shows tin−tin interactions also in solution, or a bis(stannylene) with a large distance between the tin atoms. To study the influence of a wider variety of backbones in this project, we employed dilithiated ferrocene as a building block for the synthesis of stannylenes. Base-stabilized ferrocenyl-bridged bis(stannylenes) were previously reported.34 Here, we present results of our investigations with ferrocene as the backbone and coordination compounds of these nonstabilized stannylenes.

INTRODUCTION Ever since Lappert and co-workers reported on the synthesis and structure of the first stannylene the chemistry of the heavy tetrylenes has attracted the interest of many research groups.1−4 Besides reactivity studies the interaction between the low-valent elements was in the focus of interest.5−9 The Sn−Sn bonding in distannenes can be interpreted as an interaction between a Lewis acidic and Lewis basic stannylene. Because of the vacant p-orbital and the lone pair present at the low-valent tin atom, the stannylene can react as a Lewis acid and as a Lewis base. The potential of a stannylene to form a dimer strongly depends on the steric as well as the electronic properties of the substituents connected at the tin atom. Electron-donating substituents like dialkylamide or alkylalkoxides reduce the electrophilicity of the stannylene, and therefore dimerization plays a much smaller role. Because of the existing lone pair these stannylenes react as potent Lewis bases, for example, in coordination chemistry.10−14 However, silyl, alkyl, and aryl substituents do not quench the electrophilicity present at the low-valent group 14 element, and therefore these stannylenes also react as Lewis acids. Depending on the size of these substituents stannylenes very often are monomeric in solution and dimeric in the solid state.15−24 Sekiguchi published a silylsubstituted stannylene exhibiting a rare case of a stannylene that stays dimeric in solution.25 Since 119Sn NMR spectroscopy in solution and in the solid state is a valuable analytic method in this type of chemistry, the equilibrium between dimer and monomer was investigated.26 Distannenes show a resonance at much higher field in comparison to monomeric stannylenes. © XXXX American Chemical Society

Received: February 5, 2018

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DOI: 10.1021/acs.inorgchem.8b00317 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Chart 1. Synthesized Cyclic Distannenes and Bis(stannylene) Together with 119Sn NMR Chemical Shifts δ and J(119Sn,117Sn) Coupling Constants in Solution at Room Temperature31−33

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RESULTS AND DISCUSSION Dilithiated ferrocene, which was synthesized following a literature procedure simply by deprotonation of ferrocene with butyllithium, was reacted in toluene at −40 °C with 1 equiv of electrophilic tin(II)halide [Ar*SnCl]2.35,36 From this mixture the ditin-substituted ferrocene 1 was isolated by crystallization from a hexane solution as large burgundy crystals in a yield of 62% (Scheme 1). Scheme 1. Synthesis of the Bis(stannylene) 1a

Figure 1. ORTEP of the molecular structure of 1. Hydrogen atoms are omitted, ellipsoids are drawn at the 50% probability level. Selected distances [Å] and angles [deg]: C1−Sn 2.223(2), C2−Sn 2.149(2), C1−Sn−C2 97.3(1).

Table 1. 119Sn NMR Spectroscopic Data for 1 and 2 in Solution and in the Solid State a

Ar* = 2,6-Trip2C6H3; Trip = 2,4,6-iPr3C6H2.

The stannylene 1 was characterized by elemental analysis, single-crystal X-ray diffraction, 119Sn and 57Fe Mössbauer spectroscopy, and NMR spectroscopy in solution as well as in the solid state. The molecular structure of 1 in the solid state is shown in Figure 1 along with selected interatomic distances and angles. For further crystallographic data and refinement details, see the Supporting Information. The most prominent feature of the molecular structure in the solid state is the absent interaction between the stannylene tin atoms. In contrast to the solid-state structure, the 119Sn chemical shift of 871 ppm observed in the room-temperature 119 Sn{1H} NMR spectrum of 1 in toluene-d8 is indicative of a distannene with Sn−Sn interaction (Chart 1).31,32 However, because of the large line width (2.7 kHz) and low signal-tonoise ratio we could not observe tin−tin coupling. Consequently, to investigate the change between solution- and solid-state structures, temperature-dependent 119Sn NMR spectroscopic measurements and 119Sn solid-state NMR spectroscopic experiments were performed (Table 1). In Figure 2 the 119Sn{1H} NMR spectra in solution at different temperatures are presented. There is consensus that temperature effects on 119Sn NMR chemical shifts are small, unless there are concomitant changes in structure.37 For example, in earlier work the temperature dependence of δ(119Sn) of naphthalene- and xanthene-bridged distannenes and a bis(stannylene) were investigated.32 For these systems, the chemical shifts showed a maximum change of 120 ppm in the range between −80 and +120 °C; that is, ∂δ(119Sn)/∂T < 0.6 ppm/K. In contrast, δ(119Sn) of 1 changed by 541 ppm between −80 and +100 °C, with ∂δ(119Sn)/∂T = 3.0 ppm/K. This sensitivity against temperature exceeds some of the

a1

compound

1

2

solution at 26 °C −80 to 100 °C ∂δ/∂T solid state

871 ppm 456−997 ppm 3.0 ppm/K 1111 ppm

449 ppma 437−468 ppm 0.2 ppm/K 536, 561 ppm

J(119Sn,117Sn) = 6087 Hz.

established chemical shift thermometers and indicates a significant change in structure.38 Because the 119Sn CP/MAS NMR spectrum of solid 1 yields a δ(119Sn) of 1111 ppm, we assign the high-temperature species in solution to the prevalent bis(stannylene). In contrast, the species at −80 °C appears to be mostly a cyclic distannene structure. Because of the results of our reactivity studies (vide infra) and the fact that 1 is still soluble even at low temperature, we believe that in solution at low temperature the tin−tin bond forms intramolecularly and not between two separate molecules, which would produce an oligomeric structure. Therefore, we formulate the intramolecular tin−tin bond formation reaction as depicted in Scheme 2. In the series of temperature-dependent 119Sn NMR spectroscopic measurements at −20 °C the 119Sn NMR signal is very broad (Figure 2). We interpret this broad resonance as an indicator for the spin−lattice relaxation time T1 being at a minimum.32 Given the results of the temperature-dependent NMR study, it is surprising that the high-temperature isomer was also found as the result of the crystal structure analysis. In the packing of the bis(stannylene) molecules we identified 16 short intermolecular hydrogen−hydrogen distances in the range of 2.127−2.339 Å, which might point toward hydrogen−hydrogen bonding.39−43 This suggests that the Sn−Sn bond strength in the distannene is weak enough to be compensated by intermolecular C−H···H−C contacts. The B


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Inorganic Chemistry

satellites, which show a 119Sn−117Sn coupling constant of 6087 Hz, are indicators for a distannene geometry in solution at room temperature. Interestingly, this chemical shift value does not change dramatically upon changing the temperature from −80 °C (437 ppm) to 100 °C (468 ppm), and therefore the molecule should adopt the geometry of a distannene also at 100 °C. To compare the molecular structure in solution with the solid-state structure a solid-state 119Sn cross-polarization (CP)/ magic-angle spinning (MAS) NMR spectrum of a sample of 2 was recorded. In this spectrum two signals at 536 and 561 ppm were found, also indicating a distannene structure. This suggests that, in comparison to the solution, 2 adopts a structure with reduced symmetry in the solid state with two crystallographically inequivalent tin atoms. Crystals suitable for single-crystal structure analysis of 2 were not obtained. In contrast to the trip-terphenyl-substituted derivative 1 no evidence was found for a bis(stannylene) structure in the case of the less bulky mesityl−terphenyl-substituted compound 2, neither in solution nor in the solid state (Scheme 3). The reduced bulkiness of the mesityl−terphenyl (Ar′) substituent in 2 appears to be responsible for the higher stability of the tin− tin bonding in comparison to molecule 1. Scheme 3. Bis(stannylene) Versus Cyclic Distannenea

a

Ar′ = C6H3-2,6-Mes2; Mes = C6H2-2,4,6-Me3.

Both substances 1 and 2 were also investigated by 119Sn and Fe Mössbauer spectroscopy (Figure 3, Table 2, Supporting Information). The 119Sn Mössbauer spectra of 1 and 2 could be reproduced with single signals, however, with distinctly different spectroscopic data. The bis(stannylene) 1, comprising tin with coordination number two [δ = 2.69(1) mms−1], has a higher isomer shift as compared to the cyclic distannene 2 in 57

Figure 2. 119Sn{1H} NMR spectra of 1 in toluene-d8 solution at 186.50 MHz and at different temperatures.

Scheme 2. Bis(stannylene) Versus Cyclic Distannenea

a

Ar* = C6H3-2,6-Trip2; Trip = C6H2-2,4,6-iPr3.

ferrocenyl-substituted stannylene reported here represents a rare case of a stannylene exhibiting a dimeric distannene structure in solution at low temperature and a monomeric stannylene structure in the solid state and in solution at high temperature. To investigate the influence of the size of the bulky substituents on the distannene-bis(stannylene) equilibrium, we synthesized the mesityl−terphenyl-substituted ferrocenyl derivative following the procedure of 1 (Scheme 1) and studied the structure in solution and the solid state. In solution, the mesityl-substituted ferrocenyl stannylene 2 exhibits a signal in the 119Sn{1H} NMR spectrum at 449 ppm. In comparison to 1 and other published distannenes and bis(stannylene) molecules, the 119Sn chemical shift of 2 in combination with the

Figure 3. Experimental (data points) and simulated (solid lines) 119Sn Mössbauer spectra of 2 (top) and 1 (bottom) at 5 K. C


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Inorganic Chemistry Table 2. Fitting Parametersa of 57Fe and 119Sn Mössbauer Spectroscopic Measurements of 1 and 2 at 5 K compound 57

1- Fe 1-119Sn 2-57Fe 2-119Sn

δ (mm·s−1)

ΔEQ (mm·s−1)

Γ (mm·s−1)

0.48(1) 2.69(1) 0.45(1) 2.31(1)

2.36(1) 4.46(2) 2.27(1) 2.96(2)

0.29(1) 1.17(3) 0.30(2) 1.49(3)

moiety and platinum by using an oxidative addition of Pt(0) compounds into a tin−tin bond. Since we were also interested in the coordination chemistry of nonstabilized stannylenes we reacted the low-valent tin compound 1 with 1 equiv of transition-metal complexes of nickel and palladium with the metals in oxidation state zero (Schemes 4 and 6). In the case of Scheme 4. Reaction of the Bis(stannylene) 1 with 1 equiv of Ni(cod)2 or Pd(nbe)3a

δ = isomer shift; ΔEQ = electric quadrupole splitting; Γ = experimental line width.

a

which tin is triply coordinated [δ = 2.31(1) mms−1]. These data indicate higher s-electron density in 1 in comparison to 2.44 Furthermore, the tin atoms in 1 show substantial electric quadrupole splitting of ΔEQ = 4.46(2) mms−1, substantiating pronounced lone-pair character at these tin atoms, even stronger than in a weakly arene-stabilized Sn(II) monocation but of the same magnitude as the value for the monomeric stannylene [Sn(Ar*-3,5-Pr2)2] (Table 3). Distannene 2 has a Table 3. 119Sn Mössbauer Spectroscopic Data for Literature Examples 45

[(Sn{CH(SiMe3)2}2)2] [Sn(C5H5)2]56 [Sn(C6H2-2,4,6-tBu3)2]57 [Sn{C6H2-2,4,6-(CF3)3}2]57 [Sn(Ar*-3,5-Pr2)2]58 [SnN(SiMe3)Ar′][PF]59

δ (mm·s−1)

ΔEQ (mm·s−1)

2.16 3.74 3.28 3.37 2.19a 3.369

2.31 0.86 1.90 1.93 4.41a 3.69

a

cod = 1,5-cyclooctadiene; nbe = norbornene.

the palladium species we were also able to isolate the product from the reaction with 2 equiv of the Pd(0) complex. Stannylene complexes 3−5 (Schemes 4 and 6) were isolated by crystallization and characterized by elemental analysis, NMR spectroscopy, and single-crystal structure analysis. In all three complexes the transition metals are coordinated by two tin atoms and a phenyl moiety of the terphenyl substituent. This type of coordination was also found in the case of the Ni(0) complex with the distannene A (Chart 1).31 Dark red crystals of distannene 1 were reacted in benzene with 1 equiv of [Ni(cod)2] at 40 °C. Over a period of one week the color changed from red to black and from a pentane solution of the reaction mixture dark violet crystals of 3 were obtained at −40 °C. In the 119Sn{1H} NMR spectrum of the nickel complex 3 two signals at 62 and 520 ppm exhibiting a 119Sn−119/117Sn coupling constant of 1545 Hz were found. As in the distannene nickel(0) complex with a xanthene backbone, the two stannylene tin atoms of ligand 1 show different coordination behavior: Sn2 acts as an acceptor and Sn1 as a donor in coordination with the nickel atom.31 By comparison with the literature-known, xanthene-bridged species we can assign the 119 Sn NMR signal at 62 ppm to the acceptor stannylene Sn2 and the signal at 520 ppm to the donor stannylene Sn1 (see Scheme 4 for numbering). This assignment is corroborated by results of quantum chemical calculations (vide infra). The tin− tin coupling constant of 1545 Hz is comparable with the distannene nickel complex featuring a xanthene backbone (1720 Hz).31 The result of the crystal structure determination and refinement are presented in the Supporting Information, and the molecular structure of the nickel complex 3 is shown in Figure 4, along with selected interatomic distances and angles. The nickel atom is coordinated by the two tin atoms with distances of 2.4304(5) Å (Ni1−Sn1) and 2.7047(5) Å (Ni1− Sn2). These distances are closely related to the ones observed in the xanthene-bridged structure [2.4085(4), 2.6939(4) Å].31 The longer distance is associated with the acceptor tin atom, whereas the shorter value can be assigned to the tin donor atom.31 Furthermore, the sum of angles around the tin atoms

a

Ar* = C6H3-2,6-(C6H2-2,4,6-iPr3)2; Ar′ = C6H2{C(H)Ph2}2Me-2,4,6; PF = Al{OC(CF3)3}4, at 90 K.

more symmetric coordination with a much lower quadrupole splitting parameter of ΔEQ(2) = 2.96(2) mms−1. Lappert presented in 1976 119Sn Mössbauer data of his famous bis[bis(trimethylsilyl)methyl]tin, which is in the solid state a dimeric molecule showing a bond between the tin atoms (Table 3). This distannene exhibits a signal in the 119Sn Mössbauer spectrum with an isomer shift of 2.16 mm s−1 and a quadrupole splitting of 2.31 mm s−1.45 Effects due to the substituent at the tin and the C−Sn−C angle aside, monomeric stannylenes with coordination number two at the tin atom show signals at higher isomer shifts and with larger quadrupole splitting in comparison to distannenes.46 The ferrocenyl derivatives 1 and 2 exhibit in the case of the two-coordinate bis(stannylene) compound 1 the expected higher isomer shift in comparison to the three-coordinate tin atoms in the cyclic distannene 2. This tendency is corroborated by the much higher quadrupole splitting in 1. In the case of the 57Fe Mössbauer data both spectra could be reproduced well with single signals at isomer shifts of δ = 0.48(1) mms−1 (1) and δ = 0.45(1) mms−1 (2), the typical doublet spectra for diamagnetic low-spin Fe(II), S = 0. The isomer shifts and quadrupole splitting values of 1 and 2 are almost equal, indicating comparable 4s electron densities at the iron nuclei and lie in the range typical for substituted and unsubstituted ferrocene-based coordination compounds.47−55 Tin coordination compounds of ferrocenyl-bridged tin ligands were published by Jurkschat and Herberhold.34,60,61 Jurkschat reacted a base-stabilized bis(stannylene) ligand with carbonyl complexes of chromium and tungsten. Herberhold synthesized cyclic distannide complexes with a ferrocenyl D


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Inorganic Chemistry

Figure 5. ORTEP of the molecular structure of 4. Hydrogen atoms and isopropyl groups are omitted, ellipsoids are drawn at the 50% probability level. Selected distances [Å] and angles [deg]: Sn1−Sn2 3.4756(6), Pd−Sn2 2.6164(5), Pd−Sn1 2.4939(5), Sn2−C3 2.225(4), Sn2−C4 2.181(5), Sn1−C2 2.118(5), Sn1−C1 2.188(5), Pd−C5 2.605(4), Pd−C6 2.241(4), Pd−C7 2.485(6), Sn2−Pd−Sn1 85.668(15), C4−Sn2−C3 98.68(18), C2−Sn1−C1 105.45(19), C1− Sn1−Pd 143.68(14), C2−Sn1−Pd 110.72(13), C4−Sn2−Pd 109.86(11), C3−Sn2−Pd 96.89(12).

Figure 4. ORTEP of the molecular structure of 3. Hydrogen atoms and isopropyl groups are omitted; ellipsoids are drawn at the 50% probability level. Selected distances [Å] and angles [deg]: Sn1−Sn2 3.1982(3), Sn1−Ni1 2.4300(4), Sn2−Ni1 2.6986(4), Sn2−C1 2.229(2), Sn2−C2 2.196(3), Sn1−C3 2.222(3), Sn1−C4 2.118(3), Ni1−C5 2.085(2), Ni1−C6 2.126(2), Ni1−C7 2.161(2), Ni1−C8 2.215(2), Ni1−C9 2.220(3), Ni1−C10 2.250(3), Ni1−Sn1−Sn2 55.3(1), Ni1−Sn2−Sn1 47.8(1), Sn2−Ni1−Sn1 77.0(1), C2−Sn2− Sn1 86.7(1), C4−Sn1−Sn2 99.4(1).

acceptor character; Sn1 features a trigonal planar geometry with a sum of angles of 360°, which is in line with the donor interaction with the Pd atom. The distance between the tin atoms of 3.4756(6) Å is much larger than the comparable distance in complex 3 and also in other cyclic distannenes. Besides tin coordination, the palladium atom is also stabilized by an interaction with a phenyl moiety of the terphenyl substituent. Three Pd−C distances lie in the range of 2.241(4)−2.605(4) Å, and the distances to the other carbon atoms of the ring are much longer. This type of arene coordination was also found in Pd(0) coordination compounds with monodentate aryl phosphines, which are highly active Suzuki catalysts.70,71 The coordination compound 4 was further characterized by NMR spectroscopy. In contrast to the solidstate structure exhibiting two different tin coordination modes we found for complex 4 in the room-temperature 119Sn{1H} NMR solution spectrum only one signal at 583 ppm with satellites due to coupling with the 117Sn nucleus of 2180 Hz. A dynamic interplay of the two stannylene units as shown in Scheme 5 could be a plausible explanation for the detection of only one 119Sn NMR signal. To further investigate this exchange (Scheme 5) of stannylene and terphenyl units in the coordination sphere of the palladium atom we recorded low-temperature 119Sn NMR spectra. At −80 °C two 119Sn

corroborates this interpretation: acceptor tin atom Sn2, carrying a lone pair, shows a sum of angles of 296.3° and the donor tin Sn1 of 359.9°, exhibiting the well-known trigonal planar stannylene coordination mode.62 In our coordination chemistry studies of stannylene-based Sn−P Lewis pairs we presented an example for the change of a stannylene−nickel coordination from donor to acceptor stannylene. The Ni−Sn bond length changes in these examples from donor to acceptor stannylene from 2.398(1) to 2.674(1) Å.30 Literature examples for typical stannylene−nickel coordination exhibit Ni−Sn bond lengths that are comparable to the donor−stannylene interaction.13,63,64 The coordination of the nickel atom at the aromatic ring of the terphenyl moiety is comparable with the xanthene literature example and other benzene Ni(0) complexes.31,65−67 The homologous palladium complex was synthesized by reacting the distannene with olefin complex Pd(nbe)3 (nbe = norbornene). The yield of the ditin palladium coordination compound 4 (80%) is much higher than the yield of the nickel complex (35%). Single crystals of the Pd(0) compound were obtained from a pentane solution at −40 °C. As in the nickel case, the palladium atom coordinates to the two stannylene units and the terphenyl moiety. In Figure 5 the molecular structure is shown together with selected distances and angles. Like in the other example for distannene coordination at a transition metal, the two tin units coordinate at the palladium atom in a donor (Sn1) and acceptor (Sn2) mode. The shorter distance [2.4939(5) Å] was found between the donor stannylene Sn1 and Pd, which lies in the range of other stannylene palladium coordination compounds [2.481(2)− 2.533(1) Å], whereas the acceptor tin palladium contact exhibits a larger value of 2.6164(5) Å.68,69 Further evidence for this Sn1-acceptor and Sn2-donor interpretation can be obtained by regarding the geometry of the substituents surrounding the tin atoms: Sn1 exhibits a trigonal pyramidal geometry with a sum of angles of 308°, corroborating a lone pair at Sn2 and the

Scheme 5. Dynamic Exchange of Coordinated Aryl Moieties and Tin Coordination Mode in the Palladium Complex 4

E


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Inorganic Chemistry NMR signals at 639 and 550 ppm, which are in accordance with the solid-state structure, were found. The dynamic switch of the coordination mode of both tin atoms goes along with an exchange of the aromatic moiety coordinated at the transition metal. Since we were not able to unambiguously assign the signals in the 1H NMR spectrum we can only ascertain an increasing number of signals in the 1H NMR spectrum upon cooling, which is an indicator for a deceleration of the dynamic exchange. To evaluate the possibility of coordinating two metals at the distannene we reacted compound 1 with 2 equiv of the Pd(0) olefin complex (Scheme 6) in tetrahydrofuran (THF) at −40 °C. From this mixture we isolated by crystallization red crystals of the dipalladium complex 5 with a yield of 93%.

Figure 6. ORTEP of the molecular structure of 5. Hydrogen atoms, isopropyl groups, and noncoordinating phenyl groups are omitted; ellipsoids are drawn at the 50% probability level. Selected distances [Å] and angles [deg] for both molecules in the asymmetric unit: Sn1− Sn2 3.1009(3), 3.0940(3), Sn1−Pd1 2.5451(4), 2.5456(4), Sn1−Pd2 2.6205(4), 2.6253(4), Sn2−Pd1 2.6155(4), 2.6227(3), Sn2−Pd2 2.5514(4), 2.5521(4), C1−Sn1 2.192(4), 2.191(3), C2−Sn1 2.160(4), 2.154(3), C3−Sn2 2.199(4), 2.197(4), C4−Sn2 2.150(3), 2.169(3), C9−Pd1 2.421(4), 2.604(4), C10−Pd1 2.259(4), 2.299(4), C11−Pd1 2.560(4), 2.382(4), C7−Pd2 2.305(4), 2.284(3), C8−Pd2 2.326(4), 2.375(3), Sn1−Pd1−Sn2 73.85(1), 73.42(1), Sn2−Pd2− Sn1 73.66(1), 73.48(1), Pd1−Sn1−Pd2 96.49(1), 96.85(1), Pd1− Sn1−Sn2 54.11(1), 52.24(1), Pd2−Sn1−Sn2 52.15(1), 54.44(1), Pd2−Sn2−Pd1 96.46(1), 96.63(1), Pd2−Sn2−Sn1 54.19(1), 52.08(1), Pd1−Sn2−Sn1 52.03(1), 54.34(1).

Scheme 6. Reaction of the Bis(stannylene) with 2 equiv of [Pd(nbe)3]

substituted by hydrogen atoms. The density functional theory (DFT)-optimized geometry of 3′ (Figure 7) was in good

Figure 6 shows the molecular structure of 5 in the solid state. Each palladium atom is coordinated by two stannylene units. In analogy to complex 4 the shorter Pd−Sn distances point toward the donor stannylene interaction, whereas the longer Pd−Sn bonds belong to the interaction with the palladium atom acting as a donor and the tin atom as an acceptor. The lengths of the interactions can be compared with compound 4 and stannylene−palladium coordination described in the literature.13,68,69 Although both tin atoms act as a donor as well as an acceptor in 5, the Sn−Sn distances of 3.1009(3) and 3.0940(3) Å (two molecules in the asymmetric unit) are the shortest ones found in the three isolated coordination compounds. Both palladium atoms are also coordinated by phenyl moieties of the terphenyl substituent and show interatomic Pd−C distances which lie in the range of the values found for 4.70,71 As expected, in the 119Sn{1H} NMR spectrum in solution one signal for the symmetric tin atoms was observed exhibiting a 119−117Sn coupling constant of 1875 Hz. Comparable structural motives for Pd2Sn2-coordination compounds were found in the literature.13,72 However, in these cases the Pd−Pd distances lie in the range of 2.583(1)− 2.798(1) Å and are much shorter than the values found in 5 [3.867(1), 3.854(1) Å]. In these stannylene donor coordination compounds the Sn−Pd bonds are longer [2.687(1)−2.700(2) Å] than the values found for 5. To further evaluate the bonding situation in the ditin coordination compounds a derivative of 3, model system 3′, was investigated by quantum chemical calculations. In 3′ the isopropyl groups of the noncoordinating aryl moieties of 3 were

Figure 7. DFT-optimized geometry of 3′ and selected NLMOs representing the donor−acceptor interactions between Ni and both Sn.

agreement with the solid-state structure of 3 concerning the inner Sn2Ni-core. The results of the natural bond orbital (NBO) analysis support the interpretation of the donor− acceptor bonding in 3: Sn1 therefore acts as donor toward Ni, with its lone pair directed at the Ni center (Figure 7b), whereas Sn2 acts as an acceptor for an electron pair donated from the Ni atom (Figure 7a). In the latter case, the respective natural localized molecular orbital (NLMO) revealed that 8% of the electron pair residing in a Ni d-orbital is donated into an empty Sn p-orbital. Furthermore, on the basis of the optimized geometry of 3′, calculations of the 119Sn NMR shifts were performed. The obtained values of 544 (Sn1) and 24 ppm (Sn2) are in excellent accordance with their measured 119Sn NMR spectroscopic counterparts of 520 and 62 ppm (for Sn1 and Sn2, respectively). They also confirm that the acceptor tin atom Sn2 exhibits the resonance at higher field, whereas the F


Article

Inorganic Chemistry donor tin atom Sn1 appears at lower field. Overall, the theoretical results clearly support the interpretation of the nonstabilized stannylenes acting as z-type acceptor as well as donor ligands in coordination compounds.

15 ms, and spinal64 decoupling. Referencing was achieved by the substitution method: an external sample of CHCl3 in acetone was spun at 2 kHz, and the external magnetic field was adjusted such that the 1H chemical shift of CHCl3 matched a predetermined chemical shift with respect to external 1% tetramethylsilane (TMS) in CHCl3. MAS spectra were analyzed using Herzfeld−Berger Analysis (HBA).75 X-ray Crystal Structure Analysis. X-ray data were collected with a Bruker Smart APEX II diffractometer with graphite-monochromated Mo Kα radiation. The programs used were Bruker’s APEX2 v2011.8− 0 including SADABS for multiscan absorption correction and SAINT for data reduction and SHELXS for structure solution as well as WinGX suite of programs v1.70.01 including SHELXL for structure refinement.76−79 Details of the structure refinement and solution were placed in the Supporting Information. Mö ssbauer Spectroscopy. A 57Co/Rh and a Ca119mSnO3 source were available for the 57Fe and 119Sn Mössbauer spectroscopic investigations. The samples were placed within thin-walled glass containers at a thickness of ∼10 mg Sn(Fe)/cm2. In the case of the 119 Sn measurements a palladium foil of 0.05 mm thickness was used to reduce the tin K X-rays concurrently emitted by this source. The measurement was performed in a continuous-flow cryostat system (Janis Research Co LLC) at 5 K. The source was kept at room temperature. The temperature was controlled by a resistance thermometer (±0.5 K accuracy). Fitting of the spectra was performed with the Normos-90 software package.80 The counting time was 11 d for the 119Sn and 5 d for the 57Fe spectra. The 57Fe Mössbauer spectra were placed in the Supporting Information. Quantum Chemical Calculations. DFT calculations were performed with Gaussian09.81 For model system 3′ the isopropyl groups of the noncoordinating aryl moieties in 3 were substituted by hydrogen atoms. The molecular structure of 3′ was optimized using the BP86 functional, along with the implemented def2-TZVP basis set for all atoms, except Sn.82−85 For the tin atoms, Stuttgart Dresden effective core potentials were employed, in combination with optimized valence basis sets as implemented in Gaussian09.86,87 Dispersion corrections were included by adding the D3 version of Grimme’s dispersion with Becke-Johnson damping.88 The geometry optimization was performed without imposing any symmetry constraints, and the structure obtained was confirmed as a true minimum by calculating analytical frequencies, which gave one spurious imaginary frequency. Natural bond orbitals were obtained using the NBO 6.0 software.89−91 Plots were generated with the software Chemcraft.92 On the basis of the optimized geometry, NMR calculations were performed using ADF with the GGA revPBE-D3(BJ) functional and ZORA TZ2P basis set for all atoms.93−101 [(2,6-Trip2)C6H3Sn]2(C5H4)2Fe (1). 1,1′-Dilithioferrocene (3:2-adduct with tetramethylethylenediamine (TMEDA) (108 mg, 0.131 mmol) was dissolved in 20 mL of toluene and cooled to −40 °C. The solution was then added slowly to a precooled solution of Ar*SnCl (Ar* = 2,6-Trip2C6H3) (500 mg, 0.786 mmol), and the mixture was stirred at room temperature overnight. The intensely dark red colored mixture was then dried in vacuo, extracted with hexane, and filtered over diatomaceous earth. The filtrate was concentrated, and dark red crystals of compound 1 were obtained overnight at ambient temperature. Yield: 337 mg (62%) 1H NMR (C6D6, 26 °C, 400.13 MHz): δ (ppm) 1.12 (d, 24H, 3J(1H−1H) = 6 Hz, ortho-CH(CH3)2), 1.26 (d, 24H, 3J(1H−1H) = 6 Hz, ortho-CH(CH3)2), 1.31 (d, 3 1 J( H−1H) = 7 Hz, 24H, para-CH(CH3)2), 2.88 (m, 4H, paraCH(CH3)2), 3.27 (d, 8H, ortho-CH(CH3)2), 3.48 (m, 4H, Cp-CH), 3.94 (m, 4H, Cp-CH), 7.15 (s, 8H, Trip-CH), 7.27 (s, 6H, Aryl-CH). 13 C{1H} NMR (C6D6, 26 °C, 125.76 MHz): δ (ppm) 22.9 (orthoCH(CH3)2), 24.4 (para-CH(CH3)2), 26.7 (ortho-CH(CH3)2), 31.1 (ortho-CH(CH3)2), 34.9 (para-CH(CH3)2), 70.8 (Cp-CH), 77.5 (CpCH), 113.4 (Cp-CSn), 121.6 (Trip-CH), 126.1 (Aryl-CH), 130.4 (Aryl-CH), 136.4 (Trip-C), 145.6 (Aryl-C), 147.9 (ortho-C-iPr), 149.2 (para-C-iPr), 172.9 (Aryl-C). 119Sn{1H} NMR (C6D6, 26 °C, 93.28 MHz): δ (ppm) 871 (s, Δν1/2 = 2.7 kHz), 119Sn CP/MAS NMR (111.92 MHz): δiso = 1111, δ11 = 2805, δ22 = δ33 = 265. Anal. Calcd (%) for C82H106FeSn2: C, 71.11; H, 7.71. Found: C, 70.67; H, 7.90.

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CONCLUSION Nonstabilized stannylenes were synthesized by nucleophilic substitution between 1,1′-dilithioferrocene and 2 equiv of terphenyl tinchloride. Depending on the size of the terphenyl substituent a cyclic distannene or a bis(stannylene) was isolated and characterized in the solid state by crystal-structure analyses, 119 Sn Mössbauer spectroscopy, as well as 119Sn solid-state NMR spectroscopy. In the cyclic distannene, which was formed using a mesityl-substituted terphenyl moiety, the tin−tin bond remains intact in solution even at temperatures as high as 100 °C. In contrast, the triisopropyl-substituted terphenyl derivative exhibits a bis(stannylene) structure in the solid state but shows a temperature-dependent equilibrium in solution between tin−tin bond formation and cleavage: cooling the bis(stannylene) to −80 °C results in formation of the cyclic distannene. This dynamic bis(stannylene) reacts as a chelating ligand and coordinates nickel or, depending on stoichiometry, one or two palladium atoms in oxidation state zero. In these coordination compounds the nonstabilized stannylenes act as donor as well as z-type acceptor ligands and in the monopalladium complex exhibit a dynamic switch from donor to acceptor behavior.

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EXPERIMENTAL SECTION

General Information. All manipulations were performed under an argon atmosphere using standard Schlenk techniques or an MBraun Glovebox. Diethyl ether and benzene were distilled from sodium/ benzophenone, while n-hexane was obtained from an MBRAUN solvent purification system. Benzene-d6 was also distilled from sodium. In addition, all solvents were repeatedly degassed by several freeze− pump−thaw cycles and stored in a glovebox. [2,6-Mes2C6H3SnCl]2 (Mes = mesityl), [2,6-Trip2C6H3SnCl]2 (Trip = C6H2-2,4,6-iPr3), 1,1′dilithioferrocene, and [Pd(nbe)3] were synthesized following literature procedures.16,35,36,73,74 All other compounds were purchased commercially (Aldrich) and used without further purification. Elemental analysis was performed by the Institut für Anorganische Chemie, Universität Tübingen, using a Vario MICRO EL analyzer. NMR Spectroscopy. NMR spectra were recorded on a Bruker DRX250 NMR spectrometer (1H 250.13 MHz; 13C 62.90 MHz; 119Sn 93.28 MHz) equipped with a 5 mm ATM probe head, a Bruker AvanceII+400 NMR spectrometer (1H 400.11 MHz; 13C 100.61 MHz) equipped with a 5 mm quad nucleus probe (QNP) head or a Bruker AvanceII+500 NMR-spectrometer (1H 500.13 MHz; 13C 125.76 MHz; 119Sn 186.50 MHz) equipped with a 5 mm ATM or a 5 mm TBO probe head and a setup for variable temperature. Routinely, “room-temperature” NMR spectra were obtained at 26 °C. The chemical shifts are reported in δ values in parts per million relative to external SiMe4 (1H, 13C) or SnMe4 (119Sn) using the chemical shift of the solvent 2H resonance frequency and Ξ = 25.145 020% for 13C and 37.290 632% for 119Sn.42 The multiplicity of the signals is abbreviated as s = singlet, d = doublet, t = triplet, quint = quintet, sept = septet, and m = multiplet or unresolved. The proton and carbon signals were assigned by detailed analysis of 1H, 13C{1H}, 1H−1H COSY, 1H−13C HSQC, 1H−13C HMBC, and 13C{1H} DEPT-135 NMR spectra. 119Sn cross-polarization magic-angle spinning (CP/MAS) NMR spectra were obtained on a Bruker Avance III HD 300 wide-bore NMR spectrometer operating at 300.13 (1H) and 112.0 MHz (119Sn). Powdered samples were packed in 4 mm o.d. zirconia rotors in a glovebox and spun about the magic angle at spinning frequencies in the range of 10−12 kHz. Spectra were acquired after cross-polarization using a ramp from 100 to 70% on the 119Sn channel, a contact time of G


Article

Inorganic Chemistry [(2,6-Mes2)C6H3Sn]2(C5H4)2Fe (2). Synthesis of Compound 2. 1,1′Dilithioferrocene (3:2-adduct with TMEDA) (255 mg, 0.309 mmol) was dissolved in 20 mL of toluene and cooled to −40 °C. The solution was then added slowly to a precooled solution of ArMesSnCl (ArMes = 2,6-Mes2C6H3) (867 mg, 1.850 mmol), and the mixture was stirred at room temperature overnight. The intensely dark violet colored mixture was then dried in vacuo, extracted with hexane and filtered over diatomaceous earth. The filtrate was concentrated and dark black microcrystalline material of compound 2 was obtained overnight at −40 °C. Yield: 300 mg (31%) 1H NMR (C6D6, 26 °C, 400.13 MHz): δ (ppm) 2.18 (s, 24H, ortho-CH3), 2.27 (s, 12H, para-CH3), 3.55 (s, Δν1/2 = 8 Hz, 4H, Cp-CH), (m, 4H, Cp-CH), 6.75 (s, Δν1/2 = 3 Hz, 8H, mesityl-CH), 6.97 (d, 4H, 2J(1H−1H) = 7 Hz, Ar−CH), 7.20 (t, 2H, 2J(1H-1H) = 7 Hz, Ar−CH) (all resonances without explicitly given Δν1/2 reveal Δν1/2 = 0.5−2 Hz). 13C{1H} NMR (C6D6, 26 °C, 125.76 MHz): δ (ppm) 21.5 (para-CH3), 21.9 (ortho-CH3), 71.4 (Δν1/2 = 11 Hz, Cp-CH), 77.8 (Cp-CH), 116.8 (Cp-CSn), 128.3 (Ar− CH), 128.4 (Ar−CH), 129.1 (mesityl-CH), 137.1 (para-CCH3), 137.2 (ortho-CCH3), 141.1 (ipso-CCH3), 147.8 (Ar−C), 166.6 (Ar−C (all resonances without explicitly given Δν1/2 reveal Δν1/2 = 2−3 Hz). 119 Sn{1H} NMR (C6D6, 26 °C, 93.28 MHz): δ (ppm) 449 (s + satellites, 1J(119Sn−117Sn) = 6087 Hz). 119Sn CP/MAS NMR (111.92 MHz): δiso = 561, δ11 = 1572, δ22 = 311, δ33 = −199; δiso = 536, δ11 = 1515, δ22 = 300, δ33 = −206. Anal. Calcd (%) for C58H58FeSn2: C, 66.45; H, 5.58. Found: C, 66.88; H, 5.54. Synthesis of Compound 3. Ni(COD)2 (20 mg, 0.072 mmol) was added to a solution of 1 (100 mg, 0.072 mmol) in 10 mL of benzene. The solution was stirred for one week at 40 °C displaying a color change from intensely deep red to almost black during that time. After the solvent was removed under reduced pressure, the residue was dissolved in pentane, and the solution was allowed to crystallize at −40 °C for several days. 3 was isolated as dark violet, almost black needles. Yield: 36 mg (35%). Because of the instability of compound 3 in solution we were not able to assign the signals in the 1H and 13C{1H} NMR. 119Sn{1H} NMR (C6D6, 26 °C, 93.28 MHz): δ (ppm) 62 (s + satellites, 1J(119Sn−119/117Sn) = 1545 Hz, Sn1), 520 (s + satellites, 1 119 J( Sn−119/117Sn) = 1545 Hz, Sn2). Anal. Calcd (%) for C82H106FeNiSn2: C, 68.22; H, 7.40. Found: C, 68.43; H, 7.51. Synthesis of Compound 4. Pd(nbe)3 (14 mg, 0.036 mmol) was added under stirring to a precooled solution of 1 (50 mg, 0.0361 mmol) in 5 mL of THF (−40 °C). The solution immediately changed color from intensely deep red to black, and stirring was continued for 30 min at ambient temperature. After the solvent was removed under reduced pressure, the residue was dissolved in n-pentane, and the solution was allowed to crystallize at −40 °C overnight. 4 was obtained as black crystalline material. Yield: 43 mg (80%). 1H NMR (C6D6, 26 °C, 400.13 MHz): δ (ppm) 1.09 (d, 24H, 3J(1H−1H) = 7 Hz, orthoCH(CH3)2), 1.29 (d, 24H, 3J(1H−1H) = 7 Hz, para-CH(CH3)2), 1.45 (s, Δν1/2 = 21 Hz, 24H, 3J(1H−1H) = 7 Hz, ortho-CH(CH3)2), 2.90 (m, 4H, para-CH(CH3)2), 3.11 (m, 8H, ortho-CH(CH3)2), 4.28 (s, Δν1/2 = 11 Hz, 4H, Cp-H), 7.04−7.16 (m, 12H, Ar−CH). 13C{1H} NMR (C6D6, 26 °C, 125.76 MHz): δ.(ppm) 24.1 (ortho-CH(CH3)2), 24.4 (para-CH(CH3)2), 26.4 (ortho-CH(CH3)2), 31.4 (para-CH(CH3)2), 34.5 (ortho-CH(CH3)2), 72.1 (Cp-CH), 79.1 (Cp-CH), 111.6 (Ar−C), 125.9 (Ar−C), 130.7 (ortho-Trip-C), 135.1 (ortho-CiPr), 145.5 (ipso-Trip-C), 149.1 (para-C-iPr), 165.0 (Ar−C). 119Sn{1H} NMR (C6D6, 26 °C, 93.28 MHz): δ (ppm) 583 (s + satellites, Δν1/2 = 212 Hz, 1J(119Sn−117Sn) = 2180 Hz, Sn1/Sn2). Anal. Calcd (%) for C82H106FePdSn2: C, 66.04; H, 7.16. Found: C, 65.74; H, 6.80. Synthesis of Compound 5. Pd(nbe)3 (28 mg, 0.072 mmol) was added under stirring to a precooled solution of 1 (50 mg, 0.0361 mmol) in 5 mL of THF (−40 °C). A color change of the solution from intensely deep red over blue back to red was observed. Stirring was continued for 30 min at ambient temperature. After the solvent was removed under reduced pressure, the residue was dissolved in npentane, and the solution was allowed to crystallize at −40 °C overnight to yield 5 as red needles. Yield: 58 mg (93%). 1H NMR (C6D6, 26 °C, 400.13 MHz): δ (ppm) 0.73 (d, 6H, 3J(1H−1H) = 5 Hz, Δν1/2 = 12 Hz, CH(CH3)2), 1.03 (d, 12H, 3J(1H−1H) = 7 Hz, CH(CH3)2), 1.08 (s, Δν1/2 = 15 Hz, CH(CH3)2), 1.35−1.39 (multiple

resonances, 24H, CH(CH3)2), 1.61 (d, 6H, 3J(1H−1H) = 5 Hz, Δν1/2 = 15 Hz, CH(CH3)2), 1.68 (d, 6H, 3J(1H−1H) = 5 Hz, Δν1/2 = 18 Hz, CH(CH3)2), 1.75 (d, 6H, 3J(1H−1H) = 5 Hz, Δν1/2 = 17 Hz, CH(CH3)2), 2.54 (m, 2H, CH(CH3)2), 2.69 (m, 2H, CH(CH3)2), 2.93 (m, 2H, CH(CH3)2), 3.04 (m, 2H, CH(CH3)2), 3.13(m, 4H, CH(CH3)2), 3.61 (s, 2H, Δν1/2 = 10 Hz, Cp-CH), 4.12 (s, 2H, Δν1/2 = 10 Hz, Cp-CH), 4.19 (s, 4H, Δν1/2 = 5 Hz, Cp-CH), 6.88−7.24 (multiple resonances, 14H, Aryl-CH) (all resonances without explicitly given Δν1/2 reveal Δν1/2 = 2−3 Hz) 13C{1H} NMR (C6D6, 26 °C, 125.76 MHz): δ.(ppm) 22.7 (CH(CH3)2), 23.9 (CH(CH3)2), 24.4 (CH(CH3)2), 24.8−25.1 (multiple resonances, CH(CH3)2), 25.9 (CH(CH3)2), 26.1 (CH(CH3)2), 26.4 (CH(CH3)2), 30.8 (CH(CH3)2), 31.7 (CH(CH3)2), 32.0 (CH(CH3)2), 33.9 (CH(CH3)2), 35.1 (CH(CH3)2), 68.8 (Cp-CH), 70.0 (Cp-CH), 76.7 (Cp-CH), 78.9 (Cp-CH), 90.7 (Cp-Sn), 118.1 (Ar−CH), 118.1 (Ar−CH), 120.7 (Ar−CH), 120.9 (Ar−CH), 126.0 (Ar−C), 126.3 (Ar−C), 129.1 (Ar−CH), 129.4 (Ar−CH), 136.9 (Ar−C), 139.1 (Ar−C), 140.4 (Ar−C), 142.8 (Ar−C), 146.4 (Ar−C), 146.6 (Ar−C), 147.4(Ar−C), 148.3(Ar−C), 149.2(Ar−C), 161.5(Ar−C). 119Sn{1H} NMR (C6D6, 26 °C, 93.28 MHz): δ (ppm) 208 (s + satellites, 1J(119Sn−117Sn) = 1875 Hz Sn1/Sn2). Anal. Calcd (%) for C82H106FePd2Sn2 2C5H12: C, 63.43; H, 7.52. Found: 62.62, 7.23.

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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/acs.inorgchem.8b00317. Data of crystal structure analyses, NMR spectra of published compounds, 57Fe Mössbauer spectra of 1 and 2, and details of quantum chemical calculations (PDF) Accession Codes

CCDC 1575890−1575893 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

*E-mail: [email protected]. ORCID

Lars Wesemann: 0000-0003-4701-4410 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge support by the state of BadenWürttemberg through bwHPC and the German Research Foundation through Grant No. INST 40/467-1 FUGG. L.W. thanks Dr. C. Sindlinger for support with respect to quantum chemical calculations.

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REFERENCES

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