DOI: 10.1002/chem.201500607
Communication
& Stannylenes
Formation of an Imino-Stabilized Cyclic Tin(II) Cation from an Amino(imino)stannylene Tatsumi Ochiai, Daniel Franz, Elisabeth Irran, and Shigeyoshi Inoue*[a] Dedicated to Professor Yohsuke Yamamoto on the occasion of his 60th birthday
Abstract: The novel amino(imino)stannylene 1 was prepared by conversion of HNIPr (NIPr = bis(2,6-diisopropylphenyl)imidazolin-2-imino) with one equivalent of Lappert’s tin reagent (Sn[N(SiMe3)2]2). Treatment of 1 with DMAP (4-dimethylaminopyridine) yields its Lewis acid– base adduct 2. The reaction of 1 with one equivalent of trimethylsilyl azide results in replacement of the amino group at the tin center by an N3 substituent with concomitant elimination of N(SiMe3)3 to afford dimeric [N3SnNIPr]2 (3). Remarkably, the reaction of 1 with B(C6F5)3 produces the novel tin(II) monocation 4 + [MeB(C6F5)3] comprising a four-membered stannacycle through methyl-abstraction from the trimethylsilyl group.
Figure 1. Bis(amino)stannylene (Lappert’s tin reagent) I, tin(II) monocations II, as well as III and imidazoline-2-imino complexes of main group elements (IV and V; Dip = 2,6-diisopropylphenyl, X = H, BH4, SH, Cl, Br, I).
The field of low-valent tin compounds has received much attention since the isolation of the first monomeric bis(amino)stannylene I by Lappert and co-workers (Figure 1).[1] Today, a variety of bis(amino)stannylenes is known for which the ability of the nitrogen atoms to act as s- and p-electron donors grants thermodynamic stability to the low-valent metal center.[2] Furthermore, molecular complexes of tin(II) are commonly acknowledged for their various applications in small-molecule activations. Remarkable in this regard is the synthesis and structural characterization of electron-precise compounds with an Sn=E double bond (E = O, S, Se, Te),[3] as well as related stannaimines (Sn=N).[4] Sparked by the pioneering reports of Jutzi and co-workers on half-sandwich stannocene cations, the field of low-valent tin cations has been a major topic of interest in main-group chemistry for decades.[5] However, only a few examples of divalent tin monocations are known to date.[6] One is the b-diketiminato-stabilized stannylene cation II (Figure 1).[6c] Outstanding is the report by Jones and co-workers on the amido-substituted tin(II) monocation III (Figure 1).[6d] Recently, our group has described a variety of boron,[7] aluminum (i.e. IV, Figure 1),[8] and silicon (i.e. V, Figure 1)[9] com-
plexes using the imidazolin-2-imino group. Moreover, Bertrand implemented the strongly related imidazolidin-2-iminato ligand to accomplish the isolation of very rare examples for molecular phosphorus mononitride and phosphinonitrene, as well as of a iminophosphonium salt.[10] In addition, the transition-metal chemistry of this ligand with pronounced electrondonating abilities has been thoroughly studied by Tamm and co-workers.[11] Herein, we describe the synthesis and reactivity of the amino(imino)stannylene 1. An unprecedented tin(II) monocation with a four-membered stannacycle was isolated after the reaction of 1 with B(C6F5)3. The reaction of bis(2,6-diisopropylphenyl)imidazolin-2-imine (HNIPr) with Sn[N(SiMe3)2]2 in toluene at 60 8C furnishes the novel amino(imino)stannylene 1 in near-quantitative yield (93 %, Scheme 1). The 119Sn{1H} NMR spectrum of 1 in C6D6 shows a signal at d = 208.0 ppm that is significantly shifted to higher field as compared to that of the Sn[N(SiMe3)2]2 pre-
[a] T. Ochiai, Dr. D. Franz, Dr. E. Irran, Prof. Dr. S. Inoue Institut fr Chemie, Anorganische Chemie Technische Universitt Berlin Straße des 17. Juni 135, Sekr. C2, 10623 Berlin (Germany) E-mail: [email protected]
Scheme 1. Synthesis of amino(imino)stannylene 1 and its conversion to the Lewis acid–base adduct 2 (Dip = 2,6-diisopropylphenyl, DMAP = 4-dimethylaminopyridine).
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201500607. Chem. Eur. J. 2015, 21, 1 – 5
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Communication cursor (I: d(119Sn) = 767 ppm, C6D6).[12] Interestingly, the resonance is shifted downfield (d(119Sn) = 140.9 ppm) if [D8]THF is used as the solvent (I: d(119Sn) = 602 ppm, [D8]THF).[12] We presume that the high-field signal in deuterated benzene is produced by an aggregated species with a higher-coordinate tin(II) atom. Similarly, coordination of solvent to the tin(II) atom is very likely to have an impact on the 119Sn chemical shift observed for 1 in [D8]THF. The addition of DMAP to a solution of 1 in toluene results in coordination of the Lewis base to the tin(II) center to afford 2 as verified by multinuclear NMR spectroscopy. The 119Sn{1H} NMR spectrum (C6D6) shows a singlet at d = 3.4 ppm which is shifted to lower field as compared to that of 1 but shifted upfield in comparison to that of Sn[N(SiMe3)2]2 (vide supra). X-ray diffraction analysis of 2 revealed a monomeric structure in which a trigonal-pyramidal coordination environment is observed for the tin atom (the sum of the bond angles around the tin center amounts to 2798, Figure 2). The Sn1 N3 distance Scheme 2. Formation of dimeric stannylene azide 3 (top). Related conversion of VI to stannylene azide VII as reported by Zemlyansky (bottom).[14]
lene Sn(O(CH2)2NMe2)2 (VI) with N3SiMe3, which proceeds via elimination of Me3SiO(CH2)2NMe2 (Scheme 2).[14] Theoretical calculations indicated that the formation of [(N3)Sn(m-O)(CH2)2NMe2]2 involves ligand redistribution reactions between Sn(O(CH2)2NMe2)2 and N3SiMe3.[15] Moreover, the study revealed that the formation of a stannaimine is kinetically disfavored for this system. Similar considerations may apply to our related conversion of 1 into 3. An X-ray crystallographic investigation shows 3 to be a centrosymmetric dimer, with a planar and rhombic N2Sn2 ring (Figure 3). The imino groups bridge the two tin(II) centers with Sn N bond lengths of 2.194(3) and 2.178(3) . The comparison with the Sn N bond lengths in 2 suggests a partial dative-
Figure 2. ORTEP representation of the molecular structure of 2 in the solid state. Thermal ellipsoids are at the 40 % probability level. Hydrogen atoms are omitted for clarity. Dip groups are depicted as stick models. Selected bond lengths [] and bond angles [8]: Sn1 N3 2.0588(13), Sn1 N4 2.1647(12), Sn1 N5 2.3550(13), N1 C1 1.4116(19), N2 C1 1.4124(19), N3 C1 1.262(2); N3-Sn1-N4 103.12(5), N3-Sn1-N5 85.31(5), N4-Sn1-N5 90.86(5).
of 2.0588(13) is shorter than the Sn1 N4 distance of 2.1647(12) , which suggests that the tin center interacts more strongly with the nitrogen atom of the iminato ligand than it does with the nitrogen atom of the amino group. In accordance with the higher coordination number of the tin(II) center the Sn1 N4 distance in 2 is increased in comparison to that in Sn[N(SiMe3)2]2 (2.088(6) , 2.096(1) ).[1c] The Sn1 N5 distance of 2.3550(13) is in the range of Sn N bond lengths reported for typical pyridine-stabilized stannylenes.[13] Inspired by the literature-known syntheses of rare stannaimines[4] we treated 1 with N3SiMe3. As a result, the distannylene diazide 3 was obtained in 32 % yield after recrystallization from THF. The formation of 3 can be reasoned by the cleavage of the bond between the tin atom and the amino group with release of N(SiMe3)3 (Scheme 2). Zemlyansky and co-workers described the synthesis of the dimeric tin(II) azide [(N3)Sn(mO)(CH2)2NMe2]2 (VII) by reaction of the base-stabilized stanny&
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Figure 3. ORTEP representation of the molecular structure of 3 in the solid state. Thermal ellipsoids are at the 40 % probability level. Hydrogen atoms are omitted for clarity. Dip groups are depicted as stick models. Symmetryrelated atoms are marked by an asterisk. Selected bond lengths [] and bond angles [8]: Sn1 N1 2.194(3), Sn1 N1* 2.178(3), Sn1 N6 2.110(9), C1 N1 1.307(5), C1 N2 1.377(5), C1 N3 1.393(4); N1*-Sn1-N1 77.46(12), Sn1*-N1Sn1 102.54(12), N4-N5-N6 172.6(15), N5-N6-Sn1 116.1(7).
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Communication bond character for the tin–nitrogen interactions in 3. The structural features of the N2Sn2 ring are similar to those in [(h3Cp)Sn{m-NC(NMe2)2}]2.[16] The azido groups in 3 occupy positions above and below the face of the N2Sn2 ring presumably to avoid steric hindrance. In the 1H- and 13C{1H} NMR spectra of 3 in [D8]THF, the expected resonance pattern for the iminato ligand splits into two sets of signals. This indicates the existence of an equilibrium between dimeric 3 and a proposed monomeric species 3’, which is resolved on the NMR timescale. The 119Sn{1H} NMR spectrum shows two signals at d = 285.0 ppm and d = 38.9 ppm. The resonance at higher field is in the region of typical tin(II) azides with a three-coordinate metal center.[17] This justifies the assignment of the signal observed at d(119Sn) = 285.0 ppm to the dimeric stannylene azide 3. To clarify the unexpected resonances we applied 2D DOSY NMR spectroscopy to a solution of 3 in [D8]THF (see the Supporting Information).[18] This NMR experiment revealed two species with distinct diffusion constants. To date, only a few borane adducts of stannylenes have been structurally characterized.[19] Thus, we treated stannylene 1 with B(C6F5)3 in hexane and observed conversion to the ionic
adduct of III (Figure 1, adduct not shown) resonates at d(119Sn) = 30 ppm (CD2Cl2).[6d] The molecular structure of 4 + [MeB(C6F5)3] (toluene)0.5 comprises a planar N2SiSn cycle and the Sn1 N4 distance amounts to 2.062(2) (Figure 4). Hence, this bond length is in the range
Figure 4. ORTEP representation of the molecular structure of 4 + [MeB(C6F5)3] (toluene)0.5 in the solid state. Thermal ellipsoids are at the 40 % probability level. Dip groups and counter anion are depicted as stick models. Hydrogen atoms and toluene are omitted for clarity. Selected bond lengths [] and bond angles [8]: Sn1 N3 2.197(2), Sn1 N4 2.062(2), Si1 N3 1.769(2), Si1 N4 1.725(2), Si1 C28 1.850(3), Si1 C29 1.846(3), Si2 N4 1.730(2), C1 N1 1.362(3), C1 N2 1.362(3), C1 N3 1.331(3); N3-Sn1-N4 73.43(8), N3-Si1-N4 93.65(11), Si1-N3-Sn1 93.31(9), Si1-N4-Sn1 99.52(11).
found for typical stannylenes with bis(amido) ligands that feature a four-membered N2SiSn cycle and a two-coordinate tin(II) center.[20] In comparison, the Sn1 N3 distance (2.197(2) ) is significantly larger. This contrasts the respective bonding situation of the DMAP adduct 2, in which the distance of the imino-nitrogen atom to the tin(II) center is shorter than that of the amido-nitrogen atom. As a result, we presume a high dative-bond character between the tin(II) center and the imino-nitrogen atom. Accordingly, resonance structure A1 (Scheme 3) is one suitable description for the bonding situation in 4 + , that is, an amido-bound tin(II) cation stabilized by intramolecular coordination of an imidazolin-2-imino group. Moreover, the C1 N3 distance (1.331(3) ) of the imino moiety in 4 + is significantly longer than the respective bond lengths in 2 and 3 and possesses a value between that of a typical C N single bond and a C=N double bond.[21] This suggests that the positive charge in 4 + is in part delocalized across the adjacent imidazoline moiety. Consequently, 4 + also comprises considerable imidazolium-cation character as represented by resonance structure A2 (Scheme 3). The formulation of A1 and A2 agrees with the results of our quantum-mechanical calculations on 4 + (vide infra). The calculated WBI of the Sn Nimine bond in 4 + amounts to 0.390, which is significantly lower than that of the Sn Namine interaction (0.674). For comparison, we calculated a WBI of 0.578 for the Sn Nimine bond in the DMAP adduct 2. As expected from our X-ray study, the WBI of the CNHC Nimine bond (1.335) in 4 + is lower than that of the CNHC Nimine bond in 2 (1.737). The calculated LUMO + 1 and LUMO of 4 + essentially comprise an empty p-orbital that is centered at the tin(II) atom (see the Supporting Information, Figure S25). The electron lone-pair at
Scheme 3. Synthesis of the four-membered tin(II) cation 4 + [MeB(C6F5)3] (top) and resonance structures of 4 + (bottom; Dip = 2,6-diisopropylphenyl).
tin(II) borate salt 4 + [MeB(C6F5)3] (Scheme 3). Reasonably, the formation of this product involves the abstraction of a siliconbound methyl group from 1 by the Lewis acid affording a silylium cation, which is stabilized by intramolecular interaction with the imino moiety. The 119Sn{1H} NMR spectrum of 4 + [MeB(C6F5)3] displays a signal at d = 33.5 ppm in CD3CN. In contrast, a significantly downfield-shifted signal is found in C6H5F (d = 355.0 ppm). We conclude that acetonitrile coordinates to the tin(II) center of 4 + and that only negligible interaction between the cation and the solvent occurs in fluorobenzene solution. This is supported by the 119Sn chemical shifts observed for the three-coordinate tin(II) monocation [{HC(MeCNDip)2}Sn(OEt2)] + (II, d(119Sn) = 139.5 ppm), and its congener [{HC(MeCNDip)2}Sn] + (d(119Sn) = 197 ppm, CD2Cl2), which exhibit a very similar difference in the 119Sn chemical shifts of the higher and the lower coordinate species.[6c] Notably, the DMAP Chem. Eur. J. 2015, 21, 1 – 5
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Communication the tin center is observed for the HOMO 3 and possesses a pronounced s-orbital component. In conclusion, we synthesized the novel amino(imino)stannylene 1 from the reaction of HNIPr with Sn[N(SiMe3)2]2 and its DMAP adduct 2. Compound 1 reacts with N3SiMe3 to afford the tin(II) dimer 3 which features an azido-functionalized N2Sn2 ring. The reaction of 1 with B(C6F5)3 furnishes the borate salt 4 + [MeB(C6F5)3] in a rare type of methyl-abstraction and ringclosing reaction unprecedented for stannylenes. Furthermore, 4 + [MeB(C6F5)3] is the first isolated monocationic molecular complex of tin(II) with a four-membered stannacycle and a two-coordinate tin center. Future investigations will focus on the reactivity of 4 + versus unsaturated functionalities in organic substrates.
[7]
[8] [9] [10]
Acknowledgements We are exceptionally grateful to the Alexander von Humboldt Foundation (Sofja Kovalevskaja Program) for financial support. We thank Paula Nixdorf for collecting X-ray diffraction data, as well as Samantha Voges for the recording of DOSY NMR spectra.
[11]
Keywords: azides · boranes · cations · stannylenes · tin [12]
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COMMUNICATION & Stannylenes T. Ochiai, D. Franz, E. Irran, S. Inoue* && – && A welcome surprise: A novel amino(imino)stannylene was accessed by means of the strongly electron-donating imidazolin-2-imino group. Its reactivity with an azide, 4-dimethylaminopyridine,
Chem. Eur. J. 2015, 21, 1 – 5
and tris(pentafluorophenyl)borane was investigated. Surprisingly, conversion with the Lewis acid resulted in the formation of a cyclic two-coordinate tin(II) cation (see scheme).
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Formation of an Imino-Stabilized Cyclic Tin(II) Cation from an Amino(imino)stannylene
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& Stannylenes
Formation of an Imino-Stabilized Cyclic Tin(II) Cation from an Amino(imino)stannylene Tatsumi Ochiai, Daniel Franz, Elisabeth Irran, and Shigeyoshi Inoue*[a] Dedicated to Professor Yohsuke Yamamoto on the occasion of his 60th birthday
Abstract: The novel amino(imino)stannylene 1 was prepared by conversion of HNIPr (NIPr = bis(2,6-diisopropylphenyl)imidazolin-2-imino) with one equivalent of Lappert’s tin reagent (Sn[N(SiMe3)2]2). Treatment of 1 with DMAP (4-dimethylaminopyridine) yields its Lewis acid– base adduct 2. The reaction of 1 with one equivalent of trimethylsilyl azide results in replacement of the amino group at the tin center by an N3 substituent with concomitant elimination of N(SiMe3)3 to afford dimeric [N3SnNIPr]2 (3). Remarkably, the reaction of 1 with B(C6F5)3 produces the novel tin(II) monocation 4 + [MeB(C6F5)3] comprising a four-membered stannacycle through methyl-abstraction from the trimethylsilyl group.
Figure 1. Bis(amino)stannylene (Lappert’s tin reagent) I, tin(II) monocations II, as well as III and imidazoline-2-imino complexes of main group elements (IV and V; Dip = 2,6-diisopropylphenyl, X = H, BH4, SH, Cl, Br, I).
The field of low-valent tin compounds has received much attention since the isolation of the first monomeric bis(amino)stannylene I by Lappert and co-workers (Figure 1).[1] Today, a variety of bis(amino)stannylenes is known for which the ability of the nitrogen atoms to act as s- and p-electron donors grants thermodynamic stability to the low-valent metal center.[2] Furthermore, molecular complexes of tin(II) are commonly acknowledged for their various applications in small-molecule activations. Remarkable in this regard is the synthesis and structural characterization of electron-precise compounds with an Sn=E double bond (E = O, S, Se, Te),[3] as well as related stannaimines (Sn=N).[4] Sparked by the pioneering reports of Jutzi and co-workers on half-sandwich stannocene cations, the field of low-valent tin cations has been a major topic of interest in main-group chemistry for decades.[5] However, only a few examples of divalent tin monocations are known to date.[6] One is the b-diketiminato-stabilized stannylene cation II (Figure 1).[6c] Outstanding is the report by Jones and co-workers on the amido-substituted tin(II) monocation III (Figure 1).[6d] Recently, our group has described a variety of boron,[7] aluminum (i.e. IV, Figure 1),[8] and silicon (i.e. V, Figure 1)[9] com-
plexes using the imidazolin-2-imino group. Moreover, Bertrand implemented the strongly related imidazolidin-2-iminato ligand to accomplish the isolation of very rare examples for molecular phosphorus mononitride and phosphinonitrene, as well as of a iminophosphonium salt.[10] In addition, the transition-metal chemistry of this ligand with pronounced electrondonating abilities has been thoroughly studied by Tamm and co-workers.[11] Herein, we describe the synthesis and reactivity of the amino(imino)stannylene 1. An unprecedented tin(II) monocation with a four-membered stannacycle was isolated after the reaction of 1 with B(C6F5)3. The reaction of bis(2,6-diisopropylphenyl)imidazolin-2-imine (HNIPr) with Sn[N(SiMe3)2]2 in toluene at 60 8C furnishes the novel amino(imino)stannylene 1 in near-quantitative yield (93 %, Scheme 1). The 119Sn{1H} NMR spectrum of 1 in C6D6 shows a signal at d = 208.0 ppm that is significantly shifted to higher field as compared to that of the Sn[N(SiMe3)2]2 pre-
[a] T. Ochiai, Dr. D. Franz, Dr. E. Irran, Prof. Dr. S. Inoue Institut fr Chemie, Anorganische Chemie Technische Universitt Berlin Straße des 17. Juni 135, Sekr. C2, 10623 Berlin (Germany) E-mail: [email protected]
Scheme 1. Synthesis of amino(imino)stannylene 1 and its conversion to the Lewis acid–base adduct 2 (Dip = 2,6-diisopropylphenyl, DMAP = 4-dimethylaminopyridine).
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201500607. Chem. Eur. J. 2015, 21, 1 – 5
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Communication cursor (I: d(119Sn) = 767 ppm, C6D6).[12] Interestingly, the resonance is shifted downfield (d(119Sn) = 140.9 ppm) if [D8]THF is used as the solvent (I: d(119Sn) = 602 ppm, [D8]THF).[12] We presume that the high-field signal in deuterated benzene is produced by an aggregated species with a higher-coordinate tin(II) atom. Similarly, coordination of solvent to the tin(II) atom is very likely to have an impact on the 119Sn chemical shift observed for 1 in [D8]THF. The addition of DMAP to a solution of 1 in toluene results in coordination of the Lewis base to the tin(II) center to afford 2 as verified by multinuclear NMR spectroscopy. The 119Sn{1H} NMR spectrum (C6D6) shows a singlet at d = 3.4 ppm which is shifted to lower field as compared to that of 1 but shifted upfield in comparison to that of Sn[N(SiMe3)2]2 (vide supra). X-ray diffraction analysis of 2 revealed a monomeric structure in which a trigonal-pyramidal coordination environment is observed for the tin atom (the sum of the bond angles around the tin center amounts to 2798, Figure 2). The Sn1 N3 distance Scheme 2. Formation of dimeric stannylene azide 3 (top). Related conversion of VI to stannylene azide VII as reported by Zemlyansky (bottom).[14]
lene Sn(O(CH2)2NMe2)2 (VI) with N3SiMe3, which proceeds via elimination of Me3SiO(CH2)2NMe2 (Scheme 2).[14] Theoretical calculations indicated that the formation of [(N3)Sn(m-O)(CH2)2NMe2]2 involves ligand redistribution reactions between Sn(O(CH2)2NMe2)2 and N3SiMe3.[15] Moreover, the study revealed that the formation of a stannaimine is kinetically disfavored for this system. Similar considerations may apply to our related conversion of 1 into 3. An X-ray crystallographic investigation shows 3 to be a centrosymmetric dimer, with a planar and rhombic N2Sn2 ring (Figure 3). The imino groups bridge the two tin(II) centers with Sn N bond lengths of 2.194(3) and 2.178(3) . The comparison with the Sn N bond lengths in 2 suggests a partial dative-
Figure 2. ORTEP representation of the molecular structure of 2 in the solid state. Thermal ellipsoids are at the 40 % probability level. Hydrogen atoms are omitted for clarity. Dip groups are depicted as stick models. Selected bond lengths [] and bond angles [8]: Sn1 N3 2.0588(13), Sn1 N4 2.1647(12), Sn1 N5 2.3550(13), N1 C1 1.4116(19), N2 C1 1.4124(19), N3 C1 1.262(2); N3-Sn1-N4 103.12(5), N3-Sn1-N5 85.31(5), N4-Sn1-N5 90.86(5).
of 2.0588(13) is shorter than the Sn1 N4 distance of 2.1647(12) , which suggests that the tin center interacts more strongly with the nitrogen atom of the iminato ligand than it does with the nitrogen atom of the amino group. In accordance with the higher coordination number of the tin(II) center the Sn1 N4 distance in 2 is increased in comparison to that in Sn[N(SiMe3)2]2 (2.088(6) , 2.096(1) ).[1c] The Sn1 N5 distance of 2.3550(13) is in the range of Sn N bond lengths reported for typical pyridine-stabilized stannylenes.[13] Inspired by the literature-known syntheses of rare stannaimines[4] we treated 1 with N3SiMe3. As a result, the distannylene diazide 3 was obtained in 32 % yield after recrystallization from THF. The formation of 3 can be reasoned by the cleavage of the bond between the tin atom and the amino group with release of N(SiMe3)3 (Scheme 2). Zemlyansky and co-workers described the synthesis of the dimeric tin(II) azide [(N3)Sn(mO)(CH2)2NMe2]2 (VII) by reaction of the base-stabilized stanny&
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Figure 3. ORTEP representation of the molecular structure of 3 in the solid state. Thermal ellipsoids are at the 40 % probability level. Hydrogen atoms are omitted for clarity. Dip groups are depicted as stick models. Symmetryrelated atoms are marked by an asterisk. Selected bond lengths [] and bond angles [8]: Sn1 N1 2.194(3), Sn1 N1* 2.178(3), Sn1 N6 2.110(9), C1 N1 1.307(5), C1 N2 1.377(5), C1 N3 1.393(4); N1*-Sn1-N1 77.46(12), Sn1*-N1Sn1 102.54(12), N4-N5-N6 172.6(15), N5-N6-Sn1 116.1(7).
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Communication bond character for the tin–nitrogen interactions in 3. The structural features of the N2Sn2 ring are similar to those in [(h3Cp)Sn{m-NC(NMe2)2}]2.[16] The azido groups in 3 occupy positions above and below the face of the N2Sn2 ring presumably to avoid steric hindrance. In the 1H- and 13C{1H} NMR spectra of 3 in [D8]THF, the expected resonance pattern for the iminato ligand splits into two sets of signals. This indicates the existence of an equilibrium between dimeric 3 and a proposed monomeric species 3’, which is resolved on the NMR timescale. The 119Sn{1H} NMR spectrum shows two signals at d = 285.0 ppm and d = 38.9 ppm. The resonance at higher field is in the region of typical tin(II) azides with a three-coordinate metal center.[17] This justifies the assignment of the signal observed at d(119Sn) = 285.0 ppm to the dimeric stannylene azide 3. To clarify the unexpected resonances we applied 2D DOSY NMR spectroscopy to a solution of 3 in [D8]THF (see the Supporting Information).[18] This NMR experiment revealed two species with distinct diffusion constants. To date, only a few borane adducts of stannylenes have been structurally characterized.[19] Thus, we treated stannylene 1 with B(C6F5)3 in hexane and observed conversion to the ionic
adduct of III (Figure 1, adduct not shown) resonates at d(119Sn) = 30 ppm (CD2Cl2).[6d] The molecular structure of 4 + [MeB(C6F5)3] (toluene)0.5 comprises a planar N2SiSn cycle and the Sn1 N4 distance amounts to 2.062(2) (Figure 4). Hence, this bond length is in the range
Figure 4. ORTEP representation of the molecular structure of 4 + [MeB(C6F5)3] (toluene)0.5 in the solid state. Thermal ellipsoids are at the 40 % probability level. Dip groups and counter anion are depicted as stick models. Hydrogen atoms and toluene are omitted for clarity. Selected bond lengths [] and bond angles [8]: Sn1 N3 2.197(2), Sn1 N4 2.062(2), Si1 N3 1.769(2), Si1 N4 1.725(2), Si1 C28 1.850(3), Si1 C29 1.846(3), Si2 N4 1.730(2), C1 N1 1.362(3), C1 N2 1.362(3), C1 N3 1.331(3); N3-Sn1-N4 73.43(8), N3-Si1-N4 93.65(11), Si1-N3-Sn1 93.31(9), Si1-N4-Sn1 99.52(11).
found for typical stannylenes with bis(amido) ligands that feature a four-membered N2SiSn cycle and a two-coordinate tin(II) center.[20] In comparison, the Sn1 N3 distance (2.197(2) ) is significantly larger. This contrasts the respective bonding situation of the DMAP adduct 2, in which the distance of the imino-nitrogen atom to the tin(II) center is shorter than that of the amido-nitrogen atom. As a result, we presume a high dative-bond character between the tin(II) center and the imino-nitrogen atom. Accordingly, resonance structure A1 (Scheme 3) is one suitable description for the bonding situation in 4 + , that is, an amido-bound tin(II) cation stabilized by intramolecular coordination of an imidazolin-2-imino group. Moreover, the C1 N3 distance (1.331(3) ) of the imino moiety in 4 + is significantly longer than the respective bond lengths in 2 and 3 and possesses a value between that of a typical C N single bond and a C=N double bond.[21] This suggests that the positive charge in 4 + is in part delocalized across the adjacent imidazoline moiety. Consequently, 4 + also comprises considerable imidazolium-cation character as represented by resonance structure A2 (Scheme 3). The formulation of A1 and A2 agrees with the results of our quantum-mechanical calculations on 4 + (vide infra). The calculated WBI of the Sn Nimine bond in 4 + amounts to 0.390, which is significantly lower than that of the Sn Namine interaction (0.674). For comparison, we calculated a WBI of 0.578 for the Sn Nimine bond in the DMAP adduct 2. As expected from our X-ray study, the WBI of the CNHC Nimine bond (1.335) in 4 + is lower than that of the CNHC Nimine bond in 2 (1.737). The calculated LUMO + 1 and LUMO of 4 + essentially comprise an empty p-orbital that is centered at the tin(II) atom (see the Supporting Information, Figure S25). The electron lone-pair at
Scheme 3. Synthesis of the four-membered tin(II) cation 4 + [MeB(C6F5)3] (top) and resonance structures of 4 + (bottom; Dip = 2,6-diisopropylphenyl).
tin(II) borate salt 4 + [MeB(C6F5)3] (Scheme 3). Reasonably, the formation of this product involves the abstraction of a siliconbound methyl group from 1 by the Lewis acid affording a silylium cation, which is stabilized by intramolecular interaction with the imino moiety. The 119Sn{1H} NMR spectrum of 4 + [MeB(C6F5)3] displays a signal at d = 33.5 ppm in CD3CN. In contrast, a significantly downfield-shifted signal is found in C6H5F (d = 355.0 ppm). We conclude that acetonitrile coordinates to the tin(II) center of 4 + and that only negligible interaction between the cation and the solvent occurs in fluorobenzene solution. This is supported by the 119Sn chemical shifts observed for the three-coordinate tin(II) monocation [{HC(MeCNDip)2}Sn(OEt2)] + (II, d(119Sn) = 139.5 ppm), and its congener [{HC(MeCNDip)2}Sn] + (d(119Sn) = 197 ppm, CD2Cl2), which exhibit a very similar difference in the 119Sn chemical shifts of the higher and the lower coordinate species.[6c] Notably, the DMAP Chem. Eur. J. 2015, 21, 1 – 5
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Communication the tin center is observed for the HOMO 3 and possesses a pronounced s-orbital component. In conclusion, we synthesized the novel amino(imino)stannylene 1 from the reaction of HNIPr with Sn[N(SiMe3)2]2 and its DMAP adduct 2. Compound 1 reacts with N3SiMe3 to afford the tin(II) dimer 3 which features an azido-functionalized N2Sn2 ring. The reaction of 1 with B(C6F5)3 furnishes the borate salt 4 + [MeB(C6F5)3] in a rare type of methyl-abstraction and ringclosing reaction unprecedented for stannylenes. Furthermore, 4 + [MeB(C6F5)3] is the first isolated monocationic molecular complex of tin(II) with a four-membered stannacycle and a two-coordinate tin center. Future investigations will focus on the reactivity of 4 + versus unsaturated functionalities in organic substrates.
[7]
[8] [9] [10]
Acknowledgements We are exceptionally grateful to the Alexander von Humboldt Foundation (Sofja Kovalevskaja Program) for financial support. We thank Paula Nixdorf for collecting X-ray diffraction data, as well as Samantha Voges for the recording of DOSY NMR spectra.
[11]
Keywords: azides · boranes · cations · stannylenes · tin [12]
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COMMUNICATION & Stannylenes T. Ochiai, D. Franz, E. Irran, S. Inoue* && – && A welcome surprise: A novel amino(imino)stannylene was accessed by means of the strongly electron-donating imidazolin-2-imino group. Its reactivity with an azide, 4-dimethylaminopyridine,
Chem. Eur. J. 2015, 21, 1 – 5
and tris(pentafluorophenyl)borane was investigated. Surprisingly, conversion with the Lewis acid resulted in the formation of a cyclic two-coordinate tin(II) cation (see scheme).
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Formation of an Imino-Stabilized Cyclic Tin(II) Cation from an Amino(imino)stannylene
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