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An Open‐Shell Singlet Sni Diradical and H2 Splitting Angewandte Communications Chemie How to cite: Distannabenzene Hot Paper International Edition: doi.org/10.1002/anie.202017078 German Edition: doi.org/10.1002/ange.202017078 I An Open-Shell Singlet Sn Diradical and H2 Splitting Mahendra K. Sharma, Dennis Rottschfer, Timo Glodde, Beate Neumann, Hans- Georg Stammler, and Rajendra S. Ghadwal* Dedicated to Professor Thomas M. Klapçtke on the occasion of his 60th birthday I Ph Abstract: The first Sn diradical [(ADC )Sn]2 (4) based Ph on an anionic dicarbene (ADC = {CN(Dipp)}2CPh; Dipp = 2,6-iPr2C6H3) scaffold has been isolated as a green crystalline solid by KC8 reduction of the corre- Ph sponding bis-chlorostannylene [(ADC )SnCl]2 (3). The six-membered C4Sn2-ring of 4 containing six p-electrons shows a diatropic ring current, thus 4 may also be regarded as the first 1,4-distannabenzene derivative. DFT calcula- tions suggest an open-shell singlet (OS) ground state of 4 with a remarkably small singlet–triplet energy gap Figure 1. Structurally characterized relevant tin compounds. À1 (DEOS–T = 4.4 kcalmol ), which is consistent with À1 CASSCF (DES–T = 6.6 kcalmol and ddiradical character the first tin analog (I, Figure 1). Interestingly, while Power [8] y = 37%) calculations. The diradical 4 splits H2 at room et al. also reported one more Ge derivative but I remained temperature to yield the bis-hydridostannylene the only known example of a stable tin diradicaloid. The Ph [(ADC )SnH]2 (5). Further reactivity of 4 has been studied stability of I and other Niecke-type diradicaloids is largely with PhSeSePh and MeOTf. attributed to the presence of a 6p-electron aromatic system like benzene.[4–9] Molecules containing two unpaired electrons in two Heavier Group 14 benzene derivatives featuring a C6ÀnEn degenerate (or nearly degenerate) orbitals are called dirad- ring (E = Si, Ge, or Sn) are intriguing molecules.[10] The icals, which may have a singlet or triplet ground state.[1] Stable isolation of these species however presents a considerable diradicals are appealing synthetic targets in fundamental challenge because of their inherent high reactivity.[11] While chemistry[2] and continue to attract interest in materials several stable silabenzene and germabenzene derivatives science and beyond.[3] This is because of their intriguing have been isolated,[11,12] stable monomeric stannabenzenes electronic structure, reactivity, and physical properties. Four- (II and III)[11d,j] remained extremely rare.[13] Also, no stable membered non-Kekul ring systems (E2X2)(E= main-group distannabenzene containing a C4Sn2 ring is known to date. element, X = N or O), collectively known as Niecke-type The high reactivity of heavy benzene derivatives containing [4] diradicaloids, are classical examples of main-group singlet aC6ÀnEn ring is most likely due to their open-shell electronic diradicals.[5] In 2004, Power et al.[6] isolated the first germa- structure arising from an inefficient Cp–Ep overlap. Thus, in nium diradicaloid, (ArGeNSiMe3)2 (Ar = 2,6-(2,6- addition to kinetic stabilization using bulky substituents, the [7] iPr2C6H3)C6H3). At the same time, Lappert et al. reported use of new electron-rich ligands seems desirable in accessing stable heavier benzene derivatives.[14] We recently prepared barrelene type SnII compounds [*] Dr. M. K. Sharma, Dr. D. Rottschfer, T. Glodde, B. Neumann, (IV)[15] based on an anionic dicarbene (ADCAr)[16] Dr. H.-G. Stammler, Priv.-Doz. Dr. R. S. Ghadwal Ar (ADC = {CN(Dipp)}2CAr; Ar = aryl; Dipp = 2,6-iPr2C6H3) Molecular Inorganic Chemistry and Catalysis and subsequently decided to probe the synthetic viability of Inorganic and Structural Chemistry Center for Molecular Materials corresponding 1,4-distannabenzene derivatives. Herein, we I Ph Faculty of Chemistry, Universitt Bielefeld report the first example of a Sn compound [(ADC )Sn]2 (4) Universittsstrasse 25, 33615, Bielefeld (Germany) containing a C4Sn2 ring and describe its electronic structure E-mail: [email protected] and reactivity (Scheme 1). Homepage: www.ghadwalgroup.de The Li(ADCPh)(2) is accessible by the double deproto- Supporting information and the ORCID identification number(s) for nation of the C2-arylated 1,3-imidazolium salt (IPrPh)Cl (1) the author(s) of this article can be found under: Ph (IPr = {HCN(Dipp)}2CPh; Dipp = 2,6-iPr2C6H3) with nBuLi https://doi.org/10.1002/anie.202017078. (Scheme 1). Treatment of a freshly prepared solution of 2 with 2021 The Authors. Angewandte Chemie International Edition (DMAP)SnCl (DMAP = Me NC H N) led to the formation published by Wiley-VCH GmbH. This is an open access article under 2 2 5 4 of the anticipated bis-chlorostannylene 3 in 73% yield as the terms of the Creative Commons Attribution Non-Commercial NoDerivs License, which permits use and distribution in any a colorless crystalline solid. It should be noted that the use of medium, provided the original work is properly cited, the use is non- (DMAP)SnCl2 is essential to selectively prepare 3 as the commercial and no modifications or adaptations are made. Angew. Chem. Int. Ed. 2021, 60, 1 – 6 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH 1 These are not the final page numbers! Ü Ü Angewandte Communications Chemie bond (2.517(1) ) of 3 is longer compared with those of [17a] # (IPr)SnCl2 (2.439(3) and 2.426(2) ). The Sn1ÀC2/C3 bonds (2.229(1), 2.234(1) , respectively) of 3 are, however, [17a] shorter than that of (IPr)SnCl2 (2.341(8) ). This may be because of the greater s-donor property of ADCs in comparison with classical NHCs.[18] The transannular Sn–Sn distance in 3 is approx. 4.4 , while the C-Sn-C bond angle is 90.9(1)8 (see Table S3). The SnÀC2/C3# bonds of 4 (2.202(3) and 2.205(3) ) are slightly shorter than those of 3 (2.229(1), (2.234(1) ), IV [15] 0 (2.24 to 2.27 ), and Joness Sn compound (IPr)2Sn2 (2.297 ).[19] They are, however, longer than the SnÀC bond Scheme 1. Synthesis of 3 and 4. Diradical (A) and benzenoid (B) forms observed in the stannabenzene derivatives II and III (2.02– of 4. 2.08 ),[11d,j] each featuring a formally Sn3+ center, as well as the Sn=C bonds of stannenes (2.00–2.07 ).[20] It should, reaction of 2 with SnCl2, irrespective of the ratio, almost however, be noted that the bonds in low-valent compounds Ph [15] exclusively gives [(ADC )3Sn2]SnCl3 (IV). are always longer than those of related high-valent derivatives Compound 3 was characterized by NMR and X-ray because of the lower s-character of the bonding orbitals in the [10a,21] diffraction (Figure 2). KC8 reduction of 3 led to the formation former. The C2ÀC3 bond length of 4 (1.380(4) ) is of compound 4 (see the Supporting Information), which was comparable to that of 3 (1.376(1) ). These structural isolated as a green crystalline solid in 93% yield. The parameters of 4 indicate that the unpaired electrons on the 1H NMR spectrum of 3 displays two sets of signals (four Sn atoms (see diradical form A, Scheme 1) are weakly doublets and two septets) for the isopropyl groups along with delocalized over the C4Sn2 ring to form a 6p-electron the signals due to the aryl protons. The 119Sn{1H} NMR aromatic system (see benzenoid form B). The 13C{1H} NMR spectrum of 3 shows a sharp singlet at À62.8 ppm, which is signal for the C4Sn2 moiety of 4 (186.9 ppm) is downfield comparable with those of (NHC)SnCl2 (À59.4 to À68.7) shifted compared to that of 3 (171.5 ppm), suggesting a ring- (NHC = N-heterocyclic carbene).[17] Unlike 3, the 1H NMR current effect from six p-electrons in the former. The spectrum of 4 at 298 K (Figure S4) shows rather broad calculated Nucleus Independent Chemical Shift (NICS) resonances. The 119Sn{1H} NMR spectrum of 4 exhibits values (Table S9) of 4, (NICS(0) = À7.5 and NICS(1) = a high-field signal at À426.0 ppm (w1/2 = 187 Hz), which is À10.2 ppm), indicate the presence of a diatropic ring current [22] however rather broad compared to that of 3 (À62.8 ppm, w1/ derived from the C4Sn2 ring. 2 = 30 Hz). The broadening of NMR signals of 4 may likely be The optimized geometries of 3 and 4 (Figure S29) at the due to its open-shell electronic structure (see below). Indeed, B3LYP/6-31G(d) level of theory are fully consistent with their variable-temperature 1H NMR studies (Figure S5) show con- solid-state molecular structures determined by X-ray diffrac- tinuous sharpening of signals from 298 to 224 K. At 224 K, the tion (Figure 2). DFT calculations reveal a closed-shell singlet 1H NMR signals of 4 are resolved and can be reasonably (CS) ground state for 4, however the triplet state is only assigned to the ADCPh units (Figure S6). 3.3 kcalmolÀ1 higher in energy. Interestingly, unrestricted The solid-state molecular structures of 3 and 4 (Figure 2) DFT reveals an open-shell singlet (OS) state of 4, which is À1 reveal the presence of a six-membered C4Sn2 ring with tin 1.1 kcalmol more stable than the CS solution. The frontier atoms at the 1,4-positions. The molecule in each of 3 and 4 Kohn–Sham orbital analysis (Figure S31) of 4CS exhibits that resides at a crystallographic center of inversion. The chlorine the HOMO (À2.731 eV) mainly corresponds to a p-type atoms of 3 are positioned in a trans-fashion across the orbital of the SnÀC bonds, while the HOMOÀ1(À4.937 eV) puckered C4Sn2 ring.
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