German Edition: DOI: 10.1002/ange.201910731Gold ComplexesInternational Edition: DOI: 10.1002/anie.201910731

A Stable Cyclic (R2SnAu)3 Anion Having In-Plane s-MçbiusAromaticityLiliang Wang, Jian Xu, Mitsuo Kira,* Liping Yan, Xu-Qiong Xiao, and Zhifang Li*

Abstract: A cyclic (R2SnAu)3 anion (3@ , R2Sn = 2,2,5,5-tetrakis(trimethylsilyl)-1-stannacyclopentane-1,1-diyl) hasbeen synthesized as a stable blue salt with K+(THF)6 throughthe reaction of stable dialkylstannylene 1 with R’3PAuCl (R’=Et, Ph) followed by the reduction with KC8. Crystallographicand NMR analysis shows that the six-membered (SnAu)3 ringof 3@ is planar and highly symmetric with an equal distance ofsix Au@Sn bonds. A UV/Vis spectrum of 3@ in hexane revealsan intense absorption maximum at 598 nm. While cyclic Au3

@

with four valence electrons is known as unstable anti-aromaticanion, 3@ with three divalent tin ligands is stable s aromaticanion with an unprecedented Mçbius orbital array as predictedby the perturbation MO and CCSD analysis of 3@ .

Gold with an atomic electronic configuration of[Xe]4f145d106s1 is the most noble of metals and its chemistryis currently one of the most rapidly growing fields ofchemistry.[1] The research on the gold chemistry has beendirected towards many subjects including gold nanoparticles,small gold mono- and multi-nuclear molecules, their catalysisfor various organic reactions, and theoretical aspects of theirbonding and structure. The oxidation states of gold are usually+ 1, + 3, and + 5 but rather unusual @1 state occurs owing tothe large relativistic effects as well as its relatively highelectron affinity;[1a] as shown for alkali metal aurides such asRbAu, CsAu,[2] and (NMe4)Au,[3] Au@ often works as a heavierpseudohalide like Br@ and I@ . Though recently, a number ofmono- and polynuclear gold molecules and ions have beensynthesized and characterized by X-ray analysis, NMRspectroscopy, and so on, knowledge of the bonding natureand chemical properties has been still limited.

The chemistry of homoatomic and mixed-metal goldclusters has been developed since early 1990s. For instance,Mingos et al.[4] and Gabba" and Schmidbauer et al.[5] havereported gold/tin (I) and gold/indium (II) mixed-metal clustermolecules. Sharp et al. have found that the reaction of excesslow valent gallium compounds [Cp*Ga/GaI] with Ph3PAuClaffords Au3(0) cluster III in 2004 (Figure 1).[6] As carbon-coated homometallic gold complexes, the carbene-stabilizedtrigold cations IV and V were reported by Sadighi et al.[7] in

2012, and by Bertrand et al.[8] in 2014. The Au3+ ring is

considered as an isolobal analogue of cyclic H3+ (c-H3

+)stabilized by a three-center two-electron (3c-2e) bond or s-Hgckel aromaticity.[9]

In contrast, cyclic Au3@ (c-Au3

@) having four 6s frontierelectrons is a Hgckel anti-aromatic system like isolobal c-H3

@

and less stable than the linear isomer.[10] While gaseous Au3@

has been investigated as one of the smallest Au cluster ions,[11]

no molecular c-Au3@ ion has been known to the best of our

knowledge. We herein report the synthesis of a stable trigoldanion salt, 3@·K+(THF)6 (Figure 2). Because stannylene 1[12]

features a lower-lying LUMO (5p) than the correspondingdiamino-[13] and monoaminostannylenes[14] and has higherelectron-accepting ability, 1 serves as an excellent ligandstabilizing the Hgckel anti-aromatic c-Au3

@ in an unprece-dented manner.

Cyclic (R2SnAu)3@ (3@) is synthesized using the following

route (Scheme 1): Stannylene 1 inserts smoothly into the Au@Cl bond of phosphine-ligated gold(I) chlorides (R’3PAuCl,R’= Et, Ph) in THF at room temperature giving 2 (2a ; R’=

Figure 1. Typical known mixed-metal and homoatomic gold clusters.

Figure 2. Isolable dialkylstannylene 1 and its gold compounds.

[*] Dr. L. Wang, J. Xu, Prof. M. Kira, L. Yan, Dr. X.-Q. Xiao, Prof. Z. LiKey Laboratory of Organosilicon Chemistry and Material Technologyof Ministry of Education, Hangzhou Normal UniversityNo.2318, Yuhangtang Rd, Hangzhou, Zhejiang, 311121 (P. R. China)E-mail: mitsuo.kira.e2@tohoku.ac.jp

zhifanglee@hznu.edu.cn

Supporting information and the ORCID identification number(s) forthe author(s) of this article can be found under:https://doi.org/10.1002/anie.201910731.

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Et, 2b ; R’= Ph) as a white solid in quantitative yields(Figure 2);[15] Stannylgold 2a reacts with potassium graphite(KC8) at ambient temperature to give the corresponding tin-coated cyclic Au3

@ salt, 3@·K+(THF)6, as a dark-blue solid in37% yield. Both 2 a and 2b are available to access to 3@

similarly. When the reaction of 2a with KC8 is carried out at@30 88C for 10 hours followed by the addition of excess Et3P,complex 5 is obtained in 41 % yield as colorless crystals,suggesting the formation of intermediate 4 before theformation of cyclic trimer 3@ . The reduction of complex 5with KC8 at room temperature in THF was found to generate3@ as monitored by the 1H NMR spectrum. For the exper-imental details, see the Supporting Information.

The gold complexes 2, 3@·K+(THF)6, and 5 are isolated aspure materials and recrystallized from a cooled hexanesolution. They are all stable in the solid state under argonand stored at ambient temperatures for four weeks. Thestructures of these compounds were determined by multi-nuclear NMR spectroscopy and single-crystal X-ray diffrac-tion analysis (Figure 3, Figure 4; Supporting Information,Figure S22).[18] The Sn@Au bond lengths of 3@ (av. 2.6009-(4) c) are slightly longer than those of 2a (av. 2.559(6) c) andsimilar to those of 5 (av. 2.6043(4) c). The Au@Au bondlengths of 3@ (av. 2.9343(4) c) are significantly longer thanthose of III (2.8045(8) c),[6] IV (2.6563(6) c),[7] V(2.6708 c),[8] and trigold cluster anion [Sb3Au3Sb3]

3@ (av.2.8706 c)[19] but slightly shorter than those of trigold clusteranion [Ge18Au3]

5@ (av. 2.9730 c).[20] The Au-Au-Au angles of3@ are all within 60.000: 0.07288, indicating the Au3 ringconstitutes a regular triangle. The peripheral (SnAu)3 ring of3@ constitutes a planar distorted equilateral hexagon with themaximum deviation from the averaged ring plane of: 0.1599 c. The dihedral angle between a C-Sn-C plane ofa dialkylstannylene moiety and the Au3 plane has the averagevalue of 84.26788, indicating these two planes are perpendic-ular to each other. In the crystal structure, two neighboringAu3Sn3 rings are parallel to each other and one potassium ionsurrounded by six THF molecules is sandwiched between thetwo Au3Sn3 rings. However, interaction neither between thetwo rings nor between Au3Sn3 ring and potassium atom isexpected: the distance between the nearest two (SnAu)3 ringsand that between a (SnAu)3 ring and the nearest potassiumatom are 19.90 and 9.915 c, respectively (Supporting Infor-mation, Figure S23).

In the 119Sn NMR spectrum of 3@ in [D6]benzene, a singletsignal appears at 1435 ppm assignable to the three equivalent119Sn nuclei of 3@ . The signal is at a higher magnetic field than

that of the corresponding stannylene 1 (2323 ppm),[12a] butsignificantly at a lower field than those for 2a (530 ppm) and 5(261 ppm). The latter two signals appear as a doublet andtriplet owing to the coupling with nearby 31P nuclei; 2JPSn =

2028 and 1300 Hz, respectively.

Scheme 1. Synthesis of (R2SnAu)3@ (3@) and related reactions.

Figure 3. Molecular structure of 3@ as determined by X-ray diffraction.Ellipsoids are set at 50% probability; hydrogen atoms, solventmolecules, and the potassium cation are omitted for clarity. Selectedbond lengths [b] and angles [88]: Au1–Au2A 2.9343(4), Au1–Au2 2.9343-(4), Au1–Sn1 2.5972(4), Au1–Sn1A 2.5973(4), Au2–Au2A 2.9310(4),Au2–Sn1 2.6083(4), Au2–Sn2 2.6113(4), Sn2–Au2A 2.6113(4); Au2A-Au1-Au2 59.925(11), Sn1-Au1-Au2 55.867(9), Sn1-Au1-Au2A 115.666-(12), Sn1A-Au1-Au2A 55.869(8), Sn1A-Au1-Au2 115.668(12), Sn1-Au1-Sn1A 171.529(17), Au2A-Au2-Au1 60.036(6), Sn1-Au2-Au1 55.510(11),Sn1-Au2-Au2A 115.421(9), Sn1-Au2-Sn2 170.652(12), Sn2-Au2-Au1115.896(11), Sn2-Au2-Au2A 55.860(7), Au1-Sn1-Au2 68.623(11), Au2A-Sn2-Au2 68.279(15), C17-Sn2-C17A 90.2(3).

Figure 4. Molecular structure of 5 as determined by X-ray diffraction.Ellipsoids are set at 50% probability; hydrogen atoms are omitted forclarity. Selected bond lengths [b] and angles [88]: Au1–Sn1 2.6049(4),Au1–P1 2.3331(13), Au2–Sn1 2.6036(4), Au2–P2 2.3303(13), Sn1–C12.258(5), Sn1–C4 2.266(5); P1-Au1-Sn1 173.14(4), P2-Au2-Sn1 178.95-(4), Au2-Sn1-Au1 121.813(13), P2-Au2-Sn1 178.95(4), Au2-Sn1-Au1121.813(13), C1-Sn1-Au1 109.30(12), C1-Sn1-Au2 110.69(13), C1-Sn1-C4 88.25(17), C4-Sn1-Au1 109.99(12), C4-Sn1-Au2 111.70(13).

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The UV/Vis spectrum of 3@ in hexane shows an intenseabsorption maximum (lmax) at 598 nm with the absorptivityemax/(m

@1 cm@1) of 11750 (Supporting Information, Fig-ure S21). Noticeably, the band is broad but more red-shiftedand more intense than the n!p band of stannylene 1 (lmax =

486 nm, e = 400m@1 cm@1).[12]

First, on the basis of the perturbation molecular orbital(PMO) theory[13] it is discussed how unique the orbitalinteraction in 3@ is. As shown in Figure 5a, c-Au3

@ hasa s Hgckel three-orbital array with an even number ofnegative overlap, while 3@ has a Mçbius six-orbital array withan odd number of negative overlap (Figure 5 b). Because c-Au3

@ and 3@ are both four-electron system, c-Au3@ should be

anti-aromatic and 3@ is aromatic. In other words, a s Hgckelanti-aromatic Au3

@ system is transformed into a s Mçbiusaromatic system by introducing three stannylene ligands.

In reality, the stannylene lone-pair (n) orbitals should alsoparticipate in the frontier molecular orbitals (FMOs) of 3@ .The FMOs of the c-(SnAu)3

@ should be constructed byconsidering the interaction of 6s(Au) orbitals of c-Au3 notonly with 5p(Sn) orbitals but with lone pair [n(Sn)] orbitals ofc-Sn3 ; an n(Sn) orbital is taken as an 5s–5p hybrid orbital.Before the interaction, three 6s(Au) group orbitals are takenas c1a1’ and c1e’ (c1es and c1ea), three n(Sn) group orbitals as f1a1’

and f1e’ (f1es and f1ea), and three 5p(Sn) group orbitals as f2e’

(f2es and f2ea) and f1a2’ as shown in Figure 6. The c1e’, f1e’, andf2e’ orbitals are degenerate under D3h symmetry and repre-sented by two orthogonal orbital pairs, as shown in paren-theses by referring to the MOs of H3

+ and Walsh type MOorbitals of cyclopropane.[21] Because the distance betweenthree Sn atoms are very long, the energy separation amongthree 5p(Sn) and that among three n(Sn) orbitals will be smalland that between 5p(Sn) and n(Sn) orbitals will approach tothe HOMO–LUMO gap of the stannylene. The FMOs of thec-(SnAu)3 system with total 10-electron and 9-orbitals is thusdrawn as shown in Figure 6. In the MO interaction between6s(Au) and 5p(Sn) orbitals, doubly degenerate c1e’$f2e’

(c1es$f2es and c1ea$f2ea) interactions are symmetry-allowedand lower the c1e’ (c1es and c1ea) energy levels and raise the f2e’

(f2es and f2ea) levels, while c1a1’ and f1a2’ levels are intact inagreement with the Mçbius aromatic nature of the interac-tion. The MO mixing modifies c1es and c1ea into c1es + c1f2es

and c1ea + c1f2ea, respectively. They are able to interact furtherwith occupied n(Sn) orbitals f1es and f1ea to raise the energylevels giving finally y2es (c1es + c2f2es@c3f1es) and y2ea (c1ea +

c2f2ea@c3f1ea), where 0< c1& c3< 1; y2es and y2ea constitutesdegenerated HOMO of c-[R2SnAu]3

@ , y2e“. However,because the c1es$f1es and c1ea$f1ea interactions are bothfour-electron two-orbital interaction, roughly speaking, thereis no overall energy gain or loss by these interactions; totalstabilization of the system is brought only by the interactionbetween 5p(Sn) and 6s(Au) orbitals. Because the c1a1’$f1a1’

interaction largely lowers the f1a1’ and raises the c1a1’, the goldc1a1’ orbital may not be the HOMO of the system afterinteraction. The LUMO is predicted as f1a2’ (y1a2’) withoutinteraction with any 6s(Au) orbitals.

To understand the electronic features of 3@ more deeply,the quantum chemical calculations on a model system 3a@

(Figure 2) are performed using Gaussian 16 program packageat the CCSD/6–31 + G(d,p) level for H and SDD level for Auand Sn (see the Supporting Information for calculationdetails). At the optimized D3h structure of 3a@ , the Au@Au,Au@Sn, and Sn@Sn distances of 3a@ are 3.015, 2.698, and5.384 c, which are comparable with those of X-ray deter-mined values for 3@ , 2.933, 2.605, and 5.195 c, respectively,even though the substituents on Sn are very different between3@ and 3a@ . The Sn-Au-Sn and Au-Sn-Au angles for 3a@ are172.1 and 67.988, which are close to 171.5 and 68.688 for 3@ . TheAu3 and Sn3 rings of 3a@ as well as those of 3@ are both regulartriangles. Similarly to H3

@ ,[10] bare Au3@ has no minimum with

the regular triangle structure (D3h) at the singlet energysurface. Instead, it shows two minima with linear and anisosceles triangle structures with an acute vertex angle (C2V);the latter is 37.5 kcalmol@1 higher in energy than the former.Hypothetical c-Au3

@ (D3h) should be less stable than theoptimized c-Au3

@ (C2v). Hence, the complexation energy of

Figure 5. Representation of arbitrarily selected basis set orbitals andorbital overlap modes (++, positive overlap; @, negative overlap) of a)c-Au3 and b) planar c-(SnAu)3. The number of negative overlap for c-Au3 [(6s)3 system] and c-(SnAu)3 [(6s-5p)3 system] is 0 (even) and 3(odd), respectively, in these cases. The evenness or oddness of theoverlap is independent of the basis set taken and defines the Hfckeland Mçbius character.[21a]

Figure 6. Frontier MO diagram of c-(SnAu)3@ system that is con-

structed by the interactions of 6s orbital array of a downward c-Au3

triangle (right) with n and 5p orbital arrays of an upward c-Sn3 triangle(left). The molecular orbitals are shown using the Mulliken symbolsfor D3h molecules. As degenerate e’ type orbitals are taken a set ofsymmetric (es) and antisymmetric (ea) orbitals with respect toreflection in the syz symmetry plane.

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H2Sn to c-Au3@ (D3h) is estimated to be larger than that

calculated as DEc = E(3a@)@E[c-Au3@(C2v)]@3E(H2Sn),

where E is the total energy of each system. The DEc is180.28 kcalmol@1 or 60.1 kcal mol@1 per H2Sn coordination.The value may indicate how much the c-Au3

@ (D3h) system isstabilized by complexation of H2SnD, though it is no directindex of the aromaticity of 3a@ . While the contribution oflower-lying atomic orbitals of Au, 5p, 5d, and so on to theFMOs should not be ignored, the calculated degenerate twoHOMOs and LUMO of 3 a@ (Figure 7) are in agreement withthose predicted by qualitative MO interaction diagram asshown in Figure 6.

Natural bond orbital (NBO) analysis for optimized 3a@

shows that the Wiberg bond index (WBI) values of the Au@Snbonds are 0.644, while those of Au@Au and Sn@Sn bonds aremuch smaller with 0.183 and 0.176, respectively. Each of thesix equivalent natural Au@Sn bond orbitals is approximatelyconstructed by 37% of an Au 6sp hybrid orbital and 63% ofa Sn sp2.8 hybrid orbital with 1.5 electron occupancy. Naturalcharge of each Au is @0.333, suggesting the negative chargeresides equally on three Au atoms.[22]

To investigate the aromatic character of 3a@ , nucleus-independent chemical shifts (NICS) calculations[23, 24] wereperformed at the GIAO-B3LYP//CCSD/6–31 + G(d,p) levelfor H and SDD level for Au and Sn. At the center of the ring,NICS(0), NICS(1), and NICS(2) of 3a@ are calculated to be@18.69, @11.14, and @4.60 ppm, respectively. Compatiblewith the s-aromatic character of 3a@ , the NICS values arelarge negative and decrease with increasing the distance fromthe ring plane, while the corresponding NICS values for c-Au3

+ (D3h) are larger with the numbers of@30.90,@13.95, and@2.32 ppm at the same level.[24,25]

In conclusion, a novel stable cyclic trigold anion salt3@·K+(THF)6 was synthesized using stable dialkylstannylene1 as a ligand that features not only nucleophilic lone-pairelectrons but an electrophilic vacant 5p orbital. The resulting(R2SnAu)3

@ ring is stabilized by uncommon s Mçbius aro-maticity with an alternate [5p(Sn)@6s(Au)]3 orbital array.Further work, including discovery of novel reactions andcatalytic activity of 3@ and synthesis of other types ofstannylene-ligated gold clusters, are in progress.

Acknowledgements

This work was financially supported by the Natural ScienceFoundation of Zhejiang Province (Grant No. LY19B020007),

the National Natural Science Foundation of China (Grant No.21771048).

Conflict of interest

The authors declare no conflict of interest.

Keywords: gold complexes · low-valent compounds ·stannylenes · s-Hfckel aromaticity · s-Mçbius aromaticity

How to cite: Angew. Chem. Int. Ed. 2020, 59, 1980–1984Angew. Chem. 2020, 132, 1996–2000

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Figure 7. Calculated FMOs of 3a@ . Degenerate HOMOs and LUMOcorrespond to y2es, y2ea, and y1a2’ in the MO diagram in Figure 6.

AngewandteChemieCommunications

1983Angew. Chem. Int. Ed. 2020, 59, 1980 –1984 T 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.angewandte.org

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[22] The NBO charge on Sn and H is calculated as 0.557 and @0.279,respectively, suggesting that the charge on H2Sn: is almost zero.It is difficult to determine the oxidation numbers in this case,because three Au atoms of 3a@ (as well as 3@) are equivalent.The oxidation number of Au may be defined as@1/3, if a fractionis allowed to be used.

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[24] Badri et al. have reported the NICS values of c-Au3+ and other

metallic cluster ions calculated at various theoretical levels usingvarious basis sets (Ref. [25a]). The NICS(0), NICS(1), andNICS(2) of c-Au3

+ reported by the authors are @33.07, @14.97,and @2.70 at the B3LYP/LanL2DZ level, being in good agree-ments with those calculated by us.

[25] The s-aromaticity of c-Au3+ and related metal clusters has been

often mentioned; see for example: a) Z. Badri, C. Foroutan-Nejad, P. Rashidi-Ranjbar, Phys. Chem. Chem. Phys. 2012, 14,3471 – 3481; b) S. Pan, R. Saha, S. Mandal, P. K. Chattaraj, Phys.Chem. Chem. Phys. 2016, 18, 11661 – 11676; c) H. Tanaka, S.Neukermans, E. Janssens, R. E. Silverans, P. Lievens, J. Am.Chem. Soc. 2003, 125, 2862 – 2863; d) A. C. Tsipis, C. A. Tsipis, J.Am. Chem. Soc. 2005, 127, 10623 – 10638.

Manuscript received: August 22, 2019Revised manuscript received: November 6, 2019Accepted manuscript online: November 21, 2019Version of record online: December 17, 2019

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