ARTICLE Received 17 Sep 2013 | Accepted 15 Jan 2014 | Published 25 Feb 2014 DOI: 10.1038/ncomms4265 Planar Mo¨bius aromatic pentalenes incorporating 16 and 18 valence electron osmiums Congqing Zhu1, Ming Luo1, Qin Zhu1, Jun Zhu1,2, Paul v. R. Schleyer3, Judy I-Chia Wu3, Xin Lu1,2 & Haiping Xia1 Aromaticity, a highly stabilizing feature of molecules with delocalized electrons in closed circuits, is generally restricted to ‘Hu¨ckel’ systems with 4n þ 2 mobile electrons. Although the Mo¨bius concept extends the principle of aromaticity to 4n mobile electron species, the rare known examples have complex, twisted topologies whose extension is unlikely. Here we report the realization of osmapentalenes, the first planar Mo¨bius aromatic complexes with 16 and 18 valence electron transition metals. The Mo¨bius aromaticity of these osmapentalenes, documented by X-ray structural, magnetic and theoretical analyses, demonstrates the basis of the aromaticity of the parent osmapentalynes. All these osmapentalenes are formed by both electrophilic and nucleophilic reactions of the in-plane p component of the same carbyne carbon, illustrating ambiphilic carbyne reactivity, which is seldom observed in transition metal chemistry. Our results widen the scope of Mo¨bius aromaticity dramatically and open prospects for the generalization of planar Mo¨bius aromatic chemistry. 1 State Key Laboratory of Physical Chemistry of Solid Surfaces and Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), Department of Chemistry, Xiamen University, Xiamen 361005, China. 2 Fujian Provincial Key Laboratory of Theoretical and Computational Chemistry, Xiamen University, Xiamen 361005, China. 3 Department of Chemistry, University of Georgia, Athens, Georgia 30602, USA. Correspondence and requests for materials should be addressed to J.Z. (email: [email protected]) or to H.X. (email: [email protected]). NATURE COMMUNICATIONS | 5:3265 | DOI: 10.1038/ncomms4265 | www.nature.com/naturecommunications 1 & 2014 Macmillan Publishers Limited. All rights reserved. ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms4265 romaticity, one of the most important concepts in chemistry, denotes the ‘extra’ stabilization of molecules 1–3 H + H H + H Abecause of electron delocalization in closed circuits . [Os]1 [Os]1 Although Hu¨ckel aromatic compounds with 4n þ 2mobile + + R R electrons are common, examples of Mo¨bius 4n electrons PPh3 PPh3 aromaticity have proven to be much more difficult to realize and Metallapentalene Metallapentalene to establish4–13. Indeed, the reported Mo¨bius aromatic compounds are very limited and almost all have twisted topologies14–18.For instance, although Heilbronner pointed out in 1964 that rings with + H+ – H+ 20 atoms or more might adopt Mo¨bius geometries ‘without any 4 apparent angle or steric repulsion strain’ in his prediction , – BF – H Cl H 4 Mauksch et al. in 1998 discovered computationally a twisted much [Os]1 [Os]1 smaller ring system, the cyclononatetraenyl cation, which is a HBF H O 5 + 4 2 + R Mo¨bius aromatic annulene .ThenAjamiet al. reported the R PPh3 PPh3 ¨ first synthesis of a Mobius aromatic hydrocarbon in 2003, which 1-C l 2-BF also has a twisted topology6. 4 19 1 Very recently, we reported osmapentalyne complexes , R = COOMe, [Os] = OsCl(PPh3)2 persistent and highly unusual bicyclic systems via the strategy of the stabilization caused by a transition metal fragment20–25. [Os]′ [Os]' This metal fragment not only relieves considerable ring strain in ISE = –30.7 kcal mol–1 pentalyne, but also results in Mo¨bius aromatic stabilization of the rarely realized planar Craig type, which was first proposed by Craig by analysing the interactions of d-atomic orbitals (AOs) present in an alternating cyclic array of main group pp-dp AOs ′ ′ 26 [Os] [Os] by reference to trimeric and tetrameric phosphonitric chlorides ISE = –31.4 kcal mol–1 and was later extended to transition metal-containing 4n p metallacycles by Mauksch and Tsogoeva computationally27. Interestingly, the metal–carbon triple bond in osmapentalynes 19 ′ can shift from one five-membered ring to another .We [Os] = OsCl(PH3)2 speculated that a protonation–deprotonation process via an osmapentalene intermediate could account for this Figure 1 | The proposed aromatic metallapentalene. (a) The 16-electron tautomerization (Fig. 1a). Because the in-plane p-electrons of osmapentalene intermediate proposed in the tautomeric metal–carbon the triple bonds do not contribute to the aromaticity of triple bond shift of osmapentalyne. (b) The aromaticity of a model osmapentalynes, Mo¨bius aromaticity in osmapentalynes should osmapentalene evaluated by the two variations of the ISE method. The be rooted in osmapentalenes. Therefore, osmapentalenes were energies (computed by the B3LYP functional with the LanL2DZ basis set for also expected to exhibit Mo¨bius aromaticity. osmium and the 6-311 þþ G(d,p) basis sets for carbon and hydrogen) Although six-coordinated transition metal complexes with include zero-point energy corrections. ISE, isomerization stabilization octahedral structures are expected to follow the 18-electron energy. transition metal rule28, protonation of the carbyne carbon of osmapentalyne results in an aromatic osmapentalene having an osmium with only 16 electrons. Could the magnitude of the magnetic resonance (NMR) spectroscopy. Density functional Mo¨bius aromaticity be sufficient to overwhelm the instability theory computations were employed to investigate the associated with this 18-electron rule violation? Herein, we report mechanism for the formation of osmapentalynes 1-BF4 and 2- the actual isolation of the first 16-electron metallapentalene, a BF4 from osmapentalene 3-(BF4)2. The PPh3s were replaced by 0 rare Mo¨bius aromatic system with a planar, rather than a twisted PH3s. As shown in Fig. 3, deprotonation of osmapentalene 3 at topology. the C1 and C7 carbon atoms only have 12.1 and 7.0 kcal mol À 1 free energy reaction barriers, respectively, leading to the formation of osmapentalynes 10 and 20. Our results also Results indicate that osmapentalyne 20 is thermodynamically more Synthesis of the first 16-electron metallapentalene. We first stable than osmapentalyne 10 in line with the experimental evaluated the ‘isomerization stabilization energy’ (ISE) using observations that osmapentalyne 2-BF4 was the major product. 29,30 effective variations of the Schleyer–Pu¨hlhofer method . The structure of 3-(AlCl4)2 in the solid state (generated from The ISE values of osmapentalene in Fig. 1b, À 30.7 and the reaction of 1-Cl or 2-BF4 with AlCl3 in wet dichloromethane) À 31.4 kcal mol À 1, are very close to the corresponding ISE has been verified by X-ray diffraction (Supplementary Fig. 1 and values of benzene ( À 33.2 and À 29.0 kcal mol À 1)29,30 and even Supplementary Data 1). As shown in Fig. 2c, the metallabicycle larger than those of osmapentalyne ( À 21.2 and moiety of 3 is almost planar. The mean deviation from the least- À 19.6 kcal mol À 1)19. Therefore, we reasoned that it might be squares plane is only 0.0106 Å. The Os1–C1 (1.978 Å) and Os1– possible to detect or even to trap the intermediate, 16-electron C7 (1.926 Å) bond lengths are in the range of Os À C bond 31,32 osmapentalene. Indeed, treating either 1-Cl or 2-BF4 with lengths (1.894 À 2.080 Å) of osmabenzene . The Os1 À C4 HBF4 Á Et2O at room temperature for 30 min led in both cases (2.139 Å) bond length is longer than other Os À C bond lengths, to 16-electron osmapentalene 3-(BF4)2 (Fig. 2a), which was in line with the resonance structures depicted in Supplementary isolated quantitatively as a yellow solid. However, as a 16-electron Fig. 2, in which Os1 À C4 is always a single s-bond, similar to the 19 osmium complex, 3-(BF4)2 is prone to conversion into previous observation in osmapentalynes . Distances of C À C osmapentalynes with an 18-electron osmium centre. When bonds (1.365–1.414 Å) of the fused five-membered rings are 3-(BF4)2 was treated with Et2O, osmapentalynes 1-BF4 and between C À C single- and double-bond lengths, indicating a 1 33–42 2-BF4 were formed in a 7:93 ratio based on the H nuclear delocalized structure . 2 NATURE COMMUNICATIONS | 5:3265 | DOI: 10.1038/ncomms4265 | www.nature.com/naturecommunications & 2014 Macmillan Publishers Limited. All rights reserved. NATURE COMMUNICATIONS | DOI: 10.1038/ncomms4265 ARTICLE + – 2BF 1 BF – [Os]1 Cl– [Os]1 4 [Os] 4 HBF Et O + HBF4 Et2O + 4 2 + R R R PPh3 RT, 30 min PPh3 RT, 30 min PPh3 1-Cl 3-(BF4)2: 100% 2-BF4 Cl– P1 [Os]2 SCH3 CH SNa, CO Cl1 3 + R C7 Os1 C1 RT, 4 h PPh3 O2 C2 P3 4-Cl: 74% C8 C9 C6 C5 C4 C3 O1 P2 1-Cl 3 Cl– 2 [Os] OCH3 CH ONa, CO P1 3 + R PPh O3 C10 RT, 4 h 3 C11 5-Cl: 60% O1 C7 Os1 C1 S1 C8 C2 P3 1 2 C6 C4 R = COOMe, [Os] = OsCl(PPh ) , [Os] = Os(CO)(PPh ) C9 C5 C3 3 2 3 2 O2 P2 4 Figure 2 | Synthesis of 16- and 18-electron osmapentalenes. (a) The protonation of osmapentalyne gives 16-electron osmapentalene 3-(BF4)2. (b) Reactions of osmapentalyne 1-Cl with nucleophiles, leading to the formation of 18-electron osmapentalenes 4-Cl and 5-Cl.(c,d) X-ray structures of 3 (c) and 4 (d) (drawn with 50% probability). Phenyl moieties in PPh3 have been omitted for clarity. Selected bond lengths (Å) for 3: Os1–C1 1.978(6), Os1–C4 2.139(6), Os1–C7 1.926(6), C1–C2 1.365(9), C2–C3 1.414(9), C3–C4 1.376(9), C4–C5 1.404(9), C5–C6 1.378(9), C6–C7 1.393(9).