Aromatic chloroosmacyclopentatrienes

ABSTRACT Aromatic metallacycles are of considerable current interest. Reported aromatic metallacycles are mainly those with carbon, nitrogen, oxygen and sulfur. In this work, we report the synthesis and characterization of aromatic chloroosmacyclopentatrienes, which represent the first structurally confirmed metallaaromatic with a chlorine atom in its framework. Single-crystal X-ray diffraction studies show that these planar chloroosmacyclopentatrienes possess a very short Os–ClC distance suggesting M=ClC bond character.


INTRODUCTION
The chemical bond between an s-block metal and chlorine (such as LiCl or NaCl) is usually described as an ionic bond in textbooks [1]. Compared to s-block metals, d-block transition metals can have more types of bonding with chlorine. In addition to well-known metal halide complexes, which contain ionic or covalent σ M-Cl bonds (type I in Fig. 1), d-block transition metals can also form complexes with chlorocarbons (ClR). In coordination complexes with chlorocarbons, a dative M(η 1 -ClC) bond (type II in Fig. 1), in which the chlorocarbon serves as a 2e donating ligand, is commonly assumed [2]. A chlorine atom in chlorocarbons could also interact with d-block transition metals via other types of bonds, for example, the covalent type III in chloronium form [3] and the type IV containing a metalchlorine double bond. The organic counterparts of the covalent type III bonding were reported a long time ago [4]. For example, electrophilic addition reactions of alkenes with Cl 2 are known to occur via cyclochloronium intermediates. The covalent type IV bonding is closely related to the bonding in inorganic compounds such as FCl=O and Cl 3 CCl=O [5]. However, examples of organometallic versions of M(η 1 -Cl + C) (type III) and M=ClC (type IV) bonds are rarely reported.
Recently, the chemistry of compounds with an unusual carbon-halogen multiple bond is attracting attention both experimentally and theoretically.
We propose that complex 3 might be formed via intermediates A and B as shown in Scheme 1. The reaction of OsCl 2 (PPh 3 ) 3 (1) with 2 could initially generate the vinylidene intermediate A. Reactions of OsCl 2 (PPh 3 ) 3 (1) with terminal alkynes (HC≡CR) to form osmium vinylidene complexes OsCl 2 (=C=CHR)(PPh 3 ) 2 are well-documented reactions [29][30][31]. It has also been reported that C The Author(s) 2022. Published by Oxford University Press on behalf of China Science Publishing & Media Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited. osmium vinylidene complexes can be protonated at their β-carbon to give osmium carbyne complexes [29][30][31]. Thus, the intermediate A could be protonated at the C2 position to give the intermediate B, which may undergo an electrophilic cyclization followed by the addition of Cl − to give complex 3. Consistent with the proposed mechanism, the reaction in the presence of DCl produced the partially deuterated complex 3D (Fig. 2) with a CHD methylene group (see Figs S11-S13 for characterization data).
The structure of complex 3 has been determined by single-crystal X-ray diffraction studies, which confirmed that complex 3 is a tricyclic metallacycle   Fig. 3). The five-membered osmacycle is almost planar, as reflected by the mean deviation (0.021Å) from the least-squares plane of Os1, C1, C5, C6 and Cl1. The Os-C1 bond length of 1.903(4)Å is in the range of reported Os=CHR lengths (1.810-2.142 A, based on a search of the Cambridge Structural Database, CSD version 2020 (Nov 2019)) [32][33][34][35][36], which suggests its double-bond character. The most fascinating structural feature in metallacycle 3 is that the distance of Os-Cl1 (2.359Å) is even shorter (by 0.040Å) than that of the terminal Os-Cl3 bond (2.399Å), indicating the unusual bonding (beyond a σ -bonding) between Os and Cl. It is noted that there are only two X-ray single-crystal structures (complexes 4 and 5 shown in Fig. 2) containing short Os···ClC contacts in CSD version 2020 (Nov 2019). In these two examples, the bond lengths of Os-ClAr are 2.574(1)Å for 4 [37] and 2.539(6) A for 5 [38], which are typical dative bond lengths. Thus, complex 3 represents an interesting example that contains an unusually short Os-ClC bond. Notably, all the CSD crystal structures that contain both M-ClC and terminal M-Cl (M: transition metal) bonds have a distance of an M-ClC longer than that of a terminal M-Cl.
The nuclear magnetic resonance (NMR) spectroscopy and high-resolution mass spectroscopy (HRMS) data of 3 are consistent with the solid structure. For instance, complex 3 shows the signal of proton (C2H) at 0.57 ppm in the 1 H NMR spectrum and the signal of C2 at 62.0 ppm in the 13 C{ 1 H} NMR spectrum, confirming that C2 has an sp 3 hybridized character. The signal of metal-bonded C1 is located at 271.6 ppm in the 13  spectrum, supporting the metal-carbene character of C1. Only one sharp signal appeared at -19.4 ppm in the 31 P{ 1 H} NMR spectrum, indicating the equivalent chemical environment of the two trans PPh 3 ligands, which confirms the planar structure of fused rings. The molecular formula of complex 3 was also confirmed by HRMS data (m/z = 989.1647 for [3-Cl] + ).
As shown in scheme 1, there are three possible resonance forms that could contribute to the overall structure of complex 3: the neutral metallacycle 3, the zwitterionic form 3' and the chloride dative form 3". The resonance form 3" cannot well account for the fact that the distance of Os-Cl1 (2.359Å) in the metallacycle is much shorter than those of dative Os-ClAr bonds (2.574Å for 4 and 2.538Å for 5) and even shorter than that of Os-Cl3 (2.399 A, a typical metal chloride σ -bond). The short Os-ClC distance in 3 suggests the M-ClC has a double-bond character, which can be attributed to the contribution from the resonance form 3. Complex 3 can be regarded as a chlorometallacyclopentatriene, which is a metal analog of chlorophenium ([C 4 H 4 Cl] + ). Chlorophenium ([C 4 H 4 Cl] + ) is closely related electronically to fivemembered heteroaromatics such as thiophene or furan.
To check whether the chloroosmacyclopentatriene is aromatic or not, we calculated the nucleus-independent chemical shift (NICS) value [43], an index used to evaluate aromaticity or anti-aromaticity. The structure of 3 was optimized at PBE/def2-TZVP level [44,45] and the value of NICS(1) zz of the chlorometallacycle ring in complex 3 was calculated (at PBE/x2c-TZVPall level [44,46]) to be -9.97 (Fig. 4), suggesting that chlorometallacycle 3 has an aromatic character.
To further understand the aromaticity of 3, we have performed a theoretical study of the model complex OsCl 2 (PH 3 ) 2 (k 2 -ClC 3 H 2 Me) (3M, see Fig. 4 for its structure) at PBE/def2-TZVP level [44,45]. The optimized structure of 3M well reproduced the key structural feature of 3. The aromatic character of the chloroosmacyclopentatriene 3M is indicated by the calculated isomerization stabilization energy of -11.3 kcal/mol (the energy involved in the isomerization of 3H to form 3M, see Fig. 4). Moreover, the negative NICS(1) zz value (−7.59, calculated at PBE/x2c-TZVPall level [44,46] based on the optimized structure) of 3M also supports the aromaticity of the chloroosmacyclopentatriene.
As shown in Fig. 4, there are a total of four key occupied π -type molecular orbitals (HOMO-5, HOMO-7, HOMO-10, HOMO-13, see Fig. 4) for the metallacycle of 3M, indicating that the chlorometallacycle has eight π electrons. Thus, this planar chlorometallacyclopentatriene 3 is a unique Möbius [17,19,22,[47][48][49][50] aromatic complex. It is worth noting that the π -bonding molecular orbitals of chlorometallacycle are generated by interactions of two d orbitals of an OsX 2 L 2 fragment with the π -type molecular orbitals of a ClC 3 H 2 Me fragment. In other words, osmium can form π -bonds with the carbon-bound chlorine atom by back-donation from  the osmium d orbitals to the π -type molecular orbitals of the ClC 3 H 2 Me fragment. As a result, the unusually short M-ClC bond in 3 can be partially related to a d π -p π π -interaction between the osmium atom and the ClC unit, as indicated by the resonance form 3. Thus, chloroosmacyclopentatriene 3 represents the first structurally confirmed aromatic chlorometallacycle with an M=Cl-C character.
The complexes 10, 12 and 14 were fully characterized by NMR spectroscopy and X-ray diffraction studies (Fig. 5). They all have structural features similar to those in 3, in which the M-ClR bond is systematically shorter than the corresponding M-Cl(terminal) bond. Subtle differences in the structures of complexes 3, 10, 12 and 14 are noted (Fig. 5). The Os-ClR bond in p-Tol-substituted osmium complex 12 (2.3624(8)Å) is similar to that of Ph-substituted osmium complex 3 (2.3594(10)Å), and an appreciably shorter Os-ClR bond is observed for tBu-substituted complex 10 (2.315(3)Å) and TIPS-substituted osmium complex 14 (2.3384(8)Å). The difference in the Os-Cl1(chlorocarbon) and Os-Cl3(terminal) bond distances in tBu-substituted complex 10 is as large as 0.091Å.

CONCLUSION
In summary, OsCl 2 (PPh 3 ) 3 was found to react with o-ethynylphenyl alkynes in the presence of HCl to give chlorometallacyclopentatriene complexes. These complexes are unusual in that the M-ClC(halocarbon) bond is appreciably shorter than the M-Cl(chloride) bond, and they can be regarded as compounds possessing an M=ClC bond. The novel metallacycles exhibit aromaticity, excellent planarity and thermal stability. This study not only enriches the family of metallacyclic chemistry, but also provides the synthesis of novel metallacycles containing an M=ClC bond.  Os-C(1) 1.903(4) 1.879 (12) 1.914 (3) 1.896 (3) Os-Cl (1) Figure 5. Single-crystal X-ray structures of 10, 12 and 14 with thermal ellipsoids at 50% probability level (the phenyl groups in PPh 3 and the isopropyl groups in the Si(iPr) 3 are omitted for clarity), and selected bond distances (Å) for complexes 3, 10, 12 and 14.