Synthesis of the first aromatic chloroosmacyclopentatriene featuring a unique M=ClC bond

This study is led by Dr. Jiangxi Chen (Xiamen university, Xiamen, China), Prof. Hujun Xie (Zhejiang Gongshang University, Hangzhou, China), and Prof. Guochen Jia (The Hong Kong University of Science and Technology, Hong Kong, China).

As shown in Figure 1, there are four possible bonding types between transition metal and chloride or chlorocarbon (ClR). In type I, both the transition metal and chloride provide one electron to form a classic covalent σ M–Cl bond. A chlorocarbon could offer a lone pair to a transition metal to form a dative bond (type Ⅱ). A chlorocarbon could also bind to a transition metal in a chloronium form (III) or via a M=Cl double bond (IV). However, last possibility has not yet been reported.

The researchers have successfully synthesized and characterized a series of planar chlorometallacyclopentatrienes. As shown in Figure 2A, the reactions of OsCl₂(PPh₃)₃ and o-ethynylphenyl alkynes in the presence of excess HCl produced the desired chloroosmacyclopentatrienes. As illustrated in Figure 2B with complex 10, these complexes have three resonance structures (10, 10' and 10"), corresponding to the bonding types of IV, III, and II in Figure 1, respectively. Single crystal X-ray analysis indicated that the Os–ClC bond has the shortest bond distance among the three types of Os–Cl bonds in chloroosmacyclopentatriene. For example, in complex 10, the Os–ClC bond length is 2.315 Å, while the two Os–Cl(terminal) bonds are 2.406 and 2.539 Å. Thus, the short bond length of Os–ClC suggests an unusual multiple-bond nature between Os and ClC, and supports the presence of the M=ClR bonding type IV in Figure 1.

DFT calculations indicated that the chloroosmacyclopentatrienes are aromatic. For examples, the

NICS(0) and NICS(1)_zz values of the osmacycle of 3 are -3.04 ppm and -9.97 ppm, respectively; the isomerization stabilization energy (ISE) of the model complex 3M is -11.3 Kcal/mol.

See the article:

Aromatic chloroosmacyclopentatrienes

https://doi.org/10.1093/nsr/nwac237

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