Anti-aromatic compounds, as well as small cyclic alkynes or carbynes, are particularly challenging synthetic goals. The combination of their destabilizing features hinders attempts to prepare molecules such as pentalyne, an 8π-electron anti-aromatic bicycle with extremely high ring strain. Here we describe the facile synthesis of osmapentalyne derivatives that are thermally viable, despite containing the smallest angles observed so far at a carbyne carbon. The compounds are characterized using X-ray crystallography, and their computed energies and magnetic properties reveal aromatic character. Hence, the incorporation of the osmium centre not only reduces the ring strain of the parent pentalyne, but also converts its Hückel anti-aromaticity into Craig-type Möbius aromaticity in the metallapentalynes. The concept of aromaticity is thus extended to five-membered rings containing a metal-carbon triple bond. Moreover, these metal-aromatic compounds exhibit unusual optical effects such as near-infrared photoluminescence with particularly large Stokes shifts, long lifetimes and aggregation enhancement.

Treatment of OsX2(PPh3)3 (X = Cl, Br) with HCCCH(OH)CCH in THF produces OsX2(CH=C(PPh3)CH(OH)CCH)(PPh3)2, which reacts with PPh3 to give osmabenzenes [Os(CHC(PPh3)CHC(PPh3)CH)X2(PPh3)2]+.

Treatment of the osmabenzyne Os([triple bond]CC(SiMe(3))=C(Me)C(SiMe(3))=CH)Cl(2)(PPh(3))(2) (1) with 2,2'-bipyridine (bipy) and thallium triflate (TlOTf) produces the thermally stable dicationic osmabenzyne [Os([triple bond]CC(SiMe(3))=C(Me)C(SiMe(3))=CH)(bipy)(PPh(3))(2)](OTf)(2) (2). The dicationic osmabenzyne 2 reacts with ROH (R = H, Me) to give osmabenzene complexes [Os(=C(OR)CH=C(Me)C(SiMe(3))=CH)(bipy)(PPh(3))(2)]OTf, in which the metallabenzene ring deviates significantly from planarity. In contrast, reaction of the dicationic complex 2 with NaBH(4) produces a cyclopentadienyl complex, presumably through the osmabenzene intermediate [Os(=CHC(SiMe(3))=C(Me)C(SiMe(3))=CH)(bipy)(PPh(3))(2)]OTf. The higher thermal stability of [Os(=C(OR)CH=C(Me)C(SiMe(3))=CH)(bipy)(PPh(3))(2)]OTf relative to [Os(=CHC(SiMe(3))=C(Me)C(SiMe(3))=CH)(bipy)(PPh(3))(2)]OTf can be related to the stabilization effect of the OR groups on the metallacycle. A theoretical study shows that conversion of the dicationic osmabenzyne complex [Os([triple bond]CC(SiMe(3))=C(Me)C(SiMe(3))=CH)(bipy)(PPh(3))(2)](OTf)(2) to a carbene complex by reductive elimination is thermodynamically unfavorable. The theoretical study also suggests that the nonplanarity of the osmabenzenes [Os(=C(OR)CH=C(Me)C(SiMe(3))=CH)(bipy)(PPh(3))(2)]OTf is mainly due to electronic reasons.

National Science Foundation of China [20572099, 20872123, 20873105

Transition metal catalyzed C−H phosphorylation remains an unsolved challenge. Reported methods are generally limited in scope and require stoichiometric silver salts as oxidants. Reported here is an electrochemically driven RhIII‐catalyzed aryl C−H phosphorylation reaction that proceeds through H2 evolution, obviating the need for stoichiometric metal oxidants. The method is compatible with a variety of aryl C−H and P−H coupling partners and particularly useful for synthesizing triarylphosphine oxides from diarylphosphine oxides, which are often difficult coupling partners for transition metal catalyzed C−H phosphorylation reactions. Experimental results suggest that the mechanism responsible for the C−P bond formation involves an oxidation‐induced reductive elimination process.

Metallabenzenes[1] and metallabenzynes [2] have attracted considerable research interest in recent years. However, their higher homologues are still very rare. [3] In fact, only two well-characterized examples have been reported. One is the metallanaphthalene prepared by Paneque et al. in 2003 by oxidation of a bicyclic iridium complex.[3a] The other is the metallanaphthalyne synthesized by Jia, Lin, and co-workers in 2007 by reduction of an osmium carbyne complex with zinc and subsequent CÀCl bond activation. [3b] It is known that CÀH activation [1a, 4] is a very important method for the synthesis of organic and organometallic compounds. Recently, we reported the synthesis of osmium hydride-alkenylcarbyne complex [OsH{ C À C(PPh 3 ) = CHPh}(PPh 3 ) 2 Cl 2 ]BF 4 (1) and its formal [4+2] cycloaddition with acetonitrile to give the first late-transition-metal-containing metallapyridine and metallapyridinium compounds.[5]From our further investigation of the reactivity of 1, we herein report the selective formation of metallanaphthalene 2 and metallanaphthalyne 3 from 1 in high yields by intramolecular C À H activation of the phenyl ring under a N 2 or O 2 atmosphere, respectively (Scheme 1). Furthermore, the transformation of isolated 2 into 3 is also reported. As there is a metallabenzene unit in 2 and a metallabenzyne unit in 3, the transformation of 2 into 3 represents the first example of the conversion from metallabenzene into metallabenzyne.Heating 1 in DCE (ClCH 2 CH 2 Cl) at reflux under a N 2 atmosphere gave the (m-Cl) 3 -bridged bisosmanaphthalene 2, which was isolated as a green solid in 72 % yield (Scheme 1). The structure of 2 was confirmed by X-ray diffraction. The asymmetric unit contains two independent molecules (2 A and 2 B). A drawing of the cation 2 A is shown in Figure 1. In 2 A, two osmanaphthalene metallacycles are connected by three chlorido bridges. The OsÀCl bond lengths of the chlorido bridges are in the range 2.496(2)-2.554(2) , which are similar to those in reported dinuclear osmium complexes with three chlorido bridges.[6] The mean deviation from the least-squares plane through the C11, C12, C13, C14, and C15 chain is 0.035 , and the value through the C21, C22, C23, C24, and C25 chain is 0.026 . The Os1 atom is out of the plane of the metallacyclic carbon atoms by 0.736 (the relevant value of Os2 is 0.697 ), which is similar to the metal atom displacement in the iridanaphthalene compound reported by Paneque et al. (0.76 ).[3a] The dihedral angle between the plane through C11, C12, C13, C14, and C15 and the plane constructed by C11, Os1, and C15 is 31.78 (the relevant dihedral angle in the Os2-containing six-membered metallacycle is 30.48). These values are comparable with those found in our previously reported bisruthenabenzene Scheme 1. Preparation of 2 and 3. Figure 1. X-ray crystal structure of 2 A (ellipsoids at the 50 % probability level).[12] Counteranion and phenyl rings in PPh 3 groups are omitted for clarity. Selected bond lengths []:

The reactivities of benzynes and metal-carbyne complexes are normally associated with the triple bond units. However, we have now found that electrophiles do not attack the formal osmium-carbon triple bond of osmabenzyne complex 1. Instead, 1 undergoes electrophilic substitution reactions-the typical reactions of aromatic systems.

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