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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 []:

A romatic compounds, such as benzene and its derivatives, porphyrins, fullerenes, carbon nanotubes, and graphene, have numerous applications in biomedicine, materials science, energy science, and environmental science. Metalla-aromatics are analogues of conventional organic aromatic molecules in which one of the (hydro)carbon segments is formally replaced by an isolobal transition-metal fragment. Researchers have studied these transition-metalcontaining aromatic molecules for the past three decades, particularly the synthesis and reactivity of metallabenzenes. Another focus has been the preparation and characterization of other metalla-aromatics such as metallafurans, metallapyridines, metallabenzynes, and more. Despite significant advances, remaining challenges in this field include the limited number of convenient and versatile synthetic methods to construct stable and fully characterized metalla-aromatics, and the relative shortage of new topologies.To address these challenges, we have developed new methods for preparing metalla-aromatics, especially those possessing new topologies. Our synthetic efforts have led to a large family of closely related metalla-aromatics known as aromatic osmacycles. This Account summarizes the synthesis and reactivity of these compounds, with a focus on features that are different from those of compounds developed by other groups. These osmacycles can be synthesized from simple precursors under mild conditions. Using these efficient methods, we have synthesized aromatic osmacycles such as osmabenzene, osmabenzyne, isoosmabenzene, osmafuran, and osmanaphthalene. Furthermore, these methods have also created a series of new topologies, such as osmabenzothiazole and osmapyridyne. Our studies of the reactivity of these osma-aromatics revealed unprecedented reaction patterns, and we demonstrated the interconversion of several osmacycles.Like other metalla-aromatics, osma-aromatics have spectroscopic features of aromaticity, such as ring planarity and the characteristic bond lengths between a single and double bond, but the osma-aromatics we have prepared also exhibit good stability towards air, water, and heat. Indeed, some seemingly unstable species proved stable, and their stability made it possible to study their optical, electrochemical, and magnetic properties. The stability of these compouds results from their aromaticity and the phosphonium substituents on the aromatic plane: most of our osma-aromatics carry at least one phosphonium group. The phosphonium group offers stability via both electronic and steric mechanisms. The phosphonium acts as an electron reservoir, allowing the circulation of electron pairs along metallacycles and lowering the electron density of the aromatic rings. Meanwhile, the bulky phosphonium groups surrounding the aromatic metallacycle prevent most reactions that could decompose the skeleton.

A facile synthesis of cyclic aminodiborane (NH2B2H5, ADB) from ammonia borane (NH3·BH3, AB) and THF·BH3 has made it possible to determine its important characteristics. Ammonia diborane (NH3BH2(μ-H)BH3, AaDB) and aminoborane (NH2BH2, AoB) were identified as key intermediates in the formation of ADB. Elimination of molecular hydrogen occurred from an ion pair, [H2B(NH3) (THF)](+)[BH4](-). Protic-hydridic hydrogen scrambling was proved on the basis of analysis of the molecular hydrogen products, ADB and other reagents through (2)H NMR and MS, and it was proposed that the scrambling occurred as the ion pair reversibly formed a BH5-like intermediate, [(THF)BH2NH2](η(2)-H2)BH3. Loss of molecular hydrogen from the ion pair led to the formation of AoB, most of which was trapped by BH3 to form ADB with a small amount oligomerizing to (NH2BH2)n. Theoretical calculations showed the thermodynamic feasibility of the proposed intermediates and the activation processes. The structure of the ADB·THF complex was found from X-ray single crystal analysis to be a three-dimensional array of zigzag chains of ADB and THF, maintained by hydrogen and dihydrogen bonding. Room temperature exchange of terminal and bridge hydrogens in ADB was observed in THF solution, while such exchange was not observed in diethyl ether or toluene. Both experimental and theoretical results confirm that the B-H-B bridge in ADB is stronger than that in diborane (B2H6, DB). The B-H-B bridge is opened when ADB and NaH react to form sodium aminodiboronate, Na[NH2(BH3)2]. The structure of the sodium salt as its 18-crown-6 ether adduct was determined by X-ray single crystal analysis.

Treatment of [OsCl(2)(PPh(3))(3)] with HC[triple bond]CCH(OH)C[triple bond]CH/PPh(3) produces the osmabenzene [Os{CHC(PPh(3))CHC(PPh(3))CH}Cl(2)(PPh(3))(2)][OH] (2), which is air stable in both solution and solid state. The key intermediate of the one-pot reaction, [OsCl(2){CH=C(PPh(3))CH(OH)C[triple bond]CH}(PPh(3))(2)] (3), and the related complex [Os(NCS)(2){CHC(PPh(3))CH(OH)C[triple bond]CH}(PPh(3))(2)] (7) have been isolated and characterized, further supporting the proposed mechanisms for the reaction. Reactions of 3 with PPh(3), NaI, and NaSCN give osmabenzene 2, iodo-substituted osmabenzene [Os{CHC(PPh(3))CHCICH}I(2)(PPh(3))(2)] (4), and thiocyanato-substituted osmabenzene [Os{CHC(PPh(3))CHC(SCN)CH}(NCS)(2)(PPh(3))(2)] (5) respectively. Similarly, reaction of [OsBr(2)(PPh(3))(3)] with HC[triple bond]CCH(OH)C[triple bond] CH in THF produces [OsBr(2){CH=C(PPh(3))CH(OH)C[triple bond]CH}(PPh(3))(2)] (9), which reacts with PPh(3)/Bu(4)NBr to give osmabenzene [Os{CHC(PPh(3))CHC(PPh(3))CH}Br(2)(PPh(3))(2)]Br (10). Ligand substitution reactions of 2 produce a series of new stable osmabenzenes 11-17. An electrochemical study shows that osmabenzenes 2, 12, and 14-17 have interesting different electrochemical properties due to the different co-ligand. The oxidation potentials of complexes 2, 12, 16, and 17 with Cl, NCS, and N(CN)(2) ligands gradually positively shift in the sequence of Cl

A number of recently discovered nucleophilic boron compounds, such as boryl anions and borylenes, are breaking the rules regarding boron and boron‐containing compounds and their reputation as Lewis acids/electrophiles. In a similar fashion, the B−H bonding pair electrons in boranes also show nucleophilicity which is ascribed to the lower electronegativity of boron relative to that of hydrogen. However, this nucleophilicity of the B−H bond has received far less attention. Explorations of the nucleophilicity of the B−H bonding pair electrons have led to the formation of B−H−B bonded units and B−H⋅⋅⋅H−Y dihydrogen bonds, based on which new chemistry has been uncovered, including the elucidation of the mechanism of formation of aminodiborane (ADB), the diammoniate of diborane (DADB), and lithium or sodium salts of octahydrotriborates (B3H8−), as well as the development of more convenient and straightforward synthetic routes to these reagents. Moreover, the recognition of the nucleophilic properties of the B−H bonding pair electrons will also help to more deeply understand the different mechanisms operating in hydroboration reactions.

We report the hydroboration of CO with catecholborane catalyzed by a series of bis(phosphinite) pincer ligated nickel thiolate complexes. Turnover frequencies (TOFs) up to 2400 h were achieved at room temperature under an atmospheric pressure of CO. This represents the highest TOF value known to date for the reduction of CO to the methoxide level under mild conditions.

A dearomatized PN3P*-nickel hydride complex has been prepared using an oxidative addition process. The first nickel-catalyzed hydrosilylation of CO2 to methanol has been achieved, with unprecedented turnover numbers. Selective methylation and formylation of amines with CO2 were demonstrated by such a PN3P*-nickel hydride complex, highlighting its versatile functions in CO2 reduction.