Hydroboration and Hydrogenation of an Osmium–Carbon Triple Bond: Osmium Chemistry of a Bis-σ-Borane

Abstract

Complex [OsHCl(≡CPh)(IPr)(P i Pr 3 )]OTf (1; IPr = 1,3-bis(2,6-diisopropylphenyl)imidazolylidene, OTf = CF 3 SO 3 ) replaces the chloride ligand by a hydroxo group to give [OsH(OH)(≡CPh)(IPr)(P i Pr 3 )]OTf (2), which undergoes hydroboration and hydrogenation of its metal-carbon triple bond. The hydroboration products depend upon the used reagent. Treatment of 2 with Na [BH 4 ] leads to the bis-σ-borane complex OsH 2 (η 2 , η 2 -H 2 BCH 2 Ph)(IPr)(P i Pr 3 ) (3), whereas pinacolborane (HBPin) affords the arene compound [Os(η 6 -C 6 H 5 CH 2 Bpin)H(IPr)(P i Pr 3 )]OTf (4). The hydrogenation of the triple bond of 2 occurs under 4 atm of H 2 and yields the toluene derivative [Os(η 6 -C 6 H 5 CH 3 )H(IPr)(P i Pr 3 )]OTf (5).Isolobality 1 often relates organometallics to organic functionalities. Hydroboration 2 and related processes, such as hydrogenation, 3 of carbon-carbon multiple bonds are powerful tools in organic synthesis. Analogous reactions with the organometallics counterparts, where a unit of the multiple carbon-carbon bond has been replaced by a transition metal and its associated ligands, are also known and represent a relevant group of transformations with exceptional possibilities in organometallic chemistry. However, in contrast to the purely organic processes, they have received scarce attention. 4 This is due in a part to their high complexity which is associated not only with the variety of transition metal and metal ions but also with the wide range of known co-ligands. These Lewis bases subtly govern the electron density of the metal center. Thus, they determine the behavior of a particular complex, 5 although they do not participate directly in its reactions.It is well established that the first step for the cleavage of a σ-bond is its coordination to an unsaturated transition metal compound to form a σ-complex. Thus, σ-B-H species are considered the key intermediates in the metal promoted B-H bond cleavage and have been therefore prepared by direct coordination to the metal center. 6 In contrast to the large number of dihydrogen derivatives and silane compounds, which have been studied, few examples of B-H complexes have been isolated and characterized, in particular those having most electrophilic B-alkyl moieties 7 and third row metals such as osmium. 8 In this communication, we demonstrate that σ-derivatives of the type Os(η 2 ,η 2 -H 2 Balkyl) can be prepared by hydroboration of Os-alkylidene complexes, when the coligands of the starting compound and the hydroboration reagent are appropriately selected.Hydride ligands of cationic complexes show protic character. 9 Thus, they generally undergo deprotonation. 10 In agreement with this, the hydride-alkylidyne cations [Os(η 5 -C 5 H 5 )H(≡CPh)(PR 3 )] + react with Brønsted bases to afford the corresponding neutral derivatives Os(η 5 -C 5 H 5 )(≡CPh)(PR 3 ) (PR 3 = P i Pr 3 , 11 P i Pr 2 [C(CH 3 )=CH 2 ] 12 ). The five-coordinate hydride-alkylidyne derivative [OsHCl(≡CPh)(IPr)(P i Pr 3 )]OTf (1; IPr = 1,3-bis(2,6-diisopropylphenyl)...

Dissimilarity in the Chemical Behavior of Osmaoxazolium Salts and Osmaoxazoles: Two Different Aromatic Metalladiheterocycles

“… 33 In 2015, we isolated the five-coordinate hydride-osmium-hydroxy complex [OsH(OH)(≡CPh)(IPr)(P i Pr 3 )]OTf (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazolylidene, OTf = CF 3 SO 3 ), as a result of a chloride by hydroxide replacement. 34 This asymmetrical unsaturated species, supported on an unusual NHC-Os-P i Pr 3 skeleton, 35 possesses three potential reactive points in addition to the metal center: the hydride ligand, the hydroxide group, and the alkylidyne unit. As expected for a hydroxo acid, the hydroxide ligand expresses its duality in the respective nucleophilicity and electrophilicity of the oxygen and hydrogen atoms.…”

“…Among the reported complexes, hydride-osmium-hydroxy derivatives are the most surprising and fascinating, since the reductive elimination of water is a reaction generally favored from a thermodynamic point of view . In 2015, we isolated the five-coordinate hydride-osmium-hydroxy complex [OsH­(OH)­(CPh)­(IPr)­(P i Pr 3 )]­OTf (IPr = 1,3-bis­(2,6-diisopropylphenyl)­imidazolylidene, OTf = CF 3 SO 3 ), as a result of a chloride by hydroxide replacement . This asymmetrical unsaturated species, supported on an unusual NHC-Os-P i Pr 3 skeleton, possesses three potential reactive points in addition to the metal center: the hydride ligand, the hydroxide group, and the alkylidyne unit.…”

“…Thus, it reacts with aldehydes to give carboxylate derivatives and molecular hydrogen . For its part, the osmium–alkylidyne bond undergoes hydroboration and hydrogenation reactions , as well as insertion into the Os–H bond . The presence of three positions with organometallic reactivity, one of them carrying an oxygen atom, makes the complex an excellent candidate to attempt multicomponent reactions directed toward the synthesis of metallaoxazoles.…”

Tuning the Nature and Formation of Bis(dihydrogen)–Osmium Species

“…The perpendicular plane is formed by the chelate hydroxycarbene–NHC ligand, which acts with C(1)–Os(1)–C(6) bite angles of 79.4(5) and 79.6(5)°, the coordinated hydrogen molecule situated trans to the NHC carbon atom C(1), and the triflate anion disposed trans to the hydroxycarbene carbon atom C(6) (O(2)–Os(1)–C(6) = 169.8(4) and 166.9(4)°). The Os(1)–C(1) distances of 2.093(10) and 2.087(10) Å compare well with those previously reported for other Os–NHC complexes, whereas the Os(1)–C(6) bond lengths of 1.900(12) and 1.919(12) Å support the Os–hydroxycarbene formulation . The H(01)–H(02) bond lengths of 0.9(1) and 1.0(1) Å are about 0.35 Å shorter than the separation between H(01) and H(02) in the related phenyl-pyridine complex 3 and suggest a Kubas-type character for the dihydrogen, which was confirmed by the DFT-optimized structure (Figure b).…”

Terminal Mo Carbide and Carbyne Reactivity: H2 Cleavage, B–C Bond Activation, and C–C Coupling

“…Direct hydrogenation with H 2 is well-known for other types of multiply bonded transition-metal complexes, including terminal nitrides, − oxides, − carbenes, − and carbynes. − More recently, methane C–H insertion into a Ti carbyne was reported for a system that also subsequently underwent dehydrogenative C–C coupling to olefins. , While for carbide 1 hydrogenation occurs under relatively mild conditions, attempts toward productive C–C coupling from any formed methylidene species were hampered by an undesired conversion to 4 , even at reduced temperatures. Alternatively, we considered that heterolytic H 2 activation via a frustrated Lewis pair (FLP)-type mechanism could potentially be promoted by addition of a suitable Lewis acid to support the hydride as a counterpart to the methylidyne. − However, we found that BPh 3 (computed Δ G H – = 36 kcal/mol in MeCN) reacted directly with the carbide, while the bulkier but less Lewis acidic mesityl derivative BMes 3 (Δ G H – = 22 kcal/mol in MeCN) did not produce any measurable effect, even though both of these boranes satisfy the hydricity requirements for productive H 2 cleavage with carbide 1 (see the Supporting Information for details).…”