Functionalization of 1,3-diphosphacyclobutadiene cobalt complexes via Si–P bond insertion

Abstract

Abstract We report the synthesis of functionalized 1,3-bis(diphosphacyclobutadiene) complexes via the insertion of carbon-oxygen bonds of ethers, esters, aldehydes and amides into the P–Si bond of silylated complexes. Reactions of [K(tol)2][Co(η 4-P2C2R2)2] [[K(tol)2][1a]: R=tBu, [K(tol)2][1b]: R=tPent (=tert-pentyl)] with Me3SiCl afford the trimethylsilyl-substituted derivatives [Co(η 4-P2C2R2SiMe3)(η 4-P2C2R2)] (2a, b, R=tBu, tPent). The Me3Si group is connected to a phosphorus atom of one of the 1,3-diphosphacyclobutadiene ligands. 2a, b readily react with organic substrates containing C–O single and C=O double bonds at ambient temperature. [Co(η 4-P2C2R2(CH2)4OSiMe3)(η 4-P2C2R2)] (3a, b) are formed by reaction of 2a, b with traces of THF. They can also be isolated by reacting the THF solvates [K(thf)2{Co(P2C2 tBu2)2}] ([K(thf)2][1a]) and [K(thf)3{Co(P2C2 tPent2)2}] ([K(thf)3][1b]) with Me3SiCl in toluene or THF. The adamantyl-substituted complex [Co(η 4-P2C2Ad2(CH2)4OSiMe3)(η 4-P2C2Ad2)] (3c) was prepared analogously from [K(thf)4{Co(P2C2Ad2)2}] and Me3SiCl. [K(thf)2][1a] reacts cleanly with Ph3SnCl affording [Co(η 4-P2C2 tBu2SnPh3)(η 4-P2C2 tBu2)] (4) in high yield. Reaction of 2a with styrene oxide affords [Co(η 4-P2C2 tBu2PhC2H3OSiMe3)(η 4-P2C2 tBu2)] (5) as a single regioisomer. By contrast, multinuclear NMR spectroscopic studies indicate mixtures of two isomeric insertion products 6/6′ and 7/7′, respectively, which result from the insertion of 1,2-epoxy-2-methylpropane and 1,2-epoxyoctane. Moreover, these monitoring studies show that reactions of 2a with acyclic ethers afford alkyl substituted complexes such as [Co(η 4-P2C2 tBu2Et)(η 4-P2C2 tBu2)] (8) and alkylsilyl ethers. Reaction of 2a with γ-butyrolactone gives [Co(η 4-P2C2 tBu2(CH2)3C(O)OSiMe3)(η 4-P2C2 tBu2)] (9) via cleavage of the endocyclic C–O single bond of the lactone. Benzaldehyde and acetone cleanly react with 2a to [Co(η 4-P2C2 tBu2CH(Ph)OSiMe3)(η 4-P2C2 tBu2)] (10) and [Co(η 4-P2C2 tBu2CMe2OSiMe3)(η 4-P2C2 tBu2)] (11), while the sterically more demanding ketones 3-pentanone and acetophenone selectively yield the known hydride complex [Co(η 4-P2C2 tBu2)2H] (A). Phenyl isocyanate reacts with 2a at elevated temperature to form [Co(η 3-P2C2 tBu2CON(Ph)SiMe3)(η 4-P2C2 tBu2)] (12) with a functionalized η 3-coordinated ligand. [K(tol)2][1a], [K(tol)2][1b], 2a, 2b, 3a–c, 4, 5, and 9–12 were isolated and characterized by multinuclear NMR spectroscopy, UV/Vis spectroscopy and elemental analysis. [K(tol)2][1b], 2a, 2b, 3c, 4, 5, and 9–12 were additionally characterized by X-ray crystallography.