Cyclooligomerization Reactions of Different Phosphaalkynes in the Presence of [W(CO)5thf] − A Route to Tungsten-Stabilised Phosphorus Heterocycles

Isolation of an Elusive Phosphametallacyclobutadiene and Its Role in Reversible Carbon−Carbon Bond Cleavage

The reactivity of phosphaalkynes, the isolobal and isoelectronic congeners to alkynes, with metal alkylidyne complexes is explored in this work. Treating the tungsten alkylidyne [^t BuOCO]W≡C^t Bu(THF)₂ (1) with phosphaalkyne (10) results in the formation of [O₂ C(^t BuC=)W{η² -(P,C)-P≡C-Ad}(THF)] (13-^t Bu_THF ) and [O₂ C(AdC=)W{η² -(P,C)-P≡C-^t Bu}(THF)] (13-Ad_THF ); derived from the formal reductive migratory insertion of the alkylidyne moiety into a W-C_arene bond. Analogous to alkyne metathesis, a stable phosphametallacyclobutadiene complex [^t BuOCO]W[κ² -C(^t Bu)PC(Ad)] (14) forms upon loss of THF from the coordination sphere of either 13-^t Bu_THF or 13-Ad_THF . Remarkably, the C-C bonds reversibly form/cleave with the addition or removal of THF from the coordination sphere of the formal tungsten(VI) metal center, permitting unprecedented control over the transformation of a tetraanionic pincer to a trianionic pincer and back. Computational analysis offers thermodynamic and electronic reasoning for the reversible equilibrium between 13-^t Bu/Ad_THF and 14.

1,3-Diphosphacyclobutene Cobalt Complexes

The synthesis and characterization of rare 1,3-diphosphacyclobutene transition-metal complexes is described. Reactions of the cobalt-hydride complex [Co(P₂ C₂ tBu₂ )₂ H] (G) with nBuLi, tBuLi, or PhLi afforded [Li(solv)_x {Co(η³ -P₂ C₂ tBu₂ HR)(η⁴ -P₂ C₂ tBu₂ )}] (1: R=nBu, (solv)_x =(Et₂ O)₂ ; 2: R=tBu, (solv)_x =(thf)₂ ; 3: R=Ph, (solv)_x =(Et₂ O)(thf)₂ ), with an η³ -coordinated 1,3-diphosphacyclobutene ligand as a result of organyl-anion attack at one of the phosphorus atoms of the bis(1,3-diphosphacyclobutadiene) backbone. In contrast to the reactions with PhLi, the aryl-magnesium compounds p-tolyl magnesium chloride and p-fluorophenyl magnesium bromide deprotonate [Co(P₂ C₂ tBu₂ )₂ H] to give the magnesium salt [Mg(MeCN)₆ ][Co(η⁴ -P₂ C₂ tBu₂ )₂ ]₂ (4), which contains a bis(1,3-diphosphacyclobutadiene)-cobaltate anion. The [Co(η⁴ -P₂ C₂ tBu₂ )₂ ]^- anions are well separated from the octahedral [Mg(MeCN)₆ ]²⁺ cation in the molecular structure of 4. Compound 1 reacts with Me₃ SiCl to give neutral [Co(η³ -P₂ C₂ tBu₂ HnBu)(η⁴ -P₂ C₂ tBu₂ SiMe₃ )] (5, 52 % yield) with an SiMe₃ group attached to one of the P atoms of the previously unfunctionalized backbone.

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

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.

The first complexes and cyclodimerisations of methylphosphaalkyne (PCMe)

The first complexes and cyclodimerisations of methylphosphaalkyne, P[triple bond]CMe, are reported to arise from its reactions with a range of platinum(0) complexes and [W(CO)5(THF)]. A number of differences between the chemistry of this phosphaalkyne and that of its bulkier analogues have been highlighted and explained on steric grounds.