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

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(η4-P2C2R2)2] [[K(tol)2][1a]: R=tBu, [K(tol)2][1b]: R=tPent (=tert-pentyl)] with Me3SiCl afford the trimethylsilyl-substituted derivatives [Co(η4-P2C2R2SiMe3)(η4-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(η4-P2C2R2(CH2)4OSiMe3)(η4-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(P2C2tBu2)2}] ([K(thf)2][1a]) and [K(thf)3{Co(P2C2tPent2)2}] ([K(thf)3][1b]) with Me3SiCl in toluene or THF. The adamantyl-substituted complex [Co(η4-P2C2Ad2(CH2)4OSiMe3)(η4-P2C2Ad2)] (3c) was prepared analogously from [K(thf)4{Co(P2C2Ad2)2}] and Me3SiCl. [K(thf)2][1a] reacts cleanly with Ph3SnCl affording [Co(η4-P2C2tBu2SnPh3)(η4-P2C2tBu2)] (4) in high yield. Reaction of 2a with styrene oxide affords [Co(η4-P2C2tBu2PhC2H3OSiMe3)(η4-P2C2tBu2)] (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(η4-P2C2tBu2Et)(η4-P2C2tBu2)] (8) and alkylsilyl ethers. Reaction of 2a with γ-butyrolactone gives [Co(η4-P2C2tBu2(CH2)3C(O)OSiMe3)(η4-P2C2tBu2)] (9) via cleavage of the endocyclic C–O single bond of the lactone. Benzaldehyde and acetone cleanly react with 2a to [Co(η4-P2C2tBu2CH(Ph)OSiMe3)(η4-P2C2tBu2)] (10) and [Co(η4-P2C2tBu2CMe2OSiMe3)(η4-P2C2tBu2)] (11), while the sterically more demanding ketones 3-pentanone and acetophenone selectively yield the known hydride complex [Co(η4-P2C2tBu2)2H] (A). Phenyl isocyanate reacts with 2a at elevated temperature to form [Co(η3-P2C2tBu2CON(Ph)SiMe3)(η4-P2C2tBu2)] (12) with a functionalized η3-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.

1,3‐Diphosphacyclobutene Cobalt Complexes

“…As a result, this formerly planar ligand is tilted (P3‐C3‐P4/P4‐C4‐P3 planes 27.9°; the same dihedral angle amounts to 12.5° in endo ‐H‐ 1 and 7.5° in G ), and the Co1−P3 distance (2.4066(5) Å) is significantly elongated compared with Co1−P4 (2.2584(5) Å). In contrast to related trimethylsilated bis(1,3‐diphosphacyclobutadiene) complexes [Co(P 2 C 2 R 2 ) 2 (SiMe 3 )], the Me 3 Si moiety assumes an endo ‐configuration pointing towards the cobalt. This unexpected arrangement might be explained by a template effect of the lithium cation, which assumes a bridging position in the molecular structure of endo ‐H‐ 1 .…”

“…The stereochemistry is conserved in this reaction. Considering the high reactivity of closely related silylated 1,3‐diphosphacyclobutadiene complexes, a plethora of functionalized sandwich complexes should be accessible by insertion reactions in the phosphorus–silicon bond of such complexes.…”

“…Initially, we examined the reactivity of the potassium salt F toward electrophiles (Figure a) and showed that the reactions of F with HCl⋅Et 2 O, MeI, or Ph 2 PCl afforded structurally distinct neutral species G – I . However, the redox activity and the high basicity of F seemed to limit the substrate scope . Therefore, in this report we present the reaction of hydride‐complex G with lithium organyls ( n BuLi, t BuLi, and PhLi) which gave rise to unprecedented cobalt complexes 1 – 3 [Li(solv) x {Co(η 3 ‐P 2 C 2 t Bu 2 HR)(η 4 ‐P 2 C 2 t Bu 2 )}] [ 1 : R= n Bu, (solv) x =(Et 2 O) 2 ; 2 : R= t Bu, (solv) x =(thf) 2 ; 3 : R=Ph, (solv) x =(THF/Et 2 O) x ) with 1,3‐diphosphacyclobutene ligands.…”

A homoleptic tris(diphosphacyclobutadiene) tripledecker sandwich complex

“…Although these studies revealed various novel coordination compounds, the desired tris(diphosphacyclobutadiene) tripledecker complex remained elusive. Gratifyingly, subsequent studies using the silylated complex [Co(η 4 -P2C2tBu2SiMe3)(η 4 -P2C2tBu2)] (E) 17 met with success. When the latter species is reacted with [Cp*RhCl2]2 (0.5 equivalents, Scheme 1) in toluene, a brownish green reaction mixture is formed, which contains two species according to 31 P{ 1 H} NMR spectroscopic monitoring: The known sandwich compound [Cp*Rh(η 4 -P2C2tBu2)] (2) 18 can be identified as the major product by a doublet at 39.5 ppm ( 1 JRhP = 27 Hz) and 1 as the minor product by a set of two singlets at δ = 55.7 and 111.1 ppm in a 2:1 ratio.…”

A homoleptic tris(diphosphacyclobutadiene) tripledecker sandwich complex

“…Although these studies revealed various novel coordination compounds, the desired tris(diphosphacyclobutadiene) tripledecker complex remained inaccessible. Gratifyingly, subsequent studies using the silylated complex [Co(η 4 -P 2 C 2 t Bu 2 SiMe 3 )(η 4 -P 2 C 2 t Bu 2 )] ( E ) 20 met with success (Scheme 1). When the latter species is reacted with [Cp*RhCl 2 ] 2 (0.5 equivalents) in toluene, a brownish green reaction mixture is formed, which contains two species according to 31 P{ 1 H} NMR spectroscopic monitoring: The known sandwich compound [Cp*Rh(η 4 -P 2 C 2 t Bu 2 )] ( 2 ) 21 can be identified as the major product by a doublet at 39.5 ppm ( 1 J RhP = 27 Hz) and 1 as the minor product by a set of two singlets at δ = 55.7 and 111.1 ppm in a 2 : 1 ratio.…”