Insertion of Symmetric Alkynes into the σ[Pd–C(sp², Ferrocene)] Bond of Palladacycles with [C(sp², Ferrocene),N,S(Thienyl)]^– Pincer Ligands

Abstract

A study of the reactivity of [Pd({(η5‐C5H3)–CH=N–(CH2)n–(C4H3S)}Fe{η5‐C5H5})Cl], n = 2 (1e) or 1 (1f), with symmetric alkynes R1–C≡C–R1 (R1 = CO2Me, Et or Ph) has allowed us to achieve the di‐μ‐chloro‐bridged complexes [Pd({(MeO2C–C=C–CO2Me)(η5‐C5H3)–CH=N–(CH2)n–(C4H3S)}Fe{η5‐C5H5})μ‐Cl]2, n = 2 (3e) or 1 (3f), and the mononuclear derivatives [Pd({(MeO2C–C=C–CO2Me)(η5‐C5H3)–CH=N–(CH2)n–(C4H3S)}Fe{η5‐C5H5})Cl(PPh3)], n = 2 (4e) or 1 (4f), [Pd({(R1–C=C–R1)2(η5‐C5H3)–CH=N–(CH2)n–(C4H3S)}Fe{η5‐C5H5})Cl],R1 = CO2Me, n = 2 (5e) or 1 (5f), R1 = Et, n = 2 (6e) or 1 (6f), or R1 = Ph, n = 2 (7e), and [Pd({(Ph–C=C–Ph)2(η5‐C5H3)–CH=N–(CH2)–(C4H3S)}Fe{η5‐C5H5})Cl(PPh3)] (7f·1/2Ph–C≡C–Ph). With MeO2C–C≡C–CO2Me, mono‐ (3e, 3f, 4e and 4f) and bis‐insertion complexes (5e and 5f) were obtained, whereas for R1–C≡C–R1 with R1 = Et or Ph, only bis‐insertion products (6e, 6f, 7e and 7f) were isolated. These studies also proved that 1e and 1f, which have a labile Pd–S(thienyl) bond, are more prone to undergo the insertion of R1–C≡C–R1 (R1 = CO2Me, Et or Ph) than their analogues with [C(sp2, ferrocene), N, X]– [X = O, N(amine) or S(thioether)] pincer ligands.

Why is Ferrocene so Exceptional?

“…For instance, Hor's group reported nickel(0) complexation by hemilabile P , N ‐ferrocene ligands and demonstrated their ethylene oligomerization activities , . Through the use of other chelates such as N , S ligands, ferrocene has been incorporated – by the Lopez group, for instance – onto palladacycles . Key ligands that are most useful for stabilizing gold and palladium NPs are ferrocene‐ and biferrocene‐containing thiolates, and these ligands also serve as self‐assembled monolayers .…”

Insertion Reactions of Six-Membered Cyclopalladated N,N′,N″-Triarylguanidine, [Pd{κ2(C,N)-C6H3Me-3(NHC(NHAr)(═NAr))-2}(μ-Br)]2 (Ar = 2-MeC6H4) with PhC≡C—C(O)OR (R = Me and Et): A Gateway to Second Orthopalladation through Novel Rearrangements

“…Insertion reaction of small molecules such as alkenes and alkynes into the M–C bond of organometallic compounds is one of the fundamentally important reactions in the field of organometallic chemistry and catalysis. , Insertion reactions of alkynes into the Pd–C bond of [ C , E ] palladacycles (E = nitrogen, − oxygen, and sulfur) and organopalladium complexes − have been studied intensively in the past, as these reactions pave the way to numerous alkyne-inserted products, and such products upon depalladation under suitable conditions afford novel carbocycles and heterocycles, which are otherwise difficult to prepare by conventional organic syntheses. − ,,,,,,, [ C , N ] palladacycles insert up to three molecules of alkynes, and the degree and rate of alkyne insertion and the frameworks of the products differ depending upon the electronic and steric effects of the donor atoms in the C , N chelate and ancillary ligands around the palladium, as well as substituents in alkynes, among other factors.…”

Bimetallic complexes that merge metallocene and pincer-metal building blocks: synthesis, stereochemistry and catalytic reactivity

“…Two years later, they explored the reactivity of the ferrocene-based Pd(II)-pincer complexes towards alkynes (Scheme 10). 72 Interestingly, the pincer-Pd bond undergoes an insertion reaction of the alkyne, prob-Scheme 6 Reactivity of metallocene-based Pd(II)-pincer complexes. 57,[64][65][66] Perspective Dalton Transactions ably favored by the lability of the thiophene-S-Pd interaction.…”

“…58 Scheme 10 Ferrocenyl Schiff base complexes, CpFeN Imine C Cp S Thiophene -PdCl, and their reactivity with alkynes. 71,72 Perspective Dalton Transactions because the PdCl moieties can act as electron releasing substituent, vide infra Scheme 19. 86 The Fe(II)/Fe(III) oxidation potential for the monometallic specie was 0.03 V, while for complex 58 this was −0.03 V. The set of metallocene-based pincer complexes of type B was expanded following two, different methodologies to obtain the respective CpFeC 5 H 4 (CC) n (NCN)Pd(II)Cl or Pt(II)Cl (n = 0 or 1) derivates, see (Scheme 12).…”