Oxidative Activation of the Manganese Bis-vinylidene Complexes [(η⁵-C₅R₅)(CO)₂MnCCPh]₂ (R = H, Me) toward Addition of Nucleophiles

Abstract

A new approach to the activation of transition metal bis-vinylidene (μ-1,3-butadiene-1,4-diylidene) complexes toward nucleophilic addition is proposed, based on their two-electron oxidation to corresponding μ-2-butene-1,4-diylidyne compounds of a bis-carbyne type. The latter are more electrophilic and able to react with nucleophiles that are inert to the former. In line with this synthetic methodology the reaction of the manganese bis-vinylidene complexes [(η5-C5R5)(CO)2MnCCPh]2 (1, R = H; 2, R = Me) with oxygen nucleophiles (H2O and OH-) to form binuclear cyclic bis-carbene compounds [(η5-C5R5)(CO)2Mn]2(μ-C4Ph2O) (5, R = H; 6, R = Me) is induced by oxidation, 2 equiv of oxidant (Cp2FeBF4) being required to complete the reaction of 1 with water, resulting in 5 with 76% yield. The oxidation of 1 with 1 equiv of AgBF4 leads to the thermally unstable radical cation 7, which reacts with water to give an equimolar mixture of 1 and 5. Two-electron oxidation of 1 leads to the dicationic bis-carbyne complex (η5-C5H5)(CO)2Mn+⋮CC(Ph)C(Ph)C⋮Mn+(CO)2(η5-C5H5) (3), which exists as the solvate 3·2THF (11) in THF solution and reacts with water smoothly to give 5 in about 90% yield. Unlike 1, the bis-vinylidene complex 2 does not react with water even in the presence of oxidant. Complex 6 can be obtained in 15% yield by the reaction of the bis-carbyne dicationic complex (η5-C5Me5)(CO)2Mn+⋮CC(Ph)C(Ph)C⋮Mn+(CO)2(η5-C5Me5) (4) with Bu4NOH. The different reactivities of 1 and 2 are also confirmed by cyclic voltammetry. The crystal structures of 2, 5, and 6 are reported.

Ligand Redox Non‐Innocence in Transition‐Metal σ‐Alkynyl and Related Complexes

“…The reactivity of the μ-C 4 Ph 2 ligand can be switched by these redox processes, with the neutral bis(vinylidene) system being inert towards oxygen nucleophiles (H 2 O, OH -) whereas the dicationic bis(carbyne) is readily converted into furan derivatives upon reaction with H 2 O or OH -. [106] In an interesting counterpoint, reduction of the group 6 alkynyl complexes [M(CϵCPh)(CO) 3 CpЈ] either chemically (Na/K, M = Cr, CpЈ = Cp) or electrochemically (M = Cr, CpЈ = Cp; M = Mo, W, CpЈ = Cp*) also affords bis-(carbyne) complexes [CpЈ(CO) 2 MϵC-C(Ph)=C(Ph)-CϵM(CO) 2 CpЈ], albeit in very low yields for the Mo and W examples, and is thought to occur by the sequence of reduction, CO elimination, C β -C β coupling and oxidation shown in Scheme 13. [107,108] The bis(carbyne) product undergoes two reversible reduction processes, separated by around 140 mV, although the electronic characteristics of the intermediate mono-anion have not been investigated in detail.…”

Formal Insertion of o-Carborynes into Ferrocenyl C–H Bonds: A Simple Access to o-Carboranylferrocenes

“…In the reaction of 1,2-dilithio- o -carborane with 4 + [PF 6 ] − (Scheme ), 1 equiv of ferrocenium hexafluorophosphate functions as the electrophile and the other one may act as the oxidant to oxidize the nucleophilic adducts 8a ; meanwhile, 1,2-dilithio- o -carborane also serves as a base to accept the leaving proton from the cyclopentadiene of 9a (Scheme ). After initial oxidation of ferrocene by I 2 , one-pot reaction with 1-methyl- o -carborane in Scheme would enter the same reaction pathway, which can be regarded as oxidative activation of ferrocene for nucleophilic reaction …”

“…After initial oxidation of ferrocene by I 2 , 16 one-pot reaction with 1-methyl-ocarborane in Scheme 2 would enter the same reaction pathway, which can be regarded as oxidative activation of ferrocene for nucleophilic reaction. 23 In view of a much higher yield of ferrocene (4) over the coupling product 5a (52% vs 17%) in the reaction of 1,2-dilithioo-carborane with 4 + [PF 6 ] − (Scheme 3), a competitive direct redox reaction between the two substrates is likely to occur (Scheme 4), 24 which can be viewed as the reverse process of oneelectron oxidation of 4 by o-carboryne (Scheme 5). The resulting carboranyl radicals then undergo hydrogen abstraction with solvents to regenerate o-carborane (1a).…”