Stabilized Bis-ylides as a Source of Carbene Ligands in Palladium(II) and Platinum(II) Complexes

Abstract

The reaction of the α-stabilized bis-ylide [Ph3PC(CO2Me)−C(CO2Me)PPh3] with M(II) bis-solvato derivatives, cis-[M(C6F5)2(OC4H8)2] or [M(C∧X)(OC4H8)2]ClO4 (M = Pd, Pt; C∧X = orthometalated ligand), yields the ylide-carbene complexes cis-[M(C6F5)2{C(CO2Me)−C(CO2Me)(PPh3)}(PPh3)] (M = Pt 1a, Pd 1b) or [M(C∧X){C(CO2Me)−C(CO2Me)(PPh3)}(PPh3)]ClO4 (M = Pt, C∧X = o-CH2C6H4P(o-tolyl)2, 2; M = Pd, C∧X = o-CH2C6H4P(o-tolyl)2, 3; CH2C9H6N-C8,N, 4; NC5H4-o-C6H4, 5; C13H8N, 6). The reaction appears to proceed with PC bond cleavage, generating a carbene-ylide ligand [:C(CO2Me)−C(CO2Me)PPh3] and a PPh3 group, both coordinated to the metal center. The reaction of the solvato complexes [M(C∧X)(OC4H8)(PPh3)]ClO4 with the bis-ylide results in the selective formation of [M(C∧X)(PPh3)2]ClO4 (M = Pt, C∧X = o-CH2C6H4P(o-tolyl)2, 7; M = Pd, C∧X = CH2C9H6N-C8,N, 8), showing that when one vacant position is available, only the PPh3 group binds to the metal center, and the carbene fragment decomposes to OPPh3 and dimethylfumarate. On the other hand, the reaction of the bis-solvato precursors [Pd(C6F5)(L)(OC4H8)2]ClO4 with the bis-ylide yields the corresponding metallacyclopropane [Pd{trans-C(C6F5)(CO2Me)−C(PPh3)(CO2Me)}(L)(PPh3)](ClO4) (L = SC4H8, 9; PPh3, 10), probably through the initial formation of an ylide-carbene complexsimilar to 1−6followed by the migratory insertion of the carbene ligand into the Pd−C(C6F5) bond and coordination of the newly generated ylide function. The crystal structure of complex 1a·CH2Cl2 has been analyzed by X-ray diffraction.

Ylide Ligands

“…Complex (98) does not have a carbonyl group and, according with previous DFT studies, must not show conformational preferences. In fact, complex (98) has been obtained as the dl-pair, but in a similar Zr IV complex the ylide bonds as the meso form showing that the two forms are isoenergetic [202]. The anionic bis-ylide compounds [R 2 P(CH 2 ) 2 ]bonded as bridging ligands (mode VI) have been extensively studied in complexes of coinage metals, mainly in gold derivatives.…”

“…A recent report describes a similar situation in sulfur ylides (97) without characterization of the source of preferences [201]. Complex (98) does not have a carbonyl group and, according with previous DFT studies, must not show conformational preferences. In fact, complex (98) has been obtained as the dl-pair, but in a similar Zr IV complex the ylide bonds as the meso form showing that the two forms are isoenergetic [202].…”

“…The ability of ylides to behave as carbene transfer reagents has been extended recently to stabilized bis-ylides in Pd(II) or Pt(II) complexes, as shown in Scheme 11 [98]. As a function of the auxiliary ligands in the starting material, the carbene (38) is stable (both R are C 6 F 5 groups or a cyclometallated ligand) or evolves to the metallacyclopropane (39) (only one C 6 F 5 ligand in the starting compound).…”

Reactive Palladium Carbenes: Migratory Insertion and Other Carbene–Hydrocarbyl Coupling Reactions on Well‐Defined Systems

“…The putative pentafluorophenyl palladium carbene‐ylide was not detected and only complex 32 , the result of the carbene–aryl coupling, was isolated. However, the intermediacy of the palladium carbene‐ylide is strongly supported by the isolation of this type of complex from a close related palladium precursor ( 33 , Scheme b) …”

Bonding properties and bond activation of ylides: recent findings and outlook

“…This reactivity pattern can be extended to phosphorus bisylides like 88. 99 The initial purpose was the study of its bonding properties, considering that it has two prochiral centers. The reactivity found was far more interesting than expected.…”