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Synthesis and reactivity of terminal phosphido complexes of iridium(III) Bhangu, Kiran


The iridium(III) methyl dlarylphosphido complexes, Ir(CH₃)(PR₂)-[N(SiMe₂CH₂PPh₂)₂] (2a: R = phenyl, 2b: R = meta-tolyl), have been successfully prepared by transmetalation of the iridium(III) methyl iodide complex, Ir(CH₃)(I)[N(SiMe₂CH₂PPh₂)₂], with the corresponding lithium diarylphosphide. Based primarily on a nuclear Overhauser effect difference experiment, these complexes are assigned a stereochemistry intermediate between square pyramidal and trigonal bipyramidal forms. The pyramidal geometry at the phosphido ligand is evident from the ³¹P{¹H} NMR spectral data. The complex 2a affords a mixture of at least three, as yet uncharacterized complexes when heated to 60°C for 5 hours in benzene solution; however, clean formation of the planar iridium(I) methyl-diphenylphosphine complex, Ir(PCH₃Ph₂)[N(SiMe₂CH₂PPh₂)₂], 3a, takes place when 2a is exposed to light for 24 hours in benzene solution. A crossover experiment indicates that the latter reaction involves an intramolecular mechanism. The nucleophilicity of the phosphido ligand is evident from the reaction of 2a with CH₃I; the product afforded in this reaction is Ir(CH₃)(PCH₃Ph₂)(I)[N(SiMe₂CH₂PPh₂)₂], 4. A labelling experiment with CD₃I shows that the reaction is intermolecular as the product observed is Ir(CH₃)(PCD₃Ph₂)(I)[N(SiMe₂CH₂PPh₂)₂]. Exposure of 2a at room temperature to one atmosphere of H₂ produces a mixture of the iridium(III) dihydride Ir(H)₂(PHPh₂)[N(SiMe₂CH₂-PPh₂)₂], 5, and methyl hydride Ir(CH₃)(H)(PHPh₂)[N(SiMe₂CH₂PPh₂)2], 6, in 70 and 30% yields, respectively. The analogous reaction with one atmosphere of D₂ reveals that the formation of the methyl hydride complex involves an intramolecular proton abstraction by the phosphide ligand from the bound methyl group, as the minor product observed in this reaction is Ir(CH₂D)(D)(PHPh₂)[N(SiMe₂CH₂PPh₂)₂]. A mechanism is proposed involving the formation of Ir(=CH₂)(PHPh₂)[N(SiMe₂CH₂PPh₂)₂] followed by trapping with D₂ to give the methyl hydride product. The dihydride complex observed in these reactions is apparently produced by heterolytic cleavage of dihydrogen. Under excess CO, complex 2a is converted to an octahedral carbonyl complex Ir(CH3)(CO)(PPh2)[N(SiMe2CH2PPh2)2], 9; the carbonyl and the phosphide ligands in this complex are in cis arrangement. Upon removing the excess CO from the reaction mixture, another stereoisomer, 10, is produced In which the carbonyl and the phosphide ligands are trans to one another. It is suggested that the carbonyl complex 9 observed under excess CO is the kinetically favoured isomer which rearranges to the more thermodynamically stable isomer, 10, upon removal of the excess CO. Both of the carbonyl Isomers are unstable In solution at room temperature as they convert to the planar iridium(I) complex Ir(CO)[N(SiMe₂CH₂PPh₂)₂] and methyldiphenylphosphine.

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