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Hybrid multidentate ligands : amido phosphine complexes of the Group VIII metals MacNeil, Patricia Ann

Abstract

The hybrid ligands HN(SiMe₂CH₂PPh₂)₂ and HN(CgH₅CH₂)(SiMe₂CH₂PPh₂) react readily with n-butyl lithium to yield the corresponding lithium salts. Metathesis of Group VIII transition metal halides with LiN(SiMe₂CH₂PPh₂)₂ produces a variety of amido phosphine complexes. For the nickel triad, these derivatives have the formula [MCIN(SiMe₂CH₂PPh₂)₂] (M = Ni, Pd, Pt); a variety of rhodium and iridium complexes [M(L)N(SiMe₂CH₂PPh₂)₂] (M = Rh, Ir; L = CO, n²-C₈H₁₄,C₂H₄,PMe₃,PPh₃) and [M(C0D)N(CgH₅CH₂)(SiMe₂CH₂PPh₂)] (M = Rh, Ir; COD = 1,5-cyclooctadiene) have also been prepared and characterized. Single crystal x-ray structural analysis of [NiClN(SiMe₂CH₂PPh₂)₂] indicates that it adopts a distorted square planar stereochemistry, whereas the palladium analogue [PdClN(SiMe₂CH₂PPh₂)₂] is almost perfectly square planar. Based primarily on spectral data, all of these Group VIII amido phosphines are assigned square planar geometries with mutually trans phosphines, with the tridentate ligand coordinated to the metal through the amide nitrogen and both phosphine centres. The amino phosphine dichlorides, [MCI₂NH(SiMe₂CH₂PPh₂)₂] (M = Ni, Pd, Pt) are prepared from the free ligand. The nickel derivative, [NiCl₂NH(SiMe₂CH₂PPh₂)₂], has been shown by crystallographic analysis to have a very distorted tetrahedral geometry; no interaction between the NH moiety and the nickel centre is observed. Reaction of these amino phosphines with triethyl amine cleanly produces the corresponding amido phosphines. [NiClN(SiMe₂CH₂PPh₂)₂] reacts at low temperatures with Grignards or alkyl lithium reagents to yield a series of hydrocarbyl complexes [Ni(R)N(SiMe₂CH₂PPh₂)₂] (R = Me, allyl, vinyl, phenyl). These alkyl (aryl) complexes react readily at room temperature under one atmosphere of carbon monoxide to give Ni(0) species, [Ni(C0)₂N(C0R)(SiMe₂CH₂PPh₂)₂], in which the ligand chelates in a bidentate fashion (through the phosphines) owing to migration of the acyl group to the amide nitrogen. For the methyl and phenyl derivatives, the intermediate Ni(II) acyl complexes have also been isolated. As shown by crystal structural analysis, insertion of CO into the nickel vinyl bond results in an n²-acryloyl Ni(0) species in which the hybrid ligand has undergone further rearrangement to an imidate structure. Some of the rhodium and iridium amidophosphines, lM(L)N(SiMe₂CH₂PPh₂)₂] (M = Rh, L = n²-C₈H₁₄,PPh₃; M = Ir, L = n²-C₈H₁₄, C₂H₄) and [M(COD)N(C₆H₅CH₂) (SiMe₂CH₂PPh₂)] (M = Rh.Ir) are catalyst precursors for the homogeneous hydrogenation of simple olefins under mild conditions (1 atm. H2, 22°C). For the rhodium amido phosphines, olefin isomerization is a competing process with hydrogenation, whereas with [lr(n –C₈H₄)N(SiMe₂CH₂PPh₂)₂] only straightforward reduction is observed. A number of iridium(III) amido phosphine hydrides has been prepared The five-coordinate, 16-electron complex [Ir(H)₂N(SiMe₂CH₂PPh₂)₂], formed from [Ir(n² -C₈H₁₄)N(SiMe₂CH₂PPh₂)₂] under dihydrogen, reacts readily with neutral ligands to stereoselectively produce mer cis-[Ir(H)₂(L)(SiMe₂-PPh₂)₂] (L = CO, PMe₃). Oxidative addition of dihydrogen to [Ir(L)N(SiMe₂CH₂PPh₂)₂] (L = PMe₃, PPh₃) produces hydride derivatives in which the tridentate ligand is coordinated facially. X-ray analysis of fac cis-[Ir(H)₂(PMe₃)N(SiMe₂CH₂PPh₂)₂] has provided conclusive evidence for this facial stereochemistry. The amine trihydrides fac and mer [Ir(H)₃NH(SiMe₂CH₂PPh₂)₂have been characterized; their formation formally corresponds to a ligand-assisted heterolytic cleavage of dihydrogen.

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