REACTIVITY OF THE MOLECULAR-HYDROGEN COMPLEX [IRH4(PME2PH)3]BF4 TOWARD OLEFINS - THE ORIGIN OF STEREOCHEMICAL RIGIDITY OF M(PR3)3(OLEFIN)2 SPECIES

Stoichiometric protonation of IrH3P3 (P = PMe2Ph) with HBF4 in CH2C12 leads to the formation of IrH4P3+. T1 measurements along with reactivity studies suggest that this complex contains two hydride ligands and a coordinated dihydrogen molecule. Since H2 is easily lost from this species, IrH4P3+ serves as a convenient precursor to the highly reactive, unsaturated hydride species IrH2P3+. IrH4P3+ reacts with ethylene to initially form the hydride-ethylene complex cis, mer-IrH2(C2H4)P3+. Labeling studies on this complex indicate the existence of a hydrido-ethyl equilibrium partner. Added ethylene promotes the reductive elimination of ethane and produces the nonfluxional bis-ethylene complex Ir(C2H4)2P3+, which was structurally characterized. lr(C2H4)2P3+ was found to exchange with labeled C2H4, thus indicating the existence of the monoethylene species lr(C2H4)P3+. This species can be trapped with added ligands (MeCN or CH3-) and reacts with H2 to regenerate IrH2(C2H4)P3+, thus closing the catalytic cycle for ethylene hydrogenation. Allene reacts with IrH4P3+ to produce propene and initially an unsymmetrical bis-allene complex which undergoes a slow isomerization to the symmetrical bis-allene complex. Ir(C3H4)2P3+ was structurally characterized and shown to contain two uncoupled allene ligands. EHT calculations done on the model compound Ir(C2H4)2(PH3)3+ give a high barrier for rotation of one (or two) olefin(s) as well as a high barrier for pseudorotation. The origin of these high-energy barriers stems from the absence of π-acceptor ligands such as CO on the metal. The unsymmetrical bis-allene isomer is calculated to be slightly less stable than the symmetrical one. The stereochemical rigidity of the bis-allene and bis-ethylene complexes is due to the same causes. Crystallographic data for [Ir(C2H4)2-(PMe2Ph)3]BF4·0.5H2O: a = 10.580 (5) Å, b = 20.332 (12) Å, c = 27.771 (17) Å, β = 93.84 (3)° with Z = 8 in space group P21c. For [Ir(C3H4)2(PMe2Ph)3]BF4: a = 21.817 (7) Å, b = 13.999 (2) Å, c = 10.022 (1) Å, β = 92.91 (l)° with Z = 4 in space group P21/a. © 1990, American Chemical Society. All rights reserved.