The Roles of Aminocarbyne Intermediates and Intramolecular Electron Transfer in the Formation od Carbon-Carbon Bonds by the Coupling of Isocyanides
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Source and publish data:
Journal of the American Chemical Society p. 7812 - 7823 (1989)
Update date:2022-08-17
Topics:
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Authors:
Wu, Jianxin
Fanwick, Phillip E.
Kubiak, Clifford P.
Article abstract of DOI:10.1021/ja00202a023
The binuclear iridium complex Ir2(CNR)4(dmpm)2, 1, (R = 2,6-Me2C6H3, dmpm = Me2PCH2PMe2) was prepared by reduction of 2 with Na amalgam in benzene.The structure of 1, determined by X-ray diffraction, consists of two iridium atoms bridged by two cis,cis dmpm ligands and two μ-2,6-xylyl isocyanide ligands.The Ir-Ir bond length is 2.5998 (7) Angstroem.The nonbonded distance between the carbon atoms of the μ-isocyanide ligands is 2.37 (2) Angstroem.The potential coupling of the two μ-isocyanide ligands of 1, promoted by Lewis acids, wasinvestigated.Addition of 2 equiv of BH3*THF to 1 affords Ir2(μ-CN(BH3)R)2(CNR)2(dmpm)2, 2, which contains two μ-CN(BH3)R aminocarbyne groups which are not coupled.Addition of Al2Et6 to 1 in toluene gives Ir2(C2(NR)2AlEt2)(CNR)2(dmpm)2, 3, which contains a new carbon-carbon bond, d(C-C) = 1.48 (1) Angstroem, between two coupled isocyanides.The AlEt2 fragment bridges two isocyanide N atoms to form an essentially planar five-membered C2N2Al ring.The C2N2Al ring is coplanar with the two iridium atoms.Complex 3 is paramagnetic and exhibits an isotropic EPR powder spectrum, g = 2.005 at -150 deg C.Complex 3 is reversibly oxidized electrochemically to form the diamagnetic species (CNR)2(dmpm)2>, 4.E1/2(4/3) = -0.22 V vs SCE.The mechanism of isocyanide coupling leading to 3 involves electronic reconfiguration of the d9-d9 Ir02 core of 1 to the d8-d8 IrI2 "A-frame" species 4.Paramagnetic 3 is formed by single-electron transfer to 4 by AlEt4(-), formed in situ during isocyanide coupling.Crystal data for 1: space group P21; a = 10.615 (2), b = 16.883 (3), c = 15.044 (3) Angstroem; β = 94.23 (1)0; V = 2689 (2) Angstroem3; Z = 2; R = 0.033, Rw = 0.044 for 528 variables and 3229 unique data with I > 3?(I), Mo Kα radiation.Crystal data for 2: space group P21212; a = 15.905 (2), b = 16.286 (2), c = 10.528 (3) Angstroem; Z = 2; V = 2727 (1) Angstroem3; R = 0.048, Rw =0.062 for 282 variables and 2263 unique data with I > 3?(I), Mo Kα radiation.Crystal data for 3: space group P21/c; a = 11.44 (1), b = 19.072 (1), c = 25.602 (3) Angstroem; β = 102.910; V = 5446 (2) Angstroem3; Z = 4; R = 0.035, Rw = 0.040 for 550 variables and 5330 unique data with I > 3?(I), Mo Kα radiation.
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Full text of DOI:10.1021/ja00202a023