The synthesis of triangular phosphido-bridged iridium alkyne clusters
Date
2018-08-16
Authors
Dônnecke, Daniel
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Abstract
This thesis describes the synthesis and chemistry of triangular phosphido-bridged iridium clusters. The cluster [Ir3(μ-PPh2)3 (CO)6] was obtained analytically pure for the first time. In the solid state this 48 electron cluster exhibits one short iridium-iridium bond of 2.6702(3) Å and two long iridium-iridium bonds, 2.9913(3) Å on average. Two phosphido bridges rest closely within the plane of the metal triangle while the unique phosphido group, bridging the short metal-metal bond, is almost orthogonal to this plane. NMR data suggest that this structure is also adopted in solution below 183 K. At higher temperature however the phosphido bridges give rise to an average signal which is presumably due to a rapid flip-flop motion of these groups.
Addition of one molar equivalent of dimethylacetylendicarboxylate to [Ir3(μ-PPh2)3(CO)6] results in formation of [Ir3(μ-PPh2)3(CO) 6(μ-DMAD)] which contains a diiridacyclobutene. Addition of excess alkyne leads to the CO-inserted [Ir3(μ-PPh2) 3(CO)5(μ-DMAD){κ2-MeO 2CCC(CO2Me)C(O)}] which photochemically decarbonylates, to give [Ir3(μ-PPh2)3(CO)5(μ-DMAD) 2]. The 50 electron cluster [Ir3(μ-PPh2) 3(CO)5(t-BuNC)2] also reacts with dimethylacetylendicarboxylate to yield the CO-inserted [Ir3(μ-PPh2)3(CO) 3(t-BuNC)2{κ2-MeO2CCC(CO 2Me)C(O)}2] in two isomeric forms. The new CO-insertion products represent stable iridacyclobutenones which are reluctant to undergo further insertion reactions involving carbon monoxide, tert-butylisocyanide or dimethylacetylenedicarboxylate.
Addition of dimethylacetylendicarboxylate to cluster mixtures containing predominantly [Ir2Rh(μ-PPh2)3(CO) 5] and [Ir3(μ-PPh2)3(CO)6] results in selective reaction at the tri-iridium cluster which allowed for the isolation of the heterometallic cluster by chromatography. In contrast to the tri-iridium parent, [Ir2Rh(μ-PPh2)3(CO) 5] is much less reactive to dimethylacetylendicarboxylate and inert to CO. Similarly, the heterometallic [Ir2Rh(μ-PPh2) 3(CO)4(RNC)3] (R = tert-butyl; 1,1,3,3-tetramethylbutyl) are reluctant to undergo oxidative addition reactions with dimethylacetylendicarboxylate and iodomethane which readily afford addition products with the homometallic parent clusters. The kinetic difference is a consequence of electronic rather than steric factors in the clusters.
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Keywords
Iridium, Transition metal compounds