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Hydrocarbon-Soluble, Isolable Ziegler-Type Ir(0)(n) Nanoparticle Catalysts Made from [(1,5-COD)Ir(mu-O2C8H15)](2) and 2-5 Equivalents of AlEt3: Their High Catalytic Activity, Long Lifetime, and AlEt3-Dependent, Exceptional, 200 degrees C Thermal Stability

2012-04-01
Hamdemir, Isil K.
Özkar, Saim
Yih, Kuang-Hway
Mondloch, Joseph E.
Finke, Richard G.
Hydrocarbon-solvent-soluble, isolable, Ziegler-type Ir(0)(n) nanoparticle hydrogenation catalysts made from the crystallo-graphically characterized [(1,5-COD)Ir(mu-O2C8H15)](2) precatalyst and 2-5 equiv of AlEt3 (>= 2 equiv of AlEt3 being required for the best catalysis and stability, vide infra) are scrutinized for their catalytic properties of (1) their isolability and then redispersibility without visible formation of bulk metal; (2) their initial catalytic activity of the isolated nanoparticle catalyst redispersed in cydohexane; (3) their catalytic lifetime in terms of total turnovers (TTOs) of cyclohexene hydrogenation; and then also and unusually (4) their relative thermal stability in hydrocarbon solution at 200 degrees C for 30 min. These studies are of interest since Ir(0)(n) nanoparticles are the currently best-characterized example, and a model/analogue, of industrial Ziegler-type hydrogenation catalysts made, for example, from Co(O2CR)(2) and >= 2 equiv of AlEt3. Eight important insights result from the present studies, the highlights of which are that Ir(0)(n) Ziegler-type nanoparticles, made from [(1,5-COD)Ir(mu-O2C8H15)](2) and AlEt3, are (i) quite catalytically active and long-lived; (ii) thermally unusually stable nanoparticle catalysts at 200 degrees C, vide infra, a stability which requires the addition of at least 3 equiv of AlEt3 (Al/Ir = 3), but where (iii) the Al/Ir = 5 Ir(0)(n) nanopartides are even more stable, for >= 30 mm at 200 degrees C, and exhibit 100 000 TTOs of cyclohexene hydrogenation. The results also reveal that (iv) the observed nanoparticle catalyst stability at 200 degrees C appears to surpass that of any other demonstrated nanoparticle catalyst in the literature, those reports being limited to <= 130-160 degrees C temperatures; and reveal that (v) AlEt3, or possibly surface derivatives of AlEt3, along with [RCO2 center dot AlEt3](-) formed from the first equiv of AlEt3 per 1/2 equiv of [(1,5-COD)Ir(mu-O2C8H15)](2) are main components of the nanoparticle stabilizer system, consistent with previous suggestions from Shmidt, Goulon, Bonnemann, and others. The results therefore also (vi) imply that either (a) a still poorly understood mode of nanoparticle stabilization by alkyl Lewis acids such as AlEt3 is present or, (b) that reactions between the Ir(0)(n) and AlEt3 occur to give initially surface species such as (Ir-surface)(x)-Et plus (Ir-srface)(x)-Al(Et)(2)Ir, where the number of surface Ir atoms involved, x = 1-4; and (vii) confirm the literature's suggestion that the activity of Ziegler-type hydrogenation can be tuned by the Al/Ir ratio. Finally and perhaps most importantly, the results herein along with recent literature make apparent (viii) that isolable, hydrocarbon soluble, Lewis-acid containing, Ziegler-type nanoparticles are an underexploited, still not well understood type of high catalytic activity, long lifetime, and unusually if not unprecedentedly high thermal stability nanoparticles for exploitation in catalysis or other applications where their unusual hydrocarbon solubility and thermal stability might be advantageous.