Highly Active Iridium Catalysts for Alkane Dehydrogenation. Synthesis and Properties of Iridium Bis(phosphine) Pincer Complexes Based on Ferrocene and Ruthenocene

Abstract

Novel bis(phosphine) pincer complexes of iridium based on ferrocene and ruthenocene have been synthesized. The reaction of 1,3-bis((di-tert-butylphosphino)methyl)metallocenes [t-BuP,CH,PM] (M = Fe, 10; M = Ru, 13) with [Ir(COE)2Cl]2 in refluxing toluene leads to the corresponding chloro-hydrido pincer complexes as a mixture of H-endo- (M = Fe, 11; M = Ru, 14) and H-exo-IrH(Cl)[t-BuP,C,PM] (M = Fe, 12; M = Ru, 15) isomers. Treatment of compounds 11, 12 and 14, 15 with NaH in hot cyclooctane generates corresponding dihydrido complexes IrH2[t-BuP,C,PM] (M = Fe, 6; M = Ru, 7). Reactivity of the chelated iridium atom in hydrido-iridium pincer complexes and oxidation centered at the iron atom of ferrocene-based pincer complexes were studied. Complexes H-exo-IrH(Cl)(CO)[t-BuP,C,PFe] (20), Ir(CO)[t-BuP,C,PFe] (21), and Ir(CO)[2,6-(tBu2PO)2C6H3] (30) were characterized by X-ray crystallography. On the basis of νCO values for carbonyl iridium derivatives, electronic characteristics of the corresponding fragments {Ir[t-BuP,C,PM]} n (n = 0, +1) were elucidated. Complexes 6 and 7 reveal unprecedented catalytic activity in cyclooctane dehydrogenation in the presence of tert-butylethylene as a hydrogen acceptor: turnover numbers 3300, 2571, and 1843 were obtained for 6, 7, and the known complex IrH2[2,6-(tBu2PO)2C6H3] (4c), respectively, at 180 °C for 8 h. The high catalytic activity for 6 and 7 and the difference in the catalytic activity of these “isostructural” complexes are discussed in terms of steric and electronic effects.

Ruthenium complexes with ferrocene-based P,C,P pincer ligand

“…8 The NMR spectra were recorded on a Bruker AMX 400 spectrometer. The 1 H and 31 P NMR chemical shifts are given on the δ scale and were measured at 400.13 and 161.98 MHz, re spectively.…”

“…The sandwich nature of metallocene pincer ligands provides new possibilities for fine controlling the steric and electronic effects of chelated metal atoms serving as the catalytic centers, for example, by introducing sub stituents into the unmetallated cyclopentadienyl ring 9 (not only the electron density on the chelated metal atom but also the steric accessibility of this atom for the substrate can be controlled), varying the nature of the central atom of the metallocene moiety 8 and its redox state, 7, 8 and constructing planar chiral structures. The synthesis of metallocene based rhodium, 3,10 iridium, 4,8 and palla dium 5-7,9 pincer complexes was documented.…”

Pincer‐Type Iridium Complexes for Organic Transformations

“…Replacing the aryl anion by a ferrocenyl unit ( 8 ; Figure 12.3 ) improved the catalytic activity by a factor of 2 and allowed for a decrease in the reaction temperature for dehydrogenation [20] . At 180 ° C, TONs as high as 3300 have been obtained with complex 8 , while the phosphinite catalyst 5a provided a TON of 1840 under these conditions.…”

Reactivity of Pincer Complexes Toward Carbon Monoxide

“…Interestingly, analogous reaction of the (PNP)Rh(I)-O 2 adduct (43) affords complex (44) in which the carbon monoxide promotes the OÀO bond cleavage, transferring, most likely in a concerted manner, the resulting oxygen atoms to both CO and one of the phosphorus atoms in the (PNP) pincer ligand according to Scheme 2.23.…”

“…An interesting modification of the backbone of the PCP pincer ligands including ferrocene or ruthenocene motifs has been provided recently by Koridze and coworkers [44]. They reported the synthesis of the hydrido chloride complexes (105a-b) and their reactivity with carbon monoxide, affording hydrido chloride carbonyl derivatives (106a-b) and small amounts of carbonyl compounds (107a-b) (Scheme 2.49).…”