Rhodium complexes bridged

It was recognized during the development of propene hydroformylation that propene provided some stabilization for the catalyst. In the absence of the alkene, but in the presence of carbon monoxide and hydrogen, the catalyst can undergo what has been termed intrinsic deactivation. [3 3] Apparently after oxidative addition of triphenyl-phosphine to rhodium, diphenylphosphido bridged rhodium complexes are formed. 

The equilibrium concentrations of Rh2(CO)s and Rh4(CO)i2 were determined by infrared spectroscopy by monitoring the absorbance of the band at 1886.8 cm-1, which corresponds to the stretching of the bridging carbonyls of the tetrarhodium complex. More details of the experimental procedure can be found in the original papers. For our purpose, it is enough to say that the equilibrium concentrations of the rhodium complexes were quite low (< 10-3 mol dm-3), but the same was not true for the CO concentration ( 2 mol dm-3 see... 

A series of studies deals with the catalytic activity of the dinuclear thiolate-bridged rhodium complex [ Rh( i-S Bu)(CO)(TPPTS) 2] in the hydroformylation ofpropene, 1-hexene and 1-octene (Scheme 4.4) [76-80]. Turnover frequencies up to 3100 h" were detected. 

Since the hydroformylation reaction for most substrates shows a first order dependence on the concentration of rhodium hydride, the reaction becomes slower when considerable amounts of rhodium are tied up in dimers. This will occur at low pressures of hydrogen and high rhodium concentrations. Dimer formation has mainly been reported for phosphine ligands [17, 42, 45], but similar dimeric rhodium complexes from monophosphites [47] and diphosphites [33, 39] have been reported. The orange side product obtained from HRh(15)(CO)2 was characterized as the carbonyl bridged, dimeric rhodium species Rh2(15)2(CO)2 [39]. 

Pyrazine, bridged supramolecular complexes, 46 206-213, 240-243 p-Pyrazine complexes, osmium, 37 307 Pyrazolates, ligand with rhodium complex, 44 278 Pyrazoles... 

On protonation with HBF4 or CF3SO3H, the cobalt and rhodium complexes 120 behaved similarly to 118. However, because of the lack of a well-coordinating anion the initially formed 16-electron chalcogenolato intermediate dimerized to form the chalcogenolato-bridged complexes 121 (Scheme 29).54 170... 

The Influence of Potentially Bridging Groups in Homogeneous Hydrogenation with Rhodium Complexes A Study of para-Dimethylamino-Substituted Phenylphosphine Metal Complexes... 

An almost unique variation on the -peroxo coordination mode is exhibited by a rhodium dimer, [RhCl(02)(PPh3)2]2,60 which is composed of two identical subunits which have the dioxygen moiety coordinated in the -peroxo mode. These subunits are linked, not by a chlorine bridge as in other rhodium complexes such as [RhCl(CO)2]2, but via the coordinated dioxygen group as shown in (3). 

G. Dinuclear Indenyl Rhodium Complexes with Bridging OFCOT Ligands... 

Metalloporphyrins have been used for epoxidation and hydroxylation [5.53] and a phosphine-rhodium complex for isomerization and hydrogenation [5.54]. Cytochrome P-450 model systems are represented by a porphyrin-bridged cyclophane [5.55a], macrobicyclic transition metal cyclidenes [5.55b] or /3-cyclodextrin-linked porphyrin complexes [5.55c] that may bind substrates and perform oxygenation reactions on them. A cyclodextrin connected to a coenzyme B12 unit forms a potential enzyme-coenzyme mimic [5.56]. Recognition directed, specific DNA cleavage... 

The structure of 73 was substantiated spectroscopically (5 nB = 82.5 ppm) and confirmed crystallographically. A centrosymmetric chloro-bridged dimeric structure with tricoordinate copper centers was observed. The pendant character of the borane is apparent from the rather long the B- Cu (3.05 A) and B- Cl (4.06 A) distances associated with a trigonal planar geometry around boron. Compared to related mononuclear palladium and rhodium complexes (see Section IV.B), the presence of the chloro-bridge disfavors the formation of Cl-B interactions (coordination mode F) in 73. 

Finally, the generality of this bonding situation was illustrated by preparing the rhodium complex 90 upon treatment of 7b with the [Rh(p-Cl)(nbd)]2 precursor (Scheme 51).22 The bridging coordination of the PB ligand across the Rh-Cl bond was supported both experimentally (NMR spectroscopy and X-ray diffraction) and theoretically. 

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