Rhodium derivatives

A black, sparingly soluble derivative, tentatively formulated as Rh6(CO)8(PH3)8 has been isolated from the reaction of [RhtCOhCl with phosphine (89). 

Finally, both llh4(CO)i2 and Rh6(CO)i6 react with dienes to give the species Rh6(CO)ii(Diene) (Diene = 1,o-cyclooctadiene, norbornadiene, 1,4-cyclohexadiene, and 2,3-dimethyl-l, 3-butadiene). The molecular weight of lth6(CO)u (1,5-COD) has been confirmed by mass spectroscopy (92). 

The reaction of Rh6(CO)i6 with a primary amine gives the monoanionic derivative containing a carboamide group, whereas the analogous carbo-alkoxy derivatives have been obtained by reduction with alkali metal alkoxides in anhydrous alcohol (45)  

In these cases also, initial nucleophilic attack on the carbonyl group seems probable. The least stable of these derivatives is [Rh6(CO)i5Cl]-from which the chloride ion can be readily displaced by metathesis with other anions such as SCN- (45)  

The corresponding acyl derivatives, [Rh6(CO)i5(COR)]- (R = Et, Pr), were originally obtained as stable solutions of the hydronium salts by reaction of Rhi(CO)i2 with water in the presence of a mixture of carbon monoxide and an olefin such as ethylene or propylene (43)  

---

Late transition metal boratabenzene complexes can catalyze C-H activation thus, the bis(ethylene)rhodium derivatives (HsCsB-R)Rh(C2H4)2 (R = Ph, NMe2) promote boration of alkanes faster than does the Cp analog Cp Rh(C2H4)2, although the boratabenzene compounds are thermally less stable.110... 

Iridium complexes are known to be generally less active in hydrosilylation reactions when compared to rhodium derivatives, although iridium-based catalysts with bonded chiral carbene ligands have been used successfully in the synthesis of chiral alcohols and amines via hydrosilylation/protodesilylation of ketones [46-52] and imines [53-55], The iridium-catalyzed reaction of acetophenone derivatives with organosubstituted silanes often gives two products (Equation 14.3) ... 

The reactions of dihydrobilin (1,19-dideoxybiladiene-a, c) with transition metals are strongly influenced by the nature of the metal ion. Thus with Mn(OAc)3 or FeClj the corresponding metallocorrolates have been obtained in high yield, in the presence of chromium or ruthenium salts the reaction product isolated has been the metal free macrocycle, while coordination of rhodium requires the presence of an axial ligand such as a phosphine, arsine or amine [21]. Neutral pentacoordinated rhodium complexes have thus been obtained. Although analysis of the electronic spectra of the reaction mixtures demonstrated that cyclization of the open-chain precursor and formation of metallocorrolates occur even in the absence of extra ligands, no axially unsubstituted rhodium derivative has been reported. 

To study the electronic and geometrical structures of metallocorrolates further, rhodium derivatives have also been investigated by XPS [38]. The study has been extended to rhodium derivatives of mcso-tetraphenylporphyrin and oc-taethylporphyrin in order to compare different coordinative structures. Selected data are reported in Table 10. 

Table 15 reports NMR data for rhodium derivatives of octamethylcorrole [21]-... 

The cycloocta-1,5-diene rhodium derivative of hydropentalene (CXX) has as its base peak the molecular ion and also an equally intense peak corresponding to C8H7Rh+, while that attributable to C8H6Rh+ is only slightly weaker. However, the peak at m/e 102, for free pentalene, is very weak (103). 

A concentrated solution of ammonium bromide will not serve, probably because the rhodium derivative is soluble in it. 

The only electrochemically studied rhodium derivative, Rh(acac)3, exhibits an irreversible one-electron oxidation Ep = -1-1.70 V, vs. SCE, in MeCN solution) and an irreversible two-electron reduction Ep = —2.21 V, in THE solution) . The same situation holds for Ir(dik)3 complexes. At variance with the rhodium derivative, Ir(acac)3 shows a chemically reversible one-electron oxidation ( ° = -1-1.15 V, vs. SCE, in MeCN solution) " and an irreversible two-electron reduction Ep = —2.60 V, in THE solution). No crystallographic data of species containing the [Ir(acac)3]+ cation are available. ... 

The reaction of [Rh(/x-OH)(COD-l,5)]2 with Cp2TiMe2 proceeds with methane elimination to give Ti02 and the Cp rhodium derivative CpRh(COD-l,5).295... 

It is worth noting that the analogous rhodium derivative has never been obtained, although several attempts have been made to isolate it. This is even more remarkable because in the parallel carbido chemistry both the trigonal-prismatic species [M6C(CO)i5]- (M = Co " , Rh ) and the corresponding octahedral [M6C(CO)i3j species (Co - , with 86 CVE are known. Spectroscopic... 

The results of the study on the hydrogenation of different functional groups were summarized [194]. Here complexes of rhodium, palladium and nickel fixed on balls of densely cross-linked macroporous polystyrene of HAD-4 grade, on which an-thranilic acid residues were bonded, were used as catalysts. Special attention was paid to the kinetic study of cyclohexene hydrogenation by rhodium(+) derivatives and to elucidate the reaction mechanism using D2. The influence of diffusion restrictions on the reaction rate was discussed, in particular, that of the transport of hydrogen to pores and through the gas-liquid interface. 

---