ABSTRACT
Oxygenation of C-H and C=C bonds of hydrocarbons with H2O2 and O2 is an important industrial method to convert mineral oil into useful chemicals. Despite their enormous economic impact, these reactions are still not fully understood. In the early 1970s, the potential of Rh and Ir complexes for olefin oxygenation was investigated intensively. Simple inorganic salts of these metals proved to be rather useless for industrial application when compared with the traditional Wacker system. However, the appropriate choice of ligands allows the stepwise oxidation of olefins at Rh and Ir. These systems are therefore useful to study mechanistic details of substrate binding and C-O bond formation at the catalytic metal center. Insight from these model studies helps in understanding the catalytic reactions at these (and possibly other) metal centers. Further insight into the differences between the Rh system and traditional Wacker-type oxidation at Pd may lead to useful applications.
ABSTRACT
[RhI(t-Bu2-boxate)(C2H4)2] spontaneously disproportionates to the mononuclear [RhII(t-Bu2-boxate)2], whereas [RhI(Ph2-boxate)(C2H4)2] is stable against disproportionation.
ABSTRACT
The reaction of 14e [L(Me)Rh(coe)] (1; L(Me)[double bond]ArNC(Me)CHC(Me)NAr, Ar[double bond]2,6-Me(2)C(6)H(3); coe[double bond]cis-cyclooctene) with phenyl halides and thiophenes was studied to assess the competition between sigma coordination, arene pi coordination and oxidative addition of a C-X bond. Whereas oxidative addition of the C-Cl and C-Br bonds of chlorobenzene and bromobenzene to L(Me)Rh results in the dinuclear species [[L(Me)Rh(Ph)(micro-X)](2)] (X=Cl, Br), fluorobenzene yields the dinuclear inverse sandwich complex [[L(Me)Rh](2)(anti-micro-eta(4):eta(4)-PhF)]. Thiophene undergoes oxidative addition of the C-S bond to give a dinuclear product. The reaction of 1 with dibenzo[b,d]thiophene (dbt) in the ratio 1:2 resulted in the formation of the sigma complex [L(Me)Rh(eta(1)-(S)-dbt)(2)], which in solution dissociates into free dbt and a mixture of the mononuclear complex [L(Me)Rh(eta(4)-(1,2,3,4)-dbt)] and the dinuclear complex [[L(Me)Rh](2)(micro-eta(4)-(1,2,3,4):eta(4)-(6,7,8,9)-dbt)]. The latter could be obtained selectively by the 2:1 reaction of 1 and dbt. Reaction of 1 with diethyl sulfide produces [L(Me)Rh(Et(2)S)(2)], which in the presence of hydrogen loses a diethyl sulfide ligand to give [L(Me)Rh(Et(2)S)(H(2))] and catalyses the hydrogenation of cyclooctene.
ABSTRACT
Proposed as intermediates in the catalytic oxidation of olefins to ketones, 3-rhoda-1,2-dioxolanes (κ2 C1 ,O2 -2-peroxyethyl rhodium complexes) have now been prepared by oxygenation of solid [(N4 -ligand)RhI (ethene)]PF6 with air. This process leads to stable isomeric 3-rhoda-1,2-dioxolanes A and B. Upon substitution of PF6- by BPh4- only isomer B is obtained. The X-ray structure of isomer B is presented.
ABSTRACT
The synthesis and characterization of iron-sulfur clusters stabilized by dimethylsilyl-bridged cyclopentadienyl groups are reported. The thermal reaction of Me(2)Si(eta(5)-C(5)H(4))(2)Fe(2)(CO)(4) (1) with S(8) yields the tetranuclear cubane-type cluster compound [Me(2)Si(eta(5)-C(5)H(4))(2)](2)Fe(4)S(6) (4) and the pentanuclear cluster compound [Me(2)Si(eta(5)-C(5)H(4))(2)](2)Fe(5)S(12) (3) in high yields. The photochemical reaction of 1 with S(8) yields the tetranuclear cluster compound [Me(2)Si(eta(5)-C(5)H(4))(2)](2)Fe(4)S(6)(CO) (5), which contains one residual terminal carbonyl. The crystal structures of 3 and 4 have been determined. Crystal data: 3.CH(2)Cl(2), monoclinic, C2/c, a = 23.480(13) Å, b = 11.192 (4) Å, c = 17.84 (3) Å, beta = 118.58(9) degrees, V = 4118(7) Å(3), Z = 4, R = 0.078; 4, triclinic, P&onemacr;, a = 8.4787(7) Å, b = 12.9648(9) Å, c = 13.4990(9) Å, alpha = 79.857(8) degrees, beta = 75.293(8) degrees, gamma = 74.041(11) degrees, V = 1370.9(2) Å(3), Z = 2, R = 0.0447. The Fe(5)S(12) core of 3 has a bowtie structure in which a central iron atom is octahedrally coordinated by six sulfur atoms from one tetrasulfido and four disulfido groups. The structure of 4 resembles the structure of the known iron-sulfur cluster Cp(4)Fe(4)S(6). However, 4 shows a markedly enhanced thermal stability compared to Cp(4)Fe(4)S(6). In their cyclic voltammograms, 4 and 5 exhibit electrochemical behavior typical of cubane-type Cp-iron-sulfur clusters, whereas the cyclic voltammogram of 3 is quite different. The nu(CO) mode of 5 has been measured for four different oxidation states of the cluster by means of IR spectroelectrochemical methods. The Mössbauer spectra of 3 and 3(+) are in accordance with their pentanuclear structure.
