Sandwich complexes with rhodium

In addition to 38 and 39, other similar i74-l,3-diphosphacyclobutadiene complexes have been made with iron, rhodium, and iridium (40). Interesting chemistry is observed with rhodium, where two of the molecules appear to have dimerized with a metathetical extrusion of di-tert-butylacetylene to give a sandwich complex with the two rhodium atoms complexing on opposite sides of a twisted 1,2,4,5-tetraphosphacyclo-hexatriene ring (40). 

G. E. Herberich, B. Hessner, G. Huttner, and L. Zsolnai, A Triple-Decker Sandwich Complex of Rhodium with Dibora benzene as the Bridging Ligand, Angew. Chem. Int. Ed. Engl. 20, 472-473 (1981). 

Heterobimetallic complexes in which a half-sandwich complex of rhodium ill) is cormected by three bromo-bridges to Re(CO)3 were prepared from metathesis reactions of [Cp RhBr2]2 and [Re( -Br)(C0)3(C4H80)]2- The crystal structure of Cp Rh(/t-Br)3Re(CO)3 was determined by X-ray analysis. The treatment of a mixture of [(rf-CjH8)RuCl2]2 and [Cp RhCl2]2 with silver carboxylates and subsequent hydrogenation afforded mixed metal... 

The ligands triphos or/ and np in presence of compounds of iron, cobalt, nickel, rhodium, iridium and palladium, by reaction with THF solutions of white phosphorus, P, or yellow arsenic, As, form mononuclear or dinuclear sandwich complexes containing the cyclo-triphosphorus or cyclo-triarsenic units which behave as 3n-electrons rings. 

Interesting electrical properties are to be expected with the stepwise extension of this TT-system. The preparation of multilayered cyclophanes proved to be laborious [6] nevertheless new synthetic methods in transition metal chemistry of arenes have opened up a promising alternative approach via preparation of multidecker sandwich complexes (structure type D in Fig. 3). First row transition metals like chromium, iron and cobalt [51] form strong coordinative bonds with arenes when their oxidation state is low [48a] whereas second and third row elements like ruthenium, rhodium and iridium are strongly bonded towards arenes in higher oxidation states [48a, 51]. Sandwich complexes of cyclophanes can be divided into two groups ... 

Fig. 21, Reaction paths (a tof) to multidecker sandwich complexes of ruthenium, cobalt, rhodium and iridium with [2.2]paracyclophane (8) as ligand...

Ferrocene was the first organometallic guest incorporated and numerous spectroscopic and electrochemical studies have been performed on ferrocene, substituted ferrocene, and related metallocene (e.g. cobaltocene) inclusion complexes (444-469]. Half-sandwich cyclopentadienyl- and benzene-metal carbonyl complexes have also been studied quite extensively [470-479] as have // -allyl metal (palladium) complexes [480], diene metal (rhodium) complexes [481-484], acetylene cobalt carbonyl cluster complexes [485], and complexes with metal carbonyls, e.g. Fe(CO)5, Mn2(CO)io, and CoNO(CO)3 [485a]. 

Among the first organometallic complexes with nucleobases was a series of metal carbonyl complexes with these bioligands [220-222]. The structures of nucleobase-bridged carbonyl rhodium complexes were determined by Sheldrick [223], who also obtained a series of half-sandwich ruthenium complexes with nucleobases (Scheme 1.25) [224—226]. 

Cobalt and Rhodium.— Reduction of [( -C Me2)2Rh] + with Zn-HCl affords [( -C6Mee)Rh( 7 -C6H2Me6)]+ and not [( -C6Me6)2Rh]+ as previously claimed the crystal structures of the [PFg]" salts of this ( -cyclohexa-l,3-diene) complex and of the 20-electron sandwich complex [( y -CeMe6)2Co]+ have been determined. ... 

Boehm et al.100 have synthesised and studied a series of half-sandwich rhodium (III) and iridium (III) complexes, derivatives of salicylaldehyde and L-amino acid esters. The diastereoselectivity has shown strong dependence on the type of metal as well as amino acid residue. The labile configuration of the metal atoms was suggested because of changes in the diastereomers ratio with increasing temperature. Fast epimerisation at the metal atom was suggested for some S-phenylalanine complexes. 

The enantioselective catalytic 1,3-dipolar cycloaddition of linear or cyclic nitrones to enals was accomplished using the half-sandwich rhodium(III) complex S, Rc)-[(ri -C5Me5)Rh (/ )-Prophos (H20)](SbF6)2 as catalyst precursor [33, 34]. At —25°C, quantitative conversions to the cycloadducts, with up to 95% ee, were achieved (Scheme 10). The intermediate with the dipolarophile coordinated to the rhodium has been isolated and completely characterized, including the X-ray determination of its molecular structure [33, 34]. 

The reaction of 3,4-dihydroisoquinoline A-oxide (74) and methacrylonitrile in the presence of cationic half-sandwich rhodium and iridium complexes containing a chiral diphosphine ligand was analyzed. The cycloadditions occurred with excellent regio- and diastereoselectivity and low-to-moderate enantioselectivity. Analysis of the catalytic system showed the formation of two epimeric complexes 75 containing the dipolarophile methacrylonitrile. The reaction of one of the isolated diastereopure complexes 75 with 74 afforded cycloadduct 76 with high enantioselectivity. A recycling procedure was developed in order to increase the adduct/catalyst ratio <07CEJ9746>. 

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