Organorhodium chemistry

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Cyclooctadiene rhodium chloride dimer

Organorhodium chemistry is the chemistry of organometallic compounds containing a rhodium-carbon chemical bond, and the study of rhodium and rhodium compounds as catalysts in organic reactions.1

Stable organorhodium compounds and transient organorhodium intermediates are used as catalyst such as in olefin hydroformylation, olefin hydrogenation, olefin isomerization and the Monsanto process2

Description

Organometallic rhodium compounds share many characteristics with those of cobalt (see organocobalt compounds), which is also in group 9. Rhodium can exist in oxidation states of -III to +IV, but rhodium(I) and rhodium(III) are the most common. Rhodium(I) compounds (d8 configuration) can occur with square planar or trigonal bipyramidal geometries, while rhodium (III) compounds (d6 configuration) typically have an octahedral geometry.2

Examples

Important homoleptic rhodium compounds are tetrarhodium dodecacarbonyl Rh4(CO)10 and hexadecacarbonylhexarhodium Rh6(CO)16. The hexarhodium compound is less preferred due to poor solubility. Both are important catalysts in hydroformylation of alkenes often accompanied by a phosphine ligand:

\mathrm{RHC=CH_2\ +\ CO\ +\ H_2\ \xrightarrow {Rh_4(CO)_{10} / PPh_3}\ \ RCH_2CH_2CHO}

Nitrobenzene reduction is another reaction catalysed by this compound type:

\mathrm{PhNO_2\ +\ C_6H_6\ +\ 3\ CO\ \xrightarrow {Rh_6(CO)_{16}}\ \ PhNHCOPh\ +\ 2\ CO_2}

Cyclooctadiene rhodium chloride dimer [RhCl(COD)]2 is investigated for its use in C-H bond activation. Sandwich compounds of rhodium, such as rhodocene, and half-sandwich compounds like [(η5-Cp)Rh(CO)2 are well known.

Another hexarhodium complex is (C60)Rh6(C60), where the rhodium center has two large fullerene ligands around it.

Other relevant organorhodium compounds used as catalysts
Name Molecular formula CAS
(Acetylacetonato)dicarbonylrhodium(I) Rh(CO)2(C5H7O2) 14874-82-9
Bicyclo[2.2.1]hepta-2,5-diene-rhodium(I) chloride dimer C14H16Cl2Rh2 12257-42-0
Chloro(1,5-cyclooctadiene)rhodium(I) dimer C16H24Cl2Rh2 12092-47-6
(Acetylacetonato)(norbornadiene)rhodium(I) C12H15O22Rh 32354-50-0
Chloro(1,5-hexadiene)rhodium(I),dimer C12H24Rh2Cl2 32965-49-4
Acetylacetonatobis(ethylene)rhodium(I) C9H15O2Rh 12082-47-2
[1,4-Bis(diphenylphosphino)butane](1,5-cyclooctadiene)rhodium(I) tetrafluoroborate C36H40BF4P2Rh 79255-71-3

Cyclometallation

Cyclometalated rhodium compounds constitute an important class of organometallic chemistry. Although such compounds are well documented in the literature rhodium(III) cyclometalates with azo function are spare. A typical example of this category viz. novel hexacoordinated orthometalated rhodium(III) thiolato complex trans-[Rh(CNS)Cl(PPh3)2 was synthesized from benzyl 2-(phenylazo)phenyl thioether and RhCl3·3H2O in the presence of excess PPh3 via in situ C(sp2)−H and C(sp3)−S bond scissions. This is the first example for a coordination compound of (phenylazo)thiolate ligand. The mechanism of formation of orthometalated azobenzene derivative was described to proceed via initial coordination of azo-nitrogen followed by electrophilic substitution at the pendant phenyl ring. PPh3 plays a crucial role in the C(sp3)−S cleavage process. Reductive cleavage by single electron transfer (SET) mechanism is likely to be operative for the C−S bond cleavage. Unlike analogous (phenylazo)phenolato compound the orthometalated thiolato complex exhibits a fully reversible oxidative wave at 0.82 V vs Ag/AgCl and this response is supposed to be primarily centered on the thiolato sulfur atom.3

See also

  • Chemical bonds of carbon with other elements in the periodic table:
CH He
CLi CBe CB CC CN CO CF Ne
CNa CMg CAl CSi CP CS CCl CAr
CK CCa CSc CTi CV CCr CMn CFe CCo CNi CCu CZn CGa CGe CAs CSe CBr CKr
CRb CSr CY CZr CNb CMo CTc CRu CRh CPd CAg CCd CIn CSn CSb CTe CI CXe
CCs CBa CHf CTa CW CRe COs CIr CPt CAu CHg CTl CPb CBi CPo CAt Rn
Fr CRa Rf Db Sg Bh Hs Mt Ds Rg Cn Uut Fl Uup Lv Uus Uuo
CLa CCe CPr CNd CPm CSm CEu CGd CTb CDy CHo CEr CTm CYb CLu
Ac CTh CPa CU CNp CPu CAm CCm CBk CCf CEs Fm Md No Lr
Chemical bonds to carbon
Core organic chemistry Many uses in chemistry
Academic research, but no widespread use Bond unknown

References

  1. ^ Synthesis of Organometallic Compounds: A Practical Guide Sanshiro Komiya Ed. S. Komiya, M. Hurano 1997
  2. ^ a b Crabtree, Robert H. (2005). The Organometallic Chemistry of the Transition Metals (4th ed.). USA: Wiley-Interscience. ISBN 0-471-66256-9. 
  3. ^ K. Pramanik, U. Das, B. Adhikari, D. Chopra, H. Stoeckli-Evans (2008). "RhCl3-Assisted C-H and C-S Bond Scissions: Isomeric Self-Association of Organorhodium(III) Thiolato Complex. Synthesis, Structure, and Electrochemistry". Inorg. Chem. 47 (2): 429–438. doi:10.1021/ic7016006. PMID 18161963.