Rhodium Complexes with Tripodal Polyphosphines as Excellent Precursors to Systems for the Activation of H-H and C-H Bonds

Article abstract of DOI:10.1021/ja00227a022

The trigonal-bipyramidal (TBP) Rh(I) complexes <(NP3)RhCl> (1) and <(PP3)RhCl> (2) are protonated by strong acids to give, after addition of NaBPh4, the octahedral (OCT) cis-(chloride)hydrides <(NP3)Rh(H)Cl>BPh4 (3) and <(PP3)Rh(H)Cl>BPh4 (4) which, by reaction with NaBH4, yield the cis-dihydride <(NP3)RhH2>BPh4 (6) and the monohydride <(PP3)RhH> (8), respectively .By treatment of 6 in acetone with an excess of NaBH4, the monohydride <(NP3)RhH> (7) is obtained.Protonation of 8 with HOSO2CF3 followed by addition of NaBPh4 affords the Rh(III) OCT complex <(PP3)RhH2>BPh4 (9) for which the dichotomy η²-H2 versus cis-dihydride as a function of temperature has been demonstrated.Metathetical reactions of 2 with organolithium reagents give the ?-organyl complexes <(PP3)Rh(CH3)> (11) and <(PP3)Rh(C6H5)> (12) which react with CO to give the corresponding ?-acyl derivatives <(PP3)Rh(COCH3)> (13) and <(PP3)Rh(COC6H5)> (14).Decoordination of a phosphine arm of PP3 is a necessary step for the insertion reaction.The monohydride 7 undergoes electrophilic attack by MeSO3CF3 in THF to give CH4 and the ortho-metalated hydride <<(Ph2PCH2CH2)2N(CH2CH2PPhC6H4)>RhH>(SO3CF3) (15) through the intramolecular activation of a phenyl C-H bond.The structure of the iodide derivative <<(Ph2PCH2CH2)2N(CH2CH2PPhC6H4)>RhI>BPh4*C6H6*0.5CH3COCH3 (17a) was determined by X-ray crystallography.When the methylation of 7 is carried out in THF/benzene mixtures both 15 and the cis-(phenyl)hydride <(NP3)RhH(C6H5)>(SO3CF3) (22) are obtained.Decreasing the temperature or increasing the concentration of benzene favors intermolecular C-H activation over cyclometalation.Methylation of 7 in THF followed by addition of an excess of α,α,α-trifluorotoluene gives the cis-(trifluorotolyl)hydride <(NP3)RhH(C6H4CF3)>(SO3CF3) (23) regardless of the temperature.The reductive elimination of the metalated phenyl from 15 is easily promoted by monodentate ligands such as hydride, halides, pseudohalides, pyridine, and CO to form Rh(I) TBP complexes of the formula <(NP3)RhX>ⁿ⁺ (n = 0, 1).OCT complexes of rhodium(III), in which the two additional coligands are disposed in mutually cis positions, are obtained by reacting solutions of 15 with a plethora of addenda such as H2, Cl2, and CS2.As a result, cis-dihydride, cis-dichloride, and η²-CS2 derivatives are obtained.Dihydrogen elimination from 9, protonation of 11, as well as methylation of 8 give <(PP3)Rh(SO3CF3)> (24) which exists in two isomeric forms.The <(PP3)Rh>⁺ system neither intramolecularly inserts across a C-H bond from a phenyl ring nor intermolecularly activates aromatic C-H bonds.The factors that may be responsible for such a behavior are discussed.Compound 24 reacts with neutral or anionic monodentate ligands affording TBP Rh(I) complexes or oxidatively adds HSO3CF3 to give, after addition of NaBPh4, the OCT Rh(III) cis-(triflate)hydride <(PP3)RhH(SO3CF3)>BPh4 (25).