Easy access to hydride chemistry on a tripodal P-based rhodium scaffold

摘要

The TBPY-5 rhodium complex [(PhBP _3)Rh(CH _2=CH _2)(NCMe)] (1; PhBP _3 = PhB(CH _2PPh _2) _3), which contains ethylene in the equatorial plane and a labile acetonitrile ligand at one of the axial positions, provides a simple entry into the chemistry of the fac-[P _3Rh] scaffold. DFT calculations using the model compound [(MeB(CH _2PMe _2) _3)Rh(CH _2=CH _2)(NCMe)] (1′) reproduce well the crystallographic geometry of 1 and converge to the isolated conformer, in which the strong π back-donation from the metal to the π* orbital of ethylene fixes it coplanar with the equatorial plane. Oxidation of 1 is electrochemically irreversible (at 0.080 V vs SCE), and 1 was effectively oxidized with [Cp _2Fe] ~+ in acetonitrile to [(PhBP _3)Rh(NCMe) _3] ~(2+). Reaction of 1 with hydrogen in toluene gives the dihydride [(PhBP _3)Rh(H) _2(NCMe)] (3), which loses acetonitrile to give the insoluble hydride [{(PhBP _3)Rh(H)(μ-H)} _2] (4), while 1 in THF with BHT (2,6-bis(1,1-dimethylethyl)-4-methylphenol) results in the mixed-valence paramagnetic hydride [{(PhBP _3)Rh} _2(μ-H) _3] (5). Complexes 1 and 3 react with oxygen to give the dinuclear complex [{(PhBP _3)Rh(μ-O _2)} _2]. The kinetic products from the protonation of complex 1 with carboxylic acids were found to be the ethyl complexes [(PhBP _3)Rh(η ~1-C _2H _5)(κ ~2-O _2CR)] (R = Ph, Me), which establish an equilibrium in solution with the corresponding hydride complexes [(PhBP _3)Rh(H)(κ ~2-O _2CR)] (R = Ph, Me) and free ethylene. These equilibria can be shifted to the desired compound working under either an atmosphere of ethylene or vacuum. Sequential protonation of 1 and 3 with HBF _4 in acetonitrile gave cleanly the cationic hydride complex [(PhBP _3)Rh(H)(NCMe) _2]BF _4 first and then [(PhBP _3)Rh(NCMe) _3](BF _4) _2 and hydrogen. Other electrophiles such as MeOTf react immediately with 1 at one phosphorus atom to give the alkylated (P-methyl) complex [{PhB(PMe)P _2}Rh(κ ~2-O _2SOCF _3)].