Easy Access to Hydride Chemistry on a Tripodal P-Based Rhodium Scaffold

The TBPY-5 rhodium complex [(PhBP3)­Rh­(CH2CH2)­(NCMe)] (1; PhBP3 = PhB­(CH2PPh2)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-[P3Rh] scaffold. DFT calculations using the model compound [(MeB­(CH2PMe2)3)­Rh­(CH2CH2)­(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 [Cp2Fe]+ in acetonitrile to [(PhBP3)­Rh­(NCMe)3]2+. Reaction of 1 with hydrogen in toluene gives the dihydride [(PhBP3)­Rh­(H)2(NCMe)] (3), which loses acetonitrile to give the insoluble hydride [{(PhBP3)­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 [{(PhBP3)­Rh}2(μ-H)3] (5). Complexes 1 and 3 react with oxygen to give the dinuclear complex [{(PhBP3)­Rh­(μ-O2)}2]. The kinetic products from the protonation of complex 1 with carboxylic acids were found to be the ethyl complexes [(PhBP3)­Rh­(η1-C2H5)­(κ2-O2CR)] (R = Ph, Me), which establish an equilibrium in solution with the corresponding hydride complexes [(PhBP3)­Rh­(H)­(κ2-O2CR)] (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 HBF4 in acetonitrile gave cleanly the cationic hydride complex [(PhBP3)­Rh­(H)­(NCMe)2]­BF4 first and then [(PhBP3)­Rh­(NCMe)3]­(BF4)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)­P2}­Rh­(κ2-O2SOCF3)].