Synthesis and reactivity of rhodium(III) pentamethylcyclopentadienyl complexes of N–B–PTA(BH3): X-ray crystal structures of [Cp∗RhCl2{N–B}–PTA(BH3)] and [Cp∗Rh{N–B–PTA(BH3)}(η2-CH2 = CHPh)]

The reaction between 1-boranyl-1,3,5-triaza-7-phosphaadamantane ligand N–B–PTA(BH3) and [Cp∗RhCl(μ-Cl)]2 affords [Cp∗Rh{N–B–PTA(BH3)}Cl2] (3) or [Cp∗Rh{N–B–PTA(BH3)}2Cl]Cl (5) containing one or two P-bonded boronated PTA ligands. The hydride [Cp∗Rh{N–B–PTA(BH3)}H2] (8) was also obtained by reaction of 3 with NaBH4 and alternatively by direct hydroboration of [Cp∗Rh(PTA)Cl2] with excess NaBH4. Moderately slow hydrolysis of the N-boranyl rhodium complexes affords dihydrogen, H3BO3 and the corresponding PTA derivatives, including the water-soluble dihydride [Cp∗Rh(PTA)H2] (9). Finally, the reaction of 8 with electron poor alkynes gives the alkene complexes [Cp∗Rh{N–B–PTA(BH3)}(η2-CH2 = CHR)] (R = Ph, 10; C(O)OEt, 11) as a mixture of rotamers η2-coordinated to rhodium without affecting the N–BH3 moiety. The X-ray crystal structures of 3 and 10 were also obtained and are here discussed.