exo-nido-Monothio- and exo-nido-Monophosphinorhodacarboranes:  Synthesis, Reactivity, and Catalytic Properties in Alkene Hydrogenation

Reaction of Cs[7-SR-8-R‘-7,8-C 2 B 9 H 10 ] (R = Ph, R‘ = Me; R, R‘ = Ph; R = Et, R‘ = Me) with [RhCl(PPh 3 ) 3 ] in toluene−ethanol (8:1) affords [Rh(7-SR-8-R‘-7,8-C 2 B 9 H 10 )(PPh 3 ) 2 ]. Reaction of tetraalkylammonium salts of [7-PR 2 -8-R‘-7,8-C 2 B 9 H 10 ] - (R = Ph, R‘ = H; R = Ph, R‘ = Me; R, R‘ = Ph; R = Et, R‘ = Me; R = Et, R‘ = Ph; R = i Pr, R‘ = Me) with [RhCl(PPh 3 ) 3 ] in ethanol affords [Rh(7-PR 2 -8-R‘-7,8-C 2 B 9 H 10 )(PPh 3 ) 2 ]. The structure of [Rh(7-PPh 2 -8-H-7,8-C 2 B 9 H 10 )(PPh 3 ) 2 ] has been determined by crystallographic studies. Rh(I) has a normal four-coordinated square-planar geometry. The carborane ligand is bonded to the metal by means of a P−Rh and a B(11)−H−Rh bond. Two PPh 3 ligands fulfill the coordination sphere of the metal. The spectroscopic data indicate analogous structures for the rest of the complexes. [Rh(7-SPh-8-Me-7,8-C 2 B 9 H 10 )(PPh 3 ) 2 ] reacts with L = PPh 3, PMePh 2, and PEt 3 to give the salts [RhL 4 ][7-SPh-8-Me-7,8-C 2 B 9 H 10 ]. [Rh(7-PPh 2 -8-H-7,8-C 2 B 9 H 10 )(PPh 3 ) 2 ] does not react with PPh 3, but it does react with L = PMePh 2, PMe 2 Ph, and PEt 3 to give different substitution products depending on the complex:phosphine ratio: [Rh(7-PPh 2 -8-H-7,8-C 2 B 9 H 10 )(PPh 3 )L] at the ratio 1:1, [Rh(7-PPh 2 -8-H-7,8-C 2 B 9 H 10 )L 2 ] at the ratio 1:2, and [RhL 4 ][7-PPh 2 -8-H-7,8-C 2 B 9 H 10 ] at ratios higher than 1:3. Both the thioether and the phosphino families catalyze the hydrogenation of 1-hexene to n -hexane. The thioether family is an active catalyst at P = 1 bar and T = 25 °C, whereas the phosphino family requires higher temperatures and pressures. The hydrogenation of 2-hexenes proceeds much more slowly than for 1-hexene. Both catalytic systems are recoverable upon completion of reaction. [Rh(7-PPh 2 -8-H-7,8-C 2 B 9 H 10 )(PPh 3 ) 2 ] and [Rh(7-PPh 2 -8-Me-7,8-C 2 B 9 H 10 )(PPh 3 ) 2 ] catalyze the hydrogenation of the antibiotic precursor methacycline to doxycycline with high yield and very high diastereoselectivity (ca. 100%) for the pharmacologically important molecule. The other possible isomer epi -doxycycline was not observed in any case.