Rhodium(I) and iridium(I) complexes with beta-keto phosphine or phosphino enolate ligands. Catalytic transfer dehydrogenation of cyclooctane

The synthesis of new metal complexes containing the keto phosphine ligands R(2)(1)PCH(2)C(O)R(2) (R(1) = Ph, R(2) = Ph, Me, t-Bu, P-C6H4F; R(1) = i-Pr, R(2) = Ph) is described. Reaction of 1 equiv of (diphenylphosphino)acetophenone (Ph(2)PCH(2)C(O)Ph; L) with [Rh(mu-Cl)(C2H4)(2)](2) gave the dimeric complex [Rh(mu-Cl)(C2H4)(P similar to O)](2) (1) (P similar to O = eta(1)(P) coordinated). With 2 equiv of L, [RhCl(P O)(P similar to O)] (2) was obtained (P O = eta(2)(P,O)-chelated ligand). Reaction of 2 with TIPF6 afforded the cationic compound [Rh(P O)(2)][PF6] (3). The X-ray crystal structure determination of 3 . H2O shows a distorted-square-planar geometry with the two phosphorus atoms (and oxygen atoms) in cis positions. Treatment of [RhCl(CO)(PPh(3))(2)] with 1 equiv of L gave [RhCl(CO)(P similar to O)(PPh(3))] (5). In the presence of TIPF6 the cationic complex [Rh(CO)(P O)(PPh(3))][PF6] (6) was obtained. The X-ray crystal structure determination of 6 shows a slightly distorted square planar geometry with the two phosphorus atoms in trans positions. The reaction of 5 or 6 with 1 equiv of NaOMe produced the phosphino enolate complex [Rh{Ph(2)PCH - C(- O)Ph}(CO)(PPh(3))] (7). P-3l{H-1} NMR shows 7 to dissociate PPh(3) in solution, giving a 14-electron species, which proved to be particularly useful for transfer-dehydrogenation reactions. When [Rh(mu-Cl)(COE)(2)](2) (COE = cyclooctene) was reacted with 4 equiv of L under carbon monoxide, [RhCl(CO)(P similar to O)(2)] (8) was isolated. Reaction of 8 with TIPF6 gave [Rh(CO)(P O)(P similar to O)][PF6] (9), and in the presence of NaOMe the phosphino enolate complex [Rh{Ph(2)PCH - C(- O)Ph}(CO)(P similar to O)] (10) formed. In an analogous manner, [Rh{Ph(2)PCH - C(- O)Ph}(CO)L(1)] (L(1) = P(o-tolyl)(3) (11), PPh(2)(p-tolyl) (12), P(p-C6H4F)(3) (13)) were also prepared. In a similar way, L and [RhCl(PPh(3))(3)] gave [RhCl(PPh(3))(2)(P similar to O)] (14), which was reacted with TlPF6, affording [Rh(P O)(PPh(3))(2)][PF6] (15), analogous to 6. Reacting 14 or 15 with 1 equiv of NaOMe gave the phosphino enolate complex [Rh{Ph(2)PCH - C(- O)Ph}(PPh(3))(2)] (16), analogous to 7. It reacts with PhNCO or Ph(2)PCl with formation of a C-enolate-C or O-enolate-P bond, respectively. [Rh{Ph(2)PCH - C(- O)Ph}(p-C6H4F)}(PPh(3))(2)] (17) has also been reported. By reacting PPh(3), L, and TlPF6 with [Rh(mu-Cl)(NBD)](2) (NBD norbornadiene) the pentacoordinated complex [Rh(NBD)(P O)(PPh(3))][PF6] (20) was obtained. The X-ray crystal structure determination of 20 showed that the coordination geometry around the Rh atom could be described as intermediate between square pyramidal (with the O atom occupying the apical position) and trigonal bipyramidal (with the P atom and the midpoint of one olefinic bond occupying the apical positions). When the diolefin, L, and NaOMe were added in sequence to a suspension of [Rh(mu-Cl)(COE)(2)](2), [Rh{Ph(2)PCH - C(- O)Ph}(NBD)] (21) and [Rh{Ph(2)PCH - C(- O)Ph}(COD)] (22) were obtained. The related iridium complex [Ir{Ph(2)PCH - C(- O)Ph}(COD)] (26) was obtained in a similar way and reacted with H-2 to give [Ir{Ph(2)PCH - C(- O)Ph}H-2(COD)] (27). The cationic complex 20 and the phosphino enolate complexes 4, 7, and 16 catalyze the hydrogenation and the isomerization of l-hexene. The rhodium phosphino enolate carbonyl complexes 7 and 13 are very active catalysts for transfer dehydrogenation of cyclooctane with norbornene under hydrogen pressure (7 MPa) at 60-90 degrees C. In the presence of 1 equiv of a triarylphosphine, rhodium phosphino enolate complexes not containing carbon monoxide, such as 22, proved to be active catalysts for transfer dehydrogenation at 70 degrees C, under atmospheric pressure of hydrogen, with turnover numbers up to 240 per mol of rhodium/h.