Rhodium(I) and Iridium(I) Complexes with β-Keto Phosphine or Phosphino Enolate Ligands. Catalytic Transfer Dehydrogenation of Cyclooctane

The synthesis of new metal complexes containing the keto phosphine ligands R 1 2 PCH 2 C(O)R 2 (R 1 = Ph, R 2 = Ph, Me, t -Bu, p -C 6 H 4 F; 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(μ-Cl)(C 2 H 4 ) 2 ] 2 gave the dimeric complex [Rh(μ-Cl)(C 2 H 4 )(P∼O)] 2 ( 1 ) (P∼O = η 1 (P) coordinated). With 2 equiv of L, [RhCl(P⌒O)(P∼O)] ( 2 ) was obtained (P⌒O = η 2 (P,O)-chelated ligand). Reaction of 2 with TlPF 6 afforded the cationic compound [Rh(P⌒O) 2 ][PF 6 ] ( 3 ). The X-ray crystal structure determination of 3 ·H 2 O 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∼O)(PPh 3 )] ( 5 ). In the presence of TlPF 6 the cationic complex [Rh(CO)(P⌒O)(PPh 3 )][PF 6 ] ( 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 PC ( O)Ph}(CO)(PPh 3 )] ( 7 ). 31 P{ 1 H} 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(μ-Cl)(COE) 2 ] 2 (COE = cyclooctene) was reacted with 4 equiv of L under carbon monoxide, [RhCl(CO)(P∼O) 2 ] ( 8 ) was isolated. Reaction of 8 with TlPF 6 gave [Rh(CO)(P⌒O)(P∼O)][PF 6 ] ( 9 ), and in the presence of NaOMe the phosphino enolate complex [Rh{Ph 2 PCH C( O)Ph}(CO)(P∼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 -C 6 H 4 F) 3 ( 13 )) were also prepared. In a similar way, L and [RhCl(PPh 3 ) 3 ] gave [RhCl(PPh 3 ) 2 (P∼O)] ( 14 ), which was reacted with TlPF 6, affording [Rh(P⌒O)(PPh 3 ) 2 ][PF 6 ] ( 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 -C 6 H 4 F)}(PPh 3 ) 2 ] ( 17 ) has also been reported. By reacting PPh 3, L, and TlPF 6 with [Rh(μ-Cl)(NBD)] 2 (NBD = norbornadiene) the pentacoordinated complex [Rh(NBD)(P⌒O)(PPh 3 )][PF 6 ] ( 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(μ-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 1-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 °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 °C, under atmospheric pressure of hydrogen, with turnover numbers up to 240 per mol of rhodium/h.