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

Abstract

The synthesis of new metal complexes containing the keto phosphine ligands R12PCH2C(O)R2 (R1 = Ph, R2 = Ph, Me, t-Bu, p-C6H4F; R1 = i-Pr, R2 = Ph) is described. Reaction of 1 equiv of (diphenylphosphino)acetophenone (Ph2PCH2C(O)Ph; L) with [Rh(μ-Cl)(C2H4)2]2 gave the dimeric complex [Rh(μ-Cl)(C2H4)(P∼O)]2 (1) (P∼O = η1(P) coordinated). With 2 equiv of L, [RhCl(P⌒O)(P∼O)] (2) was obtained (P⌒O = η2(P,O)-chelated ligand). Reaction of 2 with TlPF6 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)(PPh3)2] with 1 equiv of L gave [RhCl(CO)(P∼O)(PPh3)] (5). In the presence of TlPF6 the cationic complex [Rh(CO)(P⌒O)(PPh3)][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{Ph2PC ( O)Ph}(CO)(PPh3)] (7). 31P{1H} NMR shows 7 to dissociate PPh3 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 TlPF6 gave [Rh(CO)(P⌒O)(P∼O)][PF6] (9), and in the presence of NaOMe the phosphino enolate complex [Rh{Ph2PCH C( O)Ph}(CO)(P∼O)] (10) formed. In an analogous manner, [Rh{Ph2PCH C( O)Ph}(CO)L1] (L1 = P(o-tolyl)3 (11), PPh2(p-tolyl) (12), P(p-C6H4F)3 (13)) were also prepared. In a similar way, L and [RhCl(PPh3)3] gave [RhCl(PPh3)2(P∼O)] (14), which was reacted with TlPF6, affording [Rh(P⌒O)(PPh3)2][PF6] (15), analogous to 6. Reacting 14 or 15 with 1 equiv of NaOMe gave the phosphino enolate complex [Rh{Ph2PCH C( O)Ph}(PPh3)2] (16), analogous to 7. It reacts with PhNCO or Ph2PCl with formation of a Cenolate−C or Oenolate−P bond, respectively. [Rh{Ph2PCH C( O)Ph}(p-C6H4F)}(PPh3)2] (17) has also been reported. By reacting PPh3, L, and TlPF6 with [Rh(μ-Cl)(NBD)]2 (NBD = norbornadiene) the pentacoordinated complex [Rh(NBD)(P⌒O)(PPh3)][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(μ-Cl)(COE)2]2, [Rh{Ph2PCH C( O)Ph}(NBD)] (21) and [Rh{Ph2PCH C( O)Ph}(COD)] (22) were obtained. The related iridium complex [Ir{Ph2PCH C( O)Ph}(COD)] (26) was obtained in a similar way and reacted with H2 to give [Ir{Ph2PCH C( O)Ph}H2(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.