P. Braunstein et al., RHODIUM(I) AND IRIDIUM(I) COMPLEXES WITH BETA-KETO PHOSPHINE OR PHOSPHINO ENOLATE LIGANDS - CATALYTIC TRANSFER DEHYDROGENATION OF CYCLOOCTANE, Organometallics, 15(26), 1996, pp. 5551-5567
The synthesis of new metal complexes containing the keto phosphine lig
ands 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 (diphenylp
hosphino)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)] wi
th 1 equiv of L gave [RhCl(CO)(P similar to O)(PPh(3))] (5). In the pr
esence of TIPF6 the cationic complex [Rh(CO)(P O)(PPh(3))][PF6] (6) wa
s obtained. The X-ray crystal structure determination of 6 shows a sli
ghtly distorted square planar geometry with the two phosphorus atoms i
n trans positions. The reaction of 5 or 6 with 1 equiv of NaOMe produc
ed 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 trans
fer-dehydrogenation reactions. When [Rh(mu-Cl)(COE)(2)](2) (COE = cycl
ooctene) 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 t
he 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)] ga
ve [RhCl(PPh(3))(2)(P similar to O)] (14), which was reacted with TlPF
6, 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 determi
nation 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 t
he P atom and the midpoint of one olefinic bond occupying the apical p
ositions). 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 relate
d 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 complex
es 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 act
ive catalysts for transfer dehydrogenation at 70 degrees C, under atmo
spheric pressure of hydrogen, with turnover numbers up to 240 per mol
of rhodium/h.