The phosphinothiolato complexes [PdCl(dppet)PPh(3)] (3), [Pd(dppet)(2)] (4), and [Pd(2)Cl(2)(dpppt-P,&mgr;-S)(2)] (6) (Hdppet = HSC(2)H(4)PPh(2); Hdpppt = HSC(3)H(6)PPh(2)) have been synthesized in good yield by various base-free ligand exchange reactions on Pd(II), but [Pd(dpppt)(2)] (5) could only be obtained as a pure product employing an oxidative route from Pd(0). Both complexes 3 ((31)P NMR, delta(P) 66.8 (dppet), 24.1 (PPh(3)); (2)J(PP) = 459 Hz) and 5 ((13)C NMR) are trans-P,P. Crystals of 3.CH(2)Cl(2) are orthorhombic (P2(1)2(1)2(1)), with a = 9.247(3) Å, b = 17.956(9) Å, c = 19.869(9) Å. In 3 the Pd-S distance of 2.270(2) Å is short compared to the very different Pd-P distances of 2.280(2) (Pd-PPh(2)R) and 2.343(2) Å (Pd-PPh(3)). Crystals of 6.CH(2)Cl(2) are monoclinic (P2(1)/n) with a = 12.701(3) Å, b = 12.040(4) Å, c = 22.495(2) Å, and beta = 97.36(1) degrees. The structure of 6 consists of two Pd(dpppt)Cl moieties meeting at an angle of 105.81(3) degrees, linked by asymmetric thiolato bridges in a syn-endo configuration. The difference in the chelate angles of dppet (85.98(9) degrees ) and dpppt (96.60(3) degrees and 97.42(3) degrees ) seems to be crucial for palladium to form an unusual mononuclear complex (3) or a binuclear complex (6). Bischelate complexes 4 and 5 are inactive, but 3 and 6 catalyze the hydroesterification of styrene with CO (30 bar) and MeOH at moderate temperatures (60 and 80 degrees C) with no additives. The velocities are slow, but with 6 and with 3 at the lower temperature no decomposition to Pd metal is observed. Only esters are produced and regioselectivities of ca. 84% toward 2-phenylpropanoic acid methyl ester are achieved.
The chiral ligand (1R,2S,5R)-1-((diphenylphosphino)methyl)-2-isopropyl-5-methylcyclohexanethiol (4) has been prepared from low-cost commercial (−)-menthone in a three-step
enantioselective synthesis. Oxidative addition of 2 equiv of this phosphinothiol ligand to
[Pd0(PPh3)4] gave the enantiopure bis(phosphinothiolate)palladium(II) complex 5, which only
exists as the trans-P,P geometrical isomer, in both the solid state and solution, owing to the
preference of the chelate rings to adopt the λ conformation in which the position of the
menthane ring does not allow a conformation of the phenyl groups compatible with the more
sterically hindered cis geometry. These stereoelectronic coordination preferences of the chiral
phosphinothiolate ligand have been confirmed by the structure of the analogous Pt(II)
complex 6, which also exhibits the trans-P,P geometry exclusively, both in the solid state
and in solution. Crystals of 5·1/2CH2Cl2 and 6·1/2CH2Cl2 are isomorphic, belonging to the
monoclinic crystal system C2. Both chiral structures show mononuclear square-planar trans
complexes with locked λ chelate ring conformations. Compound 6 represents the first example
of a structurally characterized mononuclear bis(phosphinothiolate)platinum(II) complex.
Addition of 1 equiv of ligand 4 to a solution of [PdCl2(PPh3)2] gave the less sterically hindered
complex chloro(phosphinothiolate)(triphenylphosphine)palladium(II) (7), which exhibits the
expected cis−trans equilibrium in solution, but strongly displaced to the more sterically
stable trans isomer.
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