STERIC EFFECTS OF THE 2-(DIPHENYLPHOSPHINO)PYRIDINE BRIDGING LIGAND IN THE SYNTHESIS OF BINUCLEAR RHPD COMPLEXES - CRYSTAL-STRUCTURES OF [(CN-TERT-BU)2CL2RH(MU-PH2PPY)PD(MU-CL)]2 AND [(C8H12)RH(MU-CL)(MU-PH2PPY)PDCL2]

The reaction of [Rh(COD)(mu-Cl)]2 (COD = cycloocta-1,5-diene) with 2-(diphenylphosphino)pyridine (Ph2PPy), in benzene solution, gave the complex [Rh(COD)(Ph2PPy)Cl], (1) in which the Ph2PPy acts as monodentate P-bonded ligand. The P,N chelation of the Ph2PPy to the rhodium(I) center to give the cationic complex [Rh(COD)(Ph2PPy)]X (X = ClO4, 2a; X = PF6, 2b) was achieved by treating 1 with AgClO4 or AgPF6. The uncoordinated pyridine nitrogen atom of 1 was protonated by HPF6, giving [Rh(COD)(Ph2PPyH)Cl]PF6 (3). The known A-frame complex [Rh2Cl2(Ph2PPy)2(mu-CO)] was obtained by bubbling CO into a dichloromethane solution of 1. In the presence of PPh3, the reaction of 1 with CO gave the compound [Rh(CO)(PPh3)(Ph2PPy)Cl] (4). The reaction of 1 with cis-[Pd(CN(t)Bu)2Cl2], in CH2Cl2 solution, gave nearly quantitatively the tetranuclear formal Rh(II)Pd(I) complex {[((t)BuNC)2Cl2Rh(mu-Ph2PPy)Pd(mu-Cl)]2} (5) whose structure was also established by X-ray diffraction methods. Compound 5 was also obtained by adding in succession Ph2PPy and [Rh(COD)(mu-Cl)]2, in the appropriate stoichiometric ratio, to a CH2Cl2 solution of cis-[Pd(CN(t)Bu)2Cl2]. The addition of Ph2PPy to a CH2Cl2 solution of cis-[Pd(CN(t)Bu)2Cl2] afforded the mixed isocyanide-tertiary phosphine complex cis-[Pd(CN(t)Bu)(Ph2PPy)Cl2] (6). Crystals of 5 are monoclinic, space group P2(1)/a with Z = 2 in a unit cell of dimensions a = 19.420 (8) angstrom, b = 11.700 (5) angstrom, c = 15.006 (6) angstrom, and beta = 110.80 (2)-degrees. The structure has been solved from diffractometer data by Patterson and Fourier methods and refined by full-matrix least squares on the basis of 1959 observed reflections to R and R(w) values of 0.0738 and 0.0987, respectively. The tetranuclear complex is centrosymmetric with the inversion center relating two PdRh dimeric units in which the Pd-Rh separation is 2.612 (3) angstrom. Two Cl atoms asymmetrically bridge the Pd atoms. The Ph2PPy ligand also acts as a bridge being bound to the Pd atom through the P atom and to the Rh atom through the pyridine N atom. The reaction of 1 with {Pd[CH2C(CH3)CH2]Cl}2, in CH2Cl2 solution, gave quantitatively the palladium-allyl complex {Pd[CH2C(CH3)CH2](Ph2PPy)Cl} (9), together with [Rh(COD)(mu-Cl)]2. Complexes [Pd(COD)Cl2] and [Pd(C6H5CN)2Cl2] reacted with 1 to give quantitatively the binuclear complex [(COD)Rh(mu-Cl)(mu-Ph2PPy)PdCl2] (10) characterized by IR and NMR spectroscopy. The structure of the dichloromethane solvate of 10 was determined by a X-ray diffraction study. Crystals are triclinic, space group P1BAR, with Z = 2 in a unit cell of dimensions a = 9.282 (5) angstrom, b = 10.564 (5) angstrom, c = 14.690 (6) angstrom, alpha = 84.35 (3)-degrees, beta = 86.32 (3)-degrees, and gamma = 79.01 (3)-degrees. The structure has been solved from diffractometer data by Patterson and Fourier methods and refined by full-matrix least squares on the basis of 1443 observed reflections to R and R(w) values of 0.0730 and 0.0932, respectively. The binuclear complex consists of the (COD)Rh and PdCl2 moieties held together by Ph2PPy and chloride bridges, with the Pd...Rh separation of 3.210 (4) angstrom. The compound [Pd(Ph2PPy)Cl2] (11) was isolated by reacting [Pd(COD)Cl2] or [Pd(C6H5CN)2Cl2] with Ph2PPy in the molar ratio 1:1; it is a very reactive species and was found to react with [Rh(COD)(mu-Cl)]2 to give 10. Interestingly, compound 5 was also obtained almost quantitatively, by reacting 10 with CN(t)Bu. In the bimetallic complexes 5 and 10 important bond changes in the Ph2PPy coordination to the rhodium center, with respect to 1, occurred. Reaction sequences for the formation of these compounds were proposed. The presence of unfavorable interactions between the hydrogens of COD ligand and the phenyl groups in the bimetallic intermediates containing the bridging Ph2PPy ligand P bonded to Rh was considered the factor determining the course of reaction.