Synthesis of rhodium, iridium, and palladium tetranuclear complexes directed by 2,6-dimercaptopyridine. X-ray crystal structure of [Rh4(μ-PyS2)2(cod)4] (cod = 1,5-cyclooctadiene)

Abstract

Tetranuclear diolefin complexes of the general formula [M4(μ4-PyS2)2(diolefin)4] [M = Rh, diolefin = 1,5-cyclooctadiene (cod) (1), 2,5-norbornadiene (nbd) (2), tetrafluorobenzobarrelene (tfbb) (3); M = Ir, diolefin = cod (4), PyS2 = 2,6-pyridinedithiolate) are prepared in high yield by reaction of the appropriate complex [{M(μ-Cl)(diolefin)}2] with the salt Li2PyS2 generated “in situ”. This method is also used to prepare [Pd4(μ-PyS2)2(allyl)4] (5). Alternative syntheses for these complexes are also described. The structure of 1 was conclusively determined by a single-crystal X-ray analysis. Complex 1 crystallizes in the monoclinic system, space group C2/c, with a = 10.252(1) Å, b = 17.023(2) Å, c = 23.114(3) Å, β = 99.50(1)°, and Z = 4. Refinement by full matrix least-squares gave final R = 0.028 and Rw = 0.024. Complex 1 is tetranuclear with two S,N,S-tridentate 2,6-dimercaptopyridine ligands bridging all of the four metallic centers and presents a crystallographically imposed C2 symmetry relating two “Rh2(μ4-PyS2)(cod)2” moieties. The two S atoms of each bridging ligand exhibit different coordination modes; while one is bonded to one metal, the second one is coordinated to two different rhodium centers. The shortest Rh···Rh separation is 3.1435(5) Å. Carbonylation of the rhodium diolefin complexes under atmospheric pressure gives [Rh4(μ4-PyS2)2(CO)8] (6) which maintains the molecular framework of 1. Further reaction of the carbonyl complex with PPh3 gives [Rh4(μ-PyS2)2(CO)4(PPh3)4] (7), but this complex is prepared more conveniently by reaction of Li2PyS2 with [{Rh(μ-Cl)(CO)(PPh3)}2]. The replacement of CO by PPh3 is not selective, and this complex exists in solution as a mixture of three isomers due to the relative position of the PPh3 groups. The diolefinic and carbonyl complexes are fluxional. Variable temperature 1H and 13C{1H} spectra associated with H,H-COSY experiments led to the assignment of the olefinic resonances and the conclusion that the two diolefins at the inner part of the complexes are rigid, while the two external ones undergo the fluxional behavior due to an inversion at the terminal sulfur donor atoms. This is also the origin of the fluxionality of the carbonyl complex. Deprotonation of Py(SH)2 with [Rh(acac)(cod)] (acac = acetylacetonate) can be carried out stepwise, giving the dinuclear complex [Rh2(μ-PyS2H)2(cod)2] (8), and later the tetranuclear complex 1. This method to synthesize heterotetranuclear complexes by the addition of either [Ir(acac)(cod)] or [{Ir(μ-OMe)(cod)}2] to the isolated dinuclear rhodium complex (8) has been shown to be nonselective, giving a mixture of tetranuclear complexes with the [Rh3Ir]4+, [Rh2Ir2]4+, and [RhIr3]4+ cores. The rhodium complexes undergo two reversible one-electron oxidations at a platinum bead electrode in dichloromethane separated by approximately 0.4 V at potentials E° in the ranges 0.0−0.4 and 0.4−0.8 V. The electrochemical behavior of the iridium complex is more complicated, undergoing two similar one-electron oxidations followed by a chemical reaction.