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The palladium(II) chloro methyl complexes bearing the bidentate 6-R-C 5 H 3 N-2-CH 2 SR′ (RN-SR′; R ) H, Me, Cl; R′ ) Me, t-Bu, Ph) and the potentially terdentate 2,6-(CH 2 SR′) 2 -C 5 H 3 N (S-N-S(R′); R′ ) Me, t-Bu, Ph) pyridylthioethers as ancillary ligands were synthesized, characterized, and reacted with substituted alkynes ZCtCZ (Z ) COOMe, Z′ ) COOt-Bu, Z′′ ) COOEt). The reactions were followed under second-order conditions by 1 H NMR technique, and the reaction rates were determined. The corresponding vinyl derivatives were synthesized, and in the case of the complexes [PdCl(ZCdCZMe)(MeN-SPh)] and [PdCl(ZCd CZMe)(C1N-St-Bu)] (Z ) COOMe) reaction rates for alkyne insertion yielding the corresponding butadienyl complexes were also determined. The rate of insertion of the second alkyne on the vinyl complex is more than 3 orders of magnitude lower than the first insertion rate in both the studied complexes, thereby allowing easy separation between vinyl and butadienyl derivatives and an easy preparation of mixed butadienyl esters. Furthermore, the reaction rates are strongly dependent on the steric and electronic features of the ancillary ligands. In particular, the distortion of the complex main coordination plane, induced by the substituent in position 6 of the pyridine ring, was found to significantly influence the substrate reactivity. The structures of the mono-inserted vinyl [PdCl(ZCdCZMe)(MeN-St-Bu)] (1) and the bis-inserted butadienyl [PdCl((ZCdCZ) 2 Me)(MeN-St-Bu)] (2) complexes were determined by X-ray diffraction, and the persistence of a structural distortion of the complex skeleton was observed. Moreover, the distortion may be related to facile ancillary ligand displacement, a feature that can be exploited for the synthesis of substrates that would not be easily obtained otherwise.

The rate of insertion of methyl-substituted allenes into the Pd-Me bond in chelate pyridinethioether complexes [PdCl(Me)derivatives is remarkably enhanced by the presence of a methyl group in position 6 of the pyridine ring, which induces distortion on the main coordination plane, resulting in a metal substrate more prone to allene insertion. The flexibility of the sulfur-donor chelate ligand appears to be a paramount requisite.

The formation of metallacyclopentadienyl derivatives was studied under controlled conditions, and the kinetics and mechanism of reactions between pyridylthioether olefin Pd(0) substrates and substituted alkynes of the type ZCtCZ (Z ) COOMe, COOEt, COOt-Bu) leading to the corresponding palladacyclopentadienyl species were systematically investigated. In the case of less hindered ancillary ligands the attack of the alkyne forming a reactive monoalkyne intermediate was found to be the rate-determining step. In this respect the rates of reaction were discussed in terms of the substituent-induced basicity of sulfur and nitrogen of the ancillary ligands. The associative nature of the attack was unequivocally established, and the formation of a transition state bearing a monodentate ancillary ligand was proposed. In the case of more hindered ancillary ligands a partially stabilized monoalkyne intermediate is formed irrespective of the olefin in the starting complex, and this species strongly influences the reaction progress. Formation of hexamethylmellitate under mild conditions is also observed.

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Palladium methyl complexes with potentially terdentate pyridylthioether (S-N-S(R) ) 2,6-bis(R-thiomethyl)pyridine, R ) Me, t-Bu, Ph; N-S-N ) 2[(2-pyridylmethylthio)methyl]pyridine) ligands have been prepared and characterized. Both the bidentate chloride [Pd(Me)(S-N-S(R))]Cl and the terdentate chloride-free [Pd(Me)(S-N-S(R))] + species are present in solution and display a substantially different reactivity toward allene insertion across the Pd-C bond. The structures of the complexes [Pd(Me)(S-N-S(t-Bu))]OTf and [Pd(Me)(S-N-S(t-Bu))]Cl were determined by X-ray diffraction. The chloride methyl substrates [Pd(Me)(S-N-S(R))]Cl display an enhanced reactivity in solution with respect to the allene insertion, and this reactivity was traced back to the distortion of the main coordination plane induced by the presence of an uncoordinated -CH 2 -S-R group in position 6 of the coordinating pyridine. The equilibrium position between the terdentate and the bidentate species can be modulated by addition of chloride ion, which therefore controls the overall reactivity of the system.

The reaction between palladium(0) complexes bearing potentially terdentate ligands and dimethyl acetylenedicarboxylate (DMA) to give the corresponding palladacyclopentadiene complexes was studied under kinetic conditions. The reactivity of the complexes was markedly influenced by the nature of the ancillary ligand. Thus, when pyridyldithioether (SNS) and dipyridylthioether (NSN) ligands are used, the reactivity and the rate law of the corresponding derivatives are similar to those of the unsubstituted bidentate pyridylthioether substrates and, therefore, a marked rate increase can be obtained only by reduction of the olefin steric requirement. When terdentate NNN ligands are used, an apparent difference in reactivity between the derivatives bearing the pyridine−amine−pyridine and pyridine−amine−quinoline ligands is observed. On the basis of a detailed structural study (NMR, X-ray) and on kinetic investigations, an interpretation which takes into account the flexibility of the cycle formed between the ligand and palladium is proposed. Thus, irrespective of the size of the cycle, the complexes in which the ligand forms flexible cycles undergo ring opening less easily, with a consequent reduction of reactivity. Conversely, rigid rings cannot undergo associative attack without companion ring opening, this phenomenon being crucial in favoring the alkyne attack.

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