No abstract

Reactions of [Pd(styrene)(PR3)2], generated from trans-[PdEt2(PR3)2] and styrene, with 2 equiv. of benzyl isocyanate in THF at room-temperature afforded unusual cyclic Pd-tetramers of five-membered rings consisting of organic isocyanate dimers and palladium, [Pd(PR3){-C(O)N(R)C(O)N(R)-}]4 (PR3 = PMe3, ; PR3 = PMe2Ph, ). Additionally, a cyclic trimer, (RNCO)3, (R = benzyl) was produced as a catalytic product. Treatment of the cyclic tetramer () with 4 equiv. of chelated phosphine, such as (1,2-bis(diethylphosphino)ethane) (DEPE) or (1,2-bis(dimethylphosphino)ethane) (DMPE), readily caused conversion to a metallacyclic cis-form, [Pd{N(R)C(O)N(R)C(O)}(P ∼ P)] (P ∼ P = DEPE, ; P ∼ P = DMPE, ) in quantitative yields. In contrast, reactions of Pd(0)-PR3 with 2 equiv. of Ar-NCO (Ar = Ph, p-tolyl, p-ClC6H4) afforded metallacyclic complexes having a dimeric isocyanato moiety, cis-[Pd{C(O)N(Ar)-C(O)N(Ar)}(PR3)2] (PR3 = PMe3 Ar = C6H5, ; p-MeC6H4, ; p-Cl-C6H4, ; PR3 = PMe2Ph, Ar = p-Cl-C6H4, ). Treatment of the palladacyclic complex () with an equimolar amount of chelated phosphine such as DEPE readily caused conversion to a palladacyclic cis-form, [Pd{N(Ar)C(O)N(Ar)C(O)}(DEPE)], in quantitative yield. The catalytic cyclotrimerization of benzyl isocyanate to [Pd(styrene)(PMe3)2] was achieved by varying the molar ratio of R-NCO (R = benzyl). In addition, catalytic cyclotrimerization was performed from the five-membered palladacyclic complexes or the Pd(0)-PR3 complex with excess Ar-NCO.

The oxidative addition of dihalo-organic compounds to zerovalent transition-metal complexes is a well-known fundamental reaction, which affords self-assembled transitionmetal complexes or π-conjugated polymers in catalytic reactions.1 In particular, dinuclear group 10 metal complexes with bridging aryl groups act as catalytic intermediates or potential precursors in the preparation of π-conjugated conducting polymers or electronic and optical materials. These complexes are typically prepared by the double oxidative addition of dihalo-aryl derivatives to zerovalent metal complexes. 1-3We previously reported the double oxidative addition of dihalo-aryl or dihalo-thiophene compounds to zerovalent Pd or Ni complexes to afford linear dinuclear Pd(II) or Ni(II) compounds containing bridging aryl or thiophene ligands. 4,5However, to the best of our knowledge, studies on linear disulfido-aryl-bridged dinuclear Pd(II) complexes ([Pd]-ArS-SAr-[Pd]) have not been reported yet. Therefore, we attempted to prepare novel dinuclear Pd(II) complexes containing an aryl disulfide bridge (-Ar-S-S-Ar-) by the C-X (X = halogen) oxidative addition of haloaryl disulfide compounds to zerovalent Pd complexes. However, when pchlorophenyl disulfide (Cl-p-C 6 H 4 -S-S-C 6 H 4 -p-Cl) was treated with Pd(0)-bis(phosphine) complexes, we obtained the mononuclear bis(thiolato) Pd(II)-bis(phosphine) complex, trans-[Pd(S-C 6 H 4 -p-Cl) 2 (PR 3 ) 2 ], which is an S-S oxidative addition product instead of an initially expected C-X oxidative-addition product. Herein, we report the synthesis of such mononuclear bis(thiolato) complexes. These thiolato complexes are regarded as an intermediate or precursor related to the addition of organic disulfides to the organic unsaturated molecules or the adduct formation of organic disulfides and isocynides catalyzed by late transition-metal complexes. 6-9Many reactions, except for a few cases, 8a,c for the preparation of thiolato complexes typically use zerovalent group 10 metal complexes containing low basic PPh 3 ligand as a supporting ligand. In this study, we used Pd(0) complexes containing PMe 3 or PEt 3 ligands, which are more basic than PPh 3 . Reactions of [Pd(styrene)(PR 3 ) 2 ] (PR 3 = PMe 3 , PEt 3 ), 10 which was generated from trans-[PdEt 2 (PR 3 ) 2 ] and styrene, with p-chlorophenyl disulfide in a 2:1 or 1:1 molar ratio afforded the bis(arylthiolato) Pd(II) complexes, trans-(1), PEt 3 (2)) in moderateto-good yields (Eq. (1) in Scheme 1). A one-step reaction of trans-[PdEt 2 (PMe 3 ) 2 ] with p-chlorophenyl disulfide in 2:1 molar ratio also produced complex 1 in 69% yield.The peak-intensity ratios of the aromatic hydrogens to the PR 3 hydrogens in the 1 H NMR spectra of complexes 1 and 2 strongly support the proposed structures. Spectral data ( (1)) can be formed by the S-S bond cleavage, whereas the dinuclear Pd(II) complexes (in Eq. (2)) can be formed by the C-Cl bond cleavage of the aryl disulfide, followed by subsequent dimerization accomplished by the dissociation of phosphine. The above resu...