Synthesis and Mechanism of Formation of Novel NHC-NAC Bis-Carbene Complexes of Gold(I)

Novel [Au(NHC)(DIC)]BF4 complexes (NHC=SIMes, IMes, SIPr, IPr;DIC=2,6-dimethylphenyl isocyanide) were synthesized and characterized. These substrates react with secondary amines at room temperature under mild experimental conditions to give new bis-carbene NHC-NAC derivatives. The reaction of formation of the latter complexes with piperidine, morpholine, and diethylamine was studied by NMR and UV-vis spectrometry, and a mechanism involving a concomitant attack of the amine at the metal center and at the coordinated isocyanide was proposed on the basis of a detailed kinetic study. The mechanistic network was completely resolved by means of an independent determination of the equilibrium constant of formation of the adduct [Au(NHC)- (DIC)(amine)]þ, Keq, calculated from the absorbance vs amine concentration data obtained from direct spectrophotometric titration of the starting complex [Au(NHC)(DIC)]þ with the amine. The [Au(NHC){C(NHAr)(NC5H10)}]BF4 bis-carbene complexes obtained by attack of piperidine at the [Au(NHC)(DIC)] derivatives were isolated and fully characterized. The solid-state structures of the new complexes [Au(IMes)(DIC)]BF4 and [Au(IMes){C(NHAr)(NC5H10) }]BF4 were resolved and reported

The addition of halogens and interhalogens on palladacyclopentadienyl complexes bearing quinolyl-thioether as spectator ligands. A kinetic and computational study

We have studied the oxidative addition of halogens (I2 and Br2) and interhalogens (ICl and IBr) on complexes of the type [Pd(thioquinoline)C4(COOMe)4], (thioquinoline = 8-(methylthio)quinoline, 8-(t-butylthioquinoline, 2-methyl-8 (methylthio)quinoline, 2-methyl-8-(t-butylthio)quinoline). The expected palladium(thioquinoline)-r-butadienyl derivatives have been obtained by the stoichiometric addition of I2 and Br2 to a solution of the starting palladacyclopentadienyl complexes. The bromine in excess induces the extrusion of the di-bromo-(E, E)-r-butadiene and the formation of the thioquinoline palladium(II) di-bromide species. The kinetics and mechanism of these reactions have been determined. Except for one case which was analyzed in detail by a computational study, the oxidative addition of the interhalogens ICl and IBr yields the species that is less predictable from the thermodynamic point of view. In general the computational approach justifies the reaction progress and allows an interpretative clue suggesting a kinetically governed path to the reaction products. Finally, the solid state structures of two reaction products were resolved and reporte

Synthesis of novel palladium allyl complexes bearing heteroditiopic NHC-S ligands. Kinetic study on the carbene exchange between bis-carbene palladium allyl complexes

We have synthesized several novel palladium allyl and 1,1-dimethylallyl complexes bearing different heteroditopic NHCeS ligands giving rise to a five-membered chelate ring with the metal center.We were able to synthesize some homoleptic bis-carbene allyl derivatives by taking advantage of the hemilability of the thioetheric sulfur. Attempts at preparing mixed bis-carbene complexes bearing two different heteroditopic carbenes (i.e. NHCeS and NHCePy) simultaneously coordinated to the palladium center lead to a carbene transmetalation with the formation of a statistically distributed equilibrium mixture of the two pure homoleptic and of the mixed bis-carbene palladium allyl complexes in solution. In two different cases the rate of the equilibrium reaction was measured and a mechanistic hypothesis provided. Finally, we have determined the solid state structures of a complex bearing only one NHCeS heteroditopic carbene and of the bis-carbene (NHCeS, NHCePy) palladium allyl derivatives

Reactivity of cationic gold(I) carbene complexes toward oxidative addition of bromine

We have synthesized and characterized several cationic complexes of gold(I) of the type [Au(L1)(L2)]+ (L1 = NHC, L2 = DIC; L1 = NHC, L2 = NAC; L1 = NAC, L2 = DIC; L1 = L2 = NAC; NHC = N-heterocyclic carbene; NAC = N-acyclic carbene, DIC = 2,6-dimethylphenylisocyanide). The complexes of type [Au(NHC)(DIC)]+ react with a slight excess of Br2 yielding the corresponding gold(III) species [Au(NHC)(DIC)Br2]+. The latter decompose with a rate that is modulated by the nature of the ancillary ligands. The oxidative addition of Br2 to complexes of the type [Au(NHC)(NAC)]+ and [Au(L)(NAC)]+ (L = DIC, NAC) has been also carried out and the ensuing gold(III) derivatives [Au(NHC)(NAC)Br2]+ and [Au(NAC)2Br2]+ are stable in solution whereas the complex [Au(NAC)(DIC)Br2]+ decomposes. Finally, on the basis of kinetic studies we have proposed propose a mechanism involving a fast pre-equilibrium forming an adduct containing the starting complex and Br2 followed by a slow rearrangement of the latter to yield the final gold(III) derivatives

Synthesis, characterization and a reactivity study of some allyl palladium complexes bearing bidentate hemi-labile carbene or mixed carbene/PPh3 ligands

With the aim at synthesizing novel allyl complexes that can potentially act as catalysts in the Tsuji-Trost catalyzed reaction, we have synthesized and characterized some allyl and 2-Meallyl palladium derivatives with one hemilabile bidentate or two strong mono-coordinating spectator ligands. The hemilabile ligands are constituted by one nitrogen heterocyclic carbene (NHC) fragment acting as the pivot bearing a labile wing with a pyridine nitrogen or sulfur atom as the second stabilizing atom. One of two monodentate ligands is in all cases PPh3 whereas the other is a mono- or partially coordinated hemilabile carbene. The complexes were characterized by standard spectroscopic methods and elemental analysis and in two cases by SC-XRD technique.The reactivity of two selected complexes toward the Tsuji-Trost reaction was tested by stoichiometric allyl amination carried out with piperidine and the results of such a mechanistic investigation integrated by a computational study are also reported in this paper. (C) 2016 Elsevier Ltd. All rights reserved

Synthesis of novel olefin complexes of palladium(0) bearing monodentate NHC, phosphine and isocyanide spectator ligands

We have synthesized and characterized seventeen new bis-NHC, mixed NHCâ\u80\u93phosphines or NHCâ\u80\u93isocyanides Pd(0) olefin complexes that can potentially act as catalysts. The complexes were characterized by standard spectroscopic methods and elemental analysis and in two cases by SC-XRD technique. We have analyzed with particular care the thermodynamic and kinetic conditions governing the one-pot synthesis of the mixed complexes. In this respect we tried to validate our results by a dedicated computational study on the mutual distribution of the isomers that could be potentially formed. However, the computational result is not clear-cut owing to the not significant value of the calculated Î\u94G0. Finally, in one case we have measured the rate of the exchange reaction between not particularly encumbered olefins

Synthesis and characterization of novel olefin complexes of palladium(0) with chelating bis(N-heterocyclic carbenes) as spectator ligands

We have synthesized several novel palladium(0) olefin complexes stabilized by strong σ-donating bis-chelating carbene ligands characterized by one or two CH2spacers and electron-withdrawing olefins. Although it appears obvious that the σ-donating carbenes and electron-withdrawing olefins should cooperate in the stabilization of the ensuing complexes, the limit of their coexistence was not hitherto clear. On the basis of previously measured stabilizing capability of the olefins toward Pd(0) complexes we were able to synthesize ten complexes (nine new and one synthesized by a different protocol from that of the literature). The less electron-withdrawing olefin capable of stabilizing the complex was dimethylfumarate. However, the most interesting results were obtained with the (Z)-1,2-bis(p-tolylsulfonyl)ethene (cis-sulf) which instantly isomerizes upon coordination and in the case of the derivative bearing the olefin tetramethyl ethene-1,1,2,2-tetracarboxylate (tmetc) whose decomposition in CD2Cl2yields the saturated tetramethyl ethane-1,1,2,2-tetracarboxylate (D2). The solid-state structure of the complex 4d bearing the bis-carbene 1,1′-dibenzyl-3,3′methylenediimidazol-2,2′-diylidene and the olefin (E)-1,2-bis(p-tolylsulfonyl)ethene (trans-sulf) was also determined

Addition of halogens and interhalogens on palladacyclopentadienyl complexes stabilized by pyridyl-thioether N-S spectator ligands

We have studied from the experimental and theoretical point of view the oxidative addition of halogens (I-2 and Br-2) and interhalogens (ICl and IBr) on palladiumcyclopentadienyl complexes bearing heteroditopic pyridyl-thioether spectator ligands.Addition of I-2 or of a stoichiometric amount of Br-2 to a CDCl3 solution of the starting palladacyclo-pentadienyl complexes yields the expected palladium-sigma-butadienyl derivatives. The bromide derivative in the presence of a further excess of Br-2 gives the wanted dibromo-(E, E)-sigma-butadienyl and the pyridylthioether palladium(II) dibromide species. The rates of these reactions have been determined.When the interhalogens are used as oxidizing agents the thermodynamically hampered species is formed at first. Only in the case of the reaction of IBr is the formation of the energetically hampered derivative followed by partial isomerization to the most stable complex. The rate of isomerization and the related equilibrium constant between isomers have been measured. On the basis of the experimental evidence and the computational approach we have proposed a plausible energetic path yielding the first formed unexpected species.Finally, the solid state structures of two reaction products were resolved and reported. (C) 2016 Elsevier B.V. All rights reserved

Mechanistic and Kinetic Investigation on the Formation of Palladacyclopentadiene Complexes. A Novel Interpretation Involving a Bimolecular Self Reaction of a Monoalkyne Intermediate

The stoichiometric reaction between the complex [Pd(eta(2)-dmfu)(BiPy)] (dmfu = dimethylfumarate; BiPy = 2,2'-bipyridine) and the deactivated alkynes dmbd (dimethyl-2-butynedioate) and pna (methyl (4-nitrophenyl)propynoate), providing the respective palladacyclopentadienes, was investigated. The mechanism leading to the palladacyclopentadiene derivative involves a bimolecular self-rearrangement of the monoalkyne intermediate [Pd(eta(2)-alk)(BiPy)] (alk = dmbd, pna), followed by the customary attack of the free alkyne on the intermediate [Pd(eta(2)-alk)(BiPy)] itself and on the elusive and highly reactive "naked palladium" [Pd(BiPy)(0)] formed. The alkyne pna proved to be less effective in the displacement of dmfu than dmbd. The reaction under stoichiometric equimolar conditions of the latter with [Pd(eta(2)-dmfu)(BiPy)] allows the direct determination of the bimolecular self-reaction rate constant k(c) and consequently the assessment of all the rate constants involved in the overall mechanistic network