B. Crociani

Reactions of pyridine-2-carbaldimines with chloro-bridged palladium(II) and platinum(II) 2-methylallyl dimers. Solution behaviour of the cationic complexes [M(n3-2-MeC3H4)(py-2-CHNR)]+

The reactions of pyridine-2-carbaldimines, py-2- CHue5fbNR (R = C6H4OMe-p, Me), with allylic dimers [MCl(n3-2-MeC3H4)]2 give rise to stoichiometry, concentration, solvent and temperature dependent equilibria, in which the cationic complexes [M(n3- 2-MeC3H4)(py-2-CHue5fbNR)]+ and the anion [MCl2(n3- 2-MeC3H4)]- or Cl- are involved. In general, the ligand/dimer reaction (1/1 molar ratio) yields the ionic products [M(n3-2-MeC3H4)(py-2-CHue5fbNR)]- [MCl2(n3-2-MeC3H4)], which can be isolated as solids, whereas the same reaction in a 1/0.5 molar ratio yields the species [M(n3-2-MeC3H4 )(py-2-CHue5fb NR)] Cl, which can be studied only in solution, but are easily converted into [M(n3-2-MeC3H4)(py-2-CHue5fb NR)]X in the presence of an excess of NaX (X ue5fb ClO4, BF4, BPh4). In the cationic complexes, the α- diimino ligand is σ,σ′-N,N′ chelate to the central metal. The combined conductivity measurements and electronic, IR, and 1H NMR spectral data show that (i) the cationic complexes are greatly stabilized in methanol solution; (ii) extensive ion-pairing occurs in chlorinated solvents, such as dichloromethane, chloroform, and 1,2-dichloroethane; (iii) the complexes with X ue5fb ClO4 are slightly dissociated in acetonitrile, with the following order of dissociation constants; Pd >> Pt and py-2-CHue5fbNC6H4OMe-p) py-2-CHue5fbNMe; (iv) various dynamic processes take place in solution at different rates depending on the temperature, solvent, central metal, and counteranion. In general, a low-energy process involving syn-syn, anti-anti exchange of the allylic protons occurs, which in some cases cannot be frozen out and which is interpreted in terms of formation of stereochemically non-rigid five-coordinate intermediates by association of the cationic complexes with the solvent or the counteranion. Cation-anion interactions and, probably, formation of five-coordinate species with the more coordinating anions, Cl-and [MCl2(n3-2- MeC3H4)]-, are responsible for the solvent and anion dependent 1H NMR chemical shifts of the chelate py 2-CHue5fbNC6H4OMe-p ligand. For [Pd(n3-2-MeC3H4)- (py-2-CHue5fbNC6H4OMe-p)] [PdCl2(n3-2-MeC3H4)], but not for the platinum analogue, a rather fast scrambling of the Pd(n3-2-MeC3H4) unit between the cation and anion is observed at ambient temperature in CDCl3. This and other differences in the solution behaviour between palladium and platinum derivatives can be rationalized on the basis of a higher stability (toward dissociation) of the five-membered metallacycle M(py-2-CHue5fbNR) on going from M ue5fb Pd to M ue5fb Pt.

Protonation and methylation reactions of 2-pyridyl-palladium(II) and -platinum(II) complexes

The reactions of strong acids HX and HClO4 with the 2-pyridyl complexes [PdX(μ-C5H4N-C2,N)(PPh3)]2 (X = Cl, Br), trans-[PdCl(C5H4N-C2)(PEt3)2] and [PdCl(C5H4N-C2)(dppe)] yield the N-protonated derivatives cis-[PdX2-(C5H5N-C2)(PPh3)], trans-[PdCl(C5H5N-C2)(PEt3)2]ClO4 and [PdCl-(C5H5N-C2)(dppe)]ClO4, respectively. The terminal 2-pyridyl group of trans-[PdCl(C5H4N-C2)(PEt3)2] and [PdCl(C5H4N-C2)(dppe)] also reacts with Me2SO4/NaClO4 to give trans-[PdClC5H4(l-Me)N-C2(PEt3)2]ClO4 and [PdClC5H4(l-Me)N-C2(dppe)]ClO4. Analogous N-protonation or N-methylation reactions occur with trans-[PtBr(C5H4N-C2)(L)2] (L = PEt3, PPh3). The complexes trans-[MX(C5H5N-C2)(PMe2Ph)2]ClO4 (M = Pd, X = Cl and Br; M = Pt, X = Br) exhibit restricted rotation of the protonated 2-pyridyl group around the Mue5f8C bond. This and other chemical results and spectral data, such as the 13C NMR data of the PEt3 derivatives, are interpreted in terms of a significant contribution of a carbene-like limiting structure to the electronic configuration of this new type of ligand.

Mechanisms of nucleophilic and electrophilic attack on carbon bonded palladium(II) and platinum(II) complexes

Abstract A systematic mechanistic study is reported for the formation of palladium(II) carbene complexes by nucleophilic attack of aromatic amines on isocyanide derivatives. The most prominent step of the reaction involves direct attack of the amine nitroge on the isocyanide carbon to give an intermediate which then is converted to the final carbene species by the agency of the entering amine itself which behaves as a bifunctional catalyst. The rate of the primary step is affected by the donor ability of the entering amine, by the electrophilic character of the isocyanide carbon, and by steric crowdiness around the reacting centers, with the solvent also playing an important role. The reaction system displays a high versatility through a proper choice of the substituents on the amine and isocyanide aromatic rings and of the ancillary ligands in the metal complex. A mechanistic study is also described of the cleavage of the platinum-carbon σ-bond by electrophilic attack by the proton on organoplatinum(II) complexes. The particular mechanism which is operative, viz. direct electrophilic attack at the metalue5f8carbon bond or oxidative addition/reductive elimination, appears to be the result of many factors. These include electronic and steric properties of the cleaved group and of ancillary ligands, steric configuration of the substrate, nature of the electrophile and solvating ability of the medium.

Iminophosphine - palladium(0) complexes as catalysts for the Stille reaction

The cross-coupling of iodobenzene with tributylphenylethynylstannane or tributylvinylstannane is efficiently catalysed by iminophosphine–palladium(0)–olefin complexes of the type [Pd(η2-dmf)(P-N)] (dmf, dimethylfumarate; P-N, 1-(PPh2)-C6H4-2-Cue5fbNR (R=alkyl, aryl)). The catalytic activity depends on the R substituent of the imino group: the highest reaction rates are obtained using aryl-substituted iminophosphines. Equivalent catalytic systems can be obtained using a palladium source such as Pd(OAc)2 or Pd(dba)2 (dibenzylideneacetone, dba) in combination with the iminophosphine ligands. In the coupling of iodobenzene with tributylphenylethynylstannane, the highest reaction rates are obtained using an iminophosphine/palladium molar ratio of 2:1, while in the vinylstannane–iodobenzene coupling the best P-N/Pd ratio is 1:1.

Preparation and reactions of palladium(II) complexes with C2-bonded heteroaromatic ligands trans[PdCl(RN)(PPh3)2] (RN = 2-pyridyl, 2-pirazyl, 2-pyrimidyl group). A new reaction pathway in the insertion of isocyanides into the Pdc bond of trans-[PdXR(L)2] compounds

Abstract The complexes trans -[PdCl(R N )(PPh 3 ) 2 ] (I) [R N = 2-pyridyl (2-Py), 2-pyrazyl (2-pyz), 2-pyrimidyl (2-pym) group] have been prepared in high yield by deprotonation with NEt 3 of the corresponding cationic compounds trans [PdCl(R N H) (PPh 3 ) 2 ] + (R N H = N -protonated C 2 -heteroaromatic ligand) in the presence of an excess of PPh 3 . In chlorinated solvents, complexes I undergo a slow reversible dimerization into the binuclear derivatives [PdCl(μ-R N )(PPh 3 )] 2 (II) (μ-R N = C 2 , N 1 -bridging ligand). From the 31 P NMR spectra in 1,2-dichloroethane the following dissociation constants were obtained: 1.9 mol 1 −1 (R N = 2-py), 5.1 × 10 −2 (2-pym), 6.6 × 10 −3 (2-pyz). The dimerization becomes fast and quantitative if the PPh 3 , involved in the equilibrium is removed by oxidation or by reaction with [PdCl(η 3 -2-MeC 3 H 4 )] 2 . Only the 2-pyridyl complex Ia reacts (slowly) with CO yielding the migratory insertion product trans -[PdCl{C(2-py)ue5fbO}(PPh 3 ) 2 ], together with the dimer IIa. All the complexes I undergo migratory insertion of t-butylisocyanide with formation of trans -[PdCl{C(R N ) = NCMe 3 }(PPh 3 ) 2 ]] at rates which depend on the heterocyclic group (R N = 2-py > 2-pyz ⪢ 2-pym). The reaction of the 2-pyrazyl complex Ib with CNCMe 3 has been studied in detail by conductivity measurements and by IR and 31 P NMR spectroscopy. The data suggest a complex mechanism in which insertion occurs through rearrangement of a four-coordinate intermediate [PdCl(2-pyz)(CNCMe 3 )(PPh 3 )], and through interaction of a cationic intermediate trans -[Pd(2-pyz)(CNCMe 3 )(PPh 3 ) 2 ] + (Vb) with Cl − and with the free isocyanide of the initial equilibria. The occurrence of the latter reactions is confirmed by independent experiments in which the cationic complex Vb (isolated as perchlorate salt) is treated with an equimolar amount of [AsPh 4 ]Cl or CNCMe 3 . The isocyanide-promoted insertion step represents a new mechanistic pathway for isocyanide insertion into the Pdue5f8C bond of trans -[PdXR(L) 2 ] complexes.

Preparation and reactions of 2-pyridylplatinum(II) complexes [PtCl(C5H4N-C2)(L)2] (L = tertiary phosphine) Compounds with a markedly nucleophilic pyridine nitrogen atom

Abstract The 2-pyridyl complex trans -[PtCl(C 5 H 4 N- C 2 )(PPh 3 ) 2 ] (I) can be prepared in high yield by oxidative addition of 2-chloropyridine to [Pt(PPh 3 ) 4 ]. The reaction of I with 1,2-bis(diphenylphosphino)ethane yields the cis derivative [PtCl(C 4 5 H 4 N- C 2 )(dppe)] (II). In polar solvents, the latter rearranges to the binuclear cationic complex [Pt(μ-C 5 H 4 N- C 2 , N )(dppe)] 2 Cl 2 . The reaction of I with [PdCl(η 3 -2-MeC 3 H 4 )] 2 (1/0.5 molar ratio) gives the products [PtCl(μ-C 5 H 4 N- C 2 , N )(PPh 3 )] 2 (IV) and [PdCl(η 3 -2-MeC 3 H 4 )(PPh 3 )]. 31 P NMR monitoring of the reaction suggests that PPh 3 transfer occurs in a binuclear platinum/palladium intermediate with a bridging 2-pyridyl ligand, and leads initially to a chloride-bridged species [Pt(μ-Cl)(C 5 H 4 -N- C 2 )(PPh 3 )] 2 , which then rearranges to the more stable compound IV. Complexes I and II are readly N -protonated by HCl or HClO 4 to give the cationic derivatives [PtCl(C 5 H 5 N- C 2 )(PPh 3 ) 2 ] + and [PtCl(C 5 H 5 N- C 2 )(dppe)] + . In dichloromethane or 1,2-dichloroethane, the terminal 2-pyridyl group of I and II is slowly N -alkylated by the solvent to give [PtCl{l-R)C 5 H 4 n- C 2 }(PPh 3 ) 2 ]Cl and [PtCl(l-R)C 5 H 4 N- C 2 (dppe)]Cl (R ue5fb CH 2 Cl or CH 2 CH 2 Cl).

Preparation and protonation of 2-pyrimidyl- and 2-pyrazylpalladium(II) complexes

Abstract The oxidative addition of 2-chloropyrimidine or 2-chloropyrazine to [Pd(PPh 3 ) 4 ] yields a mixture of trans -[PdCl(C 4 H 3 N 2 - C 2 )(PPh 3 ) 2 ] (I) and [PdCl(μ-C 4 H 3 N 2 - C 2 , N 1 )(PPh 3 (II) (C 4 H 3 N 2 = 2-pyrimidyl or 2-pyrazyl group). The mononuclear complexes I are quantitatively converted into the binuclear species II upon treatment with H 2 O 2 . The reaction of II with HCl gives the N -monoprotonated derivatives cis -[PdCl 2 (C 4 H 4 N 2 - C 2 )(PPh 3 )] (III), from which the cationic complexes trans -[PdCl(C 4 H 4 N 2 - C 2 )(L) (L = PPh 3 , IV; PMe 2 Ph, V; PEt 3 , VI) can be prepared by ligand substitution reactions. Reversible proton dissociation occurs in solution for III–VI. The low-temperature 1 H NMR spectra of trans -[PdCl(C 4 H 4 N 2 - C 2 )(PMe 2 Ph) 2 ]ClO 4 show that the heterocyclic moiety undergoes restricted rotation around the Pdue5f8C 2 bond and that the 2-pyrazyl group is protonated predominantly at the N 1 atom. These results and the 13 C NMR data for the PEt 3 derivatives are interpreted on the basis of a significant d π → π ★ back-bonding contribution to the palladium—carbon bond of the protonated ligands.

Insertion of isocyanides into the palladium—carbon bond of C2-palladated heterocycles. Synthesis of trans-[PdCl{C(RN)NR}(PPh3)2] complexes (RN = 2-pyridyl, 2-pyrazyl; R = alkyl or aryl group)

The titles complexes trans-[PdCl{C(RN)ue5fbNR}(PPh3)2] (RN = 2-pyridyl (2-py), R = p-C6H4OMe, Me; RN = 2-pyrazyl (2-pyz), R = p-C6H4OMe) can be prepared by reaction of the N-protonated compounds, cis-[PdCl2(RNH)(PPh3] (RNH = 2-pyridylium (2-pyH) or 2-pyrazylium (2-pyzH) group), with PPh3, followed by addition of the isocyanide CNR and deprotonation with triethylamine, in a molar ratio Pd/PPh3/CNR/NEt3 of 1/1/1/1.1. The reaction sequence involves the successive formation of the cationic intermediates trans-[PdCl(RNH)(PPh3)2]+, trans-[Pd(RNH)(CNR)(PPh3)2]2+ and trans-[Pd(RN)(CNR)(PPh3)2]+, which were isolated and characterized as perchlorate salts for RN = 2-pyridyl. In the final step the coordinated isocyanide of trans-[Pd(RN)(CNR)(PPh3)2]+ undergoes migratory insertion into the Pd-RN σ bond, promoted by the chloride ions progressively displaced by the entering neutral ligands from cis-[PdCl2(RNH)(PPh3)]. The resulting products were characterized by conventional spectroscopic techniques and, for RN = 2-pyridyl and R = p-C6H4OMe, also by ligand substitution reaction at the palladium center and by protonation and coordination of the p-methoxyphenylimino(2-pyridyl)methyl group with strong mineral acids (HClO4, HCl) and ZnCl2, respectively.

Reactions of α-diimino ligands with the chloro-bridged dimer [RhCl(COD)]2(COD=1,5-cyclooctadiene)

Abstract The reactions of α-diimino ligands N - N ′ [ N - N ′= 2,2′-bipyridine (bipy), C 5 H 4 Nue5f82ue5f8CHue5fbNR (R= C 6 H 4 OMe- p , PyCa), RNue5fbCHue5f8CHue5fbNR (R=C 6 H 4 - OMe- p , DAB)] with [RhCl(COD)] 2 give rise to stoichiometry, solvent, ligand, and temperature dependent equilibria. In general, the 1/1 ligand/dimer reaction yields the ionic product [Rh(COD)( N - N ′)] [RhCl 2 (COD)], at room temperature. For N - N ′=DAB, the ionic form is in equilibrium with the binuclear compound [{RhCl(COD)} (μ-DAB){RhCl(COD)}] (containing a σ σ,- N , N ′ bridging α-diimine), which becomes the predominant species at low temperatures. In [Rh(COD)( N - N ′)] [RhCl 2 (COD)], a fast exchange of the Rh(COD) unit between the cation and anion occurs at 30 °C for N - N ′=PyCa and DAB (but not for N - N ′=bipy). The 1/0.5 reaction leads to a product, generally formulated as Rh(COD)( N - N ′)Cl, which probably consists of an equilibrium mixture of the cationic [Rh(COD)( N - N ′)]Cl and neutral [RhCl(COD)( N - N ′)] species, in rapid interconversion even at −80 °C. The cationic complex largely predominates in polar solvents, such as methanol, from which it can be precipitated as a perchlorate salt ( N-N ′=bipy, PyCa). For [Rh(COD)(PyCa)]ClO 4 , a low-energy process occurs which involves ligand site exchange ( cis-trans isomerization) and cannot be frozen at the lowest explored temperature (−80 °C). Such dynamic behaviour is interpreted in terms of the formation of stereochemically non-rigid five- coordinate intermediates through association of the cation with the solvent or the counteranion. In [Rh(COD)( N-N ′)] + , the σ,σ′- N,N ′ chelating abilities of N-N ′ appear to increase in the order: DAB

Insertion of isocyanides into the 2-one-1-propyl-palladium(II) bond: a convenient synthetic route to C1-palladated 1-amino-3-one-1-butene compounds

Abstract Migratory insertion of isocyanides CNR (R = p -C 6 H 4 OMe, Me) into the palladium—carbon σ bond of trans -[PdCl(CH 2 COMe)(PPh 3 ) 2 ] (I) yields the 1-amino-3-one-1-butenyl complexes trans -[PdCl{C(NHR)ue5fbCHCOMe}(PPh 3 ) 2 ] (IIa, R = p -C 6 H 4 OMe; IIb, R = Me), which have been characterized by IR, 1 H and 31 P NMR spectra. The reaction of IIa with nickel acetate gives a diamagnetic bischelate complex of the type [Ni(Oue5f8N) 2 ] , in which the deprotonated 1-amino-3-one-1-butenyl moiety acts as a bidentate O,N ligand.