Jaume Granell

Cyclometallation reactions of N-(benzylidene)benzylamines with palladium compounds

Abstract The cyclometallation of p-RC6H4CHNCH2C6H2, (R = H, Cl, NO2) by PdX2 (X = Cl, AcO) has been studied. In every case the cyclometallation occurs with formation of a five-membered ring containing the methine group. The structure of these compounds [ PdX( p -RC 6 H 3 CHN CH2C6H5)]2, derived from 1H NMR spectra, are different from those reported previously. Reaction of these compounds with PEt3 gives the compounds [PdX(p-RC6H3CHNCH2C6H5)(PEt3)2] but with an excess of PPh3 only the complexes [ PdX( p -RC 6 H 3 CHN CH2C6H5)(PPh3)] are formed.

Ligand exchange reactions in cyclopalladated complexes of benzylideneanilines. Crystal structure of [Pd(C6H4CHNC6H5)Br(PPh3)2]

The ligand exchange reactions between [{[graphics omitted]C6H5)(O2CMe)}2] and the free imines p-RC6H4CHNC6H5(R = H, Cl, NMe2, or NO2)(N[graphics omitted]CH) in acetic acid have been studied. The substitution process is favoured when the incoming ligand contains electron-withdrawing substituents. From the results obtained a new mechanism involving a bis-cyclometallated complex ([graphics omitted]P[graphics omitted])] as intermediate is proposed. The dimeric cyclopalladated compounds are obtained by metallation of the imines by Pd(O2CMe)2; by reaction with phosphines PR′3 and LiBr, [[graphics omitted]C6H5)Br(PR′3)] and [Pd(p-RC6H3CHNC6H5)Br(PR3′)2] are formed (R′= Ph or Et). All the compounds have been fully characterized and their 1H and 13C n.m.r. spectroscopic data recorded. The molecular structure of [Pd(C6H4CHNC6H5)Br(PPh3)2] has been determined by a single-crystal X-ray analysis. The crystals are monoclinic, space group P21/a, a= 17.427(3), b= 24.790(4), c= 10.424(2)A, β= 98.91(2)°, and Z= 4; R= 0.053 for 6 579 reflections. The palladium atoms are five-co-ordinated, the Pd–N distance being 2.710(6)A.

Cyclopalladation of the primary amine (R)-(+)-1-(1-naphthyl)ethylamine: a new resolving agent for monodentate phosphines

Abstract The cyclopalladation of the primary amine ( R )-(+)-1-(1-naphthyl)ethylamine, available commercially, is described. The resolution of the monodentate phosphine trans -PPh 2 (2-HOC 6 H 10 ) by means of the dinuclear cyclopalladated derivative is also reported

Cyclometallation of 2-phenylaniline and (R)-α-methylbenzylamine by palladium(II) acetate. X-ray molecular structure of [Pd{2−(2′−NH2C6H4)C6H4}Br(PPh3)]

The action of palladium(II) acetate on the primary amines 2-phenylaniline and (R)-α-methylbenzylamine and subsequent treatment with LiBr allows the preparation of the corresponding cyclopalladated dimers [Pd(CN)Br]2 (1a and 1b respectively). Mononuclear compounds [Pd(CN)Br(L)], 3 (L = py−d5) and 2 (L = PPh3), have ben obtained by reaction of 1 with py-d5 and PPh3. Addition of PPh3 to chloroform solutions of 2 produces a rapid exchange between coordinated and free PPh3. 31P{1H} NMR at variable temperature suggests that this exchange proceeds for 2a through the fast equilibrium [Pd(CN)Br(PPh3)2]⇔[P(CN)Br(PPh3)], while the usual mechanism in substitution reactions in d8 planar square complexes is involved in the case of 2b. The X-ray crystal structure of 2a has been determined. 2a crystallizes in the triclinic space group P1 with a = 12.182(3), b = 10.502(2), c = 9.998(2) A, α = 99.24(2), β = 94.69(2) and γ = 99.13(2)°.

Optically active palladacycles containing imines derived from 1-(1-naphthyl)ethylamine: new resolving agents for P-chiral phosphines

Abstract The synthesis of the new palladium metallacycles containing imines derived from 1-(1-naphthyl)ethylamine is reported. These new organometallic complexes have been used to resolve the P-chiral ligand benzylcyclohexylphenylphosphine. The absolute configuration of ( R C , S P )-[PdCl{2-[HCN-CH(Me)C 10 H 6 ]-3-ClC 6 H 3 }(PBzCyPh)] has been determined by single crystal X-ray analysis.

CHIRAL CYCLOPALLADATED COMPOUNDS FOR ENANTIOMERIC PURITIES OF FUNCTIONALIZED PHOSPHINES BY MEANS OF MULTINUCLEAR NMR

Abstract The application of the cyclopalladated compound [Pd(2-{Z-( R )-CHMeN=CH-2′,6′-Cl 2 C 6 H 3 }C 6 H 4 )Cl] 2 for the enantiomeric excess determination of functionalized phosphines is presented. The X-ray structure determination of diastereomeric complex [Pd(2-{Z-( R )-CHMeN=CH-2′,6′-Cl 2 C 6 H 3 }C 6 H 4 )Cl{( R , R )-PPh 2 (2-OHC 6 H 1 0 )}] is also reported.

Reactivity of cyclometallated compounds of N-benzylideneamines. Synthesis and X-ray crystal structure of [Pd{1-CH2-2-(CHN-C6H5)-3,5-(CH3)2C6H2} (2,4-Me2C5H3N)]ClO4

Abstract The action of Ph2PCH2CH2PPh2 (dpe) on the cyclometallated compounds [{Pd( CN )Br}2] ( CN = 2-(HCNC6H5)-5-ClC6H31a, 2-(CH2NCH-2′,6′-Cl2C6H3)C6H41b, or 1-CH2-2-(CHN-C6H5)-3,5-(CH3)2C6H21c) in a 2:1 molar ratio, gives the novel neutral species [Pd( CN )Br(dpe)] (2a,b) or the ionic compound [ (dpe)]Br (3c). The action of dpe on compound 1 in a 1:1 molar ratio gives the dinuclear cyclometallated compound 4, in which two palladium atoms are bridged by the diphosphine. The ionic compounds[ (lut)(PPh3)]ClO4 6 (lut = 2,4-lutidine) were obtained by reaction between AgClO4 and acetone solutions of the cyclometallated compounds , and subsequent addition of 2,4-lutidine. [P d{1-CH 2 -2-(CHN -C6H5)-3,5-(CH3)2C6H2}(2,4-lut)]ClO4 crystallizes in the orthorhombic space group Pcab with a = 16.331(3) A; b = 18.885(3) A; c = 24.702(4) A, and Z = 8. The endo six-membered ring displays a half-skew-chair conformation, with the palladium atom out of the plane (1.086 A) defined by the other atoms.

Kinetico–mechanistic studies of C–H bond activation on new Pd complexes containing N,N′-chelating ligands

The hybrid imine/amine palladium(II) coordination complexes [PdX2(kappa2-N(imino),N(amino))](X = Cl, AcO; kappa2-N(imino),N(amino)= 4ClC6H4CHNCH2(CH2)nN(CH3)2, n= 1, 2) have been prepared in different isomeric forms which include E/Z arrangement around the C[double bond]N bond of the hybrid ligand and {Pd(kappa(2)-N(imino),N(amino))} ring conformation. The crystal structures of four of them, E-1AcO, Z-1AcO, E-2AcO and E-2Cl, have been determined and the solution behaviour in acetic acid, the common cyclometallating solvent, for all these systems studied. The complexes in acetic acid solution are shown to maintain the structure determined by X-ray crystallography, as they do in deuterated chloroform. Nevertheless, a partial opening equilibrium of the {Pd(kappa2-N(imino),N(amino))} ring is observed by NMR experiments. When the complexes are held in solution for longer periods the corresponding cyclometallated derivatives, 1AcO-CM, 2AcO-CM, 1Cl-CM and 2Cl-CM, containing the {Pd(kappa2-C,N(imino))} palladacycle are obtained, as characterized by 1H NMR spectroscopy. In these compounds the total opening of the N(amino) moiety of the ligand has occurred. The C-H bond activation process has been studied kinetico-mechanistically at different temperatures, pressures and acid concentrations; the results agree with the need of an opening of the chelate ring in [PdX2(kappa2-N(imino),N(amino))] prior to the proper cyclometallation reaction. The values of the enthalpies of activation are higher than those observed for known N-monodentated cyclometallating ligands, as should correspond to the contribution of a ligand dechelation pre-equilibrium. The entropies and volumes of activation are also indicative of this predissociation that include an important amount of contractive ordering. The presence of small amounts of triflic acid in the reaction medium accelerates the reaction to the value observed for N(imino)-monodentate systems, indicating that the full opening of the chelate ring has taken place. For the badly oriented isomeric forms of the ligand in the chelated complex (Z), the cyclometallation process is even more slow and corresponds directly to the reorganization of the ligand to its cyclopalladation-active (E) conformation.

Kinetico-mechanistic studies of cyclometalating C-H bond activation reactions on Pd(II) and Rh(II) centres: the importance of non-innocent acidic solvents in the process.

The activation of C-H bonds in homogeneous systems has been the subject of study for many years due to its involvement in important industrial catalytic processes. A large number of reviews on the different areas involved have appeared, but those dealing with kinetic studies, including activation parameters, are rather scarce due to the severe difficulties in interpreting experimental data. In this perspective, the information available from kinetico-mechanistic studies of cyclometalation reactions on Pd(II) and Rh(II) centres via C-H bond activation is considered. Experimental results from studies performed on complexes of these metal centres indicate that the historically accepted electrophilic substitution classification is not a satisfactory mechanistic term for the process occurring during the reaction. A definite acid-assisted phenomenon is evident for all the processes studied, which contradicts the expected need for a proton abstractor in the reaction. This is even more surprising when considering the expected hydrolysis of M-C bonds in such acidic media, indicating that metalation prevails under these conditions. Only the presence of coordinated acid molecules in solvolytic carboxylic acid media can explain the observations. The fine tuning between the proton abstraction capacity of a coordinated RCO(2)H molecule and its Lewis basicity results in a unique reactivity trend. DFT calculations carried out for these acid-assisted processes fully agree with the experimental trends observed.

Cyclometallation of 3,5 disubstituted N-benzylideneamines by palladium(II). Synthesis and X-ray crystal structure of [Pd(3,5-F2C6H2CHNC6H5)Br(PPh3)2]

Abstract The action of Pd(AcO) 2 on the imines C 6 R m H 5- m CHN(CH 2 ) n C 6 H 5 have been studied. Five-membered endo metallacyles were obtained from the imines 1a (R = 3,5-F 2 , n = 0), 1b (R = 3,5-F 2 , n = 1), 1c (R = 3,5-(MeO) 2 , n = 0) and 1d (R = 3,5-(MeO) 2 , n = 1), by activation of a C(aromatic)-H bond, in spite of the presence of MeO or F substituents on the carbon atom adjacent to the metallation position. 1 H NMR spectra of the acetate bridged compounds [ P d(C-N )(μ- AcO ) 2 ] ( 2a-d ) obtained show the existence of different isomers in solution. Complexes [ P dBr(C-N )( PPh 3 )] ( 4 ) and [PdBr(C-N)(PPh 3 ) 2 ] ( 5 ) can be obtained by the action of PPh 3 on the new cyclometallated compounds. [Pd(3,5-F 2 C 6 H 2 CH=NC 6 H 5 )Br(PPh 3 ) 2 ] crystallizes in the space group P 2 1 a with a = 20.224(4), b = 17.447(3), c = 12.290(2) A, β = 110.12(3)° and Z = 4. The dihedral angle between the metallated phenyl ring and the coordination plane is 86.6°. An N=CH proton and a fluorine atom ortho to the PdC bond occupy the apical positions in the coordination sphere of the palladium atom.