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

Abstract

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.

Opto‐Mechanical Effect in Photoactive Nematic Side‐Chain Liquid‐Crystalline Elastomers

“…[31] Thus, the results indicate that both isomerization processes follow the same behaviour or isokinetic relationship [32] during the transformation from trans-to-cis and cis-to-trans isomers: the inversion mechanism (see Supporting Information, Figure SI-4).…”

Synthesis and reactivity of cyclometallated platinum (II) compounds containing [C,N,N′] terdentate ligands: Crystal structures of [PtCl{(CH3)2N(CH2)3NCH(4-ClC 6H3)}], [PtCl{(CH3)2N(CH2)3NCH(2-ClC6H3)}] and [PtCl{(CH3)2N(CH2)3NCH(3-(CH3)C6H3)}]

“…Isomerization of the ligand to the adequate conformation around the C@N bond has been reported to be a key step of the C-H bond activation process [7d,7e] and occurs more readily for the more flexible propyl derivatives which are obtained as a mixture of (Z) and (E) conformers than for the ethylene analogues which isomerizes only in the presence of a base [7e]. In addition a pre-equilibrium dechelation process is needed for the C-H activation to occur [8] and this should be easier for the propylene than for the ethylene amines assuming that a greater platinum-amine distance, as obtained from crystallographic data of compounds 3, also apply to coordination compounds derived from propyleneamine. Given the advantageous features of the propyl system, a study of regioselectivity could also be realised for several 3-substituted ligands (1c, 1d, 1e and 1h) and allow us to conclude that steric effects play a decisive role in the selectivity of the C-H bond activation.…”

“…The N-(substituted benzylidene)-N 0 ,N 0 -dimethyl-1,3-propanediamines RCHN(CH 2 ) 3 NMe 2 (1a-h) were obtained from the condensation reaction between Me 2 N(CH 2 ) 3 NH 2 and the corresponding aldehyde under the usual conditions [3,[8][9][10]. In the 1 H NMR spectra a signal (ca.…”

Cyclometalation Using d-Block Transition Metals: Fundamental Aspects and Recent Trends

“…In apolar solvents, ligand dissociation from PdX 2 L 2 is hampered, though it can be accelerated by the addition of catalytic amounts of a strong acid like triflic acid. 439 The equilibrium between species of type B and type C is particularly unfavorable when using primary amines. 440,441 For some cyclopalladations, hence a 1:1 Pd(Ac) 2 /amine stoichiometry is preferred over the 1:2 ratio typically employed for tertiary amines.…”