Synthesis of mixed Ar,Ar′-BIAN ligands (Ar,Ar′-BIAN = bis(aryl)acenaphthenequinonediimine). Measurement of the coordination strength of hemilabile ligands with respect to their symmetric counterparts

Abstract

The synthesis of Ar,Ar'-BIAN ligands (Ar,Ar'-BIAN = bis(aryl)acenaphthenequinonediimine) having different aryl groups bound to the two nitrogen atoms is reported for the first time. The ligands were obtained by two different strategies: (i) by a transimination reaction starting from a symmetric Ar,Ar-BIAN ligand having aryl groups bearing strongly electron-withdrawing substituents or (ii) by a two-step-one-pot sequence. The ligands synthesized have been chosen so that the electronic difference between the two aryl groups is very large, but the steric difference is variable and, in one case, the ligand is almost sterically symmetric. The coordination strength of the new ligands towards three palladium complexes has been measured by a competition experiment following a protocol previously described by us. The coordination strength of the mixed ligands is the mean of those of the corresponding symmetric counterparts. The X-ray crystal structure of a palladium pi-allyl complex of the electronically asymmetric-sterically symmetric ligand (3,5-(CF(3))(2)C(6)H(3)),(3,5-Me(2)C(6)H(3))-BIAN has been solved, together with those of the two symmetric analogues to allow a comparison. Despite the fact that the dodecafluorinated ligand has a K(eq) value about three orders of magnitude lower than the non-fluorinated counterpart, no notable difference is observed in the N-Pd and Pd-C(allyl) distances in the three complexes. Calculations at the density functional level confirm that Pd-BIAN distances are not strictly correlated to the coordination energies, which are in qualitative agreement with the spectroscopic evidence. The bond length is thus not a good indication of the bond strength in these cases.

Catalyst activity or stability: the dilemma in Pd-catalyzed polyketone synthesis

“…For Ar-BIANs, (E,E) and (E,Z) isomers are possible regarding to the relative configuration of the aryl rings with respect to the C=N imine bonds. In general, only the E,E isomer is observed for symmetrical Ar-BIANs, 19 but a variable amount of the E,Z isomer is always found in the case of nonsymmetrical Ar,Ar′-BIAN derivatives [16][17][18]20 and for alkyl-BIANs. 22,23 For both ligands 3 and 7, the 1 H NMR spectrum recorded at room temperature in CDCl 3 and CD 2 Cl 2 , respectively, indicated the presence in solution of the E,E isomer only.Though this was expected for the latter, it is surprising for the former.…”

“…19,20 The synthesis of the new nonsymmetrical Ar,Ar′-BIAN 3 was performed by the transimination reaction between the zinc derivative of ligand 7 and 3,4,5-trimethoxy aniline (Scheme 1), similarly to what was previously described by some of us for other nonsymmetrical Ar,Ar'-BIAN derivatives. 16 The reaction proceeds quickly and the mixed ligand could be isolated in 52 % yield after chromatographic purification. It should be noted that previously mixed ligands had only been obtained by this methodology when the starting compound had two trifluoromethyl groups on each aryl ring, according to the idea that electron-withdrawing substituents on the starting Ar-BIAN should render transimination easier.…”

“…20,38 When two substituents were present, the correlation was maintained if the sum of the two individual σ was employed, whereas ligands having different substituents on the two aryl rings followed the same trend if the arithmetic average of the Hammett σ constants for the two rings was employed in the correlation. 16 In general, this corresponds to employing for both symmetrical and nonsymmetrical ligands the value of Σσ/2 (where the sum is extended to all substituents on the two rings),which is also shown in Table 2.…”

“…16 The organometallic palladium complexes, Ar-BIAN), were applied as catalysts in CO/vinyl arene copolymerization. We found that moving the substituents on the aryl rings of Ar-BIAN from the ortho to the meta positions resulted in an enhancement of productivity of more than two orders of magnitude, and a productivity value of 3.70 kg CP/g Pd was reached.…”

Synthesis, Solid‐State Structures, and EPR Spectroscopic Studies on Polycrystalline and Single‐Crystal Samples of α‐Diimine Cobalt(II) Complexes

“…[9] Recently, some results from the insertion polymerization of ethylene have been reported using tetrahedral Co II complexes with related ligands. [10] α-Diimine ligands have also been the subject of different studies regarding the relative binding strengths, [11] as well as their reactivity with main group metals. [12] [a] REQUIMTE/CQFB, Departamento de Química, FCT, Universidade Nova de Lisboa,single-crystal X-ray diffraction.…”