Kinetic study of the insertion of norbornadiene into palladium-carbon bonds of complexes containing the rigid bidentate nitrogen ligand Bis(arylimino)acenaphtene Groen, J.H.; Delis, J.G.P.; van Leeuwen, P.W.N.M.; Vrieze, K. Published in: Organometallics DOI:10.1021/om960789fLink to publication Citation for published version (APA):Groen, J. H., Delis, J. G. P., van Leeuwen, P. W. N. M., & Vrieze, K. (1997). Kinetic study of the insertion of norbornadiene into palladium-carbon bonds of complexes containing the rigid bidentate nitrogen ligand Bis(arylimino)acenaphtene. Organometallics, 16, 68-77. DOI: 10.1021/om960789f General rightsIt is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: http://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. complexes (1b-12b), bearing the bidentate nitrogen ligand bis(arylimino)acenaphthene (Ar-BIAN), have been synthesized via reaction of the corresponding neutral acylpalladium complexes Pd(C(O)R)X(Ar-BIAN) (1a-12a) with norbornadiene (nbd). For the first time, an extensive kinetic study of this migratory alkene insertion into acyl-palladium bonds of neutral complexes containing R-diimine ligands has been carried out. It has been found that under pseudo-first-order circumstances these reactions follow the rate law k obsd ) k 1 + k 2 [nbd], which shows that these reactions proceed via a pathway independent of alkene concentration (k 1 pathway) and a pathway dependent on alkene concentration (k 2 pathway). The dramatic decrease of the rate constants k 1 and k 2 upon increasing the steric bulk of the BIAN ligand and the large negative entropy of activation and low enthalpy of activation for both pathways indicate that the k 1 and k 2 pathways are closely related and involve associative processes. From the influence of solvent, X and C(O)R ligand, steric and electronic properties of the BIAN ligand, the presence of free halide and free BIAN, and the parameters of activation, mechanisms have been proposed for both pathways. The k 1 pathway may proceed via a rate-determining solvent-assisted halide or nitrogen dissociation, followed by alkene association and migratory insertion, while the k 2 pathway may occur via a rate-determining migratory alkene insertion in a contact ion pair intermediate. This species may be formed via alkene association followed by either halide or nitrogen dissociation.

Propadiene, 3-methyl-1,2-butadiene (DMA), and 2,4-dimethyl-2,3-pentadiene (TMA) reacted via migratory insertion with both neutral and ionic Pd(R)X(p-An-BIAN) (R = Me (1), C(O)Me (2); X = Cl (a), SO3CF3 (b)) complexes, containing the rigid nitrogen ligand bis(p-anisylimino)acenaphthene (p-An-BIAN), resulting in the novel and stable allylpalladium complexes Pd(η3-C3H4R)X(p-An-BIAN) (R = Me (3), C(O)Me (6)), Pd(η3-C5H8R)X(p-An-BIAN) (R = Me (4), C(O)Me (7)), and Pd(η3-C7H12R)X(p-An-BIAN) (R = Me (5), C(O)Me (8)), respectively (X = Cl (a), SO3CF3 (b)). The neutral complexes 6a and 7a reacted with carbon monoxide to form the acylpalladium complexes Pd(C(O)C3H4C(O)Me)Cl(p-An-BIAN) (9) and Pd(C(O)C5H8C(O)Me)Cl(p-An-BIAN) (10), respectively, while the analogous trifluoromethanesulfonate complexes 6b and 7b were completely inert toward CO. Complexes 9 and 10 reacted again with propadiene and DMA, respectively, to yield the allylpalladium complexes Pd(η3-C3H4C(O)C3H4C(O)Me)Cl(p-An-BIAN) (11) and Pd(η3-C5H8C(O)C5H8C(O)Me)Cl(p-An-BIAN) (12), respectively. Also insertion of norbornadiene in complex 10 was possible, yielding the ionic complex [Pd(C7H8C(O)C5H8C(O)Me)(p-An-BIAN)]Cl (13a), which reacted with AgSO3CF3 to give [Pd(C7H8C(O)C5H8C(O)Me)(p-An-BIAN)]SO3CF3 (13b). The novel complexes 9−12 are the first isolated and fully characterized complexes formed by successive insertion reactions of carbon monoxide and allenes, while 13a is the first isolated complex containing a metal-bonded ter-oligomer of carbon monoxide, an allene, and norbornadiene. The X-ray crystal structure of 7a has been determined and shows a distorted square pyramidal geometry in which the BIAN ligand is bonded to the palladium center in an unusual asymmetric fashion (Pd−N(1) = 2.144(7) Å; Pd−N(2) = 2.600(8) Å).

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The insertion reactions of the allenes propadiene and 1,2-heptadiene in the Pd-C bond of complexes (NkN)Pd(R)X Cl, Br) have been investigated. An X-ray crystal structure determination of (8-PQ)Pd{(1-3-η)-2-methylpropenyl}Cl exhibited the unexpected monodentate coordination of the nitrogen ligand. The monodentate coordination in apolar solvents and bidentate coordination in polar solvents was demonstrated by means of NOE NMR experiments. Kinetic measurements revealed that the reactions are first order in the palladium concentration and occur via an allene concentration independent and dependent pathway. Reactions of complexes containing flexible bidentate nitrogen ligands were retarded by additional free bidentate nitrogen ligand indicating that initial dissociation of a nitrogen donor is an important step in the reaction. We have strong indications that the migration of the R group to the precoordinated allene is the rate-determining step. Instead of masslaw retardation by excess X -(X ) Cl -, Br -), an enhancement of the reaction has been observed in case of the complexes (8-PQ)Pd(Me)Cl, (8-PQ)Pd(Me)Br, and (i-Pr-DAB)Pd(C(O)-Me)Cl. Flexible bidentate nitrogen ligands greatly enhance the reaction, owing to the easy formation of an accessible site on the metal center. The insertion of allenes into the Pd-C bonds of complexes containing rigid bidentate nitrogen ligands probably proceeds via initial allene association followed by either halide or nitrogen dissociation and subsequent migration of the R group to the precoordinated allene.

Novel methyl complexes [Pd(Me)(N‐N‐N)]X (N‐N‐N = flexible or rigid terdentate nitrogen ligand, X = Cl, SO3CF3, BAr′4) have been synthesized and fully characterized. All complexes readily underwent insertion of carbon monoxide resulting in the quantitative formation of complexes [Pd{C(O)Me}(N‐N‐N)]X [X = Cl (1d–6d), BAr′4 (1e–6e)]. Subsequently, complexes 2e–6e underwent quantitative insertion of norbornadiene, resulting in complexes [Pd{C7H8C(O)Me}(N‐N‐N)]BAr′4 (2f–6f). Unexpectedly, these complexes, including even those containing rigid terdentate nitrogen ligands, possess a structure in which the nitrogen ligand is coordinated in a bidentate fashion. A kinetic study of the reaction of norbornadiene with complexes 1e–6e revealed that the reactivity of complexes 1e–6e toward norbornadiene increases with increasing rigidity of the terdentate ligand, i.e. with increasing strain in the PdN3 moiety, which indicates that insertion very likely occurs via a mechanism involving nitrogen dissociation. This is fully supported by ab initio MO calculations on CO and ethylene insertion into carbon–palladium bonds of cationic model systems containing a rigid terdentate nitrogen ligand, which showed that the lowest‐energy pathway for both insertion reactions consists of substitution of one of the distal nitrogen atoms of the rigid terdentate nitrogen ligand by the substrate, followed by a rate‐determining migratory insertion of the substrate into the carbon–palladium bond.

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