Synthesis and Reactivity of Nickel and Palladium Fluoride Complexes with PCP Pincer Ligands. NMR-Based Assessment of Electron-Donating Properties of Fluoride and Other Monoanionic Ligands

Abstract

A series of complexes of the type [( iPr PCP)M-L ] ( M = N i , P d ) , c o n t a i n i n g t h e 2 , 6 -b i s -(diisopropylphosphinomethyl)phenyl ( iPr PCP) pincer ligand and simple monoanionic ligands L (F, Cl, Br, H, Me, Ph, NO 3 , and OTf), have been synthesized and characterized. The fluoride derivatives [( iPr PCP)M-F] were prepared from the halides [( iPr PCP)M-X] (M = Ni, X = Br; M = Pd, X = I) by exchange reactions with AgF or, alternatively, by protonolysis of the methyl complexes [( iPr PCP)M-Me] with Et 3 N·3HF(TREAT-HF). A survey of the 13 C NMR data for the new complexes and previously reported ones with L = OH, OMe, and NH 2 revealed significant trends that can be directly related to the electronic properties of the anionic ligands L coordinated to Ni(II) and Pd(II) centers. The chemical shift of the 13 C resonance of the metal-bound ipso carbon is very sensitive to the σ-donating capacity of L, which increases in the order L = OTf > NO 3 > F > OAc > OMe > Cl > OH ≈ Br > I > NH 2 > Ph > Me > H. In addition, the reactivity of [( iPr PCP)M-F] complexes was investigated. These readily undergo fluoride exchange with LiOMe, affording the corresponding methoxides [( iPr PCP)MOMe] and LiF. They also react with n-C 12 H 13 I to cleanly yield [( iPr PCP)M-I] and n-C 12 H 13 F. Comparison of the rates of the latter reactions and analogous ones involving bromide or chloride complexes showed that the fluorides are significantly more nucleophilic, particularly the palladium derivative. This property was applied in a palladium-catalyzed reaction for the synthesis of alkyl fluorides using AgF as the fluoride source.

Reversible Reactions of Ni and Pd Hydroxo Pincer Complexes [(iPrPCP)M–OH] with CO2: Solid‐State Study of the Decarboxylation of the Monomeric Bicarbonate Complexes [(iPrPCP)M–OCOOH] (M = Ni, Pd)

“…Such p-d non-bonding repulsions are expected to be more intense in the second-row than in firstrow transition elements. [12,21] Notwithstanding these differences, the absolute CO 2 insertion barriers are both low, 11.5 kcal/mol for Ni and 7.9 kcal/mol for Pd, and the global free-energy change is favourable for both reactions (-5.7 and -8.7 kcal/mol, respectively). Therefore both hydroxides are predicted to react easily and quantitatively with CO 2 , as observed experimentally.…”

“…We have observed previously that the position of this signal has a direct relationship with the σ-donating capacity of the ligand bound to the metal in the trans position. [12] For both the Ni and Pd carbonates, the corresponding resonance is shifted downfield by around 3 ppm relative to that of the corresponding bicarbonate complex. This difference shows that the CO 3 2-ligand is a significantly stronger σ donor than HCO 3 -, in line with the decreased basicity of the latter.…”

The Contrasting Character of Early and Late Transition Metal Fluorides as Hydrogen Bond Acceptors

“…In a recent study of nickel and palladium pincer complexes M( iPr PCP)X with a wide variety of X ligands, it was shown that the chemical shift 13 C(ipso) provides a measure of -bond strength while the difference in chemical shifts  13 C(para)   13 C(meta) probes -bonding. On these measures, fluoride was a very weak σ-bonder but exhibited appreciable -bonding 77. Nickel analogues with other pincer ligands behaved similarly.41e The results reported in the present paper provide a much more quantitative indicator of ionic character in the M-F bond of the d8 complexes than previous approaches, though this does not exclude 4-electron d-22 p repulsions.…”

Catalytic dehydrogenative borylation of terminal alkynes by POCOP-supported palladium complexes

“…15,19,24 5c was synthesized through the same method as complex 2c. Complexes 4a, 5b, 6b, 7b, 8b were synthesized according to literature procedures and precedent and were used as precatalysts (Scheme 4).…”