Synthesis of PCP‐Supported Nickel Complexes and their Reactivity with Carbon Dioxide

Abstract

The Ni amide and hydroxide complexes [(PCP)Ni(NH(2))] (2; PCP=bis-2,6-di-tert-butylphosphinomethylbenzene) and [(PCP)Ni(OH)] (3) were prepared by treatment of [(PCP)NiCl] (1) with NaNH(2) or NaOH, respectively. The conditions for the formation of 3 from 1 and NaOH were harsh (2 weeks in THF at reflux) and a more facile synthetic route involved protonation of 2 with H(2)O, to generate 3 and ammonia. Similarly the basic amide in 2 was protonated with a variety of other weak acids to form the complexes [(PCP)Ni(2-Me-imidazole)] (4), [(PCP)Ni(dimethylmalonate)] (5), [(PCP)Ni(oxazole)] (6), and [(PCP)Ni(CCPh)] (7), respectively. The hydroxide compound 3, could also be used as a Ni precursor and treatment of 3 with TMSCN (TMS=trimethylsilyl) or TMSN(3) generated [(PCP)Ni(CN)] (8) or [(PCP)Ni(N(3))] (9), respectively. Compounds 3-7, and 9 were characterized by X-ray crystallography. Although 3, 4, 6, 7, and 9 are all four-coordinate complexes with a square-planar geometry around Ni, 5 is a pseudo-five-coordinate complex, with the dimethylmalonate ligand coordinated in an X-type fashion through one oxygen atom, and weakly as an L-type ligand through another oxygen atom. Complexes 2-9 were all reacted with carbon dioxide. Compounds 2-4 underwent facile reaction at low temperature to form the κ(1)-O carboxylate products [(PCP)Ni{OC(O)NH(2)}] (10), [(PCP)Ni{OC(O)OH}] (11), and [(PCP)Ni{OC(O)-2-Me-imidazole}] (12), respectively. Compounds 10 and 11 were characterized by X-ray crystallography. No reaction was observed between 5-9 and carbon dioxide, even at elevated temperatures. DFT calculations were performed to model the thermodynamics for the insertion of carbon dioxide into 2-9 to form a κ(1)-O carboxylate product and understand the pathways for carbon dioxide insertion into 2, 3, 6, and 7. The computed free energies indicate that carbon dioxide insertion into 2 and 3 is thermodynamically favorable, insertion into 8 and 9 is significantly uphill, insertion into 5 and 7 is slightly uphill, and insertion into 4 and 6 is close to thermoneutral. The pathway for insertion into 2 and 3 has a low barrier and involves nucleophilic attack of the nitrogen or oxygen lone pair on electrophilic carbon dioxide. A related stepwise pathway is calculated for 7, but in this case the carbon of the alkyne is significantly less nucleophilic and as a result, the barrier for carbon dioxide insertion is high. In contrast, carbon dioxide insertion into 6 involves a single concerted step that has a high barrier.

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)

“…The overall energy balance computed for the formation of the nickel bicarbonate complex (ΔE = -19.4, ΔG°= -5.7 kcal/mol) is similar to those previously reported by Nova and Hazari and co-workers (-16.0 and -4.7 kcal/mol, respectively), [10] the difference probably arising from the use of diffuse functions in our calculations (see the Computational Details in the Exp. Sect.).…”

“…Calculations were simplified by replacing the iPr PCP pincer ligand with its analogue Me PCP, which contains methyl substituents instead of isopropyl on the phosphorus atoms. In the course of our work, Nova and Hazari described similar calculations for the Me PCP-Ni system, [10] and Holm and co-workers reported similar results for Ni complexes with dianionic NNN (pyridinebiscarboxamidate) ligands.…”

“…Interestingly, their stability is deeply influenced by the nature of their pincer ligands. Thus, although Ni and Pd bicarbonates containing the bulky and strongly electron-donating tBu PCP ligand are very stable, [8,10] similar compounds containing dianionic 2,6-pyridinedicarboxamidate ligands readily undergo decarboxylation to the corresponding hydroxides. [11] Kemp and Goldberg found that a Pd bi-carbonate complex containing a hemi-labile tBu PCO pincer ligand spontaneously undergoes partial decarboxylation leading to a binuclear carbonate.…”

Acrylates from Alkenes and CO2, the Stuff That Dreams Are Made of

“…A similar mechanism for attack of CO 2 at amide and hydroxide groups of nickel compounds with subsequent insertion of CO 2 into the NidN/NidO bonds via zwitterionic intermediates has been computed recently. 47 Carboxylation of Ni(II) and Pd(II) allyl complexes has also been proposed to proceed via zwitterionic intermediates resulting from the attack of CO 2 at the terminal carbon of the η 1 -coordinated allyl group. [48][49][50][51][52][53] B1 is isoelectronic to the thermodynamic insertion product in cationic olefin polymerization such as in the Brookhart systems, 54 and is fairly stable, if solvation is taken into account.…”

Oxazole

“…Nickel pincer complexes were prepared and the subsequent reactivities of these complexes with CO 2 were studied. 58 The complex [(PCP)Ni(Cl)] was refluxed in THF with excess NaNH 2 for 24 h to form [(PCP)Ni(NH 2 )] (eq 38).…”