N-heterocyclic carbene complexes of palladium in oxygen atom transfer reactions involving the making and breaking of N-O bonds

• Published in: Inorganica Chimica Acta 2017-11, Vol.468, p.285-293
• Main Authors: Cai, Xiaochen, Majumdar, Subhojit, Serafim, Leonardo F., Temprado, Manuel, Nolan, Steven P., Cazin, Catherine S.J., Captain, Burjor, Hoff, Carl D.
• Format: Article
• Language: English
• Subjects: Chemistry; Coordination compounds; Crystal structure; Cycloaddition; Enthalpy; Equivalence; Ligands; Nitrogen dioxide; Oxidation; Palladium; Phosphine oxide; Toluene
• ISSN: 0020-1693; 1873-3255
• DOI: 10.1016/j.ica.2017.05.069

[Display omitted] •Reaction of MesCNO (Mes=2,4,6-trimethylphenyl) with [Pd(IPr)(P(p-tolyl)3)] yields [Pd(IPr)(NCMes)(κ2-O-NC-Mes(–N–C(O)Mes))].•Three major steps are proposed to be involved in the reaction: i. oxidation of the coordinated phosphine ligand to phosphine oxide ii. oxygen atom transfer forming a CO bond from the N–O bond of MesCNO, and iii. cycloaddition of a final MesCNO ligand to yield product.•Addition of two equivalents of NO at low temperature to [Pd(IPr)2(η2-O2)] generates the N-bonded complex trans-[Pd(IPr)2(NO2)2]. Insight into the energetics of this reaction are probed by DFT calculations.•The computed enthalpy of binding of two moles of NO2 to form [Pd(IMe)2(NO2)2] is −112kcal/mol indicating that its preparation from [Pd(IMe)2], N2 and 2 O2 is thermodynamically favorable by −96kcal/mol.•Crystal structures of [Pd(IPr)(NCMes)(κ2-O–NC-Mes(–N–C(O)Mes))] and trans-[Pd(IPr)2(NO2)2] are reported. Reaction of three equivalents of MesCNO (Mes=2,4,6-trimethylphenyl) with one equivalent of [Pd(IPr)(P(p-tolyl)3)] in toluene yields the solid complex [Pd(IPr)(NCMes)(κ2-O–NC-Mes(–N–C(O)Mes))]. Three major steps are proposed to be involved in the reaction based on spectroscopic studies as well as literature precedents for related cycloadditions: i. oxidation of the coordinated phosphine ligand to phosphine oxide ii. oxygen atom transfer forming a CO bond from the N–O bond of MesCNO, and iii. cycloaddition of a final MesCNO ligand to yield product. Addition of two equivalents of NO at low temperature to the in situ generated peroxide complex [Pd(IPr)2(η2-O2)] generates the N-bonded complex trans-[Pd(IPr)2(NO2)2] in keeping with a literature precedent reported for similar complexes. Insight into the energetics of this reaction are probed by DFT calculations using the truncated ligand complex [Pd(IMe)2]. The computed enthalpy of binding of two moles of NO2 to form [Pd(IMe)2(NO2)2] is −112kcal/mol indicating that its preparation from [Pd(IMe)2], N2 and 2O2 is thermodynamically favorable by −96kcal/mol. Crystal structures of [Pd(IPr)(NCMes)(κ2-O–NC-Mes(–N–C(O)Mes))] and trans-[Pd(IPr)2(NO2)2] are reported.