Square Planar Cobalt(II) Hydride versus T-Shaped Cobalt(I): Structural Characterization and Dihydrogen Activation with PNP–Cobalt Pincer Complexes

Abstract

The carbazole-based pincer ligand R(CbzPNP)H (R = i Pr, t Bu) has been used for the synthesis and characterization of various low- and high-spin cobalt complexes. Upon treatment of the high-spin complexes R(CbzPNP)­CoCl (2 R -Co II Cl) with NaHBEt3, the selective formation of cobalt­(II) hydride 3 i Pr -Co II H and T-shaped cobalt­(I) complex 4 t Bu -Co I was observed, depending on the substituents at the phosphorus atoms. For an unambiguous characterization of the reaction products, a density functional theory (DFT) supported paramagnetic NMR analysis was carried out, which established the electron configuration and the oxidation states of the metal atoms, thus demonstrating the significant impact of ligand substitution on the outcome of the reaction. A distinct one-electron reactivity was found for 4 t Bu -Co I in the dehalogenation of t BuCl and cleavage of PhSSPh. On the other hand, the CoI species displayed two-electron redox behavior in the oxidative addition of dihydrogen. The resulting dihydride complex 6 t Bu -Co III (H) 2 was found to display sluggish reactivity toward alkenes, whereas the cobalt­(II) hydride 3 i Pr -Co II H was successfully employed in the catalytic hydrogenation of unhindered alkenes. The stoichiometric hydrogenolysis of 8 i Pr -Co II Bn at elevated pressure (10 bar) led to a rapid cleavage of the Co–C bond to yield hydride complex 3 i Pr -Co II H. On the other hand, treatment of 2 i Pr -Co II Cl with phenethylmagnesium chloride directly resulted in the formation of 3 i Pr -Co II H, indicating facile β-H elimination of the alkene insertion product (reversibly) generated in the catalytic hydrogenation. On the basis of these observations, a mechanistic pathway involving a key σ-bond metathesis step of the CoII-alkyl species is proposed.

Concerted aryl-sulfur reductive elimination from PNP pincer-supported Co(iii) and subsequent Co(i)/Co(iii) comproportionation

“…Examination of the literature shows that four-coordinate Co(II) complexes of anionic pincer ligands are known with both a low-spin S ¼ 1/2 conguration (square-planar geometry) and a high-spin S ¼ 3/2 conguration (pseudotetrahedral geometry). [43][44][45][46][47][48][49][50][51][52] Low-spin, square-planar Co(II) complexes give rise to paramagnetically shied 1 H NMR resonances that are broad compared to diamagnetic compounds, but are typically interpretable in terms of their relative integration and chemical (in)equivalence. High-spin Co(II) compounds tend to produce 1 H NMR spectra that are broadened beyond useful interpretation.…”

Phosphines and N‐Heterocycles Joining Forces: an Emerging Structural Motif in PNP‐Pincer Chemistry

“…For the P i Pr 2 ‐substituted ligand derivative, the dimeric µ ‐hydrido‐bridged high‐spin iron(II) complex 43 was isolated and employed as hydrogenation catalyst for the synthesis of alkanes from the corresponding alkenes and dihydrogen . A similar catalytic activity was also reported for the related mononuclear cobalt(II) pincer complex CbzC ( i PrPNP)Co(H) …”

“…For the t Bu‐substituted CbzC ( t BuPNP) ligand, a T‐shaped high‐spin cobalt(I) complex ( 56 ) has been prepared and unambiguously characterized by X‐ray diffraction and paramagnetic NMR spectroscopy . This complex was shown to homolytically cleave diphenyl disulfide to afford the cobalt(II) thiophenolato complex 57 , while a reversible reaction was observed with dihydrogen.…”

“…This complex was shown to homolytically cleave diphenyl disulfide to afford the cobalt(II) thiophenolato complex 57 , while a reversible reaction was observed with dihydrogen. The resulting dihydride 58 (Scheme ) could only be detected under H 2 ‐pressure and readily decays to 56 under standard conditions . Using the same carbazole‐based pincer ligand, the three‐coordinate copper(I) complex 59 has been prepared by the groups of Fu and Peters and shown to catalyze the coupling reactions between N ‐unsubstituted carbamates and secondary alkyl bromides upon photoexcitation with a blue LED lamp (see Scheme ) …”

Öffnung der Valenzschale: Ein T‐förmiges Eisen(I)‐Metalloradikal und sein Potential als Atomabstraktor

“…Beispiele von T‐förmigen Eisenkomplexen sind extrem selten, und unseres Wissens nach wurde das bisher einzige Beispiel eines Eisenkomplexes dieses Typs von der Caulton‐Gruppe veröffenlicht . Wir haben über die Fähigkeit des Carbazol‐basierten Liganden (PNP)H (mit (PNP)H=3,6‐Di‐ tert ‐butyl‐1,8‐bis((di‐ tert ‐butylphosphanyl)methyl)‐9 H ‐carbazol) berichtet, eine Reihe von niedrig‐koordinierten 3d‐Metallverbindungen zu stabilisieren und eine bemerkenswert langsame Kernrelaxation zu induzieren, was zu einzigartigen spektroskopischen Eigenschaften führt . Hier berichten wir über die Synthese einer “nackten” (PNP)Fe‐Spezies, ihre elektronischen Eigenschaften und ihre Reaktivität als effektiver Sauerstoffatom‐Abstraktor.…”