Manganese Chemistry of Anionic Pyrrole-Based Pincer Ligands

Abstract

The chemistry of the pyrrole-based pincer ligands, RPNP (PNP = anion of 2,5-bis­(dialkylphosphinomethyl)­pyrrole, R = Cy and t-Bu), with manganese is reported. Metallation of tBuPNP with Mn­(II) halide precursors did not afford 1:1 ligand to metal complexes but rather led to the formation of the 2:1 complex, [Mn­(κ2-N,P- tBuPNP)2]. Reduction of in situ generated tBuPNP-Mn­(II) in the presence of 2,2′-bipyridine generated the apparent, high-spin Mn­(I) complex, [Mn­(bipy)­( tBuPNP)], although metric parameters derived from crystallography demonstrated that the compound is best regarded as containing a Mn­(II) ion with a bipy radical anion. Reactions of the Mn­(I) precursor, [MnBr­(CO)5], with RPNP afforded low-spin Mn­(I) complexes of the type [Mn­(CO) n (RPNP)] (R = Cy, n = 3; R = t-Bu, n = 2). A third equivalent of CO binds reversibly to [Mn­(CO)2( tBuPNP)] but is lost readily. Pincer backbone dehydrogenation of [Mn­(CO) n (RPNP)] with 1,4-benzoquinone produced the related complexes, [Mn­(CO) n (RdPNP)] (RdPNP = anion of 2,5-bis­(dialkylphosphinomethylene)-2,5-dihydropyrrole). [Mn­(CO)2( tBudPNP)] was found to undergo protonation by (H­{OEt2}2)­(BArF 4) at the methine position to generate the cationic species, [Mn­(CO)2( tBudPNP-H)]­(BArF 4). Reduction of [Mn­(CO)2( tBuPNP)] by KC8 produced the rare, molecular Mn(0) species, K­[Mn­(CO)2( tBuPNP)]. Electron paramagnetic resonance characterization of [Mn­(CO)2( tBuPNP)]− is consistent with a metal-based radical. The chemistry of the pincer manganese species is discussed in the context of potential catalysis and compared with more commonly encountered Mn complexes containing neutral pincer ligands.

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

“…In the case of the 78‐Co , reduction of the dehydrogenated pincer complex was shown to afford the cobalt dinitrogen complex 80 comprising a formally dianionic PNP ligand with a significant π ‐radical character found on the ligand backbone . Upon reduction of 77 with KC 8 , a similar π ‐radical species has been obtained for manganese …”

“…Yet another strategy to modify the ligand framework after initial complex formation, namely oxidative backbone dehydrogenation, has been uncovered for Pyr ( R PNP) pincer complexes by Tonzetich and co‐workers , . Starting from 75 , 76 and 21 , Tonzetich and co‐workers were able to use this approach for the synthesis of the manganese, cobalt and nickel complexes 77 and 78‐M (M = Co, Ni), respectively (see Scheme ).…”

“…In these reactions, one hydrogen atom is abstracted from each methylene sidearm leaving the overall charge of the pincer ligand unaltered , . In order to identify the preferred protonation site of these dehydrogenated species, the manganese dicarbonyl complex 77 was treated with the strong Brønsted acid [H(OEt 2 ) 2 ][BAr F 4 ] . This led to the protonation of the methine bridges with concomitant formation of a neutral PNP ligand (in complex 79 ).…”

“…In the context of the aforementioned Pyr ( R PNP)‐coordinated manganese, cobalt and nickel complexes 75 , 21 and 76 (Scheme ) and the oxidative dehydrogenation of their pincer backbones (vide supra),, it was pointed out that the cobalt complex 80 bearing a formally dianionic PNP ligand was formed via reduction of 78‐Co with KC 8 (Scheme ) . For the neutral ligands Py ( R PNP), PyN ( R PNP) and TrzN ( R PNP), related dianionic PNP ligands have been prepared by deprotonating both sidearm NH‐ or methylene‐groups in each of these ligands.…”

Asymmetric bis-PNP pincer complexes of zirconium and hafnium – a measure of hemilability

“…The synthesis of [PNP Ph ]H and its subsequent lithiation has been previously reported and is analogous to the alkylated congeners. 12,17,[20][21][22][23][24][25][26][27][28][29] A variety of lithium bases have been used in the literature but in our hands the conditions outlined in Scheme 1 and the ESI † worked best. The lithium salt could be isolated from an Et 2 O solution in very good yield (86%), but even after prolonged exposure to dynamic vacuum ( Fig.…”