Synthesis and Reactivity of Manganese(II) Complexes Containing N-Heterocyclic Carbene Ligands

Abstract

A series of manganese(II) complexes containing arylsubstituted N-heterocyclic carbene (NHC) ligands have been synthesized and characterized. Chloride complexes of Mn(II) containing the NHC ligands 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) and 1,3dimesitylimidazol-2-ylidene (IMes) were prepared in straightforward fashion by direct carbene addition to MnCl 2 (THF) 1.6 . These complexes exist as chloride-bridged dimers in solution of formula [Mn 2 Cl 2 (μ-Cl) 2 (NHC) 2 ]. The monomeric complex [MnCl 2 (IMes) 2 ] has also been prepared and structurally characterized, although NMR studies are consistent with facile dissociation of one of the IMes ligands in solution.[Mn 2 Cl 2 (μ-Cl) 2 (IPr) 2 ] serves as a precursor to dimeric alkyl and aryl compounds of Mn(II) including [Mn 2 R 2 (μ-Cl) 2 (NHC) 2 ] (R = Bn, o-tolyl, and Ph) and the bridging methyl complex [Mn 2 Me 2 (μ-Me) 2 (IPr) 2 ]. Stoichiometric reactions of these hydrocarbyl species with bromocyclohexane demonstrate that they are not chemically competent in C−C coupling reactions involving alkyl electrophiles.

Stereoselective Transfer Semi‐Hydrogenation of Alkynes to E‐Olefins with N‐Heterocyclic Silylene–Manganese Catalysts

“…In all complexes, the Mn II centers are in a distorted tetrahedral coordination environment. The geometric situation is similar to those observed in the NHC‐Mn II complexes [MnCl 2 (C{N( i Pr)C(Me)} 2 ) 2 ] and [MnCl 2 (IMes) 2 ] (IMes=1,3‐dimesitylimidazol‐2‐ylidene) . However, akin to the situation in Fe II and Ge II complexes stabilized by L1 , the pyridine N atom in [L1]MnCl 2 is reluctant in coordinating to the Mn atom as indicated by the relatively long Mn(1)–N(1) distance of 3.259 Å.…”

“…The geometrics ituationi ss imilar to those observed in the NHC-Mn II complexes [MnCl 2 (C{N(iPr)C(Me)} 2 ) 2 ] [12] and [MnCl 2 (IMes) 2 ]( IMes = 1,3-dimesitylimidazol-2-ylidene). [13] However, akin to the situation in Fe II and Ge II complexes stabilized by L1, [2f, 14] the pyridine Na tom in [L1]MnCl 2 is reluctant in coordinating to the Mn atom as indicatedb yt he relativelyl ong Mn(1)-N(1) distance of 3.259 . This is in contrast to PNP pincer-type Mn II complexes with much shorter N(py)-Mn distances, [9d, 15] which can be rationalized by the stronger s-donors trengtho fS i II versus P III towards Mn.…”

Recent progress on NHC-stabilized early transition metal (group 3–7) complexes: Synthesis and applications

“…177 Solid-state magnetic susceptibility studies revealed a high spin state of 5/2, which corrects the previous analyses that claimed an intermediate spin of 3/2 (attributed to impurities or partial decomposition of the compound). 178 Reaction of MnCl2 in THF with free NHC affords the chloride-bridged dimer 179,180 These compounds are precursors to dimeric alkyl compounds dimeric alkyl compounds such as 172. Compound 172 may react with Si(OEt)4 to selectively form MeSi(OEt)3.…”

Manganese-N-heterocyclic carbene (NHC) complexes – An overview

“…More recently, Tonzetich and co-workers reported a series of manganese(II) complexes (Fig. 16) [64] that can be used as precursors for alkyl and aryl complexes. They also tested the dimeric complex in CAC cross-coupling, but with disappointing results.…”

“…A few years later, Tonzetich and coworkers reported the synthesis of the alkyl complex [Mn(IPr)(CH 2 -SiMe 3 ) 2 ] from the corresponding Cl-bridged dimer complex (Fig. 16) [64].…”