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.
Iron-hydride and iron-boryl complexes supported by a pyrrole-based pincer ligand, tBuPNP (PNP = anion of 2,5-bis(di-tert-butylphosphinomethyl)pyrrole), were employed for a detailed mechanistic study on the hydroboration of internal alkynes. Several novel complexes were isolated and fully characterized, including iron-vinyl and iron-boryl species, which represent likely intermediates in the catalytic hydroboration pathway. In addition, the products of alkyne insertion into the Fe–B bond have been isolated and structurally characterized. Mechanistic studies of the hydroboration reaction favor a pathway involving an active iron-hydride species, [FeH( tBuPNP)], which readily inserts alkyne and undergoes subsequent reaction with hydroborane to generate product. The iron-boryl species, [Fe(BR2)( tBuPNP)] (R2 = pin or cat), was found to be chemically competent, although its use in catalysis entailed an induction period whereby the iron-hydride species was generated. Stoichiometric reactions and kinetic experiments were performed to paint a fuller picture of the mechanism of alkyne hydroboration, including pathways for catalyst deactivation and the influence of substrate bulk on catalytic efficacy.
Iron(II) phenylacetylide complexes of the pyrrole-based pincer ligands, R PNP (R = Cy, t Bu; PNP = anion of 2,5bis(dialkylphosphinomethyl)pyrrole), have been prepared and characterized. Acetylide compounds of both ligands exist as squareplanar, intermediate-spin (S = 1) species in solution, but additional Lewis base (pyridine, CO, or bipyridine) coordination is required for the isolation of [Fe(CCPh)( Cy PNP)]. The iron acetylide complex of Cy PNP readily inserts phenylacetylene to produce the enynyl species, [Fe(E-C{Ph}CH{CCPh})( Cy PNP)], representing an intermediate along the pathway to alkyne dimerization. Catalytic dimerization reactions with the Cy PNPFe system lead to a mixture of E-and Z-enyne products, indicating interconversion between the iron E-and Z-enynyl species during turnover. Insertion reactions employing 1,4-diphenylbutadiyne support a pathway for interconversion between enynyl rotamers involving rearrangement of the iron enynyl species. The solid-state structures of several acetylide and enynyl species are reported, and their chemistry is discussed in the context of mechanisms for catalytic alkyne dimerization.
The synthesis and characterization of organic compounds with unusual atom or functional group connectivity is one of the main driving forces in the discovery of new synthetic methods that has raised the interest of chemists for many years. Polycarbonyl compounds are such compounds wherein multiple carbonyl groups are directly juxtaposed and influence each other’s chemical reactivity. While 1,2-dicarbonyl or 1,2,3-tricarbonyl compounds are well-known in organic chemistry, the 1,2,3,4-tetracarbonyl motif remains barely explored. Herein, we report on the synthesis of such 1,2,3,4-tetracarbonyl compounds employing a synthetic strategy that involves C-nitrosation of enoldiazoacetates, while the diazo functional group remains untouched. This strategy not only leverages the synthesis of 1,2,3,4-tetracarbonyl compounds to an unprecedented level, it also accomplishes the synthesis of 1,2,3,4-tetracarbonyl compounds, wherein each carbonyl group is orthogonally masked. Combined experimental and theoretical studies provide an understanding of the reaction mechanism and rationalize the formation of such 1,2,3,4-tetracarbonyl compounds.
Iron(ii) boryl complexes of a pincer ligand are accessible through metathesis reactions between B2R2 and phenoxide precursors. The compounds demonstrate insertion chemistry with several unsaturated substrates leading to new organometallic complexes.
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