Neutral Fe(iv) alkylidenes, including some that bind dinitrogen

Abstract

Neutral, formally Fe(IV) alkylidene species are sought as plausible olefin metathesis catalysts, and the synthesis of several is described herein. The complexes are prepared via nucleophilic attack (Nu = MeLi, PhCH2K, 2-picolyllithium, Me2PCH2Li, MePhPCH2Li, Ph2PCH2Li) at the imine of cationic [mer-{κ-C,N,C-(C6H4-yl)-2-CH=N(2-C6H4-C(iPr)=)}Fe(PMe3)3][B(3,5-CF3-C6H3)4]. In contrast, MeMgCl and mesityllithium displaced and deprotonated bound PMe3, respectively. Structural details are provided for mer-{κ-C,N,C-(C6H4-yl)-2-CH(Bn)N(2-C6H4-C(iPr))}Fe{trans-(PMe3)2}N2 and {κ-C,N,C,P-(C6H4-yl)-2-CH(CH2PMe2)N(2-C6H4-C(iPr)=)}Fe(PMe3)2.

A Chiral Macrocyclic Tetra‐N‐Heterocyclic Carbene Yields an “All Carbene” Iron Alkylidene Complex

“…the carbene between 305 and 385 ppm, and are consistent with ad iamagnetic NMR. [52][53][54][55] Only examples by Floriani [56,57] and Chirik [45] have paramagnetic NMRs with this same diazoalkane fragment. As ingle crystal X-ray analysiso f9 showedt he first "all carbene" iron complex ( Figure 5).…”

“…The Fe=Cd istance is 1.814(4) ,m aking it slightly longert han examples with iron porphyrin complexes, [54,58,59] but considerably shorter than ex-amplesbyC hirik and Wolczanski. [45,55] In recent years, the oxidation state in systemss imilar to 9 has been heavily debated. [59,60] The conventionalk nowledge was that in isostructural porphyrin compounds, the iron center goes from iron(II) to iron(IV) upon formation of alkylidene.…”

Fe(IV) alkylidenes are actually Fe(II), and a related octahedral Fe(II) “alkylidene” is a conjugated vinyl complex

“…As a consequence, neutral iron alkylidenes were produced via nucleophilic attack at the imines of cation 1 (4-R), including several with a pendant side-arm (5-R,R'). [8] No consequential reactivity was observed for these derivatives, again despite evidence of phosphine dissociation, which is likely to be necessary prior to alkylidene reactivity.…”

“…In the course of investigating Fe(IV) alkylidenes in these laboratories, [7,8] metathesis or cyclopropanation was observed. [7] In first row transition metal cationic complexes, the 3d orbitals can be contracted to the extent that metal-ligand multiple bonding is diminished.…”

“…One obvious disparity between complexes that enable olefin metathesis, and those in Scheme 1, are the substituents on the alkylidenes. Virtually every metal-carbon double bond that reacts with an olefin is of the type M=C(H)R. [14] In this manuscript, attempts at generating iron alkylidenes with hydrogen substituents are described, and two of the alkylidenes in Scheme 1, cation [mer-{-C,N,C-(2-C 6 H 4 )CH=N(1,2-C 6 H 4 )C( i Pr)=}Fe(PMe 3 ) 3 ][BAr F 4 ] (1, BAr F 4 -= [B(3,5-(CF 3 ) 2 -C 6 H 3 ) 4 ] -), [7] and neutral mer-{-C,N,C-(2-C 6 H 4 )CH(Bn)N(1,2-C 6 H 4 )C( i Pr)=}Fe(PMe 3 ) 3 (4-Bn) [8] are evaluated via computation. In addition, an unusual dianionic chelate was found to distribute charge rather than form an Fe(II) alkylidene, as a testament to Hoffmann's calculations, [6] and the ability of pyridine-imine/azaallyl species to stabilize charge.…”