Iminoborylene complexes: evaluation of synthetic routes towards BN-allenylidenes and unexpected reactivity towards carbodiimides

Abstract

The synthetic and reaction chemistries of cationic iminoborylene complexes [LnM=B=N=CR2](+), which feature a unique heterocumulene structure, have been systematically investigated. Precursors of the type CpFe(CO)2B(Cl)NCAr2 (Ar = p-Tol/Mes, 5c/d) have been generated by B-centred substitution chemistry using CpFe(CO)2BCl2 and suitable lithiated ketimines - a reaction which is found to be highly sensitive to the steric bulk at both the metal fragment and the ketimino group. Carbonyl/phosphine exchange (using PCy3 or PPh3), followed by halide abstraction allows for the generation of the cationic iminoborylenes [CpFe(PR3)(CO)(BNCAr2)](+)[BAr(X)4](-) (R = Cy, Ar = p-Tol/Mes, 12c/d; R = Ph, Ar = Mes, 13d; Ar(X) = 3,5-X2C6H3 where X = Cl, CF3) which have been characterized spectroscopically and by X-ray crystallography. The reactivity of these iminoborylene systems towards a range of nucleophiles and unsaturated substrates has been investigated. The latter includes the first examples of M=B metathesis reactivity with a carbodiimide, and results in Fe=B cleavage and formation of the isonitrile complexes [CpFe(PCy3)(CO)(CNR)](+)[BAr(Cl)4](-) (R = (i)Pr/Cy, 16/17).

Complexation and Release of N-Heterocyclic Carbene-Aminoborylene Ligands from Group VI and VIII Metals

“…Borylenes are typically actor ligands when coordinated to a transition metal center. Two- and three-coordinate terminal borylene ligands are electronically unsaturated, − and undergo a wide array of insertion, oxidation, reduction, and addition reactions involving the M–B unit. − Furthermore, the addition of ancillary ligands, including CO and isonitrile, to metalloborylenes has emerged as an important method for the metal-templated assembly and release of novel doubly base-stabilized borylene species. − Outside of the coordination sphere of a transition metal complex, free borylenes are most commonly stabilized via the coordination of two donor ligands to a monovalent boron center (Figure ; A , B ), − which are electronically analogous to “carbone”-type ligands. − The only examples of isolable two-coordinate borylenes are “push–pull” aminoborylenes bearing π-acidic N-heterocyclic carbene (NHC) ligands (Figure ; C ). , Such species are isoelectronic to Fischer carbenes and were first reported by Bertrand, Stephan and co-workers in 2014. While having a linear ground-state reminiscent of NBC heteroallenes, − these species have been demonstrated to bind CO and cleave H 2 , reactions evocative of cyclic (alkyl)­(amino)­carbenes (CAACs) and other electrophilic carbene ligands, ,, and which further support the emerging metallomimetic reactivity of low-valent boron. , Nevertheless, unlike tricoordinate borylenes, , these dicoordinate borylene species are not known to act as nucleophiles to transition metal or main group Lewis acids.…”

Hetero-trimetallic complexes comprising bridging boryl and borylene ligands: an experimental and theoretical study

“…The stabilization of these borylene ligands in the coordination sphere of transition metals is also well-studied by Aldridge and co-workers. 17 In recent years, the field of metallaborane chemistry where borylene ligands are part of the cluster has developed extensively owing to its unique bonding features; mainly metallaborane clusters comprising triply bridged borylene unit have enriched the area of borylene chemistry satisfactorily over the past 20 years. Fehlner and co-workers have isolated the first trimetallic triply bridging borylene complex [(CpCo) 3 (μ 3 -PPh)(μ 3 -BPh)] in 1988 using [Cp*Co(PPh 3 ) 2 ].…”