9-Ferrocenyl-9-borafluorene—A Redox-Switchable Main Group Lewis Acid

Abstract

9-Ferrocenyl-9-borafluorene (FcBC12H8; 3) is readily accessible from FcHgCl and BrBC12H8. The 9-borafluorenyl substituent is bent out of the plane of the cyclopentadienyl ring toward the iron center with corresponding dip angles of α* = 25.5° and 17.1° for the two crystallographically independent molecules in the asymmetric unit of 3. The degree of ligand bending is one of the highest ever measured for a ferrocenylborane derivative, thereby indicating a pronounced Fe−B interaction. In line with that, oxidation of the Fe center to its FeIII state using AgBF4 results in a substantial decrease of the dip angle in the ferricinium species [3]BF4 (α* = 6.3°). Compound 3 can therefore be considered a rare example of a redox-switchable main group Lewis acid. The stability of the molecular framework of 3 is sufficiently high to allow for the isolation and structural characterization of its pyridine-N-oxide adduct (4). Moreover, reaction of 3 with 0.5 equiv of 1,1′-dilithioferrocene (1,1′-fcLi2 × 2/3 tmeda) and subsequent limited access of O2 leads to the formation of the trinuclear mixed-valence complex Li[Fc-(BC12H8)-fc-(BC12H8)-Fc], which has been structurally characterized as 12-crown-4 adduct (Li(12-c-4)2[6]).

Anionic Organoboranes: Delicate Flowers Worth Caring for

“…In an attempt to design an even better preorganized electron trap (4), [24] our group replaced the 1,8-naphthalenediyl by a less rigid 2,2′-biphenyldiyl backbone and the two twisted diphenylboryl fragments by strongly Lewis acidic, planar 9-borafluorenyl (BFlu) moieties. [25,26] This way, the B•••B distance of 4 is adjustable to the requirements of the aimed-for B•B bond with little energy penalty by mere rotations about B-C and C-C axes. Moreover, a closer approach of the two boron atoms now imposes less of a steric burden.…”

“…[48] It was plausibly assumed that in a first general step the B=B double bond of [11] 2acts as a π nucleophile toward one molecule of MeI. The methylation of one boron atom, accompanied by two Wagner-Meerwein-type rearrangements, affords the B(sp 2 )-B(sp 3 ) intermediate [25] -, formally an adduct between [:BFlu]and MeBFlu (16) and a close relative of [10′′] -(see Scheme 11). Accompanied by B-B-bond cleavage, two different scenarios can occur: (i) If MeI is available in sufficient supply to trap [:BFlu]in a nucleophilic substitution reaction, the observed second equivalent of 16 is generated.…”

1,1′‐Diborylferrocenes from [2]Boraferrocenophanes by Boron–Boron Exchange

“…The compounds Fc(BBr 2 ) n (n = 1, 2, 4) [19] (R = H, Me) [20] as well as the title compounds 5 and 9 display corresponding values for α* between 0.9°and 18.9°t hat are indicative of weak interactions. The strongest iron boron interactions as yet were reported for 9-ferrocenyl-9-borafluorene (25.5°) [21] and for the ferrocenylborole [FcBC 4 Ph 4 ] (29.4°). [22] (9), respectively with unexpected degradation of the -BNMe 2 -BNMe 2 -bridge.…”

Synthesis and oxidation of hexafluoroisopropoxyborylferrocenes

“…Ferrocenylboranes are redox-active and can be oxidized to the respective ferrocinium compounds that should exert higher Lewis acidity. The first structural analysis of a redox-switchable system was published only recently (2008) for 9-ferrocenyl-9-borafluorene [29]. Interestingly, the higher Lewis acidity of this compound is here accompanied by smaller values of the dip angle α*.…”