Iron-Catalyzed Dehydrocoupling/Dehydrogenation of Amine–Boranes

Abstract

The readily available iron carbonyl complexes, [CpFe(CO)2]2 (1) and CpFe(CO)2I (2) (Cp = η-C5H5), were found to be efficient precatalysts for the dehydrocoupling/dehydrogenation of the amine-borane Me2NH·BH3 (3) to afford the cyclodiborazane [Me2N-BH2]2 (4), upon UV photoirradiation at ambient temperature. In situ analysis of the reaction mixtures by (11)B NMR spectroscopy indicated that different two-step mechanisms operate in each case. Thus, precatalyst 1 dehydrocoupled 3 via the aminoborane Me2N═BH2 (5) which then cyclodimerized to give 4 via an off-metal process. In contrast, the reaction with precatalyst 2 proceeded via Me2NH-BH2-NMe2-BH3 (6) as the key intermediate, affording 4 as the final product after a second metal-mediated step. The related complex Cp2Fe2(CO)3(MeCN) (7), formed by photoirradiation of 1 in MeCN, was found to be a substantially more active dehydrocoupling catalyst and not to require photoactivation, but otherwise operated via a two-step mechanism analogous to that for 1. Significantly, detailed mechanistic studies indicated that the active catalyst generated from precatalyst 7 was heterogeneous in nature and consisted of small iron nanoparticles (≤10 nm). Although more difficult to study, a similar process is highly likely to operate for precatalyst 1 under photoirradiation conditions. In contrast to the cases of 7 and 1, analogous experimental studies for the case of photoactivated Fe precatalyst 2 suggested that the active catalyst formed in this case was homogeneous. Experimental and computational DFT studies were used to explore the catalytic cycle which appears to involve amine-borane ligated [CpFe(CO)](+) as a key intermediate.

Electronic Structure and Magnetic Anisotropy of an Unsaturated Cyclopentadienyl Iron(I) Complex with 15 Valence Electrons

“…[43][44][45][46][47][48] This reaction came to our attention through arecent report by Manners and co-workers,who used the carbonyl complexes [CpFe(CO) 2 I],[ CpFe(CO) 2 ] 2 ,a nd [Cp 2 Fe 2 (CO) 3 (CH 3 CN)] as catalyst precursors. [43] In the presence of 1 (5 mol %), DMAB was completely converted within 7h into tetramethyl-1,3-diaza-2,4-diboretane (4)a nd bis(dimethylamino)borane (5), which were formed in a9 :1 ratio according to 11 BNMR spectroscopy (Scheme 2a nd Figures S3-S5). Te tramethyl-1,3-diborazane 6 was detected as an intermediate.C atalyst 1 remained active after conversion was complete as shown by the addition of fresh substrate ( Figures S9 and S10).…”

“…In an initial study of the reactivity and the catalytic properties of 1,wefound that this complex does not react with H 2 and N 2 under ambient conditions,b ut it is an active (pre)catalyst for the dehydrogenation of N,N-dimethylamineborane (DMAB). [43][44][45][46][47][48] This reaction came to our attention through arecent report by Manners and co-workers,who used the carbonyl complexes [CpFe(CO) 2 I],[ CpFe(CO) 2 ] 2 ,a nd [Cp 2 Fe 2 (CO) 3 (CH 3 CN)] as catalyst precursors. [43] In the presence of 1 (5 mol %), DMAB was completely converted within 7h into tetramethyl-1,3-diaza-2,4-diboretane (4)a nd bis(dimethylamino)borane (5), which were formed in a9 :1 ratio according to 11 BNMR spectroscopy (Scheme 2a nd Figures S3-S5).…”

Elektronische Struktur und magnetische Anisotropie eines ungesättigten Cyclopentadienyleisen(I)‐Komplexes mit 15 Valenzelektronen

“…[43][44][45][46][47][48] [43] Unter Verwendung von 1 (5 Mol-%) wurde DMAB innerhalb von 7hvollständig zu Tetramethyl-1,3-diaza-2,4-diboretan (4)und Bis(dimethylamino)boran (5) umgesetzt. Hierbei zeigte sich, dass die Verbindung unter Normalbedingungen in C 6 D 6 -Lçsung nicht mit H 2 und N 2 reagiert.…”

“…[43][44][45][46][47][48] Dieses Ergebnis ist ein gutes Vorzeichen fürd ie zukünftige Entwicklung weiterer Anwendungen von 1 und von verwandten Cyclopentadienyleisen(I)-Komplexen in der Synthese und Katalyse. Als Folge davon zeigt 1 eine langsame Relaxation der Magnetisierung sowohl in An-als auch in Abwesenheit eines externen Magnetfeldes.D arüber hinaus katalysiert 1 die Dehydrierung von Dimethylamin-Boran, eine Reaktion, fürdie nur wenige Eisenkatalysatoren bekannt sind.…”

“…Als Folge davon zeigt 1 eine langsame Relaxation der Magnetisierung sowohl in An-als auch in Abwesenheit eines externen Magnetfeldes.D arüber hinaus katalysiert 1 die Dehydrierung von Dimethylamin-Boran, eine Reaktion, fürdie nur wenige Eisenkatalysatoren bekannt sind. [43][44][45][46][47][48] Dieses Ergebnis ist ein gutes Vorzeichen fürd ie zukünftige Entwicklung weiterer Anwendungen von 1 und von verwandten Cyclopentadienyleisen(I)-Komplexen in der Synthese und Katalyse.…”