Iron Precatalysts with Bulky Tri(tert‐butyl)cyclopentadienyl Ligands for the Dehydrocoupling of Dimethylamine‐Borane

Abstract

In an attempt to prepare new Fe catalysts for the dehydrocoupling of amine-boranes and to provide mechanistic insight, the paramagnetic Fe dimeric complex [Cp'FeI] (1) (Cp'=η -((1,2,4-tBu) C H )) was used as a precursor to a series of cyclopentadienyl Fe and Fe mononuclear species. The complexes prepared were [Cp'Fe(η -Tol)][Cp'FeI ] (2) (Tol=C H Me), [Cp'Fe(η -Tol)][BAr ] (3) (BAr =[B(C H (m-CF ) ) ] ), [N(nBu) ][Cp'FeI ] (4), Cp'FeI (5), and [Cp'Fe(MeCN) ][BAr ] (6). The electronic structure of the [Cp'FeI ] anion in 2 and 4 was investigated by SQUID magnetometry, EPR spectroscopy and ab initio Complete Active Space Self Consistent Field-Spin Orbit (CASSCF-SO) calculations, and the studies revealed a strongly anisotropic S=2 ground state. Complexes 1-6 were investigated as catalysts for the dehydrocoupling of Me NH⋅BH (I) in THF at 20 °C to yield the cyclodiborazane product [Me N-BH ] (IV). Complexes 1-4 and 6 were active dehydrocoupling catalysts towards I (5 mol % loading), however 5 was inactive, and ultra-violet (UV) irradiation was required for the reaction mediated by 3. Complex 6 was found to be the most active precatalyst, reaching 80 % conversion to IV after 19 h at 22 °C. Dehydrocoupling of I by 1-4 proceeded via formation of the aminoborane Me N=BH (II) as the major intermediate, whereas for 6 the linear diborazane Me NH-BH -NMe -BH (III) could be detected, together with trace amounts of II. Reactions of 1 and 6 with Me N⋅BH were investigated in an attempt to identify Fe-based intermediates in the catalytic reactions. The σ-complex [Cp'Fe(MeCN)(κ -H BH⋅NMe H][BAr ] was proposed to initially form in dehydrocoupling reactions involving 6 based on ESI-MS (ESI=Electrospray Ionisation Mass Spectroscopy) and NMR spectroscopic evidence. The latter also suggests that these complexes function as precursors to iron hydrides which may be the true catalytic species.

Syntheses and Reactivity of Piano-Stool Iron Complexes of Picolyl-Functionalized N-Heterocyclic Carbene Ligands

“…When the reaction progress was monitored with 11 B­{ 1 H} NMR spectroscopy, 51% of the DMAB was converted to I and the linear intermediate Me 2 NH–BH 2 –NMe 2 –BH 3 ( III ) in a 1:1.2 ratio within 7 min, along with the formation of a small amount of II ; the DMAB was fully consumed within 1 h, forming mostly I along with small amounts of II and III . The catalytic activity qualifies 2 as being one of the most active Fe-based homogeneous catalysts for the dehydrogenation of DMAB . Compounds 4 and 5 display catalytic activities and product distributions comparable to those of complex 2 toward the dehydrocoupling of DMAB under the same conditions (1 mol % of 4 or 0.5 mol % of 5 ; 1 mol % of Fe; see the Supporting Information).…”

Redox-Active, Boron-Based Ligands in Iron Complexes with Inverted Hydride Reactivity in Dehydrogenation Catalysis

“…Me 2 NH-BH 3 was selected as the substrate for catalytic dehydrogenation reactions, due to the simplicity of different known reaction products and the possibility to gain mechanistic insights. In recent years, a number of iron-based catalysts was reported for the dehydrogenation of amine and ammonia boranes, ranging from simple iron phosphine complexes, , such as [(Me 3 P) 3 Fe­(H)­(H 2 CPMe 2 )], to cyclopentadienyl complexes, − pincer-type complexes with different kinds of ligands, − monomeric , and dimeric iron­(I) complexes, complexes with β-diketiminate and P 2 N 2 -type ligands …”

“…As previous reports on iron catalysts for the dehydrogenation of Me 2 NH-BH 3 provided evidence for the formation of catalytically active nanoparticles, ,, we performed a number of poisoning experiments to get further insight in the nature of the active species. Addition of mercury to the catalytically active reaction mixture did not result in a reduced activity for 3a and 3c (entries 6 and 9).…”

Ammonium and Phosphonium Salts Containing Monoanionic Iron(II) Half-Sandwich Complexes [Fe(η5-Cp*)X2]− (X = Cl − I)

“…Inorganics 2023, 11, 437 2 of 11 halido-bridged dimers [{Fe(η 5 -Cp')(µ-X)} 2 ], according to single-crystal X-ray diffraction (XRD) (Cp' = C 5 iPr 5 , X = Br; Cp' = C 5 HiPr 4 , X = Br, I; Cp' = C 5 H 2 -1,2,4-tBu 3 , X = Br, I; Cp' = C 5 (p-C 6 H 4 Et) 5 , X = Br) [26][27][28][29]. Manners and Walter independently found that [{Fe(η 5 -C 5 H 2 -1,2,4-tBu 3 )(µ-I)} 2 ] undergoes heterolytic cleavage in toluene, affording [Fe(η 5 -C 5 H 2 -1,2,4-tBu 3 )(C 7 H 8 )] + and [Fe(η 5 -C 5 H 2 -1,2,4-tBu 3 )I 2 ] − [30,31]. In the same vein, deaggregation of [{Fe(η 5 -C 5 H 2 -1,2,4-tBu 3 )(µ-I)} 2 ] was achieved through reaction with NR 4 I (R = Et, nBu), giving rise to the formation of NR 4 [Fe(η 5 -C 5 H 2 -1,2,4-tBu 3 )I 2 ] [30,31].…”

“…Manners and Walter independently found that [{Fe(η 5 -C 5 H 2 -1,2,4-tBu 3 )(µ-I)} 2 ] undergoes heterolytic cleavage in toluene, affording [Fe(η 5 -C 5 H 2 -1,2,4-tBu 3 )(C 7 H 8 )] + and [Fe(η 5 -C 5 H 2 -1,2,4-tBu 3 )I 2 ] − [30,31]. In the same vein, deaggregation of [{Fe(η 5 -C 5 H 2 -1,2,4-tBu 3 )(µ-I)} 2 ] was achieved through reaction with NR 4 I (R = Et, nBu), giving rise to the formation of NR 4 [Fe(η 5 -C 5 H 2 -1,2,4-tBu 3 )I 2 ] [30,31]. The only other closely related compound is [Fe(η 5 -C 5 H 2 -1,2,4-tBu 3 )(µ-Br) 2 Na(DME) 2 ], which Sitzmann obtained through serendipity and in trace amounts only in the preparation of [{Fe(η 5 -C 5 H 2 -1,2,4-tBu 3 )(µ-Br)} 2 ] from [FeBr 2 (DME)] and the corresponding sodium cyclopentadienide in DME [32].…”