Aluminum-Stabilized Low-Spin Iron(II) Hydrido Complexes of 1,4,7-Trimethyl-1,4,7-triazacyclononane

Abstract

We investigated herein the reactions of (Me3tacn)FeCln (1a: n = 3, 1b: n = 2) with common aluminum hydride reagents and a bulky dihydridoaluminate {Li(ether)2}{Al(OC6H3-2,6-(t)Bu2)}(μ-H)2, which yielded the diamagnetic hydrido complexes 2-4 containing Fe(II) and Al(III). In particular, the use of divalent 1b afforded excellent isolated yields. The structures of 2-4 were determined using spectroscopic and crystallographic analyses. The crystal structures showed distorted octahedral Fe centers and fairly short Fe-Al distances [2.19-2.24 Å]. The structures of cation moiety 2 and neutral complex 4 were further probed using DFT calculations, which indicated a stable low-spin Fe(II) state and strongly electron-donating nature of the (Me3tacn)FeH3 fragment toward the Al(III) center.

Chromium(II) Alkylcyclopentadienyl Complexes with Carbon or Hydride Donor Ligands

“…The short distance is in the case of the analogous molybdenum gallane derivative [(OC) 4 Mo­(μ-H) 2 Ga­(L)] “thought primarily to be due to geometric constraints imposed by the four-membered Mo­(μ-H) 2 Ga ring”. Even shorter are Al–Fe contacts close to 219 pm in the trimethylcyclononane complex [{(Me 3 tacn)­Fe­(μ-H) 3 } 2 Al] + with three bridging hydride ligands between the octahedral AlH 6 core and each low-spin Fe­(II) ion …”

“…Even shorter are Al−Fe contacts close to 219 pm in the trimethylcyclononane complex [{(Me 3 tacn)Fe-(μ-H) 3 } 2 Al] + with three bridging hydride ligands between the octahedral AlH 6 core and each low-spin Fe(II) ion. 67 The geometry of the four hydride ligands around the aluminum atoms of the tetramer 17 looks puzzling, because they are all assembled within one hemisphere around their aluminum partner (Figure 17). This arrangement looks like the molecular structure of SF 4 with one nonbonding pair of electrons in its equatorial plane.…”

Molecular Main Group Metal Hydrides

“…Titanocene and zirconocene alkyne and cumulene complexes, Cp 2 M­(RCCR) and Cp 2 M­(RCCCCR) (M = Ti, Zr) have long been known to react with aluminum hydrides (such as HAl i Bu 2 , DIBAL-H) to give various heterocycles bearing M–H–Al bridging units, and several reports in the past two decades have continued this work. − In a more unusual reaction, when the decamethyl­ziroconocene complex Cp* 2 Zr­(Me 3 SiCCSiMe 3 ) was combined with HAl i Bu 2 , compound 586 was observed, arising from an intramolecular C–H bond activation . Ruthenium and iron also known to form bridging aluminates akin to the Group 3/4 metals. − …”

Magnesium, zinc, aluminium and gallium hydride complexes of the transition metals

“…The low-spin ground state structure of 100 has been thoroughly investigated by computational methods. 174 The frontier molecular orbitals of 100 do not support bonding between Fe and Al, and the authors present the complex as being dominated by a strong donation of electron density from a {Fe–H} – fragment to {AlX 2 } + (X = O-C 6 H 3 -2,6- t Bu). This conclusion is a good general insight into the electronic structure of the late-TM(H) n Al bonding, the dimeric examples of which are summarised in 95–96 .…”