Английская Википедия:(Pentamethylcyclopentadienyl)aluminium(I)
(Pentamethylcyclopentadienyl)aluminium(I) is an organometallic compound with the formula Al(CШаблон:SubMeШаблон:Sub) ("Me" is a methyl group; CHШаблон:Sub). The compound is often abbreviated to AlCp* or Cp*Al, where Cp* is the pentamethylcyclopentadienide anion (CШаблон:SubMeШаблон:SubШаблон:Sup). Discovered in 1991 by Dohmeier et al.,[1] AlCp* serves as the first ever documented example of a room temperature stable monovalent aluminium compound. In its isolated form, Cp*Al exists as the tetramer [Cp*Al]Шаблон:Sub, and is a yellow crystal that decomposes at temperatures above 100 °C but also sublimes at temperatures above 140 °C.[1][2]
Synthesis
The earliest documented synthesis and characterization of Cp*Al was by Dohmeier et al. in 1991,[1] where four equivalents of AlCl in toluene/diethyl ether is reacted with two equivalents of 2[Mg(Cp*)Шаблон:Sub] to give [Cp*Al]Шаблон:Sub as yellow crystals:
Despite the above synthetic scheme successfully producing tetrameters of [Cp*Al]Шаблон:Sub at reasonable yields (44%), its use of AlCl proved problematic, as AlCl synthesis requires harsh conditions and its reactive nature makes storage a challenge. As such, more facile ways of synthesising the [Cp*Al]Шаблон:Sub tetramer were discovered, and required the reduction of Cp*AlXШаблон:Sub (X = Cl, Br, I) by a metal (K when X = Cl) or a metal alloy (Na/K alloys when X = Br, I):[3][4][5][6][7]
More exotic ways of synthesizing [Cp*Al]Шаблон:Sub include the controlled disproportionation of an Al(II) dialane into constituent Al(I) and Al(III) products. For example, reacting dialane [Cp*AlBr]Шаблон:Sub with a Lewis base such as pyridine the Lewis base stabilized [Cp*AlBrШаблон:Sub] and [Cp*Al]Шаблон:Sub.[8]
Monomeric Cp*Al has also been isolated in a solid Ar matrix by heating [Cp*Al]Шаблон:Sub in toluene to 133 °C and spraying the resultant vapours with Ar onto a copper block kept at 12 K.[9]
Structure and bonding
X-ray crystallographic data determined Cp*Al to exist exclusively as a tetramer in its solid state. This tetramer, [Cp*Al]Шаблон:Sub, consists of an AlШаблон:Sub tetrahedron, and the Cp* rings are [[Hapticity|ŋШаблон:Sup-coordinated]] to the aluminium(I) cation such that the planes of the CШаблон:SubMeШаблон:SubШаблон:Sup rings are approximately parallel to the opposite base of the AlШаблон:Sub tetrahedron.[1] The perpendicular distance between Al and the Cp* ring was determined through crystallography to range from 199.7 to 203.2 pm, with a mean value of 201.5 pm.[1] The Al-Al bond in [Cp*Al]Шаблон:Sub is 276.9 pm, which is slightly shorter than that of metallic aluminium, which has an Al-Al bond length of 286 pm.[1] Additionally, the Al-Al bond in [Cp*Al]Шаблон:Sub is significantly shorter than other oligomeric and polymeric Group III M(I)-ŋШаблон:Sup-Cp* compounds such as octahedral [InCp*]Шаблон:Sub (394, 336 pm), dimeric [InCp*]Шаблон:Sub (363.1 pm), and polymeric [TlCp*] (641 pm), indicating a significantly larger interaction between aluminium atoms in [Cp*Al]Шаблон:Sub than monovalent Cp* compounds of In(I) and Tl(I).[3] Additional characterization that has been performed include Raman spectroscopy, which detected a Raman active breathing vibration (AШаблон:Sub, 377 cm-1) of the AlШаблон:Sub tetrahedron in [Cp*Al]Шаблон:Sub.[1]
Natural bond orbital (NBO) analysis of [Cp*Al] and [Cp*Al]Шаблон:Sub using B3LYP/6-31G(d,p) calculated the average charge transfer per Cp* fragment to an Al atom to be 0.657 and 0.641 respectively. This is slightly higher than the charge transfers calculated on [CpAl] and [Cp*Al]Шаблон:Sub (0.630 and 0.591 respectively). NBO calculation of the HOMO-LUMO gap in [Cp*Al] also revealed a significant decreasing in the tetrameric [Cp*Al]Шаблон:Sub complex compared to the monomeric [Cp*Al] (4.36 compared to 5.49), which is consistent with density functional theory calculations of analogous systems including superatom complexes of gold, aluminium and gallium.[10] Atoms in molecules (AIM) calculations calculate the Al-Al bonding to be metallic.[11] Stabilization of [Cp*Al]Шаблон:Sub relative to [CpAl]Шаблон:Sub is thought to arise from addition of H-H interactions on the methyl groups attached to the Cp* ligand as opposed to the increased Al-Al bonding interactions.[11]
Despite its typically tetrameric form, the monomer Cp*Al has been isolated and studied in the gas-phase using gas-phase electron diffraction. In its gaseous monomeric form, the perpendicular distance between the Al to the Cp* ring was calculated to be 206.3(8) pm, which is slightly longer than tetrameric [Cp*Al]Шаблон:Sub.[2]
Reactivity
When isolated in a solid [[Hydrogen|HШаблон:Sub]] doped Ar matrix, monomeric Cp*Al has shown to form the hydride species HШаблон:SubCp*Al upon exposure to HШаблон:Sub and photolysis with a Hg lamp:[9]
At temperatures above 100 °C, [Cp*Al]Шаблон:Sub decomposes to form pentamethylcyclopentandiene (Cp*H), metallic aluminium (Al(0)) and other non-volatile Al(III) compounds.[2] The overall stability of [Cp*Al]Шаблон:Sub is unique as there is a thermodynamic affinity for tetrameric aluminium(I) compounds ([RAl]Шаблон:Sub) to disproportionate into elemental aluminium and RШаблон:SubAl. As such, a number of different novel oligomeric structures can be synthesised when using tetrameric [Cp*Al]Шаблон:Sub as a precursor.[6] For example, treatment of [Cp*Al]Шаблон:Sub with excess selenium and tellurium in mild conditions gives the unique heterocubane structures [Cp*AlSe]Шаблон:Sub and [Cp*AlTe]Шаблон:Sub respectively.[4] These heterocubane structures are extremely air and moisture sensitive, leading to its decomposition and evolution of [[Hydrogen selenide|HШаблон:SubSe]] and [[Hydrogen telluride|HШаблон:SubTe]] respectively. Analogously, reaction of [Cp*Al]Шаблон:Sub with lighter chalcogens such as [[Oxygen|OШаблон:Sub]], [[Nitrous oxide|NШаблон:SubO]] and sulfur yield [Cp*AlX]Шаблон:Sub (X = O, S).[12]
[Cp*Al]Шаблон:Sub was also the used as a precursor to synthesize the first ever stable dimeric iminoalane containing an AlШаблон:SubNШаблон:Sub heterocycle through the treatment of [Cp*Al]Шаблон:Sub with MeШаблон:SubSiNШаблон:Sub in a 1:4 molar ratio.[13] The resultant iminoalanes was characterized to contain an ideally planar AlШаблон:SubNШаблон:Sub core ring with three coordinate aluminium and nitrogen atoms. Other dimeric iminoalanes including [Cp*AlNSi(i-Pr)Шаблон:Sub]Шаблон:Sub, [Cp*AlNSiPhШаблон:Sub]Шаблон:Sub and [Cp*AlNSi(t-Bu)Шаблон:Sub]Шаблон:Sub have since been synthesized using [Cp*Al]Шаблон:Sub as a precursor through oxidative addition of an organic azide.[3]
Function as a ligand
[Cp*Al] is able to act as an atypical exotic ligand in donor-acceptor type bonds. For example, mixing [Cp*Al]Шаблон:Sub with the Lewis acidic B(CШаблон:SubFШаблон:Sub)Шаблон:Sub forms the Al-B donor-acceptor type bond, and results in the synthesis of the adduct [Cp*Al-B(CШаблон:SubFШаблон:Sub)Шаблон:Sub].[14] Analogous main-group complexes that have been synthesised and characterised include dialane complexes [Cp*Al-Al(CШаблон:SubFШаблон:Sub)Шаблон:Sub][15] and [Cp*Al-Al(t-Bu)Шаблон:Sub],[16] and group 13-group 13 complexes [Cp*Al-Ga(t-Bu)Шаблон:Sub].[16]
[Cp*Al] is also able to act as a potent ligand to transition metals. For example, treatment of [Cp*Al] with [(dcpe)Pt(H)(CHШаблон:Subt-Bu)] (dcpe = bis(dicyclohexylphosphino)ethane) yields [(dcpe)Pt(Cp*Al)Шаблон:Sub].[17] Other transition metals which use [Cp*Al] as a ligand include, but are not limited to dШаблон:Sup metal centre complexes such as [Pd(Cp*Al)Шаблон:Sub] and [Ni(Cp*Al)Шаблон:Sub],[18] and lanthanide/actinide metal centre complexes such as (CpSiMeШаблон:Sub)Шаблон:SubU-AlCp*, (CpSiMeШаблон:Sub)3Nd-AlCp* and (CpSiMeШаблон:Sub)Шаблон:SubCe-AlCp*.[3][19]
References
- ↑ 1,0 1,1 1,2 1,3 1,4 1,5 1,6 Шаблон:Cite journal
- ↑ 2,0 2,1 2,2 Шаблон:Cite journal
- ↑ 3,0 3,1 3,2 3,3 Шаблон:Cite journal
- ↑ 4,0 4,1 Шаблон:Cite journal
- ↑ Шаблон:Cite journal
- ↑ 6,0 6,1 Шаблон:Cite journal
- ↑ Шаблон:Cite journal
- ↑ Шаблон:Cite journal
- ↑ 9,0 9,1 Шаблон:Cite journal
- ↑ Шаблон:Cite journal
- ↑ 11,0 11,1 Шаблон:Cite journal
- ↑ Шаблон:Cite journal
- ↑ Шаблон:Cite journal
- ↑ Шаблон:Cite journal
- ↑ Шаблон:Cite journal
- ↑ 16,0 16,1 Шаблон:Cite journal
- ↑ Шаблон:Cite journal
- ↑ Шаблон:Cite journal
- ↑ Шаблон:Cite journal
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