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词条 (Pentamethylcyclopentadienyl)aluminium(I)
释义

  1. Synthesis

  2. Structure and Bonding

  3. Reactivity

      Function as a Ligand  

  4. References

{{Chembox
| ImageFile = (Pentamethylcyclopentadienyl)aluminium(I).png
| ImageSize = 180px
| IUPACName =
| OtherNames =AlCp*, Cp*Al
| Section1 = {{Chembox Identifiers
| CASNo = 137013-38-8
| CASNo1 = 132645-87-5
| CASNo1_Comment = (tetramer)
| PubChem =
| SMILES =
| Section2 = {{Chembox Properties
| Al=1 | C=10 | H=15
| Appearance =
| Density =
| MeltingPt =
| BoilingPt =
| Solubility =
| Section3 = {{Chembox Hazards
| MainHazards =
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}}(Pentamethylcyclopentadienyl)aluminium(I) is an organometallic compound with the formula Al(C{{sub|5}}Me{{sub|5}}) ("Me" is a methyl group; CH{{sub|3}}). The compound is often abbreviated to AlCp* or Cp*Al, where Cp* is the pentamethylcyclopentadienide anion (C{{sub|5}}Me{{sub|5}}{{sup|−}}). Discovered in 1991 by Dhmeier 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]{{sub|4}}, 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*){{sub|2}}] to give [Cp*Al]{{sub|4}} as yellow crystals:

Despite the above synthetic scheme successfully producing tetrameters of [Cp*Al]{{sub|4}} 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]{{sub|4}} tetramer were discovered, and required the reduction of Cp*AlX{{sub|2}} (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]{{sub|4}} include the controlled disproportionation of an Al(II) dialane into constituent Al(I) and Al(III) products. For example, reacting dialane [Cp*AlBr]{{sub|2}} with a Lewis base such as pyridine the Lewis base stabilized [Cp*AlBr{{sub|2}}] and [Cp*Al]{{sub|4}}.[8]

Monomeric Cp*Al has also been isolated in a solid Ar matrix by heating [Cp*Al]{{sub|4}} 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]{{sub|4}}, consists of a Al{{sub|4}} tetrahedron, and the Cp* rings are ŋ{{sup|5}}-coordinated to the aluminium(I) cation such that the planes of the C{{sub|5}}Me{{sub|5}}{{sup|-}} rings are approximately parallel to the opposite base of the Al{{sub|4}} 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]{{sub|4}} 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]{{sub|4}} is significantly shorter than other oligomeric and polymeric Group III M(I)-ŋ{{sup|5}}-Cp* compounds such as octahedral [InCp*]{{sub|6}} (394, 336 pm), dimeric [InCp*]{{sub|2}} (363.1 pm), and polymeric [TlCp*] (641 pm), indicating a significantly larger interaction between aluminium atoms in [Cp*Al]{{sub|4}} 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 (A{{sub|1}}, 377 cm-1) of the Al{{sub|4}} tetrahedron in [Cp*Al]{{sub|4}}.[1]

Natural bond orbital (NBO) analysis of [Cp*Al] and [Cp*Al]{{sub|4}} 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]{{sub|4}} (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]{{sub|4}} 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]{{sub|4}} relative to [CpAl]{{sub|4}} 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]{{sub|4}}.[2]

Reactivity

When isolated in a solid H{{sub|2}} doped Ar matrix, monomeric Cp*Al has shown to form the hydride species H{{sub|2}}Cp*Al upon exposure to H{{sub|2}} and photolysis with a Hg lamp:[9]

At temperatures above 100 °C, [Cp*Al]{{sub|4}} decomposes to form pentamethylcyclopentandiene (Cp*H), metallic aluminium (Al(0)) and other non-volatile Al(III) compounds.[2] The overall stability of [Cp*Al]{{sub|4}} is unique as there is a thermodynamic affinity for tetrameric aluminium(I) compounds ([RAl]{{sub|4}}) to disproportionate into elemental aluminium and R{{sub|3}}Al. As such, a number of different novel oligomeric structures can be synthesised when using tetrameric [Cp*Al]{{sub|4}} as a precursor.[6] For example, treatment of [Cp*Al]{{sub|4}} with excess selenium and tellurium in mild conditions gives the unique heterocubane structures [Cp*AlSe]{{sub|4}} and [Cp*AlTe]{{sub|4}} respectively.[4] These heterocubane structures are extremely air and moisture sensitive, leading to its decomposition and evolution of H{{sub|2}}Se and H{{sub|2}}Te respectively. Analogously, reaction of [Cp*Al]{{sub|4}} with lighter chalcogens such as O{{sub|2}}, N{{sub|2}}O and sulfur yield [Cp*AlX]{{sub|4}} (X = O, S).[12]

[Cp*Al]{{sub|4}} was also the used as a precursor to synthesize the first ever stable dimeric iminoalane containing a Al{{sub|2}}N{{sub|2}} heterocycle through the treatment of [Cp*Al]{{sub|4}} with Me{{sub|3}}SiN{{sub|3}} in a 1:4 molar ratio.[13] The resultant iminoalanes was characterized to contain an ideally planar Al{{sub|2}}N{{sub|2}} core ring with three coordinate aluminium and nitrogen atoms. Other dimeric iminoalanes including [Cp*AlNSi(i-Pr){{sub|3}}]{{sub|2}}, [Cp*AlNSiPh{{sub|3}}]{{sub|2}} and [Cp*AlNSi(t-Bu){{sub|3}}]{{sub|2}} have since been synthesized using [Cp*Al]{{sub|4}} 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]{{sub|4}} with the Lewis acidic B(C{{sub|6}}F{{sub|6}}){{sub|3}} forms the Al-B donor-acceptor type bond, and results in the synthesis of the adduct [Cp*Al-B(C{{sub|6}}F{{sub|6}}){{sub|3}}].[14] Analogous main-group complexes that have been synthesised and characterised include dialane complexes [Cp*Al-Al(C{{sub|6}}F{{sub|5}}){{sub|3}}][15] and [Cp*Al-Al(t-Bu){{sub|3}}],[16] and group 13-group 13 complexes [Cp*Al-Ga(t-Bu){{sub|3}}].[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)(CH{{sub|2}}t-Bu)] (dcpe = bis(dicyclohexylphosphino)ethane) yields [(dcpe)Pt(Cp*Al){{sub|2}}].[17] Other transition metals which use [Cp*Al] as a ligand include, but are not limited to d{{sup|10}} metal centre complexes such as [Pd(Cp*Al){{sub|4}}] and [Ni(Cp*Al){{sub|4}}],[18] and lanthanide/actinide metal centre complexes such as (CpSiMe{{sub|3}}){{sub|3}}U-AlCp*, (CpSiMe{{sub|3}})3Nd-AlCp* and (CpSiMe{{sub|3}}){{sub|3}}Ce-AlCp*.[3][19]

References

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2 : Organoaluminium compounds|Cyclopentadienyl complexes
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