# (Pentamethylcyclopentadienyl)aluminium(I)

(Pentamethylcyclopentadienyl)aluminium(I) is an organometallic compound with the formula Al(C$_5$Me$_5$) ("Me" is a methyl group; CH$_3$). The compound is often abbreviated to AlCp* or Cp*Al, where Cp* is the pentamethylcyclopentadienide anion (C$_5$Me$_5$$^−$). Discovered in 1991 by Dohmeier et al., AlCp* serves as the first ever documented example of a room temperature stable monovalent aluminium compound. In its isolated form, Cp*Al exists as the tetramer [Cp*Al]$_4$, and is a yellow crystal that decomposes at temperatures above 100 °C but also sublimes at temperatures above 140 °C.

## Synthesis

The earliest documented synthesis and characterization of Cp*Al was by Dohmeier et al. in 1991, where four equivalents of AlCl in toluene/diethyl ether is reacted with two equivalents of 2[Mg(Cp*)$_2$] to give [Cp*Al]$_4$ as yellow crystals:
Despite the above synthetic scheme successfully producing tetrameters of [Cp*Al]$_4$ 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]$_4$ tetramer were discovered, and required the reduction of Cp*AlX$_2$ (X = Cl, Br, I) by a metal (K when X = Cl) or a metal alloy (Na/K alloys when X = Br, I):
More exotic ways of synthesizing [Cp*Al]$_4$ include the controlled disproportionation of an Al(II) dialane into constituent Al(I) and Al(III) products. For example, reacting dialane [Cp*AlBr]$_2$ with a Lewis base such as pyridine the Lewis base stabilized [Cp*AlBr$_2$] and [Cp*Al]$_4$.
Monomeric Cp*Al has also been isolated in a solid Ar matrix by heating [Cp*Al]$_4$ in toluene to 133 °C and spraying the resultant vapours with Ar onto a copper block kept at 12 K.

## Structure and bonding

X-ray crystallographic data determined Cp*Al to exist exclusively as a tetramer in its solid state. This tetramer, [Cp*Al]$_4$, consists of an Al$_4$ tetrahedron, and the Cp* rings are η$^5$-coordinated to the aluminium(I) cation such that the planes of the C$_5$Me$_5$$^-$ rings are approximately parallel to the opposite base of the Al$_4$ tetrahedron. 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. The Al-Al bond in [Cp*Al]$_4$ is 276.9 pm, which is slightly shorter than that of metallic aluminium, which has an Al-Al bond length of 286 pm. Additionally, the Al-Al bond in [Cp*Al]$_4$ is significantly shorter than other oligomeric and polymeric Group III M(I)-η$^5$-Cp* compounds such as octahedral [InCp*]$_6$ (394, 336 pm), dimeric [InCp*]$_2$ (363.1 pm), and polymeric [TlCp*] (641 pm), indicating a significantly larger interaction between aluminium atoms in [Cp*Al]$_4$ than monovalent Cp* compounds of In(I) and Tl(I). Additional characterization that has been performed include Raman spectroscopy, which detected a Raman active breathing vibration (A$_1$, 377 cm-1) of the Al$_4$ tetrahedron in [Cp*Al]$_4$.
Natural bond orbital (NBO) analysis of [Cp*Al] and [Cp*Al]$_4$ 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]$_4$ (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]$_4$ 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. Atoms in molecules (AIM) calculations calculate the Al-Al bonding to be metallic. Stabilization of [Cp*Al]$_4$ relative to [CpAl]$_4$ 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.
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]$_4$.

## Reactivity

When isolated in a solid H$_2$ doped Ar matrix, monomeric Cp*Al has shown to form the hydride species H$_2$Cp*Al upon exposure to H$_2$ and photolysis with a Hg lamp:
At temperatures above 100 °C, [Cp*Al]$_4$ decomposes to form pentamethylcyclopentandiene (Cp*H), metallic aluminium (Al(0)) and other non-volatile Al(III) compounds. The overall stability of [Cp*Al]$_4$ is unique as there is a thermodynamic affinity for tetrameric aluminium(I) compounds ([RAl]$_4$) to disproportionate into elemental aluminium and R$_3$Al. As such, a number of different novel oligomeric structures can be synthesised when using tetrameric [Cp*Al]$_4$ as a precursor. For example, treatment of [Cp*Al]$_4$ with excess selenium and tellurium in mild conditions gives the unique heterocubane structures [Cp*AlSe]$_4$ and [Cp*AlTe]$_4$ respectively. These heterocubane structures are extremely air and moisture sensitive, leading to its decomposition and evolution of H$_2$Se and H$_2$Te respectively. Analogously, reaction of [Cp*Al]$_4$ with lighter chalcogens such as O$_2$, N$_2$O and sulfur yield [Cp*AlX]$_4$ (X = O, S).
[Cp*Al]$_4$ was also the used as a precursor to synthesize the first ever stable dimeric iminoalane containing an Al$_2$N$_2$ heterocycle through the treatment of [Cp*Al]$_4$ with Me$_3$SiN$_3$ in a 1:4 molar ratio. The resultant iminoalanes was characterized to contain an ideally planar Al$_2$N$_2$ core ring with three coordinate aluminium and nitrogen atoms. Other dimeric iminoalanes including [Cp*AlNSi(i-Pr)$_3$]$_2$, [Cp*AlNSiPh$_3$]$_2$ and [Cp*AlNSi(t-Bu)$_3$]$_2$ have since been synthesized using [Cp*Al]$_4$ as a precursor through oxidative addition of an organic azide.

### 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]$_4$ with the Lewis acidic B(C$_6$F$_6$)$_3$ forms the Al-B donor-acceptor type bond, and results in the synthesis of the adduct [Cp*Al-B(C$_6$F$_6$)$_3$]. Analogous main-group complexes that have been synthesised and characterised include dialane complexes [Cp*Al-Al(C$_6$F$_5$)$_3$] and [Cp*Al-Al(t-Bu)$_3$], and group 13-group 13 complexes [Cp*Al-Ga(t-Bu)$_3$].
[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$_2$t-Bu)] (dcpe = bis(dicyclohexylphosphino)ethane) yields [(dcpe)Pt(Cp*Al)$_2$]. Other transition metals which use [Cp*Al] as a ligand include, but are not limited to d$^{10}$ metal centre complexes such as [Pd(Cp*Al)$_4$] and [Ni(Cp*Al)$_4$], and lanthanide/actinide metal centre complexes such as (CpSiMe$_3$)$_3$U-AlCp*, (CpSiMe$_3$)3Nd-AlCp* and (CpSiMe$_3$)$_3$Ce-AlCp*.