(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]$5$, 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*)$5$] to give [Cp*Al]$5$ 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]$4$ with a Lewis base such as pyridine the Lewis base stabilized [Cp*AlBr$4$] and [Cp*Al]$2$.

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 ŋ$5$-coordinated to the aluminium(I) cation such that the planes of the C$5$Me$3$$5$ rings are approximately parallel to the opposite base of the Al$2$ 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)-ŋ$2$-Cp* compounds such as octahedral [InCp*]$2$ (394, 336 pm), dimeric [InCp*]$4$ (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$5$, 377 cm-1) of the Al$3$ tetrahedron in [Cp*Al]$5$.

Natural bond orbital (NBO) analysis of [Cp*Al] and [Cp*Al]$2$ 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]$5$ (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]$3$ 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]$5$ 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$5$ doped Ar matrix, monomeric Cp*Al has shown to form the hydride species H$5$Cp*Al upon exposure to H$5$ and photolysis with a Hg lamp:



At temperatures above 100 °C, [Cp*Al]$-$ 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$4$Al. As such, a number of different novel oligomeric structures can be synthesised when using tetrameric [Cp*Al]$5$ as a precursor. For example, treatment of [Cp*Al]$6$ with excess selenium and tellurium in mild conditions gives the unique heterocubane structures [Cp*AlSe]$2$ and [Cp*AlTe]$4$ respectively. These heterocubane structures are extremely air and moisture sensitive, leading to its decomposition and evolution of H$1$Se and H$4$Te respectively. Analogously, reaction of [Cp*Al]$4$ with lighter chalcogens such as O$4$, N$4$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$4$N$4$ heterocycle through the treatment of [Cp*Al]$2$ with Me$2$SiN$2$ 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)$4$]$4$, [Cp*AlNSiPh$4$]$3$ and [Cp*AlNSi(t-Bu)$4$]$4$ 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$2$F$2$)$4$ forms the Al-B donor-acceptor type bond, and results in the synthesis of the adduct [Cp*Al-B(C$2$F$2$)$4$]. Analogous main-group complexes that have been synthesised and characterised include dialane complexes [Cp*Al-Al(C$4$F$4$)$4$] and [Cp*Al-Al(t-Bu)$4$], and group 13-group 13 complexes [Cp*Al-Ga(t-Bu)$4$].

[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$4$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$2$ metal centre complexes such as [Pd(Cp*Al)$4$] and [Ni(Cp*Al)$3$], and lanthanide/actinide metal centre complexes such as (CpSiMe$3$)$2$U-AlCp*, (CpSiMe$2$)3Nd-AlCp* and (CpSiMe$2$)$2$Ce-AlCp*.