Structure, Thermodynamics, and Energy Content of Aluminum–Cyclopentadienyl Clusters

Abstract

We present quantum chemistry simulations of aluminum clusters surrounded by a surface layer of cyclopentadiene-type ligands to evaluate the potential of such complexes as novel fuels or energetic materials. Density functional theory simulations are used to examine the aluminum-ligand bonding and its variation as the size of the aluminum cluster increases. The organometallic bond at the surface layer arises mainly from ligand charge donation into the Al p orbitals balanced with repulsive polarization effects. Functionalization of the ligand and changes in Al cluster size are found to alter the relative balance of these effects, but the surface organometallic bond generally remains stronger than Al–Al bonds elsewhere in the cluster. In large clusters, such as the experimentally observed Al50Cp12*, this suggests that unimolecular thermal decomposition likely proceeds through loss of surface AlCp* units, exposing the strained interior aluminum core. The calculated heats of combustion per unit volume for these systems are high, approaching 60% that of pure aluminum. We discuss the possibility of using organometallic aluminum clusters as a means of achieving rapid combustion in propellants and fuels.

Growth of metalloid aluminum clusters on graphene vacancies

“…[26][27][28]36,37 In addition to organometallic bonds, these clusters contain many metal-metal bonds in which the Al is in a low formal oxidation state. Synthesis relies on AlX (X = Cl, Br, or I) monovalent aluminum halide precursors formed in a specialized cocondensation apparatus.…”

Modeling the stability and growth of metalloid clusters for energetic materials

“…[5] There has been considerable recent interest in developing metal/organic structures that address these issues and might allow for systems with fast reaction kinetics that still retain some of the high energy density of pure metals. [6,7,8,9,10,11,12] At the micron and nanoparticle scales, organic layers are being pursued as an alternative passivation to the native oxide layer, [6,9] or as a means of assembling metal particles into structured energetic materials. In recent work we have considered low-valence metalloid clusters as materials that may allow metal oxidation on even faster timescales than coated nanoparticles.…”

“…In recent work we have considered low-valence metalloid clusters as materials that may allow metal oxidation on even faster timescales than coated nanoparticles. [7,11,12] Several experimental efforts have also begun on lab-scale synthesis of ligated metalloid clusters that could be tailored for energetics applications. [13] In these proceedings we discuss simulations of the oxidation and unimolecular thermal decomposition of aluminum clusters ligated with cyclopentadienyl (Cp) and pentamethylcyclopentadienyl (Cp * ) ligands.…”

“…[13] In these proceedings we discuss simulations of the oxidation and unimolecular thermal decomposition of aluminum clusters ligated with cyclopentadienyl (Cp) and pentamethylcyclopentadienyl (Cp * ) ligands. These metalloid aluminum clusters are a prototypical system with high-energy density [7] that allows us to study basic stability and reaction processes in metalloid clusters with low-valence metals. Smaller clusters such as Al 4 Cp 4 and Al 4 Cp * 4 can be reliably synthesized in larger quantities, while larger examples such as Al 50 Cp * 12 are challenging to produce in bulk.…”

Ab initio metadynamics simulations of oxygen/ligand interactions in organoaluminum clusters

“…The oxidation kinetics and high energy density of these systems are also of interest for possible applications in hydrogen production from water 20,21 or as fuel additives. 22,23 A range of similar ligand-protected clusters are also known; ligated gold clusters in particular have received considerable attention in recent years. [25][26][27][28][29] Recently we have performed ab initio molecular dynamics simulations to examine the interaction of oxygen with four prototypical organoaluminum clusters: Al 4 Cp 4 , Al 4 Cp * 12 , as an initial investigation of their reaction pathways and the onset of chemistry.…”