Cooperation towards nobility: equipping first-row transition metals with an aluminium sword

The exploration for noble metals substitutes in catalysis has become a highly active area of research, driven by the pursuit of sustainable chemical processes. Although the utilization of base metals holds great potential as an alternative, their successful implementation in predictable catalytic processes necessitates the development of appropriate ligands. Such ligands must be capable of controlling their intricate redox chemistry and promote two-electron events, thus mimicking well-established organometallic processes in noble metal catalysis. While numerous approaches for infusing nobility to base metals have been explored, metal-ligand cooperation has garnered significant attention in recent years. Within this context, aluminium-based ligands offer interesting features to fine-tune the activity of metal centres, but their application in base metal catalysis remains largely unexplored. This perspective seeks to highlight the most recent breakthroughs in the reactivity of heterobimetallic aluminium-base-metal complexes, while also showcasing their potential to develop novel and predictable catalytic transformations. By turning the spotlight on such heterobimetallic species, we aim to inspire chemists to explore aluminium-base-metal species and expand the range of their applications as catalysts.

Coordination chemistry and structural rearrangements of the Me2PCH2AlMe2 ambiphilic ligand

Reaction of 2 equivalents of (Me₂PCH₂AlMe₂)₂ with [{RhCl(cod)}₂] (cod = 1,5-cyclooctadiene) afforded [{κ²P,P-(Me₃Al)ClMeAl(CH₂PMe₂)₂}Rh(cod)] (1), which features a κ²-coordinated bis(phosphino)aluminate anion. In compound 1, an Al-Cl substituent bridges to a molecule of AlMe₃, which could be removed in vacuo to provide [{κ²P,P-ClMeAl(CH₂PMe₂)₂}Rh(cod)] (2). By contrast, reaction of 1 equiv. of (Me₂PCH₂AlMe₂)₂ with [{RhCl(cod)}₂] yielded [Rh(cod)(μ-Cl)(Me₂PCH₂AlMe₂)] (3) as the major product, where the phosphine donor of an intact Me₂PCH₂AlMe₂ ligand is coordinated to rhodium and a chloride ligand bridges between Rh and Al. [Rh(cod)(μ-Cl)(Me₂PCH₂AlClMe)] (3A) and 2 were also formed as minor products. The aforementioned reactions were carried out in benzene or toluene, whereas the 1 : 1 reaction of (Me₂PCH₂AlMe₂)₂ with [{RhCl(cod)}₂] in THF afforded [{Rh(μ-CH₂PMe₂)(cod)}₂] (4). Reactions of (Me₂PCH₂AlMe₂)₂ with iridium(I), gold(I) and platinum(II) precursors were also explored. A 1 : 1 reaction of (Me₂PCH₂AlMe₂)₂ with [{IrCl(cod)}₂] afforded [{κ²P,P-Cl₂Al(CH₂PMe₂)₂}Ir(cod)] (5) as one of two major phosphine-containing products; unlike 3, this compound features two chlorine substituents on aluminium. For comparison, the rhodium analogue of 5, [{κ²P,P-Cl₂Al(CH₂PMe₂)₂}Rh(cod)] (6), was also synthesized via the 1 : 1 reaction of {ClAl(CH₂PMe₂)₂}₂ with [{RhCl(cod)}₂]. Reactions of (Me₂PCH₂AlMe₂)₂ with [AuCl(CO)] or [PtCl₂(cod)] also resulted in chloride-methyl group exchange between the transition metal and aluminium. However, these reactions generated free (Me₂PCH₂AlClMe)₂ accompanied by gold and ethane, or [PtMe₂(cod)], respectively. Reaction of 1.5 equivalents of (Me₂PCH₂AlMe₂)₂ with [PtMe₂(cod)] at 75 °C afforded zwitterionic [(PtMe{μ-κ¹P:κ²P,P-MeAl(CH₂PMe₂)₃})₂] (7) which features two tris(phosphino)aluminate anions bridging between PtMe units. Compounds 1-2, 3/3A, 4-7 and (Me₂PCH₂AlClMe)₂ were crystallographically characterized.