Transition Metal Carbide Complexes

Abstract

Carbide complexes remain a rare class of molecules. Their paucity does not reflect exceptional instability but is rather due to the generally narrow scope of synthetic procedures for constructing carbide complexes. The preparation of carbide complexes typically revolves around generating L n M−CE x fragments, followed by cleavage of the C−E bonds of the coordinated carbon-based ligands (the alternative being direct C atom transfer). Prime examples involve deoxygenation of carbonyl ligands and deprotonation of methyl ligands, but several other p-block fragments can be cleaved off to afford carbide ligands. This Review outlines synthetic strategies toward terminal carbide complexes, bridging carbide complexes, as well as carbide−carbonyl cluster complexes. It then surveys the reactivity of carbide complexes, covering stoichiometric reactions where the carbide ligands act as C 1 reagents, engage in cross-coupling reactions, and enact Fischer−Tropsch-like chemistry; in addition, we discuss carbide complexes in the context of catalysis. Finally, we examine spectroscopic features of carbide complexes, which helps to establish the presence of the carbide functionality and address its electronic structure.

U–C Bond Insertion, Ring-Opening, and C–H Activation in a Uranium Bis(diisopropylamino)cyclopropenylidene (BAC) Adduct

“…Carbon atom transfer is arguably the most challenging atom-transfer reaction. , While examples of O atom transfer and N atom transfer abound, − there are very few examples of the C atom transfer reaction in the literature . C atom transfer is not only of intrinsic fundamental interest, but is also relevant to the synthesis of models for the iron–molybdenum cofactor (FeMoco) of nitrogenase. − This cofactor features a Fe 4 S 3 C cubane and a MoFe 3 S 3 C cubane linked by a vertex-sharing μ 6 -carbide ligand, and efforts to develop accurate models are undoubtedly hampered by the lack of carbide-transfer reagents.…”

“…Carbon atom transfer is arguably the most challenging atomtransfer reaction. 1,2 While examples of O atom transfer and N atom transfer abound, 3−6 there are very few examples of the C atom transfer reaction in the literature. 7 C atom transfer is not only of intrinsic fundamental interest, but is also relevant to the synthesis of models for the iron−molybdenum cofactor (FeMoco) of nitrogenase.…”

“…Nonetheless, a few C atom transfer reagents have been identified in recent years. 2 For example, Cummins and coworkers demonstrated that 7-isocyano-7-azadibenzonorbornadiene (CN 2 C 14 H 10 ) can transfer a C atom to [RuCl 2 (CO)-(PCy 3 ) 2 ], forming [RuCl 2 (C)(PCy 3 ) 2 ] via loss of N 2 and anthracene. 1 Bis(diisopropylamino)cyclopropenylidene (BAC) 11 has also recently emerged as a viable C atom transfer reagent.…”

“…Uranium complexes with these ligand types offer the promise of unique insights into actinide electronic structure. − While uranium carbene complexes are reasonably well established, − isolable uranium carbyne and carbide complexes are exceptionally rare . Most examples are restricted to the gas phase, although the U 2 (μ-C), Sc 2 U­(μ 3 -C), and Sc 2 U­(μ 3 -C) 2 fragments have been stabilized within the C 80 fullerene. − …”

Iridium(VII)–Corrole Terminal Carbides Should Exist as Stable Compounds

“…In summary, multiple considerations, including those of electron affinity and singlet–triplet gaps, lead us to postulate the existence of iridium­(VII)–corrole carbides as highly stable species, a prediction of interest to not only high-oxidation-state chemistry but also to the field of transition metal carbides . Terminal carbides are exceptionally rare, with only a handful of examples to date from groups 6 and 8, including [{N­(R)­Ar} 3 Mo­(≡C:)] − , [Tp*­(CO) 2 M­(≡CLi)] (Tp* = 3,5-dimethyltris­(pyrazolyl)­borate; M = W, Mo), − [M­(≡C:)­(L) 2 (X) 2 ] (M = Ru, Os), − and [P 2 Mo­(≡C:)­(CO)] +,0,– (where P 2 is a terphenyl-diphosphine ligand). , The present predictive study, if experimentally realized, would afford the first example of a group 9 terminal carbide.…”