Cyclometalative C-H bond activation is a process that is commonly encountered in the field of organometallic chemistry. In rare earth and actinide complexes, ligand cyclometalation is most prevalent in highly reactive alkyl and hydrido species. Numerous factors promote ligand cyclometalation and influence the rate at which it occurs. This tutorial review discusses key issues relevant to ligand cyclometalation in rare earth and actinide complexes, including kinetic and mechanistic considerations. A variety of examples is presented for a wide range of ligand types and metals, the scope of which is intended to include routine cases, while also highlighting exceptional cyclometalation reactions that lead to unusual bonding modes. The reaction chemistry of cyclometalated rare earth and actinide complexes with various small molecule substrates (e.g. phenol, anilines, triethylammonium salts, alkynes, olefins, hydrogen and hydrocarbons) is also outlined.

The synthesis of a novel bis(phosphinimine) ancillary ligand based on a carbazole framework is described (HL Ph , 4a; HL Pipp , 4b; Ph = phenyl, Pipp = para-isopropylphenyl). Protonolysis with Lu(CH2SiMe3)3(THF)2 afforded the corresponding lutetium dialkyl complexes (LuL Ph (CH2SiMe3)2, 5a; LuL Pipp (CH2SiMe3)2, 5b), which were thermally sensitive and rapidly underwent intramolecular metalative alkane elimination to generate LuL Ph* (CH2SiMe3)(THF), 6a, and LuL Pipp* (CH2SiMe3)(THF), 6b, as highly reactive intermediates. Complexes 6a and 6b further decomposed, cleanly generating doubly metalated complexes LuL Ph* * (THF), 7a, and LuL Pipp* * (THF), 7b, respectively, as the thermodynamic products. Kinetic analysis of the decomposition of 5a revealed a first-order mechanism with activation parameters ΔH ⧧ = 19.2(1) kcal·mol−1 and ΔS ⧧ = −8.2(2) cal·K−1·mol−1. Compounds 4a, 4b, 6b, and 7b were characterized by single-crystal X-ray diffraction studies.

Reactivity involving metallacycle ring opening of ortho-metalated bis(phosphinimine)carbazolide complexes (L Ph -κ3 N,κ2 C P-Ph)Lu(THF) (1a) and (L Pipp -κ3 N,κ2 C P-Ph)Lu(THF) (1b); L = [1,8-(Ph2PNAr)2dmc]; Ar = Ph (L Ph ), p-isopropylphenyl (L Pipp ); dmc = 3,6-dimethylcarbazolide) is described. Reaction of 1a,b with bulky anilines (2,4,6-trimethylaniline, MesNH2; 2,4,6-triisopropylaniline, TripNH2) promoted metallacycle ring opening of two ortho-metalated P-phenyl groups to liberate the bis(anilide) products (L Ph -κ3 N)Lu(NHMes)2 (2) and (L Pipp -κ3 N)Lu(NHTrip)2 (3). Regardless of the quantity of TripNH2 or MesNH2 utilized, double ring opening always prevailed to afford the bis(anilide) product, rather than the mono(anilide). In contrast, reaction of complex 1b with the bulkier reagent 2,4,6-tri-tert-butylaniline (Mes*NH2) only caused metallacycle ring opening of one ortho-metalated P-phenyl group, affording the mono(anilide) complex (L Pipp -κ3 N,κC P-Ph)Lu(NHMes*) (4) exclusively. Complex 4 rapidly undergoes an intramolecular rearrangement whereby metalation of an N-aryl group promotes metallacycle ring opening of the ligated P-phenyl moiety to give (L Pipp -κ3 N,κC N-Pipp)Lu(NHMes*) (5) as the structural isomer. Through deuterium labeling and kinetic studies it was established that the thermal rearrangement of 4 does not proceed through an imido intermediate. Compounds 2, 3, and 5 were characterized by single-crystal X-ray diffraction studies.

A novel bis(phosphinimine)pyrrole based ligand (HL) and its synthesis are reported. Rare earth dialkyl complexes of the ligand, LLn(CH(2)SiMe(3))(2) (Ln = Er, Lu, Sc), have been prepared and found to exhibit high thermal stability in solution. The protio-ligand and dialkyl lanthanide complexes (Ln = Er, Lu) have been characterised by single crystal X-ray diffraction studies.

Two derivatives of a new bis(pyrazolyl)carbazole pincer ligand H(CzPz R ), R = i Pr and Me, and their syntheses are reported. Lutetium dialkyl complexes of the ligand (CzPz R )Lu(CH 2 SiMe 3 ) 2 , R = i Pr and Me, have been prepared and found to exhibit high thermal stability in solution. These organolutetium compounds are Lewis base free, and the solid-state structure of (CzPz iPr )Lu(CH 2 SiMe 3 ) 2 revealed that the complex is monomeric with a trigonalbipyramidal geometry. Hydrogenolysis of (CzPz iPr )Lu(CH 2 SiMe 3 ) 2 afforded a trimetallic lutetium hydride complex that possesses five bridging hydride ligands. Notably, intramolecular C−H bond activation of the CzPz iPr ligand was found to occur during the formation of this latter hydride complex, with metalation of a pyrazole carbon atom.