Synthesis and Reactivity of Cationic Triruthenium Clusters Derived from 2‐Methyl‐ and 4‐Methylpyrimidines: From Conventional Cyclometalated Ligands to Novel Types of N‐Heterocyclic Carbenes

Abstract

The methylation of the uncoordinated nitrogen atom of the cyclometalated triruthenium cluster complexes [Ru(3)(μ-H)(μ-κ(2)N(1),C(6)-2-Mepyr)(CO)(10)] (1; 2-MepyrH = 2-methylpyrimidine) and [Ru(3)(μ-H)(μ-κ(2)N(1),C(6)-4-Mepyr)(CO)(10)] (9; 4-MepyrH = 4-methylpyrimidine) gives two similar cationic complexes, [Ru(3)(μ-H)(μ-κ(2)N(1),C(6)-2,3-Me(2)pyr)(CO)(10)](+) (2(+)) and [Ru(3)(μ-H)(μ-κ(2)N(1),C(6)-3,4-Me(2) pyr)(CO)(10)](+) (9(+)), respectively, whose heterocyclic ligands belong to a novel type of N-heterocyclic carbenes (NHCs) that have the C(carbene) atom in 6-position of a pyrimidine framework. The position of the C-methyl group in the ligands of complexes 2(+) (on C(2)) and 9(+) (on C(4)) is of key importance for the outcome of their reactions with K[N(SiMe(3))(2)], K-selectride, and cobaltocene. Although these reagents react with 2(+) to give [Ru(3)(μ-H)(μ-κ(2)N(1),C(6)-2-CH(2)-3-Mepyr)(CO)(10)] (3; deprotonation of the C(2)-Me group), [Ru(3)(μ-H)(μ(3)-κ(3)N(1),C(5),C(6)-4-H-2,3-Me(2)pyr)(CO)(9)] (4; hydride addition at C(4)), and [Ru(6)(μ-H)(2){μ(6)-κ(6) N(1),N(1'),C(5),C(5'),C(6),C(6')-4,4'-bis(2,3-Me(2)pyr)}(CO)(18)] (5; reductive dimerization at C(4)), respectively, similar reactions with 9(+) have only allowed the isolation of [Ru(3)(μ-H)(μ(3)-κ(2)N(1),C(6)-2-H-3,4-Me(2)pyr)(CO)(9)] (11; hydride addition at C(2)). Compounds 3 and 11 also contain novel six-membered ring NHC ligands. Theoretical studies have established that the deprotonation of 2(+) and 9(+) (that have ligand-based LUMOs) are charge-controlled processes and that both the composition of the LUMOs of these cationic complexes and the steric protection of their ligand ring atoms govern the regioselectivity of their nucleophilic addition and reduction reactions.

Deprotonation ofC‐Alkyl Groups of Cationic Triruthenium Clusters Containing CyclometalatedC‐Alkylpyrazinium Ligands: Experimental and Computational Studies

“…The CMe group of the cationic pyrimidine‐derived complex [Ru 3 (μ‐H)(μ‐κ 2 N , C ‐pyrMe 2 )(CO) 10 ] + (HpyrMe 2 =1,2‐dimethylpyrimidinium) can also be deprotonated with a strong base 9. That reaction is the only previously reported deprotonation related to those described herein.…”

“…Complexes that can concurrently be ascribed to both groups (1) and (2), that is, those derived from C ‐ and N ‐metalated “inium” cations, were unknown before the recent description of the cationic triruthenium derivatives [Ru 3 (μ‐H)(μ‐κ 2 N , C ‐L)(CO) 10 ] + , HL= N ‐methylquinoxalinium,7 N ‐methylpyrazinium,7 N ‐methylpyrimidinium,8 1,2‐dimethylpyrimidinium,9 and 1‐methyl‐1,5‐naphthyridinium 10. The cationic ligands of these triruthenium clusters are readily attacked by anionic nucleophiles at selected C atoms of their ligand rings (some examples are depicted in Scheme )7–9 and are also prone to one‐electron reduction processes that lead to dimeric hexanuclear products 8–10. These nucleophilic attacks and reduction reactions are orbital‐controlled rather than charge‐controlled processes and lead to neutral complexes with unsaturated but nonaromatic N‐heterocyclic ligands that in some cases are N‐heterocyclic carbenes (Scheme ).…”

The coordination chemistry of organo-hydride donors: new prospects for efficient multi-electron reduction

“…19), which are nominally N-heterocyclic carbene complexes. [244][245][246][247][248] Clusters 58-64 added hydride (from K-selectridet, KBBu s 3 H) to give the corresponding dihydro-N-heterocycle-substituted clusters, in which the metal-bound carbon atom loses carbene character and becomes carbanion-like (e.g., cluster 65 in Scheme 25 and Fig. 20).…”

“…19 Examples of Ru-carbonyl clusters bearing N-heterocyclic carbene ligands by Cabeza (spheres on Ru represent terminal CO ligands). [244][245][246][247] Scheme 25 Reduction of complex 59: hydride addition upon treatment with K-Selectridet to produce 65 and heterocyclic coupling upon reduction with cobaltocene to produce 66 (spheres on Ru = CO). 245 [Ru(tpm)(Z 6 -C 6 Me 6 )] 2+ (69), was examined.…”

Crystal structure of aquadichloridobis(1-((2-methyl-1H-imidazol-1-yl)methyl)-1H-benotriazole-κN)mercury(II), C22H24Cl2HgN10O

“…Multidentate N-heterocyclic compounds are often employed as ligands to produce polymeric networks due to their rich coordination sites and various coordination modes [1][2][3][4][5][6]. Recently, our group synthesized 1-[(benzotriazol-1-yl)methyl]-1-H-1,3-(2-methyl-imdazol), and studied its coordination behavior with transitional metal halides (such as ZnCl 2 , CoCl 2 , CdI 2 , HgBr 2 ), obtaining a series of coordination compounds [7][8][9][10].…”