Abstract
We present herein anionic borate‐based bi‐mesoionic carbene compounds of the 1,2,3‐triazol‐4‐ylidene type that undergo C−N isomerization reactions. The isomerized compounds are excellent ligands for CoII centers. Strong agostic interactions with the “C−H”‐groups of the cyclohexyl substituents result in an unusual low‐spin square planar CoII complex, which is unreactive towards external substrates. Such agostic interactions are absent in the complex with phenyl substituents on the borate backbone. This complex displays a high‐spin tetrahedral CoII center, which is reactive towards external substrates including dioxygen. To the best of our knowledge, this is also the first investigation of agostic interactions through single‐crystal EPR spectroscopy. We conclusively show here that the structure and properties of these CoII complexes can be strongly influenced through interactions in the secondary coordination sphere. Additionally, we unravel a unique ligand rearrangement for these classes of anionic mesoionic carbene‐based ligands.
Highlights
We present the synthesis of the bis-triazolium salts (H2L1)OTf and (H2L2)OTf (Scheme 1), their deprotonation chemistry leading to CÀN isomerization products, and the CoII complexes of the isomerized ligands
In order to correlate the results from EPR experiments with the electronic structure and ligand field energetics of the different cobalt complexes, including a truncated form of [Co(L1N,N)2]: [Co(L1N,N)2]Tr, where the apical CH2 groups directly above and below the CoII atom were removed from the cyclohexyl substituents, we performed complete-active space self-consistent field (CASSCF) calculations, with multireference perturbation theory (NEVPT2) to take into account dynamical correlation effects, using the ORCA program.[72]
We could conclusively show that the geometry, the spin state, the spectroscopic properties, and the reactivity of these complexes can be controlled through unique agostic interactions between the substituent on the borate backbone and the cobalt center in its secondary coordination sphere
Summary
Borate-based anionic N-heterocyclic carbenes (NHCs) are well-established as an important class of ligand.[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22] Such ligands have proven to be unique in terms of their ability to support small-molecule activation at transition-metal centers, as well as for their propensity to stabilize unusual metal oxidation states.[23,24,25,26,27,28,29,30,31,32,33,34,35,36] 1,2,3-Triazol-5-ylidene-based mesoionic carbenes (MICs) are a relatively new addition to the larger class of NHCs (Figure 1 A).[37,38,39,40,41,42]. In photochemistry.[48,49,50,51,52,53,54,55,56,57] Despite this progress in the development of MICs, the field is almost exclusively dominated by neutral MICs, even though there have been some recent examples of cationic MIC ligands.[44,45,46] To the best of our knowledge, there is just one report on a borate-based anionic MIC compound of the triazolylidene-type (Figure 1 B),[58] which was synthesized from the corresponding 1,5-regioisomer of the triazole.[58,59] Given the unique electronic properties of MICs, and the well-established utility of borate-based NHCs, we were interested in developing bidentate borate-based MICs of the form LC,C (Figure 1 C). We show that the newly developed ligands can undergo unique backbone-dependent strong agostic interactions with the CoII center. Such interactions in the secondary coordination sphere of the metal center dictate its geometry, its spin state, and its chemical reactivity. A combined synthetic, crystallographic, spectroscopic (including single-crystal EPR spectroscopy), and theoretical (wave-function-based methods) approach is used to investigate the compounds and their reactivity
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