Revealing unbound β -diketiminate anions: structural dynamics from caesium complexes †‡

This study reports the ﬁ rst structural elucidation of β -diketiminate anions (BDI − ), known for strong coordination, in their unbound form within caesium complexes. β -Diketiminate caesium salts (BDICs) were synthesised, and upon the addition of Lewis donor ligands, free BDI − anions and donor-solvated Cs + cations were observed. Notably, the liberated BDI − anions exhibited an unprecedented dynamic cisoid – transoid exchange in solution

Monoanionic β-diketiminate (BDI) ligands, 1 commonly referred to as NacNac ligands, are an important class of ligands with diverse applications in multiple chemical fields. 2 Characterised by two ketiminate moieties linked by a 1,3carbon chain, these ligands can be readily obtained via the deprotonation of β-diketimines (Fig. 1A). 3 BDI ligands are known for their ability to stabilise a broad array of metal ions from the s, 4 p, 5 d 6 and f-blocks 7 in diverse oxidation states by forming strong coordination complexes. Moreover, they have been shown to enhance catalytic activity, selectivity, and stability in several processes, including olefin polymerisation 8 and copolymerization, 9 transfer hydrogenation, 10 and cross-coupling reactions. 11 BDI ligands have demonstrated potential in developing unique main group species 12 with unconventional electronic and stereochemical properties that enable novel reactivity and catalysis. 13 They have also been used in material science for crafting novel metal-organic frameworks (MOFs), 14 biomedical chemistry for developing new metal-based therapeutics, 15 and biochemistry for modelling metalloenzymes. 16 While BDI ligands commonly form stable and robust chelating complexes by donating two nitrogen atoms as κ 2 -N,N′-ligands I (Fig. 1B), 17 examples of alternative coordination modes involving partial ligand dissociation have also been observed, II-V. 18 Recent studies have underscored the importance of the dissociation properties of BDI ligands in catalysis and bond activation chemistry. For instance, BDI dissociation has been shown to boost catalytic activities in lactide polymerisation with (BDI) 2 ZrCl 2 species, 19 trigger C-F activation in hemilabile (BDI)Co(I) complexes, 18d,20 and promote unusual ligand rearrangements during N 2 activation in low-valent [(BDI)Ca(I)] 2 species. 21 Thus, understanding the dissociation and free anionic forms of BDI ligands is pivotal for gaining a deeper insight into their chemistry. However, despite their widespread use, examples of free BDI − anions have remained elusive.
In this study, we explored the free-form structures of BDI − exhibiting cisoid-transoid dynamic behaviour in solution, using various spectroscopic and crystallographic techniques. We report novel examples of unbound Dipp BDI − [(DippNCMe) 2 CH] − anions (Dipp = 2,6-iPr 2 C 6 H 3 ) and examine them in their "naked" state, free from metal-ion binding interactions. To achieve this, we selected heavy and soft alkali metal caesium (Cs), the Lewis donor solvent THF, and strongly coordinating secondary ligands such as 18-crown-6 and [2.2.2] cryptand. The selection of Cs was motivated by its larger ionic radius and higher polarisability, typically leading to weaker ligand interactions than lighter alkali metals, a behaviour discussed extensively in the context of alkali metal mediation in organometallic chemistry. 22 Our work represents the first identification of BDI − anions without metal-ion binding, offering valuable insights into their properties.
When the stronger coordinating solvent THF-d 8 was used, the 1 H NMR spectrum of 1 exhibited a noticeable broadening at room temperature. The γ-CH and Me groups resonated as singlets at δ 4.23 and 1.54 ppm, respectively. This suggests that BDI chelates Cs + , accompanied by further THF coordination. This resulted in monomers identified as [(BDI)Cs(THF) n ] 1·(THF) n , supported by DOSY NMR studies, resembling the observed behaviour of 1·(toluene) n . As the temperature decreased, resonance signals of 1·(THF) n sharpened, with the γ-CH and Me signal appearing at δ 4.21 and 1.52 ppm at 258 K, respectively. Similar to 1·(toluene) n , the NMR data for 1·(THF) n in the 333-258 K temperature range reveal a symmetrical BDI array. Interestingly, the 1 H NMR spectrum of 1·(THF) n displayed two additional sets of broad resonances at room temperature, which narrowed into two sets of signals in a 10 : 1 ratio at 258 K (40% relative to that of 1). Each new set of signals consisted of two distinct Dipp, one γ-CH, and two Me environments, indicating two new asymmetrical BDI species in solution. The significantly shielded γ-CH groups for both species (δ 3.55 ppm, overlapped) suggested discoordination of the BDI ligand (δ 3.80 ppm for γ-CH in related κ 1 -(N, arene) BDI examples). 18e Also, the two distinct Me groups resonate at δ 2.51/1.22 and δ 2.16/1.22 for the major and minor BDI species, respectively (note that two Me signals are overlapped), reflecting asymmetric BDI skeletons. We hypothesised that the partial ionisation of 1 in THF-d 8 would lead to the formation of two distinct separated ion pairs (SIPs) of the type We attempted kinetic studies to study the pathways and factors governing BDI ligand dissociation; however, our efforts were hindered by the overlapping of NMR signals and the small amount of one of the free BDI − anionic species in the studied temperature range.
To gain a better understanding of these new BDI species, we synthesised and isolated the corresponding SIP species, [(18-cr-6) 2 Cs] + [(BDI)] − 2 and [(crypt)Cs] + [(BDI)] − 3 (Fig. 3A). The addition of 18-cr-6 (2 equiv.) and crypt (1 equiv.) to 1 in toluene yielded 2 and 3 as yellow powders in isolated yields of 40 and 54%, respectively. Crystallisation from n-hexane/ toluene allowed us to obtain crystals of 2a and 3a. X-ray diffraction analysis confirmed their arrangements as solvent-separated ion-pairs ( Fig. 3B and C). In 2a, the Cs + ion is fully solvated by two molecules of 18-cr-6, whereas in 3a, Cs + is fully entrapped by a crypt molecule. Remarkably, the BDI − anion remains unbound and adopts a transoid conformation (indicated by the relative position of the Me backbone groups). Intriguingly, crystals of a second isomeric form, 2b, featuring a ciscoid conformation of the BDI − anion, were obtained by crystallising 2 in a mixture of n-hexane/THF (Fig. 3C). The metrics  (Fig. 4A), which likely involves bond rotations in the BDI backbone chain. DFT calculations considering THF solvent effects (Fig. 4B) demonstrated that the transoid conformation C, characterised in the crystal structures of 2a and 3a, is more stable than the cisoid conformation E found in 2b, by 2.6 kcal mol −1 . These calculations also suggest that rotations within the BDI − anion backbone around the C-C and C-N bonds involve almost energetically equivalent cisoid and transoid rotamers A ⇄ E. Particularly, the unbound chelating conformation A was found to be less stable by 5   tion, emphasising the dynamic behaviour and sensitivity of BDI − anions to the surrounding ionic environment.

Conclusions
In conclusion, we successfully synthesised and characterised a series of BDI complexes with Cs cations. These complexes dissociate to form solvent-separated ion-pairs with free, unbound BDI − anions in the presence of coordinating ligands such as THF, 18-cr-6, and crypt. This study provides valuable insights into the structural and dynamic features of unbound BDI − anions in cisoid and transoid conformations, and their weak interactions with counterions in solution and in solid form. Notably, the presence of Cs + cations enables weak interactions and dynamic cisoid-transoid BDI − exchange in solution, thus challenging the conventional view of BDI ligands as predominantly bound entities to metal ions. These findings have important implications for the design and utilisation of new BDI ligands in coordination chemistry, catalysis, and related areas, encouraging further studies of unbound BDI − anions within other soft metal systems.

Conflicts of interest
There are no conflicts to declare.