Anion-π and H-Bonding Interactions Supporting Encapsulation of [Ln(NO₃)_6/5]^3–/2– (Ln = Nd, Er) with a Triazine-Based Ligand

Abstract

Reaction of NdIII and ErIII nitrate salts with a 1,3,5-tris­(dipicolylamine)-triazine (dpat) ligand yielded two unprecedented examples of [Ln­(NO3)6/5]3–/2– (Ln = Nd, Er) moieties completely encapsulated by the ligands. They are found in the two new complexes, [(H3 dpat)2]­[(Nd­(NO3)6)2]·2CH3CN (1), and the related [(H3 dpat)2]­[(Er­(NO3)5)3]·3CH3CN·2H2O (2). The structures of the complexes are similar and they crystallize in the triclinic P-1 space group with a = 12.1630(3), b = 12.2694(3), c = 17.6357(5) Å, V = 2611.10(12) Å3, and a = 14.3372(4), b = 17.1271(4), c = 25.2207(7) Å, V = 5934.7(3) Å3, respectively. Anion-π interactions, which are reported here for the first time for LnIII ion complexes, hydrogen bonding interactions and π–π stacking support the formation of the encapsulated species. Evidence of the protonated dpat ligand in 1 and 2 was found through isolation of (H2 dpat)­(NO3)2. Finally, the pH-dependent ability of the ligand to extract LaIII and nitrate ions from aqueous into toluene solution is demonstrated.

Noncovalent Interactions at Lanthanide Complexes

“…promoted by the Shibasaki catalysts [Ln(binol) 3 (M) 3 ] (M=alkali metal, Ln=lanthanide, binol=1,1′‐ bi ‐2‐naphthol) [102] . Moreover, the roles of intermolecular hydrogen bonding in the improved quantum yield and excellent luminescence stability, [103] photoactivity, [104] proton conductivity, [105] extraction, [63] etc. of lanthanide complexes have also been demonstrated.…”

“…[62] Moreover, the [Eu(NO 3 ) 5 ] 2À anion is encapsulated by two triazine ligands in 6 via ionic interactions as well as anion-π interactions between O-anions of coordinated nitrates and the triazine π-system, with distances of 2.736 and 2.751 Å (Scheme 2b). [63] Noncovalent interactions in lanthanide complexes are not limited to the involvement of the secondary coordination sphere of this class of compounds, but the central Ln atom may also participate in intermolecular interactions leading to a ''rareearth'' bond or interaction. Similarly to the other types of σhole bonds, [67][68][69][70] such as noble gases (group 18), [71] halogen (group 17), [72] chalcogen (group 16), [73] pnictogen (group 15), [74,75] tetrel (group 14), [76] triel (group 13), [77] spodium (group 12) [78] and coinage (group 11) [79] bonds, the ''rare-earth'' (group 3) bond can be defined as the interaction of a rare-earth atom (group 3) that acts as the noncovalent bond donor centre for a nucleophilic (Nu) region in another molecular entity: RÀ RE•••Nu (R = atom or functional group; RE = rare-earth atom; Nu=Ha, Ch, Pn, π-system, anion, etc.).…”

“…Moreover, due to their recent experimen-Scheme 2. Lone pair-π, [62] π-π [62] and anion-π [63] interactions in europium complexes.…”

“…[162] The length of Pb 3nÀ anions of 61-63 leads to an 1D chain, which then participates in intermolecular hydrogen bond, constructing a 2D sheet. [162] According to the estimated optical band gaps (E g ) of 2.52 (61), 2.30 (62) and 2.25 eV (63), there is no relation between them and the Pb•••I distance, and the blue shift observed in 58 in compared to 61 can be accounted for the Pb•••O types of tetrel bonds in the latter compound. [162] 7.…”

“…Like the cation-π interactions, the lone pair-π, π-π and anion-π interactions are also emergent noncovalent interactions whose occurrence has already been recognized in the coordination chemistry of lanthanides. [62][63][64][65][66] Scheme 1. Hydrogen, [57] halogen [59] and chalcogen [60] bonds, as well as cation-π interactions, [58] in lanthanide complexes.…”