Coordination of 1,10-phenanthroline, 3,4,7,8-tetramethyl-1,10-phenanthroline and 2,4,6-tris(2-pyridyl)-1,3,5-triazine to MX3 (M = Ce, U; X = I, OSO2CF3) in pyridine and acetonitrile

Abstract

Complexation studies of MX3 (M=Ce, U; X=I, OTf) with 1,10-phenanthroline (phen), 3,4,7,8-tetramethyl-1,10-phenanthroline (Me4phen) and 2,4,6-tris(2-pyridyl)-1,3,5-triazine (tptz) in pyridine (py) and acetonitrile reveal the major influence of the counter-ion X and solvent on ligand coordination. By comparison with bipy and terpy analogues (bipy=2,2′-bipyridine; terpy=2,2′:6′,2″-terpyridine), the phen and tptz complexes permit to address the effects of the preorganization, basicity and softness of these bi- and terdentate ligands on the coordination behaviour of the trivalent cerium and uranium ions.Addition of phen to MX3 (M=Ce, U; X=I, OTf) in pyridine or acetonitrile led to the formation of the mono-, bis- and tris-phen adducts. The tetrakis-phen derivatives were also obtained except for X=I in the more coordinating solvent pyridine. The greater affinity of phen for M(OTf)3 than for MI3 reflects the stronger Lewis acidity of the metal centre. The X-ray crystal structures of CeI3(phen)2(py)·py (1a·py), [CeI2(phen)2(py)2][I] (1b), UI3(phen)2(py)·1.5py (1c·1.5py), [CeI2(phen)3][I]·MeCN (2a·MeCN)], [UI2(phen)3][I]·MeCN (2b·MeCN), [M(phen)4(MeCN)2][I]3·xMeCN [M=Ce (3a·2MeCN) and U (3b·4MeCN)], Ce(OTf)3(phen)2(py)2·py (4·py), Ce(OTf)3(phen)3·2py (5a·2py), Ln(OTf)3(phen)3·py (L=Pr, Nd, Sm) and [M(OTf)2(phen)4][OTf] [M=Ce (6a) and U (6b)] were determined. The OTf− anion is a better ligand than I− and the complexes are generally less dissociated in ion pairs, as shown by the bis-phen and tris-phen complexes 4 and 5 which are neutral instead of cationic in 1b and 2, and the tetrakis-phen complexes 6 which are monocationic instead of tricationic in 3.The bis-Me4phen complexes MI3(Me4phen)2(py) [M=Ce (7a) and U (7b)] were obtained by treatment of MI3 with Me4phen in pyridine, while similar reaction with Ce(OTf)3 gave Ce(OTf)3(Me4phen)2(py)2 (8a) in pyridine and [Ce(OTf)2(Me4phen)2(μ2-OTf)]2 (8b) in acetonitrile, clearly reflecting the weaker coordinating capacity of MeCN. In contrast to that observed with the MI3/phen system, the tetrakis-Me4phen derivatives [MI(Me4phen)4][I]2 [M=Ce (9a) or U (9b)] could be obtained in pyridine, as well as in acetonitrile, in line with the stronger basicity of Me4phen compared to phen. In contrast to CeI3, no difference was observed in the coordination of 4equiv of phen or Me4phen to Ce(OTf)3 and crystals of [Ce(OTf)2(Me4phen)4][OTf]·3py (10·3py) were deposited from a pyridine solution. Solvates of complexes 7–10 have been crystallographically characterized.Treatment of LnX3 (Ln=Ce, Nd; X=I, OTf) in pyridine or acetonitrile with tptz afforded mono and bis-adducts and the crystal structures of solvates of CeI3(tptz)(MeCN)2 (11a), Ce(OTf)3(tptz)2 (14), [NdI2(tptz)2(py)][I] (15), [CeI2(tptz)2(H2O)][I] (16), and 0.8CeI3(tptz)2·0.2[CeI2(tptz)2(MeCN)][I]·0.5MeCN (18) were determined. Addition of 3equiv of tptz to CeX3 did not afford tris–tptz complexes except in the case of X=I in acetonitrile, affording crystals of [CeI(tptz)3][I]2·2MeCN (20a·2MeCN). By comparison with the terpy analogues, the 1H NMR spectra and crystal structures of the tptz compounds reveal the weaker basicity and electron donating capacity of tptz. While treatment of CeI3 with tptz or terpy and treatment of UI3 with terpy led to the formation of the corresponding Lewis base adducts, UI3 reacted with 1equiv of tptz in acetonitrile to give the dinuclear uranium(IV) oxidation compound {UI3(MeCN)}2(μ-tptz–tptz) (21a) resulting from electron transfer from the UIII ion to the azine molecule, followed by dimerization of the anion-radical tptz−. In the presence of 2equiv of tptz in pyridine, the UIII→UIV oxidation did not occur with the less electron rich U(OTf)3 which was transformed into U(OTf)3(tptz)2·2py (22·2py) and {U(OTf)2(tptz)2}2(μ-OTf)2·2py (23·2py). Crystal structures of 21–23 have been determined.