Lanthanide(III)/actinide(III) differentiation in the cerium and uranium complexes [M(C5Me5)2(L)]0,+ (L=2,2′-bipyridine, 2,2′:6′,2-terpyridine):: Structural, magnetic, and reactivity studies

Treatment of [Ce(Cp*)211 or [U(Cp*)(2)I(py)] with 1 mol equivalent of bipy (Cp* = C5Me5; bipy = 2,2'-bipyridine) in THF gave the adducts [M-(Cp*)(2)I(bipy)] (M=Ce (1a), M=U (1b)), which were transformed into [M-(Cp*)(2)(bipy)] (M=Ce (2a), M=U (2b)) by Na(Hg) reduction. The crystal structures of la and 1b show, by comparing the U-N and Ce-N distances and the variations in the C-C and C-N bond lengths within the bidentate ligand, that the extent of donation of electron density into the LUMO of bipy is more important in the actinide than in the lanthanide compound. Reaction Of [Ce(Cp*)211 or [U(Cp*),I(py)] with 1 mol equivalent of terpy (terpy= 2,2':6',2-terpyridine) in THF afforded the adducts [M(Cp*)2(terpy)]l (M=Ce (3a), M=U (3b)), which were reduced to the neutral complexes [M(Cp*)(2)-(terpy)] (M=Ce (4a), M=U (4b)) by sodium amalgam. The complexes [M(Cp*)(2)(terpy)] [M(Cp*)(2)}]I (M=Ce (5a), M=U (5b)) were prepared from a 2:1 mixture of [M(Cp*),I] and terpy. The rapid and reversible electron-transfer reactions between 3 and 4 in solution were revealed by (HNMR)-H-1 spectroscopy. The spectrum of 5b is identical to that of the 1:1 mixture of [U(Cp*)(2)I(py)] and 3b, or [U(Cp*)(2)I-2] and 4b. The magnetic data for 3 and 4 are consistent with trivalent cerium and uranium species, with the formulation [M-III(Cp*)(2)(terpy(center dot-))] for 4a and 4b, in which spins on the individual units are uncoupled at 300 K and anti-ferromagnetically coupled at low temperature. Comparison of the crystal structures of 3b, 4b, and 5b with those of 3a and the previously reported ytterbium complex [Yb(Cp*)(2)(terpy)] shows that the U-N distances are much shorter, by 0.2 A, than those expected from a purely ionic bonding model. This difference should reflect the presence of stronger electron transfer between the metal and the terpy ligand in the actinide compounds. This feature is also supported by the small but systematic structural variations within the terdentate ligands, which strongly suggest that the LUMO of terpy is more filled in the actinide than in the lanthanide complexes and that the canonical forms [U-IV(Cp*)(2)(terpy(-))] land [U-IV(Cp*)(2)(terpy(2-))] contribute significantly to the true structures of 3b and 4b, respectively. This assumption was confirmed by the reactions of complexes 3 and 4 with the H-center dot and H+ donor reagents Ph3SnH and NEt3HBPh4, which led to clear differentiation of the cerium and uranium complexes. No reaction was observed between 3 a and Ph3SnH, while the uranium counterpart 3b was transformed in pyridine into the uranium(iv) compound [U(Cp*)(2){NC5H4(py)(2)}]I (6), where NC5H4(py)(2) is the 2,6-dipyridyl(hydro-4-pyridyl) ligand. Complex 6 was further hydrogenated to [U(Cp*)2-{NC5H8(py)(2)}]I (7) by an excess of Ph3SnH in refluxing pyridine. Treatment of 4a with NEt3HBPh4 led to oxidation of the terpy(center dot-) ligand and formation of [Ce(Cp*)(2)(terpy)]BPh4, whereas similar reaction with 4b afforded [U(Cp*)(2){NC5H4(py)(2)}]BPh4 (6). The crystal structures of 6, 6' and 7 were determined.