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*)(2)I] or U(Cp*)(2)I(py)] with 1 mol equivalent of bipy (Cp*=C(5)Me(5); bipy=2,2'-bipyridine) in THF gave the adducts M(Cp*)(2)I(bipy)] (M=Ce (1 a), M=U (1 b)), which were transformed into M(Cp*)(2)(bipy)] (M=Ce (2 a), M=U (2 b)) by Na(Hg) reduction. The crystal structures of 1 a and 1 b 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*)(2)I] or U(Cp*)(2)I(py)] with 1 mol equivalent of terpy (terpy=2,2':6',2'-terpyridine) in THF afforded the adducts M(Cp*)(2)(terpy)]I (M=Ce (3 a), M=U (3 b)), which were reduced to the neutral complexes M(Cp*)(2)(terpy)] (M=Ce (4 a), M=U (4 b)) by sodium amalgam. The complexes M(Cp*)(2)(terpy)]M(Cp*)(2)I(2)] (M=Ce (5 a), M=U (5 b)) were prepared from a 2:1 mixture of M(Cp*)(2)I] and terpy. The rapid and reversible electron-transfer reactions between 3 and 4 in solution were revealed by (1)H NMR spectroscopy. The spectrum of 5 b is identical to that of the 1:1 mixture of U(Cp*)(2)I(py)] and 3 b, or U(Cp*)(2)I(2)] and 4 b. The magnetic data for 3 and 4 are consistent with trivalent cerium and uranium species, with the formulation M(III)(Cp*)(2)(terpy(*-))] for 4 a and 4 b, in which spins on the individual units are uncoupled at 300 K and antiferromagnetically coupled at low temperature. Comparison of the crystal structures of 3 b, 4 b, and 5 b with those of 3 a 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(*-))]I and U(IV)(Cp*)(2)(terpy(2-))] contribute significantly to the true structures of 3 b and 4 b, respectively. This assumption was confirmed by the reactions of complexes 3 and 4 with the H(.) and H(+) donor reagents Ph(3)SnH and NEt(3)HBPh(4), which led to clear differentiation of the cerium and uranium complexes. No reaction was observed between 3 a and Ph(3)SnH, while the uranium counterpart 3 b was transformed in pyridine into the uranium(IV) compound U(Cp*)(2){NC(5)H(4)(py)(2)}]I (6), where NC(5)H(4)(py)(2) is the 2,6-dipyridyl(hydro-4-pyridyl) ligand. Complex 6 was further hydrogenated to U(Cp*)(2){NC(5)H(8)(py)(2)}]I (7) by an excess of Ph(3)SnH in refluxing pyridine. Treatment of 4 a with NEt(3)HBPh(4) led to oxidation of the terpy(*-) ligand and formation of Ce(Cp*)(2)(terpy)]BPh(4), whereas similar reaction with 4 b afforded U(Cp*)(2){NC(5)H(4)(py)(2)}]BPh(4) (6'). The crystal structures of 6, 6' and 7 were determined.