Synthesis, Electrochemical, and Computational Studies of Organocerium(III) Complexes with Ce–Aryl Sigma Bonds

Abstract

A set of organolanthanide(III) complexes featuring Ce(III)−C aryl σ-bonds have been synthesized and characterized. The syntheses follow salt metathesis reactions of the salts: ortholithiated oxazoline (Li-ortho-oxa) (L1) and Li 2 -biphenyl (L2) with t h e c o r r e s p o n d i n g m e t a l l o c e n e p r e c u r s o r s : 1 -L n ([(C 5 Me 5 ) 2 LnKCl 2 (THF)] n ) Ln = La and Ce, 2-La, ( [ ( C 5 M e 4 S i M e 3 ) 2 L a K C l 2 ( T H F ) ] n ) a n d 2 -C e ([((C 5 Me 4 SiMe 3 ) 2 CeKCl 2 )(Et 2 O) 0.5 ] n ). Electrochemical studies were performed for the organolanthanide(III) complexes. Reversible Ce(III)/Ce(IV) redox couples were observed for (C 5 Me 5 ) 2 Ce(κ 2 -ortho-oxa) (3-Ce) and [Li(DME) 3 ][(C 5 Me 5 ) 2 Ce-( b i p h e n ) ] ( 5 -C e ) c o m p l e x e s . H o w e v e r , c o m p l e x (C 5 Me 4 (SiMe 3 )) 2 Ce(κ 2 -ortho-oxa) (4-Ce) showed an irreversible Ce(III/IV) oxidation wave.A DFT-computed redox potential for 3-Ce showed good agreement with experiment. For 5-Ce, calculations indicated that the redox potential may be influenced strongly by speciation changes associated with the Li(DME) 3 + cation becoming outer-sphere. Bonding analysis suggests strong bond polarization in the title complexes (3-Ce and 5-Ce) with weights of 7−8% from Ce in the Ce−C aryl donation-bond orbitals. Attempts to isolate Ce(IV) species from these complexes resulted into the decomposition of the products into unidentified Ce(III) species, indicating that the bulky and electron-donating cyclopentadienide derivatives, ortho-oxazolide ligand, and/or the biphenylide ligand do not effectively stabilize the organocerium(IV) complexes against decomposition under the conditions employed.

Synthesis and reactivity of a uranium(iv) complex supported by a monoanionic nitrogen–phosphorus ligand

“…52 They are also close to those of the U–C alkyl single bond in [Li(THF) 4 ][U(CH 2 t Bu) 5 ] (2.47(1)–2.51(1) Å), 63 [Li(THF) 4 ][Li(THF) 2 U(CH 3 ) 6 ] (2.500(5)–2.615(5) Å), 64 UCp*(TMTAA)(CH 2 TMTS) (TMTAA = tetramethyl-tetra-aza-annulene) (2.48(1) Å), 65 [fc(NSi t BuMe 2 ) 2 U(CH 2 Ph)(OEt 2 )][BPh 4 ] (2.482(12) Å), 47 and [η 5 -1,2,4-(Me 3 C) 3 C 5 H 2 ] 2 U(η 2 -C 4 Ph 2 ) (2.448(5)–2.475(5) Å). 66 The bond angle of C1–U1–C2 is 69.1(7)°, which is close to the C–Ln–C angles in [Li(DME) 3 ][(C 5 Me 5 ) 2 Ln(biphen)] (Ln = Ce, 69.0(7)°; Ln = La, 68.55(8)°) 60 but is smaller than the C–M–C angles in other reported metallafluorenes (74°–130°). 56–61,67,68 The bond lengths of U–N amine (average of 2.735 Å) and U–N amido (average of 2.316 Å) are longer than those observed in complex 1 (2.695 and 2.249 Å, respectively), which is probably due to the steric hindrance of the biphenyl group.…”

“…66 The bond angle of C1–U1–C2 is 69.1(7)°, which is close to the C–Ln–C angles in [Li(DME) 3 ][(C 5 Me 5 ) 2 Ln(biphen)] (Ln = Ce, 69.0(7)°; Ln = La, 68.55(8)°) 60 but is smaller than the C–M–C angles in other reported metallafluorenes (74°–130°). 56–61,67,68 The bond lengths of U–N amine (average of 2.735 Å) and U–N amido (average of 2.316 Å) are longer than those observed in complex 1 (2.695 and 2.249 Å, respectively), which is probably due to the steric hindrance of the biphenyl group. Complex 3 represents a new example of a heterofluorene containing an actinide element.…”

“…Heteroatoms in the heterofluorenes could be main group elements, 56,57 transition metals, 58,59 and rare-earth metals. 60,61 With complex 1 in hand, we attempted to synthesize a uranium-containing metallafluorene species via salt metathesis. Treatment of complex 1 with one equivalent of 2,2′-dilithiobiphenyl 62 in THF at RT overnight afforded a brown turbid solution, from which complex 3 was isolated in 76% yield as a yellow crystalline solid (Scheme 2).…”

“…The bond lengths of U1-C1 (2.50(2) Å) and U1-C2 (2.482(17) Å) are shorter than the U-C aryl bond lengths in [Li] ). [56][57][58][59][60][61]67,68 The bond lengths of U-N amine (average of 2.735 Å) and U-N amido (average of 2.316 Å) are longer than those observed in complex 1 (2.695 and 2.249 Å, respectively), which is probably due to the steric hindrance of the biphenyl group. Complex 3 represents a new example of a heterofluorene containing an actinide element.…”

“…Heteroatoms in the heterofluorenes could be main group elements, 56,57 transition metals, 58,59 and rare-earth metals. 60,61 With complex 1 in hand, we attempted to synthesize a uranium-containing metallafluorene species via salt Scheme 1 Synthesis of complexes 1 and 2. Fig.…”