Dinitrogen Reduction Operated by a Samarium Macrocyclic Complex. Encapsulation of Dinitrogen into a Sm2Li4 Metallic Cage

Abstract

In spite of the considerable number of dinitrogen complexes reported since the discovery of the first example of dinitrogen fixation in [ R u ( N H~)~]~+ 30 years ago,' the chemistry of Nz still remains in its infancy. However, new perspectives for the metalpromoted activation fixation of dinitrogen are opening up, as has been recently seen in both the reversible dinitrogen activation operated by the lithium countercation of an anionic Zr(II1) complex2 and the considerable reduction of NZ performed by electron deficient low-valent early transition metals.3 In thelatter case, the variety of bonding modes of dinitrogen associated with these systems4 has been carefully investigated and r a t i~n a l i z e d .~~Low-valent lanthanides, and in particular Sm(II), are potentially good systems for studying the coordination/reduction of dinitrogen.5 Samarocene dinitrogen complex (Cp*2Sm)z(N,) provides a puzzling example where the strikingly short N-N distance: which suggests minimal N2 reduction, is in contrast to the side-on bonding mode. Unfortunately low-valent lanthanides are rare and, besides the bis(cyclopentadieny1) systems,' limited to a very few examples.* To date, ( C P *~S~)~ ( N , ) remains the sole case of coordination of dinitrogen in the lanthanide series.We describe in this paper the preparation of a low-valent Sm macrocyclic complex and its reaction with dinitrogen to form a (1) (a) Allen, A. D.; Senoff, C. V. (4) (a) Fryzuk, M. D.; Haddad, T. S.; Mylvaganam, M.; McConville, D. H.; Rettig, S. J. Scheme 1 (lXF),Li!OEPG) Sm N, Li, / ' -L i SmCI,(THF), t (OEPG)Li,(THF), -% Ar "green crystals" + / < / h i Sm 1 (THF),L~~OEPG~tetralithium hydrazide salt. The choice of the porphyrinogen tetraanion as a spectator ligand was 2-fold. First, we were interested in studying the behavior of the highly reactive Sm(I1) center in a coordination environment sterically and electronically more flexible than that provided by two cyclopentadienyl rings. Second, the alkali cations, easily retained by the porphyrinogen ligand,lob-c may play an important role in promoting molecular activation processes at the Sm(I1) center. The reduction of the mixture obtained from the reaction of SmC13(THF)3 with the colorless tetralithium salt of octaethylporphyrinogen (OEPG)9 (OEPG)L&(THF)410 under argon may be conveniently carried out in a one-pot preparation forming a gold yellow complex.ll The subsequent reduction with 1 equiv of metallic lithium proceeds at room temperature in THF under argon to yield a deep-green, extremely air-sensitive crystalline material (Scheme 1). While the extreme air-sensitivity of this species in combination with easy loss of volatile components has prevented formula determination by elemental analysis, recrystallization of this complex from THF under nitrogen yielded moderately air-stable orange-red crystals of [ (THF)zLi(OEPG)-The formation of 1 is a perfectly reproducible process, and it can be achieved both by slow recrystallization of the green solid from a dilute THF solution under N2 (5% yield) and by ex...

Highlights in the Renaissance of Amidometal Chemistry

“…Reaction of the ªgreen complexº with N 2 leads to an interesting dinitrogen compound. [80] The NÀN distance of the complexed nitrogen is 1.525(4) , which is in agreement with an NÀN single bond. This means that a four-electron reduction has taken place.…”

Highly Efficient Hydrophosphonylation of Aldehydes and Unactivated Ketones Catalyzed by Methylene‐Linked Pyrrolyl Rare Earth Metal Amido Complexes

“…However, the addition to 2'-methoxyacetophenone afforded 10 b in an excellent yield of 93 % (Table 5, entry 2), which can be attributed to the ortho-chelate effect of the substrate, which presumably facilitated the coordination of 2'-me-A C H T U N G T R E N N U N G thoxyA C H T U N G T R E N N U N G acetophenone to the active metal center. For heteroaromatic ketones, the a-hydroxy phosphonates 10 n-10 r were also obtained in good yields (Table 5, entries [14][15][16][17][18]. It is worth noting that a series of a-hydroxy diaryl phosphonates could also be obtained in high yields through additions of diethyl phosphite to substituted benzophenones in the presence of the yttrium amide 2 as a catalyst (Table 5, entries [19][20][21][22][23][24], except in the case of 2,4'-difluorobenzophenone (51 % yield, entry 23), probably due to steric hindrance of the substrate.…”

“…[13] It has been demonstrated that lanthanide complexes containing pyrrolide ligands produced unexpected structures and displayed various reactivities with regard to polymerization, reduction of N 2 , and reversible fixation of ethylene. [14] We have reported that methylene-linked pyrrolyl lanthanide amido complexes were able to catalyze additions of the NÀH bond of the amines to carbodiimides. [15] Here we report the synthesis and characterization of rare earth metal amides with methylene-linked pyrrolyl-amido ligands exhibiting h 1 and h 5 bonding modes, together with their first applications as catalysts for hydrophosphonylations of aldehydes and unactivated ketones.…”

Heterocyclopentadienyl Complexes of Group-3 Metals

“…[59] The reaction of lithium with [(Oepg)ClSm(µ 3 -Cl)Li 3 (THF) 4 ] [formed in situ by reaction of SmCl 3 (THF) 3 and OepgLi 4 ] under nitrogen resulted in nitrogen reduction and the formation of an Sm III complex containing a tetralithium hydrazide, [{(Oepg)SmLi (THF) 2 } 2 (N 2 Li 4 )] (73), presumably through a transient Sm II species which is reoxidized to Sm III by means of an electron transfer to N 2 . [60] A different series of complexes was obtained when the reaction stoichiometry of Sm and Oepg is 2:1. The reaction of 2 equivalents of SmCl 3 (THF) 3 76), was isolated.…”