Volker Lorenz

Anja Edelmann

Stephan Gießmann

Cristian Hrib

Steffen Blaurock

Dr Frank T Edelmann
email: frank.edelmann@ovgu.de

Professor Herbert W Roesky

Disiloxanediolates and Metallasilsesquioxanes of the Rare Earth Elements

Keywords: Lanthanides, Disiloxanediolates, Silsesquioxanes, Metallasilsesquioxanes, Crystal Structures Lanthanides, Disiloxanediolates, Silsesquioxanes, Metallasilsesquioxanes, Crystal Structures

This research report summarizes recent results in the chemistry of lanthanide disiloxanediolates and metallasilsesquioxanes. Both classes of compound can be regarded as realistic model compounds for silica-supported lanthanide catalysts.

Heterobimetallic siloxanediolates of the lanthanides

The chemistry of metallasiloxanes derived from silanediols, disiloxanediols and related Si-OH species continues to be an area of vigorous research activities [1-7] because such compounds are valuable precursors for metal oxides and silicates [2,3] as well as models for silica-supported heterogeneous catalysts [1,4], or are catalytically active themselves [5]. A particularly useful ligand in this field is the tetraphenyldisiloxanediolate dianion, [(Ph 2 SiO) 2 O] 2-, which gives rise to a variety of unusual and unexpected structures with alkali metals [6] and early transition metals [6,7]. The starting material 1,1,3,3tetraphenyl-1,3-disiloxanediol, Ph 2 Si(OH)OSiPh 2 (OH) (1), is readily accessible from cheap precursors. Hydrolysis of Ph 2 SiCl 2 in the presence of (NH 4 ) 2 CO 3 affords Ph 2 Si(OH)OSiPh 2 (OH) in yields up to 51%. Minor by-products in this synthesis are diphenylsilanediol, Ph 2 Si(OH) 2 (2, 6%) and hexaphenyltrisiloxanediol, Ph 2 Si(OH)O-SiPh 2 OSiPh 2 (OH) (3, 18%) [8]. Group 3 metal (Sc, Y) and lanthanide complexes containing the [(Ph 2 SiO) 2 O] 2-ligand have been investigated mainly in our laboratory [9]. 1.1 Lanthanide(III) disiloxanediolates a) Lanthanide(III) mono(disiloxanediolates) By now, the chemistry of lanthanide bis(disiloxanediolates) is well established, and some lanthanide tris(disiloxanediolates) have also been prepared and structurally characterized (vide infra) [9]. Only the synthesis of mono-substituted rare earth metal complexes derived from difunctional siloxanediolate ligands had not been achieved until recently. Thus far only one compound of this type has been synthesized and structurally

characterized [10]. A reaction of anhydrous YbCl 3 with in situ prepared (Ph 2 SiOLi) 2 O (4) (molar ratio 1:1) in THF solution gave a colorless product which analyzed well for the mono-substituted species [(Ph 2 SiO) 2 O]YbCl(THF) 3 , but an unambiguous structural characterization failed thus far due to the lack of suitable single-crystals. We then reasoned that the closely related 1,1,3,3,5,5-hexaphenyl-1,3,5-trisiloxanediolate ligand would perhaps be a better suitable candidate for preparing a mono-substituted ytterbium species.

1,1,3,3,5,5-hexaphenyl-1,3,5-trisiloxanediol (3), like its lighter homologue 1 is readily available from the same reaction, i.e. the controlled hydrolysis of dichlorodiphenylsilane [8]. The preparation of the first mono(trisiloxanediolate) complex of a lanthanide element was achieved according to Scheme 1 by allowing anhydrous YbCl 3 to react first with 2 equivalents of LiN(SiMe 3 ) 2 , followed by treatment of the intermediate "[(Me 3 Si) 2 N] 2 YbCl" with 3 in a 1:1 molar ratio. Crystallization from the concentrated reaction mixture afforded the mono(trisiloxanediolate) complex [Ph 2 Si(OSiPh 2 O) 2 ]Yb(THF)(µ-Cl) 3 Li 2 (THF) 4 (5) in the form of colorless crystals in 93% isolated yield (determined after the crystal structure had been established). Si 3 O 4 Yb ring system resulting from coordination of the 1,1,3,3,5,5-hexaphenyl-1,3,5trisiloxanediolate dianion to ytterbium in a chelating fashion. Complexes containing chelating 1,1,3,3,5,5-hexaphenyl-1,3,5-trisiloxanediolate have previously been reported e.g.

for hafnium and uranium [6,7,9]. In these cases, however, the trisiloxanediolate ligand is formed by ring-enlargement reactions starting from disiloxanediolate precursors. A prominent structural feature of compound 5 is the presence of a so-called "ate" complex resulting from retention of lithium chloride. "Ate" complexes are a common phenomenon in organolanthanide chemistry. Their formation involves retention of alkali halides formed during the course of a reaction, thereby reflecting the strong tendency of the large lanthanide ions to achieve high coordination numbers [11]. In compound 5 the situation is such that a complex of the type "[Ph 2 Si(OSiPh 2 O) 2 ]YbCl" is sterically highly unsaturated (coordination number 3 at Yb). In this case stabilitzation is achieved not only by coordination of one THF ligand but also through "ate" complex formation with retention of two equivalents of LiCl. One lithium chloride is connected to the central ytterbium via two bridging chloride ligands. The second LiCl is coordinated in a rather peculiar way by additional coordination of a siloxide oxygen atom to Li. In both cases, the coordination sphere around lithium is supplemented by two THF ligands (Fig. 1) [10]. Heterobimetallic Group 3 metal (Sc, Y) and lanthanide complexes containing two [(Ph 2 SiO) 2 O] 2-ligands form the largest and most thoroughly investigated group of compounds in this area [9]. It was found that the small Sc 3+ and Y 3+ ions form heterobimetallic complexes in which the Group 3 metal fits into the center of a twelvemembered Si 4 O 6 Li 2 inorganic ring system formed by two lithium disiloxanediolate units.

Additional chloro functionalities and solvent molecules are arranged in the trans-positions.

Accordingly these complexes have been termed "metallacrown" derivatives of scandium and yttrium [9]. In contrast, large Ln 3+ ions such as Pr 3+ , Nd 3+ or Sm 3+ do no fit into the center of the twelve-membered Si 4 O 6 Li 2 inorganic ring system but are significantly displaced, leading to a series of bis(disiloxanediolate) complexes which have been termed "inorganic lanthanide metallocenes" [9]. It was shown that this new class of heterobimetallic lanthanide disiloxanediolates (Scheme 2a) shares structural similarities with the well-known bent metallocenes containing pentamethylcyclopentadienyl (= C 5 Me 5 ) ligands (Scheme 2b). The latter form a large and well-investigated class of organolanthanides with many of them displaying high catalytic activities in various olefin transformations [11]. In both cases two bulky ligands are coordinated to the central lanthanide ion in a bent geometry, leaving room for functionalities X (X = Cl, N(SiMe 3 ) 2 , alkyl, H etc.) as well as additional solvent molecules S (S = Et 2 O, THF etc.). The preparation of heterobimetallic disiloxanediolates of scandium and yttrium was performed as illustrated in Schemes 3 and 4. In the first step, ScCl 3 (THF) 3 or anhydrous YCl 3 were allowed to react with LiN(SiMe 3 ) 2 in order to produce in situ the "ate"- In the case of holmium and erbium, the

complexes [(Me 3 Si) 2 N] 3 M(µ-Cl)Li(THF) 3 [12].

Scheme 5

In both cases no meaningful NMR spectra could be obtained due to the strongly paramagnetic nature of the Ho 3+ and Er 3+ ions, respectively. However, the structural analyses revealed that due to the larger ionic radii of Ho 3+ and Er 3+ as compared to Sc 3+ both complexes adopt the "inorganic metallocene" structural type. This means that the lanthanide metal is significantly displaced from the center of the twelve-membered Si 4 O 6 Li 2 inorganic ring system formed by two lithium disiloxanediolate units. However, the use of these intermediate size lanthanide ions leads to formation of a new structural variety in that the molecules are "unsolvated", i.e. no solvent molecules are coordinated to the lanthanides. This is in contrast to the related Pr, Nd and Sm derivatives (vide infra), in which the central lanthanide ion is large enough to leave room for an additional chelating DME ligand. Thus the new holmium and erbium derivatives represent an intermediate structural type of lanthanide bis(disiloxanediolates) between the "inorganic metallocenes" (Pr, Nd, Sm) and the "metallacrowns" (Sc, Y). At present, however, we cannot offer a plausible explanation for the differences in the solid-state structures of the Ho/Er complexes 8a and 8b on one hand and the related yttrium complex 7 on the other, especially as there are no significant differences in the ionic radii between these metals.

Two lithium disiloxanediolate units are connected to form a twelve-membered Si 4 O 6 Li 2 metallacycle to which the lanthanide ion is coordinated in an out-of-plane fashion. The coordination sphere around the lanthanide is completed by a single chloro-functionality with no room left for additional solvent molecule coordination. A first compound containing additional coordinated solvent was the samarium complex [{(Ph 2 SiO) 2 O} 2 {Li(DME)} 2 ]SmCl(DME) (9) which was prepared by allowing anhydrous SmCl 3 to react with 3 equivalents of LiN(SiMe 3 ) 2 in order to produce in situ the "ate" complex [(Me 3 Si) 2 N] 3 Sm(µ-Cl)Li(THF) 3 (10) which was then treated with 2 equivalents of 1. Praseodymium and neodymium were also employed as representatives of the larger early lanthanide elements. The ionic radius of Pr 3+ in hexacoordinate complexes

Scheme 6

A single-crystal X-ray diffraction study of 11 revealed the presence of a heterobimetallic praseodymium disiloxanediolate which fits into the concept of "inorganic metallocenes". This result clearly demonstrated the dependence of the molecular structures of these compounds on the ionic radius of the central Ln 3+ ion. In 11 the [{(Ph 2 SiO) 2 O} 2 {Li(THF) 2 }{Li(THF)}] 2-ligand system is coordinated to praseodymium in a slightly bent arrangement, because the Pr 3+ ion is too large to fit into the cavity of the metallacrown. As a consequence, the additional ligands are forced into the cis-positions. In contrast to the trans-arrangement of the chloride and solvent ligands in 6 and 7, the Cl ligands in 11 are in cis-positions. One Li is coordinated by two THF molecules, while the other one has only one THF ligand. In the latter case there is an additional weaker Li-Cl interaction with a distance of 2.667(6) Å. Due to this weak Li-Cl bond the overall structure of 11 approaches a trigonal-prismatic coordination geometry. The most important result of the crystal structure determination of 11 is the fact that this compound too can be regarded as an "inorganic metallocene" or out-of-plane metallacrown complex.

Scheme 9

The formation of 14 can be explained by the fact that an excess of LiN(SiMe 3 ) 2 (5 equivalents) was used for the preparation of 11 instead of 3 equivalents as would be required. Thus it is likely that the silylamide with LiN(SiMe 3 ) 2 failed due to the formation of complex product mixtures. Crystallization of 14 from a concentrated solution in THF yielded pale green crystals in 65% yield. An Xray analysis confirmed the presence of the silylamide-functionalized praseodymium bis(disiloxanediolate) complex. As expected, this compound too belongs to the class of "inorganic metallocene"-type lanthanide bis(disiloxanediolates) comprising an out-of-plane coordination of the Ln 3+ ion to the twelve-membered Si 4 O 6 Li 2 inorganic ring system formed by two lithium disiloxanediolate units. Only one closely related compound, the 15) had been reported earlier in the literature [9a,b]. This compound was prepared as outlined in Scheme 10.

gadolinium(III) complex [{(Ph 2 SiO) 2 O} 2 {Li(THF) 2 } 2 ]GdN(SiMe 3 ) 2 (
(THF) 2 Li(THF) 2 O Gd O O Li Si Ph Ph O Si Ph Ph O Si Ph Ph O Si Ph Ph N Me 3 Si SiMe 3 THF [(Me 3 Si) 2 N] 3 Gd(µ-Cl)Li(THF) 3 + 2 Ph 2 Si OH O SiPh 2 OH 1 15

Scheme 10

In an attempt to prepare a disiloxanediolate complex of divalent samarium, 1 was first deprotonated using NaN(SiMe 3 ) 2 in THF solution to give (Ph 2 SiONa) 2 O ( 16

Scheme 11

The X-ray study confirmed the formation of a samarium(III) disiloxanediolate complex. In the solid state, compound 17 forms a polycyclic ring system which can be formally regarded as an "ate" complex derived from the hypothetical anionic species "[Na(THF)] 3 [Sm 3+ {(Ph 2 SiO) 2 O} 3 ]". The "ate" complex results from addition of one equivalent of Me 3 SiONa(THF) to the salt-like complex. Although the mechanism of the formation of 17 remains speculative, the source of the Me 3 SiO -ligand can only be the NaN(SiMe 3 ) 2 reagent used for the deprotonation of the disiloxanediol precursor. The oxidation of samarium(II) diiodide in the course of a reaction like that shown in Scheme 11 is not unusual. A typical example is the reaction of SmI 2 with two equivalents of Li[Ph 2 P(NSiMe 3 ) 2 ] which afforded the colorless samarium(III) "ate" complex [Ph 2 P(NSiMe 3 ) 2 ] 2 Sm(µ-I) 2 Li(THF) 2 in 70% yield. [13] The low yield in the case of 17 can be traced back to the additional incorporation of the Me 3 SiO -anion, which is presumably only a side-product in this reaction.

In the complex molecule of 17 the central Sm 3+ ion is coordinated in a nearly octahedral fashion by six oxygen atoms of three chelating disiloxanediolate ligands (Fig. 5) [17] and (µ-η 8 :η 8 -COT)[Sm{N(SiMe 3 )}(THF) 2 ] 2 [18] the molecule contains an inversion center in the solid state with a Nd-(COT ring centroid)-Nd angle of 178.2°. The outer decks of the triple-decker sandwich structure in 18 are formed by twelve-membered Si 4 O 6 Li 2 inorganic ring systems, each of them being composed of two lithium tetraphenyldisiloxanediolate units. Due to their large ionic radii, the Nd 3+ ions are significantly displaced from the center of the twelve-membered rings. Quite surprisingly, the disiloxanediolate rings in 18 differ from those in the related mononuclear complexes in that the two Li + ions are differently solvated. Each of the ring ligands contains a tricoordinate Li(THF) unit and a tetracoordinate lithium with two THF ligands attached to lithium. However, the observation of only one 29 Si NMR resonance at δ = -46.4 of additional THF leads to a more symmetrical structure. It seems plausible that the reason for the different coordination modes around the Li + ions in the solid state is steric congestion in 18 which becomes evident from a space-filling view of the molecular structure. This view also gave a good impression of how phenyl groups of the opposite disiloxanediolate interlock, thereby encapsulating the central µ-η 8 :η 8 -COT ligand like "a golden ring in a silicate box" [9h]. For example, treatment of the scandium complex 6 (Scheme 3) with an excess of trimethylaluminum in toluene afforded a colorless solution from which colorless, blockshaped single crystals could be isolated in 50% yield. This reaction was initially performed with the aim of replacing the chloro ligand in trans-position by a methyl group. However, an X-ray crystal structure determination revealed the presence of a unique heterotrimetallic (Sc/Al/Li) disiloxanediolate complex 19 as illustrated in Scheme 13.

η 8 :η 8 -COT)[Sm{N(SiMe 3 ) 2 } 2 ] 2

Scheme 13

The most surprising result of this reaction was the replacement of one Li(DME) + unit in 6 by an AlMe 2 + moiety under retention of the chloro function in trans-position.

Despite the use of an excess of AlMe 3 only one lithium was replaced. Like the starting material 6, the trimetallic product 19 too adopts the metallacrown form with the central Sc 3+ ion accomodated well within the plane of the slightly puckered inorganic ring system.

As in the starting material 6, one THF ligand and the chloride ion occupy the transpositions. The central metallacrown core is efficiently shielded by eight phenyl substituents and three coordinated THF molecules, which accounts for the high solubility of the complex in toluene (Fig. 7) [9e,g]. For a first study of a reaction between an "inorganic metallocene"-type lanthanide bis(disiloxanediolate) and a Group 13 metal trialkyl we chose the praseodymium complex [{(Ph 2 SiO) 2 O} 2 {Li(DME)} 2 ]PrCl(DME) (5) as a typical representative of this series [9g].

Treatment of this starting material with trimethylindium in a THF/toluene solvent mixture resulted in a colorless reaction mixture from which colorless crystals could be isolated. The compound is virtually insoluble in hydrocarbon solvents and diethyl ether, but well-formed single-crystals suitable for X-ray diffraction could be grown from a concentrated solution in THF. The crystal structure determination revealed the formation of an unexpected anionic complex as illustrated in Scheme 15.

[

Scheme 15

According to the X-ray diffraction study (Fig. 9), the product obtained from treatment of 12 with 2 equivalents of InMe 3 is the ionic complex [Li(THF

b) Lanthanide(III) tris(disiloxanediolates)

Homoleptic lanthanide(III) tris(disiloxanediolates are quite rare. In the case of trivalent europium as central ion a homoleptic complex was obtained according th the synthetic route outlined in Scheme 17 [9a,b].

Et 2 O O Li O O O Eu O O Si O Si Li OEt 2 SiPh 2 Si O Et 2 O Li Et 2 O Si O SiPh 2 Ph 2 Ph 2 Ph 2 Ph 2 3 Ph 2 Si O SiPh 2 OH OH + Eu[N(SiMe 3 ) 2 ] 3 [LiCl(THF) 3 ] 3 1 23

Scheme 17

The X-ray structural analysis of 23 revealed the presence of a heterobimetallic metallasiloxane, in which the Eu 3+ ion is surrounded by three mono-anionic chelating According to the X-ray analysis the neodymium complex 24 contains a central core which is closely related but not identical with that of the monomeric europium derivative

23.

Three formally monoanionic lithium disiloxanediolate ligands are coordinated to Nd and the coordination sphere of two lithium ions is completed by an additional THF ligand.

In contrast to the molecular structure of 23 there are four tricoordinate disiloxanediolate oxygen atoms while two are tetracoordinate. In 24 a polymeric superstructure results from retention of lithium bromide, which connects these monomeric units to give a linear chain polymer [9b].

Cerium(IV) siloxanediolates

Due to their high oxidation potential cerium(IV) compounds are widely used in various areas of chemistry and technology. Important fields of application include organic synthesis [21], bioinorganic chemistry [22], materials science [23] and industrial catalysis (automotive three-way catalysts, oxygen storage etc.) [START_REF]Metal Ions in Biological Systems[END_REF]. Thus there is a constant demand for new, well-defined cerium(IV) species. x < 1) [26] and a cerium(IV) metallasilsesquioxane complex (cf. Section 2.2) [27]

Scheme 19

In the molecule of 27 two potassium disiloxanediolate ligands are coordinated to cerium(IV) in a slightly bent coordination geometry. Two tert-butoxide ligands remain as functionalizable groups coordinated to Ce. The central cerium is in a distorted octahedral coordination environment. One phenyl ring of each disiloxanediolate ligand is engaged in an intramolecular η 1 -π-coordination to potassium (K(1)-C( 22) 3.395(4) Å). The same reaction carried out in a molar ratio of 1:3 took an entirely different course. In this case a light yellow compound could be isolated in high yield (ca. 70%), which was shown by Xray crystallography to be the novel heterobinuclear potassium/cerium(IV) disiloxanediolate cluster [{(Ph 2 SiO) 2 O} 2 {(DME)KO t Bu}{(Ph 2 SiO 2 )K}Ce] 2 (28). The X-ray study revealed that in 28 not only the expected dianions of 1,1,3,3-tetraphenyl-1,3-disiloxanediol act as ligands, but also the dianions of diphenyl-1,1-silanediol, which originate from cleavage of the disiloxanediolate (Fig. 10). Such transformations are not uncommon in the chemistry of silanediolates, disiloxanediolates, and trisiloxanediolates [6,7,9a,9b] In the

Metallasilsesquioxanes of the lanthanides

Lanthanide(III) metallasilsesquioxanes

Polyhedral oligosilsesquioxanes of the general formula (RSiO 1.5 ) n form an interesting and unusual class of organosilicon compounds which currently have a strong impact on both catalysis research [29,30] and materials science [31]. In accordance with

Page 32 of 53

Wiley-VCH ZAAC several unique properties, the polyhedral silsesquioxanes have been termed the "smallest particles of silica possible" [32] or "small soluble chunks of silica" [33]. With respect to catalysis research the chemistry of metallasilsesquioxanes also receives considerable current interest [29,30,33,34]. Incompletely condensed silsesquioxanes such as (c- Metal complexes derived from 29 are therefore commonly regarded as "realistic" models for industrially important metal catalysts immobilized on silica surfaces. [29,30,35] It is mainly the latter aspect, i.e. the mimicking of heterogeneous metal catalysts with the use of soluble compounds, that currently attracts various research groups to the chemistry of incompletely condensed silsesquioxanes. Examples of silica-supported species which have been successfully modeled with the use of silsesquioxanes are e.g. Phillips-and Ziegler-Natta-type olefin polymerization catalysts as well as silica-supported perfluoroborato co-catalysts (Scheme 21) [29d,38].

R O M Cl Cl O Si O O O O Si O O B X F5 F 5 F5 O Zr O O R O Cr O O O O O Si Si O Si Si Si Si O O O O R R R R O Si O O O R R O O Si(CH3)3 Cr O O R F5 F5 R Si O Si Si O O O Si O O O Si O O O O Si Si O R R R R R R R O Si O Si O O O O Si Si O O O Si R R R R R O O Si Si O R Zr Zr CH 2 Ph CH2Ph F 5 X O Si O Si Si O O O O Si Si O O Si O O Si Si O R R R R R O R O M Cl Cl R O Si O Si Si O O O O Si Si O O Si O O Si Si O R R R R R O R B O R Scheme 21
The "Periodic Table of Metallasilsesquioxanes" [29b] already covers numerous metallic elements ranging from alkali metals through early and late transition metals to the lanthanide elements [29]. Information on well-characterized lanthanide silsesquioxane derivatives is still quite limited. Metallasilsesquioxanes of rare earth metals could be very important as homogeneous analogues of rare earth metal silica-supported catalysts and rare earth silicates, which are potential materials for optoelectronics [29d].

Thus this area of research is of considerable interest for several fields of chemistry including catalysis and materials science. However, investigations in the area of metallasilsesquioxanes of rare earth metals are often hampered by difficulties with crystallization and characterization of these compounds. The first metallasilsesquioxanes incorporating lanthanide elements have been described in 1994 by Herrmann et al. [39]. La, Pr, Eu, Yb) [40]. However, single crystals of these materials suitable for X-ray diffraction could not be obtained. The somewhat complicated situation is illustrated in Scheme 22. The lanthanide tris(silylamides) reacted with 2/3 equivalents of the trisilanol 29 in THF to give the lanthanide silsesquioxanes 30 which are dimeric in solution at 233 K.

Reaction of Ln[N(SiMe 3 ) 2 ] 3 with 1 equivalent of 29 in THF resulted in complete conversion of 29 into the trisilylated compound 31, as did the reaction of Ln[N(SiMe 3 ) 2 ] 3 with 2/3 equivalents of 29 in toluene [40].

29 O Si O Si Si O O O O Si Si O O OH O Si Si OH R R R R R OH R R O Si O Si Si O O O O Si Si O O Ln(THF) 2 O O Si Si O R R O R R R R R > 1 eq. LnL 3 (THF) O Si O Si Si O O O O Si Si O O O O Si Si O R R R R R O R R Si(CH 3 ) 3 Si(CH 3 ) 3
Si(CH 3 ) 3 (THF) 0.5 eq LnL 3

31

30 (Ln = Y, La, Pr, Eu, Yb) n eq. LnL 3 0.5 < n < 1 30 + 31 (THF)

toluene > 1 eq. LnL 3

31

+ HL + LnL 3 R = c-C 6 H 11 LnL 3 = Ln[N(SiMe 3 ) 2 ] 3

Scheme 22

Variable temperature 13 C NMR data of the Y derivative were consistent with a monomer/dimer equilibrium for compounds 30 in solution as depicted in Scheme 23 [40].

2 2 THF Si O Si Si O O O Si O O O Si O O O O Si Si O R R R R R R O Si O Si Si O O O O Si Si O O O O O Si Si O R R R R R R R Ln Ln THF THF R R = c-C 6 H 11 O Si O Si Si O O O O Si Si O O Ln(THF) 2 O O Si Si O R R O R R R R R

Scheme 23

The chloro-functionalized lanthanide silsesquioxane complexes 32 (Ln = Y, La) are accessible by the reaction of 30 with 1 equivalent of Me 3 SiCl according to Scheme 24 [40].

32, Ln = Y, La 30 O Si O Si Si O O O O Si Si O O Ln(THF) 2 O O Si Si O R R O R R R R R R = c-C 6 H 11 2 Me 3 SiCl O Si Si O Si Si O Si O O O O O O O Si Si O R R R R R O Me 3 Si Ln O Si O Si Si O O O O Si Si O O O O O Si Si O R R R R R R R Ln SiMe 3 Cl Cl THF THF R R

Scheme 24

Well-defined monomeric derivatives could be obtained by treatment of 30 (Ln = Y, La, Yb) with (Me 2 NCH 2 CH 2 ) 2 NMe (PMDTA) in a 1:1 molar ratio in THF solution. The products Cy 7 Si 7 O 12 Ln(PMDTA) (33,Ln = Y,La,Yb) are depicted in Scheme 25. In all these cases severe disorder problems prevented a structural characterization of the lanthanide silsesquioxanes by X-ray diffraction [40].

33, Ln = Y, La, Yb R = c-C 6 H 11 N N O Si O Si Si O O O O Si Si O O Ln O O Si Si O R R O R R R R R N Me Me Me

Me Me

Scheme 25

The ytterbium derivative Cy 7 Si 7 O 12 Yb(PMDTA) (33,Ln = Yb) was also obtained directly from Yb[N(SiMe 3 ) 2 ] 3 and 29 in the presence of PMDTA [41]. Attempts to grow single crystals of Cy 7 Si 7 O 12 Yb(PMDTA) resulted in a small number of good-quality prisms, which were shown by X-ray diffraction to be the heterobimetallic Li/Yb silsesquioxane complex [(Cy 7 Si 7 O 12 ){Cy 7 Si 7 O 11 (OSiMe 3 )}YbLi 2 (THF) 2 (MeCN)]·2.5THF (34). Compound 34 could be more rationally prepared by lithiation of Cy 7 Si 7 O 9 (OH) 2 (OSiMe 3 ) (35) with n BuLi and reaction of the resulting Cy 7 Si 7 O 9 (OLi) 2 (OSiMe 3 ) (36) with Cy 7 Si 7 O 12 Yb(THF) 2 (33,Ln = Yb). The yttrium analogue of 34 was prepared in a similar manner [41].

Arnold et al. reported the synthesis and structural characterization of an aryloxidefunctionalized samarium silsesquioxane complex [42]. Steric protection of ( [42].

t Bu O Si O Si O O O O Si Si O O Si O O Si Si O O Sm Li Si CH 3 CH 3 t Bu O O Si O Si Si Si Si O O O Si O Si O O R R R R R R R O O O Si CH 3 CH 3 Li O Ar R R R R R R R Ar = C 6 H 3 t Bu 2 -

Scheme 26

Our own efforts in this field resulted in the isolation of bimetallic gadolinium and ytterbium derivatives. These novel functionalized lanthanide silsesquioxanes were obtained as outlined in Scheme 27. Trisilanol 29 was lithiated in situ using an excess of LiN(SiMe 3 ) 2 to give 40, followed by treatment of the reaction mixture with LnCl 3 (THF) 3 (Ln = Gd, Yb) [43]. The resulting colorless crystals of 41 (Ln = Yb) were fully characterized by spectroscopic and analytical methods as well as an X-ray crystal structure determination. In this molecule a reactive ytterbium bis(trimethylsilyl)amide unit resides on a model silica surface formed by two lithium-linked silsesquioxane cages. Thus the compounds 41 (Ln = Gd, Yb) represented the first example of a mono-functional lanthanide silsesquioxane which could be of interest for catalytic applications [43].

Scheme 27

Two novel complexes of samarium-and scandium-containing silsesquioxane silanolate ligands were synthesized with the use of the closo-silsesquioxane silanolate ligand Cy 7 Si 8 O 12 O -and structurally characterized by X-ray diffraction [44]. Yellow (C 5 Me 5 ) 2 Sm[µ-Cy 7 Si 8 O 12 O] 2 Li(THF) (42), the first organolanthanide silsesquioxane complex reported in the literature, has been obtained by treatment of the "ate"-complex (C 5 Me 5 ) 2 Sm(µ-Cl) 2 Li(THF) 2 with Cy 7 Si 8 O 12 OLi (44) in a molar ratio of 1:2 (Scheme 28).

The starting material Cy 7 Si 8 O 12 OH (43) was prepared in close analogy to a published procedure [45] (44), which was used in situ for further reactions. An X-ray diffraction study revealed the presence of a heterobimetallic complex in which samarium and lithium are bridged by two silsesquioxane silanolate ligands [44].

A different synthetic approach was chosen to synthesize a related scandium silsesquioxane complex [44]. Treatment of (C 5 Me 5 )Sc(acac) 2 [46] with the free silanol pentamethylcyclopentadiene and formation of the novel binuclear scandium complex [Sc(acac) 2 (µ-Cy 7 Si 8 O 12 O)] 2 (45) (Scheme 28). Colorless single-crystals of 45 suitable for X-ray diffraction were obtained by fractional crystallization from pentane. The crystal structure consists of dimeric molecules in which two Sc(acac) 2 units are symmetrically bridged by the mono-anionic silsesquioxane silanolate ligands. Compound 45 represented the first example of a scandium complex containing silsesquioxane derivatives as ligands.

The only closely related compound is the trinuclear scandium disiloxanediolate complex [(Ph 2 Si 2 O) 2 O] 2 Sc 3 (acac) 5 which has been prepared in a similar manner by treatment of (C 5 Me 5 )Sc(acac) 2 with 1,1,3,3-tetraphenyldisiloxanediol [9b].

Scheme 28

Very recently, a tetrameric neodymium-silsesquioxane complex (46) was prepared according to Scheme 29 and structurally characterized by single-crystal X-ray diffraction.

The four Nd-silsesquioxane cages of 46 are linked by multiple Nd-O-Nd and Nd-Cl-Nd units. A preliminary study showed that the complex is active for highly cis-1,4 polymerization of isoprene (92% selectivity) in the presence of AlEt 3 and Me 3 SiCl [47].

= i Bu R OH OH R Si O R R Si O Si O R R O O Si Si R O O O Si OH Si O R R R R R Nd R Si O R R Si O O O Si O O O R Si O O O Nd R R Si O Si O R R O O Si Si R O O O Cl Si O O O Si R R Si O Si Si O Si R R O O O Si Si O R O O O R O R Si O Si O Si O R O O Si O Si O Si O R R Si O R Si O O R O Si Si R Nd O O O Na O Nd Si O Si O

Scheme 29

As mentined above, the ytterbium bis(trimethylsilylamide) derivative [{(c-C 6 H 11 ) 7 Si 7 O 12 } 2 Li 4 ]YbN(SiMe 3 ) 2 (41, Ln = Yb) was prepared by treatment of in situ prepared 40 with anhydrous YbCl 3 [43]. Surprisingly, when carried out under very similar conditions, the analogous reaction of 40 with anhydrous erbium trichloride took an entirely different course. Compound 40 was first generated by metalation of (c-C 6 H 11 ) 7 Si 7 O 9 (OH) 3 (29) with 3 equivalents of LiN(SiMe 3 ) 2 , followed by reaction with anhydrous ErCl 3 in THF (Scheme 30, Cy = cyclohexyl)) [48]. Insoluble by-products were separated by extraction with n-pentane. Crystallization of the crude product from the same solvent afforded pale pink crystals in 65% isolated yield (determined after the crystal structure had been elucidated by X-ray diffraction). Meaningful 1 H and 13 C NMR data could not be obtained due to the paramagnetism of the Er 3+ ion. The presence of eight resonances in the 29 Si NMR spectrum indicated a rather unsymmetrical molecular structure [48].

Conclusion

Significant progress has been made in the chemistry of disiloxanediolates and metallasilsesquioxanes of the rare earth elements. The 1,1,3,3-tetraphenyldisiloxane-1,3diolate ligand forms a variety of heterobimetallic (Ln/Li) complexes with rare earth elements. Especially the structural and derivative chemistry of heterobimetallic lanthanide bis(disiloxanediolates) has turned out to be surprisingly diverse. In the case of scandium and yttrium octahedral complexes were obtained, which according to X-ray diffraction studies can be best described as novel metallacrown complexes with the lithium disiloxanediolate ligand systems arranged in the equatorial plane and additional ligands in trans-positions. In contrast, larger lanthanide ions afford heterobimetallic disiloxanediolates which can be regarded as "inorganic metallocenes" or out-of-plane metallacrown complexes. Thus this ligand system provides a rational way to block one side of a larger lanthanide ion and, presumably, control the reactivity of the lanthanide. In addition, novel coordination modes and reaction pathways have been uncovered, which should stimulate further work in this area. Among the most remarkable new compounds are the COT-bridged inverse sandwich complex 18 and the first cerium(IV) disiloxanediolates 27 and 28.

In contrast, the chemistry of lanthanide metallasilsesquioxanes is still in its infancy, although in this area too important progress has been made in recent years. Such

  Scheme 1

Figure 1 .

 1 Figure 1. Molecular structure of [Ph 2 Si(OSiPh 2 O) 2 ]Yb(THF)(µ-Cl) 3 Li 2 (THF) 3 (5)[10]

  Scheme 2

  Scheme 3

Figure 2 .

 2 Figure 2. Molecular structure of 7 [9d]. For clarity, only the ipso carbon atoms of the

Figure 3 .

 3 Figure 3. Molecular structure of [{(Ph 2 SiO) 2 O} 2 {Li(THF) 2 } 2 ]ErCl•2THF (8b) [9h]. For

  two bridging chloro ligands (Fig.4).

Figure 4 .

 4 Figure 4. Molecular structure of 11 [9d]. For clarity, only the ipso carbon atoms of the

  ), which was subsequently treated with samarium diiodide [9f]. When the reaction was carried out in refluxing THF, the dark blue-green color of SmI 2 gradually faded, indicating oxidation under formation of samarium(III) species. Work-up and crystallization from the concentrated THF solution at 2 °C afforded light yellow, plate-like crystals in low yield (ca. 10%, calculated after the structure had been established). A single-crystal X-ray analysis established the formation of the unprecedented heterobimetallic samarium(III) disiloxanediolate cluster [Me 3 SiO{µ-Na(THF)} 3 Sm{µ-(Ph 2 SiO) 2 O} 3 Na(THF)] (17). Thus the overall reaction sequence can be formulated as shown in Scheme 11 [9f].

[Figure 5 .

 5 Figure 5. Molecular structure of [Me 3 SiO{µ-Na(THF)} 3 Sm{µ-(Ph 2 SiO) 2 O} 3 Na(THF)] (17)

Figure 6 .

 6 Figure 6. Molecular structure of (µ-η 8 :η 8 -COT)[Nd{(Ph 2 SiO) 2 O} 2 {Li(THF) 2 }{Li(THF)}] 2

Figure 7 .

 7 Figure 7. Molecular structure of [{(Ph 2 SiO) 2 O} 2 {Li(THF) 2 }AlMe 2 ]ScCl(THF)(19) [9e]

Figure 8 .

 8 Figure 8. Molecular structure of [{(Ph 2 SiO) 2 O} 2 {InMe 2 (OMe)} 2 InMe 2 ]Y (20) [9e]

Figure 9 .

 9 Figure 9. Molecular structure of [Li(THF) 4 ][Pr{O(SiPh 2 OInMe 2 OSiPh 2 OSiPh 2 O) 2 }] (21)

(

  Scheme 17. An interesting example in which retention of lithium halide lead to a self-

  Cerium(IV) alkoxides form a wellinvestigated class of compounds which are of interest as precursors for the MOCVD CeO 2 layers[25]. In sharp contrast, information about the homologous cerium(IV) siloxides comprising Ce-O-Si bonds is very scarce. Ce(OSiPh 3 ) 4 (DME) x (0.5 <

  unprecedented centrosymmetric cluster molecule 28 a planar K 2 O 2 ring forms the central structural unit. The central cerium atoms in 28 are in a distorted octahedral coordination environment. Two disiloxanediolate dianions act as chelating ligands towards each Ce(IV) and simultaneously bridge two potassium ions with the cerium. An unusual µ 3 ,µ 2 -bridging coordination mode was found for the dianionic diphenyl-1,1-silanediolate ligands. One oxygen from each of the two Ph 2 SiO 2 2-ligands bridges one Ce and two potassium ions, while the other oxygen forms a µ 2 -bridge to Ce and K. The overall coordination geometry around each cerium is slightly distorted octahedral. Four tert-butoxide groups originating from precursor 25 are also present in the molecular structure of 28, two of them bridging (K + , K + ) and the other two terminally bonded to cerium. Given the high isolated yield it is clear that despite its highly unusual structure compound 28 is the main product of the reaction of 25 with 3 equivalents of 26. The coordinative unsaturation of the potassium ions (coordination numbers 4 and 5, resp.) is relieved by η 1 -and η 2 -π-interactions with neighboring phenyl substituents. The respective K•••C distances are in the range of 3.173 -3.527(4)Å. The fact that the nitrate group of the starting material was completely eliminated demonstrates the usefulness of 25 as a precursor for the synthesis of novel cerium(IV) siloxide species. Due to the remaining two t BuO ligands compound 27 also represents a potentially useful synthon for other tetravalent cerium species [9c].

Figure 10 .

 10 Figure 10. Molecular structure of [{(Ph 2 SiO) 2 O} 2 {(DME)KO t Bu}{(Ph 2 SiO 2 )K}Ce] 2 (28)

C 6 H

 6 Scheme 20

  These materials were obtained by treatment of 29 with the tris(silylamides) Ln[N(SiMe 3 ) 2 ] 3 of Y and Nd. Due to their very high solubility no structural characterization of the initially formed products was possible. However, with the use of triphenylphosphine oxide as supporting ligand X-ray quality single crystals could be obtained, and a dimeric structure was established for the Y and Nd silesesquioxane et al. reported the synthesis of a series of lanthanide silsesquioxanes resulting from reactions of 29 with lanthanide tris(silylamides) Ln[N(SiMe 3 ) 2 ] 3 (Ln = Y,

  Scheme 30

Figure 11 .Compound

 11 Figure11. ORTEP view of the metallasilsesquioxane core in 47[48]

  compounds are certainly the most realistic molecular model compounds for silicasupported lanthanide catalysts. Exciting recent reports clearly indicate that silsesquioxane cages can be used to assemble unprecedented large metallasilsesquioxane clusters containing rare earth element (e.g. 46 and 47). The cerium(IV) complex 49 is still the only Recent results in the chemistry of lanthanide disiloxanediolates and metallasilsesquioxanes are reviewed. Both classes of compounds can be regarded as realistic model compounds for silica-supported lanthanide catalysts.

  THF-adducts [{(Ph 2 SiO) 2 O} 2 {Li(THF) 2 } 2 ]-HoCl•2THF (8a, pale pink rods, 70% yield) and [{(Ph 2 SiO) 2 O} 2 {Li(THF) 2 } 2 ]ErCl•2THF (8b, pale pink needles, 72% yield) were obtained according to Scheme 5 [9h].

								Cl
					THF			THF
	LnCl 3 + 4 LiN(SiMe 3 ) 2 + 2	OH Ph 2 Si	1 O SiPh 2 OH	THF -4 HN(SiMe 3 ) 2 -2 LiCl,	Ph Ph O Si Ph Si Ph	O	O	Li Li Ln	O O	Ph Si Ph O Ph Si Ph
							THF	THF
					Ln = Ho (8a), Er (8b)

Page 10 of 53 Wiley-VCH ZAAC is

	1.13 Å. The reactions leading to the first praseodymium disiloxanediolates are
	summarized in Scheme 6. [{(Ph 2 SiO) 2 O} 2 {Li(THF) 2 }{Li(THF)}]Pr(µ-Cl) 2 Li(THF) 2 (11)
	was prepared in the same way as the Sm derivative 9 and isolated in 68% yield in the form
	of colorless platelets.
										(THF) 2
										Li
										Cl	Cl	(THF)
	PrCl 3 + 3 LiN(SiMe 3 ) 2 +	2 Ph 2 Si OH OH O SiPh 2		THF -LiCl	O Ph Ph Si	O	Pr	O Li	O Ph Si	Ph
					1					Si	O	O	Si
										Ph	Ph	(THF) 2 Li	Ph	Ph
										11
									DME
				DME Cl	DME
		Si Ph Ph	O	Pr	O Li	Si Ph	Ph
		O						O
		Si	O		O	Si
	Ph	Ph		Li		Ph	Ph
					DME
						12

The coordinated LiCl can be removed by recrystallization of 11 from DME as illustrated in

	NdCl 3 + 4 LiN(SiMe 3 ) 2 + 2 Ph 2 Si OH	1 O SiPh 2 OH	THF / DME -2 LiCl, -4 HN(SiMe 3 ) 2	Ph O Ph	Ph Ph Si Si	O O Cl	(DME) Li O Nd Li	O O O	Ph Si Ph Si Ph Ph O
								O	O
									13
			Scheme 8
	An X-ray analysis revealed that the compound is isostructural with the
	praseodymium(III) complex 12. By slightly changing the reaction conditions the reaction
	of anhydrous PrCl 3 with 2 equivalents of (Ph 2 SiOLi) 2 O (4) (made in situ by treatment of 1
	with 2 equivalents of LiN(SiMe 3 ) 2 ) according to Scheme 9 a new silylamide-functionalized
	complex, [{(Ph 2 SiO) 2 O} 2 {Li(THF) 2 } 2 ]Pr[N(SiMe 3 ) 2 ] (14), became available:
									N(SiMe 3 ) 2
	Scheme PrCl 3 + 5 LiN(SiMe 3 ) 2 + 2 6. This	resulted 1 OH Ph 2 Si O SiPh 2 in OH	formation THF / DME -4 HN(SiMe 3 ) 2 -3 LiCl,	of Ph Ph O	the Ph Ph Si O O Si	neutral THF THF Li Pr Li	complex Ph O Si Ph O Ph O Si Ph
	[{(Ph 2 SiO) 2 O} 2 {Li(DME)} 2 ]PrCl(DME) (12) with heptacoordinate praseodymium. Here THF THF
									14
	again the praseodymium is coordinated to the [{(Ph 2 SiO) 2 O} 2 {Li(DME)} 2 ] 2-ligand system
	in a an out-of-plane arrangement. Thus the compound also belongs to the group of
	"inorganic metallocenes". A terminal chloride ligand and a chelating DME complete the
	coordination sphere around praseodymium. The overall coordination geometry can be best
	described as monocapped trigonal-prismatic. The analogous neodymium derivative
	[{(Ph 2 SiO) 2 O} 2 {Li(DME)} 2 ]Nd(DME)Cl (13) was obtained according to Scheme 8 in
	75% yield in the form of pale pink crystal blocks.

Page 14 of 53 Wiley-VCH ZAAC

14 was formed by a metathetical reaction of an intermediate chloro derivative with the remaining equivalent of LiN(SiMe 3 ) 2 . Thus far, however, more deliberate preparations of 14 by treatment of the chloro precursors 11 or 12

  in two steps by treatment of 29 with SiCl 4 in the presence of triethylamine, followed by hydrolysis of the intermediate chlorosilane Cy 7 Si 7 O 12 Cl. Metalation of Cy 7 Si 8 O 12 OH with methyllithium in diethylether proceeded smoothly at room temperature to afford Cy 7 Si 8 O 12 OLi

Wiley-VCH ZAAC

Biographical Sketch


Alexandra G Trambitas

Tarun K Panda

Dr Matthias Tamm
email: m.tamm@tu-bs.de

In Fagaras

R K Mission

Rare Earth Metal Complexes Supported by Ancillary Imidazolin-2-iminato Ligands

Keywords: Rare earth metals, Lanthanides, Scandium, Yttrium, Lutetium, Imidazolin-2-iminato ligands, Non-cyclopentadienyl ligands, Homogeneous catalysis

Berlin, which he declined. His research interests lie in the areas of preparative organometallic chemistry and coordination chemistry with an emphasis on the development of novel, unusual ligand systems and on the application of their transition metal complexes in homogeneous catalysis, organic synthesis and materials science.

It was the serendipitous discovery of ferrocene, [(η-C 5 H 5 ) 2 Fe], [1] in 1951 and one year later the seminal elucidation of its sandwich-type structure by E. O. Fischer [2] and G.

Wilkinson [3] that not only triggered the remarkable development of modern organotransition metal chemistry, [START_REF]Special Issue: 50th Anniversary of the Discovery of Ferrocene[END_REF][START_REF] Werner | Landmarks in Organo-Transition Metal Chemistry[END_REF] but also paved the way for the preparation of the first series of organolanthanide complexes, i. e. tris(cyclopentadienyl) complexes, [(η 5 -C 5 H 5 ) 3 M], by Wilkinson and Birmingham in 1954. [6] Ever since, the organometallic chemistry of the rare earth elements has been dominated by cyclopentadienyl ligands (Cp) or related aromatic ligands. [7][8][9][10][11][12][13][14] In recent years, however, research in this area has increasingly focussed on the development of alternative ligand sets, and a growing number of monoanionic heteroatom ligands that can be regarded as Cp-analogous ligands because of their charge and steric requirements have been playing a key role in organolanthanide and organo rare earth metal chemistry. [15,16] Anionic ligands are required to electrostatically balance the positive charge of the lanthanide ions, which tend to form rather ionic complexes due to the limited radial extension of the 4f orbitals. [START_REF] Cotton | Lanthanide and Actinide Chemistry[END_REF] Since these metals are among the largest in the periodic table, [START_REF] Shannon | [END_REF] sterically demanding ligands are useful for the isolation of soluble molecular species.

The most important ancillary ligands are monodentate alkoxides (OR), siloxides (OSiR 3 ), aryloxides (OR), [19] amides (NR 2 ), [20] and phosphoraneiminates (NPR 3 ) [21] as well as polydentate ligands such as β-diketonates, [22] β-diketiminates, [23] benzamidinates, [24] aminotroponiminates, [25] aminopyridinato, [26] bis(phosphinimino)methanide [27] and also other phosphanoamides. [28,29] Figure 1 shows a selection of important ligands that have played an important role for the application of rare earth metal complexes in homogeneous catalysis and material science.

Imidazolin-2-iminato ligands

Following several reports from N. Kuhn and co-workers, [30][31][32] we have been able to introduce monoanionic imidazolin-2-iminato ligands (Im R N, 1-R) to the growing family of ancillary ligands and were able to demonstrate that these ligands are viable systems for the preparation of catalytically active transition as well as rare earth metal complexes. [33][34][35][36][37][38][39][40][41][42][43] As terminal ligands, these systems correspond to phosphoraneiminato ligands (NPR 3 ), which were intensively investigated by Dehnicke et al. in both transition metal [44][45][46] and rare earth metal chemistry. [21] Thereby, Dehnicke proposed a bonding analogy between Cp and NPR 3 ligands, since both are monoanionic ligands that can be regarded as 2σ,4π-electron donors.

This pseudo-isolobal relationship can also be assigned to imidazolin-2-iminato ligands, in which the phosphine unit has been replaced by an N-heterocyclic carbene (Scheme 1). As indicated by the two limiting resonance structures 1A and 1B, imidazolin-2-iminato ligands can either act as 2σ,2π-electron donors (1A) or as 2σ,4π-electron donors (1B), and the ability of the imidazolium ring to stabilise a positive charge affords highly basic ligands with a strong electron donating capacity toward early transition metals or metal in a higher oxidation state. [47] Page 8 of 51 Wiley-VCH ZAAC Scheme 1. Mesomeric structures for imidazolin-2-iminato complexes and isolobal realationship between imidazolin-2-iminato, cyclopentadienyl and phosphoraneiminato complexes.

In this report, we will cover the recent developments in the organometallic chemistry of rare earth metal complexes supported by imidazolin-2-iminato or imidazolin-2-imine-hybrid ligands. Moreover, the application of the resulting complexes in different areas of homogeneous catalysis will be presented. As representatives of the rare earth metals, the elements scandium, yttrium, lutetium and gadolinium were generally chosen, since their different ionic radii cover a large spectrum of different reactivities and coordination modes. [START_REF] Shannon | [END_REF] 3 Mono(imidazolin-2-iminato) rare earth metal complexes

Dichloro complexes

The entry into the organolanthanide complexes supported by imidazolin-2-iminato was accomplished by the synthesis of dichloro complexes of the type [(Im Dipp N)MCl 2 (THF) 3 ] (2a, M = Sc; 2b, M = Lu; 2c, M = Y) and of the "ate" complex [(Im Dipp N)GdCl 2 (THF) 2 ]·[LiCl(THF) 2 ] (2d). [41,42] These complexes could be conveniently prepared by the reaction of the imine Im Dipp NH (Dipp = 2,6-diisopropylphenyl) with trimethylsilylmethyl lithium (LiCH 2 SiMe 3 ) in the presence of the appropriate anhydrous rare earth metal halide LnCl 3 (M = Sc, Y, Lu, Gd) and were isolated as colourless crystalline solids in good yield after crystallization from THF/pentane solutions (Scheme 2). The molecular structures of 2a -2d were established by X-ray diffraction analyses; as a representative, the molecular structures of the scandium complex 2a is shown in Figure 2. The solid state structures of compounds 2a -2c show a pseudo-octahedral coordination geometry around the rare earth metal atom with the two chloride ligands in trans-positions and the three THF ligands adopting a meridional arrangement, which corresponds to the mer-transconfiguration commonly observed for mono-Cp rare earth metal complexes. [10] In all cases, exceptionally short metal-nitrogen bonds together with almost linear M-N1-C1 angles are observed (Table 1). At the time of publication, [41,42] there was only one complex featuring a shorter Sc-N bond length, and that is the AlMe 3 adduct of the recently reported transient scandium-imido complex [(PNP)Sc(N-Dipp)] containing the PNP pincer ligand bis (2diisopropylphosphino-4-methylphenyl)amide. [48] The only other Sc-N bond lengths that featured bond lengths shorter than two angstroms have been reported for β-diketiminato complexes [(nacnac)ScCl(NHtBu)] and [(nacnac)Sc(CH 3 )(NHtBu)]⋅B(C 6 F 5 ) 3 with nacnac =

G d C l 3 L i C H 2 S i M e 3 M = Sc, Y, Lu; 2a -2c N N NH M C l 3 L i C H 2 S i M
[Dipp-NC(tBu)CHC(tBu)N-Dipp]. [49] The metal-nitrogen bond length in 2b was also the second shortest reported for yttrium complexes at that time; and the shortest value was observed in a tetranuclear cyclopentadienyl-yttrium complex containing µ 3 -ethylimido ligands. [50] Short Y-N distances have also been reported for yttrium-phosphoraneiminato complexes. [51] The Lu-N bond length in 2c was still the shortest ever observed for lutetium complexes, and the previous shortest Lu-N distances were found in benzamidinate and amido complexes. [50,52] In the mean time, we have obtained several complexes, in which even shorter metal-nitrogen bonds are observed, thus giving further evidence for a particularly strong metal-ligand interaction in imidazolin-2-iminato complexes (vide infra, Table 1).

The molecular structure of 2d reveals that the trans-coordinated THF ligand in 2a -2c has been replaced by a chlorine atom (Cl2). The resulting negative charge is neutralised by a [Li(THF) 2 ] + moiety, capping the chlorine atoms Cl1 and Cl2. Again the Gd-N1 distance is very short, being the shortest value ever reported for gadolinium-nitrogen systems. The previous shortest Gd-N distances were observed for five-membered amidolanthanide heterocycles. [53] Sc Cl1 Since the substituents on the imidazole heterocycle can easily be varied, numerous imidazolin-2-iminato ligands are available. [47] Following the encouraging results obtained by

Cl2

Page 11 of 51

Wiley-VCH ZAAC using Im Dipp N to synthesize the dichloro complexes 2a -2d, the synthesis and structural characterisation of the yttrium complex [(Im tBu N)YCl 2 (THF) 3 ] (3) was also achieved (Scheme 3). [54] The overall structural parameters are very similar to those reported for the related complexes 2a -2c. The metal atom is hexacoordinated with the two chlorine atoms in transposition and the three THF ligands adopting a meridional arrangement. The N1-Y interaction in 3, revealed by a very short Y-N1 bond length of 2.115(2) Å (Table 1), is even slightly stronger than that observed for the corresponding Im Dipp N complex 2c [2.1278(18) Å]. [42] Scheme 3. Preparation of the yttrium dichloro complex [(Im tBu N)YCl 2 (THF) 3 ] (3).

Bis(neosilyl) complexes

In most cases, bis(neosilyl) rare earth metal complexes have been prepared by the reaction of [M(CH 2 SiMe 3 ) 3 (THF) 2 ] with the protonated form of a desired ancillary ligand, which is accompanied by release of tetramethylsilane (TMS). [55][56][57][58] In our hands, however, the dichloro ). [43] Scheme 4. Synthesis of the bis(neosilyl) complexes 4. The solid state structures of complexes 4a -4d were determined by X-ray diffraction analyses. Since scandium has the smallest ionic radius of all rare earth elements, [START_REF] Shannon | [END_REF] the metal atom in 4a is only four-coordinate and displays a strongly distorted tetrahedral coordination sphere (Figure 3). The imidazolin-2-iminato ligand coordinates in an almost perfectly linear fashion, and the Sc-N bond length is even shorter than observed for the dichloride 2a (Table 1). [40] In contrast, complexes 4b, 4c and 4d exhibit a five-coordinate metal atom with a trigonal-bipyramidal coordination sphere, in which the THF ligands adopt the axial positions, albeit with a pronounced deviation of the O1-M-O2 angles from 180° (Figure 3). [43] In ]. [59] This high-field shift is in agreement with the trend derived for other yttrium complexes upon substitution of alkyl groups by π-donor ligands. [60]

Bis(amide) complexes

In a similar fashion as described for bis(neosilyl) rare earth metal complexes, related diamido complexes are usually prepared from the triamides [M{N(SiRMe 2 ) 2 } 3 (THF) 2 ] (R = Me, H), which release one amine HN(SiRMe 2 ) 2 upon treatment with the protonated form of an appropriate ancillary ligand. [61] Again, the reaction of the dichlorides 2a -2d with two equivalents of [Na{N(SiMe 3 ) 2 }] in THF proved to be a superior method, and the complexes 5a -5d of the composition [(Im Dipp N)M(HMDS) 2 (THF)] [HMDS = hexamethyldisilazide, N(SiMe 3 ) 2 ] could be isolated (Scheme 5). [43] Scheme 5. Preparation of the diamido complexes 5a.

The X-ray crystal structures of the yttrium and gadolinium complexes 5b and 5d could be determined; in both cases, the coordination spheres around the metal atoms could be described as distorted tetrahedral geometries by coordination of one imidazolin-2-iminato ligand, two bis(trimethylsilyl)amido groups and one THF molecule. However, in both cases,

2a -2d M = Sc, Y, Lu, Gd N N N M N(SiMe 3 ) 2 N(SiMe 3 ) 2 Dipp Dipp 2 [Na{N(SiMe 3 ) 2 }] O 5a -5d
short intramolecular C-H⋅⋅⋅M contacts are observed with the methyl groups of one HMDS ligand as commonly found for related silylamido rare earth metal complexes (Figure 4). Since the methyl groups involved in these weak agostic interactions are located trans to the THF ligand, the metal coordination spheres in 5b and 5d should preferably be described as distorted trigonal-bipyramidal with the nitrogen ligands adopting the equatorial positions. As expected, the imidazolin-2-iminato ligands exhibit significantly shorter metal-nitrogen distances in comparison with the HMDS ligands, however, they are slightly longer than those found for the neosilyl complexes 4b and 4d (Table 1).

Bis(benzyl) complexes

Benzyl (CH 2 Ph) complexes are also very common in organolanthanide chemistry and are often encountered as pre-catalysts in various rare earth metal catalyzed organic transformations, [62] and therefore, the possibility to introduce benzyl ligands was tested for the lutetium dichloride 2c as an alternative for the established preparation from tribenzyl complexes of the type [M(CH 2 Ph) 3 (THF) n ]. [63] Accordingly, the bis(benzyl) complex 6c was synthesised by treatment of 2c with two equivalents of potassium benzyl (KCH 2 Ph) (Scheme

). In a similar fashion as described for complexes 4b -4d (vide supra), the solid state structure of 6c reveals a five-coordinate lutetium metal, with the ligands forming a distorted trigonal-bipyramidal coordination sphere (Figure 5). The two benzyl ligands are coordinated in a monohapto fashion, whereas the O1-Lu-O2 angle of 155.82(6)° deviates significantly from a linear orientation, which can be mainly attributed to the steric demand of the diisopropylphenyl substituents of the imidazolin-2-iminato ligand. This ligand coordinates to the lutetium atom in an almost linear fashion with a very short Lu-N1 bond of 2.0752( 18) Å, which even falls below the values found for 2c and 4c (Table 1).

Scheme 6. Preparation of bis(benzyl) complex [(Im Dipp N)Lu(CH 2 Ph) 2 (THF) 2 ] (6c).

Pentamethylcyclopentadienyl complexes

In view of the isolobal relationship between imidazolin-2-iminato and cyclopentadienyl (Cp) ligands (vide supra), the reaction of the dichlorides 2a -2d with pentamethylcyclopentadienyl potassium (KCp*) was also pursued. In order to introduce two Cp* ligands, the complexes 2 were treated with two equivalents of KCp*. [43] However, the spectroscopic characterisation indicated the formation of mono(pentamethylcyclopentadienyl) complexes

[(Im Dipp N)Sc(Cp*)Cl(THF)] (7a), [(Im Dipp N)Y(Cp*)Cl(THF) 2 ] (7b), and
[(Im Dipp N)Lu(Cp*)Cl(THF)] (7c).
In contrast, the elemental analysis of the product, obtained from the reaction of 2d with two equivalents of KCp*, revealed that [(Im Dipp N)Gd(Cp*) 2 (THF)] ( 8) has formed and that the larger gadolinium ion is able to accommodate two Cp* ligands (Scheme 7). [43] N

N N M Cl (THF) n KCp* Dipp Dipp M = Sc, n = 1 M = Y, n = 2 M = Lu, n = 1 7a -7c N N N Gd THF Dipp Dipp 2 KCp* 8 2a -2d
Scheme 7. Synthesis of the pentamethylcyclopentadienyl complexes 7a -7c and 8.

The 1 H NMR spectra of the scandium and lutetium complexes 7a and 7c are very similar, indicating the coordination of just one sterically demanding Cp* ligand and one THF ligand.

In both complexes, the THF ligand gives rise to three resonances (two for the α-H's and one for the β-H's), indicating a non-fluxional behaviour and strong coordination to the metal atom, which results in the formation of configurationally stable, chiral-at-metal complexes. [START_REF]These structures are closely related to chiral three-legged half-sandwich transition metal complexes of the type [(Cp)ML 1 L 2 L 3[END_REF] Consequently, the non-fluxional behaviour has an impact also on the Dipp substituents and therefore the 1 H and 13 C NMR exhibits two signals for the diastereotopic isopropyl groups as The molecular structure of 7a and 7c deduced from the NMR spectroscopy could be additionally established by single-crystal X-Ray diffraction analysis (Figure 6), revealing a pseudo-tetrahedral geometry around the metal atoms (Sc and Lu). Yet again, the Sc-N and Lu-N bond lengths of 1.9575 (10) and 2.0697( 18) Å are very short (Table 1). The highly paramagnetic nature of the gadolinium complexes prohibited the use of NMR spectroscopy for the characterisation of 8; however, its solid state structure could be established by X-ray diffraction, confirming that a complex with two Cp* ligands had formed. The coordination sphere around the Gd atom is completed by the imidazolin-2-iminato and one THF ligand to afford a distorted pseudo-tetrahedral geometry. The presence of two bulky Cp* rings leads to an elongation of the Gd-N bond length [2.2388(13) Å] in comparison with the previously described imidazolin-2-iminato gadolinium complexes such as 2d, 4d, 5d (vide supra), and [(Im Dipp N)Gd(η 8 -C 8 H 8 )(THF) 2 ] (vine infra). [42]

Carboranyl complexes

Despite the charge difference between monoanionic cyclopentadienyl ligands and dicarbollide dianions, the latter ones seem to be additional candidates for the development of a cyclopentadienyl-free rare earth metal chemistry. [65][66][67] Such metal/charge combinations would have an impact on the properties of the resultant metal complexes. With this in mind, and in collaboration with the group of Z. Xie, [68][69][70][71][72][73][74][75][76][77][78][79] we aimed at the synthesis of imidazolin-2iminato rare earth metal complexes bearing additional carboranyl ligands. Two synthetic methods, the salt metathesis route and the acid-base approach were examined. [80] Scheme 8. Synthesis of the carboranyl complexes 9a -9c through the salt metathesis route.

Scheme 8 shows the salt metathesis reaction between the dichlorides 2a -2c and the sodium salt Na 2 (C 2 B 9 H 11 ), [66] which proceeds cleanly at 60 °C in THF solution to afford the complexes [(Im Dipp N)M(η 5 -C 2 B 9 H 11 )(THF) 2 ] (9). The solid state structures of the complexes 9a and 9b were determined by X-ray diffraction analyses, and the molecular structure of 9a is

shown in Figure 7. The coordination spheres around the metal atoms (Sc and Y) can be described as distorted tetrahedra or as three-legged piano-stool geometries, respectively. The metal atoms are bound in η 5 -fashion to the pentagonal η 5 -C 2 B 3 bonding face of the carborane.

In addition to the imidazolin-2-iminato and the carboranyl ligand, two THF molecules are bound to the scandium and yttrium atoms. The M-N bond lengths in 9a [1.9466( 12) Å] and 9b [2.0829( 14) Å] are considerably shorter than in the starting materials 2a and 2b. [41,42] In fact, complex 9b features the shortest yttrium-nitrogen bond length reported to date for imidazolin-2-iminato yttrium complexes (Table 1). [42,43] C28 C29 An acid-base route was followed for the introduction of a carboranyl ligand containng an additional appended Me 2 NCH 2 CH 2 sidearm. [73] Coordination of this functional group is expected to stabilize the resulting carboranyl complexes and to prevent ligand redistribution reactions. The bis(neosilyl) complexes of 4a -4c proved to be suitable starting materials for the reaction with the zwitterionic compound [nido-(Me 2 NHCH 2 CH 2 )C 2 B 9 H 11 ], which afforded -after elimination of two equivalents of TMS -the desired products [(Im Dipp N)M(η 1 :η 5 -(Me 2 NCH 2 CH 2 )C 2 B 9 H 10 }(THF)] (10a -10c) in moderate yields (Scheme 9). [80] 10a -10c were characterised by X-ray diffraction analyses, and the molecular structure of 10a is shown in Figure 8. In all cases, the carborane ligand binds in a chelating fashion with the metal atoms being

Cyclooctatetraenyl complexes

The second most important carbocyclic ligand in organo-f-element chemistry is the 10π-

electron aromatic cyclooctatetraene dianion C 8 H 8 2-
, which is well suited to form stable mononuclear complexes with the rare earth elements. [14,15,81] In a similar fashion as described for the carboranyl complexes 9, the substitution of the two chloride ligands in complexes 2 by ). [41,42] Scheme 10. Preparation of the cyclooctatetraenyl (COT) complexes 11.

The molecular structures of 11a -11d could be established by X-ray diffraction analyses.

The COT ligand in 11a -11d is essentially η 8 -bound, albeit in a slightly distorted fashion.

Figure 9 displays the solid state structure of compounds 11a and 11b. The coordination sphere at the scandium atom is completed by one THF ligand to afford a two-legged piano-stool geometry, whereas the coordination spheres at yttrium, lutetium and gadolinium are completed by two additional THF ligands to give three-legged piano-stool geometries. In all cases, the metal-nitrogen bond lengths are marginally elongated in comparison to complexes 2, which is accompanied by a greater deviation the M-N1-C1 angles from linearity (Table 1).

This observation might be a result of the strong δ-interaction between the metal atoms and the cyclooctatetraene dianion, which can act as a 10π-electron donor. [81] Page It is important to note that imidazolin-2-iminato ligands can be regarded as monoanionic analogues of dinegative imido ligand, RN 2-, because of their ability to act as 2σ,4π-electron donors (vide supra, Scheme 1), and therefore, the rare earth metal complexes presented in this review article can be regarded as structural models for elusive terminal imido rare earth metal complexes, in which the imido group is generally found to bind in bridging or capping fashion. [82] Consequently, the [(Im Dipp N)Sc(η 8 -C 8 H 8 )] fragment in 11a can be regarded as isolobal and isoelectronic with the 16-electron titanium imido complexes of the type [(RN)Ti(η 8 -C 8 H 8 )] (R = tBu, Dipp). [83] The reactivity of these unique one-legged pianostool("pogo-stick") imido complexes towards a variety of organic substrates such as CO 2 , CS 2 , isocyanates and isothiocyanates was studied by Mountford and coworkers, and the Ti(NR) moiety was shown to undergo a number of cycloaddition-elimination reactions. [84] Accordingly, the reaction of 11a with similar substrates was studied, since the 16-electron [(Im Dipp N)Sc(η 8 -C 8 H 8 )] fragment, provided by elimination of the THF ligand, could be expected to exhibit similar reactivity. [42] Exposure of a toluene solution of 11a to CO 2 produced a brownish insoluble precipitate that might be tentatively described as a scandium oxo species, considering the formation of oligomeric [(η 8 -C 8 H 8 )Ti=O] from [(η 8 -C 8 H 8 )Ti(NtBu)] and CO 2 . [84] In contrast, a stable cycloaddition product could be isolated from the reaction of 11a with the isothiocyanate Xy-NCS to afford complex 12 with an unusual thioureato-N,N' moiety (Scheme 11, Figure 10). This result was in contrast to the observation made for the reaction of [(η 8 -C 8 H 8 )Ti(NDipp)] with Xy-NCS, which was reported to result in the formation of an N,S-bound cycloaddition product, in agreement with its decomposition to afford the carbodiimide Dipp-N=C=N=Xy. [84] Scheme 11. Synthesis of the [2+2] cycloaddition product 12. 4 Bis(imidazolin-2-iminato) rare earth metal complexes

Monochloro complexes

As mentioned before, the imidazolin-2-iminato ligands can be regarded as monodentate analogues of cyclopentadienyls, because of their ability to act as 2σ,4π-electron donors, C 5 R 5 .

Consequently, bis(imidazolin-iminato) complexes of the type [(Im Dipp N) 2 MR(THF) n ] might be regarded as analogues of metallocene or lanthanocene complexes of the type [(Cp) 2 MR(THF) n ], which have found widespread use in homogeneous catalysis. [13] Following the same strategy as described for the preparation of the dichloro complexes 2a -2c, [41,42] the reaction of anhydrous rare earth metal halides MCl 14b, M = Y; 14c, M = Lu) in very good yields (Scheme 12). [85] Scheme 12. Synthesis of the bis(imidazolin-2-iminato) complexes 14.

X-ray diffraction analysis reveals in all cases -Figure 11 shows the yttrium complex 14b

-a five-coordinated metal atom, which resides in a trigonal-bipyramidal coordination sphere, with the THF ligands adopting the axial positions. The orientation of the three anionic equatorial ligands, the two iminato ligands and the chlorine atom, is nearly perfectly trigonalplanar with the sum of the N-M-N and N-M-Cl angles being almost 360°. It should be noted that the related lanthanocene complexes tend to form dinuclear complexes or cluster of higher nuclearity, in which the metal atoms are bridged by chloride atoms. [7] The imidazolin- Interestingly, the related reaction between Im Dipp NH, YbI 2 and LiCH 2 SiMe 3 did not lead to the formation of an ytterbium(II) complex containing the [(Im Dipp N) 2 Yb] complex fragment, but instead the Yb(III) complex [(Im Dipp N) 2 YbI(THF) 2 ] (15) was isolated as a yellow crystalline material. The reaction is accompanied by the formation of a grey precipitate, presumably Yb metal, indicating that 15 has formed by disproportionation (Scheme 13). [9] We assume that the electron richness of the imidazolin-2-iminato ligand prevents the isolation of a stable divalent lanthanide complex. 15 was also characterized by Xray diffraction analysis, revealing a molecular structure, which is very similar to the fivecoordinate structures established for the chloro complexes 14 (Figure 11). As expected, the Yb-N bond distances are short [2.109(3) For 16b and 16c, the complex composition was additionally proved by X-ray diffraction analysis. The two complexes (16b and 16c) are isostructural, and the coordination geometry at the metal atoms is similar to that found for the starting materials 14, although the introduction of the neosilyl ligand produces a significantly stronger distortion from an ideal trigonalbipyramidal orientation. The yttrium compound 16b was additionally characterized by 89 Y NMR spectroscopy, giving rise to a singlet at -800.1 ppm. Comparison with the resonance observed for the corresponding bis(neosilyl) complex 4b at +813.8 ppm indicates a high-field shift of more than 1600 ppm. [43] As reported by Schaverien and Hanusa, [60] the 89 Y NMR resonance is expected to shift progressively to higher field as σ-donating alkyl groups, for instance from [Y(CH 2 SiMe 3 ) 3 (THF) 2 ], [59] are sequentially substituted by groups of increased electronegativity and π-donating ability. [58c,86] Consequently, the observed dramatic high-field shift upon introduction of a second imidazolin-2-iminato ligand produces once again proof for the strong electron-donating ability of these ligand system. Since the mono-and bis(imidazolin-2-iminato) rare earth metal complexes described above might be regarded as analogues of mono-and bis(cyclopentadienyl) complexes, tris(imidazolin-2-iminato) complexes might be regarded as isolobal to tris(cyclopentadienyl) complexes [(Cp) 3 M] [7] and to homoleptic tris(phosphoraneiminato) complexes [M(NPR 3 ) 3 ]. [21] For the latter, however, only dinuclear species such as [M(NPPh 3 ) 3 ] 2 (M = Y, La) were reported, which can be used as initiators for the ring-opening polymerisation of lactones. [87] The acid-base reaction between the rare earth metal tris(neosilyl) complexes M(CH 2 SiMe 3 ) 3 (THF) 2 ] (M = Sc, Y, Lu) and three equivalents of 1,3-diisopropyl-4,5dimethylimidazolin-2-imine (Im iPr NH) in hexane cleanly afforded the tris(imidazolin-2iminato) complexes 17 with release of tetramethylsilane (Scheme 15). [88] The molecular structures of 17a -17c were established by X-ray diffraction analyses, revealing the presence of dimeric complexes of the type [M 2 (Im iPr N) 6 (THF) n ] in the solid state. The scandium complex 17a (n = 0) forms a centrosymmetric dimer of [Sc 2 (Im iPr N) 6 ] with the two scandium atoms connected by two bridging µ 2 -Im iPr N groups to generate a Sc 2 N 2 square. In contrast, the dimeric complex units in [Y 2 (Im iPr N) 6 (THF)] (17b) and [Lu 2 (Im iPr N) 6 (THF)] (17c) contain two metal atoms each, which display different coordination geometries, since one metal atom coordinates an additional THF molecule to acquire a coordination number of five, whereas the other atom remains in a distorted tetrahedral environment (see Figure 13 for the molecular structure of 17b). In all complexes, the terminal metal-nitrogen distances are shorter than those to the bridging nitrogen atoms; however, they are longer than observed for mono-and bis(imidazolin-2-iminato) complexes (Table 3). 14, revealing that a monomeric complex has formed, in which the yttrium atom attains a coordination number of five and exhibits a coordination sphere that adopts an intermediate position between the perfectly trigonal-bipyramidal and square-pyramidal geometries. [88] For comparison, the analogous monuclear cyclopentadienyl complex [(Cp) 3 Y] ccoordinates only one solvent molecule upon crystallization from THF solution. [89] Scheme 16. Synthesis of the homoleptic complex [(Im tBu N) 3 Y] ( 18).

Constrained-Geometry ligands

Imidazolin-2-imines (Im R N) are not only valuable ligand precursors in their own right, but they also serve as useful building blocks for the design and preparation of novel multidentate poly(imidazolin-2-imine) ligands. [90][91][92] The tethering of an imidazolin-2-imine unit to a cyclopentadienyl ring through a Me 2 Si bridge leads to cyclopentadienyl-imidazolin-2-imine ligands, which can be expected to form "constrained-geometry" ansa-cyclopentadienyl-amido complexes, despite the overall neutral charge of the imine moiety. The imidazolin-2-iminotetramethylcyclopentadiene ligand 19-H can be obtained as a yellow oil from the reaction of 5-(chlorodimethylsilyl)-1,2,3,4-tetramethyl-1,3-cyclopentadiene with two equivalents of 1,3diisopropyl-4,5-dimethyl-imidazolin-2-imine (Im iPr NH). One equivalent of the imine serves as a hydrochloric acid scavenger and precipitates from the reaction mixture as the corresponding 2-aminoimidazolium chloride, Im iPr NH•HCl. [93] Scheme 17. Synthesis of the imine-functionalized cyclopentadiene 19-H. 20) in good yields (Scheme 18). [94] 20a -20c were characterized by X-ray diffraction analyses; all complexes exhibit a three-legged piano-stool geometry with a distorted pseudo-tetrahedral environment around the metal atoms. As a Consequently, these complexes are structurally closely related to well-established and commercialized "constrained-geometry" ansa-cyclopentadienyl-amido complexes containing tetravalent group 4 metals, [95] which also require two monoanionic ligands to attain an overall neutral charge. In contrast, related "true" Cp-amido rare earth metal complexes contain only one only one additional anionic ligand as exemplified by the related neosilyl complexes [(η 5 :η 1 -C 5 Me 4 -SiMe 2 -N-C 6 H 5 )Ln(CH 2 SiMe 3 )(THF) 2 ] [96] [(η 5 :η 1 -C 5 Me 4 -SiMe 2 -NtBu)Y(CH 2 SiMe 3 )(THF)] [97] and [(η 5 :η 1 -C 5 Me 4 -SiMe 2 -NXy)Lu(CH 2 SiMe 3 )(THF)] (Xy = 2,6-dimethylphenyl). [98]

iminato)pyridine pincer ligands

Bridging two imidazolin-2-imine moieties by a 2,6-dimethylpyridine unit affords the tridentate pincer ligand bis(imidazolin-2-imino)pyridine (TL tBu ), which forms stable chelate complexes with numerous 3d-transition metals. [92a,99] Since TL tBu can be expected to bind metals in an ylidic amido-type fashion (vide supra), we envisaged that this ligand should also be well suited for the complexation of hard rare earth metal ions. Thus, the reaction with the ). [100] The solid-state structures of compounds 21a and 21c were established by X-ray diffraction analysis, revealing that the coordination polyhedra in both cases are formed by chelation through the two imine and the pyridine nitrogen atoms of the TL tBu ligand and additional coordination of three chloride atoms (see Figure 16 for the molecular structure of 21a). In both complexes, however, the TL tBu ligand is asymmetrically attached to the metal atoms, and additional short M•••H contacts are observed between the metal ions and a methyl group from Page 34 of 51 Wiley-VCH ZAAC one of the tert-butyl groups, which can be regarded as a remote, [101] or secondary [102]

Catalytic applications

The number of applications of rare earth metal complexes containing noncyclopentadienyl ligands has become comparable to the plethora of lanthanocene-catalysed homogeneous reactions. [13,28b,103-105] This clearly results from the fact that a great number of novel ancillary ligands for the complexation of rare earth metals have been reported in recent Page 35 of 51 Wiley-VCH ZAAC years (see Figure 1). [21][22][23][24][25][26][27][28][29] The introduction of the imidazolin-2-iminato ligands to this growing family of ancillary ligands, as versatile alternatives to cyclopentadienyl ligands, led to the formation of viable system for the preparation of catalytically active transition as well as rare earth metal complexes. [33][34][35][36][37][38][39][40][41][42][43] The combination of highly basic nitrogen ligands with Lewisacidic rare earth metal ions should lead to the formation of active catalysts for ring-opening polymerisations. Accordingly, when the ring-opening polymerisation reaction of εcaprolactone with the half-sandwich cyclooctatetraene complex 12b was investigated, high catalytic activity was observed, and polylactones are quantitatively formed in less than 5 min. [106] In addition, studies of the intramolecular hydroamination/cyclisation reactions using catalytic amounts of the bis(neosilyl) complexes 4b and 4c and the "constrained-geometry" complexes 20a -20c (Scheme 20, reaction a), were also performed. [43,94] The main focus of the investigation was to evaluate their catalytic activity and to compare them with lanthanocene catalysts [13,105,107] and other non-Cp rare earth metal complexes. [108][109][110][111] Scheme 20. (a) hydroamination/cyclisation reaction of terminal aminoalkene (b)

hydrosilylation reaction of alkenes

The anaerobic hydroamination/cyclisation of aminoalkenes and aminoalkynes catalyzed by complexes 4b and 4c (5 mol%) proceeds to completion and is conveniently monitored by 1 H NMR spectroscopy. [43] The transformation is effective in the formation of five-membered heterocycles from primary amines at room temperature or higher temperature, respectively.

The substrates, bearing bulky geminal substituents in the β-position to the amino group Page 36 of 51 Wiley-VCH ZAAC (Thorpe-Ingold effect), [112] could be cyclised within a few minutes in contrast to those bearing less bulky substituents, where the reaction time is increasing and higher reaction temperatures are required. A comparison of the activity of complexes 4b and 4c with non-cyclopentadienyl complexes, for example [La{N(SiHMe 2 ) 2 } 2 {CH(PPh 2 NSiMe 3 )}], [108e] shows a significantly higher activity for the former complexes. In comparison with the corresponding metallocene catalysts, for example [(Cp*) 2 Ln{CH(SiMe 3 ) 2 }] [113] (Ln = La, Sm, Nd), the rates for the hydroamination/cyclisation reaction using as catalysts 4b and 4c are rather slow. [43] In addition, the catalytic activity of the "constrained-geometry" compounds 20a -20c was also evaluated in hydroamination/cyclisation reactions. Yet again, all substrates were converted into cyclic products under mild conditions using low catalyst loadings of 2 to 6 mol%. However, in comparison with the bis(neosilyl) complexes 4b and 4c, compounds 20a -20c show lower catalytic activities. [43,94] Besides the intramolecular hydroamination reaction, the bis(neosilyl) 4b and 4c were also screened for hydrosilylation reactions and proved to be highly efficient pre-catalysts for the intermolecular hydrosilylation of 1-hexene and 1-octene using a small excess (4%) of phenylsilane (PhSiH 3 ) and 2 mol% catalyst loadings (Scheme 20, reaction b). In all cases, the reaction went essentially to completion in less than five minutes at ambient temperature as judged by 1 H NMR spectroscopy. Full selectivity for the anti-Markovnikov (AM) products was achieved, and no side reactions were observed. With regard to reaction temperature, catalyst loadings and yields, the activities of compounds 4b and 4c are higher than those reported for metallocene complexes, e. g. for the hydrosilylation of 1-hexene in the presence of [Me 2 Si(C 5 Me 5 ) 2 SmCH(SiMe 3 ) 2 ] at room temperature. [114] Moreover, the activities of 4b and 4c, fall in the range that was recently observed for closely related organoyttrium catalysts of the type [{PhC(NAr) 2 }Y(CH 2 SiMe 3 ) 2 (THF)] and [{Me 2 NC(NAr) 2 }Y(CH 2 SiMe 3 ) 2 (THF)] with sterically demanding amidinate and guanidinate ligands, which were shown to be excellent catalysts for the hydrosilylation of various terminal alkenes with phenylsilane and which have displayed higher catalytic activity than any other rare earth or early transition metal catalysts. [115]

Conclusion

This overview gives ample evidence for the suitability of imidazolin-2-iminato ligands to form particularly short and strong metal-nitrogen bonds. Experimental and theoretical studies reveal an analogy between imidazolin-2-iminato and conventional imido complexes, and accordingly, the complexes presented in this paper can be regarded as accurate models for elusive mononuclear imido rare earth metal complexes. Despite the presence of highly polarised metal-nitrogen bonds, imidazolin-2-iminato ligands proved to be suitable ancillary ligands for the preparation and generation of efficient catalysts for ring-opening polymerisation, hydroamination and hydrosilylation reactions, thus demonstrating that this ligand class represents a very useful addition to the growing number of non-cyclopentadienyl ligands in organo rare earth metal and organolanthanide chemistry. 176. 85(12) 176. 4(2) 170. 85(12) 166.08 (17) 164.9(4)/166.1 (5) 178.27 (16) 169. 72(9) 171.24 (17) 174.57 (11) 172.45 (10) 175.11 (13) 151. 5(15) 162.27 (18) 162.

Figure 1 .

 1 Figure 1. Selection of monoanionic ancillary ligands in rare earth metal chemistry.

Scheme 2 .

 2 Scheme 2. Preparation of the imidazolin-2-iminato rare earth metal dichlorides 2.

Figure 2 .

 2 Figure 2. ORTEP diagrams of 2a and 2d with thermal displacement parameters drawn at 50%

complexes 2

 2 proved to be superior starting materials, and their reaction with two equivalents of LiCH 2 SiMe 3 afforded the bis(neosilyl) complexes [(Im Dipp N)Sc(CH 2 SiMe 3 ) 2 (THF)] (4a), [(Im Dipp N)M(CH 2 SiMe 3 ) 2 (THF) 2 ] (4b, M = Y; 4c, M = Lu), and [(Im Dipp N)Gd(CH 2 SiMe 3 ) 2 (THF) 2 ] (4d) in high yields (Scheme 4

Figure 3 .

 3 Figure 3. ORTEP diagram of 4a (top) and 4b (bottom) with thermal displacement parameters

Figure 4 .

 4 Figure 4. ORTEP diagram of 5d with thermal displacement parameters drawn at 50%

Figure 5 .

 5 Figure 5. ORTEP diagram of 6c with thermal displacement parameters drawn at 50%

Figure 6 .

 6 Figure 6. ORTEP diagram of 7a and 8 with thermal displacement parameters

Figure 7 .

 7 Figure 7. ORTEP diagram of 9a with thermal displacement parameters

Figure 8 .

 8 Scheme 9. Preparation of carboranyl complexes 10a -10c through the acid-base route.

  the cyclooctatetraenyl (COT) complexes of the type [(Im Dipp N)M(η 8 -C 8 H 8 )(THF) 2 ] 11b -11d for M = Y, Lu, Gd, whereas the smaller scandium atom in complex [(Im Dipp N)Sc(η 8 -C 8 H 8 )(THF)] (11a) allows the coordination of just one additional THF molecule (Scheme 10

Figure 9 .

 9 Figure 9. ORTEP diagrams of 11a (top) and 11b (bottom) with thermal displacement

Figure 10 .

 10 Figure 10. ORTEP diagram of 12 with thermal displacement parameters

  3 (M = Sc, Y, Lu) with two equivalents of the imine Im Dipp NH and two equivalents of trimethylsilylmethyl lithium (Me 3 SiCH 2 Li) in THF afforded the imidazolin-2-iminato complexes [(Im Dipp N) 2 MCl(THF) 2 ] (14a, M = Sc;

2 -Figure 11 .

 211 Figure 11. ORTEP diagram of 14b with thermal displacement parameters drawn at 50%

Scheme 13 .

 13 Scheme 13. Synthesis of the bis(imidazolin-2-iminato) ytterbium(III) complex 15.

Scheme 14 .

 14 Scheme 14. Synthesis of the neosilyl complexes 16.

Figure 12 .

 12 Figure 12. ORTEP diagram of 16b with thermal displacement parameters drawn at 50%

Figure 13 .

 13 Scheme 15. Synthesis of the complexes [(Im R N) 6 M(THF) n ] (17).

Figure 14 .

 14 Figure 14. ORTEP diagram of 18 with thermal displacement parameters

  Treatment of 19-H with rare earth metal trialkyl complexes [Ln(CH 2 SiMe 3 ) 3 (THF) 2 ] (Ln = Sc, Y, Lu) in hexane at ambient temperature generated the cyclopentadienide ligand 19, which coordinates in a chelating fashion to afford the solvent-free complexes [(η 5 :η 1 -C 5 Me 4 -SiMe 2 -NIm iPr )M(CH 2 SiMe 3 ) 2 ] (

  molecular structure of the lutetium complex 20c is shown in Figure15.Coordination of the tethered imine sidearm leads to a pronounced charge separation as indicated by long C1-N1 bonds and by an orthogonal orientation of the imidazole plane with respect to the plane containing N1, the metal atom and the centroid of the Cp ring.

Scheme 18 .Figure 15 .

 1815 Scheme 18. Synthesis of Cp-imidazolin-2-imine complexes 20.

  anhydrous trichlorides MCl 3 (M = Y, Er, Lu) in THF afforded the mononuclear complexes [(TL tBu )MCl 3 ] (M = Y, 21a; M = Er, 21b; M = Lu, 21c) in excellent yields (Scheme 19

Figure 16 .

 16 Scheme 19. Synthesis of the complexes [(TL tBu )MCl 3 ] (21).

Table 1 .

 1 Metal-nitrogen bond lengths [Å] and M-N-C angles [°] for mono(imidazolin-

Page 10 of 51 Wiley-VCH ZAAC

Page 13 of 51 Wiley-VCH ZAAC

	addition, the yttrium and lutetium complexes 4b and 4c are isostructural, and both exhibit M-
	N bonds that are shorter than those found in the starting materials 2b -2c (Table 1). Complex
	4b was additionally characterized by 89 Y NMR spectroscopy, showing a resonance at 813.8
	ppm, which is up-field from the resonance at 882.7 ppm reported for
	[Y(CH 2 SiMe 3 ) 3 (THF) 2

Page 15 of 51 Wiley-VCH ZAAC

Table 2 .

 2 Metal-nitrogen bond lengths [Å] and M-N-C angles[°] for bis(imidazolin-2-iminato)

	4(2)
	166.6(2)/168.1(2)
	163.51(19)
	166.7(2)
	162.89(19)

Table 3 .

 3 Metal-nitrogen bond lengths [Å] for tris(imidazolin-2-iminato) complexes.

	2.003(2)	2.014(2)	178.2(2)	176.9(2)
	2.166(2)	2.166(2)	172.6(2)	172.5(2)
	2.124(4)	2.137(4)	178.1(4)	175.8(4)
	2.109(3)	2.104(3)	179.1(3)	177.6(3)
	2.1701(15)	2.1993(15)	179.55(14)	169.35(14)
	2.1333(17)	2.1636(16)	179.24(17)	169.30(16)
	M-N terminal	M-N bridging

Institute for Organic Chemistry under the guidance of Prof. Dr. Henning Hopf.

After finishing her chemistry studies in Romania, she returned to Braunschweig to work 2007 -2010, her research was financially supported through a scholarship from the Evonik Foundation. awarded a postdoctoral fellowship from the Japan Society for the Promotion of Science (JSPS)


Carsten Ludwig Schmidt

Robert Dinnebier

Martin Jansen

(Ba 6 O)(ReN 3 ) 2 : Synthesis, crystal structure and physical properties

Keywords: Nitrides, Rhenium, Crystal structure, X-ray powder diffraction Nitrides, Rhenium, Crystal structure, X-ray powder diffraction

Reacting a mixture of Ba and Re (3:1) at 850 °C under flowing nitrogen has yielded the nitride-oxide (Ba6O)(ReN3)2 ( 3 R (No. 148); a = 8.1178(2) Å, c = 17.5651(4) Å; V = 1002.43(5) Å 3 ; Z = 6).

According to a structure refinement on X-ray powder data, this compound is isostructural to a recently described nitride-oxide of osmium of analogous composition. The structure consists of sheets of trigonal ReN3 units and trigonal antiprismatic Ba6O groups. The Ba-O distance of 2.73 Å is close to the sum of the respective ionic radii. The trigonal ReN3 5-nitride anion displays a Re-N bond length of 1.94 Ả, and is planar within the limits of experimental error. The constitution of the anion was confirmed by IR and Raman spectroscopy. The nitride-oxide is stable up to 1000 °C, semiconducting (σ = 4.57*10 -3 Ω -1 cm -1 at RT), and paramagnetic down to 25 K. A Curie-Weiss analysis resulted in a magnetic moment of µ = 0.68 µB per rhenium atom.

Introduction

Recently, we reported on the synthesis and characterization of (Ba6O)(OsN3)2, the first nitridoosmate [1]. It exhibits the peculiarity of containing oxygen, although its incorporation was (originally) not intended. Obviously, a hypothetic nitridoosmate of composition "Ba3OsN3" shows an extreme affinity to oxygen, gettering even traces of oxygen from the flow of purified nitrogen. This phenomenon cannot be understood in terms of structural stability since the by ternary transition metal nitrides widely adopted Ba3FeN3 type of structure [2] would also provide a well suitable configuration for "Ba3OsN3", as is indicated by the fact that the actual structure of (Ba6O)(OsN3)2 is closely related to the former. Extensive quantum mechanical calculations have proven electronic effects to rather be responsible. In order to check if such a phenomenon is unique and restricted to osmium only, we have run similar syntheses for the system Ba/Re/N/O, indeed yielding the isostructural rhenate (Ba6O)(ReN3)2.

Experimental Section

Synthesis

Ba metal (purified by sublimation) and Re metal were mixed in a 3:1 ratio and filled in a pre-dried (treated in vacuum at T = 1000 °C) Ta finger tube. The reactants were heated up to T = 850 °C in an atmosphere of nitrogen for 50 h. Nitrogen of quality 4.8 was used and standard drying conditions were applied. After 12, 24 and 36 h, the powder sample was re-ground and analyzed via powder X-ray diffraction. Ba2N was found as intermediate phase. After the repeated treatments, no indications for the presence of Re metal or Ba2N were given. The sample is a microcrystalline black powder which hydrolyzes quickly in moist air.

X-ray powder diffraction

X-ray powder diffraction data of (Ba6O)(ReN3)2 were collected at room temperature with a Stoe Stadi-P transmission diffractometer (primary beam Johannson-type Ge monochromator for Mo-Kα1-radiation, linear PSD) with the sample sealed in a glass capillary of 0.3 mm diameter (Hilgenberg, glass No. 50). The powder pattern was recorded for 20 h in the range from 2-60° 2θ with a step width of 0.01° 2θ using a linear position sensitive detector with an opening of approximately 12° 2θ. Structure determination and refinement was performed using the program TOPAS Version 3.0 (Bruker AXS, 2007). The peak profiles and precise lattice parameters were determined by a LeBail fit [3] using the fundamental parameter approach of TOPAS [4]. Agreement factors (R-values) of the final Rietveld refinement (Figure 1) are listed in Table 1, the coordinates are given in Table 2, and a selection of intra-and inter-molecular distances and angles is given in Table 3. Further details of the crystal structure investigations are available from the Fachinformationszentrum Karlsruhe, D-76344 Eggenstein-Leopoldshafen (Germany), on quoting the depository number CSD-419636, the name of the authors, and citation of the paper.

Thermal analysis

Simultaneous DTA/TG/MS (STA 409, Netzsch) was performed on a sample placed in a corundum crucible under flowing argon (100 ml/min) with a heating rate of 10 K/min.

Spectroscopy

Infrared spectroscopy was performed on a FT-IR spectrometer (IFS 113v, Bruker) from 400 cm -1 to 4000 cm -1 with KBr as the solid matrix. For the Raman experiments the powdered sample was filled in a 1.0 mm lithiumborate glass capillary (Hilgenberg, glass No 14). The measurement was performed with an excitation line of 632.817 nm (Labram 010, single grating). The acquisition time was between 5 to 300 sec with a laser power between 0.4 and 0.004 W.

Electrical conductivity measurement

Measurements were performed on compacted polycrystalline powder pellets (diameter 5 mm, thickness 0.8 mm, pressed with 920 MPa) using the standard four-point technique.

Energy-dispersive X-ray analysis

EDX analysis was performed on a XL30 TMP (Philips Electron Optics GmbH) equipped with an energy-dispersive detector S-UTW(Li) (EDAX). The powder sample was placed on a sample-holder in a glove-box filled with argon. The sample was attached to the machine via a transfer-chamber, providing inert conditions.

Magnetic measurement

Magnetic susceptibilities were measured with a MPMS7 SQUID Magnetometer (Quantum design) between 2 and 300 K. Powder samples were filled in Suprasil quartz glass tubes filled with helium. The data were corrected for the magnetization of the empty sample holder, and for the diamagnetic contributions of the constituent ions by fitting the data according to χg (korr) = χg -χ0. χ0 was obtained by fitting the experimental data χg = χ0 + Cg/(T-θ).

Results and Discussion

The title compound was synthesized by solid-state reaction from elemental barium and rhenium under flowing nitrogen gas. The detailed mechanism of its formation is unknown, but Ba2N has been proven to be an intermediate which got consumed in the course of repeated grinding and annealing cycles. EDX analysis on the black microcrystalline powder confirmed the Ba:Re ratio of 3:1, and proved the presence of N and O. The Raman data of the title compound is in good agreement with the data provided for Na4ReN3 [5]. Our spectrum comprised nearly the same bands but also some additional ones, not mentioned previously [positions in cm -1 : 184, 225, 362, 427, 859, 949, 994]. The strong signal at 949 cm -1 is indicative for Re-N multiple bonds. The IR spectrum showed two strong absorptions at 847 cm -1 and 650 cm -1 (shoulder) which are characteristic for a trigonal-planar ReN3 group. Similar IR spectra have been reported for (Ba6O)(OsN3)2 [1] and Ba3FeN3 [2]. No indications for N-H or O-H vibrations were noticable. Under argon, the compound is stable up to T = 1000 °C (mass loss < 0.5 %), as seen from TG analysis. Massive decomposition starts at T > 1050 °C, and up to T = 1300 °C a total mass loss of 3.7 % is recorded. BaO, elemental Re, and Ba2N were identified as solid residues by X-ray powder diffraction. The X-ray powder pattern of the compound (Figure 1) was found to be similar to the one of previously described (Ba6O)(OsN3)2, and both compounds proved to be isostructural. Relevant data concerning the crystal structure is found in tables 1-3. The structure is sketched in Figure 2. The constitutional building blocks are the trigonal antiprismatic Ba6O group and the trigonal planar ReN3 moiety. The Ba-O distance is 2.75 Å, the Ba-Ba distances of the Ba6 unit are split in a longer (4.03 Å) and a shorter (3.70 Å) one. These two different sets of distances are commonly observed in related trigonal antiprismatic Ba6N groups and were also found in (Ba6O)(OsN3)2. They are indicative for a filled Ba6 cluster. Concerning the ReN3 building block, the Re-N distance is 1.91 Å, the N-Re-N angle is found to be 120. These units are packed in ABC sequence with a shift of 2/3 1/3 1/3 in each step. The Curie-Weiss law applies in the range from T = 350 K down to T = 25 K. The Weiss constant is θ = -16 K. The obtained magnetic moment µ = 0.68 µB is small, indicating significant spin-orbit coupling.

The title compound is a semiconductor. At temperatures T < 100 K, the material is insulating with the resistivity ρ exceeding 1.0*10 6 Ω, while the specific conductance at RT is σ = 4.57*10 -3 Ω -1 cm -1 . Applying the Arrhenius model from T = 100 K up to 300 K, has yielded an activation energy of Ea = 0.016 eV. This low value is in accordance with the black colour of the material.

This result is suggesting that the incorporation of oxygen cannot be accounted for by discussing VEC's. We rather regard the capability of osmium and rhenium to adopt higher oxidation states the reason. This is an indispensible precondition for including additional anions, which is not fulfilled by e.g. iron.

Figure 2 .Figure 1 .Figure 2 .

 212 Figure 2. The building blocks ([Ba6O] 10+ trigonal antiprisms and trigonal pyramidal ReN3 5-anions) are packed in ABC sequence with a shift of 2/3 1/3 1/3 each (Re dark green, Ba light green, N blue, O red). The empty trigonal antiprisms from layer to layer formed by six Ba atoms are drawn in transparent grey.

Table 1 .

 1 Crystallographic

		data for (Ba6O)(ReN3)2
			(Ba6O)(ReN3)2
	Space group	3 R (No. 148)
	Cell parameters / Å	a = 8.1178(2)
			c = 17.5651(4)
	Cell volume / Å 3	1002.43(5)
	Z		6
	T / K	300
	Formula weight / g/mol	1296.49
	ρcalc / g/cm 3	5.85
	Radiation source/λ / Å	0.7093000
	Capillary diameter / mm	0.3
	Rexp / % [a]	1.44
	Rwp / % [a]	4.52
	Rp /% [a]	3.92
	Rbragg / % [a]	1.93
	GoF	3.14
	a)	as defined in TOPAS [3]

Table 2 .

 2 Atomic positions and isotropic thermal parameter of (Ba6O)(ReN3)2

	Atom	Wyck.	x/a	y/b	z/c	B / Å 2
	Ba	18f	0.3210(3)	0.2313(2)	0.4180(3)	0.3(3)
	Re	6c	0	0	0.2494(4)	0.3(3)
	N	18f	0.249(2)	0.217(2)	0.2494(4)	0.3(3)
	O	3b	0	0	1/2	0.3(3)

Table 3 .

 3 Selected

		bond lengths (Å) and angles (°) for (Ba6O)(ReN3)2
	Re-N	1.91(2)
	Re-Ba	3.485(1)
		3.767(6)
		3.822(6)
	Ba-N	2.76(2)
		2.85(1)
		3.01(1)
		3.08(1)
		3.20(2)
	Ba-O	2.738(3)
	Ba-Ba	3.703(6)
		3.885(6)
		4.033(3)
	N-Os-N	120.0

We thank A. Schulz, E. Brücher and G. Siegle for measuring the Raman, the magnetic and the conductivity data. Support by the Fonds der Chemischen Industrie (FCI) and the German National Merit Foundation (scholarships for C.L. Schmidt) is gratefully acknowledged.