Clusters of lanthanide alkoxides, aryloxides, and macrocyclic polyaryloxides

2. CLUSTERS OBTAINED UNDER NONHYDROLYTIC CONDITIONS

2.1 Clusters of lanthanide alkoxides, aryloxides, and macrocyclic polyaryloxides

Lanthanide alkoxide complexes are arguably one of the most extensively studied classes of lanthanide-containing compounds. Frequently, polynuclear complexes of the cluster type are obtained from the reactions aiming at the synthesis of mononuclear complexes. It is fair to state that the production of such clusters is more due to serendipity than by design, as is the case of most lanthanide-containing cluster compounds. The formation of polynuclear cluster species is a general reflection of the propensity of the large metal ions to maximize their coordination number and the capacity of the alkoxide ligands to bridge in order to satisfy the ligands’ steric and electrical requirement. Not surprisingly, the identity of the cluster compounds depends on both the size of the lanthanide ions and the nature of the alkoxide ligands. In addition to the interesting and diverse molecular structures observed, the research directed at the synthesis of lanthanide alkoxides is driven by the uses of such species

as precursors for advanced materials and their potential of promoting useful chemical transformations. Such efforts have produced numerous alkoxide clusters of various compositions (simple alkoxide or aryloxide ligands vs sterically crowded and/or multidentate alkoxides or aryloxides). These clusters exhibit a great variety of structural patterns and nuclearity. From the extensive research that has been accomplished, a number of general conclusions can be made:

(1) Formation of cluster structures is a general phenomenon in lanthanide alkoxide chemistry unless sterically crowded ligands are utilized. When small and sterically unhindering alkoxides are used, individual R–OR (R¼rare earths; RO or OR¼alkoxo or aryloxo) units are prone to aggregation via bridging interactions of the ligands.

Such cluster-forming bridging interactions also fulfill the desire of the ligand to achieve steric and electronic ‘‘saturation.’’

(2) Presence of small nonalkoxide ligands such as oxo, hydroxo, hydrido, halo, and chalcogenido groups is commonly observed in lanthanide alkoxide clusters. These small anionic species are important in facil- itating the formation and maintaining the structural integrity of the cluster structures.

(3) Sterically crowded multidentate, macrocyclic aryloxide ligands tend to engender different chemistry as a result of the constrained and more rigid disposition of their aryloxido moieties in the macrocyclic structure. The linking group between the individual aryloxide units within the macrocycle may also participate in synergetic metal coordination.

Reviews of the coordination chemistry of this particular family of lanthanide-containing compounds are available, but they are generally limited to the chemistry of the simplest forms of alkoxide and aryloxide ligands (Boyle and Ottley, 2008). Interested readers are referred to such reviews for more detailed discussion. With just a brief summary of the key observations and main conclusions regarding these ‘‘simple’’ alkoxides, this contribution will be focusing on the discussion of alkoxide clusters that appeared in the literature since the most recent reviews, those featuring small-unit, cluster core-building ligands, and cluster complexes of sterically demanding macrocyclic polyaryloxides.

2.1.1 Clusters of lanthanide alkoxides

Simple lanthanide alkoxide clusters contain only alkoxide ligands and possibly additional organic ancillary ligands. The core structures of these clusters have been thoroughly discussed in a recent review (Boyle and Ottley, 2008). The common core structures are collected in Figure 1. The nuclearity of the various homoleptic ‘‘R(OR)₃’’ complexes ranges from mono- to decanuclear. The geometries around these metals have been

found to encompass simple trigonal planar, tetrahedral, trigonal bipyramidal, square pyramidal, octahedral, and higher-order coordination geometries. In these motifs, the alkoxide ligands display a variety of coordination modes, from terminal monodentate [R(OR)], bridging [R(m-OR)R], triply bridging [R3(m3-OR)], and more rarely quadruply bridging [R₄(m4-OR)] interactions. The different nuclearity of the resulting clusters reflects the degree of steric and electronic saturation required at each metal center.

The second type of cluster compounds of the lanthanide alkoxides features one or several small-unit, inorganic ligands including oxo, hydroxo, halo, and other groups as part of the cluster core structure.

Frequently, a combination of such ligands is found in the same cluster species. In a sense, this ‘‘central spherical charge density’’ drives the assembly of the polynuclear complexes; without their participation, no clusters are formed or species of completely different structures result.

RO RO

RO OR R R

R R

RO RO RO

R R

R R R R

J I

R R

R R R R

R R R

R R

OR OR

OR OR OR

OR OR

OR OR OR

OR OR

OR Solv

Solv Solv

Solv

Solv Solv

Solv O OR

O O

O O OR

OR OR

R R

O O R

R R

FIGURE 1 Commonly observed central core structures ofR(OR)xclusters (redrawn after Boyle and Ottley, 2008).

Cluster-type oligomeric lanthanide oxoalkoxides are common, although their formation is generally unpredictable in terms of product nuclearity and/or the arrangement of the ancillary ligands around the cluster core. The ancillary ligands play important roles, either serving to control the degree of hydrolysis of the metal center, and therefore the degree of condensation and nuclearity of the final cluster species or modulating the sterics around the metal center to control the degree of aggregation, or both. Generally speaking, the physicochemical properties of oxoalkoxides (solubility, the behavior in solutions, volatility, etc.) differ considerably from the properties of the corresponding alkoxides (Caulton and Hubert-Pfalzgraf, 1990; Turova, 2004). They have been used success- fully as precursors of oxide materials produced by sol-gel technology.

The hydrolysis of alkoxide ligands is certainly one of the possible routes to the production of oxo and hydroxo ligands. In fact, these steps underlie the all-important sol-gel processes. However, the oxo and hydroxo groups may also be due to solvent degradation under the reaction conditions as O-containing solvents such as tetrahydrofuran (THF), ether, or dimethoxymethane are typically utilized in the preparation of lanthanide alkoxides. In many cases, however, ether byproducts are isolated, suggesting that the alkoxide ligands themselves are the source of the oxo group. Obtained by nonhydrolytic condensation, these oxoalkoxides display reactivity patterns and properties—high solubility, low volatility—different from those of alkoxides (Hubert-Pfalzgraf, 2003).

The reaction of GdCl3with KOBu^tin THF afforded the tetranuclear cluster [Na2(OBu^tGd)4(m3-OBu^t)8(m6-O)] (Schumann et al., 1990). Its structure consists of an oxygen-centered octahedron build-up by two Na and four Gd atoms, connected by eight face-capping and four terminal OBu^t groups (Figure 2, left). Using sterically less hindered methoxide, a cyclopentadienyl coligand was necessary, and the reaction led to the isolation of a pentanuclear cluster [(Cp)5R₅(m5-O)(m3-OMe)4(m-OMe)4]

Na O

O Gd

FIGURE 2 Crystal structures of [Na2(OBu^tGd)4(m3-OBu^t)8(m6-O)] (left) and [(Cp)5Gd5(m5-O)(m3-OMe)4(m-OMe)4] (right) (redrawn after Schumann et al., 1990).

(R¼Yb, Gd). X-ray structure determination shows a square pyramidal assembly of Gd atoms with an oxygen atom in the center of the base and eight bridging methoxide groups, four of which triply bridging and face-capping and the remaining four each bridging two adjacent metal atoms within the basal plane. In addition, each of the metal atoms is coordinated to a Cp ligand (Figure 2, right).

The reaction between Dy chips or Dy5O(OPrⁱ)₁₃and HOPrⁱafforded an octanuclear cluster [Dy₄(m₃,²-OR⁰)₃(m,²-OR⁰)₂(¹-OR⁰)₄(m,¹-OR⁰)₃]₂with R⁰¼C2H4OPrⁱ (Le Bris et al., 2006). Isostructural with its Y8 derivative (Hubert-Pfalzgraf et al., 2004), the cluster can be described as the assembly of two tetranuclear units by bridging monodentate ¹-alkoxide ligands (Figure 3).

The cluster complex [La3(m3,²-mmp)2(m2,²-mmp)3(mmp)4] (Figure 4, top) has been prepared by the reaction of [La{N(SiMe3)2}3] (R⁰⁰⁰¼N (SiMe3)2) with Hmmp (Hmmp¼HOCMe2CH2-OMe, 1-methoxy-2- methylpropan-2-ol) (Aspinall et al., 2007). The crystal structure (Figure 4) shows that three La atoms are linked by face- and edge- bridging mmp ligands, so that each La atom is bonded to the alkoxide groups of two face-bridging and two edge-bridging ligands.

The heterometallic cluster [NaLa3(m₃-OH)(m₃,²-mmp)2(m₂,²- mmp)4(mmp)3]4 (Figure 4, bottom) was isolated from the reaction of [La(NO₃)₃(tetraglyme)] with Na(mmp). The three La atoms of this cluster are linked by face- and edge-bridging and ‘‘dangling’’ mmp ligands as well as by a face-bridging m3-OH which is unsymmetrically bound to the three La atoms.

Dy O C FIGURE 3 Crystal structure of [Dy4(m3,²-OR⁰)3(m,²-OR⁰)2(¹-OR⁰)4(m,¹-OR⁰)3]2. For clarity, the OR⁰groups displaying a¹-coordination mode are limited to the Cacarbon atom (redrawn after Le Bris et al., 2006).

Using donor-functionalized alkoxy derivatives, a tetranuclear oxo–hydroxo cluster of net composition Lu4(O)(OH)(OR)9was obtained unexpectedly (Anwander et al., 1997). The resulting Lu4O15‘‘core’’ adopts a ‘‘butterfly’’ rather than a tetrahedral geometry (Figure 5). Other previously reported oxo-centered butterfly RE4units include Ce4(m₄-O)(m₃-OPrⁱ)2(m₂- OPrⁱ)4(OPrⁱ)7(HOPrⁱ) (Yunlu et al., 1991) and [Y4(m₃-OBu^t)2(m-OBu^t)4

(m-Cl)₂(OBu^t)4Li4(m-OBu^t)2]2(Evans et al., 1988a).

The cluster [Cp5Y5(m₅-O)(m₃-OMe)4(m-OMe)₄] (Evans and Sollberger, 1986) is comprised of a square pyramid of yttrium atoms each of which is coordinated to one Cp group. Upon each triangular face of the square

C O La Na

FIGURE 4 Crystal structures of [La3(m3,²-mmp)2(m2,²-mmp)3(mmp)4] (upper left) and its core structure (upper right) and [NaLa3(m3-OH)(m3,²-mmp)2(m2,²-mmp)4(mmp)3]4

(bottom left) and its cluster core structure (bottom right) (redrawn after Aspinall et al., 2007).

pyramid is a triply bridging methoxide ligand and attached to each vertex of the square base is a doubly bridging methoxide group. An oxide ligand lies in the interior of the structure, interacting with five yttrium atoms;

each metal atom has a formal coordination number of 10 (Figure 6).

Alkoxide ligands can also come in rather unusual form. A 12-mem- bered cyclen-type macrocycle, 1,4,7,10-tetrakis(2-hydroxyethyl)- 1,4,7,10-tetraazacyclododecane (H4L1), has been found to support the

Lu Lu2

Lu3

Lu1 Lu4 O20 O19 O11

O10

O12

O17

O3 O13

O15 O2

O6 O5

O1 O

FIGURE 5 Crystal structure of Lu4(O)(OH)(OR)9(OR¼OCMe2CH2OMe) (left) with its butterfly-shaped Lu4O15core (right) (redrawn after Anwander et al., 1997).

Y O C

FIGURE 6 Crystal structure of [Cp5Y5(m5-O)(m3-OMe)4(m-OMe)4] (redrawn after Evans and Sollberger, 1986).

assembly of a tetranuclear cluster [(La(H2L1)){La(NO₃)2(m3-OH)}2(La (HL1))](NO₃) and a pentanuclear cluster [{La(NO3)2}3(La(L1))₂(m5-OH)]

(Thompson et al., 2003).

The crystal structure of the tetranuclear complex is assembled from one [La(H2L1)]^þand two [La(NO3)2]^þcations, molecule [La(HL1)], and oneNO₃ and two bridging [m₃-OH]anions. The [La(NO3)2]^þspecies are themselves bridged by the two [m₃-OH]groups and the alkoxide site (Figure 7, left). The tetranuclear core forms a rare scoop-like motif (Figure 7, right) as opposed to other more frequently observed tetranuclear cluster core structures, such as distorted cubane, tetrahedron, butterfly, or square-shaped assemblies.

The pentanuclear cluster is built from two [Nd(m4-L1)]anions, three [Nd(NO3)2]^þcations, and them5-OHanion which ‘‘glues’’ the five cations together to form a square pyramidalC_4varrangement of the Nd(III) ions (Figure 8).

The assembly of clusters is the result of a complex interplay of several factors. It is evident that deprotonation of the alkoxide pendant arms of the chelating ligand promotes the ability of alkoxide sites to form bridges to lanthanide ions. The nature of the product is dependent on the relative Lewis acidity of the lanthanide ions, the harder Lewis acids being more able to displace the hydroxyl protons from the protonated ligand.

In addition to the frequent incorporation of oxo and hydroxo groups into cluster core structures, halo ligands have also been found in many lanthanide alkoxide clusters. An early example is [Nd6(m6-Cl)(m3-OPrⁱ)2

(m-OPrⁱ)9(OPrⁱ)6] (Andersen et al., 1978), a hexanuclear cluster obtained from the reaction of NdCl3with NaOPrⁱin HOPrⁱ. The six neodymium

La O H N C

FIGURE 7 Crystal structure of [(La(H2L1)){La(NO3)2(m3-OH)}2(La(HL1))](NO3) (left) and its tetranuclear core containing two triply bridging hydroxo groups (right) (redrawn after Thompson et al., 2003).

atoms form a trigonal prism centered about am₆-Clion (Figure 9). There are a total of 17 OPrⁱgroups, six being terminally coordinated to the Nd atoms, nine being edge bridging, and two being capping for the trigonal faces of the prism.

A triyttrium cluster Y3(m3-OBu^t)(m3-Cl)(m-OBu^t)3Cl(THF)2was obtained from the reaction of YCl3with NaOtBu in THF (Evans and Sollberger,

Nd Cl O

FIGURE 9 Crystal structure of Nd6(m6-Cl)(m3-OPrⁱ)2(m-OPrⁱ)9(OPrⁱ)6] featuring an interstitialm6-Clligand (redrawn after Andersen et al., 1978).

Nd N O

FIGURE 8 Crystal structure of [{Nd(NO3)2}3(Nd(L1))2(m5-OH)] (left) and its pentanuclear core containing am5-hydroxo group (right) (redrawn after Thompson et al., 2003).

1988). The yttrium atoms in this molecule form a triangle that has doubly bridging alkoxide groups along each edge, am₃-OBu^tgroup on one side of the Y3plane, and am₃-Cl ligand on the other side (Figure 10). One metal is coordinated to a terminal chloride ion and to a terminal alkoxide group.

Each of the remaining metal atoms is coordinated to a terminal alkoxide group and a molecule of THF.

Partial desolvation of THF in toluene led to the complete conversion into a tetradecanuclear species formulated as Y14(OBu^t)28Cl10O2(THF)2

comprised of four trimetallic units of Y3(m3-OBu^t)(m3-X)(m-Z)3(X¼Cl, O;

Z¼Cl, OBu^t). The core structure is similar to that of the trinuclear array described above. The four trimetallic subunits are connected by twom-Cl ions, am4-O ion and a [(m-OBu^t)2Y(m-Cl)]2group to give the dimer {[Y3(m3- OBu^t)(m3-Cl)(m-OBu^t)3(OBu^t)3(THF)2]2(m-Cl)[Y3(m3-OBu^t)(m-OBu^t)2(m-Cl) (OBu^t)2](m₄-O)[(m-OBu^t)2Y(m-Cl)]}₂(Figure 11).

Slightly changing the reaction conditions to a 1:2 molar ratio of YCl3

and LiOBu^t produced the dimer [Y4(m₃-OBu^t)(m-OBu^t)4(OBu^t)4(m₄-O) (m-Cl)₂Li4(m-OBu^t)2]2, in which the Y4 core has a butterfly arrangement about am₄-O group (Figure 12). Interestingly, by replacing YCl₃for LaCl₃ under otherwise identical conditions, a different trinuclear cluster

Y Cl O C

FIGURE 10 Crystal structure of Y3(m3-OBu^t)(m3-Cl)(m-OBu^t)3Cl(THF)2featuring am3-Cl ligand opposite to am3-OBu^t(redrawn after Evans and Sollberger, 1988).

La3(m₃-OBu^t)2(m-OBu^t)(OBu^t)4(THF)2 was obtained wherein a triply bridging OBu^tgroup replaces the m₃-Cl ligand in the Y₃species. These observations clearly indicate the sensitive nature of the reactions.

Y Cl O C FIGURE 12 Core structure of the tetranuclear cluster [Y4(m3-OBu^t)(m-OBu^t)4(OBu^t)4(m4- O)(m-Cl)2Li4(m-OBu^t)2]2. The four Y atoms are in a butterfly arrangement around them4-O group (redrawn after Evans and Sollberger, 1988).

Y Cl O C FIGURE 11 Crystal structure of Y14(OBu^t)28Cl10O2(THF)2whose four trimetallic subunits are connected by twom-Clions, am4-O ion, and a [(m-OBu^t)2Y(m-Cl)]2group (redrawn after Evans and Sollberger, 1988).

2.1.2 Clusters of lanthanide aryloxides

Besides the use of alkoxide ligands described above, many aryloxides have also been used to prepare organolanthanide compounds, often affording polynuclear species. Among the attractive features of these ligands is the fact that steric bulk can be manipulated through the sub- stituents on the aromatic ring without significantly altering the electronic structure of the ligand and/or the coordinating ability of the O atom.

Direct reactions of elemental lanthanoids and HOArBu^t₂ under high-temperature conditions followed by crystallization from THF or hexane afforded hydrocarbon-soluble, polymetallic, lanthanoid aryloxide clusters ½Nd₃ðOArBu^t₂Þ₉ðTHFÞ₄

^4THF, ^½La⁴^ðOArBu^t²^Þ¹²^ðOH²^Þ, ^and

½Nd₃ðOArBu^t₂Þ₉ðHOArBu^t₂Þ₂ðTHFÞ

^HOArBu^t² ^(HOArBu^t² ^¼^3;^5-Bu^t²

C₆H₃OH) (Deacon et al., 2004). The structure of½La₄ðOArBu^t₂Þ₁₂ðOH₂Þis remarkable: Four six-coordinate La ions arranged in a square plane with each vertex doubly bridged by twoOArBu^t₂ligands and a further terminal OArBu^t₂ ligand bound to each lanthanum (Figure 13, right). A single oxygen atom lies on an inversion center in the middle of the square.

The square planar La4O core of the structure is similar to that of [K5{(CH2¼CHSiO2)8}2La4(m4-OH)] (Igonin et al., 1993), an unusual lanthanum vinylsiloxanolate complex in which the central oxygen was assigned as a hydroxide.

The reactions of Sm, Eu, and Yb metals with the potential bidentate ligand 2-methoxyphenol were studied (Carretas et al., 2004). The Eu and Yb aryloxides were synthesized by dissolution of the metals in liquid NH3, whereas the Sm aryloxide was obtained by metal vapor synthesis.

Recrystallization of Yb(OC6H4OMe)3 from THF/pentane produced [Yb₆(m₃-OH)₄(m-OC₆H₄--OMe)₁₀(OC₆H₄--OMe)₂(OC₆H₄OMe)₂]

^4THF

(Figure 14). The six ytterbium atoms form a nearly planar arrangement, being joined together by four triply bridging hydroxo ligands and

La O O

FIGURE 13 Crystal structures of½Nd3ðOArBu^t₂Þ₉ðTHFÞ₄

4THF (left) and

½La4ðOArBu^t₂Þ₁₂ðOH2Þ(right) (redrawn after Deacon et al., 2004).

by 10 bridging-chelating aryloxides. All metal atoms are octacoordinate with a distorted-square antiprismatic geometry. Alternatively, the cluster core structure may be viewed as two Yb4(m3-OH)2units sharing two Yb atoms (Yb1 and Yb2); the tetranuclear motif is a common one, with the four metal atoms being coplanar and with two triangular faces capped by two triply bridging hydroxo groups, one above and one below the plane.

The reaction of Eu metal with isopropanol produces presumably a reactive species ‘‘Eu(OPrⁱ)2’’ which, in turn, can be reacted with phenols, 2,6-dimethylphenol (HOArMe2) and 2,6-diisopropylphenol (HOArPrⁱ₂), to afford the cluster species Hx[Eu8O6(OArMe2)12(OPrⁱ)8] and H₅½Eu₅O₅ðOArPrⁱ₂Þ₆ðMeCNÞ₈ (MeCN¼acetonitrile) (Evans et al., 2000).

The octanuclear complex has a cubic arrangement of europium ions with O14

O16 O2 O4

O5 O3

O1 O7 O10 O11 O15

O8 O6

O9 O12

O13

Yb4 Yb3

Yb4′

Yb2 Yb3′

Yb1

FIGURE 14 Crystal structure of [Yb6(m3-OH)4(m-OC6H4--OMe)10(OC6H4-- OMe)2(OC6H4OMe)2] (reproduced with permission from Carretas et al., 2004).

face-bridgingm4-O donor atoms, edge-bridging m-O(phenoxide/phenol) ligands, and terminal O(isopropoxide/2-propanol) ligands (Figure 15, upper left). The pentanuclear cluster is a mixed-valence species and has a square pyramidal europium core with four Eu(II) ions at the basal positions and one Eu(III) ion at the apex (Figure 15, upper right). By tweaking the reaction conditions, including temperature and solvent, a nonanuclear cluster formulated as H18{[Eu₉O₈(OArMe₂)₁₀(THF)₇] [Eu₉O₉(OArMe₂)₁₀(THF)₆]} can be crystallized which contains two types of capped cubic arrangements of europium ions in the solid state (Figure 15, bottom).

Roesky and coworkers reported unusual tetradecanuclear clusters formulated as [R₁₄(o-O2NC6H4O)24(m4-OH)2(m3-OH)16] whereR¼Dy, Er, Tm, Yb (Figure 16; Burgstein and Roesky, 2000). The cluster core of

Eu O C

FIGURE 15 Crystal structures of Hx[Eu8O6(OArMe2)12(OiPr)8], H5½Eu5O5ðOArPrⁱ₂Þ₆ ðMeCNÞ₈, and the two different types of nonanuclear clusters in

H18{[Eu9O8(OArMe2)10(THF)7][Eu9O9(OArMe2)10(THF)6]} (clockwise from upper left) (redrawn after Evans et al., 2000).

[R₁₄(m₄-OH)2(m₃-OH)16] is the same as those described below when dike- tonate ligands are used as the ancillary ligands. In fact, this tetradecanuclear cluster core motif is a common one which may be used to construct cluster assemblies of even more sophisticated structures.

Phenolates modified with other functional groups have also been utilized for the controlled hydrolytic assembly of lanthanide clusters (Costes et al., 2001; Tang et al., 2006). For example, using 2-hydroxy-3-methoxyben- zaldehyde oro-vanillin (HVan)—a multidentate ligand that may be viewed as a modified phenolate upon deprotonation—cationic trinuclear Gd and Dy hydroxo clusters, [(Van)3R₃(OH)2X2(OH2)4]^2þ(R¼Gd,X¼NO₃; R¼Dy, X¼Cl) (Figure 17), have been obtained and their magnetic properties studied. The cluster core is a trigonal bipyramid with the twom3-OH groups occupying the axial positions. Using its phenoxo group, a Van (deprotonatedo-vanillin) ligand bridges two metal atoms along each side of the triangle. Aldehyde and methoxy groups also coordinate to the metal atoms. Both clusters display complex magnetic behavior. The Gd cluster shows an antiferromagnetic behavior attributable to the three (Gd, Gd) pairs, while features typical of a single-molecule magnet (SMM) are observed for the thermally populated excited state of the Dy cluster.

R O N C FIGURE 16 Crystal structure of the tetradecanuclear cluster [R14(o-O2NC6H4O)24

(m4-OH)2(m3-OH)16] formed with nitrophenolate ligands (redrawn after Burgstein and Roesky, 2000).

Dalam dokumen S.M. KAUZLARICH, Davis, USA G.H. LANDER , Karlsruhe, Germany M.F. REID, Christchurch, New Zealand (Halaman 149-172)