Carboxylates
Carboxylates have long been recognized as effective ligands for coordinating with lanthanides, encompassing basic carboxylates like formate and acetate, as well as (poly)amino(poly) carboxylates This ligand-controlled hydrolytic method facilitates the assembly of lanthanide hydroxide clusters, showcasing the versatility and importance of carboxylates in lanthanide chemistry.
3-TCAH Thiophene-3-carboxylic acid Fig 10 bipy 2,2 0 -Bipyridine Fig 5
H 2 O 3 P t Bu Tert-butyl phosphonic acid Fig 11
H 2 pmp N-Pipe-ridinomethane-1-phosphonic acid Fig 36
H 8 TBC8A p-Tert-butylcalix[8]arene Fig 37 See Fig 37
Fig 25 Fig 31 Fig 32 HAcc 1-Amino-cyclohexanel-carboxylic acid Fig 2
Hhtp (Z)-3-Hydroxy-3-phenyl-1-(thiophen-2-yl)prop-2- en-1-one
HL 3-Fluoro-4-(trifluoromethyl)benzoic acid Fig 5
Hnmc Ortho ring-functionalized 1-phenylbutane-1,3-dione ligand 1
Hnpd Ortho ring-functionalised 1-phenylbutane-1,3-dione ligand 2
HO i Bu Isobutyl alcohol Fig 35
HO 2 C t Bu Pivalic acid Fig 6
Fig 11 Fig 34 Fig 35 Hthd 2,2,6,6-Tetramethylheptane-3,5-dione Fig 30 ina Isonicotinate Fig 4
Fig 12 Fig 14 i PrNH 2 Isopropylamine Fig 35
L-threonine carboxylates, as shown in Figure 18, are noted for their structural and functional sophistication Traditionally, lanthanide carboxylate complexes have been synthesized under highly acidic conditions (pH 3-4) due to hydrolysis concerns However, Zheng et al investigated lanthanide coordination chemistry with α-amino acids at significantly higher pH levels, revealing a rich array of "high-pH" coordination chemistry for lanthanides This research has led to the characterization of polynuclear lanthanide complexes, expanding our understanding of their chemistry.
L 8 4-Amino-3,5-dimethyl-1,2,4-triazole Fig 47 mdeaH 2 N-Methyldiethanolamine Fig 6
O-btd 4-Hydroxo-2,1,3-benzothiadiazolate Fig 24 o-van 3-Methoxysalicylaldehydato anion Fig 6
PepCO 2 H 2-[{3-(((tert-butoxycarbonyl)amino)methyl)ben- zyl}-amino]acetic acid
The study highlights the role of amino acid ligands, such as phenylcarboxylic acid and proline, in stabilizing polyhedral lanthanide-oxo/hydroxo core motifs These ligands contribute to enhanced water solubility of the resulting complexes at lower pH levels, facilitating deprotonation and hydrolysis when a base is introduced However, not all carboxylate ligands are suitable for supporting hydroxide complexes, as many lanthanide complexes with these ligands tend to precipitate before achieving higher pH levels Additionally, researchers may be cautious of creating insoluble lanthanide oxide/hydroxide precipitates, leading them to avoid high-pH conditions in their experiments.
The ligand-controlled hydrolytic method is now a standard technique for synthesizing lanthanide hydroxide cluster complexes The type of cluster species formed is highly sensitive to the choice of supporting ligands, leading to significant variations Additionally, the final structure of the clusters can be influenced by other reaction conditions, including the presence of extra ligands or reactants, even if these components are not included in the final products.
2.1.1 Tetra-, Penta-, and Heptanuclear Clusters
Long et al synthesized tetranuclear lanthanide hydroxide cluster complexes, specifically [Ln4(μ3-OH)4(Acc)6(H2O)7(ClO4)](ClO4)7·11H2O (Ln = Dy, Yb), utilizing the amino acid-like ligand HAcc to regulate lanthanide hydrolysis The cluster features a core comprising four Ln3+ ions and four hydroxo groups forming a distorted cubane structure, with carboxylate groups from the organic ligand bridging the edges Aqua ligands and a monodentate perchlorate complete the coordination spheres In contrast, reactions with lighter lanthanides La3+ and Nd3+ yielded trinuclear complexes of the form [Ln3(Acc)10(H2O)6](ClO4)9·4H2O, where three lanthanide ions are linearly arranged and connected by carboxylate groups The differences in product formation are attributed to the size and Lewis acidity of the lanthanide ions, with heavier ions like Dy3+ and Yb3+ exhibiting greater Lewis acidity, facilitating hydrolysis compared to the larger, less acidic La3+ and Nd3+.
Using nicotinic acid in a similar capacity, Zheng et al obtained and structurally characterized isostructural tetranuclear complexes of the formula [Ln 4 (μ3-OH) 4 (Hnic) 5 (H 2 O) 12 ](ClO 4 ) 8 (LnẳEu, Gd; Hnicẳpyridinium nicotinate)
The cluster core resembles the previously mentioned distorted cubane, but only five out of the six edges of the Ln4 tetrahedron are connected by the carboxylate group of the zwitterionic ligand To accommodate the coordination of the two distinct lanthanide ions, additional aqua ligands are utilized.
The hydrolysis of isonicotinate (ina) as a supporting ligand led to the formation of a tetranuclear complex, [Dy4(μ3-OH)4(ina)6(py)(CH3OH)7](ClO4)2py4CH3OH, which shares a core structure with a complex derived from nicotinic acid This complex features bridging through a carboxylate group from isonicotinate, along with the coordination of seven methanol molecules and one pyridine molecule.
Fig 2 Structure of: (a) [Dy 4 ( μ 3 -OH) 4 (Acc) 6 (H 2 O) 7 (ClO 4 )] 7+ and (b) [La 3 (Acc) 10 (H 2 O) 6 ] 9+ Reprinted with the permission from [18] Copyright 2011 Royal Society of Chemistry
The structure of the cluster complex [Eu4(μ3-OH)4(Hnic)5(H2O)12]8+ is illustrated in Fig 3, showcasing its unique metal coordination This complex has been demonstrated to exhibit properties typical of a single-molecule magnet, highlighting its significance in the field of molecular magnetism.
In the ligand-supported assembly of hydroxide clusters, incorporating organic co-ligands beyond coordinating solvents is common For instance, Zhao et al synthesized a tetranuclear complex, [Dy4(μ3-OH)2(L)10(bipy)2(H2O)2], utilizing both the ligand L (HLẳ3-fluoro-4-(trifluoromethyl)benzoic acid) and the chelating bipy (2,2'-bipyridine) alongside hydroxo and aqua ligands This complex features a parallelogram-shaped core formed by four coplanar lanthanide atoms linked by two μ3-OH groups In this structure, two edges are bridged by carboxylate groups from different L ligands, while the remaining edges are connected by a carboxylate group and a μ2-H2O molecule The coordination sphere is completed by either a bipy or a monodentate L ligand, highlighting the intricate interactions in lanthanide hydroxide complexes.
Recent studies have unveiled two new series of tetranuclear hydroxide clusters with a shared core motif Murray et al introduced isostructural complexes of the formula [Ln4(μ3-OH)2(o-van)4(O2CtBu)4(NO3)2]·CH2Cl2·1.5H2O, where Ln represents Gd or Dy, o-van denotes the 3-methoxysalicylaldehydato anion, and O2CtBu refers to pivalate Concurrently, Powell et al reported five additional isostructural complexes characterized by the formula [Ln4(μ3-OH)2(mdeaH)2(O2CtBu)8], with mdeaH2 being N-methyldiethanolamine and Ln representing Tb.
The crystal structures of the complexes in both series, featuring Dy, Ho, Er, and Tm, demonstrate the role of pivalate in stabilizing the cluster core, as illustrated in Fig 6.
Long et al successfully isolated the complex [Dy5(μ3-OH)6(Acc)6(H2O)10]Cl9·24H2O using 1-amino-cyclohexanel-carboxylic acid (Acc) and DyCl3, which contrasts significantly with the tetranuclear species obtained from Dy(ClO4)3 While previous studies have highlighted the significant anion-template effects on cluster nuclearity, it is important to note that the anions do not engage in metal coordination within these complexes Therefore, the precise influence of anions on the reactions conducted under identical conditions remains to be elucidated.
The core of the cluster features five Dy 3+ ions arranged in a trigonal bipyramidal geometry, which can also be interpreted as two distorted cubanes connected by a shared trimetallic face Each triangular metal face is complemented by a μ3-OH group, and the non-equatorial metal edges are linked by an Acc carboxylate group Additionally, the coordination sphere of each Dy 3+ ion is finalized with two aqua ligands.
Collison et al described two isostructural heptanuclear complexes, specifically [Ln7(OH)6(thmeH2)5(thmeH)(tpa)6(MeCN)2](NO3)2, where Ln represents Gd and Dy, and thmeH3 denotes tris The structural representation of [Dy4(μ3-OH)2(L)10(bipy)2(H2O)2] is shown in Figure 5, with permission from the Royal Society of Chemistry, copyright 2014.
The synthesis of triphenylacetic acid (tpaH) was conducted under solvothermal conditions, utilizing a combination of lanthanide nitrate hydrates, thmeH3, tpaH, and triethylamine in acetonitrile The resulting cluster core features seven coplanar Ln3+ ions arranged in a disc-like hexagon, surrounded by six peripheral ions.
Ln 3+ ions occupying the vertices of the hexagon and the remaining Ln 3+ ion sitting at the center of hexagon and connecting the peripheral metal ions through sixμ3-
OH groups Alternatively this cluster core can be viewed as two of the coplanar tetranulcear units, such as those shown in Figs.5and6, joined together by twoμ3-
OH groups In effect, the sixμ3-OH groups are alternatingly above and below the
Fig 7 Structure of [Dy 5 ( μ 3 -OH) 6 (Acc) 6 (H 2 O) 10 ] 9+ Reprinted with the permission from [24] Copyright 2012 American Chemical Society
Diketonates
Diketonate-based ligands are widely used in lanthanide coordination chemistry, particularly in the assembly of lanthanide hydroxide cluster complexes Recent studies have identified various cluster core motifs, with planar tetranuclear and square pyramidal pentanuclear structures being the most common In contrast, lanthanide hydroxide clusters supported by carboxylate-based ligands display significantly greater structural diversity compared to those using diketonate ligands.
The 24-cubane cluster core structure in the complex [Er60(L-thre)34(μ6-CO3)8(μ3-OH)96(μ2-O)2(H2O)18]30+ is illustrated in Fig 18(a) This figure also demonstrates the formal assembly of a discrete sodalite cage utilizing cluster cubane units as secondary building units (SBUs) in Fig 18(b) Additionally, Fig 18(c) presents the structure of the cationic 60-metal complex This content is reprinted with permission from the American Chemical Society, copyright 2009.
2.2.1 Clusters with Nuclearity Smaller Than Nine
Trinuclear lanthanide hydroxide cluster complexes are rare, with a notable example being the complex [Dy3(OH)(teaH2)3(paa)3]Cl2·MeCN·4H2O In this structure, deprotonated triethanolamine (teaH3) and N-(2-pyridyl)-acetoacetamide (paaH) work together to stabilize a cuboidal or incomplete cubane core featuring [Dy3(μ3-OH)].
The three Dy 3+ ions, along with the μ3-OH group, create a pyramid structure that is surrounded by diketonate ligands Each edge connecting the Dy ions is bridged by an ethanoxide O atom from a teaH 2 ligand, which also coordinates with its N atom and two ethanol OH groups to the same lanthanide ion Additionally, each lanthanide ion is chelated by two ketonate O atoms from a paa ligand.
The four-shell organization of 104 lanthanide atoms is depicted, showcasing the construction of the cluster core through square pyramidal secondary building units (SBUs) Additionally, the cationic complex [Ln 104 (μ3-OH) 168 (μ4-O) 30] 84+ is illustrated, highlighting the coordination and passivation of the cluster core by acetate ligands This information is reprinted with permission from the American Chemical Society, copyright 2014.
Tetranuclear cluster motifs are either a distorted cubane or a planar arrangement of 4 lanthanide ions with twoμ3-OH groups Both motifs, already discussed above, have seen frequent occurrence in the literature.
Zheng et al performed a systematic investigation into the use of acetylacetonate (acac) as a protective ligand for the assembly of hydroxide cluster complexes in organic solutions They presented a structure of isostructural tetranuclear complexes, specifically Ln4(μ3-OH)2(μ3-OCH3)2(CH3OH)2(acac)8, where Ln represents elements such as Nd and Sm.
Fig 20 Structure of representative core motifs in lanthanide-oxo/hydroxo cluster complexes supported by diketonate-based ligands
The side and top views of the complex [Dy3(OH)(teaH2)3(paa)3]2+ illustrate its structural features, as shown in Figure 21 The cubane cluster core of the compound [Ln4(μ3-OH)2(μ3-OCH3)2]8+ is characterized by coordination involving two μ3-OH and two μ3-OCH3 groups (Figure 22a) Additionally, each Ln3+ ion is chelated by two acac ligands, with two of the lanthanide ions also coordinating with a methanol molecule (Figure 22b).
MacLellan et al synthesized a series of tetranuclear hydroxide cluster complexes using 1-phenylbutane-1,3-dione ligands functionalized with nitro (Hnpd and Hnmc), methoxy (Hmmc), or fluoro (Hfpp) groups The complexes, including [Er 4 (μ3-OH) 4 (H 2 O) 2 (npd) 8 ], [Ln 4 (μ3-OH) 4 (nmc) 8 ] (Ln = Gd, Tb, Dy, and Er), [Er 4 (μ3-OH) 4 (mmc) 8 ], and [Er 4 (μ3-OH) 4 (H 2 O) 2 (fpp) 8 ], were prepared in methanol through a reaction involving lanthanide chloride hydrates, diketone ligands, and trimethylamine, which facilitates the hydrolysis of the lanthanide hydrates.
The complexes [Er4(μ3-OH)4(H2O)2(npd)8] and [Er4(μ3-OH)4(nmc)8] share a common cubane cluster core, as illustrated in Fig 23 In these structures, each Er3+ ion is coordinated with two diketonate ligands, either npd or nmc Notably, in the complex [Er4(μ3-OH)4(H2O)2(npd)8], two of the four Er3+ ions are additionally bonded to one aqua ligand alongside the two diketonate ligands, as depicted in Fig 23a.
Several tetranuclear complexes featuring various diketonate ligands with a consistent planar cluster motif have been identified The structures of [Er4(dbm)6(O-btd)4(μ3-OH)2] and [Er4(dbm)4(O-btd)6(μ3-OH)2] are illustrated in Fig 24, where dbm represents dibenzoylmethanide and O-btd denotes 4-hydroxo-2,1,3-benzothiadiazolate Both complexes exhibit a planar rhomboid cluster core, surrounded by a mix of bridging–chelating O-btd ligands and chelating-only dbm ligands.
Using a combination of Hacac and H 2 L 6 ẳN,N 0 -bis(salicylidene)-1,2- cyclohexanediamine, Sun et al obtained four isostructural complexes of the
The structures of the cubane cluster core and the complex [Ln4(μ3-OH)2(μ3-OCH3)2(CH3OH)2(acac)8] are depicted in Figure 22 This complex, which includes elements such as Samarium (Sm) and Gadolinium (Gd), follows the common formula [Ln4(μ3-OH)2(L6)2(acac)6]xH2L6yCH3CNzH2O The information is reprinted with permission from the Royal Society of Chemistry, copyright 2011.
The synthesis involved the gradual addition of a methanolic solution of lanthanide acetylacetonate hydrate to an acetonitrile solution of H2L6, followed by refluxing the mixture In the resulting rhomboid cluster core, two opposite edges are bridged by L6 phenol O, while the other two edges are connected by both L6 phenol O and an O atom from the chelating-bridging acac ligand Additionally, each lanthanide ion is coordinated by a chelating-only acac ligand.
Urbatsch et al reported the synthesis of two tetranuclear complexes, formulated as [Ln4(μ3-OH)2{(μ-O)-k 2 -htp}2{(μ-O)2-k 2 -htp}2(k 2 -htp)6] (where Ln represents Nd and Eu), utilizing (Z)-3-hydroxy-3-phenyl-1-(thiophen-2-yl)prop-2-en-1-one (Hhtp), a thiophene-containing β-diketone, as a supporting ligand The core of the rhomboid cluster is coordinated with the diketonate ligands through three distinct modes.
Fig 23 Structure of: (a) [Er 4 ( μ 3 -OH) 4 (H 2 O) 2 (npd) 8 ] and (b) [Er 4 ( μ 3 -OH) 4 (nmc) 8 ] Reprinted with the permission from [48] Copyright 2011 Royal Society of Chemistry
The structures of [Er4(dbm)6(O-btd)4(OH)2] and [Er4(dbm)4(O-btd)6(OH)2] demonstrate unique bridging configurations, where opposite edges are bridged by two oxygen atoms from the same htp ligand, while the other two edges are connected by one htpO atom Each of these htp ligands chelates a lanthanide ion, and the remaining six htp ligands serve solely as chelating agents, completing the octacoordinate sphere for each lanthanide ion.
Fig 25 Structure of [Dy 4 ( μ 3 -OH) 2 (L 6 ) 2 (acac) 6 ] Reprinted with the permission from [50] Copy- right 2011 American Chemical Society
Fig 26 (a) Schematic illustration of the Hhtp ligand and (b) structure of [Eu 4 ( μ 3 -OH) 2 {( μ -O)-k 2 - htp} 2 {( μ -O) 2 -k 2 -htp} 2 (k 2 -htp) 6 ] Reprinted with the permission from [51] Copyright 2012 Wiley-VCH Verlag GmbH & Co
Together with the above tetranuclear complexes, a pentanuclear complex
The compound Er5(μ3-OH)4(μ4-OH)(μ-η2-htp)4(η2-htp)6 was successfully isolated The formation of this larger cluster, despite identical reaction conditions, may be attributed to the varying sizes of the lanthanide ions, specifically comparing Er3+ to Nd3+.
Phosphonates and Sulfonates
Building on the successful use of carboxylate and diketonate ligands for assembling high-nuclearity lanthanide hydroxide clusters, chemists are now exploring other oxygen-based ligands, including phosphonates and sulfonates These ligands are being investigated for their potential to form cluster complexes with unique structures and properties, either used independently or in conjunction with different types of ligands.
Fig 32 Structure of [Dy 14 ( μ 4 -OH) 2 ( μ 3 -OH) 16 ( μ - η 2 -acac) 8 ( η 2 -acac) 16 ] Reprinted with the per- mission from [59] Copyright 2011 Royal Society of Chemistry
The treatment of lanthanide nitrate hydrates with pyridine, pivalic acid, and t-butyl phosphonic acid in isobutanol under reflux resulted in the formation of tetranuclear lanthanide hydroxide complexes, represented by the formula [pyH]4[Ln4(μ3-OH)(O3PtBu)3(HO3PtBu)(O2CtBu)2(NO3)6] where Ln includes Gd, Tb, Dy, Ho, and Er The Gd3+ complex features a tetranuclear core that resembles a μ3-OH-bridged trinuclear cuboidal unit, linked to a fourth metal ion through three μ3-O3PtBu2 ligands These phosphate ligands facilitate coordination and bridging between adjacent lanthanide ions Additionally, the structure includes one μ2-HO3PtBu and two μ2-O2CtBu ligands along the cuboidal edges, bridging pairs of lanthanide ions, while each lanthanide ion is chelated by an NO3 anion, with the fourth metal's coordination sphere completed by three chelating groups.
Winpenny et al successfully synthesized three isostructural octanuclear complexes with the formula [Ln8(O3PtBu)6(μ3-OH)2(H2O)2(HOiBu)(O2CtBu)12](NH3iPr)2 by replacing pyridine with isopropylamine to facilitate hydrolysis These complexes include lanthanides such as Gd, Dy, and Tb, utilizing isopropylamine (iPrNH2) and isobutyl alcohol (HOiBu) in their composition.
The arrangement of eight Ln 3+ ions forms a horseshoe-like structure, interconnected by six O3P t Bu 2 ligands, two μ2-O2C t Bu, and four μ-η2-O2C t Bu ligands The coordination spheres are further enhanced by bidentate chelating O2C t Bu ligands, HO i Bu molecules, and aqua ligands This cluster motif can also be interpreted as two μ3-OH-containing cuboidal units linked by two O3P t Bu 2 and one μ-η2-O2C t Bu ligands, with an additional lanthanide ion on each end of the double-cuboidal configuration.
Fig 33 Structure of: (a) the pentadecanuclear core with a templating chloride ion and (b) [Tb 15 ( μ 3 -OH) 20 (PepCO 2 ) 10 (dbm) 10 Cl] 4+ Reprinted with the permission from [60] Copyright
Cao et al synthesized two isostructural nonanuclear complexes, specifically [Ln9(μ2-OH)(Hpmp)12(ClO4)(H2O)26](ClO4)13·18H2O, where Ln represents Nd or Pr This synthesis utilized N-piperidinomethane-1-phosphonic acid (H2pmp) as a supporting ligand, with the process involving the addition of NaOH to an aqueous solution containing H2pmpHCl.
Fig 34 Structure of the anionic cluster complex [Gd 4 ( μ 3 -OH)(O 3 P t Bu) 3 (HO 3 P t Bu)(O 2 C t Bu) 2 (NO 3 ) 6 ] 4 Reprinted with the permission from [62] Copyright 2014 Royal Society of Chemistry
The octanuclear core features a distinctive lotus-leaf-shaped arrangement, showcasing bridging atoms, including one μ2-OH group This structure is represented by the compound [Ln8(O3PtBu)6(μ3-OH)2(H2O)2(HOiBu)(O2CtBu)12](NH3iPr)2, as illustrated in Figure 35 The image is reproduced with permission from the Royal Society of Chemistry, copyright 2013.
12 phosphonate bridging ligands The coordination spheres are completed with ClO4 anion and aqua ligands (Fig.36).
Phosphate serves as an ancillary ligand in the formation of polynuclear lanthanide hydroxide complexes For instance, Hong et al reported two isostructural decanuclear complexes, [Ln 10 (TBC8A) 2 (PhPO 3 ) 4 (OH) 2 (HCO 3 )(HCOO)(DMF) 14 ](H 6 TBC8A)xDMFyCH3OH, using a combination of lanthanide nitrate hydrate, H 8 TBC8A, and H 2 PhPO 3 in a DMF/methanol solvent system In these complexes, ten Ln 3+ ions are coordinated by two TBC8A 8 ligands, which adopt a cup conformation, with their lower-rim phenoxide O atoms stabilizing the lanthanide ions The structural integrity is further enhanced by four PhPO 3 2 ligands, two OH groups, one HCO 3 anion, and one HCOO ligand The crystal lattice features alternating arrangements of the [Ln 10 (TBC8A) 2 (PhPO 3 ) 4 (OH) 2 (HCO 3 )(HCOO)(DMF) 14 ] 2+ cations and (H 6 TBC8A) 2 anions Additionally, Zhang et al isolated a hexanuclear complex, [Yb 6 (μ6-O)(μ3-OH)8(mds)4(H2O)6], by reacting Yb2O3 with methylenedisulfonic acid under hydrothermal conditions, where six Yb 3+ ions are arranged in a μ6-O-centered octahedron, each capped by a μ3-OH group.
The structure is organized into a one-dimensional column through the coordination of two opposing Yb 3+ ions via mds coordination Each disulfonate ligand connects to a Yb 3+ ion using its two sulfonate groups, with one oxygen from each group coordinating to a Yb 3+ from different cluster units Additionally, the bridging Yb 3+ ion is coordinated by two aqua ligands The remaining four Yb 3+ ions exhibit two distinct coordination types: one is coordinated by a chelating mds ligand and an aqua ligand, while the other is coordinated solely by a chelating mds ligand.
Polyoxometalates
A new class of ligands, known as polyoxometalates (POMs), is gaining attention for their role in assembling lanthanide hydroxide complexes POMs are appreciated for their easy synthesis and adjustable chemical properties, making them effective in various applications They have been shown to facilitate numerous organic transformations, catalyze water-splitting processes, and contribute to the development of innovative memory devices.
The rising popularity of polyoxometalates (POMs) in lanthanide coordination can be attributed to two key factors Firstly, POMs are anionic structures rich in oxygen atoms, which create strong electrostatic attractions with lanthanide ions, making them effective protecting ligands and templating anions for lanthanide cluster assembly It's important to note that not all examples exhibit the polyhedral Ln-O/OH motif typical of lanthanide oxide or hydroxide clusters; some hydroxo groups may actually be associated with POM ligands rather than lanthanide ions, and many species feature lanthanide atoms that are further apart than expected for conventional clusters Secondly, the weakly coordinating nature of POMs enhances the Lewis acidity of lanthanide ions in Ln-POM combinations, potentially increasing catalytic efficiency in Lewis acid-promoted reactions.
Zhang et al reported three isostructural lanthanide tungstobismuthate complexes
Na x H 22-x {(BiW 9 O 33 ) 4 (WO 3 )[Bi 6 (μ3-O) 4 (μ2-OH) 3 ][Ln 3 (H 2 O) 6 (CO 3 )]}nH2O (LnẳPr, Nd, La) from an aqueous reaction involving Na 12 [Bi 2 W 22 O 74 (OH) 2 ] 44H 2 O, a lanthanide chloride hydrate, Na 9 [BiW 9 O 33 ]16H 2 O, and Na 2 CO 3
The structure depicted in Fig 39 illustrates a trigonal planar arrangement of three Ln 3+ ions coordinated around a μ3-CO3 2- group, with each oxygen atom linking two lanthanide ions along the triangle's edges Additionally, each Ln 3+ ion is coordinated with two aqua ligands, forming the [Ln3(H2O)6(CO3)]7+ motif, which is subsequently encapsulated by four surrounding entities.
[BiW 9 O 33 ] 9 anions, three of which being directly connected to the [Ln3(H2O)6(CO3)] 7+ core with the fourth one through one [Bi6(μ3-O)4(μ2-OH)3] 7+ unit.
By reacting Na 2 WO 4 2H 2 O, oxalic acid, and lanthanide chloride, without or with the presence KCl in an aqueous solution at pH 7.5, Chen et al obtained
The compound Na 10 [Ln 2 (C 2 O 4 )(H 2 O) 4 (μ2-OH)(W 4 O 16 )] 2 30H 2 O features a core structure comprising four lanthanide ions arranged in a rectangular formation Each longer side of the rectangle is connected by a C 2 O 4 2 ligand, while the shorter side is linked by a μ2-OH group Additionally, each lanthanide ion is coordinated with two aqua ligands This core is further stabilized by two W 4 O 16 8 units that bridge the lanthanide ions along the shorter side through O-Ln coordination, enhancing the overall structural integrity of the compound.
In the complex structure, four lanthanide ions form a square, with each side connected by a C2O4^2- ligand, and notably, there are no aqua ligands or hydroxo groups present Each lanthanide ion is coordinated through O-coordination to a W5O18^6- capping ligand.
Another series of tetranuclear Ln-POM complexes, formulated as [PMo V 8Mo VI
Dolbecq and colleagues reported the compound [PMo V 8 Mo VI 4 O 36 ] 11, which features a tetrahedral arrangement of four [Ln(H 2 O) 4 (OH)] 2+ units, where Ln represents lanthanides such as La, Ce, Nd, and Sm The anion acts as a structural support for these units, facilitating their attachment in a stable configuration.
POM ligands effectively encapsulate lanthanide ions, providing protection and facilitating their attachment to surfaces A notable instance of this coordination was demonstrated by Wang et al., showcasing the dual functionality of POM ligands in lanthanide ion interactions.
The structure of the complex {(BiW 9 O 33 ) 4 (WO 3 )[Bi 6 ( μ 3 -O) 4 ( μ 2 -OH) 3 ][Pr 3 (H 2 O) 6 (CO 3 )]} 22 is illustrated in Fig 39 (a), alongside its building blocks shown in Fig 39 (b), with hydrogen atoms and lattice solvent molecules omitted for clarity This content is reprinted with permission from the Royal Society of Chemistry, copyright 2012.
Fig 40 Structure of: (a) {[Eu 2 (C 2 O 4 )(H 2 O) 4 ( μ 2 -OH)(W 4 O 16 )] 2 } 10 (top) and cluster core (bot- tom) and (b) {[Eu(C 2 O 4 )(W 5 O 18 )] 4 } 20 (top) and the cluster core (bottom) Reprinted with the permission from [76] Copyright 2014 American Chemical Society
The structures of [PMo V 8 Mo VI 4 O 36 ] 11 and {PMo V 8 Mo VI 4 O 36 [Ln(H 2 O) 4 (OH)] 4 } 5+ illustrate the coordination of the [Ln(H 2 O) 4 (OH)] 2+ unit, highlighting the triangular face utilized for this purpose Additionally, the chain structure of the anionic complex Na 10 [Ln 6 (H 2 O) x {As 4 W 44 (OH) 2 (proline) 2 O 151 }]nH2O, featuring Ln ions (Ln = Tb, Dy, Nd) linked by hydrated Ln 3+ ions, is also demonstrated.
Organic ligands play a crucial role in coordinating metal ions from various Ln-POM SBUs, as illustrated by the structure of the anionic complex unit K20Li2[Ln3(μ3-OH)(H2O)8(AsW9O33)(AsW10O35)(mal)]·17H2O (where Ln = Dy, Tb, Gd, Eu, Sm and mal = malate) This complex features two μ3-OH group-bridged cuboidal building blocks linked by two mal ligands, with the entire motif situated between one {AsW9O33} and one {AsW10O35} unit, the latter also coordinated by a mal ligand The coordination sphere of the Ln3+ ion, chelated by the mal ligand, is completed by two aqua ligands, while the other two Ln3+ ions are each coordinated by three aqua ligands.
Fig 42 Structure of: (a) the [Ln 4 (H 2 O) 16 {As 4 W 44 (OH) 2 (proline) 2 O 151 }] 16 building block and (b) [Ln 6 (H 2 O) x {As 4 W 44 (OH) 2 (proline) 2 O 151 }] 10 Reprinted with the permission from [78] Copyright 2013 Royal Society of Chemistry
Fig 43 Structure of {[Ln 3 ( μ 3 -OH)(H 2 O) 8 (AsW 9 O 33 )(AsW 10 O 35 )(mal)] 2 } 22 Reprinted with the permission from [79] Copyright 2015 Royal Society of Chemistry
Davoodi et al synthesized the anionic octanuclear complex [(SiW 10 Sm 2 O 38 ) 4 (W 3 O 8 )(OH) 4 (H 2 O) 2 ] 26 by using an aqueous mixture of samarium chloride, Na2CO3, KCl, and Na10[A-α-SiW 9 O 34 ]xH2O This complex decomposed slowly in a concentrated solution, resulting in the final product The structure consists of eight Sm3+ ions surrounding a template anion [W 3 O 8 (OH) 4 (H 2 O) 2 ] 2, which is further encapsulated by four [SiW 10 O 38 ] 12 ligands.
Patzke et al reported the synthesis of hexadecanuclear lanthanide polyoxotungstate complexes, specifically formulated as [Ln 16 As 16 W 164 O 576 (OH) 8 (H 2 O) 42 ] 80, where Ln represents lanthanide ions such as Eu, Gd, Tb, Dy, and Ho This complex was produced through an aqueous reaction involving K 14 [As 2 W 19 O 67 (H 2 O)], lanthanide nitrate hydrate, NaCl, and CsCl In this structure, each of the 16 Ln 3+ ions is capped by a {AsW 9 O 33 } unit, and these units are interconnected by 20 tungstate anions, eight hydroxide groups, and four cesium cations, with aqua ligands completing the coordination spheres of the Ln 3+ ions.
Miscellaneous Ligands
Recent studies have revealed structurally intriguing lanthanide hydroxide cluster complexes that are supported by unconventional ligands Notably, Alikberova et al identified two hexanuclear complexes, specifically [Ln6(H2O)23(OH)10], showcasing the diversity of these compounds in lanthanide chemistry.
The compound I 8 8H 2 O(LnẳLa, Nd) is synthesized by reacting La 2 (CO 3 ) 3 6H 2 O and Nd 2 O 3 with an aqueous HI solution A similar octahedral hexanuclear cluster complex, utilizing only H2O-based ligands, has been previously identified through the hydrolysis of lanthanide salts This cluster core resembles the structure of Yb6(μ6-O)(μ3-OH)8(mds)4(H2O)6, featuring six Ln 3+ ions arranged in an octahedral formation around a μ6-OH group, with each face capped by a μ3-OH group The coordination spheres of five Ln 3+ ions are completed with four aqua ligands, while the sixth ion is coordinated by three aqua ligands and one OH group.
Recent studies have identified hexanuclear complexes featuring a triazole ligand, specifically 4-amino-3,5-dimethyl-1,2,4-triazole (L8) Cheng et al described a series of these complexes, characterized by the formula [Ln6(μ6-O)(μ3-OH)8(L8)4(H2O)14]Cl82L86H2O, where Ln represents lanthanides such as Er, Ho, and Dy In these complexes, ligand L8 effectively bridges four equatorial Ln Ln edges within an octahedral core, while the remaining coordination sites are occupied by aqua ligands or a combination of aqua and chloro ligands.
Batten et al reported the synthesis of two tetradecanuclear lanthanide hydroxide complexes, specifically [Gd14(CO3)13(ccnm)9(OH)(H2O)6(phen)13(NO3)](CO3)2.5(phen)0.5 and [Dy14(CO3)13(ccnm)10(OH)(H2O)6(phen)13], utilizing a combination of 1,10-phenanthroline (phen) and carbamoylcyanonitrosomethanide (ccnm) as protective ligands While these complexes share an identical cluster core, they exhibit slight variations in their peripheral coordination ligands and anions The structural details of the core and the cationic Gd3+ complex are illustrated in Fig 48, where 14 Gd3+ ions are interconnected by one μ3-OH group and 13 CO3^2- anions.
Gd 3+ ions are completed by ccnm, phen, aqua, and chelating NO 3 ligands (Fig.48b).
Fig 44 Structure of the [(SiW 10 Sm 2 O 38 ) 4 (W 3 O 8 )(OH) 4 (H 2 O) 2 )] 26 cluster core Reprinted with the permission from [80] Copyright 2012 Elsevier
Fig 46 Structure of [La 6 (H 2 O) 23 (OH) 10 ] 8+ Reprinted with the permission from [82] Copyright
The structure of the [Ln 16 As 16 W 164 O 576 (OH) 8 (H 2 O) 42 ] 80 cluster core is illustrated in Fig 45, featuring a color code where blue represents Ln, yellow denotes As, green indicates W, red signifies O, orange is used for Cs, and purple triangular planes depict the W 3 O 13 triads within the {AsW 9 O 33 } units This image is reprinted with permission from the Royal Society of Chemistry, copyright 2011.
This chapter explores lanthanide hydroxide cluster complexes supported by various ligands, including carboxylate, diketonate, phosphate, sulfonate, and POM It emphasizes the synthetic methods for producing these cluster species and highlights the unique core motifs of the clusters The diversity of ligands allows for effective control over lanthanide hydrolysis and showcases a wide range of cluster structures, demonstrating the significant potential for further synthetic advancements and material discoveries in lanthanide coordination chemistry Key conclusions from this work and previous studies are also presented.
Fig 47 Structure of: (a) the hexanuclear [Ln 6 ( μ 6 -O)( μ 3 -OH) 8 ] 8+ cluster core and (b) [Er 6 ( μ 6 -O) ( μ 3 -OH) 8 (L 8 ) 4 (H 2 O) 14 ] 8+ Reprinted with the permission from [84] Copyright 2009 Royal Society of Chemistry
Fig 48 Structure of: (a) the cluster core [Gd 14 (CO 3 ) 13 (OH)(NO 3 )] 14+ and (b)[Gd 14 (CO 3 ) 13 (ccnm) 9 (OH)(H 2 O) 6 (phen) 13 (NO 3 )] 5+ Reprinted with the permission from [85]Copyright 2012 Royal Society of Chemistry
The high pH at which reactions occur significantly influences the complex products formed, particularly when comparing lanthanide coordination with similar ligands at lower pH levels The formation of hydroxo intermediates, generated through the deprotonation of aqua ligands, is crucial for the assembly of cluster species.
Hydrolysis can take place in both aqueous and organic environments, utilizing hydrated lanthanide complexes, salts, or oxides This process is facilitated by the addition of inorganic or organic bases and can occur under ambient pressure or hydro/solvothermal conditions.
To facilitate hydrolysis in an aqueous solution, supporting ligands—primarily organic compounds with oxygen-based functional groups—must ensure a sufficient level of water solubility for their complexes For instance, amino acids have demonstrated effective support for lanthanide hydrolysis, whereas hydrated complexes formed with simple carboxylic acids tend to be insoluble, leading to precipitation before hydrolysis can take place.
Supporting ligands can function independently or alongside other ligands Typically, an O-based ligand, including inorganic variants, is necessary when the accompanying supporting ligand lacks O-based functional groups The characteristics, including nuclearity and structure, of lanthanide hydroxide clusters are significantly influenced by the choice of ancillary ligands.
In addition to the supporting ligand, several key factors influence the reaction outcome, including the specific characteristics of the lanthanide ions, which may differ significantly despite the common belief that lanthanide chemistry is uniform due to lanthanide contraction Furthermore, the template effects of certain small anions and the involvement of transition metal ions also play a crucial role in shaping the reaction dynamics.
6 Highly sophisticated lanthanide-containing clusters can be formally constructed by using smaller and recognizable cluster units as formal secondary building units (SBUs).
Research on lanthanide hydroxide clusters is expected to expand, revealing novel materials with potential applications yet to be explored Despite recent advancements, critical questions remain regarding the chemistry's scope, the reliability of synthetic methods, and the influence of experimental conditions on structure–property relationships Future research should focus on evaluating how different supporting ligands, metal ions, and experimental settings affect reaction outcomes The overarching aim is to establish a robust, versatile approach to these unique lanthanide-containing materials, ultimately leading to discoveries applicable in catalysis, magnetism, optics, biomedicine, and other cutting-edge technologies.
Acknowledgements We thank US National Science Foundation (ZZ; Grant CHE-1152609) and National Natural Science Foundation of China (YNZ; Grant No 21401121) for financial support of this work.
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