Lanthanoid-Anilato Complexes and Lattices †

: In this review, we describe all the structurally characterized complexes containing lanthanoids (Ln, including La and group 3 metals: Y and Lu) and any anilato-type ligand (3,6-disubstituted-2,5-dihydroxy-1,4-benzoquinone dianion = C 6 O 4 X 22− ). We present all the anilato-Ln compounds including those where, besides the anilato-type ligand, there is one or more coligands or solvent molecules coordinated to the lanthanoid ions. We show the different structural types observed in these compounds: from discrete monomers, dimers and tetramers to extended 1D, 2D and 3D lattices with different topologies. We also revise the magnetic properties of these Ln-anilato compounds, including single-molecule magnet (SMM) and single-ion magnet (SIM) behaviours. Finally, we show the luminescent and electrochemical properties of some of them, their gas/solvent adsorption/absorption and exchange capacity and the attempts to prepare them as thin films.


Introduction
The field of coordination polymers (CPs) and metal organic frameworks (MOFs) with lanthanoids (Ln) is gaining interest in this decade since they may show gas and solvent exchange and adsorption/absorption and present interesting magnetic and luminescent properties and can, therefore, be used to prepare optical and magnetic sensors of gases, contaminants and different chemical species [1][2][3].
Among the many different ligands that can be used to construct these CPs and MOFs, anilatotype ligands (3,6-disubstituted-2,5-dihydroxy-1,4-benzoquinone dianion = C6O4X2 2-, Scheme 1a) are becoming very popular since these ligands present some interesting properties: (i) They show different coordination modes as: monodentate (1kO), bidentate (1k 2 [4,5]. (ii) They can act as linear bridges connecting two metal atoms (Scheme 1a) to generate many different coordination polymers [4]. (iii) They couple (antiferro)magnetically the metal centers when they are transition metal ions and the coupling can be modulated by changing X [6]. (iv) They provide a good magnetic isolation when bridging lanthanoids (as a result of the negligible overlap with the 4f orbitals), giving rise to singlemolecule and single-ion magnet behaviours (SMM and SIM). (v) They can be reduced by one or two electrons to their semiquinone and cathecolate forms (Scheme 1b), resulting in an increase in the magnetic coupling and ordering temperatures [7]. (vi) They are topologically equivalent to the wellknown oxalato ligand (C2O4 2-) and they are able to form similar monomeric complexes [8,9] as well as extended 1D, 2D and 3D lattices although with much larger cavities and channels [4,5,10,11]. Although anilato and its derivatives (Scheme 1a) have been combined with transition metals since the 1950s [12], the use of lanthanoids with anilato ligands was not developed until the 21st century. Surprisingly, there are only three reports in the 20th century. The first one, published in 1983 by Raymon et al. [13] describes a compound with Pr(III) and chloranilate (X = Cl). The second one, published in 1987 by Robl et al. [14] presents a couple of Y(III) compounds with chloranilate and bromanilate (X = Br). The third report was published in 1996 by Robson, Abrahams et al. [15] and contains a Ce(III) compound with dhbq 2− (X = H).
The first complete and systematic study was performed by Robson, Abrahams et al. in 2002 [16]. In this seminal article, the authors prepared and structurally characterized a total of 19 Ln-anilato compounds (and one with Sc) using dhbq 2− (X = H) and chloranilato (X = Cl). Since then, almost 150 Ln-anilato compounds have been prepared, as we will show in this review.
The structures and properties of homometallic coordination polymers prepared with anilato ligands and transition metals (and even p-and s-block metals) were revised in a very complete study in 2002 by Kitagawa and Kawata [4]. More recently, in 2017, Mercuri et al. [17] performed a complete revision, focusing on the magnetic and conducting properties, of homo-and heterometallic complexes and coordination polymers with anilato and transition metals. Finally, we have very recently revised the heterometallic anilato-based 2D and 3D lattices with transition metals [18].
Surprisingly, as far as we know, no revision of the almost 150 prepared lanthanoid-anilato compounds has been published to date. Therefore, here we revise all the structurally characterized Ln-anilato compounds. We will show the different anilato-type ligands used (Scheme 1a) and their magnetic and luminescent properties. We will also show the gas and solvent adsorption/absorption and the solvent exchange capacity of some of them as well as the attempts to reduce the anilato bridge in some dimers. Finally, we will show the delamination of some of the layered lattices into thin films with promising properties.
This review is organized into seven different sections: In Section 1, we introduce the anilato-type ligands and their properties as well as their capacity to coordinate in a bis-bidentate way and to act as bridges connecting lanthanoid ions in coordination complexes and polymers. In Section 2, we will show and describe all the reported structures: (i) discrete monomers, dimers and tetramers; (ii) zigzag and ladder-type chains; (iii) hexagonal, rectangular and square layers; and (iv) 3D structures. In Section 3, we will show the magnetic properties of some of these compounds, focusing on their (in most cases, field-induced) single-molecule magnet (SMM) and single-ion magnet (SIM) behaviours. In Section 4, we will show their optical properties, including luminescence in the visible and NIR regions. In Section 5, we will show the porosity, gas and solvent adsorption/absorption and solvent exchange capacity of some of the layered Ln-anilato materials. In Section 6, we will show their redox properties and, finally, in Section 7, we will show how it is possible to easily delaminate some of the layered compounds to prepare thin films with nanometric thickness.
In order to classify all the reported Ln-anilato-based compounds, we will present them in different tables grouped by type of structure and dimensionality (Tables 1 to 7), by anilato derivative ligand (Tables 8 to 12) and by Ln(III) ion (Tables 13 to 27). We hope that these tables will help in finding any compound and all those with the same Ln(III) ion, the same ligand, or the same (or related) structure.

Two-Dimensional (2D) Lattices
Two-dimensional lattices are, by far, the most common ones in the Ln-anilato family of compounds, with more than one hundred known examples (Tables 2 to 6). In order to rationalize these 2D lattices, we have classified them according to their topology and shape of the rings forming the layers. As can be seen in Tables 2 to 6, the most recurrent ones are the hexagonal 3,6-gon and square 4,4-gon topologies.
The 3,6-gon topology is, by far, the most abundant one, with more than 90 reported examples (Tables 2 to 5). In this topology, each Ln(III) is connected to three other Ln(III) ions through anilato bridges giving rise to regular (Figure 4a and Table 2) or distorted (Figure 4b and Table 3) hexagonal rings with the typical honey comb hexagonal structure. In some of these lattices the hexagons are so distorted that they look like rectangles (with two Ln-Ln-Ln angles close to 180°). These rectangular six-membered rings may adopt a brick-wall structure (Figure 4c and Table 4) or a herringbone one ( Figure 4d and Table 5).  Table 6). In these 2D lattices, each Ln(III) is connected to four Ln(III) ions by anilato bridges giving rise to square rings that form a chessboard square structure.
As observed by Robson, Abrahams et al. in their seminal work in 2002 [16], the distortions of the hexagonal rings are due to the differences in the spatial orientations of the anilato ligands around the Ln(III) ions, which, in turn, depend on the coordination geometry and on the position occupied of the two or three coordinated solvent molecules (L). As we will see below, the size of the Ln(III) ion and the size and shape of the coordinated solvent molecules also play a key role in the final structure. The different orientations result in different angles between the anilato rings and the average plane of the layer. Thus, in the regular hexagons, the anilato rings appear tilted ca. 45° with respect to the layer (Figure 5a), whereas in the distorted hexagons, there are two possible dispositions: four anilato with their rings almost parallel to the layer (face-on, FO) and two rings almost perpendicular to it (edge-on, EO) (Figure 5b) or the opposite (i.e., 2 FO + 4 EO, Figure 5c). In the rectangular layers, there are two FO and four EO ones ( Figure 5d). As can be seen in Figure 5, the diagonals of the regular hexagons are around 16.6 Å, whereas in the distorted hexagons, the diagonals are of ca. 15, 17 and 19 Å (depending on the distortions). Finally, the rectangular rings show two large distances (of ca. 18-20 Å) and a much shorter one (of ca. 7 Å). Two important aspects of these 2D lattice are: (i) the planarity (or lack of planarity) of the four-, six-, or eight-membered rings and (ii) the relative disposition of two consecutive layers along the direction perpendicular to the layers (eclipsed or alternated). The eclipsed disposition implies the formation of channels (hexagonal or rectangular) that may contain crystallization solvent molecules that might be removed and/or exchanged, as we will show below.

Two-Dimensional Regular Hexagonal Lattices
The most common 2D structural type is the hexagonal honey comb lattice (Table 2 and Figure  4a), where each Ln(III) ion is connected to three other through bis-bidentate anilato bridges, also observed with transition metals [39,40]. The main difference is that due to their higher ionic radii, the coordination numbers of the Ln(III) ions are higher (usually eight or nine) and there are two or three coordination positions occupied by other ligands (L). These additional coligands are, in most cases, H2O or a solvent with a high capacity to coordinate Ln(III) ions such as dimethyl sulfoxide (dmso), dimethylformamide (dmf), or dimethylacetamide (dma) [41,42]. The most important consequences of this increase in the coordination number are (i) the lack of planarity of the hexagonal rings (and, therefore, of the honey comb layers) and/or (ii) the presence of distortions in the hexagonal rings.
The lack of planarity was observed in the series of compounds reported by Robson [44] and recently completed by us [45] with Pr (43), Nd (44), Sm (45), Eu (46), Tb (47), Dy (48) and Tm (49) ( Table 2). In this series of isostructural compounds, the hexagons are very regular but they are not planar. They show three equal Ln···Ln distances of ca. 16.6 Å along the diagonals of the hexagons (Figure 5a) and show a chair conformation (Figure 6a) with Ln-Ln-Ln angles (°) close to those of a tetrahedron ( Table 2).
As noted by Robson et al. [16], this lack of planarity is due to the coordination of three water molecules that occupy one triangular face of the tricapped trigonal prismatic (TCTPR-9) coordination geometry of the Ln(III) ions ( Figure 6b). We call this distribution as 300 in Table 2 to indicate that  there are three solvent molecules on the upper triangular face of the trigonal prism (3--) and no solvent molecules on the central (-0-) nor lower (--0) positions (of course, the 300 disposition is equivalent to the 003 one since the trigonal prism can be upside down). Each of the three dhbq 2− ligands occupies a central capping position (-1-) and one vertex of the lower triangular face (--1) (therefore, each anilato is coordinated at the 011 positions). This disposition results in a propeller-like orientation of the anilato ligands, giving rise to corrugated hexagonal layers (Figure 6c). These layers are packed in an alternated way, preventing the formation of hexagonal channels perpendicular to the layer. Interestingly, the water molecules in this layered structure form a Ln2(H2O)18 cage with six coordinated water molecules and twelve crystallization ones (Figure 6d). These cages connect two nonconsecutive layers and cross the hexagonal hole of the layer in between them. As we will show below, the 12 crystallization water molecules can be easily removed in a reversible way and even exchanged with other different solvents such as MeOH, EtOH, dmf, HCOOH and CH3COOH.  ( a ) The geometry was determined with the program SHAPE [29][30][31][32][33][34][35][36]. TCTPR-9 = tri-capped trigonal prism, CSAPR-9 = capped square antiprism; ( b ) dmf = dimethylformamide, fma = formamide; ( c ) largest Ln-Ln-Ln angle in the hexagon. ( d ) packing: EC = eclipsed, AL = alternated.
The hexagonal rings are even more bent than in the dhbq 2− series (Figure 7a), resulting in smaller Ln-Ln-Ln angles (around 100°, see Table 2) and in more corrugated layers (Figure 7b). The lack of planarity in this series is also attributed to the spatial disposition of the chlorocyananilato ligands around the Ln(III) ions, although now, the coordination geometry is a distorted capped square antiprism (CSAPR-9, Figure 7c) with the solvent molecules occupying two positions of the capped square face and one in the other square face, (021 in our notation, Table 2  Finally, there is an additional example of regular hexagonal rings with chloranilato, Er(III) and formamide (fma) as coligand: [Er2(C6O4Cl2)3(fma)6]·4fma·2H2O (56) (Figure 8a) [28]. The coordination geometry around the Er(III) ion is also a distorted CSAPR-9 with the solvent molecules showing a 021 disposition (Figure 8b), although now they are not located on the same triangular face, as observed in the [Ln2(C6O4ClCN)3(dmf)6]·nG series (Figure 7c). The three anilato ligands show the same distribution (110, 011 and 002) although, of course, cannot occupy the same positions. Interestingly, as a result of this change in the disposition of the solvent molecules and the anilato ligands, the hexagonal rings are now almost planar (Figure 8c), the Ln-Ln-Ln angles are close to the ideal value of 120° (Table 2) and the layers are much flatter (Figure 8d) although are also packed in an alternated way.  (64) and Eu (65) [15,16], later enlarged with a EtOH solvate of the Gd(III) derivative: [Gd2(C6O4Cl2)3(H2O)6]·2EtOH (57) by Zucchi et al. [49] and recently completed by us [28,45] with Ln(III) = Er (66), La (67), Sm (68), Dy (69) and Ho (70) (Table 3). Interestingly, in this series of compounds, Robson et al. found different structural types (phases) depending on the size of the Ln(III) ion and the number of crystallization water molecules (n): Type I structure corresponds to a coordination number of ten and is only observed for the largest ion, La(III), with n = 7. This phase is not a (3,6)-phase, but a double (3,4) one, and will be described below (compound 128).
Type II structure is formed when the coordination number is nine. It is observed for most of the intermediate Ln(III) ions. There are three different type II structures: Type IIa is originally found for the large lanthanoids, i.e., Ce (61 and 62), Pr (58), Nd (59) and, surprisingly, for the smaller Tb (60) ion (Table 3). This structure has ≈ 12 crystallization water molecules and shows distorted hexagonal rings with two FO and four EO anilato rings ( Figure 9a). The Ln(III) ions present a TCTPR-9 geometry with the coordinated water molecules occupying one vertex of each of the triangular faces and one of the capping positions (disposition 111) (Figure 9b). The three anilato ligands show dispositions of the type 200, 011 and 011. Interestingly, the Ce(III) derivative shows a single-crystal to single-crystal phase transition from a monoclinic C2/m phase (61) to a triclinic P-1 one (62) upon losing a small amount of crystallization water [16]. The monoclinic phase also presents a similar distorted hexagonal lattice and a similar TCTPR-9 coordination geometry with a 111 disposition of the coordinated water molecules (Table 3). Interestingly, the Gd(III) derivative has also been obtained as both, the monoclinic C2/m phase in a EtOH solvate (57) [49] and as the triclinic P-1 phase with water as crystallization solvent (64) [16].
Type IIb structure was originally obtained for Y (63), Gd (64) and Eu (65) with n ≈ 10 ( Table 3) and is very similar to type IIa. The main difference is the presence of four FO and two EO anilato rings in the distorted hexagonal cavities (Figure 9c  Finally, Robson et al. also noted that there is a third type II phase that they called Type IIc that had only been observed in less hydrated compounds such as [Pr2(C6O4Cl2)3(EtOH)6]·2EtOH (96) [13] and [Y2(C6O4Cl2)3(H2O)6]·≈6H2O (97) [14]. This phase has later been observed in other compounds with small lanthanoids and chloranilato or bromanilato as ligands (see Table 4). It shows very distorted hexagonal cavities that can be considered as rectangles and will be described in the next section with the rectangular 3,6 lattices.
As already noted by Robson et al. [16], the number of crystallization water molecules in these series is not easy to determine. Furthermore, the exact number may depend on the storage conditions of the single crystals. In order to shed some light on this matter, we have recently completed the [Ln2(C6O4Cl2)3(H2O)6]·nH2O series and performed a study of the crystal structures of all the members of the series [45]. This study showed that if the single crystals are taken directly from the mother liquor and covered with grease to prevent any solvent loss, then the series [Ln2(C6O4Cl2)3(H2O)6]·nH2O presents up to four different water contents: (i) n = 14, for the four largest ions (La to Nd); (ii) n = 12, for the intermediate ion (Sm to Ho); (iii) n = 10, for Er and (iii) n = 8, for the two smallest ions (Yb and Tm). Compounds with n = 14 and 12 (from La to Ho) are isostructural and their structure corresponds to the structural type IIa described by Robson et al. [16]. The only known example with n = 10 (Ln = Er) shows different unit cell parameters and, therefore, is a different crystalline phase (not a solvate). This phase corresponds to the structural type IIb described by Robson et al. [16]. Finally, for n = 8 (Ln = Tm and Yb), the structure shows more important changes since now, the coordinated water molecules occupy different positions (two on one triangular face and one on the other, with a 201 disposition) and the hexagonal rings are so distorted that they look like rectangles. This phase corresponds to the ones reported by Robl for [Y2(C6O4X2)3(H2O)6]·nH2O with X/n = Cl/6.6 (97) and Br/6 (101) [14] and will be described with the rectangular phases in the next section.   Tables 3 and 4) [45]. Although the general trend is that the number of crystallization water molecules decreases as the size of the Ln(III) decreases, there are some exceptions. These exceptions can be explained by the fact that the solvent molecules occupy the interlayer space and the inner space in the distorted hexagonal cavities and the exact number can change easily (as observed in the series with chloranilato). Despite the many different water contents, we only observe three different crystal phases along the series. The two largest ions (La and Ce) show a structure very similar to type IIa observed for chloranilato, with the same distorted hexagonal rings ( Figure 10a) and with the same 111 disposition of the coordinated water molecules and 200, 011 and 011 dispositions for the anilato ligands ( Figure 10b).
The intermediate Ln(III) ions (from Pr to Er) show a second type of unit cell parameters and a structure very similar to that of La and Ce (type IIa structure with distorted hexagonal rings, Figure  10c) and the same distribution of the coordinated water molecules around the Ln(III) ion ( Figure 10d). Finally, the two smallest Ln(III) ions (Tm and Yb) present a completely different structure with less crystallization water molecules and rectangular rings, similar to those observed with chloranilato (Table 4). These rectangular lattices will be described in the next section.  Table 3) [50,51]. In this series, the Ln(III) ions are octacoordinated due to the large size of the coordinated solvent (dmso) and small size of the Ln(III) ions. This role of the size of the Ln(III) ion is clearly demonstrated by the fact that the large lanthanoids (from La to Gd) coordinate a third dmso molecule and form rectangular layers that will be described in the next section. The coordination geometry in compounds 82-86 is a triangular dodecahedron (TDD-8) and the two dmso molecules are coordinated in trans positions with O-Ln-O bond angles of ca. 150°, in order to reduce the steric hindrance of the bulky dmso molecules (Figure 11b). A similar structure with the same coordination geometry around the Ln(III) ions is also observed in three closely related compounds prepared with the asymmetric ligand chlorocyananilato (X = Cl and CN) and Yb(III) or a 1:1 mixture of Yb/Er: [Yb2(C6O4ClCN)3(dmso)4]·2H2O (87) [46], [Yb2(C6O4ClCN)3(dmso)4]·dmso (88) and [YbEr(C6O4ClCN)3(dmso)4]·dmso (89) [37] and in two related compounds with chloranilato and Er(III) with dmso and dimethylacetamide (dma) as solvents: [Er2(C6O4Cl2)3(dmso)4]·2dmso·2H2O (90) [28] and [Er2(C6O4Cl2)3(dma)4] (91) ( Table 3) [28].
There are three very recently reported distorted hexagonal lattices prepared with tert-butylanilato (X = t-Bu) and dma, formulated as: [Ln2(C6O4(t-Bu)2)3(dma)4] with Ln = La (92), Pr (93) and Nd (94) (Figure 11c and Table 3) [52]. The coordination geometry is also a TDD-8 but in contrast to compounds 82-91, in 92-94 the two dma ligands occupy cis positions with O-Ln-O angles of ca. 83° ( Figure 11d). The smaller steric hindrance of dma compared to dmso may be at the origin of this different disposition. A second difference is the distortion of the hexagonal cavities, much more pronounced in compounds 92-94 ( Figure 11c) probably due to the steric effect of the bulky tert-butyl groups in the anilato ligands. There is a final example formulated as [Gd(dhbq)Cl(thf)2] (95) that shows a quite original distorted hexagonal lattice where one of the anilato bridges is replaced by a double Cl − one [53]. This change implies that two of the six sides of the hexagons are much shorter than the other four (4.49 vs. 8.63 Å and 8.64 Å, Figure 12a). In this compound, each Gd(III) ion is coordinated to two bis-bidentate bridging dhbq 2− ligands, two bridging Clligands and two thf molecules ( Figure 12b) in a very distorted square antiprism geometry (SAPR-8, Table 3). The two Cl − ligands occupy cis positions on the same square face of the square antiprism (20), whereas the thf molecules are located one on each square face (11) and the two dhbq 2− ligands are located with 11 and 02 dispositions ( Figure 12b). This spatial orientation gives rise to very distorted but almost planar hexagonal rings with the coordinated thf molecules pointing almost perpendicular to the ring plane ( Figure 12c).

2D Rectangular Lattices
When the distortion of the hexagons becomes more important, the largest Ln-Ln-Ln angles increase and reach values in the range ca. 160-170°. The hexagonal cavities look like rectangles, although keeping the same 3,6 topology (Figure 4c,d). As can be seen in Table 4 and Table 5, these large distortions are common and, in fact, were already observed in [Pr2(C6O4Cl2)3(EtOH)6]·2EtOH (96), the first Ln-anilato compound reported in 1983 by Raymond et al. [13].

Other 2D (Square) Lattices
As mentioned above, besides the 3,6-gon topology, although less common, there are also some 2D lattices with (3,4)-, (4,4)-and even mixed (3,4)+(3,8)-topologies ( Table 6). The only reported example with the (3,4) topology is also the only known example with a coordination number of ten: [La2(C6O4Cl2)3(H2O)6]·≈7H2O (128) [16]. Although this compound has the general formula [Ln2(anilato)3(L)n]·mG, in fact, the structure is very original since the La(III) ions are coordinated to three bidentate chloranilato ligands, two terminal water molecules and two bridging water molecules connecting two La(III) ions (Figure 16a). The coordination geometry of the La(III) ions is a sphenocorona (JSPC-10) [36] and the structure is formed by large squares containing four La(III) centers connected by bis-bidentate anilato ligands (as in other 4,4-lattices) and small rectangles with two anilato bridges and two double aquo-bridges (Figure 16d). Although there are already twenty reported Ln-anilato compounds with the 4,4 topology (Table 6), the first compound with this topology, (H3O)[Dy(C6O4(CN)Cl)2(H2O)]·4H2O (129), was not reported until 2018 [46]. In this compound, the Dy(III) ions show a capped square antiprism (CSAPR-9) geometry with a water molecule in the capped position and four chlorocyananilato ligands occupying the closest positions of both square faces (Figure 16b). Accordingly, the four anilato ligands extend along four orthogonal directions, giving rise to a chessboard-like square lattice ( Figure  4e) although it is not planar given the pushing effect of the water molecule (Figure 16e). The same structure has also been recently reported with the same ligand, but with NEt2H2 + instead of H3O + as cation, in (NEt2H2)[Dy(C6O4(CN)Cl)2]·2dmf·2.5H2O (130) [38]. In contrast with all the 3,6 lattices of the type [Ln2(anilato)3(L)n], which are neutral, the 4,4 lattices are anionic since they are formed by two dianionic anilato ligands and one Ln(III) ion: [Ln(anilato)2] − . The presence of cations in the structure represents an additional factor that may play a role in determining the final structure as we will see below. Although in compounds 129 and 130, the cations are H3O + and NEt2H2 + , respectively, Robson et al. [57] have recently reported a large series with NEt4 + cations and chloranilato formulated as:  (Table 6). In this series, the coordination geometry is square antiprism (SAPR-8) with the Ln(III) ion coordinated by four bidentate anilato ligands that also occupy the closest positions of both square faces and extend along four orthogonal directions (Figure 16c) giving rise to a planar square lattice (Figure 16f) since now there is no additional water molecule. The role of the lanthanoid size is again evidenced in this series. Thus, for large lanthanoids such as La, Ce and Nd, there is room for an extra water molecule coordinated to the Ln(III) ions resulting in the series (NEt4)[Ln(C6O4Cl2)2(H2O)] with Ln = Ce (146), La (147) and Nd (148) [58]. In this series, the coordination geometry is capped square antiprism (CSAPR-9), as observed in compounds 129 and 130, with the water molecule located on the capped position (Figure 16b). An additional interesting aspect of this series is the single-crystal to single-crystal transformation observed in the Nd(III) derivative upon removal of the coordinated water molecule [58].   [37] and Dy/7 (150) [38]. In these (3,4)+ (3,8) lattices, each Ln is connected to three other Ln through anilato bridges that extend in three orthogonal directions with a T-shape, generating eight-membered rings with four face-on (FO) and four edge-on (EO) anilato ligands together with four-membered rings with four EO anilato ligands ( Figure 17a) [37,38]. The coordination geometry around the Ln(III) ions is also a CSAPR-9 with the solvent molecules occupying three vertices of the capped square face (030) and the anilato ligands with 110, 002 and 002 dispositions (Table 6 and Figure 17b).

Three Dimensional (3D) Lattices
As can be seen in Table 7, there are only seven reported 3D Ln-anilato compounds, all with chloranilato and six of them reported in 2019. Except compounds 156 and 157, they all show the wellknown adamantane lattice (Figure 18a), although with large distortions. Most of the reported 3D lattices have been reported in a recent interesting study by Hua and Bondaruk [54] regarding the role of the geometry and size of the counter-cations on these anionic 3D lattices. Interestingly, the 3D-diamond lattice has the same basic formula as the 2D 4,4-lattices: [Ln(anilato)2] − and, therefore, it also needs a charge-balancing cation. The study of Hua and Bondaruk shows that the cation plays a key role in determining the dimensionality and the final topology since the cations establish, besides the electrostatic interactions with the anionic lattice, many different interactions such as H-bonds, π-π, Cl-π, halogen-H and CH-π interactions [54].  [16], although a preliminary report had been published in 2000 [59]. This compound shows the typical adamantane structure although with distortions in the tetrahedral environment, that appears elongated along its C2 axis. The coordination geometry around the Y atom is distorted TDD-8 (Figure 18b). In this compound, the presence of H3O + as counterion is proposed based on the lack of other cations and on charge-balance arguments [16].
A remarkable fact is that compounds (   156) shows a very original 3D structure with three unique Ce(III) ions nonacoordinated by four bidentate chloranilato ligands and a water molecule [54]. Ce1 and Ce2 show both a CSAPR-9 geometry although the water molecule is located on the capped square face in Ce1 (010, Figure 20a) or in the basal square face in Ce2 (001, Figure 20b). In contrast, Ce3 presents a geometry in between TCTPR-9 and MFF (Muffin) [35] with the water molecule coordinated on one of the triangular faces (100, Figure 20c). The structure can be described as a 3D-noq anionic lattice formed by three-, four-and five-membered rings with PPh3Me + cations inserted in the larger cavities (Figure 20d). Finally, compound (PPh3Me)[Er(C6O4Cl2)2]·1/2H2O (157) shows a unique 3D 4,4′-c structure with four-and five-membered rings (Figure 21a) and net point symbol {4.5 2 .7 3 }{4.5 3 .7 2 } with PPh3Me + cations located in the five-membered rings [54]. In this compound, there are two unique Er(III) ions with different coordination geometries. Er1 ion is surrounded by four chloranilato ligands and a water molecule in a CSAPR-9 geometry with the water molecule located on the capped square face (010, Figure 21b), whereas Er2 is surrounded by four chloranilato ligands in a TDD-8 geometry without any additional coligand (Figure 21c).

Anilato-Type Ligands
A second way to classify the above Ln-anilato compounds is based on the anilato-type ligand. With this criterion, we can see that there are nineteen compounds with lanthanoids and the ligand dhbq 2− (Table 8). Among these nineteen compounds, three are dimers, one shows a distorted hexagonal 2D lattice and the remaining fifteen compounds present a regular hexagonal 2D lattice.  [53] ( a ) thf = tetrahydrofurane, Tp -= HB(pz)3 -= hydrotris(pyrazolyl)borate.

Lanthanoid Metal Ions
If we classify all the Ln-anilato compounds based on the lanthanoid metal ion, we can see that there are eighteen compounds with Y(III) (

Single-Molecule and Single-Ion Magnets
Most of the magnetically characterized Ln-anilato compounds show the expected magnetic properties of isolated Ln(III) ions with the corresponding decrease in the χmT product when the temperature is decreased as a result of the depopulation of the excited levels that appear due to the ligand field. This behaviour confirms the absence of noticeable magnetic interactions through the anilato bridges when connecting Ln(III) ions. In fact, only when connecting transition metals, anilato bridges show weak antiferromagnetic interactions (that can be modulated by the X group) [6,61]. Although this lack of magnetic interactions may appear as a disadvantage from the magnetic point of view, the good magnetic isolation provided by the anilato ligands precludes the fast relaxation of the magnetization and allows the synthesis of Ln-anilato complexes and coordination polymers behaving as single-molecule magnets (SMMs) and single-ion magnets (SIMs) with slow relaxation of the magnetization. SMMs and SIMs are attracting many interest recently since they present memory effects and quantum phenomena that may find applications in data storage, quantum computing and spintronics [62][63][64]. SMMs and SIMs retain their spin orientation and show a slow relaxation of the magnetization at low temperatures that may follow different mechanisms such as: (i) Orbach (O), (ii) Raman (R), (iii) direct (D) and (iv) quantum tunnelling (QT). The O, R and D mechanisms are temperature dependent and the D and QT mechanisms are field dependent [62][63][64]. Usually, when several mechanisms are operative, the relaxation time is determined by the faster process and, thus, QT is operative at low temperatures (when no DC field is applied), whereas Orbach mechanism is the dominant one at high temperatures. In some cases, the application of a DC field (usually below 3000 Oe) suppress the fast relaxation through the QT mechanism. In these compounds, called fieldinduced-SMM of field induced-SIM (FI-SMM or FI-SIM), the application of a DC field is required to observe the slow relaxation of the magnetization and the direct mechanism may contribute to the relaxation of the magnetization. The equation used to reproduce the relaxation rate (the inverse of the relaxation times, τ) as a function of the temperature and/or DC field includes the O, R, D and QT mechanisms as follow [62][63][64]: Surprisingly, the possibility to prepare Ln-anilato-based SMMs and SIMs has not been exploited until very recently (Table 28) [20]. Although only the structure of compound 4 was reported, compound 4′ is isostructural to the Y analogue [(Tp)2Y2(C6O4(CH3)2)]·1.2CH2Cl2 (11) [20]. Compound 4′ shows an almost temperature independent slow relaxation of the magnetization that follows a quantum tunnelling mechanism when no DC field is applied at low temperatures and when a DC field of 1600 Oe is applied, both compounds (4 and 4′) show slow relaxation of the magnetizations with an Orbach relaxation mechanism for 4′ with Ueff = 47 K and Orbach and Raman mechanisms for 4 with Ueff = 24 K [20].
Compound [(Tp)2Dy2(C6O4Cl2)]·2CH2Cl2 (4) was almost simultaneously reported twice more (compounds 5 and 6 in Table 28) by Slageren et al. [22] and by Ishikawa et al. [19]. Both reports confirm the FI-SMM behaviour of compound 4 and complete the magnetic studies with different applied DC fields and temperature ranges. Furthermore, Slageren et al. [22] showed that when the chloranilato bridge is chemically reduced using cobaltocene, the Dy and Tb dimers (4 and 24, respectively) behave as SMM even with no applied DC field. Slageren also showed that when a DC field of 1000 Oe is applied, the reduced Dy derivative shows two different relaxation processes [22]. The energy barriers obtained by Slageren et al., even when the choranilato bridge was reduced, are similar to those obtained by Boskovic et al. [20] (Table 28). The third study, made by Ishikawa et al. [19], was performed with a DC field of 950 Oe and also showed a similar energy barrier for the high temperature data although now the relaxation times was fit to a model including only Raman and Direct mechanisms [19]. In this study, Ishikawa et al. also reported the monomer [Co(Cp)2][Dy(Tp)2(C6O4Cl2)] (1), that also behaves as a FI-SMM with an energy barrier of 49.4 K when a DC field of 1500 Oe is applied. Under this DC field, the relaxation of the magnetization of compound 1 follows Raman and Direct mechanisms [19].
Boskovic et al. [24] reported in 2019 a similar dimer to compound 4 but prepared with bromanilato instead of chloranilato: [(Tp)2Dy2(C6O4Br2)] (18). Compound 18 is also a FI-SMM and presents a low energy barrier of 7 K when a DC field of 390 Oe is applied. As observed in dimer 4, when the bromanilato bridge in dimer 18 is reduced, the compound behaves as a SMM with zero applied DC field. The reduced dimer 18 also shows low energy barriers of 10.4 and 10.6 K for DC fields of 0 and 380 Oe, respectively. In all cases, the relaxation of the magnetization follows Orbach, Raman and direct mechanisms [24].
In addition, in 2019, Ishikawa et al. [25] reported the Er and Yb derivatives with chloranilato (compounds 22 and 29, respectively, Table 28). Both compounds behave as FI-SMM and show energy barriers of 25.9 and 22.3 K, respectively, when a DC field of 1000 Oe is applied. In both cases, the relaxation times can be very well reproduced using Orbach and Raman mechanisms [25].
As can be seen in Table 28, all the above-mentioned SMM and FI-SMM complexes contain the coligand (Tp − = hydrotris(pyrazolyl)borate = HB(pz)3). The only reported dimer with SMM behaviour without the Tpcoligand is compound [Eu1.96Dy0.04(C6O4(CN)Cl)3(H2O)10]·6H2O (31) [27]. This dimer shows a zigzag structure (Figure 2d) and contains a bridging chlorocyananilato ligand. Compound 31 is also the only doped dimer with SMM behaviour (contains Eu doped with 2% of Dy) and is also the only Ln-anilato dimer showing SMM without applying a DC field or reducing the bridging anilato ligand. This compound presents an energy barrier of 25.3 K and the relaxation of the magnetization could be fit to an Orbach and Direct mechanisms, although in this case, the frequency maxima appear at high frequencies and could not be observed [27].
Besides these discrete complexes (nine dimers and one monomer, Table 28), there are also six Ln-anilato 2D lattices showing FI-SMM or even SMM (in one case). Two of these six 2D lattices are isostructural 4,4-nets showing FI-SMM behaviour: (NEt4)[Dy(C6O4Cl2)2] (135) and (NEt4)[Gd(C6O4Cl2)2] (136) [57]. In both cases, the measurements were performed with a DC field of 1000 Oe, although no analysis of the relaxation mechanisms nor the energy barrier was reported [57]. The observation of slow relaxation in compound 136 is very surprising since Gd(III) is an isotropic ion and is very unusual to observe SMM or FI-SMM behaviour in Gd(III) complexes [65]. The exact mechanism to explain this unusual behaviour is not clear yet.
The four remaining 2D lattices showing slow relaxation of the magnetization are 3,6-networks ( Table 28). Three of them contain Dy(III) and bromanilato: [Dy2(C6O4Br2)3(H2O)6]·8H2O (80) [45,55], [Dy2(C6O4Br2)3(dmso)4]·2dmso·2H2O (83) [51,55] and [Dy2(C6O4Br2)3(dmf)6] (125) [55], whereas the fourth one contains Eu doped with Dy: [Dy0.04Eu1.96(C6O4Br2)3(dmso)6]·2dmso (123) [27]. Compounds 80, 83 and 125 are three closely related compounds that, in fact, show reversible interconversion upon solvent exchange (see below) [55].  The relaxation of the magnetization follows an Orbach mechanism in compound 80, whereas in compound 83 it follows Orbach, direct and quantum tunnelling mechanisms. Since both compounds show the same structure, Ln(III) ion and ligand, we can attribute these differences to the changes in the coordination geometry of the Dy(III) ions (TCTPR-9 in 80 vs. TDD-8 in 83). These changes in the coordination number and geometry are due to the different sizes of the coligands (H2O in 80 vs. dmso in 83). Despite showing a different structure, compound 125 is closely related to compounds 80 and 83 (the only difference is that it contains dmf as coligand, instead of H2O or dmso). Compound 125 also shows slow relaxation of the magnetization when a DC field of 1000 Oe is applied (Figure 22a), although now there are two different relaxation processes: (i) a slow relaxation process (SR) with an energy barrier of 36 K and a relaxation following Orbach and direct mechanisms and (ii) a fast relaxation (FR) process with an energy barrier of 11.4 K that relaxes following an Orbach mechanism (Figure 22b) [55]. Finally, compound 123 is the only 2D lattice showing slow relaxation of the magnetization with zero applied DC field. This compound shows an energy barrier of 40.9 K [27].

Optical Properties
Besides magnetic properties, all these Ln-anilato compounds may also show interesting optical properties, such as luminescence, very common in many Ln-containing compounds [66]. Furthermore, some anilato ligands such as chlorocyananilato and nitranilato, may also show luminescence in solid state [23,67] and in solution [68,69], even when they are not coordinated. Table  29 shows all the structurally characterized Ln-anilato compounds showing luminescence with the corresponding ligand-and lanthanoid-based emission bands.
The first luminescence study on a Ln-anilato compound (and the only one with chloranilato) dates back to 2004 when Kaizaki et al. [26] performed a study of the Yb(III) dimer: [(YbTp)2(C6O4Cl2)]·2CH2Cl2 (28). This compound shows a strong 4f-4f emission at ca. 1000 nm attributed to ligand-to-metal energy transfer from the triplet state of the chloranilato ligand to the excited 4f state of the Yb(III) ion [26]. The strong absorption band at 560 nm observed in this compound was attributed to the chloranilato ligand. In contrast, the analogue dimers with Tb(III) or Eu(III) did not show any emission, suggesting the presence of a back-transfer from the Ln(III) ions to the chloranilato ligand [26].
A very interesting report also by Mercuri et al. [48] showed NIR emission in a family of three 2D lattices formulated as  [48]. The measurements performed on suspensions of nanosheets of these compounds show that the emission of the Ln(III) ions are similar to those of the crystals but the ligand emission appears blue-shifted at ca. 460 nm, close to the value observed for the free ligand in dilute solution. When these nanosheets are drop-casted, the Ln(III) emission remains unchanged but the ligand emission appears at ca. 680-720 nm, close to the value observed in the crystals [48].

Gas/Solvent Adsorption/Absorption and Solvent Exchange
Very recently, gas and solvent adsorption and even solvent exchange have been reported in some Ln-anilato compounds (Table 30). The first observation of gas adsorption in a Ln-anilato compound was reported in the series [Ln2(dhbq)3(H2O)6]·18H2O with Ln(III) = Ho (35), La (36), Gd (37), Yb (38), Lu (39), Y (40), Er (41), Ce (42), Pr (43), Nd (44), Sm (45), Eu (46), Tb (47), Dy (48) and Tm (49) [45]. As described above (Figure 6d), this series contains a cluster of 18 water molecules, plus six crystallization extra water molecules, that can be easily removed when the samples are heated to 80 °C or simply under vacuum at room temperature (the total number of removed water molecules oscillates between 18 and 22) [45]. The removal of the water molecules leads to a colour change of the samples and to an almost complete collapse of the structure, as shown by the X-ray powder diffraction of the evacuated samples (Figure 24a). Studies of the reversibility of this process showed that it is fully reversible and when the dehydrated samples are immersed in water, they recover the original structure (Figure 24a). Moreover, when the dehydrated samples are immersed in different solvents, these solvents enter in the interlayer space and, in some cases, the samples recover the crystallinity, although the new structure is not the same as the hydrated pristine sample (Figure 24b).  The evacuation of the water molecules in compounds 35-38 and 41-49 leaves empty hexagonal cavities where, besides other solvents, it is also possible to insert gases (Figure 25a). Thus, the dehydrated compounds show CO2 uptake with a maximum of ca. one CO2 molecule per hexagonal cavity at 0 °C and 100 kPa (Figure 25b) [45].
Similar results have also been observed in the corresponding series with chloranilato, formulated as [Ln2(C6O4Cl2)3(H2O)6]·nH2O, with Ln(III) = Pr (58) [45]. The main difference is that in these two series, the structure does not collapse upon dehydration (only some reflections are lost), probably because now, in contrast with the dhbq 2− series, the water molecules do not play any structural role since they are located in the distorted hexagonal or rectangular cavities and in the interlayer space. There are two other very recently published studies showing gas/solvent uptake in Ln-anilato compounds [52,57]. The first one is a very interesting study performed on compound (NEt4)[Y(C6O4Cl2)2] (132), a square 2D-4,4 anionic lattice (Figure 16f) that shows reversible CS2, I2 and Br2 uptake, while keeping the crystallinity [57]. This uptake capacity has allowed the synthesis of compounds (NEt4)[Y(C6O4Cl2)2]·nG with nG = 1.43 CS2 (131), 1.87 I2 (133) and 0.91 Br2 (134) [57]. Additionally, compound 132 can also uptake N2, H2, CO2 and CH4 in its square channels, with high binding enthalpies, among the highest reported for porous coordination polymers, as clearly evidenced by the corresponding adsorption isotherms and by a neutron diffraction study that allowed to determine the location of the adsorbed molecules and their interactions with the lattice [57].
Finally, we have very recently reported three examples of direct solvent exchange without the need to evacuate the pristine compounds [55]. This study shows that it is possible to exchange the solvent molecules, even the coordinated ones, by immersing compounds [Dy2(C6O4Br2)3(H2O)6]·8H2O (80), [Dy2(C6O4Br2)3(dmf)6] (83), or [Dy2(C6O4Br2)3(dmso)4]·2dmso·2H2O (125) in any of the two other solvents ( Figure 26). Interestingly, the three compounds present slow relaxation of the magnetization with different relaxation mechanisms and relaxation times ( Table 28) that can be easily modified by simple immersion in the desired solvent [55].

Redox Studies
An additional interesting aspect of the anilato ligands is the possibility to be chemically or electrochemically reduced by one or two electrons to form the semiquinoid and cathecolato forms, respectively (Scheme 1b). This possibility has been studied in several of the reported dimers in order to try to increase the magnetic exchange through the anilato bridge and to improve the SMM properties of these dimers. Interestingly, all the reported compounds with reduced anilato bridges are dimers and have been reported in the last three years (Table 31).
A third study, published a few months later on the closely related compounds [(LnTp)2(C6O4X2)]·G with Ln/X/G = Dy/Cl/2CH2Cl2 (5), Y/Cl/2CH2Cl2 (8), Tb/Cl/2CH2Cl2 (24) and Gd/Cl/2CH2Cl2 (26) [22] showed very similar results to those previously reported on the same series of compounds although now, the authors were able to prepare the reduced products with cobaltocene as cation and could measure an antiferromagnetic coupling of -4.17 cm -1 in the Gd(III) derivative, confirming that the magnetic coupling is much higher when the anilato bridge is in its radical semiquinone form [22].
If we look at the Ln(III) ions combined with anilato ligands, we can see that Y(III), Dy(III) and Er(III) are the most used lanthanoids, with eighteen compounds of each. There are twelve compounds with Ce(III) and Yb(III), ten compounds with Nd(III), Eu(III) and Gd(III); eight compounds with Pr(III), Tb(III) and Ho(III); seven with La(III); and three with Tm(III) and Lu(III). That Dy(III), Er(III) and Y(III) are the most used ones to date is not casual. Y(III) was largely used in the first studies since it crystallizes very easily, whereas Dy(III) and Er(III) are being used more recently in the search of SMMs and SIMs.
The good magnetic isolation provided by the anilato ligands constitutes an advantage to obtain slow relaxation of the magnetization of the Ln(III) ions. Thus, a total of fourteen single-molecule and single-ion magnets have been prepared to date with Ln(III) and anilato ligands. These fourteen examples include one monomer, seven dimers and six 2D lattices (two hexagonal 3,6-gon, two herringbone 3,6-gon and two square 4,4-gon lattices). In most cases, these compounds behave as fieldinduced-SMM or SIM since the application of a DC field (usually around 1000 Oe) is needed to suppress the fast relaxation of the magnetization through a quantum tunnelling mechanism. In a few dimers, the anilato bridge has been reduced, increasing the magnetic coupling and giving rise to SMM with no DC field applied. Doping an Eu layer with 2% of Dy(III) also results in a SMM behaviour with no applied DC field. As expected, most of these SMMs and SIMs have been prepared with Dy(III) ions, although there is one example with Yb(III), one with Er(III) and even one with the isotropic Gd(III) ion.
Given the well-known luminescence on many Ln(III) compounds and the, less known, luminescence shown by some anilato ligands, in the last three years a total of seventeen Ln-anilato compounds have been reported to present luminescence either from the ligand, from the Ln(III) ion, or from both. In most cases, the emission from the ligand, centred at around 590-720 nm, coexist with the NIR emission of the Ln(III) ions (in the Yb, Er, Ho and Nd compounds). Interestingly, the luminescence is essentially maintained (although with slight shifts in the emission bands) when the compounds are delaminated and prepared as drop-casted thin films.
Also, recently, some of these layered coordination polymers have shown a remarkable capacity to release the solvent molecules (usually water) located between the layers by gentle heating or under vacuum at room temperature. This dehydration results in a collapse of the crystal structure that can be easily fully recovered by immersing the dehydrated compounds in water. Furthermore, besides this reversible water release/uptake, some dehydrated compounds also show the capacity to adsorb other solvents, keeping the original structure or giving rise to novel crystal structures. Interestingly, the dehydrated compounds can also adsorb gas molecules such as H2, N2, CH4 and CO2.
A very recent study has also shown a remarkable solvent exchange capacity in three layered compounds without the need to previously evacuate the original solvent molecules. This result opens the way to the synthesis of different solvates starting from one on them.
The capacity of the anilato ligands to be reduced to the corresponding semiquinone and cathecolato forms has also been checked in some Ln-anilato dimers, electrochemically and chemically, resulting in an increase of the magnetic coupling between the Ln(III) ions and in the observation of slow relaxation of the magnetization even with zero applied DC field.
Last, but not least, these layered materials can be easily delaminated and drop-casted in thin films with a few monolayers thickness and micrometric lateral dimensions. Interestingly, the optical properties are retained in these thin films.
The future directions of these Ln-anilato based materials are many and very promising. Thus, the great variety of structures will soon increase with the synthesis of novel compounds with other coordinating solvents and coligands, specially, with bridging ones. Furthermore, some very recent results have shown the possibility to modulate, at will, the dimensionality of these, in some cases, porous materials. This capacity may lead to the synthesis of 2D and 3D structures with predetermined porous sizes and dimensionalities. The use of different X groups in the anilato rings will allow a modulation not only of the size but also the nucleophilicity and hydrophilicity of the cavities, channels and interlaminar spaces created in these structures. The use of different templating cations in the anionic Ln-anilato lattices, constitute another very interesting tool to control the dimensionality and properties of these materials.
Besides the structural aspects, the magnetic properties of these materials constitute a second interesting aspect worth to investigate. The use of different very asymmetric coligands may lead to an increase in the anisotropy of the Ln(III) coordination geometry that may result in an increase in the energy barriers and blocking temperatures of these SMMs and SIMs.
The luminescence displayed by many Ln(III) ions and their sensitivity to changes in their coordination environments is another important aspect to exploit and develop. Furthermore, given the gas absorption capacity and solvent or ligand exchange ability shown by some of these materials, they are promising candidates to prepare devices with sensing capacity towards different gas, solvent and ionic species. The capacity to be delaminated into large thin films with a few nanometers thickness represents an additional advantage for this purpose.
Finally, the redox activity of the anilato ligand adds additional potentialities to these materials since the reduced materials might be good electronic conductors or semiconductors (specially, if combined with Ce(III), Eu(III) or Yb(III) that have two accessible oxidation states) [70] and may also show promising magnetic properties as already shown by some Ln(III)-anilato-Ln(III) dimers with reduced anilato bridges. Finally, they may also find applications as redox/luminescent sensors since their electronic and optical properties change dramatically upon reduction of the anilato ligands.