Lanthanide induced variability in localised CoII geometries of four triangular L3Co3IILnIII complexes

Four tetranuclear heterobimetallic triangle complexes [L3Co3Dy(NO3)2(H2O)(MeOH)5](NO3) (C1), [L3Co3Gd(NO3)3(MeOH)4] (C2), [L3Co3La(NO3)2(H2O)6](NO3)(H2O) (C3), and [L3Co3TbCl(NO3)2(H2O)0.5(MeOH)3.5] (C4), where H2L = 1,4-bisformylnaphthalene-2,3-diol, have been synthesised and structurally characterised. Each complex crystallises with a complete molecule in the asymmetric unit (Z′ = 1) and displays near perfect octahedrality in two out of three CoII centres. The third CoII ion assumes a different coordination geometry in each complex: six-coordinate octahedral in C1, six-coordinate with a distortion towards trigonal prismatic in C2, five-coordinate trigonal bipyramidal in C3, and five-coordinate square pyramidal in C4; which has been attributed to increasing lanthanide cation size, coupled with a non-macrocyclic coordination environment. Continuous Shape Measurement (CShM) calculations and octahedral distortion parameter calculations were performed, using the SHAPE and OctaDist software packages, respectively, in order to aid in the assessment of each metal centre's local coordination geometry. The preliminary magnetic investigation of C3 found χmT = 9.4 cm3 K mol−1 at 300 K and M = 7.1 μB at 1.8 K, which are approximately two thirds the maximum theoretical values for three CoII ions and indicates the presence of a relatively large zero-field splitting parameter (D/kB = 65 K) operative in each CoII ion rather than exchange coupling between the CoII centres.

While some of the complexes discussed in these reports have been structurally characterised, many have not. In particular, the aldehyde containing intermediate complexes which selfassemble prior to condensation/macrocyclisation have only been successfully isolated, with full structural characterisation reported, for three older examples (Refcodes: HICLIP 31 and XUVVIV/XUVVOB 47 ) and four recently reported Ni 3 Ln examples (WARKIN, WARKEJ, WARKOT, and WARKUZ). 49 MacLachlan et al. 50 established a method to obtain heptanuclear Zn II clusters from the reaction of a pre-formed macrocyclic hexa-phenolate ligand with seven equivalents of Zn(OAc) 2 $2H 2 O, which was later shown to be capable of forming the Zn 3 LaO 6 type complex by Nabeshima et al. 51 Nabeshima further showed that the 3 + 3 condensation reaction can be carried out in the presence of two equivalents of zinc per equivalent dialdehyde, yielding a hexanuclear complex contained within the expected macrocycle. 31 While following our interest in exploring the magneto-structural correlations of non-macrocyclic dialdehyde ligated TM 3 Ln type complexes, four Co II 3 Ln complexes have been synthesised with notably different Co II coordination geometries. We now report the self-assembly and structural characterisation of four non-macrocyclic triangle complexes containing Co II 3 Ln III O 6 cores, each of which features two octahedrally coordinated Co II centres and a third Co II featuring a variation in geometry. Preliminary magnetic measurements have been carried out on the La III containing complex, C3, and are briey discussed.

Results and discussion
The four L 3 Co 3 Ln complexes reported here were prepared from a 3 : 1 : 3 mixture of methanolic solution of cobaltous nitrate, a lanthanide salt, and the relatively simple ligand, 1,4bisformylnaphthalene-2,3-diol (H 2 L). Addition of the cobalt solution caused rapid dissolution of the ligand suspension regardless of order, the same effect was not observed when the lanthanide solution was added prior to the cobalt solution. The reaction mixture required stirring overnight as the reaction proceeded through a (non-product) precipitation stage which redissolved upon further stirring. The lanthanide salts utilised were Dy(NO 3 , and TbCl 3 $6H 2 O (C4). While these complexes initially appeared to be stable in air, the crystallinity of some samples was observed to be fragile to desolvation.
Each lanthanide is bound equatorially in an O 6 environment by three catecholate (equivalent to six phenolate) groups in an approximately planar arrangement, selected distances and angles can be found in Table 1. The prearrangement of the three ligand molecules around the lanthanide centre brings the aldehyde groups into proximity akin to the imine nitrogens of a salen moiety (Fig. 1). The two adjacent aldehyde and phenolate groups form an O4 environment to bind to the equatorial sites of the cobalt centres while the remaining axial coordination sites of each metal are bound by a mixture of solvent molecules or anions. Complexes C1-C3 all crystallised in the triclinic space group P 1, while complex C4 crystallised in the monoclinic space group C2/c. Only C1 and C2 contain three 6coordinate Co II centres, with C3 and C4 each containing one heavily distorted 5-coordinate Co II centre. Of the four structurally characterised Co II 3 Ln type complexes reported by Mashima and Nozaki et al. 48 all but one Co II centre has near perfect octahedral geometry. A single Co II ion in the Co 3 Ce complex features a near square-pyramidal geometry. In order to assess the local coordination geometry of each Co II centre, Continuous Shape Measurement (CShM) calculations 52,53 were employed alongside octahedral distortion calculations (hDi, D, z, and S)  using the OctaDist soware package, 54 the values of these parameters can be found in Table 2.

Molecular structures
Crystal structure and renement details for C1-C4 can be found in Table S1 of the ESI. † The crystal structure of C1 (Fig. 2) shows the complex takes a non-planar arrangement and is the only complex reported here to contain three near perfect six-coordinate octahedral Co II centres with CShM(OC-6)/S values of 1.586/66.70 (Co2), 1.634/ 68.80 (Co3), and 0.733/38.29 (Co4).
The Dy III centre of C1 occupies a nine-coordinate environment where the three non-equatorial donors are an axially coordinated methanol and an axially coordinated m 2 -NO 3 group, binding opposite sides of the Co 3 plane. The Co2 and Co3 cobalt centres of C1 are axially capped with solvent type molecules, whereas Co4 features an axially coordinated m 1 -NO 3 group as well as a methanol cap. In order to achieve charge neutrality, the total metal charge of +9 (Dy 3+ and 3 Â Co 2+ ) is partially balanced by 3 Â L 2À and the two coordinated NO 3 À groups, with the nal negative charge originating from a non-coordinated NO 3 À anion, giving features extensive intermolecular and intra-complex hydrogen bonding (Fig. 3). The non-coordinated nitrate group forms strong hydrogen bonds from its O26 oxygen to a methanol cap of Co2 (O26/H13 ¼ 1.989(10)Å) and the water cap of Co3 (O26/H15A ¼ 1.904(10)Å). A symmetry generated complex provides two further hydrogen bonds, stemming from the H14* proton of a methanol cap of Co2* (O27/H14* ¼ 2.496(9)Å, and O28/ H14* ¼ 2.100(8)Å). There are also two strong hydrogen bonds formed between the two units complex by the methanol cap of Co4* with the water cap of Co3 (O15/H17* ¼ 2.008(7)Å), and the proton of the Co3 water cap acting as a hydrogen bond donor     to the aldehyde binding Co4* in the symmetry generated molecule (O9*/H15B ¼ 2.078(8)Å). Complex C2 crystallised in the triclinic space group P 1 with the formula [L 3 Co 3 Gd(MeOH) 4 (NO 3 ) 3 ] (Fig. 4). C2 is signicantly more planar than C1 and features two near perfectly octahedral cobalt centres (Co2 and Co3) each axially coordinated by two methanol molecules with CShM(OC-6)/S values of 1.263/59.27 (Co2) and 0.301/29.48 (Co3). The remaining cobalt centre, Co4, is also six-coordinate but is capped by a m 2 -NO 3 group leading to a heavily distorted octahedral geometry tending towards D 3h trigonal prismatic with CShM(OC-6, TPR-6)/S values of 4.914, 9.173/115.79. At the heart of C2 is a tencoordinate Gd III ion, capped either side of the Co 3 plane by a m 2 -NO 3 group, with a pseudo-hexagonal bipyramidal coordination geometry.
Similar to C1 and C2, complex C3 (Fig. 5)  The La III ion in the centre of C3 is coordinated by two m 2 -NO 3 groups and a water molecule, resulting in an 11-coordinate centre with the approximate C 5v symmetry of a capped pentagonal antiprism with a CShM(JCPAPR-11) value of 4.655. The nal negative charge required to balance the +9 charge of the metal centres, as in C1, is present as a non-coordinated nitrate. This non-coordinated nitrate species is crystallographically disordered over two spatially similar sites with freely rened occupancies of 0.47(2) (site A) and 0.53(2) (site B) (Fig. 6, inset). A water cap of Co3 forms strong hydrogen bonds to three of the four possible oxygen atoms belonging to the disordered nitrate (setting the cut-off distance to 2.50Å). The inset of Fig. 6 shows both non-coordinated nitrate sites and the hydrogen bonds they form with H15A: O31B/H15A ¼ 1.848(9)Å, O33B/H15A ¼ 2.281(9)Å, and O33A/H15A ¼ 2.154(9)Å, with an O31A/H15A distance of 2.905(8)Å.
A second unit complex is generated by inversion symmetry (Fig. 6) which forms a symmetric set of hydrogen bonds between the N1 nitrate cap of La1 and O13 water cap of Co2 in each unit complex, resulting in an intramolecular hydrogen bond (O19/ H13A ¼ 1.936Å) and an intermolecular hydrogen bond (O21*/ H13B ¼ 1.951Å). The crystal structure of C3 was also found to contain diethyl ether molecules across at least 4 sites in the asymmetric unit. These lattice solvents were heavily disordered and unstable even to isotropic renement, thus have been modelled within a solvent mask using the Olex2 implementation of BYPASS, containing 64 electrons in 192Å 3 per asymmetric unit which ts well with 1.5 diethyl ether molecules (63 electrons). These solvate species were not observed in the   microanalytical data, but are consistent with the observation of the crystalline sample rapidly crumbling upon sitting in air.
The crystal structure of C4 (Fig. 7) revealed a relatively nonplanar system featuring one ve-coordinate and two sixcoordinate cobalt centres. Although the preparation of C3 reported here also utilised a non-nitrate lanthanide salt (La(OAc) 3 $6H 2 O), C4 exclusively features a mixture of nitrate and non-nitrate (chloride) anions. Both Co2 and Co3 are octahedral, with CShM(OC-6)/S values of 0.738/43.66 (Co2) and 0.947/56.12 (Co4), with Co4 being axially coordinated by two methanol molecules. Co2 has an axially coordinated methanol on one side of the plane, however the other cap is present as a crystallographically disordered 1 : 1 methanol/water species. Co3 is unambiguously capped by a single chloride anion originating from the TbCl 3 $H 2 O salt with an overall formula of [L 3 - This vecoordinate cobalt centre sits in an approximately C 4v square pyramidal coordination geometry with a CShM(SPY-5) value of 1.734.
The crystal lattice was found to contain two solvent accessible voids equivalent to 14 electrons in 68Å 3 and 8 electrons in 24Å 3 when modelled within a solvent mask using Olex2, which corresponds to approximately 1.5 water molecules and 1 water molecule, respectively, per asymmetric unit and is consistent with the microanalytical data for C4see experimental details.
Based on the general formulation L 3 Co 3 Ln(anion) 3 + solvents, cobalt is expected to be in the +2 oxidation state (+3 from Ln III , À2 from L, and À1 from each anion, giving an overall charge of À6 to be balanced by three cobalt centres). The same result is evidenced by further examining the average equatorial Co-O bond lengths in Table 1, which are consistent with the longer bonds expected for Co II . Further, the bond valency 55,56 of each cobalt centre has been assessed in PLATON 57,58 and all sit in the range of 2.18-2.36.
The structural differences between the four complexes are quite striking and in the rst instance may be attributed to the differing sizes of the central lanthanide cation. For instance, the structure of C1, with a small Dy III ion at its centre, is dictated by the cobalt centres, each one adopting an ideal octahedral geometry. The differences between the larger lanthanide cations in complexes C2-C4 is more subtle, with no clear or obvious reason for the different cobalt geometries, other than they are now unable to form idealised octahedral geometries.
p-p stacking interactions were analysed in Olex2 (ref. 59) with aromatic centroid-centroid distances listed in Table 3 as well as the cross-sectional molecular area, calculated in the Co 3 -L 3 plane using the inbuilt ChemCra utility.
Complex C3, containing the largest trivalent lanthanide ion (La III ), has the largest calculated molecular area as well as the longest range p-p stacking interaction (as determined by Olex2). It also features the longest average Co II -Ln III and Co II -Co II distances, however it interestingly also has the shortest Ln III -Ln III distance ( Table 3). The same trend in ionic radii vs. cross-sectional molecular area is not followed for the other three complexes. Complex C4 contains Tb III which is smaller than Gd III (C2) but larger than Dy III (C1), however C4 has the second largest calculated molecular area which may be attributed to the way in which the chloride cap of Co3 in C4 inuences the planarity and shape of the complex and hence the crystal packing.
The crystal packing diagrams of C1 and C2 viewed along the crystallographic a-axis (Fig. 8) show that these complexes pack similar to a two-dimensional layered system along the ab-plane with clear inter-layer separations along the c-axis. Although C3 has the same metric symmetry and space group as C1 and C2, the geometry of the unit cell is considerably different. Where C1 and C2 have similar lengths in a and b and both a and b angles are near 90 , C3 has similar lengths in b and c with the corresponding a and g angles being near 90 . The shorter length of c in C3 relative to the other two triclinic structures results in a greater proportion of a third molecule, whose centre of mass sits outside of the unit cell, occupying the space to ensure Z ¼ 2.

Magnetic measurement
The DC magnetic susceptibility and magnetisation measurements have been performed to examine the potential Co-Co interactions and/or zero-eld splitting effects present within this complex. As complexes C1-C4 each feature a Co II centre with different coordination geometries, this initial study of C3 is not necessarily representative of the other three complexes. Fig. 9A shows the c m T product reaching 9.4 cm 3 K mol À1 at 300 K, which claried the presence of three S Co II ¼ 3/2 spins with g ¼ 2.59. The experimental magnetisation (Fig. 9B) at 7 T was 7.1 m B , which corresponds to ca. 60% of the theoretical limit (11.7 m B ). If antiferromagnetic interactions were dominant, the experimentally measured magnetisation would be approximately one third of the theoretical value and the c m T product at 1.8 K would approach one third of the value at 300 K. Hence the measured value of 7.1 m B indicates antiferromagnetic interactions are relatively small for this system. Part of the gradual decrease in the c m T(T) prole is likely to originate from the single-ion nature of the spatially separated Co II centres. To evaluate the zero-eld splitting (ZFS) of the Co II ions, simulations were performed using MAGPACK 60,61 to better comprehend the magnetic properties of C3.
An approximation based on a three-fold symmetry is applied to the magnetic analysis, to avoid overparameterization. Optimization of the ZFS D value for each cobalt(II) ion with neglecting any E value gave D/k B ¼ 65 K with g avg ¼ 2.55. Calculated c m T(T) and M(H) proles reproduced the experimental results well (the solid lines superposed in Fig. 9). This D value is relatively large, but possible in comparison with the literature values. [62][63][64][65] The large D value seems to be related to the ve-coordinate cobalt(II) ion (Co4, Fig. 5). On the other hand, the exchange coupling model using a spin-Hamiltonian based on an antiferromagnetic triangle, H ¼ À2J(S 1 S 2 + S 2 S 3 + S 3 S 1 ), cannot reproduce the experimental M-H data (Fig. S2b, ESI †).

Conclusions
Four new non-macrocyclic heterometallic triangular Co II 3 Ln III clusters have been prepared by the 1 : 3:3 reaction of Dy(NO 3 ) 3 $6H 2 O, Gd(NO 3 ) 3 $6H 2 O, La(OAc) 3 $6H 2 O, or TbCl 3 -$6H 2 O with Co(NO 3 ) 2 $6H 2 O and H 2 L in methanol. Single-crystal X-ray diffraction quality crystals were obtained by the vapour diffusion of diethyl ether into concentrated methanolic solutions of C1-C4, allowing complete and unambiguous structural characterisation. The coordination geometry of each metal centre was examined by continuous shape measurements (CShM) using the SHAPE soware package as well as OctaDist to calculate a series of octahedral distortion parameters. While C1 was found to contain three near-perfectly octahedral Co II centres, the remaining three complexes each contained one non-octahedral Co II ion. This non-octahedral site was found to be strongly distorted towards trigonal prismatic in C2, with ve coordinate centres observed for C3, trigonal bipyramidal, and C4, square pyramidal. The magnetic study of C3 claried a ground-state spin projection of S Tot ¼ 3/2 where antiferromagnetic contributions to the Co-Co interactions are weak. Throughout the literature, magnetic characterisation for the macrocyclic analogues of this type of complexes have been limited to Ni II , Cu II , and Zn II with d 8 , d 9 , and d 10 electron congurations, respectively. While the Co II based systems reported here do not crystallise with the same regularity (in terms of local coordination geometries) as previously published families of macrocyclic analogues, their potentially unique magnetic properties make for an interesting avenue of research, particularly with the inclusion of high spin d 7 metal centres.

Conflicts of interest
There are no conicts of interest to declare.