Investigating the Conformations of a Family of [M2L3]4+ Helicates Using Single Crystal X-ray Diffraction

We present five new dinuclear triple helicate compounds of types [Mn2L3](ClO4)4, [Co2L3](BF4)4, [Ni2L3](BF4)4, [Cu2L3](BF4)4, and [Zn2L3](BF4)4, where L is a previously reported semi-rigid ligand incorporating two α-diimine primary donor groups and two secondary 4-pyridyl donor groups. All complexes have been characterized in both solution and the solid state. Single crystal X-ray diffraction studies were used to probe the variation in the respective helical structures as the coordinated metal ion was altered, including the effect on the orientations of the secondary binding domains. The influence of the metal ion size, the spin state in the case of Fe(II), and the presence of Jahn-Teller distortions on the overall helical structure has been investigated. These results form a basis for the design and construction of new large metallosupramolecular architectures which manifest properties associated with the constituent helical metalloligand units.


Introduction
Metallosupramolecular assemblies have received very considerable attention over recent decades, affording a multitude of properties that can be tailored to a range of applications [1][2][3][4][5][6]. In the quest for greater control over the characteristics of these architectures, the design of increasingly complicated supramolecular topologies is of considerable ongoing interest. A variety of synthetic strategies have been developed in this regard, including the use of the subcomponent self-assembly approach [7], the molecular library approach [8,9], the symmetry interaction approach [10], and the molecular paneling approach [11], to name a few.
Recently, our group has been interested in the metalloligand approach for constructing metallosupramolecular entities [12][13][14]. This method employs a coordination complex (or complexes) with secondary binding domains as building units for achieving larger self-assembled structures with both discrete [12][13][14] and continuous [15,16] architectures. The approach is to design ligands that form a complex with a primary metal while keeping additional (secondary) coordination donor sites free. The primary metal acts to direct the coordination vectors of the secondary binding sites so that they are aligned for coordination with secondary metal centers in forming the final structure [12][13][14][15][16][17][18][19][20][21]. Using the metalloligand approach, it is thus usually possible to transfer, at least in part, the physical properties of the metalloligand units, such as luminescence and various magnetic behaviors, to the larger final architecture.
To date, the majority of the metalloligands that have been reported are mononuclear, with higher nuclearity metalloligands being comparatively rare [22][23][24][25][26]. Currently, there are a few examples of dinuclear metalloligands [20][21][22], as well as helical metalloligands [26]. However, to the best of our knowledge, no dinuclear triple helicate metalloligands have so far been incorporated in the synthesis of larger supramolecular architectures, though potential candidates have been identified [27].
have so far been incorporated in the synthesis of larger supramolecular architectures, though potential candidates have been identified [27].
We have recently shown how the magnetic properties of a tripodal mononuclear Fe(II) metalloligand, originally in the high-spin state, undergoes a spin transition when a cubic cage is formed using Ni(II) as the secondary metal ion [20]. In contrast, the substitution of Pd(II) for Ni(II) resulted in the cage maintaining its Fe(II) centers high-spin [21]. In another study, we analyzed in some detail how the presence of a spin transition in a Fe(II) dinuclear triple helicate influenced the structure of the molecule, including the respective orientations of the secondary binding domains [28].
Herein we present five new dinuclear triple helicates of type [M2L3]X4, where M = Mn(II), Co(II), Ni(II), Cu(II) or Zn(II), L is a previously reported ligand (Figure 1) [28], and X is either BF4 − or ClO4 − . For this series of helicates, we analyzed the effect that the choice of the primary metal ion has on the respective complex structures, including on the orientations of the secondary binding domains. In particular, SCXRD has been employed to probe the effect that the coordinated primary metal ion has on the respective helicate structures with respect to metal ion size, Fe(II) spin state, and the presence of Jahn-Teller distortions. This work represents a precursor study; the secondary coordinating sites of the helicate metalloligands will be later employed for the synthesis of larger metallosupramolecular architectures.

Synthesis and Characterisation
The reaction of the different M(II) salts (M = Mn, Co, Ni, Cu, and Zn) with previously reported L [28] in MeCN was followed by slow vapor diffusion of diethyl ether into the respective reaction mixtures to yield suitable crystals for SCXRD analysis in each case. The high-resolution electrospray ionization mass spectra ( The SEM images of all complexes indicated that the above crystals undergo decay and cracking upon loss of their solvents ( Figure S6). The EDS analyses confirmed the presence of the expected elements for all compounds.

Crystallography
Synchrotron SCXRD confirmed the formation of all five helicates (

Synthesis and Characterisation
The reaction of the different M(II) salts (M = Mn, Co, Ni, Cu, and Zn) with previously reported L [28] in MeCN was followed by slow vapor diffusion of diethyl ether into the respective reaction mixtures to yield suitable crystals for SCXRD analysis in each case. The high-resolution electrospray ionization mass spectra (HR ESI-MS) of all complexes yielded m/z values corresponding to [M 2 L 3 (X) 2 ] 2+ (M = Mn, Co, Ni, Cu, and Zn, with X = BF 4 - 4 , the major peak occurred at m/z 579.7500, which corresponds to the 3+ species and is in agreement with the simulated m/z value of 579.8025. The isotopic patterns for all other charge states are also in good agreement with their experimental values. The SEM images of all complexes indicated that the above crystals undergo decay and cracking upon loss of their solvents ( Figure S6). The EDS analyses confirmed the presence of the expected elements for all compounds.

Crystallography
Synchrotron SCXRD confirmed the formation of all five helicates ( Figure 2)  for low-intensity electron density, most likely arising from disordered and low occupancy solvents in the structure. Each homochiral helicate (∆∆ or ΛΛ) found in the asymmetric unit is related by inversion to its enantiomeric opposite. The coordination environment of the M(II) centers is composed of three imidazoleimine donor groups, with six nitrogens (three imine and three imidazole donors) coordinating to give a distorted octahedral arrangement.
Molecules 2023, 28, x FOR PEER REVIEW 3 of 13 group with one helicate molecule, four tetrafluoroborate or perchlorate anions, four acetonitrile and one diethyl ether and zero or one water molecules in various occupancies present in the asymmetric unit. Solvent masks were applied in all four structures accounting for low-intensity electron density, most likely arising from disordered and low occupancy solvents in the structure. Each homochiral helicate (ΔΔ or ΛΛ) found in the asymmetric unit is related by inversion to its enantiomeric opposite. The coordination environment of the M(II) centers is composed of three imidazoleimine donor groups, with six nitrogens (three imine and three imidazole donors) coordinating to give a distorted octahedral arrangement. All crystal structures display the same packing arrangement as that found previously for [Fe2L3](BF4)4 ( Figure 2) [28]. The special case of [Cu2L3](BF4)4, while displaying the same packing motif, had a tripled unit cell volume (see below). The three bis-bidentate ligands (A, B, and C) in all helicate structures are associated with different sets of intermolecular interactions. Ligand A forms a continuous 1D chain, stabilized through hydrogen bonding between the pyridyl N atom and imidazole CH group, as well as π-π interactions between adjacent helicates. The section of ligand B that is proximal to metal 1 (M1) is also shown to participate in offset π-π interactions with an equivalent terminal group of a neighboring helicate of opposite chirality. The distal component of ligand B close to metal 2 (M2) does not engage in any π-π stacking due to the neighboring rings being in an unfavorable alignment, affected by the position of counterions. The pyridyl group of ligand C at the M2 end has no intermolecular contacts with adjacent helicates; however, the pyridyl group residing at the M1 end of the helicate lies close to the central O atom of ligand A from an adjacent helicate with opposite chirality.
The asymmetric unit for [Cu2L3](BF4)4 consists of three helicate molecules and twelve tetrafluoroborate anions. Positions of nineteen acetonitriles, four diethyl ethers, and one water molecule, with various occupancies, were found in the asymmetric unit. No residual voids required the implementation of a solvent mask. The same packing motif is present as found for the other crystal structures, despite the lattice being made up of three crystallographically inequivalent helicates ( Figure 3). The three helicates in the asymmetric unit differ by their association with other helicate units in the lattice and the configuration of Jahn-Teller elongated axes in the coordination spheres. Each helicate has three non-equivalent ligands in the crystal structure, allowing the possibility for the Jahn-Teller elongated axis to occur in one of three orientations at each metal center. Since both All crystal structures display the same packing arrangement as that found previously for [Fe 2 L 3 ](BF 4 ) 4 ( Figure 2) [28]. The special case of [Cu 2 L 3 ](BF 4 ) 4 , while displaying the same packing motif, had a tripled unit cell volume (see below). The three bis-bidentate ligands (A, B, and C) in all helicate structures are associated with different sets of intermolecular interactions. Ligand A forms a continuous 1D chain, stabilized through hydrogen bonding between the pyridyl N atom and imidazole CH group, as well as π-π interactions between adjacent helicates. The section of ligand B that is proximal to metal 1 (M1) is also shown to participate in offset π-π interactions with an equivalent terminal group of a neighboring helicate of opposite chirality. The distal component of ligand B close to metal 2 (M2) does not engage in any π-π stacking due to the neighboring rings being in an unfavorable alignment, affected by the position of counterions. The pyridyl group of ligand C at the M2 end has no intermolecular contacts with adjacent helicates; however, the pyridyl group residing at the M1 end of the helicate lies close to the central O atom of ligand A from an adjacent helicate with opposite chirality.
The asymmetric unit for [Cu 2 L 3 ](BF 4 ) 4 consists of three helicate molecules and twelve tetrafluoroborate anions. Positions of nineteen acetonitriles, four diethyl ethers, and one water molecule, with various occupancies, were found in the asymmetric unit. No residual voids required the implementation of a solvent mask. The same packing motif is present as found for the other crystal structures, despite the lattice being made up of three crystallographically inequivalent helicates ( Figure 3). The three helicates in the asymmetric unit differ by their association with other helicate units in the lattice and the configuration of Jahn-Teller elongated axes in the coordination spheres. Each helicate has three non-equivalent ligands in the crystal structure, allowing the possibility for the Jahn-Teller elongated axis to occur in one of three orientations at each metal center. Since both metal centers are also inequivalent, there are nine possible configurations that might be brought about by Jahn-Teller elongations manifesting along different coordination axes of the two metal centers (Figure 3c), with two of these configurations expressed in the crystal structure (Figure 3c-f). In the discussion that follows, the distinct helicate cations will be referred to as Cu1-2, Cu3-4, and Cu5-6, with the numbers referring to the labels given to each Cu atom in the crystal structure (ligands A, B and C were, respectively, labeled as D, E, and F in Cu3-4, and G, H, and I in Cu5-6). Cu1-2 forms connections through linkages (across inversion centers) along ligand A to enantiomeric opposite Cu1-2 units, giving rise to 1D chains composed entirely of Cu1-2 units. On the other hand, Cu3-4 and Cu5-6 comprise a different category of 1D chains, wherein Cu3-4 and Cu5-6 alternate along the 1D chain defined by ligand A; specifically, the repeating pattern of Cu atoms . . . 3-4 . . . 6-5 . . . 3-4 . . . 6-5 . . . describes the chains. Two distinct chains are found for the Cu3-4 and Cu5-6 pair, one for which Cu3-4 is ∆∆ and connects to Cu5-6 units, which are ΛΛ, and another where ΛΛ Cu3-4 units connect to ∆∆ Cu5-6 helicates (Figure 3a,b). Another key difference in the three inequivalent helicates in this structure is the configuration of Jahn-Teller elongated axes at each metal center (Figure 3c-f). The Jahn-Teller elongated axis of each metal will be described here with the general notation IN id -Cu#-JN im , where I and J refer to the ligands to which the relevant donor atoms belong, and the subscripts "im" and "id" refer to the imidazole and imine N donors respectively. In Cu1-2, the configuration expressed by CN id -Cu1-BN im /AN im -Cu2-CN id is observed. The same configuration is observed in Cu3-4, although for Cu5-6, the Jahn-Teller elongation occurred according to AN id -Cu1-CN im /AN im -Cu2-CN id . In all three helicates, the even-numbered metal centers (which hold similar positions in the lattice) share the same Jahn-Teller configuration, while the elongated axis of the odd-numbered metal centers differed only at Cu5. In all coordination spheres, the Jahn-Teller elongated axis contains one of the ligand C coordination bonds, which may be related to the relative steric "freedom" of this ligand in the supramolecular arrangement, having no strong contacts with neighboring helicates.
Molecules 2023, 28, x FOR PEER REVIEW 4 of 13 metal centers are also inequivalent, there are nine possible configurations that might be brought about by Jahn-Teller elongations manifesting along different coordination axes of the two metal centers (Figure 3c), with two of these configurations expressed in the crystal structure (Figure 3c-f). In the discussion that follows, the distinct helicate cations will be referred to as Cu1-2, Cu3-4, and Cu5-6, with the numbers referring to the labels given to each Cu atom in the crystal structure (ligands A, B and C were, respectively, labeled as D, E, and F in Cu3-4, and G, H, and I in Cu5-6). Cu1-2 forms connections through linkages (across inversion centers) along ligand A to enantiomeric opposite Cu1-2 units, giving rise to 1D chains composed entirely of Cu1-2 units. On the other hand, Cu3-4 and Cu5-6 comprise a different category of 1D chains, wherein Cu3-4 and Cu5-6 alternate along the 1D chain defined by ligand A; specifically, the repeating pattern of Cu atoms …3-4…6-5…3-4…6-5… describes the chains. Two distinct chains are found for the Cu3-4 and Cu5-6 pair, one for which Cu3-4 is ΔΔ and connects to Cu5-6 units, which are ΛΛ, and another where ΛΛ Cu3-4 units connect to ΔΔ Cu5-6 helicates (Figure 3a,b). Another key difference in the three inequivalent helicates in this structure is the configuration of Jahn-Teller elongated axes at each metal center (Figure 3c-f). The Jahn-Teller elongated axis of each metal will be described here with the general notation INid-Cu#-JNim, where I and J refer to the ligands to which the relevant donor atoms belong, and the subscripts "im" and "id" refer to the imidazole and imine N donors respectively. In Cu1-2, the configuration expressed by CNid-Cu1-BNim/ANim-Cu2-CNid is observed. The same configuration is observed in Cu3-4, although for Cu5-6, the Jahn-Teller elongation occurred according to ANid-Cu1-CNim/ANim-Cu2-CNid. In all three helicates, the evennumbered metal centers (which hold similar positions in the lattice) share the same Jahn-Teller configuration, while the elongated axis of the odd-numbered metal centers differed only at Cu5. In all coordination spheres, the Jahn-Teller elongated axis contains one of the ligand C coordination bonds, which may be related to the relative steric "freedom" of this ligand in the supramolecular arrangement, having no strong contacts with neighboring helicates. The octahedral distortion parameters measuring the average length of coordination bonds and the sum of angular distortion values represented by Σ and Θ were calculated using OctaDist (Table 1) [29]. The largest metal ion in the series is Mn(II), which gave average coordinate bond lengths of 2.26 Å for both metal centers.  Figure S7)  As in the previous report discussing [Fe 2 L 3 ](BF 4 ) 4 [28], the parameters pitch and yaw have been calculated for the present systems and describe the orientations of chelate groups relative to the intermetallic axis. Pitch describes the back-and-forth rocking orientation of a chelate ring, while yaw refers to the side-to-side twisting of the chelate group. Details for the calculation of these parameters are given in Supplementary Materials (S4). The pitch values show remarkable constancy across the present series of complexes, with the measured angles falling in the range of 0.8-1.3 • (Figure 4a,b, Table S2). This is contrasted orientation of a chelate ring, while yaw refers to the side-to-side twisting of the chelate group. Details for the calculation of these parameters are given in Supplementary  Materials (S4). The pitch values show remarkable constancy across the present series of complexes, with the measured angles falling in the range of 0.8-1.3° (Figure 4a,b, Table  S2). This is contrasted by the reported pitch values for both the [HS-HS] and [HS-LS] structures of [Fe2L3](BF4)4, for which the values varied from −6.6° to 8.6° between the two structures [28]. The latter is undoubtedly related to the effect of spin equilibrium on the [Fe2L3](BF4)4 structure, and it was shown that a decrease in pitch at Fe2 from 2.6 to −2.9° was associated with the full relaxation to the [HS-LS] state. Unexpectedly, the Jahn-Teller elongations in all three [Cu2L3](BF4)4 helicates had no significant impact on the pitch. In   4 helicates led to intermetallic distances of 11.67, 11.71, and 11.54 Å for Cu1-2, Cu3-4, and Cu5-6, respectively, with the longer two distances arising for the two helicates of Jahn-Teller configuration C id -Cu1-B im /A im -Cu2-C id (Figure 3c-f  Since the structure connecting the coordinating imidazoleimine domain and the secondary pyridyl group is rigid, the orientation of each secondary binding domain is linked directly to the orientation of the conjoined chelate ring. As such, the pitch and yaw associated with each chelate ring give rise to a complementary orientation of the secondary bonding axis. Parameters describing the orientation of secondary bonding axes have been employed to investigate the effect of the size and electronic configuration of the primary metal ion, as occurred in our previous work [20,21,28]. The "secondary bond axis" (2BA) parameter describes the angular deviation of the secondary coordination axis (as defined by the axis between N and opposite C in the pyridyl group) from the intermetallic axis [28]. On the other hand, the "mutual bond axis angle" (MBA) describes the orientation of secondary coordination axes (proximal to one metal site) to each other [20,21]. These parameters provide complementary information and may be used to predict the derivative supramolecular topologies that might be targeted by these metalloligands. To exemplify the structural conformations which are highlighted by the parameters 2BA, the helicate structures of [Mn2L3](ClO4)4 and [Ni2L3](BF4)4 have been overlaid ( Figure 6). The terminal pyridyl groups, which display consistent positions, bear similar values for 2BA. On the other hand, the notable differences in 2BA at the M1 end of ligands C and B, as well as the differences in values at M2 for ligands A and B, give rise to poorly overlapped groups in the figure, with larger differences arising from larger differences in the parameter values. Since the structure connecting the coordinating imidazoleimine domain and the secondary pyridyl group is rigid, the orientation of each secondary binding domain is linked directly to the orientation of the conjoined chelate ring. As such, the pitch and yaw associated with each chelate ring give rise to a complementary orientation of the secondary bonding axis. Parameters describing the orientation of secondary bonding axes have been employed to investigate the effect of the size and electronic configuration of the primary metal ion, as occurred in our previous work [20,21,28]. The "secondary bond axis" (2BA) parameter describes the angular deviation of the secondary coordination axis (as defined by the axis between N and opposite C in the pyridyl group) from the intermetallic axis [28]. On the other hand, the "mutual bond axis angle" (MBA) describes the orientation of secondary coordination axes (proximal to one metal site) to each other [20,21]. These parameters provide complementary information and may be used to predict the derivative supramolecular topologies that might be targeted by these metalloligands. To exemplify the structural conformations which are highlighted by the parameters 2BA, the helicate structures of  (Figure 5a), except in the cases associated with differentiated spin states and Jahn-Teller distortions. At the M1 end of the helicate, the MBA values were tightly grouped, while the M2 end gave rise to a wide spread of MBA values, with the angle between the secondary coordination axes of ligands A and B being systematically larger than the other two by some margin (Figure 7d).  The contrasting trends at either end of the helicates in yaw, as well as 2BA and MBA, are likely related to the intermolecular contacts present in the crystal packing arrangement, which are consistent across the series. The intermolecular interactions present in the terminals of ligands A and B at M1 contribute some constraint on the helicate, and the strain imparted to the helicate by these contacts may be compensated by

SCXRD
All single crystal data were collected employing the MX1 beamline at the Australian Synchrotron using silicon double crystal monochromatic radiation (λ = 0.71073 Å) [30]. For each structure, two collections were taken, each consisting of a sweep through θ of 360 • but differing by the setting of κ to 0 • or 180 • . XDS software (version Jan 10, 2022) [31] was used for data integration, processing, scaling, and the merging of raw datasets. Absorption corrections were then applied at the synchrotron using SADABS (version 2.05) [32,33]. Structures were solved with ShelXT (version March 2018) [34]

Refinement Details
Full-matrix least-squares refinements were carried out using a suite of SHELX programs (version March 2018) [35,36] via the Olex2 interface [37]. Several structures exhibited orientational disorder in the helicate molecule, particularly in the linker segments of some ligands, terminal pyridine groups, and in chelate rings in rare cases. The restraints DFIX, DANG, SADI, FLAT, and RIGU and the constraint EADP were implemented where appropriate to model the disorder in various moieties, and where the electron density could not be satisfactorily modeled using one part, linker and pyridine groups were split and refined against free variables. Counter ions and solvents also exhibited disorder in some cases, which was addressed using the restraints SADI, DFIX, DANG, and RIGU, and the constraint EADP. In particular cases, solvents and anions were modeled using idealized rigid bodies [38]. Some groups were not able to be modeled anisotropically due to unreasonable anisotropic displacement parameters or inhibiting the shift convergence to zero, so they were left isotropic. Solvent masks were applied for [Mn 2 L 3 ](ClO 4 ) 4 , [Co 2 L 3 ](BF 4 ) 4 , [Ni 2 L 3 ](BF 4 ) 4 , and [Zn 2 L 3 ](BF 4 ) 4 to account for residual electron density, which could not be modeled, arising from an unknown contribution of solvent molecules in disordered arrangements. Hydrogen atoms were fixed in idealized positions using a riding model but were removed in some cases to allow the convergence of shift towards zero.

Conclusions
In this report, we present  4 helicates. Clearly, the coordination environment of the metal centers in each helicate contributes to the orientation of bis-chelating groups and hence also to the orientation of the secondary binding domains. We propose that the detailed analysis of the structural features of the present series of dinuclear helicates provides a foundation for future studies involving the design and synthesis of new large metallosupramolecular architectures that exhibit properties derived from the constituent helical metalloligand units. Ongoing studies of this type will be undertaken by our group in the future.  Table S1: Crystallographic data table for compounds presented in this study; Table S2: Selected geometric parameters describing structural orientation information for the dinuclear triple helicates; S4: Chelate ring angles.