Templating metallocycles with a macrocycle: synthesis, structures and magnetic studies of {Cr11M2} complexes

Addition of 1,4,8,11-tetrazacyclotetradecane (cyclam) to a reaction that produces octametallic rings when simpler amines are used, produces {Cr11M2} “pretzels” (M = ZnII or CuII) where the cyclam coordinates to the MII ion which then sits at the centre of a twelve-metal macrocycle. Magnetic studies were fitted using the finite-temperature Lanczos method (FTLM), and the results demonstrate that exchange interactions are transferable from previous exchange-coupled CrIII rings.

1 Electronic Supplementary Material (ESI) for Dalton Transactions. This journal is © The Royal Society of Chemistry 2022

Synthesis
All reagents were obtained from Aldrich. The syntheses of complexes were carried out in Erlenmeyer Teflon ® FEP flasks supplied by Fisher.
[Zn(cyclam)][Cr 11 ZnF 15 (O 2 C t Bu) 22 ] 1: 2ZnCO 3 .3Zn(OH) 2 (1.13 g, 2.10 mmol), deionized water (1 g, 55 mmol), and t BuCO 2 H (15 g, 147 mmol) were heated to 140 ºC with stirring for 1 h. Cyclam (0.8 g, 4.0 mmol) was added to the mixture and it was stirred for a further 30 mins. The reaction was cooled to room temperature and CrF 3 .4H 2 O (5.0 g, 28 mmol) was added to the solution and the reaction was stirred at 160 ºC for 5 h. The solution was cooled to room temperature, acetone (100 mL) was added, and the reaction stirred overnight. The product was filtered and washed with a large quantity of acetone. The product was extracted into THF (200 mL), filtered and the filtrate's solvent was removed in vacuo, affording the title product as a green solid. The product was purified by column chromatography on silica gel using toluene as an eluent to remove the impurity {Cr 8 } until a clear solution was obtained. Then, the polarity was increased by mixing toluene and ethyl acetate (10:1 v/v) to remove the second band, which is the desired product. This ratio of mixed solvent was added until the second band was collected and evaporated to dry under reduced pressure. Yield 0.20 g (2.3%, calculated from the available Cr v/v) mixture by slow evaporation of the solvents and gave X-ray quality crystals in two days.

B. Crystallography
Data for 1 were collected on an Agilent Supernova 4-circle diffractometer with a microfocus MoKα (λ=0.71073Å) source and an Eos CCD detector at a temperature of 150 K. Data for 2 were collected using a dual source Rigaku FR-X rotating anode diffractometer with VariMAX TM optics and a HyPix-6000HE detector, using CuK α (λ=1.54184Å) at a temperatures of 150 K. All data were and reduced using CrysAlisPro v41. S1 Absorption correction was performed using empirical methods (SCALE3 ABSPACK) based upon symmetry-equivalent reflections combined with measurements at different azimuthal angles. S1 The structures were solved using ShelXT and refined against all F 2 values using ShelXL implemented through Olex2 v1.5. S2,S3 Both compounds show significant disorder. In 1 the whole structure had to be split into two parts from the need to split some of the metal sites e.g. Cr8/Cr8A. For five-coordinate Zn site within the ring and the ADP for this site is large. There is additional electron density in the position for a sixth ligand. The site was therefore modelled as a mixed Zn-Cr site, which results in much more amenable thermal parameters. We included the sixth coordination site as a fluoride and we tidy the site occupancy of the fluoride to the Cr occupancy (as 5 coordinate Cr(III) is rare).
This results in Zn being present when Cr is not in PART 2, resulting in two different Zn coordination environments. The Zn was therefore split 50:50 into parts 1 and 2 to complete the connectivity table, while Cr was modelled in part 3 with the bonds to the nearest neighbours added separately. Overall, this is not entirely satisfactory, but it is the only model we could create that would make chemical sense; The Cr needs an F to charge balance. This would fit better as an O but that would mean it was either a neutral water (unlikely given the proximity of the hydrogen bonds), a terminal OH (highly unusual) or an oxide (but the bond distance is too long. The bond distance is also too long for F, but this could be attributed to poor precision in the region being modelled.).
In addition, F6 and F6A was refined to have similar distances, which has resulted in a more satisfactory refinement of their thermal parameters.
For 2 there are two molecules in the asymmetric unit. In one of the molecules F8 has large ADPs and refines well as a 100% occupied oxygen. However, if it were an oxygen it would be either water, OH or O. Water would result in unbalanced charges and unsatisfactory hydrogen bonding to the cation. Terminal OH is unlikely and the bond distance is too long for O. If it were partially occupied F, given that the site refines very well as 100% Cr, the charges would not balance. Therefore, we settled as a 100% fluoride with a large ADP.
Crystallographic data and refinement parameters are given in Table S1. CCDC 2211253-2211254 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB21EZ, UK; fax: (+44)1223-336-033; or deposit@ccdc.cam.ac.uk).

C. Magnetic measurements
Variable-temperature (2.0-300 K) direct current (dc) magnetic susceptibility measurements under an applied field of 1000 G and variable-field (0-7.0 T) magnetization measurements at low temperatures (2.0 and 4 K) were carried out for 1 and 2 constrained in eicosane with a Quantum Design MPMS-XL7 SQUID magnetometer. The susceptibility data were corrected for the diamagnetism of the constituent atoms, the eicosane and the sample holder.

D. EPR spectroscopy
Continuous wave Q-band (~34 GHz) and X-band (~9.5 GHz) EPR spectra were recorded with a Bruker EMX580 spectrometer. The continuous wave data were collected on polycrystalline powders at 5 K using liquid helium cooling. All continuous wave spectra were field corrected using a 'Strong Pitch' standard (g = 2.0028) and all powder samples were checked for any polycrystalline nature, by measuring multiple random rotations of the sample tube. Spectral simulations were performed using the EasySpin 5.2.30 simulation software S6 unless otherwise stated.
6 Figure S1: CW Q-band (ca. 34 GHz) EPR spectrum of 1 as a powder at 5 K (black) and simulation (blue). The break-down of the simulations and parameters used are as in Figure 3. Insert shows ow field section at higher receiver gain. The experiment was performed with a frequency of 34.092926 GHz.
7 Figure S2: CW X-band (ca. 9.5 GHz) EPR spectrum of 2 as a powder at 5 K (black) and simulation (red). The break-down of the simulation for each spin state (above the dashed line) is system 3 (S = ½, isolated Cu(II), blue), system 4 (S = 2, green), system 5 (S = 1, purple) and system 6 (S = 1, orange). The signal from O 2 is in the dashed blue oval. The experiment was performed with a frequency of measured at 9.388738 GHz.
8 Figure S3: CW Q-band (ca. 34 GHz) EPR spectrum of 2 as a powder at 5 K (black) and simulation (red). The break-down of the simulations and parameters used are as in Figure 4. The experiment was performed with a frequency of 34.120297 GHz.