Putting a New Spin on Supramolecular Metallacycles: Co3 Triangle

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Putting a New Spin on Supramolecular Metallacycles: Co triangle and Co square bearing tetrazine-based radicals as bridges 4

Dimitris I. Alexandropoulos, Brian S. Dolinar, Kuduva R. Vignesh, and Kim R. Dunbar J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b06925 • Publication Date (Web): 21 Jul 2017 Downloaded from http://pubs.acs.org on July 21, 2017

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Journal of the American Chemical Society

Putting a New Spin on Supramolecular Metallacycles: Co3 triangle and Co4 square bearing tetrazine-based radicals as bridges Dimitris I. Alexandropoulos, Brian S. Dolinar, Kuduva R. Vignesh and Kim R. Dunbar* Department of Chemistry, Texas A&M University, College Station, TX 77842-3012, United States. Supporting Information Placeholder ABSTRACT: The synthesis of two new radical bridged compounds [Co3(bptz)3(dbm)3]·2toluene (1) and [Co4(bptz)4(dbm)4]·4MeCN (2) (bptz = 3,6-bis(pyridyl)-1,2,4,5tetrazine; dbm = 1,3-diphenyl-1,3-propanedione) is reported. The presence of the ligand-centered radical has been confirmed by Xray crystallography and SQUID magnetometry. These complexes are the first metallacycles bearing nitrogen heterocyclic radicals as bridges. Magnetic studies reveal strong antiferromagnetic metal···radical coupling with coupling constants of J = -67.5 cm-1 and J = -66.8 cm-1 for 1 and 2, respectively. DFT calculations further support the strong antiferromagnetic coupling between CoII ions and bptz radicals and confirm S = 3, and S = 4, spin ground state for 1, and 2. Coordination-driven self-assembly,1 which is based on the structural versatility of metal ions and the directionality of metalligand interactions to construct delicate supramolecular2 architectures, has received tremendous attention over the past few decades due to the potential applications of molecular nanoscale materials in both applied and fundamental areas of research.3 In addition to their interesting physicochemical properties, supramolecules exhibit fascinating complex structures ranging from molecular rhomboids to hexagons and from trigonal pyramids and prisms to a variety of polynuclear cages.1 Among the myriad examples of metallacyclic motifs, molecular squares are the most stable and abundant because the requirement for 90˚ angles between the metal corners and the ligand edges of a molecular square are easily satisfied when square planar or octahedral metal building blocks are combined with linear organic linkers, as first described by Fujita1f and Stang1a-d. Although molecular squares are the most prevalent architectures, molecular triangles are possible when less rigid ligands are employed. In this case, the flexibility of the organic linker reduces the ring strain of the trinuclear assemblies by allowing the ligands to bend. In this vein, a number of molecular triangles have been reported4. The choice of the organic linker is of paramount importance in this chemistry because it dictates the structure and topology as well as the properties and applications of the resulting compounds. The 3,6-disubstituted tetrazine ligands such as 3,6bis(pyridyl)-1,2,4,5-tetrazine (bptz) and 3,6-bis(pyrimidyl)1,2,4,5-tetrazine (bmtz)5, are attractive candidates for the construction of supramolecular aggregates, as these ligands are sufficiently flexible to allow for the formation of polygons of various topologies by adopting non-linear conformations.6 Indeed, the ability of the bptz ligand to form molecular squares and pentagons with various transition metal ions (i.e. NiII and FeII), with anion-π interactions playing a decisive role in the formation and identity of the metallacycles, has been previously demonstrated by our

group.6 Recently we capitalized upon the non-innocent redox to prepare behavior of bmtz and bptz5 7 {[Co(tpma)]2(bmtz)}(O3SCF3)3 (tpma = tris-(2pyridylmethyl)amine) and {Cp2Co}{Dy(tmhd)3]2(bptz)}8 (tmhd = 2,2,6,6-tetramethyl-3,5-heptanedionate) complexes, that contain the one electron radical anion of these ligands. The strategy of exploring radical bridged metal complexes as candidates for high spin molecules and single molecule magnets (SMMs) is a very successful one,9 owing to the fact that direct exchange coupling from the overlap between magnetic orbitals of the ligand and metals is much stronger as compared to indirect superexchange interactions between metal spin centers through diamagnetic linkers.10 Indeed, a variety of bimetallic radical bridged compounds have been reported to date,9 but the field is relatively small compared to the vast amount of data reported for innocent closed-shell bridging ligands. With these thoughts in mind, we investigated the self-assembly reactions between CoII metal ions and the radical anion form of the bptz ligand. The β-diketonate 1,3-diphenyl-1,3propanedionate was selected as an ancillary chelating ligand to prevent extended structures from forming. Herein, we report the high yield syntheses, structures and magnetic properties of a rare example of a molecular triangle [Co3(bptz)3(dbm)3]·2toluene (1) and a molecular square [Co4(bptz)4(dbm)4]·4MeCN (2), bearing the radical anion of bptz. These results constitute the first examples of tetrazine radical bridged metallacycles and only the second report of radical bridged polygonal molecules, the other example being the (Cp2Co)6[Mn6(N,OL)6] (N,OL = radical form of 4,5bis(pyridine-2-carboxamido)-1,2-catechol) compound reported by Harris and co-workers. 10b Compound 1 was prepared by the reaction of equimolar quantities of Co(dbm)2 and bptz in the presence of one equivalent of Cp2Co in THF. The solvent was removed under vacuum and the residue was recrystallized from toluene/Et2O. The crystal structure of 1, shown in Figure 1 (top), consists of three CoII ions in a distorted octahedral environment at the vertices of a molecular triangle with three bptz radical ligands spanning each edge. Four coordination sites are occupied by two chelating bptz N-donor ligands, in an anti orientation, with the other two being filled by one chelating dbm O-donor ligand. The oxidation state of Co atoms is established as 2+ by charge balance considerations and bond valence sum (BVS)11 calculations (SI). All Ntz-Co-Ntz bond angles are close to 90˚ (88.0-92.5˚) which are much wider than those required for the formation of an equilateral triangle (60˚). The necessity for more acute angles is met by the Co···Co···Co vertex angles which correspond to an isosceles triangle. The flexibility of the bptz ligands which bow outward to alleviate any angle strain (average dihedral angles between tetrazine and pyridyl rings ~10˚)

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allows for the formation of the closed triangular structure. The intramolecular Co…Co cross-ligand separations are ~6.71Å, with the closest intermolecular Co…Co contact being 9.09Å. To our knowledge, this is the first example in which bptz is used to form molecular triangles.

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1.384(8) Å and 1.383(8) Å for 1 and 2, respectively. These values are significantly longer than the N-N distances expected for the neutral form of the ligand (~1.33 Å), consistent with the presence of a radical bptz ligand.8, 12

Figure 2. Temperature dependence of χΜT for 1 (top) and 2 (bottom) measured at 0.1 T. (inset) J-coupling scheme employed for the elucidation of magnetic exchange interactions in 1 and 2.

Figure 1. Structures of 1 (top) and 2 (bottom). H atoms are omitted for clarity. Colors: CoII, magenta; O, red; N, blue; C, gray. Compound 2 was isolated from the same reaction that led to the isolation of compound 1 but in the presence of MeCN instead of THF/toluene, a clear indication of the supramolecular nature of this chemistry. The crystal structure of 2, shown in Figure 1 (bottom), reveals four pairs of CoII ions and bptz ligands arranged in the familiar molecular square motif. The four bptz radicals occupy the edges and act as bridges between the metal ions which occupy the vertices of the polygon. Each Co atom is in the 2+ oxidation state with a distorted octahedral geometry, as in the case of 1. Both Ntz-Co-Ntz bond angles (94.8-98.3˚) and Co···Co···Co vertex angles (88.7-90.7˚) are close to the ideal 90˚ angles required for a square. Similar to the structure of 1, bptz ligands in 2 are not planar but they bow inward this time (average dihedral angles between tetrazine and pyridyl rings ~5.8˚). The Co…Co crossligand separations are ~6.82 Å, and the Co…Co cross-cavity distances are 9.54 and 9.68 Å. The closest intermolecular Co…Co contact is 9.12 Å. The fact that one MeCN molecule is encapsulated in the cavity of 2 is an excellent indication that this stabilizes and likely templates the formation of the square as opposed to the triangle. An important structural characteristic of both complexes is the NN intra-tetrazine bond distances, the averages of which are

The static direct current (dc) magnetic properties of 1 and 2 were measured from 2.0-300 K in a 0.1 T applied field (Figure 2). For 1, the experimental χΜT value (6.58 cm3 K mol-1) at 300 K is in good agreement with the theoretical one (6.75 cm3 K mol-1) for three non-interacting high-spin, S =3/2, CoII ions and three S = 1/2 bptz radicals (g=2.0). Upon cooling, the χΜT product steadily increases to a maximum of 7.71 cm3 K mol-1 at 65.0 K and then decreases sharply to a value of 1.36 cm3 K mol-1 at 2.0 K. Complex 2 exhibits similar behavior, with χΜT increasing from 9.08 cm3 K mol-1 at 300 K to a maximum of 12.28 cm3 K mol-1 at 60.0 K, and then decreasing to 2.72 cm3 K mol-1 at 2.0 K. The 300 K value for 2 is in excellent agreement with the spin-only (g = 2) value of 9.00 cm3 K mol-1 for four pairs of non-interacting CoII ions and bptz radicals. The shape of the curves indicates dominant antiferromagnetic exchange interactions between the CoII ions and the bptz radicals corresponding to an S = 3 and S = 4 ground state for 1 and 2, respectively. The lack of saturation in the magnetization vs field plots of complexes 1 and 2 (Fig. S3-S4) indicates the presence of magnetic anisotropy and/or population of low-lying excited states. This is further supported by the reduced magnetization data (Fig. S5-S6) where the isofield lines do not superimpose, indicating significant D values for both complexes. The magnetic susceptibility data for compounds 1 and 2 were fit using the PHI program13 in order to evaluate the strength of the intramolecular magnetic exchange interactions. For complex 1, the spin Hamiltonian used is shown below and it is a modified version of a fitting model used by Layfield for a Co3 triangle bridged by one radical.14


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෡ = −2‫ܬ‬1 ൫ܵԦ1 ∙ ܵԦ‫ ܣ‬+ ܵԦ‫ܵ ∙ ܣ‬Ԧ2 + ܵԦ2 ∙ ܵԦ‫ ܤ‬+ ܵԦ‫ܵ ∙ ܤ‬Ԧ3 + ܵԦ3 ∙ ܵԦ‫ ܥ‬+ ܵԦ‫ܵ ∙ ܥ‬Ԧ1 ൯ ‫ܪ‬ 3

1 − 2‫ܬ‬2 ൫ ܵԦ1 ∙ ܵԦ2 + ܵԦ2 ∙ ܵԦ3 + ܵԦ3 ∙ ܵԦ1 ൯ + ‫ ܦ‬൮෎ ൬ܵ‫݅ݖ‬2 − ܵԦ݅2 ൰൲ 3 3

‫ܥ‬

+ ߤ‫ ݋ܥ݃ ܤ‬൭෍ ܵԦ݅ ൱ ‫ ܪ‬+ ߤ‫ ݀ܽݎ݃ ܤ‬൭෍ ܵԦ݅ ൱ ‫ܪ‬ ݅=1

݅=1

݅=‫ܣ‬

෡ describe the isotropic The first and second terms in ‫ܪ‬ Coradical and CoCo exchange interactions, where J1 and J2 are the corresponding Coradical and the CoCo exchange parameters. To avoid over-parameterization, we considered all three CoII ions and all three radicals to be equivalent (S1 = S2 = S3 = SCo = 3/2 and SA = SB = SC = Srad = 1/2). The third term represents the axial zero-field splitting, D, assuming parallel anisotropic tensors for all metal centers. The last two terms account for the Zeeman interactions, including both the CoII and the radical contributions, where µB is the Bohr magneton, gCo and grad are the corresponding electronic Lande factors for CoII and bptz radical, and H is the magnetic field. Similar studies were performed for 2 and extensive discussion is provided in the SI. The best fit parameters are summarized in Table 1. Table 1. Parameters obtained from PHI fitting and DFT/ab initio calculations for 1 and 2. PHI fitting Complex gCo D (cm-1) J1 (cm-1) J2 (cm-1) J3 (cm-1)

1 2.26 +38.5 -67.5 +6.0 -

Calculations 2 2.22 +42.5 -66.8 +9.2 -

1 2.26 +38.1 -55.6 +2.3 -3.3

2 2.30 +48.6 -56.8 +4.2 -4.4

These results reveal weak ferromagnetic coupling between the CoII ions and strong antiferromagnetic exchange interactions between the CoII centers and bptz radicals. A similar JCorad value (-62.5 cm-1) was observed for the Co2 dimer bridged by bmtz radical previously reported by our group.7 The large and positive D and g > 2 values obtained for CoII are in good agreement with values reported for other octahedral CoII ions.15 Attempts to fit the susceptibility data of 1 including an additional J-coupling constant, to consider any possible radicalradical (J3 = Jradrad) magnetic exchange interactions, in the above spin Hamiltonian did not improve the fit and gave almost identical JCorad (-67.9 cm−1) and JCoCo (+ 6.0 cm−1) values. The Spin-Hamiltonian parameters were further elucidated by Density Functional Theory (DFT) using the B3LYP functional16 and ab initio calculations using CASSCF and NEVPT2 methods (SI) to rationalize the observed magnetic properties. Both DFT and ab initio calculations yield the following spin ground state configurations for the CoII ions [(dxz)2(dxy)2(dyz)1(dx2-y2)1(dz2)1] and the d-orbital ordering is shown in Figure 3. DFT calculations accurately reproduce the sign of the experimentally determined JCorad and JCoCo coupling values, but slightly underestimate the magnitude (Table 1). The Jradrad coupling constants were calculated to be -3.3 cm-1 and -4.4 cm-1 for 1 and 2, respectively. These antiferromagnetic interactions are relatively weak compared to the more dominant JCorad and JCoCo interactions, justifying the omission of Jradrad from the Hamiltonian used to fit the experimental data (vide supra). The calculations also confirm a spin ground state S = 3 and S = 4 for 1 and 2, respectively. These spin states can be achieved with a spin-up configuration on all CoII ions and a spin-down configuration on all

Figure 3. Ab initio computed d-orbital ordering for CoII ions in 1 and 2. radical ions in both complexes.17 The spin density diagrams (Figure 4) illustrate the parallel alignment between the eg magnetic orbitals of CoII ions and the π* orbitals of bptz radicals. The unpaired electron from the Co-based dyz orbital overlaps with the π* orbital of the bptz radical resulting in the observed strong antiferromagnetic exchange between the Co ions and bptz radicals.18

Figure 4. DFT computed spin density plots for the S = 3 (left) and S = 4 (right) spin state configurations of 1 and 2, respectively. The red and green iso-density surfaces (0.006 e- bohr-3) indicate positive and negative spin phases. The CASSCF (values in parentheses) and NEVPT2 calculations yielded gCo values of 2.26 (2.29) and 2.30 (2.39), and D values of 38.1 (50.5) cm-1 and 48.6 (72.1) cm-1 for CoII ions in 1 and 2, respectively. The sign of both the CASSCF and NEVPT2 computed D values are consistent with the experimentally determined values. Furthermore, the magnitudes of the NEVPT2 computed g and D values are in good agreement with the experimentally determined values. The simulated susceptibility curves, constructed using PHI, are in good agreement with the experimental data, validating the computational method used here (Figure 2). The eigenvalue plots were examined to account for the calculated sign and magnitude of the D value of CoII ions (Figure 3). The orbital splitting pattern of CoII ions in both complexes suggests that the most feasible single electron transition is between dxy → dyz orbitals (different |±ml| levels) leading to a positive D value.19 The major positive contribution to the D value is essentially from the first four quartet excited states (Table S6). In summary, we report two new CoII metallacycles bearing the radical form of bptz, which, to our knowledge, are the first of their



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kind. The reaction solvent plays a decisive role in the formation and identity of the resulting polygons. X-ray crystallographic studies reveal that the less polar THF/toluene solvents favor the formation of Co3 triangle, while the more polar MeCN solvent induces formation of Co4 square. In addition to their interesting structures, the compounds exhibit strong antiferromagnetic coupling between the CoII ions and the bptz radicals. In order to further explore the magnetic properties of such compounds, the chemistry will be extended to other magnetic metal ions of the 3d transition series as well as heavier elements. In addition, the possibility of exploiting these closed architectures as receptors for small organic molecules for donor-acceptor and sensor applications is a promising avenue.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Experimental details and computational details (PDF) Crystal structures of compounds 1 and 2 (CIF).

AUTHOR INFORMATION

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7. Woods, T. J.; Ballesteros-Rivas, M. F.; Ostrovsky, S. M.; Palii, A. V.; Reu, O. S.; Klokishner, S. I.; Dunbar, K. R., Chem. Eur. J., 2015, 21, 10302-10305. 8. Dolinar, B. S.; Gómez-Coca, S.; Alexandropoulos, D. I.; Dunbar, K. R., Chem. Commun., 2017, 53, 2283-2286. 9. Demir, S.; Jeon, I.-R.; Long, J. R.; Harris, T. D., Coord. Chem. Rev., 2015, 289-290, 149-176. 10. (a) DeGayner, J. A.; Jeon, I.-R.; Harris, T. D., Chem. Sci., 2015, 6, 6639-6648; (b) Jeon I.-R.; Harris, T. D., Chem. Commun., 2016, 52, 1006-1008; (c) Jeon I.-R.; Park, J. G.; Xiao, D. J.; Harris, T. D., J. Am. Chem. Soc., 2013, 135, 16845-16848. 11. Lui, W.; Thorp, H. H., Inorg. Chem., 1993, 32, 4102-4105. 12. (a) Schwach, M.; Hausen, H.-D.; Kaim, W., Inorg. Chem., 1999, 38, 2242-2243; (b) Maekawa, M.; Miyazaki, T.; Sugimoto, K.; Okubo, T.; Kuroda-Sowa, T.; Munakata, M.; Kitagawa, S., Dalton Trans., 2013, 42, 4258-4266. 13. Chilton, N. F.; Anderson, R. P.; Turner, L. D.; Soncini, A.; Murray, K. S., J. Comput. Chem., 2013, 34, 1164-1175. 14. Moilanen, J. O.; Chilton, N. F.; Day, B. M.; Pugh, T.; Layfield, R. A., Angew. Chem., Int. Ed., 2016, 55, 5521-5525. 15. Murrie, M., Chem. Soc. Rev., 2010, 39, 1986-1995. 16. Becke, A. D., J. Chem. Phys., 1993, 98, 5648-5652. 17. Vignesh, K. R.; Langley, S. K.; Murray, K. S.; Rajaraman, G., Chem. Eur. J., 2015, 21, 2881-2892. 18. Gass, I. A.; Tewary, S.; Rajaraman, G.; Asadi, M.; Lupton, D. W.; Moubaraki, B.; Chastanet, G.; Létard, J.-F.; Murray, K. S., Inorg. Chem., 2014, 53, 5055-5066. 19. Gómez-Coca, S.; Cremades, E.; Aliaga-Alcalde, N.; Ruiz, E., J. Am. Chem. Soc., 2013, 135, 7010-7018.

Corresponding Author [email protected]

ACKNOWLEDGMENTS We gratefully acknowledge funding from the National Science Foundation under Grant No. CHE-1310574. K. R. D. is also grateful to the Robert A. Welch Foundation (A-1449). The X-ray diffractometer and SQUID magnetometer used in this research were purchased with funds provided by the Texas A&M University Vice President of Research. We would like to thank the HPRC at Texas A&M University for providing computing resources.

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Table of Contents

Two new CoII metallacycles bearing the radical form of bptz ligand are reported, which, to our knowledge, are the first of their kind. The solvent plays a decisive role in the formation and identity of the resulting polygons, a clear indication of the supramolecular nature of this chemistry.


Putting a New Spin on Supramolecular Metallacycles: Co3 Triangle

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