Building Diatomic and Triatomic Superatom Molecules Anouck M

Letter

pubs.acs.org/NanoLett

Building Diatomic and Triatomic Superatom Molecules Anouck M. Champsaur, Alexandra Velian, Daniel W. Paley, Bonnie Choi, Xavier Roy,* Michael L. Steigerwald,* and Colin Nuckolls* Department of Chemistry, Columbia University, New York, New York 10027 United States

*S Supporting Information

ABSTRACT: In this study, we have developed a method to − create Co6Se8 superatoms in which we program the metal ligand bonds. We exclusively form the Co6Se8 core under simple reaction conditions with a facile separation of products that contain differential substitution of the core. The combination of Co2(CO)8 and PR3 with excess Se gives the differentially and directionally substituted superatoms, Co6Se8(CO)x(PR3)(6−x). The CO groups on the superatom can be exchanged quantitatively with phosphines and isonitriles. Substitution of the CO allows us to manipulate the type and length of chemical bridge between two redox- active superatomic centers in order to modulate intersuperatomic coupling. Linking two superatoms together allows us to form the simplest superatom molecule: a diatomic molecule. We extend the superatom molecule concept to link three superatoms together in a linear arrangement to form acyclic triatomic molecules. These superatom molecules have a rich electrochemical profile and chart a clear path to a whole family of superatom molecules with new and unusual collective properties. KEYWORDS: Superatoms, nanoscale atoms, superatom molecules, nanoscale building blocks

− n this study, we create the superatomic1 4 version of the approaches have relied on self-assembly and shape comple- I simplest molecules, diatomic and linear triatomic molecules. mentarity; here we describe reactions of the superatoms in The constituent superatoms are atomically defined clusters of which we manipulate the metal−ligand bonds by intentionally metal chalcogenides that are capped with passivating two- including labile surface-ligands. We prepare a series of electron ligands. The key to being able to discretely link these − superatoms, Co6Se8(CO)x(PR3)(6−x) (x =1 5, R = Et, Ph) superatoms into superatom molecules (SMs) was to control the and show that they are stereochemically robust. These new reaction chemistry of the capping ligands and to thereby superatoms undergo CO-displacement reactions with neutral program the intersuperatom bonding. Having control over the geometry, distance, and linker between the superatoms is two-electron donors (phosphines and isocyanides) and these crucial to further the understanding of how to promote reactions depend on the ligand arrangements of the initial “ ” electronic and magnetic coupling within SMs. Previous studies superatoms. These inner-sphere reactions allow us to form on colloidal semiconductors have gone to great lengths to link diatomic and linear triatomic SMs (Figure 1b). − them together into discrete “nanocrystal molecules”5 11 Superatoms having neutral cores capped with phosphine because the nanopositioning and linking of these electronic ligands are typically synthesized so that each ligand coordinates and magnetic nano-objects allows collective properties to to a metal atom to form, for example, M6E8(PR3)6 structures − emerge. The study here provides a means to synthesize ∼1nm (M = Cr, Co, Mo, Rh; E = S, Se, Te).16 20 Substitution superatoms and to couple them together into molecular form reactions of these phosphines are difficult because the M−P fi with atomic precision and having a well-de ned surface bonds tend to display low chemical reactivity at moderate definition/bonding. “ temperatures and indiscriminate reactions at higher temper- Prior studies on superatoms have focused on the outer- atures; the latter thermal reactions typically yield solid-state sphere” reaction chemistry of cobalt chalcogenide clusters and products.21,22 Moreover, the differentiation of one of the created solid-state materials via either the transfer of electrons − from the cluster to acceptors such as fullerenes12 14 or the phosphines from another in a symmetrically substituted metal molecular recognition of fullerenes by fluxional phenanthrene chalcogenide cluster is a vexing challenge. ligands (Figure 1a).15 In these cases, the essential structure of neither the inorganic core of the superatoms nor the ligand Received: June 16, 2016 shell changes. As such, superatoms in this context form Revised: July 6, 2016 extended solid-state materials with tunable properties. Those Published: July 13, 2016

Figure 1. (a) Previously studied outer-sphere reactions of Co6Se8(PR3)6 superatoms. (b) We study here the inner-sphere reactions of Co6Se8(CO)x(PR3)(6−x).

ﬀ Figure 2. Molecular structure of di erentially substituted superatoms: (a) trans-Co6Se8(CO)2(PEt3)4, (b) cis-Co6Se8(CO)2(PEt3)4, and (c) Co6Se8(CO)(PEt3)5. Atom labels are shown (a). Hydrogen atoms are omitted for clarity, and thermal ellipsoids are set at 50% probability level.

a Scheme 1. Mono- and Disubstitution of trans-Co6Se8(CO)2(PEt3)4

aThe formation of products can be controlled by the stoichiometry of the ligand.

Here we sought a bottom-up approach to SMs by developing stoichiometry governs the average value of x. We separated simple solution-phase chemistry to synthesize electrically particular superatoms using conventional silica gel chromatog- neutral and directionally substituted superatoms in one step. raphy, relying on the distinct and diagnostic 1H NMR for each We were inspired by the rich reaction chemistry developed for of the ligand substitution patterns around the Co6Se8 core 23−29 fi Re6Se8L6 clusters. Our important nding is a simple and (detailed in the Supporting Information). mild synthesis of heteroleptic Co6Se8 superatoms. The We focus on three basic Co6Se8 superatom building blocks: subsequent separation of different regioisomers is facile and two with two CO groups in either a cis or trans arrangement efficient. We further demonstrate that the reaction chemistry of and the monocarbonylated superatom, (Figure 2) because they trans- and cis-dicarbonylated superatoms offers a clear path to constitute fundamental blocks with which to build diatomic and linear, bent, cyclic, and oligomeric SMs. triatomic SMs as well as one- and two-dimensional materials. ff Our synthesis of di erentially substituted Co6Se8 superatoms The Co6Se8 superatom core is built from a slightly distorted fi 30−32 re nes previous methods: the combination of Co2(CO)8 octahedron of six cobalt atoms each face of which is capped by fi and PR3 with a signi cant excess of Se produces the family of a selenium atom. The superatomic cores of substituted superatoms, Co6Se8(CO)x(PR3)(6−x). We require Co6Se8(CO)x(PR3)(6−x) are isostructural and they show no − − − some amount of phosphine to passivate the Co6Se8 core, but clear systematic trend in the Co Co, Co Se, or Co P the stoichiometric excess of Se effectively decreases the amount distances (Table S1). of phosphine available to displace the carbonyl groups and thus By modifying the reaction and purification conditions, we modulates x. While mixtures of the carbonylated superatoms also isolate and crystallographically characterize both mer- and are produced regardless of the initial reagent ratios, the PR3/Se fac-Co6Se8(CO)3(PEt3)3, cis- and trans-Co6Se8(CO)4(PPh3)2,

5274 DOI: 10.1021/acs.nanolett.6b02471 Nano Lett. 2016, 16, 5273−5277 Nano Letters Letter

Figure 3. Molecular structure of (a) trans-Co6Se8(CNC6H4NC)2(PEt3)4, (b) cis-Co6Se8(CNC6H4NC)2(PEt3)4, and (c) the doubly oxidized − diatomic SM, [Co12Se16(PEt3)10(CNC6H4NC)][PF6]2. Hydrogen atoms and the PF6 anion have been omitted for clarity. Thermal ellipsoids are set at 50% probability level. Atom labels and colors are the same as in Figure 2. fac-Co6Se8(CO)3(PPh3)3, and Co6Se8(CO)5(PPh3)(Figure higher-dimensional materials with either of the CO-substituted S1). We also extended these reactions to tellurium: the same superatoms. reaction conditions yield a distribution of We create the simplest SMs, diatomic and acyclic triatomic 1 Co6Te8(CO)x(PEt3)(6−x), as determined by H NMR. Figure SMs, by replacing the CO group with chemical bridges. S2 shows the crystal structures of the isolated cis and trans Appropriate ditopic ligands link two redox-active superatomic disubstituted cobalt telluride clusters. The tellurium-containing centers, and varying these ligands modulates intersuperatomic superatoms are more difficult to chromatograph due to their coupling. The linking chemistry of the cis- and trans- greater sensitivity to ambient conditions. dicarbonylated superatoms is similar; in making the SMs, we The CO ligands are chemically active and the substitution focus only on the chemical reactions of trans- reaction is induced photochemically. This behavior is similar to Co6Se8(CO)2(PEt3)4 isomer. In the SM aufbau, the trans- that of metal carbonyl complexes undergoing dissociation and Co Se (CO) (PEt ) is a link and Co Se (CO)(PEt ) is a cap. substitution of CO ligands upon photolysis.33 Scheme 1 shows 6 8 2 3 4 6 8 3 5 We chose 1,4-phenylene diisocyanide (CNC6H4NC), a rigid the substitution reactions in which two electron donors such as linear linker. phosphines and isocyanides replace the CO of the trans-CO From the building blocks trans-Co6Se8(CNC6H4NC)2- superatom. Room temperature irradiation of a THF solution of (PEt ) , 1,4-phenylene diisocyanide, and Co Se (CO)(PEt ) , the superatom with a 200−400 nm broadband lamp allows a 3 4 6 8 3 5 we synthesized a diatomic SM Co Se (PEt ) (CNC H NC) selective substitution of CO groups. When we irradiate a 12 16 3 10 6 4 and linear triatomic SM Co Se (PEt ) (CNC H NC) solution of trans-Co Se (CO) (PEt ) containing two equiv- 18 24 3 14 6 4 2 6 8 2 3 4 (Figure 1b). We prepare the superatom in Figure 3ain alents (per superatom) of the incoming electron donor ligand, we obtain the product in quantitative yields. When we treat the quantitative yields from the photolysis of trans- Co6Se8(CO)2(PEt3)4 in the presence of excess 1,4-phenylene same superatom with less than 2 stoichiometric equivalents of 34 the ligand, we obtain a mixture of the monosubstituted and diisocyanide. We can form cis-Co6Se8(CNC6H4NC)2(PEt3)4 similarly from its cis-CO precursor (Figure 3b). 1H NMR disubstituted products (Scheme 1). Figure S3 displays the fi structures of the differentially substituted superatoms trans- con rms formation of the diatomic SM and the linear triatomic i SM (Figures S4 and S5). Co6Se8( PrNC)2(PEt3)4 and trans-Co6Se8(Et2PPhSMe)2- Figure 3c displays the crystal structure of the diatomic SM in (PEt3)4, obtained from treating trans-Co6Se8(CO)2(PEt3)4 with two equivalents of ligand. its doubly oxidized form [Co12Se16(PEt3)10(CNC6H4NC)]- There are two important findings in these reactions. (1) The [PF6]2. We isolate this by treating two equivalents of the stereochemistry of either the cis- or trans- arrangement of the oxidized superatom [Co6Se8(CO)(PEt3)5][PF6]withone CO groups is maintained in these reactions. The implication is equivalent of linker (see Supporting Information for details). that the phosphine ligands are not fluxional during this process. The diatomic SM is symmetrical about the linker with (2) The reactions only substitute the CO groups, not the crystallographically equivalent Co6Se8 superatomic cores. phosphines. These two findings demonstrate that selective Figure S6 displays the UV−vis spectrum for the monomeric chemistry is possible to build up SMs and one-, two-, and superatom (trans-Co6Se8(CNC6H4NC)2(PEt3)4), the diatomic

5275 DOI: 10.1021/acs.nanolett.6b02471 Nano Lett. 2016, 16, 5273−5277 Nano Letters Letter

Figure 4. Cyclic voltammogram (CV) of (a) monomeric superatoms, mono-CNPhMe2 (blue) and trans-CNPhMe2 (red), (b) diatomic SM, and (c) triatomic SM in 0.1 M TBAPF6 in THF with a 150 mV/s scan rate. (d) CV of triatomic SM in 0.1 M TBAPF6 in nitrobenzene with a 150 mV/s scan rate.

SM, and triatomic SM. Each of the spectra has a long tail that analysis of the diatomic SM redox pairs, we assign the first ∼− extends into the near-IR. oxidation event (at E1/2 0.8 V in Figure 4c) in the trimer as We used electrochemical methods35 to investigate electronic the 0 to +3 transition. We therefore attribute the pronounced coupling between the cores of the diatomic and linear triatomic splitting behavior of the linear triatomic SM to the existence of SMs. In order to understand how the electronic properties of two types of superatoms, bridging and terminal, whose redox isocyanide-bridged superatom oligomers compare to those of a potentials differ. The smooth redox peaks of the dimer arise single Co6Se8 core, we prepared two monomers as points of because each is a terminal unit oxidized at the same potential. reference: mono- and di-substituted superatoms with 2,6- Magnetic susceptibility measurements of the oxidized dimethylphenyl isocyanide (See Supporting Information for superatom [Co6Se8(CO)(PEt3)5][PF6], as well as the doubly synthetic details). The isocyanide-substituted superatoms oxidized diatomic SM [Co12Se16(PEt3)10(CNC6H4NC)][PF6]2 behave similarly to their CO-substituted analogues (Figure elucidate their open-shell characteristics (Figure S8). Specifi- S7); the strong pi-acceptor ligands bonded to the Co6Se8 core cally, [Co6Se8(CO)(PEt3)5][PF6] shows a temperature-inde- 3 make it less electron rich than the parent Co6Se8(PEt3)6 (Table pendent magnetic susceptibility (XMT) with XMT = 0.45 cm K/ S2). The cyclic voltammograms of both species are shown in mol at T = 3 K, suggesting that the core contains one unpaired Figure 4a and as expected, the Ep values (i.e., the peak value of electron with a g-factor greater than 2 (spin-only value S = 1/2 the potential) of each superatom differ and are dependent upon of an unpaired electron is 0.375 cm3K/mol for g = 2). The 3 the ligand environment. doubly oxidized diatomic SM has XMT = 0.91 cm K/mol at T = The electrochemistry of the diatomic SM and the linear 3 K. This result suggests that the diatomic SM contains two triatomic SM is rich in information. While the neutral diatomic noninteracting spins, each having XMT of the monomeric SM exhibits three reversible oxidation peaks (Figure 4b), the species. neutral triatomic SM exhibits discernible electrochemical In summary, we have found a simple method to synthesize splitting (Figure 4c). This splitting is even more pronounced and isolate Co6Se8 superatoms that are substituted with both when the voltammogram is taken in nitrobenzene (Figure 4d), phosphine and carbonyl ligands. The CO ligands can be a solvent with a much higher dielectric constant, known to replaced with two electron donors such as phosphines and ε ε better stabilize charged species ( THF = 7.58 vs nitrobenzene = isocyanides. From the trans-CO superatom and monocarbony- 34.82).36 We determined that each of the three reversible lated superatom we can create diatomic SMs and linear oxidation peaks in the CV of the neutral diatomic SM (Figure triatomic SMs that are linked together with diisocyanides. 4b) corresponds to a two-electron event as follows: under the Forming these new SMs is an important step toward studying same conditions we measured the open circuit potential of the SMs in single molecule and materials applications. The method diatomic SM dication as −0.671 V versus the ferrocene/ described here gives access to the vast number of magnetic and ferrocenium redox couple. This potential corresponds to the electroactive superatoms whose intersuperatom coupling can be species formed after the first oxidation of the neutral dimer. We easily varied by changing the linker. For example, we are can therefore assign each redox couple of the diatomic SM to exploring mixed Se/Te systems, which we can access by the following states: 0/+2, +2/+4, and +4/+6. From our covalently linking Co6Se8 and Co6Te8 carbonylated supera-

5276 DOI: 10.1021/acs.nanolett.6b02471 Nano Lett. 2016, 16, 5273−5277 Nano Letters Letter toms. The hetero diatomic superatoms offer a unique (12) Roy, X.; Lee, C. H.; Crowther, A. C.; Schenck, C. L.; Besara, T.; opportunity to study the magnetic coupling between two Lalancette, R. A.; Siegrist, T.; Stephens, P. W.; Brus, L. E.; Kim, P.; − spins as the linker is modified. Moreover, the reaction of the Steigerwald, M. L.; Nuckolls, C. Science 2013, 341 (6142), 157 160. trans-CO superatom and a ditopic linker would give rise to (13) Lee, C. H.; Liu, L.; Bejger, C.; Turkiewicz, A.; Goko, T.; oligomeric SM chains. Arguello, C. J.; Frandsen, B. A.; Cheung, S. C.; Medina, T.; Munsie, T. J. S.; D’Ortenzio, R.; Luke, G. M.; Besara, T.; Lalancette, R. A.; Siegrist, T.; Stephens, P. W.; Crowther, A. C.; Brus, L. E.; Matsuo, Y.; ■ ASSOCIATED CONTENT Nakamura, E.; Uemura, Y. J.; Kim, P.; Nuckolls, C.; Steigerwald, M. L.; *S Supporting Information Roy, X. J. Am. Chem. Soc. 2014, 136 (48), 16926−16931. (14) Turkiewicz, A.; Paley, D. W.; Besara, T.; Elbaz, G.; Pinkard, A.; The Supporting Information is available free of charge on the − ACS Publications website at DOI: 10.1021/acs.nano- Siegrist, T.; Roy, X. J. Am. Chem. Soc. 2014, 136 (45), 15873 15876. lett.6b02471. (15) Choi, B.; Yu, J.; Paley, D. W.; Trinh, M. T.; Paley, M. V.; Karch, J. M.; Crowther, A. C.; Lee, C. H.; Lalancette, R. A.; Zhu, X. Y.; Kim, Synthetic details, characterization, X-ray diffraction, P.; Steigerwald, M. L.; Nuckolls, C.; Roy, X. Nano Lett. 2016, 16 (2), magnetometry, and crystallographic (CIF) files.(PDF) 1445−1449. (16) Fenske, D.; Ohmer, J.; Hachgenei, J. Angew. Chem., Int. Ed. Engl. 1985, 24 (11), 993−995. ■ AUTHOR INFORMATION (17) Saito, T.; Yamamoto, N.; Yamagata, T.; Imoto, H. J. Am. Chem. Corresponding Authors Soc. 1988, 110 (5), 1646−1647. * (18) Steigerwald, M. L.; Siegrist, T.; Stuczynski, S. M. Inorg. Chem. E-mail: [email protected].. − *E-mail: [email protected]. 1991, 30 (10), 2256 2257. *E-mail: [email protected]. (19) Hessen, B.; Siegrist, T.; Palstra, T.; Tanzler, S. M.; Steigerwald, M. L. Inorg. Chem. 1993, 32 (23), 5165−5169. Notes (20) Thiele, G.; You, Z. L.; Dehnen, S. Inorg. Chem. 2015, 54 (6), The authors declare no competing financial interest. 2491−2493. (21) Stuczynski, S. M.; Kwon, Y. U.; Steigerwald, M. L. J. Organomet. ■ ACKNOWLEDGMENTS Chem. 1993, 449 (1−2), 167−172. − ́ ́ ́ (22) Kamiguchi, S.; Imoto, H.; Saito, T. Chem. Lett. 1996, 7, 555 We thank Dr. Raul Hernandez Sanchez for useful discussions. 556. Support for this research was provided by the Center for (23) Leduc, L.; Padiou, J.; Perrin, A.; Sergent, M. J. Less-Common Precision Assembly of Superstratic and Superatomic Solids, an Met. 1983, 95 (1), 73−80. NSF MRSEC (award number DMR-1420634), and the Air (24) Leduc, L.; Perrin, A.; Sergent, M. Acta Crystallogr., Sect. C: Cryst. Force Office of Scientific Research (award number FA9550-14- Struct. Commun. 1983, 39 (Nov), 1503−1506. 1-0381). B.C. is supported by the NSF Graduate Research (25) Long, J. R.; Williamson, A. S.; Holm, R. H. Angew. Chem., Int. − Fellowship under Grant DGE 11-44155. The Columbia Ed. Engl. 1995, 34 (2), 226 229. University Shared Materials Characterization Laboratory (26) Zheng, Z. P.; Long, J. R.; Holm, R. H. J. Am. Chem. Soc. 1997, 119 (9), 2163−2171. (SMCL) was used extensively for this research. Single crystal − ff (27) Zheng, Z. P.; Holm, R. H. Inorg. Chem. 1997, 36 (23), 5173 X-ray di raction was performed at the Shared Materials 5178. Characterization Laboratory at Columbia University. Use of (28) Gabriel, J. C. P.; Boubekeur, K.; Uriel, S.; Batail, P. Chem. Rev. the SMCL was made possible by funding from Columbia 2001, 101 (7), 2037−2066. University. We are grateful to Columbia University for support (29) Orto, P. J.; Nichol, G. S.; Wang, R. Y.; Zheng, Z. P. Inorg. Chem. of this facility. 2007, 46 (21), 8436−8438. (30) Steigerwald, M. L.; Siegrist, T.; Stuczynski, S. M. Inorg. Chem. ■ REFERENCES 1991, 30 (26), 4940−4945. (31) Diana, E.; Gervasio, G.; Rossetti, R.; Valdemarin, F.; Bor, G.; (1) Khanna, S. N.; Jena, P. Phys. Rev. B: Condens. Matter Mater. Phys. − − Stanghellini, P. L. Inorg. Chem. 1991, 30 (2), 294 299. 1995, 51 (19), 13705 13716. (32) Gervasio, G.; Kettle, S. F. A.; Musso, F.; Rossetti, R.; (2) Claridge, S. A.; Castleman, A. W.; Khanna, S. N.; Murray, C. B.; − − Stanghellini, P. L. Inorg. Chem. 1995, 34 (1), 298 305. Sen, A.; Weiss, P. S. ACS Nano 2009, 3 (2), 244 255. (33) Strohmeier, W. Angew. Chem., Int. Ed. Engl. 1964, 3 (11), 730. (3) Qian, M. C.; Reber, A. C.; Ugrinov, A.; Chaki, N. K.; Mandal, S.; (34) We treat the starting material with an excess of the linker (5 Saavedra, H. M.; Khanna, S. N.; Sen, A.; Weiss, P. S. ACS Nano 2010, − equivalents) to prevent undesired oligomerization and subsequently 4 (1), 235 240. remove the leftover linker by trituration. (4) Luo, Z. X.; Castleman, A. W. Acc. Chem. Res. 2014, 47 (10), − − (35) Creutz, C. Prog. Inorg. Chem. 1983, 30,1 73. 2931 2940. (36) Barriere, F.; Camire, N.; Geiger, W. E.; Mueller-Westerhoff, U. (5) Alivisatos, A. P.; Johnsson, K. P.; Peng, X. G.; Wilson, T. E.; T.; Sanders, R. J. Am. Chem. Soc. 2002, 124 (25), 7262−7263. Loweth, C. J.; Bruchez, M. P.; Schultz, P. G. Nature 1996, 382 (6592), 609−611. (6) Mirkin, C. A.; Letsinger, R. L.; Mucic, R. C.; Storhoff, J. J. Nature 1996, 382 (6592), 607−609. (7) Li, Y. L.; Liu, Z. Y.; Yu, G. M.; Jiang, W.; Mao, C. D. J. Am. Chem. Soc. 2015, 137 (13), 4320−4323. (8) Wang, Y. F.; Wang, Y.; Breed, D. R.; Manoharan, V. N.; Feng, L.; Hollingsworth, A. D.; Weck, M.; Pine, D. J. Nature 2012, 491 (7422), 51−U61. (9) Jones, M. R.; Mirkin, C. A. Nature 2012, 491 (7422), 42−43. (10) Glotzer, S. C.; Solomon, M. J. Nat. Mater. 2007, 6 (8), 557− 562. (11) Yin, S. N.; Yang, S. Y.; Wang, C. F.; Chen, S. J. Am. Chem. Soc. 2016, 138 (2), 566−573.

5277 DOI: 10.1021/acs.nanolett.6b02471 Nano Lett. 2016, 16, 5273−5277