Recent advances in the synthesis of atomically-precise nanoclusters (APNCs) have resulted in a remarkable increase in the number of structurally-characterized clusters. [1][2][3][4][5][6] Despite this wealth of work, however, structurally characterized AP-NCs exist for only a handful of transition metal (Cu, [7][8][9][10][11][12][13] Ag, 5,[14][15][16] Au, 5,14,17 Pd, [18][19] and Zn 20 ) and main group elements (Al, [21][22][23][24] Ga, 23,25 Ge, [26][27][28][29][30] In, [31][32] and Sn 26,28,[33][34][35][36] ). Expansion to the other transition metals, such as Co, could lead to novel magnetic materials, which could have applications in catalysis, imaging, and quantum computing. [37][38][39] However, metallic cobalt nanomaterials are highly air-sensitive, which renders them a challenge to isolate and characterize. Several different strategies have been employed to protect these nanomaterials from unwanted oxidation, including reductive annealing to improve Co crystallinity, 40 dispersion in polymer, [41][42][43][44] coating with gold, [45][46] or embedding on a support, such as graphite [47][48] or silica. [49][50] Passivation of nanomaterials with a protective "shell" comprised of anionic and/or neutral donor ligands is another viable strategy for imparting air stability. The most common passivating ligands for APNCs are thiolates (RS -); 2, 5 however, carbon monoxide, [18][19] hydrides 7,[9][10]12 and acetylides 11,[13][14][15][16][17] have also been employed.
In 2017, Barrabés and co-workers reported the synthesis of the thiolate-protected cobalt APNCs, Cox(SR)m (R = CH2CH2Ph), via reaction of CoCl2 with RSH and NaBH4 in THF/H2O. 51 This material was characterized by UV-vis spectroscopy, XPS, STEM, and XANES; however, single crystals for X-ray diffraction were not forthcoming. On the basis of MALDI analysis, the authors suggested the "formation of cobalt clusters in a range of 25-30 cobalt atoms" 51 and offered Co25(SR)18 and Co30(SR)16 as two potential formulations to fit this criterion. Given the rarity of atomically-precise cobalt nanoclusters, we endeavored to reproduce the reported synthesis and further study these unique materials. Herein, we report that the major product of this reaction is actually the thiolateprotected Co(II) T3 supertetrahedron, [Co10(SR)16Cl4], and not a Co(0)-containing APNC, as originally reported.
The 2017 synthesis of Cox(SR)m followed a modified Brust protocol (Scheme 1). [51][52] CoCl2⋅6H2O (1 equiv) was dehydrated at 150 °C and then dissolved in tetrahydrofuran (10 mL). PhCH2CH2SH (3 equiv) was added to the blue solution and stirred for 30 minutes, resulting in a color change to dark blue. NaBH4 (9 equiv), dissolved in H2O (2 mL) and chilled to 0 °C, was then quickly added to the reaction mixture. The solution was stirred for 1 h and subsequently filtered and washed with methanol. The solid was then extracted with CH2Cl2, resulting in a pink solution containing the proposed Cox(SR)m clusters. A yield was not reported.
We attempted to repeat the original synthesis as closely as possible; however, we made a few minor changes to the procedure to allow for in situ spectroscopic monitoring. Specifically, we replaced the THF and H2O with THF-d8 and D2O, respectively, and we performed the reaction in a J. Young NMR tube under an inert gas atmosphere (Figures S4 andS5). Under these conditions, we are able to successfully reproduce the deep blue solution previously reported to form upon addition of PhCH2CH2SH to CoCl2. Interestingly, upon addition of a D2O solution of NaBH4 (9 equiv) we observe a color change to dark green. This solution then slowly turned dark brown, concomitant with the deposition of a grey-brown solid. A 1 H NMR spectrum of the reaction mixture after 30 min reveals the presence of three diagnostic resonances at -10.02, 103.22, and 120.11 ppm (Figure S4), which are assignable to the cobalt(II)-thiolate cluster, [Co10(SR)16Cl4] (1) (see below). Complex 1 is the only major product observed in the reaction mixture, demonstrating that the transformation is remarkably chemoselective.
To facilitate the isolation of 1 we repeated the above procedure in the absence of water and in an inert atmosphere glove box (Scheme 2). Work-up of this reaction mixture results in the isolation of dark brown crystals of the cobalt-thiolate cluster [Co10(SR)16Cl4] (1) in 37% yield. Also formed in this reaction is a grey-brown solid, whose appearance is consistent with that of NaCl, but which is contaminated with small amounts of a Co-containing product. We believe the modest yield of this reaction is due to the presence of excess thiol (see below), which impedes the crystallization process. [53][54] Complex 1 is a rare example of an open-shell, chalcogenidestabilized T3 supertetrahedral cluster. [55][56] Comparable chalcogenide-stabilized supertetrahedra, such as [Cd10(SCH2CH2OH)16][X]4 (X = ClO4 -, NO3 -, SO4 2-) [57][58][59] We next endeavored to synthesize complex 1 via a rational route (Scheme 2). Given that NaBH4 appears to be acting solely as a base during the formation of 1, we rationalized that the reaction protocol could be simplified by substitution of PhCH2CH2SH/NaBH4 with NaSCH2CH2Ph. Thus, reaction of CoCl2⋅1.5THF with 1.6 equiv of NaSCH2CH2Ph in THF resulted in the formation of a green solution, which gradually turned dark brown over the course of 5 h, concomitant with the deposition of a grey powder. Work-up of the reaction mixture allowed for the isolation of 1 as a dark brown crystalline solid. When synthesized in this fashion complex 1 can be isolated in 81% yield. Magnetic susceptibility data were also collected on a microcrystalline sample of 1 (Figure 3). At 300 K, complex 1 exhibits an effective magnetic moment of 7.36 B.M., lower than the anticipated spin-only effective magnetic moment (12.25 B.M.), and indicative of moderate antiferromagnetic coupling between cobalt centers. Dance also reported antiferromagnetic coupling between the Co centers in [NMe4]2[Co4(SPh)10] (average J = -17 cm -1 ). 54 Finally, the magnetization curve M vs. H is linear, implying that complex 1 is a simple paramagnet (Figure 3), and shows no hysteresis at any temperature. We also endeavored to examine the effect of reaction stoichiometry on the formation of 1. The reaction of CoCl2•1.5THF with 1 equiv of NaSR still results in the formation of 1, but with a significantly reduced yield (ca. 16%). Similarly, reaction of CoCl2•1.5THF with 2 equiv of NaSR (Figure S7) resulted in the formation of large number of paramagnetic, Co-containing products, including complex 1 (but in insignificant amounts). Not surprisingly, we were unsuccessful in our attempts to isolate any products from reaction mixture. From these experiments, we hypothesize that Cl -must play an important role in directing the self-assembly of 1. Presumably, the use of greater than 1.6 equiv of thiolate per Co results in a deficiency of Cl -, which prevents the assembly of 1 and results in formation of a broad distribution of clusters. Previous workers have also noticed that the speciation of Co(II)-thiolates is highly dependent on reaction stoichiometry. [53][54] We also briefly examined the chemical properties of complex 1. It is soluble in benzene, toluene, and CH2Cl2, but insoluble in MeCN, Et2O, and alkanes. Complex 1 is soluble in THF, but partially decomposes over the course of 5 h, as evidenced by the deposition of a brown solid on standing in this solvent (Figure S6). Attempted dissolution of 1 in py-d5 results in immediate formation of a green solution that contains no resonances assignable to 1, concomitant with deposition of a brown solid (Figure S8). While 1 clearly reacts with pyridine, we have been unable to determine the identity of the product(s) formed.
The reaction of CoCl2 by NaBH4, both in the presence or absence of a passivating ligand, has been studied extensively. [49][50][66][67][68][69][70] In the absence of a passivating ligand, these reductions result in the formation of finely-divided Co(0) (in non-aqueous solvents) or Co2B (under aqueous conditions). 66 In the presence of a passivating ligand, or in the presence of surfactant, the results are more complicated. In one instance, this reaction resulted in the formation of simple Co(II) thiolate complexes, [69][70] while in other cases authentic Co(0) nanoparticles were generated. [49][50][66][67][68] Given this past precedent, as well as our own experiments, we believe that the 2017 synthesis initially resulted in formation of 1, and not Cox(SCH2CH2Ph)m-type nanoclusters, as originally suggested. However, complex 1 then decomposed upon exposure to air and water during work-up, likely generating a mixture of CoxOy(SCH2CH2Ph)m-type clusters. Consistent with this hypothesis, exposure of complex 1 to air, as a CH2Cl2 solution, results in a color change from deep brown to coral. A UV-vis spectrum of this solution features absorptions at 404, 493, and 611 nm (Figure S19). These values are very similar to those reported in 2017 for Cox(SCH2CH2Ph)m, demonstrating that the original material requires O2 for its formation, and is therefore unlikely to contain any Co(0) character.
We have re-examined the synthesis of thiolate-protected cobalt APNCs by reaction of CoCl2 with NaBH4 and PhCH2CH2SH. Despite efforts to faithfully reproduce the reported procedure, we are unable to detect the presence of a cobalt(0)-containing APNC. Instead, we isolated the intriguing Co(II) cluster, [Co10(SR)16Cl4]. This complex represents the first example of a thiolate-protected Co(II) T3 supertetrahedron. We believe that [Co10(SR)16Cl4] was also being formed in the original synthesis; however, the cluster likely reacted with oxygen and water during work-up, giving a mix of CoxOy(SCH2CH2Ph)m-type clusters. This result highlights the challenges inherent in the generation of low-valent cobalt nanoclusters, including the need for rigorous exclusion of air during their synthesis, work-up, and characterization.