Reactivity of Cobalt Clusters Co_n^±/0 with Dinitrogen: Superatom Co₆⁺ and Superatomic Complex Co₅N₆⁺

Abstract

We report a joint experimental and theoretical study on the reactions of cobalt clusters (Co n ±/0 ) with nitrogen using the customized reflection time-of-flight mass spectrometer combined with a 177.3 nm deepultraviolet laser. Comparing to the behaviors of neutral Co n (n = 2−30) and anionic Co n − clusters (n = 7−53) which are relatively inert in reacting with nitrogen in the fast-flow tube, Co n + clusters readily react with nitrogen resulting in adducts of one or multiple N 2 except Co 6 + which stands firm in the reaction with nitrogen. Detailed quantum chemistry calculations, including the energetics, electron occupancy, and orbital analysis, wellexplained the reasonable reactivity of Co n + clusters with nitrogen and unveiled the open-shell superatomic stability of Co 6 + within a highly symmetric (D 3d ) structure. The D 3d Co 6+ bears an electron configuration of a half-filled superatomic 1P orbital (i.e., 1S 2 1P 3 ||1D 0 ), a large α-highest occupied molecular orbital (HOMO)−lowest unoccupied molecular orbital (LUMO) gap, symmetric multicenter bonds, and reasonable electron delocalization pertaining to metallic aromaticity. Topology analysis by atom-in-molecule illustrates the interactions between Co n + and N 2 corresponding to covalent bonds, but the Co−N interactions in cationic Co 2 + N 2 and Co 6 + N 2 clusters are apparently weaker than those in the other systems. In addition, we identify a superatomic complex Co 5 N 6 + which exhibits similar frontier orbitals as the naked Co 5 + cluster, but the alpha HOMO−LUMO gap is nearly double-magnified, which is consistent with the high-abundance peak of Co 5 N 6 + in the experimental observation. The enhanced stability of such a ligand-coordinated superatomic complex Co 5 N 6 + , along with the superatom Co 6 + with aromaticity, sheds light on special and general superatoms.

Plasma‐Assisted Dinitrogen Activation on Small Cobalt Clusters: Co4N9+ with Enhanced Stability

“…Previously published studies based on infrared photodissociation spectroscopy have revealed that the cobalt clusters readily react with N 2 to form Co n N 2 + (n=2–17) [16] . Among the cobalt clusters Co n + (n=2–22), Co 4 + and Co 5 + have been illustrated to exhibit a relatively larger reaction rate with N 2 molecules [14f,17] . However, rare reports showed the formation of odd‐nitrogen cobalt nitrides by far to the best of our knowledge.…”

“…Figure 3 presents the optimized structures of Co 3,4 N m + (m=0–10) clusters (for details see Figures S3–S5). The nascent Co 3 + has the lowest energy structure of the isosceles triangle while Co 4 + is a tetrahedron [14f,19] . Interestingly, the doping of a single N atom in forming Co 3 N 1 + and Co 4 N 1 + exhibits similar N‐capping on a triangular hollow site; whereas the other odd‐N species Co 3 N 2m‐1 + display significant differences with the Co 4 N 2m‐1 + .…”

“…Figure 1 presents the typical mass spectra of pure Co n + (n≤10) clusters in reacting with plasma‐activated N 2 molecules under different laser power densities controlled by the flash‐lamp voltages. Under the relatively low laser power (c.a., 3.0 mJ), diverse small Co n N m + clusters are generated, including Co 1 N 6,8 + , Co 2 N 2 + , Co 3 N 2,4,6 + and Co 4 N 4,6,8 + series (Figure 1a), showing the size‐dependent preference of N 2 adsorption on the Co n + clusters [14f] . Notably, a few odd‐nitrogen products were also observed in this spectrum, such as Co 3 N 7 + , Co 4 N 9 + and Co 5 N 9 + which become dominant in the mass spectra when increasing laser power density (c.a., 12 mJ, corresponding to 470 V flash‐lamp voltage), as shown in Figure 1b.…”

Probing the binding and activation of small molecules by gas-phase transition metal clustersviaIR spectroscopy

“…A quantum chemical analysis of the system reveals a particular stable open-shell superatomic structure of D 3d symmetry for Co 6 + causing the low reactivity. 234 The often observed increase of reactivity at low temperature points to the initial formation of a weakly bound molecular complex. This forms the precursor to the activated irreversible dissociation into atomic N on the cluster surface, alternatively molecular N 2 can be released again from the molecular complex.…”

A stable superatomic Cu6(SMPP)6 nanocluster with dual emission

“…The successful synthesis of such Cu NCs includes Cu 146 Se 73 (PPh 3 ) 30 , 5 Cu 72 Se 36 (PPh 3 ) 20 , 6 Cu 52 Se 26 (PPh 3 ) 16 , 6 Cu 32 Se 16 (PPh 3 ) 12 , 6 some small copper NCs such as Cu 4 NCs, 7,8 Cu 6 NCs, [9][10][11][12][13][14] Cu 8 NCs 14,15 Cu 12 NCs, 16 Cu 13 NCs, 16 Cu 14 NCs, 15,17 Cu 15 NCs, 18 as well as some other copper NCs involving hydrides. [19][20][21][22][23][24][25][26] Among them, the Cu 6 NCs stabilized by various ligands attract the most research interest, such as Cu 6 (MPP) 6 , 14 [Cu 6 (mna) 6 ] 6− , 12 Cu 6 (dmpymt) 6 , 10 Cu 6 (SC 7 H 4 NO) 6 , 13 and Cu 6 (SC 5 H 3 NSiMe 3 ) 6 , 9 allowing such sixatom metal NCs as model systems, [27][28][29][30][31][32][33][34][35] just as Au 25 [SR] 18 , [36][37][38] to fully understand the relationship between structure and property, and to explore stable ones for cluster-genetic materials. [39]…”