Determination of the Ni–Ni Bonding Strength in Metal-String Complexes Using Head-to-Head Nanorods and Electrochemical Surface-Enhanced Raman Spectroscopy

Abstract

We report the bonding strength of nickel ions in trinickel extended metal atom chains (EMACs) and dinickel complexes using Raman, surface-enhanced Raman scattering (SERS), and electrochemical SERS (ECSERS). By using the redox ability of gold and silver nanoparticles during plasmonic excitation, the bonding strength and the valence state of metal ions can be determined. For dinickel complexes, we assign the Raman band at 322 cm–1 to Ni2+–Ni3+ stretch in [Ni2(TPG)4]­BF4 (TPG = N,N′,N″-triphenylguanidinate, [Ni2]5+) and 327 cm–1 for Ni2+–Ni1+ stretch of [Ni2]3+ moieties in Ni5(camnpda)4. For trinickel EMACs, no band is assigned to Ni3 symmetric stretch νNi3 sym in the neutral form Ni3(dpa)4X2 (dpa = dipyridyl amido and X = NCS, Cl). In the reduced form, the ECSERS curves show the band at 242 cm–1, which also appeared at gold nanoparticle SERS measurement, assigned to νNi3 sym for [Ni3]5+ core. The trinickel complexes were reduced by gold nanosphere, and this νNi3 sym band is further enhanced with SERS measurements when gold nanorods were used and the trinickel EMACs served as bridging compounds on both ends. On increasing the applied voltage in ECSERS to +1.3 V, complexes were oxidized and one additional band at 351 cm–1 appeared. This new band is assigned to νNi3 sym of [Ni3]7+ in [Ni3(dpa)4X2]+. Great vibrational frequency indicates that one electron from the metal σ* orbital instead of ligand was removed, leading to a three metal center bond. Distinct from the vibrational band wavenumber obtained in dinickel complexes, we confirm that [Ni3]5+,7+ has delocalized electronic structures.

Facet-Dependent Reduction Reaction of Diruthenium Metal–String Complexes by Face-to-Face Linked Gold Nanocrystals

“…For rods of size 12.1 ± 0.7 × 29.4 ± 2.7 nm 2 aspect ratio 2.4 ± 0.2, the longitudinal plasmonic mode lies at 627 nm and is red-shifted to 772 nm when connected by complexes. Long chains are depicted in the SEM images; the complexes anchor on both ends of the AuNRs by chlorides in solution, forming head-to-head chains. , Ying et al reported that metallaynes with sulfide moieties can serve as connectors for gold nanoparticle assemblies . Orendorff and Murphy reported that AuNR has {110}/{100} facets on the sides and {100}/{111}­on the heads, and the bridging molecules mostly anchor on the {111} facets .…”

“…The absorption during the redox processes can be recorded in situ to assist in assigning the electronic structures. 19,20 Diruthenium possesses high polarizability; thus, intense Raman signal is expected. This provides SERS detection with great sensitivity.…”

“…Because of complications in this metal–string molecular system, electrochemical SERS (ECSERS), which combines SERS and an electrochemical technique, is used to obtain the Ru–Ru bonding strength and then to identify their valence states. The absorption during the redox processes can be recorded in situ to assist in assigning the electronic structures. , Diruthenium possesses high polarizability; thus, intense Raman signal is expected. This provides SERS detection with great sensitivity.…”

Study of Electronic and Vibrational Structures of Reduced, Neutral, and Oxidized Ni3(dpa)4X2 Using Density Functional Theory and Raman Spectroscopy

“…Wu et al employed an electrochemical surface-enhanced Raman scattering (ECSERS) technique and assigned a new band at 350 cm –1 , appearing from oxidation of Ni 3 (dpa) 4 Cl 2 under applied voltages, to the symmetric stretching mode of Ni 3 υ Ni 3 ,sym of the [Ni 3 ] 7+ core. In addition, the reduction potential of Ni 3 (dpa) 4 X 2 is smaller than the oxidation potential of gold nanoparticles (AuNPs); , the complex was reduced to [Ni 3 ] 5+ when simply attached to gold nanoparticles.…”

“…Likewise, Kitagawa et al 23 examined the exchange constants, electronic structures, magnetic properties, and electron conductivities of Ni 5 (tdpa) 4 Cl 2 (tdpa = tripyridyldiamido) and Ni 7 (teptra) 4 Cl 2 (teptra = tetrapyridyltriamido) with the B3LYP functional. Wu et al 24 employed an electrochemical surface-enhanced Raman scattering (ECSERS) technique and assigned a new band at 350 cm −1 , appearing from oxidation of Ni 3 (dpa) 4 Cl 2 under applied voltages, to the symmetric stretching mode of Ni 3 υ Ni 3 ,sym of the [Ni 3 ] 7+ core. In addition, the reduction potential of Ni 3 (dpa) 4 X 2 is smaller than the oxidation potential of gold nanoparticles (AuNPs); 18,25 the complex was reduced to [Ni 3 ] 5+ when simply attached to gold nanoparticles.…”