Circular dichroism and magnetic circular dichroism spectra of tetrahedral cobalt(II) complexes of thiophenolate, o-xylene-.alpha.,.alpha.'-dithiolate, and L-cysteine-containing oligopeptides

2018

Chem. Mater.

The synthesis and characterization of a CdS-based molecular cluster, [Cd10S4(SPh)16]4– (Cd10), with site-specific substitution of Cd2+ with Mn2+ impurities are reported. The formation of the Cd10 cluster from the smaller [Cd4(SPh)10]2– (Cd4) cluster involves a metastable intermediate cluster, [Cd8S(SPh)16]2– (Cd8), that is detected by electrospray ionization mass spectrometry (ESI-MS). To account for this unexpected intermediate, we propose a complex equilibrium between Cd4, Cd8, and Cd10 exists that we exploit to introduce Mn2+ impurities at both core and surface cation sites of the Cd10 lattice. We demonstrate through two synthetic procedures that differ only in the sequence in which Mn2+ is introduced to the reaction dictates its speciation in the cluster. Introducing dopants at an early stage of the synthesis prevents full conversion of Cd8 to Cd10; however, it yields core doped Cd10 clusters. Addition of Mn2+ ions after the preparation of Cd10 yields only surface doped clusters. The composition of the doped clusters is systematically characterized by ESI-MS and exhibits speciation-dependent peak intensities. Photoluminescence (PL) spectra of the Mn2+-centered 4T1 → 6A1 transition also exhibits significant differences in peak position and PL lifetimes that are consistent with the expected variation in ligand field strength experienced by these two metal sites. However, ESI-MS and PL collected on “aged” samples indicate slow displacement of Mn2+ from core sites. This study provides new insights to the growth mechanism of clusters that remained rather elusive and demonstrates how the cluster surface dynamics and cluster equilibria can be exploited for precise doping of these well-defined semiconductor analogues.

Section: Resultsmentioning

confidence: 99%

Site-Specific Doping of Mn²⁺ in a CdS-Based Molecular Cluster

Kato

2018

Chem. Mater.

“…Previously reported ligand field parameters for Co 2+ in the Co-4 clusters are Dq = −58.8 meV and B = 68.1 meV (|Dq|/B = 0.86), and in the Co-1 cluster are Dq = −50.0 meV and B = 79.7 meV (|Dq|/B = 0.63). 32,47,48 Using eqs 1 and 2, we We also observe a gradual shift in the energy of the visible LMCT transition at ∼2.7 eV toward the analogous LMCT transition in Co-4 (∼2.9 eV) with increasing nominal Co 2+ content (see Supporting Information, Figure S4). These results indicate that Co 2+ is incorporated into the M-4 clusters.…”

Section: Resultsmentioning

confidence: 74%

“…The bands between ∼2.6 and 3.8 eV are assigned as LMCT transitions from ligand-based π-orbitals of the PhS b − and PhS t − to the partially filled t 2 orbitals of the Co 2+ ion based on previous assignment of the spectra of Co-1 and Co-4 clusters. 41,47 The structured band centered at ∼1.8 eV shown on an expanded energy axis in Figure 1 is the 4 A 2 (F) → 4 T 1 (P) ligand field transition of the pseudotetrahedral Co 2+ ion. 41 The energy and fine structure of the 4 T 1 (P) transition is similar for both (Zn 1−x Co x )-4 and (Cd 1−x Co x )-4 clusters, but not with respect to either the pure Co-4 or Co-1 clusters.…”

Section: Resultsmentioning

confidence: 99%

Cation Exchange in Small ZnS and CdS Molecular Analogues

Pittala

2015

Inorg. Chem.

The simplest means of altering the chemistry and electronic structure of any material, from molecular clusters to single crystals, is by the introduction of chemical impurities. We present a systematic study of the cation exchange reaction involving Co 2+ ions with metal benzenethiolate clusters, [M 4 (SPh) 10 ] 2− (M = Zn, Cd), yielding diluted m a g n e ti c c l us t e r s h a v i n g t h e g e n er a l f o r m ul a [(M 1−x Co x ) 4 (SPh) 10 ] 2− . This method allows high concentrations of doping at the molecular level without forming concentrated magnetic clusters such as [Co 4 (SPh) 10 ] 2− . Changes in the electronic structure of the molecular species containing on average <1 Co 2+ per cluster were observed and characterized by a variety of analytical (high-resolution electrospray mass spectrometry) and spectroscopic techniques (electronic absorption including stopped-flow kinetics, luminescence, and paramagnetic 1 H NMR). The mass spectrometry results strongly suggest that the cation exchange reaction with Co 2+ is thermodynamically favored for the [Zn 4 (SPh) 10 ] 2− cluster compared to the [Cd 4 (SPh) 10 ] 2− clusters at room temperature. The rate of the cation exchange is orders of magnitude faster for the [Cd 4 (SPh) 10 ] 2− cluster than for [Zn 4 (SPh) 10 ] 2− and is governed by ligand interconversion processes. This simple room temperature cation exchange into molecular clusters is a model reaction that provides important structural information regarding the effect of Co 2+ doping on the cluster stability.

“…Both terminal and μ-SPh – ligands have stronger ligand fields compared to μ 4 -S 2– . Thus, Co 2+ coordinated to μ 3 -S 2– and μ-SPh – ligands within the Cd 10 core should cause this transition to red-shift in comparison to strictly surface SPh – coordination. − The band shape of the 4 T 1 (P) transition differs vastly among the coordination environments of Co 2+ in Co 1 , Co 4 , and Co:CdS nanocrystals with increased intensity in the highest energy band observed only in the Co 4 cluster and is attributed to bridging SPh – ligands. ,, The observation of the overall slightly red-shifted 4 T 1 (P) transition for the 10% Co:Cd 10 -SA compared to 10% Co:Cd 10 -R in Figure a suggests that the Co 2+ is coordinated to sulfide ligands in the core, which provide a weaker ligand field than either terminal or bridging thiophenolate ligands. The 4 T 1 (P) absorption for Co:Cd 10 -SA is qualitatively more similar to the Co:CdS transition in terms of band shape than Co:Cd 10 -R.…”

Section: Results and Discussionmentioning

confidence: 99%

“…Electronic absorption spectroscopy was used as a dopant-specific probe of the Co 2+ speciation within the Cd 10 clusters as the Co 2+ -centered excited states have been shown previously to be very sensitive to the unique ligand field environments imposed by core versus surface sites. − Figure a displays the electronic absorption spectra of the Co:Cd 10 -SA clusters with increasing dopant concentration. The spectra of all pure and doped Cd 10 clusters are dominated by an intense feature in the UV region that is assigned to the ligand-to-metal charge transfer of the host cluster and intraligand π → π* of the thiophenolate ligands .…”

Section: Results and Discussionmentioning

confidence: 99%

Core-Doped [(Cd_1–xCo_x)₁₀S₄(SPh)₁₆]^4– Clusters from a Self-Assembly Route

Denhardt

2021

Inorg. Chem.

The incorporation of substitutional Co2+ impurities in [Cd10S4(SPh)16]4– (Cd10) molecular clusters prepared by the self-assembly method where Na2S is the sulfur precursor and a redox method where elemental S is the sulfur precursor is studied. The Co2+ ions provide unique spectroscopic and chemical handles to monitor dopant speciation during cluster formation and determine what role, if any, other cluster species play during Cd10 cluster formation. In contrast to the redox method that produces exclusively surface-exchanged Co2+-doped Cd10 (Co:Cd10), the preparation of Cd10 by the self-assembly method in the presence of Co2+ ions results in Co2+ incorporation at both the surface and core sites of the Cd10 cluster. Electrospray ionization mass spectrometry (ESI–MS) analysis of the dopant distribution for the self-assembly synthesis of Co:Cd10 is consistent with a near-Poissonian distribution for all nominal dopant concentrations albeit with reduced actual Co2+ incorporation. At a nominal Co2+ concentration of 50%, we observe incorporation of up to seven Co2+ ions within the Cd10 self-assembled cluster compared to a maximum of only four Co2+ dopants in the Cd10 redox clusters. The observation of up to seven Co2+ dopants must involve substitution of at least three core sites within the Cd10 cluster. Electronic absorption spectra of the Co2+ ligand field transition in the heavily doped Co:Cd10 clusters display clear deviation with the surface-doped Co2+-doped Cd10 clusters prepared by the redox method. We hypothesize that the coordination of Co2+ and S2– ions in solution prior to cluster formation, which is possible only with the self-assembly method, is critical to the doping of Co2+ ions within the Cd10 cores.