Multinuclear Cu(I) Clusters Featuring a New Triply Bridging Coordination Mode of Phosphaamidinate Ligands

Research on Synthesis, Structure, and Catalytic Performance of Tetranuclear Copper(I) Clusters Supported by 2-Mercaptobenz-zole-Type Ligands

Tetrahedral copper(I) clusters [Cu4(MBIZ)4(PPh3)2] (2), [Cu4(MBOZ)4(PPh3)4] (6) (MBIZ = 2-mercaptobenzimidazole, MBOZ = 2-mercaptobenzoxazole) were prepared by regulation of the copper-thiolate clusters [Cu6(MBIZ)6] (1) and [Cu8(MBOZ)8I]- (5) with PPh3. With the presence of iodide anion, the regulation provided the iodide-containing clusters [CuI4(MBIZ)3(PPh3)3I] (3) and [CuI4(MBOZ)3(PPh3)3I] (7). The cyclic voltammogram of 3 in MeCN (0.1 M nBu4NPF6, 298 K) at a scan rate of 100 mV s-1 shows two oxidation processes at Epa = +0.11 and +0.45 V with return waves observed at Epc = +0.25 V (vs. Fc+/Fc). Complex 3 has a higher capability to lose and gain electrons in the redox processes than complexes 2, 4, 4', 6, and 7. Its thermal stability was confirmed by thermogravimetric analysis. The catalytic performance of 3 was demonstrated by the catalytic transformation of iodobenzenes to benzonitriles using AIBN as the cyanide source. The nitrile products show potential applications in the preparation of 1,3,5-triazine compounds for organic fluorescence materials.

Luminescent Tetranuclear Copper(I) and Gold(I) Heterobimetallic Complexes: A Phosphine Acetylide Amidinate Orthogonal Ligand Framework for Selective Complexation

The synthesis of phosphine acetylide amidinate stabilized copper(I) and gold(I) heterobimetallic complexes was achieved by reacting ligand [{Ph2PC≡CC(NDipp)2}Li(thf)3] (Dipp=2,6-N,N'-diisopropylphenyl) with CuCl and Au(tht))Cl, yielding the eight membered ring [{Ph2PC≡CC(NDipp)2}2Cu2] and the twelve membered ring [{Ph2PC≡CC(NDipp)2}2Au2]. {Ph2PC≡CC(NDipp)2}2Cu2] features a Cu2 unit, which is bridged by two amidinate ligands, served as a metalloligand to synthesize the heterobimetallic CuI/AuI complexes [{(AuX)Ph2PC≡CC(NDipp)2}2Cu2] (X=Cl, C6F5). In these reactions, the central ring structure is retained. In contrast, when the twelve membered ring [{Ph2PC≡CC(NDipp)2}2Au2] was reacted with CuX (X=Cl, Br, I and Mes), the reaction led to the rearrangement of the central ring structure to give [{(AuX)Ph2PC≡CC(NDipp)2}2Cu2] (X=Cl, Br, I and Mes), which feature the same the eight membered Cu2 ring as above. These compounds were also synthesized by a one-pot reaction. The luminescent heterobimetallic complexes were further investigated for their photophysical properties.

Coordination chemistry of the CuZ site in nitrous oxide reductase and its synthetic mimics

Atmospheric nitrous oxide (N2O) has garnered significant attention recently due to its dual roles as an ozone depletion agent and a potent greenhouse gas. Anthropogenic N2O emissions occur primarily through agricultural disruption of nitrogen homeostasis causing N2O to build up in the atmosphere. The enzyme responsible for N2O fixation within the geochemical nitrogen cycle is nitrous oxide reductase (N2OR), which catalyzes 2H+/2e- reduction of N2O to N2 and H2O at a tetranuclear active site, CuZ. In this review, the coordination chemistry of CuZ is reviewed. Recent advances in the understanding of biological CuZ coordination chemistry is discussed, as are significant breakthroughs in synthetic modeling of CuZ that have emerged in recent years. The latter topic includes both structurally faithful, synthetic [Cu4(µ4-S)] clusters that are able to reduce N2O, as well as dicopper motifs that shed light on reaction pathways available to the critical CuI-CuIV cluster edge of CuZ. Collectively, these advances in metalloenzyme studies and synthetic model systems provide meaningful knowledge about the physiologically relevant coordination chemistry of CuZ but also open new questions that will pose challenges in the near future.

Computational Comparative Analysis of Small Atomically Precise Copper Clusters

Atomically precise copper clusters (APC) have attracted attention for their promise in sensing, water remediation, and electrochemical technologies. However, smaller-sized APCs and the evolution of their properties as a function of size and composition are not clearly understood. Here, we have performed an investigation into the electronic structure, geometry, and optical properties of small atomically precise copper clusters using density functional theory (DFT) and time-dependent DFT. Through comparative analysis, we show that the electronic structures of the experimentally characterized clusters, Cu₄(PN(C₆H₅)₂CH)₄ and Cu₄(SN₂C₇H₁₁)₄, are similar with the closed-shell superatom character 1S²1P². By changing the ligand on Cu₄(PN(C₆H₅)₂CH)₄ and Cu₄(SN₂C₇H₁₁)₄, there were no major changes observed in the tetrahedral Cu₄ core geometry, electronic structure, or optical spectra. However, a change in the anchor atom causes an increase in the electronic gap and induces a hypochromic shift in the onset peak in the optical spectrum of the small clusters. Increasing the copper core size showed small changes Cu-Cu bond lengths, lower electronic gap values, and a bathochromic shift in the optical spectra. Computational results not only provide detailed physical insight into APCs but also aid in identifying compound compositions of small atomically precise nanoclusters from data collected in the experiment.

Solid-phase synthesis and photoactivity of Ru-polypyridyl visible light chromophores bonded through carbon to semiconductor surfaces

1,10-Phenanthroline (phen) was grafted to either indium tin oxide (ITO), fluorine-doped tin oxide (FTO), or titanium dioxide (TiO₂) semiconductors (SC's) by electrochemical reduction of 5-diazo-phen. The phen ligand is bonded to the semiconductor at C5, and it can be handled in air. The semiconductor-phen (SC-phen) complexes displace both CH₃CN ligands from either cis-[Ru(Mebipy)₂(CH₃CN)₂]²⁺ (Mebipy = 4,4'-methyl-2,2'-bipyridine), cis-[Ru(^tBubipy)₂(CH₃CN)₂]²⁺ (^tBubipy = 4,4'-tert-butyl-2,2'-bipyridine), or cis-[Ru(pheno)(bipy)(CH₃CN)₂]²⁺ (bipy = 2,2'-bipyridine; pheno = 1,10-phenanthroline-5,6-dione) dissolved in DCM/THF (4 h, 70 °C) to form the corresponding surface-bound SC-[(phen)Ru(bipyridyl)₂]²⁺ chromophores. The identities of the SC-[(phen)Ru(Mebipy)₂]²⁺, SC-[(phen)Ru(^tBubipy)₂]²⁺, and SC-[(phen)Ru(pheno)(bipy)]²⁺ (SC = ITO, FTO or TiO₂) chromophores were confirmed by X-ray photoelectron spectroscopy (XPS); inductively coupled plasma mass spectrometry (ICP-MS); UV-vis and reflectance infrared spectroscopies; and cyclic voltammetry (CV). The data were compared to analogous Ru-polypyridyl control compounds dissolved in solution. A facile ketone-amine condensation solid-phase synthesis reaction between SC-[(phen)Ru(pheno)(bipy)]²⁺ and [Ru(1,10-phenthroline-5,6-diamine)(bipy)₂]²⁺ in ethanol (80 °C, 1 h) formed the dinuclear, bound chromophore SC-[(phen)(bipy)Ru(tpphz)Ru(bipy)₂]⁴⁺ (tpphz = tetrapyrido[3,2-a:2',3'-c:3'',2''-h:2''',3'''-j]phenazine). Photoelectrochemical oxidation of hydroquinone and triethylamine under acidic, neutral, or basic conditions showed that the SC-chromophore photoanodes are active, and that TiO₂-[(phen)Ru(Mebipy)₂]²⁺ is the most active and stable under basic- and neutral conditions. The dinuclear chromophore SC-[(phen)(bipy)Ru(tpphz)Ru(bipy)₂]⁴⁺ was most active and stable under potentiostatic conditions in acid.

Impact of Electronic and Steric Changes of Ligands on the Assembly, Stability, and Redox Activity of Cu4(μ4-S) Model Compounds of the CuZ Active Site of Nitrous Oxide Reductase (N2OR)

Model compounds have been widely utilized in understanding the structure and function of the unusual Cu₄(μ₄-S) active site (Cu_Z) of nitrous oxide reductase (N₂OR). However, only a limited number of model compounds that mimic both structural and functional features of Cu_Z are available, limiting insights about Cu_Z that can be gained from model studies. Our aim has been to construct Cu₄(μ₄-S) clusters with tailored redox activity and chemical reactivity via modulating the ligand environment. Our synthetic approach uses dicopper(I) precursor complexes (Cu₂L₂) that assemble into a Cu₄(μ₄-S)L₄ cluster with the addition of an appropriate sulfur source. Here, we summarize the features of the ligands L that stabilize precursor and Cu₄(μ₄-S) clusters, along with the alternative products that form with inappropriate ligands. The precursors are more likely to rearrange to Cu₄(μ₄-S) clusters when the Cu(I) ions are supported by bidentate ligands with 3-atom bridges, but steric and electronic features of the ligand also play crucial roles. Neutral phosphine donors have been found to stabilize Cu₄(μ₄-S) clusters in the 4Cu(I) oxidation state, while neutral nitrogen donors could not stabilize Cu₄(μ₄-S) clusters. Anionic formamidinate ligands have been found to stabilize Cu₄(μ₄-S) clusters in the 2Cu(I):2Cu(II) and 3Cu(I):1Cu(II) states, with both the formation of the dicopper(I) precursors and subsequent assembly of clusters being governed by the steric factor at the ortho positions of the N-aryl substituents. Phosphaamidinates, which combine a neutral phosphine donor and an anionic nitrogen donor in the same ligand, form multinuclear Cu(I) clusters unless the negative charge is valence-trapped on nitrogen, in which case the resulting dicopper precursor is unable to rearrange to a multinuclear cluster. Taken together, the results presented in this study provide design criteria for successful assembly of synthetic model clusters for the Cu_Z active site of N₂OR, which should enable future insights into the chemical behavior of Cu_Z.