A review of complexes of ambivalent and ambiphilic Lewis acid/bases with symmetry signatures and an alternative notation for these non-innocent ligands

Recent advances on the development of NO-releasing molecules (NORMs) for biomedical applications

Nitric oxide (NO) is an important biological messenger as well as a signaling molecule that participates in a broad range of physiological events and therapeutic applications in biological systems. However, due to its very short half-life in physiological conditions, its therapeutic applications are restricted. Efforts have been made to develop an enormous number of NO-releasing molecules (NORMs) and motifs for NO delivery to the target tissues. These NORMs involve organic nitrate, nitrite, nitro compounds, transition metal nitrosyls, and several nanomaterials. The controlled release of NO from these NORMs to the specific site requires several external stimuli like light, sound, pH, heat, enzyme, etc. Herein, we have provided a comprehensive review of the biochemistry of nitric oxide, recent advancements in NO-releasing materials with the appropriate stimuli of NO release, and their biomedical applications in cancer and other disease control.

Coordinatively Unsaturated Nickel Nitroxyl Complex: Structure, Physicochemical Properties, and Reactivity toward Dioxygen

For its important roles in biology, nitrogen monoxide (·NO) has become one of the most studied and fascinating molecules in chemistry. ·NO itself acts as a "noninnocent" or "redox active" ligand to transition metal ions to give metal-NO (M-NO) complexes. Because of this uncertainty due to redox chemistry, the real description of the electronic structure of the M-NO unit requires extensive spectroscopic and theoretical studies. We previously reported the Ni-NO complex with a hindered N3 type ligand [Ni(NO)(L3)] (L3- denotes hydrotris(3-tertiary butyl-5-isopropyl-1-pyrazolyl)borate anion), which contains a high-spin (hs) nickel(II) center and a coordinated 3NO-. This complex is very stable toward dioxygen due to steric protection of the nickel(II) center. Here, we report the dioxygen reactivity of a new Ni-NO complex, [Ni(NO)(I)(L1″)], with a less hindered N2 type bis(pyrazolyl)methane ligand, which creates a coordinatively unsaturated ligand environment about the nickel center. Here, L1″ denotes bis(3,5-diisopropyl-1-pyrazolyl)methane. This complex is also described as a hs-nickel(II) center with a bound 3NO-, based on spectroscopic and theoretical studies. Unexpectedly, the reaction of [Ni(NO)(I)(L1″)] with O2 yielded [Ni(κ2-O2N)(L1″)2](I3), with the oxidation of both 3NO- and the I- ion to yield NO2- and I3-. Both complexes were characterized by X-ray crystallography, IR, and UV-Vis spectroscopy and theoretical calculations.

The Biologically Relevant Coordination Chemistry of Iron and Nitric Oxide: Electronic Structure and Reactivity

Nitric oxide (NO) is an important signaling molecule that is involved in a wide range of physiological and pathological events in biology. Metal coordination chemistry, especially with iron, is at the heart of many biological transformations involving NO. A series of heme proteins, nitric oxide synthases (NOS), soluble guanylate cyclase (sGC), and nitrophorins, are responsible for the biosynthesis, sensing, and transport of NO. Alternatively, NO can be generated from nitrite by heme- and copper-containing nitrite reductases (NIRs). The NO-bearing small molecules such as nitrosothiols and dinitrosyl iron complexes (DNICs) can serve as an alternative vehicle for NO storage and transport. Once NO is formed, the rich reaction chemistry of NO leads to a wide variety of biological activities including reduction of NO by heme or non-heme iron-containing NO reductases and protein post-translational modifications by DNICs. Much of our understanding of the reactivity of metal sites in biology with NO and the mechanisms of these transformations has come from the elucidation of the geometric and electronic structures and chemical reactivity of synthetic model systems, in synergy with biochemical and biophysical studies on the relevant proteins themselves. This review focuses on recent advancements from studies on proteins and model complexes that not only have improved our understanding of the biological roles of NO but also have provided foundations for biomedical research and for bio-inspired catalyst design in energy science.

Noncovalent Axial I⋅⋅⋅Pt⋅⋅⋅I Interactions in Platinum(II) Complexes Strengthen in the Excited State

Coordination compounds of platinum(II) participate in various noncovalent axial interactions involving metal center. Weakly bound axial ligands can be electrophilic or nucleophilic; however, interactions with nucleophiles are compromised by electron density clashing. Consequently, simultaneous axial interaction of platinum(II) with two nucleophilic ligands is almost unprecedented. Herein, we report structural and computational study of a platinum(II) complex possessing such intramolecular noncovalent I⋅⋅⋅Pt⋅⋅⋅I interactions. Structural analysis indicates that the two iodine atoms approach the platinum(II) center in a "side-on" fashion and act as nucleophilic ligands. According to computational studies, the interactions are dispersive, weak and anti-cooperative in the ground electronic state, but strengthen substantially and become partially covalent and cooperative in the lowest excited state. Strengthening of I⋅⋅⋅Pt⋅⋅⋅I contacts in the excited state is also predicted for the sole previously reported complex with analogous axial interactions.

Thiolate-Thione Redox-Active Ligand with a Six-Membered Chelate Ring via Template Condensation and Its Pt(II) Complexes

A new template condensation reaction has been discovered in a mixture of Pt(II), thiobenzamide, and base. Four complexes of the general form [Pt(ctaPh^R)₂], R = CH₃ (1a), H (1b), F (1c), Cl (1d), cta = condensed thioamide, have been prepared under similar conditions and thoroughly characterized by ¹H NMR and UV-vis-NIR spectroscopy, (spectro)electrochemistry, elemental analysis, and single-crystal X-ray diffraction. The ligand is redox active and can be reduced from the initial monoanion to a dianionic and then trianionic state. Chemical reduction of 1a with [Cp₂Co] yielded [Cp₂Co]₂[Pt(ctaPh^CH3)₂], [Cp₂Co]₂[1a], which has been similarly characterized with the addition of EPR spectroscopy and SQUID magnetization. The singly reduced form containing [1a]^1-, (nBu₄N)[Pt(ctaPh^CH3)₂], has been generated in situ and characterized by UV-vis and EPR spectroscopies. DFT studies of 1b, [1b]^1-, and [1b]^2- confirm the location of additional electrons in exclusively ligand-based orbitals. A detailed analysis of this redox-active ligand, with emphasis on the characteristics that favor noninnocent behavior in six-membered chelate rings, is included.

Lewis Acidic PSbP Pincer Ligand in Pt-Catalyzed 1,6-Enyne Cycloisomerization: A Theoretical Study

The progressively improved heterobimetallic antimony transition metal complex PSbP-Pt (I1) provides superior activity in catalyzed 1,6-enyne cycloisomerization. Our DFT calculations demonstrate that the noninnocent character of the antimony ligand enhances the self-activation of the catalyst precursor through a substrate-aided intramolecular chloride migration, which triggers subsequent reaction. Designed alternative redox noninnocent active species with strong electron-withdrawing groups also show promising catalytic ability due to an electron-deficient antimony ligand, which lowers the typical reaction barrier for the cycloisomerization of 1,6-enyne.

Synthesis, structure and palladium coordination of ambiphilic, pyridine- and phosphine-tethered N-boryl imine ligands

New classes of ambiphilic ligands incorporating N-boryl imines can be generated in two steps from aldehydes, and their structure modulated by the ligand motif.

Multicomponent Mechanochemical Synthesis of Cyclopentadienyl Titanium tert-Butoxy Halides, Cp x TiX y (O t Bu)4-(x+y) (x, y = 1, 2; X = Cl, Br)

Titanium tert-butoxy halides of the formula Cp _x TiX _y (O ^t Bu)_4-(x+y) (x, y = 1, 2; X = Cl, Br) have been prepared thorough milling the reagents without solvent. In the case of the chloride derivatives, Cp₂TiCl₂ is used as a starting material; in the case of the bromides, a mixture of LiCp, TiBr₄, and Li[O ^t Bu] is used. The stoichiometric ratios of the starting materials are reflected in the major products of the reactions. Single-crystal X-ray structures are reported for Cp₂TiCl(O ^t Bu), Cp₂TiBr(O ^t Bu), and CpTiBr₂(O ^t Bu), as well as for Cp₂TiCl(O ⁱ Pr) and a redetermination of Cp₂TiCl(OMe). The tert-butoxy derivatives are notable for their nearly linear Ti-O-C angles (>170°) that reflect Ti-O π-bonding, an interpretation supported with density functional theory calculations.

From Conventional Lewis Acids to Heterogeneous Montmorillonite K10: Eco-Friendly Plant-Based Catalysts Used as Green Lewis Acids

The concept of green chemistry began in the USA in the 1990s. Since the publication of the 12 principles of this concept, many reactions in organic chemistry have been developed, and chemical products have been synthesized under environmentally friendly conditions. Lewis acid mediated synthetic transformations are by far the most numerous and best studied. However, the use of certain Lewis acids may cause risks to environmental and human health. This Review discusses the evolution of Lewis acid catalyzed reactions from a homogeneous liquid phase to the solid phase to yield the expected organic molecules under green, safe conditions. In particular, recent developments and applications of biosourced catalysts from plants are highlighted.

The mechanism of directed Ni(ii)-catalyzed C-H iodination with molecular iodine

This computational study reveals electrophilic cleavage pathways for substrates with N,N-bidentate directing centers in Ni(ii)-catalyzed C–H iodination with molecular iodine.

The density functional theory method is used to elucidate the elementary steps of Ni(ii)-catalyzed C(sp²)–H iodination with I₂ and substrates bearing N,N′-bidentate directing centers, amide-oxazoline (AO) and 8-aminoquinoline (AQ). The relative stability of the lowest energy high- and low-spin electronic states of the catalyst and intermediates is found to be an important factor for all of the steps in the reaction. As a result, two-state reactivity for these systems is reported, where the reaction is initiated on the triplet surface and generates a high energy singlet nickelacycle. It is shown that the addition of Na₂CO₃ base to the reaction mixture facilitates C–H activation. The presence of I₂ in the reaction provides the much needed driving force for the C–H activation and nickelacycle formation and ultimately reacts to form a new C–I bond through either a redox neutral electrophilic cleavage (EC) pathway or a one-electron reductive cleavage (REC) pathway. The previously proposed Ni(ii)/Ni(iv) and homolytic cleavage pathways are found to be higher in energy. The nature of the substrate is found to have a large impact on the relative stability of the lowest electronic states and on the stability of the nickelacycle resulting from C–H activation.

Tuning of intramolecular charge transfer properties and charge distributions in ferrocene-appended catechol derivatives by chemical substitution

In this study, we report intramolecular charge transfer (ICT) properties and charge distributions in a series of FcC derivatives (FcC = 4-ferrocenylcatecholate where Fc = ferrocene and C = catecholate). This series consists of a previously reported complex FcV (4-ferrocenylveratrole) and newly synthesized complexes FcA (4-ferrocenylcatechol bis(acetate) and Pt((t)Bu2bpy)(FcC) ((t)Bu2bpy = 4,4'-di-tert-butyl-2,2'-dipyridyl). An electrochemical analysis of Pt((t)Bu2bpy)(FcC) using cyclic voltammetry revealed two well-defined, reversible waves which were assigned to the sequential oxidation of the Pt((t)Bu2bpy)(C) and Fc moieties. The potential splitting between the waves (524 mV) indicated that there was an electronic interaction between both moieties. ICT property and charge distribution of [Pt((t)Bu2bpy)(FcC)]˙(+) were rationalized by comparison with the [FcV]˙(+) and [FcA]˙(+) (4-ferrocenylcatechol bis(acetate)). DFT calculations and UV-vis-NIR spectroscopy revealed that [Pt((t)Bu2bpy)(FcC)]˙(+), [FcV]˙(+), and [FcA]˙(+) were ferrocenium (Fc(+))-centered rather than semiquinone ligand-centered and that these complexes exhibited ICT transition bands from the catechol-derivatized framework to the Fc(+) moiety in the near infrared (NIR) region. Both the electronic coupling parameter (HAB) and delocalization parameter (α) increased in value as the electron-donating strength of the substituent groups in the catechol-derivatized framework increased (OCOCH3 ([FcA]˙(+)) < OCH3 ([FcV]˙(+)) < O(-) ([Pt((t)Bu2bpy)(FcC)]˙(+))). The electronic interactions between the organometallic center and the non-innocent framework were tuned by changing the substituents. The potential energy surfaces of the Fc(+) derivatives, obtained using two-state Marcus-Hush theory, can be modulated by changing the energy level of the molecular orbitals of the appended catechol-derivatized moieties.

Activation of Strong Boron-Fluorine and Silicon-Fluorine σ-Bonds: Theoretical Understanding and Prediction

The oxidative addition of BF3 to a platinum(0) bis(phosphine) complex [Pt(PMe3)2] (1) was investigated by density functional calculations. Both the cis and trans pathways for the oxidative addition of BF3 to 1 are endergonic (ΔG°=26.8 and 35.7 kcal mol(-1), respectively) and require large Gibbs activation energies (ΔG°(≠)=56.3 and 38.9 kcal mol(-1), respectively). A second borane plays crucial roles in accelerating the activation; the trans oxidative addition of BF3 to 1 in the presence of a second BF3 molecule occurs with ΔG°(≠) and ΔG° values of 10.1 and -4.7 kcal mol(-1), respectively. ΔG°(≠) becomes very small and ΔG° becomes negative. A charge transfer (CT), F→BF3, occurs from the dissociating fluoride to the second non-coordinated BF3. This CT interaction stabilizes both the transition state and the product. The B-F σ-bond cleavage of BF2Ar(F) (Ar(F)=3,5-bis(trifluoromethyl)phenyl) and the B-Cl σ-bond cleavage of BCl3 by 1 are accelerated by the participation of the second borane. The calculations predict that trans oxidative addition of SiF4 to 1 easily occurs in the presence of a second SiF4 molecule via the formation of a hypervalent Si species.

SO₂--yet another two-faced ligand

Experimentally known adducts of SO2 with transition metal complexes have distinct geometries. In the present paper, we demonstrate by a bonding analysis that this is a direct consequence of sulfur dioxide acting as an acceptor in one set, square-planar complexes of d(8) and linear two-coordinated complexes of d(10) transition metals, and as a donor with other compounds, well-known paddle-wheel [Rh2(O2CCF3)4] and square-pyramidal [M(CO)5] (M = Cr, W) complexes. Bonding energy computations were augmented by the natural bond orbital (NBO) analysis and energy decomposition analysis (EDA). When the SO2 molecule acts as an acceptor, bonding in the bent coordination mode to the axial position of the d(8) or the d(10) metal center, the dominant contributor to the bonding is LAO(S) (Lewis Acidic Orbital, mainly composed of the px-orbital of the S atom) as an acceptor, while a dz(2) orbital centered on the metal is the corresponding donor. In contrast, the distinct collinear (or linear) coordination of the SO2 bound at the axial position of [Rh2(O2CCF3)4] and/or [M(CO)5] is associated with a dominant donation from a lone pair localized on the sulfur atom, σ*(Rh-Rh) and/or empty LAO(M) (mainly composed of the dz(2) orbital of the metal), respectively, acting as an acceptor orbital. The donor/acceptor capabilities of the SO2 molecule were also checked in adducts with organic Lewis acids (BH3, B(CF3)3) and Lewis bases (NH3, N(CH3)3, N-heterocyclic carbene).

Solution and solid-state photoreductive elimination of chlorine by irradiation of a [PtSb](VII) complex

In our search for novel main-group-based redox-active platforms for solar fuel production, we have synthesized Cl2Sb(IV)Pt(III)Cl3(o-dppp)2 (2, o-dppp = o-(Ph2P)C6H4)), a complex featuring a highly oxidized [PtSb](VII) core. This thermally stable complex quickly evolves chlorine upon irradiation with a Xe lamp, leading to [Cl2Sb(IV)Pt(I)Cl(o-dppp)2] (1) as the photoproduct. This photoreduction is very efficient, with a maximum quantum yield of 13.8% when carried out in a 4.4 M solution of 2,3-dimethyl-1,3-butadiene in CH2Cl2. Remarkably, 2 also evolves chlorine when irradiated in the solid state under ambient conditions in the absence of a trap.

Redox and anion exchange chemistry of a stibine-nickel complex: writing the L, X, Z ligand alphabet with a single element

According to the covalent bond classification (CBC) method, two-electron donors are defined as L-type ligands, one-electron donors as X-type ligands, and two-electron acceptors as Z-type ligands. These three ligand functions are usually associated to the nature of the ligating atom, with phosphine, alkyl, and borane groups being prototypical examples of L-, X- and Z-ligands, respectively. A new SbNi platform is reported in which the ligating Sb atom can assume all three CBC ligand functions. Using both experimental and computational data, it is shown that PhICl2 oxidation of (o-(Ph2P)C6H4)3SbNi(PPh3) (1) into [(o-(Ph2P)C6H4)3ClSb]NiCl (2) is accompanied by a conversion of the stibine L-type ligand of 1 into a stiboranyl X-type ligand in 2. Furthermore, the reaction of 2 with the catecholate dianion in the presence of cyclohexyl isocyanide results in the formation of [(o-(Ph2P)C6H4)3(o-O2C6H4Sb)]Ni(CNCy) (4), a complex featuring a nickel atom coordinated by a Lewis acidic, Z-type, stiborane ligand.

Dual descriptor and molecular electrostatic potential: complementary tools for the study of the coordination chemistry of ambiphilic ligands

The possibility to retrieve the coordinating properties of ligands by a combined dual descriptor and molecular electrostatic potential analysis is shown, yielding a potentially predictive tool of their ambiphilicity and selectivity.