Isocyanide Ligands Research Articles

(Invited) Photophysical Properties and Photodynamics in CdSe Quantum Dots as Photocatalysts and Photosensitizers

Interactions between dye molecules and the semiconductor quantum dots (QDs) has a strong effect on both solar-to-electrical and solar-to-chemical energy conversion processes. In both types of applications, the charge transfer from the photoexcited QD to the molecule play a crucial role. Applying DFT-based non-adiabatic molecular dynamics, we have revealed ultrafast hole transfer (~10 fs) from the CdSe QD to the Ru(II) dye, following by a slow component (~100 fs) associated with the redistribution of population throughout QD states. The ultrafast interfacial QD-to-dye hole transfer is rationalized by strong non-adiabatic couplings between the QD surface states and the high frequency vibrational modes of isocyanide ligands in the dye. We also have studied the effect of the complex heterostructures of Janus QDs, where a half of the QD is made from CdSe and another half is from PbSe forming an interface along a specific crystalline direction. Our calculations show that the Pb-enriched (111) crystalline direction of the interface enhances optical response of the QDs. The hole relaxation is much slower than the electron relaxation, followed by the hole transfer to the dye. The hole transfer is sensitive to the polarity of the environment, rather than the ways of dye’s attachment to the surface. We also found that in nonstoichiometric QDs, a loss of a passivating ligand affects the electronic structure in the same way as injection of an electron in Cd-rich QDs or hole injection in Se-rich QDs. Moreover, electron injection to the Se-rich QDs results in highly optically active redshifted transitions associated with the introduced charge. Overall, our calculations provide atomistic insights into QD-to-molecule charge transfer, giving researchers an additional handle for enhancing QD photophysical properties by means of the QD stoichiometry, interface structure, and QD-molecule engineering.

Long Excited-State Lifetimes in Three-Coordinate Copper(I) Complexes via Triplet-Triplet Energy Transfer to Pyrene-Decorated Isocyanides.

There has been much effort to improve excited-state lifetimes in photosensitizers based on earth-abundant first-row transition metals. Copper(I) complexes have gained significant attention in this field, and in most cases, sterically driven approaches are used to optimize their lifetimes. This study presents a series of three-coordinate copper(I) complexes (Cu1-Cu3) where the excited-state lifetime is extended by triplet-triplet energy transfer. The heteroleptic compounds feature a cyclohexyl-substituted β-diketiminate (CyNacNacMe) paired with aryl isocyanide ligands, giving the general formula Cu(CyNacNacMe)(CN-Ar) (CN-dmp = 2,6-dimethylphenyl isocyanide for Cu1; CN-pyr = 1-pyrenyl isocyanide for Cu2; CN-dmp-pyr = 2,6-dimethyl-4-(1-pyrenyl)phenyl isocyanide for Cu3). The nature, energies, and dynamics of the low-energy triplet excited states are assessed with a combination of photoluminescence measurements at room temperature and 77 K, ultrafast transient absorption (UFTA) spectroscopy, and DFT calculations. The complexes with the pyrene-decorated isocyanides (Cu2 and Cu3) exhibit extended excited-state lifetimes resulting from triplet-triplet energy transfer (TTET) between the short-lived charge-transfer excited state (3CT) and the long-lived pyrene-centered triplet state (3pyr). This TTET process is irreversible in Cu3, producing exclusively the 3pyr state, and in Cu2, the 3CT and 3pyr states are nearly isoenergetic, enabling reversible TTET and long-lived 3CT luminescence. The improved photophysical properties in Cu2 and Cu3 result in improvements in activity for both photocatalytic stilbene E/Z isomerization via triplet energy transfer and photoredox transformations involving hydrodebromination and C-O bond activation. These results illustrate that the extended excited-state lifetimes achieved through TTET result in newly conceived photosynthetically relevant earth-abundant transition metal complexes.

Synthesis and Structure of Copper(I) Halide Complexes with Halogen-Functionalized Isocyanide Ligands

Eleven copper(I) halide complexes [CuX(CNR)3] (X = Cl, Br; R = C6H3-2,6-Cl2, C6H3-2-Cl-6-Me, C6H3-2,6-Br2, C6H3-2-Br-6-Me; X = I, R = C6H3-2,6-Cl2, C6H3-2-Cl-6-Me, C6H3-2-Br-6-Me) bearing halogen-functionalized aromatic isocyanide ligands were synthesized. The compounds were characterized by atomic emission spectroscopy (Cu), IR, 1H NMR spectroscopy and high-resolution mass spectrometry. The structure of nine compounds was determined by X-ray diffraction. Structural studies allow to reveal that the supramolecular structure of the complexes is based on weak C–H···X (X = Cl, Br, I) interactions and π-stacking of the aromatic rings of isocyanide ligands.

Molecular-Scale Visualization of Steric Effects of Ligand Binding to Reconstructed Au(111) Surfaces.

Direct imaging of single molecules at nanostructured interfaces is a grand challenge with potential to enable new, precise material architectures and technologies. Of particular interest are the structural morphology and spectroscopic signatures of the adsorbed molecule, where modern probes are only now being developed with the necessary spatial and energetic resolution to provide detailed information at the molecule-surface interface. Here, we directly characterize the adsorption of individual m-terphenyl isocyanide ligands on a reconstructed Au(111) surface through scanning tunneling microscopy and inelastic electron tunneling spectroscopy. The site-dependent steric pressure of the various surface features alters the vibrational fingerprints of the m-terphenyl isocyanides, which are characterized with single-molecule precision through joint experimental and theoretical approaches. This study provides molecular-level insights into the steric-pressure-enabled surface binding selectivity as well as its effect on the chemical properties of individual surface-binding ligands.

Platinum(0)-η2-1,2-(E)ditosylethene Complexes Bearing Phosphine, Isocyanide and N-Heterocyclic Carbene Ligands: Synthesis and Cytotoxicity towards Ovarian and Breast Cancer Cells.

A wide range of platinum(0)-η2-(E)-1,2-ditosylethene complexes bearing isocyanide, phosphine and N-heterocyclic carbene ancillary ligands have been prepared with high yields and selectivity. All the novel products underwent thorough characterization using spectroscopic techniques, including NMR and FT-IR analyses. Additionally, for some compounds, the solid-state structures were elucidated through X-ray diffractometry. The synthesized complexes were successively evaluated for their potential as anticancer agents against two ovarian cancer cell lines (A2780 and A2780cis) and one breast cancer cell line (MDA-MB-231). The majority of the compounds displayed promising cytotoxicity within the micromolar range against A2780 and MDA-MB-231 cells, with IC50 values comparable to or even surpassing those of cisplatin. However, only a subset of compounds was cytotoxic against cisplatin-resistant cancer cells (A2780cis). Furthermore, the assessment of antiproliferative activity on MRC-5 normal cells revealed certain compounds to exhibit in vitro selectivity. Notably, complexes 3d, 6a and 6b showed low cytotoxicity towards normal cells (IC50 > 100 µM) while concurrently displaying potent cytotoxicity against cancer cells.

Controlling the Photophysical Properties of a Series of Isostructural d6 Complexes Based on Cr0, MnI, and FeII.

Development of first-row transition metal complexes with similar luminescence and photoredox properties as widely used RuII polypyridines is attractive because metals from the first transition series are comparatively abundant and inexpensive. The weaker ligand field experienced by the valence d-electrons of first-row transition metals challenges the installation of the same types of metal-to-ligand charge transfer (MLCT) excited states as in precious metal complexes, due to rapid population of energetically lower-lying metal-centered (MC) states. In a family of isostructural tris(diisocyanide) complexes of the 3d6 metals Cr0, MnI, and FeII, the increasing effective nuclear charge and ligand field strength allow us to control the energetic order between the 3MLCT and 3MC states, whereas pyrene decoration of the isocyanide ligand framework provides control over intraligand (ILPyr) states. The chromium(0) complex shows red 3MLCT phosphorescence because all other excited states are higher in energy. In the manganese(I) complex, a microsecond-lived dark 3ILPyr state, reminiscent of the types of electronic states encountered in many polyaromatic hydrocarbon compounds, is the lowest and becomes photoactive. In the iron(II) complex, the lowest MLCT state has shifted to so much higher energy that 1ILPyr fluorescence occurs, in parallel to other excited-state deactivation pathways. Our combined synthetic-spectroscopic-theoretical study provides unprecedented insights into how effective nuclear charge, ligand field strength, and ligand π-conjugation affect the energetic order between MLCT and ligand-based excited states, and under what circumstances these individual states become luminescent and exploitable in photochemistry. Such insights are the key to further developments of luminescent and photoredox-active first-row transition metal complexes.

Rational Design of Palladium(II) Indenyl and Allyl Complexes Bearing Phosphine and Isocyanide Ancillary Ligands with Promising Antitumor Activity.

A new class of palladium-indenyl complexes characterized by the presence of one bulky alkyl isocyanide and one aryl phosphine serving as ancillary ligands has been prepared, presenting high yields and selectivity. All the new products were completely characterized using spectroscopic and spectrometric techniques (NMR, FT-IR, and HRMS), and, for most of them, it was also possible to define their solid-state structures via X-ray diffractometry, revealing that the indenyl fragment always binds to the metal centre with a hapticity intermediate between ƞ3 and ƞ5. A reactivity study carried out using piperidine as a nucleophilic agent proved that the indenyl moiety is the eligible site of attack rather than the isocyanide ligand or the metal centre. All complexes were tested as potential anticancer agents against three ovarian cancer cell lines (A2780, A2780cis, and OVCAR-5) and one breast cancer cell line (MDA-MB-231), displaying comparable activity with respect to cisplatin, which was used as a positive control. Moreover, the similar cytotoxicity observed towards A2780 and A2780cis cells (cisplatin-sensitive and cisplatin-resistant, respectively) suggests that our palladium derivatives presumably act with a mechanism of action different than that of the clinically approved platinum drugs. For comparison, we also synthesized Pd-ƞ3-allyl derivatives, which generally showed a slightly higher activity towards ovarian cancer cells and lower activity towards breast cancer cells with respect to their Pd-indenyl congeners.

Synthesis and reactivity of air stable Ni(II) complexes with isocyanides and dialkyldithiophosphate ligands: acyclic diaminocarbene formation.

A library of new neutral and cationic Ni(II) complexes containing isocyanide ligands and mono- or dialkyl-dithiophosphate have been easily prepared and fully characterized. The synthesis of the neutral complexes unfolds through the alkyl transfer from one alkyldithiophosphate leaving group coordinated to the Ni(II) complex. The alkyl transfer is controlled by steric factors and is highly solvent-dependent. These complexes shown to constitute excellent precursors to obtain new families of air stable Ni(II)-based acyclic diaminocarbene complexes (Ni(II)-ADCs) by nucleophilic attack with various alkyl-substituted amines. Remarkably, the ADC is only produced at one of the isocyanide ligands, keeping the other isocyanide unreacted. This was subsequently exploited to prepare the unprecedented neutral and cationic dinuclear Ni(II) complexes containing a bridging bis-carbene ligand using piperazine.

Gold(I) Chloride Complexes with 4-Halo-Substituted Phenyl Isocyanide Ligands

Gold(I) Chloride Complexes with 4-Halo-Substituted Phenyl Isocyanide Ligands

Manipulation of 1D and 2D self-assembly via geometry modulation of adamantane isocyanide Pt(II) complexes.

Two cationic luminescent cyclometalated Pt(II) complexes with adamantane-based isocyanide ligands are reported. This work provides important insights for the manipulation of the 1D and 2D self-assembly of Pt(II) complexes by controlling the geometry.

Influence of strong π-acceptor ligands on Cr-K-edge X-ray absorption spectral signatures and consequences for the interpretation of surface sites in the Phillips catalyst.

X-ray absorption spectroscopy (XAS) has been central to the study of the Phillips polymerization catalyst (CrO3/SiO2). As Cr K-edge XAS signatures are sensitive to the oxidation state, geometry and types of ligands on surface (active) sites, the superposition of these effects makes their interpretation challenging. Notably, CO has been particularly used as a reductant to generate low valent Cr sites from CrO3/SiO2 and as a structural IR probe for analysing reduced Cr surface sites. Hence, it is essential to establish a solid understanding of the spectroscopic impact of CO on low-valent Cr sites. We thus built a series of fully characterized low-valent Cr molecular compounds bearing isoelectronic isocyanide ligands in place of CO, with the goal of understanding the effect of the coordination of π-acceptor ligands on the XANES signature of Cr sites. Cr K-edge spectra supplemented with DFT calculations elucidate the effect of the coordination of π-acceptor ligands on XAS signatures, giving a sharp resonance at the white line while modifying the fine structure due to short Cr-C distances and stability of low-spin Cr(ii/iii) species. The isocyanide references allow the deconvolution of the XAS spectra of the reduced CrO3/SiO2 catalyst by evaluating the types of surface species and relative amounts of bound CO at different CO pressures and temperatures.

Luminescent and Photoredox-Active Molybdenum(0) Complexes Competitive with Isoelectronic Ruthenium(II) Polypyridines.

Ruthenium(II) complexes with chelating polypyridine ligands are among the most frequently investigated compounds in photophysics and photochemistry, owing to their favorable luminescence and photoredox properties. Equally good photoluminescence performance and attractive photocatalytic behavior is now achievable with isoelectronic molybdenum(0) complexes. The zero-valent oxidation state of molybdenum is stabilized by carbonyl or isocyanide ligands, and metal-to-ligand charge transfer (MLCT) excited states analogous to those in ruthenium(II) complexes can be established. Microsecond MLCT excited-state lifetimes and photoluminescence quantum yields up to 0.2 have been achieved in solution at room temperature, and the emission wavelength has become tunable over a large range. The molybdenum(0) complexes are stronger photoreductants than ruthenium(II) polypyridines and can therefore perform more challenging chemical reductions. The triplet nature of their luminescent MLCT states allows sensitization of photon upconversion via triplet-triplet annihilation, to convert low-energy input radiation into higher-energy output fluorescence. This review summarizes the current state of the art concerning luminescent molybdenum(0) complexes and highlights their application potential. Molybdenum is roughly 140 times more abundant and far cheaper than ruthenium, hence this research is relevant in the greater context of finding more sustainable alternatives to using precious and rare transition metals in photophysics and photochemistry.

Luminescent and Photoredox‐Active Molybdenum(0) Complexes Competitive with Isoelectronic Ruthenium(II) Polypyridines

AbstractRuthenium(II) complexes with chelating polypyridine ligands are among the most frequently investigated compounds in photophysics and photochemistry, owing to their favorable luminescence and photoredox properties. Equally good photoluminescence performance and attractive photocatalytic behavior is now achievable with isoelectronic molybdenum(0) complexes. The zero‐valent oxidation state of molybdenum is stabilized by carbonyl or isocyanide ligands, and metal‐to‐ligand charge transfer (MLCT) excited states analogous to those in ruthenium(II) complexes can be established. Microsecond MLCT excited‐state lifetimes and photoluminescence quantum yields up to 0.2 have been achieved in solution at room temperature, and the emission wavelength has become tunable over a large range. The molybdenum(0) complexes are stronger photoreductants than ruthenium(II) polypyridines and can therefore perform more challenging chemical reductions. The triplet nature of their luminescent MLCT states allows sensitization of photon upconversion via triplet‐triplet annihilation, to convert low‐energy input radiation into higher‐energy output fluorescence. This review summarizes the current state of the art concerning luminescent molybdenum(0) complexes and highlights their application potential. Molybdenum is roughly 140 times more abundant and far cheaper than ruthenium, hence this research is relevant in the greater context of finding more sustainable alternatives to using precious and rare transition metals in photophysics and photochemistry.

Mutual Placement of Isocyanide and Phosphine Ligands in Platinum(II) Complexes [PtHal2L1L2] (Hal = Cl, Br, I; L1,L2 = CNCy, PPh3) Leads to Highly-Efficient Photocatalysts for Hydrosilylation of Alkynes

A series of platinum complexes featuring phosphine and isocyanide ligands [PtX2(PPh3)(CNCy)] (X = Cl, Br, and I) as well as their parent phosphine [PtX2(PPh3)2] and isocyanide [PtX2(CNCy)2] analogues have been prepared and evaluated as catalysts for the photocatalytic hydrosilylation of alkynes. Under violet light irradiation (λmax = 400 nm), phosphine–isocyanides complexes [PtX2(PPh3)(CNCy)] gave high yields of silylated products (product yield up to 99%, TONs up to 1.98 × 103). The blue light irradiation (λmax = 450 nm) was more suitable for the parent phosphine complexes [PtX2(PPh3)2], which showed comparable efficiency (product yield up to 99%, TON up to 1.98 × 103), while isocyanide complexes [PtX2(CNCy)2] were not active.

Synthesis and crystal structure of bis-(tert-butyl isocyanide-κC)[5,10,15,20-tetra-kis-(4-chloro-phen-yl)porphyrinato-κ4N]iron(II).

In the title compound, [FeII(C44H24Cl4N4)(C5H9N)2] or [FeII(TClPP)(t-BuNC)2] [where TClPP and t-BuNC are 5,10,15,20-tetra-kis-(4-chloro-phen-yl)porphyrinate and tert-butyl isocyanide ligands, respectively], the metal ion lies on an inversion center and is octa-hedrally coordinated by the N atoms of the porphyrin ring in the equatorial plane and by carbon atoms of the trans t-BuNC ligands in the axial sites. The Fe-N bond length of 2.0074 (14) Å suggests a low-spin complex (S = 0). The crystal packing of the title compound is sustained by C-H⋯Cl, C-H⋯N and C__H⋯Cg (Cg = the centroid of a pyrrole ring of the TClPP porphyrinate) inter-actions, leading to a three-dimensional network. The Hirshfeld surface (HS) analysis indicates that 61.4% of the inter-molecular inter-actions are from H⋯H contacts while other contributions are from C⋯H/H⋯C, O⋯H/H⋯O and N⋯H/H⋯N inter-actions, which comprise 21.3%, 13.3% and 3.6% of the HS, respectively.

Three-coordinate monoanions of rhodium(1–) and iridium(1–): Isolable examples of coordinatively-unsaturated metalate anions

Metalate complexes – those that possess metal centers in formally negative oxidation states – are typically coordinatively and electronically saturated organometallic species. The reduced nature of the metal centers within these complexes is well understood to be responsible for their high reactivity as metal-based nucleophiles. While the chemistry of such complexes is extensive, there are very few examples where coordinative or electronic unsaturation accompanies highly-reduced d-orbital manifolds as a means to augment reactivity. Reported here is the isolation of the 16e–, d10 rhodium and iridium metalate anions, [M(CNArDipp2)3]–, where coordinative unsaturation is achieved through the use of encumbering m-terphenyl isocyanide ligands. Reactivity studies with electrophiles demonstrate that the Rh and Ir centers in these metalates retain significant nucleophilic character, which enables the formation of unusual metal/main-group-element species on account of low coordination numbers.

Isocyanide Ligands Promote Ligand-to-Metal Charge Transfer Excited States in a Rhenium(II) Complex.

A metal-to-ligand charge transfer with mixed intraligand character is observed for the rhenium hexakisarylisocyanide complex [Re(CNAr)6]PF6 (CNAr = 2,6-dimethylphenylisocyanide, λmax = 300 nm). Upon oxidation to [Re(CNAr)6](PF6)2, the dominant low energy optical transition is a ligand-to-metal charge transfer (LMCT) mixed with intraligand transitions (λmax = 650 nm). TD-DFT was used to identify the participating ligand-based orbitals in the LMCT transition, revealing that the majority of the donor orbital is based on the aryl ring (85%) as opposed to the CN bond (14%). For both [Re(CNAr)6]+ and [Re(CNAr)6]2+, structural characterization by X-ray diffraction reveals deviations from Oh geometry at the central Re ion, with larger reduction in symmetry observed for Re(II). For [Re(CNAr)6]+, these structural changes lead to a broadening of the strong ν(C≡N) stretch (2065 cm-1), as the degeneracy of the T1u IR-active mode is broken. Furthermore, a shoulder is observed for this ν(C≡N) stretch, resulting from deviation of the C-N-Ar bond from linearity. By contrast, [Re(CNAr)6]2+ has two weak bands in the ν(C≡N) region (2065 and 2121 cm-1). DFT calculations indicate that reduction of symmetry at the central rhenium ion manifests in the decrease in intensity and the observed split of the ν(C≡N) band. Stability of both complexes are limited by light-induced decomposition where Re(I) dissociates a isocyanide ligand upon irradiation and Re(II) absorbance decays under ambient light. These data provide new insights to the electronic structure of [Re(CNAr)6]2+, enhancing our understanding of LMCT excited states and the versatility of isocyanide ligands.

Bisisocyanide cyclometallated platinum(II) complexes: synthesis, structure, photophysical properties, and mechanochromic behavior

A series of cyclometallated platinum(II) complexes [Pt(ppy){CNAr}2]X with two isocyanide ligands (Hppy = 2-phenylpyridine, Ar = C6H2-2,4,6-Me3, C6H3-2-Cl-6-Me , C6H3-2,6-Cl2, C6H4-4-NMe2, C6H4-4-Me, C6H4 4-Cl, C6H4-4-Br, C6H4-4-I, C6H4-4-CF3, C6H4-3-CF3; X = BF4, OTf) was synthesized by the reaction of the [{Pt(ppy)Cl}2] dimer with isocyanides (yield 52-70%). The structure of the resulting complexes was determined using mass spectrometry, 1H, 13C{1H}, 195Pt{1H}, 1H-1H COSY, 1H-1H NOESY, 1H-13C HSQC, and 1H-13C HMBC NMR spectroscopy in solution and solid-state CP/MAS 13C and 195Pt NMR spectroscopy, IR spectroscopy and X-ray diffraction analysis in the solid phase. The photophysical properties of the obtained complexes in the solid phase and the mechanochromic luminescence behavior were studied. In the solid phase, all synthesized compounds phosphoresce in the green or orange range of visible light, while photoluminescence quantum yields reach 26%. Green phosphors exhibit a reversible mechanochromic luminescence change achieved by mechanical grinding (green to orange) and solvent adsorption (orange to green).

Phenylimido complexes of rhenium: fluorine substituents provide protection, reactivity, and solubility.

Reactions of [Re(NPhF)Cl3(PPh3)2] ({NPhF}2- = p-fluorophenylimide) with a variety of alkyl and aryl isocyanides have been studied. Different reactivity patterns and products have been obtained depending on the steric and electronic properties of the individual ligands. This involves the formation of 1 : 1 and 1 : 2 exchange products of Re(V) with the general formulae mer-[Re(NPhF)Cl3(PPh3)(isocyanide)] and cis- or trans-[Re(NPhF)Cl3(isocyanide)2]. The stability of the obtained products is correlated with the substitution pattern of the isocyanide ligands. The products have been studied by single-crystal X-ray diffraction and spectroscopic methods, including IR and multinuclear NMR spectroscopy as well as mass spectrometry. The use of partially fluorinated starting materials and ligands allows the modulation of the solubilities of the starting materials and the products as well as the monitoring of the reactions by means of 19F NMR. The attachment of the CF3 or F substituent on the isocyanides gives control over the steric bulk and the electronic properties of the ligands and, thus, their reactivity.

Multicenter interactions and ligand field effects in platinum(II) tripyrrindione radicals.

The tripyrrin-1,14-dione biopyrrin, which shares the scaffold of several naturally occurring heme metabolites, is a redox-active platform for metal coordination. We report the synthesis of square planar platinum(II) tripyrrindiones, in which the biopyrrin binds as a tridentate radical and the fourth coordination position is occupied by either aqua or tert-butyl isocyanide ligands. These complexes are stable through chromatographic purification and exposure to air. Electron paramagnetic resonance (EPR) data and density functional theory (DFT) analysis confirm that the spin density is located predominantly on the tripyrrindione ligand. Pancake bonding in solution between the Pt(II) tripyrrindione radicals leads to the formation of diamagnetic π dimers at low temperatures. The identity of the monodentate ligand (i.e., aqua vs. isocyanide) affects both the thermodynamic parameters of dimerization and the tripyrrindione-based redox processes in these complexes. Isolation and structural characterization of the oxidized complexes revealed stacking of the diamagnetic tripyrrindiones in the solid state as well as a metallophilic Pt(II)-Pt(II) contact in the case of the aqua complex. Overall, the properties of Pt(II) tripyrrindiones, including redox potentials and intermolecular interactions in solution and in the solid state, are modulated through easily accessible changes in the redox state of the biopyrrin ligand or the nature of the monodentate ligand.