Main Group Molecular Switches with Swivel Bifurcated to Trifurcated Hydrogen Bond Mode of Action

Artificial molecular machines have captured the full attention of the scientific community since Jean-Pierre Sauvage, Fraser Stoddart, and Ben Feringa were awarded the 2016 Nobel Prize in Chemistry. The past and current developments in molecular machinery (rotaxanes, rotors, and switches) primarily rely on organic-based compounds as molecular building blocks for their assembly and future development. In contrast, the main group chemical space has not been traditionally part of the molecular machine domain. The oxidation states and valency ranges within the p-block provide a tremendous wealth of structures with various chemical properties. Such chemical diversity─when implemented in molecular machines─could become a transformative force in the field. Within this context, we have rationally designed a series of NH-bridged acyclic dimeric cyclodiphosphazane species, [(μ-NH){PE(μ-N^tBu)₂PE(NH^tBu)}₂] (E = O and S), bis-P^V₂N₂, displaying bimodal bifurcated R²₁(8) and trifurcated R³₁(8,8) hydrogen bonding motifs. The reported species reversibly switch their topological arrangement in the presence and absence of anions. Our results underscore these species as versatile building blocks for molecular machines and switches, as well as supramolecular chemistry and crystal engineering based on cyclophosphazane frameworks.

Pre-arranged building block approach for the orthogonal synthesis of an unfolded tetrameric organic-inorganic phosphazane macrocycle

Inorganic macrocycles remain challenging synthetic targets due to the limited number of strategies reported for their syntheses. Among these species, large fully inorganic cyclodiphosphazane macrocycles have been experimentally and theoretically highlighted as promising candidates for supramolecular chemistry. In contrast, their hybrid organic-inorganic counterparts are lagging behind due to the lack of synthetic routes capable of controlling the size and topological arrangement (i.e., folded vs unfolded) of the target macrocycle, rendering the synthesis of differently sized macrocycles a tedious screening process. Herein, we report-as a proof-of-concept-the combination of pre-arranged building blocks and a two-step synthetic route to rationally enable access a large unfolded tetrameric macrocycle, which is not accessible via conventional synthetic strategies. The obtained macrocycle hybrid cyclodiphosphazane macrocycle, cis-[μ-P(μ-N^tBu)]₂(μ-p-OC₆H₄C(O)O)]₄[μ-P(μ-N^tBu)]₂ (4), displays an unfolded open-face cavity area of 110.1 Ų. Preliminary theoretical host-guest studies with the dication [MeNC₅H₄]₂²⁺ suggest compound 4 as a viable candidate for the synthesis of hybrid proto-rotaxanes species based on phosphazane building blocks.

A chiral phosphazane reagent strategy for the determination of enantiomeric excess of amines

Methods for measuring enantiomeric excess (ee) of organic molecules by NMR spectroscopy provide rapid analysis using a standard technique that is readily available. Commonly this is accomplished by chiral derivatisation of the detector molecule (producing a chiral derivatisation agent, CDA), which is reacted with the mixture of enantiomers under investigation. However, these CDAs have almost exclusively been based on carbon frameworks, which are generally costly and/or difficult to prepare. In this work, a methodology based on the readily prepared inorganic cyclodiphosph(iii)azane CDA ClP(μ-N^tBu)₂POBorn (Born = endo-(1S)-1,7,7-trimethylbicyclo[2.2.1]heptan-2-yl) is shown to be highly effective in the measurement of ee’s of chiral amines, involving in situ reaction of the chiral amines (R*NH₂) with the P–Cl bond of the CDA followed by quaternization of the phosphorus framework with methyl iodide. This results in sharp ³¹P NMR signals with distinct chemical shift differences between the diastereomers that are formed, which can be used to obtain the ee directly by integration. Spectroscopic, X-ray structural and DFT studies suggest that the NMR chemical shift differences between diastereomers is steric in origin, with the sharpness of these signals resulting from conformational locking of the bornyl group relative to the P₂N₂ ring induced by the presence of the P(v)-bonded amino group (R*NH). This study showcases cheap inorganic phosphazane CDAs as simple alternatives to organic variants for the rapid determination of ee.

The simple inorganic cyclodiphosph(iii)azane chiral derivatisation agent ClP(μ-^tBuN)₂POBorn (Born = endo-(1S)-1,7,7-trimethylbicyclo[2.2.1]heptan-2-yl) is shown to be effective in the measurement of ee’s of chiral amines using ³¹P NMR spectroscopy.

Recent advances in organophosphorus-chalcogen and organophosphorus-pincer based macrocyclic compounds and their metal complexes

The design and development of phosphorus based macrocycles containing one or more other heteroatoms is of crucial importance for the enhancement of modern synthetic chemistry. In recent years focus on phosphorus based macromolecules has led to intriguing and innovative structures with a variety of applications, including photophysical and host-guest properties, and in organic synthesis. This article summarizes the recent advancements in the synthesis of macrocycles that consist of organophosphorus-chalcogen (P-E, P[double bond, length as m-dash]E; E = O, S, Se) and organophosphorus-pincer based macrocyclic ligands and their transition metal complexes with emphasis given to synthetic methodologies. The reactions involve the modification of simple macrocycles with phosphorus sources or phosphorus-based chalcogenating reagents. Transition metal complexes of phosphine-based macrocyclic pincer ligands and their reactivity are also included.

The Coordination Chemistry of the N-Donor-Substituted Phosphazanes

Phosph(III)azanes, featuring the heterocyclobutane P₂ N₂ ring, have now been established as building blocks in main-group coordination and supramolecular compounds. Previous studies have largely involved their use as neutral P-donor ligands or as anionic N-donor ligands, derived from deprotonation of amido-phosphazanes [RNHP(μ-NR)]₂ . The use of neutral amido-phosphazanes themselves as chelating, H-bond donors in anion receptors has also been an area of recent interest because of the ease by which the proton acidity and anion binding constants can be modulated, by the incorporation of electron-withdrawing exo- and endo-cyclic groups (R) and by the coordination of transition metals to the ring P atoms. We observed recently that the effect of P,N-chelation of metal atoms to the P atoms of cis-[(2-py)NHP(μ-N^t Bu)]₂ (2-py=2-pyridyl) not only pre-organises the N-H functionality for optimum H-bonding to anions but also results in a large increase in anion binding constants, well above those for traditional organic receptors like squaramides and ureas. Here, we report a broader investigation of ligand chemistry of [(2-py)NHP(μ-^t NBu)]₂ (and of the new quinolyl derivative [(8-Qu)NHP(μ-N^t Bu)]₂ (8-Qu=8-quinolyl). The additional N-donor functionality of the heterocyclic substituents and its position has a marked effect on the anion and metal coordination chemistry of both species, leading to novel structural behaviour and reactivity compared to unfunctionalized counterparts.

N-Bridged Acyclic Trimeric Poly-Cyclodiphosphazanes: Highly Tuneable Cyclodiphosphazane Building Blocks

We have synthesized a completely new family of acyclic trimeric cyclodiphosphazane compounds comprising NH, Nⁱ Pr, N^t Bu and NPh bridging groups. In addition, the first NH-bridged acyclic dimeric cyclophosphazane has been produced. The trimeric species display highly tuneable characteristics so that the distance between the terminal N(H)R moieties can be readily modulated by the steric bulk present in the bridging groups (ranging from ≈6 to ≈10 Å). Moreover, these species exhibit pronounced topological changes when a weak non-bonding NH⋅⋅⋅π aryl interaction is introduced. Finally, the NH-bridged chloride binding affinities have been calculated and benchmarked along with the existing experimental data available for monomeric cyclodiphosphazanes. Our results underscore these species as promising hydrogen bond donors for supramolecular host-guest applications.

Charge-assisted phosph(v)azane anion receptors

Coordination of Cu(i) or Pd(ii) to seleno-cyclodiphosph(v)azanes of the type [RNH(Se)P(μ-NtBu)]2 results in positively charged anion receptor units which have increased anion affinity over the neutral seleno-phosph(v)azanes, due to the increase in electrostatic interactions between the receptor and the guest anions. The same effect is produced by replacement of one of the P[double bond, length as m-dash]Se units by a P-Me+ unit.

Therapeutic potential of dithiocarbamate supported gold compounds

Chrysotherapy or aurotherapy, the use of gold as medicine, is two thousand years old. Hitherto, numerous diverse gold stabilizing ligands for instance vitamins, pyridine, phosphines, naphthylamine and xanthanes have been developed and their 'chelating effect' in addition to their anti-proliferative properties have been extensively studied. Recent advances in the field of bioinorganic chemistry have led to the design of biologically relevant metal complexes with appropriate fine-tuned ligands such as metallic conjugates of dithiocarbamates (DTCs). DTC compounds have been recognised to possess diverse applications and have demonstrated interesting biological properties. For instance, the chemoprotective and antitumour properties of gold metal ions and DTC compounds respectively, presents an innovative and effective approach to cancer management. This review presents therefore the therapeutic potential of DTC ligand systems as a support for gold compounds. The importance of dithiocarbamate supported gold compounds as potential therapeutic agents is highlighted with emphasis on the therapeutic potential of gold(iii) and gold(i) dithiocarbamate derivatives.

Mechanochemistry as an emerging tool for molecular synthesis: what can it offer?

Mechanochemistry is becoming more widespread as a technique for molecular synthesis with new mechanochemical reactions being discovered at increasing frequency. Whilst mechanochemical methods are solvent free and can therefore lead to improved sustainability metrics, it is more likely that the significant differences between reaction outcomes, reaction selectivities and reduced reaction times will make it a technique of interest to synthetic chemists. Herein, we provide an overview of mechanochemistry reaction examples, with 'direct' comparators to solvent based reactions, which collectively seemingly show that solid state grinding can lead to reduced reaction times, different reaction outcomes in product selectivity and in some instances different reaction products, including products not accessible in solution.

The chameleonic reactivity of dilithio bis(alkylamido)cyclodiphosph(iii)azanes with chlorophosphines

Analogous reactions do not necessarily give analogous products.

The First Synthesis of the Sterically Encumbered Adamantoid Phosphazane P4 (N(t) Bu)6 : Enabled by Mechanochemistry

All reported attempts to synthesize the tert-butyl-substituted adamantoid phosph(III)azane P4 (N(t) Bu)6 have failed, leading to the classification of this molecule as inaccessible and a literature example of steric control in chemistry of phosphorus-nitrogen compounds. We now demonstrate that this structure is readily accessible by a solvent-free mechanochemical milling approach, highlighting the importance of mechanochemical reaction environments in evaluating chemical reactivity.

Low temperature isolation of a dinuclear silver complex of the cyclotetraphosphane [ClP(μ-PMes*)]2

The reaction of the cyclotetraphosphane [ClP(μ-PMes*)]2 (, Mes* = 2,4,6-tri-tert-butylphenyl) with Ag[Al(OR(F))4] (R(F) = CH(CF3)2) resulted in a labile, dinuclear silver complex of , which eliminates AgCl above -30 °C. Its properties were investigated by spectroscopic methods, single crystal X-ray diffraction and DFT calculations.

Cyclodiphosphazanes: options are endless

This short review describes the transition metal chemistry of cyclodiphosphazanes.

A cyclodiphosphazane based pincer ligand, [2,6-{μ-((t)BuN)2P((t)BuHN)PO}2C6H3I]: Ni(II), Pd(II), Pt(II) and Cu(I) complexes and catalytic studies

Synthesis and late-transition metal complexes of pincer capable cyclodiphosphazane, 2,6-{μ-((t)BuN)2P((t)BuHN)PO}2C6H3I (1) are described. The condensation of 2-iodoresorcinol with cis-{ClP(μ-N(t)Bu)2PN(H)(t)Bu} produced a difunctional derivative 1 in good yield. The treatment of Ni(COD)2, Pd2(dba)3·CHCl3 or Pt(PPh3)4 with 1 afforded pincer complexes [2,6-{μ-((t)BuN)2P((t)BuHN)PO}2C6H3MI] (2 M = Ni; 3 M = Pd and 4 M = Pt). The reaction of complex 3 with copper halides resulted in the formation of heterobimetallic complexes bridged by rhombic {Cu(μ-X)}2 units, [{{Cu(μ-X)}2}{μ-((t)BuN)2P((t)BuHN)PO}2C6H3PdI] (5 X = I and 6 X = Br). The crystal structures of 1-3, and 6 were established by single X-ray diffraction studies. The palladium complex 3 was tested for catalytic P-arylation of diphenylphosphine oxide (Ph2P(O)H) under microwave irradiation. Moderate to good catalytic activity was observed with aryl bromides.

Gold(i) complexes of bisphosphines with bis(azol-1-yl)methane backbone: structure of a rare dinuclear gold(i) complex [(Au2Cl){CH2(1,2-C3H2N2PPh2)2}3Cl]

This paper describes the synthesis of gold(i) complexes of bisphosphines based on bis(azol-1-yl)methane viz. bis(imidazol-1-yl)methane, bis(pyrazol-1-yl)methane and bis(1,2,4-triazol-1-yl)methane.

Axial ligand modified high valent tin(iv) porphyrins: synthesis, structure, photophysical studies and photodynamic antimicrobial activities on Candida albicans

Photophysical studies, fluorescence imaging, single crystal X-ray structure analysis and DFT calculations revealed that compounds2and3show enhanced phototoxicity towardsCandida albicanscompared to compound1.

Dinuclear Cu(I) complexes of pyridyl-diazadiphosphetidines and aminobis(phosphonite) ligands: synthesis, structural studies and antiproliferative activity towards human cervical, colon carcinoma and breast cancer cells

The copper(i) complexes containing phosphorus donor ligands such as diazadiphosphetidine, cis-{(o-OCH2C5H4N)P(μ-N(t)Bu)}2 (1) and aminobis(phosphonite), C6H5N{P(OC6H3(OMe-o)(C3H5-p))2}2 (2, PNP), have been synthesized. Treatment of 1 with copper iodide afforded the 1D coordination polymer [{Cu(μ-I)}2{(o-OCH2C5H4N)P(μ-N(t)Bu)}2]n (3). Treatment of 3 with 2,2'-bipyridine (bpy) and 1,10-phenanthroline (phen) produced mixed-ligand complexes [(L)2Cu2{(o-OCH2C5H4N)P(μ-N(t)Bu)}2][I]2 (4 L = bpy; 5 L = phen) in good yields. The reaction of 2 with copper iodide yielded a rare tetranuclear copper complex [(CuI)2C6H5N(PR2)2]2 (6), which on subsequent treatment with various pyridyl ligands produced binuclear complexes [{Cu(μ-I)(py)}2(μ-PNP)] (7), [Cu2(μ-I)(bpy)2(μ-PNP)]I (8), [Cu2(μ-I)I(bpy)(μ-PNP)] (9), [Cu2(phen)(bpy)(μ-PNP)](OTf)2 (10), [Cu2(μ-I)I(phen)(μ-PNP)] (11) and [Cu2(μ-I)(phen)2(μ-PNP)]I (12), in an almost quantitative yield. The new copper(i) complexes (4, 5 and 7-12) were tested for anti-cancer activity against three human tumor cell lines. Compounds 5, 10 and 12 showed in vitro antitumor activity 5-7 fold higher than cisplatin, the most used anticancer drug. These three most potent compounds (5, 10 and 12) were chosen for detailed study to understand their mechanism of action. The copper(i) compounds studied in the present investigation were found to inhibit tumor cell growth by arresting cells at the S-phase of the cell cycle. The characteristic nuclear morphology of treated cells showed signs of DNA damage. The experimental evidence clearly indicated that these compounds initiated apoptosis, which is mediated through the p53 pathway.

Quaternization and oxidation reactions of cyclodiphosphazane derivatives and their copper(I) and gold(I) complexes

The reactions of cyclodiphosphazane derivatives cis-{(t)BuN(H)P(μ-N(t)Bu)2PN(H)(t)Bu} (1), cis-{MeN(C4H8N)P(μ-N(t)Bu)2P(NC4H8Me)} (2) and {(Me2NCH2CH2O)P(μ-N(t)Bu)2P(OCH2CH2NMe2)} (3) with methyl iodide and methyl triflate and their subsequent reactions with elemental sulfur and selenium are reported. Interestingly, the reactions of 1-3 with an excess of methyl iodide resulted in quaternising only one phosphorus atom in cis-[{(t)BuNHP(μ-N(t)Bu)2P(CH3)NH(t)Bu}](I) (4), two exocyclic nitrogen atoms and one of the phosphorus atoms in cis-{(Me2NC4H8N)P(μ-N(t)Bu)2P(CH3)(NC4H8NMe2)}](I)3 (7) and only two exocyclic nitrogen atoms in cis-[{(Me3NCH2CH2O)P(μ-N(t)Bu)2P(OCH2CH2NMe3)}](I)2 (8). The reaction of 1 with one equiv. of methyl triflate produced cis-[{(t)BuN(H)P(μ-N(t)Bu)2P(CH3)N(H)(t)Bu}]OTf (5), whereas the same reaction in a 1 : 2 molar ratio afforded cis-{(t)BuN(H)P(CH3)(μ-N(t)Bu)2P(CH3)N(H)(t)Bu}(OTf)2 (6). Compounds 4 and 5 showed poor solubility in water, whereas 7 and 8 were high melting crystalline solids with moderate to good water solubility. Treatment of 4 with either elemental sulfur or selenium gave both cis- and trans-chalcogenide derivatives. Similar reactions of 7 and 8 produced both mono- and bischalcogenides. Reactions between 4 or 7 and CuI yielded dinuclear complexes, cis-[{Cu2(μ-I)3((t)BuN(H)P)(μ-N(t)Bu)2(P(CH3)N(H)(t)Bu)]2}(I)] (15) and cis-[{Cu2(μ-I)3[(Me2NC4H8N)P(μ-N(t)Bu)2P(CH3)(NC4H8NMe2)]2}(I)5] (16), while the reaction of 8 with CuI produced a coordination polymer [{Cu2(μ-I)3(μ-N(t)BuP)2(OCH2CH2NMe3)2}I]∞ (17), containing triiodo-bridged [Cu2(μ-I)3] linkers. The molecular structures of several of these compounds were confirmed by single crystal X-ray diffraction studies. The Cu(I)···Cu(I) distance of 2.55 Å in 15 is quite short and is the same as that found in copper metal and also in complexes containing [Cu2(μ-I)3] linkers. All the metal complexes exhibit strong intra-, inter- or both intra- and inter-molecular hydrogen bonding interactions.

Human cancerous and healthy cell cytotoxicity studies of a chiral μ-dicarbene-digold(I) metallamacrocycle

A novel eighteen membered chiral macrocyclic dicarbene-digold complex [(μ-diNHC)Au(I)]2[OTF]2 (8-(+/-)) was synthesized and characterized. Starting with enantiopure diNHC imidazolium salt ligand precursors enables access to the enantiopure versions of the digold(I) metallamacrocycles, 8-(+) and 8-(-). In vitro cytotoxicity studies indicate 8-(+/-) is moderately cytotoxic to both healthy and cancerous cell-lines, with no specificity. Confocal microscopy indicates the digold metallamacrocycle penetrates the cell membrane and causes cell death via apoptosis, as evidenced by DNA electrophoresis. The complex 8-(+/-) is characterized by a combination of NMR techniques (gDQCOSY, gHSQC, gHMBC and ROESY), single crystal X-ray diffraction, and combustion analysis.

Water Soluble Phosphane-Gold(I) Complexes. Applications as Recyclable Catalysts in a Three-component Coupling Reaction and as Antimicrobial and Anticancer Agents

Water-soluble compounds of the type [AuCl(PR3)] with alkyl-bis-(m-sulfonated-phenyl)-(mC6H4SO3Na)2 and dialkyl-(m-sulfonated-phenyl)-(mC6H4SO3Na) (R = nBu, Cp) phosphanes have been prepared. Dialkyl-phosphane compounds generate water-soluble nanoparticles of 10-15 nm radius when dissolved in water. These air-stable complexes have been evaluated as catalysts in the synthesis of propargylamines via a three-component coupling reaction of aldehydes, amines and alkynes in water. The antimicrobial activity of the new complexes against Gram-positive and Gram-negative bacteria and yeast has been evaluated. The new compounds display moderate to high antibacterial activity. The more lipophilic compounds are also potent against fungi. Their cytotoxic properties have been analyzed in vitro utilizing human Jurkat T-cell acute lymphoblastic leukemia cells. Compounds with dialkyl-(m-sulfonated-phenyl)-(mC6H4SO3Na) phosphanes displayed moderate to high cytotoxicity on this cell line. Death cell mechanism occurs mainly by early apoptosis. The catalytic/biological activity of the previously described compound with commercial m-trisulfonated-triphenylphosphine [AuCl(TPPTS)] (6) has been also evaluated to compare the effects of the higher basicity and lipophilicity of the alkyl- and di-alkyl-(m-sulfonated-phenyl) phosphanes on these new compounds.