PBP bridged [3]ferrocenophane: a bisphosphanylborane with a redox trigger

At the limits of bisphosphonio-substituted stannylenes

Donor stabilization of Sn(II) and Pb(II) halides with 1,1'-ferrocenylene bridged bisphosphanes has been explored for Fe(C5H4P(C6H5)2)2 (dppf), and Fe(C5H4PH(C4H9))2. These bisphosphanes are reacted with SnBr2 and PbCl2 with and without additional Lewis acid (AlCl3) forming acyclic and cyclic donor adducts from which the latter represent bisphosphoniotetrylenes. Since dynamic exchange in solution is observed, characterization includes solution and solid-state NMR in addition to SC-XRD, amended by DFT calculations.

Heteroatom-boron-heteroatom-doped π-conjugated systems: structures, synthesis and photofunctional properties

The potency of heteroatom-doping in reshaping optoelectronic properties arises from the distinct electronegativity variations between heteroatoms and carbon atoms. By incorporating two heteroatoms with differing electronegativities (e.g., B = N), not only is the architectural coherence of π-conjugated systems retained, but also dipolar traits are introduced, accompanied by unique intermolecular interactions absent in their all-carbon analogs. Another burgeoning doping strategy, featuring the heteroatom-boron-heteroatom motif (X-B-X, where X = N, O), has captured growing attention. This configuration's coexistence of the boron-heteroatom unit and an isolated heteroatom stimulates mutual modulation in the dipole of the boron-heteroatom unit and the heteroatom's electronegativity. In this Feature article, we present an encompassing survey of XBX-doped π-conjugated systems, elucidating how the integration of the X-B-X unit induces transformative structural and property changes within π-conjugated systems.

Gradual Donor Stabilization of a Transient Ferrocene Bridged Bisphosphanyl Phosphenium Cation

A transient phosphenium cation embedded into a [3]ferrocenophane scaffold was formed via chloride abstraction. The cation has been trapped with phosphane, carbene, and silylene donors resulting in stable adducts or bond activation of the ferrocenophane bridge. In the absence of donors, dimerization of the phosphenium cation to the corresponding dication is observed or P-C bond activation with migration of a substituent leading to a putative phosphoniodiphosphene. Using 1,3-di-tert-butylimidazol-2-silylene as the donor, further reaction of the initially formed chlorosilane leads to activation of a P-P bond of the ferrocenophane scaffold with ring expansion of the ansa-bridge. The donor formation and bonding situation are investigated by density functional theory calculations as well as experimental methods (e.g., NMR spectroscopy and X-ray crystallography).

C2-Symmetric P-Stereogenic Ferrocene Ligands with Heavier Chalcogenophosphinous Acid Ester Donor Sites

tert-Butyl-substituted diphospha[2]ferrocenophane was used as a stereochemically confined diphosphane to investigate the addition of various dichalcoganes (R2Ch2; Ch = S, Se, Te and R = Me, Ph). Bischalcogenophosphinous acid esters bearing four soft donor sides were obtained as a mixture of rac and meso diastereomers and characterized by means of multinuclear NMR and X-ray analysis. The coordination chemistry of multidentate ligand 3b was explored toward d¹⁰ coinage metal centers (Cu(I), Ag(I), and Au(I)), yielding various bimetallic complexes.

Chalcogen-Transfer Rearrangement: Exploring Inter- versus Intramolecular P-P Bond Activation

tert-Butyl-substituted diphospha[2]ferrocenophane has been used as a stereochemically confined diphosphane to explore the addition of O, S, Se and Te. Although the diphosphanylchalcogane has been obtained for tellurium, all other chalcogens give diphosphane monochalcogenides. The latter transform via chalcogen-transfer rearrangement to the corresponding diphosphanylchalcoganes upon heating. The kinetics of this rearrangement has been followed with NMR spectroscopy supported by DFT calculations. Intermediates during rearrangement point to a disproportionation/synproportionation mechanism for the S and Se derivatives. Cyclic voltammetry together with DFT studies indicate ferrocene-centred oxidation for most of the compounds presented.

Unusual Racemization of Tertiary P-Chiral Ferrocenyl Phosphines

Tertiary phosphines are generally known to withstand inversion under moderate conditions. In this work, a remarkable racemization process of three P-chiral ferrocenyl phosphines is reported. Subjected to conventional column chromatography as highly enantioenriched compounds, they greatly experienced racemization when collected at the column outlet within minutes. Initially, attention was drawn to this unusual inversion behavior after observing that the superb enantiomeric excess of these ligands (>95 % ee in all cases) was almost lost in their corresponding ruthenium(II) complexes. Successively excluding possible racemization causes, these P-chiral ferrocenyl phosphines were found to undergo a significant, acid-catalyzed racemization process at room temperature within a few minutes. This process is mainly observed during standard column chromatography by using conventional silica or alumina, but can also be triggered deliberately by addition of certain acids. Therefore, the stereochemical preservation of P-chiral phosphines during their purification may per se not always be guaranteed, since column chromatography is the most frequently used technique for purifying such types of compounds.

Bulky 1,1'-bisphosphanoferrocenes and their coordination behaviour towards Cu(i)

Two bulky mesityl substituted dppf-analogues Fe(C₅H₄PMes₂)₂ (Mes = 2,4,6-Me₃C₆H₂, 1) and Fe(C₅H₄PMes₂)(C₅H₄PPh₂) (Mes = 2,4,6-Me₃C₆H₂, Ph = C₆H₅, 3) have been prepared and their properties as donor ligands have been explored using heteronuclear NMR spectroscopy and in particular via¹J_P-Se coupling, cyclic voltammetry and DFT calculations. Based on the results obtained, a series of mono- and dinuclear Cu(i) complexes have been prepared with these new diphosphane ligands using Br^-, I^-, and BF₄^- as counter anions. For the very bulky ligand 1 rare and unprecedented double bridging complexation modes have been observed containing two non-planar Cu₂Br₂ units, while for the other dinuclear complexes planar Cu₂Br₂ units have been found. The Cu(i) complexes of 1 and 3 were then used as catalysts for CO₂-fixation reaction with terminal alkynes, and complexes with ligand 3 were found to be more efficient than those with 1. DFT calculations performed on compounds 1, 3 and their Cu(i) complexes were able to verify the trend of these catalytic reactions.

Hydrophosphination of boron-boron multiple bonds

Five compounds containing boron–boron multiple bonds are shown to undergo hydrophosphination reactions with diphenylphosphine in the absence of a catalyst. With diborenes, the products obtained are highly dependent on the substitution pattern at the boron atoms, with both 1,1- and 1,2-hydrophosphinations observed. With a symmetrical diboryne, 1,2-hydrophosphination yields a hydro(phosphino)diborene. The different mechanistic pathways for the hydrophosphination of diborenes are rationalised with the aid of density functional theory calculations.

Compounds containing boron–boron double and triple bonds are shown to undergo uncatalysed hydrophosphination reactions with diphenylphosphine.

Bis-[3]Ferrocenophanes with Central >E-E'< Bonds (E, E'=P, SiH): Preparation, Properties, and Thermal Activation

A series of bis-[3]ferrocenophanes of the general type Fe(C5H4E')2E-E(E'C5H4)2Fe (E=P, SiH and E'=PtBu, NneoPentyl, NSi(CH3)3) with an isolobal molecular framework have been prepared and characterized by heteronuclear NMR spectroscopy and X-ray crystallography. The thermal dissociation behavior with respect to homolytic fission of the central bond generating phosphorus centered radicals was investigated using EPR spectroscopy and quantum chemical calculations.

Structural and spectroscopic analysis of a new family of monomeric diphosphinoboranes

We present a series of amino- and aryl(diphosphino)boranes R₂PB(R'')PR'₂, where R₂P, R'₂P = tBu₂P, tBuPhP, Ph₂P, Cy₂P, and R'' = iPr₂N, Ph, which were obtained via the metathesis reaction of iPr₂NBBr₂ or PhBBr₂ with selected lithium phosphides. The structures of isolated diphosphinoboranes were characterized in the solid state and in solution by means of X-ray diffraction and NMR spectroscopy, respectively. The utility of these P-B-P species as ligands for transition metal complexes was tested in the reaction with [(COD)PtMe₂]. Moreover, we carried out DFT calculations to elucidate bonding interactions and philicity of the reactive centers as well as to analyze conformations of the studied species. Electronic and steric properties of substituents on P and B atoms were found to have a strong influence on the structures of the obtained compounds. Three main types of diphosphinoboranes were distinguished, based on the strength of P-B π-interaction within the molecule: (i) application of strong electron-donating substituents on P-atoms and electron-accepting phenyl groups on B atoms led to the structure with one double P[double bond, length as m-dash]B and one single P-B bond and diverse planar and pyramidal geometry of phosphanyl groups; (ii) reduction of the donor ability of phosphanyl groups gave diphosphanylboranes with delocalized P-B-P π-interactions; (iii) introduction of amino groups with strong donor abilities on B atoms canceled P-B π-interactions and allowed compounds with two very long P-B bonds and two pyramidal phosphanyl groups to be obtained.

Controllable access to P-functional [3]ferrocenophane and [4]ferrocenophane frameworks

Condensation of secondary 1,1'-diaminoferrocenes with phosphorus trihalides (PCl3 or PBr3) yielded either P-halo-1,3-diaza-2-phospha[3]ferrocenophanes or 1,1'-diaminoferrocenediyl-bis(dichlorophosphines), respectively. The latter provide controlled access to both [3]- and [4]ferrocene frameworks. Thus, reductive coupling with magnesium gave diaza-diphospha[4]ferrocenophane-annelated tetraphosphetanes, while a reaction with LiNMe2 produced a P-chloro-diazaphospha[3]ferrocenophane. Condensation of diaminoferrocenes and aminodichlorophosphines were mostly unselective, but afforded in one case a 3-amino[3]ferrocenophane. All reaction products were characterised by spectroscopic and single-crystal XRD studies. DFT studies indicate that the product selectivity in the reactions studied depends on a combination of kinetic and thermodynamic effects, which correlate subtly with the steric bulk of the N-substituents. Cyclic voltammetry measurements revealed that the ferrocenophanes can undergo multiple oxidation events, the first of which may according to DFT studies be located in both the ferrocene and aminophosphine units. The quantum chemical studies provided also insight into stereochemical aspects like ring strain in the ferrocenophane units.

Strained azabora[2]ferrocenophanes

The first BN-bridged [2]ferrocenophanes were synthesized and their structures and strain enthalpies were investigated.