Dibenzo[b,f]phosphepines: Novel Phosphane–Olefin Ligands for Transition Metals

Abstract

New, stable heterobidentate phosphane–olefin ligands based on the dibenzo[b,f]phosphepine backbone are reported together with their redox properties and coordination chemistry to rhodium(I). The X-ray crystal structures and DFT calculations show different conformations for the P-phenyl (6a) and P-mesityl (6b) derivatives. Cyclic voltammetry (vs Fc/Fc+) of 6a supported by UV–vis spectroelectrochemistry showed two cathodic waves, a reversible one at E 1/2 = −2.62 V (I f/I b = 1.0) and a quasi-reversible (I f/I b ≈ 1.2) one at E 1/2 = −3.03 V. Reduction with sodium afforded a mixture of the radical anion [6a •]−, characterized by EPR spectroscopy, and dianion [6a]2–, for which an X-ray crystal structure was obtained. Both 6a and 6b bind to RhI centers, giving rise to 3:1 (8a) and 2:1 (8b) ligand:Rh complexes, respectively. Two dibenzo[b,f]phosphepines in 8a and 8b act as heterobidentate ligands in which both the phosphorus atom and the olefinic double bond coordinate to rhodium, but the third ligand in 8a binds as a monodentate P-donor. The cyclic voltammogram of 8b showed two close-lying waves, a one-electron reversible wave at −1.45 V and a two-electron quasi-reversible wave at −1.80 V (I f/I b ≈ 1.3). 8a showed a reversible wave at −1.71 V and irreversible waves at lower potentials.

[d]-Carbon–carbon double bond engineering in diazaphosphepines: a pathway to modulate the chemical and electronic structures of heteropines

“…S3 † ), which are much smaller than the torsion angles of BZ-P , BTD-P and AN-P , as well as those of non-aromatic heteropines. 52 – 56 Such small torsion angles are reminiscent of those in aromatic borepins (ranging from 1–20°) that adopt planar conformations. 41 – 47 MI exhibits weaker electronic confinement; its incorporation on the heteropine core thus promotes a more planar conformation compared to the other substituents.…”

“…Unique to this synthetic scheme are the mild reaction conditions and the absence of reactive intermediates, thereby enabling the installation of strong electron-withdrawing substituents, such as FBZ, BTD, and MI, which were inaccessible through previous synthetic protocols of heteropines and P-containing cyclic materials. 33 – 47 , 52 – 63 …”

“…Previous studies have focused on their fundamental characteristics, including their conformational change, chemical reactivity and electronic structure. 33 – 40 With the exception of Group 13-element-containing aromatic derivatives, 41 – 52 these heteropines generally adopt “boat-like” structures and constantly undergo conformational inversion in solution, severely limiting backbone π-conjugation. 52 – 56 …”

Isomeric Dithienophosphepines: The Impact of Ring Fusion on Electronic and Structural Properties

“…Especially the fusion of (hetero)aromatic rings to the [ b , f ]‐bonds proved to be a desirable motif as the precursors for this strategy are easily made from simple starting materials . Only recently, the annulation strategy was exploited to furnish dibenzo[ b , f ]phosphepines (DBPs, B ) and diazaphosphepines ( D ), both showing remarkable stability. The bottleneck of this chemistry is the variety of derivatization that can be applied to these systems, which up to now required mostly the preinstallation of suitable functionalities and carrying them through the entire synthetic sequence .…”

“…For the realization of a-DTP and b-DTP we combined the strategyr eported by Lammertsma and co-workers for DBP [7] with the synthetic route towards thiophene-fused borepines describedb yT ovar andc o-workers. [11] In detail, DTPs were synthesized startingf rom isomeric (Z)-dithienylethenes [12] 2 and 3 allowing for the incorporation of phosphorusa te ither the aor the b-positions of the thiophene moieties( Scheme 2, left).…”