π-Excess σ2P ligands: synthesis of biaryl-type 1,3-benzazaphosphole hybrid ligands and formation of P^P′–M(CO)4 chelate complexes

Abstract

Acid-catalyzed cyclocondensations of 2-phosphanylanilines 1 with substituted benzaldehydes or heteroaryl aldehydes open a convenient route to new biaryl-type 1H-1,3-benzazaphosphole hybrid ligands 2a-f with o-phosphanylphenyl, pyridyl, imidazolyl, thienyl or o-methoxyphenyl donor groups (in addition to the σ(2)P donor) and to bromophenyl substituted benzazaphospholes 2g,h. Excess aldehyde leads to concomitant reductive N-alkylation, as shown by formation of 3h besides 2h. The reactions proceed via dihydrobenzazaphospholes 4, which can be detected under mild conditions. The aromaticity-driven dehydrogenation does not liberate dihydrogen but is accomplished by transfer hydrogenations, mainly by reduction of some of the aldehyde. N-Secondary 2-phosphanylanilines 5 also react with aldehydes to form the corresponding N-substituted benzazaphospholes 6. The formation of (P^P')M(CO)4-chelate complexes 8a (M = Cr) and 9a,b (M = Mo) was demonstrated by reaction with M(CO)4(norbornadiene). The crystal structure of 9a, determined in addition to the solution structure elucidation by multinuclear NMR spectra, confirms the chelate formation and reveals a trigonal environment for the low coordinated phosphorus, with the P-Mo(0) vector bent out of the benzazaphosphole ring plane by 14.4° (0.57 Å), together with axial chirality of the molecules in the racemic crystals by twisting of the benzazaphosphole and phenyl π-planes around the common C(2)-C(21) bond.

Impact of high π-density on the coordination properties of π-excess aromatic neutral σ2P ligands – P(π)-donor bonds to Ag+and HgCl2

“…2), grown from concentrated THF solution, showed a P,P′-chelate structure as expected. However, whereas in the recently reported Mo(0)(CO) 4 (P,P′)-chelate complex with 4 the zero-valent metal is only slightly (by 14.4°) tilted out of the ring plane with shorter ring-P-Mo than Ph 2 P-Mo(0) distances and marginal changes within the ligand, 11 the HgCl 2 complex 5 coordinates mercury almost perpendicular to the benzazaphosphole ring plane with much longer Hg-P3 (2.6978(7) Å) than Hg-P1 bonds (2.4219(5) Å) and altered P-C and C-N bond lengths within the ring. The angle of the Hg-P vector to the ring plane of 80.3°(deviation from plane 2.62 Å) and the angles C2-P-Hg (86.40(8)) and C3a-P-Hg (98.92(8)), average 92.7°, hint at a π-donor bond from the ring-P atom to Hg(II).…”

“…The NMR spectra of the yellow solid, with a marked upfield 31 P coordination chemical shift in C 6 D 6 (Δδ 31 = −84.8 ppm), indicated complex formation, but attempts to grow single crystals for a detailed structural characterization failed. A crystalline 1,3benzazaphosphole-HgCl 2 complex was finally obtained from the recently reported σ 3 P,σ 2 P-hybrid benzazaphosphole 4 11 and HgCl 2 in THF (Scheme 2). XRD analysis of the yellow single crystals of 5 ( Fig.…”

Electron‐Rich Aromatic 1,3‐Heterophospholes – Recent Syntheses and Impact of High Electron Density at σ2P on the Reactivity

“…Structure determination gave evidence of an unusual η 1 ‐π P ‐coordination of the low‐coordinated phosphorus at Hg II in the crystal state 102. Whereas the Mo(CO) 4 ‐ P , P ′‐chelate complex 78 , obtained from the same ligand with Mo(CO) 4 (NBD) (NBD – norbornadiene), shows the metal only slightly bent out of the ring plane (14.4°) with a short M–P ring distance, strengthened by the PPh 2 donor group in the cis position,40 the angle of the Hg–P bond to the ring plane in 77 is 80.3° and the Hg–P ring and P=C bonds are lengthened. Coordination at the center of the P=C bond as known for η 2 ‐phosphaalkene6 and phosphabenzene complexes8 can be excluded.…”

“…Nevertheless, yields were usually around 50 % or even higher. In the presence of excess aldehyde, primary 2‐phosphanylanilines undergo concomitant ring closure and reductive N ‐substitution by a CH 2 R group 40. Use of excess formaldehyde in this way opened a convenient route to 10a ,34a whereas treatment with o ‐phthalaldehyde gave access to the new NCH 2 ‐bridged tetracyclic 1,3‐benzazaphosphole 12 , with a planar structure and thus a more extended π system than in 2‐arylbenzazaphospholes with twisted π planes (Scheme ) 34b…”

Synthesis of a Luminescent Azaphosphole

“…Metalated 1,3‐benzazaphospholes I (M = Cr, Mo, W) were isolated as thermally stable and air‐stable complexes 15. 1,3‐Benzazaphospholes have also been developed as chelating ligands forming complexes II (M = Cr and Mo) 16. A tungsten–pentacarbonyl complex III was prepared from coplanar tetracyclic benzazaphosphole 17.…”