Aromatic 1<i>H</i>-[1,2]Diphosphole with a Planar Tricoordinated Phosphorus, Plus η²-Coordination Mode between Ruthenium(0) and a Phosphaalkene

Abstract

1-(2,4,6-Tri- tert -butylphenyl)-1 H -[1,2]diphosphole ( 12 ) was isolated from the unintentional and unexpected dehydrohalogenation of 1,3-bis(chloro(2,4,6-tri- tert -butylphenyl)phosphino)propane ( 5 ) with DBN. 1 H -[1,2]Diphosphole 12 has substantial bond delocalization within the diphosphole system. Remarkably, the shortest P−C bond in the ring (1.725(4) Å) is observed for a nominal P−C single bond with the tricoordinated phosphorus atom. The P−P distance (2.0750(18) Å) in 12 is closer to the value for a P P bond than for a P−P bond. The sum of the bond angles at the tricoordinated phosphorus atom in 12 is 351.3°, making 12 the most planar 1 H -[1,2]diphosphole known to date. The 31 P NMR spectrum of 12 contains very similar chemical shifts for the tri- and dicoordinated phosphorus atoms (90.83 and 133.80 ppm), with a large P−P coupling constant ( 2 J PP = 528.2 Hz). This is further evidence of significant delocalization of the lone pair electrons of the tricoordinated phosphorus into the ring system and is consistent with the aromaticity of the 1 H -[1,2]diphosphole ring in 12 . The reaction between ruthenium carbonyl and 8-{(2,4,6-tri- tert -butylphenyl)-[3-(2,4,6-tri- tert -butylphenylphosphanylidene)propyl]phosphanyl}-2,3,4,6,7,8-hexahydropyrrolo[1,2- a ]pyrimidine ( 8 ), which is the second product from the unexpected dehydrohalogenation, resulted in the formation of complex 27, with a novel η 2 -coordination mode between ruthenium and the phosphaalkene. The P−C bond in the three-membered ring of 27 is substantially lengthened by η 2 -coordination, from 1.650(3) Å in the parent ligand to 1.775(3) Å in complex 27 .