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

1-(2,4,6-Tri-tert-butylphenyl)-1H-[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. 1H-[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 1H-[1,2]diphosphole known to date. The 31P 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 (2JPP = 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 1H-[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 η2-coordination mode between ruthenium and the phosphaalkene. The P−C bond in the three-membered ring of 27 is substantially lengthened by η2-coordination, from 1.650(3) Å in the parent ligand to 1.775(3) Å in complex 27.