Nucleophilic Addition of Phosphines to Rhenium Allenylidenes. Unprecedented Double P-H Bond Activation to GIve an h1-P-phospha-1-butadienyl Ligand

Tertiary phosphines PMe3-xPhx (x = 0 – 2) react with the rhenium allenylidene [(triphos)(CO)2Re(C=C=CPh2)]OTf (1) yielding kinetic g-phosphonioalkynyl complexes [(triphos)(CO)2Re{CºCCPh2(PMe3-xPhx)}]OTf [x = 0, (2); 1, (4); 2 (7)]. Increasing the temperature converts the g-phosphonioalkynyl compounds into the thermodynamically stable a-phosphonioallenyl derivatives [(triphos)(CO)2Re{C(PMe3-xPhx)=C=CPh2}]OTf [x = 0, (3); 1, (5); 2 (6)]. The reactions of 1 with secondary, PHPh2, and primary, PH2CH2Fc (Fc = ferrocenyl), phosphines have been found to proceed with a similar mechanism, followed by single or double P-H bond cleavage. The g-phosphonioalkynyl [(triphos)(CO)2Re{CºCCPh2(PHPh2)}]OTf (9) and the a-phosphonioallenyl [(triphos)(CO)2Re{C(PHPh2)=C=CPh2}]OTf (10) have been intercepted by in situ NMR spectroscopy. On increasing the temperature, 10 undergoes a selective 1,3-P,C-H shift to give the a-phosphoniobutadienyl derivative [(triphos)(CO)2Re{C(=PPh2)CH=CPh2}]OTf (8). With the primary phosphine PH2CH2Fc, the a-phosphoniobutadienyl complex [(triphos)(CO)2Re{C(=PHFc)CH=CPh2}]OTf (11) was initially formed which transforms into the h1-P-phosphabutadienyl complex [(triphos)(CO)2Re{P(CH2Fc)=CHCH=CPh2}]OTf (12) upon heating at 50 °C following a second P-H bond activation and C,P-metal bonding isomerisation. Compound 1/BPh4 has been authenticated by an X-ray diffraction analysis. The structure of this complex shows the metal center to be surrounded by a fac triphos ligand, by two mutually cis carbonyl groups and by the allenylidene ligand in a slightly distorted octahedral geometry.