Comparative behaviours of phospha-alkynes and alkynes at electron-rich phosphinic metal centres

The coordination chemistry of phospha-alkynes (PCR) at electron-rich and sterically hindered {M(diphosphine)2} [M=Mo(0), W(0), Re(I), Fe(II)] centres, in which the PCR ligand adopts the very rare and electronically unfavoured end-on (η1) coordination mode, as well as at the less sterically demanding {Rh(triphos)}+ site, are described and compared with those exhibited by alkynes at the same metal centres. At the former sites, the η1-PCR ligand behaves as a weak π-acceptor and is typically activated to α-nucleophilic addition to give phospha-alkene, phosphine and phosphinidene oxide products, although activation towards electrophilic (protic) addition has also been recognized upon hydrometalation (insertion into a metal–H bond to give a phospha-alkenyl species). At the same metal sites, the η2-coordination of alkynes is unfavoured (on both electronic and steric grounds) and these substrates undergo rearrangements (H-shifts) towards η1-bonded alkynyl and vinylidene derivatives, or towards a less sterically demanding η2-allene species, and these products are activated towards β-electrophilic addition (protonation) on account of their π-electron withdrawal ability which contrasts with the behaviour of the η1-P-ligated phospha-alkyne and derivatives. The alkyne and phospha-alkyne insertions into an Fe–H bond are also compared, as well as their cycloaddition reactions (cyclotri- and cyclodimerisations, respectively) at the {Rh(triphos)}+ centre which exhibits open coordination sites for η2-ligation. The above reactions are discussed in terms of both stereochemical and electronic effects.