Complexes of azaphospholes:: Synthesis and structure of pentacarbonyl-(η1)-2-phosphaindolizine)chromium(0), -molybdenum(0), and -tungsten(0)

The complex chemical behaviour of 2-phosphaindolizines 1 (1,3-azaphospholo[1,5-a]pyridines) towards metal carbonyl compounds was studied. (eta(1)-2-Phosphaindolizine)M(CO)(5) complexes 2-4 (M = Cr, Mo, W) were formed from 1 and [(THF)M(CO)(5)], the cis-L2Cr(CO)(4) complex 5f from 1f and tetracarbonyl(norbornadiene) chromium(0). The reaction of 2-phosphaindolizines 1e, 1f, or 1g with tricarbonyl(cycloheptatriene)molybdenum(0) or tricarbonyl(mesitylene)tungsten(0) yielded sigma-complexes of the types L2M(CO)(4) or L3M(CO)(3) rather than isolable pi-complexes. In one case a strong upfield signal (delta(31)P = 6.1) was observed with a coordination shift of Delta delta = -161.7, which is typical for pi-coordination. Prolonged reaction or work-up led, however, to dismutation yielding 1g and the iac-L3Mo(CO)(3) complex 6g. X-ray structure analysis of 2a indicates an increased 10 pi-delocalization compared with 1a and a changed conformation of the acyl substituent. The influence of substituents and metals on the P-31 and C-13 complexation chemical shifts and coupling constants is discussed.