Tetracarbonyl diazaphosphole complexes of group 6B metals: the role of steric effects
Thermal substitution reactions of [M(CO)5L1c] with cis diazaphosphole derivatives (L1= 3,4-dihydro-2,3,4,5-tetraphenyl-2H-1,2,3-diazaphosphole, L1c and L1t denote cis and trans L1 respectively) have yielded a mixture of cis and trans(2 : 3 ratio) tetracarbonyl complexes in which the ligands are bonded through the phosphorus atom. In contrast the reactions of [M(CO)5L1t] with the bulkier trans diazaphosphole isomers (L1t or L2t, L2= 5-benzyl-3,4-dihydro-2,3,4-triphenyl-2H-1,2,3-diazaphosphole) have only afforded the trans disubstituted tetracarbonyl [M(CO)4(Lt)2]. On the other hand, L1t and L2t have reacted with [M(CO)5L1c] affording the mixed-phosphine derivatives trans-[M(CO)4L1cLt](Lt= L1t or L2t). The norbornadiene complex [Mo(CO)4(nbd)] produced only cis-[Mo(CO)4(L1c)2] when treated with L1c in cyclohexane at room temperature, whereas with L1t no tetracarbonyl complexes (cis or trans) have been observed. In addition it has been found that cis(or trans)-[Mo(CO)4(L1c)2] thermally interconverts by a dissociative pathway to the trans(or cis)-[Mo(CO)4-(L1c)2] to give an equilibrium composition (cis : trans= 2 : 3) whereas trans-[Mo(CO)4(Lt)2] did not isomerize. All these different stereochemical results are tentatively explained by invoking fluxional or rigid five-co-ordinate intermediates in which only steric factors play a decisive role in determining the geometry and then the products distribution. The nature of the complexes has been established essentially by i.r. and 1H n.m.r. spectra. An X-ray study on cis-[Mo(CO)4(L1c)2] is also reported.