Isocyanide Cycloaddition and Coordination Processes at Trigonal Phosphinidene-Bridged MoRe and MoMn Complexes

Heterometallic phosphinidene complexes are appealing species for the construction of novel organophosphorus ligands thanks to the high reactivity expected from the combination of M-P multiple bonding and the intrinsically different electronic and coordination preferences of the distinct metals. In a preliminary study, we found that the heterobimetallic complex [MoReCp(μ-PMes*)(CO)6] (Mes* = 2,4,6-C6H2tBu3) reacted with CN(p-C6H4OMe) via [2+1]-cycloaddition to form a novel azaphosphallene complex. We have now examined in detail the reactions of the above complex and those of its MoMn analogue with different isocyanides, which turned out to be strongly dependent on experimental conditions and on the size of the substituent at the isocyanide. All the products formed follow from one or several of the following reaction pathways: (i) CO substitution by CNR; (ii) addition of CNR at the group 7 metal centre; and (iii) [2+1] cycloaddition of isocyanide at a Mo=P bond to form azaphosphallene groups, with the former process being dominant in reactions at room temperature and for the Mn system. In contrast, low-temperature reactions of the Re system favoured the addition processes, with the [2+1] cycloaddition at Mo=P bonds only taking place at substrates without metal-metal bonds and when the size of the CNR group does not cause unbearable steric clashes when placed in between the Cp and Mes* groups.