Comparative Insertion Reactivity of CO, CO2, tBuCN, and tBuNC into Thorium– and Uranium–Phosphorus Bonds

A study of the comparative reactivity of CO, CO2, tBuCN, and tBuNC with (C5Me5)2An­[P­(H)­Mes]2 (An = Th, U) has been undertaken. While CO2 and tBuNC form identical products with both metals, namely (C5Me5)2An­[κ2(O,O)-O2CPH­(Mes)]2 and (C5Me5)2An­[η2(tBuNCPMes]­(CNtBu), respectively, differing results are obtained with CO and tBuCN. The reaction of tert-butylnitrile with (C5Me5)2U­[P­(H)­Mes]2 in a 2:1 ratio leads to double insertion into the U–P bonds and elimination of H2PMes, forming the diketimido complex (C5Me5)2U­[κ2(N,N)-(NCtBu)2P­(Mes)]. This is a case in which the analogous reaction with (C5Me5)2Th­[P­(H)­Mes]2 affords a different product, (C5Me5)2Th­[PH­(Mes)]­[κ2(P,N)-N­(H)­C­(CMe3)­P­(Mes)]. The reaction of 1 atm of CO with (C5Me5)2U­[P­(H)­Mes]2 results in double insertion with proton migration from one phosphido ligand to one of the CO carbons to form (C5Me5)2U­[(κ2(O,O)-OCPMesC­(O)­(H)­P­(H)­Mes]. This is in contrast to the previously published result of the reaction between (C5Me5)2Th­[P­(H)­Mes]2 and CO, in which the product is similar, but both protons from the phosphido ligands migrate to one carbon atom, resulting in (C5Me5)2Th­[κ2(O,O)-OC­(H)2P­(Mes)­C­(O)­P­(Mes)]. Density functional theory calculations demonstrate that the mechanisms are quite similar and therefore a similar product is formed, except uranium is less acidic, and the final C–H bond formation does not occur.