Regioselective and reversible carbon–nitrogen bond formation: Synthesis, structure and reactivity of ureato-bridged complexes [Mo2(NAr)2(µ-X){µ-ArNC(O)NAr}(S2CNR2)2] (Ar = Ph, p-tol; X = S, NAr; R = Me, Et, Pr)

Abstract

Reaction of ArNCO with syn-[MoO(µ-O)(S₂CNR₂)]₂ or syn-[MoO(µ-NAr)(S₂CNR₂)]₂ at 110 °C leads to the facile formation of bridging ureato complexes [Mo₂(NAr)₂(µ-NAr){µ-ArNC(O)NAr}(S₂CNR₂)₂] (Ar = Ph, p-tol; R = Me, Et, Pr), formed upon substitution of all oxo ligands and addition of a further equivalent of isocyanate across one of the bridging imido ligands. Related sulfido-bridged complexes [Mo₂(NAr)₂(µ-S){µ-ArNC(O)NAr}(S₂CNR₂)₂] have been prepared from syn-[Mo₂O₂(µ-O)(µ-S)(S₂CNR₂)₂]. When reactions with syn-[MoO(µ-NAr)(S₂CNEt₂)]₂ were followed by NMR, intermediates were observed, being formulated as [Mo₂O(NAr)(µ-NAr){µ-ArNC(O)NAr}(S₂CNEt₂)₂], which at higher temperatures convert to the fully substituted products. A crystallographic study of [Mo₂(N-p-tol)₂(µ-S){µ-p-tolNC(O)N-p-tol}(S₂CNPr₂)₂] reveals that the bridging ureato ligand is bound asymmetrically to the dimolybdenum centre—molybdenum–nitrogen bonds trans to the terminal imido ligands being significantly elongated with respect to those cis—a result of the trans-influence of the terminal imido ligands. This trans-influence also leads to a trans-effect, whereby the exchange of aryl isocyanates can occur in a regioselective manner. This is followed by NMR studies and confirmed by a crystallographic study of [Mo₂(N-p-tol)₂(µ-N-p-tol){µ-p-tolNC(O)NPh}(S₂CNEt₂)₂]—the PhNCO occupying the site trans to the terminal imido ligands. Ureato complexes also react with PhNCS, initially forming [Mo₂(NAr)₂(µ-S){µ-ArNC(O)NAr}(S₂CNR₂)₂], resulting from exchange of the bridging imido ligand for sulfur, together with small amounts of [Mo₂(NAr)₂(µ-S)(µ-S₂)(S₂CNEt₂)₂], containing bridging sulfide and disulfide ligands. The ureato complexes [Mo₂(NAr)₂(µ-S){µ-ArNC(O)NAr}(S₂CNR₂)₂] react further with PhNCS to give [Mo₂(NAr)₂(µ-S)₂(S₂CNR₂)₂]_n (n = 1, 2), which exist in a dimer–tetramer equilibrium. In order to confirm these results crystallographic studies have been carried out on [Mo₂(N-p-tol)₂(µ-S)(µ-S₂)(S₂CNEt₂)₂] and [Mo₂(N-p-tol)₂(µ-S)₂(S₂CNPr₂)₂]₂.