Catalytic Production of Isocyanates via Orthogonal Atom and Group Transfers Employing a Shared Formal Group 6 M(II)/M(IV) Redox Cycle

Abstract

Under an atmosphere of CO, the Mo(IV) imido complex Cp*Mo[N(iPr)C(Me)N(iPr)](NSiMe3) (Cp* = η5-C5Me5) (1) serves as a catalyst for production of an isocyanate via metal-mediated nitrene group transfer in benzene solution under mild conditions (55 °C, 10 psi) according to RN3 + CO → N2 + RNCO. Mechanistic and structural studies support a catalytic cycle for nitrene group transfer involving formal Mo(II) monocarbonyl and Mo(IV) (κ2-C,N)-isocyanate intermediates. These results complement an earlier finding that catalytic production of isocyanates can alternatively proceed through oxygen-atom transfer and an isomeric Mo(IV) (κ2-C,O)-isocyanate according to N2O + CNR → N2 + RNCO.

Atom‐ and Group Transfers to MIV Oxo and Imido Complexes, (η5‐C5Me5)M[N(iPr)C(Me)N(iPr)](E) (M = Mo, W; E = O, NSiMe3): Orthogonal Generation of a MVI Terminal Nitride via Inter‐ligand Silyl Group Migration

“…From these data, it can be seen that the gross molecular structure of 7 is similar to that of 5 . However, the W1–N3 bond length is noticeably longer at 1.7063(14) Å and this value can be compared to the W–N imido bond length of 1.763(4) Å previously determined for 4 3. Finally, as Scheme also reveals, reaction of 4 in pentane solution with N 2 O (15 psi) provided a near quantitative yield of 7 .…”

“…As Scheme and the solid‐state structural data presented in Figure 1 and Table 1 reveal, this material proved to be the mononuclear Mo VI nitrido, silyloxide, Cp*Mo[N( i Pr)C(Me)N( i Pr)](N)(OSiMe 3 ) ( 5 ) 6. A notable structural feature of 5 is the Mo1–N3 bond length of 1.6728(11) Å that is indicative of a strong formal triple bond interaction and which can be compared to the formal Mo=N double bond length of 1.758(3) Å determined for the terminal imido 3 3. In similar fashion, the Mo1–O1 single bond length of 1.9942(9) Å can be compared with the formal Mo=O double bond length of 1.7033(19) Å reported previously for the terminal oxo 1 2a.…”

“…As previously reported, the group 6 metal terminal oxo and imido complexes, 1 – 4 , can be readily prepared in high yield from the corresponding group 6 dinuclear ‘end‐on‐bridged' dinitrogen complexes, {Cp*M[N( i Pr)C(Me)N( i Pr)]} 2 (μ‐η 1 :η 1 ‐N 2 ) where M = Mo ( I ) and W ( II ),5 through either OAT using nitrous oxide, N 2 O, in the case of the terminal oxo complexes, 1 and 2 , or NGT using trimethylsilylazide, N 3 SiMe 3 , in the case of the terminal imido complexes, 3 and 4 , according to Scheme 2,3…”

“… Synthesis of group 6 M IV terminal oxo and imido complexes, 1 – 4 according to previously published procedures 2,3…”

“…Strategies to develop catalytic processes for ‘small molecule' (e.g., N 2 , N 2 O, CO 2 and CO) fixation that are based on transition‐metal mediated X–Y multiple bond activation, cleavage, and subsequent X‐ and Y‐atom and group transfers to substrates must invariably contend with the potential formation of strong metal‐nitrogen and metal‐oxygen bonds within transition metal nitrido, imido, and oxo species, [L n M(E)] (E = N, NR, and O), that can represent thermodynamic dead‐ends 1. Recently, we have shown that orthogonal catalytic production of isocyanates, RNCO, can be accomplished under mild conditions according to the two different catalytic cycles, a and b , that proceed using the second‐ and third‐row group 6 metal terminal oxo and imido complexes, Cp*M[N( i Pr)C(Me)N( i Pr)](O) (Cp* = η 5 ‐C 5 Me 5 ) where M = Mo ( 1 ) and W ( 2 ) and Cp*M[N( i Pr)C(Me)N( i Pr)](NSiMe 3 ) where M = Mo ( 3 ) and W ( 4 ), respectively, according to Scheme 2,3. We have further speculated that the success of both catalytic transformations encompassed by a and b rests with the unique M II /M IV formal metal oxidation state redox couple through which the collection of intermediate species involved in these cycles proceed.…”