Nitrene Metathesis and Catalytic Nitrene Transfer Promoted by Niobium Bis(imido) Complexes

Abstract

We report a metathesis reaction in which a nitrene fragment from an isocyanide ligand is exchanged with a nitrene fragment of an imido ligand in a series of niobium bis(imido) complexes. One of these bis(imido) complexes also promotes nitrene transfer to catalytically generate asymmetric dialkylcarbodiimides from azides and isocyanides in a process involving the Nb(V)/Nb(III) redox couple.

1,2-Addition and cycloaddition reactions of niobium bis(imido) and oxo imido complexes

“…1.7-1.8Å), while the imido bond distance of 1.816(1)Å is among the longest distances reported for terminal Nb imido bonds and is similar to the distances observed in related bis(imido) complexes. 21 Similarly, the Nb-N(3)-C(30) bond angle of 156.1(1) is among the most bent observed in Nb terminal imido complexes. Thus, the effect of p-loading in this system primarily manifests in the metrical parameters of the imido group; however, this is not an indication that the imido group is expected to be more reactive than the oxo group.…”

“…We are interested in developing p-loaded Nb bis(imido) and oxo imido systems that can carry out complex hydrofunctionalizations of unsaturated substrates, such as hydroborations and hydrosilations, using 1,2-addition and elimination reactions across imido or oxo groups. We have previously described the synthesis of Nb bis(imido) complexes: 21,67 p-loading effects in these compounds were shown to engender reactivity across their Nb-N p-bonds 7,21 relative to the chemistry observed with related mono(imido) Nb systems. 7,[57][58][59][60][61][62][63][64] Here we describe their 1,2-addition and [2 + 2] cycloaddition 68 reactivity with a variety of small molecule substrates including dihydrogen, silanes, boranes, carbon disulde, and carbon dioxide.…”

Group 5 chemistry supported by β-diketiminate ligands

“…It was hypothesized that introduction of a second imido group would serve to increase reactivity across the imido groups by creating a "π-loading" effect, 45,63 increasing the polarization of the imido bonds by electronically saturating the niobium d π orbitals. 64 In a recent report, the reactions of bis(imido) complexes 136, 139, and 141 with isocyanides were described. In order to access the more reactive 4-coordinate bis(imido) species 136, pyridine was removed using B(C 6 F 5 ) 3 .…”

“…5-coordinate bis(imido) complexes 134 and 135 could be accessed either from reduction of 80 and trapping with azide 49a or from reaction of 80 with LiNH t Bu, 45 as shown in Scheme 38. 64 As shown in Scheme 41, compound 136 reacted with arylisocyanide substrates to exchange the nitrene fragments from the isocyanide substrate and an imido group in order to generate compounds 142a-c. Contrastingly, in the presence of primary and secondary alkylisocyanide substrates and excess tert-butylazide, compound 136 catalyzed nitrene transfer to form asymmetric carbodiimides. Under these conditions, 136 could be accessed in situ, but degraded to 126 faster than it could be isolated from the byproduct Py·B(C 6 F 5 ) 3 .…”

Olefin‐Supported Rhenium(III) Terminal Oxo Complexes Generated by Nucleophilic Addition to a Cyclopentadienyl Ligand

“…[15] Thet opic of "p-loading" and its effect on the reactivity of metal-ligand multiple bonds has been explored previously in our groups,m ost notably in d 0 group 5bis(imido) systems. [17] Thecalculated LUMO of 3 + further helped to rationalize the behavior of this complex. This orbital is antibonding with respect to the oxo ligand and Re center, while the Cp ligand is forming some bonding interaction to the metal at two of its carbon atoms,a nd the remainder of the ligand is either antibonding or nonbonding with respect to Re.T he presence of metal-ligand antibonding interactions in the LUMO of 3 + involving both the oxo and Cp ligands aligns with our experimental observation that both groups can readily undergo nucleophilic attack, resulting in aloss of or decrease in coordination to the metal.…”