A Transient V^III–Alkylidene Complex: Oxidation Chemistry Including the Activation of N₂ to Afford a Highly Porous Honeycomb‐Like Framework

Abstract

Terminal vanadium alkylidenes [1][2][3][4] still constitute a rare class of complexes, and this type of functional group, when combined with this metal, has been far less explored than its heavier congeners niobium [5,6] and tantalum. [6] Part of this limitation revolves around the ability of vanadium to populate low-valent oxidation states, especially the + 3 state. As a result, alkylation reactions stemming from the high-valent metal complex often lead to reduction as opposed to the more prototypical transmetalation/a-hydrogen abstraction processes encountered in Nb V and Ta V alkylidenes.[6] Out of the few terminal vanadium alkylidene complexes known (e.g. V III ,[1] V IV , [3] and V V[4] ), only the high-valent system appears to be highly reactive for ring-opening metathesis polymerization (ROMP).[4] Therefore, an entry to the V V alkylidene group directly from a V III precursor would represent an attractive synthetic strategy for this metal given the vast quantity of low-valent vanadium halide precursors available. In general, heat, photolysis, Lewis bases, or Brønsted bases can incite a-hydrogen abstraction to form Schrock-like MÀC multiple bonds. [6,7] More recently, a one-electron oxidation step has been also shown to promote a-hydrogen abstraction. [7] When the metal is in a highoxidation state, powerful Lewis acids such as Al(CH 3 ) 3 can also instigate a-hydrogen abstraction as well as trap the MÀC multiple bond.[ [8][9][10] Although a handful of vanadium alkylidenes have been generated by treatment of a bis-alkyl precursor with PR 3 , [1] the concept of using two-electron oxidants to promote a-hydrogen abstraction, to the extent of providing a facile entry to the V V alkylidene unit, has not been realized.Herein we demonstrate that p acids or two-electron oxidants can ultimately lead to a-hydrogen abstraction when the metal in question is a low-valent V III bis-neopentyl species. As a result, a transient, four-coordinate V III neopentylidene can be generated and trapped with a series of two-electron oxidants to afford a family of V V alkylidenes. In the case where the p acid promoting the a-hydrogen abstraction is sterically crowded, N 2 becomes the substrate of choice to afford the first vanadium bridging end-on dinitrogenalkylidene species. The solid state packing diagram for this complex displays a highly "porous" honeycomb-like framework along the activated N 2 unit. During the course of our investigation, we also discovered that the elusive V III alkylidene generated in these set of reactions can be trapped with a chelate p acid such as 2,2'-bipyridine (bpy).When the bis-alkyl precursor, readily prepared from a one-pot reaction of Li(PNP) and [VCl 3 (thf) 3 ] followed by alkylation with the appropriate stoichiometry of LiCH 2 tBu, [11] was treated with N 2 CPh 2 in diethyl ether, a gradual reaction ensued over a period of 12 h to afford the V V alkylidenediphenylmethylene hydrazido [12] complex [(PNP)V( = CHtBu)(N 2 CPh 2 )] (2) in 62 % yield after workup of the reaction mixture (Scheme 1). Li...

Reactivity and Structure of Complexes of Small Molecules: Dinitrogen

“…, with a short NN bond distance of 1.246(6)Å, as well as short imide VN bond (1.757(3)Å). 97 , 98 The lack of N 2 displacement by tBuCN is consistent with the proposal of V V centers. The same group later showed that N 2 can coordinate a V II complex featuring bulky NacNac and ArOligands, leading to two antiferromagnetically coupled V III centers bridged by the reduced end-on N 2 ligand (scheme 25).…”

Pushing the Bounds of Olefin Metathesis with Vanadium

“…[66] Mindiola has reported the preparation of several vanadium(III), vanadium(IV), and vanadium(V) alkylidenes supported by β-diketiminate (nacnac) or PNP-pincer ligands, however no olefin metathesis activity was reported for these species. [67][68][69][70][71]…”

“…The following year, a vanadium(V) alkylidene was prepared through oxidation of a vanadium(III) precursor with a Wittig reagent ( II , Chart 1), however this complex demonstrated no reactivity towards NBE [66] . Mindiola has reported the preparation of several vanadium(III), vanadium(IV), and vanadium(V) alkylidenes supported by β‐diketiminate (nacnac) or PNP‐pincer ligands, however no olefin metathesis activity was reported for these species [67–71] …”

Synthesis, structures, and catalytic reactivity of bis(N-heterocyclic carbene) supported diphenyldiazomethane and 1-azidoadamantane complexes of nickel

“…[4][5][6][7][8][9][10][11][12] While N 2 elimination from diazoalkanes is generally facile upon reaction with metal complexes, the isolation and characterization of metal-bound diazoalkane adducts is possible and has yielded a plethora of complexes spanning multiple coordination modes. [6,7,13,14] Five coordination modes are possible for a diazoalkane moiety bound to a single transition metal center (Chart 1): κ 1 -N end-on coordination involving the terminal N (type A), [15][16][17][18][19][20][21][22] η 2 -N,N side-on (type B), [23][24][25][26][27][28] η 2 -C,N side-on (type C), κ 2 -C,N metallacyclic (type D), and κ 1 -C coordination (type E). Coordination of a diazoalkane to more than one metal atom is also possible.…”