Synthesis and small molecule chemistry of the niobaziridine-hydride functional group

Abstract

Chapter 1. Synthesis and Divergent Reactivity of the Niobaziridine-Hydride Functional Group The synthesis, characterization and reactivity of the niobaziridine-hydride complex Nb(H)([eta]²-t- ]Bu(H)C=NAr)(N[Np]Ar)₂ (la-H; Np = neopentyl, Ar = 3,5-Me₂C₆H₃) is discussed. The niobaziridine-hydride functional group in complex la-H serves as a protecting group for the reactive three-coordinate d² species, Nb(N[Np]Ar)₃ (2a), via reversible C-H activation of the 3- ]H-containing N[Np]Ar ligand. At elevated temperatures, complex la-H rapidly converts to the neopentylimido complex, Nb(NNp)(Ar)(N[Np]Ar)₃ (3a), which is the product of N[Np]Ar ligand C'-N oxidative addition by putative three-coordinate 2a. Although not observed directly, the evidence for the intermediacy of 2a in N[Np]Ar ligand C-H and C-N activation processes has been obtained through isotopic labeling (H/D) studies. To further ascertain the propensity of la- H to serve as a masked form of 2a, its reactivity with small-molecule substrates was surveyed. Treatment of la-H with nitrous oxide (N₂0) or triphenylphosphine oxide (OPPh₃), readily generated the oxo Nb(V) complex, ONb(N[Np]Ar)₃, thus establishing la-H as a source of the potent two-electron reductant 2a. Complex la-H was also found to effect the two-electron reduction of a host of other inorganic substrates. However, when treated with certain unsaturated organic molecules, insertion into the Nb-H moiety of complex la-H is observed in which the niobaziridine ring is left intact. Based on synthetic studies, a coordinatively induced, C-H bond reductive elimination mechanism is proposed for reactions between la-H and small molecules.

(cont.) This mechanistic proposal accounts for both the observed insertion and two-electron reduction behavior exhibited by niobaziridine-hydride la-H. To extend the generality of niobaziridine- hydride functional group as a protecting group for three-coordinate Nb(NR₂)₃ species, the complexes Nb(H)([eta]²-Me₂C=NAr)(N[i-Pr]Ar)₂ (lb-H) and Nb(H)([eta]²-Ad(H)C=NAr)(N[CH₂- Ad]Ar)₂ (1c-H) were synthesized. The thermal behavior of niobaziridine-hydrides lb-H and lc- 1-I is compared and contrasted to that of the N-neopentyl variant la-H. Chapter 2. Activation of Elemental Phosphorus: Synthesis of an Anionic Terminal Phosphide of Niobium The niobazinidine-hydride complex Nb(H)( [eta]²-t-Bu(H)C=NAr)(N[Np]Ar)₂ (la-H; Np neopentyl, Ar = 3,5-Me₂C₆H₃) was found to react quantitatively with elemental phosphorus (P₄) to provide the bridging diphosphide complex ([mu]₂:[eta]²,[eta]²-P₂)[Nb(N[Np]Ar)₃]₂ ([mu]-P₂)[2a]₂. Reductive cleavage of ([mu]-P₂)[2a]₂ with sodium amalgam afforded the sodium salt of the terminal niobium phosphide anion, [PNb(N[Np]Ar)₃)]⁻ ([2a-P]⁻), which is best formulated as containing a Nb-P triple bond. The phosphorus atom of [2a-P]⁻ has proven to be nucleophilic and is readily functionalized upon addition of an electrophile. Treatment of [2a-P]⁻ with trimethylstannyl chloride provided the terminal phosphinidene complex Me₃SnP=Nb(N[Np]Ar)₃ which contains a P-Sn single bond. However, treatment of [2a-P]⁻ with ClP(t-Bu)₂ or ClP(Ph)₂ provided niobium- complexed [eta]²-phosphinophosphinidene complexes, which contain considerable P-P multiple bonding character. Thus, substantial electronic reorganization of the Nb [equal] P moiety in [2a-P]⁻ is induced upon functionalization.

(cont.) The tendency for the Nb[equal]P unit in [2a-P]- to undergo electronic reorganization has been exploited, resulting in the synthesis of a complexed [eta]²-P,P- diphosphaorganoazide (PPNR) species, which eliminates a 'P2' unit when heated. Furthermore, treatment of the phosphido anion [2a-P]⁻ with divalent group 14 salts affords complexes of the formulation ([mu]₂:[eta]₃,[eta]₃ -cyclo-EP₂)[Nb(N[Np]Ar)₃]₂ (E = Ge, Sn, Pb). The bridging cyclo-EP₂ units in these complexes can be considered as neutral 2[pi]-electron, three-membered rings isolobal to the cyclopropenium ion. The molecular and electronic structure of anion [2a-P]⁻ and several of its derivatives are discussed. Chapter 3. Isovalent Pnictogen for O(Cl) Exchange Mediated by Terminal Pnictide Anions of Niobium Reported herein is a new, metathetical P for O(Cl) exchange mediated by an anionic niobium phosphide complex which furnished phosphaalkynes (RC [equal] P) from acyl chlorides (RC(O)Cl) under mild conditions. The niobaziridine hydride complex, Nb(H)(t-Bu(H)C=NAr)(N[Np]Ar)₂ (la-H, Np = neopentyl, Ar = 3,5-Me₂C₆H₃), has been shown in chapter 2 to react with elemental phosphorus (P₄) affording the [mu]-diphosphide complex, ([mu]²:[eta]²,[eta]²-P₂)[Nb(N[Np]Ar)₃]₂ (([mu]- P)[2a]₂), which can be subsequently reduced by sodium amalgam to the anionic, terminal phosphide complex, [Na][PNb(N[Np]Ar)₃] (Na[2a-P]).

(cont.) Treatment of Na[2a-P] with either ]pivaloyl (t-BuC(O)Cl) or 1-adamantoyl (1-AdC(O)Cl) chloride provides the thermally unstable, niobacycles, (t-BuC(O)P)Nb(N[Np]Ar)₃ (2-t-Bu) and (1-AdC(O)P)Nb(N[Np]Ar)₂ (2-1-Ad) which are intermediates along the pathway to ejection of the known phosphaalkynes t-BuC-P (3- t-Bu) and 1-AdC [equal] P (3-1-Ad). Phosphaalkyne ejection from 2-t-Bu and 2-1-Ad proceeds with formation of the niobium(V) oxo complex ONb(N[Np]Ar)₃ (2a-0) as a stable byproduct. Preliminary kinetic measurements for fragmentation of 2-t-Bu to 3-t-Bu and 2a-0 in C₆D₆ solution are consistent with a first order process. Separation of volatile 3-t-Bu from 2a-0 after thermolysis has been readily achieved by vacuum transfer in yields of 90%. Pure 2a-0 is recovered after vacuum transfer and can be treated with 1.0 equivalent of triflic anhydride (Tf₂O, Tf = SO₂CF₃) to afford the bistriflate complex, Nb(OTf)₂(N[Np]Ar)₃ (2a-(OTf)₂), in high yield. Complex 2a-(OT'f)₂ provides direct access to la-H upon reduction with magnesium anthracene, thus completing a cycle of element activation, small-molecule generation via metathetical P-atom transfer and deoxygenative recycling of the final niobium(V) oxo product. Extension of this metathetical P for O(C1) exchange to the synthesis of novel phosphaalkynes is discussed. In addition, the analogous N for O(C1) exchange reaction for the synthesis of organonitriles from the niobium nitrido anion, [NNb(N[Np]Ar)₃]⁻ ([2a-N]⁻) has been developed. Nitrido anion [2a- N]⁻ is obtained in a heterodinuclear N₂ scission reaction which employs the molybdenum trisamide system as a reaction partner.

(cont.) Treatment of [2a-N]⁻ with acyl chloride substrates rapidly furnishes organonitriles concomitant with the formation of niobium oxo 2a-0. Deoxgenative recycling of 2a-0 to a niobium complex appropriate for heterodinuclear N2 scission has been developed as well. Utilization of ¹⁵N-labeled ¹⁵N₂ gas in this chemistry has afforded a series of ¹⁵N-labeled organonitriles which have been characterized by solution ¹⁵N NMR. While, no intermediate complexes are observed during the organonitrile formation process, synthetic and computational studies on model systems provide strong evidence for the intermediacy of niobacyclic species.