Small molecule activation by diamidophosphine complexes of vanadium, niobium and tantalum

A series of diamidophosphine ligand precursors, abbreviated RR ′[NPN]Li2(S) (where [NPN] = (R ′NSiMe2CH2)2PR; R = Ph, Cy; R′ = Ph, Mes, Me; S = THF, Et2O, dioxane), were isolated in high yield. These ligands stabilize reactive complexes of the group five metals vanadium, niobium and tantalum. The sensitivity of these ligands to oxygen and water was illustrated by the reaction of CyPh [NPN]Li2(OEt2) with H2O to form ( CyPh[NPN]Li3)O. In this complex, two equivalents of [NPN] sandwich in situ formed Li2O.;The metathesis reaction of RR′[NPN]Li 2(S) and VCl3(THF)3 afforded the chloride-bridged dimers (RR′[NPN]VCl)2. Reduction of these vanadium(III) chlorides with KC8 produced binuclear vanadium complexes with bridging nitrides. These reactions were complicated by the formation of RR′[NPN]V=NR′ and concomitant ligand decomposition. Metathesis of CyPh[NPN]Li 2(OEt2) and NbCl3(dme) (dme = dimethoxyethane) under 1 atm. of N2 gave CyPh[NPN]NbCl(dme). Under 4 atm. of N2, CyPh[NPN]NbCl(dme) and (CyPh [NPN]NbCl)(μ-N2) were produced. Pressure controlled reactivity was also observed in the reaction of PhPh[NPN]Li 2(THF)2 and NbCl3(dme).;The complex CyPh[NPN]NbCl2 formed readily from CyPh[NPN]Li2(OEt2) and NbCl 4(THF)2. CyPh[NPN]NbCl2 was unstable in solution; A Nb(V) imide, CyPh[NPN]NbCl(=NPh), a heterocyclic ring, CyP(CH2SiMe2)2NPh, and a paramagnetic impurity formed spontaneously. Similarly, reduction of PhPh[NPN]NbCl2 with KC8 produced PhPh [NPN]NbCl(=NPh).;Metathesis of CyPh[NPN]Li2(OEt2) with NbCl2Me3 afforded the highly unstable complex CyPh[NPN]NbMe3. Attempts to hydrogenate this species and isolate a niobium hydride were unsuccessful. In contrast, the analogous tantalum complex, CyPh[NPN]TaMe3 was hydrogenated to produce the bimetallic tetrahydride complex (CyPh[NPN]Ta)2(μ-H) 4. This complex reacts readily with N2 to form (CyPh [NPN]Ta)2(μ-H)2(μ-η2:η 1-N2), in which molecular nitrogen is asymmetrically bound. Isotopic labeling studies suggest (CyPh[NPN]Ta)2(μ-H) 2 is the reactive intermediate that activates molecular nitrogen.;Density functional theory was employed to investigate these reactions. B3LYP/LANL2DZ level calculations showed phosphorus donors in optimized structures for niobium analogues of trimethyl complexes ‘NPN’MMe3 (‘NPN’ = (CH3NSiH2CH2)2 PCH3) do not coordinate. Energetics and Kohn-Sham orbitals of the conversion of ‘NPN’Li2(OMe2) to ‘NPN’MMe 3 to (‘NPN’M)2(H)4 and finally the putative dihydride complex (‘NPN’M)2(H)2 are discussed.;Reactions of (RPh[NPN]Ta)2(μ-H)4 (R = Cy, Ph) with primary and secondary phosphines are examined. Addition of one equivalent of HPR′2 (R′ = Ph, Cy) to the tetrahydrides gave the tantalum phosphides (RPh[NPN]Ta) 2(μ-H)3(μ-PR′2). Addition of one equivalent of H2PR″ (R″ = Cy, Ad) to (RPh[NPN]Ta)2(μ-H) 4 generated the corresponding phosphinidenes (RPh[NPN]Ta) 2(μ-H)2(μ-PR″). Mechanistic insights were offered by several labeling studies. For example, addition of HPCy 2 to the deuterated complex (PhPh[NPN]Ta)2(μ-D) 4 produced the isotopomer (PhPh[NPN]Ta)2(μ-H)(μ-D) 2(μ-PCy2). These results also implicated (RPh[NPN]Ta) 2(μ-H)2 as a possible intermediate. Attempts to isolate the dihydride intermediate by trapping with PMe3 promoted the o-metalation of ligand N-Ph rings.;Activation of non-phosphine small molecules by (RPh[NPN]Ta) 2(μ-H)4 complexes is also described. Reaction of the tetrahydride complexes with terminal alkynes gives μ-alkyne structures of the form ( RPh[NPN]Ta)2(μ-H)2(μ-RCCH). Addition of internal alkynes, however, leads only to complex decomposition. Addition of hydrazine to (CyPh[NPN]Ta)2(μ-H)4 gave (CyPh[NPN]Ta)2(μ-H)2(μ-NH) 2 with two bridging imides. Addition of Me2NNH2, however, produced Me2NH and (RPh[NPN]Ta)2(μ-H) 3(μ-N), with one bridging nitride.