Imido Complexes Derived from the Reactions of Niobium and Tantalum Pentachlorides with Primary Amines:  Relevance to the Chemical Vapor Deposition of Metal Nitride Films

Reactions of niobium and tantalum pentachlorides with tert-butylamine (≥6 equiv) in benzene afford the dimeric imido complexes [NbCl2(NtBu)(NHtBu)(NH2 tBu)]2 (90%) and [TaCl2(NtBu)(NHtBu)(NH2 tBu)]2 (79%). The niobium complex exists as two isomers in solution, while the tantalum complex is composed of three major isomers and at least two minor isomers. Analogous treatments with isopropylamine (≥7 equiv) give the monomeric complexes NbCl2(NiPr)(NHiPr)(NH2 iPr)2 (84%) and TaCl2(NiPr)(NHiPr)(NH2 iPr)2 (84%). The monomeric complexes are unaffected by treatment with excess isopropylamine, while the dimeric complexes are cleaved to the monomers MCl2(NtBu)(NHtBu)(NH2 tBu)2 upon addition of excess tert-butylamine in chloroform solution. Treatment of niobium and tantalum pentachlorides with 2,6-diisopropylaniline affords insoluble precipitates of [NH3(2,6-(CH(CH3)2)2C6H3)]2[NbCl5(N(2,6-(CH(CH3)2)2C6H3))] (100%) and [NH3(2,6-(CH(CH3)2)2C6H3)]2[TaCl5(N(2,6-(CH(CH3)2)2C6H3))] (100%), which react with 4-tert-butylpyridine to afford the soluble complexes [4-t-C4H9C5H4NH]2[NbCl5(N(2,6-(CH(CH3)2)2C6H3))] (45%) and [4-t-C4H9C5H4NH]2[TaCl5(N(2,6-(CH(CH3)2)2C6H3))] (44%). Sublimation of [NbCl2(NtBu)(NHtBu)(NH2 tBu)]2, MCl2(NiPr)(NHiPr)(NH2 iPr)2, and [NH3(2,6-(CH(CH3)2)2C6H3)]2[MCl5(N(2,6-(CH(CH3)2)2C6H3))] leads to decomposition to give [MCl3(NR)(NH2R)]2 as sublimates (32−49%), leaving complexes of the proposed formulation MCl(NR)2 as nonvolatile residues. By contrast, [TaCl2(NtBu)(NHtBu)(NH2 tBu)]2 sublimes without chemical reaction. Analysis of the organic products obtained from thermal decomposition of [NbCl2(NtBu)(NHtBu)(NH2 tBu)]2 showed isobutylene and tert-butylamine in a 2.2:1 ratio. Mass spectra of [NbCl2(NtBu)(NHtBu)(NH2 tBu)]2, [TaCl2(NtBu)(NHtBu)(NH2 tBu)]2, and [NbCl3(NiPr)(NH2 iPr)]2 showed the presence of dimeric imido complexes, monomeric imido complexes, and nitrido complexes, implying that such species are important gas phase species in CVD processes utilizing these molecular precursors. The crystal structures of [4-t-C4H9C5H4NH]2[NbCl5(N(2,6-(CH(CH3)2)2C6H3))], [NbCl3(NiPr)(NH2 iPr)]2, [NbCl3(N(2,6-(CH(CH3)2)2C6H3))(NH2(2,6-(CH(CH3)2)2C6H3))]2, and [TaCl3(N(2,6-(CH(CH3)2)2C6H3))(NH2(2,6-(CH(CH3)2)2C6H3))]2 were determined. [4-t-C4H9C5H4NH]2[NbCl5(N(2,6-(CH(CH3)2)2C6H3))] crystallizes in the space group P21/c with a = 12.448(3) Å, b = 10.363(3) Å, c = 28.228(3) Å, β = 94.92(1)°, V = 3628(5) Å3, and Z = 4. [NbCl3(NiPr)(NH2 iPr)]2 crystallizes in the space g