Cr(iPrNC(Me)NiPr)3
The amidinato complex Cr(iPrNCMeNiPr)2 is monomeric and sublimes at 50°C/10-3 torr (according to other data, sublime with <1% residue between 120 and 165 °C at 0.05 Torr). It is thermally stable at >300 °C under an inert atmosphere (N2, Ar). It has been synthesized by the treatment of anhydrous CrCl3 with 3 equivalents of 1,3-diisopropylacetamidinatolithium in THF at room temperature producing Cr(iPrNCMeNiPr)3 in 65% yield.
The amidinato complexes Cr(iPrNC(Me)NiPr)3 and Cr(tBuNC(Me)NtBu)2(X) (X = 3,5-dimethylpyrazolate or 3,5-dimethyl-1,2,4-triazolate) are volatile and sublime with <1% residue between 120 and 165 °C at 0.05 Torr; they are thermally stable at >300 °C under an inert atmosphere (N2, Ar). They have been synthesized by the treatment of anhydrous CrCl3 with 2 or 3 equivalents of 1,3-di-tert-butylacetamidinatolithium or 1,3-diisopropylacetamidinatolithium in THF at room temperature, giving Cr(tBuNCMeNtBu)2(Cl)(THF) in 78% yield, Cr(iPrNCMeNiPr)3 in 65% yield. Treatment of Cr(tBuNC(CH3)NtBu)2(Cl)(THF) with the potassium salts 3,5-disubstituted pyrazolates or 3,5-disubstituted 1,2,4-triazolates ligands (X) afforded Cr(tBuNCMeNtBu)2(X) in 65–70% yields. X-Ray crystal structure analyses of Cr(tBuNCMeNtBu)2(3,5-dimethylpyrazolato) and Cr(tBuNCMeNtBu)2(3,5-dimethyl-1,2,4-triazolato) revealed η2-coordination of the ligands.
Due to the good volatility and high thermal stability, these compounds are promising precursors for the ALD growth of chromium-containing thin films. [961]
Dimeric [Cr(tBuNC(CH3)NEt)2]2 sublimes without decomposition at 90°C/10-3 torr), while monomeric Cr(iPrNCMeNiPr)2 and Cr(tBuNCMeNtBu)2 sublime at 50°C/10-3 torr). Such a difference in sublimation temperatures indicates that [Cr(tBuNC(CH3)NEt)2]2 may sublime as a dimeric species. [Cr(tBuNC(CH3)NEt)2]2 exhibits a very different structure than the other dimeric Cr amidinate complexes: it contains a metal-metal quadruple bond with a Cr-Cr bond distance of 1.9601 Å. It is diamagnetic in the solid state, but exhibits paramagnetism in solution arising from dissociation of [Cr(tBuNCMeNEt)2]2 into monomeric Cr(η2-tBuNCMeNEt)2 according to NMR studies, Evans’ method solution magnetic moment measurements and a freezing point depression molecular weight measurement. The combined data indicate that Cr-Cr bond in [Cr(tBuNCMeNEt)2]2 is weak, in contrast to the literature claiming that Cr-Cr quadruple bonds are strong and persist in solution and the solid state.
[Cr(tBuNC(CH3)NEt)2]2 was suggested to be a promising precursor for the growth of semiconductor films with Cr2 magnetic sites because of its structure containing Cr-Cr bonds. For instance, high magnetic moments could be achieved in Cr-doped GaN films (demonstrated to have magnetic ordering temperatures above 400 K). [962]