Synthesis, structure, properties, volatility, and thermal stability of molybdenum(II) and tungsten(II) complexes containing allyl, carbonyl, and pyrazolate or amidinate ligands

Treatment of M(allyl)(Cl)(CO)2(py)2 (M = Mo, W) with 1 equiv. of potassium pyrazolates in tetrahydrofuran at −78 °C afforded M(allyl)(R2pz)(CO)2(py)n (R2pz = 3,5-disubstituted pyrazolate; n = 1, 2) in 68–81% yields. X-ray crystal structure analyses of Mo(allyl)((CF3)2pz)(CO)2(py)2 and W(allyl)(tBu2pz)(CO)2(py) revealed η1- and η2-coordination of the (CF3)2pz and tBu2pz ligands, respectively. Analogous treatment of Mo(allyl)(Cl)(CO)2(NCCH3)2 with 1 equiv. of tBu2pzK in tetrahydrofuran at −78 °C afforded [Mo(allyl)(tBu2pz)(CO)2]2 in 79% yield. An X-ray crystal structure analysis of [Mo(allyl)(tBu2pz)(CO)2]2 showed a dimeric structure bridged by two μ-η2:η1-tBu2pz ligands. Treatment of M(allyl)(Cl)(CO)2(py)2 with 1 equiv. of lithium 1,3-diisopropylacetamidinate or lithium 1,3-di-tert-butylacetamidinate in diethyl ether at −78 °C afforded M(allyl)(iPrNC(Me)NiPr)(CO)2(py) and M(allyl)(tBuNC(Me)NtBu)(CO)2(py), respectively, in 68–78% yields. The new complexes were characterized by spectral and analytical methods and by X-ray crystal structure determinations. M(allyl)(iPrNC(Me)NiPr)(CO)2(py) adopt pseudo-octahedral geometry about the metal centers, with the 1,3-diisopropylacetamidate ligand nitrogen atoms spanning one axial site and one equatorial site of the octahedron. By contrast, M(allyl)(tBuNC(Me)NtBu)(CO)2(py) adopt pseudo-octahedral structures in which the two 1,3-di-tert-butylacetamidinate ligand nitrogen atoms span two equatorial coordination sites. Sublimation of M(allyl)(tBuNC(Me)NtBu)-(CO)2(py) at 105 °C/0.03 Torr afforded ⩽7% yields of M(allyl)(tBuNC(Me)NtBu)(CO)2, along with sublimed M(allyl)(tBuNC(Me)NtBu)(CO)2(py). W(allyl)(tBuNC(Me)NtBu)(CO)2 exists in the solid state as a 16-electron complex with distorted square pyramidal geometry. Many of the new complexes undergo dynamic ligand site exchange in solution, and these processes were probed by variable temperature 1H NMR spectroscopy. The volatilities and thermal stabilities were evaluated to determine the potential of the new complexes for use as precursors in thin film growth experiments.

Graphical abstractThe synthesis, structure, dynamic NMR spectra, volatility, and thermal stability of the molybdenum(II) and tungsten(II) complexes M(allyl)(X)(CO)2(py)n and [M(allyl)(X)(CO)2]2 are described, where X is an amidinate or pyrazolate ligand and n = 0, 1, or 2.Figure optionsDownload full-size imageDownload as PowerPoint slide