Base-Free and Bisphosphine Ligand Dialkylmanganese(II) Complexes as Precursors for Manganese Metal Deposition

The
solid-state structures and the physical, solution magnetic,
solid-state magnetic, and spectroscopic (NMR and UV/vis) properties
of a range of oxygen- and nitrogen-free dialkylmanganese­(II) complexes
are reported, and the solution reactivity of these complexes toward
H2 and ZnEt2 is described. The compounds investigated
are [{Mn­(μ-CH2SiMe3)2}∞] (1), [{Mn­(CH2CMe3)­(μ-CH2CMe3)2}2{Mn­(μ-CH2CMe3)2Mn}] (2), [Mn­(CH2SiMe3)2(dmpe)]
(3; dmpe = 1,2-bis­(dimethylphosphino)­ethane), [{Mn­(CH2CMe3)2(μ-dmpe)}2] (4), [{Mn­(CH2SiMe3)­(μ-CH2SiMe3)}2(μ-dmpe)] (5), [{Mn­(CH2CMe3)­(μ-CH2CMe3)}2(μ-dmpe)] (6), [{Mn­(CH2SiMe3)­(μ-CH2SiMe3)}2(μ-dmpm)]
(7; dmpm = bis­(dimethylphosphino)­methane), and [{Mn­(CH2CMe3)­(μ-CH2CMe3)}2(μ-dmpm)] (8). Syntheses for 1–4 have previously been reported, but the solid-state
structures and most properties of 2–4 had not been described. Compounds 5 and 6, with a 1:2 dmpe/Mn ratio, were prepared by reaction of 3 and 4 with base-free 1 and 2, respectively. Compounds 7 and 8 were
accessed by reaction of 1 and 2 with 0.5
equiv or more of dmpm per manganese atom. An X-ray structure of 2 revealed a tetrametallic structure with two terminal and
six bridging alkyl groups. In the solid state, bisphosphine-coordinated 3–8 adopted three distinct structural
types: (a) monometallic [LMnR2], (b) dimetallic [R2Mn­(μ-L)2MnR2], and (c) dimetallic
[{RMn­(μ-R)}2(μ-L)] (L = dmpe, dmpm). Compound 3 exhibited particularly desirable properties for an ALD or
CVD precursor, melting at 62–63 °C, subliming at 60 °C
(5 mTorr), and showing negligible decomposition after 24 h at 120
°C. Comparison of variable-temperature solution and solid-state
magnetic data provided insight into the solution structures of 2–8. Solution reactions of 1-8 with H2 yielded manganese metal, demonstrating
the thermodynamic feasibility of the key reaction steps required for
manganese­(II) dialkyl complexes to serve, in combination with H2, as precursors for metal ALD or pulsed CVD. In contrast,
the solution reactions of 1–8 with
ZnEt2 yielded a zinc–manganese alloy with an approximate
1:1 Zn/Mn ratio.