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 towards H2 and ZnEt2 is described. The dialkyl 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 published, 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 or more equivalents 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, bis(phosphine)-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 or 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. By contrast, the solution reactions of 1-8 with ZnEt2 yielded a zinc-manganese alloy with an approximate 1:1 Zn:Mn ratio.
Wilkinson’s manganese(I) ethylene hydride complex trans-[(dmpe)2MnH(C2H4)] (10) can react as a source of a low-coordinate manganese(I) ethyl complex. This is illustrated in the reactivity of 10 towards a variety of reagents in this work (vide infra). The proposed low-coordinate intermediate, [(dmpe)2MnEt] (13), was not observed spectroscopically, but could be trapped using isonitrile ligands; reaction of 10 with CNR (R = tBu, o-xylyl) afforded the manganese(I) ethyl complexes [(dmpe)2MnEt(CNR)] (14a: R = tBu, 14b: R = o-xylyl). Ethyl complex 14a did not react further with CNtBu at 80 °C. By contrast, complex 14b reacted with excess o-xylyl isonitrile to form 1,1 insertion products, including the iminoacyl complex [(dmpe)Mn(CNXyl)3{C(=NXyl)CEt(=NXyl)}] (15, Xyl = o-xylyl). Complexes 14a-b and 15, as well as previously reported 10, were crystallographically characterized, and DFT calculations were employed to probe the accessibility of cis ethylene hydride and ethyl isomers of 10.
Reaction of the ethylene hydride complex trans-[(dmpe)2MnH(C2H4)] (10) with H2SiEt2 at 20 °C afforded the silylene hydride [(dmpe)2MnH(=SiEt2)] (16Et2) as the trans isomer. By contrast, reaction of 10 with H2SiPh2 at 60 °C afforded [(dmpe)2MnH(=SiPh2)] (16Ph2) as a mixture of the cis (major) and trans (minor) isomers, featuring a Mn–H–Si interaction in the former. The reaction to form 16Ph2 also yielded [(dmpe)2MnH2(SiHPh2)] (18Ph2); [(dmpe)2MnH2(SiHR2)] {R = Et (18Et2) and Ph (18Ph2)} were accessed cleanly by reaction of 16R2 with H2. Both 16Et2 and 16Ph2 engaged in unique reactivity with ethylene, generating the silene hydride complexes cis-[(dmpe)2MnH(R2Si=CHMe)] {R = Et (19Et2) and Ph (19Ph2)}. Compounds trans-16Et2, cis-16Ph2, and 19Ph2 were crystallographically characterized, and bonding in 16Et2 and 19Et2 was probed computationally.
trans-[(dmpe)2MnH(C2H4)] (10) reacted with primary hydrosilanes H3SiR (R = Ph, nBu) at 60 °C to afford ethane and the manganese disilyl hydride complexes [(dmpe)2MnH(SiH2R)2] (20Ph: R = Ph, 20Bu: R = nBu). 20R reacted with ethylene to form silene hydride complexes [(dmpe)2MnH(RHSi=CHMe)] (19Ph,H: R = Ph, 19Bu,H: R = nBu). Compounds 19R,H reacted with a second equivalent of ethylene to generate [(dmpe)2MnH(REtSi=CHMe)] (19Ph,Et: R = Ph, 19Bu,Et: R = nBu), resulting from apparent ethylene insertion into the silene Si–H bond. Furthermore, in the absence of ethylene, silene complex 19Bu,H slowly isomerized to the silylene hydride complex [(dmpe)2MnH(=SiEtnBu)] (16Bu,Et). Reactions of 20R with ethylene likely proceed via low-coordinate silyl {[(dmpe)2Mn(SiH2R)] (17Ph: R = Ph, 17Bu: R = nBu)} or silylene-hydride {[(dmpe)2MnH(=SiHR)] (16Ph,H: R = Ph, 16Bu,H: R = nBu)} intermediates accessed from 20R by H3SiR elimination. DFT calculations and high temperature NMR spectra support the accessibility of these intermediates, and reactions of 20R with isonitriles or N-heterocyclic carbenes yielded the silyl isonitrile complexes [(dmpe)2Mn(SiH2R)(CNR')] (21a-d: R = Ph or nBu; R' = o-xylyl or tBu), and NHC-stabilized silylene-hydride complexes [(dmpe)2MnH{=SiHR(NHC)}] (22a-d: R = Ph or nBu; NHC = 1,3-diisopropylimidazolin-2-ylidene or 1,3,4,5-tetramethyl-4-imidazolin-2-ylidene), respectively, all of which were crystallographically characterized.
Manganese silyl dihydride complexes [(dmpe)2MnH2(SiHR2)] {R = Ph (18Ph2) or Et (18Et2)} and [(dmpe)2MnH2(SiH2R)] {R = Ph (18Ph) or nBu (18Bu)} were generated by exposure of silylene hydride complexes, [(dmpe)2MnH(=SiR2)] (16R2), and disilyl hydride complexes, [(dmpe)2MnH(SiH2R)2] (20R), respectively, to H2 at room temperature. In solution, 18R and 18R2 exist as an equilibrium mixture of a central isomer with a meridional H–Si–H arrangement of the silyl and hydride ligands {this isomer may be considered to contain an η3-coordinated silicate (H2SiR3–) anion}, and a transHSi isomer with trans-disposed hydride and nonclassical hydrosilane ligands (the latter is the result of significant but incomplete hydrosilane oxidative addition). Additionally, DFT calculations indicate the thermodynamic accessibility of lateralH2 and transH2 isomers with cis- and trans-disposed silyl and dihydrogen ligands, respectively. Compounds 18Ph2 and 18Ph crystallized as the central isomer, whereas 18Bu crystallized as the transHSi isomer. Bonding in the central and transHSi isomers of 18R and 18R2 was further investigated through 29Si_edited 1H–1H COSY solution NMR experiments to determine both the sign and magnitude of J29Si,1H coupling (negative and positive values of J29Si,1H are indicative of dominant 1-bond and 2-bond coupling, respectively). These experiments afforded J29Si,1H coupling constants of –47 Hz for η3-(H2SiR3) in the central isomer of 18Et2 (calcd. –40 to –47 for 18R and 18R2), –38 to –54 Hz for η2-(R3Si–H) in the transHSi isomer of 18R and 18R2 (calcd. –26 to –47 Hz), and 5 to 9 Hz for the terminal manganese hydride ligand in the transHSi isomer of 18Et2, 18Ph, and 18Bu (calcd. 12 to 14 Hz for 18R and 18R2), experimentally supporting the nonclassical nature of bonding in the central and transHSi isomers.
Exposure of disilyl hydride complexes 20R to diisopropylcarbodiimide {C(NiPr)2} afforded manganese(I) amidinylsilyl complexes [(dmpe)2Mn{κ2-SiHR(NiPrCHNiPr)}] {R = Ph (25Ph,H) or nBu (25Bu,H)}. DFT calculations and analysis of XRD bond metrics suggest that the structure of 25R,H involves a contribution from a resonance structure featuring a neutral base-stabilized silylene and an anionic amido donor on manganese. Reactions of 20R, as well as the silylene hydride complex 16Et2, with CO2 yielded the manganese(I) formate complex trans-[(dmpe)2Mn(CO)(κ1-O2CH)] (26), with a polysiloxane byproduct. Compound 26 was found to undergo reversible CO2 elimination at room temperature, and was only stable under an atmosphere of CO2. Complexes 25R,H and 26 were crystallographically characterized.
Silyl, silylene, and silene complexes in this work were accessed via reactions of [(dmpe)2MnH(C2H4)] (10) with hydrosilanes, in some cases followed by ethylene. Therefore, ethylene (C2H4 and C2D4) hydrosilylation was investigated using [(dmpe)2MnH(C2H4)] (10) as a pre-catalyst, resulting in stepwise conversion of primary to secondary to tertiary hydrosilanes. Various catalytically active manganese-containing species were observed during catalysis, including silylene and silene complexes, and a catalytic cycle is proposed. The proposed catalytic cycle is unusual due to the involvement of silylene hydride and silene hydride complexes, potentially as on-cycle species.
The reaction of [(dmpe)2MnH(C2H4)] (10) with H2 at 60 °C afforded ethane and the dihydrogen hydride complex [(dmpe)2MnH(H2)] (11), which has previously been prepared by an alternative route. Complex 10 reacted with hydroborane reagents 9-BBN or HBMes2 at 60 °C to afford EtBR2 and Mn(I) borohydride complexes [(dmpe)2Mn(μ-H)2BR2] (29: R2 = C8H14, 30: R = Mes); two intermediates were observed in each of these reactions. Deuterium labelling experiments using the deuterated hydroborane DBMes2 suggest that this reaction proceeds via the 5-coordinate ethyl isomer of 10; [(dmpe)2MnEt] (13). By contrast, exposure of 10 to BH3∙NMe3 required a higher temperature (90 °C) to yield [(dmpe)2Mn(μ-H)2BH2] (28), and ethylene was formed as the reaction byproduct; this reaction presumably proceeded by ethylene substitution. Deuterium incorporation into both the MnH and BH environments of 28 was observed under an atmosphere of D2 at 90 °C. Reactions of 10 with free dmpe yielded ethylene and a mixture of [{(dmpe)2MnH}2(μ-dmpe)] (31) and [(dmpe)2MnH(κ1-dmpe)] (32), which could be isolated by washing/recrystallization or sublimation, respectively. Similar reactivity was observed between 10 and HPPh2, which afforded ethylene and [(dmpe)2MnH(HPPh2)] (33) at 90 °C. Exposure of 10 to HSnPh3 yielded the manganese(II) stannyl hydride complex [(dmpe)2MnH(SnPh3)] (34) along with ethylene and, presumably, additional unidentified products. However, the mechanism for formation of 34 is unclear, it could not be isolated in pure form due to decomposition to form various species including SnPh4, and the mechanism of the decomposition process remains obscure. Previously reported complex 11, along with new complexes 28-31 and 33-34, were crystallographically characterized.
This work provides valuable insights to unusual metal–ligand bonding motifs and reactions, and as such contributes to the fundamental understanding of organometallic chemistry. / Dissertation / Doctor of Philosophy (PhD) / The focus of this work is the synthesis and investigation of manganese-containing complexes with Mn–P, Mn–C, Mn–H, and/or Mn–Si linkages. Many of these complexes feature unusual bonding motifs, including the first group 7 complexes bearing an unstabilized silylene (:SiR2) ligand and the first 1st row transition metal complexes bearing an unstabilized silene (R2Si=CR2) ligand. Variable temperature Nuclear Magnetic Resonance (NMR) spectroscopy and X-ray crystallography were employed to investigate the structures of these complexes, while Density Functional Theory (DFT) calculations and trapping experiments were employed to understand the mechanisms for various unusual chemical transformations. Some of the complexes were evaluated for activity towards catalytic hydrosilylation of ethylene. This work provides valuable insights to unusual metal–ligand bonding motifs and reactions, and as such contributes to the fundamental understanding of organometallic chemistry.
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/25227 |
| Date | January 2020 |
| Creators | Price, Jeffrey S. |
| Contributors | Emslie, David J. H., Chemistry |
| Source Sets | McMaster University |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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