[(dmpe)2MnH(C2H4)] as a Source of a Low-Coordinate Ethyl Manganese(I) Species: Reactions with Primary Silanes, H2, and Isonitriles

Reactions of [(dmpe)2MnH(C2H4)] with hydrogermanes to form germylene, germyl, hydrogermane, and germanide complexes

Reactions of the ethylene hydride complex trans-[(dmpe)2MnH(C2H4)] (1) with secondary hydrogermanes H2GeR2 at 55-60 °C afforded the base-free terminal germylene hydride complexes trans-[(dmpe)2MnH(GeR2)] (R = Ph; 2a, R = Et; 2b). Room temperature reactions of 2a or 2b with an excess of the primary hydrogermanes H3GeR' (R' = Ph or nBu) afforded trans-[(dmpe)2MnH(GeHR')] (R' = Ph; 3a, R' = nBu; 3b) in rapid equilibrium with small amounts of 2a/b, as well as the digermyl hydride complex mer-[(dmpe)2MnH(GeH2R')2] {R' = Ph (4a) or nBu (4b)} and the trans-hydrogermane germyl complex trans-[(dmpe)2Mn(GeH2R')(HGeH2R')] {R' = Ph (5a) or nBu (5b)}. Pure 3b was isolated from the reaction of 2b with H3GenBu, whereas 3a decomposed readily in solution in the absence of free H3GePh, and a pure bulk sample was not obtained. Reactions of 1 with H3GeR' (R' = Ph or nBu) also proceeded at 55-60 °C to afford mixtures of 3a/b, 4a/b and 5a/b, accompanied by remaining 1. However, upon continued heating to consume 1, various unidentified manganese-containing intermediates were formed, ultimately affording the germanide complex [{(dmpe)2MnH}2(μ-Ge)] (6) in 17-49% spectroscopic yield. Pure trans,trans-6 was isolated in 27% yield from the reaction of 1 with H3GenBu, and it is notable that this reaction involves stripping of all four substituents from the hydrogermane. Complexes 2a, 3a, and 6 were crystallographically characterized, and the nature of the MnGe bonding in these species (as well as in 2b and 3b) was probed computationally.

Trans Ligand Determines the Stability of Paramagnetic Manganese(II) Hydrides of the Type trans-[MnH(L)(dmpe)2]+ Where L is PMe3, C2H4, or CO

Paramagnetic metal hydride (PMH) complexes play important roles in catalytic applications and bioinorganic chemistry. 3d PMH chemistry has largely focused on Ti, Mn, Fe, and Co. Various Mn^II PMHs have been proposed as intermediates in catalysis, but isolated Mn^II PMHs are limited to dimeric high-spin Mn^II structures with bridging hydrides. In this paper, a series of the first low-spin monomeric Mn^II PMH complexes are generated by chemical oxidation of their Mn^I analogues. This series is of the type trans-[MnH(L)(dmpe)₂]^+/0 where the trans ligand L is PMe₃, C₂H₄, or CO [dmpe is 1,2-bis(dimethylphosphino)ethane], and the thermal stability of the Mn^II hydride complexes was found to be strongly dependent on the identity of the trans ligand. When L is PMe₃, the complex is the first example of an isolated monomeric Mn^II hydride complex. In contrast, when L is C₂H₄ or CO, the complexes are only stable at low temperatures; upon warming to room temperature, the former decomposed to afford [Mn(dmpe)₃]⁺, accompanied by ethane and ethylene, whereas the latter eliminated H₂, generating [Mn(MeCN)(CO)(dmpe)₂]⁺ or a mixture of products including [Mn(κ¹-PF₆)(CO)(dmpe)₂], depending on the reaction conditions. All PMHs were characterized by low-temperature electron paramagnetic resonance (EPR) spectroscopy, and stable [MnH(PMe₃)(dmpe)₂]⁺ was further characterized by UV-vis and IR spectroscopy, Superconducting Quantum Interference Device magnetometry, and single-crystal X-ray diffraction. Noteworthy spectral properties are the significant EPR superhyperfine coupling to the hydride (∼85 MHz) and an increase (+33 cm^-1) in the Mn-H IR stretch upon oxidation. Density functional theory calculations were also employed to gain insights into the acidity and bond strengths of the complexes. Mn^II-H bond dissociation free energies are estimated to decrease in the series of complexes from 60 (L = PMe₃) to 47 kcal/mol (L = CO).

Reactions of [(dmpe)2MnH(C2H4)]: synthesis and characterization of manganese(i) borohydride and hydride complexes

Reactions of trans-[(dmpe)₂MnH(C₂H₄)] (1) with BH₃(NMe₃), 9-BBN, and HBMes₂ yielded the manganese(i) borohydride complexes [(dmpe)₂Mn(μ-H)₂BR₂] (3: R = H, 4: R₂ = C₈H₁₄, 5: R = Mes). The reaction of 1 with BH₃(NMe₃) proceeds via ethylene substitution. By contrast, a detuerium labelling study indicates that the reaction of 1 with HBMes₂ involves initial isomerization of 1 to an unobserved 5-coordinate ethyl intermediate, [(dmpe)₂MnEt], which reacts with the hydroborane to afford EtBR₂ and [(dmpe)₂MnH], followed by reaction with a second equivalent of hydroborane to generate 5 (an analogous pathway is likely followed for other base-free hydroboranes such as 9-BBN). Identification of 3-5 as κ²-borohydride complexes, as opposed to boryl dihydride or hydroborane hydride isomers, is supported by ¹¹B NMR spectroscopy, X-ray diffraction, and Atoms in Molecules calculations. Two byproducts were observed in the syntheses of 3-5: [{(dmpe)₂MnH}₂(μ-dmpe)] (6) and [(dmpe)₂MnH(κ¹-dmpe)] (7). These complexes were independently prepared by exposure of 1 to free dmpe under an atmosphere of Ar or H₂, and the generality of this synthetic route was demonstrated by the reaction of 1 with PMe₃ (under H₂) to form [(dmpe)₂MnH(PMe₃)] (8). Complexes 6-8 can exist as isomers with either a trans or a cis relationship between the hydride and κ¹-coordinated phosphine ligands on manganese. trans to cis isomerization of 6-8 is photochemically induced, whereas the reverse reaction occurs under thermal conditions. X-ray crystal structures were obtained for 3-5, trans,trans-6, cis,cis-6, trans-7, and trans-8.

Interconversion and reactivity of manganese silyl, silylene, and silene complexes

Interconversions between manganese silylene and silene complexes are reported, including those involving the first spectroscopically observed silene complexes with an SiH substituent, and their involvement in ethylene hydrosilylation is discussed.

Manganese disilyl hydride complexes [(dmpe)₂MnH(SiH₂R)₂] (4^Ph: R = Ph, 4^Bu: R = ⁿBu) reacted with ethylene to form silene hydride complexes [(dmpe)₂MnH(RHSi Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> CHMe)] (6^Ph,H: R = Ph, 6^Bu,H: R = ⁿBu). Compounds 6^R,H reacted with a second equivalent of ethylene to generate [(dmpe)₂MnH(REtSi Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> CHMe)] (6^Ph,Et: R = Ph, 6^Bu,Et: R = ⁿBu), resulting from apparent ethylene insertion into the silene Si–H bond. Furthermore, in the absence of ethylene, silene complex 6^Bu,H slowly isomerized to the silylene hydride complex [(dmpe)₂MnH( Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> SiEtⁿBu)] (3^Bu,Et). Reactions of 4^R with ethylene likely proceed via low-coordinate silyl {[(dmpe)₂Mn(SiH₂R)] (2^Ph: R = Ph, 2^Bu: R = ⁿBu)} or silylene hydride {[(dmpe)₂MnH( Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> SiHR)] (3^Ph,H: R = Ph, 3^Bu,H: R = ⁿBu)} intermediates accessed from 4^R by H₃SiR elimination. DFT calculations and high temperature NMR spectra support the accessibility of these intermediates, and reactions of 4^R with isonitriles or N-heterocyclic carbenes yielded the silyl isonitrile complexes [(dmpe)₂Mn(SiH₂R)(CNR′)] (7a–d: R = Ph or ⁿBu; R′ = o-xylyl or ^tBu), and NHC-stabilized silylene hydride complexes [(dmpe)₂MnH{ Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> SiHR(NHC)}] (8a–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. Silyl, silylene and silene complexes in this work were accessed via reactions of [(dmpe)₂MnH(C₂H₄)] (1) with hydrosilanes, in some cases followed by ethylene. Therefore, ethylene (C₂H₄ and C₂D₄) hydrosilylation was investigated using [(dmpe)₂MnH(C₂H₄)] (1) 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.