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

Abstract

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 PMe 3 , C 2 H 4 , 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 PMe 3 , the complex is the first example of an isolated monomeric Mn II hydride complex. In contrast, when L is C 2 H 4 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 H 2 , generating [Mn(MeCN)(CO)(dmpe) 2 ] + or a mixture of products including [Mn(κ 1 -PF 6 )(CO)(dmpe) 2 ], depending on the reaction conditions. All PMHs were characterized by low-temperature electron paramagnetic resonance (EPR) spectroscopy, and stable [MnH(PMe 3 )(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 = PMe 3 ) to 47 kcal/mol (L = CO).

Terminal Hydride Complex of High-Spin Mn

“…reported low-spin Mn terminal hydrides [e.g., for [(dmpe) 2 MnH(L)] + , ν(Mn−H) > 1700 cm −1 ]. 53,56 A similar observation has been noted, for example, for the high-spin (S = 1) TpCoH [ν(Co−H) = 1669 cm −1 ; Tp = tris(pyrazolyl)borate]. 21 EPR Spectroscopy.…”

“…Orientation-selective ENDOR spectra of the 2-D pattern of ( tBu L)MnD, collected from 5.35 to 7.15 kG (Figure 5), show broadening and splitting due to hyperfine anisotropy of the sharp doublet seen at the lowest field. Superimposed on the experimental spectra are EasySpin simulations (in red) calculated with the parameters for the electron-spin Hamiltonian described in Figure 3b 53 As anticipated, the simulations show that the unique (T 3 ) axis lies along the unique (Z) axis of the ZFS tensor, which must in turn lie along the Mn−D bond. In addition, we again note that the pattern shows no 2 H quadrupole splitting, as expected.…”

“…17,18,73,76 Conspicuously, the value of a iso ( 1 H) established for ( tBu L)-MnH appears to be considerably lower to that reported for low-spin (S = 1/2) [(dmpe) 2 MnH(L)] + complexes (∼85 MHz; |ρ| ≈ 0.06). 53 We emphasize, however, that ascertaining the 1 H hyperfine couplings from analysis of the EPR spectra of the latter was compromised by the presence of extensive 31 P hyperfine splittings and broadened lines. As such, these interactions could not be determined with precision; a iso ( 1 H) for [(dmpe) 2 MnH(L)] + could be as low as ∼40 MHz, which would be more usual for low-spin metal hydrides.…”

“…We have consequently been drawn to the chemistry of terminal, high-spin Mn 2+ hydrides, which are isoelectronic to high-spin Fe 3+ hydrides but considerably less oxidizing and so, presumably, more stable. Although a number of Mn hydrides have been reported, only the polynuclear {[ tBu3 CpMn] 4 [MnH 6 ]} and the recently reported [(dmpe) 2 MnH­(L)] + are open-shell; both are low-spin ( S = 1/2) at the hydride-bound Mn.…”

Isolation of an Osmium(III) Hydrido Complex

“…Emslie and Morris recently reported for a series of manganese hydrido complexes that the ligand trans to the hydride (PMe 3 , C 2 H 4 , CO) had a significant impact on the relative BDFE MnÀ H of Mn I vs. Mn II hydrides. [26] In the current case, the bipy ligand might qualitatively be considered as intermediate with respect to this manganese series. While further variation of the ligand framework is required for full rationalization, our observations apparently indicate pK a /potential compensation between the acidic (OsÀ H) and the redox active sites.…”