σ- vs π-Bonding in Manganese(II) Allyl Complexes

Abstract

Reaction of two equivalents of K­[1,3-(SiMe3)2C3H3] (= K­[A′]) with MnCl2 in THF produces the allyl complex A′2Mn­(thf)2; if the reaction is conducted in ether, the solvent-free heterometallic manganate species K2MnA′4 is isolated instead. With the related allyl K­[1,1′,3-(SiMe3)3C3H2] (= K­[A″]), reaction with MnCl2 in THF/TMEDA produces the corresponding adduct A″2Mn­(tmeda). In the solid state, both A′2Mn­(thf)2 and A″2Mn­(tmeda) are monomeric complexes with σ-bonded allyl ligands (Mn–C = 2.174(2) and 2.189(2) Å, respectively). In contrast, K2MnA′4 is a two-dimensional coordination polymer, in which two of the allyl ligands on the Mn cation are σ-bonded (Mn–C = 2.197(6), 2.232(7) Å) and the third is π-bonded (Mn–C = 2.342(7)–2.477(7) Å). Both σ-allyls are π-coordinated to potassium cations, promoting the polymer in two directions; the π-allyl ligand is terminal. Density functional theory (DFT) calculations indicate that isolated high-spin (C3R2H3)2Mn (R = H, SiMe3) complexes would possess π-bound ligands. A mixed hapticity (π-allyl)­(σ-allyl)­MnE structure would result with the addition of either a neutral ligand (e.g., THF, MeCN) or one that is charged (Cl, H). Both allyl ligands in a bis­(allyl)manganese complex are expected to adopt a σ-bonded mode if two THF ligands are added, as is experimentally observed in A′2Mn­(thf)2. The geometry of allyl–Mn­(II) bonding is readily modified; DFT results predict that [(C3H5)­Mn]+ and (C3H5)­MnCl should be σ-bonded, but the allyl in (C3H5)­MnH is found to exhibit a symmetrical π-bonded arrangement. Some of this behavior is reminiscent of that found in bis­(allyl)magnesium chemistry.

The Stabilization of Three-Coordinate Formal Mn(0) Complex with NHC and Alkene Ligation

“…80 In the crystal structure of 3, the h 2 :h 2dvtms ligand presents in a syn form. The Mn-C(alkene) distances also fall into a narrow range of 2.101( 1 81 The C(alkene)-C(alkene) distance (1.432(2) Å ) of 3 is comparable with those in 1. Thus, among these structure data, the C(alkene)-C(alkene) distances in 1 and 3 indicate strong backdonation from their formal Mn(0) center to the dvtms ligand, but the backdonation is not strong enough to allow the assignment of these complexes as manganacyclopropanes.…”

Closing the Gap: Preparation and Characterization of the First Half-Open and Open Manganocene Complexes

“…Homoleptic manganese allyl complexes have recently been prepared, but the base-free bis­(allyl)manganese compounds remain elusive. In contrast, heteroleptic manganese allyl complexes such as [{η 3 -(1,3-Me 3 Si) 2 C 3 H 3 }­Mn­{C­(SiMe 3 ) 3 }­(L)] (L = thf, PMe 3 , N,N ′-dimethylaminopyridine (dmap), quinuclidine) and [{η 3 -(1,3-Me 3 Si) 2 C 3 H 3 }­Mn­{C­(SiMe 3 ) 3 }] are accessible …”

Synthesis and Reactivity of Manganese(II) Complexes Containing N-Heterocyclic Carbene Ligands

“…Although manganese is most frequently encountered in the +2 oxidation state, the majority of its organometallic chemistry concerns the univalent state. , Such Mn­(I) compounds exhibit almost exclusively low-spin d 6 configurations and typically feature very strong-field ligands such as carbonyl. Complexes of divalent manganese containing metal–carbon bonds are known, albeit much less common. − The bonding in these high-spin Mn­(II) complexes is believed to be much more ionic than that in Mn­(I) compounds, invoking parallels with Grignard reagents. , Even in those Mn­(II) complexes containing strong-field ligands such as cyclopentadienyl, the ionic nature of metal–carbon bonding persists due to the large spin-pairing energy of manganese. Thus, high-spin configurations are observed for most Mn­(II) organometallics. , These species exhibit no spin-allowed ligand field transitions and broadened or nearly featureless NMR spectra, resulting in few reported studies featuring their detailed solution characterization.…”