Comparative Binding of H₂, N₂, and Related Ligands to [Mn(CO)₃(PCy₃)₂]⁺ and Other 16e Electrophiles. N₂ Does Not Coordinate, and H₂ Is the Most Versatile Weak Ligand

Abstract

The photochemical reaction of Mn(CO)5Br with PCy3 in toluene proceeds with the fast evolution of CO. The complex formed, MnBr(CO)3(PCy3)2, 1, readily reacts with NaA (A = B[C6H3(3,5-CF3)2]4) to form the dark green salt [Mn(CO)3(PCy3)2][A], 2, which has an agostic interaction of a cyclohexyl C−H with manganese. The light yellow η2-H2 complex, [Mn(H2)(CO)3(PCy3)2][A], 3, forms at room temperature by placing solutions of 2 under 1 atm of H2. The H2 ligand is labile and readily dissociates when the H2 atmosphere is removed in vacuo. 31P NMR clearly shows that at 25 °C under 1 atm of H2, 3 exists in equilibrium with 2. At −78 °C and under <1 atm of H2, only 3 is observed. The synthesis of the new tricarbonyl complex [Mn(CO)3(PCy3)2]+ provides an excellent system of comparison for the binding of small molecules to similar known 16e fragments such as M(CO)3(PCy3)2 (M = Cr, Mo, W, Re+) and M(CO)(dppe)2 (M = Mo, Mn+). The cationic manganese complex 2 appears to give somewhat more stable binding of H2 than the isoelectronic neutral chromium congener, which would not have been expected on the basis of relative back-bonding ability of the metal centers. Thus it is clear that increased σ-donation more than compensates for decreased back-bonding in the relative metal−H2 bonding energies. Surprisingly, binding of N2, ethylene, or silanes to 2 was not observed in solution even at low temperature and SO2 binds only weakly. By comparison with other similar systems, it is clear that H2 becomes an increasingly better ligand than N2 as the electrophilicity of the metal increases. Thus nonclassical H2 is a more versatile ligand than most classical ligands in its ability to adjust to a larger range of electronic situations.

Chelating Phosphane–Boranes as Hemilabile Ligands – Synthesis of[Mn(CO)3(η2‐H3B·dppm)][BArF4] and [Mn(CO)4(η1‐H3B·dppm)][BArF4]

“…The structural metrics are similar to those already discussed for III, [16] as expected for this isoelectronic pair and are consistent with a relatively weak δ-agostic Mn-H-B interaction [ [19] and slightly shorter than observed for neutral Cr(CO) 5 [27] in part a consequence of the polarised Bδ + ···Hδ -group interacting with the cationic metal centre. The addition of CO to 2 is reversible.…”

Taming the Cationic Beast: Novel Developments in the Synthesis and Application of Weakly Coordinating Anions

“…[376,377] Interestingly,for the formation of the 6 ] 4+ . [368] Angewandte Chemie Reviews bis-cyclometalated carbene analogue [Ir(I t Bu') 2 ] + in which both I t Bu NHCs (I t Bu = 1,3-bis(tertiarybutyl)imidazole-2-ylidene) feature C À Hactivated t Bu groups,the simple reaction of Ag[PF 6 ]w ith Ir(I t Bu') 2 Cl forms the cation, and the [PF 6 ] À ion is sufficiently weakly coordinating. Bromide abstractionb yNa[B(Ar CF3 ) 4 ]toform an agostically stabilizedm anganese cation and displacement of the agostic interactionb ydihydrogen.…”

“…Bromide abstractionb yNa[B(Ar CF3 ) 4 ]toform an agostically stabilizedm anganese cation and displacement of the agostic interactionb ydihydrogen. [368] Angewandte Chemie Reviews bis-cyclometalated carbene analogue [Ir(I t Bu') 2 ] + in which both I t Bu NHCs (I t Bu = 1,3-bis(tertiarybutyl)imidazole-2-ylidene) feature C À Hactivated t Bu groups,the simple reaction of Ag[PF 6 ]w ith Ir(I t Bu') 2 Cl forms the cation, and the [PF 6 ] À ion is sufficiently weakly coordinating. [378] Furthermore,t he [Ir(I t Bu') 2 ] + cation was shown to be readily transformed to the parent bis-NHC iridium dihydride [Ir(I t Bu) 2 (H) 2 ] + upon exposure to dihydrogen.…”

Solid State and Gas Phase NMR Studies of Immobilized Catalysts and Catalytic Active Nanoparticles

“…The first of these compounds was a tungsten complex, discovered by Kubas already in 1984 [53,54]. Later a whole series of different transition metal polyhydrides of the type M-D m (D 2 ) n with hydrogen distances varying between 0.8 and 1.7 Å were synthesized [51,[55][56][57][58][59]. Here, n gives the number of hydride and m the number of dihydrogen type ligands.…”