Experimental and Theoretical Studies of Bonding and Oxidative Addition of Germanes and Silanes, EH_4-nPh_n (E = Si, Ge; n = 0−3), to Mo(CO)(diphosphine)₂. The First Structurally Characterized Germane σ Complex

Abstract

Reaction of GeH 4 and GeH 3 Ph with the agostic complex Mo(CO)(dppe) 2 (dppe ) Ph 2 PC 2 H 4 -PPh 2 ) provides germane σ complexes Mo(CO)(η 2 -GeH 4-n Ph n )(dppe) 2 (n ) 0, 1). The coordination in these complexes has been assigned as (η 2 -Ge-H) on the basis of NMR and IR spectroscopy and by comparison to the analogous complexes of silanes. When the more electron-rich phosphine depe (depe ) Et 2 PC 2 H 4 PEt 2 ) is used, oxidative addition (OA) products MoH(GeH 3 )(CO)(depe) 2 and MoH(GeH 2 Ph)(CO)(depe) 2 are isolated (NMR and X-ray evidence). However, when the secondary organogermane GeH 2 Ph 2 is used in the depe system, the η 2 -complex Mo(CO)(η 2 -GeH 2 Ph 2 )(depe) 2 is obtained. This complex was characterized by X-ray crystallography and NMR and IR spectroscopy. The Mo(CO)(η 2 -GeH 3 Ph)(dppe) 2 and Mo(CO)(η 2 -GeH 2 Ph 2 )(depe) 2 complexes were found to be in tautomeric equilibrium with their OA products in solution. Structure and bonding comparisons are made to the analogous silane complexes, e.g., Mo(CO)(η 2 -SiH 2 Ph 2 )(depe) 2 , the X-ray structure for which is also reported. The Ge-H bonds undergo OA much more easily than Si-H, and to obtain further insight into the activation processes, ab initio DFT calculations have been performed on Mo(CO)(EH 4-n vin n )(dhpe) 2 model complexes (E ) Si, Ge; n ) 0-3; dhpe ) H 2 PCH 2 CH 2 PH 2 ; vin ) CHdCH 2 ) and also the analogous H 2 complex. Because the ease of the whole OA process is a balance between the E-H bonding energy and Mo-E bonding energy, it can be concluded that the factor that makes OA of the Ge-H bond easier than that for Si-H is the relative weakness of the Ge-H bond, despite the fact that the Mo-Ge bond is also weaker. This competition between both factors is also seen for OA of H 2 , for which although the Mo-H bonding energy is much higher than Mo-Si and Mo-Ge bonding energies, the H-H bond is also significantly stronger than the Si-H and Ge-H bonds. In general, the ease of OA of molecular hydrogen is between that of germanes and silanes. Calculations show that for alkanes the OA is much more difficult because the loss of the high C-H bond energy (comparable to or greater than that for H-H) is not as well compensated for by the energy of formation of the Mo-C bond due to the weakness of the Mo-C bond.

Synthetic, Structural, and Theoretical Investigations of Alkali Metal Germanium Hydrides—Contact Molecules and Separated Ions

“…Depending on the alkali metal and the crown ether diameter, the hydrides display either contact molecules or separated ions in the solid state, providing a unique structural insight into the geometry of the obscure GeH 3 À ion. Germyl derivatives displaying M À Ge bonds in the solid state are of the general formula [M( [18]crown-6)A C H T U N G T R E N N U N G (thf)GeH 3 ] with M = K (1) and M = Rb (4). The compounds display an unexpected geometry with two of the GeH 3 hydrogen atoms closely approaching the metal center, resulting in a partially inverted structure.…”

“…[18] Main-group derivatives are limited to [{(tBuMe 2 Si) 2 HGeLi} 2 ], obtained by the reaction of (tBuMe 2 Si) 2 GeH 2 with tBuLi in THF, [19] and [M 2 {Ge(H)Ar'} 2 ] (M= Li, Na, K; Ar' = C 6 H 3--2,6-(C 6 H 3--2,6-iPr 2 ) 2 ), prepared by the treatment of Ar'(H)GeGe(H)Ar' with the metals in toluene. [20] We here present the first family of discrete alkali metal germyl derivatives.…”

“…Compounds 1 and 4 display a "pseudo-octahedral" metal environment composed of an [18]crown-6 macrocycle, arranged in an approximate equatorial plane, with a THF donor and a germyl anion in the axial positions, resulting in a metal coordination number of eight. The trans angles are 164.61 (7) and 161.36 (7) and rubidium-oxygen ( [18]crown-6) distances range from 2.802(3) to 2.914(3) (1) and 2.847(5) to 2.970 (4) (4), while the metal-THF distances are 2.844(3) and 2.832(3) for 1, and 3.079(4) for 4. For both compounds the germyl hydrogen atoms could be located in the electron difference map and were included in the refinement by using constraints (DFIX in SHELXTL).…”

“…Each asymmetric unit contains two independent molecules. Each cation is surrounded by two [18]crown-6 macrocycles, resulting in a coordination number of twelve and cesium-oxygen distances between 3.174(2) and 3.542(2) . Hydrogen atoms in the anion could not be located or placed on calculated positions, since the germanium atom lies on a center of symmetry.…”

“…The formation of contact molecules or separated ions is mainly driven by the "fit" between the alkali metal radius and the diameter of the crown cavity. With [18]crown-6 as a well-known match for potassium and subsequent formation of a contact molecules, the combination of rubidium and [18]crown-6 has been known to afford both contact molecules and separated ions, as demonstrated with a rubidium silanide, for which two separated ions [Rb( [18] 3 ] have been identified in the asymmetric unit. [30] In accordance with this result, 1.5 equivalents of [18]crown-6 were utilized in the reaction mixture, but only the contact molecule 4 and uncoordinated crown were isolated.…”