Bicyclo[1.1.0]tetragermane-2,4-diide Diradicaloid

Abstract

The synthesis, structure, and reactivity of the bicyclo[1.1.0]tetragermane-2,4-diide compound [(ADC)Ge₂]₂ (3), which features a Ge₄ core bridged by two anionic dicarbene frameworks (ADC = PhC{N(Dipp)C}₂; Dipp = 2,6-iPr₂C₆H₃), are reported. Treatment of an alkyne-functionalized amidine Me₃SiC≡CN(Dipp)C(Ph)═N(Dipp) (1) with GeCl₄ affords [(ADC)GeCl₃(GeCl₄)] (2). KC₈ reduction of 2 yields 3 as a Venetian red crystalline solid. DFT calculations reveal a singlet ground state for 3 with the singlet-triplet energy gap of 14 kcal mol^-1. CASSCF (complete active space self-consistent field) calculations suggest a modest diradical character (β = 9%) for 3. Compound 3 readily reacts with TEMPO (2,2,6,6-tetramethylpiperidinyloxyl) to yield the Ge─Ge bond-cleaved product, [(ADC)Ge(Ge-TEMPO)]₂ (4). Treatment of 3 with Fe₂(CO)₉ gives [(ADC)Ge(Ge{Fe(CO)₄})]₂ (5).

Keywords: Carbene; Cluster; Diradicaloid; Germanium; Main group; Stretched bond.

Figure 1

Schematic illustration of cyclobutanediyls I‐C, bicyclo[1.1.0]butanes II‐C, and their heavier Group 14 congeners II‐E^SB and II‐E^LB, featuring a bridgehead short‐bond (SB) and long‐bond (LB), respectively (substituents omitted except R for II‐E). Niecke's diradicaloid III and selected germanium diradicaloids IV–VII. Representative examples of germanium diradicaloid (propellane‐type) cluster compounds IX–XIII.

Figure 2

a) Synthesis of compound 2. b) Donor–acceptor perspective for 2. c) Solid‐state molecular structure of 2. Aryl groups are shown as wireframes and H atoms are omitted for clarity. Thermal ellipsoids are shown at 50%. Selected bond lengths (Å) and angles (°): C2─C3 1.373(4), Ge1─C2 1.944(3), Ge2─C3 1.995(3), Ge1─Cl1 2.112(1), Ge1─Cl3 2.117(1), Ge2─Cl4 2.164(1), Ge2─Cl5 2.337(1); C3─C2─Ge1 129.0(2), C2─C3─Ge2 130.7(2), C2─Ge1─Cl1 116.1(1), C3─Ge2─Cl4 124.2(1), C3─Ge2─Cl5 85.0(1), Cl5─Ge2─Cl6 177.3(1).

Scheme 1

a) Synthesis of compound [(ADC)Ge₂]₂ (3). b) An alternative canonical form of 3.

Figure 3

Solid‐state molecular structure of 3. Aryl groups are shown as wireframe models. H atoms and minor occupied disordered atoms as well as solvent molecules are omitted for clarity. Ellipsoids are shown at 50% (Selected bond lengths and angles are given in Table 1).

Figure 4

a) Molecular graph of 3 obtained from QTAIM analysis (PBE0/def2‐TZVPP). The small green, red, and blue spheres indicate BCPs (bond critical points), RCPs (ring critical points), and CCP (cage critical point), with electron density (ρ(r_BCP) in a. u.) and Laplacian of electron density (∇² ρ(r) in a. u.) (in parentheses). Contour plots of ∇² ρ(r) (solid lines correspond to positive values and dashed to negative values) for the b) Ge3─C35─C36─Ge4 c) Ge1─Ge4─Ge2, and (d) Ge1─C2─C3─Ge2 planes.

Figure 5

a) Selected molecular orbitals (with energies in eV at PBE0/def2‐TZVPP) for 3. b) FOD plot (isosurfaces 0.007 a.e. in yellow) for 3.

Figure 6

CAS(2,2) orbitals for 3 according to SS‐CASSCF calculations [occupation number].

Scheme 2

a) Selected canonical structures (A, B, and C) for 3. b) Reactions of 3 with TEMPO and Fe₂(CO)₉ to 4 and 5, respectively.

Figure 7

Solid‐state molecular structures 4 and 5. Aryl groups are shown as wireframes and H atoms and occupied disordered atoms are omitted for clarity. Thermal ellipsoids are shown at 50%. Selected bond lengths (Å) and angles (°) for 4: Ge1─O1 1.858(1), Ge1─Ge3 2.568(1), Ge2─Ge4 2.529(1), Ge1⋯Ge2 3.337(1), Ge3⋯Ge4 3.302(2); Ge1─Ge4─Ge2 82.4(1); for 5: Ge1⋯Ge2 3.482(1), Ge1─Ge4 2.462(1), Ge1─Ge3 2.594(1), Ge3─Ge4 2.967(1), Ge1─Fe1 2.360(1), Ge2─Fe2 2.363(4); Ge1─Ge4─Ge2 87.4(1), Ge1─Ge4─Ge3 56.2(1), Ge1─Ge3─Ge4 52.0(1), Ge4─Ge1─Ge3 71.8(1), Ge3─Ge4─C36 65.7(2), Ge2─Ge4─Ge3─Ge1 117.3(1).