Metal-coordinated distibene and dibismuthene dications – isoelectronic analogues of butadiene dications

We report the synthesis and solid-state structures of DMAP-coordinated ([L(DMAP)GaPn] 2 [OTf] 2 ; Pn = Sb 3, Bi 4) and base-free dipnictene dications ([LGaPn] 2 [BArF x ] 2 , Pn = Sb: x = 24, 5a; 20, 5b; Bi: x = 24, 6a; 20, 6b). Quantum chemical calculations indicate that the dications 5 2+ and 6 2+ represent isoelectronic analogues of the butadiene dication.

The 1 H NMR spectra of DMAP-substituted dications 3 and 4 show singlets for the g-H atoms and the Me groups and two septets and four doublets for the i-Pr groups. The rotation of the Ga-N DMAP bonds in 3 and 4 is restricted, resulting in magnetically inequivalent aromatic protons of the DMAP ligand (Table S1, ESI †). In contrast, the base-free dications in 5 and 6 show only one septet and two doublets for the i-Pr groups, indicating trigonal-planar coordinated Ga centres or rapid inversion processes in the case of a pyramidal coordination sphere in solution. The g-H resonances are shifted to lower field (Table S1, ESI †) due to an increased Lewis acidity of the Ga centres within the base-free dications of 5 and 6 compared to 1-4. Electron density is therefore effectively pulled away from the ligand backbone and the dipnictene p system towards the Ga atoms. The coordination of an electropositive gallanediyl (LGa) ligand seems to be essential for the stabilisation of the dipnictene dications as was observed in the isolation of silylene-carbonyl complexes. 30 The pp* transition in the dications 5a and 6a is red-shifted compared to their neutral counterparts (UV/Vis: 431 (1) to 471 (5a); 505 (2) to 542 (6a) nm; Fig. S41-S44, S66, ESI †), indicating electronic changes within the p system. 11b Single crystals of 1 and 2 ( Fig. S45 and S46, ESI †) were obtained from saturated CH 2 Cl 2 and fluorobenzene solutions upon storage at À30 1C, while those of 3, 4, 5a, and 6a ( Fig. S47-S50, ESI †) were grown by slow diffusion of toluene or benzene into CH 2 Cl 2 solutions of 3, 4, 5a, and 6a, respectively. The molecular units of 5a and 6a are disordered. Crystals of 5b and 6b were grown by layering their 1,2-difluorobenzene solutions with n-hexane (Fig. 1). Compounds 1 and 2 crystallise in the monoclinic space group C2/c, and those of 3, 4, 5b, and 6b in the triclinic space group P% 1, respectively. The molecular units of all dipnictenes are located on a special position Scheme 2 Synthesis of DMAP-coordinated and base-free dipnictene dications 3-6. with a centre of inversion at the midpoint between both pnictogens atoms.
A comparison of the bond energy analyses for the homolytic and heterolytic cleavage of the Ga-Sb and Ga-Bi bonds shows that in all cases they are electron sharing bonds ( Fig. S54 and S55, ESI †). The difference in the Ga-Pn and Pn-Pn bond lengths is small overall (Table 1), whereas the charges on the pnictogen atoms differ significantly. The base-free dications 5 2+ and 6 2+ have significantly higher positive charges on the pnictogen atoms ( Fig. S64 and S65, ESI †) compared to the neutral compounds (1,2) and the DMAP-stabilised dications (3 2+ , 4 2+ ), which have similar charges. In addition, the ellipticity at the bond critical points of the Pn-Pn double bonds is higher in dications 5 2+ and 6 2+ , indicating an increase in the p bond character ( Fig. S58 and S60, ESI †).
As mentioned above, dications I and II were synthesised by a two-step oxidation reaction of the corresponding neutral NHCcoordinated dipnictenes. Therefore, the corresponding neutral compounds 5 and 6 were calculated for comparison. The reduction of the dipnictene dications to the neutral analogues leads to an increase in both the ellipticity at the bond critical points and the bond order of the Ga-Pn bonds, while both the ellipticity and the bond order decrease for the Pn-Pn bonds ( Fig. S58 and S60, ESI †), which is reflected in a decrease of the Ga-Pn and an increase of the Pn-Pn bond lengths by more than 0.1 Å. In addition, the IQA analyses show an increase in the covalent part of the total interatomic interaction energies for the Ga-Pn bond and a decrease for the Pn-Pn bond ( Fig. S62 and S63, ESI †). The twofold reduction also leads to a greater increase in the electron density at the Pn atoms compared to the Ga atoms. In other words, the charge changes in oxidation and reduction essentially take place at the Pn centre.
Base-coordinated and base-free dipnictene dications were structurally characterised. Quantum chemical calculations and UV-vis spectroscopy studies reveal a weak p backbonding contribution from the Pn 2 unit to the LGa ligand. Due to the large size of the frontier orbitals, this interaction is much smaller than expected for similar systems composed of lighter elements. The neutral molecules 5 and 6 can be considered as isoelectronic analogues of butadiene and the cations 5 2+ and 6 2+ represent analogues of the butadiene dication.
M. W. performed the experiments, C. W. the single crystal X-ray diffraction, and G. H. the quantum chemical calculations. Y. S. and A. G. assisted with the single crystal X-ray diffraction experiments. The work was supervised by S. S. The manuscript was written with contributions from all authors. All authors approved the final version of the manuscript.

Conflicts of interest
There are no conflicts to declare.