Calix[4]pyrrolato gallate: square planar-coordinated gallium(iii) and its metal–ligand cooperative reactivity with CO2 and alcohols

Forcing a priori tetracoordinate atoms into planar configuration represents a promising concept for enhanced reactivity of p-block element-based systems. Herein, the synthesis, characterization, and reactivity of calix[4]pyrrolato gallates, constituting square planar-coordinated gallium(iii) atoms, are reported. Unusual structural constraint-induced Lewis acidity against neutral and anionic donors is disclosed by experiment and rationalized by computations. An energetically balanced dearomatization/rearomatization of a pyrrole unit enables fully reversible metal–ligand cooperative capture of CO2. While alcohols are found unreactive against the gallates, a rapid and selective OH-bond activation can be triggered upon protonation of the ligand. Secondary ligand–sphere modification adds a new avenue to structurally-constrained complexes that unites functional group tolerance with unconventional reactivity.

We herewith report on the synthesis, characterization, and reactivity of two calix [4]pyrrolato gallates(III) (Fig. 1D) and their GaLC reversible addition of CO 2 and protic substrates. They represent the rst examples of monoanions with square planarcoordinated gallium(III) atoms and showcase a new direction for reactivity modulation by secondary ligand-sphere modication.

Results and discussion
Reacting the tetrasodium salts of two differently substituted calix [4]pyrrolato ligands 36 with the gallium(III) source tetraphenylphosphonium tetrachloridogallate ([PPh 4 ][GaCl 4 ]) 37 in 1,2-dimethoxyethane (DME) at room temperature over 3 h resulted in a colorless precipitate ( Fig. 2A). Aer extraction with dichloromethane, pale rose solids were isolated in fair yields (69 and 57%). The 1 H NMR spectra of the obtained compounds showed a singlet at around 5.7 ppm and two sets of aliphatic resonances originating from the methyl and ethyl groups, respectively. These characteristics indicate effective solution-phase D 2d molecular symmetry, consistent with a square planar environment around the gallium atom. Single crystals obtained from dichloromethane/n-pentane mixtures at À40 C suitable for SCXRD analyses conrmed the NMR spectroscopical data and unveiled the square planar arrangement of the four N-donor sites enclosing the gallium(III) atom (Fig. 2B)  This structural arrangement was analyzed by various computational techniques (for all computational details see the ESI). † Structural optimization with density functional theory (DFT) conrmed the square planar structure as the energetic global minimum. Constrained structural optimization of [ Me 1] À with the valence angles around the gallium atom xed to 109.5 gave a conformer 581 kJ mol À1 higher in energy as the fully relaxed structure (see Fig. S8 in the ESI). † In comparison, constraining the non-cyclic tetrapyrrolato gallate, which has an effectively tetrahedral ground state, to square planarity costs 166 kJ mol À1 . 20 This nicely illustrates the substantial support of the macrocyclic calix [4]pyrrolato ligand and its electronic properties (s-accepting, p-donating) 39 as being crucial for the experimental realization of this conguration.
The anion [ Me 1] À features an energetically low-lying LUMO (1.48 eV), having its largest coefficient (0.754 within a natural atomic orbital basis) for the 4p z orbital of the gallium atom (Fig. 2C). In contrast, the HOMO is entirely located in the ligand backbone (Fig. 2C). This agrees with the ndings for the corresponding aluminum(III) and silicon(IV) compound and matches early theoretical work from the von Schleyer group. 19 This electronic structure should impart the isolated gallates with biphilic character (vide infra). To probe the global Lewis acidity of [ Me 1] À , its solution phase (dichloromethane) uoride ion affinity (FIA solv ) was computed. It amounts to 149 kJ mol À1 ([ Et 1] À : 132 kJ mol À1 ). 40 This value is slightly reduced compared to the analogous aluminate (196 kJ mol À1 ), though still substantial given the anionic charge state of [ Me 1] À . For comparison, the FIA solv value of the tetrahedral tetrapyrrolato gallate is 58 kJ mol À1corresponding to an increase in Ga-F bond strength of 90 kJ mol À1 mainly based on geometric effects! Following these computational results [PPh 4 ][ Me 1] was reacted with one equivalent of the uoride ion donor tetrabutylammonium diuorotriphenylsilicate (TBAT) in acetonitrile at room temperature (Fig. 3A). Workup aer 4 h afforded an orange solid in 80% yield. Its 1 H NMR spectrum showed two doublets for the pyrrolic b-protons and four signals for the methyl groups. Intriguingly, the signal of the endo-methyl groups next to the uoride is split into a doublet (3.1 Hz), indicating through-space uorine-proton spin-spin coupling. This doublet splitting was also found for the signal of the carbon atom of the respective methyl groups (18.3 Hz). The 19 F NMR spectrum contained a resonance at À146 ppm, signicantly broadened due to this coupling. 15 In total, this is consistent with the formation of the dianionic [ Me 1-F] 2À , which was further conrmed by mass spectrometry. These results also agree with the calculated lower FIA solv of uorotriphenylsilane (85 kJ mol À1 ) in comparison to the value of [ Me 1] À (see above) and with the overall negative solution-phase Gibbs free reaction energy of À65 kJ mol À1 for the uoride transfer reaction from diuorotriphenylsilicate to [ Me 1] À in acetonitrile. Additional experimental proof for the Lewis acidic reactivity of [ Me 1] À was found when [PPh 4 ][ Me 1] was crystallized from tetrahydrofuran (THF) as solvent. One THF coordinating to the gallium atom was observed in the solid-state structure with a Ga-O distance of 202.3 pm (Fig. 2B) Aer exposing both gallates dissolved in dichloromethane-d 2 to 5 bar of CO 2 at room temperature, the solutions turned intensively yellow within several hours. This color change indicates the dearomatization of one pyrrole moiety (marked with an asterisk in the following formulae). The 1 H NMR spectra showed the clean emergence of a new C 1 symmetric species, which was unambiguously identied as the gallium-ligand cooperative addition product of CO 2 to [ Me 1] À and [ Et 1] À , respectively (Fig. 4, see the ESI for details). † Interestingly, the conversion to [ R 1*-CO 2 ] À was found almost quantitative for the methyl case (96%), whereas a 2 : 1 ratio of starting material to addition product was observed for the ethyl gallate under the chosen reaction conditions. The Gibbs free activation energy for the addition reaction of CO 2 to [ Me 1] À was calculated to DG ‡ r ¼ 90 kJ mol À1 , and the overall Gibbs free reaction energy to DG r ¼ 6 kJ mol À1 , which conforms with the experimental observations. Further, the computational simulations reproduced the decreased binding strength of CO 2 to [ Et 1] À (DG r ¼ 18 kJ mol À1 ). This may be rationalized by the slightly decreased Lewis acidity on the FIA solv scale of the gallium atom in [ Et 1] À (see above) as well as by a larger stabilization of [ Me 1*-CO 2 ] À in a dichloromethane solution as found by COSMO-RS 41-43 calculations (see Chapter S11 in the ESI). †   In agreement with these thermoneutral computed values, the application of reduced pressure to solutions of [PPh 4 ][ R 1*-CO 2 ] over several hours at room temperature resulted in the quantitative removal of CO 2 from the systems and in the selective reformation of [ R 1] À (see Fig. S1 and S2 in the ESI). † Element-ligand cooperative (ELC) binding of CO 2 is a widely studied reaction with various transition metals [44][45][46][47][48][49][50] and p-block elements [51][52][53][54][55][56][57][58][59] and was also investigated in small molecule contexts other than ELC. 35,[60][61][62][63][64][65][66] To the best of our knowledge, the described reaction represents the rst example for reversible CO 2 xation by a gallium(III) platform. This is achieved by the balanced energetics in dearomatization/rearomatization of one of the pyrrole rings of the calix [4]pyrrolato ligand.
We next turned our attention to protic substrates, namely alcohols, as their ready activation by the calix [4]pyrrolato aluminate was already noticed by our group. 24  , respectively, no reaction occurred, contrasting the behavior of the corresponding aluminates, i.e., the formation of an oxygenelement bond with the proton being transferred to the 2-position of one of the pyrrole rings (Fig. 5B, le part). We reasoned that protonation of the ligand backbone might serve as a powerful strategy for second-sphere reactivity modulation in structurally constrained complexes. Such tactics are well-known in an alternative context, 67 for example, applied by the Corey group in enantioselective Diels-Alder catalysis. [68][69][70] To probe this hypothesis computationally, the a-pyrrole protonated, neutral [H-Me 1*] was optimized by DFT and its electronic structure was examined. The HOMO is now concentrated almost exclusively in the pyrrole unit opposite the protonation site, which itself is hosting the LUMO (Fig. 5A). The LUMO+1 resembles the former LUMO (cf. Fig. 2C) with the dominant contribution of the 4p z orbital of the gallium atom. The calculated solution phase FIA of [H-Me 1*], which was 149 kJ mol À1 for [ Me 1] À , is now boosted to 191 kJ mol À1 , pointing to an augmented Lewis acidity.
Motivated by these computational results [PPh 4 ][ Me 1] and [PPh 4 ][ Et 1] were treated with bis(triuoromethyl)sulfonimide (HNTf 2 ) in dichloromethane-d 2 as solvent. The 1 H NMR spectra of the reaction mixtures showed an unselective product mixture. Computations disclosed a potential decomposition channel initiated aer the formation of [H-Me 1*] by the cleavage of the exopyrrolic C-C bond originating from the protonbearing carbon atom of the dearomatized pyrrole ring (see Chapter S8 in the ESI for details). † However, a selective transformation was observed when THF-d 8 was used as solvent. The 1 H NMR data revealed the dearomatization of one of the pyrrole rings, besides a slightly broadened singlet resonance at 5.02 ppm, assigned to the new proton in the 2-position of the dearomatized pyrrole ring. Apparently, the Lewis basic properties of THF suppress the undesired pathway mentioned above. Indeed, remodeling the C-C bond cleavage transition state in the presence of an explicit THF molecule coordinating to Ga in [H-Me 1*], resulted in a signicant increase in Gibbs free activation energy of 64 kJ mol À1 (see the ESI for details). † When repeating the reaction with HNTf 2 in a mixture of dichloromethane-d 2 and diethyl ether (2 : 1 % vol), the same 1 H NMR characteristics were found, supporting the Lewis base stabilization rationale.
Subsequently, the reaction with i PrOH was repeated with the in situ generated [H-Me 1*]$Et 2 O in the dichloromethane/diethyl ether mixture. Aer stirring at room temperature for 1 min, removing the solvent, and extracting with n-pentane/diethyl ether (4 : 1 % vol), a pale-yellow solid was isolated in 72% yield. Its 1 H NMR spectrum showed the formation of a symmetric species with two dearomatized pyrrole rings due to protonation at the 2-positions and an isopropyl group with shied signals compared to free i PrOH. These features align with the formation of the neutral O-H bond addition product [HH-Me 1**-O i Pr] by gallium-ligand cooperativity (Fig. 5B,

Conclusions
In conclusion, we report on the synthesis, characterization, and reactivity of two calix [4]pyrrolato gallates(III), which feature a nearly ideal square planar coordination environment around their gallium atoms. Structural constraint empowers Lewis acidic reactivity by p z -type LUMO-lowering, primarily at gallium. This was experimentally proven with donors like THF or uoride anions, opposing Coulomb repulsion in the latter case. Reversible metal-ligand cooperative addition of CO 2 is achieved via the dearomatization/rearomatization of one of the pyrrole units. The associated equilibrium can be ne-tuned by substituent variation in the ligand backbone. Isopropanol as a model for protic substrates did not react with the gallates in the rst place, but a rapid reaction was triggered upon activation with a Brønsted acid. Overall, the reactivity of the squareplanar gallate is inherently lower compared to the corresponding aluminatein line with related reports. 18 This observation can be explained by the lower global Lewis acidity of gallium. Notably, second-sphere ligand modication by protonation is identied as a powerful handle to circumvent this limitation. This strategy will account for a more general approach to increase the reactivity of p-block element complexes of the calix [4]pyrrolato ligand and broadens the substrate scope for more challenging bond activation reactions.

Data availability
Crystallographic data has been deposited at the CCDC under 2175981-2175983. Further datasets supporting this article have been uploaded as part of the ESI. †

Author contributions
L. G. and L. M. S. devised the project and designed the experiments. L. M. S carried out the calculations, did the experimental work together with E. E. and N. R., and analyzed the data. L. G. and L. M. S. wrote the manuscript.

Conflicts of interest
There are no conicts to declare.