Synthesis, characterization, and reactivity. Inspired by recent work of the groups of Beckmann, Cornella and Tan on the utilization of the s-hydrindacene skeleton31 to stabilize mono-coordinated Group 14 and 15 compounds,32–39 we prepared the mono-substituted gallium(I) compound 2 from the respective lithium precursor 134,40 and GaCp*41 in a one-step salt-metathesis reaction,42 Scheme 1. 2 is formed quantitatively according to its 1H and 13C NMR spectra, which together with IR spectroscopy and elemental analysis provide evidence for a gallium(I) compound; the 1H NMR and IR spectra are essentially devoid of conceivable signals of a gallium hydride thus evidencing the absence of Ga–H bonds and a singlet resonance at 174.1 ppm in the 13C NMR spectrum agrees well with gallium(I)-bound ipso carbon atom.13 The mono-coordinate nature of the gallium(I) centre was unambiguously supported by single-crystal X-ray diffraction analysis, Fig. 2a and S3. The Ga–C bond length of 2.097(10) Å is in good agreement with a Ga(I)–C(sp2) single bond as previously reported by Power and co-workers (2.03(1) to 2.046(8) Å)13. The distances between the gallium(I) centre and the closest carbon atoms of the flanking 9-fluorenyls (Ga–C22 3.182(9) Å and Ga–C48 3.062(10) Å) are significantly shorter than the sum of the van der Waals radii of carbon and gallium (3.57 Å)43 indicating attractive interactions as previously reported for related mono-coordinate Bi(I),34,37 Ge(I),35 and Pb(I)38 compounds.
To substantiate the oxidation state + I also experimentally, 2 was reacted with methyl iodide, hexafluorobenzene, and 1-azidoadamantane, respectively, Scheme 1. Methyl iodide readily undergoes oxidative addition affording the gallium(III) compounds 3 in 75% crystalline yield, which could be identified by its distinct 1H and 13C NMR resonances as well as single-crystal X-ray diffraction analysis. The molecular structure, Fig. 2b and S4, features an almost planar trigonal gallium(III) centre with two gallium–carbon (Ga–C1 1.963(3) Å, Ga–C2 1.994(4) Å) and one gallium–iodine bonds (2.5319(7) Å); the bond lengths are comparable to previously reported aryl gallium(III) iodide compounds.44,45 The oxidative addition of hexafluorobenzene requires prolonged reaction times at elevated temperatures, but following previous reports on the enhancement of C–F bond activation reactions by the addition of 4-dimethylaminopyridine46 allowed the isolation of 4 in 53% crystalline yield after only two hours reaction time at room temperature. In the solid state, Figure S5, 4 features a distorted tetrahedral environment around the gallium(III) centre with Ga–C and Ga–F bond lengths in good agreement with previous reports.47,48 The 1H, 13C, and 19F NMR spectra are consistent with the solid-state structure on the NMR time scale. The reaction of 2 with 1-azidoadamantane cleanly affords the gallium(III) tetraazagallole 5 in 65% yield as single-crystals, which allowed establishing its molecular structure, Figure S6. 5 features an essentially planar GaN4 ring with Ga–N single bonds (1.835(2) and 1.858(2) Å) and N–N bond lengths (1.272(3) to 1.379(3) Å) that fall in between the values expected for single and double bond thus suggesting electron delocalization as previously reported.49,50
Inspired by work of the West and Roesky group on bora-3 and aluminacyclopropenes,51 respectively, we reasoned that 2 might be a suitable precursor to isolate the hitherto unknown three-membered gallacyclopropenes. 2 indeed readily reacts with terminal alkynes at room temperature, Scheme 2, in a clean reaction affording 6 and 7 as the only products, which feature characteristic NMR resonance patterns. The 1H NMR spectra are devoid of proton resonances accounting for either the free acetylenes (phenylacetylene 2.72 ppm, 3,3-dimethylbutyne 1.89 ppm) or Group 13 cyclopropenes, which resonate due to their aromatic character at lower field, i.e., around 8 ppm (boracyclopropene)52 and 8.82 ppm (aluminacyclopropene)51. However, a new resonance appears as a sharp singlet at 5.18 (phenyl) and 5.31 ppm, respectively, i.e., the typical region of olefinic protons, but at higher field compared to gallium(III) vinyl compounds that usually resonate between 6.1 and 7.9 ppm depending on the substituents on both carbon and gallium.53,54 Furthermore, the 13C NMR spectra feature an unusual resonance at 174.1 and 184.2 ppm. The reaction is not limited to terminal alkynes and diphenylacetylene also inserts into the Ga–C bond of 2 affording 8 in 52% yield within 3 days at room temperature. The molecular solid-state structures of 6–8 were unambiguously established by single-crystal X-ray diffraction, Figs. 2c and S7-S9, and explain the observed NMR data. In all three compounds, the coordination number of one at gallium is retained and 6–8 are best descripted as the products of a carbometalation reaction, i.e., an insertion of the alkyne into the gallium(I)–carbon bond by retention of the oxidation state + I. This is remarkable as low-valent relatives preferentially undergo oxidative addition reactions leading to the more stable B(III), Al(III), and Ga(III) compounds.55 In contrast, redox-invariant insertion reactions, which are valuable synthetic methods,56–58 remain yet limited to s-block organometallics, Group 13 elements in the oxidation state + III, like Ziegler’s Aufbau reaction,59,60 and transition metals.61,62 Here, the carbometalations give rise exclusively to the syn-addition products and in case of the terminal alkynes, the gallium and hydrogen atoms are vicinal. In comparison with gallium(III) vinyl compounds,53,54 the Ga–C bonds (2.051(4) to 2.0711(19) Å) are slightly longer consistent with a more electron-rich gallium ion in the oxidation state + I. The absence of Ga–H groups was again verified by 1H NMR spectroscopy and further evidence of the low oxidation state is given by the subsequent reaction of 6 with methyl iodide that readily affords the oxidative addition product 9 in 56% yield, whose molecular structure was unambiguously established by single-crystal X-ray diffraction analysis, Figure S10.
Theoretical calculations. To gain insight into the electronic structure of the mono-coordinate gallium(I) compounds 2 as well as 6–8 and to understand the exclusive formation of 6–8, respectively, density functional theory (DFT) calculations at the wB97XD/def2TZVP//wB97XD/def2SVP level of theory63,64 have been conducted. The highest occupied molecular orbital (HOMO) of 2 represents the lone pair of electrons at gallium, while the empty Ga 4p orbital (in plane with the π orbitals of the Ga-bound phenyl ring) contributes most to the lowest unoccupied molecular orbital (LUMO), Fig. 3a; the second Ga 4p orbital (orthogonal to the π orbitals of the Ga-bound phenyl ring) interacts with the π-system of the flanking fluorenyl groups as illustrated by the degenerate LUMO + 3 and LUMO + 4, Figure S11. These attractive non-covalent interactions, Figure S14, stabilize 2 and are in line with the comparably short Ga–C contacts in the solid-state structure discussed above. A natural population analysis (NPA) revealed partial charges of -0.61 e and 0.73 e for C1 and Ga, respectively, in account for a polarized bond, and the Wiberg bond index of 0.39 suggests a single bond.
The thermochemistry of the formation of three conceivable isomers from the reactions of 2 with alkynes, i.e., the observed vinyl galanediyls 6–8, gallacyclopropenes 6iso-8iso as well as the C–H activation products 6iso'-7iso', Scheme S1, has been calculated. In each case, formation of 6–8 is exergonic by 112.6 to 128.4 kJ mol− 1 and energetically preferred compared to the other two isomers in line with the experimental observations. The relevant frontier molecular orbitals of the vinyl galanediyls 6–8 are comparable, Fig. 3b and S12-13, and those of 6 are discussed as an example next. Similar to 2, the HOMO of 6 features the nonbonding pair of electrons at gallium and the LUMO is mainly composed of the empty 4p orbital of gallium that is orthogonal oriented relative to the plane of the vinyl unit, while the second 4p orbital (in plane with the vinyl unit) contributes to the LUMO + 3 (0.7 eV), Figure S13. A non-covalent interaction analysis, Figure S15, revealed π-π-interactions between the gallium p orbital and the π-system of the fluorenyl groups as weak C–H∙∙∙π-interactions that help stabilizing the molecule. The Wiberg bond index of 0.38 and partial charges (NPA) of -0.63 e for Ga and 0.73 e for C3 suggest a polarized single bond.