How long are Ga⇆Ga double bonds and Ga–Ga single bonds in dicationic gallium dimers?

Abstract

Syntheses and characterization of two salts [(L)GaGa(L)][pf]₂ ([pf]⁻ = [Al(OR^F)₄]⁻; R^F = C(CF₃)₃) are reported. They include the first dicationic digallene [(L)Ga⇆Ga(L)]²⁺ (L = CDP^Ph = C(PPh₃)₂) and a digallane [(L)Ga–Ga(L)]²⁺ (L = [NacNac^Mes]⁻). The CDP^Ph-supported digallene dication includes a trans-bent [L–GaGa–L]²⁺ bond that is analogous to neutral R–GaGa–R molecules and related to Robinson's famous “Digallyne” [R–GaGa–R]²⁻. The dicationic digallane [(L)Ga–Ga(L)]²⁺ is analogous to the widely used “Jones magnesium dimer”, but includes a very short Ga^II–Ga^II single bond.

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An understanding of the nature and strength of covalent bonds is at the heart of chemistry. Using the standard notion of bond strengths and lengths, a single bond should be longer and weaker than a double or triple bond. Yet, more generally and following the Carter–Goddard–Malrieu–Trinquier model, the different binding modes in formally doubly bonded R_xE=ER_x systems depend on the energy difference between singlet and triplet state of the two fragments: ER_x.^1,2 For most substituents R and elements E, the singlet state is energetically more favourable than the respective triplet state.³ Thus, formal double bonds between heavier elements are better described as two dative bonds “R_xE⇆ER_x” between filled s-orbitals and empty p-orbitals of two singlet carbene-analogue fragments.^1,4–6 In this respect, group 13 elements are somewhat notorious.^7,8 Several compounds exhibit unusual GaGa bonds and have triggered fundamental discussions about the nature of covalent bonds and the concept of bond orders in general.^7–11 The digallium bonding in such compounds is illustrated in Fig. 1, i.e. digallanes, formal digallenes and digallynes. Note that only the incorporation of Na⁺ ions into the cluster core in Robinson's formal digallyne Na₂[Ar*GaGaAr*], one of the most discussed organometallic compounds,^7,8,11,12 impedes Coulomb explosion into two [Ar*Ga]⁻ anions.

Fig. 1 Non-classical formal double bonds between two R–Ga^I fragments (digallene; top left), two-electron reduction, giving a formal digallyne (top middle),^1,4–6 structures of known digallenes¹⁴ (Ar′ = 2,6-(Dipp)₂C₆H₃; Ar* = 2,6-(Tripp)₂C₆H₃) and digallynes.^11,14,15 The structures of neutral digallanes (top right) and of the cationic compounds presented herein are also shown (R = alkyl, aryl, amido; L = CDP^Ph; Y = N donor groups).

In this work, we show that Ga–Ga single and formal double bonds form within dications (Fig. 1). Ion pairing is avoided by using the weakly coordinating anion [pf]⁻ = [Al{OC(CF₃)₃}₄]⁻.¹³

Syntheses and molecular structures. The Ga⁺ source used in this work is [Ga(PhF)₂][pf].¹⁶ Upon addition of a mesityl-substituted β-diketiminate ([NacNac^Mes]⁻) ligand, Ga⁺ disproportionates, giving the dicationic digallane [{Ga(NacNac^Mes)}₂][pf]₂·1.5oDFB ([1][pf]₂·1.5oDFB) (eqn (1)) which is isostructural to neutral Mg^I dimers.¹⁷ Eqn (2) shows that the dicationic digallene [{Ga(CDP^Ph)}₂][pf]₂ ([2][pf]₂) forms with the neutral ligand L = hexaphenylcarbodiphosphorane (CDP^Ph)¹⁸ as electron-rich, four-electron ligand, under conservation of the oxidation state of Ga. Obviously, [NacNac^Mes]⁻ is more strongly coordinating than neutral CDP^Ph, inducing disproportionation of metastable Ga⁺¹⁹ (characterization in ESI†).‡

The molecular structure of the ecliptic dication 1²⁺ is shown in Fig. 2. Despite several NacNacGa compounds being known,²⁰1²⁺ is the first featuring a Ga–Ga bond. The intact dimer 1²⁺ exists in solution, as confirmed by DOSY NMR experiments (see ESI†). This is in line with quantum chemical calculations, which suggest that the dissociation is both endergonic in the gas phase and in oDFB (see below, eqn (5). Application of [In(PhF)₂][pf] in the reaction leading to 2²⁺ yielded [In(CDP^Ph)][pf] including a monomeric complex cation. However, the synthesis was not reproducible and the crystal structure is only discussed in the ESI.† In contrast to 1²⁺, DOSY NMR experiments did not allow to clarify whether 2²⁺ dissociates in solution (see ESI†). Its centrosymmetric molecular structure is displayed in Fig. 3. To the best of our knowledge, 2²⁺ is the first isolated Ga^I–CDP complex. The structure contains a Ga₂-dumbbell and a coplanar C^CDP–Ga–Ga–C^CDP moiety. Each Ga atom is coordinated by one CDP^Ph ligand in a trans-bent fashion that is typical for non-classical formal double bonds.^3–6,9

Fig. 2 Molecular structure of 1²⁺ in [1][pf]₂·1.5oDFB. Only one of the two crystallographically independent 1²⁺ units is shown. Hydrogen atoms are omitted for clarity. Thermal ellipsoids are set at 50% probability level, the mesityl groups are shown as a wireframe model (black: C; blue: N; beige: Ga).

Fig. 3 Molecular structure of 2²⁺ in [2][pf]₂. Hydrogen atoms are omitted for clarity. Thermal ellipsoids are set at 50% probability level, the phenyl groups are shown as a wireframe model (black: C; purple: P; beige: Ga).

Comparison with neutral digallanes and digallenes. The main structural parameters of the digallane 1²⁺ and digallene 2²⁺ are compared with related neutral species in Table 1. The correlation between Ga–Ga bond length and formal bond order is rather poor, underpinning the difficulty to assign bond orders in such compounds. The average Ga–Ga bond length in 1²⁺ (238.22(9) pm) is remarkably short and only comparable to those in neutral five-membered Ga^II NHC dimer analogues, e.g. [Ga^II{[N(Dipp)C(Me)]₂}]₂ (236.34(9) pm)²¹ or [Ga^II{[N(^tBu)CH]₂}]₂ (233.3(1) pm).²² The Ga–Ga bond in the singly bonded 1²⁺ is only slightly longer than that in Robinson's formal digallyne²² and significantly shorter than in other digallanes(4). This is somewhat surprising, since Coulomb repulsion should lead to a bond lengthening in the dicationic dimer. Neutral dimers [{M(NacNac)}₂] are also known for M = Zn^I,²³ and M = Mg^I.²⁴ Ga^II is isoelectronic to Zn^I and, ignoring the filled d orbitals, analogous to Mg^I. The Mg–Mg bond length in [{Mg^I(NacNac^Mes)}₂], is almost 43 pm longer²⁴ than the Ga–Ga bond in 1²⁺. The positive charge residing on Ga, leading to contracted orbitals and, in addition, d-block contraction probably account for the shorter bonds. Accordingly, the corresponding Zn^I dimer [{Zn^I(NacNac^Mes)}₂] has a similar M–M bond length (238.13(8) pm²³) to the gallium dimer. However, the dihedral angle in the Ga dimer (84.65(8)°) is even higher than in the respective Zn (45.0(1)°) and Mg dimer (43.94(7)°).

Table 1 Structural comparison of 1²⁺ and 2²⁺ with related neutral Ga-dimers (NHC = :C(N{ⁱPr}CMe)₂; tmp = 2,2,6,6-tetramethylpiperidino)

Entry^a	Compound	d Ga–Ga [pm]	d E–Ga [pm]	E–Ga–Ga angle [°]	Ref.
a Includes as subscript the formally (!) assigned GaGa bond order of the respective compound.
1Ga≡Ga	Na2[Ar*GaGaAr*]	231.9(3), 232.4(1)	204(2) av. (E = C)	131.0(4) av. (E = C)	11 and 15
			204.1(5) av. (E = C)	131.07(17) av. (E = C)
2Ga–Ga	[HC(^tBu)N]2GaGa[N(^tBu)CH]2	233.3(1)	183.8(6) av. (E = N)	134.8(2) av. (E = N)	22
3Ga=Ga	[(^tBu2MeSi)(NHC)GaGa(NHC)(SiMe^tBu2)]	234.1(3)	209.7(4) (E = C)	110.99(13) (E = C)	25
				125.88(17) (E = Si)
4Ga≡Ga	Na2[Ar′GaGaAr′]	234.7(1)	205.9(5) (E = C)	130.7(1) (E = C)	14
5Ga–Ga	[(NacNac^Mes)GaGa(NacNac^Mes)]^2+ (1^2+)	238.22(9)	184.68(18) av. (E = N)	128.99(6) av. (E = N)	This work
6Ga–Ga	(tmp)2GaGa(tmp)2	252.5(1)	190.1(4) av. (E = N)	120.7(1) av. (E = N)	26
7Ga–Ga	[(Me3Si)2HC]2GaGa[CH(SiMe3)2]2	254.1(1)	199.5(7) av. (E = C)	122.0(2) av. (E = N)	27
8Ga=Ga	[Ar′GaGaAr′]	262.68(7)	202.5(3) (E = C)	123.16(7) (E = C)	14
9Ga=Ga	[(CDP^Ph)GaGa(CDP^Ph)]^2+ (2^2+)	269.01(3)	200.52(9) (E = C)	118.18(3) (E = C)	This work
10Ga=Ga	Ph2P(DippN)2GaGa(NDipp)2PPh2	278.73(12)	207.9(2) av. (E = N)	107.70(6) av. (E = N)	28

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The Ga–Ga distance in 2²⁺ is longer than in a neutral digallene (entry 8 in Table 1), probably as a result of Coulomb repulsion between the positively charged [Ga(CDP^Ph)]⁺ fragments. The putative digallynes display shorter Ga–Ga bonds, due to the influence of the bridging Na atoms and the additional electrons in bonding orbitals (Fig. 1). In addition, the digallene in entry 3, stabilized by silyl- and NHC ligands,²⁵ has a Ga–Ga distance similar to the Ga–Ga distance in putative digallynes, possibly due to the donation of electron density of the NHC-ligands into a Ga–Ga-bonding orbital.

The digallene dimer in entry 10 displays the longest Ga–Ga bond of the compounds presented in Table 1.²⁸

Calculated thermodynamics. Eqn (3) and (4) include the formation energetics of 2²⁺ and its (hypothetical) indium-analogue, which in our experiments remained monomeric in the solid state. The CDP^Ph complexation is favourable to the monocation (eqn (3)). Dimerization in eqn (4) is slightly favourable, but only in a polar oDFB solution (ε_r = 13.38;²⁹ RI-BP86(D3BJ)/def2-TZVPP).

In agreement with the experiments, DFT calculations suggest that the Ga^II–Ga^II single bond in 1²⁺ is stronger by ca. 200 kJ mol⁻¹ compared to the formal Ga^I⇆Ga^I double bond in 2²⁺ (see eqn (4) and (5), in line with previous calculations that point to a bond order below unity for neutral digallenes.^14,30

Bonding in the dicationic digallane and digallene. QTAIM charges δ of selected atoms in 1²⁺ and 2²⁺, bond path ellipticities (ε_BCP), electron densities on bond critical points (ρ_BCP) and the Ga–Ga Wiberg bond indices (WBI), are summarized in Table 2. Both, ρ_BCP(Ga–Ga) and WBI, are higher for 1²⁺ compared to 2²⁺. Additionally, the HOMO-LUMO gap is significantly higher in 1²⁺ (2.6 vs. 2.1 eV; RI-BP86(D3BJ)/def2-TZVPP), again indicating a stronger Ga–Ga bond. The QTAIM charge of +1.09 on the formal Ga^II atoms suggests significant delocalization of the positive charge over the ligand in 1²⁺. This, along with the contracted s and p orbitals in the dication, presumably accounts for the surprisingly short Ga–Ga bond. Ellipticity ε_BCP and WBI unequivocally classify the Ga–Ga bond as a single bond. By contrast, the bonding situation in the dicationic digallene is amazingly complex (resonance forms in Fig. 4, frontier orbitals in ESI†). The C^CDP–Ga bonds can either be described as purely dative bonds (forms IV and VII in Fig. 4), leaving the positive charge on the Ga-centres, or as covalent bonds, formally shifting the positive charge to the ligand (forms I, V, VI).

Table 2 QTAIM charges, electron densities, bond path ellipticities and WBIs for selected atoms and bonds in 1²⁺ and 2²⁺ (RI-BP86(D3BJ)/def2-TZVPP). For 2²⁺, the gas phase-optimized structure and the scXRD structure (values in parentheses) were analyzed^a

a For reasoning, see section 8.4 in ESI.

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Fig. 4 Limiting resonance forms I–III and arrow-based descriptions IV–VII of 2²⁺.

Invoking covalent double bonds between C^CDP and Ga formally leaves a negative charge on the Ga atoms (forms II and III). Interestingly, the C^CDP–Ga distance in 2²⁺ is short at 200.6(1) pm and is even shorter than the covalent C^terphenyl–Ga bond in the first neutral digallene (202.5(3) pm).¹⁴ This indicates partial transfer of the positive charge to the CDP ligands as evident from entry 1 in Table 2 and is further supported by the nearly trigonal planar coordination mode around C^CDP (∑_{bond angles}(C^CDP) = 356.26(6)°). The simplified picture of a central sp²-hybridized C^CDP atom and a delocalization of the positive charge along the P–C^CDP–P axis as in form I would explain the observation. However, the calculated QTAIM charge on C^CDP is negative at about −2 (Table 2 and forms IV–VII). This suggests that the C^CDP–Ga bond also exhibits considerable electrostatic character, accounting for the surprisingly short C^CDP–Ga bond. In fact, EDA-NOCV analysis confirms that the orbital-based and electrostatic contributions are equally important to describe the C^CDP–Ga bond (see ESI†). The C–Ga bond in 2²⁺ has a WBI less than unity, underlining the contribution of resonance forms IV and VII. Additionally, ε_BCP(C^CDP–Ga) of ca. 0.11 (Table 2) and the EDA-NOCV analysis of [Ga(CDP^Ph)]⁺ indicate that both the σ- and the π-electron pairs of C^CDP contribute to the formation of the C^CDP–Ga bond (forms II, III and V–VII). Thus, the bonding situation in 2²⁺ needs to be described by multiple resonance forms but is best visualized by resonance forms V–VII.

The non-classical double bond character of the GaGa bond in 2²⁺ is clearly confirmed by an EDA-NOCV analysis: The HOMO of 2²⁺ has pronounced Ga lone pair character (see ESI†) and accordingly EDA-NOCV analysis suggests that the most important interaction between the Ga atoms is the reciprocal donation of lone pair electron density into an empty 4p orbital. The resulting deformation densities Δρ_(1)–(2) for the interaction between the [Ga(CDP^Ph)]⁺ fragments are shown in Fig. S-63 (ESI†).

In summary, we demonstrated that formation of single and formally double GaGa bonds is possible in dimeric, dicationic gallium compounds. Coulomb explosion can be prevented by employing electron rich ligands and the weakly coordinating [pf]⁻ anion. 1²⁺ is a dicationic digallane with very short GaGa bond, isostructural to a Jones Mg dimer. 2²⁺ is the first dicationic digallane: Its bonding situation is complicated and has to be described by multiple resonance forms, best V–VII. It also represents the first univalent gallium-carbodiphosphorane-complex.

The authors acknowledge support by the Fonds of the Chemical Industry, the DFG in normal procedure, the state of Baden-Württemberg through bwHPC and the DFG through grant no. INST 40/467-1 and 575-1 FUGG (JUSTUS1 and 2 cluster).

Conflicts of interest

There are no conflicts to declare.

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Footnotes

† Electronic supplementary information (ESI) available. CCDC 2220000–2220002. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d2cc06377h

‡ In oDFB solutions of 2²⁺, we reproducibly observed the formation of ca. 12% [H–CDP^Ph]⁺. The amount of protonated ligand is solvent-dependent. We suggest that the super basic ligand and Ga⁺ initiate intra-or intermolecular C–H bond activation of the ligand (see ESI†).

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