Stack by Stack: From the Free Cyclopentadienylgermanium Cation Via Heterobimetallic Main-Group Sandwiches to Main-Group Sandwich Coordination Polymers.

Heterobimetallic cationic sandwich complexes [M(μ-Cp)M'Cp]+ of group 13 (M=Ga, In) and group 14 (M'=Ge, Sn) elements have been prepared as [WCA]- salts (WCA=Al(ORF )4 ; ORF =OC(CF3 )3 ). Their molecular structures include free apical gallium or indium atoms. The sandwich complexes were formed in the reactions of [M(HMB)]+ [WCA]- (HMB=C6 Me6 ) with the free metallocenes [M'Cp2 ]. Their structures are related to known stannocene and stannocenium salts; the unprecedented germanium analogues, namely the free germanocenium cation [GeCp]+ and the corresponding triple-decker complex cation [CpGe(μ-Cp)GeCp]+ , are described herein. By variation of the reaction conditions, these sandwich complexes can be transformed into the group 13/14 mixed cationic coordination polymer [{In(HMB)(μ-SnCp2 )}n ][WCA]n . This polymeric chain motif was also successfully replicated by the synthesis of complexes [{Ga/In(HMB)(μ-FeCp2 )}n ][WCA]n containing FeCp2 as a bridging ligand.

Introduction

The stabilisation of highly reactive and subvalent main‐group compounds is a topic of great current interest, often realised by kinetic stabilisation imparted by sterically demanding ligand frameworks. Yet, the complexity of the employed ligand system is often proportional to the novelty of the aimed‐for compound. The cyclopentadienide (Cp−) ligand and the more electron‐rich alkyl‐substituted CpX derivatives represent exceptions to this generalised observation. As anionic 6π aromatics, they allow for the isolation of a plethora of subvalent neutral and cationic p‐block compounds with often undisturbed η5‐coordination modes. Due to their rich coordination chemistry, the following will focus on the most prominent archetypes of this ligand system, that is, the parent Cp− and permethylated cyclopentadienide Cp*−, although sterically more demanding CpR− residues bearing, for example, phenyl, isopropyl, tert‐butyl moieties, have attracted increasing attention. An overview of the known examples of homoleptic neutral and cationic Cp− and Cp*− compounds involving elements from groups 13–15 is shown in Scheme 1.

Scheme 1

Schematic overview of a selection of known homoleptic neutral and cationic Cp− and Cp*− compounds of the p‐block elements and the isolobal group 2 analogues.

By using a specialised reactor, Schnepf and Schnöckel generated the first structurally authenticated subvalent Al and Ga halide complexes, which were used in the synthesis of subvalent clusters. With Cp− and especially Cp*−, the subvalent metal–metal‐bonded clusters [M(CpX)] (M=Al, n=4; M=Ga, n=6) were accessed for the first time, culminating in the unique metalloid cluster Al50(Cp*)20. The compound InCp includes a polymeric solid‐state structure, and its reactivity and coordination behaviour have rarely been reported. In contrast to the related metal alkyls, silyls and amides, the [M(CpX)] oligomers are in equilibrium with their monomer M(CpX). Thus, [Ga(Cp*)]6 is used as a precursor for the introduction of monomeric GaCp* moieties. Therefore, simpler routes to the corresponding [MCp(*)] aggregates needed to be developed.[ , ] Group 13 metallocenes were subsequently used to generate many transition‐metal or main‐group coordination compounds, as well as small clusters, including MCp* as a ligand. Cyclopentadienides also stabilise the highly Lewis acidic cations [MIIICp(*) 2]+ (M=B, Al, Ga) with varying binding modes from η1,η3/η5 (M=B, Ga) to η5,η5 (M=Al). The isolobal neutral alkaline‐earth metal analogues [MCp(*) 2] (Cp: M=Be, Mg, Ba; Cp*: M=Be, Mg, Ca, Ba ) are also known and show an increasing tendency for oligomerisation when descending the group. For group 14 elements, neutral metallocenes ([MCp*2]: M=Si, Ge,[ , ] Sn, Pb ), metallocenium cations ([MCp*]+: M=Si, Ge, Sn, Pb ) and even homometallic triple‐decker metallocenium cations ([Cp*M(μ,η5:η5‐Cp*)MCp*]+: M=Sn, Pb ) are known. For the parent unsubstituted Cp− ligand, the coordination chemistry is generally much less developed, as both the electronic as well as the steric stabilisation provided by this ligand are inferior, which makes Cp−‐substituted compounds much more reactive. For group 14 elements, apart from the neutral metallocenes ([MCp2]: M=Ge, Sn, Pb ), only [SnCp]+ and the corresponding quadruple‐decker complex [Sn3Cp4]2+ are known. For group 15 elements, only the Cp*−‐substituted highly Lewis acidic metallocenium monocations ([MCp*2]+: M=P, As, Sb ) and dications ([MCp*]2+: M=P, As ) have been reported, whereas the Cp−‐substituted congeners are unknown.

Isolobal analogies

The Cp− ligand is isoelectronic with benzene, and hexamethylbenzene (HMB) corresponds to Cp*−. Similarly to [M(η5‐Cp(*))]( (M=main‐group element, n=formal oxidation state), the corresponding [M(η6‐C6R6)]( units (R=H, Me) are nido clusters, sharing an isolobal relationship. Yet, such compounds formally exchanging Cp(*)− for an arene are scarce. The cationic group 2 arene complexes of Harder and Hill and co‐workers represent one example. Our recently developed dicationic complexes [M(HMB)]2+ (M=Ca, Sr) as o‐difluorobenzene (oDFB) solvates can be directly compared with the corresponding Cp*− complexes. However, the arene/Cp(*)− structural relationships are most evident when comparing neutral group 13 Cp*− complexes with the cationic arene complexes, for example, [Ga(η5‐Cp*)] versus [Ga(η6‐HMB)]+. The first group‐13 arene complexes were reported by Schmidbaur et al., formed by the reaction between mixed‐valence GaI[GaIIIX4] (X=Cl, Br, I) and neutral arenes. However, these always showed close contact with the respective tetrahalogallate counterion. Only recently we isolated the first subvalent, truly free monoarene complexes as WCA (WCA=Al(ORF)4; ORF=OC(CF3)3) salts, [MI(HMB)][Al(ORF)4] (M=Ga (1), In (2)), by direct oxidation of the elemental metals with stable [arene]⋅+ radical‐cation solutions. With these in hand, we were interested in exploring the coordination chemistry of [M(HMB)]+, that is, comparing the similarities and differences with the group 13 Cp*− compounds. For [MCp*] (M=Al, Ga, In), widespread applications as Lewis bases[ , , ] with coordination modes comparable to CO but with σ‐donating capabilities exceeding even those of the carbonyl ligand have been reported. Because the [M(HMB)]+ cations represent their cationic equivalent, we reasoned that they could potentially act as cationic Lewis bases related to NO+, which is isolobal with CO. Similar relations have been reported for N‐heterocyclic phosphenium ions, isolobal with N‐heterocyclic carbenes and with a coordination behaviour closely related to NO+.

Results and Discussion

Orienting reactions to use [MI(HMB)]+ as NO+‐like Ligands

To probe the potential Lewis basicity of the complex cations [Ga(HMB)]+ in 1 and [In(HMB)]+ in 2, the salts were treated with a variety of common Lewis acids A (e.g., B(C6F5)3) or in situ generated Lewis acidic transition‐metal fragments (e.g., [□‐Fe(CO)4], [□‐W(CO)5]). All of the reagents A employed have shown precedence for the formation of A–MCp* Lewis acid–base adducts.[ , , ] However, in all the reactions between 1 or 2 and A, the starting materials were re‐isolated, as proven by NMR and single‐crystal XRD (scXRD) analyses. Comparison of the frontier orbital energies of the donor sites in cationic [Ga(HMB)]+ and neutral GaCp* underlined this experimental finding: All the frontier orbitals of the former are lower in energy than those of the latter, due to the positive charge. Thus, the metal‐centred lone‐pair donor orbital (HOMO−2) is 5.3 eV lower in energy in [Ga(HMB)]+ than in GaCp*, and confirms its observed negligible Lewis basicity. By contrast, the orbital energies and spatial projections shown in Figure 1 indicate favourable acceptor properties of [MI(HMB)]+. We therefore set out to synthesise so far unknown heteroleptic group 13 complex salts.

Figure 1

Comparison of the frontier orbitals of GaCp* and [Ga(HMB)]+ calculated at the BP86‐D3(BJ)/def‐SV(P) level of theory.

Using M+ in [M(HMB)]+ (M=Ga, In) as π acceptors for metallocene ligands

Syntheses

As a first attempt, 1 was combined with GaCp* to potentially yield a Ga−Ga‐bonded Lewis acid–base adduct (Scheme 2 a). GaCp* was generated in situ by the reaction of [Ga(PhF)][Al(ORF)4] (n=2,3) with LiCp* followed by filtration to separate the product from the sparingly soluble Li[Al(ORF)4]. However, both, experimental and DFT studies showed that GaCp* coordination by the Lewis basic Ga atom is less favoured than coordination by the negatively polarised aromatic Cp* moiety. Thus, the reaction with [Ga(HMB)]+ proceeded by substitution of the neutral HMB to yield the inverse sandwich cation [GaCp*Ga][Al(ORF)4] (3; see Scheme 2 a). The same cation is part of the [B(ArF)4]− (ArF=C6H3(CF3)2) salt developed by Fischer and co‐workers. Following these findings, we started to investigate the underdeveloped coordination chemistry of M+/[M(HMB)]+ (M=Ga, In) with unsubstituted Cp units in other metallocenes, specifically the neutral group 14 metallocenes [GeCp2] and [SnCp2], because they exhibit a σ‐donor‐type lone pair and a π‐donor‐type Cp− moiety. Depending on the conditions, two reaction products, 4 and 5, were isolated from the reaction between 1 and [SnCp2]. Firstly, the HMB‐stabilised stannocenium cation salt [Sn(HMB)Cp][Al(ORF)4] (4) was isolated from the in vacuo concentrated reaction mixture in oDFB (see Scheme 2 b). Apparently, salt 4 is formed by the abstraction of a Cp− ring by Ga+ followed by evaporation of the formed monomeric GaCp, which is known to be rather volatile. In the absence of a vacuum and by layering of the reaction mixture with apolar pentane, the heterobimetallic sandwich cation salt [Ga(μ‐Cp)SnCp][Al(ORF)4] (5) formed (Scheme 2 c).

Scheme 2

Reactivity of 1 towards GaCp* (a), [SnCp2] (b.c) and [GeCp2] (c). Unless explicitly stated otherwise, all reactions were performed at ambient temperature with oDFB as solvent.

Thus, in direct competition, unexpectedly, GaCp appears to be a better π‐donor ligand towards [Sn(Cp)]+ than free HMB. The analogous reaction with 1 and [GeCp2] resulted in the isolation of the salt [Ga(μ‐Cp)GeCp][Al(ORF)4] (6; Scheme 2 c). The structural data suggest considerable distortion of the neutral M′Cp2 (M′=Ge, Sn) moiety in 5 and 6 as a result of the coordination to Ga+. By changing to the corresponding indium salt 2 as the starting material, we expected much less distortion and possibly the formation of higher aggregates. In addition, indium salt 2 was expected to be less likely to abstract a Cp− moiety from the parent group 14 metallocene, while possibly retaining both HMB and the metallocene in the coordination sphere as weakly bound ligands. However, the reaction between 2 and [M′Cp2] at ambient temperature revealed widely different reactivity patterns compared with 1. For example, the reaction between 2 and [GeCp2] gave a product mixture in which the only identifiable indium‐containing product was the sandwich cation [In(μ‐Cp)GeCp][Al(ORF)4] (9) from (Scheme 3 a). By contrast, the reaction between 2 and [SnCp2] at −20 °C followed by crystallisation at low temperature (Scheme 3 c) gave a defined reaction product, the cationic multi‐decker sandwich coordination polymer [{In(HMB)(μ‐SnCp2)}][Al(ORF)4] (10), which served as inspiration for the work discussed later below.

Scheme 3

Reactivity of 2 towards [GeCp2] (a) and [SnCp2] (b,c). Unless explicitly stated otherwise, all reactions were performed at ambient temperature with oDFB as solvent.

The reaction with [GeCp2] (Scheme 3 a) also furnished the unprecedented triple‐decker cation [CpGe(μ‐Cp)GeCp][μ‐F‐{Al(ORF)3}2] (8) as an inseparable side product. We therefore set out to selectively synthesise 8, since [GeCp2] currently represents the only structurally characterised compound that includes the GeCp unit. For this, the unprecedented free cation [GeCp]+ was accessed by the reaction of 2 equivalents of the strong silylium “ion‐like” Lewis acid TMS‐F‐Al(ORF)3 (TMS=Me3Si) with [GeCp2], which resulted in the separation of TMS‐Cp and the formation of [GeCp][μ‐F‐{Al(ORF)3}2] (7) as the major product (Scheme 4 a). Further reaction of 7 with [GeCp2] (Scheme 4 b) afforded the corresponding triple‐decker cation [CpGe(μ‐Cp)GeCp][μ‐F‐{Al(ORF)3}2] (8).

Scheme 4

Synthesis of the unprecedented germanocenium cation [GeCp]+ (a) and triple‐decker germanium sandwich cation [CpGe(μ‐Cp)GeCp]+ (b). Unless explicitly stated otherwise, all reactions were performed at ambient temperature with oDFB as solvent.

Molecular structures of 3–9

The structure of the inverse sandwich cation [GaCp*Ga][Al(ORF)4] (3) is closely related to the known [B(ArF)4]− (ArF=C6H3(CF3)2) salt and is reported only in the Supporting Information (Figure S16 and Table S1). Crystals from the in vacuo concentrated reaction mixture of 1 and (SnCp2] (Scheme 2 b) revealed the HMB‐stabilised stannocenium cation salt [SnCp(HMB)][Al(ORF)4] (4; cf. the known toluene adduct [SnCp(C7H8)][Al(ORF)4] ). Because all crystals showed an inherent superstructure at 100 K, data had to be collected at 170 K, which induced strong disorder such that the refinement stopped at wR2=0.32. Thus, the data on 4 is only included in the Supporting Information (Figure S17 and Table S2). Layering the reaction mixture containing 1 and [SnCp2] with pentane yielded crystals of [Ga(μ‐Cp)SnCp][Al(ORF)4] (5), with coordination of Ga+ by a Cp− moiety of SnCp2 and an apical free Ga atom (see Figure 2 a and Scheme 2 c). Such a structural motif is rare and only found in transition‐metal–Cp fragments, for example, in [Tl(μ‐Cp)FeCp]+ or [Tl(μ‐CpB)CoCp] ([CpB]2−=[C3B2Me5]2−, pentamethyldiborolyl).

Figure 2

Molecular structures of the cations in a) [Ga(μ‐Cp)SnCp][Al(ORF)4] (5), b) [Ga(μ‐Cp)GeCp][Al(ORF)4] (6), c) [GeCp][μ‐F{Al(ORF)3}2] (7) and d) [CpGe(μ‐Cp)GeCp][μ‐F{Al(ORF)3}2] (8). Counterions and hydrogen atoms have been omitted for clarity. The thermal displacement ellipsoids were set at the 50 % probability level. For 5 and 7, only the majority component of the disordered cations is shown.

The cation structures can be described either as Ga+ coordinated by a neutral SnCp2 unit or as a stannocenium cation [SnCp]+ coordinated by the Cp− moiety of GaCp. Both descriptions are viable and partly represent the nature of the obtained cation. Calculation of the partial charges by using the population analytical methods QTAIM, NBO or PABOON revealed consistent results for all variations of the obtained heterobimetallic sandwich cations [M(μ‐Cp)M′Cp]+ (M=Ga, In; M′=Ge, Sn; for details of the population analyses, see the Supporting Information). In 5, for example, the splitting into formal Ga+ and SnCp2 gives atoms‐in‐molecules (AIM) charges of +0.60 and +0.40, whereas the alternative separation into GaCp and [SnCp]+ fragments gives AIM charges of +0.22 and +0.78, respectively. Therefore, the positive charge is delocalised over the entire molecular ion. The asymmetric unit of 5 contains two cations, both of which are disordered over two positions by a 180° rotation of the [Ga(μ‐Cp)SnCp]+ moiety perpendicular to the M−M′ axis resulting in lower precision of the bond distances. The central Sn atom closely interacts with the terminal η5‐Cp− moiety with d Sn‐C,avg.=2.482(15) Å (d Sn‐C,range=2.414(16)–2.577(11) Å) and a Sn–centroid (ct) distance of d Sn‐ct1,avg.=2.176(9) Å (d Sn‐ct1,range=2.144(8)–2.204(8) Å; Table 1). This is 0.23 Å shorter than the corresponding distance in neutral [SnCp2] with d Sn‐ct1,avg.=2.406 Å, only slightly longer than that in free [SnCp]+ (d Sn‐ct,avg.=2.122 Å ) and virtually identical to the related outer distances in [Sn3Cp4]2+ (d Sn‐ct1,avg.=2.180 Å ). The distance between the gallium atom and the ring centroid of the bridging Cp− moiety is, with d Ga‐ct2,avg.=2.160(8) Å (d Ga‐ct2,range=2.162(6)–2.177(10) Å; d Ga‐C,avg.=2.470(14) Å, d Ga‐C,range=2.43(2)–2.53(2) Å), notably longer than that in the corresponding Lewis acid–base adducts formed between GaCp and B(C6F5)3 or [Cr(CO)5] (d Ga‐ct=1.884 and 1.946 Å, respectively). Note that structures including GaCp without coordination of the gallium atom are unknown. Finally, the distance between the ring plane (pl) of the bridging Cp− moiety and Sn is, with d Sn‐pl2,avg.=2.767(7) Å (d Sn‐pl2,range=2.7596(9)–2.784(6) Å), shorter by 0.1 Å than that in the comparable [Sn3Cp4]2+. Note that here the coordination deviates slightly from perfect η5 coordination, hence the distance between the group 14 element and the ring plane of the bridging Cp− was chosen for discussion here and later. The two Cp− rings of the central SnCp2 unit are tilted against each other by 54°, which is less acute than in free [SnCp2] (46.7°).

Table 1

Selected interatomic distances in the cations of 5–8.

	Distance [Å]	5	6	7	8
		[Ga(μ‐Cp)SnCp]+	[Ga(μ‐Cp)GeCp]+	[GeCp]+	[{CpGe}2(μ‐Cp)]+
terminal Cp	Ge/Sn–Crange	2.414(16)–2.577(11)	2.237(17)–2.360(14)	2.162(12)–2.284(10)	2.292(2)–2.424(2)
	Ge/Sn–Cavg.	2.482(15)	2.290(15)	2.227(9)	2.378(2)
	Ge/Sn–ct1	2.176(9)	1.949(8)	1.884(5)	2.037(1)
bridging Cp	Ge/Sn–pl2	2.784(6)	2.646(7)	–	2.534(4)
	Ga–Crange	2.43(2)–2.53(2)	2.469(12)–2.477(12)	–	–
	Ga–Cavg.	2.47(1)	2.472(13)	–	–
	Ga–ct2	2.160(8)	2.161(6)	–	–

–

–

–

–

–

–

–

The same structural motif was obtained when layering the reaction mixture of 1 and [GeCp2 ] with pentane at ambient temperature. The molecular structure is shown in Figure 2 b) and shows the coordination of Ga+ by a Cp− moiety of GeCp2 to give [Ga(μ‐Cp)GeCp][Al(ORF)4] (6). The asymmetric unit of 6 contains one cation, which is located at a special position with a mirror plane. The central Ge atom in 6 again shows close interactions with the η5‐bound terminal Cp−. With d Ge‐C,avg.=2.290(15) Å (d Ge‐C,range=2.237(17)–2.360(14) Å), they are on average 0.15 Å shorter than that in neutral [GeCp2] (d Ge‐C,avg.=2.517(7) Å, d Ge‐C,range=2.347(7)–2.730(7) Å).

Crystals of [GeCp][μ‐F‐{Al(ORF)3}2] (7) and [CpGe(μ‐Cp)GeCp][μ‐F‐{Al(ORF)3}2] (8) were obtained by layering the respective reaction mixtures with pentane. The cationic part of 7 is disordered over two positions. The apical Ge atom falls almost in the Cp ring plane of the second part (major orientation of the molecular structure is shown in Figure 2 c; for the cation disorder, see Figure S47 in the Supporting Information), similarly to the reported molecular structure of [SnCp][μ‐F‐{Al(ORF)3}2]. Attempts to generate a quadruple‐decker dication [CpGe(μ‐GeCp2)GeCp]2+ by the reaction of 7 with [GeCp2] proved unfruitful and yielded the triple‐decker monocation [CpGe(μ‐Cp)GeCp][μ‐F‐{Al(ORF)3}2] (8; see Figure 2 d). Cation 8 represents the first multi‐decker germanocenium cation. Access to a quadruple‐decker dication might have been futile due to crystal packing/lattice energy effects induced by the use of the larger counterion [μ‐F‐{Al(ORF)3}2]− instead of [Al(ORF)4]−. Comparison of the distance between Ge and the ring centroid of the terminal Cp− in 6 (d Ge‐ct1=1.949(8) Å) with those for 7 (d Ge‐ct=1.884(5) Å) and 8 (d Ge‐ct1=2.037(1) Å) shows similar shifts to those noted above for the corresponding Sn analogues. The distances of Ga to the bridging Cp− of d Ga‐C,avg.=2.472(13) (d Ga‐C,range=2.469(12)–2.477(12) Å, d Ga‐ct2=2.161(6) Å) in 6 are identical to those found in 5, and the distance of Ge to the ring plane of the bridging Cp− moiety is, with d Ge‐pl2=2.646(7) Å, longer by 0.1 Å than that in 8. The tilt angle between the two Cp− moieties in 6 (44.3°) lies between those of neutral [GeCp2] (50.4°) and 8 (41.2°).

The reaction of the [In(HMB)]+ complex salt 2 with [GeCp2] (Scheme 3 a) led, after layering with pentane, to the above described triple‐decker cation salt 8, accompanied by the known decomposition of the [Al(ORF)4]− anion to the fluoride‐bridged [μ‐F‐{Al(ORF)3}2]− counterion, and [In(μ‐Cp)GeCp][Al(ORF)4] (9). The latter showed a superstructure and prevented refinement of the data at wR 2>0.45 (cf. Figure S53 and Table S7 in the Supporting Information). However, the constitution of the cationic part can clearly be assigned to the proposed species (due to the inferior quality of the data, the structural features are presented only in the Supporting Information). Similar reactivity was observed in the reaction between 2 and [SnCp2]. Crystallisation of the reaction mixture by layering with pentane resulted in at least three crystallographically inequivalent species, all of which showed heavy superstructures prohibiting generation of a satisfactory structure model, thereby allowing for only minor structural information on the analysed crystalline material. However, all of the characterised species are salts showing the presence of either [Al(ORF)4]− or [μ‐F‐{Al(ORF)3}2]− and contain heavy‐element MCp fragments in ring or chain motifs in which In and Sn are indistinguishable. Furthermore, no coordination of HMB was observed. As both free [M′Cp]+ cations are stable towards the counterion, the degradation of [Al(ORF)4]− is presumably mediated by In+ in solution. This might be seen in analogy with the increased Lewis acidity of the heavier InX3 compared with the lighter GaX3 [ΔFIA=5 (X=F), 24 (Cl), 34 (Br), 24 kJ mol−1 (I); FIA=fluoride ion affinity]: only the heavier In+/metallocene couple induces decomposition, and not the lighter couple with Ga+.

Rationalising the observed structures by DFT methods

The structural motifs found in the heterobimetallic sandwich cations in 5, 6 and 9 with free apical group 13 elements are unprecedented. In particular, GaCp is known as an exceptionally good σ donor, and solid‐state structures including GaCp or GaCp* without coordination at the Ga atom are scarce. Therefore, coordination of GaCp through the Ga lone‐pair orbital to the group 14 element to form the M−M′‐bonded species [CpGa‐M′Cp]+ seemed viable. However, DFT studies at the BP86‐D3(BJ)/def‐SV(P) level of theory showed that both M−M′‐bonded (M=Ga; M′=Ge/Sn) molecular ions as well as the inverse‐constituted sandwich cations [M′(μ‐Cp)MCp]+ are unfavourable, respectively, by a moderate ΔR G gas=+48/48 kJ mol−1 and a much clearer ΔR G gas=+87/+93 kJ mol−1 [see Eqn. 1], which supports the experimental findings for this structural motif.

NMR spectroscopic characterisation in oDFB and solution thermodynamics study by DFT

NMR spectra of 4–6

NMR investigation of a solution of crystalline 4 (Scheme 2 b) revealed a 119Sn chemical shift of δ=−2380 ppm, just between the chemical shift of a free stannocenium cation (δ=−2430 ppm) and neutral stannocene (δ=−2199 ppm). The 1H NMR spectrum showed the expected 1:1 ratio between HMB and Cp−. The NMR spectra of a reaction mixture of 1 and [SnCp2] revealed a different species to be present in solution: the considerable downfield shift of the 119Sn NMR resonance to δ=−2308 ppm, comparable to that of the quadruple‐decker [Sn3Cp4][Al(ORF)4]2 (δ=−2318 ppm ), together with the 71Ga NMR signal at δ=−540 ppm (cf. the signals for free GaCp, δ=−708 ppm, and 1, δ=−712 ppm ) suggest that the sought‐after coordination of Ga+ to [SnCp2] is present in solution. However, in all the spectra, and also for the following related compounds, only one resonance corresponding to both Cp− units was observed, which indicates the highly fluctional nature of the compounds in solution [cf. Eqn. (1) as one path for exchange]. Thus, the initially isolated 4 was presumably formed upon concentration of the reaction mixture under reduced pressure with removal of volatile GaCp. This is also supported by the NMR spectra of a dried and redissolved reaction mixture that suggested partial removal of a Cp‐containing moiety (ratio Cp−/HMB=1.5:1), no detectable 71Ga NMR resonance and an intermediate 119Sn NMR resonance at δ=−2340 ppm. Layering of this reaction mixture with pentane led to a mixture of crystals of 4 and 5 (SCXRD). Isolated 4 did not react with another equivalent of 1 to abstract the second Cp ring with the formation of the hypothetical dication [Sn(HMB)2]2+. Furthermore, redissolved pure 5 did not liberate GaCp under reduced pressure, further supporting the nature of the GaCp moiety as bound to the [SnCp]+ subunit in solution. The experimental data for the reaction of 1 and [SnCp2], with the two reaction products 4 and 5, suggest that this reaction is close to the thermodynamic equilibrium. Similar reactivity was observed for the reaction of 1 with [GeCp2] (Scheme 2 c). NMR investigation of the reaction mixture containing 6 showed a singlet resonance in the 71Ga NMR spectrum at δ=−670 ppm, which is in the low‐field region of reported GaCp shifts. However, this is upfield‐shifted by 100 ppm compared with the resonance observed from the reaction mixture of 1 with [SnCp2].

Reaction energetics for 4–6 by DFT calculations

The calculated reaction thermodynamics, including the solvation energies (at the BP86‐D3(BJ)/def‐SV(P) level, COSMO solvation), support these conclusions. The conversions of 1 and [M′Cp2] (M=Ge/Sn) into the corresponding sandwich cations [Ga(μ‐Cp)M′Cp]+ are exergonic by only ΔR G solv=−7/−6 kJ mol−1 for the two metallocenes [Eqn. 2], whereas the subsequent generation of the HMB‐stabilised metallocenium cation is favourable by ΔR G solv=−5/−6 kJ mol−1 [Eqn. 3].

Therefore, when working under reduced pressure, GaCp is displaced by HMB and readily removed from the reaction mixture, resulting in the formation of 4. In contrast, upon layering with pentane, the increasingly apolar reaction medium induces the crystallisation of 5.

NMR data and DFT energetics towards 9

Although the calculated thermodynamics (at the BP86‐D3(BJ)/def‐SV(P) level, COSMO solvation) for the reactions of 2 in Equation 4 are essentially identical to those for Equation (2), owing to its polymeric nature, InCp is much less volatile and is not easily removed from the reaction mixture under reduced pressure.

Thus, the reaction between 2 and [GeCp2] revealed a strikingly different reactivity. NMR spectroscopic investigation of the reaction mixture disclosed in the 19F and 27Al NMR spectra, the partial degradation of the counterion [Al(ORF)4]− through the formation of 25 % of [μ‐F‐{Al(ORF)3}2]− and an estimated 5 % of [F‐Al(ORF)3]− with respect to [Al(ORF)4]− and a strong downfield shift of the resonance in the 115In NMR spectrum to δ=−1099 ppm (cf. δ=−1302 ppm for 2 ). Similar behaviour was observed for the reaction between 2 and [SnCp2]. Here, the NMR spectroscopic investigation again showed anion decomposition through the formation of the fluoride‐bridged anion as well as the [F‐Al(ORF)3]− anion. The 119Sn NMR spectrum showed a single resonance at δ=−2179 ppm, close to the chemical shift of undisturbed [SnCp2], and an 115In NMR resonance at δ=−1105 ppm, close to that observed for the reaction with [GeCp2], which indicates a similar reactivity of 2 with the two group 14 metallocenes.

Overall, the DFT calculations and experimental observations suggest that the π‐donating capabilities of the MCp (M=Ga, In) fragment and HMB are rather similar. No heteroleptic complexes of Ga+ or clustering to multi‐decker sandwich units were observed. HMB was readily liberated in all cases. The high volatility of GaCp also creates difficulties as it can be easily removed from the reaction mixture, but only in the presence of HMB.

From isolated sandwich cations to coordination polymers

When repeating the reaction between 2 and [SnCp2] at −20 °C, subsequent crystallisation of the in vacuo concentrated solution at −30 °C led to crystals again showing a superstructure in the solid state. Fortunately, the collected data could be refined to a satisfactory level, and the molecular structure disclosed coordination of [SnCp2] as a bridging ligand for intact [In(HMB)]+ fragments to form the all‐π coordination polymer cation [{In(HMB)(μ‐SnCp2)}][Al(ORF)4] (10), as shown in Scheme 5 a and Figure 3 a. To extend the structural motif of these cationic coordination polymers and to investigate whether similar structures would also be formed when using a prototype transition‐metal metallocene, the group 13 HMB complex salts 1 and 2 were also treated with ferrocene [FeCp2]. Indeed, the reactions proceeded well and allowed isolation of zig‐zag‐type coordination polymers in the solid state for both Ga and In in the form [{M(HMB)(μ‐FeCp2)}][Al(ORF)4] (M=Ga (11), In (12); Scheme 5 b,c and Figure 3 b).

Scheme 5

Overview of reactions yielding cationic coordination polymers 10–12. Unless explicitly stated otherwise, all reactions were performed at ambient temperature with oDFB as solvent.

Figure 3

Molecular structures of the infinite coordination polymers with n=∞: a) [{In(HMB)(μ‐SnCp2)}][Al(ORF)4] (10, shown n=3) and b) [{M(HMB)(μ‐FeCp2)}][Al(ORF)4] (M=Ga (11), In (12); shown n=2). Counterions have been omitted for clarity. Thermal displacement ellipsoids are drawn at the 50 % probability level.

Molecular structures of 10–12

The distances between the Sn and Cp− moieties in 10 are, with d Sn‐Ct=2.396(17) Å (d Sn‐C,avg.=2.662(16) Å, d Sn‐C,range=2.583(16)–2.783(13) Å; Table 2), almost identical to those in free [SnCp2], which is in agreement with the observed 119Sn NMR resonance described below. Furthermore, the angle between the ring planes of the two Cp− moieties in the SnCp2 subunits (45.7°) closely resembles that in free [SnCp2] (46.7°). The distance between indium and the ring centroid of the η6‐bound HMB is, with d In‐ct(HMB)=2.807(11) Å, longer by 0.3 Å than that in free 2 (cf. d In‐ct(HMB)=2.509(5) Å) , but 0.1 Å shorter than the distance to the ring centroid of the coordinated Cp− moieties of SnCp2 (d In‐ct(Cp)=2.90(2) Å). Note that the rather tight coordination of the [GaCl4]− counterion reported by Schmidbaur for [Ga(HMB][GaCl4] (2.522 Å) elongates the Ga–HMB ring centroid distance by a similar amount of roughly 0.3 Å compared with the non‐interacting 1 (2.262 Å). It thus appears reasonable to propose that polymer formation is only possible by using the large and very weakly coordinating [Al(ORF)4]− anion employed throughout this study, otherwise coordination of the counterion, as in the case of [GaCl4]−, appears to be more favourable.

Table 2

Selected interatomic distances in the cations 10–12.

	10	11	12
[{M(HMB)(μ‐M′Cp2)}n]n+	M=In, M′=Sn	M=Ga, M′=Fe	M=In, M′=Fe
M′–Crange [Å]	2.583(16)–2.783(13)	2.036(3)–2.046(3)	2.030(4)–2.051(4)
M′–Cavg. [Å]	2.662(16)	2.041(3)	2.039(3)
M′–ct(Cp) [Å]	2.396(17)	1.645(2)	1.644(2)
M′–pl(Cp) [Å]	2.376(24)	–	–
M–ct(Cp) [Å]	2.897(20)	3.083(2)	3.064(2)
M–C(HMB)range [Å]	3.045(10)–3.203(14)	2.797(3)–2.838(3)	3.006(3)–3.051(3)
M–C(HMB)avg. [Å]	3.138(15)	2.814(3)	3.025(3)
M–ct(HMB)avg. [Å]	2.807(11)	2.438(1)	2.678(1)

[{M(HMB)(μ‐M′Cp2)}]

–

–

For both polymers 11 and 12, the bridging [FeCp2] is undistorted when compared with free [FeCp2], with d Fe‐ct(Cp)=1.645(2) and 1.644(2) Å, respectively. The distances between the group 13 element and the coordinated Cp− moieties are, for both Ga and In, despite the larger radius of the latter, almost identical, with d Ga/In‐ct(Cp)=3.083(2)/3.064(2) Å, even being slightly shorter by 0.02 Å for the heavier indium in 12. Only one related coordination polymer has been reported in the past, generated by the reaction of GaCl3 with excess [FeCp2] to form [{Ga(μ‐FeCp2)}][GaCl4] among other products. Here, the subvalent gallium is also coordinated by the counterion [GaCl4]− forming 3D networks. In this coordination polymer, the gallium–centroid distances are, with d Ga‐ct(Cp)=2.999 Å, in good agreement with the distances in 11.

Solution reactivity and NMR spectra of 11 and 12

An NMR spectroscopic investigation of solutions of isolated crystalline material of 11 and 12 showed the presence of HMB and Cp in a 1:2 ratio in the respective 1H NMR spectra. The 71Ga and 115In NMR spectra showed singlet resonances at δ=−700 and −1244 ppm, which are slightly shifted downfield compared with those in free 1 (δ Ga=−711 ppm) and 2 (δ In=−1302 ppm). The NMR resonances corresponding to Cp− differed greatly between the two samples: for the In complex 12, the respective resonances appeared as sharp singlets at δ H=4.19 ppm and δ C=69.4 ppm, whereas the Ga complex 11 showed broad resonances for both the proton (δ H=9.60 ppm, ω 1/2=200 Hz) and carbon resonances (δ C=78.9 ppm), which suggests partial electron transfer to form a paramagnetic FeIII species. This reactivity was also observed when the more electron‐deficient salt [Ga(PhF)][Al(ORF)4] (n=2,3) was treated with [FeCp2]: this resulted in a redox reaction and the formation of the ferrocenium salt [FeCp2][Al(ORF)4] together with elemental Ga0. Upon reaction of 1 with [FeCp2] at −10 °C, no change in colour was observed. However, upon warming the reaction mixture to 50 °C, a grey precipitate (possibly Ga0) formed over time. Both [FeCp2][Al(ORF)4] and 11 crystallised from this reaction mixture.

Comparing π‐donor strength of HMB and [M′Cp2]

With the reactivity described above and by comparison of the interatomic distances between the group 13 elements and bound aromatics in the molecular structures, a rough estimate of the π‐donor strength of HMB and [M′Cp2] (M′=Ge, Sn, Fe) can be obtained. HMB and the group 14 metallocenes seem to be similar in coordination strength, as seen in the generation of either the HMB‐stabilised stannocenium salt 4 or the heterobimetallic sandwich cations 5, 6 and 9. The distance between the group 13 element and the ring centroid of HMB can be used as a probe for the coordinative strength of the bound metallocene in the isolated heteroleptic polymeric complexes 10 and 12. The longer In–ctHMB distance (by 0.12 Å) observed in 10 in comparison with 12 suggests increased donor capabilities of [SnCp2] compared with [FeCp2].

Conclusions

The coordination behaviour of the subvalent [Ga(HMB)]+ and [In(HMB)]+ cations in the salts [M(HMB)][Al(ORF)4] has been explored. The isolobal analogy between [M(HMB)]+ and the corresponding neutral MCp* is not mirrored in their reactivity. No coordination of the lone‐pair orbital to Lewis acidic main‐group and transition‐metal fragments was observed. DFT calculations suggested that M−M′ bound species might be relevant for fast exchange processes on the NMR timescale. By contrast, the π‐accepting ability of these cations was investigated through reactions with the group 14 metallocenes [M′Cp2] (M′=Ge, Sn), which yielded a series of novel heterobimetallic sandwich complexes. Compound 1 cleanly formed sandwich cations [Ga(μ‐Cp)M′Cp]+ (M′=Sn (5), Ge (6)) with apical exo Ga atoms. These heterometallic sandwich cations might serve as precursors for the simultaneous introduction of σ‐donating group 13 Cp fragments and highly Lewis acidic group 14 metallocenium cations to generate heterometallic main‐group or transition‐metal push–pull complexes, which will be investigated in the future. Compound 2 exhibits strikingly different reactivities: upon pairing with the two group 14 metallocenes, partial decomposition of the counterion was observed at ambient temperature. Thus, only at reduced temperature was the unprecedented main‐group coordination polymer [{In(HMB)(μ‐SnCp2)}][Al(ORF)4] (10) accessible, this polymer showing coordination of In by both HMB and an essentially undisturbed SnCp2. This structural motif was extended to the transition‐metal metallocene [FeCp2], which formed the related coordination polymers [{M(HMB)(μ‐FeCp2)}][Al(ORF)4] (M=Ga (11), In (12)).

Experimental Section

Full details of the experimental procedures and DFT calculations can be found in the Supporting Information.

Deposition numbers 2000267, 2000268, 2000269, 2000270, 2000271, 2000272, 2000273, 2000274 and 2000275 contain the supplementary crystallographic data for this paper. These data are provided free of charge by the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service www.ccdc.cam.ac.uk/structures.

Conflict of interest

The authors declare no conflict of interest.

As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re‐organized for online delivery, but are not copy‐edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors.

Supplementary

Click here for additional data file.