The scientific study on heterometallic bonding nature between main group and transition metals is one of the core works in contemporary inorganic chemistry. The relatively earlier works in this field were the synthesis of coordination compounds of transition metals with the heavier carbene analogous R₂E: (E = Si, Ge, Sn; R = amido, alkyl, aryl) [1,2]. The latter possess the nonbonded electron pair with high s-character, and are able to act as donor–acceptor ligands in their transition metal complexes. Up to now, only a small number of complexes with metallic Ge-Fe bonds have been found. Among these compounds, the ferrogermylenes A, germylene→Fe complexes B, ferro-germylynes C, and ferrogermanes D have different bonding configurations (Scheme 1). In the known Ge-Fe molecules, the ferrogermylenes A were observed with relatively long Ge-Fe distances in the range of 2.41–2.50 Å [3-7]. In contrast, the coordinate bonding R₂Ge: →Fe in σ-donor/π-acceptor complexes B shows obviously short Ge-Fe distances in the range of 2.19–2.33 Å [2,8-12]. These species can also be regarded as Ge-Fe doubly bonded compounds because of their isoelectronic structure of "Ge=Fe" moiety, if the Ge←Fe backbond is taken into account. Pandey and co-workers presented a systematic theoretical study that π bonding contributions to the covalent bonding in the metallogermylenes A are much less than those in Fischer-type metal germylyne complexes [13-24]. In this work, a computational investigation on a model molecule of ferrogermylynes C with Ge-Fe multiple bond feature showed a really short Ge-Fe distance of 2.09 Å. Additionally, a ferrogermane molecule [Ph₃Ge–Fe(CO)(PPh₃)Cp] as shown in D has sp³ hybridization around the Ge atom and a Ge−Fe distance of 2.38 Å, which is between the corresponding distances in A and B.

Now the experimental reports on ferrogermylenes involving the structural description are limited to only a few cases [4,5,25]. They were assembled by salt metathesis reactions between the corresponding β-diketiminato germylene chlorides and the Cp-Fe carbonylates M[FeCp(CO)₂] (M = Na, K). It is necessary to synthesize more complexes containing heterometallic bonding and explore the essential features of metallic interaction [26,27], particularly the contributions of π back-donation from transition metals to heavy main group atoms.

Recently, we isolated a silylene–germylene compound coupled through intermetallic nucleophilic substitution using a germylene anion [28]. Now we further develop a new lithium germylidenide 2b and report the successful nucleophilic attack on iron halide complex CpFe(CO)₂I and β-diketiminato Fe^Ⅱ chloride complex 6 producing three ferrogermylene complexes 3a, 3b and 4a, and the exploration on their heterometallic bonding features.

The β-diketiminato germylene chlorides 1a [29] and 1b [30] (Scheme 2) were proved accessible in our previous work. Their further reduction reactions with lithium furnished lithium germylidenides 2a [28] and 2b, respectively, and the isolable byproduct 5 (Scheme 2) [31]. The new lithium germylidenide 2b was structurally characterized by NMR spectroscopy and XRD analysis. The molecular structure of 2b (Fig. 1) confirms a highly similar structure with that of 2a. The Ge atom lies within a five-membered C₃NGe-ring and coordinates end-on to the Li⁺ cation, which is further coordinated by three THF molecules to reach a tetrahedral coordination sphere. This is different from the structures of side-on coordinated germylidenide salts in earlier reports [32-38]. A singlet at δ = −0.56 ppm in the ⁷Li NMR spectrum of 2b is very close to that of the known lithium germylidenides 2a (δ = −0.53) [26].

Salt metathesis reactions of germylidenides 2a and 2b with electrophilic iron halide complex FeCp(CO)₂I gave the Ge–Fe bonded complexes 3a and 3b in moderate isolated yields (Scheme 3). Two iron complexes were comprehensively characterized by ¹H and ¹³C NMR spectroscopy, elemental analysis, and X-ray crystallography. In practice, 3a and 3b have the similar ¹H NMR spectra in C₆D₆, in which, the characteristic singlets at δ = 4.00 and δ = 3.97 ppm are assigned to the respective cyclopentadienyl attached to the iron atom. Nevertheless, the ¹H NMR spectrum of 3b displays one set of singlet at δ = 2.25 ppm for protons on the γ-C methyl group. The important chemical shifts for CO and Cp groups in the ¹³C NMR spectra of 3a and 3b are very similar with the value of the reported Ge-Fe complex (δ for CO: 3a: δ = 214.8 ppm, 3b: 215.0 ppm, (Piso)GeFeCp(CO)₂: 216.5 ppm, Piso⁻ = [ArNC(^tBu)NAr]⁻; δ for Cp: 3a: 83.5 ppm, 3b: 83.3 ppm, (Piso)GeFeCp(CO)₂: 84.5 ppm) [4]. In the IR spectra of 3a and 3b, the absorption of stretching vibrations of carbonyls (3a: = 2019 and 1967 cm⁻¹; 3b: = 2013 and 1959 cm⁻¹) are uniform to each other, but higher than those (1964 and 1921 cm⁻¹) of (Piso)GeFeCp(CO)₂, thus indicating that molecules 3a and 3b have weaker Fe→CO back-donation and possess more electron-poor valence shell on Fe. Furthermore, the UV–vis spectra of 3a and 3b in hexane severally display strong absorption bands at λ = 618 and 602 nm, respectively.

The dark green crystals of 3a and 3b in the same monoclinic space group P2₁/c were obtained from the respective concentrated hexane solutions. The corresponding molecular structures are depicted in Fig. 2. There are two independent molecules in the structure cell of 3b that have no significant geometric differences, so only one metric parameter is discussed. The molecular structures of 3a and 3b confirm the three-coordinate environment of the Ge centers. A cyclopentadienyl ligand, two carbonyls, and the anionic germylene ligand are coordinated to the Fe atoms of 3a and 3b. In contrast with planar five-membered C₃NGe-rings of the precursors 2a and 2b, ferrogermylene complexes 3a and 3b exhibit distorted pyramidal Ge centers and slightly puckered five-membered C₃NGe-rings. The angles between the Ge-Fe bond and the Ge1-C1-N1 plane (3a: 123.0° and 3b: 114.4°) are larger than those in reported digermylenes with the same azadiene anion L⁻ of 3a (LGe−GeL), and smaller than that in 2a. The azadiene C₃N backbone of the five-membered heterocycles in 3a and 3b are nearly planar, but Ge atoms are out of planes by 12.7° and 9.3°, respectively. The negative nucleus-independent chemical shift values [NICS(1) for the C₃NGe-ring in 3a, −11.6; in 3b, −10.5] confirm the aromatic C₃NGe-rings in two compounds, which can provide delocalized molecular orbitals on Ge side for a further π overlap with d orbitals of Fe and improve the electronic exchange during the metallic bonding. Therefore, the Ge-Fe distances in 3a and 3b are just related to the metallic Ge-Fe bond. The Ge–Fe bond lengths in 3a (2.4142(5) Å) and 3b (2.4415(11) Å) are comparable to those in reported species (Piso)GeFeCp(CO)₂ (2.442 Å, Piso⁻ = [ArNC(^tBu)NAr]⁻), and (^ArNacnac)GeFeCp(CO)₂ (2.496 Å, Ar=2, 6-ⁱPr₂C₆H₃) [4,5], but obviously longer than those in germylene→Fe complexes B [2,8-11] and the corresponding distances in ferrogermane derivatives D [39-41]. The Ge-Fe bond length in 3b is around 3 pm longer than that in 3a, indicating an accurate adjustment of electron density in molecular orbitals during bonding. Different from two phenyls and an electron-donating methyl on azadiene backbone of 3b, the electron-withdrawing effect of three phenyl groups of 3a can help to bring about the slightly electron-poor C₃NGe-ring. This should enhance the π back-donation from the 3d shell of Fe to the π* molecular orbitals of C₃NGe-ring and cause a little stronger Ge-Fe interaction in 3a than that in 3b, although this π backbonding is actually weak in both molecules. In fact, the Ge-Fe interactions only lead to a very slight difference in the C—O bond lengths of 1.147(3) and 1.149(3) Å in 3a, as well as 1.149(8) and 1.155(8) Å in 3b. The fact of longer Ge-N bond lengths of 3a (1.9571(18) Å) and 3b (1.995(5) Å) than those in precursors 2a (1.891(2) Å) and 2b (1.833(7) Å) should also be due to the Ge←Fe π backbond.

Very recently, we introduced a facile preparative procedure to obtain a three-coordinate β-diketiminato Fe^Ⅱ chloride 6 (Scheme 4) [42]. The reaction of 6 with lithium germylidenide 2a furnished the novel low-coordinate Ge^I-Fe^I complex 4a, which has been isolated in the form of brownish red crystals in 43% yield. On the other hand, the reaction of 2b with 6 led to a mixture of unknown products. The iron complex 4a was completely characterized by ¹H and ¹³C NMR spectroscopy, elemental analysis, and X-ray crystallography. According to the ¹H NMR spectrum of 4a, the found proton integrals can be assigned for the azadiene ligand, the β-diketiminate ligand and one THF molecule, and they are not the proton resonances for precursors 2a and 6.

Compound 4a crystallizes in the triclinic space group P1 and exhibits a distorted tetrahedral Fe-centered coordination sphere with the heterometallic Ge→Fe bond (Fig. 3). Additionally, two N atoms of the β-diketiminate ligand and an oxygen of THF molecule are coordinated with the Fe atom, resulting in a total of 14 valence electrons centered on Fe. The interior angle summation (539.9°) of C₃NGe ring in 4a is almost equal to that of the planar pentagonum (540°), indicating a planar five-membered ring. Contrary to 3a and 3b, the coordinatively unsaturated Fe1 center in 4a is nearly coplanar with the C₃NGe plane, only exhibiting a bent angle of 18.5°. Furthermore, the six-membered C₃N₂Fe ring of 4a presents a twist-boat conformation along the Fe1-C5 axis. The Ge1−Fe1 distance of 2.5120(6) Å is evidently longer not only than the corresponding distances in germylene→Fe complexes B (2.19–2.33 Å), but also than those in 3a and 3b. This is mainly attributed to the electron deficient Fe atom in 4a, which leads to the weaker π back-donation from Fe to C₃NGe ring. Similarly, the bond lengths of Fe1-N2 (1.987(3) Å) and Fe1-N3 (2.027(3) Å) in 4a are also elongated than those (both of 1.960 Å) in precursor 6, whose absolute planar C₃N₂Fe ring has a better π-delocalization due to the coordination character of β-diketiminate ligand. The structural distinctions such as central bond lengths and bent conformation indicate that the Ge-Fe bond in 4a is not a simply heterometallic Ge^I-Fe^I σ coordination.

The selected molecular orbitals (MOs) of 3a, 3b and 4a by DFT calculations are shown in Fig. 4. A delocalized molecular orbital on the C₃NGe-ring of 3a has a stable energy level of −4.24 eV, and is found to be the HOMO. The σ bonding characteristics are clearly visible in the portion of the HOMO-13 distributed between Ge and Fe. The HOMO and HOMO-7 of 3b are very similar to those of 3a. Additionally, HOMO-6 of 3b also has a significant distribution between Ge-Fe, but its characteristics are distinct from HOMO-7, while it is comparable to π type. This π-like orbital properties are also reflected in the HOMO-10 of the 3a. The contribution of Fe to these bonding orbitals displays d orbital characters, despite the deformed shape of the molecular orbitals. Different from 3a and 3b, the HOMO of 4a located at Fe atom, while the HOMO-2 derives from the overlap of the delocalized molecular orbital on the C₃NGe ring and one d orbital of Fe. This is most likely caused by the closeness of Fe and C₃NGe-ring to a plane, which induces a wider delocalization and what can be regarded as a π backbonding from Fe to Ge. Moreover, HOMO-6 presents a σ bonding interaction between the Ge and Fe atoms, and its energy level is −5.58 eV, which is somewhat higher than the σ type orbitals of 3a and 3b.

In conclusion, new ferrogermylene complexes 3a, 3b and 4a have successfully been isolated by the salt metathesis reaction of lithium germylidenides with electrophilic iron halide complex CpFe(CO)₂I and β-diketiminato Fe^Ⅱ chloride, respectively. The crystal structures of new compounds show clearly that the type of ferrogermylenes A have very long heterometallic Ge-Fe bond lengths, which are affected by the changes of the coordination environment around Ge and Fe centers. The structural and IR characterizations suggest that only by accepting the presence of the Ge←Fe π back-donation in molecules 3a, 3b and 4a, the observation of Ge-Fe bond lengths dependence on different ligand substituents can be reasonably explained. The computational works of 3a, 3b and 4a on frontier molecular orbitals and their comparison of energy states confirmed that the Ge-Fe interactions in these molecules feature the specific σ and π mixed heterometallic bonding orbitals. Owing to coordinatively unsaturated Fe center with only 14 VE, 4a presents a much weaker Ge←Fe π bonding contribution than those in 3a and 3b. Altogether, the elongation of Ge-Fe bond lengths in ferrogermylene molecules means the simultaneous diminished Ge→Fe σ donation and Ge←Fe π backbonding, not just establishing the heterometallic σ bond without the π backbond.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by National Science Foundation of China (No. 22273072), the Shaanxi Provincial Enterprise Joint Fund (No. 2021JLM-31) and the Natural Science Basic Research Plan in Shaanxi Province of China (No. 2021JM-311). We thank the measurement of crystals on Bruker D8 VENTURE PHOTON Ⅱ diffractometer. The calculations were performed at the National Demonstration Center for Experimental Chemistry Education (Northwest University).

Supplementary materials

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2023.108691.