2.1 Structural arrangements of ene-diamido and diimino ligands in binuclear compounds

We have centered our attention on structurally characterized compounds that contain the ene-diamido or the diimino fragments as a bridge between two metal atoms. Clusters of higher nuclearity are not considered in this paper [21–24]. From a search in the Cambridge Structural Database [25] three different coordination modes are identified (see Fig. 1). They are, respectively, μ-η²-(N, N′),η²-(N, N′), I, μ-η²-(N, N′),η²-(N, C), II, and μ-η²-(N, N′),η⁴-(N, N′, C, C′), III. The bridging ligand in the coordination modes I and III behaves as a eight electron donor. However, in I the bridge has the dianionic ene-diamido character, while in mode III displays a neutral diimino nature. In the latter coordination mode, two nitrogen σ lone pairs and two π contributions of C=N bonds are donated to each metal respectively. Finally, the ligand in II is also neutral and behaves as a six electron donor. Four electrons are donated to one metal through the nitrogen σ combinations of diimino, whereas the third pair of electrons come from the π contribution of one C=N bond and are donated to the second metal (Fig. 1).

On the basis of the structural features, the complexes may be distinguished in seven different categories. Tables 1 and 2 show a simplified scheme for each structural type, the complex formula and the corresponding CCDC refcode. Table 1 displays the examples in which the bridging ligand is of C₆H₄(NR)₂-o type, whereas Table 2 shows those in which the bridge does not contain the arylic backbone.

The general arrangements of the bridging ligand, displayed in Fig. 1, are encountered in both Tables 1 and 2, although with some sub-distinctions. In I the ligand bridges the two metals through the N atoms and maintains its molecular plane perpendicular to the M–M bond (μ-η²-(N, N′),η²-(N, N′) bonding type). In mode IIa the diazabutadiene ligand binds one metal through the N=C bond, while the second metal atom is coordinated by both nitrogen atoms (μ-η²-(N, N′),η²-(N, C) bonding type). The modes IIb and IIc, which differ for the presence or absence of M–M bond, are similar to that of IIa but the diazabutadiene bridge is not unique, but there is an additional bridge X. The latter coordinates both metals through a single atom (e.g., CO, H or halide) or two linked atoms (e.g. acetylene type ligand). Alternatively, the additional bridge is a second diazabutadiene ligand. In mode IIIa the diazabutadiene molecule is bent towards one metal atom and the short M–C contacts are indicative of a bonding interaction. Again, types IIIb and IIIc feature the same coordination mode of IIIa, μ-η²-(N, N′),η⁴-(N, N′, C, C′), and have an additional bridging ligand X. Their main difference is the presence or absence of a direct metal–metal bond, respectively.

In order to gain a general overview of the structural and electronic features of the coordinated 1,4-diaza-1,3-butadiene skeleton in these binuclear complexes, representative models for each subclass were selected and theoretical calculations were carried out followed by a bonding analysis.

2.2 Ene-diamido μ-η²-(N, N′),η²-(N, N′) bridging ligand

Recently, the MO distribution of [Ru₂{μ-C₆H₄(NH)₂-o}(μ-dppm)(CO)₂(PPh₃)₂] complex, in which the o-phenylenediamido ligand symmetrically bridges the two metal atoms (μ-η²-(N, N′),η²-(N, N′), type I), has been analyzed by DFT methods [20]. Model compound [Ru₂{μ-C₆H₄(NH)₂-o}(μ-H₂PCH₂PH₂)(PH₃)₂(CO)₂] has been adopted and the bonding scheme, previously discussed by EHMO method [44] has been consistently confirmed. Curiously enough, from the inspection of Tables 1 and 2 one can see that the μ-η²-(N, N′),η²-(N, N′) arrangement is specific for o-phenylenediamido ligand and not for structurally characterized examples containing the 1,4-diaza-1,3-butadiene ligand (that is, without the arylic bakbone). In fact, the ligands of type R,R′-DAD adopt the bridging coordination modes II or III exclusively.

In order to gain further information about the bonding capabilities of the o-phenylenediamido ligand, calculations on the simplest model compounds [M₂{μ-C₆H₄(NH)₂-o}(CO)₆] (M = Fe, 1a; Ru, 2a) were performed with the coordination mode μ-η²-(N, N′),η²-(N, N′), I. The optimized structure of the iron derivative 1a is displayed in Fig. 2, while its computed structural parameters are reported in Table 3. A good correlation with the experimental values of the [Fe₂{μ-C₆H₄(NH)(NR)-o}(CO)₆] complexes was found. The calculated C–C and N–C lengths are in agreement with a dianionic ene-diamido character of the C₆H₄(NH)₂-o ligand. Also, the results for 2a are consistent with the geometry of the [Ru₂{μ-C₆H₄(NH)₂-o}(PPh₃)₂(CO)₄] complex (see Section 5). For both, 1a and 2a derivatives, the MO analysis previously reported for o-phenylendiamido ruthenium complexes of type I is well suited and does no require further comment. The computed HOMO for the model 1a comes from interaction of the filled π₃* orbital with the 1b₁ metal combination and is largely ligand centered, whereas HOMO-1 accounts for the direct Fe–Fe interaction. The computed charges of both iron atoms are 1.27 and the Fe–Fe interaction is a non-polarized metal–metal bond in a d⁷–d⁷ L₃M–ML₃ system.

The ruthenium derivative [Ru₂(DAD)(CO)₆], 2b, (DAD = HNCHCHNH) was previously optimized [20]. The suitable results obtained after simplification of the o-phenylendiamido group by DAD ligand prompted us to optimize as well the simplest model compound [Fe₂(DAD)(CO)₆], 1b, with the DAD ligand arranged as in I and without symmetry constrains. The resulting structure of 1b is shown in Fig. 3. The computed structural parameters are close to those of 1a and are also collected in Table 3 for the appropriate comparison. No noteworthy differences can be observed after the oversimplification of the o-phenylendiamido ligand (dismissal of the aryl backbone).

2.3 Diimino μ-η²-(N, N′),η²-(N, C) bridging ligand

Concerning the coordination mode μ-η²-(N, N′),η²-(N, C) (type II), the model compounds [M₂(DAD)(CO)₆] (M = Fe, 1c; Ru, 2c) were optimized without symmetry restrictions. The resulting structures are displayed in Fig. 4. The computed structural parameters for the iron derivative 1c are grouped in Table 4. Bond distances and angles show a reasonable agreement when compared with those found experimentally. The calculated N–C bonds are characterized by two different distances of 1.299 and 1.405 Å, N1–C1 and N2–C2, respectively. The former distance is close to that of a typical double N=C bond, while the latter is slightly shorter than the computed NC bond in the ene-diamido complex 1a. These values are in conformity with a diimino description of the diazabutadiene bridging ligand. Selected calculated bond lengths and angles of compound [Ru₂(DAD)(CO)₆], 2c, are only included as Supplementary Material because no structural data are available for comparison. However, it is important to highlight that the computed IR spectrum of 2c in the ν(CO) region (2054, 2012, 1992, 1983, 1958 and 1956 cm^–1) fits quite well with the reported IR spectra of [Ru₂(R,R′-DAD)(CO)₆] complexes [35].

The bridging diimino ligand in these [M₂(DAD)(CO)₆] complexes behaves as a six electron donor to the d⁸–d⁸ L₃M–ML₃ system, behaving the dihapto bonded C=N bond as the equivalent of a two electron donor. Each metal displays pseudo-octahedral and trigonal bipyramidal coordination environments at the atoms Fe1 and Fe2, respectively (see 1c, Fig. 4). As a possible interpretation, the metal–metal bond can be considered of dative nature. According to the latter, the metal–metal interaction stems from the donation of a filled σ-ML₅ hybrid onto an empty σ-ML₄ one. The calculated HOMO for 1c, shown in Fig. 5, supports the viewpoint. The computed metal charges (0.41 and 0.12 for the Fe1 and Fe2 atoms in 1c, respectively) account well for the presence of a polarized metal–metal bond.

2.4 Comparison of the coordination modes μ-η²-(N, N′),η²-(N, N′), type I, and μ-η²-(N, N′),η²-(N, C), type II, in compounds with the same general formulation [M₂(DAD)(CO)₆] (M = Fe, Ru)

The optimizations of iron and ruthenium binuclear complexes with similar formulation [M₂(DAD)(CO)₆] and different coordination modes μ-η²-(N, N′),η²-(N, N′), I, and μ-η²-(N, N′),η²-(N, C), II, computed at the same level of theory, allow us to adequately compare their structural and energetic features (1b vs. 1c and 2b vs. 2c). Regarding the bond distances, the C–C bond length of the metallacycle is longer in compounds 1c and 2c (diimino μ-η²-(N, N′),η²-(N, C), coordination mode II) than the same value in 1b and 2b (for example, 1.429 versus 1.339 Å for M = Fe), in agreement with the ene-diamido formulation of the bridge (mode I) in the latter complexes. Additionally, both C–N distances in the complexes of type c (diimino) are shorter than those found for b complexes (for example, 1.299 and 1.405 versus 1.433 Å for M = Fe). These structural differences accompany also to different formal charges of the metal atoms. Comparison of the iron charges in compounds 1b (0.79 for the two atoms) and 1c (0.41 and 0.12 for the Fe1 and Fe2 atoms, respectively) confirms the different formulation of the bridge.

Concerning the energies, the [M₂(DAD)(CO)₆] isomers of type c are found to be slightly more stable than those of type b. The energy differences are computed to be 2.5 and 3.0 kcal mol^–1, for M = Fe and Ru, respectively. This fact suggests that the μ-η²-(N, N′),η²-(N, C) coordination mode for the 1,4-diaza-1,3-butadiene ligand is slightly privileged with respect to the μ-η²-(N, N′),η²-(N, N′) one. However, when the bridging ligand contains the o-phenylenediamido group the structural situation is that represented by I, according to the experimental evidence. A plausible explanation for this fact is the following: generally, the H–C–C–H (or R′–C–C–R′) torsion angle of coordinated diimine ligands are close to 0°. However, the computed H–C–C–H torsion angle for DAD in the complexes 1c and 2c is ca. 17°, a value that is not compatible with the planar o-phenylene backbone.

2.5 μ-η²-(N, N′),η⁴-(N, N′, C, C′) bridging ligand

The chemical oxidation of [Ru₂{μ-C₆H₄(NH)₂-o}(μ-dppm)(CO)₂(PPh₃)₂] complex affords [Ru₂{μ-C₆H₄(NH)₂-o}(μ-dppm)(CO)₂(PPh₃)₂](PF₆)₂, in which the C₆H₄(NH)₂-o ligand adopts the μ-η²-(N, N′),η⁴-(N, N′, C, C′) coordination mode (III) and behaves as an eight-electron donor in the diimino form. The structural characterization of the latter compound, together with a theoretical analysis of the simplest model compound [Ru₂(DAD)(CO)₆]²⁺, has recently appeared [20]. In order to generalize the attributes of the bonding mode III, the formally isoelectronic model compound [Mn₂(DAD)(CO)₆], 3, has also been studied for a convenient comparison. The optimized geometry of 3 is depicted in Fig. 6. Table 5 shows selected structural data (experimental and calculated) corresponding to the known [Mn₂(R,R′-DAD)(CO)₆] complexes. In general, there is a satisfactory agreement between the calculated and experimental geometric parameters, the bond distances agree within 0.05 Å, except for the Mn–Mn separation. Although the latter is clearly overestimated, its value of 2.723 Å is still consistent with a direct metal–metal bond [45]. The C–C and N–C lengths of 1.399 and 1.379 Å, respectively, are in conformity with a diimino constitution of the DAD ligand.

The frontier MOs of the diimino DAD ligand include two filled σ lone pairs (in phase σ_ip and out phase σ_op) that correspond to the HOMO and HOMO-1 of the molecule; two filled, lower in energy, π₁ and π₂ combinations and two empty π* combinations (π₃* and π₄*). Consequently, the bonding capabilities of this ligand arises from the two nitrogen σ lone pairs and the two filled π MOs, making possible the donation to the metals of eight electrons. On the basis of a FMO analysis, essentially, the qualitative picture of the bonding in 3 may be described as the ligand-to-metal donation of the two σ lone pairs to Mn1 atom and of the two π combinations to Mn2 atom. The different Mn1–N and Mn2–N distances accounts well with this explanation. Moreover, the specific orientation of the MnL₃ fragment at the atom Mn2 atom is well rationalized with the backdonation from one of the well known d_π metal hybrids (2e levels) of a C_3v–ML₃ fragment [46] into the empty π₃* level of the bridging ligand.

The two-electron chemical oxidation of ruthenium complexes of type I determines the reorganization of the C₆H₄(NH)₂-o ligand to the structural type III. This experimental fact has been studied theoretically in the model compounds [Ru₂(DAD)(CO)₆] and [Ru₂(DAD)(CO)₆]²⁺. However, the removal of two electrons in the binuclear system may formally occur by the simple elimination of one ligand acting as 2e donor. For example, Vrieze and coworkers reported the dissociation of CO from [Ru₂(R,R′-DAD)(CO)₆] complexes (coordination type I) to give [Ru₂(R,R′-DAD)(μ-CO)(CO)₄] (coordination type III). Analogously, the same authors observed the formation of [Ru₂(R,R′-DAD)(HCCH)(CO)₄] compounds (coordination type III) by interaction of species [Ru₂(R,R′-DAD)(CO)₆] with acetylene. Both types of compounds have been studied through the respective models [Ru₂(DAD)(μ-CO)(CO)₄], 4, and [Ru₂(DAD)(μ-HCCH)(CO)₄], 5. Full geometry optimizations were performed for 4 and 5 (both with imposed symmetry Cs) and the resulting final geometries are shown in Fig. 7. Selected calculated parameters and, for comparison, selected experimental data from X-ray crystallography have been collected in Tables 6 and 7, respectively. The geometrical features of the diimino ligand in 4 and 5 are similar and adequately match the reported structures. Furthermore, the computed IR bands concerning the CO region (2029, 2000, 1964, 1958, 1848 cm^–1 for 4 and 2020, 2001, 1958, 1956 cm^–1 for 5) correctly reproduce the experimental trends.

Both compounds were analyzed theoretically in an early work [17]. Our results are in agreement with the bonding description presented in that paper except for the proposed lack of the metal–metal bond. This fact may be re-interpreted on the basis of our results. At variance with complex 2c in which the metal–metal interaction appears in the HOMO (see Fig. 5), an analogs level for complex 4 is found in the HOMO-7 and it is significantly delocalized (Fig. 8). Moreover, the higher filled MOs feature also some antibonding metal–metal character and determine the poor metal–metal overlap population, pointed out by the authors [17]. Metal–metal bonds are frequently difficult to investigate and new approaches are continuously emerging [47,48]. Although no additional deeper studies have been performed, on the basis of our results the existence of a metal–metal interaction in 4 is a conclusion preferable to that of its non-existence. In fact, the computed Ru–Ru bond length in 4 is slightly shorter than in 2c (2.819 Å), where the Ru–Ru bond is not questionable.

The experimental evidence of CO dissociation in [Ru₂(R,R′-DAD)(CO)₆] complexes has a theoretical confirmation from the present calculations. In 2c, the dissociated CO molecule is precisely the ligand that displays the longest M–CO distance. The computed energy for the process represented in Eq. (1) is –10.64 kcal mol^–1. This value agrees with the experimental observation of dissociation of CO upon refluxing the [Ru₂(R,R′-DAD)(CO)₆] complexes in toluene [40]. The LUMO of 4 is characterized by a hybrid directed toward the created vacant position (see Fig. 8).

Finally, a representative example of the structures of type IIIc has been chosen. The model compound [(CO)₂MeRu(μ-DAD)(μ-I)Ru(CO)₂], 6, has been optimized with no symmetry constrains. Fig. 9 shows the resulting structure and Table 8 summarizes selected structural parameters and their comparison with the experimental data [39].

The calculated geometry of 6 is in good agreement with the experimental structure of complex [(CO)₂MeRu(μ-ⁱPr,H-DAD)(μ-I)Ru(CO)₂]. In addition, the computed carbonyl IR bands (2030, 2017, 1977 and 1988 cm^–1) are in accord with the experimental values. Again, the bond distances support the diimino formulation of the DAD ligand, which behaves as a 8e donor. Thus, the complex features a d⁶-RuL₄ fragment (at Ru2) saturated from the nitrogen σ lone pairs and a d⁸-RuL₃ fragment (at Ru1) which is η⁴ coordinated by the π system of DAD (four π electrons, donation and backdonation). No metal–metal bond exists in view of the long Ru–Ru distance (3.063 Å experimental and 3.150 Å computed).