N-Heterocyclic germylenes and stannylenes of the type [Fe{(η⁵-C₅H₄)NR}₂E] with bulky alkyl substituents

2.1 Crystal structures

Surprisingly, only two examples of structurally characterised 1,1′-di(alkylamino)ferrocenes have been reported to date, namely the benzyl-substituted derivatives [Fe(η⁵-C₅H₄–NHCH₂-p-C₆H₄-NMe₂)₂] and [Fe(η⁵-C₅H₄–NHCH₂-p-C₆H₄–OMe)₂] [33]. Single-crystal X-ray diffraction studies were therefore performed for [Fe(η⁵-C₅H₄–NHAd)₂] (Fig. 2) and [Fe(η⁵-C₅H₄–NHtBu)₂] (Fig. 3).

Fig. 2:

Molecular structure of [Fe(η⁵-C₅H₄–NHAd)₂] in the crystal.

Fig. 3:

Molecular structure of [Fe(η⁵-C₅H₄–NHtBu)₂] in the crystal.

The cyclopentadienyl rings are almost coplanar in each case (see below). Their conformation is synperiplanar in the case of [Fe(η⁵-C₅H₄–NHtBu)₂] (torsion angle N–C_ipso–C_ipso–N 7.2°) and synclinal in the case of [Fe(η⁵-C₅H₄–NHAd)₂] (torsion angle N–C_ipso–C_ipso–N 72.9°). This differs markedly from the antiperiplanar arrangement observed for [Fe(η⁵-C₅H₄–NHCH₂-p-C₆H₄–X)₂] (X=NMe₂, OMe) [33]. The almost fully eclipsed conformation of [Fe(η⁵-C₅H₄–NHtBu)₂] coincides with intramolecular hydrogen bonding between the two NH units (not shown in Fig. 3). Due to this attractive interaction, the cyclopentadienyl rings are tilted by 2.7° in that direction which brings the two C_ipso atoms closer together. On top of that the C_ipso–N bond vectors are inclined slightly towards the Fe atom by −3.1 and −2.2° (cg–C_ipso–N 176.9 and 177.8°, cg=cyclopentadienyl ring centroid). This allows an N1···N2 distance of 3.046(3) Å, ca. 0.30 Å shorter than the interplanar distance between the Cp rings in ferrocene. The average N–H···N angle is ca. 112°, which is at the low end of the values commonly observed for such hydrogen bonds. These structural data point to a weak intramolecular interaction [34], [35], [36], which is not at all unusual for diamines [37]. Hydrogen bonding is absent in the structure of [Fe(η⁵-C₅H₄–NHAd)₂]. In this case, the cyclopentadienyl ring tilt angle is merely 1.5°. However, in contrast to the tert-butyl congener the wedge opens up in the direction of the C_ipso atoms and the C_ipso–N bond vectors are pointing away from the Fe atom by 4.7 and 3.3°.

We have also determined the structure of the closely related aminoiminoferrocene derivative [Fe(η⁵-C₅H₄–NHAd)(η⁵-C₅H₄–N=Ad′)] (Fig. 4). The molecules are aggregated as centrosymmetric dimers by intermolecular N–H···N hydrogen bonds, whose metrical parameters [N1···N2 2.989(5) Å, N–H···N 161(3)°] indicate a significantly stronger interaction than that discussed above. The cyclopentadienyl ring tilt angle is 3.9° in this case. Similar to the diamine [Fe(η⁵-C₅H₄–NHAd)₂], the wedge opens up in the direction of the C_ipso atoms and the C_ipso–N bond vectors are pointing away from the Fe atom by 3.6 and 1.8°. The N_imine atom exhibits two substantially different C–N bond lengths, viz. 1.415(5) and 1.289(5) Å, in accord with a single and a double bond between sp² hybridised carbon and nitrogen atoms. The corresponding interatomic distances of the N_amine atom are 1.380(5) and 1.459(5) Å for the bond to the sp² and sp³ hybridised carbon atom, respectively, which compares well to the values determined for the diamines [Fe(η⁵-C₅H₄–NHAd)₂] [1.388(8) and 1.399(8) Å for N–C(sp²) vs. 1.458(8) and 1.449(8) Å for N–C(sp³)] and [Fe(η⁵-C₅H₄–NHtBu)₂] [1.411(4) and 1.414(3) Å for N–C(sp²) vs. 1.474(3) and 1.463(4) Å for N–C(sp³)]. The C–N_amine–C and C–N_imine–C angle is 120.7(3) and 117.8(3)°, respectively. This similarity of the bond angles at the N_amine and N_imine atoms is not uncommon. For example, the C–N_amine–C angles of [Fe(η⁵-C₅H₄–NHCH₂-p-C₆H₄–NMe₂)₂] are 117.9(4)°. This value lies in the narrow range of the C–N_imine–C angles [116.4(2)–120.7(2)°, average value 118.5°] found for the two independent molecules of [Fe(η⁵-C₅H₄–N=CH-p-C₆H₄–NMe₂)₂] present in the crystal [33].

Fig. 4:

Molecular structure and aggregation of [Fe(η⁵-C₅H₄–NHAd)(η⁵-C₅H₄–N=Ad′)] in the crystal. Hydrogen bonds are indicated by broken lines.

We next come to the two structurally characterised germanium(II) compounds of this study, viz. [{Fe(η⁵-C₅H₄–NtBu)₂}Ge] (Fig. 5) and [μ₂-{Fe(η⁵-C₅H₄–NAd)₂}Ge₂(μ₃-O)(GeCl₂)] (Fig. 6).

Fig. 5:

Molecular structure of [{Fe(η⁵-C₅H₄–NtBu)₂}Ge] in the crystal.

Fig. 6:

Molecular structure of [μ₂-{Fe(η⁵-C₅H₄–NAd)₂}Ge₂(μ₃-O)(GeCl₂)] in the crystal. Selected bond angles (deg): Ge1–O1–Ge2 89.75(12), Ge1–O1–Ge3 133.41(15), Ge2–O1–Ge3 136.82(15), N1–Ge1–N2 83.19(13), N1–Ge1–O1 78.35(12), N2–Ge1–O1 79.06(12), N1–Ge2–N2 82.40(13), N1–Ge2–O1 77.74(12), N2–Ge2–O1 78.64(13), O1–Ge3–Cl1 94.66(9), O1–Ge3–Cl2 94.29(9), Cl1–Ge3–Cl2 94.84(5).

The molecular structure of the N-heterocyclic germylene [{Fe(η⁵-C₅H₄–NtBu)₂}Ge] is similar to that of the neopentyl and trimethylsilyl congeners recently described by us [16]. Pertinent metric parameters are collected in Table 1. For conciseness, the data for the corresponding stannylenes are also included.

The N atoms are trigonal planar in each case. The cyclopentadienyl rings are tilted only slightly, the angle being close to the value of 7.4° reported for the almost unstrained [3]ferrocenophane in each case [38]. The germanium bond angles of the ferrocene-based germylenes in Table 1 are very similar to the values reported for Lappert’s iconic acyclic diaminogermylene [(Me₃Si)₂N]₂Ge (ca. 107°) [39] and the closely related compound [(iPrMe₂Si)₂N]₂Ge (ca. 105°) [40]. This is strongly reminiscent of the fact that the N–C–N bond angles of ferrocene-based N-heterocyclic carbenes of the type [{Fe(η⁵-C₅H₄–NR)₂}C] (ca. 120°) [19], [20] are essentially identical with the values determined for acyclic diaminocarbenes [41], [42], [43].

The unusual compound [μ₂-{Fe(η⁵-C₅H₄–NAd)₂}Ge₂(μ₃-O)(GeCl₂)] contains three Ge^II atoms, which are each three-coordinate and exhibit a trigonal pyramidal configuration (sum of angles: 240.6, 238.8 and 283.8° for Ge1, Ge2 and Ge3, respectively). The oxygen atom is in a trigonal-planar bonding environment (sum of angles 360.0°). The germanium atoms Ge1 and Ge2 are each bridged by the oxygen atom and both nitrogen atoms, which results in rather acute germanium bond angles close to 80°. The bond angles of the third germanium atom are ca. 95°. The molecule has approximate C_s symmetry (the Fe atom lies only 0.1 Å out of the plane of the three Ge atoms). It can be formally viewed as GeO stabilised by adduct formation with dichlorogermylene (acting as a Lewis acid) and the diaminogermylene [{Fe(η⁵-C₅H₄–NAd)₂}Ge] (acting as a Lewis base). The Ge–N bond lengths lie in the narrow range from 2.074(3) to 2.098(3) Å [average value 2.084(3) Å] and are thus ca. 0.2 Å longer than those of [{Fe(η⁵-C₅H₄–NtBu)₂}Ge], whose Ge^II atom is only two-coordinate. They are similar to those of the Ge₂N₂ core [average value 2.074(4) Å] of the aggregational diaminogermylene dimer [(CH₂NBz)₂Ge]₂ (Bz=benzyl), whose Ge^II atoms are also in a tricoordinate bonding environment [44]. The Ge–O bond lengths are 1.928(3) and 1.933(3) Å for the two nitrogen-bonded germanium atoms. These values agree well with those reported for the OSn^II₃ core of the bis(diaminostannylene)-stabilised SnO (ca. 2.08–2.13 Å) [31], considering the difference of the covalent radii of germanium (1.20 Å) and tin (1.39 Å) [45]. The corresponding Ge–O distance of 1.897(3) Å determined for the chlorine-bonded germanium atom is slightly, but significantly, shorter. For comparison, the Ge–O bond lengths of the polymeric dichlorogermylene adduct [GeCl₂(μ₂-1,4-dioxane)]_∞, which contains tetracoordinate Ge^II atoms, are 2.3989(12) Å [46], whereas the digermylene [Ar(SiMe₃)NGe]₂O [Ar=C₆H₂(CHPh₂)₂Me-2,6,4], whose oxygen and germanium atoms are exclusively dicoordinate, has Ge–O bond lengths of 1.8088(16) and 1.8154(15) Å and Ge–N bond lengths of 1.875(2) and 1.878(2) Å [47]. The Ge₂N₂ unit of [μ₂-{Fe(η⁵-C₅H₄–NAd)₂}Ge₂(μ₃-O)(GeCl₂)] is not planar. The folding is due to the fact that both germanium atoms are also bonded to the oxygen atom. The bridging oxygen atom causes a rather short distance of these two germanium atoms of 2.7240(6) Å, only ca. 13% longer than the sum of the covalent radii (2.40 Å) [45] and much less than the sum of the van der Waals radii (4.58 Å) [36].

Finally, we come to the structure of the stannylene [{Fe(η⁵-C₅H₄–NtBu)₂}Sn] (Fig. 7), which is very similar to that of the trimethylsilyl congener (see Table 1).

Fig. 7:

Molecular structure of [{Fe(η⁵-C₅H₄–NtBu)₂}Sn] in the crystal.

The tin bond angles of these ferrocene-based stannylenes (ca. 103°) are very close to those reported for the acyclic diaminostannylenes [(Me₃Si)₂N]₂Sn (ca. 105°) [48] and [(PhMe₂Si)₂N]₂Sn (ca. 102°) [49]. A comparison of the bond parameters of the germylenes of this study with their tin homologues (Table 1) reveals two trends which are well-known in this area of chemistry [5]. First, the Ge–N bonds are shorter than the Sn–N bonds by ca. 0.2 Å, which is in accord with the difference of the covalent radii of germanium (1.20 Å) and tin (1.39 Å) [47]. Second, the bond angles of germanium are wider than those of tin (by ca. 3°), which is in accord with Bent’s rule [50], [51].

4.1 [{Fe(η⁵-C₅H₄–NtBu)₂}Ge]

nBuLi (0.50 mL of a 1.60 M solution in hexane, 0.80 mmol, 2.6 equivalents) was added dropwise to a stirred solution of [Fe(η⁵-C₅H₄–NHtBu)₂] [27] (100 mg, 0.30 mmol) in hexane (5 mL) cooled to 0°C, leading to the formation of [Fe(η⁵-C₅H₄–NLitBu)₂] as a red precipitate. After 15 min the solid was isolated by filtration, washed with hexane (3×5 mL) and dried under vacuum. Yield 76 mg (73%). [Fe(η⁵-C₅H₄–NLitBu)₂] (76 mg, 0.22 mmol) was dissolved in THF (5 mL) and [GeCl₂(1,4-dioxane)] (52 mg, 0.22 mmol) was added to the stirred solution. After 20 h volatile components were removed under vacuum. The solid residue was extracted with hexane (10 mL) and the extract reduced to dryness under vacuum, leaving the product as a yellow solid. Yield 85 mg (95%). – ¹H NMR (C₆D₆): δ=1.39 (s, 18 H, tBu), 3.94 (m, 8 H, C₅H₄). – ¹³C{¹H} NMR (C₆D₆): δ=34.5 (CMe₃), 58.8 (CMe₃), 67.5, 70.5 (2×CH), 109.1 (C_ipso). – C₁₈H₂₆N₂FeGe (398.9): calcd. C 54.20, H 6.57, N 7.02; found C 54.31, H 6.67, N 6.79.

4.2 [{Fe(η⁵-C₅H₄–NtBu)₂}Sn]

[Fe(η⁵-C₅H₄–NLitBu)₂] (prepared as described above; 81 mg, 0.24 mmol) was dissolved in a mixture of benzene (12 mL) and THF (2 mL). SnCl₂ (45 mg, 0.24 mmol) was added and the mixture stirred for 1 h. Volatile components were removed under vacuum. The solid residue was extracted with hexane (10 mL) and the extract reduced to dryness under vacuum, leaving the product as an orange solid. Yield 102 mg (97%). – ¹H NMR (C₆D₆): δ=1.31 (s, 18 H, tBu), 3.94 (m, 8 H, C₅H₄). – ¹³C{¹H} NMR (C₆D₆): δ=34.9 (CMe₃), 59.8 (CMe₃), 66.6, 71.3 (2×CH), 112.4 (C_ipso). – ¹¹⁹Sn NMR (C₆D₆): δ=544. – C₁₈H₂₆N₂FeSn (445.0): calcd. C 48.59, H 5.89, N 6.30; found C 49.00, H 6.08, N 5.65.

4.3 [Fe(η⁵-C₅H₄–NHAd)(η⁵-C₅H₄–N=Ad′)]

A solution of [(Me₃Si)₂N]₂Pb (545 mg, 1.00 mmol) in benzene (4 mL) was added to a solution of [Fe(η⁵-C₅H₄–NHAd)₂] (500 mg, 1.03 mmol) in benzene (5 mL). The solution was stirred for 16 h. The grey solid was removed by filtration. The filtrate was reduced to dryness under vacuum. The brown residue was washed with hexane (2 mL) and recrystallised from toluene, which furnished the product as an orange-brown microcrystalline solid. Yield 320 mg (64%). – ¹H NMR (C₆D₆): δ=1.47–2.21 (m, 29 H, Ad/Ad′), 3.15 (s, 1 H, NH), 3.25, 3.40 (2 s, 2×1 H, Ad/Ad′), 4.01 (m, 2 H, C₅H₄), 4.03 (m, 4 H, C₅H₄), 4.17 (m, 2 H, C₅H₄). – ¹³C{¹H} NMR (C₆D₆): δ=28.2, 32.1, 33.2, 35.3, 36.8, 37.7, 38.3, 38.9, 39.5, 44.0 (10×Ad/Ad′ CH and CH₂), 59.7 (cyclopentadienyl CH), 61.0 (CHNH), 63.6, 64.7, 65.5 (3×cyclopentadienyl CH), 105.8, 110.3 (2×C_ipso), 181.7 (C=N). – C₃₀H₃₈N₂Fe·H₂O (500.5): calcd. C 71.99, H 8.06, N 5.60; found C 72.76, H 8.03, N 5.47.

4.4 Crystal structure determinations

For each data collection a single crystal was taken from the mother liquor, mounted on a micro-mount and all geometric and intensity data were taken from this sample. Data collections were carried out using MoKα radiation (λ=0.71073 Å) on a Stoe IPDS2 diffractometer equipped with a 2-circle goniometer and an area detector or on a Stoe StadiVari diffractometer equipped with a 200 K Dectris Pilatus detector. The data sets were corrected for absorption (by integration), Lorentz and polarisation effects. The structures were solved by direct methods (Sir 2008) [54] and refined using alternating cycles of full-matrix least-squares refinements against F² (Shelxl2014/7) [55]. H atoms were included to the models in calculated positions with the 1.2 fold isotropic displacement parameter of their bonding partner. Experimental details for each diffraction experiment are given in Table 2.

CCDC 1546004–1546009 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre viawww.ccdc.cam.ac.uk/data_request/cif.