A diketiminate-bound diiron complex with a bridging carbonate ligand

Abstract

Reduction of carbon dioxide by a diiron(I) complex gives μ-carbonato-κ³ O:O′,O′′-bis­{[2,2,6,6-tetra­methyl-3,5-bis­(2,4,6-triisopropyl­phenyl)heptane-2,5-diiminate(1−)-κ² N,N′]iron(II)} toluene disolvate, [Fe₂(C₄₁H₆₅N)₂(CO₃)]·2C₇H₈, a diiron(II) species with a bridging carbonate ligand. The asymmetric unit contains one diiron complex and two cocrystallized toluene solvent mol­ecules that are distributed over three sites, one with atoms in general positions and two in crystallographic sites. Both Fe^II atoms are η²-coordinated to diketiminate ligands, but η¹- and η²-coordinated to the bridging carbonate ligand. Thus, one Fe^II center is three-coordinate and the other is four-coordinate. The bridging carbonate ligand is nearly perpendicular to the iron–diketiminate plane of the four-coordinate Fe^II center and parallel to the plane of the three-coordinate Fe^II center.

Comment

The reduction of CO₂ is of great inter­est in the scientific community because of its relevance to biological (Ragsdale & Kumar, 1996 ▶) and proposed manmade (Lewis & Nocera, 2006 ▶) energy conversions. CO₂ is also a promising one-carbon chemical feedstock (Aresta & Dibenedetto, 2007 ▶). Recently, the Peters group reported an inter­esting reductive cleavage of CO₂ to bridging oxide (O²⁻) and carbonyl (CO) groups in a diiron complex (Lu et al., 2007 ▶). Our group reported a related reaction, where eight equivalents of CO₂ react with five equivalents of the diiron(I) complex [L ^t-BuFe]₂(μ-N₂) to give two equivalents of an iron(I) dicarbonyl complex and four equivalents of an unusual diiron(II) complex, (II), with a bridging carbonate ligand (Sadique et al., 2008 ▶). In that work, L ^t-Bu represents the anionic bidentate β-diketiminate ligand 2,2,6,6-tetra­methyl-3,5-bis­(2,4,6-triisopropyl­phenyl­imido)-4-heptyl. We report here the structure of a very similar car­bon­ate complex that is derived from CO₂ using an identical method, but uses a differently substituted β-diketiminate ligand.

The work described here uses L ^t-Bu′, a supporting β-diket­iminate ligand containing an aryl substituent with isopropyl groups in the 2, 4 and 6 positions. This ligand, shown in Fig. 1 ▶, has been described previously, including its synthesis (Sadique et al., 2007 ▶). Here, we used this ligand in place of L ^t-Bu, the 2,6-diisopropyl­phenyl β-diketiminate ligand from the previous CO₂ study (Sadique et al., 2008 ▶). The diiron(I) dinitro­gen precursor was synthesized in close analogy with the synthesis of its L ^t-Bu analog, and it reacted rapidly with gaseous CO₂. The title carbonate product, (I), crystallized as orange blocks. The ¹H NMR spectrum of (I) is very similar to that of (II), except for the presence of additional peaks due to the 4-isopropyl groups.

Figure 1.

The synthesis and connectivity of (I) (R = ⁱPr) and (II) (R = H). Thus, compound (II) differs from (I) only in the lack of isopropyl groups in the para positions of the aryl rings in the β-diketiminate ligands.

A displacement ellipsoid plot of the structure of (I) is shown in Fig. 2 ▶ and the core of the mol­ecule is shown in Fig. 3 ▶. The β-diketiminate–Fe1 unit, which is η²-coordinated to the carbonate bridge, is essentially perpendicular to the carbonate plane and to the other β-diketiminate ligand, with inter­planar angles of 85.49 (5) and 89.74 (4)°, respectively. It is inter­esting to compare the structures of the new compound (I) with that of (II) reported earlier (Sadique et al., 2008 ▶). The triclinic unit cell and intensity statistics of (I) indicate the space group P with Z = 2. Therefore, there is only one mol­ecule per asymmetric unit, in contrast with (II), which has two crystallographically independent mol­ecules with different carbonate binding modes. In (II), one mol­ecule has η² binding of car­bon­ate to each Fe atom, while the other mol­ecule has η² binding to one Fe atom and η¹ binding to the other. In (I), on the other hand, only the latter bridging mode is seen. This is not surprising, because the larger β-diketiminate ligand in (I) would be expected to prevent the more sterically crowded η²:η² bridging.

Figure 2.

The structure of (I), showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 50% probability level and H atoms have been omitted for clarity. Only one orientation of each of the disordered components is shown.

Figure 3.

The core of (I), showing the perpendicular arrangement of the β-diketiminate rings.

Despite the increased steric hindrance in (I), the Fe2—O3 distance of 1.8563 (11) Å is shorter than the analogous distance of 1.881 (1) Å in the η²:η¹ form of (II). This correlates with a significantly more obtuse angle at O3 of 138.24 (11)° in (I) versus 128.64 (11)° in (II), suggesting that (I) has more π-bonding contribution in the Fe2—O3 bond. Other bond lengths and angles are similar between (I) and (II).

Experimental

All manipulations were carried out using standard Schlenk or high-vacuum techniques to prevent reaction with oxygen or moisture. Solvents were dried by passing them through activated alumina, or by vacuum distillation from sodium benzophenone ketyl. Carbon dioxide was freed from oxygen by three cycles of degassing at 77 K, and dried over activated mol­ecular sieves. The precursor L ^t-Bu′FeNNFeL ^t-Bu′ was synthesized from L ^t-Bu′FeCl by addition of KC₈ under an N₂ atmosphere, using the same procedure as that reported for analogs with other β-diketiminate ligands (Smith et al., 2006 ▶). For the preparation of (I), L ^t-Bu′FeNNFeL ^t-Bu′ (12 mg, 9.8 µmol) was dissolved in benzene-d ₆ (0.5 ml), and the tube was cooled in liquid nitro­gen and the headspace evacuated. Carbon dioxide (7.75 ml, 65 mbar) was condensed into the tube and the tube warmed to room temperature. The ¹H NMR spectrum showed the formation of (I), with singlets (relative integrations in parentheses) at δ 51 (1H), 37 (18H), 24 (2H), 9 (4H), −11 (12H), −54 (12H) and −58 (4H). The solution was transferred into a glove-box, dried under vacuum, redissolved in pentane and cooled to 233 K to give orange crystals of (I).

Crystal data

• [Fe₂(C₄₁H₆₅N)₂(CO₃)]·2C₇H₈

• M _r = 1527.88

• Triclinic,

• a = 14.7703 (7) Å

• b = 15.5535 (8) Å

• c = 21.6991 (11) Å

• α = 77.533 (3)°

• β = 79.815 (3)°

• γ = 70.941 (3)°

• V = 4569.9 (4) ų

• Z = 2

• Mo Kα radiation

• μ = 0.37 mm⁻¹

• T = 100 K

• 0.27 × 0.25 × 0.16 mm

Data collection

• Bruker SMART APEXII Platform CCD area-detector diffractometer

• Absorption correction: multi-scan (SADABS; Sheldrick, 2008a ▶) T _min = 0.660, T _max = 0.746

• 71339 measured reflections

• 26363 independent reflections

• 19303 reflections with I > 2σ(I)

• R _int = 0.034

Refinement

• R[F ² > 2σ(F ²)] = 0.042

• wR(F ²) = 0.105

• S = 1.02

• 26363 reflections

• 1199 parameters

• 275 restraints

• H-atom parameters constrained

• Δρ_max = 0.42 e Å⁻³

• Δρ_min = −0.36 e Å⁻³

H atoms were positioned geometrically, with C—H = 0.95–1.00 Å, and refined with relative isotropic displacement parameters of U _iso(H) = 1.2U _eq(C) for methine, methylene and aromatic H atoms, and 1.5U _eq(C) for methyl H atoms. There are three disordered moieties in the diiron mol­ecule, two isopropyl groups and one tert-butyl group. The isopropyl group containing atoms C103, C113 and C123 was modeled over three positions, and the site occupancies refined to 0.511 (4), 0.275 (4) and 0.215 (4), while their sum was constrained to unity. A two-position disorder model proved insufficient due to the presence of a third site in the difference Fourier map, which manifested itself by residual peaks in an isotropic refinement or by elongated dis­place­ment ellipsoids in an anisotropic refinement of the same. All three positions of the group were restrained to have similar bond lengths and angles, and the anisotropic displacement parameters of the positionally similar atoms were constrained to be equal. The disorder of the second isopropyl group (atoms C134, C145, and C155) was modeled over two positions with site occupancies of 0.542 (7) and 0.458 (7). The bond lengths and angles were restrained to be similar and the anisotropic displacement parameters were constrained to be equal. The disorder of the tert-butyl group (atoms C14, C64, C74 and C84) was modeled over two positions and refined to site occupancies of 0.845 (2) and 0.155 (2). Despite the small percentage of the minor component of the disorder, its neglect left three residual peaks of electron density of 1.0–1.2 e Å⁻³ in positions geometrically appropriate for another orientation of the group. The bond lengths and angles of the two groups were restrained to be similar, and the anisotropic displacement parameters of related atoms were con­strained to be equal.

There are three independent cocrystallized toluene solvent mol­ecule sites, all of which required disorder modeling. The first (C17–C77) was modeled over two general positions with site occupancies of 0.608 (5) and 0.392 (5). Both orientations were restrained to have the same bond lengths and angles, and the minor component of the disorder was restrained to be flat. Additionally, the anistropic displacement parameters between one pair of promixal atoms (C37 and C77′) were restrained to be similar. The second toluene mol­ecule (C18–C78) was modeled over a crystallographic inversion center with component occupancies of 0.50 each by symmetry. The mol­ecule was restrained to be flat and the anisotropic displacement parameters of positionally close symmetry-equivalent atoms (C18 and C58, and C38 and C78) were constrained to be equal. The disorder model for the third toluene mol­ecule (C19–C79) was over a crystallographic inversion center (0.50 each by symmetry) and additionally over two general positions, giving unique atom occupancies of 0.313 (8) and 0.187 (8). The latter model was required to account for two positions of electron density on each side of the inversion center. The two unique mol­ecular positions were restrained to be flat, and the bond lengths and angles of both were restrained to be similar. A variety of restraints and constraints were also imposed on the anisotropic displacement parameters, due to the proximity of atomic positions between atoms of the general position disorder and those at symmetry-related positions.

Data collection: APEX2 (Bruker, 2004 ▶); cell refinement: SAINT (Bruker, 2008 ▶); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008b ▶); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008b ▶); molecular graphics: SHELXTL (Sheldrick, 2008b ▶); software used to prepare material for publication: SHELXTL.

Table 1. Selected geometric parameters (Å, °).

Fe1—N11	1.9754 (11)
Fe1—N21	1.9763 (11)
Fe1—O2	2.0293 (10)
Fe1—O1	2.0692 (10)
Fe2—O3	1.8563 (11)
Fe2—N14	1.9427 (11)
Fe2—N24	1.9816 (11)
C1—O1	1.2715 (17)
C1—O2	1.2745 (17)
C1—O3	1.2897 (17)

N11—Fe1—N21	97.44 (4)
N11—Fe1—O2	126.06 (5)
N21—Fe1—O2	118.41 (5)
N11—Fe1—O1	125.21 (4)
N21—Fe1—O1	125.81 (4)
O2—Fe1—O1	64.41 (4)
O3—Fe2—N14	152.19 (5)
O3—Fe2—N24	111.46 (5)
N14—Fe2—N24	96.34 (5)
O1—C1—O2	118.19 (12)
O1—C1—O3	122.12 (13)
O2—C1—O3	119.68 (13)
C1—O1—Fe1	87.85 (8)
C1—O2—Fe1	89.54 (8)
C1—O3—Fe2	138.24 (11)
C21—N11—Fe1	123.30 (9)
C12—N11—Fe1	108.05 (8)
C41—N21—Fe1	123.05 (9)
C13—N21—Fe1	110.10 (8)
C24—N14—Fe2	124.38 (9)
C15—N14—Fe2	109.48 (8)
C44—N24—Fe2	124.31 (9)
C16—N24—Fe2	106.86 (8)