An Inverted-Sandwich Dichromium(I) Complex Stabilized by Guanidinate Ligands

Abstract

Reduction of the four-coordinate “ate” species [(RC(NAr)₂Cr(µ-Cl)₂Li(THF)₂] (1) (R = diisopropylamine; Ar = 2,6-diisopropylphenyl) with potassium graphite (KC₈) in toluene leads to the formation of a toluene-bridged inverted-sandwich dichromium(I) complex, (µ-η⁶:η⁶-C₇H₈)[Cr{RC(NAr)₂}]₂ (2). X-ray analysis confirms the dinuclear nature of (2). The compound [C₆₉H₁₀₄Cr₂N₆], crystallized in the monoclinic space group, P2₁/c, has the following cell parameters: a  =  15.108(3) b  =  29.155(6) c  =  17.486(4) Å, β  =  101.19(3)°, V  =  7555(3) A³, and Z  =  4.

1. Introduction

Stabilization and isolation of low-valent and low-coordinate transition metal complexes has recently attracted more attention and is of particular importance to us not only from a structural point of view but also due to their highly reactive nature towards small molecules [1,2]. The rarity of these metal complexes could be attributed to their electronic unsaturation and thus they need the employment of sterically crowded ligands. Recently, guanidinates have received considerable attention as ancillary ligands for the stabilization of not only a wide variety of metal centers from throughout the periodic table but also for such coordinatively unsaturated metal complexes due to their easy syntheses and the possibility of steric and electronic modification [3,4,5,6,7,8]. We have previously shown that slight steric variation in the –NR₂ group present on the back of the NCN moiety plays a crucial role in determining the oxidation state of the reduced metal centers [7]. Our interest in guanidinate complexes focuses on the synthesis and reactivity of Cr(I) complexes. Reduction of the divalent chromium precursor, [(RC(NAr)₂Cr(µ-Cl)₂Li(THF)₂] (R = diisopropylamine; Ar = 2,6-diisopropylphenyl) with KC₈ in tetrahydrofuran (THF) has previously led to a monomeric Cr(0) complex [7]. Since we were interested in the isolation of Cr^I species, we replaced THF with toluene as the reaction medium and herein describe the synthesis and characterization of the rare example of dichromium inverted-sandwich complexes, which is also the first example of such species being stabilized by guanidinate ligands. Other known examples have been previously reported for closely related β-diketiminate and amidinate ligands. The synthesis of these inverted-sandwich-type dinuclear complexes has been achieved either by potassium graphite or magnesium reduction of the corresponding metal chlorides in aromatic solvents or due to the cleavage of the Cr-Cr quintuple bond upon reaction with terminal alkynes [9,10,11,12].

2. Results and Discussion

Reduction of a divalent Cr–ate complex, [(RC(NAr)₂Cr(µ-Cl)₂Li(THF)₂] (1) (R = diisopropylamide; Ar = 2,6-diisopropylphenyl), with two equivalents of freshly prepared potassium graphite (KC₈) in toluene and after workup in hexane affords an inverted-sandwich compound (µ-η⁶:η⁶-C₇H₈)[Cr{RC(NAr)₂}]₂ (2), in which a toluene molecule bridges two Cr centers in a µ-η⁶:η⁶ fashion (Scheme 1). Compound (2) was obtained in 89% yield as a red crystalline material. The ¹H NMR spectrum of (2) exhibits broad signals typical for a paramagnetic compound (Figure S1) and a magnetic moment of 6.58 µB that is comparable to other Cr^I inverted-sandwich complexes (ranging between 6.45 and 7.41 µB) [9,10,11].

Since the compound is paramagnetic and the NMR studies were not very conclusive, crystals suitable for X-ray analysis were grown from hexane solution. Single-crystal X-ray analysis confirmed the dinuclear nature of the compound (Figure 1). The two N–C–N–Cr four-member rings are arranged in an orientation with an N1–Cr1–Cr2–N5 torsion angle of 95.75°. The N–Cr–N bond angles and Cr–N bond distances [64.23(15) and 64.90(14)° and 2.059(4), 2.054(4), 2.046(4) and 2.049(4) Å, respectively] in (2) are comparable to those in (1) [65.28(10)° and 2.0453(19) Å, respectively] [7]. The nearly identical C–N bond lengths [N1–C8 1.366(6), N2–C8 1.355(5) and N3–C8 1.386(6) Å of one ligand and N4–C39 1.352(5), N5–C39 1.357(5) and N6–C39 1.394(5) Å of the other ligand] and the sum of the N–C–N bond angles around C8 and C39 were approximately 360°, confirming the presence of sp²-hybridized nitrogen and carbon atoms. This may be attributed to the role of the lone pair of non-coordinating N-atoms in the π system of the ligand that can result in an increased electron density at the Cr centers and thus may lead to stronger binding of the ligand. The bridging arene ring is not planar, as indicated by inequivalent distances from the two Cr atoms, with metal-to-centroid distances of 1.736 and 1.752 Å. The Cr1–C distances to the toluene ring range from 2.194(5) to 2.339(5) Å, with Cr1–C3 being the shortest and Cr1–C5 the longest distance being to one of the methyl-group-substituted carbon atoms of the ring. The Cr2–C distances to the toluene ring range from 2.199(5) [Cr2–C2] to 2.329(5) Å [Cr2–C6]. This shows that both chromium atoms lean toward the same edge, C2–C3, and as a result the whole molecule is distorted. The average C–C distance for the bridging toluene molecule was determined to be 1.434(6) Å. These bond lengths are comparable with the reported values found for the μ-η⁶:η⁶-arene ligands in inverted-sandwich dichromium compounds [9,10,11,12], but slightly longer than those reported for free benzene (1.397(1) Å) [13] and toluene (1.39(1) Å) [14].

3. Experimental

All manipulations were performed with rigorous exclusion of oxygen and moisture in Schlenk-type glassware either on a dual-manifold Schlenk line or in a N₂-filled glove box (mBraun 120-G) with a high-capacity recirculator (<0.1 ppm O₂). Solvents were dried by distillation from sodium wire/benzophenone. CrCl₂ was purchased from ABCR GmbH. Deuterated solvents were obtained from Cambridge Isotope Laboratories and were degassed, dried and distilled prior to use. The ¹H NMR spectrum was recorded on a Varian 400 MHz at ambient temperature. The chemical shifts are reported in ppm relative to the internal TMS. Elemental analyses (CHN) were determined using a Vario EL III instrument. Compound (1) was synthesized following the literature procedures [7]. The effective magnetic moment was determined using a Sherwood Scientific Magnetic Susceptibility Balance. X-ray crystal structure analysis was performed with a STOE-IPDS II equipped with an Oxford Cryostream low-temperature unit. Structure solution and refinement was accomplished using SIR97 [15], SHELXL2018 [16,17], WinGX [18] and Olex2 [19]. One of the isopropyl groups shows disorder that leads to a relatively large displacement parameter. A toluene solvate molecule was so badly disordered that it could not be modeled with constraints and was squeezed using the squeeze command in Olex2. An Olex2 solvent mask was used to calculate the void space and the electron count. All parameters reported in the CIF take a toluene solvate molecule into account, leading to a couple of alerts in the checkCIF report. No absorption correction was applied to the data and some of the reflections at certain angles were omitted during the refinement. CCDC-2347002 contains the supplementary crystallographic data for this paper. The crystallographic data as collected by single-crystal analysis are presented in

Table 1. Crystallographic data of the compound.

Empirical Formula	C76H112Cr2N6
Formula weight	1213.71
crystal system	monoclinic
space group	P21/c
a [Å]	15.108(3)
b [Å]	29.155(6)
c [Å]	13.9971(8)
α [deg]	90
β [deg]	101.19(3)
γ [deg]	90
V, [Å3]	7555(3)
Z	4
crystal size, [mm3]	0.33 × 0.28 × 0.09
ρcalcd, [g cm−3]	1.067
µ, [mm−1] (Mo Kα)	0.329
T, [K]	133(2)
2θ range, [deg]	2.74–49.39
no. of reflections unique	14,233
no. of reflections obs. [I > 2σ (I)]	5576
no. of parameters	739
wR2 (all data)	0.1280
R value [I > 2σ (I)]	0.0683
Goodness of fit	0.786

. These data can be obtained free of charge at https://www.ccdc.cam.ac.uk/ accessed on 4 October 2024 (or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: +44-1223-336-033; e-mail: [email protected]).

Synthesis: [(RC(NAr)₂Cr(µ-Cl)₂Li(THF)₂] (0.460 g, 0.62 mmol) in toluene (15 mL) was added to two equivalents of freshly prepared KC₈ (0.169 g, 1.34 mmol) in toluene (10 mL) at −20 °C. After stirring for ca. 10 min, the solution was brought to room temperature and further stirred for 24 h. Toluene was evaporated under vacuum and the product was dissolved in hexane (20 mL). The solution was filtered and the filtrate was reduced in vacuum to ca. one-third to afford the product as a red crystalline material at −25 °C. Yield: 0.31 g (88.8%) C₆₉H₁₀₄Cr₂N₆ (1121.60): Calcd. C 73.89, N 7.49, H 9.35; found C 73.56, N 7.33 H 9.13; ¹H-NMR (C₆D₆, 298 K): δ = 0.88 (s), 1.23 (s), 1.40 (s), 2.10 (s), 3.46 (s), 5.57 (br s), 6.48 (br s), 6.96 (br tr), 11.11 (br s), 12.05 (br s) ppm.

4. Conclusions

In conclusion, a rare example of an inverted-sandwich dichromium(I) complex stabilized by guanidinate ligands was isolated. X-ray analysis confirms the dimeric nature of the compound in which a µ-η⁶:η⁶-coordinated toluene molecule bridges the two chromium centers.