The crystal structure of Hexakis(diethylamido)dimolybdenum, Mo2(NEt2)6

Tobias Brauner; Christoph Wagner; Kurt Merzweiler

doi:10.1515/ncrs-2021-0138

Open Access Published by De Gruyter (O) May 18, 2021

The crystal structure of Hexakis(diethylamido)dimolybdenum, Mo₂(NEt₂)₆

Tobias Brauner , Christoph Wagner and Kurt Merzweiler

From the journal Zeitschrift für Kristallographie - New Crystal Structures

https://doi.org/10.1515/ncrs-2021-0138

Abstract

C₂₄H₆₀Mo₂N₆, trigonal, R3‾ (no. 148), a = 16.7680(8) Å, c = 9.7523(4) Å, V = 2374.7(2) Å³, Z = 3, R_gt(F) = 0.0205, wR_ref(F²) = 0.0517, T = 213 K.

CCDC no.: 2080037

The molecular structure is shown in the figure (Hydrogen atoms were omitted for clarity).

Table 1:

Data collection and handling.

Crystal:	Yellow needle
Size:	0.56 × 0.07 × 0.07 mm
Wavelength:	Mo K_α radiation (0.71073 Å)
μ:	0.81 mm⁻¹
Diffractometer, scan mode:	STOE IPDS 2, ω-scan
θ_max, completeness:	26.0°, >99%
N(hkl)_measured, N(hkl)_unique, R_int:	7139, 1039, 0.062
Criterion for I_obs, N(hkl)_gt:	I_obs > 2 σ(I_obs), 1007
N(param)_refined:	49
Programs:	SHELX [11], [12], Olex2 [13], Diamond [14], X-Area [15]

Table 2:

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å²).

Atom	x	y	z	U_iso*/U_eq
Mo	0.6667	0.3333	0.44755 (2)	0.02368 (11)
N	0.73519 (10)	0.46460 (10)	0.49924 (15)	0.0315 (3)
C1	0.76858 (15)	0.48126 (14)	0.6415 (2)	0.0414 (4)
H1	0.7292	0.4263	0.6961	0.050*
H2	0.7624	0.5322	0.6789	0.050*
C2	0.86725 (17)	0.50409 (19)	0.6573 (3)	0.0597 (6)
H3	0.8843	0.5141	0.7534	0.072*
H4	0.8738	0.4534	0.6229	0.072*
H5	0.9071	0.5594	0.6056	0.072*
C3	0.76863 (14)	0.54991 (13)	0.4211 (2)	0.0401 (4)
H6	0.7527	0.5344	0.3244	0.048*
H7	0.8359	0.5854	0.4277	0.048*
C4	0.72989 (19)	0.61003 (16)	0.4690 (3)	0.0585 (6)
H8	0.7549	0.6651	0.4128	0.070*
H9	0.6633	0.5761	0.4605	0.070*
H10	0.7467	0.6272	0.5640	0.070*

Source of material

Mo₂(NEt₂)₆ was synthesized from LiNEt₂ and MoCl₃(THF)₃ [1] by a slightly modified method according to Chisholm et al. [2]. Yellow single crystals of Mo₂(NEt₂)₆ were obtained by sublimation at 150 °C and 10-5 mbar.

¹H-NMR (400 MHz, C₆D₆): δ [ppm] = 1.11 (t, 36H, CH₃³J_(HH) = 7.0 Hz); 3.50 (q, 24H, CH₂³J_(HH) = 7.0 Hz), ¹³C-NMR (100 MHz, C₆D₆): δ [ppm] = 15.5 (s, 12 C, CH₃); 51.9 (s; 12 C; CH₂).

Experimental details

Details of the crystal structure determination are collected in Table 1 and the results of structure calculation are listed in Table 2 with atomic coordinates.

The hydrogen atoms were placed on calculated positions and refined with a riding model. Their U_iso values were set to 1.2 U_eq for CH₂ groups and 1.5 for CH₃ groups of the parent carbon atoms.

Comment

Since the first synthesis of Mo₂(CH₂SiMe₃)₆ around five decades ago [3], a large number of Mo₂R₆ compounds comprising Mo–Mo triple bonds were synthesized and characterized by X-ray diffraction methods [4]. However, in the case of dialkylamide derivatives Mo₂(NR₂)₆ X-ray structure determinations apart from that of the parent compound Mo₂(NMe₂)₆ [5] are still rare. Some more structural data are available from complexes with μ₂-bridging bis-amido ligands like Mo₂(R′NCH₂CH₂NR′)₃ (R′ = Me [6], R′ = i–Pr [7]), and mixed organyl/dialkylamide ligands, e.g. Mo₂R′₂(NMe₂)₄ (R′ = benzyl, o-tolyl, p-tolyl [8], ethyl [9]).

The preparation of the title compound Mo₂(NEt₂)₆ was reported in 1976 [2]. Pale yellow single crystals suitable for X-ray diffraction were grown by sublimation in high vacuum.

Mo₂(NEt₂)₆ crystallizes in the trigonal system, space group R3‾ with three formula units per unit cell. The crystal structure consists of discrete molecules Mo₂(NEt₂)₆ without any unusual short intermolecular contacts. The Mo₂(NEt₂)₆ units are situated around a crystallographic center of inversion (Wyckoff site 3a). The asymmetric unit comprises one third of the molecule with the Mo atom situated on the 3‾ axis and the remaining atoms at general positions. Thus, the Mo₂(NEt₂)₆ unit exhibits crystallographically imposed S₆ point symmetry, which implies a staggered conformation of the Mo(NEt₂)₃ fragments. Generally, the staggered conformation is preferred for Mo₂R₆ compounds with terminally binding ligands R [4]. In the case of μ₂-bridging ligands steric strain usually forces eclipsed arrangements, i.e. Mo₂(RNCH₂CH₂NR)₃ [10] (R = Me [5], i–Pr [6]).

Mo₂(NEt₂)₆ exhibits a Mo–Mo distance of 2.2277(4) Å which is close to that in Mo₂(NMe₂)₆ (2.21(2) Å) [1]. Comparable distances are found in the dimolybdenum hexaalkoxides Mo₂(OR)₆ R = CH2tBu [9] with 2.222(2) Å and R = C(CF₃)₂CH₃ [10] with 2.2263(4) Å. Furthermore, the Mo–N distances (1.9723(1) Å) and the N–Mo–N angles (113.70(4)°) are in accordance with the observations in Mo₂(NMe₂)₆ (1.97(2)–2.00(1) Å and (113.5(2)–115.2(2)°). Like in the case of the NMe₂ derivative, the nitrogen atoms adopt a nearly planar coordination (sum of the angles around N: 359.6°). The dihedral angle between the Mo₂N plane and NC₂ plane is 16° and thus markedly larger than in case of Mo₂(NMe₂)₆ (0.3–3.6°). Obviously, this is a result of increased steric crowding in the NEt₂ derivative. The orientation of the ethyl groups with respect to the Mo–Mo triple bonds allows for a differentiation into proximal and distal groups. In the case of the proximal ethyl groups the Mo–N–C angles (133.1(1)°) are significantly larger than in the distal case (114.0(1)°). Similar disparities were observed for Mo₂(NMe₂)₆ (132° and 116°, resp.) [2].

Corresponding author: Kurt Merzweiler, Naturwissenschaftliche Fakultät II, Institut für Chemie, Martin-Luther-Universität Halle Wittenberg, Kurt-Mothes-Straße 2, 06120Halle (Saale),Germany, E-mail: kurt.merzweiler@chemie.uni-halle.de

Author contributions: All the authors have accepted responsibility for the entire content of this submitted manuscript and approved submission.
Research funding: We acknowledge the financial support within the funding programme Open Access Publishing by the German Research Foundation (DFG).
Conflict of interest statement: The authors declare no conflicts of interest regarding this article.

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Received: 2021-04-12

Accepted: 2021-04-26

Published Online: 2021-05-18

Published in Print: 2021-07-27

This work is licensed under the Creative Commons Attribution 4.0 International License.