Crystal growth and physical properties of an antiferromagnetic molecule: trans-di­bromido­tetrakis(aceto­nitrile)chromium(III) tribromide, [CrBr₂(NCCH₃)₄](Br₃)

The synthesis, crystal structure determination, magnetic properties and bonding interaction analysis of a novel 3d transition-metal com­plex, [CrBr₂(NCCH₃)₄](Br₃), are reported. Single-crystal X-ray diffraction results show that [CrBr₂(NCCH₃)₄](Br₃) crystallizes in space group C2/m (No. 12) with a symmetric tribromide anion and the powder X-ray diffraction results show the high purity of the material specimen. X-ray photoelectron studies with a combination of magnetic measurements demonstrate that Cr adopts the 3+ oxidation state. Based on the Curie–Weiss analysis of magnetic susceptibility data, the Néel tem­per­ature is found to be around 2.2 K and the effective moment (μ_eff) of Cr³⁺ in [CrBr₂(NCCH₃)₄](Br₃) is ∼3.8 µ_B, which agrees with the theoretical value for Cr³⁺. The direct current magnetic susceptibility of the molecule shows a broad maximum at ∼2.3 K, which is consistent with the theoretical Néel tem­per­ature. The maximum tem­per­ature, however, shows no clear frequency dependence. Combined with the observed upturn in heat capacity below 2.3 K and the corresponding field dependence, it is speculated that the low-tem­per­ature magnetic feature of a broad transition in [CrBr₂(NCCH₃)₄](Br₃) could originate from a crossover from high spin to low spin for the split d orbital level low-lying states rather than a short-range ordering solely; this is also supported by the molecular orbital diagram obtained from theoretical calculations.

Computing details

Data collection: APEX2 (Bruker, 2016); cell refinement: SAINT (Bruker, 2016); data reduction: SAINT (Bruker, 2016); program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2016 (Sheldrick, 2015b); molecular graphics: OLEX2 (Dolomanov et al., 2009); software used to prepare material for publication: OLEX2 (Dolomanov et al., 2009).

(I)

Crystal data

C8H12Br2CrN4·2(Br1.5)	F(000) = 574
Mr = 615.77	Dx = 2.199 Mg m^−3
Monoclinic, C2/m	Mo Kα radiation, λ = 0.71073 Å
a = 14.700 (7) Å	Cell parameters from 1704 reflections
b = 10.319 (5) Å	θ = 2.4–29.2°
c = 6.308 (3) Å	µ = 11.35 mm^−1
β = 103.685 (11)°	T = 296 K
V = 929.8 (8) Å^3	Block
Z = 2	0.05 × 0.05 × 0.05 mm

Data collection

Bruker APEXII CCD diffractometer	1356 reflections with I > 2σ(I)
φ and ω scans	Rint = 0.047
Absorption correction: empirical (using intensity measurements) (SADABS; Bruker, 2016)	θmax = 35.0°, θmin = 2.4°
Tmin = 0.328, Tmax = 0.747	h = −23→22
2081 measured reflections	k = −16→16
2081 independent reflections	l = −9→10

Refinement

Refinement on F^2	0 restraints
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F^2 > 2σ(F^2)] = 0.046	H-atom parameters constrained
wR(F^2) = 0.119	w = 1/[σ^2(Fo^2) + (0.0513P)^2 + 0.1136P] where P = (Fo^2 + 2Fc^2)/3
S = 1.02	(Δ/σ)max < 0.001
2081 reflections	Δρmax = 1.10 e Å^−3
50 parameters	Δρmin = −1.04 e Å^−3

Special details

Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Refinement. Refined as a 2-component twin.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Br1	0.13203 (3)	0.000000	0.31830 (8)	0.04281 (15)
Cr1	0.000000	0.000000	0.000000	0.02459 (18)
Br2	0.500000	0.000000	1.000000	0.04678 (19)
Br3	0.32621 (5)	0.000000	0.81435 (10)	0.06414 (19)
N1	0.0644 (2)	0.1369 (2)	−0.1416 (4)	0.0362 (5)
C1	0.1024 (2)	0.2079 (3)	−0.2237 (5)	0.0394 (7)
C2	0.1525 (4)	0.2989 (4)	−0.3295 (8)	0.0694 (14)
H2A	0.198665	0.253323	−0.385914	0.104*
H2B	0.182906	0.362535	−0.225586	0.104*
H2C	0.109245	0.341118	−0.447071	0.104*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Br1	0.0382 (3)	0.0476 (3)	0.0409 (2)	0.000	0.00585 (18)	0.000
Cr1	0.0279 (4)	0.0193 (3)	0.0311 (4)	0.000	0.0162 (3)	0.000
Br2	0.0671 (5)	0.0364 (3)	0.0424 (3)	0.000	0.0240 (3)	0.000
Br3	0.0616 (4)	0.0755 (4)	0.0542 (3)	0.000	0.0115 (3)	0.000
N1	0.0431 (14)	0.0278 (11)	0.0450 (13)	−0.0043 (9)	0.0251 (11)	0.0025 (10)
C1	0.0492 (18)	0.0301 (13)	0.0455 (16)	−0.0065 (12)	0.0247 (14)	−0.0027 (12)
C2	0.104 (4)	0.0435 (19)	0.082 (3)	−0.026 (2)	0.063 (3)	−0.002 (2)

Geometric parameters (Å, º)

Br1—Cr1	2.4402 (10)	Br2—Br3	2.5460 (13)
Cr1—N1^i	2.022 (2)	N1—C1	1.120 (4)
Cr1—N1	2.022 (2)	C1—C2	1.450 (4)
Cr1—N1^ii	2.022 (2)	C2—H2A	0.9600
Cr1—N1^iii	2.022 (2)	C2—H2B	0.9600
Br2—Br3^iv	2.5461 (13)	C2—H2C	0.9600
Br1^i—Cr1—Br1	180.0	N1^iii—Cr1—N1	91.34 (14)
N1^ii—Cr1—Br1	89.95 (9)	N1^i—Cr1—N1	180.0
N1^ii—Cr1—Br1^i	90.05 (9)	N1^ii—Cr1—N1^iii	180.00 (16)
N1^iii—Cr1—Br1^i	89.95 (9)	Br3—Br2—Br3^iv	180.0
N1^i—Cr1—Br1	90.05 (9)	C1—N1—Cr1	176.5 (2)
N1—Cr1—Br1	89.95 (9)	N1—C1—C2	179.4 (4)
N1^i—Cr1—Br1^i	89.95 (9)	C1—C2—H2A	109.5
N1—Cr1—Br1^i	90.05 (9)	C1—C2—H2B	109.5
N1^iii—Cr1—Br1	90.05 (9)	C1—C2—H2C	109.5
N1^ii—Cr1—N1	88.66 (14)	H2A—C2—H2B	109.5
N1^ii—Cr1—N1^i	91.34 (14)	H2A—C2—H2C	109.5
N1^iii—Cr1—N1^i	88.66 (14)	H2B—C2—H2C	109.5

Symmetry codes: (i) −x, −y, −z; (ii) x, −y, z; (iii) −x, y, −z; (iv) −x+1, −y, −z+2.