[CrBr₂(NH₃)₄][CrBr₄(NH₃)₂], a new chromium(III) complex

Green single crystals of trans-tetra­ammine­dibromido­chrom­ium(III) trans-diamminetetra­bromidochromate(III), [CrBr₂(NH₃)₄][CrBr₄(NH₃)₂], are found to contain two symmetry-independent sixfold coordinated Cr^III cations on centres of inversion. The structure is composed of octa­hedral trans-[CrBr₂(NH₃)₄]⁺ cations and octa­hedral trans-[CrBr₄(NH₃)₂]^- anions, and adopts a distorted CsCl-type lattice. The cations and anions are linked by N-H...Br inter­actions. This is the first example in which both ions are mixed ammine-bromide Cr^III complexes.

S1. Comment

There is ongoing interest in chromium(III) complexes, owing to the importance of chromium in catalytic processes and its magnetic and optical properties. The often-found sixfold coordination of Cr^III with an essentially octahedral ligand arrangement is due to the high stability of the d³ configuration. In particular, ammonia complexes are the most numerous Cr^III derivatives and the most extensively studied, including the pure ammine [Cr(NH₃)₆]³⁺, for example [Cr(NH₃)₆][CuCl₅] (Ohba et al., 1995), and the mixed ammine–acid types, [Cr(NH₃)_6-xA_x]^(3-x)+ (x = 0–6), for example trans-[Cr(NH₃)₄Cl₂]I (Brencic et al., 1985) and Hg₆As₄[CrBr₆]Br (Olenev et al., 2003).

The synthesis and crystal structure of the title novel compound trans-tetraamminedibromochromium(III) trans-diamminetetrabromochromate(III), [CrBr₂(NH₃)₄][CrBr₄(NH₃)₂], is the result of exploratory investigations of mixed ammine–acid Cr^III complexes. Although the trans-tetraamminedibromochromium(III) cation was first mentioned (but without a crystal-structure report) more than three decades ago (Glerup & Schäffer, 1976), the trans-diamminetetrabromochromate(III) anion is observed here for the first time. To the best of our knowledge, the occurrence of Cr^III both as a complex cation and a complex anion in one compound is extremely rare. The only cases are [Cr(NH₃)₆][Cr(CN)₆] (Nielsen et al., 1986) and [Cr(NH₃)₆][Cr(NH₃)₂F₄][BF₄]₂ (Göbbels & Meyer, 2000). [Cr(NH₃)₆][Cr(CN)₆] was crystallized from [Cr(NH₃)₆]³⁺ and [Cr(CN)₆]^3- in aqueous solution under mild conditions. However, when these ammonia and cyanide complexes are present in aqueous solution the formation of mixed ammine–acid complexes will not occur. [Cr(NH₃)₆][Cr(NH₃)₂F₄][BF₄]₂, on the other hand, was obtained by the reaction of elemental Cr, B and NH₄F at 573 K, resulting in the mixed ammine–acid anion [Cr(NH₃)₂F₄]^-. For the synthesis of the title compound, an alternative non-aqueous route was used, using Cr₂(NCN)₃ as a starting material, together with NH₄Br, and this provides a stable and `naked' Cr³⁺ cation. Thus, the structurally stable compound [CrBr₂(NH₃)₄][CrBr₄(NH₃)₂] is achieved as a function of the synthetic route. The title compound can be also considered as the first real mixed ammine–acid Cr^III cation–anion compound, namely, [Cr(NH₃)_6-xA_x][Cr(NH₃)_6-yB_y] (x/y = 1–5, x + y = 6).

The title compound crystallizes in the centrosymmetric triclinic space group P1. The two symmetry-independent Cr^III atoms from the [CrBr₂(NH₃)₄]⁺ cation and the [CrBr₄(NH₃)₂]^- anion are positioned on the 1d and 1g inversion centres, while the N and Br atoms are on general positions. In the complex anion, the Cr^III atom (Cr1) is bonded to two apical NH₃ groups and four equatorial Br^- ions (Fig. 1). In the complex cation, the Cr^III atom (Cr2) is surrounded by four equatorial NH₃ groups and two apical Br^- ions. The coordinations of the Cr^III ions deviate from regular octahedral symmetry due to the different Cr—N and Cr—Br bond lengths. Comparison of the two complex ions shows that the Cr—N lengths are quite similar and that the equatorial Cr—Br bonds exhibit slightly longer distances than the axial ones (Table 1). A rigid-body correction of both octahedral entities according to the procedure of Schomaker & Trueblood (1968) is possible and it yields, for the Cr1-based complex anion, rather small bond increases of 0.007 Å for the Br atoms and 0.005 Å for the N atoms (R = 0.043). For the Cr2-based complex cation, the increases are likewise small, namely ca 0.005–0.006 Å for both Br and N atoms (R = 0.066). Based on the bond angles around the Cr^III centres, both octahedral entities are very close to D_4h symmetry, despite the triclinic space group. There is seemingly no electronic reason to distort away from D_4h because of the d³ configuration.

The crystal structure can be considered a typical ionic one, with a distorted [CsCl] motif (Fig. 2). It is quite common for the CsCl structure to be adopted when both ions are similar in size, and this is also found for [Cr(NH₃)₆][Cr(CN)₆] (Nielsen et al., 1986). The elongation of the b axis (b/a = 1.15) is caused by the non-regular octahedral ions, with CrBr₂(NH₃)₄]⁺ a lengthened and [CrBr₄(NH₃)₂]^- a flattened octahedron.

Besides the dominating Coulomb attractions, neighbouring Cr^III complex ions are also connected to each other via numerous weak N—H···Br hydrogen bonds (Table 2). These are likely to be responsible for the fact that the c/a ratio is 1.67, such that the cations and anions are not able to approach each other more closely.

S2. Experimental

Single crystals of [CrBr₂(NH₃)₄][CrBr₄(NH₃)₂] were synthesized via a typical solid-state reaction. A mixture of Cr₂(NCN)₃ and NH₄Br (1:10 molar ratio, 1.0 g total weight) was sealed in a silica tube, annealed at 723 K for a week and furnace-cooled. The synthesized products were a mixture of green [CrBr₂(NH₃)₄][CrBr₄(NH₃)₂] crystals and a black powder with undetermined composition. The crystals used for analysis were selected manually on the basis of colour and morphology. Attempts to synthesize the compound from a mixture of CrBr₃ and NH₄Br were unsuccessful.

S3. Refinement

All H atoms were located by difference Fourier synthesis and refined as riding atoms, with N—H = 0.87 (3) Å and with free but identical U_iso(H) values for all H atoms bonded to the same N atom.

Computing details

Data collection: SMART (Bruker, 2007); cell refinement: SAINT (Bruker, 2007); data reduction: SAINT (Bruker, 2007); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: PLATON (Spek, 2003); software used to prepare material for publication: enCIFer (Allen et al., 2004).

Figures

	Fig. 1. The structure of the title compound, showing the coordination around the two independent Cr^III ions. Displacement ellipsoids are drawn at the 50% probability level and H atoms are shown as small spheres of arbitrary radii. [Symmetry codes: (i) -x, -y + 1, -z + 1; (ii) -x + 1, -y, -z.]
	Fig. 2. A view of the crystal packing along the a axis, showing a distorted [CsCl]-type lattice. The [CrBr2(NH3)4]^+ cations are shown as darker octahedra (top and bottom) and the [CrBr4(NH3)2]^- anions as lighter octahedra (centre). Dashed lines indicate N—H···Br hydrogen bonds.

trans-tetraamminedibromidochromium(III) trans-diamminetetrabromidochromate(III)

Crystal data

[CrBr2(NH3)4][CrBr4(NH3)2]	Z = 1
Mr = 685.66	F(000) = 318
Triclinic, P1	Dx = 2.836 Mg m^−3
Hall symbol: -P 1	Mo Kα radiation, λ = 0.71073 Å
a = 5.9697 (5) Å	Cell parameters from 4174 reflections
b = 6.8410 (5) Å	θ = 3.0–28.3°
c = 9.9092 (8) Å	µ = 16.26 mm^−1
α = 86.184 (2)°	T = 293 K
β = 87.300 (2)°	Plate, green
γ = 84.293 (2)°	0.08 × 0.07 × 0.05 mm
V = 401.45 (6) Å^3

Data collection

Bruker SMART CCD area-detector diffractometer	1992 independent reflections
Radiation source: fine-focus sealed tube	1764 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.027
ϕ and ω scans	θmax = 28.3°, θmin = 3.0°
Absorption correction: multi-scan (SADABS; Sheldrick, 1996)	h = −7→7
Tmin = 0.283, Tmax = 0.443	k = −8→9
4174 measured reflections	l = −13→13

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.025	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.061	H atoms treated by a mixture of independent and constrained refinement
S = 1.06	w = 1/[σ^2(Fo^2) + (0.031P)^2] where P = (Fo^2 + 2Fc^2)/3
1992 reflections	(Δ/σ)max = 0.005
97 parameters	Δρmax = 0.55 e Å^−3
15 restraints	Δρmin = −1.05 e Å^−3

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

	x	y	z	Uiso*/Ueq
Br1	0.26713 (5)	0.71166 (5)	0.58607 (3)	0.0256 (1)
Cr2	0.00000	0.50000	0.50000	0.0170 (2)
N2	0.1788 (5)	0.4956 (4)	0.3136 (3)	0.0269 (8)
N3	0.1954 (5)	0.2491 (4)	0.5726 (3)	0.0280 (8)
Br2	0.26157 (6)	0.30696 (5)	−0.07410 (3)	0.0296 (1)
Br3	0.70997 (6)	0.00656 (5)	−0.22550 (3)	0.0286 (1)
Cr1	0.50000	0.00000	0.00000	0.0165 (2)
N1	0.7286 (5)	0.1718 (4)	0.0734 (3)	0.0244 (8)
H21	0.280 (8)	0.400 (6)	0.331 (6)	0.123 (15)*
H22	0.252 (9)	0.599 (5)	0.303 (6)	0.123 (15)*
H23	0.097 (9)	0.468 (9)	0.253 (5)	0.123 (15)*
H31	0.309 (6)	0.213 (9)	0.522 (4)	0.106 (13)*
H32	0.249 (9)	0.271 (9)	0.651 (3)	0.106 (13)*
H33	0.114 (8)	0.157 (7)	0.593 (6)	0.106 (13)*
H11	0.854 (6)	0.153 (7)	0.034 (4)	0.099 (12)*
H12	0.728 (9)	0.146 (9)	0.156 (3)	0.099 (12)*
H13	0.676 (8)	0.294 (5)	0.053 (6)	0.099 (12)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Br1	0.0246 (2)	0.0258 (2)	0.0282 (2)	−0.0061 (1)	−0.0023 (1)	−0.0081 (1)
Cr2	0.0176 (3)	0.0148 (3)	0.0186 (3)	−0.0006 (2)	−0.0008 (2)	−0.0030 (2)
N2	0.0294 (16)	0.0264 (15)	0.0246 (13)	−0.0034 (12)	0.0043 (11)	−0.0031 (11)
N3	0.0289 (15)	0.0224 (14)	0.0316 (14)	0.0022 (11)	−0.0032 (12)	0.0011 (12)
Br2	0.0312 (2)	0.0189 (2)	0.0378 (2)	0.0035 (1)	−0.0074 (1)	0.0004 (1)
Br3	0.0345 (2)	0.0281 (2)	0.0235 (2)	−0.0062 (1)	0.0073 (1)	−0.0049 (1)
Cr1	0.0187 (3)	0.0136 (3)	0.0172 (3)	−0.0013 (2)	−0.0004 (2)	−0.0024 (2)
N1	0.0250 (14)	0.0242 (14)	0.0256 (13)	−0.0059 (11)	−0.0034 (11)	−0.0061 (11)

Geometric parameters (Å, º)

Cr1—Br2	2.5047 (4)	N2—H23	0.83 (5)
Cr1—Br3	2.5100 (4)	N3—H32	0.88 (4)
Cr1—N1	2.073 (3)	N3—H33	0.84 (5)
Cr2—Br1	2.4736 (4)	N3—H31	0.85 (4)
Cr2—N2	2.090 (3)	N1—H11	0.83 (4)
Cr2—N3	2.084 (3)	N1—H12	0.83 (3)
N2—H21	0.86 (5)	N1—H13	0.88 (4)
N2—H22	0.87 (4)
Br1—Cr2—N2	90.48 (8)	Cr2—N3—H31	115 (4)
Br1—Cr2—N3	90.48 (8)	Cr2—N3—H32	109 (4)
N2—Cr2—N3	90.97 (11)	Cr2—N3—H33	110 (3)
Br2—Cr1—Br3	90.02 (1)	H31—N3—H32	106 (4)
Br2—Cr1—N1	89.36 (8)	H31—N3—H33	112 (5)
Br3—Cr1—N1	89.81 (8)	H32—N3—H33	103 (5)
Cr2—N2—H21	100 (4)	Cr1—N1—H11	111 (3)
Cr2—N2—H22	109 (4)	Cr1—N1—H12	106 (4)
Cr2—N2—H23	111 (4)	Cr1—N1—H13	106 (3)
H21—N2—H22	105 (4)	H11—N1—H12	115 (5)
H21—N2—H23	111 (5)	H11—N1—H13	108 (5)
H22—N2—H23	120 (6)	H12—N1—H13	112 (6)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N1—H11···Br2^i	0.83 (4)	2.88 (4)	3.617 (3)	149 (4)
N1—H12···Br1^ii	0.83 (3)	2.80 (4)	3.520 (3)	147 (5)
N1—H13···Br2^iii	0.88 (4)	2.81 (4)	3.573 (3)	146 (4)
N2—H21···Br1^ii	0.86 (5)	2.88 (5)	3.623 (3)	146 (4)
N2—H22···Br3^iii	0.87 (4)	2.78 (4)	3.578 (3)	153 (5)
N3—H31···Br1^ii	0.85 (4)	2.78 (4)	3.533 (3)	148 (5)
N3—H32···Br2^iv	0.88 (4)	2.76 (3)	3.594 (3)	159 (4)

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