A new cluster complex, (NH₄)₆[Mo₄Se₄(CN)₁₂]·6H₂O, containing a tetranuclear mixed-valence molybdenum core

The title compound, hexa­ammonium tetra-μ₃-selenido-tetra­kis­(tri­cyano­molybdenum) hexahydrate, is isostructural with the Mo/S, W/S and W/Se analogues. The structure contains disordered cyclic hydrogen-bonded [{(NH₄)(H₂O)}₃]³⁺ cations and [Mo₄Se₄(CN)₁₂]⁶⁻ cluster anions with 3m symmetry. The cation assembly consists of alternating ammonium and water mol­ecules linked by N—H...O hydrogen bonds. The anion has a typical cubane cluster structure. The cations and anions are linked together by hydrogen bonds involving the terminal N atoms of the CN groups.

S1. Comment

Cuboidal clusters with bridging chalcogenide ligands are known for a wide variety of transition metals (Holm, 1992; Coucouvanis, 1991; Saito, 1995, 1997; Shibahara, 1991, 1993; Müller, 1986). Previously, we have prepared and structurally investigated some salts of alkali metal cations and cyanide cluster anions of the general formula [M₄Q₄(CN)₁₂]^n-, where M = Mo, Q = Te, or M = W, Q = S, Se or Te (Fedin, Kalinina et al., 1999). All these compounds belong to triclinic, monoclinic or orthorhombic crystal systems and the anions are slightly distorted from ideal T_d point symmetry.

Recently, we have found that Mo/W/S/Se anions can give high symmetry cubic crystal structures with six ammonium cations and six water molecules. Three of them, for M = Mo/Q = S and M = W/Q = S or Se, were structurally investigated at room temperature (Fedin, Samsonenko et al., 2000). They appeared to be isostructural, the anions having the ideal T_d point symmetry for metal and chalcogen atoms. Unfortunately, our attemts to find the H atom positions was unsuccessful, due to the disorder of the cations.

In the present article, we report the synthesis and low temperature crystal structure investigation of (NH₄)₆[Mo₄Se₄(CN)₁₂]·6H₂O, (I), which is isostructural with the Mo/S, W/S and W/Se analogues.

The structure of (I) contains an unusual [{(NH₄)(H₂O)}₃]³⁺ cation (Fig 1a). This is a ring of alternating ammonium and water molecules linked by NH···O hydrogen bonds. The ring is not planar; it has a chair conformation. The cation assembly is disordered relative to the pseudo-rotation around the centre of the ring in such a way that each position is statistically occupied by one half of NH₄⁺ and one half of H₂O. Therefore, the central atom, O1, has a mixed atomic scattering factor, 0.5 N + 0.5O. Due to the low temperature of the experiment we have found the positions of the H atoms. Two of them, H1 and H3, are fully occupied. They correspond to the water molecule or two of the four H atoms of the NH₄⁺ cations. The position of H2 is half-occupied and corresponds to the other two H atoms of the ammonium cations. The N···O hydrogen-bond distance is 2.80 (2) Å, with an angle of 166 (5)° at the H atom.

The anion (Fig. 1 b) has a typical cubane cluster structure and possesses the highest possible point symmetry, T_d. Mo—Mo and Mo—Se distances are close to those found for [W₄Se₄(CN)₁₂]^6- (Fedin, Kalinina et al., 1999; Fedin, Samsonenko et al., 1999).

The cations and anions are linked together by hydrogen bonds involving the terminal N atoms of the CN groups (Fig. 2). For each disordered water/ammonium site, there is a hydrogen bond with O/N···N = 2.86 (3) Å and an angle of 174 (17)° at the H1 atom.

S2. Experimental

A mixture of Mo₃Se₇Br₄ (1.00 g, 0.862 mmol) and KCN (1.00 g, 15.3 mmol) was heated in a sealed Pyrex tube at 703 K for 48 h. The product was added to water (20 ml) and the mixture was refluxed for 1 h and filtered. The potassium salt isolated by addition of methanol was dissolved in CH₃COONH₄ (1M, 10 ml) and the mixture was allowed to stand at 293 K for 5–7 d. Dark red-brown octahedral crystals were isolated by filtration, washed with methanol and dried in air (yield 0.10 g, 12%). Analysis calculated for C₁₂H₃₆Mo₄N₁₈O₆Se₄: C 11.74, H 2.95, N 20.53%; found: C 11.54, H 3.00, N 20.20%; IR (KBr): 2132 (CN) cm^-1; UV-visible absorption spectrum of an aqueous solution, λ (ε in M^-1cm^-1): 340 (9680), 460 (1950) sh, 530 s h, 690 (310) nm.

S3. Refinement

The largest difference peak is near the anion, 0.6 Å from Se1. The deepest hole is near the disordered cation, 1.36 Å from H3.

Computing details

Data collection: SMART (Bruker, 1998); cell refinement: SAINT (Bruker, 1998); data reduction: SAINT; program(s) used to solve structure: SHELXTL (Sheldrick, 1998); program(s) used to refine structure: SHELXTL; molecular graphics: SHELXTL; software used to prepare material for publication: SHELXTL and local programs.

Figures

	Fig. 1. View of the structure of (a) the cation and (b) the anion of (I). The disordered positions of the H atoms are shown as dashed circles and displacement ellipsoids are shown at the 50% probability level.
	Fig. 2. The crystal packing in (I). Hydrogen bonds are shown as dashed lines.

hexaammonium tetra-µ₃-selenido-tetrakis[tricyanomolybdenum] hexahydrate

Crystal data

(NH4)6[Mo4Se4(CN)12]·6H2O	Dx = 2.257 Mg m^−3
Mr = 1228.19	Mo Kα radiation, λ = 0.71073 Å
Cubic, Pn3m	Cell parameters from 4887 reflections
Hall symbol: -P 4bc 2bc 3	θ = 2.9–28.6°
a = 12.180 (1) Å	µ = 5.44 mm^−1
V = 1806.9 (3) Å^3	T = 160 K
Z = 2	Octahedron, black
F(000) = 1172	0.20 × 0.18 × 0.15 mm

Data collection

Siemens SMART CCD diffractometer	468 independent reflections
Radiation source: fine-focus sealed tube	384 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.039
Detector resolution: 8.125 pixels mm^-1	θmax = 28.7°, θmin = 2.4°
ω rotation with narrow frames scans	h = −7→16
Absorption correction: multi-scan (SADABS; Sheldrick, 1997)	k = −16→15
Tmin = 0.784, Tmax = 0.928	l = −15→15
10436 measured reflections

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.068	Only H-atom coordinates refined
wR(F^2) = 0.184	w = 1/[σ^2(Fo^2) + (0.0111P)^2 + 84.4748P] where P = (Fo^2 + 2Fc^2)/3
S = 1.48	(Δ/σ)max = 0.007
468 reflections	Δρmax = 0.97 e Å^−3
33 parameters	Δρmin = −1.13 e Å^−3
4 restraints	Extinction correction: SHELXTL (Sheldrick, 1998), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.00004 (14)

Special details

Experimental. none

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

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

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

	x	y	z	Uiso*/Ueq	Occ. (<1)
Mo1	0.33379 (12)	0.33379 (12)	0.16621 (12)	0.0184 (7)
Se1	0.13358 (15)	0.36642 (15)	0.13358 (15)	0.0191 (7)
C1	0.3450 (11)	0.5115 (16)	0.1550 (11)	0.031 (4)
N1	0.3495 (12)	0.6041 (14)	0.1505 (12)	0.046 (5)
O1	0.1056 (9)	0.6695 (14)	0.1056 (9)	0.033 (3)	0.50
N2	0.1056 (9)	0.6695 (14)	0.1056 (9)	0.033 (3)	0.50
H1	0.117 (11)	0.749 (4)	0.117 (11)	0.030*
H2	0.029 (9)	0.64 (2)	0.116 (9)	0.030*	0.50
H3	0.159 (4)	0.641 (15)	0.159 (4)	0.030*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Mo1	0.0184 (7)	0.0184 (7)	0.0184 (7)	−0.0006 (6)	0.0006 (6)	0.0006 (6)
Se1	0.0191 (7)	0.0191 (7)	0.0191 (7)	0.0005 (7)	−0.0005 (7)	0.0005 (7)
C1	0.031 (6)	0.032 (10)	0.031 (6)	−0.013 (5)	0.005 (8)	0.013 (5)
N1	0.057 (8)	0.024 (9)	0.057 (8)	0.006 (6)	0.005 (11)	−0.006 (6)
O1	0.035 (5)	0.028 (8)	0.035 (5)	−0.001 (5)	−0.002 (7)	−0.001 (5)
N2	0.035 (5)	0.028 (8)	0.035 (5)	−0.001 (5)	−0.002 (7)	−0.001 (5)

Geometric parameters (Å, º)

Mo1—Mo1^i	2.886 (4)	Mo1—C1^v	2.17 (2)
Mo1—Mo1^ii	2.887 (4)	Se1—Mo1^i	2.502 (3)
Mo1—Mo1^iii	2.887 (4)	Se1—Mo1^ii	2.502 (3)
Mo1—Se1	2.502 (3)	C1—N1	1.13 (2)
Mo1—Se1^i	2.502 (3)	O1—H1	0.99 (3)
Mo1—Se1^ii	2.503 (3)	O1—H2	0.99 (3)
Mo1—C1	2.17 (2)	O1—H3	0.99 (3)
Mo1—C1^iv	2.17 (2)
Mo1^i—Mo1—Mo1^ii	60.0	C1—Mo1—Mo1^iii	138.5 (3)
Mo1^i—Mo1—Mo1^iii	60.0	C1^v—Mo1—Mo1^iii	95.1 (5)
Mo1^ii—Mo1—Mo1^iii	60.0	C1^iv—Mo1—Se1^i	83.9 (3)
Se1^i—Mo1—Mo1^i	54.78 (6)	C1—Mo1—Se1^i	161.9 (5)
Se1—Mo1—Mo1^i	54.78 (6)	C1^v—Mo1—Se1^i	83.9 (3)
Se1^ii—Mo1—Mo1^i	102.98 (7)	C1^iv—Mo1—Se1	83.9 (3)
Se1^i—Mo1—Mo1^ii	102.98 (7)	C1—Mo1—Se1	83.9 (3)
Se1—Mo1—Mo1^ii	54.78 (6)	C1^v—Mo1—Se1	161.9 (5)
Se1^ii—Mo1—Mo1^ii	54.78 (6)	C1^iv—Mo1—Se1^ii	161.9 (5)
Se1^i—Mo1—Mo1^iii	54.78 (6)	C1—Mo1—Se1^ii	83.9 (3)
Se1—Mo1—Mo1^iii	102.98 (7)	C1^v—Mo1—Se1^ii	83.9 (3)
Se1^ii—Mo1—Mo1^iii	54.78 (6)	C1^iv—Mo1—C1	82.6 (8)
Se1^i—Mo1—Se1	106.52 (8)	C1^iv—Mo1—C1^v	82.6 (8)
Se1^i—Mo1—Se1^ii	106.52 (8)	C1—Mo1—C1^v	82.6 (8)
Se1—Mo1—Se1^ii	106.52 (8)	Mo1^i—Se1—Mo1	70.4 (1)
C1^iv—Mo1—Mo1^i	95.1 (5)	Mo1^i—Se1—Mo1^ii	70.4 (1)
C1—Mo1—Mo1^i	138.5 (3)	Mo1—Se1—Mo1^ii	70.4 (1)
C1^v—Mo1—Mo1^i	138.5 (3)	N1—C1—Mo1	179 (2)
C1^iv—Mo1—Mo1^ii	138.5 (3)	H1—O1—H2	116 (10)
C1—Mo1—Mo1^ii	95.1 (5)	H1—O1—H3	99 (10)
C1^v—Mo1—Mo1^ii	138.5 (3)	H2—O1—H3	115 (10)
C1^iv—Mo1—Mo1^iii	138.5 (3)

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

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O1—H1···N1^vi	0.99 (3)	1.88 (4)	2.86 (3)	174 (17)
O1—H2···O1^vii	0.99 (3)	1.82 (3)	2.80 (2)	166 (5)

Symmetry codes: (vi) −x+1/2, −y+3/2, z; (vii) −z, x+1/2, y−1/2.