supplementary materials


hy2617 scheme

Acta Cryst. (2013). E69, m193-m194    [ doi:10.1107/S1600536813005898 ]

(2-Amino-7-methyl-4-oxidopteridine-6-carboxylato-[kappa]3O4,N5,O6)(ethane-1,2-diamine-[kappa]2N,N')(1H-imidazole-[kappa]N3)nickel(II) dihydrate

S. S. Baisya and P. S. Roy

Abstract top

In the title complex, [Ni(C8H5N5O3)(C2H8N2)(C3N2H4)]·2H2O, a tridentate 2-amino-7-methyl-4-oxidopteridine-6-carboxylate (pterin) ligand, a bidentate ancillary ethane-1,2-diamine (en) ligand and a monodentate 1H-imidazole (im) ligand complete a distorted octahedral geometry around the NiII atom. The pterin ligand forms two chelate rings. Both the en and im ligands are arranged nearly orthogonally relative to the pterin ligand [dihedral angles between the mean planes of the en and pterin ligands and of the im and pterin ligands are 84.62 (9) and 85.14 (9)°, respectively]. N-H...N, N-H...O, O-H...N and O-H...O hydrogen bonds link the complex molecules and lattice water molecules into a three-dimensional network.

Comment top

Increasing attention is being paid nowadays towards the metalloenzymes requiring both a pterin and a transition metal (Basu & Burgmayer, 2011; Burgmayer, 1998; Fitzpatrick, 2003; Fukuzumi & Kojima, 2008; Kaim et al., 1999). This has, in turn, catalysed research work on the coordination chemistry of the bicyclic N-heterocycles called pteridines in general, and an important member of this family named pterin in particular. Literature survey reveals the existence of only a couple of structurally characterized Ni(II)–pterin complexes (Baisya & Roy, 2013; Crispini et al., 2005) and no related quaternary complex. The present effort is concerned with the title quaternary complex, possessing a tridentate pterin ligand, a bidentate σ-donor ligand like en and a monodentate σ-donor ligand like im.

The molecular structure (Fig. 1) represents a mononuclear NiII centre in a distorted octahedral coordination geometry, with two N atoms (N6 and N7) of the en ligand, a pyrazine ring N atom (N1) of the pterin ligand and an imidazole ring N atom (N8), forming the equatorial plane. The two pterin O atoms (O1 and O3) occupy the longer axial positions, with the phenolate O3 constituting the longest axial bond [2.2722 (14) Å]. The pterin ligand forms two five-membered chelate rings having small bite angles [75.91 (6) and 77.50 (6)°], instead of only one per pterin ligand for an earlier case (Crispini et al., 2005). This factor is responsible to a large extent for the observed distortion here from regular octahedral geometry. Accordingly, the O1—Ni1—O3 axis shows maximum deviation [153.37 (5)°] from linearity. Again, closest approach to linearity [174.10 (7)°] is observed for the N7—Ni1—N8 axis, which is associated with both the im and en ligands. Here each such ligand tries to achieve near orthogonality with respect to the pterin ligand [dihedral angles between the mean planes of the en and pterin ligands and of the im and pterin ligands are 84.62 (9) and 85.14 (9)°, respectively], for minimizing the steric repulsion. In line with the earlier observations on related copper complexes (Odani et al., 1992), the pyrazine ring N atom (N1) is located in the equatorial plane. The corresponding short Ni1—N1 distance [1.9787 (16) Å] indicates dπ–pπ interaction between the pterin ring and the NiII atom (d8), with further assistance from the nearby π-donating phenolate and carboxylate O atoms (Kohzuma et al., 1988). The pterin ligand is coordinated in its binegative form as an O,N,O-donor, as evident from the charge balance of this Ni(II) complex. The significantly shorter nature of the O3—C6 [1.271 (2) Å] and N5—C5 [1.332 (2) Å] bonds could be rationalized in terms of electron-shuffling ability of the pterin ring (Baisya & Roy, 2013; Beddoes et al., 1993; Russell et al., 1992).

In the crystal, intermolecular N—H···N, N—H···O, O—H···N and O—H···O hydrogen bonds (Table 1) link the complex molecules and lattice water molecules into a three-dimensional network (Fig 2). The lattice water molecules play a decisive role for the crystal packing.

Related literature top

For the importance of pterin in metalloenzymes, see: Basu & Burgmayer (2011); Burgmayer (1998); Fitzpatrick (2003); Fukuzumi & Kojima (2008); Kaim et al. (1999). For the structures of related nickel complexes, see: Baisya & Roy (2013); Crispini et al. (2005). For the structures of related copper complexes, see: Odani et al. (1992). For the electron-shuffling ability of the pterin unit as well as its donor groups and the affect on the geometric parameters of related complexes, see: Beddoes et al. (1993); Kohzuma et al. (1988); Russell et al. (1992). For the synthesis of the pterin ligand, see: Wittle et al. (1947). For refinement of H atoms, see: Cooper et al. (2010).

Experimental top

2-Amino-4–hydroxy-7-methylpteridine-6-carboxylic acid sesquihydrate (C8H7N5O3.1.5H2O) was obtained by published procedure (Wittle et al., 1947). The title complex was synthesized by the dropwise addition of an aqueous alkaline solution (NaOH: 11 mg, 0.275 mmol) of the pterin ligand (31 mg, 0.125 mmol) to a warm (311 K; paraffin oil bath) aqueous reaction mixture containing NiSO4.7H2O (35 mg, 0.125 mmol), ethane-1,2-diamine (7.5 mg, 0.125 mmol) and 1H-imidazole (14 mg, 0.2 mmol); final volume was 45 ml. The pH value was adjusted to 10.3 and the mixture was stirred for 3 h; final pH was 9.7. The orange coloured solution was transferred to a 100 ml beaker and allowed to stand at room temperature. Orange crystals appeared after 4 days (yield: 40%), which were suitable for single-crystal X-ray diffraction. Sample for analytical purpose could be obtained by filtration, repeated washing with small quantities of water and drying in vacuo over silica gel. Analysis, calculated for C13H21N9NiO5: C 35.31, H 4.80, N 28.52%; found: C 35.72, H 4.70, N 28.07%.

Refinement top

The H atoms were all located in a difference map, but those attached to C atoms were repositioned geometrically. The H atoms were initially refined with soft restrains on bond lengths and angles to regularize their geometry (C—H = 0.93–0.98, N—H = 0.86–0.89, O—H = 0.82 Å) and Uiso(H) = 1.2–1.5Ueq(parent atom), after which the positions were refined with rigiding constrains (Cooper et al., 2010).

Computing details top

Data collection: APEX2 (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: CRYSTALS (Betteridge et al., 2003); molecular graphics: CAMERON (Watkin et al., 1996); software used to prepare material for publication: CRYSTALS (Betteridge et al., 2003).

Figures top
[Figure 1] Fig. 1. The molecular structure of the title compound. Displacement ellipsoids are drawn at the 30% probability level.
[Figure 2] Fig. 2. The crystal packing diagram of the title compound, viewed along the a axis. Dotted lines indicate hydrogen bonds.
(2-Amino-7-methyl-4-oxidopteridine-6-carboxylato-κ3O4,N5,O6)(ethane-1,2-diamine-κ2N,N')(1H-imidazole-κN3)nickel(II) dihydrate top
Crystal data top
[Ni(C8H5N5O3)(C2H8N2)(C3H4N2)]·2H2OF(000) = 1840
Mr = 442.08Dx = 1.638 Mg m3
Orthorhombic, PbcnMo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2n 2abCell parameters from 4231 reflections
a = 13.484 (2) Åθ = 1.4–28.3°
b = 8.8741 (15) ŵ = 1.13 mm1
c = 29.959 (5) ÅT = 293 K
V = 3584.9 (10) Å3Plate, orange
Z = 80.24 × 0.24 × 0.03 mm
Data collection top
Bruker Kappa APEXII CCD
diffractometer
3521 reflections with I > 2σ(I)
Graphite monochromatorRint = 0.030
φ and ω scansθmax = 28.3°, θmin = 1.4°
Absorption correction: multi-scan
(SADABS; Sheldrick, 1996)
h = 1717
Tmin = 0.77, Tmax = 0.97k = 1111
19640 measured reflectionsl = 2438
4231 independent reflections
Refinement top
Refinement on F2Primary atom site location: structure-invariant direct methods
Least-squares matrix: fullHydrogen site location: difference Fourier map
R[F2 > 2σ(F2)] = 0.036H-atom parameters constrained
wR(F2) = 0.091 Method = Modified Sheldrick w = 1/[σ2(F2) + (0.04P)2 + 3.53P],
where P = [max(Fo2,0) + 2Fc2]/3
S = 0.95(Δ/σ)max = 0.001
4231 reflectionsΔρmax = 0.62 e Å3
253 parametersΔρmin = 0.32 e Å3
0 restraints
Crystal data top
[Ni(C8H5N5O3)(C2H8N2)(C3H4N2)]·2H2OV = 3584.9 (10) Å3
Mr = 442.08Z = 8
Orthorhombic, PbcnMo Kα radiation
a = 13.484 (2) ŵ = 1.13 mm1
b = 8.8741 (15) ÅT = 293 K
c = 29.959 (5) Å0.24 × 0.24 × 0.03 mm
Data collection top
Bruker Kappa APEXII CCD
diffractometer
4231 independent reflections
Absorption correction: multi-scan
(SADABS; Sheldrick, 1996)
3521 reflections with I > 2σ(I)
Tmin = 0.77, Tmax = 0.97Rint = 0.030
19640 measured reflectionsθmax = 28.3°
Refinement top
R[F2 > 2σ(F2)] = 0.036H-atom parameters constrained
wR(F2) = 0.091Δρmax = 0.62 e Å3
S = 0.95Δρmin = 0.32 e Å3
4231 reflectionsAbsolute structure: ?
253 parametersFlack parameter: ?
0 restraintsRogers parameter: ?
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/Ueq
Ni10.461327 (17)0.49322 (3)0.372395 (8)0.0275
O10.54402 (10)0.60856 (16)0.32134 (5)0.0335
C10.63525 (15)0.5742 (2)0.31846 (6)0.0292
O20.69402 (11)0.62442 (18)0.29021 (5)0.0423
C20.67092 (14)0.4597 (2)0.35277 (6)0.0262
N10.59680 (11)0.40936 (17)0.37761 (5)0.0254
C70.61252 (13)0.3066 (2)0.40867 (6)0.0252
C40.70658 (13)0.2460 (2)0.41611 (6)0.0246
N20.78491 (11)0.29914 (18)0.39231 (5)0.0286
C30.76772 (14)0.4043 (2)0.36124 (6)0.0273
C80.85670 (16)0.4622 (2)0.33676 (8)0.0407
H1110.91800.42840.34940.0629*
H1130.85230.42980.30690.0630*
H1120.85650.56760.33630.0619*
N30.71925 (12)0.13478 (18)0.44623 (5)0.0293
C50.63537 (14)0.0864 (2)0.46626 (6)0.0297
N40.54093 (12)0.14309 (19)0.46268 (5)0.0312
C60.52732 (13)0.2575 (2)0.43450 (6)0.0268
O30.44423 (10)0.32158 (16)0.42815 (5)0.0329
N50.64372 (14)0.0319 (2)0.49338 (6)0.0414
H1720.59180.06340.50680.0504*
H1710.70030.06680.49830.0503*
N60.32580 (14)0.6051 (2)0.36971 (7)0.0478
C90.3190 (2)0.7106 (3)0.40720 (9)0.0610
C100.4178 (2)0.7830 (3)0.41328 (10)0.0556
N70.49397 (14)0.66577 (19)0.41780 (6)0.0367
H2110.55480.70390.41360.0564*
H2120.49500.63350.44520.0559*
H2020.41670.84760.43840.0688*
H2010.43170.84020.38670.0695*
H1920.26690.78390.40350.0736*
H1910.30430.65400.43520.0746*
H1810.27420.54340.36740.0732*
H1820.32280.65460.34420.0727*
N80.41434 (14)0.3313 (2)0.32768 (6)0.0383
C110.41880 (17)0.1814 (2)0.33182 (8)0.0416
N90.38881 (18)0.1135 (3)0.29413 (9)0.0652
C120.3599 (3)0.2243 (3)0.26639 (9)0.0699
C130.3786 (3)0.3547 (3)0.28659 (9)0.0733
H2610.36710.44680.27430.0892*
H2510.33440.21340.24060.0848*
H2410.38060.01530.28920.0825*
H2310.43950.12960.35680.0527*
O40.37876 (13)0.4892 (2)0.50415 (6)0.0543
H2710.33790.44360.51950.0824*
H2720.38900.43400.48330.0825*
O50.38933 (16)0.7745 (2)0.28388 (6)0.0685
H2810.36450.74000.26050.1040*
H2820.44460.74010.28170.1049*
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
Ni10.02605 (14)0.02545 (14)0.03107 (14)0.00016 (9)0.00317 (9)0.00076 (9)
O10.0339 (7)0.0321 (7)0.0344 (7)0.0011 (6)0.0042 (6)0.0073 (6)
C10.0361 (10)0.0262 (9)0.0253 (9)0.0028 (8)0.0023 (7)0.0001 (7)
O20.0456 (9)0.0479 (9)0.0334 (7)0.0033 (7)0.0059 (6)0.0132 (7)
C20.0310 (9)0.0226 (8)0.0250 (8)0.0023 (7)0.0013 (7)0.0009 (7)
N10.0259 (7)0.0252 (8)0.0250 (7)0.0000 (6)0.0009 (6)0.0005 (6)
C70.0274 (9)0.0237 (8)0.0245 (8)0.0002 (7)0.0010 (7)0.0007 (7)
C40.0271 (9)0.0238 (8)0.0228 (8)0.0001 (7)0.0000 (7)0.0030 (7)
N20.0280 (8)0.0257 (8)0.0320 (8)0.0009 (6)0.0034 (6)0.0004 (6)
C30.0297 (10)0.0236 (8)0.0286 (9)0.0025 (7)0.0039 (7)0.0027 (7)
C80.0323 (10)0.0358 (11)0.0541 (13)0.0004 (9)0.0119 (9)0.0109 (10)
N30.0282 (8)0.0304 (8)0.0293 (8)0.0033 (6)0.0010 (6)0.0046 (6)
C50.0340 (10)0.0275 (9)0.0275 (9)0.0017 (8)0.0013 (7)0.0023 (7)
N40.0290 (8)0.0321 (8)0.0324 (8)0.0002 (6)0.0042 (6)0.0064 (7)
C60.0263 (9)0.0281 (9)0.0258 (8)0.0016 (7)0.0007 (7)0.0003 (7)
O30.0255 (6)0.0372 (7)0.0359 (7)0.0023 (6)0.0030 (5)0.0058 (6)
N50.0376 (10)0.0412 (10)0.0454 (10)0.0058 (8)0.0078 (8)0.0188 (8)
N60.0336 (10)0.0475 (11)0.0624 (13)0.0043 (8)0.0076 (9)0.0034 (9)
C90.0523 (15)0.0690 (18)0.0617 (16)0.0258 (14)0.0095 (13)0.0033 (14)
C100.0660 (17)0.0399 (13)0.0610 (16)0.0132 (12)0.0007 (13)0.0132 (11)
N70.0413 (10)0.0360 (9)0.0329 (9)0.0008 (8)0.0013 (7)0.0027 (7)
N80.0422 (10)0.0324 (9)0.0402 (9)0.0059 (8)0.0084 (8)0.0020 (7)
C110.0428 (12)0.0323 (11)0.0498 (13)0.0009 (9)0.0073 (10)0.0025 (9)
N90.0756 (16)0.0430 (12)0.0769 (16)0.0070 (11)0.0090 (13)0.0200 (11)
C120.112 (3)0.0476 (15)0.0503 (15)0.0073 (16)0.0420 (16)0.0115 (12)
C130.122 (3)0.0437 (14)0.0544 (16)0.0051 (16)0.0438 (17)0.0012 (12)
O40.0545 (10)0.0558 (11)0.0525 (10)0.0125 (8)0.0175 (8)0.0034 (8)
O50.0879 (14)0.0667 (12)0.0509 (10)0.0327 (11)0.0241 (10)0.0153 (9)
Geometric parameters (Å, º) top
Ni1—O12.1519 (14)N5—H1720.853
Ni1—N11.9787 (16)N5—H1710.837
Ni1—O32.2722 (14)N6—C91.465 (3)
Ni1—N62.0812 (19)N6—H1810.888
Ni1—N72.0949 (17)N6—H1820.881
Ni1—N82.0638 (17)C9—C101.491 (4)
O1—C11.270 (2)C9—H1920.964
C1—O21.242 (2)C9—H1910.997
C1—C21.523 (3)C10—N71.468 (3)
C2—N11.324 (2)C10—H2020.945
C2—C31.418 (3)C10—H2010.962
N1—C71.320 (2)N7—H2110.896
C7—C41.395 (2)N7—H2120.869
C7—C61.452 (2)N8—C111.338 (3)
C4—N21.359 (2)N8—C131.338 (3)
C4—N31.348 (2)C11—N91.342 (3)
N2—C31.338 (2)C11—H2310.922
C3—C81.497 (3)N9—C121.346 (4)
C8—H1110.957N9—H2410.890
C8—H1130.940C12—C131.329 (4)
C8—H1120.936C12—H2510.850
N3—C51.351 (2)C13—H2610.910
C5—N41.373 (2)O4—H2710.825
C5—N51.332 (2)O4—H2720.807
N4—C61.333 (2)O5—H2810.834
C6—O31.271 (2)O5—H2820.808
O1—Ni1—N175.91 (6)C7—C6—O3118.92 (16)
O1—Ni1—O3153.37 (5)N4—C6—O3123.85 (16)
N1—Ni1—O377.50 (6)Ni1—O3—C6108.69 (11)
O1—Ni1—N6101.58 (7)C5—N5—H172118.4
N1—Ni1—N6173.26 (7)C5—N5—H171118.4
O3—Ni1—N6105.01 (7)H172—N5—H171122.9
O1—Ni1—N790.29 (6)Ni1—N6—C9109.29 (15)
N1—Ni1—N791.70 (7)Ni1—N6—H181113.4
O3—Ni1—N791.95 (6)C9—N6—H181113.8
N6—Ni1—N782.01 (8)Ni1—N6—H182108.1
O1—Ni1—N891.65 (7)C9—N6—H182110.0
N1—Ni1—N894.18 (7)H181—N6—H182101.8
O3—Ni1—N888.82 (7)N6—C9—C10108.2 (2)
N6—Ni1—N892.13 (8)N6—C9—H192112.9
N7—Ni1—N8174.10 (7)C10—C9—H192112.0
Ni1—O1—C1115.85 (12)N6—C9—H191109.6
O1—C1—O2125.32 (18)C10—C9—H191107.0
O1—C1—C2114.83 (16)H192—C9—H191107.0
O2—C1—C2119.84 (17)C9—C10—N7109.3 (2)
C1—C2—N1111.47 (16)C9—C10—H202110.1
C1—C2—C3130.01 (16)N7—C10—H202111.5
N1—C2—C3118.51 (16)C9—C10—H201107.5
C2—N1—Ni1121.72 (13)N7—C10—H201108.3
C2—N1—C7120.55 (16)H202—C10—H201110.0
Ni1—N1—C7117.63 (12)C10—N7—Ni1108.13 (14)
N1—C7—C4121.65 (16)C10—N7—H211111.1
N1—C7—C6117.12 (16)Ni1—N7—H211112.2
C4—C7—C6121.23 (16)C10—N7—H212109.4
C7—C4—N2119.28 (16)Ni1—N7—H212112.0
C7—C4—N3120.28 (16)H211—N7—H212104.0
N2—C4—N3120.44 (16)Ni1—N8—C11128.20 (15)
C4—N2—C3118.19 (16)Ni1—N8—C13126.88 (17)
C2—C3—N2121.72 (16)C11—N8—C13104.8 (2)
C2—C3—C8122.07 (17)N8—C11—N9110.8 (2)
N2—C3—C8116.19 (17)N8—C11—H231125.8
C3—C8—H111113.0N9—C11—H231123.4
C3—C8—H113108.0C11—N9—C12106.2 (2)
H111—C8—H113109.6C11—N9—H241128.0
C3—C8—H112110.4C12—N9—H241125.3
H111—C8—H112108.7N9—C12—C13107.5 (2)
H113—C8—H112107.0N9—C12—H251126.4
C4—N3—C5115.09 (15)C13—C12—H251126.1
N3—C5—N4128.71 (17)N8—C13—C12110.6 (2)
N3—C5—N5116.80 (17)N8—C13—H261124.9
N4—C5—N5114.49 (17)C12—C13—H261124.4
C5—N4—C6117.14 (16)H271—O4—H272104.5
C7—C6—N4117.19 (16)H281—O5—H28299.5
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
N5—H171···O4i0.842.503.193 (3)140
N5—H172···N4ii0.852.132.984 (3)177
N6—H182···O50.882.283.099 (3)154
N7—H211···N2iii0.902.413.298 (2)173
N7—H212···O4iv0.872.533.210 (3)136
N9—H241···O5v0.892.153.024 (3)168
O4—H271···N3vi0.822.022.837 (2)169
O4—H272···O30.812.072.859 (2)167
O5—H281···O2vii0.832.002.822 (2)170
O5—H282···O10.812.142.789 (2)138
Symmetry codes: (i) x+1/2, y+1/2, z+1; (ii) x+1, y, z+1; (iii) x+3/2, y+1/2, z; (iv) x+1, y+1, z+1; (v) x, y1, z; (vi) x1/2, y+1/2, z+1; (vii) x+1, y, z+1/2.
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
N5—H171···O4i0.842.503.193 (3)140
N5—H172···N4ii0.852.132.984 (3)177
N6—H182···O50.882.283.099 (3)154
N7—H211···N2iii0.902.413.298 (2)173
N7—H212···O4iv0.872.533.210 (3)136
N9—H241···O5v0.892.153.024 (3)168
O4—H271···N3vi0.822.022.837 (2)169
O4—H272···O30.812.072.859 (2)167
O5—H281···O2vii0.832.002.822 (2)170
O5—H282···O10.812.142.789 (2)138
Symmetry codes: (i) x+1/2, y+1/2, z+1; (ii) x+1, y, z+1; (iii) x+3/2, y+1/2, z; (iv) x+1, y+1, z+1; (v) x, y1, z; (vi) x1/2, y+1/2, z+1; (vii) x+1, y, z+1/2.
Acknowledgements top

The authors are grateful to the UGC, New Delhi, for financial assistance (SAP–DRS program). Thanks are due to the CSMCRI, Bhavnagar, Gujrat, India, for the X-ray structural data and elemental analysis data, and the University of North Bengal for infrastructure.

references
References top

Baisya, S. S. & Roy, P. S. (2013). Acta Cryst. E69, m99–m100.

Basu, P. & Burgmayer, S. J. N. (2011). Coord. Chem. Rev. 255, 1016–1038.

Beddoes, R. L., Russell, J. R., Garner, C. D. & Joule, J. A. (1993). Acta Cryst. C49, 1649–1652.

Betteridge, P. W., Carruthers, J. R., Cooper, R. I., Prout, K. & Watkin, D. J. (2003). J. Appl. Cryst. 36, 1487.

Bruker (2007). APEX2 and SAINT. Bruker AXS Inc., Madison, Wisconsin, USA.

Burgmayer, S. J. N. (1998). Struct. Bond. 92, 67–119.

Cooper, R. I., Thompson, A. L. & Watkin, D. J. (2010). J. Appl. Cryst. 43, 1100–1107.

Crispini, A., Pucci, D., Bellusci, A., Barberio, G., Deda, M. L., Cataldi, A. & Ghedini, M. (2005). Cryst. Growth Des. 5, 1597–1601.

Fitzpatrick, P. F. (2003). Biochemistry, 42, 14083–14091.

Fukuzumi, S. & Kojima, T. (2008). J. Biol. Inorg. Chem. 13, 321–333.

Kaim, W., Schwederski, B., Heilmann, O. & Hornun, F. M. (1999). Coord. Chem. Rev. 182, 323–342.

Kohzuma, T., Odani, A., Morita, Y., Takani, M. & Yamauchi, O. (1988). Inorg. Chem. 27, 3854–3858.

Odani, A., Masuda, H., Inukai, K. & Yamauchi, O. (1992). J. Am. Chem. Soc. 114, 6294–6300.

Russell, J. R., Garner, C. D. & Joule, J. A. (1992). J. Chem. Soc. Perkin Trans. 1, pp. 1245–1249.

Sheldrick, G. M. (1996). SADABS. University of Göttingen, Germany.

Sheldrick, G. M. (2008). Acta Cryst. A64, 112–122.

Watkin, D. J., Prout, C. K. & Pearce, L. J. (1996). CAMERON. Chemical Crystallography Laboratory, Oxford, England.

Wittle, E. L., O'Dell, B. L., Vandenbelt, J. M. & Pfiffner, J. J. (1947). J. Am. Chem. Soc. 69, 1786–1792.