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The structure of the title compound, (C2H10N2)[WOS3], consists of ethyl­ene­diammonium dications and tetra­hedral [WOS3]2− dianions, which are linked with the aid of four varieties of hydrogen bond, namely N—H...O, N—H...S, C—H...O and C—H...S. The strength and number of these hydrogen bonds affect the W—O and W—S bond distances.

Supporting information

cif

Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270106056393/av3050sup1.cif
Contains datablocks I, global

hkl

Structure factor file (CIF format) https://doi.org/10.1107/S0108270106056393/av3050Isup2.hkl
Contains datablock I

CCDC reference: 641779

Comment top

As part of an ongoing research programme, we are investigating the reactions of (NH4)2[WS4] with organic amines and have structurally characterized several organic ammonium tetrathiotungstates (Srinivasan et al., 2002, 2003a,b, 2005; Srinivasan, Näther et al., 2006a,b; Srinivasan, Naik et al., 2006a). Our recent results in this area have revealed that these compounds exhibit a rich structural chemistry in terms of the N—H···S interactions between the organic ammonium cation and the tetrathiotungstate anion (Srinivasan, Näther et al., 2006a,b). In almost all these compounds, the WS4 tetrahedron is slightly distorted with one or two of the W—S bonds elongated, which can be attributed to the strength and numbers of these S···H interactions. A competing hydrogen-bond acceptor such as oxygen in oxotrithiotungstate [WOS3]2- compounds offers the possibility to investigate the occurrence of N—H···O as well as N—H···S interactions in the same compound. Although the synthetic aspects of [WOS3]2- (Müller & Diemann 1968; McDonald et al., 1983), spectral characteristics (Müller et al., 1969; 1981), reactivity studies (Ansari et al., 1988) and medical applications (Mason et al., 1989) have been reported in the literature, only two structurally characterized monooxotrithiotungstates, K3[WOS3]Cl (Krebs et al., 1972) and (pipH)4[WOS3][WS4] (pipH is piperidinium) (Siemeling et al., 2006), are known to date. Both reported compounds are double salts and contain anions such as Cl- or [WS4]2- in addition to [WOS3]2- in their structures. In the present work, we describe the structure of the organic ammonium oxotrithiotungstate (enH2)[WOS3], (I) (enH2 = ethylenediammonium), which does not contain any anion other than [WOS3]2-. The title compound was prepared by following an analogous procedure used previously for the corresponding tetrathio compound (enH2)[WS4] (Srinivasan et al., 2002).

The structure of (I) consists of ethylenediammonium dications and tetrahedral [WOS3]2- dianions, with all atoms located in general positions (Fig. 1). Both (pipH)2[WS4] and the double salt (pipH)4[WOS3][WS4] crystallize in the monoclinic space group P21/n (Siemeling et al., 2006), while the tetrathio analogue of (I) (enH2)[WS4] crystallizes in the chiral orthorhombic space group P212121 (Srinivasan et al., 2002). The geometric parameters of the (enH2)2+ cation in (I) (Table 1) are in good agreement with those observed in other compounds containing the same cation (Srinivasan et al., 2002, 2003c). The bond angles around W are very close to the ideal tetrahedral angle and scatter in a small range between 108.63 (14) and 110.36 (6)°. The observed W—O and W—S bond distances are in very good agreement with those reported in the double salt (pipH)4[WOS3][WS4] (Siemeling et al., 2006). Analysis of the crystal structure showed the presence of four varieties of weak interactions, namely N—H···O, N—H···S, C—H···O and C—H···S bonds. Each cation is hydrogen bonded to six different [WOS3]2- anions. The S and O atoms of [WOS3]2- function as H-atom acceptors, while the cation acts as an H-atom donor through its ammonium and methylene groups. Each anion is linked to six different cations with the aid of the same four varieties of hydrogen bonds (Fig. 2). In (I), all H atoms involved in hydrogen bonding are singly shared donors excepting H1N1, which functions as a bifurcated donor. As a result of the hydrogen-bonding pattern, the cations and anions are organized into alternating layers, resulting in the formation of a three-dimensional hydrogen-bonded network (Fig. 3).

A total of nine hydrogen bonds (Table 2) comprising six S···H and three O···H interactions are observed, and all these contacts are shorter than the sum of their van der Waals radii (Bondi, 1964). A comparison of the geometric parameters of (I) with those of K3[WOS3]Cl and (pipH)4[WOS3][WS4] (Table 3) serves to illustrate the importance of hydrogen bonding in organic ammonium thiotungstates. In K3[WOS3]Cl with no hydrogen bonding, the W—S distances scatter in a narrow range between 2.196 (6) and 2.208 (5) Å. The difference Δ between the longest and shortest W—S bond is 0.012 Å, which is much less than the values in (I) and (pipH)4[WOS3][WS4], with the maximum Δ value of 0.0399 Å. It is to be noted that the structure of (pipH)4[WOS3][WS4] was determined at 220 K and the compound also contains an additional [WS4]2- anion in its structure. In (I), the shortest W—S bond length of 2.1791 (15) Å is observed for S3, which is involved in a single bifurcated S···H contact at a relatively long distance of 2.77 Å accompanied by a small N—H···S angle, which can explain the observed short W—S distance. In (pipH)4[WOS3][WS4], the shortest W—S bond of 2.1666 (9) Å was explained as being due to the absence of any short S···H contact. The other two W—S bond distances in (I), at 2.2010 (14) and 2.2029 (14) Å, are identical within experimental error. Atom S2 is engaged in two short N—H···S contacts, while S1 has three hydrogen-bonding interactions (two short N—H···S and one C—H···S contact), which can explain the elongation of these bonds as compared to W—S3. The observation of longer W—S bonds at 2.2009 (10) and 2.2065 (10) Å in (pipH)4[WOS3][WS4] was attributed to a singly shared hydrogen bond by each of these S atoms. [please clarify meaning]

The W—O bond distance in (I) [1.778 (4) Å] is slightly longer than that for K3[WOS3]Cl [1.76 (1) Å]. An elongation of the W—O bond with respect to related compounds, which do not exhibit O···H contacts, is indicated by the observed W—O stretching vibration in the IR spectrum of (I), which occurs as a strong signal centered at 828 cm-1. For the fully alkylated organic ammonium compound [(C2H5)4N]2[WOS3] (McDonald et al., 1983) and the pure inorganic Cs2[WOS3] (Müller et al., 1969) the W—O vibrational frequencies are at higher energies at 885 and 870 cm-1, respectively. A value of 793 cm-1 was found in (pipH)4[WOS3][WS4] (Siemeling et al., 2006). In this double salt, the N—H···O contacts are slightly shorter (1.89 and 1.93 Å) and are accompanied by larger D—H···A angles of 170 and 173°, respectively, than those observed in (I). Although the W—O distances of (I) and (pipH)4[WOS3][WS4] are nearly the same, it is to be noted that the O atom in (I) is involved in three hydrogen bonds, which includes two singly shared N—H···O interactions and a singly shared C—H···O contact.

The occurrence of a C—H···O and a C—H···S contact at 2.57 and 2.88 Å, respectively, in (I) prompted us to reinvestigate the known structures of (enH2)[WS4] and (pipH)4[WOS3][WS4] for the presence of additional weak interactions. Scrutiny of the structure of (enH2)[WS4] revealed that in addition to the reported N—H···S bonds, the [WS4]2- unit is involved in three weak C—H···S contacts at 2.89, 2.92 and 2.95 Å with corresponding D—H···A angles of 148, 131 and 143°, respectively. In (pipH)4[WOS3][WS4], the [WOS3]2- unit exhibits three weak C—H···S contacts (2.87, 2.93 and 2.96 Å) with D—H···A angles of 127, 133 and 156°, respectively. The C—H···O distance at 2.76 Å seems to be too long for significant interactions (Jeffrey, 1997). Presently, there is no direct evidence to show that C—H···S bonds affect the W—S bond distances. However, the observation of several C—H···S contacts in many tetrathiotungstates by us recently (Srinivasan, Naik et al., 2006a,b) indicates that these weak interactions may play an important role in the structural chemistry of thiotungstates.

A comparison of (I) with that of the corresponding tetrathio compound (enH2)[WS4] reveals that both structures are nearly identical (Fig. 4). A careful analysis of the overlaid structures demonstrates that when one S atom in (enH2)[WS4] is replaced by an O atom, the O atom occupies the position of that S atom in (enH2)[WS4] (Table 3), which exhibits two singly shared N—H···S contacts. It is to be noted that in (enH2)[WS4] the other three S atoms are involved in at least one bifurcated N—H···S bond. Hence, the preference of this position by the O atom can be explained due to its competing nature to engage in stronger hydrogen bonding. The incorporation of O also results in a C—H···O contact. In the tetrathio compound this bond corresponds to the weak C—H···S interaction at 2.89 Å. The structural similarity of the tetrathio and oxotrithio compounds can also explain the nearly similar hydrogen-bond surroundings around the S atoms that exhibit the longest W—S bond distances, in both compounds. It is interesting that the introduction of an O atom in the place of one S in the tetrathio compound results in about a two and half times increase in the magnitude of Δ from 0.0092 Å in (enH2)[WS4] to 0.0238 Å in (I). This observation adds more credence to our recent postulate that the magnitude of Δ can be considered as a useful measure of the distortion of the WS4 tetrahedron in organic ammonium tetrathiotungstates (Srinivasan, Näther et al., 2006a).

In summary, we have shown that when an S atom in (enH2)[WS4] is replaced by an O atom, the resulting structure is nearly identical to that of the tetrathio compound, with the O atom occupying a position favorable for stronger N—H···O interactions. The O-atom substitution is also accompanied by a pronounced increase in the Δ value for the oxotrithio compound. In addition, the introduction of asymmetry in the form of an O atom in the WS4 tetrahedron leads to the observation of more varieties of hydrogen bonds in a single compound. More examples of structurally characterized organic ammonium oxotrithiotungstates are essential for a better understanding of the importance of the weaker C—H···O and C—H···S interactions in the structural chemistry of oxotrithiotungstates. Efforts in this direction are currently underway in our laboratories.

Related literature top

For related literature, see: Ansari et al. (1988); Bondi (1964); Jeffrey (1997); Krebs et al. (1972); Müller & Diemann (1968); Müller et al. (1969, 1981); Mason et al. (1989); McDonald et al. (1983); Siemeling et al. (2006); Srinivasan et al. (2002, 2003a, 2003b, 2003c, 2005); Srinivasan, Näther, Dhuri & Bensch (2006a, 2006b); Srinivasan, Naik, Näther & Bensch (2006a, 2006b).

Experimental top

(NH4)2[WOS3] (664 mg, 2 mmol) was dissolved in water (10 ml) containing a few drops of aqueous ammonia and filtered. To the clear yellow filtrate, en (0.3 ml) was added dropwise at room temperature and the reaction mixture was kept aside for crystallization. After a day, crystalline blocks started to separate out slowly. These were isolated by filtration, washed well with ice-cold water (2 ml), 2-propanol (20 ml) and ether (20 ml), and air dried (yield 500 mg). The complex is quite stable in air and was analysed satisfactorily. νW - O 828 cm-1, νW - S 484 and 455 cm-1.

Refinement top

The positions of the C– and N-bound H atoms were located in a difference map; their bond lengths were set to ideal values and were refined using a riding model (C—H = 0.97 Å and N—H = 0.89 Å). The largest peak in the residual electron density map is located 0.78 Å from W1 and the deepest hole is located 0.79 Å from W1

Computing details top

Data collection: DIF4 (Stoe & Cie, 1998); cell refinement: DIF4; data reduction: REDU4 (Stoe & Cie, 1998); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: DIAMOND (Brandenburg, 1999); software used to prepare material for publication: CIFTAB in SHELXTL (Bruker, 1998).

Figures top
[Figure 1] Fig. 1. The structure of the constituent ions of (I), showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 50% probability level.
[Figure 2] Fig. 2. A view of the surroundings of the [WOS3]2- anion showing the linking of each anion to six different cations via N—H···O, N—H···S, C—H···O and C—H···S interactions. Hydrogen bonds are shown as dashed lines. [Symmetry codes: (i) x + 1/2, -y + 3/2, z + 1/2; (ii) -x + 1/2, y + 1/2, -z + 1/2; (iii) -x + 3/2, y + 1/2, -z + 1/2; (iv) x + 1, y, z; (v) x + 1/2, -y + 3/2, z - 1/2.]
[Figure 3] Fig. 3. The packing of (I), viewed along the a axis, showing the three-dimensional hydrogen-bonded network. N—H···O, N—H···S, C—H···O and C—H···S bonds are shown as dashed lines.
[Figure 4] Fig. 4. An overlay diagram showing the structural similarity of (enH2)[WOS3] (blue in the online version of the journal) and (enH2)[WS4] (red).
ethylenediammonium oxotrithiotungstate(VI) top
Crystal data top
(C2H10N2)[WOS3]F(000) = 664
Mr = 358.15Dx = 2.665 Mg m3
Monoclinic, P21/nMo Kα radiation, λ = 0.71073 Å
a = 8.8191 (14) ÅCell parameters from 100 reflections
b = 10.7923 (18) Åθ = 20.1–30.5°
c = 9.3793 (14) ŵ = 13.58 mm1
β = 90.631 (12)°T = 293 K
V = 892.7 (2) Å3Block, yellow
Z = 40.19 × 0.18 × 0.17 mm
Data collection top
Stoe AED-II four-circle
diffractometer
2052 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tubeRint = 0.027
Graphite monochromatorθmax = 30.0°, θmin = 2.9°
ω/θ scansh = 012
Absorption correction: numerical
(X-SHAPE; Stoe & Cie, 1998)
k = 115
Tmin = 0.091, Tmax = 0.135l = 1313
3022 measured reflections4 standard reflections every every 120 min min
2602 independent reflections intensity decay: none
Refinement top
Refinement on F2Secondary atom site location: difference Fourier map
Least-squares matrix: fullHydrogen site location: inferred from neighbouring sites
R[F2 > 2σ(F2)] = 0.026H-atom parameters constrained
wR(F2) = 0.070 w = 1/[σ2(Fo2) + (0.0334P)2 + 2.4809P]
where P = (Fo2 + 2Fc2)/3
S = 1.08(Δ/σ)max = 0.001
2602 reflectionsΔρmax = 1.79 e Å3
85 parametersΔρmin = 2.13 e Å3
0 restraintsExtinction correction: SHELXL97, Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4
Primary atom site location: structure-invariant direct methodsExtinction coefficient: 0.0020 (2)
Crystal data top
(C2H10N2)[WOS3]V = 892.7 (2) Å3
Mr = 358.15Z = 4
Monoclinic, P21/nMo Kα radiation
a = 8.8191 (14) ŵ = 13.58 mm1
b = 10.7923 (18) ÅT = 293 K
c = 9.3793 (14) Å0.19 × 0.18 × 0.17 mm
β = 90.631 (12)°
Data collection top
Stoe AED-II four-circle
diffractometer
2052 reflections with I > 2σ(I)
Absorption correction: numerical
(X-SHAPE; Stoe & Cie, 1998)
Rint = 0.027
Tmin = 0.091, Tmax = 0.1354 standard reflections every every 120 min min
3022 measured reflections intensity decay: none
2602 independent reflections
Refinement top
R[F2 > 2σ(F2)] = 0.0260 restraints
wR(F2) = 0.070H-atom parameters constrained
S = 1.08Δρmax = 1.79 e Å3
2602 reflectionsΔρmin = 2.13 e Å3
85 parameters
Special details top

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 (Å2) top
xyzUiso*/Ueq
W10.23427 (2)0.62410 (2)0.188568 (19)0.02179 (8)
S10.03782 (16)0.61605 (15)0.33097 (16)0.0339 (3)
S20.23135 (19)0.46562 (14)0.04054 (15)0.0339 (3)
S30.2303 (2)0.79721 (14)0.06900 (17)0.0378 (3)
O10.4038 (4)0.6187 (4)0.2927 (5)0.0327 (8)
N10.5718 (5)0.8387 (5)0.2825 (5)0.0292 (9)
H1N10.61530.86270.36410.044*
H2N10.50820.77650.29940.044*
H3N10.52090.90190.24430.044*
N20.9214 (6)0.9070 (5)0.2668 (6)0.0383 (12)
H1N20.87550.92980.34660.057*
H2N20.98970.96390.24320.057*
H3N20.96750.83450.28050.057*
C10.6906 (7)0.7969 (6)0.1820 (6)0.0340 (12)
H1A0.64230.77180.09320.041*
H1B0.74140.72490.22200.041*
C20.8076 (7)0.8948 (7)0.1506 (7)0.0413 (15)
H2A0.85870.87390.06280.050*
H2B0.75710.97380.13660.050*
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
W10.02061 (10)0.02227 (11)0.02250 (11)0.00107 (8)0.00015 (6)0.00098 (8)
S10.0291 (6)0.0346 (7)0.0383 (7)0.0028 (6)0.0095 (5)0.0052 (6)
S20.0447 (8)0.0270 (6)0.0300 (6)0.0008 (6)0.0050 (6)0.0065 (5)
S30.0516 (9)0.0268 (7)0.0349 (7)0.0049 (6)0.0023 (6)0.0070 (6)
O10.0265 (17)0.0297 (19)0.042 (2)0.0019 (17)0.0037 (16)0.0007 (18)
N10.027 (2)0.028 (2)0.033 (2)0.0030 (17)0.0015 (18)0.0030 (19)
N20.030 (2)0.038 (3)0.047 (3)0.003 (2)0.004 (2)0.004 (2)
C10.033 (3)0.036 (3)0.033 (3)0.003 (2)0.002 (2)0.007 (2)
C20.034 (3)0.053 (4)0.037 (3)0.001 (3)0.001 (2)0.012 (3)
Geometric parameters (Å, º) top
W1—O11.778 (4)N2—H1N20.8900
W1—S32.1791 (15)N2—H2N20.8900
W1—S12.2010 (14)N2—H3N20.8900
W1—S22.2029 (14)C1—C21.508 (9)
N1—C11.487 (7)C1—H1A0.9700
N1—H1N10.8900C1—H1B0.9700
N1—H2N10.8900C2—H2A0.9700
N1—H3N10.8900C2—H2B0.9700
N2—C21.479 (8)
O1—W1—S3108.63 (14)C2—N2—H3N2109.5
O1—W1—S1109.15 (14)H1N2—N2—H3N2109.5
S3—W1—S1109.75 (7)H2N2—N2—H3N2109.5
O1—W1—S2108.95 (15)N1—C1—C2113.5 (5)
S3—W1—S2109.96 (6)N1—C1—H1A108.9
S1—W1—S2110.36 (6)C2—C1—H1A108.9
C1—N1—H1N1109.5N1—C1—H1B108.9
C1—N1—H2N1109.5C2—C1—H1B108.9
H1N1—N1—H2N1109.5H1A—C1—H1B107.7
C1—N1—H3N1109.5N2—C2—C1112.2 (5)
H1N1—N1—H3N1109.5N2—C2—H2A109.2
H2N1—N1—H3N1109.5C1—C2—H2A109.2
C2—N2—H1N2109.5N2—C2—H2B109.2
C2—N2—H2N2109.5C1—C2—H2B109.2
H1N2—N2—H2N2109.5H2A—C2—H2B107.9
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
N1—H1N1···S2i0.892.683.496 (5)153
N1—H1N1···S3i0.892.773.353 (5)125
N1—H2N1···O10.891.942.801 (6)163
N1—H3N1···S1ii0.892.473.317 (5)159
N2—H1N2···S2i0.892.503.375 (6)168
N2—H2N2···O1iii0.891.952.816 (7)165
N2—H3N2···S1iv0.892.483.356 (6)167
C1—H1A···S1v0.972.883.666 (6)138
C2—H2A···O1v0.972.573.474 (8)155
Symmetry codes: (i) x+1/2, y+3/2, z+1/2; (ii) x+1/2, y+1/2, z+1/2; (iii) x+3/2, y+1/2, z+1/2; (iv) x+1, y, z; (v) x+1/2, y+3/2, z1/2.

Experimental details

Crystal data
Chemical formula(C2H10N2)[WOS3]
Mr358.15
Crystal system, space groupMonoclinic, P21/n
Temperature (K)293
a, b, c (Å)8.8191 (14), 10.7923 (18), 9.3793 (14)
β (°) 90.631 (12)
V3)892.7 (2)
Z4
Radiation typeMo Kα
µ (mm1)13.58
Crystal size (mm)0.19 × 0.18 × 0.17
Data collection
DiffractometerStoe AED-II four-circle
diffractometer
Absorption correctionNumerical
(X-SHAPE; Stoe & Cie, 1998)
Tmin, Tmax0.091, 0.135
No. of measured, independent and
observed [I > 2σ(I)] reflections
3022, 2602, 2052
Rint0.027
(sin θ/λ)max1)0.704
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.026, 0.070, 1.08
No. of reflections2602
No. of parameters85
H-atom treatmentH-atom parameters constrained
Δρmax, Δρmin (e Å3)1.79, 2.13

Computer programs: DIF4 (Stoe & Cie, 1998), DIF4, REDU4 (Stoe & Cie, 1998), SHELXS97 (Sheldrick, 1997), SHELXL97 (Sheldrick, 1997), DIAMOND (Brandenburg, 1999), CIFTAB in SHELXTL (Bruker, 1998).

Selected geometric parameters (Å, º) top
W1—O11.778 (4)N1—C11.487 (7)
W1—S32.1791 (15)N2—C21.479 (8)
W1—S12.2010 (14)C1—C21.508 (9)
W1—S22.2029 (14)
O1—W1—S3108.63 (14)S3—W1—S2109.96 (6)
O1—W1—S1109.15 (14)S1—W1—S2110.36 (6)
S3—W1—S1109.75 (7)N1—C1—C2113.5 (5)
O1—W1—S2108.95 (15)N2—C2—C1112.2 (5)
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
N1—H1N1···S2i0.892.683.496 (5)153
N1—H1N1···S3i0.892.773.353 (5)125
N1—H2N1···O10.891.942.801 (6)163
N1—H3N1···S1ii0.892.473.317 (5)159
N2—H1N2···S2i0.892.503.375 (6)168
N2—H2N2···O1iii0.891.952.816 (7)165
N2—H3N2···S1iv0.892.483.356 (6)167
C1—H1A···S1v0.972.883.666 (6)138
C2—H2A···O1v0.972.573.474 (8)155
Symmetry codes: (i) x+1/2, y+3/2, z+1/2; (ii) x+1/2, y+1/2, z+1/2; (iii) x+3/2, y+1/2, z+1/2; (iv) x+1, y, z; (v) x+1/2, y+3/2, z1/2.
Comparative geometric parameters (Å) for K3[WOS3]Cl (A), (pipH)4[WOS3][WS4] (B), (enH2)[WOS3] (C) and (enH2)[WS4] (D). top
ParameterABCD
W-O/S1.76 (1)1.776 (2)1.778 (4)2.1927 (14)
W-S2.196 (6)2.1666 (9)2.1791 (15)2.1852 (13)
W-S2.197 (6)2.2009 (10)2.2010 (14)2.1851 (14)
W-S2.208 (5)2.2065 (10)2.2029 (14)2.1943 (13)
Δ0.0120.03990.02380.0092
Notes: (A) Krebs et al. (1972); (B) Siemeling et al. (2006), pipH is piperidinium; (C) this work; (D) Srinivasan et al. (2002), enH2 is ethylenediammonium.
 

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