Electrochemical synthesis and a redetermination of Ni(acac)₂(H₂O)₂·H₂O

The title complex, bis­(acetyl­acetonato)­di­aqua­nickel(II) monohydrate, [Ni(C₅H₇O₂)₂(H₂O)₂)]·H₂O, was easily synthesized in high yield by electrochemical dissolution of nickel metal in an acetyl­acetone and ethanol solution via direct hydro­lysis of the electrolyte solution at a temperature of 333 K. The crystal structure shows octahedral coordination of nickel with two trans aqua ligands. An uncoordinated water mol­ecule engages in hydrogen bonding.

Key indicators

• Single-crystal X-ray study
• T = 293 K
• Mean [sigma](C-C) = 0.005 Å
• H-atom completeness 91%
• R factor = 0.040
• wR factor = 0.111
• Data-to-parameter ratio = 14.9

checkCIF results

No syntax errors found

ADDSYM reports no extra symmetry


 Alert Level C:



REFLT_03
         From the CIF: _diffrn_reflns_theta_max           26.01
         From the CIF: _reflns_number_total               2659
         TEST2: Reflns within _diffrn_reflns_theta_max
         Count of symmetry unique reflns         2832
         Completeness (_total/calc)             93.89%
          Alert C: < 95% complete

PLAT_731  Alert C Bond    Calc     0.82(4), Rep   0.816(18) ....       2.22 s.u-Ratio
                     O5   -H1      1.555   1.555

PLAT_731  Alert C Bond    Calc     0.80(5), Rep     0.81(2) ....       2.50 s.u-Ratio
                     O5   -H2      1.555   1.555

PLAT_731  Alert C Bond    Calc     0.80(4), Rep   0.804(19) ....       2.11 s.u-Ratio
                     O6   -H3      1.555   1.555

PLAT_731  Alert C Bond    Calc     0.82(4), Rep   0.819(19) ....       2.11 s.u-Ratio
                     O6   -H4      1.555   1.555

PLAT_735  Alert C D-H     Calc     0.82(4), Rep   0.819(19) ....       2.11 s.u-Ratio
                     O6   -H4      1.555   1.555

PLAT_735  Alert C D-H     Calc     0.82(4), Rep   0.816(18) ....       2.22 s.u-Ratio
                     O5   -H1      1.555   1.555

PLAT_735  Alert C D-H     Calc     0.80(4), Rep   0.804(19) ....       2.11 s.u-Ratio
                     O6   -H3      1.555   1.555

PLAT_735  Alert C D-H     Calc     0.80(5), Rep     0.81(2) ....       2.50 s.u-Ratio
                     O5   -H2      1.555   1.555

PLAT_735  Alert C D-H     Calc     0.80(5), Rep     0.81(2) ....       2.50 s.u-Ratio
                     O5   -H2      1.555   1.555

General Notes



FORMU_01  There is a discrepancy between the atom counts in the
          _chemical_formula_sum and the formula from the _atom_site* data.
          Atom count from _chemical_formula_sum:C10 H20 Ni1 O7
          Atom count from the _atom_site data:  C10 H18 Ni1 O7

CELLZ_01
         From the CIF: _cell_formula_units_Z    2
         From the CIF: _chemical_formula_sum  C10 H20 Ni O7
         TEST: Compare cell contents of formula and atom_site data

         atom    Z*formula  cif sites diff
         C         20.00     20.00    0.00
         H         40.00     36.00    4.00
         Ni         2.00      2.00    0.00
         O         14.00     14.00    0.00
          Difference between formula and atom_site contents detected.
          WARNING: H atoms missing from atom site list. Is this intentional?


 0 Alert Level A = Potentially serious problem

 0 Alert Level B = Potential problem

 10 Alert Level C = Please check

---

S1. Comment

The paramagnetic complex Ni(acac)₂(H₂O)₂ (acac = acetylacetonato) has been a subject of interest to chemists for the past two decades (Abernathy & Sharp, 1997; Sharp et al., 1997; Miller et al., 2000; Lorh et al., 1999), following the earlier report of trinuclear molecules in the crystal structure of Ni(acac)₂ (Bullen, 1956). Paramagnetic transition metal ions in solution can produce large nuclear magnetic resonance (NMR) relaxation enhancements of nuclear spins in ligand species and in solvent molecules. This phenomenon, called NMR paramagnetic relaxation enhancement or NMR–PRE, has been used widely to probe the structure, dynamics and magnetic properties of dissolved paramagnetic species. Crystal structures of suitable paramagnetic complexes provide structural data to aid NMR–PRE study (Sharp et al., 1997).

Recently, electrochemical dissolution of a nickel anode in acetylacetone and ethanol solution was carried out in our laboratory (Zhou et al., 2000a,b, 2001). Some water was added to the electrolyte to produce hydrolysis and prepare nanometer NiO by a sol–gel process at room temperature. The complex Ni(acac)₂(H₂O)₂ crystallized out of the solution.

The molecular structure is essentially the same as that reported by Montgomery & Lingafelter (1964) for this complex without water of crystallization. Though Misra et al. (1979) considered it was not possible to produce a single-crystal of Ni(acac)₂(H₂O)₂, it was very easy for us to obtain by using an electrochemical method. Because of our interest in sol–gel chemistry, the electrolyte solution with some added water was heated to 333 K for 30 min. Crystals of hydrated Ni(acac)₂(H₂O)₂^.H₂O, (I), were also obtained and the crystal structure has been determined. Each of the two products can be obtained preferentially by controlling the hydrolysis temperature carefully. Harlow & Pfluger (1973) obtained crystals of the hydrate by using a completely different method previously. The structure reported here is of greater precision.

Selected bond lengths and angles are given in Table 1, and the molecular structure is shown in Fig. 1, with a packing diagram shown in Fig. 2. The uncoordinated water molecule is involved in hydrogen bonding, and further hydrogen bonding links coordinated water molecules to acac ligands in adjacent molecules (Table 2).

In the structure without water of crystallization, the nickel ion has acac O atoms at 2.021 and 2.014 Å and water O atoms at 2.139 Å in a precisely centrosymmetric arrangement. In the hydrated structure, the corresponding distances are 1.994 (2)–2.031 (2) Å for acac, and 2.075 (2) and 2.080 (2) Å for water ligands, respectively, the nickel ion lying in a general position but retaining an octahedral coordination.

S2. Experimental

Metallic nickel was electrochemically dissolved in the presence of tetrabutylammonium bromide as supporting electrolyte. Pure nickel foil (AR 99.99%, 2 × 4 × 0.05 cm) was used as cathode and anode. The electrochemical reaction was carried out in a cell without separating the cathode and anode space. 250 ml (2.5 mol) ethanol was poured into the electrolysis bath, as solvent and as reagent. During the electrolysis process, 5 ml (0.05 mol) acetylacetone was gradully added dropwise to the electrolyte. The potential across the anode was adjusted so that a current of 0.2 A passed through the cell for 6 h. Crystals of the title compound were produced.

S3. Refinement

H atoms bonded to carbon were constrained with a riding model. H atoms of the coordinated water molecules were refined with isotropic displacement parameters, and with restraints on O—H bond lengths, but no H atoms were included for the uncoordinated water.

Computing details

Data collection: AFC-5R Diffractometer Control Software (Rigaku, 1988); cell refinement: AFC-5R Diffractometer Control Software; data reduction: MolEN/VAX in CAD-4 Operations Manual (Enraf-Nonius, 1977); program(s) used to solve structure: SHELXTL (Sheldrick, 1997); program(s) used to refine structure: SHELXTL; molecular graphics: SHELXTL; software used to prepare material for publication: SHELXTL.

Figures

	Fig. 1. The molecular structure of (I) with ellipsoids at the 50% probability level. H atoms have been omitted for clarity.
	Fig. 2. The packing viewed along the a axis. Hydrogen bonds are shown as dashed lines.

bis(acetylacetonato)diaquanickel(II) monohydrate

Crystal data

[Ni(C5H7O2)2(H2O)2)]·H2O	Z = 2
Mr = 310.97	F(000) = 328
Triclinic, P1	Dx = 1.432 Mg m^−3
a = 7.5572 (15) Å	Mo Kα radiation, λ = 0.71073 Å
b = 9.0637 (18) Å	Cell parameters from 20 reflections
c = 10.997 (2) Å	θ = 15.4–22.9°
α = 74.17 (3)°	µ = 1.37 mm^−1
β = 84.50 (3)°	T = 293 K
γ = 89.74 (3)°	Plate, blue green
V = 721.2 (2) Å^3	0.50 × 0.40 × 0.30 mm

Data collection

Rigaku AFC-5R diffractometer	2459 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.036
Graphite monochromator	θmax = 26.0°, θmin = 1.9°
ω/2θ scans	h = −9→9
Absorption correction: ψ scan (North et al., 1968)	k = −11→0
Tmin = 0.462, Tmax = 0.664	l = −13→13
3104 measured reflections	3 standard reflections every 300 reflections
2659 independent reflections	intensity decay: 1.0%

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)] = 0.040	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.111	H atoms treated by a mixture of independent and constrained refinement
S = 1.04	w = 1/[σ^2(Fo^2) + (0.0803P)^2 + 0.2124P] where P = (Fo^2 + 2Fc^2)/3
2659 reflections	(Δ/σ)max < 0.001
179 parameters	Δρmax = 0.67 e Å^−3
4 restraints	Δρmin = −0.46 e Å^−3

Special details

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
Ni	0.24504 (4)	0.29439 (3)	0.04144 (3)	0.03533 (15)
O1	0.2319 (3)	0.1089 (2)	0.1948 (2)	0.0528 (5)
O2	0.2283 (3)	0.4374 (2)	0.15143 (19)	0.0474 (4)
O3	0.2598 (3)	0.1575 (2)	−0.07507 (19)	0.0469 (4)
O4	0.2615 (3)	0.4816 (2)	−0.11120 (18)	0.0487 (5)
O5	0.5198 (3)	0.2944 (2)	0.0411 (3)	0.0545 (5)
O6	−0.0299 (3)	0.2886 (2)	0.0472 (3)	0.0539 (5)
O7	0.2386 (3)	−0.1316 (2)	0.0761 (2)	0.0604 (6)
C1	0.2184 (6)	−0.0386 (4)	0.4070 (3)	0.0734 (10)
H1A	0.2282	−0.1195	0.3659	0.110*
H1B	0.1092	−0.0518	0.4615	0.110*
H1C	0.3169	−0.0418	0.4567	0.110*
C2	0.2199 (4)	0.1143 (3)	0.3079 (3)	0.0465 (6)
C3	0.2078 (4)	0.2477 (4)	0.3483 (3)	0.0539 (7)
H3A	0.1957	0.2351	0.4355	0.065*
C4	0.2120 (4)	0.3980 (3)	0.2707 (3)	0.0435 (6)
C5	0.1985 (5)	0.5279 (4)	0.3320 (3)	0.0646 (8)
H5A	0.2031	0.6239	0.2674	0.097*
H5B	0.2957	0.5241	0.3829	0.097*
H5C	0.0881	0.5183	0.3849	0.097*
C6	0.2857 (6)	0.0686 (4)	−0.2568 (4)	0.0744 (10)
H6A	0.2746	−0.0277	−0.1925	0.112*
H6B	0.3971	0.0737	−0.3079	0.112*
H6C	0.1899	0.0775	−0.3095	0.112*
C7	0.2788 (4)	0.1984 (3)	−0.1944 (3)	0.0461 (6)
C8	0.2918 (5)	0.3496 (4)	−0.2695 (3)	0.0558 (7)
H8A	0.3085	0.3645	−0.3569	0.067*
C9	0.2821 (4)	0.4810 (3)	−0.2264 (3)	0.0458 (6)
C10	0.2940 (5)	0.6360 (4)	−0.3221 (3)	0.0642 (9)
H10A	0.2851	0.7147	−0.2786	0.096*
H10B	0.1988	0.6451	−0.3753	0.096*
H10C	0.4059	0.6471	−0.3734	0.096*
H1	0.562 (5)	0.265 (5)	−0.019 (3)	0.072 (12)*
H2	0.562 (8)	0.378 (4)	0.034 (7)	0.18 (3)*
H3	−0.073 (6)	0.364 (4)	0.061 (5)	0.102 (16)*
H4	−0.046 (7)	0.278 (7)	−0.023 (3)	0.13 (2)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Ni	0.0428 (2)	0.0247 (2)	0.0380 (2)	0.00266 (13)	−0.00507 (14)	−0.00718 (14)
O1	0.0803 (14)	0.0281 (9)	0.0469 (11)	0.0038 (9)	−0.0054 (10)	−0.0053 (8)
O2	0.0679 (12)	0.0309 (9)	0.0438 (10)	0.0024 (8)	−0.0056 (9)	−0.0107 (8)
O3	0.0640 (12)	0.0308 (9)	0.0468 (11)	0.0025 (8)	−0.0048 (9)	−0.0126 (8)
O4	0.0736 (13)	0.0314 (9)	0.0390 (10)	0.0034 (8)	−0.0050 (9)	−0.0065 (8)
O5	0.0440 (10)	0.0443 (11)	0.0833 (17)	0.0032 (9)	−0.0090 (10)	−0.0304 (11)
O6	0.0449 (10)	0.0436 (11)	0.0800 (16)	0.0045 (8)	−0.0060 (10)	−0.0285 (11)
O7	0.0758 (14)	0.0364 (10)	0.0716 (14)	0.0035 (9)	−0.0062 (11)	−0.0195 (10)
C1	0.108 (3)	0.0433 (17)	0.0556 (19)	0.0071 (17)	0.0002 (19)	0.0051 (14)
C2	0.0551 (15)	0.0355 (13)	0.0433 (14)	0.0019 (11)	−0.0042 (12)	−0.0013 (11)
C3	0.0724 (19)	0.0478 (16)	0.0391 (14)	0.0031 (14)	−0.0042 (13)	−0.0081 (12)
C4	0.0501 (14)	0.0421 (14)	0.0394 (13)	0.0024 (11)	−0.0047 (11)	−0.0131 (11)
C5	0.090 (2)	0.0513 (18)	0.0586 (19)	0.0061 (16)	−0.0085 (17)	−0.0254 (15)
C6	0.115 (3)	0.056 (2)	0.061 (2)	0.009 (2)	−0.014 (2)	−0.0289 (17)
C7	0.0514 (14)	0.0426 (14)	0.0480 (16)	0.0074 (11)	−0.0116 (12)	−0.0164 (12)
C8	0.076 (2)	0.0508 (17)	0.0411 (15)	0.0061 (15)	−0.0100 (14)	−0.0123 (13)
C9	0.0508 (14)	0.0408 (14)	0.0433 (15)	0.0028 (11)	−0.0085 (11)	−0.0062 (11)
C10	0.091 (2)	0.0477 (17)	0.0444 (16)	−0.0005 (16)	−0.0078 (15)	0.0037 (13)

Geometric parameters (Å, º)

Ni—O2	1.9958 (19)	C2—C3	1.397 (4)
Ni—O3	2.008 (2)	C3—C4	1.395 (4)
Ni—O1	2.025 (2)	C3—H3A	0.930
Ni—O4	2.030 (2)	C4—C5	1.506 (4)
Ni—O6	2.073 (2)	C5—H5A	0.960
Ni—O5	2.076 (2)	C5—H5B	0.960
O1—C2	1.253 (4)	C5—H5C	0.960
O2—C4	1.256 (3)	C6—C7	1.513 (4)
O3—C7	1.256 (3)	C6—H6A	0.960
O4—C9	1.263 (3)	C6—H6B	0.960
O4—O6^i	2.903 (3)	C6—H6C	0.960
O5—H1	0.816 (18)	C7—C8	1.392 (4)
O5—H2	0.81 (2)	C8—C9	1.396 (4)
O6—H3	0.804 (19)	C8—H8A	0.930
O6—H4	0.819 (19)	C9—C10	1.505 (4)
C1—C2	1.511 (4)	C10—H10A	0.960
C1—H1A	0.960	C10—H10B	0.960
C1—H1B	0.960	C10—H10C	0.960
C1—H1C	0.960
O2—Ni—O3	177.73 (7)	O1—C2—C1	115.8 (3)
O2—Ni—O1	91.65 (8)	C3—C2—C1	118.6 (3)
O3—Ni—O1	90.55 (8)	C4—C3—C2	126.5 (3)
O2—Ni—O4	87.82 (8)	C4—C3—H3A	116.8
O3—Ni—O4	89.99 (8)	C2—C3—H3A	116.8
O1—Ni—O4	179.09 (8)	O2—C4—C3	125.8 (3)
O2—Ni—O6	89.69 (9)	O2—C4—C5	115.4 (3)
O3—Ni—O6	89.76 (9)	C3—C4—C5	118.8 (3)
O1—Ni—O6	89.64 (10)	C4—C5—H5A	109.5
O4—Ni—O6	91.09 (10)	C4—C5—H5B	109.5
O2—Ni—O5	90.20 (9)	H5A—C5—H5B	109.5
O3—Ni—O5	90.42 (9)	C4—C5—H5C	109.5
O1—Ni—O5	88.52 (10)	H5A—C5—H5C	109.5
O4—Ni—O5	90.75 (10)	H5B—C5—H5C	109.5
O6—Ni—O5	178.15 (9)	C7—C6—H6A	109.5
C2—O1—Ni	124.88 (18)	C7—C6—H6B	109.5
C4—O2—Ni	125.48 (17)	H6A—C6—H6B	109.5
C7—O3—Ni	127.00 (17)	C7—C6—H6C	109.5
C9—O4—Ni	126.21 (17)	H6A—C6—H6C	109.5
C9—O4—O6^i	117.13 (18)	H6B—C6—H6C	109.5
Ni—O4—O6^i	107.63 (10)	O3—C7—C8	125.3 (3)
Ni—O5—O7^ii	127.64 (11)	O3—C7—C6	115.1 (3)
Ni—O5—H1	108 (3)	C8—C7—C6	119.7 (3)
Ni—O5—H2	113 (5)	C7—C8—C9	126.4 (3)
H1—O5—H2	108 (6)	C7—C8—H8A	116.8
Ni—O6—H3	112 (4)	C9—C8—H8A	116.8
Ni—O6—H4	102 (4)	O4—C9—C8	125.0 (3)
H3—O6—H4	114 (5)	O4—C9—C10	115.9 (3)
C2—C1—H1A	109.5	C8—C9—C10	119.0 (3)
C2—C1—H1B	109.5	C9—C10—H10A	109.5
H1A—C1—H1B	109.5	C9—C10—H10B	109.5
C2—C1—H1C	109.5	H10A—C10—H10B	109.5
H1A—C1—H1C	109.5	C9—C10—H10C	109.5
H1B—C1—H1C	109.5	H10A—C10—H10C	109.5
O1—C2—C3	125.6 (3)	H10B—C10—H10C	109.5

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

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O6—H4···O7^iii	0.82 (2)	2.20 (5)	2.799 (3)	130 (5)
O5—H1···O7^ii	0.82 (2)	2.08 (3)	2.781 (3)	144 (4)
O6—H3···O4^i	0.80 (2)	2.13 (2)	2.903 (3)	161 (5)
O5—H2···O2^iv	0.81 (2)	2.65 (5)	3.237 (3)	131 (6)
O5—H2···O4^iv	0.81 (2)	2.22 (4)	2.929 (3)	147 (6)

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