metal-organic compounds
trans-Bis(N,N-diethylethylenediamine)nickel(II) dibromide
aDepartment of Chemistry, Tulane University, 6400 Freret Street, New Orleans, Louisiana 70118-5698, USA
*Correspondence e-mail: donahue@tulane.edu
The structure of the title compound, [Ni(C6H16N2)2]Br2 or [Ni(Et2en)2]Br2 (Et2en is asymmetric N,N-diethylethylenediamine), containing an NiII atom (site symmetry ) in square-planar NiN4 coordination, is described and contrasted with related structures containing NiII in octahedral coordination with axial X− ligands (X− = variable anions). The dialkylated N atom has an appreciably longer bond length to the NiII atom [1.9666 (13) Å] than does the unsubstituted N atom [1.9202 (14) Å]. The Ni—N bond lengths in [Ni(Et2en)2]Br2 are significantly shorter than corresponding values in tetragonally distorted [Ni(Et2en)2X2] compounds (X = −O2CCF3, OH2, or −NCS), which have a triplet ground state. The in these axially ligated [Ni(Et2en)2X2] compounds populates the metal-based dx2-y2 orbital, which is Ni—N antibonding in character. Each Et2en ligand in each [Ni(Et2en)2]2+ cation forms a pair of N—H⋯Br hydrogen bonds to the Br− anions, one above and below the NiN4 square plane. Thus, a ribbon of alternating Br− pairs and [Ni(Et2en)2]2+ cations that are canted at 65° relative to one another is formed by hydrogen bonds.
Related literature
The synthesis of a broad variety of Ni(Et2en)2X2 compounds is described by Goodgame & Venanzi (1963). The compounds containing NiII in octahedral coordination with axial X ligands have been structurally characterized for X = −O2CCF3 (Senocq et al., 1999), −NCS (Lever et al., 1983) and H2O with non-coordinated Cl− counter-anions (Ihara et al., 1991). [Ni(Et2en)2][ClO4]2 containing a square-planar centrosymmetric cation has been identified as having triclinic (Ikeda et al., 1995; Narayanan & Bhadbhade, 1998) and monoclinic (Hayami et al., 2009) polymorphs.
Experimental
Crystal data
|
Refinement
|
Data collection: APEX2 (Bruker, 2009); cell SAINT (Bruker, 2008); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008a); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008a); molecular graphics: SHELXTL (Sheldrick, 2008a); software used to prepare material for publication: SHELXTL.
Supporting information
https://doi.org/10.1107/S1600536810050403/wm2433sup1.cif
contains datablocks I, global. DOI:Structure factors: contains datablock I. DOI: https://doi.org/10.1107/S1600536810050403/wm2433Isup2.hkl
Orange diamondoid crystals of [Ni(Et2en)2]Br2 grew by diffusion of tBuOMe vapor into a dry methanol solution in a sealed vial. The MeOH solution was prepared by stirring an excess of powdered [Ni(Et2en)2]Br2 in several mL of dry MeOH for a period of five minutes. This heterogeneous mixture was then passed though a pad of packed Celite to remove all undissolved material and produce a homogeneous filtrate.
H-atoms were identified in the final
Their positions were refined with isotropic thermal parameters.Complexes of the general type Ni(Et2en)2X2 (Et2en = asymmetric N,N-diethylethylenediamine; X- = variable anions) were first synthesized by Goodgame & Venanzi (1963) as compounds which, depending on the particular identity of X-, might reveal triplet and singlet spin states in close enough energetic proximity that a thermal distribution between them could be observed. The compounds with X = halide, -O2CR, NO2- and -NCS were formulated with a tetragonally-distorted octahedral coordination around NiII with axial X- ligands, while those with X- = ClO4-, BF4-, BPh4- and NO3- were recognized as being complexes with NiII in square-planar coordination with noncoordinating X- counterions. Apart from a tendency for the F- , Cl- and Br- compounds to adsorb ambient moisture and form the corresponding hydrates, the Ni(Et2en)2X2 compounds are readily prepared and handled. The ease with which X- is varied and the amenability of the Ni(Et2en)2X2 compound set to straightforward
and UV-vis spectroscopic measurements, disposes it as a useful vehicle for teaching the spectrochemical series in an undergraduate laboratory context.In the course of a revised and expanded laboratory experiment with Ni(Et2en)2X2 compounds at Tulane University, undergraduate students produced diffraction quality crystal samples of the unhydrated bromide compound by using the vial-in-vial vapor diffusion technique. Following a data collection at 100 K, structure solution and
of this bromide compound revealed the NiII atom to have square-planar coordination in the crystalline state (Scheme 1, Figure 1) rather than octahedral coordination as assumed by Goodgame & Venanzi (1963). The Ni···Br interatomic distances are 4.3048 (6) and 5.0032 (8) Å, which are too large to be compatible with any bonding interaction between them. Despite the noncoordination of Br- in the the possibility of weak axial interaction by Br- with the NiII atom in solution is not precluded.The [Ni(Et2en)2]2+ cation resides on an inversion center in C2/c such that only two independent and appreciably different Ni—N bond lengths occur (1.9202 (14), 1.9666 (13) Å). The longer Ni—N interatomic distance found for the dialkylated nitrogen atom may be plausibly attributed to steric effects exerted by the ethyl groups. Square planar [Ni(Et2en)2]2+ has also been structurally characterized as the perchlorate salt (Ikeda et al., 1995; Narayanan & Bhadbhade, 1998; Hayami et al., 2009) and observed to have similar Ni—N bond lengths of 1.930 (3) and 1.976 (2) Å (Ikeda et al., 1995).
The Ni—N bond lengths found for square-planar [Ni(Et2en)2]2+ contrast with those observed in related structures with axial ligands. The corresponding Ni—N bond lengths are 2.065 (2) and 2.262 (2) Å where X = -O2CCF3 (Senocq et al., 1999), 2.064 (3) and 2.271 (3) Å where X = OH2 (Ihara et al., 1991), and 2.083 (2) and 2.318 (2) Å (averaged values for two independent molecules) where X = -NCS (Lever et al., 1983). The longer Ni—N bond lengths in these latter compounds are due to a triplet
in which the dx2-y2 orbital is singly occupied. This orbital, which is antibonding in character with respect to metal and ligand, is unoccupied in square-planar [Ni(Et2en)2]2+, thus accounting for the pronounced shortening observed in its Ni—N bond lengths.Although the Br- ions do not have a bonding interaction with the NiII atoms in the title
they participate in a one-dimensional ribbon of hydrogen bonds, the formation of which is undoubtedly the principal factor governing the pattern of crystal packing. As illustrated in Figure 2, adjacent [Ni(Et2en)2]2+ cations are inclined at an angle of 65° and form a pseudo herringbone (or zigzag) pattern in the plane of the a and c axes. Two bromide anions are positioned between adjacent nickel complexes, one above and one below the square-planar complex cations. The pronounced canting of the [Ni(Et2en)2]2+ cations orients each NH2 group on each Et2en ligand to form two hydrogen bonds, one above and one below the square plane. Thus, each [Ni(Et2en)2]2+ cation forms four hydrogen bonds, two above and two below the square plane at opposite ends of the nickel complex cation. Approximate squares of hydrogen bonds are formed, with nitrogen atoms and bromide anions on opposing vertices and sides ~3.4 Å in length . Figure 3 presents an alternative rendering of this hydrogen bonding pattern with all the carbon atoms of the Et2en ligands removed for clarity.The synthesis of a broad variety of Ni(Et2en)2X2 compounds is described by Goodgame & Venanzi (1963). The compounds containing NiII in octahedral coordination with axial X ligands have been structurally characterized for X = -O2CCF3 (Senocq et al., 1999), -NCS (Lever et al., 1983) and H2O with non-coordinated Cl- counter-anions (Ihara et al., 1991). [Ni(Et2en)2][ClO4]2 containing a square-planar centrosymmetric cation has been identified as having triclinic (Ikeda et al., 1995; Narayanan & Bhadbhade, 1998) and monoclinic (Hayami et al., 2009) polymorphs.
Data collection: APEX2 (Bruker, 2009); cell
SAINT (Bruker, 2008); data reduction: SAINT (Bruker, 2008); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008a); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008a); molecular graphics: SHELXTL (Sheldrick, 2008a); software used to prepare material for publication: SHELXTL (Sheldrick, 2008a).[Ni(C6H16N2)2]Br2 | F(000) = 920 |
Mr = 450.95 | Dx = 1.647 Mg m−3 |
Monoclinic, C2/c | Mo Kα radiation, λ = 0.71073 Å |
Hall symbol: -C 2yc | Cell parameters from 6699 reflections |
a = 12.837 (3) Å | θ = 2.5–28.3° |
b = 11.162 (3) Å | µ = 5.45 mm−1 |
c = 13.244 (3) Å | T = 100 K |
β = 106.543 (4)° | Diamondoid, orange |
V = 1819.2 (8) Å3 | 0.05 × 0.05 × 0.05 mm |
Z = 4 |
Bruker APEXI CCD diffractometer | 2130 independent reflections |
Radiation source: fine-focus sealed tube | 2029 reflections with I > 2σ(I) |
Graphite monochromator | Rint = 0.024 |
φ and ω scans | θmax = 28.3°, θmin = 2.5° |
Absorption correction: multi-scan (SADABS; Sheldrick, 2008b) | h = −17→16 |
Tmin = 0.623, Tmax = 0.772 | k = −14→14 |
7870 measured reflections | l = −17→17 |
Refinement on F2 | Primary atom site location: structure-invariant direct methods |
Least-squares matrix: full | Secondary atom site location: difference Fourier map |
R[F2 > 2σ(F2)] = 0.019 | Hydrogen site location: difference Fourier map |
wR(F2) = 0.050 | All H-atom parameters refined |
S = 1.06 | w = 1/[σ2(Fo2) + (0.0311P)2 + 1.2408P] where P = (Fo2 + 2Fc2)/3 |
2130 reflections | (Δ/σ)max = 0.001 |
153 parameters | Δρmax = 0.57 e Å−3 |
0 restraints | Δρmin = −0.43 e Å−3 |
[Ni(C6H16N2)2]Br2 | V = 1819.2 (8) Å3 |
Mr = 450.95 | Z = 4 |
Monoclinic, C2/c | Mo Kα radiation |
a = 12.837 (3) Å | µ = 5.45 mm−1 |
b = 11.162 (3) Å | T = 100 K |
c = 13.244 (3) Å | 0.05 × 0.05 × 0.05 mm |
β = 106.543 (4)° |
Bruker APEXI CCD diffractometer | 2130 independent reflections |
Absorption correction: multi-scan (SADABS; Sheldrick, 2008b) | 2029 reflections with I > 2σ(I) |
Tmin = 0.623, Tmax = 0.772 | Rint = 0.024 |
7870 measured reflections |
R[F2 > 2σ(F2)] = 0.019 | 0 restraints |
wR(F2) = 0.050 | All H-atom parameters refined |
S = 1.06 | Δρmax = 0.57 e Å−3 |
2130 reflections | Δρmin = −0.43 e Å−3 |
153 parameters |
Experimental. The diffraction data were collected in three sets of 606 frames (0.3 deg. width in ω at φ = 0, 120 and 240 deg. A scan time of 10 sec/frame was used. |
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds 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 > 2σ(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. |
x | y | z | Uiso*/Ueq | ||
Br1 | 0.0000 | 0.913257 (18) | 0.2500 | 0.01466 (7) | |
Br2 | 0.0000 | 0.469242 (19) | 0.2500 | 0.01838 (7) | |
Ni1 | −0.2500 | 0.7500 | 0.0000 | 0.01079 (7) | |
N1 | −0.31881 (11) | 0.67603 (11) | 0.09965 (10) | 0.0131 (3) | |
N2 | −0.39293 (11) | 0.80349 (12) | −0.07621 (11) | 0.0141 (3) | |
C1 | −0.43989 (13) | 0.69026 (14) | 0.05604 (13) | 0.0163 (3) | |
C2 | −0.46277 (14) | 0.80667 (14) | −0.00423 (13) | 0.0178 (3) | |
C3 | −0.29344 (13) | 0.54467 (14) | 0.11697 (12) | 0.0160 (3) | |
C4 | −0.29416 (14) | 0.48022 (15) | 0.01586 (13) | 0.0189 (3) | |
C5 | −0.27917 (14) | 0.74426 (15) | 0.20169 (12) | 0.0183 (3) | |
C6 | −0.31232 (17) | 0.69197 (17) | 0.29384 (14) | 0.0238 (4) | |
H1N | −0.4012 (17) | 0.868 (2) | −0.1105 (17) | 0.020 (5)* | |
H2N | −0.4197 (18) | 0.748 (2) | −0.1241 (17) | 0.023 (5)* | |
H1A | −0.4657 (16) | 0.6263 (19) | 0.0066 (16) | 0.015 (5)* | |
H1B | −0.4752 (18) | 0.6886 (17) | 0.1114 (17) | 0.018 (5)* | |
H2A | −0.538 (2) | 0.811 (2) | −0.045 (2) | 0.030 (6)* | |
H2B | −0.4442 (17) | 0.8744 (19) | 0.0408 (16) | 0.017 (5)* | |
H3A | −0.2203 (16) | 0.5385 (17) | 0.1685 (14) | 0.010 (4)* | |
H3B | −0.3443 (15) | 0.5112 (18) | 0.1468 (14) | 0.012 (4)* | |
H4A | −0.231 (2) | 0.498 (2) | −0.0054 (17) | 0.037 (6)* | |
H4B | −0.293 (2) | 0.394 (2) | 0.033 (2) | 0.037 (6)* | |
H4C | −0.3579 (19) | 0.492 (2) | −0.0417 (18) | 0.031 (6)* | |
H5A | −0.2023 (19) | 0.746 (2) | 0.2170 (17) | 0.023 (5)* | |
H5B | −0.3036 (18) | 0.8251 (19) | 0.1900 (17) | 0.018 (5)* | |
H6A | −0.291 (2) | 0.744 (2) | 0.349 (2) | 0.036 (6)* | |
H6B | −0.388 (3) | 0.680 (3) | 0.281 (2) | 0.052 (9)* | |
H6C | −0.280 (2) | 0.615 (3) | 0.313 (2) | 0.040 (7)* |
U11 | U22 | U33 | U12 | U13 | U23 | |
Br1 | 0.01334 (11) | 0.01570 (11) | 0.01413 (11) | 0.000 | 0.00262 (8) | 0.000 |
Br2 | 0.01588 (12) | 0.01737 (12) | 0.01981 (12) | 0.000 | 0.00175 (8) | 0.000 |
Ni1 | 0.00960 (13) | 0.01277 (13) | 0.01037 (13) | −0.00131 (9) | 0.00344 (9) | 0.00080 (9) |
N1 | 0.0117 (6) | 0.0162 (6) | 0.0112 (6) | −0.0021 (5) | 0.0029 (5) | 0.0002 (5) |
N2 | 0.0135 (6) | 0.0150 (6) | 0.0141 (6) | −0.0002 (5) | 0.0042 (5) | 0.0013 (5) |
C1 | 0.0110 (7) | 0.0213 (8) | 0.0174 (7) | −0.0017 (6) | 0.0052 (6) | 0.0013 (6) |
C2 | 0.0139 (8) | 0.0218 (8) | 0.0196 (8) | 0.0024 (6) | 0.0075 (6) | 0.0015 (6) |
C3 | 0.0172 (7) | 0.0159 (7) | 0.0148 (7) | −0.0021 (6) | 0.0043 (6) | 0.0022 (6) |
C4 | 0.0207 (8) | 0.0157 (7) | 0.0197 (8) | −0.0023 (6) | 0.0051 (6) | −0.0012 (6) |
C5 | 0.0211 (8) | 0.0211 (8) | 0.0132 (7) | −0.0071 (6) | 0.0058 (6) | −0.0032 (6) |
C6 | 0.0303 (10) | 0.0288 (9) | 0.0149 (8) | −0.0087 (7) | 0.0105 (7) | −0.0026 (7) |
Ni1—N2i | 1.9202 (14) | C2—H2B | 0.95 (2) |
Ni1—N2 | 1.9202 (14) | C3—C4 | 1.518 (2) |
Ni1—N1 | 1.9666 (13) | C3—H3A | 0.993 (19) |
Ni1—N1i | 1.9666 (13) | C3—H3B | 0.933 (19) |
N1—C1 | 1.505 (2) | C4—H4A | 0.96 (2) |
N1—C3 | 1.505 (2) | C4—H4B | 0.99 (3) |
N1—C5 | 1.508 (2) | C4—H4C | 0.96 (2) |
N2—C2 | 1.483 (2) | C5—C6 | 1.519 (2) |
N2—H1N | 0.84 (2) | C5—H5A | 0.95 (2) |
N2—H2N | 0.88 (2) | C5—H5B | 0.95 (2) |
C1—C2 | 1.509 (2) | C6—H6A | 0.92 (3) |
C1—H1A | 0.96 (2) | C6—H6B | 0.95 (3) |
C1—H1B | 0.97 (2) | C6—H6C | 0.96 (3) |
C2—H2A | 0.96 (3) | ||
N2i—Ni1—N2 | 180.0 | N2—C2—H2B | 109.5 (12) |
N2i—Ni1—N1 | 93.51 (6) | C1—C2—H2B | 112.1 (12) |
N2—Ni1—N1 | 86.49 (6) | H2A—C2—H2B | 110.1 (18) |
N2i—Ni1—N1i | 86.49 (6) | N1—C3—C4 | 112.36 (13) |
N2—Ni1—N1i | 93.51 (6) | N1—C3—H3A | 107.0 (11) |
N1—Ni1—N1i | 180.00 (7) | C4—C3—H3A | 109.9 (11) |
C1—N1—C3 | 108.44 (11) | N1—C3—H3B | 107.9 (12) |
C1—N1—C5 | 109.81 (12) | C4—C3—H3B | 110.8 (12) |
C3—N1—C5 | 110.62 (12) | H3A—C3—H3B | 108.7 (15) |
C1—N1—Ni1 | 108.08 (9) | C3—C4—H4A | 111.6 (14) |
C3—N1—Ni1 | 113.09 (10) | C3—C4—H4B | 104.9 (14) |
C5—N1—Ni1 | 106.74 (9) | H4A—C4—H4B | 109 (2) |
C2—N2—Ni1 | 109.36 (10) | C3—C4—H4C | 115.2 (14) |
C2—N2—H1N | 108.6 (14) | H4A—C4—H4C | 110.1 (19) |
Ni1—N2—H1N | 120.3 (15) | H4B—C4—H4C | 106 (2) |
C2—N2—H2N | 107.2 (14) | N1—C5—C6 | 115.20 (13) |
Ni1—N2—H2N | 106.2 (14) | N1—C5—H5A | 105.6 (14) |
H1N—N2—H2N | 104 (2) | C6—C5—H5A | 110.5 (13) |
N1—C1—C2 | 108.55 (12) | N1—C5—H5B | 108.7 (13) |
N1—C1—H1A | 107.5 (12) | C6—C5—H5B | 109.8 (13) |
C2—C1—H1A | 107.6 (12) | H5A—C5—H5B | 106.6 (19) |
N1—C1—H1B | 111.3 (13) | C5—C6—H6A | 108.2 (16) |
C2—C1—H1B | 110.8 (12) | C5—C6—H6B | 114.6 (19) |
H1A—C1—H1B | 110.9 (17) | H6A—C6—H6B | 106 (2) |
N2—C2—C1 | 104.89 (13) | C5—C6—H6C | 111.4 (16) |
N2—C2—H2A | 109.1 (15) | H6A—C6—H6C | 111 (2) |
C1—C2—H2A | 111.1 (14) | H6B—C6—H6C | 105 (2) |
Symmetry code: (i) −x−1/2, −y+3/2, −z. |
D—H···A | D—H | H···A | D···A | D—H···A |
N2—H2N···Br1i | 0.88 (2) | 2.47 (2) | 3.3524 (15) | 176.3 (19) |
N2—H1N···Br2i | 0.84 (2) | 2.64 (2) | 3.4381 (15) | 157.9 (19) |
Symmetry code: (i) −x−1/2, −y+3/2, −z. |
Experimental details
Crystal data | |
Chemical formula | [Ni(C6H16N2)2]Br2 |
Mr | 450.95 |
Crystal system, space group | Monoclinic, C2/c |
Temperature (K) | 100 |
a, b, c (Å) | 12.837 (3), 11.162 (3), 13.244 (3) |
β (°) | 106.543 (4) |
V (Å3) | 1819.2 (8) |
Z | 4 |
Radiation type | Mo Kα |
µ (mm−1) | 5.45 |
Crystal size (mm) | 0.05 × 0.05 × 0.05 |
Data collection | |
Diffractometer | Bruker APEXI CCD |
Absorption correction | Multi-scan (SADABS; Sheldrick, 2008b) |
Tmin, Tmax | 0.623, 0.772 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 7870, 2130, 2029 |
Rint | 0.024 |
(sin θ/λ)max (Å−1) | 0.666 |
Refinement | |
R[F2 > 2σ(F2)], wR(F2), S | 0.019, 0.050, 1.06 |
No. of reflections | 2130 |
No. of parameters | 153 |
H-atom treatment | All H-atom parameters refined |
Δρmax, Δρmin (e Å−3) | 0.57, −0.43 |
Computer programs: APEX2 (Bruker, 2009), SAINT (Bruker, 2008), SHELXS97 (Sheldrick, 2008a), SHELXL97 (Sheldrick, 2008a), SHELXTL (Sheldrick, 2008a).
D—H···A | D—H | H···A | D···A | D—H···A |
N2—H2N···Br1i | 0.88 (2) | 2.47 (2) | 3.3524 (15) | 176.3 (19) |
N2—H1N···Br2i | 0.84 (2) | 2.64 (2) | 3.4381 (15) | 157.9 (19) |
Symmetry code: (i) −x−1/2, −y+3/2, −z. |
Acknowledgements
JPD and JTM gratefully acknowledge Tulane University for support of the Tulane Crystallography Laboratory. Stephen Ashe, Cecilia Burns, Greyson Durr, Andrew Kronfol, Camilla Munson, Whitley Muskwe, Clifford Nelson, Kristy Nguyen, Sarah Oertling, Beau Pritchett and Michael Soforenko (the undergraduate students in the fall 2010 teaching of CHEM 323 at Tulane University) are thanked for their effort in providing the crystalline sample used in this study.
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Complexes of the general type Ni(Et2en)2X2 (Et2en = asymmetric N,N-diethylethylenediamine; X- = variable anions) were first synthesized by Goodgame & Venanzi (1963) as compounds which, depending on the particular identity of X-, might reveal triplet and singlet spin states in close enough energetic proximity that a thermal distribution between them could be observed. The compounds with X = halide, -O2CR, NO2- and -NCS were formulated with a tetragonally-distorted octahedral coordination around NiII with axial X- ligands, while those with X- = ClO4-, BF4-, BPh4- and NO3- were recognized as being complexes with NiII in square-planar coordination with noncoordinating X- counterions. Apart from a tendency for the F- , Cl- and Br- compounds to adsorb ambient moisture and form the corresponding hydrates, the Ni(Et2en)2X2 compounds are readily prepared and handled. The ease with which X- is varied and the amenability of the Ni(Et2en)2X2 compound set to straightforward magnetic susceptibility and UV-vis spectroscopic measurements, disposes it as a useful vehicle for teaching the spectrochemical series in an undergraduate laboratory context.
In the course of a revised and expanded laboratory experiment with Ni(Et2en)2X2 compounds at Tulane University, undergraduate students produced diffraction quality crystal samples of the unhydrated bromide compound by using the vial-in-vial vapor diffusion technique. Following a data collection at 100 K, structure solution and refinement of this bromide compound revealed the NiII atom to have square-planar coordination in the crystalline state (Scheme 1, Figure 1) rather than octahedral coordination as assumed by Goodgame & Venanzi (1963). The Ni···Br interatomic distances are 4.3048 (6) and 5.0032 (8) Å, which are too large to be compatible with any bonding interaction between them. Despite the noncoordination of Br- in the crystal structure, the possibility of weak axial interaction by Br- with the NiII atom in solution is not precluded.
The [Ni(Et2en)2]2+ cation resides on an inversion center in C2/c such that only two independent and appreciably different Ni—N bond lengths occur (1.9202 (14), 1.9666 (13) Å). The longer Ni—N interatomic distance found for the dialkylated nitrogen atom may be plausibly attributed to steric effects exerted by the ethyl groups. Square planar [Ni(Et2en)2]2+ has also been structurally characterized as the perchlorate salt (Ikeda et al., 1995; Narayanan & Bhadbhade, 1998; Hayami et al., 2009) and observed to have similar Ni—N bond lengths of 1.930 (3) and 1.976 (2) Å (Ikeda et al., 1995).
The Ni—N bond lengths found for square-planar [Ni(Et2en)2]2+ contrast with those observed in related structures with axial ligands. The corresponding Ni—N bond lengths are 2.065 (2) and 2.262 (2) Å where X = -O2CCF3 (Senocq et al., 1999), 2.064 (3) and 2.271 (3) Å where X = OH2 (Ihara et al., 1991), and 2.083 (2) and 2.318 (2) Å (averaged values for two independent molecules) where X = -NCS (Lever et al., 1983). The longer Ni—N bond lengths in these latter compounds are due to a triplet electronic configuration in which the dx2-y2 orbital is singly occupied. This orbital, which is antibonding in character with respect to metal and ligand, is unoccupied in square-planar [Ni(Et2en)2]2+, thus accounting for the pronounced shortening observed in its Ni—N bond lengths.
Although the Br- ions do not have a bonding interaction with the NiII atoms in the title crystal structure, they participate in a one-dimensional ribbon of hydrogen bonds, the formation of which is undoubtedly the principal factor governing the pattern of crystal packing. As illustrated in Figure 2, adjacent [Ni(Et2en)2]2+ cations are inclined at an angle of 65° and form a pseudo herringbone (or zigzag) pattern in the plane of the a and c unit cell axes. Two bromide anions are positioned between adjacent nickel complexes, one above and one below the square-planar complex cations. The pronounced canting of the [Ni(Et2en)2]2+ cations orients each NH2 group on each Et2en ligand to form two hydrogen bonds, one above and one below the square plane. Thus, each [Ni(Et2en)2]2+ cation forms four hydrogen bonds, two above and two below the square plane at opposite ends of the nickel complex cation. Approximate squares of hydrogen bonds are formed, with nitrogen atoms and bromide anions on opposing vertices and sides ~3.4 Å in length . Figure 3 presents an alternative rendering of this hydrogen bonding pattern with all the carbon atoms of the Et2en ligands removed for clarity.