Synthesis and theoretical analysis of the structures and bonding in five new neutral square planar bis[2,4-di(aryl)-1,3,5-triazapentadienato]nickel(II) complexes

Article abstract of DOI:10.1016/j.ica.2005.09.022

Solvothermal reactions in methanol of nickel acetate tetrahydrate, Ni(OAc)₂ ? 4H₂O, with benzonitrile derivatives NC(C₆H₄)X, where X is one of the electron withdrawing substituents -CN, -NO₂, or -CF₃, located at the m- or p-positions relative to -CN, yield complexes of the general formula Ni{HNC(R)-NC(R)-NH}₂. More specifically, 3-nitrobenzonitrile, 4-nitrobenzonitrile, 1,3-dicyanobenzene, 1,4-dicyanobenzene, and ααα-trifluoro-p-toluonitrile are found to react with Ni(OAc)₂ ? 4H₂O to yield Ni{HNC(R)-NC(R)-NH} ₂, where R = 3-(NO₂)C₆H₄, 4-(NO ₂)C₆H₄, 3-(CN)C₆H₄, 4-(CN)C₆H₄, or 4-(CF₃)C₆H ₄, respectively. Analogous reactions of nitriles lacking electron withdrawing groups do not occur under similar conditions. Solid-state structures have been determined for the complexes with p-NO₂, p-CN, and p-CF₃ substituents on the phenyl rings. In addition, we describe density functional theory (DFT) and natural bonding orbital theory (NBO) studies on a simplified analog of these compounds, aimed at understanding their molecular bonding. It is shown that the new compounds for which solid-state structures have been determined are model examples of coordination compounds containing robust ω-bonds.

Full text of DOI:10.1016/j.ica.2005.09.022

Inorganica Chimica Acta 359 (2006) 1169–1176
www.elsevier.com/locate/ica
Synthesis and theoretical analysis of the structures and bonding in
five new neutral square planar
bis[2,4-di(aryl)-1,3,5-triazapentadienato]nickel(II) complexes
a
b
b
b
Ilia A. Guzei , Karen R. Crozier , Kendric J. Nelson , Jocelyn C. Pinkert ,
b
b
b,
*
Nicholas J. Schoenfeldt , Katherine E. Shepardson , Robert W. McGaff
a
University of Wisconsin-Madison, 1101 University Avenue, Madison, WI 53706, USA
University of Wisconsin-La Crosse, 4021 Cowley Hall, 1725 State Street, La Crosse, WI 54601, USA
b
Received 3 August 2005; accepted 16 September 2005
Available online 2 November 2005
This paper is dedicated to Professor Paul J. Taylor on the occasion of his retirement.
Abstract
Solvothermal reactions in methanol of nickel acetate tetrahydrate, Ni(OAc)2 Æ 4H2O, with benzonitrile derivatives NC(C6H4)X, where
X is one of the electron withdrawing substituents –CN, –NO2, or –CF3, located at the m- or p-positions relative to –CN, yield complexes
of the general formula Ni{HN@C(R)–N@C(R)–NH}2. More specifically, 3-nitrobenzonitrile, 4-nitrobenzonitrile, 1,3-dicyanobenzene,
1,4-dicyanobenzene, and aaa-trifluoro-p-toluonitrile are found to react with Ni(OAc)2 Æ 4H2O to yield Ni{HN@C(R)–N@C(R)–
NH}2, where R = 3-(NO2)C6H4, 4-(NO2)C6H4, 3-(CN)C6H4, 4-(CN)C6H4, or 4-(CF3)C6H4, respectively. Analogous reactions of nitriles
lacking electron withdrawing groups do not occur under similar conditions. Solid-state structures have been determined for the com-
plexes with p-NO2, p-CN, and p-CF3 substituents on the phenyl rings. In addition, we describe density functional theory (DFT) and
natural bonding orbital theory (NBO) studies on a simplified analog of these compounds, aimed at understanding their molecular bond-
ing. It is shown that the new compounds for which solid-state structures have been determined are model examples of coordination com-
pounds containing robust x-bonds.
ꢀ 2005 Elsevier B.V. All rights reserved.
Keywords: Crystal structures; Coordination compounds; Metallocycles; Condensation reactions; Density functional theory; Natural bonding orbital
theory
1. Introduction
coordination compounds. Recently, we have been studying
reactions of nickel(II) acetate tetrahydrate, Ni(OAc)2 Æ
4H2O (1), with various benzonitrile derivatives in order to
understand the reaction system and elucidate structural fea-
tures of the resultant mononuclear metal complexes.
In 2001 Kryatov reported the solid state structure of the
neutral complex Ni{HN@C(Me)–N@C(Me)–NH}2 (2),
and its synthesis from acetonitrile and nickel complex [Ni2-
(l–OH)2(tmpa)2]2+ {tmpa = tris(2-pyridylmethyl)amine}
[9]. The formation and structural characterization of cat-
ionic (protonated) analogs of 2 by reaction of NiCl2 Æ 2H2O
with nitriles in the presence of 2-propanone oxime has also
been reported [10]. Several other structurally related nickel
Metal-promoted reactions of nitriles have, as a whole,
proven to be to be a significant tool for the synthesis of a vari-
ety of diverse compounds, and several reviews on this topic
have appeared in the literature within the past decade
[1–8]. Our work in this area has focused on the application
of solvothermal (solventothermal) synthetic techniques to
reactions of nitriles with transition metal sources as a means
for preparation of both molecular and extended-structure
*
Corresponding author. Tel.: +1 608 785 8656; fax: +1 608 785 8281.
E-mail address: mcgaff.robe@uwlax.edu (R.W. McGaff).
0020-1693/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2005.09.022

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I.A. Guzei et al. / Inorganica Chimica Acta 359 (2006) 1169–1176
complexes have been reported [11–18]. Importantly, the
latter were prepared through reactions of pre-formed ligands
with metal ion sources rather than through in situ condensa-
tion of a simple molecule such as acetonitrile. As a whole, the
ligands in nickel complex 2 and its analogs referred to above
are representative of a common structural motif in transition
metal coordination chemistry [19].
The previously reported synthesis of 2 is specific for
the bimetallic nickel reagent employed by the authors,
since a series of simpler (monometallic) Ni(II) starting
materials failed to react with acetonitrile under analogous
conditions [9]. In the reactions that produce cationic ana-
logs of 2, a simple monometallic nickel(II) starting mate-
rial is employed, but the presence of a ketoxime is
required [10]. In an extension of our earlier investigations
involving reactions of simple transition metal complexes
with aromatic nitriles [20,21], we have found that 1 reacts
in methanol solution under mild solvothermal conditions
with benzonitrile derivatives NC(C6H4)X, where X is one
of the electron withdrawing substituents –CN, –NO2, or
–CF3, located at either the meta or para positions relative
to –CN, to yield neutral complexes analogous to 2. No
other reagents are added to the reaction mixture.
PTFE-lined autoclaves and heating at 110 ꢁC. Reaction
times were five days unless otherwise stated. Unless other-
wise indicated, products were isolated by filtration on frit-
ted glass discs and washed with methanol, toluene, and
diethyl ether then dried.
2.2.2. Preparation of bis[2,4-di(m-nitrophenyl)-1,3,
5-triazapentadienato]nickel(II) (3)
In a typical reaction, 1 (25.6 mg, 0.103 mmol) and 3-
nitrobenzonitrile (91.0 mg, 0.614 mmol) reacted to produce
thin yellow crystals, unsuitable for X-ray crystallography.
Attempts to obtain diffraction-quality crystals via a variety
of recrystallization techniques and manipulation of reac-
tion conditions were unsuccessful. Yield: 19.7 mg, 28.0%.
Anal. Calc. for C28H20N10O8Ni: C, 49.22; H, 2.95; N,
20.50. Found: C, 49.24; H, 2.87; N, 20.33%. IR (KBr):
3349(w), 3328(w), 3092(w), 2922(w), 2859(w), 1982(w),
1913(w), 1847(w), 1737(w), 1591(m), 1544(sh), 1525(vs),
1508(vs), 1480(vs), 1466(sh), 1422(s), 1347(vs), 1299(vs),
1274(sh), 1176(m), 1097(m), 1078(sh), 1044(m), 1000(w),
958(w), 913(m), 872(m), 810(m), 803(m), 749(s), 722(s),
1
703(vs), 670(w), 637(w) cmꢀ1. H NMR (DCON(CD3)2):
7.84 [t, 4H, H(Ar), JHH = 8 Hz overlapping with s(br),
4H, NH], 8.41 [m, 4H, H(Ar)], 8.60 [m, 4H, H(Ar)],
8.91 [t, 4H, H(Ar), JHH = 1.9 Hz] ppm. 13C NMR
(DCON(CD3)2): 163.3, 149.2, 140.7, 134.9, 130.9, 125.7,
123.1 ppm.
2. Experimental
2.1. Methods and instrumentation
Infrared spectra were recorded on a Midac Prospect-IR
instrument. The MALDI mass spectrum was recorded on a
Bruker Reflex II instrument, employing a-cyano-4-
hydroxycinnamic acid as a matrix. All NMR spectra were
recorded on a Bruker AC-300 instrument operating at
2.2.3. Preparation of bis[2,4-di(p-nitrophenyl)-1,3,
5-triazapentadienato]nickel(II) (4)
In a typical reaction, 1 (34.0 mg, 0.137 mmol) and 4-
nitrobenzonitrile (121.3 mg, 0.819 mmol) reacted to pro-
duce well formed red needle-shaped crystals suitable for
single crystal X-ray analysis. The crystals were collected
by filtration on a fritted glass disc, washed with copious
quantities of methanol, ethanol, toluene, and diethyl ether
and dried to afford 4 (36.1 mg, 38.7%). Anal. Calc. for
C28H20N10O8Ni: C, 49.22; H, 2.95; N, 20.50. Found: C,
48.88; H, 3.13; N, 20.26%. IR (KBr): 3361(w), 3347(w),
3335(m), 3113(w), 3094(w), 3077(w), 2935(w), 2840(w),
2446(w), 1597(m), 1549(vs), 1524(vs), 1480(vs), 1431(s),
1397(m), 1344(vs), 1297(s), 1237(w), 1185(w), 1158(w),
1109(s), 1040(m), 1011(s), 944(w), 865(s), 853(s), 813(s),
779(w), 768(w), 743(w), 707(vs), 665(m), 651(w), 628(w)
1
75.468 MHz for 13C spectra and 300 MHz for H spectra.
Because of the limited solubility of 3–7 and 13C–19F cou-
pling in 7, large numbers of scans were needed to record
useful spectra. Elemental analyses were performed by Gal-
braith Laboratories, Knoxville, TN, USA or Desert Ana-
lytics, Tucson, AZ, USA. Methanol (anhydrous, 99.8%),
ethanol (absolute, anhydrous), toluene (HPLC grade),
diethyl ether (anhydrous), ethylenediamine (99%),
nickel(II) acetate tetrahydrate (99.998%), 4-nitrobenzonit-
rile (97%), 3-nitrobenzonitrile (98%), 1,4-dicyanobenzene
(98%), 1,3-dicyanobenzene (98%), 4-cyanophenol (95%),
aaa-trifluoro-p-toluonitrile (99%), benzonitrile (99.9%),
KBr, NMR solvents {DCON(CD3)2 and (CD3)2SO} and
silica gel (230–400 mesh) were obtained from commercial
sources and used as received. Reported yields are based
upon nickel starting material 1.
cmꢀ1 1H NMR (DCON(CD3)2): 7.72 [s(br), 4H, NH],
.
16.45 [s, 16H, H(Ar)] ppm. 13C NMR (DCON(CD3)2):
163.3, 149.6, 145.0, 129.9, 124.4 ppm.
2.2.4. Preparation of bis[2,4-di(m-cyanophenyl)-1,3,
5-triazapentadienato]nickel(II) (5)
2.2. Synthesis
In a typical reaction, 1 (34.0 mg, 0.136 mmol) and 1,3-
dicyanobenzene (104.9 mg, 0.819 mmol) reacted to produce
extremely thin orange crystals, unsuitable for X-ray crys-
tallography. Attempts to obtain diffraction-quality crystals
via a variety of recrystallization techniques and manipula-
tion of reaction conditions were unsuccessful. Elemental
analysis and 1H NMR both indicated that the crystals
2.2.1. General synthetic methods
Unless otherwise stated, all manipulations were per-
formed under aerobic conditions. Syntheses were carried
out by combining the reactants with 5.0 mL of methanol
in glass vials, stirring, and then placing the vials in

I.A. Guzei et al. / Inorganica Chimica Acta 359 (2006) 1169–1176
1171
consisted of 5 co-crystallized with one molecule of metha-
nol per molecule of the nickel complex. Yield of
5 Æ CH3OH: (27.5 mg, 31.8% based upon starting 1). Anal.
Calc. for C33H24N10ONi: C, 62.38; H, 3.81; N, 22.05.
Found: C, 62.50; H, 3.49; N, 22.28%. IR (KBr): 3439(br),
3330(s), 3073(w), 3039(w), 2851(w), 2226(s), 1971(w),
1907(w), 1838(w), 1808(w), 1720(m), 1603(m), 1558(m),
1547(vs), 1488(sh), 1476(vs), 1441(sh), 1404(vs), 1320(sh),
1300(s), 1259(w), 1216(w), 1199(w), 1173(m), 1131(w),
1093(m), 1054(m), 1000(w), 980(w), 956(w), 905(m),
18.2%) as a pale yellow powder. Anal. Calc. for
C32H20F12N6Ni: C, 49.58; H, 2.60; N, 10.84. Found: C,
49.53; H, 2.59; N, 10.78%. IR (KBr): 3348(m), 3075(w),
2963(w), 2937(w), 1618(m), 1594(m), 1541(s), 1523(sh),
1491(sh), 1474(s), 1435(sh), 1403(m), 1327(vs), 1313(sh),
1287(sh), 1261(w), 1238(w), 1170(s), 1124(s), 1111(s),
1069(s), 1038(m), 1015(s), 980(sh), 945(w), 856(s), 830(m),
773(m), 745(m), 693(s), 634(w), 626(w) cmꢀ1 1H NMR
.
((CD3)2SO): 7.46 [s(br), 4H, NH], 7.85 [d, 8H, H(Ar),
JHH = 8 Hz], 8.16 [d, 8H, H(Ar), JHH = 8 Hz] ppm. 13C
NMR ((CD3)2SO) 162.4, 141.7 [q, JFC = 1 Hz], 129.9 [q,
JFC = 32 Hz], 128.2, 125.1 [q, JFC = 5 Hz], 124.2 [q,
JFC = 272 Hz] ppm. The quartet at 141.7 ppm was partially
resolved.
805(s), 782(w), 730(vs), 679(s), 622(m) cmꢀ1 1H NMR
.
((CD3)2SO): 7.36 [s(br), 4H, NH], 7.72 [t, 4H, H(Ar),
JHH = 8 Hz], 7.99 [m, 4H, H(Ar)], 8.34 [m, 4H, H(Ar)],
8.42 [t, 4H, H(Ar), JHH = 1.5 Hz] ppm. 13C NMR
((CD3)2SO): 161.6, 138.7, 133.5, 132.3, 130.9, 129.6,
118.7, 111.3 ppm. MALDI-MS m/z = 603.0 (calc. 603.1),
C32H21N10Ni+.
2.3. X-ray crystallography
2.3.1. General methods for data collection, structure solution,
and refinement
2.2.5. Preparation of bis[2,4-di(p-cyanophenyl)-1,3,
5-triazapentadienato]nickel(II) (6)
Evaluation of crystals and data collection were per-
formed on a Bruker CCD-1000 diffractometer with Mo
In a typical reaction, 1 (8.6 mg, 0.035 mmol) and 1,4-
dicyanobenzene (26.1 mg, 0.204 mmol) reacted to produce
6 as an orange-yellow solid. Yield: 3.0 mg, 14%. The very
small scale was employed in order to minimize the pres-
ence of solid byproducts and unreacted 1,4-dicyanoben-
zene that otherwise complicate the purification process
for 6. Anal. Calc. for C32H20N10Ni: C, 63.71; H, 3.34;
N, 23.22. Found: C, 63.76; H, 2.93; N, 22.68%. IR
(KBr): 3329(m), 3076(w), 2951(w), 2229(s), 1931(w),
1807(w), 1716(m), 1607(m), 1580(s), 1536(vs), 1489(s),
1457(vs), 1397(s), 1306(s), 1288(sh), 1248(w), 1199(m),
1161(w), 1115(m), 1042(m), 1017(m), 973(w), 945(m),
˚
Ka (k = 0.71073 A) radiation and a diffractometer to crys-
tal distance of 4.9 cm. Crystals were selected under oil un-
der ambient conditions, attached to nylon loops, and
mounted in a stream of cold nitrogen at 100(2) K, then cen-
tered in the X-ray beam. Data were collected to survey the
reciprocal space to the extent of a full sphere to a resolution
˚
of 0.80 A. The resulting highly redundant datasets were
corrected for Lorentz and polarization effects. Absorption
corrections were based upon fitting a function to the empir-
ical transmission surface as sampled by multiple equivalent
measurements [22]. Successful solutions for all structures
by the direct methods provided most non-hydrogen atoms
from the E-maps. The remaining non-hydrogen atoms were
located in alternating series of least-squares cycles and
difference Fourier maps. All non-hydrogen atoms were
refined with anisotropic displacement coefficients. All
hydrogen atoms were included in the structure factor calcu-
lations at idealized positions and were allowed to ride on
the neighboring atoms with relative isotropic displacement
coefficients. Further details regarding the determination of
structures for 4, 6, and 7 are given in the following sections
and in Table 1.
854(s), 832(w), 798(w), 761(s), 699(m), 645(w) cmꢀ1. H
1
NMR ((CD3)2SO): 7.47 [s(br), 4H, NH], 7.96 [d, 8H,
H(Ar), JHH = 9 Hz], 8.15 [d, 8H, H(Ar), JHH = 9 Hz]
ppm. 13C NMR ((CD3)2SO): 161.9, 141.7, 132.2, 128.2,
118.7, 112.3 ppm. Layering of diethyl ether above an eth-
ylenediamine solution of 6, prepared by heating a mixture
of ethylenediamine and 6 to reflux under argon, resulted
over the course of several days in the formation of single
crystals suitable for X-ray analysis.
2.2.6. Preparation of bis[2,4-di(p-trifluoromethylphenyl)-
1,3,5-triazapentadienato]nickel(II) (7)
In a typical reaction, 1 (25.9 mg, 0.104 mmol) and aaa-
trifluoro-p-toluonitrile (104.7 mg, 0.6119 mmol) reacted
over the course of two weeks to produce yellow needle-
shaped crystals of 7, co-crystallized with two molar equiv-
alents of the starting nitrile and three molar equivalents of
methanol as established by X-ray analysis. These crystals
were re-dissolved in diethyl ether and the resulting solution
was loaded onto a 2.54 · 20 cm silica gel column. A broad
pale yellow band eluted in diethyl ether and contained 7
along with some of the nitrile starting material. Removal
of all traces of aaa-trifluoro-p-toluonitrile was accom-
plished by heating at approximately 110 ꢁC under dynamic
vacuum over the course of two days to afford 7 (14.7 mg,
2.3.2. Determination of the structure of 4
A red crystal with approximate dimensions 0.46 ·
0.38 · 0.16 mm3 was selected. The initial cell constants were
obtained from three series of x scans at different starting an-
gles. Each series consisted of 20 frames collected at intervals
of 0.3ꢁ in a 6ꢁ range about x with an exposure time of 30 s
per frame. A total of 53 reflections were obtained. The
reflections were successfully indexed by an automated
indexing routine built in the SMART program. The final cell
constants were calculated from a set of 6160 strong reflec-
tions from the actual data collection. A total of 10430 data
were harvested by collecting three sets of frames with 0.25ꢁ
scans in x with an exposure time of 90 s per frame.

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Table 1
Crystal data and structure refinement [22] for 4, 6, and 7
Compound
4
6 (plus co-crystallized solvent) 7 (plus co-crystallized nitrile and solvent)
Empirical formula
Formula weight
Temperature (K)
C28H20N10NiO8
683.25
100(2)
C32H20N10Ni Æ solvent
603.29
100(2)
C50H36F18N8NiO2
1181.58
100(2)
˚
Wavelength (A)
0.71073
0.71073
0.71073
Crystal system
Space group
Unit cell dimensions
monoclinic
P2/n
triclinic
triclinic
ꢀ
ꢀ
P1
P1
˚
a (A)
5.3646(7)
11.4362(15)
21.972(3)
90
91.781(2)
90
1347.4(3)
2
1.684
7.1236(14)
11.398(2)
11.659(2)
69.297(3)
82.724(4)
85.182(3)
877.6(3)
8.2162(6)
˚
b (A)
10.8864(7)
14.5746(10)
83.4660(10)
83.4320(10)
70.4900(10)
877.6(3)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
3
˚
Volume (A )
Z
1
1
Dcalc (Mg/m3)
1.142
0.586
310
0.43 · 0.32 · 0.20
1.88–26.38ꢁ
ꢀ7 6 h 6 8,
ꢀ14 6 k 6 14,
ꢀ14 6 l 6 14
5510
1.613
0.519
598
0.46 · 0.35 · 0.31
1.99–26.38
ꢀ10 6 h 6 10,
ꢀ13 6 k 6 13,
ꢀ18 6 l 6 18
12539
Absorption coefficient (mmꢀ1
F(000)
)
0.796
700
Crystal size (mm3)
0.46 · 0.38 · 0.16
2.57–26.39
ꢀ6 6 h 6 6,
ꢀ14 6 k 6 14,
ꢀ27 6 l 6 27
10430
h Range for data collection (ꢁ)
Index ranges
Reflections collected
Independent reflections [Rint
Completeness to h = 26.38 (%)
Absorption correction
Maximum and minimum transmission
Refinement method
]
2716 [0.04381]
98.2
multiscan with SADABS
0.8832 and 0.7110
3369 [0.0326]
93.8
multiscan with SADABS
0.8917 and 0.7866
4944 [0.0223]
99.2
multiscan with SADABS
0.8556 and 0.7962
full-matrix least-squares on F2 full-matrix least-squares on F2 full-matrix least-squares on F2
Data/restraints/parameters
Goodness-of-fit on F2
2716/0/214
1.123
3369/0/197
1.097
4944/73/369
1.047
Final R indices [I > 2r(I)]
R indices (all data)
Largest difference peak and hole (e A
R1 = 0.0559, wR2 = 0.1261
R1 = 0.0703, wR2 = 0.1310
0.660 and ꢀ0.380
R1 = 0.0462, wR2 = 0.1214
R1 = 0.0500, wR2 = 0.1239
1.014 and ꢀ0.378
R1 = 0.0486, wR2 = 0.1200
R1 = 0.0568, wR2 = 0.1264
0.945 and ꢀ0.942
ꢀ3
˚
)
ꢀ
The systematic absences in the diffraction data for 4
were uniquely consistent for the space group P21/n that
yielded chemically reasonable and computationally stable
results of refinement [22]. The complex occupies a crystal-
lographic inversion center.
P1 that yielded chemically reasonable and computationally
stable results of refinement [22]. The complex occupies a
crystallographic inversion center.
There was one solvate molecule present in the asymmet-
ric unit. A significant amount of time was invested in
attempting to identify and refine the disordered molecule.
Bond length restraints were applied to model this molecule
but the resulting isotropic displacement coefficients sug-
gested that the molecule was mobile. In addition, the
refinement was computationally unstable. Option
SQUEEZE of program PLATON [23,24] was used to correct
the diffraction data for diffuse scattering effects and to iden-
tify the solvate molecule. PLATON calculated the upper limit
of volume that can be occupied by the solvent molecule to
2.3.3. Determination of the structure of 6
A
red crystal with approximate dimensions
0.43 · 0.32 · 0.20 mm3 was selected. The initial cell con-
stants were obtained from three series of x scans at differ-
ent starting angles. Each series consisted of 20 frames
collected at intervals of 0.3ꢁ in a 6ꢁ range about x with
an exposure time of 10 s per frame. A total of 44 reflections
were obtained. The reflections were successfully indexed by
an automated indexing routine built in the SMART program.
The final cell constants were calculated from a set of 4508
strong reflections from the actual data collection. A total of
5510 data were harvested by collecting three sets of frames
with 0.3ꢁ scans in x with an exposure time of 30 s per
frame.
3
˚
be 263.4 A , or 30.0% of the unit cell volume. The program
calculated 57 electrons in the unit cell for the diffuse
species.
2.3.4. Determination of the structure of 7
A
yellow crystal with approximate dimensions
The systematic absences in the diffraction data for 6
0.46 · 0.35 · 0.31 mm3 was selected. The initial cell con-
stants were obtained from three series of x scans at differ-
ent starting angles. Each series consisted of 20 frames
ꢀ
were consistent for the space groups P1 and P1. The E-sta-
tistics strongly suggested the centrosymmetric space group

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1173
collected at intervals of 0.3ꢁ in a 6ꢁ range about x with the
exposure time of 10 s per frame. A total of 54 reflections
were obtained. The reflections were successfully indexed
by an automated indexing routine built in the SMART pro-
gram. The final cell constants were calculated from a set
of 5820 strong reflections from the actual data collection.
A total of 12539 data were harvested by collecting four sets
of frames with 0.25ꢁ scans in x with an exposure time of
103 s per frame.
haps more important factor contributing to our results is
the use of aromatic nitriles (benzonitrile derivatives) bear-
ing strongly electron-withdrawing substituents at meta or
para positions relative to the reacting nitrile functional
group. It appears that the presence of these electron with-
drawing groups is required for the reaction represented in
Scheme 1 to occur under the conditions we employ. The
reaction was not observed (as judged by the lack of any sol-
ids or observable color change in the product mixture fol-
lowing treatment under conditions similar to those under
which 3–7 were synthesized) for unsubstituted benzonitrile
or 4-cyanophenol, a benzonitrile derivative bearing an elec-
tron-donating –OH group at the para position relative to
–CN. This provides some further support for generaliza-
tion of the mechanism described by the authors of the ori-
ginal report [9] to the present work. This mechanism was
proposed to involve ligand formation via condensation of
an acetamidine molecule, generated in situ, with either
the acetonitrile solvent, acetamide, or another acetamidine
molecule. All three of these possibilities are reasonable, and
the proposed mechanism is indeed based upon solid litera-
ture precedent [18,25–31]. Regardless of the identity of the
species reacting with acetamidine, attack at the C@N car-
bon by nitrogen is required for the ultimate formation of
the ligand. Clearly, this step of the overall process would
be facilitated by any group that decreases the electron den-
sity at the carbon atom under attack.
The systematic absences in the diffraction data were con-
ꢀ
sistent for the space groups P1 and P1. The E-statistics
ꢀ
strongly suggested the centrosymmetric space group P1
that yielded chemically reasonable and computationally
stable results of refinement [22]. The metal complex occu-
pies a crystallographic inversion center. There are also
two molecules of methanol and two molecules of aaa-tri-
fluoro-p-toluonitrile per molecule of the Ni complex in
the unit cell. The trifluoromethyl group on C(8) is disor-
dered over three positions in a 44:43:13 ratio and was re-
fined with an idealized geometry.
3. Results and discussion
3.1. Preparation of compounds 3–7
Five neutral metallocyclic complexes have been prepared
in ‘‘one-pot’’ syntheses employing reactions of 1 with six
molar equivalents of benzonitrile derivatives substituted
with the strong electron withdrawing groups –CN, –NO2,
or –CF3 at the meta or para positions relative to the reacting
cyano groups in the starting nitrile. The reaction and gener-
alized structure for the products are shown in Scheme 1.
The formation of these compounds represents a general-
ization of the acetonitrile condensation reaction involving
nickel reported in the literature [9] in that a simple mono-
metallic nickel precursor is employed, rather than the
(bimetallic) [Ni2(l-OH)2(tmpa)2]2+. Further, five new nitr-
iles are observed to undergo reactions analogous to that
observed for acetonitrile in the previous work. In contrast
to the original report, solvothermal synthesis in an (appar-
ently) non-reactive solvent is employed in the formation of
3–7, rather than a solvolysis reaction occurring at ambient
or slightly elevated temperature. An additional and per-
3.2. Characterization of products and X-ray structural
determinations on 4, 6 and 7
We were successful in obtaining diffraction-quality sin-
gle crystals of 4, 6, and 7 and determining their structures
via single crystal X-ray methods. Molecular drawings of
4, 6, and 7 are shown in Figs. 1–3, respectively. Selected
bond distances and angles are presented in Table 2. The
compositions of 3 and 5, which do not crystallize well,
are inferred by comparison of their infrared and NMR
spectra to those of 4, 6, and 7, and by elemental analysis.
We also acquired a MALDI (matrix assisted laser desorp-
tion/ionization) mass spectrum for 5, which co-crystallizes
with one molecule of methanol per molecule of 5, that
clearly showed a peak corresponding to the molecular ion
R
N
Ni
N
R
HN
HN
NH
NH
+ byproducts
Ni(OAc)2 4H2O + 6 NC(C6H4)X
R
R
3: NC(C6H4)X = 3-nitrobenzonitrile, R = 3-(NO2)C6H4
4: NC(C6H4)X = 4-nitrobenzonitrile, R = 4-(NO2)C6H4
5: NC(C6H4)X = 1,3-dicyanobenzene, R = 3-(CN)C6H4
6: NC(C6H4)X = 1,4-dicyanobenzene, R = 4-(CN)C6H4
7: NC(C6H4)X =
-trifluoro-p-toluonitrile, R = 4-(CF3)C6H4
Scheme 1.

1174
I.A. Guzei et al. / Inorganica Chimica Acta 359 (2006) 1169–1176
Fig. 3. A molecular drawing of 7 shown with 50% probability ellipsoids.
Only the preferred orientation of the disordered CF3 group at C(8) is
shown. Selected dihedral angles: N(1)–C(1)–C(2)–C(3) = 37.0(3)ꢁ, N(3)–
C(9)–C(10)–C(15) = ꢀ51.4(3)ꢁ.
Fig. 1. A molecular drawing of 4 shown with 50% probability ellipsoids.
Selected dihedral angles: N(1)–C(1)–C(2)–C(3) = 14.7(4)ꢁ, N(5)–C(8)–
C(9)–C(14) = 16.6(4)ꢁ.
estingly, in the three complexes the phenyl rings are tilted
in two different ways; in 4, atoms C(3) and C(14) are on
the opposite sides of the nickel heterocycle Ni–N–C–N–
C–N, Fig. 1, whereas in 6 and 7 the corresponding atoms
(C(3) and C(15) in 6, C(3) and C(15) in 7) are on the same
side of the nickel coordination plane. The Ni–N bond dis-
tances and N–Ni–N angles are in good agreement with
those reported for three relevant Ni(II) complexes, Table
2. The N–Ni–N bite angles are slightly smaller than the int-
erligand N–Ni–N angles in all cases, but all angles are close
to 90ꢁ.
3.3. Density functional theory and natural bonding orbital
studies
In order to understand the molecular bonding in the
complexes described here we performed density functional
theory (DFT) studies of a simplified analog of 3–7,
bis[1,3,5-triazapentadienato]nickel(II) (8), Fig. 4 at the
PBE1PBE/6-311+G* level of theory [32]. The geometrical
parameters of the optimized structure are in excellent
agreement with experimental data from this work and
other selected papers, Table 2. Calculated metal–ligand dis-
Fig. 2. A molecular drawing of 6 shown with 50% probability ellipsoids.
Selected dihedral angles: N(1)–C(1)–C(2)–C(3) = 41.6(3)ꢁ, N(5)–C(9)–
C(10)–C(15) = ꢀ19.0(3)ꢁ.
˚
plus one proton. The observed isotopic pattern closely
matched a calculated isotopic pattern, providing further
support for the formulation of 5.
Complexes 4, 6, and 7 occupy crystallographic inversion
centers in their respective lattices, thus idealized molecular
symmetry (D2h) exceeds crystallographic symmetry (Ci) in
these cases. While the coordination environment of each
Ni atom is square planar, the entire complexes deviate
from planarity as the phenyl substituents are tilted relative
to the six-membered heterocycles Ni–N–C–N–C–N. Inter-
tances can exceed the experimental values by over 0.1 A,
however in the case of 4, 6, and 7 the DFT distances differ
from the observed parameters only by an average of
˚
0.009 A, emphasizing a good choice of the level of theory.
The metal–ligand donor–acceptor interactions in 8 were
studied with the natural bonding orbital theory analysis
[33]. In the 16-electron complex 8 the central Ni metal is
sd-hybridized with the hybridized orbitals oriented at 90ꢁ
to each other. The two pairs of opposite Ni–N bonds are
best described as 3-center 4-electron hypervalent x-bonds.

I.A. Guzei et al. / Inorganica Chimica Acta 359 (2006) 1169–1176
1175
Table 2
Literature, experimental, and theoretical parameters for selected square-planar complexes of Ni(II)
˚
Ni–N (A)
N–Ni–N(bite) (ꢁ)
N–Ni–N (interligand) (ꢁ)
Reference
[11]
Bis[2,4-di(dimethylamino)-1,3,5-triazapentadienato]nickel(II)
1.848
88.81
91.19
1.854
Bis[2,4-dimethyl-1,3,5-triazapentadienato]nickel(II)
1.850
89.31
90.69
[9]
1.851
Bis[2,4-diamino-1,3,5-triazapentadienato]nickel(II)
1.852
88.62
91.38
[15]
1.859
4
6
7
8
1.846(2)
1.853(2)
1.8570(17)
1.8650(17)
1.847(2)
1.856(2)
1.862a
89.37(11)
89.60(7)
89.14(9)
88.80a
90.63(11)
90.40(7)
90.86(9)
91.20a
This work
This work
This work
This work
a
Calculated values for 8 with D2h geometry.
the new compounds for which solid-state structures have
been determined are model examples of coordination com-
pounds containing robust x-bonds.
H
H
H
N
Ni
N
H
N
Ni
N
N
N
N
N
N
N
N
N
H
H
H
H
H
H
5. Supplementary material
H
H
Crystallographic data for the structures of compounds
4, 6, and 7 have been deposited with the Cambridge Crys-
tallographic Data Centre as CCDC271166, CCDC271167,
and CCDC271168, respectively. Copies of the data can be
obtained free of charge on application to The Director,
CCDC, 12 Union Road, Cambridge CB2 1EZ, UK, fax:
int. code +44 1223 336 033, e-mail: data_request@ccdc.
cam.ac.uk or www.ccdc.cam.ac.uk.
H
H
H
H
Fig. 4. Two possible resonance structures of 8. Both structures show pairs
of 3-center, 4-electron x-bonds. A routine natural resonance theory
analysis of 8 failed due to a very high number of energetically similar
possible resonance structures.
The x-bonds are usually highly ionic, collinear, and char-
acterized by very strong nL ! rꢁM–L donor–acceptor inter-
actions. All of these criteria are met in 8. The Ni–N
bonds are strongly polarized toward the nitrogen atoms,
consistent with the natural charge of +0.95 on the nickel
and ꢀ0.78 on each of the four nitrogen atoms. The N–
Ni–N triads are perfectly linear, which is also observed
experimentally in 4, 6, and 7, where the Ni atoms occupy
crystallographic inversion centers. The donor–acceptor
interaction between the sp2 nitrogen lone pair and the anti-
bonding orbital of the opposite Ni–N r-bond is an enor-
mous 103.3 kcal/mol for each N–Ni–N triad. Thus,
complexes 4, 6, and 7 are model examples of coordination
compounds containing robust x-bonds.
Acknowledgments
The authors acknowledge the donors of The Petroleum
Research Fund, administered by the ACS, as well as the
Faculty Research Committee and College of Science and
Allied Health at the University of Wisconsin-La Crosse
for financial support of this work. Further acknowledge-
ment is made to Dr. Martha Vestling, Department of
Chemistry, University of Wisconsin-Madison, for acquir-
ing and assisting in the interpretation of the mass
spectrum.
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