Chelating tris(amidate) ligands: Versatile scaffolds for nickel(ii)

Article abstract of DOI:10.1039/b914301g

The synthesis and characterization of nickel complexes supported by a family of open-chain, tetradentate, tris(amidate) ligands, [N(o-PhNC(O)R) ₃]^3- ([L^R]^3- where R = ⁱPr, ^tBu, and Ph) is described. The complexes [Ni(L ^iPr)]^-, [Ni(L^tBu)]^-, and [Ni(L ^Ph)(CH₃CN)]^- have been characterized by solution-state spectroscopic methods and single crystal X-ray diffraction. Each ligand gives rise to a different primary coordination sphere about the nickel centre. These studies indicate that the ligands' acyl substituents can be used to regulate the coordination mode of the amidate donors to nickel and the coordination number of the nickel centres. In addition, the ability of these complexes to bind cyanide has been explored. These experiments demonstrate that only one of these complexes, [Ni(L^iPr)]^-, is able to irreversibly bind cyanide and can be used to assemble [Et₄N] ₃[Ni(L^iPr)(μ₂-CN)Co(L^iPr)], a cyanide bridged, heterobimetallic complex. The synthesis and characterization of the cyanide containing complexes, including magnetic susceptibility studies, are described. The Royal Society of Chemistry.

Full text of DOI:10.1039/b914301g

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Chelating tris(amidate) ligands: versatile scaffolds for nickel(II)†
Matthew B. Jones,a Brian S. Newell,b Wesley A. Hoffert,b Kenneth I. Hardcastle,a Matthew P. Shoresb and
Cora E. MacBeth*a
Received 16th July 2009, Accepted 15th October 2009
First published as an Advance Article on the web 17th November 2009
DOI: 10.1039/b914301g
The synthesis and characterization of nickel complexes supported by a family of open-chain,
i
tetradentate, tris(amidate) ligands, [N(o-PhNC(O)R)3]3- ([LR]3- where R = Pr, tBu, and Ph) is
described. The complexes [Ni(LiPr)]-, [Ni(LtBu)]-, and [Ni(LPh)(CH3CN)]- have been characterized by
solution-state spectroscopic methods and single crystal X-ray diffraction. Each ligand gives rise to a
different primary coordination sphere about the nickel centre. These studies indicate that the ligands’
acyl substituents can be used to regulate the coordination mode of the amidate donors to nickel and the
coordination number of the nickel centres. In addition, the ability of these complexes to bind cyanide
has been explored. These experiments demonstrate that only one of these complexes, [Ni(LiPr)]-, is able
to irreversibly bind cyanide and can be used to assemble [Et4N]3[Ni(LiPr)(m2-CN)Co(LiPr)], a cyanide
bridged, heterobimetallic complex. The synthesis and characterization of the cyanide containing
complexes, including magnetic susceptibility studies, are described.
Introduction
Nickel complexes supported by amidate-based ligand systems
have been used to investigate the role of nickel in biological
systems1–9 and observe high-valent nickel species.9–18 The majority
of these studies have employed either macrocyclic9–11 or open-
chain chelating ligands1–5,12–16 that stabilize Ni(II) ions in square
Chart 1 Possible coordination modes of amidate donors.
planar coordination geometries. Open-chain ligands that contain
substituted phenylene units into the ligand backbone, can be used
amidate donors and stabilize Ni(II) ions in alternative coordination
to stabilize metals with unique structural properties.26,27 These
geometries have received less attention but have been used to pre-
studies suggested that the ligand’s acyl substitutents could be used
pare nickel complexes that display unique coordination geometries
to regulate exogenous anion binding26 and, in the case of Al(III)
(i.e., trigonal pyramidal17–19) and biomimetic reactivities.7,8
Ligand systems that incorporate amidate donors are attractive
scaffolds for several reasons: (1) The amide functional group
([RNHC(O)R¢]) is readily synthesized in high yields from amine
complexes,27 enforce mononuclear complex formation.
Herein, we report the synthesis and characterization of nickel
complexes supported by a series of these tris(amidate) ligands
i
([N(o-PhNC(O)R)3]3-, R = Pr, tBu, and Ph) and demonstrate that
precursors, making ligands of this type highly modular. (2)
the ligands’ amidate acyl substituents can be used to control both
Substituents on amide-based ligands can be varied to regulate
the coordination number of the nickel ion and the coordination
both electronic and steric features of the resulting transition
mode (Chart 1) of the amidate donors in the resulting metal
metal complexes.17,20–25 (3) Amide functional groups are chemically
complexes. In addition, we describe the ability of these nickel
robust. However, a significant challenge in the application of these
complexes to bind cyanide and show that, in one case, complexes
ambidentate ligands to metal complex design is to control the
of this type can be used to assemble a heterobimetallic complex.
binding mode (Chart 1) so as to afford predictable coordination
geometries.20
Given the modular nature of open-chain amidate ligands, our
laboratory has been exploring the coordination chemistry of
tris(amidate) ligand systems ([N(o-PhNC(O)R)3]3-) derived from
the tris(2-aminophenyl)amine (N(o-PhNH2)3) ligand scaffold. We
have demonstrated that these systems, which incorporate ortho-
Results and discussion
Synthesis
The syntheses of the ligands H3LiPr (H3LiPr = N(o-PhNHC(O)iPr)3
and H3LtBu (H3LtBu = N(o-PhNHC(O)tBu)3) have been re-
cently reported.26,27 The phenyl derivative, H3LPh (H3LPh = N(o-
PhNHC(O)Ph)3), is prepared in good yield (85%) using an anal-
ogous synthetic strategy (see Experimental section for complete
details).
The nickel complexes PPh4[Ni(LiPr)], PPh4[Ni(LtBu)], and
PPh4[Ni(LPh)(CH3CN)] are synthesized using the general route
outlined in Scheme 1. In a typical preparation, the ligand
is reacted with a slight excess (3.1 equivalents) of a metal
aDepartment of Chemistry, Emory University, Atlanta, GA, USA. E-mail:
cmacbet@emory.edu; Fax: 01 404-727-6586; Tel: 01-404-727-7033
bDepartment of Chemistry, Colorado State University, Fort Collins, CO,
USA. E-mail: shores@lamar.colostate.edu; Tel: 01-970-491-7235
† Electronic supplementary information (ESI) available: Crystallographic
data for all complexes, electrochemistry, and additional magnetic data
and fits. CCDC reference numbers 738758–738762. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/b914301g
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 401–410 | 401
©

Scheme 1 General synthetic preparation of (A) PPh4[Ni(LiPr)], (B) PPh4[Ni(LtBu)], and (C) PPh4[Ni(LPh)(CH3CN)].
Table 1 Selected bond lengths (A) and angles (◦) for [Ni(LiPr)]-,
˚
˚
Ni–Namine bond length of 2.0189(15) A. The Namidate–Ni–Namidate
[Ni(LtBu)]-, and [Ni(LPh)]-
bond angles (127.19(7), 118.29(7), and 112.68(7)◦) are close to the
idealized value of 120◦ expected for a perfect trigonal pyramid. The
nickel ion rises ~0.15 A above the equatorial plane defined by the
[Ni(LiPr)]-
[Ni(LtBu)]-
[Ni(LPh)]-
˚
three N-amidate donors toward the vacant axial coordination site.
The isopropyl substituents of the ligand are positioned above the
equatorial plane so that they completely surround this open axial
coordination site. This type of arrangement has been observed
in other nickel complexes supported by open-chain tris(amidate)
ligands.17,18
The geometry of four-coordinate metal centres can be quan-
titatively evaluated using the t4 parameter recently described by
Houser and co-workers.28 This parameter is useful, as the extreme
values of 0.0 and 1.0 correspond to idealized tetrahedral and
square planar geometries, respectively, and idealized intermediate
geometries (seesaw and trigonal pyramidal) fall between these two
values. The t4 value of 0.82 exhibited by [Ni(LiPr)]- is close to the
idealized t4 value expected for an idealized trigonal pyramidal
complex (0.85).
Unlike the nickel ion in [Ni(LiPr)]-, the equatorial plane about
the nickel centre in [Ni(LtBu)]- (Fig. 1B) consists of two N-amidate
donors and one O-amidate donor. The two coordinated N-amidate
donors are positioned so that their tert-butyl substituents are
significantly shielding the nickel centre. The third arm of the ligand
coordinates through an O-amidate donor, which results in the
formation of a seven-membered chelate ring within the complex.
This coordination mode positions the tert-butyl substituent of
the O-amidate donor a greater distance from the nickel centre,
reducing steric strain. The bond lengths of the NCO moiety (N–C
Ni–N1
Ni–N2
Ni–N3
Ni–N4
Ni–N5
Ni–O3
N1–Ni–N2
N1–Ni–N3
N1–Ni–N4
N1–Ni–O3
N1–Ni–N5
N2–Ni–N3
N2–Ni–N4
N3–Ni–N4
Ni–N5–C40
2.0189(15)
1.9542(15)
1.9570(16)
1.9493(16)
2.043(2)
1.976(2)
2.009(2)
2.080(3)
2.078(3)
2.056(3)
2.039(3)
2.012(3)
1.9167(19)
84.25(9)
82.16(9)
86.32(6)
85.39(7)
84.85(7)
79.33(12)
80.01(11)
80.13(12)
102.78
175.14(13)
112.35(13)
124.93(13)
113.45(12)
170.4(4)
112.68(7)
127.19(7)
118.29(7)
110.16(9)
hydride and transmetallated in situ with NiBr2. These steps
yield the corresponding potassium salts of the metal complexes
(e.g., K[Ni(LiPr)]) and two equivalents of the KBr by-product.
The potassium salts are isolable but difficult to crystallize.
However, in situ salt metathesis with tetraphenylphosphonium
bromide ([PPh4]Br) readily affords the tetraphenylphosphonium
salts (e.g., PPh4[Ni(LiPr)]) in reasonable yields (Scheme 1). For each
complex, the tetraphenylphosphonium salt and potassium salt
exhibit nearly identical spectroscopic (IR, UV-visible absorption,
1
and H NMR) signatures, indicating the counter cation is not
significantly altering the coordination mode of the ligand. The
tetraphenylphosphonium salts can be recrystallized to produce
analytically pure materials.
˚
1.290(4) and C–O 1.302(3) A) comprising the O-amidate donor
indicate that the anionic charge is delocalized throughout this unit.
In this coordination environment, the nickel ion is nearly coplanar
Structural characterization
˚
with the equatorial donors and deviates only 0.032 A from the
The nickel complexes PPh4[Ni(LiPr)], PPh4[Ni(LtBu)], and
PPh4[Ni(LPh)(CH3CN)] have been characterized by single-crystal
X-ray diffraction. The molecular structures of these complexes are
illustrated in Fig. 1 and selected bond lengths and angles are given
in Table 1. Crystallographic data and refinement parameters are
listed in Table 2.
The Ni(II) ion in [Ni(LiPr)]- possesses a distorted trigonal
pyramidal coordination geometry. The nickel centre is coordinated
by the three N-amidate donors, which make up the equatorial
plane, and the tertiary amine donor of the ligand backbone
(Fig. 1A). Together these donors form three five-membered chelate
rings about the nickel ion. The complex displays Ni–Namidate
plane.
The overall geometry is distorted trigonal pyramidal (t4
=
˚
0.71). The Ni—Namidate bond lengths (1.976(2) and 2.009(2) A) in
[Ni(LtBu)]- are longer than the Ni–Namidate bond lengths observed
in [Ni(LiPr)]- (Table 1). The Ni–Oamidate bond length in [Ni(LtBu)]- is
the shortest bond within the nickel centre’s primary coordination
˚
sphere (1.9167(19) A).
In contrast to both [Ni(LiPr)]- and [Ni(LtBu)]-, in which the
nickel ions are four-coordinate 16 e- species, the nickel complex
of the [LPh]3- ligand is isolated as an 18 e-, five-coordinate com-
plex, [Ni(LPh)(CH3CN)]- with a coordinated acetonitrile ligand.
(Fig. 1C). The Ni–Namidate bond lengths of 2.078(3), 2.056(3), and
Ph
-
˚
˚
2.039(3) A exhibited by [Ni(L )(CH3CN)] are all longer than
bond lengths of 1.9542(15), 1.9570(16), and 1.9493(16) A and a
402 | Dalton Trans., 2010, 39, 401–410
This journal is The Royal Society of Chemistry 2010
©

Table 2 Crystallographic data for [Ni(LiPr)]-, [Ni(LtBu)]-, [Ni(LPh)]-, [Ni(LiPr)(CN)]2-, and [NiCo(LiPr)2(CN)]3-
PPh4[Ni(LiPr)]·
PPh4[Ni(LtBu)]·
DMF·0.6Et2O
PPh4[Ni(LPh)-
(CH3CN)]·2CH3CN
[Et4N]2[Ni(LiPr)-
(CN)]
[Et4N]3[NiCo(LiPr)2-
(CN)]
2DMF
Empirical formula
Crystal system
Space group
C60H67N6NiO5P
Triclinic
P1
C62.40H72N5NiO4.6
Triclinic
P1
P
C69H56N7NiO3P
Monoclinic
P21/c
10.9218(3)
25.9666(9)
20.1405(7)
90
90.153(17)
90
5711.9(3)
4
C47H73N7NiO3
Monoclinic
P21/n
11.412(6)
18.653(10)
43.91(2)
C85H126CoN12NiO6
Rhombohedral
R3
19.0019(17)
19.0019(17)
21.787(4)
90
90
120
6812.7(15)
3
¯
¯
˚
a/A
11.3871(1)
16.0284(2)
16.8718(2)
115.3630(10)
102.2640(10)
91.6840(10)
2693.69(5)
2
14.4575(8)
15.1016(8)
15.4786(8)
64.786(3)
72.689(3)
77.867(3)
2905.3(3)
2
˚
b/A
˚
c/A
a (◦)
b (◦)
g (◦)
V/A
Z
90
96.676(14)
90
9285(8)
3
˚
8
Crystal size/mm
T/K
Reflections collected
0.50 ¥ 0.32 ¥ 0.30
173(2)
39239
0.52 ¥ 0.40 ¥ 0.18
173(2)
42454
0.33 ¥ 0.26 ¥ 0.20
173(2)
60804
0.42 ¥ 0.20 ¥ 0.15
172(2)
159728
0.13 ¥ 0.13 ¥ 0.05
172(2)
41833
Indep. reflns (Rint
)
10982 (0.0400)
1.036
R1 = 0.0438
wR2 = 0.1162
R1 = 0.0519
wR2 = 0.1221
11735 (0.0581)
1.041
R1 = 0.0505
wR2 = 0.1414
R1 = 0.0745
wR2 = 0.1633
10176 (0.0786)
1.021
R1 = 0.0673
wR2 = 0.1795
R1 = 0.1053
wR2 = 0.1910
11486 (0.0843)
1.056
R1 = 0.0793
wR2 = 0.1873
R1 = 0.0925
wR2 = 0.1954
8677 (0.0971)
1.021
R1 = 0.0733
wR2 = 0.1488
R1 = 0.1447
wR2 = 0.1663
Goodness-of-fit on F2
Final R indices [I > 2s(I)]
R indices (all data)
Fig. 1 Thermal ellipsoid diagrams of (A) [Ni(LiPr)]-, (B) [Ni(LtBu)]-, and (C) [Ni(LPh)(CH3CN)]- drawn at 35% probability. Hydrogen atoms and counter
cations have been removed for clarity.
2
the Ni–Namidate bond lengths observed in [Ni(LiPr)]- and [Ni(LtBu)]-
Specifically, hexacoordinate, mononuclear tris(k -amidate) alu-
˚
(Table 1). In addition, the Namine–Ni bond length (2.080 A) in
minium(III) complexes were isolated and characterized with both
[Ni(LPh)(CH3CN)]- is longer than the Namine–Ni bond lengths
the [LiPr]3- and [LtBu
]
ligands. However, attempts to isolate the
3-
observed in the four-coordinate species, presumably due to the
analogous complex using the methyl ligand derivative, [LMe]3-,
were unsuccessful and lead only to the formation of multinuclear
species. These results suggest that larger acyl substituents are
required to ensure the formation of mononuclear complexes with
these ligands.
trans influence exerted by the acetonitrile ligand. The Namine
–
Ni–NNCCH3 bond angle is nearly linear (175.14(13)◦) allowing the
phenyl substituents of the ligand to orient about the coordinated
acetonitrile in a bowl-like cavity structure.
Taken together, these structural data suggest that the relative size
of the ligands’ amidate acyl substituents can significantly influence
both the coordination mode of the ligand and the coordination
number of the resulting transition metal complexes. To further
probe this trend, attempts were made to synthesize the methyl
congener of this series using the H3LMe ligand (where [H3LMe] =
N(o-PhNHC(O)CH3)3). Unfortunately, these experiments led only
to complicated reaction mixtures. We have observed similar results
when attempting to prepare aluminium-containing analogues.27
Spectroscopic characterization
The nickel complexes have been characterized by 1H NMR,
FT-IR, and UV-visible absorption spectroscopies. All three com-
plexes ([Ni(LiPr)]-, [Ni(LtBu)]-, and [Ni(LPh)(CH3CN)]-) exhibit
paramagnetically-shifted 1H NMR spectra. The 1H NMR (25 ◦C,
CD3CN) spectra of [Ni(LiPr)]- and [Ni(LPh)(CH3CN)]- are indica-
tive of pseudo C3-symmetric species in solution (see Experimental
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 401–410 | 403
©

section). In contrast, the spectrum of [Ni(LtBu)]-, under identical
experimental conditions is significantly more complicated, exhibit-
ing seventeen paramagnetically-shifted resonances consistent with
a lower symmetry (C1) species. Variable temperature (-60 to 30 ◦C)
1H NMR spectroscopy was conducted on both [Ni(LiPr)]- and
[Ni(LtBu)]- in d6-acetone and confirmed non-fluxional solution-
state behaviour of these species over this range of temperatures.
The magnetic moments of [Ni(LiPr)]-, [Ni(LtBu)]-, and
[Ni(LPh)(CH3CN)]- were obtained using solution phase 1H NMR
methods.29,30 The four-coordinate species, [Ni(LiPr)]- and
[Ni(LtBu)]-, exhibit meff values of 3.03 and 3.37 mB, respec-
tively, indicative of S = 1 ground states. The five-coordinate
[Ni(LPh)(CH3CN)]- exhibits a meff value of 3.27 mB (25◦, d6-DMSO)
similar to other five-coordinate Ni(II) species having high-spin,
S = 1 ground-states.31–37 The UV-visible absorption spectra for
[Ni(LiPr)]-, [Ni(LtBu)]-, and [Ni(LPh)(CH3CN)]- were recorded at
room temperature as N,N-dimethylformamide (DMF) solutions.
The four-coordinate complexes, [Ni(LiPr)]- and [Ni(LtBu)]-, exhibit
similar spectra with lmax values at 525 and 520 nm, respectively.
The five-coordinate species, [Ni(LPh)(CH3CN)]-, exhibits a broad
absorbance centred at 707 nm. A similar broad absorbance
(694 nm) has been observed and its transitions assigned (3E¢(F) →
3A1¢(F), 3A2¢(F)) for a high-spin trigonal bipyramidal Ni(II)
complex containing an N2O3 donor set.32,38
Fig. 2 Thermal ellipsoid diagrams of the two independent anions found in
the asymmetric unit of [Et4N]2[Ni(LiPr)(CN)]; (A) [Ni1(LiPr)(CN)]2- and (B)
[Ni2(LiPr)(CN)]2- drawn at 35% probability. Hydrogen atoms and counter
˚
cations have been removed for clarity. Selected bond lengths (A) and angles
(◦) for (A) Ni1–N1 2.091(5), Ni1–N2 2.027(4), Ni1–N3 2.083(4), Ni1–N4
2.122(5), Ni1–C31 2.018(6), N4–Ni1–C31 158.9(2), N3–Ni1–N1 143.25,
N2–Ni1–C31 116.7(2), N4–Ni1–N3 76.29(18), N4–Ni1–N1 80.77(18),
Ni1–C31–N5 167.2(6); and (B): Ni2–N9 2.139(4), Ni2–N8 2.023(5),
Ni2–N7 2.101(5), Ni2–N6 2.089(5), Ni2–C62 2.019(7), N9–Ni2–C62
169.0(2), N6–Ni2–N7 133.76(17), N6–Ni2–N8 106.21(18), N7–Ni2–N8
106.65(18), Ni2–C62–N10 173.6(6).
and square pyramidal geometries can be quantified by using the t5
parameter defined by Addison and Reedijk.40 The value of t5 varies
between 0.0 (for idealized square pyramidal geometry) and 1.0 (for
idealized trigonal bipyramidal geometry). Applying this structural
parameter to [Ni1(LiPr)(CN)]2- and [Ni2(LiPr)(CN)]2- gives rise to t5
values of 0.26 and 0.59, respectively, illustrating their intermediate
geometries. Co-crystallization of geometrical40–46 or polytopal
isomers46–48 within the same unit cell has been observed for
other transition metal systems. Co-crystallization of these species
is rare, however, because compounds with different molecular
structures typically possess different crystallization kinetics and
crystal lattice packing energies.49 These structural data suggest that
the two geometries observed for [Ni(LiPr)(CN)]2- in the solid-state
are very similar in energy and that, in solution, a distribution of
these geometries must exist. In solution, [Ni(LiPr)(CN)]2- displays
a single, quazi-reversible electrochemical event (50 mV s-1) in its
cyclic voltammogram centred at -251 mV (Fig. S1 in ESI†). We
tentatively assign this process to the NiII/III couple.
When the [Ni(LtBu)]- complex is treated with cyanide, a
slight colour change from orange to red-orange is observed.
This reaction, however, does not give rise to a single, well-
defined product in solution. Both solution-state IR and 1H
NMR spectroscopy indicate that cyanide does interact to some
extent with the nickel centre. For example, in the solution-state
FT-IR spectrum (CH3CN) a major nCN stretching band appears
at 2109 cm-1, suggesting coordination of the cyanide ligand
in a terminal fashion,39 similar to what is observed for the
isolated [Ni(LiPr)(CN)]2- species (vide supra). However, several
lower intensity, higher frequency bands also appear (2150 cm-1
and 2186 cm-1) that cannot be definitely assigned. The 1H NMR
spectrum (CD3CN) also reveals the presence of at least two major
species in solution. One of the species is paramagnetic and exhibits
a spectrum nearly identical to the [Ni(LtBu)]- starting material.
The second species is diamagnetic and exhibits three distinct tBu
resonances. These data are consistent with the existence of a
solution-state equilibrium between a diamagnetic, square planar
complex and a paramagnetic species. We propose that cyanide
Cyanide coordination
The differences in coordination number and environment exhib-
ited by [Ni(LiPr)]-, [Ni(LtBu)]-, and [Ni(LPh)(CH3CN)]- prompted us
to explore the ability of these complexes to coordinate exogenous
ligands. Specifically, we sought to explore the ability of these
complexes to bind cyanide because it is a small, linear donor.
The orange [Ni(LiPr)]- complex reacts readily with one equivalent
of tetraethylammonium cyanide ([Et4N]CN) to produce a green
solution. We formulate the nickel-containing product of this
reaction to be [Ni(LiPr)(CN)]2- by spectroscopic match to an
authentic sample prepared directly from the protio ligand in a
one-pot procedure (see Experimental section for details).
The dianionic cyanide complex, [Ni(LiPr)(CN)]2-, exhibits a
cyanide stretch in the IR spectrum at 2112 cm-1 consistent with a
terminally bound cyanide ligand.39 This complex is paramagnetic
and exhibits a meff value of 3.25 mB (25◦, d6-DMSO) indicating
a high-spin, S = 1 system (vide supra). This species can be
recrystallized by the slow diffusion of diethyl ether into a
concentrated DMF solution of the complex to afford X-ray quality
crystals. Results of the single crystal X-ray diffraction studies
carried out on [Et4N]2[Ni(LiPr)(CN)] are shown in Fig. 2. Crystals
of [Et4N]2[Ni(LiPr)(CN)] form in a P21/n space group with Z = 8.
There are two crystallographically independent and geometrically
dissimilar [Ni(LiPr)(CN)]2- units within the asymmetric unit cell.
Both nickel centres are five-coordinate and contain the same
donor atoms within their primary coordination sphere. Each Ni(II)
ion is coordinated by three N-amidate and one tertiary amine
donor of the chelating ligand and a terminal cyanide ligand. In
one of the anions, [Ni1(LiPr)(CN)]2- (Fig. 2A), the nickel centre
displays a distorted square pyramidal geometry. The other nickel
centre, [Ni2(LiPr)(CN)]2- (Fig. 2B), exhibits a distorted trigonal
bipyramidal coordination geometry. For five-coordinate species,
the degree of distortion between idealized trigonal bipyramidal
404 | Dalton Trans., 2010, 39, 401–410
This journal is
The Royal Society of Chemistry 2010
©

coordination to the nickel centre in [Ni(LtBu)]- may cause one
of the coordinated ligand arms to dissociate to afford a square
planar species. Similar solution-state equilibria have recently been
observed for nickel complexes supported by scorpionate ligands.50
The [Ni(LPh)(CH3CN)]- complex also reacts with cyanide to pro-
duce a complex reaction mixture that contains both paramagnetic
and diamagnetic products. This result was somewhat surprising
as the solid-state data obtained for [Ni(LPh)(CH3CN)]- clearly
demonstrates the ability of this ligand to support five-coordinate
Ni(II) species. One possible explanation for this result is that the
association constant for acetonitrile binding to [Ni(LPh)]- is greater
than the association constant for cyanide. Further studies probing
these observations are underway in our laboratories.
fitted |D| and TIP values are significantly larger than might
be expected. The former may be due to weak intermolecular
interactions being incorporated into the phenomenological D
parameter (although no obvious exchange pathways are present
in the crystal structures); the latter may be indicative of low lying
excited spin states, and similar values have been reported for a
Ni(II) porphyrin complex.53 Nevertheless, we considered two other
alternatives. The potential coupling of a Ni(I) center (S = 12 ) with
a radical dianionic ligand did not result in any reasonable fits to
the data. Alternatively, antiferromagnetic coupling of a high spin
1
Ni(III) center (S = 3/2) with a radical tetraanionic ligand (S =
)
afforded large intramolecular exchange coupling (J ~ -5000 cm-21)
and a reasonable gNi of ~2.3, but only when the radical g was fixed
at 2.00; even then, the fits were inferior to the original scenario.
To examine magnetic anisotropy in more detail, we collected
magnetization data at various fields between 2 and 35 K (Fig. 3, in-
set). The data collected at different fields do not overlay each other,
and deviate significantly from the Brillouin function expected
for S = 1, indicating significant zero-field splitting. However,
modeling the data with ANISOFIT54 does not afford satisfactory
fits, even when we restrict fitting to the lowest temperatures
(< 14 K). The lack of agreement is likely due to the presence of low
lying excited spin states; unfortunately ANISOFIT requires well
isolated ground spin states to give the best fits. We conclude that
an S = 1 Ni(II) ion with significant (but complicated) zero-field
splitting is operative in these compounds.
Solid-state magnetic properties of selected mononuclear Ni
complexes
The unusual ligand field environment for Ni(II) ions pro-
vided by the tris-amidate ligand architecture may influence
magnetic anisotropy parameters, which are of interest in the
field of molecular magnetism.51,52 Thus, solid-state magnetic
susceptibility studies were undertaken for Ph4P[Ni(LiPr)] and
[Et4N]2[Ni(LiPr)(CN)]. A plot of the temperature dependence of
cMT for [Et4N]2[Ni(LiPr)(CN)] (obtained at 0.1 T) appears in Fig. 3.
Here, the cMT value of 1.46 emu K mol-1 at 300 K is significantly
larger than that expected for a mononuclear Ni(II) complex with
S = 1 and g = 2.00 (1.00 emu K mol-1). The product decreases
gradually to 1.32 emu K mol-1 at 50 K, followed by a steep drop to
0.56 emu K mol-1 at 2 K. Qualitatively similar data are obtained
for Ph4P[Ni(LiPr)] (ESI Fig. S2†).
Formation of a heterobimetallic complex
The ability of [Ni(LiPr)]- to irreversibly bind cyanide in solution is
unique for this series of complexes. We next sought to investigate
whether [Ni(LiPr)(CN)]2- could be used to form heterobimetallic
complexes. Our motivation for this study is based upon the
widespread utility of terminal cyanide complexes in the assembly
of molecular, cyanide-bridged clusters via the “building block
approach.”51,52,55–57 Cyanide complexes with terminal cyanide
ligands or open binding sites are often used as capping or
blocking groups in the construction of these species. Thus, the
reaction of green [Ni(LiPr)(CN)]2- with a four-coordinate, trigonal
pyramidal Co(II) complex of the same ligand, [Co(LiPr)]-,26 im-
mediately produces a reddish-violet solution. The product of this
reaction is isolated as a reddish-purple solid in good yield (83%).
FT-IR studies suggest a bridging coordination mode of the cyanide
ligand, as the product exhibits a single nCN (KBr) stretch at
2126 cm-1. The increase in cyanide stretching frequency observed
for the product compared to the mononuclear nickel cyanide
precursor, is consistent with the formation of a cyanide-bridged
species.55
Crystals of X-ray quality were grown by the diffusion of
diethyl ether into a concentrated DMF solution of the prod-
uct. The molecular structure of the heterobimetallic complex,
[Et4N]3[CoNi(LiPr)2(m2-CN)] is shown in Fig. 4. The trianionic
complex crystallizes in a rhombohedral space group (R3) and
adopts 3-fold symmetry. The crystals contained poorly resolved
solvent peaks and there was some disorder in the pendant
arms of the tetraethylammonium cations. Nevertheless, the
[CoNi(LiPr)2(m2-CN)]3- unit is well-resolved. To be confident that
the correct molecular geometry had been assigned, the nickel
and cobalt atoms were exchanged as well as the carbon and
Fig.
3
Temperature dependence of magnetic susceptibility for
[Et4N]2[Ni(LiPr)(CN)] obtained at a measuring field of 0.1 T. Best fits to
the data (red line) give g = 2.295, D = -23.03 cm-1, E = -2.60 cm-1, TIP =
475 ¥ 10-6 emu mol-1, and relative error f = 0.005. Inset: Magnetization of
[Et4N]2[Ni(LiPr)(CN)] as a function of reduced magnetic field.
Fitting the susceptibility data to a magnetic model, we first con-
sidered the most likely scenario, in which unpaired spin localizes
on the Ni center (i.e. Ni(II), S = 1). For Ph4P[Ni(LiPr)], the best fit
affords g = 2.35, D = -19.44 cm-1, E = -1.46 cm-1, temperature in-
dependent paramagnetism (TIP) = 808 ¥ 10-6 emu mol-1, and rela-
tive error f = 0.018. Similarly, the best fit for [Et4N]2[Ni(LiPr)(CN)]
gives g = 2.295, D = -23.03 cm-1, E = -2.60 cm-1, TIP = 475 ¥
10-6 emu mol-1, and relative error f = 0.005. In both cases, the
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 401–410 | 405
©

toward the bridging cyanide. This type of co-linear arrangement
of atoms within multimetallic assemblies has been previously
observed in binuclear Cu(II) cryptates68,69 and in Co(II) cyanide
clusters assembled with capping ligands.70 These results suggest
that transition metal complexes supported by [LR]3- type ligands
maybe useful as capping species in the assembly of larger molecular
clusters.71
Magnetism of [Et4N]3[CoNi(LiPr)2(l2-CN)]
A plot of the temperature dependence of cMT (obtained at 0.1 T)
appears in Fig. 5. The cMT value at 300 K is 5.07 emu K mol-1,
which again is significantly larger than expected for non-
interacting S = 1 Ni(II) and S = 3/2 Co(II) spin centres
(2.875 emu K mol-1). The product decreases linearly (due to Co(II)
orbital moment and/or TIP) to 3.59 emu K mol-1 at 70 K. At
lower temperatures, the susceptibility drops off more rapidly to
0.86 emu K mol-1 at 3 K. Using JulX,72 and assuming no zero-field
splitting, the best fit to the data indicates weak antiferromagnetic
coupling between the Co(II) and Ni(II) ions (J = -2.04 cm-1). This
is consistent with expected weak superexchange (through cyanide
p* orbitals) between the singly-occupied molecular orbitals of the
constituent ions, which for Ni(II) and Co(II) ions should have the
same symmetries.51,73 A large TIP value is observed, consistent
both with the Ni-CN complex [Et4N]2[Ni(LiPr)(CN)] as well as
distortion of the local coordination sphere around the Co(II),
which is known to give large TIP and Zeeman contributions to
the susceptibility.74
Fig. 4 Thermal ellipsoid diagrams (35%) of (A) [CoNi(LiPr)2(m2-CN)]3-
and (B) the core structure of [CoNi(LiPr)2(m2-CN)]3-. Hydrogen atoms
and counter ions have been removed for clarity. Selected bond lengths
◦
˚
(A) and angles ( ): Co–N1 2.354(9); Co–N3 2.035(4); Co–C11 2.029(13);
Ni–N4 2.390(9); Ni–N5 2.030(4); Ni–N2 2.063(12); C11–N2 1.167(7);
N1–Co–C11 180.00(1); C11–N2–Ni 180.00(1); C11–Co–N3 105.59(11);
N3–Co–N3¢ 113.06(9); N3–Co–N1 74.41(11); N2–Ni–N5 105.86(11);
N5–Co–N5¢ 112.83(9); N5–Ni–N4 74.14(11).
nitrogen atoms of the bridging cyanide ligand and their final
positions verified after least squares analysis by critical evaluation
of their respective atomic displacement parameters. Note that
the orientation of the cyanide bridging ligand in [CoNi(LiPr)2(m2-
CN)]3- is different from that observed in the [Et4N]2[Ni(LiPr)(CN)]
precursor, i.e., the cyanide ligand is coordinated to the nickel centre
via the nitrogen atom. This type of cyanide flipping or linkage
isomerism has been observed in the formation of other bridged
cyanide clusters.58
The Ni(II) and Co(II) centres both display distorted trigonal
bipyramidal coordination geometries with M(II)–Namidate bond
length similar to those observed in the mononuclear complexes.
Fig.
5 Temperature dependence of magnetic susceptibility for
[Et4N]3[CoNi(LiPr)2(m2-CN)] obtained at a measuring field of 0.1 T. As-
suming no zero-field splitting (D and E fixed at 0), best fits to the data (red
line) give J = 2.04 cm-1, g = 2.15, TIP = 6600 ¥ 10-6 emu mol-1, and relative
error f = 0.11. Inset: Magnetization of [Et4N]3[CoNi(LiPr)2(m2-CN)] as a
function of reduced magnetic field.
˚
Each metal centre displays relatively long (< 2.3 A) M(II)–Namine
bond lengths.59 Long M(II)–Ntertiary amine bond distances have been
observed in other dinuclear Ni(II)60–64 and Co(II)65–67 complexes
supported by sterically demanding ligands. The M(II)–Napical
elongation observed in [CoNi(LiPr)2(m2-CN)]3- is likely to be due
to the geometric distortions that occur upon formation of the
bimetallic complex. Specifically, the two [LiPr]3- ligands which cap
the Co(II) and Ni(II) centres are positioned so that the isopropyl
substituents are interlocked about the bridging cyanide ligand,
As with the mononuclear complexes, magnetization data (inset,
Fig. 4) clearly show significant magnetic anisotropy; however,
this data does not yield reasonable fits using ANISOFIT. In
addition to the issues described above for the mononuclear Ni(II)
precursor, it has been shown that fitting magnetization data with
Co(II) complexes can be difficult due to the mixing of low-lying
excited states into the ground state,75 as well as the presence of
Zeeman effects. Evidence for the latter behavior is shown in the
ESI, Fig. S3,† where the susceptibility data at higher fields tracks
≡
forcing a completely linear N1–Co–C N–Ni–N4 arrangement
of atoms within in the molecule. In addition, both the Co(II)
˚
and Ni(II) centres are distorted ~0.55 A away from the trigonal
planes formed by the N-amidate donors of the [LiPr]3- ligands
406 | Dalton Trans., 2010, 39, 401–410
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The Royal Society of Chemistry 2010
©

under those obtained at lower fields.76 If D is included in the
fits to the susceptibility data (ESI, Fig. S4 and S5†), the quality
improves somewhat when the parameter is allowed to refine freely,
affording D = 20 cm-1. As |D| values for Co(II) complexes are
typically 1 cm-1 to 5 cm-1,77 this large value would appear to trace
back to the presence of Ni(II), similar to the mononuclear species.
In spite of the rather weak coupling, the apparently large values of
magnetic anisotropy merit further study. Efforts along these lines
are underway.
samples were prepared under an N2 atmosphere. Solution state
magnetic moments were measured using the Evans method.29,30
Mass spectra were recorded in the Mass Spectrometry Center
at Emory University on a JEOL JMS-SX102/SX102A/E mass
spectrometer. Cyclic voltammetric experiments were carried out
using a CH Instruments (Austin, TX) Model 660C potentiostat.
All electrochemistry experiments were conducted in DMF with
0.20 M tetrabutylammonium hexafluorophosphate as the sup-
porting electrolyte. Electrochemical experiments were conducted
using a three-component cell consisting of a Pt auxiliary electrode,
a non-aqueous reference electrode (Ag/AgNO3), and a glassy
carbon working electrode. All electrochemical measurements are
referenced and reported versus the ferrocene/ferrocenium couple.
PPh4CN was prepared using a literature procedure.78 The ligand
Conclusions
In summary, nickel(II) complexes supported by a family of chelat-
ing tris(amidate) ligands, [N(o-PhNC(O)R)3]3- ([LR]3- where R =
iPr, tBu, and Ph), have been prepared and characterized. The nickel
centres in [Ni(LiPr)]- and [Ni(LtBu)]- exhibit similar coordination ge-
ometries but different primary coordination spheres. In [Ni(LiPr)]-,
the four-coordinate Ni(II) ion is ligated by a tertiary amine and
three N-amidate donors of the ligand. In contrast, the nickel centre
in [Ni(LtBu)]- is coordinated by a tertiary amine, one O-amidate
donor, and two N-amidate donors of the ligand. The phenyl
ligand derivative, [LPh]3-, stabilizes nickel(II) as a five-coordinate
solvento adduct, [Ni(LPh)(CH3CN)]-. We propose that the amidate
substituents are regulating the coordination motifs observed in this
series of complexes. The ability of these nickel complexes to bind
cyanide has also been investigated. Only one of the complexes,
[Ni(LiPr)]-, irreversibly binds cyanide to form an isolable cyanide
complex, [Ni(LiPr)(CN)]2-. [Ni(LiPr)(CN)]2- is capable of adopting
either trigonal bipyramidal or square pyramidal geometries in
the solid-state and can be used to assemble a cyanide-bridged,
heterobimetallic complex. The unusual ligand fields presented
by this family of ligands engender interesting, if complicated,
magnetic behaviour in the Ni(II) complexes studied, and merit
more in-depth investigation. Overall, these studies demonstrate
that these chelating tris(amidate) ligands offer highly tuneable and
versatile scaffolds for Ni(II).
26
27
precursor, N(o-PhNH2)3,26 and the ligands H3LiPr and H3LtBu
were synthesized using previously published procedures.
2,2¢,2¢¢-trisphenylamidotriphenylamine, H3LPh
A
suspension of N(o-PhNH2)3 (2.01 g, 6.93◦ mmol) in
dichloromethane (DCM, 40 mL) was lowered to 0 C under an
atmosphere of N2. Triethylamine (3.09 mL, 22.2 mmol) was then
added, followed by benzoyl chloride (2.57 mL, 22.2 mmol). The
mixture was stirred for 90 min and was allowed to warm to room
temperature. The reaction mixture was washed with aqueous HCl
(0.1 M, 100 mL), dried over magnesium sulfate, and concentrated
in vacuo; yielding a green oil. Crystals of the product were obtained
by layering petroleum ether onto a concentrated DCM solution
and cooling to -40 ◦C (3.53 g, 85%). 1H NMR (d, CD3CN,
300 MHz): 9.16 (s, 3H, NH), 7.66 (t, 3H, J = 4.2 Hz, ArH),
7.45 (t, 3H, J = 7.5 Hz, ArH), 7.40 (d, 6H, J = 7.2 Hz, ArH),
7.30 (t, 6H, J = 7.5 Hz, ArH), 7.10 (t, 6H, J = 3.9 Hz, ArH),
6.90 (m, 3H, ArH). 13C NMR (d, CD3CN, 300 MHz): 166.55,
139.46, 135.43, 133.09, 133.03, 129.50, 128.52, 127.45, 126.69,
126.13, 125.76. HRESI-MS: C39H31O3N4 m/z Calcd. 603.23907
Found 603.23914 [M+1]+. FTIR (KBr, cm-1): n(NH) 3273, n(CO)
1655.
Experimental
[Ph4P][Ni(LiPr)]
General considerations
To a solution of H3LiPr (H3LiPr = N(o-PhNHC(O)iPr)3) (117 mg,
0.23 mmol) in dry DMF (3 mL) was added solid potassium
hydride (31 mg, 0.77 mmol). When hydrogen gas evolution ceased
(~ 40 min), NiBr2 (51 mg, 0.23 mmol) was added as a solid and the
reaction stirred for four hours. Tetraphenylphosphonium bromide
(98 mg, 0.23 mmol) was added to the deep orange solution as
a solid. After stirring for 1 h, the solution was concentrated
in vacuo to yield an orange solid. The resulting orange solid was
then dissolved in CH3CN and filtered through a sintered glass
frit to remove KBr. The filtrate was concentrated under reduced
pressure and the product isolated as an orange solid. X-ray quality
crystals were obtained by the slow diffusion of diethyl ether into
a DMF solution of the complex to give orange crystals (109 mg,
All manipulations were carried out using standard Schlenk tech-
niques or conducted in an MBraun Labmaster 130 drybox under
a dinitrogen atmosphere. All reagents used were purchased from
commercial vendors and used as received unless otherwise stated.
Anhydrous solvents were purchased from Sigma-Aldrich and fur-
ther purified by sparging with Ar gas followed by passage through
activated alumina columns. Anhydrous NiBr2 was purchased from
Strem Chemical, Inc. Deuterated solvents were purchased from
Sigma Aldrich or Cambridge Isotope Laboratories, Inc. and
degassed and dried according to standard procedures prior to
use. Elemental analyses were performed by Columbia Analytical
Services, Tucson, AZ or Atlantic Microlab, Inc., Norcross, GA.
1H and 13C NMR spectra were recorded on a Varian Mercury
300 MHz spectrometer at ambient temperature. Chemical shifts
were referenced to residual solvent peaks. Infrared spectra were
recorded as KBr pellets on a Varian Scimitar 800 Series FT-IR
spectrophotometer. UV-Visible absorption spectra were recorded
on a Cary 50 spectrophotometer using 1.0 cm quartz cuvettes. All
1
68%). H NMR (d, CD3CN, 300 MHz): 21.38 (br), 19.98 (sh),
+
7.64 m, 16H and 7.86 t, 4H -PPh4 , 2.16 (br). FTIR (KBr, cm-1):
n: 3059, 3020, 2959, 2926, 2866, 1677 (CODMF), 1604 (CO), 1578,
1476, 1440, 1386, 1295, 1276, 1207, 1108, 1040, 967, 767, 724,
690, 527. meff = 3.03 mB (Evans Method, (CD3)2SO, 298 K).
l
max(e, M-1 cm-1) (DMF): 401(794) 525 (69) Anal. Calcd (found)
Dalton Trans., 2010, 39, 401–410 | 407
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©

for Ph4P[NiLiPr]·2DMF: C, 69.17 (69.35); H, 6.48 (6.35); N, 8.07
0.14 mmol) was added as a solid and the reaction stirred for
4 h. Tetraethylammonium bromide (29 mg, 0.14 mmol) was
added to the deep orange solution as a solid and stirred for
1 h. Tetraethylammonium cyanide (21 mg, 0.14 mmol) was then
added as a solid and the reaction stirred for one hour. Solvent
was removed in vacuo and the brown-green solid was washed with
THF to yield a light green solid. Green, X-ray quality crystals
were obtained by vapor diffusion of diethyl ether into DMF
(97 mg, 85%). 1H NMR (d, CD3CN, 300 MHz): 37.41 (br), 12.70
(sh), 10.56 (sh), 5.34 (sh), 3.12 (q, (CH3-CH2)4N), 1.16 (t, (CH3-
CH2)4N), -7.38 (sh), -15.49(sh). FTIR (KBr, cm-1): n(CN) 2112.
meff = 3.25 mB (Evan’s Method, (CD3)2SO, 298 K). lmax(e, M-1cm-1)
(MeCN): 654(32). Anal. Calcd (found) for [Et4N]2[N(LiPr)(CN)]:
C, 66.98 (66.65); H, 8.73 (8.45); N, 11.63 (11.64). Electrochemistry
(DMF, 0.2 M [n-Bu4N]PF6) E1/2 = -251 mV (vs. Fc/Fc+), DEp =
115 mV, ipf/ipr = 0.76.
(8.15).
[Ph4P][Ni(LPh)(MeCN)]
To a stirred solution of H3LPh (H3LPh = N(o-PhNHC(O)Ph)3)
(361 mg, 0.60 mmol) in DMF (5 mL) was added KH (79 mg,
1.98 mmol) as a solid. When H2 evolution ceased (~30 min),
NiBr2 (131 mg, 0.60 mmol) was added as a solid. The pale
yellow solution turned orange as the metal salt dissolved over a
period of four hours. Tetraphenylphosphonium bromide (251 mg,
0.60 mmol) was added as a solid. After 30 min of stirring, solvent
was removed under reduced pressure. The resulting yellow oil was
dissolved in acetonitrile (40 mL) and filtered to remove KBr. The
filtrate was concentrated under reduced pressure, and the resulting
yellow-green solid was recrystallized from acetonitrile (409 mg,
66%). X-ray quality crystals were obtained by vapor diffusion
of diethyl ether into a concentrated acetonitrile solution of the
complex. 1H NMR (d, CD3CN, 300 MHz): 48.36 (br), 21.14
[Et4N]3[Ni(LiPr)(l2-CN)Co(LiPr)]
(sh), 12.41 (sh), 9.18 (sh), 7.73 t, 4H and 7.53 m, 16H -PPh4 ,
To a solution of (Et4N)2[NiN(o-PhNC(O)iPr)3CN] (117 mg,
0.14 mmol) in DMF (~ 4 mL) was added Et4N[CoN(o-
PhNC(O)iPr)3] (95 mg, 0.14 mmol) as a DMF solution. After 2 h of
stirring, diethyl ether (~ 5 mL) was added to precipitate the product
as a reddish-purple solid. The solid was collected by filtration,
washed with acetonitrile (~ 5 mL) and diethyl ether (~ 5 mL), and
dried in vacuo (175 mg, 83%). X-ray quality crystals were grown by
slow diffusion of diethyl ether into a concentrated DMF solution
+
-8.63 (sh), -29.87 (br). FTIR (KBr, cm-1) n: 3056, 3021, 2927,
2247 (CH3CN), 1596 (CO), 1584, 1552, 1473, 1442, 1357, 1109,
1041, 997, 914, 754, 723, 690, 527. meff = 3.27 mB (Evans Method,
DMSO-d6, 298 K). lmax(e, M-1cm-1) (DMF): 707(16). Sample for
elemental analysis was prepared by the diffusion of diethyl ether
into a concentrated DMF solution of the complex. The presence
1
of one DMF solvent per complex was confirmed by H NMR
+
1
and integrated versus the PPh4 counterion. Anal. Calcd (found)
of the product. H NMR (d, (CD3)2SO, 400 MHz): 37.29 (br),
for Ph4P[NiN(o-PhNC(O)Ph)3(MeCN)]·DMF: C, 73.45 (73.32);
21.25 (br), 20.15 (br), 17.72 (sh), 12.96 (br), 12.65 (sh), 10.68 (sh),
9.34 (br), 7.58 (sh), 6.36 (sh), 5.55 (sh), 3.21 (q, (CH3-CH2)4N),
1.16 (t, (CH3-CH2)4N), 0.39 (br), -1.37 (br), -7.16 (sh), -15.68
(sh). FTIR (KBr, cm-1): n(CN) 2126. lmax (e, M-1cm-1) (DMF):
572(281), 834(40). Anal. Calcd (found) for [Et4N]3[(NiN(o-
PhNC(O)iPr)3)(CoN(o-PhNC(O)iPr)3)CN]: C, 66.98 (66.65); H,
8.73 (8.45); N, 11.63 (11.64).
H, 5.17 (5.01); N, 7.56 (7.67).
[Ph4P][Ni(LtBu)]
To a solution of H3LtBu (H3LtBu = N(o-PhNHC(O)tBu)3) (426 mg,
0.79 mmol) in dry DMF (50 mL) was added solid potassium
hydride (104 mg, 2.59 mmol). A colorless precipitate formed.
When all of the solid dissolved, NiBr2 (172 mg, 0.79 mmol)
was added as a solid and the reaction stirred for four hours.
Tetraphenylphosphonium bromide (329 mg, 0.79 mmol) was
added to the deep orange solution as a solid. After stirring
for 1 h, the solution was concentrated in vacuo. The resultant
orange powder was dissolved in MeCN and filtered to yield the
final product as an orange solid. X-ray quality crystals could be
obtained by the slow diffusion of diethyl ether into DMF to give
orange crystals (271 mg, 37%). 1H NMR (d, CD3CN, 300 MHz):
53.99 (br), 39.64 (br), 31.00 (br), 25.07 (br), 22.44 (sh), 21.93 (br),
21.27 (br), 17.15 (sh), 14.32 (br), 13.70 (sh), 9.89 (sh), (7.90 t,
Cyanide binding to [Ph4P][Ni(LtBu)3]
To an orange solution of [Ph4P][Ni(LtBu)] (41 mg, 0.04 mmol) in
acetonitrile (4 mL) was added tetraphenylphosphonium cyanide
(16 mg, 0.04 mmol) as a solid and the reaction stirred for one hour.
Solvent was removed in vacuo and the orange-red solid was washed
with THF (3 ¥ 2 mL) to yield a dark red-orange solid (56 mg, 98%).
1H NMR (d, CD3CN, 400 MHz): diamagnetic resonances: 8.22,
7.92, 7.75, 7.55, 7.36, 7.22, 7.07, 6.96, 6.78, 6.52, 6.06, 1.27, 1.23,
0.93. paramagnetic resonances: 53.29, 39.49, 30.79, 25.60, 22.16,
21.77, 21.35, 17.02, 14.52, 13.62, 10.13, -1.48, -2.99, -5.04, -18.74,
-19.28. FTIR (MeCN, cm-1): 2975, 2868, 2187, 2150, 2109, 1590,
1293, 1224, 1187, 1165, 1110, 763, 725, 693, 529. lmax(e, M-1cm-1)
(MeCN): 408(sh). Attempts to recrystallize this species afforded
only starting materials, i.e., [Ph4P][Ni(LtBu)] and PPh4[CN].
+
4H and 7.70 m, 16H -PPh4 ), 0.17 (br), -1.51 (sh), -3.1 (br),
-5.09 (sh), -18.8 (br), -19.4 (sh). FTIR (KBr, cm-1) n: 3057, 2949,
2919, 2861, 1597 (CO), 1557, 1475, 1439, 1387, 1328, 1244, 1195,
1172, 1108, 1045, 997, 951, 756, 724, 691, 528, 483. meff = 3.37 mB
(Evans Method, CD3CN, 298 K). lmax (e, M-1cm-1) (DMF): 520
(119). Anal. Calcd (found) for Ph4P[NiN(o-PhNC(O)tBu)3]·DMF:
C, 71.29 (70.88); H, 6.58 (6.61); N, 6.93 (6.80).
X-ray crystallographic studies
Suitable crystals were coated with Paratone-N oil, suspended on a
small fiber loop and placed in a cooled nitrogen gas stream at 173 K
on a Bruker D8 APEX II CCD sealed tube diffractometer with
[Et4N]2[Ni(LiPr)(CN)]
To a solution of H3LiPr (68 mg, 0.14 mmol) in dry DMF (3 mL)
was added solid potassium hydride (18 mg, 0.45 mmol). When the
reaction mixture was completely homogeneous, NiBr2 (30 mg,
graphite monochromated Mo-Ka (0.71073 A) radiation. Data
˚
were measured using a series of combinations of phi and omega
scans with 10 s frame exposures and 0.5◦ frame widths. Data
408 | Dalton Trans., 2010, 39, 401–410
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The Royal Society of Chemistry 2010
©

collection, indexing and initial cell refinements were all carried
out using APEX II software.51 Frame integration and final cell
refinements were done using SAINT software.79 The final cell pa-
rameters were determined from least-squares refinement on 2159
reflections. The structure was solved using Direct methods and
difference Fourier techniques (SHELXTL, V6.12).80 Hydrogen
atoms were placed in their expected chemical positions using the
HFIX command and were included in the final cycles of least
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Acknowledgements
We thank Rui Cao and Sheri Lense for assistance with X-ray
crystallography and Prof. K. S. Hagen for helpful discussions.
Leslie A. Alexander is acknowledged for her initial synthesis of
[Et4N][Ni(LtBu)]. This work was supported by the Emory Univer-
sity Research Council (URC) and the donors of the American
Chemical Society Petroleum Research Fund (ACS PRF).
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