Tripodal phenylamine-based ligands and their CoII complexes

Article abstract of DOI:10.1021/ic701289y

The syntheses of two phenylamine-based ligand systems, N(o-PhNH ₂)₃ and N(o-PhNHC(O)ⁱPr)₃, are reported. These ligands readily coordinate to Co^II to form monomeric complexes. X-ray diffraction studies

Full text of DOI:10.1021/ic701289y

Inorg. Chem. 2007, 46, 8117−8119
Tripodal Phenylamine-Based Ligands and Their CoII Complexes
Matthew B. Jones and Cora E. MacBeth*
Department of Chemistry, 1515 Dickey DriVe, Emory UniVersity, Atlanta, Georgia 30322
Received June 29, 2007
The syntheses of two phenylamine-based ligand systems, N(o-
PhNH2)3 and N(o-PhNHC(O)iPr)3, are reported. These ligands
readily coordinate to CoII to form monomeric complexes. X-ray
In this context, we were particularly interested in inves-
tigating a tripodal tetraamine ligand in which the three ligand
arms are comprised of o-phenylamine moieties. The incor-
poration of o-phenylamine groups into this ligand backbone
is intriguing for several reasons: (1) The o-phenylamine
ligand backbone should be less flexible upon metal ion
chelation than its alkylamine counterparts.6 (2) Chelating
o-phenylenediamine units7 and o-phenylenediamine deriva-
tives8 have the potential to act as noninnocent or redox-active
ligands. (3) The tetradentate tripodal ligands of the general
form [N(o-PhL)3], where L ) OH9 or PR2,10 have been
known for some time, but the coordination chemistry of [N(o-
PhNH2)3] and its derivatives has, to the best of our
knowledge, yet to be investigated with first-row late-
transition-metal ions.11
In this work, we present the syntheses of the tetraamine
ligand tris(2-aminophenyl)amine, N(o-PhNH2)3, and its tria-
midoamine derivative, 2,2′,2′′-tris(isobutylamido)tripheny-
lamine [N(o-PhNHC(O)iPr)3]. The CoII complexes of both
of these ligands have been prepared and characterized.
The tetraamine ligand N(o-PhNH2)3 was synthesized in
multigram quantities by first preparing tris(2-nitrophenyl)-
amine via an SNAr reaction.12 The Pd-catalyzed reduction
of this species afforded N(o-PhNH2)3 in high yield (Scheme
1). The triamidoamine analogue was readily prepared by
acylation of a N(o-PhNH2)3 unit with isobutyryl chloride
-
diffraction studies establish that the [N(o-PhNC(O)iPr)3]3 ligand
stabilizes the CoII ion in a trigonal-monopyramidal coordination
environment. The axial coordination site in this complex is
accessible and, upon cyanide coordination, generates an electro-
chemically active species.
Tetraamine, tripodal ligand systems have been widely
employed in many areas of inorganic chemistry.1 Tripodal,
trianionic triamidoamines2 derived from tetraamine precur-
sors have received particular attention because of the ability
of this ligand type to support metal complexes with uncom-
mon coordination geometries,3 unique inorganic functional-
ities,4 and diverse catalytic capabilities.5 The most common
building block for these types of triamidoamine systems are
tripodal tetraamine fragments that contain a tertiary amine
substituted by three alkylamine moieties, such as tris(2-
aminoethyl)amine (tren).1 We have been interested in explor-
ing alternative building blocks for this type of ligand as a
means of varying the electronic features of the resulting
transition-metal centers.
* To whom correspondence should be addressed. E-mail: cora.macbeth@
emory.edu.
(1) Blackman, A. G. Polyhedron 2005, 24, 1-39.
(2) Schrock, R. R. Acc. Chem. Res. 1997, 30, 9-16.
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F. Coord. Chem. ReV. 1999, 193, 913-939.
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Chem., Int. Ed. Engl. 1992, 104, 1510-1512. (b) Pinkas, J.; Wang,
T.; Jacobson, R. A.; Verkade, J. G. Inorg. Chem. 1994, 33, 4202-
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1450-1452. (b) Cummins, C. C.; Schrock, R. R. Inorg. Chem. 1994,
33, 395-396. (c) Freundlich, J. S.; Schrock, R. R.; Cummins, C. C.;
Davis, W. M. J. Am. Chem. Soc. 1994, 116, 6476-6477. (d) Cummins,
C. C.; Schrock, R. R.; Davis, W. M. Angew. Chem., Int. Ed. Engl.
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Brito, F. Coord. Chem. ReV. 1999, 193-195, 857-911. (b) Zanello,
P. Metal Complexes Containing Redox-active Ligands. Inorganic
Electrochemistry: Theory, Practice, and Application; Royal Society
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6, 72-76.
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1966, 88, 2727-2730. (b) Redshaw, C.; Rowan, M. A.; Homden, D.
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(11) Silylated derivatives of the N(o-PhNH2)3 ligand, [N(o-PhNHSiEt3)3]
and [N(o-PhNHSiMe3)3], have been investigated with early transition
metals. See: Baumann, R. Group 4 Complexes Containing Tridentate
Diamido Donor Ligands. Organometallic Chemstry and Catalysis.
Ph.D. Thesis, Massachusetts Institute of Technology, Cambridge, MA,
Feb 1999.
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10.1021/ic701289y CCC: $37.00
Published on Web 09/01/2007
© 2007 American Chemical Society
Inorganic Chemistry, Vol. 46, No. 20, 2007 8117

COMMUNICATION
Scheme 1. Syntheses of N(o-PhNH2)3 and N(o-PhNHC(O)iPr)3
Figure 1. Thermal ellipsoid diagrams of [Co(N(o-PhNH2)3)Br]BPh4 (A)
and Et4N[Co(N(o-PhNC(O)iPr)3)] (B) drawn at 35% probability. Counterions
and H atoms have been omitted for clarity. Selected bond lengths (Å) and
angles (deg) for [Co(N(o-PhNH2)3Br]BPh4: Co1-Br1 2.3875(5), Co1-
N1 2.338(2), Co-N2 2.056(3), Co-N3 2.069(3), Co-N4 2.073(3), Br1-
Co1-N1 178.42(6), N3-Co1-N4 113.87(10), N2-Co1-N3 117.49, N2-
Co1-N4 111.60(10). Selected bond lengths (Å) and angles (deg) for
Et4N[Co(N(o-PhNC(O)iPr)3]: Co1-N1 2.115(8), Co1-N2 1.946(7), Co1-
N3 1.965(9), Co1-N4 1.966(4), N2-Co1-N3 115.5(4), N2-Co1-N4
121.4(3), N3-Co-N4 120.5(3), N1-Co1-N2 84.7(3), N1-Co1-N3 85.1-
(3), N1-Co1-N4 84.1(3).
Scheme 2. Syntheses of [Co(N(o-PhNH2)3)Br]BPh4 and
Et4N[Co(N(o-PhNC(O)iPr)3]
sites are occupied by the tertiary amine of the ligand and a
Br- ion. The CoII center is distorted 0.5 Å out of the trigonal
plane toward the Br- ion. [Co(N(o-PhNH2)3)Br]BPh4 is
3
paramagnetic and displays a high-spin S ) /2 ground state
in solution (µeff ) 4.37, THF-d8) at 25 °C. The complex is
stable for several months under dry anaerobic environments
as a crystalline solid or in solution (THF).
To evaluate the coordination properties of the triamidoam-
ine N(o-PhNHC(O)iPr)3 ligand, its CoII complex was pre-
pared. N(o-PhNHC(O)iPr)3 was reacted with 3 equiv of
potassium hydride in N,N-dimethylformamide (DMF) and
then transmetalated with CoBr2 to yield a deep-blue solution.
Salt metathesis with tetraethylammonium bromide (Et4NBr)
yielded Et4N[Co(N(o-PhNC(O)iPr)3)] in 83% yield with
concomitant formation of 3 equiv of potassium bromide
(Scheme 2). X-ray-quality teal crystals of Et4N[Co(N(o-
PhNC(O)iPr)3)] were obtained by vapor diffusion of diethyl
ether into a DMF solution of the Co complex. The crystal
structure of Et4N[Co(N(o-PhNC(O)iPr)3)] contains two crys-
tallographically independent molecules in the unit cell. The
geometrical properties of these two complexes are indistin-
guishable within experimental error. The solid-state structure
of one of the molecules of Et4N[Co(N(o-PhNC(O)iPr)3)] is
depicted in Figure 1B.15 The complex displays a trigonal-
monopyramidal coordination geometry about the Co ion. The
trigonal coordination plane is composed of the three anionic
N-amidate donors. The average Co-Neq distance in this plane
is 1.959(9) Å. The Co ion is slightly distorted (0.182 Å) out
of the trigonal plane toward the vacant axial coordination
site. The opposite axial site is occupied by the tertiary amine
donor and exhibits a Co-N1 bond distance of 2.115(8) Å.
The vacant coordination site on the CoII ion is surrounded
by the three amidate isopropyl groups of the ligand. The
(Scheme 1) to afford (N(o-PhNHC(O)iPr)3 in good yield
(70%) as a white microcrystalline solid.
We evaluated the structural features of [N(o-PhNH2)3] by
preparing its CoII complex as outlined in Scheme 2. The
cationic complex [Co(N(o-PhNH2)3)Br]BPh4 was prepared
by metalation of the neutral tetraamine ligand with CoBr2
in a MeOH/THF mixture followed by in situ salt metathesis
with 1 equiv of NaBPh4. The resulting indigo complex was
purified by recrystallization by slow diffusion of diethyl ether
into a THF solution of the metal salt to give the analytically
pure [Co(N(o-PhNH2)3)Br]BPh4 in good yield (73%). Crys-
tals suitable for X-ray diffraction measurements were grown
by vapor diffusion of diethyl ether into a concentrated
acetonitrile solution of the complex. The solid-state molecular
structure of [Co(N(o-PhNH2)3)Br]BPh4 was determined and
is depicted in Figure 1A.14 The CoII ion is five-coordinate
and displays a trigonal-bipyramidal coordination environment
with a τ value of 1.0.13 The equatorial plane is composed of
the three primary amine N donors, and the axial coordination
(13) τ is a structural index parameter used to measure trigonality in five-
coordinate complexes (τ ) 0 for perfect square-pyramidal geometry;
τ ) 1 for perfect trigonal-bipyramidal geometry). Addison, A. W.;
Rao, T. N.; Reedijk, J.; van Rijn, J.; Verschoor, G. C. J. Chem. Soc.,
Dalton Trans. 1984, 1349-1356.
(14) Crystal data for [Co(N(o-PhNH2)3)Br]BPh4: C42H38BBrCoN4, M )
748.41, tetragonal, space group I4(I)/a, a ) 34.5691(18) Å, b )
34.5691(18) Å, c ) 13.4536(10) Å, R ) 90°, V ) 16077.4(17) Å3, Z
) 16, Fcalcd ) 1.237 Mg/m3, F(000) ) 6160, λ(Mo KR) ) 0.710 73
Å, µ(Mo KR) ) 1.453 mm-1, crystal dimensions 0.29 × 0.28 × 0.18
mm3. The final R is 0.0600 (wR2 ) 0.1274). 11 742 total reflections,
10 021 independent (Rint ) 10.92%) with I > 2σ(I).
(15) Crystal data for Et4N[Co(N(o-PhNC(O)iPr)3)]: C38H53CoN5O3, M )
686.78, monoclinic, space group P2(1), a ) 14.609(11) Å, b ) 15.421-
(12) Å, c ) 15.668(12) Å, â ) 90.116(16)°, V ) 3530(5) Å3, Z ) 4,
F
calcd ) 1.292 Mg/m3, F(000) ) 1468, λ(Mo KR) ) 0.710 73 Å, µ(Mo
KR) ) 0.530 mm-1, crystal dimensions 0.14 × 0.11 × 0.04 mm3.
The final R is 0.0855 (wR2 ) 0.1905). 34 250 total reflections, 10 133
independent (Rint ) 17.79%) with I > 2σ(I).
8118 Inorganic Chemistry, Vol. 46, No. 20, 2007

COMMUNICATION
addition of more than 1 equiv of [Et4N]CN causes no
additional change in the visible absorption spectrum. These
data are consistent with the formation of a 1:1 CN-/[Co-
(N(o-PhNC(O)iPr)3)]- complex, which we formulate as
[Et4N]2[Co(N(o-PhNC(O)iPr)3)(CN)]. This formulation is
supported by a Job’s plot (Figure S1 in the Supporting
Information) and Fourier transfer infrared (FTIR) spectros-
copy of the magenta complex, which displays a transition at
2112 cm-1 consistent with the formation of a terminal cyano
complex.18 The binding constant of the CN- ligand to -[Co-
(N(o-PhNC(O)iPr)3)]- was determined to be ca. 6 × 102 from
the titration data (see Figure S2 in the Supporting Informa-
tion).
[Et4N]2[Co(N(o-PhNC(O)iPr)3)(CN)] was also examined
by cyclic voltammetry (Figure 2, inset). [Et4N]2[Co(N(o-
PhNC(O)iPr)3)(CN)] displays a single, reversible electro-
chemical event at -0.145 V (∆Ep ) 0.095 V; ipcipa-1 ) 0.95)
vs Fc0/Fc+. We tentatively assign this one-electron redox
process to the CoII/CoIII couple. The observation that both
CN- and acetonitrile bind to the Co center in [Co(N(o-PhNC-
(O)iPr)3)] but DMF and Br- do not indicates that the cavity
surrounding the Co ion is limiting exogenous ligand binding
to small or linear donors. Additional characterization studies
of [Et4N]2[Co(N(o-PhNC(O)iPr)3)(CN)] and its one-electron
oxidation product are in progress in our laboratories.
Figure 2. UV-visible absorption data for the titration of Et4N[Co(N(o-
PhNC(O)iPr)3)] with 0.00, 0.25, 0.50, 0.75, and 1.0 equiv of Et4NCN. (Inset)
Cyclic voltammogram of [Et4N]2[Co(N(o-PhNC(O)iPr)3)(CN)] recorded in
DMF with 0.10 M [TBA]PF6 recorded at 10 mV/s.
groups forming this cavity are oriented so that the methine
protons of the isopropyl groups are positioned inside the
protective pocket above the Co ion. The trigonal-monopy-
ramidal coordination environment observed in Et4N[Co(N(o-
PhNC(O)iPr)3)] is relatively rare for Co ions but has been
observed with other tetradentate, face-capping ligand systems
on both CoI 16 and CoII 17 centers.
Et4N[Co(N(o-PhNC(O)iPr)3)] displays a high-spin elec-
3
tronic configuration (S ) /2) at 25 °C in DMSO-d6 with
µeff ) 4.69 µB. The electrochemical properties of the complex
were investigated utilizing cyclic voltammetry, but the
complex did not display any significant or reversible
electrochemical events at 25 °C in DMF with n-tetrabuty-
lammonium hexafluorophosphate ([TBA]PF6) as the sup-
porting electrolyte.
We speculated that Et4N[Co(N(o-PhNC(O)iPr)3)] should
have an open and accessible axial coordination site given
the geometry it displayed in the solid state. This assumption
was supported by the observation that Et4N[Co(N(o-PhNC-
(O)iPr)3)] exhibited a dramatic color change, from blue to
magenta, upon dissolution in acetonitrile. Acetonitrile binding
to the complex is reversible, however, because the complex
reverts to its original blue color and displays its original
solution-state (DMF) optical properties after it is dried under
reduced pressure.
In an attempt to generate an isolable five-coordinate metal
complex, Et4N[Co(N(o-PhNC(O)iPr)3)] was dissolved in
DMF and treated with increasing quantities of tetraethylam-
monium cyanide ([Et4N]CN). The UV-visible absorption
data from the titration experiment are shown in Figure 2.
The visible absorption spectrum changes after each addition
until exactly 1 equiv of [Et4N]CN has been added. The
In summary, we have reported the syntheses of a new type
of tetradentate tripodal ligand system and its triamidoamine
derivative. The neutral tetraamine ligand, N(o-PhNH2)3,
forms a monocationic five-coordinate Co complex, while the
trianionic [(N(o-PhNC(O)iPr)3]3- derivative stabilizes the CoII
ion in an uncommon trigonal-monopyramidal geometry.
Exogenous ligand binding in the unoccupied axial site of
this complex may be controlled by the cavity surrounding
the vacant coordination site. Efforts to understand these
effects and investigate the coordination chemistry of these
ligands with other transition metals are ongoing in our
laboratories.
Acknowledgment. The authors thank Dr. Kenneth Hard-
castle, Dr. Xikui Fang, and Rui Cao in the Emory X-ray
Crystallography Center for Crystallographic Assistance,
Emory University, for financial support of this work.
Supporting Information Available: Details for all ligand and
complex syntheses, spectroscopic experiments, and X-ray crystal-
lographic details for Et4N[Co(N(o-PhNC(O)iPr)3)] and [Co(N(o-
PhNH2)3)Br]BPh4 (CIF format). This material is available free of
charge via the Internet at http://pubs.acs.org.
IC701289Y
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Inorganic Chemistry, Vol. 46, No. 20, 2007 8119