Bis(α-diimine)nickel complexes: Molecular and electronic structure of three members of the electron-transfer series [Ni(L)2]z (z = 0, 1+, 2+) (L = 2-phenyl-1,4-bis(isopropyl)-1,4-diazabutadiene). A combined experimental and theoretical study

Article abstract of DOI:10.1021/ic700407m

From the reaction of Ni(COD)₂ (COD = cyclooctadiene) in dry diethylether with 2 equiv of 2-phenyl-1,4-bis(isopropyl)-1,4-diazabutadiene (L^Ox)⁰ under an Ar atmosphere, dark red, diamagnetic microcrystals of [Ni^II(L^?)₂] (1) were obtained where (L^?)^1- represents the π radical anion of neutral (L^Ox)⁰ and (L^Red)^2- is the closed shell, doubly reduced form of (L^Ox)⁰. Oxidation of 1 with 1 equiv of ferrocenium hexafluorophosphate in CH ₂Cl₂ yields a paramagnetic (S = 1/2), dark violet precipitate of [Ni^I(L^Ox)₂](PF₆) (2) which represents an oxidatively induced reduction of the central nickel ion. From the same reaction but with 2 equiv of [Fc](PF₆) in CH ₂Cl₂, light green crystals of [Ni^II(L ^Ox)₂(FPF₅)]-(PF₆) (3) (S = 1) were obtained. If the same reaction was carried out in tetrahydrofuran, crystals of [Ni^II(L^Ox)_2- (THF)(FPF₅)](PF ₆)·THF (4) (S = 1) were obtained. Compounds 1, 2, 3, and 4 were structurally characterized by X-ray crystallography: 1 and 2 contain a tetrahedral neutral complex and a tetrahedral monocation, respectively, whereas 3 contains the five-coordinate cation [Ni^II(L^Ox) ₂(FPF₅)]⁺ with a weakly coordinated PF ₆^- anion and in 4 the six-coordinate monocation [Ni ^II(L^Ox)₂(THF)(FPF₅)]⁺ is present. The electro- and magnetochemistry of 1-4 has been investigated by cyclic voltammetry and SQUID measurements. UV-vis and EPR spectroscopic data for all compounds are reported. The experimental results have been confirmed by broken symmetry DFT calculations of [Ni^II(L^?) ₂]⁰, [Ni^I(L^Ox)₂] ⁺, and [Ni^II(L^Ox)₂]²⁺ in comparison with calculations of the corresponding Zn complexes: [Zn ^II(^tL^Ox)₂]²⁺, [Zn ^II(^tL^Ox)(^tL^?)] ⁺, [Zn^II(^tL^?)₂] ⁰, and [Zn^II(^tL^?)( ^tL^Red)]^- where (^tL ^Ox)⁰ represents the neutral ligand 1,4-di-tert-butyl-1,4- diaza-1,3-butadiene and (^tL^?)^1- and ( ^tL^Red))^2- are the corresponding one- and two-electron reduced forms. It is clearly established that the electronic structures of both paramagnetic monocations [Ni^I(L^Ox) ₂]⁺ (S = 1/2) and [Zn^II(^tL ^Ox)(^tL^?)]⁺ (S = 1/2) are different.

Full text of DOI:10.1021/ic700407m

Inorg. Chem. 2007, 46, 5327−5337
Bis(
Three Members of the Electron-Transfer Series [Ni(L)2]z (z
(L 2-Phenyl-1,4-bis(isopropyl)-1,4-diazabutadiene). A Combined
Experimental and Theoretical Study
r-diimine)nickel Complexes: Molecular and Electronic Structure of
)
0, 1 , 2+)
+
)
Nicoleta Muresan,† Krzysztof Chlopek,‡ Thomas Weyhermuller,† Frank Neese,§ and Karl Wieghardt*,†
1
Max-Planck-Institut fu¨r Bioanorganische Chemie, Stiftstrasse 34-36, D-45470 Mu¨lheim an der
Ruhr, Germany, Forschungszentrum Karlsruhe GmbH, Institut fu¨r Nanotechnologie,
Postfach 3640, D-76021 Karlsruhe, Germany, and Institut fu¨r Physikalische und Theoretische
Chemie, Wegelerstrasse 12, D-53115 Bonn, Germany
Received March 2, 2007
From the reaction of Ni(COD)2 (COD
) cyclooctadiene) in dry diethylether with 2 equiv of 2-phenyl-1,4-bis(isopropyl)-
1,4-diazabutadiene (LOx)0 under an Ar atmosphere, dark red, diamagnetic microcrystals of [NiII(L•)2] (1) were obtained
-
0
2-
where (L•)1 represents the
π
radical anion of neutral (LOx
)
and (LRed
)
is the closed shell, doubly reduced form
of (LOx)0. Oxidation of 1 with 1 equiv of ferrocenium hexafluorophosphate in CH2Cl2 yields a paramagnetic (S
)
1/2), dark violet precipitate of [NiI(LOx)2](PF6) (2) which represents an oxidatively induced reduction of the central
nickel ion. From the same reaction but with 2 equiv of [Fc](PF6) in CH2Cl2, light green crystals of [NiII(LOx)2(FPF5)]-
(PF6) (3) (S
(THF)(FPF5)](PF6)
)
1) were obtained. If the same reaction was carried out in tetrahydrofuran, crystals of [NiII(LOx)2-
‚
THF (4) (S ) 1) were obtained. Compounds 1, 2, 3, and 4 were structurally characterized by
X-ray crystallography: 1 and 2 contain a tetrahedral neutral complex and a tetrahedral monocation, respectively,
-
whereas 3 contains the five-coordinate cation [NiII(LOx)2(FPF5)]+ with a weakly coordinated PF6 anion and in 4 the
six-coordinate monocation [NiII(LOx)2(THF)(FPF5)]+ is present. The electro- and magnetochemistry of 1
−4 has been
investigated by cyclic voltammetry and SQUID measurements. UV
−vis and EPR spectroscopic data for all compounds
are reported. The experimental results have been confirmed by broken symmetry DFT calculations of [NiII(L•)2]0,
+
+
[NiI(LOx)2]+, and [NiII(LOx)2]2 in comparison with calculations of the corresponding Zn complexes: [ZnII(tLOx)2]2
,
0
[ZnII(tLOx)(tL•)]+, [ZnII(tL•)2]0, and [ZnII(tL•)(tLRed)]- where (tLOx
)
represents the neutral ligand 1,4-di-tert-butyl-1,4-
diaza-1,3-butadiene and (tL•)1 and (tLRed
clearly established that the electronic structures of both paramagnetic monocations [NiI(LOx)2]+ (S
[ZnII(tLOx)(tL•)]+ (S 1/2) are different.
)
are the corresponding one- and two-electron reduced forms. It is
1/2) and
-
2-
)
)
Introduction
discovered mainly by X-ray crystallography that these ligands
can adopt three different oxidation levels, namely the neutral
The coordination chemistry of N,N′-disubstituted R-di-
imines with main group metal ions (Li, Mg, Al, Ga, In) has
been investigated in detail in the past.1-12 It has been
(4) Cloke, F. G. N.; Hanson, G. R.; Henderson, M. J.; Hitchcock, P. B.;
Raston, C. L. J. Chem. Soc., Chem. Commun. 1989, 100.
(5) Cloke, F. G. N.; Dalby, C. I.; Henderson, M. J.; Hitchcock, P. B.;
Kennard, C. H. L.; Lamb, R. N.; Raston, C. L. J. Chem. Soc., Chem.
Commun. 1990, 1394.
(6) Thiele, K.-H.; Lorenz, V.; Thiele, G.; Zo¨nnchen, P.; Scholz, J. Angew.
Chem., Int. Ed. Engl. 1994, 33, 1372.
(7) Gardiner, M. G.; Hanson, G. R.; Henderson, M. J.; Lee, F. C.; Raston,
C. L. Inorg. Chem. 1994, 33, 2456.
(8) Rijnberg, E.; Richter, B.; Thiele, K.-H.; Boersma, J.; Veldman, N.;
Spek, A. L.; van Koten, G. Inorg. Chem. 1998, 37, 56.
(9) Pott, T.; Jutzi, P.; Neumann, B.; Stammler, H.-G. Organometallics
2001, 20, 1965.
* To whom correspondence should be addressed. E-mail: wieghardt@
mpi-muelheim.mpg.de. Phone: ++49-208-306 3610. Fax: ++49-208-306
3952.
† Max-Planck-Institut fu¨r Bioanorganische Chemie.
‡ Institut fu¨r Nanotechnologie.
§ Institut fu¨r Physikalische und Theoretische Chemie.
(1) van Koten, G.; Vrieze, K. AdV. Organomet. Chem. 1982, 21, 151.
(2) Richter, S.; Daul, C.; von Zelewsky, A. Inorg. Chem. 1976, 15,
943.
(3) Corvaja, C.; Pasimeni, L. Chem. Phys. Lett. 1976, 261.
10.1021/ic700407m CCC: $37.00
Published on Web 05/27/2007
© 2007 American Chemical Society
Inorganic Chemistry, Vol. 46, No. 13, 2007 5327

Muresan et al.
Scheme 1. Ligands and Abbreviationsa
the (L•)1- ligand. It is therefore quite surprising that despite
our above apparently clear understanding of the molecular
and electronic structures of the main group metal (and ZnII)
complexes this was until very recently not the case for similar
transition metal complexes of R-diimines. Thus, all neutral,
tetrahedral, and diamagnetic bis(R-diimine)metal complexes
of Ni (and paramagnetic Fe) have been described as Ni(0)
(d10)15-20 or Fe(0) (d8)21-23 complexes containing two neutral,
diamagnetic R-diimine ligands (LOx). We24,25 and others26
have shown recently for the first time that the nickel
complexes contain in fact a central Ni(II) ion with a high
spin d8 electron configuration (SNi ) 1) which is coupled
intramolecularly antiferromagnetically to two ligand π
radicals (L•)1- (Srad )1/2): their electronic structures are thus
best described as [NiII(L•)2] (St ) 0) and not [Ni0(LOx)2] (St
) 0).
It is an interesting observation that these neutral complexes
[NiII(L•)2] can be reversibly twice oxidized yielding a mono-
and a dication and, in some cases, also twice reduced
affording a mono- and dianion, respectively. Thus, these
complexes are members of an electron-transfer series.15 The
molecular and electronic structures of these ionic members
have not been studied in the past. Here we report the
syntheses and characterization of the three-membered se-
ries: [NiII(L•)2]0, [NiII(L•)(LOx)]+, and [NiII(LOx)2]2+ where
a Red numbers represent av bond lengths in Å.
R-diimines (LOx)0, monoanionic π radicals (L•)1-, and
reduced dianionic R-diamides (L )
Red 2- as shown in Scheme
1. Interestingly, X-ray crystallography has unambiguously
demonstrated that the three ligand oxidation levels can be
identified by their varying C-N and C-C bond lengths: for
(LOx)0 the average C-N and C-C bond distances are at 1.29
( 0.01 and 1.47 ( 0.01 Å, respectively, whereas those for
the π radical monoanions are at 1.34 ( 0.01 and 1.38 (
0.01 Å, and finally, those in the dianions are at 1.40 ( 0.01
and 1.36 ( 0.01 Å (Scheme 1).
(LOx)0, (L•)1-, and (LRed 2-
represent the redox members of
)
the asymmetric R-diimine ligand 2-phenyl-1,4-bis(isopropyl)-
1,4-diaza-1,3-butadiene (Schemes 1 and 2).
We establish their molecular and electronic structures
experimentally and also by density functional theoretical
(DFT) calculations. In order to calibrate this methodology
for the three nickel complexes 1, 2, and 3 (Scheme 2), we
have also calculated and optimized the geometry of the
corresponding Zn complexes from refs 2, 7, and 8.
Raston et al.7 and van Koten et al.8 have synthesized and
structurally characterized the following members of an
electron-transfer series: K2[ZnII(tLRed)2], K[ZnII(tLRed)(tL•)],
[ZnII(tL•)2], [ZnII(tLOx)(tL•)](OTf), [ZnII(tLOx)2](OTf)2 where
(tLOx)0, (tL•)1-, and (tLRed 2-
represent the three oxidation
)
levels of 1,4-di-tert-butyl-1,4-diaza-1,3-butadiene. The most
important observations are that the ZnN4 polyhedron is in
all cases tetrahedral and that in the two paramagnetic species,
namely the monoanion and the monocation, respectively, the
two ligands exist in differing oxidation levels, which is
clearly borne out by the observed structural parameters. Thus,
the unpaired electron is localized on one monoanionic (tL•)1-
ligand; no delocalization of this electron over both ligands
is observed. The same is true for tetrahedral, paramagnetic
complexes [MIII(LRed)(L•)]0 (M ) Al, Ga).1-12
Experimental Section
All air-sensitive materials were manipulated under argon using
standard Schlenk line procedures or a glovebox. Bis(cyclooctadi-
ene)nickel(0) and ferrocenium hexafluorophosphate were purchased
from Aldrich. All chemicals were used without further purification.
(15) Baloh, A. L.; Holm, R. H. J. Am. Chem. Soc. 1966, 88, 5201.
(16) tom Dieck, H.; Svoboda, M.; Kopf, J. Z. Naturforsch., B: Chem. Sci.
1978, 33, 1381.
(17) Walther, D. Z. Anorg. Allg. Chem. 1974, 405, 8.
(18) (a) Svoboda, M.; tom Dieck, H. Z. Naturforsch., B: Chem. Sci. 1981,
36, 814. (b) tom Dieck, H.; Svoboda, M.; Greiser, T. Z. Naturforsch.,
B: Chem. Sci. 1981, 36, 823.
(19) Go¨rls, H.; Walther, D.; Siedler, J. Cryst. Res. Technol. 1987, 22, 1145.
(20) Bonrath, W.; Po¨rschke, K. R.; Mynott, R.; Kru¨ger, C. Z. Naturforsch.,
B: Chem. Sci. 1990, 45, 1647.
Recent DFT calculations on these latter complexes have
reliably reproduced these structures.13,14 The calculated and
experimental EPR parameters agree nicely, giving confidence
that the unpaired electron resides in a π* ligand orbital of
(21) tom Dieck, H.; Bruder, H. J. Chem. Soc., Chem. Commun. 1977, 24.
(22) Walther, D.; Kreisel, G.; Kirmse, R. Z. Anorg. Allg. Chem. 1982, 487,
149.
(23) Bart, S. C.; Hawrelak, E. J.; Lobkovsky, E.; Chirik, P. Organometallics
2005, 24, 5518.
(24) Blanchard, S.; Neese, F.; Bothe, E.; Bill, E.; Weyhermu¨ller, T.;
Wieghardt, K. Inorg. Chem. 2005, 44, 3636.
(25) Chlopek, K.; Bothe, E.; Neese, F.; Weyhermu¨ller, T.; Wieghardt, K.
Inorg. Chem. 2006, 45, 6298.
(10) Baker, R. J.; Farley, R. D.; Jones, C.; Mills, D. P.; Kloth, M.; Murphy,
D. M. Chem.sEur. J. 2005, 11, 2972.
(11) Pott, T.; Jutzi, P.; Kaim, W.; Schoeller, W. W.; Neumann, B.;
Stammler, A.; Stammler, H.-G.; Wanner, M. Organometallics 2002,
21, 3169.
(12) (a) Baker, R. J.; Farley, R. D.; Jones, C.; Kloth, M.; Murphy, D. M.
Chem. Commun. 2002, 1196. (b) Baker, R. J.; Farley, R. D.; Jones,
C.; Kloth, M.; Murphy, D. M. Dalton Trans. 2002, 3844.
(13) Tuononen, H. M.; Armstrong, A. F. Dalton Trans. 2006, 1885.
(14) Tuononen, H. M.; Armstrong, A. F. Inorg. Chem. 2005, 44, 8277.
(26) Khusniyarov, M. M.; Harms, K.; Burghaus, O.; Sundermeyer, J. Eur.
J. Inorg. Chem. 2006, 15, 2985.
5328 Inorganic Chemistry, Vol. 46, No. 13, 2007

Bis(r-diimine)nickel Complexes
Scheme 2. New Complexes of Nickel and Zinc Complexes from
References 7 and 8
stirred for 1 h. A light green precipitate formed which was washed
with n-hexane (2 × 5 mL) and dried in vacuo. Yield: 0.20 g (80%).
X-ray quality crystals were obtained by slow diffusion of n-pentane
into a concentrated solution of 3 in CH2Cl2. Anal. Calcd for
C28H40N4F12NiP2: C, 43.04; H, 5.12; N, 7.17; Ni, 7.51; P, 7.84.
Found: C, 43.01; H, 5.46; N, 6.95; Ni, 7.25; P, 7.54.
[NiII(LOx)2(THF)(FPF5)](PF6)‚THF (4). To a solution of 1 (200
mg, 0.4 mmol) in THF (10 mL) was added ferrocenium hexafluo-
rophosphate (260 mg, 0.8 mmol) with stirring for 1.5 h at 20 °C.
The dark red color of the solution immediately turned to light green.
The resulting solution was filtered, and the filtrate was concentrated
to ∼1 mL by evaporation of the solvent under reduced pressure.
n-Hexane (10 mL) was added, and the resulting suspension was
stirred for 1 h. A slightly green precipitate was formed, washed
with n-hexane, and dried in vacuo. Yield: 0.15 g (80%). X-ray
quality crystals were obtained by slow diffusion of n-pentane
into a concentrated solution of 4 in THF. Anal. Calcd for
C36H56N4F12NiO2P2: C, 47.72; H, 6.10; N, 6.05. Found: C, 47.55;
H, 6.08; N, 5.82.
X-ray Crystallographic Data Collection and Refinement of
the Structures. A dark red single crystal of 1, a dark blue crystal
of 2, and greenish yellow crystals of 3 and 4‚THF were coated
with perfluoropolyether, picked up with nylon loops, and mounted
in the nitrogen cold stream of the diffractometer. A Bruker-Nonius
Kappa-CCD diffractometer equipped with a Mo-target rotating-
anode X-ray source and a graphite monochromator (Mo KR, λ )
0.71073 Å) was used. Final cell constants were obtained from least-
squares fits of all measured reflections. Intensity data of 2, 3, and
4 were corrected for absorption using intensities of redundant
reflections.
The structures were readily solved by direct methods and
subsequent difference Fourier techniques. The Siemens ShelXTL28
software package was used for solution and artwork of the structure;
ShelXL9729 was used for the refinement. All non-hydrogen atoms
were refined anisotropically, except for a disordered tetrahydrofuran
solvent molecule in 4 which was split on three positions and
isotropically refined. Hydrogen atoms were placed at calculated
positions and refined as riding atoms with isotropic displacement
parameters. Crystallographic data of the compounds are listed in
Table 1.
The ligand 2-phenyl-1,4-bis(isopropyl)-1,4-diazabutadiene was
synthesized according to published procedures.27
[NiII(L•)2] (1). To a solution of Ni(COD)2 (1 g, 3.63 mmol) (COD
) cyclooctadiene) in diethyl ether (20 mL) under an argon
blanketing atmosphere was added a solution of 2-phenyl-1,4-bis-
(isopropyl)-1,4-diazabutadiene (1.58 g, 7.27 mmol) in diethyl ether
(10 mL). The reaction mixture was stirred overnight at room
temperature. The color of the solution changed from yellow to deep
red. The solvent was removed by evaporation under reduced
pressure to give a dark red precipitate which was washed with
acetonitrile and dried in vacuo. Yield: 1.40 g (78%). X-ray quality
crystals were obtained by slow evaporation of the solvent from a
concentrated solution of 1 in acetonitrile. Anal. Calcd for
C28H40N4Ni: C, 68.47; H, 8.15; N, 11.41; Ni, 11.97. Found: C,
68.31; H, 8.21; N, 11.64; Ni, 11.22.
1H NMR (400 MHz, toluene-d8): δ (ppm) ) 1.75 (d, 3J ) 6.31,
6H, iPr-CH3), 1.91 (d, 3J ) 6.49, 6H, iPr-CH3), 1.89 (d, 3J ) 6.31,
3
3
6H, iPr-CH3), 1.90 (d, J ) 6.49, 6H, iPr-CH3), 2.65 (sept, J )
6.49, 2H, iPr-CH), 3.66 (sept, 3J ) 6.31, 2H, iPr-CH), 6.9 (m, 4H,
C6H5), 7.05 (m, 4H, C6H5), 7.16 (m, 2H, C6H5), 8.75 (s, 2H, NCH).
[NiI(LOx)2] PF6 (2). To a solution of 1 (200 mg, 0.40 mmol) in
CH2Cl2 (10 mL) was added ferrocenium hexafluorophosphate (130
mg, 0.40 mmol) with stirring for 1.5 h at 20 °C. The dark red color
of the solution immediately turned to dark violet. The resulting
solution was filtered, and the filtrate was concentrated to ∼1 mL
by evaporation of the solvent under reduced pressure. n-Hexane
(10 mL) was added, and the resulting suspension was stirred for 1
h. A dark violet precipitate was formed, washed with n-hexane (2
× 5 mL), and dried in vacuo. Yield: 0.20 g (80%). X-ray quality
crystals were obtained by slow diffusion of n-pentane into a
concentrated solution of 2 in THF. MS (ESI, pos ion, CH2Cl2),
m/z ) 490 {2 - PF6}+. Anal. Calcd for C28H40N4F6NiP: C, 52.5;
H, 6.29; N, 8.8; F, 17.93; Ni, 9.23; P, 4.87. Found: C, 52.76; H,
6.24; N, 8.89; F, 17.83; Ni, 9.37; P, 4.76.
Disorder was found in all four structures. The asymmetric unit
in 1 contains two crystallographically independent molecules of
which the one containing Ni(1) had disordered isopropyl groups
which were split on two positions in a 56:44 and 73:27 ratio,
respectively. Corresponding atoms were restrained to have the same
distances and thermal displacement parameters. The structure of 2
was fully refined in the enantiomorphic space groups P3121 (No.
152) and P3221 (No. 154). The R-factor and Flack30 parameters
were slightly lower for the solution in P3121, and therefore, this
space group was chosen. The PF6- anion lying on a crystallographic
2-fold axis was found to be disordered and was split on two
positions in a 69:31 ratio. The P-F and F-F distances were
restrained to be equal within errors, and equivalent thermal
displacement parameters were used for neighboring split atoms.
Similar problems occurred in the structure of 3 in which the
[NiII(LOx)2(FPF5)](PF6) (3). To a solution of 1 (300 mg, 0.61
mmol) in CH2Cl2 (10 mL) was added ferrocenium hexafluorophos-
phate (400 mg, 1.22 mmol) with stirring for 1.5 h at 20 °C. The
dark red color of the solution immediately turned to light green.
The resulting solution was filtered, and the filtrate was concentrated
to ∼1 mL by evaporation of the solvent under reduced pressure.
n-Hexane (10 mL) was added, and the resulting suspension was
-
isopropyl group attached to N(24), and the uncoordinated PF6
anion was split in a 91:9 ratio. A total of 25 restraints was used to
equal distances and displacement parameters using EADP, SADI,
(28) ShelXTL V.5; Siemens Analytical X-Ray Inst. Inc.: Madison, WI, 1994.
(29) Sheldrick, G. M. ShelXL97; University of Go¨ttingen: Go¨ttingen,
Germany, 1997.
(27) Armesto, D.; Bosch, P.; Gallego, M. G.; Martin, J. F.; Ortiz, M. J.;
(30) (a) Flack, H. D. Acta Crystallogr. 1983, A39, 876-881. (b) Bernadelli,
Perez-Ossorio, R.; Ramos, A. Org. Prep. Proced. Int. 1987, 19, 181.
G.; Glack, H. D. Acta Crystallogr. 1985, A41, 500-511.
Inorganic Chemistry, Vol. 46, No. 13, 2007 5329

Muresan et al.
Table 1. Crystallographic Data for 1, 2, 3, and 4‚THF
1
2
3
4‚THF
chem formula
fw
C28H40N4Ni
491.35
C28H40F6N4NiP
636.32
C28H40F12N4NiP2
781.29
C36H56F12N4NiO2P2
925.50
space group
a, Å
b, Å
c, Å
P1h, No. 2
10.1190(4)
9.9954(3)
26.5990(8)
91.161(5)
90.020(5)
99.519(5)
2652.7(2)
4
P3121, No. 152
9.1845(3)
9.1845(3)
30.6806(12)
90
90
120
2241.33(14)
3
P21/c, No. 14
18.8774(5)
9.7865(2)
18.6485(5)
90
106.408(3)
90
3304.88(14)
4
P21/n, No. 14
12.5937(4)
19.4208(6)
18.1200(6)
90
109.208(5)
90
4185.1(2)
4
R, deg
â, deg
γ, deg
V, Å
Z
T, K
100(2)
100(2)
100(2)
100(2)
F calcd, g cm-3
reflns collected/2Θmax
unique reflns/I > 2σ(I)
no. params/restraints
λ, Å/µ(KR), cm-1
R1a/GOFb
1.230
1.414
1.570
1.469
41996/52.00
10392/7730
615/40
0.71073/7.53
0.0504/1.058
0.0861
25078/65.00
5405/4818
197/30
0.71073/7.65
0.0536/1.081
0.1197
84831/60.00
9630/7136
461/25
0.71073/7.79
0.0418/1.018
0.0845
74161/60.00
12180/9675
566/63
0.71073/6.31
0.0640/1.073
0.1540
wR2c (I > 2σ(I))
residual density, e Å-3
+0.49/-0.41
+0.89/-0.70
+0.73/-0.50
+1.18/-0.67
2
2
2
2
2
a Observation criterion: I > 2σ(I). R1 ) ∑||Fo| - |Fc||/∑|Fo|. b GOF ) [∑[w(Fo - Fc )2]/(n - p)]1/2
.
c wR2 ) [∑[w(Fo - Fc )2]/∑[w(Fo )2]]1/2 where
w ) 1/σ2(Fo ) + (aP)2+ bP, P ) (Fo + 2Fc )/3.
2
2
2
and SAME instructions of ShelXL. Compound 4 showed disorder
of the uncoordinated PF6 anion and both the coordinated and
10-7 in the maximum element of the DIIS error vector). The
geometries were considered converged after the energy change was
less than 5 × 10-6 Eh, the gradient norm and maximum gradient
element were smaller than 1 × 10-4 Eh/Bohr and 3 × 10-4 Eh/
Bohr, respectively, and the root-mean square and maximum
displacements of atoms were smaller than 2 × 10-3 Bohr and 4 ×
10-3 Bohr, respectively. The starting geometries were those of the
crystal structures of 1, 2, and 3 and those for the Zn complexes
from ref 8. Corresponding orbitals35 and density plots were obtained
by the program Molekel.36
We describe our computational results of nickel and zinc
complexes containing noninnocent ligands using the broken sym-
metry (BS) approach first proposed by Ginsberg37 and standardized
by Noodleman.38,39 The BS approach has been successfully applied
for transition-metal-containing complexes in the past in the groups
of Wieghardt, Neese,40-44 and others.38,45
-
uncoordinated THF molecules. The PF6- split positions were refined
with above-mentioned restraints from which a 84:16 ratio resulted.
The coordinated THF was refined with a split ratio of 73:27, and
the uncoordinated one was split on three positions (41:33:26) using
EADP and SAME restraints (63 restraints).
Physical Measurements. Electronic spectra of complexes in
CH2Cl2 solution and corresponding spectra from electrochemical
measurements were recorded on an HP 8452A diode array
spectrophotometer (range 200-1000 nm) or a Perkin-Elmer double-
beam photometer (range 200-2000 nm). Cyclic voltammograms
and coulometric experiments were performed with an EG&G
potentiostat/galvanostat, model 273A. Variable field (1.0 T),
variable temperature (4-300 K) magnetization data were recorded
on a SQUID magnetometer (MPMS Quantum Design). The
experimental magnetic susceptibility data were corrected for
underlying diamagnetism by use of tabulated Pascal constants.
X-band EPR spectra were recorded on a Bruker ESP 300
spectrometer.
Calculations. All calculations were performed by using the
ORCA program package.31 The geometry optimizations were
carried out at the B3LYP level32 of DFT. The all-electron Gaussian
basis sets were those reported by the Ahlrichs group.33,34 Triple-ú
quality basis sets with one set of polarization functions on the nickel
and nitrogen atoms were used (TZVP).34 The carbon and hydrogen
atoms were described by slightly smaller polarized split-valence
SV(P) basis sets that are double-ú quality in the valence region
and contain a polarizing set of d-functions on the non-hydrogen
atoms.33 The auxiliary basis sets for all complexes used to expand
the electron density in the calculations were chosen to match the
orbital basis. The SCF calculations were tightly converged (1 ×
10-8 Eh in energy, 1 × 10-7 Eh in the density change, and 1 ×
Since for some of the complexes studied in this work one can
obtain broken-symmetry (BS) solutions to the spin-unrestricted
(35) Neese, F. J. Phys. Chem. Solids 2004, 65, 781.
(36) Molekel. Molekel, AdVanced InteractiVe 3d-Graphics for Molecular
Sciences; available at http://www.cscs.ch/molekel/.
(37) Ginsberg, A. P. J. Am. Chem. Soc. 1980, 102, 111.
(38) Noodleman, L.; Peng, C. Y.; Case, D. A.; Mouesca, J. M. Coord.
Chem. ReV. 1995, 144, 199.
(39) (a) Noodleman, L.; Case, D. A.; Aizman, A. J. Am. Chem. Soc. 1988,
110, 1001.(b) Noodleman, L.; Davidson, E. R. Chem. Phys. 1986,
109, 131. (c) Noodleman, L.; Norman, J. G.; Osborne, J. H.; Aizman,
C.; Case, D. A. J. Am. Chem. Soc. 1985, 107, 3418. (d) Noodleman,
L. J. Chem. Phys. 1981, 74, 5737.
(40) (a) Bachler, V.; Olbrich, G.; Neese, F.; Wieghardt, K. Inorg. Chem.
2002, 41, 4179. (b) Ghosh, P.; Bill, E.; Weyhermu¨ller, T.; Neese, F.;
Wieghardt, K. J. Am. Chem. Soc. 2003, 125, 1293.
(41) (a) Herebian, D.; Bothe, E.; Neese, F.; Weyhermu¨ller, T.; Wieghardt,
K. J. Am. Chem. Soc. 2003, 125, 9116. (b) Herebian, D.; Wieghardt,
K.; Neese, F. J. Am. Chem. Soc. 2003, 125, 10997.
(42) Slep, L. D.; Mijovilovich, A.; Meyer-Klaucke, W.; Weyhermu¨ller, T.;
Bill, E.; Bothe, E.; Neese, F.; Wieghardt, K. J. Am. Chem. Soc. 2003,
125, 15554.
(43) Chlopek, K.; Bothe, E.; Neese, F.; Weyhermu¨ller, T.; Wieghardt, K.
Inorg. Chem. 2006, 45, 6298.
(44) Patra, A. K.; Bill, E.; Bothe, E.; Chlopek, K.; Neese, F.; Weyhermu¨ller,
T.; Stobie, K.; Ward, M. D.; McCleverty, J. A.; Wieghardt, K. Inorg.
Chem. 2006, 45, 7877.
(31) Neese, F. In ORCA, an Ab Initio, Density Functional and Semiem-
pirical Electronic Structure Program Package, Version 2.4, ReVision
36; Max-Planck Institut fu¨r Bioanorganische Chemie: Mu¨lheim/Ruhr,
Germany, May 2005.
(32) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Becke, A. D. J.
Chem. Phys. 1986, 84, 4524. (c) Lee, C. T.; Yang, W. T.; Parr, R. G.
Phys. ReV. B 1988, 37, 785.
(33) Scha¨fer, A.; Horn, H.; Ahlrichs, R. J. Chem. Phys. 1992, 97, 2571.
(34) Scha¨fer, A.; Huber, C.; Ahlrichs, R. J. Chem. Phys. 1994, 100, 5829.
(45) (a) Remenyi, C.; Kaupp, M. J. Am. Chem. Soc. 2005, 127, 11399. (b)
Bencini, A.; Carbonera, C.; Dei, A.; Vaz, M. G. F. Dalton Trans.
2003, 1701.
5330 Inorganic Chemistry, Vol. 46, No. 13, 2007

Bis(r-diimine)nickel Complexes
Table 2. Selected Bond Lengths (Å) in 1, 2, 3, and 4
1
2
3
4
Ni1-N1
Ni1-N4
N1-C2
C2-C3
C3-N4
θ, degc
1.940(2)a
1.924(2)
1.338(3)
1.409(4)
1.322(4)
78.9(3)
1.938(2)a
1.934(2)
1.336(3)
1.418(4)
1.313(3)
1.932(2)b
1.914(3)
1.333(4)
1.420(4)
1.322(4)
78.4(2)
1.936(2)b
1.917(3)
1.338(4)
1.411(4)
1.319(4)
1.987(2)
1.979(2)
1.305(3)
1.461(3)
1.294(3)
70.0(2)
2.032(2)
2.013(2)
1.291(3)
1.499(3)
1.266(2)
1.993(2)
1.997(2)
1.293(2)
1.496(3)
1.272(2)
2.084(3)
2.043(2)
1.289(4)
1.496(4)
1.267(4)
2.095(2)
2.044(2)
1.292(3)
1.490(4)
1.262(4)
86.6
a Molecule 1 with two ligands (L•)1- b Molecule 2 with two (L•)1- ligands. c Dihedral angle between two metallacycles Ni-N-C-C-N.
.
Kohn-Sham equations, we will adopt the following notation: the
given system (molecule, ion) is divided into two fragments. The
notation BS(m,n) then refers to a broken symmetry state with m
unpaired spin-up electrons on fragment 1 and n unpaired spin-down
electrons essentially localized on fragment 2. In most cases,
fragments 1 and 2 correspond to the metal and the ligand,
respectively. Note that in this notation a standard high-spin open-
shell solution would be written down as BS(m+n,0). In general,
the BS(m,n) notation refers to the initial guess to the wavefunction.
The variational process does, however, have the freedom to
converge to a solution of the form BS(m-n,0) where effectively
the n-spin-down electrons pair with n < m spin-up electrons on
the partner fragment. Such a solution is then a standard Ms = S )
(m - n)/2 unrestricted Kohn-Sham solution. As explained
elsewhere,35 the nature of the solution is investigated via the
corresponding orbital transformation which via the corresponding
orbital overlaps displays whether the system is to be described as
a spin-coupled or a closed-shell solution.
Complex 1 was oxidized by 1 equiv of ferrocenium
hexafluorophosphate in CH2Cl2 solution yielding a dark
violet precipitate of [NiI(LOx)2](PF6) (2) after evaporation of
the solvent and addition of n-hexane. From variable tem-
perature (4-300 K) magnetic susceptibility measurements
(SQUID) a temperature-independent magnetic moment of
2.1 µB was established for a solid sample of 2. Thus, 2
1
possesses an S ) /2 ground state.
If 2 equiv of ferrocenium hexafluorophosphate was
employed in the above reaction, a light green precipitate of
[NiII(LOx)2(FPF5)](PF6) (3) was obtained in 80% yield.
Variable temperature magnetic susceptibility measurements
exhibit a temperature-independent magnetic moment of 3.2
µB (10-300 K) indicating an S ) 1 ground state for 3.
Recrystallization of 3 from dry tetrahydrofuran solution
affords light green crystals of [NiII(LOx)2(FPF5)(THF)](PF6)‚
THF (4) which are again paramagnetic (µ ) 3.1 µB (10-
300 K); S ) 1).
Results
2. Crystal Structure Determinations. The crystal struc-
tures of 1, 2, 3, and 4 have been determined at 100(2) K.
Table 1 summarizes the crystallographic details, and Table
2 gives selected bond distances of the five-membered chelate
rings Ni-N-C-C-N. Figure 1 displays a single neutral
molecule in crystals of 1, Figure 2 exhibits an ion pair in
crystals of 2, and Figure 3 shows that of a [NiI(LOx)2(FPF5)]+
monocation in crystals of 3, whereas in Figure 4 the
monocation [NiII(LOx)2(THF)(FPF5)]+ in 4 is shown.
1. Synthesis and Characterization of Complexes. The
reaction of Ni(COD)2 (COD ) cyclooctadiene) at 20 °C with
2 equiv of 2-phenyl-1,4-bis(isopropyl)-1,4-diazabutadiene,27
(LOx)0, in diethyl ether under an Ar blanketing atmosphere
affords dark red microcrystals of [NiII(L•)2] (1) in 78% yield
after evaporation of the solvent. As judged from its normal
1H NMR spectrum (see Experimental Section), 1 is diamag-
netic with an S ) 0 ground state.
Figure 1. Structure of one of the two crystallographically independent
neutral molecules in crystals of 1 (ellipsoids are drawn at the 50% probability
level).
Figure 2. Structures of the monocation and the corresponding well-
separated PF6 anion in crystals of 2 (ellipsoids at the 50% probability lev-
el).
Inorganic Chemistry, Vol. 46, No. 13, 2007 5331

Muresan et al.
correlated (C2 axis), bidentate chelate rings Ni-N-C-C-N
which form a dihedral angle of 70°. The av C-N distance
at 1.300 ( 0.01 Å and the corresponding elongated C-C
bond length at 1.461 ( 0.01 Å are indicative of the presence
of two identical neutral ligands (LOx)0 (R-diimine) which
render the central nickel ion monovalent (+I). Thus, one-
electron oxidation of 1 results in an oxidatively induced
reduction of the central nickel ion; eq 1 and, conversely, the
reduction of 2 represents a reductively induced oxidation of
the central nickel ion.
[NiII(L•) ] [NiI(LOx) ]+
≡
+ e-
(1)
2
2
(1)
(2)
Figure 3. Structure of the five-coordinate monocation [NiII(LOx)2(FPF5)]+
in crystals of 3 (ellipsoids are at the 50% probability level).
Interestingly, the structure of the monocation [ZnII(tL•)(tLOx)]-
(OTf) in ref 8 clearly exhibits two inequivalent ligands with
differing C-N and C-C distances. The av C-N length of
1.324 Å for the first and 1.271 Å for the second ligand (C-
C: 1.406 Å for the first and 1.474 Å (!) for the second)
unambiguously demonstrate the presence of a neutral ligand
(tLOx)0 and of a single π radical anion (tL•)1-. These
experimental results imply that the electronic structure of
the monocation in the nickel complex 2 cannot be represented
as [NiII(L•)(LOx)]+ with localized ligand oxidation levels
(L•)1- and (LOx)0. Due to the fact that the two five-membered
chelate rings are nearly orthogonal with respect to each other,
the differing oxidation levels of the two ligands, if they exist,
should be localized. This is different in similar square planar
nickel complexes of D2h symmetry (monoanion or monoca-
tion)41 where electron delocalization of class III prevails and
the two ligands display identical C-N and C-C bond lengths
that are the arithmetric mean values between those of one
(L•)1- and one (LOx)0 (or [LRed]2-).
Figure 4. Structure of the six-coordinate monocation [NiII(LOx)2(THF)-
(FPF5)]+ in crystals of 4.
The structure of 3 is composed of the monocation
[NiII(LOx)2(F-PF6)]+ (Figure 3) and another well-separated
PF6- anion. The cation possesses two neutral (LOx)0 ligands
Crystals of 1 consist of distorted tetrahedral, neutral
molecules [NiII(L•)2]. There are two crystallographically
independent sites occupied by four such molecules in the
unit cell of the triclinic space group P1h (Z ) 4). The
geometrical features (Figure 1) of these two independent
molecules are identical within experimental error. It is clearly
established that both ligands of these molecules are bound
as monoanionic radicals to a nickel(II) center. The average
C-N bond length of the five-membered chelates at 1.336
( 0.01 and 1.319 ( 0.01 Å, respectively, and the av C-C
bond length at 1.414 ( 0.01 Å are typical for (L•)1- radicals
(see above). The two best chelate planes Ni-N-C-C-N
in each of the two neutral molecules in 1 form dihedral angles
of 78.9° and 78.4°, respectively, which is close to the ideal
tetrahedral angle of 90°.
It is remarkable that the structure of tetrahedral [ZnII(tL•)2]
in ref 2 displays very similar geometric features to those of
the two identical Zn-N-C-C-N chelate rings: the av
C-N bond length at 1.330 Å and the av C-C bond distance
at 1.389 Å indicate again the presence of two ligand π radical
monoanions.
-
and a single F-coordinated PF6 anion. The Ni‚‚‚F bond at
2.452(2) Å is very weak. Thus, the nickel center in the cation
is five-coordinate (4 + 1), and the coordination polyhedron
is best described not as square base pyramidal (τ ) 0.0) but
trigonal bipyramidal (τ ) 1.0) with a τ value of 0.88 or 0.85
depending on the choice of the apical ligand (F or N21).46
The C-N bonds of both ligands are short at av 1.280 Å,
indicating the presence of CdN double bonds whereas the
av C-C bond of the five-membered chelate rings is long at
1.497 Å (C-C single bond). Thus, the oxidation level of
both ligands is as in (LOx)0 rendering the central nickel ion
divalent. These ligand dimensions are very similar to those
reported for [ZnII(tLOx)2]OTf (av C-N 1.251 Å; C-C 1.465-
(11) Å).8 Oxidation of 2 to 3 is thus a metal-centered process.
Crystals of 4 are composed of a monocation [NiII(LOx)2-
(THF)(FPF5)]+, well-separated PF6- anions, and an additional
THF molecule of crystallization. The monocation shown in
Figure 4 possesses two closed-shell neutral ligands (LOx)0
As shown in Figure 2, crystals of 2 consist of well-
separated monocations [Ni(LOx)2]+ and hexafluorophosphate
anions. The monocation possesses a distorted tetrahedral
environment composed of two identical, crystallographically
(46) τ is a geometric parameter, and it was introduced for pentacoordinated
structures as a measure of trigonality. τ is 0 for a perfectly square-
pyramidal geometry, while it becomes unity for a perfectly trigonal-
bipyramidal geometry. Addison, A. W.; Nageswara Rao, T. J. Chem.
Soc., Dalton Trans. 1984, 1349.
5332 Inorganic Chemistry, Vol. 46, No. 13, 2007

Bis(r-diimine)nickel Complexes
Figure 5. Cyclic voltammogram of 1 in THF at 25 °C (conditions: 0.10
M [N(n-Bu)4]PF6 supporting electrolyte; scan rate 25 mV s-1; glassy carbon
working electrode).
Figure 6. Electronic spectra of 1, 2, and 3 in CH2Cl2 solution at 20 °C.
as is readily deduced from their CdN and C-C bond
distances. The Ni‚‚‚F bond at 2.318(2) Å which is shorter
than its analogue in 3 despite the increase of the coordination
number from five to six by binding of a sixth THF ligand in
4.
The average Ni-N bond distance increases from 1 (1.930
Å) to 2 (1.983 Å) as one might expect for two species with
the same tetrahedral polyhedron but decreasing oxidation
state of the metal (+II in 1, but +I in 2). Upon increasing
the coordination number from 4 in 1 to 5 in 3, and to 6 in
4, keeping the metal oxidation state constant at +II is
accompanied by an increase of the av Ni-N bond length
(1.930 f 2.009 f 2.066 Å).
3. Electro- and Spectroelectrochemistry (UV-vis; EPR).
The cyclic voltammogram of 1 in a tetrahydrofurane solution
containing 0.10 M [N(n-Bu)4]PF6 supporting electrolyte
recorded at 25 °C at a glassy carbon working electrode is
shown in Figure 5. Two one-electron-transfer waves are
observed at E11/2 ) -0.82 V versus the ferrocenium/ferrocene
Figure 7. X-Band EPR spectrum of a liquid nitrogen shock frozen CH2-
Cl2 solution of 2 (10 K) (conditions: frequency 9.434 GHz; power 5 µW;
modulation 8 G). See Table 4 for simulation parameters for the two
subspectra a and b.
Table 3. Electronic Spectra of Complexes in CH2Cl2 Solution at 20 °C
couple (Fc+/Fc) and E2 ) -0.53 V. From coulometric
1/2
complex
λ )
max, nm (ꢀ, 104 M-1 cm-1
measurements it was established that both processes cor-
respond to two successive one-electron oxidations of 1.
Interestingly, the first wave in the CV is fully reversible
whereas the second wave displays a more irregularly shaped
wave in accordance with the observation that one-electron
oxidation of the monocation [NiI(LOx)2]+ affords a cationic
species [NiII(LOx)2(FPF6)(THF)]+ with an increased coordina-
tion number. The process may therefore be described as an
EC-mechanism (electron transfer followed by a chemical
reaction). It is noteworthy that reduction of the ligands with
1
2
3
350sh, 420(0.3), 495(1.5), 700(0.08)
350sh, 559(0.87), 757sh(0.4), ∼1800(0.2)
297(0.68)
Table 4. X-band EPR Spectra of 2 in Frozen CH2Cl2 Solution at 77 K
isomer
gx
gy
gz
a
b
calcd
2.066
2.070
2.085
2.210
2.198
2.244
2.444
2.359
2.370
the freezing procedure, the [a]/[b] ratio changes indicating
a thermodynamic equilibrium in solution with rapid trans-
formation rates. It is well-established that heat transfer rates
in liquid N2 differ markedly from those in liquid n-pentane/
N2 (it is slow in the former and faster in the latter process).
As noted previously, distorted tetrahedral complexes contain-
ing two asymmetric bidentate ligands exist as two geo-
metrical isomers provided that the dihedral angle between
the two five-membered chelate rings are *90°. In other
words, the distance between the centers of the phenyl rings
of the two chelates in [Ni(LOx)2]+ is different in A from that
in B.
formation of (LRed 2-
is not observed in the potential range
)
0 to -1.7 V versus Fc+/Fc.
Figure 6 displays the electronic spectra of 1, 2, and 3 in
CH2Cl2 solution, and Table 3 summarizes the data. We have
also recorded the X-band EPR spectrum of paramagnetic 2
(S ) 1/2) in frozen CH2Cl2 solution at 10 K which is displayed
in Figure 7.
As has been reported previously by us for a similar nickel
complex, the spectrum could only be satisfactorily simulated
by taking into account two subspectra, a and b. Both spectra
display rather similar but not identical gx, gy, gz values (Table
4). The ratio of species [a]/[b] was found to be 60:40 when
the solution of 2 was shock-frozen with liquid N2 and 45:55
in a liquid n-pentante/liquid dinitrogen bath. Upon changing
We note that the g-anisotropy of the paramagnetic mono-
cations a and b in solution of 2 is quite large. This is
interpreted as a clear indication that the unpaired electron
Inorganic Chemistry, Vol. 46, No. 13, 2007 5333

Muresan et al.
resides in Ni d-orbital rather than in a π* ligand orbital. Thus,
two electron configurations must be considered: (a) a
genuine Ni(I) complex with a d9 electron configuration as
in distorted tetrahedral [NiI(LOx)2]+ containing two closed-
shell 1,2-diimine ligands (LOx)0 or (b) a nickel(II) complex
with a d8 electron configuration (SNi ) 1) and a single
localized or delocalized ligand π radical monoanion [NiII(L•)-
(LOx)]+ T [NiII(LOx)(L•)]+ where the ligand spin (Srad ) 1/2)
couples intramolecularly antiferromagnetically with the
1
nickel ion (SNi ) 1) yielding the observed S ) /2 ground
state. The present EPR spectra alone do not allow to
unequivocally distinguish between choices a and b.
4. DFT Calculations. a. Optimized Geometries. Here we
first develop a detailed picture of the electronic structures of
the zinc complexes [ZnII(tL•)2], [ZnII(tLOx)(tL•)]+, [Zn(tLOx)2]2+,
and [ZnII(tL•)(tLRed)]- and then of the nickel complexes [NiII-
(L•)2], [NiI(LOx)2]+, and [NiII(LOx)2]2+. As pointed out above,
the molecular structures of 5, 6, and 8, and of 1, 2, and 3
have been determined by X-ray crystallography at cryogenic
temperatures.
The geometry of 5 was optimized under the assumptions
of either an S ) 1 state or a broken-symmetry BS(1,1)
antiferromagnetically coupled singlet state with the B3LYP
functional. The two structures were found to be essentially
identical and within the scatter of the relaxation procedure
also of identical energy (e.g., within 0.1 kcal/mol). Thus,
we conclude that there are two unpaired electrons in 5 that
are essentially uncoupled. Examination of the spin-density
profiles indicates that the two unpaired electrons are situated
on the ligands that are thus in a π-radical state. Interestingly,
the calculated C-N and C-C bond lengths in all three states
are identical (see Figure 8) and in excellent agreement with
experiment. These data support the presence of two π-rad-
icals in 5. The calculated av Zn-N bond lengths are
overestimated by ∼0.05 Å, a result which is typical for the
B3LYP functional. The calculated dihedral angle between
the two metallacycles at 80.2° is also in excellent agreement
with experiment.
For neutral [Ni(L•)2] (1), the ground state geometry was
calculated for a BS(2,2) Ms ) 0 state. In this instance, a
stationary state on the potential energy surface was located
where the calculated bond distances within the two equivalent
ligands are in excellent agreement with experiment and
indicative of the presence of two ligand π radicals (L•)1-.
The Ni-N distances are again overestimated by ∼0.08 Å.
We also calculated a closed-shell S ) 0 state and a high-
spin, open-shell S ) 2 state for 1 which were both found to
be higher in energy by 15.7 and 16.0 kcal mol-1, respectively,
than the BS (2,2) Ms ) 0 state. We refrain from a discussion
of these solutions. In summary, the present geometry
optimizations shown in Figure 8 for 1 and 5 are in excellent
accord with experiment and indicate the presence of two
Figure 8. Experimental (red) and calculated (green) lengths (Å) in 5 (top)
and 1 (bottom).
ligand π radical monoanions and a Zn(II) (SZn ) 0) and a
Ni(II) (SNi ) 1), respectively.
In a very similar fashion, we have calculated and optimized
the geometry of the monocations in 6 and 2. Attempts to
find broken symmetry solutions BS(2,1) for 2 and BS(1,0)
1
for 6 failed. In both cases, the S ) /2 solution was found
only. Thus, attempts to calculate [NiII(L•)(LOx)]+ with a high-
spin Ni(II) ion and an antiferromagnetically coupled ligand
radical (L•)1- reverted to the S ) 1/2 (Ni(I), d9) solution (see
below). The results are shown in Figure 9. The calculated
structure of the monocation in 6 indicates the presence of
two different ligands, namely a π radical monoanion (tL•)1-
and a neutral ligand (tLOx)0. The C-N and C-C distances
are different in both ligands. They agree very well with the
experimental data. Thus, 6 clearly contains a cation [ZnII-
(tLOx)(tL•)]+ where the unpaired electron is localized on one
monoanion (tL•)1-. In stark contrast, the structure of the
cation in 2 contains two equivalent ligands with four short
C-N bonds (1.29 Å) and two long C-C bond lengths at
1.47 Å which correspond to CdN double and C-C single
bonds, respectively, rendering the oxidation level of both
ligands (LOx)0. Consequently, the oxidation state of the central
nickel ion must be +I. These calculated data are again in
excellent agreement with experiment.
The optimized geometry of the diamagnetic, closed-shell
dication [Zn(tLOx)2]2+ (7) S ) 0 is shown in Figure 10. The
agreement between experiment8 and calculation is only
reasonable due to the low quality of the X-ray structure of
5334 Inorganic Chemistry, Vol. 46, No. 13, 2007

Bis(r-diimine)nickel Complexes
Figure 9. Experimental (red) and calculated (green) bond lengths (Å) in
the monocations of 6 (top) and 2 (bottom). θ represents the dihedral angle
between the two M-N-C-C-N chelate rings.
Figure 10. Experimental (red) and calculated (green) bond lengths (Å)
in the dication [ZnII(tLOx)2]2+ in 7 (top) and hypothetical [NiII(LOx)2]2+ (S
) 1) (bottom). The experimental data for the latter were taken from the
crystal structure of 3.
7, but it is nonetheless unambiguously established that the
ligands are at the (tLOx)0 oxidation level. Similarly, the
calculations of the hypothetical, paramagnetic dication [NiII-
(LOx)2]2+ (S ) 1) show the presence of the two neutral (LOx)0
ligands. The C-N and C-C distances are in excellent
agreement with those measured in 3 containing the five-
coordinate monocation [NiII(LOx)2(FPF5)]+ and 4 with a six-
coordinate monocation [NiII(LOx)2(THF)(FPF5)]+.
Finally, we have optimized the geometry of the paramag-
netic monoanion [ZnII(tLRed)(tL•)]- (S )1/2). The results are
shown in Figure 11 where it is clearly seen that the two
ligands exist in two different, localized oxidation levels,
namely (tLRed 2- and (tL•)1-. This result is in excellent
)
agreement with experiment8 and calculations for neutral
[AlIII(tLRed)(tL•)] in ref 13.
Figure 11. Experimental (red) and calculated (green) bond lengths (Å)
in the monoanion [ZnII(tLRed)(tL•)]-.
b. Electronic Structures. A qualitative bonding scheme
derived from the spin-unrestricted BS calculation of 1 is
shown in Figure 12. Five orbitals which are mainly of metal
d character are identified. Three of these orbitals are found
in the spin-up and the spin-down manifolds; they are doubly
occupied. The other two nickel-based orbitals originate from
the t2 set and occur only in the spin-up manifold. These two
orbitals are thus singly occupied with parallel spins. This
pattern defines a high-spin Ni(II) configuration (SNi ) 1) at
the metal center. In addition to these two metal-centered
SOMOs, two ligand-centered orbitals are identified in the
spin-down manifold which are not populated in the spin-up
manifold, thus leading to the observed overall Ms ) 0 BS
state. These orbitals correspond to two symmetry-adapted
combinations of the SOMO of the two ligand radicals. Thus,
the basic electronic structure description of 1 features a high-
spin Ni(II) ion which is strongly antiferromagnetically
coupled to two ligand-centered π radicals.24,25 The calculated
spatial overlaps for these corresponding metal-ligand couples
are at S ) 0.54 and 0.52, indicative of the antiferromagnetic
nature of these interactions. The magnetic exchange coupling
constant J was calculated to be -1250 cm-1 according to
the Yamaguchi approach47 (Hˆ HDvV ) -2JSˆA‚SˆB). This
calculated large value is in accord with the experimental
Inorganic Chemistry, Vol. 46, No. 13, 2007 5335

Muresan et al.
Figure 14. Spin density plot of 5 as derived from (a) S ) 1 (left) and (b)
S ) 0 (right) DFT calculations.
Figure 15. Spin density plot of the monocation [ZnII(tLOx)(tL•)]+ in 6 (S
1
) /2) as derived from DFT calculations.
breakdown of the spin density into atomic contributions via
a spin population analysis supports the presence of two
unpaired electrons at the Ni ion with positive spin and two
unpaired electrons with negative spin localized on both ligand
π radicals.
Figure 12. Qualitative MO diagram of the corresponding orbitals35 of
magnetic pairs of 1 as derived from the broken symmetry DFT calculations
(B3LYP).
It is now very instructive to compare these results with
those obtained for the analogous zinc complex 5. Five
predominantly metal d orbitals which are each doubly
occupied were identified indicating the presence of a Zn(II)
ion (d10, SZn ) 0). In addition, two singly occupied ligand-
based π orbitals are identified which possess parallel spin
orientation in the S ) 1 state but are antiparallel in the S )
0 state as shown in the spin density plot of Figure 14. Since
the singlet and triplet states are nearly degenerate, the two
spins in 5 are essentially uncoupled.
Figure 13. Spin density plot of 1 as derived from BS DFT calculations
together with values of the spin density of the Mulliken spin population
analysis.
It is now very interesting that for the monocation [ZnII-
(tL•)(tLOx)]+ in 6 we identify again five doubly occupied
metal d orbitals as expected for a Zn(II) ion (d10, SZn ) 0)
and, in addition, one singly occupied orbital at the ligand π
radical (tL•)1-. The spin density plot shown in Figure 15
clearly establishes that only this ligand orbital carries spin
density. The ligand (tLOx)0 is a closed-shell, neutral ligand
finding that 1 possesses a singlet ground state up to 20 °C
with no indication of thermal population of higher spin states.
Keeping in mind the caveats pointed out in refs 24 and
25, it is possible to analyze the spin density in 1. The
corresponding plot for 1 is shown in Figure 13 which nicely
visualizes the antiparallel spin alignment between the high-
spin Ni(II) (positive spin density in red) and the radical
ligands (negative spin density in yellow). An approximate
Ox
without spin density (SL ) 0).
In stark contrast is the calculated electronic structure of
the corresponding monocation [NiI(LOx)2]+ in 2. As displayed
in Figure 16a, five predominantly metal d orbitals are clearly
identified, the first four of which are doubly occupied
whereas the fifth d-orbital is only singly occupied. This is
indicative of the presence of a paramagnetic Ni(I) ion (d9;
(47) (a) Soda, T.; Kitagawa, Y.; Onishi, T.; Takano, Y.; Shigeta, Y.; Nagao,
H.; Yoshioka, Y.; Yamaguchi, K. Chem. Phys. Lett. 2000, 319, 223.
(b) Yamaguchi, K.; Takahara, Y.; Fueno, T. In Applied Quantum
Chemistry; Smith, V. H., Eds.; Reidel: Dordrecht, 1986; p 155.
(48) Lo¨wdin, P.-O. AdV. Quantum Chem. 1970, 5, 185.
5336 Inorganic Chemistry, Vol. 46, No. 13, 2007

Bis(r-diimine)nickel Complexes
(tLRed 2- and a paramagnetic monoanion (tL•)1- (SRad ) 1/2).
)
The ligand centered radical is localized at the one (tL•)1-
ligand.
Conclusions
Three members of the electron-transfer series [Ni(L)2]z (z
) 0, 1+, 2+) have been synthesized and structurally and
spectroscopically characterized (UV-vis, EPR, electro- and
magnetochemically). In addition, broken symmetry DFT
calculations have been performed for each member in order
to elucidate their electronic structures. The computational
methodology employed has been calibrated by calculations
of the corresponding series [ZnII(tL)2]z (z ) 2+, 1+, 0, 1-)
where the results were found to be in excellent agreement
with experiment in ref 8. The most salient results of this
work may be summarized as follows.
(a) The tetrahedral, neutral complexes [NiII(L•)2] and
[ZnII(tL•)2] contain both a divalent metal ion and two
monoanionic R-diimine ligand π radicals. In the diamagnetic
nickel(II) complex, the central high spin metal ion (SNi ) 1)
is strongly antiferromagnetically coupled to two ligand π
1
radicals (Srad ) /2) yielding an St ) 0 ground state. In
contrast, the tetrahedral Zn complex (SZn ) 0) contains two
essentially uncoupled π radical ligand anions which render
this species paramagnetic.
(b) One-electron oxidation of the neutral species yields
the tetrahedral monocations: [NiI(LOx)2]+ and [ZnII(tLOx)-
1
(tL•)]+ both with an St ) /2 ground state but with differing
electronic structures. The nickel complex contains a central
1
Ni(I) ion (d9, SNi ) /2) and two neutral R-diimine ligands
LOx. Thus, the electrochemical or chemical one-electron
oxidation of the neutral complex [NiII(L•)2] involves oxidation
of both ligand radicals (one electron goes to the oxidant and
one to the Ni(II) ion) and a one-electron reduction of the
Ni(II) ion. In contrast, the zinc monocation [ZnII(tLOx)(tL•)]+
consists of a diamagnetic Zn(II) ion (d10, SZn ) 0), one
anionic ligand π radical (tL•)1-, and one neutral oxidized
Figure 16. (a) Qualitative MO diagram of the corresponding orbitals of
2 as derived from DFT calculations. (b) Spin density plot of the monocation
in 2 (S ) /2).
1
1
SNi ) /2). All ligand orbitals are either doubly or not
1
R-diimine ligand (tLOx) yielding an Srad ) St ) /2 ground
occupied at all as is expected for two closed-shell diamag-
netic (LOx)0 ligands. Accordingly, the spin density shown in
Figure 16b is localized at the nickel center; no spin density
is observed on the ligands. Thus, the electronic structures
of the monocations of Zn in 6 and of nickel in 2 are very
different: [ZnII(tL•)(tLOx)]+ versus [NiI(LOx)2]+.
state.
(c) The tetrahedral dications [Ni(LOx)2]2+ and [ZnII(tLOx)2]2+
consist of a central Ni(II) ion (SNi ) 1) and a diamagnetic
Zn(II) ion (d10, SZn ) 0) and two neutral, diamagnetic
R-diimine ligands, respectively.
Finally, the hypothetical tetrahedral dication [NiII(LOx)2]2+
contains five predominantly metal d orbitals, the first three
of which are doubly occupied whereas the remaining two d
orbitals are only singly occupied. This is representative for
a paramagnetic Ni(II) ion (d8, SNi ) 1) in a tetrahedral ligand
environment composed of two neutral (LOx) ligands.
For the sake of completeness, the electronic structure of
the monoanion [ZnII(tLRed)(tL•)]- is best described as a Zn(II)
ion (d10, SZn ) 0) coordinated to a diamaganetic dianion
Acknowledgment. We are grateful to the Fonds der
Chemischen Industrie for financial support.
Supporting Information Available: X-ray crystallographic
CIF-files for compounds 1, 2, 3, and 4 and listings of DFT
calculations (bond distances and Mulliken population analyses) of
zinc and nickel complexes. This material is free of charge via the
Internet at http://pubs.acs.org.
IC700407M
Inorganic Chemistry, Vol. 46, No. 13, 2007 5337