Dicopper(I) complexes with reduced states of 3,6-bis(2′-pyrimidyl)-1,2,4,5-tetrazine: Crystal structures and spectroscopic properties of the free ligand, a radical species, and a complex of the 1,4-dihydro form

Article abstract of DOI:10.1021/ic001228q

The complexes {(μ-bmtz^·-)[Cu(PPh₃)₂] ₂}(BF₄) (1) and {(μH₂bmtz)[Cu (PPh₃)₂]₂}(BF₄)₂ (2) (bmtz = 3,6-bis(2′-pyrimidyl)-1,2,4,5-tetrazine and H₂bmtz = 1,4-dihydro-3,6-bis(2′-pyrimidyl)-1,2,4,5-tetrazine) were obtained as stable materials that could be crystallized for structure determination. 1·2 CH₂Cl₂: C₈₄H₇₀BCl₄ Cu₂F₄N₈P₄; monoclinic, C2/c; a = 26.215(7) A, b = 22.122(6) A, c = 18.114(5) A, β = 133.51(1)°; Z = 4. 2·CH₂Cl₂: C₈₃H₇₀B₂ Cl₂Cu₂F₈N₈P₄; triclinic, P1; a = 10.948(2) A, b = 12.067(2) A, c = 30.287(6) A, α = 93.82(3)°, β = 94.46(3)°, γ = 101.60(3)°; Z = 2. Bmtz itself was also structurally characterized (C₁₀H₆N₈; monoclinic, P2₁/c; a = 3.8234(8) A, b = 10.147(2) A, c = 13.195(3) A, β = 94.92(3)°; Z = 2). Whereas the radical complex ion contains a planar tetrazine ring in the center, the 1,4-dihydrotetrazine heterocycle in the corresponding complex of H₂bmtz is considerably folded. Both systems exhibit slight twists between the tetrazine and the pyrimidine rings. The intra-tetrazine distances are characteristically affected by the electron transfer, as is also evident from a comparison with the new structure of free bmtz; the bonding to copper(I) changes accordingly. Spectroscopy including X- and W-band EPR of the radical species confirms that the electron addition is mainly to the tetrazine ring.

Full text of DOI:10.1021/ic001228q

Inorg. Chem. 2001, 40, 2263-2269
2263
Dicopper(I) Complexes with Reduced States of 3,6-Bis(2′-pyrimidyl)-1,2,4,5-tetrazine:
Crystal Structures and Spectroscopic Properties of the Free Ligand, a Radical Species, and
a Complex of the 1,4-Dihydro Form
Markus Glo1ckle,† Klaus Hu1bler,† Hans-Ju1rgen Ku1mmerer,‡ Gert Denninger,‡ and
Wolfgang Kaim*,†
Institut fu¨r Anorganische Chemie, Universita¨t Stuttgart, Pfaffenwaldring 55,
D-70550 Stuttgart, Germany, and Physikalisches Institut, Universita¨t Stuttgart, Pfaffenwaldring 57,
D-70550 Stuttgart, Germany
ReceiVed NoVember 6, 2000
The complexes {(µ-bmtz•-)[Cu(PPh3)2]2}(BF4) (1) and {(µ-H2bmtz)[Cu(PPh3)2]2}(BF4)2 (2) (bmtz ) 3,6-bis(2′-
pyrimidyl)-1,2,4,5-tetrazine and H2bmtz ) 1,4-dihydro-3,6-bis(2′-pyrimidyl)-1,2,4,5-tetrazine) were obtained as
stable materials that could be crystallized for structure determination. 1‚2 CH2Cl2: C84H70BCl4Cu2F4N8P4;
monoclinic, C2/c; a ) 26.215(7) Å, b ) 22.122(6) Å, c ) 18.114(5) Å, â ) 133.51(1)°; Z ) 4. 2‚CH2Cl2:
C83H70B2Cl2Cu2F8N8P4; triclinic, P1h; a ) 10.948(2) Å, b ) 12.067(2) Å, c ) 30.287(6) Å, R ) 93.82(3)°, â )
94.46(3)°, γ ) 101.60(3)°; Z ) 2. Bmtz itself was also structurally characterized (C10H6N8; monoclinic, P21/c;
a ) 3.8234(8) Å, b ) 10.147(2) Å, c ) 13.195(3) Å, â ) 94.92(3)°; Z ) 2). Whereas the radical complex ion
contains a planar tetrazine ring in the center, the 1,4-dihydrotetrazine heterocycle in the corresponding complex
of H2bmtz is considerably folded. Both systems exhibit slight twists between the tetrazine and the pyrimidine
rings. The intra-tetrazine distances are characteristically affected by the electron transfer, as is also evident from
a comparison with the new structure of free bmtz; the bonding to copper(I) changes accordingly. Spectroscopy
including X- and W-band EPR of the radical species confirms that the electron addition is mainly to the tetrazine
ring.
Introduction
general,10 to undergo stepwise reduction and protonation, leading
to 1,4-dihydro derivatives11 after a 2e/2H+ process. Yet, the
ability of bmtz to bind metal centers via the azo-imine chelating
groups should remain because there are four such moieties in
bmtz with only two being engaged in the reductive protonation.
Thus, bmtz is a multifunctional system where coupling of
electron and proton transfer and interaction with two metal
centers can occur.
The bmtz ligand is related to 3,6-bis(2′-pyridyl)-1,2,4,5-
tetrazine (bptz);12 bptz and the isomeric 3,6-bis(4′-pyridyl)-
1,2,4,5-tetrazine13 have been used as strong acceptor ligands12-21
and, more recently, as components of supramolecular struc-
tures.22-24 Nitrogen-rich unsaturated ligands such as bptz25-28
Coordination compounds between copper(I) as a structurally
flexible template center and multifunctional organic ligands have
been used to construct a large variety of unusual structures in
supramolecular chemistry.1,2 Examples include catenates3 and
molecular knots,4 grids,5 helicates,6 and decks.7 To extend this
scope, we have recently presented the new potentially tetrakis-
bischelate ligand 3,6-bis(2′-pyrimidyl)-1,2,4,5-tetrazine (bmtz)8
and described its capacity for inducing strong metal-metal
interaction in dinuclear FeIIFeIII or RuIIRuIII mixed-valent
situations.8,9
As a highly electron-deficient and multiply nucleophilic
molecule, bmtz can be expected, like 1,2,4,5-tetrazines in
† Institut fu¨r Anorganische Chemie.
(10) Neugebauer, F. A.; Krieger, C.; Fischer, H.; Siegel, R. Chem. Ber.
1983, 116, 2261.
‡ Physikalisches Institut.
(1) Munakata, M.; Wu, L. P.; Kuroda-Sowa, T. AdV. Inorg. Chem. 1999,
(11) Hausen, H.-D.; Hornung, F.-M.; Ketterle, M.; Schwach, M.; Kaim,
W. Unpublished results.
46, 173.
(2) Swiegers, G. T.; Malefetse, T. J. Chem. ReV. 2000, 100, 3483.
(3) (a) Livoreil, A.; Dietrich-Buchecker, C.; Sauvage, J.-P. J. Am. Chem.
Soc. 1994, 116, 9399. (b) Baumann, F.; Livoreil, A.; Kaim, W.;
Sauvage, J.-P. J. Chem. Soc., Chem. Commun. 1997, 35.
(4) Chambron, J.-C.; Dietrich-Buchecker, C. O.; Nierengarten, J.-F.;
Sauvage, J.-P.; Solladie´, N.; Albrecht-Gary, A. M.; Meyer, M. New
J. Chem. 1995, 19, 409.
(5) (a) Baxter, P. N. W.; Lehn, J.-M.; Fischer, J.; Yoinou, M.-T. Angew.
Chem. 1994, 106, 2432; Angew. Chem., Int. Ed. Engl. 1994, 33, 2284.
(b) Bassani, D. M.; Lehn, J.-M.; Fromm, K.; Fenske, D. Angew. Chem.
1998, 110, 2534; Angew. Chem., Int. Ed. Engl. 1998, 37, 2364.
(6) Hasenknopf, B.; Lehn, J.-M.; Boumediene, N.; Dupont-Gervais, A.;
Van Dorsselaer, A.; Kneisel, B.; Fenske, D. J. Am. Chem. Soc. 1997,
119, 10956.
(12) Kohlmann, S.; Ernst, S.; Kaim, W. Angew. Chem. 1985, 97, 698;
Angew. Chem., Int. Ed. Engl. 1985, 24, 684.
(13) Kaim, W.; Kohlmann, S. Inorg. Chem. 1990, 29, 1898.
(14) Kaim, W.; Kohlmann, S. Inorg. Chem. 1987, 26, 68.
(15) Kaim, W.; Kohlmann, S. Inorg. Chem. 1987, 26, 1469.
(16) Ernst, S. D.; Kaim, W. Inorg. Chem. 1989, 28, 1520.
(17) Kaim, W.; Kohlmann, S. Inorg. Chem. 1990, 29, 2909.
(18) Klein, A.; Hasenzahl, S.; Kaim, W.; Fiedler, J. Organometallics 1998,
17, 3532.
(19) Poppe, J.; Moscherosch, M.; Kaim, W. Inorg. Chem. 1993, 32, 2640.
(20) Roche, S.; Thomas, J. A.; Yellowlees, L. J. J. Chem. Soc., Chem.
Commun. 1998, 1429.
(21) Glo¨ckle, M.; Fiedler, J.; Katz, N. E.; Garcia Posse, M.; Cutin, E.;
Kaim, W. Inorg. Chem. 1999, 38, 3270.
(7) Schwach, M.; Hausen, H.-D.; Kaim, W. Chem.sEur. J. 1996, 2, 446.
(8) Kaim, W.; Fees, J. Z. Naturforsch. 1995, 50b, 123.
(9) Ketterle, M.; Fiedler, J.; Kaim, W. J. Chem. Soc., Chem. Commun.
1998, 1701.
(22) Campos-Ferna´ndez, C. S.; Cle´rac, R.; Dunbar, K. R. Angew. Chem.
1999, 111, 3685; Angew. Chem., Int. Ed. 1999, 38, 3477.
(23) Withersby, M. A.; Blake, A. J.; Champness, N. R.; Cooke, P. A.;
Hubberstey, P.; Scho¨der, M. J. Am. Chem. Soc. 2000, 122, 4044.
10.1021/ic001228q CCC: $20.00 © 2001 American Chemical Society
Published on Web 04/06/2001

2264 Inorganic Chemistry, Vol. 40, No. 10, 2001
Table 1. Crystallographic and Refinement Data
Glo¨ckle et al.
bmtz
1‚2CH2Cl2
2‚CH2Cl2
empirical formula
fw
temp (K)
wavelength (Å)
cryst syst
space group
a (Å)
C10H6N8
238.23
173(2)
0.710 73
monoclinic
P21/c (No. 14)
3.8234(8)
10.147(2)
13.195(3)
90
94.92(3)
90
510.1(2)
2
1.551
0.108
C84H70BCl4Cu2F4N8P4
1671.05
173(2)
0.710 73
monoclinic
C2/c (No. 15)
26.215(7)
22.122(6)
18.114(5)
90
C83H70B2Cl2Cu2F8N8P4
1674.95
173(2)
0.710 73
triclinic
P1h (No. 2)
10.948(2)
12.067(2)
30.287(6)
93.82(3)
94.46(3)
101.60(3)
3893.8(14)
2
b (Å)
c (Å)
R (deg)
â (deg)
133.51(1)
90
7619(3)
γ (deg)
V (Å3)
Z
F
4
calcd (g cm-3
)
1.457
0.845
1.008
R1 ) 0.0763
Rw ) 0.2168
1.429
0.767
1.196
R1 ) 0.0763
Rw ) 0.1952
µ (mm-1
)
GOF
1.052
R1 ) 0.0426
Rw ) 0.1334
final R indices [I > 2 σ(I)] (all data)a
2
2
2
a R1 ) (∑||Fo| - |Fc||)/∑|Fo|. Rw ) {∑[w(|Fo| - |Fc| )2]/∑[w(Fo )2]}1/2
.
and the recently described 2,2′-azobis(5-chloropyrimidine)29 are
known to form radical complexes rather easily.
been used up and the volume of the solution was reduced to about
one-fourth. Treatment with pentane produced a red oil that produced a
red solid after addition of little dichloromethane and diethyl ether.
Washing with pentane, drying in vacuo, dissolution in 8 mL of
acetonitrile, filtration from insoluble material, and removal of the solvent
gave analytically pure 1 as a very air- and moisture-sensitive red
material in 57% yield (260 mg). Anal.Calcd for C82H66BCu2F4N8P4
(1501.28 g/mol): C, 65.60; H, 4.43; N, 7.46. Found: C, 64.89; H,
4.32; N, 7.47.
In this report we describe the crystal structure of bmtz as
well as structural and spectroscopic studies of two dicopper(I)
complexes of the reduced forms of bmtz, viz., the radical ion
complex {(µ-bmtz•-)[Cu(PPh3)2]2}(BF4) (1) and the correspond-
ing 1,4-dihydro species {(µ-H2bmtz)[Cu(PPh3)2]2}(BF4)2 (2)
(H2bmtz ) 1,4-dihydro-3,6-bis(2′-pyrimidyl)-1,2,4,5-tetrazine).
The crystallographic results thus allowed us to structurally
compare compounds of all three components within the redox
system bmtz0/•-/2-. In addition to the structural information,
spectroscopic data were obtained to assess the electronic
structures of the compounds.
{(µ-H2bmtz)[Cu(PPh3)2]2}(BF4)2‚CH2Cl2 (2‚CH2Cl2). To 52.4 mg
(0.167 mmol) of [Cu(CH3CN)4](BF4) in 15 mL of dried dichlo-
romethane was added 88.0 mg (0.336 mmol) of PPh3 dissolved in 9
mL of CH2Cl2. After the mixture was stirred for 5 h at room
temperature, a solution of 20.0 mg (0.083 mmol) of H2bmtz8 in 8 mL
of CH2Cl2 was added and the orange solution was stirred for 16 h.
Addition of pentane precipitated the product, which was recrystallized
from dichloromethane/pentane to yield 100 mg (72%) of a bright-orange
material with moderate air sensitivity. Anal. Calcd for C83H70B2Cl2-
Cu2F8N8P4 (1674.95 g/mol): C, 59.52; H, 4.21; N, 6.69. Found: C,
59.58; H, 4.05; N, 6.69. 1H NMR (CD2Cl2): δ ) 8.80 (br, 4H, H4,4′,6,6′),
7.92 (br s, 2H, N1,4-H), 7.76 (t, 2H, H5,5′), 7.43-7.37, and 7.29-7.17
Experimental Section
Instrumentation. X-band EPR spectra at about 9.5 GHz were
recorded on a Bruker System ESP 300 equipped with a Bruker ER035M
gaussmeter and a HP 5350B microwave counter. W-band EPR spectra
at about 94 GHz were obtained with a Bruker ELEX SYS E680
spectrometer. 1H NMR spectra were taken on a Bruker AC 250
spectrometer. UV/vis absorption spectra were recorded on a Bruins
Instruments Omega 10 spectrophotometer. Cyclic voltammetry was
carried out at 50 mV/s standard scan rate in dichloromethane/0.2 M
Bu4NPF6 at 203 K, using a three-electrode configuration (glassy carbon
or Pt working electrode, Pt counter electrode, Ag/AgCl reference) and
a PAR 273 potentiostat and function generator. The ferrocene/
ferrocenium couple served as the internal reference.
{(µ-bmtz)[Cu(PPh3)2]2}(BF4) (1). A mixture containing 72.4 mg
(0.304 mmol) of bmtz,8 159.3 mg (0.607 mmol) of PPh3, and 19.4 mg
(0.305 mmol) of activated copper powder in 15 mL of dried dichlo-
romethane was stirred for 1 day at room temperature. (Activation of
Cu was accomplished by washing with hydrochloric acid, degassed
water, methanol, acetone, and diethyl ether). To this reddish solution
was then added in 8 mL of CH2Cl2 a preparation of [Cu(CH3CN)2-
(PPh3)2](BF4), obtained from 95.6 mg (0.304 mmol) of [Cu(CH3CN)4]-
(BF4) and 159.3 mg (0.607 mmol) of PPh3. The color of the solution
turned deep red immediately. After 43 h all the copper powder had
3
3
(m, 60H, phenyl-H); J4,5 ) J5,6 ) 4.9 Hz.
Crystallography. Single crystals were obtained through slow
evaporation of a saturated nitromethane solution (bmtz, dark-red rods,
0.5 mm × 0.2 mm × 0.2 mm) or through slow condensation of pentane
into a solution of 1 or 2 in dichloromethane at 8 °C (1‚2 CH2Cl2, dark-
red rods, 0.6 mm × 0.2 mm × 0.2 mm) or 22 °C (2‚CH2Cl2, orange
platelets, 0.4 mm × 0.4 mm × 0.15 mm).
Siemens P3 and P4 diffractometers with Mo KR radiation (λ )
0.710 73 Å) were used for data collection at 173 K. Intensity
measurements were performed using ω scans. The structures were
solved by direct methods with refinement by full-matrix least squares
of F2 using SHELXTL 5.1. Absorption correction was carried out via
ψ scan for compound 2 only (0.749 < T < 0.894).
Crystallographic and refinement data are summarized in Table 1.
All atoms of bmtz were refined using anisotropic temperature factors.
For 1‚2 CH2Cl2 the hydrogen atoms of aromatic rings were refined
freely; their isotropic temperature factors were set at those of the bound
carbon atoms, multiplied by 1.2. The hydrogen atoms of the dichlo-
romethane solvate molecules were introduced at ideal positions. The
tetrafluoroborate anion was found disordered; two alternative positions
with 67% and 33% weighting were used for the refinement. For 2‚
CH2Cl2 all hydrogen atoms were introduced at ideal positions; their
isotropic temperature factors were set at those of the bound carbon
atoms, multiplied by 1.2. The CH2Cl2 solvate molecule was found
disordered; two alternatives (each of 50% probability) with common
(24) Bu, X.-H.; Morishita, H.; Tanaka, K.; Biradha, K.; Furusho, S.;
Shionoya, M. J. Chem. Soc., Chem. Commun. 2000, 971.
(25) Schwach, M.; Hausen, H.-D.; Kaim, W. Inorg. Chem. 1999, 38, 2242.
(26) Kaim, W.; Ernst, S.; Kohlmann, S.; Welkerling, P. Chem. Phys. Lett.
1985, 118, 431.
(27) Kaim, W.; Kohlmann, S. Inorg. Chem. 1986, 25, 3442.
(28) Klein, A.; McInnes, E. J. L.; Scheiring, T.; Zalis, S. J. Chem. Soc.,
Faraday Trans. 1998, 94, 2979.
(29) Doslik, N.; Sixt, T.; Kaim, W. Angew. Chem. 1998, 110, 2125; Angew.
Chem., Int. Ed. 1998, 37, 7, 2403.
-
chlorine positions were used in the refinement. One of the BF4 ions

Dicopper(I) Complexes
Inorganic Chemistry, Vol. 40, No. 10, 2001 2265
Figure 1. Molecular structure of bmtz in the crystal.
Table 2. Bond Lengths [Å] and Angles [deg] of bmtza
N1-C5
N1-C1
C1-C2
N2-N3
N2-C6
1.3330(16)
1.3436(17)
1.382(2)
1.3296(15)
1.3393(17)
C2-C3
N3-C6A
C3-N4
N4-C5
C5-C6
1.380(2)
1.3437(16)
1.3412(17)
1.3351(17)
1.4930(16)
Figure 2. Molecular structure of {(µ-bmtz)[Cu(PPh3)2]2}•+ in the
crystal of 1‚2CH2Cl2.
C5-N1-C1
N1-C1-C2
N3-N2-C6
C3-C2-C1
N2-N3-C6A
N4-C3-C2
C5-N4-C3
115.49(12)
122.50(13)
117.26(11)
116.66(12)
117.90(11)
122.52(13)
115.59(12)
N1-C5-N4
N1-C5-C6
N4-C5-C6
N2-C6-N3A
N2-C6-C5
N3A-C6-C5
127.25(12)
117.04(11)
115.71(11)
124.85(11)
118.01(11)
117.15(11)
a Symmetry transformations used to generate equivalent atoms: (A)
-x, -y + 1, -z + 2.
was also disordered; refinement was performed with two alternative
positions with 56.4% and 43.6% weighting factor.
Results and Discussion
Syntheses and Structures. The dark-red ligand bmtz as
synthesized by the reaction of 2-cyanopyrimidine with hydra-
zine8 could be crystallized from nitromethane for X-ray dif-
fraction. Reaction of bmtz with 2 equiv of [Cu(PPh3)4](BF4)
yielded a very labile species assumed to be {(µ-bmtz)[Cu-
(PPh3)2]2}2+; it disintegrates above 210 K. Facile dissociation
from the weakly basic bmtz and enhanced reducibility through
coordination are considered responsible for this lability.
Reaction of bmtz with Cu, PPh3, and [Cu(PPh3)2(CH3CN)2]-
(BF4) in dry CH2Cl2 under argon produced the air- and moisture-
sensitive red radical complex {(µ-bmtz•-)[Cu(PPh3)2]2}(BF4)
(1), which was obtained in single crystal form as bis-dichlo-
romethane solvate for X-ray diffraction from pentane/dichlo-
romethane. The above route was chosen to avoid using hydrated
Cu(BF4)2 in a comproportionation reaction,25 which yields
dihydro derivatives as unwanted side products. The higher
sensitivity of 1 in comparison to the sensitivity of the bptz
analogue25 is attributed to the presence of additional nucleophilic
N centers for protonation-assisted degradation processes.
The light-orange compound {(µ-H2bmtz)[Cu(PPh3)2]2}(BF4)2
(2) was obtained from [Cu(PPh3)2(CH3CN)2](BF4) and H2-
bmtz8,11 in dichloromethane.
Figure 3. View of the structure of {(µ-bmtz)[Cu(PPh3)2]2}•+ along
the C2-C2A axis.
a
Table 3. Selected Distances [Å] in 1‚2CH2Cl2 and 2‚CH2Cl2
{(µ-bmtz)[Cu(PPh3)2]2}•+
Cu-N1
Cu-N2
Cu-P1
Cu-P2
N1-C1
N1-C5
N2-C6
2.133(6)
1.994(5)
2.225(2)
2.238(2)
1.332(9)
1.346(8)
1.327(8)
N2-N3
N3-C6A
N4-C5
N4-C3
1.380(7)
1.321(8)
1.319(9)
1.322(9)
Cu-CuA
6.680(2)
{(µ-H2bmtz)[Cu(PPh3)2]2}2+
Cu1-N1
Cu1-N2
Cu1-P1
Cu1-P2
Cu2-N5
Cu2-N6
Cu2-P3
Cu2-P4
N1-C4
N1-C1
N2-C5
N2-N3
2.072(5)
2.167(5)
2.228(2)
2.279(2)
2.089(5)
2.212(6)
2.264(2)
2.269(2)
1.341(8)
1.343(8)
1.288(8)
1.426(7)
N3-C10
N4-C3
N4-C4
N5-C9
N5-C8
N6-C10
N6-N7
N7-C5
N8-C9
N8-C6
1.376(8)
1.335(9)
1.335(8)
1.345(8)
1.347(9)
1.275(8)
1.420(7)
1.396(8)
1.327(8)
1.333(10)
Crystallographic data of bmtz and of the compounds 1‚2 CH2-
Cl2 and 2‚CH2Cl2 are summarized in Table 1, and selected bond
data are listed in Tables 2-4. Figures 1-5 illustrate the
molecular structures of the bmtz-containing species.
Cu1-Cu2
7.116(3)
a Symmetry transformations used to generate equivalent atoms: (A)
1
1
-x + /2, -y + /2, -z + 2.
The structure of the free ligand bmtz (Figure 1, Table 2) is
not esssentially different from that of the related bptz.28 Because
of the replacement of the two remaining ortho CH groups in
the outer rings by N, the dihedral angle between these rings
and the central tetrazine decreases from 19.1° in bptz28 to 12.5°
in bmtz. Stacking of the aromatic systems in the crystal occurs;
however, the closest interatomic contact at 3.05 Å between N3
and N3A of neighboring molecules lies in the region of the
sum of the van der Waals radii.
The molecular structure of the dinuclear radical cation in 1‚
2 CH2Cl2 (Figures 2 and 3, Table 3) shows some significant
changes in the bridging ligand with regard to the bond lengths
of the central tetrazine ligand (see below; Table 5). The
orientation of the six-membered rings is only slightly altered
relative to bmtz (Figure 3), and the dihedral angle is slightly
increased from 12.5° to 16.6°. In comparison, the related {(µ-
bptz)[Cu(PPh3)2]2}(BF4) exhibits only a 5.8° interplanar angle.25

2266 Inorganic Chemistry, Vol. 40, No. 10, 2001
Glo¨ckle et al.
Table 4. Selected Bond Angles [deg] in 1‚2CH2Cl2 and 2‚CH2Cl2
{(µ-bmtz)[Cu(PPh3)2]2}•+
N2-Cu-N1
N2-Cu-P1
N1-Cu-P1
78.8(2)
115.53(18)
113.50(17)
N2-Cu-P2
N1-Cu-P2
P1-Cu-P2
112.00(18)
104.19(17)
123.36(8)
{(µ-H2bmtz)[Cu(PPh3)2]2}2+
N1-Cu1-N2
N1-Cu1-P1
N2-Cu1-P1
N1-Cu1-P2
N2-Cu1-P2
P1-Cu1-P2
77.8(2)
N5-Cu2-N6
N5-Cu2-P3
N6-Cu2-P3
N5-Cu2-P4
N6-Cu2-P4
P3-Cu2-P4
77.1(2)
129.49(16)
121.32(15)
99.65(16)
98.01(16)
120.09(7)
111.77(18)
109.72(16)
115.16(18)
106.34(16)
125.50(8)
Scheme 1
Figure 4. Molecular structure of {(µ-H2bmtz)[Cu(PPh3)2]2}2+ in the
crystal of 2‚CH2Cl2.
Scheme 2
The twisted structure (Figure 3) results in a skewed arrange-
ment at the metal centers where the P1-Cu-P2 plane makes
an 80.3° angle with the tetrazine plane. The coordination
geometry at the copper centers is distorted tetrahedral. Whereas
the Cu-P distances are rather similar, the Cu-N distances vary
much more significantly in 1‚2 CH2Cl2 (∆ ) 0.139 Å) than in
{(µ-bptz)[Cu(PPh3)2]2}(BF4) (∆ ) 0.052 Å).25 In both cases,
the bonds to the tetrazine-N atoms are shorter than those to
the pyridyl or pyrimidyl nitrogen centers. This general result is
not unexpected because the spin and thus the added charge are
concentrated in the tetrazine ring, as will be shown below;
however, the strongly dissymmetric bonding of copper(I) to the
nitrogen atoms of bmtz•- in 1‚2 CH2Cl2 is remarkable. At 1.994-
(5) Å the Cu-N(tetrazine) bond is shorter even than that in
related copper(I) complexes of azo ligands30 and their anion
radicals.29,31 The weaker basicity of pyrimidine N vs pyridine
N centers14,16 is obviously not compensated by a better acceptor
effect because the singly occupied molecular orbital (SOMO),
au (Scheme 2), has vanishingly small contributions from the
peripheral rings.8,14
As a consequence of the tighter binding to the tetrazine ring,
the copper atoms in 1‚2 CH2Cl2 are separated by 6.680 Å in
comparison to 6.743 Å for {(µ-bptz)[Cu(PPh3)2]2}(BF4).25
The crucial and expected10,32 special feature in the structure
of complex 2‚CH2Cl2 is the pronounced boat conformation of
the 1,4-dihydro-1,2,4,5-tetrazine ring in coordinated H2bmtz
(Figures 4 and 5). The dihedral angle between planes intersecting
at the sp3 centers N3 and N7 (Figure 5) is 144.3°, compared
Figure 5. View of the structure of {(µ-H2bmtz)[Cu(PPh3)2]2}2+ along
the N3-N7 axis. Phenyl rings and H atoms at N3 and N7 are omitted
for clarity.
with 148° of free 1,4-dihydro-1,2,4,5-tetrazine.10 As in 1‚2 CH2-
Cl2, the pyrimidyl rings are twisted by about 15° relative to the
corresponding planes of the central ring, and the copper(I)
centers are distorted tetrahedral; in all instances (1 and 2), the
sum of angles at the metal atoms lie around 646°, i.e., half-
way between the values for an ideal tetrahedron (657°) and a
trigonal pyramid (630°).
The copper-nitrogen bonds are lengthened in 2‚CH2Cl2
relative to those of 1‚2 CH2Cl2, those to N(pyrimidine) by about
0.05 Å, and those to N(tetrazine) by about 0.2 Å. This result
reflects the removal of negative charge by protonation. Neither
Coulombic effects nor strong back-donation are now available
for enhanced bonding to the bridging ligand; accordingly,
compound 2 dissociates rapidly in acetonitrile. At 7.116 Å the
Cu-Cu distance is appreciably lengthened relative to that in
1‚2 CH2Cl2 (6.680 Å). Intra- and intermolecular hydrogen
bonding is insignificant because of intermolecular shielding by
the PPh3 ligands and unfavorable intramolecular orientation
resulting in distances greater than 2.5 Å between H(Ntetrazine
)
and neighboring Npyrimidine, although the N4-N7 and N3-N8
distances lie at 2.73 Å. Unusually short contacts involving the
phenyl rings could not be observed for either dicopper complex
(Figures 3 and 5), which rules out the π/π interaction found in
other dinuclear compounds between phenylphosphanecopper(I)
fragments and N heterocycles.25,33-35
(30) Kaim, W.; Kohlmann, S.; Jordanov, J.; Fenske, D. Z. Anorg. Allg.
Chem. 1991, 598/599, 217.
(31) Moscherosch, M.; Field, J. S.; Kaim, W.; Kohlmann, S.; Krejcik, M.
J. Chem. Soc., Dalton Trans. 1993, 211.
(32) Kaim, W. ReV. Chem. Intermed. 1987, 8, 247.
(33) Vogler, C.; Hausen, H.-D.; Kaim, W.; Kohlmann, S.; Kramer, H. E.
A. Rieker, J. Angew. Chem. 1989, 101, 1734; Angew. Chem., Int. Ed.
Engl. 1989, 28, 1659.

Dicopper(I) Complexes
Inorganic Chemistry, Vol. 40, No. 10, 2001 2267
Scheme 3
Table 5. Bond Lengthsa in the Central Tetrazine Ring for Three
Oxidation States of bmtz
much less negative potential than the free bmtz•- ligand (Table
6). This result illustrates the strong Coulombic effect by addition
of two cationic metal centers to the site of electron addition.
The reduction of 1 is totally irreversible at a very negative
potential, confirming an extremely large stability range of about
1.6 V for the isolable radical complex intermediate. Such values
corresponding to comproportionation constants Kc > 1020 have
similarly been observed for the corresponding bptz complex
where the absolute potentials lie at more negative values.25
Surprisingly, complex 2 is also reversibly reducible at 203
K, although at a rather negative potential (Table 6). Free H2-
bmtz is only irreversibly reduced.8 Both 1 and 2 exhibit
irreversible oxidation processes at about +0.9 V vs ferrocene/
ferrocenium, which are attributed to the oxidation of copper(I),
PPh3, and, for 2, the H2bmtz ligand.
compound
d(N-N) d(C-NX) d(C-NY)
X
Y
bmtz
1.330(2) 1.339(2) 1.344(2)
1.380(7) 1.321(8) 1.327(8)
b
b
H
b
Cu
Cu
{(µ-bmtz)[Cu(PPh3)2]2}•+ c
{(µ-H2bmtz)[Cu(PPh3)2]2}2+ d 1.426(7) 1.376(8) 1.288(8)
1.420(7) 1.396(8) 1.275(8)
a In Å. b Lone pair. c From 1‚2CH2Cl2. d From 2‚CH2Cl2.
Table 6. Electrochemical Data of Ligands and Complexesa
compound
E1/2(ox) E1/2(red) tempb solventc
-1.12 -2.17d 298 CH3CN
ref
bmtz•-
8
H2bmtz
0.28 -2.23d 298 CH3CN 8, 11
1
2
-0.26e -1.90d 203 CH2Cl2 this work
e
-1.39 203 CH2Cl2 this work
{(µ-bptz)[Cu(PPh3)2]2}(BF4) -0.35e -2.06d 298 CH2Cl2 25
Absorption Spectroscopy. The radical complex 1 exhibits
long-wavelength absorptions at 510(sh) and 430 nm (ꢀ ) 6100
M-1 cm-1). They are assigned to metal-to-ligand charge transfer
(MLCT) transitions from the filled d orbitals of the copper(I)
centers (d10 configuration) to the still half-unoccupied π* orbital
(au) of bmtz. For the labile {(µ-bmtz)[Cu(PPh3)2]2}2+ the MLCT
band occurs at much lower energy at 727 nm, in agreement
with a more stabilized empty acceptor orbital on bmtz. Accord-
ingly, the MLCT band is still further shifted to higher energies
in complex 2; the band lies at 400 nm (ꢀ ) 3100 M-1 cm-1).
Because the tetrazine-centered au MO (Scheme 2) is now fully
occupied, this transition must occur to the next unoccupied MO
(b1u), which has appreciable contributions from the pyrimidine
rings. It is not surprising that corresponding dicopper(I)
complexes bridged by 2,2′-bipyrimidine have similar absorption
spectra,33-35 confirming the nonparticipation of the dihydrotet-
razine part of the bridging ligand of 2 in the creation of low-
lying excited charge-transfer states.
EPR Spectroscopy (X and W Bands). The paramagnetic
complex 1 exhibits partially resolved hyperfine structure in the
X-band spectrum at ambient temperature. A tentative simulation
involving 63,65Cu (I ) 3/2), 31P (I ) 1/2), and 14N (I ) 1) isotopes
is shown in Figure 6, and the data are summarized in Table 7.
Continuous wave electron nuclear double resonance (CW-
ENDOR) was not possible because the ESR transition could
not be saturated; a differentiation between the 63Cu and 65Cu
isotopes34,36 could not be observed because of the unfavorable
ratio of the line width to copper coupling constant.
a From cyclic voltammetry at 50 or 100 mV/s. Potentials E in V vs
ferrocene/ferrocenium. b In K. c Electrolyte, 0.1 or 0.2 M Bu4NPF6.
d Peak potential for irreversible process. e Further irreversible oxidation
at about +0.9 V.
The bond lengths within the tetrazine ring vary quite
systematically in the series bmtz, 1‚2CH2Cl2, and 2‚CH2Cl2, as
outlined in Table 5. In agreement with changing bond orders
as illustrated by the electron pair formulas (Scheme 3) the N-N
bond lengthens continuously from bond order 1.5 to a single
bond, and the corresponding C-NX bond, which gets protonated
in H2bmtz, exhibits a less pronounced behavior. As a counter-
move, the other C-N bond (C-NY in Table 5) shortens to an
imine double bond, binding the metal. In contrast to the central
tetrazine ring, the 2-pyrimidyl rings exhibit no significant
structural changes in that series, which suggests that their system
is not involved in the accommodation of the added electrons.
This result parallels the observations made for the partially
characterized bptz system, although the pyridine substituents
there25 have been replaced by the better accepting pyrimidine
rings in bmtz. Spectroscopic studies, especially EPR hyperfine
interaction, will confirm these structural results.
Cyclic Voltammetry. Considering the lability of {(µ-bmtz)-
[Cu(PPh3)2]2}2+ above 210 K, low-temperature measurements
in dichloromethane solution were used for systems 1 and 2.
The data are summarized in Table 6. The radical complex 1 is
reversibly oxidized at 203 K to {(µ-bmtz)[Cu(PPh3)2]2}2+ at a
(34) Vogler, C.; Kaim, W.; Hausen, H.-D. Z. Naturforsch. 1993, 48b, 1470.
(35) Schwach, M.; Hausen, H.-D.; Kaim, W. Chem.sEur. J. 1996, 2, 446.
(36) Kaim, W.; Moscherosch, M. J. Chem. Soc., Faraday Trans. 1991,
87, 3185.

2268 Inorganic Chemistry, Vol. 40, No. 10, 2001
Glo¨ckle et al.
ref
Table 7. EPR Data of Paramagnetic Speciesa
compound
aN/aN’
a(65Cu/63Cu)
ap
giso
solvent
b
bmtz•- c
1d
_/0.52
0.609/0.486
f
0.605/0.463
0.215/_
2.0040
2.0053e
2.0033
2.0055h
2.0032
THF
8
0.824/0.769
f
0.769
f
0.910
0.691
CH3CN
CH2Cl2
CH2Cl2
CH2Cl2
this work
this work
37, 40
34
2•- f
{(µ-bptz)[Cu(PPh3)2]2}•+ g
0.758 g
{(µ-bpym)[Cu(PPh3)2]2}•+ c
0.691/0.646
a Coupling constants a in mT. X-band data unless stated otherwise. b Coordinated (N) and uncoordinated (N′) nitrogen atoms of the tetrazine
ring. c At 298 K. d At 273 K, copper isotope coupling unresolved (values from computer simulation). e W-band data at 20 K (powder): g1 ) g2 )
2.0062, g3 ) 2.0021. f At 200 K, hyperfine coupling not resolved. g At 300 K, no differentiation between the copper isotopes. h 245 GHz data at
4 K in frozen acetone/ethanol (5/1) solution: g1 ) g2 ) 2.0067, g3 ) 2.0026.
Figure 7. W-band EPR spectrum of 1 (powder) at 20 K (top) and
graphical simulation with the data from Table 7 (bottom).
Figure 6. X-band EPR spectrum of 1 in acetonitrile at 273 K (top)
and graphical simulation with the data from Table 7 (bottom).
g anisotropy of copper(I)-containing radical complexes.40 The
W-band spectrum of 1 at 20 K (Figure 7) exhibits an axial
symmetry with g components similar to those determined by
245 GHz EPR for {(µ-bptz)[Cu(PPh3)2]2}(BF4).40 The axial
splitting stands in contrast to the rhombic splitting observed
for related dicopper(I) complexes bridged by azo-containing
anion radical ligands.40 We attribute this effect to the unique
localization of spin at the tetrazine ring.
Compound 2 could be reversibly reduced only at low
temperatures (Table 6). In fluid solution at 200 K and in the
glassy frozen state at 117 K, 2•- gave only an unresolved EPR
line of about 10 mT total width. The isotropic g value of 2.0033
points to a predominantly ligand-centered spin with more C than
N participation, in agreement with the occupation of the b1u
MO by the unpaired electron. The numerous additional hyperfine
coupling from 1H and 14N centers of the pyrimidine rings would
explain the absence of resolution due to numerous overlapping
hyperfine lines.
The coupling constants are comparable37 to those of {(µ-
bptz)[Cu(PPh3)2]2}•+ (Table 7), confirming the spin localization
at the two different kinds of nitrogen atoms in the tetrazine ring
with secondary spin transfer to the coordinated metal centers
and their additionally bound donor atoms (31P). A π/σ hyper-
conjugation mechanism has been invoked for this spin transfer,34
and the conformational sensitivity of this mechanism may
explain the difference in a(31P) between bmtz•- and bptz•-
compounds (Table 7). The slightly larger copper coupling
constants for 1 are in agreement with the structurally confirmed
stronger Cu-Ntetrazine bonding.
The coupling constants of 63,65Cu and 31P are rather small
when compared with typical values of a(63.65Cu) ≈ 8 mT for
copper(II) species38 or with the isotropic hyperfine constants
ao ) 213.92 mT (63Cu), 228.92 mT (65Cu), and 474.79 mT
(31P).39 The hyperfine results thus confirm the predominantly
ligand-centered spin in complex 1.
Additional evidence comes from the isotropic g value of
2.0053, which lies close to that of free bmtz•- (2.0040).8 The
absence of any g component features in the low-temperature
(120 K) X-band spectrum of 1 due to unresolved overlap from
hyperfine lines prompted a study at W-band frequency (94
GHz). High-field EPR has proven valuable for determining the
Although the series of crystallographically and spectroscopi-
cally characterized compounds in Scheme 3 does not constitute
a simple redox triad, i.e., two consecutive redox pairs,41 they
(40) Barra, A.-L.; Brunel, L.-C.; Baumann, F.; Schwach, M.; Moscherosch,
M.; Kaim, W. J. Chem. Soc., Dalton Trans. 1999, 3855.
(41) (a) Boyd, D. C.; Connelly, N. G.; Garcia Herbosa, G.; Hill, M. G.;
Mann, K. R.; Mealli, C.; Orpen, A. G.; Richardson, K. E.; Rieger, P.
H. Inorg. Chem. 1994, 33, 960. (b) Bartlett, I. M.; Carlton, S.;
Connelly, N. G.; Harding, D. J.; Hayward, O. D.; Orpen, A. G.; Ray,
C. D.; Rieger, P. H. J. Chem. Soc., Chem. Commun. 1999, 2403. (c)
Baik, M.-H.; Ziegler, T.; Schauer, C. K. J. Am. Chem. Soc. 2000,
122, 9143.
(37) Kaim, W.; Kohlmann, S. Inorg. Chem. 1987, 26, 1469.
(38) Weil, J. A.; Bolton, J. R.; Wertz, J. E. Electron Paramagnetic
Resonance; Wiley: New York, 1994.
(39) Goodman, B. A.; Raynor, J. B. AdV. Inorg. Chem. Radiochem. 1970,
13, 135.

Dicopper(I) Complexes
Inorganic Chemistry, Vol. 40, No. 10, 2001 2269
illustrate how chemical reactivity and structure are connected
with electron transfer. The lability of both the ligand-oxidized
{(µ-bmtz)[Cu(PPh3)2]2}2+ and the 1,4-dihydro form (2) of the
dicopper(I) system contrasts with the dissociative stability of
the radical intermediate 1, which, despite sensitivity toward
protons, oxygen, and other electrophiles in solution, can be
viewed as an interesting new kind of paramagnetic component
in solids. For instance, several metal complexes with the
tetrazine-containing verdazyl radical function have been de-
scribed very recently.42-44
Acknowledgment. Support from Deutsche Forschungsge-
meinschaft, Volkswagenstiftung, and Fonds der Chemischen
Industrie is gratefully acknowledged. We thank in particular
Graduiertenkolleg “Magnetische Resonanz” for its support.
Supporting Information Available: Details of the X-ray structure
determinations in CIF format. This material is available free of charge
via the Internet at http://pubs.acs.org.
IC001228Q
(43) Brook, D. J. R.; Fornell, S.; Noll, B.; Yee, G. T.; Koch, T. H. J. Chem.
Soc., Dalton Trans. 2000, 2019.
(42) Brook, D. J. R.; Fornell, S.; Stevens, J. E.; Noll, B.; Koch, T. H.;
(44) Hicks, R. G.; Lemaire, M. T.; Thompson, L. K.; Barclay, T. M. J.
Am. Chem. Soc. 2000, 122, 8077.
Eisfeld, W. Inorg. Chem. 2000, 39, 562.