Comparative structural coordination chemistry of two tricyclic bisamidines

Article abstract of DOI:10.1016/j.ica.2010.08.015

The tricyclic bisamidines L1 and L2 are designed to be preconstrained so as to present synperiplanar donor sites for metal coordination. Their very different bite angles of 35° and 70° result from the incorporation of two five-membered instead of six-membered rings in their respective backbones. Distinct coordination preferences in a variety of metal complexes have now been confirmed by X-ray structural studies. While L1 afforded monodentate, symmetrical and unsymmetrical chelating as well as bimetallic bridging modes in its complexes, L2 has been found exclusively in a bidentate chelating mode.

Full text of DOI:10.1016/j.ica.2010.08.015

Inorganica Chimica Acta 364 (2010) 185–194
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Comparative structural coordination chemistry of two tricyclic bisamidines
a,
Jingwei Li a, Daniel W. Widlicka a, Kerry Fichter a, David P. Reed a, Gary R. Weisman a, , Edward H. Wong
⇑
⇑⇑
,
Antonio DiPasquale b, Katie J. Heroux b, James A. Golen c, Arnold L. Rheingold b
a Department of Chemistry, University of New Hampshire, Durham, NH 03824, USA
b Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, CA 92093, USA
c Department of Chemistry and Biochemistry, University of Massachusetts Dartmouth, North Dartmouth, MA 02747, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 13 August 2010
The tricyclic bisamidines L1 and L2 are designed to be preconstrained so as to present synperiplanar
donor sites for metal coordination. Their very different bite angles of 35° and 70° result from the
incorporation of two five-membered instead of six-membered rings in their respective backbones. Dis-
tinct coordination preferences in a variety of metal complexes have now been confirmed by X-ray
structural studies. While L1 afforded monodentate, symmetrical and unsymmetrical chelating as well
as bimetallic bridging modes in its complexes, L2 has been found exclusively in a bidentate chelating
mode.
Dedicated to Professor Arnold L. Rheingold
on the occasion of his 70th birthday.
Keywords:
Tricyclic bisamidine
Metal complexes
Crystal structures
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Unsaturated bidentate nitrogen-based ligands are important
chelators in metal coordination chemistry providing many com-
plexes of interest for their catalytic, bioinorganic, photochemical,
and photophysical applications. These are epitomized by 2,2-
bipyridyl [1–4] and related ligands like phenathroline [5,6], biimi-
dazoles [7,8] and bioxazolines [9–11]. We have reported a bisami-
dine L1 with synperiplanar donor sites enforced by its tricyclic
backbone constraints. Due to its relatively small idealized bite an-
gle of 35° [12–14], it has been found to adopt monodentate, sym-
metrical and unsymmetrical chelating as well as bridging
coordination modes in its metal complexes (Fig. 1) [12]. We now
describe new Pd(II), Zn(II), Ag(I), and Hg(II) complexes of this li-
gand whose structures further confirm this versatility in metal
binding. Since the tricyclic nature of L1 allows significant tuning
of its ideal bite angle through variations in ring sizes, we have pre-
pared a new bisamidine, L2, featuring three six-membered rings in
its backbone (Fig. 1) whose lone pairs are expected to be consider-
ably more convergent (idealized bite angle 70°) [14]. We have pre-
pared and determined structures of five of these metal complexes
to confirm its predilection for the chelation coordination mode.
2.1. General
Bisamidine L1 was prepared as described by Reed and Weis-
man [15]. N,N0-bis-(3-aminopropyl)-1,2-ethylenediamine was
purchased from Sigma–Aldrich Chemical Co. Dithiooxamide was
obtained from Fluka Chemical Co. All other reagents and solvents
were also obtained from commercial sources and used without
further purification. Deuterated solvents were obtained from
Cambridge Isotope Laboratories and stored over 3 Å molecular
sieves.
All 1H and 13C{1H} NMR spectra were acquired on either a Var-
ian Mercury 400 MHz spectrometer operating at 399.75 and
100.51 MHz respectively or on a Varian UnityINOVA 500 MHz spec-
trometer operating at 500 and 125.67 MHz respectively. IR spectra
were recorded using KBr pellets on a Nicolet MX-1 FT spectropho-
tometer. ESI-MS was performed on a Thermofinnigan LCQ Mass
Spectrometer coupled to a Picoview electrospray source. FAB-MS
was performed on the JEOL JMS-AX505HA Mass Spectrometer at
the University of Notre Dame. Electronic spectra were measured
using a Cary 219 spectrophotometer. Elemental analysis was per-
formed at Atlantic Microlab Inc., Norcross, GA.
All reactions were performed in standard Schlenk glassware un-
der a nitrogen atmosphere. Recrystallizations were conducted in
closed containers without special precautions to exclude either
air or moisture. Bulk solvent removal was by rotary evaporation
under reduced pressure and trace solvent removal from solids
was by vacuum pump evacuation.
⇑
Corresponding author. Tel.: +1 603 862 1550; fax: +1 603 862 4278.
Corresponding author. Tel.: +1 603 862 1550; fax: +1 603 862 4278.
E-mail addresses: gary.weisman@unh.edu (G.R. Weisman), ehw@cisunix.unh.
⇑⇑
edu (E.H. Wong).
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.08.015

186
J. Li et al. / Inorganica Chimica Acta 364 (2010) 185–194
(a)
(b)
N
N
N
N
N
N
N
L1
L2
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
M
M
M
M
M
(c)
N
N
N
N
N
N
N
N
L1
L2
70°
M
35°
M
Fig. 1. (a) Tricyclic bisamidine L1 and L2; (b) L1 coordination modes; and (c) Ideal bite angles for L1 and L2.
Idealized bite angles for ligands L1 and L2 were calculated using
the molecular mechanics program MM2 in CHEM3D
Caution! Perchlorate salts of metal complexes containing or-
ganic ligands in organic solvents are potentially explosive.
Although no problems were encountered by us, cautious handling
of only small amounts of these compounds should be the rule.
100.51 MHz, ref. center line of CDCl3 set at 77.3 ppm) d 21.52,
45.00, 47.66, 48.06, 148.04; 1H NMR (CD3CN, 399.75 MHz): d
1.73–1.78 (m, 4H), 3.18–3.20 (m, 4H), 3.18 (s, 4H), 3.35 (ꢀt, 4H);
13C{1H} NMR (CD3CN, 100.51 MHz, ref. center line of CD3CN set
.
at 117.62)
d 21.53, 44.38, 47.45, 47.63, 148.03; IR (KBr):
1601 cmꢁ1
(N@C–C@N);
UV–Vis
(CH3CN):
248 nm
(e
= 7500 Mꢁ1 cmꢁ1); Anal. Calc. for C10H16N4ꢂ(H2O)0.8: C, 58.12;
H, 8.58; N, 27.11. Found: C, 58.18; H, 8.66; N, 26.79%. (This material
was of sufficient purity for metal complexations. Purification by
sublimation twice (130 °C/ꢀ20 mTorr) afforded a lighter yellow
product. Recrystallization from hexane or heptane followed by
sublimation gave a pure white product.)
2.2. Experimental
2.2.1. 2,3,4,6,7,9,10,11-Octahydro-pyrazino[1,2-a:4,3-a0]dipyrimidine
(L2)
A 250 mL three-necked round-bottomed flask equipped with a
reflux condenser with N2 inlet tube, a fritted gas dispersion tube
(initially closed) and a pressure-equalizing addition funnel was
charged with dithiooxamide (2.00 g, 16.6 mmol) suspended in
absolute EtOH (30 mL). A solution of N,N0-bis-(3-aminopropyl)-
1,2-ethylenediamine (2.90 g, 16.6 mmol) in absolute EtOH
(20 mL) was added by syringe. The nitrogen manifold exit line
was routed through two scrubbing towers charged with 30% aque-
ous NaOH solution in order to trap the gases evolved. The reaction
mixture was heated at 80 °C for 2 h. The reaction mixture was then
cooled to room temperature and EtOH (30 mL) was added in order
to immerse the fritted gas dispersion tube. Residual gaseous
byproducts were purged from the solution by entrainment with
nitrogen for 24 h. Solvent was then removed by rotary evaporation
leaving a thick dark-red liquid, which was dissolved in CHCl3
(100 mL) and filtered through glass wool. CHCl3 was removed by
rotary evaporation and toluene (100 mL) was added followed by
heating. The near-boiling mixture was filtered through glass wool.
The toluene extracts were azeotropically distilled over a Dean-
Stark trap for 2 days. Evaporation of the resulting bright dark-yel-
low solution under reduced pressure afforded a yellow solid
(1.74 g, 55% yield). 1H NMR (CDCl3, 399.75 MHz): d 1.85 (ꢀp, 4H,
Japp = ꢀ6 Hz, N–CH2–CH2–CH2–N@), 3.21 (ꢀt, 4H, Japp = ꢀ6 Hz, N–
CH2–CH2–CH2–N@), 3.22 (s, 4H, N–CH2–CH2–N), 3.54 (ꢀt, 4H,
Japp = ꢀ6 Hz, N–CH2–CH2–CH2–N@); 13C{1H} NMR (CDCl3,
A crude mixture obtained prior to the use of the toluene azeotro-
pic distillation procedure was shown by NMR and MS analysis to be
a mixture of L2 and A. Spectra for A: 1H NMR (CDCl3, 360.15 MHz,
TMS): d 1.8 (br, s, 2H, NH2), 1.72 (m, 2H, NCH2CH2CH2N@,
J = 6.7 Hz), 1.85 (p, 2H, NCH2CH2CH2NH2), 2.72 (t, 2H, CH2NH2,
J = 6.6 Hz), 3.21–3.28 (m, 2H), 3.47–3.60 (m, 8H). 13C{1H} NMR
(CDCl3, 90.56 MHz, ref. central line of CDCl3 at d 77.23) d 21.23
(NCH2CH2CH2N@), 30.79 (NCH2CH2CH2NH2), 39.05, 45.06, 45.10,
45.16, 46.92, 47.44, 147.80, 152.72. MS (EI): m/z 211.3 (M+1).
2.2.2. 1,4-Bis-(3-aminopropyl)-2,3-piperazinedione (B)
A crude mixture of L2 and A was dissolved in D2O. NMR analysis
was consistent with the formation of B. 1H NMR (D2O, 360.15 MHz,
ref. CH3CN set at 2.05 ppm) d 1.74 (p, 4H), 2.62 (t, 4H), 3.49 (t), 3.65
(s, 4H); 13C{1H} NMR (D2O, 90.56 MHz, ref. center peak of CH3CN
set at 1.7 ppm) d 29.85, 38.65, 44.83, 46.04, 159.20. This NMR sam-
ple was concentrated by rotary evaporation. EtOH (2 mL) was
added the sample was again rotary evaporated to ensure the re-
moval of residual D2O. NMR of this sample in CDCl3 was consistent
with a mixture of A and B, indicating partial dehydration.
2.2.3. 1,4-Bis-(2-aminoethyl)-2,3-piperazinedione (C)
L1 was dissolved in H2O and left for 14 h. The solution was con-
centrated by rotary evaporation, the residue was dissolved in

J. Li et al. / Inorganica Chimica Acta 364 (2010) 185–194
187
CHCl3, and the solution was dried over Na2SO4. Concentration of
the filtrate after the removal of the drying agent afforded a white
waxy solid: m.p.: 111–112 °C; 1H NMR (CDCl3, 360.15 MHz) d
1.27 (br s, 4H, NH2), 2.95 (t, 4H, J = 6.3 Hz, CH2CH2NH2), 3.54 (t,
4H, J = 6.3 Hz, CH2CH2NH2), 3.64 (s, 4H, NCH2CH2N); 13C NMR
(CDCl3, 90.56 MHz, ref. central line of CDCl3 set at 77.23 ppm) d
39.92, 45.60, 50.76, 158.02; IR (KBr) 3387.7 (NH asym.), 3317
(NH sym.), 1667 cmꢁ1; MS (CI, isobutane) m/z 183.2 (M-18+1) [16].
in the precipitation of a white solid. After filtration, the product
was washed with additional MeOH and dried under reduced pres-
sure to give a white powder. Clear, colorless cubic crystals were
harvested after Et2O diffusion into a DMF solution of this powder
over several days (112 mg, 65% yield). 1H NMR (CD3CN,
399.75 MHz): d 1.93 (m, 4H), 3.37 (t, 4H), 3.42 (s, 4H), 3.51 (t,
4H); 13C{1H} NMR (CD3CN, 100.52 MHz, ref. CD3CN set at
117.6 ppm) d 20.1, 44.9, 46.6, 47.4, 148.0; MS (EI): 429.3 amu
{[Hg(L2)Cl]+}; IR (KBr): 1608 cmꢁ1 (N@C–C@N); Anal. Calc. for
Hg(C10H16N4)Cl2: C, 25.90; H, 3.48; N, 12.08; Cl, 15.29. Found: C,
25.99; H, 3.50; N, 11.93; Cl, 15.10%.
2.2.4. [Pd(L2)2](BF4)2, complex 1
An amount of Pd(CH3CN)4(BF4)2 (250 mg, 0.563 mmol) was dis-
solved in CH3CN (5 mL). Addition of 4.4 mL (222 mg, 0.5 mmol)
CH3CN of this solution by syringe to L2 (194 mg, 1.01 mmol) in a
flask resulted in a yellow solution. The reaction mixture was stirred
at room-temperature for 4 h. The solution was then transferred to
test tubes for crystallization by slow Et2O diffusion. Yellow needles
(236 mg, 71% yield) of complex 1 were harvested. 1H NMR (CD3CN,
399.75 MHz): d 1.95 (m, 4H), 3.39 (t, 4H), 3.40 (t, 4H), 3.45
(s, 4H); 13C{1H} NMR (CD3CN, 100.51 MHz, ref. CD3CN set at
117.6 ppm): d 20.1, 46.3, 46.8, 48.7, 155.0; IR (KBr): 1612 cmꢁ1
(N@C–C@N); ESI-MS: 557.1 amu {[Pd(L2)2(BF4)]+}; Anal. Calc. for
Pd(C10H16N4)2(BF4)2ꢂ(H2O)1.5: C, 34.74; H, 5.10; N, 16.20. Found:
C, 34.78; H, 4.84; N, 16.30%.
2.2.9. [Hg(L1)Cl2]2, complex 6
An amount of HgCl2 (38.4 mg, 0.103 mmol) was dissolved in
5 mL methanol. Upon addition of a methanol solution of L1
(84.5 mg, 0.515 mmol in 5 mL) a white precipitate formed. After
filtration, this solid was washed with methanol and dried under re-
duced pressure to give product as a white powder (34.5 mg, 86%
yield). This was recrystallized from DMSO by ether diffusion to give
1
clear cubic crystals. H NMR ((CD3)2SO): d 3.73–3.80 (AA0XX0, 4H),
3.55–3.70 (AA0XX0, 4H), 3.41 (s, 4H); 13C{1H} NMR ((CD3)2SO, ref.
center line of (CD3)2SO set at 39.51 ppm) d 43.4, 51.7, 52.7,
154.7; IR (KBr): 1624, 1543 cmꢁ1 (N@C–C@N); Anal. Calc. for
Hg(C8H12N4)Cl2: C, 22.05; H, 2.78; N, 12.86. Found: C, 22.22; H,
2.72; N, 12.67%.
2.2.5. [Pd2(L1)4]Br4, complex 2
An amount of Pd(PhCN)2Br2 (114 mg, 0.241 mmol) and 2.20
equivalents of L1 (90.8 mg, 0.553 mmol) were dissolved in 7 mL
DMSO to give an orange solution. After stirring for 30 min, diethyl
ether was slowly diffused in to give a yellow solid (134 mg, 23%
yield). 1H NMR (CD3OD, 499.775 MHz): d 3.66 (s, 4H), 3.89–3.94
(AA0XX0, 4H), 4.19–4.24 (AA0XX0, 4H); 13C{1H} NMR (CD3OD,
125.67 MHz, ref. CD3CN set at 117.6 ppm): d 42.52, 49.47, 54.47,
153.63; IR (KBr): 1605, 1533 cmꢁ1 (N@C–C@N); UV–Vis (DMSO):
2.2.10. [Cu(L2)3](ClO4)2, complex 7
Addition of an amount of Cu(ClO4)2ꢂ(H2O)6 (37.2 mg,
0.101 mmol) to a solution of L2 (58.3 mg, 0.303 mmol) in MeCN
(5 mL) resulted in the formation of a light blue solution. After stir-
ring for 1 h, the solution was transferred to small vials for crystal-
lization by diethyl ether diffusion. Clear, green X-ray quality
crystals were harvested after several days.
A total mass of
315 nm
(
e
= 50,200 Mꢁ1 cmꢁ1);
ESI-MS:
1106.5 amu
51.0 mg (60% yield) of the crystals were collected. IR (KBr pellet):
1615 cmꢁ1 (N@C–C@N); Anal. Calc. for Cu(C10H16N4)3(ClO4)2ꢂ
(H2O): C, 42.03; H, 5.88; N, 19.61; Cl, 8.27. Found: C, 42.05; H,
5.83; N, 19.35; Cl, 8.18%.
{[Pd2(L1)4]Br3]+}; Anal. Calc. for Pd2(C8H12N4)2Br4ꢂ(H2O)4: C,
30.47; H, 4.48; N, 17.77; Br, 25.34. Found: C, 30.05; H, 4.55; N,
17.71; Br, 24.99%.
2.2.6. Zn(L1)3(NO3)2, complex 3
2.2.11. [Cu(L2)2](ClO4)2, complex 8
An amount of Zn(NO3)2ꢂ6H2O (34.1 mg, 0.114 mmol) and a
slight excess of three equivalents of L1 (59.0 mg, 0.360 mmol) were
dissolved in acetonitrile (5 mL) with stirring. The clear solution
was subjected to ether diffusion. Clear rhombic crystals (54.6 mg,
0.0801 mmol, 70%) of 3 grew after 3 days. 1H NMR (CD3CN): d
3.41 (s, 4H), 3.55–3.73 (AA0XX0, 4H), 3.75–3.93 (AA0XX0, 4H);
13C{1H} NMR (CD3CN, ref. CD3CN set at 117.6 ppm): d 44.93,
53.02, 53.61, 156.64; IR (KBr): 1641, 1628, 1582, 1547 cmꢁ1
(N@C–C@N); Anal. Calc. for Zn(C8H12N4)3(NO3)2: C, 42.27; H,
5.32; N, 28.75. Found: C, 42.09; H, 5.37; N, 28.97%.
An amount of Cu(ClO4)2ꢂ(H2O)6 (58.2 mg, 0.157 mmol) and two
equivalents of L2 (59.6 mg, 0.310 mmol) were combined in a flask.
A blue precipitate formed immediately when EtOH (5 mL) was
added into the flask. The product was collected by filtration and
washed with additional MeOH, dried under reduced pressure to
give a blue powder (65.7 mg, 66% yield). Clear, dark blue needle
crystals were harvested after Et2O diffusion into a CH3CN solution
of this powder over several days. IR (KBr): 1623 cmꢁ1 (N@C–C@N);
Anal. Calc. for Cu(C10H16N4)2(ClO4)2: C, 37.13; H, 4.99; N, 17.32; Cl,
10.96. Found: C, 37.61; H, 5.00; N, 17.45%.
2.2.7. Zn(L2)3(ClO4)2, complex 4
2.2.12. [Ag(L2)2](BF4), complex 9
Addition of an amount of Zn(ClO4)2ꢂ6H2O (38.6 mg,
0.104 mmol) to a solution of L2 (61.2 mg, 0.318 mmol) in CH3CN
(20 mL) formed a colorless solution. After stirring for 2 h, this solu-
tion was transferred to vials for crystallization by diethyl ether dif-
fusion. X-ray quality colorless crystals of 4 were harvested over
several days (44 mg, 50% yield). 1H NMR (CD3CN, 399.75 MHz): d
1.70–1.85 (2m, 3 ꢃ 4H), 3.02–3.60 (m, 3 ꢃ 12H); 13C{1H} NMR
(CD3CN, 100.53 MHz, ref. CD3CN set at 117.7 ppm) d 20.4, 42.5,
46.6, 47.3, 147.3; IR (KBr): 1614 cmꢁ1 (N@C–C@N); Anal. Calc. for
Zn(C10H16N4)3(ClO4)2: C, 42.84; H, 5.75; N, 19.78; Cl, 8.43. Found:
C, 42.61; H, 5.81; N, 19.71; Cl, 8.64%.
Addition of AgBF4 (28.9 mg, 0.148 mmol) to a solution of L2
(62.1 mg, 0.323 mmol) in MeCN (6 mL) generated a light yellow
solution. This MeCN solution was transferred to test tubes for
recrystallization by ether diffusion. Light yellow crystals
(38.9 mg, 45% yield) were harvested from the solution after several
days in the dark. 1H NMR (CD3CN, 399.75 MHz, ref. center line of
CD2HCN set at 1.96 ppm): d 1.85 (m, 4H), 3.30 (t, 4H), 3.32 (s,
4H), 3.46 (t, 4H); 13C{1H} NMR (CD3CN, 100.52 MHz, ref. CD3CN
set at 117.6 ppm) d 20.7, 46.95, 46.98, 47.6, 148.4; IR (KBr):
1626, 1596 cmꢁ1 (N@C–C@N); Anal. Calc. for Ag(C10H16N4)2(BF4):
C, 41.47; H, 5.57; N, 19.35. Found: C, 41.58; H, 5.64; N, 19.28%.
2.2.8. [Hg(L2)2](HgCl4), complex 5
2.2.13. [Ag(L2)2](BPh4), complex 10
Addition of an amount of HgCl2 (95.3 mg, 0.351 mmol) to a
solution of L2 (71.1 mg, 0.370 mmol) in MeOH (15 mL) resulted
Addition of a solution of AgBF4 (48.8 mg, 0.251 mmol) in EtOH
(2 mL) to a solution of L2 (99.2 mg, 0.516 mmol) in EtOH (3 mL)

188
J. Li et al. / Inorganica Chimica Acta 364 (2010) 185–194
produced a cloudy light yellow solution. After stirring for 3 h in the
dark, a solution of NaBPh4 (88.1 mg, 0.257 mmol) in EtOH was
added to exchange the counter anion BF4ꢁ. Immediately, a white
precipitate was formed. The white precipitate was filtered off
and washed with additional EtOH, followed by drying under vac-
uum in the dark to give a white powder (144 mg, 71% yield). Clear,
colorless cubic crystals were harvested after Et2O diffusion into a
DMF solution of this white powder in the dark over several days.
1H NMR (CD3)2SO, 399.75 MHz): d 1.75 (m, 4H), 3.25 (t, 4H), 3.29
(s, 4H), 3.37 (t, 4H), 6.78 (t, 4H), 6.92 (s, 4H), 7.17 (m, 4H);
13C{1H} NMR ((CD3)2SO, 100.52 MHz, ref. center line of (CD3)2SO
set at 40.2 ppm) d 20.8, 46.7, 47.0, 47.6, 122.3, 126.0, 136.0,
Table 1
Crystal data for the new metal complexes of L1 and L2. For full details, see the Supplementary
material.
Pd2L14Br4 (2)
36H54Br4N18Pd2
1271.41
P2(1)/n
yellow
11.7786(7)
17.2310(10)
23.4789(14)
90.00
102.0240(10)
90.00
ZnL13(NO3)2 (3)
ZnL23(ClO4)2 (4)
Chemical formula
C
C24H36N14O6Zn
682.04
Pbcn
colorless
17.5946(8)
9.1257(4)
18.3195(8)
90.00
C30H48Cl2N12O8Zn
841.07
Molecular weight
Space group
Color
a (Å)
b (Å)
P2(1)/n
colorless
9.7049(7)
37.191(3)
9.8276(7)
90.00
c (Å)
a
(°)
b (°)
(°)
90.00
90.00
93.4270(10)
90.00
c
V (Å3)
Z
4660.7(5)
4
1.812
173(2)
4.250
9761
543/0
0.0493
0.0795
0.1168
2941.4(2)
4
1.549
173(2)
0.901
3454
3540.8(5)
4
1.578
100(2)
0.913
8222
478/0
0.0391
0.0441
Dcalc (g/cm3)
T (K)
l
(Mo K
Unique data, Rint
Parameters/restraints
R1 (I > 2 (I))
a
) (mmꢁ1
)
204/0
r
0.0418
0.0493
0.1458
R1 (all data)
wR2 (all data)
0.1032
Hg2L12Cl4 (6)
HgL22HgCl4 (5)
CuL23(ClO4)2 (7)
Chemical formula
Molecular weight
Space group
Color
a (Å)
b (Å)
C
16H24Cl4Hg2N8
C20H32Cl4Hg2N8
927.52
C30H48Cl2CuN12O8
839.24
P2(1)/n
blue
9.5258(11)
37.083(4)
10.0098(12)
90.00
95.262(2)
90.00
3521.0(7)
4
1.583
100(2)
0.841
8205
488/4
0.0460
871.41
P1
ꢀ
P2(1)/n
colorless
16.995(3)
9.5747(14)
17.531(3)
90.00
113.641(2)
90.00
2613.2(7)
4
2.358
100(2)
12.172
5928
307/0
0.0292
colorless
8.6045(2)
8.6444(2)
8.8147(2)
108.4530(10)
96.805(2)
109.4500(10)
567.77(2)
1
2.549
173(2)
13.996
2671
c (Å)
a
(°)
b (°)
c
(°)
V (Å3)
Z
Dcalc (g/cm3)
T (K)
l
(Mo K
Unique data, Rint
Parameters/restraints
R1 (I > 2 (I))
a
) (mmꢁ1
)
137/0
0.0237
r
R1 (all data)
0.0251
0.0331
0.0500
wR2 (all data)
0.0549
0.0685
0.1212
CuL22(ClO4)2 (8)
AgL22BPh4 (10)
Ag2L13(BPh4)2 (12)
Chemical formula
Molecular weight
Space group
Color
a (Å)
b (Å)
C
40H68Cl4Cu2N16O18
C44H52AgBN8
811.62
P1
C72H76Ag2B2N12
1346.81
P1
1329.98
P2(1)/n
blue
13.6639(15)
8.7736(10)
21.757(2)
90.00
94.121(2)
90.00
2601.6(5)
2
1.698
100(2)
1.113
6031
361/0
ꢀ
ꢀ
colorless
10.1692(16)
11.3691(17)
17.836(3)
71.899(2)
82.114(3)
88.589(3)
1941.2(5)
2
1.389
100(2)
0.563
8705
colorless
10.418(3)
21.453(6)
28.921(8)
108.846(4)
91.832(5)
95.322(5)
6078(3)
4
1.472
100(2)
0.701
27084
c (Å)
a
(°)
b (°)
c
(°)
V (Å3)
Z
Dcalc (g/cm3)
T (K)
l
(Mo K
Unique data, Rint
Parameters/restraints
R1 (I > 2 (I))
a
) (mmꢁ1
)
487/0
0.0534
0.0664
0.1296
1585/0
0.0471
0.0664
r
0.0377
0.0439
0.1078
R1 (all data)
wR2 (all data)
0.1170

J. Li et al. / Inorganica Chimica Acta 364 (2010) 185–194
189
148.3; IR (KBr): 1602 cmꢁ1 (N@C–C@N); Anal. Calc. for
Ag(C10H16N4)2(BC24H20)[Ag(BC24H20)]0.9 C, 65.88; H, 5.90; N,
9.37. Found: C, 65.59; H, 5.82; N, 9.14%.
ethylenediamine (Scheme 1) [15]. It the case of L2, it was found
that post-reaction azeotropic distillation with toluene was neces-
sary in order to avoid contamination of the product by hydrolysis
products (vide infra). The resulting solid had a yellow coloration
due to traces of residual dithiooxamide. Although this was suffi-
ciently pure for complexation studies by virtue of elemental and
spectral analyses, it can be further purified by recrystallization
from hexane or heptane followed by sublimation to give white
crystalline L2.
Our initial attempts to synthesize L2 by the above dithiooxa-
mide method were frustrated by the presence of an unexpected
byproduct. NMR and mass spectra were consistent with contami-
nation of L2 by mixed amide–amidine A (Scheme 2), the result of
apparent partial hydrolysis of L2. This hypothesis was confirmed
by a series of NMR-scale experiments. When the L2/A mixture
was dissolved in D2O, proton and carbon NMR spectra showed
the formation of fully hydrolyzed diamino-oxamide B. Attempted
isolation of B by azeotropic removal of water with EtOH gave a
mixture of A and B, showing that the hydrolysis is readily revers-
ible (Scheme 2). With this observation in hand, the L2/A mixture
was azeotropically distilled with toluene over a Dean-Stark trap,
giving L2 and thus leading to the modified procedure for the syn-
thesis of L2 noted above. The fact that this hydrolysis problem
had to be overcome was a little surprising since no hydrolysis of
L1 had been observed in the past and no special precautions had
been taken during its synthesis. As a control experiment, a small
:
2.2.14. [Ag2(L1)4](BF4)2, complex 11
Addition of an amount of AgBF4 (156 mg, 0.800 mmol) to a solu-
tion of L1 (268 mg, 1.63 mmol) in MeCN (20 mL) generated a light
yellow solution. The reaction mixture was stirred overnight. After
centrifugation, the MeCN supernatant was transferred to test tubes
for recrystallization in the dark by ether diffusion. Colorless cubic
crystals (308 mg, 74% yield) were harvested from the solution after
several days. 1H NMR (CD3CN, 399.75 MHz, ref. center line of
CD2HCN set at 1.96 ppm): d 3.37 (s, 4H), 3.53–3.58 (t, 4H), 3.79–
3.85 (t, 4H); 13C{1H} NMR (CD3CN, 100.52 MHz, ref. CD3CN set at
117.6 ppm) d 44.21, 51.98, 53.90, 156.19; IR (KBr): 1630 cmꢁ1
(N@C–C@N); Anal. Calc. for Ag(C8H12N4)2BF4: C, 36.74; H, 4.62; N,
21.42. Found: C, 36.58; H, 4.83; N, 21.16%.
2.2.15. [Ag2(L1)3](BPh4)2, complex 12
Addition of an amount of AgBF4 (151 mg, 0.778 mmol) to a solu-
tion of L1 (274 mg, 1.67 mmol) in MeCN (40 mL) yielded a colorless
solution with a brown precipitate. After stirring for 6 h in the dark,
one equivalent of NaBPh4 (267.0 mg, 0.780 mmol) was added to
exchange the counteranion. Then the solution was stirred for 2 h
in the dark. After filtering the insoluble brown solid off, the filtrate
was concentrated to 20 mL. Then 100 mL of ethanol was added to
produce a white precipitate. This white precipitate was collected
and washed with additional EtOH, then dried under vacuum over-
night to give a white solid (435 mg, 74% yield). Clear, colorless
crystals were harvested after Et2O diffusion into CH3CN solution
of this white solid in the dark over several days. 1H NMR (CD3CN,
399.75 MHz, ref. center line of CD2HCN set at 1.96 ppm): d 3.38
(s, 3 ꢃ 4H), 3.57 (t, 3 ꢃ 4H), 3.84 (t, 3 ꢃ 4H), 6.87 (t, 8H), 7.02 (t,
16H), 7.29 (m, 16H); 13C{1H} NMR (CD3CN, 100.52 MHz, ref. CD3CN
set at 117.6 ppm) d 44.0, 51.8, 54.0, 122.1, 125.9, 136.0, 156.1; IR
S
NH2
S
1)
H2N
NH NH2
NH NH2
N
N
N
N
EtOH, reflux, 2 h
2) azeotropic distillation
toluene
(-H2O)
(KBr):
1621 cmꢁ1
(N@C–C@N);
Anal.
Calc.
for
L2
Ag2(C8H12N4)3(BC24H20)2: C, 64.21; H, 5.69; N, 12.48. Found: C,
64.21; H, 5.58; N, 12.50%.
Scheme 1. Synthesis of L2.
2.3. X-ray crystallography
A summary of data collection and refinement results are pre-
sented in Table 1. Further details can be found in the CIF files
deposited with CCDC. Software employed for structure determina-
tion include: APEX2 Version 2.2/SHELXTL (Bruker AXS Inc., 2007),
AINT Version 7.34a (Bruker AXS Inc., 2007), SADABS Version
2007/2 (Sheldrick, Bruker AXS Inc.), XPREP Version 2005/2 (Sheld-
rick, Bruker AXS Inc.). The Bruker suite of programs APEX2/SHEL-
XTL, SAINT, SADABS, XPREP may be obtained from Bruker AXS
Inc., 5467 East Cheryl Parkway, Madison WI 53711. XS Version
2008/1 and XL Version 2008/1 [17].
X-ray crystal structure figures were prepared using Crystal-
Maker 8.2 for Mac (CrystalMaker Software Ltd., Centre for Innova-
tion and Enterprise, Oxford University Begbroke Science Park,
Sandy Lane, Yarnton, Oxfordshire, OX5 1PF, UK; http://www.crys-
talmaker.com) Hydrogens, counterions and solvents have been re-
moved for clarity. For all heavy atoms, 50% thermal ellipsoids are
shown.
3. Results and discussion
3.1. Synthesis
The bisamidine L2 was prepared in a similar procedure as L1 by
the reaction of dithiooxamide and N,N0-bis-(3-aminopropyl)-1,2-
Scheme 2. Hydrolysis studies of L1 and L2.

190
J. Li et al. / Inorganica Chimica Acta 364 (2010) 185–194
Scheme 3. Preparation of the metal complexes of L1 and L2.
sample of L1 was dissolved in D2O. In a matter of hours, L1 was
completely converted to diamino-oxamide C (NMR; kinetics have
not been carried out). It was possible in this case to isolate the fully
hydrolyzed product, C, and it was subsequently shown that it could
be dehydrated back to L1 using azeotropic distillation with toluene
(Scheme 2). Qualitatively, L2 is more sensitive towards hydrolysis
than L1. While it is not clear whether this is a kinetic or a thermo-
dynamic effect, both sequential hydrolyses appear to be com-
pletely reversible.
All described metal complexes were readily prepared in good
yields by room-temperature reactions between L1 and L2 and
the respective metal precursor in appropriate solvents such as ace-
tonitrile, DMSO, methanol, or ethanol (Scheme 3). Resulting crude
products were typically recrystallized by diethyl ether diffusion
into a sample solution.
In contrast to the facile synthesis of the bimetallic cuprous
[Cu2(L1)3](BF4)2 complex from [Cu(CH3CN)4]BF4 [12], attempts to
prepare Cu(I) complexes of L2 were unsuccessful as the initially
beige-colored solids were readily oxidized to Cu(II) products dur-
ing purification procedures. The inability of L2 to bridge cuprous
centers and its stable chelation of Cu(II) likely contribute to this
oxidation propensity.
AA0MM0XX0 where the JAM type long-range couplings are small),
one apparent pentet (N–CH2–CH2–CH2–N@ : XX0 of AA0MM0XX0
where the JAM long-range couplings are small), and a singlet (N–
CH2–CH2–N). Its 13C{1H} NMR spectrum also suggests a C2v time-
averaged geometry with only four upfield signals between d 21.5
and 48.1 and the amidine resonance at d 148.0.
All characterized complexes of L1 and L2 exhibit strong N@C–
C@N stretches between 1525 and 1670 cmꢁ1 in their IR spectra.
However, both symmetric and asymmetric stretches are clearly re-
solved in only two of these compounds.
3.2.1. Palladium complexes 1 and 2
A proton NMR spectrum of the [Pd(L2)2](BF4)2 complex 1 in
CD3CN suggests retention of the averaged bisamidine C2v symme-
try albeit with downfield chemical shifts. The amidine 13C{1H}
NMR signal is also shifted downfield by 8.5 ppm from the free li-
gand value. Its monomeric nature in methanol solution was con-
firmed by an ESI-MS study which revealed a parent peak at
577.1 amu consistent with the [Pd(L2)2](BF4)+ species. By contrast,
the dimeric nature of complex 2, [Pd2(L1)4]Br4, in methanol was
supported by its ESI-MS spectrum which featured a parent peak
+
at 1106.5 amu for the [Pd2(L1)4]Br3 solution species. This is also
3.2. Spectral characterization
in agreement with its solid-state (vide infra) structure which has
approximate D4 symmetry with an average twist angle of around
35° for the four bridging bisamidines. Interestingly, proton NMR
spectra of the [Pd2(L1)4]Br4 complex 2 in either d6-DMSO or d4-
methanol displayed only a singlet and two apparent triplets. This
is indicative of an averaged bisamidine C2v symmetry due to a fac-
ile dynamic process. Although significant broadening of these sig-
nals was observed in d4-methanol, no decoalescence was observed
down to ꢁ90 °C. Since no bisamidine exchange was found between
free ligand and this complex, this fluxional process most likely in-
A solid-state FT-IR spectrum of bisamidine L1 displays strong
symmetric and asymmetric N@C–C@N stretches at 1631 and
1616 cmꢁ1 respectively [13,14].
The solid-state FT-IR spectrum of new bisamidine ligand L2
shows a broad and strong N@C–C@N band at 1601 cmꢁ1 and its
*
electronic spectrum exhibits an intense
p–p
band at 248 nm (
e
7500 Mꢁ1 cmꢁ1). The proton NMR spectrum of ligand L2 features
two apparent triplets (N–CH2–CH2–CH2–N@ : AA0 and MM0 of

J. Li et al. / Inorganica Chimica Acta 364 (2010) 185–194
191
Table 2
Selected bond data for the new metal complexes of L1 and L2. For full structural details, see Supplementary material.
Complex
Metal–N
distance (Å)
Coordinated
chelate
amidine N–metal– (Å)
Coordinated amidine
C@N (imino) distance
Coordinated amidine
C–N (amino) distance
(Å)
Coordinated
amidine
C–N (amino)–C
angle
Bisamidine
N@C–C@N
torsion
N
angle (°)
bite angle (°)
sum (°)
Pd2Ll4Br4 (2)
ZnLl3(NO3)2 (3)
2.006(avg)
1.304(avg)
1.295
1.287
1.290(avg)
1.299
1.334(avg)
1.337
1.335
1.352(avg)
1.330
358.7(avg)
357.6
355.2(avg)
357.4(avg)
356.8
7.0–20.6
3.6
4.8
2.9–19.2
8.8
1.976 (g1
)
)
2.079 (g2
83.53
75.7(avg)
ZnL23(ClO4)2 (4)
[HgL1Cl2]2 (6)
2.151(avg)
2.129
2.946
1.280
1.374
344.9
[HgL22]HgCl4 (5) 2.251(avg)
73.4(avg)
73.3-79.4
1.294(avg)
1.290(avg)
1.340(avg)
1.351(avg)
358.2(avg)
358.2(avg)
ꢁ1.2–12.3
CuL23(ClO4)2 (7)
2.324(ax)
5.6–20.9
2.027(eq)
CuL22(ClO4)2 (8)
AgL22BPh4 (10)
Ag2Ll3(BPh4)2
(12)
1.965(avg)
2.299(avg)
2.104–2.209 (l2
82.99(avg)
71.5(avg)
1.300(avg)
1.288(avg)
1.295(avg)
1.337(avg)
1.357(avg)
1.359(avg)
358.7(avg)
357.9(avg)
351.8(avg)
13.7(avg)
8.7–20.9
0.7
)
2.171–2.622 (g2
)
74.0
1.284(avg)
1.28(avg)
1.30(avg)
1.363(avg)
1.36(avg)
1.38(avg)
350.2(avg)
351(avg)
350(avg)
4.3
6.9(avg)
2.095–2.109
2.725–2.820 (unsymmetrical-
g2
)
(avg) = average value; (ax) = axial; (eq) = equatorial; (
g
1) = monodentate; (
g
2) = chelating; ( 2) = bimetallic bridging.
l
volves the intramolecular synchronized twisting of all bridging li-
gands about its Pd–Pd axis to attain an averaged D4h symmetry.
Table 2. Full details of the structural determinations and bond data
can be found in the Supplementary material.
While both L1 and L2 are preconstrained so as to present a lim-
ited range of synperiplanar N@C–C@N torsion angles centered
about zero (Table 2), new ligand L2 is more conformationally flex-
ible because of the distal six-membered rings. Two basic types of
conformations are observed in the chelated L2 ligands of the struc-
tures reported herein. In one type, the central methylene of each –
CH2–CH2–CH2– distal ring ‘‘flap” is distorted in the same direction
away from the mean plane of the bisamidine unit of the ligand (de-
noted SYN for the purpose of further discussion). Symmetry is C1
rather than Cs in this case. In the second type, the two flaps are dis-
torted in opposite directions relative to the mean plane (denoted
ANTI) and the ligand has approximate C2 symmetry. There is no
apparent correlation between the type of conformation and the
magnitude of the observed N@C–C@N torsion angles in the coordi-
nation complexes. Both SYN and ANTI are observed to have torsion
angles ranging up to approximately 20°. In one case (vide infra),
disorder between these two conformational types is observed. It
is likely that the energy difference between such conformations
is small, and possible that the preference is determined by packing.
3.2.2. Zinc complexes 3 and 4
Since the solid-state structure of Zn(L1)3(NO3)2, 3, features one
chelating and two monodentate ligands while its proton and C-13
NMR spectra revealed a single set of ligand signals indicative of C2v
symmetry, a facile dynamic process must be occurring in solution
to average all three bisamidines. Interestingly, complex
4
Zn(L2)3(ClO4)2 displays two sets of unequal upfield bisamidine b-
CH2 multiplets (1.4:1 ratio) in its room-temperature proton NMR
spectrum. Variable-temperature studies between ꢁ40 and +70 °C
in CD3CN revealed a 1.3:1 ratio at ꢁ40° that increased with tem-
perature till near coalescence at +70 °C (see Supplementary mate-
rial spectra 1 and 2). One plausible explanation of this behavior
may be the presence of a 2:1 as well as 3:1 complex in solution
with the former increasingly favored by a higher temperature. Fur-
ther studies are needed to confirm this hypothesis.
3.2.3. Mercury complexes 5 and 6
The Hg(L2)2(HgCl4) complex 5 displayed solution spectral
behavior consistent with a symmetrical bisamidine environment.
Absence of 13C–199Hg satellites in its C-13 NMR spectrum is consis-
tent with facile ligand exchange. Although the X-ray structure of
the dimeric complex 6, [Hg(L1)Cl2]2, displayed an unsymmetri-
cally-chelating ligand, its simple solution NMR spectra, again with
absence of 13C–199Hg satellites, indicated fast ligand exchange.
3.3.1. The [Pd2(L1)4]Br4 complex 2
In this dimeric structure approximating D4 symmetry, the two
palladium centers are held at a distance of 2.741(6) Å by four
bridging bisamidines which exhibit a twist angle of about 35°
down the Pd–Pd axis (Fig. 2). All Pd–N distances are very similar,
ranging only from 1.998 to 2.014 Å. An essentially square-planar
PdN4 coordination sphere is found around each metal center. The
lengthened average C@N bond length of 1.30 Å (compared to the
uncoordinated diimino C@N distance of 1.27–1.28 Å, vide infra) is
consistent with strong Pd–N bonding due to amidine conjugation.
The diimine (N@C–C@N) dihedral angles are splayed out from 7° to
21° to accommodate metal bridging. A dipalladium complex
bridged by two bioxazolines has been structurally characterized
and here the Pd–Pd separation was found to be substantially longer
at 2.894 Å [18]. A formamidinate-bridged palladium(II) dimer of D4
symmetry has been reported in the literature [19]. Its Pd–Pd sepa-
ration is substantially shorter at 2.555(1) Å due to its smaller NCN
bridging units.
3.2.4. Silver complexes 9–12
Like both the mercuric complexes, silver(I) complexes 9–12;
Ag(L2)2BF4, Ag(L2)2BPh4, Ag2(L1)4(BF4)2, and Ag2(L1)3(BPh4)2 all
display a single set of bisamidine proton signals indicative of aver-
aged C2v ligand geometry and facile exchange with any added free
ligand. Consistent with this rapid exchange, no 13C–107,109Ag satel-
lites were observed. As described in the next section, in the solid-
state complexes 10 and 12, Ag(L2)2BPh4 and Ag2(L1)3(BPh4)2, were
found to have very different coordination modes.
3.3. X-ray structural studies
Crystal structure data for the nine new complexes are summa-
rized in Table 1. Relevant bond lengths and angles are collected in

192
J. Li et al. / Inorganica Chimica Acta 364 (2010) 185–194
Fig. 4. Structure of Zn(L1)3(NO3)2, complex 3. Nitrates omitted.
By contrast, only one bisamidine in complex 3, Zn(L1)3(NO3)2, is
chelating while the remaining two are essentially monodentate
(Zn(1)ꢂ ꢂ ꢂN(3) separation is over 3.00 Å), yielding a pseudo-tetrahe-
dral Zn(II) coordination sphere (Fig. 4). A C2 axis passes through the
metal and chelating ligand with N–Zn–N bite angle of 83.5(1)°, far
from the idealized 35° for the free ligand. The chelating Zn–N dis-
tance of 2.079(2) Å is significantly longer than the monodentate
Zn–N distance of 1.976(2) Å. Within the monodentate L1, the coor-
dinated C@N is elongated to 1.295(3) Å compared to 1.276(3) Å for
the uncoordinated C@N bond, while the N–C bond is significantly
shorter (1.337(3) Å) upon coordination compared to the uncoordi-
nated side (1.379(3) Å). The sum of C–N–C bond angles around the
bound amidine ring amino nitrogen N(2) (357.6(3)°) is close to pla-
narity compared to a much more pyramidalized N(4) (344.7(3)°) in
the uncoordinated amidine. These data are fully consistent with
associated amidine polarization upon metal coordination.
Fig. 2. Structure of [Pd2(L1)4]Br4, complex 2. Bromides not shown.
3.3.3. The [Hg(L2)2]HgCl4 and [Hg(L1)Cl2]2 complexes 5 and 6
The structure of the [Hg(L1)Cl2]2 complex 6 has inversion sym-
metry with two bridging chlorides, one terminal chloride, and an
unsymmetrically-chelating bisamidine at each five-coordinate
mercuric center (Fig. 5). The two disparate Hg–N bond-lengths
are 2.129(4) and 2.946(4) Å. As a result, the strongly-bound ami-
dine C@N is slightly lengthened to 1.299(5) Å compared to
1.280(6) Å for the weakly-bound C@N group, while the amidine
C–N bond shortens significantly to 1.330(5) Å from 1.374(5) Å.
Consistent with increased amidine polarization, this amino N’s
C–N–C angles sum to a near-planar 356.7(6)°.
Fig. 3. Structure of Zn(L2)3(ClO4)2, complex 4. Perchlorates not shown.
The complex 5, [Hg(L2)2]HgCl4, structure features a mercury
coordination geometry that approximates a trigonal bipyramid
3.3.2. The Zn(L1)3(NO3)2 and Zn(L2)3(ClO4)2 complexes 3 and 4
The six-coordinate complex 4, Zn(L2)3(ClO4)2, structure approx-
imates overall D3 symmetry with a trigonal twist angle of 45°
(Fig. 3). An average Zn–N distance of 2.15 Å and bite angle of
75.7° are close to the idealized L2 metal chelation geometry. The
ligand is not severely-distorted from planarity as indicated by an
average amino C–N–C angle sum of 357°. An average C@N distance
of 1.29 Å and C–N of 1.35 Å are observed. All three of the L2 ligands
in the complex adopt SYN conformations with N@C–C@N torsion
angles of the same sign ranging from 2.9° to 19.2°. Three structural
reports of Zn(II) complexes with three chelating biimidazoles or
bioxazolines have appeared [20–22]. All these have very similar
N–Zn–N bite angles of 76–79°, Zn–N distances of 2.16–2.26 Å,
C@N bond lengths of 1.26–1.33 Å, and N@C–C@N torsion angles
of 0.5–8.8°.
Fig. 5. Structure of [Hg(L1)Cl2]2, complex 6.

J. Li et al. / Inorganica Chimica Acta 364 (2010) 185–194
193
Fig. 6. Structure of [Hg(L2)2]HgCl4, complex 5.
Fig. 8. Structure of Cu(L2)2(ClO4)2, complex 8. Only one perchlorate shown.
(s = 0.825) [23] with one weak equatorial coordination of the
HgCl42ꢁ dianion through a Cl bridge at 3.045(5) Å (Fig. 6). The axial
N(2)–Hg(1)–N(5) angle is 170.3(2)° while equatorial N(1)–Hg(1)–
N(6) angle is 120.8(2)°. The chelating bisamidines have bite angles
of 73°, close to the idealized value of 75° for L2. Hg–N bond dis-
tances within the chelate ring, however, are somewhat unsymmet-
rical with Hg(1)–N(1) significantly longer at 2.329(5) Å compared
to Hg(1)–N(2) at 2.151(4) Å, for example. The average C–N–C angle
sum at the amino N’s is 358.3°. The ligand with the HgCl4 moiety
perched above is ANTI with a N@C–C@N torsion angle of ꢁ1.2°,
while the other is SYN with a 12.3° torsion angle.
2.84 Å, likely due to its smaller idealized bite angle. One of the
unsymmetrically-chelating L2 ligands adopts the SYN conforma-
tion with a relatively large N@C–C@N torsion angle of 20.1°. The
other exhibits two-site torsional disorder in one of the distal six-
rings (SYN and ANTI, indicated by the two half-occupancy carbons
C(17)/C(17a) connected through two-tone bonds). The symmetri-
cally chelating L2 adopts a SYN conformation with a N@C–C@N tor-
sion angle of ꢁ5.6°.
The monomeric 2:1 complex 8, Cu(L2)2(ClO4)2, features a very
distorted square pyramidal Cu(II) (s = 0.27) with two chelating
equatorial ligands and a weak axial perchlorate coordination at
2.817 Å (Fig. 8). Strong Cu–N bonding is indicated by the short
average distance of 1.965 Å, lengthened C@N of 1.300 Å and short-
ened amino-C of 1.337 Å. One of the ligands adopts the SYN and the
other the ANTI conformation, with similar N@C–C@N torsion an-
gles of 12.1° and 15.2° respectively. This structure contrasts dra-
matically with the reported dimeric structure of [Cu(L1)2]2(ClO4)4
which has a Cu–Cu distance of 2.775(1) Å supported by four bridg-
ing bisamidines [12].
3.3.4. The Cu(L2)3(ClO4)2 and Cu(L2)2(ClO4)2 complexes 7 and 8
The 3:1 complex 7 Cu(L2)3(ClO4)2 has a similar structure to the
previously-reported Cu(L1)3(ClO4)2 featuring a tetragonal geome-
try with Jahn–Teller elongations along the N(5)–Cu(1)–N(12) axis
(Fig. 7). One symmetrically and two unsymmetrically-chelating
bisamidines are found in each case, though the weaker axial Cu–
N bonds are substantially elongated in Cu(L1)3(ClO4)2 at 2.81 and
3.3.5. The Ag(L2)2BPh4 and Ag2(L1)3(BPh4)2 complexes 10 and 12
A severely-distorted tetrahedral silver coordination is revealed
in the structure of complex 10, Ag(L2)2BPh4, with two chelating
bisamidines with N–Ag–N bite angles nearing 72° (Fig. 9). Each li-
Fig. 7. Structure of Cu(L2)3(ClO4)2, complex 7. Perchlorates omitted.
Fig. 9. Structure of Ag(L2)2BPh4, complex 10. Anion omitted.

194
J. Li et al. / Inorganica Chimica Acta 364 (2010) 185–194
Co(II), Pd(II), Cu(I), Cu(II), Ag(I), Zn(II), Cd(II), Hg(II), as well as sev-
eral lanthanide(III) complexes (La, Eu, Gd, and Lu) [24]. The design
of L2 with two six-membered bisamidine rings in its backbone to
provide significantly more convergent lone pairs was intended to
favor the chelation mode. Indeed, this has been the only experi-
mentally-observed coordination mode in X-ray structures of its
Cu(II), Zn(II), Hg(II), Ag(I), Eu(III) and Gd(III) complexes [14]. These
data confirm our premise that tuning of the coordination chemis-
try of this family of tricyclic bisamidines can be readily attained
through ring size variations. Finally, in all complexes studied, coor-
dination of the amidine moiety results in lengthening of its C@N
bond, shortening of its C–N bond, and a less pyramidal amine
nitrogen, entirely consistent with anticipated coordination-in-
duced polarization of the amidine unit.
Appendix A. Supplementary material
CCDC 777814, 777815, 777816, 777817, 777818, 777819,
777820, 777821 and 777822 contains the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
http://www.ccdc.cam.ac.uk/data_request/cif. Supplementary data
associated with this article can be found, in the online version, at
doi:10.1016/j.ica.2010.08.015.
Fig. 10. Structure of Ag2(L1)3(BPh4)2, complex 12. Anions not shown.
gand is flattened with an amine bond angle sum of 358°. The two
ligands are conformationally distinct in the complex, one having
the SYN and the other the ANTI conformation, with N@C–C@N tor-
sion angles of ꢁ8.7° and 20.9° respectively.
References
By contrast, the two independent Ag2(L1)3(BPh4)2 dimeric units
in the X-ray structure of complex 12 both feature three distinct L1
coordination modes: symmetrical bridging, unsymmetrical chelat-
ing, as well as unsymmetrical bridging (Fig. 10).
[1] A.P. Smith, C.L. Fraser, Compr. Coord. Chem. II 1 (2004) 1.
[2] B.-H. Ye, M.-L. Tong, X.-M. Chen, Coord. Chem. Rev. 249 (2005) 545.
[3] O. Maury, H. Le Bozec, Acc. Chem. Res. 38 (2005) 691.
[4] V. Balzani, G. Bergamini, F. Marchioni, P. Ceroni, Coord. Chem. Rev. 250 (2006)
1254.
[5] C.R. Luman, F.N. Castellano, Compr. Coord. Chem. II 1 (2004) 25.
[6] R.D. Hancock, D.L. Melton, J.M. Harrington, F.C. McDonald, R.T. Gephart, L.L.
Boone, S.B. Jones, N.E. Dean, J.R. Whitehead, G.M. Cockrell, Coord. Chem. Rev.
251 (2007) 1678.
[7] M. Tadokoro, K. Nakasuji, Coord. Chem. Rev. 198 (2000) 205.
[8] J.C. Yoder, J.P. Roth, E.M. Gussenhoven, A.S. Larsen, J.M. Mayer, J. Am. Chem.
Soc. 125 (2003) 2629.
[9] D. Rechavi, M. Lemaire, Chem. Rev. (Washington, DC, US) 102 (2002) 3467.
[10] S. Dagorne, S. Bellemin-Laponnaz, A. Maisse-Francois, Eur. J. Inorg. Chem.
(2007) 913.
Only one of these units will be discussed. The Ag(1)–Ag(2) dis-
tance of 2.8749(6) Å is shorter than that found in metallic silver
(2.89 Å) and considerably shorter than their van der Waals sum
of 3.44 Å. BIIM-bridged as well as unbridged Ag2 structures have
been reported [20]. Silver–silver separations of 2.83–3.08 Å are ob-
served. Here in Ag2(L1)3(BPh4)2, Ag(2) is five-coordinate while
Ag(1) has a T-shaped coordination sphere augmented by a weak
interaction with a bridging amidine nitrogen (N(6)). As a result,
disparate Ag–N bond lengths are observed; symmetrical bridging
Ag–N distances range from 2.104(3) to 2.209(3) Å, and unsymmet-
rical bridging is indicated by Ag(1)–N(5) at 2.095(3) Å and Ag(2)–
N(6) at 2.820(3) Å. However, the approach angle for N(6) to Ag(2)
is far from ideal. In fact, this nitrogen is better oriented for a very
long chelation to Ag(1) at a 3.086 Å distance. Less exaggerated
unsymmetrical chelation is seen in the pair of Ag(2)–N(9) and
Ag(2)–N(10) bond lengths of 2.244(3) and 2.551(3) Å.
[11] A. Pfaltz, Asymmetric Synth. (2007) 131.
[12] D.W. Widlicka, E.H. Wong, G.R. Weisman, K.C. Lam, R.D. Sommer, C.D.
Incarvito, A.L. Rheingold, Inorg. Chem. Commun. 3 (2000) 648.
[13] D.W. Widlicka, MS Thesis, Department of Chemistry, University of New
Hampshire, Durham, NH, USA, 2002.
[14] J. Li, MS Thesis, Department of Chemistry, University of New Hampshire,
Durham, NH, USA, 2007.
[15] D.P. Reed, G.R. Weisman, Org. Synth. 78 (2002) 73.
[16] I. Saikawa, T. Hori, K. Momoi, Y. Kobane, T. Yasuda, M. Miyahara, S. Tai, H.
Tsuda, M. Doi, S. Kishimoto, in: Toyama Chemical Co., Ltd., Japan, Patent
Application: JP, 1979, p. 10.
[17] G.M. Sheldrick, Acta Crystallogr., Sect. A A64 (2008) 112.
[18] A. Scarel, J. Durand, D. Franchi, E. Zangrando, G. Mestroni, C. Carfagna, L.
Mosca, R. Seraglia, G. Consiglio, B. Milani, Chem.-Eur. J. 11 (2005) 6014.
[19] F.A. Cotton, J. Gu, C.A. Murillo, D.J. Timmons, J. Am. Chem. Soc. 120 (1998)
13280.
[20] R.-L. Sang, L. Xu, Eur. J. Inorg. Chem. (2006) 1260.
[21] R.Q. Zou, Q. Xu, Acta Crystallogr., Sect. E: Struct. Rep. E61 (2005) m1075.
[22] G.K. Patra, I. Goldberg, A. Sarkar, S. Chowdhury, D. Datta, Inorg. Chim. Acta 344
(2003) 7.
[23] A.W. Addison, T.N. Rao, J. Reedijk, J. Van Rijn, G.C. Verschoor, J. Chem. Soc.,
Dalton Trans. (1984) 1349.
[24] K.A. Fichter, MS Thesis, Department of Chemistry, University of New
Hampshire, Durham, NH, USA, 2004.
4. Summary and conclusions
With the characterization of these eleven new complexes
including nine X-ray structural studies, the tricyclic bisamidines
L1 and L2 have been shown to have extensive metal coordination
chemistry. To date L1, with divergent donor lone pairs, has been
found in monodentate, symmetrical and unsymmetrical chelating,
as well as symmetrical and unsymmetrical metal-bridging modes
in its solid-state structures. These include Mn(II), Fe(II), Ru(II),