Synthesis and crystallographic characterization of dinuclear silver complexes supported by N,N′-diarylformamidinates

Article abstract of DOI:10.1016/S0020-1693(01)00522-9

Synthesis of two new disilver(I) complexes of diarylformamidinate bearing o-ethoxy (1) and m-butoxy substituents (2) is described. Crystallographic characterization revealed that each Ag center is supported primarily by two N centers in approximate co-linear geometry. Secondary coordination of ethereal O center to Ag was also observed in molecule 1. Conformational isomerism exists in the crystal of 2, and is attributed to the low energy-barrier free rotation of aryls. Crystal data: compound 1 crystallizes in orthorhombic space group Pbca, a = 11.392(1), b = 15.007(1), c = 19.363(2) ?, Z = 4; compound 2 crystallizes in tetragonal space group P4₂/n, a = 24.152(2), c = 14.371(2) ?, Z = 8.

Full text of DOI:10.1016/S0020-1693(01)00522-9

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Inorganica Chimica Acta 321 (2001) 200–204
Note
Synthesis and crystallographic characterization of dinuclear silver
complexes supported by N,N%-diarylformamidinates
Steven Radak a,b, Yuhua Ni a, Guolin Xu a, Kelly L. Shaffer c, Tong Ren a,c,
*
a Department of Chemistry, Uni6ersity of Miami, Coral Gables, FL 33124, USA
b Department of Chemistry/Earth Science/Physics, Miami-Dade Community College, North Campus, Miami, FL 33167, USA
c Department of Chemistry, Florida Institute of Technology, Melbourne, FL 32901, USA
Received 4 May 2001; accepted 8 June 2001
Abstract
Synthesis of two new disilver(I) complexes of diarylformamidinate bearing o-ethoxy (1) and m-butoxy substituents (2) is
described. Crystallographic characterization revealed that each Ag center is supported primarily by two N centers in approximate
co-linear geometry. Secondary coordination of ethereal O center to Ag was also observed in molecule 1. Conformational
isomerism exists in the crystal of 2, and is attributed to the low energy-barrier free rotation of aryls. Crystal data: compound 1
,
crystallizes in orthorhombic space group Pbca, a=11.392(1), b=15.007(1), c=19.363(2) A, Z=4; compound 2 crystallizes in
,
tetragonal space group P42/n, a=24.152(2), c=14.371(2) A, Z=8. © 2001 Elsevier Science B.V. All rights reserved.
Keywords: N,N%-Diarylformamidinates; Dinuclear silver complexes; Crystallographic characterization
1. Introduction
synthesis and structural characterization of two novel
disilver(I) compounds with X as o-ethoxy (1) and m-n-
butoxy (2).
There has been a renewed interest in dinuclear and
polynuclear Ag compounds, especially their lumines-
cent properties [1–4] and the nature of Ag···Ag interac-
2. Experimental
tion
communicated both the structure and solid state
fluorescence of
bis(m-N,N%-h2-N,O-h2-N%,O%-di(o-
(argentophilicity)
[4,5].
Previously,
we
All commercial reagents used were of analytical
reagent grade and were used without further purifica-
tion. 1H and 13C NMR spectra were recorded on a
methoxyphenyl)formamidinato)disilver(I) [1], where the
bridging formamidinates support the Ag2 core through
the formation of both NꢀAg covalent bonds and sec-
ondary O···ꢁAg dative bonds. The identical crystal
structure was later reported by Busch et al. [6]. In order
to improve both the solubility and emission property of
this type of disilver(I) compound, we investigated the
synthesis of analogous disilver(I) complexes with diaryl-
formamidinates (DArF) bearing a variety of phenyl
substituents (Scheme 1) [7]. Reported herein are the
* Corresponding author. Tel.: +1-305-284 6617; fax: +1-305-284
1880.
Scheme 1. Line structure of Ag2(diarylformamidinate)2; X is the
phenyl substituent.
E-mail address: tren@miami.edu (T. Ren).
0020-1693/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0020-1693(01)00522-9

S. Radak et al. / Inorganica Chimica Acta 321 (2001) 200–204
201
Table 1
aromatic), 3.52 (q, 8H, CH2O), 0.81 (t, 12H, CH3).
UV–Vis: umax (nm) (m, M−1 cm−1): 252(sh), 298(sh),
325 (42 300). IR (KBr pellet, cm−1): 3071(w), 3020(w),
2973(w), 2927(w), 2875(w), 1660(w), 1547(s), 1496(s),
1450(w), 1387(w), 1341(m), 1298(w), 1236(s), 1201(m),
1119(s), 1041(m), 917(w), 797(w), 753(m) and 649(w).
Data for 2: FAB MS (m/e, based on 107Ag): 895
Crystal data and structure refinement for compounds 1 and 2
Compound
1
2
Empirical formula
Formula weight
C
782.4
300(2)
0.71073
34H38N4O4Ag2
C42H54N4O4Ag2
894.6
300(2)
Temperature (K)
Wavelength (Mo Ka)
0.71073
1
3
[MH+]. H NMR ((CD3)2CO): l 8.31 (t, 2H, JAgꢀH
=
,
(A)
14.7 Hz, NꢀCHꢀN), 7.06 (t, 4H, aromatic), 6.67 (m,
8H, aromatic), 6.48 (m, 4H, aromatic), 3.81 (t, 8H,
CH2O), 1.64 (m, 8H, CH2), 1.42 (m, 8H, CH2), 0.93 (t,
12H, CH3). UV–Vis: umax (nm) (m, M−1 cm−1): 311
(49 300). IR (KBr pellet, cm−1): 2956(m), 2932(m),
2871(w), 1586(s), 1555(vs), 1469(s), 1376(m), 1345(s),
1256(m), 1193(s), 1154(s), 1069(w), 1026(w), 976(w),
843(w), 769(w) and 699(w).
Crystals of compounds 1 and 2 were grown via vapor
diffusion of hexanes into a CH2Cl2 solution and slow
diffusion of CH3OH into an acetone solution, respec-
tively. X-ray intensity data were measured at 300 K on
a Bruker SMART 1000 CCD-based X-ray diffractometer
with crystals affixed to a quartz fiber by epoxy glue.
Data were measured using omega scans of 0.3° per
frame such that a hemisphere was collected. A total of
1271 frames were collected with a final resolution of
Crystal system
Space group
orthorhombic
Pbca
tetragonal
P42/n
Unit cell dimensions
,
a (A)
11.392(1)
15.007(1)
19.363(2)
3310.2(5)
4
1.570
1.226
1584
24.152(2)
24.152(2)
14.371(2)
8382(2)
,
b (A)
,
c (A)
3
,
V (A )
Z
8
Dcalc (g cm−3
)
1.418
0.978
3680
0.44×0.04×0.04
1.19–25.00
7382 [Rint=0.0494]
v (mm−1
F(000)
)
Crystal size (mm)
q Range (°)
Independent reflections
0.27×0.23×0.07
2.10–25.00
2912
[Rint=0.0235]
Data/restraints/parameter 2912/0/199
s
Final R indices
[I\2|(I)]
R indices (all data)
7382/0/470
R1=0.021,
wR2=0.052
R1=0.031,
wR2=0.056
1.014
R1=0.064,
wR2=0.161
R1=0.130,
wR2=0.237
1.086
,
0.75 A. No decay was indicated by the recollection of
Goodness-of-fit on F2
the first 50 frames at the end of data collection. The
®
frames were integrated with the Bruker SAINT soft-
ware package [10] using a narrow-frame integration
algorithm, which also corrects for the Lorentz and
polarization effects. Absorption corrections were ap-
plied using SADABS supplied by George Sheldrick.
Structures of 1 and 2 were solved and refined using
the Bruker SHELXTL® (Version 5.1) Software Package
[11–13] in the space groups Pbca and P42/n, respec-
tively. Positions of all non-hydrogen atoms were lo-
cated by direct method. The asymmetric unit of 1
contains one half of an independent molecule that is
related to the other half via a crystallographic inversion
center. The asymmetric unit of 2 contains the halves of
two independent molecules and each half is related to
the other half via an inversion center. With all non-hy-
drogen atoms being anisotropic and all hydrogen atoms
in calculated position and riding mode the structures
were refined to convergence by least-squares method on
F2, SHELXL-93, incorporated in SHELXTL PCV 5.03.
Data collection and structural refinement parameters
for 1 and 2 are given in Table 1.
Bruker AVANCE300 NMR spectrometer, with chemi-
cal shifts (l) referenced to the residual proton(s) in the
deuterated solvents. Infrared spectra were recorded on
a Perkin–Elmer 2000 FT-IR spectrometer using KBr
disks. UV–Vis spectra were obtained with a Perkin–
Elmer Lambda-900 UV–Vis spectrophotometer in
CH2Cl2 solution. Formamidine ligands were prepared
by the condensation reaction between triethylorthofor-
mate and appropriate aniline in 1:2 molar ratio accord-
ing to
a
literature procedure [8]. The disilver
compounds were prepared and purified as described for
bis(m - N,N% - h2 - N,O - h2 - N%,O% - di(o - methoxyphenyl)-
formamidinato)disilver(I) [1,9]. Slow addition of con-
centrated aqueous ammonia to AgNO3 crystals (1.70 g,
10 mmol) resulted in the precipitation of an off-white
solid, which gradually disappears with the further addi-
tion (total volume 10 ml). Appropriate formamidine (10
mmol) was added to the clear solution and a white
precipitate was formed immediately upon the addition
of ethanol (1–2 ml). The mixture was vigorously stirred
for 3 h before being filtered. The off-white solid col-
lected was rinsed with copious amount of H2O and
CH3OH and dried in vacuo. Yields based on AgNO3: 1,
82% and 2, 35%.
3. Results and discussion
Synthetic procedure for compounds 1 and 2 is
modified from that of Ag2(dialkylformamidinate)2 [1,9]
and the reaction sequence is shown in Scheme 2. The
cause of the low yield for 2 is unclear presently. It
Data for 1: FAB MS (m/e, based on 107Ag): 783
1
3
[MH+]. H NMR (C6D6): l 8.39 (t, 2H, JAgꢀH=12.6
Hz, NꢀCHꢀN), 6.81(m, 12H, aromatic), 6.66 (m, 4H,

202
S. Radak et al. / Inorganica Chimica Acta 321 (2001) 200–204
N,N%-h2-N,O-h2-N%,O%-di(o-methoxyphenyl)formamidi-
appears that 2 is generally less stable in both solution
and solid state in comparison with the Ag2 compounds
with o-substituted formamidinate. Perhaps, part of the
product formed initially was degraded to side prod-
uct(s) that were soluble in the reaction mixture.
Molecular structures of 1 and 2 determined by X-ray
diffraction study are shown in Figs. 1 and 2, respec-
tively, and the selected bond lengths and angles are
given in Table 2. In both 1 and 2, each Ag center forms
two AgꢀN bonds that are trans to each other. The
NꢀAgꢀN angles deviate approximately 10° from the
180° angle expected for an idealized two-coordination
geometry. The Ag···Ag distances are 2.7533(4), 2.748(1)
,
nato)disilver(I) (2.783(1) A) [1], but are longer than
those observed in bis(N,N%-di(p-tolyl)formamidi-
,
nato)disilver(I) (2.705(1) A) [14] and bis(N,N%-diphenyl-
,
triazenato)disilver(I) (2.669(1) A) [15]. All the afore-
,
mentioned AgꢀAg distances are at least 0.1 A shorter
than those reported for disilver compounds supported
,
by phosphine ligands (2.890–3.095 A), where the exis-
tence of argentophilicity was suggested based on spec-
troscopic evidence [4].
Previous study of Ag2 compound supported by N,N%-
di(o-methoxyphenyl)formamidinate ligands revealed
that the ethereal oxygen centers form the dative bonds
to Ag(I) centers in addition to the N-coordination to
Ag(I) [1]. It is clear from Fig. 1 that similar interactions
exist in 1: there are two covalent bonds from N1 and
N2 centers and two dative bonds from O1 and O2
centers for each Ag center. The existence of the latter is
,
and 2.742(2) A for molecules 1, 2a and 2b, respectively,
which are identical within experimental errors. These
bond lengths are slightly shortened from that of bis(m-
,
verified by the AgꢀO distances (2.658(3) and 2.645(3) A
for Ag1ꢀO1 and Ag1ꢀO2, respectively), which are sig-
nificantly smaller than the sum of the Ag and O van der
Scheme 2. Synthesis of disilver(I) compounds: (i) NH4OH (excess);
(ii) HDArF, EtOH (or MeOH).
,
Waals radii, 3.24 A [1]. To enable the formation of the
O···ꢁAg dative bond without serious steric repulsion
between the O-alkyl groups, the aryl groups are all
canted in respect to the N1ꢀAg1ꢀN2 plane, as shown in
Fig. 1(b). It is interesting to note that both aryls from
the same ligand are canted in the same direction in 1,
which yields an effective C2h point symmetry with
Ag1ꢀAg1A vector as the C2 axis. In the previously
reported
bis(m-N,N%-h2-N,O-h2-N%,O%-di(o-methoxy-
phenyl)formamidinato)disilver(I), on the other hand,
one of the aryls is pointed above the NꢀAgꢀN plane,
while the other from the same ligand is pointed beneath
the plane, and an effective D2 symmetry for the
molecule is resulted in [1].
While the coordination geometry around Ag2 core in
2 is similar to that observed for other Ag-formamidi-
nate complexes [1,14,16], a conformational isomerism
exists in the solid state due to the subtle difference in
the arrangement of the peripheral substituents. As illus-
trated in Scheme 3, three stable conformations exist for
diarylformamidine bearing an m-alkoxy substituent,
which are defined as (a) s-cis,s-cis- (s-cis-/s-trans- are
defined between the NꢀC (methine) bond and the ring
CꢀC bond prioritized by the OR group), (b) s-cis,s-
trans-, and (c) s-trans,s-trans- (for conformational de-
scriptors see [17]). Three conformations can
inter-convert among themselves by rotating either one
or both aryls 180° around NꢀC(phenyl) bond. In the
crystal of 2, there are two independent molecules, 2a
and 2b, and the former adopts the s-trans, s-trans-con-
formation while the latter adopts the s-cis,s-cis- confor-
mation. As a result, 2a is best described as a molecular
rod extended in the direction parallel to the Ag···Ag
vector, while 2b is a molecular rod extended in the
Fig. 1. ORTEP plots of molecule 1 at 30% probability level: (a) top
view; (b) side view.

S. Radak et al. / Inorganica Chimica Acta 321 (2001) 200–204
203
Fig. 2. ORTEP plots of two independent molecules of 2 at 30% probability level: (a) 2a; (b) 2b.
Table 2
Selected bond lengths and angles of molecules 1, 2a and 2b
1
2
Molecule 2a
Molecule 2b
Bond lengths
Ag(1)ꢀAg(1A)
Ag(1)ꢀN(1)
Ag(1)ꢀN(2)
N(1)ꢀC(1)
2.7533(4)
2.1453(17)
2.1523(17)
1.318(3)
Ag(1)ꢀAg(1A)
Ag(1)ꢀN(1)
Ag(1)ꢀN(2)
N(1)ꢀC(1)
2.7479(14)
2.088(7)
2.095(7)
1.338(11)
1.339(11)
Ag(2)ꢀAg(2A)
Ag(2)ꢀN(3)
Ag(2)ꢀN(4)
N(3)ꢀC(22)
N(4)ꢀC(22)
2.7423(15)
2.116(6)
2.119(7)
1.329(11)
1.321(11)
N(2)ꢀC(1)
1.318(3)
N(2)ꢀC(1)
Bond angles
N(1)ꢀAg(1)ꢀN(2)
N(1)ꢀAg(1)ꢀAg(1A)
N(2)ꢀAg(1)ꢀAg(1A)
N(1)ꢀC(1)ꢀN(2)
168.54(6)
85.50(4)
83.15(5)
124.2(2)
N(1)ꢀAg(1)ꢀN(2)
N(1)ꢀAg(1)ꢀAg(1A)
N(2)ꢀAg(1)ꢀAg(1A)
N(1)ꢀC(1)ꢀN(2)
169.9(3)
84.3(2)
85.7(2)
126.1(8)
N(3)ꢀAg(2)ꢀN(4)
N(3)ꢀAg(2)ꢀAg(2A)
N(4)ꢀAg(2)ꢀAg(2A)
N(4)ꢀC(22)ꢀN(3)
169.4(3)
84.9(2)
84.6(2)
125.5(7)
Scheme 3. Possible confirmations of di(m-alkoxyphenyl)formamidine: (a) s-cis,s-cis-, two bonds defining s-cis are shown in bold; (b) s-cis,s-trans-,
two bonds defining s-trans are shown in bold, (c) s-trans,s-trans-.

204
S. Radak et al. / Inorganica Chimica Acta 321 (2001) 200–204
direction orthogonal to the AgꢀAg vector. Coexistence
of 2a and 2b in 1:1 ratio implies the absence of en-
ergetic preference for either conformation. It is also
interesting to note that the free ligand di(m-bu-
toxyphenyl)formamidine was crystallized in the s-cis,s-
trans-conformation [18].
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Crystallographic data for the structural analysis have
been deposited with the Cambridge Crystallographic
Data Centre, CCDC Nos. 162958 and 162959 for com-
pounds 1 and 2, respectively. Copies of this information
may be obtained free of charge from The Director,
CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK
(fax: +44-1223-336-033; e-mail: deposit@ccdc.cam.
ac.uk or www: http://www.ccdc.cam.ac.uk).
Acknowledgements
The authors thank the University of Miami for
providing support for this work (start-up fund and the
funding for the CCD-diffractometer). Early work per-
formed at the Florida Institute of Technology was
partially supported by the Florida Solar Energy Center.
The Miami-Dade Community College Bridges Grant
(NIGMS R025GM50083-03) sponsered the sabbatical
leave of SR.
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Crystallogr., submitted for publication.
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