Syntheses, structures, and electronic interactions of dicyanamide/tricyanomethanide-bridged binuclear organometallic complexes

Article abstract of DOI:10.1021/ic025879t

Dicyanamide-bound mononuclear compounds Cp(dppe)FeN(CN)₂ (3) and Cp(PPh₃)₂RuN(CN)₂ (4) were isolated in high yields by the reactions of Cp(dppe)FeCl (1) and Cp(PPh₃)₂RuCl (2), respectively, with excess sodium dicyanamide. Compounds 3 and 4 are excellent precursors for the design of dicyanamide-bridged binuclear complexes [{Cp(dppe)Fe}₂N(CN)₂](SbF₆) (5) and [{Cp(PPh₃)₂Ru}₂N(CN)₂] (SbF₆) (6) by the incorporation with 1 and 2, respectively. Controlling oxidation of 5 with ferrocenium hexafluorophosphate afforded the mixed-valence compound [{Cp(dppe)Fe}₂N(CN)₂](PF₆)₂ (5a) which exhibits a broad absorption band in the near-infrared region (centered at 1500 nm, ε = 750 cm^-1 M^-1) due to the intervalence charge transfer of Robin and Day class II mixed-valence system. Tricyanomethanide-bound mononuclear compounds Cp(dppe)FeC(CN)₃ (7) and Cp(PPh₃)₂RuC(CN)₃ (8) were prepared by the same methods as 3 and 4 using potassium tricyanomethanide as the starting material instead. The tricyanomethanide-bridged binuclear complexes [{Cp(dppe)Fe}₂C(CN)₃](CF₃SO₃) (9) and [{Cp(PPh₃)₂- Ru}₂C(CN)₃](SbF₆) (10) were prepared by the reactions between 7 and 1 and between 8 and 2, respectively. Cyclic voltammograms of the dicyanamide/tricyanomethanide-bridged binuclear complexes showed stepwise reversible one-electron oxidation waves with the potential separation of the two redox couples in the range 0.14-0.25 V, indicating the demonstrably electronic communication is operative between the organometallic components through a dicyanamide/tricyanomethanide spacer with metal...metal distances more than 7.8 A. Furthermore, the electronic coupling transmitted by the tricyanomethanide is appreciably greater than that by the dicyanamide. The complexes 3-10 were characterized by elemental analysis, IR, UV-vis, ¹H and ³¹P NMR, and ES-MS. The crystal structures of 3 and 5-9 were determined by X-ray crystallography.

Full text of DOI:10.1021/ic025879t

Inorg. Chem. 2003, 42, 633−640
Syntheses, Structures, and Electronic Interactions of Dicyanamide/
Tricyanomethanide-Bridged Binuclear Organometallic Complexes
,†,‡,§
Li-Yi Zhang,† Lin-Xi Shi,† and Zhong-Ning Chen*
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of
Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China, State Key Laboratory of
Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of
Sciences, Shanghai 200032, China, and State Key Laboratory of Coordination Chemistry,
Nanjing UniVersity, Nanjing 210093, China
Received July 17, 2002
Dicyanamide-bound mononuclear compounds Cp(dppe)FeN(CN)2 (3) and Cp(PPh3)2RuN(CN)2 (4) were isolated in
high yields by the reactions of Cp(dppe)FeCl (1) and Cp(PPh3)2RuCl (2), respectively, with excess sodium
dicyanamide. Compounds 3 and 4 are excellent precursors for the design of dicyanamide-bridged binuclear complexes
[{Cp(dppe)Fe}2N(CN)2](SbF ) (5) and [{Cp(PPh3)2Ru}2N(CN)2](SbF ) (6) by the incorporation with 1 and 2,
6
6
respectively. Controlling oxidation of 5 with ferrocenium hexafluorophosphate afforded the mixed-valence compound
[{Cp(dppe)Fe}2N(CN)2](PF ) (5a) which exhibits a broad absorption band in the near-infrared region (centered at
6 2
1500 nm, ꢀ ) 750 cm-1 M-1) due to the intervalence charge transfer of Robin and Day class II mixed-valence
system. Tricyanomethanide-bound mononuclear compounds Cp(dppe)FeC(CN)3 (7) and Cp(PPh3)2RuC(CN)3 (8)
were prepared by the same methods as 3 and 4 using potassium tricyanomethanide as the starting material
instead. The tricyanomethanide-bridged binuclear complexes [{Cp(dppe)Fe}2C(CN)3](CF SO ) (9) and [{Cp(PPh3)2-
3
3
Ru}2C(CN)3](SbF ) (10) were prepared by the reactions between 7 and 1 and between 8 and 2, respectively.
6
Cyclic voltammograms of the dicyanamide/tricyanomethanide-bridged binuclear complexes showed stepwise reversible
one-electron oxidation waves with the potential separation of the two redox couples in the range 0.14−0.25 V,
indicating the demonstrably electronic communication is operative between the organometallic components through
a dicyanamide/tricyanomethanide spacer with metal‚‚‚metal distances more than 7.8 Å. Furthermore, the electronic
coupling transmitted by the tricyanomethanide is appreciably greater than that by the dicyanamide. The complexes
3−10 were characterized by elemental analysis, IR, UV−vis, 1H and 31P NMR, and ES-MS. The crystal structures
of 3 and 5−9 were determined by X-ray crystallography.
Introduction
assembly molecules that consist of an organic unsaturated
conjugated spacer linking two redox-active metal termini
could permit electron flow to occur along the molecular
backbone. The mediated electronic effect between the redox-
active termini through an organic spacer is usually evaluated
by electrochemical measurements. When different redox
states are stably present at both ends of the spacer, an odd
electron-containing species or mixed-valence compound can
be generated by an electrochemical or/and chemical redox
Transition metal complexes that exhibit electronic delo-
calization are of current interest because of their potential
application as molecular wires which are essential for the
assembly of nanoscale electronic devices.1-6 The linear
* Corresponding author. E-mail: czn@ms.fjirsm.ac.cn. Fax: +86-591-
379-2346.
† Fujian Institute of Research on the Structure of Matter.
‡ State Key Laboratory of Organometallic Chemistry.
§ State Key Laboratory of Coordination Chemistry.
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10.1021/ic025879t CCC: $25.00 © 2003 American Chemical Society
Published on Web 12/20/2002
Inorganic Chemistry, Vol. 42, No. 2, 2003 633

Zhang et al.
method, and a wealth of decisive information about through-
bridge electron transfer can be recognized by UV-vis-NIR
spectrometry.7-20
and characterized by spectroscopic measurements and X-ray
crystallography. Herein are described the preparation and
characterization of the dicyanamide/tricyanomethanide-
containing mono- or dinuclear organometallic complexes
together with a mixed-valence binuclear complex [{Cp-
(dppe)Fe}2N(CN)2](PF6)2 (5a).
Dicyanamide (N(CN)2-)21,22 and tricyanomethanide (C(C-
- 23,24
N)3 )
are versatile for the design of polymeric metal
architectures by self-assembly. They behave frequently as
µ- or/and µ3-bridges linking two or three metal centers to
produce 1D, 2D, or 3D extended aggregates. The bridging
array MsNtCsNsCtNsM′/MsNtCsC(CN)sCtNs
M′ is nonlinear owing to the sp2 hybridization of the middle
nitrogen/carbon atom with M‚‚‚M′ separation more than 7.5
Å. However, it is essential to detect the capability of
dicyanamide/tricyanomethanide as a spacer to mediate
electronic effect between two redox-active organometallic
centers.25,26 Thus, a series of dicyanamide/tricyanomethanide-
containing mono- and binuclear complexes were prepared
Experimental Section
Material and Reagents. All operations were performed in an
atmosphere of dry argon by using Schlenk and vacuum techniques.
Solvents were dried by standard methods and distilled prior to use.
The reagents sodium dicyanamide (NaN(CN)2), potassium tricya-
nomethanide (KC(CN)3), thallium cyclopentadienide, dicyclopen-
tadiene, triphenylphosphine (PPh3), 1,2-bis(diphenylphosphino)-
ethane (dppe), potassium triflate (KCF3SO3), and sodium hexafluoro-
antimonate (NaSbF6) were purchased from commercial sources
(Acros, Fluka, and Aldrich Chemicals Co.). The organometallic
compounds Cp(dppe)FeCl (1)27 and Cp(PPh3)2RuCl (2)28 were
prepared by the reported procedures.
Cp(dppe)FeN(CN)2 (3). Compound 1 (1.0 mmol, 545.0 mg) and
NaN(CN)2 (3.0 mmol, 267.0 mg) were added into 20 mL of
methanol which was stirred at room temperature for 2 h to give a
deep red solution. The solvent was removed in vacuo, and the
residue was dissolved in 5 mL of dichloromethane. After taken by
filtration, the filtrate was purified by chromatography using
aluminum oxide column. Elution with dichloromethane gave the
pure product. Yield: 75%. Layering petroleum ether onto the 1,2-
dichloroethane solution afforded the product as crystals. Anal. Calcd
for C33H29FeN3P2‚0.5C2H4Cl2: C, 64.32; H, 4.92; N, 6.62. Found:
C, 64.68; H, 4.72; N, 6.44. IR spectrum (KBr, cm-1): ν 2266 (m,
N(CN)2), 2224 (w, N(CN)2), 2158 (s, N(CN)2). 1H NMR spectrum
(CDCl3): δ 7.77-7.26 (m, 20H, C6H5), 4.14 (s, 5H, C5H5), 3.73
(s, 2H, C2H4Cl2), 2.28 (d, 4H, P(CH2)2P). 31P NMR spectrum
(CDCl3): δ 99.4 (s). UV-vis (λmax/nm (ꢀ, cm-1 M-1)): 235
(58000), 315 (3900), 383 (1400).
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Cp(PPh3)2RuN(CN)2 (4). To a dichloromethane (20 mL) solu-
tion of Cp(PPh3)2RuCl (0.20 mmol, 145.2 mg) was added a
methanol (5 mL) solution of NaN(CN)2 (0.50 mmol, 44.5 mg) with
the color change from orange into pale yellow. After the solution
was stirred at room temperature for 4 h, the solvents were
evaporated in vacuo to leave a residue which was dissolved in 3
mL of dichloromethane. After taken by filtration, the filtrate was
layered with petroleum ether to give pale yellow crystals of the
product. Yield: 88%. Anal. Calcd for C43H35N3P2Ru: C, 68.25;
H, 4.66; N, 5.55. Found: C, 68.44; H, 4.25; N, 5.36. IR spectrum
(KBr, cm-1): ν 2270 (s, N(CN)2), 2229 (m, N(CN)2), 2164 (s,
N(CN)2). 1H NMR spectrum (CDCl3): δ 7.31-7.15(m, 30H, C6H5),
4.20 (s, 5H, C5H5). 31P NMR spectrum (CDCl3): δ 43.3 (s). UV-
vis (λmax/nm (ꢀ, cm-1 M-1)): 231 (17000), 351 (1100).
[{Cp(dppe)Fe}2N(CN)2](SbF6) (5). To a methanol (5 mL)
solution of 1 (0.10 mmol, 55.5 mg) was added first a methanol (5
mL) solution of 3 (0.10 mmol, 58.5 mg), and then sodium
hexafluoroantimonate (0.11 mmol, 28.5 mg). The solution color
changed rapidly into deep red. After the solution was stirred at
room temperature for 2 h, the methanol was evaporated in vacuo
to leave a residue which was dissolved in 2 mL of dichloromethane.
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634 Inorganic Chemistry, Vol. 42, No. 2, 2003

Dicyanamide/Tricyanomethanide-Bridged Complexes
After taken by filtration, the product was crystallized by layering
petroleum ether onto the dichloromethane solution. Yield: 80%.
Anal. Calcd for C64H58N3F6P4Fe2Sb‚CH2Cl2‚2CH3OH: C, 54.03;
H, 4.60; N, 2.82. Found: C, 53.93; H, 4.28; N, 2.58. ES-MS (m/
z): 1339 ([{Cp(dppe)Fe}2N(CN)2(SbF6)+]), 1104 ([{Cp(dppe)-
Fe}2N(CN)2]+), 561 ([Cp(dppe)FeNCN]+), 519 ([Cp(dppe)Fe]+).
IR spectrum (KBr, cm-1): ν 2299 (m, N(CN)2), 2208 (s, N(CN)2),
658 (s, SbF6). 1H NMR spectrum (CDCl3): δ 7.63-7.15 (m, 40H,
by the color change from orange-red into pale yellow. The solvents
were removed in vacuo, and the residue was dissolved in 3 mL of
dichloromethane. The product was isolated as a precipitate by
layering petroleum ether onto the solution. After taken by filtration,
the product was recrystallized in 1,2-dichloroethane-petroleum
ether to give yellow crystals. Yield: 88%. Anal. Calcd for
C45H35N3P2Ru: C, 69.22; H, 4.52; N, 5.38. Found: C, 69.72; H,
1
4.05; N, 5.08. IR spectrum (KBr, cm-1): ν 2175 (s, C(CN)3). H
C6H5), 4.19 (s, 10H, C5H5), 2.25 (s, 8H, P(CH2)2P). UV-vis (λmax
/
NMR spectrum (CDCl3): δ 7.34-7.09 (m, 30H, C6H5), 4.29 (s,
nm (ꢀ, cm-1 M-1)): 234 (52000), 316 (39000), 479 (800).
5H, C5H5). 31P NMR spectrum (CDCl3): δ 42.1 (s). UV-vis (λmax
nm (ꢀ, cm-1 M-1)): 234 (45000), 340 (2700).
/
[{Cp(dppe)Fe}2N(CN)2](PF6)2 (5a). To a dichloromethane (10
mL) solution of [{Cp(dppe)Fe}2N(CN)2](PF6) (0.10 mmol, 125 mg)
was added ferrocenium hexafluorophosphate (0.10 mmol, 33.1 mg)
with stirring at room temperature for 2 h. The solution was then
concentrated, leaving 2 mL of the volume. Diffusion of diethyl
either gave a precipitate which was washed with diethyl ether three
times and redissolved in 10 mL of dichloromethane with stirring
for 2 h. The solution was concentrated again and diffused with
diethyl ether to precipitate the product which was disposed
repeatedly for three times by the same procedure. Crystallization
in dichloromethane-petroleum gave the pure product. Yield: 51%.
Anal. Calcd for C64H58F12N3P6Fe2‚0.5CH2Cl2: C, 53.91; H, 4.14;
N, 2.92. Found: C, 53.99; H, 3.77; N, 2.77. IR spectrum (KBr,
cm-1): ν 2233 (m, N(CN)2), 2187 (m, N(CN)2), 2087 (m, N(CN)2),
839 (s, PF6). UV-vis-NIR (λmax/nm (ꢀ, cm-1 M-1)): 230 (41000),
317 (4400), 1550 (750).
[{Cp(PPh3)2Ru}2N(CN)2](SbF6) (6). To a dichloromethane (5
mL) solution of 2 (0.10 mmol, 72.6 mg) was added first a
dichloromethane (5 mL) solution of 4 (0.1 mmol, 75.7 mg), and
then a methanol (2 mL) solution of sodium hexafluoroantimonate
(0.11 mmol, 28.5 mg). The solution was stirred at room temperature
for 4 h with the solution color changing from orange into earthy
yellow. The solvents were removed in vacuo to leave a yellow
residue which was dissolved in 3 mL of dichloromethane. After
taken by filtration, the filtrate was layered with petroleum ether
for crystallization of the product. Yield: 75%. Anal. Calcd for
C84H70N3F6P4Ru2Sb: C, 59.94; H, 4.19; N, 2.50. Found: C, 59.78;
H, 4.26; N, 2.39. ES-MS (m/z): 1448 ([{Cp(PPh3)2Ru}2N(CN)2]+),
731 ([Cp(PPh3)2RuNCN]+), 691 ([Cp(PPh3)2Ru]+), 626 ([(PPh3)2-
Ru]+). IR spectrum (KBr, cm-1): ν 2295 (m, N(CN)2), 2208 (s,
N(CN)2), 658 (s, SbF6). 1H NMR spectrum (CDCl3): δ7.29-7.10
(m, 60H, C6H5), 4.24 (s, 10H, C5H5). 31P NMR spectrum (CDCl3):
δ 41.4 (s). UV-vis (λmax/nm (ꢀ, cm-1 M-1)): 234 (88000), 321
(6100).
Cp(dppe)FeC(CN)3 (7). To a dichloromethane (15 mL) solution
of 1 (0.20 mmol, 111.0 mg) was added a methanol solution of
potassium tricyanomethanide (0.40 mmol, 51.6 mg) with rapid color
change from deep dark red into red. After the solution was stirred
at room temperature for 2 h, the solvents were removed to leave a
red residue which was dissolved in 5 mL of dichloromethane. After
taken by filtration, the filtrate was chromatographed by an aluminum
oxide column, and the red band was collected using dichlo-
romethane-methanol (50:1) as eluate. Yield: 81%. Anal. Calcd
for C35H29FeN3P2: C, 68.98; H, 4.80; N, 6.89. Found: C, 68.48;
H, 4.77; N, 6.59. IR spectrum (KBr, cm-1): ν 2168 (s, C(CN)3).
1H NMR spectrum (CDCl3): δ 7.69-7.27 (m, 20H, C6H5), 4.23
(s, 5H, C5H5), 2.30 (d, 4H, P(CH2)2P). 31P NMR spectrum
(CDCl3): δ 98.7 (s). UV-vis (λmax/nm (ꢀ, cm-1 M-1)): 234
(44000), 319 (3000), 470 (700).
[{Cp(dppe)Fe}2C(CN)3](CF3SO3) (9). To a dichloromethane
(10 mL) solution of 7 (0.10 mmol, 60.9 mg) was added first a
dichloromethane (5 mL) solution of 1 (0.10 mmol, 55.5 mg), and
then a methanol (3 mL) solution of potassium trifluoromethane-
sulfonate (0.12 mmol, 22.6 mg). After the solution was stirred at
room temperature for 4 h, the solvents were evaporated in vacuo,
and the residue was dissolved in 3 mL of dichloromethane. After
taken by filtration, the filtrate was layered with petroleum ether to
crystallize the product. Yield: 85%. Anal. Calcd for C67H58F3-
Fe2N3O3P4S: C, 62.98; H, 4.57; N, 3.29. Found: C, 63.69; H, 4.27;
N, 3.19. ES-MS (m/z): 1128 ([{Cp(dppe)Fe}2C(CN)3]+), 607 ([Cp-
(dppe)FeC(CN)3]+), 517 ([Cp(dppe)Fe]+). IR spectrum (KBr,
cm-1): ν 2201 (w, C(CN)3), 2179 (s, C(CN)3), 1277 (s, CF3SO3),
1174 (m, CF3SO3), 1032 (m, CF3SO3). 1H NMR spectrum
(CDCl3): δ 7.74-7.19 (m, 40H, C6H5), 4.14 (s, 10H, C5H5), 2.48
(d, 4H, P(CH2)2P), 2.07 (s, 4H, P(CH2)2P). 31P NMR spectrum
(CDCl3): δ 97.1 (s). UV-vis (λmax/nm (ꢀ, cm-1 M-1)): 230
(65000), 268 (27000), 472 (1000).
[{Cp(PPh3)2Ru}2C(CN)3](SbF6) (10). Compounds 8 (0.10
mmol, 78.1 mg) and 2 (0.10 mmol, 72.6 mg) were put into a 25
mL Schlenk flask and dissolved in 10 mL of dichloromethane with
stirring. A methanol (5 mL) solution of sodium hexafluoroanti-
monate (0.12 mmol, 31.5 mg) was then added, and the solution
was stirred at room temperature for 4 h with the color change from
orange-red into earthy yellow. The solvents were removed in
vacuo to leave a residue which was dissolved in 3 mL of dichloro-
methane. After filtration, the filtrate was layered with petroleum
ether to afford the product as a precipitate which was washed
with diethyl ether three times. The product was recrystallized
in dichloromethane-petroleum ether. Yield: 81%. Anal. Calcd
for C86H70F6N3P4Ru2Sb‚0.5CH2Cl2: C, 59.38; H, 4.09; N,
2.40. Found: C, 59.61; H, 3.90; N, 2.72. ES-MS (m/z): 1470
([{Cp(PPh3)2Ru}2C(CN)3]+), 691 ([Cp(PPh3)2Ru]+). IR spectrum
(KBr, cm-1): ν 2202 (m, C(CN)3), 2185 (s, C(CN)3), 656 (s, SbF6).
1H NMR spectrum (CDCl3): δ 7.47-7.06 (m, 60H, C6H5), 4.33
(s, 10H, C5H5). 31P NMR spectrum (CDCl3): δ 42.3 (s). UV-vis
(λmax/nm (ꢀ, cm-1 M-1)): 234 (88000), 321 (6100).
X-ray Crystallography. Crystals suitable for X-ray diffraction
were grown by layering petroleum ether onto the 1,2-dichloroethane
solution for 3‚1/2C2H4Cl2 and 8‚H2O, by diffusion of petroleum
ether into the dichloromethane-methanol (4:1) solutions for 5‚CH2-
Cl2‚2CH3OH and 6‚2CH2Cl2‚1/2CH3OH‚3/2H2O, and by layering
hexane onto the dichloromethane solutions for 7 and 9. Crystal-
lographic parameters and details for data collection and refinement
were summarized in Table 1 for 3‚1/2CH2ClCH2Cl, 5‚CH2-
Cl2‚2CH3OH, and 6‚2CH2Cl2‚1/2CH3OH‚3/2H2O and in Table 2
for 7, 8, and 9. Full crystallographic data are provided in the
Supporting Information.
Cp(PPh3)2RuC(CN)3 (8). To a dichloromethane (15 mL) solu-
tion of 2 (0.20 mmol, 145.2 mg) was added a methanol (4 mL)
solution of potassium tricyanomethanide (0.40 mmol, 51.6 mg).
The solution was stirred at room temperature for 4 h accompanied
Single crystals sealed in capillaries with mother liquor were
measured on a SIEMENS SMART CCD diffractometer, and the
reflection data were collected at 293 K by ω scan technique using
graphite-monochromated Mo KR (λ ) 0.71073 Å) radiation. The
Inorganic Chemistry, Vol. 42, No. 2, 2003 635

Zhang et al.
Table 1. Crystallographic Data for Complexes 3‚1/2C2H4Cl2,
5‚CH2Cl2‚2CH3OH, and 6‚2CH2Cl2‚3/2CH3OH‚1/2H2O
dichloromethane are 0.50, respectively. For 7, the carbon (C51,
C52, C53, C54, C55, and C56) atoms in one of the phenyl groups
were fixed as rigid bodies, with C-C distances of 1.390 Å. For 9,
the trifluoromethanesulfonate exhibits a statistical distribution with
the occupancy factors of 0.50 for atoms S, C, O1, O2, O3, F1, F2,
and F3, respectively.
6‚2CH2Cl2‚
3‚1/2C2H4Cl2 5‚CH2Cl2‚2CH3OH 1/2CH3OH‚3/2H2O
empirical
formula
fw
space group P1h
a, Å
C34H31ClFeN3P2 C67H68Cl2F6Fe2-
N3O2P4Sb
C
86.5H79Cl4F6N3O2-
P4Ru2Sb
634.86
1489.47
1896.10
P1h
Physical Measurements. Elemental analyses (C, H, N) were
performed on a Perkin-Elmer model 240C automatic instrument.
The electrospray mass spectra (ES-MS) were recorded on a Finngan
LCQ mass spectrometer using dichloromethane-methanol as
mobile phase. The UV-vis spectra in dichloromethane solutions
were measured on a Perkin-Elmer Lambda 25 UV-vis spectrom-
eter. IR spectra were recorded on a Magna750 FT-IR spectropho-
tometer with KBr pellet. 1H and 31P NMR measurements were made
on a Bruker AM500 spectrometer with SiMe4 as the internal
reference and 85% H3PO4 as external standard, respectively. The
cyclic voltammogram was obtained by use of a potentiostat/
galvanostat model 263A in 1 mM dichloromethane solutions
containing 0.1 M (Bu4N)PF6 as supporting electrolyte at a scan
rate of 100 mV s-1. Platinum and glassy graphite were used as
working and counter electrodes, respectively, the potentials were
referenced to Ag/AgCl, and under the present experimental condi-
tions, the ferrocenium/ferrocene couple was observed at 0.585 V.
P21/c
9.5782(3)
13.2975(2)
18.5124(4)
29.1127(4)
15.3230(5)
15.4279(5)
21.5190(7)
84.380(1)
82.280(1)
65.179(1)
4570.5(3)
2
b, Å
c, Å
11.0580(3)
14.4083(4)
88.529 (1)
82.181(1)
88.924(1)
1511.20(8)
2
R, deg
â, deg
γ, deg
V, Å3
Z
102.137(1)
7006.4(2)
4
F
calcd, g/cm3 1.395
1.412
1.019
0.71073
1.378
0.863
0.71073
µ, mm-1
radiation
(λ, Å)
0.722
0.71073
temp, K
293(2)
0.0608
293(2)
0.0946
0.2200
1.133
293(2)
0.0681
0.1885
1.177
R1(Fo)a
2
wR2(Fo )b 0.1396
GOF
1.157
2
2
2
a R1 ) ∑|Fo - Fc|/∑Fo. b wR2 ) ∑[w(Fo - Fc )2]/∑[w(Fo )]1/2
Table 2. Crystallographic Data for Complexes 7, 8‚H2O, and 9
.
Results and Discussion
7
8‚H2O
9
Dicyanamide/tricyanomethanide-containing mononuclear
organometallic compounds were prepared in high yields by
the reactions between chloride-containing compounds and
excess sodium dicyanamide or potassium tricyanomethanide.
The reactions of the dicyanamide/tricyanomethanide-contain-
ing mononuclear compounds with the chloride-containing
organometallic components 1 or 2 in equimolar ratios led to
the isolation of dicyanamide/tricyanomethanide-bridged bi-
nuclear complexes. The binuclear arrays of 5, 6, 9, and 10
could also be accessible by the direct reactions between the
chloride-containing mononuclear components (1 or 2) and
sodium dicyanamide/potassium tricyanomethanide in 2:1
molar ratios. Nevertheless, attempts to isolate pure products
of the heterobinuclear species [Cp(dppe)FeN(CN)2RuCp-
(PPh3)2]+ and [Cp(dppe)FeC(CN)3RuCp(PPh3)2]+ were un-
successful by the combination of the dicyanamide- (3 or 4)
or tricyanomethanide-containing (7 or 8) synthons with the
chloride-containing components (1 or 2) because the products
were contaminated by the impurity of the homobinuclear
species which was extremely difficult to remove by crystal-
lization as well as by chromatography.
The IR spectra of complexes 3-10 showed characteristic
stretching vibration bands of dicyanamide and tricya-
nomethanide, respectively. Relative to ν(N(CN)2) values of
the free ligand (2287, 2229, and 2181 cm-1), those of
mononuclear compounds 3 (2266, 2224, and 2158 cm-1) and
4 (2270, 2229, and 2164 cm-1) indicated a lower frequency
shift due to the σ donation from the ligand to the metal center
upon coordination. The ν(N(CN)2) values of the dicyana-
mide-bridged binuclear complexes 5 (2299 and 2208 cm-1)
and 6 (2295 and 2208 cm-1), however, showed higher
frequency shifts compared with those of mononuclear
compounds 3 and 4. Those shifts reflect the integrated
electronic effect of σ donation from the bridging ligand to
the organometallic centers as well as π back-donation from
empirical
formula
fw
space group
a, Å
C35H29C10FeN3P2
C45H37N3P2Ru
C67H58F3Fe2-
N3O3P4S
1277.8
609.40
P1h
9.5641(2)
11.8008(1)
14.2658(3)
82.780(1)
84.37
71.671(1)
1513.36(5)
2
798.79
P21/n
10.5627(2)
22.5231(1)
17.3953(4)
C2/c
16.4730(2)
18.0376(3)
20.8622 (1)
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
Z
97.422(1)
93.348(1)
4103.75(12)
4
1.293
6188.28(13)
4
1.372
F
calcd, g/cm3
1.337
0.633
µ, mm-1
0.496
0.664
radiation (λ, Å)
temp, K
0.71073
293(2)
0.0688
0.1460
1.113
0.71073
293(2)
0.0677
0.1968
1.234
0.71073
293(2)
0.0678
0.1569
1.122
R1(Fo)a
2
wR2(Fo )b
GOF
a R1 ) ∑|Fo - Fc|/∑Fo. b wR2 ) ∑[w(Fo - Fc )2]/∑[w(Fo )]1/2
.
2
2
2
data intensity was corrected for LP factors, and the SADABS
technique was applied for absorption corrections. The metal atoms
were determined by Patterson procedure for 9 whereas by direct
methods for other compounds. The remaining non-hydrogen atoms
were located from the successive difference Fourier syntheses. The
structures were refined on F2 by full-matrix least-squares method
using the SHELXTL-97 program package.29 The non-hydrogen
atoms were refined anisotropically whereas the hydrogen atoms
were generated geometrically and refined with isotropic thermal
parameters.
The refinements of 3‚1/2C2H4Cl2, 5‚CH2Cl2‚2CH3OH, and 6‚
2CH2Cl2‚1/2CH3OH‚3/2H2O were performed by fixing the C-Cl
(1.760 ( 0.005 Å) and C-O (1.420 ( 0.005 Å) distances of the
solvate 1,2-dichloroethane, dichloromethane, and methanol. The
problem of crystal quality for 5‚CH2Cl2‚2CH3OH resulted in a
relative high R factor (0.0946). For 6‚2CH2Cl2‚1/2CH3OH‚3/2H2O,
the occupancy factors of atoms C03, Cl1, and Cl2 in the solvate
(29) Sheldrick, G. M. SHELXL-97, Program for the Refinement of Crystal
Structures; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
636 Inorganic Chemistry, Vol. 42, No. 2, 2003

Dicyanamide/Tricyanomethanide-Bridged Complexes
Figure 3. Perspective view of the complex cation of 6 with atom
numbering scheme. Thermal ellipsoids are shown at the 30% probability
level.
Figure 1. Perspective view of complex 3 with atom numbering scheme.
Thermal ellipsoids are shown at the 30% probability level.
Table 3. Selected Bond Distances (Å) and Angles (deg) for Complexes
3, 5, and 6
3
5
6
Fe1-N1
1.928(4)
Fe1-N1
Fe2-N2
1.942(12) Ru1-N1
1.945(13) Ru2-N2
2.218(4) Ru1-P1
2.187(5) Ru1-P2
2.206(4) Ru2-P3
2.213(4) Ru2-P4
2.100(15) Ru1-C131
2.090(15) Ru1-C132
2.091(14) Ru1-C133
2.082(15) Ru1-C134
2.067(15) Ru1-C135
2.083(17) Ru2-C136
2.045(17) Ru2-C137
2.077(18) Ru2-C138
2.073(7)
2.077(8)
2.328(2)
2.343(2)
2.330(2)
2.332(2)
2.226(9)
2.198(8)
2.186(8)
2.215(8)
2.236(8)
2.237(10)
2.233(10)
2.210(10)
2.198(9)
2.195(9)
1.129(10)
1.133(11)
1.311(13)
1.307(13)
Fe1-P1
Fe1-P2
2.2066(12) Fe1-P1
2.2111(12) Fe1-P2
Fe2-P3
Fe2-P4
Fe1-C51
Fe1-C52
Fe1-C53
Fe1-C54
Fe1-C55
2.088(5)
2.055(5)
2.061(5)
2.090(5)
2.086(5)
Fe1-C91
Fe1-C92
Fe1-C93
Fe1-C94
Fe1-C95
Fe2-C96
Fe2-C97
Fe2-C98
Fe2-C99
Fe2-C100
N1-C1
2.08(2)
Ru2-C139
2.102(17) Ru2-C140
1.157(17) N1-C1
1.136(17) N2-C2
1.302(19) N3-C1
1.299(19) N3-C2
N1-C1
N2-C2
N3-C1
N3-C2
1.138(5)
1.141(7)
1.311(6)
1.310(7)
N2-C2
Figure 2. Perspective view of the complex cation of 5 with atom
numbering scheme. Thermal ellipsoids are shown at the 30% probability
level.
N3-C1
N3-C2
N1-Fe1-P1 93.48(12) N1-Fe1-P1 86.3(3)
N1-Ru1-P1 89.73(19)
N1-Ru1-P2 88.17(19)
N1-Fe1-P2 86.56(11) N1-Fe1-P2 95.0(4)
the organometallic center to the ligand. Upon one-electron
oxidation, the IR spectra showed that the ν(N(CN)2) of 5a
(2233, 2187, and 2087 cm-1) exhibited significant shifts to
low frequencies relative to those of its reduced form 5 (2299
and 2208 cm-1). This shift reflects a great electronic flow
that occurred from the bridging ligand to the metal centers
upon oxidation. Compared with the ν(C(CN)3) of the free
ligand (2179s cm-1), those of the mononuclear complexes
7 (2168s cm-1) and 8 (2175s cm-1) show a slight shift to
lower frequencies. The ν(C(CN)3) in the binuclear complexes
9 (2202m and 2185m cm-1) and 10 (2201m and 2179s cm-1)
display a higher frequency shift relative to the mononuclear
complexes 7 and 8, reflecting the electronic effect from both
σ donation and π back-bonding effects upon the formation
of a bridging array.
P1-Fe1-P2 87.10(5)
P1-Fe1-P2 86.96(16) P1-Ru1-P2 101.15(8)
N2-Fe2-P3 87.4(3)
N2-Fe2-P4 92.8(3)
N2-Ru2-P3 88.4(2)
N2-Ru2-P4 90.1(2)
P3-Fe2-P4 86.66(15) P3-Ru2-P4 101.25(8)
C1-N1-Fe1 168.7(12) C1-N1-Ru1 174.2(7)
C2-N2-Fe2 173.8(12) C2-N2-Ru2 177.0(7)
N1-C1-N3 171.3(15) N1-C1-N3 167.7(10)
C1-N3-C2 123.3(13) C1-N3-C2 127.2(9)
N2-C2-N3 171.6(15) N2-C2-N3 167.7(10)
C1-N1-Fe1 171.3(4)
N1-C1-N3 172.1(5)
C1-N3-C2 121.6(5)
N2-C2-N3 173.0(6)
the other end (N2-C2 ) 1.141(7), C2-N3 ) 1.310(7) Å).
This phenomenon is more easily understood in complexes 5
and 6 once a symmetric bridging array M-NCNCN-M (M
) Fe and Ru for 5 and 6, respectively) is formed. Compared
with the Fe-N, Fe-P, Fe-C, and C-N distances in
mononuclear iron(II) compound 3, those in the dicyanamide-
bridged binuclear iron(II) complex 5 show simply a slight
difference (Table 3). As depicted in Figures 2 and 3, the
bridging array M-NCNCN-M (M ) Fe and Ru for 5 and
6, respectively) adopts a “V”-type conformation because of
sp2 hybridization of the middle nitrogen atom of the
dicyanamide, where the C-N-C angle (123.3(13)° for 5
and 127.2(9)° for 6) shows an appreciably larger deviation
from 120° than that in mononuclear complex 3 (121.6(5)°).
The ORTEP plots of the dicyanamide-containing com-
plexes 3, 5, and 6 are depicted in Figures 1, 2, and 3,
respectively. Selected bond distances and angles of the three
compounds are gathered in Table 3 for the purpose of
comparison. In the terminal dicyanamide-bound complex 3,
the N-C bond distances (N1-C1) 1.138(5), C1-N3 )
1.311(6) Å) in the Fe-bonding end are the same as those in
Inorganic Chemistry, Vol. 42, No. 2, 2003 637

Zhang et al.
Figure 6. Perspective view of the complex cation of 9 with atom
numbering scheme. Thermal ellipsoids are shown at the 30% probability
level.
Table 4. Selected Bond Distances (Å) and Angles (deg) for Complexes
7, 8, and 9
7
8
9
Figure 4. Perspective view of complex 7 with atom numbering scheme.
Thermal ellipsoids are shown at the 30% probability level.
Fe1-N1
Fe1-P1
Fe1-P2
Fe1-C51
Fe1-C52
Fe1-C53
Fe1-C54
Fe1-C55
N1-C1
N2-C2
N3-C3
C1-C4
1.921(4)
2.2234(15) Ru1-P1
2.2177(15) Ru1-P2
Ru1-N1
2.081(8) Fe1-N1
2.335(2) Fe1-P1
2.325(2) Fe1-P2
2.188(9) Fe1-C51
2.215(9) Fe1-C52
2.223(10) Fe1-C53
2.208(9) Fe1-C54
2.184(9) Fe1-C55
1.147(11) N1-C1
1.155(16) N2-C2
1.163(15)
1.906(4)
2.2019(16)
2.1994(15)
2.081(5)
2.084(6)
2.084(6)
2.078(6)
2.070(6)
1.148(6)
1.145(15)
2.096(6)
2.067(6)
2.065(5)
2.088(6)
2.107(6)
1.154(6)
1.141(9)
1.143(9)
1.407(7)
1.426(9)
Ru1-C71
Ru1-C72
Ru1-C73
Ru1-C74
Ru1-C75
N1-C1
N2-C2
N3-C3
C1-C4
1.378(15) C1-C3
1.426(19) C2-C3
1.424(18)
1.392(7)
1.403(14)
C2-C4
C2-C4
C3-C4
1.421(10) C3-C4
N1-Fe1-P1 94.60(13) N1-Ru1-P1 89.95(19) N1-Fe1-P1 85.05(13)
N1-Fe1-P2 87.42(13) N1-Ru1-P2 87.87(19) N1-Fe1-P2 90.16(13)
P1-Fe1-P2 86.64(5)
C1-N1-Fe1 173.0(4)
N1-C1-C4 177.3(6)
N2-C2-C4 177.6(8)
N3-C3-C4 179.1(7)
C1-C4-C2 120.9(6)
C1-C4-C3 119.4(5)
C2-C4-C3 119.7(5)
P1-Ru1-P2 101.20(8) P1-Fe1-P2 86.81(6)
C1-N1-Ru1 174.5(7) C1-N1-Fe1 174.4(4)
N1-C1-C4 178.6(11) N1-C1-C3 177.0(6)
N2-C2-C4 178.9(17) N2-C2-C3 180.000(3)
N3-C3-C4 178.0(16)
C1-C4-C2 121.1(10) C1-C3-C2 119.4(4)
C1-C4-C3 119.1(10) C1*-C3-C2 119.4(4)
C2-C4-C3 121.1(10) C1-C3-C1* 121.3(7)
9 become shortened about 0.02 Å upon the formation of a
bridging array FeC(CN)3Fe, where the Fe-N-C angles
(173.0(4)-174.4(4) Å) deviate appreciably from linearity.
The tricyanomethanide is planar, and the sum of the bond
angles (C-C-C) at the central carbon atom is equal to 360°
because of the sp2 hybridization.31,32 The N-C-C angles
(177.0(6)-180°) are close to 180° because of the sp
hybridization of the cyano carbon atoms, in which the N-C
and C-C distances are in the ranges 1.141-1.154 and
1.392-1.426 Å, respectively, showing a typical triple and
conjugated single-double bonding character.31,32 The Fe‚‚
‚Fe separation through a bridging tricyanomethanide is 7.883
Å, slightly shorter than that through a bridging dicyanamide
observed in 5 (8.088 Å).
Figure 5. Perspective view of complex 8 with atom numbering scheme.
Thermal ellipsoids are shown at the 30% probability level.
While the carbon atoms of the dicyanamide are sp hybridized,
the N-C-N angles (167.7-172.1°) deviate moderately from
linearity,25,30 especially in dicyanamide-bridged binuclear
ruthenium(II) complex 6 (167.7(10)°) which exhibits the
most severe deviation among the three structurally character-
ized dicyanamide-containing complexes. The C-N-Fe angle
is in the range 177.0(7)-168.7(12)°, deviating also from
linearity. The metal‚‚‚metal separations through a bridging
dicyanamide are 8.088 and 8.501 Å, respectively, for
complexes 5 and 6.
The perspective views of tricyanomethanide-containing
complexes 7-9 are shown in Figures 4-6, respectively.
Selected bond distances and angles of the three compounds
are gathered in Table 4 for the purpose of comparison.
Relative to the Fe-N (1.921(4) Å) and Fe-P (average 2.221
Å) bond lengths of mononuclear complex 7 with terminal
tricyanomethanide, those (Fe-N ) 1.906(4) Å; av Fe-P )
2.201 Å) of tricyanomethanide-bridged binuclear complex
The redox chemistry of 3-10 was investigated by cyclic
voltammetry, and the redox potentials are presented in Table
5. The mononuclear compounds 3, 4, 7, and 8 afford a single
reversible one-electron oxidation process, while the dicy-
anamide/tricyanomethanide-bridged binuclear complexes 5,
(31) Dixon, D. A.; Calabrese, J. C.; Miller, J. S. J. Am. Chem. Soc. 1986,
108, 2582.
(32) Andersen, P.; Klewe, B.; Thom, E. Acta Chem. Scand. 1967, 21, 1530.
(30) Jurgens, B.; Irran, E.; Schneider, J.; Schnik, W. Inrog. Chem. 2000,
39, 665.
638 Inorganic Chemistry, Vol. 42, No. 2, 2003

Dicyanamide/Tricyanomethanide-Bridged Complexes
Table 5. Cyclic Voltammogram Data for Complexes 3-10a
b
E
1/2(ox1)/(∆EP)
E1/2(ox2)/(∆EP)
∆E1/2
Kc
3
4
5
5a
6
7
8
9
10
0.520(0.099)
0.938(0.104)
0.545(0.090)
0.563(0.062)
0.922(0.113)
0.670(0.099)
1.134(0.097)
0.742(0.085)
1.123(0.110)
0.686(0.081)
0.700(0.064)
1.100(0.120)
0.141
0.137
0.178
240
210
1020
0.915(0.082)
1.365(0.102)
0.173
0.242
841
12333
Figure 8. Near-infrared spectrum of 5a in dichloromethane.
a Potential data in volts vs Ag/AgCl are from single scan cyclic
voltammograms recorded at 25 °C. Detailed experimental conditions
are given in the Experimental Section. b The comproportionation con-
stants, Kc, were calculated by the formula Kc ) exp(∆E1/2/25.69) at 298
K.
the wider conjugate scale of the tricyanomethanide compared
with the bridging dicyanamide.
Mixed-valence compound 5a was isolated as a stable solid
species by the controlled oxidation of 5 with ferrocenium
hexafluorophosphate in dichloromethane. The cyclic volta-
mmogram showed it displays the same electrochemical
behavior as its reduced form 5 affording two reversible one-
electron redox couples (Table 5). In dichloromethane solu-
tion, 5a exhibits a broad absorption band (Figure 8) in the
near-infrared region centered at about 1500 nm (ꢀ ) 750
cm-1 M-1). The disappearance of such a band for 5 supports
the assignment of this near-infrared band to intervalence
charge transfer (IT) of the mixed-valence system in 5a.
Although further measurements of this intriguing IT band
were hindered by the insufficient stability of 5a in other
organic solvents, application of Hush’s theoretical analysis33
of the IT band to 5a is still possible. The half-width (∆ν1/2)
is related to the energy of the IT band (νmax) by the equation34
νmax - ν0 ) (∆ν1/2)2/2310, where ν0 is the internal energy
difference between 5 and 5a with different oxidation states
and can be estimated by the difference in the redox potential
∆E1/2. For 5a, the difference in potentials from the cyclic
voltamogram is 0.137 V which corresponds to ν0 ) 1100
cm-1. With the use of this equation, the calculated (∆ν1/2
Figure 7. Cyclic voltammograms of dicyanamide-bridged dinuclear
complexes 5 and 6 and tricyanomethanide-bridged complexes 9 and 10
recorded in 0.10 M dichloromethane solution of (Bu4N)PF6 at a scan rate
of 0.10 V/s.
6, 9, and 10 exhibit stepwise one-electron redox behavior
(Figure 7). Obviously, an electronic coupling is operative
between the redox termini through the bridging dicyanamide
or tricyanomethanide. By comparison of the oxidation
potential separation (∆E1/2) between the two redox processes,
it is observed that the electronic communications in the
complexes 6 (∆E1/2 ) 0.178 V) and 10 (∆E1/2 ) 0.242 V)
with Cp(PPh3)2Ru as redox termini are appreciably greater,
respectively, than those in the corresponding complexes 5
(∆E1/2 ) 0.141 V) and 9 (∆E1/2 ) 0.173 V) with Cp(dppe)-
Fe as redox-active centers. This is easy to understand
considering that ruthenium can afford a better π back-
donation than iron to the bridging ligand. Moreover, the
electron transfer mediated by bridging tricyanomethanide is
more efficient relative to that by bridging dicyanamide25 in
view of the larger ∆E1/2 in the tricyanomethanide-bridged
)
calcd is estimated to be 3580 cm-1, while the corresponding
observed (∆ν1/2)obsd is 4850 cm-1. Although the width of the
intervalence band at half-height for the µ-dicyanamide
molecule is somewhat greater than that calculated from
Hush’s equation derived for valence-trapped species, the ratio
of 1.35 between the observed and calculated ∆ν1/2 is typical
for mixed-valence compounds of the class II type of the
Robin and Day classification.34-36 Another expression for
the interaction parameter R2 can be utilized to estimate the
degree of ground-state delocalization in a mixed-valence
complex, R2 ) 4.24 × 10-4[(ꢀmax∆ν1/2)/(νmaxd2)]],36 where
d is the separation between two redox centers. Using the
(33) Hush, N. S. Prog. Inorg. Chem. 1967, 8, 391.
(34) (a) Dowling, N.; Henry, P. M.; Lewis, N. A.; Taube, H. Inorg. Chem.
1981, 20, 2345. (b) Colbran, S. B.; Robinson, B. H.; Simpson, J.
Organometallics 1983, 2, 952. (c) Dowling, N.; Henry, P. M. Inorg.
Chem. 1982, 21, 4088.
(35) Robin, M. B.; Day, P. AdV. Inorg. Chem. Radiochem. 1967, 10, 247.
(36) (a) Callahan, R. W.; Keene, F. R.; Meyer, T. J.; Salmon, D. J. J. Am.
Chem. Soc. 1977, 99, 1064. (b) Colbert, M. C. B.; Lewis, J.; Long,
N. J.; Raithby, P. R.; White, A. J. P.; Williams, D. J. J. Chem. Soc.,
Dalton Trans. 1997, 99.
binuclear complexes 9 (∆E1/2 ) 0.173 V) and 10 (∆E1/2
)
0.242 V) than those in the dicyanamide-bridged complexes
5 (∆E1/2 ) 0.141 V) and 6 (∆E1/2 ) 0.178 V). This
phenomenon could be elucidated by the shorter MM
distances through the bridging tricyanomethanide as well as
Inorganic Chemistry, Vol. 42, No. 2, 2003 639

Zhang et al.
intramolecular Fe‚‚‚Fe distance (8.1 Å) from the structural-
analysis for 5, a value for R2 of 3.5 × 10-3 is calculated for
mixed-valence complex 5a. This value of R2 is in the range
calculated for Class II mixed-valence compounds.6b,8d,35-37
In 1982, Taube and co-workers38 reported the electro-
chemical data and near-infrared spectra of the mixed-valence
compound [{(NH3)5Ru}2N(CN)2]5+, where it showed a small
value of the comproportionation constant (Kc ) 340) but a
relatively large extinction coefficient (ꢀ ) 2800 cm-1 M-1)
for the intervalence band. Replacing trans ammonias by
pyridine or by isonicotinamide decreases the electronic
coupling (Kc ) 340-375; ꢀ ) 2800-2310 cm-1 M-1),
revealing that the dominant coupling mechanism is πd(Ru)-
π*(ligand) delocalization. In the dicyanamide-bridged organo-
metallic compounds 5 and 6, the comproportionation con-
stants Kc are 240 and 1020 (Table 5), respectively. Relative
to that observed in the dinuclear ruthenium complex [{NH3)5-
Ru}2N(CN)2](PF6)2 (E1/2 ) 0.149 V, Kc ) 340), the electronic
coupling between two ruthenium centers through a bridging
dicyanamide in the organometallic compound [{Cp(PPh3)2-
Ru}2N(CN)2](SbF6) (6) (E1/2 ) 0.178 V, Kc ) 1020) be-
comes stronger. Obviously, the electronic effect caused by
the auxiliary ligands exerts an important role in the elec-
tronic delocalization of the dinuclear ruthenium com-
pounds. In organometallic compound 6, the PPh3 and Cp
ligands permit a higher degree of πd(Ru)-π*(N(CN)2) back-
bonding which results in a stronger electronic communica-
tion between the ruthenium centers through the dicyanamide
bridge.
Conclusions
An efficient synthetic route is described for the preparation
of dicyanamide/tricyanomethanide-bridged binuclear organo-
metallic complexes by the incorporation between two metal
components, one containing dicyanamide/tricyanomethanide
with potential bridging group dicyanamide/tricyanomethanide,
the other affording substitutable coordinated chloride. It has
been demonstrated that the dicyanamide/tricyanomethanide
could mediate an efficient electronic coupling between two
organometallic redox termini with metal‚‚‚metal separation
more than 7.8 Å. The electronic coupling for the bridging
array of tricyanomethanide is appreciably better than that
for dicyanamide. The mixed-valence compound 5a, belong-
ing to a class II mixed-valence species, affords an interva-
lence transition band in the near-infrared region.
Acknowledgment. We are greatly grateful for the finan-
cial supports from NSF of China (20171044 and 20273074),
the project for “hundred talents” from Chinese Academy of
Sciences, and the startup foundation for scholars studying
abroad from the ministry of education, China.
Supporting Information Available: X-ray crystallographic files
in CIF format for the structure determinations of compounds 3 and
5-9. This material is available free of charge via the Internet at
http://pubs.acs.org.
(37) Sutton, J. E.; Krentzien, H.; Taube, H. Inorg. Chem. 1982, 21, 2842.
(38) (a) Crutchley, R. J. AdV. Inorg. Chem. 1994, 41, 273. (b) Nelsen, S.
F. Chem. Eur. J. 2000, 6, 581.
IC025879T
640 Inorganic Chemistry, Vol. 42, No. 2, 2003