Phenylcyanamidoruthenium scorpionate complexes

Article abstract of DOI:10.1021/ic302535h

Nine [Ru(Tp)(dppe)L] complexes, where Tp is hydrotris(pyrazol-1-yl)borate, dppe is ethylenebis(diphenylphosphine), and L is (4-nitrophenyl)cyanamide (NO₂pcyd^-), (2-chlorophenyl)cyanamide (2-Clpcyd ^-), (3-chlorophenyl)cyanamide (3-Clpcyd^-), (2,4-dichlorophenyl)cyanamide (2,4-Cl₂pcyd^-), (2,3-dichlorophenyl)cyanamide (2,3-Cl₂pcyd^-), (2,5-dichlorophenyl)cyanamide (2,5-Cl₂pcyd^-), (2,4,5-trichlorophenyl)cyanamide (2,4,5-Cl₃pcyd^-), (2,3,5,6-tetrachlorophenyl)cyanamide (2,3,5,6-Cl₄pcyd^-), and (pentachlorophenyl)cyanamide (Cl₅pcyd^-), and the dinuclear complex [{Ru(Tp)(dppe)}₂(μ-adpc)], where adpc ^2- is azo-4,4-diphenylcyanamide, have been prepared and characterized. The crystal structures of [Ru(Tp)(dppe)(Cl₅pcyd)] and [{Ru(Tp)(dppe)}₂(μ-adpc)] reveal the Ru^II ion to occupy a pseudooctahedral coordination sphere in which the cyanamide ligand coordinates to Ru^II by its terminal nitrogen atom. For both complexes, the cyanamide ligands are planar, indicating significant π mixing between the cyanamide and phenyl moieties as well as the azo group in the case of adpc^2-. The optical spectra of the nominally ruthenium(III) species [Ru(Tp)(dppe)L]⁺ were obtained through spectroelectrochemistry measurements and showed an intense near-IR absorption band. Time-dependent density functional theory calculations of these species revealed that oxidation of the ruthenium(II) species led to species where partial oxidation of the cyanamide ligand had occurred, indicative of noninnocent character for these ligands. The spin densities reveal that while the 3-Clpycd species has substantial Ru^II(3-Clpycd⁰) character, the Cl₅pycd species is a much more localized ruthenium(III) complex of the Cl₅pycd monoanion. Some bond order and charge distribution data are derived for these ruthenium(III) species. The near-IR band is assigned as a quite complex mixture of d-d, 4d_π to L(NCN) MLCT, and L(NCN) to Ru 4d LMCT with even a scorpionate ligand component. Spectroelectrochemistry was also performed on [{Ru(Tp)(dppe)} ₂(μ-adpc)] to generate the mixed-valence state. The intense intervalence transition that is observed in the near-IR is very similar to that previously reported for [{Ru(trpy)(bpy)}₂(μ-adpc)]²⁺, where trpy is 2,2′:6′,2″-terpyridine and bpy is 2,2′-bipyridine, and by analogy identifies [{Ru(Tp)(dppe)} ₂(μ-adpc)]⁺ as a delocalized mixed-valence complex.

Full text of DOI:10.1021/ic302535h

Article
pubs.acs.org/IC
Phenylcyanamidoruthenium Scorpionate Complexes
†
†
†
‡
‡
Carmen Harb, Pavel Kravtsov, Mohommad Choudhuri, Eric R. Sirianni, Glenn P. A. Yap,
,§
,†
A. B. P. Lever,* and Robert J. Crutchley*
†
Department of Chemistry, Carleton University, Ottawa, Ontario K1S 5B6, Canada
Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716, United States
Department of Chemistry, York University, Toronto, Ontario M3J 1P3, Canada
‡
§
*
S Supporting Information
ABSTRACT: Nine [Ru(Tp)(dppe)L] complexes, where Tp
is hydrotris(pyrazol-1-yl)borate, dppe is ethylenebis-
(
(
diphenylphosphine), and L is (4-nitrophenyl)cyanamide
−
−
NO pcyd ), (2-chlorophenyl)cyanamide (2-Clpcyd ), (3-
2
−
chlorophenyl)cyanamide (3-Clpcyd ), (2,4-dichlorophenyl)-
cyanamide (2,4-Cl pcyd ), (2,3-dichlorophenyl)cyanamide
−
2
−
(
2,3-Cl pcyd ), (2,5-dichlorophenyl)cyanamide (2,5-
2
−
Cl pcyd ), (2,4,5-trichlorophenyl)cyanamide (2,4,5-
2
−
Cl pcyd ), (2,3,5,6-tetrachlorophenyl)cyanamide (2,3,5,6-
Cl pcyd ), and (pentachlorophenyl)cyanamide (Cl pcyd ),
3
−
−
4
5
and the dinuclear complex [{Ru(Tp)(dppe)} (μ-adpc)],
2
where adpc2 is azo-4,4-diphenylcyanamide, have been prepared and characterized. The crystal structures of [Ru(Tp)(dppe)-
−
II
(
Cl pcyd)] and [{Ru(Tp)(dppe)} (μ-adpc)] reveal the Ru ion to occupy a pseudooctahedral coordination sphere in which the
5
2
II
cyanamide ligand coordinates to Ru by its terminal nitrogen atom. For both complexes, the cyanamide ligands are planar,
indicating significant π mixing between the cyanamide and phenyl moieties as well as the azo group in the case of adpc . The
optical spectra of the nominally ruthenium(III) species [Ru(Tp)(dppe)L] were obtained through spectroelectrochemistry
2−
+
measurements and showed an intense near-IR absorption band. Time-dependent density functional theory calculations of these
species revealed that oxidation of the ruthenium(II) species led to species where partial oxidation of the cyanamide ligand had
occurred, indicative of noninnocent character for these ligands. The spin densities reveal that while the 3-Clpycd species has
II
0
substantial Ru (3-Clpycd ) character, the Cl pycd species is a much more localized ruthenium(III) complex of the Cl pycd
5
5
monoanion. Some bond order and charge distribution data are derived for these ruthenium(III) species. The near-IR band is
assigned as a quite complex mixture of d−d, 4d to L(NCN) MLCT, and L(NCN) to Ru 4d LMCT with even a scorpionate
π
ligand component. Spectroelectrochemistry was also performed on [{Ru(Tp)(dppe)} (μ-adpc)] to generate the mixed-valence
2
state. The intense intervalence transition that is observed in the near-IR is very similar to that previously reported for
2
+
[
[
{Ru(trpy)(bpy)} (μ-adpc)] , where trpy is 2,2′:6′,2″-terpyridine and bpy is 2,2′-bipyridine, and by analogy identifies
2
+
{Ru(Tp)(dppe)} (μ-adpc)] as a delocalized mixed-valence complex.
2
INTRODUCTION
This study will demonstrate that, by a careful choice of
ligands, partial neutralization of the complex charge can occur
without a catastrophic loss of mixed-valence properties. The
■
Donor−acceptor polymeric materials have found application in
1
a number of devices such as Mach−Zehnder modulators, solar
+
7
2
3
neutral analogues to the complexes [Ru(trpy)(bpy)L] , where
L is a substituted phenylcyanamide ligand, trpy is 2,2′:6′,2″-
terpyridine, and bpy is 2,2′-bipyridine, and the dinuclear
energy cells, and polymer spintronics. For modulator
applications, these materials are usually prepared by doping
organic donor−acceptor molecules into a host polymer.
Donor−acceptor molecules must be neutral so that, under
the influence of a large potential field, their dipole moments
cause molecules to align in the host polymer and so express
nonlinear-optical properties. In coordination chemistry, there
are many examples of dinuclear donor−acceptor systems,
otherwise called mixed-valence complexes, that possess proper-
ties that could find application in electrooptic polymeric
materials except for a significant problem. The vast majority of
2+
8
2−
complex [{Ru(trpy)(bpy)} (μ-adpc)] , where adpc is azo-
2
4
,4′-diphenylcyanamide, will be prepared. Upon oxidation,
III
these complexes possess strong π interactions between Ru
and the cyanamide group, which for the dinuclear complex is
the superexchange pathway for metal−metal coupling and
results in its mixed-valence properties. In previous studies, we
9
have found that the coordination of anionic π-donor atoms to
ruthenium decouples the ruthenium acceptor wave function
4
−6
mixed-valence complexes are charged,
and making neutral
mixed-valence complexes without losing the properties of the
desired mixed-valence state is a synthetic challenge.
Received: November 19, 2012
Published: January 22, 2013
©
2013 American Chemical Society
1621
dx.doi.org/10.1021/ic302535h | Inorg. Chem. 2013, 52, 1621−1630

Inorganic Chemistry
Article
from the cyanamide superexchange pathway and that this
results in weak coupling between metal ions. Neutralization of
the complex charge may be achieved by using an anionic
auxiliary ligand whose charge is delocalized and so possesses
poor π-donor properties. Scorpionate ligands successfully
achieve this result. Hydrotris(pyrazol-1-yl)borate (Tp ) is a
commercially available anionic tridentate ligand that coordi-
filtered off using a Celite packed column and the filtrate was rotary
evaporated to approximately 20 mL. Anhydrous diethyl ether (60 mL)
was added to the flask to precipitate out the crude product. The crude
product was collected, washed with diethyl ether, and recrystallized
from 2:1:1 CH Cl /ether/hexane first by dissolving the product in a
2
2
minimum volume of CH Cl , then by adding diethyl ether using half
10
−
2
2
of that volume, and finally by adding an equivalent volume of hexane.
The crystals were collected and dried under vacuum. Yield: 0.327 g
−1
1
nates to a metal ion through its pyrazolyl groups. The complex
(
55.9%). IR (KBr pellets): ν(NCN) 2180, ν(BH) 2489 cm . H
III/II
[
Ru(Tp) ] possesses a Ru
couple of 0.45 versus normal
NMR (300 MHz, DMSO-d ): δ 8.06 (2H, d), 7.78 (1H, d), 7.72 (2H,
2
6
11
hydrogen electrode (NHE), which, in comparison with that
of other ruthenium homoleptic complexes of ammine and
d), 7.41 and 7.38 (10H, two overlapping br s peaks), 7.24, (2H, t),
7
.03 and 7.02 [(4H, t) and (2H, d), overlapping t and d peaks,
−
II
respectively], 6.75 (4H, t), 6.26 (2H, t), 6.10 (2H, d), 5.76 (CH Cl of
terpyridine, shows that Tp stabilizes the Ru oxidation state
2
2
3
+/2+
crystallization, s), 5.45 (1H, d), 5.28 (1H, t), 5.00−4.00 (1H, br s),
3.10 (4H, m). Anal. Calcd for C42.2 BRu: C, 56.85; H,
4.34; N, 14.14. Found: C, 57.06; H, 4.31; N, 14.07.
better than ammine ([Ru(NH ) ]
= 0.05 V vs NHE) but
3
6
3
+/2+
H N Cl0.4O P
38.4 9 2 2
significantly less than 2,2′;6′,2″-terpyridine ([Ru(trpy) ]
=
2
1
2
II
1
.25 V vs NHE). In order to stabilize the Ru oxidation state
1
Preparation of [Ru(Tp)(dppe)(2-Clpcyd)]· / CH Cl . A method
II
2
2
2
further, two coordination sites around Ru will be occupied by
,2-bis(diphenylphosphino)ethane (dppe).
analogous to that above was used. Yield: 0.15 g (51.8%). IR (KBr
pellets): ν(NCN) 2181, ν(BH) 2481 cm . H NMR (300 MHz,
13
−1
1
1
DMSO-d ): δ 8.03 (2H, d), 7.73 (1H, d), 7.42 (10H, overlapping
6
peaks: 2H, t; 4H, t; 4H, t), 7.23 (4H, t), 7.03 (7H, overlapping peaks:
1
(
H, d; 4H, t; 2H, d), 6.70 (5H, overlapping peaks: 1H, t; 4H, t), 6.38
1H, t), 6.24 (2H, t), 6.04 (1H, d), 5.76 (s), 5.37 (1H, d), 5.24 (1H,
t), 4.66 (1H, s), 3.05 (4H, t). Anal. Calcd for C42.5H N P BCl Ru: C,
39
8
2
2
5
6.31; H, 4.34; N, 12.36. Found: C, 56.52; H, 3.97; N, 11.96.
Preparation of Ru(Tp)(dppe)(3-Clpcyd)·0.4CH Cl . A method
2
2
analogous to that above was used. Yield: 0.17 g (52.1%). IR (KBr
Nine mononuclear complexes [Ru(Tp)(dppe)L] and a
−1
1
pellets): ν(NCN) 2173, ν(BH) 2476 cm . H NMR (300 MHz,
DMSO-d ): δ 8.03 (2H, d), 7.73 (1H, d), 7.42 (10H, overlapping
peaks: 2H, t; 4H, t; 4H, t), 7.23 (4H, t), 7.03 (6H, overlapping peaks:
dinuclear complex [{Ru(Tp)(dppe)} (μ-adpc)] have been
2
1
6
prepared and characterized by H NMR, IR, UV−vis, and
near-IR (NIR) spectroscopies, cyclic voltammetry, elemental
analysis, and crystallography. Density functional theory (DFT)
calculations of the mononuclear ruthenium(III) complexes
revealed the electronic properties of the ruthenium cyanamide
4
6
H, t; 2H, d), 6.70 (5H, overlapping peaks: 1H, t; 4H, t), 6.39 (1H, d),
.23 (2H, t), 6.10 (1H, t), 6.03 (1H, d), 5.76 (s), 5.37 (1H, d), 5.24
(1H, t), 4.65 (1H, s), 3.04 (4H, t). Calcd for C42.4H38.8N P BCl Ru:
C, 56.71; H, 4.35; N, 12.48. Found: C, 56.95; H, 4.15; N, 12.23.
8
2
1.8
1
chromophore. The mixed-valence properties of [{Ru(Tp)-
Preparation of Ru(Tp)(dppe)(2,3-Cl
2 2 2 2
pcyd)· / CH Cl . A meth-
+
od analogous to that above was used. Yield: 0.14 g (50.4%). IR (KBr
pellets): ν(NCN) 2181, ν(BH) 2476 cm . H NMR (300 MHz,
DMSO-d ): δ 8.03 (2H, d), 7.73 (1H, d), 7.42 (10H, overlapping
(
dppe)} (μ-adpc)] are compared with those of the previously
2
−1
1
8
3+
studied ion, [{Ru(trpy)(bpy)} (μ-adpc)] .
2
6
peaks: 2H, t; 4H, t; 4H, t), 7.23 (4H, t), 7.03 (6H, overlapping peaks:
4H, t; 2H, d), 6.70 (5H, overlapping peaks: 1H, t; 4H, t), 6.59 (1H, d),
6.25 (2H, t), 5.93 (1H, d), 5.76 (s), 5.37 (1H, d), 5.24 (1H, t), 4.65
EXPERIMENTAL SECTION
Ruthenium(III) chloride trihydrate (reagent grade, Pressure Chemical
■
Co.), triphenylphosphine (PPh ; ReagentPlus, 99%, Sigma-Aldrich),
(1H, s), 3.04 (4H, t). Calcd for C42.5
4.07; N, 11.91. Found: C, 54.47; H, 3.88; N, 11.68.
Preparation of Ru(Tp)(dppe)(2,5-Cl pcyd)·0.8CH
od analogous to that above was used. Yield: 0.16 g (52.3%). IR (KBr
H N P BCl Ru: C, 54.25; H,
38 8 2 3
3
potassium hydrotris(pyrazol-1-yl)borate hydrate (KTp; 98%, Strem
Chemicals), and 1,2-bis(diphenylphosphino)ethane (dppe; 99%,
Aldrich) were used as received. Thallium salts of (4-nitrophenyl)-
Cl . A meth-
2 2
2
−
−
−1
1
cyanamide (NO pcyd ), (2-chlorophenyl)cyanamide (2-Clpcyd ), (3-
pellets): ν(NCN) 2181, ν(BH) 2473 cm . H NMR (300 MHz,
DMSO-d ): δ 8.03 (2H, d), 7.73 (1H, d), 7.42 (10H, overlapping
2
−
chlorophenyl)cyanamide (3-Clpcyd ), (2,4-dichlorophenyl)cyanamide
2,4-Cl pcyd ), (2,3-dichlorophenyl)cyanamide (2,3-Cl pcyd ), (2,5-
6
−
−
(
peaks: 2H, t; 4H, t; 4H, t), 7.23 (4H, t), 7.03 (7H, overlapping peaks:
1H, d; 4H, t; 2H, d), 6.70 (4H, t), 6.47 (1H, d), 6.02 (1H, s), 5.76 (s),
5.37 (1H, d), 5.24 (1H, t), 4.65 (1H, s), 3.04 (4H, t). Calcd for
2
2
−
dichlorophenyl)cyanamide (2,5-Cl pcyd ), (2,4,5-trichlorophenyl)-
2
−
cyanamide (2,4,5-Cl pcyd ), (2,3,5,6-tetrachlorophenyl)cyanamide
3
−
−
(
2,3,5,6-Cl pcyd ), (pentachlorophenyl)cyanamide (Cl pcyd ), and
C H N P BCl3.6Ru: C, 53.19; H, 4.03; N, 11.59. Found: C, 53.09;
42.8 38.6 8 2
4
5
2
−
azo-4,4-diphenylcyanamide (adpc ) were prepared from literature
H, 3.76; N, 11.42.
8
,14−17
methods.
Tetrabutylammonium hexafluorophosphate (TBAH)
[Ru(Tp)(dppe)(2,4-Cl pcyd)]·CH Cl . A method analogous to
2 2 2
was synthesized by combining 0.1 M aqueous solutions of
tetrabutylammonium bromide (95+%, Aldrich) and ammonium
hexafluorophosphate (ReagentPlus, 99%, Sigma-Aldrich). The result-
ing TBAH powder was twice recrystallized from 1:1 ethanol/water and
vacuum-dried at 110 °C. N,N′-Dimethylformamide (DMF; Sigma-
Aldrich, ChromosolvPlus, 99.9%, HPLC grade) was used as received.
that above was used. Yield: 0.300 g (49.9%). IR (KBr pellets):
−1 1
ν(NCN) 2171, ν(BH) 2486 cm . H NMR (300 MHz, DMSO-d ): δ
6
8.04 (2H, d), 7.75 (1H, d), 7.41 and 7.39 (10H, two overlapping br s
peaks), 7.23 (2H, t), 7.06, 7.05, and 7.04 [(4H, t), (1H, d), and (2H,
d) overlapping t, d, and d peaks, respectively], 6.75 and 6.74 [(1H, d)
and (4H, m) overlapping d and m peaks, respectively], 6.25 (2H, t),
18
19
The reagent complexes Ru(PPh ) Cl , Ru(Tp)(PPh ) Cl, and
5.94 (1H, d), 5.76 (CH Cl of crystallization, s), 5.39 (1H, d), 5.25
2 2
3
3
2
3 2
20
Ru(Tp)(dppe)Cl were prepared according to literature methods.
(1H, t), 5.00−4.00 (1H, br s), 3.06 (4H, m). Anal. Calcd for
Preparation of [Ru(Tp)(dppe)(4-NO pcyd)]·0.2CH Cl . To a
00 mL round-bottomed flask equipped with a stir bar and containing
degassed DMF (200 mL) was added [Ru(Tp)(dppe)Cl] (0.500 g,
.67 mmol) under positive argon pressure. The solution was refluxed
under argon until [Ru(Tp)(dppe)Cl] dissolved. Tl[4-NO pcyd]
C
43
H
39
N
8
Cl
4
P
2
BRu: C, 52.52; H, 3.99; N, 11.39. Found: C, 52.89; H,
2
2
2
5
3.82; N, 11.51.
1
[Ru(Tp)(dppe)(2,4,5-Cl pcyd)]· / C H O. A method analogous
to that above was used. Yield: 0.380 g (60.9%). IR (KBr pellets):
ν(NCN) 2189, ν(BH) 2473 cm . H NMR (300 MHz, DMSO-d ): δ
3
2 4 10
0
−1 1
2
6
(
0.245 g, 0.67 mmol, fw 366.36 g/mol) was then added, and the
8.05 (2H, d), 7.76 (1H, d), 7.39 and 7.36 (10H, two overlapping br s
peaks), 7.26 and 7.24 [(1H,s) and (2H, t) overlapping s and t peaks,
respectively], 7.04 and 7.02 [(4H, t) and (2H, d) overlapping t and d
peaks, respectively], 6.75 (4H, t), 6.24 (2H, t), 6.05 (1H, s), 5.39 (1H,
mixture was refluxed for 3 h under argon, during which time the
solution changed from dark orange to bright orange and a white
precipitate of TlCl formed. After cooling at 4 °C overnight, TlCl was
1
622
dx.doi.org/10.1021/ic302535h | Inorg. Chem. 2013, 52, 1621−1630

Inorganic Chemistry
Article
d), 5.26 (1H, t), 5.00−4.00 (1H, br s), 3.06 (4H, m), 3.39 and 1.09
4 10
recovery of the original complex spectrum. Elemental analyses
were performed by Canadian Microanalytical Services Ltd.,
Delta, British Columbia, Canada.
(
C H O of crystallization, q and t peaks, respectively). Anal. Calcd for
C H N Cl O P BRu: C, 54.48; H, 4.26; N, 11.55. Found: C, 54.65;
44
41
8
3
0.5 2
H, 4.15; N, 11.57.
Crystallography. Crystals of [RuTp(dppe)(Cl pcyd)] and
5
Preparation of [Ru(Tp)(dppe)(2,3,5,6-Cl pcyd)]·0.2CH Cl . A
4
2
2
[
{RuTp(dppe)} (μ-adpc)]·2H O were grown by slow diffusion
2 2
method analogous to that above was used. Yield: 0.362 g (56.0%). IR
−1
1
of diethyl ether into a concentrated DMF solution of each
complex. The diffraction data were collected on a Bruker-AXS
Smart Apex CCD diffractometer using Mo Kα radiation (λ=
0.71073 Å) at 200(2) K using an ω-scan technique and
corrected for absorption using equivalent reflections. The unit-
cell parameters, equivalent reflections, and systematic absences
were uniquely consistent for the reported space groups. Direct
methods were used to solve the structures, and refinement was
carried out with full-matrix least-squares procedures. The
diruthenium complex structure was located on an inversion
center. Anisotropic refinement was performed on all non-
hydrogen atoms. Borate protons were located from the electron
(
KBr pellets): ν(NCN) 2198, ν(BH) 2479 cm . H NMR (300 MHz,
DMSO-d ): δ 8.02 (2H, d), 7.72 (1H, d), 7.35 (10H, two overlapping
6
br s peaks), 7.22 (2H, t), 7.05, 7.02, and 7.00 [(4H, t), (2H, d) and
(
(
5
1H, s) overlapping t, d, and s peaks, respectively], 6.73 (4H, t), 6.22
2H, t), 5.76 (CH Cl of crystallization, s), 5.31 (1H, d), 5.22 (1H, t),
2
2
.00−4.00 (1H, br s), 3.04 (4H, br dd). Anal. Calcd for
N Cl P BRu: C, 51.49; H, 3.62; N, 11.38. Found: C,
C4
2.2
H
35.4
8
4.4 2
5
1.82; H, 3.81; N, 10.99.
Preparation of [Ru(Tp)(dppe)(Cl pcyd)]. A method analogous
5
to that above was used. Yield: 0.352 g (52.5%). IR (KBr pellets):
−1 1
ν(NCN) 2188, ν(BH) 2459 cm . H NMR (300 MHz, DMSO-d ): δ
6
8
7
.03 (2H, d), 7.73 (1H, d), 7.35 (10H, two overlapping br s peaks),
.23 (2H, t), 7.05 and 7.02 [(4H, t) and (2H, d), overlapping t and d
peaks, respectively], 6.73 (4H, t), 6.22 (2H, t), 5.33 (1H, d), 5.23 (1H,
t), 5.00−4.00 (1H, br s), 3.04 (4H, br dd). Anal. Calcd for
C H N Cl P BRu: C, 50.35; H, 3.42; N, 11.18. Found: C, 50.08;
density map and refined in position with 1.2U of the attached
eq
boron atom. All other hydrogen atoms were treated as idealized
contributions. Scattering factors are contained in the
42
34
8
5 2
H, 3.16; N, 11.11.
24
SHELXTL, version 6.12, program library. The data set for
Preparation of [{Ru(Tp)(dppe)} (μ-adpc)]·DMF·2H O. To a
2
2
[
{RuTp(dppe)} (μ-adpc)]·2H O was treated with SQUEEZE
2 2
5
00 mL round-bottomed flask equipped with a stir bar and degassed
to model the disordered water molecules of hydration as
DMF (200 mL) was added [Ru(Tp)(dppe)Cl] (0.500g, 0.67 mmol)
under argon. The solution was refluxed and stirred until [Ru(Tp)-
25
diffused contributions. CIF documents for both complexes
have been deposited with the CSD under depositary numbers
900736 and 900737.
(
dppe)Cl] dissolved. Under positive argon pressure, Tl [adpc] (0.223
2
g, 0.33 mmol) was then added, and the mixture was refluxed overnight.
The dark-red reaction mixture was filtered through a Celite packed
column in order to separate the fine TlCl particles, the filtrate was
rotary evaporated to a minimum volume, and the product complex
precipitated by ether diffusion. Yield: 0.500 g (88.7%). IR (KBr
Oscillator Strength Calculation. Because of the non-
Gaussian ligand-to-metal charge-transfer (LMCT) band shape,
26
calculation of the oscillator strength using eq 1,
−1
1
f = 4.61 × 10−9εmaxΔν
pellets): ν(NCN) 2174, ν(BH) 2470 cm . H NMR (300 MHz,
1/2
(1)
DMSO-d ): δ 8.04 (4H, d), 7.75 (2H, d), 7.43 and 7.42 (20H, two
6
overlapping br s peaks), 7.31 (4H, d), 7.23 (4H, t), 7.07 (4H, d), 7.04
where εmax is the maximum extinction coefficient in M cm−1
−1
(
(
8H, t), 6.76 (8H, t), 6.25 (4H, t), 6.18 (4H, d), 5.38 (2H,d), 5.25
−1
and Δν1 is the bandwidth at half-peak-height in cm , would
/2
2H, t), 5.00−4.00 (2H, br s), 3.32 (H O associated with the
2
lead to an inexact value. In order to obtain a more accurate
value, multiple Gaussian bands were fitted to the NIR band
envelope using Jandel Scientific Peakfit v3.0 software. A
representative fit is shown in the Supporting Information
complex), 3.06 (8H, m), 7.95, 2.89, and 2.73 (DMF of crystallization,
three singlet peaks). Anal. Calcd for C H N O P B Ru : C, 58.23;
87
87 19
3
4
2
2
H, 4.88; N, 14.83. Found: C, 58.32; H, 4.68; N, 14.48.
(
Figure S1). The NIR band oscillator strength was then
PHYSICAL MEASUREMENTS
■
calculated according to eq 2, which sums the oscillator strength
from each of the Gaussian bands to give the total oscillator
strength of the NIR transition.
BOMEM Michelson MB100 FT-IR and Cary 5 UV−vis−NIR
spectrophotometers were used to record IR and electronic
1
spectra, respectively. H NMR spectra of dimethyl-d sulfoxide
6
(
DMSO-d ) solutions were recorded using a Bruker 300 Ultra
f = 4.61 × 10−9
6
∑
εiΔνi
Shield spectrometer at room temperature. Cyclic voltammetry
studies were performed with a Ω Metrohm Autolab
PGSTAT30 potentiostat/galvanostat on argon-saturated sol-
utions of the complexes in DMF with 0.1 M TBAH as the
supporting electrolyte. A three-electrode arrangement consist-
ing of a platinum disk working electrode (BAS, 1.6 mm
diameter), a platinum-wire auxiliary electrode, and a silver-wire
quasi-reference electrode was used. The electrochemical cell
consisted of a double-jacketed glass container with an internal
i
(2)
In the above equation, ε and Δν are the extinction coefficient
i
i
−1
at λ and bandwidth at half-peak-height in cm , respectively,
max
of each Gaussian band.
DFT Calculations. DFT calculations utilized the Gaussian
09 (revisions B.01 and C.01) program. Optimized geometries
2
7
were calculated using the B3LYP exchange-correlation func-
28
29−31
tional with the LANL2DZ basis set
for all elements
21
volume of 15 mL. Ferrocene (E° = 0.665 V vs NHE) and
except ruthenium, chlorine, and phosphorus, for which the
def2-TZVP triple-ζ basis set was used. Tight self-consistent-
field convergence criteria (10 au) were used for all
3
2
cobaltocenium hexafluorophosphate (E° = −0.589 V vs
2
1
−8
NHE) were used for calibration.
An optically transparent thin-layer electrochemical cell
arrangement was used to conduct spectroelectrochemical
calculations. The wave functions were stable (checked with
Stable keyword), and the stationary points were truly minima
(Freq keyword check).
2
2,23
studies.
Indium−tin oxide-coated glass served as working
and counter electrodes, and AgCl/Ag was used as the reference
electrode. The solvents and supporting electrolyte were the
same as those used for cyclic voltammetry. For the
spectroelectrochemical studies, the spectroscopic changes
observed upon oxidation are reversible with greater than 95%
Molecular orbital (MO) compositions were calculated using
3
3,34
32,35−38
the AOMix program
using the Mulliken scheme.
3
5−38
Atomic charges were calculated using the Mulliken
natural population analysis methods (MPA and NPA,
and
3
9
respectively), as implemented in Gaussian 09, revisions B.01
1
623
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Inorganic Chemistry
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and C.01. Extended charge decomposition analysis was
Table 1. Crystal Data and Structure Refinement for
[Ru(Tp)(dppe)(Cl pcyd)] and [{RuTp(dppe)} (μ-
34,40
41,42
performed using AOMix-CDA.
The PCM
was used to
5
2
model solvation for methanol as the solvent.
adpc)]·2H O
2
43−45
Time-dependent DFT (TD-DFT)
was used to calculate
empirical formula
fw (g/mol)
C H BC N P Ru
C H B N O P Ru
84 80 2 18 2 4 2
42
34
15
8
2
the energies and intensities of the 80 lowest-energy electronic
transitions of both complexes. These were converted with the
1001.84
200(2)
0.71073
1721.30
46
temperature (K)
wavelength (Å)
cryst syst
200(2)
SWizard program into simulated spectra using Gaussian
−1
0.71073
functions with half-widths of 2500 cm .
orthorhombic
monoclinic
space group
Pccn
P2(1)/n
RESULTS AND DISCUSSION
■
unit cell
dimensions
For this study, mononuclear and dinuclear complexes were
synthesized under argon to avoid oxidation, and it was
important to ensure that the reagent complex [Ru(Tp)(dppe)-
Cl] was completely dissolved before the addition of the
cyanamide thallium salt; otherwise, yields were significantly less,
and the product was difficult to purify. Mononuclear complexes
were, for the most part, readily soluble in organic solvents.
However, with increasing numbers of chloro substituents on
the phenylcyanamide ligand, slight heating was required for
a (Å)
20.198(4)
25.050(5)
16.714 (4)
90.00
12.035(3)
25.181(6)
15.913(4)
90.00
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
90.00
109.513(5)
90.00
90.00
3
volume (Å ), Z
8456(3), 8
1.574
4545.3(18), 2
1.258
density (calcd) (
3
1
Mg/m )
dissolution. H NMR spectra of the complexes showed
abs coeff (mm−1)
0.806
4080
0.456
1768
chemical shifts (see the Experimental Section) belonging to
Tp, dppe, and phenylcyanamide ligands with integrations
consistent with the complex stoichiometry. In the case of
pentachlorophenyl)cyanamide complex, the presence of the
cyanamide ligand was confirmed by the strong ν(NCN) band
observed in the complex’s IR spectrum. The ν(NCN) band is
observed in the range of 2171−2198 cm for the complexes of
this study.
Crystallographic data for the complexes [Ru(Tp)(dppe)-
F(000)
cryst size
0.22 × 0.22 × 0.11
2.27−15.40
0.16 × 0.11 × 0.03
2.42−17.36
θ range for data
collection (deg)
(
limiting Indices
−26 ≤ h ≤ 26, −33 ≤ k ≤ −14 ≤ h ≤ 14, −29 ≤ k ≤
30, −22 ≤ l ≤ 22 29, −18 ≤ l ≤ 18
10491 8006
−1
reflns collected
indep reflns
abs corrn
5450 [R(int) = 0.0573]
multiscan
4593 [R(int) = 0.0685]
multiscan
(
Cl pcyd)] and [{Ru(Tp)(dppe)} (μ-adpc)] are given in Table
, and Figures 1 and 2 show ORTEP drawings of the
max and min
transmn
0.9166 and 0.8426
0.9878 and 0.9306
5
2
1
refinement method full-matrix least-squares on full-matrix least-squares on
complexes, respectively. Tables 2 and 3 present the bond
2
2
F
F
lengths and angles for [Ru(Tp)(dppe)(Cl pcyd)] and [{Ru-
5
data/restraints/
parameters
5450/0/535
8006/0/499
(
Tp)(dppe)} (μ-adpc)], respectively. For both complexes, the
2
II
Ru ions occupy a pseudooctahedral coordination sphere in
which the cyanamide moiety coordinates to Ru by its terminal
nitrogen atom and the cyanamide moiety is approximately in
the plane of the phenyl ring of the cyanamide ligand. The
planarity of the phenylcyanamide and adpc ligands has been
observed in other crystal structures and is ascribed to π mixing
GOF on F2
1.001
1.023
II
a
R1
0.0573
0.0685
b
wR2
0.1428
0.1280
a
R1 = ∑||F | − |F ||/∑|F |. wR2 = (∑w(|F | − |F |) /∑w|F | )1/2.
b
2
2
o
c
o
o
c
o
8
,15
between the cyanamide and phenyl π systems.
In Tables 2
and 3, the cyanamide moiety is approximately linear, as is the
angle describing coordination of its terminal nitrogen atom to
II
Ru . This may suggest significant π interactions, but when the
coordination sphere is not sterically constrained, smaller bond
angles have been observed. For example, [Pd(trpy)(2,6-
+
II
dichlorophenylcyanamide)] possesses a Pd −NCN bond
47
angle of 137.9(4)°. Ruthenium(II) cyanamide bond lengths
in Tables 2 and 3 are similar to those seen in other
7
,48
ruthenium(II) complexes.
III/II
Table 4 gives the Ru
couples of the mononuclear
[
Ru(Tp)(dppe)L] complexes, where L is a substituted
phenylcyanamide ligand, as well as the vis−NIR electronic
0
,+
absorption data for [Ru(Tp)(dppe)L] . Cyclic voltammetry of
the [Ru(Tp)(dppe)L] complexes showed a single reversible
wave in the range of 0.7−1 V. This couple shifts to more
positive potentials with increasing numbers of chloro
substituents on the phenylcyanamide ligand, which is consistent
Figure 1. ORTEP diagram of [Ru(Tp)(dppe)(Cl pcyd)] with 50%
5
probability ellipsoids. Hydrogen atoms are omitted for clarity. Color
with an overall reduction of ligand donor properties. The
code: Ru, red; P, orange; N, blue; B, pink; C, gray; Cl, green.
III/II
formal assignment of this wave to a Ru
couple is
complicated by the noninnocence of the cyanamide ligand.
Oxidation of [Ru(Tp)(dppe)L] generates an intense NIR band
cyanamide complexes and previously assigned to a LMCT of
7
,49
the ruthenium(III) cyanamide chromophore.
However, for
(
Table 4 and Figure 3) similar to that seen in other ruthenium
the complexes of this study (Table 4), the oscillator strength of
1
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As will be discussed in detail below, the TD-DFT
calculations of the optical spectra of these two species provide
excellent agreement with the experimental UV−vis energies of
these two species albeit TD-DFT predicts too high an intensity,
54
which we have previously noted.
The percent composition of the frontier molecular orbitals
provides insight into how these systems are constructed at the
electronic molecular level. These α and β manifolds are shown
in Figures 4 and 5.
The α-HOMO orbital in both complexes is comprised of a π-
antibonding interaction between Ru 4dπ and the HOMO of the
NCN ligand (specified as HOFO, the HOMO of the fragment
NCN ligand). It is the locus of the unpaired electron having an
overlap of 0.91 (mono, 0.84 penta) with the β-LUMO of the
complex.
The α-LUMO in both species is, somewhat unexpectedly, the
σ* antibonding interaction between Ru 4d and two phosphorus
and two scorpionate nitrogen atoms.
Figure 2. ORTEP diagram of [{RuTp(dppe)} (μ-adpc)]·2H O with
2
2
5
0% probability ellipsoids. Water molecules of hydration and hydrogen
atoms are omitted for clarity. The color legend is as in Figure 1.
Of particular relevance is the β-LUMO of these species. This
is comprised primarily of an antibonding interaction between
the Ru 4dπ β hole and the HOFO of the NCN ligand (Table
5). The corresponding bonding interaction is the β-HOMO in
the monochloro species but is the β-HOMO−1 in the
pentachloro species. These are strongly mixed Ru 4dπ and π-
L(NCN) orbitals and are important for the bonding between
ruthenium and L(NCN). This important bonding contribution
provided by the β manifold is absent from the α manifold
because there is no α-4dπ hole. This then explains why the
Mayer bond order between Ru 4d and the NCN ligand is much
greater in the β manifold than the α manifold (Table 5). This is
not reflected in the Mayer bond orders with the other two
ligands because, as shown in Figures 4 and 5, there is little
involvement of these ligand orbitals with the β-4dπ hole.
The significantly larger contribution of the L(NCN) HOFO
to the β-LUMO in the monochloro species (41.9%) relative to
the pentachloro species (33.1%) argues for much greater
covalency in the former. This is supported by the spin densities
this NIR band decreases with increasing π-donor properties of
2+
15
the cyanamide ligand, while for [Ru(NH ) L]
and dinuclear
3
5
4
+
22
[
{Ru(NH ) } (μ-dicyd)] complexes, the ruthenium(III)
3
5 2
cyanamide NIR band oscillator strength increased with
increasing donor properties of the cyanamide group and the
strength of ruthenium(III) cyanamide π bond.
5
0
To obtain further insight into these species, DFT
calculations were carried out on the two extreme members of
the mononuclear complexes, [Ru(Tp)(dppe)L], where L = 3-
−
−
Clpcyd and Cl pcyd , henceforth, for simplicity, referred to as
5
the monochloro and pentachloro cyanamide species. To
provide as accurate an assessment as possible, consistent with
computer cost and time, we employed the B3LYP functional,
5
1
which behaves well with ruthenium species, and the
LANL2DZ basis set for all atoms except ruthenium,
phosphorus, and chlorine, where the triple-ζ def2-TZVP basis
32,52
set, downloaded from EMSL,
was used. The resulting bond
distances agreed well with those expected based on the X-ray
III
structural data (see the Supporting Information), e.g., d(Ru −
(
NPA) shown in Table 6. The L(NCN) ligand is non-
NCN) = 1.95 Å (monochloro) and 1.96 Å (pentachloro)
compared with an average of 1.97 Å observed in six similar
55
innocent, and when the parent ruthenium(II) species is
oxidized, there is also partial oxidation of the L(NCN) ligand.
Thus, the net single spin is shared almost equally between
ruthenium and L(NCN) in the monochloro species but is
much more localized on ruthenium in the pentachloro species.
Figure 6 shows the MPA spin density of the monochloro
species (that for the pentachloro species is shown in the
1
5,17,53
ruthenium(III) complexes.
Of particular note, one of the
imidazole rings of the scorpionate ligand lies exactly in the
plane of the dicyanamide ligand in the DFT-optimized
geometry. This is not observed in the X-ray structure of the
ruthenium(II) species perhaps because of packing forces or
weaker interligand coupling via ruthenium.
Table 2. Selected Crystal Structure Data for [Ru(Tp)(dppe)(Cl pcyd)]
5
a
Bond Lengths/Å
Ru−N(7)
Ru−N(1)
Ru−N(5)
Ru−N(3)
2.040(4)
2.089(3)
2.131(3)
2.134(4)
Ru−P(1)
Ru−P(2)
2.2812(13)
2.2878(13)
N(7)−C(36)
N(8)−C(36)
N(8)−C(42)
1.173(5)
1.296(6)
1.361(5)
a
Bond Angles/deg
N(7)−Ru−N(5)
N(1)−Ru−N(5)
N(7)−Ru−N(3)
N(1)−Ru−N(3)
N(5)−Ru−N(3)
N(7)−Ru−P(1)
88.87(13)
85.03(13)
88.68(14)
88.73(13)
83.39(13)
90.40(10)
N(1)−Ru−P(1)
N(5)−Ru−P(1)
N(7)−Ru−P(2)
N(1)−Ru−P(2)
N(3)−Ru−P(2)
P(1)−Ru−P(2)
92.28 (10)
97.63(10)
93.58(10)
92.41(10)
93.80(10)
85.22(4)
C(36)−N(7)−Ru
C(36)−N(8)−C(42)
N(7)−C(36)−N(8)
166.2(4)
127.4(4)
166.0(5)
a
Estimated standard deviations are in parentheses.
1
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Table 3. Selected Crystal Structure Data for [{RuTp(dppe)} (μ-adpc)]·2H O
2
2
a
Bond Lengths/Å
Ru−N(7)
Ru−N(1)
Ru−N(5)
Ru−N(3)
2.043(5)
2.092(5)
2.122(5)
2.132(6)
Ru−P(1)
Ru−P(2)
2.2826(18)
2.2628(18)
N(7)−C(36)
N(8)−C(36)
N(8)−C(42)
N(9)−C(39)
N(9)−N(9A)
1.155(7)
1.288(8)
1.378(7)
1.522(9)
1.154(10)
a
Bond Angles/deg
N(7)−Ru−N(5)
N(1)−Ru−N(5)
N(7)−Ru−N(3)
N(1)−Ru−N(3)
N(5)−Ru−N(3)
N(7)−Ru−P(1)
86.8(2)
N(1)−Ru−P(1)
N(7)−Ru−P(2)
N(1)−Ru−P(2)
N(3)−Ru−P(1)
N(5)−Ru−P(2)
P(2)−Ru−P(1)
92.82(14)
94.61(15)
92.36(15)
95.10(16)
95.61(14)
85.19(6)
C(36)−N(7)−Ru
160.6(6)
120.8(5)
172.4(7)
107.6(9)
85.83(19)
85.5(2)
C(36)−N(8)−C(42)
N(7)−C(36)−N(8)
N(9A)−N(9)−C(39)
87.5(2)
84.1(2)
94.45(15)
a
Estimated standard deviations are in parentheses.
Table 4. Cyclic Voltammetry, NIR, and Absorption Data for
Supporting Information). It is very similar in appearance to the
π-HOMO, which is the SOMO in this complex.
0
,+
[
Ru(Tp)(dppe)L)] Complexes
These oxidized species are then visualized as resonance
[
Ru(Tp)(dppe)
III
−
II
0
+
hybrids between Ru −L(NCN) and Ru −L(NCN) , with
the latter more important in the monochloro species and the
former more important in the pentachloro species. This is also
seen clearly in the occupancies of the L(NCN) β-HOFO
(Table 5), being substantially less than 1 and indeed
approaching 0.5 in the monochloro species; i.e., the ligand is
[
Ru(Tp)(dppe)L)]
L)]
[
Ru(Tp)(dppe)
0
/+
b
b
L
L)] (V)
0.72
MLCT
LMCT
-Clpcyd−
2
377 (1720)
375 (2120)
382 (1400)
372 (2100)
377 (2180)
383 (1690)
1120 (4510)
0.078]
[
3
2
2
2
2
2
-Clpcyd−
0.78
0.80
0.82
0.84
0.87
0.87
0.89
0.93
951 (4000)
0.076]
0
[
close to being described as L(NCN) .
−
−
−
,4-Cl pcyd
997 (4460)
0.076]
2
Thus, the covalent bonding between the metal and L(NCN)
ligand is greater in the monochloro species than in the
pentachloro species. In terms of the discussion above about the
intensity of the visible region absorption, greater mixing
between Ru 4dπ and π* L(NCN) in the primary donor and
acceptor MOs involved in the NIR transition, in the
monochloro species, will lead to a reduction in the degree of
net charge transfer and, hence, a weaker transition intensity in
the monochloro species relative to the pentachloro species, as is
indeed observed.
[
,3-Cl pcyd
973 (4930)
0.092]
2
[
,5-Cl pcyd
967 (7470)
0.13]
2
[
,4,5-
987 (8700)
[0.17]
−
Cl pcyd
3
,3,5,6-
Cl pcyd
379 (sh, 2600) 929 (9390)
[0.17]
−
4
−
Cl pcyd
379 (sh, 5140) 956 (10700)
5
[
0.19]
946 (13100)
0.20]
−
c
4
-NO pcyd
457 (2590)
The experimental UV−vis and predicted TD-DFT spectra
are shown in Figure 7. There is excellent agreement, between
theory and experiment, insofar as the energies and band
envelope are concerned, but the TD-DFT greatly over-
emphasizes the intensities especially for the monochloro
2
[
a
b
in nm (εmax in M−1
cm ) [oscillator strength f]; sh = shoulder; DMF solution. Mostly
vs NHE, DMF solution, and 0.1 M TBAH. λ
max
−
1
c
intraligand charge transfer.
54
species. Such predicted enhanced intensities are not unusual.
We suppose that the energy/band shape agreement does
provide evidence that def2-TZVP DFT analysis is sound. We
note in passing that a somewhat poorer fit between the
experimental and TD-DFT predicted spectra is noted if
LANL2DZ is used for phosphorus and chlorine instead of
def2-TZVP. It is especially important in the pentachloro species
to use def2-TZVP for chlorine.
The detailed assignments are shown in the Supporting
Information in Table S1. The very intense visible region
transition is β-HOMO−1 to β-LUMO in the pentachloro
species and β-HOMO to β-LUMO in the monochloro species.
Although excitation occurs from a different frontier molecular
orbital, in fact, β-HOMO−1 (pentachloro) and β-HOMO
(
monochloro) involve primarily the same β-HOFO of the
L(NCN) ligand, which is also the primary orbital in β-LUMO.
Because the same ligand orbital is involved in both the ground
and excited states, overlap is considerable, hence leading to an
intense transition. Different 4d orbitals are involved in the
ground and excited states (see the Supporting Information).
Further, we recall that β-HOFO of the L(NCN) ligand is only
Figure 3. Spectroelectrochemistry of [Ru(Tp)(dppe)(Cl pcyd)] in
DMF, 0.1 M TBAH, under increasing oxidation potentials (0−0.6 V vs
AgCl/Ag).
5
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Figure 4. Percent composition of frontier molecular orbitals of monochloro species: α manifold (left); β manifold (right). Color code: Ru, black;
NCN ligand, red; scorpionate ligand, green; diphosphine ligand, yellow. The white space identified by --- is the HOMO−LUMO gap. The α-HOMO
is 207, and the β-HOMO is 206.
Figure 5. Percent composition of frontier molecular orbitals of pentachloro species: α manifold (left); β manifold (right). Color code: Ru, black;
NCN ligand, red; scorpionate ligand, green; diphosphine ligand, yellow. The white space identified by --- is the HOMO−LUMO gap. The α-HOMO
is 239, and the β-HOMO is 238.
a
a,b
Table 5. Selected Properties of the Mono- and
Table 6. NPA Spin Densities and Charges of the Mono-
and Pentachlorocyanamide Species
Pentachlorocyanamido Species
property
Cl−NCN
Cl −NCN
location
Cl−NCN
Cl −NCN
5
5
b
b
b
α-Ru−L(NCN) Mayer bond order
β-Ru−L(NCN) Mayer bond order
α-4d valence population
0.269 (0.317)
0.280 (0.337)
0.625 (0.697)
3.96
ruthenium
NCN
0.54 (−0.45)
0.42 (−0.16)
0.05 (0.21)
0.62 (−0.40)
0.33 (−0.26)
0.07 (0.22)
b
0.67 (0.776)
3.95
3.42
97.5
55.7
scorpionate
diphosphine
β-4d valence population
3.35
−0.02 (1.34)
−0.02 (1.43)
c
a
b
α-HOFO L(NCN) occupancy
97.4
Spin (charge). DFT calculations using b3lyp/def2-tzvp for
phosphorus, chlorine, and ruthenium atoms. PCM = DMF.
c
β-HOFO L(NCN) occupancy
64.6
e
d
d
%
%
4d Ru contribution to the β-LUMO
46.9
53.2
L(NCN) contribution to the β-
42.3
34.2
e
is most likely due to one or more vibronic transitions where the
electronic state is coupled to an ν(NCN) vibration.
At higher energy, broad absorption is seen arising from
LUMO
a
DFT calculations, using b3lyp/def2-tzvp for chlorine, phosphorus,
and ruthenium atoms. PCM = DMF. Interfragment Mayer bond
order between the L(NCN) ligand fragment and the rest of the
molecule. 39 in the mono species; 71 in the penta species. β hole in
Ru 4dπ. See Figures 5 and 6.
b
−
1
multiple transitions, many of which, below 30000 cm ,
terminate on the same β-LUMO and originate from
scorpionate and diphosphine ligand orbitals, i.e., ligand-to-
ligand charge transfer. They tend to be weak because the
ligand−ligand overlap is small.
As shown by DFT calculations, oxidation of the ruthenium-
(II) complexes of this study leads to species where partial
oxidation of the cyanamide ligand has occurred, indicative of
the noninnocent character for these ligands. The degree of
cyanamide ligand oxidation depends upon the relative stability
of the cyanamide ligand and the ruthenium ion. For the easily
c
d
e
partially occupied. Thus, this transition can be defined as a
mixture of d−d, L(NCN) to Ru 4d LMCT, and indeed 4d to
L(NCN) metal-to-ligand charge-transfer (MLCT), thus not
easily described! We also note from Figures 4 and 5 that the
scorpionate ligand contributes to the relevant MOs and so is
involved in this transition.
While DFT analysis reveals only one electronic transition in
the region around 12000 cm , the experimental envelope
clearly indicates extra absorption on the high-energy side. This
−1
2−
oxidized 1,4-dicyanamidebenzene dianion (dicyd ) and its
substituted derivatives, DFT calculations showed that it is
17
1
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Inorganic Chemistry
Article
effect of the Ru(Tp)(dppe) coordination sphere on metal−
metal coupling.
The cyclic voltammogram of [{Ru(Tp)(dppe)} (μ-adpc)]
Figure S5 in the Supporting Information) showed two
2
(
reversible couples at 0.67 and 0.81 V vs NHE (Table 4), and
this gives a comproportionation constant of only K = 230,
c
which is far less than that observed for [{Ru(trpy)(bpy)} (μ-
2
adpc)]2+ (K = 1.3 × 10 ). This implies significantly weaker
13
8
c
metal−metal coupling for the mixed-valent [{Ru(Tp)-
dppe)} (μ-adpc)] complex. However, neutral dinuclear
+
(
2
complexes upon oxidation are subject to ion-pairing effects,
which reduce the comproportionation constant and therefore
make K a poor measure of metal−metal coupling. This is
c
illustrated by the electrolyte-dependent comproportionation
constant of bis(fulvalene)dinickel, which in dichloromethane
4
gives K = 4.1 × 10 for a 0.1 M [NBu ]Cl electrolyte but K =
c
4
c
12
57
5
.4 × 10 for 0.1 M [NBu ][B(C F ) ]. A more accurate
4 6 5 4
gauge of metal−metal coupling is derived from spectroelec-
trochemical studies.
Figure 8 shows the vis−NIR absorption changes associated
+
with the formation of [{Ru(Tp)(dppe)} (μ-adpc)] in DMF.
2
Figure 6. MPA spin density for the monochloro species.
only because of a strong solvation of the [{Ru(NH ) } (μ-
3
5 2
dicyd)]3 complex that the Ru ion predominates instead of
+
III
−
56
radical anion dicyd . In contrast, the complex ion [{Ru-
+
−
(
trpy)(thd)} (μ-dicyd)] , where thd is 2,2,6,6-tetramethyl-3,5-
2
55b
heptanedione,
shows an electron paramagnetic resonance
−
(
EPR) spectrum of a radical dicyd bridging ligand and
therefore has oxidation states [Ru , dicyd , Ru ]. The first
II
−
II
2−
8
oxidation of free adpc occurs at 0.48 V vs NHE, which is
2
−
Figure 8. Spectroelectrochemistry of [{Ru(Tp)(dppe)} (μ-adpc)] in
approximately 0.9 V more positive than that of dicyd . The
positive shift in potential is largely due to the electron-
2
DMF, 0.1 M TBAH, under increasing oxidation potentials (0.0−0.93
V vs AgCl/Ag).
2
−
withdrawing properties of the azo group of adpc . Cyclic
voltammetry and spectroelectrochemical studies of the
+
dinuclear complex ion [{Ru(Tp)(dppe)} (μ-adpc)] were
These spectroscopic changes are strikingly similar to those of
2
3
+
performed to explore its mixed-valence properties and the
[{Ru(trpy)(bpy)} (μ-adpc)] and strongly suggests that
2
Figure 7. Electronic spectra of the monochloro (left) and pentachloro (right) species. Experimental data recorded in DMF (black) and TD-DFT-
calculated spectra (green). Note: The experimental data use the left-hand molar absorbance axis but are multiplied by a factor of 7 for the
monochloro species and a factor of 2 for the pentachloro species.
1
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Inorganic Chemistry
Article
metal−metal coupling in the mixed-valence state is largely
AUTHOR INFORMATION
Corresponding Author
E-mail: robert_crutchley@carleton.ca (R.J.C.), blever@yorku.
ca (A.B.P.L.).
Notes
■
unperturbed by a Ru(Tp)(dppe) coordination sphere com-
pared with that of Ru(trpy)(bpy). Indeed, [{Ru(Tp)-
*
+
(
dppe)} (μ-adpc)] shows an intense NIR absorption centered
2
−1
−1
at 1932 nm (ε = 6480 M cm ), which is not much different
3+
from the intervalence band of [{Ru(trpy)(bpy)} (μ-adpc)]
2
The authors declare no competing financial interest.
1
−1
8
(
λmax = 1920 nm with ε = 10000 M− cm ) in DMF. By
+
analogy with the latter complex, [{Ru(Tp)(dppe)} (μ-adpc)]
2
ACKNOWLEDGMENTS
■
is suggested to possess a delocalized mixed-valence state with
This work was supported by the Natural Sciences and Research
Council of Canada and Carleton University. Computational
work was made possible by the facilities of the Shared
Hierarchical Academic Research Computing Network, Ontario,
Canada (http://www.sharcnet.ca).
−1
resonance exchange integral Had = 2600 cm (half of the
58
energy of the intervalence transition).
CONCLUSION
■
Nine mononuclear [Ru(Tp)(dppe)L] complexes, where L is a
substituted phenylcyanamide ligand, and the dinuclear complex
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