Tricarbonylrhenium(I) Complexes of Dinucleating Redox-Active Pincer Ligands

Article abstract of DOI:10.1021/acs.organomet.8b00013

Two homobimetallic tricarbonylrhenium(I) complexes of new redox-active dinucleating pincer ligands have been prepared to assess the impact of spacer size on the first oxidation potentials with respect to mononucleating analogues and on intramolecular elec

Full text of DOI:10.1021/acs.organomet.8b00013

Article
Cite This: Organometallics XXXX, XXX, XXX−XXX
Tricarbonylrhenium(I) Complexes of Dinucleating Redox-Active
Pincer Ligands
James R. Gardinier,*,† Jeewantha S. Hewage,† Brian Bennett,‡ Denan Wang,† and Sergey V. Lindeman†
†Department of Chemistry, Marquette University, Milwaukee, Wisconsin 53201-1881, United States
‡Department of Physics, Marquette University, Milwaukee, Wisconsin 53201-1881, United States
S
* Supporting Information
ABSTRACT: Two homobimetallic tricarbonylrhenium(I) complexes of
new redox-active dinucleating pincer ligands have been prepared to assess
the impact of spacer size on the first oxidation potentials with respect to
mononucleating analogues and on intramolecular electronic communica-
tion. The new pincer ligands feature two tridentate NNN- sites each
composed of two pyrazolyl flanking donors and a diarylamido anchor that
are either directly linked (to form a central benzidene core, H2(L1)) or
linked via a para-phenylene group (to form a para-terphenyldiamine core, H2(L2)). The bimetallic complexes of the
deprotonated ligands, [fac-Re(CO)3]2(μ-L1), 1, and [fac-Re(CO)3]2(μ-L2), 2, were fully characterized in solution and the solid
state including by single-crystal X-ray diffraction for 1. The electrochemical properties of each depended strongly on solvent and
electrolyte. Complex 1 exhibits two one-electron oxidations in all electrolyte-containing solutions but with separations between
first and second oxidation potentials, ΔE1/2, between 119 and 316 mV depending on conditions. On the other hand, cyclic
voltammetry of 2 showed one two-electron oxidation in DMF with NBu4PF6 as an electrolyte but two one-electron oxidations
with a maximal separation in ΔE1/2 of 96 mV in CH2Cl2 with NBu4B(C6F5)4 as an electrolyte. The oxidized complexes 1n+ and
2n+ (n = 1, 2) were prepared by chemical oxidation and were studied spectroscopically (UV−vis/NIR, EPR). The mono-oxidized
complex 1+ behaves as a Robin−Day Class III species, while 2+ is a Robin−Day Class II species that shows thermal valence
trapping at 77 K by EPR spectroscopy. As suggested from theoretical studies using DFT methods, the oxidized complexes
maintain considerable ligand radical character, so their electronic structures can be formulated as (CO)3ReI(μ-Ln+)ReI(CO)3 (n =
1 or 2).
groups and on the length of the intervening polyphenylene
spacer.7d,9,10 In contrast, the electrochemical properties of 2°
diamine derivatives (with R = alkyl, aryl and R′ = H) that may
be suitable amido ligand precursors in metal complexes are less
studied in organic chemistry because they are not well-behaved.
These organic derivatives decompose on oxidation to form
dinitroxyl radicals (R′ = O) or intractable polymeric species.11
However, in cases of ortho-phenylenediamines, replacement of
hydrogen(s) with a metal (with R = alkyl, aryl, R′ = metal) has
indeed provided numerous robust complexes with interesting
electronic properties and remarkable chemical reactivity.12−14
Metal derivatives of para-substituted (poly)phenylenediamines
are scarce15 as they are expected to be highly reactive, moisture-
sensitive species like other simple metal diarylamides. We
conjectured that the addition of Lewis donors at either ortho-
positions of an arylamine or the β- positions of an alkylamine
could give stable dinucleating metal pincer derivatives like the
known mononucleating diaryl-, dialkyl, or N-aryl-N-alkyl-
amines.16
INTRODUCTION
■
Contemporary interest in metal complexes of redox-active
ligands1 is stimulated by their potential use in technological
applications (redox flow batteries,2 electrochromic materials3),
in catalytic applications,4 or simply by fundamental interest in
bonding and electron transfer. In such complexes, the usual
electrochemical and reaction chemistry mediated by the metal
center is augmented by the ligand’s added capacity to either
accept or donate one electron. The electron accepting
5
properties of the bipyridyl (bipy) moiety in [Ru(bipy)3]2+ or
the electron donating behavior of the porphyrin in heme or
other metalloporphyrin systems6 are prominent examples.
Clearly, for some reactions or electrochemical applications,
the transfer of more than one electron onto or from a redox-
active ligand would be desirable. For this purpose, we have
initiated investigations into metal complexes bound to ligands
capable of multiple oxidations or reductions. One potentially
attractive class of such multiply redox-active ligands would be
organic (oligo)phenylenediamines of the form (RR′N)2(μ-
C6H4)n, where R = R′ = alkyl, n = 1−7 or R = R′ = aryl, n = 1−
3. These organic compounds are well-known electron donors7
and in certain cases have been implemented in various
technological and molecular electronic applications.8 The 3°
amine derivatives undergo two successive one-electron
oxidations at potentials that depend on the nitrogen’s R, R′
We have been examining the chemical and electrochemical
properties of metal complexes of pincer-type ligands that have
diarylamido anchors and pyrazolyl flanking donors.17−20 The
Received: January 8, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.8b00013
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Chart 1. Previously Studied Complex 0Me and the Three New Complexes in This Work19
cavity resonating at 9.63 GHz, an Oxford instruments ITC503
temperature controller, and a ESR-900 helium flow cryostat. The
spectra were recorded using 100 kHz field modulation unless
otherwise specified. Spectra were simulated by using the EasySpin
program.39
electron-rich diarylamido group can act as a single-electron
donor or as a Brønsted base. Further, the ligand’s arylpyrazolyl
arms confer structural flexibility to the metal complexes and can
accommodate either fac- or mer- conformations,18,19 or can
even dissociate to open a reactive site on the metal.20 We
previously reported that fac-Re(CO)3(L0Me), 0Me (left, Chart 1,
where X = Me)19b, and related derivatives19a showed a quasi-
reversible one-electron oxidation near 0 V vs Fc/Fc+, a result
that was intriguing because neither the free ligand nor the metal
Precursors and Ligands. H(L0Ph). A solution of 30 mL of C6H6 and
10 mL of absolute ethanol that was purged with argon 15 min was
transferred via cannula to a mixture of 0.797 g (2.022 mmol) of
H(L0Br), 0.370 g (3.03 mmol) of PhB(OH)2, and 0.234 g (0.202
mmol) of Pd(PPh3)4 under argon. Next, 10 mL of an argon-purged 2
M Na2CO3 aqueous solution was added via cannula transfer. After the
magnetically stirred biphasic mixture had been heated at 80 °C for 16
h under argon, the mixture was cooled to room temperature and was
poured into 100 mL of H2O. The aqueous and organic fractions were
separated. The aqueous layer was extracted with two 50 mL portions
of ethyl acetate. The combined organic layers were dried over MgSO4
and filtered. After removing solvents under vacuum, the oily residue
was subjected to column chromatography on silica gel using a 20 vol %
solution of ethyl acetate in hexanes as an eluent. Solvent was removed
from the second band (Rf = 0.45) to give 0.630 g (80% yield) of
H(L0Ph) as a colorless solid after drying under vacuum 1 h. Mp, 81−
+
precursor (or (solvent)3Re(CO)3 fragment) alone exhibits
reversible oxidation behavior (especially at such a mild
potential). We have now successfully prepared the dinucleating
pincer complexes (middle and right, Chart 1) and herein report
on the effect that the para-arylenediamine spacer has on their
electronic properties and provide a comparison to those of their
monometallic counterparts, fac-Re(CO)3(L0X).
EXPERIMENTAL SECTION
■
Syntheses. General Considerations. Chemicals. Solvents for
syntheses, spectroscopic characterization, or electrochemical studies
were dried by conventional means and distilled under argon prior to
use. Solvents used in organic workup or chromatographic separations
were used as received from commercial sources. The compounds
H(L0Br),17a Pd(PPh3)4,21 and Re(CO)5Br22were prepared by literature
methods. All other chemicals were used as received from commercial
sources.
1
82 °C. H NMR (CDCl3): δH 7.77 (dd, J = 2.4, 0.05 Hz, 1H, H5pz),
7.75 (dd, J = 1.8, 0.5 Hz, 1H, H3pz), 7.73 (dd, J = 2.4, 0.5 Hz, 1H,
H5pz), 7.70 (dd, J = 1.8, 0.5 Hz, 1H, H3pz), 7.58−7.53 (m, 3H, Ar),
7.47−7.39 (m, 6H, Ar and NH), 7.31 (tt, J = 7.3, 1.9 Hz, 1H, Ar), 7.20
(d, J = 1.5 Hz, 1H, Ar), 7.09 (dd, J = 8.3, 1.9 Hz, 1H, Ar), 6.48 (t, J =
2.1 Hz, 1H, H4pz), 6.42 (t, J = 2.1 Hz, 1H, H4pz), 2.35 (s, 3H, CH3)
ppm. 13C NMR (CDCl3): δC 140.8, 140.6, 137.1, 133.6, 133.2, 131.7,
131.1, 130.2, 130.0, 129.7, 128.9, 127.1, 126.9, 126.6, 125.9, 123.8,
120.3, 120.4, 117.6, 106.84, 106.77, 20.7 ppm.
Instrumentation and Characterization. Melting point determi-
nations were made on samples contained in glass capillaries using an
Electrothermal 9100 apparatus and are uncorrected. Midwest
MicroLab, LLC (Indianapolis, IN), performed all elemental analyses.
1H and 13C NMR spectra were recorded on a Varian 400 MHz
spectrometer. Chemical shifts were referenced to solvent resonances at
δH 7.26 and δC 77.16 for CDCl3, δH 2.05 for acetone-d6, δH 2.50 and
δC 39.51 for DMSO-d6, and δH 2.75 and δC 29.76 for DMF-d7.
Electronic absorption (UV−vis/NIR) measurements were made on a
Cary 5000 instrument. Abbreviations for NMR and UV−vis: br
(broad), sh (shoulder), m (multiplet), ps (pseudo-), s (singlet), d
(doublet), t (triplet), q (quartet), p (pentet), sept (septet). Solution
magnetic moments were measured by the Evan’s method.23 IR spectra
were recorded for samples as KBr pellets in the 4000−500 cm−1 region
on a Nicolet Magna-IR 560 spectrometer. Electrochemical measure-
ments were collected under a nitrogen atmosphere for samples as 0.1
mM solutions in CH2Cl2 with either 0.1 M NBu4PF6 or 0.1 M
NBu4[B(C6F5)4] as the supporting electrolyte or in DMF with 0.1 M
NBu4PF6, as described in the main text. A three-electrode cell
composed of a Ag/AgCl electrode (separated from the reaction
medium with a semipermeable polymer membrane filter), a platinum
working electrode, and a glassy carbon counter electrode was used for
the voltammetric measurements. Data were collected at scan rates of
50, 100, 200, 300, 400, and 500 mV/s. With this set up, the ferrocene/
ferrocenium couple had an E1/2 value of +0.52 V in CH2Cl2 and +0.55
V in DMF at a scan rate of 200 mV/s, consistent with the literature
values.24 Solid state magnetic susceptibility measurements were
performed using a Johnson-Matthey MSB-MK1 instrument. Dia-
magnetic corrections were applied using tabulated values of Pascal’s
constants.25 EPR spectra were obtained as solutions ∼ 0.2 mM in
CH2Cl2 using a Bruker ELEXYS E600 equipped with an ER4116DM
N,N′-Di(p-tolyl)-4,4′-benzidine, IA. Under argon, 40 mL of dry
deoxygenated toluene was added via cannula transfer to a mixture of
4.00 g (9.84 mmol) of 4,4′-diiodobiphenyl, 2.64 g (24.7 mmol, 2.5
equiv) of p-toluidine, 2.84 g (29.6 mmol, 3 equiv) of NaOtBu, 0.6 mL
(2 mol %) of PtBu3, and 0.181 g (0.20 mmol, 2 mol %) of Pd2(dba)3.
After the reaction mixture had been heated at reflux 15 h under argon,
solvents were removed by vacuum distillation. The resulting solid was
washed with 150 mL of deionized water, three 50 mL portions of
Et2O, and then was vacuum-dried to afford 3.53 g of impure IA as a
gray solid. The solid was added to 300 mL of ethyl acetate, and the
mixture was heated at reflux 5 min, filtered hot, allowed to cool to
room temperature 1 h, then placed in a −20 °C freezer overnight. The
thin plate-like crystals were collected by filtration, washed with Et2O,
and dried under vacuum. Additional crops of crystals were obtained by
reducing the mother liquor by rotary evaporation to 50% volume and
collecting the crystals after cooling, as above. A total of 2.65 g (74%
yield) of IA was obtained as a lustrous ivory-colored solid. Solubility:
ca. 0.014 M in ethyl acetate. Mp, 232−233 °C. 1H NMR (DMSO-d6):
δH 8.07 (s, 2H, NH), 7.45 (d, J = 8.7 Hz, 4H), 7.06 (d, J = 8.8 Hz,
4H), 7.05 (d, J = 8.8 Hz, 4H), 7.00 (d, J = 8.7 Hz, 4H), 2.23 (s, 6H,
CH3) ppm. 13C NMR (DMSO-d6): δC 142.6, 140.7, 131.0, 129.6,
128.6, 126.4, 117.5, 116.3, 20.3 ppm.
N,N′-Di(2-bromo-p-tolyl)-2,2′-dibromo-4,4′-benzidine, IB. A sol-
ution of 0.59 mL (11 mmol, 4 equiv) of Br2 in 20 mL of DMF was
added dropwise to a cold (0 °C) solution of 1.04 g (2.84 mmol) of IA
in 10 mL of DMF. After complete addition, the mixture was stirred at
0 °C for 1 h, the cold bath was removed, and the mixture was stirred
an additional 4 h. Then, the supernatant of 10 mL of a saturated
aqueous Na2S2O3 solution was added. The resulting biphasic mixture
B
DOI: 10.1021/acs.organomet.8b00013
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Article
was poured into 100 mL of dilute NaHCO3, and the pale brown
precipitate was collected by gravity filtration. The pale brown solid was
washed with 100 mL of deionized water and was vacuum-dried to
H, 3.05 (3.11); N, 10.60 (10.87). IR (KBr): νCO 2017, 1911, 1874. 1H
NMR (DMF-d7): δH: 8.77 (d, J = 2.3 Hz, 1H, pz), 8.65 (d, J = 1.9 Hz,
1H, pz), 8.64−6.6 (m, 2H, pz), 8.00 (d, J = 6.7 Hz, 1H, Ar), 7.98 (d, J
= 6.9 Hz, 1H, Ar), 7.94 (d, J = 2.0 Hz, 1H, Ar), 7.80 (d, J = 7.5 Hz,
2H, Ar), 7.65 (dd, J = 8.9, 2.0 Hz, 2H, Ar), 7.49−7.42 (m, 3H, Ar),
7.30 (t, J = 7.3 Hz, 2H, Ar), 6.82 (t, J = 2.4 Hz, 1H, pz), 6.76 (t, J = 2.4
Hz, 1H, pz), 2.38 (s, 3H) ppm. 1H NMR (acetone-d6): δH: 8.50 (d, J =
2.7 Hz, 1H, pz), 8.42 (d, J = 2.2 Hz, 1H, pz), 8.37(d, J = 2.5 Hz, 2H,
pz), 7.96 (d, J = 4.8 Hz, 1H, Ar), 7.94 (d, J = 5.3 Hz, 1H, Ar), 7.78 (d,
J = 2.2 Hz, 1H, Ar), 7.71 (br d, J = 7.4 Hz, 2H), 7.57 (dd, J = 8.8, 2.3
Hz, 1H, Ar), 7.44−7.38 (br t, 2H), 7.35 (d, J = 1.4 Hz, 1H, Ar), 7.29−
7.24 (m, 2H, Ar), 6.72 (t, J = 2.5 Hz, 1H, pz), 6.52 (t, J = 2.5 Hz, 1H,
pz), 2.37 (s, 3H) ppm. 13C NMR (DMF-d7): δC: 197.3, 195.9, 195.8,
147.8, 146.2, 144.6, 144.1, 139.9, 132.8, 132.2, 132.1, 129.8, 129.2,
129.1, 128.9, 128.7, 126.7, 126.1, 125.9, 124.8, 121.5, 121.3, 116.6,
108.43, 108.42, 20.0 ppm. 13C NMR (acetone-d6): δC: 197.5, 196.4,
196.1, 148.4, 147.0, 144.6, 144.1, 140.7, 132.8, 132.7, 132.2, 130.3,
129.8, 129.6, 129.5, 129.4, 127.1, 126.7, 126.5, 125.1, 122.3, 121.8,
117.2, 108.8, 108.7, 20.5 ppm. UV−vis λmax, nm (ε, M−1 cm−1), DMF:
375 (26 500), 268 (31 000). Crystals suitable for single-crystal X-ray
diffraction were grown by layering a CH2Cl2 solution with hexane and
allowing solvents to diffuse 1 d.
1
leave 1.76 g (91% yield) of IB as a beige solid. Mp, 198−199 °C. H
NMR (CDCl3): δH 7.74 (d, J = 2.1 Hz, 2H), 7.44 (br s, 2H), 7.35 (dd,
J = 8.5, 2.1 Hz, 2H), 7.25 (d, J = 8.5 Hz, 2H), 7.20 (d, J = 8.5 Hz, 2H),
7.07 (dd, J = 8.1, 1.5 Hz, 2H), 6.36(s, 2H, NH), 2.32 (s, 6H, CH3)
ppm. 13C NMR (CDCl3): δC 139.96, 137.21, 133.79, 133.59, 133.28,
131.00, 129.03, 126.29, 119.87, 116.75, 115.49, 113.70, 20.65 ppm.
N,N′-Di(2-pyrazol-1-yl-p-tolyl)-2,2′-di(pyrazol-1-yl)-4,4′-benzi-
dine, H2(L1). A 20 mL aliquot of dry, distilled, and argon-purged p-
xylenes was added by syringe to a mixture of 1.504 g (2.21 mmol) of
IB, 1.055 g (15.50 mmol, 7 equiv) of pyrazole, 2.14 g (15.5 mmol) of
K2CO3, and 0.50 mL (0.4 g, 4.58 mmol) of (N,N′-dimethylethylene-
diamine = DMED) previously placed under argon. Then, 0.200 g (1.05
mmol) of CuI was added under an argon blanket. After the resulting
mixture had been heated at reflux 2 d under argon, the mixture was
cooled to room temperature and solvent was removed by vacuum
distillation. The solid residue was then dissolved in a biphasic mixture
of 50 mL of ethyl acetate and 50 mL of H2O. The aqueous and organic
layers were separated. The aqueous layer was extracted with three 25
mL portions of ethyl acetate. The combined organic layers were dried
over MgSO4 and filtered. Volatiles were removed under vacuum to
give a dark brown oil that was subjected to flash chromatography on
silica gel. First, elution with hexanes removed residual (trace) xylene.
Then, 3:1 hexane:ethyl acetate was used to remove unreacted and
partially reacted starting materials. Finally, 1:1 hexanes:ethyl acetate
was used to elute the desired band (Rf = 0.53) which, after removing
solvent under vacuum, gave 0.830 g (63%) of H2(L1) as a pale brown
(0Ph)(BF4). A 20 mL aliquot of dry, argon-purged CH2Cl2 was added
by syringe to a mixture of 0.0343 g (0.052 mmol) of 0Ph and 0.0061 g
(0.052 mmol) of NOBF4 under argon. After the resulting purple
solution had been stirred 2 h, solvent was removed by vacuum. The
purple solid was washed with 5 mL of Et2O and dried under vacuum
to give 0.030 g (77%) of (0Ph)(BF4) as a purple solid. Mp, > 350 °C
dec μeff (solid, 295 K) = 1.7 μB. IR (KBr): νCO 2033, 1906 cm−1. UV−
vis (CH2Cl2): nm (ε, M−1 cm−1) 773 (1800), 627 (1200), 395 (5340).
[Re(CO)3]2(μ-L1), 1. A mixture of 0.127 g (0.20 mmol) of H2(L1)
and 0.164 g (0.40 mmol) of Re(CO)5Br in 10 mL of toluene was
heated at reflux under argon 10 min. Then, 0.28 mL of a solution 1.47
M NEt4OH in MeOH (0.40 mmol) was added by syringe (while hot
and under argon); a bright yellow solution formed immediately. After
a few minutes, a yellow precipitate began forming. The mixture was
heated an additional 12 h, then was cooled to room temperature. The
yellow solid was collected by filtration, was washed with two 15 mL
portions of MeOH, one 15 mL portion of Et2O, and was dried under
vacuum 1 h. Yield: 0.211 g (89%). Mp, 260−261 °C (Dec.). Anal.
Calcd (Found) for Re2C44N10H30O6: C, 45.28 (45.54); H, 2.59 (2.78);
1
solid. Mp, 85−86 °C. H NMR (CDCl3): δH 7.74 (m, 8H), 7.50 (s,
2H), 7.40 (s, 4H), 7.39 (d, J = 6.6 Hz, 2H), 7.20 (s, 2H), 7.09 (d, J =
8.1 Hz, 2H), 6.47, (t, J = 1.9 Hz, 2H, pz), 6.42 (t, J = 1.9 Hz, 2H, pz),
1
2.35 (s, 6H, CH3) ppm. H NMR (acetone): δH 9.27 (s, 2H, NH),
8.14 (dd, J = 2.5, 0.4 Hz, 2H, pz), 7.99 (dd, J = 2.4, 0.4 Hz, 2H, pz),
7.79 (d, J = 1.9 Hz, Ar), 7.75 (d, J = 1.8 Hz, Ar), 7.73 (d, J = 2.3 Hz,
pz), 7.59 (dd, J = 8.6, 2.3 Hz, 2H, Ar), 7.43 (d, J = 8.6 Hz, 2H, Ar),
7.41 (dd, J = 8.3, 1.9 Hz, 2H, Ar), 6.51 (t, J = 2.1 Hz, 2H, pz), 6.47 (t, J
= 2.1 Hz, 2H, pz), 2.33 (s, 6H, CH3) ppm. 13C NMR (CDCl3): δc
140.85, 140.66, 136.77, 133.70, 132.08, 131.64, 131.06, 130.22, 130.00,
128.95, 127.38, 126.27, 125.87, 123.19, 120.18, 117.88, 106.89, 106.80,
20.75 ppm.
1
N, 12.00 (11.84). IR (KBr): νCO 2013, 1905, 1882 cm−1. H NMR
H2(L2). An argon-purged (15 min) mixture of 35 mL of benzene
and 12 mL of ethanol was added via cannula to a mixture of 1.217 g
(3.09 mmol, 2.5 equiv) of H(L0Br), 0.205 g (1.23 mmol) of 1,4-
phenylenediboronic acid, and 0.028 g (0.25 mmol, 20 mol %) of
Pd(PPh3)4 previously placed under argon. Then 15 mL of an argon-
purged 2 M Na2CO3 aqueous solution was added by cannula and the
mixture was heated at 80 °C for 15 h. Then, the organic layer was
separated and the aqueous layer was washed with two 20 mL aliquots
of ethyl acetate. The combined organic fractions were dried with
anhydrous MgSO4, filtered, and evaporated to leave a brown solid. The
solid was washed with two 20 mL portions of ethanol, 20 mL of Et2O,
and was dried under vacuum to give 0.757 g (87% yield) of H2(L2) as
(DMF-d7): δH 8.74 (d, J = 2.7 Hz, 2H, pz), 8.64 (d, J = 2.4 Hz, 2H,pz),
8.62 (d, J = 2.26 Hz, 2H, pz), 8.61 (d, J = 2.6 Hz, 2H,pz), 7.98 (d, J =
8.3 Hz, 2H, Ar), 7.96 (d, J = 8.8 Hz, 2H, Ar), 7.74 (dd, J = 8.8, 2.1 Hz,
2H, Ar), 7.45 (s, 2H, Ar), 7.28 (dd, J = 8.3, 1.7 Hz, 2H, Ar), 7.21 (s,
2H, Ar), 6.82 (t, J = 2.5 Hz, pz), 6.77 (t, J = 2.4 Hz, 2H, pz), 2.37 (6H,
CH3) ppm. 13C NMR (DMSO-d6): δc 196.66, 195.55, 195.20, 146.02,
145.31, 144.16, 143.73, 132.48, 131.87, 130.81, 128.72, 128.50, 127.52,
124.72, 124.21, 120.26, 119.91, 116.40, 108.11, 108.06, 20.06. UV−vis
λmax, nm (ε, M−1 cm−1), CH2Cl2: 402 (22 400), 238 (36 350). Single
crystals of 1·1.59DMF·0.41Et2O suitable for X-ray diffraction studies
were grown by layering a 50% Et2O/MeOH mixture on a DMF
solution and allowing solvents to diffuse 2 d.
1
a pale yellow solid. Mp, 235−236 °C. H NMR (CDCl3): δH 8.26 (s,
1(BF4)2. In a manner like that described for (0Ph)(BF4) above, a
mixture of 0.0353 g (0.030 mmol) of 2 and 0.0071 g (0.061 mmol) of
NOBF4 yielded 0.0341 g (84%) of 1(BF4)2 as a blue/black solid. Mp,
> 350 °C dec μeff (solid, 295 K) = 2.9 μB. IR (KBr): νCO 2028, 1936,
1914 cm−1. UV−vis (CH2Cl2): cm−1 (ε, M−1 cm−1) 654 (45 600), 552
(31 000), 227 (231 000).
1(BF4). A mixture of 0.0232 g (19.9 μmol) of 1 and 0.0267 g (19.9
μmol) of 1(BF4)2 in 20 mL of CH2Cl2 was stirred 30 min at room
temperature; then solvent was removed to give 0.0361 g (72% yeld) of
1(BF4) as a red solid. μeff (Evan’s, CD2Cl2, 295 K) = 1.6 μB. IR (KBr):
νCO 2011, 1900 cm−1. UV−vis (CH2Cl2): cm−1 (ε, M−1 cm−1) 739
(53 500), 660 (4400), 535 (42 800).
2H, NH), 7.78 (d, J = 2.4 Hz, 2H), 7.76 (d, J = 1.9 Hz, 2H), 7.73 (d, J
= 2.4 Hz, 2H), 7.70 (d, J = 1.8 Hz, 2H), 7.60 (s, 4H, Ar), 7.59 (d, J =
2.1 Hz, 2H), 7.48 (dd, J = 8.6, 2.1 Hz, 2H), 7.42 (dd, J = 8.6, 1.9 Hz,
4H), 7.20 (d, J = 1.8 Hz, 2H), 7.10 (dd, J = 8.2, 1.7 Hz, 2H), 6.48 (t, J
= 2.4 Hz, 2H), 6.42 (t, J = 2.3 Hz, 2H), 2.35 (s, 6H) ppm. 13C NMR
(CDCl3): δC 140.9, 140.7, 138.6, 137.3, 133.6, 132.6, 131.8, 131.3,
130.3, 130.1, 129.9, 129.0, 127.0, 126.8, 126.0, 123.7, 120.5, 117.7,
107.0, 106.9, 20.8 ppm.
Metal Complexes. Re(CO)3(Me,Ph), 0Ph. A mixture of 0.201 g
(0.514 mmol) of H(L0Ph), 0.209 g (0.514 mmol) of Re(CO)5Br, 0.35
mL of 1.47 M (solution in MeOH, 0.514 mmol) NEt4OH and 15 mL
of toluene was heated at reflux 12 h under argon. Solvents were
removed by vacuum distillation. The yellow residue was washed with
two 10 mL portions of methanol and then was dried under vacuum 2 h
to give 0.252 g (74%) of 0Ph as a yellow powder. Mp, 251−252 °C
(Dec.). Anal. Calcd (Found) for Re2C50N10H34O6: C, 50.90 (51.26);
[Re(CO)3]2(μ-L2), 2. By using the method described for 1 above, a
mixture of 0.115 g (0.163 mmol) of H2(L2), 0.132 g (0.33 mmol, 2
equiv) of Re(CO)5Br, 0.22 mL of 1.47 M (solution in MeOH, 0.33
mmol) NEt4OH, and 10 mL of toluene gave 0.121 g (60%) of 2 as a
C
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a
yellow powder. Mp, 330 °C (Dec.). Anal. Calcd (Found) for
Scheme 2. Preparation of H2(L2)
C50H34N10O6Re2: C, 48.30 (48.66); H, 2.76 (2.95); N, 11.27
1
(11.10). IR (KBr): νCO 2013, 1901, 1876 cm−1. H NMR (DMF-
d7): δH 8.94 (dd, J = 2.7, 0.7 Hz, 2H, pz), 8.82 (dd, J = 2.4, 0.7 Hz, 2H,
pz), 8.79 (br d, J = 2.6 Hz, 4H, pz), 8.16 (d, J = 6.2 Hz, 2H, Ar), 8.15
(s, 2H, Ar), 8.14 (d, J = 5.5 Hz, 2H, Ar), 8.04 (s, 4H, Ar), 7.87 (dd, J =
8.9, 2.2 Hz, 2H, Ar), 7.62 (d, J = 1.9 Hz, 2H, Ar), 7.48 (dd, J = 8.5, 1.8
Hz, 2H, Ar), 6.99 (t, J = 2.5 Hz, 2H, pz), 6.92 (t, J = 2.5 Hz, 2H,pz),
2.55 (s, 6H) ppm. 13C NMR (DMSO-d6): δc 196.85, 195.37, 195.18,
146.77, 145.25, 144.19, 143.73, 136.96, 132.58, 131.94, 131.27, 129.14,
128.50, 128.42, 127.42, 126.05, 125.26, 124.40, 120.94, 120.46, 116.01,
108.25, 108.20, 20.19. UV−vis λmax, nm (ε, M−1 cm−1), CH2Cl2: 403
(8300), 227 (10 300). Attempts to obtain crystals suitable for single-
crystal X-ray diffraction by a number of conditions only produced
microcrystalline solids.
a
Key: (i) Pd(PPh3)4, C6H6, EtOH, Na2CO3(aq).
diiodoarene with toluidine. Subsequent bromination afforded
the sparingly soluble tetrabromo derivative which was then
subjected to CuI-catalyzed amination with pyrazole to give
H2(L1). Attempts to produce H2(L1) by Ni(0)-catalyzed
homocoupling of H(L0Br)17a (i.e., the compound on the left of
Scheme 2) was unsuccessful; only starting materials were
recovered. H2(L2) was obtained in high yield by the Suzuki
coupling reaction between p-benzenediboronic acid and 2 equiv
of H(L0Br) (Scheme 2). The new mononucleating pincer ligand
H(L0Ph) was prepared similarly by the Suzuki coupling method
but used an equimolar ratio of H(L0Br) and PhB(OH)2.
The tricarbonylrhenium(I) complexes were prepared by
heating a mixture of ligand, Re(CO)5Br, and (NEt4)(OH) (1
Re and 1 OH− per N−H bond on a given ligand) in toluene at
reflux 12 h. The bimetallic complexes precipitate as bright
yellow solids that can be purified by filtration and drying under
vacuum. On the other hand, the monometallic complexes are
appreciably soluble in toluene. So, solvent is removed by
vacuum distillation and the yellow residue was washed with
methanol to remove the byproduct (NEt4)(Br), and the
analytically pure yellow solids 0x (x = Me, Ph) are obtained
after drying under vacuum. The IR spectra for solid samples
(ATR) or for KBr pellets of each 0Ph, 1, and 2 show CO
stretching bands characteristic of the fac-Re(CO)3 moiety
similar to that reported for 0Me.19 That is, each shows three
bands (an A″ and two A′ modes) for the local Cs symmetric
Re(CO)3NAr(Npz)2 unit. Somewhat surprisingly, given the
different electronic properties of the complexes discussed later,
the substitution pattern of the pincer ligand has little impact on
the CO stretching frequency; the average CO stretching
frequencies are statistically equivalent ranging between 1930(2)
and 1934(2) cm−1.
2(BF4)2. By using the method described for (0Ph)(BF4), a mixture of
0.0345 g (0.028 mmol) of 2 and 0.0065 g (0.056 mmol) of
(NO)(BF4) yielded 0.0329 g (84%) of 2(BF4)2 as a black solid. Mp, >
350 °C dec μeff (solid, 295 K) = 2.7 μB. IR (KBr): νCO 2034, 1938,
1914 cm−1. UV−vis (CH2Cl2): cm−1 (ε, M−1 cm−1) 817 (149 000),
227 (54 700).
Crystallography. X-ray intensity data from a yellow plate of 0Ph
and a light-yellow prism of 1·1.59DMF·0.41Et2O were collected at
100.0(1) K with an Oxford Diffraction Ltd. Supernova diffractometer
equipped with a 135 mm Atlas CCD detector using Mo(Kα) radiation.
Raw data frame integration and Lp corrections were performed with
CrysAlis Pro (Oxford Diffraction, Ltd.).26 Final unit cell parameters
were determined by least-squares refinement of 13 376 and 25 309
reflections of 0Ph and 1·1.59DMF·0.41Et2O, respectively, with I >
2σ(I) for each. Analysis of the data showed negligible crystal decay
during collection in each case. Direct methods structure solutions,
difference Fourier calculations, and full-matrix least-squares refine-
ments against F2 were performed with SHELXTL.27 Numerical
absorption corrections based on Gaussian integration over a
multifaceted crystal model were applied to the data in each
experiment. All non-hydrogen atoms were refined with anisotropic
displacement parameters. Hydrogen atoms were placed in geometri-
cally idealized positions and included as riding atoms. The X-ray
crystallographic parameters and further details of data collection and
structure refinements are given in Table S1.
RESULTS AND DISCUSSION
■
Syntheses. Schemes 1 and 2 outline the preparative routes
to the new ligands, H2(L1) and H2(L2), respectively. H2(L1) is
synthesized in a three-step sequence starting from 4,4-
diiodobiphenyl (Scheme 1). Thus, N,N′-di-p-tolyl-p-benzidene
was prepared in high yield by a Pd-catalyzed amination of the
Structures. The fac-binding mode was confirmed by single-
crystal X-ray diffraction studies of 0Ph and 1·1.59DMF·
0.41Et2O, whose structures are given in Figures 1 and 2,
respectively. Unfortunately, suitable crystals of 2 could not be
grown despite exhaustive effort. The metrics of the ReN3C3
moieties in each complex (Table 1) are nearly identical to those
a
Scheme 1. Preparation of H2(L1)
a
Key: (i) Pd2dba3, PtBu3, NaOtBu, toluene, reflux 15 h; (ii) 4 equiv
Br2, DMF, 0 °C 4 h; (ii) xs Hpz, K2CO3, N,N′-DMED, CuI, p-xylene,
reflux 2 d.
Figure 1. Structure of 0Ph with thermal ellipsoids drawn at 50%
probability and hydrogen atoms removed for clarity.
D
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with expectations based on solid state structures, or idealized
models of the complexes as drawn in Chart 1. That is, there are
two sets of resonances for pyrazolyl ring hydrogens for the
inequivalent heterocycles as well as the predicted number of
resonances for aryl and methyl hydrogens. Moreover, the 13C
NMR spectrum of each compound shows three resonances for
the symmetrically inequivalent CO groups.
Theoretical Calculations. Before discussion of the
experimental electronic properties, it will be useful to describe
qualitative aspects of the molecular orbital diagrams found from
DFT calculations; full details regarding theoretical calculations
are given in the Supporting Information. Figures 3 and S7
display the frontier orbitals of 0Ph, obtained from calculations at
the M06/Def2-SV(P) level of theory. For this low symmetry
species, a coordinate system is adopted for convenience such as
to have Re as the origin and the z-axis to be coincident with the
Re−C bond that is trans- to the diarylamido nitrogen, NAr. The
x-axis is then arbitrarily assigned coincident with the Re−C
bond that is trans- to the nitrogen of the pyrazolyl bound to the
tolyl group, NpzMe, and the y-axis is aligned with the final Re−C
bond trans- to the nitrogen of the pyrazolyl bound to the
biphenyl, NpzPh. In this way, the dyz orbital participates in π
interactions with the diarylamido nitrogen’s py orbital, and with
two carbonyl π* orbitals, one trans- to NAr and the other trans-
to NpzPh. The dxz orbital participates in π interactions with π*
orbitals of carbonyl groups trans- to NAr and to NpzMe. Likewise,
the dxy orbital has π interactions with π* orbitals of carbonyl
groups trans- to the two rhenium-bound pyrazolyl nitrogens,
NpzMe and NpzPh. The HOMO (second from left, Figure 3) is
mainly a πL orbital29 centered on the toluidinyl arm of the
diarylamido fragment that is weakly mixed (in an antibonding
fashion) with orbitals of the Re(dyz)−CO π interaction. The
HOMO(-N) (N = 1−3) (N = 1 is shown to left of Figure 3,
others in Figure S7) are mainly metal-centered orbitals with
dπ−pπ bonding interactions involving the Re-CO fragments.
The corresponding antibonding dπ−pπ interactions (associated
with the Re-CO fragments) are found in LUMO(+N) (N =
11−13) (Figure S7). The LUMO and LUMO(+1) are π*
orbitals on the pincer scaffold (near and far right, Figure 3),
while LUMO(+N) (N = 3, 4) are mainly π* orbitals involving
Figure 2. Structure of 1·1.59DMF·0.41Et2O with solvate molecules
and hydrogen atoms omitted for clarity. Thermal ellipsoids are drawn
at 50% probability.
reported earlier for 0Me. The average Re−N distance of
2.161(2) Å in 0Me,19b 2.164(3) Å in 0Ph, and 2.158(3) Å in 1
are the same, but the average Re−C distance of 1.928(3) Å in
0Me is slightly longer than 1.917(3) Å in 0Ph or 1.921(3) Å in 1.
The C−O bond distances are rather unremarkable, averaging
1.15 Å, in line with other rhenium(I) carbonyls.19 The amido
nitrogen(s) of each 0x and 1 are similar, being nearly planar
with a sum of angles about N1 (or N2) of 356° (but
asymmetric such as to be an acute triclinic pyramid derived
from an 8% folded trigonal plane28). Also, the six-membered
ReN3C2 chelate rings are not planar but have twist-boat
conformations such that there is a 40(3)° dihedral angle
between attached pz and aryl rings and with the rhenium atom
folded along the NAr···Npz hinge (e.g., N1···N11 or N1···N12 in
0Ph). The dihedral angles between rings in the biphenyl
moieties in 0Ph (19°) and 1 (20°) are similar. For the latter, the
rhenium centers are located on opposite sides of the mean
plane of the connecting biphenyl unit. The monometallic
complexes 0Me and 0Ph are soluble in many organic solvents
except MeOH, Et2O, and light alkanes. On the other hand, the
bimetallic complexes 1 and 2 are freely soluble in dmso or
DMF, but are only slightly soluble in CH2Cl2, and are insoluble
in most other organic solvents. Thus, comparisons of solution
properties between mono- and bimetallic complexes were
limited to either DMF or (dilute) CH2Cl2 solutions. The
1
2
2
2
solution H NMR spectra of the new complexes are in accord
the carbonyl groups. The σ* interactions involving dx −y and dz
Table 1. Selected Bond Distances (Å) and Angles (deg) for 0Ph and 1·1.59DMF·0.41Et2O
0Ph
1·1.59DMF·0.41Et2O
bond distances (Å)
Re1−N1
Re1−N11
Re1−N21
Re1−C51
Re1−C52
Re1−C53
C51−O1
C52−O2
2.157(3)
2.178(3)
2.158(3)
1.917(4)
1.913(4)
1.922(4)
1.150(4)
1.165(4)
1.147(4)
Re1−N1
Re1−N11
Re1−N21
Re1−C14
Re1−C15
Re1−C16
C14−O1
C15−O2
C16−O3
2.160(2)
Re2−N2
Re2−N41
Re2−N51
Re2−C44
Re2−C45
Re2−C46
C44−O4
C45−O5
C46−O6
2.162(2)
2.170(2)
2.143(2)
1.917(3)
1.927(3)
1.920(3)
1.154(4)
1.147(4)
1.150(4)
2.152(2)
2.163(2)
1.917(3)
1.923(3)
1.923(3)
1.151(4)
1.155(4)
1.154(4)
C53−O3
bond angles (deg)
N1−Re1−N11
N1−Re1−N21
N21−Re1−N11
C52−Re1−C51
C51−Re1−C53
C52−Re1−C53
79.07(10)
81.26(10)
85.06(10)
89.39(15)
87.77(14)
88.24(15)
N1−Re1−N21
N11−Re1−N1
N11−Re1−N21
C14−Re1−C15
C14−Re1−C16
C16−Re1−C15
79.20(9)
80.87(9)
84.48(9)
89.80(14)
90.68(13)
89.01(12)
N2−Re2−N41
N51−Re2−N2
N51−Re2−N41
C44−Re2−C45
C44−Re2−C46
C46−Re2−C45
79.14(9)
81.38(9)
83.52(9)
90.16(13)
90.15(13)
88.49(13)
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Figure 3. Frontier orbitals of 0Ph from DFT calculations (M06/Def2-SV(P)).
orbitals are much higher energy, found in LUMO (+N = 22 and
27).
Selected frontier orbitals for 1 and 2 are depicted in Figures
4, S8, and S9. The molecular orbitals of 1 and 2 are similar and
Figure 5. Electronic spectrum of 1 (blue solid line) and 2 (violet
dashed line) in CH2Cl2. The nonzero long wavelength absorbance and
relatively low ε for all bands in the spectra of 2 are due to the low
solubility in this solvent (a suspension is formed).
Electrochemistry. The electrochemical properties of the
rhenium complexes are summarized in Table 2, and cyclic
voltammograms of CH2Cl2 solutions of the new complexes are
shown in Figure 6 (data for DMF solutions are found in Figure
S1 and Table S1). The complex 0Ph shows a single reversible
one-electron oxidation wave at 0.57 V vs Ag/AgCl which is
close to that at 0.55 V for 0Me, expectedly showing only a small
dependence on the nature of the hydrocarbon (Me vs Ph)
attached to the 4-aryl position of the ligand. Bimetallic complex
1 shows two reversible one-electron oxidation waves for 1+/1
and 12+/1+ couples. The first oxidation in 1 is 248 mV more
negative than that for 0Ph under the same conditions
(electrolyte) which reflects the electron-rich nature of the
benzidene linker. The separation between oxidation waves, ΔE,
in the voltammograms is typically used to measure the
equilibrium constant for comproportionation, Kcom (eq 1),
and can provide a qualitative measure of the strength of
electron
Figure 4. Frontier orbitals of 1 (left) and 2 (right) from DFT
calculations (M06/Def2-SV(P)).
can be qualitatively derived from linear combinations of orbitals
in 0Ph or 0Me. That is, the HOMO and HOMO(-1) in 1 and 2
are ligand-centered πL orbitals weakly mixed with bonding
combinations of Re dyz and CO π* orbitals. However, the
HOMOs in 1 and 2 are constructed from out-of-phase
combinations of HOMOs of monomer units, as illustrated by
examination of the phases of py orbitals on the diarylamido
nitrogens, NAr (second from bottom, Figure 4). The HOMO(-
1) of each 1 and 2 is the in-phase−phase combination of
monomer HOMOs (again seen by inspection of the phases of
NAr py orbitals, bottom Figure 4). Similar to 0Ph, the LUMO
and LUMO(+N) (N = 1−4) in both 1 and 2 are π* orbitals
centralized on the bridging ligand. Thus, the first oxidation of
each could be predicted to be heavily ligand-centered, while the
lowest energy absorption would have mainly π−π* character
mixed with, in order of decreasing contribution, n−π*, and
metal−ligand charge transfer (MLCT) character.
2+
2+
1 + 1 ⇄ 21+ Kcom = (1+)2/[(1)(1 )]
(1)
communication in a mixed valence species.30,31 However, the
value of ΔE and hence Kcom varies substantially depending on
the experimental setup. For 1, ΔE varies from 316 mV (Kcom
∼
105) in medium polarity solvent CH2Cl2 with a non-ion pairing
electrolyte NBu4[B(C6F5)4]32 to only 119 mV (Kcom ∼ 102) in
the highly polar solvent DMF that contains an electrolyte with
better ion pairing abilities NBu4PF6. The former setup with
larger Kcom suggests that the one-electron oxidized species 1+
would be near the Robin−Day33 Class II/Class III border (Kcom
≥ 106, Class III) for a mixed valence compound, while the
latter, in electrolyte-containing DMF, suggests 1+ to be a Class
II mixed valence compound. Similar observations have been
made for purely organic N,N,N,N-(tetraaryl)benzidene deriva-
tives, which have varying ΔE’s between 215 and 300 mV (Kcom
between 103 and 104) in electrolyte-containing CH2Cl2.34−37
Noteworthy, these organic (tetraaryl)benzidine cation radicals
are fully delocalized (Class III) in CH2Cl2 without electrolyte,
based on spectroscopic measurements and theoretical stud-
ies.34−37 For 2, the first oxidation potential is ca. 200 mV more
positive than in 1, showing that this diamine is less electron-rich
UV−vis. The electronic spectrum of each 0Ph, 1, and 2 in
either CH2Cl2 or DMF are similar to that reported earlier for
0Me in CH2Cl2 with sets of three overlapping medium to high
intensity (4000 < ε < 45 000 M−1 cm−1) bands in the near-UV
to blue range 330−450 nm that gives rise to the yellow color of
the complexes; see Figure 5. TD-DFT calculations (Tables S5−
S7) indicate these low energy bands arise due to HOMO to
LUMO(+N) (N = 0−4) transitions which may be described as
combinations of intraligand π−π*charge transfer (owing to
spatial separation of orbitals), n−π*, and MLCT in character,
as predicted above.
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Table 2. Electrochemical Data from Cyclic Voltammetry Experiments of Complexes in CH2Cl2 with Various Supporting
Electrolytes
a
b
b
c
complex
electrolyte
NBu4PF6
V vs Ag/AgCl , Eox1 (Esep, mV)
Eox2 (Esep, mV)
Kcom
0Me
0Ph
1
0.552 (138)
0.569 (148)
0.321 (147)
0.224 (69)
NBu4PF6
NBu4PF6
0.549 (140)
0.540 (73)
7.85 × 103
2.51 × 105
2.24 × 101
4.37 × 101
NBu4[B(C6F5)4]
NBu4PF6
d
d
e
2
0.519
0.598 (176)
NBu4[B(C6F5)4]
0.408 (89)
0.504 (93)
a
b
c
Average values of (Epa + Epc)/2 from scan rates of 50, 100, 200, 300, 400, and 500 mV/s. Peak potential separation (Epa − Epc). Kcom = e(ΔE·F/RT)
T = 295 K. From square wave voltammogram. Width of combined bands.
,
d
e
Figure 6. Left: Cyclic voltammograms of 0Ph, 1, and 2 in CH2Cl2 with NBu4PF6 as an electrolyte. Owing to low solubility of 2, the data were
multiplied by a factor of 6 to be on scale. Right: Cyclic (blue) and square wave (red) voltammograms of 1 and 2 in in CH2Cl2 with NBu4[B(C6F5)4]
as an electrolyte.
Figure 7. X-band EPR spectra (experimental, top black lines) of 2(BF4) in CH2Cl2 at 295 K (left) and 77 K (right). Instrumental parameters: 295 K:
Freq = 9.463 GHz; Power = 1.6 mW, modulation 7 G; 77 K: Freq = 9.434 GHz; Power = 5.056 mW, modulation 3 G. Simulated spectra (red,
bottom traces); see text.
than that in 1. Moreover, ΔE is only about 100 mV (Kcom
∼
distinctly and deeply colored: (12+)(BF4)2 is blue, and
(22+)(BF4)2 is green. These colors arise in part by characteristic
pi-radical bands (β-HOMO(-N) to SOMO (β-LUMO)
transitions) found in the visible region of the electronic
101) in CH2Cl2 with NBu4[B(C6F5)4], which reduces to ∼80
mV with NBu4PF6. In DMF, only one two-electron oxidation
wave is observed at a potential (0.63 V) only slightly negative of
those found for monometallic 0x (x = Me, 0.65 V; x = Ph, 0.71
V). Thus, according to cyclic voltammetry, 2+ would be
predicted to be a Robin−Day Class I mixed valence species in
electrolyte-containing CH2Cl2 and could not be formed in
DMF in the presence of electrolyte. Thus, it appears from the
current data and the literature of (tetraaryl)benzidine cation
radicals that the electrochemical data only give the lower limit
for electron communication in mixed valent species.
spectrum: λmax = 645 nm (ε 41 700 M−1 cm−1) for 12+, λmax
=
806 nm (ε 15 200 M−1 cm−1) for 22+. The IR spectra of the
dioxidized solids show a shift in the C−O stretching bands to
higher frequency versus those for the non-oxidized counter-
parts, reflecting a lower capacity for metal−CO back-bonding
in the former. The change in average energy of the C-O
stretching bands on oxidation, ΔνCO(avg), was 28 cm−1 on
traversing from 1 (1933 cm−1) to 12+ (1961 cm−1) and 33 cm−1
between 2 (1933 cm−1) and 22+ (1963 cm−1) which is
comparable to 29 cm−1 for 0Ph (1934 cm−1) to 0Ph+ (1963
cm−1). As a means of comparison, ΔνCO(avg) was ≈100 cm−1
Oxidation. To further investigate the electronic behavior of
1 and 2, the complexes were each subjected to chemical
oxidation. The reaction of each complex with 2 equiv of
(NO)(BF4) in CH2Cl2 gave (12+)(BF4)2 and (22+)(BF4)2,
respectively. It is noteworthy that, while the starting materials
have low solubility in CH2Cl2, the oxidized derivatives (both
singly and doubly oxidized) are quite soluble. The doubly
oxidized derivatives are paramagnetic in the solid at 295 K (μeff
2.8 0.1 μB) but are EPR silent both in solution at 295 K or as
frozen glasses at 77 K. Each doubly oxidized complex is also
+
on traversing CpRe(CO)3 to CpRe(CO)3 , a heavily rhenium-
centered oxidation.38
EPR. With the dioxidized derivatives in hand, comproportio-
nation reactions were used to study the properties of the singly
oxidized derivatives, 1+ and 2+. First, mixing equimolar
solutions of EPR silent 1 or 2 with their EPR silent doubly
oxidized counterparts (12+ or 22+) produced EPR-active
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Figure 8. Left: Spectrophotometric titration of 2 (bottom spectrum with black line) with 22+ (spectrum shown overlaid as a blue dashed line) to give
2+ (top spectrum shown in green). Inset: Absorbance at 2000 nm as a function of added 22+. Right: Deconvolution of low energy band into three
Gaussian curves (black lines); see also Figure S7. The experimental spectrum is in blue. Sum of Gaussian fits is the red dashed line.
solutions of 1+ or 2+. The left of Figure 7 shows the X-band
EPR spectrum of a CH2Cl2 solution of 2+ recorded at 295 K.
assigned as charge resonance for 1+ and as intervalence charge
transfer (IVCT) for 2+. The charge resonance assignment for
bands in 1+ is aligned with voluminous literature on Robin−
Day Class III radical cations of benzidene34−37 variants (like the
aforementioned N,N,N′N′-tetraarylbenzidene derivatives). The
assignment of an IVCT band for 2+ is based on the EPR data
that suggests Class II behavior. Such an assignment is bolstered
by extrapolation of the electrochemical data to conditions
without electrolyte and on results of time-dependent density
functional theory (TD-DFT) calculations that show overlap
between β-HOMO and β-LUMO and a highly solvent-
dependent very low energy CT transition. Hush analysis of
the NIR bands was used to extract information about electronic
coupling.40 The asymmetric, structured low-energy NIR bands
in each experimental spectrum could be fit by deconvolution
(right Figure 8 and Figure S5) as a sum of three Gaussian-
shaped curves of decreasing intensity separated by about 1300−
1600 cm−1 perhaps part of a vibronic progression (coupled with
CC/CN stretching modes)41 or due to conformational
effects42 (low energy rotamers for 1+ and 2+) or both. For 1+,
the electronic coupling matrix element was determined directly
from the experimental band maximum, HAB = νmax/2 = 3715
cm−1, a value in the range found for Robin−Day Class III
tetraarylbenzidene derivatives.7,10,34−36 The asymmetric IVCT
band in the spectra of 2+ was subject to Hush analyses using eqs
2 and 340 The transition dipole moment, μ12, was calculated by
summing over the three Gaussian curves used to fit the IVCT
band. The lowest
1
The eleven-line signal for the S = /2 species was adequately
simulated39 as axial with gx,y = 2.011 and gz = 1.998 and
hyperfine coupling of the electron to two rhenium nuclei with
ARe1 = 1.14 mT, ARe2 = 1.25 mT (I 185Re (37.4%),187Re
5
(62.6%) = /2), indicating full delocalization of the electron.
The 295 K spectrum of 1+ (Figure S2) is simulated as axial with
gx,y = 2.011, gz = 1.996, ARe1 = 1.16 mT, and ARe2 = 1.04 mT,
values nearly identical to those of 2+. Interestingly, the
spectrum of 2+ at 77 K (right of Figure 7) is an
inhomogeneously broadened six-line derivative signal (giso
=
2.012; ARe = 141 MHz) that is very similar to that of
monometallic (0Me)+ (at 10 K, giso = 2.015, ARe = 124 MHz)
reported previously19b or that of (0Ph)+ at 77 K (giso = 2.016,
ARe = 146 MHz, Figure S3b). An alternate EPR model for the
spectra of 2+ at 77 K of a rhombic signal with gx,y,z = 2.027,
1.998, 1.981 and hyperfine coupling of the electron to one
nitrogen (ANx,y,z = 4.53, 4.43, 4.08 mT) and one rhenium
(ARe,x,y,z = 0.40, 0.38, 0.84 mT), shown in the bottom right of
Figure 7, could be fit to the spectrum but is less satisfactory
since it gives nitrogen hyperfine coupling constants that are an
order of magnitude higher than those of pure organic
derivatives. Another possible model, with hyperfine coupling
of the electron to two nitrogen atoms (which required two
disparate AN’s and much larger anisotropic broadenings), was
even less satisfactory (higher root-mean-square deviations from
experimental data), while that with the electron coupled to two
rhenium centers was wholly unsatisfactory. Thus, it appears
most likely that the electron in 2+ is localized on one end of the
molecule at low temperature (i.e., is thermally valence-trapped).
Unfortunately, the low temperature spectrum of 1+ was not
suitably structured for successful modeling (Figure S2b).
NIR. The identities of 1+ and 2+ were further supported by
spectrophotometric titrations. That is, UV−vis/NIR spectra
were recorded after substoichiometric quantities of doubly
oxidized derivatives in CH2Cl2 were added to dilute solutions of
the appropriate non-oxidized derivatives 1 or to a suspension of
2 in CH2Cl2. As displayed for 2 in Figure 8 and for 1 in Figure
S4, the spectrum shows the appearance of distinctive new bands
for the singly oxidized species 2+ (or 1+) that stop growing in
intensity when 1 equiv of doubly oxidized species is added to
the solution of the appropriate charge-neutral compound. In
each case, new bands are found in the NIR region that are
μ (D) = 0.09584
ε(υ)∂υ/ν
∫
max
(
)
12
(2)
(3)
HAB (cm−1) = μ νmax/eR
12
energy component of the structured IVCT band from
deconvolution gave νmax of 4376 cm−1. In eq 3, μ12 was
converted to non-SI units 1D = 10−18 statC·cm. The
elementary charge constant, e, is 4.803 × 10−10 statC. Finally,
the effective charge separation distance, R, is in units of cm. The
values for R in diamine cation radicals have been the subject of
2
much scrutiny and values of
/
the nitrogen−nitrogen
3
separation has given reasonable agreement with other
experimental data.10b,43 For 2+, a value of R (2/3 N−N) =
9.47 × 10−8 cm (from geometry optimized structure) gave HAB
= 502 cm−1. As an estimated lower limit, the use of R (Re−Re)
H
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Article
Figure 9. Plots of spin density isosurfaces (magenta, 0.025 isovalue) for (0Ph)+ (top left), 1+ (bottom left), and 12+ (S = 1, bottom right) with spin
density values from Mulliken population analysis given in green.
= 16.43 × 10−8 cm gave HAB of 289 cm−1. In either case, the
HAB values are smaller than, but in line with, other Class II
mixed valence species such as HAB (2/3 N−N) = 705 cm−1 in
E,E-1,4-bis{4-[di(4-n-butoxyphenyl)amino]styryl]-2,5-dicyano-
benzene or HAB (2/3 N−N) = 735 cm−1 in 1,4-bis{4-[di(4-
methoxyphenyl)amino]phenylethynyl}benzene.35b
provided singly and doubly oxidized derivatives 1n+ and 2n+ (n
= 1, 2) that permitted spectroscopic characterization and
assignment of Robin−Day classes to the n = 1 cases. The
complex 1+ is a Robin−Day Class III mixed valence compound,
whereas complex 2+ is a Class II species that was thermally
valence trapped at 77 K by EPR spectroscopy. Thus, the
interjection of the additional benzene ring in 2+ versus 1+
profoundly dampens electronic coupling, reducing HAB by
nearly an order of magnitude. One could project that a p-
quarterphenylenediamine-linked pincer would be an electroni-
cally isolated, Robin−Day Class I species. Unfortunately,
solubility issues have hindered the preparation of such a
compound. Nonetheless, these results and prediction are on par
with the recent studies on dialkylamine-capped oligo-phenyl-
enes that show electronic saturation when three phenylenes
bridge two nitrogen centers, which is a result of the disparate
energy of nitrogens and the oligophenylene bridge.9 It may be
possible to increase the electronic communication in these di-
pincer systems by using more electron-deficient metals,
favoring electron delocalization away from the bridge or by
restricting rotation of the central arylene units, which are
avenues for future study.
Theoretical. The results of DFT calculations are in
qualitative agreement with experimental observations for the
(0Ph)n+, 1n+, 2n+ (n = 0, 1, or 2) series in that the redox events
are mainly ligand-centered but are increasingly delocalized onto
the rhenium centers with charge of the complex. Thus, the
electronic structure of (0Ph)+ is best described as an oxidized
ligand bound to rhenium(I) (0Ph·+)ReI(CO)3. Examination of
spin density isosurface shows a majority of the spin (82%) is
located on the ligand, in particular the amido nitrogen atom
and on the carbon atoms at the positions ortho- and para- to
the amido nitrogen on the tolyl ring (top of Figure 9).
Similarly, 1+ and 12+ (or 2+ and 22+) are best described as
mainly ligand-centered radicals of the formulations: (OC)3ReI-
(L·+)-ReI(CO)3 and (OC)3ReI-(L··2+)-ReI(CO)3, respectively.
Mulliken population analyses (M06/def2-SV(P)) shows that
the spin density of 1+ is weighted disparately to one side of the
complex, exemplified by different values for each atom of the
rhenium amido fragment (Re1 0.17, N1 0.38; Re 2 0.02, N2
0.05, Figure 9, left). For 12+ (S = 1), the spin density is more
evenly delocalized along the ligand fragment onto both
rhenium atoms (Re1 0.22, N1 0.42; Re2 0.25, N2 0.40, Figure
9, right). Similar observations are found for 2+ and 22+.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.8b00013.
CONCLUSIONS
■
Two bimetallic rhenium complexes of new dinucleating pincer
ligands have been prepared. Each of these new complexes
exhibits solvent- and electrolyte-dependent electrochemical
behavior. Complex 1, constructed from a benzidene core,
exhibits two reversible one-electron oxidations. Complex 2,
with a p-terphenylenediamine core, exhibits one two-electron
oxidation in DMF but has two one-electron oxidations in
CH2Cl2. The solvent- and electrolyte-dependent oxidation
potentials in both 1 and 2 further underscore the difficulties in
assigning Robin−Day classifications to mixed valent derivatives
(in this case, the one-electron oxidized species) using
electrochemical techniques. Chemical oxidations of 1 and 2
Electrochemical data, electronic absorption spectra, and
details of computational studies (PDF)
Molecular structures from DFT calculations (XYZ)
Accession Codes
CCDC 1590573 and 1590574 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
I
DOI: 10.1021/acs.organomet.8b00013
Organometallics XXXX, XXX, XXX−XXX

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Article
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AUTHOR INFORMATION
Corresponding Author
*E-mail: james.gardinier@marquette.edu.
■
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ORCID
James R. Gardinier: 0000-0001-7554-924X
Denan Wang: 0000-0003-3603-6788
Author Contributions
All authors have approved the final version of this manuscript.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
J.R.G. thanks Marquette University for support.
■
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