X-ray crystallography and electrochemistry reveal electronic and steric effects of phosphine and phosphite ligands in complexes RuII(κ4-bda)(PR3)2 and RuII(κ3-bda)(PR3)3 (bda?=?2,2′-bipyridine-6,6′-dicarboxylato)

Article abstract of DOI:10.1016/j.poly.2018.12.033

We have examined coordination of PR₃ = triphenylphosphine, triethylphosphine, triisopropyl phosphite, trimethyl phosphite, and 1,3,5-triaza-7-phosphaadamantane (PTA) to the fragment Ru^II(bda) to better understand how different phosphine and phosphite ligands influence the electronic and structural properties of the Ru^II complexes. PTA and P(OMe)₃ afforded complexes with three phosphorus ligands bound to Ru, with the bda being tridentate (κ³-N,N,O) in complexes 4 and 5; for the other three phosphorus ligands, even in the presence of >2 equiv, only Ru^II(κ⁴-bda)(PR₃)₂ species 1–3 were seen. Both experimental and computational methods were used to study the complexes. Steric effects are the main factor determining whether bis- or tris(PR₃) complexes are formed. Cyclic voltammetry studies of the complexes revealed an increase in Ru^III/II potential upon having another phosphorus ligand in the equatorial position. Computational studies predict that the additional phosphine ligand in the equatorial plane of 4 engages in significant orbital mixing with the ruthenium center that results in lower energy bonding as compared to the axial phosphine ligands. This work provides the first evaluation of phosphorus ligand steric and electronic effects on the Ru^II(bda) fragment.

Full text of DOI:10.1016/j.poly.2018.12.033

Accepted Manuscript
X-ray crystallography and electrochemistry reveal electronic and steric effects
II
4
of phosphine and phosphite ligands in complexes Ru (κ -bda)(PR ) and
3
2
II
3
Ru (▲κ -bda)(PR ) (bda = 2,2'-bipyridine-6,6'-dicarboxylato)
3
3
Sima Yazdani, Braden E. Silva, Thomas C. Cao, Arnold L. Rheingold, Douglas
B. Grotjahn
PII:
S0277-5387(18)30838-6
https://doi.org/10.1016/j.poly.2018.12.033
POLY 13650
DOI:
Reference:
To appear in:
Polyhedron
Received Date:
Accepted Date:
18 September 2018
16 December 2018
Please cite this article as: S. Yazdani, B.E. Silva, T.C. Cao, A.L. Rheingold, D.B. Grotjahn, X-ray crystallography
II
4
and electrochemistry reveal electronic and steric effects of phosphine and phosphite ligands in complexes Ru (κ -
II
3
bda)(PR ) and Ru (▲κ -bda)(PR ) (bda = 2,2'-bipyridine-6,6'-dicarboxylato), Polyhedron (2018), doi: https://
3
2
3 3
doi.org/10.1016/j.poly.2018.12.033
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X-ray crystallography and electrochemistry reveal electronic and steric effects of
II
4
II
3
phosphine and phosphite ligands in complexes Ru ( -bda)(PR ) and Ru ( -bda)(PR )
3
2
3 3
(bda = 2,2'-bipyridine-6,6'-dicarboxylato)
a
a
a
b
Sima Yazdani , Braden E. Silva , Thomas C. Cao , Arnold L. Rheingold , Douglas B.
a
’
Grotjahn *
aDepartment of Chemistry and Biochemistry, 5500 Campanile Drive, San Diego State
University, San Diego, CA 92182
bDepartment of Chemistry and Biochemistry, University of California, San Diego, La Jolla, CA
2093-0385
9
th
In honor of Bill Jones on his 65 birthday – let there be many more
Abstract
We have examined coordination of PR = triphenylphosphine, triethylphosphine, triisopropyl
3
phosphite, trimethyl phosphite, and 1,3,5-triaza-7-phosphaadamantane (PTA) to the fragment
II
Ru (bda) to better understand how different phosphine and phosphite ligands influence the
II
electronic and structural properties of the Ru complexes. PTA and P(OMe) afforded complexes
3
3
with three phosphorus ligands bound to Ru, with the bda being tridentate ( -N,N,O) in
complexes 4 and 5; for the other three phosphorus ligands, even in the presence of >2 equiv, only
II
4
Ru ( -bda)(PR ) species 1-3 were seen. Both experimental and computational methods were
3
2
used to study the complexes. Steric effects are the main factor determining whether bis- or
tris(PR ) complexes are formed. Cyclic voltammetry studies of the complexes revealed an
3
increase in RuIII/II potential upon having another phosphorus ligand in the equatorial position.
Computational studies predict that the additional phosphine ligand in the equatorial plane of 4
1

engages in significant orbital mixing with the ruthenium center that results in lower energy
bonding as compared to the axial phosphine ligands. This work provides the first evaluation of
II
phosphorus ligand steric and electronic effects on the Ru (bda) fragment.
Keywords: Ruthenium, phosphine and phosphite ligands, X-ray crystallography, DFT,
electrochemistry.
1
. Introduction
Phosphines and phosphites [P(OR) ] are commonly employed as ancillary ligands for
3
transition metal complexes because of their ability to stabilize low valent metal centers, as well
as their ability to fine-tune the stereoelectronic features of their transition metal complexes [1-2].
In recent literature, novel ruthenium complexes supported by the dianionic tetradentate bda
ligand (bda = 2,2’-bipyridine-6,6’-dicarboxylate) have been shown to be especially potent for
catalytic water oxidation. The chelating nature of the bda ligand along the equatorial plane
6
results in a distorted octahedral geometry in its d metal complexes, with the remaining two
coordination sites situated in a diaxial configuration [3-5]. Therefore, the primary route towards
modification of bda-supported complexes is substitution of the diaxial ligands L in Ru(bda)(L)2.
Interest in catalytic properties has inspired the synthesis and characterization of a vast array of
Ru(bda)(L) complexes with axial ligands L coordinated through N, S, and C atoms [3-5]. To our
2
knowledge, examples of Ru(bda)(L) where L = PR have yet to be reported.
2
3
II
In this work, we describe the synthesis and characterization of five Ru (bda)PR complexes
3
bearing phosphine or phosphite diaxial ligands (Figure 1, 1-5), and investigate the electronic and
steric effects of phosphorus ligands on the backbone bda ligand and structure of Ru complexes.
2

In addition, we attempt to rationalize the preference for phosphorus ligands to occupy a third
equatorial position (for instance PTA [2] and trimethyl phosphite) while others are restricted to
coordination of only two ligands in the axial positions.
Steric effects of phosphines were quantified by Tolman in 1977 based on the Corey-Pauling-
Koltun model of ligand-metal complexes, leading to the proposal of the well-known Tolman
cone angle (θ) [1]. Also, the electron donating ability of phosphorus-containing ligands (PR3)
was studied by Tolman’s electronic parameter (TEP), defined as the frequency of the A carbonyl
1
mode of (R P)Ni(CO) complexes [1]. The steric and electronic profiles of a phosphine, as
3
3
measured by θ and TEP, respectively, can impart a significant degree of control over the
outcome of transition-metal-mediated reactions. More recent examples of detailed studies on a
variety of phosphorus ligands are those of Suresh and co-workers [6, 7], who used a combined
approach of quantum and molecular mechanics to estimate and separate the steric effects of a
PR ligand from its electronic effects. By means of a stereoelectronic plot, one may select ligands
3
as part of designing an organometallic catalyst [2, 6]. The electronic effects of substituted
phosphines were quantified in terms of the molecular electrostatic potential minimum (V ) by
min
Suresh and Koga [6, 7]. In a recent review, Kühl had compared various methods used for
predicting the electronic effects of phosphine ligands and supported the use of (V ) as a
min
parameter for the quantification of electronic effects of phosphine ligands [8]. Therefore, in our
work we selected various phosphine and phosphite ligands to examine the electronic and steric
effects on Ru(bda)(PR ) complexes (Scheme 1).
3
n
3

N
N
N
N
N
i
PPh3
N
PEt3
P(O Pr)3
P
P(OMe)3
N
N
N
N
N
N
N
O
O
O
Ru
O
O
O
O
O
O
O
Ru
Ru
Ru
P
Ru
O
O
O
O
O
O
O
O
P
O
O
N
P(OMe)3
P(OMe)3
i
PPh3
PEt3
P(O Pr)3
N
N
N
N
N
1
2
3
4
5
Scheme 1. Complexes described in this work.
2
2
. Materials and Methods
.1. Physical measurements
1
31
H NMR and P NMR spectra were taken on Varian 500 MHz Inova or 400 MHz VNMRS
NMR spectrometers. Suitable single crystals for X-ray crystallography were grown using vapor
diffusion of chloroform into methanolic solutions of complexes 1, 2, 3, and 4. X-ray crystal
structure determinations of complexes were obtained with Bruker single-crystal diffractometers
with CCD detectors and low-temperature cryostats with hi-flux Cu and Mo radiation sources.
Cyclic voltammetry (CV) measurements were carried out on CH instruments CHI760E and
CHI600C potentiostats, with glassy carbon working electrode (diameter – 3 mm), and Pt
+
counter-electrode, with Ag /AgCl reference electrode.
2
.2.
Synthesis
4

Synthesis of 6,6’-dimethyl-2,2’-bipyridine. A mixture of Ni(PPh ) Br (12.95 g, 0.01743
3
2
2
mol), Bu NI (21.46 g, 0.05811 mL) and Zn dust (3.81 g, 0.0583 mol) in this order was added to a
4
flask which was then charged with THF (210 mL) and mixture was stirred under N atmosphere
2
at room temperature, after which 2-bromo-6-methylpyridine (10.0 g, 0.0581 mol) was added, and
the mixture was refluxed overnight. The solvents were removed by rotary evaporation, and the
1
crude product checked by HNMR. Diethyl ether (200 mL) and 10% NH OH (50 mL) were
4
added to the crude reaction and the mixture was stirred for 1 h, after which solids were removed
by filtration and purified by column chromatography on silica gel using ethyl acetate and hexane
yielding 6,6’-dimethyl-2,2’-bipyridine as a white solid (4.20 g, 60%).
Synthesis of 2,2’-bipyridine-6,6’-dicarboxylic acid [5]. The white solid 6,6’-dimethyl-2,2’-
bipyridine (4.00 g, 0.0217 mol) was added to concentrated sulfuric acid (80 mL) cooled by an ice
bath. Chromium trioxide (13.02 g, 0.1302 mol) was slowly added over 1 h, and the reaction
mixture stirred at room temperature for 24 h. Ice was added and the mixture was filtered and the
solid washed with water and dry in oven under vacuum (4.76 g, 90 %).
Synthesis of Ru (bda)(dmso) [5]. Under nitrogen atmosphere a mixture of 2,2’-bipyridine-
2
6
,6’-dicarboxylic acid (H bda) (1.001 g, 4.099 mmol) and Ru(dmso) Cl (1.984 g, 4.095 mmol)
2 4 2
was added to a 20 mL vial and methanol (5 mL) was added to the vial, followed by Et N (0.3
3
mL), and the mixture refluxed for 24 h. The reaction mixture was allowed to cool, filtered and
the solid washed with methanol (20 mL). The product dried under vacuum and the compound
was obtained as red-brown solid (1.23 g, 60 %).
Synthesis of Ru (bda)(PPh ) (1). A 20 mL vial in the glove box was charged with
3
2
Ru(bda)(dmso) (0.1000 g, 0.1994 mmol) and methanol (2.5 mL) was added. Another vial was
2
charged with triphenylphosphine (0.1046 g, 0.3988 mmol) and methanol (2.5 mL) was added.
5

The two suspensions were mixed together. The resulting mixture sonicated for 1 h, then was
3
1
stirred overnight. The completion of reaction was verified by P NMR and filtered to give
product (0.156 g, 92 %).
II
Synthesis of Ru(bda)(PEt ) (2). The Ru (bda)(dmso) complex (0.0500 g, 0.0997 mmol)
3
2
2
were dissolved in methanol (10 mL) then triethylphosphine (0.0271 mL, 0.2004 mmol) was
added. The reaction mixture was stirred 12 h at room temperature. The suspension was filtered,
and the brown filtrate solution was concentrated under vacuum. The residue was recrystallized
from methanol (0.0348 g, 60%).
i
II
Synthesis of Ru(bda)(P(O Pr) ) (3). Under N atmosphere in a 20 mL vial Ru
3
2
2
(
bda)(dmso) (0.050g, 0.0996 mmol) was added and CH Cl (3.5 mL) was added. In a separate
2
2
2
vial, triisopropyl phosphite (0.049 mL, 3.988 mmol) was added to CH Cl (3.5 mL) and then the
2
2
solution was added to the content of the other vial. The reaction mixture was stirred for 12 h. The
1
reaction was filtered, and the filtrate was concentrated to dryness and product checked by H and
31P NMR (0.0547 g, 72%).
II
II
Synthesis of Ru (bda)(PTA) (4). Under N atmosphere to Ru (bda)(dmso) (0.0311 g,
3
2
2
0
.06200 mmol) was added methanol (1 mL). In a separate vial the 1,3,5-tiaza-7-
phosphaadamantane (0.0342 g, 0.1550 mmol) dissolved in methanol (1.5 mL total amount of
methanol) and two precursor solutions were combined and the resulting mixture stirred at room
temperature for 0.5 h. The mixture was filtered, and filtered solid was dried under vacuum. The
obtained orange solid was washed with methanol (0.5 mL) (0.0475 g, 86%).
Formation of Ru (bda)(P(OMe) ) (5). In the glovebox, a resealable NMR tube was charged
3
3
II
with Ru (bda)(dmso) (0.010 g, 0.0199 mmol) and 2 equiv of trimethyl phosphite (0.0047 mL,
2
6

0
.0399 mmol), then 0.5 mL of CD OD, were added and the NMR tube was sealed. Outside the
3
glovebox, the tube was placed in a rotating oven in 30 °C for 24 h which resulted a clear yellowish
solution of combination of 5 and Ru (bda)(dmso)(P(OMe) ) complexes in 1:1 molar ratio, along
3
with liberated dmso. For comparison, we set up another reaction with excess of trimethyl phosphite
(0.0070 mL, 0.0598 mmol), which formed 5 and dmso after 2 d in a rotating oven at 30 °C. The
1
31
complexes were characterized by H and P NMR data (Table 2) but not isolated.
2
.3.
Crystal structure determination
Suitable single crystals from the methanol solutions of complexes 1, 2, 3, and 4 were obtained
mostly by diffusion technique. Selected crystallography information is presented in Table 1.
Table 1. Bond lengths [Å] and angles [°] obtained by X-ray crystallography
Complex
Ru-O
1
2
3
4a
4b
Bond lengths
Å]
2
.191(3)
.223(3)
.946(4)
.954(4)
.371(12)
.376(12)
2.194(16)
2.208(4)
2.139(15)
-
[
2
1
1
2
2
2.194(16)
1.944(19)
1.944(19)
2.372(6)
2.372(6)
124.05(8)
163.92(3)
81.40(11)
85.14(5)
2.217(6)
1.986(12)
1.987(2)
2.353(4)
2.384(4)
125.28(15)
164.55(12)
80.21(4)
86.06(4)
-
-
2.1402(18)
2.0187(17)
2.349(6)
2.334(6)
-
-
-
Ru-N
Ru-P
2.322(6)
-
Angles [°]
O-Ru-O
P-Ru-P
N-Ru-N
124.58(11)
167.16(4)
81.22(15)
-
175.98(2)
77.01(3)
86.92(5)
90.51(2), 91.82(2)
-
8
6.03(8)
92.56(4)
-
O-Ru-P
O-Ru-N
8
7
9.31(8)
87.34(4)
77.28(7)
89.18(5)
89.70(5)
7.59(13)
77.22(11)
156.35(6)
-
-
1
57.83(14)
158.67(7)
157.48(5)
-
a = axial ligands; b = equatorial ligand
7

3
3
.
Results and discussion
.1.
Synthesis and isolation
Reactions for making Ru complexes performed at room temperature afforded high yields (up
to 92%). In some cases the crude products were further purified by recrystallization. The reaction
of PTA was the fastest, being finished in 0.5 h, whereas the reaction of P(OMe) was the slowest,
3
requiring 2 d to complete.
Complexes 1, 2, and 3 were isolated as disubstituted Ru(bda)(L) species. Addition of an
2
excess of phosphorus ligands did not result in trisubstituted complexes analogous to 4; for
example, using 3 equiv of PEt , only bis(PEt ) species 3 and no tris(PEt ) species could be detected
3
3
3
1
31
in H and P NMR spectra (estimated detection limit, 5% yield). In contrast, when making
complex 4, if only 2 equiv of PTA was used, we observed ca. 2/3 of product 4 and unreacted
Ru(bda)(dmso) species. Interestingly, when making 5, if only 2 equiv of P(OMe) was used, we
2
3
observed both 5 and partially reacted Ru(bda)(dmso)[P(OMe) ] in a ratio of 1 : 1.
3
3
.2.
NMR and X-ray characterization
Complexes 1-5 were characterized by NMR spectroscopy (Table 2), and complexes 1-4 were
isolated and characterized by X-ray crystallography (Table 1 and Figure 1).
1
The H NMR spectral data for 1 are illustrative. The signals for bda protons indicate symmetry
consistent with tetradentate coordination mode. The spectrum shows one set of resonances for the
bda ligand, with each signal representing two protons [8.29 (d), 8.27 (d), 7.63 (t)], and a multiplet
(7.37-7.06 ppm) for the triphenylphosphine axial ligands (Fig. 2a in supporting information).
8

3
1
The P NMR data for 1-3 show a single sharp signal, whereas for 4 and 5, a doublet and triplet
1
are seen, consistent with one unique P coupled to two other, equivalent phosphines. The H NMR
data for 4 in CD OD (Table 2 and Fig. S4a) are also distinct from those for 1-3, in that the aromatic
3
region shows six sharp signals instead of only three. In the reaction containing 5 observed in
1
31
CD OD solution, the H and P NMR signals for the non-bda nuclei are sharp (Figure S5), whereas
3
1
the H NMR signals for the bda protons are broadened, possibly because the protic solvent CD OD
3
-
interacts with the negative charge on the dangling CO moiety.
2
1
31
a
Table 2. H and P NMR spectra of compounds 1-5 in CD OD.
3
1H
31P
bda
PR3
1
8.29 (dd, J = 8.0, 1.1, 2H),
7.40 – 7.30 (m, 6H),
7.26 – 7.13 (m, 14H),
30.8 (s) c
b
7
.64 (t, J = 7.8, 2H)
b
7
.14 – 7.05 (m, 12H)
2
8.43 (dd, J = 8.0, 1.1, 2H),
1.27 (qt, J = 7.5, 2.9, 12H),
0.66 (p, J = 7.6, 18H)
16.4 (s)
8
7
.15 (dd, J = 7.7, 1.2, 2H),
.95 (t, J = 7.9, 2H)
3 d
4 e
8.38 (dd, J = 8.0, 1.1, 2H),
4.62 – 4.56 (m, 6H)
1.01 (d, 36H)
116.5 (s)
8
8
.08 (dd, J = 7.8, 1.1, 2H),
.00 (t, J = 7.8, 2H)
8.80 (d, J = 8.2, 1H)
4.85, 4.67 (two d, J = 13.1, total 6H)
4.57 (s, 6H),
4.42, 4.32 (two d, J = 13.1, total 12H), = 38.7)
3.73, 3.60 (two d, J = 15.3, total 12H)
-51.6 (t) and -
55.1 (d) ( J
2
8
8
8
8
7
.51 (d, J = 8.6, 1H)
.46 (d, J = 8.0, 1H)
.30 (d, J = 7.6, 1H)
.14 (t, J =7.8, 1H)
.55 (d, J = 7.9, 1H)
PPcis
5 d,f ~8.56 (broad, ~1H)
3.44 (t, J = 5.2, 18H),
3.92 (d, J = 10.5, 9H)
136.27 (t) and
~8.35 (broad, ~1H)
~8.27 (broad, ~1H)
~8.02 (broad, ~2H)
~7.51 (broad, ~1H)
121.76 (d)
2
( J
= 68.0)
PPcis
9

(a) Chemical shifts  in ppm, coupling constants J in Hz. (b) One 2H signal for bda included
under signals for PPh ligands. (c) in DMSO-d . (d) in CD OD. (e) in CDCl . (f) in
3
6
3
3
reaction solution, CD OD solvent.
3
The solved X-ray structures revealed that compounds 1, 2, and 3 have C symmetry,
2
v
orthorhombic crystal system and Pbcn and Pbca space group with molecular formula Ru(bda)L2.
The complex 4 is monoclinic and C 2/c space group.
As illustrated in Figure 1, the RuL (Fig. 1a-c) complexes are in a distorted octahedral
2
configuration with N, N, O, O, atoms of bda occupying the equatorial square plane, whereas
phosphine and phosphite ligands bound to Ru are in the axial positions. Notably, in complex 4
three PTA ligands are coordinated with the metal center in axial and equatorial positions, forcing
one of the carboxylates away from metal center. As may be expected, compound 4 exhibits
different bond lengths and angles compared to compounds 1-3. The O-Ru-P and P-Ru-P angles
are larger in 4 than any of the other reported complexes, on average 1.1° and 10.8°, respectively,
and the O-Ru-N (156°) angle is small compared to those of other complexes Also, the Ru-N bonds
are longer and Ru-P bonds are shorter in 4 as compared to compounds 1-3, on average (0.12 Å and
0
.03 Å, respectively). Additionally, it is interesting to note that the equatorial Ru-P distance in 4
is on average 0.02 Å shorter than the axial Ru-P bond lengths, likely a result of mutual trans
influence of the two axially disposed ligands.
The fact that complexes 1-3 were formed exclusively even when excess ligand was present,
whereas 4 and 5 appeared to be formed exclusively even when insufficient ligand was present, is
rather remarkable. We explain the dichotomous behavior based on the cone angle of phosphorus
1
0

ligand: the cone angles for PPh , PEt , P(OiPr) , P(OMe) and PTA are 145°, 132°, 128°, 107° and
3
3
3
3
1
03°, respectively [1, 2]. In our hands, PTA and P(OMe) gave Ru(bda)L complexes, and these
3 3
two ligands have the smallest cone angles. In contrast, the electronics of the five phosphine and
phosphite ligands (TEP for PEt = 2061.7, PPh = 2068.9, PTA = 2069, P(OiPr) = 2075.9, and
3
3
3
-1
P(OMe) = 2079.5 cm ) [1, 2] do not correlate to the reactivity we observe here. Therefore, we
3
conclude that steric effect (as measured by cone angle) determines whether bis- or tris(PR3)
complexes are formed.
However, another way to explain the preference for PTA to coordinate in both axial and
equatorial positions it is useful to consider parameters designed to rank steric and electronic
properties of phosphine ligands. Based on investigations of Suresh and co-workers [6, 7], Vmin for
the ligands PEt , PPh , PTA, and P(OMe) are -43.55, -34.07, -33.69 and -26.12 kcal/mol,
3
3
3
respectively. Phosphines possessing a more negative Vmin values are expected to show stronger
electron donating properties, while phosphines with less negative Vmin values are expected to show
more electron-withdrawing properties [6]. Additionally, steric effects of substituents on
phosphorus influence the electron donating/withdrawing effects of the overall phosphine ligand.
For example, changes in the bulkiness of substituents attached to phosphorus can alter the p-
3
character of the sp -hybridized lone pair electrons of the phosphorus atom; which would lead to
an overall increase in the Vmin value. We note that PPh and PTA have very similar V values
3
min
(and also similar TEP, see previous paragraph), yet here react very differently.
1
1

1
2
3
4
Figure 1. X-ray crystal structure of complexes 1 to 4. Selected bond lengths [Å] and angles
°] are listed in Table 1.
[
1
2

3
.3.
Electrochemistry
Redox properties of 1-4 were investigated using cyclic voltammetry (CV). CV was first
conducted in aqueous potassium phosphate solution (0.1 M, µ = 0.1 and pH = 7) (Fig. 2 and Fig
S6). A summary of electrochemical results for 1-4 is presented in Table 3. At pH 7 a clearly
reversible RuII/III couple was observed in the CV of 2 and 3. As shown in Table 3, the redox
wave of RuII/III shows an increasing trend as PEt < PPh < P(O Pr) < PTA from 0.25 V to 0.95
i
3
3
2
V. As we expected, the oxidation potential of compound 4 is higher than that of the other
complexes in the series. We attribute this to strong π-interactions between the additional
phosphine ligand and the Ru center and to the fact that in complex 4, the Ru center is formally
cationic, whereas in 1-3, it is neutral.
The coordination chemistry of PTA has been investigated thoroughly [2]; Peruzzini and co-
workers suggest that PTA and phosphite ligands are comparable in their σ-bonding and π-
bonding abilities. Our electrochemistry data in Table 3 show that E of complex 4 is 0.26 V
½
more positive than E of compound 3 probably because of cationic Ru in the complex 4.
1
/2
1
3

Figure 2. Cyclic voltammagrams of complexes 2 and 3 (red trace) versus background (in blue) in
.1 M potassium phosphate buffer / CF CH OH (4 : 1) solution (pH = 7.0). CV of compound 2
0
3
2
1
4

shows ΔE = 0.0857 V, forward current = 5.25e-6 A and reverse current = -3.48e-6 A. Compound
p
3
shows ΔE = 0.351 V, forward current = 6.88e-5 A and reverse current = -6.02e-5 A.
p
Table 3. Electrochemical features of 1-4 (0.5 mM) in 0.1 potassium phosphate buffer /
CF CH OH (4:1) solution (pH = 7.0).
3
2
E1/2OX (V vs
Ag/AgCl)
Ru(II/III)
Complex
Eonset
II
Ru (bda)(PPh )
0.45
0.25
0.69
0.95
1.4
1.2
1
3
2
II
Ru (bda)(PEt )
3
2
II
Ru (bda)(P(OiPr) )
3
2
II
Ru (bda)(PTA)
1.1
3
For further investigation of the redox behavior of these compounds, CV spectra were
recorded in mixtures of potassium phosphate buffer / CF CH OH with three different pH values.
3
2
Figure 3 shows the pH dependent behavior of compounds 2 and 3. An increase in E oxidation
1
/2
peak with increase of pH observed for complex 3. Furthermore, as shown in Figure 3 at acidic
(blue line) and basic (green line) pH, two waves are observed, where the second one is smaller
II/III
and RuIII/IV. For complexes 1 and 4
and appears at more positive E which may assign for Ru
1
/2
we were not able to perform CV at different pH values because of poor solubilities.
1
5

Figure 3. Comparison of CVs in present of compounds 2 and 3; blue line pH = 1, red line pH =
7
, and green line pH = 10-11 in mixed potassium phosphate / CF CH OH solution showing two
3 2
reversible waves of RuIII/II and Ru IV/III redox couples.
However, the cyclic voltammetry result for complex 1 did not clearly show redox waves for
RuII/III and/or RuIII/IV couples in aqueous solution, possibly because of low solubility in aqueous
1
6

solvent. We investigated propylene carbonate (PC) as a polar aprotic solvent to solubilize
complex 1 during the electrochemistry experiments [9]. Figure 4 illustrates the cyclic
n
voltammograms of 1 mM 1 in 0.1 M 0.1 M Bu NPF /PC with water added in varying amounts.
4
6
In the absence of water and in presence of water clearly a wave is observed at E = 0.45 V for
1
/2
redox of RuIII/II. In fact, after adding water, the potential of the II/III couple did not change [9].
n
Figure 4. CVs of 1 mM of 1 in 0.1 M Bu NPF /PC by addition of increasing amounts of water,
4
6
as illustrated in legend of the figure. The dashed line shows the background in the absence of 1.
3
.4.
DFT computations
All molecular structures were optimized in the Gaussian16 Revision D.01 [10] program with
the B3LYP density functional [11-13] with the cc-pVDz basis set [13] for all main group atoms
1
7

and the Stuttgart-Dresden (SDD) [13-15] effective core potential for ruthenium. All stationary
points were verified by frequency calculations and full population analysis. All calculations were
carried out on isolated gas-phase species.
II
Table 4. Computed HOMO – LUMO gaps for Ru (bda)(L) species (this work) and Tolman
2
,3
electronic parameters for L (literature [1]).
-1
Complex
HOMO/LUMO PR TEP (cm )
3
gap (eV)
II
1
2
3
, Ru (bda)(PPh )
2.669
2.653
2.760
2.656
2.613
2068.9
2061.7
2075.9
3
2
II
, Ru (bda)(PEt )
3
2
II
, Ru (bda)(P(OiPr) )
3
2
II
Ru (bda)(PTA)
2
II
3
2069
4
, Ru ( - bda)(PTA)
3
II
To aid in our study of these Ru (bda)(PR ) complexes, we investigated the calculated
3
2
molecular orbitals in order to probe the extent of bonding interactions between Ru and the PR3
ligands. We initially began by optimizing the set of isolated compounds 1-4, as well as species
not observed experimentally, complexes corresponding to 1-3 containing an additional
II
4
phosphine ligand in the equatorial plane with an unbound carboxylate, and the complex Ru ( -
bda)(PTA) where both bda carboxylates are bound to ruthenium. However, we were unable to
2
II
3
locate a minimized structure of the type Ru ( -bda)(PR ) when PR is PPh , PEt , or P(OiPr) ,
3
3
3
3
3
3
which we attributed to unfavorable steric interactions.
Orbital analysis of compounds 1-3 reveal similar bonding features, including the highest
occupied molecular orbitals (HOMO) being centered on carboxylate oxygens and the ruthenium
center with σ-symmetry, and lowest unoccupied orbitals being mainly located on the bipyridine
moiety of the bda (Figs. 5 and 6, Table 4). Comparing complexes 1-3 of similar structure, the
HOMO-LUMO gap for compound 3 was found to be bigger than other complexes) (Table 4)
1
8

-1
which can be rationalized by P(OiPr) having the largest TEP value (2075.9 cm ) (Table 4).
3
Turning now to compound 4, analysis showed the HOMO to be a σ-interaction between bda
nitrogens and ruthenium, as well as PTA orbitals. The LUMO was again mainly centered on the
bipyridine part of the bda ligand, however in this case showing some π anti-bonding character
between bda nitrogens and ruthenium. Analysis of lower energy orbitals in compound 4 (Figures
4
and 5) show low energy (HOMO – 5 and HOMO -3) orbitals with π-bonding character
between the equatorial PTA and ruthenium. Interestingly, the HOMO -2 orbital shows π anti-
bonding character between the axial PTA ligands and ruthenium, as well as some density on the
equatorial PTA, however in this case with non-bonding character with respect to the ruthenium.
The HOMO -1 orbital is π anti-bonding in character between the equatorial PTA and ruthenium.
Overall, there is a net π bonding interaction between the equatorial PTA ligand and the
ruthenium, which also may help explain the favorability of three PTA ligands to coordinate.
Figure 5. Selected molecular orbitals for 1 to 4.
1
9

Figure 6. Selected molecular orbitals for 4.
. Conclusions
4
-
Here we report synthesis of Ru based complexes using the 2,2’-bipyridine-6-6’-dicarboxylic
acid scaffold. In this work we used one triarylphosphine, one trialkylphosphine, two phosphites,
and the unique ligand PTA, each presenting a different combination of steric and electronic effects.
For complexes 1 to 3, phosphine and phosphite ligands occupy the two axial coordination sites
4
available on the fragment Ru( -bda). In contrast, complexes 4 and 5 possess an additional
phosphine ligand occupying the equatorial position, displacing a carboxylato of bda, resulting in
3
Ru( -bda)(L) . Notable is that P(OMe) and P(OiPr) are similar electronically, but the larger
3
3
3
steric demand of the latter results in an L complex; similarly, PTA and PPh are similar
2
3
electronically, but the larger steric demand of latter results in an L complex. Moreover, P(OMe)
2
3
is less electron-rich than PTA and seems to react more slowly (2 d vs. 0.5 h) with Ru(bda)(dmso)2,
2
0

but the two ligands share similar cone angles and both give L complexes as final products, where
3
the presumed L intermediate is not detected during reaction.
2
Comparison of the redox potentials for the II/III couple in Ru(bda)(PR ) reflects the donor
3
2
abilities of PEt (0.25 V), PPh (0.45 V) and P(OiPr) (0.69 V). The II/III redox potential for the
3
3
3
tris(PTA) complex was more positive, likely because the Ru center bears a formal positive charge
thanks to ionization of the Ru-carboxylate bond enforced by the presence of the third, equatorial
phosphine. Computational scrutiny of 4 suggests that there is a net π bonding interaction between
the equatorial PTA ligand and the ruthenium, which also helps explain the relatively positive
potential of the II/III couple.
II
This work reports the first complexes of phosphorus ligands on the Ru (bda) fragment and
points to future exploration of reactivity.
Appendix A. Supplementary data
CCDC 1868484-1868487 contains the supplementary crystallographic data for 1-4. These data
can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the
Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44)
1
223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
5
. Acknowledgements
We thank San Diego State University for initial support of this work. The later portions of this
research were supported by the U.S. Department of Energy, Office of Science, Office of Basic
Energy Sciences, under Award DE-SC0018310. Special thanks to Dr. Diane K. Smith for
2
1

invaluable help with electrochemical methods and Aaron Nash and Daniel Sattler for support
during this work.
6
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2
4

[16]
II
This work provides the first evaluation of phosphorus ligand binding to the Ru (bda) fragment.
3
The smallest ligand formed a ( -N,N,O)-bda(PR ) complex, whereas for the other three
3
3
4
phosphorus ligands, only ( -bda)(PR ) species were seen. Cyclic voltammetry,
3
2
crystallography, and computations evaluate some of the electronic and structural differences
between the two structural types.
[17]
2
5