Synthesis and structure of palladium(II) complexes supported by bis-NHC pincer ligands for the electrochemical activation of CO2

Article abstract of DOI:10.1016/j.poly.2017.06.018

A series of bis-NHC pincer complexes of palladium(II) have been prepared and characterized. These pyridyl-spaced dicarbene complexes ([(PDC^R)Pd(MeCN)](PF₆)₂) were synthesized with substituents of varying steric bulk at the

Full text of DOI:10.1016/j.poly.2017.06.018

Accepted Manuscript
Synthesis and Structure of Palladium(II) Complexes Supported by bis-NHC
Pincer Ligands for the Electrochemical Activation of CO2
Gabriel A. Andrade, John L. DiMeglio, Eric T. Guardino, Glenn P.A. Yap, Joel
Rosenthal
PII:
S0277-5387(17)30436-9
DOI:
http://dx.doi.org/10.1016/j.poly.2017.06.018
POLY 12699
Reference:
Polyhedron
To appear in:
Received Date:
Accepted Date:
26 April 2017
15 June 2017
Please cite this article as: G.A. Andrade, J.L. DiMeglio, E.T. Guardino, G.P.A. Yap, J. Rosenthal, Synthesis and
Structure of Palladium(II) Complexes Supported by bis-NHC Pincer Ligands for the Electrochemical Activation of
CO2, Polyhedron (2017), doi: http://dx.doi.org/10.1016/j.poly.2017.06.018
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Synthesis and Structure of Palladium(II) Complexes Supported by bis-
NHC Pincer Ligands for the Electrochemical Activation of CO2
Gabriel A. Andrade, John L. DiMeglio, Eric T. Guardino, Glenn P. A. Yap, and Joel Rosenthal*
Department of Chemistry and Biochemistry, University of Delaware, Newark, DE 19716
Abstract
A series of bis-NHC pincer complexes of palladium(II) have been prepared and characterized. These
pyridyl-spaced dicarbene complexes ([(PDCR)Pd(MeCN)](PF6)2) were synthesized with substituents of
varying steric bulk at the wingtip positions, which include R = methyl, ethyl, isopropyl, cyclohexyl,
mesityl and 2,6-diisopropylphenyl. The synthesis of this library of complexes was accomplished either by
direct metallation of the prerequisite pyridyl-spaced bis-imidazolium proligands with Pd(OAc)2 or via
treatment with Ag2O to afford the corresponding silver carbenes, which were then transmetallated onto
palladium. Solid-state structures for each of the [(PDCR)Pd(MeCN)](PF6)2 derivatives were obtained via
X-ray crystallography and allowed for the steric properties of each PDCR ligand to be evaluated by two
methods. These analyses, which included calculation of the percent buried volume (%VBur) and solid
angles of the PDCR ligands, served to characterize the steric environment around the palladium center in
each of the complexes that was prepared. Finally, voltammetry and controlled potential electrolysis
studies were performed to characterize the redox chemistry of the [(PDCR)Pd(MeCN)](PF6)2 derivatives
and assess if they could electrocatalyze the reduction of CO2. The influence of the steric properties of
the PDCR ligand on the electrochemistry of the resulting complexes [(PDCR)Pd(MeCN)](PF6)2 is also
discussed.
Keywords: N-heterocyclic carbene, sterics, percent buried volume, solid angles, carbon dioxide
activation

1. Introduction
Palladium pincer complexes supported by pyridine spaced bis-N-heterocyclic carbene (NHC) ligands,
have been successfully employed for many catalytic applications including Mizoroki-Heck coupling,1
Suzuki,2 and Sonagashira2 coupling reactions. Palladium complexes supported by pincer ligands have
also been recently reviewed for their broad use in catalytic applications.3 Ligands incorporating NHCs are
of interest due to their being strong σ-donors and their ability to form kinetically inert bonds with many
transition metals.4
Pincer complexes of palladium have also been studied for the reduction of carbon dioxide. Such
efforts have been inspired by prior work by Dubois and coworkers, which demonstrated that complexes
of palladium that are supported by tridentate phosphine ligands (triphos) display fast kinetics for CO2
reduction with good selectivity for production of CO.5 Despite the excellent kinetics and selectivity
observed for Dubois’ catalysts, the instability of the Pd(triphos) platform under electrocatalytic
conditions limits the maximum turnover numbers (TONs) observed for these catalysts which range from
~10–150.6,7 The labile nature of the flexible phosphine ligand, coupled with the general lability of low
valent palladium centers, introduces the possibility for side reactions, which are believed to be
responsible for the rapid loss in activity that is observed for Pd(triphos) electrocatalysts.8
It has been speculated that palladium complexes supported by inert carbene ligands might be more
robust platforms for the electrocatalytic reduction of CO2.9 Along these lines, Wolf and coworkers have
recently shown that palladium complexes incorporating pyridine and lutidine spaced bis-NHC pincer
ligands can promote the electrocatalytic reduction of CO2 to CO.10 In related studies it was also shown
that elaborating the substitution on the backbone of these bis-NHC pincer complexes impacts the ability
of these systems to activate CO2.11 It has also been shown that varying the substituent at the para-
position of the pyridine ring impacts the ability of these types of complexes to promote CO2 reduction,12
however, the substitution pattern on the imidazole, also known as the wingtip position of the bis-NHC
ligand, has not been synthetically explored for CO2 electrocatalyst development. Indeed, each of the
palladium bis-NHC pincer complexes employed for CO2 electrolysis that have been described to date
have incorporated n-butyl substituents at the wingtip positions. Our lab has also investigated metal
complexes supported by NHC ligands for small molecule activation,13,14 and recently undertook an effort
to understand how the wingtip substituent of bis-NHC pincer complexes might influence the ability of
such systems to activate CO2. As such, herein we report the preparation and characterization of a set of
pyridyl spaced dicarbene (PDCR) ligand scaffolds with wingtips of varying steric bulk, since steric

Figure 1. The general naming and numbering scheme for the set of palladium complexes discussed in this work.
shielding of the metal has been reported to be important for palladium cross-coupling catalysis
supported by NHC ligands,15 as well as for zinc complexes for CO2 activation.16 To understand the effect
of different wingtip steric profiles, a series of pyridyl spaced dicarbene complexes (Figure 1a) or
[(PDCR)Pd(X)](Y) were prepared, where R, X, and Y denote the substitution on wingtip of the ligand, the
ancillary ligand coordinated to palladium, and the outer sphere counter anion, respectively. Through
analysis of the solid-state structures of these architectures, we were able to determine how steric
shielding of the palladium center by the wingtip substituent of the PDCR ligand correlates with the ability
of such platforms to activate CO2.
2. Experimental
General Materials and Methods. Reactions were performed in oven-dried round-bottomed flasks
unless otherwise noted. Reagents and solvents were purchased from Sigma Aldrich, Acros, Fisher,
Strem, Pressure Chemical or Cambridge Isotopes Laboratories. [Pd(Br)2(MeCN)2] was synthesized using a
published procedure.17 Solvents for synthesis were of reagent grade or better and were dried by passage
through activated alumina and then stored over 4 Å molecular sieves prior to use. Column
chromatography was performed with 40-63 µm silica gel with the eluent reported in parentheses.
Analytical thin-layer chromatography (TLC) was performed on precoated glass plates and visualized
using UV light. The following compounds were previously reported and were prepared according to
published methods: 1-(2,4,6,-trimethylphenyl)-1H-imidazole,18 1-(2,6-diisopropylphenyl)-1H-imidazole,18
1-(isopropyl)-1H-imidazole,19 2,6-di(1H-imidazol-1-yl)pyridine,20 2,6-bis[3-(mesityl)imidazolium]pyridine
dibromide (5),21 2,6-bis[3-(2,6-diisopropylphenyl)imidazolium]pyridine dibromide (6),21 and
[PDCMePd(Br)]Br.22

1
Compound Characterization. H NMR and 13C NMR spectra were recorded at 25 °C on a Bruker 400
MHz spectrometer. Proton spectra are referenced to the residual proton resonance of the deuterated
solvent (CDCl3 = δ 7.26; CD3CN = δ 1.94; (CD3)2SO = δ 2.50) and carbon spectra are referenced to the
carbon resonances of the solvent (CDCl3 = δ 77.16; CD3CN = δ 1.32, 118.26; (CD3)2SO = δ 39.52). All
chemical shifts are reported using the standard δ notation in parts-per-million; positive chemical shifts
are to higher frequency from the given reference. LR-GCMS data were obtained using an Agilent gas
chromatograph consisting of a 6850 Series GC System equipped with a 5973 Network Mass Selective
Detector. Low resolution MS data was obtained using either a LCQ Advantage from Thermofinnigan or a
Shimadzu LC/MS-2020 single quadrupole MS coupled with an HPLC system, with dual ESI/APCI source.
High-resolution mass spectrometry analyses were either performed by the Mass Spectrometry
Laboratory in the Department of Chemistry and Biochemistry at the University of Delaware or at the
University of Illinois at Urbana-Champaign.
X-Ray Crystallography and Structural Analyses. Crystals were mounted using viscous oil onto a
plastic mesh and cooled to the data collection temperature. Data were collected on a Bruker-AXS APEX II
DUO CCD diffractometer with Cu-Kα radiation (λ = 1.54178 Å) and was focused by Goebbel mirrors for
[(PDCCy)Pd(Br)]Br. All other data were collected with Mo-Kα radiation (λ = 0.71073 Å) monochromated
with graphite. Unit cell parameters were obtained from 36 data frames, 0.5° ω, from three different
sections of the Ewald sphere. The unit-cell parameters and systematic absences were uniquely
consistent to the reported space groups for [(PDCDipp)Pd(MeCN)](PF6)2, [(PDCCy)Pd(Br)3],
[(PDCCy)Pd(MeCN)](PF6)2, and proligand 4; and for Pna21 and Pnam [Pnma] for
[(PDCMes)Pd(MeCN)](PF6)2. No symmetry higher than triclinic was observed in [(PDCEt)Pd(Br)]Br,
[((PDCCy)AgBr)2Ag]Br,
[(PDCCy)Pd(Br)]Br,
[(PDCDipp)Pd(Br)]Br,
[(PDCiPr)Pd(MeCN)](PF6)2,
[(PDCDipp)Pd(MeCN)](PF6)2, and [(PDCMe)Pd(MeCN)](PF6)2. In all cases wherein pairs of space groups are
possible, excepting [(PDCMes)Pd(MeCN)](PF6)2, the centrosymmetric space group option yielded
chemically reasonable and computationally stable results of refinement. In the case of
[(PDCMes)Pd(MeCN)](PF6)2, only the noncentrosymmetric space group yielded chemically reasonable and
computationally stable results of refinement. The data were treated with multi-scan absorption
corrections. The structures were solved using direct methods or intrinsic phasing and refined with full-
matrix, least-squares procedures on F2. The compound molecule resides on a mirror plane in
[(PDCDipp)Pd(MeCN)](PF6)2. Two compound molecules (Z’ = 2) were located in the asymmetric units of
[(PDCEt)Pd(Br)]Br and [((PDCCy)AgBr)2Ag]Br.

–
In the case of [(PDCDipp)Pd(MeCN)](PF6)2, the slight but significant disorder in the PF6 leads to a
level B alert in the checkCIF report with a second alert caused by an AFIX 33 methyl H-atoms that
require a large DAMP value because of their proximity to a mirror plane. The co-crystallized chloroform
solvent molecule in [(PDCCy)Pd(Br)]Br was modeled isotropically with refined site occupancy of 54/46.
An ethyl group was found disordered in [(PDCEt)Pd(MeCN)](PF6)2 and was modeled with rigid-bond
restraints with refined site occupancy of 56/44. An acetonitrile molecule of solvation was located in the
asymmetric unit of [(PDCMes)Pd(MeCN)](PF6)2, and one of two counterions was found disordered in two
positions with a refined site occupancy of 70/30. The disorder in the counterions in proligand 4, and, the
solvent molecules in [((PDCCy)AgBr)2Ag]Br, precluded satisfactory modeling and were treated as diffused
contributions.23
All non-hydrogen atoms, excluding the disordered solvent in [(PDCDipp)Pd(Br)]Br, were refined with
anisotropic displacement parameters. All hydrogen atoms were treated as idealized contributions with
geometrically calculated positions and with Uiso equal to 1.2, or 1.5 for methyl, Ueq of the attached atom.
Atomic scattering factors are contained in various versions of the SHELXTL program library. The CIFs
have been deposited at the Cambridge Crystallographic Data Centre: CCDC 1539130, [(PDCCy)Pd(Br)]Br;
CCDC 1539131, [(PDCCy)Pd(MeCN)](PF6)2; CCDC 1539132, [(PDCDipp)Pd(Br)]Br; CCDC 1539133,
[(PDCDipp)Pd(MeCN)](PF6)2;
CCDC
1539134,
[(PDCEt)Pd(MeCN)](PF6)2;
CCDC
1539135,
[(PDCiPr)Pd(MeCN)](PF6)2; CCDC 1539136, [(PDCCy)Pd(Br)3]; CCDC 1539137, [(PDCMes)Pd(MeCN)](PF6)2;
CCDC 1539138, [(PDCMe)Pd(MeCN)](PF6)2; CCDC 1539139, proligand 4; CCDC 1539140,
[(PDCEt)Pd(Br)]Br; CCDC 1539141, [((PDCCy)AgBr)2Ag]Br.
Electrochemical Measurements. All electrochemistry was performed using either a CHI-620D
potentiostat/galvanostat or a CHI-760D bipotentiostat. Cyclic voltammetry was performed using a
standard three-electrode configuration. CV scans were recorded for quiescent solutions using a glassy
carbon working disk electrode (3.0 mm diameter CH Instruments) and a platinum wire auxiliary
electrode. All potentials were measured against a Ag/AgCl reference electrode. CV experiments were
performed in DMF using 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) as the supporting
electrolyte. Concentrations of analytes were 1.0 mM. Controlled Potential Electrolysis (CPE) experiments
were carried out using a platinum mesh counter electrode and the same working and reference
electrodes as above. CPE experiments were performed at 1.0 atm in an air tight single compartment cell
saturated with CO2. The electrolysis cell was sparged with DMF saturated CO2 for approximately 30

minutes, following which time the system was sealed and electrolysis initiated. To determine Faradaic
efficiencies for CO production, the headspace of the electrolysis cell was sampled periodically by
manually removing 1.0 mL aliquots using a gas-tight syringe. These aliquots were analyzed by manual
injection into a gas-sampling loop of a Shimadzu GC-2014 gas chromatograph (GC). This GC was
equipped with two 10 port injection valves in line with 2m HaySepT 80/100 columns. Quantification of
CO was accomplished using a flame ionization detector (FID) with methanizer after passage through a
3m HaySepD 80/100 column using helium (99.999%) as the carrier gas. Quantification of H2 was
accomplished using a thermal conductivity detector (TCD) after passage through a packed MolSieve 5A
60/80 column, using argon as the carrier gas (99.999%).
Synthesis and Characterization
1-Cyclohexyl-1H-imidazole. This compound was prepared by amending a previously published
method.24 Ammonium chloride (10.7 g, 0.200 mol), glyoxal (40 %, 15.3 mL, 0.134 mol), formaldehyde
(30 %, 16.0 mL, 0.134 mol), and deionized water (250 mL) were combined in a 1 L round bottom flask. In
a separate flask, a solution of cyclohexylamine (11.5 mL, 0.100 mol) in deionized water (100 mL) was
acidified with phosphoric acid (85 %, 15 mL). The acidified amine was added to the ammonium chloride
solution. After heating the resulting colorless solution at reflux for 12 hours, the clear orange solution
was cooled to room temperature and subsequently poured into a 1 L beaker that was half-filled with ice.
The resulting mixture was made basic with sodium hydroxide (40 %) until the pH was in the range of 10
– 12. The product was then extracted into ethyl acetate and the organic layer was washed with water
and dried over sodium sulfate. Following removal of the solvent under reduced pressure, the desired
product was purified via vacuum distillation (105 ˚C) to give 7.5 g of the title compound as a colorless oil.
1
Yield = 50 %. H NMR (400 MHz, CDCl3, 25 ˚C) δ/ppm: 7.53 (s, 1H), 7.04 (s, 1H), 6.95 (s, 1H), 3.90 (tt, J =
11.8, 3.8 Hz, 1H), 2.12-2.09 (m, 2H), 1.92-1.87 (m, 2H), 1.79-1.70 (m, 1H), 1.67-1.57 (m, 2H), 1.45-1.35
(m, 2H), 1.29-1.18 (m, 1H). 13C NMR (101 MHZ, CDCl3, 25 ˚C) δ/ppm: 135.43, 129.03, 117.10, 56.90,
34.54, 25.54, 25.34. HR-EI-MS [M]+ m/z: calculated for C9H14N2 150.1157; found, 150.1162.
Synthesis of the pyridyl-spaced bis-imidazolium based proligands:
2,6-Bis[3-(ethyl)imidazolium]pyridine dibromide (2). The preparation of this compound was
based on previously published methods.21,25 2,6-Bis(1-imidazolyl)pyridine (0.201 g, 0.095 mmol),
bromoethane (0.70 ml, 0.94 mmol), and 3.0 mL of xylenes were added to a thick-walled glass tube. The
tube was sealed with a screw top cap equipped with a rubber o-ring and the vessel was heated at 150 °C

for 4 days. After cooling the reaction mixture to room temperature, the resulting material was triturated
several times with ether and the solid was collected to yield 0.386 g of an off white solid. Yield = 94 %.
1H NMR (400 MHz, DMSO-d6, 25 °C) δ/ppm: 10.74 (s, 2H), 8.88 (s, 2H), 8.61 (t, J = 8.1 Hz, 1H), 8.29 (d, J =
8.1 Hz, 2H), 8.21 (s, 2H), 4.40 (q, J = 7.3 Hz, 4H), 1.55 (t, J = 7.3 Hz, 6H). 13C NMR (101 MHz, DMSO, 25 °C)
δ/ppm: 145.31, 144.84, 135.66, 123.52, 119.39, 114.12, 45.15, 15.03. HR-LIFDI-MS [M–Br]+ m/z:
calculated for C15H19N5Br 348.0824; found, 348.0830.
2,6-Bis[3-(isopropyl)imidazolium]pyridine dibromide (3). This compound was prepared by
amending a previously published method.26 Isopropyl imidazole (1.40 g, 12.7 mmol) was added to 2,6-
dibromopyridine (1.50 g, 6.33 mmol) in a thick-walled glass tube. The neat mixture was stirred and
heated at 150 °C for 4 days. After cooling the reaction to room temperature, the resulting brown solid
was suspended in dichloromethane leaving the title compound as an insoluble white solid (2.24 g),
which was isolated by vacuum filtration. Yield = 77%. All analytical data matched that previously
reported for this compound.26 1H NMR (400 MHz, d6-DMSO, 25 °C) δ/ppm: 10.60 (s, 2H), 8.90 and 8.31 (2
× t, J = 1.94 Hz, 2H), 8.62 (t, J = 8.16 Hz), 8.32 (d, J = 8.16 Hz, 2H), 4.87 (septet, J = 6.66 Hz, 2H), 1.63 (d, J
= 6.66 Hz, 12H).
2,6-Bis[3-(cyclohexyl)imidazolium]pyridine dibromide (4). This compound was prepared by
amending a previously published method.26 2,6-Dibromopyridine (8.36 g, 0.035 mol) and 1-Cyclohexyl-
1H-imidazole (10.59 g, 0.706 mol) were mixed in a 50 mL thick-walled glass tube. The vessel was sealed
and heated at 150 ˚C for 4 days. After cooling the reaction to room temperature, the desired product
was purified from the resulting black residue by dissolving it in a minimal amount of CH2Cl2 and then
1
cooling the solution to 0 °C to deliver 9.6 g of the title compound as a white solid. Yield = 51 %. H NMR
(400 MHz, DMSO-d6, 25 ˚C) δ/ppm: 10.60 (s, 2H), 8.91 (s, 2H), 8.62 (t, J = 8.1, 1H), 8.13-8.31 (m, 4H),
4.54-4.43 (m, 2H), 2.17 (d, J = 9.0, 4H), 1.95-1.89 (m, 8H), 1.70 (d, J = 12.5, 2H), 1.48-1.38 (m, 4H), 1.32-
1.22 (m, 2H). 13C NMR (101 MHz, DMSO, 25 ˚C) δ/ppm: 145.35, 144.71, 134.65, 122.16, 119.52, 114.33,
59.80, 32.28, 24.56, 24.37. X-ray quality crystals of this compound were obtained by exchanging the Br–
–
counter anions to PF6 , followed by vapor diffusion of ether into a saturated acetonitrile solution. A
labeled thermal ellipsoid plot for 2,6-Bis[3-(cyclohexyl)imidazolium]pyridine dihexafluorophosphate can
be found in the Supporting Information (Figure S1).
General Procedure (A): Synthesis of [(PDCR)PdBr]Br complexes via direct metalation using
Pd(OAc)2, This procedure was adapted from a previously published method.1 Pd(OAc)2 (1 equiv) and the
pyridyl-spaced bis-imidazolium proligand (1 equiv) were added to a round bottom flask, which was then

evacuated. DMSO (10 ml), which had been saturated with N2 for 30 minutes, was added to the reaction
flask via syringe, and the headspace was evacuated. The septum sealing the flask was then reinforced
with copper wire and the reaction was heated at 150 °C for 3 hours. After cooling the reaction to room
temperature, the resulting mixture was filtered through celite and the solvent was removed under
reduced pressure. The crude material was then purified via column chromatography on silica with the
eluent mixture indicated for each compound detailed below.
General Procedure (B): Synthesis of [(PDCR)PdBr]Br complexes via transmetalation. This
procedure was adapted from previously published methods.21,27 The pyridyl-spaced bis-imidazolium
proligand (1 equiv) and Ag2O (1 equiv) were added to a round bottom flask along with 15 mL of DMSO.
The resulting solution was shielded from light with aluminum foil and stirred for 12 hours at room
temperature. [Pd(Br)2(MeCN)2] (1.05 equiv) was then added to the reaction, which was stirred for an
additional 3 hours. The reaction was then filtered through celite and the solvent was removed under
reduced pressure. The crude material was then purified via column chromatography on silica using the
eluent mixture indicated for each compound detailed below.
[(PDCEt)Pd(Br)]Br This compound was prepared via General Procedure A from proligand 2
(0.259 g, 0.060 mmol) and Pd(OAc)2 (0.135 g, 0.060 mmol). The title compound was purified on silica
using MeOH and CH2Cl2 (1:4) as the eluent to deliver 180 mg of an off white solid. Yield = 56%. X-ray
quality crystals of this complex were grown via vapor diffusion of ether into a concentrated
CH2Cl2:MeOH solution. 1H NMR (400 MHz, DMSO-d6, 25˚C) δ/ppm: 8.58 (t, J = 8.2 Hz, 1H), 8.47 (d, J = 2.2
Hz, 2H), 8.02 (d, J = 8.2 Hz, 2H), 7.80 (d, J = 2.2 Hz, 2H), 4.56 (q, J = 7.2 Hz, 4H), 1.38 (t, J = 7.1 Hz, 6H). 13
C
NMR (101 MHz, DMSO, 25˚C) δ/ppm: 165.48, 150.26, 146.54, 123.91, 118.43, 108.83, 45.34, 16.49. HR-
LIFDI-MS [M–Br]+ m/z: calculated for C15H17N5PdBr 453.9697; found, 453.9684. X-ray quality crystals of
this compound were obtained by vapor diffusion of pentane into a saturated chloroform solution. A
labeled thermal ellipsoid plot for [(PDCEt)Pd(Br)]Br can be found in the Supporting Information (Figure
S2).
[(PDCIpr)Pd(Br)]Br This compound was prepared via General Procedure A from proligand 3
(0.400 g, 0.875 mmol) and Pd(OAc)2 (0.196 g, 0.875 mmol). The title compound was purified on silica
using increasing concentrations of MeOH in CH2Cl2 (from 0% – 20% MeOH in CH2Cl2) as the eluent to
deliver 320 mg of an off white solid. Yield = 65%. 1H NMR (400 MHz, DMSO-d6, 25˚C) δ/ppm: 8.58 (t, J =
8.23 Hz, 1H), 8.55 and 7.98 (2 × d, J = 1.95 Hz, 2H), 8.04 (d, J = 8.23 Hz, 2H), 5.78 (septet, J = 6.83 Hz, 2H),
1.47 (d, J = 6.83 Hz, 12H). 13C NMR (101 MHz, CDCl3, 25˚C) δ/ppm: 164.76, 150.29, 146.52, 120.39,

118.88, 108.75, 52.05, 39.52, 22.38. HR-ESI-MS [M–Br]+ m/z: calculated for C17H21N5BrPd 480.0015;
found, 480.0016.
[(PDCCy)Pd(Br)]Br This compound was prepared via General Procedure A from proligand 4
(0.802 g, 1.50 mmol) and Pd(OAc)2 (0.335 g, 1.50 mmol). The title compound was purified on silica using
MeOH and CH2Cl2 (1:5) as the eluent to deliver 408 mg of a yellow solid. Yield = 43%. This complex was
also prepared via General Procedure B from proligand 4 (0.336 g, 0.630 mmol), Ag2O (0.145 g, 0.630
mmol) and [Pd(Br)2(MeCN)2] (0.229 g, 0.660 mmol). The title compound was then purified on silica using
MeOH and CH2Cl2 (1:10) as the eluent to deliver 0.193 g of an off-white solid. Yield = 55%. 1H NMR (400
MHz, DMSO-d6, 25˚C) δ/ppm: 8.58 (t, J = 8.2 Hz, 1H), 8.51 (d, J = 2.2 Hz, 2H), 8.01 (d, J = 8.2 Hz, 2H), 7.95
(d, J = 2.2 Hz, 2H), 5.44 (t, J = 12.1 Hz, 2H), 2.12 – 1.61 (m, 14H), 1.53 – 1.11 (m, 7H). 13C NMR (101 MHz,
DMSO, 25˚C) δ/ppm: 165.20, 150.32, 146.51, 120.73, 118.66, 108.83, 58.64, 32.65, 25.01, 24.51. HR-ESI-
MS [M–Br]+ m/z: calculated for C23H29N5PdBr 562.0638; found, 562.0651. X-ray quality crystals of this
compound were obtained by vapor diffusion of pentane into a saturated chloroform solution. A labeled
thermal ellipsoid plot for [(PDCCy)Pd(Br)]Br can be found in the Supporting Information (Figure S3).
[(PDCMes)Pd(Br)]Br This compound was prepared via General Procedure A from proligand 5
(1.01 g, 1.66 mmol) and Pd(OAc)2 (0.372 g, 1.66 mmol). The title compound was then purified on silica
using a solvent gradient from pure CH2Cl2 to MeOH/CH2Cl2 (1:4) to deliver 0.390 g of an off-white solid.
Yield = 40%. This complex was also prepared via General Procedure B from proligand 5 (0.505 g, 0.829
mmol), Ag2O (0.192 g, 0.829 mmol) and [Pd(Br)2(MeCN)2] (0.303 g, 0.870 mmol). The title compound
was then purified on silica using MeOH and CH2Cl2 (1:4) to deliver 0.396 g of an off-white solid. Yield =
1
67% yield. H NMR (400 MHz, DMSO-d6, 25˚C) δ/ppm: 8.73 and 7.70 (2 × d, J = 2.22 Hz, 2H), 8.68 (t, J =
8.17 Hz, 1H), 8.20 (d, J = 8.17 Hz, 2H), 6.94 (s, 4H), 2.25 (s, 6H), 1.96 (s, 12H). 13C NMR (101 MHz, CD3CN,
25˚C) δ/ppm: 171.16, 151.90, 147.01, 140.47, 135.38, 135.27, 129.51, 125.76, 119.55, 118.26, 110.21,
21.00, 17.91. HR-ESI-MS [M–Br]+ m/z: calculated for C29H29N5PdBr 632.0641; found, 632.0640.
[(PDCDipp)Pd(Br)]Br This compound was prepared via General Procedure B from proligand 6
(0.129 g, 0.186 mmol), Ag2O (0.043 g, 0.19 mmol) and [Pd(Br)2(MeCN)2] (0.068 g, 0.19 mmol). The title
compound was then purified on silica using MeOH and CH2Cl2 (1:6.5) to deliver 0.093 g of an off white
1
solid. Yield = 63%. H NMR (400 MHz, DMSO-d6, 25˚C) δ/ppm: 8.73 (d, J = 2.2 Hz, 2H), 8.68 (t, J = 8.2 Hz,
1H), 8.18 (d, J = 8.3 Hz, 2H), 7.88 (d, J = 2.1 Hz, 2H), 7.38 (t, J = 7.8 Hz, 2H), 7.20 (d, J = 7.8 Hz, 4H), 2.39
(p, J = 6.8 Hz, 4H), 1.10 (d, J = 6.8 Hz, 12H), 1.05 (d, J = 6.8 Hz, 12H). 13C NMR (101 MHz, DMSO, 25˚C)
δ/ppm: 169.38, 150.45, 146.30, 144.23, 133.78, 130.05, 126.49, 123.54, 118.77, 109.76, 28.03, 24.36,

22.93. HR-ESI-MS [M–2Br]2+ m/z: calculated for C37H44N6Pd 339.1326; found, 339.1330. X-ray quality
crystals of this compound were obtained by vapor diffusion of pentane into a saturated chloroform
solution. A labeled thermal ellipsoid plot for [(PDCCy)Pd(Br)]Br can be found in the Supporting
Information (Figure S4).
General Procedure for synthesis of [(PDCR)Pd(MeCN)](PF6)2 complexes. A combination of 1
equivalent of [(PDCR)Pd(Br)]Br and 2 equivalents of AgPF6 were dissolved in dry MeCN within a round
bottom flask in a N2 filled glovebox. The reaction vessel was then sealed with a rubber septum,
reinforced with copper wire and shielded from light with aluminum foil. The reaction vessel was then
taken out of the glovebox and heated in an oil bath at 70 °C for 3 hours. After cooling the reaction to
room temperature, the mixture was filtered through celite to remove the AgBr precipitate and the
filtrate was concentrated in vacuo. The resulting crude material was then recrystallized from
acetonitrile/ether to yield a white solid, which was collected via vacuum filtration. X-ray quality crystals
of each complex were grown by vapor diffusion of ether into concentrated acetonitrile solutions.
[(PDCMe)Pd(MeCN)](PF6)2 This complex was prepared via the general method described above
from [(PDCMe)Pd(Br)]Br (0.125 g, 0.0240 mmol) and AgPF6 (0.125 g, 0.0490 mmol) to deliver 0.139 g of
the title compound as a white solid. Yield = 83%. 1H NMR (400 MHz, DMSO-d6, 25˚C) δ/ppm: 8.62 (t, J =
8.2 Hz, 1H), 8.47 (d, J = 2.2 Hz, 2H), 8.03 (d, J = 8.3 Hz, 2H), 7.74 (d, J = 2.1 Hz, 2H), 3.93 (s, 6H). 13C NMR
(101 MHz, DMSO, 25˚C) δ/ppm: 165.68, 150.99, 147.52, 125.39, 118.45, 109.18, 99.56, 36.92, 1.25. HR-
ESI-MS [M–MeCN–2PF6]2+ m/z: calculated for C13H11N5Pd 172.5105; found, 172.5093
[(PDCEt)Pd(MeCN)](PF6)2 This complex was prepared via the general method described above
from [(PDCEt)Pd(Br)]Br (0.235 g, 0.0440 mmol) and AgPF6 (0.223 g, 0.0880 mmol) to deliver 0.269 g of
the title compound as a white solid. Yield = 87%. 1H NMR (400 MHz, CD3CN, 25˚C) δ/ppm: 8.44 (t, J = 8.3
Hz, 1H), 7.90 (d, J = 2.2 Hz, 2H), 7.67 (d, J = 8.3 Hz, 2H), 7.42 (d, J = 2.3 Hz, 2H), 4.24 (q, J = 7.3 Hz, 4H),
1.52 (t, J = 7.3 Hz, 6H). 13C NMR (101 MHz, CD3CN, 25˚C) δ/ppm: 166.54, 152.45, 149.23, 124.54, 119.45,
110.44, 47.14, 16.64, 1.32. Elemental Analysis (C17H20N6PdP2F12 + CH3OH: C 29.34; H, 3.28; N, 11.41,
Found C, 29.48; H, 2.86, N, 11.67) HR-ESI-MS [M–2PF6]2+ m/z: calculated for C17H20N6Pd 207.0388; found,
207.0387
[(PDCIpr)Pd(MeCN)](PF6)2 This complex was prepared via the general method described above
from [(PDCIpr)Pd(Br)]Br (0.80 g, 1.4 mmol) and AgPF6 (0.72 g, 2.9 mmol) to deliver 0.660 g of the title
1
compound as a white solid. Yield = 63%. H NMR (400 MHz, CD3CN, 25˚C) δ/ppm: 8.45 (t, J = 8.28 Hz,

1H), 7.94 and 7.51 (2 × d, J = 2.16 Hz, 2H), 7.69 (d, J = 8.28 Hz, 2H), 4.71 (septet, J = 6.73 Hz, 2H), 1.59 (d,
J = 6.73 Hz, 12H). 13C NMR (101 MHz, DMSO, 25˚C) δ/ppm: 163.91, 150.95, 147.56, 120.45, 119.37,
109.13, 51.86, 39.52, 22.57, 1.24. ESI-MS [M–2PF6]2+m/z: calculated for C19H24N6Pd 221.0549; found,
221.0552.
[(PDCCy)Pd(MeCN)](PF6)2. This complex was prepared via the general method described above
from [(PDCCy)PdBr]Br (0.333 g, 0.500 mmol) and AgPF6 (0.262 g, 1.00 mmol) to deliver 0.422 g of the
title compound as a white solid. Yield = 92%. 1H NMR (400 MHz, CD3CN, 25˚C) δ/ppm: 8.45 (t, J = 8.2 Hz,
1H), 7.93 (d, J = 1.9 Hz, 2H), 7.66 (d, J = 8.3 Hz, 2H), 7.50 (d, J = 2.0 Hz, 2H), 4.30 (tt, J = 12.2, 3.8 Hz, 2H),
2.14 – 1.96 (m, 12H), 1.81 (ddd, J = 24.5, 12.1, 2.9 Hz, 7H), 1.51 – 1.25 (m, 7H). 13C NMR (101 MHz,
CD3CN, 25˚C) δ/ppm: 165.61, 152.33, 149.20, 121.34, 119.63, 110.34, 61.42, 33.92, 26.15, 25.30. HR-ESI-
MS [M–2PF6]2+ m/z: calculated for C25H32N6Pd 261.0856; found, 261.0857.
[(PDCMes)Pd(MeCN)](PF6)2 This complex was prepared via the general method described above
from [(PDCMes)PdBr]Br (0.34 g, 0.48 mmol) and AgPF6 (0.24 g, 0.96 mmol) to deliver 0.302 g of the title
compound as a white solid. Yield = 70%. 1H NMR (400 MHz, CD3CN, 25˚C) δ/ppm: 8.55 (t, J = 8.24 Hz,
1H), 8.13 and 7.39 (2 × d, J = 2.10 Hz, 2H), 7.83 (d, J = 8.24 Hz, 2H), 7.07 (s, 4H), 2.29 (s, 6H), 2.07 (s,
12H). 13C NMR (101 MHz, CD3CN, 25˚C) δ/ppm: 169.02, 152.74, 149.41, 141.54, 135.86, 133.84, 130.09,
125.68, 120.29, 111.08, 20.90, 17.64. ESI-MS [M–PF6]+ m/z: calculated for C31H32N6PF6Pd 739.1371;
found, 739.1377.
[(PDCDipp)Pd(MeCN)](PF6)2 This complex was prepared via the general method described above
from [(PDCDipp)PdBr]Br (0.350 g, 0.439 mmol) and AgPF6 (0.222 g, 0.877 mmol) to deliver 0.421 g of the
title compound as a white solid. Yield = 99%. 1H NMR (400 MHz, CD3CN, 25˚C) δ/ppm: 8.61 (t, J = 8.3 Hz,
1H), 8.18 (d, J = 2.2 Hz, 2H), 7.88 (d, J = 8.4 Hz, 2H), 7.57 – 7.50 (m, 4H), 7.39 (d, J = 7.8 Hz, 4H), 2.52
(hept, J = 6.8 Hz, 4H), 1.99 (s, 3H), 1.18 (d, J = 6.9 Hz, 12H), 1.15 (d, J = 6.8 Hz, 12H). 13C NMR (101 MHz,
CD3CN, 25˚C) δ/ppm: 168.81, 152.79, 149.61, 146.43, 133.23, 132.69, 126.60, 125.35, 120.24, 111.24,
29.18, 24.17, 23.94. HR-ESI-MS [M–PF6]+ m/z: calculated for C37H44N6PF6Pd 823.2299, found 823.2303.
[((PDCCy)Ag(Br))2Ag]Br This silver NHC complex was synthesized using a modified literature
procedure.27 2,6-Bis[3-(cyclohexyl)imidazolium]pyridine dibromide (0.107 g, 0.200 mmol) and Ag2O
(0.046 g, 0.20 mmol) were combined in a round bottom flask to which 25 mL of CH2Cl2 was added. The
reaction flask was shielded from light using aluminum foil and stirred at room temperature for 12 hours.
The resulting mixture was then filtered through celite and the solvent was removed under reduced

pressure to yield the title compound (0.067 g, 60 % yield) as an off-white powder. Single crystals of the
title compound were obtained by diffusion of pentane into a saturated CH2Cl2 solution in an aluminum
1
foil covered vial. H NMR (400 MHz, DMSO-d6, 25˚C) δ/ppm: δ 8.50 (t, J = 8.0 Hz, 1H), 8.42 (s, 2H), 8.06
(d, J = 8.1 Hz, 2H), 7.79 (s, 2H), 3.88 (s, 2H), 1.69 – 1.28 (m, 13H), 1.05 (dd, J = 23.5, 16.4 Hz, 5H), 0.94 –
0.54 (m, 4H). HR-LIFDI-MS [M+H]+ m/z: calculated for C23H30N5AgBr 564.0724; found, 564.0734.

3. Results and Discussion
The synthetic pathway employed to prepare the pyridyl-spaced dicarbene palladium complexes
utilized in this study is shown in Scheme 1. The synthesis began with preparation of the substituted
imidazoles via condensation reactions with the properly substituted primary amines, glyoxal, and
ammonium chloride under acidic conditions. The N-alkyl substituted imidazoles were purified via
distillation while the N-aryl substituted derivatives were purified through sublimation. The substituted
imidazoles were then reacted with 2,6-dibromopyridine under neat conditions at elevated temperatures
for several days to afford the required bis-imidazolium proligands containing ancillary methyl (1),
isopropyl (3), cyclohexyl (4), mesityl (5), or 2,6-diisopropylphenyl groups (6). The pyridyl spaced bis-
imidazolium with ethyl substituents (2) was prepared via alkylation of 2,6-di(1H-imidazol-1-yl)pyridine
with ethylbromide. Each of the bis-imidazolium proligands were initially metallated by treatment with
Pd(OAc)2 in DMSO at 150 °C. the resultant [(PDCR)Pd(Br)]Br complexes could be purified by column
chromatography on silica, however, the yields for these metallation reactions were modest, ranging
from approximately 40–60%.
A side product isolated upon reaction of the bis-imidazolium proligand bearing cyclohexyl
groups with Pd(OAc)2 shed light on a limitation of this metallation procedure. Crystallization of this
Scheme 1. Synthetic pathways used to prepare the library of [(PDCR)Pd(MeCN)](PF6)2 complexes used in this
study.

Figure 2. (a) Solid-state crystal structure of complex [(PDCCy)Pd(Br)3], with thermal ellipsoids shown at 50% probability. All
hydrogen atoms except for H11 have been omitted for clarity. (b) Solid-state crystal structure of [((PDCCy)AgBr)2Ag]Br with
thermal ellipsoids shown at 30 % probability. All hydrogen atoms and an outersphere Br– counter anion are omitted for
clarity.
byproduct, revealed a complex in which a PdII(Br)3 core was bound to a single NHC on the PDCCy scaffold.
Figure 2 shows the thermal ellipsoid plot for this complex, in which a single NHC and three bromides
bind the square planar PdII center. The second imidazolium unit on the ligand backbone remains
protonated and serves as the counter cation to the anionic [(NHC)Pd(Br)3]– unit. Analysis of the solid-
state structure shown in Figure 2a showed that the interaction between the imidazolium proton on C11
and the nearest bromide ligand (Br1) is negligible,28,29 as the closest interaction between Br1 and H11
was measured to 2.864 Å, with a Pd–Br1–H11 angle of 71.35°. The presence of protonated ligand in the
isolated side product suggests that a limitation of this metallation procedure may be the mismatched
pKa(DMSO) values of acetate (~13)30 and imidazolium (~20)31, which precludes complete deprotonation of
the bis-imidazolium proligands.
In order to circumvent the formation of the unwanted side product shown in Figure 2a, we also
prepared the desired [(PDCR)Pd(Br)]Br complexes via transmetalation from the corresponding silver
carbene intermediates. This protocol involved treatment of the staring pyridyl-spaced bis-imidazolium
derivatives with Ag2O to produce the corresponding silver-carbene intermediates, which were then
transmetallated onto [Pd(Br)2(MeCN)2] in a one pot, two-step reaction. Each of the [(PDCR)Pd(Br)]Br
complexes of Scheme 1 were easily prepared using this method, utilizing much milder conditions than
the Pd(OAc)2 route described above.
Although the silver transmetalation strategy was generally carried out without isolation of the
silver–PDCR intermediate, this species was prepared, isolated and characterized for the cyclohexyl-

Figure 3. (a) The general numbering scheme used to label the [(PDCR)Pd(NCMe)](PF6)2 thermal ellipsoid plots,
except for the R = Mes and Dipp complexes, which contain mirror planes down the center of the structures
such that C1=C11, N1=N5, C2=C10, C3=C9, N2=N4, etc. (b) Solid-state crystal structures of each
–
[(PDCR)Pd(MeCN)](PF6)2 complex. Thermal ellipsoids are show at 50 % probability. All hydrogen atoms and PF6
counterions have been omitted for clarity.
appended PDCCy ligand by reacting the proligand and Ag2O in CH2Cl2 in a reaction vessel that was
protected from light. After filtration the silver-carbene containing product was isolated and crystallized.
The solid-state structure of this complex ([(PDCCy)AgBr)2Ag]Br) is shown in Figure 2b. The solid-state
structure shows a central unit containing two PDCCy ligands bridging three AgI atoms and an outersphere
bromide anion. The central Ag center (Ag2) is coordinated in trans-fashion by two separate PDCCy
carbenes through C11 and C24. The central Ag2 center is in proximity to Ag1 and Ag3 and displays a
Ag⋯Ag interaction (~3 Å) with both of these terminal silver atoms (Ag1 and Ag2). The distal separation
of Ag1–Ag2 (2.998 Å) and Ag2–Ag3 (3.068 Å) is slightly more than two times the covalent radius of Ag
(2.90 Å).32 Each of the PDCCy ligands that is bound to Ag2 bridges to the terminal Ag centers (Ag2 and
Ag3). In addition to being coordinated to a PDCCy carbene, both Ag2 and Ag3 contain a bromide ligand.
The NHC–Ag–Br bond angles are nearly linear. We note that bonding metrics similar to those observed
for [((PDCCy)AgBr)2Ag]Br have been reported for an analogous pyridine spaced bis-NHC complex of Ag33
as well as other Ag NHC complexes.34-36
Using the silver-transmetallation route we successfully prepared each of the [(PDCR)Pd(Br)]Br
complexes of Scheme 1. Solid-state structures for the [(PDCEt)Pd(Br)]Br, [(PDCCy)Pd(Br)]Br, and
[(PDCDipp)Pd(Br)]Br complexes, which are reproduced in the Supporting Information (Figures S2 – S4)
were also obtained and are similar to that of [(PDCMe)Pd(Br)]Br, which has been reported previously.1
Each of the [(PDCR)Pd(Br)]Br, complexes was converted to the corresponding solvato complex by
treating with two equivalents of AgPF6 in acetonitrile and removal of the precipitated AgBr. Single
crystals of each of the resulting [(PDCR)Pd(MeCN)](PF6)2, were obtained and analyzed by X-ray
crystallography and he resulting solid-state structures are shown in Figure 3; selected bond metrics are
reported in Table 1. All six [(PDCR)Pd(MeCN)](PF6)2 structures display a pseudo square planar geometry
at palladium with a C1–Pd–C11 bond angle approaching 160°. As can be seen in Table 2, the bond
lengths between the palladium center, PDCR ligand and acetonitrile solvato ligand are similar for the

Table 1. Selected bonding metrics for the series of [(PDCR)Pd(MeCN)](PF6)2 complexes shown in Figure 3.
[(PDCR)Pd(MeCN)](PF6)2
R = Me
2.030
1.961
2.031
2.003
1.338
1.347
158.85
178.97
R = Et
2.029
1.966
2.034
2.004
1.325
1.335
158.31
177.49
R = iPr
2.039
1.961
2.040
2.002
1.339
1.336
158.45
178.93
R = Cy
2.038
1.963
2.040
2.003
1.329
1.334
158.46
173.86
aR = Mes
2.016
1.957
2.035
1.999
1.338
1.337
157.94
178.23
aR = Dipp
2.019
1.959
2.019
2.008
1.347
1.347
158.52
180.00
C1–Pd (Å)
N3–Pd (Å)
C11–Pd (Å)
N6–Pd (Å)
C2–C3 (Å)
C9–C10 (Å)
C1–Pd–C11 (°)
N3–Pd–N6 (°)
aThe mesityl and diisopropylphenyl complexes lie on a mirror plane in the solid-state, which makes C1–Pd
equivalent to C11–Pd, and C2–C3 equivalent to C9–C10 for these two structures.
entire [(PDCR)Pd(MeCN)](PF6)2 series. When taken as a whole, these results suggest that variation of the
wingtip substituent on the PDCR ligand does not induce significant electronic differences for the ligand
backbone.
Although each of the PDCR ligands adopt similar structures when bound to palladium(II)
the varying wingtip substituents induce distinct levels of steric demand about the pyridyl-spaced
dicarbene ligand periphery. Two methods that have been developed to quantitate the steric properties
of ligands are calculation of percent buried volume (%VBur)
37 and the use of solid angles.38 Both analyses
can be completed with applications, which are freely available, 38,39 and utilize xyz coordinate files, which
we readily obtained from the crystal data for the [(PDCR)Pd(MeCN)](PF6)2 series. The %VBur is a measure
of the percentage of space occupied by a ligand within a sphere, of radius of 3.5 Å, centered at the
metal. In assessing the steric properties of the PDCR ligands, %VBur analyses were performed for each
[(PDCR)Pd(MeCN)](PF6)2 complex of Figure 3, following removal of the acetonitrile ligand. As such, each
of the %VBur(PDCR) values listed in Table 2 involved direct analysis of the [(PDCR)Pd]2+ cores for each of
the six complexes of Figure 3. Inspection of the %VBurPDCR values in Table 2 clearly shows that this
parameter is surprisingly similar (51.5–53.8%) for each of the PDC ligands containing alkyl (methyl, ethyl,
isopropyl and cyclohexyl) wingtip groups, indicating that increasing the size or branching of an alkyl
wingtip does not significantly perturb the steric bulk around the Pd center for the (PDCR)Pd platforms.
By contrast, substitution of the PDC wingtips with more sterically demanding mesityl and 2,6-
diisopropylphenyl groups results in a dramatic increase in the %VBurPDCR values. The mesityl group
(%VBur(PDCMes) = 57.1%) adds nearly 5% to the percent buried volume compared to the average alkyl
appended [(PDCR)Pd]2+ complex, while the 2,6-diisopropylphenyl (%VBur(PDCDipp) = 63.4%) adds nearly
11% compared to the average alkyl appended [(PDCR)Pd]2+ derivative. As such, these analyses indicate
that the Pd center is significantly more sterically encumbered for the [(PDCMes)Pd(MeCN)](PF6)2 and
[(PDCDipp)Pd(MeCN)](PF6)2 complexes.

Analyses involving solid angles were also conducted to assess the steric properties of the
[(PDCR)Pd(MeCN)](PF6)2 complexes.38 The solid angle paradigm is equivalent to placing a metal complex
within a sphere while replacing the metal center of the complex with a point source of light. The extent
to which a ligand shields the outer sphere from the light source/metal center is then determined as the
percentage of the shadowed areas over the surface area of the full sphere. Conducting this analysis for
the [(PDCR)Pd]2+ units delivered solid angles for each of the PDCR ligands. As shown in Table 3, the trends
observed for the G(PDCR) values are akin to the %VBur(PDCR) calculations described above. Each of the
[(PDCR)Pd(MeCN)](PF6)2 complexes with alkyl wingtips display similar G(PDCR) values that range from
51.45–54.63% and show a slight increasing trend as the size and branching of the alkyl substituent
increases. By contrast, the G(PDCR) values obtained for PDCMes and PDCDipp are larger than those for the
alkyl pyridyl-spaced dicarbenes by approximately 8% and 15%, respectively. A graphical representation
of the solid angles for the PDCR complexes with smallest ([(PDCMe)Pd(MeCN)](PF6)2) and largest
([(PDCDipp)Pd(MeCN)](PF6)2) wingtip substituents are shown in blue in Figure 4 and highlights the
difference in steric properties between the largest and smallest PDCR scaffolds we have considered in
this study.
Figure 4. Images obtained from the Solid
G
application for [(PDCMe)Pd(MeCN)](PF6)2 and
[(PDCDipp)Pd(MeCN)](PF6)2 complexes. The shaded blue regions represent the portion of the sphere
surrounding these complexes that is shielded by the corresponding PDCR ligands.

Figure 5. Cyclic voltammograms (CVs) recorded for (a) [(PDCEt)Pd(NCMe)](PF6)2, (b) [(PDCCy)Pd(NCMe)](PF6)2, (c)
[(PDCMes)Pd(NCMe)](PF6)2, and (d) [(PDCDipp)Pd(NCMe)](PF6)2 in DMF containing 0.1 M TBAPF6 under an atmosphere
of N2 (black traces) or CO2 (red traces).
Given our ongoing interest in electrochemistry and CO2 activation,40–45 we probed the ability of
the [(PDCR)Pd(MeCN)]2+ platform to reduce CO2. Voltammetric and controlled potential electrolysis
(CPE) experiments were carried out for [(PDCR)Pd(MeCN)](PF6)2 in which the wingtip groups were either
ethyl, cyclohexyl, mesityl or 2,6-diisopropylphenyl. Cyclic voltammograms (CVs) were recorded for each
of these complexes in DMF containing 0.1 M TBAPF6 as supporting electrolyte using a glassy carbon
working electrode. As shown in Figure 5, CVs were recorded under an atmosphere of N2 (black traces) or
CO2 (red traces).
Each complex displays two reduction waves under N2 and peak potentials for these redox events
are provided in Table 4. These peak potentials are similar to those observed for the previously studied
Table 2. Calculated values correlating to steric bulk around the metal center for each of the
[(PDCR)Pd(MeCN)](PF6)2 complexes of Figure 3.
[(PDCR)Pd(MeCN)](PF6)2
R = Me
51.45
17.46
68.91
67.42
51.5
R = Et
53.21
17.47
70.68
68.29
52.8
R = iPr
54.63
17.49
72.12
69.05
53.8
R = Cy
54.51
17.47
71.98
68.77
51.9
aR = Mes
62.60
17.13
79.73
73.89
57.1
aR = Dipp
69.43
17.35
86.78
80.53
63.4
G(L)PDC (%)
G(L)MeCN (%)
Sum G(L) (%)
G(L)Complex (%)
VBur(PDCR) (%)
G-values and %VBur(PDCR) values were calculated using the applications Solid G and SambVca, respectively.
Table 3. Electrochemistry data recorded for [(PDCR)Pd(MeCN)](PF6)2 complexes
Complex
aEred(1)
–0.90 V
–0.95 V
–0.96 V
–0.94 V
aEred(2)
–1.21 V
–1.28 V
–1.25 V
–1.25 V
bIp
~0
~0
5.17
7.10
[(PDCEt)Pd(NCMe)](PF6)2
[(PDCCy)Pd(NCMe)](PF6)2
[(PDCMes)Pd(NCMe)](PF6)2
[(PDCDipp)Pd(NCMe)](PF6)2
aRecorded in DMF containing 0.1 M TBAPF6 under an atmosphere of N2. Difference in current response at Ered(2) under CO2
versus under N2. All potentials referenced to Ag/AgCl.

[PDCnBuPd(MeCN)](BF4)2.10 The redox behavior for the two complexes of Figure 5 with alkyl wingtips
([(PDCEt)Pd(MeCN)](PF6)2 and [(PDCCy)Pd(MeCN)](PF6)2) is virtually unchanged under CO2, suggesting
these complexes do not electrochemically active carbon dioxide. By contrast, the redox behavior for the
two complexes containing the more sterically encumbered PDCMes and PDCDipp ligands both show
modest current enhancements at the second reduction wave (Ered(2) = –1.25 V versus Ag/AgCl) under
CO2. The magnitude of the current enhancements are 5.1 A and 7.1 A for [(PDCMes)Pd(MeCN)](PF6)2
and [(PDCDipp)Pd(MeCN)](PF6)2 respectively (Table 3).
CPE experiments were performed on the library of complexes shown in Figure 3. Each complex
was dissolved in DMF containing 0.1 M TBAPF6 that was saturated with CO2. When the CPE was
conducted at potentials more negative than –1.3 V versus Ag/AgCl, which is where the second reduction
feature is observed for the [(PDCR)Pd(MeCN)](PF6)2 complexes, the glassy carbon working electrode
became passivated. Inspection of the working electrode after the CPE showed a dark film had formed on
the electrode during the experiment and that this film suppressed redox processes at the electrode.
Similar behavior has also been observed for a previously studied [(PDCnBu)Pd(Br)]Br complex.10 When the
above CPE experiments were repeated at slightly less negative potentials (i.e., the Ered(2) values from
Table 4), fouling of the working electrode surface was avoided, however, these experiments only
produced trace amounts of CO, even upon addition of Lewis acids such as Mg(ClO4)2 to the electrolysis
solution, which is known to aid CO2 activation.46 Addition of weak Brønsted acids such as
trifluoroethanol also did not improve the catalysis observed for the [(PDCR)Pd(MeCN)](PF6)2 complexes.
We note that similar catalytic behavior has been reported for [(PDCnBu)Pd(Br)]Br and suggests that
pyridyl-spaced dicarbene complexes of this type are not sufficiently reducing enough to activate CO2
without forming insoluble deposits on carbon electrode surfaces.
Summary and Conclusions
In this study, we have reported the synthesis of a library of [(PDCR)Pd(MeCN)](PF6)2 complexes,
where R (i.e., the wingtip position of the pyridyl-spaced dicarbene ligand) was systematically varied to
include methyl, ethyl, isopropyl and cyclohexyl alkyl groups, as well as bulky mesityl or 2,6-
diisopropylphenyl groups. The synthesis of these complexes was accomplished via two complementary
routes; direct metallation of the pyridyl-spaced bis-imidazolum proligands (1–6) with Pd(OAC)2 in DMSO
produced the desired [(PDCR)Pd(Br)]Br complexes, however, unwanted side products of the form
[(PDCR)Pd(Br)3] were also obtained and identified by X-ray crystallography. By contrast, treatment of
proligands 1–6 with Ag2O to generate the corresponding silver bis-NHC complexes, which were then

transmetallated with Pd(Br)2(MeCN)2 to generate the same [(PDCR)Pd(Br)]Br complexes under much
milder conditions and in higher yields. Metathesis of each [(PDCR)Pd(Br)]Br with AgPF6 in MeCN
ultimately delivered each of the [(PDCR)Pd(MeCN)](PF6)2 complexes. Analysis of the solid-state
structures for each these systems revealed that the steric environment about the Pd center is similar for
each of the PDCR complexes with alkyl substituents at the ligand wingtip positions. By contrast, the steric
environment about the Pd center for the PDCMes and PDCDipp is much more encumbered as evidenced
from percent buried volume (%VBur) and solid angle calculations. Voltammetry experiments
demonstrated the steric contributions of the PDCR wingtip positions impacts the ability of the
[(PDCR)Pd(MeCN)](PF6)2 complexes to activate CO2. The Pd complexes supported by PDCMes and PDCDipp
show the most significant current enhancement under CO2, however, these systems form catalytically
inactive deposits at the electrode surface during CPE experiments, which obviates bulk scale CO2
electrolysis experiments. Although the complexes described herein are not effective for the bulk scale
electrochemical reduction of CO2, we believe the understanding this study provides on the effect of
wingtip sterics and related molecular design principles will be instructive for future efforts aimed at
developing catalysts for the electrochemical activation of CO2.
Acknowledgements
G.A.A. was supported through an NSF sponsored LSAMP “bridge to the doctorate fellowship”.
Research reported in this publication was supported by the National Science Foundation through
CAREER award CHE-1352120 and by the University of Delaware Research Foundation. NMR and other
data were acquired at the University of Delaware using instrumentation supported by NIGMS (5 P30
GM110758-02) at the NIH and CHE-1048367 from the NSF.
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