Synthesis and reactivity studies of a [Cp?Rh] complex supported by a methylene-bridged hybrid phosphine-imine ligand

Article abstract of DOI:10.1016/j.jorganchem.2020.121294

[Cp?Rh] complexes (Cp? = η⁵-pentamethylcyclopentadienyl) supported by bidentate chelating ligands are useful in studies of redox chemistry and catalysis, but little information is available for derivatives bearing “hybrid” [P,N] chelates. Here,

Full text of DOI:10.1016/j.jorganchem.2020.121294

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Synthesis and reactivity studies of a [Cp*Rh] complex supported by a methylene-
bridged hybrid phosphine-imine ligand
Julie A. Hopkins, Davide Lionetti, Victor W. Day, James D. Blakemore
PII:
S0022-328X(20)30196-0
DOI:
https://doi.org/10.1016/j.jorganchem.2020.121294
JOM 121294
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 30 January 2020
Revised Date: 10 April 2020
Accepted Date: 11 April 2020
Please cite this article as: J.A. Hopkins, D. Lionetti, V.W. Day, J.D. Blakemore, Synthesis and reactivity
studies of a [Cp*Rh] complex supported by a methylene-bridged hybrid phosphine-imine ligand, Journal
of Organometallic Chemistry (2020), doi: https://doi.org/10.1016/j.jorganchem.2020.121294.
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Synthesis and Reactivity Studies of a [Cp*Rh] Complex Supported
by a Methylene-Bridged Hybrid Phosphine-Imine Ligand
Julie A. Hopkins,# Davide Lionetti,#,† Victor W. Day,# and James D. Blakemore#,*
Department of Chemistry, University of Kansas, 1567 Irving Hill Road, Lawrence, KS 66045, United
States
Supporting Information Placeholder
Abstract
[Cp*Rh] complexes (Cp* = η5-pentamethylcyclopentadienyl) supported by bidentate chelating ligands are
useful in studies of redox chemistry and catalysis, but little information is available for derivatives bearing
“hybrid” [P,N] chelates. Here, the preparation, structural characterization, and chemical and electrochemical
properties of a [Cp*Rh] complex bearing the κ2-[P,N]-2-[(diphenylphosphino)methyl]pyridine ligand (PN) are
reported. Cyclic voltammetry data reveal that [Cp*Rh(PN)Cl]PF6 (1) undergoes a chemically reversible, net
two-electron reduction at –1.28 V vs. ferrocenium/ferrocene, resulting in generation of a rhodium(I) complex
1
(3) that is stable on the timescale of the voltammetry. However, H and 31P{1H} NMR studies reveal that
chemical reduction of 1 generates a mixture of products over a 1 h timescale; this mixture forms as a result of
deprotonation of the methylene group of 1 by 3 followed by further reactivity. The analogous complex
[Cp*Rh(PQN)Cl]PF6 (2; PQN = κ2-[P,N]-8-(diphenylphosphino)quinoline) does not undergo self-
deprotonation or further reactivity upon two-electron reduction, confirming the reactivity of the acidic
backbone methylene C–H bonds in the PN complexes. Comparison of the electrochemical properties 1 and 2
also shows that the extended conjugated system of PQN contributes to an additional ligand-centered redox
event
for
2
that
is
absent
for
1.
catalyze production of H2. The complexes are
supported by the η5-pentamethylcyclopentadienyl
(Cp*) ligand, as well as an additional bidentate
ligand. The parent catalyst in this family,
[Cp*Rh(bpy)Cl]+ (A, Chart 1), is supported by the
workhorse 2,2´-bipyridyl (bpy) ligand. This
complex was reported in 1987 by Kölle and
Grätzel as a catalyst for H2 production,4 and by
other groups to serve as a catalyst for NAD+
reduction to NADH.5 For many years, this catalyst
was presumed to operate via a rhodium(III)
hydride intermediate, although such a species had
not been isolated or fully characterized. New
interest in this species was piqued by reports from
our group6 and others7 showing that exposure of
the reduced form of A, Cp*Rh(bpy), to weak
proton sources results in generation of an isolable
1. Introduction
Identification of ligand structure-function
relationships is an important strategy for rational
design of organometallic catalysts. In redox
chemistry, substitution of one ligand for another
often results in significant changes to the
properties of the target metal complex, for example
in shifted reduction potential, reactivity, or
basicity.1 Such shifts can often lead to marked
changes in catalytic effectiveness, especially in
molecular catalysis of small-molecule activation
reactions requiring effective management of both
protons (H+) and reducing equivalents (e–). Efforts
to understand how ligands impact transfer of H+/e–
form the basis of strategies aimed at tuning of
catalysts and improving catalytic efficiencies.2
However, these efforts remain challenging due to
the propensity of both metal centers and ligands to
become directly involved in reactivity.3
compound
bearing
the
endo-η4-
pentamethylcyclopentadiene ligand (η4-Cp*H),
generated by transfer of H+ to the Cp* ring.8 9 (n.b.,
,
We have been working to understand the
reactivity properties of
the endo face of the ring is defined as the one
facing the metal center). Related reactivity studies
have demonstrated that the exposure of the
compound bearing (η4-Cp*H) to a sufficiently
a model family of
organometallic rhodium complexes that can serve,
in several cases, to couple H+/e– equivalents and

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strong acid results in quantitative generation of H2
and regeneration of (η5-Cp*). Inspired by this
rather uncommon chemistry, we have been
investigating synthesis and reactivity of other
[Cp*Rh] complexes supported by various
bidentate ligands
diimine ligands and dppb. In particular, use of
PQN favors generation of an isolable [RhIII–H]
species that can, unlike its dppb analogue, undergo
protonolysis and evolve H2 (see Scheme 1).
Moreover, 2 displays especially rich reductive
electrochemistry, undergoing
a
two-electron
reduction to form Cp*Rh(PQN) at –1.19 V vs.
Fc+/0
(all
potentials
referenced
to
+
+
ferrocenium/ferrocene, denoted hereafter as Fc+/0)
that can be further reduced by one electron at –
Rh
N
Rh
N
Cl
Ph2P
Cl
Ph2P
[Cp*Rh(PN)Cl]+
[Cp*Rh(PQN)Cl]+
(1)
(2)
+
+
Rh
Rh
Cl
Cl
N
Ph2P
N
PPh2
2.26 V vs. Fc+/0.
[Cp*Rh(bpy)Cl]+
[Cp*Rh(dppb)Cl]+
(A)
(B)
Scheme 1. H2 evolution chemistry with the Cp*Rh(PQN)
platform.
Chart 1. Series of half-sandwich rhodium complexes.
Despite these features, the only [Cp*Rh]
system supported by a hybrid [P,N] chelating
ligand that has been investigated for its reactivity
properties is this example bearing PQN.15 Because
of the especially useful properties engendered by
this donor set, we imagined that probing the
synthesis and reactivity profile of [Cp*Rh] species
supported by other hybrid [P,N] ligands could be a
useful strategy for understanding the reactivity
properties of this family of compounds. Along this
line, our attention was drawn to the κ2-[P,N]-2-
Along this line, we have shown that reduction
and protonation of a [Cp*Rh] complex (B) bearing
the 1,2-bis(diphenylphosphino)benzene (dppb)
ligand affords access to a stable rhodium(III)
hydride.10 Unlike the [η4-Cp*H] complexes
mentioned above, this [RhIII–H] complex is
remarkably resistant to protonolysis.6,7,11,12 This
result agrees with the high apparent stability of
other half-sandwich rhodium hydride complexes
studied in different contexts,13 as well as the
reactivity properties of rhodium hydrides
supported by other ligand sets.14 However, the
high stability of the dppb-supported hydride
renders it somewhat less applicable to model
studies of more reactive [Cp*Rh] systems.
[(diphenylphosphino)methyl]pyridine
commonly known as PN; this ligand is chelating,
much like PQN, but presents an
ligand,
alkyldiarylphosphine donor and an imine donor for
binding to a central metal. Importantly, although
the coordination chemistry of PN ligands has been
studied with a variety of transition metals,16 only a
few Rh complexes supported by PN are known.17
In particular, we were pleased to find that
structural data from X-ray diffraction (XRD)
analysis was available in the Cambridge Structural
More recently, we have reported the synthesis
and reactivity of a [Cp*Rh] complex (2) supported
by
a
hybrid
ligand,
8-
(diphenylphosphino)quinoline (PQN).15 PQN
presents a triarylphosphine donor and an imine
donor with appropriate placement for binding to
[Cp*Rh] in a five-membered metallacycle. Use of
PQN engenders intriguing properties that are
intermediary between those of systems bearing
Database18
for
[Cp*Rh(PN)Cl]PF6
(1),19
suggesting that this compound could be prepared.
However, no information on the synthetic methods

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needed for preparation of 1 or information
regarding its properties are available, encouraging
further work to establish how its reactivity
compares with other members of our family of
complexes from previous studies.
literature procedure.20 The PN ligand was
synthesized by the method of Hung-Low.21
Deuterated NMR solvents were purchased from
Cambridge Isotope Laboratories; CD3CN was
dried over molecular sieves and C6D6 was dried
over sodium/benzophenone. 1H, 13C, and 31P NMR
spectra were collected on 400 or 500 MHz Bruker
spectrometers and referenced to the residual
Here, we report the synthetic procedures,
characterization data, and reactivity studies of the
[Cp*Rh] complex 1 supported by PN. New
structural data for 1 from XRD are compared both
to that previously available19 as well as to our prior
data for the PQN-ligated analogue 2;15 these
analyses suggest that PN is a slightly more
effective donor than PQN to Rh. Consistent with
this structural data, 1 undergoes a net two-electron
reduction near –1.28 V vs. Fc+/0, a value that is
significantly more negative than that of 2, resulting
in generation of a transient rhodium(I) complex (3)
that is stable on the timescale of voltammetry. 3
displays no further reductions in our accessible
solvent window. Chemical reduction of 1 with
Na(Hg), however, does not yield 3 as an isolable
compound; rather, a mixture of products forms as a
result of further reactivity leading to generation of
several observable compounds that has been
studied by 1H and 31P{1H} nuclear magnetic
resonance (NMR). Taken together, the results
suggest that the highly basic nature of 3 and the
acidic backbone C–H bonds of PN engender
unexpected reactivity upon chemical reduction of 1
that is unavailable in 2, which lacks acidic
backbone protons. Avoidance of acidic moieties in
supporting ligands is thus a strategy that can guide
future work with highly basic [Cp*Rh] complexes
intended for study of H+/e– management in
catalysis.
1
protio-solvent signal22 in the case of H and 13C.
Heteronuclear NMR spectra were referenced to the
appropriate external standard following the
recommended scale based on ratios of absolute
frequencies (Ξ).23
2.2 Electrochemistry
Electrochemical experiments were carried out
in a nitrogen-filled glove box. 0.10 M tetra(n-
butylammonium)hexafluorophosphate
(Sigma-
Aldrich; electrochemical grade) in acetonitrile
served as the supporting electrolyte. Measurements
were made with a Gamry Reference 600 Plus
Potentiostat/Galvanostat using a standard three-
electrode configuration. The working electrode
was the basal plane of highly oriented pyrolytic
graphite (HOPG) (GraphiteStore.com, Buffalo
Grove, Ill.; surface area: 0.09 cm2), the counter
electrode was a platinum wire (Kurt J. Lesker,
Jefferson Hills, PA; 99.99%, 0.5 mm diameter),
and a silver wire immersed in electrolyte served as
a pseudo-reference electrode (CH Instruments).
The reference was separated from the working
solution by a Vycor frit (Bioanalytical Systems,
Inc.). Ferrocene (Sigma Aldrich; twice-sublimed)
was added to the electrolyte solution at the
conclusion of each experiment (~1 mM); the
midpoint potential of the ferrocenium/ferrocene
couple (denoted as Fc+/0) served as an external
standard for comparison of the recorded potentials.
Concentrations of analyte for cyclic voltammetry
were typically 1 mM.
2. Experimental
2.1 General Considerations
2.3 Crystallography
All manipulations were carried out in dry N2-
filled gloveboxes (Vacuum Atmospheres Co.,
Hawthorne, CA) or under N2 atmosphere using
standard Schlenk techniques unless otherwise
noted. All solvents were of commercial grade and
dried over activated alumina using a PPT Glass
Contour (Nashua, NH) solvent purification system
prior to use, and were stored over molecular
sieves. All chemicals were from major commercial
suppliers and used as received after extensive
drying. [Cp*RhCl2]2 was prepared according to
A full hemisphere of diffracted intensities (560
5-second frames with an ω scan width of 1.00°)
was measured for a single-domain crystal of 1
using graphite-monochromated Mo Kα radiation (λ
= 0.71073 Å) on a Bruker SMART APEX CCD
Single Crystal Diffraction System. X-rays were
provided by a fine-focus sealed X-ray tube
operated at 50 kV and 35 mA. Preliminary lattice
constants were determined with the Bruker
program SMART.24
The Bruker program