Versatile synthesis of PR2NR′ 2 ligands for molecular electrocatalysts with pendant bases in the second coordination sphere

Article abstract of DOI:10.1021/om201168h

Two 1,5-diaza-3,7-diphosphacyclooctane (P₂N₂) ligands with alkyl-substituted phosphines have been synthesized via a versatile method that allows for improved control of the phosphine substituent. The methyl- and benzyl-substituted phosphine P₂N₂ ligands (P ^Me₂N^Ph₂ and P^Bn ₂N^Ph₂) were synthesized and characterized by ³¹P{¹H} NMR, ¹H NMR, and elemental analysis, and their corresponding [Ni(P^R₂N^Ph ₂)₂](BF₄)₂ complexes were synthesized and characterized by ³¹P{¹H} NMR, ¹H NMR, and electrochemistry. The structure of the complex [Ni(P ^Me₂N^Ph₂)₂](CF ₃SO₃)₂ was characterized by X-ray crystallography.

Full text of DOI:10.1021/om201168h

Communication
pubs.acs.org/Organometallics
Versatile Synthesis of PR NR′2 Ligands for Molecular Electrocatalysts
2
with Pendant Bases in the Second Coordination Sphere
Michael D. Doud, Kyle A. Grice, Alyssia M. Lilio, Candace S. Seu, and Clifford P. Kubiak*
Department of Chemistry and Biochemistry, University of California San Diego, 9500 Gilman Drive MC 0358, La Jolla, California
92093, United States
S
* Supporting Information
ABSTRACT: Two 1,5-diaza-3,7-diphosphacyclooctane
(P2N2) ligands with alkyl-substituted phosphines have been
synthesized via a versatile method that allows for improved
control of the phosphine substituent. The methyl- and benzyl-
substituted phosphine P2N2 ligands (PMe2NPh and PBn2NPh
)
2
2
1
were synthesized and characterized by 31P{1H} NMR, H
NMR, and elemental analysis, and their corresponding
1
[Ni(PR NPh2)2](BF4)2 complexes were synthesized and characterized by 31P{1H} NMR, H NMR, and electrochemistry. The
2
structure of the complex [Ni(PMe2NPh2)2](CF3SO3)2 was characterized by X-ray crystallography.
The catalytic reductions of protons to H2 and of small
molecules such as CO2 and N2 pose significant chemical
challenges. Transformations of these molecules are often
hindered by high kinetic barriers for the formation of key
intermediates; however, these reactions can be facilitated by
proton-coupled electron transfer (PCET) reduction processes.1
Many enzymes take advantage of PCET to manage electron
transfers at low overpotentials.2 One particularly interesting
example is the [FeFe]hydrogenase enzyme (A), which contains
an azadithiolate ligand whose nitrogen base is poised to deliver
or accept a proton during the reversible oxidation of H2, as
shown in Figure 1.3 Many groups have developed both
of H2 oxidation,5 proton reduction to H2,6 O2 reduction,7 and
more recently the oxidation of formate.8
One reason PR NR′ systems exhibit high electrocatalytic
2
2
performances lies in their ability to take on multiple
conformations that orient the nitrogen bases in ideal positions
for assisting proton-coupled processes.9 The utility of these
complexes is additionally expanded by the fact that both the
phosphine and amine substituents of the ligand can be varied in
order to tune the thermodynamic properties and reactivity of
the complexes. The basicity of the pendant amine can be tuned
via substitution at R′,10 while the redox potentials of the metal
center can be modified via substitution at R and to a lesser
extent at R′. More donating substituents at R generally lead to
more negative reduction potentials for the metal complexes.
The steric properties of R also affect the bite angles and
dihedral angles of the two ligands on the metal center, which
have a substantial effect on the hydricities (hydride donation
abilities) of the corresponding [NiH(PR NR′ )2]+ species and
2
2
the reduction potentials of the metal complexes.11 The
development of new PR NR′ catalysts has been hampered by
2
2
the limited availability of starting materials for the ligand
synthesis. This arises from the fact that the first step in the
literature synthesis is the dihydroxymethylation of a primary
phosphine (Scheme 1).12
Primary phosphines are highly air-sensitive, noxious, toxic,
and often pyrophoric chemicals whose commercial availability
in bulk quantities is limited to cyclohexyl- and phenyl-
phosphine. While some primary phosphines can be synthesized
via alkylation of PH3 or via the Arbuzov reaction followed by
reduction, these routes require that the primary phosphine be
isolated from the reactions prior to use in the synthesis of P2N2
Figure 1. Structures of the [FeFe]hydrogenase active site (A) and the
[Ni(PR NR′2)2]2+ cation (B).
2
structural and functional mimics of this active site, incorporat-
ing various pendant bases and studying their ability to support
hydrogen transfer and bonding in H2 activation and other
reactions.4 One of the most successful functional models of the
hydrogenase enzyme to date is the [Ni(PR NR′ )2]2+ system
2
2
(B), also shown in Figure 1. Different variants have shown
Received: November 22, 2011
Published: January 31, 2012
excellent electrocatalytic rates for the proton-coupled processes
© 2012 American Chemical Society
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dx.doi.org/10.1021/om201168h | Organometallics 2012, 31, 779−782

Organometallics
Communication
Scheme 1. Traditional PR NR′ Ligand Synthesis
2
2
ligands.13 Ideally, ligand syntheses should use stable and
commercially available reactants, be concise, and be scalable for
Figure 2. PR NPh ligands with methyl (5) and benzyl (6) phosphine
substituents synthesized from THPCl.
2
2
the production of large quantities.
Here, we report a new synthetic route to P2N2 ligands that
meets the criteria above for convenience, cost, simplicity, and
scalability. A report by Griffiths and co-workers indicates that
alkylbis(hydroxymethyl)phosphines (4) can be synthesized
utilizing a cheap, commercially available, and air-stable reagent:
tetrakis(hydroxymethyl)phosphonium chloride (THPCl, 1).14
This route removes the need to synthesize or utilize a primary
phosphine. We demonstrate its use in the synthesis of PR NPh
alkyl-substituted P2N2 ligands affect the reduction potentials of
the associated [Ni(PR NPh2)2]2+ complexes. The electro-
2
chemical properties of such complexes with R = Ph, Cy are
well documented in the literature.5−9 The nickel complexes
[Ni(PR NPh2)2(CH3CN)n](BF4)2 (R = Me, n = 0, 7; R = Bn, n
2
= 1, 8) were synthesized analogously to the previously reported
complexes (Scheme 3).
2
2
These complexes typically give cyclic voltammograms
consisting of two reversible one-electron redox processes
associated with E°(NiII/I) and E°(NiI/0), exemplified by the
CV of [Ni(PMe2NPh2)2](BF4)2 (7) shown in Figure 3.8 Table 1
compares the reduction potentials in benzonitrile for previously
reported complexes with those synthesized in this work. The
methyl-substituted phosphine ligands on [Ni(PMe2NPh2)2]2+ (7)
ligands with methyl and benzyl substituents on the phosphine
(Scheme 2).
Dehydroxymethylation of THPCl (1) under basic conditions
in neat triethylamine or KOH in ethanol yields tris-
(hydroxymethyl)phosphine (THP, 2).15 This phosphine reacts
with alkyl halides, yielding the alkyltris(hydroxymethyl)-
phosphonium halide salt (3). Methyl iodide and benzyl
chloride were successfully used as alkylating agents for this
reaction. This selection of compounds demonstrates the
versatility of the synthesis: alkylating agents can be chosen
irrespective of the halide leaving group and with concern for
safety, utility, and expense. After alkylation, the compound is
once more dehydroxymethylated in neat triethylamine to form
4. From here the synthesis of the PR NPh ligands 5 and 6
shift both reduction potentials of the nickel complex more
+/0
negative against the FeCp2
reference in comparison to
[Ni(PPh2NPh2)2]2+ (9) and the previously reported [Ni-
(PCy2NPh2)2]2+ (10).8 The electron-donating character of the
alkylphosphines (R = Me, Bn) is expected to be greater than
that of the aryl phosphine (R = Ph). However, it is notable that
the the benzyl-substituted P2N2 complex 8 is reduced at nearly
the same potential as the phenyl-substituted complex 9. The
decrease in steric bulk at R as compared to the cyclo-
hexylphosphine-substituted P2N2 ligands may have the effect of
reducing the distortion toward a tetrahedral geometry in the
Ni(II) state. The partial tetrahedral distortion in complexes
ligated with the bulky cyclohexylphosphine P2N2 ligands lowers
the NiII/I reduction potential and stabilizes the Ni(I) state. Less
bulky alkyl substituents may favor a square-planar geometry in
the Ni(II) state, which raises the NiII/I reduction potential and
2
2
proceeds in the same fashion as the traditional ligand synthesis,
as 4 is treated with 1 equiv of a primary amine (R′NH2; aniline
for 5 and 6) to form the corresponding PR NR′ . The overall
2
2
synthesis from 1 gave yields of 63% for 5 and 13% for 6 (Figure
2).
Ligands 5 and 6 were isolated and characterized by elemental
16
1
analysis, H NMR, and 31P{1H} NMR. We have used these
ligands to prepare their homoleptic nickel complexes, and their
cyclic voltammograms were examined to study how these P-
Scheme 2. PR NR′ Synthesis via Dehydroxymethylation and Alkylation of THPCl (1)
2
2
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Communication
Scheme 3. Synthesis of Homoleptic Nickel Complexes of PR NR′ Ligands
2
2
Figure 4. X-ray crystal structure of [Ni(PMe2NPh2)2](CF3SO3)2 (11)
with protons, cocrystallized solvent, and counterions excluded for
clarity. Ellipsoids are shown at the 50% probability level. Selected bond
distances (Å) and angles (deg): Ni−P1 = 2.2393(11), Ni−P1′ =
2.2393(11), Ni−P2 = 2.2088(12), Ni−P2′ = 2.2089(12); P1−Ni−P2
= 83.16(4), P1−Ni−P1′ = 99.64(6), P1′−Ni−P2′ = 83.16(5), P2−Ni−
P2′ = 96.06(6). P1 and P1′ and P2 and P2′ are related by symmetry.
Figure 3. Cyclic voltammogram of 2.0 mM [Ni(PMe2NPh2)2](BF4)2
(7) in benzonitrile/0.2 M Bu4NPF6 with a 1 mm glassy-carbon
working electrode. Scan rate: 0.1 V s−1.
differs significantly from the reported structures of the
[Ni(PBn2NPh2)2(CH3CN)]2+ and [Ni(Pn‑Bu2NPh2)2(CH3CN)]2+
cations, both of which exhibit distorted-trigonal-bipyramidal
Table 1. Reduction Potentials for the Series of Complexes
[Ni(PR NPh2)2(CH3CN)](BF4)2 in Benzonitrile
2
structures.15 The structure of 11 deviates slightly from a
b
Complex
E°(NiII/I) (V vs Cp2Fe+/0
)
E°(NiI/0) (V vs Cp2Fe+/0
)
a
b
b
square-planar geometry, with the Ni center lying on a
crystallographic 2-fold axis and displaying a dihedral angle of
16.16° between the two P−Ni−P planes.
7 (Me, Ph)
8 (Bn, Ph)
9 (Ph, Ph)
−1.01
−0.78
−0.79
−0.60
−1.30
−1.16
−1.00
−1.10
b
b
b
b
c
c
We have demonstrated a versatile synthetic route for the
production of P-alkyl P2N2-type ligands that avoids the use of
primary phosphines. This synthesis gives unprecedented
control over the substitution at the phosphorus atoms in the
ligand, which directly modifies the donating character of the
ligand and the redox potentials of the associated metal
10 (Cy, Ph)
a
b
c
7 does not bear a CH3CN ligand. This work. Reference 8.
destabilizes the Ni(I) state.10 Indeed, from Table 1 it can be
seen that as the steric bulk of the phosphine groups increases
(Me < Ph < Bn < Cy), the reduction potential E°(NiII/I
)
complexes. The homoleptic nickel complexes of the PMe2NPh
2
becomes less negative (Me < Ph < Bn < Cy).
and PBn2NPh ligands have been synthesized and characterized,
2
The importance of the steric effect exerted by the phosphine
substituent is illustrated by the X-ray crystal structure of the
[Ni(PMe2NPh2)2]2+ complex. X-ray-quality crystals were ob-
tained for [Ni(PMe2NPh2)2](CF3SO3)2 (11) (Figure 4) by vapor
diffusion of diethyl ether into acetonitrile. The noncoordinative
nature of both the [BF4]− and [CF3SO3]− anions allow for
and the reduction potentials of the two complexes have been
compared with previously reported PR NPh complexes (R =
2
2
Ph, Cy). A homoleptic nickel PMe2NPh complex has also been
2
characterized by X-ray crystallography.
This new route to P2N2 ligands invites the synthesis of P2N2
ligands with a multitude of substituents on the phosphine. We
are in the process of employing this control to impart further
functionality on P2N2 ligands and their metal complexes. While
previous studies have attempted to attach P2N2 catalysts to
electrode surfaces, they have been limited by the traditional
synthesis, which has largely restricted ligand functionalization
to the amine.17 Attaching the amine to a surface limits the
flexibility of the pendant base, which has been shown to be
reasonable comparisons between the [Ni(PR NR′ )]2+ cations.
2
2
This structure differs from most previous [Ni(PR NR′ )2]2+
2
2
complexes in that acetonitrile is not coordinated to the metal
center in the crystal structure;8−10 this was previously observed
only for the [Ni(PCy2NBz2)2]2+ complex.9 The reduced steric
influence exerted by the methyl substituent yields a distorted-
square-planar structure for the [Ni(PMe2NPh2)2]2+ cation, which
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Organometallics
Communication
critical to the electrocatalytic activity of the P2N2 complexes.10
By combining the synthesis described in this report with
electrode surface modification techniques, the freedom of the
pendant base should be preserved while attachment of an
appropriate functional group to the phosphine substituent will
enable a direct connection between the electrode and the
catalytic site.
Ibrahim, S. K.; De Gioia, L.; Davies, S. C.; Yang, X.; Sawers, G.;
Pickett, C. J. Nature 2005, 433, 610−613.
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(6) Wilson, A. D.; Shoemaker, R. K.; Miedaner, A.; Muckerman, J. T.;
DuBois, D. L.; DuBois, M. R. Proc. Natl. Acad. Sci. U.S.A. 2007, 104,
6951−6956.
In addition, we are using this synthesis to modify the
(7) Yang, J. Y.; Bullock, R. M.; Dougherty, W. G.; Kassel, W. S.;
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electrocatalytic activity of PR NR′ complexes of nickel and
2
2
other metals. By increasing the electron-donating character of
the phosphine substituent, the redox potentials of the metal
(8) Galan, B. R.; Schoffel, J.; Linehan, J. C.; Seu, C.; Appel, A. M.;
̈
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PR NR′2 complexes can be pushed to more negative values, and
2
the hydricities of the corresponding [NiH(PR NR′ )]+ com-
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D. L. Organometallics 2007, 26, 3918−3924.
2
2
plexes should also increase.18 This thermodynamic property
can be used to predict whether a complex will oxidize H2 or
reduce protons under the given reaction conditions.19 Our goal
is to find a similar balance in the new complexes with regard to
formate oxidation and carbon dioxide reduction.8 Adding more
electronically donating and less sterically bulky substituents
increases the hydride donation ability and should push formate
oxidation catalysis toward its microscopic reverse, the single
proton-coupled two-electron reduction of carbon dioxide to
formate or further reduction products. Further studies of P2N2
ligands synthesized by this method and their corresponding
metal complexes will be reported in due course.
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̈
21, 1409−1412.
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ASSOCIATED CONTENT
* Supporting Information
(16) (a) The P2N2 ligand PMe2NPh has been synthesized previously,
■
2
S
but spectral data were not reported. See: Markel, V. G.; Jin, G. Y.;
̈
Schoener, C. Tetrahedron Lett. 1980, 31, 1409−1412. (b) PBn2NPh2 and
its nickel complexes have been synthesized via the traditional P2N2
synthesis. See: Kilgore, U. J.; Stewart, M. P.; Helm, M. L.; Dougherty,
W. G.; Kassel, W. S.; DuBois, M. R.; DuBois, D. L.; Bullock, R. M.
Inorg. Chem. 2011, 50, 10908−10918.
Text and figures giving experimental details and cyclic
voltammograms for compounds prepared in this paper and a
CIF file giving X-ray crystal data for 11. This material is
available free of charge via the Internet at http://pubs.acs.org.
(17) Le Goff, A.; Artero, V.; Jousselme, B.; Tran, P. D.; Guillet, N.;
AUTHOR INFORMATION
Corresponding Author
*E-mail: ckubiak@ucsd.edu.
■
́ ́
Metaye, R.; Fihri, A.; Palacin, S.; Fontecave, M. Science 2009, 326,
1384−1387.
(18) Berning, D. E.; Noll, B. C.; DuBois, D. L. J. Am. Chem. Soc.
1999, 121, 11432−11447.
Notes
(19) Berning, D. E.; Miedaner, A.; Curtis, C. J.; Noll, B. C.; DuBois,
M. R.; DuBois, D. L. Organometallics 2001, 20, 1832−1839.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This material is based upon work supported by the Air Force
Office of Scientific Research through the MURI program under
AFOSR Award No. FA9550-10-1-0572. Financial support of
the National Science Foundation (Grant No. CHE-0741968) is
gratefully acknowledged. Professor Arnold Rheingold and Dr.
Curtis Moore are acknowledged for assistance with crystallog-
raphy. Dr. Yongxuan Su is acknowledged for mass spectrometry
services. Dr. D. L. DuBois and Dr. A. M. Appel from PNNL are
gratefully acknowledged for helpful discussions. Dr. U. J.
Kilgore is acknowledged for independent verification of the
PMe2NPh synthesis.
2
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