Supporting Information
Selectivity for HCO
2
-
over H
2
in the electrochemical catalytic
reduction of CO
2
by (POCOP)IrH
2
Samantha I. Johnson, Robert J. Nielsen,* William A.
Goddard, III,
Materials and Process Simulation Center (MSC) and J
oint Center for Artificial
Photosynthesis (JCAP) California Institute of Techn
ology, Pasadena CA 91125
*smith@wag.caltech.edu
Appendix 1: Scheme S1 – Reduction of the cationic a
cetonitrile complex
..................1
Appendix 2: Figure S1 – HOMO of the Ir
I
complex
..................................................
...2
Appendix 3: pKa calculations with water clusters
...................................................
.......3
Appendix 4: Scheme S2 – Quaternary amine POCOP
..................................
.....4
Appendix 5: Scheme S3 – Additional transition state
s for protonation
..................5
Appendix 6: Figure S2 – Structures along the proton
ation reaction coordinate
.......6
Appendix 7: Figure S3 – Hydricities compared to oth
er hydridic compounds
.........7
Appendix 8: Geometries of Molecules
...................................................
...........................8
Appendix 9: Calculation Details
...................................................
....................................9
1
Appendix 1: Calculation of the doubly reduced aceto
nitrile complex
Scheme S1: Free energies calculated in acetonitrile
at -1.2V vs SHE. Reduction with loss of solvent i
s
preferred to a two-electron reduction of the solven
to complex as previously proposed,
1
which leads to
a reduced acetonitrile adduct.
In previous work
1
a doubly reduced cationic solvento complex was cal
culated, as shown
below. However, the geometry showed a bent acetonit
rile, suggesting that the acetonitrile
had been reduced, not the metal center. When this c
omplex is recalculated with our
methods, we compute a reduction potential of 01.8,
which is inconsistent with the
experimentally observed reduction potential.
2
Appendix 2
Figure S1: HOMO of Ir
I
complex
The HOMO 01 and HOMO of POCOP0Ir
I
hydride anion (
Mol
7
) is shown. The high
electron density in axial positions explains why oxygen cannot coordinate simultaneously
in a transition state analogous to
TS
4
.
HOMO -1
HOMO
3
Appendix 3: Justification for Calculations with Wat
er Clusters
The calculated free energy and pKa for the auto0dis
sociation of water is used to justify
our use of an explicit four0water cluster (plus con
tinuum solvation) in transition state
calculations. A neutral 4H
2
O and anionic OH
0
3H
2
O cluster were used.
4 H
2
O (liq)
H
+
(1M) + OH
0
3H
2
O (1 M)
<G
calc
= 20.2 kcal/mol
<G
exp
= 19.05 kcal/mol
4
Appendix 4
(a)
(b)
Scheme S2: (a) Structures of quaternary amine POCOP
complexes, (b) Free
energies of reactions featuring the full ligand ver
sus the truncated ligand.
Experimentally a quaternary amine solvation handle
(1,10piperazinium) was added to the
standard (POCOP) ligand in order to aid with solvat
ion. In our calculations, for
simplicity, we eliminated this handle, but validate
d two calculations to ensure that our
free energies would be similar. The first is the lo
ss of acetonitrile in water and the second
is the free energy of reaction with CO
2
to form formate, involving a change in overall
charge. These can be seen in Scheme S2b.
The difference in free energies of these reactions
does not exceeds 0.5 kcal/mol, which is
well within the error of DFT. Thus we feel comforta
ble in using the simplified ligand
scaffold.
5
Appendix 5
Scheme S3 – Free energies for protonation via the Y
-shaped cluster
Protonation via the Y0shaped cluster used previousl
y for protonation has a higher barrier
than the square shaped cluster, showing that water
orientation is significant.
6
Appendix 6
Figure S2 –
Figures of images along the intrinsic reaction coor
dinate calculation. A.
Point on the reverse path; B. The transition state;
C. Point on the forward path. All bond
lengths in Angstroms. For reference, the spectator
Ir0H bond length is 1.70 Å.
A.
B.
C.
7
Appendix 7
Figure S3 – Hydricities of (POCOP) Ir compared to o
ther hydridic compounds
In Figure S3, the first reduction potential vs ferr
ocene of several P
4
Ni
2
and Pd
3
compounds are plotted against their measured hydric
ites in acetonitrile, denoted by the
blue and yellow diamonds. Dubois and coworkers note
d that the first half0wave one
electron reduction potential correlated linearly wi
th the resulting measured hydricity. The
point marked by the red “X” is that calculated for
(POCOP)Ir(H)
2
(NCCH
3
) for the (III/II)
couple vs ferrocene. The value for this does not li
e on the line established by the Pd and
Ni compounds, which means that while the Ir complex
has a calculated hydricity near
some of the more reactive Pd and Ni compounds, more
energy is required to gain the
same return in hydricity. This indicates an interes
ting relationship between hydricity and
structure.
References
1.
Cao, L.; Sun, C.; Sun, N.; Meng, L.; Chen, D.
Dalton Trans.
2013
,
42
, 5755.
2.
Berning, D. E.; Miedaner, A.; Curtis, C. J.; Noll,
B. C.; Rakowski DuBois, M. C.;
DuBois, D. L.,
Organometallics
2001
,
20
, 183201839.
3.
Raebiger, J. W.; Miedaner, A.; Curtis, C. J.; Mille
r, S. M.; Anderson, O. P.;
DuBois, D. L.,
J. Am. Chem. Soc.
2004
,
126
, 550205514.
40
50
60
70
-2.5
-2
-1.5
-1
-0.5
0
∆
∆
∆
∆
G(H
-
) [kcal/mol]
E(II/I) vs Fc/Fc
+
(P4)Ni
(P4)Pd
(PoCoP)Ir(III/II)
8
Appendix 8: Geometries of Molecules
Optimized molecular geometries can be found in the
supplemental file SI_POCOP_Ir.xyz
In order to open these files for viewing and analys
is, the authors suggest a program as
Mercury, which can be found at
http://www.ccdc.cam.ac.uk/pages/Home.aspx
free of
charge.
9
Appendix 9: Calculation Details
10