Fast transfer hydrogenation using a highly active orthometalated heterocyclic carbene ruthenium catalyst

Article abstract of DOI:10.1016/j.jorganchem.2005.07.001

The free carbene 1,3,4-triphenyl-4,5-dihydro-1H-1,2,4-triazol-5-ylidene reacts with trans,cis-RuHCl(PPh₃)₂(ampy) (ampy = 2-(aminomethyl)pyridine) affording an orthometalated N-heterocyclic carbene complex characterized by an X-ray di

Full text of DOI:10.1016/j.jorganchem.2005.07.001

Journal of Organometallic Chemistry 690 (2005) 5570–5575
www.elsevier.com/locate/jorganchem
Fast transfer hydrogenation using a highly active
orthometalated heterocyclic carbene ruthenium catalyst
a,
b
b
*
Walter Baratta , Jan Sch u¨ tz , Eberhardt Herdtweck ,
b
a
Wolfgang A. Herrmann , Pierluigi Rigo
a
Dipartimento di Scienze e Tecnologie Chimiche, Universit a` di Udine, Via Cotonificio 108, I-33100 Udine, Italy
b
Department Chemie, Technische Universit a¨ t M u¨ nchen, Lichtenbergstrasse 4, D-85747 Garching, Germany
Received 10 May 2005; received in revised form 24 June 2005; accepted 4 July 2005
Available online 8 August 2005
Abstract
The free carbene 1,3,4-triphenyl-4,5-dihydro-1H-1,2,4-triazol-5-ylidene reacts with trans,cis-RuHCl(PPh ) (ampy) (ampy =
3
2
2
-(aminomethyl)pyridine) affording an orthometalated N-heterocyclic carbene complex characterized by an X-ray diffraction study.
This compound in presence of NaOH shows very high catalytic activity for the transfer hydrogenation of several ketones to alcohols
ꢀ1
using 2-propanol as hydrogen source, affording TOF values up to 120,000 h (at 50% conversion).
2005 Elsevier B.V. All rights reserved.
ꢀ
Keywords: Ruthenium; Carbene; Orthometalation; Transfer hydrogenation; X-ray structure
1
. Introduction
co-catalysts and long reaction times are required. Re-
lated carbene complexes of rhodium and iridium also
show catalytic activity in transfer hydrogenation [8].
In a recent work, we have isolated highly efficient
catalysts for transfer hydrogenation bearing 2-(amino-
methyl)pyridine (ampy) in combination with a diphos-
The isolation of stable free N-heterocyclic carbenes
(
NHCs) by Arduengo et al. [1] in 1991 led to a new
attention to the carbene chemistry and in particular
for the use of these ligands in homogeneous catalysis
ꢀ
[
2]. NHC transition metal complexes, in which the
phine [PP] [9a] or a cyclometalated phosphine [PC]
ꢀ
carbene ligands are two electron donors with slight
p-back-bonding, are thermally stable and robust to deg-
radation favoring the generation of efficient and long-
living catalysts. As regards ruthenium, most of the
known NHC-catalysts have been applied to olefin
metathesis [3], C–C alkyne coupling [4] and hydrogena-
tion [5] reactions. It should be noted that, despite ruthe-
nium complexes have proven to be excellent catalysts for
hydrogen transfer reactions from alcohols to ketones [6],
only a few ruthenium based catalysts, containing car-
bene ligands, have been reported [7]. For these systems
high concentration of ruthenium pre-catalysts or base
ligand, namely [(2-CH -6-MeC H )PPh ] [9b,9c]. For
2
6
3
2
the latter system, displaying a metal-carbon r bond,
the formation of a five-membered metallacycle [10]
leads to a robust complex with an electron rich metal
center. Apparently, the ampy ligand shows a high li-
gand acceleration in the transfer hydrogenation cata-
lyzed by phosphino ruthenium(II) complexes, with
respect to the other nitrogen ligands [9a,9b]. On the
other hand, the NHC carbenes are considered to be-
have similarly to tertiary phosphines, but bind to the
metal center more strongly and are excellent electron
donors. Moreover, it is now well-established that aryl-
and alkyl-substituted N-heterocyclic carbene ligands
undergo facile intramolecular C–H bond activation,
*
Corresponding author. Tel.: +39 432 558889; fax: +39 432 558803.
ꢀ
E-mail address: inorg@dstc.uniud.it (W. Baratta).
leading to cyclometalated [EC] complexes (E = C of
0
022-328X/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2005.07.001

W. Baratta et al. / Journal of Organometallic Chemistry 690 (2005) 5570–5575
5571
ꢀ
carbene) [11]. Although different [CC] ruthenium
complexes have been isolated [11a], only the arene spe-
cies RuCl[CC](p-cymene) is catalytically active for al-
kene and alkyne C–C coupling reactions [12].
In view of these findings, we wished to examine
whether the combination of ampy with NHC ligands
in a ruthenium complex would afford active hydrogen
transfer catalysts and we have investigated the coordina-
tion chemistry of the commercially available free
carbene 1,3,4-triphenyl-4,5-dihydro-1H-1,2,4-triazol-5-
ylidene. As a matter of fact, with this ligand in combina-
tion with ampy we have obtained a new orthometalated
ꢀ
[
CC] ruthenium complex that is an extremely active
catalyst for the transfer hydrogenation of ketones
ꢀ
1
(
TOF up to 120,000 h ).
2
. Results and discussion
Fig. 1. ORTEP drawing of complex 1 in the solid state, using thermal
ellipsoids at the 20% probability level for clarity. Selected bond length
(
2
A˚ ) and angles (ꢁ): Ru–Cl 2.5040(6), Ru–P 2.2818(6), Ru–N(4)
.144(2), Ru–N(5) 2.221(2), Ru–C(1) 1.970(2), Ru–C(12) 2.070(2),
Treatment of the monohydride complex trans,cis-
RuHCl(PPh ) (ampy) [9a] with an equimolar amount
3
2
and Cl–Ru–P 93.86(2), Cl–Ru–N(4) 81.67(5), Cl–Ru–N(5) 81.63(6),
Cl–Ru–C(1) 175.12(7), Cl–Ru–C(12) 98.96(6), P–Ru–N(4) 175.52(5),
P–Ru–N(5) 102.94(6), P–Ru–C(1) 90.48(6), N(4)–Ru–N(5) 76.16(8),
P–Ru–C(12) 88.06(6), N(4)–Ru–C(1) 94.00(8), N(4)–Ru–C(12)
of the free carbene 1,3,4-triphenyl-4,5-dihydro-1H-
,2,4-triazol-5-ylidene in refluxing toluene afforded the
cyclometalated heterocyclic carbene derivative 1 which
1
9
2.95(8), N(5)–Ru–C(1) 99.56(8), N(5)–Ru–C(12) 168.94(8), C(1)–
was isolated in high yield (Eq. 1)
Ru–C(12) 78.96(9).
H
Ph
N
Ph P
N
N
3
Ru
Cl
+
N
Ph
N
Ph
Ph P
3
NH2
C(12) (78.96(9)ꢁ) angles for the two five-membered cycles
with a relatively large P–Ru–N(5) 102.94(6) angle for the
weakly coordinated amino nitrogen.
ð1Þ
1
N
In the H NMR spectrum of 1 two broad doublets at
Ph
Ph
d 4.22 and 3.20 (J(HH) = 17.6 Hz) are for the nonequiv-
toluene
reflux
N
N
Ru
+ PPh3 + H2
alent geminal CH protons, whereas the signals at d 2.59
2
Ph P
Cl
3
and 2.05 are for the NH protons, as proven by the addi-
2
NH2
tion of a D O solution of NaOH to 1 (CDCl ) that leads
2
3
1
quickly to the disappearance of the amino protons, indi-
13
1
In this reaction one PPh is displaced by the carbene
3
cating that a fast H/D exchange occurs. In the C{ H}
NMR spectrum the low-field signals at d 199.3 and 170.6
are for the Ru–C carbene and the orthometalated car-
with concomitant orthometalation of a phenyl group
and dihydrogen extrusion. The formation of the five-
membered chelate ring leads to the thermally stable
complex 1. In order to establish the former formulation
with six different donor ligands coordinated to the metal
center, an X-ray structural analysis has been carried out
on a single crystal of 1 and a perspective view of the
complex is depicted in Fig. 1.
ꢀ
bon, respectively, in agreement with other [CC] ruthe-
nium systems [14]. The CH signal of the ampy ligand is
2
at d 48.3, very close to that of related ampy complexes
[
9] and the free ligand (d 47.8).
The orthometalated heterocyclic carbene compound
1
is an efficient catalytic precursor for the transfer
The ruthenium atom is in a distorted octahedral envi-
hydrogenation of ketones to alcohols with 2-propanol
as hydrogen donor and in presence of a base (Eq. 2)
ronment with PPh trans to the pyridine nitrogen and the
3
NH group trans to the orthometalated carbon. The Ru–
2
O
OH
OH
O
C(1) and Ru–C(12) lengths are typical for Ru-heterocy-
clic carbene and Ru-aryl bond distances. The ampy
1
0.05 mol %
ð2Þ
+
+
Ph
NaOH 2 mol % Ph
˚
ligand presents a Ru–N(4) distance of 2.144(2) A for
the pyridine nitrogen, and a relatively long Ru–N(5) dis-
Thus, when 1 (0.05 mol%) is used with NaOH as co-cat-
alyst (2 mol%), acetophenone (0.1 M solution) is quickly
reduced to 1-phenylethanol at reflux, with a TOF value
˚
tance (2.221(2) A) for the NH group, in agreement with
2
the trans influence [13] of the phenyl ligand. Complex 1
shows small N(4)–Ru–N(5) (76.16(8)ꢁ) and C(1)–Ru–
ꢀ1
of 110,000 h at 50% conversion (Table 1).

5572
W. Baratta et al. / Journal of Organometallic Chemistry 690 (2005) 5570–5575
0
Table 1
isolated yield, starting from 1.50 g of 3 -methoxyace-
tophenone (0.2 M in 2-propanol) and using the complex
1 (0.05 mol%). The catalytic activity of 1 has also been
compared with that of the related ruthenium derivatives
containing the ampy ligand [9a,9b]. Under identical
catalytic conditions, the precursor trans,cis-RuHCl-
a
Catalytic transfer hydrogenation of ketones using complex 1
-1 c
Ketone
Alcohol
Conversion
TOF (h )
b
%
(min)
O
OH
9
9
9
(5)
110000
(
PPh ) (ampy) exhibits a lower activity for the reduction
3 2
ꢀ1
of acetophenone (TOF = 28,000 h ) [9a] than 1. The
latter is also more active than the cyclometalated phos-
phine derivative RuCl(CO)[(2-CH -6-MeC H )PPh ]-
O
OH
Cl
0
(10)
50000
2
6
3
2
ꢀ
1
(
ampy) (TOF = 60,000 h ) [9b], suggesting that the
Cl
ꢀ
bidentate carbene [CC] with the ampy ligand is a par-
ticular favorable combination for obtaining a highly ac-
tive catalytic species. Under our standard conditions
O
O
OH
OMe
OMe
9
9
8
8
(10)
(2)
70000
(
NaOH 2 mol% relative to acetophenone), control
experiments in the absence of 1 result in about 2%
reduction of the ketone after 1 h. Furthermore, when
the catalysis with 1 is carried out at 25 ꢁC instead of
OH
OMe
OMe
OMe
OMe
120000
8
2 ꢁC, 1-phenylethanol is formed in small amount (3%
conv. after 1 h). As regards the mechanism, it is likely
that in the basic 2-propanol media the complex 1 leads
to the corresponding monohydride derivative through
the isopropoxide/b-elimination route, as reported for
other ruthenium chloride catalytic precursors [9a,16],
since in absence of base no activity for 1 has been ob-
served. Furthermore, the high performance of 1 can be
O
OH
9
9
(5)
100000
O
OH
9
9
6
6
(10)
(15)
70000
50000
attributed to the presence of the RuH/NH motif [17]
2
in addition to the orthometalated heterocyclic carbene
that allows stability to the ruthenium center, avoiding
facile oxidation or degradation and favoring long-living
catalytic species.
In conclusion, we have successfully isolated a new
heterocyclic carbene ruthenium catalyst capable of per-
forming the transfer hydrogenation of several ketones
with very high activity. Work is currently in progress
to expand the chemistry of ruthenium complexes con-
taining ancillary orthometalated heterocyclic carbenes
for homogeneous catalysis.
O
OH
a
Conditions: reactions were carried out at 82 ꢁC, ketone 0.1 M in
-propanol, ketone/Ru/NaOH = 2000/1/40.
2
b
The conversion was determined by GC.
c
Turnover frequency (moles of ketone converted to alcohol per mole
of catalyst per hour) at 50% conversion.
Following this protocol a number of different ke-
tones, i.e., alkylaryl, cyclic and dialkyl substrates, can
be quantitatively converted to the corresponding alco-
hols within a few minutes and with TOF values that
are among the highest reported in the literature [9,15].
3. Experimental
0
0
Particularly fast reduction is achieved with 3 ,4 -dimeth-
oxyacetophenone and cyclohexanone, which show TOF
All reactions were carried out under an argon atmo-
sphere using standard Schlenk techniques. Solvents were
carefully dried by conventional methods and distilled
under argon before use, whereas chemicals were pur-
chased from Aldrich and used without further purifica-
tion. The complex trans,cis-RuHCl(PPh ) (ampy) [9a]
ꢀ
1
values of 120,000 and 100,000 h , respectively, the first
being converted in less than 2 min. It should be noted
that 5-hexen-2-one is selectively reduced at the carbonyl
group without hydrogenation or isomerization of the
carbon–carbon double bond, allowing this procedure
to be applied for the synthesis of unsaturated alcohols.
Since complex 1 can be easily obtained from the com-
mercially available free carbene and displays a signifi-
cantly higher performance (TOF values) respect to the
reported metal carbene hydrogen transfer catalysts
3
2
and 1,3,4-triphenyl-4,5-dihydro-1H-1,2,4-triazol-5-yli-
dene [18] were prepared according to the procedures
reported in the literature. NMR measurements were car-
ried out using a Bruker AC 200 spectrometer. Chemical
1
13
shifts, in ppm, are relative to TMS for H and C, and
3
1
[
7,8], it holds promise for a broad application in the
reduction of carbonyl compounds. As example, 3 -meth-
oxy-1-phenylethanol has been easily obtained in 83%
to external 85% H PO for P. Elemental analysis (C,
3 4
0
H, N) was performed by the Microanalytical Labora-
tory of the Technische Universit a¨ t M u¨ nchen.

W. Baratta et al. / Journal of Organometallic Chemistry 690 (2005) 5570–5575
5573
3
.1. Synthesis of complex 1
suspension of trans,cis-RuHCl(PPh ) (ampy)
Table 2
Crystallographic data for 1 Æ 2CDCl
3
A
1 Æ 2CDCl
3
3
2
(
1
242 mg, 0.314 mmol) and 1,3,4-triphenyl-4,5-dihydro-
H-1,2,4-triazol-5-ylidene (103 mg, 0.346 mmol) was
Formula
Fw
Color habit
Crystal dimensions (mm )
Crystal system
C46H37D Cl N PRu
1044.02
Yellow/prism
0.30 · 0.38 · 0.46
Triclinic
2
7
5
stirred at ꢀ60 ꢁC in 6 mL of toluene for 20 min. The
mixture was then stirred at room temperature for 2 h
and refluxed for 1.5 h. Cooling the suspension to
3
ꢀ
Space group
P1 (no. 2)
˚
ꢀ
30 ꢁC afforded a precipitate which was collected by fil-
a (A)
12.9019(1)
13.2558(1)
15.3039(1)
98.4528(3)
109.8214(3)
107.0775(2)
2263.51(3)
2
173
1.532
0.835
1056
1.67–25.28
± 15, ± 15, ± 18
51,646
8208/0.041
7239
8208/0/724
0.0277/0.0642
0.0341/0.0670
1.036
˚
tration and washed once with 4 mL of toluene and twice
with 5 mL diethyl ether and recrystallized from chloro-
form to give a yellow crystalline product. Yield:
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
2
6
8
09 mg (83%). Anal. Calc. for C H ClN PRu: C,
44 37 5
˚
3
5.79; H, 4.64; N, 8.72. Found: C, 66.01; H, 4.67; N,
V (A )
1
.93%. H NMR (200.1 MHz, CDCl , 20 ꢁC): d 8.73
Z
3
T (K)
(
s, 1H; NCH), 7.81–6.51 (m, 32H, aromatic protons),
ꢀ
3
2
D
calc (g cm
)
4
.22 (br d, J(HH) = 17.6 Hz, 1H; CH ), 3.20 (br d,
ꢀ1
2
l (mm
)
2
J(HH) = 17.6 Hz, 1H; CH ), 2.59 (br, 1H; NH ), 2.05
2
2
F(000)
h Range (ꢁ)
Index ranges (h, k, l)
No. of rflns. collected
No. of indep. rflns./Rint
No. of obsd. rflns. (I > 2r(I))
No. of data/restraints/params
1
3
1
(
br, 1H; NH2). C{ H} NMR (50.3 MHz, CDCl ,
3
2
0 ꢁC): d 199.3 (s; Ru–C carbene), 170.6 (s; Ru–C phe-
nyl), 166.3, 162.6, 151.8, 150.4, 142.7, 138.0–119.6,
1
3
11.0 (aromatic carbons), 48.3 (d, J(CP) = 5.3 Hz;
3
1
1
CH2).
P{ H} NMR (81.0 MHz, CDCl , 20 ꢁC): d
3
a
5
0.4 (s).
R /wR (I > 2r(I))
1
2
a
R
1
/wR
2
(all data)
2
a
GOF (on F )
Largest diff. peak and hole (e A
3
.2. Single crystal X-ray structure determination of
˚
ꢀ3
)
+0.46/ꢀ0.44
compound 1 Æ 2CDCl3
P
P
P
P
a
2
o
2
2
2
2
1=2
R
1
=
(jF
o
j ꢀ jF
c
2
j)/ jF
o
j; wR2 ¼ f ½wðF F Þ ꢁ= ½wðF Þ ꢁg
;
c
o
P
2
o
2
c
1=2
GOF ¼ f ½wðF F Þ ꢁ=ðn ꢀ pÞg
.
Crystal data and details of the structure determina-
tion are presented in Table 2. Suitable single crystals
for the X-ray diffraction study were grown from CDCl3.
A clear yellow prism was stored under perfluorinated
ether, transferred in a Lindemann capillary, fixed, and
sealed. Preliminary examination and data collection
were carried out on an area detecting system (NONIUS,
MACH3, j-CCD) at the window of a rotating anode
refinements with 734 parameters were carried out by
minimizing
P
2
o
2
2
wðF ꢀ F Þ with the SHELXL-97 weighting
c
scheme and stopped at shift/err <0.001. The final resid-
ual electron density map showed no remarkable fea-
tures. Neutral atom scattering factors for all atoms
and anomalous dispersion corrections for the non-
hydrogen atoms were taken from International Tables
for Crystallography. All calculations were performed
on an Intel Pentium II PC, with the STRUX-V system,
including the programs PLATON, SIR92, and SHELXL-97
[19]. A disorder over two positions (1/2:1/2 each) of
(
NONIUS, FR951) and graphite monochromated Mo
˚
Ka radiation (k = 0.71073 A). The unit cell parameters
were obtained by full-matrix least-squares refinement
of 8202 reflections. Data collection was performed at
1
73 K (OXFORD CRYOSYSTEMS) within a h-range
of 1.67ꢁ < h < 25.28ꢁ, measured with nine data sets in
rotation scan modus with Du/Dx = 1.0ꢁ. A total num-
ber of 51,646 intensities were integrated. Raw data were
corrected for Lorentz, polarization, and, arising from
the scaling procedure, for latent decay and absorption
effects. After merging (Rint = 0.041) a sum of 8208 (all
data) and 7239 [I > 2r(I)], respectively, remained and
all data were used. The structure was solved by a com-
bination of direct methods and difference Fourier syn-
thesis. All non-hydrogen atoms were refined with
anisotropic displacement parameters. All hydrogen
atoms were found and refined with individual isotropic
displacement parameters. The deuterium atoms of the
both solvent molecules CDCl could be resolved clearly.
3
Crystallographic data (excluding structure factors) for
the structure reported in this paper have been depo-
sited with the Cambridge Crystallographic Data Centre
as supplementary publication no. CCDC-279017 [1 Æ
2CDCl ]. Copies of the data can be obtained free of
3
charge on application to CCDC, 12 Union Road, Cam-
bridge CB2 1EZ, UK (fax: (+44)1223 336 033; e-mail:
deposit@ccdc.cam.ac.uk).
3.3. Catalytic activity of complex 1
two disordered solvent molecule CDCl were placed in
The ruthenium complex 1 (4.0 mg 5.0 lmol) was dis-
3
ideal positions (riding model). Full-matrix least-squares
solved in 5 mL of 2-propanol. The ketone (2 mmol) was

5574
W. Baratta et al. / Journal of Organometallic Chemistry 690 (2005) 5570–5575
dissolved in 18.6 mL of 2-propanol and the solution was
heated to reflux under argon. By addition of 0.4 mL of a
(b) R. Noyori, S. Hashiguchi, Acc. Chem. Res. 30 (1997) 97;
c) J.E. B a¨ ckvall, J. Organomet. Chem. 652 (2002) 105.
7] (a) A.A. Danopoulos, S. Winston, W.B. Motherwell, Chem.
(
[
0
.1 M solution of NaOH in 2-propanol and 1 mL of the
Commun. (2002) 1376;
solution containing the ruthenium complex the reduc-
(
b) M. Poyatos, J.A. Mata, E. Falomir, R.H. Crabtree, E. Peris,
tion of the ketone starts immediately (complex 1
Organometallics 22 (2003) 1110;
0
.05 mol%, NaOH 2 mol%) and the yield was deter-
(c) J. Louie, C.W. Bielawski, R.H. Grubbs, J. Am. Chem. Soc.
1
23 (2001) 11312;
mined by GC analysis.
(
d) S. Burling, M.K. Whittlesey, J.M.H. Williams, Adv. Synth.
Catal. 347 (2005) 591.
8] (a) M. Albrecht, R.H. Crabtree, J. Mata, E. Peris, Chem.
Commun. (2002) 32;
0
3
.4. Preparation of 3 -methoxy-1-phenylethanol using
complex 1
[
(
b) E. Mas-Marz a´ , M. Poyatos, M. Sana u´ , E. Peris, Organomet-
allics 23 (2004) 323;
c) M. Albrecht, J.R. Miecznikowski, A. Samuel, J.W. Faller,
R.H. Crabtree, Organometallics 21 (2002) 3596;
d) J.R. Miecznikowski, R.H. Crabtree, Organometallics 23
(2004) 629.
0
3
-methoxyacetophenone (1.50 g, 10 mmol) was dis-
(
solved in 46 mL of 2-propanol and refluxed under ar-
gon. By addition of 1 mL of a 2-propanol solution of
(4.0 mg 5.0 lmol) and 2 mL of a 0.1 M solution of
(
1
NaOH, the reaction starts immediately and was com-
pleted after 15 min. The solution was concentrated and
the residue was dissolved in 20 mL of ether, dried over
[9] (a) W. Baratta, E. Herdtweck, K. Siega, M. Toniutti, P. Rigo,
Organometallics 24 (2005) 1660;
(
b) W. Baratta, P. Da Ros, A. Del Zotto, A. Sechi, E. Zangrando,
P. Rigo, Angew. Chem. Int. Ed. 43 (2004) 3584;
(c) W. Baratta, A. Del Zotto, G. Esposito, A. Sechi, M.
Toniutti, E. Zangrando, P. Rigo, Organometallics 23 (2004)
6264.
Na SO and purified through silica gel chromatography.
2
4
The product was obtained pure as pale yellow oily li-
quid. Yield: 1.26 g (83%).
[
[
10] (a) A.D. Ryabov, Chem. Rev. 90 (1990) 403;
(
b) I. Omae, Coord. Chem. Rev. 32 (1980) 235.
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J.M.J. Williams, Organometallics 23 (2004) 4537, and references
therein;
Acknowledgements
This work was supported by the Consorzio Universi-
tario del Friuli in sponsorship of a fellowship for J.S.,
the Ministero dellꢀUniversit a` e della Ricerca Scientifica
e Tecnologica (MURST) and the Bayerische For-
schungsstiftung. We are grateful to Dr. K. Siega for
the catalytic measurements and Dr. M. Toniutti for
the synthesis of the ruthenium precursor.
(b) K. Abdur-Rashid, T. Fedorkiw, A.J. Lough, R.H. Morris,
Organometallics 23 (2004) 86;
(
(
(
c) R. Dorta, E.D. Stevens, S.P. Nolan, J. Am. Chem. Soc. 126
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