Reactivity of the terminal oxo species ((tBu2PCH 2SiMe2)2N)RhO

Article abstract of DOI:10.1039/c3dt31972e

Reactivity of the 4-coordinate molecule (PNP)RhO (PNP is ( ^tBu₂PCH₂SiMe₂)₂N) towards CO proceeds stepwise, first forming an η²-CO₂ complex, by a mechanism which involves a preliminary adduct of CO on Rh, then a second CO displaces CO₂. Reaction of the oxo complex with CO ₂ occurs in time of mixing even at low temperature to form (PNP)Rh(η²-CO₃), with no intermediate detectable. DFT calculations indicate an initial bond formation between the oxo center and the CO₂ carbon. Reaction of (PNP)RhO with H₂ occurs only at a 1 : 2 molar stoichiometry, to ultimately form (PNP)Rh(H)₂ and free H₂O. No intermediate reaches detectable population even at -60 °C, but DFT mapping of various possible mechanisms on the singlet energy surface shows that the nearly equi-energetic (PNP)Rh(H₂O) and (PNP)RhH(OH) are formed, but only the latter readily adds the second molecule of H₂ to proceed to the observed products; these reactions thus both involve heterolytic splitting of H₂. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3dt31972e

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Reactivity of the terminal oxo species
((tBu2PCH2SiMe2)2N)RhO†‡
Cite this: Dalton Trans., 2013, 42, 6745
Nikolay P. Tsvetkov, José G. Andino, Hongjun Fan, Alexander Y. Verat and
Kenneth G. Caulton*
Reactivity of the 4-coordinate molecule (PNP)RhO (PNP is (tBu2PCH2SiMe2)2N) towards CO proceeds step-
wise, first forming an η2-CO2 complex, by a mechanism which involves a preliminary adduct of CO on Rh,
then a second CO displaces CO2. Reaction of the oxo complex with CO2 occurs in time of mixing even at
low temperature to form (PNP)Rh(η2-CO3), with no intermediate detectable. DFT calculations indicate an
initial bond formation between the oxo center and the CO2 carbon. Reaction of (PNP)RhO with H2
occurs only at a 1 : 2 molar stoichiometry, to ultimately form (PNP)Rh(H)2 and free H2O. No intermediate
reaches detectable population even at −60 °C, but DFT mapping of various possible mechanisms on the
singlet energy surface shows that the nearly equi-energetic (PNP)Rh(H2O) and (PNP)RhH(OH) are formed,
but only the latter readily adds the second molecule of H2 to proceed to the observed products; these
reactions thus both involve heterolytic splitting of H2.
Received 28th August 2012,
Accepted 13th March 2013
DOI: 10.1039/c3dt31972e
www.rsc.org/dalton
ligand oxidized (i.e., less than an octet), this oxyl character
helps to define how low a value of q in an MOq+ moiety will
Introduction
Terminal oxo complexes with higher (>6) d electron count are still support such oxidation of an “oxo” ligand above its typical
a continuing challenge.1,2 We reported recently3 the synthesis charge of 2−. Recent work with the PNP−1 ligand on other
and characterization of a remarkable molecule, (PNP)RhO, metals has suggested the reactivity of (PNP)RhO might not be
where PNP is (tBu2PCH2SiMe2)2N−1: superficially trivalent localized at the unusual oxo ligand,6,7 but may involve the
rhodium but only 4 coordinate and nearly planar, and also a Rh/O bond and may also include both spin delocalization and
ground state triplet, for a d6 configuration.4 The synthesis of even new bond formation to the PNP nitrogen. We report here
this molecule starts from a “(PNP)Rh equivalent,” since this on the reaction of (PNP)RhO with seemingly the simplest of
3-coordinate monovalent Rh structure is not the ground state, reagents, H2, as well as with simple Lewis bases and with CO
but instead exists (Scheme 1) as the fully characterized form5 or CO2. A relevant precedent8,9 will be compared to (PNP)RhO
t
where one Bu C–H bond has oxidatively added to the metal, in detail below.
abbreviated (PNP*)RhH. This molecule is in equilibrium, at a
rate of ∼103 s−1, with the authentic (PNP)Rh structure, which
leads to unusual fluxionally-averaged NMR at 25 °C. The oxi-
dants Me3NO, pyridine N-oxide, and even N2O all oxidize this
rhodium source to give the same product, (PNP)RhO. We
report here a still more convenient reagent for this synthesis.
This triplet molecule has a considerable amount (50%) of spin
density on the oxo center. Since spin density on oxo leaves that
Indiana University, Department of Chemistry, 800 East Kirkwood Avenue,
Bloomington, Indiana, USA. E-mail: caulton@indiana.edu; Fax: +1 812-855-8300;
Tel: +1 812-855-4798
†Dedicated to David Cole-Hamilton, for his career-long visionary approaches to
challenging problems.
‡Electronic supplementary information (ESI) available: Computational and spec-
troscopic results are available. See DOI: 10.1039/c3dt31972e
Scheme 1
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(PCP)Rh(CO2) pincer complex,10 which also shows NMR
spectra indicating Cs symmetry. The IR spectrum of (PNP)Rh
(CO2) in pentane shows two strong absorptions at 1805 and
Results
Reactivity of (PNP)RhO with non-redox N-donors
Because of its thermolability (isomerizing3 at and above 1847 cm−1; the values for the (PCP)Rh(CO2) pincer complex are
−20 °C to add one Bu methyl CH bond across the Rh/O bond, 1798 and 1592 cm−1 hence remarkably lower.
t
to form [N(SiMe2CH2PtBu2)(SiMe2CH2PtBu(CMe2CH2)]Rh(OH),
Scheme 1), all reactions of (PNP)RhO have employed freshly
prepared solutions of the molecule, generally synthesized from
N-methyl morpholine N-oxide in THF. This synthetic oxo trans-
fer reaction is complete in less than 5 h at −30 °C where the
t
rate of attack on Bu by the RhO functionality is insignificant.
This represents an improved synthesis compared to the N2O
reagent reported earlier.3
Low temperature NMR study
Samples of (PNP)RhO were combined 1 : 1 in THF with
Is a simple adduct (PNP)Rh(CO)O or (PNP)Rh(CO2) detectable
as an intermediate in the reaction of (PNP)RhO with CO?
When (PNP)RhO (synthesized from N2O) is combined with
substoichiometric CO (1 : 0.8 mole ratio) at −196 °C and
PhCN or with 4-dimethylaminopyridine at −30 °C. No diamag-
netic peaks were observed by 31P NMR, indicating no adduct
formation or reaction. By its synthetic method, it obviously
does not bind neat THF or either donor atom of equimolar
warmed minimally to mix CO with the fluid solution in d8-
N-methyl morpholine; its chemical shifts in THF are identical
THF, and monitored with progressive warming of the NMR
to those in benzene or toluene.
probe, one sees 10% (PNP)Rh(CO) already at −80 °C. At
−50 °C, (PNP)RhO is still observed by 1H NMR but brief
Reactivity of (PNP)RhO with PMe3
shaking and warming of the glass surface to dissolve CO from
the headspace shows detectable (PNP)Rh(CO2). Even at 0 °C,
Phosphines are generally an “easy” target for oxo transfer, and
we find a facile reaction with (PNP)RhO. Reaction of (PNP)RhO
some (PNP)RhO remains, and (PNP)Rh(CO2) is still present,
with substoichiometric PMe3 in THF occurs at −30 °C in less
indicating a deficiency of dissolved CO. Finally, after shaking
at 25 °C, the CO2 complex is gone and the amount of (PNP)Rh-
than 5 min to give complete consumption of phosphine with
formation of free OPMe3. At this point, unreacted (PNP)RhO
(CO) has increased. Thus, any (PNP)Rh(CO)O, if it is indeed an
remains present, and under these conditions (PNP)Rh(PMe3)O,
intermediate, disappears rapidly and never reaches detectable
if it is an intermediate at all, is not detected. No (PNP)Rh
population, but (PNP)Rh(CO2) is an intermediate. It also
appears that there is no detectable species (PNP)Rh(“C2O3”)
which contains both CO and CO2, and that this reaction is well
(PMe3), also synthesized independently for comparison, was
formed.
described as one where CO displaces CO2 from III.
Reactivity of (PNP)RhO with CO
Reaction of (PNP)RhO with 1 atm CO in THF beginning at
−30 °C proceeds over 1 h (Scheme 1) with visible color change.
DFT analysis of the CO reaction
NMR assay after 1–12 h shows the production of (PNP)Rh(CO). We envision that all reactions reported here might occur via
To prove that the observed reactivity was not due to the avail- the available thermal population of a singlet isomer of the
able N-methyl morpholine present from the synthesis of (PNP)- ground state triplet (PNP)RhO. Certainly 5-coordinate RhIII
RhO, we prepared the oxo species from N2O, then reacted this species here (i.e., an initial adduct of (PNP)RhO with any sub-
as above with CO. The product was unchanged: (PNP)Rh(CO). strate L) always have their singlet state far below their triplet,
We established that there is no reaction between authentic due to the larger coordination number (vs. 4 in (PNP)RhO)
(PNP)Rh(CO) and 1 atm CO2 over 24 h at 25 °C in benzene. In splitting the d orbitals more. All calculated energies reported
contrast, (PNP)Rh(CO2) (see below) is completely consumed by here use singlet (PNP)RhO as the reference state; from the
1 atm CO within 180 min at 25 °C in benzene to produce only triplet, all reaction energies will be less exothermic by the
(PNP)Rh(CO). The monocarbonyl (PNP)Rh(CO) does not add triplet → singlet promotion energy.11
more CO over 24 h under 1 atm CO.
DFT(B3LYP) calculation of the minimum energy geometry
of the full set of atoms in (PNP)Rh(CO2) optimized (B, Fig. 1)
to an η2-structure (consistent with the NMR conclusion of Cs
symmetry) with the Rh-bound C/O bond longer than the
Independent synthesis of a potential intermediate: (PNP)Rh-
(CO2)
(PNP*)RhH reacts in time of mixing in benzene with CO2 (1 unbound one by 0.05 Å. The angle N–Rh–O, 175.4°, is con-
atm) to give complete conversion to a single product, whose siderably larger than N–Rh–C, 141.1°, and the Rh/O distance,
1H and 31P NMR spectra indicate Cs symmetry. This rules out 2.211 Å, is longer than the Rh/C distance, 2.006 Å. The OCO
structures where CO2 is C-bound (I) or O-bound (II) to Rh, and angle is 141.5°. This same minimum was also reached starting
is consistent with the more common mode of binding (III), from a C2v geometry I where only carbon was bound to Rh and
provided that the η2-CO2 does not undergo rapid rotation all the angles at this carbon were about 120°. The reaction
about its bond to Rh. This is the structure proposed for a energy to form (PNP)Rh(CO2) is −66.4 kcal mol−1 (Fig. 1).
6746 | Dalton Trans., 2013, 42, 6745–6755
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(PNP)Rh(CO)O, A, in angle C/Rh/O, the atoms which must
form a bond, but the C/O distance at the TS is still a very long
2.44 Å, hence qualifying as early. (PNP)Rh(CO)(O), A, has a CO
stretching frequency of 1944 cm−1, which indicates consider-
able back donation to CO (cf. 1932 cm−1 in (PNP)Rh(CO)13).
The Rh–N bond in species A is long (2.23 Å), but the Rh/O dis-
tance has lengthened only 0.02 Å from that in (PNP)RhO. The
N–Rh–O angle has decreased 23° in the process of binding CO.
We also sought a TS for direct conversion of (PNP)RhO +
free CO to the CO2 complex without any preliminary Rh/C
interaction. This revealed that attack of the CO vector on oxo
perpendicular to the RhvO bond merely collapsed towards
the (PNP)Rh(CO)O adduct A.
Fig. 1 Structures and electronic energies (kcal mol−1 vs. (PNP)RhO + CO) for
attack on (PNP)RhO by one CO. See ESI‡ for detailed structural parameters.
Reactivity of (PNP)RhO with CO2
Regarding the mechanism of this reaction, we identified
(Fig. 1) stationary state adduct A, a carbonyl complex of tri-
valent Rh: (PNP)Rh(CO)(O). The electronic energy of this species,
+0.5 kcal mol−1, means that its formation is unfavorable when
the entropy penalty for adduct formation is considered.12 This
may be one factor which causes the reaction to be slower than
that of (PNP)RhO with CO2 (see below): low population of the
primary product adduct. The species B, (PNP)Rh(η2-CO2), is
formed from adduct A in a highly exothermic step. The reac-
tion energy for (PNP)Rh(η2-CO2) reacting with additional CO to
There are synthetic reports of reactive capture of CO2 by both
oxo and hydroxo species, but no detailed mechanistic
studies.14–16
The reaction of dark orange (PNP)RhO with CO2 (1 atm)
was carried out in THF. The reaction occurs in time of mixing
at −30 °C, with color bleaching to pale orange, and shows the
diamagnetic product to have C2v symmetry, with one tBu
(virtual triplet), one SiMe (singlet) and one (virtual triplet) CH2
1H NMR chemical shift, and one 31P chemical shift (111 Hz
coupling to rhodium). The infrared spectrum of the isolated
product shows a CvO stretching vibration at 1701 cm−1, con-
sistent with pendant CvO of17 a bidentate carbonate in (PNP)-
Rh(CO3). The CI-mass spectrum shows nothing heavier than
the cation of protonated (PNP)RhO, indicating loss of CO2
under the ionization conditions.
form (PNP)Rh(CO) and free CO2 is −39 kcal mol−1 11
which
,
explains why it is difficult to detect (PNP)Rh(η2-CO2). We
sought a TS for the conversion of A to B by an intramolecular
C1/O1 bond formation step. This revealed (Scheme 2) structure
TSA-B, at an energy of +7.3 kcal mol−1, hence only 6.8 kcal
mol−1 above the adduct (PNP)Rh(CO)O, A; this low TSA-B
energy is consistent with the Hammond postulate that a very
exothermic reaction should have a low barrier…and an early
TSA-B, which explains why (PNP)Rh(CO)O is not detected experi-
mentally. Structurally,11 this TSA-B has its biggest change from
Low temperature NMR study
When (PNP)RhO (synthesized from N2O) is combined with
substoichiometric CO2 at −196 °C and warmed minimally
while mixing CO2 with the fluid solution in d8-THF, and moni-
tored with progressive warming of the NMR probe, one sees
already at −80 °C full conversion of (PNP)RhO to (PNP)Rh-
(CO3); this more complete conversion (vs. the CO study
reported above) is presumably due in part to the greater
molarity of CO2 (less volatile) than CO. At −50 °C, 1H NMR
confirms that all (PNP)RhO has been consumed, and there is
no change in the solution composition up to 0 °C, then also at
25 °C. This reaction thus has a very low barrier and no inter-
mediate reaches detectable concentration.
DFT analysis of the CO2 reaction
A
DFT(B3LYP) geometry optimization of (PNP)Rh(κ2-CO3)
(Fig. 2) gave only one geometry, D, C2v symmetric, and with a
reaction energy of formation of −34.3 kcal mol−1. This struc-
ture has the two C/O distances involving the two Rh-bound car-
bonate oxygens longer (by 0.1 Å) than that to the terminal
oxygen. The two angles NRhO average 149°, and differ by only
5°. The Rh/O distances are similar to those from Rh to OH
ligands. The quite favorable reaction energy indicates that the
terminal oxo group is nucleophilic. Geometry optimization
Scheme 2
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starting from the κ2-CO3 structure; subsequent TS search then
gave the κ1-CO3 structure, indicating that the retention of the
C/O bond is energetically dominant. This allows the con-
clusion that this [2 + 2] mechanism is not energetically viable
on the singlet surface.
Reactivity of (PNP)RhO towards H2
The observed reactivity of M/O bonds is dramatically different
for early, oxophilic metals and later transition metals, due to
the reduction of M/O bond strength as one moves to the right
in the transition series due to population of the π*MO orbital.
TiO2 does not react with H2, but CuO does. Alcohols evolve
hydrogen with Ti hydrides, but IrIIIOR,18 RuIIOR19,20 and
CuIOR21 species react with H2 to form the metal hydride and
liberate ROH. Examples of M/O multiple bonds in M(O)n
reacting with H2 are rare22 (exceptions, with very high
barriers and slow rates, are OsO4 and MnO4−); molybdenum
terminal oxo species do not react with H2 apparently due to
thermodynamic preferences. We now report an example of a
terminal oxo rhodium complex reacting easily with H2 to form
water.
Reaction of (PNP)RhO with 1 atm H2 goes to completion
Fig. 2 Structures and electronic energies (kcal mol−1) for attack on (PNP)RhO
by CO2. See ESI‡ for detailed structural parameters.
1
already at −30 °C (τ ∼ 10 h) in THF to form a molecule of C2v
2
symmetry which we identify as (PNP)Rh(H)2 by comparison to
the known NMR spectral data.3 The liberated water is seen as a
broad peak at 2.5 ppm. If (PNP)RhO is combined with 1 atm
H2 at −100 °C in THF, low temperature NMR observations
beginning at −90 °C show no new species until the conversion
of the oxo complex to (PNP)Rh(H)2 begins (−30 °C). In particu-
lar, there is no adduct (PNP)Rh(H2)(O) formed to a detectable
extent. This is in agreement with adduct formation being
endothermic, hence also endergonic, from the DFT calcu-
lations (see below) and the lack of adducts of (PNP)RhO with
amines and nitriles (see above).
The reaction of (PNP)RhO with H2 is somewhat slower than
its reaction with CO or CO2. The evaluation of the reaction of
(PNP)RhO with H2 is therefore potentially complicated by the
fact that (PNP)RhO transforms slowly even at −20 °C to give
(Scheme 1) isomeric (PNP*)Rh(OH), which was shown in-
dependently3 to react with H2 to give (PNP)Rh(H)2 and water at
25 °C. Relative rates thus become important, and so a d8 THF
solution of (PNP)RhO was prepared at −30 °C, then divided
into two parts, only one of which was combined with 1 atm
H2, and both were held at −40 °C for 12 h. NMR analysis (done
quickly at −20 °C, to prevent conversion of (PNP)RhO to
(PNP*)Rh(OH)) after that time showed full conversion under
H2 to (PNP)Rh(H)2, while the control showed primarily
unchanged (PNP)RhO together with only traces (<5%) of
(PNP*)Rh(OH). This confirms that the oxo species reacts faster
with dissolved H2 than it does by intramolecular isomerization
in forming (PNP*)Rh(OH).
beginning from a square pyramidal structure of (PNP)Rh(κ2-
CO3) yielded the same C2v geometry discussed above.
Geometry optimization beginning from a monodentate car-
bonate structure yielded a stationary state C, with κ1-carbonate,
but this is much (40.4 kcal mol−1) less stable than the κ2-car-
bonate structure. Thus, 4-coordinated and 14 valence electron
RhIII is not favored; another negative factor about this struc-
ture is the charge separation (E) involved. A TS1-C, between
singlet (PNP)RhO, 1, and singlet κ1-carbonate C was located
for the direct addition of CO2 via only its carbon to the term-
inal oxo ligand. This attack trajectory is remarkably facile;
lying only 8.3 kcal mol−1 in electronic energy above the separ-
ated particles. The TΔS penalty of about 10–12 kcal mol−1 at
273 K will still leave a barrier which is consistent with this
(observed to be rapid) reaction at −78 °C. The structure of this
TS shows (see ESI‡) it to be very early, with little stretching of
the Rh/O bond, very little lengthening of the bonds from those
in free CO2 (1.18 Å), and a still-planar geometry around Rh.
Only the angle OCO in the approaching CO2 shows consider-
able progress towards product, being 149.7°.
Synthesis and characterization of the potential intermediate
(PNP)Rh(H2O)
A search for a TS1-D connecting species 1 and free CO2 to form
the bidentate carbonate D directly by a [2 + 2] addition of one
CvO bond across the Rh/O bond shows an extreme rise of An independent synthesis of the potential intermediate (PNP)-
energy (up to +56 kcal mol−1) in a constrained geometry scan Rh(H2O) was carried out (see Scheme 1 for D2O analog) by
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addition of stoichiometric water in THF to (PNP*)RhH. The −30 °C under 1 atm H2 to yield a half life for conversion of
product molecule has C2v proton and phosphorus NMR spec- 5 h. This means that (PNP)Rh(H2O), or (PNP)RhH(OH), which
tral features (ESI Fig. p S4‡). Addition of equimolar D2O to is in rapid equilibrium with it (see below), is a competent
(PNP)Rh(H2O) shows, within 5 minutes, disappearance of a intermediate for the reaction of (PNP)RhO with H2; if the RDS
resonance at 1.8 ppm and growth of a signal at 2.5 ppm attrib- for the overall reaction involves the first molecule of H2 (as
uted to free HDO, consistent with rapid H/D exchange; this established by DFT results below), this is consistent with the
confirms the assignment of the 1.8 ppm signal to the protons lack of accumulation of detectable population of (PNP)Rh
of coordinated water. In fact, this product of reaction of equi- (H2O).
molar (PNP)Rh(H2O) with D2O shows a 31P{1H} NMR spectrum
Addition of 5-fold excess water to N2O-derived (PNP)RhO
with multiplets due to Rh(OD2), Rh(OHD) and Rh(OH2) isoto- under 1 atm H2 in THF shows no reaction at −30 °C, showing
pomers, resolvable due to an H/D isotope effect (0.04 ppm per that water is not a catalyst for this hydrogenation reaction.
deuteron) on the phosphorus chemical shift. The three 31P{1H} This also shows (see below) that (PNP)RhO shows no net reac-
NMR doublets (from coupling to Rh with JPRh = 148 Hz) show tion (e.g., proton transfer, forming (PNP)Rh(OH)2) with water
an intensity ratio of 1 : 2 : 1 due to equal populations of H and under these conditions.
D in the solution. Finally there is a second set of weak P/Rh
We have tested whether the oxo complex has the ability to
doublets (with JPRh = 194 Hz) with P chemical shift displaced oxidize ethanol, which might lead to (PNP)Rh(H2O) and
by only 0.03 ppm which we assign to a species (PNP)Rh(water) MeCHO. Under our standard conditions (−30 °C in THF),
hydrogen bonded to a second water molecule.23
there is no reaction over 48 h. Not only is there no oxidation of
The possibility that this product is not (PNP)Rh(H2O) but is alcohol, but this shows that there is no addition of the O–H
instead Cs symmetric (PNP)Rh(H)(OH), was raised by their bond across the Rh/O bond, which might have formed (PNP)-
similar thermodynamic stability (see below). If true, (PNP)Rh- Rh(OEt)(OH). In this experiment, only unchanged reactants
(H)(OH) would have to show rapid fluxional exchange between are recovered.
Rh–H and O–H at 25 °C; this was tested by recording its 1H
NMR spectrum at −90 °C. There, and also at −60 and −30 °C,
no new species was observed (e.g., there is no second isomer,
Computational study of possible
intermediates, energetics and
hydrogenolysis mechanism
Mechanism of (PNP)RhO interacting with the first H2
molecule
(PNP)Rh(H)(OH)) and there was no decoalescence of a hydride
from a hydroxyl signal; those would both be negative chemical
shifts, so their coalesced peak would not be at the observed
+1.8 ppm, further arguing against the structural assignment as
(PNP)Rh(H)(OH), and thus consistent with the observed struc-
ture having the aquo ligand on monovalent Rh. Establishing
experimentally the preferred structure of this species is impor-
tant since the DFT calculations (see below) agree with experi-
ment in having the aquo isomer more stable than (PNP)Rh(H)
(OH).
Reaction of (PNP*)RhH with D2O was carried out in THF to
establish where the hydride ligand of the reagent complex is
located in the product. 2H NMR of the resulting product
(Scheme 1), taken within 5 min of formation shows only the
signal at ∼1.8 ppm due to the deuterons already assigned to
the coordinated water ligand. This means that the hydride of
(PNP*)RhH returns to its carbon faster than any possible
scrambling with the acidic water deuterons. This shows that
approaching D2O does not behave as an acid towards the Rh–C
bond (forming ∼CH2DRh(H)(OD)) or towards the Rh–H bond
(forming Rh(HD)(OD)). Indeed since (PNP*)RhH equilibrates5
with authentic (PNP)Rh at a rate of 103 s−1 at 25 °C, it is prob-
able that the lack of H/D mixing originates from D2O attacking
T-shaped (PNP)Rh.
We have considered options where the two H of the arriving
H2 migrate to Rh and O, to Rh and N and to N and O (Fig. 3).
Among these, only the electronic energy of the first H2 adduct
lies higher than the separated particles, and overall the com-
plete reaction is highly exothermic. DFT(B3LYP) calculations
Reactivity of (PNP)Rh(H2O) with H2
Is (PNP)Rh(H2O) an intermediate in the hydrogenolysis reac-
tion? Reaction of (PNP)Rh(H2O) with 1 atm H2 goes to com-
pletion in d8 THF at 25 °C to form (PNP)Rh(H)2. The released
water is detected at 2.5 ppm. This reaction was monitored
Fig. 3 Structures and electronic energies (kcal mol−1) for sequential reaction of
(PNP)RhO with 2 H2. See ESI‡ for detailed structural parameters. Bold arrows
quantitatively (both product growth and reagent decay) at show the lowest energy path and TS energies are beside arrows.
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on candidate intermediates for the reaction of singlet (PNP)- to rhodium; thus, one H interacts with Rh on the lowest
RhO with H2 showed (Fig. 3) a minimum for a possible energy path for bimolecular encounter of (PNP)RhO with H2
primary product which is an η2-H2 complex, 2, with a negligi- having maximum opportunity to begin bond formation to
bly lengthened H/H distance, but this is endothermic com- oxygen.
pared to the reagents (ΔE is +6.9 kcal mol−1 vs. singlet (PNP)-
Mechanism of reaction with the second H2 molecule
RhO, 1); the Rh/O distance in this structure is negligibly
lengthened from that in (PNP)RhO. Remarkably,
a
RhV The reaction (Fig. 3) of a second molecule of H2 was con-
species, 3, is a branch point for H migration to N (forming 6) sidered with each of species 4 and 5, to release H2O and form
or O (forming 4) downstream of the H2 complex 2. Forming (PNP)Rh(H)2, 10. The reaction energy for a second molecule of
species 3 is found to be the RDS of the overall reaction. Start- H2 to convert the aquo complex 5 to (PNP)Rh(H)2 and free
ing geometry optimization from a structure where the H–H water is −29.3 kcal mol−1 (Fig. 3). Experimentally, the reverse
vector lies parallel to the Rh/O vector of 1 yields a trivalent reaction of operationally unsaturated (PNP)Rh(H)2 with H2O
metal species (PNP)Rh(H)(OH), 4, with no residual H/H bond was tested independently, and no reaction (equimolar in THF)
and new Rh–H and O–H bonds in a square pyramidal geome- was detected over 24 h at 25 °C. This is consistent with the
try. This species 4 is much more stable (by 52.2 kcal mol−1
)
DFT calculation for the thermodynamics of forming (PNP)Rh-
than the reagents. However, slightly more stable (at −55.5 kcal (H)2 and water, and serves to show that the absence of weak
mol−1) than this Rh(III) species is the monovalent, square acid (water)/weak base (hydride on (PNP)Rh(H)2) reactivity has
planar aquo complex (PNP)Rh(H2O), 5, containing a pyramidal a thermodynamic, not merely a kinetic origin.
oxygen; the Rh–OH2 distance is very long (2.27 Å).
Concerning direct attack of H2 on (PNP)Rh(H2O), 5, we were
Formal [2 + 2] addition of the H–H bond across the Rh–N unable to find a minimum energy structure with H2 cis to H2O
bond of (PNP)RhO gives (PN(H)P)RhH(O), 6, in Fig. 3 but this in a species (PNP)Rh(H2)(H2O). The lack of a minimum for
lies at considerably higher energy than the previously addition of H2 to planar (PNP)Rh(H2O) can be attributed to
described products. This molecule has only a very long N/Rh the energetic gain from Rh/H2 binding being less than the
distance (2.48 Å), but its angle HRhO is 114.3°, nonlinear due energy to distort the N–Rh–O angle of 5; such distortion is
to some N/Rh bonding interaction. More competitive however needed to rehybridize the filled z2 orbital to form
a
is the isomeric monovalent, planar species (PN(H)P)Rh(OH), 7, sterically accessible acceptor orbital to receive the H–H
which lies 55.8 kcal mol−1 below the reagents. The similar sigma bond electron density, and thus diminish the filled
stability of both trivalent Rh (4) and monovalent Rh (7) z2/H2 repulsion associated with approach of H2 to a planar
suggests that forming an O–H bond is the dominant thermo- (PNP)Rh(H2O). For a similar reason, intact H2 does not find
dynamic stabilizing factor; oxidation of metal (RhI vs. RhIII) is an energetic minimum bonded to planar, d8 (PNHP)Rh(OH),
not the controlling factor.
7. Direct conversion of 5 to 10 is thus not mechanistically
Note that each pair among 4, 5, 6, and 7 are simply con- viable.
nected by 1,2 hydrogen migrations. TS energies show that 4
It therefore appears that the minimum energy path for com-
and 7 are in facile equilibrium and nearly thermoneutral. bining with a second H2 is to rely on the facile equilibrium
However, the activation energy for conversion of 3 to 4 is forming 4 from aquo complex 5, then add H2 to form 8
smaller than for 3 to 6, making 6 and 7 mechanistically irrele- (Fig. 3). There is only a low barrier for one of the H of coordi-
vant. TS6–7 involves two main motions: large decrease (vs. 6) of nated H2 in 8 to migrate to OH, forming the H2O complex 9
the angle H1–Rh–O1 and a 0.1 Å lengthening of the Rh–O1 dis- (i.e., α-H migration). Species 9 apparently suffers some destabili-
tance. The O1–H1 distance at TS6–7, 1.77 Å, is still long, the zation due to the trans hydrides, which is what makes loss of
Rh–H1 distance is as short as it was in 6 and thus this is not at H2O to form 10 highly exergonic. A search for TS9–10 showed
all a structure where H1 bridges Rh and O1. TS4–7 has a fully structures where the departing water proton hydrogen bonds
developed terminal hydride Rh–H2 distance and a significantly to one hydride. Regarding possible hydrogen bonding, (PNP)-
1
(by 0.1 Å) shortened Rh–N distance, characteristic of a fully Rh(H)2 with equimolar H2O in THF showed no change in H
developed migrated H2 but with angle H2–Rh–O1 still much NMR chemical shifts at a 1 : 1 stoichiometry.
larger than in 4.
We sought TS8–9 for H migrating from the anticipated more
The reaction of H2 with OsO4 is accelerated by stoichio- labile H of the H2 ligand. TS8–9 has the dihydrogen fully
metric nucleophile,22 which suggests that an empty metal rotated from what it was in 8, and H3–H4 lengthened by
orbital is not required; in fact, the impact of the nucleophile 0.25 Å, and with O1 migrated towards H4. H4–O1 has shor-
there is to change from a thermodynamically uphill reaction to tened to 1.27 Å (vs. the fully formed OH distance in the
a favorable one, which also has the expected impact of lower- hydroxyl of 0.97 Å) and both H3 and H4 are closer to Rh than
ing the activation energy. With this in mind, can (PNP)Rh- in 8. Especially noteworthy is that Rh–O1 has lengthened by
(H2O) also form from (PNP)RhO without the intermediacy of 0.15 Å, and in response the Rh–N distance has shortened by
any Rh/H interaction? We have sought a TS1–5 for attack of H2 0.05 Å vs. 8.
on the oxo of (PNP)RhO from a direction trans to the RhO
We have also evaluated a direct path from (PNP)Rh(H2O) to
vector (Fig. 3). This search for direct attack of H2 on O, without (PNP)Rh(H)2 by a dissociative mechanism: capture of T-shaped
any Rh/H interaction collapses to species 4, where one H binds (PNP)Rh, the product of unimolecular H2O dissociation from
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(PNP)Rh(H2O), by H2. The bond dissociation energy of H2O 12 cm−1 higher than that calculated (identical to that observed
from (PNP)Rh(H2O) is +12.7 kcal mol−1 (see Fig. 3). The elec- experimentally) for (PNP)Rh(CO), 1932 cm−1, suggesting sig-
tronic energy rise for dissociating water from (PNP)Rh(H2O) is nificant π basicity of Rh in the oxo carbonyl. This is perhaps
probably a good estimate for its transition state free energy, due to 4 electron repulsions from both amide and oxo in
since entropy will not develop significantly until they become (PNP)RhO(CO).
translationally independent, so the energy of the Rh/OH2 bond
Because it is multistep (Fig. 3), and because of H–H bond
rupture product must be compared to the TS4–5 energy of scission absent from COx reactions, the H2 reaction is inher-
−38.7. Within DFT energy errors, those two energies remain ently more complex. We find that the second H2 molecule
competitive, and probably do not allow decisive conclusion as encounters a high barrier to directly attack (PNP)Rh(H2O) 5.
to the fastest route for the second addition of H2, to form The calculations show that energy rises about 20 kcal mol−1 as
(PNP)Rh(H)2.
H2 approaches (PNP)Rh(H2O), so it is preferable to isomerize
the aquo complex back to (PNP)RhH(OH), 4 which then binds
(weakly) H2, and then collapses to (PNP)Rh(H)2 with loss of
water. In short, it is better for H2 to bind to unsaturated tri-
valent, vs. monovalent, rhodium here, and the RhIII(H2) complex
8, once formed, benefits from the high Bronsted acidity of
coordinated H2 to protonate hydroxide intramolecularly.
Species 4 is thus a mechanistic branch point for reaction with
the second H2 molecule. Overall the reaction of (PNP)RhO with
H2 on this path occurs as two heterolytic cleavages of H2.
However, because of the low bond dissociation energy of the
Rh/O bond in (PNP)Rh(OH2), reaction with the second H2 mole-
cule by this Rh/O dissociative mechanism may be energeti-
cally competitive, and here the second step with H2 is
oxidative addition.
Reactivity of (PNP)RhO towards H2O
Remarkably, there is no reaction between (PNP)RhO and equi-
molar water at −30 °C in THF over 48 h. This puts new limits
on possible Bronsted basicity of the terminal oxo ligand in this
molecule, and also confirms the result, established above, that
rhodium in (PNP)RhO is not very Lewis acidic, since it is
unable to even coordinate water under these conditions. We
feel that the low temperatures employed would not signifi-
cantly retard the rate of any proton transfer reaction, so our
result is a thermodynamic statement.
Discussion and conclusions
A 5d metal isoelectronic analog of (PNP)RhO, with a
phenyl-based P/N pincer, [tBu2PCH2(C6H3)C2H4NMe2]PtO+, or
(PCN)PtO+, has been reported8 and shows reactivity analogous
to ours here with CO and with H2. These authors report that it
is endergonic for this 4 coordinate species to bind acetone,
hence it too is not very Lewis acidic. Its mode of self-annihil-
ation (slower than for our 4d metal analog) is by oxo atom
transfer to the phosphorus, in spite of the potentially oxidiz-
From the results reported here and earlier,5 (PNP*)RhH is
established to be a useful synthon (i.e., an equivalent of (PNP)
Rh) for reaction with a variety of reagents of very different char-
acter (Scheme 3).
In spite of its operational unsaturation, (PNP)RhO
(Scheme 1) is not a strong Lewis acid, being inert to conven-
tional Lewis bases as strong as a donor-substituted pyridine.
Its reaction with CO2 is not a redox reaction and is perhaps
best viewed as nucleophilic oxo attacking the electrophilic
carbon of CO2. (PNP)RhO forms simple adducts with H2 or CO
only weakly. Examination3 of the LUMO of singlet (PNP)RhO
shows it to be primarily x2 − y2, hence sterically inaccessible,
which helps account for the poor Lewis acidity of this species
in the planar structure. The reactions with CO and H2 are both
redox reactions and have enough overall driving force that they
may proceed in spite of weak enthalpies of binding to (PNP)-
RhO, with heavy involvement of oxo/substrate bond formation
at the transition state. On the other hand, the calculated CO
stretching frequency of (PNP)RhO(CO), 1944 cm−1, is only
t
able C–H bonds in the Bu groups on the phosphine. The key
difference with our example is that the platinum case has a
singlet ground state.
A potential product not observed from (PNP)RhO and H2O
is (PNP)Rh(O2) + H2. The calculated reaction energy for this
is +57.9 kcal mol−1 (calculated with reference to singlet
(PNP)RhO), consistent with its non-observation. Regarding
further oxidation of metal, (PNP)Rh(O)2 was found to be
18.1 kcal mol−1 less stable than (PNP)Rh(O2), so RhV is not a
viable option. That (PNP)RhO cannot oxidize water (true in
certain of these possible products) is perhaps no surprise;
what is surprising is that it does not engage in proton trans-
fer, given the basicity to be expected from both the oxo
group22 [compare also its reaction with CO2], and even from
amide N and from Rh. It is also noteworthy that there is no
reaction of water at silicon. We also considered the possi-
bility that H2O might effect nucleophilic attack on the oxo of
(PNP)RhO to give (PNP)Rh(H)2 with release of O2. DFT calcu-
lation shows that this reaction has an energy of +44.4 kcal
mol−1, and is thus unfavorable. Our no-reaction result is dra-
matically different than the reactivity8 of (PCN)PtO+, which
reacts with water by simple proton transfer to form (PCN)Pt-
+
Scheme 3
(OH)2 .
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{1H} NMR (25 °C, C6D6): −19.2 (dt, J = 45 and 157), 57.0 (dd,
J = 45 and 145).
Experimental
General considerations
Reaction of (PNP)RhO with PMe3. Into a sample of (PNP)-
All manipulations were performed using standard Schlenk RhO in a J-Young tube (generated by procedure 2 in d8 THF) 1
techniques or in an argon-filled glove box unless otherwise equivalent of PMe3 was vacuum transferred. NMR showed
noted. Pentane and THF were purified using an Innovative complete conversion of (PNP)RhO into the known complex,
Technologies solvent purification system Pure Solv 400-6-MD. (PNP)Rh(PMe3) and OPMe3. Depending upon the temperature
Benzene-d6 and d8-THF were dried under Ph2CO/Na and each used, this reaction, when performed in the presence of excess
vacuum transferred and stored in the glove box under argon. N2O or NMO (the former transfers its oxygen faster), recycles
Water (H2O and D2O) was degassed by freeze–pump–thaw rhodium into (PNP)RhO, which then has some tendency to
technique before use. Hydrogen (H2), carbon monoxide (CO), isomerize into (PNP*)Rh(OH), which has no oxidizing ability.
carbon dioxide (CO2), and dinitrogen monoxide (N2O) were
Independent synthesis of Me3PO. 0.01 mL of PMe3 (7.4 mg,
purchased, as highest purity available, through commercial 0.097 mmol) in 0.5 ml of C6D6 was freeze–pump–thaw
vendors and used without further purification. All reactions degassed and 760 mm Hg of O2 (∼1 equiv.) was added on a
with gases were accomplished with a vacuum line equipped vacuum line. After all frozen solid melted, the tube was quickly
with manometer (0–760 Torr) for accurate dosage. (PNP*)RhH shaken. Slow conversion into Me3PO was observed (12% after
1
was prepared following the published synthesis.24 N-Methyl- 24 h). H NMR (25 °C, C6D6): 0.88 (d, J = 12.8). 31P{1H} NMR
morpholine N-oxide (NMO) was dried by azeotropic distillation (25 °C, C6D6): 31.5 (s).
of DMF and water away from a suspension of the monohydrate
Reaction of (PNP)RhO with CO2. The sample of (PNP)RhO
in DMF, followed by solvent removal from the residue, in generated by procedure 1 was freeze–pump–thaw degassed
vacuo. NMR chemical shifts are reported in ppm relative to and 760 mm Hg of CO2 was added on a vacuum line. The flask
protio impurities in the deutero solvents. Coupling constants was placed a Dewar filled with acetone at −40 °C and the con-
are given in Hz. 31P NMR spectra are referenced to external tents allowed to melt and left stirring for 30 min at −30 °C.
standards of H3PO4. NMR spectra were recorded with a Varian The solution color changed from dark orange to orange. All
Unity INOVA instrument (400 MHz 1H; 162 MHz 31P). “PNP” is volatiles were removed in vacuum starting at −20 °C and allow-
t
N(SiMe2CH2PBu2 )2. The fact that (PNP*)Rh(OH) (the intramo- ing the temperature to increase up to 25 °C. The residual
lecular rearrangement product of (PNP)RhO, which forms at orange solid was dissolved in deuterated benzene. NMR
trace amounts overnight at −40 °C) was never seen in the CO showed complete conversion into the new diamagnetic mole-
1
and CO2 reactions described below shows that this intramole- cule (PNP)RhCO3. H NMR (25 °C, C6D6): 0.09 (s, SiMe, 12 H),
t
cular conversion is slower than the reactions with the gaseous 0.87 (t, J = 4.5, CH2, 4H), 1.32 (t, J = 6.6, Bu, 36H). 31P{1H}
reagents.
NMR (25 °C, C6D6): 49.9 (d, J = 111). IR (pentane solution):
1701 cm−1. CI-MS (THF solution, m/z): Obs. 551.2156 [M −
CO3]+. C22H52NP2RhSi2 Theory: 551.2169, and Obs. 568.2222
Generation of (PNP)RhO
Procedure 1, from (PNP*)RhH and NMO (N-methylmorpho- [M − CO2 + H]+ C22H53NOP2RhSi2 Theory: 568.2196. There is
line N-oxide). 25 mg of (PNP*)RhH (0.0453 mmol) and 5.3 mg no trace of (PNP)Rh(CO2) product observed here, which proves
of NMO (N-methylmorpholine N-oxide) (0.0453 mmol) were that all (PNP*)RhH had converted to (PNP)RhO, since this
placed into a Schlenk flask and cooled in a Dewar filled with hydride rapidly reacts (see below) with CO2 to give (PNP)Rh-
acetone at −40 °C. 3 mL of THF were added dropwise via (CO2). In the separate low temperature search for intermedi-
syringe. After one hour of stirring at −20 to −30 °C, formation ates using oxo complex from procedure 2, species (PNP)Rh(N2)
of (PNP)RhO is complete. 1H NMR spectra are identical with is unreactive with CO2.
those reported.3
In another experiment at lower temperature, already
Procedure 2, from (PNP*)RhH and N2O. 25 mg of (PNP*) at −80 °C the amounts of (PNP)Rh(CO3) and (PNP)Rh(N2)
RhH (0.0453 mmol) and 0.5 mL of d8 THF were placed into a are equal, and this shows no change up to 0 °C. There is
J-Young tube and the solution was freeze–pump–thaw thus a very low barrier to form the η2-CO3 species. This means
degassed and 760 mm Hg of N2O (∼2 equiv.) was added on a that dissolved [CO2] exceeded that of [Rh], due in part to the
vacuum line. The J-Young tube was immediately placed in a lower volatility of CO2 during low temperature sample
Dewar filled with acetone at −40 °C. After all frozen solid preparation.
melted, the tube was quickly shaken. Formation of (PNP)RhO
and (PNP)RhN2 was observed in time of mixing at − 30 °C in a (CO2). (PNP*)RhH (0.0100 g, 0.018 mmol) was dissolved in
1 : 1 ratio by 1H NMR.
C6D6 (0.6 ml) in a J-Young NMR tube. This solution was
Reaction of (PNP*)RhH with CO2, forming (PNP)Rh-
Synthesis of (PNP)Rh(PMe3). To 25 mg of (PNP*)RhH freeze–pump–thawed three times and 1 atm of CO2 was added.
(0.045 mmol) and 0.5 mL of C6D6 was added 0.0047 mL of Spectroscopic assay showed complete conversion to a single
1
PMe3 (0.0453 mmol, 3.5 mg). NMR showed complete conver- product. 31P{1H} NMR (C6D6): δ 51.96 (d, JRhP 122.6 Hz).
sion into the new diamagnetic complex (PNP)Rh(PMe3). 1H 1H NMR (C6D6): δ 1.40 (vt, 3JHP 13.0 Hz, 18 H, t-Bu), 1.37 (vt, 3JHP
3
NMR (25 °C, C6D6): 0.50 (s, SiMe, 12 H), 0.90 (t, J = 4.2, CH2, 12.9 Hz, 18 H, t-Bu), 0.88 (vt, JHP 9.2 Hz, 2 H, Si–CH2–P), 0.85
3
4 H), 1.31 (t, J = 5.6, tBu, 36 H), 1.38 (d, J = 6.9, PMe3, 9 H). 31
P
(vt, JHP 9.2 Hz, 2 H, Si–CH2–P), 0.17 (s, 6 H, Si–CH3), 0.11 (s,
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6 H, Si–CH3). Solvent was stripped in vacuum, the residue dis- indeed available CO in the headspace, but not dissolved, and
solved in pentane and IR recorded: νCO = 1805, 1847 cm−1
.
this finally displaces N2 from (PNP)Rh(N2).
Reaction of (PNP)Rh(CO2) with CO. Independent evaluation
of rate of CO addition to (PNP)Rh(CO2). A sample of
Reaction of (PNP)RhO with CO
Room temperature product analysis. The sample of (PNP)- 0.045 mmol (PNP)Rh(CO2) in 0.5 mL C6D6 was freeze–pump–
RhO generated (“standard method”) in a flask by procedure 1 thaw degassed and 760 mm Hg of CO (0.090 mmol) was added
in d8 THF was transferred with Teflon cannula (to minimize on a vacuum line. After all frozen solid melted, the tube was
heating of the solution) into a J-Young NMR tube capped with quickly shaken. Full conversion into known (PNP)Rh(CO) was
a septum and immersed in a Dewar filled with acetone cooled observed in less than 3 h of end-over-end agitation at 25 °C. In
at −40 °C. A plastic syringe (3 mL) was used to push the solu- another experiment, there was 11% conversion after
tion by argon from the flask into the NMR tube. The solution 10 minutes of agitation.
was frozen in liquid nitrogen and the septum was quickly
Low temperature monitoring of reaction of (PNP)Rh(CO2)
replaced with a Teflon screw cap. The tube was freeze–pump– with CO. A sample of (PNP)Rh(CO2) synthesized from 25 mg
thaw degassed (−40 °C for melting) and 760 mm Hg of CO (PNP*)RhH (0.045 mmol) and 1 equivalent of CO2 in toluene
(∼3.5 equiv.) was added on a vacuum line. The contents were d8 was freeze–pump–thaw degassed and 760 mm Hg of CO
allowed to melt and shaken vigorously and NMR after (0.090 mmol) was added to the frozen solid on a vacuum line.
5 minutes at 25 °C showed complete consumption of (PNP) Multinuclear NMR at −30 °C did not show any new adduct
RhO and formation of the known (PNP)Rh(CO).13
It was necessary to establish how CO reacts with the intra- Rh(CO).
(PNP)Rh(“C2O3”), but only slow conversion into known (PNP)
molecular rearrangement product of (PNP)RhO, (PNP*)Rh-
Synthesis of (PNP)Rh(H2O). 25 mg of (PNP*)RhH
(OH). Therefore, (PNP*)Rh(OH) was reacted in a 1 : 2 mole (0.0453 mmol) and 0.5 mL of d8 THF was placed into a
ratio with CO in d8 toluene was studied beginning at −60 °C. J-Young tube and 0.8 mg (0.045 mmol) of H2O was added via
At the lowest observation temperatures, there is already some syringe. NMR showed complete conversion into the new dia-
(PNP)Rh(CO) (probably due to local overheating upon mixing, magnetic aquo complex. 1H NMR (25 °C, d8 THF): −0.05 (s,
t
since the spectrum still shows unconsumed (PNP*)Rh(OH)) SiMe, 12 H), 0.56 (t, J = 4.2, CH2, 4 H), 1.29 (t, J = 5.5, Bu, 36
but also an AMX pattern in the 31P{1H} NMR spectrum which H), 1.8 (2H, H2O). 31P{1H} NMR (25 °C, THF d8): 52.8 (d, J =
we attribute to a simple monocarbonyl adduct where the phos- 148). When the analogous reaction was run with D2O, deuter-
phine tBu group is still metallated, and the hydroxyl group is ium NMR assay shows only a weak (∼10%) signal due to deu-
still intact. This last point is supported by seeing a signal at terium at the tBu chemical shift.
−4.8 ppm characteristic of the hydroxyl proton.25 Already at
Reaction of (PNP)Rh(H2O) with H2. A sample of (PNP)Rh-
−40 °C, there is less unreacted (PNP*)Rh(OH), but the AMX (H2O) synthesized from (PNP*)RhH and 1 equivalent of H2O in
intermediate is strong. Spectra then recorded at 25 °C show d8 THF was freeze–pump–thaw degassed and 760 mm Hg of
complete disappearance of the AMX intermediate, with for- H2 (∼2 equiv.) was added on a vacuum line. After all frozen
mation of only (PNP)Rh(CO); this is the same product as from solid melted, the tube was quickly shaken. Full conversion
(PNP)RhO and two moles of CO, so care must be exercised to into known (PNP)Rh(H)2 was observed after 3 h of end-on-end
avoid reaction temperatures where (PNP)RhO converts to agitation at 25 °C; liberated water is partly responsible for con-
(PNP*)Rh(OH).
version of some (PNP)Rh fragment to degradation products, if
Reaction of (PNP)RhO with CO with low temperature NMR left dissolved together too long here. The reaction of (PNP)Rh
monitoring. The sample of (PNP)RhO generated by procedure (OH2) with 1 atm H2 was also monitored at −30 °C by 31P NMR
2 was freeze–pump–thaw degassed and 760 mm Hg of CO was in THF to follow the decrease of reagent complex and growth
added to the frozen solid on a vacuum line. The flask was of (PNP)Rh(H)2. Aliquots were taken periodically up to
placed in a Dewar filled with acetone at −90 °C and allowed to 400 minutes, then at 24 hours; assay was by taking a liquid
melt. NMR starting from −80 °C showed simultaneous decay aliquot and rapidly (within 10 min,) recording the 31P{1H}
of (PNP)RhO and conversion into the known diamagnetic mole- NMR of each aliquot in a probe held at +25 °C; this assay is
cule (PNP)Rh(CO). Intermediate (PNP)Rh(CO2) was detected fast compared to the rate of previous observations of this reac-
between −50 °C and 0 °C.
tion, all executed at +25 °C. The half-life for the reaction at
Production, in this experiment, of (PNP)RhO (together with −30 °C was found to be ∼330 minutes.
equimolar (PNP)Rh(N2)) at −80 °C in d8 THF was confirmed by
Reaction of (PNP)RhO with H2 with low temperature NMR
1H NMR prior to adding gaseous carbon monoxide. The suc- monitoring. A sample of (PNP)RhO was generated by the pre-
cessful detection of (PNP)Rh(CO2) depends on the fact that vious “standard procedure.” The tube was then held at −40 °C
substoichiometric dissolved CO is available (lack of agitation and 760 mm Hg of H2 (∼2 equiv.) was added on a vacuum line.
of head space CO into solution), and this is further confirmed NMR spectra starting from −80 °C showed simultaneous decay
by the observation of unreacted (PNP)RhO at temperatures as of (PNP)RhO and conversion into the known diamagnetic mole-
warm as 0 °C. Finally after shaking vigorously at 25 °C, not cule (PNP)Rh(H)2. No intermediate was seen.
only is (PNP)Rh(CO2) gone, but the ratio of (PNP)Rh(CO) to
Addition of water to (PNP)RhO. 0.0005 mL of H2O (0.5
(PNP)Rh(N2) has changed to 8 : 1, indicating that there was equivalents to the Rh) was vacuum transferred to the solution
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of (PNP)RhO generated from 29 mg of (PNP*)RhH and N2O in is the reason why it is possible to find a temperature where
0.5 mL of d8 THF at −30 °C during 1 h. The solution contain- this path to (PNP)Rh(H)2 is slower than the reaction via H2
1
ing water was monitored by H and 31P NMR and showed no addition to (PNP)RhH(OH).
reaction even after 24 h at −30 °C. An additional 0.001 mL of
H2O was then added by vacuum transfer and no reaction was
seen after 24 h at −30 °C. The H2O 1H NMR peak is seen at
2.7 ppm (it became taller after the second addition of H2O,
Acknowledgements
showing that it had not been frozen to ice) together with para- This work was supported by the National Science Foundation
magnetic (PNP)RhO. Overall observation time for no reaction (NSF-CHE-0544829).
was over 48 hours at −30 °C. Higher temperature observations
are precluded by the background reaction (Scheme 1) of
addition of one methyl C–H bond across the Rh/O bond.
Notes and references
Attempts to establish possible small 1H NMR chemical shift
changes due to equilibrium formation of a hydrogen bonded
pair {(PNP)RhO; H2O} were frustrated by the strong T−1 chemi-
cal shift dependence due to the paramagnetism of (PNP)RhO.
1 K. P. O’Halloran, C. Zhao, N. S. Ando, A. J. Schultz,
T. F. Koetzle, P. M. B. Piccoli, B. Hedman, K. O. Hodgson,
E. Bobyr, M. L. Kirk, S. Knottenbelt, E. C. Depperman,
B. Stein, T. M. Anderson, R. Cao, Y. V. Geletii,
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K. Morokuma, S. Wu, P. Kögerler and C. L. Hill, Inorg.
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2 E. Spaltenstein, R. R. Conry, S. C. Critchlow and
J. M. Mayer, J. Am. Chem. Soc., 1989, 111, 8741–8742.
3 A. Y. Verat, H. Fan, M. Pink, Y. S. Chen and K. G. Caulton,
Chem.–Eur. J., 2008, 14, 7680–7686.
4 Compare this to the isoelectronic triplet (PNP)RuCl.
L. A. Watson, O. V. Ozerov, M. Pink and K. G. Caulton, J.
Am. Chem. Soc., 2003, 125, 8426.
5 A. Y. Verat, M. Pink, H. Fan, J. Tomaszewski and
K. G. Caulton, Organometallics, 2008, 27, 166–168.
6 A. Walstrom, M. Pink, N. P. Tsvetkov, H. Fan, M. Ingleson
and K. G. Caulton, J. Am. Chem. Soc., 2005, 127, 16780–16781.
7 A. Walstrom, L. A. Watson, M. Pink and K. G. Caulton,
Organometallics, 2004, 23, 4814–4816.
Computational details
All calculations were carried out on the full set of atoms
present, using Density Functional Theory with the B3LYP26–28
functional and the 6-31G** basis set with no symmetry restric-
tions, which has been shown to work well for many transition
metal-containing systems.29 All transition metals were rep-
resented using the Los Alamos basis set (LACVP).30,31 Energies
of the optimized structures were reevaluated by additional
single-point calculations on each optimized geometry using
Dunning’s correlation-consistent triple-ζ basis set,32 cc-pVTZ
(-f). Since we calculate all intermediates as singlets here, we
chose to use the B3LYP functional in spite of its overstabiliza-
tion of higher spin states (i.e., the singlet was previously3 cal-
culated to lie 15.3 kcal mol−1 above the triplet using a single
determinant method). The B3LYP choice was also bench-
marked by its calculation of ΔG° > 0 for the addition of H2 to
(PNP)RhO. In any event, our calculations fully establish mech-
anism on the singlet surface. A similar situation was recently
reported33 for S = 3/2 Mo[N(tBu)Ar]3 reacting with nitriles to
finally give doublet products; on the quartet surface, binding
the nitrile was repulsive, and the mechanism was mapped out
on the doublet surface.
8 E. Poverenov, I. Efremenko, A. I. Frenkel, Y. Ben-David,
L. J. W. Shimon, G. Leitus, L. Konstantinovski,
J. M. L. Martin and D. Milstein, Nature, 2008, 455, 1093–
1096.
9 I. Efremenko, E. Poverenov, J. M. L. Martin and D. Milstein,
J. Am. Chem. Soc., 2010, 132, 14886–14900.
Efforts to find a TS for direct addition of H2 to the terminal 10 A. Vigalok, Y. Ben-David and D. Milstein, Organometallics,
oxo of (PNP)RhO: step-scans, beginning from (PNP)Rh(H2O) 1996, 15, 1839–1844.
but with the H/H distance fixed between 1.54 to 0.9 Å never 11 See ESI.‡
reached a maximum, and the energy rises fast since the O/H 12 L. A. Watson and O. Eisenstein, J. Chem. Educ., 2002, 79,
distances remain within bonding range, hence exhibit repul-
sion. A step-scan search with two fixed OH distances in the 13 M. J. Ingleson, M. Pink and K. G. Caulton, J. Am. Chem.
range 0.74 to 1.37 Å showed no mutual approach of the two H, Soc., 2006, 128, 4248–4249.
but instead showed one H bridging O and Rh, with the other 14 C. E. Housmekerides, D. L. Ramage, C. M. Kretz,
1269–1277.
H terminal on O. A search for a TS1–4 for [2 + 2] addition of the
H–H bond directly across the Rh/O bond collapsed towards 3,
J. T. Shontz, R. S. Pilato, G. L. Geoffroy, A. L. Rheingold and
B. S. Haggerty, Inorg. Chem., 1992, 31, 4453–4468.
hence showed no new approach trajectory distinct from 15 C. E. Housmekerides, R. S. Pilato, G. L. Geoffroy and
passing through (PNP)Rh(H2)O. The calculations show that
(PNP*)Rh(OH) is more stable than both singlet and triplet
A. L. Rheingold, J. Chem. Soc., Chem. Commun., 1991, 563–
566.
(PNP)RhO, as observed experimentally, and providing another 16 M. R. Churchill, G. Davies, M. A. El-Sayed and
benchmark to the utility of the B3LYP functional. Finally (see J. P. Hutchinson, Inorg. Chem., 1982, 21, 1002–1007.
ESI‡), the energy of the H2 adduct of (PNP*)Rh(OH) lies so 17 J. Fujita, A. E. Martell and K. Nakamoto, J. Chem. Phys.,
high (+20.3 kcal mol−1 relative to (PNP)RhO + free H2) that this
1962, 36, 339–345.
6754 | Dalton Trans., 2013, 42, 6745–6755
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