Charge-Tuned Rh(I) Complexes
Organometallics, Vol. 28, No. 7, 2009 2063
Scheme 1. Formation of the Mono-cationic Complex 2 and
the Di-cationic Complex 3 by Protonation of the Zwitterionic
Metalate 1
population on the Rh atom. Nonetheless, in different complexes
with the same donor atoms and the same coordination geometry,
the CO stretching frequency can be used, with confidence, as a
measure of the electronic densities.
The rhodate complex [Rh(CO)2I2]- (the “Monsanto” catalyst)4
is taken as reference in comparing kinetic constant values; until
now, the highest ones have been determined for carbonyl
complexes containing bis(imino)carbazolide5 and N-heterocyclic
carbene6 ligands. It can be assumed that for geometrically
analogous complexes whose CO stretching frequencies are
equal, the electron density on the Rh atom is the same and the
variation of the sterics of ligand substituents allows the
observation of steric effects alone.7 For RhI carbonyl complexes,
the oxidative addition is often followed by the migratory CO
insertion, forming an acetyl complex. This reaction seems faster
when steric demand is high8,9 but slower for high electron
density complexes.10 Apparently, rate-enhancing electronic
effects in the migratory CO insertion are observed when strong
π-donor ligands are used.11 In this paper we report kinetic and
thermodynamic studies of the oxidative addition of iodomethane
to the zwitterionic metalate12 [Rh(EtSNS)(CO)]13 (1) and its
protonated derivatives:14 [Rh(HEtSNS)(CO)]X (2 · X) and
[Rh(H2EtSNS)(CO)]X2 (3 · X2) [X ) PF6, OTf, NO3; EtSNS )
EtNC(S)Ph2PdNPPh2C(S)NEt-].15 In the case of [Rh(EtSN-
S)(CO)], 1H NMR was useful to obtain real second-order kinetic
parameters, while in most literature cases, pseudo-first-order rate
constants were determined.16 In the case of the monoprotonated
compounds 2 · X, a quantitative correlation between their CO
stretching frequency with different counterion and the kinetic
constant was observed, showing a long-range electronic effect
due to the different hydrogen-bonding acceptor properties of
the anion. The biprotonated complexes were found, as expected,
to react very slowly.
Results and Discussion
As previously reported,13,14 the S,N,S-κ3 complex [Rh(EtSN-
S)(CO)] (1) can be prepared by reaction of HEtSNS with
[Rh(CO)2Cl]2 in the presence of t-BuOK or by bubbling CO in
a solution of the S,S-κ2 complex [Rh(EtSNS)(cod)] (cod ) 1,5-
cyclooctadiene). During these reactions, the unstable S,S-κ2
intermediate [Rh(EtSNS)(CO)2] was observed, monitoring the
solution reaction by FTIR (νCO: 2075, 2009 cm-1). The
coordination of the nitrogen atom raises the electron density of
the Rh, as inferred from the lower CO stretching frequency of
1 [νCO (CH2Cl2): 1967 cm-1]. Compound 1 is a biprotic base
and can be protonated, affording cationic species [Rh(HEtSN-
S)(CO)]+ (2) and [Rh(H2EtSNS)(CO)]2+ (3), in which protons
bind to the nitrogen atoms of the thioamidyl functions [pKa in
CH2Cl2 ) 6.5(3) and 4.8(4)]14 (Scheme 1). Compound 3 can
be conveniently prepared by using an excess of acid, while 2 is
obtained by mixing 1 and 3 in 1:1 molar ratio.
The electronic distribution varies dramatically, as evidenced
by the C-S and C-N bond distances14 and by the 31P{1H}
NMR chemical shift values (Table 1). In the case of 2 · X (X )
PF6, OTf, NO3), two 31P resonances are present,14 suggesting
that proton exchange is very slow (exchange can be fast, i.e.,
when X ) CF3COO-; δ ) 27.9 ppm, unpublished results). In
turn, protonation influences the electron density on the Rh(CO)
system, as reflected by the infrared CO stretching frequencies
(Table 1).
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The Mulliken populations for the Rh atom were calculated
at the BS-0 level for 1, 2, and 3 in the gas phase and varies
from -0.338 to -0.263 au (Table 2), showing that the
protonation of one thioamidic function induces a 13% decrease
in the electronic population for the monoprotonation and a 22%
decrease for the biprotonation (a decrease of 8% and 16%,
respectively, has been found in solution simulation). This
decrease in the calculated population is in agreement with the
trend found experimentally for the CO stretching frequencies.
A more detailed calculation has been carried out at the BS-I
level, in both the gas and solution phase, to confirm the BS-0
data. The results are consistent with the above-mentioned trend
(see Table S1 of the Supporting Information).
As suggested by a referee of this paper, it would be of great
interest to understand if two complexes with essentially the same
electron density at the metal could have different CO stretching
frequencies if the strength of back-donation to CO differs (for
example depending on whether a π-acceptor or π-donor ligand