Phosphine-Modified Cobalt-Catalyzed Hydroformylation System
Organometallics, Vol. 26, No. 2, 2007 353
in eq 2 is proceeding near the diffusion-controlled limit,
The current work focuses on the effect of phosphine substitu-
tion on the thermodynamics for the key hydrogen activation
step in eq 3,
2 Co(CO)4 + H2 f 2 HCo(CO)4
(2)
Co2(CO)6L2 + H2 ) 2 HCo(CO)3L
(3)
as was found for the three examples in which the kinetics of
such a radical process have been rigorously established. The
underlying assumption is that trimolecular radical-based hy-
drogen activation processes will exhibit essentially no activation
barrier, as long as the net reaction process is thermodynamically
favorable. The enthalpy change for the reaction in eq 2 is quite
favorable, ∆H ) -14 kcal/mol, on the basis of bond energy
considerations using the Co-H bond energy, 59 kcal/mol, that
was determined by NMR methods.6 At lower CO pressure, a
faster CO-dependent pathway to the hydride becomes domi-
nant.12 The kinetics of the CO-dependent pathway have been
investigated by Wegman and Brown, who found them to be
consistent with a radical chain process that is catalyzed by Co-
(CO)4.13 Both the CO-dependent and the CO-independent
pathways for the production of HCo(CO)4 involve the Co(CO)4
radical. Our own unpublished kinetic data that were measured
while determining the thermodynamics5 nicely corroborate
Ungva´ry’s results, as demonstrated in Figures 4 and 5 in the
Supporting Information.
where L ) (p-CF3C6H4)3P and n-Bu3P. In addition, it was hoped
that the NMR measurements could shed some light on the effect
of phosphine substitution on the strength of the Co-Co bond.
In comparison with our previous measurement for L ) CO,
these phosphine ligands span a range of Lewis basicities and
differ considerably in their back-bonding characteristics when
compared with CO. Accordingly, data of this type should be of
considerable value in providing experimental benchmarks
for theoretical investigations of cobalt carbonyl hydride chemis-
try.25-30 The latter field spans an impressive range of reactions
including the hydroformylation of olefins to aldehydes,21-23 the
Reppe carbonylation of olefins to carboxylic acids,31 the
homologation of methanol to higher alcohols,32-34 the BASF
process for the carbonylation of methanol to acetic acid,2 and
the hydrogenation of CO to methanol.35 The cobalt hydroformy-
lation system based on the n-Bu3P ligand was patented by
Slaugh and Mullineaux36,37 and is considered representative of
the Shell hydroformylation process.22,36-39 We also report here
on the solvent effect for hydride formation in eq 3 for this
important phosphine ligand.
The Co(CO)4 radicals that are produced by the thermolysis
of Co2(CO)8 in eq 1 undergo facile hydrogen atom transfer
reactions6 and have been proposed to promote many of the
reactions of HCo(CO)4 that are of central importance to the
cobalt-catalyzed olefin hydroformylation process.14 Importantly,
it has been shown that the reaction of HCo(CO)4 with the type
of unactivated olefins that are typically used in the hydroformy-
lation reaction such as 1-hexene or 1-octene requires Co2(CO)8
promotion. The kinetic order in this very significant case was
established and shown to be 0.5 order in Co2(CO)8.15 In contrast,
activated olefins that are typified by styrene follow a different
pathway involving a radical pair mechanism that is not
influenced by the presence of Co2(CO)8.16 The ubiquitous 0.5
order in Co2(CO)8 that has been rigorously established13,15,17-19
in most of the cases involving unactivated substrates is difficult
to reconcile with the conventional Heck-Breslow style reaction
mechanism20 for the hydroformylation reaction.20-24 Accord-
ingly, despite the intense activity over the past 60 years on this
important organocobalt system, further work on these reactions
that involve the olefin substrate is strongly called for and will
be investigated in more detail by the high-pressure operando
NMR method in future studies.
Experimental Section
Equilibrium reactions were examined in situ using a General
Electric GN 300/89 NMR spectrometer equipped with a toroid
detector pressure probe built in-house.40 The pressure vessel portion
of the probe was machined from Be-Cu alloy (Brush-Wellman
alloy 25) and has an internal volume of 8 mL. Probe heating was
accomplished by means of an outer-jacketed electrical furnace that
fits snugly around the pressure vessel and is powered from a
Sorensen DCR150-3B power supply and computer-controlled to
within (0.1 °C using a copper-constantan thermocouple built into
the furnace.
Equilibrium experiments were performed by initially loading
solvents and cobalt complexes into the pressure vessel prior to
assembly under a purified helium atmosphere in a glovebox.
Reactive and/or inert gases were then admitted to the desired
pressures. Pressures were monitored using a strain-gauge pressure
transducer (Omega, model PX302-5KGV) and were controlled by
means of an ISCO model 100DM high-pressure syringe pump. The
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