Equilibration between a Phosphine−Cobalt Complex and an Analogous Complex Containing an N-Heterocyclic Carbene:  The Thermodynamics of a Phosphine−Carbene Exchange Reaction

Abstract

The reaction of CpCo(PPh 3 )Me 2 with the N-heterocyclic carbene 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) does not result in complete substitution of the PPh 3 , as expected, but rather in an equilibrium mixture of CpCo(PPh 3 )Me 2 , IPr, CpCo(IPr)Me 2 , and PPh 3 . Determination of the equilibrium constants for this unusual reaction over the temperature range 30-50 °C shows that ∆H°) -24.4 ( 1.7 kJ mol -1 , ∆S°) -65.8 ( 5.5 J mol -1 K -1 , and thus that it is entropy factors arising from steric crowding in CpCo(IPr)-Me 2 that prevent complete substitution. Evidence for steric crowding is found in a comparison with the crystal structures of CpCo(PPh 3 )Me 2 , CpCo(IPr)Me 2 , and CpCo(IPr)(CO). The smaller, more basic PMe 3 does displace IPr completely from CpCo(IPr)Me 2 .Phosphine complexes are used extensively in catalytic processes, generally stabilizing reaction intermediates through their electronic and steric properties. 1 N-Heterocyclic carbenes (A, R ) alkyl, aryl

N‐Heterocyclic Carbene Functionalized Iridium Phosphinidene Complex [Cp*(NHC)IrPMes*]: Comparison of Phosphinidene, Imido, and Carbene Complexes

“…[4,6,7] Phosphine substitution in the established Rubased Grubbs catalysts [(PR 3 ) 2 Cl 2 RuCHR'] by imidazol-2-ylidenes, has already provided new catalytic systems with superior activities in olefin metathesis. [2,6] Interestingly, these Ru complexes represent the only known examples of a new class of compounds that contain both an alkylidene moiety as well as an N-heterocyclic carbene ligand.…”

Cobalt-mediated oligomerization reactions of amino-substituted acetylene derivatives

“…Complex 3 0 d possesses a C 2 axis passing through the middle of Co-Co bond and C1-C1A vector. The bonding parameters of 3 0 d show that the Co1-C1 and Co1-C1A are rather short (1.82-1.83 Å ) in comparison to other cobalt-carbenes (1.88-2.10 Å ) [13]. The almost equal bond distances between cobalt (Co1 and Co1A) and the C carbyne atoms (C1 and C1A) suggest partial double bond character due to electron delocalization between cobalt atoms, C1 and C1A.…”

“…However, the reaction of the diethylaminoacetylene derivative 1b with CpCo(CO) 2 in a 1:3 ratio at 115°C did not give the trimetallic cluster 4 0 b, but the dinuclear species 3 0 b was obtained in 41% yield (Scheme 2) [10]. Compound 3 0 b, a brown oil, was characterized by 1 H, 13 C NMR spectroscopy as well as by mass spectrometry. The 1 H NMR spectrum of 3 0 b shows a triplet (d = 1.21 ppm) and a quartet (d = 3.03 ppm) in addition to the Cp resonance (d = 5.03 ppm).…”

“…The 1 H NMR spectrum of 3 0 b shows a triplet (d = 1.21 ppm) and a quartet (d = 3.03 ppm) in addition to the Cp resonance (d = 5.03 ppm). In the 13 group along with the signal for the Cp resonance (d = 86.42 ppm). The 13 C resonances of the two different quarternary carbon atoms of the C-NEt 2 and C-SPh groups could not be resolved, possibly owing to broadening through coupling with the quadrupolar cobalt atoms.…”

Catalysis and Organometallic Chemistry of Monometallic Species

“…228 An equilibrium between a tertiary phosphine and an N-heterocyclic carbene has been observed for [Co(η 5 -C 5 H 5 )Me 2 (PPh 3 )] and thermodynamic parameters determined. 229 Related rhodium() and iridium() complexes of the non-cyclic carbene bis(diisopropylamino)carbene have been isolated for the first time. IR spectroscopy of rhodium() carbonyl carbene complexes indicate that this class of carbene is the most basic carbene ligand known to date.…”