Abstract:Although pentadienediyl complex 6b does
not
react when heated for months in refluxing toluene, it
rapidly converts to the rhodocenium product
7b
+
when
subjected to electrochemical or chemical oxidizing conditions. Oxidation of the indenyl pentadienediyl complex
6a likewise affords rhodocenium complex
7a
+
.
“…Methylation of the second Cp ring as in [Cp* 2 Rh] + or [Cp*Cp Me4 Rh] + (Cp Me4 = η 5 ‐tetramethlycyclopentadienide) decreases the reduction potential further but, while preventing dimerization, still fails to chemically stabilize the corresponding neutral rhodocenes against other follow‐reactions , . Higher stabilities of reduced rhodocenium ions were reported for congeners with the Cp''' (Cp''' = 1,2,3‐tri tert butylcyclopentadienide) ligand, culminating in the observation of two chemically reversible reductions of the [Cp'''Cp*Rh] + cation . Again, sizeable cathodic shifts of the reduction potentials on replacing the Cp ligand by Cp* were noted (Table ).…”
As an extension of our continuing work in metallocenium chemistry, we report here on new functionalized rhodocenium salts. In contrast to isoelectronic cobaltocenium compounds, rhodium as a 4d metal allows synthetic routes via prefunctionalized cyclopentadienyl half-sandwich precursors, thereby facilitating access to monofunctionalized rhodocenium salts containing substituents comprising methyl, trimethylsilyl, carboxylate and carboxylate ester as well as amide derivatives. Synthetic aspects, scope and limitations, as well as spectroscopic ( 1 H/ 13 C-NMR, IR, HR-MS), and structural (XRD) properties
IntroductionAfter almost 70 years of research in metallocene chemistry, still relatively little is known about cobaltocenium, rhodocenium and iridocenium salts that are isoelectronic and as stable and redox responsive as their ubiquitous ferrocene congeners. The main reason for the underdeveloped chemistry of these metallocenium salts is their intrinsic cationic nature, making it quite difficult to access functionalized derivatives by standard chemical reactions developed for arenes. However, in recent years [a] mers and subsequently polymerized to the corresponding sidechain functionalized redox-responsive polymers.In this contribution, we aim at developing rhodocenium chemistry a bit further, in part based on our experience in cobaltocenium chemistry, [1,2] and disclose here a first chemoselective synthesis of rhodocenium monocarboxylic acid hexafluoridophosphate which is a key synthon for other monosubstituted rhodocenium salts accessible by standard carboxylic acid reactivity. The structural, spectroscopic and electrochemical properties of these new rhodocenium derivatives are compared with those of their cobaltocenium congeners.
Results and Discussion
SynthesisIn general, rhodocenium chemistry is somewhat different from cobaltocenium chemistry, as is often observed when comparing the reactivities and stabilities of analogous compounds of 3d vs. 4d/5d metals in a homologous group of elements. Successful synthetic routes to cobaltocenium compounds cannot simply Scheme 1. Synthesis of compounds.
“…Methylation of the second Cp ring as in [Cp* 2 Rh] + or [Cp*Cp Me4 Rh] + (Cp Me4 = η 5 ‐tetramethlycyclopentadienide) decreases the reduction potential further but, while preventing dimerization, still fails to chemically stabilize the corresponding neutral rhodocenes against other follow‐reactions , . Higher stabilities of reduced rhodocenium ions were reported for congeners with the Cp''' (Cp''' = 1,2,3‐tri tert butylcyclopentadienide) ligand, culminating in the observation of two chemically reversible reductions of the [Cp'''Cp*Rh] + cation . Again, sizeable cathodic shifts of the reduction potentials on replacing the Cp ligand by Cp* were noted (Table ).…”
As an extension of our continuing work in metallocenium chemistry, we report here on new functionalized rhodocenium salts. In contrast to isoelectronic cobaltocenium compounds, rhodium as a 4d metal allows synthetic routes via prefunctionalized cyclopentadienyl half-sandwich precursors, thereby facilitating access to monofunctionalized rhodocenium salts containing substituents comprising methyl, trimethylsilyl, carboxylate and carboxylate ester as well as amide derivatives. Synthetic aspects, scope and limitations, as well as spectroscopic ( 1 H/ 13 C-NMR, IR, HR-MS), and structural (XRD) properties
IntroductionAfter almost 70 years of research in metallocene chemistry, still relatively little is known about cobaltocenium, rhodocenium and iridocenium salts that are isoelectronic and as stable and redox responsive as their ubiquitous ferrocene congeners. The main reason for the underdeveloped chemistry of these metallocenium salts is their intrinsic cationic nature, making it quite difficult to access functionalized derivatives by standard chemical reactions developed for arenes. However, in recent years [a] mers and subsequently polymerized to the corresponding sidechain functionalized redox-responsive polymers.In this contribution, we aim at developing rhodocenium chemistry a bit further, in part based on our experience in cobaltocenium chemistry, [1,2] and disclose here a first chemoselective synthesis of rhodocenium monocarboxylic acid hexafluoridophosphate which is a key synthon for other monosubstituted rhodocenium salts accessible by standard carboxylic acid reactivity. The structural, spectroscopic and electrochemical properties of these new rhodocenium derivatives are compared with those of their cobaltocenium congeners.
Results and Discussion
SynthesisIn general, rhodocenium chemistry is somewhat different from cobaltocenium chemistry, as is often observed when comparing the reactivities and stabilities of analogous compounds of 3d vs. 4d/5d metals in a homologous group of elements. Successful synthetic routes to cobaltocenium compounds cannot simply Scheme 1. Synthesis of compounds.
“…For example, reductive elimination of bonds cd affords the vinylcyclopropene complex, elimination of bonds bc yields a metallacyclohexadiene, and coupling of bonds ad generates a coordinated cyclopentadiene; examples of all these reactions have been observed previously. [2][3][4][5][6][7][8][9][10][11][12][13][14][15] We believe this model is useful for understanding the chemistry described in this paper, in which we describe other skeletal contortions and substituent gymnastics observed in these systems. Skeletal rearrangements have been observed previously during the reactions of vinylcyclopropenes with transition metal complexes.…”
Section: Introductionmentioning
confidence: 95%
“…The ring opening of vinylcyclopropenes by transition metal complexes to give metallacyclohexadienes and related compounds is now well-established. [2][3][4][5][6][7][8][9][10][11][12][13][14][15] Metallacyclohexadienes are compounds of considerable interest, yet few isolated examples exist, and their reaction chemistry is poorly defined. They have been invoked as key intermediates in the Do ¨tz reaction, 16 and iridacyclohexadiene species, obtained by a different route, 17 were shown to undergo a facile deprotonation at the R-carbon to afford the first examples of iridabenzene complexes, leading to an extensive chemistry for these compounds.…”
On treatment with [Ir(PMe 3 ) 2 (acac)] at room temperature, 1,2,3-triphenyl-3-vinylcyclopropene undergoes ring opening accompanied by rearrangement to give, instead of the expected 1,2,3-triphenyliridacyclohexadiene complex, a crystallographically characterized 1,2,4-triphenyliridacyclohexadiene complex containing cis-phosphine ligands. Studies with 2 H-and doubly 13 C-labeled vinylcyclopropenes, the syntheses and characterization of which are also reported, show that this process involves a rearrangement of the carbon skeleton and not a substituent shift. The corresponding 1,2-diphenyl-3-vinylcyclopropene undergoes iridacyclohexadiene formation without any rearrangement. On heating at 90 °C, each iridacycle converts to its corresponding isomer containing trans-phosphine ligands without any skeletal or substituent rearrangement of the metallacycle, as evidenced by absence of change in the labeling pattern. At higher temperatures, further rearrangement occurs in the case of each metallacycle, which does not alter the metallacyclohexadiene backbone, but rather exchanges the substituents of the R and R′ carbon atoms. This rearrangement is shown to occur even when there is no driving force due to relief of steric effects. Mechanisms for each rearrangement are proposed and discussed.
“…3-Vinyl-1-cyclopropenes of general structure 1 (Scheme ) undergo thermal and photochemical ring expansion to cyclopentadienes and indenes. − Transition metal complexes also effect this ring expansion catalytically and stoichiometrically, to give cyclopentadienes, η 4 -cyclopentadiene complexes, and η 5 -cyclopentadienyl complexes. − On treatment with carbonyl-bearing metal complexes, cyclohexadienones, η 4 -cyclohexadienone complexes, and phenols have been prepared. − In several of these closures, pentadienediyl intermediates 2 resulting from ring opening of the vinylcyclopropenes have been observed , and have been shown to be precursors to the ring-closed η 4 -cyclopentadiene products 3 . − ,− Use of selectively deuterated compounds has demonstrated the stereochemistry of both the ring-opening and -closure reactions. , While metallacyclohexadiene intermediates 4 may be energetically accessible in the reaction, , ring-closure reactions do not require such intermediates and have been shown to occur directly from nonplanar intermediates 2 . ,, 1 …”
Under conditions of kinetic control, 1,2-diphenyl-3-vinyl-1-cyclopropene undergoes ring
opening with the [Rh(Cl)(PMe3)2] fragment to give two isomeric η3:η1-1,3-pentadienediyl
compounds: the expected 1,2-diphenyl isomer, and the 2,3-diphenyl isomer resulting from
an apparent skeletal rearrangement reaction. The latter complex has been characterized
by X-ray crystallography. Both complexes underwent ring closure to give the same
1,2-diphenylcyclopentadienyl complex on treatment with silver ion. Addition of a third
equivalent of trimethylphosphine to the 2,3-diphenyl isomer produced two meridional
rhodacyclohexadienes, which exhibit facile solvent-dependent chloride dissociation. In
contrast, phosphine added reversibly to the 1,2-diphenyl isomer to give only the chloride-dissociated compound, and the tris(phosphine) product could only be isolated after anion
exchange with hexafluorophosphate. No deprotonation to give rhodabenzene complexes could
be achieved. The mechanism of rearrangement is proposed to involve a carbocation
rearrangement during the ring-opening reaction and is compared to other metal-promoted
reactions of vinylcyclopropenes.
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