“…Initial reductive elimination of CMe 4 from the cis form of the complex followed by the intramolecular oxidative addition of an O–Me bond to the tungsten center results in the formation of 16e Cp*W(NO)(Me)(P(O)(OMe) 2 ) that then dimerizes so that the tungsten centers can attain the favored 18e configuration. Most interestingly, the product of this rearrangement is unlike those of previously reported Arbuzov rearrangements in that both the Me group and the resulting phosphonate ligand remain attached to the same metal center. − …”
The complexes trans-Cp*W(NO)(CHCMe)(H)(L) (Cp* = η-CMe) result from the treatment of Cp*W(NO)(CHCMe) in n-pentane with H (∼1 atm) in the presence of a Lewis base, L. The designation of a particular geometrical isomer as cis or trans indicates the relative positions of the alkyl and hydrido ligands in the base of a four-legged piano-stool molecular structure. The thermal behavior of these complexes is markedly dependent on the nature of L. Some of them can be isolated at ambient temperatures [e.g., L = P(OMe), P(OPh), or P(OCH)CMe]. Others undergo reductive elimination of CMe via trans to cis isomerization to generate the 16e reactive intermediates Cp*W(NO)(L). These intermediates can intramolecularly activate a C-H bond of L to form 18e cis complexes that may convert to the thermodynamically more stable trans isomers [e.g., Cp*W(NO)(PPh) initially forms cis-Cp*W(NO)(H)(κ-PPhCH) that upon being warmed in n-pentane at 80 °C isomerizes to trans-Cp*W(NO)(H)(κ-PPhCH)]. Alternatively, the Cp*W(NO)(L) intermediates can effect the intermolecular activation of a substrate R-H to form trans-Cp*W(NO)(R)(H)(L) complexes [e.g., L = P(OMe) or P(OCH)CMe; R-H = CH or MeSi] probably via their cis isomers. These latter activations are also accompanied by the formation of some Cp*W(NO)(L) disproportionation products. An added complication in the L = P(OMe) system is that thermolysis of trans-Cp*W(NO)(CHCMe)(H)(P(OMe)) results in it undergoing an Arbuzov-like rearrangement and being converted mainly into [Cp*W(NO)(Me)(PO(OMe))], which exists as a mixture of two isomers. All new complexes have been characterized by conventional and spectroscopic methods, and the solid-state molecular structures of most of them have been established by single-crystal X-ray crystallographic analyses.
“…Initial reductive elimination of CMe 4 from the cis form of the complex followed by the intramolecular oxidative addition of an O–Me bond to the tungsten center results in the formation of 16e Cp*W(NO)(Me)(P(O)(OMe) 2 ) that then dimerizes so that the tungsten centers can attain the favored 18e configuration. Most interestingly, the product of this rearrangement is unlike those of previously reported Arbuzov rearrangements in that both the Me group and the resulting phosphonate ligand remain attached to the same metal center. − …”
The complexes trans-Cp*W(NO)(CHCMe)(H)(L) (Cp* = η-CMe) result from the treatment of Cp*W(NO)(CHCMe) in n-pentane with H (∼1 atm) in the presence of a Lewis base, L. The designation of a particular geometrical isomer as cis or trans indicates the relative positions of the alkyl and hydrido ligands in the base of a four-legged piano-stool molecular structure. The thermal behavior of these complexes is markedly dependent on the nature of L. Some of them can be isolated at ambient temperatures [e.g., L = P(OMe), P(OPh), or P(OCH)CMe]. Others undergo reductive elimination of CMe via trans to cis isomerization to generate the 16e reactive intermediates Cp*W(NO)(L). These intermediates can intramolecularly activate a C-H bond of L to form 18e cis complexes that may convert to the thermodynamically more stable trans isomers [e.g., Cp*W(NO)(PPh) initially forms cis-Cp*W(NO)(H)(κ-PPhCH) that upon being warmed in n-pentane at 80 °C isomerizes to trans-Cp*W(NO)(H)(κ-PPhCH)]. Alternatively, the Cp*W(NO)(L) intermediates can effect the intermolecular activation of a substrate R-H to form trans-Cp*W(NO)(R)(H)(L) complexes [e.g., L = P(OMe) or P(OCH)CMe; R-H = CH or MeSi] probably via their cis isomers. These latter activations are also accompanied by the formation of some Cp*W(NO)(L) disproportionation products. An added complication in the L = P(OMe) system is that thermolysis of trans-Cp*W(NO)(CHCMe)(H)(P(OMe)) results in it undergoing an Arbuzov-like rearrangement and being converted mainly into [Cp*W(NO)(Me)(PO(OMe))], which exists as a mixture of two isomers. All new complexes have been characterized by conventional and spectroscopic methods, and the solid-state molecular structures of most of them have been established by single-crystal X-ray crystallographic analyses.
“…We have recently begun an investigation into the possibility of forming persistent iron-centered radicals of the type η 5 -Cp‘Fe(CO)L (Cp‘ = Cp, substituted Cp; L = bulky phosphines) . The 17-electron compounds CpFe(CO) 2 and Cp*Fe(CO) 2 can be generated photochemically from the corresponding dimers and have been investigated using time-resolved IR spectroscopy, but are exceedingly reactive and recombine rapidly to form the corresponding 18-electron dimers (eq 1) …”
mentioning
confidence: 99%
“… Attempts to stabilize substituted monomeric iron compounds with sterically demanding phosphines have generally failed. For instance, while abstraction of the hydridic hydrogen atom from CpFe(CO)(PPh 3 )H with the trityl radical almost certainly forms CpFe(CO)(PPh 3 ), the latter and its dimer are both unstable and only the diiron compound Cp 2 Fe 2 (CO) 3 (PPh 3 ) is formed (eq 2). 6a,c …”
mentioning
confidence: 99%
“…We also describe routes to the hydrides, Cp ⧧ Fe(CO) 2 H and Cp † Fe(CO) 2 H, and to the new series of substituted hydrides Cp † Fe(CO)LH (L = PMe 3 , PMe 2 Ph, PMePh 2 , PPh 3 ), all potential precursors to new 17-electron compounds of the type Cp‘Fe(CO)L. We find that both [Cp ⧧ Fe(CO) 2 ] 2 and [Cp † Fe(CO) 2 ] 2 undergo slight thermal homolysis in solution to give the monomeric species Cp ⧧ Fe(CO) 2 and Cp † Fe(CO) 2 , respectively, identified by spectroscopy and by chemical reactions, characteristic of 17-electron compounds, 1a,b which will be described in a subsequent paper. A preliminary report of aspects of this work has appeared 6a…”
The known compound
[Cp‡Fe(CO)2]2 and the
new, more soluble
[Cp†Fe(CO)2]2
(Cp‡ = η5-C5Ph5; Cp† =
η5-C5Ph4(p-tolyl))
are prepared via a new, very effective route involving
hydridic
hydrogen atom abstraction from the hydrides,
Cp‡Fe(CO)2H and
Cp†Fe(CO)2H, by the
trityl
radical. The dimers dissociate thermally to the corresponding
17-electron monomers, Cp‡Fe(CO)2 and
Cp†Fe(CO)2, which probably assume
“planar” (C
2
v
)
structures with OC−Fe−CO bond angles of ∼90°.
“…Until recently, the iron triad had yet to yield examples of persistent metal-centered radicals, and we therefore initiated an investigation into the chemistry of iron-centered radicals of the type η 5 -Cp‘‘Fe(CO)L • (Cp‘‘ = Cp, substituted Cp; L = bulky ligands) . It was known that the 17-electron compounds CpFe(CO) 2 • and Cp*Fe(CO) 2 • may be formed photochemically from the corresponding dimers as in eq 1, but they are exceedingly reactive and recombine rapidly to form the corresponding 18-electron dimers (eq 1) …”
The 18-electron dimers
[Cp‘Fe(CO)2]2 (Cp‘ =
η5-C5Ph5,
η5-C5Ph4(p-tolyl)),
which undergo
only very slight thermal dissociation to the corresponding 17-electron
monomers Cp‘Fe(CO)2
•,
nonetheless exhibit reactivity patterns which reflect the chemistry of
the monomers. Thus,
reactions with several organic halides RX give
Cp‘Fe(CO)2R and Cp‘Fe(CO)2X,
consistent
with initial halogen atom abstraction by
Cp‘Fe(CO)2
• followed by coupling of the
resulting
carbon-centered radical with a second molecule of
Cp‘Fe(CO)2
•. Reactions of
[(η5-C5Ph4(p-tolyl)}Fe(CO)2]2 with small
phosphines L result in displacement of
η5-C5Ph4(p-tolyl)
radicals
rather than formation of isolable 17-electron compounds
[{η5-C5Ph4(p-tolyl)}Fe(CO)L•,
while
reaction with the isonitrile t-BuNC results in
disproportionation to the salt
[{η5-C5Ph4(p-tolyl)}Fe(t-BuNC)3][{η5-C5Ph4(p-tolyl)}Fe(CO)2],
possibly via the substituted species (η5-C5Ph4(p-tolyl)Fe(CO)(t-BuNC)•.
Reactions of
[{η5-C5Ph4(p-tolyl)}Fe(CO)2]2
with phosphites
P(OR)3 do give the substituted radical species
{η5-C5Ph4(p-tolyl)}Fe(CO){P(OR)3}•,
but
Arbuzov products such as
{η5-C5Ph4(p-tolyl)}Fe(CO)2Me,
{η5-C5Ph4(p-tolyl)}Fe(CO)2{P(O)(OMe)2}, and
{η5-C5Ph4(p-tolyl)}Fe(CO){P(OMe)3}{P(O)(OMe)2}
are the major products
formed when R = Me and, to a lesser extent, Et. The Arbuzov
reaction is hindered when R
is large, and the compound
{η5-C5Ph4(p-tolyl)}Fe(CO){P(O-i-Pr)3}•
is a persistent radical
which has been characterized by, inter alia, EPR
spectroscopy. The EPR spectrum of a
frozen solution in benzene can be analyzed in terms of a uniaxial
g matrix (g
∥ = 1.993,
g
⊥ =
2.103) with isotropic 31P hyperfine coupling of 37 G (3.7
mT), typical of d7 organometallic
radicals in which the unpaired electron is essentially confined to a
metal d
z
2
orbital.
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