Two-Electron and One-Electron Reduction of the Indenyl Complex [Mn(.eta.5-C9H7)(CO)3] and Reversible Counterion-Controlled Comproportionation of [Mn(.eta.5-C9H7)(CO)3] and [Mn(.eta.3-C9H7)(CO)3]2- To Give [Mn(.eta.5-C9H7)(CO)3]-
“…The separation between the waves is 150 mV for 1 and 280 mV for 2 . As argued by Cooper, these small potential differences suggest marked stabilization of the final reduction products ( 13 and 14 ) by an η 5 → η 3 hapticity shift concomitant with the second reduction and also supports η 5 -indenyl ligation in the first reduction product [( η 3 -Ind)CpMo(PR 3 ) 2 ] + .…”
Section: Discussionsupporting
confidence: 56%
“…More recently, Cooper reported the chemical, spectroscopical, and electrochemical study of the system [IndMn(CO) 3 ] 0/-/2- . The expected η 3 -Ind coordination is assigned to the dianion (18e species), but the electrochemical evidence supports the 19e monoanionic intermediate [IndMn(CO) 3 ] - with η 5 -Ind coordination . In close parallel, the 2e reduction of [IndFe(CO) 3 ] + occurs in a stepwise fashion to give ultimately the anion [( η 3 - Ind)Fe(CO) 3 ] - .…”
The dicationic complexes [IndCpMoL2]2+ (Ind = indenyl; Cp = cyclopentadienyl; 1, L2 =
dppe; 2, L = PMe3; 4, L2 = Bipy; 5, L2 = tBu2bipy; 6, L2 = H2biim; 7, L = CO and CNMe; 8,
L = CNMe; 9, L = CNtBu; 10, L = NCMe and NMF; 11, L = NMF) were synthesized from
[IndCpMoL‘2]2+ (L‘ = CO, NCMe), [IndCpMo(NCMe)Cl]BF4, or IndCpMoCl2 by ligand
substitution. The neutral complexes (η3-Ind)CpMoL2 (13, L2 = dppe; 14, L = PMe3; 15, L2 =
Bipy; 16, L = CO and CNMe; 17, L = CNtBu) were obtained upon reduction of the respective
dications with Cp2Co. (η
3-Ind)CpMo(dppe) (13) was also prepared by deprotonation of the
cyclopentadiene cation [IndMo(η4-C5H6)(dppe)]BF4
(12) with NEt3. The cyclic voltammograms
of the dications present one reversible 2e reduction step from Mo(IV) to Mo(II) except for 1,
2, and 6, in which cases two reversible 1e waves are observed. The ring-slipped complexes
(η
3-Ind)CpMoL2 and the parent dications present similar CV's. The X-ray crystal structures
of [IndCpModppe][BF4]2 and (η
3-Ind)CpMo(dppe) confirm the indenyl ring planar η5
coordination and slip-folded η3, respectively.
“…The separation between the waves is 150 mV for 1 and 280 mV for 2 . As argued by Cooper, these small potential differences suggest marked stabilization of the final reduction products ( 13 and 14 ) by an η 5 → η 3 hapticity shift concomitant with the second reduction and also supports η 5 -indenyl ligation in the first reduction product [( η 3 -Ind)CpMo(PR 3 ) 2 ] + .…”
Section: Discussionsupporting
confidence: 56%
“…More recently, Cooper reported the chemical, spectroscopical, and electrochemical study of the system [IndMn(CO) 3 ] 0/-/2- . The expected η 3 -Ind coordination is assigned to the dianion (18e species), but the electrochemical evidence supports the 19e monoanionic intermediate [IndMn(CO) 3 ] - with η 5 -Ind coordination . In close parallel, the 2e reduction of [IndFe(CO) 3 ] + occurs in a stepwise fashion to give ultimately the anion [( η 3 - Ind)Fe(CO) 3 ] - .…”
The dicationic complexes [IndCpMoL2]2+ (Ind = indenyl; Cp = cyclopentadienyl; 1, L2 =
dppe; 2, L = PMe3; 4, L2 = Bipy; 5, L2 = tBu2bipy; 6, L2 = H2biim; 7, L = CO and CNMe; 8,
L = CNMe; 9, L = CNtBu; 10, L = NCMe and NMF; 11, L = NMF) were synthesized from
[IndCpMoL‘2]2+ (L‘ = CO, NCMe), [IndCpMo(NCMe)Cl]BF4, or IndCpMoCl2 by ligand
substitution. The neutral complexes (η3-Ind)CpMoL2 (13, L2 = dppe; 14, L = PMe3; 15, L2 =
Bipy; 16, L = CO and CNMe; 17, L = CNtBu) were obtained upon reduction of the respective
dications with Cp2Co. (η
3-Ind)CpMo(dppe) (13) was also prepared by deprotonation of the
cyclopentadiene cation [IndMo(η4-C5H6)(dppe)]BF4
(12) with NEt3. The cyclic voltammograms
of the dications present one reversible 2e reduction step from Mo(IV) to Mo(II) except for 1,
2, and 6, in which cases two reversible 1e waves are observed. The ring-slipped complexes
(η
3-Ind)CpMoL2 and the parent dications present similar CV's. The X-ray crystal structures
of [IndCpModppe][BF4]2 and (η
3-Ind)CpMo(dppe) confirm the indenyl ring planar η5
coordination and slip-folded η3, respectively.
“…Furthermore, cyclic voltammetry suggests that the η 5 to η 3 ring slip equilibrium occurs rapidly on the electrochemical time scale . Since that first example, several more examples of reductively induced haptotropic shifts of the indenyl ligand have been reported. , One particularly interesting example involves the stepwise reduction of (η 5 -Ind)Mn(CO) 3 to its dianion . The relatively small difference between the first and second reduction potentials (0.25 V) was taken as evidence that the 1e – reduction intermediate is (η 5 -Ind)Mn(CO) 3 – , and the 2e – reduction product is (η 3 -Ind)Mn(CO) 3 2– , having undergone a haptotropic shift that markedly stabilizes the dianionic reaction product.…”
Complexes of the type R Cp 2 Ti(C 2 Fc) 2 (where R Cp = cyclopentadienyl or a substituted cyclopentadienyl) have high molar absorptivity Fe II to Ti IV metal-to-metal charge transfer (MMCT) absorptions and have been investigated for possible use as photoredox catalysts. Relative to Cp 2 Ti(C 2 Fc) 2 , the MMCT shifts to higher or lower energy when R Cp is either a stronger or weaker donor ligand than Cp, respectively. The indenyl ligand (Ind) has been described in the literature as both a stronger and weaker donor ligand than Cp. Herein, we report the preparation of Ind 2 Ti(C 2 Fc) 2 and the complex with CuBr bound between the two alkynes, Ind 2 Ti(C 2 Fc) 2 CuBr, in order to determine the effect that replacing Cp with Ind has on the MMCT absorption energy. The complexes are characterized by NMR and UV− vis spectroscopy, cyclic voltammetry, and X-ray crystallography in the case of Ind 2 Ti(C 2 Fc) 2 CuBr. Relative to Cp 2 Ti(C 2 Fc) 2 , the Fe III/II potential of Ind 2 Ti(C 2 Fc) 2 shifts cathodically and the Ti IV/III potential shifts anodically, resulting in a lower energy MMCT absorption. The Ti IV/III reduction wave is also chemically irreversible, with i pa /i pc approaching unity as the scan rate is increased from 10 to 500 mV/s. Examination of the literature reviewed in this report demonstrates that, in all cases, replacement of Cp with Ind leads to a cathodic shift of the potential for metal oxidation. However, replacement of Cp with Ind typically leads to an anodic shift of the reduction potential, leading some to suggest that Ind is a poorer e − donor than Cp. In the context of the literature, these results are interpreted as indicating that Ind is a better electron donor than Cp, but that reduction induces an η 5 to η 3 haptotropic shift that is rapid on the electrochemical time scale, stabilizing the reduction product, resulting in an anodic shift of the Ti IV/III reduction. Furthermore, these opposing effects are not unique to this system.
“…When the former mode of reactivity is realized, deprotonation of a neutral transition metal hydride yields an anionic metal complex as shown in eq 1. Examples of low-valent organometallic anions are numerous; many have been employed in organic synthesis as well as in the preparation of more highly functionalized organometallic species. − …”
Treatment of (η 5 -C 5 Me 5 )(PMe 3 )IrH 2 (1) with tert-butyllithium gives (η 5 -C 5 Me 5 )(PMe 3 )Ir-(H)(Li) (2) as a bright yellow solid. NMR evidence indicates that the lithium iridate 2 is aggregated in benzene, is converted to a single symmetrical species in THF, and is present as a dimer in DME. Treatment of 2 with 3,3-dimethylbutane trifluoromethanesulfonate-1,2-syn-d 2 (3-syn-d 2 ) gave the alkylated hydridoiridium complex 4a-anti-d 2 , which was converted to the corresponding chloride Cp*(PMe 3 )Ir(CHDCHDCMe 3 )(Cl) (4c-anti-d 2 ) by treatment with CCl 4 . Analysis of this material by NMR spectroscopy showed that it was contaminated with e15% syn isomer. The alkylation therefore proceeds with predominant inversion of configuration at carbon, indicating that the major pathway is an S N 2 displacement and not an outer-sphere electron-transfer reaction. Protonation studies carried out on iridate 2 with organic acids of varying pK a allowed us to estimate that the pK a of the dihydride 1 falls in the range 38-41, making it less acidic than DMSO and more acidic than toluene. This represents the least acidic transition metal hydride whose pK a has been quantitatively estimated. Treatment of 2 with main group electrophiles allowed the preparation of several other hydridoiridium derivatives, including Cp*(PMe 3 )Ir(SnPh 3 )(H) (5a), Cp*(PMe 3 )Ir(SnMe 3 )(H) (5b), and Cp*(PMe 3 )Ir(BR 2 )(H) (6a, R ) F; 6b, R ) Ph). Reaction of 2 with acid chlorides and anhydrides leads to acyl hydrides Cp*(PMe 3 )Ir(COR)(H), and fluorocarbons also react, giving products such as Cp*(PMe 3 )Ir(C 6 F 5 )(H) in the case of hexafluorobenzene as the electrophile.
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