ISSN:
0947-6539
Keywords:
cobalt compounds
;
cyclopentadienyl ligands
;
metallocenes
;
peralkylations
;
rhodium compounds
;
Chemistry
;
General Chemistry
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
The acidity of methyl protons in the 18-electron cationic complexes [MCp*Cp]1PF6- and [MCp2*]+PF6- (M = Co or Rh, Cp = η5-C5H5, Cp* = η5- C5Me5) has been used for novel syntheses of starburst organometallic complexes. [CoCp*Cp]+PF6- was deprotonated at -30 °C with Si2Me6NK (1 equiv) in THF to give the fulvene complex [CoICp(η4-C5Me4CH2)]. This was not stable at 20°C, but was characterized by 1H and 13C NMR at -30 °C. The complexes [MCp*Cp]+PF6- (M = Co or Rh) reacted with excess base (KOH or tBuOK) and alkyl halides (RX = CH3I, C2H5I, CH2=CHCH2Br, or PhCH2Br) to give decasubstituted complexes (i.e., two hydrogens of each methyl group are replaced by two R groups). Distinct endo and exo alkyl groups are observed in the new complexes by NMR at room temperature. Coalescence occurs at higher temperatures; this indicates that the bulky alkyl groups are rotating. The calculated activation parameters (ΔG≠) are 71.3±0.8, 70.3±0.8, and 81.0±0.8 kJmol-1 for [Co(C5iPr5)Cp]+PF6-, [Rh(C5iPr5)-Cp]+PF6- (in C6D5NO2), and [Co{C5-(CHEt2)5}Cp]+PF6 (in o-C6H4Cl2), respectively. The single directionality of these ligands is confirmed by the X-ray crystal structure of [Co(C5iPr5)Cp]+PF6. This structure clearly shows the “paddle wheel” conformation adopted in the C5iPr5 ligand, due to steric hindrance between adjacent isopropyl groups and the staggered conformation of the two rings. The relative E° values measured by cyclic voltammetry show that the electron-donating properties of the new C5(CHR2)5 ligand are between those of C5H5 and C5Me5- With the decamethylcobalticinium salt [CoCp2*]+PF6-, the permethylation reactions with base and CH3 I did not go to completion, even under forcing conditions and with repeated reaction (the maximum number of methyl groups introduced was 14). On the other hand, [RhCp2*]+PF6-, in which the distance between rings is larger than in the Co analogue, reacted with excess KOH and CH3I to give [Rh(C5iPr5)2]+PF6 in 55% yield; only one diastereoisomer was detected by NMR (ΔG≠ = 85.5±0.8 kJ mol-1 in C6D5NO2). The C5iPr5 sandwich complexes are extremely robust, even at the 19-electron (CoII) stage. The 20-electron K+ salt of the CoI anion was regiospecifically protonated by H2O at the Cp ligand to give [Co(C5iPr5)(η4-C5H6)] and was decomplexed at 190°C to K+(C5iPr5)-.
Additional Material:
3 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/chem.19950010608
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