User:Circador/Cyclobutadieneiron tricarbonyl/Bibliography

Bibliography

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• Dietmar Seyferth 2003 essay in Organometallics
  • This is a source from a peer-reviewed journal that is reliable, verifiable, independent secondary source so it is good
  • predicting and first synthesis
    • great theoretical interest because of instability and debate over preferred ground state
      • square triplet, square singlet, or rectangular singlet (Jahn-Teller distortion)
    • 1956 "The Possible Existence of TM complexes of CBD" by Longuet-Higgins and Orgel
      • backbonding delta bond, pi bonds and sigma bond hypothesized
      • using isoelectronic series, (C4H4)Ni(CO)2 hypothesized or C4H4AuCl2
      • also hypothesized as an intermediate in certain precedented reactions, like Reppe synthesis of cyclooctatetraene (no benzene was ever observed), where molecular formula was found to be C7H4O3Fe, and that could be rewritten as the compound
    • 3 years later, its 1959, Hubel reacts Fe3(CO)12 with diphenylacetylene, find light yellow, crystalline solid with low melting point and sublimes in vacuum
      • x-ray crystal structure by Dodge and Schomaker reveals that it is the tetraphenyl substituted version
      • but this is a synthetic roadblock because you can't functionalize it anymore
    • Criegee also interested in 'cyclobutadiene problem' and tried to isolate it but got the dimer instead (as it is very reactive)
      • he followed the (C4H4)Ni(CO)2 prediction and tried to react the tetramethylcyclobutenedichloride with Ni(CO)4 and gave formula C8H12NICl2, and soluble in chlorinated organic solvents
      • so they did it, with confirmation of x-ray structure by Dunitz, Mez, Mills and Shearer.
      • it's actually dimeric
      • but again, it's substituted, it can't be further functionalized, could only displace the ligand or displace the bridging chlorides
    • 1957, Pettit starts independent career and quickly interested in organoiron chemistry,
      • Nathan Bauld, a colleague sought his advice to make benzocyclobutadiene iron complex, so gave Pettit's student Emerson with trans-dibromobenzocyclobutene, and they got the benzocyclobutadiene compound which then spurred investigating cis-3,4,-dichlorocyclobutene with Fe2(CO)9 in pentane at 30C for 2 h
        • after filtration, pentane removal, vacuum distillation yields yellow liquid with low BP undervacuum, and cyrstals from pentane with low melting point 26 C
        • NMR backs it up, with single peak AND elemental analysis agrees with (C4H4)Fe(CO)3 AND IR shows only two CO bands
        • but is it aromatic?
    • Now we can find ways of making substituted versions and alternative syntheses
      • Michael rosenblum one-pot system: react photo-alpha-pyrone with iron pentacarboonyl --> low yield and can be separated but also the process inherently destroys the compounds because CO is photolabile
        • Could also make the cyclobutadiene, cyclopentadienyl cobalt (but also forms a mixture of products, as well as Rhodium and vanadium species (CBD)(Cp)V(CO)2 from CpV(CO)4
      • Need a more general strategy since 3,4-dihalocyclobutene/metal carbonyl doesn't work for metals not Ni or Fe --> Amiet, Reeves, Pettit synthesis using sodium salts of metal carbonyl anions
        • 3,4-dichlorocyclobutene with Na2Fe(CO)4
        • opens the door to Ru, Mo, W, and also tetramethylCBD versions of Fe, Cr, Mo, W
        • Cobalt dimer could also be prepared which then leads to the final product after oxidation by I2
    • Actual Structure and Bonding
      • it is square planar BUT two independent C-C distances 1.420 and 1.430 BUT these are also within 2 standard deviations of the average
      • C-H hydrogen atoms slightly bent away from carbonyl groups
      • Electronic structure probed by low-energy photoelectron spectrum has assigned ALL 9 bands
      • NMR studies looking at 13C-1H coupling constants agree that there is square geometry with equal C-C bonds
    • Bonding
      • sigma, MO1 of CBD can donate into metal s, pz, dz2
      • pi bond to py and dyz or to px and dxz
      • finally delta backbonding from dx2-y62 to MO4
    • Quantum Chemical calculations
      • ???
    • Reactivity
      • can use photolability of the CO ligands to make dinitrogen and dihydrogen complexes, need to be stabilized in noble gas matrices
      • intramolecular cycladdition the CBD can even react with olefinic ligands bound to the Fe for intramolecular cycloaddition
      • photoinduced oxidative addition with hydrocarbon glass to form unsaturated 16e- d6 that then adds into triethylsilane, for example
      • in situ generation of cyclobutadiene need to oxidize iron center with CAN and trap with methylpropiolate, forming the dewar benzene derivative
        • does it tell us anything about the ground state of CBD? it behaves like a singlet diene so far
        • reacting with dimethyl maleate, you still only get the ENDO product with cis configuration, (endo, cis-dicarbomethyoxybicyclohexene)
          • would not be expected to have complete retention if it had triplet character
          • also look at ref 102b for suggestion that ground state is rectangular
        • reacting with dimethylfumarate, meanwhile only trans-dicarbomethoxybicyclohexene is observed
      • in situ generation of cyclobutadiene of synthetic utility
        • new way of making hemi-Dewar biphenyl
        • new synthesis of cubane 1,3-dicarboxylic acid
        • access to homocubyl system
        • Uwe Benz making linear oligomers for high MW organoiron and cobalt
  • Listing of substituted complexes, Ref 67 in 1982 lists them
    • undergoes lots of Friedel-crafts type reactions to yield substituted versions --> similar to what people did with ferrocene
    • cyclobutadiene complexes of V, Nb, Ta, Cr, Mo, W, Mn, Ru, Os, Rh, Ir, Ni, Pd, Pt reported REF 128
• Molecular orbital studies on cyclobutadienemetal complexes: the concept of metalloaromaticity | Inorganic Chemistry (acs.org)
  • while CBD normally unstable, TM complexes of cyclobutadiene remain quite stable
  • it even acts aromatic, so how can we understand this bonding motif esp metal-ring bonding
  • explained between metal and ring bonding and differentiated from cyclopentadienide ion
    • formally, total symmetry is Cs but locally D4h in the CBD ring and C3v for the M(CO)3 fragment
    • this allows us to separately analyze the M-L bonding interactions
    • d-orbital splitting according to dxz, dyz most stabilized by pi bonding interactions, then dxy, for this problem the dx2-y2 is nonbonding, and the dz2 takes on sigma antibonding, and
  • so how do you reconcile the singlet nature and the nature of normal cyclobutadiene vs difference observed in CBD Fe fragment?
    • strong mixing of d pi and degenerate eg orbitals, then if we consider the total Pi system in bonding orbitals, there is 6 electrons
    • this depends solely on the metal-ring bonding interactions
    • not quite the traditional aromaticity, instead call it 'metalloaromaticity'
  • the role of the CO then stabilizes the orbitals that interact the least with the CBD ring, this has been previously talked about Hoffman et al. for M(CO)3 / M(C6H6) fragments
  • Okay, can we further substantiate these claims with isolectronic metals?
    • let's look at different metals, like CbCr(CO)4 or CbNi(CO)2
      • Cr 3d levels are higher in energy than Fe 3d levels (less electronegativity), which decreases CBD mixing and increases CO interactions
      • Also the number of carbonyls in either C2v or C4v geometry now compete with the xz, yz orbitals for
• Unraveling the stability of CBD complexes using aromaticity markers
  • using aromaticity analysis using isolobal fragments, CH+, CO, and Be
  • looking at induced ring currents
• See 1977 Avi Efraty Chemical Reviews Article Cyclobutadienemetal complexes | Chemical Reviews (acs.org)
  • preparation
    • preparation from cyclobutene and cyclobutane derivatives
      • halocyclobutene
        • really only limited by potential dihalocyclobutene precursors
      • halocyclobutane
      • photo-alpha-pyrone
      • cis-3,4-carbonyldioxycyclobutene
    • preparation from alkynes and alkadiynes
      • alkynes + metal carbonyls
      • macrocyclic alkadiynes + metal carbonyl
      • alkynes with complexes and salts
      • using alkyne precursors
    • preparation from pi-ligand transfer reactions
  • reactivity
    • reactions that don't affect CBD
    • reactions that affect CBD into other ligands
    • reactions that substitute CBD
  • displacement of CBD
    • destroying CBD ligand
    • generating/using free CBD
  • structure and bonding
    • compared to cyclopentadienyl reveals that in complexes C(CBD)-C(CBD) > C(Cp)-C(Cp) AND M-C(CBD) < M-C(Cp), which agrees with original Longuet-Higgins and orgel paper
    • Higgins and Orgel, Green, and Cotton! (REF 284, 285)
      • G. E. Coates, M. L. H. Green, and K. Wade, “Organometallic Compounds", Vol. 2, M. L. H. Green, Ed., 3rd ed, Methuen, London, 1968, pp 68-70.
      • F. A. Cotton, “Chemical Application of Group Theory”, 2nd ed, Wiley, New York, N.Y., 1971, pp 239-241.
  • characterization
    • vibrational spectroscopy
      • can look at CO stretching frequencies
      • look at Andrews and Davison (REF 292, 293, 294)
        • D. C. Andrews and g. Davison, J. Organomet. Chem., 36, 349 (1972).
        • V. T. Aleksanyan and . N. Nefedova, Zh. Strokt. Khim., 14, 839 (1973)
        • . Andrews and G. Davison, J. Organomet. Chem., 76, 373 (1974)
    • UV/Vis spectroscopy not really rigorously studied
    • Mossbauer
      • has been used before, REF 296 and REF 297
      • R. Grubbs, R. Breslow, R. Herber, and S. J. Lippard, J. Am. Chem. Soc., 89, 6864 (1967)
      • R. H. Herber, R. B. King, and. N. Ackermann, J. Am. Chem. Soc., 96, 5437 (1974).
    • NMR spectroscopy
      • reveal that four ring positions are magnetically equivalent BUT of course can be explained low-energy barrier of rotation faster than NMR timescale
        • tetrakis(p-tolyl)cyclobutadienemetal complxes REF 158, 165 however feature near perfect AB quartet pattern for aromatic protons which infers presence of cyclobutadiene ligand
          • A. Efraty and P. M. Maitlis, J. Am. Chem. Soc., 89, 3744 (1967).
          • A. Efraty, Ph.D. Dissertation, McMaster University, Hamilton, Ontario, Canada, 1967.
        • mostly just a singlet at 6.09 ppm
      • interestingly partially substituted cyclobutadienes don't show any H-H couplings between vicinal OR transannular protons, so literally just singlets
        • protons of five-membered ring significantly deshielded vs 4-membered
      • see REF 299, Preston and Davis for 1H and 13C NMR
        • H.G. Preston, Jr., and J. C. Davis, Jr., J. Am. Chem. Soc., 88, 1585 (1966) .
      • see REF 305 for 13C NMR of 57Fe and 13C satellites
        • P. S. Nielsen, R. S. Hansen, and H. J. Jakobsen, J. Organomet. Chem., 114, 145 (1976).
    • Mass Spec, ya need volatility
      • characteristic ionization/decomposition pathways have been characterized, with the peak corresponding to the cation absent