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Transition metal isocyanide complexes

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Technetium (99mTc) sestamibi is used in nuclear medicine imaging.[1]

Transition metal isocyanide complexes are coordination compounds containing isocyanide ligands. Because isocyanides are relatively basic, but also good pi-acceptors, a wide range of complexes are known. Some isocyanide complexes are used in medical imaging.

Scope of isocyanide ligands

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structure of Os3(CO)9(CNCH2)3CMe.[2]

Several thousand isocyanides are known, but the coordination chemistry is dominated by a few ligands.[3] Common isonitrile ligands are methyl isocyanide, tert-butyl isocyanide, phenyl isocyanide, and cyclohexylisocyanide.

Isocyanides are electronically similar to CO, but for most R groups, isocyanides are superior Lewis bases and weaker pi-acceptors. Trifluoromethylisocyanide is the exception, its coordination properties are very similarly to those of CO.

Because the CNC linkage is linear, the cone angle of these ligands is small, so it is easy to prepare polyisocyanide complexes. Many complexes of isocyanides show high coordination numbers, e.g. the eight-coordinate cation [Nb(CNBu−t)6I2]+.[4] Very bulky isocyanide ligands are also known, e.g. C6H3-2,6-Ar2-NC (Ar =aryl).[5]

Di- and triisocyanide ligands are well developed, e.g., (CH2)n(NC)2. (CH2)n((NC)2. Usually steric factors force these ligands to bind to two separate metals, i.e., they are binucleating ligands.[6] Chelating diisocyanide ligands require elaborate backbones.[7]

Synthesis

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Structure of Fe(tert-BuNC)5. Notice that some C-N-C angles strongly deviate from 180°, a characteristic of low-valent isocyanide complexes.[8]

Because of their low steric profile and high basicity, isocyanide ligands often install easily, e.g. by treating metal halides with the isocyanide. Many metal cyanides can be N-alkylated to give isocyanide complexes.[9]

Reactions

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Typically, isocyanides are spectator ligands, but their reduced and oxidized complexes can prove reactive by virtue of the unsaturated nature of the ligand

The first metal carbene complex, Chugaev's red salt, was not recognized as such until decades after its preparation.[10]

Cationic complexes are susceptible to nucleophilic attack at carbon. In this way, the first metal carbene complexes where prepared. Because isocyanides are both acceptors and donors, they stabilize a broader range of oxidation states than does CO. This advantage is illustrated by the isolation of the homoleptic vanadium hexaisocyanide complex in three oxidation states, i.e., [V(CNC6H3-2,6-Me2)6]n for n = -1, 0, +1.[11]

Because isocyanides are more basic donors ligands than CO, their complexes are susceptible to oxidation and protonation. Thus, Fe(tBuNC)5 is easily protonated, whereas its counterpart Fe(CO)5 is not:[8]

Fe(CNR)5 + H+ → [HFeL5]+
Fe(CO)5 + H+ → no reaction

Some electron-rich isocyanide complexes protonate at N to give aminocarbyne complexes:[12]

LnM-CNR + H+ → [LnM≡CN(H)R]+

Isocyanides sometimes insert into metal-alkyl bonds to form iminoacyls.[13]

Structure and bonding

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Isocyanide complexes often mirror the stoichiometry and structures of metal carbonyls. Like CO, isocyanides engage in pi-backbonding. The M-C-N angle provides some measure of the degree of backbonding. In electron-rich complexes, this angle is usually deviates from 180°. Unlike CO, cationic and dicationic complexes are common. RNC ligands are typically terminal, but bridging RNC ligands are common. Bridging isocyanides are always bent. General trends can be appreciated by inspection of the homoleptic complexes of the first row transition metals.

Homoleptic complexes

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1st Transition Series
Complex colour electron config. structure comments
[V(CNC6H3-2,6-Me2)6]− green d6, 18e− octahedral[14] Cs+ salt
[V(CNC6H3-2,6-Me2)6]0 purple d5 octahedral[14]
[V(CNC6H3-2,6-Me2)6]+ red d4 octahedral[14] PF6− salt
[V(CNC6H3-2,6-Me2)7]+ red d4, 18e− monocapped trigonal prism[15] iodide salt
[Cr(CNPh)6]3+ orange d3 octahedral[16]
[Cr(CN-t-Bu)7]2+ orange d4, 18e− octahedral[17]
[Cr(CNPh)6]0, 18e− d6 octahedral[18] many analogues
[Cr(CNMe)6]+OTf− yellow-brown d5 octahedral[19]
[Mn(CNPh)6]+ yellow d6, 18e− octahedral[20]
[Fe(CNMe)5]0 colourless d8, 18e− trigonal bipyramidal
[Fe2(CNEt)9]0 yellow d8 confacial bioctahedral[21] see Fe2(CO)9
[Fe(CNMe)6]2+ colourless d6, 18e− octahedral
[Co2(CN-t-Bu)8]0 red-orange d9 pentacoordinated with bridging isocyanides[22] see Co2(CO)8
[Co(CN-t-Bu)5]+ yellow d8, 18e− trigonal bipyramidal[23]
[Co(CNC6H3-2,6-Me2)4]− red d6, 18e− tetrahedral[24] see Co(CO)4−
[Ni(CNMe)4]0 colourless d10, 18e− tetrahedral see Ni(CO)4
[Ni(CNC6H3-2,6-iPr2)4]2+ yellow d8 square planar[25] see [Ni(CN)4]2-
[Ni4(CN-t-Bu)7]0 red d10 cluster[26]
[Cu(CNMe)4]+ colourless d10, 18e− tetrahedral analogous [Cu(CO)4]+ is unknown

IR spectroscopy

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The νC≡N band in isocyanides is intense in the range of 2165–2110 cm−1.[27] The value of νC≡N is diagnostic of the electronic character of the complex. In complexes where RNC is primarily a sigma donor ligand, νC≡N shifts to higher energies vs free isocyanide. Thus, for [Co(CN−t−Bu)5]+, νC≡N = 2152, 2120 cm−l.[23] In contrast, for the electron-rich species Fe2(CNEt)9, νC≡N = 2060, 1920, 1701, 1652 cm−l.[28]

See also

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References

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  1. ^ Underwood, S. R.; Anagnostopoulos, C.; Cerqueira, M.; Ell, P. J.; Flint, E. J.; Harbinson, M.; Kelion, A. D.; Al-Mohammad, A.; Prvulovich, E. M.; Shaw, L. J.; Tweddel, A. C. (1 February 2004). "Myocardial perfusion scintigraphy: the evidence". European Journal of Nuclear Medicine and Molecular Imaging. 31 (2): 261–291. doi:10.1007/s00259-003-1344-5. PMC 2562441. PMID 15129710.
  2. ^ Constable, Edwin C.; Johnson, Brian F.G.; Khan, Fatima K.; Lewis, Jack; Raithby, Paul R.; Mikulcik, Patrizia (1991). "Synthesis and crystal and molecular structure of Os3(CO)9[1,1,1-tris(isocyanomethyl)ethane]: A novel tris-isocyanide ligand capped triosmium cluster". Journal of Organometallic Chemistry. 403 (1–2): C15–C18. doi:10.1016/0022-328X(91)83111-G.
  3. ^ Patil, Pravin; Ahmadian-Moghaddam, Maryam; Dömling, Alexander (2020-09-29). "Isocyanide 2.0". Green Chemistry. 22 (20): 6902–6911. doi:10.1039/D0GC02722G. ISSN 1463-9270.
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  5. ^ Drance, Myles J.; Sears, Jeffrey D.; Mrse, Anthony M.; Moore, Curtis E.; Rheingold, Arnold L.; Neidig, Michael L.; Figueroa, Joshua S. (2019). "Terminal coordination of diatomic boron monofluoride to iron". Science. 363 (6432): 1203–1205. Bibcode:2019Sci...363.1203D. doi:10.1126/science.aaw6102. PMID 30872521. S2CID 78094683.
  6. ^ Harvey, P. (2001). "Chemistry, Properties and Applications of the Assembling 1,8-Diisocyano-p-menthane, 2,5-Dimethyldiisocyanohexane and 1,3-Diisocyanopropane Ligands and Their Coordination Polynuclear Complexes". Coordination Chemistry Reviews. 219–221: 17–52. doi:10.1016/S0010-8545(00)00415-X.
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  12. ^ Pombeiro, Armando J.L; Fátima, M.; Guedes Da Silva, C. (2001). "Coordination Chemistry of CNH2, The Simplest Aminocarbyne". Journal of Organometallic Chemistry. 617–618: 65–69. doi:10.1016/S0022-328X(00)00641-0.
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  14. ^ a b c Barybin, Mikhail V.; Young, Victor G.; Ellis, John E. (1998). "Syntheses and Structural Characterizations of the First 16-, 17-, and 18-Electron Homoleptic Isocyanide Complexes of Vanadium: Hexakis(2,6-dimethylphenyl Isocyanide)vanadium(I, 0, −I)1". Journal of the American Chemical Society. 120 (2): 429–430. doi:10.1021/ja9729239.
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  17. ^ Dewan, John C.; Mialki, William S.; Walton, Richard A.; Lippard, Stephen J. (1982). "Crystal Structure of the Homoleptic Seven-coordinate Complex Heptakis(tert-butyl Isocyanide)chromium(II) Hexafluorophosphate, [Cr(CN-t-Bu)7](PF6)2". Journal of the American Chemical Society. 104: 133–136. doi:10.1021/ja00365a025.
  18. ^ Ljungström, Evert; Hämäläinen, Reijo; Turpeinen, Urho; Andresen, Arne F.; Smidsrød, Olav; Pontchour, Cha-On; Phavanantha, Pathana; Pramatus, Supanich; Cyvin, Bjørg N.; Cyvin, Sven J. (1978). "The Crystal Structure of Hexakis(phenyl isocyanide)chromium(0), Cr(CNC6H5)6". Acta Chemica Scandinavica. 32a: 47–50. doi:10.3891/acta.chem.scand.32a-0047.
  19. ^ Bohling, David A.; Mann, Kent R. (1984). "X-ray Structural Characterization of [Cr(CNPh)6]CF3SO3, [Cr(CNPh)6](PF6)2, and [Cr(CNPh)6](SbCl6)3.CH2Cl2. Completion of a Unique Series of Complexes in which the Metal Attains Four Different Oxidation States While Maintaining Identical Ligation". Inorganic Chemistry. 23 (10): 1426–1432. doi:10.1021/ic00178a025.
  20. ^ Ericsson, Märta-Stina; Jagner, Susan; Ljungström, Evert; Tørneng, E.; Woldbæk, T.; Strand, T. G.; Sukhoverkhov, V. F. (1980). "The Crystal Structure of Hexakis(phenylisocyanide)manganese(I) Tri-iodide, [Mn(CNC6H5)6]I3". Acta Chemica Scandinavica. 34a: 535–540. doi:10.3891/acta.chem.scand.34a-0535.
  21. ^ J.-M. Bassett, M. Green, J. A. K. Howard, F. G. A. Stone (1978). "Formation of nona(ethyl isocyanide)diiron from penta(ethyl isocyanide)iron and reaction of penta(tert-butyl isocyanide)iron with diphenylacetylene; x-ray crystal structures of nona(ethyl isocyanide)diiron and tris(tert-butyl isocyanide){1,4-bis-(tert-butylimino)-2,3-diphenylbuta-1,3-diene}iron". J. Chem. Soc., Chem. Commun.: 1000-1. doi:10.1039/C39780001000.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  22. ^ Barker, Geoffrey K.; Galas, Anita M. R.; Green, Michael; Howard, Judith A. K.; Stone, F. Gordon A.; Turney, Terence W.; Welch, Alan J.; Woodward, Peter (1977). "Synthesis and Reactions of Octakis(t-butyl Isocyanide)dicobalt and Pentakis(t-butyl Isocyanide)ruthenium; X-ray Crystal and Molecular Structures of Co2(ButNC)8 and Ru(Ph3P)(ButNC)4". Journal of the Chemical Society, Chemical Communications (8): 256. doi:10.1039/C39770000256.
  23. ^ a b King, R. B.; Saran, Mohan Singh (1972). "Isocyanide-metal complexes. I. Reactions of the pentakis(tert-butyl isocyanide)cobalt cation with some tertiary phosphines and arsines". Inorganic Chemistry. 11 (9): 2112–2119. doi:10.1021/ic50115a025.
  24. ^ Leach, Patricia A.; Geib, Steven J.; Corella, Joseph A.; Warnock, Garry F.; Cooper, N. John (1994). "Synthesis and Structural Characterization of [Co{CN(2,6-C6H3Me2)]−, the First Transition Metal Isonitrilate". Journal of the American Chemical Society. 116 (19): 8566–8574. doi:10.1021/ja00098a017.
  25. ^ Kanters, J. A.; Nijs, H. L. L. M.; Van Der Sluis, P. (1989). "The Structure of the Tetrahydrofuran Solvate of Tetrakis(2,6-diisopropylphenylisocyanide)nickel(II) Diperchlorate". Acta Crystallographica Section C Crystal Structure Communications. 45: 16–18. doi:10.1107/S0108270188009497.
  26. ^ Day, V. W.; Day, R. O.; Kristoff, J. S.; Hirsekorn, F. J.; Muetterties, E. L. (1975). "Fluxional, Catalytically Active Metal Cluster, Heptakis(tert-butylisocyanide)tetranickel". Journal of the American Chemical Society. 97 (9): 2571–2573. doi:10.1021/ja00842a061.
  27. ^ Stephany, R. W.; de Bie, M. J. A.; Drenth, W. (1974). "A 13C-NMR and IR study of isocyanides and some of their complexes". Organic Magnetic Resonance. 6 (1): 45–47. doi:10.1002/mrc.1270060112.
  28. ^ Bassett, Jean-Marie; Green, Michael; Howard, Judith A. K.; Stone, F. Gordon A. (1978). "Formation of Nona(ethyl isocyanide)di-iron from Penta(ethyl isocyanide)iron and Reaction of Penta(t-butyl isocyanide)iron with Diphenylacetylene; X-ray Crystal Structures of Nona(ethyl Isocyanide)di-iron and Tris(t-butyl isocyanide)]1,4-bis-(t-butylimino)-2,3-diphenylbuta-1,3-diene]iron". Journal of the Chemical Society, Chemical Communications (22): 1000. doi:10.1039/C39780001000.
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