Binding and redox properties of iron(II) bonded to an oxo surface modeled by calix[4]arene

Article abstract of DOI:10.1021/ic991467h

The syntheses of the parent compounds [{p-Bu(t) - calix[4] - (O)₂(OR)₂}Fe - L] [R = Me, L = THF, 5; R = Bu(n), L = THF, 6; R = PhCH₂, L = THF, 7; R = SiMe₃, L = none, 8] have been performed by reacting the protonated form of the dialkylcalix[4]arene with [Fe₂Mes₄] [Mes = 2,4,6-Me₃C₆H₂]. All of them undergo one-electron oxidative functionalization. By use of different oxidizing agents, the following iron(III) derivatives have been obtained: [{p-Bu(t) - calix[4] - (O)₂(OR)₂}Fe - X] [X = Cl, R = Me, 9; X = I, R = Me, 10] and [{p-Bu(t) - calix[4] - (O)₂(OR)₂}₂Fe₂(μ-X] [X = O, R = Me, 11; X = O, R = Bu(n), 12; X = S, R = Me, 13], 9 and 10 being particularly appropriate for a further functionalization of the metal. The last three display typical antiferromagnetic behavior [J = - 78.6 cm^-1, 11; J = - 64.1 cm^-1, 13]. In the case of 7 and 8, the reaction with O₂ led to the dealkylation of one of the alkoxo groups, with the formation of a dimeric iron(III) derivative {[μ-p-Bu(t) - calix[4] - (O)₃(OR)}₂Fe₂] [R = PhCH₂, 14; R = SiMe₃, 15] [J = - 9.8 cm^-1]. The reaction of the parent compounds with Bu(t)NC and diazoalkanes led to the formation of [Fe = C] functionalities supported by a calix[4]arene oxo surface. The following compounds have been isolated and characterized: {[p-Bu(t) - calix[4] - (O)₂(OR)₂}Fe = CNBu(t)] [R = SiMe₃, 16, v(CN) = 2175 cm^-1], {[p-Bu(t) - calix[4] - (O)₂(OR)₂}Fe = CPh₂] [R = Me, 17; R = PhCH₂, 18; R = SiMe₃, 19]. The three carbene complexes 17 - 19 display quite an unusual high-spin state, which is a consequence of the formation of a weak π interaction between the metal and the carbene carbon, as confirmed by the extended Huckel calculations. The carbene functionality has been removed from the iron center in the reaction with O₂ and HCl. The proposed structures have been supported by X-ray analyses of complexes 8, 9, 12, 14, 16, 17, and 19.

Full text of DOI:10.1021/ic991467h

2604
Inorg. Chem. 2000, 39, 2604-2613
Binding and Redox Properties of Iron(II) Bonded to an Oxo Surface Modeled by
Calix[4]arene
Vittorio Esposito, Euro Solari, and Carlo Floriani*
Institut de Chimie Mine´rale et Analytique, BCH, Universite´ de Lausanne,
CH-1015 Lausanne, Switzerland
Nazzareno Re
Facolta` di Farmacia, Universita` degli Studi “G. D’Annunzio”, I-66100 Chieti, Italy
Corrado Rizzoli and Angiola Chiesi-Villa
Dipartimento di Chimica, Universita` di Parma, I-43100 Parma, Italy
ReceiVed December 21, 1999
The syntheses of the parent compounds [{p-But-calix[4]-(O)2(OR)2}Fe-L] [R ) Me, L ) THF, 5; R ) Bun,
L ) THF, 6; R ) PhCH2, L ) THF, 7; R ) SiMe3, L ) none, 8] have been performed by reacting the protonated
form of the dialkylcalix[4]arene with [Fe2Mes4] [Mes ) 2,4,6-Me3C6H2]. All of them undergo one-electron oxidative
functionalization. By use of different oxidizing agents, the following iron(III) derivatives have been obtained:
[{p-But-calix[4]-(O)2(OR)2}Fe-X] [X ) Cl, R ) Me, 9; X ) I, R ) Me, 10] and [{p-But-calix[4]-(O)2-
(OR)2}2Fe2(µ-X] [X ) O, R ) Me, 11; X ) O, R ) Bun, 12; X ) S, R ) Me, 13], 9 and 10 being particularly
appropriate for a further functionalization of the metal. The last three display typical antiferromagnetic behavior
[J ) -78.6 cm-1, 11; J ) -64.1 cm-1, 13]. In the case of 7 and 8, the reaction with O2 led to the dealkylation
of one of the alkoxo groups, with the formation of a dimeric iron(III) derivative {[µ-p-But-calix[4]-(O)3(OR)}2Fe2]
[R ) PhCH2, 14; R ) SiMe3, 15] [J ) -9.8 cm-1]. The reaction of the parent compounds with ButNC and
diazoalkanes led to the formation of [FedC] functionalities supported by a calix[4]arene oxo surface. The following
compounds have been isolated and characterized: {[p-But-calix[4]-(O)2(OR)2}FedCNBut] [R ) SiMe3, 16,
νCN ) 2175 cm-1], {[p-But-calix[4]-(O)2(OR)2}FedCPh2] [R ) Me, 17; R ) PhCH2, 18; R ) SiMe3, 19]. The
three carbene complexes 17-19 display quite an unusual high-spin state, which is a consequence of the formation
of a weak π interaction between the metal and the carbene carbon, as confirmed by the extended Hu¨ckel calculations.
The carbene functionality has been removed from the iron center in the reaction with O2 and HCl. The proposed
structures have been supported by X-ray analyses of complexes 8, 9, 12, 14, 16, 17, and 19.
Introduction
positions of square planar iron; (iii) the presence of two weakly
binding ether groups can mimic variation in the coordination
Although a variety of chemical transformations are assisted
or catalyzed by iron-oxo surfaces,1 their simulation by iron
compounds is almost nonexistent. One preliminary attempt
would be to disclose how much an oxygen donor atom
environment can affect the redox properties of the iron center
and its tendency to form iron-carbon bonds.2,3 To this purpose,
we have considered as ligand the bisalkylated calix[4]arene
dianion (Chart 1). This ligand has a number of characteristics
making the simulation of an oxo surface very likely:4-6 (i) the
preorganized quasi-planar arrangement of four oxygen donor
atoms; (ii) the cavity and the substituents at the two oxygen
atoms can be used for protecting the accessibility to the axial
(2) For related molecular approaches to oxo surfaces binding organome-
tallic functionalities see the following. (a) Chisholm, M. H. Chemtracts:
Inorg. Chem. 1992, 4, 273. (b) Kla¨ui, W. Angew. Chem., Int. Ed. Engl.
1990, 29, 627. (c) Nagata, T.; Pohl, M.; Weiner, H.; Finke, R. G.
Inorg. Chem. 1997, 36, 1366. (d) Pohl, M.; Lyon, D. K.; Mizuno, N.;
Nomiya, K.; Finke, R. G. Inorg. Chem. 1995, 34, 1413.
(3) Silver, J. Chemistry of Iron; Blackie: Glasgow, U.K., 1993. Pearson,
A. J. Iron Compounds in Organic Synthesis; Academic: New York,
1994. Whitmire, K. H.; Kerber, R. C.; Fagan, P. J.; Akita, M. In
ComprehensiVe Organometallic Chemistry II; Abel, E. W., Stone, F.
G. A., Wilkinson, G., Eds.; Pergamon: Oxford, 1995; Vol. 7, Chapters
1-4.
(4) Floriani, C. Chem.sEur. J. 1999, 5, 19.
(5) (a) Giannini, L.; Caselli, A.; Solari, E.; Floriani, C.; Chiesi-Villa, A.;
Rizzoli, C.; Re, N.; Sgamellotti, A. J. Am. Chem. Soc. 1997, 119,
9198. (b) Giannini, L.; Caselli, A.; Solari, E.; Floriani, C.; Chiesi-
Villa, A.; Rizzoli, C.; Re, N.; Sgamellotti, A. J. Am. Chem. Soc. 1997,
119, 9709. (c) Caselli, A.; Giannini, L.; Solari, E.; Floriani, C.; Re,
N.; Chiesi-Villa, A.; Rizzoli, C. Organometallics 1997, 16, 5457. (d)
Zanotti-Gerosa, A.; Solari, E.; Giannini, L.; Floriani, C.; Re, N.; Chiesi-
Villa, A.; Rizzoli, C. Inorg. Chim. Acta 1998, 270, 298. (e) Castellano,
B.; Solari, E.; Floriani, C.; Re, N.; Chiesi-Villa, A.; Rizzoli, C.
Organometallics 1998, 17, 2328. (f) Castellano, B.; Solari, E.; Floriani,
C.; Re, N.; Chiesi-Villa, A.; Rizzoli, C. Chem. Eur. J. 1999, 5, 722.
(g) Castellano, B.; Solari, E.; Floriani, C.; Scopelliti, R.; Re, N. Inorg.
Chem. 1999, 38, 3406.
* To whom correspondence should be addressed.
(1) (a) Thomas, J. M.; Thomas, W. J. Principles and Practice of
Heterogeneous Catalysis; VCH: Weinheim, Germany, 1997. (b)
Mechanisms of Reactions of Organometallic Compounds with Surfaces;
Cole-Hamilton, D. J., Williams, J. O., Eds.; Plenum: New York, 1989.
(c) Kung, H. H. Transition Metal Oxides: Surface Chemistry and
Catalysis; Elsevier: Amsterdam, The Netherlands, 1989. (d) Hoff-
mann, R. Solid and Surfaces, A Chemist’s View of Bonding in Extended
Strucures; VCH: Weinheim, Germany, 1988. (e) Catalyst Design,
Progress and PerspectiVes; Hegedus, L., Ed.; Wiley: New York, 1987.
(f) Bond, G. C. Heterogeneous Catalysis, Principles and Applications,
2nd ed.; Oxford University Press: New York, 1987.
10.1021/ic991467h CCC: $19.00 © 2000 American Chemical Society
Published on Web 05/18/2000

Properties of Iron(II)
Inorganic Chemistry, Vol. 39, No. 12, 2000 2605
immediately, and the suspension was stirred a few minutes, by which
time all starting material had been consumed. The solid was collected
and dried (10-6 mbar, 85 °C, 48 h), yielding 4 as a white fluffy powder
(103 g, 97%). Anal. Calcd for C50H72O4Si2: C, 75.70; H, 9.15. Found:
C, 75.55; H, 9.12. 1H NMR (C6D6, 200 MHz, 298 K, ppm): δ 8.12 (s,
2H, OH); 7.23 (s, 4H, ArH); 6.95 (s, 4H, ArH); 4.50 (d, 4H, J ) 13
Hz ArCH2Ar); 3.32 (d, 4H, J ) 13 Hz ArCH2Ar); 1.46 (s, 18H, C(CH3);
Chart 1
1
0.87 (s, 18H, C(CH3) 0.36 (s, 18H, Si(CH3). H NMR (CDCl3, 200
MHz, 298 K, ppm): δ 7.17 (s, 2H, OH); 7.02 (s, 4H, ArH); 6.75 (s,
4H, ArH); 4.22 (d, 4H, J ) 13 Hz, ArCH2Ar); 3.19 (d, 4H, J ) 13 Hz
ArCH2Ar); 1.51 (s, 18H, C(CH3); 1.20 (s, 18H, C(CH3); 0.37 (s, 18H,
Si(CH3). X-ray quality crystals were grown in CHCl3/MeOH solutions.
Synthesis of 5. [Fe2Mes4] (9.19 g, 15.65 mmol) was added to a pale-
yellow solution of 1 (21.20 g, 31.3 mmol) in THF (230 mL). The
resulting red suspension was stirred for 12 h during which time it
gradually become a brown suspension with a microcrystalline precipitate
in a slightly exothermic process. Volatiles were distilled off, and
n-pentane (110 mL) was added to the solid residue. 5 was collected as
a white powder (22.24 g, 89%). Anal. Calcd for C50H66FeO5: C, 74.80;
H, 8.28. Found: C, 74.99; H, 8.27.
number of the metal according to the nature of the axial ligands.
In this report we focus on the synthesis of the iron(II)
dimethoxycalix[4]arene parent compound, its oxidation to iron-
(III) derivatives, and the formation of iron-carbon multiple
bonds7 in carbene and isocyanide derivatives.
Experimental Section
Synthesis of 6. [Fe2Mes4] (2.50 g, 4.25 mmol) was added to a pale-
yellow solution of 2 (6.48 g, 8.51 mmol) in THF (120 mL), and the
resulting red solution was stirred for 18 h. A brown solution was
obtained. Volatiles were distilled off, and n-pentane (40 mL) was added
to the solid residue. 6 was collected as a white powder (6.05 g, 80%).
Anal. Calcd for C56H78FeO5: C, 75.82; H, 8.86. Found: C, 75.71; H,
8.92.
Synthesis of 7. [Fe2Mes4] (5.46 g, 9.27 mmol) was added to a
solution of 3 (15.38 g, 18.55 mmol) in THF (200 mL), and the resulting
red suspension was stirred overnight. A brown solution was obtained.
Volatiles were distilled off, and n-pentane (110 mL) was added to the
solid residue. 7 was collected as a white powder (14.43 g, 81%). Anal.
Calcd for C62H74FeO5: C, 77.97; H, 7.81. Found: C, 77.87; H, 7.95.
Synthesis of 8. [Fe2Mes4] (6.43 g, 10.91 mmol) was added to a
solution of 4 (17.32 g, 21.83 mmol) in THF (230 mL), and the resulting
red suspension was stirred for 4 h during which time it gradually become
a brown solution. Volatiles were distilled off, and n-pentane (90 mL)
was added to the solid residue. 8 was collected as a white powder (15.77
g, 85%). Anal. Calcd for C50H70FeO4Si2: C, 70.89; H, 8.33. Found:
C, 70.80; H, 8.51. X-ray quality crystals were grown in toluene
solutions.
Synthesis of 9 (Method A). HgCl2 (905 mg, 3.33 mmol) was added
to a clear-green solution of 5 (2.68 g, 3.33 mmol) in toluene (100 mL),
resulting immediately in a deep-purple suspension. A portion of
insoluble material was filtered off (HgCl), while the filtrate was
concentrated to dryness, leaving a deep-purple solid residue to which
n-pentane (40 mL) was added. Complex 9‚(thf)2 was collected as a
deep-purple powder (1.84 g, 61%). Anal. Calcd for C54H74ClFeO6: C,
71.24, H, 8.19. Found: C, 70.86, H, 7.87. X-ray quality crystals were
grown in Et2O/toluene solution. The crystals contain Et2O and toluene
of crystallization.
Synthesis of 9 (Method B). PhCH2Cl (248 mg, 1.96 mmol) was
rapidly added to a stirred clear-green solution of 5 (1.57 g, 1.96 mmol)
in toluene (100 mL), resulting immediately in a deep-purple solution.
Volatiles were distilled off, leaving a deep-purple solid residue to which
n-pentane (40 mL) was added. Complex 9‚(toluene)0.5 was collected
as a deep-purple powder (1.05 g,65%). Anal. Calcd for C49.5H62-
ClFeO4: C, 73.19, H, 7.69. Found: C, 73.18, H, 7.96.
General Procedure. All reactions were carried out under an
atmosphere of purified nitrogen. Solvents were dried and distilled before
use by standard methods. Infrared spectra were recorded with a Perkin-
Elmer FT 1600 spectrophotometer. NMR spectra were recorded on an
AC-200 Bruker spectrometer. Elemental analyses were performed using
an EA 1110 CHN elemental analyzer by CE Instruments. Unless
otherwise indicated, all commercial reagents were used as received.
Compounds 1-3,8 Ph2CN2,9 and [Fe2Mes4]10 were prepared according
to published procedures. The solvent content of the solids has been
confirmed, in many cases, by CG analysis of the products derived from
their thermal decomposition.
Magnetic susceptibility measurements were made on a MPMS5
superconducting quantum interference device susceptometer (Quantum
Design Inc.) operating at a magnetic field strength of 1 kOe. Corrections
were applied for diamagnetism calculated from Pascal constants.11
Effective magnetic moments were calculated as µeff ) 2.828(øVT)1/2
,
where øV is the magnetic susceptibility per vanadium. Fitting of the
magnetic data to the theoretical expression was performed by minimiz-
ing the agreement factor, defined as ∑[øiobsdTi - øicalcdTi]2/(øiobsdTi)2
through a Levenberg-Marquardt routine.
Extended Hu1ckel Calculations. Extended Hu¨ckel calculations were
performed with the CACAO molecular orbital program,12 with param-
eters taken from ref 13.
Synthesis of 4. N,O-bis(trimethylsilyl)acetamide (BSA) (36.1 mL,
147.6 mmol) was rapidly added to a CH3CN suspension (800 mL) of
p-But-calix[4]arene‚(CH2Cl2) (98.49 g, 134.2 mmol) under a stream
of nitrogen while stirring. A white microcrystalline solid formed
(6) (a) Giannini, L.; Solari, E.; Dovesi, S.; Floriani, C.; Re, N.; Chiesi-
Villa, A.; Rizzoli, C. J. Am. Chem. Soc. 1999, 121, 2784. (b) Giannini,
L.; Guillemot, G.; Solari, E.; Floriani, C.; Re, N.; Chiesi-Villa, A.;
Rizzoli, C. J. Am. Chem. Soc. 1999, 121, 2797. (c) Giannini, L.;
Dovesi, S.; Solari, E.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C. Angew.
Chem., Int. Ed. 1999, 38, 807. (d) Giannini, L.; Solari, E.; Floriani,
C.; Re, N.; Chiesi-Villa, A.; Rizzoli, C. Inorg. Chem. 1999, 38, 1438.
(e) Dovesi, S.; Solari, E.; Scopelliti, R.; Floriani, C. Angew. Chem.,
Int. Ed. Engl. 1999, 38, 2388. (f) Caselli, A.; Solari, E.; Scopelliti,
R.; Floriani, C. J. Am. Chem. Soc. 1999, 121, 8296.
Synthesis of 10. Iodine (370 mg, 1.46 mmol) was added to a clear-
green solution of 5 (2.34 g, 2.91 mmol) in toluene (100 mL). The
resulting deep-violet solution was stirred overnight. Volatiles were
distilled off, and n-pentane (40 mL) was added to the solid residue.
Complex 10 was collected as a deep-violet powder (1.55 g, 62%). Anal.
Calcd for C46H58FeIO4: C, 64.42, H; 6.82. Found: C, 64.58; H, 6.90.
Synthesis of 11. A clear-green solution of 5 (4.816 g, 6.00 mmol)
in toluene (100 mL) was cooled to -30 °C and exposed to purified O2
while being stirred. The color gradually turned deep-red, and in few
minutes a red precipitate appeared. After 12 h O2 was replaced with
N2 and the suspension was allowed to warm to room temperature.
Volatiles were distilled off, and n-pentane (80 mL) was added to the
red residue. Complex 11‚(C5H12) was collected as a red powder (2.88
(7) A preliminary communication on the Fe-calixarene carbene func-
tionality has been published. Giusti, M.; Solari, E.; Giannini, L.;
Floriani, C.; Chiesi-Villa, A.; Rizzoli, C. Organometallics 1997, 16,
5610.
(8) Arduini, A. Casnati, A. In Macrocycle Synthesis; Parker, P., Ed.;
Oxford University Press: New York, 1996; Chapter 7.
(9) Smith, L. I.; Howard, K. L. Organic Syntheses; Wiley: New York,
1955; Vol. 3, p 351.
(10) Klose, A.; Solari, E.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C.; Re,
N. J. Am. Chem. Soc. 1994, 116, 9123.
(11) Boudreaux, E. A.; Mulay, L. N. Theory and Applications of Molecular
Paramagnetism; Wiley: New York, 1976; pp 491-495.
(12) Mealli, C.; Proserpio, D. M. J. Chem. Educ. 1990, 67, 3399.
(13) Alvarez, S. Tables of Prameters for Extended Hu¨ckel Calculations;
Universitat de Barcelona: Barcelona, Spain, 1993.

2606 Inorganic Chemistry, Vol. 39, No. 12, 2000
Esposito et al.
g, 67%). Anal. Calcd for C97H128Fe2O9: C, 75.18; H, 8.32. Found: C,
75.11; H, 8.32. X-ray quality crystals were grown in toluene solutions.
Synthesis of 12. A clear, green-brown solution of 6 (5.694 g, 6.99
mmol) in toluene (100 mL) was cooled to -30 °C and exposed to
purified O2 while being stirred. The color gradually turned deep-red.
After 6 h O2 was replaced with N2 and the suspension was allowed to
warm to lab temperature. Volatiles were distilled off, and n-pentane
(100 mL) was added to the red residue. Complex 12 was collected as
a red powder (4.29 g, 75%). Anal. Calcd for C104H140Fe2O9: C, 75.88;
H, 8.57. Found: C, 75.77; H, 8.74. X-ray quality crystals were grown
in toluene solutions.
Scheme 1
Synthesis of 13. Sulfur (109 mg, 0.43 mmol) was added to a clear-
green solution of 5 (2.34 g, 2.91 mmol) in toluene (100 mL), and the
resulting deep-violet solution was stirred overnight. Volatiles were
distilled off, and n-pentane (40 mL) was added to the solid residue.
Complex 13 was collected as a deep-violet powder (1.92 g, 76%). Anal.
Calcd for C92H116Fe2O8S: C, 73.98, H, 7.83. Found: C, 73.90; H, 7.81.
Synthesis of 14. A brown solution of 7 (2.34 g, 2.44 mmol) in Et2O
(100 mL) was cooled to -30 °C and exposed to purified O2 while
being stirred. The color gradually turned deep-red. After 8 h O2 was
replaced with N2 and the suspension was allowed to warm to room
temperature. Volatiles were distilled off, and n-pentane (30 mL) was
added to the orange residue. Complex 14 was collected as an orange
powder (1.30 g, 67%). Anal. Calcd for C51H59FeO4: C, 77.36; H, 7.51.
Found: C, 77.49; H, 7.92. X-ray quality crystals were grown in toluene
solutions.
Synthesis of 15. A brown solution of 8 (4.14 g, 4.89 mmol) in Et2O
(120 mL) was cooled to -30 °C and exposed to purified O2 while
stirring. The color gradually turned deep-purple, and a crystalline
precipitate formed. After 12 h O2 was replaced with N2 and the
suspension was allowed to warm to lab temperature. Volatiles were
distilled off, and n-pentane (30 mL) was added to the solid residue.
Complex 15‚(Et2O)1.5 was collected as a deep-purple powder (2.06 g,
47.6%). Anal. Calcd for C53H76FeO5.5Si: C, 71.92; H, 8.65. Found:
C, 71.97; H, 8.32. X-ray quality crystals were grown in toluene
solutions.
Synthesis of 16. Complex 8 (3.25 g, 3.84 mmol) was added to a
toluene solution (100 mL) of ButNC (638 mg, 7.67 mmol), and the
resulting deep-yellow solution was stirred overnight. Volatiles were
distilled off, and n-pentane (110 mL) was added to the solid residue.
Complex 17 was collected as a yellow powder (2.50 g, 64%). Anal.
Calcd for C55H79FeNO4Si2: C, 71.09, H, 9.05; N, 1.51. Found: C,
71.01, H, 8.57; N, 1.53. IR (Nujol, νmax/cm-1): 2175 (m). X-ray quality
crystals were grown in THF/toluene solutions.
Synthesis of 17. Solid Ph2CN2 (0 °C, 691 mg, 3.56 mmol) was added
in one portion to a cold (-30 °C), clear-green solution of 5 (2.86 g,
3.56 mmol) in toluene (100 mL). The color turned quite soon to deep-
green. It was allowed to warm to lab temperature over 5 h. Volatiles
were distilled off, and n-pentane (40 mL) was added to the green
residue. Complex 17 was collected as a green powder (2.59 g, 81%).
Anal. Calcd for C59H68FeO4: C, 79.00; H, 7.64. Found: C, 78.98; H,
7.51. Crystals suitable for X-ray analysis were obtained from CH2Cl2.
Synthesis of 18. Complex 7 (2.98 g, 3.11 mmol) was added in one
portion to a cold (-30 °C) solution of Ph2CN2 (654 mg, 3.37 mmol)
in n-hexane (80 mL). The resulting deep-red suspension was stirred
for 2 h, then allowed to come to room temperature over 5 h, during
which time it gradually turned deep-green. The precipitate was collected
and dried, yielding 18 as a green powder (2.46 g, 75%). Anal. Calcd
for C71H76FeO4: C, 81.28; H, 7.30. Found: C, 80.99; H, 7.02.
Sometimes the solid contains hydrocarbons of crystallization, i.e., C6H14
or C5H12.
(150 mL), and the resulting deep-green solution was stirred for 36 h at
which time a pale-green precipitate was obtained. It was collected and
dried as a pale green powder, 20 (2.29 g, 68%). Anal. Calcd for C64H74-
ClFeNO4: C, 75.91; H, 7.37; N, 1.38. Found: C, 76.01; H, 7.26; N,
1.12.
Reaction of 17 with O2. A solution of 17‚(C5H12) (3.17 g, 3.28
mmol) in toluene (100 mL) was exposed to purified O2 while being
stirred. The starting green solution gradually turned deep-amber.
Volatiles were distilled off, and n-pentane (40 mL) was added to the
red residue. Complex 11‚(C5H12)2 was collected as a red powder (2.14
g, 80%). Anal. Calcd for C102H140Fe2O9: C, 75.54; H, 8.70. Found:
C, 75.76; H, 8.35. Benzophenone was obtained as a byproduct in a
stoichiometric amount. A gas absorption measurement gave a molar
2
ratio Fe/O2 of /3.
X-ray Crystallography for Complexes 8, 9, 12, 14, 16, 17, and
19. Single crystals suitable for X-ray diffraction were grown from
common organic solvents (Table 1). Data for 8 and 16 were collected
on a Kuma KM4CCD, data for 17 were collected on a Rigaku AFC6S
diffractometer, and those for 9, 12, 14, and 19 were collected on a
MAR345 image plate using Mo KR radiation. The solutions and
refinements were carried out using the programs SHELX7614 and
SHELX93.15 The details of the X-ray data collection, structure solution,
and refinement are given in the Supporting Information.16
Results and Discussion
(A) Starting Compounds: [Fe{p-But-calix[4]-(O)2(OR)2].
The Experimental Section details the large-scale synthesis of
the bisalkylated derivatives of p-But-calix[4]arene, namely,
p-But-calix[4]-(O)2(OR)2 (1-4), which are not available from
the literature.8 The use of the bisalkylated form is compulsory
for three major reasons: (i) for binding iron in its +2 oxidation
state, (ii) for keeping a coordination site or a metal functionality
available for reactivity studies, and (iii) for protecting the [FeO4]
moiety from dimerization, which prevents reactivity at the metal
center.17
The parent complexes 5-8 are easily accessible from the
reaction of the free ligands 1-4 and [Fe2Mes4]10 (Scheme 1).
The alternative synthesis from FeCl2 and the alkali salts of 1-4
should not be pursued because of the difficulty in freeing the
final complex from [MCl], particularly when [Cl-] remains
bonded to iron(II) (see below, complex 20). Depending on the
nature of R, the iron(II)-calix[4]arene complexes can be
obtained in the unsolvated (complex 8) or solvated (5-7) form,
though in the latter case the solvent molecule is only weakly
bonded. All monomeric complexes 5-8 display a magnetic
behavior according to a d6 high-spin state (see below). The
structures of 5-8 are exemplified by a report on the X-ray
Synthesis of 19. Complex 8 (3.14 g, 3.71 mmol) was added in one
portion to a cold (-30 °C) solution of Ph2CN2 (719 mg, 3.71 mmol)
in n-pentane (80 mL). The resulting deep-red suspension was allowed
to come to lab temperature over 5 h, during which time it gradually
turned deep-green. The precipitate was collected and dried, yielding
19 as a green powder (1.61 g, 43%). Anal. Calcd for C63H80FeO4Si2:
C, 74.67; H, 7.98. Found: C, 74.24; H, 7.77. Crystals suitable for X-ray
analysis were obtained from a pentane solution.
(14) Sheldrick, G. M. SHELX76, program for crystal structure determi-
nation; University of Cambridge: Cambridge, England, 1976.
(15) Sheldrick, G. M. SHELXL93, program for crystal structure refinement;
University of Go¨ttingen: Go¨ttingen, Germany, 1993.
(16) See paragraph at the end of paper regarding Supporting Information.
(17) Olmstead, M. M.; Sigel, G.; Hope, H.; Xu, X.; Power, P. P. J. Am.
Chem. Soc. 1985, 107, 8087.
Synthesis of 20. Pyridinium chloride (0.38 g, 3.31 mmol) was added
in one portion to a solution of 18 (C5H12) (3.51 g, 3.63 mmol) in Et2O

Properties of Iron(II)
Inorganic Chemistry, Vol. 39, No. 12, 2000 2607
Table 1. Experimental Data for the X-ray Diffraction Studies of Crystalline Complexes 8, 9, 12, 14, 16, 17, and 19
complex
formula
8
9
12
14
16
17
19
C50H70FeO4Si2‚
C46H58ClFeO4‚
0.5C7H8‚
0.5C4H10O
16.969(3)
13.510(3)
21.526(4)
90
107.61(3)
90
4703.6(18)
4
C104H140Fe2O9
C102H118Fe2O8
‚4C7H8
C55H79FeNO4Si2‚
C59H68FeO4
‚3CH2Cl2
C63H80FeO4Si2
C7H8
C4H8O
a, Å
21.759(3)
19.105(3)
12.776(2)
90
92.30(2)
90
19.295(4)
20.942(4)
24.841(5)
90
12.648(3)
13.492(3)
18.286(4)
83.68(3)
72.04(3)
66.63(3)
2724.6(13)
1
1952.3
P1 (No. 2)
-130
0.710 69
1.190
12.283(2)
14.951(2)
18.122(2)
84.66(2)
80.03(2)
66.54(2)
3005.7(9)
2
1002.4
P1 (No. 2)
-130
0.710 69
1.108
13.115(2)
21.265(3)
12.573(2)
100.57(2)
117.53(2)
76.64(2)
3013.3(10)
2
1151.8
P1 (No. 2)
-135
0.710 69
1.270
11.195(2)
21.762(4)
25.408(4)
100.53(3)
97.59(1)
89.68(3)
6031.4(19)
4
1013.3
P1 (No. 2)
-130
0.710 69
1.116
b, Å
c, Å
R, (deg)
â, (deg)
γ, (deg)
V, Å3
90
90
5306.8(14)
4
10038(3)
4
Z
fw
939.3
849.4
1645.9
Pbcn (No. 60)
-130
0.710 69
1.089
3.38
space group
temp, °C
λ, Å
C2/c (No. 15)
-130
0.710 69
1.176
P21/c (No. 14)
-130
0.710 69
1.200
F
calc, g cm-3
µ, cm-1
3.68
4.18
3.20
3.30
5.6
3.29
transm coeff
3.68
4.18
3.38
3.20
3.30
5.6
3.29
Ra
0.056
0.086
0.063
0.166
0.918
3532
0.064
0.108
0.073
0.067
wR2
0.136
0.218
0.168
0.279
0.225
0.165
GOF
1.059
1.090
1.059
1.072
1.016
1.092
N obsdc
5069
5226
7566
7758
3599
14 202
24 241
14 202
1258
N-independentd
N-refinement e
variables
6211
9401
7483
10 888
7566
10 248
7758
8718
5069
5226
6475
6941
290
505
514
618
613
652
a Calculated on the observed reflections. b Values in square brackets refer to the “inverted” structure. c N-obsd is the total number of independent
reflections having I > 2σ(I). d N-independent is the number of independent reflections. e N-refinement is the number of reflection used in the
refinement having I > 0 for 12 and 17 and I > 2σ(I) for 8, 9, 14, 16, 19.
Scheme 2
Chart 2
For discussing the X-ray structural analysis of the prototypes
of each class of compounds reported in this paper, we refer to
the numbering scheme in Chart 2 while a selection of the
structural parameters of complexes 8, 9, 12, 14, 16, 17, and 19
is reported in Table 2 and the conformational parameters are
listed in Table 3.
The structures of complexes 8 and 9 are displayed in Figures
1 and 2, respectively.18 In complex 8 the O4 core shows little,
yet significant, tetrahedral distortions (Table 3). The macrocyclic
ligand is square-planarly coordinated to iron, the metal out-of-
plane distance being 0.010(1) Å. Complex 9 possesses a
crystallographically imposed C2 symmetry, the 2-fold axis
running along the Fe-Cl(1) direction. The coordination poly-
hedron around the metal is nearly a trigonal bipyramid, with
the O(1), O(3), Cl(1) atoms defining the equatorial plane and
analysis of 8 (see below), while the structure of 5 is given in
the Supporting Information.
(B) One-Electron Oxidation of [Fe{p-But-calix[4]-(O)2-
(OR)2]. The parent compounds 5-8, regardless of the coordina-
tion number of the metal, i.e., 4 or 5, are prone to be oxidized
to iron(III) derivatives. Scheme 2 displays a number of these
reactions, specifically the transformations to iron(III) halides
and iron(III) oxo functionalities. The former reactions leading
to 9 and 10 follow the usual free radical type pathway and give
rise to five-coordinate iron(III) derivatives, displaying the typical
high-spin state of a d5. Complexes 9 and 10 are particularly
appropriate for pursuing further functionalization of the iron
center.
(18) Complex 8 crystallizes with one toluene solvent molecule that is hosted
in the calixarene cavity. The calixarene cavity of complex 9 is
statistically occupied by toluene and Et2O molecules in a complex/
toluene/Et2O molar ratio of 1/0.5/0.5.

2608 Inorganic Chemistry, Vol. 39, No. 12, 2000
Esposito et al.
Table 2. Selected Bond Distances (Å) and Angles (deg) for 8, 9, 12, 14, 16, 17, and 19
8
Fe(1)-O(1)
1.825(2)
Fe(1)-O(2)
2.205(2)
Si(1)-O(2)
Fe(1)-O(2)
1.684(2)
2.208(4)
9
Fe(1)-Cl(1)
Fe(1)-O(3)
2.235(1)
1.806(4)
Fe(1)-O(1)
Fe(1)-O(4)
1.787(4)
2.168(3)
12a
14b
16
Fe(1)-O(1)
Fe(1)-O(4)
1.808(4)
2.207(4)
Fe(1)-O(2)
Fe(1)-O(5)
2.214(4)
1.765(1)
Fe(1)-O(3)
1.825(4)
177.3(1)
Fe(1)-O(5)-Fe(1)′
Fe(1)-O(1)
Fe(1)-O(4)
1.782(3)
2.092(3)
Fe(1)-O(2)
Fe(1)-O(4)′
2.225(2)
1.914(2)
Fe(1)-O(3)
1.831(3)
Fe(1)-O(1)
1.875(4)
2.093(6)
179.2(5)
Fe(1)-O(2)
Si(1)-O(2)
2.396(4)
1.674(5)
Fe(1)-O(3)
Si(2)-O(4)
1.874(5)
1.678(4)
Fe(1)-O(4)
N(1)-C(51)
2.381(5)
1.152(8)
Fe(1)-C(51)
Fe(1)-C(51)-N(1)
17
2.228(6)
1.449(10)
125.1(6)
Fe(1)-O(1)
1.852(5)
1.943(8)
119.2(6)
Fe(1)-O(2)
Fe(1)-O(3)
1.831(4)
1.485(10)
115.8(7)
Fe(1)-O(4)
2.258(6)
Fe(1)-C(47)
C(48)-C(47)
C(54)-C(47)
Fe(1)-C(47)-C(54)
Fe(1)-C(47)-C(48)
C(48)-C(47)-C(54)
19
molecule
molecule
molecule
A
molecule
A
B
B
Fe(1)-O(1)
1.818(2)
1.807(3)
1.958(5)
1.667(3)
1.469(7)
1.826(2)
1.826(2)
1.973(5)
1.689(3)
1.476(6)
Fe(1)-O(2)
2.203(2)
2.915(2)
1.709(3)
1.464(6)
123.6(4)
118.3(4)
2.457(2)
2.434(2)
1.689(2)
1.459(7)
121.3(4)
116.6(4)
Fe(1)-O(3)
Fe(1)-O(4)
Fe(1)-C(45)
Si(2)-O(4)
C(52)-C(45)
Fe(1)-C(45)-C(46)
Si(1)-O(2)
C(46)-C(45)
Fe(1)-C(45)-C(52)
C(46)-C(45)-C(52)
118.2(4)
122.1(4)
a ′ ) -x, y, 1.5 - z. b ′ ) -x, 1 - y, 1 - z.
Table 3. Comparison of Relevant Conformational Parameters within Calixarene for Complexes 8, 9, 12, 14, 16, 17, and 19
19
molecule
molecule
8
9
12
14
16
17
A
B
(a) Distances (Å) of Atoms from the O4 Mean Planea
O(1)
O(2)
O(3)
O(4)
Fe
0.185(2)
-0.185(2)
0.185(2)
-0.185(2)
0.010(1)
-0.232(3)
0.229(3)
-0.230(3)
0.233(3)
0.478(1)
-0.252(4)
0.250(4)
-0.249(4)
0.251(4)
0.442(1)
-0.361(2)
0.339(2)
-0.310(2)
0.332(2)
0.407(1)
-0.169(4)
0.171(4)
-0.173(4)
0.172(4)
0.382(1)
-0.209(4)
0.212(4)
-0.218(4)
0.215(4)
0.553(2)
-0.274(3)
0.295(3)
-0.296(3)
0.271(3)
0.583(1)
-0.229(3)
0.229(3)
-0.232(3)
0.232(3)
0.552(1)
(b) Dihedral Angles (deg) between Planar Moietiesb
E-A
E-B
E-C
E-D
A-C
B-D
134.4(1)
110.4(1)
134.4(1)
110.4(1)
91.3(1)
133.8(1)
109.1(1)
134.6(1)
118.1(1)
91.5(1)
139.9(1)
103.3(1)
138.4(1)
105.0(1)
98.2(1)
152.5(1)
114.1(1)
125.4(1)
112.8(1)
97.9(1)
135.7(1)
96.5(1)
140.2(2)
94.9(1)
95.9(2)
168.6(2)
131.9(2)
111.6(3)
133.4(1)
110.2(2)
94.7(2)
140.6(1)
98.7(1)
151.0(1)
91.3(1)
111.6(1)
170.0(2)
137.0(1)
97.8(1)
139.7(1)
98.8(1)
96.7(1)
163.4(2)
139.1(1)
132.8(2)
151.7(2)
133.0(1)
137.6(3)
(c) Contact Distances (Å) between Para Carbon Atoms of Opposite Aromatic Ringsc
C(4)‚‚‚C(17)
C(10)‚‚‚C(24)
9.246(4)
7.218(3)
9.102(8)
7.565(6)
9.555(7)
6.614(8)
9.358(5)
7.558(5)
9.659(9)
5.714(10)
8.971(10)
7.365(15)
9.934(7)
5.572(6)
9.524(7)
6.034(7)
a For complex 8 O(30 and O(4) should be read O(1)′ and O(2)′, respectively; prime denotes a transformation of -x, y, 0.5 - z. b E (reference
plane) refers to the least-squares mean plane defined by the C(7), C(14), C(21), C(28) bridging methylenic carbons. c For complex 8 C(17) and
C(24) should be read C(4)′ and C(10)′, respectively; prime denotes a transformation of -x, y, 0.5 - z.
the O(2) and O(4) atoms at the axial positions. The metal is
displaced by 0.028(1) Å from the equatorial plane. The Fe-
Cl(1) vector forms a dihedral angle of 3.1(1)° with the normal
to the O4 mean plane. The short Fe-O(1) and Fe-O(3) bond
distances in both 8 and 9 support a significant metal-oxygen
π bonding, which is absent in the cases of O(2) and O(4) (Table
2). The calixarene macrocycles assume the usual elliptical
conformation, the A and C rings being slightly pushed outward
and the B and D rings inward, relative to the cavity (Table 3).
The asymmetry of the cavity is monitored by the separation
between the opposite C(4), C(17), and C(10), C(24) para carbon
atoms. In this conformation a toluene molecule is accommodated
inside the cavity as a guest.
The oxidation of 5-8 with dioxygen is strongly dependent
on the R substituent at the alkylated oxygen (Scheme 2). In the
case of Me and Bun the reaction led, independent of the reaction
conditions, to the corresponding µ-oxo derivatives 11 and 12.
The latter compounds are similar to the µ-oxo derivatives of
iron(III) supported by polydentate or macrocyclic ligands,19
including the magnetic and structural properties (see below and

Properties of Iron(II)
Inorganic Chemistry, Vol. 39, No. 12, 2000 2609
Figure 1. SCHAKAL view of complex 8. Disorder affecting the
toluene guest molecule has been omitted for clarity. A prime denotes
a transformation of -x, y, 0.5 - z.
Figure 3. SCHAKAL view of complex 12. Disorder affecting the B
and D butyl groups has been omitted for clarity. A prime denotes a
transformation of -x, y, 1.5 - z.
Figure 2. SCHAKAL view of complex 9. Disorder involving the guest
molecules has been omitted for clarity.
the Experimental Section). The reaction of 5 with elemental
sulfur (Scheme 2) parallels the formation of the µ-oxo com-
plex,20,21 leading to 13. An interesting comparison can be made
between the magnetic properties of 11 and 13. The reaction of
7 and 8 with dioxygen follows a completely different pathway,
leading to the formation of the iron(III) dimers 14 and 15. The
reaction mechanism is rather difficult to determine, though a
similar dealkylation has been observed whenever when we tried
to increase the oxidation state or in general the specific charge
of the metal (Scheme 3).5 The antiferromagnetic properties of
the iron(III) dimer have been analyzed and discussed in the
following section. The structures of 12 and 14 are displayed in
Figures 3 and 4.
Figure 4. SCHAKAL view of complex 14. A prime denotes a
transformation of -x, 1 - y, 1 - z.
Scheme 3
Complex 12 (Figure 3) possesses a crystallographically
imposed C2 symmetry, the 2-fold axis running through the O(5)
µ-oxo oxygen atom, perpendicular to the Fe-O(5)-Fe direction.
(19) (a) Kurtz, D. M., Jr. Chem. ReV. 1990, 90, 585 and references therein.
(b) Haselhorst, G.; Wieghardt, K.; Keller, S.; Schrader, B. Inorg. Chem.
1993, 32, 520.
(20) Berno, P.; Floriani, C.; Chiesi-Villa, A.; Guastini, C. J. Chem. Soc.,
Dalton Trans. 1989, 551.
(21) (a) Floriani, C.; Fachinetti, G. Gazz. Chim. Ital. 1973, 103, 1317. (b)
Mitchell, P. C. H.; Parker, D. A. J. Inorg. Nucl. Chem. 1973, 35, 1385.
Mitchell, P. C. H.; Parker, D. A. J. Chem. Soc., Dalton Trans. 1976,
1821. (c) Dorfman, J. R.; Girerd, J. J.; Simhon, E. D.; Stack, T. D. P.;
Holm, R. H. Inorg. Chem. 1984, 23, 4407 and references therein. (d)
Corazza, F.; Floriani, C.; Zehnder, M. J. Chem. Soc., Dalton Trans.
1987, 709.
Complex 14 consists of centrosymmetric dimers (Figure 4). In
both complexes iron exhibits five-coordination through the
oxygen atoms of the tetrahedrally distorted O4 core (Table 3)
and the O(5) bridging oxo group in 12 or the O(4)′ oxygen atom
from the symmetry-related calixarene in 14. Both coordination

2610 Inorganic Chemistry, Vol. 39, No. 12, 2000
Esposito et al.
Figure 6. SCHAKAL view of complex 17. Disorder affecting the C
ring and the methylene chloride guest molecule has been omitted for
clarity.
Figure 5. SCHAKAL view of complex 16.
Scheme 4
diazoalkanes as the source of the corresponding carbenoids is
quite appealing. The possibility of binding a carbene over a
metal-oxo surface bears a particular significance because of
the relationship with those surfaces active in the Fischer-
Tropsch synthesis.22 The reaction of 4-8, regardless of the
substituent at the oxygen atoms and the solvent bonded at the
metal, led to the loss of N2 from the diazoalkane. Only in the
case, however, of diphenyldiazomethane was it possible to
isolate quite stable carbene derivatives,23 namely, 17-19, which
display an unusual thermal stability. Their reactivity is very
limited toward conventional substrates like olefins and acetyl-
enes. Although complexes 17-19 are quite stable, the synthesis
of differently substituted carbenes, i.e., :CPhH, (COOR)2C:, and
(Ph)(COOR)C:, was unsuccessful.
The structures of 16, 17, and 19 are displayed in Figures 5,
6, and 7 and 8, respectively. In the crystals of 19 two
crystallographically independent molecules showing a slightly
different geometry are present. Hereafter, the two molecules
will be referred as molecule 19A (Figure 7) and 19B (Figure
8). The five-coordination around iron is achieved through the
four oxygen atoms from the calixarene and the C(51), C(47),
and C(45) carbon atoms for 16, 17, and 19, respectively. In all
complexes the O4 core shows remarkable tetrahedral distortions
(Table 3), so the coordination polyhedron around the metals
should be better described as a distorted trigonal bipyramid with
the O(2), O(4), C(51); O(1), O(3), C(47); and O(1), O(3), C(45)
atoms defining the equatorial plane for 16, 17, and 19,
respectively. The distortions from an ideal symmetry is mainly
indicated by the value of the angles involving the apical oxygen
polyhedra resemble a trigonal bipyramid, with O(1), O(3), O(5)
(in 12) or O(1), O(3), O(4)′ (in 14) defining the equatorial plane,
while O(2) and O(4) are in the axial sites. The metal is displaced
by 0.007(1) and 0.043(2) Å from the O4 mean plane in 12 and
14, respectively. The Fe-O(5) (12) and Fe-O(4)′ (14) vectors
form dihedral angles of 11.3(1) and 1.3(1)°, respectively, with
the normal to the corresponding O4 mean planes. The Fe-O
bond distances vary according to the degree of π-bonding
formation between the metal and the different oxygen donor
atoms. The elliptical conformation of the calixarene macrocycle
is similar to those observed in the previous complexes,5,6 the
main difference consisting of the increase of the dihedral angle
between the B and D rings in 12.
(C) [FedC] Functionality. The synthetic approach to the
Fe-C multiple bond functionalities has been devised by reacting
4-8 with isocyanides and diazoalkanes. The reaction of 8 with
ButNC led to the corresponding isocyanide derivative 16,
displaying a quite considerable increase in the C-N stretching
vibration (2175 cm-1), according to a highly electron-poor
center. The structural parameters (Scheme 4) confirm a limited
contribution by the iron ion to the formation of an Fe-C
multiple bond. The reaction of the iron(II) complexes with
(22) Herrmann, W. A. In Applied Homogeneous Catalysis with Organo-
metallic Compounds; Cornils, B., Herrmann, W. A., Eds.; VCH:
Weinheim, Germany, 1996; Vol. II, Chapter 3, pp 747-762.
(23) For iron(II)-carbene functionality supported by a porphyrin ligand
see the following. (a) Mansuy, D.; Lange, M.; Chottard, J. C.; Bartoli,
J. F.; Chevrier, B.; Weiss, R. Angew. Chem. 1978, 90, 828. (b) Mansuy,
D. Pure Appl. Chem. 1980, 52, 681. (c) Latos-Grazynski, L.; Cheng,
R.-J.; La Mar, G. N.; Balch, A. L. J. Am. Chem. Soc. 1981, 103, 4270.
(d) Balch, A. L.; Cheng, R.-J.; La Mar, G. N.; Latos-Grazynski, L.
Inorg. Chem. 1985, 24, 2651. (e) Brothers, P. J.; Collman, J. P. Acc.
Chem. Res. 1986, 19, 209. (f) Mansuy, D. Pure Appl. Chem. 1987,
59, 759. (g) Artaud, I.; Gregoire, N.; Battioni, J.-P.; Dupre, D.; Mansuy,
D. J. Am. Chem. Soc. 1988, 110, 8714. (h) Artaud, I.; Gregoire, N.;
Leduc, P.; Mansuy, D. J. Am. Chem. Soc. 1990, 112, 6899. (i) Wolf,
J. R.; Hamaker, C. G.; Djukic, J.-P.; Kodadek, T.; Woo, L. K. J. Am.
Chem. Soc. 1995, 117, 9194. (j) Ziegler, C. J.; Suslick, K. S. J. Am.
Chem. Soc. 1996, 118, 5306. (k) Mansuy, D.; Mahy, J. P. In
Metalloporphyrins Catalyzed Oxidations; Montanari, F., Casella, L.,
Eds.; Kluwer: Dordrecht, The Netherlands, 1994; p 175.

Properties of Iron(II)
Inorganic Chemistry, Vol. 39, No. 12, 2000 2611
observed in disubstituted calixarene metal derivatives, the
macrocycle assumes an elliptical cross section conformation,5,6
as indicated by the dihedral angles the aromatic rings form with
the “reference” plane and by the values of the distances between
opposite para carbon atoms (Table 3).
The diphenylcarbene derivatives 17-19 are characterized by
a very high thermal stability and kinetic inertness. They can be
decomposed only by acids or oxygen. In the former case, the
anionic iron(II) complex 20 has been obtained from the reaction
of 17 with PyHCl. The protonation of the carbene carbon is
followed by the homolytic cleavage of the Fe-C σ bond, thus
forming the pyridinium cation [PyCHPh2]+, 20. It should be
mentioned at this stage that there was intrinsic instability of
the Fe-C σ bond when we tried the alkylation of complexes 9
and 10. The structure of 20 has been supported by X-ray
analysis, which is not included in the present report because
the structural parameters of the anion are quite similar to those
of 9. The reaction of 17 with O2 produces benzophenone and
the dianion µ-oxo complex 11. The same reaction has been
observed for other metal-carbene functionalities supported by
macrocyclic ligands.25
Figure 7. SCHAKAL view of molecule A in complex 19.
(D) Magnetic Susceptibility Analysis. The magnetic sus-
ceptibilities of complexes 5-19 were measured in the temper-
ature range 1.9-300 K, and those of 11, 13, and 14 are shown
in Figure 9 as a function of temperature.
(D.1) Iron(II)-Iron(III) Complexes. The magnetic moments
of 5-8 and 16-19 are essentially constant in the whole range
of temperature, with room-temperature values of about 4.7-
5.1 µB, and show only a small decrease below 20 K (due to
zero-field splitting). This behavior is typical of high-spin, S )
2, Fe(II) monomeric species.26
The magnetic moments of 9 and 10 are constant in the whole
range of temperature, with room-temperature values of ca. 5.9
5
µB, and are typical of high-spin, S ) /2, Fe(III) monomeric
Figure 8. SCHAKAL view of molecule B in complex 19. Disorder
affecting the D ring has been omitted for clarity.
species.26
The temperature dependence of the magnetic moments of 11-
15 shows a steady decrease from 300 to 1.9 K (Figure 9) and
is typical of antiferromagnetic coupled Fe(III) dimers. The data
for 11, 13, and 14 were fitted with a theoretical equation26 based
atoms [O(1)-Fe-O(3), 169.8(1)° in 16; O(2)-Fe-O(4),
162.6(2) and 164.9(1)° in 17 and 19A, respectively]. The metal
is displaced by 0.017(2), 0.037(1), 0.251(2), and 0.004(2) Å
from the O4 cores of 16, 17, 19A, and 19B, respectively. The
Fe-C(51), Fe-C(47), and Fe-C(45) vectors are nearly per-
pendicular to the O4 core, and the dihedral angles are formed
with the normal top, the O4 mean planes being 1.1(2), 3.4(2),
7.4(2), and 0.60(2)° for 16, 17, 19A, and 19B, respectively. The
shortest Fe-O bond distances (Table 2) correspond to the
formation of significant π bonding, in agreement with the
approximate linearity of the Fe-O-C bond angles [ranging
from 150.1(4) to 172.1(3)° for Fe-O(1)-C(1) in 16 to
Fe-O(3)-C(20) in 19A, respectively]. A large difference has
been observed in the Fe-O(2) distance between 19A and 19B
[2.203(2) vs 2.457(2) Å]. The Fe-C bond distances involving
both the isocyanide ligand (2.093(6) Å, complex 16) and the
carbene ligands (1.943(8), 1.958(5), 1.973(5) Å for complexes
17, 19A, and 19B, respectively) are remarkably longer than the
values usually observed.23,24 In complexes 17 and 19 this
lengthening could be ascribed to the intraligand steric hindrance
involving R hydrogen atoms of the carbene phenyl rings and
the O(1) and O(3) deprotonated oxygen atoms (complex
16, O(1)‚‚‚H(49), 2.39 Å; O(3)‚‚‚H(59), 2.59 Å; complex
19A, O(1)‚‚‚H(47), 2.52 Å, O(3)‚‚‚H(57), 2.38 Å; complex 19B,
O(1)‚‚‚H(47), 2.49 Å, O(3)‚‚‚H(57), 2.49 Å). As usually
5
on the Heisenberg model H ) -2JS1S2 (S1 ) S2 ) /2):
2
2Ng2µB
ødim
)
[exp(x) + 5 exp(3x) + 14 exp(6x) +
kT
30 exp(10x) + 55 exp(15x)]/[1 + 3 exp(x) + 5 exp(3x) +
7 exp(6x) + 9 exp(10x) + 11 exp(15x)]
where ø ) J/(kT).
To obtain good fits, we included a correction for a small
quantity of monomeric Fe(III) impurities that were assumed to
obey Curie’s law. The following equation is therefore used for
the total susceptibility,
Ng′2µB S(S + 1)
2
1
2
ø ) (1 - p)ødim + p
3kT
where S ) 5/2, g′ is the g factor of the impurity (assumed to be
2.00), and p is the monomeric impurity fraction. The calculated
best-fit parameters are g ) 1.91, J ) -78.6 cm-1, and p )
(25) (a) Klose, A.; Solari, E.; Re, N.; Floriani, C.; Chiesi-Villa, A.; Rizzoli,
C. Chem. Commun. 1997, 2297. (b) Klose, A.; Solari, E.; Floriani,
C.; Geremia, S.; Randaccio, L. Angew. Chem. 1998, 110, 155. (c)
Klose, A.; Solari, E.; Hesschenbrouck, J.; Floriani, C.; Re, N.; Geremia,
S.; Randaccio, L. Organometallics 1998, 18, 360.
(26) (a) O’Connor, C. J. Prog. Inorg. Chem. 1982, 29, 203. (b) Kahn, O.
Molecular Magnetism; VCH: Weinheim, Germany, 1993.
(24) (a) Riley, P. E.; Davis, R. E.; Allison, N. T.; Jones, W. M. Inorg.
Chem. 1982, 21, 1321. (b) Bauer, D.; Harter, P.; Herdtweck, E. J.
Chem. Soc., Chem. Commun. 1991, 829.

2612 Inorganic Chemistry, Vol. 39, No. 12, 2000
Esposito et al.
Figure 10. Orbital interaction diagram for complex 17.
(E) Theoretical Calculations. Extended Hu¨ckel calcula-
tions29 were performed on the [{p-But-calix[4]-(O)2(OMe)2}-
Fe] fragment, simplifying the calix[4]arene ligand with two
phenoxo and two anisoles placed in a C2V geometry. This
simplified model retains the main features of the whole ligand;
in particular, the geometrical constraints on the O4 set of donor
atoms have been maintained by fixing the geometry of the
phenoxo and anisole molecules to the experimental X-ray values.
The frontier orbitals of the [{p-But-calix[4]-(O)2(OMe)2}-
Fe] fragment are reported on the left of Figure 10 and consist
of four low-lying metal-based orbitals. We can distinguish three
lower lying orbitals, i.e., the 1b1(dxz), pointing in the plane
2
containing the methylated oxygen, the 1a1(dz ), and the 1a2(dxy).
Because of the stronger interaction with two phenoxo ligands
in the yz plane, the 1b2(dyz) is almost 1 eV higher in energy
2
2
than the other dπ orbital, 1b1(dxz). The 2a1(dx y ) orbital pointing
toward the four oxygen ligands is still higher in energy.
However, its energy strongly depends on the iron to oxygen
distances involving the substituted O(2) and O(4) atoms, which
are quite sensitive to the nature of the substituent R and vary
from 2.23 Å for RdCH3 in 17 to 2.32 Å for RdSi(CH3)3 in
2
2
19A. In particular, the 2a1(dx y ) orbital shows strong decreases
in energy on lengthening the above iron to oxygen distances
such that it is higher than the 1b2(dyz) by almost 1 eV in 17 but
by less than 0.4 eV in 19.
Next, we consider the simplest carbene complex [{p-But-
calix[4]-(O)2(OMe)2}FedCH2] as a model of the diphenyl-
carbenes 17-19. The bonding between two moieties is displayed
by the interaction orbital diagram in Figure 10. On the extreme
right of Figure 10 the frontier orbitals of CH2, i.e., the σ-donor
a1 (sp2 hybrid) and the p-acceptor b2(py), are presented. Because
of the quintet spin state of for this complex, the bonding between
the metal fragment and the CH2 unit, illustrated in the second
column of Figure 10, is quite different from that arising from
the usual molecular orbital interpretation of carbene complexes.30
Two main interactions are still observed between the σ-donor
Figure 9. Magnetic susceptibilities (O) and magnetic moments (b)
as a function of the temperature for 11 (a), 13 (b), and 14 (c).
0.5% for 11; g ) 1.94, J ) -64.1 cm-1, and p ) 0.6% for 13;
and g ) 2.07, J ) -9.8 cm-1, and p ) 1.9% for 14.
It is noted that the µ-oxo Fe(III)-Fe(III) dimer 11 shows
strong antiferromagnetic coupling with a value of J (-78.6
cm-1) in the range reported for several other µ-oxo Fe(III)-
Fe(III) dimers.27 A much smaller antiferromagnetic coupling
(J ) -9.8 cm-1) is instead observed for 14 in the range found
for other phenoxo alkoxo or hydroxo-bridged Fe(III) dimers.28
(28) Snyder, B. S.; Patterson, G. S.; Abrahamson, A. J.; Holm, R. H. J.
Am. Chem. Soc. 1989, 111, 5214 and references therein.
(29) (a) Hoffmann, R.; Lipscomb, W. N. J. Chem. Phys. 1962, 36, 2179.
(b) Hoffmann, R. J. Chem. Phys. 1963, 39, 1397.
(27) Gorun, S. M.; Lippard, S. J. Inorg. Chem. 1991, 30, 1625 and
(30) Hoffmann, P. In Transition Metal Carbene Complexes; Verlag
Chemie: Weinheim, Germany, 1983; p 113.
references therein.

Properties of Iron(II)
Inorganic Chemistry, Vol. 39, No. 12, 2000 2613
interact with the higher-lying 1b2(dyz) so that the preferred
carbene orientation is expected to be that in the Fe-O(2)-O(4)
plane. This orientation, however, cannot be achieved because
of the intraligand steric hindrance, which forces the complex
to the experimentally observed orientation with the carbene in
the Fe-O(1)-O(3) plane. As a consequence of this sterically
imposed orientation, the interaction between the π level of the
carbene and the low-lying metal 1b1(dxz) is quite weak, leading
to a relatively low-energy π* metal-carbene antibonding orbital,
which allows its population to favor the high-spin state. This
picture is confirmed by our extended Hu¨ckel calculations on
the [{p-But-calix[4]-(O)2(OMe)2}FedCH2] model complex,
which cannot have any intraligand steric hindrance. Separate
calculations have been performed on the singlet states of the
two possible carbene conformations, and that with the carbene
in the Fe-O(2)-O(4) plane resulted in a configuration that is
ca. 22 kcal/mol more stable than the experimental conformation.
Moreover, the molecular orbital diagram for the two conforma-
tions reported in Figure 11 illustrates clearly that the hypothetical
conformation with the carbene in the Fe-O(2)-O(4) plane
shows a stronger metal-carbene p interaction and much higher
lying π*(Fe-C) orbital, which would probably favor a con-
ventional low-spin carbene with the usual iron-carbon double
bond character. This arrangement can be described as a sterically
driven high-spin state.
Figure 11. Orbital interaction diagram of two conformations.
2
orbital a1 of CH2 and the σ-acceptor metal orbital 1a1(dz ), and
between the π-acceptor b2 of CH2 and the π-donor 1b1(dxz).
However, one of the four unpaired electrons occupies the π*
metal-carbene antibonding orbital, thus reducing the metal-
carbon bond order. This is in agreement with the experimental
evidence showing long Fe-C bond distances (1.943 Å in 17,
1.958 Å in 19A, and 1.973 Å in 19B), which are closer to a
single bond than to a double bond. This arrangement is probably
favored by the particular orientation imposed by the steric
hindrance between the carbene phenyl rings and the methyl or
trimethylsilyl groups on the O(2) and O(4) oxygens. Indeed,
for both complexes 17 and 19, the X-ray structures show that
the carbene plane lies essentially in the xz symmetry plane
passing through Fe, O(1), and O(3), the orientation relieving
the above steric interaction. However, this is not the preferred
carbene orientation expected on the basis of frontier orbital
patterns of the metal fragment.
Conclusions
The reactivity of the iron-bis-O-alkylated calix[4]arene
explored in this paper gives significant insight on the redox
behavior of iron(II) in a square-planar environment made up of
oxygen donor atoms only. The oxidation state (III) seems to
be, under the explored conditions, the only accessible one. In
the meantime, the parent compound, when reacted with diaz-
oalkanes, led to the formation of FedC functionalities in
metallacalix[4]arene derivatives. This result revealed how such
a functionality, which is a key intermediate in the Fischer-
Tropsch synthesis, can be easily bound to an iron-oxo surface.
Acknowledgment. We thank the “Fonds National Suisse de
la Recherche Scientifique” (Bern, Switzerland, Grant No. 20-
53336.98), Action COST D9 (European Program for Scientific
Research, OFES No. C98.008), and Fondation Herbette (Uni-
versity of Lausanne, N. Re) for financial support.
In principle the π-acceptor orbital of CH2 can interact with
either the 1b2(dyz) or 1b1(dxz) metal orbitals, leading to different
orientations of the carbene moiety with the CH2 unit lying in
the xz or yz plane, i.e., the Fe-O(2)-O(4) or Fe-O(1)-O(3)
plane, respectively. According to the general rules for orbital
interactions,31 the carbene π-acceptor level should preferentially
Supporting Information Available: ORTEP drawings, details of
the X-ray data collection, structure solution and refinement, and tables
giving crystal data, atomic coordinates, isotropic and anisotropic
displacement parameters, and bond lengths and angles for 8, 9, 12, 14,
16, 17, and 19. This material is available free of charge via the Internet
at http://pubs.acs.org.
(31) Albright, T. A.; Burdett, J. K.; Whangbo, M. H. Orbital Interaction
in Chemistry; Wiley: New York, 1985.
IC991467H