Organometallic isocyanocyclopentadienides: A combined synthetic, spectroscopic, structural, electrochemical, and theoretical investigation

Article abstract of DOI:10.1021/om049842n

The synthesis of organometallic isocyanocyclopentadienides, which results in aromatic isocyanides incorporating nonbenzenoid π-systems was investigated. The analysis of physical, chemical, electrochemical and spectroscopic properties shows that the electronic influence of the ferrocenyl moiety was similiar to aryl substituents. The electrochemical properties states that [Cr(CNR)₆]^0,1+,2+ were consistent, with the isocyanocymantrene, a stronger π-acid than isocyanoferrocene. The electronic properties of the isocyanocyclopentadienide ligand, which can be turned to a substantial extent by varying the nature of the metal fragment bound to its ring, were also elaborated.

Full text of DOI:10.1021/om049842n

Organometallics 2004, 23, 2927-2938
2927
Or ga n om eta llic Isocya n ocyclop en ta d ien id es: A
Com bin ed Syn th etic, Sp ectr oscop ic, Str u ctu r a l,
Electr och em ica l, a n d Th eor etica l In vestiga tion
Thomas C. Holovics,† Stephan F. Deplazes,† Masaharu Toriyama,‡
Douglas R. Powell,† Gerald H. Lushington,† and Mikhail V. Barybin*,†
Department of Chemistry, University of Kansas, 1251 Wescoe Hall Drive,
Lawrence, Kansas 66045, and College of Pharmacy, Nihon University,
7-7-1 Narashinodai, Funabashi-shi, Chiba 274-8555, J apan
Received March 2, 2004
This article reports on the chemistry of two organometallic isocyanocyclopentadienides,
which represent an emerging new class of aromatic isocyanides incorporating nonbenzenoid
π-systems. Interaction of aminoferrocene with a mixture of phenyl formate/phenol followed
by subsequent dehydration of the resulting ferrocenylformamide with POCl3 produced air
and thermally stable isocyanoferrocene (CNFc, Fc ) ferrocenyl) in a high yield. Treating
lithiocymantrene, LiCm (Cm ) (η5-C5H4)Mn(CO)3), with tosyl azide afforded thermally
sensitive cymantrenyl azide. Without isolation, CmN3 was reduced by NaBH4 to form
aminocymantrene, which was converted into air stable but thermally and light sensitive
isocyanocymantrene, CNCm. Combining 6 equiv of CNR (R ) Fc, Cm) with bis(naphthalene)-
chromium(0) afforded Cr(CNR)6. Successive one-electron oxidations of Cr(CNR)6 with Ag+
produced the corresponding paramagnetic [Cr(CNR)6]+ and [Cr(CNR)6]2+. The compounds
[Cr(CNR)6]0,1+,2+ (R ) Fc, Cm) are remarkable due to the incorporation of seven transition
metal atoms within relatively compact ML6 motifs. The physical, chemical, electrochemical,
and spectroscopic properties of the structurally characterized series [Cr(CNFc)6]0,1+,2+ indicate
that the electronic influence of the ferrocenyl moiety, often compared to an alkyl group, is
in fact more similar to that of aryl substituents. Electrochemical properties of [Cr(CNR)6]0,1+,2+
(R ) Fc, Cm) are consistent with isocyanocymantrene being a substantially stronger π-acid
than isocyanoferrocene. This conclusion was unambiguously corroborated by a DFT analysis
of the Frontier molecular orbitals of CNFc and CNCm. Unpaired spin delocalization within
odd-atom, nonbenzenoid aromatic π-systems of [Cr(CNR)6]1+,2+ (R ) Fc, Cm) was studied
by multinuclear paramagnetic NMR and contrasted with patterns observed for similar
complexes incorporating benzenoid and even-atom, nonbenzenoid aromatic moieties.
In tr od u ction
extensive work in these areas continues to rely on a
fundamentally quite limited pool of isocyanide mol-
ecules.1-5 Aryl isocyanides, CNAr, are generally more
thermally and air sensitive than alkyl isocyanide con-
geners thereof.6 For instance, CNPh and a number of
its derivatives deteriorate rapidly upon exposure to air
and isomerize to the corresponding thermodynamically
favored cyanides at 40-50 °C.7
The electronic advantage of aryl over alkyl iso-
cyanides stems from communication between the π-
systems of the isocyano group and the aromatic moiety
within the former species.2,8 Isocyanoferrocene9 and 1,1′-
diisocyanoferrocene,10 which may be regarded as organo-
metallic derivatives of the elusive11 isocyanocyclopenta-
dienide anion, were the only known examples of non-
Organic isocyanides (:CtN-R), also referred to as
isonitriles or carbylamines, are among the few isolable
species possessing a lone electron pair on a carbon atom.
The most general route to these highly reactive sub-
stances involves formylation of the corresponding pri-
mary amine, H2NR, followed by dehydration of the
resulting formamide, H(O)CNHR.1 Given the important
role of isocyanides in organic and organometallic syn-
thesis,1,2 catalysis,2a-c materials science,3 drug discov-
ery,4 and diagnostic medicine,5 it is surprising that the
* To whom correspondence should be addressed. E-mail: mbarybin@
ku.edu.
† University of Kansas.
‡ Nihon University.
(1) Ugi, I. Isonitrile Chemistry; Academic Press: New York, 1971.
(2) (a) Singleton, E.; Oosthuizen, H. E. Adv. Organomet. Chem. 1983,
22, 209-309. (b) Treichel, P. M. Adv. Organomet. Chem. 1973, 11, 21-
86. (c) Yamamoto, Y. Coord. Chem. Rev. 1980, 32, 193-233. (d)
Carnahan, E. M.; Protasiewicz, J . D.; Lippard, S. J . Acc. Chem. Res.
1993, 26, 90-97. (e) Weber, L. Angew. Chem., Int. Ed. 1998, 37, 1515-
1517. (f) Hahn, F. E. Angew. Chem., Int. Ed. Engl. 1993, 32, 650-665.
(3) (a) Cornelissen, J . L. M.; Rowan, A. E.; Nolte, R. J . M.;
Sommerdijk, N. A. J . M. Chem. Rev. 2001, 101, 4039-4070. (b) Nakano,
T.; Okamoto, Y. Chem. Rev. 2001, 101, 4013-4038.
(4) (a) Ugi, I.; Werner, B.; Do¨mling, A. Molecules 2003, 8, 53-66.
(b) Weber, L. Curr. Med. Chem. 2002, 9, 1241-1253. (c) Do¨mling, A.
Curr. Opin. Chem. Biol. 2002, 6, 306-313. (d) Ugi, I. Pure Appl. Chem.
2001, 73, 187-191. (e) Do¨mling, A. Curr. Opin. Chem. Biol. 2000, 6,
318-323.
(5) Sharma, V.; Piwnica-Worms, D. Chem. Rev. 1999, 99, 2545-
2560.
(6) Malatesta, L. Prog. Inorg. Chem. 1959, 1, 283-379.
(7) Meier, M.; Mueller, B.; Ruechardt, C. J . Org. Chem. 1987, 52,
648-652.
10.1021/om049842n CCC: $27.50 © 2004 American Chemical Society
Publication on Web 05/13/2004

2928 Organometallics, Vol. 23, No. 12, 2004
Holovics et al.
benzenoid aryl isocyanides prior to our recent com-
munications.12,13 The chemistry of isocyanoferrocene has
remained practically unexplored since discovery of the
compound in the late 1980s.9 This may be attributed to
the fact that only tedious and very low-yield (8-12%
starting from ferrocene) syntheses of aminoferrocene
were available until recently.14 Furthermore, conversion
of H2NFc to CNFc (Fc ) ferrocenyl) has been reported
to be poorly reproducible.9a The isolated complexes of
CNFc have been limited to (OC)5Cr(CNFc) and (OC)4Fe-
(CNFc).9 Notably, the electronic properties of isocyano-
ferrocene as a ligand have been suggested to be similar
to those of methyl isocyanide on the basis of electro-
chemical characteristics of (OC)5Cr(CNR) (R ) Fc, Me)
and the long-accepted fact15 that ferrocenyl is a slightly
stronger electron-donating substituent than methyl.9b,c
Despite many parallels in the chemistries of ferrocene
and arenes, consideration of ferrocenyl’s π-system as a
potential electron acceptor has not been addressed
experimentally. This is not surprising given that the
five-membered rings of Cp2Fe already possess an effec-
tive negative charge of ca. 0.35.15
Exp er im en ta l Section
Gen er a l P r oced u r es, Sta r tin g Ma ter ia ls, a n d Equ ip -
m en t. Unless specified otherwise, all operations were per-
formed under an atmosphere of 99.5% argon purified by
passage through columns of activated BASF catalyst and
molecular sieves. All connections involving the gas purification
system were made of glass, metal, or other materials imper-
meable to air. Solutions were transferred via stainless steel
needles (cannulas) whenever possible. Standard Schlenk
techniques were employed with a double manifold vacuum line.
Solvents, including deuterated solvents, were freed of impuri-
ties by standard procedures and stored under argon.
Solution infrared spectra were recorded on a Thermo Nicolet
Avatar 360 FTIR spectrometer with samples sealed in 0.1 mm
gastight NaCl cells. NMR samples were analyzed on Bruker
DRX-400 or Bruker Avance 500 spectrometers. 1H and 13C
chemical shifts are given with reference to residual 1H and
13C solvent resonances relative to SiMe4. Such referencing
eliminated bulk susceptibility effects for paramagnetic samples.
Two-dimensional NMR techniques (DQF-COSY, 1H-13C HMQC,
and H-13C HMBC)16 were employed to obtain unambiguous
1
assignments of 1H and 13C NMR resonances. The aromatic
hydrogen resonances are labeled in reference to the corre-
sponding carbon atoms. 14N NMR chemical shifts are refer-
enced to liquid NH3 at 25 °C. In the variable-temperature NMR
studies, the console temperature was calibrated using a
thermocouple immersed into an NMR tube containing pure
solvent (CD2Cl2). Melting points are uncorrected and were
determined for samples in sealed capillary tubes. Elemental
analyses were carried out by Desert Analytics, Tucson, AZ.
Phenyl formate,17 acetic-formic anhydride,18 V(CO)6,19 [Et4N]-
[V(CO)6],20 tosyl azide,21 bis(η6-naphthalene)chromium(0),22
and aminoferrocene14b were prepared according to literature
procedures. Other reagents were obtained from commercial
sources and freed of oxygen and moisture before use, if
required.
The traditional methods of tuning properties of aryl
isocyanides have revolved around the π-system of
benzene for decades and are based on C-H substitution
at the aromatic ring.1,2,6,8 Altering the nature of the
metal fragment in η5-bound isocyanocyclopentadienyl
complexes would constitute a fundamentally new ap-
proach for modulating electronic and structural char-
acteristics of aromatic isocyanides. Herein, we report
on the efficient synthesis and properties of isocyano-
ferrocene and of the hitherto unknown isocyano-
cymantrene, CNCm {Cm ) cymantrenyl or (η5-C5H4)-
Mn(CO)3}. Heptanuclear, binary complexes of these
organometallic isocyanocyclopentadienides, namely,
[Cr(CNR)6]0,1+,2+ (R ) Fc, Cm), are described as well. A
preliminary account of a portion of this work has been
communicated.12
Ma gn etic Su scep tibility Mea su r em en ts. Solid-state vol-
ume magnetic susceptibilities were measured on a J ohnson
Matthey MSB-1 balance at ambient temperature and con-
verted into the corresponding molar susceptibilities in the
usual manner.23 Samples were packed into gastight tubes
(0.400 cm o.d. × 0.324 cm i.d.) to a depth of ca. 3 cm in a
drybox. The air correction of 0.029 × 10-6 was applied to
volume susceptibilities of all samples packed under argon.
Diamagnetic corrections applied to the molar susceptibilities
of the paramagnetic substances are reported as ødiam. These
corrections were obtained by adding contributions from the
[BF4]- or [SbF6]- ions23 to experimentally determined molar
susceptibilities of diamagnetic Cr(CNFc)6 or Cr(CNCm)6. The
molar susceptibilities of Cr(CNFc)6 and Cr(CNCm)6 at 25 °C
were measured to be -403.4 × 10-6 and -300.2 × 10-6 cm3
mol-1, respectively.
(8) Selected examples: (a) Chen, J .; Calvet, L. C.; Reed, M. A.; Carr,
D. W.; Grubisha, D. S.; Bennett, D. W. Chem. Phys. Lett. 1999, 313,
741-748. (b) Wagner, N. L.; Laib, F. E.; Bennett, D. W. Inorg. Chem.
Commun. 2000, 3, 87-90. (c) Henderson, J . I.; Feng, S.; Bein, T.;
Kubiak, C. P. Langmuir 2000, 16, 6183-6187. (d) Hanack, M.;
Kamenzin, S.; Kamenzin, C.; Subramanian, L. Synth. Met. 2000, 110,
93-102.
(9) (a) Knox, G. R.; Pauson, P. L.; Willison, D.; Solca´niova´, E.; Toma.
Organometallics 1990, 9, 301-306. (b) El-Shihi, T.; Siglmu¨ller, F.;
Herrmann, R.; Carvalho, M. F. N. N.; Pombeiro, A. J . L. J . Organomet.
Chem. 1987, 335, 239-247. (c) El-Shihi, T.; Siglmu¨ller, F.; Herrmann,
R.; Carvalho, M. F. N. N.; Pombeiro, A. J . L. Port. Electrochim. Acta
1987, 5, 179-185.
Syn th esis of F cNHC(O)H (1). Approximately 3 mL of a
65/35 mol % mixture of phenyl formate/phenol17 was added to
solid FcNH2 (0.958 g, 4.77 mmol) in one portion at room
(10) Van Leusen, D.; Hessen, B. Organometallics 2001, 20, 224-
226.
(11) Banert, K.; Ko¨ehler, F.; Meier, B. Tetrahedron Lett. 2003, 44,
3781-3783.
(16) Levitt, M. H. Spin Dynamics. Basics of Nuclear Magnetic
Resonance; J ohn Wiley & Sons, Ltd: New York, 2001.
(17) Yale, H. L. J . Org. Chem. 1971, 36, 3228-3240.
(18) Krimen, L. I. Org. Synth. 1970, 50, 1-3.
(12) Barybin, M. V.; Holovics, T. C.; Deplazes, S. F.; Lushington, G.
H.; Powell, D. R.; Toriyama, M. J . Am. Chem. Soc. 2002, 124, 13668-
13669.
(19) (a) Liu, X.; Ellis, J . E. Inorg. Synth. 2004, 34, 96-103. (b) Ellis,
J . E.; Faltynek, R. A.; Rochfort, G. L.; Stevens, R. E.; Zank, G. A. Inorg.
Chem. 1980, 19, 1082-1085.
(13) Robinson, R. E.; Holovics, T. C.; Deplazes, S. F.; Lushington,
G. H.; Powell, D. R.; Barybin, M. V. J . Am. Chem. Soc. 2003, 125,
4432-4433.
(20) Barybin, M. V.; Pomije, M. K.; Ellis, J . E. Inorg. Chim. Acta
1998, 269, 58-62.
(14) Improved syntheses of aminoferrocene and 1,1′-diaminoferro-
cene have recently been published: (a) ref 10. (b) Bildstein, B.; Malaun,
M.; Kopacka, H.; Wurst, K.; Mitterbo¨ck, M.; Ongania, K.-H.; Opromolla,
G.; Zanello, P. Organometallics 1999, 18, 4325-4336. (c) Kavallieratos,
K.; Hwang, S.; Crabtree, R. H. Inorg. Chem. 1999, 38, 5184-5186. (d)
Shafir, A.; Power, M. P.; Whitener, G. D.; Arnold, J . Organometallics
2000, 19, 3978-3982.
(21) Regitz, M.; Hocker, J . Organic Syntheses; J ohn Wiley: New
York, 1973; Collect. Vol. V, pp 179-183.
(22) (a) Pomije, M. K.; Kurth, C. J .; Ellis, J . E.; Barybin, M. V.
Organometallics 1997, 16, 3582-3587. (b) Ku¨ndig, E. P.; Timms, P.
L. J . Chem. Soc., Dalton. Trans. 1980, 991-995.
(23) (a) Earnshaw, A. Introduction to Magnetochemistry; Academic
Press: New York, 1968. (b) Kahn, O. Molecular Magnetism; VCH
Publishers: New York, 1993.
(15) Nesmeyanov, A. N.; Perevalova, E. G.; Gibin, S. P.; Grandberg,
K. I.; Kozlovsky, A. G. Tetrahedron Lett. 1966, 22, 23-81-2387.

Organometallic Isocyanocyclopentadienides
Organometallics, Vol. 23, No. 12, 2004 2929
temperature under argon with vigorous stirring. A slightly
exothermic reaction occurred, affording a brown solution
within a minute. After 4 h of stirring at room temperature,
complete consumption of FcNH2 was confirmed by TLC using
neat Et2O as eluent. All volatiles were removed under vacuum
(5 × 10-3 Torr) at T < 50 °C (important!) using short-path
distillation equipment. The residual oil was dissolved in Et2O
and passed through a 30 cm column packed with Florisil. An
intensely orange solution was collected. All Et2O was removed,
leaving an orange oil, which was recrystallized from Et2O,
providing a 79% yield of crystalline orange 1 (0.860 g, 3.75
mmol), which was spectroscopically (IR, 1H and 13C NMR)
identical to bona fide ferrocenylformamide.9a Mp: 92-94 °C
(lit. 86-87 °C9a).
CH2Cl2. After stirring for 20 min, the reaction mixture was
washed sequentially with 200 mL of saturated aqueous
Na2CO3 and 100 mL of H2O, dried over sodium sulfate,
concentrated, and chromatographed on silica gel. A trace
amount of cymantrene was eluted with hexanes, while the
subsequent elution with Et2O provided a yellow solution of 3.
Solvent removal followed by drying under vacuum for 2 h
afforded analytically pure, bright yellow 3 (5.100 g, 20.64
mmol) in an 80% yield (or a 55% overall yield based on the
starting amount of cymantrene used in the above synthesis of
CmNH2). Mp: 97-100 °C dec. Anal. Calcd for C9H6NMnO4:
C, 43.75; H, 2.45; N, 5.67. Found: C, 44.06; H, 2.52; N, 5.71.
IR (CH2Cl2): νNH 3415 w; νCO 2022 s, 1935 vs, 1706 m cm-1
.
In dichloromethane solutions at 25 °C, 3 exists as a 9:1 mixture
of two conformational isomers. The following NMR data refer
to the major isomer. 1H NMR (500 MHz, CD2Cl2, 25 °C): δ
4.64 (s, 2H, C5H4, H3,4), 5.13 (s, 2H, C5H4, H2,5), 7.33 (s, 1H,
NH). 8.20 (s, 1H, formyl H) ppm. 13C{1H} NMR (125.8 MHz,
CD2Cl2, 25 °C): δ 73.4 (C5H4, C2,5), 79.6 (C5H4, C3,4), 111.1
(C5H4, C1), 159.6 (formyl C), 225.4 (Mn{CO}3) ppm.
Syn th esis of CNF c (2). This procedure is highly reliable
and constitutes a modified version of the poorly reproducible
synthesis reported by Knox et al.9a Phosphorus oxychloride
(0.915 mL, 9.78 mmol) was added at once to a stirred solution
i
of 1 (2.222 g, 9.70 mmol) and freshly distilled Pr2NH (4.08
mL, 29.1 mmol) in 50 mL of CH2Cl2 at ambient temperature.
After stirring for 10 h, the reaction mixture was quenched with
100 mL of 10% aqueous potassium carbonate. The organic
layer was separated, washed with distilled water (2 × 50 mL),
and dried over MgSO4 without protection from air. Filtration
of the resulting orange solution followed by solvent removal
afforded crude CNFc as an orange solid. This solid was
dissolved in hexanes and passed through a 25 cm column of
Florisil using a 1:1 mixture of hexanes/Et2O. The solvent was
removed in vacuo to provide a 93% yield of spectroscopically
pure, microcrystalline, peach-colored 2 (1.907 g, 9.04 mmol).
Mp ) 73-75 °C (lit. 76-77 °C9a). IR (CH2Cl2): νCN 2122 vs
cm-1. 1H NMR (400 MHz, CDCl3, 25 °C): δ 4.13 (ψ-t, 2H, C5H4,
H3,4), 4.32 (s, 5H, C5H5), 4.57 (ψ-t, 2H, C5H4, H2,5) ppm. 13C{1H}
NMR (100.6 MHz, CDCl3, 25 °C): δ 67.2 (C5H4, C2,5), 67.2
(C5H4, C3,4), 71.0 (C5H5), 174.0 (CNFc) ppm. 14N NMR (36.2
MHz, CDCl3, 25 °C): δ 172.1 (W1/2 ) 28 Hz) ppm.
Syn th esis of CNCm (4). Phosphorus oxychloride (0.91 mL,
9.73 mmol) was added at once to a stirred yellow solution of 2
(2.000 g, 8.09 mmol) and Et3N (5.60 mL, 40.2 mmol) in 150
mL of CH2Cl2 at ambient temperature. After stirring for 1 h,
the reaction was quenched with 300 mL of 10% aqueous
sodium carbonate. The organic layer was separated, washed
with distilled water (2 × 150 mL), and dried over Na2SO4
without protection from air. The yellow solution was filtered
and evaporated to dryness under reduced pressure. The
residue was passed through a 20 cm column of Florisil using
a 1:1 mixture of hexanes/CH2Cl2 to elute a fast moving yellow
band. The solvent was removed in vacuo (5 × 10-3 Torr) at 0
°C (important!) to afford tan-yellow 4 in an 81% yield (1.500
g, 6.55 mmol). Mp ) 26-29 °C dec. IR (CH2Cl2): νCN 2133 m;
1
νCO 2034 s 1952 vs cm-1. H NMR (500 MHz, CD2Cl2, 25 °C):
δ 4.66 (s, 2H, C5H4, H3,4), 5.11 (s, 2H, C5H4, H2,5) ppm. 13C{1H}
NMR (125.8 MHz, CD2Cl2, 25 °C): δ 80.7 (C5H4, C3,4), 81.7
(C5H4, C2,5), 91.8 (C5H4, C1), 168.9 (CNCm), 223.6 (CO) ppm.
14N NMR (36.2 MHz, CDCl3, 25 °C): δ 164.3 (W1/2 ) 41 Hz)
ppm. Elemental analysis of 4 was not sought due to the poor
stability of its samples at ambient temperature.
Syn th esis of Cm NH2. A 1.60 M solution of nBuLi in
hexanes (23.5 mL, 37.60 mmol) was added dropwise to a cold
(-78 °C), pale yellow solution of cymantrene (7.680 g, 37.63
mmol) in 150 mL of THF. After 1 h of vigorous stirring at -78
°C, the yellow-orange reaction mixture was treated with a
solution of p-toluenesulfonyl azide (7.424 g, 37.64 mmol) in
75 mL of THF. The resulting deep red-orange mixture was
allowed to warm to room temperature for 13 h in the dark.
Formation of cymantrenyl azide, CmN3, was confirmed by
FTIR (νNN 2129 m; νCO 2021 s, 1934 vs cm-1). All solvent was
subsequently removed in vacuo, and the oily residue was
redissolved in ca. 150 mL of EtOH. To this solution NaBH4
(7.424 g, 37.64 mmol) dissolved in 150 mL of EtOH was
carefully added at room temperature. A vigorous exothermic
reaction occurred. After the gas evolution had ceased, the
orange mixture was extracted with Et2O (300 mL). The
ethereal layer was washed with 300 mL of H2O and then
treated with 300 mL of water acidified to pH ) 3 with 1.0 M
HCl. The aqueous layer was separated and neutralized to pH
) 8 with 1.0 M NaOH. Extraction with Et2O afforded an
orange-yellow solution of CmNH2, which was dried over
Na2SO4, filtered, and concentrated to dryness to provide
orange-yellow CmNH2 (5.689 g, 25.97 mmol) in a 69% yield.
Mp ) 74-76 °C (lit. 77-77.5 °C24). IR (Et2O): νNH 3460 w,
3386 w; νCO 2012 s, 1921 vs cm-1. The product may contain a
small amount of cymantrene and can be further purified by
column chromatography or sublimation,24 if desired. Such a
purification, however, is not necessary for the synthesis of
CmNHC(O)H.
Syn th esis of Cr (CNF c)6 (5). An orange solution of 2 (1.900
g, 9.00 mmol) in 50 mL of THF was added to a brown solution
of bis(η6-naphthalene)chromium(0) (0.427 g, 1.39 mmol) in 70
mL of THF via cannula at room temperature. Within 15 h of
stirring, a deep orange-red solution/slurry formed. Heptane
(20 mL) was introduced to the reaction mixture, and all but
about 40 mL of the solvent was removed under vacuum. The
precipitate was filtered off and washed thoroughly with
pentane (4 × 15 mL) to remove naphthalene and any unre-
acted 2. After drying in vacuo for 3 h, microcrystalline, orange-
red 5 (1.696 g, 1.29 mmol) was isolated in a 93% yield. Mp:
256-257 °C dec. Anal. Calcd for C66H54N6CrFe6: C, 60.13; H,
4.13; N, 6.38. Found: C, 60.22; H, 3.89; N, 6.18. [M + 1]+ Calcd
for C66H54N652Cr56Fe6: 1320.2. Found: 1320.5. IR (CH2Cl2):
νCN 1971 vs br cm-1 1H NMR (400 MHz, CD2Cl2, 25 °C): δ
.
4.18 (s, 2H, C5H4, H3,4), 4.27 (s, 2H, C5H4, H2,5), 4.34 (s, 5H,
C5H5) ppm. 13C{1H} NMR (100.6 MHz, CD2Cl2, 25 °C): δ 67.6
(C5H4, C2,5), 67.6 (C5H4, C3,4), 70.8 (C5H5) ppm. 14N NMR (36.2
MHz, CD2Cl2, 25 °C): δ 310.4 (W1/2 ) 77 Hz) ppm.
Syn th esis of Cr (CNCm )6 (6). A yellow solution of 4 (1.500
g, 6.55 mmol) in 60 mL of THF was added to a brown solution
of bis(η6-naphthalene)chromium(0) (0.330 g, 1.07 mmol) in 60
mL of THF via cannula at -78 °C with vigorous stirring. The
reaction mixture was allowed to warm gradually to room
temperature. Within 15 h of stirring, a deep red solution
formed. All but ca. 10 mL of the solvent was removed under
vacuum, and 50 mL of heptane was added to precipitate a
microcrystalline, red solid. An additional 10 mL of the solvent
was removed, and 50 mL of pentane was added to the mixture.
The precipitate was filtered off and washed thoroughly with
Syn th esis of Cm NHC(O)H (3). Crude CmNH2 (5.689 g,
25.97 mmol), obtained in the above synthesis, was dissolved
in 30 mL of CH2Cl2 and treated dropwise with a solution of
excess acetic-formic anhydride18 (67.40 mmol) in 30 mL of
(24) Cais, M.; Narkis, N. J . Organomet. Chem. 1965, 3, 188-199.

2930 Organometallics, Vol. 23, No. 12, 2004
Holovics et al.
pentane (4 × 15 mL) to remove naphthalene and any unre-
acted free ligand. After drying in vacuo for 3 h, microcrystal-
line, scarlet 6 (1.390 g, 0.974 mmol) was isolated in a 91%
yield. Mp: 118-120 °C dec. Anal. Calcd for C54H24N6Cr-
Mn6O18: C, 45.47; H, 1.70; N, 5.89. Found: C, 45.12; H, 1.70;
(CO) ppm. 14N NMR (36.2 MHz, CD2Cl2, 22 °C): δ 866.2 (W1/2
) 1170 Hz) ppm. µeff(24.0 °C) ) 1.99 µB (ødiam ) -380.2 × 10-6
cm3 mol-1).
Attem p ted Oxid a tion of 6 w ith V(CO)6. A yellow solution
of V(CO)6 (0.051 g, 0.233 mmol) in 30 mL of CH2Cl2 was added
to an orange-red solution of 6 (0.300 g, 0.210 mmol) in 100
mL of CH2Cl2 with stirring at -55 °C. The mixture was
warmed to room temperature. Formation of only trace amounts
of 6+[V(CO)6] was observed by FTIR. However, after ca. 5 h,
no cation 6+ could be detected at all and production of
[V(CO)6]- with concomitant depletion of V(CO)6 became more
pronounced. Combining equimolar solutions of 6+[SbF6] and
[Et4N][V(CO)6] in CH2Cl2 generated essentially the same FTIR
pattern in νCN and νCO stretching regions.
N, 5.65. IR (CH2Cl2): νCN 1947 m br; νCO 2030 s, 1937 vs cm-1
.
1H NMR (500 MHz, CD2Cl2, 25 °C): δ 4.63 (s, 2H, C5H4, H3,4),
4.87 (s, 2H, C5H4, H2,5), ppm. 13C{1H} NMR (125.8 MHz,
CD2Cl2, 25 °C): δ 78.7 (C5H4, C2,5), 80.2 (C5H4, C3,4), 101.4
(C5H4, C1), 224.9 (CO) ppm. 14N NMR (36.2 MHz, CD2Cl2, 25
°C): δ 169.1 (W1/2 ) 1115 Hz) ppm.
Syn th esis of [Cr (CNF c)6][BF 4] (5+[BF 4]). A solution of
5 (0.980 g, 0.743 mmol) in 120 mL of CH2Cl2 was added to
solid AgBF4 (0.145 g, 0.745 mmol) at once with stirring at room
temperature. The color of the reaction mixture changed from
orange-red to orange-brown within minutes. The mixture was
stirred for 6 h at ambient temperature, then filtered through
a 3 cm plug of Celite. An additional 50 mL of CH2Cl2 was
employed to wash the filter cake until the washings were
colorless. All but ca. 35 mL of the solvent was removed under
vacuum, and 35 mL of heptane was added to precipitate a
microcrystalline, saddle-brown solid. This solid was filtered
off, washed with pentane (2 × 20 mL), and dried in vacuo for
2 h to afford saddle-brown 5+[BF4] (1.020 g, 0.726 mmol) in a
98% yield. Mp: 205 °C dec. Anal. Calcd for C66H54N6BCr-
F4Fe6: C, 56.42; H, 3.87; N, 5.98. Found: C, 56.00; H, 3.86;
N, 5.53. IR (CH2Cl2): νCN 2053 vs; νBF 1065 m br cm-1. 1H NMR
(400 MHz, CD2Cl2, 25 °C): δ 1.32 (s, 2H, C5H4, H2,5), 3.98 (s,
5H, C5H5), 5.73 (s, 2H, C5H4, H3,4) ppm. 13C{1H} NMR (100.6
MHz, CD2Cl2, 25 °C): δ 71.7 (C5H5), 77.5 (C5H4, C3,4), 109.2
(C5H4, C2,5) ppm. 14N NMR (36.2 MHz, CD2Cl2, 25 °C): δ 863.5
(W1/2 ) 531 Hz) ppm. µeff(24.5 °C) ) 1.78 µB (ødiam ) -442.4
× 10-6 cm3 mol-1).
Syn th esis of [Cr (CNF c)6][BF 4]2 (52+[BF 4]2). CH2Cl2 (50
mL) was introduced into a flask containing a solid mixture of
5+[BF4] (0.500 g, 0.356 mmol) and AgBF4 (0.071 g, 0.365 mmol)
at room temperature. Within minutes, the initially brown
reaction mixture acquired a greenish hue. After stirring for
24 h at room temperature, all but 25 mL of the solvent was
removed from the forest-green solution under vacuum. Hep-
tane (ca. 30 mL) was added with stirring to precipitate a
beautiful, microcrystalline, forest-green solid. The brownish
supernatant was decanted. The solid was washed with pentane
(2 × 30 mL) until the washings were absolutely colorless and
dried in vacuo for 1 h to afford 52+[BF4]2 (0.480 g, 0.322 mmol)
in a 90% yield. Compound 52+[BF4]2 decomposes above 250 °C
without melting. Anal. Calcd for C66H54N6B2CrF8Fe6: C, 53.14;
H, 3.65; N, 5.63. Found: C, 52.47; H, 3.41; N, 5.50. IR
(CH2Cl2): νCN 2131 vs, 2160 m sh; νBF 1065 s br cm-1. 1H NMR
(400 MHz, CD2Cl2, 25 °C): δ -1.04 (s, 2H, C5H4, H2,5), 3.83 (s,
5H, C5H5), 7.75 (s, 2H, C5H4, H3,4) ppm. 13C{1H} NMR (100.6
MHz, CD2Cl2, 25 °C): δ 72.6 (C5H5), 97.3 (C5H4, C3,4), 158.5
(C5H4, C2,5) ppm. 14N NMR (36.2 MHz, CD2Cl2, 25 °C): δ 1044.4
(W1/2 ) 130 Hz) ppm. µeff(24.5 °C) ) 2.76 µB (ødiam ) -482.4
× 10-6 cm3 mol-1).
Syn th esis of [Cr (CNF c)6][V(CO)6] (5+[V(CO)6]). A yellow
solution of V(CO)6 (0.037 g, 0.169 mmol) in 30 mL of CH2Cl2
was added to an orange-red solution of 5 (0.220 g, 0.167 mmol)
in 100 mL of CH2Cl2 with stirring at -55 °C. The reaction
mixture acquired an orange-brown color within minutes. Upon
warming to ambient temperature for 30 min, the mixture was
filtered through a 3 cm plug of Celite. An additional 50 mL of
CH2Cl2 was employed to wash the filter cake until the
washings were colorless. After removing all but ca. 10 mL of
the solvent under vacuum, 20 mL of heptane was added to
the filtrate to precipitate a microcrystalline, brown solid. The
product was filtered, washed with pentane (3 × 20 mL), and
dried in vacuo to afford saddle-brown 5+[V(CO)6] (0.242 g,
0.157 mmol) in a 94% yield. Compound 5+[V(CO)6] decomposes
at ca. 250 °C without melting. IR (CH2Cl2): νCN 2053 vs; νCO
1853 vs cm-1. The 1H and 13C NMR (CD2Cl2, 25 °C) spectra
for the cation in 5+[V(CO)6]- were essentially identical to the
corresponding patterns reported above for 5+[BF4]-.
Syn th esis of [Cr (CNCm )6][SbF 6] (6+[SbF 6]). A colorless
solution of AgSbF6 (0.159 g, 0.463 mmol) in 60 mL of CH2Cl2
was added to a red solution of 6 (0.600 g, 0.421 mmol) in 60
mL of CH2Cl2 with vigorous stirring at room temperature. The
reaction mixture turned dark green over a period of 1 h and
then was filtered through a 3 cm plug of Celite. An additional
50 mL of CH2Cl2 was used to wash the filtercake until the
washings were colorless. The filtrate was concentrated to ca.
10 mL under vacuum. Addition of heptane (40 mL) followed
by removal of about 10 mL of the solvent produced a green
precipitate. This solid was filtered, washed with pentane (3 ×
15 mL), and dried in vacuo for 2 h to afford microcrystalline,
lime-green 6+[SbF6] (0.652 g, 0.392 mmol) in a 93% yield. Mp:
226-229 °C dec. Anal. Calcd for C54H24N6CrF6Mn6O18Sb: C,
39.02; H, 1.46; N, 5.06. Found: C, 38.83; H, 1.42; N, 4.68. IR
Syn th esis of [Cr (CNCm )6][SbF 6]2 (62+[SbF 6]2). A color-
less solution of AgSbF6 (0.047 g, 0.137 mmol) in 40 mL of
CH2Cl2 was added to a green solution of 6+[SbF6] (0.216 g,
0.130 mmol) in 30 mL of CH2Cl2 with vigorous stirring at room
temperature. The color of the reaction mixture changed from
green to dark blue-green over a period of 1 h. The mixture
was filtered through a 4 cm plug of Celite. An additional 40
mL of CH2Cl2 was employed to wash the filtercake until the
washings were colorless. The filtrate was concentrated to about
10 mL, layered with 100 mL of heptane, and kept at -35 °C
overnight. The resulting crystals were decanted and dried
under vacuum for 4 h to provide moss-green 62+[SbF6]2 (0.211
g, 0.111 mmol) in an 85% yield. Compound 62+[SbF6]2 decom-
poses at 162 °C without melting. Anal. Calcd for C54H24N6-
CrF12Mn6O18Sb2: C, 34.17; H, 1.27; N, 4.43. Found: C, 33.99;
H, 1.15; N, 4.33. IR (CH2Cl2): νCN 2156 m br; νCO 2034 s, 1961
vs; νSbF 630 s cm-1. 1H NMR (500 MHz, CD2Cl2, 24 °C): δ -3.74
(s, 2H, C5H4, H2,5), 6.46 (s, 2H, C5H4, H3,4), ppm. 13C{1H} NMR
(125.8 MHz, CD2Cl2, 24 °C): δ 91.6 (C5H4, C3,4), 202.5 (CO),
207.3 (C5H4, C2,5), ppm. 14N NMR (36.2 MHz, CD2Cl2, 24 °C):
δ 1021.3 (W1/2 ) 878 Hz) ppm. µeff(24.0 °C) ) 3.01 µB (ødiam
)
-460.2 × 10-6 cm3 mol-1).
X-r a y Cr ysta llogr a p h ic Ch a r a cter iza tion of 5‚CH2Cl2,
6, 5+[V(CO)6]‚CH2Cl2, a n d 52+[BF 4]2‚CH2Cl2. X-ray quality
crystals of 5‚CH2Cl2 were obtained from a nearly saturated
solution of 5 in CH2Cl2 maintained at -30 °C for two weeks.
Crystals of 6, 5+[V(CO)6]‚CH2Cl2, and 52+[BF4]2‚CH2Cl2 were
grown at 4 °C by carefully layering pentane over CH2Cl2
solutions of these complexes. All manipulations with the
crystals prior to transfer to the goniometer were performed
in air. Intensity data for all samples were collected using a
Bruker APEX CCD area detector mounted on a Bruker D8
goniometer employing graphite-monochromated Mo KR radia-
tion (λ ) 0.71073 Å). The samples were cooled to 100(2) K.
The space groups were determined by systematic absences
(CH2Cl2): νCN 2073 m br; νCO 2029 s, 1950 vs; νSbF 596 s cm-1
.
1H NMR (400 MHz, CD2Cl2, 22 °C): δ 0.17 (s, 2H, C5H4, H2,5),
5.95 (s, 2H, C5H4, H3,4), ppm. 13C{1H} NMR (100.6 MHz,
CD2Cl2, 22 °C): δ 85.0 (C5H4, C3,4), 145.0 (C5H4, C2,5), 217.4

Organometallic Isocyanocyclopentadienides
Organometallics, Vol. 23, No. 12, 2004 2931
Ta ble 1. Cr ysta l Da ta , Da ta Collection , Solu tion , a n d Refin em en t for 5‚CH2Cl2, 6, 5+[V(CO)6]‚CH2Cl2, a n d
5
2+[BF 4]2‚CH2Cl2
5‚CH2Cl2
6
5+[V(CO)6]‚CH2Cl2
5
2+[BF4]2‚CH2Cl2
empirical formula
fw
C
67H56Cl2CrFe6N6
C
54H24CrMn6N6O18
C
73H56Cl2CrFe6N6O6V
C67H56B2Cl2CrF8Fe6N6
1576.80
plate, green
0.39 × 0.33 × 0.04
monoclinic
P21/n
19.0172(12)
21.7693(14)
30.9281(19)
90
94.422(2)
90
12765.8(14)
8, 2
1.641
1.643
6368
1403.18
1426.43
block, red
0.28 × 0.20 × 0.16
triclinic
P1h
12.422(3)
12.638(3)
16.810(3)
94.215(4)
91.765(4)
93.017(4)
2626.6(10)
2, 1
1622.18
cryst habbit, color
cryst size (mm)
cryst syst
space group
a (Å)
plate, red
0.20 × 0.15 × 0.04
triclinic
prism, black
0.24 × 0.17 × 0.05
triclinic
P1h
P1h
9.7777(11)
10.6494(12)
14.6628(17)
92.226(2)
95.533(2)
111.283(2)
1411.5(3)
1, 0.5
13.0914(7)
16.6234(9)
16.8244(9)
68.977(2)
77.423(2)
87.490(2)
3333.3(3)
2, 1
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z, Z′
F
calc (Mg m-3
)
1.651
1.828
714
1.804
1.678
1416
1.616
1.696
1642
µ (mm-1
F(000)
)
temp (K)
100(2)
100(2)
100(2)
100(2)
θ range (deg)
2.06-30.02
2.01-26.00
1.50-26.00
1.42-26.00
no. of reflns collected
no. of unique reflns
10507
16688
21148
71211
7176
0.0191
9971
0.0273
12396
0.0268
24833
0.0372
a
Rint
max./min. transmn
no. of data/restraints/params
no. of reflns with I > 2σ(I)
R1b; wR2c
0.9305/0.7113
7176/43/409
5999
0.0353; 0.0884
1.023
0.7751/0.6508
9971/0/766
8886
0.0365; 0.1074
1.047
0.9200/0.6864
12 396/20/856
10032
0.0345; 0.0880
0.942
0.9372/0.5666
24 833/11/1685
17519
0.0493; 0.1330
0.974
GOF on F2
largest diff peak/hole (e‚Å-3
)
0.768/-0.340
1.091/-0.380
1.185/-0.579
3.171/-1.457
Rint ) ∑|Fo - Fo |/∑|Fo |. R1 ) ∑||Fo| - |Fc||/∑|Fo|. c wR2 ) [∑(w(Fo - Fc2)2)/∑(w(Fo2)2)]1/2
.
a
2
2
2
b
2
and/or statistical tests and verified by subsequent refinements.
FcH+/FcH couple was 89 mV at the scan rate of 100 mV/s.
This peak-to-peak separation is identical to that previously
observed for the FcH+/FcH couple at 100 mV/s in a very similar
electrochemical setup, which employed a 0.2 M solution of
[nBu4N][PF6] in CH2Cl2 as a supporting electrolyte.26 IR
compensation was achieved before each CV run by measuring
the uncompensated solution resistance followed by incremental
compensation and circuit stability testing. Background cyclic
voltammograms of the electrolyte solution were recorded before
adding the analytes. The half-wave potentials (E1/2) were
determined as averages of the cathodic and anodic peak
potentials of reversible couples and are referenced to the FcH+/
FcH couple.27 No significant difference (<0.01 V) between
external and internal referencing with FcH+/FcH was docu-
mented.
Com p u t a t ion a l Wor k . Electronic structure calculations
were conducted on the compounds 3 and 4 at the all-electron
density functional theory level using the Gaussian 98 program.
The computations were performed using Becke’s three-
parameter hybrid exchange functional28 with the LYP correla-
tion functional.29 The standard 6-31G split valence basis set30
was employed in both cases, including a single d polarization
function on all heavy atoms and a single p function on all
hydrogen atoms.31 The initial molecular geometries were
obtained by extracting the coordinates of single CNFc and
CNCm moieties from the crystal structures of 5‚CH2Cl2 and
6. The geometries of the C-N-C fragments were refined
through quantum chemical optimization at the theory level
described above. The ferrocenyl portion of CNFc was frozen
during the calculations, but in a subsequent validation step,
the unsubstituted ring was twisted by 36° relative to the
All structures were solved by direct methods and refined by
full-matrix least-squares methods on F2. Non-hydrogen atoms
were refined anisotropically. Hydrogen atom positions were
initially determined by geometry and refined by a riding model.
Hydrogen atom displacement parameters were set to 1.2 times
the displacement parameters of the bonded atoms. All cal-
culations employed the SHELXTL V5.0 suite of programs
(SHELXTL-Plus V5.0, Siemens Industrial Automation, Inc,
Madison, WI). In all cases, the data were corrected for
absorption by a semiempirical method.25 For 52+[BF4]2‚CH2Cl2,
a spurious peak of 3.17 e‚Å-3 was located 0.878 Å from Fe(6B)
in the final difference map. The above peak is likely to be a
consequence of a slight disorder of this iron atom. Notably,
the Fe-C distances observed for the corresponding ferro-
cenyl unit are very similar to those found in all other ferro-
cenyl moieties within 52+[BF4]2‚CH2Cl2. Crystal data, data
collection, solution, and refinement information for 5‚CH2Cl2,
6, 5+[V(CO)6]‚CH2Cl2, and 52+[BF4]2‚CH2Cl2 are summarized
in Table 1. Full description of the crystallographic work is
available in the Supporting Information. All thermal ellipsoids
are drawn at the 50% probability level.
Electr och em ica l Mea su r em en ts. Cyclic voltammetric
(CV) experiments on 2 × 10-3 M solutions of 2, 5, 6, and V(CO)6
in CH2Cl2 were conducted at room temperature using an
EPSILON (Bioanalytical Systems Inc., West Lafayette, IN)
electrochemical workstation. The electrochemical cell was
placed in an argon-filled Vacuum Atmospheres drybox. Tetra-
butylammonium hexafluorophosphate (0.1 M solution in CH2Cl2)
was used as a supporting electrolyte. Cyclic voltammograms
were recorded at 22 ( 2 °C using a three-component system
consisting of a platinum working electrode, a platinum wire
auxiliary electrode, and a glass encased nonaqueous silver/
silver chloride reference electrode. The reference Ag/Ag+
electrode was monitored with the ferrocenium/ferrocene couple.
(26) Zietlow, T. C.; Klendworth, D. D.; Nimry, T.; Salmon, D. J .;
Walton, R. A. Inorg. Chem. 1981, 20, 947-949.
(27) Conelly, N. G.; Geiger, W. Chem. Rev. 1996, 96, 877-910.
(28) Becke, A. D. J . Chem. Phys. 1993, 98, 5648-5652.
(29) (a) Miehlich, B.; Savin, A.; Stoll, H.; Preuss, H. Chem. Phys.
Lett. 1989, 157, 200-206. (b) Lee, C.; Yang W.; Parr, R. G. Phys. Rev.
B 1988, 37, 785-789.
Under the experimental conditions employed, ∆Epa
,
of the
pc
(25) Sheldrick, G. M. SADABS. Program for Empirical Absorption
Correction of Area Detector Data; University of Go¨ttingen, Germany,
2000.
(30) Hariharan, P. C.; Pople, J . A. Mol. Phys. 1974, 27, 209-214.
(31) Frisch, M. J .; Pople, J . A.; Binkley, J . S. J . Chem. Phys. 1984,
80, 3265-3269.

2932 Organometallics, Vol. 23, No. 12, 2004
Holovics et al.
Sch em e 1
Sch em e 2
substituted ring in order to confirm that ring rotations play a
negligible role in the structure and energetics of the Frontier
orbitals. All computational parameters not explicitly specified
above were set to their default values.
Sch em e 3
Resu lts a n d Discu ssion
Syn th etic Wor k . Several greatly improved syntheses
of aminoferrocene, which afford multigram quantities
of H2NFc in up to 52% yields starting from ferrocene,
have recently been published.14 Our facile preparation
of thermally stable, peach-colored isocyanoferrocene (2)
involved treatment of solid FcNH2 with a 65/35 mol %
mixture of phenyl formate/phenol, followed by dehydra-
tion of the resulting ferrocenylformamide (1) with
strictly 1.0 equiv of POCl3 in the presence of iPr2NH in
CH2Cl2 (Scheme 1). The superior formylating potency
of phenyl formate as compared to EtOC(O)H permitted
us to conduct the formylation step at ambient temper-
ature, thereby preventing previously encountered9a
extensive thermal decomposition of 1. A solution of
excess acetic-formic anhydride in CH2Cl2 proved equally
efficient in formylating aminoferrocene at ca. 20 °C.
While syntheses of isocyanides from the corresponding
formamides often employ excess dehydrating agents,1
it is imperative to avoid exposing 1 to even a slight
excess of POCl3 to ensure the essentially quantitative
conversion of 1 into 2. This observation might explain
the earlier report by others claiming that dehydration
of FcNHCHO with POCl3 afforded “variable, nonrepro-
ducible (25-90%) yields” of CNFc.9a
To demonstrate the feasibility of varying the transi-
tion metal moiety bound to the isocyanocyclopentadienyl
ring, synthesis of isocyanocymantrene (4) was sought.
The choice of the “Mn(CO)3” unit was in part influenced
by the fact that, among the cyclopentadienyl complexes
of metals, cymantrene derivatives are second only to
those of ferrocene in terms of synthetic applications.32
Aminocymantrene, the most obvious precursor to 4, can
be prepared via a laborious, six-step reaction sequence,
CmH f CmC(O)C6H4-p-Cl f CmC(O)OH f CmC(O)Cl
f CmC(O)N3 f CmNHC(O)OCH2Ph f CmNH2, in
11%-26% yields.24,33
Ta ble 2. Ch a r a cter istic IR,a 13C NMR,b a n d 14N
NMRc Da ta for 2 a n d 4 a t 25 °C
ν
CN, cm-1
δ{13CNR}, ppm
δ{C14NR}, ppm
2
4
2122
2133
174.0
168.9
172.1
164.3
a
b
In CH2Cl2. In CDCl3 vs Me4Si. c In CDCl3 vs liquid NH3.
yield (starting from cymantrene) after workup. In
dichloromethane solution at 25 °C, 3 exists as a 9:1
mixture of two conformational isomers, presumably due
to the restricted rotation35 around the Cm(H)N-C(O)H
bond. Dehydration of 3 under standard conditions
(Scheme 2) provided thermally and light sensitive, tan-
yellow 4. For long-term storage, compound 4 should be
cooled to ca. -30 °C to avoid substantial thermal
decomposition of its samples.
The entire synthesis of 4 from cymantrene can be
conveniently monitored by IR in the νCO, νNN, νNH, and
νCN stretching regions. Unlike CNPh,6,7 both 2 and 4
are air stable for practical purposes and do not show a
tendency to rearrange into the corresponding nitriles.
Isocyanides 2 and 4 can be easily distinguished from
FcCN and CmCN, respectively, on the basis of the
characteristic features in their IR (νCN), 13C NMR
(δ{CNR}), and 14N NMR (δ{CNR}) spectra (Table 2).
Combining 6 equiv of 2 with Cr(η6-naphthalene)222 in
THF afforded crystalline, orange-red Cr(CNFc)6 after
workup (5) (Scheme 3). Oxidation of 5 with 1.0 equiv of
AgBF4 in CH2Cl2 gave saddle-brown microcrystals of
[Cr(CNFc)6][BF4] (5+[BF4]), subsequent treatment of
which with another equivalent of AgBF4 in CH2Cl2
provided [Cr(CNFc)6][BF4]2 (52+[BF4]2) as a sparkling,
forest-green solid (Scheme 3). High yields of microcrys-
talline, scarlet Cr(CNCm)6 (6), lime-green [Cr(CNCm)6]-
[SbF6] (6+[SbF6]), and moss-green [Cr(CNCm)6][SbF6]2
(62+[SbF6]2) were obtained from 4 in a similar fashion
(Scheme 3).
In search for a more practical route to CmNH2, we
treated in situ-generated lithiocymantrene with tosyl
azide to afford somewhat thermally sensitive CmN3
(Scheme 2). Without isolation, cymantrenyl azide was
reduced to CmNH2 in a good yield using NaBH4.34 The
crude amine was then formylated with acetic-formic
anhydride to give pure CmNHC(O)H (3) in a 55% overall
Infrared and magnetic properties of Cr(CNR)6,
[Cr(CNR)6]+, and [Cr(CNR)6]2+ (R ) Fc, Cm) are fully
consistent with their formulations as low-spin, octahe-
dral complexes of Cr(0), Cr(I), and Cr(II), respectively
(Table 3). The energy of the “T1u”-like νCN band increases
upon going from 5 to 5+ to 52+. This indicates sequential
oxidation of the chromium rather than the iron atoms.
(32) Ginzburg, A. G. Chemistry of Cymantrene. Russ. Chem. Rev.
1993, 62, 1098-1118.
(33) Our attempts to scale-up the reported procedures by a factor
of 5 resulted in the overall yield reduction from 26 to 11%.
(34) Notably, recent attempts by others to reduce cymantrenyl azide
with NaBH4 have failed to generate any CmNH2: Su¨nkel, K.; Birk,
U.; Soheili, S.; Stramm, C.; Teuber, R. J . Organomet. Chem. 2000, 599,
247-255.
(35) Kessler, H. Angew. Chem., Int. Ed. Engl. 1970, 9, 219-235.

Organometallic Isocyanocyclopentadienides
Organometallics, Vol. 23, No. 12, 2004 2933
Ta ble 3. IRa a n d Ma gn etic Da ta b for 5z a n d 6z (z )
0, 1+, 2+)
ν
CN, cm-1
ν
CO, cm-1
µeff(25 °C), µB
5
6
1971
1947
2053
2073
2131
2156
diamagnetic
diamagnetic
1.78
1.99
2.76
2030, 1937
2029, 1950
2034, 1961
5+ c
6+ d
52+ c
62+ d
3.01
a
b
d
In CH2Cl2. Solid state. c [BF4]- salt. [SbF6]- salt.
The same trend in νCN was documented for 6, 6+, and
2+. The C-NR stretching frequencies of 5, 5+, 6, and
6
6+ are significantly depressed with respect to those of
the corresponding free isocyanides, reflecting substan-
tial back-bonding interactions within these complexes.36
In addition, the energy of the asymmetric νCO band
increases by 11-13 cm-1 upon each one-electron oxida-
tion of 6 to successively generate 6+ and 62+. Thus, the
carbonyl stretching frequencies are quite sensitive to
the chromium oxidation state in [Cr(CNCm)6]z (z ) 0,
1+, 2+). Compounds 5+[BF4], 6+[SbF6], 52+[BF4]2, and
F igu r e 1. ORTEP (50%) diagram of 5‚CH2Cl2. Hydrogen
atoms and solvent are omitted for clarity. Selected bond
distances (Å) and angles (deg): Cr-C1 1.930(2), Cr-C2
1.941(2), Cr-C3 1.941(2), C1-N1 1.175(3), C2-N2 1.183(3),
C3-N3 1.177(3), av C-N-C 161.5.
by eqs 2 and 3 could not be detected at all, while the
νCO band at 1853 cm-1 due to [V(CO)6]- appeared more
prominent compared to that observed shortly after
mixing the reagents. Both of these observations are
consistent with slow thermal decomposition of V(CO)6
in dichloromethane at ambient temperature to generate
[V(CO)6]-.19
6
2+[SbF6]2 are paramagnetic both in the solid state and
in solution, and their magnetic moments are in accord
with low-spin d5 configuration of 5+ and 6+ and low-
spin d4 configuration of 52+ and 62+
.
The remarkable air, light, and thermal stabilities of
5, 6, 5+, 6+, 52+, and 62+ constitute compelling evidence
that the nonbenzenoid, η5-cyclopentadienyl substituents
in 2 and 4 are much closer to a benzenoid aryl rather
than an alkyl group in terms of their electronic influ-
ence. Indeed, species [Cr(CNAr)6]0,1+,2+ are generally air
and thermally stable in the solid state.36 On the
contrary, the only two binary alkyl isocyanides of Cr(0)
known, namely, Cr(CNtBu)6 and Cr(CNC6H11)6, are very
air and light sensitive.22b,37 Furthermore, species
[Cr(CNAlkyl)6]+ have been observed only electrochemi-
cally and are too thermally unstable to be isolated.38
Similar to [Cr(CNAr)6]2+, 52+ and 62+ are reluctant to
add another isocyanide ligand, whereas seven-coordi-
Cr ysta llogr a p h ic Stu d ies. Complexes 5z and 6z (z
) 0, 1+, 2+) are highly unusual owing to the incorpora-
tion of seven transition metals within relatively compact
ML6 motifs. In addition, compounds 5, 5+, and 52+
feature seven electroactive metal centers (vide infra).
To the best of our knowledge, only one other species
containing six ferrocenyl groups separated from the
central metal ion by no more than two atoms is known.39
The X-ray structure of 5‚CH2Cl2 is shown in Figure 1.10
The metric parameters of its nearly perfectly octahedral
Cr(CN)6 core are in accord with those observed for other
binary isocyanides of chromium(0).37,40 The average
C-N-C(Fc) angle in 5 is 162°. Such a degree of bending
at the nitrogen atom is also typical for zerovalent,
octahedral complexes of bulky aryl isocyanides (aryl )
2,6-Me2C6H4, 2,6-iPr2C6H4).40a,41 Notably, the average
C-N-C(tBu) angle in Cr(CNtBu)6 is 153°.37 The less
pronounced bending of aryl-substituted isocyanide ligands
in their low-valent complexes has been attributed to
partial delocalization of the back-donated electron den-
sity into the aromatic rings that can be described by
the linear resonance structures MdCdN+dAryl-.2a,b
t
nate [Cr(CNAlkyl)7]2+ (alkyl ) Bu, C6H11) are well
documented.38
Complex 5 was readily oxidized by vanadium(0)
hexacarbonyl in CH2Cl2 to afford 5+[V(CO)6] in a nearly
quantitative yield (eq 1). The electron transfer was
complete within minutes as judged by FTIR of the
reaction mixture. The spectroscopic characteristics of
the cations within 5+[V(CO)6] and 5+[BF4] are identical.
On the other hand, the FTIR spectrum of the mixture
obtained by combining equimolar solutions of 6 and
V(CO)6 in CH2Cl2 (eq 2) was dominated by bands due
to the neutral starting materials and indicated the
presence of only trace amounts of ions 6+ and [V(CO)6]-.
The same FTIR pattern in νCO and νCN regions was
generated by treating 6+[SbF6] with 1 equiv of [Et4N]-
[V(CO)6] (eq 3). Thus, the redox equilibrium shown in
eq 2 is greatly shifted to the left. Over time, the already
minute presence of cation 6+ in the mixtures described
(39) This is
a
Cd(II) complex of 1,4,7,10,13,16-hexa(ferrocenyl-
Lloris, J . M.;
(36) (a) Bullock, J . P.; Mann, K. R. Inorg. Chem. 1989, 28, 4006-
4011. (b) Treichel, P. M.; Essenmacher, G. J . Inorg. Chem. 1976, 15,
146-150.
methyl)-1,4,7,10,13,16-hexaaza-cyclooctadecane:
Martinez-Man˜ez, R.; Pardo, T.; Soto, J .; Padilla-Tosta, M. E. J . Chem.
Soc., Dalton Trans. 1998, 2635-2641.
(37) Acho, J . A.; Lippard, S. J . Organometallics 1994, 13, 1294-
(40) (a) Anderson, K. A.; Scott, B.; Wherland, S.; Willett, R. D. Acta
Crystallogr. 1991, C47, 2337-2339. (b) Ljungstro¨m, E. Acta Chem.
Scand. 1978, A32, 47-50.
1299.
(38) (a) Mialki, W. S.; Wigley, D. E.; Wood, T. E.; Walton, R. A. Inorg.
Chem. 1982, 21, 480-485. (b) Dewan, J . C.; Mialki, W. S.; Walton, R.
A.; Lippard, S. J . J . Am. Chem. Soc. 1982, 104, 133-136.
(41) Barybin, M. V.; Young, V. G., J r.; Ellis, J . E. J . Am. Chem. Soc.
2000, 122, 4678-4691, and references therein.

2934 Organometallics, Vol. 23, No. 12, 2004
Holovics et al.
F igu r e 2. ORTEP (50%) diagram of 6. Hydrogen atoms
and solvent are omitted for clarity. Selected bond distances
(Å) and angles (deg): Cr-C1 1.951(2), Cr-C2 1.910(2), Cr-
C3 1.930(2), Cr-C4 1.907(2), Cr-C5 1.926(2), Cr-C6
1.937(2), C1-N1 1.167(3), C2-N2 1.179(3), C3-N3 1.168(3),
C4-N4 1.193(3), C5-N5 1.178(3), C6-N6 1.175(3), C1-
N1-C10 169.6(2), C2-N2-C20 155.6(2), C3-N3-C30
176.3(3), C4-N4-C40 139.5(2), C5-N5-C50 157.3(2), C6-
N6-C60 173.7(2).
F igu r e 3. ORTEP (50%) diagram of 5+[V(CO)6]‚CH2Cl2.
Hydrogen atoms and solvent are omitted for clarity.
Selected bond distances (Å) and angles (deg): Cr-C1
1.971(3), Cr-C2 1.994(3), Cr-C3 1.979(3), Cr-C4 1.957(3),
Cr-C5 1.963(3), Cr-C6 1.971(3), V-C71 1.935(3), V-C72
1.959(3), V-C73 1.961(3), V-C74 1.974(3), V-C75 1.953(3),
V-C76 1.953(3), C1-N1 1.158(3), C2-N2 1.157(3), C3-
N3 1.164(3), C4-N4 1.160(3), C5-N5 1.159(3), C6-N6
1.163(3), C71-O1 1.148(4), C72-O2 1.150(3), C73-O3
1.151(4), C74-O4 1.147(4), C75-O5 1.160(4), C76-O6
1.149(4), C1-N1-C10 167.4(3), C2-N2-C20 176.9(3), C3-
N3-C30 176.2(3), C4-N4-C40 167.0(3), C5-N5-C50
169.7(3), C6-N6-C60 169.9(3).
Ta ble 4. Selected Aver a ge Metr ic P a r a m eter s for
6, 5, 5+, a n d 52+a
Cr-C, Å
C-NR, Å
C-N-C, deg
6
5
1.93(2)
1.18(1)
162(14)
162(4)
171(4)
175(3)
1.937(7)
1.972(13)
2.019(17)
1.178(5)
1.160(3)
1.150(5)
5+
52+ b
a
Numbers in parentheses constitute the standard deviations
increase upon oxidation of 5 to 5+. These facts imply a
lower extent of back-bonding within less electron-rich
5+ as compared to 5. The Cr-C, C-NR, and C-N-C(R)
values obtained for 5+ are very similar to those recently
determined by us for [Cr(CN6Az)6]+ (6Az ) 6-azulenyl),
another binary Cr(I) complex of a nonbenzenoid aro-
matic isocyanide.13 The V-C bond lengths within the
nearly octahedral anion of 5+[V(CO)6] vary from 1.935(3)
to 1.974(3) Å and are statistically shorter than those
reported for neutral V(CO)6 (1.993(2)-2.005(2) Å).43 On
the other hand, they are either indistinguishable from
or only marginally longer than the V-C distances found
for several other structurally characterized salts of
[V(CO)6]-.44 The C-O bond lengths in 5+[V(CO)6]
(1.147(4)-1.160(4) Å) are well within the range of the
C-O distances previously documented for [V(CO)6]-.44
Compound 5+[V(CO)6] belongs to a fundamentally in-
triguing class of ion pairs A+|[V(CO)6]-, some of which
have been shown to undergo interionic electron transfer
upon photoexcitation.45
b
of the mean. Both crystallographically independent cations
within 52+[BF4]2‚CH2Cl2 are considered.
The shortest Fe‚‚‚Fe distance within 2 is 6.29 Å, which
is only 0.93 Å greater than that in zwitterionic Fc4B.42
Figure 2 displays the molecular structure of 6. While
the average Cr-C and C-NCm bond distances and
C-N-C angle for 6 are virtually identical to the
corresponding values found for 5, complex 6 exhibits
greater variations among the individual parameters of
the same chemical nature (Figure 2, Table 4). The lack
of crystallographically imposed symmetry in 6 coupled
with the somewhat greater bulk of CNCm compared to
CNFc may account for the above observation. A very
similar phenomenon was noted for the structure of
Cr(CNtBu)6, in which all bulky CNtBu ligands are
crystallographically unique.37 Indeed, in Cr(CNtBu)6,
the Cr-C distances vary from 1.87(1) to 1.98(1) Å and
the C-N-C angles range from severely bent, 136.1(9)°,
to nearly linear, 175(1)°.37 In both 6 and Cr(CNtBu)6,
smaller C-N-C angles correspond to shorter Cr-C
bonds. The shortest Cr‚‚‚Mn and Mn‚‚‚Mn distances
within 6 are 5.61 and 6.14 Å, respectively.
One of two crystallographically unrelated, 16-electron
dications 52+ within 52+[BF4]2‚CH2Cl2 is shown in
Figure 4. As summarized in Table 4, the Cr-C bonds
continue to elongate, while the C-NFc bonds be-
Crystallographic characterization of 5+[V(CO)6] (Fig-
ure 3) confirmed the electron transfer from 5 to V(CO)6
upon mixing the two electroneutral compounds (eq 1).
The data for 5+ are very similar to those recently
determined by us for [Cr(CN2Az)6]+ (2Az ) 2-azulenyl).
Table 4 shows that the Cr-C bonds lengthen, the
C-NFc distances shorten, and the C-N-C(Fc) angles
(43) Bellard, S.; Rubinson, K. A.; Sheldrick, G. M. Acta Crystallogr.
1979, B47, 271-274.
(44) (a) Doyle, G.; Eriksen, K. A.; Van Engen, D. Organometallics
1985, 4, 2201-2206. (b) Calderazzo, F.; Pampaloni, G.; Lanfranchi,
M.; Pelizzi, G. J . Organomet. Chem. 1985, 296, 1-13. (c) Calderazzo,
F.; Pampaloni, G.; Vitali, D.; Zanazzi, P. F. J . Chem. Soc., Dalton Trans.
1982, 1993-1997. (d) Silverman, L. D.; Corfield, P. W. R.; Lippard, S.
J . Inorg. Chem. 1981, 20, 3106-3109. (e) Wilson, R. D.; Bau, R. J .
Am. Chem. Soc. 1974, 96, 7601-7602.
(42) Cowan, D. O.; Shu, P.; Hedberg, F. L.; Rossi, M.; Kistenmacher,
T. J . J . Am. Chem. Soc. 1979, 101, 1304-1307.
(45) Marlin, T. W.; Homoelle, B. J .; Spears, K. G. J . Phys. Chem. A
2002, 106, 1152-1166.

Organometallic Isocyanocyclopentadienides
Organometallics, Vol. 23, No. 12, 2004 2935
Figu r e 6. Cyclic voltammogram of 6 in 0.1 M [nBu4N][PF6]/
CH2Cl2 vs FcH/FcH+. Scan rate ) 100 mV/s.
Ta ble 5. E1/2 P oten tia lsa (in V) for [Cr (CNR)6]z/z+1
ver su s [F cH]0/[F cH]+
F igu r e 4. ORTEP (50%) diagram of 52+[BF4]2‚CH2Cl2. The
[BF4]- anions, solvent, and hydrogen atoms are omitted
for clarity. One of two independent cations is shown.
Selected bond distances (Å) and angles (deg): CrA-C1A
2.043(4), CrA-C2A 2.010(4), CrA-C3A 1.974(7), CrA-C4A
2.035(4), CrA-C5A 2.013(4), CrA-C6A 2.004(4), C1A-N1A
1.158(5), C2A-N2A 1.149(5), C3A-N3A 1.155(4), C4A-
N4A 1.154(4), C5A-N5A 1.151(5), C6A-N6A 1.151(5),
C1A-N1A-C10A 169.8(4), C2A-N2A-C20A 175.5(4),
C3A-N3A-C30A 171.7(4), C4A-N4A-C40A 175.2(4),
C5A-N5A-C50A 178.2(5), C6A-N6A-C60A 176.5(4).
R
couple
C6H11
Fc
Phc
Cm
b
[Cr(CNR)6]0/1+
-1.54
-0.77
-0.97
-0.35
-0.83
-0.21
-0.53
-0.04
[Cr(CNR)6]1+/2+
All measurements were performed in CH2Cl2/[nBu4N][PF6] to
a
b
ensure quantitative comparison. Ref 48. c Ref 49a.
electron-rich Cr0/CrI systems, whereas the relative
contribution of σ(RNCfCr) donation to the donor/
acceptor ratio of CNR should increase upon oxidation.
Taking into account both data rows of Table 5, one may
suggest that CNCm is a significantly stronger π-accep-
tor and, perhaps, a slightly weaker σ-donor than CNFc.
To further corroborate this conclusion, we considered
DFT-generated molecular orbitals of 2 and 4. From
Figure 7 it is apparent that virtual MOs of 4 capable of
back-bonding (LUMO, LUMO+3, and LUMO+6; LUMO
) lowest unoccupied molecular orbital) show substan-
tially greater stabilization compared to the correspond-
ing virtual MOs of 2 (LUMO, LUMO+2, LUMO+3),
thereby making CNCm a stronger π-acid than CNFc.
In addition, the HOMO-7 (HOMO ) highest occupied
molecular orbital) of 4, localized primarily on the
terminal carbon atom (lone pair) and antibonding with
respect to the C-NR bond, is ca. 0.5 eV more stabilized
than the similar molecular orbital (HOMO-7) of 2 as
illustrated in Figure 7. Therefore, CNCm would indeed
be a somewhat weaker σ-donor than CNFc.
The difference in reactivity of 5 and 6 toward V(CO)6
is easily rationalized by comparing E1/2 of the couples
5/5+ and 6/6+ (Table 5) to that of V(CO)6/[V(CO)6]-
under identical electrochemical conditions. We observed
a half-wave potential of -0.54 V (∆Epa,pc ) 117 mV) for
the V(CO)6/[V(CO)6]- process in CH2Cl2/[nBu4N][PF6],50
which is certainly high enough to ensure essentially
complete27 oxidation of 5 to 5+ according to eq 1. On
the contrary, only partial oxidation of 6 by V(CO)6 might
be expected at best. Of note, applying the Nernst
equation to the quasi-reversible systems 6/6+ and V(CO)6/
[V(CO)6]- and approximating their E°′ potentials with
the corresponding E1/2 values27 leads to an underesti-
mation of the completeness of the actual chemical redox
process described by eq 2.
Figu r e 5. Cyclic voltammogram of 5 in 0.1 M [nBu4N][PF6]/
CH2Cl2 at negative potentials vs FcH/FcH+. Scan rate )
100 mV/s.
come shorter in the oxidation process 5 f 5+ f 52+
.
Thus, the first two oxidations of 5 are chromium-
centered. The trends in the bond distances and angles
presented in Table 4 nicely parallel those documented
for [Cr(CNPh)6]z (z ) 0, 1+, 2+)40b,46 and the isoelec-
tronic series [V(CN-2,6-Me2C6H3)6]z (z ) 1-, 0, 1+).41
Electr och em ica l a n d DF T Stu d ies. Cyclic voltam-
mograms of 5 and 6 in CH2Cl2/[nBu4N][PF6] solutions
exhibit two one-electron, successive, quasi-reversible47
anodic waves (∆Epc,pa ) 79-88 mV at ν ) 100 mV/s,
ic/ia ≈ 1.0; Figures 5 and 6). These waves correspond to
the chromium-centered oxidations consecutively gener-
ating the cations [CrI(CNR)6]+ and [CrII(CNR)6]2+ (R )
Fc, Cm). The electrochemical data summarized in Table
5 imply that the donor/acceptor ratio2b of the isocyanide
ligands decreases substantially in the order CNC6H11
, CNFc < CNPh , CNCm.
The influence of π-back-bonding on E1/2 values of the
The above electrochemical analysis indicates that the
donor/acceptor characteristics of isocyanoferrocene are
Crz/Crz+1 couples should be greatest in the case of the
(46) Bohling, D. A.; Mann, K. R. Inorg. Chem. 1984, 23, 1426-1432.
(47) Qualitatively, these waves can be considered reversible given
that, under the experimental conditions employed, reversible couple
FcH/FcH+ exhibits ∆Epc,pa ) 89 mV at ν ) 100 mV, and this peak-to-
peak separation increases upon increase in the scan rate. See also ref
26.
(48) Mialki, W. S.; Wigley, D. E.; Wood, T. E.; Walton, R. A. Inorg.
Chem. 1982, 21, 480-485.
(49) (a) Bullock, J . P.; Mann, K. R. Inorg. Chem. 1989, 28, 4006-
4011. (b) Treichel, P. M.; Essenmacher, G. J . Inorg. Chem. 1976, 15,
146-150.
(50) See Supporting Information.

2936 Organometallics, Vol. 23, No. 12, 2004
Holovics et al.
Figu r e 8. Cyclic voltammogram of 5 in 0.1 M [nBu4N][PF6]/
CH2Cl2 vs FcH/FcH+ showing all accessible oxidation steps.
Scan rate ) 100 mV/s.
non-diffusion-controlled process involving the oxidation
product adsorbed or precipitated onto the electrode
surface.53 Such an adsorption was very likely to be a
consequence of poor solubility of the highly charged
cation [Cr(CNFc)6]8+ in CH2Cl2. Upon reduction of the
surface-confined octa-cation [Cr(CNFc)6]8+ on the re-
verse scan, the resultant 52+ dissolved and was further
reduced under diffusion control to sequentially generate
5+ and 5. Importantly, practically no loss of current was
documented for the Cr-centered reductions as compared
to the corresponding oxidation processes on the forward
scan. The cyclic voltammogram displayed in Figure 8
remained essentially unchanged upon completion of
several full cycles. The average E1/2 potential of the
Fe(II)/Fe(III) transformations for [Cr(CNFc)6]z/z+1 is
estimated to be 0.26 eV. Without commenting on the
thermodynamic significance of this value, we note that
it is very similar to E1/2 ) 0.25 eV (∆Epa,pc ) 150 mV at
ν ) 100 mV/s) observed in the electrochemical oxidation
of free CNFc under the same conditions.
Mu ltin u clea r P a r a m a gn etic NMR Stu d ies. The
paramagnetic complexes 5+, 6+, 52+, and 62+ give
F igu r e 7. Selected molecular orbitals of 2 (left) and 4
(right) and their corresponding energies calculated at the
6-31G (D, F) level. For clarity, structures of orbitals with
no appreciable density on the isocyano group are not
shown.
1
relatively narrow H, 13C, and 14N NMR signals. This
2
3
fact is consistent with their ideal T (5+ and 6+) and T
(52+ and 62+) ground states characterized by short
electron spin relaxation times, T1e.54,55 The 1H para-
magnetic shifts for 5z and 6z (z ) 1+, 2+), determined
relative to the chemical shifts of the corresponding
nuclei in the diamagnetic zerovalent 5 or 6, exhibit
approximately Curie behavior at 200 K < T < 300 K
(i.e., ∆δ 1/T and ∆δ{T)∞} ≈ 0) and are practically
contact in origin because of the high symmetry of the
complexes (e.g., Figure 9). Small J ahn-Teller distor-
tions expected for these cations are very likely to be
dynamic on the NMR time scale.54
much closer to those of aryl rather than alkyl iso-
cyanides. In accord with this finding, the E1/2 potential
of the elusive [Cr(CNMe)6]0/1+ couple in CH2Cl2 was
predicted to be -1.67 eV vs FcH/FcH+,51 which is 0.7
eV more negative than that of the [Cr(CNFc)6]0/1+ sys-
tem. Thus, isocyanoferrocene is a substantially stronger
π-acceptor compared to isocyanomethane, so the previ-
ously suggested9b,c electronic similarity between CNFc
and CNMe only applies to the donor properties of these
isocyanides. The enhanced π-acidity of 2 with respect
to CNMe is associated with the possibility of partial
delocalization of back-donated electron density into the
ferrocenyl moieties of CNFc. Figure 7 illustrates that
CNFc’s LUMO and LUMO+2 are especially suited for
such an interaction.
1
The H paramagnetic shifts for the C5H4 rings of 5z
and 6z (z ) 1+, 2+) are large and occur in both
directions (Table 6). This suggests unpaired spin delo-
calization within their π-systems.54 Given that the
nature of the substituent R′ in diamagnetic (η5-C5H4R′)-
Application of higher potentials to a solution of 5
produced a broad anodic peak at Ep,a ) 0.42 eV (Figure
8) with a shape indicative of a diffusion-controlled
process. This wave corresponds to the oxidation of six
(52) For the [Cr(CNFc)6]z system, the Cr(II)/Cr(III) oxidation (if it
is possible at all) is expected to occur at a much more positive potential
as compared to the Fe(II)/Fe(III) processes. See ref 49.
(53) Bond, A. M.; Colton, R.; Mahon, P. J .; Snook, G. A.; Tan, W. T.
J . Phys. Chem. B 1998, 102, 1229-1234.
(54) LaMar, G. N., Horrocks, W. D., J r., Holm, R. H., Eds. NMR of
Paramagnetic Molecules; Academic Press: New York, 1973.
(55) For a discussion of theory of the paramagnetic shift for strong
field d5 complexes, see: McGarvey, B. R.; Batista, N. C.; Bezerra, C.
W. B.; Schultz, M. S.; Franco, D. W. Inorg. Chem. 1998, 37, 2865-
2872, and references therein.
ferrocenyl substituents in 52+ 52
Interestingly, a larger
.
response was observed on the reverse cathodic scan. The
symmetrical shape of the reduction wave suggests a
(51) Bursten, B. E. J . Am. Chem. Soc. 1982, 104, 1299-1304.

Organometallic Isocyanocyclopentadienides
Organometallics, Vol. 23, No. 12, 2004 2937
F igu r e 10. Observed directions of the 1H, 13C, and 14N
paramagnetic shifts for the nuclei in [Cr(CNFc)6]+/2+ (left),
[Cr(CNCm)6]+/2+ (center), and [Cr(CN2Az)6]+ (right). The
symbols “-” and “+” denote upfield and downfield shifts,
respectively, relative to chemical shifts of the nuclei in the
corresponding diamagnetic zerovalent Cr(CNR)6 or free
CNR.
The presence of unpaired spin in the π-system of the
aromatic substituent of a coordinated ligand is not
necessarily evidence for metal-ligand π-covalency.54,57,58
Nevertheless, we have recently demonstrated through
DFT studies that M(dπ)fL(pπ*) back-bonding is indeed
an important contributor to the mechanism of unpaired
spin delocalization in binary isocyanide complexes of
Cr(I).13 Because of correlation effects, ligand-to-metal
σ-interaction would favor placing excess negative spin
on the ligating isocyanide carbon atoms in 5z and 6z (z
) 1+, 2+).41 The unpaired spin generated in this fashion
may enter the π-system of the ligands via the atomic
exchange coupling process (vide supra) operating in
reverse. A similar mechanism was originally proposed
by Drago and Fitzgerald,58a who doubted that un-
paired spin density in the π-systems of the aryl sub-
stituents in [Ni(NCPh)6]2+ was a consequence of nickel-
benzonitrile π-bonding.59 Thus, one has to be cautious
invoking metal-ligand π-covalency to explain the NMR
patterns obtained for complexes 52+ and 62+, which ex-
hibit only marginal M(dπ)fL(pπ*) interactions at best.
F igu r e 9. Variable-temperature 1H NMR spectra of
5+[BF4] in CD2Cl2 at 500 MHz.
Ta ble 6. 1H, 13C, a n d 14N NMR P a r a m a gn etic
Sh iftsa (in p p m ) for 5+, 6+, 52+, a n d 62+
H2,5
H3,4
C5H5
C2,5
C3,4 C5H5
CO
N
5+ b -2.95 +1.55 -0.36
+41.6
+66.3
+90.9 +29.7 +1.8
+128.6 +11.4
+9.9 +0.9
+6.3
+553.1
-7.5 +697.1
+734.0
6+ c -4.7
+1.32
52+ b -5.31 +3.57 -0.51
62+ c -8.67 +1.83
-22.4
852.2
a
In CD2Cl2 at 25 °C, see Schemes 1 and 2 for atom-labeling
legend. vs 5. c vs 6.
b
Mn(CO)3 has very little effect on the 13C chemical shift
of the carbonyl groups (e.g., compare the 13C NMR
spectra of 3, 4, and 6 reported herein), substantial 13C
paramagnetic shifts observed for the carbonyl carbon
nuclei in 6+ and 62+ are noteworthy (Table 6). The
magnitudes of these paramagnetic shifts are also con-
sistent with “through-bond” rather than “through-space”
interactions between the CO groups and the chromium
Con clu d in g Rem a r k s
centers in 6+ and 62+
.
Efficient syntheses of isocyanoferrocene and iso-
cyanocymantrene from readily available starting ma-
terials have been established. These compounds repre-
sent an emerging new class12,13 of aromatic isocyanides
incorporating nonbenzenoid π-systems. The physical,
chemical, electrochemical, and spectroscopic properties
of the structurally characterized series [Cr(CNFc)6]0,1+,2+
indicate that the electronic influence of the ferrocenyl
moiety, often considered to be similar to an alkyl group,
is more similar to that of aryl substituents. Conse-
quently, the donor/acceptor characteristics of the CNFc
ligand are close to those of aryl isocyanides but quite
different from those of alkyl isocyanides, including
CNMe. In addition, we demonstrated that electronic
properties (especially π-acidity) of the isocyanocyclo-
pentadienide ligand can be tuned to a substantial extent
by varying the nature of the metal fragment bound to
its ring. Redox chemistry of [Cr(CNFc)6]z (z > 2),
carbonyl substitution transformations of [Cr(CNCm)6]z
(z ) 0, 1+, 2+), and chemistry of other hitherto
unknown organometallic isocyanocyclopentadienides are
For benzenoid π-systems, unpaired spin in the p
orbital of an aromatic carbon atom polarizes (unpairs)
electrons of the corresponding C(sp2)-H σ-bond via an
atomic exchange coupling mechanism.54 This leads to
paramagnetic shifts of the 13C and the corresponding
1H resonances occurring in opposite directions in the
absence of substantial pseudo-contact interactions. Also,
the unpaired spin signs and, hence, contact shifts
alternate throughout benzenoid aromatic frameworks.54
Figure 10 summarizes directions of the paramagnetic
1
shifts observed for the H, 13C, and 14N nuclei in 5z and
6z (z ) 1+, 2+). Surprisingly, the 3,4-13C nuclei of the
nonbenzenoid C5H4 rings in 5z and 6z (z ) 1+, 2+)
undergo downfield paramagnetic shifts, which is in
conflict with negative unpaired spin density in the
p orbitals of these carbon atoms predicted from the
1H NMR spectra of the complexes. This discrepancy
might be due to the fact that the 13C paramagnetic shift
of a carbon atom within an aromatic scaffold depends
not only on the unpaired spin density in its p orbital
(which has zero coefficient at the nucleus) but also on
spin densities at the neighboring carbon nuclei.54 Such
an explanation is supported by the observation that
the resonances for carbon atoms at the junctions of five-
and seven-membered rings in [Cr(CN2Az)6]+ (2Az )
2-azulenyl), which are analogous to the 3,4-C atoms in
5+ and 6+, do exhibit upfield paramagnetic shifts, as
expected (Figure 10).50,56
(56) Robinson, R. E.; Holovics, T. C.; Deplazes, S. F.; Barybin, M.
V. Manuscript in preparation.
(57) Drago, R. S. Physical Methods in Chemistry; Surfside Scientific
Publishers: Gainesville, FL, 1992; Chapter 12.
(58) (a) Fitzgerald, R. J .; Drago, R. S. J . Am. Chem. Soc. 1967, 89,
2879-2883. (b) La Mar, G. N.; Sherman, E. O.; Fuchs, G. A. J . Coord.
Chem. 1972, 1, 289-296.
(59) Kluiber, R. W.; Horrocks, W. D., J r. Inorg. Chem. 1966, 5, 152-
154.

2938 Organometallics, Vol. 23, No. 12, 2004
Holovics et al.
Su p p or tin g In for m a tion Ava ila ble: Complete tables of
atomic coordinates, bond distances, angles, and anisotropic
displacement parameters for 5‚CH2Cl2, 6, 5+[V(CO)6]‚CH2Cl2,
and 52+[BF4]2‚CH2Cl2; cyclic voltammogram of [Et4N][V(CO)6];
1H, 13C, and 2D NMR spectra of [Cr(CN2Az)6][BF4]. This
material is available free of charge via the Internet at
http://pubs.acs.org.
currently under investigation and will be reported in
due course.
Ack n ow led gm en t. Financial support of this work
was provided by Kansas NSF EPSCoR, Kansas Tech-
nology Enterprise Corporation, and the University of
Kansas. M.V.B. is grateful to KU Center for Research
for the New Faculty GRF Award and to Professor David
R. Benson for fruitful discussions and suggestions
during manuscript preparation.
OM049842N