Syntheses and structural characterizations of the first 16-, 17-, and 18-electron homoleptic isocyanide complexes of vanadium: Hexakis(2,6-dimethyl-phenyl isocyanide)vanadium(I, 0, -I)1

Full text of DOI:10.1021/ja9729239

J. Am. Chem. Soc. 1998, 120, 429-430
429
homoleptic isocyanide of vanadium is the dicationic species
[V(CN-t-Bu)6]2+ 15
Syntheses and Structural Characterizations of the
First 16-, 17-, and 18-Electron Homoleptic Isocyanide
Complexes of Vanadium: Hexakis(2,6-dimethyl-
phenyl isocyanide)vanadium(I, 0, -I)1
.
The recent availability by a conventional route of the highly
labile (naphthalene)vanadium(0) complexes, V(η6-C10H7R)2 (R
) H, Me),16 which function as useful sources of atomic vanadium
in chemical reactions,16b proved to be of crucial importance in
our initial synthesis of 2, from which 1 and 3 were derived in
nearly quantitative yields. In a typical preparation of 2, a dark
red-brown solution of bis(1-methylnaphthalene)vanadium (2.22
g, 6.62 mmol) in heptane/THF (130 mL/130 mL, 0 °C) was mixed
with 2,6-dimethylphenyl isocyanide (6.11 g, 46.6 mmol) in
heptane/THF (70 mL/30 mL, 0 °C), whereupon a deep purple
solution rapidly formed. The reaction mixture was stirred for 12
h, and removal of about 150 mL of the solvent mixture in vacuo
gave beautiful dichroic green-purple microcrystals. These were
separated by filtration, washed with pentane, dried in vacuo, and
recrystallized from THF/heptane to afford 4.50 g (81% yield) of
very air-sensitive and golden-purple 2. Our most efficient
conversion of 2 to 1 involved stirring a solution of 2 in THF
with a suspension of cesium graphite, CsC8,17 at -50 °C.
Following filtration and recrystallization from THF/heptane, an
84% yield of unsolvated cesium salt of 1 was obtained as free
flowing and very air-sensitive brown microcrystals. Oxidation
of 2 with 1 equiv of [Fe(C5H5)2][PF6] in THF at -70 °C gave a
deep orange-red solution of 3. Following solvent removal and
recrystallization from THF/heptane, a 92% yield of deep red and
moderately air-sensitive, microcrystalline 3[PF6] was isolated as
a pure substance.18
Mikhail V. Barybin, Victor G. Young, Jr., and John E. Ellis*
Department of Chemistry, UniVersity of Minnesota
Minneapolis, Minnesota 55455
ReceiVed August 19, 1997
There has been considerable interest in exploring the properties
of neutral and anionic homoleptic organic isocyanide metal
complexes with the hope that these species might prove to be
useful in organic, organometallic, and inorganic syntheses, as are
analogous metal carbonyls.1-2 Corresponding isocyanides have
received far less attention due to their relative inaccessibility,
especially for the very early transition metals. For example,
neutral isocyanide complexes of this type are well established
for many of the group 6-10 transition metals,3 but only recently
have Cooper and co-workers devised syntheses for the first anionic
homoleptic isocyanide complexes of the group 7-9 metals.4-6
Although neutral (M ) V) or anionic (M ) V, Nb, Ta) pure
carbonyls of the group 5 transition metals have been known for
many years, prior attempts to access analogous isocyanide
complexes of these elements have been unsuccessful.7 For
example, procedures that work well for the syntheses of other
[ML6]- (M ) V, Ta; L ) CO, PF3),8 related neutral group 6
metal isocyanides, M(CNR)6,9 or [Co(CNAr)4]- (Ar ) 2,6-
dimethylphenyl), the first reported homoleptic isonitrilate,10
invariably failed with VCl3(THF)3 and CNAr and provided only
uncharacterized tarry isocyanide reduction products.11 We now
report on the syntheses, isolation, and structural characterization
of the initial vanadium compounds of this class, as well as the
previously unknown monocation, [V(CNAr)6]Z [Z ) -1 (1), 0
(2), and +1 (3)]. Compound (2) is, perhaps, of most interest in
that it is presently the only known stable 17-electron metal(0)
isocyanide complex and is thereby analogous to the long-
established V(CO)6.12 Compounds 1 and 3 are also the first
isocyanide analogues of the stable [V(CO)6]- 13 and exceedingly
unstable [V(CO)6]+,14 respectively. The only prior example of a
Spectroscopic and magnetic properties of 1, 2, and 3 are fully
in accord with their formulations as mononuclear six-coordinate
complexes containing low-spin d6, d5, and d4 vanadium, respec-
tively. For example, Cs 1 is diamagnetic in solution and the solid
1
state and has H and 13C NMR spectra for equivalently bound
2,6-dimethylphenyl groups18 that closely resemble those of
Yamamoto’s isoelectronic Cr(0) complex.9 Mineral oil mull
infrared spectra in the ν(CN) region show one intense peak with
a frequency (1802 cm-1) that is very similar to that previously
reported for [Co(CNAr)4]-.4 Exposure of solutions of 1 to trace
amounts of air caused it to be rapidly converted to 2.
Compound 2 is paramagnetic in THF and in the solid state
with µeff ) 1.76 µB (298 K), a value that is virtually identical
with µeff ) 1.78 µB reported for V(CO)6.19 While V(CO)6
undergoes facile disproportionation in THF20 and is easily reduced
by cobaltocene or decamethylcobaltocene,21 2 is unreactive toward
these species at room temperature. As in the case of V(CO)6,22
2 is ESR silent at room temperature. Also, sharp NMR signals
are observed for 2 in C6D6. Both of these features are consistent
(1) Highly Reduced Organometallics. Part 46. For Part 45, see: Fischer,
P. J.; Ahrendt, K. A.; Young, V. G., Jr.; Ellis, J. E. Organometallics In press.
(2) (a) Cotton, F. A.; Wilkinson, G. AdVanced Inorganic Chemistry, 5th
ed.; J. Wiley and Sons: New York, 1988. (b) Collman, J. P.; Hegedus, L. S.;
Norton, J. R.; Finke, R. G. Principles and Applications of Organotransition
Metal Chemistry; University Science Books: Mill Valley, CA, 1987. (c)
Hegedus, L. S. Transition Metals in the Synthesis of Complex Organic
Molecules; University Science Books: Mill Valley, CA, 1994.
(3) Yamamoto, Y. Coord. Chem. ReV. 1980, 32, 193.
(4) Warnock, G. F.; Cooper, N. J. Organometallics 1989, 8, 1826.
(5) Corella, J. A.; Thompson, R. L.; Cooper, N. J. Angew. Chem., Int. Ed.
Engl. 1992, 31, 83.
(6) Utz, T. L.; Leach, P. A.; Geib, S. J.; Cooper, N. J. Chem. Commun.
1997, 947.
(7) Barybin, M. V.; Ellis, J. E. Unpublished research.
(8) (a) Barybin, M. V.; Pomije, M. K.; Ellis, J. E. Inorg. Chim. Acta In
press. (b) Ellis, J. E.; Warnock, G. F.; Barybin, M. V.; Pomije, M. K. Chem.
Eur. J. 1995, 1, 521.
(9) Yamamoto, Y.; Yamazaki, H. J. Organomet. Chem. 1985, 282, 191.
(10) Leach, P. A.; Geib, S. J.; Corella, J. A.; Warnock, G. F.; Cooper, N.
J. J. Am. Chem. Soc. 1994, 116, 8566.
(11) 2,6-Dimethylphenyl isocyanide also reacted with sodium metal or
sodium naphthalenide in THF to form deep red tars, which had IR spectra
close to those of products obtained from reductions carried out in the presence
of VCl3(THF)3 and those reported for polymerized phenyl isocyanide:
Onitsuka, K.; Yonai, K.; Takei, F.; Joh, T.; Takahashi, S. Organometallics
1994, 13, 3862 and references therein.
(12) Natta, G. Ercoli, R.; Calderazzo, F.; Alberola, A.; Corradini, P.;
Allegra, G.; Atti Acad. Naz. Lincei, Classe Sci. Fis. Mater. Nat. 1959, 27,
107.
(13) Ercoli, R.; Calderazzo, F.; Alberola, A. J. Am. Chem. Soc. 1960, 82,
2966.
(14) Bond, A. M.; Colton, R. Inorg. Chem. 1976, 15, 2036.
(15) Silvermann, L. D.; Corfield, P. W. R.; Lippard, S. J. Inorg. Chem.
1981, 20, 3106.
(16) (a) Pomije, M. K.; Kurth, C. J.; Ellis, J. E.; Barybin, M. V.
Organometallics 1997, 16, 3582. (b) Barybin, M. V.; Ellis, J. E. Unpublished
research.
(17) Bergbreiter, D. E.; Killough, J. M. J. Am. Chem. Soc. 1978, 100, 2126.
(18) Satisfactory elemental analyses were obtained for 1-3. Selected
spectral data for Cs 1: IR ν(CN), (THF) 2028 vw br, 1823 vs br, 1772 sh br
cm-1 1H NMR (300 MHz, CD3CN, 25 °C) δ 2.39 (s, 6H, o-Me), 6.69 (t,
;
3JH-H ) 7.5 Hz, 1H, p-H), 6.90 (d, 3JH-H ) 7.5 Hz, 2H, m-H) ppm; 13C{1H}
NMR (75.5 MHz, CD3CN, 25 °C) 20.3 (CH3), 122.9 (p-C), 128.5 (m-C), 133.2
(o-C) ppm. For IR 2: ν(CN), (mineral oil mull) 2006 w sh, 1929 vs br, 1845
1
m sh; (THF) 2008 w sh, 1939 vs, 1952 w sh; H NMR (300 MHz, C6D6 25
3
°C) δ -2.26 (t, JH-H ) 7.3 Hz, 1H, p-H), 9.36 (s, 6H, o-Me), 13.84 (d,
3JH-H ) 7.3 Hz, 2H, m-H) ppm; 13C{1H} NMR (75.5 MHz, C6D6, 23 °C)
-4.9 (CH3), 101.5 (m-C), 176.9 (p-C) ppm. For 3 [PF6]: IR ν(CN), (THF)
2142 w, 2033 vs, 1995 m sh.
(19) Calderazzo, F.; Cini, R.; Corrandini, P.; Ercoli, R.; Natta, G. Chem.
Ind. (London) 1960, 79, 500.
(20) Hieber, W.; Winter, E.; Schubert, E. Chem. Ber. 1962, 95, 196.
(21) (a) Calderazzo, F.; Pampaloni, G. J. Chem. Soc., Chem. Commun.
1984, 1249. (b) Calderazzo, F.; Pampaloni, G.; Pelizzi, G.; Vitali, F.
Polyhedron 1988, 7, 2039.
S0002-7863(97)02923-5 CCC: $15.00 © 1998 American Chemical Society
Published on Web 01/01/1998

430 J. Am. Chem. Soc., Vol. 120, No. 2, 1998
Communications to the Editor
pendent V(CN-2,6-xylyl) units. Structures for 1 and 3 are
qualitatively similar to that of 2, except the vanadium atoms are
in general positions and counterions are present. Substantial
scatter is observed in the V-C distances in 1 and 3, vide infra;
however, the inner V(CN)6 cores of all three species are best
regarded as being essentially octahedral. For example, com-
pounds 1-3 have nearly linear VCN bonds and average cis and
trans C-V-C angles that are well within the range of values
expected for octahedral structures.27d Compound 2 exhibits a
possible small tetragonal distortion from a regular octahedron,
very similar in magnitude to that observed for V(CO)6.22c Average
V-C and nitrile C-N distances for 1-3 are 1.98(3), 2.026(7),
and 2.07(2) Å and 1.20(2), 1.186(5), and 1.169(6) Å, respectively.
The lengthening of the M-C distances and concomitant shorten-
ing of the nitrile C-N distances with increasing metal oxidation
state in proceeding from 1 to 3 are consistent with the aryl
isocyanides functioning as good acceptor ligands28 and correlate
nicely with the aforementioned infrared ν(CN) values observed
for 1-3.18 Exactly the same trends have been observed for the
isoelectronic chromium complexes [Cr(CNPh)6]Z, for Z ) 0 to
+2, respectively.26 Also noteworthy is the fact that the average
C-N distance for the vanadate 1 is statistically indistinguishable
from analogous values of 1.20(2) and 1.20(3) Å previously
reported for the other structurally characterized homoleptic
isocyanide metalates, [Mn(CNAr)5]- 6 and [Co(CNAr)4]-,10 re-
spectively. There is appreciable bending at the isocyanide
nitrogens in 1-3 with corresponding CNC angles averaging 158-
(10), 163(4), and 173(2)°, respectively. Counterion (Cs+) per-
turbation of 1 undoubtedly contributes to the large spread in CNC
angles (and to a lesser extent the structural parameters of the
V(CN)6 core) observed for the vanadate and makes a simple
analysis difficult at best.29 However, the CNC angles observed
for 2 and 3, which are relatively unperturbed by the [PF6]-
counterion, do correlate well with the corresponding V-C and
C-N distances and infrared ν(CN) values of these species in the
expected manner. These data support prior views that η1-RNC
ligands should become increasingly bent as the metal center
becomes more electron rich, provided other factors, such as the
steric bulk of the ligand, the coordination number of the metal,
and so on, remain essentially constant.28 Ultimately, this nitrile
bending may result in reductive coupling of adjacent coordinated
isocyanides, which was first observed by Lippard30 and is now a
well-known phenomenon of the early transition metals.30,31
Whether the bulky nature of the arylisocyanide ligands employed
in this study prevents the latter from occurring in 1-3 is an
important issue. Also, extension of this research to the group 4
and heavier group 5 elements is currently underway in this
laboratory.
Figure 1. Molecular structure of 2 showing the labeling scheme at 50%
probability ellipsoids; hydrogens are omitted for clarity. Selected bond
lengths (Å) and angles (deg): V-C(1) 2.034(3), V-C(2) 2.022(4),
V-C(3) 2.023(4), C(1)-N(1) 1.184(4), C(2)-N(2) 1.183(4), C(3)-N(3)
1.194 (4), N(1)-C(7) 1.400(4), N(2)-C(15) 1.408(4), N(3)-C(23) 1.406-
(4), av ring C-C 1.39(1), av ring C-Me 1.504(5) Å, V-C(1)-N(1)
175.9(3), V-C(2)-N(2) 175.6(3), V-C(3)-N(3) 178.5(3), C(1)-N(1)-
C(7) 168.1(3), C(2)-N(2)-C(15) 158.8(3), C(3)-N(3)-C(23) 161.5(3),
av cis C-V-C 90(3), trans C-V-C 180.0.
with the very short electron spin relaxation time expected for a
1
homoleptic octahedral low-spin d5 complex.23 The H and 13C
NMR resonances18 show strong paramagnetic shifts that are
essentially contact in origin due to the approximately octahedral
symmetry of the molecule.24 HETCOR NMR spectra allowed
unambiguous assignments of 13C NMR resonance positions of
the equivalent 2,6-dimethylphenyl groups present in 2. Solution
and mull infrared spectra of 2 are unexceptional and are similar
to those reported for the octahedral Cr(0) complex, Cr(CNAr)6.9
Compound 2 is rapidly oxidized in air to give 3, which is also
air sensitive, but much less so than 1 or 2.
Compound 3 is paramagnetic in the solid state, with µeff
)
2.73 µB (298 K), which is close to the corresponding value of
2.75 µB reported for the Cr(II) salt, [Cr(CNcyclohexyl)6][PF6]2,
also an octahedral low-spin d4 complex.25 The IR spectrum of 3
[PF6]18 is similar to that reported for the Cr(I) complex, [Cr-
(CNPh)6][PF6].26
Single-crystal X-ray studies on 1-3 were carried out to confirm
their existence and obtain structural data for these novel species.27
Figure 1 shows the molecular structure of 2, which contains a
vanadium atom on an inversion center resulting in three inde-
(22) (a) Pratt, D. W.; Myers, R. J. J. Am. Chem. Soc. 1967, 89, 6740. (b)
Rubinson, K. A. J. Am. Chem. Soc. 1976, 98, 5188. (c) Bellard, S.; Rubinson,
K. A.; Sheldrick, G. M. Acta Crystallogr., Sect. B. 1979, 35, 271.
(23) Drago, R. S. Physical Methods in Chemistry; W. B. Saunders:
Philadelphia, PA, 1977; Chapter 12.
Acknowledgment. We dedicate this article to Professor Roald
Hoffmann on the occasion of his 60th birthday. We thank the National
Science Foundation and donors of the Petroleum Research Fund,
administered by the American Chemical Society, for financial support
of this research. The authors are grateful to Professor Kent Mann for
useful discussions, Dr. Jason Halfen for assistance with the X-ray
structural characterization of 2, and Ms. Christine Lundby for expert
assistance in the preparation of this manuscript.
(24) (a) LaMar, G. N.; Horrocks, W. D., Jr.; Holm, R. H., Eds.; NMR of
Paramagnetic Molecules; Academic Press: New York, 1973. (b) The isotropic
shifts observed for 2 are consistent with predominantly dπ-π* unpaired
electron spin delocalization via spin polarization. Detailed analyses will be
reported separately: Barybin, M. V.; Ellis, J. E. Unpublished results.
(25) Wigley, D. E.; Walton, R. A. Organometallics 1982, 1, 1322.
(26) Bohling, D. A.; Mann, K. R. Inorg. Chem. 1984, 23, 1426.
(27) (a) Crystal data for Cs1: C54H54CsN6V, Mw ) 970.88, triclinic, P1h,
dark dichroic green-brown block, a ) 12.0077(2) Å, b ) 14.3698(1) Å, c )
17.1379(2) Å, R ) 66.534(1)°, â ) 79.878(1)°, γ ) 83.106(1)°, V ) 2666.23-
(5) Å3, Z ) 2, final residual, R ) 0.0524, with GOF ) 1.020 on F2. (b)
Crystal data for 2: C54H54N6V, Mw ) 837.97, triclinic, P1h, deep purple plate,
a ) 10.7588(3) Å, b ) 11.2539(2) Å, c ) 11.8350(3) Å, R ) 117.760(1)°,
â ) 109.184(1)°, γ ) 92.153(2)°, V ) 1166.05(5) Å3. Z ) 1, final residual,
R ) 0.0630, with GOF ) 1.010 on F2. (c) Crystal data for 3 [PF6]‚THF:
C58H62F6N6OPV, Mw ) 1055.05, monoclinic, P21/c, irregular red block, a )
15.6942 (4) Å, b ) 20.8924(6) Å, c ) 19.6343(5) Å, â ) 98.749(1)°, V )
6363.0(3) Å3, Z ) 4, final residual, R ) 0.0658, with GOF ) 1.067 on F2.
(d) For 1: av V-C-N 177(2)°, av cis C-V-C 91(3)°, av trans C-V-C
176.0 (7). For 2: av V-C-N 177(2)°, av cis C-V-C 90(3)°, av trans
C-V-C 180.0°. For 3: av V-C-N 176(2)°, av cis C-V-C 90(3)°, av trans
C-V-C 177(2).
Supporting Information Available: Complete crystallographic and
additional analytical data for 1-3, except structure factor tables (22 pages).
See any current masthead for ordering and Internet access instructions.
JA9729239
(28) Singleton, E.; Oosthuizen, H. E. AdV. Organomet. Chem. 1983, 22,
209.
(29) Details of the cesium cation-vanadate interaction are given in the
Supporting Information and will be discussed in a separate publication:
Barybin, M. V.; Ellis, J. E. Manuscript in preparation.
(30) Carnahan, E. M.; Protasiewicz, J. D.; Lippard, S. J. Acc. Chem. Res.
1993, 26, 90 and references therein.
(31) Collazo, C.; Rodewald, D.; Schmidt, H.; Rehder, D. Organometallics
1996, 15, 4884 and references therein.