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The First Complex with a 24 Triple Bond between Vanadium Atoms in a Ligand Framework of Fourfold SymmetryЧ[V2{(p-CH3C6H4)NC(H)N(p-C6H4CH3)}4].

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automated Enraf-Nonius CAD 4 diffractometer with Mo,, (0.71073 A)
radiation. The space group was Pi (No. 2), with a = 9.444(2) 8, b =
10.773(2) A, c =13.220(2) A, I= 68.72(1)", 1 = 81.49(1)'. y = 84.32(1)",
Y = 1238 A3, Z = 2, and pC4,=
= 1.240 gcm-'. Two standard reflections
(025, 312). measured every hour, gave ~ 2 intensity
deviation over the
duration of data collection. Of 6010 measured reflections, 4356 were observed and 3616 with I > 20(I) were used for structure solution and refinement employing the SDP computer programs by Enraf-Nonius. The final
cycle of least-squares refinement of 262 parameters gave residuals of
R = 0.058 and R , = 0.064 and greatest unassigned electron density of
details of the crystal structure investigation are
0.880 e- . k 3Further
available on request from the Fachinformationszentrum Karlsruhe,
Gesellschaft fur wissenschaftlich-technische Information mbH, D-W-7514
Eggenstein-Leopoldshafen 2, on quoting the depository number CSD56191, the names of the authors. and the journal citation.
[lo] For a thorough compilation of data of metal P-diketonate complexes see:
R. C. Mehrotra, R. Bohra. D. P . Gaur, Metal 1-Diketonates and Allied
Derivarives, Academic Press, New York, 1978.
Ill] J. A. T. Norman, G. P. Pez. J. Chem. Suc. Chem. Commun. 1991,971.
[12] a) P. Van der Sluis, A. L. Spek, K. Timmer, H. A. Meinema, Acta Cr.vslullo@-. Secl. C 46, 1990, 1741; b) R. Gardiner, D. W. Brown. P. S. Kirlin,
A. L. Rheingold, Chem. Muter. 1991, 3. 1053.
The First Complex with a 02n4 Triple Bond
between Vanadium Atoms in a Ligand Framework
of Fourfold SymmetryIvz {(p-CH, c6H,)Nc(H)N(p-C6H,cH,)) 41* *
By F Albert Cotton,* Lee M . Daniels, and Carlos A . Murillo*
In memoriam Rosa Viquez de Murillo
The chemistry of compounds containing metal-to-metal
multiple bonds has been widely developed in the last two
decades. Many of the compounds structurally characterized
possess frameworks of structure type 1 ( n refers to the bond
order between the metal atoms). However, progress in the
related chemistry of metals in the 3d transition series, with
the exception of chromium, has been slow.['' Only one other
complex with a first-row metal with this structure is known,
A triple bond
between vanadium atoms has been claimed for two compounds
in which the coordination number of the metal
atom is greater than in a structure of type 1. The latter
structure is probably best described as an edge-sharing bioc-
[ '1
R = H. OMe
Prof. Dr. F. A. Cotton, Dr. L. M. Daniels,
Prof. Dr. C. A. Murillo ['I
Laboratory for Molecular Structure and Bonding
Texas A & M University
College Station, TX 77843 (USA)
Permanent address: Department of Chemistry
University of Costa Rica
Ciudad Universitaria (Costa Rica)
We are grateful to the Vicerrectoria de Investigacion, U.C.R. (Grant
No. 115-87-516) for support of work at the University of Costa Rica and
to the National Science Foundation for support of work at Texas A & M
University. We also thank Dr. Hong Chen for assistance with some of the
spectroscopic data.
Angew. Chem. I n f . Ed. Engl. 31 (1992) N o . 6
tahedron. We now report the structural characterization of
the first dinuclear vanadium(n) compound of this type, tetrakis( N, N'-di-p-tolylformamidato)divanadium(rl).
Theoretical calculations have clearly indicated the possible
existence of molecules of type 1, with vanadium-to-vanadium
triple bonds between 2.0 and 2.1 8, in length.[41This presented a challenge for the synthetic chemist. Unfortunately, the
chemistry of vanadium(n) had not been widely investigated
until recent years.[51We thus decided to undertake a comprehensive study of the chemistry of V" with the goal of eventually characterizing a compound of the type 1.[6'
In addition to the high instability towards air of most of
the V" compounds, one of the major difficulties in their
preparation is the lack of good starting materials.[71Early
work indicated that N-donors such as amines bind to vanadium@) quite strongly.['. However, reactions of vanadium
complexes containing monocoordinated amines, such as
pyridine, or chelating amines, such as tetramethylendiamine
(TMEDA), led mainly to mononuclear vanadium(r1) compounds. In no single instance did such starting materials
provide any of the type 1 compounds, even when suitable
ligands were used.[sd1
It became clear that an amine-free starting material was
needed. A potentially bridging ligand with only N-donor
groups was very appealing, but we did not want to generate
a complex with a forced configuration that could raise a
question as to the presence of a metal-to-metal bond. We
decided that the N,N'-di-p-tolylformamidato (dfm) ligand
was a good candidate. It has two potential N-donor centers
and is able to adopt various configurations depending on the
requirements of the metal atoms.
Compounds in which dfm (or a closely related ligand)
bridges a metal-to-metal bond have been structurally characterized for M = Cr,['] Mo,["I W,19] Re,["] Ru,["] OS,''~]
C O , [ ~Rh,[I4]
and Ir.[15]Compounds with a similar framework but without metal-to-metal bonds are also known for
M = Ni and Pd.[I6] The dfm ligand is also capable of forming a bridge between Cu' or Ag' ions, although in this case
the stoichiometry is different, namely [M,(dfm),].[' 71 The
metal-to-metal distances vary from 1.930(2) A in [Cr,(dfm),]
to 2.705(6) 8, in [Ag,(dfm,)]. The torsion angle (N-M-M-N)
also changes from a very few degrees to about 27' in the
cation [Ni,(dfm),]+ .[16]
Dfm can also act as a chelating ligand, the most relevant
example of which is found in the mononuclear vanadium(I1)
compound [V(dfm),(py),] formed by the reaction of
[vCl,(py),] (py = pyridine) with Li(dfm) in toluene.[7d1
The title compound, [V2(dfm),] . toluene has been fully
characterized by various physical and chemical techniques
including X-ray crystallography.["l It is of the same type as
other dinuclear formamidato complexes having four bridging
ligands. Its structure, depicted in Figure 1, is homologous to
that of the dimolybdenum['Ol and ditungsten['] complexes.
The vanadium(r1)-to-vanadium(1r) separation of 1.978(2) 8,
is the second shortest bond length found for the [M,(dfm),]
species. Only the Cr, unit of [Cr,(dfm),] has a shorter M-M
distance (1.930(2) 8,). This bond is roughly 0.3 8, shorter
than the vanadium-to-vanadium distance found in the complexes of type 2 and 0.47 8, shorter that the V-V bond in
The vanadium-to-nitrogen
separations of 2.092(4) A and 2.120(4)8, are also much
shorter than the typical distance of 2.2 8, found in a host of
mononuclear vanadium(1r) compounds in which there is
pyridine or tmeda.[7.81This is probably an indication of the
higher basicity of the dfm ligand. The M-N bond is only
slightly longer than that found in [Cr,(dfm),] (average
2.047 A). The vanadium-to-vanadium distance and the small
Verlagsgesellschaft mbH, W-6940 Weinheim. 1992
Fig. 1. ORTEP perspective drawing of the molecular structure of [VZ(dfm)J.
toluene with ellipsoids at the 30% probability level. The carbon atoms of the
tolyl groups are drawn as arbitrarily sized spheres.
"formal shortness ratio"[" (FSR) supports the presence of a
Fdirly strong bond. The FSR of 0.808 is very similar to those
of the homologous series with metal-metal quadruple
bonds: 0.814 (Cr), 0.840 (Mo), and 0.838 (W). However, a
striking difference is found in the magnetic anisotropy, Ax of
the M-M bond. The change in the chemical shift of the
methine proton is very large. Relative to the nickel analogue
A6 for the title complex is 4.08. The calculated Ax value of
7300 x
m3 molecule- * is the highest known for any of
the dinuclear [M,(dfm),] compounds.[191
The electronic absorption spectrum (toluene solution)
shows two bands (24700, 21 300 cm-') and a shoulder
(1 8 200 cm- ') which following some preliminary SCF-XaSW calculations[201can probably be assigned to the N 6,
n + 6*, and n -+ F transitions, respectively. These are on the
edge of a rapidly rising absorption into the UV which probably arises from one or more L -+ M charge-transfer transitions.
Experimental Procedure
All syntheses were carried out under an inert atmosphere (argon) and with dry
and deoxygenated solvents. [Vz(dfm),] ' toluene was prepared by the reduction
of VCl,(thf), [21] (0.80 g, 2.14 mmol) with one equivalent of NaHBEt, in
20 mL of THF at -70 "C and further addition of a cold suspension of Li(dfm)
(4.28 mmol) in 20 mL of THF. After the temperature of the bath reached 20 "C,
the red solution was stirred for 30 min. The solvent was then removed under
vacuum. The dried solid was extracted with 35 mL of toluene and the solution
was kept at -70°C overnight. The long fiberlike, copper-colored crystals
formed were filtered and quickly washed with small amounts of hexane (0.55 g,
52% yield). Upon recrystallization from toluene and layering with hexane, red
block-shaped crystals of [V,(dfm),] . toluene formed. Both crystalline forms
gave the same NMR spectrum; that of the copper-colored crystals showed only
a negligible amount of toluene. The compound is diamagnetic (by NMR). 'H
NMR (200 MHz, C,D,, 25 "C): 6 = 1.99 (s, -CH,), 6.00 and 6.63 (d, -C6H4-),
10.24 (s, -NCHN-). UV/VIS(toluene): >.ma, [nm] = 550 (b. sh), 470. 405.
Received: January 14. 1992 [ZS12OIE]
German version: Anger,.. Chem. 1992, 104, 795
CAS Registry numbers:
[V2(dfm)J, 140633-84-7;[Vz(dfm,)]. toluene, 140633-85-8;[VCl,(thf),], 1955906-9; Li(dfm). 75344-35-3.
[l] F. A. Cotton, R. A. Walton, Multiple Bonds B e t w e n Metol Atoms. 1st ed..
Wiley, New York. 1982, 2nd ed., in press.
121 F. A. Cotton, R. Poli, Inorg. Chem. 1987, 26, 3652-3653.
[3] a) F. A. Cotton, M. Millar. J. Am. Chem. SOC. 1977, 99, 7886-7891;
b) F. A. Cotton, G . E. Lewis. G. N. Mott, Inorg. Chem. 1983,22,560-561.
141 F. A. Cotton, M. P. Diebold. I. Shim, Inorg. Chem. 1985, 24, 1510-1516.
[S] F. A. Cotton. G . Wilkinson, Advanced Inorgunic ChemisfrJ, 5th ed., Wiley,
New York, 1988. pp. 675-676.
Verla~sgesellsrhafrmhH. W-6940 Weinherm,1992
[6] For further discussion see for example F. A. Cotton. L. R. Falvello. R.
Llusar, E. Libby. C. A. Murillo, W Schwotzer, Inorg. Clrem. 1986, 25.
3423-3428, and references cited therein.
[7] a) Ref. [6]; b) F. A. Cotton, C. A. Murillo. ing. Cienc. Quim. 1985, 9, 1-2.
c) F. A. Cotton. E. Libby, C. A. Murillo, G. Valle, Inorg. Synfh. 1990,27.
306-310. d) F. A. Cotton. R. Poli, Inorg. C / r i n i . Acru 1988, 141, 91-98.
e) F. A. Cotton. L. M. Daniels, M. L. Montero, C. A. Mnrillo, unpublished results.
[XI See for example a) P. Dapporto, F. Mani, C. Mealli, fnorg. Chem. 1978,17,
1323 -1329; b) J. J. H. Edema, A. Meetsma, S . Gambarotta, J. Am. Chem.
Soc. 1989, I f f , 6878--6880; c) J. J. H. Edema, W. Stauthamer, F. van Bolhuis, S. Gambarotta, W. J. 3. Smeets, A. L. Spek. Inorg. Chem. 1990, 29,
1302-1306; d) J. J. H. Edema, A. Meetsma, F. van Bolhuis, S. Gambarotta, ihrd. 1991, 3032056-2061 ; e) J. J. H. Edema, S. Gambarotta. A.
Meetsma, A. L. Spek, N . Veldman, ihid. 1991, 30, 2062-2066; f) J. J. H.
Edema. S. Gambarotta. S. Hao, C. Bensimon, ihid. 1991,30,2584-2586.
[9] F. A. Cotton, T. Ren, .l
Am. Chen?. SOC.,1992, I I 4 , 2237-2242.
[lo] F. A. Cotton, X. Feng. M. Matusz, Inorg. Chem. 1989, 28, 594-601.
[Ill F. A. Cotton. T. Ren. J. Am. Chem. Soc., 1992. l f 4 , 2495-2501.
[12] F. A. Cotton, T. Ren, Inorg. Chem. 1991, 30, 3675-3679.
1131 F. A. Cotton, T. Ren, I. L. Eglin, Inorg. Chem. 1991, 30, 2559-2563.
[14] J. L. Bear, C:L. Yao, R. S. Lifsey, J. D. Korp, K. M. Kadish, Inorg. C/iem.
1991,30, 336-340.
[15] F. A. Cotton. R. Poli. Pobhedron 1987. 6 , 1625-1628.
(161 F. A. Cotton. M. Matusz, R. Poli. X. Feng, J. Am. Chem. Soc. 1988, tf0,
1144- 1154.
[17] F. A. Cotton. X. Feng, M. Matusz, R. Poli, J. Am. Chrm. SOC.1988, IfO,
7077 -7083.
[18] Crystal data for V2N,C,,H,, . C,H,, T = 298 K: M, =1087.2, space
group P4/n, u = 13.214(6). c = 17.427(5) A, V = 3043(3) A3, Z = 2.
e,,,,, =1.19 gcm-'. ki(CuK,) = 29.4cm-', Rigaku AFCSR, 4" 5 20 I
120". 4971 reflections were collected; 2282 unique, 1179 having I > 3 4 4 .
The data were corrected for absorption ($-scans) and an overall decay of
14.4%. Initial positions were taken from the homologous structure of
[W2(dfm)]. toluene [9]. The coordinates and isotropic temperature factor
for the methine H atom were included in the refinement. Positions for
other hydrogen atoms in the complex were calculated. Disordered solvent
molecules reside on the fourfold axis between the metal complexes. The
toluene molecules were modeled as two rigid groups with the methyl
groups pointing in opposite directions along the fourfold axis. The final
cycles of refinement led to R = 0.049 and R,, = 0.068. Further details of
the crystal structure investigation may be obtained from the Cambridge
Crystallographie Data Centre, University Chemical Laboratory, Lensfield
Road, GB-Cambridge CB2 1EW (UK) on quoting the names of the authors and the journal citation.
[19] The magnetic anisotropy was calculated using the McConnell equation
A6 = Ax [(l - 3cos*O)/12xr3],where A6 = 4.08, 0 = 90", r = 3.62 A. See
ref. 191 for further discussion.
1201 The SCF-Xa-SW calculations were carried out for the model system
[V2(HNCHNH),] and will be reported elsewhere. Also see the work on
[MoZ(HPO,),I2- in M. D. Hopkins, V. M. Miskowski, H. B. Gray. J. Am.
Chem. SOC.1986, 108,959-963.
[21] L. Manzer, Inorx. Synth. 1982, 21, 135-140.
How High is the Barrier for the Valence
Isomerization of Cyclobutadiene?**
By Giinther Maier,* Reinhard Wolf;
and Hans-Otto Kalinowski
"Our current knowledge about cyclobutadiene['I allows
this chapter to be terminated. It is now secured textbook material," wrote one of us (G.M.) three years ago.[ldlThis statement did not remain unchallenged,r21although even at the
time two existing gaps were indicated. The first concerns the
UV spectrum of cyclobutadiene that still is not clearly established. The spectrum must show an absorption maximum
Prof. Dr. G. Maier, Dip1.-Chem. R. Wolf, Dr. H.-0 Kalinowski
Institut fur Organische Chemie der Universitat
Heinrich-Buff-Ring 58, D-W-6300 Giessen (FRG)
Small rings, Part 73. This research was supported by the Fonds der
Chemischen Industrie. -Part 72: G. Maier, D. Volz, J. Neudert, Synthesis,
in press.
Angew. Chem. Int. Ed. Engl. 31 (1992) No. 6
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bond, symmetry, fourfold, framework, complex, first, atom, triple, ch3c6h4, vanadium, c6h4ch3, ligand
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