Angewandte
Chemie
As a consequence of the distorted square-planar coordi-
nation geometry about both chromium atoms of 3 and the cis-
chelating nature of the nacnac ligands, the phenyl group and
the hydride are necessarily in a cis arrangement. Thus, the
C
ipso-Cr-H angles are 79.58 and 83.08, respectively, and the
ꢀ
nonbonded Cipso H separation is a modest 2.57 . In view of
this close proximity and the potential formation of a rather
ꢀ1
[7]
ꢀ
strong (DC-H = 113 kcalmol ) aromatic C H bond, reduc-
tive elimination of a molecule of benzene from 3 might be
expected to be facile. However, 3 is indefinitely stable at room
temperature, and its thermolysis proceeded very slowly even
at 808C (approximate half life greater than one month).
Among the multiple products of its decomposition were
1
benzene (identified by H NMR spectroscopy) and [{({iPr2-
(C6H3)}2nacnac)Cr}2(m-h1:h1-CH2(CH2)3O)] (4; identified by
X-ray diffraction, see the Supporting Information); the latter
ꢀ
is the result of a C O bond activation of the THF solvent.
Repetition of the thermolysis in [D18]octane revealed very
Figure 2. Temperature dependence of the molar magnetic susceptibil-
ity (cM, open circles, line is the fit with a Curie–Weiss expression) and
the effective magnetic moment (meff, filled circles) of 5.
little decomposition of 3 up to 1208C.
ꢀ
Hypothetical C H bond formation in 3 without loss of
benzene might form a benzene complex. Chromium has a
pronounced tendency for the formation of h6-arene com-
is not at all clear. Possible reasons include the presence of a
direct metal–metal bond in 3, which prevents the develop-
ment of overlap between the valence orbitals of the phenyl
ipso carbon atom and the hydride ligand by coulombic
repulsion, or the widely differing spin states of 3 and 5 (i.e., a
dramatic example of “spin blocking”).[13] These questions, as
well as the further exploration of molecules akin to 3 and 5,
are the subject of current research.
plexes,[8] and the stability of this structural motif (DCr-Ar
=
39 kcalmolꢀ1)[9] should be another contribution to the ther-
modynamic driving force for the reductive elimination. The
envisioned compound can indeed be prepared, albeit not by
the path suggested. Instead, magnesium reduction of [{({iPr2-
(C6H3)}2nacnac)Cr}2(m-Cl)2] in THF in the presence of a small
amount of benzene yielded a black solution, from which
crystals of [{({iPr2(C6H3)}2nacnac)Cr}2(m-h6:h6-C6H6)] (5)
could be isolated in 58% yield. The molecular structure of 5
is shown in Figure 1b.[4] Two {(nacnac)CrI} fragments are
joined by a bridging benzene ligand, which keeps the two
Cr atoms at a distance of 3.515(2) .[10] Thermolysis of 5 also
required high temperatures (over 808C), eventually yielding
benzene and 4 without any evidence of 3 being formed.
Complex 5 is a paramagnet with an unusually large room-
temperature magnetic moment (meff = 7.4(1)mB per dimer at
293 K). This value is close to the spin-only moment for a
strongly coupled system with sixunpaired electrons ( S = 3,
meff = 6.93mB). Two magnetically independent chromium ions
would result in moments of either 8.4mB (two S = 5/2 ions) or
5.5mB (two S = 3/2 ions), both of which are inconsistent with
the measured moment. The results of variable-temperature
magnetic measurements on 5 are shown in Figure 2.
Experimental Section
3: [({iPr2(C6H3)}2nacnac)Cr(Ph)(thf)] (500 mg, 0.808 mmol) was dis-
solved in pentane (50 mL) and placed in an ampule. The ampule was
evacuated and filled with H2 (1 atm), and the solution was allowed to
stir 10 h, during which time the solution became cloudy. The hydrogen
and pentane were removed in vacuo. The product was extracted with
pentane (25 mL) to give a brown solution. The byproduct filtered off
was identified as [{({iPr2(C6H3)}2nacnac)Cr}2(m-H)2] (2). The solution
was concentrated and cooled to ꢀ308C to yield brown crystals of 3
1
(142 mg, 17% yield). H NMR (400 MHz, [D8]THF, 295 K): d = 23.2
(br), 7.05 (br), 6.41 (br), 2.69 (br), 0.30 ppm (br). IR (KBr): n˜ = 3058
(w), 2959 (s), 2929 (s), 2867 (m), 1526 (s), 1461 (s), 1435 (s), 1392 (s),
1314 (s), 1524 (m), 1174 (m), 1098 (w), 1020 (w), 932 (w), 796 (m),
760 cmꢀ1 (w). UV/Vis (pentane): lmax (e) = 622 (236mꢀ1 cmꢀ1), 689 nm
(259mꢀ1 cmꢀ1). meff = 2.4(1)mB (293 K); m.p. 2248C (decomp); LIFDI-
MS m/z (%): 1016.6 (100) [M+].[5] Elemental anal. calcd (%) for
C64H88N4Cr2: C 75.55, H 8.72, N 5.51; found: C 74.09, H 8.26, N 5.25.
5: [({iPr2(C6H3)}2nacnac)Cr(m-Cl)]2 (2.00 g, 1.96 mmol) was dis-
solved in THF (75 mL). Excess Mg (500 mg, 20.6 mmol), excess
benzene (1 mL, 11.3 mmol), and a catalytic amount of napthalene
(5 mg, 0.04 mmol) were added, and the solution was allowed to stir
overnight. The color gradually changed from deep green to black. The
solvent was removed in vacuo and the product was extracted with
cold THF (25 mL). Concentration and cooling to ꢀ308C yielded
black crystals of 5 (1.23 g, 58% yield). 1H NMR (400 MHz, [D8]THF,
295 K): d = 98.2 (br), 12.0 (br), 8.88 (br), 2.59 ppm (br). IR (KBr): n˜ =
3052 (w), 2953 (s), 2925 (m), 2865 (m), 1537 (s), 1520 (s), 1457 (s),
1433 (s), 1407 (s), 1360 (m), 1312 (s), 1258 (w), 1226 (w), 1173 (m),
1103 (w), 1023 (w), 932 (w), 791 (m), 758 (m), 724 cmꢀ1 (w). UV/Vis
(pentane): lmax (e) = 485 (4392mꢀ1 cmꢀ1), 865 nm (991mꢀ1 cmꢀ1);
m.p. 2908C (decomp). MS m/z (%): 469.3 (100) [(M+ꢀC6H6)/2], 954.5
Its molar magnetic susceptibility followed the Curie–
Weiss law, with a temperature-independent moment of 7.4mB.
Saturation magnetization measurements at 5 K (see the
Supporting Information) were consistent with a spin ground
state of S = 3. The only reasonable interpretation of these
results posits extremely strong (j J j > 500 cmꢀ1) ferromagnetic
coupling between the two metal centers (CrI, d5, S = 3/2),
mediated by the bridging benzene ligand.[11]
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Complexes 3 and 5 are isomers related by a simple C H
bond formation/cleavage. However, we have not observed
any interconversion between them; thus our belief—based on
bond-strength considerations[12]—that 5 is the thermodynami-
cally more stable isomer remains tentative. The kinetic
barrier separating 3 and 5 is surprisingly high, and its origin
Angew. Chem. Int. Ed. 2007, 46, 6692 –6694
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim