A stable alkyl hydride of a first row transition metal

Article abstract of DOI:10.1039/b301943h

Hydrogenolysis of a Cr(III) dialkyl precursor produced a binuclear chromium complex with a bridging hydride and a bridging alkyl; this structurally characterized organometallic compound is thermally very stable and does not undergo the expected reductive elimination of alkane.

Full text of DOI:10.1039/b301943h

A stable alkyl hydride of a first row transition metal
Leonard A. MacAdams, Gerald P. Buffone, Christopher D. Incarvito, James A. Golen, Arnold L.
Rheingold and Klaus H. Theopold*
Department of Chemistry and Biochemistry, Center for Catalytic Science and Technology, University of
Delaware, Newark, DE 19716, USA. E-mail: theopold@udel.edu; Fax: (302)831-6335; Tel: (302)831-1546
Received (in Purdue, IN) 18th February 2003, Accepted 28th February 2003
First published as an Advance Article on the web 10th April 2003
Hydrogenolysis of a Cr(III) dialkyl precursor produced a
binuclear chromium complex with a bridging hydride and a
bridging alkyl; this structurally characterized organome-
tallic compound is thermally very stable and does not
undergo the expected reductive elimination of alkane.
relatively long for a dinuclear CrII complex,6 but the possibility
of significant metal–metal bonding cannot be dismissed. It is
supported by the low magnetic moment of 2 (meff = 2.5(1) mB
at 294 K, i.e. 1.8(1) mB per Cr), possibly augmented by
antiferromagnetic coupling mediated by the bridging ligands.
In view of the unusual chemical nature of 2 and the
diminished utility of NMR spectroscopy for its characterization,
it was important to rule out alternative structures, such as a
mixed-valent (CrII, CrIII) m-alkylidene. While the determination
of hydrogen atom positions by X-ray diffraction is always
compromised by a systematic error,7 the detection of H(1)–H(3)
in a difference map leaves little doubt about their presence.
Furthermore, the gross structural features, bond distances and
magnetic moment of 2 are very similar to those of a class of CrII
dimers including—inter alia—[(2,6-Me2Ph)2nacnacCr)2(m-
H)2] (3).8 Dihydride 3 was the product of an attempt to
synthesize [(2,6-Me2Ph)2nacnacCr(CH2CH3)2]; while this dia-
lkyl appeared to be formed as an intermediate, it apparently
suffered facile b-hydrogen elimination. Both the formation of 2
and 3 presumably involve binuclear reductive eliminations (of
SiMe4 for 2 and C2H6 or H2 for 3). The molecular structure of
3 is shown in Fig. 2;9 like 2, it is a dinuclear Cr–Cr-bonded
compound (Cr–Cr = 2.6207(7) Å, meff = 1.2(1) µB/Cr). The
metal–ligand distances in both compounds are essentially
identical and the effective magnetic moments of 2 and 3 are
similar in magnitude and consistent with significant metal–
metal interactions. This correspondence in physical properties
strongly suggests that 2 and 3 belong in the same class and that
the organic ligand of 2 is a bridging alkyl rather than a m-
alkylidene. Additional evidence for this was provided by
Organometallic compounds containing both an alkyl group (R)
and a hydride ligand (H) in adjacent positions are generally
unstable, decomposing by facile reductive elimination of alkane
(R–H).1 Exceptions to this rule are certain compounds contain-
ing transition metals of the second and third row, which allow
the direct observation of C–H oxidative addition.2 In contrast,
the M–C and M–H bonds of first row transition metals are
thought to be too weak to compensate for the loss of the C–H
bond, and since the intrinsic activation barriers to C–H
oxidative addition/reductive elimination are apparently small,
the thermodynamically favored alkane elimination proceeds
rapidly. Stable alkyl hydrides of first row transition metals are
thus exceedingly rare.3 Herein we report the synthesis and
structural characterization of a dinuclear chromium alkyl
hydride that is surprisingly stable.
In the context of our investigation of paramagnetic chromium
alkyls as models for heterogeneous ethylene polymerization
catalysts,4 we have prepared [(2,6-Me2Ph)2nacnacCr(CH-
2SiMe3)2] (1, (2,6-Me2Ph)2nacnac = N,NA-bis(2,6-dimethyl-
phenyl)-2,4-pentanediiminato). 1 is a pseudotetrahedral CrIII
alkyl; despite its coordinative unsaturation (11-electron config-
uration) it does not catalyze the polymerization of ethylene in
the absence of a co-catalyst. Reaction of 1 in pentane with
excess hydrogen (1 atm, RT) yielded a red solution; standard
work-up of the product gave [(2,6-Me2Ph)2nacnacCr)2(m-
CH2SiMe3)(m-H)] (2 )† in high yield (see Scheme 1). The 1
H
NMR spectrum of 2 exhibited several isotropically shifted and
broadened resonances, as expected of a paramagnetic com-
pound. The surprising nature of 2 was revealed by a crystal
structure determination, the result of which is shown in Fig.
1.5‡
2 is a dinuclear chromium complex in which each chromium
is coordinated by one nacnac ligand. The two chromium atoms
are joined by a bridging hydride and a bridging trimethylsi-
lylmethyl group. The hydride (H(1)) and the hydrogen atoms of
the methylene group (H(2), H(3)) were located and their
positions refined. The coordination geometry about each
chromium is best described as distorted square planar and the
dihedral angle between the two nacnac planes is 43.9°,
presumably due to steric interactions between the aryl sub-
stituents and the m-alkyl. The Cr–Cr distance of 2.6026(9) Å is
Fig. 1 The molecular structure of 2. Selected distances [Å] and angles [°]:
Cr(1)–Cr(2) 2.6026(9), Cr(1)–C(43) 2.288(4), Cr(2)–C(43) 2.190(4),
Cr(1)–H(1) 1.67(7), Cr(2)–H(1) 1.78(7), Cr(1)–N(1) 2.080(4), Cr(1)–N(2)
2.026(3), Cr(2)–N(3) 2.062(4), Cr(2)–(N4) 2.061(4); Cr(1)–C(43)–Cr(2)
71.01(13), Cr(1)–H(1)–Cr(2) 97(4); Cr(1)–C(43)–Si(1) 144.4(3), Cr(2)–
C(43)–Si(1) 105.1(2), N(1)–Cr(1)–N(2) 89.0(1), N(3)–Cr(2)–N(4)
88.4(1).
Scheme 1 Hydrogenolysis of [(2,6-Me2Ph)2nacnacCr(CH2SiMe3)2] (1 ).
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was stirred overnight at room temperature, whereupon its color had changed
to dark red. After evaporation of the pentane the residual solid was extracted
with Et2O. Cooling of the concentrated Et2O solution to 230 °C yielded
dark red crystals (0.500 g, 95% yield) of 2 . 1 H NMR (C6D6): d = 14.7 (br),
6.4 (br), 4.7 (br), 2.6 (br) ppm; IR (KBr): n = 3056 (m), 3016 (m), 2954 (s),
2915 (s), 2852 (m), 1533 (s), 1435 (s), 1384 (s), 1285 (m), 1259 (m), 1241
(s), 1182 (s), 1094 (m), 1022 (m), 979 (s), 857 (s), 761 (s), 673 (m) cm 21
;
UV/Vis (Et2O): lmax (e) = 480 (663 M 21 cm 21 ) nm; mp: 194–197 °C;
meff (294 K) = 2.5(1) µB; MS (15 eV) : m/z 802 (M+); elemental analysis
calcd (%) for C46H62Cr2N4Si1: C 68.80, H 7.78, N 7.00; found: C 68.72, H
7.66, N 6.82.
3 : [(2,6-Me2Ph)2nacnacCrCl2(THF)2] (0.440 g, 0.768 mmol) was
dissolved in 10 mL Et2O and the solution cooled to 230 °C. A 3.0 M Et2O
solution of EtMgBr (0.533 mL, 1.6 mmol) was added via syringe, resulting
in a rapid color change from red to green. The solution was warmed to room
temperature and stirred overnight, whereupon the color had changed to a
dark orange. Filtering, concentrating, and cooling to 230 °C yielded orange
crystals of 3. 1 H NMR (C6D6): d = 19.3 (br), 13.7 (br), 7.0 (br), 6.8 (br),
2.0 (br) ppm; IR (KBr): n = 3055 (m), 3014 (m), 2958 (s), 2917 (s), 2856
(m), 1522 (s), 1454 (s), 1378 (s), 1260 (m), 1240 (s), 1180 (s), 1096 (m),
1022 (m), 851 (s), 762 (s) cm 21; meff (294 K) = 1.7(1) µB; MS (15 eV) :
m/z 716 (M+); elemental analysis calcd (%) for C42H52Cr2N4: C 70.36, H
7.31, N 7.81; found: C 68.88, H 7.63, N 6.25.
‡ CCDC 196911 and 205747. See http://www.rsc.org/suppdata/cc/b3/
b301943h/ for crystallographic data in .cif or other electronic format.
Fig. 2 The molecular structure of 3 . Selected distances [Å] and angles [°]:
Cr(1)–Cr(1A) 2.6207(7), Cr(1)–(H1) 1.77(3), Cr(1)–H(1A) 1.76(3), Cr(1)–
N(1) 2.0179(18), Cr(1)–N(2) 2.0268(19); Cr(1)–H(1)–Cr(1A) 95.9(9);
N(1)–Cr(1)–N(2) 90.56(8).
1 J. Halpern, Acc. Chem. Res., 1982, 15, 332–338.
2 (a) A. H. Janowicz and R. G. Bergman, J. Am. Chem. Soc., 1982, 104,
352; (b) J. K. Hoyano and W. A. G. Graham, J. Am. Chem. Soc., 1982,
104, 3723; (c) W. D. Jones and F. J. Feher, Organometallics, 1983, 2,
562; (d) P. O. Stoutland, R. G. Bergman, S. P. Nolan and C. D. Hoff,
Polyhedron, 1988, 7, 1429; (e) B. A. Arndtsen, R. G. Bergman, T. A.
Mobley and T. H. Peterson, Acc. Chem. Res., 1995, 28, 1154; (f) R. H.
Crabtree, J. Chem. Soc., Dalton Trans., 2001, 2437.
3 (a) K. Jonas and G. Wilke, Angew. Chem., Int. Ed. Engl., 1969, 8, 519;
(b) R. A. Henderson and K. E. Oglieve, Chem. Commun., 1999, 2271;
(c) a closely related dinuclear TiIII alkyl hydride has been prepared: J. R.
Hagadorn and M. J. McNevin, Organometallics, 2003, 22, 609.
4 (a) K. H. Theopold, Eur. J. Inorg. Chem., 1998, 15; (b) W.-K. Kim, M.
J. Fevola, L. M. Liable-Sands, A. L. Rheingold and K. H. Theopold,
Organometallics, 1998, 17, 4541; (c) L. A. MacAdams, W.-K. Kim, L.
M. Liable-Sands, I. A. Guzei, A. L. Rheingold and K. H. Theopold,
Organometallics, 2002, 21, 952; (d) K. H. Theopold, L. A. MacAdams,
C. Puttnual, G. P. Buffone and A. L. Rheingold, Polym. Mater. Sci.
Eng., 2002, 86, 310.
labeling experiments coupled with mass spectroscopy. Thus,
the mass spectrum (EI, 15 eV) of 2 exhibited a molecular ion at
m/z 802 (C46H62Cr2N4Si) with the requisite chromium isotope
pattern. The product of the reaction of 1 with D2 showed a
molecular ion at m/z 803, consistent with the formation of
[(2,6-Me2Ph)2nacnacCr)2(m-CH2SiMe3)(m-D)] (2-d1).10 GC-
MS analysis of the volatile products of the reaction of 2 in
toluene with D2O was consistent with the formation of
(CH3)3SiCH2D as the sole organic product; in particular, there
was no indication of the formation of (CH3)3SiCHD2, the
expected product of deuterolysis of an alkylidene species.
Based on all these observations the assignment of 2 as an alkyl
hydride complex is unambiguous.
2 is remarkably stable. For example, heating of a toluene-d8
solution of 2 to 100 °C for several days did not produce any
signs of decomposition. Prolonged photolysis did not induce
any chemical change either. Finally, reaction of 2 with excess
H2 or D2 had no discernible effect. To our knowledge, such
resistance to alkane reductive elimination is unprecedented. We
are considering two possible rationalizations of this phenome-
non. First, it is conceivable that the inorganic product of the
alkane elimination—presumably [((2,6-Me2Ph)2nacnacCr)2]—
is too unstable thermodynamically to allow its formation. The
other possibility is that the reductive elimination faces an
insurmountable kinetic barrier. Maybe electron–electron repul-
sion with the metal–metal bond prevents the development of
overlap between the valence orbitals of the hydride and the alkyl
group. However, we note that steric interactions force the core
of 2 into a butterfly configuration; thus the dihedral angle
between the planes defined by Cr(1)–C(43)–Cr(2) and Cr(1)–
H(1)–Cr(2) is 47°. The nonbonded C(43)…H(1) distance of
2.74 Å and the C(43)–Cr–H(1) angles of 86° are characteristic
of a cis-relationship of these two ligands; they provide no
structural evidence for a repulsive interaction with a Cr–Cr
bond. We are conducting further experiments and calculations
to explore the unusual stability of 2 and to delineate the
reactivity of this class of dinuclear Cr(II) organometallics.
This research was supported by a grant from the U.S.
National Science Foundation.
5 2 : C46H62Cr2N4Si, M = 803.09, monoclinic, P21/n, a = 12.33630(10)
Å, b = 20.9724(3) Å, c = 17.6709(3) Å, b = 106.0100(10)°, V =
21
4394.52(10) Å3, Z = 4, Dcalc = 1.214 g cm
, q = 1.54–25.00°,
MoKa l = 0.71073 Å, T = 173 K, 16789 reflections, µ = 0.557 mm
21
, max and min transmission: 0.9211 and 0.8734, solved by direct
methods and refined by full-matrix least-squares procedures using
SHELXTL (5.1), 490 parameters, R = 0.0687, wR = 0.1912. CDCC
reference number 196911.
6 (a) F. A. Cotton and R. A. Walton, Multiple Bonds Between Metal
Atoms, 1st edn., Wiley, New York, 1982, p. 150; (b) J. J. H. Edema and
S. Gambarotta, Comments Inorg. Chem., 1991, 11, 195; (c) R. A.
Heintz, R. L. Ostrander, A. L. Rheingold and K. H. Theopold, J. Am.
Chem. Soc., 1994, 116, 11387.
7 (a) J. D. Dunitz, X-Ray Analysis and the Structure of Organic
Molecules, Ithaca, Cornell University Press, 1979, p. 391; (b) R. Bau
and M. H. Drabnis, Inorg. Chim. Acta, 1997, 259, 27.
8 Similar compounds featuring a related nacnac ligand are [(2,6-i
Pr2Ph)2nacnacCr)2(m-CH3)2] (Cr–Cr 2.597(10) Å, Cr–Navg 2.09 Å, Cr–
Cavg 2.21 Å, meff = 2.5(1) µB) and [(2,6-iPr2Ph)2nacnacCr)2(m-H)2]
(Cr–Cr 2.676(13) Å, Cr–Navg 2.05 Å, Cr–Havg 1.71 Å, meff = 3.5(1) mB);
these will be included in a forthcoming full paper; L. A. MacAdams,
PhD thesis, University of Delaware, 2002.
9 3 : C42H52Cr2N4, M = 716.88, monoclinic, C2/c, a = 23.4971(13) Å,
b = 10.7216(6) Å, c = 15.0645(9) Å, b = 92.4850(10)°, V = 3791.6(4)
21
Å3, Z = 4, Dcalc = 1.256 g cm
, q = 2.09–28.28°, MoKa l =
0.71073 Å, T = 150 K, 11853 reflections, µ = 0.607 mm 21 , max and
min transmission: 0.9145 and 0.8882, solved by direct methods and
refined by full-matrix least-squares procedures using SHELXTL (5.1),
221 parameters, R = 0.0471, wR = 0.1375. CDCC reference number
205747.
Notes and references
†
2 : [(2,6-Me2Ph)2nacnacCr(CH2SiMe3)2] (1, 0.700 g, 1.32 mmol) was
10 A comparison of the IR spectra of 2 and 2-d1 did not reveal any
significant differences; presumably the hydride (deuteride) stretches are
obscured by ligand bands.
placed in a large ampoule and dissolved in 25 mL pentane. The solution was
degassed and the ampoule was filled with H2 (1 atm). The reaction mixture
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