Lewis acid stabilized methylidene and oxoscandium complexes

Article abstract of DOI:10.1021/ja806635x

The methylidene scandium complex (PNP)Sc(μ₃-CH₂)(μ₂-CH₃)₂[Al(CH₃)₂]₂ (PNP = N[2-P(CHMe₂)₂-4-methylphenyl]₂^-) can be prepared from the reaction of (PNP)Sc(CH₃)₂ and 2 equiv of Al(CH₃)₃. The Lewis acid stabilized methylidenes candium complex has been crystallographically characterized, and its bonding scheme analyzed by DFT. In addition, we report preliminary reactivity studies of the Sc-CH₂ ligand with substrates such as H₂NAr and OCPh₂. While the former results in an Bronsted acid-base reaction, the latter reagent produces the olefin H₂C=CPh₂ along with the novel oxoscandium complex (PNP)Sc(μ₃-O)(μ₂-CH₃)₂[Al(CH₃)₂]₂, quantitatively. Copyright

Full text of DOI:10.1021/ja806635x

Published on Web 10/11/2008
Lewis Acid Stabilized Methylidene and Oxoscandium Complexes
Jennifer Scott, Hongjun Fan, Benjamin F. Wicker, Alison R. Fout, Mu-Hyun Baik, and
Daniel J. Mindiola*
Department of Chemistry, Molecular Structure Center, and the School of Informatics, Indiana UniVersity,
Bloomington, Indiana 47405
Received August 21, 2008; E-mail: mindiola@indiana.edu
Scheme 1. Synthesis of Complexes 1-6a
Tebbe’s reagent, Cp2Ti[µ2-CH2)(µ2-Cl)Al(CH3)2], can be categorized
as a quintessential organometallic reagent due to its role in catalytic
olefin metathesis1 and stoichiometric methylidene group-transfer
processes.2 It has been instrumental in the design of more efficient
catalysts as well as the synthesis of natural products and commodity
chemicals and materials.3 The preparation of Tebbe’s reagent involves
treatment of Cp2TiCl2 with 2 equiv of Al(CH3)3, resulting in the
elimination of CH4 and AlCl(CH3)2.2a Likewise, the analogue Cp2Ti[µ2-
CH2)(µ2-CH3)Al(CH3)2] is prepared by addition of Al(CH3)3 to
Cp2Ti(CH3)2.2a Although originally speculated by Tebbe,2a the role
of Al(CH3)3 along the deprotonation step has been addressed by Grubbs
(Ti)4 and Schrock (Ta).5 Surprisingly, for over 30 years the report of
a scandium (or lanthanide) methylidene complex analogous to Tebbe’s
reagent has not been realized.6 In fact, scandium alkylidene complexes
are unknown to date, while their lanthanide relatives have been scarcely
documented.6-8 We were curious as to whether Al(CH3)3 could
stabilize a highly polarized ScdCH2 ligand as it does in the case of
Ti. Here, we describe the preparation of such a ligand, its isolation,
a For simplicity, the PNP cartoon represents N[2-P(CHMe2)2-4-methyl-
and use as a Brønsted base or as a group 3 Wittig reagent.7a,b
When (PNP)ScCl2 (1, PNP- ) N[2-P(CHMe2)2-4-methylphenyl]2)9,10
is treated with 2 equiv of LiCH3 ·LiBr complex, the dimethyl
(PNP)Sc(CH3)2 (2) is isolated in 58% yield after workup of the reaction
mixture (Scheme 1).10,11 Although the 31P NMR spectrum of 2 is rather
featureless (0.4 ppm, ∆ν1/2 ) 80 Hz), the 1H NMR spectrum clearly
reveals the resonance for the equivalent dimethyl ligands (0.51 ppm).
The solid state structure collected on a single crystal of 2 portrays a
highly skewed (PNP)Sc scaffold (P-Sc-P, 146.58(3)°) with a
ScsCH3 distance of 2.207(3) Å.11 Treating 2 with 2 equiv or an excess
of Al(CH3)3 in benzene at 25 °C resulted in the clean, gradual formation
-
phenyl]2
.
one CH2 ligands between Sc and two Al atoms to form a ScC3Al2 set
of planar rings. Overall, the gross features of the two fused four-atom
planar rings resembles Cavell’s lanthanide complex having a formal
SmdC bond.8,12a The ScsCH2 distance (2.3167(17) Å) is slightly
longer than the average ScsCH3 bond length (∼2.2 Å)13a-d but much
shorter than methide abstracted Sc · · ·CH3 distances (∼2.7 Å).13b-d
The methylidene hydrogens, as well as the bridging and terminal
methyl hydrogens of this complex, were also located and refined
isotropically. The methylidene hydrogens are not equivalent in the solid
state, with one of them pointing more toward Sc, whereas one of the
bridging methyls suggests an R-hydrogen agostic interaction taking
place with the metal center (2.33(2) Å, Figure 1).11 Whether or not
Al(CH3)3 abstracts the methyl in 2 to promote deprotonation is
speculative at present, but we favor the formation of an intermediate
(PNP)Sc[µ2-CH2)(µ2-CH3)Al(CH3)2] (A) based on our variable tem-
perature 27Al NMR spectra. The room temperature 27Al NMR spectra
1
of a new product along with methane extrusion, as inferred by H
NMR spectroscopy. The new resonance in the 31P NMR spectrum of
this complex was uninformative due to the broad line widths.11 The
1H NMR spectrum was also rather uninformative at room temperature,
revealing a very broad peak centered at -0.20 ppm. However, the
notable presence of a singlet at 0.69 ppm integrating to two protons
suggested the formation of a methylidene unit. The latter CH2
resonance was unambiguously correlated to the 13C NMR resonance
at 28.8 ppm by HMQC NMR experiments. Cooling the NMR solution
of this newly isolated product to -50 °C shifted the CH2 resonance
to 0.75 ppm in the 1H NMR spectrum and also resolved the resonance
at -0.20 ppm to suggest the formation of an organometallic complex
having three inequivalent sCH3 environments with the same ratio of
hydrogens (6:6:6). Due to these interesting spectroscopic signatures,
the structural connectivity of this new scandium complex was
elucidated by single crystal X-ray diffraction studies.
Figure 1 depicts the molecular structure of the first mononuclear
scandium methylidene, (PNP)Sc(µ3-CH2)(µ2-CH3)2[Al(CH3)2]2 (3),
stabilized by two Al(CH3)3 ligands in a pseudo transoid fashion
(AlsCH2sAl, 152.77(11)°).11,12 The Sc atom in the structure of 3 is
confined in an octahedral geometry due to bridging of two CH3 and
Figure 1. Molecular structures of complexes 3 (left) and 6 (right) with thermal
ellipsoid at the 50% probability level. iPr methyls on P, solvent, and H-atoms
(with the exception of R-hydrogens) have been excluded for clarity.
9
14438 J. AM. CHEM. SOC. 2008, 130, 14438–14439
10.1021/ja806635x CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
exception being the shorter Sc-O bond length 2.008(2) Å, thus leading
to a tucked-in appearance of the ScOC2Al2 skeleton. The latter distance
is longer than scandium alkoxide distances (∼1.9 Å) but shorter than
dative Scr:OR2 resulting from a coordinating ether (∼2.2 Å).16
Herein, credible evidence for formation of a scandium methylidene
is presented. The nucleophilic nature of the methylidene carbon,
coupled with the large ionic radius of the Sc(III) ion, allows for this
rare motif to coordinate two Al(CH3)3 ligands, one of which appears
to be dissociating to permit interesting reactivity. We have also
demonstrated that the methylidene ligand can be smoothly protonated
with a weak acid such as H2NAr or transferred to an organic group
such as a ketone, to yield the corresponding olefin and novel scandium
oxo ligand (also stabilized by two Al(CH3)3). Therefore, Tebbe’s
original strategy to trap group 4 alkylidene compounds can now be
expanded to the earliest of the transition metals.
Figure 2. (Left) HOMO-1 orbital for the optimized structure of complex 3
with isodensity at 0.05 au. (Right) Most plausible resonance structure for the
Sc(µ3-CH2)(µ2-CH3)2Al2 motif.
of 3 display two broad Al environments (154 and 50 ppm), where the
former resonance is virtually identical to that of free Al(CH3)3.11 Upon
cooling to -30 °C, the two resonances coalesce into one broad signal
at 57 ppm (∆ν1/2 ) 4900 Hz), consistent with the resting state of 3
having equivalent Al environments. Therefore, we propose that 3 is
most likely undergoing dissociation into putative A and free Al(CH3)3
in solution state at room temperature.
To address the bonding scheme in complex 3, DFT methods were
relied upon to compute the frontier orbitals and natural bond order
about the Sc(µ3-CH2)(µ2-CH3)2Al2 scaffold.11,14 The HOMO-1 stipu-
lates that the p orbital on the methylidene carbon does not form a π
interaction with Sc (Figure 2). Likewise, the computed Mayer bond
order of 0.83 is too small to suggest a strong interaction and formally
assign it as a ScdCH2 ligand (X-ray: 2.317 Å, calcd: 2.284 Å). In
addition, HOMO-6 and HOMO-7 present highly ionic bridging methyl
groups confined between Sc and Al.11 The computed Mayer bond
orders of 0.54 for both Sc-µ2-C-Al units and 0.42 and 0.44 for each
Al-µ2-C-Sc further corroborate this proposition. Based on these data,
we propose that the two fused four-membered rings in 3 adopt a
canonical structure depicted in Figure 2. To account for this type of
geometry, the bridging methyls and methylene groups must all be
involved in 3-centered-2-electron bonding (Figure 2).14
Given our proposal that 3 equilibrates to A and Al(CH3)3, we
inquired whether such a system would be amenable to reactivity similar
to Tebbe’s reagent. Complex 3 can be protonated with an excess of
H2NAr (Ar ) 2,6-iPr2C6H3) to quickly form the anilide-methyl
(PNP)Sc(NHAr)(CH3) (4) intermediate9 and after 12 h ultimately
produce the bis-anilide (PNP)Sc(NHAr)2 (5) quantitatively (Scheme
1).11 The connectivity of complex 5 has been inferred by single crystal
X-ray diffraction analysis and NMR spectral data and can be prepared
independently by the reaction of (PNP)ScCl2 with 2 equiv of
LiNHAr.11 When a solution of 3 was added to OCPh2 in benzene, an
immediate reaction occurred. Examination of the mixture by 31P and
1H NMR evinced clean formation of a new C2 symmetric Sc complex.
Notably, the methylidene resonance originally present in 3 was absent,
suggesting that this ligand has been transformed or replaced. However,
the resonances corresponding to the bridging and terminal methyl
moieties in the two Al(CH3)3 groups were fluxional even to -50 °C,
unlike those observed for complex 3. Based on this evidence and the
formation of the terminal olefin, H2CdCPh215 (verified by 1H NMR
spectroscopy), we propose the new product to be the Lewis acid
stabilized scandium oxo complex (PNP)Sc(µ3-O)(µ2-CH3)2[Al(CH3)2]2
(6), depicted in Scheme 1. The solid state structure of 6 further supports
our proposed connectivity, revealing a three-coordinate oxo-ligand
bridged by one Sc and two Al atoms (Figure 2). The metrical
parameters of 6 closely resemble those for 3, with the only significant
Acknowledgment. We thank the NSF (CHE-0348941), the Sloan
and the Dreyfus Foundation for financial support of this research. J.S.
acknowledges NSERC for a postdoctoral fellowship. Dr. Frank Gao
is acknowledged for aid with multinuclear NMR spectroscopic studies.
Supporting Information Available: Experimental preparation and
reactivity (all compounds), computational studies, and crystallographic
details for compounds 2, 3, 5, and 6. This material is available free of
charge via the Internet at http://pubs.acs.org.
References
(1) Tebbe, F. N.; Parshall, G. W.; Ovenall, D. W. J. Am. Chem. Soc. 1979,
101, 5074.
(2) For seminal examples:(a) Tebbe, F. N.; Parshall, G. W.; Reddy, G. S. J. Am.
Chem. Soc. 1978, 100, 3611. (b) Petasis, N. A.; Hu, Y.-H. Curr. Org. Chem.
1997, 1, 249. (c) Petasis, N. A.; Hu, Y.-H. J. Org. Chem. 1997, 62, 782.
(d) Petasis, N. A.; Lu, S.-P.; Bzowej, E. I.; Fu, D.-K.; Staszewski, J. P.;
Akritopoulou-Zanze, I.; Patane, M. A.; Hu, Y.-H. Pure Appl. Chem. 1996,
68, 667. (e) Pine, S. H. Org. React. 1993, 43, 1. (f) Cannizzo, L. F.; Grubbs,
R. H. J. Org. Chem. 1985, 50, 2386–2387.
(3) (a) Fu¨rstner, A. AdV. Synth. Catal. 2002, 344, 567. (b) Schrock, R. R. AdV.
Synth. Catal. 2002, 344, 571. (c) Schrock, R. R. Chem. ReV. 2002, 102,
145. (d) Trnka, T. M.; Grubbs, R. H. Acc. Chem. Res. 2001, 34, 18. (e)
Grubbs, R. H. AdV. Synth. Catal. 2007, 349, 34. (f) Schrock, R. R. AdV.
Synth. Catal. 2007, 349, 41.
(4) Ott, K. C.; deBoer, E. J. M.; Grubbs, R. H. Organometallics 1984, 3, 223.
(5) Schrock, R. R.; Sharp, P. R. J. Am. Chem. Soc. 1978, 100, 2389.
(6) Giesbrecht, G. R.; Gordon, J. C. Dalton Trans. 2004, 2387.
(7) (a) Dietrich, H. M.; To¨rnroos, K. W.; Anwander, R. J. Am. Chem. Soc.
2006, 128, 9298. (b) Masuda, J. D.; Jantunen, K. C.; Ozerov, O. V.; Noonan,
K. J. T.; Gates, D. P.; Scott, B. L.; Kiplinger, J. L. J. Am. Chem. Soc.
2008, 130, 2408. (c) Schumann, H.; Muller, J. J. Organomet. Chem. 1979,
169, C1.
(8) (a) Aparna, K.; Ferguson, M.; Cavell, R. G. J. Am. Chem. Soc. 2000, 122,
726. (b) Cavell, R. G.; Babu, R. P. K.; Aparna, K. J. Organomet. Chem.
2001, 617, 158. (c) Liddle, S. T.; McMaster, J.; Green, J. C.; Arnold, P. L.
Chem. Commun. 2008, 1747.
(9) Scott, J.; Basuli, F.; Fout, A. R.; Huffman, J. C.; Mindiola, D. J. Angew.
Chem., Int. Ed., 2008, http://dx.doi.org/anie.200803325.
(10) A similar pincer analogue of scandium has been reported. (a) Fryzuk, M. D.;
Giesbrecht, G.; Rettig, S. J. Organometallics 1996, 15, 3329. (b) Fryzuk,
M. D.; Giesbrecht, G. R.; Rettig, S. J. Can. J. Chem. 2000, 78, 1003.
(11) See Supporting Information.
(12) A methylidene ligand bridged by two HB(C6F5)2 has been reported for
Zr(IV). (a) von H. Spence, R. E.; Parks, D. J.; Piers, W. E.; MacDonald,
M.-A.; Zaworotko, M. J.; Steven J. Rettig, S. J. Angew, Chem. Int. Ed.
1995, 34, 1230. A Sc methylidene has been proposed along a Lewis acid
promoted cyclometallation pathway. (b) Conroy, K. D.; Hayes, P. G.; Piers,
W. E.; Parvez, M. Organometallics 2007, 26, 4464.
(13) (a) Thompson, M. E.; Baxter, S. M.; Bulls, A. R.; Burger, B. J.; Nolan,
M. C.; Santarsiero, B. D.; Schaefer, W. P.; Bercaw, J. E. J. Am. Chem.
Soc. 1987, 109, 203. (b) Hayes, P. G.; Piers, W. E.; McDonald, R. J. Am.
Chem. Soc. 2002, 124, 2132. (c) Piers, W. E.; D; Emslie, D. J. H. Coord.
Chem. ReV. 2002, 233. (d) Mountford, P.; Ward, B. D. Chem. Commun.
2003, 1797.
(14) The bonding in a pentacoordinate methylidene carbon has been investigated
theoretically. Radius, U.; Silverio, S. J.; Hoffmann, R.; Gleiter, R.
Organometallics 1996, 15, 3737.
(15) Adam, W.; Baeza, J.; Liu, J.-C. J. Am. Chem. Soc. 1972, 94, 2000.
(16) Bond lengths were obtained from the Cambridge Structural Database.
JA806635X
9
J. AM. CHEM. SOC. VOL. 130, NO. 44, 2008 14439