Evidence for the existence of a terminal imidoscandium compound: Intermolecular C-H activation and complexation reactions with the transient Sc=NAr species

Article abstract of DOI:10.1002/anie.200803325

Don't blink or you'll miss it: A transient imidoscandium complex (see scheme) can be generated by several routes. The imido ligand can activate the C-H bond in pyridine and benzene, and it can be complexed with Al(CH ₃)₃ to yield an imide zwitterion. A combination of isotopic labeling, reactivity, and theoretical studies strongly endorse formation of a terminal imido ligand at scandium. (Chemical Equation Presented)

Full text of DOI:10.1002/anie.200803325

Communications
DOI: 10.1002/anie.200803325
Group 3 Imides
Evidence for the Existence of a Terminal Imidoscandium Compound:
ꢀ
Intermolecular C H Activation and Complexation Reactions with the
Transient Sc NAr Species**
=
Jennifer Scott, Falguni Basuli, Alison R. Fout, John C. Huffman, and Daniel J. Mindiola*
Terminal imide ligands in early-transition-metal chemistry
have recently undergone a dramatic renaissance, given their
potential applications in processes such as group transfer and
catalysis.[1] Absent from this extensive list are Group 3
transition-metal imides, an antithesis given the inherent
affinity of the highly electropositive metal ions for a hard
donor such as nitrogen. To date, complexes of Group 3
transition-metal elements (including the lanthanides) with
terminal imido ligands have been neither directly detected
nor isolated; their existence during the formation of a narrow
list of dinuclear or polynuclear bridging imides has only been
speculated.[2–4] The inability to isolate a terminal imide
linkage may be due to the discrepancy in energy between
the lanthanide and imide-nitrogen orbitals,[4] rendering this
type of bond highly polarized and thus prohibiting the formal
ligand type has been recently demonstrated to be an ideal
ancillary support in the preparation of an unprecedented
bridging phosphinidene moiety on lutetium(III).[6] For us,
assembling the PNP ancillary ligand and ScIII to form
[(PNP)ScCl2] (1) in 95% yield proved straightforward by
treatment of Li(PNP) with [ScCl3(thf)3] in toluene at 708C
over 48 h.[7,8] Bright yellow 1 can be readily transmetalated
with LiNHAr (Ar= 2,6-iPr2C6H3) to afford [(PNP)Sc-
(NHAr)Cl] (2) in 76% yield (Scheme 1). To incorporate a
=
ꢁ
M NR or M NR bond that is prototypical among most early
transition metals. As a result, such a mismatch in orbital
energies should bestow unprecedented nucleophilic behavior
to the imido nitrogen atom when coordinated to an ion such
as a lanthanide. Herein, we present credible evidence for the
existence of a terminal scandium imido complex by applying a
combination of isotopic labeling and reactivity studies of a
=
transient Sc NR complex, evidenced by the intermolecular
ꢀ
C H activation of pyridine and benzene as well as complex-
ation with Al(CH3)3.
The fact that we can generate transient, reactive titanium
alkylidynes of the type [(PNP)Ti CR] (PNP = N[2-P-
ꢀ
Scheme 1. Synthesis of complexes 1–5, subsequent C H activation of
pyridine to form 4 and complexation of 3 with Al(CH3)3 to produce 5.
A proposed mechanism is also depicted. Complexes 2–5 have been
characterized by X-ray crystallography. The PNP caricature represents
N[2-P(CHMe2)2-4-methylphenyl]2ꢀ; Ar=2,6-iPr2C6H3.
ꢁ
(CHMe2)2-4-methylphenyl]2, R = Ph, SiMe3, and tBu, among
other groups)[5] encouraged the search for an isolobal
ꢁ
{(PNP)Sc NR} fragment, owing to the comparable atomic
radii between titanium(IV) and scandium(III) when weighed
against the other Group 3 congeners. Likewise, the PNP
good leaving group for deprotonation of the anilide, precursor
2 was alkylated with LiCH3 to produce the yellow anilide
methyl complex [(PNP)Sc(NHAr)(CH3)] (3) in 72% yield
subsequent to recrystallization from hexane at ꢀ358C
(Scheme 1).[7] Although the 31P NMR spectroscopic resonan-
[*] Dr. J. Scott, F. Basuli, A. R. Fout, Dr. J. C. Huffman,
Prof. Dr. D. J. Mindiola
Department of Chemistry and the Molecular Structure Center
Indiana University, Bloomington, IN 47405 (USA)
Fax: (+1)812-855-8300
ces for compounds 1–3 are unremarkable (7–2 ppm, Dn1/2
>
90 Hz), presumably owing to effective coupling of the
quadrupolar 45Sc scalar with the 31P nucleus, the 1H and
13C NMR spectra were clearly consistent with the proposed
E-mail: mindiola@indiana.edu
[**] The authors 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. Hongjun
Fan is thanked for performing DFT calculations and Dr. John
Tomaszewski is also acknowledged for aid with multinuclear NMR
spectroscopic studies. Supporting information for this article is
available on the WWW under http://www.angewandte.org or from
the author.
1
connectivity.[7] Notably, the H NMR spectrum of compound
3 reveals an NH proton (d = 5.98 ppm) in addition to a methyl
group resonating upfield at d = 0.28 ppm. Likewise, solid-
state structural data for single crystals of 2[9] and 3[10] were
collected (Figure 1). These molecular structures are related
and both portray highly skewed {(PNP)Sc} scaffolds with the
ꢀ
Sc Nanilide bonds being shorter (2 2.0073(17), 3 2.0298(13) ꢀ)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200803325.
than the Sc NPNP bonds (2 2.1385(16), 3 2.1568(12) ꢀ).[7]
ꢀ
8502
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Angewandte
Chemie
goes an a-hydrogen abstraction step to furnish a terminal
scandium imido intermediate, [(PNP)Sc = NAr(py)] (A),
which rapidly promotes activation of the adjacent ortho-
hydrogen of pyridine to give 4 (Scheme 1). Since complex 3
fails to extrude alkane under analogous conditions in the
absence of a donor such as pyridine, an associative pathway to
form putative 3-py most likely transpires along the reaction
coordinate to A.
The proposed synthetic pathway in Scheme 1 is further
complimented by an independent route to 4, explicitly via a
masked scandium imide. Accordingly, when one equivalent of
Al(CH3)3 in hexane is added to 3, the formation of a new
species is observed. In the 1H NMR spectrum, a broad
resonance integrating to nine hydrogen atoms (d = 0.2 ppm)
is observed, as well as a broad resonance in the 31P NMR
spectrum centered at d = 2.4 ppm (which resolves into two
broad resonances at d = 2.1 and 0.0 ppm upon cooling the
mixture to ꢀ508C).[7] To determine the connectivity of this
new product, single-crystal X-ray diffraction studies were
relied upon; these data showed a zwitterionic scandium imide
[(PNP)Sc{N(Ar)Al(CH3)3}] (5; Scheme 1).[14] The metrical
parameters in the solid-state structure of 5 (Figure 2) are
Figure 1. The molecular structures of complexes 2–4. Methyl groups of
isopropyl substituents on phosphorous, solvent, and hydrogen atoms
(except a-hydrogen atoms) have been excluded for clarity.
Each anilide nitrogen atom lies in the plane formed by its
substituents, and the orientation of the isotropically refined a-
NH hydrogen atom is anti with respect to the N9-Sc-X groove
(X = Cl, CH3). The Sc···H distances (2 2.19(2), 3 2.277(17) ꢀ),
coupled with the obtuse Sc-N-C32 angles (2 155.93(14)8, 3
155.41(11)8), suggest significant a-hydrogen agostic interac-
tions in the solid state, which is in accordance with these
systems partaking in Nanilide!Sc p donation.
Only with exhaustive thermolysis (1008C for several days
in C6D6), does compound 3 eventually break down into a
mixture of intractable materials. It was found, however, that
complex 3 reacts readily with pyridine (py) at room temper-
ature to form the pyridyl anilide complex [(PNP)Sc(NHAr)-
(h2-NC5H4)] (4) concomitant with methane elimination.[7]
Diagnostic spectroscopic features for complex 4 include an
anilide NH resonance at d = 6.18 ppm in the 1H NMR
spectrum, as well as four inequivalent pyridyl resonances in
the d = 8.3–6.6 ppm region.[7] The 13C NMR spectrum unmis-
takably exposes a pyridyl carbon resonance at 217 ppm—
identical to the chemical shift reported for the alkylidene
Figure 2. The molecular structures of complexes 5 and 8. Methyl
groups of isopropyl substituents on phosphorous, solvent, and hydro-
gen atoms (except a-hydrogen atoms) have been excluded for clarity.
comparable to those of the isoelectronic titanium complex
[(PNP)Ti{C(tBu)Al(CH3)3}],[15] an alkylidyne analogue of
2
[11]
=
ꢀ
pyridyl derivative [(PNP)Ti CHtBu(h -NC5H4)].
Single-
Tebbeꢁs reagent. The Sc N imido bond in 5 is much shorter
crystal structural analysis of 4 also corroborates our NMR
(1.9366(14) ꢀ) than in precursors 2, 3, and 4, as well as
significantly contracted relative to that observed for the only
reported scandium m2-imido complex [({C5H4(CH2)2NMe2}Sc-
{m2-NC(Ph)C6H10})2] (2.056(2) ꢀ).[3] DFT calculations and
molecular orbital analysis converged well to the solid-state
structure observed for 5.[7] The natural bond order (NBO)
spectroscopic data, presenting a coordinatively saturated
scandium center bearing an h2-pyridyl ligand (Sc N
ꢀ
[12]
ꢀ
2.229(3), Sc C 2.243(3) ꢀ, Figure 1).
Intuitively, we would expect formation of 4 from 3 to
occur by a s-bond metathesis step, such as those reported in
the literature.[13] However, isotopic labeling studies using
precursor 3 and [D5]py suggest otherwise, cleanly producing
the isotopomer [(PNP)Sc(NDAr)(h2-NC5D4)] ([D5]4).
Examination of the volatiles reveals only formation of CH4,
not CDH3, thereby hinting that a-hydrogen abstraction
precedes ortho-metalation of the pyridine substrate. In the
isotopic labeling studies, no deuterium incorporation is
observed into the anilide aryl framework or components of
the PNP ancillary ligand when the final mixture is assayed by
ꢀ
calculations indicate a much stronger interaction Sc1 N2
ꢀ
(1.16) than that with the PNP amide nitrogen atom (Sc1 N1
0.50).[16] Likewise, the NBO for the bridging Al1 C39 of 0.48
ꢀ
ꢀ
is clearly smaller than that for the terminal Al CH3 group
(0.90) and is thus consistent with a more ionic C39 group
shared between scandium and aluminum. Inspection of the
molecular orbitals for 5 confirms the ionic nature of the m2-
CH3 ligand (HOMOꢀ8, Figure 3) as well as a p bond
=
(HOMOꢀ2) arising from Sc NAr orbital overlap
1
2
a combination of H and H NMR spectroscopy. Therefore,
(Figure 3).[7] As anticipated for a d0 metal center, the
the isotopic labeling studies suggest that compound 3 under-
LUMO is purely metal based (Figure 3).
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Communications
form [(PNP)Sc(NHAr)(C6H5)] (8) as the major product (6:
12 hrs, 708C, 45% yield of isolated product; 7: 24 hrs, 708C,
64% yield of isolated product). Other observable products in
the reaction mixture are minor and have not been identified.
Upon treatment with C6D6, a-hydrogen abstraction in 6 and 7
ꢀ
once again precedes C H bond activation of the solvent, as
isotopically enriched [(PNP)Sc(NDAr)(C6D5)] ([D6]8) is
produced (Scheme 2). No deuterium is incorporated into
1
the leaving alkane, as implied by H and 2H NMR spectra of
the reaction mixture. The formation of 8 from 6 and 7 implies
that a putative low-coordinate imidoscandium intermediate
=
[(PNP)Sc NAr] (B) is likely produced along the reaction
coordinate (Scheme 2). Therefore, the chemistry of putative
ꢀ
B seems to resemble intermolecular 1,2-C H bond additions
involving Group 4–6 imide complexes.[1,19] Compound 6 is far
more reactive than 7, undergoing gradual decomposition in
solution or as a solid at 258C. Therefore, compound 6 must be
rapidly isolated after immediate formation from LiCH2C-
(CH3)3 and 2 in hexane at ꢀ358C. Independently, complex 8
can be prepared in 74% yield from 2 and LiC6H5 in toluene.[7]
Diagnostic features include two broad overlapping resonan-
ces in the 31P NMR spectrum at d = 3.5 and 3.0 ppm, as well as
Figure 3. Selected frontier orbitals for 5 depicting isodensity at 0.05 au.
Formation of complex 5 likely traverses through a hypo-
thetical zwitterion [(PNP)Sc(NHAr)(m2-CH3Al(CH3)3)],
which would then eliminate methane owing to the inherent
superbasicity of the putative anion.[17] Piers and co-workers
reported an analogous methide abstraction reaction with the
surrogate [({ArNC(CH3)}2CH)Sc(NHR){m2-CH3B(C6F5)3}]
(Ar= 2,6,-iPr2C6H3, R = tBu, 2,6-iPr2C6H3), but were unable
to promote elimination of methane via imido bond forma-
tion.[18] We cannot refute, however, the possibility of a smaller
Al(CH3)3 unit coordinating first to the anilide nitrogen atom
in 3 before alkane elimination furnishes the imido linkage.[2f]
Complex 5 is indeed a masked form of a terminal scandium
imido compound, given that treatment with two equivalents
(or neat) pyridine results in immediate activation of the ortho
1
an anilide proton resonance at d = 6.39 ppm in the H NMR
spectrum. The solid-state structure of 8[20] clearly portrays a
ꢀ
ꢀ
phenyl ligand (Sc Cipso 2.275(2) ꢀ) resulting from C H
activation of the arene (Figure 2). Analogous to the molecular
structures of 2–4, the anilide hydrogen atom points away from
the aryl moiety, while the phosphine arms of the PNP ligand
are highly distorted from a transoid orientation (P-Sc-P
146.787(19)8).
The work presented herein therefore supports the exis-
tence of a terminal scandium imido ligand. Systems such as 4–
7 appear to be synthons of a reactive terminal scandium imido
ligand, either by means of decomplexation with Lewis bases, a
thermally induced 1,3-hydrogen shift, or by a-hydrogen
abstraction. Our findings demonstrate, through experimental
evidence, that imidoscandium complexes can be generated
ꢀ
C H bond of pyridine to form 4, concurrent with formation of
the adduct pyAl(CH3)3 (Scheme 1).[7] Complex 4 is also found
to be a resting state for intermediate A (Scheme 1), as the
isotopomer [D5]4 can exchange with proteo-pyridine at 908C
over 8 h and vice versa (Scheme 1).
ꢀ
under mild conditions and can promote the activation of C H
bonds, including that of benzene. Mechanistic and computa-
tional studies derived from this new type of polarized ligand
will be presented in due course.
Incorporation of a more sterically encumbering group in a
scaffold such as {(PNP)Sc(NHAr)}+ facilitates a-hydrogen
abstraction. Accordingly, treatment of 2 with the alkyl
reagents LiCH2X(CH3)3 (X = C, Si) results in rapid formation
of the anilide alkyl derivatives [(PNP)Sc(NHAr){CH2X- Experimental Section
See the Supporting Information for full synthetic, spectroscopic, and
structural details for all new compounds (2–8).
CCDC 692694, 692695, 692696, 692697, 692698 (2–5, 8) contain
the supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
(CH3)3}] (6 X = C, 7 X = Si; Scheme 2). As opposed to 3,
ꢀ
compounds 6 and 7 undergo C H activation of benzene to
Received: July 8, 2008
Published online: October 1, 2008
ꢀ
Keywords: abstraction · C H activation · imido ligands ·
metalation · scandium
.
ꢀ
Scheme 2. Intermolecular C H activation of benzene promoted by
intermediate B to produce 8.
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Angew. Chem. Int. Ed. 2008, 47, 8502 –8505

Angewandte
Chemie
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[14] Crystal data for 5: C44H73AlN2P2Sc, M = 763.92, monoclinic,
space group C2/c, a = 24.3573(10), b = 22.7846(9), c =
18.4427(8) ꢀ, b = 114.9920(10)8, V= 9276.8(7) ꢀ3, Z = 8,
1calcd = 1.094 Mgm3ꢀ, T= 150(2) K, MoKa = 0.71073 ꢀ, absorp-
tion coefficient 0.276 mmꢀ1, F(000) = 3320, 45314 reflections
collected, 9461 independent reflections, Rint = 0.0299, GoF =
1.025, R = 0.0354 [I > 2s(I)], wR2 = 0.0901 [I > 2s(I)].
[15] B. C. Bailey, A. R. Fout, H. Fan, J. C. Huffman, D. J. Mindiola,
Angew. Chem. 2007, 119, 8394; Angew. Chem. Int. Ed. 2007, 46,
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=
[3] D. J. Beetstra, A. Meetsma, B. Hessen, J. H. Teuben, Organo-
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[16] The Sc N distance is comparable to the Pauling and Scho-
maker–Stevenson predicted bond length of 1.923 ꢀ for a double
bond using covalent radii with corrections for electronegativities.
L. Pauling, The Nature of the Chemical Bond, 3rd ed., Cornell
University Press, Ithaca, NY, 1960.
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Am. Chem. Soc. 2007, 129, 5302.
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P. J. Walsh, F. J. Hollander, R. G. Bergman, Organometallics
1993, 12, 3705; P. Royo, J. Sanchez-Nieves, J. Organomet. Chem.
2000, 597, 61; S. Y. Lee, R. G. Bergman, J. Am. Chem. Soc. 1995,
117, 5877; C. P. Schaller, C. C. Cummins, P. T. Wolczanski, J. Am.
Chem. Soc. 1996, 118, 591; J. L. Polse, R. A. Andersen, R. G.
Bergman, J. Am. Chem. Soc. 1998, 120, 13405; D. F. Schafer II,
P. T. Wolczanski, J. Am. Chem. Soc. 1998, 120, 4881; H. M. Hoyt,
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Chem. Soc. 1994, 116, 4133; T. R. Cundari, T. R. Klinckman, P. T.
Wolczanski, J. Am. Chem. Soc. 2002, 124, 1481; P. J. Walsh, F. J.
Hollander, R. G. Bergman, J. Am. Chem. Soc. 1988, 110, 8729;
J. L. Bennett, P. T. Wolczanski, J. Am. Chem. Soc. 1997, 119,
10696; L. M. Slaughter, P. T. Wolczanski, T. R. Klinckman, T. R.
Cundari, J. Am. Chem. Soc. 2000, 122, 7953.
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130, 2408.
[7] See the Supporting Information.
[8] M. D. Fryzuk, G. Giesbrecht, S. J. Rettig, Organometallics 1996,
15, 3329.
[9] Crystal data for 2: C38H58ClN2P2Sc, M = 685.21, monoclinic,
space group P21/c, a = 20.6443(16), b = 11.3633(9), c =
17.3145(13) ꢀ, b = 107.585(2)8, V= 3872.0(5) ꢀ3, Z = 4, 1calcd
=
1.175 Mgmꢀ3, T= 150(2) K, MoKa: 0.71073 ꢀ, absorption coef-
ficient 0.369 mmꢀ1, F(000) = 1472, 22269 reflections collected,
11154 independent reflections, Rint = 0.0710, GoF = 0.810, R =
0.0436 [I > 2s(I)], wR2 = 0.0701 [I > 2s(I)].
[10] Crystal data for 3: C39H61N2P2Sc, M = 664.80, monoclinic, space
group P21/c, a = 20.653(3), b = 11.3736(14), c = 17.400(2) ꢀ, b =
107.674(3)8, V= 3894.4(8) ꢀ3, Z = 4, 1calcd = 1.134 Mgmꢀ3, T=
150(2) K, MoKa: 0.71073 ꢀ, absorption coefficient 0.369 mmꢀ1
,
F(000) = 1440, 69288 reflections collected, 11386 independent
reflections, Rint = 0.0845, GoF = 0.901, R = 0.0355 [I > 2s(I)],
wR2 = 0.0778 [I > 2s(I)].
[11] B. C. Bailey, H. Fan, J. C. Huffman, M.-H. Baik, D. J. Mindiola, J.
Am. Chem. Soc. 2006, 128, 6798.
[12] Crystal data for 4: C45.50H68N3P2Sc, M = 763.93, monoclinic,
space group P21/c, a = 25.0198(9), b = 20.7333(8), c =
[20] Crystal data for 8: C44H63N2P2Sc, M = 726.86, triclinic, space
¯
group P1, a = 11.7183(6), b = 12.2735(6), c = 17.2532(8) ꢀ, a =
74.9370(10),
b = 73.8470(10),
g = 62.5370(10)8,
V=
19.0601(7) ꢀ, b = 90.2720(10)8, V= 9887.2(6) ꢀ3, Z = 8, 1calcd
=
2089.52(18) ꢀ3, Z = 2, 1calcd = 1.155 Mgmꢀ3
,
T= 150(2) K,
1.026 Mgmꢀ3, T= 150(2) K, MoKa: 0.71073 ꢀ, absorption coef-
ficient 0.243 mmꢀ1, F(000) = 3304, 74624 reflections collected,
19728 independent reflections, Rint = 0.0312, GoF = 1.043, R =
0.0602 [I > 2s(I)], wR2 = 0.1923 [I > 2s(I)].
MoKa: 0.71073 ꢀ, absorption coefficient 0.284 mmꢀ1, F(000) =
784, 23174 reflections collected, 7905 independent reflections,
R
int = 0.0319, GoF = 1.013, R = 0.0357 [I > 2s(I)], wR2 = 0.0859
[I > 2s(I)].
Angew. Chem. Int. Ed. 2008, 47, 8502 –8505
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
8505