Accelerated ligand metalation in a β-diketiminato scandium dimethyl complex activated with bis(pentafluorophenyl)borane

Article abstract of DOI:10.1021/om700435v

Equimolar reactions of LScMe₂ (L = (Ar)NC(tBu)CHC(tBu)N(Ar); Ar = 2,6-iPr₂-C₆H₃) and HB(C₆F ₅)₂ proceed through an isolable ion pair 2 to a metalated scandium borate 3 with loss of methane. Multinuclear NMR experiments confirm the proposed structure, which was also verified by synthesis via alternative routes and derivatization. Deuterium labeling studies offer insight into the mechanism of methane loss, which occurs through C-H activation of an abstracted methide rather than the direct intramolecular metalation commonly observed for β-diketiminato scandium alkyl complexes. A large primary kinetic isotope effect of k_H/k_D = 8.7(6) was observed for the decomposition of 2, which corroborates the proposed mechanism while also implicating a highly reactive four-membered scandocycle intermediate, LSc((H)CH₂B(C₆F₅)₂). Reaction of 2 equiv of HB(C₆F₅)₂ with LScR₂ yields μ₂-hydridoborate complexes [LScCH₃] [(μ-H) ₂B(C₆F₅)2] (5), while 4 equiv of borane react to afford the bis-μ₂-hydridoborate complex [LSC][(μ-H) ₂B(C₆F₅)₂]₂, 6.

Full text of DOI:10.1021/om700435v

4464
Organometallics 2007, 26, 4464-4470
Accelerated Ligand Metalation in a â-Diketiminato Scandium
Dimethyl Complex Activated with Bis(pentafluorophenyl)borane
Korey D. Conroy, Paul G. Hayes,‡ Warren E. Piers,* and Masood Parvez
Department of Chemistry, UniVersity of Calgary, 2500 UniVersity DriVe N.W., Calgary,
Alberta, T2N 1N4, Canada
ReceiVed May 3, 2007
Equimolar reactions of LScMe2 (L ) (Ar)NC(tBu)CHC(tBu)N(Ar); Ar ) 2,6-iPr2-C6H3) and
HB(C6F5)2 proceed through an isolable ion pair 2 to a metalated scandium borate 3 with loss of methane.
Multinuclear NMR experiments confirm the proposed structure, which was also verified by synthesis via
alternative routes and derivatization. Deuterium labeling studies offer insight into the mechanism of
methane loss, which occurs through C-H activation of an abstracted methide rather than the direct
intramolecular metalation commonly observed for â-diketiminato scandium alkyl complexes. A large
primary kinetic isotope effect of kH/kD ) 8.7(6) was observed for the decomposition of 2, which
corroborates the proposed mechanism while also implicating a highly reactive four-membered scandocycle
intermediate, LSc((H)CH2B(C6F5)2). Reaction of 2 equiv of HB(C6F5)2 with LScR2 yields µ2-hydridoborate
complexes [LScCH3][(µ-H)2B(C6F5)2] (5), while 4 equiv of borane react to afford the bis-µ2-hydridoborate
complex [LSc][(µ-H)2B(C6F5)2]2, 6.
Chart 1
Introduction
The established reactivity of electrophilic boranes1 as co-
catalyst activators for early metal organometallic compounds
is dominated by abstraction, yielding cationic species that exhibit
enhanced catalytic activity toward olefinic substrates as dem-
onstrated by olefin polymerization2 and hydroamination.3 Typi-
cally, perfluoroaryl boranes such as B(C6F5)3 are employed, due
to their high Lewis acidity and the weakly coordinating nature
of the resultant anions. On the other hand, use of the bis-
pentafluorophenyl borane, HB(C6F5)2,4 gives ion pairs that are
less chemically innocent due to the presence of a reactive H-B
function and the tighter ion-pairing allowed due to the lower
steric bulk of the anion. For example, treatment of Cp2ZrR2 (R
) CH3, CH2Ph, and CH2SiMe3) with various equivalencies of
HB(C6F5)2 can lead to loss of RH5 and formation of a reactive
four-membered metallocycle, which can be stabilized by PMe3
(I, Chart 1). Analogous chemistry with dialkyl titanocenes
resulted in redox processes and generation of Ti(III) complex-
es.6 Schrock’s methyl methylidene tantalocene complex,
Cp2Ta(dCH2)CH3,7 reacts with HB(C6F5)2 to form another d0
borane-stabilized methylidene complex (II, Chart 1).8
The organometallic chemistry of the group 3 metals, while
advancing rapidly,9 is still devoid of well-characterized examples
of metal-to-element multiple-bonded compounds.10 Given the
rich chemistry observed when HB(C6F5)2 was reacted with group
4 and 5 systems bearing two reactive hydrocarbyl units, and
particularly the potential to generate masked alkylidenes, we
have conducted a study of the reactivity of HB(C6F5)2 with the
well-defined dimethyl scandium complex [(ArNC(tBu)CHC-
(tBu)NAr)]Sc(CH3)2 (Ar ) 2,6-iPr2-C6H3), 1,11 in the hopes that
a masked scandium methylidene species might be generated.
While mechanistic studies implicate such a species, it is highly
* To whom correspondence may be addressed. Tel: 403-220-5746.
E-mail: wpiers@ucalgary.ca.
‡ Current address: Department of Chemistry and Biochemistry, Univer-
sity of Lethbridge, E866 University Hall, Lethbridge, Alberta, T1K 3M4,
Canada.
(1) Piers, W. E. AdV. Organomet. Chem. 2005, 52, 1-77.
(2) (a) Chen, E.; Marks, T. J. Chem. ReV. 2000, 100, 1391-1434. (b)
Hou, Z.; Luo, Y.; Li, X. J. Organomet. Chem. 2006, 691, 3114-3121. (c)
Bochmann, M. J. Chem. Soc., Dalton Trans. 1996, 255-270. (d) Arndt,
S.; Okuda, J. AdV. Synth. Catal. 2005, 347, 339-354. (e) Hou, Z.;
Wakatsuki, Y. Coord. Chem. ReV. 2002, 231, 1-22. (f) Jordan, R. F. AdV.
Organomet. Chem. 1991, 32, 325-87. (g) Bochmann, M. J. Organomet.
Chem. 2004, 689, 3982-3998. (h) Kretschmer, W. P.; Meetsma, A.; Hessen,
B.; Schmalz, T.; Qayyum, S.; Kempe, R. Chem.-Eur. J. 2006, 12, 8969-
8978.
(3) (a) Mu¨ller, T. E.; Beller, M. Chem. ReV. 1998, 98, 675-703. (b)
Beller, M.; Seayad, J.; Tillack, A.; Jiao, H. Angew. Chem., Int. Ed. 2004,
43, 3368-3398. (c) Hultzsch, K. C. AdV. Synth. Catal. 2005, 347, 367-
391. (d) Bambirra, S.; Tsurugi, H.; van Leusen, D.; Hessen, B. Dalton Trans.
2006, 1157-1161.
(5) (a) Spence, R. E. v. H.; Parks, D. J.; Piers, W. E.; MacDonald, M.
A.; Zaworotko, M. J.; Rettig, S. J. Angew. Chem., Int. Ed. Engl. 1995, 34,
1230-1233. (b) Spence, R. E. v. H.; Piers, W. E. Organometallics 1995,
14, 4617-4624. (c) Spence, R. E. v. H.; Piers, W. E.; Sun, Y.; Parvez, M.;
MacGillivray, L. R.; Zaworotko, M. J. Organometallics 1998, 17, 2459-
2469.
(6) Chase, P. A.; Piers, W. E.; Parvez, M. Organometallics 2000, 19,
2040-2042.
(7) Schrock, R. R.; Sharp, P. R. J. Am. Chem. Soc. 1978, 100, 2389-
2399.
(8) Cook, K. S.; Piers, W. E.; Rettig, S. J. Organometallics 1999, 18,
1575-1577.
(9) (a) Skinner, M. E. G.; Tyrrell, B. R.; Ward, B. D.; Mountford, P. J.
Organomet. Chem. 2002, 647, 145-150. (b) Zeimentz, P. M.; Arndt, S.;
Elvidge, B. R.; Okuda, J. Chem. ReV. 2006, 106, 2404-2433. (c) Mountford,
P.; Ward, B. D. Chem. Commun. 2003, 1797-1803. (d) Emslie, D. J.; Piers,
W. E. Coord. Chem. ReV. 2002, 233-34, 129-155.
(4) (a) Parks, D. J.; Spence, R. E. v H.; Piers, W. E. Angew. Chem., Int.
Ed. Engl. 1995, 34, 809-811. (b) Parks, D. J.; Piers, W. E.; Yap, G. P. A;
Organometallics 1998, 17, 5492-5503. (c) In benzene solution, bis-
pentafluorophenyl borane exhibits a dimer/monomer equilibrium. Since it
reacts via the monomer, we write its formula this way throughout the present
article.
(10) Giesbrecht, G. R.; Gordon, J. C. Dalton Trans. 2004, 2387-2393.
(11) Hayes, P. G.; Piers, W. E.; Lee, L. W. M.; Knight, L. K.; Parvez,
M.; Elsegood, M. R. J.; Clegg, W. Organometallics 2001, 20, 2533-2544.
10.1021/om700435v CCC: $37.00 © 2007 American Chemical Society
Publication on Web 07/25/2007

â-Diketiminato Scandium Dimethyl Complex
Organometallics, Vol. 26, No. 18, 2007 4465
Scheme 1
reactive and rapidly activates a C-H bond in an N-aryl
the greater donating ability of the fluorine lone pairs and the
strongly donating hydride effectively exclude any contact
between scandium and the C-H bonds of the abstracted
methide. The scandium hydride distance is quite elongated at
2.08(2) Å, implying that the hydride is covalently bound to
boron (cf. B-H1 ) 1.24 Å) and donating σ-electron density to
scandium.
isopropyl group of the ligand framework.
Results and Discussion
Equimolar Reactions of LScMe2 with HB(C6F5)2. The
â-diketiminato scandium dimethyl complex 1 is prepared
according to literature procedures11 by salt metathesis of LScCl2
with excess LiCH3 in toluene. Monitoring the reaction of 1 with
an equimolar amount of HB(C6F5)2 in d8-toluene at 0 °C by 1H
NMR spectroscopy reveals the formation of a C2-symmetric
product, 2, corresponding to the ion pair resulting from methide
abstraction from 1 by the highly Lewis acidic borane, and no
observable byproducts (Scheme 1). Complex 2 is stable for a
number of hours at 0 °C; however, raising the solution
temperature to 25 °C facilitates the clean conversion of 2 to a
new Cs symmetric product, assigned as 3 (Vide infra), with
observed loss of methane (0.16 ppm) over a period of 1 h.
X-ray quality crystals of ion pair 2 were obtained from
concentrated hexane solutions at -35 °C, and the structure is
shown in Figure 1. The structure of 2 depicts a six-coordinate
distorted octahedral scandium center with two ortho fluorine
contacts from the C6F5 groups at 2.340(2) and 2.351(2) Å, which
correlate with previous examples of d0 group 3 metals with
structurally characterized ortho-fluorine contacts such as 2.390(4)
Å for [(ArNC(tBu)CHC(tBu)NAr)ScCH3][CH3B(C6F5)3]12 and
2.366(3) Å for [Cp2Y][MeB(C6F5)3].13 The scandium lies out
of the plane of the ligand by over 1.1 Å, which is a general
feature of scandium â-diketiminato complexes,11 and the degree
of this out-of-plane bonding provides qualitative insight into
the degree of steric congestion at scandium. As expected, the
sterically more demanding substituent occupies the less con-
gested exo position, allowing for further electronic stabilization
through Sc-F interactions, while the scandium methyl occupies
the more crowded endo site. Previous examples11 of methide
abstraction from 1 by B(C6F5)3 reveal contact ion pairs via
coordination of the abstracted methyl group at scandium as well
as by an ortho-fluorine. In the less sterically encumbered 2,
It is evident that the strong ion pairing persists in solution,
as treatment of 2 with PMe3 at 0 °C, followed by warming to
room temperature, proceeds smoothly to 3 and uncoordinated
phosphine exclusively, with no appreciable change in reaction
rate. Apparently, the alkylhydridoborate binds strongly enough
Figure 1. Thermal ellipsoid diagram of 2. Ligand aromatic carbons
and all hydrogen atoms except H1 are removed for clarity. Selected
bond distances (Å): Sc-N1, 2.151(2); Sc-N2, 2.060(2); Sc-C1,
2.193(3); Sc-F1, 2.351(2); Sc-F6, 2.340(2); Sc-H1, 2.08(2).
Distance of Sc from N1-C4-C5-C6-N2 mean plane (Å):
1.13(2). Selected bond angles (deg): N1-Sc-N2, 94.28(8); N1-
Sc-C1, 103.63(10); N2-Sc-B1, 107.30(9); C1-Sc-F1, 153.36(9);
C1-Sc-F6, 79.99(9); N1-Sc-H1, 86.7(7); N2-Sc-H1,
143.4(6).
(12) Hayes, P. G.; Piers, W. E.; McDonald, R. J. Am. Chem. Soc. 2002,
124, 2132-2133.
(13) Song, X. J.; Thornton-Pett, M.; Bochmann, M. Organometallics
1998, 17, 1004-1006.

4466 Organometallics, Vol. 26, No. 18, 2007
Conroy et al.
Scheme 2
Figure 2. 300 MHz NMR spectrum of the methyne region of 3 at
room temperature in d8-toluene. The broad multiplet ([) at ∼2.68
ppm is part of the quartet (1JH-B ) 68 Hz) for the hydridoborate
(CH3(H)B(C6F5)2).
to preclude association by phosphine even when a large excess
of PMe3 is employed.
While 2 exhibits C2 symmetry in solution, the NMR features
of 3 are decidedly more complex. Compound 3 is formed as a
kinetic mixture of two isomers in a ratio of 85:15 (determined
from integrating the â-diketiminato backbone proton of the two
species), possibly representing the exo and endo cyclometalated
positions.11,14 Heating the mixture at 50 °C for 1 h converts the
minor isomer to the major isomer quantitatively, which is stable
Table 1. kobsd and Half-Lives for Methane Loss
compd
T (°C)
k
obsd (s-1
)
t1/2 (h)
1
for hours at temperatures > 80 °C. The H NMR spectrum of
1
2
2
2
65
9.6
21
29
43
29
35
3.59 × 10-4
4.83 × 10-5
1.64 × 10-4
4.16 × 10-4
1.67 × 10-3
4.76 × 10-5
9.77 × 10-5
0.53
4.28
1.27
0.53
0.15
4.23
1.32
the major isomer of 3 exhibits four distinct resonances: three
septets and one multiplet for the four methyne C-H’s from
the isopropyl groups (Figure 2). Furthermore, there are two
singlets corresponding to the tert-butyl groups and an additional
broad resonance upfield of 0 ppm, which integrates as three
hydrogens. Although the observed loss of CH4 is reminiscent
of the reaction of HB(C6F5)2 with Cp2ZrMe2 to produce I,5c
the scandocyclic analogue would be expected to exhibit more
symmetric structural features. We thus assign 3 as the metalated
species shown. The asymmetry of 3, formally chiral at scandium,
is also borne out in the 19F NMR spectrum as two diastereotopic
C6F5 groups are observed. The 11B{1H} NMR spectrum reveals
a sharp singlet in the borate region at -20.6 ppm, which splits
2
d6-2
III
for the isopropyl methyl substituents. One additional broad
singlet at 1.02 ppm is assigned to the borate methyl group. The
1H spectral features of 4 are analogous to the previously reported
[LSc][CH3B(C6F5)3]2.11
The 19F NMR spectrum exhibits two distinct borate groups
present in a 1:2 ratio. The tetrakis-pentafluorophenyl borate has
sharp multiplets, whereas the methylhydridoborate anion exhibits
broad unresolved signals presumably due to hindered rotation
induced by strong ortho-fluorine contacts at scandium. Reaction
of 1 with [Ph3C][B(C6F5)4] forms the previously reported
scandium monocation, which, when treated with HB(C6F5)2, also
affords 4 (Scheme 2). Samples of 4 prepared by either route
depicted in Scheme 2 are spectroscopically identical with the
exception of residual triphenylethane present in samples pre-
pared by route B.15
1
into a doublet (1JB-H ) 68 Hz) in the H-coupled experiment.
Two-dimensional NMR experiments were also performed,
with the most informative data emanating from a 1H-11B
HMQC experiment; two distinct B-H cross-peaks are observed
corresponding to the borohydride at 2.95 ppm and a broad
upfield resonance at -0.13 ppm. In the section of the 1H NMR
spectrum shown in Figure 2, one of the quartets of borohydride
multiplets is evident at 2.68 ppm, the rest being buried under
the isopropyl methyne resonances. The 13C DEPT-135 spectrum
Mechanistic Studies. Qualitatively, the rate of conversion
of 2 to 3 is significantly faster than methane-releasing metalative
process in closely related systems. For example, neutral complex
1 undergoes ligand metalation with a half-life of 0.5 h at 65
°C,11 while cationic [LScMe][MeB(C6F5)3] exhibits a half-life
for methane release of 1.83 h at 35 °C.16 This suggests that a
direct ligand metalation mechanism may not be occurring in
the conversion of 2 to 3, so the reaction was examined in more
detail via a series of kinetic experiments in which the process
1
exhibits one inverted CH2 group at 33 ppm, and the H-1H
COSY has no off-diagonal peaks associated with the broad
singlet at -0.13 ppm. Unfortunately, all attempts to obtain X-ray
quality crystals by exhaustive manipulation of solvent systems
and experimental conditions were unsuccessful, and so the
structural assignment of the major isomer of 3 as depicted in
Scheme 1 is not absolutely proven.
The identity of 3 was further confirmed by its independent
preparation; reaction of the metalated derivative of 111 with
HB(C6F5)2 (Scheme 1) provides a mixture of two products, the
major one of which was clearly identified as 3. The minor
component was assigned to the product resulting from abstrac-
tion of the metalated isopropyl methylene unit. Heating this
mixture in d8-toluene leads to quantitative conversion to the
thermodynamic product 3. Furthermore, 3 was derivatized
through reaction with the anilinium salt [PhN(H)Me2][B(C6F5)4]
in d8-toluene, affording a C2V-symmetric scandium cation, 4,
with no evidence for PhNMe2 coordination (Scheme 2). The
1H NMR spectrum of 4 shows only one isopropyl methyne
septet, one singlet for the tert-butyl groups, and two doublets
1
was followed by H NMR spectroscopy. Monitoring in situ
prepared solutions of 2 in d6-benzene at 29 °C shows clean first-
order conversion with a rate of 4.16 × 10-4 s-1 and a half-life
of 0.5 h (Table 1) by integrating the baseline-resolved ligand
backbone signals. The conversion of 2 to 3 was monitored at a
variety of temperatures (10-43 °C), and an Eyring plot (Figure
3) was constructed, affording activation parameters (∆Sq )
-13(2) eu; ∆Hq ) 18.3(6) kcal mol-1; ∆Gq ) 22.2 kcal mol-1).
Cyclometalation in a related scandium borate,17 III, exhibits a
(15) Dicationic organoscandium derivatives: Ward, B. D.; Bellemin-
Laponnaz, S.; Gade, L. H. Angew. Chem., Int. Ed. 2005, 44, 1668-1671.
(16) Hayes, P. G.; Piers, W. E.; Parvez, M. Organometallics 2005, 24,
1173-1183.
(14) Knight, L. K.; Piers, W. E.; Fleurat-Lessard, P.; Parvez, M.;
McDonald, R. Organometallics 2004, 23, 2087-2094.
(17) Hayes, P. G. Ph.D. Thesis, University of Calgary, 2004.

â-Diketiminato Scandium Dimethyl Complex
Organometallics, Vol. 26, No. 18, 2007 4467
This large kH/kD value is not typical of “normal” σ-bond
metathesis processes,18 but is similar to the value of 9.1(6)
observed in another system where the C-H bond partnering in
the methane-eliminating σ-bond metathetical event was found
within a methyl borate group.19 In that instance, the large
magnitude of the KIE was rationalized on the basis of it being
a composite of both primary and secondary isotope effects.
Whatever the explanation, the large kH/kD observed here is
indicative of methane loss via cleavage of a borate methyl
C-H/D bond, as depicted in Scheme 3. This intimate mecha-
nism of methane loss is supported by monitoring the deuterium
incorporation into the evolved methane and complex 3 isoto-
pomers. Starting from d6-2, a direct ligand metalation (path A)
would yield CHD3 and have no proton resonance for the
resultant methyl group on the borate (i.e., d3-3 would be the
expected product). In contrast, cleavage of one of the C-D
bonds of the abstracted borate methyl with the remaining methyl
group on scandium to produce methane should extrude CD4
exclusively (path B). Rapid cyclometalation of the resultant four-
membered scandocycle to form d2-3 would thus exhibit a proton
resonance that integrates as 1H for the borate methyl group.
The aforementioned primary kinetic isotope effect is suggestive
that path B is operative, as no C-D bonds are broken in path
A.
Figure 3. Eyring plot for the thermal decomposition of 2.
Activation parameters obtained: ∆Sq ) -13(2) eu; ∆Hq ) 18.3-
(6) kcal mol-1; ∆Gq ) 22.2 kcal mol-1
.
To ensure reproducibility, a number of experiments monitor-
ing the decomposition of d6-2 each provided clear evidence that
path B is dominant at room temperature, although because of
the large isotope effect on path B, path A is detectably
competitive via the observation of CD3H.20 Nonetheless integra-
Figure 4. Typical first-order plots for the thermal decomposition
of 2 and d6-2 at 29 °C and 0.0347 M.
slightly higher enthalpy of activation (i.e., 21.5(2) kcal mol-1),
while the entropies of activation observed for these two systems
are slightly different (i.e., -7.0(7) eu versus -13(2) eu for 2),
with the more negative value for 2 implying a somewhat more
ordered transition state. Both sets of data are consistent with a
σ-bond metathesis transformation involving C-H cleavage/
formation processes. However, deuterium labeling experiments
suggest that the mechanism of methane loss in 2 does not
involve direct metalation of the Sc-C bond with an isopropyl
C-H bond. Deuterium labeled d6-1 was prepared in an
1
tion of the methane and borate methyl resonances in the H
NMR spectrum (employing a 15 s relaxation delay to allow for
complete relaxation of the methane protons) gives a d2-2:CD3H
ratio of 91:9, mirroring the relative rates of path B versus path
A in this system. Taking into account the kH/kD, in the all-proteo
system, path B dominates by about 2 to 3 orders of magnitude.
While the scandocycle IV is not observed directly, its
presence, as implied by the spectroscopic evidence supporting
path B, is interesting. The fact that no appreciable amount of
IV accumulates during the course of the reaction implies that,
even though it is masked as a borane adduct, the “ScdCH2”
moiety is a very reactive species. Installation of a more
metalation-resistant ancillary may permit isolation of this borane-
ligated scandium alkylidene function.
Chart 2
Synthesis of LScRn[(µ-H)2B(C6F5)2]2-n (n ) 0, 1). Reactions
of 1 with greater than 1 equiv of HB(C6F5)2 were also
investigated. Complex 1 reacts cleanly with 2 equiv of
HB(C6F5)2 in THF to yield the well-defined (µ-H)2B(C6F5)2
product 5 (Scheme 4) in moderate yields, as well as CH3B-
(C6F5)2.21 When four or more equivalents of HB(C6F5)2 are
allowed to react with 1, both methyl groups are exchanged for
dihydridoborate ligands and the bis-[(µ-H)2B(C6F5)2] product,
6, is obtained. Compound 6 can also be prepared through
addition of two further equivalents of HB(C6F5)2 to 5, with loss
of another CH3B(C6F5)2 molecule.
analogous fashion to 1 with the exception that LiCD3‚LiI was
used in place of LiCH3. The H NMR spectrum of 1 and d6-1
1
in d8-toluene are identical notwithstanding the absence of the
scandium methyl peak at 0.00 ppm in d6-1, which is present in
the 2H NMR spectrum taken in C7H8. Qualitatively, it is
immediately apparent that reaction of d6-1 with HB(C6F5)2,
affording dn-3, is much slower than in the unlabeled system;
under the same conditions where 3 is produced quantitatively
(i.e., 80 min at room temperature), d6-2 underwent <10%
conversion to d2-3. However, heating a sample of d6-2 in d6-
benzene at 47 °C for ∼30 min resulted in clean, complete
conversion to d2-3. Quantitatively, a primary kinetic isotope
effect of 8.7(6) (Figure 4) was observed when the two reactions
were followed side by side.
(18) 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, 203-219.
(19) Zhang, S.; Piers, W. E.; Gao, X.; Parvez, M. J. Am. Chem. Soc.
2000, 122, 5499-5509.
(20) (a) We observe no incorporation of D into the N-aryl isopropyl
groups,20b,c so it is unlikely that the observed CD3H arises due to some
other deuterium scrambling process. (b) Fekl, U.; Goldberg, K. I. J. Am.
Chem. Soc. 2002, 124, 6804-6805. (c) Fekl, U.; Kaminsky, W.; Goldberg,
K. I. J. Am. Chem. Soc. 2003, 125, 15286-15287.
(21) Dagorne, S.; Guzei, I. A.; Coles, M. P.; Jordan, R. F. J. Am. Chem.
Soc. 2000, 122, 274-289.

4468 Organometallics, Vol. 26, No. 18, 2007
Conroy et al.
Scheme 3
Scheme 4
contacts, there is also a donation from F5 of the exo
[H2B(C6F5)2] substituent to scandium at 2.370(5) Å (Vide supra).
This fluoride serves to cap one of the faces of the tetrahedron
formed by the two â-diketiminato nitrogen atoms and the two
hydridoborate ligands. This close contact in the exo site is
consistent with this position being more sterically accessible
than the endo position, where no close Sc-F interactions are
observed. Again, this contact is observed exclusively in the solid
state; low-temperature NMR studies at -90 °C do not exhibit
coalescence of the C6F5 groups in the 19F NMR time-averaged
spectrum. The scandium center lies 1.3 Å out of the plane of
Only one diastereomer of 5 is observed in solution, as
determined by variable-temperature NMR spectroscopy. Pre-
sumably, the bulky hydridoborate group occupies the less
sterically hindered exo position in compound 5. The 11B{1H}
NMR spectrum exhibits a single sharp, upfield resonance at all
temperatures measured, which becomes a triplet (1JH-B ) 69
1
1
Hz) in the H-coupled experiment. The H NMR indicates a
ligand resonance pattern consistent with the presence of two
chemically distinct nonancillary ligands. In the case of product
6, where two diastereotopic [(µ-H)2B(C6F5)2] ligands are present,
two resonances in the room-temperature 11B{1H} NMR spec-
trum indicate that exchange of these moieties is slow on the
NMR time scale. The static nature of the structure is also
manifested in the 1H NMR spectrum, where the pattern observed
for the isopropyl resonances demonstrates the reduction in
symmetry expected for the slow exchange regime.
Crystals of 6 suitable for X-ray diffraction analysis were
grown from cold hexanes, and a CrystalMaker depiction of the
structure is shown in Figure 5, along with selected metrical
parameters. In addition to the four Sc-H and two Sc-N
Figure 5. Thermal ellipsoid diagram of 6. Ligand aromatic groups
and all hydrogen atoms except those associated with boron are
removed for clarity. Selected bond lengths (Å): Sc-N1, 2.052(2);
Sc-N2, 2.148(2); Sc-F5, 2.370(5); Sc-C4, 2.591(2); Sc-C5,
2.630(2); Sc-C6, 2.764(2). Distance of Sc from N1-C4-C5-
C6-N2 mean plane (Å): 1.309(2). Selected bond angles (deg):
N1-Sc-N2, 96.18(6); N1-Sc-B1, 135.85(6); N1-Sc-B2,
112.48(6); N2-Sc-B1, 103.73(6); N2-Sc-B2, 114.39(6);
Sc-N1-C4, 96.58(11); Sc-N2-C6, 103.54(12).

â-Diketiminato Scandium Dimethyl Complex
Organometallics, Vol. 26, No. 18, 2007 4469
were collected on a Bruker P4/RA/SMART 1000 CCD diffracto-
meter using Mo KR radiation at -100 °C.
the ligand, which is 0.2 Å further than that observed for 2 and
is consistent with the greater steric congestion at the metal center
in 6. Metrical parameters associated with the â-diketiminato
ligand are unremarkable.
Synthesis of LSc(CD3)2, d6-1. This compound was prepared in
an identical fashion to that previously described for 1 with the
1
exception that LiCD3‚LiI was used in place of LiCH3.1 The H
Three of the four bridging hydrides were located in the
electron density map and exhibit Sc-H distances ranging
from 2.063(1) to 2.202(1) Å. Previously reported Sc-H
spectrum matched 1, except no resonances were observed for the
scandium methyl groups, which resonate at the same position in
2
2
the H NMR spectrum. H NMR (C7H8): δ (s, 0.00).
distances in scandium borohydride species of 2.17-2.19 Å
22
Synthesis of [LScCD3][CD3(H)B(C6F5)2], d6-2. A 50 mL flask
charged with d6-1 (0.15 g, 0.26 mmol) and HB(C6F5)2 (0.088 g,
0.26 mmol) was attached to a swivel frit apparatus. The entire
assemblage was evacuated, and toluene (25 mL) was vacuum
distilled into the flask. After warming to room temperature, the
reaction was allowed to stir at room temperature for 20 min,
whereupon an orange solution was obtained. Toluene was removed
in Vacuo, and hexamethyldisiloxane (15 mL) was added. The
mixture was sonicated (15 min), cooled to - 35 °C (30 min), and
filtered. The solvent was removed to give a yellow powder, which
was recrystallized from hexanes (0.16 g, 0.17 mmol, 65%). 1H NMR
(C7D8): δ 6.91-6.77 (ov m, 6H; C6H3), 6.20 (s, 1H; CH), 3.40
(sp, 2H; CHMe2), 2.97 (q, 1H; ScCD3(H)B(C6F5)2, 1JH-B ) 69 Hz),
2.55 (sp, 2H; CHMe2), 1.22 (d, 6H; CHMe2), 1.15 (d, 6H; CHMe2),
1.09-1.01 (ov m, 30H; CHMe2, NCCMe3). 13C{1H} NMR
(C7D8): δ 176.4 (NCCMe3), 149.4 (Cipso), 142.5, 141.8, 141.1, 137.5
(C6H3), 96.2 (CH), 45.1 (CMe3), 32.1 (CMe3), 31.3, 30.8 (CHMe2),
29.3, 28.4, 26.4, 25.0 (CHMe2), 24.0 (ScCD3) (CD3(H)B(C6F5)2
not observed). 19F NMR (C7D8): δ -131.0, (o-F), -158.3, (p-F),
for [Li(THF)2][Cp*(C2B9H11)ScH]2
and 2.03
Å
for
(C5H3(SiMe3)2)2Sc(BH4)23 suggest that the hydrides in 3 are
tightly bound to the borate centers and the scandium center is
best depicted as a bis-hydridoborate-stabilized dication. Attempts
to remove an HB(C6F5)2 unit by treating with a Lewis base24
were unsuccessful.
In reactions of Cp2ZrR2 with multiple equivalents of
HB(C6F5)2 we invoked a stepwise path involving Cp2Zr(H)R
intermediates that add a second equivalent of HB(C6F5)2 to give
the observed Cp2Zr[(µ-H)2B(C6F5)2]R products. This is unlikely
to be the case in these scandium â-diketiminato systems, since
all attempts to generate hydrido complexes supported by this
ligand system have led to ligand fragmentation processes
initiated by hydride transfer to an imino carbon.14 It thus seems
likely that the path by which compound 5 forms involves
interaction of the second borane equivalent with 2 directly,
possibly by hydride abstraction from the [(H)CH3B(C6F5)2]
anion. An analogous sequence from 5 delivers compound 6.
1
-162.6, (m-F). 11B NMR (C7D8): δ -17.2 (d, 1B; JH-B ) 69
Hz).
Experimental Section
Synthesis of [κ3-LSc][CH3(H)B(C6F5)2], 3. A 50 mL flask
charged with 1 (0.350 g, 0.608 mmol) and HB(C6F5)2 (0.210 g,
0.608 mmol) was attached to a swivel frit apparatus. The entire
assemblage was evacuated, and toluene (25 mL) was vacuum
distilled into the flask. After warming to room temperature, the
reaction was allowed to stir at room temperature for 80 min,
whereupon a dark orange solution was obtained. Toluene was
removed in Vacuo, and hexamethyldisiloxane (15 mL) was added.
The mixture was sonicated (15 min) and cooled to -35 °C (30
min), and the solid yellow product was removed by filtration. The
product was recrystallized from hexanes (0.374 g, 0.413 mmol,
General Procedures. All manipulations were performed either
in an Innovative Technologies System One inert atmosphere
glovebox or on greaseless vacuum lines equipped with Teflon
needle valves (Kontes) using swivel-frit-type glassware. Toluene,
THF, and hexanes were dried and purified using the Grubbs/Dow
purification system25 and stored in evacuated bombs. Bromobenzene
and d5-bromobenzene were predried over CaH2, and hexamethyl-
disiloxane, d8-THF, d6-benzene, and d8-toluene were dried and
stored over sodium/benzophenone. All were distilled prior to use.
The following reagents were synthesized using literature proto-
cols: LSc(CH3)2 1,11 HB(C6F5)2, and DB(C6F5)2.4a [PhN(H)Me2]-
[B(C6F5)4] and [Ph3C][B(C6F5)4] were received as generous gifts
from NOVA Chemicals Ltd. All other materials were obtained from
Aldrich and either used as received or dried and distilled prior to
use.
1
68%). H NMR (C6D6): δ 7.11-6.64 (ov m, 6H; C6H3), 5.75 (s,
1H; CH), 3.19 (sp, 1H; CHMe2), 3.09 (m, 1H; CH2CHMe), 2.95
(q, 1H; ScCH3(H)B(C6F5)2, 1JH-B ) 68 Hz), 2.82 (sp, 1H; CHMe2),
2.55 (sp, 1H; CHMe2), 1.45 (d, 3H; CHMe2), 1.33 (d, 3H; CHMe2),
1.13 (d, 3H; CHMe2), 1.01-0.97 (ov m, 30H; CHMe2, NCCMe3),
0.82 (d, 3H; CHMe2), -0.13 (br s, 3H; ScCH3(H)B(C6F5)2,).
13C{1H} NMR (C7D8): δ 174.8, 174.5 (NCCMe3), 145.5, 143.9
(Cipso), 143.4, 137.6, 136.9, 135.3, 129.3, 128.5, 126.0, 125.7, 124.0,
122.5 (C6H3), 101.3 (CH), 42.9, 39.3 (CMe3), 32,1, 31.5 (CMe3)
21.2, 29.2, 28.4, 27.8 (CHMe2), 25.1, 25.0, 24.8, 24.4 (2), 24.2,
23.4 (2) (CHMe2) (CH3(H)B(C6F5)2 not observed). 19F NMR
(C7D8): δ -130.0, -130.5 (o-F), -159.9, -160.3 (p-F), -164.3,
-164.7 (m-F). 11B NMR (C7D8): δ -20.6 (d, 1B; 1JH-B ) 68 Hz).
Anal. Calcd for C48H56N2BF10Sc: C, 63.59; H, 6.23; N, 3.09.
Found: C, 63.97; H, 6.19; N, 2.94.
Samples were analyzed by NMR spectroscopy on Bruker AMX-
300 and DRX-400 spectrometers at room temperature unless
otherwise specified. 1H and 13C were referenced to Si(CH3)4 through
2
the residual peaks of the employed solvent, H spectra to external
Si(CD3)4 at 0.0 ppm, 11B spectra to external BF3‚OEt2 at 0.0 ppm,
and 19F spectra to CFCl3 using an external standard of hexafluo-
robenzene (δ -163.0 ppm) in C6D6. NMR data are provided in
ppm; 13C resonances for the C6F5 groups are not reported. The
3
coupling constants (i.e., JH-H) for the isopropyl groups on the
ligand range between 6.4 and 7.2 Hz and, thus, are not reported in
the NMR analysis of the compounds. Elemental analyses were
performed by Mrs. Roxanna Smith and Mrs. Dorothy Fox in the
microanalytical laboratory at the University of Calgary. X-ray data
Synthesis of [LSc][CH3(H)B(C6F5)2][B(C6F5)4], 4. A 5 mm
NMR tube was charged with 3 (0.017 g, 0.019 mmol), and [PhN-
(H)Me2][B(C6F5)4] and d8-toluene (0.5 mL) were added. The tube
was shaken vigorously for 10 min and then allowed to settle,
whereupon a dark orange oil collected at the bottom of the tube.
The d8-toluene solution containing PhNMe2 was decanted, and the
oil was dissolved in d5-bromobenzene. 1H NMR (C6D5Br): δ 7.31-
7.17 (ov m, 6H; C6H3), 6.57 (s, 1H; CH), 2.25 (br sp, 4H, CHMe2),
1.31-1.24 (ov m, 30H; CHMe2, NCCMe3), 1.02 (br s, 3H; CH3-
(H)B(C6F5)2,) 0.89 (d, 6H; CHMe2). 13C{1H} NMR (C6D5Br): δ
173.2 (NCCMe3), 152.3 (Cipso), 149.9, 142.0, 141.7, 140.3, 133.8,
(C6H3), 97.6 (CH), 47.9 (CMe3), 47.0 (CHMe2), 32.1 (CMe3), 27.7
(22) Bazan, G. C.; Schaefer, W. P.; Bercaw, J. E. Organometallics 1993,
12, 2126-2130.
(23) (a) Lappert, M. F.; Singh, A.; Atwood, J. L.; Hunter, W. E. J. Chem.
Soc., Chem. Commun. 1983, 206-207. (b) See also: Basuli, F.; Tomasze-
wski, J.; Huffman, J. C.; Mindiola, D. J. Organometallics 2003, 22, 4705-
4714.
(24) Hu¨sgen, N. S.; Luinstra, G. A. Inorg. Chim. Acta 1997, 259, 185-
196.
(25) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J. Organometallics 1996, 15, 1518-1520.

4470 Organometallics, Vol. 26, No. 18, 2007
Conroy et al.
Table 2. Data Collection and Structure Refinement Details
for 2 and 6
was attached to a swivel frit apparatus. The entire assemblage was
evacuated, and toluene (30 mL) was vacuum distilled into the flask.
After warming to room temperature, the reaction mixture was
allowed to stir for 30 min and filtered and solvent removed under
vacuum to afford a yellow powder. Hexanes (15 mL) were added,
and the resultant suspension was sonicated for 15 min. The volume
was reduced to 5 mL, cooled to -78 °C for 30 min, and back-
filtered. The solvent was removed in Vacuo to afford 6 as a light
2
6
formula
C49H60BF10N2Sc
C59H57B2F20N2Sc‚
C6H14
fw
922.76
triclinic
P1h
1326.82
monoclinic
P21/n
13.4712(1)
18.5810(2)
26.1801(2)
90
90.899(1)
90
6552.3(1)
4
cryst syst
space group
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
Z
12.233(5)
12.595(4)
17.932(8)
99.87(2)
92.75(2)
117.27(2)
2394.1(2)
2
1
yellow powder (0.089 g, 0.072 mmol, 72%). H NMR (C6D6): δ
6.99 (t, 2H; C6H3, JH-H ) 7.7 Hz), 6.87-6.83 (m, 4H; C6H3), 5.55
(s, 1H; CH), 3.08 (sp, 2H; CHMe2), 2.52 (sp, 2H; CHMe2), 2.48
(q, 4H; Sc[(µ-H)2B(C6F5)2]2, 1JH-B ) 67 Hz), 1.34 (d, 6H; CHMe2),
1.13 (d, 6H; CHMe2), 1.04 (d, 6H; CHMe2), 1.00 (d, 6H; CHMe2),
0.82 (s, 18H; NCCMe3). 13C{1H} NMR (C7D8): δ 177.0
(NCCMe3), 142.4 (Cipso), 141.0, 140.6, 128.3, 125.3, 124.7 (C6H3),
82.2 (CH), 45.0 (CMe3), 31.6 (CHMe2), 30.7 (CMe3), 28.7
(CHMe2), 25.8, 25.4, 25.1, 24.6 (CHMe2). 19F NMR (C7D8): δ
-127.5, -130.3 (o-F), -156.6, -156.8 (p-F), -162.3, -163.4
(m-F). 11B NMR (C7D8): δ -13.6 (t, 1B; 1JH-B ) 67 Hz), -15.5
(t, 1B; 1JH-B ) 67 Hz). Anal. Calcd for C59H57N2B2F20Sc: C, 57.12;
H, 4.63; N, 2.26. Found: C, 56.81; H, 4.55; N, 2.07.
Kinetic Isotope Effect Measurements. The following is a
general procedure for the kinetic measurements of conversion from
2 to 3 as monitored by 1H NMR spectroscopy. A 5 mm NMR tube
was charged with 1 or d6-1 (0.010 g, 1.74 µmol) and HB(C6F5)2
(0.006 g, 1.74 µmol), sealed with a rubber septum, and cooled to
-78 °C. d6-Benzene (0.5 mL) was added via syringe, yielding a
0.0347 M solution, which was slowly warmed to room temperature,
shaken briefly to facilitate mixing, and then placed in the magnet,
whereupon acquisition commenced following a brief pause for
temperature equilibration. Data were analyzed by monitoring the
disappearance of the backbone peak of 2 relative to 3 and plotted
using the first-order equation. Each experiment was repeated a
minimum of two times.
T, K
173(2)
173(2)
λ, Å
0.71073
1.280
968
0.71073
1.345
2744
F
calc, g/cm3
F(000)
µ, mm-1
0.227
0.210
cryst size, mm3
transmn factors
θ range, deg
no. of data/restraints/
params
0.32 × 0.08 × 0.05
0.989-0.931
3.2-25.0
8372/0/586
0.27 × 0.22 × 0.20
0.959-0.946
2.5-30.0
18 604/0/822
GoF
1.00
1.02
R1 (I > 2σ(I))
wR2 (all data)
residual density, e/Å3
0.043
0.097
0.19 and -0.26
0.057
0.144
0.40 and -0.42
(CH3(H)B(C6F5)2), 25.8 (CHMe2). 19F NMR (C6D5Br): δ -129.6
(o-F; CH3(H)B(C6F5)2), -131.1 (o-F; B(C6F5)4), -151.5 (p-F;
CH3(H)B(C6F5)2), -157.2 (m-F; CH3(H)B(C6F5)2), -161.6 (p-F;
B(C6F5)4), -165.4 (m-F; B(C6F5)4). 11B{1H} NMR (C6D5Br): δ
-16.6 (s, 2B).
Synthesis of [LScCH3][(µ-H)2B(C6F5)2], 5. HB(C6F5)2 (0.160
g, 0.466 mmol) and 1 (0.135 g, 0.233 mmol) were dissolved in
THF (15 mL) and stirred for 15 min, whereupon solvent was
removed under vacuum to give an oily yellow solid. Hexanes (20
mL) were added and the reaction mixture was filtered. The solvent
was removed in Vacuo to afford 5 as a yellow solid, which was
Single-Crystal X-ray Analyses. Crystals of 2 and 6 were coated
with Paratone 8277 oil and mounted on a glass fiber. Measurements
were made on a Nonius Kappa CCD diffractometer (University of
Calgary) using graphite-monochromated Mo KR radiation for all
measurements. Table 2 gives further details, and the crystallographic
information files are available as Supporting Information.
1
recrystallized from cold hexanes (0.112 g, 0.123 mmol, 53%). H
NMR (C7D8, 285 K): δ 7.17-6.81 (ov m, 6H; C6H3), 5.49 (s, 1H;
CH), 3.46 (sp, 2H; CHMe2), 2.77 (q, 2H; Sc[(µ-H)2B(C6F5)2], 1JH-B
) 69 Hz), 2.61 (sp, 2H; CHMe2), 1.46 (d, 6H; CHMe2), 1.38 (d,
6H; CHMe2), 1.10 (d, 6H; CHMe2), 0.98 (d, 6H; CHMe2), 0.87 (s,
18H; NCCMe3), 0.18 (ScMe). 13C{1H} NMR (C7D8), 285 K): δ
173.9 (NCCMe3), 142.4 (Cipso), 140.1, 139.4, 126.3, 124.2, 123.6
(C6H3), 87.8 (CH), 43.9 (CMe3), 32.3 (CHMe2), 31.0 (CMe3), 28.1
(CHMe2), 26.1, 25.9, 24.2, 23.7 (CHMe2) (ScMe not observed).
19F NMR (C7D8): δ -131.2 (o-F), -157.5 (p-F), -163.7 (m-F).
11B NMR (d8-THF): δ -10.9 (t, 1B; 1JH-B ) 69 Hz). Anal. Calcd
for C48H58N2BF10Sc: C, 63.45; H, 6.43; N, 3.08. Found: C, 63.15;
H, 6.29; N, 2.96.
Acknowledgment. Funding for this work came from the
Natural Sciences and Engineering Research Council in the form
of a Discovery Grant to W.E.P. and scholarships to K.D.C.
(PGS-A and CGS-D) and P.G.H. (PGS-A and PGS-B). K.D.C.
also thanks Alberta Ingenuity for a Studentship Award. P.G.H.
acknowledges the Alberta Heritage Foundation for a Ralph
Steinhauer Award and the Sir Izaak Walton Killam Foundation
for Doctoral Fellowships.
Supporting Information Available: Crystallographic data
(CIF) for 2 and 6. This material is available free of charge via the
Internet at http://pubs.acs.org.
Synthesis of [LSc][(µ-H)2B(C6F5)2]2, 6. A 50 mL flask charged
with 1 (0.058 g, 0.10 mmol) and HB(C6F5)2 (0.140 g, 0.40 mmol)
OM700435V