Synthesis and structures of cationic aluminum and gallium amidinate complexes

Article abstract of DOI:10.1021/ja992104j

Aluminum and gallium amidinate complexes, {RC(NR')₂}MMe₂ (R, R' = alkyl; M = Al, Ga), react with the 'cationic activators' [Ph₃C][B(C₆F₅)₄] and B(C₆F₅)₃ to yield cationic Al and Ga alkyl species whose structures are strongly influenced by the steric properties of the amidinate ligand. The reaction of acetamidinate Al complexes {MeC(NR')₂}AlMe₂ (R' = (i)Pr, 1a; R' = Cy, 3a) with 0.5 equiv of [Ph₃C][B(C₆F₅)₄] or B(C₆F₅)₃ yields {MeC(NR')₂}₂Al₂Me₃⁺ (R' = (i)Pr, 2a⁺; R' = Cy, 4a⁺) as the B(C₆F₅)₄^- or MeB(C₆F₅)₃^- salts. X-ray crystallographic analyses establish that 2a⁺ and 4a⁺ are double-amidinate-bridged dinuclear cations, in which the two metal centers are linked by μ-η¹,η¹ and μ-η¹-*h² amidinate bridges. NMR studies show that 2a⁺ undergoes two dynamic processes in solution: (i) a μ-η¹,η¹/μ-η¹,η² amidinate exchange and (ii) Me exchange between the two metal centers. The reaction of {MeC(N(i)Pr)₂}GaMe₂ (1b) with 0.5 equiv of B(C₆F₅)₃ yields {MeC(N(i)Pr)₂}₂Ga₂Me₃⁺ (2b⁺), whose structure and dynamic properties are similar to those of 2a⁺. The reaction of the bulkier (i)Bu-substituted amidinate complexes {(t)BuC(N(i)Pr)₂}MMe₂ (M = Al, 6a; M = Ga, 6b) with 0.5 equiv of [Ph₃C][B(C₆F₅)₄] yields {(t)BuC(N(i)Pr)₂}MMe₂·{(t)BuC(N(i)Pr)₂}MMe⁺ (M = Al, 7a⁺; M = Ga, 7b^-) as the B(C₆F₅)₄^- salts, the former of which is thermally unstable. An X-ray crystallographic analysis establishes that 7b⁺ is a single- amidinate-bridged dinuclear cation, in which the two metal centers are linked by a μ-η¹,η² amidinate bridge. NMR data establish that the structures of 7a⁺ and 7b⁺ are similar and both species are rigid in solution. 6a and 6b also react with B(C₆F₅)₃ to yield [7a][MeB(C₆F₅)₃] and [7b][MeB(C₆F₅)₃], respectively, which decompose by C₆F₅^- transfer to yield {(t)BuC(N(i))Pr)₂}M(Me)(C₆F₅) (M = Al, 9a; M = Ga, 9b) and boron species. The 'super-bulky' amidinate complexes {(t)BuC(N(t)Bu)₂}MMe₂ (M = Al, 12a; M = Ga, 12b) react with 1 equiv of [Ph₃C][B(C₆F₅)₄] to yield {(t)BuC(N(i)Bu)₂}MMe⁺ (M = Al, 13a⁺; M = Ga, 13b⁺) as the B(C₆F₅)₄^- salts. The salts [13a][B(C₆F₅)₄] and [13b][B(C₆F₅)₄] are thermally unstable and could not be isolated. However, the NMR data for 13a⁺ and 13b⁺ in C₆D₅Cl are consistent with base-free, three-coordinate structures or labile, four-coordinate solvated cations. These results provide a starting point for understanding the mechanism and reactivity trends in ethylene polymerization catalyzed by cationic Al amidinate species.

Full text of DOI:10.1021/ja992104j

274
J. Am. Chem. Soc. 2000, 122, 274-289
Synthesis and Structures of Cationic Aluminum and Gallium
Amidinate Complexes
Samuel Dagorne,† Ilia A. Guzei,‡ Martyn P. Coles,† and Richard F. Jordan*,†
Contribution from the Departments of Chemistry, The UniVersity of Iowa, Iowa City, Iowa, 52242, and
Iowa State UniVersity, Ames, Iowa, 50011
ReceiVed June 21, 1999
Abstract: Aluminum and gallium amidinate complexes, {RC(NR′)2}MMe2 (R, R′ ) alkyl; M ) Al, Ga),
react with the “cationic activators” [Ph3C][B(C6F5)4] and B(C6F5)3 to yield cationic Al and Ga alkyl species
whose structures are strongly influenced by the steric properties of the amidinate ligand. The reaction of
acetamidinate Al complexes {MeC(NR′)2}AlMe2 (R′ ) iPr, 1a; R′ ) Cy, 3a) with 0.5 equiv of [Ph3C][B(C6F5)4]
+
i
-
-
or B(C6F5)3 yields {MeC(NR′)2}2Al2Me3 (R′ ) Pr, 2a+; R′ ) Cy, 4a+) as the B(C6F5)4 or MeB(C6F5)3
salts. X-ray crystallographic analyses establish that 2a+ and 4a+ are double-amidinate-bridged dinuclear cations,
in which the two metal centers are linked by µ-η1,η1 and µ-η1,η2 amidinate bridges. NMR studies show that
2a+ undergoes two dynamic processes in solution: (i) a µ-η1,η1/µ-η1,η2 amidinate exchange and (ii) Me exchange
between the two metal centers. The reaction of {MeC(NiPr)2}GaMe2 (1b) with 0.5 equiv of B(C6F5)3 yields
{MeC(NiPr)2}2Ga2Me3+ (2b+), whose structure and dynamic properties are similar to those of 2a+. The reaction
of the bulkier tBu-substituted amidinate complexes {tBuC(NiPr)2}MMe2 (M ) Al, 6a; M ) Ga, 6b) with 0.5
equiv of [Ph3C][B(C6F5)4] yields {tBuC(NiPr)2}MMe2‚{tBuC(NiPr)2}MMe+ (M ) Al, 7a+; M ) Ga, 7b+) as
the B(C6F5)4- salts, the former of which is thermally unstable. An X-ray crystallographic analysis establishes
that 7b+ is a single-amidinate-bridged dinuclear cation, in which the two metal centers are linked by a µ-η1,η2
amidinate bridge. NMR data establish that the structures of 7a+ and 7b+ are similar and both species are rigid
in solution. 6a and 6b also react with B(C6F5)3 to yield [7a][MeB(C6F5)3] and [7b][MeB(C6F5)3], respectively,
-
which decompose by C6F5 transfer to yield {tBuC(NiPr)2}M(Me)(C6F5) (M ) Al, 9a; M ) Ga, 9b) and
boron species. The “super-bulky” amidinate complexes {tBuC(NtBu)2}MMe2 (M ) Al, 12a; M ) Ga, 12b)
react with 1 equiv of [Ph3C][B(C6F5)4] to yield {tBuC(NtBu)2}MMe+ (M ) Al, 13a+; M ) Ga, 13b+) as the
B(C6F5)4- salts. The salts [13a][B(C6F5)4] and [13b][B(C6F5)4] are thermally unstable and could not be isolated.
However, the NMR data for 13a+ and 13b+ in C6D5Cl are consistent with base-free, three-coordinate structures
or labile, four-coordinate solvated cations. These results provide a starting point for understanding the mechanism
and reactivity trends in ethylene polymerization catalyzed by cationic Al amidinate species.
Introduction
tion.3 Recent developments in this area include (i) cationic
polymerization of isobutylene and isobutylene/isoprene by Cp2-
Neutral aluminum complexes, (AlX3)2 (X ) halide, alkyl,
alkoxide, etc.), are widely used as reagents or catalysts for Lewis
acid-mediated reactions (Friedel-Crafts, Diels-Alder, etc.),
alkylating agents, initiators for cationic polymerizations, and
cocatalysts/activators in transition-metal-catalyzed olefin poly-
merizations.1 Additionally, neutral Al alkyls catalyze the oli-
gomerization of ethylene to R-olefins at elevated temperatures
and ethylene pressures.2 Cationic aluminum complexes are of
potential interest for many of these applications because of the
possibility that the increased electrophilicity resulting from the
cationic charge may enhance substrate coordination and activa-
Al+,3a (ii) ring-opening polymerization of propylene oxide by
+ 3b
(salen)Al(MeOH)2+, (acen)Al(MeOH)2
,
{η3-MeN((CH2)2-
NMe2)2}AlMe2+, and {η3-HRN(CH2)2NR′(CH2)2NR}AlCl+,3c,d
(iii) ring-opening polymerization of D,L-lactide by {HRN(CH2)2-
NR′(CH2)2NR}AlCl+,3d and (iv) catalysis of enantioselective
Diels-Alder reactions by chiral cationic boron species.3e
Additionally, cationic Al alkyls have been reported to polymer-
ize ethylene, but the active species in these reactions have not
yet been identified.4
(2) (a) Ziegler, K., Gellert, H. G. Justus Liebigs Ann. Chem. 1950, 567,
195. (b) Skinner, W. A.; Bishop, E.; Cambour, P.; Fuqua, S.; Lim, P. Ind.
Eng. Chem. 1960, 52, 695. (c) Skup´ınska, J. Chem. ReV. 1991, 91, 613. (d)
Al-Jarallah, A. M.; Anabtawi, J. A.; Siddiqui, M. A. B.; Aitani, A. M.;
Al-Sa’doun, A. W. Catal. Today 1992, 14, 42.
(3) (a) Bochmann, M.; Dawson, D. M. Angew. Chem., Int. Ed. Engl.
1996, 35, 2226. (b) Atwood, D. A.; Jegier, J. A.; Rutherford, D. J. Am.
Chem. Soc. 1995, 117, 6779. (c) Jegier, J. A.; Atwood, D. A. Inorg. Chem.
1997, 36, 2034. (d) Emig, N.; Nguyen, H.; Krautscheid, H.; Reau, R.;
Cazaux, J. B.; Bertrand, G. Organometallics 1998, 17, 3599. (e) Hayashi,
Y.; Rohde, J. J.; Corey, E. J. J. Am. Chem. Soc. 1996, 118, 5502. (f) For
a review, see: Atwood, D. A. Coord. Chem. ReV. 1998, 176, 407.
(4) (a) Coles, M. P.; Jordan, R. F. J. Am. Chem. Soc. 1997, 119, 8125.
(b) Ihara, E.; Young, V. G., Jr.; Jordan, R. F. J. Am. Chem. Soc. 1998, 120,
8277. (c) Bruce, M.; Gibson, V. C.; Redshaw, C.; Solan, G. A.; White, A.
J. P.; Williams, D. J. Chem. Commun. 1998, 2523.
* Address correspondence to this author at The Department of Chemistry,
The University of Chicago, 5735 S. Ellis Ave., Chicago, IL 60637.
† The University of Iowa.
‡ Iowa State University.
(1) (a) Starowieyski, K. B. In Chemistry of Aluminium, Gallium, Indium
and Thallium, Downs, A. J., Ed.; Chapman & Hall: London, UK, 1993;
pp 322-371. (b) Eisch, J. J. In ComprehensiVe Organometallic Chemistry
II; Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds.; Elsevier: Oxford,
UK, 1995; Vol. 11, Chapter 6. (c) Matyjaszewski, K., Ed. Cationic
Polymerizations: Mechanisms, Synthesis and Applications; Marcel Dek-
ker: New York, 1996. (d) Kennedy, J. P.; Marechal, E. Carbocationic
Polymerization; Wiley-Interscience: New York, 1982. (e) Eisch, J. J. In
ComprehensiVe Organometallic Chemistry II; Abel, E. W., Stone, F. G.
A., Wilkinson, G., Eds.; Elsevier: Oxford, UK, 1995; Vol. 1, Chapter 10.
10.1021/ja992104j CCC: $19.00 © 2000 American Chemical Society
Published on Web 12/23/1999

Cationic Al and Ga Amidinate Complexes
J. Am. Chem. Soc., Vol. 122, No. 2, 2000 275
-
Chart 1
Amidinate ligands, RC(NR′)2 (VII), are interesting candi-
dates for application as ancillary ligands in {L-X}AlR+ species
because their steric and metal-binding properties can be modified
by variation of the amidinate substituents, and they form
compact four-membered {RC(NR′)2}Al chelate rings resulting
in sterically open Al centers.9 We recently reported that {RC-
(NR′)2}AlMe2 complexes react with cationic activators to yield
cationic Al species which polymerize ethylene.4a,10 Here we
describe the structures and dynamic properties of the cationic
Al species formed in these systems. As will be seen, the
chemistry of cationic Al amidinate species is more complicated
than that of cationic complexes based on I and II due to the
ability of amidinate ligands to adopt several types of chelating
and bridging bonding modes. The synthesis and structures of
cationic gallium species derived by methyl abstraction from
{RC(NR′)2}GaMe2 complexes are also described. The Ga
cations are considerably less reactive than the Al analogues,
due to the lower polarity of the Ga-Me bonds vs the Al-Me
bonds, and thus serve as useful models for the Al systems.11
Moreover, cationic Ga alkyls are of interest for synthetic and
catalytic applications involving polar substrates because of the
relative stability of Ga-R bonds (vs Al-R) toward hydrolysis
and electrophilic cleavage.12
Results
i
Reaction of {MeC(NR′)2}AlMe2 (1a, R′ ) Pr; 3a, R′ )
Cy) with B(C6F5)3 or [Ph3C][B(C6F5)4]. The reaction of 1a
with 0.5 equiv of B(C6F5)3 or [Ph3C][B(C6F5)4] in C6D5Cl (10
min, 23 °C) yields the dinuclear cationic complex {MeC(Ni-
Pr)2}2Al2Me3 (2a+) as the MeB(C6F5)3 or B(C6F5)4 salt,
respectively (100% by 1H NMR vs an internal standard, Scheme
1). For [2a][B(C6F5)4], the presence of 1 equiv of Ph3CMe (vs
2a+) was also observed by 1H NMR. [2a][MeB(C6F5)3] and [2a]-
[B(C6F5)4] do not react further with excess B(C6F5)3 or [Ph3C]-
[B(C6F5)4] at 23 °C as shown by 1H and 19F NMR. [2a][MeB-
(C6F5)3] slowly decomposes in CD2Cl2 (t1/2 ≈ 5 h, 23 °C) and
C6D5Cl (t1/2 ≈ 18 h, 23 °C) to unidentified species, whereas
[2a][B(C6F5)4] is stable in those solvents at 23 °C for several
days. [2a][MeB(C6F5)3] and [2a][B(C6F5)4] were isolated as
analytically pure colorless solids in 83% and 78% yield,
respectively, by generation in CH2Cl2, removal of volatiles, and
pentane washing.13 Initial NMR studies suggested that 2a+
adopts a Me-bridged structure.4a However, more extensive NMR
and crystallographic results establish that 2a+ adopts an amidi-
nate-bridged structure (vide infra).
+
-
-
Three-coordinate cationic Al alkyls of general type {L-X}-
AlR+ that are stabilized by monoanionic bidentate L-X-
ligands are attractive targets for investigation because they
represent a reasonable balance of electrophilicity and stability.
The use of bidentate stabilizing ligands may alleviate complica-
tions from ligand redistribution reactions, a common feature in
Al chemistry.1a In contrast, simple two-coordinate AlR2+ species
(R ) Me, Et) appear to react rapidly with B(C6F5)4-,5 one of
the most robust “noncoordinating” anions known,6 while four-
coordinate AlR2(NR3)2+ and related AlR2L2+ species may need
to undergo ligand dissociation prior to substrate binding in many
cases. We have described the synthesis and reactivity of {L-
X}AlR+ species containing N,N-diisopropylaminotroponiminate
(ATI, I, Chart 1) and N,N-diaryldiketiminate ligands (II) which
form five- and six-membered chelate rings with Al.4b,7 Neutral
{L-X}AlR2 complexes based on I and II react readily with
the “cationic activators” [Ph3C][B(C6F5)4], B(C6F5)3 and [NHMe2-
Ph][B(C6F5)4] to yield cationic products whose structures are
strongly influenced by the steric properties of the L-X- ligand
and the nature of the Al-R alkyl group. Thus far, three structural
types have been characterized: base-free three-coordinate
species III, base-stabilized four-coordinate cations IV, and
dinuclear Me-bridged cations V. Base-free (ATI)AlR+ (R )
Et, iBu) species polymerize ethylene,4b,8 while base-free
{diketiminate}AlR+ species undergo an unusual cycloaddition
reaction with ethylene and other unsaturated hydrocarbons to
give bicyclic complexes of type VI.7
Similarly, the reaction of 3a with 0.5 equiv of [Ph3C]-
[B(C6F5)4] in hexane (3 d, 23 °C) quantitatively yields [{MeC-
(NCy)2}2Al2Me3][B(C6F5)4] ([4a][B(C6F5)4]), which is isolated
as an analytically pure solid by filtration (61%, Scheme 1). The
1H NMR spectrum of [4a][B(C6F5)4] (CD2Cl2) is very similar
to that of [2a][B(C6F5)4], except for the Cy resonances. [4a]-
[B(C6F5)4] was obtained as colorless crystals by crystallization
from a mixed solvent system (10/10/1/1 hexane/pentane/C6D5-
Cl/ClCD2CD2Cl, see Experimental Section).
(9) (a) Edelmann, F. T. Coord. Chem. ReV. 1994, 137, 403. (b) Barker,
J.; Kilner, M. Coord. Chem. ReV. 1994, 133, 219.
(10) Representative result: {tBuC(NiPr)2}AlMe2/[Ph3C][B(C6F5)4] (tolu-
ene, 100 °C, 6 atm ethylene); activity ) 2708 g PE/(mol‚h‚atm).
(11) øAl ) 1.6 and øGa ) 1.8.
(5) Bochmann, M.; Sarsfield, M. J. Organometallics 1998, 17, 5908.
(6) (a) Strauss, S. H. Chem. ReV. 1993, 93, 927. (b) Bochmann, M. J.
Chem. Soc., Dalton Trans. 1996, 255. (c) Turner, H. W. Eur. Pat. Appl. 0
277 004, 1988.
(12) Several cationic Ga methyl complexes are stable in H2O, which is
ascribed to the high stability of the GaMe2 fragment. (a) Shriver, D. F.;
+
(7) (a) Radzewich, C. E.; Coles, M. P.; Jordan, R. F. J. Am. Chem. Soc.
1998, 120, 9384. (b) Radzewich, C. E.; Guzei, I. A.; Jordan, R. F. J. Am.
Chem. Soc. 1999, 121, 8673. (c) Radzewich, C. E.; Jordan, R. F. Manuscript
in preparation.
(8) [(ATI)AlEt][B(C6F5)4] (toluene, 100 °C, 6 atm ethylene); activity )
1375 g PE/(mol‚h‚atm).
Parry, R. W. Inorg. Chem. 1962, 1, 835. (b) Tobias, R. S.; Sprague, M. J.;
Glass, G. E. Inorg. Chem. 1968, 7, 1714. (c) Olapinsky, H.; Weidlein, J. J.
Organomet. Chem. 1973, 54, 87.
(13) Compound [2a][B(C6F5)4] was synthesized by slow addition of 1a
to a solution of [Ph3C][B(C6F5)4] and by addition of [Ph3C][B(C6F5)4] to a
solution of 1a. Both procedures afforded [2a][B(C6F5)4] in ca. 78% yield.

276 J. Am. Chem. Soc., Vol. 122, No. 2, 2000
Dagorne et al.
Chart 2
(6) Å).15 Thus, there is little π-delocalization within the N(1)-
C(10)-N(2) unit, and N(1) acts as a µ-amide donor and N(2)
acts as an imine donor. The Al(2)-N(1) (1.979(1) Å) and Al-
(1)-N(1) (2.027(1) Å) distances are slightly longer than the
Al-N bond distances in the µ-amido complex (Me2Al(µ-
NHAd))2 (Ad ) adamantyl, 1.97(2) Å average).16 The Al(2)-
N(2) distance (1.927(1) Å) is similar to those in neutral chelated
amidinate complexes (e.g., 3a, 1.92(2) Å average)17 but is
slightly shorter than the Al-Nimine bond distance observed in
neutral Al imine complexes (e.g., dimethyl{N-(p-tolyl)-2-(p-
tolylimino)propylamino-N,N′}aluminum, 1.979(6) Å, A, Chart
2).18 The N(1)-C(10)-N(2) angle (109.6(1)°) is typical for a
chelated acetamidinate Al species (e.g., 3a, 110.4(2)°).17 The
µ-η1,η2 bridge in 4a+ represents a new amidinate bonding mode.
However, several examples of cluster compounds that contain
triply bridging µ-η1,η1,η1 amidinates are known, including Os3-
(µ-H)(CO)9{PhNC(Ph)NH} (B, Chart 2) and Os3(µ-H)(CO)9-
{HNC(Me)NH}.19
+
Figure 1. Molecular structure of the {MeC(NCy)2}2Al2Me3 cation
(4a+) in [4a][B(C6F5)4]. Hydrogen atoms are omitted.
Scheme 1
In contrast, the N(3)-C(24) and N(4)-C(24) distances
(1.360(2), 1.332(2) Å) within the µ-η1,η1 amidinate are inter-
2
mediate between the N-Csp single and double bond distances,
indicating that there is significant π-delocalization within the
N(4)-C(24)-N(3) unit. The Al(1)-N(4) distance (1.959(1) Å)
and the N(4)-C(24)-N(3) angle (119.0(1)°) are similar to the
values in the dimeric µ-η1,η1 amidinate complex ({MeC-
(NMe)2}AlMe2)2 (1.92(5) Å and 118.5(1)°).20 The Al(2)-N(3)
bond distance (1.866(1) Å) is ca. 0.1 Å shorter than the Al-
(2)-N(4) bond distance.
The Al centers in 4a+ adopt distorted tetrahedral geometries.
The acute N(1)-Al(2)-N(2) bite angle (69.51(6)°) is normal
for chelated Al amidinate complexes (e.g., 3a, 69.96(8)°)17 and
is compensated for by the opening of the N(1)-Al(2)-C(3)
(119.52(7)°) and N(2)-Al(2)-C(3) (114.32(7)°) angles. The
N(2)-Al(2)-N(3) (108.64(6)°) and N(1)-Al(2)-N(3) (109.30-
(6)°) angles are close to the ideal tetrahedral angle of 109.47°.
The bond angles at Al(1) are closer to the ideal tetrahedral value
than those at Al(2) because Al(1) has only η1-bonded ligands.
The Al-C bond distances (1.954(2) Å average) are similar to
those in 3a (1.95(7) Å average).17
Molecular Structure of [4a][B(C6F5)4]. [4a][B(C6F5)4]
-
crystallizes as discrete 4a+ and B(C6F5)4 ions with no close
cation-anion contacts. The structure of the B(C6F5)4- anion is
normal. The 4a+ cation (Figure 1) is a dinuclear species with
C1 symmetry. The two Al centers are linked by two different
amidinate bridges: a µ-η1,η2 amidinate (N(1)-C(10)-N(2)),
which is bonded to Al(1) through the bridging nitrogen N(1)
and to Al(2) by both N(1) and N(2), and a µ-η1,η1 amidinate
(N(3)-C(24)-N(4)), in which the two nitrogens are bonded to
different Al centers. Due to their different bonding modes, the
µ-η1,η2 and µ-η1,η1 amidinate units exhibit significant differ-
ences in bond distances and angles.
Molecular Structure of [2a][B(C6F5)4]. [2a][B(C6F5)4] was
obtained as colorless crystals by crystallization from a mixed
solvent system (10/1 hexane/toluene, see Experimental Section).
The X-ray structural analysis for [2a][B(C6F5)4] was imprecise
due to poor crystal quality but allowed determination of the
connectivity and approximate metrical parameters. [2a][B(C6F5)4]
crystallizes as discrete 2a+ and B(C6F5)4 ions, with no close
-
(15) Levine, I. R. J. Chem. Phys. 1963, 38, 2326.
(16) Waggoner, K. M.; Power, P. P. J. Am. Chem. Soc. 1991, 113, 3385.
(17) Coles, M. P.; Swenson, D. C.; Jordan, R. F. Organometallics 1997,
16, 5184.
(18) Kanters, J. A.; Van Mier, G. P. M.; Nijs, R. L. L. M.; Van Der
Steen, F.; Van Koten, G. Acta Crystallogr. C 1988, 44, 1391.
(19) (a) Burgess, K.; Holden, D. H.; Johnson, B. F. G.; Lewis, J. J. Chem
Soc., Dalton Trans. 1983, 1199. (b) Deeming, A. J.; Peters, R. J. Organomet.
Chem. 1980, 202, C39.
The C-N bond distances in the µ-η1,η2 amidinate are
significantly different. The N(1)-C(10) distance (1.435(2) Å)
2
is slightly shorter than the normal N-Csp single bond distance
(1.47 Å),14 and the N(2)-C(10) distance (1.289(2) Å) is
2
comparable to the normal N-Csp double bond distance (1.27-
(14) Sutton, L. E. Interatomic Distances and Configuration in Molecules
and Ions; Special Publications No. 18; The Chemical Society: London,
1965.
(20) Hausen, H. D.; Gerstner, F.; Schwarz, W. J. Organomet. Chem.
1978, 145, 277.

Cationic Al and Ga Amidinate Complexes
J. Am. Chem. Soc., Vol. 122, No. 2, 2000 277
1
) -66 °C in H NMR, Figure 3) but does not affect the third
(lower field) Al-Me resonance. Process (i) also causes the two
sets of amidinate NCMeN resonances to broaden and coalesce
1
(Tcoal. ) -66 °C in H NMR, Figure 4). Additionally, process
(i) causes the four iPr-CH 13C NMR resonances to broaden and
collapse as illustrated in Figure 5. The iPr-CH line shapes could
only be simulated adequately by assuming a pairwise exchange
of resonances 1 and 4 and of resonances 2 and 3 in Figure 5.23
These results indicate that process (i) permutes the two amidinate
ligands of 2a+ but does not permute the two ends of a given
amidinate ligand. The most reasonable mechanism for this
process (Scheme 2) involves a reversible slippage of the µ-η1,η2
amidinate to a µ-η1,η1 bonding mode and the formation of
intermediate species C. Species C has C2V symmetry as drawn
in Scheme 2 but may well adopt a puckered Cs-symmetric
structure analogous to that of ({MeC(NMe)2}AlMe2)2.20 The
two high-field Al-Me resonances (δ -0.54, -0.75) are assigned
to the AlMe2 unit, and the low-field Al-Me resonance (δ -0.17)
is assigned to the AlMe unit of 2a+. The free energy barriers
for process (i) calculated from the Al-Me and NCMeN 1H NMR
line shapes and the NCMeN 13C NMR line shapes are identical
(∆Gq ) 9.5(7) kcal/mol).24 Activation parameters for process
(i) were obtained from an Eyring plot based on rate constants
Figure 2. Molecular structure of the {MeC(NiPr)2}2Al2Me3 cation
(2a+) in [2a][B(C6F5)4]. Hydrogen atoms are omitted. Key bond lengths
(Å): N(3)-C(15), 1.35(1); N(4)-C(15), 1.31(1); N(1)-C(7), 1.43-
(1); N(2)-C(7), 1.27(1); Al(2)-N(1), 2.018(3); Al(1)-N(1), 1.973-
(9). Key bond angles (deg): N(1)-Al(1)-N(2), 70.0(4); N(1)-C(7)-
N(2), 110.6(9); N(4)-C(15)-N(3), 118.8(9).
+
cation-anion contacts. The structure of the B(C6F5)4- anion is
normal. The structure of 2a+ (Figure 2) is closely analogous to
that of 4a+.
1
determined from simulations of the NCMeN region of the H
Solution Structure and Dynamic Behavior of [2a][B(C6F5)4]
and [2a][MeB(C6F5)3]. The solution behavior of 2a+ was
investigated in detail because the NMR spectra of this cation
are simpler and more informative than those of 4a+, which are
NMR spectrum (Figure 6, ∆Hq ) 9.9(5) kcal/mol; ∆Sq ) 0(3)
eu).25,26 The ∆Hq value for process (i) is lower than the
enthalpies of Al-N dissociation reported for R3N‚AlR3 amine
adducts (∆H ≈ 20-30 kcal/mol).27 Process (i) is related to the
intramolecular exchange of µ-η1,η1 amidinate and chelating
amidinate groups in Pd bis-amidinate species.28
1
complicated by the presence of the Cy groups. The H, 13C,
11B, and 19F NMR spectra of [2a][B(C6F5)4] are identical to
those of [2a][MeB(C6F5)3] in CD2Cl2 from -85 to 23 °C, with
the exception of the anion resonances. In both cases, resonances
characteristic of free anion are observed.21 In particular, the 1H
spectrum of [2a][MeB(C6F5)3] in CD2Cl2 contains a broad
(22) (a) The effect of coupling to quadrupolar nuclei (e.g., 14N, 99.36%
1
abundance, I ) 1) on the spectra of spin
/ nuclei is described in the
2
following: Harris, R. K. Nuclear Magnetic Spectroscopy; Longman
Scientific & Technical: Essex, UK, 1986; pp 139-140. In the case of 13C-
-
singlet for the MeB(C6F5)3 group at δ 0.48, which is
14N coupling, the C line shape is determined by the product T1,
×
13
14N
characteristic of the free MeB(C6F5)3- anion. Thus, cation/anion
interactions are not significant for these salts under these
conditions.
J
N. When 10πT1, N × J
N ) 1, the 13C-14N coupling is completely
13C-14
14
13C-14
collapsed, and a sharp singlet is observed for the 13C NMR resonance.
14
13C-14
Increasing the T1, N × J
N product decreases the extent of collapse of
13C-14N coupling; a broad singlet is observed for intermediate values (i.e.,
1
The H and 13C NMR spectra of 2a+ at -85 °C in CD2Cl2
14N
13C-14
N
14N
< 10), and a triplet is observed when T1,
2 < 10πT1, × J
×
contain three Al-Me signals, two sets of CMe signals, eight sets
of iPr-CH3 signals, and four sets of iPr-CH signals. These data
are consistent with the C1 symmetry observed in the solid-state
i
J
> 20. For 2a+, the broadening of one Pr-CH resonance (δ 50.3,
13C-14
∆ν ) N12 Hz vs ∆ν ) 7 Hz (average) for the other iPr-CH resonances) and
of one set of NCMeN resonances (δ 179.4, ∆ν ) 10 Hz and δ 14.1, ∆ν )
16 Hz vs ∆ν ) 6 and 8 Hz (average) for the other NCMeN resonances of
i
structure. In addition, in the 13C spectrum of 2a+, one Pr-CH
14
the same types) is ascribed to a larger T1, value for the four-coordinate
N
resonance and one set of NCMeN resonances are significantly
broader than the other resonances of these types. This broaden-
ing is ascribed to incomplete collapse of 13C-14N coupling and
implies that one nitrogen is in a more symmetric environment
than the others and hence has a longer quadrupolar relaxation
time.22 This feature is consistent with the presence of one four-
coordinate nitrogen and three three-coordinate nitrogens, as
nitrogen vs the three-coordinate nitrogens. (b) Higher symmetry at N
14
14N
dramatically increases T1, N. For example, T1, g 2000 ms for [NEt4]I
14
but T1, N ) 38 ms for NH3. See: Lehn, J. M.; Kintzinger, J. P. In Nitrogen
NMR; Witanowski, M., Webb, G. A., Eds.; Plenum: London, 1973; Chapter
3, pp 124-127.
(23) Note that the difference in the line widths of the four iPr-CH signals
at -85 °C does not complicate the simulation of the spectra.
(24) Barriers for process (i) were calculated from the coalescence of the
AlMe2, NCMeN, and NCMeN resonances using standard formulas for a
two-site, equal population exchange process: i.e., Tcoal. ) coalescence
temperature; kcoal. ) exchange rate constant at Tcoal. ) (2.22)(∆ν); ∆Gq )
(4.576)Tcoal.(10.32 + log(Tcoal./kcoal.)). For 1H NMR: AlMe2 coalescence,
∆ν ) 74.2 Hz, kcoal. ) 165 s-1, Tcoal. ) 207(2) K, ∆Gq ) 9.5(7) kcal/mol;
NCMeN coalescence, ∆ν ) 63.7 Hz, kcoal. ) 142 s-1, Tcoal. ) 207(2) K,
∆Gq ) 9.5(7) kcal/mol. For 13C NMR: AlMe2 coalescence, ∆ν ) 254.6
Hz, kcoal. ) 565 s-1, Tcoal. ) 217(2) K, ∆Gq ) 9.5(7) kcal/mol; NCMeN
1
observed in the solid state. The Al-Me JCH values (113, 114,
116 Hz) are consistent with the presence of three terminal Al-
Me groups. In contrast, the Me-bridged dinuclear cation {(ATI)-
1
AlMe)}2(µ-Me)+ exhibits a large JCH value (133 Hz) for the
µ-Me group and a normal 1JCH value (118 Hz) for the terminal
Me groups.4b Thus, on the basis of -85 °C NMR data, it is
likely that 2a+ retains its solid-state structure in CD2Cl2 solution.
As the sample temperature is raised from -85 °C, two
dynamic processes can be detected for [2a][MeB(C6F5)3] by 1H
and 13C NMR that are identified as processes (i) and (ii) in
Scheme 2. The lower energy process (i) causes broadening and
coalescence of the two highest field Al-Me resonances (Tcoal.
coalescence, ∆ν ) 53.9 Hz, kcoal. ) 120 s-1, Tcoal. ) 203(2) K, ∆Gq
)
,
9.5(7) kcal/mol; NCMeN coalescence, ∆ν ) 268.1 Hz, kcoal. ) 595 s-1
Tcoal. ) 217(2) K, ∆Gq ) 9.5(7) kcal/mol.
(25) The activation parameters were obtained from the Eyring equation:
ln(k/T) ) (-∆Hq/R)/T + 23.76 + ∆Sq/R, using a least-squares fit of ln(k/
T) vs 1/T (m ) -∆Hq/R, b ) 23.76 + ∆Sq/R). (b) The uncertainties in m
and b were used to estimate the uncertainties in ∆Hq and ∆Sq by standard
2
2
error propagation methods: (σ∆Hq
) )
) (-R)2(σm)2, and (σ∆Sq ) (-R)2-
(σb)2.26
-
(21) (a) The 13C, 11B, and 19F NMR spectra for the B(C6F5)4 anion of
(26) Skoog, D. A.; Leary, J. J. Principles of Instrumental Analysis, 4th
ed.; Saunders College: New York, 1992; pp A13-14.
(27) Henrickson, C. H.; Duffy, D.; Eyman, D. P. Inorg. Chem. 1968, 7,
1047.
[2a][B(C6F5)4] are identical to those for [Ph3C][B(C6F5)4]. (b) The 1H, 13C,
11B, and 19F NMR spectra for the MeB(C6F5)3- anion of [2a][MeB(C6F5)3]
are identical to those for [NBu3(CH2Ph)][MeB(C6F5)3].

278 J. Am. Chem. Soc., Vol. 122, No. 2, 2000
Dagorne et al.
Scheme 2
Figure 4. (a) NCMeN region of the 1H NMR spectrum of 2a+ (CD2-
Cl2) from -85 to -45 °C. (a) Experimental spectra and (b) simulated
spectra. Processes (i) and (ii) are defined in the text. Spectra were
simulated assuming the following values for the exchange rate
constants: ki ) rate constant for process (i) at T and kii ) rate constant
for process (ii) at T; at T ) -85 °C, ki ) kii ) 0 s-1; at T ) -71 °C,
ki ) 85 s-1, kii ) 0 s-1; at T ) -66 °C, ki ) 178 s-1, kii ) 0 s-1; at
T ) -45 °C, ki ) 1400 s-1, kii ) 20 s-1
.
Process (ii) also causes the Pr-CH 13C NMR resonances to
broaden further and ultimately coalesce to a singlet at ca. -56
°C, as illustrated in Figure 5. Process (ii) thus permutes all three
Al-Me groups and permutes the ends of the amidinate ligands,
and it must therefore involve exchange of the Me groups
between the Al centers. A reasonable mechanism for this process
(Scheme 2) is the formation of intermediate C followed by
migration of a Me group from the AlMe2 center to the AlMe
center. The free energy barrier for process (ii), ∆Gq ) 11.7(3)
kcal/mol,29 was calculated from the Al-Me 1H NMR line shape
changes. This value is intermediate between the barriers for
bridge/terminal Me exchange in Al2Me6 (15.4(2) kcal/mol in
toluene)30 and {(ATI)AlMe)}2(µ-Me)+ (∆Gq ) 8.5(4) kcal/mol
i
1
Figure 3. Al-Me region of the H NMR spectrum of 2a+ (CD2Cl2)
from -85 to -7 °C. (a) experimental spectra and (b) simulated spectra.
Processes (i) and (ii) are defined in the text. Spectra were simulated
assuming the following values for the exchange rate constants: ki )
rate constant for process (i) at T and kii ) rate constant for process (ii)
at T; at T ) -85 °C, ki ) kii ) 0 s-1; T ) -71 °C: ki ) 85 s-1, kii )
0 s-1; at T ) -66 °C, ki ) 178 s-1, kii ) 0 s-1; at T ) -45 °C, ki )
1400 s-1, kii ) 20 s-1; at T ) -23 °C, ki ) 12 400 s-1, kii ) 150 s-1
;
at T ) -7 °C, ki ) 45 000 s-1, kii ) 530 s-1
.
Above -50 °C, a second, higher energy process (process (ii)
in Scheme 2) is detected for [2a][MeB(C6F5)3] which causes
the AlMe2 and AlMe resonances to broaden and ultimately
(28) Barker, J.; Cameron, N.; Kilner, M.; Mahoud, M. M.; Wallwork,
S. C. J. Chem. Soc., Dalton Trans. 1986, 1359.
1
coalesce to a singlet (Tcoal. ) -12 °C in H NMR, Figure 3).

Cationic Al and Ga Amidinate Complexes
J. Am. Chem. Soc., Vol. 122, No. 2, 2000 279
Figure 6. Eyring Plot for process (i) for 2a+.
Figure 5. (a) iPr-CH region of the 13C{1H} spectrum of 2a+ (CD2Cl2)
from -85 to -35 °C. (a) Experimental spectra and (b) simulated
spectra. Processes (i) and (ii) are defined in the text. Spectra were
simulated assuming the following values for the exchange rate
constants: ki ) rate constant for process (i) at T and kii ) rate constant
for process (ii) at T; at T ) -85 °C, ki ) kii ) 0 s-1; at T ) -71 °C,
ki ) 85 s-1, kii ) 0 s-1; at T ) -66 °C, ki ) 178 s-1, kii ) 0 s-1; at
Figure 7. Eyring Plot for process (ii) for 2a+.
T ) -56 °C, ki ) 605 s-1, kii ) 7 s-1; at T ) 35 °C, ki ) 2500 s-1
,
Scheme 3
kii ) 63 s-1. The apparent monotonic shift of resonances 1-4 to lower
field as the temperature is increased was not incorporated in the
simulations.
in CD2Cl2).31 The activation parameters for process (ii) were
obtained from an Eyring plot based on rate constants determined
from simulation of the Al-Me region of the 1H NMR spectrum
(Figure 7, ∆Hq ) 9.7(5) kcal/mol; ∆Sq ) -9(2) eu).
The possibility that the dynamic behavior of 2a+ involves
dissociation of 2a+ into the base-free {MeC(NiPr)2}AlMe+
cation and 1a can be ruled out by NMR observations. Thus,
1
the H NMR spectrum of [2a][MeB(C6F5)3] in CD2Cl2 at 23
°C in the presence of 1 equiv of 1a contains sharp resonances
for [2a][MeB(C6F5)3] and 1a in a 1/1 ratio. This result
establishes that intermolecular exchange of [2a][MeB(C6F5)3]
and 1a is slow on the NMR time scale under conditions where
processes (i) and (ii) are both rapid on the NMR time scale.
The lack of intermolecular [2a][MeB(C6F5)3]/1a exchange
(29) The free energy barrier for process (ii) was calculated from the
coalescence of the AlMe2 and AlMe 1H NMR resonances using the graphical
method for a two-site, unequal population exchange process described in
the following: Shanan-Atidi, H.; Bar-Eli, K. H. J. Phys. Chem. 1970, 74,
961. Tcoal. ) 261(2) K, kcoal. ) 522 s-1, ∆Gq ) 11.7(3) kcal/mol. Similar
estimates for the exchange barrier for process (ii) were obtained from the
analysis of the line broadening of the AlMe resonance between 223 and
239 K (∆Gq ≈ 11.7 kcal/mol).
contrasts with the fast intermolecular exchange observed for
{(ATI)AlMe)}2(µ-Me)+ and (ATI)AlMe2 at 23 °C.4b
Reaction of [2a][MeB(C6F5)3] with Lewis Bases. The
reaction of [2a][MeB(C6F5)3] with 1 equiv of NMe2Ph in CD2-
Cl2 (12 h, 23 °C) yields a 1/1 mixture of the amine adduct
[{MeC(NiPr)2}Al(Me)(NMe2Ph)][MeB(C6F5)3] ([5a][MeB-
(30) Ham, N. S.; Mole, T. In Progress in NMR Spectroscopy; Emsley,
J. W., Feeney, J., Sutcliffe, V., Eds.; Pergamon: New York, NY, 1969;
Vol. 4, p 128.
1
(C6F5)3]) and 1a (85% by H NMR, Scheme 3). [5a][MeB-
(31) The free energy barrier for the bridge/terminal Me exchange in
{(ATI)AlMe)}2(µ-Me)+ was estimated from the line broadening of the
bridging Al-Me resonance at 190 K (∆Gq ≈ 8.5(4) kcal/mol).
(C6F5)3] is stable in CD2Cl2 at 23 °C for several days and is
thus more robust than [2a][MeB(C6F5)3]. The 1H and 13C NMR

280 J. Am. Chem. Soc., Vol. 122, No. 2, 2000
Dagorne et al.
spectra of [5a][MeB(C6F5)3] (CD2Cl2) contain characteristic
resonances for the free MeB(C6F5)3- anion along with one Al-
Me signal, one NCMeN signal, and two iPr-CH3 signals, which
is consistent with Cs symmetry at Al. The coordination of NMe2-
Scheme 4
1
Ph is also evidenced by H and 13C NMR. The o-Ph and p-Ph
1H NMR resonances (δ 7.47, 7.51) and the NMe 13C NMR
resonance (δ 46.0) of the coordinated NMe2Ph are shifted
downfield from the corresponding resonances of free NMe2-
Ph.32 The identity of 5a+ was confirmed by generation of [5a]-
[B(C6F5)4] by a protonolysis route. The reaction of 1a with 1
equiv of [HNMe2Ph][B(C6F5)4] in CD2Cl2 (15 min, 23 °C) yields
[5a][B(C6F5)4] (100% by 1H NMR, Scheme 3) and methane.33
Reaction of {MeC(NiPr)2}GaMe2 (1b) with Cationic
Activators. The reaction of 1b with 0.5 equiv of B(C6F5)3 in
C6D5Cl (15 min, 23 °C) yields the dinuclear cation {MeC(Ni-
+
-
1
Pr)2}2Ga2Me3 (2b+) as the MeB(C6F5)3 salt (100% by H
NMR vs an internal standard, Scheme 1). [2b][MeB(C6F5)3]
does not undergo further reaction with excess B(C6F5)3 at 23
°C and is more stable in C6D5Cl at 23 °C (t1/2 ≈ 5 d) than its
Al analogue [2a][MeB(C6F5)3]. The salt [2b][MeB(C6F5)3] was
isolated as an analytically pure colorless solid (71%) by
generation in 10/1 hexane/CH2Cl2 mixture followed by several
pentane washes. The variable-temperature NMR data for 2b+
parallel the data for 2a+ and establish that the two species have
analogous structures and dynamic properties.34
The composition of [7a][B(C6F5)4] was established by chemi-
cal derivatization. The reaction of [7a][B(C6F5)4] with 1 equiv
of NMe2Ph in CD2Cl2 (10 min, 23 °C) yields a 1/1 mixture of
6a and the amine adduct [{tBuC(NiPr)2}Al(Me)(NMe2Ph)]-
[B(C6F5)4] ([8a][B(C6F5)4], Scheme 4).36 Interestingly, this
reaction is much faster than the reaction of 2a+ and NMe2Ph
In contrast, 1b reacts readily with [Ph3C][B(C6F5)4] in C6D5-
Cl (10 min, 23 °C), but neither 2b+ nor Ph3CCH3 is observed
as products. Similarly, 1b reacts with [HNMe2Ph][B(C6F5)4] in
C6D5Cl (1 h, 23 °C), but neither {MeC(NiPr)2}Ga(Me)(NMe2-
Ph)+ nor CH4 is observed. Thus, in contrast to the case for the
corresponding Al reactions, it appears that Ph3C+ and HNMe2-
Ph+ attack the Ga amidinate unit rather than the Ga-Me bonds
of 1b.
1
(12 h, 23 °C). The H and 13C NMR data for [8a][B(C6F5)4]
are similar to those for [5a][B(C6F5)4].
The 11B and 19F NMR spectra of [7a][B(C6F5)4] in C6D5Cl
-
1
establish that the B(C6F5)4 anion is free in solution. The H
and 13C NMR spectra of [7a][B(C6F5)4] are unchanged between
-60 and 60 °C, indicating that, unlike 2a+, 7a+ is not fluxional
over this temperature range.37 The 1H and 13C NMR spectra of
Reaction of {tBuC(NiPr)2}AlMe2 (6a) with [Ph3C][B(C6F5)4].
The methyl abstraction chemistry of the tBu-substituted amidi-
nate complexes {tBuC(NiPr)2}MMe2 (M ) Al, 6a; M ) Ga,
6b) was investigated in an effort to disfavor the formation of
t
[7a][B(C6F5)4] contain three Al-Me signals, two Bu signals,
and four Pr-CH signals, which is consistent with the presence
i
t
of two inequivalent Al centers and overall C1 symmetry. These
data do not allow a definitive structural assignment for [7a]-
dinuclear Al species. Replacing the MeC unit of 1a by a BuC
unit increases the steric interactions between the amidinate
substituents, which in turn increases the steric crowding at the
metal center.17
1
[B(C6F5)4]. However, the facts that (i) the H NMR Al-Me
chemical shifts of 7a+ are very similar (δ -0.34, -0.39, and
-0.42) whereas those for 2a+ are very different (δ -0.17, -0.54
and -0.75), (ii) 7a+ is rigid in solution while 2a+ is fluxional,
and (iii) 7a+ reacts much more rapidly with NMe2Ph than does
2a+ suggest that the structure of 7a+ is different from those of
2a+ and 4a+. In fact, the NMR spectra of 7a+ are nearly
identical to those of the analogous dinuclear Ga cation [{tBuC-
(NiPr)2}GaMe‚{tBuC(NiPr)2}GaMe2]+ (7b+), in which the two
metal centers are connected by a µ-η1,η2 bridging amidinate
(vide infra). Thus, it is likely that 7a+ adopts a similar structure,
as illustrated in Scheme 4.
The reaction of 6a with 0.5 equiv of [Ph3C][B(C6F5)4] in
C6D5Cl (10 min, 23 °C) yields a 1/1 mixture of Ph3CCH3 and
the dinuclear species [{tBuC(NiPr)2}AlMe‚{tBuC(NiPr)2}-
AlMe2]+ (7a+) as the B(C6F5)4 salt ([7a][B(C6F5)4], 90% by
-
1H NMR, Scheme 4).35 [7a][B(C6F5)4] does not undergo further
reaction with excess [Ph3C][B(C6F5)4] at 23 °C in C6D5Cl. [7a]-
[B(C6F5)4] decomposes in C6D5Cl (t1/2 ≈ 3 h, 23 °C) and C6D6
(t1/2 ≈ 7 h, 23 °C) and therefore could not be isolated in pure
form.
Reaction of {tBuC(NiPr)2}GaMe2 (6b) with [Ph3C]-
[B(C6F5)4]. The reaction of 6b with 0.5 equiv of [Ph3C]-
[B(C6F5)4] in C6D5Cl (15 min, 23 °C) yields the dinuclear cation
[{tBuC(NiPr)2}GaMe‚{tBuC(NiPr)2}GaMe2]+ (7b+) as the
B(C6F5)4- salt ([7b][B(C6F5)4], 100% by 1H NMR vs an internal
(32) Data for NMe2Ph. 1H NMR (CD2Cl2): δ 7.23 (t, 3J ) 7.5, 2H,
3
3
m-Ph), 6.75 (d, J ) 7.6, 2H, o-Ph), 6.71 (t, J ) 7.2, 1H, p-Ph), 2.95 (s,
6H, NMe). 13C NMR (CD2Cl2): δ 151.3 (ipso-Ph), 129.4 (Ph), 116.8 (Ph),
113.0 (Ph), 40.7 (NMe).
(33) The 1H and 13C NMR data for [5a][B(C6F5)4] are identical to those
for [5a][MeB(C6F5)3] except for the anion resonances.
(34) (a) In particular, the widely divergent Ga-Me 1H NMR chemical
shifts for 2b+ at -85 °C (δ 0.43, -0.04, -0.27) parallel those observed
for 2a+ at -85 °C (δ -0.17, -0.54, -0.75). (b) The NMR line shape
changes for 2b+ between -85 and 23 °C parallel those for 2a+ and are
consistent with the occurrence of processes (i) and (ii) via the intermediate
D, as illustrated in Scheme 2. (c) The free energy barrier for process (ii)
(∆Gq ) 14.8(4) kcal/mol) calculated from the Ga-Me 1H NMR line shape
changes for 2b+ is higher than that for 2a+ (∆Gq ) 11.7(3) kcal/mol).
(35) Both the addition of [Ph3C][B(C6F5)4] to a solution of 6a and the
addition of 6a to a solution of [Ph3C][B(C6F5)4] yield [7a][B(C6F5)4] (90%
(36) The generation of [8a][B(C6F5)4] by the reaction of 6a with [HNMe2-
Ph][B(C6F5)4] confirmed this assignment.
(37) (a) Due to the low stability of [7a][B(C6F5)4] in CD2Cl2, NMR
spectra were recorded in CD2Cl2 from -60 to -30 °C and C6D5Cl from
-30 to 60 °C. (b) [7a][B(C6F5)4] rapidly decomposes at 60 °C (t1/2 ) 15
min), which precluded NMR studies at higher temperatures. (c) Additionally,
1
the H and 13C NMR spectra of a 1/1 mixture of [7a][B(C6F5)4] and 6a at
23 °C in C6D5Cl contain sharp resonances of the two complexes in a 1/1
ratio, which shows that intermolecular exchange between [7a][B(C6F5)4]
and 6a is slow on the NMR time scale under these conditions.
1
by H NMR).

Cationic Al and Ga Amidinate Complexes
J. Am. Chem. Soc., Vol. 122, No. 2, 2000 281
tBu) complexes.41 The amidinate that chelates Ga(2) (N(4)-
C(18)-N(3)) is normal; i.e., it is symmetrically bonded to Ga-
(2), it exhibits delocalized NCN π-bonding, and the N(4)-
C(18)-N(3) angle (109.2(2)°) is in the expected range. The
Ga-C bond distances (1.950(3) Å average) are slightly shorter
t
than those in {tBuC(NR′2)2}GaMe2 (R′ ) Cy, Bu; 1.98(2) Å
t
average)41 and [Me2Ga(NH2 Bu)2]Br (1.97(6) Å average).42
Solution Structure of [7b][B(C6F5)4]. The 1H, 13C, 11B, and
19F NMR spectra of [7b][B(C6F5)4] in CD2Cl2 are essentially
unchanged between -60 and 60 °C and indicate that the
-
B(C6F5)4 anion is free in solution and that the 7b+ cation is
1
not fluxional over this temperature range.43 The H and 13C
NMR spectra of 7b+ contain three Ga-Me resonances at very
similar chemical shifts (δ 0.55, 0.45, 0.35), two sets of CtBu
i
i
Figure 8. Molecular structure of the {tBuC(NiPr)2}GaMe‚{tBuC(Ni-
resonances, four sets of Pr-CH resonances, and eight Pr-CH3
Pr)2}GaMe2 cation (7b+) in [7b][B(C6F5)4]. Hydrogen atoms are
+
1
resonances.44 The GaMe JCH values for 7b+ (125, 123, 123
omitted.
Hz) are consistent with the presence of three terminal Ga-Me
groups. These data are consistent with the C1-symmetric
structure observed for 7b+ in the solid state. The rigidity of
7b+ in solution and the similarity of the Ga-Me 1H NMR
chemical shifts parallel the properties of the Al analogue 7a+
but contrast with the dynamic behavior and divergent Al-Me
and Ga-Me 1H NMR chemical shifts observed for 2a+ and 2b+.
Thus, it is clear that 7b+ and 7a+ adopt analogous structures.
Reaction of [7b][B(C6F5)4] with NMe2Ph. The reaction of
[7b][B(C6F5)4] with 1 equiv of NMe2Ph in C6D5Cl (15 min, 23
°C) yields a 1/1 mixture of 6b and the amine adduct [{tBuC-
(NiPr)2}Ga(Me)(NMe2Ph)][B(C6F5)4] ([8b][B(C6F5)4], Scheme
4).45 The 1H and 13C NMR spectra of [8b][B(C6F5)4] (C6D5Cl)
are very similar to those of [8a][B(C6F5)4].
Chart 3
standard, Scheme 4).38 [7b][B(C6F5)4] does not undergo further
reaction with excess [Ph3C][B(C6F5)4] at 23 °C in C6D5Cl.
Unlike the Al analogue [7a][B(C6F5)4], [7b][B(C6F5)4] is stable
at 23 °C in C6D5Cl for several days and was isolated as an
analytically pure yellow solid (64%) by generation in benzene
followed by several hexane and pentane washes. [7b][B(C6F5)4]
was obtained as pale yellow crystals by crystallization from a
mixed solvent system (10/10/1 hexane/pentane/C6D5Cl, see
Experimental Section), and its structure was determined by
X-ray crystallography.
i
Reaction of {tBuC(NR′)2}AlMe2 (R′ ) Pr, 6a; R′ ) Cy,
10a) with B(C6F5)3. Initial studies suggested that the reaction
of 6a or 10a with B(C6F5)3 yields {tBuC(NR′)2}Al(Me)(MeB-
(C6F5)3) ion pairs.4a However, further studies have shown that
these reactions are more complicated. The reaction of 6a with
1 equiv of B(C6F5)3 in C6D5Cl or CD2Cl2 (20 min, 23 °C) yields
a 1/1 mixture of {tBuC(NiPr)2}Al(Me)(C6F5) (9a) and MeB-
Molecular Structure of [7b][B(C6F5)4]. [7b][B(C6F5)4]
crystallizes as discrete 7b+ and B(C6F5)4 ions with no close
-
cation-anion contacts. The structure of the B(C6F5)4- anion is
normal. The molecular structure of 7b+ is illustrated in Figure
8. 7b+ has overall C1 symmetry and is an adduct of {tBuC(Ni-
Pr)2}GaMe+ and {tBuC(NiPr)2}GaMe2. The two Ga centers are
linked by a µ-η1,η2 bridging amidinate ligand (N(1)-C(6)-
N(2)) through N(2). The µ-η1,η2 amidinate unit is very similar
to those in the dinuclear Al cations 2a+ and 4a+. The C(6)-
N(1) and C(6)-N(2) distances (1.285(3) and 1.459(3) Å) are
significantly different, indicating that the π-bonding within the
N-C-N unit is localized. N(1) bonds as an imine to Ga(1).
The Ga(1)-N(1) distance (2.007(2) Å) is comparable to those
observed in neutral Ga imine complexes (e.g., N-methylsalicy-
laldiminate gallium dimethyl, E, Chart 3, 2.019(7) Å).39
N(2) bonds to Ga(1) and Ga(2) as an unsymmetrical µ-amide.
The Ga(1)-N(2) distance (2.155(2) Å) is similar to those in
bulky gallium amine adducts (e.g., Me3Ga(NtBuH2), 2.12(1)
Å).40 The Ga(2)-N(2) distance (2.037(2) Å) is comparable to
those in bulky gallium µ-amide complexes, e.g., {Me2Ga(µ-
NH(1-Ad))}2 (2.02(4) Å average) and {Me2Ga(µ-NHPh)}2
(2.03(7) Å).16 The N(1)-C(6)-N(2) angle (108.6(2)°) is similar
to those in 2a+ (110.6(9)°) and 4a+ (109.6(1)°).
1
(C6F5)2 (100% by H NMR, Scheme 5). The 9a/MeB(C6F5)2
1
mixture was characterized by H, 11B, and 19F NMR. The 13C
and 19F NMR spectra of the mixture each contain two sets of
1
C6F5 signals (2/1 intensity ratio in 19F NMR). The H and 13C
NMR spectra for 9a contain one Al-Me resonance, one iPr-CH
resonance, and two iPr-CH3 resonances, which is consistent with
Cs symmetry at Al. The NMR data for MeB(C6F5)2 are
consistent with literature data (11B NMR, δ 72).46
The reaction of 6a and B(C6F5)3 at low temperature in CD2-
1
Cl2 was monitored by 11B and H NMR to determine if the
formation of cationic complexes precedes the formation of 9a
(41) Dagorne, S.; Young, V. G., Jr.; Jordan, R. F. Manuscript in
preparation.
(42) Atwood, D. A.; Jones, R. A.; Cowley, A. H.; Bott, S. G.; Atwood,
J. L. J. Organomet. Chem. 1992, 425, C1.
(43) (a) [7b][B(C6F5)4] rapidly decomposes in C6D5Cl at 60 °C (t1/2
)
30 min). (b) The 1H NMR spectrum of a 1/1 mixture of [7b][B(C6F5)4]
and 6b in CD2Cl2 contains sharp resonances of the two complexes between
-60 and 60 °C, which indicates that intermolecular exchange between 7b+
and 6b is slow on the NMR time scale.
(44) As for 2a+, the 13C NMR spectrum of 7b+ contains one set of
NCtBuN resonances and one iPr-CH resonance that are significantly broader
than the other resonances of the same type, which is consistent with the
presence of one four-coordinate nitrogen and three three-coordinate nitro-
gens.
The two Ga centers in 7b+ adopt distorted tetrahedral
geometries similar to those in {tBuC(NR′)2}GaMe2 (R′ ) Cy,
(38) Slow addition of 6b to a solution of [Ph3C][B(C6F5)4] and addition
of [Ph3C][B(C6F5)4] to a solution of 6b. Both afford [7b][B(C6F5)4] in ca.
64% yield.
(45) In contrast, the reaction of 6b with [HNMe2Ph][B(C6F5)4] yields
undentified species.
(39) Bregadze, V. I.; Furmanova, N. G.; Golubinskaya, L. M.; Kompan,
O. Y.; Struchkov, Y. T. J. Organomet. Chem. 1980, 192, 1.
(40) Atwood, D. A.; Jones, R. A.; Cowley, A. H., Bott, S. G.; Atwood,
J. L. J. Organomet. Chem. 1993, 453, 24.
(46) (a) Qian, B.; Ward, D. L.; Smith, M. R. Organometallics 1998, 17,
3070. For comparison, the11B NMR resonance for MeBPh2 appears at δ
70.6 in CD2Cl2. See: Wrackmeyer, B.; No¨th, H. Chem. Ber. 1976, 109,
1075.

282 J. Am. Chem. Soc., Vol. 122, No. 2, 2000
Dagorne et al.
Scheme 5
Figure 9. Molecular structure of {tBuC(NCy)2}Al(Me)(C6F5) (11a).
and MeB(C6F5)2. The 1H and 11B NMR spectra of the mixture
-
after 15 min at -60 °C contain characteristic MeB(C6F5)3
Pr)2}Ga(Me)(C6F5) (9b) and MeB(C6F5)2 after 7 h in C6D5Cl
or 12 h in C6D6.50 Thus, [7b][MeB(C6F5)3] is more stable than
the Al analogue [7a][MeB(C6F5)3] but nevertheless could not
be isolated in pure form. The NMR spectra of 9b are very
similar to those of the Al analogues 9a and 11a. In addition,
the 13C NMR spectrum of 9b contains a sharp ipso-C6F5
resonances, which is consistent with the formation of cationic
1
Al species. The H NMR spectrum also contains resonances
1
for 7a+ (ca. 80% by H NMR) that are identical to those of
[7a][B(C6F5)4], and resonances for unidentified species (20%
1
by H NMR). Attempts to isolate [7a][MeB(C6F5)3] were not
successful due to the rapid conversion to 9a and MeB(C6F5)2
above 0 °C. Thus, the reaction of 7a with B(C6F5)3 proceeds
by initial Me- abstraction to generate ionic species which
decompose by C6F5- transfer (Scheme 5). Analogous degrada-
tion products were observed in the reaction of the â-diketiminate
Al complex {HC(CMeN(p-tolyl)}2AlMe2 and B(C6F5)3,46 and
in group 4 metal complexes.47
2
resonance (δ 115.6, t, JCF ) 45 Hz) showing the presence of
a C6F5 group not bound to boron and thus consistent with the
presence of a Ga-C6F5 group. In contrast, B-ipso-C6F5 reso-
nances are normally very broad if detected at all.
Synthesis and Structure of {tBuC(NtBu)2}AlMe2 (12a). The
difference in structures of 2a+, 2b+, and 4a+ (double amidinate
bridging) vs 7a+ and 7b+ (single amidinate bridging) suggested
that utilization of “super-bulky” amidinate ligands might allow
generation of mononuclear cationic Al and Ga species. Accord-
ingly, the synthesis and methyl abstraction chemistry of {tBuC-
(NtBu)2}MMe2 (12a and 12b) were investigated.
Similarly, the reaction of {tBuC(NCy)2}AlMe2 (10a) with 1
equiv of B(C6F5)3 in CD2Cl2 (20 min, 23 °C) yields a 1/1
mixture of {tBuC(NCy)2Al(Me)(C6F5) (11a) and MeB(C6F5)2
1
1
(100% by H NMR). The H NMR data for 11a are similar to
the data for 9a except for the NCy resonances. In addition, 11a
was characterized by X-ray crystallography, which confirmed
the structures of both 11a and 9a. The molecular structure of
11a is illustrated in Figure 9. Compound 11a adopts a distorted
tetrahedral structure similar to that of 10a.17 The C6F5 ring is
normal. The Al-C6F5 bond distance (Al-C(31), 2.011(3) Å)
is longer than the terminal Al-Ph bond distances in Al2Ph6
(1.95(8) Å average).48 The Al-CH3 bond distance (Al-C(1),
1.941(3) Å) is similar to those of 10a (1.954(2) Å average).17
Reaction of {tBuC(NiPr)2}GaMe2 (6b) with B(C6F5)3. The
reaction of 6b with 1 equiv of B(C6F5)3 in C6D5Cl (15 min, 23
°C) yields [7b][MeB(C6F5)3] (100% by 1H NMR vs an internal
standard, Scheme 5) along with 0.5 equiv of unreacted B(C6F5)3.
The 1H NMR spectrum of the product mixture contains
resonances for [7b][MeB(C6F5)3] which are identical to those
t
The Bu-amidinate reagent Li[tBuC(NtBu)2] was generated
by the reaction of tBuLi with tBuNdCdNtBu (Et2O, 0 °C) and
reacted in situ with 1 equiv of AlMe2Cl (Et2O, 0 °C) to yield
{tBuC(NtBu)2}AlMe2 (12a, Scheme 6).51 Complex 12a is highly
air-sensitive and was isolated as a colorless crystalline solid by
1
sublimation (70 °C, <0.001 mmHg). The H and 13C NMR
spectra of 12a (C6D6) at 23 °C are consistent with Cs or higher
symmetry at Al. The highest m/e peak in the EI mass spectrum
of 12a corresponds to the [AlMe2]+ ion, which contrasts with
the EI mass spectra of 1a, 3a, 6a, and 10a, which all contain
peaks for [{RC(NR′)2}AlMe]+ ions.17 This difference suggests
t
-
that the bulky BuC(NtBu)2 ligand is more labile than the
-
i
t
-
i
MeC(NR)2 (R ) Pr, Cy) and BuC(NR)2 (R ) Pr, Cy)
ligands.
for [7b][B(C6F5)4] except for the anion resonances, and the 11
B
The molecular structure of 12a was determined by X-ray
crystallography and is illustrated in Figure 10. Compound 12a
adopts a distorted tetrahedral structure similar to that of 10a.
However, the tBu-N-C bond angles in 12a (139.6(2)° average)
NMR spectrum contains a resonance at δ -14 characteristic of
-
the MeB(C6F5)3 anion and a very broad resonance at δ 61
corresponding to B(C6F5)3.49 The [7b][MeB(C6F5)3]/B(C6F5)3
mixture quantitatively evolves to a 1/1 mixture of {tBuC(Ni-
(50) [7b][MeB(C6F5)3] is also unstable in the absence of B(C6F5)3 in
C6D5Cl and is converted to 9b (10 h, 23 °C, 90% by 1H NMR) and
unidentified boron species.
(47) Yang, X.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1994, 116,
10015.
(48) Malone, J. F.; McDonald, W. S. J. Chem. Soc., Dalton Trans. 1972,
(51) (a) For synthesis of mono-amidinate group 13 dialkyl complexes
by salt metathesis routes, see: Kottmair-Maieron, D.; Lechter, R.; Weidlein,
J. Z. Anorg. Allg. Chem. 1991, 593, 111. (b) For the synthesis of 12b, see
ref 41.
2646.
(49) A 11B NMR spectrum of B(C6F5)3 in C6D5Cl at 80 °C confirmed
this assignment.

Cationic Al and Ga Amidinate Complexes
J. Am. Chem. Soc., Vol. 122, No. 2, 2000 283
Chart 4
Attempts to generate the amine adduct [{tBuC(NtBu)2}Al-
(Me)(NMe2Ph][B(C6F5)4] by reaction of 13a+ with 1 equiv of
NMe2Ph or by reaction of 12a with 1 equiv of [HNMe2Ph]-
[B(C6F5)4] led to the formation of unidentified species. In
contrast, the reaction of 13b+ with 1 equiv of NMe2Ph in C6D5-
Cl (10 min, 23 °C) yields the amine adduct [{tBuC(NtBu)2}-
Ga(Me)(NMe2Ph)][B(C6F5)4] ([14b][B(C6F5)4], 100% by 1H
NMR vs an internal standard, Scheme 6). [14b][B(C6F5)4] is
stable in C6D5Cl for several days at 23 °C, which contrasts with
the apparent low stability of the Al analogue, which could not
be observed. However, numerous attempts to isolate [14b]-
[B(C6F5)4] in pure form were unsuccessful due to its oily nature.
Figure 10. Molecular structure of {tBuC(NtBu)2}AlMe2 (12a). Hy-
drogen atoms are omitted.
Scheme 6
Discussion
Influence of Amidinate Steric Properties on the Structures
of Cationic Al and Ga Complexes. Aluminum and gallium
{RC(NR′)2}MMe2 complexes undergo methyl abstraction by
the “cationic activators” [Ph3C][B(C6F5)4] and B(C6F5)3 to yield
cationic products. The stoichiometry of the reactions and the
structures of the cations that are produced are determined
primarily by the steric properties of the amidinate ligands. The
acetamidinate complexes {MeC(NR′)2}AlMe2 (R′ ) iPr, 1a; R′
) Cy, 3a) react with 0.5 equiv of [Ph3C][B(C6F5)4] or B(C6F5)3
to yield the double-amidinate-bridged dinuclear cations 2a+ and
4a+, in which the two metal centers are linked by µ-η1,η1 and
µ-η1,η2 amidinate bridges. The Ga complex {MeC(NiPr)2}-
GaMe2 (1b) reacts with 0.5 equiv of B(C6F5)3 to yield the
analogous dinuclear cationic Ga alkyl species 2b+. The bulkier
tBu-substituted amidinate complexes {tBuC(NiPr)2}MMe2 (M
) Al, 6a; M ) Ga, 6b) react with 0.5 equiv of [Ph3C][B(C6F5)4]
or B(C6F5)3 to yield the single-amidinate-bridged dinuclear
cations 7a+ and 7b+, in which the two metal centers are linked
by a µ-η1,η2 amidinate. These dinuclear cations are presumed
to form by initial generation of base-free {RC(NR′)2}MMe+
species and subsequent trapping by {RC(NR′)2}MMe2. In
contrast, the “super-bulky” amidinate complexes {tBuC(NtBu)2}-
MMe2 (M ) Al, 12a; M ) Ga, 12b) react with 1 equiv of
[Ph3C][B(C6F5)4] to yield {tBuC(NtBu)2}MMe+ cations 13a+
and 13b+. The available data for 13a+ and 13b+ are consistent
with base-free, three-coordinate structures or labile, four-
coordinate solvated cations; however, these species decompose
rapidly in solution to unidentified products, which hindered
characterization.
are larger than the corresponding Cy-N-C bond angles in 10a
(131.9(5)° average) due to the severe steric crowding between
the tBu-C and N-tBu substituents in 12a. As a result, the tBu-
N-Al bond angles in 12a (127.7(1)° average) are smaller than
the Cy-N-Al bond angles in 10a (133.9(5)° average).
Reaction of {tBuC(NtBu)2}MMe2 (12a, M ) Al; 12b, M
) Ga) with [Ph3C][B(C6F5)4]. The reaction of 12a or 12b with
1 equiv of [Ph3C][B(C6F5)4] in C6D5Cl (10 min, 23 °C) yields
a 1/1 mixture of [{tBuC(NtBu)2}MMe][B(C6F5)4] ([13a]-
[B(C6F5)4], M ) Al; [13b][B(C6F5)4], M ) Ga) and Ph3CCH3
1
(100% by H NMR vs an internal standard, Scheme 6). The
13a+ and 13b+ cations decompose to unidentified species at
23 °C in C6D5Cl (13a+, t1/2 ≈ 1 h; 13b+, t1/2 ≈ 7 h) and C6D6
(13a+, t1/2 ≈ 2 h; 13b+, t1/2 ≈ 10 h), and thus [13a][B(C6F5)4]
and [13b][B(C6F5)4] could not be isolated in pure form.
The 11B, 19F, and 13C NMR spectra of [13a][B(C6F5)4] and
[13b][B(C6F5)4] (C6D5Cl) establish that the B(C6F5)4- anion is
free in solution. The 1H NMR spectra of 13a+ and 13b+ (C6D5-
t
Cl) contain two Bu signals in a 2/1 ratio and one MMe signal
(13a+, δ -0.04; 13b+, δ 0.35). The MMe resonances are shifted
significantly downfield from the MMe resonances of 12a and
12b (δ -0.66; δ -0.17), which is consistent with more electron-
deficient metal centers in 13a+ and 13b+ vs 12a and 12b. These
data are consistent with base-free, three-coordinate 13a+ and
13b+ cations or solvated, four-coordinate 13a+‚C6D5Cl and
13b+‚C6D5Cl cations. For comparison, three-coordinate â-diketim-
inate {HC(CMeN(2,6-iPr2-Ph)}AlR+ complexes have been
characterized crystallographically.7b,c
Comparison of key bond angles in {RC(NR′)2}AlMe2 com-
plexes as a function of the C-R and N-R′ substituents provides
insight to the role of steric effects in the methyl abstraction
reactions (Chart 4).52 Replacement of the Me group at the central
carbon of a {MeC(NR′)2}AlMe2 (R′ ) iPr, Cy)53 complex by a
tBu group decreases the R′-N-Al angle by ca. 10°, to ca. 134°.
(52) The Ga analogues exhibit very similar structural trends.
(53) The steric bulk of the N-Cy substituent is very similar to that of the
N-iPr substituent; see ref 17.

284 J. Am. Chem. Soc., Vol. 122, No. 2, 2000
Dagorne et al.
Chart 5
Chart 6
bite angle (ca. 69°; I in Chart 6) and compact size of the {RC-
(NR′)2}Al unit facilitate the binding of two {RC(NR′)2}Al units
to a single nitrogen center in the µ-η1,η2 amidinate-bridged
structures. In contrast, the larger N-Al-N bite angles and
concomitant increased crowding in the (ATI)Al (ca. 83°; J in
Chart 6) and {HC(CMeNAr)2}Al (ca. 96°; K in Chart 6) systems
disfavor N-bridged structures.
It should also be noted that there are significant electronic
differences between the amidinate and the ATI and diketiminate
systems, as illustrated by the localized structures in Chart 6.
The nitrogen lone pair in a RC(NR′)2}M unit is associated with
a four-electron π-system, while those in (ATI)Al and {HC-
(CMeNAr)2}Al units are associated with ten- and six-electron
π-systems, respectively. The increased π-delocalization and
potential aromatic character in the latter systems may decrease
the nucleophilicity of the N centers and disfavor N-bridged
structures.
Similarly, replacement of the N-Cy (or N-iPr) substituents of a
{tBuC(NR′)2}AlMe2 (R′ ) Pr, Cy) complex by a Bu group
decreases the R′-N-Al angle a further 7°, to ca. 127°. Thus,
the degree of steric crowding at Al increases dramatically as
tBu substituents are incorporated into the amidinate ligand. The
steric crowding at the metal centers in {tBuC(NtBu)2}MMe+
and {tBuC(NtBu)2}MMe2 prevents the formation of the corre-
sponding dinuclear adduct.
i
t
The difference in the structures of the double-amidinate-
bridged cations 2a+, 2b+, and 4a+ and the single-amidinate-
bridged cations 7a+ and 7b+ can also be traced to steric factors.
As illustrated in Chart 5, in an idealized amidinate structure
with 120° bond angles at the C and N centers, the nitrogen sp2
donor orbitals project in parallel directions, which favors
µ-η1,η1-bridged structures (F, Chart 5). However, increased
steric interactions between the amidinate substituents will tend
to decrease the N-C-N angle, which in turn will favor chelated
or µ-η1,η2-bridged structures (G or H, Chart 5). For example,
formamidinates and acetamidinates are well known to form
transition metal complexes with µ-η1,η1-bridged structures of
type F.9a There are also several examples of dinuclear group
13 complexes with µ-η1,η1 amidinate bridges, including [{µ-
η1,η1-MeC(NMe)2}2MMe2]2 (M ) Al, Ga)20 and [{µ-η1,η1-
HC(NCy)2}2InMe]2.54 In these cases, the amidinate ligands
contain at least one sterically small substituent and the N-C-N
angles are ca. 118°. In contrast, the bulkier systems {RC(NR′)2}-
MMe2 (R ) Me, tBu; R′ ) iPr, Cy, tBu; M ) Al, Ga) all adopt
monomeric chelated amidinate structures with N-C-N angles
of ca. 109°.17,41 The µ-η1,η1 and µ-η1,η2 amidinate ligands in
2a+ and 4a+ are characterized by N-C-N angles of ca. 119°
and 109°, respectively. In 4a+, the shortest H-H distance
between the CMe and the Cy substituents of the µ-η1,η2
amidinate (N(1)-C(10)-N(2), Figure 1) is 2.24 Å, which is
only slightly less than the sum of the van der Waals radii (ca.
2.4 Å). However, in the µ-η1,η1 amidinate of 4a+, much shorter
CMe/Cy H-H contacts (1.88 Å) are observed, indicating that
significant steric interactions between the CMe and Cy groups
are present due to the larger N-C-N angle. As noted above,
changing the CR group from Me to tBu significantly increases
the CR/NR′ steric interactions, which apparently precludes
formation of µ-η1,η1-bridged structures for 7a+ and 7b+.
Al versus Ga. In several cases, parallel methyl abstraction
chemistry is observed for Al and Ga amidinate compounds. The
Al and Ga {MeC(NiPr)2}MMe2 complexes 1a and 1b react with
B(C6F5)3 to yield 2a+ and 2b+, and the Al and Ga {tBuC-
i
t
(NR′)2}MMe2 (R′ ) Pr, Bu) complexes react with B(C6F5)3
and [Ph3C][B(C6F5)4] to yield the analogous pairs of complexes
7a+ and 7b+, and 13a+ and 13b+. In contrast, while the Al
complexes {RC(NiPr)2}AlMe2 (1a, R ) Me; 6a, R ) tBu) react
with [HNMe2Ph][B(C6F5)4] to yield the cationic amine adducts
5a+ and 8a+ and methane, the Ga analogues 1b and 6b do not
undergo Ga-Me protonolysis by this reagent. Additionally,
while 1a cleanly reacts with [Ph3C][B(C6F5)4] to yield 2a+, the
Ga analogue 1b does not undergo Me- abstraction by Ph3C+.
These reactivity differences are ascribed to the lower polarity
of the Ga-Me bonds vs the Al-Me bonds in these systems.
The acid HNMe2Ph+ apparently reacts with the amidinate
ligands of 1b and 6b, although the products of these reactions
have not been identified. Similarly, the Ph3C+ ion may attack
the amidinate ligand of 1b rather that the Ga-Me bond;
t
however, steric crowding in the Bu-substituted analogue 6b
directs the attack of Ph3C+ to a Ga-Me bond. As initially
anticipated, the Ga cations (2b+, 7b+, and 13b+) are more stable
than their Al counterparts (2a+, 7a+, and 13a+), which was of
significant assistance in characterization of the latter systems.
Dynamic and Reactivity Properties of 2a+ and 7a+. The
dynamic and reactivity properties of the two types of dinuclear
Al cations, i.e., 2a+ vs 7a+, are very different. The double-
amidinate-bridged dinuclear cation 2a+ undergoes two different
fluxional processes: µ-η1,η1/µ-η1,η2 amidinate exchange (pro-
cess (i)) and Al-Me exchange between the two Al centers
(process (ii)), while 7a+ is nonfluxional up to 60 °C. However,
intermolecular exchange is not observed between 2a+ and 1a,
nor between 7a+and 6a, which shows that neither dinuclear
cation dissociates to mononuclear species easily. The dinuclear
cations are cleaved by Lewis bases and thus are potential source
of {RC(NiPr)2}AlMe+ or {RC(NiPr)2}AlMe(L)+ species, but
the double-amidinate-bridged 2a+ is significantly less reactive
than the single-amidinate-bridged 7a+. Thus, 2a+ slowly reacts
Bonding Trends in Dinuclear [{L-X}MR‚{L-X}AlR2]+
Species. The µ-η1,η1 and µ-η1,η2 amidinate-bridged structures
+
observed for the Al and Ga {RC(NR′)2}2M2Me3 cations
contrast with the Me-bridged structures observed for the
analogous aminotroponiminate and diketiminate species {(ATI)-
AlMe}2(µ-Me)+ and ({HC(CMeNAr)2}AlMe)2Me+ (Ar ) 2-
tBu-Ph).4b,7c In all three cases, the three-center, four-electron
N-bridges are probably electronically favored over the two-
center, two-electron methyl bridges. However, the acute N-Al-N
(54) Zhou, Y.; Richeson, D. S. Inorg. Chem. 1996, 35, 1423.

Cationic Al and Ga Amidinate Complexes
J. Am. Chem. Soc., Vol. 122, No. 2, 2000 285
for C6D5Cl appear at δ 134.2, 129.3 (t), 128.3(t), and 126.0(t). 11B
with 1 equiv of NMe2Ph (12 h, 23 °C) to yield a 1/1 mixture of
the amine adduct 5a+ and 1a, while 7a+ reacts very rapidly
with NMe2Ph (<10 min at 23 °C) to yield the corresponding
amine adduct and the neutral precursor. The higher reactivity
of 7a+ vs 2a+ toward cleavage to mononuclear species may
contribute to the higher activity of 7a+ vs 2a+ in ethylene
polymerization, although further investigation of this issue is
required.4a
NMR chemical shifts are reported vs BF3‚Et2O (0.1 M in C6D5Cl). 19
F
NMR chemical shifts are reported vs CFCl3 in CDCl3. All coupling
constants are reported in hertz. Mass spectra were obtained using the
direct insertion probe (DIP) method on a VG Analytical Trio I
instrument operating at 70 eV. Elemental analyses were performed by
Desert Analytics Laboratory (Tucson, AZ).
[{MeC(NiPr)2}2Al2Me3][MeB(C6F5)3] ([2a][MeB(C6F5)3]). A solu-
tion of B(C6F5)3 (770 mg, 1.50 mmol) in CH2Cl2 (20 mL) was added
to {MeC(NiPr)2}AlMe2 (1a, 600 mg, 3.00 mmol). The reaction mixture
was stirred for 30 min at 23 °C, and the volatiles were removed under
vacuum, leaving a colorless oily residue. Trituration with pentane
Importance of the Anion. As for cationic transition metal
alkyl species, the stability of the cationic Al and Ga amidinate
alkyl species is strongly influenced by the counterion properties.
Compounds [7a][B(C6F5)4] and [7b][B(C6F5)4] are more stable
in solution than [7a][MeB(C6F5)3] and [7b][MeB(C6F5)3], due
1
afforded pure [2a][MeB(C6F5)3] as a white solid (910 mg, 83%). H
3
NMR (CD2Cl2, 23 °C): δ 3.79 (sept, J ) 6.6, 4H, CHMe2), 2.31 (s,
6H, CMe), 1.28 (d, 3J ) 6.5, CHMe2), 0.48 (br s, 3H, MeB), -0.38 (br
-
to the C6F5 transfer in the latter salts. The choice of an
1
3
s, 9H, AlMe). H NMR (CD2Cl2, -20 °C): δ 3.75 (br sept, J ) 5.9,
4H, CHMe2), 2.28 (s, 6H, CMe), 1.24 (br s, 24H, CHMe2), 0.52 (br s,
3H, MeB), -0.15 (br s, 3H, AlMe), -0.57 (br s, 6H, AlMe2). 1H NMR
(CD2Cl2, -85 °C): δ 3.79 (br sept, 1H, CHMe2), 3.67 (br sept, 3H,
CHMe2), 2.33 (s, 3H, CMe), 2.15 (s, 3H, CMe), 1.30 (m, 9H, CHMe2),
1.18 (m, 6H, CHMe2), 1.02 (m, 9H, CHMe2), 0.55 (br s, 3H, MeB),
-0.17 (br s, 3H, AlMe), -0.54 (br s, 3H, AlMe), -0.75 (s, 3H, AlMe).
13C NMR (CD2Cl2, 23 °C): δ 182.0 (CMe), 148.6 (d, 1JCF ) 235, C6F5),
appropriate anion is critical for cation stability in these systems,
and thus is important for potential application of these cations.
Conclusions
Aluminum and gallium {RC(NR′)2}MMe2 complexes react
with “cationic activators” to yield cationic alkyl species whose
structures are strongly influenced by the steric properties of the
amidinate ligand. Three types of structure are observed: (i)
fluxional double-amidinate-bridged dinuclear species {RC-
(NR′)2}2M2Me3+ containing µ-η1,η1 and µ-η1,η2 amidinates, (ii)
rigid single-amidinate-bridged dinuclear species {RC(NR′)2}-
MMe‚{RC(NR′)2}MMe2+ containing a µ-η1,η2 amidinate, and
(iii) thermally unstable base-free (three-coordinate) or solvated
(four-coordinate) “{RC(NR′)2}MMe+ ” species. The preference
for amidinate-bridged structures contrasts with preference for
the Me-bridged structures observed for the {(ATI)AlMe}2(µ-
Me)+ and ({HC(CMeNAr)2}AlMe)2Me+ (Ar ) 2-tBu-Ph)
systems. The results reported here provide a starting point for
understanding the mechanism and reactivity trends in ethylene
polymerization catalyzed by cationic Al species. Studies con-
cerning the influence of the amidinate electronic properties and
the nature of the Al-R group on the structures and reactivity of
cationic Al amidinate species will be reported in due course.
1
1
137.9 (d, JCF ) 243, C6F5), 136.8 (d, JCF ) 246, C6F5), 129.7 (br s,
ipso-C6F5), 50.5 (d, 1JCH ) 139, CHMe2), 23.4 (q, 1JCH ) 127, CHMe2),
17.8 (q, 1JCH ) 130, CMe), 9.2 (MeB), -5.6 (br q, 1JCH ) 114, AlMe).
13C NMR (CD2Cl2, -85 °C): δ 182.4 (MeC), 179.4 (MeC), 148.7 (d,
1
1
1JCF ) 238, C6F5), 137.8 (d, JCF ) 242, C6F5), 135.3 (d, JCF ) 236,
C6F5), 130.5 (br s, ipso-C6F5), 50.3 (CHMe2), 50.1 (CHMe2), 49.5
(CHMe2), 48.8 (CHMe2), 23.4 (CHMe2), 22.9 (CHMe2), 22.8 (CHMe2),
22.6 (CHMe2), 22.2 (CHMe2), 22.0 (CHMe2), 21.8 (CHMe2), 21.6
(CHMe2), 16.6 (MeC), 14.1 (MeC), 9.4 (MeB), -3.8 (AlMe), -6.6
(AlMe), -6.6 (AlMe). 11B NMR (CD2Cl2, 23 °C): δ -14 (br). 19F
NMR (CD2Cl2): δ -131.6 (6F, o-C6F5), -163.6 (3F, p-C6F5), 166.3
(6F, m-C6F5). Anal. Calcd for C38H46Al2BF15N4: C, 50.23; H, 5.10;
N, 6.17. Found: C, 50.46; H, 4.92; N, 6.09.
[{MeC(NiPr)2}2Al2Me3][B(C6F5)4] ([2a][B(C6F5)4]). A solution of
[Ph3C][B(C6F5)4] (402 mg, 0.436 mmol) in CH2Cl2 (5 mL) was cannula-
transferred at 23 °C to {MeC(NiPr)2}AlMe2 (1a, 173 mg, 0.872 mmol),
and the mixture was stirred for 30 min. The volatiles were removed
under vacuum, leaving a colorless oily residue. The residue was
triturated with pentane to afford a white solid. The mixture was filtered
through a glass frit, and the solid was washed with pentane (3 × 5
mL) and dried under vacuum, affording pure [2a][B(C6F5)4] (324 mg,
78%). The NMR spectra of [2a][B(C6F5)4] exhibit resonances for 2a+
that are identical to those for [2a][MeB(C6F5)3] (see above) and
resonances for the B(C6F5)4- anion that are listed here. 13C NMR (C6D5-
Cl):58 δ 149.0 (d, 1JCF ) 241, C6F5), 138.8 (d, 1JCF ) 245, C6F5), 136.9
Experimental Section
General Procedures. All experiments were carried out under N2
using standard Schlenk techniques or in a Vacuum Atmospheres
glovebox. Benzene, toluene, pentane, hexanes, and diethyl ether were
distilled from Na/benzophenone and stored under N2 prior to use. CH2-
Cl2, CD2Cl2, ClCD2CD2Cl, and C6D5Cl were distilled from CaH2 and
stored under vacuum prior to use. The Al and Ga amidinate dimethyl
complexes 1a, 3a, 6a, 10a, 1b, 6b, and 12b were prepared by salt
metathesis procedures.17,41 [Ph3C][B(C6F5)4] was prepared by a literature
procedure.55,56 [NHMe2Ph][B(C6F5)4] and B(C6F5)3 were obtained as a
gift from Boulder Scientific.57 All other chemicals were purchased from
Aldrich and used as received, except NMe2Ph which was dried over
molecular sieves (4 Å) prior to use.
1
(d, JCF ) 243, C6F5). 11B NMR (C6D5Cl): δ -16.5 (br s). 19F NMR
(C6D5Cl): δ -132.5 (m, 8F, o-C6F5), -163.2 (t, 3JFF ) 20, 4F, p-C6F5),
-167.0 (br s, 8F, m-C6F5). Anal. Calcd for C43H43Al2BF20N4: C, 48.69;
H, 4.09; N, 5.28. Found: C, 48.91; H, 4.22; N, 5.19.
[{MeC(NiPr)2}2Ga2Me3][MeB(C6F5)3] ([2b][MeB(C6F5)3]). A CH2-
Cl2 solution (1 mL) of {MeC(NiPr)2GaMe2 (1b, 236 mg, 0.98 mmol)
was added by cannula to a hexane solution (15 mL) of B(C6F5)3 (250
mg, 0.490 mmol). The reaction mixture was stirred for 3 h at 23 °C,
resulting in a white suspension. The mixture was filtered through a
glass frit. The colorless solid was washed with hexane (3 × 5 mL) and
dried under vacuum to afford pure [2b][MeB(C6F5)3] as a colorless
solid in 71% yield. 1H NMR (C6D5Cl): δ 3.25 (br, 4H, CHMe2), 1.69
(s, 6H, MeC), 1.03 (br s, 3H, MeB), 0.78 (br, 24H, CHMe2), 0.12 (br,
3H, GaMe), -0.32 (br, 6H, GaMe2). 1H NMR (C6D5Cl, 60 °C): δ 3.36
(sept, 3J ) 6.5, 4H, CHMe2), 1.77 (s, 6H, MeC), 0.98 (br s, 3H, MeB),
0.87 (d, 24H, 3J ) 6.1, CHMe2), -0.14 (br, 9H, GaMe). 1H NMR (CD2-
Cl2, -85 °C): δ 3.71 (br, 3H, CHMe2), 3.55 (br, 1H, CHMe2), 2.14
(s, 3H, MeC), 2.10 (s, 3H, MeC), 1.25 (br, 6H, CHMe2), 1.11 (br, 9H,
CHMe2), 0.98 (br, 6H, CHMe2), 0.92 (br, 3H, CHMe2), 0.55 (br, 3H,
MeB), 0.43 (s, 3H, GaMe), -0.04 (s, 3H, GaMe), -0.27 (s, 3H, GaMe).
1H, 13C, and 11B NMR spectra were obtained on a Bruker AMX-
360 spectrometer, in Teflon-valved or flame-sealed tubes, at ambient
probe temperature (23 °C) unless otherwise indicated. 19F NMR spectra
1
were obtained on a Bruker AC-300 spectrometer. H and 13C NMR
chemical shifts are reported vs SiMe4 and were determined by reference
1
to the residual solvent peaks. The residual H NMR resonances for
C6D5Cl appear at δ 7.14, 7.00, and 6.96, and the 13C NMR resonances
(55) (a) Chien, J. C. W.; Tsai, W. M.; Rausch, M. D. J. Am. Chem. Soc.
1991, 113, 8570. (b) See also ref 4b.
(56) Data for [Ph3C][B(C6F5)4]. 1H NMR (C6D5Cl): δ 7.68 (t, J ) 7.2,
3H, p-Ph), 7.31 (t, J ) 7.6, 6H, m-Ph), 7.08 (d, J ) 7.2, 6H, o-Ph). 11B
NMR (C6D5Cl): δ -16.5 (br s). 19F NMR (C6D5Cl): δ -132.5 (m, 8F,
3
o-C6F5), -163.2 (t, JFF ) 20, 4F, p-C6F5), -167.0 (br s, 8F, m-C6F5).
(57) Data for B(C6F5)3. 19F NMR (C6D5Cl): δ -128.4 (br, 6F, o-C6F5),
-143.2 (br, 3F, p-C6F5), -160.6 (br, 6F, m-C6F5). 11B NMR (C6D5Cl, 80
°C): δ 61 (br).
-
(58) The ipso-B(C6F5)4 resonance was obscured by the solvent reso-
nances.

286 J. Am. Chem. Soc., Vol. 122, No. 2, 2000
13C NMR (C6D5Cl, 23 °C):59 δ 179.7 (MeC), 150.1 (d, JCF ) 238,
Dagorne et al.
149.1 (d, 1JCF ) 245, C6F5), 136.6 (d, 1JCF ) 245, C6F5), 135.3 (d, 1JCF
) 243, C6F5), 126.4 (br, ipso-C6F5), 46.7 (CHMe2), 45.7 (CHMe2),
45.3 (CHMe2), 45.0 (CHMe2), 41.5 (CCMe3), 40.0 (CCMe3), 28.1
(CCMe3), 28.0 (CCMe3), 25.8 (CHMe2), 25.6 (CHMe2), 25.5 (CHMe2),
24.4 (CHMe2), 23.5 (CHMe2), 22.6 (CHMe2), 21.8 (CHMe2), 21.7
(CHMe2), -3.1 (AlMe), -3.5 (AlMe), -4.0 (AlMe). 11B NMR (C6D5-
Cl): δ -16.5 (br s). 19F NMR (C6D5Cl): δ -132.5 (m, 8F, o-C6F5),
1
1
1
C6F5), 139.2 (d, JCF ) 242, C6F5), 136.7 (d, JCF ) 236, C6F5), 51.5
(CHMe2), 24.0 (CHMe2), 17.3 (MeC), 11.3 (MeB), 0.4 (GaMe), 0.4
(GaMe). 13C NMR (CD2Cl2, -85 °C): δ 179.8 (MeC), 176.0 (MeC),
148.7 (d, 1JCF ) 238, C6F5), 137.8 (d, 1JCF ) 242, C6F5), 135.3 (d, 1JCF
) 236, C6F5), 128.7 (br s, ipso-C6F5), 50.6 (CHMe2), 50.4 (CHMe2),
49.3 (CHMe2), 48.6 (CHMe2), 24.5 (CHMe2), 23.2 (CHMe2), 22.9
(CHMe2), 22.7 (CHMe2), 22.2 (CHMe2), 22.0 (CHMe2), 21.5 (CHMe2),
16.8 (MeC), 15.4 (MeC), 9.4 (MeB), 0.9 (GaMe), -0.75 (GaMe), -3.1
(GaMe). 11B NMR (C6D5Cl): δ -14 (br s). 19F NMR (C6D5Cl): δ
3
-163.2 (t, JFF ) 20, 4F, p-C6F5), -167.0 (br, 8F, m-C6F5).
Generation of [7a][MeB(C6F5)3] from 6a and B(C6F5)3 at -60
°C. An NMR tube was charged with {tBuC(NiPr)2}AlMe2 (6a, 30.0
mg, 0.0125 mmol) and B(C6F5)3 (63.9 mg, 0.0125 mmol), and CD2Cl2
(0.5 mL) was added by vacuum transfer at -78 °C. The tube was
immersed in an acetone/dry ice bath (-78 °C) and was flame-sealed
under vacuum at this temperature. The tube was removed from the
-78 °C bath and vigorously shaken for 30 s, yielding a colorless
solution, and was immediately inserted into the NMR probe which had
been precooled at -60 °C. The probe was maintained at -60 °C for
3
-131.9 (m, 6F, o-C6F5), -164.7 (t, JFF ) 20, 3F, p-C6F5), -167.3
(m, 6F, m-C6F5). Anal. Calcd for C38H46BF15Ga2N4: C, 45.90; H, 4.67;
N, 5.64. Found: C, 46.13; H, 4.86; N, 5.23.
[{MeC(NCy)2}2Al2Me3][B(C6F5)4] ([4a][B(C6F5)4]). A slurry of
{MeC(NCy)2}AlMe2 (3a, 200 mg, 0.720 mmol) and 0.5 equiv of [Ph3C]-
[B(C6F5)4] (331 mg, 0.359 mmol) in hexane (5 mL) was vigorously
stirred at 23 °C for 3 d, yielding a colorless slurry. The mixture was
filtered through a glass frit, and the colorless solid was washed several
times with pentane and dried under vacuum to afford [4a][B(C6F5)4]
1
20 min. A H NMR spectrum was then recorded which showed that
[7a][MeB(C6F5)3] had formed in 80% yield. The 1H and 13C NMR data
of [7a][MeB(C6F5)3] are identical to those of [7a][B(C6F5)4] except for
1
as a colorless solid in 61% yield. H NMR (C6D5Cl): δ 3.10 (br, 4H,
-
NCH)), 1.92 (s, 6H, CMe), 1.80-1.45 (br, 16H, Cy), 1.40-0.80 (br,
24H, Cy), -0.43 (br s, 9H, AlMe). Anal. Calcd for C55H59Al2BF20N4:
C, 54.10; H, 4.88; N, 4.59. Found: C, 54.46; H, 4.92; N, 4.71.
Generation of [{MeC(NiPr)2}Al(Me)(NMe2Ph)][B(C6F5)4] ([5a]-
[B(C6F5)4]). A solution of [HNMe2Ph][B(C6F5)4] (853 mg, 0.106 mmol)
in CD2Cl2 (5 mL) was added to a vial containing {MeC(NiPr)2}AlMe2
(1a, 211 mg, 106 mmol). The resulting solution was transferred to an
NMR tube and maintained at 23 °C for 15 min. NMR spectra were
recorded and indicated that 100% conversion to [5a][B(C6F5)4] had
the MeB(C6F5)3 resonances, which are listed here.
Data for MeB(C6F5)3-. H NMR (CD2Cl2, -60 °C): δ 0.57 (br s,
1
MeB). 13C NMR (CD2Cl2, -60 °C): δ 147.7 (d, JCF ) 238, C6F5),
1
1
1
136.5 (d, JCF ) 242, C6F5), 135.8 (d, JCF ) 236, C6F5), 126.4 (br s,
ipso-C6F5), 15.3 (q, JCH ) 129, MeB). 11B NMR (CD2Cl2, -60 °C):
1
δ -14 (br s).
[{tBuC(NiPr)2}GaMe2‚{tBuC(NiPr)2}GaMe][B(C6F5)4] ([7b][B(C6-
F5)4]). A mixture of {tBuC(NiPr)2}GaMe2 (6b, 200 mg, 0.706 mmol)
and [Ph3C][B(C6F5)4] (326 mg, 0.353 mmol) in benzene (1 mL) was
stirred for 1 h at 23 °C. Hexane (10 mL) was added, resulting in the
formation of two layers (deep red bottom layer and yellow top layer).
The mixture was stirred at 23 °C for 2 d, resulting in the formation of
a yellow solid. The mixture was filtered, and the solid was washed
with pentane (3 × 5 mL) and dried under vacuum to afford pure [7b]-
[B(C6F5)4] (361 mg, 64%). 1H NMR (CD2Cl2, -60 °C): δ 4.75 (septet,
3J ) 6.1, 1H, CHMe2), 4.59 (septet, 3J ) 6.5, 1H, CHMe2), 4.53-4.42
(m, 2H, CHMe2), 1.73 (s, 9H, CMe3), 1.65 (s, 9H, CMe3), 1.68-1.27
(m, 24H, CHMe2), 0.55 (s, 3H, GaMe), 0.45 (s, 3H, GaMe), 0.35 (s,
3H, GaMe). 13C NMR (C6D5Cl): δ 186.7 (CCMe3), 184.4 (CCMe3),
149.0 (d, 1JCF ) 241, C6F5), 138.8 (d, 1JCF ) 245, C6F5), 136.9 (d, 1JCF
) 243, C6F5), 52.3 (CHMe2), 51.9 (CHMe2), 47.7 (CHMe2), 46.9
(CHMe2), 41.2 (CMe3), 39.4 (CMe3), 29.1 (CMe3), 28.8 (CMe3), 27.4
(CHMe2), 26.4 (CHMe2), 26.0 (CHMe2), 25.7 (CHMe2), 24.6 (CHMe2),
1
3
occurred. H NMR (CD2Cl2): δ 7.63 (t, J ) 7.9, 2H, m-Ph), 7.51 (t,
3J ) 7.3, 1H, p-Ph), 7.47 (d, 3J ) 7.9, 2H, o-Ph), 3.58 (sept, 3J ) 6.4,
2H, CHMe2), 3.20 (s, 6H, NMe2Ph), 2.17 (s, 3H, CMe), 1.03 (d, 3J )
6.5, 6H, CHMe2), 0.92 (d, 3J ) 6.4, 6H, CHMe2), -0.30 (s, 3H, AlMe).
13C NMR (CD2Cl2): δ 182.0 (CMe), 148.6 (d, 1JCF ) 245, C6F5), 143.7
1
1
(ipso-NMe2Ph), 138.6 (d, JCF ) 245, C6F5), 136.7 (d, JCF ) 243,
C6F5), 131.4 (NMe2Ph), 129.8 (NMe2Ph), 125.1 (br, ipso-C6F5), 120.9
(NMe2Ph), 46.7 (NMe2Ph), 46.0 (CHMe2), 24.7 (CHMe2), 24.6
(CHMe2), 12.7 (CMe), -13.4 (br, AlMe).
Generation of [{MeC(NiPr)2}Al(Me)(NMe2Ph)][MeB(C6F5)3]
([5a][MeB(C6F5)3]). NMe2Ph (7.3 mg, 0.032 mmol) was added to a
solution of [({MeC(NiPr)2}Al)2Me3][MeB(C6F5)3] ([2a][MeB(C6F5)3],
27.0 mg, 0.032 mmol) in CD2Cl2 (0.6 mL). The mixture was stirred
for 15 min and transferred to an NMR tube. The tube was maintained
at 23 °C, and the reaction was monitored by 1H NMR. The NMR spectra
showed that complete conversion to a 1/1 mixture of [5a][MeB(C6F5)3]
and {MeC(NiPr)2}AlMe2 had occurred after 12 h. The NMR spectra
for [5a][MeB(C6F5)3] are identical to those for [5a][B(C6F5)4], except
for the MeB(C6F5)3- resonances which are listed here. 1H NMR (CD2-
1
23.6 (CHMe2), 23.2 (CHMe2), 22.1 (CHMe2), 3.6 (q, JCH ) 125,
GaMe), 1.3 (q, JCH ) 123, GaMe), 1.3 (q, JCH ) 123, GaMe). 11B
1
1
NMR (C6D5Cl): δ -16.5 (br s). 19F NMR (C6D5Cl): δ -132.5 (m,
3
8F, o-C6F5), -163.2 (t, JFF ) 20, 4F, p-C6F5), -167.0 (br s, 8F,
m-C6F5). Anal. Calcd for C49H55BF20GaN2: C, 47.83; H, 4.51; N, 2.28.
Cl2): δ 0.48 (br s, 3H, MeB). 13C NMR (CD2Cl2): δ 148.6 (d, 1JCF
)
Found: C, 48.24; H, 4.37; N, 2.13.
1
1
235, C6F5), 137.9 (d, JCF ) 243, C6F5), 136.8 (d, JCF ) 246, C6F5),
Generation of [{tBuC(NiPr)2}Al(Me)(NMe2Ph)][B(C6F5)4] ([8a]-
[B(C6F5)4]). (a) From [7a][B(C6F5)4]. [{tBuC(NiPr)2}AlMe2‚{tBuC-
(NiPr)2}AlMe][B(C6F5)4] ([7a][B(C6F5)4]) was generated in situ in
C6D5Cl in a valved NMR tube as described above, and 1 equiv of NMe2-
Ph (10.3 µL, 0.0883 mmol) was added at 23 °C by syringe. The tube
was vigorously shaken and maintained at 23 °C for 10 min. NMR
spectra were then recorded and showed that complete conversion to a
1/1 mixture of [7a][B(C6F5)4] and 6a had occurred.
129.7 (br s, ipso-C6F5). 11B NMR (CD2Cl2): δ -14 (br).
Generation of [{tBuC(NiPr)2}AlMe2‚{tBuC(NiPr)2}AlMe][B(C6F5)4]
([7a][B(C6F5)4]). An NMR tube was charged with {tBuC(NiPr)2}AlMe2
(6a, 21.2 mg, 0.0820 mmol) and [Ph3C][B(C6F5)4] (40.7 mg, 0.0410
mmol), and C6D5Cl (0.5 mL) was added by vacuum transfer at -78
°C. The tube was flame-sealed under vacuum, warmed to 23 °C, and
vigorously shaken. A pale brown solution formed, and the tube was
maintained at 23 °C for 15 min. A 1H NMR spectrum was recorded at
25 °C, showing that a 1/1 mixture of Ph3CCH3 and [7a][B(C6F5)4] had
formed. Numerous attempts to isolate [7a][B(C6F5)4] in pure form were
unsuccessful due to the thermal instability of this species.
(b) From 6a. [8a][B(C6F5)4] was generated by the procedure outlined
above for [{MeC(NiPr)2}Al(Me)(NMe2Ph)][B(C6F5)4] ([5a][B(C6F5)4]),
using 208 mg (0.0865 mmol) of {tBuC(NiPr)2}AlMe2 (6a) and 694
mg (0.0866 mmol) of [HNMe2Ph][B(C6F5)4]. Quantitative conversion
Data for Ph3CCH3. 1H NMR (C6D5Cl): δ 7.15-7.00 (m, 15H, Ph),
2.03 (s, 3H, CH3). 13C NMR (C6D5Cl): δ 150.1 (ipso-Ph), 129.0 (Ph),
128.0 (Ph), 126.1 (Ph), 52.6 (CCH3), 30.6 (CH3).
Data for [7a][B(C6F5)4]. 1H NMR (C6D5Cl): δ 4.05-3.75 (m, 4H,
CHMe2), 1.15 (s, 9H, CMe3), 1.14 (s, 9H, CMe3), 1.00-0.89 (m, 24H,
CHMe2), -0.36 (s, 3H, AlMe), -0.39 (s, 3H, AlMe), -0.42 (s, 3H,
AlMe). 13C NMR (CD2Cl2, -85 °C): δ 188.2 (CCMe3), 187.6 (CCMe3),
1
1
to [8a][B(C6F5)4] was observed by H and 13C NMR. H NMR (CD2-
3
Cl2): δ 7.61-7.48 (m, 5H, NPh), 4.13 (sept, J ) 6.3, 2H, CHMe2),
3.21 (s, 6H, NMe2Ph), 1.42 (s, 3H, CMe3), 1.00 (d, 3J ) 6.2, 6H,
CHMe2), 0.89 (d, J ) 6.3, 6H, CHMe2), -0.11 (s, 3H, AlMe). 13C
3
1
NMR (CD2Cl2): δ 189.3 (CMe3), 148.6 (d, JCF ) 245, C6F5), 144.4
1
1
(ipso-NMe2Ph), 138.6 (d, JCF ) 245, C6F5), 136.7 (d, JCF ) 243,
C6F5), 131.2 (NMe2Ph), 129.8 (NMe2Ph), 125.1 (br, ipso-C6F5), 121.1
(NMe2Ph), 47.1 (NMe2Ph), 46.5 (CHMe2), 29.3 (CMe3), 26.9 (CHMe2),
25.5 (CHMe2), 12.7 (CMe), -10.9 (br, AlMe). 11B NMR (C6D5Cl): δ
-
(59) The ipso-MeB(C6F5)3 resonance was obscured by the solvent
resonances.

Cationic Al and Ga Amidinate Complexes
J. Am. Chem. Soc., Vol. 122, No. 2, 2000 287
Table 1. Selected Bond Lengths (Å) and Angles (deg) for
[{MeC(NCy)2}2Al2Me3][B(C6F5)4] ([4a][B(C6F5)4])
added at 23 °C by syringe. The tube was vigorously shaken and
maintained at 23 °C for 10 min. NMR spectra were then recorded and
1
showed that complete conversion to [8b][B(C6F5)4] had occurred. H
Al(1)-N(1)
Al(1)-N(4)
Al(1)-C(1)
Al(1)-C(2)
N(4)-C(24)
N(3)-C(24)
2.027(1)
1.959(1)
1.949(2)
1.971(2)
1.332(2)
1.360(2)
Al(2)-N(1)
Al(2)-N(2)
Al(2)-N(3)
Al(2)-C(3)
N(1)-C(10)
N(2)-C(10)
1.979(1)
1.927(1)
1.866(2)
1.942(2)
1.435(1)
1.289(2)
3
NMR (C6D5Cl): δ 7.21 (d, J ) 6.1, 2H, p-PhMe2N), 7.15-7.03 (m,
3
8H, o- and m-PhMe2N), 4.03 (septet, J ) 6.1, 2H, CHMe2), 2.68 (s,
3
3H, NMe2Ph), 1.20 (s, 9H, CMe3), 0.67 (d, J ) 6.1, 6H, CHMe2),
3
0.52 (d, J ) 5.8, CHMe2), 0.18 (s, 3H, GaMe). 13C NMR (C6D5Cl):
1
δ 185.7 (s, CCMe3), 149.0 (d, JCF ) 241, C6F5), 145.2 (s, ipso-
1
1
PhMe2N), 138.8 (d, JCF ) 245, C6F5), 136.9 (d, JCF ) 243, C6F5),
N(1)-Al(1)-N(4)
N(4)-Al(1)-C(1)
N(4)-Al(1)-C(2)
N(1)-Al(1)-C(1)
N(1)-Al(1)-C(2)
C(1)-Al(1)-C(2)
N(4)-C(24)-N(3)
Al(1)-N(1)-Al(2) 104.17(6) Al(2)-N(1)-C(9)
Al(1)-N(1)-C(9) 114.0(1) Al(2)-N(1)-C(10)
Al(1)-N(1)-C(10) 111.2(1)
98.11(6) N(2)-Al(2)-N(1)
105.33(7) N(1)-Al(2)-N(3)
119.80(7) N(1)-Al(2)-C(3)
116.81(7) N(2)-Al(2)-N(3)
103.49(7) N(2)-Al(2)-C(3)
112.97(8) N(3)-Al(2)-C(3)
69.51(6)
1
1
130.4 (d, JCH ) 163, p- or m-PhMe2N), 120.0 (d, JCH ) 161,
o-PhMe2N), 46.8 (d, 1JCH ) 138, CHMe2), 46.4 (q, 1JCH ) 141, NMe2-
Ph), 28.6 (q, 1JCH ) 129, CCMe3), 26.7 (q, 1JCH ) 128, CHMe2), 26.3
(q, 1JCH ) 129, CHMe2), -7.1 (q, 1JCH ) 129, GaMe). 11B NMR (C6D5-
Cl): δ -16.5 (br s). 19F NMR (C6D5Cl): δ -132.5 (m, 8F, o-C6F5),
109.30(6)
119.52(7)
108.64(6)
114.32(7)
123.06(7)
109.6(1)
123.3(1)
86.82(9)
3
-163.2 (t, JFF ) 20, 4F, p-C6F5), -167.0 (br, 8F, m-C6F5).
119.0(2)
N(1)-C(10)-N(2)
Generation of {tBuC(NiPr)2}Al(Me)(C6F5) (9a) from 6a and
B(C6F5)3. An NMR tube was charged with {tBuC(NiPr)2}AlMe2 (6a,
30.0 mg, 0.0125 mmol) and B(C6F5)3 (63.9 mg, 0.0125 mmol), and
CD2Cl2 (0.5 mL) was added by vacuum transfer at -78 °C. The tube
was flame-sealed under vacuum, warmed to 23 °C, vigorously shaken,
and maintained at 23 °C. A 1H NMR spectrum was recorded
immediately and showed that quantitative conversion to a 1/1 mixture
of {tBuC(NiPr)2}Al(Me)(C6F5) (9a) and MeB(C6F5)2 had occurred.
Data for 9a. 1H NMR (CD2Cl2): δ 4.13 (septet, 3J ) 6.2, 2H,
CHMe2), 1.43 (s, CMe3), 1.10 (d, 3J ) 6.2, 6H, CHMe2), 0.96 (d, 3J )
6.2, 6H, CHMe2), -0.43 (s, 3H, AlMe). 13C NMR (CD2Cl2):60 δ 181.3
(CCMe3), 46.0 (CHMe2), 39.6 (CCMe3), 29.3 (CMe3), 26.4 (CHMe2),
25.5 (CHMe2), -8.6 (br, AlMe). 19F NMR (CD2Cl2): δ -124.1 (m,
2F, o-C6F5), -153.6 (m, 1F, p-C6F5), -162.2 (m, 2F, m-C6F5).
Table 2. Selected Bond Lengths (Å) and Angles (deg) for
[{tBuC(NiPr)2}GaMe‚{tBuC(NiPr)2}GaMe2][B(C6F5)4]
([7b][B(C6F5)4])
Ga(1)-N(1)
Ga(1)-N(2)
Ga(1)-C(1)
Ga(1)-C(2)
N(1)-C(6)
N(2)-C(6)
2.007(2)
2.155(2)
1.954(3)
1.946(3)
1.285(3)
1.459(3)
Ga(2)-N(2)
Ga(2)-N(3)
Ga(2)-N(4)
Ga(2)-C(14)
N(4)-C(18)
N(3)-C(18)
2.037(2)
1.987(2)
1.973(2)
1.951(3)
1.338(3)
1.346(3)
N(1)-Ga(1)-N(2)
N(2)-Ga(1)-C(1)
N(2)-Ga(1)-C(2)
N(1)-Ga(1)-C(1)
N(1)-Ga(1)-C(2)
C(1)-Ga(1)-C(2)
64.73(9) N(3)-Ga(2)-N(4)
67.10(9)
1
Data for MeB(C6F5)2. H NMR (CD2Cl2): δ 1.67 (s, 3H, MeB).
116.6(1)
117.9(1)
113.7(1)
111.9(1)
119.3(1)
N(3)-Ga(2)-N(2)
114.57(9)
1
1
13C NMR (C6D5Cl): δ 147.9 (d, JCF ) 249, C6F5), 143.9 (d, JCF
)
N(3)-Ga(2)-C(14) 122.1(1)
1
247, C6F5), 137.6 (d, JCF ) 254, C6F5), 114.9 (br s, ipso-C6F5), 16.7
(q, 1JCH ) 129, MeB). 11B NMR (C6D5Cl): δ 72 (br s). 19F NMR (C6D5-
Cl): δ -129.8 (m, 4F, o-C6F5), -147.7 (m, 2F, p-C6F5), -161.7 (m,
4F, m-C6F5).
N(4)-Ga(2)-N(2)
114.21(9)
N(4)-Ga(2)-C(14) 119.9(1)
N(2)-Ga(2)-C(14) 112.1(1)
Ga(1)-N(2)-Ga(2) 107.08(9) Ga(2)-N(2)-C(11) 113.9(2)
Generation of {tBuC(NiPr)2}Ga(Me)(C6F5) (9b) from 6b and
B(C6F5)3. An NMR tube was charged with {tBuC(NiPr)2}GaMe2 (6b,
50.0 mg, 0.176 mmol) and B(C6F5)3 (90.0 mg, 0.176 mmol), and C6D5-
Cl (0.5 mL) was added by vacuum transfer at -78 °C. The tube was
warmed to 23 °C and vigorously shaken, resulting in a colorless
solution. The tube was maintained at 23 °C, and the reaction was
monitored by 1H NMR. The NMR spectra showed that the reaction of
6b and B(C6F5)3 initially yields [7b][MeB(C6F5)3], which is gradually
converted to {tBuC(NiPr)2}Ga(Me)(C6F5) (9b) and MeB(C6F5)2. Com-
plete conversion to 9b and MeB(C6F5)2 was observed after 7 h at 23
°C.
Ga(1)-N(2)-C(11) 117.3(2)
Ga(1)-N(2)-C(6)
N(4)-C(18)-N(3)
Ga(2)-N(2)-C(6)
N(1)-C(6)-N(2)
112.5(1)
108.6(2)
87.4(1)
109.2(2)
Table 3. Selected Bond Lengths (Å) and Angles (deg) for
{tBu(NCy)2}Al(C6F5)(Me) (11a)
Al-N(1)
Al-N(2)
N(1)-C(2)
1.905(2)
1.893(2)
1.344(3)
Al-C(1)
Al-C(31)
N(2)-C(2)
1.941(3)
2.011(3)
1.348(3)
N(1)-Al-N(2)
N(2)-Al-C(1)
N(2)-Al-C(31)
Al-N(1)-C(11)
C(11)-N(1)-C(2)
N(1)-C(2)-N(2)
69.84(9)
C(1)-Al-C(31)
N(1)-Al-C(1)
N(1)-Al-C(31)
Al-N(2)-C(21)
C(21)-N(2)-C(2)
116.8(2)
119.8(1)
111.4(1)
133.4(2)
132.3(2)
1
3
Data for 9b. H NMR (C6D5Cl): δ 4.09 (septet, J ) 6.1 Hz, 2H,
CHMe2), 1.29 (s, 9H, CMe3), 1.05 (d, 3J ) 6.5, 6H, CHMe2), 0.95 (d,
3J ) 6.1, 6H, CHMe2), 0.32 (s, 3H, GaMe). 13C NMR (C6D5Cl): δ
176.9 (s, CCMe3), 149.7 (d, 1JCF ) 239, C6F5), 139.9 (br s, C6F5), 136.8
120.6(1)
109.7(1)
137.2(2)
131.7(2)
107.8(2)
1
1
(d, JCF ) 255, C6F5), 46.6 (d, JCH ) 134, CHMe2), 39.3 (s, CMe3),
1
1
29.2 (q, JCH ) 127, CMe3), 26.4 (q, JCH ) 119, CHMe2), 25.5 (q,
1JCH ) 125, CHMe2), -4.4 (q, JCH ) 123, GaMe). 19F NMR (C6D5-
1
Table 4. Selected Bond Lengths (Å) and Angles (deg) for
{tBu(NtBu)2}AlMe2 (12a)
Cl): δ -123.4 (m, 2F, o-C6F5), -155.5 (m, 1F, p-C6F5), -162.6 (m,
2F, m-C6F5).
Al-N(1)
Al-N(2)
N(1)-C(3)
1.904(2)
1.908(2)
1.347(2)
Al-C(1)
Al-C(2)
N(2)-C(3)
1.969(2)
1.967(2)
1.348(2)
Generation of {tBuC(NCy)2}Al(Me)(C6F5) (11a) from 10a and
B(C6F5)3. An NMR tube was charged with {tBuC(NCy)2}AlMe2 (10a,
21.0 mg, 0.0125 mmol) and B(C6F5)3 (63.9 mg, 0.0125 mmol), and
CD2Cl2 was added by vacuum transfer. The tube was flame-sealed under
vacuum, warmed to 23 °C, and vigorously shaken. A 1H NMR spectrum
was recorded immediately and showed that quantitative formation of
a 1/1 mixture of {tBuC(NCy)2}Al(Me)(C6F5) (11a) and MeB(C6F5)2
had occurred.
N(1)-Al-N(2)
N(2)-Al-C(1)
N(2)-Al-C(2)
Al-N(1)-C(4)
C(12)-N(2)-C(3)
N(2)-C(3)-N(1)
68.73(7)
C(1)-Al-C(2)
N(1)-Al-C(1)
N(1)-Al-C(2)
Al-N(2)-C(12)
C(4)-N(1)-C(3)
115.9(1)
117.44(9)
114.6(1)
128.1(1)
139.9(2)
116.7(1)
115.13(9)
127.3(1)
139.4(2)
106.0(2)
1
Data for 11a. H NMR (CD2Cl2): δ 3.21 (br, 2H, NCH)), 1.85-
1.65 (br, 8H, Cy), 1.50-1.05 (br, 12H, Cy), 1.30 (s, 9H, CMe3), -0.78
(br s, 3H, AlMe). 19F NMR (CD2Cl2): δ -123.5 (m, 2F, o-C6F5),
-152.5 (m, 1F, p-C6F5), -162.9 (m, 2F, m-C6F5).
-16.5 (br s). 19F NMR (C6D5Cl): δ -132.5 (m, 8F, o-C6F5), -163.2
3
(t, JFF ) 20, 4F, p-C6F5), -167.0 (br, 8F, m-C6F5).
t
{tBuC(NtBu)2}AlMe2 (12a). A colorless solution of BuCdNdCt-
Generation of [{tBuC(NiPr)2}Ga(Me)(NMe2Ph)][B(C6F5)4] ([8b]-
[B(C6F5)4]). [{tBuC(NiPr)2}GaMe2‚{tBuC(NiPr)2}GaMe][B(C6F5)4] ([7b]-
[B(C6F5)4]) was generated in situ in C6D5Cl in a valved NMR tube as
described above, and 1 equiv of NMe2Ph (10.3 µL, 0.0883 mmol) was
Bu (2.00 g, 13.0 mmol) in Et2O (35 mL) was cooled to 0 °C, and
(60) The AlC6F5 and MeB(C6F5)2 resonances are significantly overlapped
and therefore are not listed here.

288 J. Am. Chem. Soc., Vol. 122, No. 2, 2000
Dagorne et al.
Table 5. Summary of Crystal Data for Compounds [4a][B(C6F5)4], [7b][B(C6F5)4], 11a, and 12a
complex
[4a][B(C6F5)4]
[7b][B(C6F5)4]
11a
12a
formula
fw
C55H59Al2BF20N4
1220.83
C49H55BF20Ga2N4
1230.22
C24H34AlF5N2
472.51
C15H33AlN2
268.41
crystal size (mm)
d (calc), Mg/m3
crystal system
space group
a, Å
b, Å
c, Å
0.45 × 0.40 × 0.34
0.46 × 0.43 × 0.42
0.56 × 0.18 × 0.17
0.43 × 0.42 × 0.37
1.448
1.536
1.266
1.006
triclinic
P1h
triclinic
P1h
14.0186(7)
18.8356(9)
21.689(1)
101.326(1)
90.239(1)
108.236(1)
5320.1(4)
monoclinic
P21/n
13.161(2)
15.336(2)
12.441(2)
trigonal
P3221
9.3599(4)
9.3599(4)
35.044(2)
12.2334(8)
12.4767(8)
18.800(1)
92.832(1)
102.235(1)
90.305(1)
2800.6(3)
2
R, deg
â, deg
99.29(2)
γ, deg
V, Å3
2478.1(6)
4
2658.8(2)
6
Z
4
T (K)
173(2)
173(2)
200(2)
173(2)
diffractometer
radiation, λ (Å)
2θ range (deg)
index ranges: h; k; l
no. of reflns
no. of unique reflns
Rint
Bruker CCD-1000
Mo KR, 0.710 73
4.68 < 2θ < 52.74
-15,14; (15; 0,23
19 923
11 240
0.019
Bruker CCD-1000
Mo KR, 0.710 73
3.32 < 2θ < 52.74
(17; (23; 0,27
47 676
21 513
0.025
Enraf-Nonius CAD4
Mo KR, 0.710 10
4.0 < 2θ < 50.0
-15,2; -18,1; (14
5189
4240
0.021
Bruker CCD-1000
Mo KR, 0.710 73
5.02 < 2θ < 56.58
-12,11; -12,9; -46,12
12 044
4083
0.020
µ, mm-1
0.160
1.121
0.133
0.104
transmission coefficients (%)
structure solution
data/restraints/parameters
GOF on F2
R indices (I > 2σ(I))d,e
0.650/0.627
direct methodsa
21 513/0/1403
1.021
R1 ) 0.0388
wR2 ) 0.0927
R1 ) 0.0619
wR2 ) 0.1017
2.164
0.962/0.957
direct methodsa
4083/30/236
1.061
R1 ) 0.0494
wR2 ) 0.1312
R1 ) 0.0571
wR2 ) 0.1357
0.337
direct methodsa
11 240/0/744
1.011
R1 ) 0.0360
wR2 ) 0.0796
R1 ) 0.0635
wR2 ) 0.0868
0.268
direct methodsb,c
4240/0/425
1.109
R1 ) 0.0408
wR2 ) 0.0829
R1 ) 0.0926
wR2 ) 0.1063
0.221
R indices (all data)d,e
max resid density (e/Å3)
a SHELXTL-Version 5.1, Bruker Analytical X-ray Systems, Madison, WI. b SHELXTL-Plus Version 5, Siemens Industrial Automation, Inc.,
Madison, WI. c MULTAN, Multan80., University of York, York, England. d R1 ) ∑||Fo| - |Fc||/∑|Fo| e wR2 ) [∑[w(Fo - Fc2)2]/∑[w(Fo )2]]1/2
,
2
2
where w ) q/σ2(Fo ) + (aP)2 + bP.
2
tBuLi (7.62 mL of 1.7 M solution in pentane, 13.0 mmol) was added
dropwise by syringe. The mixture was allowed to warm to room
temperature and was stirred for 1 h, resulting in a white slurry. The
mixture was cooled to 0 °C, and a colorless solution of AlClMe2 (1.20
g, 13.0 mmol) in Et2O (10 mL), which was also cooled to 0 °C, was
added dropwise. The resulting mixture was allowed to warm to room
temperature and was stirred for 12 h, affording a slurry of a white solid
in a pale yellow solution. The mixture was filtered. The volatiles were
removed from the filtrate under vacuum to afford a colorless solid as
a crude product. Sublimation of the crude product (70 °C, <0.001
mmHg) to a -78 °C coldfinger afforded {tBuC(NtBu)2}AlMe2 (1.3 g,
An NMR tube was charged with {tBuC(NtBu)2}GaMe2 (12b, 20 mg,
0.064 mmol) and [Ph3C][B(C6F5)4] (58 mg, 0.064 mmol), and C6D5Cl
(0.5 mL) was added by vacuum transfer at -78 °C. The tube was flame-
sealed under vacuum, warmed to 23 °C, and vigorously shaken. A dark
brown solution formed. The tube was maintained at 23 °C for 30 min.
A
1H NMR spectrum was recorded and showed that quantitative
conversion to a 1/1 mixture of [13b][B(C6F5)4] and Ph3CCH3 had
1
occurred. H NMR (C6D5Cl): δ 1.15 (s, 9H, CCMe3), 1.11 (s, 18H,
NCMe3), 0.35 (s, 3H, GaMe). 13C NMR (C6D5Cl): δ 191.4 (CCMe3),
149.0 (d, 1JCF ) 241, C6F5), 138.8 (d, 1JCF ) 245, C6F5), 136.9 (d, 1JCF
) 243, C6F5), 56.4 (NCMe3), 38.1 (CCMe3), 33.6 (NCMe3), 30.4
(CCMe3), 0.2 (GaMe). 11B NMR (C6D5Cl): δ -16.5 (br s). 19F NMR
1
37%) as colorless block crystals. H NMR (C6D6): δ 1.34 (s, 18H,
NCMe3), 1.20 (s, 9H, CCMe3), -0.21 (s, 6H, AlMe2). 13C NMR
(C6D6): δ 181.9 (NCN), 53.4 (NCMe3), 37.5 (CCMe3), 34.0 (NCMe3),
31.6 (CCMe3), -7.1 (AlMe2). EI-MS (m/z): 57 (AlMe2+, 46). Anal.
Calcd for C15H33AlN2: C, 67.10; H, 12.41; N, 10.44. Found: C, 67.43;
H, 12.54; N, 10.70.
(C6D5Cl): δ -132.5 (m, 8F, C6F5), -163.2 (t, JFF ) 20, 4F, C6F5),
3
-167.0 (br, 8F, C6F5).
Generation of [{tBuC(NtBu)2}Ga(Me)(NMe2Ph)][B(C6F5)4] ([14b]-
[B(C6F5)4]). A solution of [{tBuC(NtBu)2}GaMe][B(C6F5)4] ([13b]-
[B(C6F5)4]) in C6D5Cl (0.5 mL) was generated in an NMR tube as
described above. The tube was maintained at 23 °C for 30 min, and 1
equiv of NMe2Ph (4.1 µL, 0.064 mmol) was added at 23 °C by syringe.
The tube was maintained at 23 °C for 10 min. NMR spectra were
recorded and showed that quantitative conversion to [14b][B(C6F5)4]
had occurred. 1H NMR (C6D5Cl): δ 7.18 (t, 3J ) 7.2, 4H, m-Ph), 6.90
(t, 3J ) 7.6, 2H, p-Ph), 6.77 (d, 3J ) 8.6, 4H, o-Ph), 2.66 (s, 3H, NMe),
1.32 (s, 18H, NCMe3), 1.16 (s, 9H, CCMe3), 0.12 (s, 3H, GaMe). 13C
NMR (C6D5Cl): δ 190.5 (CCMe3), 149.0 (d, 1JCF ) 241, C6F5), 145.5
Generation of [{tBuC(NtBu)2}AlMe][B(C6F5)4] ([13a][B(C6F5)4]).
An NMR tube was charged with {tBuC(NtBu)2}AlMe2 (12a, 17 mg,
0.064 mmol) and [Ph3C][B(C6F5)4] (58 mg, 0.064 mmol), and C6D5Cl
(0.5 mL) was added by vacuum transfer at -78 °C. The tube was flame-
sealed under vacuum, warmed to 23 °C, and vigorously shaken. A dark
brown solution formed. The tube was maintained at 23 °C for 5 min.
A
1H NMR spectrum was recorded and showed that quantitative
conversion to a 1/1 mixture of [13a][B(C6F5)4] and Ph3CCH3 had
1
1
1
occurred. H NMR (C6D5Cl): δ 1.22 (s, 9H, CCMe3), 1.20 (s, 18H,
(ipso-Ph), 138.8 (d, JCF ) 245, C6F5), 136.9 (d, JCF ) 243, C6F5),
129.4 (p- or m-Ph), 128.4 (p- or m-Ph), 119.9 (o-Ph), 55.9 (NCMe3),
46.4 (NMe), 36.5 (CCMe3), 34.0 (NCMe3), 31.1 (CCMe3), -3.4
(GaMe). 11B NMR (C6D5Cl): δ -16.5 (br s). 19F NMR (C6D5Cl): δ
NCMe3), -0.04 (s, 3H, AlMe). 13C NMR (C6D5Cl): δ 189.5 (CCMe3),
149.0 (d, 1JCF ) 241, C6F5), 138.8 (d, 1JCF ) 245, C6F5), 136.9 (d, 1JCF
) 243, C6F5), 54.5 (NCMe3), 36.1 (CCMe3), 34.3 (NCMe3), 32.0
(CCMe3), -2.9 (AlMe). 11B NMR (C6D5Cl): δ -16.5 (br s). 19F NMR
3
-132.5 (m, 8F, o-C6F5), -163.2 (t, JFF ) 20, 4F, o-C6F5), -167.0
3
(C6D5Cl): δ -132.5 (m, 8F, C6F5), -163.2 (t, JFF ) 20, 4F, C6F5),
(br s, 8F, m-C6F5).
-167.0 (br, 8F, C6F5).
NMR Simulations. NMR spectra were simulated using the software
package gNMR (version 3.6 for Macintosh, Cherwell Scientific
Generation of [{tBuC(NtBu)2}GaMe][B(C6F5)4] ([13b][B(C6F5)4]).

Cationic Al and Ga Amidinate Complexes
J. Am. Chem. Soc., Vol. 122, No. 2, 2000 289
Publishing Limited), in which the evaluation of static spectra is based
on general NMR theory61 and the evaluation of dynamic spectra
involving chemical exchange is based on the standard Louiville
[{tBuC(NiPr)2}GaMe‚{tBuC(NiPr2}GaMe2][B(C6F5)4] ([7b][B(C6-
F5)4]): crystals were grown from a 10/10/1 hexane/pentane/C6D5Cl
mixed solvent system. The salt was dissolved in C6D5Cl (0.1 mL), and
a 1/1 mixture of hexane/pentane (2 mL) was slowly added to form a
top layer. The two layers were allowed to diffuse at 23 °C for 2 d to
afford large crystals which formed at the interface of the two layers.
All non-H atoms were refined anisotropically, and all H-atoms were
placed in ideal positions and refined as riding atoms with relative
isotropic displacement coefficients. There are two independent mol-
ecules of [7b][B(C6F5)4] in the asymmetric unit. The two 7b+ cations
differ in the rotational conformation around the Ga-N(µ-amidinate) bond
but exhibit very similar bond lengths and angles, while the two
representation of quantum mechanics, as described by Binsch.62,63
Lorentzian line shape was assumed for all the calculated spectra.
A
X-ray Crystallography. The structure of [2a][B(C6F5)4] was
determined by V. G. Young, Jr. (University of Minnesota). The structure
of 11a was determined by D. C. Swenson (University of Iowa), and
the structures of [4a][B(C6F5)4], [7b][B(C6F5)4], and 12a were deter-
mined by I. A. Guzei (Iowa State University). Crystal data, data
collection details, metrical parameters, and solution refinement proce-
dures are collected in Tables 1-5, except those for [2a][B(C6F5)4],
which are listed in the Supporting Information.
-
B(C6F5)4 anions are identical.
[{MeC(NiPr)2}2Al2Me3][B(C6F5)4] ([2a][B(C6F5)4]): crystals were
grown by crystallization from a 10/1 mixture of hexane and toluene (5
mL). Pure [2a][B(C6F5)4] (100 mg) was dissolved in toluene (0.5 mL)
in a sample tube, and hexane (5 mL) was added very slowly to form
a top layer. The two layers were allowed to diffuse at 23 °C for 5 d to
afford small crystals of poor quality which formed at the interface of
the two layers. Crystallographic details are given in the Supporting
Information. The dearth of observed data precluded a fully anisotropic
refinement. Only the Al atoms were refined anisotropically. The cell
constants and metrical parameters should be considered approximate
because of the presence of a satellite crystal; however, the atom
connectivity is established to be analogous to that of [2a][B(C6F5)4].
[{MeC(NCy)2}2Al2Me3][B(C6F5)4] ([4a][B(C6F5)4]): crystals were
grown by crystallization from a mixed solvent system of 10/10/1/1
hexane/pentane/C6D5Cl/ClCD2CD2Cl. Pure [4a][B(C6F5)4] (100 mg)
was dissolved in a 1/1 mixture of C6D5Cl and ClCD2CD2Cl (0.2 mL),
and a 1/1 mixture of hexane and pentane (2 mL) was added very slowly
to form a top layer. The two layers were allowed to diffuse at 23 °C
for 3 d to afford large crystals which formed at the interface of the
two layers. All non-H atoms were refined anisotropically, and all
H-atoms were placed in ideal positions and refined as riding atoms
with relative isotropic displacement coefficients.
{tBuC(NCy)2}Al(Me)(C6F5) (11a): crystals were grown from a
saturated pentane solution at -78 °C. All non-H atoms were refined
anisotropically, and all H-atoms were refined isotropically.
{tBuC(NtBu)2}AlMe2 (12a): crystals were grown by slow sublima-
tion of crude 12a (70 °C, <0.001 mmHg, 1 d). Pure 12a was collected
as colorless crystals on a -78 °C cold probe. All non-H atoms were
refined anisotropically, and all H-atoms were placed in ideal positions
and refined as riding atoms with relative isotropic displacement
coefficients. There is positional disorder in the structure: atoms C(5),
C(6), and C(7) are equally disordered over two positions each in a
65/35 ratio. The distances between the tertiary carbon atoms and the
disordered methyl carbons were restrained to be identical within
0.002 Å.
Acknowledgment. This work was supported by the Depart-
ment of Energy (DE-FG02-88ER13935) and the Eastman
Chemical Co. We thank V. G. Young, Jr. (University of
Minnesota), and D. C. Swenson (University of Iowa) for the
X-ray structure determinations of [2a][B(C6F5)4] and 11a,
respectively.
Supporting Information Available: Tables of atomic
coordinates, isotropic displacement parameters, anisotropic
displacement parameters, and bond distances and bond angles
for [2a][B(C6F5)4], [4a][B(C6F5)4], [7b][B(C6F5)4], 11a, and 12a,
and details concerning crystal data, data collection, and solution
and refinement for [2a][B(C6F5)4] (PDF). This material is
available free of charge via the Internet at http://pubs.acs.org.
(61) Pople, J. A.; Schneider, W. G.; Bernstein, H. J. High-Resolution
Nuclear Magnetic Resonance; McGraw-Hill: New York, NY, 1959.
(62) (a) Binsch, G. In Dynamic Nuclear Magnetic Resonance Spectros-
copy; Jackman, L. M., and Cotton, F. A., Eds.; Academic Press: London,
1975; p 45. (b) Steigel, A. In NMR, Basic Principles and Progress; Diehl,
P., Fluck, E., Kosfeld, R., Eds.; Springer-Verlag: Berlin, 1978; Vol. 15, p
1.
(63) (a) Binsch, G. J. Am. Chem. Soc. 1969, 91, 1304. (b) Stephenson,
D. S.; Binsch, G. J. Magn. Reson. 1978, 30, 625.
JA992104J