Participation of the α,α′-diiminopyridine ligand system in reduction of the metal center during alkylation

Article abstract of DOI:10.1021/ja020485m

The reaction of {[2,6-(i-Pr)₂PhN=C(Me)]₂ (C₅H₃N)}MnCl₂ with alkylating agents formed a dinuclear Mn(I) derivative via ligand reductive coupling. In the case of the trivalent Cr analogue, a similar reaction afforded reduction toward Cr(II) but also alkylation at the pyridine ring para position followed by an unprecedented cycloaddition that generated a tricyclic system.

Full text of DOI:10.1021/ja020485m

Published on Web 09/13/2002
Participation of the r,r′-Diiminopyridine Ligand System in
Reduction of the Metal Center during Alkylation
Hiroyasu Sugiyama,† Ghazar Aharonian,† Sandro Gambarotta,*,†
Glenn P. A. Yap,† and Peter H. M. Budzelaar*,‡
Contribution from the Department of Chemistry, UniVersity of Ottawa,
Ottawa, Ontario K1N 6N5, Canada, and the Department of Inorganic Chemistry,
UniVersity of Nijmegen, ToernooiVeld 1, 6525 ED Nijmegen, The Netherlands
Received April 3, 2002
Abstract: The reaction of {[2,6-(i-Pr)2PhNdC(Me)]2(C5H3N)}MnCl2 with alkylating agents formed a dinuclear
Mn(I) derivative via ligand reductive coupling. In the case of the trivalent Cr analogue, a similar reaction
afforded reduction toward Cr(II) but also alkylation at the pyridine ring para position followed by an
unprecedented cycloaddition that generated a tricyclic system.
divalent precursor,5 and thus the ability of this ligand to afford
reduction of the metal center might be central to catalytic
Introduction
The discovery made by Gibson and co-workers,1 Brookhart
and co-workers,2 and Bennett3 a few years ago that an R,R′-
diiminopyridine {[2,6-(i-Pr)2PhNdC(Me)]2(C5H3N)} ligand may
provide late metals with a very high catalytic activity for olefin
polymerization has marked a milestone in the neverending saga
of Ziegler-Natta catalysis. Subsequent studies on the behavior
of this ligand system have pointed out an unanticipated
involvement of the R,R′-diiminopyridine ligand into the reactiv-
ity of its metal complexes.4 The anionization of the ligand
resulting from the direct alkylation of the pyridine ring and the
consequent diminishment of the metal coordination number
might be the key to explain the success of this ligand to sustain
such a high level of catalytic activity. The ligand also promotes
reduction of the metal center, providing examples of rare low-
valent metal alkyl derivatives4a,5,6 in principle expected to be
inactive. However, the monovalent Co alkyl derivative recently
reported displays catalytic activity comparable to that of the
performance. Furthermore, experimental7 and theoretical8 studies
have clearly demonstrated that this particular ligand system
cannot be considered as an ancillary one since resonance
structures with a considerable extent of metal-to-ligand charge
transfer may play a rather significant role in the stabilization of
the low-valent derivatives. Moreover, an interesting parallel can
be drawn with the 1,3-diiminoarene ligand [2,6-(i-Pr)2PhNd
C(Me)]2(C6H4) that, similar to the pyridine congener, can also
be alkylated but even deprotonated at one or two of the methyl
groups directly attached to the imino functions.9 Although this
behavior has never been reported in the case of the R,R′-
diiminopyridine ligand, these observations offer additional
possibilities for explaining the unique reactivity patterns pro-
moted by this remarkable ligand system and its ability to sustain
a high level of catalytic activity.
In this paper we report the isolation and characterization of
a monovalent manganese compound whose structure sheds some
light on the intriguing behavior of the complexes of this
remarkable ligand system. In addition, we have investigated a
similar Cr(III) derivative, finding not only reduction of the metal
center but also an unprecedented type of ligand reorganization
and coupling.
* Corresponding authors: e-mail sgambaro@science.uottawa.ca and
budz@sci.kun.nl.
† University of Ottawa.
‡ University of Nijmegen.
(1) (a) Britovsek, G. J. P.; Bruce, M.; Gibson, V. C.; Kimberley, B. S.; Maddox,
P. J.; Mastroianni, S.; McTavish, S. J.; Redshaw, C.; Solan, G. A.;
Stro¨mberg, S.; White, A. J. P.; Williams, D. J. J. Am. Chem. Soc. 1999,
121, 8728. (b) Britovsek, G. J. P.; Dorer, B. A.; Gibson, V. C.; Kimberley,
B. S.; Solan, G. A. (BP Chemicals Limited), WO 99/12981, 1999 [Chem
Abstr. 1999, 130, 252793]. (c) Britovsek, G. J. P.; Mastroianni, S.; Solan,
G. A.; Baugh, S. P. D.; Redshaw, C.; Gibson, V. C.; White, A. J. P.;
Williams, D. J.; Elsegood, M. R. J. Chem. Eur. J. 2000, 6, 2221. (d) Bruce,
M.; Gibson, V. C.; Redshaw, C.; Solan, G. A.; White, J. P. A.; Williams,
D. J. Chem. Commun. 1998, 2523. (e) Britovsek, G. J. P.; Gibson, V. C.;
Kimberley, B. S.; Maddox, P. J.; McTavish, S. J.; Solan, G. A.; White, A.
J. P.; Williams, D. Chem. Commun. 1998, 849.
Results and Discussion
As previously described, alkylations of {[2,6-(i-Pr)2PhNd
C(Me)]2(C5H3N)}MnCl2 afforded Mn-alkyl derivatives formu-
(5) (a) Kooistra, M.; Knijnenburg, Q.; Smits, J. M. M.; Horton, A. D.;
Budzelaar, P. H. M.; Gal, A. W. Angew. Chem. 2001, 40, 4719. (b) Gibson,
V. C.; Humphries, M.; Tellmann, K.; Wass, D.; White, A. J. P.; Williams,
D. J. Chem. Commun. 2001, 2252.
(2) (a) Small, B. L.; Brookhart, M. Polymer Prepr. (Am. Chem. Soc., DiV.
Polym. Chem.) 1998, 39, 213. (b) Small, B. L.; Brookhart, M. J. Am. Chem.
Soc. 1998, 120, 7143. (c) Small, B. L.; Brookhart, M.; Bennett, A. M. A.
J. Am. Chem. Soc. 1998, 120, 4049. (d) Dias, E. L.; Brookhart, M.; White,
P. S. Chem. Commun. 2001, 423.
(3) (a) Bennett, A. M. A. (DuPont) WO 98/27124, 1998 [Chem Abstr. 1998,
129, 122973x]. (b) Bennett, A. M. A. CHEMTECH 1999 (July), 24-28.
(4) (a) Reardon, D.; Conan, F.; Gambarotta, S.; Yap, G. P. A.; Wang, Q. J.
Am. Chem. Soc., 1999, 121, 9318. (b) Bruce, M.; Gibson, V. C.; Redshaw,
C.; Solan, G. A.; White, A. J. P.; Willams, D. J. J. Chem. Soc., Chem.
Commun. 1998, 2523.
(6) Reardon, D.; Aharonian, G.; Gambarotta, S.; Yap, G. P. A. Organometallics
2002, 21, 786.
(7) De Bruin, B.; Bill, E.; Bothe, E.; Weyhermuller, T.; Wieghardt, K. Inorg.
Chem. 2000, 39, 2936.
(8) Budzelaar, P. M. H.; De Bruin, B.; Gal, A. W.; Wieghardt, K.; Van Lenthe,
J. H. Inorg. Chem. 2001, 40, 4649.
(9) (a) Nuckel, S.; Burger, P. Organometallics 2000, 19, 3305. (b) Nuckel, S.;
Burger, P. Organometallics 2001, 20, 4345.
9
12268
J. AM. CHEM. SOC. 2002, 124, 12268-12274
10.1021/ja020485m CCC: $22.00 © 2002 American Chemical Society

R,R′-Diiminopyridine Ligand System in Alkylation
A R T I C L E S
N(2)-C(19) ) 1.367(9) Å, C(19)-C(20) ) 1.450(11) Å] are
as expected and do not suggest the presence of any particular
feature. Within each monomeric moiety the coordination
geometry around the Mn atom is severely flattened tetrahedral
[N(1)-Mn-N(2) ) 73.1(3)°, N(1)-Mn-N(3) ) 140.3(3)°,
N(1)-Mn-C(35) ) 108.1(3)°, N(2)-Mn-N(3) ) 74.6(2)°,
N(2)-Mn-C(35) ) 151.0(3)°] and is defined by the three
nitrogen donor atoms of the ligand [Mn-N(1) ) 2.301(6) Å,
Mn-N(2) ) 2.073(6) Å, Mn-N(3) ) 2.226(6) Å] and the C
atom of the alkyl group [Mn-C(35) ) 2.097(9) Å].
There are two main features in complex 2. First, the
manganese was reduced to the monovalent state. Second, the
formation of the C-C bond via coupling of the two former
methyl groups implies a formal elimination of two hydrogen
atoms. This complex behavior is easily explained if we assume
that the reaction may proceed through two distinct reaction
pathways (Scheme 1). As a first possibility, the reaction
produces a monovalent {[2,6-(i-Pr)2PhNdC(Me)]2(C5H3N)}-
MnR where the ligand did not undergo any transformation. This
species was previously isolated and characterized for R ) Me.6
This “straight” reduction of the metal center is not terribly
surprising given that alkyllithium reagents are known to be able
to work as reducing agents. In a second pathway, anionization
of the ligand may occur on a divalent manganese adduct by
removing one proton from one of the two methyl groups and
forming a C-C double bond at the expenses of one of the two
imino functions. This process, similar to that occurring in the
[2,6-(i-Pr)2PhNdC(Me)]2(C6H4) ligand,9 does not modify the
metal oxidation state. However, subsequent reductive coupling
of the C-C double bonds from two identical units forms the
dimeric 2, restores the imino functions, and provides the two
electrons necessary for the reduction of the two divalent
manganese atoms to the final monovalent state. The lability of
the hydrogen atoms of the methyl groups attached to the imine
function in diimine pyridinato complexes of Rh and Ir was
previously demonstrated on the basis of the facile H,D exchange
observed in the presence of NaOMe/methanol-d1.9b
Figure 1. Thermal ellipsoid plot of 2, drawn at 30% probability.
lated as {[2,6-(i-Pr)2PhNdC(Me)]2(C5H3N)}MnMe and {[2,6-
(i-Pr)2PhNdC(Me)]2(C5H3N)}Mn(CH2SiMe3)][Li(OEt2)4] (1),
respectively, containing the metal in the formal mono- and
zerovalent states.6 There was no indication in these particular
cases that the diiminopyridine ligand was directly involved in
assisting the reduction of the metal center as it was instead
observed in the case of the vanadium analogue.4 Simply in the
case of the formally zerovalent 1, the C-C distances formed
by the two peripheral methyl groups attached to the imino
functions were unusually short, although this was not sufficient
to conclusively decide about the presence of an authentic C-C
double bond. During its isolation we have observed the
formation of a substantial amount of dark-colored material.
After several recrystallizations from toluene/hexane mixtures
we have now obtained an analytically pure crystalline solid
formulated as {[2,6-(i-Pr)2PhNdC(Me)](C5H3N)[2,6-(i-Pr)2-
PhNdC(CH2)]}2[Mn(CH2SiMe3)]2 (2).
The solid-state structure revealed the connectivity and the
metal oxidation state. The complex contains two monovalent
Mn-CH2SiMe3 units each surrounded by the diiminopyridine
ligand (Figure 1). The two identical moieties are connected by
a C-C single bond [C(21)-C(21a) ) 1.562(15) Å] between
two former methyl groups, each attached to one of the two imino
functions [C(21)-C(20) ) 1.521(10) Å, C(20)-N(3) ) 1.344-
(9) Å; C(14)-C(13) ) 1.491(11) Å, C(14)-N(1) ) 1.317(9)
Å] of each Mn-ligand unit. All the other bond distances
[C(14)-C(15) ) 1.474(11) Å, N(2)-C(15) ) 1.362(9) Å,
This behavior prompted us to reconsider the other complex
previously isolated from the same reaction mixture and initially
Scheme 1
9
J. AM. CHEM. SOC. VOL. 124, NO. 41, 2002 12269

A R T I C L E S
Sugiyama et al.
Figure 2. Observed and calculated bond lengths, in angstroms, for diiminopyridine complexes. Observed values and averaged observed values are shown
in italic type.
formulated as the zeroValent [{[2,6-(i-Pr)2PhNdC(Me)]2
(C5H3N)}MnR][Li(ether)4] (1).6 It is tempting to suggest that
the same divalent and monodeprotonated precursor of the
dimeric 2 (Scheme 1) may alternatively undergo further depro-
tonation. As a result, a divalent -ate 2,6-(i-Pr)2PhNCdCH2)]2-
(C5H3N)}MnR][Li(ether)4] complex with manganese surrounded
by a dianionic ligand might be formed. In view of the shortness
of the “C-CH3” bonds mentioned earlier, this seems more likely
to be the correct formulation for 1. However, the possibility
that the complex might also be another monoValent species with
only one deprotonated methyl group and the double bond equally
disordered over the two positions cannot be ruled out on the
basis of the analytical and crystallographic data alone.
Electron transfer from metal to ligand seems to be a recurring
feature in diiminopyridine complexes. The calculations show
that L′MnMe, although formally Mn(I), is best described as
having a ligand radical anion antiferromagnetically coupled to
a high-spin Mn(II) center. The deprotonated-ligand complexes
[L′ H]MnCl and [L′ H]MnMe show significant spin density
at the imine C and N atoms and the terminal methylene group,
apparently due to electron transfer from Mn(II) to the ligand,
induced by the strong ligand field of the amide functionality.
Since of these atoms only the methylene carbons are sterically
accessible, this might explain the observed easy dimerization.
In contrast, the complex of the doubly deprotonated ligand does
not show appreciable spin density at the ligand, presumably
because of the weak π-acceptor character of this dianionic
ligand.
To clarify this point, we carried out DFT optimizations of
the structures of L′MnCl2, L′MnCl, L′MnMe, [L′ H]MnCl,
[L′
(HNdC(Me))2C5H3N model ligand and [L′
H]MnMe, and [L′ - 2H]MnMe-, where L′ is the 2,6-
The involvement of the ligand in assisting the reduction of
manganese was substantially different from that observed in the
case of vanadium. Thus, investigating the behavior of the
chromium analogues was the next obvious step. The reaction
of CrCl3(THF)3 with [2,6-(i-Pr)2PhNdC(Me)]2(C5H3N) in tolu-
ene was a straightforward complexation affording the corre-
sponding {2,6-bis[2,6-(i-Pr)2PhNdC(Me)]2(C5H3N)}CrCl3‚(CH2-
Cl2)1.33 (3) that was isolated in crystalline form from methylene
chloride. The complex is isostructural with the vanadium species
reported before,4 with comparable bond distances and angles
(Figure 3). Different from vanadium, however, the catalytic
H] and [L′
2H] are its singly and doubly deprotonated forms. Figure 2
shows calculated bond lengths for the most relevant species and
compares them with those observed in the X-ray structures
reported previously for complexes of the “real” ligand [2,6-(i-
Pr)2PhNdC(Me)]2(C5H3N) (L).6 The agreement is good except
for the M-Nimine bond lengths, which are consistently too short
in the calculations by ca. 0.05-0.1 Å. This is to be expected,
since a CdNH group is a much better donor than a CdNAr
group for both steric and electronic reasons. Concerning the
important (former) C-Me bonds, the X-ray bond lengths for 1
agree well with those predicted for [L′ - 2H]MnMe- but not
for an average of the bond lengths predicted for a singly
activity was surprisingly low (2.6 kg of PE mol-1 h-1 atm-1
)
under the usual reaction conditions employed for this family of
catalysts (MAO:Cr ) 70-1000, 1 atm of ethylene, room
temperature) yet producing polyethylene of comparable quality
and with the usual bimodal distribution [Mw ) 57 000; PD )
33]. Given the close structural analogy with the vanadium
derivative, the low activity of 3 can be ascribed to either the
deprotonated ligand (e.g., 1.429 Å for [L′
H]MnCl). Also,
the similarity to the deprotonated arene analogue reported by
Burger9 (included in Figure 2) is remarkable. Thus, we believe
that the formulation of 1 as an -ate complex of Mn(II) is the
correct one.
9
12270 J. AM. CHEM. SOC. VOL. 124, NO. 41, 2002

R,R′-Diiminopyridine Ligand System in Alkylation
A R T I C L E S
Scheme 3
Figure 3. Thermal ellipsoid plot of 3, drawn at 30% probability.
1.523(6) Å, C(16)-C(17)-C(34) ) 114.1(3)°] and (b) cycload-
dition to an identical ring of a second unit forming two C-C
single bonds [C(18)-C(16a) ) 1.563(5) Å] and linking the ring
meta carbon atoms. The resulting 6,6,6-tricyclo type of frame
bears one negative charge in each of the two residual π-systems
present between the nitrogen atoms and the adjacent imine
functions in each of the two former pyridine rings. The long
C-C distances [C(13)-C(14) ) 1.517(5) Å, C(20)-C(21) )
1.500(5) Å] formed by the methyl groups with the carbon atoms
engaged in the formation of the two imino functions clearly
indicated the presence of single bonds, thus ruling out the
possibility of deprotonation as observed in the case of manga-
nese. The C-N distances are rather short [C(13)-N(1) ) 1.313-
(4) Å, C(20)-N(3) ) 1.322(5) Å] and in good agreement with
the presence of a substantial C-N double-bond character. The
coordination geometry around the chromium center is nearly
square-planar [N(1)-Cr-N(2) ) 76.75(10)°, N(1)-Cr-N(3)
) 153.09(11)°, N(1)-Cr-C(41) ) 103.06(13)°, N(2)-Cr-N(3)
) 76.80(1)°, N(2)-Cr-C(41) ) 169.23(16)°, N(3)-Cr-C(41)
) 103.84(13)°] as expected for a divalent chromium metal and
is defined by the three ligand nitrogen atoms and the carbon
atom of the alkyl group [Cr-N(1) ) 2.114(3) Å, Cr-N(2) )
2.051(3) Å, Cr-N(3) ) 2.109(3) Å, Cr-C(41) ) 2.153(4) Å].
The formation of 4 is a complex reaction that can be
rationalized in three main steps (Scheme 3). The first step is
probably reduction of the metal center by 1 equiv of alkylating
agent. Easy reduction is in line with the results obtained with
Mn6 and Co5 and is also observed for Cr in combination with
Me3Al (vide infra). Reaction with a further 2 equiv of BzMgCl
then results in alkylation at the metal atom and at the pyridine
para position, the latter being related to the ortho coupling
previously observed with vanadium.4a Whether ring alkylation
involves direct attack of BzMgCl at the pyridine ring or
alkylation at Cr followed by migration to the ring cannot be
established at this point. In this respect, it should be mentioned
that the ability of uncoordinated pyridine ring to undergo
nucleophilic attack is well established.11 Finally, the square-
planar Cr(II) product dimerizes to the observed tricyclic
structure. In support of this interpretation, calculations on model
system [L′ + H]CrMe (where [L′ + H] has a hydrogen atom
added to Cpara) predict that the highest spin density on the ligand
is at the pyridine meta carbons (Figure S-1 in Supporting
Information). Also, the X-ray structure reported for [2,6-
Scheme 2
Figure 4. Thermal ellipsoid plot of 4, drawn at 30% probability.
metal d3 electronic configurations,10 possibly determining
instability of the M-alkyl functions, or to metal reduction with
or without ligand assistance. To probe this point, we have
studied the behavior of 3 with both strong and weak alkylating
agents.
The reaction of 3 with 2 equiv of PhCH2MgCl triggered a
very unusual transformation through which the new complex
{[2,6-(i-Pr)2PhNC(Me)]2(4-PhCH2C5H3N)Cr(CH2Ph)}2-
(toluene) (4) was formed (Scheme 2). The magnetic moment
clearly indicated that the two chromium atoms are in the divalent
state with the two metals magnetically uncoupled. Similar
reaction carried out with MeMgI only gave intractable materials.
Complex 4 is dinuclear (Figure 4) and consists of two CrCH2-
Ph moieties [Cr-C(41) ) 2.153(4) Å] chelated by a large
hexadentate dianion arising from two separate processes: (a)
alkylation at the pyridine ring para position [C(17)-C(34) )
(11) See for example (a) Gilman, H.; Gregory, W. A.; Spatz, S. M. J. Org.
Chem. 1951, 16, 1788. (b) Nunn, A. J.; Schofiel, K. J. Chem. Soc. 1952,
(10) Margl, P.; Deng, L.; Ziegler, T. Organometallics 1999, 18, 5701.
589.
9
J. AM. CHEM. SOC. VOL. 124, NO. 41, 2002 12271

A R T I C L E S
Sugiyama et al.
(2) Å]. The ligand system is apparently intact. No special
deformation is observed in the pyridine ring. The long C-C
distances [C(13)-C(14) ) 1.510(3) Å] formed by the methyl
group with the imino carbon atoms [C(13)-N(1) ) 1.295(3)
Å] clearly indicated that no deprotonation occurred in this case.
The coordination geometry around the chromium center is
square pyramidal and very similar to those of the Co and Fe
supercatalysts [N(1)-Cr-N(2) ) 76.82(4)°, N(1)-Cr-N(1a)
) 151.63(8)°, N(1)-Cr-Cl(1) ) 100.45(4)°, N(2)-Cr-Cl(1)
) 59.23(6)°] with the metal center being coplanar with the three
N-donor atoms and one of the two chlorine atoms. The second
chlorine lies on the axis nearly perpendicular to the molecular
plane [Cl(2)-Cr-N(1) ) 96.79(4)°, Cl(2)-Cr-N(2) ) 97.40-
(6)°, Cl(2)-Cr-Cl(1) ) 103.37(3)°].
Given the fact that the ligand in 5 is apparently intact and
neutral, the connectivity clearly assigned a formal divalent
oxidation state to the chromium atom. This is also in good
agreement with the magnetic moment that was as expected for
the d4 high-spin electronic configuration. Thus, the formation
of the divalent 5 clearly indicates that even a mild alkylating
agent may promote the reduction of the metal center. This is
especially striking in the case of trivalent Cr when we consider
the unusually high stability and chemical inertness of the Cl2-
CrMe moiety12 capable of resisting the attack of HCl and whose
stability should increase even further while in combination with
the diiminato ligand. Thus, ligand involvement in the reduction
remains a distinct possibility. Given also that 5, while in
combination with a variety of cocatalyts, is totally inactive
toward olefin polymerization, the easy reduction of the metal
center explains the lack of activity of 3.
Figure 5. Thermal ellipsoid plot of 5, drawn at 30% probability.
(i-Pr)2PhNdC(Me)]2(C5H3N)CoCH2SiMe3 shows a close con-
tact (3.5 Å) between the pyridine rings of two neighboring
molecules,5a indicating that a close approach of these rings is
sterically feasible. Driving force for the dimerization would then
be the conversion of one net C-C π-bond per monomer to a
σ-bond, coupled with a delocalization of the ligand negative
charge to the imine nitrogens.
From what is presented above, it appears that reduction is a
rather general trend in this chemistry. It may be that reduction
initially takes place at the ligand, forming a ligand radical anion.
This may result in a stable situation (as in {[2,6-(i-Pr)2PhNd
C(Me)]2(C5H3N)}MnMe and complex 2), whereas in others the
radical anion can in turn reduce the metal (as has presumably
happened in the formation of 4). On the other hand, the nature
of the transition metal seems to determine the type of further
transformations undergone by the ligand. Thus, this intriguing
behavior does not necessarily have implications for the catalytic
performances of these species and questions remain open about
the role of the Al cocatalyst (a mild alkylating agent). Supporting
a very high catalytic activity is a primary characteristic of the
diiminopyridine ligand. However, the effect of the reduction
of the metal center on the catalytic activity of this family of
complexes is unclear and controversial in the case of the Co
supercatalyst.1e,5 In the case of the highly active vanadium,4
the MAO cocatalyst alkylated the pyridine ortho position and
anionized the ligand, but no reduction toward the inactive
divalent state apparently occurred under these conditions. Thus,
we were puzzled by the low catalytic activity displayed by 3.
In other words, the low activity may be the result of (1) metal
reduction, (2) the d3 electronic configuration, or (3) some ligand
transformation. To probe these possibilities we have thus reacted
3 with Me3Al and isolated the resulting complex. To keep the
conditions as close as possible to those employed for the
polymerization, the reaction was carried out in toluene with Me3-
Al. The reaction of 3 with Me3Al afforded a dark-brown
compound that was isolated in crystalline form as extremely
air-sensitive paramagnetic and pyrophoric crystals. The formula-
tion of {[2,6-(i-Pr)2PhNdC(Me)]2(C5H3N)}Cr(µ-ClAlMe3)2 (tolu-
ene) (5) was provided by the X-ray crystal structure and was in
good agreement with analytical and magnetism data.
Conclusions
In this paper we have described the isolation of new
complexes of the popular diiminopyridinato ligand system where
the ligand has undergone two different types of unprecedented
transformations. The first, C-C bond formation as observed in
the case of 2, proceeds via deprotonation of a methyl group
forming a C-C double bond. This reductive coupling is
accompanied by a formal reduction of the metal oxidation state.
The second consists of pyridine ring alkylation that, in the
particular case of chromium, also triggered a complex rear-
rangement and dimerization. Even in this case, the metal was
reduced. In both scenarios the ligand became anionic. Reduction
of the metal center as observed in the case of vanadium,
chromium, manganese, and cobalt seems to be a trend in the
organometallic derivatives of this ligand. In turn, the stabilization
of low-valent metal alkyls may be explained by a great ability
of this ligand system to accept and efficiently delocalize
electronic charge. The reduction of the metal center as observed
in the case of the treatment of 3 with Me3Al is particularly
eloquent and reiterates that destabilization of higher-valent metal
alkyls occurs in favor of lower oxidation states. Thus questions
continue to rise about the nature of the catalytically active
species and metal oxidation state, given that in the case of early
transition metals lower oxidation states are commonly expected
to be less active if active at all. Thus, further studies aimed at
elucidating the effect of this particular ligand system on the
stability of the M-C bond will be central both to understand
The structure of 5 (Figure 5) consists of a simple diimine
pyridinato CrCl2 complex where each of the two chlorine atoms
bridge one Me3Al unit [Cr-Cl(1) ) 2.3435(9) Å, Cr-Cl(2) )
2.5415(8) Å, Cl(1)-Al(1) ) 2.348(2) Å, Cl(2)-Al(2) ) 2.313-
(12) Kauffmann, K.; Beirich, C.; Hamsen, A.; Moller, T.; Philipp, C.; Wing-
bermulhe, D. Chem. Ber. 1992, 125, 157.
9
12272 J. AM. CHEM. SOC. VOL. 124, NO. 41, 2002

R,R′-Diiminopyridine Ligand System in Alkylation
A R T I C L E S
1039 (m), 982 (w), 935 (w), 891 (w), 839 (w), 796 (s), 777 (m), 760
(w), 723 (m), 700 (w), 654 (w). Analytical data for the formula
C33H43N3Cl3Cr (without solvent) were obtained upon crushing the
crystalline solid and gently heating in vacuo for 1 h. Anal. Calcd
the catalytic performance of this family of catalysts and to the
design of even more potent catalysts.
Experimental Section
(Found): C 61.92 (61.73), H 6.77 (6.61), N 6.56 (6.48). µeff ) 3.83µBM
.
All operations were performed under inert atmosphere by using
standard Schlenk-type techniques. MnCl2(THF)2,13 CrCl3(THF)3,13 [2,6-
(i-Pr)2PhNdC(Me)]2(C5H3N),14 LiCH2Si(CH3)315 and {[2,6-(i-Pr)2PhNd
Preparation of {[2,6-(i-Pr)2PhNC(Me)]2(4-PhCH2C5H3N)Cr-
(CH2Ph)}2‚(toluene) (4). A suspension of CH2Cl2-free 3 (213 mg, 0.33
mmol) in toluene (5 mL) at -36 °C was treated with an Et2O solution
of C6H5CH2MgCl (0.51 M, 2 mL, 1.0 mmol). The color changed
immediately from green to reddish-brown and then gradually to dark
green. The mixture was stirred at ambient temperature for 4 h and then
filtered and extracted with toluene (20 mL × 3) to remove the white
precipitate of MgCl2. The dark green extracts were concentrated to the
volume of ca. 5 mL, from which dark green crystals of 4 (54 mg, 0.035
mmol, 21%). were obtained by slow addition of hexane (5 mL) for a
period of 2 days. Anal. Calcd (Found) for C101H122N6Cr2: C 79.59-
(79.51), H 8.07(7.97), N 5.51(5.48). IR (NaCl, Nujol mull, cm-1) 3062
(m), 3022 (m), 1591 (s), 1515 (m), 1489 (m), 1439 (s), 1334 (w), 1320
(w), 1306(s), 1276 (s), 1253 (w), 1242 (w), 1224 (m), 1206 (m, sh),
1175 (m), 1149 (s), 1117 (w), 1094 (m), 1056 (w), 1029 (w), 1004
(w), 977 (s), 959 (m), 934 (w), 900 (w), 877 (w), 862 (m), 853 (w),
820 (w), 800 (s), 778 (m), 767 (m), 762 (s), 739 (m), 726 (w), 711(w),
6
C(Me)]2(C5H3N)}MnCl2 were prepared according to published pro-
cedures. A solution of PhCH2MgCl in ether was prepared and titrated
according to standard procedures. Infrared spectra were recorded on a
Mattson 9000 and Nicolet 750-Magna FTIR instruments from Nujol
mulls prepared in a drybox. Samples for magnetic susceptibility
measurements were weighed inside a drybox equipped with an
analytical balance and sealed into calibrated tubes. Magnetic measure-
ments were carried out with a Gouy balance (Johnson Matthey) at room
temperature. Magnetic moments were calculated following standard
methods,16 and corrections for underlying diamagnetism were applied
to data.17 Elemental analyses were carried out with a Perkin-Elmer 2400
CHN analyzer. Data for X-ray crystal structure determination were
obtained with a Bruker diffractometer equipped with a Smart CCD
area detector.
Isolation of {[2,6-(i-Pr)2PhNC(dCH2)]2(C5H3N)}Mn(CH2SiMe3)]-
[Li(OEt2)4] (1) and {[2,6-(i-Pr)2PhNdC(Me)](C5H3N)[2,6-(i-Pr)2PhNd
C(CH2)]}2[Mn(CH2SiMe3)]2 (2). Solid LiCH2Si(CH3)3 was added (0.32
g, 3.4 mmol) to an orange suspension of {[2,6-(i-Pr)2PhNdC(Me)]2-
(C5H3N)}MnCl2 (1.0 g, 1.7 mmol) in toluene (80 mL) at room
temperature. A fast reaction was observed during which the color of
the mixture changed from orange to dark brown. The suspension was
stirred for an additional 4 h, after which, the solvent was removed in
vacuo. The resulting brown residue was dissolved in freshly distilled
diethyl ether (80 mL), centrifuged to eliminate a small amount of pale
colored solid, and the resulting solution was concentrated to about 50
mL. Dark blue-black solid precipitated upon standing overnight at room
temperature. The dark brown mother liquor was placed at 4 °C, in which
yellow-brown crystals of 1 were isolated (0.6 g, 0.6 mmol, 35%). Anal.
Calcd (Found) for C53H94LiMn N3O4Si: C 68.65 (68.86), H 10.22-
(10.34), N 4.53 (4.27). IR (Nujol mull, cm-1) ν 1912 (w), 1851 (w),
1790 (w), 1643 (s), 1589 (s), 1570 (m), 1364 (s), 1321 (w), 1261 (s),
1192 (w), 1094 (s), 1021 (s), 929 (w), 861 (m), 802 (s), 721 (m), 691
(m), 663 (m). µeff ) 3.91µBM. The solid separated from the solution
was dissolved in toluene and layered with hexane, affording dark
crystals of 2 (0.55 g, 0.44 mmol, 52%). Anal. Calcd. (Found) for
C74H106N6Si2Mn2: C 71.35 (71.64), H 8.58 (8.67), N 6.75 (6.27). IR
(Nujol mull, cm-1): 1646 (s), 1566 (m), 1531 (s), 1495 (s), 1366 (s),
1317 (w), 1268 (s), 1251 (s), 1229 (s), 1194 (w), 1164 (s), 1136 (w),
1111 (s), 1092 (s), 1046 (s), 1026 (s), 993 (s), 956 (s), 875 (s), 823 (s),
785 (m), 773 (w), 764 (m), 748 (s), 727 (s), 692 (s), 671 (w), 619 (w).
698 (s). µeff ) 4.75µBM
.
Preparation of Cr(C33H43N3){ClAl(CH3)3}2‚(toluene) (5). A sus-
pension of CH2Cl2-free 3 (652 mg, 1.02 mmol) in cold (0 °C) toluene
(30 mL) was treated with a heptane solution of Al(CH3)3 (2.0 M, 5
mL, 10 mmol) over a period of 30 min, affording a dark-green
homogeneous solution. The solution gradually changed the color to
dark-brown, and separated crystalline 5 (623 mg, 0.741 mmol, 73%)
was obtained upon standing at room temperature. Anal. Calcd for
C46H69N3Al2Cl2Cr: C 65.70 (65.58), H 8.27(8.19), N 5.00(4.93). IR
(NaCl, Nujol mull, cm-1) 3078 (w), 3062 (w), 2361 (m), 2343 (m),
1573 (s), 1320 (w), 1270 (s), 1211 (m), 1172 (s), 1103 (w), 1058 (w),
1033 (w), 982 (w), 936 (w), 842 (w), 813 (m), 802 (m), 795 (m), 776
(m), 756 (w), 734 (s), 698 (s), 678 (s). µeff ) 4.83µBM
.
X-ray Crystallography: Structural Determination of 2, 3, 4, and
5. These compounds consistently yielded crystals that diffract weakly,
and the results presented are the best of several trials. The data crystals
were selected, mounted on thin, glass fibers using paraffin oil and cooled
to the data collection temperature for 2, 3, and 4 and in a Lindemann
capillary for 5. Data were collected on a Bruker AXS SMART 1k CCD
diffractometer using 0.3° ω-scans at 0°, 90°, and 180° in φ. Initial unit-
cell parameters were determined from 60 data frames collected at
different sections of the Ewald sphere. Semiempirical absorption
corrections based on equivalent reflections were applied.18
Systematic absences in the diffraction data sets and unit-cell
parameters were consistent with P21/c (no.13) for 2, P1 (no. 1) and
P-1 (no. 2) for 3, C2/c (no. 15) and Cc (no. 9) for 4, and Pnma (no.
62) and Pn21a [Pna21 (no. 33)] for 5. Solution in the centrosymmetric
space groups yielded chemically reasonable and computationally stable
results of refinement. The structures were solved by direct methods,
completed with difference Fourier syntheses and refined with full-matrix
least-squares procedures based on F2.
The compound molecule was located on a 2-fold axis in 4, on a
mirror plane in 5, and at an inversion center in 2. Cocrystallized toluene
solvent molecules were located each on a 2-fold axis in 4 and on a
mirror plate in 5. Three symmetry-unique but chemically equivalent
compound molecules were located in the asymmetric unit of 3
cocrystallized with four symmetry-unique molecules of methylene
chloride solvent. All non-hydrogen atoms, except those of the solvent
molecule in 5, which were refined isotropically with idealized geometry,
were refined with anisotropic displacement coefficients. All hydrogen
atoms were treated as idealized contributions. All scattering factors
are contained in several versions of the SHELXTL program library
µeff ) 5.65µBM
.
Preparation of {2,6-bis[2,6-(i-Pr)2PhNdC(Me)]2(C5H3N)}CrCl3‚
(CH2Cl2)1.33 (3). A solution of 2,6-bis[1-(2,6-diisopropylphenylimino)-
ethyl]pyridine (2.0 g, 4.16 mmol) in toluene (100 mL) was treated with
CrCl3(THF)3 (1.55 g, 4.16 mmol). The mixture was refluxed overnight
(110 °C). The solution was evaporated to dryness and the solvent was
replaced with 20 mL of methylene chloride, upon which the color of
the solution changed to dark green. The solution was filtered and layered
with hexane, yielding green crystals of 3 over a period of 2 weeks.
(2.4 g, 3.3 mmol, yield 79%). IR (Nujol mull, cm-1): 3168 (m), 2954
(s), 2879 (s), 1620 (w), 1576 (m), 15.14 (m), 1464 (s), 1379 (s), 1323
(w), 1259 (m), 1209 (w), 1171 (w), 1149 (w), 1101 (m), 1055 (w),
(13) Manzer, L. E. Inorg. Synth. 1982, 21, 138.
(14) Johnson, L. K.; Killian, C. M.; Brookhart, M. S. J. Am. Chem. Soc. 1995,
117, 6414.
(15) Tessier-Youngs, C.; Beachley, O. T., Jr. Inorg. Synth. 1986, 24, 95.
(16) Mabbs, M. B.; Machin, D. Magnetism and Transition Metal Complexes;
Chapman and Hall; London, 1973.
(17) Foese, G.; Gorter, C. J.; Smits, L. J. Constantes Selectionne´es Diamagne´-
tisme, Paramagnetisme, Relaxation Paramagnetique; Masson: Paris, 1957.
(18) Blessing, R. Acta Crystallogr. 1995, A51, 33-38.
9
J. AM. CHEM. SOC. VOL. 124, NO. 41, 2002 12273

A R T I C L E S
Sugiyama et al.
Table 1. Crystal Data and Structure Analysis Results
2
3
4
5
formula
MW
crystal system
space group
a (Å)
C74H106N6Si2Mn2
1245.71
monoclinic
P2(1)/c
17.479(3)
12.0607(19)
18.368(3)
110.800(3)
3619.9(10)
2
C
34.3H45.67Cl5.67N3Cr
C101H12N6Cr2
1524.05
monoclinic
C2/c
23.5634(10)
18.8452(7)
21.9494(8)
111.4240(10)
9073.3(6)
4
C46H69Al2Cl2CrN3
840.90
753.29
monoclinic
P2(1)/n
18.124(2)
16.440(1)
37.861(3)
95.932(2)
11220(2)
12
orthorhombic
Pnma
17.968(2)
14.715(1)
19.712(2)
b (Å)
c (Å)
â (deg)
V (Å3)
5211.9(8)
4
Z
radiation (KR Mo)
0.710 73
203
1.143
0.425
1340
0.0818
0.1541
1.088
0.710 73
203
1.338
0.738
4716
0.0702
0.2064
1.022
0.710 73
203
1.116
0.287
3272
0.0677
0.1765
1.044
0.710 72
293
1.072
0.386
1800
0.0868
0.2663
1.049
T (K)
D
µ
calcd (g cm-3
)
calcd (cm-1
)
F000
R
2 a
Rw
GoF
2
a R ) Σo - Fc/ΣFoRw ) [(Σ(Fo - Fc)2 /ΣwFo )]1/2
.
Table 2. Selected Bond Distances and Anglesa
2
3
4
5
Mn-C(35)
2.097(9)
2.301(6)
2.073(6)
2.226(6)
1.317(9)
Cr-Cl(1)
Cr-Cl(2)
Cr-Cl(3)
Cr-N(1)
Cr-N(2)
2.295(2)
Cr-C(41)
Cr-N(1)
Cr-N(2)
Cr-N(3)
C(13)-N(1)
C(13)-C(14)
C(16)-C(18a)
2.153(4) Cl(1)-Cr
2.114(3) Cl(2)-Cr
2.3435(9)
Mn-N(1)
2.350(2)
2.273(2)
2.156(6)
1.996(5)
2.161(6)
1.291(8)
2.5415(8)
2.1428(16)
2.000(2)
2.3482(16)
2.3130(12)
1.295(3)
1.510(3)
1.969(6)
2.026(3)
100.45(4)
159.23(6)
100.45(4)
103.37(3)
151.63(8)
148.08(5)
124.96(4)
96.79(4)
97.40(6)
96.79(4)
Mn-N(2)
2.051(3) N(1)-Cr
Mn-N(3)
2.105(3) N(2)-Cr
N(1)-C(14)
1.313(4) Cl(1)-Al(1)
1.517(5) Cl(2)-Al(2)
1.563(5) N(1)-C(13)
1.563(5) C(13)-C(14)
1.523(6) Al(1)-C(19)
1.516(5) Al(2)-C(20)
169.23(16) Cl(1)-Cr-N(1)
103.06(13) Cl(1)-Cr-N(2)
153.09(11) Cl(1)-Cr-N(1a)
103.84(13) Cl(1)-Cr-Cl(2)
76.65(10) N(1)-Cr-N(1a)
C(14)-C(13)
C(14)-C(15)
N(2)-C(15)
1.491(11) Cr-N(3)
1.474(11) N(3)-C(20)
1.362(9)
1.367(9)
C(20)-C(21)
Cl(1)-Cr-N(2)
1.500(10) C(18)-C(16a)
N(2)-C(19)
C(19)-C(20)
C(20)-N(3)
C(21)-C(21a)
C(35)-Mn-N(2)
N(3)-Mn-N(1)
C(35)-Mn-N(1)
N(3)-Mn-C(35)
C(20)-C(21)-C(21a)
171.59(17)
176.38(8)
152.1(2)
100.35(15)
107.24(16)
92.74(15)
95.08(16)
82.20(16)
C(17)-C(34)
C(15)-C(16)
1.450(11) Cl(2)-Cr-Cl(3)
1.344(9)
1.562(15) Cl(1)-Cr-N(1)
151.0(3)
140.3(3)
108.1(3)
111.4(3)
111.2(8)
N(1)-Cr-N(3)
C(41)-Cr-N(2)
C(41)-Cr-N(1)
N(1)-Cr-N(3)
C(41)-Cr-N(3)
N(1)-Cr-N(2)
C(17)-C(34)-C(40)
Cr-C(41)-C(47)
Cl(1)-Cr-N(3)
Cl(2)-Cr-N(1)
Cl(2)-Cr-N(3)
Cl(2)-Cr-N(2)
113.8(3)
119.2(3)
Cr-Cl(1)-Al(1)
Cr-Cl(2)-Al(2)
Cl(2)-Cr-N(1)
Cl(2)-Cr-N(2)
Cl(2)-Cr-N(1a)
a Distances are given in angstroms; angles are given in degrees.
Acknowledgment. This work was supported by the Natural
Science and Engineering Council of Canada (NSERC).
with the latest used version being v.6.12 (Sheldrick, G. M. Bruker AXS;
Madison, WI, 2001). Crystallographic data and relevant bond distances
and angles are reported in Tables 1 and 2.
Supporting Information Available: Complete crystallo-
graphic data (CIF) for complexes 2, 3, 4, and 5. This material
is available free of charge via the Internet at http://pubs.acs.org.
Calculations. Geometry optimizations of the minima of the reaction
path were performed with the GAMESS-UK package19 using the
UB3LYP hybrid density functional20 with split-valence basis sets:
LANL2DZ (small core pseudopotential) on Mn/Cr21 and 3-21G(*) for
all other atoms.22 Geometries were optimized without constraints.
JA020485M
(20) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Lee, C.; Yang, W.;
Parr, R. G. Phys. ReV. B 1988, 37, 785. (c) Miehlich, B.; Savin, A.; Stoll,
H.; Preuss, H. Chem. Phys. Lett. 1989, 157, 200.
(21) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299.
(22) (a) Binkley, J. S.; Pople, J. A.; Hehre, W. J. J. Am. Chem. Soc. 1980, 102,
939. (b) Gordon, M. S.; Binkley, J. S.; Pople, J. A.; Pietro, W. J.; Hehre,
W. J. J. Am. Chem. Soc. 1982, 104, 2797. (c) Frisch, M. J.; Pople J. A.;
Binkley, J. S. J. Chem. Phys. 1984, 80, 3265, and references therein. (d)
Dobbs, K. D.; Hehre, W. J. J. Comput. Chem. 1986, 7, 359; Ibid. 1987, 8,
861, 880.
(19) GAMESS-UK is a package of ab initio programs written by M. F. Guest,
J. H. van Lenthe, J. Kendrick, K. Schoffel, and P. Sherwood, with
contributions from R. D. Amos, R. J. Buenker, H. J. J. van Dam, M. Dupuis,
N. C. Handy, I. H. Hillier, P. J. Knowles, V. Bonacic-Koutecky, W. von
Niessen, R. J. Harrison, A. P. Rendell, V. R. Saunders, A. J. Stone, D. J.
Tozer, and A. H. de Vries. The package is derived from the original
GAMESS code due to M. Dupuis, D. Spangler and J. Wendoloski, NRCC
Software Catalog, Vol. 1, Program QG01 (GAMESS), 1980.
9
12274 J. AM. CHEM. SOC. VOL. 124, NO. 41, 2002