Synthesis and reactivity of two-coordinate ni(i) alkyl and aryl complexes

Article abstract of DOI:10.1021/ja4095236

Reaction of [(IPr)Ni(μ-Cl)]₂ (1-Cl; IPr = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) with ClMg{CH(SiMe ₃)₂}·Et₂O affords (IPr)Ni{CH(SiMe ₃)₂} (2), a two-coordinate Ni(I) alkyl complex. An analogous two-coordinate aryl derivative, (IPr)Ni(dmp) (dmp = 2,6-dimesitylphenyl), can be similarly prepared from Li(dmp) and 1-Cl. Reaction of 2 with alkyl bromides gives the three-coordinate Ni(II) alkyl halide complex (IPr)Ni{CH(SiMe₃)₂}Br. Evidence for a radical mechanism is presented to explain the reaction of 2 with alkyl halides.

Full text of DOI:10.1021/ja4095236

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Communication
Synthesis and Reactivity of Two-Coordinate Ni(I) Alkyl and Aryl Complexes
Carl A Laskowski, Donald J. Bungum, Steven M Baldwin,
Sarah A Del Ciello, Vlad M. Iluc, and Gregory L Hillhouse
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 15 Nov 2013
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Synthesis and Reactivity of Two-Coordinate Ni(I) Alkyl and Aryl Complexes
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Carl A. Laskowski,‡ Donald J. Bungum,§ Steven M. Baldwin, Sarah A. Del Ciello, Vlad M. Iluc,† and
Gregory L. Hillhouse*
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Gordon Center for Integrative Science, Department of Chemistry, The University of Chicago, Chicago, Illinois 60637, USA
Supporting Information Placeholder
crystals of 2 were grown from hexamethyldisiloxane (HMDSO)
ABSTRACT: Reaction of [(IPr)Ni(ꢀ-Cl)]2 (1-Cl; IPr = 1,3-
and confirmed the identification of 2 as a two-coordinate complex
(Figure 1). The crystal structure of 2 reveals a nearly linear C–Ni–
C core (174.81(10)°) and Ni–C bond lengths of 1.910(2) and
1.968(3) Å for the IPr and alkyl groups, respectively. The alkyl
bond length seen in 2 is somewhat shorter than those found in the
three-coordinate Ni(I) alkyl complexes (PPh3)Ni[(C,N:κ2-
bis(2,6-diisopropylphenyl)imidazolin-2-ylidene)
with
ClMg{CH(SiMe3)2}·Et2O affords (IPr)Ni{CH(SiMe3)2} (2), a
two-coordinate Ni(I) alkyl complex. An analogous two-coordinate
aryl derivative, (IPr)Ni(dmp) (3; dmp = 2,6-dimesitylphenyl), can
be similarly prepared from Li(dmp) and 1-Cl. Reaction of 2 with
alkyl bromides gives the three-coordinate Ni(II) alkyl halide com-
plex (IPr)Ni{CH(SiMe3)2}Br (5). Evidence for a radical mecha-
nism is presented to explain the reaction of 2 with alkyl halides.
C(SiMe3)2(SiMe2-2-C5H4N)]
(2.025(4)
Å)7a
and
(tBu2PCH2CH2PtBu2)Ni(CH2CMe3) (1.982(3) Å).7b
A
related
Ni(I) silyl complex, (tBu2PCH2CH2PtBu2)Ni(SiHMes2), has also
been reported.12 There is no agostic interaction in 2 between Ni
and the α-CH moiety (which was located in the X-ray difference
map) as indicated by the long 2.26(3) Å Ni–H distance.
Ni-based alkyl-alkyl coupling has emerged as a powerful tool
for the formation of C–C bonds.1 In contrast to Pd, for which β-
hydride elimination is facile, Ni alkyls show decreased propensity
to β-hydride eliminate,2 possibly due to a thermodynamic prefer-
ence for alkyl complexes over olefin hydride complexes.3 The
recent development of Ni-based stereoconvergent alkyl-alkyl
coupling reactions strongly suggests a more complicated mecha-
nism is occurring than the “simple” Pd-based oxidative addi-
tion/transmetallation/reductive elimination mechanism often pro-
posed.4 One could envision a radical mechanism proceeding from
either a Ni(II) or Ni(I) alkyl. While Ni(II) alkyls are plentiful and
their reactions with alkyl halides have been studied,5,6 Ni(I) alkyls
are quite rare. We have had previous success isolating reactive
low-coordinate Ni(I) species including one of the few structurally
characterized Ni(I) alkyls.7 Herein we report the synthesis of two-
coordinate Ni(I) alkyl and aryl complexes supported by the N-
Scheme 1
heterocyclic
carbene
(NHC)
ligand
1,3-bis(2,6-
diisopropylphenyl)imidazolin-2-ylidene (IPr). IPr and related
NHC’s have recently been shown to support Ni-based alkyl cou-
pling reactions,8 thus IPr seemed to present a relevant scaffold for
preparing a Ni(I) alkyl complex and studying its coupling reactivi-
ty with alkyl halides.
It has been demonstrated that Sigman’s d9-d9 dimer
[(IPr)Ni(ꢀ-Cl)]2 (1-Cl)9 can serve as a precursor to two-coordinate
d9 Ni amides.10 Attempts to use an analogous salt-metathesis
strategy with 1-Cl to prepare two-coordinate Ni(I) alkyls incorpo-
rating small alkyl groups lacking β-hydrogens proved ineffective.
For example, reaction of 1-Cl with Mg(CH2Ph)2 gave only the
homocoupled product dibenzyl and the known Ni(0) dimer
[(IPr)Ni]2 (Scheme 1).11 Use of a more sterically encumbered
alkyl proved capable of avoiding the homocoupling reaction.
Combination of 1-Cl with ClMg{CH(SiMe3)2}·Et2O in a 1:2 stoi-
chiometric fashion (Et2O solution) affords the two-coordinate
Ni(I) alkyl complex (IPr)Ni{CH(SiMe3)2} (2) as a yellow powder
Figure 1. Thermal ellipsoid plot of 2 with all hydrogen atoms
removed for clarity except on C(41). Ellipsoids shown at 50%
probability. Selected metrical parameters for 2: Ni-C(1) =
1.910(2), Ni-C(41) = 1.968(3) Å; C(1)-Ni-C(41) = 174.81(10)°.
1
in 82% isolated yield (Scheme 1). H-NMR analysis revealed a
paramagnetic spectrum consistent with the solution magnetic
moment of 1.9 ꢀB as measured via Evans’ method. X-ray quality
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in 62% isolated yield as an orange solid (Scheme 2). Complex 4 is
1
2
3
4
5
6
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8
paramagnetic with a solution magnetic moment of 2.10 ꢀB as
determined via Evans’ method. X-ray quality crystals were grown
from diethyl ether solution and revealed a distorted trigonal planar
geometry with the sum of angles at Ni = 360° (Figure 4). A slight
elongation of the Ni–CNHC bond to 1.950(2) Å and of the Ni–Calkyl
bond to 2.008(3) Å was observed when compared with 2. The
ability of the 13-electron Ni(I) alkyl complex 2 to coordinate a
third ligand is consistent with its electron deficient character.
Scheme 2
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Figure 2. Thermal ellipsoid plot of 3·(C7H8) with all hydrogen
atoms and toluene of solvation removed for clarity. Ellipsoids
shown at 50% probability. Selected metrical parameters for 3: Ni-
C(1) = 1.923(2), Ni-C(31) = 1.944(2) Å; C(1)-Ni-C(31) =
175.97(8)°.
Analogous metathesis chemistry leads to the formation of a
two-coordinate Ni(I) terphenyl complex. Reaction of 1-Cl with
Li(dmp) (dmp = 2,6-dimesitylphenyl) afforded (IPr)Ni(dmp) (3)
as a bright-yellow solid in 70% isolated yield. The solution mag-
netic moment of 3 (1.80 ꢀB) is indicative of a 1-electron paramag-
net. Crystals of 3 were obtained from HMDSO solution and re-
vealed a nearly linear C–Ni–C angle of 175.97(8)° (Figure 2). The
Ni–C bond lengths of 1.923(2) and 1.944(2) Å for the NHC and
aryl substituents are similar to those observed in 2 with only a
slightly contracted Ni–Caryl bond with corresponding elongation
of the Ni–CNHC bond.
Inasmuch as other formally “Ni(I)” alkyl complexes have been
shown, upon inspection, to be in fact Ni(II) complexes with re-
duced ligands,5g,5h,13 DFT studies (B3LYP/LANL2DZ) of 2 and 3
were undertaken to examine their electronic structures and to
confirm the assignments as Ni(I) alkyl and aryl complexes. Single
point calculations done on the geometries obtained from the X-ray
structures revealed SOMOs that are localized on Ni composed of
Ni 3d(z2) mixed with Ni 4s. The Ni in each complex dominates
the unpaired-spin density with Mulliken spin densities of 1.01 and
1.00 for 2 and 3, respectively (see Figure 3). No other atom in
either case has an unpaired spin density greater than 0.01. The
pure Ni character of the SOMO is consistent with an assignment
as Ni(I) and likely contributes to the stabilities of 2 and 3.
Figure 4. Thermal ellipsoid plot of 4 with all hydrogen atoms
removed for clarity except on C(31). Ellipsoids shown at 50%
probability. Selected metrical parameters for 4: Ni-C(1) =
1.950(2), Ni-C(41) = 1.855(2), Ni-C(31) = 2.008(3) Å; C(1)-Ni-
C(31) = 135.12(8), C(1)-Ni-C(41) = 108.23(9), C(31)-Ni-C(41) =
116.35(9)°.
Reactions of 2 with secondary alkyl halides were investigated
since they might shed mechanistic light on the stereospecificity of
cross-coupling reactions previously reported.4a-j Addition of 1-
bromo-1-phenylethane to a solution of 2 resulted in immediate
reaction yielding a dark, inky-turquoise solution. Cross coupling
to yield 1,1-bis(trimethylsilyl)-2-phenylpropane was not observed,
but instead GC-MS and 1H-NMR spectroscopic analysis of the
organic products revealed the homocoupled product, 2,3-
diphenylbutane. NMR analysis of the reaction also revealed a new
diamagnetic IPr-containing product inconsistent with either
[(IPr)Ni]2 or 1-Br. X-ray quality crystals of the new IPr-
containing product were grown via diffusion of hexanes into a
benzene solution, and analysis of the blue crystals revealed the
product to be
a three-coordinate Ni(II) alkyl complex,
(IPr)NiBr{CH(SiMe3)2}, 5 (Figure 5). 5 exhibits a distorted Y-
shaped geometry in which the Br forms the stem of the Y. Com-
pared with 2, significant shortening of the NHC (1.814(8) Å) and
alkyl (1.902(8) Å) Ni–C bonds is observed. This shortening likely
arises from a combination of two complimentary factors. Firstly a
decrease in bond lengths upon going from Ni(I) to Ni(II) and sec-
ondly the loss of the trans influence of the alkyl ligand upon go-
Figure 3. Unpaired-spin density plots for 2 (left) and 3 (right).
Although 2 is sterically protected from dimerization, it reacts
with the small Lewis base pivaloisocyanide to afford the three-
coordinate Ni(I) alkyl complex, (IPr)Ni(CNtBu){CH(SiMe3)2} (4)
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Journal of the American Chemical Society
ing from two- to three-coordinate. The other structurally charac-
terized example of a Ni bis(trimethylsiyl)methyl complex is
1
(PMe3)2NiCl{CH(SiMe3)2},14 with Ni–C at 1.972(8) Å, and while
the Ni–Calkyl bond in 5 is significantly shorter, it is comparable to
other three-coordinate Ni(II) alkyls10,15 which are T-shaped com-
plexes with the alkyl group forming the stem of the T in all cases.
The T-shaped configuration has been calculated to be the elec-
tronically preferred geometry for d8 three-coordinate metal com-
plexes with σ-donor ligands,16 but the large steric profiles of the
ligands in 5 are likely responsible for its observed Y-shaped ge-
ometry.
Scheme 4
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Scheme 3
The formation of the tetrakis(trimethylsilyl)ethane was some-
what surprising. One explanation for its formation would be for a
species such as a Ni(III) dialkyl bromide complex to eliminate
(Me3Si)2CHBr that subsequently reacts with another equivalent of
2 to yield the homocoupled product (and either 1-Br or 5 depend-
ing on the mechanism). To test this hypothesis 2 was directly
reacted with (Me3Si)2CHBr under the standard reaction condi-
tions. Clean, rapid conversion of 2 to 5 was observed along with
formation of 1,1,2,2-tetrakis(trimethylsilyl)ethane (Scheme 5).
Silicon is known to stabilize both α- and β-radicals with estimates
of this effect being around 1-3 kcal/mol for an α-Si radical.17 In
addition, Si has been shown to increase the rate of α-Br abstrac-
tion 3.3x for Bu3Ge· and 6.6x for Bu3Sn· relative to a bromoal-
kane.18 This effect would likely be enhanced by the presence of
two α-Si groups as the effect of Si-substitution has been shown to
be additive;17c even when one α-Si is proposed to have no stabili-
zation effect on α-radicals, two α-Si groups were found to be
stabilizing.19 Therefore, halogen abstraction from (Me3Si)2CHBr
by 2 is expected to be rapid while oxidative addition or SN2 dis-
placement would both be strongly inhibited by the very bulky
SiMe3 groups. The possibility of bromide abstraction to form a
carbocation seems unlikely as silicon tends to destabilize α-
cations and this reaction type is only observed when the cation is
stabilized.20
n
n
Figure 5. Thermal ellipsoid plot of 5·(1/2 C6H6) with all hydrogen
atoms except on C(28) and molecules of solvation removed for
clarity. Ellipsoids shown at 50% probability. Selected metrical
parameters for 5: Ni-C(1) = 1.814(8), Ni-C(28) = 1.902(8), Ni-
Br(1) = 2.247(2) Å; C(1)-Ni-C(28) = 93.8(4), C(1)-Ni-Br(1) =
141.0(3), C(28)-Ni-Br(1) = 124.9(2)°.
To investigate the possibility of alkyl group scrambling during
the course of reaction of 2 with alkyl halides, 2 was reacted with
(Me3Si)2CDBr prepared from CDBr3 according to the literature
synthesis of (Me3Si)2CHBr.21 Reaction with 2 afforded 5-d0 and
1,1,2,2-tetrakis(trimethylsilyl)ethane-d2. No alkyl scrambling was
observed ruling out the transient formation of a dialkyl Ni(III).
Bimolecular oxidative addition, which has recently been proposed
in another system,22 is also ruled out as no monodeteuro homo-
coupling products are observed.
The isolation of 5 along with 2,3-diphenylbutane suggests a
radical mechanism for oxidative addition of alkyl halides. The
ability of the phenyl group to stabilize a radical would allow the
radical to persist long enough for homocoupling to occur. Without
the stabilization, the radical could react with 5 to either abstract
·CH(SiMe3)2 to form the heterocoupled product or form a Ni–C
bond to form a dialkyl Ni(III) complex from which reductive
elimination to form the heterocoupled product could occur. To
probe this further, other alkyl bromides were examined.
Scheme 5
Reaction of 2 with benzyl bromide resulted in a mixture of
products (Scheme 4). Among the Ni-containing products were
obtained 1-Br and 5 in a ~ 4:1 ratio. Analysis of the organic prod-
ucts revealed the expected heterocoupled product 1,1-
bis(trimethylsilyl)-2-phenylethane as the major product, but also
dibenzyl and 1,1,2,2-tetra(trimethylsilyl)ethane as byproducts (~
4:2:1 relative ratio). Benzyl bromide would form a less stable
radical than (1-bromoethyl)benzene as it forms a primary instead
of a secondary radical. Additionally the smalr steric profile of
benzyl radical would make formation of a four-coordinate Ni
intermediate more facile. Therefore, a mixture of products is rea-
sonable from the reaction of benzyl bromide.
Further evidence for a radical pathway was seen in reaction of 2
with the unactivated alkyl bromides. Reaction of a primary alkyl
bromide, 1-bromobutane, with 2 resulted in slow consumption of
the alkyl bromide over the course of 2 days with formation of 1-
Br. The major organic product from this reaction was
(Me3Si)2CH2 with 0.7 equiv of butene isomers formed. 2-
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(3) Breitenfeld, J.; Vechorkin, O.; Corminboeuf, C.; Scopelliti, R.; Hu,
X. L. Organometallics 2010, 29, 3686.
bromobutane reacts slowly over the course of hours to give a 2:1
mixture of 1-Br and 5 along with (Me3Si)2CH2 and 0.8 equiv of
butene isomers relative to 1-Br. Menthyl bromide reacts over the
course of 4 days yielding 1-Br as the major Ni-containing prod-
uct. Once again (Me3Si)2CH2 is formed, however the other organ-
ic products were not identified. For none of these unactivated
alkyl bromides was a cross-coupled product observed. The isola-
tion of a Ni(II) alkyl bromide from reaction of 2 with alkyl bro-
mides is strong evidence for a radical oxidative addition mecha-
nism. The fact that alkyl bromides capable of forming stabilized
radicals react faster than simple alkyl halides further supports this
suggestion.
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In conclusion, the first two-coordinate Ni alkyl and aryl com-
plexes have been synthesized and crystallographically character-
ized. Study of the reactivity of 2 has led to the isolation and crys-
tallographic characterization of a three-coordinate Ni(II) alkyl
complex. These reactivity studies strongly implicate a radical
mechanism for the reaction of Ni(I) alkyl complexes with alkyl
halides. How these findings relate to Ni-catalyzed alkyl-alkyl
coupling is less clear since the large steric profiles of the ligands
in this particular system could bias its reactivity.
ASSOCIATED CONTENT
Supporting Information
Experimental, spectroscopic, computational, and analytical details
(PDF); complete crystallographic details for 2, 3, 4, and 5 (CIF).
This material is available free of charge via the internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
g-hillhouse@uchicago.edu
Present Addresses
‡
3M Corporate Research Materials Laboratory, 3M Center, St.
Paul, MN 55144.
§
Department of Philosophy, St. Louis University, St. Louis, MO
63108.
† Department of Chemistry and Biochemistry, University of Notre
Dame, Notre Dame, IN 46556.
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ACKNOWLEDGMENT
This work was supported by the National Science Foundation
through grants CHE-1266281 (to G.L.H.) and CHE-1048528
(CRIF MU instrumentation), and a Beckman Scholars Fellowship
from the Arnold and Mable Beckman Foundation (to D.J.B.).
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