Three-Coordinate NiII: Tracing the Origin of an unusual, facile Si-C(sp3) bond cleavage in [(tBu2PCH 2SiMe2)2N]Ni+

Article abstract of DOI:10.1021/ja108426f

All attempts to synthesize (PNP)Ni(OTf) form instead (^tBu ₂PCH₂SiMe₂NSiMe₂OTf)Ni(CH ₂P^tBu₂). Abstraction of F^- from (PNP)NiF by even a catalytic amount of B

Full text of DOI:10.1021/ja108426f

ARTICLE
pubs.acs.org/JACS
Three-Coordinate NiII: Tracing the Origin of an Unusual,
Facile Si-C(sp3) Bond Cleavage in [(tBu2PCH2SiMe2)2N]Niþ
Benjamin C. Fullmer, Hongjun Fan, Maren Pink, John C. Huffman, Nikolay P. Tsvetkov, and
Kenneth G. Caulton*
Department of Chemistry, Indiana University, Bloomington, Indiana 47405, United States
S Supporting Information
b
ABSTRACT: All attempts to synthesize (PNP)Ni(OTf) form instead (tBu2PCH2-
SiMe2NSiMe2OTf)Ni(CH2PtBu2). Abstraction of F- from (PNP)NiF by even a
catalytic amount of BF3 causes rearrangement of the (transient) (PNP)Niþ to
analogous ring-opened [(tBu2PCH2SiMe2NSiMe2F)]Ni(CH2PtBu2). Abstrac-
tion of Cl- from (PNP)NiCl with NaB(C6H3(CF3)2)4 in CH2Cl2 or C6H5F
gives (PNP)NiB(C6H3(CF3)2)4, the key intermediate in these reactions is
(PNP)Niþ, [(PNP)Ni]þ, in which one Si-C bond (together with N and two
P) donates to Ni. This makes this Si-C bond subject to nucleophilic attack by F-,
triflate, and alkoxide/ether (from THF). This σSi-C complex binds CO in the
time of mixing and also binds chloride, both at nickel. Evidence is offered of a
“self-healing” process, where the broken Si-C bond can be reformed in an
equilibrium process. (PNP)Niþ reacts rapidly with H2 to give (PN(H)P)NiHþ,
which can be deprotonated to form (PNP)NiH. A variety of nucleophilic attacks
(and THF polymerization) on the coordinated Si-C bond are envisioned to
occur perpendicular to the Si-C bond, based on the character of the LUMO of
(PNP)Niþ.
e were surprised to be able to synthesize1 three T-shaped
species (PNP)M, M = Fe, Co, and Ni, where PNP is the
of the two silver isotopes (each I = 1/2).24 We therefore tried a
route beginning with monovalent, three-coordinate (PNP)Ni.
Redox Approach to “(PNP)Ni(OTf)”. 1. Product Character-
ization. The reaction of paramagnetic (PNP)Ni with equimolar
AgOTf proceeds similarly in benzene and in THF with formation
of a black solid (Ag0). The phosphorus-containing material
remains soluble; the 31P{1H} NMR spectrum, measured on
material isolated after a reaction time of 12 h, shows an AX
pattern with a large (82 ppm) difference in chemical shifts, and a
W
tridentate ligand [(tBu2PCH2SiMe2)2N]-1, all of which have
very low valence electron count. Much catalytic chemistry has
been developed around such three-coordinate late transition
metal species (or their synthons), including olefin polymeriza-
tion and catalytic C-C bond formation between polar sub-
strates.2-11 Cationic three-coordinate species have been espe-
cially effective in this application andfor olefinpolymerization.12-20
We wondered what reactivity changes would happen when we
moved from d8 (PNP)Co to the isoelectronic cation (PNP)Niþ. In
search of a precursor to such an electrophilic species, (PNP)
Ni(O3SCF3),21,22 we found that halide metathesis from (PNP)
NiCl with NaOTf (OTf = F3CSO3-) or Me3SiOTf was not
successful. We therefore turned to an oxidative synthetic approach,
the reaction of (PNP)Ni with AgO3SCF3. We report here the
outcome of this reaction, which has led to the discovery of what we
now establish to be reversible Si-C bond formation/scission in the
unusual isolable species (PNP)Niþ (Scheme 1).
0
JPP large enough (213 Hz) to indicate trans phosphines in a
single diamagnetic product. While the two P are inequivalent,
there is a molecular mirror plane containing Ni, two P, and N
t
since there are only two SiMe chemical shifts and two Bu
chemical shifts, the latter each being doublets (not virtual trip-
lets). It is difficult to envision a structure that makes the two P
inequivalent, but a single crystal structure determination of this
compound, crystallized from pentane (Figure 1) reveals that
there has been Si-CH2 bond scission, and the silicon has
captured the triflate as its new substituent. Triflate is not coor-
dinated to the metal, but the resulting tBu2PCH2 moiety is now
bidentate on NiII. Despite their functional difference, the two
phosphorus have essentially the same distance from Ni. The
tBu2PCH2 ligand (Scheme 2) can be thought of either as a
’ RESULTS
As mentioned in the introduction, preparation of (PNP)Ni
(OTf) is not simple. Stirring of AgOTf with (PNP)NiCl for 2
days in THF shows essentially no conversion to the triflate23 but
only the production of increasing amounts of a silver phosphine
complex, identifiable by its large 31P{1H} NMR doublet for each
Received: September 17, 2010
Published: February 9, 2011
r
2011 American Chemical Society
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Scheme 1
Scheme 2
Scheme 3
chemical shifts and a small (27 Hz) PP0 coupling constant. Over a
period of hours in benzene, this species declines in intensity as
signals of the crystallographically characterized product grow.
One of the intermediate AX 31P chemical shifts lies within 2 ppm
of that for the final product (i.e., very similar environments in
intermediate and final product), but the other lies 38 ppm
downfield from the tBu2PCH2 ligand of the final product. What
is the intermediate on the way to the final product? Its 1H NMR
spectrum shows Cs symmetry, and the inequivalent phosphorus
sites suggest that the Si-CH2 cleavage has already occurred. We
can account for the small PP0 coupling by avoiding having the
two P mutually trans, but at least three structures qualify, all of
which have the Si-O bond already formed (Scheme 3): A and B
differ only in whether the Ni-P bond has been retained or
broken. A is the least motion product, which keeps the CH2 cis to
the N it departed from. B is similar but is unusual in having NiII be
only three-coordinate. C uses triflate to temporarily maintain
coordination number 4 at Ni and awaits displacement of this O
by the bulky tBu2P nucleophile. Both A and B have Cs symmetry,
1
so this will not be a distinguishing feature of the observed H
NMR spectrum; C has no symmetry, due to the chiral sulfur, and
is inconsistent with the molecular mirror symmetry indicated by
the proton NMR. Why is the crystallographic product thermo-
dynamically preferred to one of the suggested intermediate
structures? The simplest explanation is that trans phosphines in
Figure 1 minimize the mutual repulsion between four tBu groups
in A, which we suggest is the actual structure of the intermediate.
After a 12-h reaction time, the ratio of intermediate A to the
crystallographic product is 1:9. Heating (60 ꢀC) a pure (from
selective crystallization) sample of the crystallographic product
in benzene shows the reappearance of the intermediate, reaching
relative populations of 1:9. This proves that the relative stability
of the two is quite close and that the two are in equilibrium, even
if they are connected by a significantly high barrier. All reactivity
studies of (tBu2PCH2SiMe2NSiMe2OTf)Ni(CH2PtBu2) reported
here are thus done with this 1:9 equilibrium mixture.
3. Mechanistic Generalities. Immediate observation of black
silver metal implies that electron transfer is fast. To think of the
detailed first mechanistic step, it may be that an adduct, (PNP)
NiAgOTf, forms with a dative bond from dz2 of Ni to Agþ, and
this collapses by inner sphere electron transfer to something
where the contained atomic Ag0 must now aggregate. Either of
these species has an odd number of electrons so it would be 31P
NMR silent. However, we find that reaction of (PNP)Ni with
equimolar [Cp*2Fe]OTf in benzene gives results identical to
those with silver triflate: both the intermediate and the final
product are seen within 1 h of combining the reagents. Thus, any
Figure 1. ORTEP view (50% probabilities) of the structure of [tBu2-
PCH2SiMe2NSiMe2(O3SCF3)]Ni[tBu2PCH2] with hydrogens omitted
for clarity. Unlabeled atoms are carbons. Selected structural parameters:
Ni-C7, 1.9598(14) Å; Ni-N1, 1.9784(11) Å; Ni-P1, 2.1933(4) Å;
Ni-P2, 2.2018(4) Å; P1-C7, 1.7516(14)Å; C7-Ni-P2, 97.72(4)ꢀ;
Ni-P1-C7, 58.30(5)ꢀ; Si1-O1-S1, 135.57(7)ꢀ; N1-Ni-P1,
49.74(3)ꢀ.
P-ylide with donation from the PdC bond and from a P-Ni σ
bond or as a C-metalated phosphine accompanied by P lone pair
donation; in either case the ligand is a 3-electron donor (neutral
formalism) and the nickel is divalent. The nickel is rigorously
planar and deviations from “square” planar are the angle N1-
Ni-C7, which is 169.09(5)ꢀ, and also P1-Ni-P2, at 147.216-
(15)ꢀ. A 1H NMR doublet with the unusual chemical shift of -
0.5 ppm is assigned to the CH2 protons of the ylide ligand.
2. Identification of an Intermediate. Earlier time observa-
tions (5 min after reagent mixing) of the solution 31P NMR
spectrum show that the reaction involves one detectable inter-
mediate, having an AX 31P{1H} NMR pattern with very different
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Ag/Ni direct bonding is not uniquely responsible for the Si-C
cleavage and the Si-O bond formation. Instead it occurs follow-
ing outer sphere electron transfer, within a species of composi-
tion (PNP)Niþ, partnered somehow with triflate.
point) intensity 4 peak at -0.26 ppm attributed to the coalesced
CH2 groups. This negative chemical shift for the CH2 protons is
consistent with M/CH connectivity in an M/C(sp3) species32-34
and shares that feature with (tBu2PCH2SiMe2NSiMe2OTf)Ni
(CH2PtBu2). Altogether, these indicate that an ether-free struc-
ture is preferred thermodynamically over coordination of THF.
After 12 h at room temperature, this solution has transformed
into a transparent glassy solid of polymeric THF.35 The 31P{1H}
NMR spectrum of this sample now shows narrow lines as an AX
pattern with chemical shifts located within 4 ppm of those of
(tBu2PCH2SiMe2NSiMe2OTf)Ni(CH2PtBu2), consistent with
Other Synthetic Approaches. We next carried out reactions
designed to make authentic (PNP)NiOTf by a nonredox route:
following synthesis of (PNP)NiF (see Experimental Section),
this was reacted with equimolar Me3SiOTf in benzene at 25 ꢀC.
There is a rapid color change, and NMR spectra recorded within
10 min of mixing show only the presence of the kinetic (i.e., P
mutually cis, A) isomer (tBu2PCH2SiMe2NSiMe2OTf)Ni(CH2-
PtBu2); after 3 h, the kinetic isomer and the crystallographic
isomer are equally populated. This we believe indicates that
either (PNP)NiOTf itself or the cation in any species [(PNP)
Niþ][SiMe3F(OTf)-] is exceptionally reactive at the silicon of
PNP, to permit triflate to attack as a nucleophile. Our belief that a
cationic Ni species must be involved comes from the observation
that (PNP)NiCl is recovered unchanged after 2 days in the
presence of equimolar NaOTf in THF: free triflate does not
attack Si in four-coordinate (PNP)NiCl.
0
cleavage of the ligand Si-C bond. The JPP value, 198 Hz, is
large enough to indicate trans positioning of these two nuclei. We
suggest that the R group in the resulting (tBu2PCH2SiMe2-
NSiMe2OR)Ni(CH2PtBu2) is the poly-THF chain. On the other
hand, (PNP)NiBArF survives for at least 6 days in CD2Cl2
4
solvent, so there is no chloride abstraction by this electrophilic
nickel species.
Because of the polymerization of THF, we moved to more
inert solvents (for a preliminary account, see ref 1). Reaction of
Si-C Cleavage Mechanism. Why/how does this unusual
reaction occur, to form a structure isomeric with the more
conventional product, (PNP)Ni(OTf)? Why is this fundamen-
tally rearranged species formed, and not simply (PNP)Ni
(OTf)?25-27
(PNP)NiCl with 5-fold excess of NaBArF in benzene (ArF =
4
3,5-(CF3)2C6H3) gives no change over 15 h; a polar solvent is
needed. NMR of the product formed from this reaction in
CD2Cl2 shows three proton chemical shifts but absolutely no
31P{1H} NMR signal at 25 ꢀC. Since the proton chemical shifts
were in the normal region, we felt that the absence of a phos-
phorus NMR signal was not due to paramagnetism. The 31P{1H}
NMR spectrum recorded at -60 ꢀC shows two equal intensity
signals, each about 88 Hz wide at half height. At -40 ꢀC these
have each broadened significantly, indicating that the lack of 31P
NMR signal is due to room temperature being at the coalescence
temperature. The room temperature 1H NMR spectrum shows
apparent C2v symmetry (although the tBu signal is a doublet, not
a virtual triplet), and only two aryl signals are seen (ortho and
para), suggesting all four aryl rings are equivalent. Since one
possible structure would have one pendant phosphine and
achieve an 18-electron configuration by η6 binding of one aryl
ring, we looked at low temperature 1H and 19F NMR spectra in
CD2Cl2. This showed no aryl region decoalescence by 1H NMR
and only a single signal by 19F NMR. The mechanism of
fluxionality leading to the coalescence at 25 ꢀC was probed by
We initially expected that the electrophilicity of (PNP)Niþ
would have its influence on the N-Si bond. Although Si-O
bond formation might have been anticipated, the fact that it
happens with the weakly nucleophilic triflate is surprising. In
close proximity to this (PNP)Niþ cation is triflate, which at least
in benzene can be quite tightly ion paired (hence proximate for
significant time) and also nucleophilic. It thus becomes capable
of unusual triflate behavior, attack on Si. This still requires an
explanation of selectivity of the bond cleaved: Si-N vs Si-C.
Here selectivity is perhaps controlled by the potential for product
stabilization: Si/N cleavage leaves nitrogen with a single sub-
stituent, which is oxidation (an imine, D), while Si-C cleavage
leaves CH2, which can be stabilized by binding to nickel. It is also
relevant that, in (PNP)Niþ, this is no ordinary CH2, but one
carrying an electrophilic substituent: four-coordinate phosphorus
and thus analogous to a phosphonium cation. This stabilizes the
evolving carbanion by ylide character.
adding NaBArF4 to a CD2Cl2 solution of (PNP)NiBArF . This
4
showed only one kind of aryl ring (para and ortho signals) and
one signal for CF3, indicating that free and any hypothetical
“coordinated” BArF are indistinguishable. Since the 19F NMR
4
spectrum did not decoalesce at -60 ꢀC, and since the 31P{1H}
0
NMR spectrum could be resolved to yield a JPP value of 38 Hz
at -55 ꢀC, we abandoned the idea of coordinated arene and
considered instead that both phosphorus are bound to Ni, but
not in a trans stereochemistry. To establish additionally that the
Toward Naked (PNP)Niþ. Given our suspicion that cationic
(PNP)Niþ might have exceptionally electrophilic character,9,28-31
we sought to make this species in the absence of a nucleophilic
anion. Reaction of (PNP)Ni with equimolar [Cp*2Fe][B(C6-
F5)4] in d8-THF was carried out to evaluate the possibility of
coordination of THF to (PNP)Niþ. Reaction of (PNP)Ni with
[Cp*2Fe]BPhF4 (PhF = C6F5) in THF immediately gives Cp*2Fe
and a product with no 31P{1H} NMR signal, ruling out the
formation of the adduct (PNP)Ni(THF)þ. The primary product
(PNP)NiBArF product neither is paramagnetic nor has some
4
cleavage of the PNP backbone, it was reacted with CO. This
reaction went to completion in the time of mixing, to form a
species whose characteristics are fully in agreement with C2v
symmetric (PNP)Ni(CO)þ, with a νCO value of 2044 cm-1
.
Such a high νCO indicates high electrophilicity of the T-shaped
(PNP)Niþ moiety.36,37 Note that the 1-electron reduced species
(PNP)Ni(CO) has a νCO of 1940 cm-1 38
.
first produced here (31P NMR silent) is suggested to be (PNP)
Solvent choice is essential to the synthesis of (PNP)Niþ from
(PNP)NiCl. Whereas chloride is abstracted from (PNP)NiCl by
1
NiBPhF . The H NMR spectrum of this solution shows a
4
t
doublet for Bu (hence not trans stereochemistry), one singlet
NaBArF in fluorobenzene or in dichloromethane, it is not in
4
for SiMe, and a broad (hence fluxional, near the decoalesence
THF. Indeed (PNP)Niþ reacts with NaCl in THF to form
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summary, the ground state structure of singlet (PNP)Niþ
involves a highly activated Si-CH2 bond (stretched to 2.07
Å), which is susceptible to attack even by a nucleophile as weak as
triflate. This structure naturally explains the puzzling selectivity
for cleavage of the Si-CH2 bond rather than the Si-N bond.
The nickel in (PNP)Niþ is thus highly electrophilic, which is the
origin of the unusual ligand backbone coordination and cleavage;
Si-C bond cleavage stabilizes a species with an activated Si-C
bond and the Ni/C bond that must form upon Si-C bond
scission is already attached to nickel. Attack (backside?) by X-
on this silicon will complete the formation of the primary
product, (tBu2PCH2SiMe2NSiMe2X)Ni(CH2PtBu2), detected by
NMR spectroscopy. This transannular interaction of the Si-C
bond with nickel has its precedents in agostic Si-C bonds,39 as
well as a transannular interaction in the olefin metathesis inter-
mediate metallacycle (R3P)Cl2RuCH2CH2CH2.40
Figure 2. DFT geometry optimized structure of (left) (PNP)Niþ,
showing Mayer bond orders of two Si-C bonds, and (right) one
bond-cleaved species. Hydrogens not shown.
(PNP)NiCl. This is apparently due to a combination of the
higher solubility of NaCl in THF, as well as the fact that the
electrophilicity of sodium in NaBArF4 is diminished by coordina-
2. T-Shaped and Si-C-Cleaved Alternatives. DFT(PBE)
calculations on several other structures revealed some surprises.
Three-coordinate T-shaped (PNP)Niþ is calculated to have a
triplet state nearly degenerate with the singlet;41 we have found
that the higher spin state is very often favored in PNP species as a
result of the low coordination number leaving numerous orbi-
tion of THF to naked Naþ. If [(PNP)Ni]BArF is synthesized
4
from (PNP)NiCl and NaBArF4 in CD2Cl2 and then separated by
filtration from solid NaCl, and then the resulting (PNP)Niþ is
1
dissolved in THF, the H NMR signature of (PNP)Niþ is
1
tals empty. Indeed, the normal H NMR chemical shifts of
evident immediately after dissolution. At longer times, this
THF solution of (PNP)Niþ shows polymerization of the THF
to a solid but still narrow-line 31P{1H} NMR spectrum as an AX
(PNP)Niþ at 25 ꢀC show that the triplet state is not responsible
for the lack of 31P NMR signal at 25 ꢀC. The singlet here lies
much closer to the ground state than it does for the isoelectronic
cobalt analogue, Co(PNP), where the singlet lies 21 kcal/mol
higher. Geometry optimization of the singlet species where the
Si-CH2 bond has been cleaved showed that this (Figure 2, right)
is only 13.9 kcal/mol higher than the triplet three-coordinate
species. The Ni-CH2 bond has been fully formed in this species,
and the interesting feature is how silicon responds to being three-
coordinate. The silicon is coplanar with its three substituents and
is eclipsed with the amide nitrogen plane and hence suitable for π
overlap with the amide lone pair. In fact this Si-N bond length is
0.14 Å shorter than the other one in this structure, consistent
with Si-N multiple bonding. Geometry optimization from a
starting geometry where the SiMe2 group of the Si-C cleaved
product (Figure 2, right) is rotated 90ꢀ, to diminish such π
bonding, minimizes to reform the Si-CH2 bond, yielding the
σSi-C complex42-45 discussed above. A triplet state was also
located for the Si-C cleaved species. It has a pseudotetrahedral
coordination geometry, like classic four-coordinate NiII triplets,
presumably because of occupancy of an x2-y2 orbital, which is
strongly σ* in a planar structure. However, its energy, þ43.4
kcal/mol, makes it clear that the chemistry here must proceed
further on the singlet surface.
0
pattern with JPP = 200 Hz. At shorter polymerization reaction
observation times, the lines in the AX NMR pattern have
multiple, closely spaced components, which we attribute to
heterogeneity of molecular weights coming from varying poly-
mer chain lengths attached to the pendant silyl group: (tBu2-
PCH2SiMe2NSiMe2ORn)Ni(CH2PtBu2), where Rn are these
diverse polymer chain lengths.
An indication of the steric protection offered by the PNP
1
ligand is available in that the SiMe H NMR signals of a 1:1
mixture of (PNP)Niþ and (PNP)NiCl in CD2Cl2 shows sepa-
rate resonances at 25 ꢀC. Thus there is neither formation of a
halide-bridged species nor exchange of the chloride between the
two nickels, which is rapid on the 1H NMR time scale.
Insight from DFT Energies and Structures. 1. Ground State
Structure. Survey of a variety of structures for (PNP)Niþ by
DFT(PBE) calculations gave the surprising result that the global
minimum singlet structure is Figure 2 (left). This has short
distances from both Si1 (2.53 Å) and C1 (2.22 Å) to Ni; the
metal satisfies its unsaturation by recruiting the Si-C bonding
electron density as a donor. This is appropriately called a σSiC
complex. In trying to understand how such an unconventional
structure might be a minimum and be even more stable than the
T-shaped alternative, we recognized that phosphorus in this
structure has migrated and also reoriented its lone pair, to overlap
better with the empty x2-y2 orbital that lies along the y axis
(trans to N); in doing so, it pulls both Si and CH2 closer to nickel,
a donation from the Si-C bond to the lobe of the nickel x2-y2
directed along the x axis. Thus the phosphorus lone pair seeks out
the best direction for overlap, not simply pointing toward the
metal nucleus. Symptomatic of this orientation is that phos-
phorus is nearly coplanar with nickel and the two tBu quaternary
carbons. This C-Si distance has lengthened 0.16 Å compared to
the other corresponding bond in this structure, and the Si-C1-
P2 angle is large (135ꢀ). The angles Ni-P2-C(tBu) are all very
large (122ꢀ), indicative of distortion of bonding at P2. Mayer
bond orders are shown in Figure 2 and support the idea that short
distances indeed involve partial chemical bond formation. In
X-ray Diffraction Structure Determination of [(PNP)Ni]BArF4..
The results and conclusions from this computational study of
(PNP)Niþ were subsequently confirmed by an X-ray structure
determination of the product from reaction of anhydrous Na-
BArF with (PNP)NiCl in fluorobenzene solvent (Figure 3);
4
once formed, this species can be dissolved without change in
CH2Cl2, and it can be formed even with stoichiometric NaBArF .
4
The X-ray result confirms the short contacts between Ni and
CH2 carbon and one Si and lengthening of that Si-C
bond.22,46-49 The unusual angles Ni-P-C(tBu) are also con-
firmed; the unconventional phosphorus has the shorter distance
to nickel. The short contacts to Ni come at the expense of the
unusual angle Ni-N1-Si1 = 89.3ꢀ, without distorting the H-
C-H and Me-Si-Me angles at the unusual C and Si more than
5ꢀ from 109ꢀ. It is noteworthy that this Si-C bond donation is
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Figure 4. LUMO of the σSi-C structure found for (PNP)Niþ.
we suspected that the two products were [(tBu2PCH2SiMe2-
NSiMe2F)]Ni(CH2PtBu2)] and its adduct with the available BF3
attached to the SiF fluorine. The fluorine NMR spectrum of the
product solution shows one broadened quartet, assigned to the
BF3 on SiF, one broadened F at 1/3 the intensity, assigned to that
Si-F, and (at higher intensity) one sharp doublet of multi-
plets, assigned to the fluorine in boron-free [(tBu2PCH2SiMe2-
NSiMe2F)]Ni(CH2PtBu2); the doublet splitting is due to P, and
the smaller splitting is due to the 6 protons of the near SiMe2
Figure 3. ORTEP view (50% probabilities) of the structure of
[(tBu2PCH2SiMe2)2N]Niþ, from its BArF4 salt, with hydrogens omitted
for clarity. Unlabeled atoms are carbons. Selected structural parameters:
Ni1-N1, 1.866(2) Å; Ni1-P1, 2.1201(8) Å; Ni1-P2, 2.189(6) Å;
Ni-C3, 2.158(3) Å; Ni1-Si1, 2.494(2) Å; Ni-Si2, 3.135(1) Å; Si2-
C14, 1.854(6) Å; Si1-C3, 2.069(4) Å; C3-P1, 1.840(3) Å; C14-P2,
1.818(17) Å; P1-Ni1-P2, 124.6(6)ꢀ; Si1-N1-Ni1, 89.27(13)ꢀ;
Si2-N1-Ni1, 122.81(14)ꢀ; N1-Si1-C3, 103.92(16)ꢀ; P1-C3-
Si1, 135.50(18)ꢀ.
1
energetically preferred to any C-H agostic donation or to ligand
tBu C-H cleavage, either heterolytic (H migrating to amide N)
or oxidative addition to metal.
group. Indeed this SiMe is a doublet in its H NMR spectrum
with the same J value (6 Hz) found in the 19F NMR spectrum.
The fact that the fluorine and 31P and 1H NMR spectra of this
mixture show separate resonances for [(tBu2PCH2SiMe2N-
SiMe2F)]Ni(CH2PtBu2)] and its BF3 adduct means that any
migration of BF3 to a different Si-F is slow on the NMR time
scale; this is supported by the fact that P-F coupling is retained
(i.e., spin correlation persists) in the signal of one of the two
phosphorus nuclei in the BF3 adduct. The ESIþ mass spectrum
of this sample, dissolved in THF, shows a molecular ion for
protonated [(tBu2PCH2SiMe2NSiMe2F)]Ni(CH2PtBu2), with
the nickel isotope pattern but a stronger peak for protonated
[(tBu2PCH2SiMe2NH)]Ni(CH2PtBu2), a species where the
SiMe2F group has been removed and replaced by one proton.
While the BF3 adduct discussed above does not survive ESI-MS
injection conditions in this THF solution, the identity of this
more abundant N/Si-cleaved ion is supported by crystals formed
The phosphorus site exchange detected by 31P NMR is then
explained by reversing the roles of the two phosphine arms
rapidly at room temperature via the T-shaped structure. We
located a transition state between the σSi-C complex and the
T-shaped (see Supporting Information), verified by one imagi-
nary frequency, which is associated with motion along the Ni-C1
line. It lies above the σSi-C complex isomer by 11.9 kcal/mol, a
magnitude that is consistent withthe NMR coalescence. ThisTSis
not the T-shaped species, since that is already verified to be a
minimum. The misdirection of the P2 lone pair becomes evident
with the loss of the Si-C donation since the Ni-P2 distance is
longer (by 0.16 Å) than in either of the adjacent minima. On going
to the TSfrom the σSi-C complex, the Ni-C1 distance isthe most
changed parameter, lengthened by 0.69 Å, with Ni-Si lengthened
by 0.28 Å and C1-Si shortened by 0.12 Å. At the TS, —P1-Ni-
P2 has increased by 28ꢀ, and —Ni-N-Si1 has increased by 13.6ꢀ.
in a reaction of equimolar (PNP)NiF and BF3 OEt2.
3
A single crystal X-ray structure determination of this product
shows (Figure 5) that the silyl group has been lost, probably as
SiMe2F2, and the amide nitrogen has been doubly protonated,
hence the compound is the BF4 salt [(tBu2PCH2SiMe2NH2)]
Ni(CH2PtBu2)BF4. This is the 1:2 product of reaction of (PNP)
0
The small JPP value in the decoalesced spectrum is due to these two
nuclei being much less than 180ꢀ apart.
The LUMO of the σSi-C complex (Figure 4) shows that
orbital has Si-C participation; however, this is not on the back
side of silicon but is instead along the Si-C internuclear line, and
thus, as observed experimentally, a nucleophile (triflate or fluo-
ride or THF) will attack perpendicular to that bond, not at the
back side of the silicon, anti to the Si-C bond.
Niþ with adventitious HF in the available BF3 OEt2. The struc-
3
ture of this molecule shows it to be a hydrogen-bonded pair, with
one fluorine hydrogen-bonded to one NH2 proton. The second
NH2 proton hydrogen bonds to a different symmetry-related
BF4, and thus both F2 and F4 are involved in hydrogen bonding
in a chain motif. The B-F distances to each of these two
fluorines is longer by 0.024 Å (2.4 σ) and 0.039 Å (3.9 σ) than to
the other two uninvolved F. The two phosphorus nuclei are
located trans. Nickel is coplanar with its attached P1, P2, N1, and
C12 (angles sum to 359.98ꢀ). The P2-C12 distance in
the tBu2PCH2 ligand is shorter than that in the five-membered
ring, and the P2-Ni distance is shorter than that in the five-
membered ring.
BF3 Catalysis of Si-C Cleavage. We wanted to probe the
hypothesis that even a weaker nucleophile could cleave the
stretched Si-C bond of naked (PNP)Niþ. One approach to
this would be to use a Lewis acid to abstract fluoride from
(PNP)NiF. If the Lewis acid would then shuttle fluoride to
silicon and cleave the ligand Si-C bond, the Lewis acid would be
regenerated (Scheme 4), making the rearrangement/isomeriza-
tion catalytic in Lewis acid. We chose BF3 OEt2 for this purpose.
3
Indeed, in time of mixing at 25 ꢀC in benzene, 30 mol %
BF3 OEt2 converts (PNP)NiF completely into two molecules
DFT(PBE) calculations on the species in Scheme 4 show that
BF4- coordinates to (PNP)Niþ, so separated ions are unneces-
sary for the catalyzed fluoride transfer mechanism. The DFT
geometry optimization shows that the final product adduct with
BF3 is calculated to be more stable, by 17.2 kcal/mol in free
energy, when the BF3 binds to the amide nitrogen (Figure 6a),
3
(9:1 mol ratio), each with a 31P{1H} NMR spectrum that is an
AX pattern with additional splitting due to 19F. Both products
0
show a JPP value large enough to qualify as having transoid
location of the phosphorus nuclei. Given the product ratio and
the lack of a signal for BF3 OEt2 in the 19F NMR of the product,
3
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Scheme 4. BF3 Catalysis of F Migration, Showing Reaction Energies (kcal/mol)
product, (PN(H)P)NiHþ, exhibits a hydride triplet, and the
triflate salt slowly precipitates from the reaction solution as pale
yellow needles. The product formed has Cs symmetry from its
proton and phosphorus NMR spectra (the latter a doublet with
selective hydride coupling, establishing the presence of a single
hydride), which indicates the surprising reformation of a pincer
ligand by Si-C coupling. The 1H NMR spectrum of this isolated
solid, dissolved in THF, shows a broadened intensity 1 signal at
2.9 ppm, which we attribute to formation of an amine still
coordinated to nickel (Scheme 5).
The structure of [(PN(H)P)NiH]OTf was established by a
single crystal X-ray diffraction study. While a fully satisfactory
refinement model could not be found (see Supporting Infor-
mation), the connectivity was established and shows a planar
four-coordinate (PN(H)P)NiHþ moiety, with triflate hydrogen-
bonded to the amine proton. The overall idealized symmetry is
Cs, consistent with the spectroscopic data.
Figure 5. ORTEP view (50% probabilities) of the structure of
t
[tBu2PCH2SiMe2NH2(BF4)]Ni[tBu2PCH2] with hydrogens and Bu
methyls omitted for clarity. Unlabeled atoms are carbons. Selected struc-
tural parameters: Ni1-C12, 1.956(5) Å; Ni1-N1, 1.992(4) Å; Ni1-
P2, 2.1466(14) Å; Ni1-P1, 2.2272(15) Å; B1-F1, 1.359(9) Å; B1-F3,
1.360(9) Å; B1-F4, 1.383(9) Å; B1-F2, 1.399(8) Å; Si1-N1,
1.772(5) Å; C12-P2, 1.741(6) Å; C3-P1, 1.833(5) Å; C12-Ni1-
N1, 162.0(2)ꢀ; C12-Ni1-P2, 49.95(16)ꢀ; N1-Ni1-P2, 112.33(13)ꢀ;
C12-Ni1-P1, 104.49(17)ꢀ; N1-Ni1-P1, 93.21(13)ꢀ; P2-Ni1-P1,
154.44(6)ꢀ.
2. Mechanistic Studies. The reassembly of the cleaved ligand
is surprising on multiple counts. Apparently carbon of the CH2-
PtBu2 ligand is nucleophilic, retaining an ability to attack
the silicon carrying the good leaving group triflate. Support
for this comes from our ability to establish that it is the
(less populated) isomer with this carbon cis to the pendant
silyl group that disappears faster under H2, detectable since
the equilibrium between the two isomers is slower than the
reaction of the cis isomer with H2. The reaction is heterolytic
splitting of H2, made possible in part by the Bronsted
basic amide nitrogen. Conventional wisdom certainly would
have this happening from an H2 complex, since these are
known to be Bronsted acidic. If such an Ni(H2) inter-
mediate protonates nitrogen, the resulting reduced amide/
silicon donation may make four-coordinate silicon become
quite electrophilic and hence subject to attack by even a weak
carbon nucleophile. Recall also that the triflate-loss species
(Figure 2, right), where the SiMe2 plane is perpendicular to
the NiP2N plane, is not a stationary state but collapses to form
a bond between that Si and the ylidic CH2 carbon; thus any
removal of triflate from that silicon plausibly leads it to seek
out the electron density of that carbon. Once formed, an NH
group could hydrogen bond to neighboring silyl triflate, enhancing
its leaving group character. We suggest that hydrogenolysis of the
Ni-CH2 bond of (tBu2PCH2SiMe2NSiMe2OTf)Ni(CH2PtBu2)
rather than to the fluorine of the Si-F group (Scheme 4); such a
structure could also be in accord with our spectroscopic data. The
lesser thermodynamic stability of BF3 on F than on N is reflected
in the structural parameters of each calculated structure (see
Supporting Information). Boron is much less pyramidal when it
is attached to F on Si than when it is on N, and the B-F(Si)
distance is longer than the other three (terminal) B-F distances
by 0.49 Å. We found a stationary state (Figure 6b) that is an ion
pair between BF4- and the σSi-C complex geometry of (PNP)
-
Niþ, with the BF4 lying above the coordination plane and
hydrogen-bonded to a number of aliphatic hydrogens, as well as
interacting with nickel by one fluorine; this structure is an
attractive intermediate for delivery of fluorine to silicon in
nonpolar benzene. Note the σSi-C complexation already evident
in this structure.
Reaction of (tBu2PCH2SiMe2NSiMe2OTf)Ni(CH2PtBu2) with
H2. We next investigated how the Ni-CH2 bond of (tBu2-
PCH2SiMe2NSiMe2OTf)Ni(CH2PtBu2) might react with H2.
1. Product Characterization. (tBu2PCH2SiMe2NSiMe2OTf)
Ni(CH2PtBu2) in benzene reacts slowly under 1 atm of H2, with
the reaction essentially complete within 7 days at 25 ꢀC. The
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Figure 6. DFT geometry-optimized structures and energies (kcal/mol) of species relevant to Scheme 4.
Scheme 5
never occurs during the observed reaction, since once formed, a
tBu2PMe ligand should survive unaltered.
The slow rate of that reaction is then attributed to the difficulty of
removing triflate from silicon in b, Scheme 5. This may be slowly
initiated by acidic impurities, including the glass surface, but then
might become autocatalytic once some of the acid [(PN(H)P)
NiH]OTf is formed.
How can this ligand reassembly be “triggered” by H2? We
suggest (Scheme 5) that the Si-C bond formation happens
independent of the H2 and that the role of H2 is simply to trap the
reassembled isomeric form. That is, we propose that the Si-C
cleaved neutral species is in spectroscopically undetectable
equilibrium with authentic (PNP)Niþ (b, Scheme 5). The obser-
vations establish that it is the cis isomer that reacts fast with H2
and that the trans isomer only slowly disappears as a result of its
slower rate of isomerization to the cis form (a, Scheme 5). Thus
the species in the rapid but endergonic equilibrium with (PNP)
Niþ is the cis isomer, where Si is proximate to the CH2. The
reassembly to form (PNP)Niþ is thus a reaction where triflate
finally shows its leaving group character, since its departure from
silicon is responsible for reforming the Si-C bond in the rever-
sible equilibrium that is shifted by H2. It is also true, since
(tBu2PCH2SiMe2NSiMe2OTf)Ni(CH2PtBu2) slowly polymerizes
THF, that this supports the presence of a trace equilibrium
amount of (PNP)Niþ.
To try to carry out a one-pot conversion of (PNP)Niþ with H2
and base to give (PNP)NiH, we also employed a base often
considered50 to be proton-specific, DBU (Scheme 6). In a preli-
minary control experiment, addition of DBU to a CD2Cl2
solution of [(PNP)Niþ]BArF4 in a 1:1 mol ratio gives immediate
conversion to a species with an AX 31P{1H} NMR spectrum,
0
with a JPP = 30 Hz, indicative of the two P being cis in a species
like that of (tBu2PCH2SiMe2NSiMe2X)Ni(CH2PtBu2) where
X = triflate (Scheme 6). Remarkably, this indicates that DBU
functions as a nucleophile, binding to onesilicon to give an iminium
substituent on that arm of backbone-cleaved PNP ligand. This
solution evolves, with a half-life of ∼12 h, with growth of a new
AX 31P{1H} NMR pattern with the large JPP (205 Hz) con-
0
sistent with the two phosphorus nuclei mutually trans, thus
indicating trans-(tBu2PCH2SiMe2NSiMe2DBU)Ni(CH2PtBu2)þ.
Each isomer has spectra in agreement with Cs symmetry, thus
two tBu doublets and two SiMe singlets, together with a doublet
for each isomer at negative chemical shifts, characteristic of a
CH2PR2 ligand. If this mixture of cis- and trans-(tBu2PCH2-
SiMe2NSiMe2DBU)Ni(CH2PtBu2)þ is subjected to 1 atm of H2
there is slow (9 d) disappearance of both isomeric cations
to form a mixture of (PNP)NiH and (PN(H)P)NiHþ. This
hydrogenolysis reaction is much slower than that of (PNP)Niþ
with H2, suggesting that there is an equilibrium as in Scheme 5
and that the excess DBU present shifts the equilibrium away
from (PNP)Niþ, hence slowing the overall conversion. For
comparison, equimolar NEt3 fails to deprotonate (PN(H)P)
NiHþ in an attempted one-pot synthesis from (PNP)Niþ and
NEt3 under H2; this also shows that NEt3 fails to attack the
Deprotonation of (PN(H)P)NiHþ. To test the hypothesis in c,
Scheme 5, reaction of [(PNP)Ni]BArF with 1 atm of H2 in
4
CH2Cl2 was observed to occur rapidly, to form the BArF4 salt of
the same cation discussed above, (PN(H)P)NiHþ. In its reaction
with H2, (PNP)Niþ thus shows addition of nucleophilic H- to
nickel, not to the Si-C bond. This cation was deprotonated
rapidly by LiNiPr2 in THF to give the neutral (PNP)NiH. While
this heterolytic splitting of H2 (vs oxidative addition) is to be
expected (Scheme 5c) on divalent nickel, it gets an assist from the
Bronsted basic character of the amide nitrogen. With this now
established as a rapid reaction of the intact (PNP)Niþ species,
this becomes attractive not only as the thermodynamic driving
force for the slow reaction of H2 with (tBu2PCH2SiMe2NSiMe2-
OTf)Ni(CH2PtBu2) but also for the mechanism described above.
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Scheme 6
Information). The several isomer energies are very densely
packed for nickel. Calculations (Figure 7) show that, beyond
what we considered earlier,51 isoelectronic but lower valent
(PNP)Co does indeed have a σSiC complex as a minimum, lower
in energy than the T-shaped singlet by 7.5 kcal/mol. It is
also lower than the oxidatively added species (PNP*)CoH by
2.5 kcal/mol. However it is higher in energy than triplet T-shaped
(PNP)Co by 15.7 kcal/mol, consistent with the fact that this is
the observed structure. Moreover, the (at least half) occupancy of
all d orbitals in the triplet state makes the σSi-C structure
implausible. In general, one needs to consider other factors for
Si-C coordination: the x2-y2 orbital must be empty (singly
occupied is not sufficient), the metal must be in an oxidation state
with satisfactory energy match between a d orbital and the σSi-C
bond orbital (i.e., the metal must be very electron-poor or
electrophilic), and the energy of the intramolecular C-H
cleavage isomer must be higher. Thus, going to more electron-
rich rhodium (Figure 8), the singlet T-shaped structure is
6.7 kcal/mol more stable that the triplet (due probably to larger
d-orbital splitting moving down a group) but the σSi-C structure
is higher in energy (but by a negligible 0.90 kcal/mol) than
(PNP*)RhH; this last structure is what is found experimentally.
We therefore speculate as follows. This effect of larger d orbital
splitting destabilizing the triplet state energy may favor the σ
complexation for (PNP)Mþ where M is Pd or Pt, if C-H
oxidative addition is still less stable there. d8 (PNP)Au2þ is even
more likely to adopt a σSi-C structure. The d6 configuration is
another way to leave x2-y2 empty, but our calculations (Figure 9)
show that the preference for low-coordinate 3d species to be high
spin trumps the σSi-C structure. Thus, although the σSi-C com-
plex isomer is competitive with various singlet and triplet Fe2þ
structures, quintet T-shaped (PNP)Feþ is most stable of all, so
the orbital half-occupancy in this structure precludes any σSi-C
complex being the ground state. Consistent with the lowest
reducing power of the 3d metal, the C-H oxidatively added
isomer is never energetically competitive. The d6 species Ru2þ
shows the quintet to be least stable, due to the large d orbital
splitting, but here the oxidatively added singlet is the global
minimum. Experimentally, for d6 (PNP)Ru(N2)(OTf), hetero-
lytic splitting of an H-C(sp3) bond52 dominates over Si-C
cleavage. The σSi-C structure of (PNP)Ruþ is competitive for
both singlet and triplet but is not the ground state. As is evident
from all these calculations, the best general conclusion is that
these late metal, high d electron count (PNP)Mqþ species have a
great variety of redox and geometric isomeric structures at com-
petitively similar energies, especially given the errors involved
in the calculations.
silicon of (PNP)Niþ. A 10-fold excess of NEt3 gives only
90% deprotonation.
’ DISCUSSION
We report four related (tBu2PCH2SiMe2NSiMe2X)Ni(CH2-
PtBu2) species here, with X = F, OTf, O-polymer, and FBF3. The
difference in 31P chemical shifts in a given species varies by only
7 ppm among these, and JPP varies by 50 Hz, or 20%. The
homology of these species is thus clear.
0
The pincer backbone bond cleavage described here shows a
vulnerability of the ligand, particularly the Si-C bond, to
nucleophilic attack. This redirects use of this electrophilic pincer
complex to non-nucleophilic reaction conditions; H2 is one
example. At the same time, the reformation of this bond in
Scheme 5, reaction b shows a remarkable thermodynamic proxi-
mity of the intact pincer; this equilibrium represents an unusual
“self-healing” behavior which decreases the vulnerability to
pincer backbone damage.
Consistent with the idea that this Si-C cleavage reaction
derives from the electrophilicity of the N-Ni unit as a whole, we
have previously observed Si-C bond cleavage,51 forming E,
where the oxidant (eq 1) was ferricinium cation and the trapping
nucleophile was intramolecular (nitride). This can now be
seen as (PNP)CoNþ trapping electrophilic silicon with nitride,
as well as with phosphorus. In brief, d4 singlet (PNP)CoNþ is a
candidate for σSi-C complexation, which can trigger cleavage of
that bond.
’ EXPERIMENTAL SECTION
All reactions were performed in an argon glovebox or on a Schlenk
line using standard air-sensitive techniques. Solvent distillation was
carried out using either Na/benzophenone, CaH2, 4 Å molecular sieves,
a Grubbs-type purification system, or a combination of the four. Solvents
were degassed and stored in airtight glassware. [(tBu2PCH2SiMe2)2N]
Ni was prepared following the published38 synthesis. 1H NMR chemical
shifts are reported in ppm relative to protio impurities in the deuterated
solvents. 31P{1H} spectra are referenced to external standards of 85%
H3PO4 (at 0 ppm). NMR spectra were recorded with a Varian Gemini
2000 (300 MHz 1H; 121 MHz 31P), or a Varian Unity Inova instrument
(400 MHz 1H; 162 MHz 31P). Mass spectra were acquired on a PE-Sciex
API III triple quadrupole spectrometer. Gas reactions were carried out
on a calibrated gas line with the solution being first degassed.
Since 3d metals are harder to oxidize than their heavier group
analogues, this is an additional reason why some of the oxidation
occurs at the nickel pincer amide N in contrast to 4d and 5d
analogues; that is, the HOMO of the oxidized material has
greater amide participation for the 3d case. To the extent that the
nitrogen is partially oxidized, it becomes a poor leaving group.
This contributes to selectivity for Si-C, not N-C, bond rupture.
Broadened Consideration of σSiC Complexation. A key
question is the generality of this unusual phenomenon of Si-C σ
complexation. We have probed this with DFT(PBE) calculations
on some relevant comparison cases (Figures 7, 8, and 9, where
all cases are on the same energy scale; see also Supporting
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Figure 7. DFT energies of isomeric isoelectronic cobalt and nickel species in two spin states.
and dissolved in minimal dichloromethane. This solution was then
layered with pentane and slow diffusion over 2 days produced crystals
1
for X-ray diffraction studies. H NMR (25 ꢀC, CD2Cl2): 7.73 ppm
(s, ArH, 8H), 7.57 ppm (s, ArH, 4H), 1.50 ppm (d, JPH = 10.8 Hz, tBu,
36H), 0.43 ppm (s, SiMe, 12H), -0.22 ppm (vbs, CH2, 4H). 31P{1H}
NMR (25 ꢀC, CD2Cl2): no signal. 19F NMR (25 ꢀC, CD2Cl2):
-63.1 ppm (s). 31P{1H} NMR (-55 ꢀC, CD2Cl2): 102.5 ppm (d,
JPH = 35 Hz), 41.3 ppm (d, JPH = 41 Hz). This reaction, carried out in
CH2Cl2 over 30 min, gave an isolated yield of 83%.
[(tBu2PCH2SiMe2)2N]Ni(CO)]B[3,5-(CF3)2C6H3]4. A solution
of [(tBu2PCH2SiMe2)2N]NiCl 15 mg (0.030 mmol) in 3 mL of fluoro-
benzene was prepared. To this solution was added 30 mg (0.034 mmol)
of NaB[3,5-(CF3)2C6H3]4. An atmosphere of CO was placed over the
solution. The solution changed color from red to yellow within 12 h. The
solvent was stripped to dryness, and CD2Cl2 was added for NMR
analysis. This solution was then placed in a solution IR cell for IR ana-
lysis. 1H NMR (25 ꢀC, CD2Cl2): 7.74 ppm (s, ArH, 8H), 7.58 ppm (s,
ArH, 4H), 1.43 ppm (t, JPH = 7.6 Hz, tBu, 36H), 1.27 ppm (t, JPH = 6 Hz,
CH2, 4H), 0.28 ppm (s, SiMe, 12H). 31P{1H} NMR (25 ꢀC, CD2Cl2):
90.2 ppm (s). 19F NMR (25 ꢀC, CD2Cl2): -63.2 ppm (s). IR (Dichloro-
methane): 2044 cm-1. This reaction, repeated using isolated [(tBu2-
PCH2SiMe2)2N]Ni]B[3,5-(CF3)2C6H3]4 in CH2Cl2, gave an isolated
yield of 90%.
Figure 8. DFT energies of isomeric (PNP)Rh complexes, including one
triplet species.
Reaction of (PNP)Ni with [Cp*2Fe]B(C6F5)4. Fifteen milli-
grams (0.030 mmol) of (PNP)Ni was dissolved in 1 mL of d8-THF in
a J-Young NMR tube. To this solution was added 30 mg (0.030 mmol)
of Cp*2FeB(C6F5)4. An immediate color change was visible from pale
[(tBu2PCH2SiMe2)2N]NiF. Fifteen mg (0.028 mmol) of (PNP)
NiCl was dissolved in 10 mL THF in a Schlenk flask. To this solution
were added 12.6 mg (0.084 mmol) of anhydrous Me4NF and 42.6 mg
(0.28 mmol) of CsF, and this was stirred for 12 h. The solution color
changed from red to a golden yellow during this time. The solution was
filtered, the solvent was stripped, and the resulting solid was dissolved
in C6D6 for NMR spectral analysis. Subsequently dissolving the solid
in minimal toluene, then slowly removing the toluene under vacuum
1
yellow to orange. The H and 31P{1H} NMR spectra were taken of
the solution after 1 h. 1H NMR (25 ꢀC, d8-THF): 1.43 ppm (d, JPH
=
13.5 Hz, tBu, 36H), 0.40 ppm (s, SiMe, 12H), 0.27 ppm (bs, CH2, 4H).
There is no detectable signal in the 31P{1H} NMR spectrum at this time,
consistent (see below) with the species identity (PNP)Niþ. After 12 h
the solution had polymerized and a 31P{1H} NMR spectrum was taken
1
resulted in crystal formation. H NMR (25 ꢀC, C6D6): 1.47 ppm (t,
of the solid mass. 31P{1H} NMR (25 ꢀC, d8-THF): 54.6 ppm (d, JPP
198 Hz), -33.8 (d, JPP = 198 Hz).
=
JPH = 7.5 Hz, tBu,36H,), 0.38 ppm (t, JPH = 4.8 Hz, CH2, 4H), 0.25 (s,
SiMe, 12H). 31P{1H} NMR (25 ꢀC, C6D6): 34.2 ppm (d, JFP = 28.3 Hz).
19F NMR (25 ꢀC, C6D6): -160.8 ppm (t, JPF = 27.6 Hz).
[(tBu2PCH2SiMe2)2N]Ni]B[3,5-(CF3)2C6H3]4. A solution of
[(tBu2PCH2SiMe2)2N]NiCl, 15 mg (0.030 mmol), was prepared
in 3 mL of fluorobenzene. To this solution was added 30 mg
(0.034 mmol) of NaB[3,5-(CF3)2C6H3]4. The solution changed
color from red to orange within 12 h. The solvent was stripped to
dryness, and pentane was added. (PNP)NiBArF4 oiled out in pentane
and was separated by pipet. This oil was then dissolved in CD2Cl2,
and NMR spectra were recorded. This was then stripped to dryness
[(tBu2PCH2SiMe2NSiMe2OTf)]Ni(CH2PtBu2)]. 1. From (PNP)
Ni þ AgOTf. To a solution of 15 mg (0.030 mmol) of [(tBu2-
PCH2SiMe2)2N]Ni in 1 mL of C6D6 was added 8 mg (0.031 mmol)
of AgOTf in a vial at 25 ꢀC. The solution darkened, and a black
precipitate formed after 30 s. The solution was filtered, and the filtrate
was orange (distinctly different from the pale yellow of the starting
material). After 48 h the solution was stripped to dryness, and minimal
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Figure 9. DFT energies of isomeric, isoelectronic Fe vs Ru PNP complexes in three spin states.
toluene was added. Toluene was slowly pumped off at 25 ꢀC. Small
crystals formed suitable for X-ray diffraction studies.
18H), 1.23-1.26 ppm (m, CH2, 4H), 0.48 ppm (s, SiMe, 6H), 0.39 ppm
(s, SiMe, 6H), -22.06 ppm (t, JPH = 66.1 Hz, Ni-H, 1H). N-H proton
was not located. 31P{1H} NMR (25 ꢀC, CD2Cl): 63.2 ppm. 19F NMR
(25 ꢀC, CD2Cl2): -63.2 ppm (s).
2. From (PNP)Ni þ [FeCp*2]OTf. To a solution of 15 mg (0.030 mmol)
of [(tBu2PCH2SiMe2)2N]Ni in 0.5 mL of C6D6 was added 11 mg
(0.033 mmol) of [FeCp*2]OTf (1:1) in a J-Young NMR tube at 25 ꢀC.
NMR spectra were taken at various time increments. These spectra
were similar to those found with AgOTf. The equilibrium ratio of 1:9
between isomers A and B was reached at 48 h. Also detected was the
signal at 3.9 ppm of Cp*2Fe.
[(tBu2PCH2SiMe2)2N]NiH. 1. From [(PN(H)P)NiH]OTf and Et3N
in D8THF. Fifteen milligrams (0.023 mmol) of [(tBu2PCH2SiMe2)2
NH]NiH(OTf) was dissolved in 0.5 mL of deuterated THF. To this
solution was added a 10-fold excess (3.2 μL) of Et3N. The NMR of the
1
resulting yellow solution showed 90% conversion to (PNP)NiH. H
3. From (PNP)NiF þ Me3SiOTf. To a solution of 15 mg (0.029 mmol)
of [(tBu2PCH2SiMe2)2N]NiF in 0.5 mL of C6D6 was added 3-fold
excess of Me3SiOTf in a J-Young NMR tube at 25 ꢀC. The solution went
from golden brown to orange over 48 h. The spectra at this time were
similar to those of the 1:9 equilibrium ratio between isomers A and B.
Isomer A: 1H NMR (25 ꢀC, C6D6): 1.13 ppm (d, JPH = 14.7 Hz, tBu,
NMR (25 ꢀC, d8-THF): 1.30 ppm (t, JPH = 6.3 Hz, tBu, 36H), 0.95 ppm
(t, JPH = 6.6 Hz, CH2, 4H), 0.61 ppm (s, SiMe, 12H), -21.30 ppm
(t, JPH = 66 Hz, NiH, 1H). 31P{1H} NMR (25 ꢀC, d8-THF): 73.2 ppm.
CI-MS (THF solution, m/z): obsd 507.2548 [M]þ C22H53NNiP2Si2;
theory 507.2540.
2. From [(PN(H)P)Ni(H)]BArF and LiNiPr2 in THF. A solution of
4
18H), 1.09 ppm (d, JPH = 10 Hz, Bu, 18H), 0.64 ppm (s,SiMe, 6H),
(PN(H)P)NiH(BArF ) in CD2Cl2 obtained from 20 mg (0.039 mmol)
t
4
0.54 ppm (s,SiMe, 6H), 0.41 ppm (d, JPH = 6 Hz,CH2, 2H), 0.31 ppm
(d, JPH = 9.6 Hz,CH2, 2H). 31P{1H} NMR (25 ꢀC, C6D6): 61.6 ppm (d,
JPP = 27 Hz), 5.3 ppm (d, JPP = 27 Hz). Isomer B: 1H NMR (25 ꢀC,
C6D6): 1.2 ppm (d, JPH = 14.7 Hz, tBu, 18H), 1.1 ppm (d, JPH = 13 Hz,
tBu, 18H), 0.69 ppm (s,SiMe, 6H), 0.48 ppm (d, JPH = 12 Hz,CH2, 2H),
0.43 ppm (s,SiMe, 6H), -0.5 ppm (d, JPH = 5 Hz,CH2, 2H). 31P{1H}
NMR (25 ꢀC, C6D6): 58.4 ppm (d, JPP = 213 Hz), -34.0 ppm
(d, JPP = 213 Hz).
of (PNP)NiCl, 35 mg of NaB[3,5-(CF3)2C6H3]4, and 1 atm H2 (see
above) was dried in vacuum and dissolved in 3 mL of THF. Next, 4.2 mg
(0.039 mmol) of LiNiPr2 was added. After 10 min of stirring, all volatiles
were removed in vacuum. The residue was extracted with 2 ꢀ 10 mL of
pentane and filtered through a glass filter. Pentane was removed in a
vacuum, and the yellow solid was dissolved in C6D6 for NMR analysis.
1H NMR (25 ꢀC, C6D6): 1.23 ppm (t, JPH = 6.4 Hz, tBu, 36H), 0.82 ppm
(t, JPH = 4.8 Hz, CH2, 4H), 0.40 ppm (s, SiMe, 12H), -21.19 ppm
(t, JPH = 68 Hz, NiH, 1H). 31P{1H} NMR (25 ꢀC, C6D6): 73.8 ppm.
3. From (PNP)NiCl þ NaBH4. Fifteen milligrams (0.030 mmol) of
(PNP)NiCl was dissolved in 1 mL of THF in a J-Young NMR tube. To
this solution was added a slight excess 1.5 mg (0.039 mmol) of NaBH4.
[(tBu2PCH2SiMe2)2NH]NiH(OTf). A solution of (tBu2PCH2-
SiMe2NSiMe2OTf)]Ni(CH2PtBu2) (15 mg, 0.023 mmol) in 0.5 mL
of C6D6 was freeze-pump-thaw degassed. To this J-Young NMR
tube was added 760 mmHg of H2 on a vacuum line. After 1 week of
end-over-end agitation, the solution had changed color from orange
to yellow and a colorless precipitate had formed. The solution was
filtered, and the solid was dissolved in deuterated THF for NMR
spectral analysis. This solution was then layered with pentane to
allow for slow diffusion. From this solution, colorless needles were
grown for X-ray diffraction analysis. 1H NMR (25 ꢀC, d8-THF)
2.80 ppm (s, NH, 1H), 1.37 ppm (t, JPH = 7.5 Hz, tBu, 18H), 1.28 ppm
1
The H and 31P{1H} NMR were recorded after 12 h. These spectra
show production of known (PNP)Ni and (PNP)NiH (apparent from
hydride triplet at -21.3 ppm). Also a tBu doublet in the 1H NMR along
with a broad signal in the 31P{1H} is consistent with production of a
small amount of metal-free phosphine-borane adduct; this side reaction
correlates with the appearance of a black solid, attributed to nickel metal.
Reaction of (PNP)NiF with BF3 Et2O, forming [(tBu2PCH2-
3
SiMe2NSiMe2F)]Ni(CH2PtBu2)] and its BF3 adduct. 1. With a
t
(t, JPH = 7.2 Hz, Bu, 18H), 0.89 ppm (t, JPH = 6.3 Hz, CH2, 2H),
catalytic amount of BF3 Et2O. Fifteen milligrams (0.030 mmol) of
(PNP)NiF was dissolved in 1 mL of C6D6 in a J-Young NMR tube. To
0.59 ppm (s,SiMe, 6H), 0.34 ppm (s,SiMe, 6H), -22.37 ppm (t,
JPH = 68.4 Hz, NiH, 1H). The second CH2 peak is obscured by the
tBu peaks. 31P{1H} NMR (25 ꢀC, d8-THF): 62.9 ppm.
3
this solution was added 1 μL of BF3 Et2O (0.01 mmol, ∼30 mol %).
3
[(tBu2PCH2SiMe2)2NH]NiH(BArF ). A solution of 20 mg (0.039
The solution color changed from golden brown to orange red in 5 min.
4
1
mmol) of (PNP)NiCl and 35 mg (0.039 mmol) of NaB[3,5-(CF3)2-
C6H3]4 in 0.5 mL of CD2Cl2 was agitated end-over-end for 1 h to form
At 30 min the H and 31P{1H} NMR of the solution showed two
products (see NMR below). This solution was stripped to dryness and
dissolved in THF for mass spectral analysis. The solution was then
stripped to dryness again and redissolved in C6D6 for analysis by 1H and
31P{1H} NMR, to establish product lifetime. [(tBu2PCH2SiMe2N-
SiMe2F)]Ni(CH2PtBu2)] 1H NMR (25 ꢀC, C6D6): 1.34 ppm (d,
[(PNP)Ni]BArF . The solution was freeze-pump-thaw degassed, and
4
760 mmHg of H2 was added on a vacuum line. In time of mixing the
solution had changed color from red to yellow. NMR analysis showed
1
complete conversion into (PN(H)P)NiH(BArF ). H NMR (25 ꢀC,
4
t
t
CD2Cl2): 7.76 ppm (br.s, Ar-o, 8 H), 7.59 ppm (br.s, Ar-p, 4 H),
1.33 ppm (t, JPH = 7.0 Hz, tBu, 18H), 1.32 ppm (t, JPH = 7.0 Hz, tBu,
JPH = 14.2 Hz, Bu, 18H), 1.21 ppm (d, JPH = 12.3 Hz, Bu, 18H),
0.45 ppm (d, JFH = 6 Hz, SiMe, 6H), 0.41 ppm (s, SiMe, 6H), -0.47 ppm
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Journal of the American Chemical Society
ARTICLE
(d, JPH = 4.2 Hz, CH2, 2H); the second CH2 is not positively identified
due to overlap with silyl methyl groups. 31P{1H} NMR (25 ꢀC, C6D6):
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62.3 ppm (dd, JPP = 232, JPF = 6.2, Hz), -24.3 ppm (dd, JPP = 232, JPF
=
30.8, Hz). 19F{1H} NMR (25 ꢀC, C6D6): -116.3 ppm (m, JPF = 30.5
1
Hz, JHF 6 Hz). ESI-MS (m/z): 525.29 [M þ H]þ, 450.26 [M]þ. H
NMR of BF3 adduct (25 ꢀC, C6D6): 1.29 ppm (d, JPH = 14.5 Hz, tBu,
18H), 1.19 ppm (d, JPH = 12.8 Hz, tBu, 18H), 0.75 ppm (d, JFH = 5 Hz,
SiMe, 6H), 0.59 ppm (s,SiMe, 6H), -0.07 ppm (d, JPH = 6 Hz, CH2,
2H) the second CH2 is not positively identified. 31P{1H} NMR of BF3
adduct (25 ꢀC, C6D6): 45.1 ppm (d, JPP = 190 Hz), -48.7 ppm (d, JPP
=
191 Hz). 19F{1H} NMR of BF3 adduct (25 ꢀC, C6D6): -125.1 ppm
(q, JFB = 30 Hz), -125.6 ppm (s).
2. With equimolar BF3 Et2O. The experiment was carried out with
3
3 μL BF3 Et2O (1:1) ratio, to establish the identity of the minor product
3
above as a BF3 adduct. After 30 min H and 31P{1H} NMR were
1
recorded, which showed complete conversion to the BF3 adduct pro-
duced in smaller mole fraction above. Crystals were grown by slow
evaporation of solvent over several days and were identified by X-ray
diffraction as the product of hydrolysis of the BF3, from released HF.
Reaction with DBU and Its Impact on Hydrogenation
Reactivity. A solution of 20 mg (0.039 mmol) of (PNP)NiCl and
35 mg (0.039 mmol) of NaB[3,5-(CF3)2C6H3]4 in 0.5 mL of d8-THF
1
was stirred for 24 h. According to both 31P and H NMR there is no
reaction. THF was removed in vacuo, and the residue was redissolved in
0.5 mL of CD2Cl2. 1H and 31P NMR showed partial conversion (∼50%)
into (PNP)Ni[3,5-(CF3)2C6H3]4 after 24 h with vigorous stirring. Next,
0.0043 mL (4.4 mg, 0.029 mmol) of DBU (1,8-diazabicyclo[5.4.0]
undec-7-ene) was added via syringe. The solution was freeze-pump-
thaw degassed, and 760 mmHg of H2 was added on a vacuum line. 31
P
NMR of the solution in 10 min showed formation of the kinetic product
of attack of BDU at silicon. 31P{1H} NMR (25 ꢀC, CD2Cl2): 59.5 and
3.1 ppm (both d, J = 30 Hz). In 4 days one could see formation of the
second product, the DBU attack product on silicon with the two P
mutually trans. 31P{1H} NMR (25 ꢀC, CD2Cl2): 55.0 and -38.2 ppm
(both d, J = 205 Hz) together with some amount of the deprotonation
product of (PN(H)P)NiHþ, (PNP)NiH.
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S
Supporting Information. Full computational details along
b
with crystallographic details in CIF format. This material is
available free of charge via the Internet at http://pubs.acs.org.
(31) Castonguay, A.; Sui-Seng, C.; Zargarian, D.; Beauchamp, A. L.
Organometallics 2006, 25, 602.
’ AUTHOR INFORMATION
(32) Fischbach, A.; Bazinet, P. R.; Waterman, R.; Tilley, T. D.
Organometallics 2008, 27, 1135.
(33) Liang, L.-C.; Lin, J.-M.; Hung, C.-H. Organometallics 2003, 22,
Corresponding Author
caulton@indiana.edu
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’ ACKNOWLEDGMENT
We thank the National Science Foundation (NSF CHE-
0544829) for financial support. We thank Prof. Daniel Mindiola
for gifts of several useful oxidants for this work and Dr. Michael
Ingleson for useful discussion.
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