Basicities of nickel and allyl in [Ni(η3-C3H5)(PhP(CH2 CH2PPh2)2]+ and comparison of the effects of allyl and methyl on the basicity of nickel

Article abstract of DOI:10.1021/om0100535

Kinetic studies on the reaction between [Ni(η³-C₃H₅)(triphos)]⁺ (triphos = {Ph₂PCH₂CH₂}₂-PPh) and [lutH]⁺ (lut = 2,6-dimethylpyridine) in MeCN show that initial protonation of the nickel or allyl is followed by equilibration of the proton between the two sites. Analysis of the data allows calculation of the basicities of the nickel and allyl sites. This, together with earlier work on [NiMe(triphos)]⁺, shows the nickel site is (2.5-79) × 10⁶ times more basic when coordinated to allyl than to methyl.

Full text of DOI:10.1021/om0100535

Organometallics 2001, 20, 2579-2582
2579
Ba sicities of Nick el a n d Allyl in
[Ni(η3-C3H5)(P h P (CH2CH2P P h 2)2]+ a n d Com p a r ison of th e
Effects of Allyl a n d Meth yl on th e Ba sicity of Nick el
William Clegg, Graham Cropper, Richard A. Henderson,*
Christopher Strong, and Benjamin Parkinson
Department of Chemistry, Bedson Building, University of Newcastle,
Newcastle-upon-Tyne, NE1 7RU, U.K.
Received J anuary 23, 2001
Kinetic studies on the reaction between [Ni(η3-C3H5)(triphos)]+ (triphos ) {Ph2PCH2CH2}2-
PPh) and [lutH]+ (lut ) 2,6-dimethylpyridine) in MeCN show that initial protonation of the
nickel or allyl is followed by equilibration of the proton between the two sites. Analysis of
the data allows calculation of the basicities of the nickel and allyl sites. This, together with
earlier work on [NiMe(triphos)]+, shows the nickel site is (2.5-79) × 106 times more basic
when coordinated to allyl than to methyl.
In tr od u ction
Exp er im en ta l Section
All manipulations in the synthetic and kinetic aspects of
this work were performed under an atmosphere of dinitrogen
using Schlenk and syringe techniques, as appropriate. Triphos,
lutidine, NaBPh4, C3H5MgBr, and NiCl2‚6H2O were purchased
from Aldrich and used as received.
All solvents were dried and distilled under dinitrogen
immediately prior to use. MeCN was distilled from CaH2, thf
from Na/benzophenone, CH2Cl2 from P2O5, and diethyl ether
from Na. [lutH]BPh4 was prepared by the method reported in
the literature.7
The protonation of transition metal complexes con-
taining coordinated hydrocarbon residues is a funda-
mental reaction of importance in industrial processes1
and in the action of certain metalloenzymes.2 In general,
protonation at both the metal and carbon-based ligands
can be slow,3,4 and establishing which is the initial site
of protonation is often complicated by the ability of
protons to move between adjacent carbon and metal
sites. Depending on the complex, initial protonation at
the metal or the carbon-based ligand has been ob-
served,5 and there is currently no reliable method of
predicting the kinetically or thermodynamically pre-
ferred site of protonation.
P r ep a r a tion of [Ni(η3-C3H5)(tr ip h os)]BP h 4. To a sus-
pension of [NiCl(triphos)]BPh4 (0.75 g, 0.79 mmol) in thf (10
mL) was added C3H5MgBr (2.0 mL of 2 M solution in thf; 4.0
mmol). The solution turned dark red almost immediately and
was stirred overnight at room temperature. The next day any
excess Grignard reagent was destroyed by dropwise addition
of methanol. All volatiles were then removed in vacuo, and
the residue was dissolved in a minimum of CH2Cl2, then
filtered through Celite to remove insoluble material. Slow
diffusion of methanol into the solution produced red needles
of the product, which were removed by filtration, washed with
methanol, and then dried in vacuo. Yield ) 0.26 g; 35%. Anal.
Calcd for C61H58BNi P3: C, 76.8; H, 6.1. Found: C, 76.5; H,
6.0. 31P{1H} NMR (THF): 112.1 (t, J PP ) 48.6 Hz, central P);
48.2 (d, J PP ) 48.6 Hz, terminal P). 1H NMR (CDCl3): 6.3-
7.9 (m, 45, Ph of triphos and BPh4-); 5.0 (quin, 1, J HH ) 8 Hz,
CH2CHCH2); 3.3 (d, 4, J HH ) 8 Hz, CH2CHCH2); 2.6, 2.4 (br,
8, -CH2CH2-). The positions, multiplicities, and coupling
constants attributable to the η3-C3H5 ligand are fully in accord
with values in the literature for Ni(η3-allyl) complexes.8
Kin etic Stu d ies. All kinetic studies were performed on an
Applied Photophysics stopped-flow SX.18V spectrophotometer,
modified to handle air-sensitive solutions. The temperature
was maintained at 25.0 °C using a Grant LTD6G recirculating
thermostat tank.
Before it is possible to confidently predict whether
protonation is at the metal or hydrocarbon ligand, it is
necessary to understand (i) the factors that control the
rates of protonation and (ii) the relative basicities of the
two sites. We report herein kinetic studies on the
equilibrium protonation of [Ni(η3-C3H5)(triphos)]+ {triph-
os ) PhP(CH2CH2PPh2)2} with [lutH]+ (lut ) 2,6-
dimethylpyridine) in MeCN, which allows us to calculate
the difference in basicities of the nickel and η3-allyl sites.
These results (together with those of earlier studies6 on
[NiMe(triphos)]+) allow us to show that allyl and methyl
ligands have markedly different effects on the basicity
of the nickel site.
(1) Elschenbroich, C.; Salzer, A. Organometallics, A Concise Intro-
duction, 2nd ed.; VCH: Weinheim, 1992; Chapter 17, and references
therein.
(2) Evans, D. J .; Henderson, R. A.; Smith, B. E. In Bioinorganic
Catalysis, 2nd ed.; Reedijk, J ., Bouwman, E., Eds.; Marcel Dekker:
New York, 1999; Chapter 7, p 153, and references therein.
(3) Kramarz, K. W.; Norton, J . R. Prog. Inorg. Chem. 1994, 42, 1,
and references therein.
The kinetics were studied in dry MeCN under pseudo-first-
order conditions with [lutH+] and [lut] in at least a 10-fold
excess over the concentration of complex. Mixtures of [lutH]-
(4) Henderson, R. A. Angew Chem., Int. Ed. 1996, 35, 946, and
references therein.
(5) Åkermark, B.; Martin, J .; Nystro¨m, J .-E.; Stro¨mberg, S.;
Svensson, M.; Zetterberg, K.; Zuber, M. Organometallics 1998, 17,
5367, and references therein.
(6) Henderson, R. A.; Oglieve, K. E. J . Chem. Soc., Chem. Commun.
1999, 2271.
(7) Gro¨nberg, K. L. C.; Henderson, R. A.; Oglieve, K. E. J . Chem.
Soc., Dalton Trans. 1998, 3093.
(8) J olly, P. W.; Wilke, G. The Organic Chemistry of Nickel; Academic
Press: New York, 1974; Chapter VI, p 335, and references therein.
10.1021/om0100535 CCC: $20.00 © 2001 American Chemical Society
Publication on Web 05/08/2001

2580 Organometallics, Vol. 20, No. 12, 2001
Clegg et al.
BPh4 and lut were prepared from stock solutions of the two
reagents. All solutions were used within 1 h of preparation.
Under all the conditions described herein the absorbance-
time curves were a single exponential, with an initial absor-
bance which is that of [Ni(η3-C3H5)(triphos)]+ and a final
absorbance corresponding to the equilibrium mixture of [Ni-
(η3-C3H5)(triphos)]+ and [NiH(η3-C3H5)(triphos)]2+. The associ-
ated rate constants (kobs) were determined by a computer fit
to the exponential absorbance-time curve. In all cases the
curve was an exponential for more than 4 half-lives. The
dependence of kobs on [lutH+] and [lut] was determined
graphically, as presented in the Results and Discussion section.
X-r a y Cr ysta l Str u ctu r e Deter m in a tion of [Ni(SP h )-
(tr iph os)]BP h4. Crystallographic data for [Ni(η3-C3H5)(triphos)]-
BPh4: a ) 11.8500(5) Å, b ) 14.2338(6) Å, c ) 15.9500(7) Å,
R ) 80.287(2)°, â ) 69.588(2)°, and γ ) 80.221(2)° with Z ) 2
in space group P1. R1(F) ) 0.0298 for 8829 reflections with I
> 2σ(I), and Rw(F2) ) 0.0769 for all 10 316 unique data and
616 parameters; GOF ) 1.023 on F2. The data collection and
structure determination followed standard procedures, with
F igu r e 1. Structure of the cation in [Ni(η3-C3H5)(triphos)]-
BPh4 with 50% probability ellipsoids; triphos H atoms are
omitted. Important bond distances (Å) and angles (deg):
Ni-P(1) ) 2.1897(4), Ni-P(2) ) 2.1830(4), Ni-P(3) )
2.2935(4); Ni-C(36) ) 1.9898(15), Ni-C(35) ) 2.0618(16),
Ni-C(37) ) 2.0853(16); P(1)-Ni-P(2) ) 88.490(16), P(1)-
Ni-P(3) ) 104.448(17), P(2)-Ni-P(3) ) 90.154(16), P(1)-
Ni-C(36) ) 116.59(5), P(2)-Ni-C(36) ) 118.90(5), P(3)-
Ni-C(36) ) 128.75(6).
a
Bruker AXS SMART CCD diffractometer and Mo KR
radiation (θmax ) 28.6°, 44 882 reflections measured, 23 463
unique, semiempirical absorption corrections based on sym-
metry-equivalents), direct methods, and full matrix least
squares refinement on all unique F2 values. Programs were
Bruker SMART (data collection), SAINT (integration), and
SHELXTL (structure solution).
Resu lts a n d Discu ssion
Str u ctu r e of [Ni(η3-C3H5)(tr ip h os)]+. [Ni(η3-C3H5)-
(triphos)]BPh4 was prepared, in modest yield (ca. 30%),
by the route shown in eq 1.
of acid is crucial in ensuring that an equilibrium
reaction is observed. Use of stronger acids associated
with conjugate bases that can coordinate (e.g., anhy-
drous HCl) results in evolution of propene and formation
of [NiCl(triphos)]BPh4, probably because of coordination
of Cl- and/or addition of more than one proton to the
allyl complex.
triphos
[NiCl(triphos)]BPh
8
NiCl2‚6H2O
4
NaBPh4
V LiSPh
[Ni(SPh)(triphos)]BPh4 (1)
V C3H5MgBr
[Ni(η3-C3H5)(triphos)]BPh4
The exponential shape of the curves indicates that the
reaction exhibits a first-order dependence on the con-
centration of [Ni(η3-C3H5)(triphos)]+. This is confirmed
by experiments in which the concentration of [Ni(η3-
C3H5)(triphos)]+ was varied from 0.05 to 0.25 mmol
dm-3 while keeping the concentrations of [lutH]+ (10
mmol dm-3) and lut (5 mmol dm-3) constant. Under
these conditions the observed rate constant did not vary,
The structure of the cation in crystals of [Ni(η3-C3H5)-
(triphos)]BPh4 is shown in Figure 1. The coordination
geometry of the nickel is best considered as a distorted
tetrahedron comprising the three phosphorus atoms and
the central carbon of the η3-C3H5 residue. The important
bond lengths and angles are listed in Figure 1 and show
that there is nothing unusual about the structure.9
The crystal structure confirms the presence of the η3-
kobs ) 0.05 s-1
.
The dependence of kobs on the concentrations of
[lutH]+ and [lut] is complicated. Analysis of the kinetic
data is accomplished by considering two conditions: (i)
the variation of kobs with [lutH+]/[lut] at constant [lutH+]
(Figure 2, main) and (ii) variation of kobs with [lutH+]
at constant [lutH+]/[lut] (Figure 2, inset). Analysis of
these data gives the rate law in eq 2.
1
C3H5 ligand that was indicated by the H NMR spec-
trum. The angle C(35)-C(36)-C(37) ) 118° is in the
normal range,10 and the Ni-C distances are close to the
mean values reported for η3-C3H5 ligands.11
Rea ction of [Ni(η3-C3H5)(tr ip h os)]+ w ith [lu tH]+.
The reaction between [Ni(η3-C3H5)(triphos)]+ and an
excess of [lutH]+ and lut in MeCN is an equilibrium
process. This is most evident when the reaction is
studied on a stopped-flow apparatus. The absorbance-
time curves are exponential, but, most notably, the
magnitude of the curves increases with increasing
[lutH+]/[lut] (up to a maximum value when [lutH+]/[lut]
) ca. 2), as progressively more [Ni(η3-C3H5)(triphos)]+
is protonated. It is important to stress that the choice
3
+
)
-d[Ni(η -C3H5)(triphos) ]
dt
{(2.1(0.3) × 102[lutH+] + (1.0(0.2)}[Ni(η3-C3H5)(triphos)+]
(8.0(0.3) × 103[lutH+] + (4.0(0.2) × 102[lut]
(2)
The rate law shown in eq 2 is too complicated for a
single equilibrium protonation, but rather corresponds
to two coupled equilibria:12 the first step is protonation
of [Ni(η3-C3H5)(triphos)]+ and the second a unimolecular
rearrangement of the protonated species as shown in
(9) J olly, P. W.; Wilke, G. The Organic Chemistry of Nickel; Academic
Press: New York, 1974; Chapter VI, p 331, and references therein.
(10) Cotton, F. A.; Luck, R. L. Inorg. Chem, 1989, 28, 3210.
(11) Orpen, A. G.; Brammer, L.; Allen, F. H.; Kennard, O.; Watson,
D. C.; Taylor, R. J . Chem. Soc., Dalton Trans. 1989, 51.
(12) Wilkins R. G. Kinetics and Mechanisms of Reactions of Transi-
tion Metal Complexes, 2nd ed.; VCH: Weinheim, 1991; p 34.

Basicities of Nickel and Allyl
Organometallics, Vol. 20, No. 12, 2001 2581
F igu r e 2. Kinetics for the reaction of [Ni(η3-C3H5)(triphos)]+ with mixtures of [lutH]+ and lut in MeCN at 25.0 °C. All
kinetics were measured at λ ) 350 nm. Main figure: dependence of 1/kobs[lut] on [lutH+]/[lut] at constant [lutH+]. Data
corresponds to [lutH+] ) 1-20 mmol dm-3, [lut] ) 2.5 mmol dm-3 (2); [lutH+] ) 1-20 mmol dm-3, [lut] ) 5.0 mmol dm-3
(b); [lutH+] ) 1-20 mmol dm-3, [lut] ) 10.0 mmol dm-3 (9). Inset: dependence of 1/kobs[lut] on [lutH+] at constant [lutH+]/
[lut]. Data points correspond to [lutH+]/[lut] ) 1.0 (b) and [lutH+]/[lut] ) 2.0 (9). Lines and curves are those defined by
eq 2.
F igu r e 3. Mechanism for the protonation of [Ni(η3-C3H5)(triphos)]+ with [lutH]+ in MeCN.
Figure 3. The rate law associated with this mechanism
is shown in eq 3, when k-2 > k2.
and the alternative in which initial protonation of nickel
is followed by proton transfer to carbon. Studies on the
protonation of a variety of alkene, alkyne,5,13,14 and η3-
allyl15,16 complexes show that no generalizations can be
made about whether the metal or ligand is protonated
first. Nonetheless, the study on [Ni(η3-C3H5)(triphos)]+
still allows us to calculate the difference in basicities of
the nickel and carbon sites.
3
+
)
-d[Ni(η -C3H5)(triphos) ]
dt
{k1k2[lutH+] + (k2 + k-2)}[Ni(η3-C3H5)(triphos)+]
k1[lutH+] + k-1[lut]
Ba sicities of th e Nick el a n d η3-Allyl Sites. The
(3)
1
pKa of the protonated complex can be calculated
lutH
Comparison of eqs 2 and 3 yields k2 ) 0.03 ( 0.01 s-1
,
directly. In MeCN, the pKa
of [lutH]+ is 15.4,17 and
since K1 ) KalutH/Ka , pKa1 ) 16.7 can be calculated. Our
mechanism proposes that this is the pKa of coordinated
1
k-2 ) 0.97 ( 0.01 s-1, k1 ) (8.0 ( 0.3) × 103 dm3 mol-1
s-1, and k-1 ) (4.0 ( 0.2) × 102 dm3 mol-1 s-1. The
equilibrium constants K1 ) k1/k-1 ) 20 and K2 ) k2/k-2
) 3.1 × 10-2 are simply calculated from these rate
constants. Unfortunately, the relatively poor solubility
of [Ni(η3-C3H5)(triphos)]+ means we have been unable
to confirm the values of K1 and K2 using NMR spec-
troscopy.
(13) Henderson, R. A.; Oglieve, K. E. J . Chem. Soc., Chem. Commun.
1991, 584.
(14) Oglieve, K. E.; Henderson, R. A. J . Chem. Soc., Dalton Trans.
1991, 3295.
(15) Henderson, R. A.; Oglieve, K. E. J . Chem. Soc., Dalton Trans.
1994, 767.
(16) Henderson, R. A.; Hughes, D. L.; Macdonald, C. J .; Oglieve, K.
E. Inorg. Chim. Acta 1997, 259, 107, and references therein.
(17) Cauquis, G.; Deronzier, A.; Serve, D.; Vieil, E. J . Electroanal.
Chem. Interfacial Electrochem. 1975, 60, 205.
The kinetics described above do not allow us to
distinguish between the mechanism shown in Figure 3

2582 Organometallics, Vol. 20, No. 12, 2001
Clegg et al.
3
propene. We can calculate the pKa for the protonation
results presented herein into context.18 The effect of
formally replacing a Me group for η3-C3H5 is similar to
(i) changing a CO for PPh3 in [MnH(CO)5] (pKa ) 14.2)
and [MnH(CO)4(PPh3)] (20.4) or [CoH(CO)4] (8.4) and
[CoH(CO)3(PPh3)] (15.4); (ii) replacing C5H5 by C5Me5
in [FeH(η5-C5H5)(CO)2] (19.4) and [FeH(η5-C5Me5)(CO)2]
(26.3); and (iii) changing the metal as in [FeH2(CO)4]
(11.4) and [RuH2(CO)4] (18.7) or [PdH{P(OMe)3}4]+
(10.6) and [PtH{P(OMe)3}4]+ (18.5).
The large difference in basicities observed in [NiR-
(triphos)]+ (R ) Me or η3-C3H5) stands in stark contrast
to the effect methyl and allyl residues have in organic
molecules. As illustrated by the following aqueous pKa’s,
methyl and allyl groups have a similar affect on the
acidities19 of adjacent carboxylate {MeCO2H (4.8) and
CH2dCHCO2H (4.2)} and amine groups {MeNH3+ (10.7)
and CH2dCHCH2NH3+ (9.5)}. It seems most likely that
the η3-coordination of the allyl group in the metal
complex is the origin of the increased basicity of the
nickel site in [Ni(η3-C3H5)(triphos)]+. Obviously this
binding mode is unavailable to organic molecules.
Interestingly, if this proposal is correct, then it follows
that the metal site in a complex containing an η1-allyl
ligand would be ca. 106 times less basic than the
analogue containing an η3-allyl ligand.
at the other site (Ni). Although we do not observe the
direct protonation of this site, the unobserved protolytic
equilibrium constant K3, as defined by eq 4, is related
to K1 and K2 by the simple relationship K3 ) K1K2 )
3
0.62, and thus pKa ) 15.2.
K3
[Ni(η3-C3H5)(triphos)+] + [lutH+] y
\z
[Ni(η3-C3H5)(triphos)]2+ + lut (4)
Thus, the difference in the basicities of the nickel and
η3-allyl is only ca. 30-fold. That this difference must be
small is self-evident since the proton can move between
the two sites. Indeed, a small difference in the basicities
of metal and ligand must be a general feature of all
complexes that show such proton-transfer processes.
One further feature from this study is that, irrespec-
tive of whether K1 or K3 corresponds to protonation of
the η3-C3H5 group, it is clear that upon coordination to
the {Ni(triphos)} site propene becomes at least a 1033
stronger acid.
The results of these studies allow us to compare the
effect that η3-C3H5 and Me groups have on the basicity
of the nickel. Recently,6 we showed that the reaction of
[NiMe(triphos)]+ with anhydrous HCl (pKa ) 8.9) in
MeCN involves initial protonation of the nickel to form
Kinetic studies on the equilibrium protonation reac-
tion of [Ni(η3-C3H5){(Ph2PCH2CH2)2PPh}]+ with [lutH]+
(lut ) 2,6-dimethylpyridine) shows that the basicities
of the nickel and allyl sites differ by less than 30.
Furthermore, comparison of [NiR{(Ph2PCH2CH2)2PPh}]+
(R ) Me or η3-C3H5) shows that η3-allyl makes the nickel
site ca. 106 times more basic than methyl.
the spectroscopically detectable [Ni(H)Me(triphos)]2+
.
Ni
Analysis of the data gives pKa ) 8.8 for [NiH(Me)-
(triphos)]2+. Comparison with the results presented
herein shows that the nickel in [Ni(η3-C3H5)(triphos)]+
is (2.5-79) × 106 times more basic than in [NiMe-
(triphos)]+. Clearly, [NiH(η3-C3H5)(triphos)]2+ is a closed
shell complex, while [Ni(H)Me(triphos)]2+ is a 16-
electron species. Nonetheless, the difference in pKa’s for
structurally so similar complexes is noteworthy.
Su p p or tin g In for m a tion Ava ila ble: This material is
available free of charge via the Internet at http://pubs.acs.org.
OM0100535
Although the acidities of metal-hydrides have been
determined in MeCN, this is the first time that the
influence of different hydrocarbon residues on the
acidities has been quantified. It is useful to put the
(18) Kristjansdottir, S. S.; Norton, J . R. Transition Metal Hy-
drides: Recent Advances in Theory and Experiment; Dedieu, A., Ed.;
VCH: New York, 1992; Chapter 9, and references therein.
(19) Clayden, J .; Greeves, N.; Warren, S.; Wothers, P. Organic
Chemistry OUP: Oxford, 2000; Chapter 8.