Kinetic study of the insertion of norbornadiene into palladium-carbon bonds of complexes containing the rigid bidentate nitrogen ligand bis(arylimino)acenaphthene

Article abstract of DOI:10.1021/om960789f

The ionic [Pd(C₇H₈C(O)R)(Ar-BIAN)]X (R = Me, Et, iPr, Ph; X = Cl, Br, I; Ar = p-An, P-FC₆H₄, P-BrC₆H₄, p-Tol, Ph, o,o′-Me₂C₆H₃,o,o′-iPr ₂C₆H₃) complexes (1b-12b), bearing the bidentate nitrogen ligand bis(arylimino)acenaphthene (Ar-BIAN), have been synthesized via reaction of the corresponding neutral acylpalladium complexes Pd(C(O)R)X(Ar-BIAN) (1a-12a) with norbornadiene (nbd). For the first time, an extensive kinetic study of this migratory alkene insertion into acyl-palladium bonds of neutral complexes containing α-diimine ligands has been carried out. It has been found that under pseudo-first-order circumstances these reactions follow the rate law k_obsd = k₁ + k₂[nbd], which shows that these reactions proceed via a pathway independent of alkene concentration (k₁ pathway) and a pathway dependent on alkene concentration (k₂ pathway). The dramatic decrease of the rate constants k₁ and k₂ upon increasing the steric bulk of the BIAN ligand and the large negative entropy of activation and low enthalpy of activation for both pathways indicate that the k₁ and k₂ pathways are closely related and involve associative processes. From the influence of solvent, X and C(O)R ligand, steric and electronic properties of the BIAN ligand, the presence of free halide and free BIAN, and the parameters of activation, mechanisms have been proposed for both pathways. The k₁ pathway may proceed via a rate-determining solvent-assisted halide or nitrogen dissociation, followed by alkene association and migratory insertion, while the k₂ pathway may occur via a rate-determining migratory alkene insertion in a contact ion pair intermediate. This species may be formed via alkene association followed by either halide or nitrogen dissociation.

Full text of DOI:10.1021/om960789f

68
Organometallics 1997, 16, 68-77
Kin etic Stu d y of th e In ser tion of Nor bor n a d ien e in to
P a lla d iu m -Ca r bon Bon d s of Com p lexes Con ta in in g th e
Rigid Bid en ta te Nitr ogen Liga n d
Bis(a r ylim in o)a cen a p h th en e
J ohannes H. Groen, J ohannes G. P. Delis, Piet W. N. M. van Leeuwen, and
Kees Vrieze*
Anorganisch Chemisch Laboratorium, J . H. van ’t Hoff Research Instituut, Universiteit van
Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands
Received September 16, 1996X
The ionic [Pd(C7H8C(O)R)(Ar-BIAN)]X (R ) Me, Et, iPr, Ph; X ) Cl, Br, I; Ar ) p-An,
p-FC6H4, p-BrC6H4, p-Tol, Ph, o,o′-Me2C6H3, o,o′-iPr2C6H3) complexes (1b-12b), bearing the
bidentate nitrogen ligand bis(arylimino)acenaphthene (Ar-BIAN), have been synthesized via
reaction of the corresponding neutral acylpalladium complexes Pd(C(O)R)X(Ar-BIAN) (1a -
12a ) with norbornadiene (nbd). For the first time, an extensive kinetic study of this migratory
alkene insertion into acyl-palladium bonds of neutral complexes containing R-diimine ligands
has been carried out. It has been found that under pseudo-first-order circumstances these
reactions follow the rate law kobsd ) k1 + k2[nbd], which shows that these reactions proceed
via a pathway independent of alkene concentration (k1 pathway) and a pathway dependent
on alkene concentration (k2 pathway). The dramatic decrease of the rate constants k1 and
k2 upon increasing the steric bulk of the BIAN ligand and the large negative entropy of
activation and low enthalpy of activation for both pathways indicate that the k1 and k2
pathways are closely related and involve associative processes. From the influence of solvent,
X and C(O)R ligand, steric and electronic properties of the BIAN ligand, the presence of
free halide and free BIAN, and the parameters of activation, mechanisms have been proposed
for both pathways. The k1 pathway may proceed via a rate-determining solvent-assisted
halide or nitrogen dissociation, followed by alkene association and migratory insertion, while
the k2 pathway may occur via a rate-determining migratory alkene insertion in a contact
ion pair intermediate. This species may be formed via alkene association followed by either
halide or nitrogen dissociation.
In tr od u ction
retical17-22 investigations have been carried out on
insertion reactions of CO into alkyl-metal bonds, but
studies of insertion reactions of alkenes into acyl-metal
bonds are less abundant.8,23-30 Sen et al. succeeded in
isolating complexes obtained after alkene insertion into
The insertion reaction of alkenes into palladium-
carbon bonds is a crucial process in many palladium-
catalyzed organic reactions.1-3 A recent example is the
copolymerization of CO and alkenes, resulting in the
formation of polyketones. This process is proposed to
proceed via alternating migratory insertion reactions of
CO and alkenes into alkyl- and acyl-palladium bonds,
respectively.4-13 Many experimental14-16 and theo-
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Principles and Applications of Organotransition Metal Chemistry;
University Science Books: Mill Valley, CA, 1987.
* To whom correspondence should be addressed.
X Abstract published in Advance ACS Abstracts, December 15, 1996.
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S0276-7333(96)00789-3 CCC: $14.00 © 1997 American Chemical Society

Insertion of Norbornadiene into Pd-C Bonds
Organometallics, Vol. 16, No. 1, 1997 69
standard Schlenk techniques. Solvents were dried and stored
under nitrogen. Pd(DBA)2 (DBA ) dibenzylideneacetone),40
Ar-BIAN,41 Pd(Me)X(Ar-BIAN),27 Pd(C(O)Me)Cl(p-An-BIAN)
(1a ),27 Pd(C(O)Me)Cl(p-Tol-BIAN) (9a ),27 Pd(C(O)Me)Cl(o,o′-
iPr2C6H3-BIAN) (12a),27 [Pd(C7H8C(O)Me)(p-An-BIAN)]Cl (1b),27
[Pd(C7H8C(O)Et)(p-An-BIAN)]Cl (4b),27 and [Pd(C7H8C(O)Me)-
(o,o′-iPr2C6H3-BIAN)]Cl (12b)27 were prepared according to the
literature. All other starting chemicals were used as com-
mercially obtained. 1H and 13C NMR spectra were recorded
on a Bruker AMX 300 spectrometer (300.13 and 75.48 MHz,
respectively) at 20 °C, unless noted otherwise. Chemical shifts
are in ppm relative to TMS as external standard (s ) singlet,
d ) doublet, dd ) doublet of doublets, t ) triplet, pst )
pseudotriplet, q ) quartet, sep ) septet, m ) multiplet, br )
broad). IR spectra were obtained on a Bio-Rad FTS-7 spec-
trophotometer and mass spectra on a J EOL J MS SX/SX102A
four-sector mass spectrometer fitted with a 10 µm tungsten
FD emitter or equipped with a fast atom bombardment source.
acyl-palladium bonds by using trans-[Pd(C(O)R)(NCMe)-
(PPh3)2]BF4 and trans-Pd(C(O)R)Cl(PPh3)2 (R ) Me, Ph)
as starting compounds.23 Later, also other groups
isolated and characterized complexes obtained after
alkene insertion by using acylpalladium complexes with
bidentate phosphorus24,25,29 and nitrogen8,26-28,30,31
ligands. In these cases intramolecular chelation through
the carbonyl group stabilizes the products by preventing
decomposition via â-hydrogen elimination.
Although several complexes obtained after alkene
insertion into acyl-metal bonds have been isolated,
mechanistic and kinetic studies are very scarce. In a
classic paper, Thorn and Hoffmann analyzed the energy
requirements for insertion of ethylene into a hydride-
platinum bond from three-, four-, and five-coordinate
intermediates.32 According to their calculations, inser-
tion from a four-coordinate intermediate is preferred.
More recent calculations support these results.33,34 In
accord with this finding, intramolecular alkene inser-
tions into carbalkoxy-palladium bonds have been shown
to proceed from four-coordinate intermediates.35 Very
recently, Brookhart et al. reported the formation of
[Pd(C2H4C(O)Me)(phen)]BAr4 (Ar ) 3,5-(CF3)2C6H3)
upon warming a solution of [Pd(C(O)Me)(η2-C2H4)-
(phen)]BAr4 from -78 to -46 °C, which also shows that
insertion occurs from a four-coordinate species.31 How-
ever, both intra-36 and intermolecular37 alkene inser-
tions into hydride-platinum bonds have been proposed
to occur from five-coordinate intermediates. Further-
more, the stability of five-coordinate organoplatinum
and -palladium complexes containing an alkene38 and
the reactivity of acylpalladium complexes containing
both rigid bi- and terdentate nitrogen ligands toward
alkenes suggest that insertion from five-coordinate
species may also be a low-energy pathway.27,39 The only
kinetic study of alkene insertion reactions into acyl-
metal bonds has been carried out by Sen et al.23
However, the use of acylpalladium complexes bearing
rather easily dissociating monophosphines resulted in
complex kinetics. To gain more insight into the mech-
anism of alkene insertion reactions, we carried out a
kinetic study of the alkene insertion into acyl-pal-
ladium bonds and used complexes of the type Pd-
(C(O)R)X(Ar-BIAN), bearing the bidentate nitrogen
ligand bis(arylimino)acenaphthene. It has been shown
that these complexes react readily and cleanly with
unsaturated molecules such as strained alkenes27 and
allenes.30
P r ep a r a tion of Com p lexes 2a -8a , 10a , a n d 11a . The
complexes Pd(C(O)R)Cl(p-An-BIAN) (R ) Et (4a ), iPr (5a ), Ph
(6a )) were prepared by using the following procedure. To a
solution of Pd(DBA)2 (0.50 mmol) and p-An-BIAN (0.55 mmol)
in 25 mL of acetone was added the corresponding carboxylic
acid chloride (0.55 mmol). After being stirred for 2 h at 20
°C, the solution was evaporated and the residue was washed
with diethyl ether (2 × 20 mL). Column chromatography over
silica 60 with dichloromethane/diethyl ether (4/1) as eluent
resulted in the product (45-71% yield). The acylpalladium
complexes Pd(C(O)Me)X(p-An-BIAN) (X ) Br (2a ), I (3a )), Pd-
(C(O)Me)Cl(p-XC6H4-BIAN) (X) F (7a ), Br (8a )), Pd(C(O)Me)-
Cl(Ph-BIAN) (10a ), and Pd(C(O)Me)Cl(o,o′-Me2C6H3-BIAN)
(11a ) were synthesized by using essentially the following
procedure. A solution of the corresponding methylpalladium
complex (0.5 mmol) in 25 mL of dichloromethane was stirred
under a CO atmosphere at 20 °C. After 30 min the solution
was filtered through Celite filter aid. The residue was
extracted with dichloromethane (5 mL), and the combined
filtrates were evaporated to dryness. The products were
washed with diethyl ether (2 × 20 mL) and dried in vacuo (72-
96% yield). No correct microanalyses were obtained due to
the presence of irreproducible amounts (1-2 equiv) of dichloro-
methane.
P d (C(O)Me)Br (p-An -BIAN) (2a ). 1H NMR (300.13 MHz,
CDCl3, δ): 8.05 d (8.2 Hz), H5; 7.46 pst, H4; 7.27 m (4 H), H10
;
7.04 d (8.8 Hz), H9; 6.7 br, H3; 3.91 s, OMe; 2.25 s, C(O)Me.
13C NMR (75.48 MHz, CDCl3, δ): 223.2, C12; 159.7, C11; 144.7,
C8; 131.9, C6; 131.7, C5; 129.1, C4; 127.1, C2; 125.6, C3; 123.4,
C
10; 115.2, C9; 56.2, OMe; 36.0, C(O)Me; not observed C1, C7.
IR (KBr): 1709 cm-1, ν(CO). MS: found, m/ z 542 (calcd for
C
28H23N2O3Pd, 542).
P d (C(O)Me)I(p-An -BIAN) (3a ). 1H NMR (300.13 MHz,
CDCl3, δ): 8.05 d (8.3 Hz), H5; 7.48 pst, H4; 7.24 d (8.9 Hz),
H
10; 7.05 d (8.9 Hz), H9; 7.0 br, H3; 3.91 s, OMe; 2.25 s, C(O)Me.
13C NMR (75.48 MHz, CDCl3, δ): 222.6, C12; 159.6, C11; 144.9,
Exp er im en ta l Section
C8; 132.0, C6; 131.6, C5; 129.2, C4; 127.2, C2; 125.7, C3; 123.5,
C
10; 115.3, C9; 56.3, OMe; 40.2, C(O)Me; not observed C1, C7.
Gen er a l Com m en ts. All manipulations were carried out
under an atmosphere of purified dry nitrogen by using
IR (KBr): 1705 cm-1, ν(CO). MS: found m/ z 542 (calcd for
C
28H23N2O3Pd, 542).
P d (C(O)Et)Cl(p-An -BIAN) (4a ). 1H NMR (300.13 MHz,
(31) Rix, F. C.; Brookhart, M.; White, P. S. J . Am. Chem. Soc. 1996,
118, 4746.
(32) Thorn, D. L.; Hoffmann, R. J . Am. Chem. Soc. 1978, 100, 2079.
(33) Trzcinska, B. M.; Fackler, J . P., J r.; Anderson, A. B. Organo-
metallics 1984, 3, 319.
(34) Fujimoto, H.; Yamasaki, T.; Mizutani, H.; Koga, N. J . Am.
Chem. Soc. 1985, 107, 6157.
(35) Samsel, E. G.; Norton, J . R. J . Am. Chem. Soc. 1984, 106, 5505.
(36) van Leeuwen, P. W. N. M.; Roobeek, C. F.; Frijns, J . H. G.;
Orpen, A. G. Organometallics 1990, 9, 1211.
(37) Clark, H. C.; J ablonski, C. R.; Halpern, J .; Weil, T. A.; Matovani,
A. Inorg. Chem. 1974, 13, 1541.
(38) Albano, V. G.; Natile, G.; Panunzi, A. Coord. Chem. Rev. 1994,
133, 67 and references therein.
CDCl3, δ): 8.04 d (8.4 Hz), H5; 7.48 pst, H4; 7.27 m (4 H), H10
;
7.03 d (8.9 Hz), H9; 6.8 br, H3; 3.89 s, OMe; 2.80 q (7.3 Hz),
CH2Me; 0.69 t (7.3 Hz), CH2Me. 13C NMR (75.48 MHz, CDCl3,
-20 °C, δ): 228.1, C12; 170.7, 165.0, C1; 159.4, 159.0, C11; 144.6,
C7; 141.2, 138.6, C8; 132.0, 131.4, C5; 131.7, C6; 129.1, C4; 127.0,
126.6, C2; 125.8, 125.2, C3; 124.0, 122.8, C10; 115.6, 114.7, C9;
56.2, 56.1, OMe; 42.6, CH2Me; 9.9, CH2Me. IR (KBr): 1703
cm-1, ν(CO). MS: found, m/ z 556 (calcd for C29H25N2O3Pd,
556).
(39) Ru¨lke, R. E.; Kaasjager, V. E.; Kliphuis, D.; Elsevier, C. J .; Van
Leeuwen, P. W. N. M.; Vrieze, K.; Goubitz, K. Organometallics 1996,
15, 668.
(40) Rettig, M. F.; Maitlis, P. M. Inorg. Synth. 1977, 17, 134.
(41) van Asselt, R.; Elsevier, C. J .; Smeets, W. J . J .; Spek, A. L.;
Benedix, R. Recl. Trav. Chim. Pays-Bas 1994, 113, 88.

70 Organometallics, Vol. 16, No. 1, 1997
Groen et al.
P d (C(O)iP r )Cl(p-An -BIAN) (5a ). 1H NMR (300.13 MHz,
pst, H4; 7.19 d (7.3 Hz), H3; 7.12 d (8.6 Hz), H9; 5.96 dd (4.8,
2.7 Hz), 5.45 dd (4.8, 3.0 Hz), H14,15; 3.94 s, OMe; 2.98 br, 2.33
br, H13,16; 2.53 d (5.9 Hz), H18; 2.44 s, C(O)Me; 2.11 d (5.9 Hz),
CDCl3, δ): 8.03 d (7.9 Hz), H5; 7.47 pst, H4; 7.28 m (4 H), H10
;
7.04 d (8.7 Hz), H9; 6.7 br, H3; 3.90 s, OMe; 2.98 sep (7.0 Hz),
CHMe2; 0.98 d (7.0 Hz), CHMe2. 13C NMR (75.48 MHz, CDCl3,
-20 °C, δ): 230.8, C12; 170.2, 164.3, C1; 158.6, 158.5, C11; 143.7,
C7; 140.7, 137.9, C8; 131.1, 130.5, C5; 130.9, C6; 128.2, C4; 126.5,
126.0, C2; 125.1, 124.5, C3; 123.3, 122.5, C10; 114.7, 114.0, C9;
55.5, 55.4, OMe; 47.3, CHMe2; 18.5, CHMe2. IR (KBr): 1702
cm-1, ν(CO). MS: found, m/ z 570 (calcd for C30H27N2O3Pd,
570).
P d (C(O)P h )Cl(p-An -BIAN) (6a ). 1H NMR (300.13 MHz,
CDCl3, δ): 8.04 m (2 H), H5; 7.98 d (7.9 Hz), Ho; 7.5 m (4 H),
4,10; 7.37 t (7.2 Hz), Hp; 7.24 pst, Hm; 7.06 d (8.1 Hz), H10; 6.8
H
12; 1.60 d (9.0 Hz), 1.24 d (9.0 Hz), H17. IR (KBr): 1600 cm-1
,
ν(CO). MS: found, m/ z 634 (calcd for C35H31N2O3Pd, 634).
[P d (C7H8C(O)iP r )(p-An -BIAN)]Cl (5b). 1H NMR (300.13
MHz, CDCl3, δ): 8.16 d (8.3 Hz), H5; 7.55 pst, H4; 7.38 d (8.7
Hz), H10; 7.12 d (8.7 Hz), H9; 7.00 d (7.0 Hz), H3; 5.96 dd (4.8,
2.5 Hz), 5.46 dd (4.8, 2.9 Hz), H14,15; 3.92 s, OMe; 3.06 sep (6.8
Hz), CH (iPr); 3.01 br, 2.30 br, H13,16; 2.51 d (5.7 Hz), H18; 1.95
d (5.7 Hz), H12; 1.53 d (8.7 Hz), H17 (signals of Me (iPr) are
overlapping with signal of the other H17); 1.25 d (6.8 Hz), 1.02
d (6.8 Hz), Me (iPr). IR (KBr): 1599 cm-1, ν(CO). MS: found,
m/ z 662 (calcd for C37H35N2O3Pd, 662).
H
m (6 H), H3,9; 3.90 s, 3.82 s, OMe. 13C NMR (75.48 MHz,
CDCl3, δ): 219.6, C12; 171.2, 165.5, C1; 160.0, 159.3, C11; 144.8,
C7; 140.9, 138.9, C8; 136.3, Cips; 132.2 Cp; 131.9, C6; 131.9,
131.4, C5; 130.6, Co; 129.1, C4; 128.2, Cm; 127.3, 127.0, C2;
125.8, 125.3, C3; 124.5, 122.9, C10; 115.3, 115.0, C9; 56.3, OMe.
IR (KBr): 1680 cm-1, ν(CO). MS: found, m/ z 604 (calcd for
33H25N2O3Pd, 604).
P d (C(O)Me)Cl(p-F C6H4-BIAN) (7a ). 1H NMR (300.13
MHz, CDCl3, δ): 8.10 d (8.3 Hz), H5; 7.52 pst, H4; 7.30 dd (13.2
Hz, 8.7 Hz), H10; 7.23 d (8.7 Hz), H9; 6.8 br, H3; 2.27 s, C(O)Me.
13C NMR (75.48 MHz, CDCl3, δ): 222.4, C12; 164.1, C1; 160.8,
[P d (C7H8C(O)P h )(p-An -BIAN)]Cl (6b). 1H NMR (300.13
MHz, CDCl3, δ): 8.18 d (8.3 Hz), H5; 7.90 d (8.0 Hz), Ho; 7.71
t (7.4 Hz), Hp; 7.56 pst, Hm; 7.51 pst, H4; 7.43 d (8.7 Hz), H10
;
7.29 br, H3; 7.17 d (8.7 Hz), H9; 6.12 dd (5.2, 2.7 Hz), 5.52 dd
(5.2, 3.1 Hz), H14,15; 3.96 s, OMe; 3.06 d (5.4 Hz), H18; 3.05 br,
2.39 br, H13,16; 2.07 dd (5.4, 1.5 Hz), H12; 1.70 d (9.1 Hz), 1.26
d (9.1 Hz), H17. IR (KBr): 1598 cm-1, ν(CO). MS: found, m/ z
696 (calcd for C40H33N2O3Pd, 696).
[P d (C7H 8C(O)Me)(p -F C6H 4-BIAN)]Cl (7b ). 1H NMR
(300.13 MHz, CDCl3, 0 °C, δ): 8.16 d (8.3 Hz), H5; 7.54 pst,
H4; 7.53 pst, H10; 7.32 pst, H9; 7.03 d (7.3 Hz), H3; 5.96 dd (5.3,
C
C
11; 145.1, C8; 132.2, C5; 132.0, C6; 129.3, C4; 126.7, C2; 125.9,
C3; 123.7 d (6.0 Hz), C10; 117.5 br, C9; 34.0, C(O)Me; not
observed C7. IR (KBr): 1711 cm-1, ν(CO). MS: found, m/ z
518 (calcd for C26H17N2OF2Pd, 518).
2.9 Hz), 5.50 dd (5.3, 3.1 Hz), H14,15; 2.96 br, 2.39 br, H13,16
2.51 s, C(O)Me; 2.47 d (6.2 Hz), H18; 2.10 dd (6.2, 1.7 Hz), H12
1.71 d (8.9 Hz), 1.27 d (8.9 Hz), H17
;
;
,
.
IR (KBr): 1596 cm-1
P d (C(O)Me)Cl(p-Br C6H4-BIAN) (8a ). 1H NMR (300.13
MHz, CDCl3, δ): 8.11 d (8.3 Hz), H5; 7.67 d (8.3 Hz), H10; 7.54
pst, H4; 7.21 d (8.3 Hz), H9; 6.9 br, H3; 2.27 s, C(O)Me. 13C
NMR (75.48 MHz, CDCl3, δ): 221.6, C12; 145.1, C8; 133.6, C5;
132.3, C4; 132.1, C6; 129.5, C10; 126.7, C2; 126.1, C3; 123.6, C9;
122.2, C11; 34.0, C(O)Me; not observed C1, C7. IR (KBr): 1707
cm-1, ν(CO). MS: found, m/ z 640 (calcd for C26H17N2OBr2-
Pd, 640).
ν(CO). MS: found, m/ z 610 (calcd for C33H25N2OF2Pd, 610).
[P d (C7H8C(O)Me)(p-Br C6H4-BIAN)]Cl (8b). 1H NMR
(300.13 MHz, CDCl3, -20 °C, δ): 7.98 d (8.3 Hz), H5; 7.61 d
(8.5 Hz), H10; 7.47 pst, H4; 7.03 d (8.5 Hz), H9; 6.94 d (7.0 Hz),
H3; 6.13 dd (5.4, 2.8 Hz), 5.94 dd (5.4, 3.1 Hz), H14,15; 2.96 br,
1.78 br, H13,16; 2.75 d (6.5 Hz), H18; 2.36 s, C(O)Me; 2.03 d (6.5
Hz), H12; 1.45 d (9.3 Hz), 1.32 d (9.3 Hz), H17. IR (KBr): 1601
cm-1, ν(CO). MS: found, m/ z 732 (calcd for C33H25N2OBr2-
Pd, 732).
P d (C(O)Me)Cl(P h -BIAN) (10a ). 1H NMR (300.13 MHz,
[P d(C7H8(C(O)Me)(p-Tol-BIAN)]Cl (9b). 1H NMR (300.13
MHz, CDCl3, δ): 8.15 d (8.3 Hz), H5; 7.53 pst, H4; 7.41 d (8.0
Hz), H10; 7.30 d (8.0 Hz), H9; 7.08 d (7.3 Hz), H3; 5.96 dd (5.1,
CDCl3, δ): 8.06 d (8.3 Hz), H5; 7.54 pst, H4; 7.44 m (6 H), H10,11
;
7.31 d (7.8 Hz), H9; 6.7 br, H3; 2.24 s, C(O)Me. 13C NMR (75.48
MHz, CDCl3, -20 °C, δ): 224.5, C12; 170.5, 166.0, C1; 148.0,
C7; 145.7, 145.0, C8; 132.3, 131.7, C5; 131.7, C6; 130.8, 129.9,
2.7 Hz), 5.43 dd (5.1, 3.1 Hz), H14,15; 2.99 br, 2.40 br, H13,16
;
C
10; 129.2, 129.1, C4; 128.5, 128.3, C3; 126.7, 126.3, C2; 126.0,
2.53 s, C(O)Me; 2.50 s, Me, 2.45 d (7.2 Hz), H18; 1.99 d (7.2
Hz), H12; 1.68 d (8.9 Hz), 1.28 d (8.9 Hz), H17. IR (KBr): 1594
cm-1, ν(CO). MS: found, m/ z 602 (calcd for C35H31N2OPd,
602).
125.8, C11; 121.6, 121.1, C9; 34.0, C(O)Me. IR (KBr): 1708
cm-1, ν(CO). MS: found, m/ z 482 (calcd for C26H19N2OPd,
482).
P d (C(O)Me)Cl(o,o′-Me2C6H 3-BIAN) (11a ). 1H NMR
(300.13 MHz, CDCl3, δ): 8.09 d (8.2 Hz), 8.05 d (8.2 Hz), H5;
7.50 m (2 H), H4; 7.22 m (6 H), H10,11; 6.78 d (6.7 Hz), 6.62 d
(6.8 Hz), H3; 2.42 s, 2.34 s, Me; 2.20 s, C(O)Me. 13C NMR
(75.48 MHz, CDCl3, δ): 221.4, C12; 169.9, 166.9, C1; 145.6,
[P d (C7H8C(O)Me)(P h -BIAN)]Cl (10b). 1H NMR (300.13
MHz, CDCl3, δ): 8.14 d (8.3 Hz), H5; 7.65 pst, H10; 7.50 pst,
H4; 7.43 d (7.6 Hz), H9; 6.96 d (7.3 Hz), H3; 5.94 dd (5.0, 2.7
Hz), 5.45 dd (5.0, 3.0 Hz), H14,15; 2.97 br, 2.43 br, H13,16; 2.51 s,
C(O)Me; 2.41 d (6.4 Hz), H18; 2.02 d (6.4 Hz), H12; 1.72 d (9.0
Hz), 1.28 d (9.0 Hz), H17; signal of H11 is overlapping with
signal of H4. IR (KBr): 1591 cm-1, ν(CO). MS: found, m/ z
574 (calcd for C33H27N2OPd, 574).
[P d(C7H8C(O)Me)(o,o′-Me2C6H3-BIAN)]Cl (11b). 1H NMR
(300.13 MHz, CDCl3, δ): 8.22 d (8.3 Hz), H5; 7.53 pst, H4; 7.28
m (6H), H10,11; 6.69 d (7.3 Hz), H3; 5.92 dd (5.3, 2.9 Hz), 5.37
dd (5.3, 3.1 Hz), H14,15; 3.55 br, 2.99 br, H13,16; 2.52 s, C(O)Me;
2.34 s, 2.31 s, Me; 1.73 d (8.9 Hz), 1.24 d (8.9 Hz), H17; 1.69 dd
(6.7, 2.0 Hz), H12; signal of H18 is overlapping with signals of
Me. IR (KBr): 1700 cm-1, ν(CO). MS: found, m/ z 630 (calcd
for C37H33N2O3Pd, 630).
Kin etics. The rates of the reactions of norbornadiene with
complexes 1a -12a , resulting in complexes 1b-12b, were
followed spectrophotometrically by repetitive scanning of the
spectrum at suitable times at a fixed wavelength, where the
difference between the absorbances of educt and product was
largest. The reactions were started by addition of norborna-
diene to a 4.3 × 10-4 M solution of palladium complex in a
quartz cell, in the thermostated cell compartment of a Perkin-
Elmer Lambda 5 spectrophotometer, with a temperature
accuracy of (0.5 °C. The use of at least a 10-fold excess of
norbornadiene over complex ensured pseudo-first-order kinet-
144.9, C8; 144.5, C7; 132.3, 131.7, C5; 131.9, C6; 129.9, C10
;
129.8, 129.3, C4; 129.0, C9 (signal of other C9 is overlapping
with signals of C4); 128.1, 127.3, C3; 127.8, 127.2, C2; 124.9,
C
11; 33.4, C(O)Me; 19.1, 18.6, Me. IR (KBr): 1704 cm-1, ν(CO).
MS: found, m/ z 538 (calcd for C30H27N2OPd, 538).
P r ep a r a tion of Com p lexes 2b, 3b, a n d 5b -11b. To
solutions of 2a , 3a , and 5a -11a (0.10 mmol) in dichloro-
methane (20 mL) was added norbornadiene (0.11 mmol). After
being stirred at 20 °C for 2 h, each solution was evaporated to
dryness and the residue was washed with diethyl ether (2 ×
20 mL) and dried in vacuo, resulting in 2b, 3b, and 5b-11b
(74-87% yield), which were too unstable to allow microanaly-
sis and 13C NMR.
[P d (C7H8C(O)Me)(p-An -BIAN)]Br (2b). 1H NMR (300.13
MHz, CDCl3, δ): 8.15 d (8.3 Hz), H5; 7.54 pst, H4; 7.42 d (8.7
Hz), H10; 7.21 d (7.3 Hz), H3; 7.14 d (8.7 Hz), H9; 5.98 dd (5.2,
2.8 Hz), 5.44 dd (5.2, 3.1 Hz), H14,15; 3.94 s, OMe; 3.02 br, 2.31
br, H13,16; 2.58 d (5.9 Hz), H18; 2.51 s, C(O)Me; 2.01 d (5.9 Hz),
H
12; 1.59 d (9.0 Hz), 1.27 d (9.0 Hz), H17. IR (KBr): 1599 cm-1
,
ν(CO). MS: found, m/ z 634 (calcd for C35H31N2O3Pd, 634).
[P d (C7H8C(O)Me)(p-An -BIAN)]I (3b). 1H NMR (300.13
MHz, CDCl3, δ): 8.12 d (8.3 Hz), H5; 7.52 d (8.6 Hz), H10; 7.51

Insertion of Norbornadiene into Pd-C Bonds
Organometallics, Vol. 16, No. 1, 1997 71
ics in all runs. All reactions obeyed a first-order rate law for
at least 4 half-lives of the reaction. The rate constants kobsd
were calculated from the slopes of plots of ln{(At - A∞)/(A0 -
A∞)} vs time (A0 ) absorbance after mixing of the reagents,
A∞ ) absorbance at completion of the reaction).
1a -12a with norbornadiene led to insertion of the
alkene into the acyl-palladium bond, resulting in the
formation of complexes 1b-12b (eq 3). This reaction
Resu lts
Syn t h esis a n d Ch a r a ct er iza t ion of Com p lexes
1-12. Carbon monoxide inserts readily into the methyl-
palladium bond of complexes of the type Pd(Me)X(Ar-
BIAN) to give acylpalladium compounds 1a -3a and
7a -12a in high yields (eq 1). The complexes Pd(C(O)R)-
Cl(p-An-BIAN) (R ) Et (4a ), iPr (5a ), Ph (6a )) were
synthesized via the oxidative addition of the corre-
sponding carboxylic acid chloride to Pd(DBA)2 in the
presence of p-An-BIAN in acetone (eq 2).
occurred within 2 h, and the products were formed in
high yields (74-87%). The novel alkylpalladium com-
1
plexes 2b, 3b, and 5b-11b were characterized by H
NMR, IR, and mass spectroscopy. Unfortunately, these
complexes are too unstable to allow microanalysis or 13C
NMR. Formation of 2b, 3b, and 5b-11b is clear from
the well-defined pattern of the inserted norbornadiene
in the 1H NMR. In all cases a coupling constant
3J (H12,H18) of about 6 Hz is observed, indicating a syn
addition of Pd-C(O)R to the exo face of the alkene. This
syn-exo addition of the alkene is in agreement with
previous observations.23,27,28,30,39,45 The low CO stretch-
ing frequency of about 1600 cm-1 indicates that the
carbonyl oxygen atom is coordinated to the metal center,
forming a five-membered palladacycle, which is a com-
mon feature for complexes (bearing uni- and bidentate
ligands) formed via insertion of alkenes into a pal-
ladium-acyl bond.8,23,27,28,30,39,45 The observed high
equivalent conductances of 15-26 Ω-1 cm2 mol-1 in
dichloromethane at 20 °C for complexes 2b, 3b, and 5b-
11b are in agreement with an ionic structure, whereas
those for the neutral precursors 2a , 3a , and 5a -11a
The acylpalladium compounds 1a -3a and 7a -12a
are stable toward decarbonylation in the solid state as
well in solution at 20 °C (no trace of decarbonylated
methylpalladium complexes after 16 h in dichloro-
methane at 20 °C). However, at higher temperature
slow decarbonylation occurred. When a solution of 1a -
3a and 7a -12a was refluxed in dichloromethane for 2.5
h, 10-20% of the corresponding decarbonylated
methylpalladium complex and 80-90% unreacted
acylpalladium complex were present. The novel acylpal-
ladium complexes 2a -8a , 10a , and 11a were character-
are 0.02-0.08 Ω-1 cm2 mol-1
.
As reported for similar acyl- and alkylpalladium
complexes,27,44 all novel acyl- and alkylpalladium com-
pounds, except for 11a , show fluxional behavior on the
1H NMR time scale: the complexes are in fast exchange
at 20 °C and give one averaged signal for the protons
on both halves of the BIAN ligand. In the case of the
acylpalladium complexes mechanisms via halide dis-
sociation,28,44 nitrogen dissociation,43 and CO de-
insertion44 have been proposed to explain the isomer-
ization, whereas isomerization of the alkylpalladium
complexes has been proposed to proceed via site ex-
change of the acetylnorbornene moiety, aided by coor-
dination of the halide.27
Kin etics. Similar to the conditions used for the
synthesis of complexes 1b-12b , the reaction of nor-
bornadiene (nbd) with the neutral acylpalladium com-
plexes 1a -12a in dichloromethane occurred cleanly and
quantitatively under pseudo-first-order conditions, i.e.
when a large excess of norbornadiene, [nbd] . [Pd], was
employed. Under these conditions, the reaction could
be followed spectrophotometrically, and in all cases
isosbestic points were obtained. The reaction could also
be followed by 1H NMR, which did not show the
1
ized by H and 13C NMR, mass, and IR spectroscopy.
Unfortunately, no correct microanalyses were obtained
due to the presence of irreproducible amounts of dichlo-
romethane, varying from 1 to 2 equiv. Formation of
these complexes is clear from the resonance of the
carbonyl group in the 13C NMR (at about 220 ppm) and
the observation of a CO stretching frequency (in the
region of 1680-1720 cm-1) in the IR. These data agree
well with those reported for other acylpalladium com-
plexes bearing bidentate nitrogen ligands.27,28,30,39,42-44
The reaction of the neutral acylpalladium complexes
(42) de Graaf, W.; Boersma, J .; van Koten, G. Organometallics 1990,
9, 1479.
(43) Delis, J . P. G.; van Leeuwen, P. W. N. M.; Vrieze, K.; Veldman,
N.; Spek, A. L.; Fraanje, J .; Goubitz, K. J . Organomet. Chem. 1996,
514, 125.
(44) van Asselt, R.; Vrieze, K.; Elsevier, C. J . J . Organomet. Chem.
1994, 480, 27.
(45) Dekker, G. P. C. M.; Elsevier, C. J .; Vrieze, K.; van Leeuwen,
P. W. N. M.; Roobeek, C. F. J . Organomet. Chem. 1992, 430, 357.

72 Organometallics, Vol. 16, No. 1, 1997
Groen et al.
Ta ble 1. Ra te Con sta n ts k1 a n d k2 a n d th e En th a lp y a n d En tr op y of Activa tion of th e Rea ction of
Com p lexes P d (C(O)R)X(Ar -BIAN) (1a -12b) w ith Nor bor n a d ien e (Sta n d a r d Devia tion s in P a r en th eses)a
entry no.
1
compd
R
X
Ar
102k1, s-1
102k2, s-1 M-1
∆Hq,d kJ mol-1
∆Sq,d J mol-1 K-1
1a
Me
Cl
p-An
0.39(2)
4.62(10)
k1: 37.9(22)
k2: 48.4(18)
k1: -140(6)
k2: -126(6)
2b
3c
4
1a
1a
2a
Me
Me
Me
Cl
Cl
Br
p-An
p-An
p-An
0.33(5)
0.17(3)
0.47(6)
5.01(22)
6.15(15)
7.23(29)
k1: 43.2(21)
k2: 40.5(43)
k1: 43.7(40)
k2: 45.0(17)
k1: 50.9(38)
k2: 43.9(28)
k1: 50.5(37)
k2: 38.6(15)
k1: 31.5(76)
k2: 25.6(28)
k1: -142(7)
k2: -127(15)
k1: -137(14)
k2: -111(6)
k1: -121(13)
k2: -124(10)
k1: -124(13)
k2: -146(5)
k1: -187(26)
k2: -180(10)
5
6
7
8
3a
4a
5a
6a
Me
Et
I
p-An
p-An
p-An
p-An
0.55(8)
0.25(3)
0.17(2)
0.22(4)
8.10(36)
2.64(13)
1.77(7)
Cl
Cl
Cl
iPr
Ph
6.12(20)
9
10
11
12
13
14
7a
8a
9a
10a
11a
12a
Me
Me
Me
Me
Me
Me
Cl
Cl
Cl
Cl
Cl
Cl
p-FC6H4
p-BrC6H4
p-Tol
0.75(4)
0.64(9)
0.56(5)
0.83(7)
0.33(3)
0.05(1)
11.63(19)
22.72(42)
5.26(26)
8.70(32)
1.58(6)
Ph
o,o′-Me2C6H3
o,o′-iPr2C6H3
0.40(1)
a
b
d
Conditions: dichloromethane solvent; 19.5 °C; [Pd] ) 3.8 × 10-4 M, unless noted otherwise. THF solvent. c 2-Me-THF solvent. k1
and k2 in these columns refer to the data for the respective kinetic pathways.
F igu r e 2. Pseudo-first-order rate constants (kobsd) of the
reaction of 1a with nbd in dichloromethane/acetonitrile
mixtures as a function of the concentration of nbd. Condi-
tions: 19.5 °C; [Pd] ) 4.3 × 10-4 M.
F igu r e 1. Pseudo-first-order rate constants (kobsd) of the
reaction of 1a with nbd in different solvents as a function
of the concentration of nbd. Conditions: 19.5 °C; [Pd] )
4.3 × 10-4 M.
presence of any intermediate in the temperature range
of -80 to +20 °C. A plot of the observed rate constants
kobsd (which were calculated from the slope of plots of
ln{(At - A∞)/(A0 - A∞)} vs time) vs the concentration of
nbd resulted in straight lines with an intercept with the
y axis, indicating that the kinetics obey the typical rate
law kobsd ) k1 + k2[nbd]. The values of the observed
rate constant kobsd of the reactions of 1a -12a with nbd
have been collected in Table S1 (Supporting Informa-
tion). The values of the first-order rate constant k1 and
the second-order rate constant k2 at 19.5 °C, obtained
from linear regression analysis of the experimental data,
along with the values of the parameters of activation
∆Hq and ∆Sq, determined from the values of k1 and k2
measured at different temperatures (for these values,
see Table S2 in the Supporting Information), have been
collected in Table 1.
In flu en ce of th e Solven t. To study the influence
of the solvent on the kinetics, the insertion reaction of
nbd into the carbon-palladium bond of 1a was carried
out in different solvents at 19.5 °C (Figure 1). The use
of solvents with comparable polarity, i.e. dichloromethane,
THF, and 2-Me-THF, resulted in small differences in
both k1 and k2 (Table 1, entries 1-3). Interestingly,
carrying out the reaction in acetone and acetonitrile led
to curved-line plots. The same observation was made
when using mixtures of dichloromethane and aceto-
nitrile as solvent (Figure 2). Increasing the percentage
of acetonitrile in a dichloromethane solution from 0%
v/v to 17% v/v led to an increase of both k1 and k2.
Further increase of the acetonitrile/dichloromethane
ratio resulted in curved-line plots.
In flu en ce of th e C(O)R a n d X Liga n d s. From
Figures 3 and 4 it is clear that the pseudo-first-order
rate constant kobsd of the reaction of nbd with Pd-
(C(O)R)X(p-An-BIAN) in dichloromethane at 19.5 °C
depends on both the nature of the halide X and the
nature of the C(O)R ligand. The second-order rate
constant k2 increases in the order X ) Cl < Br < I and
in the order R ) iPr < Et < Me < Ph. The first-order
rate constant k1 is less sensitive to variation of X or
C(O)R. The values of k1 are within experimental error
equal for R ) Et, iPr, and Ph and somewhat larger for
R ) Me. Also, variations in the value of k1 upon varying
the X ligand are small and are within experimental
error.
In flu en ce of th e Ar Gr ou p . To examine the influ-
ence of the BIAN ligand on the observed rate constant
of the reaction of nbd with Pd(C(O)Me)Cl(Ar-BIAN) in
dichloromethane at 19.5 °C, both steric and electronic
properties of the aryl groups of the Ar-BIAN ligand were

Insertion of Norbornadiene into Pd-C Bonds
Organometallics, Vol. 16, No. 1, 1997 73
F igu r e 3. Pseudo-first-order rate constants (kobsd) of the
reaction of Pd(C(O)Me)X(p-An-BIAN) (X ) Cl (1a ), Br (2a ),
I (3a )) with nbd as a function of the concentration of nbd.
Conditions: dichloromethane solvent; 19.5 °C; [Pd] ) 4.3
× 10-4 M.
F igu r e 6. Pseudo-first-order rate constants (kobsd) of the
reaction of Pd(C(O)Me)Cl(p-XC6H4-BIAN) (X ) OMe (1a ),
F (7a ), Br (8a ), Me (9a ), H (10a )) with nbd as a function of
the concentration of nbd. Conditions: dichloromethane
solvent; 19.5 °C; [Pd] ) 4.3 × 10-4 M.
F igu r e 7. Hammett plot for the second-order rate con-
stants k2 of the reaction of Pd(C(O)Me)Cl(p-XC6H4-BIAN)
(X ) OMe (1a ), F (7a ), Br (8a ), Me (9a ), H (10a )) with nbd
as a function of the σ value of the para substituents X on
the aryl groups of the BIAN ligand. Conditions: dichloro-
methane solvent; 19.5 °C; [Pd] ) 4.3 × 10-4 M.
F igu r e 4. Pseudo-first-order rate constants (kobsd) of the
reaction of Pd(C(O)R)Cl(p-An-BIAN) (R ) Me (1a ), Et (4a ),
iPr (5a ), Ph (6a ) with nbd as a function of the concentration
of nbd. Conditions: dichloromethane solvent; 19.5 °C; [Pd]
) 4.3 × 10-4 M.
the Hammett σ values46 of the substituents at the para
position of the aryl groups of the ligand resulted in a
straight line with a slope (F) of 1.4 ( 0.1 (Figure 7). A
similar plot for k1 did not show a clear-cut dependence,
because of the small variations and the relatively large
experimental errors in the values of k1.
In flu en ce of Excess F r ee p-An -BIAN a n d F r ee
Ch lor id e. In Figures 8 and 9 the influence of addition
of excess free p-An-BIAN on the values of k1 and k2,
respectively, is shown. Interestingly, the k1 pathway
is strongly retarded by addition of small amounts of free
ligand (in the absence of free p-An-BIAN k1 ) (0.39 (
0.02) × 10-2 s-1; with 1 equiv or more of p-An-BIAN k1
≈ 0.09 × 10-2 s-1). However, the k2 pathway is only
slightly retarded by addition of free ligand. Increasing
the free chloride concentration from 0.0 M to 4.3 × 10-3
M (0 to 10 equiv with respect to the palladium concen-
tration), while maintaining a constant ionic strength,
influenced neither k1 nor k2.
F igu r e 5. Pseudo-first-order rate constants (kobsd) of the
reaction of Pd(C(O)Me)Cl(Ar-BIAN) (Ar ) Ph (10a ), o,o′-
Me2C6H3 (11a ), o,o′-iPr2C6H3 (12a )) with nbd as a function
of the concentration of nbd. Conditions: dichloromethane
solvent; 19.5 °C; [Pd] ) 4.3 × 10-4 M.
varied (Table 1, entries 1, 9-14). Figure 5 clearly shows
a dramatic influence of the steric bulk of the aryl groups
on the values of k1 and k2, which both increase in the
order Ar ) o,o′-iPr2C6H3 , o,o′-Me2C6H3 , Ph. The
influence of the electronic properties of the BIAN ligand
was determined by varying the para substituent on the
aryl groups of the ligand (Figure 6). While the small
variations in k1 are within experimental error, the value
of k2 increases in the order Ar ) p-An < p-Tol < Ph <
p-FC6H4 < p-BrC6H4. A plot of the value of log k2 vs
Discu ssion
Insertions of unsaturated molecules ()un) in square-
planar organoplatinum and -palladium complexes of
the type M(R)X(L)2 may occur via an associative path-
way (Scheme 1, pathway II) and a dissociative pathway.
(46) Hammett, L. P. Physical Organic Chemistry: Reaction Rates,
Equilibria, and Mechanisms, 2nd ed.; McGraw-Hill: New York, 1970.

74 Organometallics, Vol. 16, No. 1, 1997
Groen et al.
Sch em e 1. P ossible Mech a n ism s for In ser tion of
a n Un sa tu r a ted Molecu le (u n ) in Com p lexes of th e
Typ e M(R)X(L)2
F igu r e 8. First-order rate constants k1 of the reaction of
1a with nbd as a function of the ratio [p-An-BIAN]/[1a ].
Conditions: dichloromethane solvent; 19.5 °C; [Pd] ) 4.3
× 10-4 M.
M). For example, the contribution of k1 to the observed
rate constant of the reaction of 1a with nbd in dichlo-
romethane at 19.5 °C is 0.39 × 10-2 s-1, while the
contribution of k2 to kobs is (4.62 × 10-2) × 0.061 ) 0.28
× 10-2 s-1
.
Th e Alk en e-In d ep en d en t k1 P a th w a y. Similar to
substitution reactions in square-planar complexes, the
k1 pathway can be assigned either to a dissociative
pathway, i.e. pathways I and III in Scheme 1, or to an
associative solvento pathway.51-56 Strong evidence
supports the view, however, that in the present system
the k1 pathway may be an associative process rather
than a dissociative process. (i) The value of k1 decreases
significantly with the increasing steric demand of the
BIAN ligand, i.e. in the range Ph-BIAN . o,o′-Me2C6H3-
BIAN . o,o′-iPr2C6H3-BIAN, while the opposite is
expected in the case of a dissociative pathway. (ii) The
low value of the enthalpy of activation (31.5-50.9 kJ
mol-1) and the large negative value of the entropy of
activation (-121 to -187 J mol-1 K-1) are consistent
with an associative process. These parameters of
activation are comparable with those reported for as-
sociative substitution reactions in palladium(II) com-
plexes, e.g. ∆Hq ) 51 ( 1 kJ mol-1 and ∆Sq ) -114 (
4 J mol-1 K-1 for the nucleophilic substitution reaction
of trans-Pd(C6F5)2(tht)2 (tht ) tetrahydrothiophene)
with 2-methylpyridine57 and ∆Hq ) 41 ( 4 kJ mol-1 and
∆Sq ) -172 ( 8 J mol-1 K-1 for the solvent-assisted
dissociation of the acetate ligand in Pd(Et)OOCMe-
(PMe3)2.58
F igu r e 9. Second-order rate constants k2 of the reaction
of 1a with nbd as a function of the ratio [p-An-BIAN]/[1a ].
Conditions: dichloromethane solvent; 19.5 °C; [Pd] ) 4.3
× 10-4 M.
In the latter case association of the substrate may be
preceded by dissociation of either the neutral ligand L
or the anionic ligand X (Scheme 1, pathways I and III,
respectively). Studies on insertion reactions of both
carbon monoxide47-49 and alkenes23 in neutral com-
plexes bearing phosphine ligands have revealed that
insertion predominantly occurs via dissociation of one
of the phosphine ligands. For complexes containing the
rigid bidentate nitrogen ligand BIAN, dissociation of one
of the nitrogen atoms might seem to be more difficult.27
However, complexes of the type Pd(R)X(Ar-BIAN) ex-
hibit a high reactivity toward CO,27 norbornadiene,27
and allenes,30,50 which indicates the presence of low-
energy pathways for insertion reactions in these com-
plexes.
From the results it is clear that insertion of nor-
bornadiene into the acyl-palladium bond of complexes
Pd(C(O)R)X(Ar-BIAN) (1a -12a ) proceeds via (at least)
two pathways, which are both first order with respect
to the concentration of palladium: a pathway indepen-
dent of the alkene concentration with a rate constant
k1 (the so-called k1 pathway) and a pathway with a rate
constant k2, which is first order with respect to the
concentration of alkene (the so-called k2 pathway). The
contributions of both pathways are about the same in
the case of the lowest used concentration of nbd (0.061
The first possible solvento pathway (Scheme 2, path-
way A) involves a rate-determining associative substitu-
tion of the halide X by solvent, i.e., the making of the
solvent-palladium bond is more important than the
breaking of the halide-palladium bond.53,54 Since most
reactions were performed in dichloromethane, which is
(51) Roulet, R.; Gray, H. B. Inorg. Chem. 1972, 11, 2101.
(52) Romeo, R.; Uguagliati, P.; Belluco, U. J . Mol. Catal. 1975, 1,
325.
(53) Cross, R. J . Adv. Inorg. Chem. 1989, 34, 219 and references
therein.
(54) Romeo, R. Comments Inorg. Chem. 1990, 11, 21 and references
therein.
(55) Romeo, R.; Arena, G.; Monsu` Scolaro, L.; Plutino, M. R.; Bruno,
G.; Nicolo`, F. Inorg. Chem. 1994, 33, 4029.
(56) Romeo, R.; Monsu` Scolaro, L.; Nastasi, N.; Arena, G. Inorg.
Chem. 1996, 35, 5087.
(57) Minniti, D. Inorg. Chem. 1994, 33, 2631.
(58) Kawataka, F.; Kayaki, Y.; Shimizu, I.; Yamamoto, A. Organo-
metallics 1994, 13, 3517.
(47) Garrou, P. E.; Heck, R. F. J . Am. Chem. Soc. 1976, 98, 4115.
(48) Anderson, G. K.; Cross, R. J . J . Chem. Soc., Dalton Trans. 1979,
1246.
(49) Anderson, G. K.; Cross, R. J . Acc. Chem. Res. 1984, 17, 67.
(50) Delis, J . P. G.; Groen, J . H.; Vrieze, K.; van Leeuwen, P. W. N.
M.; Veldman, N.; Spek, A. L. To be submitted for publication.

Insertion of Norbornadiene into Pd-C Bonds
Organometallics, Vol. 16, No. 1, 1997 75
positioned trans to the acyl ligand,61 which is favorable
in view of a subsequent migratory insertion (pathway
B). Migratory insertion and rearrangements result in
formation of the product. Alternatively, substitution of
the halide X by the apically coordinated nitrogen atom
in species VI may occur (pathway C). This reaction,
which can be expected to occur rather easily due to the
relatively large trans influence of the precoordinated
alkene positioned trans to the halide ligand, results via
formation of species II in the product as proposed in
pathway A.
Sch em e 2. Th r ee P ossible P a th w a ys for th e k1
P a th w a y of th e Alk en e In ser tion in Acylp a lla d iu m
Com p lexes Bea r in g Ar -BIAN
Pathways A-C are in agreement with the following
observations.
(i) Increasing of the coordinating ability of the solvent,
i.e. increasing the acetonitrile percentage in a dichloro-
methane solution, leads to an increase of k1.
(ii) The nature of neither the halide X nor the C(O)R
ligand in the complexes Pd(C(O)R)X(Ar-BIAN) influ-
ences the first-order rate constant k1 significantly.
(iii) The value of k1 is independent of the presence of
free halide.
Considering the pathway via partial halide dissocia-
tion (pathway A), the remarkable retardation of the k1
pathway upon addition of free ligand may be explained
by substitution of the weakly coordinated solvent in
species I by BIAN (eq 4). This may result in species X,
a moderately polar solvent, it is likely that the dissoci-
ated halide remains in the neighborhood of the complex,
forming contact ion pair intermediate I.27,39,59 Substitu-
tion of the weakly coordinated solvent molecule by the
alkene results in species II, from which a migration of
the acyl ligand to the precoordinated alkene may occur.
Migratory insertion reactions from this kind of ionic
complex are known to proceed very easily.23-25,27,28,31,60
Complete halide dissociation in species III results in the
product. Two alternative pathways (Scheme 2, path-
ways B and C) proceed via a rate-determining associa-
tive substitution of a nitrogen atom by solvent. This
involves dissociation of the nitrogen atom positioned
trans to the acyl ligand, which has a larger trans
influence than the halide ligand.61 Since the BIAN
ligand is a rigid ligand, the dissociated nitrogen atom
will remain in the vicinity of the palladium center,
which will result in intermediate IV. Coordination of
a bidentate ligand in such an asymmetric fashion has
been reported very recently for the BIAN ligand30 and
has been observed also for other rigid nitrogen
ligands.62,63 Species IV may be considered to be a
T-shaped intermediate in which the fourth coordination
site is occupied by a weakly coordinating solvent mol-
ecule. A cis-trans isomerization, which is known to
proceed rather quickly in this kind of T-shaped inter-
mediate,54 followed by an alkene association leads to
species VI. In this species the nitrogen atom, which has
a larger trans influence than the halide ligand, is
in which both nitrogen ligands are coordinated in a
unidentate fashion.64-66 Since substitution of one of the
ligands in species X by nbd can be expected to be more
difficult than in the case of species I, a decrease of the
value of k1 might be observed. Also, in the case of the
alternative pathways B and C free BIAN may substitute
the coordinated solvent in species IV or V, resulting in
the formation of less reactive complexes. It must be
noted, however, that upon addition of free p-An-BIAN
to a solution of 1a in CDCl3, in the presence or in the
absence of nbd, species of the type X have not been
1
observed in the H NMR in the temperature range of
-80 to +20 °C. This is to say that if the supposed
intermediates in Scheme 2 were in fast preequilibrium,
we could not explain the retarding influence of free
BIAN.
Th e Alk en e-Dep en d en t k2 P a th w a y. Analogous
to the alkene-independent k1 pathway, the large nega-
tive value of the entropy of activation (-180 to -111 J
mol-1 K-1) and the observed decrease of the value of k2
(64) Very recently, an X-ray structure of [Pd(Me)(p-An-BIAN)2]SO3-
CF3 has been determined. The X-ray structure shows one BIAN ligand
coordinated in a bidentate fashion, while the other BIAN ligand is
coordinated in a unidentate fashion.65 The observed equivalence of both
ligands in the 1H NMR in the temperature range -80 to +20 °C
indicates also the presence of a species containing two unidentate
coordinated BIAN ligands, i.e. a species similar to species X. Further-
more, coordination of two bidentate nitrogen ligands in a unidentate
fashion has been observed recently for [Rh(η2:η2-nbd)(iPr-6-Me-
(59) Oslinger, M.; Powell, J . Can. J . Chem. 1973, 51, 274.
(60) Kawataka, F.; Shimizu, I.; Yamamoto, A. Bull. Chem. Soc. J pn.
1995, 68, 654.
(61) Appleton, T. G.; Clark, H. C.; Manzer, L. E. Coord. Chem. Rev.
1973, 10, 335.
(62) Hansson, S.; Norrby, P.-O.; Sjo¨gren, M. P. T.; A° kermark, B.;
Cucciolito, M. E.; Giardano, F.; Vitagliano, A. Organometallics 1993,
12, 4940.
PyCa)2]SO3CF3 (iPr-6-Me-PyCa
) 2-(N-2-propanecarbaldimino)-6-
methylpyridine).66
(65) Groen, J . H.; de J ong, B. J .; Vrieze, K.; Smeets, W. J . J .; Spek,
A. L. To be submitted for publication.
(63) Fanizzi, F. P.; Maresca, L.; Natile, G.; Lanfranchi, M.; Tirip-
icchio, A.; Pacchioni, G. J . Chem. Soc., Chem. Commun. 1992, 333.
(66) Haarman, H. F.; Bregman, F. R.; Ernsting, J .-M.; Elsevier, C.
J .; Vrieze, K.; Veldman, N.; Spek, A. L. Organometallics, in press.

76 Organometallics, Vol. 16, No. 1, 1997
Groen et al.
Sch em e 3. Th r ee P ossible P a th w a ys for th e k2
P a th w a y of th e Alk en e In ser tion in Acylp a lla d iu m
Com p lexes Bea r in g Ar -BIAN
however, by dissociation of the nitrogen atom positioned
trans to the acyl ligand. This results in species IX,
which subsequently may isomerize to species VI. It
should be noted that VI also may be formed directly
from VIII. However, this must involve dissociation of
the nitrogen atom positioned trans to the halide ligand,
which can be considered to be more difficult. Formation
of VI may be followed by either a rate-determining
migratory insertion followed by rearrangements (path-
way B) or substitution of the halide X by the apically
coordinated nitrogen atom (pathway C). The latter
reaction results via formation of species II in the product
as proposed in pathway A. Also, pathways B and C are
at first sight attractive ones for the k2 pathway, since
they both explain the unobserved mass-law retardation
upon addition of free halide and free ligand and the
increase of k2 upon increasing the electron-withdrawing
character of the Ar-BIAN ligand in a way similar to that
mentioned for pathway A. Furthermore, increase of the
electron-withdrawing properties of the BIAN ligand may
weaken the nitrogen-palladium bonds, leading to a
more facile formation of species IX. However, pathway
B does not explain the observed rate enhancement upon
increasing the polarity of the solvent and is not in line
with the dependence of the value of k2 of the halide
ligand. Therefore, we prefer a pathway proceeding via
formation of contact ion pair intermediate II, i.e. path-
way A and/or C.
Interestingly, a retardation of the k2 pathway is
observed upon increasing the electron-releasing prop-
erty of the migrating C(O)R group, while maintaining
a constant steric demand, i.e. in the range R ) Ph >
iPr; the opposite is a more common feature in migratory
insertion reactions.21,22,49,68-73 Comparison of the pa-
rameters of activation indicates that the relatively high
reactivity of Pd(C(O)Ph)Cl(p-An-BIAN) (6a ) is attribut-
able to a more favorable enthalpy of activation rather
than entropy of activation. This is not unexpected, since
all complexes can be assumed to react via a mechanism
with similar intermediates. The relatively low enthalpy
of activation for 6a may be explained by considering the
five-coordinate species VIII. This species may be more
stabilized than those derived from 1a , 4a , and 5a , which
contain the more electron-releasing C(O)Me, C(O)Et,
and C(O)iPr ligands, respectively. Higher CO insertion
rates into phenyl-metal bonds compared to insertion
rates into alkyl-metal bonds have been observed earlier
for rhodium(III)74 and palladium(II)75 complexes, but a
higher reactivity of M-C(O)Ph bonds compared to
M-C(O)R bonds (R ) Me, Et, iPr) toward alkene
insertion reactions has never been reported before.
When the norbornadiene insertion in 1a is performed
in acetone, acetonitrile, and mixtures of dichloromethane
with large amounts of acetonitrile, the reactions are no
longer first-order with respect to the nbd concentration
upon increasing the steric demand of the BIAN ligand
indicate an associative process. Taking also into ac-
count the first order with respect to the nbd concentra-
tion, the k2 pathway may proceed via the square-
pyramidal intermediate VIII (Scheme 3), resulting in
the formation of the contact ion pair intermediate II
(pathway A). A rate-determining migratory insertion
followed by halide dissociation leads to the product. This
pathway is an attractive one for the k2 pathway, since
it explains the following observations.
(i) Increase of the polarity of the solvent leads to a
rate enhancement of the k2 pathway. Polar solvents will
stabilize the highly polarized contact ion pair II by
efficient solvation.
(ii) The k2 rate constant increases in the range X )
Cl < Br < I, i.e. with decreasing Pd-X bond strength.67
Weakening of the halide-palladium bond will result in
a more facile formation of intermediate II.
(iii) No mass-law retardation upon addition of free
halide is observed, which is in line with a pathway in
which no (complete) halide dissociation is involved
before or during the rate-determining step.
(iv) A Hammett plot for the k2 rate constant as a
function of the σ values of the para substituents on the
aryl groups of the BIAN ligand results in a straight line,
with a positive slope (F) of 1.4 ( 0.1, indicating that the
rate of the k2 pathway increases with the electron-
withdrawing character of the para substituent on the
aryl groups of the BIAN ligand. It is known that the
resulting decrease of electron density on the metal
center leads to an increase of the stability of five-
coordinate species, such as intermediate VIII, in the
case of complexes bearing sterically demanding rigid
bidentate nitrogen ligands.38,63
(68) Kubota, M.; Blake, D. M.; Smith, S. A. Inorg. Chem. 1971, 10,
1430.
(69) Stille, J . K.; Regan, M. T. J . Am. Chem. Soc. 1974, 96, 1508.
(70) Stille, J . K.; Huang, I. J . Am. Chem. Soc. 1974, 96, 1518.
(71) Blake, D. M.; Winkelman, A.; Chung, Y. L. Inorg. Chem. 1975,
14, 1326.
(72) Calderazzo, F. Angew. Chem., Int. Ed. Engl. 1977, 16, 299.
(73) Ziegler, T.; Versluis, L.; Tschinke, V. J . Am. Chem. Soc. 1986,
108, 612.
(74) Bassetti, M.; Sunley, G. J .; Maitlis, P. M. J . Chem. Soc., Chem.
Commun. 1988, 1012.
Two alternative pathways (Scheme 3, pathways B and
C) also involve the formation of species VIII, followed,
(67) Carares, J . A.; Coco, S.; Espinet, P.; Lin, Y.-S. Organometallics
1995, 14, 3058.
(75) Hoare, J . L.; Cavell, K. J .; Hecker, R.; Skelton, B. W.; White,
A. H. J . Chem. Soc., Dalton Trans. 1996, 2197.

Insertion of Norbornadiene into Pd-C Bonds
Organometallics, Vol. 16, No. 1, 1997 77
but start to exhibit saturation kinetics. This will result
in more complex kinetics, and insertion reactions in
these solvents were not studied any further.
solvent-assisted in the case of the alkene-independent
pathway. In the case of the alkene-dependent pathway
alkene association may precede the dissociation step.
Con clu sion
Ack n ow led gm en t. This work was supported by the
Netherlands Foundation for Chemical Research (SON)
with financial support from the Netherlands Organiza-
tion for Scientific Research (NWO).
The stability and reactivity of palladium complexes
containing the bidentate nitrogen ligand Ar-BIAN have
made it possible to carry out for the first time an
extensive kinetic study of the migratory insertion of
norbornadiene into carbon-palladium bonds of neutral
acylpalladium complexes. The reaction proceeds via an
alkene-independent pathway and an alkene-dependent
pathway. Both pathways are closely related and involve
associative processes. In both cases the mechanism may
involve halide or nitrogen dissociation, which may be
Su p p or tin g In for m a tion Ava ila ble: Tables giving pri-
mary kinetic data for the reactions of 1a -12a with nbd (8
pages). Ordering information is given on any current mast-
head page.
OM960789F