Isocyanide insertion into the palladium-carbon bond of complexes containing bidentate nitrogen ligands: A structural and mechanistic study

Article abstract of DOI:10.1021/om970096e

Insertion of the isocyanides 2,6-dimethylphenyl isocyanide (DIC), teri-butyl isocyanide (TIC) and tosylmethyl isocyanide (TosMIC) into the Pd-Me bond of complexes (N?N)Pd(Me)Cl (N?N = 2,2′-bipyridine (bpy), 1,10-phenanthroline (phen), 2,2′-bipyrimidine (bpm)) afforded quantitatively (N?N)Pd(C(=N-R)Me)Cl (R = 2,6-Me₂C₆H₃, t-Bu, CH₂Tosyl). The course of the reaction has been shown to proceed via the intermediates [(N?N)Pd(CN-R)(Me)]Cl (N?N = bpy, phen; R = 2,6-Me₂C₆H₃, t-Bu, CH₂Tosyl), which have been characterized at 250 K by NMR and IR spectroscopies and conductivity measurements. The mechanism of the insertion reaction involves substitution of the halide by the isocyanide followed by a rate-determining methyl migration to the precoordinated isocyanide. Kinetic measurements of the isocyanide insertion reaction, which provided the reaction rate constants of the isocyanide association (k₁), isocyanide dissociation (k_-1), and methyl migration reaction (k₂), have demonstrated that the migration rate of the methyl group to the precoordinated isocyanide increases with increasing electrophilicity of the isocyanide. Methyl migration in the intermediate complexes [(N?N)Pd(CN-R)(Me)]Cl (N?N = bpy, phen; R = 2,6-Me₂C₆H₃, t-Bu, CH₂Tosyl) also occurs in the solid state. The complexes (N?N)Pd(C(=N-R)Me)X (N?N = bpy, phen; R = t-Bu, CH₂Tosyl; X = Cl, Br) show a fluxional behavior due to a site exchange of the nitrogen donor atoms. Spin saturation transfer ¹H NMR experiments showed that the mechanism of this process involves Pd-N bond breaking and subsequent isomerization via a Y-shaped intermediate.

Full text of DOI:10.1021/om970096e

2948
Organometallics 1997, 16, 2948-2957
Isocya n id e In ser tion in to th e P a lla d iu m -Ca r bon Bon d of
Com p lexes Con ta in in g Bid en ta te Nitr ogen Liga n d s: A
Str u ctu r a l a n d Mech a n istic Stu d y
J ohannes G. P. Delis, Peter G. Aubel, Kees Vrieze,* and
Piet W. N. M. van Leeuwen
Anorganisch Chemisch Laboratorium, J . H. van ’t Hoff Instituut, Universiteit van Amsterdam,
Nieuwe Achtergracht 166, NL-1018 WV Amsterdam, The Netherlands
Nora Veldman and Anthony L. Spek†
Bijvoet Center for Biomolecular Research, Vakgroep Kristal- en Structuurchemie, Universiteit
Utrecht, Padualaan 8, NL-3584 CH Utrecht, The Netherlands
Franciscus J . R. van Neer
Instituut voor Technische Scheikunde, Universiteit van Amsterdam, Nieuwe Achtergracht 166,
NL-1018 WV Amsterdam, The Netherlands
Received February 10, 1997X
Insertion of the isocyanides 2,6-dimethylphenyl isocyanide (DIC), tert-butyl isocyanide (TIC)
and tosylmethyl isocyanide (TosMIC) into the Pd-Me bond of complexes (NkN)Pd(Me)Cl
(NkN ) 2,2′-bipyridine (bpy), 1,10-phenanthroline (phen), 2,2′-bipyrimidine (bpm)) afforded
quantitatively (NkN)Pd(C(dNsR)Me)Cl (R ) 2,6-Me2C6H3, t-Bu, CH2Tosyl). The course of
the reaction has been shown to proceed via the intermediates [(NkN)Pd(CN-R)(Me)]Cl (NkN
) bpy, phen; R ) 2,6-Me2C6H3, t-Bu, CH2Tosyl), which have been characterized at 250 K by
NMR and IR spectroscopies and conductivity measurements. The mechanism of the insertion
reaction involves substitution of the halide by the isocyanide followed by a rate-determining
methyl migration to the precoordinated isocyanide. Kinetic measurements of the isocyanide
insertion reaction, which provided the reaction rate constants of the isocyanide association
(k1), isocyanide dissociation (k-1), and methyl migration reaction (k2), have demonstrated
that the migration rate of the methyl group to the precoordinated isocyanide increases with
increasing electrophilicity of the isocyanide. Methyl migration in the intermediate complexes
[(NkN)Pd(CN-R)(Me)]Cl (NkN ) bpy, phen; R ) 2,6-Me2C6H3, t-Bu, CH2Tosyl) also occurs
in the solid state. The complexes (NkN)Pd(C(dNsR)Me)X (NkN ) bpy, phen; R ) t-Bu,
CH2Tosyl; X ) Cl, Br) show a fluxional behavior due to a site exchange of the nitrogen donor
1
atoms. Spin saturation transfer H NMR experiments showed that the mechanism of this
process involves Pd-N bond breaking and subsequent isomerization via a Y-shaped
intermediate.
In tr od u ction
alkenes.12-14 The mechanism of CO insertion into
Pd-C bonds has been studied extensively for many
systems both experimentally15-19 and theoretically.20-24
Carbon monoxide insertion into metal-carbon bonds
constitutes a key step in many transition-metal-
catalyzed processes.1-4 One of these involves the pal-
ladium-catalyzed copolymerization of CO and alkenes5-11
involving a perfectly alternating insertion of CO and
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G. J . Am. Chem. Soc. 1995, 117, 5263-5274.
* To whom correspondence should be addressed.
† To whom correspondence concerning crystallographic data should
be addressed.
X Abstract published in Advance ACS Abstracts, May 15, 1997.
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S0276-7333(97)00096-4 CCC: $14.00 © 1997 American Chemical Society

Isocyanide Insertion into the Palladium-Carbon Bond
Organometallics, Vol. 16, No. 13, 1997 2949
It has been generally accepted that precoordinated CO
and the hydrocarbyl group should be cis in the reacting
complex15,25,26 and that the hydrocarbyl group migrates
to CO rather than vice versa.27,28 Our study on the
carbonylation of organopalladium compounds, such as
(LkL)Pd(Me)X, showed that the choice of ligands has a
large influence on the reaction. Unexpectedly, it ap-
peared that insertion of CO into the Pd-C bond of the
neutral complexes (NkN)Pd(Me)X,12,18 of which the
bidentate nitrogen ligands are both flexible and rigid
with a small bite angle, is usually faster than insertion
of CO into the Pd-C bond of the analogous complexes
(PkP)Pd(Me)X, of which the diphosphine ligands are
flexible with a small or large bite angle.29 Surprisingly,
the insertion of CO in the complex {σ3-N,N′,N′′-2,2′:6′,2′′-
terpyridyl}methylpalladium(II) chloride containing a
terdentate nitrogen ligand is very fast.30
The CO insertion process in the case of (NkN)Pd-
(Me)X may involve halide or nitrogen dissociation upon
coordination of CO. The latter possibility might well
occur as structural studies on Pd-η3-allyl complexes
and (2,9-dimethyl-1,10-phenanthroline)Pt(X)2L com-
plexes (L ) C2H4, CO, PPh3, ONPh) revealed monoden-
tate coordination of a nitrogen ligand for flexible ligands
as well as for rigid ones.31-33
To obtain more insight into the mechanism of CO
insertion, we turned our attention to insertion of the
isoelectronic isocyanides. Isocyanides might allow the
observation of intermediate complexes and variation of
electronic and steric factors by variation of the R group
on the nitrogen atom, and moreover, it would be of great
interest to look at the possibility for the direct synthesis
of the polyimine analogue of polyketone. Since it is
known that complexes having bidentate nitrogen ligands
allow fast insertions of CO and olefins,12-14,18,32,34 we
wished to explore the prospect of copolymerization of
isocyanides and olefins. Isocyanide insertion into Pd-C
bonds has been extensively studied,35-37 but mainly for
complexes containing phosphine ligands.38-43 We de-
scribe in this report, of which a preliminary account has
already appeared,44 the insertion of isocyanides into the
Pd-Me bond of (NkN)Pd(Me)Cl complexes, the charac-
terization of the reaction intermediates, and a detailed
kinetic study of the isocyanide association, isocyanide
dissociation, and methyl migration reaction.
Exp er im en ta l Section
Ma ter ia l a n d Ap p a r a tu s. All manipulations have been
carried out in an atmosphere of purified, dry nitrogen using
standard Schlenk techniques. Solvents were dried and stored
under nitrogen. 1H and 13C NMR spectra were recorded on a
Bruker AMX 300 and DRX 300 spectrometer (300.13 and 75.48
MHz respectively). Elemental analyses were carried out by
our Institute. Mass spectrometry was carried out on a J EOL
J MS SX/SX102A four-sector mass spectrometer coupled to a
J EOL MS-MP 7000 data system. IR spectra were recorded
on a Bio-Rad FTS-7.
The complexes (bpm)Pd(Me)Cl,10 (bpy)Pd(Me)Cl,18 and (phen)-
Pd(Me)Cl18 were synthesized according to previously reported
procedures. The isocyanides 2,6-dimethylphenyl isocyanide
(DIC), tert-butyl isocyanide (TIC), and tosylmethyl isocyanide
(TosMIC) were purchased from Fluka and used without
purification.
Gen er a l P r oced u r e for In ser tion of DIC a n d TosMIC
P r ovid in g Com p lexes (NkN)P d (C(dNsR)Me)Cl (NkN )
bp y (1), p h en (2), bp m (3); R ) 2,6-Me2C6H3 (a ), CH2-Tosyl
(c)). To a solution of (NkN)Pd(Me)Cl (100 mg, 0.22 mmol) in
dichloromethane (20 mL) the appropriate isocyanide (0.22
mmol) was added, after which the mixture was stirred for 2
h. The volume of the solution was concentrated to 5 mL, and
hexane (30 mL) was added. The crystalline material was
collected by centrifugation.
(bp y)P d (C(dNs2,6-Me2C6H3)Me)Cl (1a ). Yield: 84 mg;
0.19 mmol; 86% 1H NMR (300 MHz, CDCl3) (numbering
3
scheme of the protons is presented in Figure 1): δ 9.15 (d, J
) 4.9 Hz, 1H, H6), 9.09 (d, 3J ) 4.9 Hz, 1H, H6′), 8.04 (m, 4H,
3
H4, H4′, H3, H3′), 7.53 (m, 2H, H5, H5′), 6.98 (d, J ) 7.4 Hz,
2H, Hmeta), 6.84 (dd, 3J ) 7.4 Hz, 1H, Hpara), 2.29 (s, 3H,
C(dNR)CH3), 2.26 (s, 6H, (CH3)2Ph). 13C NMR (75.48 MHz,
CDCl3): δ 183.7 (CdN), 155.6, 152.6 (C6, C6′), 151.9, 149.6,
148.1, 139.3, 139.1, 128.0, 127.0, 126.3, 122.7, 122.4, 121.7
(Cphen, CAr), 29.1 (C(dNR)CH3), 19.5 ((CH3)2Ph). Elemental
Analysis Found (calcd) for C20H20ClN3Pd‚CH2Cl2: C, 46.91
(47.66); H, 4.17 (4.19); N, 7.88 (7.94). FAB MS Found (calcd)
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for C20H20ClN3Pd: 444 (444). IR ν(CdN) (KBr): 1623 cm-1
.
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86, 5043-5044.
(p h en )P d (C(dNs2,6-Me2C6H3)Me)Cl (2a ). Yield: 93 mg;
3
0.20 mmol; 89%. 1H NMR (300 MHz, CDCl3): δ 9.45 (d, J )
3
3
3.9 Hz, 1H, H6), 9.41 (d, J ) 3.9 Hz, 1H, H6′), 8.55 (d, J )
(26) Flood, T. C.; J ensen, J . E.; Statler, J . A. J . Am. Chem. Soc. 1981,
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3
3
8.2 Hz, 1H, H4), 8.44 (d, J ) 8.2 Hz, 1H, H4′), 7.96 (d, J )
3
3
(27) van Leeuwen, P. W. N. M.; Roobeek, C. F.; van der Heijden, H.
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8.2 Hz, 1H, H7), 7.95 (d, J ) 8.2 Hz, 1H, H7′), 7.87 (dd, J )
3
8.2, 4.8 Hz, 1H, H5), 7.79 (dd, J ) 8.2 Hz, 1H, H5′), 6.99 (d,
(28) van Leeuwen, P. W. N. M.; Roobeek, K. F. Recl. Trav. Chim.
Pays.-Bas 1995, 114, 73-75.
3J ) 7.4 Hz, 2H, Hmeta), 6.84 (dd, J ) 7.4 Hz, 1H, Hpara), 2.38
3
(s, 3H, C(dNR)CH3), 2.31 (s, 6H, (CH3)2Ph). 13C NMR (75.48
MHz, CDCl3): δ 183.4 (CdN), 152.5, 150.0 (C6, C6′), 148.1,
146.7, 144.3, 138.2, 137.8, 130.0, 129.4, 128.0, 127.8, 127.1,
(29) Dekker, G. P. C. M.; Elsevier, C. J .; Vrieze, K.; van Leeuwen,
P. W. N. M. Organometallics 1992, 11, 1598-1603.
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K.; Veldman, N.; Spek, A. L. Organometallics 1997, 16, 551-562.
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2950 Organometallics, Vol. 16, No. 13, 1997
Delis et al.
p h en (2), bp m (3); R ) t-Bu (b)). To a solution of (NkN)-
Pd(Me)Cl (100 mg, 0.22 mmol) in dichloromethane (20 mL) at
0 °C, TIC (19 mg, 0.22 mmol) was added. The solution was
stirred for 15 min, after which it was warmed to room
temperature and stirred for another 16 h. The volume of the
solution was concentrated to 5 mL, and hexane (30 mL) was
added. The crystalline material was collected by centrifuga-
tion.
(bp y)P d (C(dNsC(CH 3)3)Me)Cl (1b). Yield: 75 mg; 0.19
mmol; 82%. 1H NMR (300 MHz, CDCl3) (numbering scheme
is presented in Figure 1): δ 9.08 (d, 3J ) 4.7 Hz, 1H, H6),
3
8.53 (d, J ) 4.7 Hz, 1H, H6′), 8.09 (m, 4H, H4, H4′, H3, H3′),
7.52 (m, 2H, H5, H5′), 2.49 (s, 3H, C(dNR)CH3), 1.62 (s, 9H,
C(CH3)3). 13C NMR (75.48 MHz, CDCl3): δ 183.4 (CdN), 155.0,
152.6 (C6, C6′), 151.3, 148.7, 138.9, 126.5, 126.0, 122.9, 122.7,
121.6 (Cphen, CAr), 63.5 (C(CH3)3), 32.2 (C(CH3)3), 29.4 (C(dNR)-
CH3). FAB MS Found (calcd) for C16H20N3ClPd: 396 (396).
IR ν(CdN) (KBr): 1636 cm-1
.
(p h en )P d (C(dNsC(CH3)3)Me)Cl (2b). Yield: 70 mg; 0.17
3
mmol; 76%. 1H NMR (300 MHz, CDCl3): δ 9.37 (d, J ) 4.7
3
3
Hz, 1H, H6), 8.79 (d, J ) 4.7 Hz, 1H, H6′), 8.60 (d, J ) 8.2
3
3
Hz, 1H, H4), 8.51 (d, J ) 8.2 Hz, 1H, H4′), 8.02 (d, J ) 8.2
3
Hz, 1H, H7), 8.00 (d, J ) 8.2 Hz, 1H, H7′), 7.88 (m, 2H, H5,
F igu r e 1. Isocyanide insertion in (NkN)Pd(Me)Cl com-
plexes.
H5′), 2.69 (s, 3H, C(dNR)CH3), 1.71 (s, 9H, C(CH3)3). 13C NMR
data (75.48 MHz, CDCl3); δ CdN not observed, 151.1, 149.3
(C6, C6′), 146.2, 144.2, 137.7, 137.4, 129.8, 129.2, 127.4, 126.5,
125.2 (Cphen, CAr), 32.2 (C(CH3)3), 30.2 (C(dNR)CH3). FAB MS
126.9, 125.3, 125.1, 122.4, 117.4, 117.2 (Cphen, CAr), 29.4
(C(dNR)CH3), 19.7 ((CH3)2Ph). Elemental Analysis Found
(calcd) for C22H20ClN3Pd‚1/2CH2Cl2: C, 52.69 (52.91); H, 4.33
(4.15); N, 7.93 (8.23). FAB MS Found (calcd) for C22H20ClN3-
Found (calcd) for
C18H20N3ClPd: 420 (420). IR ν(CdN)
(KBr): 1637 cm-1
.
Pd: 468 (468). IR ν(CdN) (KBr): 1629 cm-1
.
(bp m )P d (C(dNsC(CH3)3)Me)Cl (3b). Yield: 61 mg; 0.15
mmol; 70%. 1H NMR (300 MHz, CDCl3): δ 9.30 (dd, J ) 5.0
3
(bp y)P d (C(dNsCH2Tosyl)Me)Cl (1c). Yield: 85 mg;
4
3
4
3
0.17 mmol; 76%. 1H NMR (300 MHz, CDCl3): δ 9.02 (d, J )
Hz, J ) 2.2 Hz, 1H, H6′), 9.24 (dd, J ) 5.0 Hz, J ) 2.2 Hz,
3
4
3
1H, H4), 9.17 (dd, J ) 5.0 Hz, J ) 2.2 Hz, 1H, H4′), 8.76 (d,
5.1 Hz, 1H, H6), 8.56 (d, J ) 5.1 Hz, 1H, H6′), 8.10 (m, 4H,
3J ) 5.0 Hz, 1H, H6), 7.71 (d, J ) 5.0 Hz, 1H, H5′), 7.67 (d,
3
3
H4, H4′, H3, H3′), 7.52 (m, 2H, H5, H5′), 7.88 (d, J ) 8.2 Hz,
3J ) 5.0 Hz, 1H, H5), 2.48 (s, 3H, C(dNR)CH3), 1.63 (s, 9H,
C(CH3)3). 13C NMR (75.48 MHz, CDCl3): δ 190.5 (CdN), 161.4,
160.0, 159.6, 159.3, 158.5, 156.7, 124.5, 124.2 (Cbpm), 68.3
(C(CH3)3), 32.9 (C(CH3)3), 32.8 (C(dNR)CH3). FAB MS Found
(calcd) for C14H18N5ClPd: 398 (398). IR ν(CdN) (KBr): 1648
3
3
2H, Hortho), 7.33 (d, J ) 8.2 Hz, 2H, Hmeta), 5.82 (d, J ) 13.9
3
Hz, 1H, CH2SO2), 5.27 (d, J ) 13.9 Hz, 1H, CH2SO2), 2.44 (s,
3H, Ph-CH3), 2.44 (s, 3H, C(dNR)CH3). 13C NMR (75.48
MHz, CDCl3): δ CdN not observed, 155.3, 152.5, 139.1, 138.8,
129.3, 128.6, 127.1, 126.4, 122.0, 121.0 (Cphen, CAr), 31.1
(C(dNR)CH3), 21.5 (Ph-CH3). Elemental Analysis Found
(calcd) for C20H20ClN3O2SPd: C, 46.43 (47.25); H, 4.02 (3.97);
N, 8.07 (8.26). FD MS Found (calcd) for C20H20ClN3O2SPd:
cm-1
.
Syn th esis of (bp m )P d (C(dNsC(CH3)3)Me)Br . The com-
plex (bpm)Pd(C(dNst-Bu)Me)Cl (100 mg, 0.25 mmol) and KBr
(166 mg, 1.4 mmol) were dissolved in acetone (40 mL) and
stirred for 2 h. The solvent was evaporated, and the residue
was washed twice with THF. The volume of the solvent was
concentrated and hexane (30 mL) was added, providing a
yellow crystalline material, which was collected by centrifuga-
tion. Yield: 100 mg; 0.22 mmol; 90%. 1H NMR (300 MHz,
509 (508). IR ν(CdN) (KBr): 1617 cm-1
.
(p h en )P d (C(dNsCH2Tosyl)Me)Cl (2c). Yield: 89 mg;
3
0.17 mmol; 76%. 1H NMR (300 MHz, CDCl3): δ 9.28 (d, J )
3
3
4.6 Hz, 1H, H6), 8.81 (d, J ) 4.6 Hz, 1H, H6′), 8.59 (d, J )
3
3
8.2 Hz, 1H, H4), 8.54 (d, J ) 8.2 Hz, 1H, H4′), 8.02 (d, J )
3
8.2 Hz, 1H, H7), 8.01 (d, J ) 8.2 Hz, 1H, H7′), 7.89 (m, 2H,
3
4
H5, H5′), 7.89 (d, 3J ) 8.2 Hz, 2H, Hortho), 7.34 (d, 3J ) 8.2 Hz,
CDCl3): δ 9.42 (dd, J ) 5.0 Hz, J ) 2.2 Hz, 1H, H6′), 9.25
3
4
3
3
3
(dd, J ) 4.6 Hz, J ) 2.0 Hz, 1H, H4), 9.16 (dd, J ) 4.7 Hz,
2H, Hmeta), 5.99 (d, J ) 13.9 Hz, 1H, CH2SO2), 5.33 (d, J )
13.9 Hz, 1H, CH2SO2), 2.71 (s, 3H, Ph-CH3), 2.43 (s, 3H,
C(dNR)CH3). 13C NMR (75.48 MHz, CDCl3): δ CdN not
observed, 154.0, 149.7 (C6, C6′), 145.1, 145.0, 138.8, 138.5,
130.5, 130.1, 129.9, 129.2, 127.9, 127.4, 126.3, 125.8 (Cphen, CAr),
32.3 (C(dNR)CH3), 22.1 (Ph-CH3). Elemental Analysis Found
(calcd) for C22H20ClN3O2SPd: C, 48.84 (49.63); H, 3.81 (3.79);
N, 7.70 (7.89). FAB MS Found (calcd) for C22H20ClN3O2SPd:
4J ) 2.2 Hz, 1H, H4′), 8.78 (d, J ) 3.2 Hz, 1H, H6), 7.72 (d,
3
3J ) 4.8 Hz, 1H, H5′), 7.68 (d, J ) 4.8 Hz, 1H, H5), 2.50 (s,
3
3H, C(dNR)CH3), 1.62 (s, 9H, C(CH3)3). 13C NMR (75.48 MHz,
CDCl3): δ CdN not observed, 160.8, 160.3, 158.7, 157.8, 157.6,
156.9, 123.8, 123.6 (Cbpm), 55.4 (C(CH3)3), 32.4 (C(CH3)3), 30.3
(C(dNR)CH3). FAB MS Found (calcd) for C14H18N5BrPd: 442
(442). IR ν(CdN) (KBr): 1648 cm-1
.
532 (532). IR ν(CdN) (KBr): 1618 cm-1
.
Ch a r a ct er iza t ion of t h e In t er m ed ia t e Com p lexes
[(NkN)P d (CN-R)(Me)]Cl (NkN ) bp y (4), p h en (5); R )
2,6-Me2C6H 3 (a ), t-Bu (b ), CH 2-Tosyl (c)) by 1H NMR
Sp ectr oscop y. To a solution of (NkN)Pd(Me)Cl (5 mg, 13
µmol) in 0.5 mL of CDCl3 was added the appropriate amount
of a stock solution of isocyanide (13 µmol) in CDCl3 at 250 K
after which the color of the solution turned from yellow to
colorless. The solution was transferred to a precooled 5 mm
NMR tube. The intermediates 4a , 4b, and 5a at temperatures
higher than 273 K turned out to be in equilibrium with the
starting complex and isocyanide, as intermediate complex 4a
could be observed together with starting complex 1 and
uncoordinated DIC. Intermediate complex 4b could be ob-
served together with starting complex 1 and uncoordinated
TIC, and intermediate 5a could be observed together with
(bp m )P d (C(dNsCH2Tosyl)Me)Cl (3c). Yield: 90 mg;
0.18 mmol; 80%. 1H NMR (300 MHz, CDCl3): δ 9.22 (m, 3H,
3
4
C-Hbpm), 9.92 (dd, J ) 5.6, J ) 2.2 Hz, 1H, C-Hbpm), 7.74 (t,
3J ) 4.9 Hz, 1H, H5), 7.73 (t, 3J ) 4.9 Hz, 1H, H5′), 7.86 (d, 3J
3
) 8.2 Hz, 2H, Hortho), 7.35 (d, J ) 8.2 Hz, 2H, Hmeta), 5.93 (d,
3J ) 13.4 Hz, 1H, CH2SO2), 5.20 (dq, J ) 13.4 Hz, J ) 1.3
Hz, 1H, CH2SO2), 2.45 (d, 5J ) 1.3 Hz, 3H, C(dNR)CH3), 2.45
(s, 3H, Ph-CH3). 13C NMR (75.48 MHz, CDCl3): δ 196.2
(CdN), 160.4, 160.0, 159.4, 156.3, 124.7, 124.3 (C-H), 161.4,
159.8 (Cq), 145.0, 136.0, 130.0, 129.1 (CAr), 77.7 (CH2SO2), 31.4
(C(dNR)CH3), 22.1 (Ph-CH3). FAB MS Found (calcd) for
3
5
C
18H18ClN5O2SPd: 510 (510). IR ν(CdN) (KBr): 1619 cm-1
.
Gen er a l P r oced u r e for th e In ser tion of TIC P r ovid in g
Com p lexes (NkN)P d (C(dNsR)Me)Cl (NkN ) bp y (1),

Isocyanide Insertion into the Palladium-Carbon Bond
Organometallics, Vol. 16, No. 13, 1997 2951
starting complex 2 and uncoordinated DIC. This equilibrium
was shifted to the right for complexes 4c, 5b, and 5c, as they
are the only observed species in the temperature ranging from
250 to 320 K.
where Cj is the concentration of component j and n is the
number of the components.
The least-squares fitting was performed using a modified
Powell algorithm45 to find the minimum in the objective
function.
The concentration of the components were calculated via
numerical integration of a set of coupled ordinary differential
equations describing the concentrations as a function of time.
The governing equations are the following in the case of the
isocyanide insertion:
Elemental analyses and mass spectrometry could not be
performed since complexes 4a -c and 5a -c reacted in the solid
state to give complexes 1a -c and 2a -c, respectively, at room
temperature.
[(bp y)P d (CN-2,6-Me2C6H3)(Me)]Cl (4a ). 1H NMR (300
3
MHz, CDCl3, 223 K): δ 9.46 (d, J ) 8.8 Hz, 1H, H3), 9.43 (d,
3
3J ) 8.8 Hz, 1H, H3′), 8.60 (d, J ) 4.8 Hz, 1H, H6), 8.56 (d,
k2
9
2 + DIC yk1z 5a
8 2a
3
3
3J ) 4.8 Hz, 1H, H6′), 8.38 (t, J ) 7.7 Hz, 1H, H4), 8.34 (t, J
3
3
k-1
) 7.7 Hz, 1H, H4′), 7.72 (t, J ) 6.1 Hz, 1H, H5), 7.66 (t, J )
3
3
6.1 Hz, 1H, H5′), 7.36 (t, J ) 7.4 Hz, 1H, Hpara), 7.23 (d, J )
7.4 Hz, 2H, Hmeta), 2.54 (s, 6H, (CH3)2Ph), 1.12 (s, 3H, Pd-
CH3).
d[2]
dt
) -k1[2][DIC] + k-1[5a ]
[(bp y)P d (CN-C(CH3)3)(Me)]Cl (4b). 1H NMR (300 MHz,
3
3
CDCl3, 223 K): δ 9.30 (d, J ) 8.2 Hz, 1H, H3), 9.25 (d, J )
d[DIC]
dt
3
3
) -k1[2][DIC] + k-1[5a ]
8.2 Hz, 1H, H3′), 8.48 (d, J ) 5.6 Hz, 1H, H6), 8.46 (d, J )
5.6 Hz, 1H, H6′), 8.33 (m, 2H, H4, H4′), 7.69 (m, 2H, H5, H5′),
1.69 (s, 9H, C(CH3)3), 0.93 (s, 3H, Pd-CH3).
d[5a ]
dt
[(bp y)P d (CN-CH2Tosyl)(Me)]Cl (4c). 1H NMR (300
MHz, CDCl3, 223 K): δ 8.90 (br, 1H, H6), 8.73 (br, 1H, H6′),
8.31 (br, 2H, H4, H4′), 8.10 (br, 1H, H3), 7.83 (br, 1H, H3′),
7.67 (br, 1H, H5), 7.47 (br, 1H, H5′), 8.00 (br, 2H, Hortho), 7.47
(br, 2H, Hmeta), 6.11 (s, 2H, CH2SO2), 2.42 (s, 3H, Ph-CH3),
0.66 (s, 3H, Pd-CH3).
) k1[2][DIC] - k-1[5a ] - k2[5a ]
d[2a ]
) k2[5a ]
dt
The differential equations were solved numerically with the
EPISODE method.45 This algorithm is suited for integration
of stiff equations, i.e., in the case of the occurrence of both very
fast and slow chemical reactions.
[(p h en )P d (CN-2,6-Me2C6H3)(Me)]Cl (5a ). 1H NMR (300
MHz, CDCl3, 223 K): δ 9.01-8.92 (m, 4H, H6, H6′, H5, H5′),
3
3
8.21 (d, J ) 7.3 Hz, 1H, H4), 8.17 (d, J ) 7.3 Hz, 1H, H4′),
8.28 (s, 2H, H7, H7′), 2.61 (s, 6H, (CH3)2Ph), 1.32 (s, 3H, Pd-
CH3).
Sp in Sa tu r a tion Tr a n sfer Mea su r em en ts. The lattice
relaxation times were obtained using standard inversion
recovery methods with 10 data points and a 90°/180° pulse
width. The T1 was measured at the lowest and highest
temperature for which a spin saturation transfer measurement
was carried out and varied from ca. 1.5-3 s in the temperature
range from 273 to 334 K. The temperature of the NMR sample
was checked externally with CH3OH with an accuracy of 0.6
°C. The spin saturation transfer measurements were carried
out using the Forse´n-Hoffman method46 with a (Td-π/2)n
pulse sequence (presaturation time Td ) 0.1-25 s, presatu-
ration power 70 dB, relaxation delay d1 ) 25 s, eight scans
per data point).
[(p h en )P d (CN-C(CH3)3)(Me)]Cl (5b). 1H NMR (300
MHz, CDCl3, 223 K): δ 8.98-8.86 (m, 4H, H6, H6′, H5, H5′),
3
3
8.24 (d, J ) 7.3 Hz, 1H, H4), 8.12 (d, J ) 7.3 Hz, 1H, H4′),
8.24 (s, 2H, H7, H7′), 1.79 (s, 9H, C(CH3)3), 1.11 (s, 3H, Pd-
CH3).
[(p h en )P d (CN-CH2Tosyl)(Me)]Cl (5c). 1H NMR (300
3
MHz, CDCl3, 223 K): δ 9.21 (d, J ) 8.0 Hz, 1H, H3), 8.92 (d,
3J ) 8.0 Hz, 1H, H3′), 8.50 (m, 2H, H6, H6′), 8.15-7.96 (m,
4H, H4, H4′, Hortho), 7.85 (s, 2H, H7, H7′), 7.50 (d, 3J ) 7.8 Hz,
2H, Hmeta), 6.22 (s, 2H, CH2SO2), 2.49 (s, 3H, Ph-CH3), 0.90
(s, 3H, Pd-CH3).
Rea ction in th e Solid Sta te of th e In ter m ed ia te Com -
p lexes 4a -c a n d 5a -c P r ovid in g th e Com p lexes 1a -c
a n d 2a -c. The intermediate complexes were synthesized as
described above, after which precooled (250 K) hexanes (5 mL)
was added. The crystalline material was collected by cen-
trifugation and dried in vacuo. The conversion of intermediate
complexes in the solid state to the products at 293 or 320 K
Cr ysta l Str u ctu r e Deter m in a tion of 1a . Crystal and
refinement data for complex 1a and experimental details of
the crystal structure determination have been published in a
preliminary account of this work.44 Atomic coordinates, bond
lengths, bond angles, and thermal parameters have been
deposited in the Supporting Information and at the Cambridge
Crystallographic Data Centre.
1
was followed by H NMR spectroscopy in precooled CDCl3.
Kin etic Mea su r em en ts of In ser tion of Isocya n id e by
1H NMR Sp ectr oscop y. The intermediate complexes in
CDCl3 were synthesized as described above, after which the
NMR tube was transferred to a prethermostated NMR spec-
trometer. The relative concentrations of the compounds in the
solution were determined as a function of time.
Resu lts
The isocyanides 2,6-dimethylphenyl isocyanide (DIC),
tert-butyl isocyanide (TIC), and tosylmethyl isocyanide
(TosMIC) inserted quantitatively into the Pd-C bond
of the complexes (bpy)Pd(Me)Cl (bpy ) 2,2′-bipyridine)
(1), (phen)Pd(Me)Cl (phen ) 1,10-phenanthroline) (2),
and (bpm)Pd(Me)Cl (bpm ) 2,2′-bipyrimidine) (3) when
1 equiv of isocyanide was added to a solution of the
starting complex in dichloromethane (see Figure 1). The
insertion reactions of DIC and TosMIC have been
carried out at 294 K, while the insertion reactions of
TIC with complexes 1, 2, and 3 were carried out by
addition of TIC to a solution of the starting complex in
dichloromethane at 273 K and slow warming of the
reaction mixture from 273 to 294 K, since otherwise
Deter m in a tion of th e Kin etic Con sta n ts for Rea ction
of 2 w ith DIC. The reaction rate constants of the elementary
isocyanide association, dissociation, and methyl migration
steps with the rate constants k1, k-1, and k2, respectively, were
determined for each set of data by fitting the composition of
the reaction mixture by a nonlinear regression. The composi-
1
tion of the reaction mixture was determined by means of H
NMR during the reaction time. The minimum objective
function, which was the sum of squares of the residual
concentrations of 1, DIC, 5a , and 2a , is the difference between
the observed and calculated value:
n
(45) Press, W. H. Numerical Recipes in Fortran; Cambridge Uni-
versity Press: New York, 1992.
(46) Mann, B. E. J . Magn. Reson. 1976, 21, 17-23.
(Cjobs - Cjcalcd
)
2
φ )
∑
i)1

2952 Organometallics, Vol. 16, No. 13, 1997
Delis et al.
uncharacterized side products were formed. The reac-
tion time strongly depends on the isocyanides; TosMIC,
DIC, and TIC required 1, 3, and 16 h, respectively,
before total conversion of the starting complex was
reached. The reactions are highly selective under the
mentioned conditions, since formation of other products
has not been observed except for reaction of 3 with DIC
in dichloromethane, which provided several uncharac-
terized side products in addition to the expected inser-
tion product.
The solid products (bpy)Pd(C(dNR)Me)Cl (1a -c),
(phen)Pd(C(dNR)Me)Cl (2a -c), and (bpm)Pd(C(dNR)-
Me)Cl (3b,c) (R ) 2,6-Me2C6H3 (a ), t-Bu (b), and
tosylmethyl (c)) are stable under an atmosphere of
nitrogen for weeks, while decomposition occurs in air
within several days.
We have used the strongly σ-coordinating bidentate
nitrogen ligands bpy, phen, and bpm, since other
R-diimine ligands like bis(arylimino)acenaphthene (Ar-
BIAN) and 1,4-diazabutadiene (R-DAB) are substituted
by isocyanides. Substitution of the bidentate nitrogen
ligands bpy, phen, and bpm has been observed when
more than 1 equiv of isocyanide was added to the
reaction mixture.
It appears that there is a difference in the σ-coordi-
nating capabilities between the ligands phen, bpy, and
bpm. Competition experiments showed that addition
of 1 equiv of bpy to a solution of complex 3b in CDCl3
resulted in an immediate substitution of bpm by bpy,
and as expected, addition of 1 equiv of bpm to a solution
of 1b in CDCl3 did not give substitution of bpy by bpm.
Analogous experiments with phen and bpy and their
complexes demonstrated that coordination of phen
toward Pd(II) is favored with respect to bpy. Therefore,
we can conclude that the σ-coordinating capabilities
increase in the order bpm < bpy < phen.
F igu r e 2. ORTEP plot at 50% probability level of complex
(bpy)Pd(C(dNs2,6-Me2C6H3)Me)Cl (1a ).
Ta ble 1. Bon d Dista n ces (Å) for
(bp y)P d (C(dNs2,6-Me2C6H3)Me)Cl (1a ) (w ith Esd ’s
in P a r en th eses)
Pd(1)-Cl(1)
Pd(1)-N(1)
Pd(1)-N(2)
Pd(1)-C(11)
N(1)-C(1)
N(1)-C(5)
N(2)-C(6)
N(2)-C(10)
N(3)-C(11)
N(3)-C(13)
C(1)-C(2)
C(2)-C(3)
C(3)-C(4)
C(4)-C(5)
C(5)-C(10)
2.3081(18)
2.146(5)
2.066(5)
1.976(5)
1.323(9)
1.348(7)
1.320(8)
1.380(7)
1.258(8)
1.416(9)
1.386(9)
1.350(10)
1.376(10)
1.399(8)
1.482(9)
C(6)-C(7)
1.380(9)
1.387(9)
1.380(10)
1.359(9)
1.510(10)
1.406(8)
1.399(9)
1.401(11)
1.504(10)
1.370(11)
1.378(10)
1.386(11)
1.498(9)
1.733(9)
1.700(8)
C(7)-C(8)
C(8)-C(9)
C(9)-C(10)
C(11)-C(12)
C(13)-C(14)
C(13)-C(18)
C(14)-C(15)
C(14)-C(20)
C(15)-C(16)
C(16)-C(17)
C(17)-C(18)
C(18)-C(19)
Cl(2)-C(21)
Cl(3)-C(21)
Ta ble 2. Bon d An gles (d eg) for
(bp y)P d (C(dNs2,6-Me2C6H3)Me)Cl (1a ) (w ith Esd ’s
in P a r en th eses)
The products 1a -c, 2a -c and, 3b,c have been fully
characterized by 1H and 13C NMR and IR spectroscopies,
mass spectroscopy, and elemental analysis and by an
X-ray structure determination of complex 1a .
Cl(1)-Pd(1)-N(1)
Cl(1)-Pd(1)-N(2)
Cl(1)-Pd(1)-C(11)
N(1)-Pd(1)-N(2)
96.69(13) C(7)-C(8)-C(9)
172.77(14) C(8)-C(9)-C(10)
89.55(18) N(2)-C(10)-C(5)
78.91(19) N(2)-C(10)-C(9)
118.9(7)
119.9(6)
115.4(5)
121.5(6)
123.1(5)
128.5(5)
The 1H NMR signal of the methyl group on the
palladium shifted upon insertion of the isocyanide from
ca. 1 ppm in the starting complex to ca. 2.5 ppm in the
carbo imine product, which has also been observed in
the case of insertion of CO into the Pd-Me bond of
complexes containing bidentate nitrogen ligands.12,18
The CdN stretching frequencies of the products 1a -c,
2a -c, and 3b,c occurred in the IR spectrum around
1630 cm-1. In the 13C NMR spectra, the 13C chemical
shift of the CdN fragments could be observed around
184 ppm.47-51 The two methyne protons of the tosyl-
methyl group have been observed as two doublets in the
1H NMR spectrum with a large coupling constant
between those two protons of 2J ) 13.9 Hz. The
magnetic inequivalency of these two protons indicates
a hindered rotation of the carbo imine group around the
Pd-C bond.
N(1)-Pd(1)-C(11) 173.5(2)
C(5)-C(10)-C(9)
Pd(1)-C(11)-N(3)
N(2)-Pd(1)-C(11)
Pd(1)-N(1)-C(1)
Pd(1)-N(1)-C(5)
C(1)-N(1)-C(5)
Pd(1)-N(2)-C(6)
95.1(2)
127.1(4)
113.3(4)
119.0(5)
126.4(4)
Pd(1)-C(11)-C(12) 112.3(4)
N(3)-C(11)-C(12)
N(3)-C(13)-C(14)
N(3)-C(13)-C(18)
119.2(5)
121.0(6)
117.6(5)
Pd(1)-N(2)-C(10) 115.4(4)
C(6)-N(2)-C(10) 117.9(5)
C(11)-N(3)-C(13) 127.4(5)
C(14)-C(13)-C(18) 120.8(6)
C(13)-C(14)-C(15) 117.5(6)
C(13)-C(14)-C(20) 121.7(6)
C(15)-C(14)-C(20) 120.8(6)
C(14)-C(15)-C(16) 121.9(7)
C(15)-C(16)-C(17) 119.6(7)
C(16)-C(17)-C(18) 121.0(7)
C(13)-C(18)-C(17) 119.0(6)
C(13)-C(18)-C(19) 120.9(7)
C(17)-C(18)-C(19) 120.1(6)
N(1)-C(1)-C(2)
C(1)-C(2)-C(3)
C(2)-C(3)-C(4)
C(3)-C(4)-C(5)
N(1)-C(5)-C(4)
N(1)-C(5)-C(10)
C(4)-C(5)-C(10)
N(2)-C(6)-C(7)
C(6)-C(7)-C(8)
122.0(6)
120.1(7)
118.8(6)
119.3(6)
120.9(6)
116.1(5)
123.0(5)
123.5(5)
118.3(6)
Cl(2)-C(21)-Cl(3)
115.6(4)
Molecu la r St r u ct u r e (b p y)P d (C(dNs2,6-Me2-
C6H3)Me)Cl (1a ). Crystals suitable for X-ray diffrac-
tion of complex 1a were obtained from slow diffusion of
hexanes into a solution of the complex in dichlo-
romethane. The molecular structure is presented in
Figure 2, while the bond lengths and bond angles of the
non-hydrogen atoms are collected in Tables 1 and 2.
This structure displays a square-planar geometry about
the palladium atom with a bidentate coordinated bpy
ligand, the chloride atom, and the carbon atom of the
(47) Dupont, J .; Pfeffer, M. J . Chem. Soc., Dalton Trans. 1990,
3193-3138.
(48) Veya, P.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C. Organome-
tallics 1994, 13, 441-450.
(49) Onitsuka, K.; J oh, T.; Takahashi, S. J . Organomet. Chem. 1994,
464, 247-251.
(50) Onitsuka, K.; J oh, T.; Takahashi, S. Angew. Chem., Int. Ed.
Engl. 1992, 31, 851-852.
(51) Zografidis, A.; Polborn, K.; Beck, W.; Markies, B. A.; van Koten,
G. Z. Naturforsch. 1994, 49b, 1494-1498.

Isocyanide Insertion into the Palladium-Carbon Bond
Organometallics, Vol. 16, No. 13, 1997 2953
inserted isocyanide. It clearly shows that the plane
C(12)-C(11)-N(3) makes an angle of 82.1(6)° with the
coordination plane of palladium. This is analogous to
the complex (bpy)Pd(C(O)Me)Cl, which shows that the
C(O)Me plane is perpendicular to the coordination
plane.18 The Pd-N(1) distance (2.146(5) Å) is longer
than the Pd-N(2) distance (2.066(5) Å), showing that
the C(dNsR)Me group has a larger trans influence than
the chloride atom. The N(3)-C(13) distance of
1.258(8) Å is comparable to other carbo imine bonds of
palladium complexes formed by isocyanide insertion.49,51
mixture leads to a decrease in conductivity, as the ions
recombine to the nonconducting species 1a -c and 2a -
c.
The intermediate complexes [(NkN)Pd(CN-R)(Me)]-
Cl (NkN ) bpy (4), phen (5); R ) 2,6-Me2C6H3 (a ), t-Bu
(b), and tosylmethyl (c)), as shown in eq 1, have been
characterized by 1H NMR spectroscopy (see Experimen-
tal Section) at 223 K, since at 273 K a fluxional process
occurs in which both halves of the bidentate nitrogen
ligands bpy and phen become magnetically equivalent
on the NMR time scale.
It is interesting to note that the crystal lattice of
complex 1a contains one dichloromethane molecule per
complex molecule, in which one hydrogen of dichlo-
romethane is bonded to N(3) of the carbo imine group
via a hydrogen bridge (N(3)-H(21) ) 2.253(9) Å).
The temperature at which the product forms is
dependent on the isocyanide used, as intermediates
4a -c and 5a -c react to form the products 1a -c and
2a -c at 273 K for R ) tosylmethyl, at 283 K for R )
2,6-Me2C6H3, and at 293 K for R ) tert-butyl.
An interesting observation is that at temperatures
higher than 273 K, intermediates 4a , 4b, and 5a (see
eq 1) are in equilibrium with the starting complex and
isocyanide, while this equilibrium is shifted to the right
for complexes 4c, 5b, and 5c in the temperature range
from 250 to 320 K (see Experimental Section).
On the basis of these measurements, a mechanism
for the isocyanide insertion into the palladium-carbon
bond of (NkN)Pd(Me)Cl complexes may be postulated,
as shown in eq 1.
Id en tifica tion of In ter m ed ia tes of th e Isocya -
n id e Migr a tor y In ser tion Rea ction . We monitored
the reactions of complex 1 and 2 with the isocyanides
DIC, TIC, and TosMIC by IR and NMR spectroscopies
and conductivity measurements. After addition of DIC,
TIC, or TosMIC to a solution of the complexes 1 or 2 in
dichloromethane at 273 K, the IR spectrum shows a very
intense vibration around 2200 cm-1 characteristic of an
isocyanide σ-coordinated to the palladium atom,40-42,48,51
1
while the H NMR spectrum shows the formation of a
new complex with a methyl signal around 1 ppm,
characteristic of a methyl σ-coordinated to a Pd cen-
ter.12,18,34 Slowly warming the reaction mixture from
273 to 294 K shows a decreasing intensity of the
σ-coordinated isocyanide and an increasing intensity of
the imine CdN vibration around 1630 cm-1 in the IR
It should be mentioned that intermediate 4a could
not be synthesized quantitatively at low temperature
(250 K). Since free bpy ligand also (10% with regard to
complex 4a ) appeared in the reaction mixture, probably
intermediates are also formed containing two σ-coordi-
nated isocyanides in addition to intermediate 4a. Warm-
ing of the reaction mixture from 250 to 293 K, however,
results in quantitative formation of product 1a . Ad-
ditionally, quantitative formation of the intermediates
[(bpm)Pd(CN-t-Bu)(Me)]Cl and [(bpm)Pd(CN-CH2-
Tosyl)(Me)]Cl appeared to be impossible, as free ligand
(50% with regard to the intermediate complexes) could
also be observed in the reaction mixture of complex 3
with TIC and TosMIC at 270 K.
The intermediate complexes 4a -c and 5a -c can be
precipitated from a solution in dichloromethane when
hexane is added. Interestingly, these intermediates in
the solid state quantitatively react to products 1a -c and
2a -c, although more slowly than in solution. Inter-
mediates 4b and 5b convert within a few minutes in
the solid state to the products 1b and 2b, respectively,
upon heating to 320 K, while no product is formed when
the solid is kept at room temperature. However, the
intermediates 4a and 5a convert quantitatively to the
products 1a and 2a , respectively, within 2 days at room
temperature, while 4c and 5c in the solid state react
quantitatively to 1c and 2c within 1 h at room temper-
ature. Such a reaction in the solid state has been
observed before for the complex [(PhNC)(Et3P)2PdCCPd-
(PEt3)2(CNPh)]Cl2.41
1
spectrum. In the H NMR spectrum, decreasing inten-
sity of the signal around 1 ppm and increasing intensity
of the signal around 2.4 ppm from the products 1a -c
or 2a -c can be observed.
Addition of DIC, TIC, or TosMIC to complexes 1 or 2
in acetonitrile at 273 K showed an increase of the
conductivity from around 4 to 160 Ω-1 cm2 mol-1 in a
few seconds, whereas in dichloromethane and chloro-
form the conductivity increases from around 0.04 to 20
and 2 Ω-1 cm2 mol-1, respectively. This indicates that
during the reaction in acetonitrile, as well as in dichlo-
romethane, the chloride is substituted by the isocyanide
to form an ionic intermediate containing a σ-coordinated
isocyanide (see eq 1). The low conductivity value for
(1)
Kin etics of th e Migr a tor y In ser tion Rea ction .
Detailed kinetics of the isocyanide insertion into the
Pd-C bond of (NkN)Pd(R)X complexes have been car-
ried out. Determination of the migration rates of the
methyl group to the precoordinated isocyanide, as
described by the reaction constant k2 (see eq 1), could
readily be performed by measuring the concentration
decay of the intermediate complexes 4c, 5c, and 5b in
the intermediate complexes in the moderately polar
solvent chloroform might indicate the existence of
contact ion pairs. Subsequent warming of the reaction
1
time by means of H NMR spectroscopy in a tempera-

2954 Organometallics, Vol. 16, No. 13, 1997
Delis et al.
Ta ble 3. Migr a tion Ra tes k2 for Com p lexes 4c a n d
5a -c (w ith Esd ’s in P a r en th eses)
Ta ble 4. Va lu es of k1 a n d k-1 for Com p lex 5a (w ith
Esd ’s in P a r en th eses)
com-
∆Hq (kJ
∆Sq (J K-1 ∆Gq (kJ
T (K)
k1 (M-1 s-1
)
k-1 (s-1
)
298
mol-1
)
pound T (K)a
k2 (s-1
)
mol-1
)
mol-1
)
288.9
294.1
296.2
299.2
304.4
309.6
0.0027(7)
0.0037(6)
0.0070(21)
0.0088(51)
0.019(12)
0.020 (22)
0.00064(17)
0.0012(3)
0.0022(8)
0.0035(28)
0.011(9)
4c
264.2 0.00071(1)
268.3 0.001345(9)
273.4 0.00343(4)
277.6 0.0060(2)
282.7 0.0144(2)
288.9 0.00084(2)
294.1 0.00196(3)
98.4(2.1) 68.1(7.6)
78.1(4.3)
87.0(3.8)
0.012(23)
5a
Nitr ogen Don or Atom Site Exch a n ge in (NkN)-
P d (C(dNR)Me)Cl Com p lexes. Irradiation in the H
296.2 0.00253(5) 110.0(1.9) 77.2(6.3)
299.2 0.0043(1)
1
NMR spectrum of the H6 proton adjacent to the nitrogen
of the bpy, phen, or bpm in complexes 1b, 1c, 2b, 2c,
and 3b caused a spin saturation transfer to the H6′
proton and vice versa. Exchange of protons H6 and H6′
with one another could be confirmed with EXSY 1H
NMR measurements, which can be explained by a
process in which both nitrogen atoms of the bidentate
nitrogen ligand change coordination sites, as shown in
eq 2. Overlap of signals in the 1H NMR spectrum
304.4 0.0096(4)
309.6 0.018(1)
294.1 0.000190(5)
299.2 0.00039(1)
304.4 0.00081(2) 108.4(2.1) 52.3(7.1)
309.6 0.00186(7)
314.7 0.00360(8)
5b
92.8(4.2)
83.9(0.8)
5c
277.6 0.00085(7)
282.7 0.00175(4)
288.9 0.00392(5)
293.0 0.00664(7)
87.1(0.4) 10.7(1.4)
a
The standard deviation in T is 0.6 K.
impeded a detailed study of the nitrogen donor atom
exchange for complexes 1a , 2a , and 3c. However, it is
reasonable to assume that the process shown in eq 2
also occurs for these complexes. Fluxional behavior of
this type has been reported before for the analogous
complexes (bpy)Pd(C(O)Me)Cl13 and (Ar-BIAN)Pd(C(O)-
Me)Cl,52 but evidence for the mechanism of this process
could not be obtained for these compounds. A similar
isomerization has been found by Pregosin et al. and
Ba¨ckvall et al.53 for the complexes (NkN)Pd(η3-allyl)X.
In complexes where NkN ) bpm, it could be shown that
the process occurs via Pd-N bond breaking and subse-
quent isomerization, as has also been demonstrated for
the complex [Rh(bpm)(nbd)]OTf.54
F igu r e 3. Nonlinear regression fitting of the composition
of the reaction mixture 2 with DIC at 294.1 K. The
experimental values for the relative concentrations of the
components 2, 2a , DIC, and 5a are displayed by the
markers, while the result of the calculations are displayed
by the extended lines.
Therefore, we have investigated complex 3b contain-
ing the ligand bpm in more detail to obtain more insight
into the mechanism of the nitrogen donor exchange in
ture range from 264 to 314 K (see Table 3). A logarith-
mic plot of these concentrations against time resulted
in a straight line, which showed that the reactions are
perfectly first order in palladium concentration for at
least three half-lifes. Unfortunately, overlap of several
1
(NkN)Pd(C(dNR)Me)Cl complexes. The H NMR spec-
trum of complex 3b shows four signals for H4, H6, H4′,
and H6′ (the numbering scheme of the protons is
1
1
presented in Scheme 1), of which the position in the H
signals in the H NMR spectrum upon addition of TIC
1
NMR spectrum has been determined by H NOE dif-
to complex 1 prevented determination of the kinetic data
in this case. Since intermediate complex 5a is in
equilibrium with complex 2 and free DIC, the reaction
rate constants of the elementary steps, i.e., isocyanide
association (k1), dissociation (k-1), and methyl migration
(k2), have been determined by fitting the composition
of the reaction mixture by a nonlinear regression in the
temperature range from 289 to 304 K. The composition
of the reaction mixture was determined by 1H NMR
spectroscopy. The result of one of these calculations for
the reaction of 2 with DIC at 294.1 K is displayed in
Figure 3. The values for k1 and k-1 are collected in
Table 4, whereas the values for k2 are shown in Table
3. The thermodynamic activation parameters ∆Hq, ∆Sq,
and ∆G298q (see Table 3) were obtained from an Eyring
plot. Since the standard deviations in the values for k1
and k-1 are rather large, no values for the thermo-
dynamic activation parameters have been calculated.
1
ference and H decoupling experiments at 264 K. The
methyl protons of the C(dN-t-Bu)Me group reveal a
large NOE interaction with proton H6 of the bpm ligand
cis to this group. The influence of the carbo imine group
also results in a relatively large chemical shift difference
between H6 and the other three protons H6′, H4, and
H4′. From an EXSY 1H NMR measurement of complex
3b, it could be inferred that protons H4, H6, H4′, and
H6′ are in exchange with one another.
The rates of the site exchange have been measured
by the Forse´n-Hoffman method46 for complexes 1b, 1c,
(52) van Asselt, R.; Vrieze, K.; Elsevier, C. J . J . Organomet. Chem.
1994, 480, 27-40.
(53) Gogoll, A.; O¨ rnebro, J .; Grennberg, H.; Ba¨ckvall, J . E. J . Am.
Chem. Soc. 1994, 116, 3631-3632.
(54) Haarman, H. F.; Bregman, F. R.; Ernsting, J . M.; Elsevier, C.
J .; Veldman, N.; Spek, A. L.; Vrieze, K. Organometallics 1997, 16, 54-
67.

Isocyanide Insertion into the Palladium-Carbon Bond
Organometallics, Vol. 16, No. 13, 1997 2955
Sch em e 1
and Pd-Cl(1) distances of complex 1a are equal within
the standard deviations as compared to those observed
for complex (bpy)Pd(C(O)Me)Cl. The equal Pd-N(1)
distances in complex 1a and (bpy)Pd(C(O)Me)Cl indicate
that the trans influence of the C(O)Me group and the
C(dNs2,6-Me2C6H3) group are comparable.
Kin etics of th e In ser tion Rea ction . From the
results, it is clear that the mechanism of isocyanide
insertion involves coordination of the isocyanide to the
metal via displacement of the chloride atom, followed
by a rate-determining migration of the methyl group to
the precoordinated isocyanide. Such a mechanism has
been proposed before for isocyanide insertion into pal-
ladium- and platinum-carbon bonds.39,41,58,59 The
complex trans-[PdCl(2-pyrazyl)(PPh3)2]59 undergoes mi-
gratory insertion of TIC via Pd-X bond breaking,
although there are indications for a mechanism involv-
ing Pd-P bond breaking.
It has been proposed that multiple insertion of iso-
cyanides into Pd-C bonds of (Ph2P-C2H4-PPh2)Pd-
(CHdCHCO2Me)Br60 occurs via a five-coordinate tran-
sition state, while it has been proven that such a
reaction with X(R3P)2PdCCPd(PR3)X complexes takes
place via Pd-X bond breaking.41
As substitution of the ligand bpm occurs upon addi-
tion of the isocyanides TIC and TosMIC to complex 3
at 250 K, also in this case other mechanisms may occur
involving Pd-N bond breaking instead of Pd-X bond
breaking.
The values for k1 and k-1 could only be determined
in the case of reaction of 2 with DIC, which showed that
the rate of isocyanide association (k1 ) 0.0037(6) M-1
s-1 at 294.1 K, [DIC] ) 0.03 M) is comparable with the
rate of isocyanide dissociation (k-1 ) 0.0012(3) s-1 at
294.1 K). As the pre-equilibrium in eq 1 for reactions
of 1 with TosMIC and 2 with TIC and TosMIC lies close
to the respective intermediates, we might conclude that
in these cases the value for K1 is much larger than that
for reaction of 2 with DIC.
The rate of methyl migration to the precoordinated
isocyanide, with rate constant k2, increases along the
series TIC < DIC < TosMIC. The migration rate is
enhanced by R groups on the isocyanide having electron-
withdrawing properties, such as a tosylmethyl group,
and it is reduced by an electron-donating R group, such
as tert-butyl. In earlier studies, it has already been
shown that p-chlorophenyl isocyanide inserts with a
higher rate than methyl isocyanide into the Pd-R′ bond
of (PR3)2Pt(R′)X (R′ ) CH3, C6H5; X ) Br, I),58 and
analogously, the insertion of isocyanides with electron-
withdrawing R groups into the Pd-acetylide bond of the
complexes X(R3P)2PdCCPd(PR3)X is faster than the
insertion of isocyanides having electron-donating R
groups.41
As can be seen from Table 3, the migration step
involves relatively high activation enthalpies and posi-
tive activation entropies. It can be expected that solvent
molecules are more strongly ordered around the ionic
intermediates 4 and 5 than around the neutral products
1a -c and 2a -c. During the migration step, the com-
Ta ble 5. Kin etic Da ta for Nitr ogen Don or
Exch a n ge P r ocesses (w ith Esd ’s in P a r en th eses)
∆Hq
∆Sq
∆Gq
298
compound
(kJ mol-1
)
(J K-1 mol-1
)
(kJ mol-1
)
1b
1c
2b
2c
3b
78.2(1.9)
33.6(1.5)
82.6(1.4)
49.3(1.7)
79.6(2.5)
7.7(1.6)
-158.1(5.2)
15.8(4.7)
75.9(2.4)
80.7(3.0)
77.9(2.8)
81.2(3.3)
71.2(5.1)
-107.1(5.3)
28.1(8.6)
2b, 2c, and 3b at different temperatures. The thermo-
dynamic activation parameters for the mentioned com-
plexes, derived from Eyring plots, have been collected
in Table 5. The exchange rates are not influenced by
the concentration of the complexes in CDCl3 nor by the
addition of Cl- or free bidentate nitrogen ligand.
Discu ssion
In ser tion of Isocya n id e in to P d -C Bon d s of
(NkN)P d (Me)Cl Com p lexes. The insertion of the
isocyanides DIC, TIC, and TosMIC into the Pd-C bond
of complexes 1, 2, and 3 resulted in the quantitative
formation of the monoinsertion products 1a -c, 2a -c,
and 3b,c. This is rather surprising since in the case of
complexes having phosphine ligands, multiple insertions
take place rather than monoinsertion.41,42,49,55-57 Quan-
titative product formation requires addition of exactly
1 equiv of the isocyanide with respect to the palladium
complex, because an excess causes substitution of the
bidentate ligand.
The structure shown in Figure 2 of complex 1a
exhibits some large similarities with the structure of
complex (bpy)Pd(C(O)Me)Cl.18 The Pd-N(1), Pd-N(2),
(55) van Baar, J . F.; Klerks, J . M.; Overbosch, P.; Stufkens, D. J .;
Vrieze, K. J . Organomet. Chem. 1976, 112, 95-103.
(56) Tanase, T.; Ohizumi, T.; Kobayashi, K.; Yamamoto, Y. Orga-
nometallics 1996, 15, 3404-3411.
(57) Ito, Y.; Ihara, E.; Murakami, M.; Shiro, M. J . Am. Chem. Soc.
1990, 112, 6446-6447.
(58) Treichel, P. M.; Wagner, K. P.; Hess, R. W. Inorg. Chem. 1973,
12, 1471-1477.
(59) Bertani, R.; Berton, A.; Di Dianca, F.; Crociani, B. J . Organomet.
Chem. 1986, 303, 283-299.
(60) Otsuka, S.; Ataka, K. J . Chem. Soc., Dalton Trans. 1976, 327-
334.

2956 Organometallics, Vol. 16, No. 13, 1997
Delis et al.
plex converts from an ionic species into a neutral species
giving rise to disorder in the orientation of the solvent
molecules and a positive value for ∆S. This indicates
that the transition state of the migration reaction
resembles the neutral final state and might, therefore,
look like a five-coordinate neutral species having the
halide atom coordinated to the metal center, as proposed
previously for isocyanide insertion into (PR3)2Pt(R)X
complexes.58
The interesting observation that product formation
occurs from intermediates 4a -c and 5a -c even in the
solid state shows that in the crystal lattice movement
of the methyl group to the precoordinated isocyanide
and subsequent movement of the chloride atom from the
second coordination sphere to the palladium atom
occurs, albeit at a lower rate.
3b as a function of the R group on the carbo imine
moiety, the bidentate nitrogen ligand, concentration of
the carbo imine complexes in CDCl3, and the presence
of free bidentate nitrogen ligand and Cl- in a solution
of the carbo imine complexes in CDCl3. A t-Bu sub-
stituent on the carbo imine group gives a higher
exchange rate (∆G298q ) 75.9(2.4) kJ mol-1 for 1b) than
q
a tosylmethyl substituent (∆G298 ) 80.7(3.0) kJ mol-1
for 1c). This is understandable as the trans influence
of the carbo imine group will be enlarged by an electron-
donating t-Bu group, thereby facilitating Pd-N bond
breaking trans to the carbo imine group. The exchange
process does not occur in the complexes 1 and 2,13 which
can be explained by the smaller trans influence of a
methyl group with respect to a carbo imine group or an
acetyl group.63 This indicates that destabilization of the
initial state of the carbo imine complexes is an impor-
tant factor contributing to the exchange process.
Comparison of the rate of CO insertion18 with the rate
of isocyanide insertion into the Pd-Me bond of (NkN)-
Pd(Me)Cl complexes shows that the rates are of the
same order of magnitude. The major difference between
both reactions is that intermediate complexes such as
[(NkN)Pd(Me)(CO)]Cl have never been observed for CO
insertion reactions. Furthermore, we do not have any
experimental indication for which of the steps in the
CO insertion mechanism is the rate-determining one.
Nitr ogen Don or Atom Site Exch a n ge in (NkN)-
P d (C(dNR)Me)Cl Com p lexes. Analogous to what
has been proposed for the complexes (bpm)Pd(η3-allyl)-
X53 and [Rh(bpm)(nbd)]OTf,54 the exchange in complex
3b of the protons H4, H4′, H6, and H6′ with one another
can be explained by a mechanism (see Scheme 1) in
which during the site exchange of the two pyrimidine
rings (i.e., exchange of H4 with H4′ and H6 with H6′),
the two rings undergo an internal rotation around the
C-C bond (i.e., exchange of H6 with H4 and H6′ with
H4′). It is clear that the internal ligand rotation
necessitates Pd-N bond breaking (see Scheme 1). In
earlier studies on this exchange process,53,54 it was
proposed that internal ligand rotation may occur in a
T-shaped intermediate A (Scheme 1). If this were so,
internal ligand rotation and exchange of the pyrimidine
rings are uncoupled processes and the exchange of H6′
and H4′ is independent of the exchange of H6′ and H6.
However, spin saturation 1H NMR experiments using
a short presaturation time of 0.1 s upon irradiation of
proton H6 or H6′ showed equal rates of spin saturation
transfer to each of the protons H4, H6′, and H4′
(saturation of H6) or H4, H6, and H4′ (saturation of
H6′). For these spin saturation experiments, complex
(bpm)Pd(C(dNst-Bu)Me)Br was used, as the chemical
q
The ∆G298 values (75.9(2.4) kJ mol-1) and the ex-
change rates (k ) 0.248 s-1 in dichloromethane at 296.2
K) for complex 1b are comparable with those for the
q
complex (bpy)Pd(C(O)Me)Cl (∆G298 ) 75(4) kJ mol-1
;
k ) 0.252 s-1 in dichloromethane at 297 K), which is
consistent with the similar trans influences of the acetyl
and C(dN-2,6-Me2C6H3)Me group.
The ∆G298q values for the bpy complexes 1b,c (∆G298
q
) 75.9(2.4) kJ mol-1 for 1b and 80.7(3.0) kJ mol-1 for
1c) are comparable with those for phen complexes 2b,c
q
(∆G298 ) 77.9(2.8) kJ mol-1 for 2b and 81.2(3.3) kJ
mol-1 for 2c), whereas the ∆G298q value for complex 3b
containing bpm is lower (71.2(5.1) kJ mol-1 for 3b),
resulting in a faster exchange rate for this complex. The
higher exchange rate for complex 3b might be connected
with the lower σ-coordinating capabilities of the ligand
bpm with regard to the ligands bpy and phen, as
mentioned in the results. It should be noted that the
value for ∆Sq for complexes 1b, 2b, and 3b is very small,
while this value for the complexes 1c and 2c is largely
negative. In addition, the values for ∆Hq for 1b, 2b,
and 3b are much larger than those for 1c and 2c, which
we cannot readily explain.
Intermolecular ligand exchange as the source of the
observed exchange can be excluded, since the exchange
rates are not influenced by the concentration of the
complex in CDCl3 nor by the addition of free bidentate
nitrogen ligand. Furthermore, as addition of 0.5-2
equiv of Cl anions to a mixture of complexes 1b,c, 2b,c,
or 3b in CDCl3 at 294 K has no effect on the exchange
rate, we might conclude that Pd-Cl bond breaking is
not involved in the rate-determining step of the nitrogen
donor exchange.
1
shift differences in the H NMR spectrum between the
protons H6′, H4, and H4′ is larger than those in complex
3b. These measurements indicate that internal ligand
rotation does not occur without exchange of the two
pyrimidine rings. Therefore, we propose that subse-
quent to Pd-N1′ bond breaking trans to the carbo imine
group (this group has a larger trans influence than
chloride), cis-trans isomerization of N1 and N1′ occurs
via a Y-shaped61,62 intermediate B, which involves also
internal ligand rotation (see Scheme 1).
Con clu sion
The neutral (NkN)Pd(Me)Cl complexes undergo facile
and quantitative monoinsertion of isocyanides, which
is the first step on the road to the polyimine analogue
of polyketone. The mechanism of this reaction involves
substitution of the halide by the isocyanide followed by
a rate-determining methyl migration to the precoordi-
nated isocyanide. A kinetic study on this reaction
showed that the migration rate of the methyl group to
We have studied the nitrogen donor atom site ex-
change in the carbo imine complexes 1b,c, 2b,c, and
(61) Louw, W. J .; van Eldik, R.; Kelm, H. Inorg. Chem. 1980, 19,
2878-2880.
(63) Chen, J . T.; Yeh, Y. S.; Yang, C. S.; Tsai, F. Y.; Huang, G. L.;
Shu, B. C.; Huang, T. M.; Chen, Y. S.; Lee, G. H.; Cheng, M. C.; Wang,
C. C.; Wang, Y. Organometallics 1994, 13, 4804-4824.
(62) Kelm, H.; Louw, W. J .; Palmer, D. A. Inorg. Chem. 1980, 19,
843-847.

Isocyanide Insertion into the Palladium-Carbon Bond
Organometallics, Vol. 16, No. 13, 1997 2957
the precoordinated isocyanide increases with increasing
electrophilicity of the isocyanide.
supported in part (N.V. and A.L.S.) by the Netherlands
Foundation of Chemical Research (SON) with financial
aid from the Netherlands Organization for Scientific
Research (NWO). We thank J .-M. Ernsting for NMR
spectroscopy assistance and R. H. Fokkens and J . W.
H. Peeters for the mass spectroscopy.
The complexes (NkN)Pd(C(dNsR)Me)X show a flux-
ional behavior due to a site exchange of the nitrogen
donor atoms. The mechanism of this process involves
Pd-N bond breaking and subsequent isomerization via
a Y-shaped intermediate.
Su p p or tin g In for m a tion Ava ila ble: Tables of crystal
data, atomic coordinates, anisotropic thermal parameters,
bond lengths, bond angles, and torsion angles of 29 (6 pages).
Ordering information is given on nay current masthead page.
Ack n ow led gm en t. Professor dr E. Drent, dr W. P.
Mul (Koninklijke/Shell-Laboratorium, Amsterdam) and
professor dr C. J . Elsevier (University of Amsterdam)
are acknowledged for their helpful discussions and Shell
Research BV for financial support. This work was
OM970096E