Transcyclometalation processes with late transition metals: C(aryl)-H bond activation via noncovalent C-H···interactions

Article abstract of DOI:10.1021/ja001808m

The bis(ortho-)chelated platinum complex [PtCl(NCN)], 1 (NCN = [C₆H₃(CH₂NMe₂)₂-2,6]^-), has been used as a novel metal precursor for C-H bond activation and subsequent cyclometalation of the potentially terdentate coordinating pincer ligand PCHP (PCP = [C₆H₃(CH₂PPh₂)₂-2,6]^-). Depending on the reaction temperature various intermediates, each with unique structural features, could be isolated and identified during the formation of the final, bis(ortho-)cyclometalated product [PtCl(PCP)], 2. These results provide valuable insight into the intimate steps of this reaction for which we like to introduce the term transcyclometalation process (see note). At room temperature, a sparingly soluble dimetallic complex is formed, which comprises a η¹-monodentate C-bound NCN ligand and a bridging, P,P η²-ε²-coordinating PCHP ligand. Solution spectroscopy and X-ray analysis of this platinum dimer established a C-H bond which is interacting by intramolecular noncovalent H···Cl hydrogen bonding with the Pt-Cl motif. As a further intermediate, the formation of a trans-bis(phenyl) complex [Pt(η³-PCP)(η¹-HNCN)]Cl, 5, has been identified, which is characterized by a bis(ortho-)cyclometalated PCP ligand which is η³-P,C,P' coordinated to the platinum center as well as an η¹-monodentate C-bound NCN ligand. The equivalent HCl, which is formally released during the formation of 5, is intramolecularly trapped by one of the basic amine groups, as is apparent from the identification of precursors of 5, i.e., compounds in which either one ([3]Cl) or two ([4]X₂) of the NMe₂ groups have become protonated (see Scheme 5). Remarkably, the proton in [4]X₂ is not only bound to the nitrogen lone pair but also interacts with the filled d(z)² orbital of the nucleophilic platinum(II) center, thus disclosing complex 5 as an organometallic proton sponge that is able to sequester protons due to Pt,N-bidentate chelating properties. Prolonged exposure of the reaction mixture at 80 °C or performing the reaction at 110 °C afforded the transcyclometalated complex 2 and 1 equiv of the diaminoarene NCHN as the two ultimate products.

Full text of DOI:10.1021/ja001808m

11822
J. Am. Chem. Soc. 2000, 122, 11822-11833
Transcyclometalation Processes with Late Transition Metals: Caryl-H
Bond Activation via Noncovalent C-H‚‚‚Interactions
Martin Albrecht,† Paulo Dani,† Martin Lutz,‡ Anthony L. Spek,‡,§ and Gerard van Koten*,†
Contribution from the Debye Institute, Department of Metal-Mediated Synthesis, and
BijVoet Center for Biomolecular Research, Crystal and Structural Chemistry, Utrecht UniVersity,
Padualaan 8, 3584 CH Utrecht, The Netherlands
ReceiVed May 24, 2000
Abstract: The bis(ortho-)chelated platinum complex [PtCl(NCN)], 1 (NCN ) [C6H3(CH2NMe2)2-2,6]-), has
been used as a novel metal precursor for C-H bond activation and subsequent cyclometalation of the potentially
terdentate coordinating pincer ligand PCHP (PCP ) [C6H3(CH2PPh2)2-2,6]-). Depending on the reaction
temperature various intermediates, each with unique structural features, could be isolated and identified during
the formation of the final, bis(ortho-)cyclometalated product [PtCl(PCP)], 2. These results provide valuable
insight into the intimate steps of this reaction for which we like to introduce the term transcyclometalation
process (see note). At room temperature, a sparingly soluble dimetallic complex is formed, which comprises
a η1-monodentate C-bound NCN ligand and a bridging, P,P η2-ꢀ2-coordinating PCHP ligand. Solution
spectroscopy and X-ray analysis of this platinum dimer established a C-H bond which is interacting by
intramolecular noncovalent H‚‚‚Cl hydrogen bonding with the Pt-Cl motif. As a further intermediate, the
formation of a trans-bis(phenyl) complex [Pt(η3-PCP)(η1-HNCN)]Cl, 5, has been identified, which is
characterized by a bis(ortho-)cyclometalated PCP ligand which is η3-P,C,P′ coordinated to the platinum center
as well as an η1-monodentate C-bound NCN ligand. The equivalent HCl, which is formally released during
the formation of 5, is intramolecularly trapped by one of the basic amine groups, as is apparent from the
identification of precursors of 5, i.e., compounds in which either one ([3]Cl) or two ([4]X2) of the NMe2
groups have become protonated (see Scheme 5). Remarkably, the proton in [4]X2 is not only bound to the
2
nitrogen lone pair but also interacts with the filled dz orbital of the nucleophilic platinum(II) center, thus
disclosing complex 5 as an organometallic proton sponge that is able to sequester protons due to Pt,N-bidentate
chelating properties. Prolonged exposure of the reaction mixture at 80 °C or performing the reaction at 110 °C
afforded the transcyclometalated complex 2 and 1 equiv of the diaminoarene NCHN as the two ultimate products.
Introduction
Scheme 1
The selective activation of (unstrained) C-H bonds is a
significant key step in the synthesis of both commodity and
fine chemicals, as this offers a general method for the introduc-
tion of new functional groups.1 Since its discovery, cyclometa-
lation involving (transition) metals (Scheme 1) has been widely
applied and represents an elegant methodology for the creation
of a metal-to-carbon bond under relatively mild conditions in
terms of temperature and/or pressure.2
In particular, the (site-selective) functionalization of aromatic
rings has attracted much attention.3 Various electron-donating
heteroatoms (E) have been successfully used in the (late)
transition metal-mediated activation of C-H bonds of potentially
E,C-bidentate and “pincer”-type4 E,C,E′-terdentate coordinating
ligand precursors (E, E′ ) P, N, etc; C ) aryl or alkyl).2,4,5
However, detailed mechanistic aspects of the intimate C-H
bond activation and cleavage steps are elusive, despite the
numerous studies devoted to cyclometalation processes.6
A
primary conclusion common to most of these investigations
concerns the initial step, which comprises the substitution of a
weakly bound (frequently neutral) ligand L on the metal
precursor M by a stronger donor E (Scheme 1). Subsequent
ligand dissociation and formation of a coordinatively unsaturated
and electron deficient metal center has been proposed to create
(2) (a) Shilov, A. E.; Shul’pin, G. B. Chem. ReV. 1997, 97, 2897. (b)
Omae, I. Coord. Chem. ReV. 1984, 53, 261. (c) Dehand, J.; Pfeffer, M.
Coord. Chem. ReV. 1976, 18, 327. (d) Ryabov, A. D. Chem. ReV. 1990,
90, 403. (e) Newkome, G. R.; Puckett, W. E.; Gupta, V. K.; Kiefer, G. E.
Chem. ReV. 1986, 86, 451. f) Murai, S.; Kakiuchi, F.; Sekine, S.; Tanaka,
Y.; Kamatani, A.; Sonoda, M.; Chatani, N. Nature 1993, 366, 529. (g)
Brainard, R. L.; Nutt, W. R.; Lee, T. R.; Whitesides, G. M. Organometal-
lics 1988, 7, 2379. (h) Anklin, C. G.; Pregosin, P. S.; Wombacher, F.;
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Pregosin, P. S.; Mu¨ller, B. Inorg. Chem. 1994, 33, 2522. (j) Steenwinkel,
P.; Gossage, R. A.; van Koten, G. Chem. Eur. J. 1998, 4, 759. (k) Campora,
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Chem. 1999, 111, 199; Angew. Chem., Int. Ed. 1999, 38, 147.
* Corresponding author. Fax: +31-30-2523615. Phone: +31-30-
2533120. E-mail: g.vankoten@chem.uu.nl.
† Debye Institute, Department of Metal-Mediated Synthesis.
‡ Bijvoet Center for Biomolecular Research, Crystal and Structural
Chemistry.
§ Corresponding author pertaining crystallographic data. E-mail: a.l.
spek@chem.uu.nl.
(1) (a) Jones, W. D.; Feher, F. J. Acc. Chem. Res. 1989, 22, 91. (b)
Parshall, G. W. Acc. Chem. Res. 1975, 8, 113. (c) Halpern, J. Inorg. Chim.
Acta 1985, 100, 41. For a theoretical study, see: (d) Saillard, J.-Y.;
Hoffmann, R. J. Am. Chem. Soc. 1984, 106, 2006.
10.1021/ja001808m CCC: $19.00 © 2000 American Chemical Society
Published on Web 11/16/2000

Transcyclometalation Processes
J. Am. Chem. Soc., Vol. 122, No. 48, 2000 11823
the reactive intermediate for cyclometalation,2d which then
probably reacts along a reaction pathway that includes formation
of an arenium-type intermediate.7,8
Scheme 2
Complexes containing pincer-type ligands are especially
attractive for such investigations, because these polydentate
ligands can (i) stabilize intermediates during a reaction, which
allows their proper identification,9 and (ii) retard the reaction
which may provide insight into kinetic and thus mechanistic
aspects of the cyclometalation process.10 Recently, cyclometa-
lated Ru(II) complexes, i.e. [RuCl(NCN)(PPh3)] (NCN is the
abbreviation of the monoanionic terdentate ligand [C6H3(CH2-
NMe2)2-2,6]-), have been successfully used as metal precursors
for cycloruthenation of another PCHP ligand (PCHP is the meta-
bis(phosphino)arene ligand [C6H4(CH2PPh2)2-1,3], which rep-
resents the precursor of the monoanionic terdentate PCP ligand
[C6H3(CH2PPh2)2-1,3]-).11 The outcome of this reaction is
surprising, since this formal exchange of cyclometalated ligands
at the metal center is rarely observed and has predominantly
been reported to occur in palladium(II) complexes (Scheme 2).12
In analogy to transesterification reactions (eq 1), we like to
introduce the term transcyclometalation reaction to describe the
overall process of this reaction (eq 2).13,14
R′′-COOR + R′OH a R′′-COOR′ + ROH
M(C,E) + (CH,E′) a M(C,E′) + (CH,E)
(1)
(2)
Here, we report on our recent studies using the bis(ortho-)-
chelated complex [PtCl(NCN)] as a substrate for the transcy-
cloplatination reaction with meta-bis(phosphino)arene ligands
PCHP. Several interesting intermediates on the way to the bis-
(ortho-)chelated product [PtCl(PCP)] have been isolated and
fully characterized, in particular some containing unprecedented
noncovalent C-H‚‚‚Cl-Pt interactions.
(3) For other hydrocarbon activation processes, see, e.g.: (a) Crabtree,
R. H. Chem. ReV. 1985, 85, 2483. (b) Wick, D. D.; Goldberg, K. I. J. Am.
Chem. Soc. 1997, 119, 10235. (c) Holtcamp, M. W.; Labinger, J. A.; Bercaw,
J. E. J. Am. Chem. Soc. 1997, 119, 848. For studies toward a regioselective
control of cyclometalation reactions, see e.g.: (d) Alsters, P. L.; Engel, P.
F.; Hogerheide, M. P.; Copijn, M.; Spek, A. L.; van Koten, G. Organo-
metallics 1993, 12, 1831. (e) Valk, J.-M.; Maassarani, F.; van der Sluis, P.;
Spek, A. L.; Boersma, J.; van Koten, G. Organometallics 1994, 13, 2320.
(f) Crespo, M.; Martinez, M.; Sales, J.; Solans, X.; Font-Bardia, M.
Organometallics 1992, 11, 1288. (g) Ryabov, A. D. Inorg. Chem. 1987,
26, 1252.
Results
Heating of a toluene solution containing equimolar amounts
of the meta-diphosphinoarene ligand C6H4(CH2PPh2)2-1,3 (ab-
breviated as PCHP) and the cyclometalated complex [PtCl-
(NCN)], 1, to 110 °C leads to a rapid formation of a white
precipitate. When heating was continued for several days, most
of the solid dissolved again. Analysis of the products obtained
after workup (see Experimental Section) revealed the clean
formation of the transcyclometalated complex [PtCl(PCP)], 2,
in high yields, as is demonstrated unambiguously by the
pertinent spectroscopic data (Scheme 3).15 The second product
of this reaction which is formed in equivalent amounts was
identified as the noncoordinated meta-bis(amino)arene NCHN.
Neutral and Protonated Pt(PCP)(NCN) Complexes. De-
tailed examination of the products formed in the reaction which
was carried out in benzene at reflux temperature, i.e. at 80 °C,
provided insight in the nature of the intermediates preceding
the formation of the final products 2 and NCHN. Similar to the
reaction at 110 °C, the immediate formation of a precipitate is
observed. When the temperature is kept at 80 °C, this solid did
not dissolve even after prolonged reaction time (3 days).
Isolation of this solid and purification under neutral conditions
gave a single product, which according to NMR spectroscopic
analyses appeared to be a complex of the type [Pt(η3-PCP)(η1-
HNCN)]. This complex, [3]Cl, contains two mutually trans-C-
(4) van Koten, G. Pure Appl. Chem. 1989, 61, 1681.
(5) For E,E′ ) P: (a) Perera, S. D.; Shaw, B. L. J. Chem. Soc., Dalton
Trans. 1995, 3861. (b) Moulton, C. J.; Shaw, B. L. J. Chem. Soc., Dalton
Trans. 1976, 1020. (c) Gorla, F.; Togni, A.; Venanzi, L. M.; Albinati, A.;
Lianza, F. Organometallics 1994, 13, 1607. (d) Nemeh, S.; Jensen, C. H.;
Binamira-Soriaga, E.; Kaska, W. C. Organometallics 1983, 2, 1442. (e)
van der Boom, M. E.; Liou, S.-Y.; Ben-David, Y.; Gozin, M.; Milstein, D.
J. Am. Chem. Soc. 1998, 120, 13415. (f) Karlen, T.; Dani, P.; Grove, D.
M.; Steenwinkel, P.; van Koten, G. Organometallics 1996, 15, 5453. For
E,E′ ) N: (g) Rietveld, M. H. P.; Grove, D. M.; van Koten, G. New J.
Chem. 1997, 21, 751. (h) Djukic, J.-P.; Maisse, A.; Pfeffer, M.; de Cian,
A.; Fischer, J. Organometallics 1997, 16, 657. (i) Carre´, F.; Chuit, C.; Corriu,
R. J. P.; Mehdi, A.; Reye´, C. Angew. Chem. 1994, 106, 1152; Angew. Chem.,
Int. Ed. 1994, 33, 1097. For E ) As: (j) Bennett, M. A.; Hoskins, K.;
Kneen, W. R.; Nyholm, R. S.; Mason, R.; Hitchcock, P. B.; Robertson, G.
B.; Towl, A. D. C. J. Am. Chem. Soc. 1971, 93, 4592. For E ) N,E′ ) P:
(k) Gandelman, M.; Vigalok, A.; Shimon, L. J. W.; Milstein, D. Organo-
metallics 1997, 16, 3981. For E,E′ ) S: (l) Errington, J.; McDonald, W.
S.; Shaw, B. L. J. Chem. Soc., Dalton Trans. 1980, 2312. (m) Hall, J. R.;
Loeb, S. J.; Shimizu, G. K. H.; Yap, G. P. A. Angew. Chem. 1998, 110,
130; Angew. Chem. Int. Ed. Engl. 1998, 37, 121. (n) Huck, W. T. S.; Prins,
L. J.; Fokkens, R. H.; Nibbering, N. M. M.; van Veggel, F. C. J. H.;
Reinhoudt, D. N. J. Am. Chem. Soc. 1998, 120, 6240. (o) Bergbreiter, D.
E.; Osburn, P. L.; Liu, Y.-S. J. Am. Chem. Soc. 1999, 121, 9531.
(6) Intuitively, a reaction mechansim which is analoguous to electrophilic
aromatic substitutions has been proposed very early: Cope, A. C.; Friedrich,
E. C. J. Am. Chem. Soc. 1968, 90, 909.
(7) Canty, A. J.; van Koten, G. Acc. Chem. Res. 1995, 28, 406.
(8) Alternative pathways include e.g. oxidative addition-reductive
elimination cycles without creation of an intermediate arenium species.
(9) (a) van Koten, G.; Timmer, K.; Noltes, J. G.; Spek, A. L. J. Chem.
Soc., Chem. Commun. 1978, 250. (b) Grove, D. M.; van Koten, G.; Louwen,
J. N.; Noltes, J. G.; Spek, A. L.; Ubbels, H. J. C. J. Am. Chem. Soc. 1982,
104, 6609. (c) van Beek, J. A. M.; van Koten, G.; Smeets, W. J. J.; Spek,
A. L. J. Am. Chem. Soc. 1986, 108, 5010. (d) Gossage, R. A.; Ryabov, A.
D.; Spek, A. L.; Stufkens, D. J.; van Beek, J. A. M.; van Eldik, R.; van
Koten, G. J. Am. Chem. Soc. 1999, 121, 2488.
(13) In analogy to transamination and transesterification, a transcyclo-
metalation process involves the interconversion of one cylometalated ligand
metal complex M(C,E) into another M(C,E′) with the concomitant
consumption and formation of the corresponding arene ligands (CH,E′) and
(CH,E), respectively (see Schemes 2 and 3). The driving force in this
reaction is the difference in heteroatom-metal bond strength which is
highest for the P-M bond in the case of the late transition metals PdII, PtII,
or RuII.
(10) (a) Albrecht, M.; Gossage, R. A.; Spek, A. L.; van Koten, G. J.
Am. Chem. Soc. 1999, 121, 11898. (b) van der Boom, M. E.; Higgitt, C.
L.; Milstein, D. Organometallics 1999, 18, 2413. (c) van der Boom, M. E.;
Kraatz, H.-B.; Ben-David, Y.; Milstein, D. Chem. Commun. 1996, 2167.
(11) Dani, P.; Karlen, T.; Gossage, R. A.; Smeets, W. J. J.; Spek, A. L.;
van Koten, G. J. Am. Chem. Soc. 1997, 119, 11317.
(12) (a) Ryabov, A. D. In PerspectiVes in Coordination Chemistry;
Williams, A. F., Floriani, C., Merbach, A. E., Eds.; Verlag Helvetica
Chimica Acta: Basel, Switzerland, 1992; p 271.
(14) For early examples of transcyclometalation, although not formulated
as such, see ref 12 and also: (a) Maassarani, F.; Pfeffer, M.; Spek, A. L.;
Schreurs, A. M. M.; van Koten, G. J. Am. Chem. Soc. 1986, 108, 4222. (b)
Dupont, J.; Beydoun, N.; Pfeffer, M. J. Chem. Soc., Dalton Trans. 1989,
1715. (c) Djukic, J.-P.; Maisse, A.; Pfeffer, M. J. Organomet. Chem. 1998,
567, 65. (d) Pregosin, P. S.; Wombacher, F.; Albinati, A.; Lianza, F. J.
Organomet. Chem. 1991, 418, 249. (e) Ryabov, A. D.; van Eldik, R. Angew.
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(15) Rimmel, H.; Venanzi, L. M. J. Organomet. Chem. 1983, 259, C6.

11824 J. Am. Chem. Soc., Vol. 122, No. 48, 2000
Albrecht et al.
Scheme 3
a downfield shift of the NCH3 (0.2 ppm) and the ArCH2N (0.3
ppm) proton resonances is noted. Second, a broad signal at 11.8
ppm in the spectrum of [4]Cl2 was observed which integrated
for two protons and which disappeared completely after addition
of a few drops of D2O to this solution. Furthermore, charac-
1
teristic cross-peaks were observed in a H-1H COSY NMR
spectrum of [4]Cl2 (but not in the corresponding spectrum of
[3]Cl) between the resonance at 11.8 ppm and those attributed
to the ArCH2N and the NCH3 protons. These findings provide
evidence for the presence of two symmetry-related and het-
eroatom-stabilized H-N (ammonia) moieties in [4]Cl2.17,18 The
1H NMR spectra of solutions of [4]Cl2 in CDCl3 at different
temperatures pointed to a significant temperature dependence
of the resonances of the protons in the CH2NHMe2 substituents.
The broad signal due to the ammonia N-H proton at δH
)
11.8 ppm varies between 11.88 ppm (328 K) and 11.17 ppm
(213 K). Similar but less pronounced chemical shift differences
were observed for the benzylic ArCH2N protons (∆δ ) 0.11
ppm) and for the NCH3 groupings (∆δ ) 0.08 ppm) in the same
temperature range.
bonded monoanionic NCN and PCP ligands. In [3]Cl, both
ortho-phosphorus donors are also coordinated to the platinum
center whereas the amines are both noncoordinating while one
is in addition protonated (see Scheme 4).
The 31P NMR spectrum shows a singlet at 31.2 ppm (Table
1), which is typical for symmetrically bis(ortho-)chelate bonded
PCP-type ligands.14,16 The presence of coupling signals due to
195Pt (natural abundance 34%; 1JPPt ) 2970 Hz) unambiguously
demonstrates coordination of the phosphine donors to the metal
center. Moreover, also the (PCP) benzylic proton resonances
Exchange of the (noncoordinating) anions in [4]Cl2 from Cl-
to BF4-, i.e., the formation of [4](BF4)2, did not significantly
change the spectroscopic properties. An exception is the
1
disappearance of the signal in the H NMR spectrum at 11.8
ppm (assigned to N-H) and the presence of a new very broad
resonance at 5.8 ppm (w1/2 ca. 150 Hz). This highfield shift
presumably is a consequence of the different hydrogen acceptor
capacity of the BF4- anion with respect to Cl-. Upon cooling,
the δH values shift to higher field (∆δ ) 0.6 ppm) as described
for this proton in [4]Cl2, concomitant with a significant
narrowing (w1/2 ) ca. 35 Hz at 213 K).
3
show coupling with 195Pt (δH ) 4.15, JHPt ) 28.3 Hz; Table
2). Mutual trans configuration of the aryl units is suggested by
the chemical shift of the two ipso-carbons (located at δC ) 160.1
and 169.0 ppm, respectively; see Table 3).
The singlet due to the NCH3 protons is considerably upfield
shifted (from 3.07 ppm in 1 to 1.69 ppm in [3]Cl), which
strongly points to the absence of metal-coordinated amine/
ammonium groups in [3]Cl (cf. δH for NCH3 amounts to 2.21
Whereas [3]Cl and [4]Cl2 could not be crystallized, suitable
single crystals for an X-ray structure determination were
obtained for [4](BF4)2 (from a CH2Cl2 solution). Most evidently,
a crystallographic rotation axis (C2) is present, which is defined
by the Cipso and Cpara nuclei of both the PCP and the NCN ligand
and the platinum center. This relates the two nitrogen and the
two phosphorus donors, respectively, by symmetry and forces
the metal in the PCP and NCN aryl planes, respectively (Figure
1). Notably, the η1-coordinating NCN aryl ring does not adopt
a perpendicular orientation (vide infra) but is rotated by 76.2-
(2)° with respect to the metal coordination plane and to the
terdentate coordinating PCP ligand (for relevant bond lengths
and angles, see Table 4). Refinement of the obtained data set
converged best by using a disordered model for the position of
3
in free NCHN), as does the lack of satellites due to JHPt
coupling. Most interestingly, the equivalent HCl, which was
liberated during the cyclometalation process, has been found
to be present in [3]Cl even after thorough drying in vacuo (see
correct elemental analyses). This indicates that, in solution, [3]-
Cl contains a η1-C-bonded NCN ligand, which formally
possesses one dimethylammonium and one dimethylamine
substituent, i.e., a complex [Pt(η3-PCP)(η1-NHCN)]Cl. On the
basis of the narrow line width of the NCH3 signal at 1.69 ppm
of [3]Cl, a fast acid/base equilibrium is assumed comprising
(probably) intermolecular exchange of the acidic proton of a
dimethylammonium center to a dimethylamine substituent. This
most likely Cl--mediated proton migration process can be
slowed by cooling, as is shown by a severe broadening of the
characteristic NCH3 resonance (w1/2 ) 23 Hz) at 178 K (C2F4-
Br2 solution). However, complete decoalescence of the signal
is not observed even at these low temperatures, which points
to a very fast exchange rate, as is indeed expected for proton
transfer in amine/ammonium systems.
+
the NHMe2 moieties. In one conformation (50% population)
the nitrogen center points toward the platinum center (Figure
2a). This orientation fixes the ammonium proton between the
nitrogen and the platinum. Intramolecular chelation of nitrogen
and platinum is indicated by short N-H‚‚‚Pt distances (2.82 Å
for H‚‚‚Pt and 3.535(10) Å for N‚‚‚Pt; angle N-H‚‚‚Pt 134°).
Hence, this conformation of [4](BF4)2 shows a hydrogen-
bonding motif which involves a nucleophilic platinum(II) center
as acceptor site and represents another example of an organo-
metallic proton sponge.17,19 The alternative conformation (50%
population) is characterized by the position of the ammonium
entities pointing away from the metal center, toward the
unsubstituted part of the NCN aryl ring (Figure 2b). The
nitrogen-bound protons interact with the BF4 anions, thus
Addition of pentane to a solution of [3]Cl in CH2Cl2 induced
disproportionation of [3]Cl to the bis-protonated complex [Pt-
(η3-PCP)(η1-NHCNH)]Cl2, [4]Cl2, and neutral [Pt(η3-PCP)(η1-
NCN)], 5 (Scheme 5). Fractional precipitation gave first the
bis-protonated complex [4]Cl2 and subsequently neutral 5. Both
compounds were isolated as colorless solids.
1
The H NMR data of the complex [4]Cl2 in CDCl3 shows
(17) (a) Wehman-Ooyevaar, I. C. M.; Grove, D. M.; Kooijman, H.; van
der Sluis, P.; Spek, A. L.; van Koten, G. J. Am. Chem. Soc. 1992, 114,
9916. (b) Wehman-Ooyevaar, I. C. M.; Grove, D. M.; van der Sluis, P.;
Spek, A. L.; van Koten, G. J. Chem. Soc., Chem. Commun. 1990, 1367.
(18) Emsley, J. Chem. Soc. ReV. 1980, 9, 91.
some significant differences compared to those of [3]Cl. First,
(16) (a) Bennett, M. A.; Jin, H.; Willis, A. C. J. Organomet. Chem. 1993,
451, 249. (b) Gorla, F.; Venanzi, L. M.; Albinati, A. Organometallics 1994,
13, 43. (c) Gorla, F.; Togni, A.; Venanzi, L. M.; Albinati, A.; Lianza, F.
Organometallics 1994, 13, 1607. (d) Cross, R. J.; Kennedy, A. R.; Muir,
K. W. J. Organomet. Chem. 1995, 487, 227.
(19) Brammer, L.; Zhao, D.; Ladipo, F.; Braddock-Wilking, J. Acta
Crystallogr. 1995, B51, 632.

Transcyclometalation Processes
J. Am. Chem. Soc., Vol. 122, No. 48, 2000 11825
Scheme 4
Table 1. Selected 31P NMR Dataa
Scheme 5
complex
δP (1JPPt
)
PCHPb
-9.9
17.6 (3005)
Ab
2c
[3]Cl
[4]Cl2
5
6
7
33.1 (2970)
31.7 (2983)
32.0 (2866)
31.6 (3050)
10.5 (3041),d 18.2 (3069)d
20.5 (3092)
a From CDCl3 solution, unless stated otherwise; δ in ppm and J in
d
Hz. b From C6D6. c From ref 15. 2JPP ) 429 Hz.
1
Table 2. Selected H NMR Dataa
ArCH2P
complex
(3JHPt
)
ArCH2N
4.02
NCH3
3.07
Caryl-H
PCHP 3.41
7.51-7.36, 7.09-6.88
6.99, 6.80
1
Ab
3.85 (br) 3.62
3.90 (25.6)
4.15 (28.3) 3.37
2.10
7.69, 7.3-6.6
2c
7.96-7.06
[3]Cl
1.69
1.94
1.56
7.3-7.1
[4]Cl2 4.22 (27.6) 3.73
11.8, 7.79-7.06
7.15-7.03
5
6
7
4.14 (28.3) 3.05
4.75-3.51 5.41-1.88d 1.87, 1.37 9.93, 8.25-5.74
metal center (distance Pt‚‚‚Hbenzyl).20 Both conformations of the
[4](BF4)2 molecular geometry in the solid state are in concert
with earlier conclusions made for the positioning of the
ammonium NH and the benzylic CH2N protons in the coordina-
tion sphere of the platinum center of [3]Cl and [4]X2 in solution
(X ) Cl, BF4). From the X-ray diffraction experiment, however,
an identical result is obtained, when the single molecule
comprises both structural motifs, i.e., hydrogen bonding of one
ammonium substituent to the platinum center and the other
having a N-H‚‚‚F-mediated cation-anion hydrogen bonding
(Figure 2c). This requires a reduced crystallographic symmetry
of the molecule and a random distribution of relative orientation
of the nitrogen substituents. Theoretical considerations support
such a confirmation, since the filled dz2 orbital of neutral
platinum(II) complexes, which is pivotal for this type of
3.49
1.77
7.69-7.1, 6.80, 6.67
a From CDCl3 solution, unless stated otherwise; δ in ppm and J in
Hz. b From C6D6. c From ref 15. d Eight AB doublets centered at δH
5.41, 4.76, 4.57, 3.84, 3.78, 3.51, 2.32, and 1.88.
Table 3. Selected 13C NMR Dataa
complex
ArCH2P (2JCPt
)
ArCH2N
NCH3
C-Pt
130.5
PCHP
35.9
1
77.1
66.7
65.5
68.3
68.5, 66.0
68.2
53.7
43.8
42.7
45.6
45.3
45.1
144.5
[3]Cl
[4]Cl2
5
6
7
46.5
46.1
47.0
35.5, 34.0
169.0, 160.1
167.6, 161.0
171.1, 158.6
141.3
139.6
a From CDCl3 solution; δ in ppm and J in Hz.
(20) (a) Albinati, A.; Arz, C.; Pregosin, P. S. J. Organomet. Chem. 1988,
356, 367. (b) Albinati, A.; Pregosin, P. S.; Wombacher, F. Inorg. Chem.
1990, 29, 1812. (c) Brookhart, M.; Green, M. H. L.; Wong, L. L. In Progress
in Inorganic Chemistry; Lippard, S. J., Ed.; Wiley: New York, 1988; Vol.
36, p 1.
forming N-H‚‚‚F hydrogen bonding motifs. Instead of the
NHMe2 groups (as found in the first conformation), benzylic
protons occupy the virtual z axis of the square plane defined
by the metal-bound ligands and are located 2.67 Å from the

11826 J. Am. Chem. Soc., Vol. 122, No. 48, 2000
Albrecht et al.
Figure 1. Perspective view (50% probability) of [4](BF4)2 showing
the adopted numbering scheme. Only one of the two disordered
positions for the CH2NMe2 groups is shown (50% occupancy each).
All hydrogen atoms and the BF4- anions have been omitted for clarity.
Table 4. Selected Bond Lengths (Å) and Angles (deg) for
a
Complexes [4](BF4)2, [6]‚4C6H6, and [7]‚1/6C6H14
[4](BF4)2
[6]‚C6H6
[7]‚1/6C6H14
Bond Lengths
Pt- C51 2.096(12) 2.034(3)
2.043(4)
Pt-P
Pt-L
2.2753(12) 2.3036(6)-2.3165(6) 2.3124(10)-2.3159(10)
2.083(12) 2.4392(6)
Bond Angles
P-Pt-P 161.26(6) 177.57(2)
L-Pt-C51 180 176.48(7)
2.4084(10)
176.12(3)
178.29(13)
P-Pt-L 80.63(3) 87.45(2)-91.38(2) 89.05(3)-89.10(4)
P-Pt-C51 99.37(3) 89.30(7)-92.00(7) 90.39(14)-91.36(14)
Dihedral Angle between the Pt Coordination Plane
and the NCN Aryl Plane
76.2(2)
89.9(1)
85.5(2)
a L ) Cl ([b] and [7]), C1 ([4]).
interactions, functions rather as a 2e donor (monofunctional
Lewis base) than as a 4e donor (bifunctional Lewis base, Figure
2a).21
Neutral Pt(PCP)(NCN) Complex 5. The second product
obtained by fractional precipitation of a solution of [3]Cl is
identical to the product which is isolated after treatment of [3]-
Cl or [4]Cl2 with an excess of base (e.g., NEt3) and has been
characterized as complex 5 (Scheme 5). Due to its neutral
character, 5 is much better soluble in organic solvents than the
ionic complexes [3]Cl or [4]X2. When the spectroscopic
properties of 5 are compared to those of the corresponding ionic
complexes, the absence of acidic protons is an obvious differ-
ence. In addition, the singlets due to the NCH3 and ArCH2N
protons are shifted to higher field and are located at 1.56 and
3.05 ppm, respectively. Only small shift differences of the
Figure 2. Molecular structures of complex [4](BF4)2, showing the two
disordered models (occupancy factor 0.5 each). The carbon-bound
hydrogen atoms and also the phenyl substituents on phosphorus (except
C
ipso) have been omitted in the representations: (a) The NHMe2
groups are bent toward the metal center, giving rise to intramolecular
N-H‚‚‚Pt bonding. (b) The NMe2 groups are directed away from the
platinum nucleus toward the BF4 anion, resulting in N-H‚‚‚F hydrogen-
bonded anion-cation pairs. (c) Superposition of the two structures
shows intramolecular N-H‚‚‚Pt hydrogen bonding and also cation-
anion interactions in the same molecule.
3
benzylic ArCH2P signals (δH ) 4.14, JHPt ) 28.3 Hz) and
structure of 5 in solution. This comprises a platinum nucleus
as the center of a (distorted) square plane defined by the two
phosphorus nuclei and the two ipso-carbons from the PCP ligand
and the NCN ligand, respectively. Furthermore, the aryl plane
of the monodentate binding NCN pincer ligand seems to be
orientated perpendicular to the central aryl ring of the PCP
ligand. Such a conformation has been found in other complexes
containing two aryl units in mutual trans position and can be
justified by considering both stereochemical (vide supra) and
electronic arguments (orbital situation at the metal center and
at the aryl units).22
1
the phosphorus signal (δP ) 31.6, JPPt ) 3052 Hz) are ob-
served.
Various attempts to crystallize 5 resulted invariably in the
formation of thin needles which had a low diffractive response.
Hence, the obtained crystal data set is of a quality which is too
low for the detailed discussion of structural aspects (bond lengths
and angles). Preliminary studies suggest, however, a molecular
connectivity pattern as anticipated from investigations on the
(21) Wehman-Ooyevaar, I. C. M.; Grove, D. M.; de Vaal, P.; Dedieu,
P. A.; van Koten, G. Inorg. Chem. 1992, 31, 5484.

Transcyclometalation Processes
J. Am. Chem. Soc., Vol. 122, No. 48, 2000 11827
The neutral complex 5 contains tertiary amines and in the
presence of an acid HX therefore rapidly interconverts to the
dicationic complex [4]X2. For example, the addition of HBF4
to 5 results in the quantitative formation of [4](BF4)2. However,
in the presence of a large excess of acid, e.g., HCl, [4]X2
undergoes Pt-C bond cleavage, resulting in the formation of
the transmetalated product PtCl(PCP) with concomitant elimina-
tion of the meta-bis(ammonio)arene [C6H4{CH2NHMe2}2-1,3]2+
as a water-soluble dichloride salt.
Intermediates Preceding the Formation of Pt(PCP)(NCN),
5. When the reaction of PCHP with PtCl(NCN) is carried out
in C6H6 at room temperature (instead of 80 °C), a white powder
is formed, which has been identified as the dimetallic complex
6 (Scheme 4). This dimeric Pt2 species is hardly soluble in
CHCl3 and insoluble in C6H6. Its complicated 31P NMR
spectrum (CDCl3 solution) has been analyzed by simulations,
which point to the presence of two chemically inequivalent,
mutually trans-positioned 31P atoms (see Supporting Informa-
tion). The calculated positions of the resonance signals and of
the corresponding coupling constants (δPA ) 10.5, δPB ) 18.2
ppm; 2JPAPB ) 429 Hz, 1JPAPt ) 3041 Hz, 1JPBPt ) 3069 Hz) fit
the measured data excellently. From the 1H NMR spectra, further
evidence is gained for a low symmetry of this complex. For
example, eight different (partially overlapping) benzylic 1H
Figure 3. Perspective view (50% probability) of [6]‚4C6H6 showing
the adopted numbering scheme. Hydrogen atoms except H1, and the
included solvent molecules have been omitted for clarity. Note the short
intramolecular contacts in the macrocycle of 6 (Pt‚‚‚C1 3.568(3) Å;
Pt‚‚‚H1 2.88(3) Å) and the hydrogen-bonding motif (Cl‚‚‚H1 2.66(3)
Å; Cl‚‚‚C1 3.554(3) Å; C1-H1‚‚‚Cl 153(2)°).
platinum centers is bound to a chloride as well as to a η1-C-
bonded NCN ligand (through C51), while both nitrogen atoms
of the amino substituents are not coordinated (Table 4). The
structure clearly demonstrates the reduced symmetry of the NCN
and PCP ligands in the solid state, which parallels the dissym-
metry observed for 6 in solution (cf. the 1H NMR pattern
revealing eight different benzylic protons and two inequivalent
NCH3 groups). These observations strongly suggest that the
structure of 6 in the solid state is retained in solution. Perhaps
the most intriguing aspect of the molecular structure is the
particularly short distance found between the C1-bound proton
and chloride (H1‚‚‚Cl ) 2.66(3) Å; angle C1-H1‚‚‚Cl is
153(2)°), suggesting intramolecular noncovalent C-H‚‚‚Cl
hydrogen bonding (Figure 3).23 Additional short contacts of
particular relevance in view of a subsequent cyclometalation
reaction have been found around the metal center; i.e., the
distances Pt‚‚‚C1 (3.568(3) Å) and Pt‚‚‚H1 (2.88(3) Å) are both
slightly shorter than the sum of the corresponding van der Waals
radii. These structural features are in good agreement with earlier
reported data on related palladium complexes.24
1
signals have been found between 5.4 and 1.9 ppm. From H-
1H COSY measurements they were all identified as parts of
2
AB doublets, some of them displaying JHP-type interactions
to phosphorus. Another indication for a highly dissymmetric
complex is the two different signals due to the NCH3 protons,
located in a 1:1 ratio at 1.87 and 1.37 ppm, respectively. These
chemical shift values and the absence of any coupling reso-
nances with the 195Pt nucleus are characteristic for two different
noncoordinated amines. All of the observed proton resonances
are rather broad and suggest a dynamic behavior of the molecule.
Remarkably, one sharp singlet is found at 9.93 ppm, which
displays only a weak cross-peak in the 1H-1H COSY spectrum,
4
related to a JHH coupling with the aromatic protons of the
central phenyl unit of the PCHP ligand. Hence, this resonance
has been attributed to the C1-bound proton PCHP. Unambiguous
evidence for the assignment of this signal has been provided
by an experiment using PCDP, a modified PCHP ligand
precursor containing a deuterium substituent on carbon C1.
When PtCl(NCN), 1, is treated with equimolar amounts of
PCDP, again a white precipitate, 6-D, is formed. Its 1H and 31
P
Monitoring of the Transcyclometalation Process. The
formation of the dimetallic complex 6 has been followed by
1H and 31P NMR spectroscopy. Upon mixing of the metal
precursor PtCl(NCN), 1, and the phosphine PCHP in C6D6 at
room temperature, the signal due to free PCHP (δP ) -9.9)
disappears instantaneously (31P NMR). Instead, a new singlet
is observed at 17.6 ppm, which originates from two equivalent
NMR spectra are fully identical with those of 6 except for the
absence of the signal at 9.93 ppm in 6-D.
When compared to the line width of the other signals of 6,
the remarkably sharp signal due to the PCHP protons is pointing
to a reduced dynamic behavior in this part of the molecule.
Together with the extraordinary large downfield shift of this
aromatic proton (more than 1.2 ppm downfield from the other
aryl-bound protons in 6), an additional stabilization by weak
interactions can be deduced, e.g., owing to hydrogen bonding
of the C1-bound proton to a heteroatom. Detailed elucidation
of the structure of 6 in the solid state has been achieved by a
single-crystal X-ray structure determination.
Neutral [PtCl(NCN)(PCHP)] Complex 6. The molecular
structure of 6 in the crystal is depicted in Figure 3 and shows
a bimetallic 16-membered macrocycle comprising two PCHP
ligands bridging between two square-planar coordinated plati-
num(II) centers. An inversion center relates, e.g., the two
platinum centers to each other (Ci symmetry). Each of the
phosphines that are trans-bound to a platinum center (1JPPt
)
3005 Hz; vide infra complex 7). Consequently, formation of a
dimeric species A immediately after mixing of the reactants is
assumed (see Scheme 4). This is corroborated by the high-field
1
shift of the signals due to ArCH2N and NCH3 in the H NMR
spectrum, which is diagnostic for noncoordinating amine
ligands.9a Complex A slowly reacts in solution, as is unambigu-
ously demonstrated by 31P NMR spectroscopy; the signal at
17.6 ppm gradually disappears and concomitantly a new set of
signals appears, which corresponds to that of 6. These changes
are linear in time and suggest therefore (pseudo-) zero-order
(23) Taylor, R.; Kennard, O. J. Am. Chem. Soc. 1982, 104, 5063.
(24) Khare, G. P.; Little, R. G.; Veal, J. T.; Doedens, R. J. Inorg. Chem.
1975, 14, 2475.
(22) Terheijden, J.; van Koten, G.; Muller, F.; Grove, D. M.; Vrieze,
K.; Nielsen, E.; Stam, H. C. J. Organomet. Chem. 1986, 315, 401.

11828 J. Am. Chem. Soc., Vol. 122, No. 48, 2000
Albrecht et al.
transcyclometalated product 2 and the corresponding arene
NCHN at 110 °C after prolonged heating. This transcycloplati-
nation process of the PCHP ligand precursor is occurring with
high stereoselectivity as only one C-H bond, ortho, ortho
orientated to the phosphorus-containing substituents, is ac-
tivated.5a-f It must be noted that the transcyclometalated product
2 has been obtained quantitatively from the bis(ortho-)-
cyclometalated compound 1. The latter PtX(NCN) complex is
known to have an exceptional stability and a high resistance
toward various reagents as HCl.25 In this context, the formation
of 2 from PtCl(NCN) 1 is remarkable and represents the first
detailed example of a transcyclometalation reaction in organo-
platinum(II) chemistry.12,14d-e From this study it is obvious that
the transcyclometalation reaction is a multistep process. In the
next sections, the importance of the various unique structural
features of the various intermediates will be discussed.
Ligand Substitution and C-H Bond Activation. Previous
reports have shown that substitution of a (relatively) weakly
coordinated ligand by an incoming stronger bonding ligand
initiates the cyclometalation reaction.2 Accordingly, addition of
PCHP to the cyclometalated platinum precursor 1 is expected
to result in replacement of the hard amine ligands by phosphine
ligands, which are known to be much softer σ donors;26 cf. the
formation of [PtBr(η1-NCN)(PPh3)2]9a from [PtBr(NCN)] and
PPh3 as well as the formation of 7 in the present study. Clearly,
in these complexes, bis(ortho-)chelation of the NCN ligand does
not compensate for the stronger bonding of platinum(II) to
phosphorus ligands.27 A similar reactivity has been observed
between the platinum precursor 1 and PCHP, affording dimeric
A as the initially detectable product, in which the PCHP ligand
bridges between two platinum centers. Coordination of the two
phosphorus donor atoms of the PCHP ligand to the same
platinum nucleus, i.e., an η2-P,P-bonding mode as found in
[(PCP)Ru(PCHP)](OTf)11 apparently is not favored.28 Complex
A (symmetry D2h) is thermodynamically unstable and spontane-
ously rearranges to the conformational isomer 6, which displays
a lower symmetry (Ci). The bimetallic complex 6 is unprec-
edented, and its formation from A demonstrates the extra
stabilization brought about by formation of the C-H‚‚‚Cl
bonding. Most importantly, the noncovalent hydrogen bonding
interactions (Caryl-H‚‚‚Cl) have been established both in the solid
state (X-ray) and in solution (NMR, IR). Computational studies
have suggested that noncovalent hydrogen bonds play a
significant role in C-H bond activation.29 The dimeric complex
6 seems to illustrate this view, since it displays an extraordinary
high degree of steric and electronic preorganization for cyclo-
metalation, mainly imposed by the hydrogen-bonding motif.
First, the metal center is located in close proximity to H1 and,
more importantly, also to C1. Second, the Caryl-H bond is
considerably weakened by the interaction of the proton with
the metal-bound chloride and may be envisaged as an “acti-
vated” bond. Third, the electrophilicity of the platinum center
is significantly increased (reduced σ donation of the chloride
to the metal center due to competitive electron release into the
noncovalent Caryl-H‚‚‚Cl hydrogen bond). In contrast, the arene
system (predominantly C1) possesses an enhanced electron
Figure 4. Displacement ellipsoid plot representation (50% probability)
of [7]‚1/6C6H14 showing the adopted numbering scheme. Only one of
the two positions for the NCH3 group is shown (50% occupancy). All
hydrogen atoms and the solvent residue have been omitted.
reaction kinetics for the rate-determining step of this probably
intramolecular process.
Following the formation of 6 by 1H NMR spectroscopy
confirms these conclusions. Immediately after mixing of 1 and
PCHP, a signal pattern is observed which is fully consistent
with the presence of A only. Within the subsequent 0.5 h, all
these resonances broaden significantly, and eventually, gradual
appearance of broad resonances due to 6 (and likewise
concomitant disappearance of those assigned to A) is noted.
These new signals remain broad with exception of the signal at
9.93 ppm, assigned to the C1-bound proton PCHP (vide supra).
The formation of A and hence fast replacement of the amine
ligands by phosphorus donors has been studied in a separate
experiment. Mixing of 1 with 2 mol equiv of PPh3 under
identical conditions as applied for the preparation of 6 affords
a single product 7, containing the two phosphine ligands
mutually trans positioned and an η1-C-bonded NCN ligand
(Scheme 4). As expected, the spectroscopic properties of 7 are
very closely related to the data earlier obtained for [PtBr(η1-
NCN)(PPh3)2]9a and show also high similarities to those of A
(see Tables 1 and 2).
In contrast to A, its mononuclear analogue 7 is inherently
stable in the solid state and in solution, which allowed detailed
analyses (Tables 1-3) by a single-crystal X-ray structure
determination. The molecular structure shows a square-planar
platinum(II) complex which possesses two PPh3 ligands in
mutually trans configuration and a monodentate C-bonded NCN
ligand (Figure 4). As a result, the Pt-C51 bond is stretched
from 1.907(5) in 1 to 2.043(4) Å in 7. In contrast, the Pt-Cl
bond is hardly affected (Table 4). In addition, the NCN aryl
plane in 7 is oriented perpendicular to the coordination plane
of the platinum(II) center, which contrasts to the almost coplanar
situation in 1 (vide supra).
Discussion
The present transcyclometalation reaction has no precedent
in that various intermediate stages in the process could be
isolated and identified. Moreover, a sequence of intermediates
could be established comprising the rapid formation of A and
its conversion to dimeric 6 at room temperature, formation of
the monoprotonated bisaryl product and its redistribution to the
neutral bisarylplatinum and the bis-protonated derivatives (cf.
5 and [4]X2) below 80 °C, and finally the formation of the
(25) Albrecht, M.; van Koten, G. AdV. Mater. 1999, 11, 171.
(26) (a) Davies, J. A.; Hartley, F. R. Chem. ReV. 1981, 81, 79. (b)
Gilheany, D. G. Chem. ReV. 1994, 94, 1339.
(27) In the corresponding Ni(II) complexes, chelation of the NCN ligand
vs phosphine coordination is strongly dependent on the nature of the amines;
see: van Beek, J. A. M.; van Koten, G.; Ramp, M. J.; Coenjaarts, N. C.;
Grove, D. M.; Goubitz, K.; Zoutberg, M. C.; Stam, C. H.; Smeets, W. J. J.;
Spek, A. L. Inorg. Chem. 1991, 30, 3059.
(28) Shaw, B. L. AdV. Chem. Ser. 1982, 196, 101.
(29) Mohr, M.; Zipse, H. Chem. Eur. J. 1999, 5, 3046.

Transcyclometalation Processes
J. Am. Chem. Soc., Vol. 122, No. 48, 2000 11829
Scheme 6
coordinatively unsaturated 14e metal center.3c,33 Recoordination
of the phosphorus ligand is expected to quench the reaction
successfully, which might provide another reason for the long
reaction time required for the formation of [3]Cl (vide supra).
In the forward process, the arene moiety of the PCHP ligand
may supply additional electron density to the deficient metal
center. This is inducing an electrophilic aromatic substitution
process by the platinum center and formation of an arenium
intermediate.7,34 As evidenced in related platinum arenium
chemistry,10a a subsequent reversible 1,2-sigmatropic migration
of the proton H1 from C1 to the metal center leads to a platinum-
(IV) product. Such platinum(IV)-hydride complexes have been
shown to be reasonably stable when a strong base is coordinated
trans to the metal-bound hydride.17,21,35 In the present system,
this is achieved by intramolecular attack of the noncoordinating
tertiary amines of the NCN ligand. Alternatively, the amine
substituents assist in stabilizing the hydrogen by acting as a
base and hence by formation of a N-H‚‚‚Pt bonding motif.
This weakens the H‚‚‚Pt interaction and eventually affords a
cationic ammonium entity, concomitant with the reduction of
the metal center to platinum(II). Formally, this corresponds to
an intramolecular, nitrogen-mediated reductive elimination of
HCl from a platinum(IV) complex, affording [3]Cl. This
sequence implies that, during the cyclometalation process, Pt-
Cl bond cleavage in the metal precursor is preferred over Pt-
Caryl bond fission under the applied conditions (C6H6, 80 °C),
and hence, HCl is (formally) released as a byproduct (Scheme
4). Obviously, the intramolecular stabilization of the platinum-
(IV) intermediate prevents further migration of the hydrogen
by a second 1,2-sigmatropic shift onto the NCN ligand. A
second pathway, which is sterically rather unfavored, would
involve direct nitrogen-assisted proton abstraction from the
arenium complex, without passing through an oxidation-
reduction cycle of the platinum center (Scheme 6).
density, imposed by the stronger polarization of the C1-H1
bond (vide NMR data). Therefore, this is a predestinated system
for electrophilic substitution reactions (cf. activated arene
moiety, electrophilic platinum(II) nucleus as E+).
Cyclometalation Process. In view of this intramolecular
preorganization in 6 in the solid state as well as in solution, it
is no surprise that cyclometalation of 6 (in hot C6H6) indeed
occurs, yielding complex [3]Cl. Due to the poor solubility
properties of 6, no (spectroscopic) evidence could be obtained
whether 6 represents a true kinetic intermediate30 in the
cyclometalation process of the PCHP ligand or a resting state
(i.e., a thermodynamic “sink”, thus involving regeneration of
the higher symmetrical complex A as essential step for suc-
cessful C-H activation). Moreover, it is possible that either
prior Pt-P bond dissociation occurs leading to a 14e arylmetal
chloride species with a monodentate coordinated PCHP ligand2d,12
or a monometallic η2-E,E-coordinating intermediate is formed
which precedes the cyclometalation step.31 It is interesting to
note, however, that similar dinuclear complexes containing the
PC(Me)P ligand (type A coordination) do not form subsequently
type 6 complexes with intramolecular hydrogen bonds and also
do not undergo any cyclometalation reaction.32
Formation of [Pt(PCP)(NCN)], 5, could point to an associative
mechanism involving an octahedral platinum(IV) center com-
prising a metal-bound chloride and a hydride ion.17 However,
as follows from the present results, some other intermediates
are preceding the formation of 5, of which [Pt(PCP)(NCNH)]+,
[3]+, is an important one. This clearly corresponds to a kinetic
product as a change of the solvent leads to irreversible formation
of neutral [Pt(PCP)(NCN)], 5, and dicationic [Pt(PCP)(NH-
CNH)]2+, [4]2+
.
Notably, treatment of 5 with a large excess of an acid HX
results initially in the ionic complex [4]X2. Subsequently,
cleavage of the nonstabilized Pt-C bond is promoted and
corresponds to the formal completion of the transcyclometalation
reaction forming the product [PtX(PCP)], 2 (X ) e.g. Cl) and
the arene NCHN. This last stage probably involves an oxidative
addition-reductive elimination sequence with intermediate
formation of a platinum(IV)-hydride complex [Pt(X)(H)(PCP)-
(NCN) as indicated in Scheme 6.3c,17c
Finally it should be noted that the transcyclometalation
reaction presented here takes place under neutral conditions and
without auxiliaries. Previously used catalysts (e.g. by using
HOAc or HOAcF as a solvent)12 may be disadvantageous for
such TCM reactions, as a proper choice of the solvent system
Considering the various aspects outlined above, the following
mechanism for the cyclometalation of the PCHP ligand in 6 is
proposed (Scheme 6): eventual dissociation of a Pt-P bond in
6 affords the reactive intermediate, a strongly electron-deficient
(30) Jencks, W. P. Acc. Chem. Res. 1980, 13, 161.
(31) See e.g.: (a) Bennett, M. A.; Johnson, R. N.; Tomkins, I. B. J.
Organomet. Chem. 1977, 128, 73. (b) Karlen, T.; Dani, P.; Grove, D. M.;
Steenwinkel, P.; van Koten, G. Organometallics 1996, 15, 568. (c) Lesueur,
W.; Solari, E.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C. Inorg. Chem. 1997,
36, 3354. (d) Rybtchinski, B.; Vigalok, A.; Ben-David, Y.; Milstein, D. J.
Am. Chem. Soc. 1996, 118, 12406. (e) Baltensperger, U.; Gunter, J. R.;
Ka¨gi, S.; Kahr, G.; Marty, W. Organometallics 1983, 2, 578.
(33) (a) Grubbs, R. H.; Coates, G. W. Acc. Chem. Res. 1996, 29, 85. (b)
Huang, D.; Streib, W. E.; Bollinger, J. C.; Caulton, K. G.; Winter, R. F.;
Scheiring, T. J. Am. Chem. Soc. 1999, 121, 8087.
(34) (a) Wheland, G. W. J. Am. Chem. Soc. 1942, 64, 900. (b) Farcasiu,
D. Acc. Chem. Res. 1982, 15, 46.
(32) (a) Al-Salem, N. A.; Empsall, H. D.; Markham, R.; Shaw, B. L.;
Weeks, B. J. Chem. Soc., Dalton Trans. 1979, 1972. (b) van der Boom, M.
E.; Gozin, M.; Ben-David, Y.; Shimon, L. J. W.; Frolow, F.; Kraatz, H.-
B.; Milstein, D. Inorg. Chem. 1996, 35, 7068.
(35) (a) Rendina, L. M.; Puddephatt, R. J. Chem. ReV. 1997, 97, 1735.
(b) Rostovtsev, V. V.; Labinger, J. A.; Bercaw, J. E.; Lasseter, T. L.;
Goldberg, K. I. Organometallics 1998, 17, 4530. (c) Stahl, S. S.; Labinger,
J. A.; Bercaw, J. E. J. Am. Chem. Soc. 1996, 118, 5961.

11830 J. Am. Chem. Soc., Vol. 122, No. 48, 2000
Albrecht et al.
greatly stabilizes the transition state(s) and/or lowers the
activation energy for its formation. Addition of an acid as
catalyst implies, however, that H+ is facilitating e.g. the M-N
bond cleavage by protonation of the dissociated (free) nitrogen
ligand.
Scheme 7
Another cyclometalation mechanism involving heteroatom
induced proton abstraction in 6 before electrophilic aromatic
substitution may be discarded, since addition of weak bases
(intramolecularly present as R-NMe2 entities) or strong bases
(NaOH, K2CO3) to 6 did not accelerate the formation of [3]Cl.
Similarly, the reaction of the cationic platinum solvato complex
[Pt(NCN)(H2O)](BF4) with PCHP did not cause any product
containing a cyclometalated PCP ligand.36 These findings are
consistent with the proposed mechanism (Scheme 6) and
emphasize the crucial influence of the electronic properties of
the metal center and of the presence of a platinum-halogen
bond. The high significance of the trans C-M-X arrangement
in similar square-planar complexes is well-documented.9,37 This
is further underlined by the fact that addition of AgBF4 to 6
only affords the solvato complex [Pt(NCN)(H2O)](BF4). Its
formation is probably preceded by cleavage of the H‚‚‚Cl
hydrogen bonded assembly due to silver-mediated chloride
abstraction. This reduces the preorganization for cyclometalation
as found in dimer 6 and, more importantly, probably increases
the electrophilic character of the metal center to such an extent
that the crucial P-Pt bond cleavage and formation of the
unsaturated 14e intermediate is fully inhibited.
This cycloplatination reaction provides a mild access to
platinum(II) complexes containing two C-Pt σ-bonds. Com-
monly, cyclometalation is used for the introduction of the first
M-C σ-bond. Cases where this route can be applied for the
preparation of a second M-C σ-bond are rare.14e,38 Noteworthy,
alternative routes for the preparation of stable compounds of
the type [PtR(NCN)] (R ) aryl, alkyl, alkyne) containing two
M-C bonds exist, e.g., by reacting 1 with RLi. However, little
is known about this reaction. For example, it has been shown
that for R ) tolyl or phenylacetyl, a second M-C bond is
formed with concomitant elimination of LiCl.22 In contrast,
alkyllithium reagents (R ) allyl, Me) result in the formation of
1 only instead of the alkylated products (eq 3). In the case of
[Pt(NCN)(CtCR)], it has been shown that the reaction with
CuCl affords [PtCl(NCN)] and Cu(CtCR).39 A similar prefer-
ence for the cleavage of the Pt-C bond trans to carbon C1 by
LiCl could be responsible for the observed result.
this transcyclometalation process does not involve Ru-Cl bond
cleavage and concomitant HCl formation but rather Ru-Caryl
bond cleavage (cf. Scheme 6).10 Ultimately, both reactions
produce the transcyclometalated products but apparently via
different routes; i.e., the transcycloplatination of 1 proceeds via
Pt-Cl bond cleavage and formation of the bisarylplatinum
species, whereas transcycloruthenation apparently occurs via H
transfer. Note that both transcyclometalations have to proceed
from the neutral species. This identifies the [Pt(NCN)]+ and
the [RuCl(PPh3)]+ units as isolobal monocationic ML+ synthons.
The selectivity in cleaving either the M-Cl or the M-C bond,
viz. elimination of either first HCl or NCHN directly, probably
originates from the different nature of the relevant (metal-
centered) frontier orbitals in the corresponding ruthenium or
platinum arenium intermediates during the cyclometalation
process (Scheme 6). In a d8 metal center (platinum(II) com-
plexes), electron density perpendicular to the platinum coordina-
tion plane (viz. a dz2-type orbital) can participate in stabilizing
a proton by chelation.40
Organometallic Proton Sponges. Various similar trans-bis-
(aryl)platinum(II) complexes are known that contain aminoaryl
type ligands.22 Characteristic to these complexes is a hindered
rotation around the Pt-C σ-bond of the η1-bonded aryl ligand
and the approximately perpendicular position of the nonchelating
aryl ligands with respect to the metal coordination plane. For
complexes 3-5, a similar conformational situation has been
found, which is particularly emphasized due to the presence of
two sterically bulky ortho-substituents on the η1-coordinating
aryl unit (see Pt-C(51)). Hence, two limiting isomeric structures
for the positioning of the ammonium groups and the source of
the heteroatom stabilization of the acidic protons in the
dicationic complex [4]Cl2 exist: either the nitrogen nuclei are
directed toward the metal center (structure I, Scheme 7) or they
Transcycloplatination vs Transcycloruthenation of PCHP
Ligands. When a ruthenium(II) complex analogous to 1, [RuCl-
(NCN)(PPh3)], is used as the metal precursor for the cyclo-
metalation of PCHP, elimination of NCHN is observed; i.e.,
(36) Equimolar amounts of PCHP and [Pt(NCN)(H2O)](BF4) were stirred
in THF at reflux for several hours. As indicated by the diagnostic 31P NMR
spectrum (δP ) 41), the obtained product contains a coordinated phosphine
trans to the metal-bound carbon of the terdentate-bonded NCN ligand; for
similar reactivities, see: Maassarani, F.; Davidson, M. F.; Wehman-
Ooyevaar, I. C. M.; Grove, D. M.; van Koten, M. A.; Smeets, W. J. J.;
Spek, A. L.; van Koten, G. Inorg. Chim. Acta 1995, 235, 327.
(37) See, e.g.: (a) Grove, D. M.; van Koten, G.; Verschuuren, A. H. M.
J. Mol. Catal. 1988, 45, 169. (b) Albrecht, M.; Gossage, R. A.; Lutz, M.;
Spek, A. L.; van Koten, G. Chem. Eur. J. 2000, 6, 1431.
(38) Crespo, M.; Grande, C.; Klein, A. J. Chem. Soc., Dalton Trans.
1999, 1629.
(39) Back, S.; Gossage, R. A.; Lutz, M.; del R´ıo, I.; Spek, A. L.; Lang,
H.; van Koten, G. Organometallics 2000, 19, 3296.
(40) Hence, a 3c-4e Pt‚‚‚H‚‚‚N bond is formed,17 which promotes the
(reversible) 1,2-sigmatropic migration of the hydrogen onto the metal center.
In contrast, stabilization of a metal-bound hydrogen in the hypothetical z-axis
of the corresponding ruthenium(II) complex is much less pronounced (d6
configuration, dz2 orbital empty). Hence, the equilibrium between a cationic
ruthenium(IV) complex and an arenium (either a NCN- or PCP-arenium
system) is shifted onto the arenium side. In both arenium structures, the
M-Carenium bond is weakened. Whereas Ru-C bond fission is hampered
in the PCP ligand due to chelation of the phosphines, this bond is relatively
labile in the NCN-arenium moiety (η1-C-bonded). Consequently, Ru-C
bond breaking of the NCN-arenium ligand results in the formation of [RuCl-
(PCP)(PPh3)] and elimination of NCHN. The different reactivities of the
platinum(II) and ruthenium(II) metal precursors therefore indirectly support
the mechanism as proposed in Scheme 6.

Transcyclometalation Processes
J. Am. Chem. Soc., Vol. 122, No. 48, 2000 11831
the dicationic complex [4]X2, however, the metal center is
neutral, which reduces its donating properties and therefore
weakens the chelation strength. Another important aspect for
the effectiveness of (organometallic) proton sponges is the steric
constrain of the skeleton connecting the two donor atoms. Due
to the aryl backbone in [4]X2 and 8, the nitrogen donor is held
rigidly in a place above the platinum coordination plane, which
forces the proton to be located also in close proximity to a
mutual dz2 orbital of the metal center. This deshielding effect
is confirmed by the chemical shift values of the interlocked
proton, which is at lower field when the N-H‚‚‚Pt skeleton is
more rigid.
Figure 5. Structural similarities between organic and organometallic
proton sponges.
are oriented away from platinum (structure II). Structure I
represents an organometallic analogue of the organic “proton
sponge”,41 with one amine nitrogen substituted by an electron-
rich platinum center (Figure 5). This N,Pt-system displays an
excellent donor motif for selective proton scavenging via 3c-
4e N-H‚‚‚Pt(II) interactions and hence a rare example of an
organometallic proton sponge.17,42 Strongly related zwitterionic
platinum(II) complexes, viz. cis-[PtX(C∼N)(C∼NH)] (X ) Cl,
Br; C∼N ) C6H4{CH2NMe2}-2), 8 (Scheme 7), have been
reported, which contain unprecedented N-H‚‚‚Pt hydrogen
bonds.17 In these structures, a diagnostic 1H NMR signal of the
interlocked N-H‚‚‚Pt(II) proton appeared between 13 and 16
ppm, which showed 195Pt coupling resonances.43 The size of
these coupling constants correlates directly with the structural
position of the proton between nitrogen and platinum; i.e., the
larger the coupling constant the smaller the Pt‚‚‚H distance and
the longer the N-H bond. In analogous platinum(IV) complexes
containing a metal-bound hydride rather than a metal-stabilized
proton, a signal at ca. -20 ppm is observed.21 When these data
are compared with the observed values of [4]Cl2 (δH ) 11.8
ppm, no 1JHPt resolved), Pt‚‚‚H interactions in solution may be
deduced for [4]Cl2, which are relatively weak. This is further
Conclusion
Transcyclometalation, i.e., the substitution of one cyclo-
metalated ligand by another, has been used as a versatile
methodology which provides a novel access to metal-to-carbon
bonds. This concept has been applied for the substitution of a
bis(ortho-)cyclometalated NCN ligand on a platinum center by
a terdentate bonding PCP-type pincer ligand under neutral
conditions.
An essential prerequisite for successful transcyclometalation
in the present case is the difference in bond strength of the
platinum-heteroatom bond, which has to be stronger in the
product than in the reactant (viz. Pt-P stronger than Pt-N
bond). A reaction trajectory is favored that is similar to that of
electrophilic aromatic substitutions. Hence, transcyclometalation
is proposed to be initiated by weak C-H‚‚‚Cl-M hydrogen
bonds, which induce a remarkable substrate-reactant preorga-
nization and presumably promote the transient arenium forma-
tion.
Another intriguing intermediate that has been detected during
the transcyclometalation of 1 is the organometallic complex 5,
which displays typical characteristics of organic proton sponges.
This is a consequence of the fixed proximity of the electron-
rich platinum(II) center to the lone pairs of the noncoordinating
nitrogen nuclei. Hence, this Pt,N chelating unit is capable of
sequestering an equimolar amount of protons, leading to the
formation of mono- or bis-protonated organoplatinum com-
plexes, e.g. [4](BF4)2.
Finally it is important to note that at the present state of our
studies it is clear that the reaction profile outlined in this report
displays only one of probably seVeral different reaction mech-
anisms of the transcyclometalation process (cf. [PtCl(NCN)] vs
[RuCl(PPh)(NCN)]). However, it is very well possible that a
limited, well-defined set of routes exists. Which mechanism is
operative is strongly dependent on the nature (hardness, acidity)
of the metal center and the ligands. Moreover, a proper choice
of auxiliaries, such as catalytic or stoichiometric amounts of
AcOH, may either accelerate or inhibit transcyclometalation.
Reinvestigation of previously reported, though not as such
recognized, transcyclometalations could be a fruitful approach
to obtain valuable information about various pathways for
conversion of an aromatic C-H bond into the corresponding
aryl-metal bond.
1
corroborated by the H-1H NOESY spectrum, which clearly
revealed cross-peaks stemming from nuclear Overhouser effects
(nOe’s) between the spins of the Hmeta protons of the mono-
dentate zwitterionic NHCNH ligand (Scheme 7) and both the
NCH3 groupings and the benzylic ArCH2N protons. Similarly,
the characteristic IR data of [4]Cl2 show a broad vibration band
observed at 2480 cm-1 (assigned to the asymmetric stretching
mode νas of the N-H bond), which is higher than the strongly
interlocked N-H stretch vibration in the organometallic N-H‚
‚‚Pt proton sponge 8 (2340 cm-1 17
) but considerably lower than
the expected νas of N-H bonds in free ammonium compounds
(2700 cm-1).44 A rather weak interaction between the platinum-
(II) center and the nitrogen-bound proton is also supported by
solid-state analyses, which indicate disordered CH2NHMe2 units
in the single crystals of [4](BF4)2.
The N-H‚‚‚Pt hydrogen bond motif directly reflects the
proton sequestering capacity of the organometallic proton
sponges [4]X2 and 8 and their corresponding deprotonated
precursors. The difference in strength is best understood by
considering the situation at the platinum center: in the case of
8, a zwitterionic structure is present which comprises an
ammonium unit and a platinate, whose anionic charge increases
the donating nature of filled dz2-type orbitals significantly. In
(41) (a) Alder, R. W. Chem. ReV. 1989, 89, 1215. (b) Staab, H. A.; Saupe,
T. Angew. Chem. 1988, 100, 895; Angew. Chem., Int. Ed. Engl. 1988, 27,
865.
Experimental Section
(42) For related proton sponge exhibiting 3c-4e X-H‚‚‚Pt(II) interac-
tions, see: (a) Mart´ın, A. J. Chem. Educ. 1999, 76, 578. (b) Hedden, D.;
Roundhill, D. M.; Fultz, W. C.; Rheingold, A. L. Organometallics 1986,
5, 1336. (c) Albinati, A.; Anklin, C. G.; Ganazzoli, F.; Ru¨egg, H.; Pregosin,
P. S. Inorg. Chem. 1987, 26, 503. (d) Albinati, A.; Arz, C.; Pregosin, P. S.
Inorg. Chem. 1987, 26, 508.
(43) Pregosin, P. S.; Ru¨egg, H.; Wombacher, F.; van Koten, G.; Grove,
D. M.; Wehman-Ooyevaar, I. C. M. Magn. Reson. Chem. 1992, 30, 548.
(44) Hesse, M.; Meier, H.; Zeeh, B. Spectroscopic Methods in Organic
Chemistry; Georg Thieme Verlag: Stuttgart, Germany, 1987.
General Comments. All reactions involving free phosphines were
performed under an atmosphere of N2 using standard Schlenk tech-
niques. Pentane, toluene, and C6H6 were distilled from Na-benzophe-
none, and CH2Cl2 was distilled from CaH2. Starting materials were
obtained commercially (PPh3) or prepared according to published
procedures (PCHP,15 [PtCl(NCN)]9b). Elemental analyses were obtained
from Kolbe, Mikroanalytisches Laboratorium (Mu¨lheim, Germany).
Incorporated solvent molecules were identified by NMR spectroscopy.

11832 J. Am. Chem. Soc., Vol. 122, No. 48, 2000
Albrecht et al.
Table 5. Crystallographic Data for Complexes [4](BF4)2, [6]‚4C6H6, and [7]‚1/6C6H14
[4](BF4)2
[6]‚C6H6
[7]‚1/6C6H14
empirical formula
fw
C44H48N2P2Pt‚2BF4
1035.49
monoclinic
C88H94Cl2N4P4Pt2‚4C6H6
2105.06
monoclinic
P2l/c (No. 14)
0.25 × 0.25 × 0.13
colorless
C48H49ClN2P2Pt‚1/6C6 H14
960.74
triclinic
cryst system
space group
cryst size/mm
cryst color
unit cell dimens
a/Å
C2 (No. 5)
P1h (No. 2)
0.15 × 0.15 × 0.08
0.36 × 0.33 × 0.21
colorless
colorless
12.8866(3)
15.9008(3)
12.1765(3)
90
121.535(1)
90
2126.58(8)
2
1.617
13.0416(2)
15.5605(3)
24.1866(5)
90
92.638(1)
90
4903.08(16)
2
1.426
11.5811(3)
12.4431(3)
17.6349(5)
70.7122(10)
84.1765(11)
69.4615(13)
2245.87(10)
2
b/Å
c/Å
R/deg
â/deg
γ/deg
V/Å3
Z
D
calc/g cm-3
1.421
3.289
µ/mm-1 (Mo KR)
abs corr
3.444
3.020
PLATON (DELABS)
0.24-0.70
0.61
18 789, 3974, 3972
0.073
PLATON (MULABS)
0.61-0.68
0.65
91 192, 11 238, 9612
0.067
PLATON (MULABS)
0.34-0.43
0.65
43 828, 10 268, 9219
0.061
transm range
(sin θ/λ)m ax/Å-1
reflcns measd, unique obsd
Rint
params, restraints
R1a (obsd/all reflcns)
wR2b (obsd/all reflcns)
S
296, 95
567, 0
517, 39
0.0278/0.0278
0.0703/0.0703
1.04
0.0261/0.0362
0.0551/0.0585
1.04
0.0351/0.0408
0.1017/0.1050
1.07
resid density/e Å-3
-0.67 < 0.76
-1.24 < 1.08
-1.99 < 2.11
a R1 ) Σ||Fo| - |Fc||/Σ|Fo|. b wR2 ) [Σ[w(Fo - Fc2)2]/Σ[w(Fo )2]]1/2
.
2
2
All NMR spectra were recorded on a Varian Inova 300 spectrometer
operating at 300 MHz (1H), 75 MHz (13C), and 121 MHz (31P),
respectively, in CDCl3 solution (25 °C), unless stated otherwise. Internal
SiMe4 (1H and 13C) and external H3PO4 (31P) were used as references
(δ ) 0.00 ppm, J in Hz; see Tables 1-3). Infrared measurements were
performed on a Perkin-Elmer FT-IR spectrometer using CHCl3 as
solvent.
[PtCl(PCP)], 2. A solution of PCHP (90 mg, 190 µmol) and PtCl-
(NCN) (80 mg, 190 µmol) in toluene (10 mL) was heated to reflux
temperature for 3 days. The formed precipitate was filtered off, and all
volatiles of the filtrate were evaporated in vacuo. The white residue
(131 mg, 98%) was recrystallized from CH2Cl2 (2 mL), which was
carefully layered with pentane (10 mL). This yielded colorless crystals
of 2. The spectroscopic properties of 2 are in full agreement with the
reported data.15
[Pt(PCP)(η1-C-C6H3{CH2NMe2}-2-{CH2NHMe2}-6)]Cl, [3]Cl. A
solution of PCHP (110.8 mg, 234 µmol) and PtCl(NCN) (100.2 mg,
238 µmol) in C6H6 (10 mL) was refluxed for 3 days. The white
precipitate, which had gradually formed, was collected, washed with
cold C6H6 (2 × 1 mL), and dried in vacuo to afford [3]Cl, quantitatively.
Anal. Found (calcd for [3]Cl‚1.5C6H6): C, 63.00 (62.81); H, 5.68 (5.57);
N, 2.67 (2.76).
[Pt(PCP)(η1-C-C6H3{CH2NHMe2}2-2,6)]Cl2, [4]Cl2. To a solution
of [3]Cl (98.8 mg, 0.11 mmol) in CH2Cl2 (1 mL) was added pentane
(4 mL). The formed precipitate was collected and dried in vacuo, which
gave [4]Cl2 as a white microcrystalline solid (40.2 mg; 39%). Anal.
Found (calcd [4]Cl2‚1.5CH2Cl2): C, 51.85 (51.55); H, 4.95 (4.85); N,
2.42 (2.63).
[Pt(PCP)(η1-C-C6H3{CH2NHMe2}2-2,6)](BF4)2, [4](BF4)2. Method
A. A solution of 5 (172 mg, 0.20 mmol) in CH2Cl2 (3 mL) was treated
with HBF4 (0.10 mL, 0.6 mmol; 54% in Et2O). After being stirried for
1 h, the solution was layered with Et2O, which yielded pure [4](BF4)2
as colorless crystals (98.5 mg; 95%).
Method B. A solution of [4]Cl2 (129 mg, 0.15 mmol) in CH2Cl2 (3
mL) was treated with AgBF4 (58.4 mg, 0.30 mmol) in the absence of
light. After 0.5 h, the suspension was filtered through Celite and the
filtrate concentrated to 1 mL. This solution was layered with pentane,
which gave colorless crystals of [4](BF4)2 (133 mg, 86%). Anal. Found
(calcd [4](BF4)2‚1.25CH2Cl2): C, 47.29 (47.60); H, 4.86 (4.46); N, 2.49
(2.45).
[Pt(PCP)(η1-C-C6H3{CH2NMe2}2-2,6H)], 5. Method A. To a
solution of [3]Cl (98.8 mg, 0.11 mmol) in CH2Cl2 (1 mL) was added
pentane (4 mL). The formed precipitate was removed, and the solute
was treated with additional pentane (4 mL). This afforded 5 as a second
precipitate, which was collected and dried in vacuo (45.2 mg; 44%).
Method B. An excess of NEt3 (1 mL, 14 mmol) was added to a
solution of [3]Cl (67.1 mg, 75 µmol) in CH2Cl2 (4 mL). After the
solution was stirred for 0.5 h, water was added (3 mL) and the two
phases were separated. The aqueous layer was extracted with CH2Cl2
(2 × 3 mL), and the combined organic phases were dried over MgSO4
and subsequently evaporated to dryness, thus affording 59.3 mg (92%)
of 5. Crystal needles for X-ray structure determination were grown by
diffusion of hexane into a solution of 5 in CH2Cl2. Anal. Found
(calcd): C, 61.28 (61.46); H, 5.31 (5.39); N, 3.34 (3.26).
[PtCl(η1-C-C6H3{CH2NMe2}2-2,6)(PCHP)]2, 6. A solution of
PCHP (52.5 mg, 111 µmol) and PtCl(NCN) (48.0 mg, 114 µmol) in
C6H6 (4 mL) was stirred for 4 h. The white precipitate, which formed
gradually, was collected, washed with cold C6H6 (2 × 1 mL), and
precipitated from CH2Cl2/C6H6. Drying in vacuo afford 88.3 mg (89%)
of pure 6. Crystals which were suitable for X-ray structure determination
were obtained by slow evaporation of the (saturated) C6H6 layer of the
reaction mixture. Anal. Found (calcd): C, 58.94 (58.96); H, 5.21 (5.29);
N, 3.06 (3.13); P, 6.87 (6.91).
[PtCl(η1-C-C6H3{CH2NMe2}2-2,6)(PPh3)2], 7. Addition of PPh3
(80.3 mg, 306 mmol) to a solution of PtCl(NCN) (64.4 mg, 153 µmol)
in C6H6 (5 mL) afforded, after stirring for 2 h and removal of all
volatiles, the crude title product. Purification was achieved by repeated
crystallization of 7 from CH2Cl2/Et2O. Yield: 118.8 mg (82%). Crystals
suitable for X-ray structure determination were grown by recrystalli-
zation from CH2Cl2/hexane (gas diffusion). Anal. Found (calcd): C,
60.72 (60.92); H, 5.31 (5.22); N, 2.84 (2.96); P, 6.73 (6.55).
X-ray Structure Determination of Complexes [4](BF4)2 and 5-7.
Intensities were measured on a Nonius KappaCCD diffractometer with
rotating anode (Mo KR, λ ) 0.710 73 Å) at a temperature of 150 K.
Crystal data and details on data collection and refinement for all
complexes are summarized in Table 5. The structures were solved with
Patterson methods (DIRDIF-9745) and refined with the program
SHELXL-9746 against F2 of all reflections. Non-hydrogen atoms were
refined freely with anisotropic displacement parameters, and hydrogen
atoms were refined as rigid groups. In structure [6]‚4C6H6, the hydrogen

Transcyclometalation Processes
J. Am. Chem. Soc., Vol. 122, No. 48, 2000 11833
atom H1 was refined freely with an isotropic displacement parameter.
In structure [7]‚1/6C6H14, the hexane molecule was refined with
isotropic displacement parameters and a ratio of 1/6 with respect to
the Pt complex. The benzylamino groups at N1 of [4](BF4)2 and [7]‚
1/6C6H14 were refined with disorder models. All calculations, graphical
illustrations, and checking for higher symmetry were performed with
the PLATON47 package.
Acknowledgment. This work was supported in part (P.D.)
by the Conselho Nacional de Desenvolvimento Cient´ıfico e
Tecnolo´gico (CNPq, Brazil) and in part (M.L., A.L.S.) by the
Dutch Council for Chemical Sciences (CW-NWO).
Supporting Information Available: A figure showing the
measured and simulated 31P NMR spectrum of 6 as well as
tables of additional crystallographic and refinement parameters,
anisotropic thermal parameters, and complete nearest-neighbor
distances and angles of the complexes [4](BF4)2, 6, and 7 (PDF).
The material is available free of charge via the Internet at
http://pubs.acs.org.
(45) Beurskens, P. T.; Admiraal, G.; Beurskens, G.; Bosman, W. P.;
Garcia-Granda, S.; Gould, R. O.; Smits, J. M. M.; Smykalla, C. The
DIRDIF97 Program System; Technical report of the Crystallography
Laboratory, University of Nijmegen: Nijmegen, The Netherlands, 1997.
(46) Sheldrick, G. M. SHELXL-97 Program for crystal structure refine-
ment; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(47) Spek, A. L. Acta Crystallogr. 1990, A46, C-34.
JA001808M