(κ2-P,S)Pt(benzyl) complexes derived from 1/3-P iPr2-2-StBu-indene: Facile synthesis of carbanion- and borate-containing zwitterions

Article abstract of DOI:10.1039/b813421a

The versatile new ligand precursor 1/3-PⁱPr₂-2-S ^tBu-indene has been employed in the preparation of neutral and cationic (κ²-P,S)Pt(benzyl) complexes, as well as structurally related zwitterions in which the for

Full text of DOI:10.1039/b813421a

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(j2-P,S)Pt(benzyl) complexes derived from 1/3-PiPr2-2-StBu-indene:
facile synthesis of carbanion- and borate-containing zwitterionsw
Kevin D. Hesp,a Robert McDonald,b Michael J. Ferguson,b Gabriele Schattec and
Mark Stradiotto*a
Received (in Berkeley, CA, USA) 1st August 2008, Accepted 23rd September 2008
First published as an Advance Article on the web 6th October 2008
DOI: 10.1039/b813421a
The versatile new ligand precursor 1/3-PiPr2-2-StBu-indene has
been employed in the preparation of neutral and cationic
(j2-P,S)Pt(benzyl) complexes, as well as structurally related
zwitterions in which the formally cationic metal fragment is
counterbalanced by an uncoordinated indenide or borate frag-
ment that is contained within the ancillary ligand backbone.
the 10p-electron indenide unit functions as an uncoordinated
anionic charge reservoir to counterbalance the cationic
k2-P,N-MLn fragment, rather than as a site for metal binding.
Encouraged by the successful pairing of soft phosphine donors
in (k2-Ph2B(CH2PR2)2)PtLn zwitterions,4c,d and in our quest
to establish the first examples of zwitterionic group 10
complexes featuring heterobidentate indenide ligation, we
turned our attention to the synthesis of complementary Pt
complexes derived from new P,S-substituted indenes. We
report herein that 1/3-PiPr2-2-StBu-indene functions as a con-
venient precursor for the construction of neutral, cationic, and
zwitterionic (k2-P,S)Pt(benzyl) complexes. The versatility of
this ligation strategy is further demonstrated through the
direct transformation of the (k2-P,S-indenide)Pt(k3-benzyl)
zwitterion into a structurally related borate-based zwitterion.
Lithiation of 2-StBu-indene followed by quenching with ClPiPr2
provided 1/3-PiPr2-2-StBu-indene in 87% isolated yield. In target-
ing Pt(benzyl) derivatives due to their ability to adopt Z1, Z3, and
other coordination modes, 1/3-PiPr2-2-StBu-indene was added to
(COD)Pt(Z1-benzyl)Cl, affording (k2-3-PiPr2-2-StBu-indene)-
Pt(Z1-benzyl)Cl 1 in 68% isolated yield (Scheme 1; COD =
Zwitterionic late transition metal complexes featuring enforced
formal charge separation between a cationic metal fragment and
an anionic moiety sequestered within a coordinated ancillary
ligand have emerged as attractive alternatives to related cationic
complexes in reactivity applications, owing to their heightened
solubility in low-polarity media, increased tolerance to polar
coordinating solvents, and avoidance of counteranion effects.1
Indeed, such zwitterions can offer a useful intermediate range of
electrophilicity relative to more traditional neutral and cationic
species. Although significantly less well-explored than their group
9 relatives, zwitterionic Ni,2 Pd,3 and Pt4 complexes have proven
useful in a range of substrate transformations, including: Ni-
catalyzed ethylene polymerization;2a,d–f Pd-catalyzed CO–ethylene
co-polymerization3c and aldol condensation;3d and metal-
mediated stoichiometric C–H bond activation of amines (Pd),3b
as well as hydrocarbons (Pt).4b–d Notwithstanding these
advances, further diversification in this field is limited by a
dearth of ligation strategies for supporting group 10 zwitterionic
complexes. Nearly all such zwitterionic species reported thus far
feature borate-based ligation;2–4 to the best of our know-
ledge, non-borate group 10 zwitterions are restricted to a limited
class of Pd pincer complexes that contain tethered sulfate
groups.3a,d In this regard, the identification of alternative ligation
strategies that allow for the preparation of neutral, cationic, and
zwitterionic group 10 complexes, ideally from a convenient single
ligand precursor, represents an important goal in the quest for
further control in tuning the reactivity of group 10 metal species.
We have reported on our investigations of cationic and
formally zwitterionic complexes of Ru, Rh, and Ir, supported
1
Z4-1,5-cyclooctadiene). Monitoring of this reaction by use of H
and 31P NMR methods confirmed that 1 is formed quantitatively
as a single diastereomer, the identity of which was ascertained on
the basis of X-ray crystallographic data (see Fig. 1 and Table 1).z
While crystallographically characterized (k2-P,S)Pt(benzyl) species
have not been reported previously, the trans-disposition of chlo-
ride and phosphorus ligands in 1 mirrors that observed in a related
(k2-P,N)Pt(Z1-benzyl)Cl complex,7 in keeping with the greater
trans-directing ability of phosphorus relative to sulfur.8
Dehydrohalogenation of 1 employing NaN(SiMe3)2 pro-
duced the k2-P,S-indenide complex 2, which was isolated as an
by 3-PR2-2-NR0 -indene and mono-deprotonated indenide
2
ligands, respectively.5,6 These zwitterions are unusual in that
a Department of Chemistry, Dalhousie University, Halifax, Nova
Scotia, Canada B3H 4J3. E-mail: mark.stradiotto@dal.ca;
Fax: +1 902 494 1310; Tel: +1 902 494 7190
b X-Ray Crystallography Laboratory, Department of Chemistry,
University of Alberta, Edmonton, Alberta, Canada T6G 2G2
c Saskatchewan Structural Sciences Centre, University of
Saskatchewan, Saskatoon, Saskatchewan, Canada S7N 5C9
w Electronic supplementary information (ESI) available: Experimental
details and characterization data, including crystallographic data for 1,
2ꢀDMAP, 2ꢀB(C6F5)3(C7H8), and 3. CCDC 687090–687093. For ESI and
crystallographic data in CIF format see DOI: 10.1039/b813421a
Scheme
complexes.
1
Synthesis of cationic and zwitterionic (k2-P,S)Pt
Chem. Commun., 2008, 5645–5647 | 5645
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Fig. 1 ORTEP diagrams for 1, 2ꢀDMAP, 2ꢀB(C6F5)3(C7H8), and 3, shown with 50% ellipsoids. Selected hydrogen atoms, the toluene solvate and
ꢂ
portions of the C6F5 groups in 2ꢀB(C6F5)3(C7H8), as well as the B(C6F5)4 anion in 3, have been omitted for clarity.
obtain X-ray quality crystals of 2, the identification of
2ꢀDMAP (and indirectly 2) as a k2-P,S-indenide complex
was confirmed by use of diffraction methods (Fig. 1).z Our
assignment of 2ꢀDMAP as a formally zwitterionic6 species is
supported by the observation of C–C distances within the
carbocyclic backbone that are consistent with a delocalized
10p-electron indenide framework (Table 1). However, as noted
for k2-P,N-indenide complexes of groups 8 and 9,5b–d the
relatively short P–C3 distance in 2ꢀDMAP suggests that a
non-zwitterionic R2PQC3 resonance contributor featuring a
formal negative charge on phosphorus11 should be considered
in describing the electronic structure of this adduct.
Table 1 Selected interatomic distances (A)
1a
2ꢀDMAPb
2ꢀB(C6F5)3
3d
c,d
Pt–S
Pt–P
Pt–CH2
Pt-Cipso
Pt-Cortho
P–C3
2.3637(14)
2.2135(14)
2.096(5)
—
2.4005(7)
2.2342(7)
2.075(3)
—
2.3658(12)
2.2254(13)
2.085(5)
2.255(5)
2.354(5)
1.817(5)
1.785(5)
1.513(6)
1.517(6)
1.354(6)
1.458(6)
1.392(7)
1.370(7)
1.383(7)
1.381(7)
1.390(7)
1.402(7)
c
2.3608(10)
2.2116(10)
2.061(4)
2.269(4)
2.566(4)
1.800(4)
1.775(4)
1.505(5)
1.515(6)
1.347(5)
1.480(5)
1.382(6)
1.387(6)
1.384(6)
1.381(7)
1.373(6)
1.411(5)
—
—
1.814(6)
1.757(5)
1.502(7)
1.498(8)
1.368(7)
1.479(8)
1.394(8)
1.399(9)
1.368(9)
1.405(9)
1.393(8)
1.388(8)
1.767(3)
1.775(3)
1.399(4)
1.411(4)
1.420(4)
1.433(4)
1.412(4)
1.374(4)
1.405(4)
1.373(4)
1.405(4)
1.441(4)
S–C2
C1–C2
C1–C7a
C2–C3
C3–C3a
C3a–C4
C4–C5
C5–C6
C6–C7
C7–C7a
C7a–C3a
a
Notably, the indenide-based zwitterion 2 serves as a convenient
synthon in the preparation of alternative cationic and zwitterionic
(k2-P,S)Pt(Z3-benzyl) complexes. Treatment of 2 with B(C6F5)3
afforded the borate-based zwitterion 2ꢀB(C6F5)3 in 68% isolated
yield, while addition of H(OEt2)2B(C6F5)4 to 2 produced the
cationic complex 3 in 88% isolated yield (Scheme 1). The ability
of 2 to serve as a precursor to 3 is of particular significance, since
efforts to prepare 3 from 1 by use of Li(Et2O)2.5B(C6F5)4 were
unsuccessful. In contrast to the broad NMR spectral features that
were noted for 2 and 2ꢀDMAP at 300 K (vide supra), under similar
conditions the 1H and 13C{1H} NMR spectra for each of 2ꢀ
B(C6F5)3 and 3 are consistent with a C1-symmetric structure, with
only modest line-broadening observed in the case of 2ꢀB(C6F5)3.
Data obtained from X-ray diffraction studies involving 2ꢀB(C6F5)3
and 3 also revealed C1-symmetric connectivity for these complexes
(Fig. 1),z and enabled further structural comparisons.12 In con-
trast to the C–C bond delocalization and somewhat short P–C3
distance that are found in the k2-P,S-indenide complex 2ꢀDMAP
(vide supra), more localized single (e.g. C1–C2) and double (e.g.
C2–C3) bonds, along with typical P–C3 distances, are observed in
both 2ꢀB(C6F5)3 and 3 (Table 1). In this regard, 2ꢀB(C6F5)3 and 3
can be viewed as (k2-P,S-indene)Pt(Z3-benzyl) complexes, in
keeping with the line drawing representations in Scheme 1;
whereas in 3 the formally cationic Pt fragment is counterbalanced
b
2.3695(14). Pt–N
Pt–Cl
=
=
2.102(2). B–Cl
=
1.701(7).
d
Benzyl distances in 2ꢀB(C6F5)3 (in 3): C10–C11 1.451(8) (1.456(6));
C11–C12 1.416(7) (1.407(6)); C12–C13 1.408(7) (1.400(6)); C13–C14
1.361(8) (1.371(6)); C14–C15 1.397(9) (1.388(6)); C15–C16 1.355(8)
(1.375(6)); C11–C16 1.424(8) (1.411(6)).
analytically pure solid in 67% yield (Scheme 1). Notably, 2
represents the first isolable Pt zwitterion to feature non-borate
ancillary ligation. In contrast to the relatively sharp NMR
signals observed for 1 at 300 K, the 1H NMR spectrum of 2 at
this temperature exhibited very broad features, possibly attri-
butable to slow inversion at sulfur, and/or Z1–Z3 dynamics of
the coordinated benzyl ligand.9,10 While upon cooling to 273
K some diagnostic 1H NMR signals associated with 2 could be
assigned qualitatively, further cooling to 185 K provided no
gains in spectral resolution. Conversely, the 31P{1H} NMR
resonance for 2 remained sharp between 273–300 K, appar-
ently precluding an observable temperature-dependent equili-
brium involving 2 and other phosphorus-containing species.
Exposure of 2 to 4-dimethylaminopyridine (DMAP) afforded
the adduct 2ꢀDMAP as an analytically pure solid in 82%
isolated yield (Scheme 1). As with 2, this adduct exhibited
ꢂ
by an outer-sphere B(C6F5)4 counteranion, the indenylborate
ligation featured in 2ꢀB(C6F5)3 gives rise to a structurally analo-
gous zwitterionic (charge-neutral) complex.13 Despite these appar-
ent similarities, some structural variations within the coordinated
Z3-benzyl ligands of 2ꢀB(C6F5)3 and 3 were observed. Progressive
1
broad H NMR features (and a single sharp 31P{1H} NMR
resonance) in the range 185–300 K. While we have yet to
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(C51H43BF15PPtS, 1209.78 g molꢂ1): a = 21.679(2), b = 18.6294(17),
c = 23.275(2) A, V = 9400.0(15) A3, space group = Pbca (orthor-
hombic), Z = 8, independent reflections = 10 253 (Rint = 0.1001),
GOF = 1.060, R1 = 0.0379 (Fo2 4 2s(Fo2)), wR2 = 0.0892 (all data).
Selected crystal data for 3 (C50H36BF20PPtS, 1285.72 g molꢂ1): a =
17.6907(18), b = 17.3368(18), c = 17.7636(18) A, b =117.9280(10)1,
V = 4813.6(9) A3, space group = P21/c (monoclinic), Z = 4,
independent reflections = 11 034 (Rint = 0.0431), GOF = 1.058,
lengthening of the Pt–C linkage is noted for both complexes on
going from Pt–CH2 to Pt–Cipso to Pt–Cortho (Table 1), in keeping
with structural trends commonly observed in LnM(Z3-benzyl)
complexes.12,14 However, while the first two of these distances
are statistically equivalent in 2ꢀB(C6F5)3 and 3, the elongation of
the Pt–Cortho distance in 3 (2.566(4) A) is significantly more
pronounced than in 2ꢀB(C6F5)3 (2.354(5) A). While the potential
influence of crystal packing as a source of such structural differ-
ences cannot be discounted, it is worthy of mention that the
2
R1 = 0.0367 (Fo 4 2s(Fo2)), wR2 = 0.0934 (all data).
1 Such zwitterions are distinct from those featuring a formally
anionic metal fragment coordinated to a cationic ancillary ligand:
R. Chauvin, Eur. J. Inorg. Chem., 2000, 577.
1
observed structural trend also matches the J(PPt) values mea-
sured for 2 (5012 Hz), 2ꢀB(C6F5)3 (5264 Hz), and 3 (5317 Hz), with
the increasing 1J(PPt) value signaling a progressively weaker
interaction between Pt and the Z2-(C11QC12) moiety that occu-
pies a position trans to phosphorus in 2ꢀB(C6F5)3 and 3 (and
presumably 2).10 When considered collectively, the solid state
structural variations noted within the Z3-benzyl fragments of
2ꢀB(C6F5)3 and 3, as well as the differing solution dynamic
behavior observed for 2, 2ꢀB(C6F5)3, and 3, may presage divergent
reactivity for these complexes, attributable in part to the unique
electronic characteristics of the ancillary ligand backbone.
In summary, we have demonstrated that 1/3-PiPr2-2-StBu-
indene functions as a versatile new ligand precursor in the
synthesis of cationic [(k2-P,S)Pt(Z3-benzyl)]+Xꢂ species (3), as
well as analogous zwitterionic (charge-neutral) complexes in which
the formally cationic metal fragment is counterbalanced by an
uncoordinated indenide (2) or borate (2ꢀB(C6F5)3) fragment within
the ancillary ligand backbone. In addition to offering a new and
complementary approach for the assembly of structurally related
cationic and zwitterionic group 10 complexes, the convenience of
employing a single ligand precursor in these syntheses may offer
advantages over alternative strategies that require the preparation
of distinct ligands for supporting cations and zwitterions. Having
established the synthetic feasibility of heterobidentate indenide
ligation in group 10 chemistry, we have initiated a comparative
reactivity survey involving 2, 2ꢀB(C6F5)3, and 3, in an effort to
document the influence of the indenide, indenylborate, and indene
ligand backbone (respectively) in tuning the reactivity of structu-
rally related group 10 metal complexes. The results of these and
related investigations will be reported in due course.
2 For selected recent examples, see: (a) B. M. Boardman, J. M.
Valderrama, F. Munoz, G. Wu, G. C. Bazan and R. Rojas,
Organometallics, 2008, 27, 1671; (b) J. Cho, G. P. A. Yap and C.
G. Riordan, Inorg. Chem., 2007, 46, 11308; (c) Y. Chen, C. Sui-
Seng and D. Zargarian, Angew. Chem., Int. Ed., 2005, 44, 7721; (d)
Y. Chen, G. Wu and G. C. Bazan, Angew. Chem., Int. Ed., 2005,
44, 1108; (e) H. Y. Kwon, S. Y. Lee, B. Y. Lee, D. M. Shin and Y.
K. Chung, Dalton Trans., 2004, 921; (f) C. B. Shim, Y. H. Kim, B.
Y. Lee, Y. Dong and H. Yun, Organometallics, 2003, 22, 4272.
3 (a) R. van de Coevering, M. Kuil, A. P. Alfers, T. Visser, M. Lutz,
A. L. Spek, R. J. M. Klein Gebbink and G. van Koten, Organo-
metallics, 2005, 24, 6147; (b) C. C. Lu and J. C. Peters, J. Am.
Chem. Soc., 2004, 126, 15818; (c) C. C. Lu and J. C. Peters, J. Am.
Chem. Soc., 2002, 124, 5272; (d) R. van de Coevering, M. Kuil, R.
J. M. Klein Gebbink and G. van Koten, Chem. Commun., 2002,
1636.
4 (a) E. Khaskin, P. Y. Zavalij and A. N. Vedernikov, Angew.
Chem., Int. Ed., 2007, 46, 6309; (b) C. M. Thomas and J. C. Peters,
Organometallics, 2005, 24, 5858; (c) J. C. Thomas and J. C. Peters,
J. Am. Chem. Soc., 2003, 125, 8870; (d) J. C. Thomas and J. C.
Peters, J. Am. Chem. Soc., 2001, 123, 5100; (e) T. Marx, L.
Wesemann and S. Dehnen, Organometallics, 2000, 19, 4653.
5 (a) R. J. Lundgren, M. A. Rankin, R. McDonald and M. Stradiotto,
Organometallics, 2008, 27, 254; (b) R. J. Lundgren, M. A. Rankin,
R. McDonald, G. Schatte and M. Stradiotto, Angew. Chem., Int.
Ed., 2007, 46, 4732; (c) J. Cipot, R. McDonald, M. J. Ferguson,
G. Schatte and M. Stradiotto, Organometallics, 2007, 26, 594;
(d) M. A. Rankin, R. McDonald, M. J. Ferguson and M. Stradiotto,
Organometallics, 2005, 24, 4981.
6 Complexes of donor-substituted indenide ligands are described as
zwitterionic in that they lack conventional resonance structures that
delocalize the indenide anionic charge onto the pendant donor atoms.
7 N. Oberbeckmann-Winter, P. Braunstein and R. Welter, Organo-
metallics, 2004, 23, 6311.
8 Similar structural trends have been observed in (k2-P,S)PtX2 (X =
Ph or Cl) complexes: (a) R. Malacea, L. Routaboul, E. Manoury,
J.-C. Daran and R. Poli, J. Organomet. Chem., 2008, 693, 1469; (b)
R. Romeo, L. Monsu’ Scolaro, M. R. Plutino, A. Romeo, F.
Nicolo’ and A. Del Zotto, Eur. J. Inorg. Chem., 2002, 629.
9 Complex dynamic behavior resulting in significant NMR line-
broadening has been noted in (k2-P,S)Pt(Cl)(Z1-allyl), [(k2-P,S)Pt-
(Z3-allyl)]+Xꢂ, and related complexes, see: (a) E. Hauptman,
P. J. Fagan and W. Marshall, Organometallics, 1999, 18, 2061; (b)
M. Bressan and A. Morvillo, J. Organomet. Chem., 1986, 304, 267.
10 For discussions of Pt(benzyl) dynamics and the use of 1J(PPt)
values in related bonding analyses, see: L. E. Crascall, S. A. Litster,
A. D. Redhouse and J. L. Spencer, J. Organomet. Chem., 1990,
394, C35, and references cited therein.
We thank the Natural Sciences and Engineering Research
Council of Canada (including a Discovery Grant for MS and a
Canada Graduate Scholarship for KDH), and Dalhousie
University for their generous support of this work. We also
thank Drs Michael Lumsden and Katherine Robertson
(Atlantic Region Magnetic Resonance Center, Dalhousie)
for assistance in the acquisition of NMR data.
Notes and references
11 K. Izod, Coord. Chem. Rev., 2002, 227, 153.
12 Crystallographically characterized LnPt(Z3-benzyl) complexes are
rare: (a) A. Sonoda, P. M. Bailey and P. M. Maitlis, J. Chem. Soc.,
Dalton Trans., 1979, 346; (b) ref. 10 herein.
z Crystallographic data have been deposited for 1 (CCDC 687093),
2ꢀDMAP (CCDC 687090), 2ꢀB(C6F5)3(C7H8) (CCDC 687091), and 3
(CCDC 687092).w Selected crystal data for 1 (C26H36ClPPtS, 642.12 g
molꢂ1): a = 13.1479(3), b = 12.5326(4), c = 15.8100(5) A, b =
98.1364(17)1, V = 2578.91(13) A3, space group = P21/c (monoclinic),
Z = 4, independent reflections = 5271 (Rint = 0.0389), GOF = 1.081,
R1 = 0.0335 (Fo2 4 2s(Fo2)), wR2 = 0.0810 (all data). Selected crystal
data for 2ꢀDMAP (C33H45N2PPtS, 727.83 g molꢂ1): a = 10.6807(9), b =
14.1937(11), c = 20.6273(17) A, b =97.7681(9)1, V = 3098.4(4) A3,
space group = P21/c (monoclinic), Z = 4, independent reflections =
13 For [Z5-Cp0B(C6F5)3]ꢂ complexes of groups 4 and 6, see: (a) P. J.
Shapiro, P.-J. Sinnema, P. Perrotin, P. H. M. Budzelaar, H. Weihe,
B. Twamley, R. A. Zehnder and J. J. Nairn, Chem.–Eur. J., 2007,
13, 6212; (b) M. Bochmann, S. J. Lancaster and O. B. Robinson,
J. Chem. Soc., Chem. Commun., 1995, 2081.
14 For a selection of crystallographically characterized LnM(Z3-benzyl)
complexes (M a Pt), see: (a) J.-C. Wasilke, Z. J. A. Komon, X. Bu
and G. C. Bazan, Organometallics, 2004, 23, 4174; (b) T.-F. Wang, C.-
C. Hwu, C.-W. Tsai and Y.-S. Wen, J. Chem. Soc., Dalton Trans.,
1998, 2091, and references cited therein; (c) ref. 2e herein.
7060 (Rint = 0.0264), GOF = 1.059, R1 = 0.0235 (Fo 4 2s(Fo2)),
wR2 = 0.0573 (all data). Selected crystal data for 2ꢀB(C6F5)3(C7H8)
2
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Chem. Commun., 2008, 5645–5647 | 5647