Molecular and electronic structure of platinum bis(n-arylamino)phosphenium complexes including [Pt(phosphane)(phosphenium)-(N-heterocyclic carbene)]

Article abstract of DOI:10.1002/anie.200352326

Give and take: A cationic N-heterocyclic carbene (NHC)-phosphenium adduct is prepared (see picture; orange = P, lilac = N, gray = C, white = H). On reaction with [Pt(PPh₃)₃] it splits into its constituent parts, delivering an NHC ligand (a strong σ donor) and a phosphenium ligand (a π acceptor) from one reagent. These first platinum phosphenium complexes are characterized by short Pt-P bonds and large Pt-P coupling constants.

Full text of DOI:10.1002/anie.200352326

Angewandte
Chemie
P Ligands
Molecular and Electronic Structure of Platinum
Bis(N-arylamino)phosphenium Complexes
including [Pt(phosphane)(phosphenium)-
(N-heterocyclic carbene)]**
Ned J. Hardman, Michael B. Abrams,
Melanie A. Pribisko, Thomas M. Gilbert,
Richard L. Martin,* Gregory J. Kubas,* and
R. Tom Baker*
Dedicated to Professor M. Frederick Hawthorne
on the occasion of his 75th birthday
The application of electrophilic late-transition-metal com-
plexes in catalysis has enjoyed widespread success in recent
years.[1–6] We have been investigating electrophilic platinum
ꢀ
complexes for catalysis and hydrocarbon C H bond activa-
tion[7,8] and recently reported a simple strategy for obtaining
unsaturated electrophilic metal centers by addition of bulky
bis(N-arylamino)phosphenium cations, easily derived from
bis(N-aryl)diimines.[9] Detailed theoretical investigations[10] of
[*] Dr. N. J. Hardman, Dr. M. B. Abrams, M. A. Pribisko, Dr. R. L. Martin,
Dr. G. J. Kubas, Dr. R. T. Baker
Chemistry and Theory Divisions
Los Alamos National Laboratory
Los Alamos, NM 87545 (USA)
Fax: (+1)505-667-9905
E-mail: rlm@t12.lanl.gov
kubas@lanl.gov
bakertom@lanl.gov
Prof. T. M. Gilbert
Department of Chemistry and Biochemistry
Northern Illinois University, DeKalb, IL 60115 (USA)
[**] The authors thank the US Department of Energy's Laboratory-
Directed R&D program at Los Alamos for support of this project
and partial support of T.M.G.'s summer sabbatical at Los Alamos.
We are also grateful to the referees for a number of helpful
comments.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2004, 43, 1955 –1958
DOI: 10.1002/anie.200352326
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1955

Communications
the electronic structure of these phosphenium cations, their
donor–acceptor adducts, and transition-metal complexes
showed them to be excellent p acceptors, but poor s donors[11]
and thus complementary to N-heterocyclic carbenes (NHCs,
which are good s donors, poor p acceptors).[12]
Herein we report the synthesis, characterization, and
electronic structure of an N-heterocyclic carbene adduct
+
(CN2) of the bis(N-mesitylamino)phosphenium cation [PN2 ,
Eq. (1); mes = 2,4,6-Me3C6H2, Tf = SO2CF3]. In the reaction
of this adduct with [Pt(PPh3)3] to form the
[Pt(PPh3)(PN2)(CN2)]+ ion, we demonstrate that NHC-phos-
phenium adducts are particularly well suited for preparingthe
first platinum phosphenium complexes, supplyingboth excel-
lent s-donor and p-acceptor ligands from the same reagent.
Phosphenium cations[13] have experienced a resurgence as
ligands for transition metals[9,14] and main-group elements,[15]
Figure 1. Optimized structure (DFT) of model NHC–phosphenium
cation adduct. Orange=P, lilac=N, gray=C, white=H.
hybrid DFT (B3LYP and the 6-31G* basis
set).[23] The calculation finds a P C bond length
ꢀ
of 1.91 Š and an angle between the vector
which bisects the PN2 angle and the P–C vector
of only 1038, indicative of partial rehybridiza-
tion at the phosphorus center. These parame-
ters can be contrasted with the other reported
NHC-phosphenium cation adduct,[18] [carb:!
PPh2]AlCl4 (4, carb = 2,3-dihydro-1,3-diiso-
propyl-4,5-dimethylimidazol-2-ylidene)
in
ꢀ
which our calculated P C bond length is
1.87 Š (experimentally determined: 1.813(7) Š) and the
analogous angle involving the PPh2 unit is 1138(experimen-
tally determined: 112.78). The NHC adduct of the more
electrophilic [PPh2]+ ion may thus be better represented as an
imidazolatophosphane with a lone pair at phosphorus,
whereas the N donors in 3 stabilize the localized positive
charge on the phosphorus center. Adduct 3 is also distin-
and as p-acceptor ligands for homogeneous catalysis.[16] We
are interested in cyclic bis(N-arylamino)phosphenium cations
as “bulky CO” ligands that create electrophilic metal centers
and also offer a Lewis acid function in the metal's coordina-
tion sphere for bifunctional catalysis.[17] Given the poor s-
donor ability of phosphenium cations,[10b] stable donor–ac-
ceptor adducts (L!PN2)+ could serve as useful reagents by
supplyingboth donor and acceptor ligands to the metal. As
our previously reported PMe3 adducts were formed rever-
sibly,[9] we investigated the stronger donor NHC ligand 2.[18]
Reaction of [PK N(mes)CH2CH2NL (mes)]OTf (1) with NHC
(2; CN2)[19] afforded the donor–acceptor adduct [N2CD!
PN2]OTf (3) as a colorless solid in good yield.[20] Adduct 3 was
characterized by NMR spectroscopy and its electronic
structure determined usingdensity functional theory (DFT).
The 31P chemical shift of 3 (d = 113.7 ppm) is significantly
upfield from that of 1 (d = 188.6 ppm) whereas the carbene
carbon resonance shifts from d = 214.4 ppm in CN2 to d =
guished from adducts of diaminophosphenium cations with
electrophilic carbenes, such as [(Me3Si)2C P(NiPr2)2] which
+
=
has
d(13C) = 76.51 ppm
(JC-P = 87.6 Hz)
and
dP-C =
1.620(3) Š.[24]
Reaction of adduct 3 with [Pt(PPh3)3] displaces two
phosphane ligands to give three-coordinate [Pt(phosphane)-
(phosphenium)(carbene)]OTf [5, Eq. (2)].[25] While plati-
ꢀ
165.7 ppm in 3 and exhibits a large one-bond C P coupling
constant (199 Hz). Similar NHC–phosphinidene complexes,
albeit with two-coordinate phosphorus centers, feature JC-P
couplingconstants of approximately 100 Hz. [21] In contrast to
the PMe3 adduct,[9] variable temperature 31P and 13C NMR
spectra of 3 showed no shiftingof the resonance signals (as
would be expected for reversible adduct formation) but only a
gradual sharpening as the temperature was lowered. Hin-
num(0) phosphane complexes participate readily in oxidative
ꢀ
addition reactions, cleavage of the P C bond in 3 affords a
ꢀ
dered rotation about the N Cmes bond leads to very broad
zerovalent product and may therefore be considered a “non-
oxidative addition” reaction similar to those of tetraamino-
ethylene derivatives that afford bis(diaminocarbene) metal
complexes.[26] Complex 5 may also be obtained from stepwise
reactions of the phosphenium cation with [Pt(PPh3)3] to yield
PPh3 and [Pt(PPh3)2(phosphenium)][OTf], (6)[27] followed by
resonances in the proton NMR spectrum at 258C, as observed
previously for PN2Cl[9] and confirmed for 3 by observation of
six different mesityl methyl signals at ꢀ568C in the
13C NMR.[22] We have studied the model adduct [(MK eNN
=
CMeNMe)CL !PK (NMeCH2CH2NL Me)]+
(Figure 1)
with
1956
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Angewandte
Chemie
treatment with NHC 2 to liberate another PPh3 and afford 5
[Eq. (2)]; in contrast, [Pt(PPh3)3] did not react readily with the
NHC ligand.
Complexes 5 and 6 were characterized by elemental
analysis and NMR spectroscopy and the molecular and
electronic structure of 5 were determined by X-ray diffraction
(Figure 2)[28] and DFT calculations. The Pt-P couplingcon-
correct symmetry to interact with the PN2 empty P(pp)
orbital as observed in the HOMO of the model complex (see
Supporting Information Figure S4). A similar argument holds
for the CN2 ligand, but it is a significantly poorer p acid than
the PN2+ ligand.[31]
In conclusion, we have prepared a cationic NHC-phos-
phenium adduct from its constituents and shown that it can
replace two phosphane ligands of [Pt(PPh3)3] with strong s-
donor NHC and effective p-acceptor phosphenium ligands.
These first examples of platinum phosphenium complexes are
ꢀ
characterized by short Pt P bonds and large Pt–P coupling
constants. We are currently conductingfurther additions of
NHC-phosphenium adducts to transition metals and inves-
tigating the reactivity and catalytic activity of the resulting
complexes.
Received: July 7, 2003
Revised: December 6, 2003 [Z52326]
Keywords: heterocycles · N-heterocyclic carbenes · P ligands ·
.
platinum
Figure 2. Molecular structure of 5, thermal ellipsoids are set at 30%
probability. Hydrogen atoms and triflate counteranion are omitted for
clarity. Selected interatomic distances [Š] and angles [8]: Pt(1)-C(21)
2.037(10), Pt(1)-P(1) 2.116(3), Pt(1)-P(2) 2.316(3), P(1)-N(1)
1.622(10), P(1)-N(2) 1.638(11); C(21)-Pt(1)-P(1) 132.9(3), C(21)-Pt(1)-
P(2) 101.9(3), P(1)-Pt(1)-P(2) 125.25(11).
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stants for the phosphenium ligands in 5 (7354 Hz) and 6
(6498 Hz) were significantly larger than those for the
phosphane ligands (3795 and 4237 Hz, respectively). These
ꢀ
large coupling constants correlate with short Pt P bond
lengths; that in 5 is the shortest reported to date[29]
(2.116(3) Š) and is approximately 7% shorter than the
average bond length found in tricoordinate [Pt(PPh3)3] (ca.
2.266 Š).[30] While the structure of the NHC ligand in 5
(C2 axis bisectingC(22) and N(4) and passingthrough the
N(3)–C(Ar) vector) allows hypothetically for bindingthrough
the carbene carbon atom or the nitrogen atom of the central
ring, platinum coordination by the carbene carbon atom was
confirmed by 13C NMR spectroscopy (d = 194.5 ppm with
2
J
Pt-C = 1614 and JP-C = 84.5 Hz). The PN2 and CN2 planes of
the phosphenium and carbene ligands are nearly perpendic-
ular (87.8 and 79.28) to the distorted PtP2C trigonal plane, as
predicted by DFT calculations. The perpendicular orientation
of the PN2 and CN2 planes in 5 is also apparent in DFT studies
of the model complex in which all eight aryl substituents in 5
are replaced with methyl groups. This preference can be
understood in terms of Pt(dp)–P(pp) back bonding. The
ligand field arising from the three ligand lone-pair orbitals
constitutes a pseudo D3h environment about the platinum
center. In such a situation, the two energetically highest
d orbitals are dx2ꢀy2 and dxy in character, where the z axis is
chosen perpendicular to the plane and the x direction is
chosen to lie alongthe Pt–P
axis. Both orbitals are
phosphane
[10] a) W. W. Schoeller, U. Tubbesing, J. Mol. Struct. 1995, 343, 49;
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filled in the nominally d10 complex, but note that while the
dxy–PN2 s interaction is repulsive, the dx2ꢀy2 orbital has the
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1957

Communications
[11] Our DFT calculations indicate that, unlike CO, bis(N-arylami-
6.67, 6.86, 6.97, 7.17, 7.29, 7.37, 7.74 ppm (m, m-H of N-mes, Ph);
13C{1H} NMR (100 MHz, CD2Cl2) d = 18.13, 18.60 (o-Me of N-
mes), 21.45 (p-Me of N-mes), 51.67 (PN2CH2), 121.18, 126.00,
128.42, 129.01, 129.12, 129.29, 129.67, 129.84, 130.60, 131.53,
133.54, 133.66, 136.96, 137.42, 138.98, 194.48 ppm (ipso-carbene
no)phosphenium cations do not form stable complexes with BH3
(see SupportingInformation).
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1996, 519, 137; b) H. Nakazawa, M. Kishishita, T. Ishiyama, T.
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references therein.
1
2
C, JPt-C 1614.3 Hz, JC-P 84.5 Hz); 31P{1H} NMR (162 MHz,
CD2Cl2) d = 36.7 (dd, 1JPt-P = 3795 Hz, 2JP-P = 240 Hz, PPh3),
271.8 ppm (dd, 1JPt-P = 7354 Hz, 2JP-P = 240 Hz, PN2).
[26] M. F. Lappert, J. Organomet. Chem. 1988, 358, 185.
[27] 6: Solid PN2OTf (0.472 g, 1.0 mmol) was added slowly over 1 h
to a cold (08C) solution of [Pt(PPh3)3] (1.07 g, 1.0 mmol) in
CH2Cl2 (20 mL). The solution was warmed to ambient temper-
ature and stirred for 3h. Solvent was removed in vacuo and the
subsequent pale yellow powder was dissolved in warm toluene
(608C; 30 mL). The toluene solution was allowed to sit in the
freezer overnight upon which a pale yellow oil formed at the
bottom of the flask. The toluene was decanted off and the oil
pumped on for 5 h to yield a yellow powder. Yield 0.64 g, 49.8%.
1H NMR (400 MHz, CD2Cl2) d = 2.06 (12H, o-Me of N-mes),
2.36 (6H, p-Me of N-mes), 3.99 (4H, PN2CH2), 6.91, 7.15,
7.35 ppm (ov m, 34H, m-H of N-mes, Ph); 13C{1H} NMR
(100 MHz, CD2Cl2) 18.27 (o-Me of N-mes), 21.40 (p-Me of N-
mes), 51.99 (PN2CH2), 129.16, 130.46, 131.15, 133.86, 134.19,
134.37, 136.77, 139.35 ppm; 31P{1H} NMR (162 MHz, CD2Cl2)
[15] a) R. Oberdörfer, M. Nieger, E. Niecke, Chem. Ber. 1994, 127,
2397; b) N. Burford, P. J. Ragogna, K. N. Robertson, T. S.
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124, 382.
[16] a) B. Breit, Chem. Commun. 1996, 2071; b) B. Breit, J. Mol.
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[17] C. E. Webster, Y. Fan, M. B. Hall, D. Kunz, J. F. Hartwig, J. Am.
Chem. Soc. 2003, 125, 858.
1
2
43.9 (dd, JPt-P = 4237 Hz, JP-P = 232 Hz, PPh3), 289.0 ppm (dtr,
[18] One other phosphenium–NHC adduct (4) was prepared pre-
viously by reaction of [PPh2(NHC)]Cl with AlCl3 (31P NMR: d =
ꢀ27 ppm; carbene carbon atom not detected in 13C NMR
spectrum): N. Kuhn, J. Fahl, R. Boese, D. Blaeser, Z. Anorg.
Allg. Chem. 1999, 625, 729.
1JPt-P = 6498 Hz, 2JP-P = 232 Hz, PN2).
[28] X-ray diffraction data for [Pt(PPh3)(PN2)(CN2)]OTf (5) were
collected on a Bruker P4/CCD diffractometer (Bruker AXS)
usinrgagphite monochromatized Mo
radiation (l =
Ka
0.71073 Š) at T= 183 K. A pale yellow block of the dichloro-
methane solvate C58H56F3N5O3P2PtS·CH2Cl2 measuring0.2 ”
0.1 ” 0.1 mm3 was covered with paratone oil in an inert
atmosphere and mounted on a glass fiber. Monoclinic, P21/c
with a = 16.653(6), b = 18.830(6) c = 19.924(7) Š, a = 90, b =
[19] IUPAC name for 2 is 1,3,4-triphenyl-4,5-dihydro-1H-1,2,4-tria-
zol-5-ylidene.
[20] 3: Solid (PN2)OTf (1; 0.945 g, 2 mmol) was added slowly over
30 min to a solution of NHC 2 (594 mg, 2 mmol) in cold (08C)
CH2Cl2 (15 mL). The solution was allowed to stir overnight and
volatiles were removed in vacuo, yield 1.26 g, 82%. 1H NMR
(400 MHz, CD2Cl2): d = 2.29 (vbr, 18H, o,p-Me of N-mes), 3.67
(vbr, 4H, PN2CH2), 6.90 (vbr, 4H, m-H of N-mes), 7.25–
7.70 ppm (ov m, 15H, N-Ph and C-Ph); 13C{1H} NMR (100 MHz,
CD2Cl2) 20.07, 20.65, 21.21 (C6H2(CH3)3), 54.00 (PN2CH2),
123.46, 128.99, 129.88, 129.94, 130.29, 130.63, 130.93, 131.10,
131.21, 131.40, 132.03, 137.09, 138.20, 165.70 ppm (d, ipso
carbene C, JP-C 199.4 Hz). 31P{1H} NMR (162 MHz, CD2Cl2)
d = 113.7 ppm (br).
112.614(5), g = 908, V= 5767(3) Š3, Z = 4. 1calcd 1.513 mgmꢀ3
.
30393 reflections collected of which 8057 independent reflec-
tions with I > 2s. All non-hydrogen atoms were refined aniso-
tropically with final R1 = 0.0773, wR2 = 0.1382 for I > 2s.
CCDC-212747 (5) contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cam-
bridge Crystallographic Data Centre, 12 Union Road, Cam-
bridge CB21EZ, UK; fax: (+ 44)1223-336-033; or deposit@
ccdc.cam.ac.uk).
[21] A. J. Arduengo III, J. C. Calabrese, A. H. Cowley, H. V. R. Dias,
J. R. Goerlich, W. J. Marshall, B. Riegel, Inorg. Chem. 1997, 36,
2151.
[29] A Cambridge CSD search revealed one shorter bond of 2.114 Š.
However, this is one of two chemically identical but crystallo-
graphically independent molecules in the unit cell. This molecule
features two Pt centers bridged by hydrido and phosphido units.
[22] As noted by a referee, in the absence of a more detailed study we
ꢀ
cannot rule out hindered rotation about the P C bond as well.
ꢀ
The shortness of the Pt P bond is thus likely to be due to the
[23] DFT calculations employed the B3LYP functional and the
LANL2DZ and relativistic-effective core potential and associ-
ated basis for Pt, except that the Pt basis set was completely
uncontracted. This approach yields two p functions with nearly
identical exponents (0.6048 and 0.5982); only the former was
retained. The 6-31G* basis set was utilized for the ligands.
[24] A. Igau, A. Baceiredo, H. Grützmacher, H. Pritzkow, G.
Bertrand, J. Am. Chem. Soc. 1989, 111, 6853.
ꢀ
compensation of the short Pt H bond. Regardless, the bond
length must be reported as an average of the two distances in the
cell and is reported in the paper as 2.162(8) Š. Cf. J. Jans, R.
Naegeli, L. M. Venanzi, A. Albinati, J. Organomet. Chem. 1983,
247, C37.
[30] P. A. Chaloner, P. B. Hitchcock, G. T. L. Broadwood-Strong,
Acta Crystallogr. Sect. C 1989, 45, 1309.
[31] While steric hindrance prevented a more detailed study of these
rotational preferences, rotation of the PN2 plane 108 from
perpendicular cost approximately twice as much energy as the
same rotation for the CN2 ligand.
[25] 5: Two different procedures involved addingeither a) 2 to 6 or
b) 3 to a solution of [Pt(PPh3)3]. Both reactions were carried out
on a 1-mmol scale at ꢀ108C in CH2Cl2 (20 mL) and were
allowed to warm to room temperature over 3 h, whereupon
volatiles were removed in vacuo. The resultingyellow powder
was purified by washingwith warm (50 8C) toluene (2 ” 50 mL)
and collectingthe slightly yellow solid. Yield a) 0.58 g(44%),
b) 0.54 g(41%). Crystals suitable for X-ray diffraction studies
were grown from a saturated CH2Cl2 solution layered with
1
toluene. H NMR (400 MHz, CD2Cl2) d = 1.85, 2.27 (6H, o-Me
of N-mes), 2.46 (6H, p-Me of N-mes), 3.78 (4H, PN2CH2), 6.39,
1958
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Angew. Chem. Int. Ed. 2004, 43, 1955 –1958