New metal-only Lewis pairs: Elucidating the electronic influence of N -heterocyclic carbenes and phosphines on the dative Pt-Al bond

Article abstract of DOI:10.1021/ic300531b

The synthesis and full characterization of a new heteroleptic N-heterocyclic carbene (NHC)-phosphine platinum(0) complex and formation of its corresponding alane adduct is reported. The influence of the ligands on the Lewis basic properties was studied vi

Full text of DOI:10.1021/ic300531b

Article
pubs.acs.org/IC
New Metal-Only Lewis Pairs: Elucidating the Electronic Influence of
N-Heterocyclic Carbenes and Phosphines on the Dative Pt-Al Bond
Jurgen Bauer, Rudiger Bertermann, Holger Braunschweig,* Katrin Gruss, Florian Hupp,
̈
̈
and Thomas Kramer
Institut fur Anorganische Chemie, Julius-Maximilias-Universitat Wurzburg, Am Hubland, 97074 Wurzburg, Germany
̈
̈
̈
̈
S
* Supporting Information
ABSTRACT: The synthesis and full characterization of a new
heteroleptic N-heterocyclic carbene (NHC)−phosphine plat-
inum(0) complex and formation of its corresponding alane
adduct is reported. The influence of the ligands on the Lewis
basic properties was studied via multinuclear NMR-spectros-
copy, X-ray analyses, and density functional theory (DFT)
calculations. Consistently, the effect of changing the halogens
upon the Lewis acid properties of aluminum halides was studied by X-ray analysis and DFT calculations.
aluminum atom.11 Descending the third row of the periodic
table, Fischer delivered an example of a transition metal gallium
adduct, the complex [Cp*(Cp*Ga)2Rh−GaCl3] (11).12
INTRODUCTION
■
Currently, the concept of metal-centered Lewis basicity is
under intense investigation in organometallic research, and we
are particularly interested in unsupported metal-only Lewis pairs
(MOLPs). Lewis basic behavior of electron-rich transition
metal complexes was observed in the early 1960s by the groups
of Vaska and Wilkinson by means of rhodium and iridium
species.1,2 Subsequently, studies on the reactivity of trans-
[IrCl(CO)(PPh3)2] (1) toward protic substrates played a
central role in the development of metal-centered Lewis
basicity.3 Early attempts to apply metal-centered nucleophilicity
resulted in Lewis acid−base adducts of metals with small
molecules, for example, BF3,4 although the proposed borane
complexes were later refuted.5 Nonetheless this work led to the
development of metal-centered Lewis basicity as a synthetic
concept in inorganic chemistry.6 In the following years, half-
sandwich compounds of the general formula [CpLnM] (M =
Co, Rh, Ir) predominantly served as examples of metal-
centered Lewis basicity. For instance, the group of Werner
studied the Lewis basic properties of the complex [Cp-
(Me3P)2Co] (2), which reacted with a variety of electrophiles
(3−5).7,8 Using analogous rhodium compounds, Mayer et al.
examined half-sandwich adducts of the type [Cp(R3P)2Rh−
AlR′3] (R = Me, Et; R′ = Me, Et) (6−8) by multinuclear NMR
spectroscopy. Thereby, the Lewis acidic behavior of certain
alanes toward transition metal Lewis bases was introduced, and
kinetic and thermodynamic data were provided.9 Additionally,
[Cp(Me3P)Rh(Al2Me4Cl2)] (9) could be studied by X-ray
structural analysis. [(Cp*)(Me3P)(H)2Ir−AlPh3] (10) (Cp* =
1,2,3,4,5-pentamethylcyclopentadienyl) was reported by Berg-
mann et al. as the first structurally characterized neutral
transition metal aluminum complex. However, according to X-
ray structural analysis the Ir−Al bond of 10 is supported by two
H-atoms (Figure 1).10 Additional density functional theory
(DFT) calculations carried out by Lelj on this complex indicate
a significant charge donation from the transition metal to the
The coordination chemistry of the lightest group 13
homologue, boron, has since been studied in a variety of
different ligand classes, mainly three-coordinate boryl ligands,
because of their essential roles in the transition metal-catalyzed
hydroboration, diboration, and other boron addition reactions
to unsaturated organics.13−17 As is well-known, the reaction of
electron rich late transition metal complexes with boron halides
results in oxidative addition of a B−X bond.18,19 This was
consistently shown by our group using the complex
[(Cy3P)2Pt] (12), affording trans-(halo)(boryl) com-
plexes.20−23 Accordingly, in a recent DFT study Sakaki et al.
proposed a possible mechanism for the oxidative addition of
boranes to late transition metals.24 The formation of the final
trans-(bromo)(boryl) platinum(II) complex is enabled by
coordination of the borane to platinum(0), resulting in a
complex that can be considered as a Lewis acid−base adduct. In
conclusion it seems that for Lewis acidic molecules with
appropriate empty orbitals, oxidative addition may proceed by a
prior M→E donation.24 Similarly, the reaction of the heavier
homologues GaBr3 and GaI3 with late transition metals results
in oxidative addition products: trans-(halo)(gallyl) complexes.25
In contrast, GaCl3 follows a different reaction pathway, namely,
the formation of an unsupported Lewis acid−base adduct.25
Accordingly, Frenking et al. indicated, using DFT calculations
on late transition metal adduct complexes with the general
formula [(Me3P)2M−EX3] (M = group 10 metal; E = group 13
element; X = halogen), that M−E (E = Al, Ga) bond strength
increases in the order F < Cl < Br < I, in agreement with the
trends of Lewis acidity of boron halides and in the order F > Cl
> Br > I for heavier group 13 halides.26,27
Received: November 22, 2011
Published: May 1, 2012
© 2012 American Chemical Society
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Figure 1. Metal-complexes 3−8, 10, 11.
Herein we report the synthesis and full characterization of
the new heteroleptic NHC−phosphine−Pt(0) complex
[(IMes)(iPr3P)Pt] (13) (IMes = N,N′-bis(2,4,6-
trimethylphenyl)imidazol-2-ylidene). The reactivity toward
AlX3 was investigated, and the corresponding alane adducts
[(IMes)(iPr3P)Pt−AlX3] (14 X = Cl, 15 X = Br) were fully
characterized. To the best of our knowledge, the complex
[(IMes)(iPr3P)Pt−AlBr3] (15) is the first example of a
heteroleptic phosphine/NHC platinum metal base with the
Lewis acid AlBr3. For structural and spectroscopic comparison
the alane adducts [(iPr3P)2Pt−AlX3] (16: X = Cl, 17: X = Br)
were synthesized. Aluminum chloride was employed in these
studies as it is widely used as a strong Lewis acid.28,29 The
influence of NHC and R3P ligands on the Lewis basicity of
Pt(0) complexes was inferred on the basis of structural,
spectroscopic, and computational data. For compounds 16 and
17, both 27Al NMR and 195Pt NMR spectra were obtained,
both of which displayed a 1J(27Al−195Pt) spin−spin coupling
to the best of our knowledge the first time this coupling has
been observed.
In general, the influence of ligands can be varied in terms of
their steric and electronic properties. An elegant method for
quantification of the steric demand of various ligands is the
“buried volume” method, developed recently by Nolan and
Cavallo.34,35 Alternatively, the analysis of the Tolman’s
electronic parameters (TEP) can be used for comparison of
the electron-donor ability of the ligands.36−38 A pioneering
attempt to compare the steric and electronic properties of
several alkyl and aryl phosphines was made by Tolman,
comparing spectroscopic properties of transition metal
phosphine complexes.36 On a smaller scale, our group
examined several platinum complexes by multinuclear NMR
spectroscopic and X-ray structural analyses to verify the
properties of PiPr3 and PCy3.39−41 It was also found that the
larger and more electron-donating the phosphine, the higher
the catalyst activity in olefin metathesis.42 In particular, PCy3
turned out to be the ligand of choice in the famous “Grubbs 1st
generation catalyst”.42 Several years later the substitution of one
phosphine by one N-heterocyclic carbene led to a phosphine
NHC complex with increased catalytic activity, the so-called
“Grubbs 2nd generation catalyst”.43,44 Further attempts should
be made to vary the ligand sphere of the complexes [L2Pt] by
reducing the steric demand of the PR3 ligands. As shown below,
PiPr3 is the least σ-electron donating ligand of the mentioned
ligands (Table 1). However, these measures of steric hindrance
are not without controversy, as the %Vbur of PiPr3 is greater
than PCy3, yet the cone angle is smaller.45,46
RESULTS AND DISCUSSION
■
Synthesis of NHC-Containing Pt(0) Complexes. To
gain a better understanding of the dative bond between two
metals, our group employed the well-established transition
metal Lewis base [(Cy3P)2Pt] (12) to synthesize unsupported
MOLPs containing the d-block metal platinum and representa-
tive Lewis acidic s-, p-, and d-block metal fragments. The
reactions of the strong Lewis acids BeCl2, AlCl3, GaCl3, and
ZrCl4 with 12 resulted in MOLPs of the general formula
[(Cy3P)2Pt−ECln] (E = Be, Al, Ga, Zr, n = 2, 3, 4) (18−21)
(Figure 2).25−32 Hence, we were interested in varying, and
Table 1. TEP and %Vbur for PiPr3, PCy3, IMes, and SIMes
PiPr3
PCy3
SIMes
IMes
a
a
b
b
TEP [cm−1] (Ni(CO)3L)
2059
2056
2052
2050
c
d
d
e
e
%Vbur (LAuCl)
34.0
160
33.4
170
36.9
36.5
c
cone angle [deg]
a
b
c
d
e
Ref 34. Ref 45. Ref 46. M−L = 2.28 Å. M−L = 2.00 Å.
To vary the steric and electronic properties of homo- and
heteroleptic platinum complexes, [(iPr3P)2Pt] (24) was used as
starting material for the synthesis of the heteroleptic
platinum(0) complex [(IMes)(iPr3P)Pt] (13). To this end,
24 was stirred with a stoichiometric amount of IMes in a
ligand-exchange reaction at ambient temperature in hexane. As
24 is obtained from the tris(phosphine) complex [(iPr3P)3Pt]
(25) upon heating or in vacuo,47,48 we also reacted 25 directly
with equimolar amounts of IMes in hexane. Multinuclear NMR
spectroscopy of the reaction mixture, as well as a color change
from colorless to yellow, revealed complete consumption of the
starting materials and formation of [(IMes)(iPr3P)Pt] (13),
which was isolated by crystallization from hexane at −30 °C in
60% yield.
Figure 2. Metal-only-Lewis pairs 18−21 and 23.
preferably increasing, the electron-donating properties of
complexes of the type [L2Pt] and thus prepared new
heteroleptic 14-electron platinum(0) complexes and their
corresponding AlCl3 adducts using NHCs (N-heterocyclic
carbenes), for example, [(SIMes)(Cy3P)Pt] (22) and its alane
adduct [(SIMes)(Cy3P)Pt−AlCl3] (23), respectively (SIMes =
N,N′-bis(2,4,6-trimethylphenyl)imidazolylidene). Conse-
quently, the direct influence of NHCs on the Lewis basic
properties of the central metal was analyzed.33
Substitution of one phosphine by an NHC causes a slight
high-field shift of the 31P{1H} NMR spectroscopic resonance of
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13 to δ = 69.4 ppm (24: δ = 74.0 ppm). This is in accord with
previous findings, particularly in case of the synthesis of
angles (13 (X = P): 175.71(7)°, 26 (X = C): 177.4(3)°) and
Pt−C distances, respectively (13: 1.996(2) Å, 26: 1.959(8) Å).
To provide information on the propensity of the homoleptic
bis(phosphine) complex 24 and the new heteroleptic species
13 to form MOLPs, we subsequently targeted the synthesis of
the alane adducts [(IMes)(iPr3P)Pt−AlX3] (14: X = Cl, 15: X
= Br) and [(iPr3P)2Pt−AlX3] (16: X = Cl, 17: X = Br),
respectively.
1
[(SIMes)(Cy3P)Pt] (22) (δ = 58.4 ppm, JP−Pt = 3640 Hz)
1
from [(Cy3P)2Pt] (12) (δ = 62.3 ppm, JP−Pt = 4160 Hz) by
phosphine/carbene-exchange.33 Additionally, the coupling
1
constants are somewhat smaller (13: JP−Pt = 3897 Hz)
compared to the starting material 24 (1JP−Pt = 4104 Hz). These
spectroscopic parameters (Table 2) are in line with recent
Synthesis of Lewis Acid−Base Adducts. In analogy to
previous results, the MOLPs 14, 15 and 16, 17 were
synthesized by stirring the precursors 13 and 24 in benzene
at ambient temperature with equimolar amounts of AlCl3 or
AlBr3. These reactions yielded the expected T-shaped Lewis
acid−base adducts [(IMes)(iPr3P)Pt−AlX3] (14: X = Cl, 15: X
= Br) and [(iPr3P)2Pt−AlX3] (16: X = Cl, 17: X = Br)
according to Scheme 1.
Table 2. NMR Spectroscopic Parameters in Solution of
Compounds 12−17, 19, 24, 27
a
b
a,
c
a
a
27Al
b
31P{1H}
1JP−Pt
13C{1H}
195Pt{1H}
1JAl−Pt
12
19
13
14
15
16
17
24
27
62.3
53.5
69.4
56.9
53.5
65.5
63.1
74.0
4160
3032
3897
2988
2980
3034
3061
4104
3046
−6501
d
67.5
2034
201.5
179.7
167.9
−6290
d
68.9
42.7
68.5
44.6
2100
2200
1933
1999
d
Scheme 1. Formation of the Complexes [(IMes)(iPr3P)Pt−
AlX3] (14 = Cl, 15 = Br) and [(iPr3P)2Pt−AlX3] (16 = Cl, 17
= Br).
−5550
−5410
−6590
d
51.2
41.3
2150
a
b
c
δ in ppm. Coupling constants in Hz. Signal of carbenoid carbon.
d
Not detected.
findings, which indicate decreased Pt−P bond strengths as a
result of the enhanced trans-influence of the NHC ligand.33
The resonance of the carbenoid carbon atom in the 13C{1H}
NMR spectrum of 13 (δ = 201.5 ppm) is in the expected range
for late transition metal NHC complexes, such as [(IMes)2Pt]
(26) published by Arduengo et al. (δ = 197.5 ppm).49 An
additional measurement of the 195Pt{1H} NMR resonances
revealed a slight downfield shift for compound 13 (δ = −6290
ppm) relative to its precursor complex 24 (δ = −6590 ppm).
These findings are similar to the complex [(SIMes)(Cy3P)Pt]
(22) (δ = −6151 ppm) and its precursor complex [(Cy3P)2Pt]
(12) (δ = −6501 ppm). Overall, the spectroscopic similarities
between 22 and 13 are obvious, indicating closely related
compounds despite their different ligands.
Light yellow crystals suitable for X-ray structure determi-
nation were obtained from a hexane solution at −30 °C.
The formation of the MOLPs is confirmed by multinuclear
NMR spectroscopy in solution. For the heteroleptic adduct 14
the 31P{1H} NMR spectrum reveals a singlet at δ = 56.9 ppm
flanked by Pt satellites (1JP−Pt = 2988 Hz), and for 15 the
31P{1H} NMR spectrum shows a signal at δ = 53.5 ppm (1JP−Pt
= 2980 Hz, while the bis(phosphine) adduct 16 shows a singlet
at δ = 65.5 ppm (1JP−Pt = 3034 Hz) and 17 shows a signal at δ =
63.1 ppm (1JP−Pt = 3061 Hz). Altogether, the high-field shift of
Complex 13 crystallizes in the space group P1 and displays the
̅
expected overall linear geometry around the platinum center
(Figure 3). A comparison with the above-mentioned complex
26 reveals comparable structural features, such as C−Pt−X
1
the 31P{1H} NMR resonances and the decrease of the JP−Pt
coupling constants by about 1000 Hz in comparison to those of
the precursors 13 (δ = 69.4 ppm, 1JP−Pt = 3897 Hz) and 24 (δ
1
= 74.0 ppm, JP−Pt = 4104 Hz) is in good agreement with
analogous systems, for example, [(Cy3P)2Pt−AlCl3] (19) (δ =
53.5 ppm, 1JP−Pt = 3032 Hz) and [(Cy3P)2Pt−AlBr3] (27) (δ =
51.2 ppm, JP−Pt = 3046 Hz).31
1
195Pt{1H} and 27Al NMR Spectra of the Bisphosphine
Adducts 16, 17, 19, and 27. We were able to obtain relatively
sharp 195Pt{1H} and 27Al NMR spectra with well resolved
spin−spin coupling information for the symmetrical bis-
(phosphine) compounds 16 and 17. Although 27Al with its
high quadrupole moment is involved in the spin system, all
couplings were observable, because of the axial symmetry of the
complexes 16 and 17, leading to a small electric field gradient
Figure 3. Molecular structure of 13. Relevant bond lengths [Å] and
angles [deg]: Pt−C1 1.996(2), Pt−P 2.208(1); C1−Pt−P 175.71(7).
Ellipsoids are drawn at the 50% probability level. Ellipsoids of the
ligands and hydrogen atoms are omitted for clarity.
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Figure 4. 195Pt{1H} NMR (left) and 27Al NMR (right) spectra of 16. The experimental 195Pt{1H} NMR spectrum (left, bottom) was recorded at
107.5 MHz with 200000 scans, recycle delay 0.8 s, acquisition time 0.2 s, and processed with a line broadening factor of 30 Hz. The simulated
195Pt{1H} NMR spectrum (left, top) was simulated with the coupling constants extracted from the 27Al and 31P{1H} NMR spectra using a line width
of 470 Hz.
(EFG) at the 27Al site and a long(er) spin lattice relaxation time
T1. Scalar coupling between two spins will only appear if the
spin−lattice relaxation times, T1, of both nuclei are sufficiently
long to fulfill the relationship 2πJT1 ≫ 1. If 2πJT1 for one
nucleus, such as aluminum, is long enough, then splitting will
lines corresponding to the 1/2 states and the 3/2 states
exhibit the broadest lines. This behavior is reflected by the
experimental 195Pt{1H} spectrum in Figure 4, but could not be
simulated by the spin system analysis program.
The 27Al NMR spectrum reveals a singlet at δ = 68.5 ppm
flanked by 195Pt satellites (1JAl−Pt = 1933 Hz) for compound 16
and a similar coupling pattern at δ = 44.6 ppm (1JAl−Pt = 1999
Hz) for compound 17. Additional 27Al NMR spectra of the
PCy3 analogues 19 and 27 revealed somewhat broader singlets
at δ = 67.5 ppm (1JAl−Pt = 2034 Hz) (19) and δ = 41.3 ppm
(1JAl−Pt = 2150 Hz) (27). The chemical shifts are in the typical
range for AlX3−Lewis base adducts, while the coupling
constants of about 2000 Hz between 195Pt and 27Al proves
the connectivity of the Pt−Al unit.53 Again the relatively sharp
and resolved 27Al resonances of compound 16 and 17 indicate
that the quadrupolar relaxation plays only a minor role for the
27Al nucleus. Coupling constants for two-bond couplings
between 27Al and 31P are typically in the range of 12−30
Hz,50 but a 27Al−31P coupling could not be observed either in
the 31P{1H} nor in the 27Al NMR spectra of our above-
mentioned compounds. For compounds 19 and 27 it was not
possible to observe 195Pt{1H} NMR signals, probably because
of an increased electric field gradient at the 27Al site. Another
reason could be the slower tumbling in solution of the larger
complex resulting in broader, unresolved NMR lines.
be observed for the signal of the other nucleus as shown by
50
̈
Ohmann and Edlund. On the other hand, comparison of the
195Pt satellites in the 31P{1H} NMR spectra (measured at 202
MHz) of compound 16 and 17 shows that they are only slightly
broader than the central resonance, indicating that the chemical
shift anisotropy (CSA) at the 195Pt nuclei seem to be rather
small. Furthermore, the relative high magnitude of the coupling
constants (ca. 2000 and ca. 3000 Hz) compared with the line
width in the 195Pt NMR of about 470 Hz (16) leads to the
resolved coupling pattern.51
The 195Pt{1H} NMR spectrum reveals a triplet of sextets at δ
1
= −5550 (1JPt−P = 3034 Hz, JPt−Al = 1933 Hz) ppm for
compound 16 and a similar coupling pattern at δ = −5410
1
(1JPt−P = 3061 Hz, JPt−Al = 1999 Hz) ppm for compound 17.
The coupling pattern is due to coupling of the 195Pt (I = 1/2)
nucleus to the spin 5/2 27Al nucleus perturbed by additional
coupling to the two identical spin 1/2 31P nuclei. The chemical
shift of the respective signal is between that of the
bis(phosphine) complex 24 (δ = −6590 ppm) and the typical
shift region of trans-oxidative addition products (δ = −4780
ppm).52 As shown in Figure 4 the simulation of the 195Pt{1H}
NMR spectrum of compound 16 using the NMR spin system
analysis module “Daisy” from the NMR Software suite Topspin
3.0 pl4 by Bruker Biospin GmbH shows a triplet of sextets
which fits very well to the experiment. However the intensities
of the 1:1:1:1:1:1 sextet due to the coupling from the 27Al
nucleus are not handled correctly by the software, because of
the different T2 relaxation times of the different spin states of
the spin 5/2 nucleus 27Al, which leads to different line widths in
the coupling pattern. Normally the outer lines for the 5/2
states are smaller in width and higher in intensity than the inner
Discussion of Lewis Acid−Base Adducts. Single crystals
suitable for X-ray analysis were obtained from a toluene
solution at −30 °C (14 and 16). The complexes crystallize in
space groups P21/n (14) and C2/c (16), respectively (Figure
5). Both compounds feature the characteristic T-shaped
geometry at the platinum center observed for complexes of
the type [(Cy3P)2Pt−ECln] (E = Be, Al, Ga, Zr, n = 2, 3, 4)
(13−16) (Figure 5).30−32 A comparison of the structural data
of 16 with 14 reveals (Table 3) very similar Pt−P bond lengths
(14: 2.287(1) Å; 16: 2.308(1) and 2.324(1) Å) and P−Pt−P
(16: 166.42(1)°) and P−Pt−C1 angles (14: 165.97(1)°). The
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formation of the AlBr3 and AlI3 adducts was indicated by
multinuclear NMR spectroscopy only. For compound 27, the
31P{1H} NMR spectrum shows a singlet at δ = 51.2 ppm
flanked by 195Pt satellites (1JP−Pt = 3046 Hz).31 Compound 27
could be obtained almost quantitatively as a pale orange solid,
but in contrast to the chloro species 19, readily decomposes in
toluene solution within 2−3 days at ambient temperature,
forming trans-[(Cy3P)2PtBrX] (X = H (29), Br (30)) (Scheme
2).54
Scheme 2. Formation and Decomposition of the Complex
[(Cy3P)2Pt−AlBr3] (27).
Figure 5. Molecular structures of 14 and 16. Relevant bond lengths
[Å] and angles [deg]: 14: Pt−Al 2.376(1), Pt−P 2.287(1), Pt−C1
2.009(4), Al−Cl2 2.157(2), Al−Cl1 2.159(2), Al−Cl3 2.169(2); P−
Pt−C1 165.97(1), Cl3−Al−Pt 109.92(5), Cl1−Al−Pt 117.72(6),
Cl2−Al−Pt 111.84(6), Cl3−Al−Cl1 103.85(6), Cl3−Al−Cl2
107.65(6), Cl1−Al−Cl2 105.16(6); 16: Pt−Al 2.384(1), Pt−P1
2.308(1), Pt−P2 2.324(1); P1−Pt−P2 166.42(1), Al−Cl1 2.159(1),
Al−Cl2 2.152(1), Al−Cl3 2.155(1); P1−Pt−P2 166.42(1), Cl2−Al−
Pt 109.85(3), Cl3−Al−Pt 110.48(3), Cl1−Al−Pt 116.75(3), Cl2−Al−
Cl3 109.68(3), Cl2−Al−Cl1 104.81(3), Cl3−Al−Cl1 104.95(3).
Ellipsoids are drawn at the at the 50% probability level. Ellipsoids of
the ligands, solvent molecules, and hydrogen atoms are omitted for
clarity.
Pale yellow single crystals suitable for X-ray structural
analysis were obtained of both 17 and 27 from toluene
solutions at −30 °C, thus allowing the first structural
comparison between a LxM−ECln adduct and its bromo
analogue LxM−EBrn. Complex 17 crystallizes in the space
Pt−Al bond is oriented almost orthogonal to the L−Pt−L′ axis
(14: C1−Pt−Al 92.90(1)°; P−Pt−Al 101.09(4)°; 16: P−Pt−
Al 96.34(2)° and 97.11(2)°) and the bond lengths are nearly
identical (14: 2.376(1) Å; 16: 2.384(1) Å). Additionally, the
wider Cl1−Al−Pt angle in compound 16 (Cl1−Al−Pt
116.75(3)°, Cl2−Al−Pt 109.85(3)°, Cl3−Al−Pt 110.48(3)°)
is a result of a steric interaction between the chloride and the
proximal iPr group. Likewise, in 14 the two wider Cl−Al−Pt
angles (Cl1−Al−Pt 117.72(6)°, Cl2−Al−Pt 111.84(6)°, Cl3−
Al−Pt 109.92(5)°) can be accounted for by both repulsion
between Cl1 and Cl3 and the platinum bound ligands.
As PiPr3 is sterically somewhat less bulky than PCy3, the
corresponding base adduct 16 displays a smaller distortion of
the overall T-shaped geometry in comparison with
[(Cy3P)2Pt−AlCl3] (19) as indicated, for example, by
comparison of the P1−Pt−P2 angles (16: 166.42(1)°, 19:
162.07(2)°).31,33
Discussion of Bis(phosphine) Platinum AlBr3 Adducts. For
a structural comparison with our new compound [(iPr3P)2Pt−
AlBr3] (17) we crystallized compound [(Cy3P)2Pt−AlBr3]
(27). As mentioned above, we previously employed
[(Cy3P)2Pt] (12) for the synthesis of MOLPs of the general
formula [(Cy3P)2Pt−AlX3] (X = Cl (19), Br (27), I (28)). The
AlCl3 adduct was fully characterized by both multinuclear NMR
spectroscopy and X-ray structural analysis, whereas the
group C2/c, the analogue 27 in the space group P1,
̅
respectively. Both compounds display the expected T-shaped
geometry around the platinum center (Figure 6). A comparison
of the P2Pt moieties of the chloro species (19, 16) with their
respective bromo analogues (17, 27) reveals only minor
differences. The Pt−Al distances in compounds 19 and 27 are
virtually identical, whereas the Pt−Al distances of 16 (2.384(1)
Å) and 17 (2.368(2) Å) differ slightly by 2 pm. DFT
calculations on the optimized compounds 16, 17, 19, and 27
showed nearly identical Pt−Al distances (Table 4) and so
crystal packing effects could be the reason for the small
deviance between theory and experiment. A closer look at the
AlBr3 groups of compounds 17 and 27 reveals subtle
differences. In compound 17 the torsion angle (P1−Pt−Al−
Br1: 0.47(8)°) reveals that the AlBr3 group is eclipsed with
respect to the P2Pt moiety. As a result the Al−Br1 bond is
elongated (Al−Br1 2.325(2) Å, Al−Br2 2.316(2) Å, Al−Br3
2.313(2) Å) because of a steric interaction between the
bromide and a hydrogen atom from the nearby iso-propyl
group. In compound 27 the conformation is staggered (P1−
Pt−Al−Br2: 87.60(4)°) and so the Al−Br distances are similar
(Al−Br1 2.319(1) Å, Al−Br2 2.315(1) Å, Al−Br3 2.317 (1) Å).
Comparison of the geometry of the Pt-bound AlBr3 with
those of amine-alane adducts derived from computational55 and
experimental studies,56 suggests that the platinum base exerts a
Table 3. Structural Parameters of Compounds 13−17, 19, 23, 27
a
a
a
b
C−Pt
P−Pt
Al−Pt
L−Pt−L
[(Cy3P)2Pt−AlCl3] (19)
[(Cy3P)2Pt−AlBr3] (27)
[(SIMes)(Cy3P)Pt−AlCl3] (23)
[(IMes)(iPr3P)Pt] (13)
2.299(1), 2.313(1)
2.303(1), 2.308(1)
2.301(2)
2.386(1)
2.380(1)
2.384(2)
162.07(2)
160.09(3)
168.3(5)
1.991(9)
1.996(2)
2.009(4)
2.208(1)
175.71(7)
165.97(1)
166.42(1)
165.28(5)
[(IMes)(iPr3P)Pt−AlCl3] (14)
[(iPr3P)2Pt−AlCl3] (16)
[(iPr3P)2Pt−AlBr3] (17)
2.287(1)
2.376(1)
2.384(1)
2.368(2)
2.308(1), 2.324(1)
2.302(2), 2.323(2)
a
b
Distances in Å. Angles in deg.
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Å) are comparable to that of [tmp−AlBr3] (2.280(2)−2.300(2)
Å) (tmp = tetramethylpiperidine) and [H3N−AlBr3] (2.312 Å
mean). The Br−Al−Br angles of 17 (103.14(7)−109.56(7)°)
and 27 (99.22(4)°−109.09(4)°) and [tmp−AlBr3] (101.59(7)
°−110.02(8)°) are also in a similar range, whereas those of
[H3N−AlBr3] (116.6° mean) are significantly larger.
DFT Calculations. To provide further information as to the
Lewis basicity of the different transition metal complexes
employed for the synthesis of MOLPs, DFT calculations were
carried out (Figure 7). In addition to the optimized structures
of [(iPr3P)2Pt] (24), [(IMes)(iPr3P)Pt] (13), and [(SIMes)-
(iPr3P)Pt] (31), the complexes [(Me3P)2Pt] (32), [(IMe)-
(Me3P)Pt] (33), and [(SIMe)(Me3P)Pt] (34) were calculated
as sterically less-demanding models, as well as their
corresponding AlCl3 adducts 35−38 (Figure 5) (IMe = N,N′-
bis(methyl)imidazol-2-ylidene, SIMe = N,N′-bis(methyl)-
imidazolylidene).
Interestingly, the saturation of the NHC backbone has no
immediate effect on the basicity of the platinum center. Thus,
both simplified NHC model complexes 33 and 34 give rise to
almost identical BDEs (Table 4) upon adduct formation with
AlCl3, which are about 20 kJ/mol more negative than that of
32. This indicates an increased interaction between the
platinum and aluminum centers in NHC complexes, which is
in good agreement with previously reported results.33
Figure 6. Molecular structure of 17 and 27. Ellipsoids are drawn at the
at the 50% probability level. Ellipsoids of the ligands, solvent molecules
and hydrogen atoms are omitted for clarity. Relevant bond lengths [Å]
and angles [deg]: 17: Pt−P1 2.302(2), Pt−P2 2.323(2), Pt−Al
2.368(2), Al−Br1 2.325(2), Al−Br2 2.316(2), Al−Br3 2.325(2); P−
Pt−P 165.28(5), Br2−Al−Br3 109.56(7), Br2−Al−Br1 103.74(7),
Br3−Al−Br1 103.14(7); 27: Pt−P1 2.303(1), Pt−P2 2.308(1), Pt−Al
2.380(1), Al−Br1 2.319(1), Al−Br2 2.315(1), Al−Br3 2.317(1); P−
Pt−P 160.09(3), Br2−Al−Br3 107.45(4), Br2−Al−Br1 109.09(4),
Br3−Al−Br1 99.22(4).
similarly strong electron donation. Thus, the Al−Br distances of
17 (2.313(2)−2.325(2) Å) and 27 (2.3154(11)−2.3193(11)
Table 4. Selected Calculated Parameters of [L2Pt−AlCl3] (14, 16, 34−37) and [L2Pt] (13, 24, 30−33)
distance
14
2.455
16
2.458
35
2.459
36
2.458
37
2.451
38
2.451
a
Pt−Al
ab
,
Pt−L1
2.032
2.397
2.362
2.366
2.029
2.405
2.326
2.331
2.041
2.327
2.045
2.333
a
Pt−L2
c
WBI
Pt−Al
Pt−L1
Pt−L2
0.54
0.56
0.49
0.52
0.50
0.50
0.54
0.58
0.48
0.51
0.51
0.51
0.54
0.53
0.53
0.54
0.54
0.52
b
b
natural charge
Pt
−0.23
1.31
−0.31
1.31
−0.22
1.31
−0.34
1.31
−0.25
1.29
−0.25
1.29
Al
b
L1
0.34
0.37
0.33
0.37
0.35
0.33
b
L2
0.32
0.36
0.32
0.37
0.35
0.36
L2Pt
0.43
0.42
0.43
0.40
0.45
0.44
AlCl3
−0.43
−0.42
−0.43
−0.40
−0.45
−0.44
d
BDE
−131
48
−146
37
−130
48
−167
13
−188
−189
e
bd
,
prep (L1L2Pt)
8
8
e
d
prep (AlCl3)
100
96
99
82
86
86
d
interaction
−279
−278
−277
−262
−282
−282
c
WBI
13
24
31
32
33
34
c
b
b
WBI Pt−L1
0.66
0.60
0.68
0.61
0.62
0.64
c
WBI Pt−L2
0.61
0.58
0.60
0.61
0.64
0.63
natural charge
Pt
−0.42
0.20
−0.49
0.25
−0.41
0.18
−0.52
0.26
−0.47
0.23
−0.46
0.22
b
L1
b
L2
0.22
0.24
0.23
0.26
0.24
0.24
a
b
c
d
e
Distances in Å. L1 = NHC, phosphine; L2 = phosphine. Wiberg Bond Index. Energies in kJ/mol. Preparation energy, e.g., steric rearrangement
during the reaction.
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Figure 7. Optimized structures of the compounds 14, 16, and 35−38.
However, this trend is reversed in the case of the
synthetically employed complexes 13, 24, and 31, which
comprise much more sterically demanding phosphine and
NHC ligands. Here, the bis(phosphine) complex 13 is
determined, by means of the BDE for the Pt−Al bond in the
corresponding adduct 16, to be a stronger transition metal
Lewis base than the NHC complexes. The reason for this
reversed trend is the significantly increased preparation
enthalpy (e.g., the sterical rearrangement of the fragments
during adduct formation) in the NHC-containing complexes 13
and 31 (24: 37 kJ/mol, 13 and 31: 48 kJ/mol). Combined with
almost identical values for the interaction enthalpies, this leads
to an overall less negative BDE for the NHC complexes. Hence,
as judged by the interaction energies alone, PiPr3 appears to
exert the same electronic influence on the platinum center as
the NHCs IMes and SIMes. This finding is in stark contrast to
earlier calculations on corresponding PCy3 complexes. Here,
the bis(phosphine) complex [(Cy3P)2Pt−AlCl3] (19) revealed
a significantly reduced interaction enthalpy (ca. 20 kJ/mol) in
comparison to the mixed-ligand complexes with PCy3 and
NHC ItBu ligands (ItBu = N,N′-bis(tert-butyl)imidazol-2-
ylidene).33 Analysis of the natural charges, as calculated by
the natural bond orbital (NBO) method, reveals a uniform
trend for all investigated systems, both the models 36−38 and
the authentic complexes 14, 16, and 35. Upon adduct
formation, the overall natural charge of the neutral L2Pt
fragment (by definition natural charge = 0) in all compounds is
increased to about 0.41−0.45, because of electron donation
from the metal atom to the Lewis acid AlCl3. This positive
charge is distributed between the platinum center and the
ligands, respectively. For example in complex 14, the natural
charge of the IMes ligand is increased by 0.14 (0.20 in 13 to
0.34 in 14), whereas an almost identical increase is found for
the PiPr3 ligand (0.22 in 13 to 0.32 in 14). Therefore, again
similar electronic properties for IMes and PiPr3 are found. In
addition to the decreased negative charge of the platinum atom
(−0.42 in 13 to −0.22 in 14) this sums up to an overall positive
charge of 0.44 for the [(IMes)(iPrP3)Pt] fragment in the
MOLP 14 (Table 4).
In further calculations, the influence of the halogen atoms on
the properties of the MOLPs [(Cy3P)2Pt−AlX3] (X = Cl (19),
Br (27), and I (28)) and [(iPr3P)2Pt−AlX3] (X = Cl (16), Br
(17)) was investigated.31 Here, a clear trend was observed, that
is, the Pt−Al BDE decreases as the group is descended (Table
5). The strongest interaction was found for the AlCl3 adduct
(19: BDE = −141 kJ/mol, 16: BDE = −146 kJ/mol), followed
by the AlBr3 (27: BDE = −124 kJ/mol, 17: BDE = −126 kJ/
mol) and the AlI3 species (28, BDE = −106 kJ/mol). A
Table 5. Selected Calculated Parameters of [(Cy3P)2Pt−
AlX3] and [(iPr3P)2Pt−AlX3]
19
27
28
16
17
X
Cl
Br
I
Cl
Br
a
Pt−Al
2.459
0.52
0.5
2.462
0.54
0.49
2.461
0.55
0.48
2.458
0.52
0.50
2.459
0.53
0.49
b
WBI Pt−Al
WBI Pt−P
b
natural charge
Pt
Al
−0.31
1.31
−141
40
−0.29
1.1
−0.26
0.67
−106
51
−0.31
1.31
−146
37
−0.29
1.09
−129
42
c
BDE
−124
45
d
c
prep [L2Pt]
d
c
prep (AlX3)
98
100
98
96
98
c
interaction
−279
b
−269
−256
−278
−269
a
c
Distances in Å. Wiberg Bond Index. Energies in kJ/mol.
Preparation energy, e.g., steric rearrangement during the reaction.
d
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195Pt{1H} NMR (107.5 MHz, C6D6): δ = −6290 (1JPt−P = 3897 Hz)
ppm. Elemental analysis (%) calculated for C30H46N2PPt: C 54.53, H
7.02, N 4.24; found: C 53.89, H 6.67, N 4.06.
possible explanation for this trend is the reduced electrostatic
interaction in the adducts with less electronegative halogens.
The disparity between the natural charges of platinum and
aluminum is most distinct in 19 and 16 (1.6) and is reduced
when the chlorine is substituted by bromine (27 and 17: 1.4)
or iodine (28: 0.9). Furthermore, the preparation enthalpy of
[(Cy3P)2Pt] (12) upon adduct formation is more positive for
heavier halogens. As expected, AlI3 is the weakest Lewis acid
with respect to MOLP formation in the investigated systems
(Table 5). This result supports the earlier reports by Frenking
et al. for group 10 complexes of the type [(Me3P)2Pt−EX3] (E
= B, Al; X = Cl, Br, I).26
[(IMes)(iPr3P)Pt(AlCl3)] (14). AlCl3 (2.0 mg, 15 μmol) and
[(IMes)(iPr3P)Pt] (13) (10 mg, 15 μmol) were dissolved in benzene
(1.0 mL) at ambient temperature. A yellow solid precipitated
immediately, was filtered off and rinsed with benzene. The precipitate
was dissolved in toluene and yellow crystals were obtained after 24 h at
1
−30 °C (7 mg, 10 μmol, 65%). H NMR (500.1 MHz, C6D6): δ =
3
6.81 (m, 4H, Har), 6.13 (d, 2H, JH−H = 1 Hz, NCHCHN), 2.34 (s,
12H, o-CH3, Mes), 2.14 (s, 6H, m-CH3, Mes), 1.30−1.20 (m, 3H, CH,
iPr), 1.02−0.98 (m, 18H, CH3, iPr) ppm. 13C{1H} NMR (125.8 MHz,
C6D6): δ = 179.7 (s, NCN), 135.4 (s, ipso-Car), 134.4 (s, p-Car), 129.7
1
(s, o-Car), 128.3 (s, m-Car), 122.9 (s, NCHCHN), 25.1 (d, JC−P = 25
Hz, CH, iPr), 20.0 (s, p-CH3, Mes), 19.9 (s, CH3, iPr), 17.7 (s, o-CH3,
Mes) ppm. 27Al NMR (130.3 MHz, C6D6): δ = 68.9 (1JAl−Pt = 2100
Hz) ppm. 31P{1H} NMR (202.5 MHz, C6D6): δ = 56.9 (1JP−Pt = 2988
Hz) ppm. Elemental analysis (%) calculated for C37H50AlCl3N2PPt: C
50.37; H 5.71; N 3.18; found: C 50.11; H 5.72; N 3.51.
CONCLUSION
■
Herein, we report on the synthesis of the new heteroleptic
NHC-phosphine complex [(IMes)(iPr3P)Pt] (13) and its
characterization by multinuclear NMR spectroscopy, elemental
analysis, and single crystal X-ray diffraction analysis. Addition-
ally, the alane adducts of 13 and the homoleptic Pt(0)
compound [(iPr3P)2Pt] (24) were prepared and fully
characterized (14−17) including multinuclear NMR, structural,
and computational data. For compounds 16 and 17 both 27Al
NMR and 195Pt NMR spectra were obtained, both of which
[(IMes)(iPr3P)Pt(AlBr3)] (15). AlBr3 (8.0 mg, 30 μmol) was added to
a solution of [(IMes)(iPr3P)Pt] (13) (20 mg, 30 μmol) in toluene
(1.0 mL) at −30 °C. A yellow solid precipitated after 48 h, which was
filtered off and rinsed with toluene. The precipitate was dissolved in
toluene and yellow crystals were obtained after 24 h at −30 °C (19
1
mg, 21 μmol, 71%). H NMR (500.1 MHz, C6D6): δ = 6.74 (s, 4H,
Har), 6.26 (s, 2H, NCHCHN), 2.29 (s, 12H, o-CH3, Mes), 2.10 (s, 6H,
m-CH3, Mes), 1.80−1.69 (m, 3H, CH, iPr), 0.89−0.81 (m, 18H, CH3,
iPr) ppm. 13C{1H} NMR (125.8 MHz, C6D6): δ = 167.9 (s, NCN),
138.4 (s, ipso-Car), 136.6 (s, p-Car), 136.0 (s, o-Car), 129.5 (s, m-Car),
122.9 (s, NCHCHN), 25.6 (d, 1JC−P = 25 Hz, CH, iPr), 20.9 (s, p-CH3,
Mes), 19.6 (s, CH3, iPr), 19.2 ppm (s, o-CH3, Mes) ppm. 27Al NMR
(130.3 MHz, C6D6): δ = 42.7 (1JAl−Pt = 2200 Hz) ppm. 31P{1H} NMR
(202.5 MHz, C6D6): δ = 53.5 (1JP−Pt = 2980 Hz) ppm. Elemental
analysis (%) calculated for C37H50AlCl3N2PPt: C 38.89; H 4.90; N
3.02; found: C 38.92; H 4.85; N 3.06.
1
displayed the J(27Al−195Pt) spin−spin couplingto the best
of our knowledge this is the first time this coupling has been
observed. According to computational analysis, the PiPr3 ligand
seems to exert a similar electronic influence on the platinum
centers as the investigated NHCs, that is, IMes and SIMes. This
finding contrasts previous studies on PCy3 complexes, in which
the phosphine appeared to be a weaker electron donor than
NHC ligands. In the course of our theoretical investigations, we
further examined the MOLPs [(Cy3P)2Pt−AlBr3] (27) and
[(Cy3P)2Pt−AlI3] (28). Here, the expected trend for the
decrease of the Lewis acidity of the alanes with increase of the
atomic number of the halogen atoms was confirmed.
[(iPr3P)2Pt(AlCl3)] (16). In a J. Young NMR tube AlCl3 (4.0 mg, 30
μmol) was added to a yellow solution of [(iPr3P)2Pt] (24) (20 mg, 30
μmol) in benzene (0.6 mL). The color of the reaction mixture
lightened, and a yellow solid precipitated. The precipitate was filtered
off and rinsed with hexane. For X-ray analysis suitable crystals were
obtained from a solution in benzene at ambient temperature after 24 h
(13 mg, 20 μmol, 67%). 1H NMR (500.1 MHz, C6D6): δ = 2.48−2.40
(sep, 3JH−H = 4 Hz, 6H, CH), 1.23−1.08 (q, 3JH−H = 8 Hz, 36H, CH3)
ppm. 13C{1H} NMR (125.8 MHz, C6D6): δ = 26.0 (vt, N =
|1JC−P+3JC−P| = 25 Hz, CH, iPr), 20.9 (s, CH3, iPr) ppm.58 27Al NMR
(130.3 MHz, C6D6): δ = 68.5 (1JAl−Pt = 1933 Hz) ppm. 31P{1H} NMR
EXPERIMENTAL SECTION
■
General Considerations. All manipulations were performed in an
inert atmosphere of argon using standard Schlenk and glovebox
techniques. Solvents were distilled over alkali metal, degassed, and
stored over molecular sieves (4 Å) under argon. Deuterated solvents
were degassed by three freeze−pump−thaw cycles and stored under
argon over molecular sieves. NMR experiments were performed on a
Bruker Avance 500 (1H: 500.1 MHz; 13C: 125.8 MHz; 27Al: 130.3
(202.5 MHz, C6D6): δ = 65.5 (1JP−Pt = 3034 Hz) ppm. 195Pt{1H}
1
NMR (107.5 MHz, C6D6): δ = −5550 (1JPt−P = 3034 Hz, JPt−Al
=
1933 Hz) ppm. Elemental analysis (%) calculated for
C18H42AlCl3P2Pt: C 33.32; H 6.52; found: C 33.38; H 6.61.
MHz; 31P: 202.5 MHz; 195Pt: 107.5 MHz) apparatus. H NMR and
1
13C{1H} NMR spectra were calibrated to TMS; in the case of 27Al
NMR spectra Al(NO3)3 in D2O was used as external standard, and for
31P{1H} NMR spectra 85% H3PO4 was used. The external standard for
the 195Pt{1H} NMR spectra was Na2[PtCl6] in D2O. Elemental
analyses were performed on an Elementar vario Micro Cube elemental
analyzer and Leco Instrumente CHNS 932 elemental analyzer.
[(Cy3P)2Pt],47 [(iPr3P)3Pt]47 and IMes57 were prepared according
to known methods.
[(iPr3P)2Pt(AlBr3)] (17). In a J. Young NMR tube AlBr3 (14 mg, 52
μmol) was added to a yellow solution of [(iPr3P)2Pt] (24) (35 mg, 52
μmol) in benzene (0.7 mL) at 0 °C. The color of the reaction mixture
deepened and a yellow solid precipitated. The precipitate was filtered
off and rinsed with hexane. For X-ray analysis suitable crystals were
obtained from a solution in benzene at ambient temperature after 24 h
(15 mg, 20 μmol, 52%). 1H NMR (500.1 MHz, C6D6): δ = 2.67−2.58
(sep, 3JH−H = 4 Hz, 6H, CH), 1.19−1.14 (q, 3JH−H = 8 Hz, 36H, CH3)
ppm. 13C{1H} NMR (125.8 MHz, C6D6): δ = 25.9 (vt, N =
|1JC−P+3JC−P| = 25 Hz, CH, iPr), 20.9 (s, CH3, iPr) ppm.58 27Al NMR
(130.3 MHz, C6D6): δ = 44.6 (1JAl−Pt = 1999 Hz) ppm. 31P{1H} NMR
[(IMes)(iPr3P)Pt] (13). IMes (19 mg, 58 μmol) and [(iPr3P)3Pt]
(25) (40 mg, 58 μmol) were dissolved in hexane (1.0 mL). The
intensely yellow solution was kept at −30 °C for 5 d and yellow
(202.5 MHz, C6D6): δ = 63.1 (1JP−Pt = 3061 Hz) ppm. 195Pt{1H}
1
crystals were obtained after 3 days (29 mg, 44 μmol, 70%). H NMR
1
(500.1 MHz, C6D6): δ = 6.84−6.83 (m, 4H, m-Har), 6.21 (d, 3JH−H = 1
Hz, 2H, NCHCHN), 2.37 (s, 12H, o-CH3, Mes), 2.17 (s, 6H, p-CH3,
Mes), 1.74−1.65 (m, 3H, CH, iPr), 1.09−1.07 (m, 18H, CH3, iPr)
ppm. 13C{1H} NMR (125.8 MHz, C6D6): δ = 201.5 (s, NCN), 138.3
(s, ipso-Car), 137.4 (s, p-Car), 135.6 (s, o-Car), 128.8 (s, m-Car), 119.2 (s,
NCHCHN), 25.6 (d, 1JC−P = 24 Hz, CH, iPr), 21.3 (d, 2JC−P = 11 Hz;
CH3, iPr), 21.2 (s, p-CH3, Mes), 18.8 ppm (s, o-CH3, Mes) ppm.
31P{1H} NMR (202.5 MHz, C6D6): δ = 69.4 (1JP−Pt = 3897 Hz) ppm.
NMR (107.5 MHz, C6D6): δ = −5410 (1JPt−P = 3061 Hz, JPt−Al
=
1999 Hz) ppm. Elemental analysis (%) calculated for
C18H42AlBr3P2Pt: C 27.64; H 5.41; found: C 26.95; H 5.26.
[(Cy3P)2Pt(AlBr3)] (27). AlBr3 (8.0 mg, 29 μmol) was added to a pale
yellow solution of [(Cy3P)2Pt] (12) (21 mg, 28 μmol) in toluene (0.6
mL). The solvent from the orange solution was removed in vacuo to
obtain 27 (28 mg, 27 μmol, 98%) as a light orange solid. Suitable
crystals were obtained by means of diffusion of hexane to a toluene
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solution at −30 °C. 1H NMR (500.1 MHz, d8-tol): δ = 2.69−2.65 (m,
6H, Cy), 2.22−1.03 ppm (m, 60H, Cy). 13C{1H} NMR (125.8 MHz,
d8-tol): δ = 36.3 (vt, N = |1JC−P+3JC−P| = 25 Hz, C1, Cy), 31.6 (s, C3,
C5, Cy), 27.9 (vt, N = |2JP−C+4JP−C| = 11 Hz, C2, C6, Cy), 26.6 ppm (s,
C4, Cy).58 27Al NMR (130.3 MHz, d8-tol): δ = 41.3 (1JAl−Pt = 2150
ACKNOWLEDGMENTS
■
We are grateful to the German Science Foundation (DFG) for
financial support. J.B. is grateful to the Fonds der Chemischen
Industrie for a doctoral scholarship.
Hz) ppm. 31P{1H} NMR (202.5 MHz, d8-tol): δ = 51.2 ppm (1JP−Pt
=
3046 Hz). Elemental analysis (%) calculated for C36H66AlBr3P2Pt: C
42.28, H 6.50; found: C 42.74, H 6.77.
REFERENCES
■
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Crystal Structure Determination. The crystal data of 13, 14, 16,
and 27 were collected on a Bruker Apex diffractometer with a CCD
area detector and graphite monochromated MoKα radiation. The
structures were solved using direct methods, refined with the Shelx
software package and expanded using Fourier techniques.59 All non-
hydrogen atoms were refined anisotropically. Hydrogen atoms were
assigned to idealized positions and were included in structure factor
calculations.
Crystal Data for 13. C30H45N2PPt, Mr = 659.74, yellow block, 0.31
× 0.16 × 0.06 mm3, triclinic space group P1, a = 10.3236(18) Å, b =
̅
10.8091(18) Å, c = 15.026(3) Å, α = 78.494(4)°, β = 77.506(4)°, γ =
66.744(3)°, V = 1491.8(4) Å3, Z = 2, ρcalcd = 1.469 g·cm−3, μ = 4.776
mm−1, F(000) = 664, T = 173(2) K, R1 = 0.0203, wR2 = 0.0492, 5858
independent reflections [2θ ≤ 52.06°] and 319 parameters.
Crystal Data for 14. C37H50AlCl3N2PPt, Mr = 882.18, yellow block,
0.37 × 0.16 × 0.15 mm3, monoclinic space group P21/n, a =
10.7618(3) Å, b = 16.4437(4) Å, c = 22.3018(5) Å, β = 100.4370(10)
°, V = 3881.31(17) Å3, Z = 4, ρcalcd = 1.510 g·cm−3, μ = 3.913 mm−1,
F(000) = 1772, T = 100(2) K, R1 = 0.0288, wR2 = 0.0664, 7949
independent reflections [2θ ≤ 52.78°] and 483 parameters.
Crystal Data for 16. C18H42AlCl3P2Pt, Mr = 648.88, yellow block,
0.22 × 0.17 × 0.085 mm3, monoclinic space group C2/c, a =
31.069(8) Å, b = 10.862(3) Å, c = 17.407(5) Å, β = 118.672(3)°, V =
5154(2) Å3, Z = 8, ρcalcd = 1.672 g·cm−3, μ = 5.916 mm−1, F(000) =
2576, T = 173(2) K, R1 = 0.0172, wR2 = 0.0375, 6442 independent
reflections [2θ ≤ 56.74°] and 238 parameters.
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Crystal Data for 17. C18H42AlBr3P2Pt, Mr = 782.26, yellow block,
0.29 × 0.15 × 0.08 mm3, Monoclinic space group C2/c, a =
31.4522(17) Å, b = 10.9575(5) Å, c = 17.4716(9) Å, β = 118.731(3)°,
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Crystal Data for 27. C93H156Al2Br6P4Pt2, Mr = 2321.58, yellow
plate, 0.49 × 0.025 × 0.021 mm3, triclinic space group P1, a =
(22) Braunschweig, H.; Brenner, P.; Muller, A.; Radacki, K.; Rais, D.;
̈
̅
9.8756(11) Å, b = 11.2140(12) Å, c = 22.450(2) Å, α = 96.912(2)°, β
= 93.273(2)°, γ = 94.380(2)°, V = 2455.4(5) Å3, Z = 1, ρcalcd = 1.570
g·cm−3, μ = 5.411 mm−1, F(000) = 1166, T = 173(2) K, R1 = 0.0281,
wR2 = 0.0674, 9757 independent reflections [2θ ≤ 52.4°] and 445
parameters.
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Crystallographic data have been deposited with the Cambridge
Crystallographic Data Center as supplementary publication no.
CCDC-854859 (13), CCDC-854860 (14), CCDC-854861 (16),
CCDC-868866 (17), and CCDC-854862 (27). These data can be
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ASSOCIATED CONTENT
■
S
* Supporting Information
Cartesian coordinates of all optimized geometries as well as
CIF files giving crystallographic data of 13, 14, 16, 17, and 27.
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: h.braunschweig@mail.uni-wuerzburg.de. Fax: +49-(0)
931/31-85263. Phone: +49-(0)931/31-85260.
Notes
The authors declare no competing financial interest.
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