Syntheses and structures of terminal arylalumylene complexes

Article abstract of DOI:10.1002/anie.201310559

Terminal arylalumylene complexes of platinum [Ar-Al-Pt(PCy ₃)₂] (Ar=2,6-[CH(SiMe₃)₂] ₂C₆H₃ (Bbp) or 2,6-[CH(SiMe₃) ₂]₂-4-(tBu)C₆H₂ (Tbb)) have been synthesized either by the reaction of a dialumene-benzene adduct with [Pt(PCy₃)₂], or by the reduction of 1,2-dibromodialumanes Ar(Br)Al-Al(Br)Ar in the presence of [Pt(PCy₃)₂]. X-Ray crystallographic analysis reveals that the Al-Pt bond lengths of these arylalumylene complexes are shorter than the previously reported shortest Al-Pt distance. DFT calculations suggest that the Al-Pt bonds in the arylalumylene complexes have a significantly high electrostatic character. Quite a character: A terminal arylalumylene complex of platinum was obtained by the reaction of a dialumene-benzene adduct and [Pt(PCy₃)₂]. Reduction of 1,2-dibromodialumanes in the presence of [Pt(PCy₃)₂] also afforded the terminal arylalumylene complexes. DFT calculations suggest that the Al-Pt bonds in the arylalumylene complexes have a significantly high electrostatic character.

Full text of DOI:10.1002/anie.201310559

Angewandte
Chemie
DOI: 10.1002/anie.201310559
Main-Group Ligands
Syntheses and Structures of Terminal Arylalumylene Complexes**
Koichi Nagata, Tomohiro Agou, and Norihiro Tokitoh*
Dedicated to Professor Renji Okazaki on the occasion of his 77th birthday
Abstract: Terminal arylalumylene complexes of platinum [Ar-
Al-Pt(PCy3)2] (Ar = 2,6-[CH(SiMe3)2]2C6H3 (Bbp) or 2,6-
[CH(SiMe3)2]2-4-(tBu)C6H2 (Tbb)) have been synthesized
either by the reaction of a dialumene–benzene adduct with
[Pt(PCy3)2], or by the reduction of 1,2-dibromodialumanes
Ar(Br)Al-Al(Br)Ar in the presence of [Pt(PCy3)2]. X-Ray
ꢀ
crystallographic analysis reveals that the Al Pt bond lengths of
these arylalumylene complexes are shorter than the previously
Figure 1. Transition-metal complexes of Lewis-base-coordinated (A–C)
and Lewis-base-free (D) alumylenes. Ar=2,6-(iPr)2C6H3.
ꢀ
reported shortest Al Pt distance. DFT calculations suggest that
ꢀ
the Al Pt bonds in the arylalumylene complexes have
a significantly high electrostatic character.
aluminum moieties (i.e., D). Because the coordination of
Lewis bases may mask the intrinsic nature of the alumylene
ligands, the development of Lewis base-free alumylene
complexes is desirable so as to elucidate the bonding situation
between the alumylene and transition-metal moieties. Herein,
we report the syntheses and structures of platinum complexes
of arylalumylenes, which are the first examples of Lewis-base-
free alumylene complexes.
T
ransition-metal complexes of subvalent main-group-ele-
ment compounds attract considerable attention not only
because of their unique electronic structures, but also because
of their synthetic potential in organometallic chemistry. In
particular, complexes of group 13 metallylenes (:ER, E = B,
Al, Ga, In, and Tl) are expected to show particular bonding
interactions between the subvalent group 13 elements and
transition-metal fragments because these metallylenes pos-
sess a lone pair of electrons and two vacant p orbitals, and
may act as s-donor/p-acceptor ligands.[1] The chemistry of
borylene complexes has been extensively developed,[2]
whereas the examples of the heavier group 13 metallylene
complexes, having the formula of [M(ER)mLn] (R: anionic
monodentate ligands), have been limited for the gallium and
indium homologues, and have yet to be reported for
aluminum.[3] Although Lewis-base-coordinated terminal alu-
mylene complexes (e.g., A, B, and C; Figure 1) have been
synthesized as stable compounds,[4–7] there have been no
alumylene complexes featuring two-coordinate subvalent
Recently, we reported on the reactivity of the dialumene-
benzene adduct 2 as a synthetic equivalent of the diary-
[8,9]
=
ldialumene BbpAl AlBbp.
During the studies on the
reactivity of 2, the reaction of 2 and [Pt(PCy3)2] was
investigated with the expectation of trapping of the dialu-
mene as a p-dialumene complex of platinum.[10] The reaction
progress was monitored by 31P NMR spectroscopy, which
showed the formation of a mixture containing a new platinum
complex (dP = 69.9 ppm). Fractional crystallization of the
crude material from n-hexane at ꢀ358C yielded a small
amount (3%) of the arylalumylene complex 1a as air- and
moisture-sensitive dark red crystals (Scheme 1). The forma-
[*] K. Nagata, Dr. T. Agou, Prof. Dr. N. Tokitoh
Institute for Chemical Research, Kyoto University
Gokasho, Uji, Kyoto 611-0011 (Japan)
E-mail: tokitoh@boc.kuicr.kyoto-u.ac.jp
[**] This work was supported by JSPS KAKENHI (Nos. 22350017,
24550048, 24655028, and 24109013), Grants for Excellent Graduate
Schools, MEXT (Japan), and the “Molecular Systems Research”
project of RIKEN Advanced Science Institute. T.A. thanks the Kyoto
Technoscience Center and the Research Institute for Production
Development for the financial support. K.N. acknowledges the
support from Grants-in-Aid for JSPS Fellows from JSPS (No.
252926). The synchrotron radiation experiments were performed at
the BL38B1 beamline of the SPring-8 with the approval of the Japan
Synchrotron Radiation Research Institute (JASRI; proposal No.
2013A1183). We are grateful to Dr. K. Miura, Dr. S. Baba, and Dr. N.
Mizuno (JASRI) for the X-ray crystallographic analyses at the
SPring-8.
Scheme 1. Syntheses of the arylalumylene complexes 1a and 1b.
tion of 1a implies that 2 reacts as an arylalumylene source in
addition to the diaryldialumene synthon. After screening of
the reaction conditions, reduction of the 1,2-dibromodialu-
manes 3a[11] and 3b with KC8 in the presence of [Pt(PCy3)2]
was found to afford 1a and 1b, respectively, as the sole
products. After recrystallization from n-hexane at ꢀ358C, the
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201310559.
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arylalumylene complexes were obtained in moderate yields
(1a: 72%, 1b: 21%). The complexes 1a and 1b are stable up
to 79 and 1108C, respectively, in the solid state, but they
slowly decompose in solution, even at ꢀ358C, to give
complicated mixtures containing [Pt(PCy3)2] and PCy3.
In the 31P NMR spectra, 1a and 1b exhibit singlets
important.[6d,16] The calculated NBO corresponding to the
ꢀ
Al Pt bond is predominantly formed from the overlap of the
3s(Al) and 6s(Pt) orbitals [s(Al-Pt) = 0.87(3s3p0.03)Al +
0.50(6s6p0.036d0.02)Pt]. Meanwhile, the Pt!Al p-back dona-
tion interactions were identified as donor/acceptor interac-
tions, and the stabilization energies from the two 5d(Pt)!
3p(Al) p-back donations were estimated to be 19.86 and
4.54 kcalmolꢀ1 by using a second-order perturbation theory
accompanied by 195Pt satellites at d = 69.9 ppm (1JPPt
=
4015 Hz) and d = 69.8 ppm (1JPPt = 4033 Hz), respectively,
ꢀ
which are downfield shifted with respect to those of [Pt-
(PCy3)2] (d = 62.3 ppm, JPPt = 4160 Hz) and the structurally
analysis. The nature of the Al Pt bond in 1a was further
1
investigated in terms of the energy decomposition analy-
sis,[17,18] thus showing that the Al-Pt bonding interaction is
mainly electrostatic. The electrostatic interaction contributes
74.0% of the total attractive interactions between the BbpAl
and [Pt(PCy3)2] moieties. The breakdown of the Al-Pt orbital-
interaction energy into s and p components indicates that the
Pt!Al p-back donation significantly contributes to the
covalent bonding (s: 55.8%, p: 44.2%).
related carbonyl complex [(Cy3P)2Pt(CO)] (d = 63.7 ppm,
1JPPt = 4101 Hz).[12] Definite signals could not be observed in
the 27Al and 195Pt NMR spectra of 1a and 1b, probably
because of the signal broadening caused by the high quadru-
pole moment of the 27Al nuclei.
Molecular structures of 1a and 1b were determined by X-
ray crystallographic analyses, thereby showing that the
aluminum atoms are definitely two-coordinate and are
bound to the platinum atoms in a terminal fashion with C1-
Al1-Pt1 angles of 179.2(2) (1a) and 173.96(14)8 (1b;
Figure 2). The platinum centers adopt distorted trigonal-
In summary, the first Lewis-base-free terminal arylalu-
mylene complexes were obtained by two different routes: the
treatment of a dialumene–benzene adduct (2) with [Pt-
(PCy3)2] and the reduction of the 1,2-dibromodialumanes 3
ꢀ
in the presence of [Pt(PCy3)2]. The Al Pt bonds in the
arylalumylene complexes were shortened compared to the
ꢀ
previously reported Al Pt distances, thus indicating the
stronger bonding interactions between the alumylene and
platinum moieties. The DFT calculations suggested that the
ꢀ
Al Pt bonds in the arylalumylene complexes possess signifi-
cantly high electrostatic character and that the contribution of
the Pt!Al p-back donation to the covalent interactions is
comparable to that of the Al!Pt s-donation.
Figure 2. Molecular structures of a) 1a and b) 1b. Thermal ellipsoids
are drawn at the 50% probability level. Hydrogen atoms are omitted,
and the Bbp, Tbb, and Cy groups are shown in wireframe format for
clarity. Three Cy and two Me groups in 1a were disordered over two
positions (see the Supporting Information for details). Selected bond
lengths [ꢀ] and angles [8]: 1a: Al1–Pt1 2.2857(18), C1–Al1 2.001(6),
Pt1–P1 2.2828(17), Pt1–P2 2.2903(16); C1-Al1-Pt1 179.2(2), Al1-Pt1-P1
114.86(6), Al1-Pt1-P2 117.85(6), P1-Pt1-P2 127.29(6). 1b: Al1–Pt1
2.2829(13), C1–Al1 1.986(4), Pt1–P1 2.3071(9), Pt1–P2 2.2673(10); C1-
Al1-Pt1 173.96(14), Al1-Pt1-P1 119.14(4), Al1-Pt1-P2 109.20(4), P1-Pt1-
P2 131.56(4).
Experimental Section
All the manipulations were performed under a dry argon atmosphere
by using the Schlenk techniques and glove boxes. Solvents were
purified by the Ultimate Solvent System, Glass Contour Company[19]
(n-hexane) or by the bulb-to-bulb distillation from a potassium mirror
(C6D6 and mesitylene). [Pt(PCy3)2] was prepared according to
a literature.[20]
Reaction of 2 with [Pt(PCy3)2]: A solution of 2 (13.4 mg,
0.0124 mmol) and [Pt(PCy3)2] (17.4 mg, 0.0230 mmol) in mesitylene
(2 mL) was stirred at room temperature for 2.5 h and then at 508C for
2 h, thus affording a mixture containing 1a and [Pt(PCy3)2] in a ratio
of ca. 1.0:1.5. Small amounts of pure 1a (1.0 mg, 0.00085 mmol, 3%)
were obtained by fractional crystallization from n-hexane at ꢀ358C.
Reduction of 3a in the presence of [Pt(PCy3)2]: To a mesitylene
(5 mL) solution of 3a (13.2 mg, 0.013 mmol) and [Pt(PCy3)2]
(19.0 mg, 0.025 mmol) was added KC8 (3.8 mg, 0.028 mmol). The
mixture was stirred at room temperature for 4.5 h. After removal of
the solvents, the residue was extracted with n-hexane and filtered.
The filtrate was concentrated and stored at ꢀ358C to give 1a as dark
ꢀ
planar geometries. The Pt1 Al1 bonds of the arylalumylene
complexes [1a: 2.2857(18) ꢀ, 1b: 2.2829(13) ꢀ] are slightly
ꢀ
shortened compared to the shortest Pt Al distance previously
reported [2.327(2) ꢀ],[6d] most likely reflecting the decreased
coordination number of platinum as well as the difference in
the aluminum-bound substituents.[13]
To gain further information on the bonding situation in 1a
and 1b, density functional theory (DFT) calculations at the
M062X[14]/SDD[Pt]:6-311G(2df)[Al,P]:6-31G(d)[Si,C,H]
level of theory were performed on a real molecule of 1a. The
comparison of the optimized and experimental bond lengths
and angles of 1a shows that the DFT-optimized structure is
well-matched to that found in the single crystals. The natural
bond orbital (NBO) analysis[15] on the optimized geometry of
1a showed that the Al-Pt bond has a small Wiberg bond index
(0.59), thus indicating that the Al-Pt bond is highly ionic and
that the contribution of the covalent interaction is less
1
red crystals (22.2 mg, 0.019 mmol, 72%). m.p. 798C (dec.); H NMR
(600 MHz, C6D6): d = 0.29 (s, 36H, Si(CH3)3), 1.22–1.43 (m, 24H, Cy),
1.65–1.73 (m, 18H, Cy), 1.90–1.92 (m, 12H, Cy), 2.20–2.22 (m, 12H,
Cy), 2.75 (s, 2H, CH(SiMe3)2), 6.78 (d, 3J = 7.7 Hz, 2H, m-ArH),
7.08 ppm (t, 3J = 7.7 Hz, 1H, p-ArH); 13C{1H} NMR (151 MHz,
C6D6): d = 1.28 (s, SiMe3), 27.1 (s, C4(Cy)), 28.3 (virtual triplet, JCP
=
4.5 Hz, C2,6(Cy),), 31.2 (s, CH(SiMe3)2), 31.3 (s, 4JCPt = 24.1 Hz,
C3,5(Cy)), 41.3 (virtual triplet, JCP = 9.1 Hz, JCPt = 36.2 Hz, C1(Cy)),
2
123.9 (s, 4JCPt = 22.7 Hz, m-C(Ar)), 129.22 (s, p-C(Ar)), 149.4 (s, o-
C(Ar)), 160.0 ppm (t, 3JCP = 25.7 Hz, ipso-C(Ar)); 31P NMR
(120 MHz, C6D6): d = 69.9 ppm (s, 1JPPt = 4015 Hz); UV/vis
3882
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Chemie
(hexane): l = 447 (e 1600), 488 (e 1800) nm; UV/Vis (THF): l = 446 (e
1400), 489 (e 1500) nm; HRMS (DART-TOF, positive mode) m/z
calcd. for [C56H107AlP2Si4195Pt]+: 1175.6388; found: 1175.6412.
Macdonald, A. H. Cowley, J. Am. Chem. Soc. 1999, 121, 12113;
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1b: As described for the reduction of 3a, a mesitylene (5 mL)
solution of 3b (21.5 mg, 0.0193 mmol) and [Pt(PCy3)2] (29.1 mg,
0.0386 mmol) was treated with KC8 (5.3 mg, 0.039 mmol). After
workup and recrystallization, 1b was obtained as dark red crystals
(10.0 mg. 0.0082 mmol, 21%). m.p. 1108C (dec.); 1H NMR (600 MHz,
C6D6): d = 0.32 (s, 36H, Si(CH3)3), 1.20–1.44 (m, 24H, Cy), 1.35 (s,
9H, C(CH3)3), 1.68–1.72 (m, 18H, Cy), 1.90–1.92 (m, 12H, Cy), 2.21–
2.23 (m, 12H, Cy), 2.72 (s, 2H, CH(SiMe3)2), 6.81 ppm (s, 2H, m-
ArH); 13C{1H} NMR (151 MHz, C6D6): d = 1.30 (s, SiMe3), 27.1 (s,
C4(Cy)), 28.3 (virtual triplet, JCP = 4.6 Hz, C2,6(Cy)), 31.0 (s, CH-
(SiMe3)2), 31.2 (s, C3,5(Cy)), 31.4 (s, CMe3), 34.5 (s, CMe3), 41.4
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(virtual triplet, JCP = 8.3 Hz, C1(Cy)), 121.3 (s, 4JCPt = 22.7 Hz, m-
3
C(Ar)), 149.0 (s, p-C(Ar)), 151.1 (s, o-C(Ar)), 157.0 ppm (t, JCP
=
27.2 Hz, ipso-C(Ar)); 31P NMR (243 MHz, C6D6): d = 69.8 ppm (s,
1JPPt = 4033 Hz); UV/vis (hexane): l = 447 (e 1700), 483 (e 1900) nm;
UV/Vis (THF): l = 447 (e 1500), 483 (e 1600) nm; HRMS (DART-
TOF, positive mode) m/z calcd. for [C60H115AlP2Si4195Pt]+: 1231.7021;
found: 1231.7026.
Single crystals of 1a and 1b·hexane were obtained by cooling
their saturated solutions in n-hexane to ꢀ358C. The crystal data of 1a
was collected on a Rigaku Saturn 70 CCD diffractometer with
a VariMax Mo Optic System using a MoKa radiation (l = 0.71070 ꢀ),
while that of 1b·hexane was collected at the BL38B1 beamline of the
SPring-8 using an ADSC Quantum 315 CCD detector and Si(111)-
monochromated X-ray radiation (l = 0.85000 ꢀ). The structures were
solved with the Shelx program package.[21] Crystal data for 1a:
monoclinic, space group P21/c, ꢀ1738C, a = 13.1525(3), b =
19.5941(4), c = 24.5674(5) ꢀ, b = 96.2678(15), V= 6293.5(2) ꢀ3, Z =
4, m = 2.402 mmꢀ1 (l = 0.71070 ꢀ), 2.088 < q < 25.508, Rint = 0.0845,
completeness to qmax 99.9%, 760 parameters refined, R1 (I > 2s(I)) =
0.0456, wR2 (all data) = 0.1110, GOF = 1.018, largest diff. peak and
hole 1.917 and ꢀ1.714 eꢀꢀ3. Crystal data for 1b·hexane: triclinic,
space group P-1, ꢀ1708C, a = 12.5246(1), b = 13.9973(2), c =
21.9295(3) ꢀ, a = 89.9651(6), b = 83.0595(5), g = 73.3812(6)8, V=
3654.56(8) ꢀ3, Z = 2, m = 0.243 mmꢀ1 (l = 0.85000 ꢀ), 2.058 < q <
31.008, Rint = 0.0507, completeness to qmax 99.0%, 683 parameters
refined, R1 (I > 2s(I)) = 0.0423, wR2 (all data) = 0.1172, GOF = 1.086,
largest diff. peak and hole 1.253 and ꢀ2.204 eꢀꢀ3. CCDC-948098 (1a)
and 948113 (1b·hexane) contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.
uk/data_request/cif.
Received: December 5, 2013
Revised: January 17, 2014
Published online: March 11, 2014
Keywords: aluminum · density functional calculations · metal–
.
metal interactions · platinum · structure elucidation
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