Diboran(4)yl platinum(II) complexes

Article abstract of DOI:10.1021/ic400641a

The platinum diboran(4)yl complexes 1-3 have been prepared by the selective oxidative addition of one B-Hal bond in aryl-substituted diboranes(4) Hal ₂B₂Ar₂ (Hal = Cl, Ar = mes, dur; Hal = I, Ar = mes). Because of the electron deficiency of the remote B2 atom, all species show a rare dative Pt-B bonding interaction, whose magnitude is strongly dependent on the nature of the halide substituent.

Full text of DOI:10.1021/ic400641a

Communication
pubs.acs.org/IC
Diboran(4)yl Platinum(II) Complexes
Holger Braunschweig,* Alexander Damme, and Thomas Kupfer
Institut fur Anorganische Chemie, Julius-Maximilians Universitat Wurzburg, Am Hubland, D-97074 Wurzburg, Germany
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* Supporting Information
Chart 1. Possible Coordination Modes of the Diboran(4)yl
Ligand (Ar = mes, dur; R = Et, iPr; Hal = Br, I)
ABSTRACT: The platinum diboran(4)yl complexes 1−3
have been prepared by the selective oxidative addition of
one B−Hal bond in aryl-substituted diboranes(4)
Hal2B2Ar2 (Hal = Cl, Ar = mes, dur; Hal = I, Ar =
mes). Because of the electron deficiency of the remote B2
atom, all species show a rare dative Pt−B bonding
interaction, whose magnitude is strongly dependent on the
nature of the halide substituent.
the dative Pt−B bonding interaction increases with increasing
electron deficiency at boron as Cl < Br < I.
The oxidative addition of one B−Hal bond of Hal2B2Ar2 (Hal
= Cl, Ar = mes, dur; Hal = I, Ar = mes) proceeds readily at room
temperature within minutes upon reaction with [Pt(PEt3)3] in a
pentane solution (Scheme 1). The transformations show high
ecently, recognition of the Suzuki−Miyaura cross-coupling
Rreaction as part of the Nobel Prize award in 2010
emphasized the significance of boron reagents in organic
synthesis.1,2 Concomitantly, research in transition-metal boryl
complexes considerably increased because they function as
important intermediates in the borylation of organic substrates
via hydroboration,3 C−H activation,4 and diboration reactions.5
The latter process makes use of diboranes(4) as the boron
source, and initially proceeds by B−B bond activation to afford
cis-bisboryl complexes.5 However, our latest results have shown
that the B−B bond is not the only possible reaction site in
diboranes(4), and halide-substituted derivatives preferably react
with low-valent platinum phosphines by oxidative addition of the
B−Hal bond and formation of diboran(4)yl6 and diboran(4)-
1,2-diyl species.7 Notable exceptions are (i) Cl2B2(NMe2)2,
which shows a rather poor selectivity,8 and (ii) B2F4, which reacts
exclusively at the B−B linkage.9
Thus, aryl-substituted diboranes(4) Br2B2Ar2 (Ar = mes, dur;
mes = mesityl, dur = duryl) are converted into platinum
diboran(4)yls A with high selectivity upon reaction with
[Pt(PEt3)3], which feature a rare dative Pt−B bonding
interaction.6a,c Such an interaction is not present in the
diboran(4)yl complexes B, which were obtained via B−Hal
bond activation in Hal2B2(NMe2)2 (Hal = Br, I).6a,b Here, π
donation from the NMe2 substituent effectively lowers the
electron deficiency of the second boron center, and no further
stabilization is required. A third coordination mode (Chart 1)
was realized for the iodine derivative I2B2(NMe2)2, which could
be converted to the diboran(4)-1,2-diyl species C under forcing
conditions (165 °C, 10 min).7
Scheme 1. Synthesis of Diboran(4)yl Complexes 1−3
selectivity, and only the diboran(4)yl complexes 1−3 and the
diborane−phosphine adducts Hal2B2Ar2·PEt3 are evident in the
11B NMR spectra of the reaction mixtures.12 Because of the
release of 1 equiv of PEt3, which readily forms Lewis acid−base
adducts with electrophilic diboranes(4), 2 equiv of Hal2B2Ar2 are
required to reach quantitative conversion. After recrystallization
from concentrated CH2Cl2/pentane mixtures, the diboran(4)yls
1−3 are isolated as orange/red crystals in 24%, 28%, and 44%
yield, respectively. Higher yields were significantly thwarted by
the rather difficult removal of the Hal2B2Ar2·PEt3 byproducts.
The formation of diboran(4)yl complexes and the presence of
a dative Pt−B bonding interaction are clearly obvious in the
solution NMR spectra of 1−3. Thus, two doublets are found in
the 31P NMR spectra of 1−3 (1, δ 8.07, 11.4; 2, δ 7.70, 11.2; 3, δ
1.40, 2.89) reminiscent of an AB spin system (Table 1). The 2JP−P
(1, 333 Hz; 2, 326 Hz; 3, 226 Hz) and 1JP−Pt coupling constants
(2812−2984 Hz) are highly characteristic for platinum(II)
complexes with a trans configuration of the phosphine ligands.
All 31P NMR parameters strongly resemble those found for the
related species trans-[(Et3P)2Pt(Br){B(mes)B(mes)(Br)}]
(4)6a and trans-[(Et3P)2Pt(Br){B(dur)B(dur)(Br)}] (5; Table
1).6c The unsymmetrical nature of 1−3 is further illustrated by
Subsequently, we wondered if and how the nature of the halide
influences the dative Pt−B bonding interaction in type A
complexes. We now present our results regarding the reactivity of
11b
Cl2B2Ar2 (Ar = mes, dur)10,11a and I2B2mes2
toward
[Pt(PEt3)3], which readily afforded the diboran(4)yl complexes
trans-[(Et3P)2Pt(Cl){B(mes)B(mes)(Cl)}] (1), trans-
[(Et3P)2Pt(Cl){B(dur)B(dur)(Cl)}] (2), and trans-[(Et3P)2Pt-
(I){B(mes)B(mes)(I)}] (3). We will show that the strength of
Received: March 15, 2013
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ic400641a | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
Table 1. 11B and 31P NMR Spectroscopic Parameters of
Diboran(4)yl Complexes 1−5 in Solution (δ in ppm)
δ(11B)
δ(11B)
δ(31P)
δ(31P)
[B1]
[B2]
[P1]
[P2]
a
b
a
b
a
1
2
3
4
5
61.5
58.0
51.0
56.6
108
109
112
107
109
8.07 (1JP−Pt = 2958
11.4 (1JP−Pt = 2912
Hz; 2JP−P = 333 Hz)
Hz; 2JP−P = 333 Hz)
7.70 (1JP−Pt = 2984
Hz; 2JP−P = 326 Hz)
11.2 (1JP−Pt = 2929
Hz; 2JP−P = 326 Hz)
1.40 (1JP−Pt = 2842
2.89 (1JP−Pt = 2812
Hz; 2JP−P = 226 Hz)
Hz; 2JP−P = 226 Hz)
5.54 (1JP−Pt = 2921
Hz; 2JP−P = 333 Hz)
8.35 (1JP−Pt = 2869
Hz; 2JP−P = 333 Hz)
53.6
5.55 (1JP−Pt = 2931
8.17 (1JP−Pt = 2873
Hz; 2JP−P = 327 Hz)
Hz; 2JP−P = 327 Hz)
a
b
Figure 2. Molecular structure of the duryl-substituted diboran(4)yl
complex 2 in the solid state. Thermal ellipsoids are displayed at the 50%
probability level. For clarity, hydrogen atoms and thermal ellipsoids of
the carbon atoms have been omitted. Selected bond lengths are
presented in Table 2 and bond angles in Table 3.
CD2Cl2. C6D6.
11B NMR spectroscopy (Table 1), which reveals two broad
(fwhm 1567−1830 Hz) resonances for B1 (Pt−B; 1, δ 108; 2, δ
109; 3, δ 112) and B2 (B-Hal; 1, δ 61.5; 2, δ 58.0; 3, δ 51.0).
Similar to the results of 4 and 5,6a,c the low-field signal (B1) is
indicative of a platinum-bound boryl group. By contrast, the B2
signal appears at significantly higher field than the precursor
diboranes(4) Hal2B2Ar2 (δ 85−89),11 thus verifying the dative
Pt−B2 bonding interaction even in solution.6a,c The 11B NMR
signals of B2 already imply a considerable influence of the halide
nature on the strength of this dative bonding interaction, which
evidently becomes stronger in the order Cl (1/2: δ 61.5/58.0) <
Br (4: δ 56.6) < I (3: δ 51.0).
Table 2. Selected Bond Lengths (Å) of Diboran(4)yl
Complexes 1−8 in the Solid State
Pt1−B1
Pt1−B2
B1−B2
Pt1−X1
B2−X2
1
2.035(6)
2.038(7)
2.057(4)
2.038(3)
2.027(9)
2.073(5)
2.087(4)
2.069(7)
2.570(6)
2.547(3)
2.504(4)
2.531(3)
2.506(8)
1.657(8)
1.648(4)
1.649(6)
1.649(4)
1.67(1)
2.501(1)
1.854(6)
1.853(3)
2.260(4)
2.027(3)
2.016(9)
1.963(6)
2.234(4)
2.242(8)
2
2.5044(6)
2.7827(3)
2.621(4)
3
46a
56c
66a
76b
87
2.6276(8)
2.6470(5)
2.8226(3)
2.7990(18)
Further evidence for this result comes from X-ray diffraction
studies on 1−3 (Figures 1 and 2 and Tables 2 and 3).
1.742(8)
1.738(6)
1.725(11)
Table 3. Selected Bond Angles (deg) of Diboran(4)yl
Complexes 1−5 in the Solid State
ΣPt
P1−Pt1−P2 X1−Pt1−B1 Pt1−B1−B2 B1−Pt1−B2
1
2
3
4
5
360.15
361.10
360.13
360.12
360.91
170.25(1)
166.35(2)
168.70(3)
169.94(2)
167.78(7)
176.5(1)
172.9(1)
175.4(1)
176.2(1)
174.3(2)
87.4(1)
86.7(1)
84.2(2)
86.0(1)
84.7(5)
40.2(1)
40.2(1)
40.9(1)
40.5(1)
41.7(3)
nature of the halide substituent and increases in the order Cl < Br
< I. By contrast, the influence of the aryl substituent is almost
negligible, as deduced from the direct comparison of the pairs 1/
2 (Cl) and 4/5 (Br), which feature similar values for the mesityl
and duryl derivatives.
Figure 1. Molecular structure of the mesityl-substituted diboran(4)yl
complexes 1 (left) and 3 (right) in the solid state. Thermal ellipsoids are
displayed at the 50% probability level. For clarity, hydrogen atoms and
thermal ellipsoids of the carbon atoms have been omitted. Selected bond
lengths are presented in Table 2 and bond angles in Table 3.
In our initial communication on this topic, we evaluated the
nature of this dative bonding interaction in detail by a
combination of spectroscopy, X-ray diffraction, and density
functional theory calculations.6a In agreement with the present
study, our results clearly suggested an electron-precise bonding
situation for the diboran(4)yl ligand with two-center-two-
electron Pt1−B1 and B1−B2 bonds and a dative Pt1−B2
bonding interaction. Particularly, short Pt1−B1 and B1−B2
bond lengths in 1−5 in comparison to the classical diboran(4)yl
species B of trans-[(R3P)2Pt(X) {B(NMe2)B(NMe2)X}] (6, R =
iPr, X = Br;6a 7, R = iPr, X = I;6b 8, R = Et, X = I7) (Table 2)
without any dative bonding interaction support this picture and
most likely rule out a three-center-two-electron bonding mode of
the Pt1−B1−B2 fragment, which is expected to result in a
significant lengthening of these bonds.
Accordingly, short Pt1−B2 bond distances [1, 2.570(6) Å; 2,
2.547(3) Å; 3, 2.504(4) Å] clearly imply the presence of a dative
Pt1−B2 bonding interaction, which is most pronounced for the
iodine derivative 3. A comparison of the dative Pt1−B2 bonding
interaction with the corresponding Pt1−B1 boryl bond [1,
2.035(6) Å; 2, 2.038(7) Å; 3, 2.057(4) Å] also shows that the
strongest effect is present in the iodine species 3. Thus, the bond
elongation between the Pt1−B1 and Pt1−B2 bonds significantly
decreases from 26.2% in 1 to 21.7% in 3, which might also be
compared to the values found in the related bromine complexes 4
(24.1%)6a and 5 (23.6%).6c Obviously, the strength of the dative
Pt−B bonding interaction is directly related to the electro-
philicity of the boron center B2, which strongly depends on the
The effect of the aryl substituent on the Pt1−Hal1 bond
lengths and thus on the trans influence of the diboran(4)yl ligand
B
dx.doi.org/10.1021/ic400641a | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
Jove, D.; Batsanov, A. S.; Howard, J. A. K.; Perutz, R. N.; Marder, T. B.
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is also insignificant, as shown by the molecular structures of 1/2
[Cl; 1, 2.501(1) Å; 2, 2.504(4) Å] and 4/5 [Br; 4, 2.611(4) Å; 5,
2.625(8) Å].6a,c
All other structural parameters are unremarkable and strongly
resemble those of 46a and 5.6c The platinum centers in 1−3 adopt
a distorted square-pyramidal geometry with the B2 atom in the
apical position. At the same time, the basis defined by the atoms
Pt1, B1, Hal1, P1, and P2 remains almost regular square-planar
with ΣPt of approximately 360° (Table 3), while the Pt1−B1−B2
bond angles are rather acute and successively change from the
chlorine derivatives 1 [87.4(1)°] and 2 [86.7(5)°] to 84.2(2)° in
the iodine analogue 3. The Pt1−B1 [1, 2.035(6) Å; 2, 2.038(7)
Å; 3, 2.057(4) Å], Pt1−P1/2 [1, 2.3208(5) Å/2.3507(5) Å; 2,
2.3161(6) Å/2.3478(6) Å; 3, 2.3258(9) Å/2.3668(9) Å], and
B1−B2 [1, 1.657(8) Å; 2, 1.648(4) Å; 3, 1.649(6) Å] bond
lengths are very similar in all species 1−5 and do not show any
dependency on the nature of the halide or aryl ligands (Table 2).
In this contribution, we have presented further details on the
reactivity of aryl-substituted diboranes(4) toward low-valent
platinum species. The diboran(4)yl complexes 1−3 derived from
the electrophilic diboranes(4) Hal2B2Ar2 (Hal = Cl, Ar = mes,
dur; Hal = I, Ar = mes) also show a rare dative Pt−B bonding
interaction between the platinum(II) center and the remote
boron atom B2 both in solution and in the solid state. This
interaction is considerably influenced by the nature of the halide
substituent and strengthens in the order Cl < Br < I.
(8) Curtis, D.; Lesley, M. J. G.; Norman, N. C.; Orpen, A. G.; Starbuck,
J. J. Chem. Soc., Dalton Trans. 1999, 1687.
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Timms, P. L.; Whittell, G. R.; Marder, T. B. Chem. Commun. 1998, 319.
(10) Cl2B2Ar2 was characterized in the solid state by X-ray diffraction.
Data and a graphical representation can be found in the Supporting
Information.
(11) (a) Hunold, R. Ph.D. Thesis, Philipps-Universitat Marburg,
̈
Marburg, Germany, 1998; (b) Hommer, H.; Noth, H.; Knizek, J.;
̈
Ponikwar, W.; Schenk-Kirchner, H. Eur. J. Inorg. Chem. 1998, 1519.
(12) Braunschweig, H.; Damme, A.; Jimenez-Hall, O. C.; Kupfer, T.;
Radacki, K. Angew. Chem., Int. Ed. 2012, 51, 6267.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures and crystallographic details, graphical
representation of the molecular structure of Cl2B2dur2, and
crystallographic data of 1−3 (CCDC 928667−928669) and
Cl2B2dur2 (CCDC 928691) in CIF format. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: h.braunschweig@uni-wuerzburg.de. Fax: +49 931-31-
84623.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We are grateful to the DFG for financial support.
■
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dx.doi.org/10.1021/ic400641a | Inorg. Chem. XXXX, XXX, XXX−XXX