Conversion of trans-bromoboryl platinum complexes into their cis-analogues upon treatment with chelating bisphosphines

Article abstract of DOI:10.1021/om700976t

Oxidative addition of B-halogen bonds to Pt(O) centers generally leads to corresponding trans-halo(boryl)complexes. Here, we report on a series of unprecedented ci-bromo(boryl)species of the type cw-[(dcpe)Pt{B(Br)R)(Br)], which were obtained from corresponding complexes trans-[(Cy₃P)₂Pt(B(Br)R)(Br)] (R = Fc, Pip, Mes) (Cy = cyclohexyl; Fc = ferroceny; Pip = piperidyl, Mes = mesityl) upon treatment with the chelating bisphosphine dcpe (=bis(dicyclohexyl)phosphinoethane). In CH ₂Cl₂ solution and in the presence of PCy3, one of these products, cw-[(dcpe)Pt{B(Br)Mes)(Br)], is converted into the corresponding chloroboryl complex cis-[(dcpe)Pt{B(Cl)Mes)(Br)]. All compounds were fully characterized by multinuclear NMR spectroscopy and single-crystal X-ray analyses.

Full text of DOI:10.1021/om700976t

418
Organometallics 2008, 27, 418–422
Conversion of trans-Bromoboryl Platinum Complexes into their
cis-Analogues upon Treatment with Chelating Bisphosphines
Holger Braunschweig,* Ralph Leech, Daniela Rais, Krzysztof Radacki, and
Katharina Uttinger
Institut für Anorganische Chemie, Julius-Maximilians-UniVersität Würzburg,
Am Hubland, 97074 Würzburg, Germany
ReceiVed October 1, 2007
Oxidative addition of B-halogen bonds to Pt(0) centers generally leads to corresponding trans-
halo(boryl)complexes. Here, we report on a series of unprecedented cis-bromo(boryl)species of
the type cis-[(dcpe)Pt{B(Br)R)(Br)], which were obtained from corresponding complexes trans-
[(Cy3P)2Pt{B(Br)R}(Br)] (R ) Fc, Pip, Mes) (Cy ) cyclohexyl; Fc ) ferroceny; Pip ) piperidyl, Mes
) mesityl) upon treatment with the chelating bisphosphine dcpe ()bis(dicyclohexyl)phosphinoethane).
In CH2Cl2 solution and in the presence of PCy3, one of these products, cis-[(dcpe)Pt{B(Br)Mes)(Br)], is
converted into the corresponding chloroboryl complex cis-[(dcpe)Pt{B(Cl)Mes)(Br)]. All compounds were
fully characterized by multinuclear NMR spectroscopy and single-crystal X-ray analyses.
Chart 1. Different Types of Pt Complexes
Introduction
Boryl complexes of platinum play an important role as key
intermediates in the catalyzed diboration of alkynes, alkenes,
and other unsaturated organic compounds,1 as well as in the
C-H activation of organic substrates,2 and have thus attracted
considerable attention.3 It has been shown that in square-planar
complexes of the type [MR2P2] (M ) Ni, Pd, Pt, P ) phosphine)
the reductive elimination of R2, which is a crucial step in the
aforementioned catalytic cycles, is facilitated by the mutual cis-
arrangement of the two ligands R.4 A corresponding trans-
disposition, however, usually precludes the reductive elimination
of R2 or requires at least much harsher conditions.4c Therefore,
cis-complexes should be advantageous for possible applications
in homogeneous catalysis. The oxidative addition of B-H,
B-B, and B-E (E ) main group element) bonds to low-valent
transition metals is a commonly used method for the synthesis
of boryl complexes. In the case of reactions of B-B, B-Si, or
B-Sn bonds with Pt(0) species a cis-disposition is achieved,
for example in complexes cis-[(Ph3P)2Pt(BR2)2] (A; R2 ) Cat
()1,2-O2-C6H4), Pin ()2,3-O2-2,3-Me2-C4H6), F2, (Cl, NMe2)),4a,5
cis-[(Ph3P)2Pt(BNMe2)E] (E ) SnMe3, GeMe3),6 or cis-[(R3P)2-
Pt(BPin)(SiMe2Ph)] (R3 ) Me3, Me2Ph, Et3)7 (Chart 1).
The oxidative addition of boron-halogen (halogen ) Cl,
Br, I) bonds to zerovalent platinum species, however,
* Corresponding author. E-mail: h.braunschweig@mail.uni-wuerzburg.de.
Fax: (+49)931-888-4623. Tel: (+49)931-888-5260.
(1) (a) Beletskaya, I.; Pelter, A. Tetrahedron 1997, 53, 4957–5026. (b)
Burgess, K.; Ohlmeyer, M. J. Chem. ReV. 1991, 91, 1179–1191. (c) Crudden,
C. M.; Edwards, D. Eur. J. Org. Chem. 2003, 4695–4712. (d) Fu, G. C.;
Evans, D. A.; Muci, A. R. AdV.Catal. Processes 1995, 1, 95–121. (e)
Ishiyama, T.; Miyaura, N. J. Organomet. Chem. 2000, 611, 392–402. (f)
Ishiyama, T.; Miyaura, N. Chem. Rec. 2004, 3, 271–280. (g) Marder, T. B.;
Norman, N. C. Top. Catal. 1998, 5, 63–73. (h) Braunschweig, H.; Kupfer,
T.; Lutz, M.; Radacki, K.; Seeler, F.; Sigritz, R. Angew. Chem., Int. Ed.
2006, 45, 4048–4051.
(2) (a) Cho, J. Y.; Tse, M. K.; Holmes, D.; Maleczka, R. E., Jr.; Smith,
M. R. Science 2002, 295, 305–308. (b) Coventry, D. N.; Batsanov, A. S.;
Goeta, A. E.; Howard, J. A. K.; Marder, T. B.; Perutz, R. N. Chem. Commun.
2005, 2172–2174. (c) Ishiyama, T.; Nobuta, Y.; Hartwig, J. F.; Miyaura,
N. Chem. Commun. 2003, 2924–2925. (d) Ishiyama, T.; Miyaura, N. J.
Organomet. Chem. 2003, 680, 3–11. (e) Ishiyama, T.; Miyaura, N. Pure
Appl. Chem. 2006, 78, 1369–1375. (f) Mkhalid, I. A. I.; Conventry, D. N.;
Albesa-Jove, D.; Batsanov, A. S.; Howard, J. A. K.; Perutz, R. N.; Marder,
T. B. Angew. Chem., Int. Ed. 2006, 45, 489–491. (g) Murphy, J. M.;
Lawrence, J. D.; Kawamura, K.; Incarvito, C.; Hartwig, J. F. J. Am. Chem.
Soc. 2006, 128, 13684–13685.
(5) (a) Curtis, D.; Lesley, M. J. G.; Norman, N. C.; Orpen, A. G.;
Starbuck, J. J. Chem. Soc., Dalton Trans. 1999, 1687–1694. (b) Iverson,
C. N.; Smith, M. R. J. Am. Chem. Soc. 1995, 117, 4403–4404. (c) Lesley,
G.; Nguyen, P.; Taylor, N. J.; Marder, T. B.; Scott, A. J.; Clegg, W.;
Norman, N. C. Organometallics 1996, 15, 5137–5154. (d) Lu, N.; Norman,
N. C.; Orpen, A. G.; Quayle, M. J.; Timms, P. L.; Whittell, G. R. J. Chem.
Soc., Dalton Trans. 2000, 4032–4037. (e) Kerr, A.; Marder, T. B.; Norman,
N. C.; Orpen, A. G.; Quayle, M. J.; Rice, C. R.; Timms, P. L.; Whittell,
G. R. Chem. Commun. 1998, 319–320. (f) Iverson, C. N.; Smith, M. R.
Organometallics 1996, 15, 5155–5165. (g) Ishiyama, T.; Matsuda, N.;
Murata, M; Ozawa, F.; Suzuki, A.; Miyaura, N. Organometallics 1996,
15, 713–720. (h) Clegg, W.; Johann, T. R. F.; Marder, T. B.; Norman, N. C.;
Orpen, A. G.; Peakman, T. M.; Quayle, M. J.; Rice, C. R.; Scott, A. J.
J. Chem. Soc., Dalton Trans. 1998, 1431–1438.
(3) (a) Aldridge, S.; Coombs, D. L. Coord. Chem. ReV. 2004, 248, 535–
559. (b) Braunschweig, H. Angew. Chem., Int. Ed. 1998, 37, 1786–1801.
(c) Braunschweig, H.; Kollann, C.; Rais, D. Angew. Chem., Int. Ed. 2006,
45, 5254–5274. (d) Irvine, G. J.; Lesley, M. J. G.; Marder, T. B.; Norman,
N. C.; Rice, C. R.; Robins, E. G.; Roper, W. R.; Whittell, G. R.; Wright,
L. J. Chem. ReV. 1998, 98, 2685–2722. (e) Braunschweig, H.; Colling, M.
Coord. Chem. ReV. 2001, 223, 1–51.
(4) (a) Clegg, W.; Lawlor, F. J.; Lesley, G.; Marder, T. B.; Norman,
N. C.; Orpen, A. G.; Quayle, M. J.; Rice, C. R.; Scott, A. J.; Souza, F. E. S.
J. Organomet. Chem. 1998, 550, 183–192. (b) Komiya, S.; Abe, Y.;
Yamamoto, A.; Yamamoto, T. Organometallics 1983, 2, 1466–1468. (c)
Crabtree Robert, H. The Organometallic Chemistry of the Transition Metals;
John Wiley & Sons, Inc: Hoboken, NJ, 2005; 4th ed., pp 173–174.
(6) Habereder, T.; Noeth, H. Appl. Organomet. Chem. 2003, 17, 525–
538.
(7) Sagawa, T.; Asano, Y.; Ozawa, F. Organometallics 2002, 21, 5879–
5886.
10.1021/om700976t CCC: $40.75
2008 American Chemical Society
Publication on Web 01/16/2008

ConVersion of trans-Bromoboryl Platinum Complexes
Organometallics, Vol. 27, No. 3, 2008 419
Scheme 1. Conversion of trans-Bromoboryl Complexes into
their cis-Analogues by Addition of dcpea
generally leads to corresponding trans-platinum(halo)boryls,
e.g., trans-[(R3P)2Pt(BCl2)(Cl)] (R3 ) Me3, Me2Ph, MePh2,
Ph3),8 trans-[(Ph3P)2Pt(BR2)(Cl)] (R2 ) Cat, (Cl, NMe2)),4a,5a
trans-[(Cy3P)2Pt{B(Br)Fc}(Br)] (B; Cy ) cyclohexyl; Fc )
ferrocenyl),9 and trans-[(Cy3P)2Pt(BI2)(I)].10 Recent compu-
tational11 and experimental12 studies on these systems have
proven the R2B ligand to exert an exceptionally strong trans-
influence. Moreover, the oxidative addition of B-Br bonds
in particular allowed for the realization of novel Pd and Pt
complexes with boron-centered ligands in highly unusal
coordination modes including heterodinuclear13 and cationic
borylene complexes,12a,14 base-stabilized borylenes,15 bridg-
ing boryl complexes,16 and iminoboryl complexes.17
a
(a) R ) Fc; C6H6, RT; 1 f 2; (b) R ) Pip; Tol, 80 °C; 3 f 4; (c)
R ) Mes; C6H6, 45 °C; 5 f 6.
In the present paper we report on the reactivity of trans-
bromo(boryl) complexes of platinum toward chelating bispho-
sphines, thus establishing a fully characterized series of
unprecedented cis-bromo(boryl) species.
In continuation of our work in this area, we wondered if
corresponding cis-halo(boryl) complexes were accessible, em-
ploying chelating bisphosphine ligands. Access to these target
compounds may be achieved by (i) oxidative addition of
haloboranes to zerovalent species [(PP)PtL] (PP ) bisphosphine,
L ) labile ligand) or (ii) subsequent conversion of divalent
complexes trans-[(R3P)2Pt(BR2)(Hal)] upon reaction with suit-
able bisphosphines PP. For the higher homologues of B, i.e.,
Al, Ga, and In, the former method was successfully applied to
the synthesis of complexes of the type cis-[(dcpe)Pt(ER2)(R)]
(dcpe ) bis(dicyclohexylphosphino)ethane; E ) Al, Ga, In; R
) CH2CMe3, CH2SiMe3).18 However, attempts to exchange the
phosphines in trans-[(Me3P)2Pt(BCl2)(Cl)] (C) with dppm
()bis(diphenylphosphino)methane) led to cleavage of the Pt-B
bond and formation of cis-[(Me3P)2Pt(dppm)]Cl2 (D),5a although
similar trans–cis conversions were successfully achieved with
different sets of Pt-bound ligands (e.g., [(PP)Pt(Cl)CF3], [(dppe)-
Pt(Cl)(2-C5H4N)]; dppe ) bis(diphenylphosphino)ethane).19
Moreover, it was shown that the two monodentate phosphines
in complexes of the type cis-[(Ph3P)2Pt(BR2)2] (A) can be easily
exchanged by a chelating bisphosphine with retention of the
cis-geometry, yielding, for example, cis-[(PP)Pt(BCat)2] (E).5d
Results and Discussion
Synthesis and NMR Spectroscopic Characteristics. The
reaction of the platinum boryl complex trans-[(Cy3P)2-
Pt{B(Br)Fc}(Br)] (1) with dcpe was investigated by means of
multinuclear NMR spectroscopy. After 6 h the consumption of
the starting materials of the 1:1 mixture, formation of free PCy3,
and the appearance of two new signals could be observed in
the 31P{1H} NMR spectrum. These spectroscopic findings
indicate the exchange of PCy3 by the chelating bisphosphine
and the conversion of the trans-boryl complex 1 into a related
cis-species, with different chemical environments for the two
phosphorus nuclei (Scheme 1). Compared to the starting boryl
complex (δ (31P) ) 21.5 ppm; 1JP-Pt ) 2892 Hz)9 the two signals
are downfield shifted, and the broad resonance at 65.0 ppm
(1JP-Pt ) 1098 Hz) can be assigned to the phosphorus atom in
trans-position to the boron center, while the sharp peak at 52.0
ppm (1JP-Pt ) 4140 Hz) arises from the phosphorus atom trans
to bromide. The new compound was isolated in 71% yield as
an orange solid, and single crystals suitable for X-ray diffraction
analysis, which confirmed the expected constitution of cis-
[(dcpe)Pt{B(Br)Fc)(Br)] (2) (Vide infra, Figure 1), could be
obtained by slow evaporation of a toluene solution of the
reaction mixture.
(8) Charmant, J. P. H.; Fan, C.; Norman, N. C.; Pringle, P. G. Dalton
Trans. 2007, 114–123.
(9) Braunschweig, H.; Radacki, K.; Rais, D.; Seeler, F. Organometallics
2004, 23, 5545–5549.
(10) Braunschweig, H.; Radacki, K.; Uttinger, K. Inorg. Chem. 2007,
46, 8796–8800.
In contrast to the formation of 2, the corresponding reaction
of the aminoboryl species trans-[(Cy3P)2Pt{B(Br)Pip}(Br)] (3)
with dcpe in toluene is much slower, and heating at 80 °C for
7 days is required to reach completion of the phosphine
exchange with concomitant trans–cis rearrangement. In the
(11) Zhu, J.; Lin, Z.; Marder, T. B. Inorg. Chem. 2005, 44, 9384–9390.
(12) (a) Braunschweig, H.; Radacki, K.; Rais, D.; Scheschkewitz, D.
Angew. Chem., Int. Ed. 2005, 44, 5651–5654. (b) Braunschweig, H.;
Brenner, P.; Mueller, A.; Radacki, K.; Rais, D.; Uttinger, K. Chem.-Eur.
J. 2007, 13, 7171–7176.
(13) (a) Braunschweig, H.; Radacki, K.; Scheschkewitz, D.; Whittell,
G. R. Angew. Chem., Int. Ed. 2005, 44, 1658–1660. (b) Braunschweig, H.;
Radacki, K.; Rais, D.; Seeler, F.; Uttinger, K. J. Am. Chem. Soc. 2005,
127, 1386–1387. (c) Braunschweig, H.; Whittell, G. R. Chem.-Eur. J. 2005,
11, 6128–6133.
31P{1H} NMR spectrum a broad signal at 65.3 ppm (1JP-Pt
)
1187 Hz) and a sharp one at 53.5 ppm (1JP-Pt ) 4108 Hz) can
be detected. The boryl complex cis-[(dcpe)Pt{B(Br)Pip)(Br)]
(4) could be successfully crystallized (Vide infra, Figure 1),
yielding 58% of the pure product.
(14) Braunschweig, H.; Radacki, K.; Uttinger, K. Angew. Chem., Int.
Ed. 2007, 46, 3979–3982.
(15) (a) Braunschweig, H.; Radacki, K.; Rais, D.; Seeler, F. Angew.
Chem., Int. Ed. 2006, 45, 1066–1069. (b) Braunschweig, H.; Rais, D.;
Uttinger, K. Angew. Chem., Int. Ed. 2005, 44, 3763–3766. (c) Braunschweig,
H.; Burschka, C.; Burzler, M.; Radacki, K. Angew. Chem., Int. Ed. 2006,
45, 4352–4355. (d) Braunschweig, H.; Radacki, K.; Rais, D.; Uttinger, K.
Organometallics 2006, 25, 5159–5164. (e) Braunschweig, H.; Radacki, K.;
Uttinger, K. Eur. J. Inorg. Chem. 2007, 27, 4350–4356.
(16) Braunschweig, H.; Radacki, K.; Rais, D.; Whittell, G. R. Angew.
Chem., Int. Ed. 2005, 44, 1192–1194.
From the reaction of trans-[(Cy3P)2Pt{B(Br)Mes}(Br)] (5)
and dcpe in C6H6 after 5 h at 45 °C cis-[(dcpe)Pt{B(Br)-
Mes)(Br)] (6) precipitated, which was isolated in 58% yield and
is characterized by similar 31P{1H} NMR data, that is, a broad
singlet at 65.4 ppm (1JP-Pt ) 1187 Hz) and a doublet at 53.5
ppm (1JP-Pt ) 4108 Hz, 2JP-P ) 5 Hz). A single crystal, obtained
from a CH2Cl2/hexane mixture at -35 °C, could be analyzed
by X-ray diffraction (Vide infra, Figure 1). However, if the same
reaction is carried out in CH2Cl2 rather than C6H6, the initially
formed cis-[(dcpe)Pt{B(Br)Mes)(Br)] (6) reacts with the solvent
in the presence of PCy3 within 6 days with halogen exchange
at the boron center and formation of [(dcpe)Pt{B(Cl)Mes)(Br)]
(7). The 31P{1H} NMR spectrum of the reaction mixture features
(17) (a) Braunschweig, H.; Radacki, K.; Rais, D.; Uttinger, K. Angew.
Chem., Int. Ed. 2006, 45, 162–165. (b) Braunschweig, H.; Radacki, K.;
Rais, D.; Schneider, A.; Seeler, F. J. Am. Chem. Soc. 2007, 129, 10350–
10351.
(18) Fischer, R. A.; Weiss, D.; Winter, M.; Mueller, I.; Kaesz, H. D.;
Froehlich, N.; Frenking, G. J. Organomet. Chem. 2004, 689, 4611–4623.
(19) (a) Zanardo, A.; Michelin, R. A.; Pinna, F.; Strukul, G. Inorg. Chem.
1989, 28, 1648–1653. (b) Crociani, B.; Di Bianca, F.; Giovenco, A.; Berton,
A.; Bertani, R. J. Organomet. Chem. 1989, 361, 255–267.

420 Organometallics, Vol. 27, No. 3, 2008
Braunschweig et al.
(e5 Hz) and was thus only detected in the case of the mesityl
species 6 and 7. All of these spectroscopic characteristics as
1
well as the significant differences of the two JP-Pt couplings
observed here for each complex have ample precedence in the
literature and were reported, for example, for cis-[(R3P)2Pt-
(BPin)(SiMe2Ph)],7 [(dcpe)Pt(Cl)(CH2CMe3)],21 and [(dppe)Pt-
1
(Cl)(Me)],22 respectively. It should be noted that the JP-Pt
coupling constants for the phosphorus atom trans to boron are
extremely small (2: 1098, 4: 1187, 6: 1056, 7: 1047 Hz)
compared to other square-planar Pt(II) complexes. Even ligands
that exhibit a strong trans-influence and, thus, impose relatively
small couplings, such as in cis-[(Ph3P)2Pt(BNMe2)(GeMe3)]
(1200 Hz (P trans to Ge)),6 cis-[(Me3P)2Pt(BPin)(SiMe2Ph)]
(1374 Hz (P trans to Si)), and cis-[(dcpe)Pt(AlR2)(R)] (R )
CH2CMe3) (1222 Hz (P trans to Al)),18 give rise to greater
1
values for JP-Pt. All additional multinuclear NMR data, such
as the broad 11B{1H} NMR reasonances (2: 94, 4: 49, 6: 100,
7: 103 ppm), that are significantly deshielded with respect to
those of the starting bromoboranes are unobtrusive and meet
with the expectations.
Structural Characterization. The compounds 2 and 7
crystallize in the monoclinic P21/c, 4 in the orthorhombic Pbca,
¯
and 6 in the triclinic P1 space group. All boryl ligands (defined
Figure 1. Molecular structures of the cis-bromoboryl complexes.
Hydrogen atoms and solvent molecules are omitted for clarity.
Thermal ellipsoids are at the 50% probability level. In 7 the boryl
moiety is disordered (i.e., the boryl ligand is twisted by 8° with
respect to the position in Figure 1, due to thermal rotation/
fluctuation). Important bond lengths are shown in Table 1.
by the BR2 plane) adopt the preferential, almost perpendicular
arrangement with respect to the PtP2 plane (dihedral angles 2:
76.68°; 4: 80.20°; 6: 70.24°; 7: 78.69°), thus allowing for
maximum overlap between the occupied platinum dxy and vacant
p-orbital at boron.3d To avoid steric repulsion, the bulky mesityl
groups in 6 and 7 are twisted with respect to the BR2 planes, as
indicated by the corresponding dihedral angles of 50.20° (6)
and 46.19° (7).
The Pt-B bond distances (2: 2.044(3), 4: 2.068(3), 6:
2.132(4), 7: 2.113(10) Å) are somewhat greater than in the
corresponding starting materials trans-[(Cy3P)2Pt{B(Br)R}(Br)]
(1: 1.944(3), 3: 2.021(5), 5: 2.009(3) Å)12b and, thus, comparable
to other Pt complexes bearing phosphines in trans-position to
a boryl moiety, e.g., cis-[(Ph3P)2Pt(BCat)2] and cis-[(Ph3P)2Pt-
(BNMe2)(SnMe3)].5c,6 As expected, the Pt-Br distances in the
products are in the range of about 2.50 Å (Table 1), thus being
significantly smaller than in the corresponding trans boryl
complexes (2.6141(3)-2.6313(5) Å),12b reflecting the much
smaller trans-influence of phosphine ligands in comparison to
boryls (Vide supra).
Table 1. Important Bond Lengths (in Å) of the Compounds 2, 4, 6,
and 7
Pt-P
(trans B)
Pt-P
(cis B)
Pt-B
Pt-Br
B-Hal
2: B(Fc)Br 2.3716(8) 2.2165(7) 2.004(3) 2.4839(3) 1.988(3)
4: B(Pip)Br 2.3567(8) 2.2148(8) 2.068(3) 2.5044(3) 2.011(3)
6: B(Mes)Br 2.3632(10) 2.2225(10) 2.132(4) 2.495(4)
1.978(4)
7: B(Mes)Cl 2.367(3) 2.223(2) 2.113(10) 2.5002(10) 1.777(10)
a peak at 103 ppm, indicating the formation of Br2PCy3,20
a
broad singlet at 65.4 (1JP-Pt ) 1187 Hz), and a doublet at 53.5
2
(1JP-Pt ) 4108 Hz, JP-P ) 2 Hz) for 7. cis-[(dcpe)Pt-
{B(Cl)Mes)(Br)] (7) could be isolated in 61% yield, and crystals
suitable for X-ray analysis were grown from a CH2Cl2/hexane
mixture at -35 °C (Vide infra, Figure 1).
The most notable structural feature of the complexes 2, 4, 6,
and 7 is the pronounced difference between both Pt-P separa-
tions in each molecule. The Pt-P distances trans to the boron
centers (Table 1) are approximately 15 pm greater than those
trans to bromide, as an effect of the strong trans-influence of
the boryl group. The theoretically predicted11 and in the case
of the trans-complexes 1, 3, and 5 experimentally proven
trend12b that B(aryl) ligands impose a stronger trans-influence
than B(NR2)-based groups becomes apparent here as well,
however, to a much lesser extent, as the observed differences
in Pt-B bond lengths for 2, 4, 6, and 7 are very small and lie
almost in the range of the respective standard deviations.
In conclusion, the conversion of several trans-haloboryl
complexes to their cis-analogues was accomplished by the
reaction with an appropriate chelating phosphine, thus giving
rise to a set of unprecedented, fully characterized cis-(bro-
mo)boryl complexes, cis-[(dcpe)Pt{B(X)R)(Br)] (R ) Fc, Pip,
Scheme 2. Conversion of cis-[(dcpe)Pt{B(Br)Mes)(Br)] (6) into
cis-[(dcpe)Pt{B(Cl)Mes)(Br)] (7) in the Presence of PCy3 in
CH2Cl2
All of the aforementioned dcpe complexes 2, 4, 6, and 7
possess two chemically nonequivalent phosphorus atoms and,
thus, should exhibit two doublets in the 31P{1H} NMR spectra.
Due to unresolved coupling with the quadrupolar boron nucleus,
however, the resonances for the phosphorus atoms in trans-
position to the boryl ligand are very broad, thus precluding the
2
resolution of the expected doublet. The JP-P coupling for the
phosphorus atom trans to bromide was found to be very small
(21) Hackett, M.; Ibers, J. A.; Whitesides, G. M. J. Am. Chem. Soc.
1988, 110, 1436–1448.
(20) Godfrey, S. M.; McAuliffe, C. A.; Mushtaq, I.; Pritchard, R. G.;
Sheffield, J. M. J. Chem. Soc., Dalton Trans. 1998, 3815–3818.
(22) Appleton, T. G.; Bennett, M. A.; Tomkins, I. B. J. Chem. Soc.,
Dalton Trans. 1976, 439–446.

ConVersion of trans-Bromoboryl Platinum Complexes
Organometallics, Vol. 27, No. 3, 2008 421
[(dcpe)Pt{B(Mes)Br}(Br)] (6). Solid trans-[(Cy3P)2Pt{B(Br)-
Mes}(Br)] (5) (0.050 g, 0.048 mmol) was suspended in benzene
(0.6 mL) in a J-Young NMR tube, and dcpe (0.020 g, 0.048 mmol)
was added. After warming at 45 °C for 3 h the reaction was judged
to be complete by 31P{1H} NMR. After 2 days pale yellow
crystalline material precipitated, which was recrystallized from a
CH2Cl2/hexane mixture at -35 °C. Colorless crystals could be
Mes, X ) Br; R ) Mes, X ) Cl). The distinct differences in
the Pt-P bond lengths observed in the solid state structures
can be ascribed to the strong trans-influence of the boryl groups.
Studies directed toward the utilization of such compounds and
derivatives thereof in catalytic processes are underway in our
laboratories.
obtained after
2 days, yielding 25 mg (58%) of cis-
Experimental Section
[(dcpe)Pt{B(Br)Mes}(Br)] (6).
All manipulations were performed either under dry argon or in
Vacuo using standard Schlenk line and glovebox techniques.
Solvents (toluene, benzene, and hexane) were purified by distillation
under dry argon from appropriate drying agents (Na and Na/K alloy,
respectively) and stored under the same inert gas over molecular
sieves. Deuterated solvents were degassed by three freeze–
pump–thaw cycles and stored over molecular sieves. NMR spectra
were recorded on a Bruker Avance 500 (1H: 500.13; 31P: 202.46;
11B: 160.47; 13C: 125.77 MHz) NMR spectrometer. 1H and
13C{1H} NMR spectra were referenced to external TMS via the
residual protio solvent (1H) or the solvent itself (13C). 11B{1H}
NMR spectra were referenced to external BF3 · OEt2 and 31P{1H}
NMR spectra to 85% H3PO4. Microanalyses for C, H, and N were
performed on a Elementar Vario MICRO cube instrument.
[(dcpe)Pt{B(Br)Fc}(Br)] (2). Solid [Pt(PCy3)2] (0.100 g, 0.132
mmol) and FcBBr2 (0.053 g, 0.149 mmol) were put in a J-Young
NMR tube and dissolved in toluene (0.6 mL); the reaction was
judged to be complete by 31P{1H} NMR spectroscopy showing a
corresponding signal for trans-[(Cy3P)2Pt{B(Br)Fc}(Br)] (1). Then
dcpe (0.060 g, 0.142 mmol) was added to the deep red reaction
mixture. Overnight orange crystals formed, which were separated
and washed with benzene (2 × 0.3 mL), yielding 90 mg of cis-
[(dcpe)Pt{B(Br)Fc}(Br)] (3) (71%).
1H NMR (500 MHz, CD2Cl2, 24 °C): δ 6.75 (2 overlapping s,
2H, CmetaH, Mes), 2.56 (s, 6H, Cortho-CH3, Mes), 2.31 (m, 2H, dcpe),
2.23 (s, 3H, Cpara-CH3, Mes), 2.15 (m, 2H, dcpe), 2.04 (m, 2H,
dcpe), 1.88-0.75 (m, 42H, dcpe). 13C{1H} NMR (125.8 MHz,
CD2Cl2, 24 °C,): δ 150.0 (br s, Cipso, Mes, from 2D-HMBC), 138.6
(s, Cortho, Mes), 136.8 (s, Cpara, Mes), 128.8 (s, Cmeta, Mes), 37.5
1
3
1
(dd, JC-P ) 35 Hz, JC-P ) 2 Hz, C1, dcpe), 34.5 (d, JC-P ) 18
3
Hz, C1, dcpe), 30.4 (s, C3–5, dcpe), 29.6 (d, JC-P ) 4 Hz, C3–5,
3
3
dcpe), 29.4 (d, JC-P ) 3 Hz, C3–5, dcpe), 29.0 (d, JC-P ) 2 Hz,
C3–5, dcpe), 27.5 (d, 2JC-P ) 13 Hz, C2–6, dcpe), 27.4 (d, 2JC-P
)
2
9 Hz, C2–6, dcpe), 27.3 (d, JC-P ) 10 Hz, C2–6, dcpe), 26.8 (d,
2JC-P ) 14 Hz, C2–6, dcpe), 26.5 (d, 4JC-P ) 1 Hz, C4, dcpe), 26.3
4
(d, JC-P ) 1 Hz, C4, dcpe), 25.8 (s, Cortho-CH3, Mes), 24.6 (dd,
3
1JC-P ) 34 Hz, JC-P ) 23 Hz, P-CH2, dcpe), 21.0 (s, Cpara-CH3,
Mes), 18.8 (dd, 1JC-P ) 20 Hz, 3JC-P ) 5 Hz, P-CH2, dcpe). 11B{1H}
NMR (160 MHz, CD2Cl2, 24 °C): δ 103 (br s, ω½ ≈ 1470 Hz).
31P{1H} NMR (202.5 MHz, CD2Cl2, 24 °C): δ 65.4 (br s, 1JP-Pt
)
1
3
1056 Hz), 53.6 (d, JP-Pt ) 4025 Hz, JP-P ) 5 Hz). Anal. Calcd
for C35H59BBr2P2Pt: C, 46.32; H, 6.55. Found: C, 46.20; H, 6.32.
[(dcpe)Pt{B(Cl)Mes}(Br)] (7). Solid trans-[(Cy3P)2Pt{B(Br)-
Mes}(Br)] (5) (0.100 g, 0.096 mmol) was dissolved in CH2Cl2 (0.6
mL) in a J-Young NMR tube, and dcpe (0.040 g, 0.096 mmol)
was added. After 3 h the reaction was judged to be complete,
showing the corresponding signals for cis-[(dcpe)Pt{B(Br)Mes}-
(Br)] (6) in the 31P{1H} NMR spectrum. After 3 days the new
compound cis-[(dcpe)Pt{B(Cl)Mes}(Br)] (7) can be detected by
31P{1H} NMR spectroscopy. After another 3 days the conversion
was complete. The solvent was allowed to evaporate slowly, and
after 4 days the solid was washed with C6H6 (3 × 0.3 mL), yielding
50 mg (61%) of cis-[(dcpe)Pt{B(Cl)Mes}(Br)] (7). Crystals suitable
for an X-ray diffraction analysis could be prepared by recrystalli-
zation from a CH2Cl2/hexane mixture at -35 °C.
1H NMR (500 MHz, CD2Cl2, 24 °C): δ 4.60 (m, 1H, C5H4B),
4.55 (m, 1H, C5H4B), 4.51 (m, 1H, C5H4B), 4.43 (m, 1H, C5H4B),
4.29 (s, 5H, C5H5), 2.50-0.60 (m, 48H, dcpe). 13C{1H} NMR
(125.8 MHz, CD2Cl2, 24 °C): δ 87.2 (br s, Cipso, C5H4B, from 2D-
HMBC), 77.3 (s, C5H4B), 76.6 (s, C5H4B), 73.2 (s, C5H4B), 72.1
(s, C5H4B), 70.3 (s, C5H5), 35.8–34.2 (complex superpositions, C1,
1
dcpe), 30.1-25.5 (complex superpositions, dcpe), 26.1 (dd, JC-P
3
1
) 33 Hz, JC-P ) 23 Hz, P-CH2, dcpe), 19.3 (dd, JC-P ) 19 Hz,
3JC-P ) 6 Hz, P-CH2, dcpe). 11B{1H} NMR (160 MHz, CD2Cl2,
24 °C): δ 94 (br s, ω½ ≈ 1560 Hz). 31P{1H} NMR (202.5 MHz,
1
1
CD2Cl2, 24 °C): δ 65.0 (br s, JP-Pt ) 1098 Hz), 52.0 (s, JP-Pt
)
1H NMR (500 MHz, CD2Cl2, 23 °C): δ 6.75 (2 overlapping s,
2H, CmetaH, Mes), 2.59 (s, 6H, Cortho-CH3, Mes), 2.29 (m, 2H, dcpe),
2.22 (s, 3H, Cpara-CH3, Mes), 2.12 (m, 2H, dcpe), 1.96 (m, 2H,
dcpe), 1.88-0.75 (m, 42H, dcpe). 13C{1H} NMR (125.8 MHz,
CD2Cl2, 23 °C): δ 148.2 (br s, Cipso, Mes), 138.6 (s,Cortho, Mes),
4140 Hz). Anal. Calcd for C36H57BBr2FeP2Pt: C, 44.42; H, 5.90.
Found: C, 44.96; H, 5.60.
[(dcpe)Pt{B(Br)Pip}(Br)] (4). Solid [Pt(PCy3)2] (0.109 g, 0.144
mmol) was dissolved in toluene (0.6 mL) in a J-Young NMR tube,
and PipBBr2 (0.038 g, 0.149 mmol) was added; the reaction was
judged to be complete by 31P{1H} NMR spectroscopy showing a
signal for trans-[(Cy3P)2Pt{B(Br)Pip}(Br)] (3). Then dcpe (0.070
g, 0.166 mmol) was added to the yellow reaction mixture and heated
at 80 °C for 1 week. The white solid was removed and the solution
was allowed to evaporate slowly. After 2 weeks benzene (1 mL)
was added to the oily residue and the solvent was allowed to
evaporate slowly. Colorless solid formed after 3 days, which was
washed with hexane (3 × 1 mL), yielding 75 mg (58%) of cis-
[(dcpe)Pt{B(Br)Pip}(Br)] (4).
1
138.1 (s, Cpara, Mes), 128.8 (s,Cmeta, Mes), 37.3 (dd, JC-P ) 35
3
1
Hz, JC-P ) 2 Hz, C1, dcpe), 34.3 (d, JC-P ) 17 Hz, C1, dcpe),
3
30.1 (s, C3–5, dcpe), 29.5 (d, JC-P ) 4 Hz, C3–5, dcpe), 29.2 (d,
3JC-P ) 4 Hz, C3–5, dcpe), 28.9 (d, JC-P ) 1 Hz, C3–5, dcpe),
3
27.5 (d, 2JC-P ) 10 Hz, C2–6, dcpe), 27.4 (d, 2JC-P ) 6 Hz, C2–6,
dcpe), 27.3 (d, 2JC-P ) 10 Hz, C2–6, dcpe), 26.8 (d, 2JC-P ) 14 Hz,
4
4
C2–6, dcpe), 26.5 (d, JC-P ) 1 Hz, C4, dcpe), 26.3 (d, JC-P ) 1
Hz, C4, dcpe), 25.8 (s, Cortho-CH3, Mes), 25.1 (dd, 1JC-P ) 33 Hz,
3JC-P ) 23 Hz, P-CH2, dcpe), 21.1 (s, Cpara-CH3, Mes), 18.9 (dd,
1JC-P ) 20 Hz, JC-P ) 5 Hz, P-CH2, dcpe). 11B{1H} NMR (160
3
1H NMR (500 MHz, CD2Cl2, 23 °C): δ 4.59(m, 2H, NCH2, C-1
and C-2 Pip), 3.29 (m, 1H, NCH2, C-1 Pip), 2.97 (m, 1H, NCH2,
C-2 Pip), 2.81 (m, 1H, Pip), 2.65 (m, 1H, dcpe), 2.40-1.00 (m,
52H, Pip and dcpe). 13C{1H} NMR (125.8 MHz, CD2Cl2, 23 °C):
δ 56.1 (s, NCH2, C-1), 51.6 (s, NCH2, C-2), 36.2-34.2 (complex
superpositions, C1, dcpe), 30.8-26.0 (complex superpositions, Pip
MHz, CD2Cl2, 23 °C): δ 100 (br s, ω½ ≈ 1330 Hz). 31P{1H} NMR
(202.5 MHz, CD2Cl2, 23 °C): δ 65.7 (br s, 1JP-Pt ) 1047 Hz), 53.5
1
3
(d, JP-Pt
) 4094 Hz, JP-P ) 2 Hz). Anal. Calcd for
C35H59BBrClP2Pt: C, 48.71; H, 6.89. Found: C, 49.01; H, 6.96.
Crystal Structure Determination. The crystal data of 2, 4, 6,
and 7 were collected on a Bruker X8Apex diffractometer with CCD
area detector and multilayer mirror monochromated Mo KR
radiation. The structures were solved using direct methods, refined
with the Shelx software package (G. Sheldrick, University of
Göttingen 1997), and expanded using Fourier techniques. All non-
1
3
and dcpe), 19.7 (dd, JC-P ) 20 Hz, JC-P ) 7 Hz, P-CH2, dcpe).
11B{1H} NMR (160 MHz, CD2Cl2, 23 °C): δ 49 (br s). 31P{1H}
1
NMR (202.5 MHz, CD2Cl2, 23 °C): δ 65.3 (br s, JP-Pt ) 1187
Hz), 53.5 (s, 1JP-Pt ) 4108 Hz). Anal. Calcd for C33H58BBr2NP2Pt:
C, 44.21; H,6.52; N 1.56. Found: C, 44.70; H, 6.89; N 1.56.

422 Organometallics, Vol. 27, No. 3, 2008
Braunschweig et al.
hydrogen atoms were refined anisotropically. Hydrogen atoms were
assigned idealized positions and were included in structure factor
calculations.
T ) 100(2) K, R1 ) 0.0442, wR2 ) 0.1021, 13 550 independent
reflections [2θ e 65.88°] and 508 parameters.
Crystal data for 7: C35H59BBrClP2Pt · 2(CH2Cl2), Mr ) 1032.87,
colorless needles, 0.18 × 0.08 × 0.06, monoclinic space group P21/
c, a ) 8.6923(13) Å, b ) 17.456(2) Å, c ) 27.895(4) Å, ꢀ )
92.335(7)°, V ) 4229.0(10) Å3, Z ) 4, Fcalcd ) 1.622 g · cm-3, µ
) 4.680 mm-1, F(000) ) 2072, T ) 99(2) K, R1 ) 0.0946, wR2
) 0.1646, 8244 independent reflections [2θ e 52.22°] and 464
parameters.
Crystal data for 2: C36H57BBr2FeP2Pt, Mr ) 973.33, orange
blocks, 0.29 × 0.28 × 0.17, monoclinic space group P21/c, a )
12.1129(8) Å, b ) 14.2546(9) Å, c ) 21.6898(15) Å, ꢀ )
92.097(3)°, V ) 3742.6(4) Å3, Z ) 4, Fcalcd ) 1.727 g · cm-3
,
µ ) 6.366 mm-1, F(000) ) 1928, T ) 98(2) K, R1 ) 0.0422, wR2
) 0.0772, 11 695 independent reflections [2θ e 63.88°] and 388
parameters.
Crystal data for 4: C31H58BBr2NP2Pt · (C7H8), Mr ) 964.58,
colorless plates, 0.220 × 0.210 × 0.120, orthorhombic space group
Pbca, a ) 17.1174(4) Å, b ) 19.8926(4) Å, c ) 23.6733(6) Å, V
Acknowledgment. This work was supported by the DFG
and the Fonds der Chemischen Industrie.
) 8061.0(3) Å3, Z ) 8, Fcalcd ) 1.590 g · cm-3, µ ) 5.570 mm-1
,
Supporting Information Available: Crystallographic data (CIF
files) have been deposited with the Cambridge Crystallographic Data
Center as supplementary publication nos. CCDC 662542-5. These
data can be obtained free of charge from The Cambridge Crystal-
lographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif or
at http://pubs.acs.org.
F(000) ) 3872, T ) 100(2) K, R1 ) 0.0389, wR2 ) 0.0659, 11 403
independent reflections [2θ e 59.38°] and 418 parameters.
Crystal data for 6: C35H59BBr2P2Pt · 2.5(C6H6), Mr ) 1102.75,
¯
colorless needles, 0.18 × 0.065 × 0.065, triclinic space group P1,
a ) 8.6839(3) Å, b ) 16.0549(5) Å, c ) 17.7870(6) Å, R )
78.295(2)°, ꢀ ) 81.914(2)°, γ ) 76.596(2)°, V ) 2350.69(14) Å3,
Z ) 2, Fcalcd ) 1.558 g · cm-3, µ ) 4.786 mm-1, F(000) ) 1114,
OM700976T