Synthesis and novel reactivity of platinum phosphine-borane complexes trans-[PtH(PPhR·BH3)(PEt3)2] (R = H, Ph)

Article abstract of DOI:10.1039/b002615h

The oxidative-addition reaction of Pt(PEt₃)₃ with the phosphine-borane adducts PhPH₂·BH₃ and Ph₂PH·BH₃ leads to regioselective insertion of the Pt(PEt₃)₂ fragment into the P-H bond to afford the hydride complexes trans[PtH(PPhR·BH₃)(PEt₃)₂] (R = H 1, R = Ph 2); reaction of 2 with PhPH₂·BH₃ leads to an unusual ligand exchange reaction which generates 1 and Ph₂PH·BH₃.

Full text of DOI:10.1039/b002615h

Synthesis and novel reactivity of platinum phosphine–borane complexes
trans-[PtH(PPhR·BH3)(PEt3)2] (R = H, Ph)
Hendrik Dorn, Cory A. Jaska, Ryan A. Singh, Alan J. Lough and Ian Manners*
Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, M5S 3H6, Canada.
E-mail: imanners@alchemy.chem.utoronto.ca
Received (in Cambridge, UK) 3rd April 2000, Accepted 2nd May 2000
Published on the Web 25th May 2000
The oxidative-addition reaction of Pt(PEt3)3 with the
phosphine–borane adducts PhPH2·BH3 and Ph2PH·BH3
leads to regioselective insertion of the Pt(PEt3)2 fragment
into the P–H bond to afford the hydride complexes trans-
[PtH(PPhR·BH3)(PEt3)2] (R = H 1, R = Ph 2); reaction of
2 with PhPH2·BH3 leads to an unusual ligand exchange
reaction which generates 1 and Ph2PH·BH3.
with the electron rich Pt(0) complex Pt(PEt3)3 and a preliminary
reactivity study of the resulting phosphine–borane complexes.
Reaction of the primary phosphine–borane adduct
PhPH2·BH3 with a stoichiometric amount of Pt(PEt3)3 in
toluene at 60 °C and subsequent crystallization from hexanes
resulted in the formation of a yellow solid 1 (23% isolated yield,
quantitative by 11B and 31P NMR before workup) which was
characterized by multinuclear NMR spectroscopy in CDCl3.†
The spectroscopic data for the product was consistent with a
structure 1 in which insertion of Pt into a P–H bond rather than
the B–H bond had taken place. For example, the product
displayed two distinct sets of signals in the 31P NMR spectrum:
a doublet at d 17.9 was assigned to the PEt3 ligands attached to
platinum, showing coupling to another phosphorus nucleus (JPP
19.4 Hz) as well as 195Pt satellite signals (JPPt 2572 Hz).
Furthermore, a broad doublet at d ca. 248.7 indicated coupling
to a quadrupolar boron nucleus and one hydrogen substituent
(JPH 299 Hz) and showed 195Pt satellites with the magnitude JPPt
ca. 1440 Hz. This signal was assigned to the phosphorus of the
fragment Pt–PPhH·BH3. The 11B NMR spectrum showed a
single broad resonance at d 233.2 which is in the region typical
for BH3 adducts of phosphines. The key 1H NMR spectroscopic
feature is the low-frequency hydride resonance at d 25.74,
which appears as a doublet of triplets with a larger coupling to
one phosphorus atom (JHP 124.6 Hz) and a smaller coupling to
two phosphorus atoms (JHP 15.4 Hz) as well as 195Pt satellites
(JHPt 872 Hz). This suggested the presence of one trans
substituent (PPhH·BH3) and two cis substituents (PEt3), with
respect to the hydride ligand.
The discovery of new synthetic methods for the formation of
homonuclear or heteronuclear bonds between main group
elements is of importance for the construction of inorganic
polymer chains and also for the general development of p-block
chemistry.1,2 Transition metal-catalyzed routes represent a
particularly attractive approach to this problem.2,3 Recently we
reported a novel metal-catalyzed route to linear and cyclic
oligomeric and also high molecular weight poly(phosphinobor-
anes) which involved dehydrocoupling of phosphine–borane
adducts in the presence of late transition metal complexes
(Scheme 1).4,5 A plausible first step in these reactions involves
insertion of the transition metal into either the P–H or B–H bond
of the adduct. Subsequent steps may involve s-bond metathesis
and/or oxidative addition/reductive elimination processes. Sig-
nificantly, no homodehydrocoupling products with P–P or B–B
bonds have been detected from these reactions to date. It should
be noted that the insertion of transition metals into the P–H
bonds of secondary phosphines has ample precedent. For
example, oxidative addition of P(III)–H bonds at Pt(0) or Pd(0)
centers has been demonstrated and such processes are believed
to be a key step in the catalytic hydrophosphination of alkenes
and alkynes.6 On the other hand, a range of mono- and di-
nuclear boryl complexes have recently been reported as a result
of the ambient temperature reactions of the B–H bonds in
Me3P·BH3 or Me3P·BH2BH2·PMe3 at metal centers.7,8 In order
to investigate potential pathways during the transition metal
mediated P–B bond formation reactions we are studying the
hitherto unexplored coordination chemistry of phosphine–
borane adducts with both P–H and B–H bonds. Here, we report
the oxidative-addition reactions of Ph2PH·BH3 and PhPH2·BH3
Fig. 1 Molecular structure of 1 (30% displacement ellipsoids). Hydrogen
atoms attached to carbon are omitted. Selected bond lengths (Å) and angles
(°): Pt(1)–P(1) 2.3477(14), Pt(1)–P(2) 2.2863(13), Pt(1)–P(3) 2.2771(14),
P(1)–B(1) 1.953(7), P(1)–H(1P) 1.32(6); P(3)–Pt(1)–P(2) 168.26(5), P(3)–
Pt(1)–P(1) 92.83(5), P(2)–Pt(1)–P(1) 98.87(5), B(1)–P(1)–Pt(1) 119.8(2).
Scheme 1
DOI: 10.1039/b002615h
Chem. Commun., 2000, 1041–1042
This journal is © The Royal Society of Chemistry 2000
1041

The demonstration of a novel phosphine–borane ligand
exchange reaction at the Pt center indicates that relatively
complex reaction chemistry may need to be unravelled if a full
mechanistic understanding of late transition metal-catalyzed P–
B bond formation reactions is to take place. Detailed studies of
the reactivity of novel complexes such as 1 and 2 and analogs
directed towards this goal are in progress.
Scheme 2
We wish to acknowledge the Petroleum Research Fund
administered by the ACS for funding and the Deutsche
Forschungsgemeinschaft (DFG) for a postdoctoral fellowship
for H. D.
Notes and references
† Selected spectroscopic data: for 1: 1H NMR (300 MHz, CDCl3) d
7.76–7.70 (m, Ar), 7.26–7.19 (m, Ar), 4.39 (d, JHP 299 Hz, PH), 2.0–0.9 (br
q, BH3), 1.80 (m, CH2), 1.03 (m, CH3), 25.74 [dt, JHP(trans) = 124.6 Hz,
Fig. 2 1H NMR spectrum (300 MHz, C6D6) of the hydride region of 2
[JHP(trans) 124.4 Hz, JHP(cis) 16.9 Hz, JHPt 805 Hz, PtH].
J
HP(cis) 15.4 Hz, JHPt 872 Hz, PtH]; 11B{1H} NMR (160 MHz, CDCl3) d
233.2 (br); 31P NMR (121 MHz, CDCl3) d 17.9 (d, JPP 19.4 Hz, JPPt 2572
Hz, PEt3), 248.7 (br d, JPH 299 Hz, JPPt 1440 Hz, PHPh); IR (Nujol) 2350
(nBH), 2022 (nPtH) cm21; EIMS m/z (%): 541 (1) [M+ 2 BH3], 118 (100)
The molecular structure of compound 1 was confirmed by
single crystal X-ray analysis‡ and is consistent with the solution
NMR data. A SHELXTL drawing is shown in Fig. 1. The
geometry around the Pt(II) center is close to square-planar with
trans PEt3 ligands. Although the platinum hydride ligand was
not observed in the X-ray study its presence is clearly indicated
by the large P(3)–Pt(1)–P(2) angle of 168.26(5)° and, as
discussed, from the 1H NMR spectroscopic data. The lengths of
the bonds Pt(1)–P(2) [2.2863(13) Å] and Pt(1)–P(3)
[2.2771(14) Å] are shorter than the Pt(1)–P(1) bond [2.3477(14)
Å], presumably as a consequence of the trans influence of the
hydride ligand. A similar observation was made in the hydride
complex cis-[PtH(P(O)Ph2)(PPh2(OH))(PEt3)].9
A similar reaction was observed between the secondary
phosphine–borane adduct Ph2PH·BH3 and 1 equivalent of
Pt(PEt3)3 (Scheme 2). The orange–yellow platinum complex 2
was isolated in 67% yield. For 2, signals in the 31P NMR
spectrum are observed at d 16.7 (PEt3) and d 23.7 (PPh2·BH3)
and in the 11B NMR spectrum at d 231.4. Fig. 2 shows the
hydride region of the 1H NMR spectrum of 2 with the
characteristic dt coupling pattern which is flanked by 195Pt
satellite signals (JHPt 805 Hz).
1
[PEt3]. For 2: H NMR (300 MHz, C6D6) d 8.26–8.14 (m, Ar), 7.26–7.00
(m, Ar), 3.0–1.9 (br q, BH3), 1.48 (m, CH2), 0.93 (m, CH3), 26.75 [dt,
JHP(trans) 124.4 Hz, JHP(cis) 16.9 Hz, JHPt 805 Hz, PtH]; 11B{1H} NMR
(160 MHz, C6D6) d 231.4 (br); 31P{1H} NMR (121 MHz, C6D6) d 16.7 (d,
JPP 17.4 Hz, JPPt 2648 Hz, PEt3), 23.7 (br, JPPt 1575 Hz, PPh2); IR (Nujol)
2341 (nBH), 2009 (nPtH) cm21; EIMS m/z (%): 617 (21) [M+ 2 BH3], 62
(100) [H2PEt].
‡ Crystal data for 1: C18H39BP3Pt, M = 554.30, monoclinic, space group
C2/c, a
= 27.1904(13), b = 9.8355(7), c = 19.0955(11) Å, b =
110.777(3)°, U = 4774.6(5) Å3, Z = 8, Dc = 1.542 g cm23, m = 6.077
mm21, F(000) 2200, T = 150(1) K, crystal size 0.15 3 0.15 3 0.10 mm,
5176 independent reflections, 51791 collected. Goodness-of-fit on F2
=
1.042, final R indices [I > 2s(I)], R1 = 0.0387, wR2 = 0.0969. The
structures were solved and refined with the SHELXTL-PC V5.1 software
package.12 Refinement was by full-matrix least squares on F2 using all data
(negative intensities included). Hydrogens bonded to carbon atoms were
included in calculated positions and treated as riding atoms, hydrogens
attached to P(1) and B(1) were refined with isotropic thermal parameters.
CCDC 182/1620. See http://www.rsc.org/suppdata/cc/b0/b002615h/ for
crystallographic files in .cif format.
1 See, for example: J. E. Mark, H. R. Allcock and R. West, Inorganic
Polymers, Prentice Hall, Englewood Cliffs, NJ, 1992; I. Manners,
Angew. Chem., Int. Ed. Engl., 1996, 35, 1602.
2 For a recent review on catalytic dehydrocoupling, see: F. Gauvin, J. F.
Harrod and H. G. Woo, Adv. Organomet. Chem., 1998, 42, 363.
3 See, for example: T. Imori, V. Lu, H. Cai and T. D. Tilley, J. Am. Chem.
Soc., 1995, 117, 9931; S. M. Katz, J. A. Reichl and D. H. Berry, J. Am.
Chem. Soc., 1998, 120, 9844.
Compound 1 represents the first example of a metal complex
of a primary phosphine–borane adduct and only two other
examples of transition metal complexes of secondary phos-
phine–borane adducts analogous to 2 are known; a tetrahedral
5
iron complex, [Fe(h -C5Me5)(CO)2(PPh2·BH3)],10 and
a
4 H. Dorn, R. A. Singh, J. A. Massey, A. J. Lough and I. Manners, Angew.
Chem., Int. Ed., 1999, 38, 3321.
square-planar
palladium
complex,
[Pd(dppp)-
(C6F5)(PPh2·BH3)] [dppp = 1,3-bis(diphenylphosphino)pro-
pane], a proposed intermediate in the Pd-catalyzed coupling of
secondary phosphine–boranes with aryl halides.11 However, the
synthesis of the latter compounds involved nucleophilic
substitution steps rather than insertion of the transition metal
fragment.
The isolation of novel complexes 1 and 2 via oxidative
addition suggests that P–H bond activation may be the key
initial step in the late transition metal-catalyzed formation of P–
B bonds. In order to explore the reactivity of the complexes, a
solution of 2 in C6D6 was treated with another equivalent of
PhPH2·BH3. After 8 h at room temperature the solution had
changed from orange to yellow, however, no P–B coupling
reaction was observed by 31P NMR spectroscopy. Surprisingly,
a complete exchange of the phosphine–borane ligand had
occurred to form exclusively complex 1 together with
Ph2PH·BH3 (Scheme 3).
5 H. Dorn, R. A. Singh, J. A. Massey, J. M. Nelson, C. A. Jaska, A. J.
Lough and I. Manners, J. Am. Chem. Soc., 2000, in press.
6 P. G. Pringle and M. B. Smith, J. Chem. Soc., Chem. Commun., 1990,
1701; L.-B. Han and M. Tanaka, J. Am. Chem. Soc., 1996, 118, 1571; D.
K. Wicht, I. V. Kourkine, B. M. Lew, J. M. Nthenge and D. S. Glueck,
J. Am. Chem. Soc., 1997, 119, 5039; E. Costa, P. G. Pringle and K.
Worboys, Chem. Commun., 1998, 49.
7 Y. Kawano, T. Yasue and M. Shimoi, J. Am. Chem. Soc., 1999, 121,
11 744 and references therein.
8 For further examples of transition metal–boron compounds see: H.
Braunschweig, Angew. Chem., Int. Ed., 1998, 37, 1787; G. J. Irvine,
M. J. G. Lesley, T. B. Marder, N. C. Norman, C. R. Rice, E. G. Robins,
W. R. Roper, G. R. Whittell and L. J. Wright, Chem. Rev., 1998, 98,
2685; M. R. Smith III, Prog. Inorg. Chem., 1999, 48, 505; C. N.
Muhoro, X. He and J. F. Hartwig, J. Am. Chem. Soc., 1999, 121,
5033.
9 L.-B. Han, N. Choi and M. Tanaka, Organometallics, 1996, 15, 3259.
10 W. Angerer, W. S. Sheldrick and W. Malisch, Chem. Ber., 1985, 118,
1261.
11 A.-C. Gaumont, M. B. Hursthouse, S. J. Coles and J. M. Brown, Chem.
Commun., 1999, 63.
12 G. M. Sheldrick, SHELXTL-PC V5.1, Bruker Analytical X-ray
Systems Inc., Madison, WI, 1997.
Scheme 3
1042
Chem. Commun., 2000, 1041–1042