[{(dppe)Pt}2B7H11]: An arachno-bimetallanonaborane based on the uncommon n-B9H15 cluster framework

Article abstract of DOI:10.1039/a801093e

The first bimetallanonaborane, [{(dppe)Pt)₂B₇B₁₁], based on the uncommon n-B₉H₁₅ cluster framework, is prepared and characterized as a final product from the reaction of pentaborane (9) with [PtCl₂(dppe)].

Full text of DOI:10.1039/a801093e

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[{(dppe)Pt}2B7H11]: An arachno-bimetallanonaborane based on the uncommon
n-B9H15 cluster framework
Ramo´n Mac´ıas, Nigam P. Rath and Lawrence Barton*†
Department of Chemistry, University of Missouri-St. Louis, St. Louis, MO 63121, USA
The first bimetallanonaborane, [{(dppe)Pt}2B7B11], based on
the uncommon n-B9H15 cluster framework, is prepared and
characterized as a final product from the reaction of
pentaborane (9) with [PtCl2(dppe)].
The structure of [{(dppe)Pt}2B7H11], given in Fig. 1, consists
of a nine-vertex n-arachno framework, obtained by removal of
two adjacent vertices of connectivity six and four respectively,
from an 11-vertex closo-octadecahedron. The {(dppe)Pt}
moieties are located at the 6 and 8 positions in the n-arachno
B9H15 framework, each replacing a BH group and a bridging H
atom. There are bridging H atoms at the B(4)–B(5) and
B(3)–B(9) edges and endo-H atoms on B(3) and B(7). Only the
latter endo-H atom is observed from the X-ray structure
determination but the presence is clearly confirmed from NMR
spectral data. [{(dppe)Pt}2B7H11] is shown as structure IV and
a topological representation, showing the numbering scheme
and the endo-hydrogens, is given as structure V. Bond distances
are within the normal ranges for Pt–B, Pt–P and B–B bonding
connections. The B(4)–B(9) edge bridged by a BH2 group, at
1.739(13) Å, is similar to that observed for the same cluster
interboron connection in n-B9H152,11 and in the ruthenaborane,3
but it is much shorter than the corresponding B–B interaction in
the recently discovered azaplatinaborane, [3,3-(PMe2Ph)2-
3-PtB7H10-m-5,6-(NHR)], 1.92(2) Å,12 in which the B–B edge
is bridged by a NHEt group (structure VI). NMR data conform
to the observed molecular structure for this new biplatinabor-
ane. The 11B spectrum exhibits four resonances of relative
The coordination number pattern recognition theory for boranes
and carbaboranes, as described by Williams in 1976,1 requires
that in order to generate arachno-clusters from nido systems, a
highest connectivity vertex, adjacent to the open face, is
removed. The predicted cluster framework for an arachno
9-vertex system is illustrated by structure I and recognized as
n-arachno B9H15. In contrast, the more commonly encountered
9-vertex arachno-species are derived by removal of a low
connectivity vertex from nido-B10H14 to generate derivatives of
what are referred to as i-B9H15 (structure II) and i-C2B7H13.2
(a)
P(4)
B(5)
6
With the exception of the ruthenanonaborane, [2-(h -C6Me6)-
Pt(6)
B(4)
2-RuB8H14], (structure III)3 reported in trace yields in a
preliminary communication in 1986, all the known nine-vertex
arachno-metallaboranes have structures based on the i-B9H15
framework.4 This article describes the synthesis and complete
characterization of a diplatinanonaborane whose structure is
based on a polyhedral nine-vertex n-B9H15 arachno-cluster type
and for which the application of electron-counting rules suggest
that the species is another example of the growing list of ‘rule-
breakers’.
B(2)
B(1)
B(3)
B(9)
P(3)
B(7)
Pt(8)
P(2)
B(3)
P(1)
(b)
Derivatives of B5H9, in which an electrophilic group has
replaced a bridging H atom, are well known.5 Group 10
derivatives, [2,3-m-{MLn(X)}B5H8], where M = Ni, Pd, Pt, Ln
= (PR3)2, dppe [dppe = (Ph2PCH2)2], X = Cl, Br, are isolated
if the reaction between [B5H8]2 and MLnX2 is carried out and
worked up at low temperatures.6 Such species are generally
unstable at room temperature in solution, usually resulting in
decomposition involving reduction of the metal moiety to
elemental metal by the borane moiety. Herein, we describe the
results when the reaction mixture, Li[B5H8] and PtCl2(dppe), is
allowed to warm to room temperature, dried in vacuo for 2 days,
exposed to air and separated using TLC on silica gel following
filtration through silica gel.8 Thus [{(dppe)Pt}2B7H11] is
prepared from [2,3-m-{(dppe)Pt(Cl)}B5H8], in ca. 10% yield as
B(4)
Pt(6)
B(5)
B(7)
B(9)
B(1)
B(2)
Pt(8)
Fig. 1 (a) Projection of [{(dppe)Pt}2B7H11]], showing only the P atoms of
the chelating ligand on Pt. Ellipsoids are drawn at the 50% probability level.
In addition to a terminal H atom on each B there is an exo-terminal H at
position 3, indicated by 1H NMR spectra. (b) Alternative representation
with the ligands omitted for clarity. Selected interatomic distances (Å): from
Pt(6) to P(3) 2.285(2), to P(4) 2.278(2), to B(2) 2.190(9), to B(5) 2.192(0),
to B(7) 2.208(9); from Pt(8) to P(1) 2.289(2), to P(2) 2.295(2), to B(1)
2.227(9), to B(2) 2.267(9), to B(7) 2.328(9), to B(9) 2.302(9); interboron
values: B(4)–B(9) 1.739(13), B(3)–B(9) 1.80(2), B(3)–B(4) 1.82(2), the
other B–B distances are in the range 1.765(13)–1.883(13). Selected angles
(°) P(3)–Pt(6)–P(4) 85.83(8), P(1)–Pt(8)–P(2) 84.97(7), P(3)–Pt(6)–B(5)
174.5(2), P(4)–Pt(6)–B(7) 167.6(3), P(1)–Pt(8)–B(9) 170.3(2), P(2)–Pt(8)–
B(7) 152.1(2), B(4)–B(3)–B(9) 57.5(5).
a
yellow air-stable crystalline solid, along with
[(dppe)PtB3H7],8a [(dppe)(BH3)2],8b and what appear to be Pt
chloride salts. NMR and mass spectral data identified the yellow
solid as [{(dppe)Pt}2B7H11].9 This was confirmed by a single
crystal structure determination of the diethyl ether sequi-
solvate.10
Chem. Commun., 1998
1081

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intensity 1:4:1:1 and 1H-{11B(selective)} experiments reveal
eleven different cage H atoms of which two are bridging and
two are endo-hydrogen atoms. In addition, 31P NMR spectra
exhibit four different resonances of equal intensity; two
doublets, an overlapped doublet of doublets that is seen as an
apparent triplet, and a singlet. Each signal shows additional
coupling to 195Pt. Since P atoms in the same chelating dppe
ligand effectively do not couple, we must be observing four-
bond 4J(31P–31P) coupling constants. From Fig. 1(a), it appears
that P(3) and P(4) are effectively trans to P(2) whereas P(1) is
directed towards the cluster B(1)–B(9) connection. This
conformation suggests that the P atoms involved in 31P–31P
coupling are P(3), P(4) and P(2). Thus we assign P(2) to the
observed apparent triplet, P(3) and P(4) to the doublets and P(1)
to the singlet. The cage connectivities of the Pt atoms are
different; that for Pt(6) is three whereas that for Pt(8) is four and
this appears to be reflected in the differences in 195Pt–31P
coupling constants listed in ref. 9. Characterization of
[{(dppe)Pt}2B7H11] is completed by mass spectral data. The
most intense envelope is that for the [M 2 BH3]+ ion,
suggesting that the connection of the m-H-BH2 group is quite
fragile, perhaps accounting for the preference of i-B9H15
isomers for nonaboranes and their derivatives.
2 L. Barton, Top. Cur.. Chem., 1982, 100, 169.
3 M. Bown, X. L. R. Fontaine, N. N. Greenwood, J. D. Kennedy and M.
Thornton-Pett, J. Organomet. Chem., 1986, 315, C1.
4 J. D. Kennedy, Prog. Inorg. Chem., 1986, 36, 211; K. B. Gilbert, S. K.
Boocock and S. G. Shore, in Comprehensive Organometallic Chem-
istry, ed. G. Wilkinson, E. W. Abel and F. G. A. Stone, Pergamon,
Oxford, 1982, part 6, ch. 41, pp. 879–945; L. Barton and D. K.
Srivastava, Comprehensive Organometallic Chemistry, II, ed. G.
Wilkinson, E. W. Abel and F. G. A. Stone, Pergamon, Oxford, 1995,
vol. 1, ch. 8, pp. 275–373.
5 T. Onak, G. B. Dunks, I. W. Searcy and J. Spielman, Inorg. Chem.,
1967, 6, 1465; D. F. Gaines and T. V. Iorns, J. Am. Chem. Soc., 1967,
89, 3375; R. A. Geanangel and S. G. Shore, J. Am. Chem. Soc., 1967, 89,
6771.
6 N. N. Greenwood and J. Staves, J. Chem. Soc., Dalton Trans., 1977,
1788; N. N. Greenwood, J. D. Kennedy and J. Staves, J. Chem. Soc.,
Dalton Trans., 1978, 1146.
7 Full experimental details available as supplementary material upon
request from the authors.
8 (a) NMR data for [(dppe)PtB3H7], (CDCl3, 294–297 K) {ordered as:
relative intensity d(11B) (rel. to BF3·OEt2) [d(1H)]} 1BH 19.8 [3.28];
1
2BH2 4.1 [4.20 (2 H); 2.80 (2 H), J(195Pt–1H) ca. 63 Hz]; additional
cage d(1H) 22.64 (2m-H). d(H)(dppe) 7.69–7.41 (4C6H5), 2.37 (m
2CH2). d(31P) (CDCl3, 294–297 K, rel. 85% H3PO4) 55.4, 1J(195Pt–31P)
2541 Hz. Data compare very well with those for [(PPh3)2PtB3H7], in J.
Bould, J. D. Kennedy and W. S. McDonald, Inorg. Chim. Acta., 1992,
196, 201; (b) [(dppe)(BH3)2], (CDCl3, 294–297 K): 2BH3 239.9 [1.02;
A final point of note concerning [{(dppe)Pt}2B7H11] is that it
appears not to conform to the polyhedral skeletal electron pair
theory (PSEPT) counting rules.13 An arachno nine-vertex
cluster requires n + 3 skeletal electron pairs, which for this
cluster would be 12 electron pairs. Using the conventional
1
AAAXXA spin system, N = 15.77 Hz]; J(11B–1H) coupling constants
not resolved due to the broadness of the boron resonances. d(31P)
(CDCl3, 294–297 K, rel. 85% H3PO4) 18.9 (br).
9 Selected NMR data for [{(dppe)Pt}2B7H11], (CDCl3, 294–297 K)
{ordered as: relative intensity d(11B) (rel. to BF3·OEt2) [d(1H)]} 1BH
39.2 [3.98], 4B 15.2 [4.98, 4.80, 4.22, 4.12, 2.82 (endo) [2J(195Pt–1H)
ca. 67 Hz], 1BH2 24.7 [2.07 (m, J 5.1 Hz; endo/exo)], 1BH 224.6
[1.34]; additional cage d(1H) 20.23 (bridging), 21.74 (bridging).
d(1H)(dppe) 8.0–6.9 (4C6H5), 2.31 (m, CH2), 1.87 (m, CH2). The 11B
spectra were too broad to observe coupling constants. d(31P) (CDCl3,
228 K, rel. 85% H3PO4) 55.7 [d, P(3 or 4)], 53.1 [d of d, P(2)], 49.4 [s,
electron-counting
methods
devised
by
Wade,14
[{(dppe)Pt}2B7H11] possesses 11 skeletal electron pairs. Such
ambiguity has been observed for essentially all clusters
containing three-connectivity group 10 metal moieties.15 Some
examples of such systems include [(PPh3)2(CO)Os(PPh-
Me2)Cl(m-H)PtB5H7],15a
[(PEt3)2Pt(CMe)2B4H4]15b
and
[(PPhMe2)2PtB8H12].15c In these systems, the organometallic Pt
fragment is best regarded as a square planar 16-electron center
that contributes two orbitals and two electrons to the cluster
framework. Under this scenario, the metal moiety is not isolobal
with a conventional three orbital, two electron conical vertex
and the skeletal electron count is two electrons short of the
number required for compliance with the PSEPT. In
[{(dppe)Pt}2B7H11] this feature may be ascribed to the vertex
unit [(dppe)Pt] at the 6-position. Apparently in
[{(dppe)Pt}2B7H11], the two Pt vertices, with different con-
nectivity, contribute differently to the total skeletal electron
count for the cluster. The vertex Pt(8), which has connectivity 4
may be considered to be a ‘normal’ conical vertex conforming
to the PSEPT, although in a formal sense each [(dppe)Pt] vertex
subrogates a BH(m-H) moiety. This appears to represent a new
example of the so-called rule breakers, and such phenomena
warrant further investigation.
4
1
P(1) and 45.3 d, P(4 or 3)]; J[31P(4 or 3)–31P(2)] 12.2 Hz, J[195Pt–
4
4
31P(4 or 3)] 2658 Hz; J[31P(4 or 3)–31P(2)] + J[31P(3 or 4)–31P(2)]
19.0 Hz, 1J[195Pt–31P(2)] 2534 Hz; 1J[195Pt–31P(1)] 2419 Hz;
4J[31P(2)–31P(3 or 4)] 22.6 Hz, 1J[195Pt–31P(3 or 4)] 2609 Hz. Low
resolution MS (VG, ZAB-E: FAB in CH2Cl2, 3-nitrobenzyl alcohol
matrix) overlapped with the M 2 H2 ion, gave an apparent cutoff at m/z
1280 (calc. for 12C52 H5911B731P4198Pt2 1280). Observed and calculated
1
mass spectral parent profiles showing isotopic distribution for the [M 2
BH3] ion, with m/z (max.) = 1259, compare very well.
10 Crystal data for C58H74B7O1.5P4Pt2, [(dppe)2Pt2B7H11], M = 1384.90
¯
(includes 1.5 molecules of Et2O), triclinic, space group P1, a =
12.8918(1), b = 13.7153(1), c = 17.8679(2) Å, a = 97.118(1), b =
94.409(2), g = 108.92(1)°, U = 2942.44(5) Å3, Dc = 1.563 Mg m23
Z = 2, F(000) = 1370, m(Mo-Ka) = 4.897 mm21, T = 223(2) K. A
total of 11 509 (Rint = 0.07) independent reflections, 2qmax = 52.0° on
a Siemens CCD single-crystal X-ray diffractometer using w scans. The
final wR(F2) for all unique reflections was 0.119 with a conventional
R(F) of 0.049 [for 8526 reflections with I > 2s(I)] for 677 parameters.
CCDC 182/824.
,
We acknowledge support by the Missouri Research Board
and the ACS-PRF (grant 31001-AC3). We also acknowledge
instrumentation grants from the NSF (No. CHE-9309690 and
CHE-9318696), the DOE (Grant No. DE-FG02-92CH10499)
and the UM-St. Louis Center for Molecular Electronics. We
thank Professor Michael Gross of the Washington University
Mass Spectrometry NIH Research Resource (Grant. No.
P41RR094) for the mass spectra. Experimental details for
11 J. C. Huffman, PhD Thesis, Indiana University, 1974, p. 771; R. A.
Beaudet, Mol. Struct. Energ., 1986, 5, 417.
12 U. Do¨rfler, P. A. Salter, X. L. R. Fontaine, N. N. Greenwood, J. D.
Kennedy and M. Thornton-Pett, J. Chem. Soc., Dalton Trans.,
(7/08097B). We thank the authors for providing us with a copy of this
manuscript prior to publication.
13 K. Wade, Adv. Inorg. Chem. Radiochem., 1976, 18, 60; R. W. Rudolph,
Acc. Chem. Res., 1976, 9, 446; D. M. Mingos, Acc. Chem. Res., 1984,
17, 311.
14 J. D. Kennedy, Main Group Metal Chem., 1989, 12, 149.
15 (a) J. Bould, J. E. Crook, N. N. Greenwood and J. D. Kennedy, J. Chem.
Soc., Dalton Trans., 1991, 185; (b) G. B. Barker, M. Green, T. P. Onak,
F. G. A. Stone, C. B. Ungermann and A. J. Welch, J. Chem. Soc., Chem.
Commun., 1980, 1186; (c) S. K. Boocock, N. N. Greenwood, M. J.
Hails, J. D. Kennedy and W. S. McDonald, J. Chem. Soc., Dalton
Trans., 1981, 1415.
the
preparation
and
spectral
characterization
of
[{(dppe)Pt}2B7H11], [(dppe)PtB3H7] and [(dppe)(BH3)2] are
available upon request from the authors.
Notes and References
† E-mail: lbarton@umsl.edu
1 R. E. Williams, Adv. Inorg. Chem. Radiochem., 1976, 18, 66.
Received in Bloomington, IN, USA, 6th February 1998; 8/01093E
1082
Chem. Commun., 1998