Polyazolyl chelate chemistry. 13. An osmaboratrane

Article abstract of DOI:10.1021/om0306123

The reaction of [Os(C₆H₅)Cl(CO)(PPh₃)₂] with Na[HB(mt)₃] (mt = methimazolyl) results in benzene elimination and the formation of the first osmaboratrane [B(mt)₃Os(CO)(PPh₃)](Os→B), the crystal structure of which reveals a somewhat long transannular dative bond between osmium(0) and boron(III).

Full text of DOI:10.1021/om0306123

Organometallics 2004, 23, 913-916
913
Notes
P olya zolyl Ch ela te Ch em istr y. 13. An Osm a bor a tr a n e1
Mark R. St.-J . Foreman,† Anthony F. Hill,*,†,‡ Andrew J . P. White,† and
David J . Williams†
Department of Chemistry, Imperial College London,
South Kensington, London SW7 2AZ, U.K., and Research School of Chemistry, Institute of
Advanced Studies, Australian National University, Canberra ACT, Australia
Received September 26, 2003
Summary: The reaction of [Os(C6H5)Cl(CO)(PPh3)2] with
Na[HB(mt)3] (mt ) methimazolyl) results in benzene
elimination and the formation of the first osmaboratrane
[B(mt)3Os(CO)(PPh3)](OsfB), the crystal structure of
which reveals a somewhat long transannular dative
bond between osmium(0) and boron(III).
has now included ruthenium examples. Furthermore,
structural data are also available for a range of base-
stabilized osmium borylene complexes.9 Thus the wealth
of structural data for 2c-2e Os-B bonding provides a
suitable basis for interpreting the structural features.10
Resu lts a n d Discu ssion
In tr od u ction
The ruthenaboratrane [B(mt)3Ru(CO)(PPh3)](RufB)
(1a ) was produced from the reactions of the σ-vinyl
complexes [Ru(CHdCHR)Cl(CO)(PPh3)2] (R ) H, Ph,
CPh2OH) with the salt Na[HB(mt)3],4 and accordingly
a similar approach was explored for the preparation of
the osmium analogue. Unfortunately, the analogous
σ-vinyl complexes of osmium have yet to be isolated via
the routes employed for ruthenium: [OsHCl(CO)-
(PPh3)3] fails to hydroosmate alkynes12 and catalyzes
the conversion of bis(trans-â-styryl)mercury to ethynyl
benzene, diphenylbutadiyne, and styrene and of bis-
(phenylethynyl)mercury to diphenylbutadiyne.12,13 The
corresponding σ-phenyl complex [Os(C6H5)Cl(CO)(PPh3)2]
(2) is however readily obtained from the reaction of
[OsHCl(CO)(PPh3)3] and diphenylmercury13 and has
been shown to serve as a precursor for the introduction
of the hydrotris(pyrazolyl)borate ligand (Scheme 1).14
Although metal-boron dative bonds have long been
postulated,2,3 the only structurally authenticated ex-
ample is the ruthenaboratrane [{B(mt)3}Ru(CO)(PPh3)]-
(RufB) (1a ; mt ) methimazolyl).4 This complex in-
volves a transannular dative bond from ruthenium(0)
to the bridgehead boron(III) of a cage structure derived
from the hydrotris(methimazolyl)borate ligand, HB-
(mt)3. It might therefore be considered isoelectronic with
Parshall’s archetypal salt Na[H3BRe(CO)5](RefB)2 with
both formally being based on octahedrally coordinated
d8-metal centers. Given the lack of structural data for
metal-boron dative bonding other than for 1a , and the
caveat that “simple” ruthenium boryls had not been
structurally characterized,5-7 we have investigated the
synthesis of an osmaboratrane. In the interim, a con-
siderable body of structural data for osmium boryls has
emerged from the studies of Roper and Wright,8 and this
(8) Rickard, C. E. F.; Roper, W. R.; Williamson, A.; Wright, L. J .
Angew. Chem., Int. Ed. 1999, 38, 1110. (b) Rickard, C. E. F.; Roper,
W. R.; Williamson, A.; Wright, L. J . Organometallics 1998, 17, 4869.
(c) Rickard, C. E. F.; Roper, W. R.; Williamson, A.; Wright, L. J .
Organometallics 2000, 19, 4344. (d) Irvine, G. J .; Roper, W. R.; Wright,
L. J . Organometallics 1997, 16, 2291. (e) Rickard, C. E. F.; Roper, W.
R.; Williamson, A.; Wright, L. J . Organometallics 2002, 21, 1714. (f)
Clark, G. R.; Irvine, G. J .; Roper, W. R.; Wright, L. J . Organometallics
1997, 16, 5499.
(9) Irvine, G. J .; Rickard, C. E. F.; Roper, W. R.; Williamson, A.;
Wright, L. J . Angew. Chem., Int. Ed. 2000, 39, 948. (b) Rickard, C. E.
F.; Roper, W. R.; Williamson, A.; Wright, L. J . Organometallics 2002,
21, 4862.
† Imperial College.
‡ Australian National University.
* Corresponding author.
(1) For part 12 see: Foreman, M. R. St.-J .; Hill, A. F.; Owen, G. R.;
White, A. J . P.; Williams, D. J . Organometallics 2003, 22, 4446.
(2) Parshall, G. W. J . Am. Chem. Soc. 1964, 86, 361.
(3) For a review of metal-boron dative bonding see: Gilbert, K. B.;
Boocock, S. K.; Shore, S. G. In Comprehensive Organometallic Chem-
istry; Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds.; 1982; Perga-
mon: Oxford, Vol. 6, pp 880-886. However, the identity of such
complexes has been questioned: Braunschweig, H. Angew. Chem., Int.
Ed. 1998, 37, 1786.
(4) Hill, A. F.; Owen, G. R.; White, A. J . P.; Williams, D. J . Angew.
Chem., Int. Ed. 1999, 38, 2759.
(10) Since the majority of osmium boryl and borylene chemistry post-
dates the definitive review of metal boryls,11 Table S1 in the Supporting
Information summarizes salient data (11B NMR chemical shifts and
Os-B bond lengths).
(5) The boryl complex [Ru{BClBN(SiMe3)BClN(SiMe3)2}(CO)2(η-
C5H5)]6 and the diborane(4)yl complex [Ru{B(NMe2)B(NMe2)Br}(CO)2-
(η-C5H5)]7 have been described. In this context when considering
unbridged 2c-2e Ru-B bonds, we specifically exclude interstitial boride
clusters, ruthenaboranes, ruthenacarbaboranes, and agostic B-H-
Ru interactions, wherein delocalized bonding models are appropriate.
(6) Braunschweig, H.; Kollann, C.; Klinkhammer, K. L. Eur. J .
Inorg. Chem. 1999, 1523.
(11) 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.
(12) Bedford, R. B.; Hill, A. F.; Thompsett, A. R.; White, A. J . P.;
Williams, D. J . Chem. Commun. 1996, 1059. (b) Hill, A. F.; Wilton-
Ely, J . D. E. T. J . Chem. Soc., Dalton Trans. 1998, 3501.
(13) Rickard, C. E. F.; Roper, W. R.; Taylor, G. E.; Waters, J . M.;
Wright, L. J . J . Organomet. Chem. 1990, 389, 375.
(7) Braunschweig, H.; Koster, M.; Wang, R. Inorg. Chem. 1999, 38,
415.
10.1021/om0306123 CCC: $27.50 © 2004 American Chemical Society
Publication on Web 01/15/2004

914 Organometallics, Vol. 23, No. 4, 2004
Notes
Sch em e 1. P r op osed Mech a n ism for
Osm a bor a tr a n e F or m a tion a
high temperatures (110 °C) required for the conversion
of [Os(C6H5)(CO)(PPh3)2{κ2-N,N′-HB(pz)3}] (4) to [Os-
(C6H5)(CO)(PPh3){κ3-N,N′,N′′-HB(pz)3}].14
Spectroscopic data for 1b parallel those reported for
the ruthenium analogue.4 Thus the low frequency of ν-
(CO) suggests a zerovalent osmium center, while the
13C and 1H NMR data associated with the three mt
heterocycles indicate two chemical environments for
these. The 31P{1H} NMR spectrum comprises a single
resonance which is significantly broadened (ω1/2 ) 420
Hz), indicating trans disposed phosphorus and quadru-
polar boron nuclei. The 11B NMR spectrum consists of
a single peak at 12.4 ppm, contrasting with the chemical
shifts typically found for osmium boryls, which, with the
exception of the coordinatively unsaturated complex
[Os(BO2C6H4)(C6H4Me-2)(CO)(PPh3)2] (δB ) 26.5),8a,c
span the range δ 36-63.8-10 The move from trigonal to
tetrahedral boron coordination is typically accompanied
by an upfield shift of the 11B resonance.15
The molecular structure of 1b was confirmed by an
X-ray crystallographic study, which showed it to have
an overall structure little different from that of the
ruthenium analogue 1a (Figure 1a). The geometry about
osmium in 1b may be described as pseudo-octahedral
with angles between the cis ligands falling in the range
81.2(3)-99.99(7)°. The coordination of the phosphine
and carbonyl ligands is unremarkable for such ligands
bound to osmium(0). The feature of interest is the
metallaboratrane cage, the axle of which is provided by
the OsfB dative bond, which at 2.171(8) Å, if compared
to those observed for conventional 2c-2e Os-B bonds
in octahedral boryl complexes [2.073(7)-2.154(7) Å],10,16
is the longest so far observed. This is perhaps not
surprising given both the tetrahedral coordination at
boron and the purely σ character of the OsfB bond. The
structurally characterized boryl and borylene complexes
of osmium8-10 involve three-coordinate boron with the
capacity for a π-component to the Os-B bonding. This
is borne out by the observation that for nonmetallacyclic
six-coordinate osmium boryls, the shortest Os-B bond
is observed for [Os(BCat)(S2CNEt2)(CO)(PPh3)2] [2.073-
(7) Å, cat ) OC6H4O-2], in which the superlatively
π-dative dithiocarbamate ligand coordinates trans to the
Os-B bond. Furthermore, it is difficult to quantify the
impact of the geometric constraints of the inclusion of
the Os-B bond within the cage structure of 1b, given
that its retention requires the accommodation of adja-
cent tetrahedral (B) and octahedral (Os) geometries. The
potential strain of such an arrangement is alleviated
by a flattening of two of the mt buttresses away from
local C3v-B(mt)3Os symmetry (Figure 1b), such that the
boron atom is displaced by 0.348 and 0.243 Å from the
mean planes of the heterocycles based on S(1) and S(2),
respectively, but only 0.017 Å from the heterocycle
bound to S(3). In this respect, an alternative valence-
bond description as an osmium(II) base-stabilized boryl
complex (Chart 2) would have credence were it not for
the effective equivalence of the three B-N bond lengths
[1.564(12)-1.572(12) Å] and the remarkably low value
of ν(CO).
a
L ) PPh3.
Notably, in this synthesis of [Os(C6H5)(CO)(PPh3){HB-
(pz)3}] (3; pz ) pyrazolyl), an intermediate complex 4
with a bidentate HB(pz)3 ligand is obtained initially, and
this is only slowly converted to the tridentate product
in refluxing toluene.
Treating a red dichloromethane solution of 2 [ν(CO)
) 1907 cm-1] with Na[HB(mt)3] results in the rapid
formation of a colorless intermediate with ν(CO) ) 1934
cm-1. We have not successfully isolated this intermedi-
ate because it spontaneously converts at room temper-
ature to a new complex with ν(CO) ) 1879 cm-1. By
analogy with the formation of 4, we assume that the
intermediate is a complex of the bidentate HB(mt)3
ligand (5). Heating the mixture to reflux for 5 min
results in complete conversion to the final product,
formulated as the desired osmaboratrane [B(mt)3Os-
(CO)(PPh3)] (1b). When the reaction was carried out as
a diethyl ether suspension, very slow decolorization
occurred over 4 days. The isolated solid had two ν(CO)
absorptions (ca 1:1 mixture) at 1895 and 1869 cm-1. The
latter corresponds to the osmaboratrane (vide infra).
The former band relates to an intermediate (6), which
is however different from that observed in dichlo-
romethane since dissolution of the mixture at ambient
temperature results in the immediate conversion to 1b.
Thus the rate-limiting step in the formation of 1b from
2 would appear to be the conversion of 5 to 6. Further-
more, phosphine is not liberated in this step, suggesting
that the conversion of 5 to 6 involves the displacement
of one phosphine ligand, an inference supported by the
(15) No¨th, H.; Wrackmeyer, B. Nuclear Magnetic Resonance Spec-
troscopy of Boron Compounds; Springer-Verlag: New York, 1978.
(16) Shorter Os-B bonds are observed in five-coordinate osmium
boryls, e.g., [Os(BCat)Cl(CO)(PPh3)2], 2.019(3) Å, and [Os{B(OEt)2}-
Cl(CO)(PPh3)2], 2.076(3) Å.8b
(14) Burns, I. D.; Hill, A. F.; White, A. J . P.; Williams, D. J .; Wilton-
Ely, J . D. E. T. Organometallics 1998, 17, 1552.

Notes
Organometallics, Vol. 23, No. 4, 2004 915
Ch a r t 1. (a ) Hyd r otr is(m eth im a zolyl)bor a te a n d
(b) Tr is(m eth im a zolyl)bor a n e (m eta lla bor a tr a n e)
Coor d in a tion
Ch a r t 2. Reson a n ce F or m s for (a )
Meta lla bor a tr a n e a n d (b) Ba se-Sta bilized Bor yl
Com p lex
in 1b is of a purely σ-nature.17 We are therefore
currently investigating the generality of metallabo-
ratrane formation with respect to variations in d-
configuration, geometry, and principle quantum number
(3d, 4d, and 5d) of the metal.
Exp er im en ta l Section
Gen er a l P r oced u r es. Conventional Schlenk and vacuum
line techniques were employed for the exclusion of air. The
salt Na[HB(mt)3]18 was prepared by heating Hmt and Na[BH4]
in refluxing xylenes according to a published procedure.4 The
complex [Os(C6H5)Cl(CO)(PPh3)2] has been described previ-
ously.13 Elemental microanalytical data were obtained com-
mercially from the University of North London Analytical
Service.
F igu r e 1. (a) Molecular structure of 1b. Selected bond
distances (Å) and angles (deg): Os-B 2.171(8), Os-C
1.826(8), Os-P 2.403(2), Os-S(1) 2.424(2), Os-S(2) 2.406(2),
Os-S(3) 2.493(2), S(1)-C(1) 1.726(9), S(2)-C(7) 1.715(9),
S(3)-C(13) 1.720(8), B-N(2) 1.572(12), B-N(8) 1.572(12),
B-N(14) 1.564(12), Os-B-N(2) 110.3(5), Os-B-N(8)
111.5(6), Os-B-N(14) 111.6(6), N(2)-B-N(8) 113.0(7),
N(2)-B-N(14) 104.6(7), N(8)-B-N(14) 105.6(7). (b) View
of the structure of 1b perpendicular to the {N(2)N(8)N-
(14)} plane showing the different folding of the mt rings
away from local C3 symmetry.
Syn th esis of [B(m t)3Os(CO)(P P h 3)] (1b). A mixture of
[Os(C6H5)Cl(CO)(PPh3)2] (0.126 g, 0.147 mmol) and Na[HB-
(mt)3] (0.060 g, 0.16 mmol) in dichloromethane (20 mL) was
stirred for 30 min, by which time the solution had decolorized.
The mixture was then heated under reflux for 20 min and then
diluted with ethanol (20 mL). The solvent volume was reduced
in vacuo to ca. 10 mL, and the colorless crystals that formed
were isolated by filtration and washed with ether. Yield: 0.122
g (0.141 mmol, 96%). The complex is air stable both as a solid
and in solution. Crystallographic grade crystals were grown
by slow vertical diffusion of a dichloromethane solution of the
complex through layers of diethyl ether and hexane. IR (CH2-
Cl2): 1879 [ν(CO)]; (Nujol):* 1862vs [ν(CO)], 1562w, 1408w,
1297w, 1280w, 1261w, 1204s, 1018vs cm-1. *Values for
samples crystallized from dichloromethane and ethanol. The
solid product isolated from carrying out the synthesis in diethyl
ether suspension has ν(CO) ) 1869 cm-1; however when
dissolved in dichloromethane, a ν(CO) value of 1879 cm-1 is
observed, which is identical to that for samples crystallized
from dichloromethane and ethanol, suggesting crystal poly-
morphism. FAB-MS (nba matrix): m/z (%) 832(92) [M]+,
804(13) [M-CO]+, 719(19) [M-mt]+, 689(4) [M-mt-CO]+,
570(7) [M-PPh3]+, 542(100) [M-PPh3-CO]+. NMR (CDCl3, 25
°C) 1H: 3.65 (s, 3 H, NCH3), 3.92 (s br, 6 H, NCH3), 6.00, 6.42
(s br × 2, 1 H × 2, NCHdCH), 6.50, 6.55 (s × 2, 2 H × 2,
NCHdCH), 7.27, 7.36, 7.52 (m × 3, 15 H, C6H5) ppm. 11B: 12.4
ppm. 31P{1H}: 6.14 (s br, ω1/2 ) 3.84 ppm, 420 Hz). 13C{1H}:
133.6 [C4(C6H5)], 128.6, 127.6 [C2,3,5,6(C6H5)], 121.9, 121.4,
The disparity between the geometry of the S(3)-based
heterocycle and those based on S(1) and S(2) is reflected
in the osmium-sulfur bonding with Os-S(1) and Os-
S(2) being respectively 44 and 35σ shorter than the Os-
S(3) bond length of 2.493(2) Å. This is however not
manifest in statistically significant variations in the
corresponding C-S thione bond lengths of the hetero-
cycles [1.715(9)-1.726(9) Å].
Con clu sion s
The structural characterization of the first osmabo-
ratrane reveals an Os-B interaction that is longer than
found in octahedral osmium boryls or base-stabilized
borylenes for which a degree of OsdB multiple bonding
can be inferred. The sp3 hybridization of boron in
boratranes leaves no orbitals of suitable symmetry for
π-bonding, and so it may be assumed that the bonding
(17) We have ignored the possibility of (dπ)f(B-Nσ*) hyperconju-
gation in the absence of a full molecular orbital treatment.
(18) Garner, M.; Reglinski, J .; Cassidy, I.; Spicer, M. D.; Kennedy,
A. R. Chem. Commun. 1996, 1975.

916 Organometallics, Vol. 23, No. 4, 2004
Notes
117.5, 117.0 (C3N2), 33.3 (1C, NCH3), 32.8 (2C, NCH3) ppm
(quaternary carbon resonances CO, CS, and CP not observed
due to low solubility). Anal. Found: C, 44.86; H, 3.73; N, 9.96.
Calcd for C31H30BN6OOsPS3: C, 44.82; H, 3.64; N, 10.12.
Crystal data: [C31H30N6OBPS3Os]‚2CH2Cl2.Et2O, M ) 1074.74,
monoclinic, P21/n (no. 14), a ) 16.690(2) Å, b ) 11.1483(10)
Å, c ) 24.269(2) Å, â ) 107.365(9)°, V ) 4310.0(7) Å3, Z ) 4,
Dc ) 1.656 g cm-3, µ(Mo KR) ) 3.430 mm-1, T ) 293 K, pale
yellow prisms; 7548 independent measured reflections, F2
refinement, R1 ) 0.044, wR2 ) 0.086, 4672 independent
observed absorption corrected reflections [|Fo| > 4σ(|Fo|), 2θmax
) 50°], 460 parameters. CCDC 223925.
Ack n ow led gm en t. We gratefully acknowledge the
financial support of the E.P.S.R.C (U.K.) and the
Australian Research Council (DP034270).
Su p p or tin g In for m a tion Ava ila ble: X-ray crystallo-
graphic file in CIF format for the structure of compound 1b‚
2CH2Cl2‚Et2O; a table of selected 11B NMR and structural data
for boryl and borylene complexes of osmium post-dating ref
11. This material is available free of charge via the Internet
at http://pubs.acs.org.
OM0306123