Formation of metallaboratranes: The missing link. the first iridaboratranes, [IrH(CO)(PPh3){K3-B,S,S′-B(mt) 2R}](Ir→B) (mt = methimazolyl, R = mt, H)

Article abstract of DOI:10.1021/om040128f

The reaction of Vaska's complex with 1 equiv of Na[HB(mt)₃] or Na[H₂(mt)₂] (mt = methimazolyl) affords the iridaboratranes [IrH(CO)(PPh₃){k³-B,S,S′-B(mt) ₂R}](Ir^I→B) (R = mt, H),

Full text of DOI:10.1021/om040128f

1062
Organometallics 2005, 24, 1062-1064
Formation of Metallaboratranes: The Missing Link. The
First Iridaboratranes,
[IrH(CO)(PPh3){K3-B,S,S′-B(mt)2R}](IrfB)
(mt ) Methimazolyl, R ) mt, H)
Ian R. Crossley, Anthony F. Hill,* and Anthony C. Willis
Research School of Chemistry, Institute of Advanced Studies, Australian National University,
Canberra, Australian Capital Territories, Australia
Received November 16, 2004
Chart 1. Metallaboratranes
Summary: The reaction of Vaska’s complex with 1 equiv
of Na[HB(mt)3] or Na[H2B(mt)2] (mt ) methimazolyl)
affords the iridaboratranes [IrH(CO)(PPh3){κ3-B,S,S′-
B(mt)2R}](IrIfB) (R ) mt, H), the first examples of met-
allaboratranes in which the transannular metalfboron
bond is supported by only two methimazolyl buttresses.
The elucidation of the metallaboratrane structural
motif, in the compounds [M(CO)(PPh3){B(mt)3}](MfB)
(M ) Ru (1),1 Os (2),2 mt ) methimazolyl; Chart 1),
afforded the first unequivocal evidence for long mooted
dative metalfboron interactions.3 Within these tricyclo-
[3.3.3.0] cage structures, the transannular MfB inter-
action is supported and sterically shielded by three
methimazolyl buttresses, which constrain the d8 ML5
fragment to adopt a monovacant octahedral geometry.
The pair of metal valence electrons in such a constrained
geometry are housed in a metal-based orbital of σ
symmetry, oriented toward the Lewis acidic BIII center.
The driving force for assembly of the metallaboratrane
motif, and hence the dative interaction, may thus be
considered a function of several factors, including d
occupancy, geometric constraints, and metal σ basicity.
In seeking to explore the inherent limitations of these
factors, we have subsequently reported the preparation
of metallaboratranes from groups 9 (Rh)4,5 and 10 (Pt)6
and have thus established this geometry to be a general
motif for metals with d8 and d10 electron configurations.
The first examples of the metallaboratrane geometry
were obtained by the reaction of the hydrotris(methi-
mazolyl)borate salt Na[HB(mt)3]7 with d6 transition-
metal organyls of the type [M(R)Cl(L)(PPh3)3] (ML )
RuCO, OsCO, RhCl; R ) aryl, vinyl) to afford the d8
metallaboratranes 1, 2, and [RhCl(PPh3){B(mt)3}] (3),
respectively. Implicit in their formation is a process
which involves activation of an agostic B-H-M linkage
(vide infra) and subsequent reductive elimination of
R-H: hence, the role of the σ-organyl as a hydrogen
acceptor. However, with our recent report of the pla-
tinaboratrane salt [PtH(PPh3){B(mt)3}]Cl(PtfB) (4‚Cl)6
we have illustrated the capacity to obtain stable ex-
amples of the metallaboratrane motif directly from low-
valent, nonorganometallic precursors, 4‚Cl being pre-
pared from Na[HB(mt)3] and cis-[PtCl2(PPh3)2]. We have
thus begun to further examine the elements of group 9,
reasoning that isoelectronic precursors may be similarly
amenable to metallaboratrane formation. Moreover,
pursuant to our goal of elucidating mechanisms that
might operate, we envisaged that the incorporation of
strongly π-acidic ancillary ligands might reduce the
lability of key intermediate species and thus facilitate
their isolation. For this purpose Vaska’s complex, [IrCl-
(CO)(PPh3)2], was chosen. Herein, we report the syn-
thesis of the first iridaboratranes, [IrH(CO)(PPh3)-
{κ3-B,S,S′-B(mt)2R}](IrfB) (R ) mt, H), which exhibit
an unprecedented κ3-B,S,S′ chelation mode.
The reaction of Na[HB(mt)3] with [IrCl(CO)(PPh3)2]
in dichloromethane solution results in the formation of
a single product, which we formulate as the iridabo-
ratrane [IrH(CO)(PPh3){κ3-B,S,S′-B(mt)3}] (5) on the
basis of spectroscopic and analytical data (Scheme 1).8
While crystallographic grade crystals of 5 have thus far
proven elusive, the formulation follows unequivocally
from multinuclear NMR spectroscopic studies. Thus, the
1H NMR spectrum reveals the presence of three unique
methimazolyl environments, a single triphenylphos-
phine moiety, and a discrete metal hydride (δH -11.8),
* To whom correspondence should be addressed. E-mail: a.hill@
anu.edu.au.
(1) (a) Hill, A. F.; Owen, G. R.; White, A. J. P.; Williams, D. J. Angew.
Chem., Int. Ed. 1999, 38, 2759. (b) Foreman, M. R. St.-J.; Hill, A. F.;
Owen, G. R.; White, A. J. P.; Williams, D. J. Organometallics 2003,
22, 4446.
(2) Foreman, M. R. St.-J.; Hill, A. F.; White, A. J. P.; Williams, D.
J. Organometallics 2004, 23, 913.
(3) Gilbert, K. B.; Boocock, S. K.; Shore, S. G. In Comprehensive
Organometallic Chemistry; Abel, E. W., Stone, F. G. A., Wilkinson, G.,
Eds.; Pergamon: Oxford, U.K., 1982; Vol. 6, pp 880-886.
(4) Crossley, I. R.; Foreman, M. R. St.-J.; Hill, A. F.; White, A. J.
P.; Williams, D. J. Chem. Commun., in press.
(5) Crossley, I. R.; Hill, A. F.; Humphrey, E. R.; Willis, A. C.
Submitted for publication in Chem. Commun.
(6) Crossley, I. R.; Hill, A. F. Organometallics 2004, 23, 5656.
(7) Garner, M.; Reglinski, J.; Cassidy, I.; Spicer, M. D.; Kennedy,
A. R. Chem. Commun. 1996, 1975.
10.1021/om040128f CCC: $30.25 © 2005 American Chemical Society
Publication on Web 02/12/2005

Communications
Scheme 1. Formation of Iridaboratranesa
Organometallics, Vol. 24, No. 6, 2005 1063
the B-H bond.9 Formation of 4‚Cl is then believed to
ensue by dissociation of the remaining chloride ligand
and chelation of the pendant mt group. Complex 5
possesses no such labile ligand to allow coordination of
the third mt group.
Complex 5 thus represents a “missing link” in the
evolution of the metallaboratrane geometry and offers
the first qualitative evidence for the stability of, and
predilection for attaining, the dative metal-boron in-
teraction. Indeed, of the previously isolated tricyclo-
[3.3.3.0]metallaboratranes, it might have been argued
that it is the constrained cage geometry, inherent from
the three buttressing methimazolyl donors, which dic-
tates the spatial proximity of the boron(III) center and
metal lone pair, rather than any predisposition toward
MfB dative bonding. In contrast, the dibuttressed cage
of 5 affords greater flexibility and does not preclude
relaxation of the metal geometry to a potentially prefer-
able square-planar arrangement, with dissociation of
PPh3. That such relaxation is not observed is presum-
ably attributable to the presence of the IrfB linkage.
This would seem to confirm that the dative metal-boron
interaction is a fundamental component of the bonding
scenario within the metallaboratrane motif and is not
merely a consequence of geometry.
Encouraged by our successful synthesis of compound
5, we sought to establish whether the pendant mt group
is influential in the B-H addition step and attempted
an analogous reaction with the salt Na[H2B(mt)2]. We
note that while several examples of the anionic H2B-
(mt)2 ligand adopting a κ3-S,S′,H chelation mode in
transition-metal complexes have been reported,10 no
evidence for insertion of a metal into the agostic B-H
function has been previously observed. However, the
reaction of Na[H2B(mt)2] with [IrCl(CO)(PPh3)2] affords
a single product, in a manner analogous to that for 5,
which we have unequivocally established to be [IrH-
(CO)(PPh3){κ3-B,S,S′-B(mt)2H}] (6) by both spectro-
scopic and crystallographic analysis.11 Spectroscopically,
compound 6 reflects the characteristic traits observed
a Legend: L ) PPh3, mt ) methimazolyl, * ) proposed
intermediate.
which is believed to lie cis to the phosphine, though 31P-
1H coupling is not resolved. Notable by their absence
are resonances associated with either agostic B-H-Ir
or terminal B-H groups, their signatures being simi-
larly absent from the infrared spectrum, which does,
however, verify the retention of the carbonyl ligand (νCO
1990 cm-1). The existence of an iridium-boron bond is
inferred from the observation of a single, significantly
broadened resonance in the 31P{1H} NMR spectrum (δP
4.52, half-height width 136 Hz), due to the triphenyl-
phosphine ligand. This is consistent with those we have
observed for other metallaboratranes in which a phos-
phine lies transoid to a dative metal-boron interaction.
Having accounted for four coordination sites (CO, B, H,
P), this leaves only two remaining for mt chelation; i.e.,
we must assume a pendant mt group.
Complex 5 is the first example of an iridaboratrane,
and its isolation reinforces the emerging generality of
this structural motif. Moreover, this is the first instance
in which the B(mt)3 fragment has been observed to
adopt a bicyclo[3.3.0] cage structure at a metal center.
This is significant, since our proposed mechanism for
the formation of the platinaboratrane salt 4‚Cl, derived
by analogy to those for compounds 1-3, invokes such a
species as the penultimate product, arising from (i)
κ2-S,S′ chelation of the HB(mt)3 ligand, (ii) dissociation
of a phosphine ligand with formation of an agostic
B-H-M interaction,1b and (iii) insertion of a metal into
(9) The term “oxidative addition” is not applicable in this instance,
since the metal oxidation state is preserved.
(10) See for example: (a) Garcia, R.; Paulo, A.; Domingos, A.; Santos,
I.; Ortner, K.; Alberto, R. J. Am. Chem. Soc. 2000, 122, 11240. (b)
Garcia, R.; Paulo, A.; Domingos, A.; Santos, I. J. Organomet. Chem.
2001, 632, 41. (c) Garcia, R.; Xing, Y.-H.; Paulo, A.; Domingos, A.;
Santos, I. Dalton 2002, 4236. (d) Foreman, M. R. St.-J.; Hill, A. F.;
Tshabang, N.; White, A. J. P.; Williams, D. J. Organometallics 2003,
22, 5593. (e) Abernethy, R.; Hill, A. F.; Neumann, H.; Willis, A. C.
Inorg. Chim. Acta, in press.
(11) Data for 6 are as follows. Yield: 83%. NMR (CDCl3, 25 °C): 1H
(299.945 MHz), δH -13.4 (s br, 1 H, IrH), 3.35, 3.47 (s × 2, 3 H × 2,
3
NCH3 × 2), 6.69, 6.72 (d × 2, JHH ) 2.0 Hz, 1 H × 2, NCHdCH),
3
6.74, 6.81 (d × 2, JHH ) 1.9 Hz, 1 H × 2, NCHdCH), 7.60-7.52 (m,
6 H, PPh3), 7.43-7.35 (m, 9 H, PPh3); 13C{1H} (75.428 MHz), δC 33.8,
(8) Data for 5 are as follows. Yield: 66%. IR (KBr): νCtO 1990 νIrH
2130 cm-1. NMR (CDCl3, 25 °C): 1H (299.945 MHz), δH -11.8 (s. br.,
1 H, IrH), 3.39, 3.54, 3.58 (s × 3, 3 H × 3, NCH3), 6.54, 6.64 (d × 2,
34.4 (NCH3 × 2), 120.0, 120.3 (NCHdCH), 122.3, 122.7 (NCHdCH),
3
4
128.1 (d, JPC ) 9.6 Hz, C3,5(C6H5)), 129.8 (d, JPC ) 1.5, C4(C6H5)),
133.8 (d, 2JPC ) 13.0, C2,6(C6H5)), 135.5 (d, JPC ) 35.0 Hz, C1(C6H5)),
1
3JHH ) 2.3 Hz, 1 H × 2, NCHdCH), 6.59, 6.61 (d × 2, JHH ) 2.0 Hz,
164.6, 164.9 (CdS), 175.6 (CtO); 31P{1H} (121.420 MHz), δP 6.2 (m
br, hhw 95 Hz); 11B{1H} (96.232 MHz), δB -4.5 (s br, hhw 300 Hz).
Anal. Found: C, 42.96; H, 3.62; N, 7.23; S, 8.00. Calcd for C27H27BIrN4-
OPS2‚0.5CH2Cl2: C, 43.23; H, 3.69; N, 7.33; S, 8.39. N.B.: multiple
recrystallizations were required to completely remove residual PPh3
and provide an analytically pure sample of the dichloromethane
hemisolvate (1H NMR). Crystal data for 6‚CHCl3: [C27H27BIrN4OPS2]‚
CHCl3, Mr ) 841.05, triclinic, P1h (No. 2), a ) 9.3509(2) Å, b ) 9.7847-
(2) Å, c ) 19.1038(5) Å, R ) 92.759(2)°, â ) 99.661(1)°, γ ) 110.926(1)°,
3
1 H × 2, NCHdCH), 6.77, 6.92 (m br × 2, 1 H × 2, NCHdCH), 7.57-
7.48 (m, 6 H, PPh3), 7.41-7.32 (m, 9 H, PPh3); 13C{1H} (75.428 MHz),
δC 33.9, 34.4, 34.5 (CH3 × 3), 117.3, 118.9 (NCHdCH), 118.0 (br, NCHd
CH × 2) 122.4, 122.8 (NCHdCH), 128.1 (d, 3JPC ) 9.5 Hz, C3,5(C6H5)),
129.6 (d, 4JPC ) 2.0 Hz, C4(C6H5)), 133.8 (d, 2JPC ) 12.5 Hz, C2,6(C6H5)),
134.7 (d, 1JPC ) 36.3 Hz, C1(C6H5)), 163.6, 163.9, 166.3, 173.7 (CdS ×
3, CtO); 31P{1H} (121.420 MHz), δP 4.52 (m br, hhw 162 Hz); 11B{1H}
(96.232 MHz) δB 3.19 (s br, hhw 149 Hz). Anal. Found: C, 44.36; H,
3.80; N, 9.84; S, 11.13. Calcd for C31H31BIrN6OPS3: C, 44.66; H, 3.75;
N, 10.08; S, 11.54. N.B.: multiple recrystallizations from dichoromethane
and light petroleum were necessary to obtain satisfactory elemental
microanalytical data, which nevertheless indicate residual dichlo-
romethane (Cl, 0.57).
V ) 1598.35(7) Å3, Z ) 2, Dc ) 1.747 g cm-3, µ(Mo KR) ) 46.4 cm-1
,
T ) 200(2) K, yellow plates, 7345 independent measured reflections,
F refinement, R1 ) 0.030, wR2 ) 0.035 for 6032 independent
absorption corrected reflections (I > 3σ(I); 2θmax ) 48°), 373 parameters,
CCDC252699.

1064 Organometallics, Vol. 24, No. 6, 2005
Communications
the molecular geometry is also constrained by chelating
sulfur donors,14 with less inherent flexibility than those
of 6. Overall, however, the Ir-B separation in 6 is
consistent with the formulation of a dative interaction,
as compared with these examples and on the basis of
covalent radii considerations (∑r(IrB) ) 2.15 Å). The
Ir-S distances are largely unremarkable, a strong
inverse trans influence being observed for Ir-S2 (2.4173-
(10) Å; cf. Ir-S1 ) 2.4847(11) Å) due to the carbonyl
ligand. More unusual is the somewhat long Ir-P linkage
(2.4113(10) Å), which exceeds over 96% of those recorded
by the CCDC and is notably greater than the sum of
covalent radii (∑r(IrP) ) 2.37 Å). This would seem
counterintuitive, since a dative IrfB interaction might
be anticipated to exert an inverse trans influence.
However, a similar situation has been observed in the
dirhodaboratrane salt [Rh2{B(mt)3}2{κ-S,S′-HB(mt)3}]-
Cl,4 where lengthening of the bridging Rh-S linkages
trans to boron was apparent. For the rhodium example,
this effect is arguably consistent with an alternative
bonding description, in which the bridgehead boron is
described not as a Lewis acid but as an internally base-
stabilized σ-boryl, acting as a donor to a Rh(III) center
and thus exerting a strong trans influence. A similar
argument may be applied to 6, and indeed a marginal
(5σ) discrepancy between the two B-N (1.557(6), 1.581(6)
Å) linkages together with the relatively high carbonyl
stretching frequency (1988 cm-1) may well afford this
credence. However, it remains our belief that while such
contributions cannot be discounted, electroneutrality
dictates that these be minor components of the overall
bonding scenario. Nonetheless, we continue to explore
the possibilities and implications in this respect.
In conclusion, we have reported the synthesis of
the first iridaboratrane complexes, viz. [IrH(CO)(PPh3)-
{κ3-B,S,S′-B(mt)3}] (5) and [IrH(CO)(PPh3){κ3-B,S,S′-
B(mt)2H}] (6). These represent the first examples of the
metallaboratrane motif in which the dative metalfboron
bond is buttressed by only two methimazolyl groups,
affording the complexes greater flexibility and thus
potentially allowing relaxation to square-planar geom-
etries. The isolation of these compounds thus implies
the acquisition of the dative metalfboron interaction
to be inherently favorablesnot merely a consequence
of a constrained monovacant octahedral geometrysand
furthermore supports our proposed mechanism for the
formation of such species. Moreover, complex 6 is the
first metallaboratrane to possess a sterically unimpeded
MfB linkage; thus, there exists now viable scope for
derivatization at the metal-boron interface, a scenario
which we are currently exploring.
Figure 1. Molecular geometry of [IrH(CO)(PPh3){κ3-
B,S,S′-B(mt)2H}] (6) in 6‚CHCl3 with carbon-bound hydro-
gen atoms omitted for clarity (50% displacement ellipsoids).
Selected bond lengths (Å) and angles (deg): Ir-B )
2.210(5), Ir-C ) 1.826(5), Ir-P ) 2.411(1), Ir-S(1) )
2.485(1), Ir-S(2) ) 2.417(1), S(1)-C(11) ) 1.717(5),
S(2)-C(21) ) 1.703(5), B-N(12) ) 1.557(6), B-N(22) )
1.581(6); Ir-B-N(12) ) 107.8(3), Ir-B-N(22) ) 106.5(3),
N(12)-B-N(22) ) 107.4(3).
for 5, viz. (i) unique environments for each mt group,
(ii) a discrete Ir-H function (δH -13.4, d, JPH ) 7.6 Hz),
(iii) retention of the carbonyl ligand (νCtO 1988 cm-1),
(iv) absence of an agostic B-H-Ir group, and (v)
transoid disposition of a single PPh3 moiety and dative
IrfB interaction. Additionally, while it is not resolved
1
in the H NMR spectrum, the presence of a terminal
B-H function is verified by a weak infrared stretching
mode at 2384 cm-1
.
The molecular geometry of 6, in a crystal of the
monochloroform solvate, is illustrated in Figure 1, which
confirms an octahedral arrangement about iridium, with
a direct Ir-B bond (2.210(5) Å) buttressed by two
methimazolyl bridges. The proposed arrangement of
ancillary ligands is also confirmed, and a peak attribut-
able to the iridium hydride is also observed in the
electron density difference map, though this was refined
under restraints to an idealized position. While mean-
ingful discussion of the geometric parameters is largely
precluded, due to the absence of any suitably compa-
rable structures in the Cambridge Structural Database
(CCDC), several features are noteworthy. The iridium-
boron linkage may be compared to those observed in
σ-boryl complexes, the most closely aligned examples
of which are perhaps the 9-BBN complex [IrH2(PMe3)3-
(9-BBN)] (2.093 Å)12 and the catecholborane derivative
[IrHCl(CO)(PPh3)2(Bcat)] (2.044 Å),13 though neither is
directly comparable, each being three-coordinate at
boronsa feature which would be expected to shorten the
Ir-B separation. Perhaps more applicable are σ-carbo-
ranyl complexes, wherein the chelated B(III) vertex
might be considered to act as a Lewis acid, engaged in
dative metalfboron bonding. The few such examples
which exist typically exhibit Ir-B linkages in the range
2.09-2.13 Å, though it is noteworthy that in each case
Acknowledgment. We thank the Australian Re-
search Council (ARC) for financial support (Grant No.
DP034270).
Supporting Information Available: Figures and tables
giving full details of the crystal structure determination of 6‚
CHCl3 (CCDC 252699), including positional and thermal
parameters, and text giving synthetic procedures for 5 and 6.
This material is available free of charge via the Internet at
http://pubs.acs.org.
OM040128F
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