Metallaboratranes: Bis- and tris(methimazolyl)borane complexes of group 9 metal carbonyls and thiocarbonyls

Article abstract of DOI:10.1021/om900655k

The iridium poly(methimazolayl)borane complexes [IrH(CE)(PPh₃) {κ³-B,S,S-B(mt)₂R}](Ir→B) (mt = methimazolyl = 2-mercapto-3-methylimidazol-l-yl; E = O, S; R = mt, H) are described in detail. For R = mt, these materials are elucidated as paradigms for the final mechanistic intermediate in metallaboratrane formation, a role illustrated through hydride abstraction to afford the cationic salts [Ir(CE)(PPh ₃){κ⁴-B(mt)₃}]X(Ir→)⁸ (E = O, S; X = Cl, BF₄). The rhodium poly(methimazolyl)borate complexes [Rh(CO)(PPh₃){κ²-S,S'-HB(mt)₂R}] (R = mt, H) are also reported. These compounds are obtained in preference to the respective borane complexes (analogous to iridium); however [Rh(CO)(PPh ₃){κ²-S,S'-HB(mt)₃}] is observed to undergo facile solution-phase conversion to [Rh(CO)(PPh₃) {κ⁴-B,S,S ,S"-B(mt)₃}]Cl(Rh→B)⁸ in chlorinated solvents. The ramifications of these results, with respect to metallaboratrane formation, are discussed, substantiating previous mechanistic conjecture. In an attempt to establish an alternative route to iridaboratranes, the first isolable tris(methimazolyl)borate complex of iridium, cis,cis-[IrHCl(PPh₃)₂{κ2-S,S'-HB(mt)₃], is reported and shown not to evolve to the iridaboratrane [IrCl(PPh ₃){κ⁴-B(mt)₃}](Ir→B)⁸ under conditions that lead to the corresponding rhodaboratrane. Factors are discussed that may contribute to this fine balance between the formation of methimazolylborate and methimazolylborane complexes.

Full text of DOI:10.1021/om900655k

326 Organometallics 2010, 29, 326–336
DOI: 10.1021/om900655k
Metallaboratranes: Bis- and Tris(methimazolyl)borane Complexes of
Group 9 Metal Carbonyls and Thiocarbonyls
Ian R. Crossley,*,†,‡ Anthony F. Hill,*,† and Anthony C. Willis†
†
Research School of Chemistry, Institute of Advanced Studies, Australian National University,
Canberra, Australian Capital Territory, Australia and ‡Department of Chemistry,
University of Sussex, Brighton, United Kingdom
Received July 24, 2009
The iridium poly(methimazolyl)borane complexes [IrH(CE)(PPh3){κ3-B,S,S0-B(mt)2R}](IrfB)8
(mt=methimazolyl=2-mercapto-3-methylimidazol-1-yl; E=O, S; R=mt, H) are described in detail.
For R = mt, these materials are elucidated as paradigms for the final mechanistic intermediate in
metallaboratrane formation, a role illustrated through hydride abstraction to afford the cationic salts
[Ir(CE)(PPh3){κ4-B(mt)3}]X(IrfB)8 (E=O, S; X=Cl, BF4). The rhodium poly(methimazolyl)borate
complexes [Rh(CO)(PPh3){κ2-S,S0-HB(mt)2R}] (R = mt, H) are also reported. These compounds are
obtained in preference to the respective borane complexes (analogous to iridium); however [Rh(CO)-
(PPh3){κ2-S,S0-HB(mt)3}] is observed to undergo facile solution-phase conversion to [Rh(CO)(PPh3)-
{κ4-B,S,S0,S00-B(mt)3}]Cl(RhfB)8 in chlorinated solvents. The ramifications of these results, with respect
to metallaboratrane formation, are discussed, substantiating previous mechanistic conjecture. In an
attempt to establish an alternative route to iridaboratranes, the first isolable tris(methimazolyl)borate
complex of iridium, cis,cis-[IrHCl(PPh3)2{κ2-S,S0-HB(mt)3], is reported and shown not to evolve to the
iridaboratrane [IrCl(PPh3){κ4-B(mt)3}](IrfB)8 under conditions that lead to the corresponding rhoda-
boratrane. Factors are discussed that may contribute to this fine balance between the formation of
methimazolylborate and methimazolylborane complexes.
Introduction
and coordination chemists alike. As a consequence, the
literature contains numerous examples of these sulfur-donor
ligands acting as monodentate (κ1-S), bidentate (κ2-S,S0),
and tridentate (κ3-S,S0,S00 or κ3-S,S0,H) chelates to mid-late
transition metals in low oxidation states,4-11 with more
recent reports illustrating a capacity to ligate early (NbV,
)
in higher oxidations states.
In an organometallic context, of some note has been the
unprecedented capacity for these ligands to undergo B-H
activation at metal centers, leading to the installation of a σ-
1
The poly(methimazolyl)borate ligands HB(mt)3 and
H2B(mt)22 (mt=methimazolyl), purported to serve as “soft
analogues” of the ubiquitous poly(pyrazolyl)borates,3 have
captured the imaginations of organometallic, bioinorganic,
TaIII-V, TiIII-IV, ZrIV 12,13
and late (PtIV)14 transition metals
*To whom correspondence should be addressed. E-mail: a.hill@
anu.edu.au.
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r
pubs.acs.org/Organometallics
Published on Web 12/22/2009
2009 American Chemical Society

Article
Organometallics, Vol. 29, No. 2, 2010 327
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retrodative MfB linkage is supported by three buttressing
methimazolyl units, affording a tricyclo-[3.3.3.0] cage, have
Chart 1. Selected Metallaboratranesa
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a MLL0 = Ru(CO)(PPh3), Os(CO)(PPh3), Fe(CO)2, RhCl(PPh3),
Rh(cod)þ, Rh(PMe3þ)2þ, Rh(S2CNMe2), RhH(PPh3), Rh(PPh3)(CNC6-
H2Me3), PtH(PPh3) , PtI2; M0L00 = Pt(PPh3), Co(PPh3)þ NiBr, Pd-
(PMe3); M00L00 = Rh(DMAP), Rh(CO), PdCl, PtCl; M000 = Ni, Pd, Pt,
CuCl, AgCl, AuCl, Auþ; DMAP = N,N0-dimethylaminopyridine.
subsequently become a subject of fascination, with respect to
the processes involved in their formation and their structural
facets. In seeking to explore these features we have prepared
several more examples from groups 9 (Rh,21 Ir22) and
10 (Pt23), including both neutral and cationic, octahedral
and trigonal bipyramidal species, thereby demonstrating a
degree of generality for the metallaboratrane structural
motif. Rabinovich has isolated a rare example of a coordi-
natively unsaturated cationic cobaltaboratrane, [Co(PPh3)-
{B(mimtBu)3}]þ,24,25 while Tatsumi has described the para-
magnetic nickelaboratrane [NiCl{B(mimtBu)3}],26 although
the mechanisms by which these two unique examples form
remain unknown. Subsequently, Parkin showed that not
only could the chloride in Tatsumi’s nickeloboratrane be
metathesized with various halides and pseudohalides but
the NifB bond could be oxidatively cleaved with iodine,
haloforms, and XeF2.27a Parkin has reported mimtBu and
mimPh analogues27b of the previously reported complexes
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only structurally authenticated examples of MfB dative bonding.
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mazolyl would have the abbreviation mimMe; however the simpler “mt”
is widely used in the literature, while Bm and Tm are often used to refer
to the H2B(mt)2 and HB(mt)3 ligands, respectively.
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328 Organometallics, Vol. 29, No. 2, 2010
Crossley et al.
[RhCl(PPh3){B(mt)3}]21 and [IrH(CO)(PPh3){B(mt)3}] (1),22
in addition to the novel ferra- and palladaboratranes
[Fe(CO)2{B(mimtBu)3}]27c and [Pd(PMe3){B(mimtBu)3}],27d
the latter being an analogue of the platinaboratrane
[Pt(PPh3){B(mt)3}].23a Connelly has demonstrated that a
rhodaboratrane may also be constructed in which the methi-
mazolyl buttress is replaced by the 4-ethyl-3-methyl-5-mer-
capto-1,2,4-triazolylheterocycle.28 While it might appear
that metallaboratrane formation is a peculiarity of the geo-
metric features of HB(mt)3 and H2B(mt)2 ligands, in a highly
significant breakthrough, Bourissou has shown that metal-
laboratranes may be prepared via the direct reaction of late
transition metal complexes with preformed neutral ω-phos-
phinoboranes.29 The implications of this are far-reaching
and promise a significant broadening of the field, which is
already well underway.30
Scheme 1. Mechanistic Proposal for Rhodaboratrane and Pla-
tinaboratrane Formation (L = PPh3)a
Previously, we presented the first systematic investigation
of a series of isoelectronic rhodaboratranes, which differed in
their ancillary co-ligands and complex charge.21 In so doing,
we gleaned insights into (i) the processes involved in their
formation and (ii) their co-ligand substitution chemistry, and
we (iii) presented a qualitative bonding model. Subsequently,
quantitative theoretical studies of various metallaboratranes
have appeared,26,27,29,30 which point to variable degrees of
electron transfer from metal to boron with respect to the
metal-boron dative bond. Notably, complex charge see-
mingly exerts little influence over the capacity to maintain
the σ-retrodative RhfB linkage; indeed the cationic
rhodium(I) systems we have so far isolated appear to be
significantly more stable than their neutral counterparts.
This is further substantiated by the isolation of both neutral
and cationic metallaboratranes based on platinum(0) and
platinum(II) (Scheme 1).31
a (i) þ Na[HB(mt)3], - NaCl, - L; (ii) B-H activation; (iii) cage
closure following phosphine dissociation; (iv) cage closure following
chloride dissociation; (v) reductive deprotonation.
Scheme 2. Iridaboratrane Formation (L = PPh3)
In contrast, the nature of the ancillary ligand set would
seem to be of fundamental importance and thus warranted
further study. Indeed, we have on several occasions reached
this same conclusion, not least in our studies of the interac-
tion of Na[HB(mt)3] with Vaska’s complex, which afforded
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the unprecedented dibuttressed metallaboratrane [IrH(CO)-
(PPh3){κ3-B,S,S0-B(mt)3}](MfB)8 (1),22 apparently due to
the lack of a sufficiently labile ancillary ligand, and hence
latent coordination site, to accommodate the third methi-
mazolyl donor. The isolation of the complexes [IrH(CO)-
(PPh3){κ3-B,S,S0-BR(mt)2}](MfB)8 (R = mt 1, R = H 2,
Scheme 2) was significant in showing that the MfB
dative bond is not simply a geometric corollary of the
tricicylo-[3.3.3.0] cage topology. Given the parallels between
the chemistry of rhodium(I) and iridium(I) and the unique
character of the iridaboratranes thus far obtained, the
extended investigation of the group 9 carbonyls seemed
appropriate for assessing the mutual interplay of metal and
ancillary ligand. We thus report herein the reactions of
Na[H4-nB(mt)n] (n = 2, 3) with the complexes [MCl(CO)-
(PPh3)2] (M = Rh, Ir) and [IrCl(CS)(PPh3)2], leading to the
isolation of metallaboratrane and poly(methimazolyl)borate
(31) Our preferred method for assigning oxidation states assumes
that the boranes HB(mt)2 and B(mt)3, which are independently iso-
lable,32,33 coordinate as neutral ligands. Alternative perspectives on
oxidation state, valency, and dn configurations in metallaboratrane have
been presented: (a) Hill, A. F. Organometallics 2006, 25, 4741. (b) For a
perspective based on valency rather than oxidation states see: Parkin, G.
Organometallics 2006, 25, 4744.

Article
Organometallics, Vol. 29, No. 2, 2010 329
Table 1. Selected Infrared Data for Iridium Complexes
(L = PPh3)
Chart 2. Sulfur Dioxide Coordination Modes and Comparisons
with Metallaboratranes
complex
[IrCl(CO)L2]34
ν
CO/cm-1
1961
1988
1990
2015
2020
2034
2013
2025
2040
2025
2034
2046
2075
211841
[IrH(CO)L(HB(mt)2}]
[IrH(CO)L{B(mt)3}]
[IrCl(CO)L2(O2)]34
[IrCl(CO)L2(SO2)]35
[IrCl(CO)L2(RNSO)]a,36
[IrCl(CO)L2(CS2)]37
[IrCl(CO)L2(CSe2)]38
[IrCl(CO)L2(F2CdCF2)]39
[IrCl(CO)L2(F3CCtCCF3)]39
[IrD2Cl(CO)L2]b,34
[IrHCl2(CO)L2]34
[IrCl3(CO)L2]34
[IrH(BCat)Cl(CO)L2]40
a R = SO2C6H4Me-4. b Datum for the D2 adduct provided since the
Ir-H and CO modes couple in the H2 adduct (see ref 34).
complexes and the elucidation of processes leading to their
interconversion. Aspects of this work have contributed to
preliminary reports.21a,d
the Lewis acid boranes HB(mt)2 and B(mt)3 being neutral
isolable compounds,32,33 the values of νCO for 1 and 2 are
consistent with the description of these compounds as
adducts of iridium(I) (Table 1).34-41 Thus coordination of
a range of π-acceptor ligands (O2, CS2, CSe2, F2CdCF2,
F3CCtCCF3) to Vaska’s complex results in a substantial
increase in νCO of 55-80 cm-1, somewhat less than observed
for conventional two-fragment oxidative addenda (H2, HCl,
Cl2) for which the iridium(III) oxidation state is invoked
without debate. Perhaps the most germane entries are how-
ever the SO2 and iminooxosulfurane adducts.42 Sulfur diox-
ide was the first recognized example of what have in the
interim come to be termed “Z-type” ligands (or to use the
older and perfectly adequate term, Lewis acids). Thus
[IrCl(CO)(PPh3)2(SO2)] involves a dative bond from iridium
to a pyramidalized sulfur(IV) atom (A, Chart 2), which is
distinct from the two alternative coordination modes in
which the sulfur (B) or one SdO double bond (C) provides
Results and Discussion
Synthesis and Characterization of Iridaboratranes. The
dichloromethane-mediated reactions between equimolar
amounts of the salts Na[HnB(mt)4-n] (n=1, 2) and Vaska’s
complex [IrCl(CO)(PPh3)2] proceed readily over 3 h to
afford exclusively the complexes [IrH(CO)(PPh3){κ3-B,S,
S0-B(mt)3}] (1) and [IrH(CO)(PPh3){κ3-B,S,S0-BH(mt)2}]
(2), respectively (Scheme 2).
The formulation of 1 and 2 followed unequivocally from
spectroscopic data, based upon the key features and trends
1
that we have previously established.15,19-23 Thus, the H
NMR spectra clearly indicated the “locked” geometry asso-
ciated with the metallaboratrane cage, unique environments
being observed for each of the methimazolyl groups within
both 1 and 2. Additionally, the presence of a single triphenyl-
phosphine ligand is apparent from the aromatic proton
resonances, which integrate consistently against those of
the methimazolyl groups, and the discrete metal-hydride
(δH -11.8 1, -13.6 2), which confirms the occurrence of
B-H activation. For 1, this latter feature is further sup-
ported by the absence of any resonances associated with
either terminal (B-H) or bridging (B-H-M) borohydride
functionalities, evidence for which is also conspicuously
absent from the infrared spectrum. In the case of 2, a single
B-H infrared absorption (νBH=2384 cm-1) is observed for
the retained borohydride function, although its typically
broadened resonance could not be unambiguously identified
in the 1H NMR spectrum. The 31P{1H} NMR spectra each
exhibit a single, heavily broadened, resonance (δP 4.5, half-
height width (hhw) = 163 Hz for 1; δP = 6.2, hhw = 95 Hz
for 2), indicative of appreciable interaction with the quadru-
polar boron nucleus and consistent with a trans disposition
of these two nuclei. While we have, on occasion,23 resolved
spin-spin coupling between trans-disposed boron and phos-
phorus nuclei, this is not the case for either 1 or 2. However, a
(32) Crossley, I. R.; Hill, A. F. Manuscript in preparation.
(33) (a) The conventional Lewis base adduct dmffB(mt)3 has been
structurally characterized: Schwalbe, M.; Andrikopoulos, P. C.;
Armstrong, D. R.; Reglinski, J.; Spicer, M. D. Eur. J. Inorg. Chem.
2007, 1351. Similarly, Bailey has reported the synthesis of MeImfB(mt)3
(MeIm=N-methylimidazole).10b
(34) Vaska, L. Acc. Chem. Res. 1968, 1, 335.
(35) (a) Vaska, L.; Bath, S. S. J. Am. Chem. Soc. 1966, 88, 1333.
(b) LaPlaca, S. J.; Ibers, J. A. Inorg. Chem. 1966, 5, 405. (c) Blake, A. J.;
Ebsworth, E. A. V.; Henderson, S. G. D.; Murdoch, H. M.; Yellowlees, L. J.
Z. Kristallogr. 1992, 199, 290.
(36) (a) Herberhold, M.; Hill, A. F. J. Chem. Soc., Dalton Trans.
1988, 2027. (b) Herberhold, M.; Hill, A. F. J. Organomet. Chem. 1990, 387,
323.
(37) Baird, M. C.; Wilkinson, G. J. Chem. Soc. A 1967, 865.
(38) Kawakami, K.; Ozaki, Y.; Tanaka, T. J. Organomet. Chem.
1974, 69, 151.
(39) Parshall, G. W.; Jones, F. N. J. Am. Chem. Soc. 1965, 87, 5356.
(40) Westcott, S. A.; Marder, T. B.; Baker, M. R.T.; Calabrese, J. C.
Can. J. Chem. 1993, 71, 930.
(41) Given the low electronegativity of boron and at best modest
π-acidity of the BCat ligand, this value would appear anomalously high.
However, the trans disposition of hydride and carbonyl ligands results in
significant coupling (νIrH =2007 cm-1) of the νCO and νIrH oscillators.
This is not an issue when the hydride and carbonyl ligands are mutrually
cis, as in the case of 1 and 2. For a discussion of this phenomenon see:
Vaska, L. J. Am. Chem. Soc. 1966, 88, 4100.
(42) For reviews on the coordination chemistry of sulfur dioxide and
iminooxosulfuranes, respectively, see: (a) Ryan, R. R.; Kubas, G. J.;
Moody, D. C.; Eller, P. G. Struct. Bonding (Berlin) 1981, 46, 47. (b) Hill,
A. F. Adv. Organomet. Chem. 1994, 36, 131.
1
coupling of magnitude 8 Hz was resolved in the H NMR
spectrum of 2, which was assigned to a cis 31P-1H inter-
action, thus further defining the stereochemistry. The final
(sixth) coordination site in both 1 and 2 is occupied by the
carbonyl ligand, the retention of which is clear from the
infrared spectra (νCO 1990 1, 1988 cm-1 2). In keeping with

330 Organometallics, Vol. 29, No. 2, 2010
Crossley et al.
Figure 2. Molecular structure of 2 in a crystal of 2 CHCl3.
3
Methimazolyl hydrogen atoms are omitted; phenyl groups
simplified for clarity (50% displacement ellipsoids). Selected
bond distances (A) and angles (deg): Ir1-B1 2.210(5), Ir1-C1
1.826(5), Ir1-P1 2.411(1), Ir1-S1 2.485(1), Ir1-S2 2.417(1),
Ir1-H1 1.63(3), S1-C11 1.717(5), S2-C21 1.703(5), B1-N12
1.557(6), B1-N22 1.581(6), Ir1-B1-N12 107.8(3), Ir1-B1-
N22 106.5(3), N22-B1-N12, 107.4(3), S1-Ir1-B1 85.90(13),
S2-Ir1-B1 86.01(12), S1-Ir1-S2 91.69(4), B1-Ir1-H1 86.4
(11), Ir1-S1-C11 96.27(16), Ir1-S2-C21 97.18(15).
Figure 1. Molecular structure of [IrH(CO)(PPh3){κ3-B,S,S0-B
(mt)3}] (1) in a crystal of 1 (CH2Cl2)1.43. Methimazolyl hydro-
3
gen atoms are omitted; phenyl groups simplified for clarity
(50% thermal ellipsoids). Selected bond distances (A) and
angles (deg): Ir1-B1 2.193(6), Ir1-C1 1.811(7), Ir1-P1 2.404
(1), Ir1-S1 2.463(1), Ir1-S2 2.417(1), Ir1-H1 1.60(5), S1-C11
1.707(5), S2-C21 1.711(5), S3-C31 1.699(5), B1-N12 1.583(6),
B1-N22 1.553(7), B1-N32 1.567(7), Ir1-B1-N12 108.0(3),
Ir1-B1-N22 109.4(3), Ir1-B1-N32 117.4(3), N12-B1-
N22 106.3(4), S1-Ir1-S2 93.46(4), Ir1-S1-C11 96.0(2),
Ir1-S2-C21 97.7(2).
depicted in Figures 1 and 2, respectively, and confirm the formu-
lations based on spectroscopic and microanalytical data.
Although not shown, the chloroform solvate fits comfor-
tably within a cleft provided by the two methimazolyl
two electrons to the metal center. In both the SO2 and
iminooxosulfurane adducts of Vaska’s complex, the Lewis
acid interaction is labile and the iridium center not greatly
displaced from the almost square coordination plane of
the remaining ligands. A similar situation is observed for
[Ir(O3SMe)(CO)(PPh3)2(SO2)],43 [Ir(CO)(PPh3)(NHC6H4P-
Ph2)(SO2)],44 and the unusual complex [IrH(CO)(PTol3)2-
(SO2)] (Tol=C6H4Me-4),45 in which the CO rather than SO2
ligand assumes the axial coordination site.46 This last exam-
ple (with νCO = 2064, νIrH = 1965 cm-1) and the PPh3 ana-
logue (νCO = 2050, νIrH = 1965 cm-1) are in some respects
most akin to 1 and 2 and have been designated by Mingos46b
as d8 complexes; that is, the weak coordination of a Lewis
acid to the metal center is not taken to constitute a two-
electron oxidation or depletion in the d-configuration. In a
similar manner Gilbert has recently reported a DFT study of
the complexes [M(SO2)(PPh3)3] that describes these as having
a d10 configuration.47 Chart 2 depicts compounds in support of
this analogy. Given that the iridium SO2 complexes have
significantly higher vCO values than do 1 and 2, it would seem
especially appropriate to denote these iridaboratranes as invol-
ving monovalent d8-iridium.
groups and one phenyl ring of 2, though the CH
S
3 3 3
separations (2.78, 2.98 A) are beyond those attributable
to significant hydrogen-bonding interactions. Geometric
features associated with the carbonyl, phosphine, and
(located and refined) hydride ligand are unremarkable,
leaving us to focus on the metallaboratrane cage of 2 itself.
The interligand angles at iridium and boron are essentially
octahedral and tetrahedral, respectively. The minor devia-
tions may be readily attributable to the modest steric profile
of the hydride ligand (angles to cis ligands being marginally
acute) and the geometrical constraints of chelation
(S2-Ir1-B1 86.01(12), S1-Ir1-B1 85.90(13) A). The typi-
cally strong trans influence of the hydride ligand is mani-
fested in a significant (68 esd) lengthening of the iridium
sulfur bond to which it is trans coordinated, relative to that
trans to the carbonyl ligand. The iridium-boron separation
of 2.210(5) A is considerably longer than those between
octahedral iridium and three-coordinate boron, which span
the range 1.990-2.121 A. Notably, despite the inclusion of
large N-substituents in the iridaboratranes [IrH(CO)(PPh3)
t
{B(mtPh)3}] and [IrCl(PPh3){B(mtR)3] (R = Bu, Ph)27b
these each have shorter Ir-B separations than found in 1.
In any event, the Ir(1)-B(1) bond length in 2 is well within
the sum of the covalent radii (2.25 A).
The complexes 1 and 2 have been structurally character-
ized and details of 2 discussed in a preliminary report.22 In
the interim, structural data for the analogues [IrH(CO)
The molecular structure of 1 (Figure 1) is topologically
similar to that of 2 with a comparable Ir1-B1 bond length of
2.193(6) A. That is, the inclusion of a fourth non-hydrogen
substituent on boron does not lead to a sterically induced
lengthening of this bond; rather a modest (statistically in-
significant, 3 esd) shortening is suggested. Indeed the entire
molecular geometry of 1 is remarkably similar to that of 2;
that is, all cis interligand angles involving the hydride are
marginally acute, a trans influence is demonstrated by the
hydride ligand, and the angles about iridium and boron are
close to octahedral and tetrahedral, respectively.
t
(PPh3){κ3-B,S,S0-B(mimR)3}] (R = Bu, Ph) have become
available.27b The molecular geometries of 1 and 2 are
(43) Randall, S. L.; Miller, C. A.; Janik, T. S.; Churchill, M. R.;
Atwood, J. D. Organometallics 1994, 13, 141.
(44) Dahlenburg, L.; Herbst, K. Chem. Ber. 1997, 130, 1693.
(45) Miller, C. A.; Janik, T. S.; Churchill, M. R.; Atwood, J. D. Inorg.
Chem. 1996, 35, 3683.
(46) (a) Levison, J. J.; Robinson, S. D. J. Chem. Soc., Dalton Trans.
1972, 2013. (b) Bell, L. K.; Mingos, D. M. P. J. Chem. Soc., Dalton Trans.
1982, 673.
(47) Brust, D. J.; Gilbert, T. M. Inorg. Chem. 2004, 43, 1116.

Article
The inherent significance of compound 1, in relation to the
Organometallics, Vol. 29, No. 2, 2010 331
identified on the basis of spectroscopic data, which are
comparable to those of 1, although the CtS stretching mode
(1363 cm-1) coincides with the fingerprint region of the B
(mt)3 ligand. It is, however, noted that the yields of complex 5
are poor, and samples typically contain intractable con-
taminants. Nonetheless its identity is clear from somewhat
limited spectroscopic data. Efforts to obtain single crystals of
both 4 and 5 have proven unsuccessful, an apparent result of
reduced solution-phase stability, relative to the carbonyl
analogues; indeed, this results in the complete decomposition
of 5 within 48 h in anaerobic solutions.
ongoing study of metallaboratrane formation, is that it
serves as a paradigm for the final intermediate of our
proposed mechanism for this process (Scheme 1), from
which the κ4-B(mt)3M tricyclo-[3.3.3.0] cage can be gene-
rated through loss of a labile co-ligand and chelation of the
pendant methimazolyl donor. That bicyclo-[3.3.0]-1 can be
isolated quantitatively as an appreciably stable material
would, therefore, seem to be attributable to the absence of
any sufficiently labile ligand within its coordination sphere.
However, we have observed that on prolonged standing
(several days), NMR samples (CDCl3 solution) of 1 exhibit
evidence for the formation of a trace material (<10%),
which is apparently devoid of the hydride ligand and pos-
sesses only two unique methimazolyl environments (ratio
2:1), as established from observation of the NCH3 proton
resonances (δH 3.45 (6H), 3.26 (3H)). Moreover, while most
of the imidazolyl 1H NMR resonances for this material are
largely obscured by other aromatics (the appreciable shift to
higher frequency implying enhanced deshielding of these
nuclei), one signal is clearly observed at 7.8 ppm, in a region
that we have found to be diagnostic of cationic metalla-
boratranes. Indeed, for all such materials that we have
characterized thus far, the proton nuclei of the two equiva-
lent methimazolyl rings, lying proximal to boron (i.e.,
CH3NCHdCHNB), resonate above 7.7 ppm,1,12,13,15 in
contrast to the neutral complexes, for which this resonance
is observed below 7.0 ppm.1,10,11,14
The solution-phase behavior of complex 4 is, however,
more interesting, since it serves to demonstrate conclusively
the capacity to convert a neutral complex of the type [IrH-
(CE)(PPh3){κ3-B,S,S0-B(mt)3}] into a cationic metallabora-
trane salt, i.e., [Ir(CE)(PPh3){κ4-B,S,S0,S00-B(mt)3}]Cl (E =
O 3 Cl, S 6 Cl). When solutions of 4 in CDCl3 are left to
3
3
stand for prolonged periods, the 1H NMR resonances asso-
ciated with this material (i.e., 3 ꢀ NCH3; 6 ꢀ imidazole CH)
are observed to diminish, concomitant with the development
of signals suggestive of the cationic metallaboratrane salt
[Ir(CS)(PPh3){κ4-B,S,S0S00-B(mt)3}]Cl (6 Cl) (i.e., 2 ꢀ NCH3
3
(2:1); 4 ꢀ imidazolyl CH (2:1:2:1), Figure S1 see Supporting
Information). Under ambient conditions, this conversion pro-
ceeds slowly over the course of several days, with 6+ ultimately
accounting for >95% of the NMR active materials (4 days),
although complete conversion is never achieved. Nonetheless,
alongside the observations noted for 1 (vide supra) this process
clearly defines the final stage of metallaboratrane formation as
being that which we had presumed, i.e., dissociation/abstrac-
tion of a labile ligand to allow for chelation of the third
methimazolyl donor. Hydride ligands are not dissociatively
labile per se but are readily abstracted by electrophiles, in this
case adventitious acid. It should be noted that these cationic
complexes are isoelectronic with the previously reported and
structurally characterized neutral group 8 examples [M(CO)-
(PPh3){κ4-B(mt)3}] (M = Ru,15 Os20) and [Ru(CS)(PPh3){κ4-
B(mt)3}].19b
Given these observations and the fact that in rigorously
purified solvent no such trace materials are observed, it
appeared likley that the presence of trace DCl in the com-
mercial NMR solvent was effecting protonolysis of the
hydride ligand in 1, thereby affording a vacant coordination
site and allowing the generation of trace amounts of the salt
[Ir(CO)(PPh3){κ4-B,S,S0,S00-B(mt)3}]Cl (3 Cl). In seeking to
3
substantiate this notion we examined the interaction of 1
with stoichiometric amounts of HCl, which did result in the
generation of significant amounts of 3 Cl (>50%). How-
3
ever, clean quantitative conversion could not be effected,
presumably due to subsequent decomposition, the 1H NMR
spectra exhibiting a plethora of intractable side-products
that come to predominate over relatively short time frames.
Greater success was achieved by treating 1 with 1 equiv of
[Ph3C]BF4, which effected in excess of 95% conversion to the
The validity of these arguments has recently received
further support from Parkin’s synthesis of the carbonyl-free
metallaboratrane [IrCl(PPh3){κ4-B(mt)3}],27b obtained from
[IrCl(PPh3)(η4-C8H12)] in the manner previously described
for the rhodium analogue [RhCl(PPh3){κ4-B(mt)3}],21b,c
a
process that is clearly facilitated by the absence of the some-
what inert carbonyl ligand. Interestingly, we have also
obtained the rhodium analogue from Wilkinson’s catalyst,
which led us to investigate the reaction between Na[HB(mt)3]
and [IrCl(PPh3)3]. Unfortunately, only intractable mixtures
were obtained, a situation that was also encountered when
exploring the reaction between Na[HB(mt)3] and [IrCl(η4-
C8H12)2]2, in an attempt to synthesize [Ir(η4-C8H12){κ4-B-
(mt)3}]Cl, by analogy with the rhodium analogue.21
Unperturbed by these failures we envisioned an alternative
route to iridaboratranes, again by analogy with our earlier
work,19,20 commencing from an iridum(III) precursor. Our
initial ruthenaboratrane syntheses involved the installation
of the HB(mt)3 cage on ruthenium(II) precursors that bore
either a σ-aryl or hydride ligand that could serve as hydrogen
sink through reductive elimination of arene or dihydrogen.15
In the course of other studies we had cause to prepare the
iridium(III) complex [IrHCl2(PPh3)3], which we anticipated
would react with Na[HB(mt)3] to afford the first example of
an isolable tris(methimazolyl)borate complex of iridium.
Indeed, this reaction proceeds, in dichloromethane solution,
presumed cationic material 3 BF4 (δH = 3.45 (3H, NCH3);
3
3.48 (6H, NCH3); 7.84 (2H, CHdCH)). However, separa-
tion of the Ph3CH byproduct proved problematic, thus
further confounding unequivocal identification of the re-
maining imidazole resonances within the cluttered aromatic
region and indeed the acquisition of informative micro-
analytical data. In view of these difficulties, it seemed that
an alternative hydride precursor might allow us to defini-
tively demonstrate the proposed conversion, to which end
the thiocarbonyl analogue of 2 was chosen.
The syntheses of [IrH(CS)(PPh3){κ3-B,S,S0-B(mt)3}] (4)
and its bis-methimazolyl analogue [IrH(CS)(PPh3){κ3-B,S,
S’-BH(mt)2}] (5) are largely comparable to those of 1 and 2,
but employ [IrCl(CS)(PPh3)2]48 in place of Vaska’s complex.
Complex 4 is obtained quantitatively and unequivocally
(48) (a) Lu, G.-L.; Roper, W. R.; Wright, L. J.; Clark, G. R.
J. Organomet. Chem. 2005, 690, 972. (b) Hill, A. F.; Wilton-Ely, J. D. E. T.;
Breedlove, B. K.; Kubiak, C. P. Inorg. Synth. 2002, 33, 244. (c) Hill, A. F.;
Wilton-Ely, J. D. E. T. Organometallics 1996, 15, 3791.

332 Organometallics, Vol. 29, No. 2, 2010
Crossley et al.
Chart 3. Considered Structures for Complex 7 with Associated
Point Groups
Scheme 3. Methimazolylborate and Borane Complexes of Rho-
dium (L = PPh3)
typically an order of magnitude larger.52 The data fail to
unequivocally discriminate between A and B, and it is
perhaps noteworthy that the ESI-mass spectrum is devoid
of a molecular ion, with the heaviest attributable isotopic
envelope corresponding to [M - Cl]þ; however it should be
noted that the technique is particularly prone to inducing
metal halide ionization.
Given our recent observations on hydridoplatinabora-
trantes23 we reasoned that the dehydrochlorination of 7
might be effected by the addition of a strong non-nucleo-
philic base, e.g., DBU, thereby affording an iridium(I)31
metallaboratrane. Unfortunately, while a reaction clearly
ensues, as evident from a color change and the complete loss
of 7 upon workup, the nature of the product mixture is
complex and remains indeterminate but does not appear to
contain a metallaboratrane species.
Attempted Synthesis of [RhH(CO)(PPh3){K3-B,S,S0-
HnB(mt)3-n}] (n=1, 2). We have previously reported, at length,
the synthesis of numerous rhodaboratrane complexes, with
both neutral and cationic metal centers (Chart 1).21 How-
ever, given our isolation of iridium complexes 1 and 2, thus
far unique (supplemented by mimR analogues reported by
Parkin) in exhibiting a κ3-B,S,S’ binding mode for the
B(mt)2R (R = mt, H) ligands, the pursuit of rhodium
analogues seemed of value. Somewhat surprisingly, the
reactions of [RhCl(CO)(PPh3)2] with Na[HnB(mt)3-n] (n =
1, 2) failed to result in B-H activation, yielding instead the
simple poly(methimazolyl)borate complexes [Rh(CO)(PPh3)-
{η2-S,S0-HB(mt)2R}] (R = mt 8, H 9, Scheme 3), akin to
Connelly’s mercaptotriazolyl analogue.28 Once again, formula-
tions follow from spectroscopic data, which reveal retention of
the carbonyl ligands (νCO 1973 cm-1 8, 1976 cm-1 9) and B-H
functionalities (νBH 2307 cm-1 8, 2380, 2208 cm-1 9), which in
thecaseof9seemingly involve a bridging B-H-Mmodeinthe
over the course of 2 h to afford a single product that we
formulate as [IrHCl(PPh3)2{κ2-S,S0,-HB(mt)3}] (7), akin to
[RuH(CO)(PPh3)2{κ2-S,S0-HB(mt)3}].19 Although attempts
to obtain X-ray quality crystals of this material have thus far
proven unsuccessful, the formulation follows with confi-
dence from spectroscopic and elemental microanalytical
data. Our preferred formulation for 7 is shown in Chart 3
(A) alongside alternatives that are consistent with the bulk
composition but which may be discarded on the basis of
1
spectroscopic data. The H NMR spectrum demonstrates
the retention of the hydride ligand (δH -18.1 (dd), JPH 20.2,
14.2 Hz), the couplings to which are of magnitudes consistent
with a cis disposition to two phosphorus nuclei, a situation
confirmed by observation of two distinct resonances in the
31P NMR spectrum at 6.01 (dd, JPP 14 Hz; JPH 14 Hz) and
0.83 (dd, JPP 14 Hz; JPH 20 Hz) ppm, thereby excluding the
Cs-symmetric formulations C and E. From the sharp nature
of these two resonances and the observation of an absorption
consistent with a terminal B-H functionality in the infrared
spectrum (νBH 2433 cm-1) it may be inferred that neither
B-H activation nor two-center, two-electron coordination
has occurred (excluding C and D). The observation of three
unique methimazolyl groups in the 1H NMR spectrum also
allows us to exclude entries C and E and while the salts B49,50
and D51 have no element of symmetry (due to the chiral
C3 HB(mt)3Ir cage in B), the latter may be discounted on the
(52) The two phosphine ligands in F are chemically inequivalent, as
are the three methimazolyl groups; however trans-1JPP couplings are
typically an order of magnitude larger than cis-1JPP couplings. For
example, the complex trans-[Os(C6H5)(CO)(PPh3)2{κ2-N,N0-HB(pz)3}]
has 2JPP=301.8 Hz: Burns, I. D.; Hill, A. F.; White, A. J. P.; Williams,
D. J.; Wilton-Ely, J. D. E. T. Organometallics 1998, 17, 1552.
2
basis of the small JPP coupling, since trans couplings are
(49) Tridentate coordination of the HB(mt)3 ligand through three
sulfur donors typically results in high barriers to C3-HB(mt)3M cage
inversion50,51 rendering ligands L in HB(mt)3MXL2 diastereotopic on
NMR time scales: Foreman, M. R. St.-J.; Hill, A. F.; White, A. J. P.;
Williams, D. J. Organometallics 2003, 22, 3831.
(50) Bailey, P. J.; Dawson, A.; McCormack, C.; Moggach, S. A.;
Oswald, I. D. H.; Parsons, S.; Rankin, D. W. H. Inorg. Chem. 2005, 44,
8884.
(53) The concept of oxidation state loses its utility in highly covalent
compounds involving bonds between elements of comparable electro-
negativity; that is, the hypothetical perspective of 100% ionic bonding
has little to offer. The term “formal oxidation state” is often used to
allude to this artifice; however even this concession can lead to dichoto-
mies, e.g., in describing Hartwig’s complexes54 [RhHx(BO2C2Me4)4-x
-
(C5Me5)] (x = 1, 2) as being rhodium(V) when rhodium is more
electronegative than boron or hydrogen.
(54) Hartwig, J. F.; Cook, K. S.; Hapke, M.; Incarvito, C. D.; Fan,
Y.; Webster, C. E.; Hall, M. B. J. Am. Chem. Soc. 2005, 127, 2538.
(51) A similar topology was recently established for [RuCl(dmso)2-
{κ3-H,S,S0-HB(mt)3}], giving rise to three distinct methimazolyl environ-
ments.4g

Article
Organometallics, Vol. 29, No. 2, 2010 333
solid state, as reported previously for the complexes
[RhL2{H2B(mt)2}] (L2 = 1,5-cyclooctadiene or L = CO,
CNC6H2Me2-2,6).4d This interaction is lost in solution
Scheme 4. Relative Positions of the B-H Activation Equilibria
for Pt, Rh, and Ir
1
(CH2Cl2: νBH 2308 cm-1). The H NMR spectra of both 8
and 9, though not allowing for resolution of the borohydride
functionalities, indicate exchange of the methimazolyl sites,
which exhibit a single magnetic environment at ambient temp-
erature. Retention of a single phosphine ligand is clearly
evidenced by the observation of both a single resonance in
the respective 31P NMR spectra (δP 40.1, d, JRhP 159.7 Hz 8;
1
44.6, d, JRhP 151 Hz 9) and aromatic resonances in the H
NMR spectra that integrate consistently with the methimazolyl
signals.
Both 8 and 9 exhibit limited stability in the solid state, even
with the rigorous exclusion of air and moisture, which
might seem to suggest a predominance of the square-planar,
16-electron κ2-S,S0-HB(mt)2R (R=mt, H) forms, despite the
inferred involvement of five-coordintate κ3-S,S0,S00 and/or
κ3-S,S0H geometries in solution suggested by the equilibra-
tion process. Indeed, the solution lifetimes of 8 and 9 are
appreciably greater than in the solid state; thus, while ESI
mass-spectrometric and 13C{1H} NMR data can be readily
obtained, microanalytical data cannot. These difficulties
have also precluded the growth of X-ray quality single
crystals.
clear that the rhodium-hydride linkage is appreciably more
labile than in the iridium analogue, given the ease with which
it is lost in chlorinated solvent (vide supra). A further result of
this lability would be a tendency to impede the B-H activa-
tion step, by virtue of the reverse process (i.e., B-H elim-
ination) being facile; that is, the borate-borane equilibrium
is shifted in favor of the borate complex.
Interestingly, in solution, complex 8 is observed to under-
go a slow, albeit partial, conversion to another material, the
spectroscopic data (1H, 31P NMR) for which are entirely
consistent with it being the rhodaboratrane salt
[Rh(CO)(PPh3){B(mt)3}]Cl (10 Cl). Although its isolation
3
and comprehensive characterization have not been effected,
comparison of spectroscopic data with those for the isoelec-
tronic neutral complex [Ru(CO)(PPh3){κ4-B(mt)3}]15 further
substantiates the formulation. Given that one would
presume the intermediacy of [RhH(CO)(PPh3){κ3-B,S,S0-
B(mt)3}] in this process, by analogy with the iridium
systems, these observations together would seem to imply
that (i) B-H activation at rhodium is less facile than at
iridium and (ii) the hydride at rhodium is significantly more
reactive than that at iridium. Although the latter of these is
undisputed, the former would seem counterintuitive if one
draws the analogy between B-H and C-H “oxidative”
addition. However, given the relative Pauling electronega-
tivities of boron (2.0), carbon (2.5), hydrogen (2.2), rhodium
(2.3), and iridium (2.2), the terms “oxidative addition” and
“oxidation state” should be used with cautious cognizance of
the axioms upon which they are based. We have recently
demonstrated that the B-H activation of chelated poly-
(methimazolyl)borate ligands can be a reversible process
(Scheme 4).23c For the platinum systems studied, the rate-
limiting step was found to be dissociation of a labile ligand
(typically a phosphine) from the square-planar HB(mt)3
complex salt [Pt(PR3)2{κ2-S,S0-HB(mt)3}]Cl (R = alkyl,
aryl), this apparently being prerequisite to B-H activation
or at least to attaining a stable molecular geometry, i.e., the
octahedral [PtH(PR3){κ4-B(mt)3}]Cl. This process was im-
peded when R = Me and Et, due to the reduced lability of
these stronger σ-donors (relative to PPh3). Moreover, treat-
ment of the platinaboratrane with an excess of PR3 (R=Me,
Et) effects, quantitatively, migration of the hydride back to
boron, to afford [Pt(PR3)2{κ2-S,S0-HB(mt)3}]Cl.
Thermochemical data associated with M-H, M-C, and
M-Cl bond formation point to the process becoming more
favorable as one descends group 9.55,56 There are few such
data available for metal boryls (M-BR2)57 and none for
metal-boron dative bonds. Furthermore, since all examples
of MfBR3 bonding to date involve chelated or cage struc-
tures, such data would, were they to eventuate, include
contributions from ring/cage strain. However, since the cova-
lent radii for rhodium (142 pm) and iridium (141 pm)58 are
similar, we might assume that in the present case cage strain
and entropic considerations may be factored out. This leaves,
at least from a superficial perspective, the simple arithmetic
sum of bond dissociation energies (BDE) for the respective
MfB and M-H bonds versus the cleaved B-H bond as the
determinant factor. Comparative data for bond dissociation
energies are not available for metallaboratranes as such, since
the MfB bonds are housed within a cage. Bourissou has
however investigated in detail, both crystallographically and
computationally, the complexes [MCl2{iPr2PC6H4)2BPh}]
(M = Pd, Pt),30e [M{iPr2PC6H4)3B}] (M = Ni, Pd, Pt), and
[MCl{iPr2PC6H4)3B}] (M = Cu, Ag, Au).30c These studies
concluded that in each case the metalfboron interaction was
strongest for the 5d metal in each triad. For the group 11
examples from copper to gold there was a smooth increase in
(55) Martinho Simoes, J. A.; Beauchamp, J. L. Chem. Rev. 1990, 90,
629. (b) Pearson, R. G. Chem. Rev. 1985, 85, 41.
(56) Nolan, S. P.; Hoff, C. D.; Stoutland, P. O.; Newman, L. J.;
Buchanan, J. M.; Bergman, R. G.; Yang, G. K.; Peters, K. S. J. Am.
Chem. Soc. 1987, 109, 3143.
(57) Rablen, P. R.; Hartwig, J. F.; Nolan, S. P. J. Am. Chem. Soc.
1994, 116, 4121.
(58) Cordero, B.; Gomez, V.; Platero-Prats, A. E.; Reves, M.;
In the present discussion, the lability of the ligand set is not
an issue, given that it is invariant; rather, it is the intrinsic
lability of the metal centers that must be considered. It is
ꢀ
ꢀ
ꢀ
´
Echeverrıa, J.; Cremades, E.; Barragan, F.; Alvarez, J. Dalton Trans.
2008, 2832.

334 Organometallics, Vol. 29, No. 2, 2010
Crossley et al.
the degree of electron transfer from the metal to boron.
For the group 10 examples, the NifB interaction was found
to be stronger than the corresponding PdfB interaction, but
still weaker than for platinum. This result, which was attri-
buted to better housing of the smaller nickel center within the
cage, offset the weaker basicity of the 3d metal relative to its
4d and 5d congeners. Given that each group 9 element is more
electropositive than the corresponding elements of groups
10 and 11, it might be presumed that this trend would be
reproduced. The B-H bond dissociation enthalpy (BDE) for
H-BH2NH3 has been estimated at 430 kJ/mol, and for a
range of three-coordinate boranes BDEs span the range
440-470 kJ/mol. With recourse to data for the addition
of dihydrogen (H-H) versus catecholborane (H-BCat) to
Vaska’s complex, Hartwig and Nolan have concluded that the
resulting Ir-B bond is at least equal to or greater than that of
the corresponding iridium hydride.56 The addition of HBCat
to Vaska’s complex is exothermic by ca. 63 kJ/mol, while the
corresponding rhodium complex is unknown.59,60 It might
therefore be surmised that the driving force for the sponta-
neous formation of iridaboratranes arises similarly from the
favored formation of IrfB and Ir-H bonds. In the case of
rhodaboratrane formation the reaction leading to weaker
RhfB and Rh-H bonds does not provide sufficient impetus
for the accumulation of (spectroscopically) significant
amounts of the rhodaboratrane unless this is compensated
for bysubsequent exothermic reactionof the transienthydride
with HCl or halocarbon solvent.56
Conclusions
We have described the synthesis of the bis-methimazolyl-
buttressed iridium-borane complexes [IrH(CE)(PPh3){κ3-
B,S,S0-B(mt)2R}] (E O, S; R = mt, H) and attempts to
prepare the rhodium analogues, which have instead afforded
the more conventional borate complexes [Rh(CO)(PPh3)-
{κ2-S,S0-HB(mt)2R}] (R = mt, H). Despite appreciable air
sensitivity in the solid state, the complex [Rh(CO)(PPh3)-
{κ2-S,S0-HB(mt)3}] has been observed to slowly convert in
solution to the rhodaboratrane salt [Rh(CO)(PPh3){κ4-
B(mt)3}]Cl, the iridium analogue of which can be generated
only in the presence of a strong hydride acceptor (e.g., trityl
tetrafluoroborate). In contrast, the iridium thiocarbonyl
analogue [Ir(CS)(PPh3){κ4-B(mt)3}]Cl forms, albeit slowly
(4 days) under mild conditions in chlorinated solvent
(CDCl3). The differing facility of these conversions has been
attributed in part to the thermodynamics of the resultant
M-H and inferred MfB bonds. Moreover, this same effect
has been implicated in the apparently reduced propensity
toward B-H activation observed for the rhodium com-
pounds, as compared with iridium, since the lower dissocia-
tion energies of the Rh-H and RhfB bonds precludes the
observation of the inferred transient rhodaboratrane
[RuH(CO)(PPh3){B(mt)3}]. The equilibrium can be driven
to favor the borane, only in the presence of an irreversible
hydride acceptor. Circumstantial evidence that the BDE for
RhfB is less than that for IrfB is provided by the ready
isolation of [IrH(CS)(PPh3){HB(mt)2}], while the corre-
sponding thiocarbonyl rhodaboratrane [RhH(CS)(PPh3)-
{HB(mt)2}] has not been observed but rather evolves spon-
taneously into the unusual thioketone complex [RhH(PPh3)-
{SdC(PPh3)BH(mt)2}] via insertion of CS into the RhfB
bond.21d
Thus, only where the hydride is lost to a more facile
process (i.e., solvent exchange) is the RhfB linkage locked
in, hence the slow formation of 10 Cl, without observation
3
of the intermediate hydride complex [RhH(CO)(PPh3){κ3-B,
S,S0-B(mt)3}], which presumably exists only transiently.
These arguments are, on reflection, more widely applic-
able, in accounting for our ability to prepare a variety of
platinaboratranes,22,23 while palladaboratranes remain
rare.27d One might thus expect to observe a similar pheno-
menon with respect to any 4d-5d couplet of metallabora-
trane analogues and might thus question the comparable
facility with which the original group 8 examples [M(CO)-
(PPh3){κ4-B(mt)3}] (M = Ru, Os)9-11 were obtained. It
should, however, be noted that these neutral materials were
obtained from organometallic M(II) precursors, in which a
σ-organyl (alkyl, aryl, vinyl) served as hydride acceptor, the
elimination of RH thus driving the equilibrium in favor of
the borane complex. Furthermore, the mechanistic subtleties
by which these group 8 metallaboratranes form remain to be
elaborated given that evidence is not yet available to dis-
criminate between addition-elimination or σ-complex-
assisted61 metathesis pathways. Indeed, this phenomenon
is perhaps best illustrated by rhodium, the original rhoda-
boratrane [RhCl(PPh3){B(mt)3}]21b being obtained in under
1 h from [RhCl2(Ph)(PPh3)2], while other examples genera-
ted from 16-electron Rh(I) precursors have proven more
difficult to obtain.21
Experimental Section
General Methods. Conventional Schlenk and vacuum line
techniques were routinely employed throughout for the exclu-
sion of air. Unless otherwise stated, products were moderately
air-stable; thus analytical samples were typically prepared in air.
Solvents were distilled under prepurified nitrogen from appro-
priate drying agents. The compounds [IrCl(CO)(PPh3)3],62
[IrCl(CS)(PPh3)2],48 [RhCl(CO)(PPh3)2],63 Na[HB(mt)3],15 and
Na[H2B(mt)2]4e were prepared according to published proce-
dures. All other materials were obtained from commercial
sources and used as supplied after spectroscopic and analytical
verification of purity. All NMR spectra were recorded on a
Varian Inova 300 instrument (1H, 299.9 MHz, 13C, 75.42 MHz,
referenced to external SiMe4; 31P, 121.4 MHz, ref external 85%
H3PO4; 11B, 96.23 MHz, ref external BF3 OEt2). Carbon-13
3
NMR assignments were confirmed with recourse to 2-D corre-
lation (HMQC and HMBC) spectra. Elemental microanalytical
and mass spectrometric data were provided by the ANU analy-
tical services, the latter confirmed by simulation of isotopic
manifolds.
Synthesis of [IrH(CO)(PPh3){K3-B,S,S0-B(mt)3}] (1). A mix-
ture of [IrCl(CO)(PPh3)2] (1.00 g, 1.29 mmol) and Na[HB(mt)3]
(0.503 g, 1.34 mmol) in dichloromethane (30 mL) was stirred
under a closed atmosphere of N2 for 3 h. After allowing the
suspension to settle, the solution was filtered via cannula to a
second Schlenk flask and concentrated under reduced pressure.
(59) Although not documented in SciFinder (C43H31BClO3P2Rh),
the related complex [RhHCl(CO)(PPh3)2(BO2C2Me4)] may play a role
in the dehydrogenative borylation of alkenes with HBO2C2Me4, which is
mediated by [RhCl(CO)(PPh3)2],60 either within the catalytic cycle or as
a tangential catalyst resting state.
(60) Mkhalid, I. A. I.; Coapes, R. B.; Edes, S. N.; Coventry, D. N.;
Souza, F. E. S.; Thomas, R. L.; Hall, J. J.; Bi, S.-W.; Lin, Z.; Marder,
T. B. Dalton Trans. 2008, 1055.
(61) Perutz, R. N.; Sabo-Etienne, S. Angew. Chem., Int. Ed. 2007, 46,
2578.
(62) Collman, J. P.; Sears, C. T., Jr.; Kubota, M. Inorg. Synth. 1990,
28, 92.
(63) Evans, D.; Osborn, J. A.; Wilkinson, G. Inorg. Synth. 1990,
28, 79.

Article
Organometallics, Vol. 29, No. 2, 2010 335
The addition of excess ether effected precipitation of an off-
white solid. The solvent was removed via cannula and the solid
washed with diethyl ether and dried in vacuo. The product was
recrystallized from a mixture of CH2Cl2 and hexane. Yield:
0.930 g (87%). IR (KBr): νCtO 1990 νIrH 2130 cm-1. NMR
(CDCl3, 25 °C), 1H: 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, 3JHH 2.3 Hz, 1 H ꢀ 2,
2.0, 1 H ꢀ 2, NCHdCH), 6.72, 6.95 (d ꢀ 2, 3JHH 2.0 Hz, 1 H ꢀ 2,
NCHdCH), 7.64-7.55 (m, 6 H, PPh3), 7.43-7.33 (m, 9 H,
PPh3). 13C{1H}: δC 33.9, 34.5, 34.6 (NCH3 ꢀ 3), 116.7, 118.3,
118.4, 120.5, 121.9, 122.9 (NCHdCH), 128.0 [d, 3JPC 10.0 Hz,
2
C
3,5(C6H5)], 129.7 [s, C4(C6H5)], 134.1 [d, JPC 12.0 Hz, C2,6
-
(C6H5)], 163.6, 166.9 (CdS), M-CS not unequivocally identi-
fied. 31P{1H}: δP 6.8 (s br, hhw 156 Hz). 11B{1H}: δB 4.3 (s br,
hhw 271 Hz). Anal. Found: C, 42.84; H, 3.72; N, 9.26; S, 13.76.
3
NCHdCH), 6.59, 6.61 (d ꢀ 2, JHH 2.0 Hz, 1 H ꢀ 2,
NCHdCH), 6.77, 6.92 (m br ꢀ 2, 1 H ꢀ 2, NCHdCH),
Calcd for C31H31BIrN6PS4 (CH2Cl2)0.5: C, 42.40; H, 3.61; N,
3
7.57-7.48 (m, 6 H, PPh3), 7.41-7.32 (m, 9 H, PPh3). 13C{1H }:
9.42; S, 14.37. ESIþ MS: m/z 850.2 [M]þ, 849.3 [M - H]þ, 805.2
[M - CS - H]þ, 737.2 [M - mt]þ, 586.8 [M - PPh3]þ. Acc.
Mass: 849.08659. Calcd for [M - H]þ = C31H30BN6PS4Ir:
849.08749.
δ
C 33.9, 34.4, 34.5 (CH3 ꢀ 3), 117.3, 118.9 (NCHdCH), 118.0
3
(br, NCHdCH ꢀ 2), 122.4, 122.8 (NCHdCH), 128.1 [d, JPC
9.5, C3,5(C6H5)], 129.6 [d, JPC 2.0, C4(C6H5)], 133.8 [d, JPC
4
2
12.5, C2,6(C6H5)], 134.7 [d, JPC 36.3 Hz, C1(C6H5)], 163.6,
1
Synthesis of [IrH(CS)(PPh3){K3-B,S,S0-BH(mt)2}] (5). A mix-
ture of [IrCl(CS)(PPh3)2] (0.100 g, 0.125 mmol) and Na[HB-
(mt)3] (0.035 g, 0.134 mmol) in dichloromethane (10 mL) was
stirred under a closed atmosphere of N2 for 1.5 h. After being
allowed to settle, the solution was filtered via cannula to a
second Schlenk and concentrated at reduced pressure. The
addition of excess diethyl ether effected the precipitation of an
off-white solid; the solvent was filtered off via cannula and the
solid dried in vacuo. Yield: 0.04 g, 43%. NMR (CDCl3, 25 °C),
1H: δH -11.1 (s br, JPH 7.6 Hz, 1 H, IrH), 3.32, 3.50 (s ꢀ 2, 3 H ꢀ
2, NCH3 ꢀ 2), 6.73 (m, 1 H ꢀ 2, NCHdCH); 6.77, 6.84 (m ꢀ
2, 1 H ꢀ 2, NCHdCH), 7.67-7.56 (m, 6 H, PPh3), 7.45-7.33
(m, 9 H, PPh3). 31P{1H}: δP 9.6 (s br, hhw 112 Hz). 11B{1H}: δB
-0.5 (s br, hhw 231 Hz). ESIþ MS: m/z 737.0 [M]þ, 474.9 [M -
PPh3]þ. Acc. Mass: 739.1105. Calcd for [HM]þ =C27H28N4S3-
BPIr: 739.0935. Satisfactory elemental microanalytical data not
obtained due to solution instability.
163.9, 166.3, 173.7 (CdS ꢀ 3, CtO). 31P{1H}: δP 4.5 (m br,
hhw 163 Hz). 11B{1H}: δB 3.19 (s br, hhw 149 Hz). Anal. Found: C,
44.47; H, 3.93; N, 9.66; S, 10.74. Calcd for C31H31BIr-
N6OPS3: C, 44.66; H, 3.75; N, 10.68; S, 11.54. Crystals of a
nonstoichiometric dichloromethane solvate 1 (CH2Cl2)1.43 sui-
3
table for diffractometry were grown by slow diffusion of
hexane into a dichloromethane solution of 1. Crystal data
for 1 (CH2Cl2)1.34: [C31H31BIrN6OPS3] (CH2Cl2)1.34], M =
3
3
947.36, monoclinic, C 2/c (No. 15), a = 39.7468(7) A,
b = 9.6094(1) A, c = 20.3913(4) A, β = 99.9747(10)°, V =
7670.6(2) A3, Z = 8, Dc = 1.641 Mg m-3, μ(Mo KR) = 3.908
mm-1, T = 200(2) K, colorless needles, 6772 independent mea-
sured reflections, F refinement, R1 = 0.0288, wR2 = 0.0315 for
4591 independent absorption-corrected reflections [I > 3σ(I);
2θmax =50°], 437 parameters, CCDC 736053.
Synthesis of [IrH(CO)(PPh3){K3-B,S,S0-BH(mt)2}] (2). In a
procedure analogous to that used for the preparation of 1,
Vaska’s complex (2.000 g, 2.56 mmol) and Na[H2B(mt)2]
(0.726 g, 2.78 mmol) were stirred in dichloromethane (100 cmL)
for 3 h and then isolated as for 1 above. The product was
Formation of [Ir(CS)(PPh3){K4-B(mt)3}]Cl (6 Cl).64 An
3
NMR sample solution of 4 in CDCl3 was left to stand and
monitored periodically by H NMR spectroscopy (Figure S1,
1
Supporting Information). Optimal conversion was achieved
after 4 days. NMR (CDCl3, 25 °C), 1H: δH 3.40 (s, 3 H, NCH3),
3.48 (s, 6 H, NCH3), 6.85 (m, 1 H, NCHdCH), 7.13 (m, 2 H,
recrystallized from CH2Cl2 to yield pure 2 (CH2Cl2)0.5 as an off-
3
white solid. Yield: 1.61 g (87%). IR (CHCl3): νCtO 1988 cm-1
ν
,
IrH 2132, νBH 2384 cm-1. NMR (CDCl3, 25 °C), 1H: δH -13.4
3
NCHdCH), 7.75 (d, JHH 2.2 Hz, 1 H, NCHdCH); 8.84 (d,
3JHH 2.2 Hz, 2 H, NCHdCH), 7.58-7.43 (m, 15 H, PPh3).
(s br, 1 H, IrH), 3.35, 3.47 (s ꢀ 2, 3 H ꢀ 2, NCH3 ꢀ 2), 6.69, 6.72
(d ꢀ 2, 3JHH 2.0, 1 H ꢀ 2, NCHdCH), 6.74, 6.81 (d ꢀ 2, 3JHH
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}: δC 33.8, 34.4 (NCH3 ꢀ 2),
120.0, 120.3 (NCHdCH), 122.3, 122.7 (NCHdCH), 128.1
13C{1H}: δC 33.9 (NCH3, 2C), 34.1 (NCH3), 120.2, 125.1
3
(NCHdCH), 121.6, 124.8 (2C, NCHdCH), 128.5 [d, JPC
4
2
9.3 Hz, C3,5(C6H5)], 130.7 [s, JPC, C4(C6H5)], 133.5 [d, JPC
11.6, C2,6(C6H5)], 161.1, 163.3 (CdS), M-CS not resolved.
31P{1H}: δP -0.3 (s br, hhw 148 Hz). 11B{1H}: δB 6.9 (s br,
hhw 216 Hz).
3
4
[d, JPC 9.6, C3,5(C6H5)], 129.8 [d, JPC 1.5, C4(C6H5)], 133.8
2
1
[d, JPC 13.0, C2,6(C6H5)], 135.5 [d, JPC 35.0 Hz, C1(C6H5)],
164.6, 164.9 (CdS), 175.6 (CtO). 31P{1H}: δP 6.2 (m br, hhw 95
Hz). 11B{1H}: δB -4.5 (s br, hhw 300 Hz). Anal. Found: C,
43.37; H, 3.88; N, 7.01; S, 7.90. Calcd for C27H27BIrN4OPS2-
(CH2Cl2)0.5: C, 43.23; H, 3.69; N, 7.33; S, 8.39. Crystals of the
Synthesis of [IrHCl2(PPh3)3]. In a modification of a literature
procedure65 a mixture of [IrCl6i](NH4)2 (1.70 g, 3.85 mmol) and
PPh3 (6.00 g, 23.00 mmol) in PrOH (30 mL) was brought to
reflux for 3 days. The resulting yellow suspension was allowed to
cool to ambient temperature, then extracted with three succes-
sive 30 mL portions of diethyl ether, each of which was decanted
from the mixture. These fractions were combined and filtered to
recover any remaining suspended material. The bulk sample was
collected by filtration, washed with ether, and dried in vacuo.
The combined solids were recrystallized from CH2Cl2/hexane to
afford the desired product as a pale yellow solid, which was
characterized by comparison to literature data as the cis-mer
isomer.65 Yield: 1.405 g (35%). NMR (CDCl3, 25 °C), 1H:
δH -19.2 (dt, JPH 15, 13 Hz, 1H); 7.35-6.85 (m, 45H). 31P{1H}:
δP -2.7 (d, JPP 18, 2P); -7.9 (t, JPP 18, 1P).
chloroform solvate 2 CHCl3 suitable for diffractometry were
obtained via slow cooling of a saturated solution of 2 in chloro-
form. Crystal data for 2 CHCl3: [C27H27BIrN4OPS2] CHCl3,
3
3
3
M=841.05, triclinic, P1 (No. 2), a=9.3509(2) A, b=9.7847(2) A,
c=19.1038(5) A, R=92.759(2)°, β=99.661(1)°, γ=110.926 (1)°,
V = 1598.35(7) A3, 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, CCDC 252699.
Synthesis of [IrH(CS)(PPh3){K3-B,S,S0-B(mt)3}] (4). A mix-
ture of [IrCl(CS)(PPh3)2] (0.100 g, 0.125 mmol) and Na-
[HB(mt)3] (0.048 g, 0.128 mmol) in dichloromethane (10 mL)
was stirred under a closed atmosphere of N2 for 20 min. After
being allowed to settle, the solution was filtered via cannula to a
second Schlenk and concentrated at reduced pressure. The
addition of excess diethyl ether effected the precipitation of an
off-white solid; the solvent was filtered off via cannula and the
solid dried in vacuo. Yield: 0.07 g, 66%. IR (KBr): νIrH 2134, νCS
1363 cm-1. NMR (CDCl3, 25 °C), 1H: δH -9.3 (s br, hhw 12 Hz,
JPH 6 Hz, 1 H, IrH), 3.32, 3.56, 3.60 (s ꢀ 3, 3 H ꢀ 3, NCH3 ꢀ 3),
6.59, 6.60, (s br ꢀ 2, 1 H ꢀ 2, NCHdCH); 6.64, 6.66 (d ꢀ 2, 3JHH
Synthesis of [IrHCl(PPh3)2{K2-S,S0-HB(mt)3}] (7). A mixture
of [IrHCl2(PPh3)3] (0.500 g, 0.476 mmol) and Na[HB(mt)3]
(0.178 g, 0.476 mmol) in dichloromethane (40 mL) was stirred
(64) We note that while all other data (spectroscopic and micro-
analytical) unequivocally confirm the identity and high purity of com-
pounds 6 to 8, carbon analyses have routinely been found to be
significantly in error. Given the reproducibility of this error and the
lack of deviation for other elemental compositions (H, N, S), we must
conclude this to be a peculiarity of rhodaboratranes for which we are
currently unable to account.
(65) Vaska, L.; DiLuzio, J. W. J. Am. Chem. Soc. 1962, 84, 4989.

336 Organometallics, Vol. 29, No. 2, 2010
Crossley et al.
under a closed N2 atmosphere for 2 h. After allowing the halide
salts to settle, the solution was filtered via cannula to a second
Schlenk and concentrated under reduced pressure. The addition
of excess Et2O resulted in precipitation of the product. The
solvent was filtered off by cannula and the solid dried in vacuo.
Recrystallization from dichloromethane/hexane yielded 7 as a
pale yellow solid. Yield: 0.437 g, 83%. NMR (CDCl3, 25 °C),
1H: δH -18.1 (dd, JPH 20.2, 14.2 Hz, 1 H), 2.80, 3.38, 3.70 (s ꢀ 3,
3 H ꢀ 3, NCH3 ꢀ 3), 6.69 (d, 3JHH 2.0 Hz, 1H, NCHdCH), 6.96
(overlapping doublets (2), 3JHH 1.8, 1 H ꢀ 2, NCHdCH), 6.98,
Synthesis of [Rh(CO)(PPh3){K2-H2B(mt)2}] (9). A mixture of
[RhCl(CO)(PPh3)2] (0.200 g, 0.289 mmol) and Na[H2B(mt)2]
(0.090 g, 0.343 mmol) in dichloromethane (15 mL) was stirred
under a closed atmosphere of N2 for 1.5 h. After being allowed to
settle, the solution was filtered via cannula to a second Schlenk and
concentrated at reduced pressure. The slow addition of excess
diethyl ether effected the precipitation of an off-white solid; the
solvent was filtered off via cannula and the solid dried in vacuo.
Yield: 0.119 g, 65%. IR (KBr): νCO 1976, νBH 2380, νBHRh
2208 cm-1; (CH2Cl2): νCO 1965, νBH 2350, 2338 cm-1. NMR
(CDCl3, 25 °C), 1H: δH 3.32 (9 H, NCH3), 6.48 (d, 3JHH 1.6 Hz,
3
(d, JHH 2.0 Hz, 1H, NCHdCH); the final two imidazole
3
3 H, NCHdCH); 6.72 (d, JHH 1.6 Hz, 3 H, NCHdCH),
resonances are lost within the aromatic region 7.49-6.99 (m,
32 H, PPh3 þ 2 ꢀ NCHdCH). 13C{1H}: δC 33.2, 34.2, 35.2
(NCH3 ꢀ 3), 120.5, 121.3, 123.5 (NCHdCH) other imidazole
resonances buried under aromatics, complex unresolved aro-
matics, 152.6, 154.0, 155.6 (CdS). 31P{1H}: δP 6.0 (dd, JPP
14 Hz, JPH 13 Hz, 1P), 0.83 (dd, JPP 14 Hz, JPH 20 Hz, 1P).
ESIþ MS: m/z 1069.5 [M - Cl]þ, 805.3 [M - Cl - PPh3 - H]þ.
High res: [M - Cl]þ 1069.22, C48H47BN6P2S3Ir. Anal. Found:
C, 51.84; H, 4.49; N, 7.87; S, 8.68; Cl, 3.61. Calcd for C27H27BIr-
N4OPS2: C, 52.20; H, 4.29; N, 7.61; S, 8.71; Cl, 3.21.
7.72-7.63 (m, 6 H, PPh3), 7.34-7.23 (m, 9 H, PPh3). 31P{1H}:
δP 44.6 (d, JRhP 151.0 Hz). 11B{1H}: δB -4.7 (s br, hhw 179 Hz).
13C{1H}(CD2Cl2): δC 35.0 (NCH3), 119.6 (NCHdCH), 122.2
3
4
(NCHdCH), 128.0 [d, JPC 10 Hz, C3,5(C6H5)], 129.9 [s, JPC
1.5, C4(C6H5)], 134.3 [d, 2JPC 11.1, C2,6(C6H5)], 160.9 (CdS).
Spectroscopic Elucidation of [Rh(CO)(PPh3){K4-B(mt)3}]Cl
1
(10 Cl). NMR (CDCl3, 25 °C), H: δH 3.47 (s, 6 H, NCH3),
3
3.48 (s, 3 H NCH3), 6.74 (d, 3JHH 2.0 Hz, 1 H, NCHdCH), 6.97
(d, JHH 2.0 Hz, 2 H, NCHdCH), 8.86 (d, JHH 2.2 Hz, 2 H,
3
3
3
NCHdCH), 8.90 (d, JHH 2.2 Hz, 2 H, NCHdCH); 8.84
Synthesis of [Rh(CO)(PPh3){K2-HB(mt)3}] (8).64 A mixture
of [RhCl(CO)(PPh3)2] (0.200 g, 0.289 mmol) and Na[HB(mt)3]
(0.110 g, 0.294 mmol) in dichloromethane (15 mL) was stirred
under a closed atmosphere of N2 for 1.5 h. After being allowed
to settle, the solution was filtered via cannula to a second
Schlenk and concentrated at reduced pressure. The slow addi-
tion of excess diethyl ether effected the precipitation of an off-
white solid; the solvent was filtered off via cannula and the solid
dried in vacuo. Yield: 0.144 g, 67%. IR (KBr): νCO 1973, νBH
2307. NMR (CDCl3, 25 °C), 1H: δH 3.53, (9 H, NCH3), 6.66, (6,
3JHH 1.9 Hz, 3 H, NCHdCH); 7.11 (s br, 3 H, NCHdCH),
7.72-7.61 (m, 6 H, PPh3), 7.41-7.30 (m, 9 H, PPh3). 31P{1H}: δP
40.1 (d, JRhP 159.7 Hz). 11B{1H}: δB -1.76 (s br, hhw 300 Hz).
13C{HMQC}: δC 35.2 (NCH3), 118.8 (NCHdCH), 122.8 (2C,
NCHdCH), 128.1, 130.1, 133.4 (PPh3).
(d, 3JHH 2.2 Hz, 2 H, NCHdCH), 7.58-7.43 (m, 15 H, PPh3).
31P{1H}: δP -5.6 (m br).
Acknowledgment. We thank the Australia Research
Council (DP034270, DP0771497) for financial support.
I.R.C. gratefully acknowledges the award of a Royal
Society Fellowship.
Supporting Information Available: Full details of crystal-
lographic structure determination for the solvate of 1 (CH2-
3
Cl2)1.43 (CCDC 736053) and 2 CHCl3 (CCDC 252699) in CIF
3
1
format; H NMR spectra monitoring the progress of the reac-
tion of 4 with CDCl3. This material is available free of charge via
the Internet at http://pubs.acs.org.