Modulating reactivity in iridium bis(N-heterocyclic carbene) complexes: Influence of ring size on E-H bond activation chemistry

Article abstract of DOI:10.1039/c4dt01398k

The changes in the steric and electronic properties of N-heterocyclic carbenes (NHCs) as a function of ring size have a profound effect on the reactivity of their late transition metal complexes. Comparison of closely related complexes featuring either a saturated 5- or 6-membered NHC, reveals that the larger ring is associated with an increased propensity towards intramolecular C-H activation, but with markedly lower reactivity towards external substrates. Thus, systems of the type [IrL₂(H) ₂]⁺ give rise to contrasting chemical behaviour, primarily reflecting the differing possibilities for secondary stabilization of the metal centre by the N-bound aryl substituents: highly labile [Ir(5-Mes) ₂(H)₂]⁺ can only be studied by trapping experiments, while [Ir(6-Mes)₂(H)₂]⁺ is air and moisture stable, and unreactive towards many external reagents. With appropriate substrates, this heightened reactivity can be exploited, and in situ generated [Ir(5-Mes)₂(H)₂]⁺ is capable of intermolecular B-H and N-H activation chemistry. In the case of H ₃B·NMe₂H, this affords a rare opportunity to study amine/aminoborane coordination via single crystal neutron diffraction methods.

Full text of DOI:10.1039/c4dt01398k

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Modulating reactivity in iridium bis(N-heterocyclic
carbene) complexes: influence of ring size on
E–H bond activation chemistry†
Cite this: Dalton Trans., 2014, 43,
12288
Nicholas Phillips,a Christina Y. Tang,a Rémi Tirfoin,a Michael J. Kelly,a
Amber L. Thompson,a Matthias J. Gutmannb and Simon Aldridge*a
The changes in the steric and electronic properties of N-heterocyclic carbenes (NHCs) as a function of
ring size have a profound effect on the reactivity of their late transition metal complexes. Comparison of
closely related complexes featuring either a saturated 5- or 6-membered NHC, reveals that the larger ring
is associated with an increased propensity towards intramolecular C–H activation, but with markedly lower
reactivity towards external substrates. Thus, systems of the type [IrL2(H)2]+ give rise to contrasting chemical
behaviour, primarily reflecting the differing possibilities for secondary stabilization of the metal centre by
the N-bound aryl substituents: highly labile [Ir(5-Mes)2(H)2]+ can only be studied by trapping experiments,
while [Ir(6-Mes)2(H)2]+ is air and moisture stable, and unreactive towards many external reagents. With
appropriate substrates, this heightened reactivity can be exploited, and in situ generated [Ir(5-Mes)2(H)2]+ is
capable of intermolecular B–H and N–H activation chemistry. In the case of H3B·NMe2H, this affords a
rare opportunity to study amine/aminoborane coordination via single crystal neutron diffraction methods.
Received 12th May 2014,
Accepted 19th June 2014
DOI: 10.1039/c4dt01398k
www.rsc.org/dalton
parts. Thus, the percentage buried volumes (%Vbur)4 calculated
Introduction
for 5-Mes (also known as sIMes) and 6-Mes based on their
N-heterocyclic carbene (NHC) ligands have in recent years respective silver(I) halide complexes are 36.1 and 44.0,4f,5
been shown to be effective alternatives to traditional phos- respectively.
phine donors in the stabilization of late transition metal com-
In recent work we have been interested in developing
plexes of wide-ranging catalytic relevance.1 Their strong rhodium and iridium complexes featuring ancillary NHC
σ-donor characteristics, for example, offer benefits in terms of ligands for E–H bond activation processes (E = H, B, C, N).6 In
metal–ligand bond strength and consequent complex longev- particular, we have sought to establish routes to 14-electron
ity.2 In addition, the popular family of ligands based on the systems of the type [M(NHC)2(H)2]+ and to exploit their reactiv-
unsaturated imidazol-2-ylidene core offers the possibility for ity in amineborane dehydrogenation chemistry.6b,c With this
wide-ranging and synthetically facile variation in the steric in mind, we hypothesized that the size of the supporting
profile of the N-bound substituents. More recently, related heterocyclic ring in saturated NHCs might represent a key vari-
‘expanded ring’ NHCs, featuring saturated 6- and 7-membered able in controlling the properties of such cations, given the
heterocyclic cores have emerged, offering even stronger effects that ring size are expected to have on ligand steric/elec-
σ-donor and more sterically demanding ligand profiles.3 The tronic properties. Thus, in the current study we set out
wider NCN angle at the carbenic carbon leads not only to to establish synthetic routes to systems of the type
greater carbon 2p character in the HOMO, but is also respon- [M(NHC)2(H)2]+ featuring the 5-Mes, 6-Mes and (less peripher-
sible for the greater demands exerted by the N-substituents in ally bulky) 6-Xyl ligands. A preliminary account of two of these
6-membered NHCs compared to their 5-membered counter- compounds has previously been communicated by us.6h,i
aInorganic Chemistry Laboratory, Department of Chemistry, University of Oxford,
South Parks Road, Oxford, OX1 3QR, UK. E-mail: SimonAldridge@chem.ox.ac.uk
bISIS Facility, Rutherford Appleton Laboratory-STFC, Chilton, Didcot, Oxford OX11
0QX, UK
†CCDC 1002300–1002308. For crystallographic data in CIF or other electronic
format see DOI: 10.1039/c4dt01398k
12288 | Dalton Trans., 2014, 43, 12288–12298
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Results and discussion
Reactions of NHCs with [Ir(COE)2(μ-Cl)]2
The room-temperature reaction of free 6-Mes with dimeric
[Ir(COE)2(μ-Cl)]2 in a 6 : 1 ratio has been shown in preliminary
studies to lead to the formation of the doubly C–H activated
bis(NHC) complex Ir(6-Mes′)2H (1; Scheme 1) together with the
hydrochloride salt [(6Mes)H]Cl.6h While the corresponding
chemistry with the related carbene 6-Xyl leads to the formation
of the analogous compound Ir(6-Xyl′)2H (1′), that with 5-Mes
proceeds along markedly different lines. Thus, under similar
conditions (solvent, temperature, reaction time) no C–H acti-
vation is observed, and the planar four-coordinate Ir(I) system
Ir(5-Mes)2(COE)Cl (2) can be isolated in good yield (see
Scheme 1 and Fig. 1). More forcing conditions (70 °C, 48 h) are
required for the generation of Ir(III) species, and even then the
extent of additional ligand activation is confined to a single
C–H bond. The product so formed, Ir(5-Mes)(5-Mes′)HCl (3) is
reminiscent of the products of the reactions of M(I) precursors
(M = Rh, Ir)6e,7 with the unsaturated imidazolylidene ligand,
IMes.
Fig. 1 Molecular structures of (top left) Ir(6-Xyl’)2H (1’), (top right) Ir(5-
Mes)2(COE)Cl (2) and (bottom) Ir(5-Mes)(5-Mes’)HCl (3) as determined
by X-ray crystallography; most hydrogen atoms omitted for clarity and
thermal ellipsoids set at the 40% probability level. Key bond lengths (Å)
and angles (°): (for 1’) Ir(1)–C(2) 1.98(1), Ir(1)–C(24) 2.00(1), Ir(1)–C(22)
2.161(9), Ir(1)–C(45) 2.096(9), Ir(1)–H(11) 1.68, C(2)–Ir(1)–C(24) 177.5(5),
C(22)–Ir(1)–C(45) 167.8(4); (for 2) Ir(1)–Cl(2) 2.367(1), Ir(1)–C(28)
2.061(4), Ir(1)–C(54) 2.076(4), Ir(1)–C(55) 2.135(5), Ir(1)–C(56) 2.130(5),
C(55)–C(56) 1.427(7); (for 3) Ir(1)–Cl(2) 2.415(1), Ir(1)–C(3) 2.099(4), Ir(1)–
C(11) 2.022(5), Ir(1)–C(26) 2.049(3), Ir(1)–H(8) 1.52, Cl(2)–Ir(1)–C(3)
167.3(1), C(11)–Ir(1)–C(26) 173.1(2).
The higher (effective C2) symmetry implicit in the doubly
activated systems 1 and 1′ (cf. mono-activated 3) is reflected in
the respective 1H NMR spectra (e.g. three ortho-Me and two
para-Me signals for 1 cf. four ortho-Me and three para-Me for
3). In each of the Ir(III) compounds, the presence of the
1
hydride ligand is signaled by a very high field H NMR reson-
ance (at δH = −37.54, −36.85 and −32.85 ppm for 1, 1′ and 3,
respectively). Such chemical shifts are also consistent with the
solid-state structures determined crystallographically [Fig. 1
and ref. 6h], which feature in each case, a heavy atom skeleton
defining an approximately planar co-ordination environment
at iridium, with the hydride ligand being located effectively
trans to a vacant site.8
Ir(I) and Ir(III) systems, respectively, is consistent with the elec-
tronic properties of the two ligands. Thus, saturated 6-mem-
bered NHCs are known to be appreciably more basic than
their 5-membered analogues,3e and presumably therefore
render the iridium centre more susceptible to oxidative
addition. The steric profiles of the two ligands are evidently
less important here, since, although the extra backbone
methylene group in 6-Mes pushes the N-bound mesityl groups
further towards the metal centre, it is evident from the crystal
structure of 3, for example, that C–H activation of an ortho-
methyl group in the 5-Mes ligand can occur without introdu-
cing undue strain into the system. Thus, the Ir–C(carbene) dis-
tance associated with the activated NHC ligand in 3 [2.022(5) Å]
is, if anything, shorter than the corresponding distance invol-
ving the unactivated 5-Mes ligand [2.049(3) Å]. Moreover the
‘canting’ of the NHC heterocycle to one side to accommodate
the benzylic tether [as manifested by Ir(1)–C(11)–N angles of
123.6(3) and 128.7(3)°] is not noticeably different from the
asymmetry implicit in the binding of the unactivated NHC in
the same compound [cf. Ir(1)–C(26)–N angles of 124.6(3) and
128.2(3)°].
The differing reactivity of the 5-Mes and 6-Mes NHCs
towards [Ir(COE)2(μ-Cl)]2, generating (at room temperature)
Reversal of C–H activation chemistry via reactions with HX
While, ultimately, extrusion of the alkene ligands from [Ir-
(COE)2(μ-Cl)]2 by either 5-Mes and 6-Mes appears to implicate
activation of one or more pendant mesityl groups, subsequent
reaction of the C–H activated products with reagents of the
Scheme 1 Syntheses of 5- and 6-Mes complexes of iridium 1–3. Key
reagents and conditions: (i) [Ir(COE)2Cl]2 (0.166 equiv.), THF, 20 °C, 12 h,
52%; (ii) [Ir(COE)2Cl]2 (0.25 equiv.), THF, 20 °C, 12 h, 82%; (iii) toluene,
70 °C, 24 h, 79%.
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type HX offers a versatile route back to simple C(2) bound bis-
(NHC) complexes. Thus, for example, the reaction of 3 with
dihydrogen leads to quantitative conversion by 1H NMR to
Ir(5-Mes)2(H)2Cl (5; Scheme 2), while the corresponding
reaction with 1 yields a complex of overall composition
Ir(6-Mes)2H3 (4).
Additionally, in the case of 1, stepwise cleavage of the two
Ir–C bonds can be effected via reactions with stoichiometric
quantities of HCl (Scheme 3). Thus, reaction with a single
equivalent of the acid leads to controlled formation of Ir(6-
Mes)(6-Mes′)HCl (6), i.e. the 6-Mes analogue of compound 3. 6
is a potential intermediate in the initial synthesis of Ir(6-
Fig. 2 Molecular structures of Ir(5-Mes)2(H)2Cl (5) and the heavy atom
skeleton of Ir(6-Mes)2(H)2Cl (8) as determined by X-ray crystallography;
most hydrogen atoms omitted for clarity and thermal ellipsoids set at
the 40% probability level. Key bond lengths (Å) and angles (°): (for 5)
Ir(1)–Cl(2) 2.404(2), Ir(1)–C(3) 2.016(6), Ir(1)–C(26) 2.009(6), Ir(1)–H(11)
Mes′)2H (1) from 6-Mes and [Ir(COE)2(μ-Cl)]2, with subsequent 1.53, Ir(1)–H(12) 1.60, C(3)–Ir(1)–C(26) 176.5(2); (for 8) Ir(1)–Cl(2)
2.401(2), Ir(1)–C(3) 2.053(3), C(3)–Ir(1)–C(3’) 177.5(1).
loss of HCl (driven by the presence of excess NHC acting as a
base) presumably facilitating a second C–H activation step.
Consistently, reaction of 6 with one equivalent of 6-Mes leads
pounds 5 and 8 (Fig. 2). In the case of compound 6, which
retains a single benzylic tether, a 2 : 1 : 1 pattern is observed
for the aromatic meta-CHs of the activated NHC, together with
three ortho-methyl signals (3 : 3 : 3) and two mutually coupled
to the regeneration of the doubly activated system 1.
Reaction of 6 with a second equivalent of HCl leads to pro-
tonation of the single remaining iridium benzyl function, and
to the formation of the bis(6-Mes) stabilized (hydrido)-iridium
dichloride species 7. Alternatively, the reaction of 6 with di-
hydrogen allows access to the corresponding (dihydrido)
iridium chloride species 8, thereby completing access to a
range of systems of the type Ir(6-Mes)2HnCl3−n (n = 1–3).
The characterization of mixed hydrido/chloride complexes
5–8 relies heavily on multinuclear NMR spectroscopy and
mass spectrometry, allied to X-ray crystallography for com-
2
doublets (at δH = 2.19, 2.30 ppm, JHH = 11.5 Hz) assigned to
the IrCH2 unit. In addition, a high-field signal at δC
=
−13.2 ppm displaying a two-bond coupling to the metal-bound
hydride (2JCH = 10.1 Hz) is assigned to the IrCH2 unit in the
13C NMR spectrum. This spectrum also features two carbene
signals at δC = 207.7 and 204.0 ppm, consistent with the pres-
ence of both activated 6-Mes′ and unactivated 6-Mes donors.
Similar patterns of resonances are observed in the corres-
ponding spectra of the structurally characterized 5-Mes ana-
logue, 3. That said, the Ir–H resonance measured for 6 (δH
=
−46.50 ppm) is shifted significantly upfield compared to that
of 3 (δH = −32.85 ppm), consistent with the stronger σ-donat-
ing capabilities of the six-membered heterocycle.3e
By contrast, hydrido/dichloride and dihydrido/chloride
species 5, 7 and 8 display much simpler 1H and 13C NMR
spectra, consistent with the presence of unactivated NHC sub-
stituents. Thus, in each case, a pattern consisting of one aro-
Scheme 2 Hydrogenation of 5- and 6-Mes complexes 1 and 3. Key matic meta-CH (8H), one ortho-methyl (24H) and one para-
reagents and conditions: (i) H2 (4 atm), toluene, 20 °C, 5 h, quantitative;
(ii) vacuum, 20 °C, 4 d, ca. 25%; or TBE, 20 °C, 16 h, quantitative;
(iii) H2 (4 atm), toluene, 20 °C, 1 h, quantitative.
methyl signal (12H) is observed, together with a very high field
resonance associated with the metal-bound hydride ligand
[at δH = −32.81 (5), −49.80 (7) and −33.50 ppm (8)]. The struc-
tures of compounds 5 and 8 have been determined by X-ray
crystallography (Fig. 2) and confirm the heavy-atom skeletons
predicted spectroscopically. It is of note that the Ir–C(carbene)
distances measured for the 6-Mes system 8 [2.053(3) Å] are
markedly longer than those determined for its like-for-like
5-Mes counterpart 5 [2.009(6), 2.016(6) Å]. Such a trend pre-
sumably reflects not only the increased steric demands of the
flanking aryl substituents (%Vbur = 36.1, 44.0 for 5-Mes and
6-Mes, respectively, based on their LAuCl complexes),4f,5 but
also the greater C 2p orbital character in the carbene lone pair
as the N–C–N angle widens.1b
In contrast to the very high field resonances associated with
Scheme 3 Hydrogenation of 5- and 6-Mes complexes 1 and 3. Key
the hydride ligands in the mixed hydrido/chloride systems 7
reagents and conditions: (i) HCl (1.0 equiv.), Et2O, 20 °C, 30 min, 90%; (ii)
6-Mes, benzene-d6, quantitative by 1H NMR; (iii) HCl (1.0 equiv.), Et2O,
20 °C, 30 min, 83%; (iv) H2 (3 atm), toluene, 20 °C, 3 h, 84%.
and 8 (−49.80 and −33.50 ppm, respectively), the corres-
ponding signal for Ir(6-Mes)2H3 (4) is located at δH
=
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−9.75 ppm. 4 is also found to be stable only under an atmos-
phere of dihydrogen. Purging the system, either by placing
under continuous vacuum, or by replacement of the H2 over-
pressure with argon leads to the observation of multiple
hydride signals in the 1H NMR spectrum, including that of
doubly C–H activated system 1. Cleaner dehydrogenative con-
version of 4 back to 1 can be achieved over a 12 h period by the
addition of tert-butylethylene (TBE, 3,3-dimethylbutene;
Scheme 2). Crystals of 4 suitable for X-ray crystallography
could be obtained by re-crystallization under a dihydrogen
atmosphere, but even then the quality of the solution was poor
and only sufficient to confirm the heavy atom skeleton and a
linear C–Ir–C array.
Fig. 3 1H NMR shifts for the iridium hydride ligands in complexes of the
type Ir(6-Mes)2HnCl3−n (n = 1–3).
pyramidal geometry (Fig. 3). In the case of 4, however, the
single fluxionally-averaged resonance presumably reflects the
weighted mean of an analogous very high field signal, and a
lower field signal due to the trans-IrH2 unit. Brookhart, for
example, has recently shown that trans-Ir(III)H2 units give rise
to much lower field 1H resonances [e.g. δH = −9.07 ppm for the
hydride ligands in (PONOP)Ir(CH3)(H)2].13,14
Exposure of 4 to excess D2 (at ca. 4 atm pressure) leads to
incorporation into the Ir–H position, such that the hydride res-
1
onance in the H NMR spectrum completely disappears over a
period of ca. 14 days. At shorter reaction times the signal is
observed to broaden, although no distinct signals could be
resolved attributable to the isotopic perturbation of chemical
shift potentially observed for an IrL2(H2)H formulation.9 Deu-
terium exchange into the high field signal also appears to be
Synthesis and reactivity of cationic systems of the type
[Ir(NHC)2(H)2]+
correlated with its incorporation into the ortho-methyl posi- In preliminary work we showed that the reaction of 1 with the
tions of the 6-Mes ligand, an observation consistent (i) with conjugate acid of a very weakly basic anion (e.g. [H(OEt2)2]-
the observed reconversion of 4 to C–H activated Ir(6-Mes′)2H [BArf4], Brookhart’s acid) leads to the protonation of one of the
(1) under continuous vacuum, and (ii) with a proposed mech- benzyl functions and to the formation of [Ir(6-Mes)(6-Mes′)H]+
anism in which elimination of H2 from 4 is rapidly followed by in which the formally 14-electron metal centre is stabilized not
(or even synchronous with) intramolecular C–H activation.
by the anion but by a (highly fluxional) agostic interaction
The nature of the hydride ligands in 4 was further investi- (Scheme 4).6h Subsequent reaction of [Ir(6-Mes)(6-Mes′)H]+
gated by VT-NMR experiments. Low temperature 1H NMR with dihydrogen, proceeds in much the same way as the hydro-
measurements in toluene-d8 do not reveal any evidence of genation of 6 to 8, generating in this case the cationic di-
decoalescence in the high field signal at the lowest tempera- hydride [Ir(6-Mes)2(H)2]+ (9) which features no significant
ture accessed (193 K). The measured value of T1(min), 510 ms interaction with the [BArf4]− counter-ion.6h However, in view of
(at 300 MHz and 253 K) is comparable to (or even greater than)
those determined for classical Ir(III) hydrides [e.g. 215 ms at
400 MHz and 233 K for Ir(PCy3)2(H)2(κ2-S2CH)], but markedly
in excess of that determined for the rapidly exchanging
hydride/dihydrogen ligands in the related cation [Ir(PCy3)2(H)-
(H2)(κ2-S2CH)]+ (29 ms at 400 MHz and 213 K).10,11 As such,
these data are certainly more consistent with an Ir(III) trihy-
dride formulation for 4, Ir(6-Mes)2(H)3, rather than an Ir(I)
hydride(dihydrogen) structure. Of interest spectroscopically,
1
then, is the markedly lower field H chemical shift associated
with the three (rapidly exchanging) hydride ligands in 4, com-
pared to those for the mixed hydride/chloride species 7 and 8
(Fig. 3). The lowest energy structure calculated by DFT for the
model trihydride Ir(6-Me)2(H)3 features an approximately
square pyramidal coordination geometry, reminiscent of those
determined crystallographically for 1, 1′ and 3, and in line
with that predicted computationally by Eisenstein and co-
workers for the model bis(phosphine) system Ir(PH3)2(H)3.12
Such a geometry features two distinct hydride ligand environ-
ments, either trans to a vacant site, or to another hydride.
Scheme 4 Generation of the [Ir(6-Mes)2(H)2]+ cation via hydride or
Hydride ligands in Ir(III) systems situated opposite vacant
halide abstraction ([BArf4]− counter-ions omitted for clarity). Key
coordination sites are known to give rise to very high field
reagents and conditions: (i) [Ph3C][BArf4] (1.0 equiv.), fluorobenzene,
−30 °C, 2 h, 54%; (ii) Na[BArf4] (1.0 equiv.), fluorobenzene, −30 °C,
chemical shifts (δH < −40 ppm),8b and that measured for 7
(−49.80 ppm) is therefore consistent with the proposed square 2 h, 78%.
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the syntheses described above of a number of alternative a coordinated 5-Mes heterocycle [e.g. 106.2(4)° for 5, cf.
potential precursors [of the type Ir(6-Mes)2(H)2X], we have 115.1(3)° for 8] presumably means that the corresponding
sought to open up more convenient synthetic routes to such degree of aryl π-stabilization and steric protection of the metal
14-electron dihydride systems (Scheme 4).
centre in the putative [Ir(5-Mes)2(H)2]+ cation will be signifi-
In the event, the reaction of either Ir(6-Mes)2(H)3 (4) with a cantly reduced.
hydride abstraction agent such as [Ph3C][BArf4], or of Ir(6-
While the exploitation of 6-Mes/6-Xyl ligands in these
Mes)2(H)2Cl (8) with the halide abstractor Na[BArf4], provides systems gives rise to ‘bottle-able’ air-stable formally 14-electron
facile access to [Ir(6-Mes)2(H)2][BArf4]. Similar chemistry can be cations, a related consequence is a lack of affinity of complexes
shown to generate the related 6-Xyl derivative [Ir(6-Xyl)2(H)2]- such as 9/9′ towards external B–H/N–H containing substrates.
[BArf4] (9′), but attempts to isolate the corresponding 5-Mes Thus, even in the presence of excess H3B·NMe3 or
complex via analogous routes have proved unsuccessful. Thus, H3B·NMe2H, no evidence is seen for coordination or activation
for example, halide abstraction protocols appear to generate of these substrates. By contrast, the much more labile [Ir(5-
systems featuring Ir–F bonds, presumably via fluoride abstrac- Mes)2(H)2]+ system, while not isolable (at least in our hands),
tion from the [BArf4]− counter-ion by a highly electrophilic can be accessed in situ and trapped by coordination of the
species. In stark contrast, 9 and 9′ are extra-ordinarily robust amineborane H3B·NMe3. Thus, addition of a 1 : 1 mixture of 5
systems, being stable to exposure to both air and moisture, and H3B·NMe3 in THF to a suspension of Na[BArf4] leads to the
and able to be re-crystallized unchanged from bench (i.e. wet) formation of [Ir(5-Mes)2(H)2(κ2-H3B·NMe3)][BAr4f ] (10), the
1
chloroform in air over a period of >10 days.
identity of which is suggested by H NMR signals assigned to
Crystallographic studies of both 9 and 9′ reveal the under- the IrH2 (δH = −21.86 ppm) and fluxional BH3 moieties (δH
=
lying origins of this inertness. In each case, the putative −2.11 ppm), and by an 11B signal at δB = 13.0 ppm. The corres-
14-electron metal centre is stabilized by additional interactions ponding signals for the related IMes system, [Ir(IMes)2(H)2(κ2-
involving the π-systems of flanking aryl rings. Thus, in the case H3B·NMe3)][BArf4], are found at −21.80, −2.28 and 14.4 ppm,
of 9′, for example (Fig. 4), contacts between the iridium centre respectively.6c The structure of 10 in the solid state has also
and ipso-carbons of two of the flanking xylyl groups [2.809(5), been determined by X-ray crystallography (Fig. 5), which con-
2.818(7) Å] are comfortably within the sum of the respective firms that the elusive [Ir(5-Mes)2(H)2]+ fragment has indeed
van der Waals radii (3.87 Å).15 While some distortion of the been trapped by coordination of the H3B·NMe3 fragment, acting
metal–carbene fragment is required to bring about this sec- as a 4-electron κ2-ligand. Structural metrics for the cationic
ondary ligation at Ir(1) – as manifested by divergent Ir(1)–C(2)– component of 10 [notably the Ir⋯B and B–N distances of
N angles [129.4(5) and 112.9(5)°] – it is clear from a spacing 2.24(1)/1.55(2) Å, and the Ir⋯B–N angle of 138.5(7)°] are close
filling representation of the structure of [Ir(6-Xyl)2(H)2]+ that to those determined for the imidazolylidene analogue.6c
the resulting alignment of the two flanking xylyl rings very
14-Electron systems of the type [ML2(H)2]+ (M = Rh, Ir) have
effectively shields the coordination environment of the metal also been shown to be active catalysts for the dehydrogenation
centre trans to the hydride ligands. In addition, it is apparent of a range of B/N containing substrates,6b,c,16 and in a similar
from this depiction that the steric ‘shrouding’ relies very little vein, in situ generated [Ir(5-Mes)2(H)2]+ can be shown to be
on the presence (or otherwise) of the para-methyl groups, con- competent for the conversion of H3B·NMe2H to (H2BNMe2)2 in
sistent with the similar inertness observed for both 9 and 9′. THF. In this case, the metal-containing product isolated after
By contrast, the markedly narrower NCN angle associated with 1 h is shown by multinuclear NMR and a combination of X-ray
Fig. 4 Two views of the molecular structure of [Ir(6-Xyl)2(H)2][BArf4] (9’)
as determined by X-ray crystallography. (Left) thermal ellipsoid plot
(40% probability level): most hydrogen atoms omitted and xylyl methyl
groups shown in wireframe format for clarity; contacts between Ir(I) and
xylyl ipso carbons shown in red. (Right) space filling representation. Key
bond lengths (Å) and angles (°): Ir(1)–C(2) 2.042(6), Ir(1)–C(24) 2.049(6),
Ir(1)–H(24) 1.68, Ir(1)–H(26) 1.72, Ir(1)⋯C(16) 2.818(7), Ir(1)⋯C(38)
2.809(5), C(2)–Ir(1)–C(24) 176.1(3).
Fig. 5 Molecular structures of the cationic component of [Ir(5-
Mes)2(H)2(κ2-H3B·NMe3)][BAr4f ] (10) as determined by X-ray crystallo-
graphy; most hydrogen atoms omitted and mesityl methyl groups
shown in wireframe format for clarity. Thermal ellipsoids set at the 40%
probability level. Key bond lengths (Å) and angles (°): Ir(1)–C(2) 2.039(7),
Ir(1)–C(23) 2.040(7), Ir(1)⋯B(13) 2.24(1), B(13)–N(70) 1.55(2), C(2)–Ir(1)–
C(23) 166.2(3), Ir(1)⋯B(13)–N(70) 138.5(7).
12292 | Dalton Trans., 2014, 43, 12288–12298
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and neutron diffraction studies to be a co-crystallite of the ami-
neborane adduct [Ir(5-Mes)2(H)2(κ2-H3B·NMe2H)][BAr4f ] (11)
Conclusions
and the corresponding complex of the dehydrogenated amino- The differing steric and electronic properties of the 5- and
borane ligand H2BNMe2, i.e. [Ir(5-Mes)2(H)2(κ2-H2BNMe2)][BArf4] 6-Mes NHC ligands have been illuminated through their reac-
(12) in a ratio of ca. 4 : 6 (Fig. 6 and Table 1).6b,c,17 The co- tivity in a number of iridium systems. Both ligands possess an
crystallization of these two complexes presumably reflects the array of readily accessible ortho-methyl C–H bonds, and their
very similar binding affinities of κ2-H2BNMe2 and H3B·NMe2H differing reactivities towards [Ir(COE)2(μ-Cl)]2 – generating Ir(I)
ligands determined computationally.18
and Ir(III) complexes, respectively – presumably reflects the
The structure determined by neutron diffraction for the stronger σ-donor characteristics of the expanded ring system.
aminoborane complex component (i.e. 12) has much in Thus, under more forcing conditions, the 5-Mes system can be
common with that reported for the analogous IMes system – shown to be amenable to similar C–H activation processes
which has been characterized by the same technique.6d Thus, without appearing to introduce significant strain in the result-
both the IMes and 5-Mes systems feature a close-to-linear ing chelate ring.
Ir⋯B–N framework, and the metrics associated with the
While σ-donor capabilities might also be expected to play a
Ir(μ-H)2B unit are also very similar for the two complexes role in modulating the lability of the related (formally 14-elec-
[e.g. d(Ir⋯B) = 2.07(2) for 12 vs. 2.044(11) Å; d(Ir–Hb) = 1.83(3)/ tron) cations [IrL2(H)2]+, the starkly differing properties of
1.87(3) vs. 1.886(16)/1.822(19) Å and d(B–Hb) = 1.28(3)/1.33(4) [Ir(5-Mes)2(H)2]+ and [Ir(6-Mes)2(H)2]+/[Ir(6-Xyl)2(H)2]+ are also
vs. 1.30(2)/1.41(2) Å]. The structure of amineborane complex thought to be influenced by the closer approach of the
11, on the other hand, represents to our knowledge, the first to pendant aryl substituents to the unsaturated metal centre in
be determined for such a system by neutron diffraction. The the latter case. Thus, additional (albeit weak) stabilization of
non-linear Ir⋯B–N unit, and longer Ir⋯B and B–N separations the metal centre through interaction with the aryl π-system is
(cf. 12) are consistent with the results of X-ray studies of com- facilitated for the 6-membered heterocycles by the wider N–
plexes featuring four-coordinate amineborane ligands C–N angle and consequent closing up of the cavity defined by
[cf. 138.5(7)°, 2.24(1), 1.55(2) Å for 10]. The Ir(μ-H)2B unit the flanking substituents. The packing of the aryl groups
defining the κ2 mode of coordination is, however very similar around the metal centre which leads to highly effective screen-
to that found in 12, being essentially planar, and featuring Ir– ing from external reagents – a factor which is responsible not
(μ-H) and B–(μ-H) distances of 1.75(7)/1.87(6) and 1.21(6)/ only for the remarkable lack of reactivity of 6-Mes derivative 9
1.29(8) Å, respectively.
towards air and moisture, but also to its lack of catalytic
activity in amineborane dehydrogenation. The role of the para-
methyl groups in controlling this inertness appears to be
minor, with similar reactivity (or lack thereof) being observed
for the ortho-xylyl analogue 9′.
Experimental
General considerations
Fig. 6 Key aspects of the iridium coordination spheres of [Ir(5-
Mes)2(H)2(κ2-H3B·NMe2H)][BAr4f ] (11) and [Ir(5-Mes)2(H)2(κ2-H2B·NMe2)]-
[BArf4] (12) as determined by single crystal neutron diffraction. Thermal
ellipsoids set at the 40% probability level.
All manipulations were carried out using standard Schlenk
line or dry-box techniques under an atmosphere of argon.
With the exception of fluorobenzene, solvents were degassed
by sparging with argon and dried by passing through a
column of the appropriate drying agent using a commercially
available Braun SPS; fluorobenzene was dried by refluxing over
calcium hydride, distilled, sparged and stored over activated
molecular sieves. NMR spectra were measured in C6D6 or
CD2Cl2 which were dried over potassium or molecular sieves,
respectively, and stored under argon in Teflon valve ampoules.
NMR samples were prepared under argon in 5 mm Wilmad
507-PP tubes fitted with J. Young Teflon valves. 1H and
13C NMR spectra were recorded on Varian Mercury-VX-300 or
Bruker AVII-500 spectrometers and referenced internally to
residual protio-solvent (1H) or solvent (13C) resonances and are
reported relative to tetramethylsilane (δ = 0 ppm). 11B and
19F NMR spectra were referenced with respect to Et2O·BF3 and
CFCl3, respectively. Chemical shifts are quoted in δ (ppm) and
coupling constants in Hz. Elemental analyses were carried out
Table 1 Key structural metrics relating to amine- and aminoborane
coordination to [Ir(5-Mes)(H)2]+ obtained from neutron diffraction
studies of 11 and 12
Parametera
11
12
d(Ir⋯B)
d(B–N)
d(N–C)
2.21(4)
1.55(3)
1.54(3), 1.51(3)
0.98(6)
2.07(2)
1.40(2)
1.51(2), 1.51(2)
—
d(N–H)
d(B–Ht)
d(B–Hb)
d(Ir–Hb)
∠(Ir⋯B–N)
d(Ir–Ht)
d(Ir–C)
1.13(6)
—
1.21(6), 1.29(8)
1.75(7), 1.87(6)
125(2)
1.28(3), 1.33(4)
1.83(3), 1.87(3)
171(1)
1.53(2), 1.54(2)
2.035(7), 2.043(6)
167.6(3)
∠(C–Ir–C)
a Distances in Å, angles in degrees.
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at London Metropolitan University. Starting materials 5-Mes lography (for 1′): C40H46IrN4, Mr = 775.03, monoclinic, P21, a =
(sIMes),19 6-Xyl,3a 6Mes,3a [Ir(COE)2(μ-Cl)]2,20 Na[BArf4],21 9.0172(3), b = 15.0295(6), c = 12.2995(5) Å, β = 94.952(2)°, V =
[H(OEt2)2][BArf4],22 and [Ph3C][BAr4f ]23 were prepared by litera- 1660.6(1) Å3, Z = 2, ρc = 1.550 Mg m−3, T = 150 K, λ =
ture procedures. The syntheses of complexes 1 and 9 have 0.71073 Å, 15 631 reflections collected, 7051 independent
been communicated by us previously.6h
[R(int) = 0.090], which were used in calculations. R1 = 0.0492,
wR2 = 0.0832 for observed unique reflections [I > 2σ(I)] and R1
= 0.0877, wR2 = 0.0946 for all unique reflections. Max. and
Crystallography
Single crystal X-ray diffraction data for compounds were col- min. residual electron densities 6.96 and −5.76 e Å−3. CCDC
lected on a Nonius KappaCCD (compounds 1′, 2·OEt2, 3, 5, 9′, ref.: 1002300.
10 and 11/12) or Oxford Diffraction SuperNova diffractometer
(5-Mes)2Ir(COE)Cl (2). A solution of 5-Mes (274 mg,
(compound 8) at 150 K (100 K for 11/12). Data collection and 2.6 mmol) in THF (30 mL) was added to a stirred solution of
reduction were carried out using Collect and Denzo/Scalepack [Ir(COE)2(μ-Cl)]2 (200 mg, 0.22 mmol) also in THF (20 mL).
or CrysAlis, respectively. The structures were solved using The orange solution was stirred for 12 h and darkened to an
either Sir92 or Superflip and refined using CRYSTALS.24 orange/brown colour. Removal of the volatiles in vacuo yielded
Hydrogen atoms were generally visible in the difference map an orange solid which was extracted into diethyl ether (3 ×
and were refined with restraints before inclusion in the final 30 mL). Concentration and storage at −30 °C produced orange
refinement using a riding model.24d The Flack x parameter crystals in 82% yield (370 mg). Crystals of 2·OEt2 suitable for
was refined for the non-centrosymmetric cases,24e,25 except for X-ray crystallography were obtained by slow evaporation of a
1′ which was also twinned by rotation about the a-axis.26 diethyl ether solution at room temperature. 1H NMR (C6D6,
PLATON/SQUEEZE was used to deal with diffuse residual elec- 300 MHz, 298 K): δH 6.96 (2H, s, meta-CH Mes), 6.84 (6H, s,
tron density in the voids in 3.27 Single crystal neutron diffrac- meta-CH Mes), 3.16 (4H, m, NCH2), 3.08 (4H, m, NCH2), 2.55
tion data were collected at 100 K on the time-of-flight Laue (6H, s, para-CH3), 2.47 (6H, s, para-CH3), 2.32 (6H, s, ortho-
diffractometer SXD at the ISIS spallation neutron source.28 The CH3), 2.29 (6H, s, ortho-CH3), 2.24 (6H, s, ortho-CH3), 2.20 (6H,
X-ray structure solution was refined against the neutron diffr- s, ortho-CH3), 2.06 (2H, dd, 3JHH = 9.6 Hz, CH COE), 1.73, 1.38,
action data using SHELXL.29 Complete details of all structures 1.23 (each 4H, m, CH2 COE). 13C NMR (C6D6, 75 MHz, 298 K):
are contained within the respective CIFs which have been de- δC 207.9, 204.0 (NCN), 138.7, 137.7 (NC Mes), 137.4, 137.3
posited with the CCDC (1002300–1002308).
(para-C Mes), 134.8, 134.3, 134.1, 133.7 (ortho-C Mes), 128.9,
128.5, 128.2, 127.6 (meta-CH Mes), 51.1, 49.6 (NCH2), 36.5
(CH COE), 30.1, 27.1, 26.3 (CH2 COE), 20.8, 19.8 (para-CH3),
Syntheses
Ir(6-Xyl′)2H (1′). A solution of 6-Xyl (350 mg, 1.20 mmol) in 19.4, 18.7, 18.0, 17.3 (ortho-CH3). MS (EI +ve): m/z (%) 950.5 (4)
THF (20 mL) was added to
stirred solution of M+. Accurate mass ([C50H66N4191IrCl]+): 948.4574 (meas.),
a
[Ir(COE)2(μ-Cl)]2 (257 mg, 0.287 mmol) also in THF (20 mL). 948.4581 (calc.). Reproducible elemental microanalysis for 2
The reaction mixture was stirred at room temperature for 12 h proved difficult to obtain due to the retention of variable
and darkened to an orange/brown. Removal of the volatiles amounts of diethyl ether solvate. While attempts to obtain
in vacuo yielded a dark orange solid, which was washed crystalline samples from alternative solvents were unsuccess-
1
hexanes (3 × 15 mL) and extracted into toluene. Layering of the ful, H and 13C NMR data imply that bulk samples were >95%
toluene solution with pentane and storage at −30 °C gave red pure. Crystallography (for 2·OEt2): C54H76ClIrN4O, Mr
=
crystals suitable for X-ray crystallography in 43% yield 1024.90, monoclinic, P21/c, a = 17.3193(2), b = 18.8464(2), c =
(190 mg). 1H NMR (C6D6, 300 MHz, 298 K): δH 7.22 (2H, d, 15.1207(1) Å, V = 4908.4(1) Å3, Z = 4, ρc = 1.387 Mg m−3, T =
3
3JHH = 9.0 Hz, meta-H Xyl), 7.11 (2H, t, JHH = 9.0 Hz, para-H 150 K, λ = 0.71073 Å, 69 693 reflections collected, 11 168 inde-
Xyl), 7.06 (2H, d, 3JHH = 6.0 Hz, meta-H Xyl′), 6.90 (2H, t, 3JHH
=
pendent [R(int) = 0.050], which were used in calculations. R1 =
3
6.0 Hz, para-H Xyl′), 6.68, 6.12 (2H, d, JHH = 6.0 Hz, meta-H 0.0356, wR2 = 0.0566 for observed unique reflections [I > 2σ(I)]
Xyl′), 3.10 (2H, m, NCH2), 2.73 (2H, m, NCH2), 2.60 (4H, m, and R1 = 0.0659, wR2 = 0.0745 for all unique reflections. Max.
NCH2), 2.38 (6H, s, ortho-CH3), 2.20 (6H, s, ortho-CH3), 2.13 and min. residual electron densities 3.01 and −1.54 e Å−3
.
2
2
(2H, d, JHH = 6.3 Hz, IrCH2), 2.00 (2H, d, JHH = 6.3 Hz, CCDC ref.: 1002301.
IrCH2), 1.85 (6H, s, ortho-CH3), 1.43 (4H, m, NCH2CH2), (5-Mes)(5-Mes′)IrHCl (3). A solution of
2
(450 mg,
−36.85 (1H, s, IrH). 13C NMR (C6D6, 126 MHz, 298 K): δC 203.8 0.47 mmol) in toluene (40 mL) was heated to 70 °C for 48 h.
(NCN), 147.4 (NC Xyl), 144.8 (NC Xyl′), 137.0 (ortho-C Xyl), Volatiles were removed in vacuo and the red residue extracted
135.5 (ortho-C Xyl′), 130.2 (meta-CH Xyl), 128.7 (meta-CH Xyl′), with diethyl ether (3 × 20 mL). Concentration and storage at
127.5 (meta-CH Xyl′), 126.6 (para-CH Xyl), 125.5 (ortho-CH −30 °C yielded bright red, X-ray quality crystals in 79% yield
Xyl′), 123.2 (para-CH Xyl′), 47.9 (NCH2), 46.1 (NCH2), 22.9 (310 mg). 1H NMR (C6D6, 300 MHz, 298 K): δH 6.79 (4H, s,
(NCH2CH2), 20.1 (ortho-CH3 Xyl′), 18.7 (ortho-CH3 Xyl), 15.6 meta-CH Mes), 6.73 (1H, s, meta-CH Mes′), 6.70 (2H, s, meta-
(ortho-CH3 Xyl′), 7.8 (IrCH2). MS (EI +ve): m/z (%) 774.4 (10) CH Mes′), 6.67 (1H, s, meta-CH Mes′), 3.52 (2H, br m, NCH2),
[M − 2H]+, 776.4 (5) M+. Accurate mass ([C40H45N4191Ir]+): 3.18 (4H, s, NCH2), 2.94 (2H, m, NCH2), 2.42 (6H, s, para-
772.3238 (meas.), 772.3272 (calc.). Elemental analysis (meas.): CH3 Mes), 2.41 (2H, br, IrCH2), 2.33 (12H, s, ortho-CH3 Mes),
C 61.78; H 5.93; N 7.12. (calc.): C 61.99; H 5.98; N 7.23. Crystal- 2.18 (6H, s, ortho-CH3 Mes), 1.91 (3H, s, para-CH3 Mes′), 1.74
12294 | Dalton Trans., 2014, 43, 12288–12298
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(3H, s, ortho-CH3 Mes′), −32.84 (1H, s, IrH). 13C NMR (C6D6, Mr = 842.59, orthorhombic, Pbca, a = 17.0027(1), b = 19.4001
75 MHz, 298 K): δC 210.4, 209.9 (NCN), 137.9 (NC Mes′), 137.8 (1), c = 23.4149(2) Å, V = 7723.50(9) Å3, Z = 8, ρc = 1.449 Mg
(NC Mes), 137.5 (NC Mes′), 137.0 (para-C Mes), 136.9, 136.4 m−3, T = 150 K, λ = 0.71073 Å, 158 718 reflections collected,
(para-C Mes′), 136.1 (ortho-C Mes′), 135.8 (ortho-C Mes), 133.1 8784 independent [R(int) = 0.033], which were used in calcu-
(ortho-C Mes′), 129.6, 129.5, 129.5 (meta-CH Mes′), 129.2 (meta- lations. R1 = 0.0311, wR2 = 0.0542 for observed unique reflec-
CH Mes), 52.0, 51.3, 50.5, 49.4 (NCH2), 21.3 (para-CH3 Mes), tions [I > 2σ(I)] and R1 = 0.0643, wR2 = 0.0768 for all unique
21.3, 21.2, 21.2 (para-CH3 Mes′), 19.7, 19.4, 19.2 (ortho-CH3 reflections. Max. and min. residual electron densities 2.51 and
Mes′), 18.9 (ortho-CH3 Mes). MS (EI +ve): m/z (%) 802.4 (1) −1.75 e Å−3. CCDC ref.: 1002303.
[M − HCl]+. Elemental analysis (meas.): C 59.80, H 6.63,
(6-Mes)(6-Mes′)IrHCl (6). A solution of HCl in diethyl ether
N 6.91. (calc.): C 60.02, H 6.23, N 6.66. Crystallography (for 3): (0.06 mL of a 2.0 M solution, 0.12 mmol) was added to a
C42H52IrN4Cl, Mr = 840.57, monoclinic, P21/n, a = 15.8693(1), stirred solution of 1 (100 mg, 0.12 mmol) also in diethyl ether
b = 12.3445(1), c = 21.1450(2) Å, β = 104.697(1)°, V = 4006.8(1) (20 mL). A colour change from orange to yellow was observed
Å3, Z = 4, ρc = 1.393 Mg m−3, T = 150 K, λ = 0.71073 Å, 138 077 on addition. After 15 min, volatiles were removed in vacuo to
reflections collected, 9099 independent [R(int) = 0.024], which give the yellow product which was isolated in 90% yield
were used in calculations. R1 = 0.0282, wR2 = 0.0611 for (93 mg) and >95% purity (by NMR) without further purifi-
1
observed unique reflections [I > 2σ(I)] and R1 = 0.0424, wR2 = cation. H NMR (C6D6, 300 MHz, 298 K): δH 7.04 (1H, s, meta-
0.0732 for all unique reflections. Max. and min. residual elec- H Mes′), 6.92 (1H, s, meta-H Mes), 6.83 (1H, s, meta-H Mes),
tron densities 1.50 and −1.43 e Å−3. CCDC ref.: 1002302.
(6-Mes)2IrH3 (4). A solution of 1 (50 mg, 0.06 mmol) in meta-H Mes′), 3.33 (1H, d, JHH = 10.9 Hz, NCH2 Mes′), 2.97
toluene (10 mL) in a high pressure flask was freeze–pump– (1H, m, NCH2), 2.78 (2H, m, NCH2), 2.73 (4H, m, NCH2), 2.61
thaw degassed and back filled with dihydrogen to a pressure (3H, s, para-CH3 Mes), 2.49 (6H, s, para-CH3 Mes), 2.40 (3H, s,
6.74 (2H, s, meta-H Mes), 6.65 (2H, s, meta-H Mes), 6.64 (1H, s,
3
2
of ca. 4 atm. The reaction mixture was warmed to room temp- para-CH3 Mes′), 2.36 (3H, s, ortho-CH3 Mes), 2.30 (1H, d, JHH
erature and stirred for 3 h, over which time a colour change = 11.5 Hz, IrCH2), 2.25 (6H, s, ortho-CH3 Mes), 2.19 (1H, d,
was observed from orange to pale yellow. Conversion was 2JHH = 11.5 Hz, IrCH2), 2.18 (6H, s, ortho-CH3 Mes), 1.87 (3H, s,
judged to be quantitative by 1H NMR spectroscopy when main- ortho-CH3 Mes′), 1.70 (3H, s, ortho-CH3 Mes), 1.54 (2H, qn,
tained under a H2 atmosphere. 1H NMR (C6D6, 300 MHz, 3JHH = 5.7 Hz, NCH2CH2), 1.29 (1H, m, NCH2CH2), 1.15 (1H,
3
298 K): δH 6.90 (8H, s, meta-CH), 2.63 (8H, t, JHH = 5.4 Hz, m, NCH2CH2), −46.50 (1H, s, IrH). 13C NMR (C6D6, 75 MHz,
2
NCH2), 2.41 (12H, s, para-CH3), 2.10 (24H, s, ortho-CH3), 1.30 298 K): δC 207.4, 204.0 (both d, JCH = 6.3 Hz, NCN), 144.1
3
(4H, qn, JHH = 5.4 Hz, NCH2CH2), −9.75 (3H, s, IrH3). 13C (NC Mes′), 143.5, 143.4, 143.2 (NC Mes), 137.6, 135.9 (para-C
NMR (C6D6, 126 MHz, 298 K): δC 188.9 (NCN), 149.0 (NC Mes), Mes), 135.5 (para-C Mes′), 134.7, 134.5, 133.7, 133.6 (ortho-C
133.4 (para-C Mes), 132.9 (ortho-C Mes), 128.1 (meta-CH Mes), Mes), 129.8 (meta-CH Mes′), 129.3, 129.2, 129.0, 128.4 (meta-
44.3 (NCH2), 20.9 (NCH2CH2), 20.0 (para-CH3 Mes), 17.9 CH Mes), 126.6 (meta-CH Mes′), 126.2 (ortho-C Mes′), 48.8, 48.3
(ortho-CH3 Mes). MS (CI −ve): m/z (%) 836.4 (100) M+. Reprodu- (s, NCH2), 47.3, 47.3 (NCH2), 22.4, 21.7 (NCH2CH2), 21.3, 21.1,
cible elemental microanalysis for 4 proved impossible to 20.8 (para-CH3), 20.6 (ortho-CH3 Mes′), 20.1, 19.6, 18.8, 16.8
2
obtain due to the tendency of the compound to decompose (ortho-CH3), −13.2 (d, JCH = 10.1 Hz, IrCH2). MS (EI +ve): m/z
other than when under a dihydrogen atmosphere. Single crys- (%) 868.4 M+. Accurate mass ([C44H56N4191IrCl]+): 866.3798
tals of 4 suitable for X-ray crystallography were obtained from (meas.), 866.3799 (calc.).
toluene solution under an overpressure of dihydrogen.
However, the quality of the structure solution was poor and (0.03 mL of a 2.0 M solution, 0.06 mmol) was added to a
only sufficient to determine the heavy atom skeleton. stirred solution of 1 (25 mg, 0.03 mmol) also in diethyl ether
(6-Mes)2IrHCl2 (7). A solution of HCl in diethyl ether
(5-Mes)2IrH2Cl (5). In a Young’s ampoule, a solution of 3 (10 mL). The solution was observed to turn from orange to
(200 mg, 0.24 mmol) in toluene (15 mL) was freeze–pump– yellow and back to orange in colour. Stirring was allowed to
thaw degassed and back filled with dihydrogen to a pressure continue for 10 min after which time the volatiles were removed
of ca. 4 atm. The reaction mixture was warmed to room temp- in vacuo. The product was collected as a pale orange solid in
erature and stirred for 1 h, over which time a colour change 83% yield (22 mg) and >95% purity (by NMR) without further
1
was observed from red to yellow. Conversion was judged to be purification. H NMR (C6D6, 300 MHz, 298 K): δH 6.79 (8H, s,
1
3
quantitative by H NMR spectroscopy. X-ray quality crystals of meta-CH), 2.68 (8H, t, JHH = 6 Hz, NCH2), 2.35 (12H, s, para-
5 were isolated from a concentrated solution in diethyl ether CH3), 2.27 (24H, s, ortho-CH3), 1.40 (4H, qn, 3JHH = 6 Hz, −49.80
stored at −30 °C. H NMR (C6D6, 300 MHz, 298 K): δH 6.78 (s, (1H, s, IrH). 13C NMR (C6D6, 126 MHz, 298 K): δC 198.2 (d,
1
8H, meta-CH), 3.08 (s, 8H, NCH2), 2.34 (s, 12H, para-CH3), 2.19 2JCH = 6.7 Hz, NCN), 145.4 (NCMes), 136.8 (para-C Mes), 135.3
(s, 24H, ortho-CH3), −32.81 (s, 2H, IrH). 13C NMR (C6D6, (ortho-C Mes), 130.3 (meta-CH Mes), 50.0 (NCH2), 22.2
126 MHz, 298 K): δH 210.4 (NCN), 137.9 (NC Mes), 137.0 (para- (NCH2CH2), 20.1 (para-CH3 Mes), 19.6 (ortho-CH3 Mes). MS
C Mes), 135.8 (ortho-C Mes), 129.2 (meta-CH Mes), 50.5 (EI +ve): m/z (%) 830.3 (1) [M − 2HCl]+, 904.3 (1) M+. Accurate
(NCH2), 21.3 (para-CH3), 18.9 (ortho-CH3). MS (EI +ve): m/z (%) mass ([C44H57N4191Ir35Cl2]+): 902.3444 (meas.), 902.3566 (calc.).
842.4 (5) M+. Accurate mass ([C42H54N4191IrCl]+): 840.3639
(meas.), 840.3643 (calc.). Crystallography (for 5): C42H54IrN4Cl, toluene (20 mL) in a high pressure flask was freeze–pump–
(6-Mes)2IrH2Cl (8). A solution of 6 (50 mg, 0.06 mmol) in
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thaw degassed and back filled with dihydrogen to a pressure added at −30 °C to a stirred solution containing 5 (50 mg,
of ca. 4 atm. The reaction mixture was warmed to room temp- 0.06 mmol) and H3B·NMe3 (43 mg, 0.6 mmol) also in THF
erature and stirred for 3 h. Removal of volatiles in vacuo (20 mL). The reaction mixture was stirred whilst warming to
yielded the yellow solid product in 84% yield (44 mg). Yellow room temperature over 1 h, during which time the yellow
crystals suitable for X-ray crystallography were obtained from a colour paled. Precipitated solid was removed by filtration and
concentrated solution in diethyl ether at 20 °C. 1H NMR (C6D6, the volatiles were removed in vacuo. The remaining pale yellow
3
300 MHz, 298 K): δH 6.81 (8H, s, meta-CH), 2.57 (8H, t, JHH
=
solid was washed with hexanes (3 × 10 mL) and dried in vacuo
6.0 Hz, NCH2), 2.36 (12H, s, para-CH3), 2.15 (24H, s, ortho- (yield 52%, 54 mg). Crystals suitable for X-ray crystallography
3
CH3), 1.37 (4H, qn, JHH = 6.0 Hz, NCH2CH2) −33.05 (2H, s, were obtained by layering a fluorobenzene solution with
IrH2). 13C NMR (C6D6, 126 MHz, 298 K): δC 205.1 (NCN), 145.7 pentane and storage at 20 °C. 1H NMR (CD2Cl2, 300 MHz,
(NC Mes), 136.3 (para-C Mes), 135.1 (ortho-C Mes), 130.6 298 K): δH 7.73 (s, 8H, ortho-CH [BAr4f ]−), 7.57 (s, 4H, para-CH
(meta-CH Mes), 48.6 (NCH2), 22.8 (NCH2CH2), 22.0 (para-CH3 [BAr4f ]−), 6.80 (s, 8H, meta-CH), 3.67 (s, 8H, NCH2), 2.36 (s,
Mes), 19.9 (ortho-CH3 Mes). MS (EI +ve): m/z (%) 830.4 (1) [M − 24H, ortho-CH3), 2.24 (s, 12H, para-CH3), 1.93 (s, br, 9H,
Cl]+. Elemental analysis (meas.): C 60.88, H 6.60, N 7.06. NMe3), −2.11 (br s, 3H, BH3), −21.86 (s, 2H, IrH). 13C NMR
1
(calc.): C 60.70, H 6.71, N 6.44. Crystallography (for 8): (CD2Cl2, 75 MHz, 298 K): δC 198.8 (NCN), 162.1 (qt, JCB = 49.6
C44H56IrN4Cl, Mr = 868.63, orthorhombic, Fdd2, a = 42.9414(5), Hz, CB [BAr4f ]−), 137.9 (NC Mes), 137.7 (para-C Mes), (ortho-C
b = 21.5369(3), c = 8.42331(9) Å, V = 7790.10(15) Å3, Z = 8, ρc = Mes), 135.1 (br, ortho-CH [BAr4f ]−), 129.8 (meta-CH Mes), 129.2
1
1.481 Mg m−3, T = 150 K, λ = 1.54180 Å, 21 447 reflections col- (m, meta-C [BAr4f ]−), 124.9 (qt, JCF = 271.0 Hz, CF3 [BAr4f ]−),
lected, 2192 independent [R(int) = 0.029], which were used in 117.8 (m, para-CH [BAr4f ]−), 52.7 (NCH2), 51.4 (NMe3), 46.1,
calculations. R1 = 0.0211, wR2 = 0.0571 for observed unique 21.1 (para-CH3 Mes), 18.9 (ortho-CH3 Mes). 11B NMR (CD2Cl2,
reflections [I > 2σ(I)] and R1 = 0.0211, wR2 = 0.0571 for all 96 MHz, 298 K): δB 13.0 (s, BH3), −6.9 (s, [BAr4f ]−). 19F NMR
unique reflections. Max. and min. residual electron densities (CD2Cl2, 282 MHz, 298 K): δ −62.87 (s, [BAr4f ]−). MS (EI +ve):
1.06 and −1.01 e Å−3. CCDC ref.: 1002304.
m/z (%) 807.4 (6) [M − H3BNMe3]+, 880.5 (4) M+. Accurate mass
[(6-Xyl)2IrH2][BArf4] (9′). A solution of [(6-Xyl)(6-Xyl′)IrH]- ([C45H66N511B191Ir]+): 880.5180 (meas.), 880.5045 (calc.).
[BArf4] (40 mg, 0.02 mmol) prepared in situ from 1′ and Crystallography (for 10): C77H78IrN5B2F24, Mr = 1743.29, tri-
[H(OEt2)2][BArf4] in fluorobenzene (10 mL) was freeze–pump– clinic, P1, a = 12.5841(2), b = 17.9389(3), c = 18.2196(3) Å, α =
ˉ
thaw degassed and backfilled with dihydrogen gas to ca. 86.8231(6), β = 69.9280(7), γ = 87.6703(6)°, V = 3856.21(11) Å3,
4 atm. The reaction mixture was stirred over 1 h, during which Z = 2, ρc = 1.501 Mg m−3, T = 150 K, λ = 0.71073 Å, 60 623
time the solution turned from orange to yellow. The volatiles reflections collected, 17 332 independent [R(int) = 0.070],
were removed in vacuo and the remaining yellow solid was which were used in calculations. R1 = 0.0552, wR2 = 0.0877 for
washed with hexanes (2 × 10 mL) and dried in vacuo (yield observed unique reflections [I > 2σ(I)] and R1 = 0.1004, wR2 =
52%, 17 mg). Crystals suitable for X-ray crystallography were 0.1205 for all unique reflections. Max. and min. residual elec-
obtained by layering a fluorobenzene solution with pentane tron densities 6.73 and −4.41 e Å−3. CCDC ref.: 1002306.
and storage at 20 °C. 1H NMR (CD2Cl2, 300 MHz, 298 K):
Preparation of [(5-Mes)2IrH2(κ2-H3BNHMe2)][BAr4f ] (11) and
δH 7.72 (s, 8H, ortho-CH [BAr4f ]−), 7.55 (s, 4H, para-CH [(5-Mes)2IrH2(κ2-H2BNMe2)][BAr4f ] (12). A solution of Na[BAr4f ]
3
3
[BAr4f ]−), 7.24 (tr, JHH = 7.4 Hz, 4H, para-CH), 7.15 (d, JHH
=
(53 mg, 0.06 mmol) in THF (10 mL) was added at −78 °C to a
3
7.5 Hz, 8H, meta-CH), 3.14 (tr, JHH = 5.7 Hz, 8H, NCH2), 2.11 stirred solution containing
5
(50 mg, 0.06 mmol) and
3
(qn, JHH = 5.7 Hz, 4H, NCH2CH2), 1.83 (s, 24H, ortho-CH3), H3B·NHMe2 (35 mg, 0.6 mmol) also in THF (10 mL). The reac-
−43.82 (s, 2H, IrH). 13C NMR (CD2Cl2, 125 MHz, 298 K): δC tion mixture was stirred whilst warming to room temperature
200.6 (NCN), 162.3 (q, JCB = 49.3 Hz, CB [BAr4f ]−), 138.7 over 1 h, during which time the yellow colour paled. The solu-
1
(NC Xyl), 137.0 (ortho-C Xyl), 135.4 (br, ortho-CH [BAr4f ]−), 129.8 tion was filtered and volatiles removed in vacuo. The residue
(para-CH Xyl), 129.5 (meta-CH Xyl), 129.3 (m, meta-C [BAr4f ]−), was washed with hexanes (2 × 10 mL), dried under vacuum
1
125.2 (q, JCF = 271.0 Hz, CF3 [BAr4f ]−), 118.0 (m, para-CH and collected a pale yellow powder in 59% yield (61 mg). Crys-
[BAr4f ]−), 46.6 (NCH2), 21.3 (NCH2CH2), 18.4 (ortho-CH3). tals suitable for X-ray and neutron diffraction studies were
Elemental analysis (meas.): C 49.76; H 3.44; N 3.44. (calc.): obtained from a fluorobenzene solution layered with pentane
C 49.90; H 3.66; N 3.64. Crystallography (for 9′): C64H56BF24IrN4, and stored at 20 °C. These were shown by a combination of
Mr = 1642.29, monoclinic, P21/n, a = 22.1025(2), b = 14.5368(1), diffraction techniques to be a 4 : 6 mixture of 11 and 12. On re-
c = 23.3691(2) Å, β = 111.973(1)°, V = 6963.1(1) Å3, Z = 4, ρc = dissolution of single crystalline samples in deuterated solvent
1.567 Mg m−3, T = 150 K, λ = 0.71073 Å, 88 057 reflections col- for NMR measurements further dehydrogenation of 11 occurs,
lected, 15 868 independent [R(int) = 0.055], which were used in yielding solutions in which 12 is the overwhelmingly major
calculations. R1 = 0.0476, wR2 = 0.0777 for observed unique constituent (ca. 90%). 1H NMR (CD2Cl2, 300 MHz, 298 K):
reflections [I > 2σ(I)] and R1 = 0.0879, wR2 = 0.1074 for all δH 7.72 (s, 8H, ortho-CH [BArf4]−), 7.56 (s, 4H, para-CH [BAr4f ]−);
unique reflections. Max. and min. residual electron densities (i) signals corresponding to the major component (H2BNMe2
2.84 and −2.84 e Å−3. CCDC ref.: 1002305.
complex 12): 6.84 (s, 8H, meta-CH Mes), 3.74 (s, 8H, NCH2),
Preparation of [(5-Mes)2IrH2(κ2-H3BNMe3)][BAr4f ] (10). A 2.46 (s, 6H, NMe2), 2.36 (s, 12H, para-CH3 Mes), 1.87 (s, 24H,
solution of Na[BArf4] (53 mg, 0.06 mmol) in THF (10 mL) was ortho-CH3 Mes), −6.18 (br, 2H, BH2), −15.70 (d, 2JHH = 13.5 Hz,
12296 | Dalton Trans., 2014, 43, 12288–12298
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2H, IrH); (ii) distinct signals corresponding to the minor com-
ponent (H3B·NHMe2 complex 11): 6.88 (s, 8H, meta-CH Mes),
3.78 (s, 8H, NCH2), 2.46 (s, 6H, NMe2), 2.33 (s, 12H, para-CH3
Mes), 1.90 (s, 24H, ortho-CH3 Mes), −20.51 (s, 2H, IrH). 13C
Res., 2008, 41, 1523; (c) R. Corberán, E. Mas-Márza and
E. Peris, Eur. J. Inorg. Chem., 2009, 1700; (d) J. C. Y. Lin,
R. T. W. Huang, C. S. Lee, A. Bhattacharyya, W. S. Hwang
and I. J. B. Lin, Chem. Rev., 2009, 109, 3561; (e) S. Díez-
González, N. Marion and S. P. Nolan, Chem. Rev., 2009,
109, 3612.
3 (a) M. Iglesias, D. J. Beetstra, J. C. Knight, L. L. Ooi,
A. Stasch, S. Coles, L. Male, M. B. Hursthouse, K. J. Cavell,
A. Dervisi and I. A. Fallis, Organometallics, 2008, 27, 3279;
(b) E. L. Kolychev, I. A. Portnyagin, V. V. Shuntikov,
V. N. Khrustalev and M. S. Nechaev, J. Organomet. Chem.,
2009, 694, 2454; (c) M. Iglesias, D. J. Beetstra, B. Kariuki,
K. J. Cavell, A. Dervisi and I. A. Fallis, Eur. J. Inorg. Chem.,
2009, 1913; (d) S. Flügge, A. Anoop, R. Goddard, W. Thiel
1
NMR (CD2Cl2, 75 MHz, 298 K): δC 187.5 (NCN), 162.1 (qt, JCB
= 49.6 Hz, CB [BAr4f ]−), 138.4 (NC Mes), 137.4 (para-C Mes),
135.9 (ortho-C Mes), 135.2 (br, ortho-CH [BAr4f ]−), 129.9 (meta-
1
CH Mes), 129.2 (meta-C [BAr4f ]−), 124.9 (qt, JCF = 271.0 Hz,
CF3 [BAr4f ]−), 117.8 (m, para-CH [BAr4f ]−), 50.5 (NCH2), 41.1
(NCH3), 21.2 (para-CH3 Mes), 17.8 (ortho-CH3 Mes). 11B NMR
(CD2Cl2, 96 MHz, 298 K): δB 48.1 (br, BH2), 10.8 (s, BH3), −6.9
(s, BAr4f). 19F NMR (CD2Cl2, 282 MHz, 298 K): δ −62.89 (s,
[BAr4f ]−). MS (ESI +ve): m/z 864.5 (100) M+. Accurate mass
([C44H62N511B192Ir]+): 864.4834 (meas.), 864.4732 (calc.).
Elemental analysis (meas.): C 52.73, H 4.76, N 3.68. (calc. for a
4 : 6 co-crystallite of 11 and 12): C 52.81, H 4.36, N 4.05. X-ray
crystallography (43 : 57 co-crystal of 11 and 12):
and A. Fürstner, Chem.
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B. Kariuki, Organometallics, 2012, 31, 4118.
ˉ
C76H74.86B2F24IrN5, Mr = 1728.11, triclinic, P1, a = 12.7291(1),
b = 17.5467(2), c = 18.0256(2) Å, α = 88.273(1), β = 70.202(1), γ =
87.531(1)°, V = 3784.1(1) Å3, Z = 2, ρc = 1.517 Mg m−3, T =
100 K, λ = 0.71073 Å, 66 843 reflections collected, 17 236 inde-
pendent [R(int) = 0.027], which were used in calculations. R1 =
0.0411, wR2 = 0.0947 for observed unique reflections [I > 2σ(I)]
and R1 = 0.0486, wR2 = 0.1042 for all unique reflections. Max.
4 %Vbur: (a) A. C. Hillier, W. J. Sommer, B. S. Yong,
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5 W. A. Herrmann, S. K. Schneider, K. Öfele, M. Sakamoto
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6 (a) C. Y. Tang, W. Smith, D. Vidovic, A. L. Thompson,
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3059; (b) C. Y. Tang, A. L. Thompson and S. Aldridge,
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132, 10578; (d) C. Y. Tang, J. Lednik, D. Vidovic,
A. L. Thompson and S. Aldridge, Chem. Commun., 2011, 47,
2523; (e) C. Y. Tang, W. Smith, A. L. Thompson, D. Vidovic
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(f) C. Y. Tang, N. Phillips, J. I. Bates, A. L. Thompson,
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8096; (g) C. Y. Tang, N. Phillips, M. J. Kelly and S. Aldridge,
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and min. residual electron densities 2.80 and −1.22 e Å−3
.
CCDC ref.: 1002307. Neutron diffraction (35 : 65 co-crystal of
ˉ
11 and 12): C76H74.86B2F24IrN5, Mr = 1728.11, triclinic, P1, a =
12.689(3), b = 17.5549(16), c = 17.948(4) Å, α = 88.275(19), β =
70.175(17), γ = 87.82(2)°, V = 3757.7(14) Å3, Z = 2, ρc = 1.527 Mg
m−3, T = 100 K, λ = Laue, 7639 independent reflections, which
were used in calculations. R1 = 0.0937, wR2 = 0.2053 for
observed unique reflections [I > 2σ(I)] and R1 = 0.1060, wR2 =
0.2162 for all unique reflections. Max. and min. residual elec-
tron densities ∼12% of a carbon atom. CCDC ref.: 1002308.
Acknowledgements
EPSRC (studentships for NP, MJK, and access to the NMSF,
Swansea University); NSERC (post-doctoral fellowship for JIB).
Experiments at the ISIS Pulsed Neutron and Muon Source were
supported by a beam-time allocation from the Science and
Technology Facilities Council (RB1120229).
Notes and references
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