Synthesis and solvent dependent reactivity of chelating bis-N-heterocyclic carbene complexes of Fe(II) hydrides

Article abstract of DOI:10.1039/c2dt12048h

The synthesis and isolation of low coordinate methylenebis-(N-DIPP- imidazole-2-ylidene)iron^(II)hydrides, ((^DIPPC) ₂CH₂)FeH_2-yI_y ((DIPP = 2,6-di-isopropylphenyl, y = 1 or 0), was complicated by competitive reactions with solvent, rapid reductive elimination of H₂ and/or dissociation of the bis-N-heterocyclic carbene ligand. Addition of KH to (( ^DIPPC)₂CH₂)FeI₂ in THF/haloalkane mixtures enabled a short lived mono-hydride to be trapped by reaction with CH₂Cl₂ or cyclo-heptylbromide to form (( ^DIPPC)₂CH₂)FeI(X) (X = Cl or Br, respectively). Toluene coordination stabilises iron-mono hydride complexes as (( ^DIPPC)₂CH₂)Fe^IIH{η⁶- (toluene)} species, which can be isolated in low yield from combination of borohydride salts and ((^DIPPC)₂CH₂)FeI ₂ in toluene, including an imidazole C4 deprotonated carbene-borane, methylene(N-DIPP-imidazole-2-ylidene)(N-DIPP-4-triethyl-borane-imidazole-2- ylidene)](hydrido)(η⁶-toluene)iron. In the absence of toluene, or at short reaction times compounds with empirical formula ((^DIPPC) ₂CH₂)Fe(H)(HB(R)₃)·LiI (R = Et or sec-Bu) that function as a masked Fe^(II)-dihydride are isolated. Whilst ((^DIPPC)₂CH₂)Fe(H)(HB(R) ₃)·LiI was stable for days in Et₂O, more polar solvents (MeCN, THF) led to formation of the carbene borane adducts (( ^DIPPC)₂CH₂)(BR₃)₂. The addition of CO or cyclo-heptylbromide to ((^DIPPC)₂CH ₂)Fe(H)(HB(R)₃)·LiI formed ((^DIPPC) ₂CH₂)Fe(CO)₃ and ((^DIPPC) ₂CH₂)FeBr₂, respectively with BR₃ evolved from both reactions as a by-product. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2dt12048h

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PAPER
Synthesis and solvent dependent reactivity of chelating bis-N-heterocyclic
carbene complexes of Fe(II) hydrides†
Sergey Zlatogorsky‡ and Michael J. Ingleson*
Received 27th October 2011, Accepted 2nd December 2011
DOI: 10.1039/c2dt12048h
The synthesis and isolation of low coordinate methylenebis-(N-DIPP-imidazole-2-ylidene)iron(II)hydrides,
((DIPPC)2CH2)FeH2-yIy ((DIPP = 2,6-di-isopropylphenyl, y = 1 or 0), was complicated by competitive
reactions with solvent, rapid reductive elimination of H2 and/or dissociation of the bis-N-heterocyclic
carbene ligand. Addition of KH to ((DIPPC)2CH2)FeI2 in THF/haloalkane mixtures enabled a short lived
mono-hydride to be trapped by reaction with CH2Cl2 or cyclo-heptylbromide to form ((DIPPC)2CH2)
FeI(X) (X = Cl or Br, respectively). Toluene coordination stabilises iron-mono hydride complexes as
((DIPPC)2CH2)FeIIH{η6-(toluene)} species, which can be isolated in low yield from combination of
borohydride salts and ((DIPPC)2CH2)FeI2 in toluene, including an imidazole C4 deprotonated
carbene-borane, methylene(N-DIPP-imidazole-2-ylidene)(N-DIPP-4-triethyl-borane-imidazole-2-ylidene)]
(hydrido)(η6-toluene)iron. In the absence of toluene, or at short reaction times compounds with empirical
formula ((DIPPC)2CH2)Fe(H)(HB(R)3)·LiI (R = Et or sec-Bu) that function as a masked Fe(II)-dihydride
are isolated. Whilst ((DIPPC)2CH2)Fe(H)(HB(R)3)·LiI was stable for days in Et2O, more polar
solvents (MeCN, THF) led to formation of the carbene borane adducts ((DIPPC)2CH2)(BR3)2.
The addition of CO or cyclo-heptylbromide to ((DIPPC)2CH2)Fe(H)(HB(R)3)·LiI formed ((DIPPC)2CH2)
Fe(CO)3 and ((DIPPC)2CH2)FeBr2, respectively with BR3 evolved from both reactions as a by-product.
of azobenzene by high spin (β-diketiminate)FeH,24 reactivity
reminiscent of nitrogenases. The wider development of low
Introduction
While iron hydrides are well established in homogenous catalysis
and in hydrogenases,1–5 their intermediacy during substrate
reduction at the FeMo co-factor of the Mo-dependent nitrogen-
ase has only recently been demonstrated.6–10 A number of FeMo
co-factor states have been calculated to contain unconventional
iron hydrides where iron centres are low coordinate (coordi-
nation numbers ≤ 5) and paramagnetic.11–13 Diamagnetic iron
hydrides with coordination numbers of 5 or 6 are well
studied,14,15 however by contrast the chemistry of paramagnetic
Fe-hydrido complexes with coordination numbers ≤ 5 is under-
developed. Recent seminal work has made significant progress
by synthesising unprecedented low coordinate and paramagnetic
iron hydride complexes,16–22 with a three coordinate Fe(I)
hydride a notable achievement.23 Subsequent reactivity studies
have demonstrated that these ‘non-conventional’ iron hydrides
are effective for the reduction of a range of unsaturated moi-
eties16,19,21,24 and for hydrodefluorination of fluorocarbons.25 Of
particular note is the reductive cleavage of the N–N double bond
coordinate and/or paramagnetic iron hydrides is imperative
due to their potential importance in bioinorganic reduction pro-
cesses combined with the unprecedented reactivity recently
uncovered.
We have recently synthesised distorted tetrahedral iron(II)
complexes ligated by a bulky bidentate N-heterocyclic carbene
ligand, methylenebis-(N-DIPP-imidazole-2-ylidene) termed
(
DIPPC)2CH2, (DIPP = 2,6-di-isopropylphenyl).26 ((DIPPC)2CH2)
FeY2 complexes are high spin and have a coordination number
of 4 with a range of anionic ligands, Y. Simplistically the
(
DIPPC)2CH2 ligand represents a ‘neutral analogue’ of the mono-
anionic β-diketiminates that have facilitated the development
of low coordinate iron hydride chemistry. Hoping to utilise
the additional electronic and steric flexibility afforded by an
exchangeable anionic ligand, Y−, the synthesis of related iron
hydride complexes ((DIPPC)2CH2)FeH(Y) was targeted. A ques-
tion of particular interest was the stability of these low coordinate
paramagnetic iron mono-hydrides and Fe(II)-dihydrides. Previous
work concluded that low coordinate iron complexes have an
energetic preference for electronegative anionic ligands, with the
relatively more electropositive hydride ligand being comparably
unstable when bonded to paramagnetic, low coordinate iron
centres.20 Herein we describe our studies regarding the installa-
tion of hydrides onto the {((DIPPC)2CH2)Fe(II)} fragment, includ-
ing their solvent dependent reactivity.
‡Present address: Department of Chemistry, University of Liverpool,
Liverpool, L69 3BX, UK.
Department of Chemistry, University of Manchester, Manchester, (UK),
M13 9PL. E-mail: michael.ingleson@manchester.ac.uk;
Fax: +44 (0)161 275 4616; Tel: +44 (0)161 306580
†CCDC reference numbers 851122–851127. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/c2dt12048h
This journal is © The Royal Society of Chemistry 2012
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Method B. A J. Young’s NMR tube was charged with com-
pound 1 (40 mg, 0.051 mmol), 0.6 ml THF and benzyl bromide
(6 μl, 0.051 mmol). The tube was rotated for 16 h and then
heated to reflux for 3 h. 1H NMR spectroscopy revealed that
((DIPPC)2CH2)FeIBr was the major species present along with 1
and ((DIPPC)2CH2)FeBr2 present as minor components.
Experimental
General considerations
All reactions were performed using standard glovebox or
Schlenk line techniques, unless otherwise specified. Solvents
used were either purified by an Innovative Technology PS-MD-5
solvent purification system or distilled from appropriate drying
agents and degassed. Deuterated solvents were distilled from
appropriate drying agents and degassed. ((DIPPC)2CH2)FeI2
(compound 1) and ((DIPPC)2CH2)FeBr2 were synthesised as pre-
viously reported.26 All other materials were purchased from
commercial vendors and used as received. NMR spectra were
recorded with a Bruker AV-400 spectrometer (400 MHz 1H;
100 MHz 13C; 162 MHz, 11B 128 MHz). 1H NMR chemical
shifts are reported in ppm relative to protio impurities in the
deuterated solvents and 13C NMR using the centre line of
CD2Cl2 (or other solvent as appropriate) as internal standard.
Unless otherwise stated all NMR spectra are recorded at 293 K.
Protio-solvent suppression experiments were performed on
Varian Unity Inova instruments (flame sealed d6-dmso capil-
laries used to lock field) using pulse sequences written in-house
by Prof. Gareth Morris. Elemental analysis of air sensitive com-
pounds was performed by London Metropolitan University
service. Solution magnetic moments were recorded at 293 K
using the Evans method.27,28
Method C. Lithium triethylborohydride (128 μl of 1 M solu-
tion in THF, 0.128 mmol) was added to the stirred solution of 1
(100 mg, 0.128 mmol) at ambient temperature in the mixture of
THF (5 ml) and cyclo-heptylbromide (0.1 ml). An initial reddish
colouration disappeared within seconds to form a pale yellow-
brownish solution. After stirring for 2 h the solution was concen-
1
trated to 0.5 ml and the H NMR spectrum was recorded. This
revealed that ((DIPPC)2CH2)FeIBr was the major product along
with ((DIPPC)2CH2)FeI2 as a minor product. No ((DIPPC)2CH2)
FeBr2 was observable by 1H NMR spectroscopy.
Attempts to isolate ((DIPPC)2CH2)FeIBr from each method by
fractional recrystallisation failed, frustrating full characterisation.
Reaction of compound 1 with NaBH4: formation of 4
A J. Young’s NMR tube was charged with 1 (39 mg,
0.05 mmol), sodium borohydride (3.8 mg, 0.1 mmol) and THF
(0.6 ml). A dark red-brown solution formed upon sonication
(30 min). After overnight mixing the contents of the NMR tube
were filtered and layered with pentane, producing a small quan-
tity of X-ray quality crystals of the carbene borane adduct 4.
Compound 4 could not be isolated pure due to intractable para-
magnetic and diamagnetic by-products.
[Methylenebis(N-Dipp-imidazole-2-ylidene)]chloroiodoiron, 2
Lithium triethylborohydride (128 μl of 1 M solution in THF,
0.128 mmol) was added to the stirred solution of 1 (100 mg,
0.128 mmol) at ambient temperature in the mixture of THF
(5 ml) and DCM (1 ml). An initial reddish colouration disap-
peared within seconds to form a pale yellow-brownish solution.
After overnight stirring the off-white precipitate was isolated by
filtration from the pale green-brown supernatant and dried
in vacuo to give off-white powder (30 mg, 34%). X-ray quality
crystals were isolated from the reaction mixture after standing
overnight without stirring at ambient temperature. Characteris-
ation by NMR spectroscopy was hindered by the extremely low
solubility of this compound in organic solvents.
Reaction of 1 with LiBEt3H: formation of 5
A J. Young’s NMR tube was charged with 1 (20 mg,
0.026 mmol), MeCN (0.6 ml) and lithium triethylborohydride
(51 μl of 1 M solution in THF, 0.051 mmol), resulting in the
formation of a dark-yellow solution. After standing overnight at
ambient temperature 1H NMR spectroscopy revealed a minor
paramagnetic component and a major diamagnetic component.
The solution was concentrated to 0.3 ml and left overnight to
yield yellow X-ray quality crystals of 5:
1H NMR (400 MHz, MeCN, 293 K), δ: 7.45–7.37 (m, 2H,
C4 aromatic protons of DIPP), 7.26–7.15 (m, 4H, aromatic
protons at C3 and C5 of DIPP), 6.98, 6.89, 6.82 (app. s, 2H +
2H + 2H, N-CH2-N + N-CHvCH-N + N-CHvCH-N), 2.50
(app. s, 4H, CH(CH3)2), 1.26–0.90 (overlapping multiplets,
24H, CH(CH3)2), 0.68–0.44 (overlapping multiplets, 30H, 2 ×
B(CH2-CH3)3).
For C31H40ClFeIN4 calculated: C, 54.21, H, 5.87, N, 8.16;
found: C, 54.18, H, 5.77, N, 8.07.
[Methylenebis(N-Dipp-imidazole-2-ylidene)]bromoiodoiron
Method A. A J. Young’s NMR tube was charged with equi-
molar compound 1 (15.6 mg, 0.02 mmol) and ((DIPPC)2CH2)
11B NMR (128 MHz, 293 K), δ −13.4 (s)
1
FeBr2 (13.7 mg, 0.02 mmol) and 0.6 ml THF. Initially the H
NMR spectrum revealed no reaction, but on heating for 72 h at
((DIPPC)2CH2)Fe(H)(HBEt3)·LiI 6
1
reflux one new paramagnetic product appeared in the H NMR
spectrum, with residual 1 and ((DIPPC)2CH2)FeBr2 also observa-
ble. This new product is assigned as ((DIPPC)2CH2)FeIBr based
on a similar 1H NMR spectrum to 1 and on subsequent reactivity
studies.
Method A. Lithium triethylborohydride (0.77 ml of 1 M solu-
tion in THF, 0.77 mmol) was added to the stirred Et2O suspen-
sion (25 ml) of 1 (300 mg, 0.38 mmol) at ambient temperature,
leading to immediate formation of a dark-red solution. After
30 min stirring this was filtered, concentrated to 5 ml and stored
at −22 °C overnight. The resulting dark-red microcrystalline
1H NMR (400 MHz, THF with flame sealed d6-dmso capil-
lary, 293 K), δ: 62.4, 28.6, 5.7, −0.9, −4.7, −5.4, −17.8.
2686 | Dalton Trans., 2012, 41, 2685–2693
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[Methylene(N-Dipp-imidazole-2-ylidene)(N-Dipp-4-triethyl-
material was filtered cold (−22 °C), washed with pentane (2 ×
10 ml) and dried in vacuo to afford a dark-red solid (250 mg,
87%).
borane-imidazole-2-ylidene)](hydrido)(η6-toluene)iron, 9
Lithium triethylborohydride (0.51 ml of a 1 M solution in THF,
0.51 mmol) was added to a stirred toluene suspension (20 ml) of
1 (200 mg, 0.26 mmol) at ambient temperature, leading to
immediate formation of a dark-red solution which was stirred for
45 min. This was filtered and the solvent removed in-vacuo. The
resulting dark-red solid was extracted with pentane (50 ml) and
filtered. The filtrate was concentrated to 7 ml and kept at −22 °C
for 3 days which resulted in the precipitation of a small amount
of X-ray quality red plates.
1H NMR (300 MHz, C6D6, 293 K), δ: 4.0 (v br), 1.3, −2.7,
−5.2, −6.8, −9.0, −12 (v br), −25.2.
For C37H57BFeILiN4 calculated: C, 58.60, H, 7.58, N, 7.39;
found: C, 58.45, H, 7.49, N, 7.22.
Evans (C6D6, toluene capillary): averaged value 4.02 μB.
Method B. Lithium triethylborohydride (0.256 ml of 1 M solu-
tion in THF, 0.256 mmol) was added to a stirred toluene suspen-
sion (15 ml) of 1 (100 mg, 0.128 mmol) at ambient temperature,
leading to the immediate formation of a dark-red mixture. After
40 min stirring the dark-red solution was separated by filtration,
the solvent was removed in-vacuo, the resulting solid washed
with pentane (2 × 30 ml) and dried in vacuo to afford bronze-red
1H NMR (400 MHz, CD2Cl2, 293 K), δ: 7.63–7.59 (overlap-
ping multiplets, 8H, aromatic protons on DIPP substituents +
N-CH2-N), 6.30 (app. s, 1H, N-CHvCH-N), 6.20 (app. s, 1H,
N-CHvCH-N), 5.67–5.53 (overlapping multiplets, 1H + 2H +
2H, aromatic protons of coordinated toluene), 5.10 (s, 1H,
N-C(BEt3)vCH-N), 2.81 (JH-H = 6.81 Hz), 2.56 (JH-H = 6.94
Hz), 2.37 (JH-H = 6.47 Hz), 2.30 (JH-H = 6.64 Hz) (septets, 1H
+1H +1H +1H, 4 × inequivalent CH(CH3)2), 1.82 (s, 3H, CH3
of coordinated toluene), 1.29 (q, 6H, B(CH2-CH3)3, JH-H = 7.82
Hz), 1.21–1.05 (overlapping multiplets, 24 H, 4 × CH(CH3)2),
0.88 (t, 9H, B(CH2-CH3)3, JH-H = 7.06 Hz), −10.26 (s, 1H,
hydride).
1
powder (50 mg, 52%). The H NMR spectrum was identical to
material produced via method A.
For C37H57BFeILiN4 calculated: C, 58.60, H, 7.58, N, 7.39;
found: C, 58.47, H, 7.65, N, 7.27.
((DIPPC)2CH2)Fe(H)(HB(sec-Bu)3)·LiI 7
Lithium tris(sec-butyl)borohydride (0.2 ml of 1 M solution in
THF, 0.2 mmol) was added to a stirred benzene suspension
(15 ml) of 1 (78 mg, 0.1 mmol) at ambient temperature, leading
to the formation of a dark-red solution within 10 min. This
solution was concentrated and pentane was added (30 ml). The
resulting brown precipitate was separated by filtration, washed
with pentane (2 × 10 ml) and dried in vacuo to afford bronze-
coloured solid (50 mg, 59%).
[Methylenebis(N-Dipp-imidazole-2-ylidene)] (hydrido)
(η6-toluene)iron(II) iodide, 10
Method A. Lithium triethylborohydride (51 μl of 1 M solu-
tion in THF, 0.051 mmol) was added to the stirred toluene
suspension (5 ml) of 1 (20 mg, 0.026 mmol) at ambient tempera-
ture, leading to immediate formation of a dark-red solution
which was allowed to stir for 15 min. 12-Crown-4 (8.3 μl,
0.051 mmol) was added and the mixture stirred for 2 h, the dark
red solution was then filtered and layered with pentane (15 ml),
the mixture was left to crystallise for 7 days. A small quantity of
large dark yellow X-ray quality crystals precipitated out of the
red solution.
For C43H69BFeILiN4 calculated: C, 61.30, H, 8.25, N, 6.65;
found: C, 61.21, H, 8.09, 6.79.
1H NMR (300 MHz, C6D6, 293 K): Only four extremely
broad resonances were observed at: 7.5, 3.64, 1.44 and
0.91 ppm.
1H NMR (400 MHz, CD2Cl2, 293 K), δ: 7.85 (app. s, 2H,
N-CHvCH-N) 7.56–7.49, 7.41–7.36, 7.35–7.30 (multiplets, 2H
+ 2H + 2H, aromatic protons on DIPP substituents), 7.27–7.20,
7.19–7.12, 7.10–7.03 (multiplets), 6.93 (app. s, 2H, N-CHv
CH-N), 6.12 (s, 2H, N-CH2-N), 6.08 (app. s, 2H, aromatic
protons of coordinated toluene), 5.80 (app. s, 2H, aromatic
protons of coordinated toluene), 4.77 (app. s, 1H, aromatic proton
of coordinated toluene), 3.44 and 2.77 (app. singlets, 2H + 2H,
2 × inequivalent CH(CH3)2), 1.78 (s, 3H, CH3 of coordinated
toluene), 1.45–1.35, 1.32–1.21, 1.20–1.09, 0.90–0.84 (over-
lapped multiplets, 6H + 6H +6H +6H, 4 × inequivalent
C(CH3)2), −10.22 (s, 1H, hydride).
Evans (C6D6, toluene capillary): average 3.22 μB.
Methylenebis(N-Dipp-imidazole-2-ylidene)iron-tricarbonyl, 8
A J. Young’s NMR tube was charged with compound 6 (25 mg)
and C6D6 (0.6 ml). The red solution was degassed by three
freeze-pump-thaw cycles and exposed to 1 bar of CO. The con-
tents of the NMR tube were allowed to mix overnight under a
CO atmosphere by rotation of the tube forming a yellow-
brown solution, which was degassed and exposed to N2.
The NMR spectra indicated complete consumption of the
starting material and formation of a diamagnetic compound
along with minor paramagnetic impurities. IR spectroscopy
confirmed the major Fe(CO) containing product was consistent
with 8:
Method B. Lithium triethylborohydride (256 μl of
1 M. solution in THF, 0.256 mmol) was added to a stirred
toluene suspension (8 ml) of 1 (100 mg, 0.128 mmol) at
ambient temperature, leading to immediate formation of a dark-
red solution. This was stirred for 15 min and then 12-crown-4
(41 μl, 0.256 mmol) was added and the mixture stirred for a
further 1.5 h. Trimethylphosphine (0.64 ml of 1 M. toluene solu-
tion, 0.64 mmol) was then added. Within approx. 5 min the
IR (THF solution) νCO (cm−1): 1968, 1891, 1864.
A small quantity of X-ray quality yellow crystals deposited
from C6D6 solution on standing. Numerous attempts to obtain
pure solid material failed due to intractable paramagnetic impuri-
ties and the instability of compound 8 under vacuum.
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 2685–2693 | 2687

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initial dark-red suspension changed colour to dark yellow, stir-
ring was continued for 2 h, then the dark yellow solution was
filtered off and layered with pentane (40 ml). A dark yellow pre-
cipitate (containing X-ray quality crystals) was filtered off after
7 days and dried in vacuo to afford dark-yellow microcrystalline
solid of 10 (by NMR spectroscopy and X-ray crystallography).
Method C. Lithium triethylborohydride (51 μl of 1 M solu-
tion in THF, 0.051 mmol) was added to a stirred toluene suspen-
sion (5 ml) of 1 (20 mg, 0.026 mmol) at ambient temperature.
After stirring for 15 min 12-crown-4 (8.3 μl, 0.051 mmol) was
added and the mixture stirred for a further 1.5 h, followed by
addition of cyclooctene (10 μl, 0.77 mmol). Stirring was then
continued for 2 h and the dark red solution was filtered and
layered with pentane (15 ml). After 7 days a small number of
large dark yellow X-ray quality crystals precipitated out of the
red solution and were confirmed to be 10 by unit cell measure-
ments and 1H NMR.
Fig. 1 Formation of ((DIPPC)2CH2)FeI(X). Right, ORTEP structure of
2 (50% probability ellipsoids and hydrogens omitted for clarity) Selected
bond lengths (Å) and angles (°): Fe–C = 2.095(14) and 2.105(11),
C–Fe–C = 88.7(5).
Reaction of 1 with one eq. of p-tolyl magnesium bromide
A J. Young’s NMR tube was charged with 1 (20 mg,
0.026 mmol) and THF (0.6 ml). To this solution p-tolyl mag-
nesium bromide (26 μl of 1 M solution in THF, 0.026 mmol)
was added resulting in a rapid colour change to brown and pre-
cipitation of a small amount of light-brown solid. The precipitate
was allowed to settle and a 1H NMR spectrum recorded.
Attempts to isolate analytically pure solid failed.
between 1 and cyclo-heptylbromide (over 24 h) whilst CH2Cl2
only reacts slowly with 1 requiring three days for complete con-
sumption of 1. This disparate reactivity confirms that the
haloalkanes are intercepting an iron-hydride, tentatively assigned
as [((DIPPC)2CH2)FeI2(H)]−, 3, to produce ((DIPPC)2CH2)FeI(X).
The observed reformation of 1 on standing from the initially
formed diamagnetic mono-hydride, 3, may proceed by a ligand
scrambling mechanism, with mass balance dictating that equi-
molar 1, ((DIPPC)2CH2)FeH2 and LiI be produced, although 1 is
the only ((DIPPC)2CH2)Fe containing soluble product observed.
Confirmation that ligand scrambling can occur in related
((DIPPC)2CH2)FeXY systems was provided by mixing 1 and
((DIPPC)2CH2)FeBr2, in THF solution. This results in the for-
1H-NMR (400 MHz, THF+(CD3)2SO capillary, 293 K), δ:
59.0 30.1; 7.1; 6.4; −1.6; −4.9l −8.9; −14.2; −19.6; −27.1;
−42.8.
Results and discussion
Addition of one equivalent of LiBEt3H to ((DIPPC)2CH2)FeI2, 1,
in THF led to complete consumption of 1 and the formation of a
1
single ((DIPPC)2CH2)Fe containing product (by H NMR) which
mation of
a new paramagnetic product consistent with
was diamagnetic and displayed an hydride resonance (at
−7.6 ppm). The expected by-product THF-BEt3 was the only
observable boron containing species (by 11B NMR). Attempts to
isolate this hydride containing species (with and without
removal of the THF-BEt3 by-product) were frustrated by the
1
((DIPPC)2CH2)FeBr(I) (by H NMR), which could also be inde-
pendently prepared by addition of one equivalent of benzyl
bromide to 1.
The extension of a ligand scrambling mechanism to a mixed
hydride/iodide system would require the proposed dihydride
by-product ‘((DIPPC)2CH2)FeH2’, to be unstable, or not observa-
1
reformation of 1 (by H NMR and unit cell comparison on crys-
tallised material). The exact identity of this hydride species has
eluded us but its diamagnetic nature precludes a simple
((DIPPC)2CH2)FeH(I) formalism as all previously reported struc-
turally characterised four coordinate FeII-hydrides23 and all
((DIPPC)2CH2)FeY2 complexes synthesised to date are paramag-
netic. Support for a mono-hydride formulation for the product
from addition of one equivalent of LiBEt3H to 1 in THF was
provided by the repeating the reaction in the presence of excess
dichloromethane which rapidly led to the precipitation of poorly
soluble ((DIPPC)2CH2)FeCl(I), 2. This is consistent with the well
precedented trapping of an FeII-hydride by exchange for
halide.29 The structure of 2 (Fig. 1) is comparable to that pre-
viously reported for 1 and ((DIPPC)2CH2)FeBr2, with iodide
occupying the less sterically congested pseudo-axial site in the
distorted tetrahedral geometry.26 Analogous reactivity was
observed when one equivalent of LiBEt3H was added to 1 in
THF containing excess cyclo-heptylbromide, rapidly (< 1 h)
forming ((DIPPC)2CH2)FeI(Br). In contrast there is no reaction
1
ble by H NMR. Low coordinate, paramagnetic Fe(II) hydrides
(≤ 5) are extremely rare,16,20,23,30 with no dihydrido species,
L2Fe(II)H2, reported to date to the best of our knowledge,
suggesting a fundamental instability of these complexes. Indeed
Girolami et al., have previously concluded that (dippe)FeH2
(dippe = 1,2-bis-(diisopropyl-phosphinoethane) is unstable with
respect to H2 loss.31 This was indicated by (i) the high spin
complex (dippe)FeEt2 undergoing β-hydride elimination fol-
lowed by rapid H2 loss, and (ii) hydrogenolysis of (dippe)
Fe(CH2C6H4-para-Me)2 generating (η6-arene)Fe(dippe) com-
plexes and no Fe–H species. Exchange of dippe for the bis-NHC
chelating ligand (DIPPC)2CH2 will generate a more electron
rich iron centre due to the superior σ donor properties of NHCs,
classically this would be expected to slow reductive elimination
and potentially enhance the stability of a low coordinate Fe-
dihydride. Combination of 1 with two equivalents of KH
resulted in no reaction in arenes or Et2O due to poor solubility
2688 | Dalton Trans., 2012, 41, 2685–2693
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(1 is recovered unchanged after 24 h). Use of the more polar
solvent THF led to rapid gas evolution and the formation of
intractable products. Holland et al., have previously reported that
strong field ligands (e.g., CO) result in rapid reductive elimin-
ation from a low-coordinate Fe(II)H,16 and it is feasible that
strong field NHC ligands also induce rapid H2 reductive elimin-
ation from low coordinate ((DIPPC)2CH2)FeH2. In contrast to the
isolable Fe-(N2) complexes observed on H2 loss from β-diketimi-
nate ligated iron-hydride complexes, intractable products are
formed from addition of KH to 1 under one atmosphere of N2 or
Argon, with no evidence for N2 coordinated complexes (by IR
spectroscopy) from this and all subsequent reactions.
To facilitate the observation of any intermediate hydrido-iron
species addition of two equivalents of soluble sources of hydride
to 1 was investigated. The combination of NaBH4, LiBHEt3
or LiBH(sec-Bu)3 with 1 in THF or MeCN initially produced
short lived paramagnetic complexes which frustrated all isolation
attempts. The carbene borane adducts were ultimately the major
boron and carbene containing products observed in solution,
with 4 and 5 (Fig. 2) isolated from the addition of two equiva-
lents of NaBH4 or LiBHEt3, respectively, to 1. The structures of
4 and 5 are unremarkable, with 5 having a significantly longer
B–C bond relative to 4, as expected due to steric destabilisation
(Fig. 2).32 The carbene-borane adducts are presumably formed
from cleavage of the Fe–C bond followed by subsequent carbene
coordination to a borane species present in solution after
metathetical transfer of hydride to iron complex 1. This is con-
sistent with ‘((DIPPC)2CH2)FeH2’ complexes being unstable in
THF, with dissociation of the bis-carbene taking place, either
directly from a ((DIPPC)2CH2)FeH2 species or after loss of H2.
In contrast the reaction of 1 with two equivalents of LiBHEt3
in Et2O led to a rapid colour change to red and the formation of
one major paramagnetic complex that persisted for days in solu-
tion. The 1H NMR spectrum revealed at least six new broad
paramagnetically shifted resonances. The formation of the
expected by-product THF-BEt3 was confirmed by 11B NMR.
Filtration, followed by subsequent recrystallisation led to a
microcrystalline solid in good yield (87%), on which elemental
analysis was consistent with an iron complex with the empirical
formula of ((DIPPC)2CH2)Fe(H)(HBEt3)·LiI, 6. Replacing
LiBEt3H with LiBH(sec-Bu)3 resulted in a similar outcome,
1
generating a paramagnetic species (by H NMR) that afforded a
microcrystalline solid with an elemental composition consistent
with ((DIPPC)2CH2)Fe(H)(HB(sec-Bu)3)·LiI, 7. Whilst the unam-
biguous characterisation of 6 or 7 by X-ray diffraction has been
frustrated in our hands due to their microcrystalline nature para-
magnetic Fe(II) borohydrides are precedented with both bidentate
and tridentate supporting ligands.33,34 Attempts to confirm the
presence of B–H and Fe–H moieties in 6 and 7 by infrared spec-
troscopy (and for B–H by 11B NMR spectroscopy) in solution
were unsuccessful. Compound 6 was found to be EPR silent dis-
favouring a reduced Fe(I) S = ½ complex and more consistent
with a high or intermediate spin Fe(II) complex (supported by the
Evans method solution magnetic moments of 4.0 and 3.3 μB for
6 and 7, respectively).22 The observed solubility of 6 and 7 in
arene solvents disfavours solvent separated ion pairs, instead it is
feasible that iodide remains coordinated to Fe and the lithium
cations in 6 and 7 are coordinated to two aryl rings of the DIPP
N-substituents intra- or intermolecularly (Fig. 3). Related arene
entrapment of Na+ by an iridium-bis carbene compound and of
Li+ by (aryl)2MnI have been reported.35,36 A Li—arene coordi-
nated species is also consistent with elemental analyses being
free of etherate solvents. Based on the empirical formula,
observed paramagnetism and reactivity studies we tentatively
Fig. 2 Formation of carbene borane adducts from addition of M
[BHR3] to 1 in THF or MeCN. Inset bottom, ORTEP representations
of 4 and 5 (50% probability ellipsoids and hydrogens omitted for
clarity), selected bond lengths (Å) for 4: B1–C1 = 1.584 (5), for 5:
B1–C1 1.651(6).
Fig. 3 Schematic for the production of ((DIPPC)2CH2)FeBr2 and 8 via
addition of excess cyclo-heptylbromide and CO, respectively to 6 (pro-
posed structure of 6 also shown). Inset right, ORTEP representations of
8 (50% probability ellipsoids and hydrogens omitted for clarity).
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assign 6 and 7 as the five coordinate complex, ((DIPPC)2CH2)
Fe(H)(HBEt3)·LiI (Fig. 3 centre left).
frequencies of 8 at 1968, 1891 and 1864 cm−1 (in THF solution)
are lower than the analogous cis-phosphine ligated species (e.g.,
Compound 7 proved to be extremely sensitive to carbene dis-
sociation, forming complex mixtures on standing in Et2O that
contained carbene borane adducts (by 11B NMR). In contrast the
LiBEt3H derived product 6 was stable in Et2O for days. This dis-
parity we attribute to the greater steric pressure in the iron
coordination sphere of 7 generated by the bulkier sec-Bu groups,
with steric pressure well documented to assist carbene dis-
sociation from electron rich Fe centres.37–40 All subsequent
studies concentrated on 6 in Et2O or arene solvents, as 6 decom-
poses readily in neat THF. As the synthesis of 6 utilises LiBEt3H
as a 1 M THF solution the sensitivity to higher concentrations of
THF is attributed to enhanced solvation of ionic species (e.g.,
LiI) in neat THF enabling formation of a different Fe species by
loss of Li(solv)xI.
The presence of BEt3 in compound 6 was supported by the
addition of excess cyclo-heptylbromide to a C6D6 solution of 6.
This formed BEt3 and ((DIPPC)2CH2)FeBr2 as the only
((DIPPC)2CH2)Fe containing species. Importantly cyclo-heptane
was also observed (by 13C{1H} NMR spectroscopy) consistent
with hydride/halide exchange from an Fe species that reacts as
an Fe(II)-dihydride equivalent (Fig. 3, bottom left). The func-
tional ‘dihydride’ formulation of 6 is consistent with the dispa-
rate reactivity observed on addition of cyclo-heptylbromide to 3
which was tentatively formulated as a mono-hydride Fe species.
In contrast to 6, the latter only forms ((DIPPC)2CH2)FeBrI as
expected due to the presence of only one hydride ligand to
exchange for bromide. Further confirmation of the presence of
BEt3 in 6 was forthcoming from the exposure of a C6D6 solution
of 6 (from dissolution of analytically pure red microcrystalline
6) to one atmosphere of CO. This formed the tricarbonyl
complex ((DIPPC)2CH2)Fe(CO)3, 8, as the major Fe(CO) species
in solution (by IR spectroscopy), along with BEt3 as the only
boron containing by-product (by 11B NMR). The repeated evol-
ution of BEt3 from 6 despite the absence of any observable 11B
resonances in recrystallised 6 clearly indicates that the borane is
coordinated to the paramagnetic Fe centre in this complex. As
decomposition of 6 in arene solvents is slow at 25 °C (7 days for
full consumption of 6) the formation of 8 must proceed via
initial ligation of CO to Fe, which starts a multistep reaction that
includes borane dissociation, reduction of Fe(II) to Fe(0) (pre-
sumably by elimination of H2) and trapping of the Fe(0) species
by additional CO. The reductive elimination of H2 from Fe(II)
species initiated by Lewis base coordination (e.g., CO, N2,
CNR) is documented, including from low coordinate iron
hydrides,16,41 whilst CO coordination to Fe inducing B–H clea-
vage and subsequent reductive elimination to a Fe0(CO)x species
is also precedented.42
dppeFe(CO)3 = 1978, 1910 and 1891 cm−1 44
) consistent with
the stronger σ donor power of the NHC. Whilst compound 8 is
formed as the major product (by IR spectroscopy) it was stable
in solution only under a CO atmosphere, under an N2 atmos-
phere or dynamic vacuum 8 decomposes to an intractable
mixture.
As the persistence of LiI in solution was complicating the for-
mation of low coordinate ‘((DIPPC)2CH2)Fe-hydrides’ the reac-
tivity of 1 with LiBHR3 in toluene was investigated to favour
rapid precipitation of inorganic salt by-products and enable
access to LiI free ‘((DIPPC)2CH2)Fe(H)Y’ species. Combination
of 1 and one equivalent of LiBEt3H (1 M in THF) with toluene
1
as solvent led to an H NMR spectrum containing two major
species consistent with the paramagnetic compounds 1 and 6.
BEt3 was again observed as the only boron containing by-
product, along with the precipitation of microcrystalline
Li(THF)3(I) (by unit cell comparison).45 The observation of 1
suggests a non-stoichiometric reaction in toluene in contrast to
reactivity in neat THF, which produced the diamagnetic mono-
hydride 3. The addition of two equivalents of LiBEt3H to 1 in
toluene results in complete consumption of 1 and the formation
1
of 6 as the only observable Fe containing product (by H NMR
spectroscopy). The solvent disparity between arene/Et2O com-
pared to THF we attribute to the removal of the first equivalent
of LiI from the reaction mixture by precipitation in toluene/Et2O
solvents, converting 3 to a lower coordinate ‘((DIPPC)2CH2)
FeH(I)’ species, which then rapidly reacts with a second equival-
ent of LiBEt3H to produce 6. In contrast to Et2O solution (where
6 is stable for > 7 days) in arene solvents 6 reacts slowly to give
1
a mixture of complexes including H NMR resonances indicat-
ing η6-aryl coordination to a diamagnetic iron centre. We there-
fore subsequently sought to isolate and optimize the synthesis of
the η6-arylFe species as a potential ((DIPPC)2CH2)FeH synthon
for subsequent transformations by aryl displacement.46
Synthesis of [((DIPPC)2CH2)FeH(η6-toluene)] complexes
After combination of 1 and two equivalents of LiBEt3H in
toluene the resultant red solution was filtered, dried and washed
with pentane (to remove BEt3). This led to the extraction of
free BEt3 and surprisingly a diamagnetic pentane soluble
((DIPPC)2CH2)Fe containing product that displayed a hydride res-
onance at −10.26 ppm. Recrystallisation from pentane (10 days
−20 °C) produced complex 9 in low yield (Fig. 4 top inset),
which contained an η6 toluene ligand and a deprotonated anionic
abnormal carbene-borane, with the ((DIPPC)2CH2) ligand remain-
ing coordinated to Fe in a bidentate fashion. The η6-toluene
moiety is symmetrically capping the ((DIPPC)2CH2)FeH frag-
ment, with the C1–Fe–C5 and arene planes nearly orthogonal
(80.35°), with an arene-centroid-Fe distance of 1.577 Å. Both
values are closely comparable to the related complex [(η6-C7H8)
(μ-OH)(H)Fe{Sn[CH(SiMe3)2]2}2].47 The yaw angles for
((DIPPC)2CH2) in 9 (average 7.0°) confirm the absence of signifi-
cant strain in the metallacycle.43,48,49 The Fe–C(carbene) bonds
(1.966(5) and 1.970(5) Å) are unremarkable for octahedral
FeII,50 whilst the B–C bond length (1.647(10) Å) is elongated
The structure of 8 (Fig. 3) is trigonal-bipyramidal (angles in
the equatorial plane sum to 359.9°), with one axial site and
two equatorial sites occupied by CO. The Fe–CO bond distances
do not show any significant difference between the CO trans to
carbene and the two carbonyls in the equatorial plane. The
metallacycle is not significantly strained, with a bite angle and
average yaw angle of 86.31 and 5.7°, respectively (yaw angle is
the in-plane distortion of the NHC imposed by metallacycle
steric constraints, and is defined as the difference between the
two M–C–N angles divided by two).43 The ν(CO) stretching
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Fig. 5 Proposed mechanism for the competitive formation of 6 and 9.
Fig. 4 Schematic for the formation of compounds 9 and 10. ORTEP
representations of 9 (inset top) and 10 (inset bottom) at 50% probability
ellipsoids and hydrogens omitted for clarity apart from the Fe–H of 10
Selected bond lengths (Å) and angles (°) for 9: B1–C2 = 1.647(10) Fe–
C1 = 1.966(5) and Fe–C5 = 1.970(5); for 10 Fe–C1 1.952(3) and Fe–C5
= 1.951(3).
when the NHC is coordinated to a Lewis acidic metal centre
which will lower the pKa of the C4 and C5 protons.53
The majority of the ((DIPPC)2CH2)Fe containing material post
addition of two equivalents of LiBEt3H to 1 in toluene is 6
which is insoluble in pentane. In an attempt to remove LiI from
6 two equivalents of 12-crown-4 were introduced shortly
(∼ 10 min) after formation of 6 in toluene. Post filtration, the
1
again indicating steric weakening of the B–C bond.32 The
hydride could not be located in the penultimate Fourier differ-
ence map, but the formulation of 9 as an Fe(II) hydride is based
on the diamagnetism and the integral of one hydride resonance
observed spectroscopically on dissolution of these crystals in
CD2Cl2. The existence of 9 as an arene capped species is in con-
trast to the related (β-diketiminate)-FeH complex, 1,3-tBu2-1,3-
bis-(2,6-diisopropyl-phenylimido)FeH, which is a three coordi-
nate iron-hydride that displays no propensity to bind toluene.23
This disparity can be attributed to a more open iron centre in 9
(the N-DIPP substituent steric bulk is located further from the
iron centre relative to the bulky β-diketiminate analogue).51
Mechanistically the formation of 6 and 9 can be viewed as
addition of a second equivalent of LiBEt3H to a transient
mono hydride species, ‘((DIPPC)2CH2)FeH(I)’ (Fig. 5). LiBEt3H
can either coordinate to Fe to form 6 or deprotonate the carbene
to ultimately form 9 on capping with toluene. Deprotonation of
a metal coordinated carbene at the alkenyl position to generate
an abnormal carbene has precedent,52,53 as does deprotonation
of carbene precursors by LiBEt3H to generate carbene borane
adducts.32 The low yield of 9 suggests that the deprotonation
pathway is kinetically disfavoured, with coordination of
[HBEt3]− to iron dominating. Whilst 9 is only produced in poor
yield it is noteworthy, as numerous (NHC)M catalysed conver-
sions (including examples using (NHC)Fe based catalysis)
involve strong bases (e.g. RLi, RMgBr).54,55 Thus carbene back-
bone (alkenyl) deprotonation may be a more common decompo-
sition route in catalysis than previously realised, particularly
only species observed (by H NMR) was still consistent with 6,
indicating the continued presence of LiI. On standing there was a
gradual colour change from red to yellow and a reduction in the
intensity of resonances corresponding to 6 (over seven days)
with the concomitant slow growth of new paramagnetic and dia-
1
magnetic species in the H NMR spectra. Throughout BEt3 is
the only observed diamagnetic boron containing product, pre-
cluding B–C cleavage as reported by Holland et al., on addition
of KBEt3H to (β-diketiminate)FeCl.33 Recrystallisation over
7 days of the mixture derived from 6 and 12-crown-4 from
toluene/pentane produced [((DIPPC)2CH2)Fe(η6-toluene)H]I, 10,
in low yield (Fig. 4 bottom inset). The hydride was located in
the penultimate Fourier difference map and freely refined, it was
also confirmed by dissolution of crystals of 10 in CD2Cl2 which
showed a diamagnetic integral of one hydride resonance
(−10.4 ppm). It is noteworthy that addition of LiBHEt3 to 10
resulted in no reaction, confirming that 10 is not an intermediate
in the production of 9 (Fig. 4).
The structure of 10 is grossly similar to that of 9, with the
carbene ligand in 10 also having minimal strain in the metalla-
cycle (yaw angles of 5.9°) and similar Fe–C bonds distances and
bite angle. The two most significant differences in the structures
of 9 and 10 are associated with the disposition of the η6-toluene
ligand. In 9 the tolyl-methyl is orientated towards the ligand
backbone and there is a larger inter-plane angle of 80.3° between
the C–Fe–C and toluene aryl planes. Compound 10 in contrast
has the para-H of the toluene ligand directed towards the ligand
backbone CH2 and a smaller angle between the C1–Fe–C5 plane
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NMR spectroscopy was uninformative, producing NMR silent
soluble products on addition of the reducing equivalents. In con-
trast the reduction of (dcype)FeCl2 and α-di-imineFeX2 com-
pounds, both produce diamagnetic L2Fe(η6-arene) coordinated
species on reduction.57 The formation of ((DIPPC)2CH2)Fe(η6-
arene) complexes is clearly sterically accessible (as exemplified
by 9 and 10), and the absence of diamagnetic Fe(0) products is
therefore attributed to insufficient stabilisation of the electron
rich Fe(0) centre by carbene ligation. This is consistent with the
weak, reversible binding of NHCs to Fe(0) observed by Grubbs
et al.39 Thus redox flexible neutral bidentate ligands, of which
the α-diimines are a paradigm, are more suited to stabilisation of
reduced Fe centres being superior π acceptors relative to
((DIPPC)2CH2).
Fig. 6 Overlay of the wireframe core structures of 9 (blue) and 10
(red), ’including the nearest ligand backbone CH2—arene C distance,
annotated in black for 9 and in green for 10 (DIPP groups and hydrogens
are omitted for clarity). Inset space filling diagram (100% van der Waals
radii) of 9, showing the close contacts between B-CH2 groups and the
backbone CH2 position.
In an attempt to intercept any ((DIPPC)2CH2)Fe(II)-hydride
complexes 12-crown-4 and then cyclooctene or PMe3 (3 equiva-
lents of each) were added to 6 in d8-toluene. Addition of cyclo-
octene was envisaged to form an Fe-alkyl complex analogous to
the work of Holland et al., where insertion products are formed
from combination of [β-diketiminateFeH] and alkenes.58 How-
ever, this led to a mixture of paramagnetic and diamagnetic pro-
ducts (by 1H NMR) from which only 10 could be isolated
by recrystallisation (from toluene/pentane). Use of PMe3 in
place of cyclo-octene (to trap any transient low coordinate
hydrides as higher coordinate and more stable species) resulted
in a rapid colour change from red to yellow and a 1H NMR spec-
trum revealing complete consumption of 6 and the growth of
new paramagnetic and diamagnetic products. The 11B spectrum
revealed the major boron containing product was Me3P-BEt3,
with carbene-borane adducts also observed as minor products.
On recrystallisation from toluene/pentane over 7 days 10 was
reproducibly isolated, albeit in low yield (a maximum isolated
yield was 16%). The complex reactivity observed herein is
consistent with previous attempts to form paramagnetic iron-
hydrides ligated with different multidentate ligands using
borohydride reagents. These have been equally complicated by
competitive reaction pathways which can lead to intractable mix-
tures.33,59 Furthermore Deng et al., have recently reported that
the hydrogenation of the related complex (IPr)2FeMe2 (IPr =
2,5-diisopropyl-3,4-dimethylimidazol-1-ylidene) led to complex
mixtures that eluded isolation and characterisation.37
and the tolyl arene plane of 65.3° (Fig. 6). This results in a
greater distance between the ligand backbone CH2 and the aryl
ring for 9 relative to 10 (4.209 versus 3.806 Å). This is attributed
to the presence of the BEt3 moiety in compound 9 which results
in more steric congestion. This is manifested in; (i) a reduction
in the angle between the imidazole C3N2 plane and the aryl
plane of the DIPP substituent to 78.7° in 9 (in comparison the
average analogous angle in 10 is closer to orthogonal at 84.1°).
(ii) A greater N–CH2–N angle, which is noticeably larger in 9
(112.2(5) compared to 108.6(2)° in 10) to minimise unfavour-
able steric interactions with the proximate B–CH2 groups (Fig. 6
inset). Combined these result in more DIPP steric bulk in proxi-
mity of the Fe–H site in 9, preventing the orientation observed in
10 where the sterically larger tolyl-methyl is located away from
the ligand CH2 backbone towards the Fe–H site.
With 10 only isolable in low yields a number of alternative
routes for its synthesis were explored; (i) The immediate addition
of toluene to diamagnetic 3 (proposed to be [((DIPPC)2CH2)
1
FeH(I)2]−) in THF resulted in no change in the H NMR spec-
trum. (ii) Following the synthesis of (η6-arene)Fe complexes by
hydrogenolysis of (dippe)Fe(benzyl)2 precursors,31 compound 1
was treated with one equivalent of p-tolylMgBr in THF. This
produced a new paramagnetic product of low symmetry that
was tentatively assigned as ((DIPPC)2CH2)Fe(η1-para-tolyl)(I).
The addition of H2 (4 bar) to this reaction mixture resulted in the
formation of an intractable mixture which did not contain any
In conclusion, the installation of hydrides onto the
((DIPPC)2CH2)Fe fragment to generate low coordinate Fe(II)
-
hydrido species has been found to be non-trivial, particularly
frustrated by competitive reaction with solvents which is sum-
marized in Fig. 7: (i) polar solvents lead to carbene dissociation
from the iron centre (ii) haloalkanes result in the expected rapid
hydride/halide exchange (iii) arene solvents produce complex
mixtures from which (η6-arene) capped iron species can be iso-
lated on recrystallisation. Further complexity is introduced by:
(i) the coordinated carbene being susceptible to deprotonation
and (ii) continued coordination of iodide to iron during
attempted metathesis for hydride. The latter combined with con-
tinued borane coordination to Fe (presumably as Fe-H-BEt3) has
enabled the isolation of a functional ‘masked’ form of a four
coordinated Fe-dihydride. Our reactivity studies also indicate a
fundamental instability of low coordinate iron(II)-dihydrides
towards reductive elimination of H2 analogous to previous
unsuccessful attempts to synthesise low coordinate (four and
1
compound 10 (by H NMR). (iii) To take advantage of the pro-
pensity for iron-butyl moieties to undergo β-hydride elimination
n
two equivalents of BuLi were added to 1,31,56 however no dia-
magnetic hydride resonances were observed with currently uni-
dentified paramagnetic products formed. (iv) Komiya et al.,
developed a two step approach to [(dcype)FeH(arene)]+ (dcype =
1,2-bis(dicyclo-hexylphosphinoethane)) by initial reduction of
(dcype)FeCl2 to form (dcype)Fe(η6-arene) followed by sub-
sequent protonation.46 However, reduction of 1 using a range of
reductants, and reducing equivalents (1, 2 or excess) repeatedly
failed to produce any isolable ((DIPPC)2CH2)Fe0 products, even
in the presence of a range of Fe(0) trapping agents (e.g., N2,
toluene, benzene, naphthalene, cyclooctene). Throughout 1H
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18 S. D. Brown, M. P. Mehn and J. C. Peters, J. Am. Chem. Soc., 2005, 127,
13146.
19 E. J. Daida and J. C. Peters, Inorg. Chem., 2004, 43, 7474.
20 P. L. Holland, Acc. Chem. Res., 2008, 41, 905.
21 S. C. Bart, E. Lobkovsky and P. J. Chirik, J. Am. Chem. Soc., 2004, 126,
13794.
22 Y. Lee, R. A. Kinney, B. M. Hoffman and J. C. Peters, J. Am. Chem.
Soc., 2011, 133, 16366.
23 J. M. Smith, R. J. Lachicotte and P. L. Holland, J. Am. Chem. Soc., 2003,
125, 15752.
24 A. R. Sadique, E. A. Gregory, W. W. Brennessel and P. L. Holland,
J. Am. Chem. Soc., 2007, 129, 8112.
25 J. Vela, J. M. Smith, Y. Yu, N. A. Ketterer, C. J. Flaschenriem,
R. J. Lachicotte and P. L. Holland, J. Am. Chem. Soc., 2005, 127, 7857.
26 S. Zlatogorsky, C. A. Muryn, F. Tuna, D. J. Evans and M. J. Ingleson,
Organometallics, 2011, 30, 4974.
27 D. F. Evans, J. Chem. Soc., 1959, 2003.
28 S. K. Sur, J. Magn. Reson., 1989, 82, 169.
29 M. L. H. Green and L.-L. Wong, J. Chem. Soc., Dalton Trans., 1987,
411.
30 T. R. Dugan and P. L. Holland, J. Organomet. Chem., 2009, 694, 2825.
31 A. R. Hermes, T. H. Warren and G. S. Girolami, J. Chem. Soc., Dalton
Trans., 1995, 301.
32 Y. Yamaguchi, T. Kashiwabara, K. Ogata, Y. Miura, Y. Nakamura,
K. Kobayashi and T. Ito, Chem. Commun., 2004, 2160.
33 Y. Yu, W. W. Brennessel and P. L. Holland, Organometallics, 2007, 26,
3217.
Fig. 7 Solvent dependent summary for reactions of 1 (termed LIGFeI2
for simplicity, LIG = ((DIPPC)2CH2)).
five, respectively) Fe(II)H2 complexes chelated by diphosphine
and 2,6-di-imino-pyridine ligands.
34 M. P. Mehn, S. D. Brown, T. K. Paine, W. W. Brennessel, C. J. Cramer,
J. C. Peters and J. L. Que, Dalton Trans., 2006, 1347.
35 C. Y. Tang, A. L. Thompson and S. Aldridge, Angew. Chem. Int. Ed.,
2009, 49, 921.
36 C. Ni, J. C. Fettinger, G. J. Long and P. P. Power, Dalton Trans., 2010,
39, 10664.
37 L. Xiang, J. Xiao and L. Deng, Organometallics, 2011, 30, 2018.
38 V. Lavallo, A. El-Batta, G. Bertrand and R. H. Grubbs, Angew. Chem.,
Int. Ed., 2011, 50, 268.
39 V. Lavallo and R. H. Grubbs, Science, 2009, 326, 559.
40 R. A. Layfield, J. J. W. McDouall, M. Scheer, C. Schwarzmaier and
F. Tuna, Chem. Commun., 2011, 47, 10623.
41 Y. Gao, D. G. Holah, A. N. Hughes, G. J. Spivak, M. D. Havighurst,
V. R. Magnuson and V. Polyakov, Polyhedron, 1997, 16, 2797.
42 J. S. Figueroa, J. G. Melnick and G. Parkin, Inorg. Chem., 2006, 45,
7056.
43 C. H. Leung, C. D. Incarvito and R. H. Crabtree, Organometallics, 2006,
25, 6099.
44 S. D. Ittel, C. A. Tolma, P. J. Krusic, A. D. English and J. P. Jesson,
Inorg. Chem., 1978, 17, 3432.
45 H. Nöth and R. Waldhor, Z. Naturforsch., B: Chem. Sci, 1998, 53, 1525.
46 H. Kubo, M. Hirano and S. Komiya, J. Organomet. Chem., 1998,
556, 89.
Acknowledgements
We gratefully acknowledge the Royal Society (M.J.I for a Uni-
versity Research Fellowship), the EPSRC (Grant Number EP/
H006990/1 S. Z) and the University of Manchester for funding.
Professor G. Morris (The University of Manchester) is gratefully
acknowledged for assistance with solvent suppression NMR
spectroscopy, Dr Christopher Muryn for assistance with X-Ray
crystallography and Profs R. Winpenny and D. Collison are
thanked for useful discussions.
References
1 A. Boddien, D. Mellmann, F. Gartner, R. Jackstell, H. Junge, P. J. Dyson,
G. Laurenczy, R. Ludwig and M. Beller, Science, 2011, 333, 1733.
2 F. Gloaguen and T. B. Rauchfuss, Chem. Soc. Rev., 2009, 38, 100.
3 X. Liu, S. K. Ibrahim, C. Tard and C. J. Pickett, Coord. Chem. Rev.,
2005, 249, 1641.
4 C. Tard and C. J. Pickett, Chem. Rev., 2009, 109, 2245.
5 M. J. Corr and J. A. Murphy, Chem. Soc. Rev., 2011, 40, 2279.
6 R. Y. Igarashi, M. Laryukhin, P. C. Dos-Santos, H.-I. Lee, D. R. Dean,
L. C. Seefeldt and B. M. Hoffman, J. Am. Chem. Soc., 2005, 127, 6231.
7 D. Lukoyanov, Z.-Y. Yang, D. R. Dean, L. C. Seefeldt and
B. M. Hoffman, J. Am. Chem. Soc., 2010, 132, 2526.
8 P. C. D. Santos, R. Y. Igarashi, H.-I. Lee, L. C. Seefeldt and D. R. Dean,
Acc. Chem. Res., 2005, 38, 208.
9 L. C. Seefeldt, I. G. Dance and D. R. Dean, Biochemistry, 2004, 43,
1401.
10 P. E. Doan, J. Telser, B. M. Barney, R. Y. Igarashi, D. R. Dean,
L. C. Seefeldt and B. M. Hoffman, J. Am. Chem. Soc., 2011, 133, 17329.
11 I. Dance, J. Am. Chem. Soc., 2007, 129, 1076.
12 J. Kästner and P. E. Blöchl, J. Am. Chem. Soc., 2007, 129, 2998.
13 I. Dance, Dalton Trans., 2010, 39, 2972.
14 M. Peruzzini and R. Poli, ed., Recent Advances in Hydride Chemistry,
Elsevier, New York, 2001.
15 G. J. Kubas, Metal Dihydrogen and sigma-bond complexes, Kluwer
Academic, New York, 2001.
16 Y. Yu, A. R. Sadique, J. M. Smith, T. R. Dugan, R. E. Cowley,
W. W. Brennessel, C. J. Flaschenriem, E. Bill, T. R. Cundari and
P. L. Holland, J. Am. Chem. Soc., 2008, 130, 6624.
17 C. C. Lu, C. T. Saouma, M. W. Day and J. C. Peters, J. Am. Chem. Soc.,
2007, 129, 4.
47 J. J. Schneider, J. Hagen, N. Czap, C. Kruger, S. A. Mason, R. Bau,
J. Ensling, P. Gutlich and B. Wrackmeyer, Chem.–Eur. J., 2000, 6, 625.
48 J. A. Mata, M. Poyatos and E. Peris, Coord. Chem. Rev., 2007, 251, 841.
49 M. Poyatos, J. A. Mata and E. Peris, Chem. Rev., 2009, 109, 3677.
50 B. Li, T. Liu, C. V. Popescu, A. Bilko and M. Y. Darensbourg, Inorg.
Chem., 2009, 48, 11283.
51 J. M. Smith, R. J. Lachicotte and P. L. Holland, Chem. Commun., 2001,
1542.
52 Y. Wang, Y. Xie, M. Y. Abraham, P. Wei, H. F. Schaefer, P. v. R. Schleyer
and G. H. Robinson, J. Am. Chem. Soc., 2010, 132, 14370.
53 U. J. Scheele, S. Dechert and F. Meyer, Chem.–Eur. J., 2008, 14, 5112.
54 R. B. Bedford, M. Betham, D. W. Bruce, A. A. Danopoulos, R. M. Frost
and M. Hird, J. Org. Chem., 2006, 71, 1104.
55 T. Hatakeyama, S. Hashimoto, K. Ishizuka and M. Nakamura, J. Am.
Chem. Soc., 2009, 131, 11949.
56 I. d. l. Rios, M. J. Tenorio, M. A. J. Tenorio, M. C. Puerta and P. Valerga,
J. Organomet. Chem., 1996, 525, 57.
57 S. C. Bart, E. J. Hawrelak, E. Lobkovsky and P. J. Chirik, Organometal-
lics, 2005, 24, 5518.
58 J. Vela, S. Vaddadi, T. R. Cundari, J. M. Smith, E. A. Gregory,
R. J. Lachiotte, C. J. Flaschenriem and P. L. Holland, Organometallics,
2004, 23, 5226.
59 W. A. Chomitz and J. Arnold, Dalton Trans., 2009, 1714.
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