σ-Alane complexes of chromium, tungsten, and manganese

Article abstract of DOI:10.1021/ja2119892

Photolytic ligand displacement and salt metathesis routes have been exploited to give access to κ¹ σ-alane complexes featuring Al-H bonds bound to [W(CO)₅] and [Cp′Mn(CO) ₂] fragments, together with a related κ² complex of [Cr(CO)₄]. Spectroscopic, crystallographic, and quantum chemical studies are consistent with the alane ligands acting predominantly as σ-donors, with the resulting binding energies calculated to be marginally greater than those found for related dihydrogen complexes.

Full text of DOI:10.1021/ja2119892

Communication
pubs.acs.org/JACS
σ-Alane Complexes of Chromium, Tungsten, and Manganese
Ian M. Riddlestone,† Sian Edmonds,† Paul A. Kaufman,† Juan Urbano,† Joshua I. Bates,† Michael J. Kelly,†
̂
Amber L. Thompson,† Russell Taylor,‡ and Simon Aldridge*,†
†Inorganic Chemistry Laboratory, Department of Chemistry, University of Oxford, South Parks Road, Oxford, OX1 3QR, U.K.
‡Hull Research and Technology Centre, BP Chemicals Ltd., DL10, HRTC Saltend, Hull, HU12 8DS, U.K.
S
* Supporting Information
Scheme 1. Potential Synthetic Routes to σ-Alane Complexes
ABSTRACT: Photolytic ligand displacement and salt
metathesis routes have been exploited to give access to
κ1 σ-alane complexes featuring Al−H bonds bound to
[W(CO)5] and [Cp′Mn(CO)2] fragments, together with a
related κ2 complex of [Cr(CO)4]. Spectroscopic, crystallo-
graphic, and quantum chemical studies are consistent with
the alane ligands acting predominantly as σ-donors, with
the resulting binding energies calculated to be marginally
greater than those found for related dihydrogen com-
plexes.
Exploiting either (1) Halide Substitution at Aluminum by a
Nucleophilic Metal Hydride, or (2) Ligand Substitution at a
Transition Metal (M) by an Alane
aggregation prevalent among aluminum hydrides.26 Moreover,
the transition metal fragments employed, [Cp′Mn(CO)2] and
[W(CO)5], permit for the first time comparison of alane σ-
complexes with archetypal dihydrogen, borane, and silane
systems.1−7
ransition metal σ-complexes featuring activated but more-
T
or-less intact E−H bonds (E = H, B, or Si) have attracted
significant attention since the first dihydrogen complex was
reported in 1984.1−8 Such efforts reflect not only interest in the
fundamental electronic structure of nonclassical bonds, but also
the relevance of σ-complexes to E−H oxidative addition,2,3 and
ultimately to metal catalyzed processes for the addition of E−H
bonds to unsaturated substrates.9−11 While σ-alkane complexes
are relatively uncommon,12 coordination of H2 and of libraries
of boranes/silanes to unsaturated 16-electron fragments such as
[CpMnL2] and [W(CO)3L2] has allowed for systematic studies
of the electronic structure of such systems.2−8
Among Group 13 systems, the coordination and activation of
B−H bonds at transition metal centers has been systematically
investigated,13,14 not least due to applications in hydro-
boration, borylation, and dehydrocoupling processes.11,15,16
By contrast, the interaction of E−H bonds featuring the heavier
Group 13 elements is much less well developed despite, for
example, reports of transition metal dopants catalyzing
dihydrogen cycling from aluminum hydride materials.17,18
Thus, while Ueno, Ogino, and co-workers have reported the
coordination of an amine−gallane adduct to [W(CO)5],19 the
majority of complexes containing coordinated Al−H bonds
feature highly electrophilic early transition metals and/or
anionic hydroaluminate ligands. Examples of simple, unsup-
ported, charge neutral σ-alane complexes are very rare and
those featuring the classical [CpMnL2] and [W(CO)3L2]
fragments are unknown.20−24
The ready accessibility of amidinate and guanidinate
stabilized aluminum dihalides,27 and of the [Cp′Mn(CO)2H]−
anion,28 suggests that a halide metathesis approach (Scheme 1,
route 1) might offer a convenient synthesis of compounds
featuring a Mn−H−Al interaction. Consistently, the reaction
between K[Cp′Mn(CO)2H] and Cl2Al{(NiPr)2CPh} (Scheme
2) leads to the formation of Cp′Mn(CO)2[HAl(Cl)-
{(NiPr)2CPh}] (1), which has been characterized by standard
spectroscopic methods (see Supporting Information (SI)) and
by X-ray crystallography. Particularly indicative of the bridging
hydrogen atom is the 1H NMR resonance at δH = −13.93 ppm;
the location of H(1) could also be inferred crystallographically
in the solid state from the difference Fourier map (Figure 1).
The geometry around Al(2) approximates to trigonal
bipyramidal in a manner reminiscent of related manganese
silane complexes (with due allowance made for the constraints
imposed by the four-membered amidinate ring and the Mn−H
linkage).4,5,29 Thus, the sum of the angles subtended at Al(2)
by the Mn−Al, Al−Cl, and Al−N(6) vectors is 349.1(1)°, and
the H−Al−N(4) angle is 150.6(1)°. The identification of H(1)
as bridging the Mn−Al linkage is consistent with a metal−metal
distance [2.446(1) Å] which is markedly longer than the Fe−Al
separation measured for the closely related (but non-hydride-
containing) Fp system CpFe(CO)2[Al(Cl){(NiPr)2CPh}]
[2.340(1) Å; see SI]. Moreover, while the (O)C−M−Al angles
in the iron compound are nearly equal [80.7(1), 82.1(1)°],
In this manuscript, we report two strategies for the synthesis
of complexes containing neutral alane donor ligands (Scheme
1). Such approaches mirror the routes used for σ-borane
complexes of the type Cp′Mn(CO)2(κ1-HBR2) [Cp′ = η5-
(C5H4Me)],25 but additionally employ sterically imposing
ancillary frameworks, in order to minimize the tendency for
Received: January 6, 2012
Published: January 20, 2012
© 2012 American Chemical Society
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Scheme 2. Syntheses of Manganese σ-Alane Complexes 1
and 2
Scheme 3. Syntheses of Group 6 σ-Alane Complexes 3 and
4
a
a
a
Key reagents/conditions: (i) M = W, alane (0.5 equiv. of dimer),
hexanes, 20 °C, UV photolysis, 100 min, 8% (as single crystals); (ii) M
= Cr, alane (1.0 equiv.), toluene, 20 °C, UV photolysis, 90 min, 49%
(co-crystallized as a ca. 9:1 mixture with the κ1 complex).
toluene leads to the formation of Cp′Mn(CO)2[κ1-H2Al-
{(NDippCMe)2CH}] (2). Consistent with the spectroscopic
1
data determined for 1, the H NMR spectrum of 2 features a
a
Key reagents/conditions: (i) K[Cp′Mn(CO)2H] (1.1 equiv.), Et2O,
similarly high-field shifted signal (δH = −15.42 ppm).
Interestingly (and in contrast to reports of related κ1-borane
complexes),34−40 there appears to be slow exchange between
the bound and unbound Al−H hydrogens on the NMR time
scale at temperatures up to 75 °C. Crystallographically, 2 is
characterized by similar Al−H(Mn) and Mn−H distances to 1
[1.57(3), 1.70(2) Å for 2 vs 1.56(3), 1.72(3) Å], although the
Mn···Al separation is significantly longer [2.654(1) cf. 2.446(1)
Å], reflecting a wider Al−H−Mn angle [109(2) vs 97(1)°] and,
presumably, the greater steric requirements of the β-
diketiminato skeleton (Figure 1).
20 °C, 18 h, 17% (as single crystals); (ii) Cp′Mn(CO)3 (1.0 equiv.),
toluene, 20 °C, UV photolysis (Hg arc lamp, 1 kW), 165 min, 13% (as
single crystals; ca. 50% as microcrystalline material) (Dipp =
2,6-iPr2C6H3).
The carbonyl stretching frequencies measured for 2 [νs(CO)
= 1947; νas(CO) = 1879 cm−1] can be compared to analogous
values of 1995−1967 and 1937−1901 cm−1 for σ-complexes of
tricoordinate boranes (which possess π-acceptor capabilitie-
s),25a and 1947−1919, 1858−1820 cm−1 for tetra-coordinate
boranes.34,37 As such, these data are indicative of a dominant σ-
donor role for the alane ligand. Also consistent with such a
description is the alignment of the H2Al{(NDippCMe)2CH}
fragment in 2; a Cp centroid−Mn−Al−H torsion angle of 67.0°
is reflective of minimal overlap of the Al−H σ* orbital with the
HOMO of the metal fragment.34,37
Figure 1. Molecular structures of 1 and 2 in the solid state. H atoms
(except metal-bound H’s) omitted for clarity and thermal ellipsoids set
at the 40% probability level. Key bond lengths (Å) and angles (°): (for
1) Mn(1)···Al(2) 2.446(1), Mn(1)−H(21) 1.56(3), Al(2)−H(21)
1.72(3), Al(2)−Cl(3) 2.156(1), Al(2)−N(4) 1.923(2), Al(2)−N(6)
1.906(2), Mn(1)−H(21)−Al(2) 97(1), N(4)−Al(2)−N(6) 69.6(1);
(for 2) Mn(1)···Al(2) 2.654(1), Mn(1)−H(21) 1.57(3), Mn(1)−
C(34) 1.760(3), Mn(1)−C(42) 1.770(3), Al(2)−H(21) 1.70(2),
Al(1)−H(22) 1.59(3), Al(2)−N(3) 1.910(2), Al(2)−N(19) 1.925(2),
Mn(1)−H(21)−Al(2) 109(2), H(21)−Al(2)−H(22) 143(1).
Photolytic substitution of carbonyl ligands by Al−H donors
can also generate κ1 alane complexes featuring [M(CO)5]
fragments, and under appropriate conditions, related κ2 systems
by substitution of a second CO ligand. Thus, the reaction of
[{iPr2NC(NDipp)2}AlH2]2 with W(CO)6 generates (OC)5W-
[κ1-H2Al{(NDipp)2CNiPr2}] (3) via substitution of a single
CO ligand; for this system at least, the use of more forcing
conditions results only in the formation of the known anionic
tungsten hydride [(OC)5W(μ-H)W(CO)5]−.41,42 A κ1 mode of
coordination for 3 in solution is implied by a lowering of
symmetry within the flanking Dipp substituents (four CH3 and
those for 1 are more disparate [76.0(1), 115.7(1)°], reflecting
the accommodation of an additional hydrogen atom within the
manganese coordination sphere. A description as a three-legged
piano stool complex featuring a coordinated Al−H bond is
therefore conceivable. With this in mind, we set out to explore
alternative synthetic pathways using starting materials contain-
ing preformed Al−H bonds (i.e., route 2).
1
two CH signals in both H and 13C NMR spectra) and by the
While simple amidinato derivatives of the type [(amid)-
AlH2]n are not well-known; related dihydrides containing
sterically more encumbered heterocycles are readily available.
Thus, we probed the reactivity of the dimeric guanidinate-
stabilized alane [{iPr2NC(NDipp)2}AlH2]2, and the mono-
meric β-diketiminato derivative {HC(CMeNDipp)2}AlH2 with
metal carbonyl substrates under photolytic conditions (Scheme
3).30−33 Such chemistry provides not only a complementary
synthesis of [Cp′Mn(CO)2] alane σ-complexes, but also access
to related systems featuring [M(CO)5] and [M(CO)4]
fragments, and hence comparison with additional families of
archetypal E−H σ complexes. Thus, photolysis of an equimolar
mixture of {HC(CMeNDipp)2}AlH2 and Cp′Mn(CO)3 in
observation of distinct resonances for the terminal and bridging
Al−H hydrogens (at δH = 6.80 and −7.11 ppm). The former
signal is very broad, and could be definitively identified only via
1H{27Al} experiments; the Al−H−W signal can, however, be
resolved as a doublet (2JHH = 20.0 Hz). Moreover, as with 2,
slow fluxional exchange of the aluminum-bound hydrogens is
observed at temperatures up to 75 °C, implying that ΔG⧧ > 17
kcal mol−1; similar behavior has been reported for a gallane
fragment bound to [W(CO)5].19 Although experimental
coupling constant data for geminal AlH2 units have not been
reported, the value determined for 3 can be compared to
quantum chemical estimates for ‘free’ alanes (22.6−29.7 Hz)
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Journal of the American Chemical Society
Communication
alane binding with other σ-complexes. Such studies reveal
excellent agreement with X-ray structural data, and estimate
ligand binding energies which are comparable to/greater than
those found for H2, but markedly less than those for CO (Do =
26.9, 25.6, 54.2 kcal mol−1 for alane, H2, and CO complexes of
[Cp′Mn(CO)2] and 22.9, 14.3, 36.2 kcal mol−1 for the
corresponding [W(CO)5] systems). Moreover, the relatively
similar alane binding energies calculated for these two metal
systems (cf. much larger differences for the H2 and CO
complexes) are consistent with the minor role of π-back-
bonding in alane coordination.
In summary, we have demonstrated two synthetic
approaches to complexes of charge neutral alane donors, and
elucidated κ1 and κ2 modes of coordination at 16- and 14-
electron metal fragments, respectively. Spectroscopic and
structural data are consistent with a predominantly σ donor
role for the alane, with the marked polarity of the Al−H bond
presumably also being important. Further studies targeting
broader demonstration of this chemistry, and elucidation of
fundamental patterns of reactivity will be reported in due
course.
Figure 2. Molecular structures of 3 and 4 in the solid state. H atoms
(except metal-bound H’s) omitted and Dipp Pr groups shown in
i
wireframe format for clarity and thermal ellipsoids set at the 40%
probability level. Key bond lengths (Å) and angles (°): (for 3)
W(1)···Al(2) 2.841(1), W(1)−H(21) 1.80(3), W(1)−C(37)
2.014(4), W(1)−C(45) 2.024(3), Al(2)−H(21) 1.67(3), Al(2)−
H(22) 1.46(3), Al(2)−N(3) 1.913(2), Al(2)−N(5) 1.892(2), W(1)−
H(21)−Al(2) 110(2), H(21)−Al(2)−H(22) 123(2); (for 4)
Cr(1)···Al(2) 2.420(1), Cr(1)−H(11) 1.64, Cr(1)−H(12) 1.69,
Cr(1)−C(19) 1.892(2), Cr(1)−C(21) 1.834(3), Cr(1)−C(23)
1.846(3), Al(2)−H(11) 1.51, Al(2)−H(12) 1.65, Al(2)−N(3)
1.856(1), Cr(1)−H(11)−Al(2) 100, Cr(1)−H(12)−Al(2) 93,
H(11)−Cr(1)−H(12) 81, H(21)−Al(2)−H(12) 86.
ASSOCIATED CONTENT
* Supporting Information
■
S
Characterizing data for 1−4 and CpFe(CO)2[Al(Cl)-
{(NiPr)2CPh}]; CIF files for all X-ray crystal structures; details
of DFT calculations (including run files). This material is
available free of charge via the Internet at http://pubs.acs.org.
and rationalized in terms of the minor extent of Al−H bond
activation.43
Details of the mode of alane binding in 3 in the solid state
were established crystallographically (Figure 2). In this case, the
structure is characterized by a relatively long M−Al separation
of 2.841(1) Å (cf. 2.55 Å for the sum of the respective covalent
radii),44 which presumably indicates that the predominant
binding interaction in this tungsten system is through the
bridging hydrogen atom. A κ1 mode of interaction is consistent
with the decidedly nonlinear C(4)···Al(2)···W(1) angle
[136.9(1)°] and with markedly differing Al−H distances
[1.46(3) and 1.67(3) Å]. By contrast, the major product of
the reaction of {HC(CMeNDipp)2}AlH2 with M(CO)6 (M =
Cr, Mo) appears from 1H NMR spectroscopy to be the κ2 alane
adduct (OC)4M[κ2-H2Al{(NDippCMe)2CH}] in each case.
Coordination of both Al−H bonds is consistent with the
AUTHOR INFORMATION
Corresponding Author
Simon.Aldridge@chem.ox.ac.uk
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
BP and EPSRC for funding; EPSRC for access to the National
Mass Spectrometry Service Centre; the Oxford Chemical
Crystallography Service and the Oxford Supercomputer
Centre; NSERC and the University of Huelva for postdoctoral
fellowships (for J.I.B. and J.U.); Prof. I. Manners and Dr. G.
Whittell for assistance with the synthesis of K[Cp′Mn(CO)2H];
1
pattern of two Dipp CH3 and one CH signals observed by H
NMR spectroscopy, and with the observation of a single Al−H
resonance (at δH = −10.48 ppm for M = Cr). In the case of the
chromium compound 4, crystallographic studies confirmed that
the major (89%) component in the crystal features a κ2-alane
ligand bound to a [Cr(CO)4] moiety (Figure 2).45,46
Structurally, this mode of binding is reflected by a linear
C(17)···Al(2)···Cr(1) unit [179.5(1)°], by essentially equiv-
alent Cr−H−Al linkages, and by a Cr···Al separation [2.420(1)
Å] which is shortened with respect to the W···Al separation
measured for 3 [2.841(1) Å] by more than would be expected
simply on the basis of the differing covalent radii of chromium
and tungsten (Δd = 0.23 Å).44 Moreover, 4 also provides a
platform on which the relative σ-donor capabilities of CO and
alane ligands can be assessed; the respective trans Cr−C
distances [d(Cr−C) = 1.834(3)/1.846(3) (trans to alane) and
1.892(2) Å (trans to CO)] are consistent with the Al−H bond
being a weaker σ-donor.
Prof. J. McGrady and Dr. T. Kramer for assistance with DFT
̈
calculations; Dr. D. Vidovic for the crystallographic study of
CpFe(CO)2[Al(Cl){(NiPr)2CPh}] (SI).
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H2Al{(NDippCMe)2CH}] can be prepared free from the κ1 complex
by employing (OC)4Cr(cod) as the chromium containing starting
material.
(29) McGrady, G. S.; Sirsch, P.; Chatterton, N. P.; Ostermann, A.;
Gatti, C.; Altmannshofer, S.; Herz, V.; Eickerling, G.; Scherer, W.
Inorg. Chem. 2009, 48, 1588−1598.
(30) Bonyhady, S. J.; Collis, D.; Frenking, G.; Holzmann, N.; Jones,
C.; Stasch, A. Nat. Chem. 2010, 2, 865−869.
(31) Cui, C.; Roesky, H. W.; Hao, H.; Schmidt, H.-G.; Noltemeyer,
M. Angew. Chem., Int. Ed. 2000, 39, 1815−1817.
(32) For a recent review focusing on the use of guanidinate ligand
scaffolds in main group chemistry see: Jones, C. Coord. Chem. Rev.
2010, 254, 1273−1289.
(33) 2: A solution in toluene (100 mL) containing Cp′Mn(CO)3
(0.10 mL, 0.70 mmol) and {HC(CMeNDipp)2}AlH2 (0.31 g, 0.70
mmol) was subjected to UV photolysis for 165 min. The resulting red
solution was filtered, concentrated, and cooled to −30 °C to yield
yellow crystals of 2, suitable for X-ray crystallography. Yield (of single
crystals): 0.058 g, 13%; a significantly higher yield (ca. 50% of a >95%
1
pure compound can be obtained as a microcrystalline material). H
NMR (300 MHz, C6D6, 298 K): δH −15.42 (br s, 1H, Al−H−Mn),
1.10 (d, 3JHH = 6.9 Hz, 6H, CH3 of Dipp iPr), 1.15 (d, 3JHH = 6.6 Hz,
i
3
i
6H, CH3 of Dipp Pr), 1.47 (d, JHH = 6.6 Hz, 6H, CH3 of Dipp Pr),
1.51 (d, 3JHH = 6.9 Hz, 6H, CH3 of Dipp iPr), 1.54 (s, 6H, CH3 of β-
diketiminato backbone), 1.56 (s, 3H, CH3 of Cp’), 3.39 (sept, JHH
6.9 Hz, 2H, CH of Dipp Pr), 3.45 (sept, JHH = 6.6 Hz, 2H, CH of
3
=
i
3
i
Dipp Pr), 3.58 (m, 2H, Cp′), 3.67 (m, 2H, Cp′), 4.95 (s, 1H, γ-CH),
5.75 (br s, 1H, Al-H), 7.12−7.13 (m, 6H, ArH). 13C{1H} NMR (126
i
MHz, C6D6, 298 K): δC 13.2 (CH3 of Cp′) 23.8 (CH3 of Dipp Pr),
i
i
24.3 (CH3 of Dipp Pr), 24.6 (CH3 of Dipp Pr), 24.7 (CH3 of Dipp
iPr), 25.6 (CH3 of β-diketiminato backbone), 28.4 (CH of Dipp Pr),
i
i
29.4 (CH of Dipp Pr), 80.4 (Cp′), 80.7 (Cp′), 100.1 (Cp′), 97.7 (γ-
2554
dx.doi.org/10.1021/ja2119892 | J. Am. Chem.Soc. 2012, 134, 2551−2554