Cationic aluminum hydride complexes: Reactions of carbene-alane adducts with trityl-borate

Article abstract of DOI:10.1039/c6cc01585a

Reaction of (Idipp)AlH₃ with [Ph₃C][B(C₆F₅)₄] in toluene affords the dimeric aluminum dication [((Idipp)AlH(μ-H))₂][B(C₆F₅)₄]₂2. In contrast, the

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ARTICLE TYPE
Cationic Aluminum Hydride Complexes: Reactions of Carbene-Alane
Adducts with Trityl-Borate
Levy L. Caoa, Erika Daleya, Timothy C. Johnstone and Douglas W Stephan*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
The known carbeneꢀalane adduct (Idipp)AlH3 1, was
50 prepared via literature methods.20 Reaction of species 1 with one
equivalent of [Ph3C][B(C6F5)4] at room temperature in toluene
Scheme 1 Structurally characterized aluminumꢀhydride cations.
Reaction of (Idipp)AlH3 with [Ph3C][B(C6F5)4] in toluene
affords the dimeric aluminum dication [((Idipp)AlH(µ-
H))2][B(C6F5)4]2 2. In contrast, the reaction of (IBn)AlH3 with
[Ph3C][B(C6F5)4] in bromobenzene gives a redistribution
10 product,
the
salt
of
a
monomeric
dication
[(IBn)2AlH][B(C6F5)4]2 4.
Aluminum hydrides are important reagents in organic, inorganic
and materials chemistry. Indeed, such species are used in a
variety of roles including reagents for reductions1ꢀ6 and in
15 inorganic synthesis.7ꢀ8 Applications as components in materials
chemistry and in particular, hydrogen storage materials9ꢀ11 have
also garnered attention. From
a coordination chemistry
perspective, aluminumꢀhydride derivatives exhibit a range of
geometries resulting from the low steric demand of hydride and
20 the ability of aluminum to accommodate coordination numbers
ranging from three to six. The majority of reported aluminum
hydride species are either neutral or anionic while, in contrast,
cationic aluminumꢀhydride species are less common. A matrix
isolation study at 4 K described a [AlH]•+ radical cation which
25 was studied by EPR spectroscopy.12ꢀ13 The first fully
resulted in the immediate precipitation/crystallization of a new
55 species 2 (Scheme 2). The formation of 2 proceeds via hydride
abstraction from 1, liberating Ph3CH, followed by subsequent
dimerization and crystallization. Although this avenue of
reactivity is not widely exploited in aluminumꢀhydride chemistry,
it is known in the literature to generate both aluminum17 and
60 transition metal alkyl cations.21ꢀ22 The formation of 2 represents,
to our knowledge, the first dimeric aluminumꢀhydride dication
salt.
The complete insolubility of this product in all organic
solvents in which it was stable precluded spectroscopic
65 characterization, however compound 2 was characterized by
single crystal Xꢀray diffraction (Figure 1). Solution of the
structure via direct methods revealed that the asymmetric unit
contains a single Al center and a [B(C6F5)4]ꢀ anion, and structure
refinement confirmed the molecular structure of 2 to be that of
70 the centrosymmetric dimeric dication salt, [((Idipp)AlH(µꢀ
H))2][B(C6F5)4]2. The Al center adopts a pseudoꢀtetrahedral
geometry featuring coordinate bonds to the carbene, a terminal
hydride, and two bridging hydrides. The angles about Al range
from 102.1(8)° to 128.5(9)° and the AlꢀC bond length is 1.964(3)
75 Å. The terminal Alꢀhydride bond length refined to 1.50(2) Å and
the bridging AlꢀH distances were determined to be 1.66(2) Å and
1.71(2) Å. The resulting Al···Al distance in the cation of 2 is
2.584(2) Å, a value greater than the sum of the covalent radii of
the two atoms.23 The structure of the cation 2 is reminiscent of
80 the neutral dimeric Al(II) species reported by Jones and
characterized
[H2Al((MeNCH2CH2N(Me)CH2CH2CH2)2)][AlH4]
Alꢀhydride
cations
and
[H2Al(MeN(CH2CH2NMe2)2)][AIH4] (Scheme 1) were prepared
and reported by Atwood and coworkers in 1991.14ꢀ15 These
30 species exhibited sixꢀ and fiveꢀcoordinate aluminum centers,
respectively. In 1994, Soloveichik and coworkers described the
structure of the salt [AlH2(C4H8O)4][(C5H5)3Yb(Na)Yb(C5H5)3]
(Scheme 1) which contained
a sixꢀcoordinate aluminumꢀ
dihydride cation.16 In 2004, we employed an aluminum complex
35 of a phosphinimineꢀamine ligand to generate salts of the
aluminumꢀhydride
cation
[(iPr2C6H3N)C(Me)CHPPh2(NC6H3iPr2)AlH][B(C6F5)4].17
Roesky and coworkers exploited a bulky nonꢀcoordinating anion
to isolate the salts [H2Al(NMe3)2]2[(AlH)8(CCH2tBu)6] and
40 [H(nBu)Al(NMe3)2][(AlH)7(AlNMe3)(CCH2tBu)6] (Scheme 1) in
2005.18 These latter compounds are examples of fourꢀcoordinate
aluminum hydride cations. Most recently, Wright and coworkers
described
the
structure
of
[(1,4ꢀHꢀpyridꢀ1ꢀ
yl)4Al][(pyridine)4AlH2], which was formed from the reaction of
45 (tBuO)AlH2 and pyridine.19 In this manuscript, we describe two
reactions of carbeneꢀalane adducts with trityl cation affording the
first dimeric fourꢀcoordinate and monomeric threeꢀcoordinate
aluminum hydride dications.
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coworkers,24 which features an AlꢀAl single bond (2.6375(8) Å,
and terminal Al–H distances of 1.56(2) Å and 1.53(2) Å).
NMR spectrum of 4 in C6D5Br shows resonances attributable to
the IBn ligand but no signal was observed in the 27Al NMR
spectrum. The IR spectrum of 4 featured a signal at 1963 cmꢀ1,
which is in the same region as observed for other cationic
24
Similarly, the compound [(iPr2N(CNDipp)2AlH)]2 exhibited an
Al···Al distance of 2.675(1) Å with terminal Al–H distances of
5
1.53(2) Å. The observation of a shorter Al···Al distance in 2 in 40 aluminum hydride species.27 The solid state structure of 4 was
comparison to that in Al(II) species is consistent with the higher
oxidation state of the Al centers in 2 prompting shorter AlꢀHbridge
bonds thus drawing the Al atoms closer together.
unambiguously confirmed by Xꢀray crystallography. Compound
4 proved to be the dicationic borate salt [(IBn)2AlH][B(C6F5)4]2.
The cation of this salt is a planar monohydrido aluminum dication
containing two IBn ligands (Figure 2). The planes of the two
45 carbenes are oriented approximately orthogonal to each other,
allowing the benzyl substituents to envelop the aluminum center.
Two of the pendent arenes are positioned above and below the
pseudoꢀtrigonal coordination plane of the aluminum center. The
sum of the angles about aluminum is 359° and the CꢀAlꢀC‘ angle
50 is 113.6(2)°. The AlꢀC bond distances in 4 are 1.987(4) Å and
2.010(4) Å. The structural data also reveals that one of the benzyl
substituents on one of the IBn ligands is oriented such that a CꢀC
πꢀbond is positioned above with AlꢀC distances of 3.124(5) Å and
2.572(4) Å, respectively.
10
Figure 1 POVꢀray depiction of the dication of 2. C: black, N: blue, Al:
teal , H: gray. All ligandꢀbased H atoms, as well as the [B(C6F5)4]ꢀ anions,
are omitted for clarity. Selected bond distances and angles: AlꢀAl‘
2.584(2), AlꢀHbridg 1.71(2), 1.66(2), AlꢀHterm 1.50(2), AlꢀC 1.964(3),
15 HbridgꢀAlꢀC 102.1(8), Hbridg‘ꢀAlꢀC 104.1(8), Htermꢀ AlꢀC 128.5(9), HtermꢀAlꢀ
Hbridg 114(1), HtermꢀAlꢀHbridg‘ 117(1).
55
Figure 2 POVꢀray depiction of the dication of 4. C: black, N: blue, Al:
teal, H: gray. All ligandꢀbased H atoms as well as the [B(C6F5)4] anions
are omitted for clarity. Selected bond distances and angles: AlꢀCright
1.987(4), AlꢀCleft 2.010(4), AlꢀH 1.43(5),CrightꢀAlꢀH 131(2), Cleft
60 114(2), CrightꢀAlꢀCleft 113.6(2).
ꢀ AlꢀH
DFT calculations were performed at the B3LYP/6ꢀ311G(d,p)
level of theory to gain further insight into the electronic structure
of dication 4. The optimized geometry corresponded well with
65 the crystallographically determined structure (nonꢀhydrogen
RMSD = 0.30 Å), and a frequency calculation confirmed that that
IR active band at 1963 cmꢀ1 arises from a normal mode
comprising almost exclusively the AlꢀH stretch (See SI). Based
on natural population analysis (NPA),28ꢀ29 the Al atom of
70 compound 4 bears a natural atomic charge of +1.50 (See SI,
Figure S11 & Table S2). An NPA analysis reported for the
similar dicationic hydrido boron complex prepared by Ong and
coꢀworkers30 suggested that their boron complex was stabilized
by distribution of positive charge from the boron center onto the
75 carbodicarbene supporting ligands. The lack of such charge
redistribution in 4 suggests that it is stabilized by other effects.
The LUMO of the dication (Figure 3) is mainly located on the Al
atom and has significant contribution from the proximal arene
ring. An electronic interaction between the Al center and the
80 aromatic ring of the pendent benzyl group, inferred from the
crystal structure of 4 in addition to the delocalized nature of the
LUMO, was corroborated by the results of a natural bond orbital
Scheme 2 Synthesis of 2 and 4.
20
To probe the impact of altering the carbene, 1,3ꢀ
dibenzylimidazolꢀ2ꢀylidene (IBn) was allowed to react with
(EtMe2N)AlH3 in toluene, to readily afford (IBn)AlH3 (3) in 72%
yield after recrystallization. This species gives rise to a broad
1
25 singlet resonance in H NMR spectrum in C6D5Br at 4.38 ppm
corresponding to the aluminumꢀbound hydride centers. The 27Al
NMR spectrum of 3 shows a broad singlet resonance at 108.7
ppm. These data are comparable to those seen for related NHCꢀ
alane adducts.25ꢀ26 An Xꢀray crystallographic study of 3 (See SI,
30 Figure S3) confirmed the formulation, revealing a AlꢀCNHC bond
length of 2.059(2) Å, which falls within the range of typical Alꢀ
CNHC bonds (2.034ꢀ2.067 Å).25ꢀ26
Treatment of
3
with
a
stoichiometric amount of
[Ph3C][B(C6F5)4] in C6H5Br at room temperature results in the
35 formation of Ph3CH and a new species 4 in 46% yield. The H
1
2
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(NBO) analysis.31 Using second order perturbation theory, a
a These authors contributed equally.
45 † Electronic Supplementary
Crystrallographic data have been deposited in CCDC#: 1453308ꢀ
1453310. Experimental details and spectral data have been deposited.. See
DOI: 10.1039/b000000x/
donor acceptor interaction was identified between a bonding
NBO on the pendent arene ring and an empty lone pair NBO on
the Al center. The Alꢀbased NBO has essentially pure 3pz
character and the bonding NBO, localized between the two
carbon atoms of the arene ring that are closest to the metal center,
has πꢀsymmetry and is oriented such that it overlaps with the Alꢀ
based NBO (See SI, Figure S12 & Table S3). This interaction
may impart stabilization on the complex, accounting for its
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The above reactions of NHCꢀalane adducts with trityl borate
were shown to generate the dimeric aluminumꢀhydride dicationic
35 salt 2 as well as the monomeric threeꢀcoordinate aluminumꢀ
hydride salt 4. These observations represent the first dicationic
aluminumꢀhydride salts to be characterized. The differing nature
of these species illustrates the influence of carbene substituents
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105
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40 Notes and references
a Department of Chemistry, University of Toronto, 80 St.George St.,
Toronto, Ontario, M5S 3H6 E-mail: dstephan@chem.utoronto.ca
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