The reactivity of gallium-(i), -(ii) and -(iii) heterocycles towards Group 15 substrates: Attempts to prepare gallium-terminal pnictinidene complexes

Article abstract of DOI:10.1039/b511451a

The reactivity of a series of Ga(i), Ga(ii) and Ga(iii) heterocyclic compounds towards a number of Group 15 substrates has been investigated with a view to prepare examples of gallium-terminal pnictinidene complexes. Although no examples of such complexes were isolated, a number of novel complexes have been prepared. The reactions of the gallium(i) N-heterocyclic carbene analogue, [K(tmeda)][:Ga([N(Ar)C(H)]₂)] (Ar = 2,6-diisopropylphenyl) with cyclo-(PPh)₅ and PhNNPh led to the unusual anionic spirocyclic complexes, [(κ²P,P′-(PhP)₄)Ga([N(Ar)C(H)] ₂)]^- and [(κ²N,C-PhNN(H)(C ₆H₄))Ga([N(Ar)C(H)]₂)]^-, via formal reductions of the Group 15 substrate. The reaction of the digallane(4), [Ga([N(Ar)C(H)]₂)]₂, with (Me₃Si)N₃ afforded the paramagnetic, dimeric imido-gallane complex, [([N(Ar)C(H)] ₂)Ga(μ-N(SiMe₃))]₂, via a Ga-Ga bond insertion process. In addition, the new gallium(iii) phosphide, [GaI(P(H)Mes*)([N(Ar)C(H)]₂)], Mes* = C₆H ₂Bu^t₃-2,4,6; was prepared and treated with diazabicycloundecane (DBU) to give [Ga(DBU)(P(H)Mes*)([N(Ar)C(H)] ₂)], presumably via a gallium-terminal phosphinidene intermediate, [Ga(PMes*)([N(Ar)C(H)]₂)]. The possible mechanisms of all reactions are discussed, all new complexes have been crystallographically characterised and all paramagnetic complexes have been studied by ENDOR and/or EPR spectroscopy. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b511451a

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PAPER
www.rsc.org/dalton | Dalton Transactions
The reactivity of gallium-(I), -(II) and -(III) heterocycles towards Group 15
substrates: attempts to prepare gallium–terminal pnictinidene complexes
Robert J. Baker,a Cameron Jones,*a David P. Mills,a Damien M. Murphy,*a Evamarie Hey-Hawkinsb and
Robert Wolfa,b
Received 11th August 2005, Accepted 21st September 2005
First published as an Advance Article on the web 26th October 2005
DOI: 10.1039/b511451a
The reactivity of a series of Ga(I), Ga(II) and Ga(III) heterocyclic compounds towards a number of
Group 15 substrates has been investigated with a view to prepare examples of gallium–terminal
pnictinidene complexes. Although no examples of such complexes were isolated, a number of novel
complexes have been prepared. The reactions of the gallium(I) N-heterocyclic carbene analogue,
=
[K(tmeda)][:Ga{[N(Ar)C(H)]2}] (Ar = 2,6-diisopropylphenyl) with cyclo-(PPh)5 and PhN NPh led to
the unusual anionic spirocyclic complexes, [{j2P,Pꢀ-(PhP)4}Ga{[N(Ar)C(H)]2}]− and [{j2N,C-
PhNN(H)(C6H4)}Ga{[N(Ar)C(H)]2}]−, via formal reductions of the Group 15 substrate. The reaction
of the digallane(4), [Ga{[N(Ar)C(H)]2}]2, with (Me3Si)N3 afforded the paramagnetic, dimeric
imido–gallane complex, [{[N(Ar)C(H)•]2}Ga{l-N(SiMe3)}]2, via a Ga–Ga bond insertion process. In
•
addition, the new gallium(III) phosphide, [GaI{P(H)Mes*}{[N(Ar)C(H)]2 }], Mes* = C6H2But3-2,4,6;
was prepared and treated with diazabicycloundecane (DBU) to give [Ga(DBU){P(H)Mes*}-
=
{[N(Ar)C(H)]2}], presumably via a gallium–terminal phosphinidene intermediate, [Ga{ PMes*}-
•
{[N(Ar)C(H)]2 }]. The possible mechanisms of all reactions are discussed, all new complexes have been
crystallographically characterised and all paramagnetic complexes have been studied by ENDOR
and/or EPR spectroscopy.
compounds have been more widely explored. These include
Introduction
the kinetically stabilised Group 13–terminal imide complexes,
[ArꢀMNArꢀꢀ] M = Ga or In, Arꢀ = C6H3(C6H3Pri2-2,6)2-2,6;
Arꢀꢀ = C6H3(C6H2Me2But-2,6,4)2-2,6, which theoretical stud-
ies suggest have significant p-bonding inherent in their MN
interactions.5 Varying levels of p-bonding have also been invoked
for complexes containing interactions between gallium and the
heavier pnictogens, as for example in [(Mes)P{Ga(C6H2Pri3-
2,4,6)2}2] Mes = mesityl,6 [{But3SiP(H)Ga(l-PSiBut3)}2]7 and
[{Li(THF)3}2Ga2{As(SiPri3)}4].8
Multiple bonds between the heavy p-block elements are of
much current interest as such bonds often display very different
chemistry to their “classical” first row analogues. Whilst there
are now many examples of compounds containing “non-classical”
homo- or heteronuclear multiple bonds between the heavier Group
14 and/or Group 15 elements,1 the chemistry of compounds
possessing multiply bonded Group 13 metal centres is less well
developed. In this area, several examples of compounds incor-
porating homonuclear Group 13 metal–metal interactions with a
bond order of greater than one have been reported.1,2 The nature
of the bonding in such compounds can be difficult to explain
using standard models, and this has led to extensive debate in the
literature. The extent of this debate is perhaps best exemplified
by the unusual dianion, [Ar*GaGaAr*]2− Ar* = C6H3(C6H2Pri3-
2,4,6)2-2,6; which has been described by a number of experimental
and theoretical studies as having a Ga–Ga bond order of 1,
2 or 3.3 Compounds containing a heteronuclear multiple bond
between a Group 13 metal and another heavy p-block element
have also been studied. In this area, a handful of compounds
incorporating Group 13 metal–Group 16 double bonds have been
Most relevant to our endeavours in this area are the investi-
gations by the groups of Roesky and Power into the reactivity of
the neutral aluminium(I) and gallium(I) heterocycles, [:M{[N(Ar)-
C(Me)]2CH}], M = Al9 or Ga,10 Ar = C6H3Pri2-2,6, towards aryl
and silyl azides. These have led to terminal imide complexes,
11
=
[Ar*N M{[N(Ar)C(Me)]2CH}], but also to metal tetrazoles,
[{j2N,Nꢀ-N4(SiMe3)2}{M[N(Ar)C(Me)]2CH}],12 a gallium azide,
[{(Me3Si)2N}(N3)Ga{[N(Ar)C(Me)]2CH}],12 and several other
novel heterocyclic complexes13 via different pathways which
depend upon the metal and the azide precursor. We have de-
veloped a high yielding synthetic route to the anionic gallium(I)
heterocycle, [K(tmeda)][:Ga{[N(Ar)C(H)]2}] 1,14 which is closely
related to [:M{[N(Ar)C(Me)]2CH}] and behaves similarly to
the valence isoelectronic N-heterocyclic carbene (NHC) class
of ligand with respect to its main group and transition metal
coordination chemistry.15,16 In the current study it was proposed
to further examine the apparent analogy between 1 and both
[:M{[N(Ar)C(Me)]2CH}] and NHCs by attempting to utilise it
as a precursor to new gallium–terminal pnictinidene complexes
which could potentially exhibit Ga–pnictogen multiple bond
reported, e.g. the terminal gallium chalcogenides, [TptBuGa E]
=
(TptBu = tris(3,5-di-tert-butylpyrazoloyl)hydroborate; E = S, Se,
Te),4 though Group 13 metal–Group 15 element multiply bonded
aCentre for Fundamental and Applied Main Group Chemistry, School
of Chemistry, Main Building, Cardiff University, Cardiff, UK CF10 3AT.
E-mail: jonesca6@cardiff.ac.uk
bInstitut fu¨r Anorganische Chemie der Universita¨t Leipzig, Johannisallee 29,
04107, Leipzig, Germany
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cyclo-[{(SiMe3)3C}Ga(PBut)3]21 and [[Al{[N(Ar)C(Me)]2CH}]2(l-
P4)]22 respectively. It is interesting to note that 1 does not react with
either (PBut)3 or (PhP)4CH2.
character. A closely related paramagnetic gallium(III) heterocycle,
[GaI2{[N(Ar)C(H)•]2}],17 and a diamagnetic digallane(4), [Ga{[N-
(Ar)C(H)]2}]2,18 containing gallium(II) centres, were also enlisted
for this cause. Although no terminal gallium–pnictinidene com-
plexes resulted, a variety of novel heterocyclic gallium–pnictide
species were obtained, as discussed herein.
1
The 31P{ H} NMR spectrum of 2 in THF displays a signal
pattern indicative of an AAꢀBBꢀ spin system (dA = −80.2 ppm,
dB = −6.1 ppm; where PA are attached to Ga), as has been seen for
related heterocyclic systems, e.g. [As(PCy)4]−, Cy = cyclohexyl.23
The signals are, however, significantly broadened and as a result
efforts to accurately simulate the spectrum were unsuccessful.
It seems likely that the broadness of the signals results from
a fluxional process in solution which could involve a cleavage
and reformation of Ga–P and/or P–P bonds. Evidence for this
proposal comes from the solid state structure of the compound
(vide infra). Unfortunately, no light could be shed on the nature of
Results and discussion
N-Heterocyclic carbenes have been utilised as stabilising ligands in
the preparation of a variety of novel main group complexes.19 One
of the more interesting compound classes that has arisen from this
work are the NHC–terminal pnictinidene complexes reported by
Cowley et al., e.g. [(IMes)→ER], E = P or As; R = Ph, C6F5 or CF3,
IMes = 1,3-dimesitylimidazol-2-ylidene.20 These were prepared in
the reactions of IMes with cyclic pnictanes, e.g. cyclo-(PPh)5, and
1
the fluxional process occurring in solution as the 31P{ H} NMR
spectrum of 2 did not resolve at temperatures as low as −90 ◦C.
Crystals of 2 suitable for an X-ray diffraction study were grown
from a toluene solution and its molecular structure is depicted
in Fig. 1. This shows it to contain an anionic spirocyclic system
=
have been shown not to exist as pnictaalkenes with C E bonds
and one lone pair on the pnictogen centre, but instead as NHC–
pnictinidene adducts with two lone pairs on the pnictogen centre.
In light of the previously described analogy between 1 and NHCs,16
it seemed that its reaction with cyclic pnictanes might lead to
terminal gallium–pnictinidene complexes. In the reaction of 1 with
(PhP)5 in THF under a range of stoichiometries, this did not prove
to be the case and instead the novel ionic spirocyclic complex, 2,
was formed in low to moderate yields after recrystallisation from
toluene (Scheme 1). Monitoring of either the 1 : 1, 3 : 1 or 5 :
1 reactions by 31P NMR spectroscopy showed that 2 was the
major reaction product, and in the latter two cases it was found
after work-up that excess 1 remained unreacted. The mechanism
of this reaction seemingly involves the oxidative insertion of the
gallium(I) centre of 1 into a P–P bond of (PPh)5 with concomitant
loss of one PPh fragment. The outcome of this fragment could
not be determined but it most probably leads to cyclic oligomers,
(PPh)n. The reaction that gave 2 is related to the treatment of the
tetrameric gallium(I) diyl, [Ga{C(SiMe3)3}]4, with cyclo-(PBut)3,
and the aluminium(I) heterocycle, [:Al{[N(Ar)C(Me)]2CH}], with
P4, both of which lead to oxidative insertion of the Group 13 centre
into a P–P bond of the phosphorus reactant and formation of
6
which coordinates the potassium counter-ion via a skewed g -arene
interaction with one of the GaN2C2 heterocycle’s N-substituents.
The potassium centre is additionally chelated by two phosphorus
centres of the GaP4 ring and a molecule of tmeda. Interestingly,
the phosphorus phenyl substituents are syn, anti, anti with respect
to each other, a situation which renders all the phosphorus centres
1
chemically inequivalent. This is at odds with the solution 31P{ H}
NMR spectrum of 2 which displays only two broad signals, thus
suggesting that the GaP4 ring of the compound undergoes a rapid,
fluxional isomeric rearrangement via Ga–P and/or P–P bond
cleavage and reformation. As in previously reported coordination
complexes of the anion of 1,15,16 the Ga–N bond lengths of the
˚
GaN2C2 heterocycle in 2 [1.914 A avg.] decrease, and its N–Ga–N
Fig. 1 Molecular structure of 2 (isopropyl groups omitted for clarity).
◦
˚
Selected bond lengths (A) and angles ( ): Ga(1)–N(1) 1.927(5), Ga(1)–N(2)
1.901(5), Ga(1)–P(1) 2.3818(18), Ga(1)–P(4) 2.4230(18), K(1)–P(2)
3.590(2), K(1)–P(4) 3.314(2), P(1)–P(2) 2.201(2), P(2)–P(3) 2.233(3),
P(3)–P(4) 2.194(2), C(1)–C(2) 1.351(9); N(1)–Ga(1)–N(2) 88.5(2), N(1)–
Ga(1)–P(1) 116.53(17), N(1)–Ga(1)–P(4) 104.77(17), N(2)–Ga(1)–P(1)
115.67(16), N(2)–Ga(1)–P(4) 124.80(16), P(1)–Ga(1)–P(4) 105.52(6),
P(2)–K(1)–P(4) 54.76(5), Ga(1)–P(1)–P(2) 91.42(8), Ga(1)–P(1)–C(27)
104.8(2), P(1)–P(2)–P(3) 103.70(9), P(2)–P(3)–P(4) 91.99(9), P(3)–P(4)–
Ga(1) 98.78(8).
=
Scheme 1 i) cyclo-(PPh)5, THF; ii) PhN NPh, Et2O, [4 + 1]; iii) 1,3-H
shift.
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angle [88.5(2)◦] becomes more obtuse relative to those parameters
˚
in the free anionic ligand [Ga–N avg. = 1.970 A; N–Ga–N =
83.02(11)◦].14 It is believed this arises from the smaller covalent
radius of the Ga(III) centre in 2 compared to that of the Ga(I)
˚
centre in 1. The average Ga–P and P–P bond lengths in 1 [2.402 A
˚
and 2.209 A respectively] are comparable to those in related
t
˚
compounds, e.g. cyclo-[{(SiMe3)3C}Ga(PBu )3] [2.360 A avg. and
21
˚
˚
˚
2.217 A avg. respectively], whilst the P–K distances [3.452 A avg.]
24
are in the known range for such interactions (3.043–3.851 A).
A number of other attempts were made to form a terminal
gallium–pnictinidene complex using 1 as a precursor, but with
#
#
=
limited success. For example, its reaction with Ar P PEt3 (Ar =
C6H3Mes2-2,6), a known phosphinidene transfer reagent,25 led to
an intractable mixture of many phosphorus containing products.
The reactivity of 1 towards a variety of di-pnictenes was also
investigated and, perhaps surprisingly, no reaction was observed
t
26
=
=
with Mes*P PMes* (Mes* = C6H2Bu 3-2,4,6) or Ar*E EAr*
(E = As, Sb).27 Compound 1 did, however, react with azobenzene,
=
PhN NPh, in 1 : 1 or 2 : 1 stoichiometries to give the unusual
ionic spirocyclic system, 3, in good yields (Scheme 1). One possible
mechanism for formation of 3 involves an initial orthometallation
of azobenzene by the Ga(I) centre of 1 to give a short-lived Ga–H
=
intermediate which reduces the N N moiety to give the observed
˚
Fig. 2 Molecular structure of 3. Selected bond lengths (A) and
product. It is noteworthy that the reduction of aromatic azo-
compounds to hydrazoarenes by Group 13 hydride agents, e.g.
Bui2AlH, is known.28 In addition, gallium centres have been shown
to participate in intramolecular metallation reactions in similar
systems. An example here is the reaction of Me2GaCl with the
lithiated hydrazine, LiN(SiMe3)N(H)(Naph) (Naph = naphthyl),29
which leads to a diazagallole that is closely related to 3 and arises
from a gallium mediated C–H activation (methane elimination) at
the 2-position of the naphthyl substituent. Another possible, and
perhaps more likely mechanism involves a [4 + 1] cycloaddition
of the Ga(I) centre of 1 with the azobenzene, followed by a
rapid 1,3-migration of the ortho-aryl proton to the nitrogen centre
bearing the metallated phenyl group (Scheme 1). A precedent for
this mechanism exits with the reaction of the iminophosphane,
angles (◦): Ga(1)–N(1) 1.910(4), Ga(1)–N(2) 1.910(4), Ga(1)–N(3)
1.964(4), Ga(1)–C(38) 1.969(5), N(1)–C(1) 1.392(6), N(2)–C(2)
1.410(6), N(3)–N(4) 1.442(5), C(1)–C(2) 1.339(7); N(1)–Ga(1)–N(2)
87.78(17), N(1)–Ga(1)–N(3) 113.29(17), N(1)–Ga(1)–C(38) 128.06(18),
N(2)–Ga(1)–N(3) 116.25(17), N(2)–Ga(1)–C(38) 128.06(18), N(3)–Ga(1)–
C(38) 85.05(18), C(27)–N(3)–N(4) 114.8(4), C(33)–N(4)–N(3) 111.5(4).
centre is completed by one molecule each of diethyl ether and
tmeda. Both heterocycles containing the distorted tetrahedral
gallium centre are effectively planar and the bond lengths and
angles within the heterocycle originating from 1 are similar to those
in 2. The other heterocycle displays intracyclic N–N and N–C dis-
˚
˚
tances [1.442(5) A and 1.427(6) A respectively] that are normal for
single bonded interactions and similar to those in the diazaphos-
2
˚
=
=
EtP NMes*, with azobenzene which affords the diazaphosphole,
phole, [(Mes*)N (Et)P{j N,P-N(Ph)N(H)(C6H4)}] [1.439(8) A
2
[(Mes*)N (Et)P{j N,P-N(Ph)N(H)(C6H4)}], via [4 + 1] cycload-
and 1.420(5) A].30 Also within this heterocycle, the gallium–amide
˚
=
dition and C–H activation processes.30 Whichever mechanism is in
interaction [Ga(1)–N(3),1.964(4) A] is significantly longer than
˚
=
operation, it was found that 1 does not react with MesN NMes,
which is devoid of ortho-aryl protons.
those in the other GaN2C2 heterocycle, whilst the Ga–C bond
24
˚
length [1.969(5) A] is in the normal range.
The spectroscopic data for 3 support its proposed formulation.
Specifically, its infrared spectrum exhibits an N–H stretching
absorption at 3486 cm−1, and a broad peak was observed in its
1H NMR spectrum at d 7.47 ppm which clearly arises from the
N–H proton. Additionally, the 13C NMR spectrum of the complex
displays the expected number of resonances in the aromatic region.
Crystals of 3 suitable for X-ray diffraction were grown from a
saturated hexane solution and its molecular structure is shown in
Fig. 2. As was the case for 2, this shows the complex to contain a
spirocyclic anion. The hydrogen atom, H(4), bonded to N(4) was
located from difference maps and isotropically refined, thus allow-
ing the nitrogen centre, N(4), to be assigned as having a distorted
tetrahedral geometry, which includes coordination to the potas-
sium centre through its lone pair. This potassium also has a weaker
interaction with the p-orbital lone pair of the trigonal planar
In consideration of the aforementioned reactivity of the neutral
Al(I) and Ga(I) heterocycles, [:M{[N(Ar)C(Me)]2CH}], towards
organic azides, the reactions of 1 with azides of varying steric bulk,
RN3, R = SiMe3, 9-triptycenyl, Mes* or Ar*, were investigated. In
all cases intractable mixtures were obtained. We considered that re-
action of these azides with the digallane(4), [Ga{[N(Ar)C(H)]2}]2
4, obtainable in high yield via oxidative coupling of 1,15c could
lead to the oxidative insertion of imide fragments into the Ga–
Ga bond of 4. In this respect, we have recently shown that
4 will oxidatively add to low oxidation state transition metal
fragments.15a The reaction of 4 with 2 equivalents of (Me3Si)N3
yielded the blue-green paramagnetic complex, 5, in good yield
(79%) after stirring for 48 hours (Scheme 2). Repeating the
reaction with a large excess of (Me3Si)N3 led to the same complex
and did not appreciably increase the rate of reaction. Although,
the formation of 5 is relatively slow (as judged by the colour
change of the reaction mixture), no intermediates in its formation
2
centre, N(3), and an g -attachment to the non-metallated phenyl
ring of the diaza-ligand. The coordination sphere of the potassium
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39.9% natural abundance). This means that even the small electron
spin density (0.47%) at the gallium nucleus of 5 produces easily
observable HFCs to both gallium isotopes.
Evidence for the diradical nature of 5 can also be obtained from
its EPR spectra. For two interacting unpaired electrons (S = 12 ),
an S = 1 triplet ground state can be observed in the spectrum,
provided the coupling between the two spin systems is sufficiently
strong. In that case the zero field splitting term for the randomly
oriented triplet should produce a characteristic pattern in the
DMS = 1 region (centre field). Unfortunately, due to the intense
Scheme 2 i) Excess Me3SiN3, hexane.
could be isolated. It is clear, however, that the mechanism
involves single electron oxidations of the gallium heterocycles of 4.
Compound 5 can be considered as a dimer of the gallium–terminal
1
2
nature of the signals arising from the individual S = spins in
5, the zero field splitting parameter (D) could not be observed.
However, the DMS = 2 transition at half field was seen at 1670
G (see inset in Fig. 3). While this half field transition is extremely
=
imide complex, [{[N(Ar)C(H)]2}Ga N(SiMe3)], and can be com-
1
pared to the related dimeric, diamagnetic imidogallane, [{(g -
Cp*)GaN(xylyl)}2], which arises from the reaction of (xylyl)N3
1
2
and (g -Cp*)Ga:.31 There are also parallels between the formation
5
weak, indicating that the two S = spins are only weakly coupled,
it nevertheless confirms the diradical nature of 5. The weak
of 5 and the singlet biradicaloid germanium imide complex,
[{ArꢀGeN(SiMe3)}2], which results from the reaction of the di-
germyne, ArꢀGeGeArꢀ, with (Me3Si)N3.32 It is worthy of note that
the reactions of 4 with either (9-triptycenyl)N3, Mes*N3 or Ar*N3
were carried out but all led to intractable mixtures of products.
As 5 is paramagnetic, no meaningful data could be obtained
from its NMR spectra. However, X-band continuous wave EPR
spectra for this compound were recorded at room temperature and
77 K. Only the spectra acquired at 298 K are shown in Fig. 3, as the
anisotropic frozen solution spectrum did not yield any additional
information. Owing to the poor resolution of the first derivative
spectrum (Fig. 3a), the second derivative spectrum (Fig. 3b) was
recorded. The resulting isotropic spectrum was simulated (Fig. 3c)
based on the following spin Hamiltonian parameters; giso = 2.0035,
aH = 5.8 G, aN = 6.0 G, a69Ga = 20.4 G and a71Ga = 26.0 G. Therefore,
the strong EPR signal for 5 is centred close to that of free spin.
In addition, it is typical for paramagnetic diazabutadiene–gallium
complexes previously reported by us,33,34 in that it is dominated by
isotropic hyperfine couplings (HFC) to two equivalent 1H nuclei,
two equivalent 14N nuclei and a 69,71Ga nucleus. The relatively large
HFCs to the gallium nucleus originate from the large theoretical
isotropic hyperfine couplings for gallium (69Ga; I = 3/2, a0 =
4356 G, 60.1% natural abundance; 71Ga: I = 3/2, a0 = 5535 G,
1
2
interaction between the two S = spins is due to the fact that the
unpaired electrons are primarily localised on the diazabutadiene
backbone, and the tetrahedral bonding arrangement around the
gallium centres prevents efficient spin–spin coupling.
The molecular structure of 5 is depicted in Fig. 4 and shows
it to be dimeric with bridging imido ligands. Its gallium centres
have heavily distorted tetrahedral geometries and are slightly
displaced from the least squares planes defined by the chelating
˚
diazabutadiene ligands (0.375 A avg.). In contrast, the Ga2N2
Fig. 4 Molecular structure of 5 (isopropyl groups omitted for clarity).
◦
˚
Selected bond lengths (A) and angles ( ): Ga(1)–N(1) 1.871(3), Ga(1)–N(2)
1.900(2), Ga(1)–N(3) 2.017(2), Ga(1)–N(4) 2.012(3), Ga(2)–N(1)
1.895(2), Ga(2)–N(2) 1.876(3), Ga(2)–N(5) 2.005(3), Ga(2)–N(6)
2.030(3), C(1)–C(2) 1.392(4), C(27)–C(28) 1.407(5), Si(1)–N(1) 1.706(3),
Si(2)–N(2) 1.706(3); N(1)–Ga(1)–N(2) 90.50(11), N(1)–Ga(1)–N(3)
126.11(11), N(1)–Ga(1)–N(4) 123.45(11), N(2)–Ga(1)–N(3) 113.58(10),
N(2)–Ga(1)–N(4) 123.45(11), N(3)–Ga(1)–N(4) 83.62(11), N(1)–Ga(2)–
N(2) 90.50(11), N(1)–Ga(2)–N(5) 121.91(10), N(1)–Ga(2)–N(6)
112.90(11), N(2)–Ga(2)–N(5) 123.95(11), N(2)–Ga(2)–N(6) 127.50(11),
N(5)–Ga(2)–N(6) 83.50(11), Ga(1)–N(1)–Ga(2) 89.61(11), Ga(1)–N(1)–
Si(1) 135.09(15), Ga(2)–N(1)–Si(1) 133.12(15), Ga(1)–N(2)–Ga(2)
89.30(11), Ga(1)–N(2)–Si(2) 132.70(15), Ga(2)–N(2)–Si(2) 136.75(18).
Fig. 3 X-Band (9.360 GHz) EPR spectrum of 5 recorded at 298 K
in toluene. (a) First harmonic signal, (b) second harmonic signal, and
(c) simulated spectrum. Inset figure shows the DMS = 2 transition due to
the weakly interacting S = 1 triplet at half field.
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˚
heterocycle is effectively planar and its Ga–N bonds (1.885 A
avg.) are significantly shorter than those to the diazabutadiene
1
˚
ligand (2.014 A avg.) but longer than those seen in [{(g -
Cp*)GaN(xylyl)}2] (1.860 A avg.),31 which possesses 3-coordinate
˚
Ga centres. The N-centres of the Ga2N2 heterocycle in 5 have
distorted trigonal planar geometries (R angles = 358.3◦ avg.).
An examination of the C–C and C–N bond lengths within
the diazabutadiene ligands of 5 suggests a significant degree of
delocalisation, as has previously been seen in related paramagnetic
•
complexes employing this ligand, e.g. [GaI2{[N(Ar)C(H)]2 }].17
˚
Although the Ga · · · Ga separation [2.654(3) A] is well within the
sum of the van der Waals radii (3.8 A),31 there is no evidence for a
˚
Ga–Ga bond in the compound, as has been previously discussed
1
for similar compounds, e.g. [{(g -Cp*)GaN(xylyl)}2] [Ga · · · Ga
31
˚
separation 2.6495(6) A].
To the best of our knowledge, terminal phosphinidene com-
plexes of gallium are unknown. If they were accessible, their po-
tential use in, for example, cycloaddition and metathesis reactions
is clear. Given that transition metal–terminal phosphinidene com-
plexes can be readily made by the base assisted dehydrohalogena-
tion of primary phosphido–metal halide complexes,35 we believed
similar methodologies could be applied to appropriate Ga(III)
heterocyclic complexes. To this end, a suitable gallium phosphide
Fig. 5 X-Band cw-EPR spectrum of 6 recorded in hexane solution at
298 K. (a) First harmonic, and (b) second harmonic signals.
harmonic signal is also shown (Fig. 5b). The profile of the
spectrum was difficult to reproduce accurately in its simulation
due to the broadened line-widths observed in the spectral wings
(the mI dependency of the line-widths is influenced by the tumbling
rates of the radical in solution). The hyperfine pattern is, however,
dominated by a large isotropic coupling to the gallium isotopes,
69Ga and 71Ga, which in the case of 71Ga was estimated at 40
G (ca. 0.72% spin density on Ga) in order to account for the
overall width of the spectrum. In addition, the magnitudes of
•
precursor to such a reaction, [GaI{P(H)Mes*}{[N(Ar)C(H)]2 }]
6, was prepared in good yield by the 1 : 1 salt elimination
•
reaction between [GaI2{[N(Ar)C(H)]2 }] 7 and [LiP(H)Mes*]
(Scheme 3). Considering the steric bulk of both the Ga and
P substituents of 6, it was anticipated that its treatment with
a base would lead to HI elimination and the formation of
•
=
[Ga{ PMes*}{[N(Ar)C(H)]2 }]. Surprisingly, however, the re-
action of 6 with an excess of DBU in THF gave rise to the
diamagnetic complex, [Ga(DBU){P(H)Mes*}{[N(Ar)C(H)]2}] 8,
in good isolated yield. The formation of this complex could
1
the couplings to H and 14N in 6 were easily measured at 5.5
1
G and 5.8 G respectively, by inspection of the well resolved H
conceivably occur via the expected phosphinidene complex,
and 14N hyperfine patterns associated with the −3/2 and +3/2
Ga mI states (Fig. 5b). The 127I coupling could not, however,
be resolved. The values of the HFCs for 6 can be compared
to those previously reported by us for 7 (1H ∼5.0 G, 14N ∼5.0
G, 69,71Ga ∼25 G, equating to ca. 0.45% spin density at Ga).34
The reason behind the increased coupling to the gallium centre
in 6, relative to 7, lies with the electron withdrawing effect of
the phosphido ligand which polarises the unpaired electron from
the diazabutadiene backbone onto the gallium nucleus. We have
previously observed similar effects in closely related complexes,
•
=
[Ga{ P(Mes*)}{[N(Ar)C(H)]2 }], which undergoes a rapid in-
tramolecular single electron reduction of the diazabutadiene
ligand to give [Ga{P•Mes*}{[N(Ar)C(H)]2}], the radical P-centre
of which abstracts an atom of hydrogen from the solvent. This
would leave the unsaturated Ga-centre to be coordinated by a
molecule of DBU. Such a proposal seems reasonable considering
the well known red-ox activity of diazabutadienes.33,34
•
e.g. [GaI{P(SiMe3)2}{[N(But)C(H)]2 }].33
The frozen solution EPR spectrum of 6 at X-band did not
provide any additional resolution for the Ga hyperfine interaction
(only a broad asymmetric spectrum was observed), suggesting
that this interaction is dominated by the isotropic component. As
a result, its X-band cw-ENDOR spectra revealed a powder type
pattern regardless of the measuring field position. A representative
1H ENDOR spectrum of 6 is shown in Fig. 6. According to
Scheme 3 i) LiP(H)Mes*, THF; ii) excess DBU, THF.
The infrared spectra of both 6 and 8 show characteristic
P–H stretching absorptions at m 2397 cm−1 and 2403 cm−1
respectively. No useful NMR information could be obtained from
paramagnetic 6, but the data for 8 are consistent with its proposed
formulation. Most diagnostically, its 31P NMR spectrum shows a
doublet at high field, −148.5 ppm (1JP–H = 215 Hz), as would be
expected for a metal–primary phosphide complex.
In order to obtain more information on 6, its room temperature
isotropic EPR spectrum was acquired and is depicted in Fig. 5a.
For higher resolution of the hyperfine couplings, the second
1
the room temperature EPR spectrum, the isotropic H hyperfine
coupling was estimated to be 5.5 G (15.5 MHz). For a strongly
coupled a-proton, principal values of the hyperfine tensor will
occur at approximately a/2, a, and 3/2a, with slight deviations
from those expected due to the delocalised nature of the spin
density on the N–C–C–N unit. Depending on the magnitude of the
coupling, the broadened outer lines are often not readily observed
in the cw-ENDOR spectrum, as discussed in a recent publication.34
6 8 | Dalton Trans., 2006, 64–72
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Fig. 6 X-Band polycrystalline 1H ENDOR spectrum of 6 recorded at
10 K in CD2Cl2/C6D5CD3.
1
Therefore, the relatively small H ENDOR resonances in Fig. 6
arise primarily from the aryl substituents. The largest couplings of
4.90 MHz and 3.97 MHz can be assigned to the protons on the aryl
ring, while the smaller inner couplings at 2.35 MHz and 1.20 MHz
were assigned to the methyl protons of the aryl groups. These
couplings are in good agreement with those previously reported
by us for a series of related aryl substituted Ga–diazabutadiene
complexes.34 While the P(H)Mes* substituent clearly affects the
unpaired spin density on the gallium nucleus, it has little effect on
the already small spin density within the aryl groups.
˚
Fig. 7 Molecular structure of 6. Selected bond lengths (A) and
angles (◦): Ga(1)–I(1) 2.5512(11), Ga(1)–N(1) 1.976(5), Ga(1)–N(2)
1.972(6), Ga(1)–P(1) 2.337(2), N(1)–C(1) 1.347(9), N(2)–C(2) 1.324(5),
C(1)–C(2) 1.402(10), P(1)–H(1A) 1.32(6); N(1)–Ga(1)–N(2) 84.0(2),
N(1)–Ga(1)–I(1) 106.69(17), N(1)–Ga(1)–P(1) 123.97(17), N(2)–Ga(1)–
I(1) 110.57(18), N(2)–Ga(1)–P(1) 114.43(19), P(1)–Ga(1)–I(1) 113.49(6),
Ga(1)–P(1)–C(27) 102.0(2).
The X-ray crystal structure of 6 was obtained and its asym-
metric unit was shown to contain two crystallographically inde-
pendent molecules which have no significant geometric differences
and, thus, only one is shown in Fig. 7. Unusually, the asymmet-
ric unit also contains the reaction by-product, LiI, in the form
of the dimeric molecule, [{Li2I2(THF)3.5(OEt2)0.5}], which exhibits
coordinated solvent site disorder and has no appreciable contacts
with either of the other two molecules of 6. An X-ray crystal
structure analysis was also carried out on 8 and its molecular
structure is depicted in Fig. 8. Both 6 and 8 are monomeric
and exhibit distorted tetrahedral coordination environments for
their gallium centres. Their Ga–P bond lengths are similar
and in the normal range for gallium phosphide complexes, cf.
36
˚
˚
2.388 A avg. in [(Mes*)Ga{P(H)Mes*}2] or 2.2991(11) A in
•
[GaI{P(SiMe3)2}{[N(But)C(H)]2 }].33 The magnitudes of the C–
C and C–N distances in their diazabutadiene ligands are, however,
significantly different and suggest a degree of delocalisation over
=
the NCCN fragment of 6, and localised C C and C–N bonds in
8. There are also differences between their Ga–N(diazabutadiene)
bond lengths and their N–Ga–N angles, which are shorter and
more obtuse respectively in 8. Not surprisingly, the Ga–N(DBU)
bond length in this compound is more than 0.1 A longer than its
Ga–N(diazabutadiene) interactions.
Fig.
8
Molecular structure of
˚
8
(isopropyl groups omitted for
◦
clarity). Selected bond lengths (A) and angles ( ): Ga(1)–N(1) 1.918(3),
Ga(1)–N(2) 1.909(3), Ga(1)–N(3) 2.029(3), Ga(1)–P(1) 2.3675(12), C(1)–
C(2) 1.339(6), N(1)–C(1) 1.413(5), N(2)–C(2) 1.405(5), P(1)–H(1A)
1.33(7); N(1)–Ga(1)–N(2) 88.72(13), N(1)–Ga(1)–N(3) 109.76(14),
N(1)–Ga(1)–P(1) 115.81(10), N(2)–Ga(1)–N(3) 113.20(14), N(2)–Ga(1)–
P(1) 111.67(10), P(1)–Ga(1)–N(3) 114.96(11), Ga(1)–P(1)–C(27)
114.32(13).
˚
Conclusion
The reactivity of a series of Ga(I), Ga(II) and Ga(III) heterocyclic
compounds towards a number of Group 15 substrates has been
investigated with a view to prepare examples of gallium–terminal
pnictinidene complexes. Although no examples of such com-
plexes were isolated, a gallium–terminal phosphinidene complex,
•
=
[Ga{ P(Mes*)}{[N(Ar)C(H)]2 }], has been implicated as a short
lived intermediate in one reaction. In addition, a number of
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unusual complexes containing gallium–pnictogen bonds have
been prepared. These have highlighted the reducing ability of
the anionic gallium(I) heterocycle, [:Ga{[N(Ar)C(H)]2}]−, the
reactivity of which differs to that of valence isoelectronic NHCs in
this study. Moreover, the reactivity of the Ga–Ga bond of the digal-
lane(4), [Ga{[N(Ar)C(H)]2}]2, has been exploited in the synthesis
of the dimeric imido–gallane complex, [{[N(Ar)C(H)•]2}Ga{l-
1.14 (br, 12H CH3), 1.23 (br, 12H CH3), 2.09 (s, 12H, NCH3),
2.31 (s, 4H, NCH2), 2.99 (br. m, 4H, CH), 5.20 (br, 2H, NC2H2,
1
6.8–7.7 (m, 26H, Ar–H); 31P{ H} NMR (121.6 Mz, D8-THF): d −
80.2 (br. m), −6.1 (br. m); IR m/cm−1 (Nujol): 1663 (m), 1580 (m),
1186 (m), 1132 (m), 1100 (m), 1055 (m), 1024 (m), 739 (s), 693 (s);
(MS/MALDI/-ve) m/z: 878 [{(PhP)4}Ga{N(Ar)C(H)}2−, 100%].
Preparation of [K(tmeda)(OEt2)][{j2N,C-PhNN(H)(C6H4)}-
N(SiMe3)}]2, which can be considered as a dimer of the
•
=
Ga{[N(Ar)C(H)]2}] 3. To a solution of PhN NPh (0.065 g,
=
gallium–terminal imide, [{[N(Ar)C(H) ]2}Ga N(SiMe3)]. EN-
0.35 mmol) in diethyl ether (20 cm3) at −80 ◦C was added a
solution of [K(tmeda)][:Ga{[N(Ar)C(H)]2}] (0.40 g, 0.66 mmol)
in diethyl ether (50 cm3) over 5 min. The resultant solution was
allowed to warm to room temperature and stirred overnight.
Volatiles were then removed in vacuo and the red residue extracted
into hexane (20 cm3). Filtration, concentration and placement at
−30 ◦C overnight gave orange/red crystals of 3 (0.17 g, 57% based
on azobenzene). Mp 184–187 ◦C(decomp.); 1H NMR (400 MHz,
DOR and/or EPR spectroscopy has shown that the unpaired
electron of all paramagnetic complexes is primarily based on their
diazabutadiene ligands. Studies of the reactivity of low oxidation
state gallium heterocycles are ongoing in our laboratory.
Experimental
General considerations
2
3
C6D6): d 1.22 (br. d, JHH = 6.6 Hz, 12H, CH3), 1.27 (t, JHH
=
7.3 Hz, 6H, Et2O), 1.49 (br. d, 2JHH = 6.6 Hz, 12H, CH3), 2.08 (s,
12H, NCH3), 2.26 (s, 4H, NCH2), 3.11 (overlapping m., 4H, CH),
3.77 (q, 3JHH = 7.1 Hz, 4H, Et2O), 5.24 (s, 2H, NCH), 6.6–7.3 (m,
15H, Ar–H), 7.47 (br, 1H, NH); 13C NMR (75.57 MHz, C6D6): d
15.3 (Et2O), 23.8 (CH3), 24.1 (CH3), 28.0 (CH), 28.2 (CH), 45.6
(NCH3), 57.9 (NCH2), 65.6 (Et2O), 119.6 (NC2H2N), 123.1, 123.3,
123.7, 128.2, 128.7, 129.3, 129.5, 136.6, 137.4, 140.6, 142.5, 144.2,
148.6, 149.1 (Ar–C); IR m/cm−1 (Nujol): 3486 (br, NH), 1603 (m),
1493 (m), 1459 (s), 1382 (w), 1260 (s), 1024 (s), 801 (s), 693 (w),
668 (w); (MS/APCI) m/z: 377 [{N(Ar)C(H)}2H+, 100%].
All manipulations were carried out using standard Schlenk and
glove box techniques under an atmosphere of high purity argon.
Hexane, toluene and THF were distilled over potassium whilst
Et2O was distilled over Na/K then freeze/thaw degassed prior to
use. 1H, 13C and 31P NMR spectra were recorded on either a Bruker
DXP400 or a Jeol Eclipse 300 spectrometer and were referenced
to the residual 1H or 13C resonances of the solvent used or external
85% H3PO4, d 0.0 ppm (31P NMR). A confident assignment of the
13C NMR spectrum of 2 could not be made due to the presence
of many overlapping multiplet resonances. The continuous wave
(cw) EPR/ENDOR spectra were recorded on an X-band Bruker
ESP300E series spectrometer equipped with an ESP360 DICE
ENDOR unit, operating at 12.5 kHz field modulation in a Bruker
EN801 cavity. The ENDOR spectra were recorded at 10 K using
an Oxford instruments ESR 900 continuous flow He cryostat.
These spectra were obtained using 8 dB RF power from an ENI
A-300 RF amplifier with 75 or 250 kHz RF modulation depth.
Computer simulations were carried out using Bruker’s Simfonia
program.37 Mass spectra were recorded using a VG Fisons
Platform II instrument under APCI conditions, or were obtained
from the EPSRC National Mass Spectrometry Service at Swansea
University. IR spectra were recorded using a Nicolet 510 FT-IR
spectrometer as Nujol mulls between NaCl plates. Melting points
were determined in sealed glass capillaries under argon, and are
uncorrected. [K(tmeda)][:Ga{[N(Ar)C(H)]2}],14 cyclo-(PPh)5,38
Preparation of [{[N(Ar)C(H)•]2}Ga{l-N(SiMe3)}]2 5. To a
solution of [Ga{[N(Ar)C(H)]2}]2 (0.32 g, 0.34 mmol) in hexane
(30 cm3) at −78 ◦C was added excess Me3SiN3 (0.40 cm3)
over 5 min. The resultant solution was allowed to warm to
room temperature and stirred for 48 h, during which time the
colour changed from red to deep purple. This solution was then
concentrated to ca.10 cm3, filtered and placed at −30 ◦C overnight
to give blue-green crystals of 5 (0.29 g, 79%). Mp 182–186 ◦C
(decomp.); IR m/cm−1 (Nujol): 1455 (s), 1376 (s), 1260 (m), 1223
(w), 1096 (s), 964 (s), 799 (m), 754 (m); (MS/APCI) m/z: 377
[{N(Ar)C(H)}2H+, 100%].
•
Preparation of [GaI{P(H)Mes*}{[N(Ar)C(H)]2 }]·0.5[{Li2I2-
•
(THF)3.5(OEt2)0.5}] 6. To a solution of [GaI2{[N(Ar)C(H)]2 }]
(3.17 g, 4.5 mmol) in THF (50 cm3) at −78 ◦C was added a
solution of Mes*P(H)Li (made in situ by treating Mes*PH2 (1.27 g,
4.56 mmol) with one equivalent of BunLi in THF/hexane). The
resultant solution was allowed to warm to room temperature and
stirred for 12 h. Volatiles were then removed in vacuo and the red
residue washed with hexane (10 cm3), extracted into diethyl ether
(2 × 30 cm3) and filtered. The filtrate was concentrated and placed
[Ga{[N(Ar)C(H)]2}]2,15c [I2Ga{[N(Ar)C(H)]2}]17 and Mes*PH2
39
were synthesised by literature procedures whilst all other chemicals
were obtained from commercial sources and used as supplied.
Syntheses
◦
Preparation of [K(tmeda)][{j2P,Pꢀ-(PhP)4}Ga{[N(Ar)C(H)]2}] 2.
To a solution of cyclo-(PhP)5 (0.07 g, 0.13 mmol) in THF
(10 cm3) at −80 ◦C was added a solution of [K(tmeda)]-
[:Ga{[N(Ar)C(H)]2}] (0.10 g, 0.16 mmol) in THF (10 cm3) over
5 min. The resultant solution was allowed to warm to room
temperature and stirred overnight. Volatiles were then removed
in vacuo and the orange residue washed with hexane, extracted
into toluene (20 cm3) and filtered. Concentration and placement
of the filtrate at −30 ◦C overnight gave orange crystals of 2 (0.03 g,
at −30 C overnight to give red crystals of 6 (3.45 g, 68%). Mp
88–94 ◦C (decomp.); IR m/cm−1 (Nujol): 2397 (m, P–H), 1645 (m),
1591 (m), 1457 (s), 1382 (m), 1362 (m), 1321 (m), 1287 (w), 1253
(w), 1207 (m), 1107 (br), 799 (s), 757 (s); (MS/ES) m/z: 852 [MH+,
5%], 377 [{N(Ar)C(H)}2H+, 100%].
Preparation of [Ga(DBU){P(H)Mes*}{[N(Ar)C(H)]2}] 8. To
◦
a solution of 6 (2.70 g, 2.38 mmol) in THF (20 cm3) at −78 C
was added a solution of DBU (1 cm3) in THF (5 cm3) over
5 min. The resultant solution was warmed to room temperature
◦
31%). Mp 108–110 C (decomp.); 1H NMR (400 MHz, C6D6): d
7 0 | Dalton Trans., 2006, 64–72
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and stirred for 12 h, during which time the colour changed
from red to yellow. Volatiles were then removed in vacuo and
the residue washed with hexane (5 × 10 cm3). Extraction into
dieth◦yl ether (50 cm3), filtration, concentration and placement at
−30 C overnight yielded yellow crystals of 8 (0.99 g, 47%). Mp
◦
1
203–207 C (decomp.); H NMR (300 MHz, C6D6): d 1.14 (d,
2JHH = 7.1 Hz, 12H, CH3), 1.20 (d, JHH = 7.1 Hz, 12H, CH3),
2
1.34 (s, 18H, o-tBu), 1.35 (s, 9H, p-tBu), 1.42 (m, 6H, DBU), 1.72
(m, 2H, DBU), 2.00 (m, 2H, DBU), 2.71 (m, 6H, DBU), 3.38
(overlapping m, 4H, CH), 3.73 (d, 1JPH = 215 Hz, 1H, PH), 5.70
(s, 2H, NCH), 6.81–7.52 (m, 8H, Ar–H); 13C NMR (75.57 MHz,
C6D6): d 19.5 (CH3–Pri), 21.1 (CH3–Pri), 22.9 (DBU), 26.2 (DBU),
27.9 (CH), 28.2 (CH), 28.5 (DBU), 29.6 (DBU), 31.2 (CH3–But),
31.4 (CH3–But), 32.3 (quat. C), 32.5 (quat. C), 37.1 (DBU), 44.4
(DBU), 48.1 (DBU), 52.3 (DBU), 122.4 (CN), 122.6 (m-ArC),
124.1 (p-ArC), 132.3 (d, 2JPC = 49.9 Hz, o-Mes*), 137.0 (d, 1JPC
=
3
62.8 Hz, ipso-Mes*), 146.4 (o-ArC), 148.7 (d, JPC = 18.4 Hz,
m-Mes*), 155.7 (ipso-ArC), 159.1 (p-Mes*), 160.3 (DBU); 31P
1
NMR (121.7 MHz, C6D6): d = − 148.51 (d, JPH = 215 Hz);
IR m/cm−1 (Nujol): 2403 (w, P–H), 1645 (m), 1573 (s), 1452 (s),
1379 (m), 1360 (m), 1256 (m), 1205 (m), 1100 (m), 1036 (m), 673
(m); (MS/EI) m/z: 723 [M+ − DBU, 85%], 377 [{N(Ar)C(H)}2H+,
100%].
X-Ray crystallography
Crystals of 2, 3, 5, 6 and 8 suitable for X-ray structural determina-
tion were mounted in silicone oil. Crystallographic measurements
were made using a Nonius Kappa CCD diffractometer. The
structures were solved by direct methods and refined on F2 by
full matrix least squares (SHELX97)40 using all unique data. All
non-hydrogen atoms are anisotropic with H-atoms included in
calculated positions (riding model), except for the N–H hydrogen
of 3 and the P–H hydrogen atoms of 6 and 8, which were
refined isotropically. Crystal data, details of data collections and
refinement are given in Table 1.
CCDC reference numbers 280910–280914.
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b511451a
Acknowledgements
We thank the EPSRC (postdoctoral fellowship for RJB, stu-
dentship for DPM) for financial support and for funding the
National ENDOR Service (GR/R17980/01). Financial sup-
port from the Studienstiftung des Deutschen Volkes (PhD
grant for R.W.) and the German Academic Exchange Ser-
vice (313-ARC-XVI-02/10) is gratefully acknowledged. The EP-
SRC Mass Spectrometry Service at Swansea University is also
thanked.
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7 2 | Dalton Trans., 2006, 64–72
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