Gallium and indium compounds supported by a [PNP] pincer ligand

Article abstract of DOI:10.1016/j.ica.2010.07.064

The new bis(phosphino)amido ligand, [MePNP^Ph], that incorporates (i) an ortho-tolylene linker between nitrogen and phosphorus and (ii) phenyl substituents on phosphorus, has been synthesized as its protonated derivative, [MePNP^Ph]H,

Full text of DOI:10.1016/j.ica.2010.07.064

Inorganica Chimica Acta 364 (2010) 157–161
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Gallium and indium compounds supported by a [PNP] pincer ligand
⇑
Kevin Yurkerwich, Gerard Parkin
Department of Chemistry, Columbia University, New York, NY 10027, USA
a r t i c l e i n f o
a b s t r a c t
The new bis(phosphino)amido ligand, [MePNPPh], that incorporates (i) an ortho-tolylene linker between
nitrogen and phosphorus and (ii) phenyl substituents on phosphorus, has been synthesized as its proton-
ated derivative, [MePNPPh]H, via sequential treatment of (2-Br,4-Me-C6H3)2NH with (i) BunLi, (ii) Ph2PCl
and (iii) HCl. Deprotonation of [MePNPPh]H with BunLi in THF affords the lithium derivative which has
been isolated as both mono and bis THF adducts, [MePNPPh]Li(THF) and [MePNPPh]Li(THF)2. Treatment
of [MePNPPh]Li(THF)2 with GaCl3 and InX3 (X = Cl, Br, I) gives a series of [MePNPPh]MX2 complexes in
which the [PNP] donor binds in a ‘‘T”-shaped manner and the metal has a distorted trigonal bipyramidal
Article history:
Available online 6 August 2010
Dedicated with respect to Professor Arnold
L. Rheingold in recognition of his important
contributions to inorganic chemistry and for
his constant help and advice over many
years.
geometry. The reaction of [MePNPPh]Li(THF)2 with ‘‘GaI” yields the Ga–Ga bonded complex [j2
MePNPPh](GaI)(GaI)[ 2-MePNPPh] in which the [MePNPPh] ligand binds in a 2-P,N manner. The bis(phos-
phino)amine [MePNPPh]H may also serve as a ligand and treatment of [MePNPPh]H with GaBr3 affords
2-{[MePNPPh]H}GaBr2][GaBr4], in which the [MePNPPh]H ligand coordinates in a 2-P,P manner such
that the gallium adopts a tetrahedral geometry.
-
Keywords:
[PNP] pincer ligand
Gallium
Indium
Gallium–gallium bond
Hemilabile
j
j
[j
j
Ó 2010 Elsevier B.V. All rights reserved.
C3 symmetric tripodal ligands have found widespread use in
coordination chemistry. In this regard, we have applied tris(pyraz-
olyl)hydroborato and tris(2-mercapto-1-R-imidazolyl)hydroborato
ligands, both of which belong to the L2X Covalent Bond Classifica-
tion [1,2], to a variety of topics of interest in main group chemistry
[3–7]. For example, we have demonstrated that these ligands are
effective for binding aluminum, gallium and indium, as highlighted
by the synthesis of monovalent gallium and indium compounds,
terminal chalcogenido compounds, and catenated gallium com-
pounds [8–13]. Here we report the application of a ‘‘T”-shaped C2
symmetric ligand to gallium and indium chemistry.
[24], were reported, together with a variant that possesses a tolylene
i
linker, [MePNPPr ] [25–27].2 Since the methyl group of the tolylene
linker provides a good 1H NMR spectroscopic probe, we were inter-
ested in the [MePNPPh] hybrid that features both a tolylene linker
and phenyl substituents on phosphorus. Indeed, the bis(phos-
phino)amine [MePNPPh]H may be synthesized by appropriate modi-
i
fication of the methods used for [PNPPh]H [22] and [MePNPPr ]H
[25,26] as illustrated in Scheme 1, i.e. sequential treatment of (2-
Br, 4-Me-C6H3)2NH with (i) BunLi, (ii) Ph2PCl and (iii) HCl. The
molecular structure of [MePNPPh]H has been determined by X-ray
diffraction, as illustrated in Fig. 2.3
Bis(phosphino)amido ligands have emerged to be a significant
class of L2X ligands that provides a [PNP] donor array [14].1 The
first members of this class, namely [N(SiMe2CH2PR2)2], were de-
scribed by Fryzuk in 1982 [15–17], while the related
[N(CH2CH2PR2)2] ligand was reported in 2006 [18,19]. An important
development with respect to [PNP] ligands was the incorporation of
ortho-arylene linkers between the ligating atoms, as illustrated in
Fig. 1 [20]. The first such ligand to be described, namely [PNPPh], fea-
tures phenyl substituents on phosphorus and was reported indepen-
dently by two research groups in 2003 [21,22]. Shortly thereafter,
derivatives that possess cyclohexyl, isopropyl and isobutyl substitu-
The lithium derivative [MePNPPh]Li(THF)2 may be obtained by
deprotonation of [MePNPPh]H with BunLi in THF (Scheme 1) and
the molecular structure as determined by X-ray diffraction is illus-
trated in Fig. 3, thereby revealing that the geometry is a distorted
trigonal bipyramid. Interestingly, while crystals of the bis THF ad-
duct are obtained from solutions in THF, crystals of the mono THF
adduct may be obtained from benzene, as illustrated in Fig. 4. The
most notable structural feature of [MePNPPh]Li(THF) is that the
coordination geometry of lithium is highly distorted from tetrahe-
dral, a consequence of the preferred ‘‘T”-shaped binding mode of
the [PNP] pincer ligand. For example, the P–Li–P and N–Li–O bond
angles are 128.5(1)° and 136.3(2)°, respectively. Another indication
i
i
ents on phosphorus, namely [PNPCy] [23], [PNPPr ] [23] and [PNPBu
]
2
⇑
Corresponding author.
E-mail address: parkin@columbia.edu (G. Parkin).
[PNP] ligands that belong to the L3 class are also known. See Ref. [14].
A related ligand in which the arylene linkers are ‘‘tied” together is also known. See
Ref. [27].
1
3
For the molecular structure of [PNPPh]H, see Ref. [21].
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.07.064

158
K. Yurkerwich, G. Parkin / Inorganica Chimica Acta 364 (2010) 157–161
Fig. 1. [R0PNPR] ligands.
Fig. 2. Molecular structure of [MePNPPh]H.
Another interesting aspect pertaining to the isolation of the
mono THF adduct [MePNPPh]Li(THF) is that the ability to isolate
i
[MePNPPr ]Li(THF), rather than the five coordinate bis adduct
i
[MePNPPr ]Li(THF)2, was attributed to the greater basicity and ste-
ric bulk of the PPri2 group relative to the PPh2 group [26]. However,
since we have now demonstrated that the four-coordinate mono
THF adduct [MePNPPh]Li(THF) that features PPh2 donors may also
be isolated, it is evident that isopropyl substituents are not re-
quired for the isolation of mono THF adducts in this system.
To date, the majority of applications of [R’PNPR] ligands has fo-
cused on transition metal chemistry. Thus, there are only few re-
ports of main group metal [R0PNPR] complexes[21,26,29,30], and
there are no structurally characterized gallium and indium deriva-
tives listed in the Cambridge Structural Database [31]. Therefore, it
is noteworthy that the gallium and indium halide complexes,
[MePNPPh]GaCl2 and [MePNPPh]InX2 (X = Cl, Br, I), may be synthe-
sized by treatment of [MePNPPh]Li(THF)2 with the respective MX3
derivative (Scheme 2). The molecular structures of [MePNPPh]GaCl2
and [MePNPPh]InX2 (X = Cl, Br, I) have been determined by X-ray
diffraction, as illustrated for [MePNPPh]GaCl2 in Fig. 5. In each case,
the [PNP] donor binds in a ‘‘T”-shaped manner and the geometry of
the metal may be described as a distorted trigonal bipyramid.
Selected metrical data pertaining to the [MePNPPh]Ga and
[MePNPPh]In interactions are summarized in Table 2.
Scheme 1.
of the distortion from tetrahedral coordination is provided by the
value of 0.66 for the s4 four-coordinate geometry index [28],4
which is significantly smaller than the value of 1.0 for an idealized
tetrahedron.
While both mono and bis THF adducts of the type
i
[R’PNPR]Li(THF)x, specifically [MePNPPr ]Li(THF) [26] and
[PNPPh]Li(THF)2, [21],5 have been previously reported, the structural
characterization of [MePNPPh]Li(THF) and [MePNPPh]Li(THF)2 is
noteworthy because they are derivatives of the same [R’PNPR] ligand.
As such, the structural comparison of [MePNPPh]Li(THF) and
[MePNPPh]Li(THF)2 provides a direct indication of the differences in
bond lengths that result upon coordination of an additional ligand.
In this regard, the key structural data for [MePNPPh]Li(THF) and
[MePNPPh]Li(THF)2 are summarized in Table 1, from which it is evi-
dent that coordination of a second THF molecule increases all bond
lengths to lithium by 0.08–0.11 Å.
In contrast to the
[MePNPPh]MX2, a 2-coordination mode is manifested in dinuclear
2-MePNPPh](GaI)(GaI)[ 2-MePNPPh] (Fig. 6), a product of the
reaction of [MePNPPh]Li(THF) with ‘‘GaI” (Scheme 3). Specifically,
the [MePNPPh] ligands of [ 2-MePNPPh](GaI)(GaI)[ 2-MePNPPh
j
3-coordination mode observed in
j
[j
j
4
s
4 = [360 ꢀ (
a
+ b)]/141, where
a + b is the sum of the two largest angles. See Ref.
[28].
For a diethylether adduct, [PNPCy]Li(OEt2), see Ref. [23].
j
j
]
5

K. Yurkerwich, G. Parkin / Inorganica Chimica Acta 364 (2010) 157–161
159
Table 1
Selected bond lengths and angles for [MePNPPh]Li(THF)x (x = 1 and 2).
[MePNPPh]Li(THF)
[MePNPPh] Li(THF)2
Li–P1 (Å)
Li–P2 (Å)
Li–N (Å)
Li–O1 (Å)
Li–O2 (Å)
P1–Li–P2 (°)
P1–Li–N (°)
P2–Li–N (°)
O1–Li–O2 (°)
2.563(3)
2.484(3)
1.970(3)
1.897(3)
–
128.47(12)
76.16(10)
82.55(10)
–
2.673(2)
2.673(2)
2.053(7)
1.975(4)
1.975(4)
151.9(3)
76.0(1)
76.0(1)
114.0(3)
Fig. 3. Molecular structure of [MePNPPh]Li(THF)2.
Scheme 2.
bind in a
j
2-P,N manner via a phosphorus and nitrogen atom [32].6
2-MePNPPh](GaI)-
While both the Ga–P and Ga–N bonds within [
j
(GaI)[j
2-MePNPPh] are shorter than the corresponding values in
mononuclear [MePNPPh]GaCl2 (Table 1), the difference is more sig-
nificant for the Ga–P bonds (0.13 Å). Furthermore, the N–Ga–P bond
angle of 85.1(1)° for [j j
2-MePNPPh](GaI)(GaI)[ 2-MePNPPh] is consid-
erably less acute than the corresponding value of 79.1(2)° for
[MePNPPh]GaCl2. Thus, it is evident that the ‘‘T”-shaped conforma-
tion of the [PNP] ligand does not maximize the Ga–P bonding inter-
actions in this system. As such, the [MePNPPh] ligand exhibits
hemilability [33–36], a feature that may have ramifications with re-
spect to reactivity and its applications in catalysis.
Another interesting aspect of
[j -
2-MePNPPh](GaI)(GaI)[j2
MePNPPh] is that it possesses a Ga–Ga bond, with a structure that
resembles those of the bis(mercaptoimidazolyl)hydroborato coun-
t
t
terparts [BmMe](GaI)(GaI)[BmMe
]
and [BmBu ](GaI)(GaI)[BmBu
]
-
[37].7 Furthermore, the Ga–Ga bond length of 2.416(1) Å for [j2
MePNPPh](GaI)(GaI)[
j
2-MePNPPh] is comparable to the values of
2.414(2) and 2.423(2) Å for [BmMe](GaI)(GaI)[BmMe
]
and
t
t
[BmBu ](GaI)(GaI)[BmBu ], respectively.
Finally, the bis(phosphino)amine [MePNPPh]H may also serve as
a ligand. For example, treatment of [MePNPPh]H with GaBr3 affords
6
j
2-Coordination has only recently been observed for a uranium compound. See
Ref. [32].
7
Fig. 4. Molecular structure of [MePNPPh]Li(THF).
For related cationic compounds with Ga–Ga bonds, see Ref. [8].

160
K. Yurkerwich, G. Parkin / Inorganica Chimica Acta 364 (2010) 157–161
Fig. 6. Molecular structure of [j j
2-MePNPPh](GaI)(GaI)[ 2-MePNPPh].
latter in Fig. 7. Of particular note, the GaꢁꢁꢁN distances (2.8 Å) are
approximately 1 Å longer than the sum of covalent radii of gallium
and nitrogen [38,39]. As such, the [MePNPPh]H ligand coordinates
in a j j
2-P,P manner that is distinct from the 3-coordination modes
Fig. 5. Molecular structure of [MePNPPh]GaCl2.
that have been observed for the amine ligands [HN(Si-
Me2CH2PR2)2] and [HN(CH2CH2PR2)2] [15,40]. There are no other
structurally characterized compounds containing {j2-[R0PNPR]H}
ligands listed in the Cambridge Structural Database [31] and so
[
j
2-{[MePNPPh]H}GaBr2][GaBr4] (Scheme 4). Furthermore,
{[MePNPPh]H}GaCl2][GaCl4] is obtained via the corresponding reac-
tion of [MePNPPh]H with GaCl2. The molecular structures of [j2
{[MePNPPh]H}GaCl2][GaCl4] and 2-{[MePNPPh]H}GaBr2][GaBr4]
[ -
j2
[j
2-{[MePNPPh]H}GaX2]+ represent a new type of ligand binding
-
mode for this system. The Ga–P bond lengths for [{[MePNPPh]H}-
GaX2]+ are comparable to those of [ 2-MePNPPh](GaI)(GaI)[j2
[
j
j
-
have been determined by X-ray diffraction, as illustrated for the
Table 2
Selected bond lengths and angles for [MePNPPh] derivatives of gallium and indium.
M–P1 (Å)
M–P2 (Å)
M–N (Å)
P1–M–P2 (Å)
P1–M–N
P2–M–N
[MePNPPh]GaCl2
[MePNPPh]InCl2
[MePNPPh]InBr2
2.521(2)
2.6303(6)
2.6383(7)
2.6994(8)
2.7214(8)
2.384(2)
2.3819(6)
–
2.524(2)
2.6643(6)
2.6652(7)
2.7002(4)
2.7451(8)
2.376(2)
2.3786(6)
2.389(1)
1.948(6)
2.209(2)
2.210(2)
2.160(3)
2.175(3)
–
157.59(8)
144.37(2)
144.18(2)
143.29(2)
144.16(2)
113.38(7)
117.36(2)
–
78.5(2)
72.67(5)
72.51(6)
72.76(7)
72.31(7)
–
79.1(2)
72.82(5)
72.88(6)
70.89(7)
71.98(7)
–
a
[MePNPPh]InI2
[j
[j
[j
2-{[MePNPPh]H}GaBr2]+ [GaBr4]
2-{[MePNPPh]H}GaCl2]+ [GaCl4]
–
–
–
–
2-MePNPPh](GaI)(GaI) [
j
2-MePNPPh
]
1.925(3)
85.1(1)
a
Data for two crystallographically independent molecules.
Scheme 3.

K. Yurkerwich, G. Parkin / Inorganica Chimica Acta 364 (2010) 157–161
161
(GaI)[
j
2-MePNPPh] exhibits
j
2-P,N coordination of [MePNPPh], thus
indicating that it may serve as a hemilabile ligand.
Acknowledgment
We thank the National Science Foundation (CHE-0749674) for
support of this research and for the acquisition of an X-ray diffrac-
tometer (CHE-0619638).
Appendix A. Supplementary material
Scheme 4.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.ica.2010.07.064.
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In conclusion, [MePNPPh] represents a new member of a grow-
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Li reacts with GaCl3 and InX3 (X = Cl, Br, I) to give a series of
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Furthermore, the Ga–Ga bonded complex [j
2-MePNPPh](GaI)-