Homo- and heteroleptic complexes of four-membered group 13 metal(I) N-heterocyclic carbene analogues with group 10 metal(0) fragments

Article abstract of DOI:10.1021/ic062057d

A series of complexes between recently developed four-membered group 13 metal(I) heterocycles and group 10 metal(0) fragments have been prepared and structurally characterized. One prepared complex, [Pt{Ga[N(Ar)] ₂CNCy₂}₃] (Ar = C₆H ₃Prⁱ₂-2,6; Cy = cyclohexyl), possesses the shortest Pt-Ga bonds yet reported, the covalent components of which are suggested by theoretical studies to have significant π character.

Full text of DOI:10.1021/ic062057d

Inorg. Chem. 2007, 46, 11−13
Homo- and Heteroleptic Complexes of Four-Membered Group 13 Metal(I)
N-Heterocyclic Carbene Analogues with Group 10 Metal(0) Fragments
Shaun P. Green, Cameron Jones,* and Andreas Stasch
Center for Fundamental and Applied Main Group Chemistry, School of Chemistry, Main Building,
Cardiff UniVersity, Cardiff CF10 3AT, U.K.
Received October 27, 2006
A series of complexes between recently developed four-membered
group 13 metal(I) heterocycles and group 10 metal(0) fragments
have been prepared and structurally characterized. One prepared
gaps are large (ca. 60 kcal/mol), they are less than those in
the related six-membered heterocycles (ca. 100 kcal/mol).2
Considering this and the fact that significant π back-bonding
has been suggested in homoleptic group 13 diyl complexes
of group 10 metals, e.g., [Ni{GaC(SiMe3)3}4],1a,4 it was
decided to examine the coordination of 1 and 2 toward group
10 metal(0) fragments. The aim here was to prove the
σ-donor ability of 1 and 2 and to probe for metal-metal π
bonding in homoleptic complexes of these ligands.5 Such
complexes would be of added interest because related
homoleptic group 10 NHC complexes are finding applica-
tions in areas such as catalytic amination,6 CO2 fixation,7
etc. Our preliminary results in this direction are reported
herein.
The reactions of 1 and 2 with a range of group 10 olefin
complexes were carried out (Scheme 1). The reaction of 1
with [Ni(COD)2] (COD ) 1,5-cyclooctadiene) in either 2:1
or 4:1 stoichiometries afforded the heteroleptic complex 3
in good yield. Treatment of the Ni precursor with 2 at -80
°C led to deposition of Ni metal upon warming of the
reaction mixture to -10 °C. The differences here are likely
due to 2 being a poorer σ donor than 1 (higher s-character
lone pair), which leads to instability of the In analogue of 3,
which is presumably formed in the reaction. In contrast,
gallium and indium diyls can readily displace both ligands
from [Ni(COD)2], and the resultant homoleptic complexes,
[Ni(ER)4] (E ) Ga or In), are kinetically inert.1a,4 These
differences are compatible with the greater steric bulk of 1
and 2 compared to metal diyls. The related Pt complexes, 4
complex, [Pt
cyclohexyl), possesses the shortest Pt
the covalent components of which are suggested by theoretical
studies to have significant character.
{
Ga[N(Ar)]2CNCy2
}
]
3
(Ar
)
C6H3Pri2-2,6; Cy
Ga bonds yet reported,
)
−
π
The coordination chemistry of group 13 metal(I) com-
pounds is a rapidly emerging field. Most progress has been
-
made with metal diyls, :MIR (R ) bulky alkyl, aryl, C5Me5 ,
etc.), which have been used in the formation of numerous
complexes with p-, d-, and f-block metal fragments.1 More
recently, the coordination chemistry of the related anionic
five-membered heterocycle, [:GaI{[N(Ar)C(H)]2}]- (Ar )
C6H3Pri2-2,6), and neutral six-membered heterocycles,
[:MI{[N(Ar)C(Me)]2CH}] (M ) Al or Ga), has begun to be
explored, and analogies with both metal diyls and N-
heterocyclic carbenes (NHCs) have been identified.2 In 2006,
we reported the synthesis of the first neutral four-membered
group 13 metal(I) NHC analogues, [:M{[N(Ar)]2CNCy2}]
[M ) Ga (1) or In (2), Cy ) cyclohexyl].3 Theoretical studies
of models of the heterocycles suggested that they have a
directional lone pair at the metal center (highest occupied
molecular orbital, HOMO, of sp character) and thus the
ability to act as σ donors. In addition, the metal centers each
possess an empty p orbital (lowest unoccupied molecular
orbital, LUMO), which could act as a π acceptor in
transition-metal complexes. Although their HOMO-LUMO
(4) Uhl, W.; Benter, M.; Melle, S.; Saak, W.; Frenking, G.; Uddin, J.
Organometallics 1999, 18, 3778. N.B.: although GaCp* is a weaker
π acceptor than gallium(I) alkyls, its homoleptic group 10 complexes
have been reported. See: Jutzi, P.; Neumann, B.; Schebaum, L. O.;
Stammler, A.; Stammler, H. G. Organometallics 1999, 18, 4462.
Gemel, C.; Steinke, T.; Weiss, D.; Cokoja, M.; Winter, M.; Fischer,
R. A. Organometallics 2003, 22, 2705.
(5) An Fe complex of a four-membered Ga heterocycle has been reported,
but this was prepared via halide abstraction from an iron gallyl complex
and contained a tetrahedral Ga center, thus precluding Fe-Ga π
bonding. Jones, C.; Aldridge, S.; Gans-Eichler, T.; Stasch, A. Dalton
Trans. 2006, 5357.
* To whom correspondence should be addressed. E-mail: jonesca6@
cardiff.ac.uk.
(1) (a) Gemel, G.; Steinke, T.; Cokoja, M.; Kempter, A.; Fischer, R. A.
Eur. J. Inorg. Chem. 2004, 4161. (b) Cowley, A. H. Chem. Commun.
2004, 2369. (c) Gamer, M. T.; Roesky, P. W.; Konchenko, S. N.;
Nava, P.; Ahlrichs, R. Angew. Chem., Int. Ed. 2006, 45, 4447 and
references cited therein.
(2) (a) Baker, R. J.; Jones, C. Coord. Chem. ReV. 2005, 249, 1857. (b)
Roesky, H. W. Inorg. Chem. 2004, 43, 7284. (c) Hardman, N. J.;
Phillips, A. D.; Power, P. P. ACS Symp. Ser. 2002, 822, 2. (d) Kempter,
A.; Gemel, C.; Fischer, R. A. Chem. Commun. 2006, 1551 and
references cited therein.
(6) Caddick, S.; Cloke, F. G. N.; Hitchcock, P. B.; Lewis, A. K. K. Angew.
Chem., Int. Ed. 2004, 43, 5824.
(3) Jones, C.; Junk, P. C.; Platts, J. A.; Stasch, A. J. Am. Chem. Soc.
2006, 128, 2206.
(7) Yamashita, M.; Goto, K.; Kawashima, T. J. Am. Chem. Soc. 2005,
127, 7294.
10.1021/ic062057d CCC: $37.00
Published on Web 12/09/2006
© 2007 American Chemical Society
Inorganic Chemistry, Vol. 46, No. 1, 2007 11

COMMUNICATION
Scheme 1. Syntheses of Compounds 3-6 (Ar ) C6H3Pri2-2,6; Cy )
Cyclohexyl)
Figure 1. Molecular structure of 3 (H atoms removed for clarity). Selected
bond lengths (Å) and angles (deg): Ga1-N2 2.047(3), Ga1-N1 2.060(3),
Ga1-Ni1 2.2384(8), Ni1-C75 2.086(5), Ni1-Ga2 2.2401(9), Ga2-N4
2.041(3), Ga2-N5 2.066(3), N1-C1 1.354(5), C1-N2 1.356(5), C1-N3
1.364(5), N4-C38 1.361(5), N5-C38 1.346(5), N6-C38 1.374(5); N2-
Ga1-N1 64.26(13), Ga1-Ni1-Ga2 103.74(4), N4-Ga2-N5 64.48(13),
N1-C1-N2 107.4(3), N5-C38-N4 108.1(3).
of the slight excesses of 1 or 2 that were employed in these
reactions. However, the room temperature 31P{1H} NMR
spectrum of the reaction mixture that gave 5 displayed only
very broad peaks with no discernible 195Pt satellites in the
region 35-50 ppm. This indicates the presence of several
products that are involved in one or more fluxional processes.
The 31P{1H} NMR spectrum of the mixture that gave 4 also
indicated several products, but a singlet at δ 39.9 ppm with
195Pt satellites (1JPtP ) 3860 Hz) was assigned to this
compound.
The X-ray crystal structures9 of 3-6 were obtained, and
ORTEP diagrams for 3 and 6 are depicted in Figures 1 and
2. Those for 4 and 5 can be found in the Supporting
Information, although relevant geometrical parameters are
discussed here. The geometries of the heterocyclic ligands
in 3 and 6 are similar to that of the free heterocycle,3 although
their Ga-N distances are slightly shorter and their N-Ga-N
angles more obtuse, as would be expected upon coordination
of the heterocycle. All compounds 3-5 have distorted
tetrahedral geometries with M-M bond lengths (3, 2.319 Å
mean; 4, 2.357 Å mean; 5, 2.533 Å mean) that lie within
the known ranges for such interactions.10 In accordance with
the expected weaker donor ability of 2 compared to 1, the
and 5, were formed in low yields from the 2:1 reactions of
the group 13 heterocycles with [Pt(dppe)(C2H4)]. Upon
dissolution of isolated crystals of both complexes in toluene,
decomposition to [Pt(dppe)2] and unidentified insoluble
materials occurs. The instability of these complexes may
result from loss of the bulky group 13 ligands in solution
(vide infra). Similarly, the reactions of 1 or 2 with Pd(0)
precursors, e.g., [Pd2(dba)3] (dba ) dibenzylideneacetone),
resulted in Pd deposition below 0 °C.
More success was had in the 3:1 or 4:1 reactions of 1
with [Pt(norbornene)3], which in both cases gave the
thermally stable homoleptic complex 6 in good yield.
(N.B.: the 2:1 reaction led only to unidentified products.)
Again, the related reactions with 2 led only to metal
deposition at low temperature. Comparisons can be made
between 3-coordinate 6 and monomeric, homoleptic Pt group
13 diyl complexes, which always possess 4-coordinate Pt
centers in the solid state.1a,4 Again, the greater steric bulk of
1 relative to diyl ligands can be used to explain this
difference. It is of note that the aforementioned six-membered
Ga(I) heterocycle has been used as a ligand toward Pt(0),
but the only structurally characterized complex [Pt{Ga-
{[N(Ar)C(Me)]2CH}}2(η2-1,3-COD)] is heteroleptic.8
The spectroscopic data for 3 and 6 are as would be
expected for their proposed structures and will not be
discussed here, except to comment that a number of the
signals in the spectra of 6 are broad, presumably because of
steric crowding of its bulky ligands (vide infra), which slow
their rotation about the Pt-Ga bonds. The low solubility of
this compound in normal solvents at low temperatures
precluded a variable-temperature NMR study. Because of
the instability of 4 and 5 in solution at ambient temperature
and their poor solubility in noncoordinating solvents below
0 °C, no meaningful NMR data could be obtained for these
complexes. Because these complexes were crystallized from
toluene reaction solutions, which spent ca. 1 h at ambient
temperature, they are apparently more stable in the presence
(9) Crystal data for 3‚3THF: C94H148Ga2N6NiO3, M ) 1608.33, triclinic,
space group P1h, a ) 13.496(3) Å, b ) 18.139(4) Å, c ) 18.576(4)
Å, R ) 80.05(3)°, â ) 81.38(3)°, γ ) 82.52(3)°, V ) 4403.5(15) Å3,
Z ) 2, Dc ) 1.213 g cm-3, F(000) ) 1736, µ(Mo KR) ) 0.870 mm-1
,
T ) 150(2) K, 15 373 unique reflections [R(int) 0.0499], R (on F)
0.0663, wR (on F 2) 0.1629 [I > 2σ(I)]. Crystal data for 4‚hexane:
C106H150Ga2N6P2Pt, M ) 1904.79, triclinic, space group P1h, a )
14.172(3) Å, b ) 17.635(4) Å, c ) 22.360(5) Å, R ) 80.33(3)°, â )
88.13(3)°, γ ) 68.81(3)°, V ) 5134.3(18) Å3, Z ) 2, Dc ) 1.232 g
cm-3, F(000) ) 1996, µ(Mo KR) ) 1.957 mm-1, T ) 150(2) K,
19 932 unique reflections [R(int) 0.1050], R (on F) 0.0721, wR (on
F2) 0.1451 [I > 2σ(I)]. Crystal data for 5‚0.5hexane: C103H143In2N6P2-
Pt, M ) 1951.90, triclinic, space group P1h, a ) 15.690(3) Å, b )
18.532(4) Å, c ) 19.397(4) Å, R ) 102.36(3)°, â ) 98.51(3)°, γ )
106.69(3)°, V ) 5143.0(18) Å3, Z ) 2, Dc ) 1.260 g cm-3, F(000) )
2018, µ(Mo KR) ) 1.877 mm-1, T ) 150(2) K, 17 969 unique
reflections [R(int) 0.0451], R (on F) 0.0569, wR (on F 2) 0.1307 [I >
2σ(I)]. Crystal data for 6‚3toluene: C132H192Ga3N9Pt, M ) 2309.20,
monoclinic, space group P21/n, a ) 19.329(4) Å, b ) 33.357(7) Å, c
) 19.333(4) Å, â ) 96.36(3)°, V ) 12389(4) Å3, Z ) 4, Dc ) 1.238
g cm-3, F(000) ) 4872, µ(Mo KR) ) 1.821 mm-1, T ) 150(2) K,
21 681 unique reflections [R(int) 0.0486], R (on F) 0.0345, wR (on
F 2) 0.0761 [I > 2σ(I)].
(10) As determined from a survey of the Cambridge Crystallographic
Database, Oct 2006.
(8) Mentioned as unpublished results in ref 1a.
12 Inorganic Chemistry, Vol. 46, No. 1, 2007

COMMUNICATION
Figure 3. Representations of (a) the HOMO (-0.1394 hartrees) and (b)
the HOMO-1 (-0.1411 hartrees) of [Pt{Ga[N(C6H3Me2-2,6)]2CNMe2}3].
for calculations of this type. To shed light on the nature of
the short Pt-Ga bonds in 6, a charge decomposition analysis
(CDA) of the model complex was carried out. This technique
has been used to quantify the σ (d) and π (b) components of
metal-metal bonds.4,12 In [Pt{Ga[N(C6H3Me2-2,6)]2CNMe2}3],
the results indicate a significant (39.8% mean) π contribution
to the Pt-Ga bonds. Similar π contributions to the M-E
bonds in homoleptic diyl complexes, [M(ER)4] (M ) Ni,
Pd, or Pt; E ) B-Tl), have been calculated by CDA and
other methods and were said to be significant.1a,4,13 It should
be noted, however, that this fraction only relates to the
covalent components [d, 0.43 electrons (mean); b, 0.29
electrons (mean)] of the Pt-Ga bonds, which are polarized
(atomic charges: Pt, -0.96; Ga, +0.44) and thus have a
considerable electrostatic component. An orbital population
analysis of the model suggested that the majority of the π
contribution to the Pt-Ga bonds is derived from the bonding
combination of two nearly degenerate Ga empty p-orbital
sets (mixed with Ga lone pair sets of appropriate symmetry)
with either the Pt dxy or dx -y orbitals, giving rise to HOMO
and HOMO-1, respectively (Figure 3). The antibonding
combination of these orbitals comprise the LUMO+1 and
LUMO+2.
In summary, we have prepared and structurally character-
ized a series of group 10 metal(0) complexes with four-
membered Ga(I) and In(I) heterocycles. Crystallographic and
theoretical studies suggest that the covalent components of
the Pt-Ga bonds in one homoleptic complex possess
significant π character. Considering the synthetic applications
that homoleptic group 10 NHC complexes are finding, we
are currently examining further related chemistry of com-
plexes such as 6 and will report on this in due course.
Figure 2. Molecular structure of 6 (H atoms and isopropyl groups removed
for clarity). Selected bond lengths (Å) and angles (deg): Pt1-Ga2 2.3058-
(5), Pt1-Ga1 2.3108(6), Pt1-Ga3 2.3118(6), Ga1-N1 2.025(3), Ga1-
N2 2.034(3), Ga2-N5 2.025(3), Ga2-N4 2.030(3), Ga3-N8 2.020(3),
Ga3-N7 2.029(3), N1-C1 1.350(4), N2-C1 1.349(4), N4-C38 1.358-
(4), N5-C38 1.354(4), N7-C75 1.352(4), N8-C75 1.362(4); Ga2-Pt1-
Ga1 119.47(2), Ga2-Pt1-Ga3 118.232(18), Ga1-Pt1-Ga3 122.295(14),
N1-Ga1-N2 65.37(11), N5-Ga2-N4 65.78(11), N8-Ga3-N7 65.65-
(11), N2-C1-N1 108.6(3), N5-C38-N4 108.6(3), N7-C75-N8 108.0-
(3).
Pt-P distances in 5 (2.249 Å mean) are slightly shorter than
those in 4 (2.261 Å mean).
The structure of 6 differs from the others in that it shows
the complex to be homoleptic with a trigonal-planar Pt center.
There appears to be considerable steric crowding between
the heterocyclic ligands, two of which are nearly orthogonal
to the trigonal plane (89.0° Ga1 heterocycle; 88.2° Ga2
heterocycle), while the other (Ga3 heterocycle) subtends an
angle of 47.9° with it. This arrangement presumably relieves
the congestion at the metal center. Despite this congestion,
the Pt-Ga (3-coordinate) bond lengths (2.309 Å mean) are
the shortest yet reported and can be compared to the next
shortest bonds [Pt-Ga (2-ccordinate): 2.316 Å mean] in [Pt-
(dcpe){GaC(SiMe3)3}2] [dcpe ) bis(dicyclohexylphosphino)-
ethane].11 This, combined with fact that theoretical studies
have suggested significant Pt-Ga π bonding (0.22 electrons
versus the σ component, 0.19 electrons) in a model of the
latter complex, encouraged us to carry out preliminary
density functional theory (DFT) calculations on a model of
6, viz., [Pt{Ga[N(C6H3Me2-2,6)]2CNMe2}3].
2
2
The optimized geometry of the complex closely resembles
the structure of 6 in that two heterocyclic ligands are almost
orthogonal to the coordination plane (angles between
planes: 84.6° and 87.6°) while the third is not (angle between
planes: 55.3°). The calculated Pt-Ga and Ga-N bond
lengths (2.389 and 2.142 Å mean, respectively) are slightly
longer (ca. 3%) than the experimental values, as is common
Acknowledgment. We gratefully acknowledge financial
support from EPSRC in the form of a partial studentship for
S.P.G.
Supporting Information Available: Crystallographic CIF files
for 3-6, ORTEP diagrams for 4 and 5, full synthetic details for
3-6, spectroscopic data for 3 and 6, and full details of the DFT
calculation. This material is available free of charge via the Internet
at http://pubs.acs.org.
(11) Weiss, D.; Winter, M.; Merz, K.; Knu¨fer, A.; Fischer, R. A.; Fro¨lich,
N.; Frenking, G. Polyhedron 2002, 21, 535.
(12) Boehme, C.; Frenking, G. Chem.sEur. J. 1999, 5, 2184.
(13) Doerr, M.; Frenking, G. Z. Anorg. Allg. Chem. 2002, 628, 843.
IC062057D
Inorganic Chemistry, Vol. 46, No. 1, 2007 13