Synthesis and Structure of Novel Bridged Dinuclear Indium Complexes

Full text of DOI:10.1021/ic951241o

Inorg. Chem. 1996, 35, 1423-1424
1423
Synthesis and Structure of Novel Bridged Dinuclear Indium Complexes
Yuanlin Zhou and Darrin S. Richeson*
Department of Chemistry, University of Ottawa, Ottawa, Ontario, Canada K1N 6N5
ReceiVed September 26, 1995
Due to the Lewis acidity of three-coordinate complexes of
aluminum, gallium, and indium, the organometallic chemistry
of these metals is dominated by the appearance of four-
coordinate monomeric species and four-membered-ring dimers.1
Recently, several alkyl complexes with unusual coordination
geometries have been reported. For example, there are now
several structurally characterized polynuclear and monomeric
five- and six-coordinate complexes.2-16 The dominant profile
of aluminum among these examples is illustrated by the fact
that the first structurally characterized five- and six-coordinate
organometallic complexes of indium have been only very
recently reported.7,8
While the steric and electronic effects that dictate the
formation of transition metal dinuclear species have been heavily
investigated, similar investigations of the post-transition metals
are lacking.16 Bidentate, three atom bridging ligands such as
RNXNR- (X ) CR′, CH, or N) are among the species that
have been commonly employed in these investigations and have
been important in preparing a wide variety of dinuclear transition
metal complexes.17 Modification of the organic substituents on
these ligands should allow tuning of their electronic properties
and steric bulk.
Reaction of an ethereal suspension of InCl3 with 2 equiv of
Li(CyNCHNCy)18 (Cy ) cyclohexyl) afforded a new species
(1) in 74% yield after crystallization from diethyl ether. Both
1H and 13C NMR are consistent with the presence of one
environment for the formamidinate ligands as clearly indicated
by the single resonances for the methyne proton (7.45 ppm)
and carbon (162.1 ppm).19 Elemental analysis confirmed the
formula of this species as [In(CyNC(H)NCy)2Cl]n. On the basis
of the ready solubility of 1 in organic solvents, the comparable
ionic radius of In(III) to the transition metals, and the ability of
indium to accommodate 5- or 6-fold coordination, we anticipated
that this complex was a dinuclear species. In order to confirm
the level of aggregation for this species, the structure of 1 was
determined by single-crystal X-ray diffraction analysis, and the
result is shown in Scheme 1.20
Complex 1 crystallizes in the cubic space group Pn3hn with
six molecules in the unit cell. The molecular geometry and
atom-numbering scheme are shown in Scheme 1. The molecule
exhibits a nonbonded binuclear In-In unit (In1-In1A )
3.142(5) Å) which is bridged by four formamidinate ligands.
The In centers are in an unusual square-based pyramidal, five-
coordinate environment consisting of the four formamidinate
N atoms with a terminal chloride occupying the apical position
and completing the coordination sphere [In1a-In1-Cl1, 180.0°;
In1a-In1-N1, 78.0(4)°; Cl1-In1-N1, 102.0(4)°; N1-In1-
N1b, 156.1(6)°; N1-In1-N1c, 87.5(6)°; In1-N1-C1,
124.1(13)°; In1-N1-C2, 116.8(11)°].21 The molecule pos-
sesses a rigorous C4 axis coincident with the In-In vector as
well as 2-fold axes perpendicular to the higher order rotation
axis. The obvious twist in the five-membered In1-N1-C1-
N1A-In1A rings is likely an effort to reduce the steric
interactions between the bulky cyclohexyl substituents.
The lantern-type structure of 1 is, to our knowledge, unique
among the group 13 elements. In addition, the solid state
structure of 1 contrasts with that reported for the analogous
V(III) dimer, [V(CyNC(H)NCy)2(µ-Cl)]2.17 This species ex-
hibits one formamidinate ligand spanning the dinuclear core,
bridging Cl atoms, and one terminal formamidinate ligands
occupying a nonbridging chelating position with an overall
octahedral coordination of the V atoms.
With these facts in mind, we have initiated a systematic study
of the complexes of formamidinate and related ligand systems
with post transition metals. Herein we report, as part of our
ongoing investigation, the synthesis and characterization of a
novel family of dimeric InIII compounds of the general formula
[In(CyNC(H)NCy)nR3-n]2 (n ) 2, 1; R ) Cl, Me).
(1) For a summary of Ga and In organometallic chemistry see: Tuck, D.
G. In ComprehensiVe Organometallic Chemistry; Wilkinson, G., Stone,
F. G. A., Abel, E. W., Eds.; Pergamon Press: Oxford, England, 1982;
Vol. 1, Chapter 7.
(2) Robinson, G. H.; Sangokoya, S. A. J. Am. Chem. Soc. 1987, 109,
6852.
(3) Healy, M. D.; Barron, A. R. J. Am. Chem. Soc. 1989, 111, 398.
(4) Leman, J. T.; Barron, A. R. Organometallics 1989, 8, 1828.
(5) Self, M. F.; Pennington, W. T.; Laske, J. A.; Robinson, G. H.
Organometallics 1991, 10, 36.
(6) Lee, B.; Pennington, W. T.; Robinson, G. H. Organometallics 1990,
9, 1709.
(7) Zhou, Y.; Richeson, D. S. Organometallics 1995, 14, 3558.
(8) Leman, J. T.; Roman, H. A.; Barron, A. R. Organometallics 1993,
12, 2986.
(9) (a) Schumann, H.; Seuss, T. D.; Just, O.; Weiman, R.; Hemling, H.;
Go¨rlitz, F. H. J. Organomet. Chem. 1994, 479, 171. (b) Schumann,
H.; Go¨rlitz, F. H.; Seuss, T. D.; Wassermann, W. Chem. Ber. 1992,
125, 3. (c) Schumann, H.; Wassermann, W.; Dietrich, A. J. Orga-
nomet. Chem. 1989, 365, 11. (d) Schumann, H.; Hartmann, U.;
Wassermann, W. Polyhedron 1990, 9, 353. (e) Schumann, H.;
Hartmann, U.; Wassermann, W. Chem. Ber. 1991, 124, 1567.
(10) Reger, D. L.; Knox, S. J.; Rheingold, A. L.; Haggerty, B. S.
Organometallics 1990, 9, 2581.
(11) Reger, D. L.; Mason, S. S.; Reger, L. B.; Rheingold, A. L.; Ostrander,
R. L. Inorg. Chem. 1994, 33, 1803.
(12) Reger, D. L.; Mason, S. S.; Rheingold, A. L.; Ostrander, R. L. Inorg.
Chem. 1994, 33, 1811.
(18) The lithium salt, LiCyNCHNCy (Cy: cyclohexyl), was prepared by
the stoichiometric reaction of CyNHCHNCy with MeLi in Et2O.
CyNHCHNCy was prepared according to the published procedure:
Saeed, A. H.; Selman, A. S. J. Spectrosc. 1982, 27, 123.
(19) In a typical experiment, a Schlenk flask was charged with InCl3 (0.40
g, 1.81 mmol), LiCyNCHNCy (0.77 g, 3.62 mmol), and diethyl ether
(25 mL). Removal of solvent and crystallization in ether yielded 0.76
g (74%, 0.67 mmol) of 1. Spectroscopic data for 1. IR (Nujol, cm-1):
1622 (s), 1581 (s). 1H NMR (C6D6, ppm): 7.45 (s, NCHN, 4H);
3.25-3.10 (m, Cy, 8H); 2.1-1.2 (m, Cy, 80H). 13C NMR (C6D6,
ppm): 162.1 (s, NCHN); 57.4, 36.6, 26.0, 25.8 (4s, Cy). MP
(sealed): 157-159 °C. Anal. Calcd for C52H92Cl2In2N8: C, 55.28;
H, 8.21; N, 9.92. Found: C, 54.87; H, 8.37; N, 9.86.
(13) Leman, J. T.; Roman, H. A.; Barron, A. R. J. Chem. Soc., Dalton
Trans. 1992, 2183.
(20) Crystal data: Pn3hn (cubic), a ) 22.678(7) Å, volume ) 11663(4)
Å3, fw ) 1129.90, Z ) 6, F(000) ) 3552, Dcalc ) 0.979Mg.m-3, no.
of unique reflections ) 1729, RF ) 0.067, and Rw ) 0.086.
(21) References 6, 8, 9, 14, and 15 present crystal structures of five-
coordinate gallium and indium complexes in distorted trigonal
bipyramidal coordination environments. The values for the angle along
the pseudoaxial vectors in these reports fall in the range of 144.9-
169.7°. In all cases the methyl group is in the equatorial plane.
(14) Rettig, S. J.; Storr, A.; Trotter, J. Can. J. Chem. 1976, 54, 1278.
(15) Rettig, S. J.; Storr, A.; Trotter, J. Can. J. Chem. 1975, 53, 58.
(16) For a review, see: Cotton, F. A.; Walton, R. A. Multiple Bonds
Between Metal Atoms, 2nd ed.; J. Wiley & Sons: New York, 1982.
(17) Berno, P.; Hao, S.; Minhas, R.; Gambarotta, S. J. Am. Chem. Soc.
1994, 116, 7417 and references therein.
0020-1669/96/1335-1423$12.00/0 © 1996 American Chemical Society

1424 Inorganic Chemistry, Vol. 35, No. 6, 1996
Communications
Scheme 1
Due to the uniqueness of 4, direct comparison of the In-N
[2.226(14) Å] and In-Cl [2.383(9) Å] distances with other
Scheme 2
structurally characterized species is not straightforward.
A
majority of the reported five-coordinate In complexes have
relatively long range intramolecularly coordinated nitrogen
centers.9a-c In these examples the average In-N distance of
2.55 Å is considerably longer than the sum of the covalent radii
(2.19 Å) and than the In-N distance observed for 1. The
reported diphenyltriazenido [(C6H5N)2N] complexes, while
possessing only nonbridging, chelating ligands, have In-N bond
lengths that span the value found for 1.8 The In-Cl distance
of 1 is, as expected, slightly shorter than for some recently
reported six-coordinate complexes.11,12,13
Complex 1 can be synthesized independently by the interac-
tion of (CH3)2InCl with 2 equiv of free formamidine and
elimination of two molecules of methane (Scheme 1). Interest-
ingly, compound 1 does not react directly with Me4Sn or MeLi,
a feature likely due to the steric congestion caused by the four
cyclohexyl substituents. However, the methyl analog [In-
(CyNC(H)NCy)2(CH3)]2, 2, can be prepared by the reaction of
In(CH3)3 with 2 equiv of CyN(H)C(H)NCy and elimination of
methane.22
The stepwise formation of 2 was confirmed by the series of
reactions shown in Scheme 2. The reaction of In(CH3)3 with 1
equiv of CyN(H)C(H)NCy proceeds to yield an intermediate
cyclic species [Me2In(CyNC(H)NCy)]2 (3).23 We propose the
connectivity shown in Scheme 1 based upon the spectroscopic
data and by analogy with the reported tetrahedrally coordinated
N,N′-dimethylacetamidinate complexes of aluminum and gal-
lium, [Me2M(MeNC(Me)NMe)]2 (M ) Al, Ga).24,25 These
species exhibited centrosymmetric puckered eight-membered
ring structures. The downfield shift of the methyl protons (0.17
ppm) relative to In(CH3)3 (-0.18 ppm) is consistent with a four-
coordinate structure of 3 but does not rule out another highly
symmetrical structure.26 Support for our formulation of complex
3 is provided by its facile conversion to 2, in excellent yield
(85%), by reaction with a second molecule of formamidine and
elimination of methane.
features. In addition, the presence of a single methyl carbon
signal at -6.1 ppm is consistent with the reported five-
coordinate diphenyltriazenido-In species.8 The chemical shifts
for the methyne proton (7.67 ppm) and carbon (161.2 ppm) are
also in accord with a structure similar to that of 1.
The use of the dicyclohexylformamidinate ligand leads to a
family of unusual dinuclear indium complexes that are unique
among the elements of group 13. Without the possibility of
In-In interactions, it would appear that steric features are the
primary cause for the formation of these compounds. We are
currently expanding our work to investigate the replacement of
both the N-R and C-H groups in the formamidinate ligand
system with the goal of dissecting the features that influence
the formation of dinuclear main group organometallic com-
plexes.
We feel confident in assigning an analogous dimeric structure
to 2 based upon comparison with 1 and 3 and its spectral
(22) Spectroscopic data for 2. IR (Nujol, cm-1): 1571 (s). 1H NMR (C6D6,
ppm): 7.67 (s, NCHN, 4H); 3.15-2.96 (m, Cy, 8H); 1.95-1.15 (m,
Cy, 80H), 0.31 (s, Me, 6H). 13C NMR (C6D6, ppm): 161.2 (NCHN);
58.0, 37.1, 26.7, 26.0, (8s, Cy); -6.1 (s, Me). MP (sealed): 93-95
°C. Anal. Calcd for C54H98In2N8: C, 59.56; N, 9.07; N, 10.29.
Found: C, 59.26; H, 9.24; N, 10.33.
(23) Spectroscopic data for 3. IR (Nujol, cm-1): 1562. 1H NMR (C6D6,
ppm): 7.68 (s, NCHN, 2H); 2.97-2.83 (m, Cy, 4H); 1.75-1.00 (m,
Cy, 40H); 0.17 (s, Me, 12H). 13C NMR (C6D6, ppm): 162.4 (s,
NCHN); 57.4, 37.0, 26.0, 25.3 (4s, Cy); -5.3 (Me). Mp (sealed):
97-98 °C. Anal. Calcd for C30H58In2N4: C, 51.15; H, 8.30; N, 7.95.
Found: C, 51.33; H, 8.31; N, 7.94.
Acknowledgment. This work was supported by the Natural
Sciences and Engineering Research Council of Canada.
Supporting Information Available: Text providing experimental
details and a description of the structural solution, tables of atomic
positions, thermal parameters, crystallographic data, and bond distances
and angles, and an ORTEP drawing for compound 1 (10 pages).
Ordering information is given on any current masthead page.
(24) Gerstner, F.; Weidlein, J. Z. Naturforsch. 1978, 33b, 24.
(25) Hausen, H. D.; Gerstner, F.; Schwarz, W. J. Organomet. Chem. 1978,
145, 277.
(26) Bradley, D. C.; Dawes, H.; Frigo, D. M.; Hursthouse, M. B.; Hussain,
B. J. Organomet. Chem. 1987, 325, 55.
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