Eclipsed M2X6 compounds exhibiting very short metal-metal triple bonds

Article abstract of DOI:10.1021/ic049795r

The preparation, characterization, and electronic structure of homoleptic complexes of molybdenum and tungsten bridged by bis(alkylamido)phenylboranes, M₂[RN-B^Ph-NR]₃ (M = Mo, R = Et (1), Pr (2); M = W, R = Et (3), Pr (4)), are described. These triple metal-metal bond species (i) exhibit a nearly eclipsed ligand geometry and (ii) possess the shortest metal-metal bonds of neutral dimolybdenum and ditungsten M₂X ₆ complexes observed to date (d(Mo-Mo) = 2.1612(6) A (1); d(W-W) = 2.2351(7) A (4)).

Full text of DOI:10.1021/ic049795r

Inorg. Chem. 2004, 43, 3618−3624
Eclipsed M X Compounds Exhibiting Very Short Metal−Metal Triple
2 6
Bonds
David R. Manke, Zhi-Heng Loh, and Daniel G. Nocera*
Department of Chemistry, 6-335, Massachusetts Institute of Technology, 77 Massachusetts AVenue,
Cambridge, Massachusetts 02139-4307
Received February 16, 2004
The preparation, characterization, and electronic structure of homoleptic complexes of molybdenum and tungsten
bridged by bis(alkylamido)phenylboranes, M [RN−BPh−NR]3 (M ) Mo, R ) Et (1), iPr (2); M ) W, R ) Et (3), iPr
2
(4)), are described. These triple metal−metal bond species (i) exhibit a nearly eclipsed ligand geometry and (ii)
possess the shortest metal−metal bonds of neutral dimolybdenum and ditungsten M X complexes observed to
2
6
date (d(Mo−Mo) ) 2.1612(6) Å (1); d(W−W) ) 2.2351(7) Å (4)).
Introduction
bond.7-12 Chisholm and Armstrong have synthesized several
d3-d3 bimetallic compounds bridged by four-atom dianionic
ligands; these compounds deviate from an eclipsed geometry
by 10.7-13.9°.7-11 In one case, a triple metal-metal bond
species has been prepared with a bidentate ligand possessing
a three-atom bridge;12 this disiloxide-ligated complex exhibits
a variance of only 3° from an eclipsed conformation.
We now report a new class of triple metal-metal bond
complexes in which group 6 metals are spanned by bis-
(alkylamido)phenylboranes, PhB(RN)22-. The three-atom
bridge of this dianionic framework is structurally similar to
the widely used monoanionic amidinate ligands13-17 with the
additional feature that an electron-accepting group is situated
at the bridgehead. Despite its η2-hapticity, the bis(alkylami-
Binuclear complexes comprising d3 metal centers are
characterized by metal-metal distances of 2.2-2.4 Å and a
M2X6 coordination geometry in which the ligands assume
an ethane-like staggered conformation.1 The short metal-
metal distance has an electronic basis that finds its origins
in a bond order of three, resulting from a σ2π4 ground-state
configuration. To a first approximation, this configuration
precludes an electronic basis for the preferred ligand geom-
2
etry. One σ and two π bonds composed of pure dz and (dxz,
dyz) orbitals, respectively, form a cylindrically symmetric
triple bond that exhibits an unrestricted barrier to rotation.
The staggered conformation is presumed to arise from the
reduced steric clashing of ligands across the short metal-
metal bond. One bonding model modifies this description
by considering the mixing of higher energy orbitals into those
of the dσ and dπ sets;2 for this case, rehybridization results
in greater orbital overlap between ML3 fragments in an
eclipsed ligand conformation. The preference for an eclipsed
configuration vanishes when configuration interaction is
included in higher order calculations.3-5 Nevertheless, steric
factors enforce a staggered conformation in M2X6 complexes
with monodentate ligands.6 An eclipsed geometry is only
approached when bidentate ligands span the metal-metal
(6) Cotton, F. A.; Walton, R. A. Multiple Bonds between Metal Atoms,
2nd ed.; Clarendon Press: Oxford, 1993.
(7) Blatchford, T. P.; Chisholm, M. H.; Folting, K.; Huffman, J. C. Inorg.
Chem. 1980, 19, 3175-3176.
(8) Gilbert, T. M.; Bauer, C. B.; Bond, A. H.; Rogers, R. D. Polyhedron
1999, 18, 1293-1301.
(9) Blatchford, T. P.; Chisholm, M. H.; Huffman, J. C. Inorg. Chem. 1987,
26, 1920-1925.
(10) Armstrong, W. H.; Bonitatebus, P. J., Jr. Z. Kristallogr.sNew Cryst.
Struct. 1999, 214, 241-242.
(11) Chisholm, M. H.; Folting, K.; Hampden-Smith, M.; Smith, C. A.
Polyhedron 1987, 6, 1747-1755.
(12) Su, K.; Tilley, T. D. Chem. Mater. 1997, 9, 588-595.
(13) Keaton, R. J.; Jayaratne, K. C.; Henningsen, D. A.; Koterwas, L. A.;
Sita, L. R. J. Am. Chem. Soc. 2001, 123, 6197-6198.
(14) Littke, A.; Sleiman, N.; Bensimon, C.; Richeson, D. S. Organometallics
1998, 17, 446-451.
* To whom correspondence should be addressed. E-mail: nocera@mit.edu.
(1) Chisholm, M. H. Acc. Chem. Res. 1990, 23, 419-425.
(2) Albright, T. A.; Hoffman, R. J. Am. Chem. Soc. 1978, 100, 7736-
7737.
(3) Hall, M. B. J. Am. Chem. Soc. 1980, 102, 2104-2106.
(4) Cotton, F. A.; Stanley, G. G.; Kalbacher, B. J.; Green, J. C.; Seddon,
E.; Chisholm, M. H. Proc. Natl. Acad. Sci. U.S.A. 1977, 74, 3109-
3113.
(15) Chen, C.-T.; Rees, L.; Cowley, A. R.; Green, M. L. H. J. Chem. Soc.,
Dalton Trans. 2001, 1761-1767.
(16) Cotton, F. A.; Matusz, M.; Poli, R.; Feng, X. J. Am. Chem. Soc. 1988,
110, 1144-1154.
(17) Chisholm, M. H.; Cotton, F. A.; Daniels, L. M.; Folting, K.; Huffman,
J. C.; Iyer, S. S.; Lin, C.; Macintosh, A. M.; Murillo, C. A. J. Chem.
Soc., Dalton Trans. 1999, 1387-1391.
(5) Bursten, B. E.; Cotton, F. A.; Green, J. C.; Seddon, E. A.; Stanley, G.
G. J. Am. Chem. Soc. 1980, 102, 4579-4588.
3618 Inorganic Chemistry, Vol. 43, No. 12, 2004
10.1021/ic049795r CCC: $27.50 © 2004 American Chemical Society
Published on Web 05/20/2004

Eclipsed M2X6 Compounds
(300 MHz, C6D6, 25 °C) δ 1.000 (br s, 6H), 1.132 (t, 6H, J )
7.000 Hz), 3.021 (q, 4H, J ) 6.966 Hz), 3.273 (q, 4H, J ) 7.050
Hz), 7.2-7.5 (m, 5H). 11B NMR (96.205 MHz, C6D6, 25 °C) δ
30.088. Anal. Calcd for C14H25BLi2N2O: C, 64.17; H, 9.62; N,
10.69. Found: C, 63.94; H, 9.56; N, 10.78.
do)phenylborane ligands usually chelate a single center when
interacting with main group elements.18-23 Only recently has
the coordination chemistry of the ligand been expanded to
the transition metals, but here again only for monomeric
complexes.23-25 There are only a handful of cases in which
the ligand binds two metal centers,21,23-27 and only one
example involving a transition metal.28 In this instance, the
ligand assumes a µ-η2,η2 coordination mode with both ni-
trogens bridging two titanium metal centers. For the triply
bonded dimolybdenum and ditungsten complexes described
here, the bis(alkylamido)phenylborane ligand is µ-η1,η1. The
complexes possess nearly eclipsed ligand geometries, and
they are further distinguished by the shortest metal-metal
distances yet observed for a neutral M2X6 triple metal-metal
bond species. The consequences of the eclipsed geometry
and short metal-metal bond on the electronic structure and
reactivity of the compounds are discussed.
1
PhB(iPrNLi)2. H NMR (300 MHz, C6D6, 25 °C) δ 0.972 (d,
6H, J ) 6.700 Hz), 1.097 (d, 6H, J ) 7.000 Hz), 3.555 (hept, 2H,
J ) 6.275 Hz), 7.2-7.5 (m, 5H). 11B NMR (96.205 MHz, C6D6,
25 °C) δ 29.580.
Mo2[RN-BPh-NR]3, R ) Et (1) and R ) iPr (2). MoCl3(dme)
(100 mg) was suspended in 7 mL of toluene, and the mixture was
frozen. Lithiated diamide was dissolved in toluene in a separate
container; (PhB(EtNLi)2‚OEt2 (134 mg) addition gave a suspension,
and PhB(iPrNLi)2 (111 mg) addition gave a solution. The desired
lithiated diamide solution/suspension was added dropwise over 3
min to the partially thawed MoCl3(dme) suspension. The resulting
mixture was allowed to slowly warm to room temperature. After
stirring overnight, solvent was removed by vacuum evaporation,
and 10 mL of pentane was added. The solution was filtered through
Celite to remove LiCl. Concentration of the solution followed by
cooling to -35 °C and finally filtration afforded 28 mg of tan
crystals (23.4% crystal yield) of 1 and 33 mg of tan crystals (24.3%
crystal yield) of 2. Crystals of 1 contained cocrystallized n-hexane;
the presence of solvent caused a variable elemental analysis.
Analytical data for 1 follow. 1H NMR (300 MHz, C6D6, 25 °C) δ
0.882 (t, 18H, J ) 7.171), 3.388 (q, 12H, J ) 7.171), 7.238 (t, 3H,
J ) 7.150), 7.338 (t, 6H, J ) 7.200), 7.838 (dd, 6H, J ) 7.900,
1.300). 11B NMR (96.205 MHz, C6D6, 25 °C) δ 50.628. Anal. Calcd
for C36H59B3N6Mo2: C, 54.03; H, 7.43; N, 10.50. Found: C, 54.98;
H, 7.44; N, 10.11. Analytical data for 2 follow. 1H NMR (300 MHz,
C6D6, 25 °C) δ 1.071 (d, 36H, J ) 6.681), 3.797 (sept, 6H, J )
6.554), 7.257 (t, 3H, J ) 7.415), 7.377 (t, 6H, J ) 7.577), 7.838
(dd, 6H, J ) 6.903, 1.385). 11B NMR (96.205 MHz, C6D6, 25 °C)
δ 49.581. Anal. Calcd for C36H57B3N6Mo2: C, 54.17; H, 7.20; N,
10.53. Found: C, 54.22; H, 7.09; N, 10.64.
Experimental Section
General Procedures. All manipulations were carried out using
modified Schlenk techniques under an atmosphere of N2 or in a
Vacuum Atmosphere HE-553-2 glovebox. Solvents for synthesis
were of reagent grade or better and were dried according to standard
methods.29 Trichloro(dimethoxyethane)molybdenum(III),30 bis(eth-
ylamino)phenylborane,31 sodium heptachloropentakis(tetrahydrofu-
ran)ditungstate,32 and bis(isopropylamino)phenylborane were pre-
pared by literature methods. All other materials were used as re-
ceived. N,N’-Dilithiobis(isopropylamido)phenylborane has been
prepared previously,19 but additional analytical data for the dilithio
salt is presented here. Elemental analyses were performed at H.
Kolbe Mikroanalytisches Laboratorium.
PhB(EtNLi)2‚OEt2. PhB(EtNH)2 (500 mg) was dissolved in 7
mL of diethyl ether and frozen. Upon melting, the dropwise addition
of 2.2 mL of n-butyllithium (2.8 M in hexanes) to the cold solution
caused a white precipitate to appear after a few minutes. The room
temperature mixture was stirred overnight. The fine white powder
was isolated by filtration and washed with diethyl ether (3 × 5
mL) to yield 682 mg of PhB(EtNLi)2‚OEt2 (92% yield). 1H NMR
W2[RN-BPh-NR]3, R ) Et (3) and R ) iPr (4). Tetrahydro-
furan solutions (7 mL) containing 100 mg of NaW2Cl7(THF)5 were
frozen. A suspension of PhB(EtNLi)2‚OEt2 (79 mg) or a solution
of PhB(iPrNLi)2 of (65 mg) was added dropwise over 3 min to a
partially thawed NaW2Cl7(THF)5 solution. The mixture was allowed
to slowly warm to room temperature. After the mixture stirred
overnight, solvent was removed by vacuum evaporation, and 10
mL of pentane was added. The solution was filtered through Celite
to remove LiCl. Concentration of the solution followed by cooling
to -35 °C and finally filtration afforded 23 mg of tan crystals
(25.8% crystal yield) of 3 and 22 mg of tan crystals (22.7% crystal
yield) of 4. Analytical data for 3 follow. 1H NMR (300 MHz, C6D6,
25 °C) δ 0.969 (t, 18H, J ) 7.000 Hz), 3.379 (q, 12H, J ) 7.149
Hz), 7.2-7.4 (m, 9H), 7.604 (dd, 6H, J ) 7.745, 1.192). 11B
NMR (96.205 MHz, C6D6, 25 °C) δ 51.937. Anal. Calcd for
C30H45B3N6W2: C, 40.49; H, 5.10; N, 9.44. Found: C, 40.63; H,
(18) Fest, D.; Habben, C. D.; Meller, A.; Sheldrick, G. M.; Stalke, D.;
Pauer, F. Chem. Ber. 1990, 123, 703-706.
(19) Habben, C. D.; Heine, A.; Sheldrick, G. M.; Stalke, D. Z. Naturforsch.
1992, 47b, 1367-1369.
(20) Koch, H.-J.; Roesky, H. W.; Besser, S.; Herbst-Irmer, R. Chem. Ber.
1993, 126, 571-574.
(21) Geschwentner, M.; Noltemeyer, M.; Elter, G.; Meller, A. Z. Anorg.
Allg. Chem. 1994, 620, 1403-1408.
(22) Chivers, T.; Gao, X.; Parvez, M. Angew. Chem., Int. Ed. Engl. 1995,
34, 2549-2551.
(23) Albrecht, T.; Elter, G.; Noltemeyer, M.; Meller, A. Z. Anorg. Allg.
Chem. 1998, 624, 1514-1518.
(24) Manke, D. R.; Nocera, D. G. Inorg. Chem. 2003, 42, 4431-4436.
(25) Manke, D. R.; Nocera, D. G. Inorg. Chim. Acta 2003, 345, 235-240.
(26) Heine, A.; Fest, D.; Stalke, D.; Habben, C. D.; Meller, A.; Sheldrick,
G. M. J. Chem. Soc., Chem. Commun. 1990, 742-743.
(27) Chivers, T.; Fedorchuk, C.; Schatte, G.; Parvez, M. Inorg. Chem. 2003,
42, 2084-2093.
1
5.21; N, 9.36. Analytical data for 4 follow. H NMR (300 MHz,
C6D6, 25 °C) δ 1.145 (d, 36H, J ) 6.681 Hz), 3.892 (sept, 6H, J
) 6.518 Hz), 7.265 (t, 3H, J ) 7.415 Hz), 7.383 (t, 6H, J ) 7.252),
7.858 (dd, 6H, J ) 7.985 Hz, 1.466 Hz). 11B NMR (96.205 MHz,
C6D6, 25 °C) δ 51.500. Anal. Calcd for C36H54B3N6W2: C, 44.39;
H, 5.90; N, 8.63. Found: C, 44.30; H, 5.76; N, 8.53.
Physical Methods. 1H NMR spectra were recorded on solutions
at 25 °C within the magnetic fields of a Varian Unity 300 or
Mercury 300 spectrometers, which were located in the Department
of Chemistry Instrumentation Facility (DCIF) at MIT. Chemical
shifts are reported using the standard δ notation in ppm. 1H spectra
were referenced to residual solvent peak. 11B{1H} NMR spectra
(28) Koch, H.-J.; Roesky, H. W.; Bohra, R.; Noltemeyer, M.; Schmidt,
H.-G. Angew. Chem., Int. Ed. Engl. 1992, 31, 598-599.
(29) Armarego, W. L. F.; Perrin, D. D. Purification of Laboratory
Chemicals, 4th ed.; Butterworth-Heinmann: Oxford, 1996.
(30) Gilbert, T. M.; Landes, A. M.; Rogers, R. D. Inorg. Chem. 1992, 31,
3438-3444.
(31) Burch, J. E.; Gerrard, W.; Mooney, E. F. J. Chem. Soc. 1962, 2200-
2203.
(32) Chisholm, M. H.; Eichhorn, B. W.; Folting, K.; Huffman, J. C.;
Ontiveros, C. D.; Streib, W. E.; Van der Sluys, W. G. Inorg. Chem.
1987, 26, 3182-3186.
Inorganic Chemistry, Vol. 43, No. 12, 2004 3619

Manke et al.
Table 1. Crystallographic Data for Mo2[EtN-BPh-NEt]3 (1) and
W2[iPrN-BPh-NiPr]3 (4)
one set of polarization functions. B, N, Mo, and W were described
by a Slater-type orbital triple-ê basis set augmented by two sets of
polarization functions. Non-hydrogen atoms were assigned a
relativistic frozen core potential, with the shells up to and including
3d for Mo, 4d for W, and 1s for B, C, and N treated as core.
1
4
empirical formula
fw
C36H59B3Mo2N6
800.2
C36H57B3N6W2
974.01
T (K)
183(2)
193(2)
λ (Å)
space group
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
0.71073 Å
P21/n
19.7976(14)
9.4329(6)
23.6392(16)
90
114.356(1)
90
4021.7(5)
4
0.71073 Å
C2/c
21.0389(13)
16.3854(10)
14.2619(9)
90
126.897(1)
90
3931.8(4)
4
Results and Discussion
Synthesis. The dilithio salts of bis(alkylamido)phenyl-
boranes provide a useful reagent for the transfer of the bis-
amido ligands to metal centers. The dilithio salts are prepared
from the reaction of 2 equiv of n-BuLi with the correspond-
ing diamine in greater than 90% yield. The ethyl derivative
1
is isolated as the diethyl ether adduct. The H NMR dem-
d
calcd (Mg/m3)
1.322
0.655
0.0457
0.1222
1.645
5.879
0.0427
0.1187
onstrates two sets of ethyl peaks, one for the coordinated
ether and one for the amido ethyl groups, that integrate
equally. This NMR result is consistent with the elemental
analysis, which indicates a single ether molecule per mole-
cule of the dilithio salt. One set of ethyl resonances is well
behaved, while the second exhibits a broad singlet for the
methyl resonance. The isopropyl derivative is isolated solvent
free and displays two isopropyl methyl resonances and a
single methylene heptet in the 1H NMR. A single resonance
is observed in the 11B NMR spectra of each of the salts.
Metathesis reactions between the dilithio salts of bis-
(alkylamido)phenylboranes with monomeric metal halides
have proven effective in affording monomeric transition
metal compounds of the diamido ligands.24,25 The reaction
was employed here using Li2[RN-BPh-NR] (R ) Et,
iPr) with bimetallic metal halide starting materials
“MoCl3(dme)” or NaW2Cl7(THF)5 to give tan and crystalline
M2[RN-BPh-NR]3 compounds (M ) Mo, R ) Et (1) and
abs coeff (mm-1
)
R1a
wR2a
a R1 ) ∑||Fo| - |Fc||/∑|Fo|; wR2 ) [∑w(Fo - Fc )2/∑w(Fo )2]1/2
.
2
2
2
were collected at the DCIF on a Varian Unity 300 spectrometer
and referenced to an external BF3‚OEt2 standard at 0 ppm. UV-
vis absorption spectra were collected on an Aviv 14DS spectro-
photometer.
X-ray diffraction experiments were performed on single crystals
grown from concentrated pentane or hexanes solutions at -35 °C.
Crystals were removed from the supernatant liquid and transferred
onto a microscope slide coated with Paratone N oil, and one was
affixed to a glass fiber, coated in Paratone N oil, and cooled to
-90 °C. Data collection was performed using the Mo KR (λ )
0.71073 Å) radiation of a Bruker CCD diffractometer. The data
were processed and refined by using the program SAINT supplied
by Siemens Industrial Automation, Inc. The structures were solved
by direct methods (SHELXTL v6.10, Sheldrick, G. M., and Siemens
Industrial Automation, Inc., 2000) in conjunction with standard
difference Fourier techniques. All non-hydrogen atoms were refined
anisotropically unless otherwise noted. Hydrogen atoms were placed
in calculated positions. Two ethyl groups (C5 and C6, C7 and C8)
in 1 were disordered over two sites. The disorder was modeled
and refined to partial occupancy factors of 0.720230 for C5 and
C6 (0.27977 for C5A and C6A) and 0.583690 for C7 and C8
(0.41631 for C7A and C8A). A n-hexane molecule was present in
the asymmetric unit and was refined isotropically. DFIX instructions
were used to set the distances in the disordered ethyl groups to
those observed in the well-ordered groups, and to restrain the
isotropically refined solvent molecule. Details regarding the refined
data and cell parameters are provided in Table 1.
Computational Methods. Density functional theory (DFT)
calculations were carried out at the local density approximation
(LDA) level of theory using the Amsterdam Density Functional
program.33-35 The calculations were performed on a home-built
Linux cluster consisting of 60 processors running in parallel.
Gradient corrections were introduced using the Becke exchange
functional36 (B) and the Lee-Yang-Parr37 (LYP) correlation
functional. Relativistic corrections were included by using the scalar
zero-order regular approximation38-40 (ZORA). C and H were
described by a Slater-type orbital triple-ê basis set augmented by
iPr (2); M ) W, R ) Et (3) and Pr (4)) in yields of
i
1
23-25%. By H NMR, all compounds in solution possess
D3h symmetry, with only single sets of ethyl and phenyl peaks
observed for 1 and 3 and single sets of isopropyl and phenyl
peaks observed for 2 and 4; compounds 1-4 exhibit a single
resonance in 11B NMR spectra.
Structural Analysis. Crystals of 1 and 4 were analyzed
by X-ray diffraction analysis. Both compounds possess three
bis(alkylamido)phenylborane ligands spanning a metal-
metal triple bond. Tables 2 and 3 list selected bond lengths
and angles for compounds 1 and 4, respectively. The solid-
state structures exhibit pseudo-D3h symmetry (Figure 1),
which is consistent with the single set of alkyl resonances
1
in the H NMR spectra. These structures resemble the D3h
lantern structures reported by Cotton and co-workers for
dimers of lesser bond order.41-43 The nearly eclipsed ligand
(36) Becke, A. D. Phys. ReV. A 1988, 38, 3098-3100.
(37) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785-789.
(38) Van Lenthe, E.; Baerends, E. J.; Snijders, J. G. J. Chem. Phys. 1993,
99, 4597-4610.
(39) Van Lenthe, E.; Baerends, E. J.; Snijders, J. G. J. Chem. Phys. 1994,
101, 9783-9792.
(40) Van Lenthe, E.; Ehlers, A.; Baerends, E. J. J. Chem. Phys. 1999, 110,
8943-8953.
(33) ADF2002.01; SCM, Theoretical Chemistry, Vrije Universiteit: Am-
sterdam, The Netherlands, 2002.
(41) Cotton, F. A.; Daniels, L. M.; Falvello, L. R.; Murillo, C. A. Inorg.
Chim. Acta 1994, 219, 7-10.
(34) Fonseca Guerra, C.; Snijders, J. G.; Te Velde, G.; Baerends, E. J.
Theor. Chem. Acc. 1998, 99, 391-403.
(35) Te Velde, G.; Bickelhaupt, F. M.; Baerends, E. J.; Fonseca Guerra,
C.; Van Gisbergen, S. J. A.; Snijders, J. G.; Ziegler, T. J. Comput.
Chem. 2001, 22, 931-967.
(42) Cotton, F. A.; Daniels, L. M.; Maloney, D. J.; Murillo, C. A. Inorg.
Chim. Acta 1996, 249, 9-11.
(43) Cotton, F. A.; Daniels, L. M.; Falvello, L. R.; Matonic, J. H.; Murillo,
C. A. Inorg. Chim. Acta 1997, 256, 269-275.
3620 Inorganic Chemistry, Vol. 43, No. 12, 2004

Eclipsed M2X6 Compounds
Table 2. Selected Bond Lengths (Å) and Angles (deg) for
Mo2[EtN-BPh-NEt]3 (1)
The steric constraints imposed by the three-atom N-B-N
bridge appear to be the determinant factor in the short metal-
metal bond lengths and small torsional distortion of 1-4.
The only previous d3-d3 molybdenum dimer with a three-
atom bridge, Mo2[O2Si(OtBu)2]3, possesses an average
X-M-M-X torsion angle of 3°, an angle that is more than
twice that observed for 1. The major structural differences
between the two three-atom backbones are highlighted in
Figure 2. The trigonal boron produces an average N-B-N
angle of 117.4°, whereas the tetrahedral silicon produces an
average O-Si-O angle of 105.4°. In conjunction with the
longer Si-O bond, as compared to that of N-B, the average
O‚‚‚O separation of the disiloxide bridged complex is longer
(2.616 Å) than the N‚‚‚N separation in 1 (2.461 Å). The
Bond Lengths
Mo(1)-Mo(2)
Mo(1)-N(2)
Mo(1)-N(4)
Mo(1)-N(6)
Mo(1)-N(1)
Mo(1)-N(3)
Mo(1)-N(5)
2.1612(6)
1.983(4)
1.990(4)
1.975(4)
1.980(4)
2.002(4)
1.975(4)
N(1)-B(1)
N(2)-B(1)
N(3)-B(2)
N(4)-B(2)
N(5)-B(3)
N(6)-B(3)
1.444(7)
1.438(7)
1.429(7)
1.452(7)
1.440(7)
1.437(7)
Bond Angles
Mo(1)-Mo(2)-N(1)
Mo(1)-Mo(2)-N(3)
Mo(1)-Mo(2)-N(4)
Mo(2)-Mo(1)-N(4)
Mo(2)-Mo(1)-N(5)
Mo(2)-Mo(1)-N(6)
N(1)-B(1)-N(2)
94.56(12)
94.26(11)
94.35(12)
94.12(11)
94.57(12)
94.21(11)
117.0(3)
117.9(5)
117.3(5)
119.3(3)
124.0(4)
Mo(1)-N(6)-C(11)
Mo(2)-N(1)-C(1)
Mo(2)-N(3)-C(5)
Mo(2)-N(5)-C(9)
Mo(1)-N(2)-B(1)
Mo(1)-N(4)-B(2)
Mo(1)-N(6)-B(3)
Mo(2)-N(1)-B(1)
Mo(2)-N(3)-B(2)
Mo(2)-N(5)-B(3)
119.8(3)
119.4(3)
125.8(4)
119.6(3)
117.0(3)
116.4(3)
117.0(3)
116.6(3)
116.7(3)
116.9(3)
N(3)-B(2)-N(4)
more constrained five-membered M-N-B-N-M ring
accounts for its greater planarity and shorter M-M bond of
1. The M-M bonds of 1 and 4 are also significantly shorter
than their staggered bis(amide) analogues and than all other
monodentate anionic ligands (Table 4).46-49 The contraction
in M-M distance of 0.05-0.06 Å is greater than that
observed between molybdenum and tungsten compounds
possessing four-atom bridging ligands and their unbridged
counterparts (∆d ) -0.02 Å). It is also noteworthy that the
M-M distance in the bridging disiloxide complex is longer
than its unbridged alkoxide analogues. We attribute this
disparity between 1 and Mo2[O2Si(OtBu)2]3 to the longer
O‚‚‚O separation (owing to the longer d(Si-O)). That both
complexes possess nearly eclipsed torsional conformations
suggests that the short metal-metal bond of 1 is of steric
and not of electronic origin.
N(5)-B(3)-N(6)
Mo(1)-N(2)-C(3)
Mo(1)-N(4)-C(7)
Torsion Angles
N(1)-Mo(2)-Mo(1)-N(2)
N(3)-Mo(2)-Mo(1)-N(4)
N(5)-Mo(2)-Mo(1)-N(6)
0.2 N(3)-Mo(2)-Mo(1)-N(2) 122.0
0.4 N(3)-Mo(2)-Mo(1)-N(6) 119.9
1.3 N(5)-Mo(2)-Mo(1)-N(2) 116.8
N(1)-Mo(2)-Mo(1)-N(4) 121.4 N(5)-Mo(2)-Mo(1)-N(4) 121.6
N(1)-Mo(2)-Mo(1)-N(6) 118.3
Table 3. Selected Bond Lengths (Å) and Angles (deg) of
W2[iPrN-BPh-NiPr]3 (4)
Bond Lengths
W(1)-W(1A)
W(1)-N(1)
W(1)-N(2)
W(1)-N(3)
2.2351(7)
1.984(7)
1.985(7)
1.959(8)
N(1)-B(1A)
N(2)-B(1)
N(3)-B(2)
1.436(13)
1.468(13)
1.456(10)
Bond Angles
N(1)-W(1)-W(1A)
N(2)-W(1)-W(1A)
N(3)-W(1)-W(1A)
W(1)-N(1)-C(1)
W(1)-N(2)-C(4)
W(1)-N(3)-C(7)
93.1(2)
94.2(2)
94.0(2)
124.9(6)
118.2(6)
123.8(6)
W(1)-N(1)-B(1A)
W(1)-N(2)-B(1)
W(1)-N(3)-B(2)
N(1A)-B(1)-N(2)
N(3)-B(2)-N(3A)
114.6(6)
113.0(6)
116.6(7)
118.0(8)
118.8(12)
Electronic Structure. Density functional calculations were
performed on model molybdenum and tungsten compounds,
in which methyl groups substitute for alkyl substituents and
the geometry of a M2[MeN-BPh-NMe]3 is constrained to
D3h symmetry. Agreement between calculated and observed
Torsion Angles
structures suggests that these simplifications are reasonable.
A geometry optimization yielded metal-metal bond dis-
tances that are about 0.1 Å different from the crystallographic
data (d(Mo-Mo) ) 2.17342 Å; d(W-W) ) 2.24728 Å).
N(1)-W(1)-W(1A)-N(2A)
N(3)-W(1)-W(1A)-N(3A)
N(1)-W(1)-W(1A)-N(1A) 116.7
0.4 N(1)-W(1)-W(1A)-N(3A) 120.7
1.6 N(2)-W(1)-W(1A)-N(2A) 116.0
geometry is most directly indicated by the N-M-M-N
torsion angles, which are only 0.2-1.6°. The coordination
geometry of each metal center is nearly trigonal pyramidal,
as demonstrated by M-M-N angles of 93.1(2)-94.57(12)°.
In the bridging mode observed here, metric parameters of
the bis(alkylamido)phenylborane ligands are similar to those
when the ligands chelate a metal center.18,24,25 The N-B
distances of 1.429(7)-1.468(13) Å indicate significant N-B
π-bonding. The only exceptional difference for the ligands
in chelating versus bridging modes is the N-B-N angle,
which increases from 110.4(3)-111.2(4)° to 117.0(3)-
118.8(12)°, respectively.
Figure 3 displays the energy levels and representations of
the frontier Kohn-Sham molecular orbitals. We note that
Kohn-Sham orbitals differ from those derived from a
Hartree-Fock (HF) formalism by the inclusion of the
exchange-correlation energy;50 however, comparative analy-
sis establishes that the shapes and symmetries of the Kohn-
Sham orbitals accord well with those calculated by more
(44) Huq, F.; Mowat, W.; Shortland, A.; Skapski, A. C.; Wilkinson, G. J.
Chem. Soc., Chem. Commun. 1971, 1079-1080.
(45) Chisholm, M. H.; Cotton, F. A.; Extine, M.; Stults, B. R. Inorg. Chem.
1976, 15, 2252-2257.
(46) Chisholm, M. H.; Cotton, F. A.; Frenz, B. A.; Reichert, W. W.; Shive,
L. W.; Stults, B. R. J. Am. Chem. Soc. 1976, 98, 4469-4476.
(47) Cotton, F. A.; Stults, B. R.; Troup, J. M.; Chisholm, M. H.; Extine,
M. J. Am. Chem. Soc. 1975, 97, 1242-1243.
(48) Chisholm, M. H.; Cotton, F. A.; Murillo, C. A.; Reichert, W. W. Inorg.
Chem. 1977, 16, 1801-1808.
As is usually the case for metal-metal dimers, the most
intriguing metric is the metal-metal bond distance. The
values of 2.1612(6) and 2.2351(7) Å for 1 and 4, respectively,
are exceptional. The shortest M-M distances observed for
M2X6 species prior to this study were 2.167 Å (Mo)44 and
2.255 Å (W).45
(49) Chisholm, M. H.; Clark, D. L.; Folting, K.; Huffman, J. C.; Hampden-
Smith, M. J. Am. Chem. Soc. 1987, 109, 7750-7761.
(50) Kohn, W.; Becke, A. D.; Parr, R. G. J. Phys. Chem. 1996, 100, 12974-
12980.
Inorganic Chemistry, Vol. 43, No. 12, 2004 3621

Manke et al.
Figure 1. Solid-state structure of Mo2[EtN-BPh-NEt]3 (1) viewed (a) normal to and (b) along the metal-metal axis with thermal ellipsoids shown at the
30% probability level.
for Mo2(NMe2)6.4,5 Metal-metal character is first observed
in the HOMO - 1 for the molybdenum complex and in
HOMO - 2 for the tungsten complex; for the latter, another
set of nitrogen p based orbitals lies between it and the
HOMO. The LUMO for the molybdenum dimer is δ
bonding, with the Mo-Mo π* orbital lying immediately to
higher energy. For the tungsten case, the δ and π* orbitals
are displaced to higher energy, and a W-W σ* orbital
Figure 2. Geometrical comparison of the two known three-atom
assumes the LUMO position; this σ* orbital for the molyb-
denum complex is LUMO + 2. Large HOMO-LUMO gaps
of 3.1848 and 3.2031 eV are observed for dimolybdenum
and ditungsten complexes, respectively. The dσ and dπ of
ML3 orbitals do not exhibit significant rehybridization, and
no obvious orbital or orbital set of the ML3 fragments appears
to lead to enhanced orbital overlap in an eclipsed ligand
configuration.
The large HOMO-LUMO gap is manifested in the ab-
sorption spectra of the M2[RN-BPh-NR]3 complexes. The
absorption profile of 1 and 2 are dominated by a single,
intense band at 310 nm. A similar feature is observed in the
absorption spectrum of Mo2(NMe2)6, and it has been ascribed
to a LMCT transition. The appearance of the LMCT
bridged d3-d3 dimolybdenum complexes, Mo2[EtN-BPh-NEt]3 (1) and
Mo2[O2Si(OtBu)2]3.
Table 4. Metal-Metal Triple Bond Lengths of Eclipsed and Staggered
M2X6 Complexes
d(Mo≡Mo)/Å
d(W≡W)/Å
a
g
Mo2(NMe2)6
2.211(2)
2.190(1)
2.222(2)
W2(NMe2)6
2.294(1)
2.265(1)
2.315(2)
2.2738(8)
b
h
Mo2(DMEDA)3
Mo2(OCH2CMe3)6
W2(DMEDA)3
W2(OiPr)6
c
i
d
j
Mo2(OCMe2CMe2O)3
2.1942(6) W2(OCMe2CMe2O)3
Mo2(O2Si(OtBu)2)3
2.240(1)
2.167
e
f
k
Mo2(CH2SiMe3)6
W2(CH2SiMe3)6
2.255(2)
Mo2[EtN-BPh-NEt]3
2.1612(6) W2[iPrN-BPh-NiPr]3 2.2351(7)
a Reference 46. b Reference 7. c Reference 48. d Reference 8. e Reference
12. f Reference 44. g Reference 47. h Reference 9. i Reference 49. j Refer-
ence 11. k Reference 45.
transition of Mo2(NMe2)6 to slightly lower energy (λmax
)
traditional HF and extended Hu¨ckel approximations,51,52 and
indeed, these orbitals have found widespread use in electronic
structure descriptions.53-59 In both complexes, the HOMO
is a nitrogen p based orbital, similar to the result obtained
325 nm) is consistent with the longer Mo-Mo bond of the
unbridged dimer.60 The allowed metal-based π f π*
transition of the molybdenum dimer is believed to be masked
by the LMCT absorption. The LMCT transition shifts to
significantly higher energy for ditungsten complexes. In
W2(NMe2)6, the LMCT shifts to λmax ) 282, revealing the
π f π* transition as a shoulder (λmax ) 360 nm) on the
(51) Stowasser, R.; Hoffmann, R. J. Am. Chem. Soc. 1999, 121, 3414-
3420.
(52) Hamel, S.; Duffy, P.; Casida, M. E.; Salahub, D. R. J. Electron
Spectrosc. 2002, 123, 345-363.
(53) Lehnert, N.; Neese, F.; Ho, R. Y. N.; Que, L., Jr.; Solomon, E. I. J.
Am. Chem. Soc. 2002, 124, 10810-10822.
(57) Bachmann, J.; Nocera, D. G. J. Am. Chem. Soc. 2004, 126, 2829-
2837.
(58) Gray, T. G.; Rudzinski, C. M.; Meyer, E. E.; Nocera, D. G. J. Phys.
Chem. A 2004, 108, 3238-3243.
(59) Gray, T. G.; Veige, A. S.; Nocera, D. G. J. Am. Chem. Soc., accepted
for publication.
(54) Chen, P.; Root, D. E.; Campochiaro, C.; Fujisawa, K.; Solomon, E. I.
J. Am. Chem. Soc. 2003, 125, 466-474.
(55) Chisholm, M. H.; Gallucci, J.; Hadad, C. M.; Huffman, J. C.; Wilson,
P. J. J. Am. Chem. Soc. 2003, 125, 16040-16049.
(56) Gray, T. G.; Rudzinski, C. M.; Meyer, E.; Holm, R. H.; Nocera, D.
G. J. Am. Chem. Soc. 2003, 125, 4755-4770.
(60) Chisholm, M. H.; Clark, D. L.; Kober, E. M.; Van Der Sluys, W. G.
Polyhedron 1987, 6, 723-727.
3622 Inorganic Chemistry, Vol. 43, No. 12, 2004

Eclipsed M2X6 Compounds
Figure 3. Calculated frontier orbitals of model compounds Mo2[MeN-BPh-NMe]3 and W2[MeN-BPh-NMe]3.
trailing edge of the charge-transfer absorption band. A similar
profile is observed for ditungsten complexes 3 and 4. As
with the dimolybdenum complex, these absorption bands are
blue-shifted (λmax ) 271 and 334 nm) relative to the
unbridged W2(NMe2)6 dimer.
a donor bridgehead (e.g., amine). We have shown that this
A-D-A ligand motif supports bimetallic cores in a two-
electron mixed valence configuration.59-66 We describe
herein bimetallic centers of the transition metal series
spanned by a ligand of antithetical composition: a D-A-D
three-atom frame composed of a π-accepting boron bridge-
head adjacent to π-donor amides. This D-A-D backbone
is very constrained, thus affording compounds with excep-
tional properties for the class of triple metal-metal bond
complexes.
The chemical properties of the M2[RN-BPh-NR]3 com-
plexes are consistent with the electronic structure of Figure
4. The ligand-based HOMO and high energy of the LUMO
presage an inert reaction chemistry; this is the case. The
complexes do not react with σ-donor ligands nor do they
react with oxo and chalcogenide transfer reagents (e.g.,
nitriles, ketones, epoxides, pyridine N-oxide, selenium). The
complexes are, however, susceptible to oxidation, as indi-
cated by their pyrophoric nature when exposed to air.
The M2[RN-BPh-NR]3 complexes reported herein expand
the transition metal chemistry of bis(alkylamido)boranes from
mononuclear centers to bimetallic cores. The compounds are
complementary to those bimetallic cores that are spanned
by ligands composed of three-atom bridges in which two
π-accepting groups (e.g., fluorophosphines) are adjacent to
Acknowledgment. This work was supported by a grant
from the National Science Foundation (CHE-0132680).
(61) Heyduk, A. F.; Macintosh, A. M.; Nocera, D. G. J. Am. Chem. Soc.
1999, 121, 5023-5032.
(62) Odom, A. L.; Heyduk, A. F.; Nocera, D. G. Inorg. Chim. Acta 2000,
297, 330-337.
(63) Heyduk, A. F.; Nocera, D. G. J. Am. Chem. Soc. 2000, 122, 9415-
9426.
(64) Heyduk, A. F.; Nocera, D. G. Chem. Commun. 1999, 1519-1520.
(65) Heyduk, A. F.; Nocera, D. G. Science 2001, 293, 1639-1641.
(66) Veige, A. S.; Nocera, D. G. Chem. Commun., accepted for publication.
Inorganic Chemistry, Vol. 43, No. 12, 2004 3623

Manke et al.
bond distances and angles. This material is available free of charge
via the Internet at http://pubs.acs.org.
Supporting Information Available: Table of X-ray crystal-
lographic data for complexes 1 and 4 including a fully labeled
thermal ellipsoid plot, final atomic coordinates, equivalent isotropic
displacement parameters, anisotropic displacement parameters, and
IC049795R
3624 Inorganic Chemistry, Vol. 43, No. 12, 2004