A terminal molybdenum arsenide complex synthesized from yellow arsenic

Article abstract of DOI:10.1021/ic9016068

A terminal molybdenum arsenide complex Is synthesized In one step from the reactive As₄ molecule. The properties of this complex with its arsenic atom ligand are discussed In relation to the analogous nitride and phosphide complexes.

Full text of DOI:10.1021/ic9016068

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
Inorg. Chem. 2009, 48, 9599–9601 9599
DOI: 10.1021/ic9016068
A Terminal Molybdenum Arsenide Complex Synthesized from Yellow Arsenic
John J. Curley,† Nicholas A. Piro,† and Christopher C. Cummins*
Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139-4307.
† These authors contributed equally to this work.
Received August 25, 2009
A terminal molybdenum arsenide complex is synthesized in one
step from the reactive As4 molecule. The properties of this complex
with its arsenic atom ligand are discussed in relation to the
analogous nitride and phosphide complexes.
demonstrated for N2 and P4.8,10,14 While metallic arsenic is
unreactive toward 1, molecular As4 serves as a reactive and
soluble source of arsenic.6,15-18
Heating gray arsenic metal to ca. 650 °C provided a source
of As4 vapors, which were trapped by passage through
toluene cooled to -40 °C.6 The addition of 1 to this slurry
at -40 °C followed by warming to 20 °C led to the direct
formation of the triple bond between Mo and As, yielding 1-
As (Scheme 1). This compound was isolated as a yellow
powder in 67% yield following extraction and precipitation
from Et2O/n-pentane at -35 °C. The molecule 1-As crystal-
lizes in the chiral space group P21, with chirality induced by
the propeller orientation of the three anilide ligands about the
Mo-As core (Figure 1). This structure revealed a short
The atomic ligands of the heaviest group 15 elements (As,
Sb, and Bi) remain rarely encountered in transition-metal
chemistry.1,2 Examples of molecules that contain one of these
pnictogens as a terminal ligand are currently limited to
complexes of molybdenum, niobium, and tungsten.3-6
Impressively versatile with regard to the formation of term-
inal multiple bonds is the three-coordinate compound Mo-
(N[tBu]Ar)3 (1), which has been used as a precursor to stable
compounds containing bonds to both heavy and light atomic
ligands: EMo(N[tBu]Ar)3, where E=C-, N, P, O, S, Se, and
Te.7-12 Moreover, this d3, C3v transition-metal fragment is
isolobal to those free atoms belonging to group 15,13 provid-
ing an ideal platform for the formation of triple bonds to
these elements when they are presented to molybdenum, as
˚
Mo-As distance of 2.2248(5) A. This is comparable to, but
slightly shorter than, the known terminal arsenide complexes
˚
˚
of molybdenum [2.252(3) A] and tungsten [2.2903(11) A] that
bear a triamidoamine ligand, (Me3SiNCH2CH2)3N, and
contain a donor trans to the triple bond.3,4 The value also
agrees well with the sum of the triple-bond covalent radii for As
19
˚
˚
(1.06 A) and Mo (1.13 A).
We sought to identify the resonant frequency of the
Mo-As oscillator present in 1-As.20 Vibrational data were
acquired on PtMo(N[tBu]Ar)3 (1-P) and 1-As by both IR
*To whom correspondence should be addressed. E-mail: ccummins@
mit.edu.
and Raman spectroscopies. In the far-IR (600-200 cm-1
;
CsI pellet), the spectra of these two compounds were nearly
identical, with the exception of two resonant absorptions
located at 538 and 394 cm-1. The lower-frequency vibration
at 394 cm-1 was assigned to the Mo-As stretching mode for
1-As, whereas the band at 538 cm-1 was assigned to the
phosphide mode for 1-P. These frequencies matched the
ꢀ
(1) Balazs, G.; Gregoriades, L. J.; Scheer, M. Organometallics 2007, 26,
3058–3075.
ꢀ
(2) Johnson, B. P.; Balazs, G.; Scheer, M. Coord. Chem. Rev. 2006, 250,
1178–1195.
::
(3) Mosch-Zanetti, N. C.; Schrock, R. R.; Davis, W. M.; Wanninger, K.;
Seidel, S. W.; O’Donoghue, M. B. J. Am. Chem. Soc. 1997, 119, 11037–11048.
::
::
(4) Scheer, M.; Muller, J.; Haser, M. Angew. Chem., Int. Ed. Engl. 1996,
35, 2492–2496.
ꢀ
(5) Balazs, G.; Sierka, M.; Scheer, M. Angew. Chem., Int. Ed. 2005, 44,
4920–4924.
(14) Laplaza, C. E.; Johnson, M. J. A.; Peters, J. C.; Odom, A. L.; Kim,
E.; Cummins, C. C.; George, G. N.; Pickering, I. J. J. Am. Chem. Soc. 1996,
118, 8623–8638.
(6) Spinney, H. A.; Piro, N. A.; Cummins, C. C. J. Am. Chem. Soc. 2009,
131, in press.
(7) Peters, J. C.; Odom, A. L.; Cummins, C. C. Chem. Commun. 1997,
1995–1996.
(15) Di Vaira, M.; Midollini, S.; Sacconi, L.; Zanobini, F. Angew. Chem.,
(8) Laplaza, C. E.; Cummins, C. C. Science 1995, 268, 861–863.
(9) Curley, J. J.; Cook, T. R.; Reece, S. Y.; Muller, P.; Cummins, C. C.
Int. Ed. Engl. 1978, 17, 676–677.
::
::
(16) Scherer, O. J.; Vondung, J.; Wolmershauser, G. J. Organomet. Chem.
J. Am. Chem. Soc. 2008, 130, 9394–9405.
(10) Laplaza, C. E.; Davis, W. M.; Cummins, C. C. Angew. Chem., Int.
Ed. Engl. 1995, 34, 2042–2044.
1989, 376, C35–C38.
(17) Tan, R. B.; Comerlato, N. M.; Powell, D. R.; West, R. Angew.
Chem., Int. Ed. Engl. 1992, 31, 1217–1218.
(11) Johnson, A. R.; Davis, W. M.; Cummins, C. C.; Serron, S.; Nolan, S. P.;
Musaev, D. G.; Morokuma, K. J. Am. Chem. Soc. 1998, 120, 2071–2085.
(12) McDonough, J. E.; Mendiratta, A.; Curley, J. J.; Fortman, G. C.;
Fantasia, S.; Cummins, C. C.; Rybak-Akimova, E. V.; Nolan, S. P.; Hoff,
C. D. Inorg. Chem. 2008, 47, 2133–2141.
(18) Scherer, O. J. Acc. Chem. Res. 1999, 32, 751–762.
(19) Pyykko, P.; Riedel, S.; Patzschke, M. Chem.;Eur. J. 2005, 11, 3511–
3520.
(20) Nugent, W. A.; Mayer, J. M. Metal-Ligand Multiple Bonds: The
Chemistry of Transition Metal Complexes Containing Oxo, Nitrido, Imido,
Alkylidene, or Alkylidyne Ligands; John Wiley & Sons, Inc.: New York, 1988.
(13) Hoffmann, R. Angew. Chem., Int. Ed. Engl. 1982, 21, 711–724.
r
2009 American Chemical Society
Published on Web 09/18/2009
pubs.acs.org/IC

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
9600 Inorganic Chemistry, Vol. 48, No. 20, 2009
Curley et al.
Figure 1. Thermal ellipsoid plot (50% probability) of 1-As with hydro-
gen atoms omitted for clarity.
Figure 2. Graph that correlates selected frontier orbitals for the series of
compounds EtMo(N[tBu]Ar)3 (E = N, P, and As). Here the 1e orbital
refers to the E-Mo π bond, while 2e is the antibond; the a1 orbital is the
highest-lying orbital with pseudo-C3v symmetry. The a2 orbital is a
combination of anilide orbitals that finds no symmetry match on Mo in
a C3v field.
Scheme 1. Synthesis of 1-As
pied molecular orbital (LUMO) and LUMOþ1 orbitals are
the antibonding counterparts. These orbitals are separated
by a HOMO-LUMO gap of 2.3 eV, a qualitative measure of
the bonding interaction.25 In contrast to bonding patterns of
the lighter elements (C, N, and O), the heavier main-group
elements are characterized by a stabilized orbital of largely s
character sometimes called an “inert pair”.26,27 Therefore, we
anticipated that the orbital that contained the most arsenic
4s character would be much lower in energy relative to the
π-symmetry bonding orbitals; this orbital (HOMO-78) was
located 8.7 eV below the HOMO.
For all three compounds 1-E, the combination of p orbitals
on the anilide nitrogen atoms, which are primarily nonbond-
ing with respect to Mo, was found to have similar energy
(Figure 2, a2).28 This is consistent with the notion that the
ligand-based nonbonding molecular orbital in a pseudo-C3v-
symmetric field is not greatly altered by the atomic ligand
triply bound to Mo. Upon moving from N to P and As there
is a drastic decrease in the splitting between the π-symmetry
bonding and antibonding orbitals that is consistent with the
notion that heavier elements form less stable π bonds.29
For E=N, the π bond lies approximately 2 eV below the
HOMO. The corresponding orbital is higher in energy for E=P
and becomes the HOMO for E=As. Similar trends have been
documented for the related chalcogen series AMo(N-
[tBu]Ar)3 (A=O, S, Se, and Te).11 The orbital labeled a1 is
the highest filled orbital that is symmetric about the C3 axis.
Upon changing from N to P to As, this orbital is only slightly
destabilized. For 1-N, this orbital corresponds to the nitrogen
Stokes shifts measured by Raman spectroscopy on powder
samples of 1-As and 1-P. Additional far-IR data and Raman
spectroscopy data acquired on 14/15NtMo(N[tBu]Ar)3
lacked these signals while reproducing other features
observed in spectra of 1-E (E=As and P).
Vibrational force constants have a linear correlation with
the bond order; therefore, these values offer another means of
characterizing the Mo-E bond.21 Using the harmonic oscill-
ator approximation, we found a force constant of 3.87 mdyn/
˚
˚
A for 1-As and 4.02 mdyn/A for 1-P. These values are
˚
approximately half of the 7.86 mdyn/A value found for
NMo(N[tBu]Ar)3 (1-N).14 The series of compounds defined
by 1-E compares well to the pnictogens reported by Schrock,
˚
EMo(Me3SiNCH2CH2)3N, kMoAs = 3.47 mdyn/A, kMoP
3.74 mdyn/A, and kMoN = 7.26 mdyn/A.
=
3,22
˚
˚
These force
constants were interpreted to correspond to a M-E bond
order of 3 for each pnictogen.22 The slightly larger force
constants for the series 1-E relative to those found in the
complexes of Schrock are likely a reflection of stronger
bonds. The origin of these differences is not entirely straight-
forward but could be attributed the lack of a donor in the
position trans to the triply bonded ligand in the series 1-E.
Bonding in 1-As was further examined by quantum che-
mical calculations, using the X-ray coordinates as a starting
point for geometry optimization.23,24 This analysis shows
that the highest occupied molecular orbital (HOMO) and
HOMO-1 of 1-As are the π-symmetry bonding orbitals
between Mo and As. Correspondingly, the lowest unoccu-
(25) Albright, T. A.; Burdett, J. K.; Whangbo, M.-H. Orbital Interaction
in Chemistry; John Wiley & Sons, Inc.: New York, 1984; pp 1-25.
(26) Sidgwick, N. V. Ann. Rep. Prog. Chem. 1933, 30, 120–128.
(27) Drago, R. S. J. Phys. Chem. 1958, 62, 353–357.
(28) Anhaus, J. T.; Kee, T. P.; Schofield, M. H.; Schrock, R. R. J. Am.
Chem. Soc. 1990, 112, 1642–1643.
(21) Robinson, E. A.; Lister, M. W. Can. J. Chem. 1963, 41, 2988–2995.
(22) Johnson-Carr, J. A.; Zanetti, N. C.; Schrock, R. R.; Hopkins, M. D.
J. Am. Chem. Soc. 1996, 118, 11305–11306.
(23) 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.
(29) Power, P. P. Chem. Rev. 1999, 99, 3463–3503.
(30) The five atomic orbitals that contribute most to the highest occupied
a1-like orbital for the species 1-E are listed: N, 109 A: N(pz), 10.27%; C21(py),
9.09%; C33(px), 8.53%; C30(px), 7.32%; C18(py), 6.86%. P, 109 A: P(pz),
37.81%; Mo(dz2), 8.92%; Mo(s), 4.15%; C21(px), 3.01%; C9(py), 2.85%. As,
180 A: As(pz), 45.99%; Mo(dz2), 15.04%; Mo(s), 4.03%; C29(px), 2.88%;
As(s), 2.68%.
(24) ADF; Scientific Computing & Modelling NV: Amsterdam, The Netherlands,
2007.

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
Communication
Inorganic Chemistry, Vol. 48, No. 20, 2009 9601
lone pair; however, the nature of this orbital changes upon
descending the pnictogen series.26,27,30
Table 1. Bond Dissociation Energies (BDEs), Stretching Force Constants, Bond
Lengths, and Calculated Bond Indices for the Terminal Pnictogen Complexes 1-E
The Nalewajski-Mrozek (NM) bond index is a quantita-
tive measure of the bond order that has recently been applied
to metal-ligand multiple bonds.33 This measure was found
to correlate well with chemical intuition of bond order and
yield bond multiplicities that are relatively independent of the
chosen basis set.33 Our computed values of NM multiplicities
are reported in Table 1, alongside experimental measure-
ments of the Mo-E multiple bond. The values of the NM
bond index support our suggestion that the three complexes,
1-E, all contain a considerable degree of Mo-E multiple
bonding. The bond index was observed to decrease in the order
Mo-N . Mo-P > Mo-As. This behavior parallels our
results obtained from both vibrational spectroscopy and
qualitative molecular orbital analysis, which suggest that
the Mo-N bond in 1-N is considerably stronger than the
Mo-E bond in 1-P and 1-As. Solution-phase thermo-
chemistry data have been used to estimate the triple bond
dissociation energies (BDEs) for 1-N and 1-P as 155.3
and 92.2 kcal/mol, respectively.31,32 Calculations on this
series suggest that the Mo-As triple bond is nearly 18 kcal/
mol weaker than the M-P triple bond, in contrast to the
dramatic 60 kcal/mol weakening observed upon going from
N to P.
c
BDEa
kb
rexp
bond indexd
1-As
1-P
1-N
∼74
92.2
155.3
3.87
4.02
7.86
2.2248(5)
2.119(4)
1.651(4)
2.4853
2.5109
2.7038
a Values are in kcal/mol and experimentally determined for 1-P and
1-N;31,32 the value is calculated for 1-As (see the Supporting Infor-
mation). b Values are in mdyn/A calculated using the harmonic oscillator
˚
approximation. c Experimental bond length from X-ray diffraction
studies. d Nalewajski-Mrozek bond index.
value for ΔS of -56 eu was found for the formation of the
related μ-S complex from the terminal sulfide complex SMo-
(N[tBu]Ar)3 and 1.12 Both this μ-S complex and the related
μ-P complex, (μ-P)[Mo(N[tBu]Ph)3]2, are observed to form
at -35 °C, a temperature at which the μ-As complex is not
observed.12,36 This contrast is attributed to a less negative
enthalpy associated with the formation of 2. This, in turn, can
be ascribed to a manifestation of the inert-pair effect. This
low-energy lone pair is also likely responsible for the lack of
reactivity toward the reagents mentioned above (MeOTf,
SSbPh3, and DMDO) that would seek to engage this electron
pair in bonding interactions.
In general, the reactivity of 1-As was found to be quite
limited. For example, 1-As was not found to engage in a
reaction with MeOTf under the mild conditions that afford
methylation of 1-N.34 Nor was 1-As found to abstract a
sulfur atom from SSbPh3, a reagent that readily transfers its
chalcogen to 1-P.12 Similarly, the potent oxygen-atom-trans-
fer reagent dimethyldioxirane (DMDO) affords OPMo(N-
[tBu]Ar)3 from 1-P,35 but we were unable to obtain
OAsMo(N[tBu]Ar)3 using this methodology; only unreacted
1-As was recovered from the reaction mixture.
One reaction that 1-As was found to engage in was a
reversible capping by 1 to form the complex (μ-As)[Mo(N-
[tBu]Ar)3]2 (2, eq 1). Upon cooling of an equimolar mixture
of 1-As and 1 to -80 °C, the originally yellow-brown solution
took on a bright-purple hue, indicative of the formation of
(μ-As)[Mo(N[tBu]Ar)3]2. This color change was reversible
upon repeated cooling and warming cycles. The thermody-
namic parameters for this reversible reaction were extracted
from a van’t Hoff plot of equilibrium constants measured by
1H NMR spectroscopy in 5 °C intervals over the temperature
range -54 to -96 °C. This analysis yielded the parameters
ΔH = -12.4(1.6) kcal/mol and ΔS = -59(8) eu. The very
large negative entropy for the reaction to form 2 is consistent
with the large decrease in the degrees of freedom imposed by
the interdigitation of tert-butyl groups that is required to
accommodate the short, one-atom bridge in 2. A very similar
ðμ-AsÞ½MoðN½tBuꢀArÞ ꢀ
1 þ 1-As h
ð1Þ
3 2
Herein we have reported the preparation and X-ray crystal
structure of 1-As, a compound that contains a Mo-As triple
bond and is one of very few compounds with a bone fide one-
coordinate arsenic atom in the solid state.37 A comparison of
the Mo-E multiple bonds within the 1-E series, as well as
those reported by Schrock and Scheer,3,4,22 allows features
that are characteristic of multiple bonding to be contrasted
between the lighter and heavier elements. The results sum-
marized herein show that the characteristics of Mo-E
bonding within 1-E are largely preserved for E = As or P
and that the most pronounced differences in the Mo-E bond
are noted when comparing one of these two molecules to
those of 1-N. These results agree with the idea that the lighter
elements (C, N, and O) deviate from “normal” chemical
behavior in their unique propensity to form strong multiple
bonds.38
Acknowledgment. The authors thank the United States
National Science Foundation for support through Grant
CHE-719157 and Prof. Karsten Meyer for his generous
contribution of the As4-generating apparatus.
Supporting Information Available: Crystallographic data in
CIF format and experimental procedures and characterization
data for 1-As and 2. This material is available free of charge via
the Internet at http://pubs.acs.org.
(31) Cherry, J.-P. F.; Johnson, A. R.; Baraldo, L. M.; Tsai, Y.-C.;
Cummins, C. C.; Kryatov, S. V.; Rybak-Akimova, E. V.; Capps, K. B.;
Hoff, C. D.; Haar, C. M.; Nolan, S. P. J. Am. Chem. Soc. 2001, 123, 7271–
7286.
(32) Stephens, F. H.; Johnson, M. J. A.; Cummins, C. C.; Kryatova, O. P.;
Kryatov, S. V.; Rybak-Akimova, E. V.; McDonough, J. E.; Hoff, C. D.
J. Am. Chem. Soc. 2005, 127, 15191–15200.
(33) Michalak, A.; DeKock, R. L.; Ziegler, T. J. Phys. Chem. A 2008, 112,
7256–7263.
(36) Johnson, M. J. A.; Lee, P. M.; Odom, A. L.; Davis, W. M.; Cummins,
C. C. Angew. Chem., Int. Ed. Engl. 1997, 36, 87–91.
(37) A search of the Cambridge Structural Database on July 20, 2009,
revealed four such molecules: AstM(Me3SiNCH2CH2)3N (M=Mo and W),3,4
2,4,6-tBu3C6H2CtAs,39 and [(F3C)3BCtAs]-.40
(34) Sceats, E. L.; Figueroa, J. S.; Cummins, C. C.; Loening, N. M.;
Van der Wel, P.; Griffin, R. G. Polyhedron 2004, 23, 2751–2768.
(35) Johnson, M. J. A.; Odom, A. L.; Cummins, C. C. Chem. Commun.
1997, 1523–1524.
(38) Burdett, J. K. Chemical Bonds: A Dialog; Wiley: New York, 1997.
::
(39) Markl, G.; Sejpka, H. Angew. Chem., Int. Ed. Engl. 1986, 25, 264.
(40) Finze, M.; Bernhardt, E.; Willner, H.; Lehmann, C. W. Angew.
Chem., Int. Ed. 2004, 43, 4160–4163.