Terminal phosphide and dinitrogen molybdenum compounds obtained from pnictide-bridged precursors

Article abstract of DOI:10.1021/ic015592g

The syntheses of molybdenum terminal phosphide and dinitrogen complexes supported by N-iso-propylanilide ligands are not as straightforward as those previously reported for the analogous N-tert-butylanilide complexes. To arrive at these synthetic targets, facile, high-yielding syntheses of bridging pnictide precursors have been developed, and their reduction chemistry has been explored. Cleavage of the reduced μ-E₁ compounds was undertaken using CO and N₂, providing access to the target compounds.

Full text of DOI:10.1021/ic015592g

6860
Inorg. Chem. 2001, 40, 6860-6862
Terminal Phosphide and Dinitrogen Molybdenum Compounds Obtained from Pnictide-Bridged Precursors
John-Paul F. Cherry,† Frances H. Stephens,† Marc J. A. Johnson, Paula L. Diaconescu, and
Christopher C. Cummins*
Massachusetts Institute of Technology, Room 2-227, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139
ReceiVed September 25, 2001
Inspired by recent findings that molybdenum alkylidyne
complexes supported by ancillary alkoxide ligands can be prepared
by alcoholysis of the corresponding N-iso-propylanilide alkyli-
dynes,1 we selected as synthetic targets two new Mo-ligand
multiply bonded compounds supported by removable N-iso-
propylanilides. Because the successful syntheses of terminal
phosphide and dinitrogen compounds proved to be both interesting
and challenging, they are communicated in the present report.
Whereas the reaction of white phosphorus with three-coordinate
molybdenum compounds supported by N-tert-butylanilide ligands
has given rise exclusively to the terminal phosphide functionality,2
it is seen that employing the less sterically hindered fragment
Mo(N[i-Pr]Ar)3 (2, Ar ) 3,5-C6H3Me2) leads to a phosphide-
bridged product 22-µ-P.3 Facile combination of the desired PMo-
(N[i-Pr]Ar)3 (2-P) with reactive Mo(H)(η2-Me2CdNAr)(N[i-
Pr]Ar)2 (1) accounts for favorable formation of this bridged
phosphide.4 It was surmised that if 2-P could be accessed in the
absence of 1, it would be stable and possess synthetic utility. For
this reason, a method was sought for splitting dinuclear 22-µ-P.
Initially, it was found that nitric oxide was able to effect
quantitative bridge cleavage to give equimolar amounts of 2-P
and nitrosyl 2-NO, but the two possessed similar solubility
properties and were therefore difficult to separate. Because
[2-CO]- is isoelectronic with 2-NO, but should be separable from
2-P because of its negative charge, the one-electron reduction
and subsequent carbonylation of 22-µ-P was investigated. Em-
ploying sodium amalgam in THF under a nitrogen atmosphere
proved to be an effective regimen for high-yield isolation of the
sodium salt of the [22-µ-P]- anion (Scheme 1).
Scheme 1
The red-purple contact ion pair [Na(THF)][22-µ-P] crystallizes
in space group P21/c as large blocks from pentane. To stabilize
the sodium cation in the solid state, one aryl group flips to interact
in a π-fashion with the alkali metal (Figure 1).5 The Mo-P-Mo
bridge is thereby desymmetrized, resulting in modestly different
Mo-P distances (2.183(2) and 2.197(2) Å). The Mo-P bond
distances in the neutral6 and anionic species are similar.7 In both
22-µ-P and [Na(THF)][22-µ-P], the Mo-P distances are shorter
than those observed for (µ-P)[Mo(N[t-Bu]Ph)3]2 (2.2430(6) Å),8
attributable to steric considerations.
separated from 2-P via precipitation and subsequent filtration after
the addition of the crown ether 12-c-4 (2 equiv based on Na) to
produce [Na(12-c-4)2][2-CO]. X-ray quality yellow blocks of 2-P
were found to crystallize from pentane in the space group P212121
(Figure 2). The Mo-P distance of 2.116(3) Å is in close
agreement with those observed for previously reported PMo(N-
Addition of stoichiometric CO to a THF solution of [Na(THF)]-
[22-µ-P] at 25 °C effected quantitative cleavage of the phosphide
bridge. Because of the ionic nature of [2-CO]-, it was easily
† First and second named authors contributed equally to this work.
(1) Tsai, Y.-C.; Diaconescu, P. L.; Cummins, C. C. Organometallics 2000,
19, 5260.
(2) Laplaza, C. E.; Davis, W. M.; Cummins, C. C. Angew. Chem., Int. Ed.
Engl. 1995, 34, 2042.
[t-Bu]Ar)3 (2.119(4) Å)2 and PW(NN3) (2.162(4) Å, NN3
)
[(RNCH2CH2)3N]).9 The 31P NMR resonance at δ ) 1256 ppm
(3) Other symmetrical µ2-P1 compounds: Scheer, M.; Mu¨ller, J.; Schiffer,
M.; Baum, G.; Winter, R. Chem.sEur. J. 2000, 6, 1252. Johnson, M. J.
A.; Lee, P. M.; Odom, A. L.; Davis, W. M.; Cummins, C. C. Angew.
Chem., Int. Ed. Engl. 1997, 36, 87. Fermin, M. C.; Ho, J.; Stephan, D.
W. J. Am. Chem. Soc. 1994, 116, 6033. Strube, A.; Heuser, J.; Huttner,
G.; Lang, H. J. Organomet. Chem. 1988, 356, C9.
(4) See Supporting Information for details of this independently performed
reaction.
(5) Weiss, E. Angew. Chem., Int. Ed. Engl. 1993, 32, 1501.
(6) In the solid-state structure of 22-µ-P, the phosphorus atom lies on a
crystallographic inversion center making the Mo-P distances identical
(2.2164(4) Å). A subtle Jahn-Teller distortion breaks the symmetry of
an otherwise partially occupied doubly degenerate HOMO; one anilide
ligand is tilted much further away from the vertical than are the other
two, a distortion of approximate C2h symmetry if only core atoms are
considered. Further crystallographic information can be found in the
Supporting Information.
10.1021/ic015592g CCC: $20.00 © 2001 American Chemical Society
Published on Web 12/04/2001

Communications
Inorganic Chemistry, Vol. 40, No. 27, 2001 6861
to react with CO to produce equimolar [Na(THF)][2-CO] and
2-N. In contrast to its phosphorus analogue, [Na(THF)x][22-µ-
N] undergoes a reaction with dinitrogen to produce equimolar
[Na(THF)x][2-N2] and 2-N. This discovery provides the only
useful protocol to access [Na(THF)x][2-N2], unlike previously
described syntheses of the sodium and magnesium salts of [(N2)-
Mo(N[t-Bu]Ar)3]-.13,14 When carried out with 15N2, this experi-
ment generated exclusively unlabeled 2-N and [Na(THF)x][2-
15N2] (15N NMR, δ ) 401.3 and 348.8 ppm;15 these values are
similar to those for other dinitrogen anions13,16). Additionally, 22-
µ-15N is synthesized in 94% yield when 1 is exposed to 15N2
17
and can be reduced under an argon atmosphere to generate [Na-
(THF)x][22-µ-15N]. Low temperature (-80 °C) 15N NMR spec-
troscopy was employed to observe the anionic µ-nitrido resonance
at δ ) 688.7 ppm. Other µ-15N complexes have been examined
by 15N NMR, including the trimers (Cp*MeTa15N)3, δ ) 528
and 525 ppm,18 and {(µ-15N)Nb(N[i-Pr]Ar)2}3, δ ) 592 ppm.13
Addition of N2 (natural isotopic abundance) to a THF-d8 solution
of [Na(THF)x][22-µ-15N] produced equimolar [Na(THF)x][2-N2]
and 2-15N. The latter compound evinced a 15N NMR signal at
757.7 ppm, in accord with values observed for other N-
hydrocarbylanilide-supported terminal nitrides NMo(N[R]Ar)3 (R
Figure 1. Molecular structure of [Na(THF)][22-µ-P] (35% thermal
ellipsoids). Hydrogen atoms are omitted for clarity. Selected bond
distances (Å) and angles (°): Mo(1)-P, 2.183(2); Mo(2)-P, 2.197(2);
Mo(1)-N(1), 2.051(6); Mo(1)-N(2), 2.017(5); Mo(1)-N(3), 1.970(6);
Mo(2)-N(4), 2.059(6); Mo(2)-N(5), 2.059(6); Mo(2)-N(6), 1.959(6);
Mo(1)-P-Mo(2), 173.87(10).
) Bu, δ ) 837.9 ppm; R ) Ad, δ ) 839.8 ppm)19 as well as
other terminal d0 nitrides.20 For synthetic purposes, the separation
of [Na(THF)][2-N2] from 2-N was accomplished in 48% yield
by crystallization from ether (leaving 52% of [Na(THF)][2-N2]
and 100% of 2-N in the mother liquor). Complete separation of
the anionic N2-complex from equimolar 2-N was accomplished
by complexation of the cation with the crown ether 12-c-4 (2
equiv per Na) to produce the relatively insoluble [Na(12-c-4)2]-
[2-N2] in 97% yield.
t
2
The contact ion pair [Na(THF)3][2-N2] was found to crystallize
with molecular C3 symmetry (Figure 3). The N(2)-N(3) bond
distance was determined to be 1.186(8) Å. While a 3:1 THF/Na
ratio was observed for crystals selected freshly from the mother
liquor, drying of the crystals under vacuum produced a 1:1 THF/
Na ratio as reflected by 1H NMR integration and elemental
analysis.
Figure 2. Molecular structure of 2-P (35% thermal ellipsoids). One of
two independent molecules is shown. Hydrogen atoms are omitted for
clarity. Selected bond distances (Å) and angles (°): Mo(1)-P(1), 2.116-
(3); Mo(1)-N(1), 1.935(9); Mo(1)-N(2), 1.978(10); Mo(1)-N(3), 1.971-
(10).
Addition of a stoichiometric amount of methyl tosylate to an
ether solution of [Na(THF)][2-N2] formed 2-N2Me in 58%
isolated yield as red crystals. Similar molybdenum methyldi-
azenido complexes prepared with bulkier ligands have been
reported.14,21 Ruby red plates of 2-N2Me crystallized from
for 2-P is similar to the values of δ ) 1216 and 1346 ppm,
respectively, for the aforementioned compounds PMo(N[t-Bu]-
Ar)3 and PW(NN3). The origin of such large downfield shifts
characteristic of terminal phosphides has been elucidated in
detail.10
(13) Mindiola, D. M.; Meyer, K.; Cherry, J. P. F.; Baker, T.; Cummins, C.
C. Organometallics 2000, 19, 1622.
(14) Peters, J. C.; Cherry, J. P. F.; Thomas, J. C.; Baraldo, L. M.; Mindiola,
D. J.; Davis, W. M.; Cummins, C. C. J. Am. Chem. Soc. 1999, 121,
10053.
Sodium amalgam reduction of 22-µ-N11 proceeds with a color
change from dark purple to forest green, the latter color being
characteristic of [Na(THF)x][22-µ-N] (Scheme 1).12 This species
(which was not isolated, but rather characterized in situ) was found
(15) 15N NMR shifts are referenced to external neat nitromethane (380.2 ppm
with respect to liquid NH3 (0.0 ppm)). von Philipsborn, W.; Mu¨ller, R.
Angew. Chem., Int. Ed. Engl. 1986, 25, 383.
(16) O’Donoghue, M. B.; Zanetti, N. C.; Davis, W. M.; Schrock, R. R. J.
Am. Chem. Soc. 1997, 119, 2753.
(7) This is not unexpected, because the electron introduced upon reduction
occupies a primarily nonbonding orbital (rather than a Mo-P-Mo
bonding orbital). See Supporting Information for results of density
functional theory calculations on [Na(THF)][22-µ-P].
(17) FT-IR spectroscopy for 22-µ-N: ν(Mo14N) ) 725 cm-1; ν(Mo15N) )
706 cm-1
.
(18) Banaszak Holl, M. M.; Kersting, M.; Pendley, B. D.; Wolczanski, P. T.
Inorg. Chem. 1990, 29, 1518.
(8) Johnson, M. J. A.; Lee, P. M.; Odom, A. L.; Davis, W. M.; Cummins,
C. C. Angew. Chem., Int. Ed. Engl. 1997, 36, 87.
(19) Cherry, J. P. F.; Johnson, A. R.; Baraldo, L. M.; Tsai, Y.-C.; Cummins,
C. C.; Kryatou, S. U.; Rybak-Akimoua, E. V.; Capps, K. B.; Hoff, C.
D.; Haar, C. M.; Nolan, S. P. J. Am. Chem. Soc. 2001, 123, 7271.
(20) Odom, A. L.; Cummins, C. C.; Protasiewicz, J. D. J. Am. Chem. Soc.
1995, 117, 6613. Laplaza, C. E.; Odom, A. L.; Davis, W. M.; Cummins,
C. C.; Protasiewicz, J. D. J. Am. Chem. Soc. 1995, 117, 4999. Donovan-
Mtunzi, S.; Richards, R. L.; Mason, J. J. Chem. Soc., Dalton Trans.
1984, 469. Dilworth, J. R.; Donovan-Mtunzi, S.; Kan, C. T.; Richards,
R. L.; Mason, J. Inorg. Chim. Acta 1981, 53, L161.
(9) Zanetti, N. C.; Schrock, R. R.; Davis, W. M. Angew. Chem., Int. Ed.
Engl. 1995, 34, 2044.
(10) Wu, G.; Rovnyak, D.; Johnson, M. J. A.; Zanetti, N. C.; Musaev, D. G.;
Morokuma, K.; Schrock, R. R.; Griffin, R. G.; Cummins, C. C. J. Am.
Chem. Soc. 1996, 118, 10654.
(11) Tsai, Y.-C.; Johnson, M. J. A.; Mindiola, D. J.; Cummins, C. C.; Klooster,
W. T.; Koetzle, T. F. J. Am. Chem. Soc. 1999, 121, 10426.
(12) Electrochemically, 22-µ-N undergoes a reversible oxidation at -0.9 V
and a reversible reduction at -1.9 V (vs Fc/Fc+) in the absence of
nitrogen. In the presence of dinitrogen, two irreversible reduction waves
(-2.5 and -2.9 V) and a reversible oxidation wave (-0.6 V) were
produced.
(21) Greco, G. E.; Schrock, R. R. Inorg. Chem. 2001, 40, 3861.
(22) DeVries, N.; DeWan, J. C.; Jones, A. G.; Davison, A. Inorg. Chem.
1988, 27, 1574.
(23) Fryzuk, M. D.; Johnson, S. A. Coord. Chem. ReV. 2000, 200-202, 379.

6862 Inorganic Chemistry, Vol. 40, No. 27, 2001
Communications
Figure 3. Molecular structure of [Na(THF)3][2-N2] (35% thermal
ellipsoids). Hydrogen atoms are omitted for clarity. Selected bond
distances (Å) and angles (°): Mo-N(2), 1.824(6); N(2)-N(3), 1.186-
(8); N(3)-Na, 2.196(7); Mo-N(1), 2.023(3); N(1)-Mo-N(2), 95.89-
(9).
Figure 4. Molecular structure of 2-N2Me (35% thermal ellipsoids).
Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and
angles (°): Mo-N(1), 1.981(2); Mo-N(2), 1.936(2); Mo-N(3), 2.013-
(2); Mo-N(5), 1.760(2); N(5)-N(4), 1.244(3); N(4)-C(1), 1.448(4);
N(1)-Mo-N(5), 97.33(9); Mo-N(5)-N(4), 172.5(2); N(5)-N(4)-C(1),
119.1(2).
concentrated ether at -25 °C in the space group P1h (Figure 4).
The X-ray crystal structure revealed that in the solid state 2-N2Me
has one N-iso-propylanilide ligand flipped by 180° relative to the
other two ligands.22 The N(4)-N(5) bond distance is 1.244(3)
Å, slightly elongated relative to that of its anionic precursor, yet
remaining indicative of an NN bond order of 2.23 The Mo-N(5)-
N(4) angle is 172.5(2)°, and the N(5)-N(4)-C(1) angle is
119.1(2)°.
In summary, reaction of complex 1 with elemental phosphorus
or nitrogen smoothly generates pnictogen-atom bridged com-
plexes. The latter undergo smooth one-electron reduction followed
by bridge cleavage upon treatment with the π-acids CO or
dinitrogen, affording synthetic access to mononuclear derivatives
with terminal phosphide or dinitrogen functionality. The new
Mo-ligand multiply bonded species presage new manifolds of
chemistry because the relatively unhindered N-iso-propylanilide
ligands are protolytically replaceable.
and Lucile Packard Foundations. C.C.C. also wishes to thank the
National Science Board for awarding him the 1998 Alan T.
Waterman Award. We also thank Aaron Odom for assistance with
X-ray crystallography and V´ıctor Dura`-Vila` for assistance with
DFT calculations. The cover graphic was generated in part using
the excellent free programs PLATON (http://www.cryst.chem.uu.nl/
platon/), POV-Ray (http://www.povray.org/), and the Gimp (http://
www.gimp.org/) on a Pentium-Linux machine.
Supporting Information Available: Text giving synthetic, spectro-
scopic, and analytical data for all new compounds and DFT calculations
on [Na(THF)][22-µ-P], and tables giving crystal data, atomic coordinates,
structure solution and refinement details, bond lengths and angles, and
anisotropic thermal parameters for 22-µ-P, [Na(THF)][22-µ-P], 2-P, [Na-
(THF)3][2-N2], and 2-N2Me. This material is available free of charge
via the Internet at http://pubs.acs.org.
Acknowledgment. For support of this work, we are grateful
to the National Science Foundation (Grant CHE-9988806) and
for fellowships to C.C.C. from the Alfred P. Sloan and the David
IC015592G