Cationic manganese(I) dihydrogen and dinitrogen complexes derived from a formally 16-electron complex with a bis-agostic interaction, [Mn(CO)(Ph2PC2H4PPh2)2]+

Article abstract of DOI:10.1021/ja960499q

We have synthesized a new 16-electron precursor [Mn(CO)(dppe)₂]BAr'₄ (1) capable of binding small molecules (dppe = Ph₂PC₂H₄PPh₂; BAr'₄ = B[C₆H₃(3,5-CF₃)₂]₄). This species has yielded the first well characterized cationic manganese-H₂ complex, [Mn(H₂)(CO)(dppe)₂]BAr'₄. The complexes MnH₃(R₂PC₂H₄PR₂)₂ [R = Me (dmpe), Et (depe)] were proposed to contain H₂ ligands, and CpMn(CO)₂(H₂) was recently isolated. We also report that N₂ reaction with 1 gives an extremely labile N₂ complex characterized by ¹⁵NMR.

Full text of DOI:10.1021/ja960499q

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6782
J. Am. Chem. Soc. 1996, 118, 6782-6783
Cationic Manganese(I) Dihydrogen and Dinitrogen
Complexes Derived from a Formally 16-Electron
Complex with a Bis-Agostic Interaction,
[Mn(CO)(Ph2PC2H4PPh2)2]+
Wayne A. King,† Xiao-Liang Luo,† Brian L. Scott,†
Gregory J. Kubas,*,† and Kurt W. Zilm‡
Los Alamos National Laboratory
Chemical Science and Technology DiVision
MS-C346, Los Alamos, New Mexico 87545
Department of Chemistry, Yale UniVersity
New HaVen, Connecticut 06520
ReceiVed February 16, 1996
The synthesis and characterization of metal-η2-dihydrogen
complexes has provided great insight into the catalytic and
stoichiometric activation of H2 by transition metals.1 The fine
line of stability of H2 complexes toward either dissociation of
H2 or oxidative addition to a dihydride is governed by the degree
of backbonding the H2 receives from the metal center.1
Systematic variation of the electronegativity of the metal center
in LnM-H2 by changing the donor/acceptor properties of the
ancillary ligands and the metal allow exploration of the reaction
coordinate for addition of H2 to metals. To this end, we have
synthesized a new 16-electron precursor [Mn(CO)(dppe)2]BAr′4
(1) capable of binding small molecules (dppe ) Ph2PC2H4PPh2;
Figure 1. ORTEP drawing (50% probability ellipsoids) of [MnCO)-
(dppe)2]+. Selected bond lengths (Å): Mn(1)-C(50), 1.733(7); Mn(1)-
P(1), 2.278(2); Mn(1)-P(2), 2.280(2); Mn(1)-P(3), 2.321(2); Mn(1)-
P(4), 2.335(2); Mn(1)-H(12), 2.89(6); Mn(1)-H(18), 2.98(6). Selected
bond angles (deg): C(50)-Mn(1)-P(1), 92.3(2); C(50)-Mn(1)-P(2),
90.8(2); C(50)-Mn(1)-P(3), 96.1(2); C(50)-Mn(1)-P(4), 97.2(2);
P(1)-Mn(1)-P(4), 82.67((7); P(2)-Mn(1)-P(3), 82.90(7); P(1)-
Mn(1)-P(3), 95.68(7); P(2)-Mn(1)-P(4), 98.02(7).
of two agostic interactions from ortho-hydrogen atoms (H12,
H18) located on separate phenyl rings of mutually trans-
phosphines.7 This contrasts with the Mo(CO)(dppe)2 analogue
which contains only one such interaction.8 The double interac-
tion most likely results from the reduced Mn-P-ortho-phenyl-H
ring size as compared to the Mo system, preventing full
interaction of the C-H bond. The Mn‚‚‚H distances are much
longer (2.89(6), 2.98(6) Å) than that found for the single agostic
interaction in Cr(CO)3(PCy3)2 (2.240(1) Å)9 and similar to that
in the Mo analogue (2.98(11) Å),8 a larger-radius metal.
Although multiple agostic interactions are rare, a similar double
phenyl-H interaction has been observed in M(PPhBut2)2 (M )
Pt, Pd), which also contain two M-tert-butyl-H interactions.10
The 1H NMR resonances for the anion in 1 are unremarkable
and consistent with those observed in related [Re(PR3)2(CO)3]-
BAr′4.11 The most diagnostic 1H signal is that for the PC2H4P
bridge, which gives a broad multiplet indicative of both 31P and
2
BAr′4 ) B[C6H3(3,5-CF3)2]4 ). This species has yielded the
first well-characterized cationic manganese-H2 complex, [Mn-
(H2)(CO)(dppe)2]BAr′4. The complexes MnH3(R2PC2H4PR2)2
[R ) Me (dmpe), Et (depe)] were proposed to contain H2
ligands,3a and CpMn(CO)2(H2) was recently isolated.3b We also
report that N2 reaction with 1 gives an extremely labile N2
complex characterized by 15N NMR.
Complex 1 forms on reaction of Mn(CO)(dppe)2Br4 with the
sodium salt of the large noncoordinating anion, NaBAr′4, in CH2-
Cl2 followed by filtration to remove NaBr.5 Cationic complexes
(5) [Mn(CO)(dppe)2]BAr′4‚0.7toluene:CH2Cl2 (20 mL) was vacuum
transferred onto Mn(CO)(dppe)2Br (0.65 mmol) and NaBAr′4 (0.84 mmol)
at -78 °C. The solution was placed under Ar and allowed to warm to 25
°C. After 30 min, the CH2Cl2 was removed in vacuo from the deep-blue
solution. The solids were extracted with warm toluene (25 mL), and the
solution was reduced to 10 mL and allowed to stand for 16 h. The blue
crystalline precipitate was isolated by decantation, washing with toluene,
[Mn(CO)(L)(dppe)2][PF6] (L ) NCR or CNR) have been
prepared by reaction of Mn(CO)(dppe)2Br and TlPF6 in the
presence of L. However, reaction in the absence of L yielded
only the decomposition product, [Mn(CO)2(dppe)2][PF6].6 The
deep blue, extremely air and moisture sensitive 1 is thus
apparently stabilized by BAr′4-. It is soluble in aromatic sol-
vents and CH2Cl2 but insoluble in hydrocarbons and crystallizes
from toluene as a solvate containing 0.7 toluene after drying in
vacuo. The complex can be obtained toluene-free by preparation
in CH2Cl2 followed by filtration and solvent removal in vacuo.
The X-ray structure of the 1 cation is formally five-coordinate
square-pyramidal (Figure 1). The notable feature is the presence
1
and drying in vacuo. Yield: 0.93 g (79%). H NMR (CD2Cl2) d 7.72 (s,
C6H3(3,5-CF3)2, 8H, ortho); 7.56, (s, C6H3(3,5-CF3)2, 4H, para); 7.3-7.0-
(m), 6.16(m) (C6H5 of toluene and phosphine, 43.5H); 2.79 (m, PCH2CH2P,
8H); 2.34 (CH3, toluene, 2H); 31P NMR{1H} (CD2Cl2) d 82.6; IR (cm-1
ν(CO) 1839 (CH2Cl2), 1862 (Nujol).
)
(6) Garcia-Alonso, F. J.; Riera, V. Transition Met. Chem. 1985, 10, 19.
(7) X-ray quality crystals were grown from toluene. [Mn(CO)-
(dppe)2]BAr′4‚1.5toluene: aquamarine, rectangular, 0.21 × 0.25 × 0.45
mm, Mr ) 1881.16; space group P1h; a ) 12.956(2) Å; b ) 16.935(3) Å;
c ) 20.568(4) Å, a ) 89.50°, b ) 75.63°, g ) 86.33°; V ) 4362(1) Å3; Z
) 2; Dcalc ) 1.432 g/cm3; Siemens P4PC diffractometer; 173 K; Mo Ka
radiation (l ) 0.710 73 Å); scan method 2θ-θ; θ range for data collection
2.53 to 29.54°; total number of data measured, 13 173; number of
independent reflections, 11 341 (Rint ) 3.54%); number of observed
reflections, 7405 (F > 4.0σ(F)). The structure was solved by direct methods
and refined by full-matrix least-squares procedures to give final residuals
of R ) 0.0753 and Rw ) 0.1754. From final difference Fourier maps,
residual electron densities of 1.140 and -0.719 e/Å3 were present. Hydrogen
atoms H(12) and H(18) were located and refined with temperature factors
fixed at 0.08 Å2. All other hydrogen atoms included the final refinement
were fixed at idealized positions. All atoms other than hydrogen were refined
with anisotropic thermal parameters with the exception of toluene carbons.
(8) Sato, M.; Tatsumi, T.; Kodama, T.; Hidai, M.; Uchida, T.; Uchida,
Y. J. Am. Chem. Soc. 1978, 100, 447.
† Los Alamos National Laboratory.
‡ Yale University.
(1) (a) Kubas, G. J. Acc. Chem Res. 1988, 21, 120. (b) Jessop, P. G.;
Morris, R. H. Coord. Chem. ReV. 1992, 121, 155. (c) Heinekey, D. M.;
Oldham, W. J., Jr. Chem. ReV. 1993, 93, 913. (d) Crabtree, R. H. Angew.
Chem., Int. Ed. Engl. 1993, 32, 789. (e) Kubas, G. J.; Burns, C. J.; Eckert,
J.; Johnson, S. W.; Larson, A. C.; Vergamini, P. J.; Unkefer, C. J.; Khalsa,
G. R. K.; Jackson, S. A.; Eisenstein, O. J. Am. Chem. Soc. 1993, 115, 569.
(f) Luo, X.-L.; Kubas, G. J.; Burns, C. J.; Eckert, J. Inorg. Chem. 1994,
33, 5219.
(2) Brookhart, M.; Grant, R. G.; Volpe, A. F., Jr. Organometallics 1992,
11, 3920.
(9) Zhang, K.; Gonzalez, A. A.; Mukerjee, S. L.; Chou, S.-J.; Hoff, C.
D.; Kubat-Martin, K. A.; Barnhart, D.; Kubas, G. J. J. Am. Chem. Soc.
1991, 113, 9170.
(10) Otsuka, S.; Yoshida, T.; Matsumoto, M.; Nakatsu, K. J. Am. Chem.
Soc. 1976, 98, 5880.
(3) (a) Perthuisot, C.; Fan, M.; Jones, W. D. Organometallics 1992, 11,
3622. (b) Banister, J. A.; Lee, P. D.; Poliakoff. Organometallics 1995, 14,
3876.
(4) Reimann, R. H.; Singleton, E. J. Organomet. Chem. 1972, 38, 113.
S0002-7863(96)00499-4 CCC: $12 00 © 1996 American Chemical Society

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Communications to the Editor
J. Am. Chem. Soc., Vol. 118, No. 28, 1996 6783
55Mn coupling. This shows that 1 is fluxional at 298 K which
is consistent with the closely related neutral Mo(CO)(R2PC2H4-
PR2)2 (R ) Ph, Bz).1e,f,8 No significant changes in the PC2H4P
resonances are observed on cooling to 198 K, indicating no
reduction in symmetry, i.e., the agostic interactions have not
been frozen out. The phenyl region consists of two distinct
sets of multiplets containing both phosphine and toluene-solvate
phenyl protons. The first ranges from 7.3-7.0 ppm, and the
second is significantly upfield of the first at 6.16 ppm,
integrating as 8 H against the PC2H4P protons. The latter feature
is also present in the toluene-free system and is remarkably
result from the much more electrophilic Mn center (νCO ) 1862
cm-1 in 1 versus 1723 cm-1 in Mo(CO)(dppe)2) weakening
H-H primarily by drawing electron density away from the H-H
σ-bonding orbital rather than by backbonding.
Our attempts to evaluate the role of backbonding in 1 and
related fragments compelled us to investigate the dinitrogen
complex, [Mn(N2)(CO)(dppe)2]BAr′4, formed by placing CH2Cl2
solutions of 1 under N2 at -78 C.15 The blue solution turns
yellow on N2 addition. On warming to room temperature in a
closed system, the solution turns blue-green. 31P{1H} NMR of
[Mn(N2)(CO)(dppe)2]BAr′4 at 198 K in CH2Cl2 reveals a
resonance at 75.7 ppm. On warming to 298 K a second
resonance associated with the 16-electron complex grows in at
82.6 ppm, and the resonance associated with the N2 complex is
observed at 75.0 ppm. Both peaks broaden, indicating exchange
is beginning to take place. Integration reveals 37% of the Mn
centers contain bound N2. 1H NMR at 200 K consists of two
multiplets for PC2H4P, consistent with N2 trans to CO. The
resonance at 6.16 ppm associated with the ortho-phenyl protons
disappears on N2 addition, and a new multiplet grows in at 6.47
ppm integrating as eight protons relative to those of the anion.
1
similar to that observed for Mo(CO)[Bz2PC2H4PBz2]2. In
contrast to the latter, no collapse of the resonance into the base
line is seen down to 198 K, indicating 1 is more fluxional. The
resonance did sharpen (from 92 Hz at 298 K to 74 Hz at 198
K, FWHM, 500 MHz) and shifted slightly to 6.05 ppm,
indicating octahedrally-coordinated Mn in which the site trans
to CO interacts with8 rapidly exchanging agostic hydrogens.
The 31P{1H} NMR shows a singlet at both 298 and 198 K,
consistent with a highly fluxional complex.
The H2 complex forms by placing solutions of 1 under H2.
1
On warming to 25 °C, the H resonances associated with the
N2 complex decrease in intensity, and those associated with 1
increase. Most notably the PC2H4P resonances collapse to a
single broad multiplet with a slight upfield shoulder.
In order to determine if N2 binding is bridging or terminal,
an 15N NMR study was performed on the complex prepared
with 99.9% 15N-enriched N2 (Isotec Inc). At 200 K, two
multiplets occur at -27.9 and -56.2 ppm (external reference:
nitromethane). Although these could not be assigned (Mn-NN
vs Mn-NN) because broadening from 55Mn coupling destroyed
31P and 15N coupling information, two resonances are consistent
with terminal rather than µ-N2. On warming to 300 K, the
resonances shift to -25.1 and -56.1 ppm and broaden slightly.
Precipitation from CH2Cl2 with pentane at -78 °C was used
to trap [Mn(N2)(CO)(dppe)2]BAr′4 in the solid state.16 This gave
an opportunity to observe νNN and νCO in IR of solid mulls to
confirm terminal rather than µ-N2 as in [Mo(CO)(depe)2]2(µ-
N2)17 and also to gauge backbonding. Both νNN and νCO (2167
and 1911 cm-1) are much higher than those for Mo(N2)(CO)-
(dppe)2 (2090 and 1809 cm-1), confirming significantly less
backbonding in [Mn(N2)(CO)(dppe)2]BAr′4 than Mo(N2)(CO)-
(dppe)2. By comparison, MnH(N2)(dmpe)2 is much more basic
with νNN ) 1947 cm-1,3 which is 220 cm-1 lower than for Mn-
12
[Mn(H2)(CO)(dppe)2]BAr′4 is light yellow with solubility
similar to 1. The H2 is extremely labile and readily dissociates
when the H2 atmosphere is removed or the complex is warmed.
At 298 K, the H resonances of PC2H4P show two multiplets
1
consistent with H2 trans to CO. The upfield phenyl resonance
collapses into the phenyl multiplet at 7.3-7.0 ppm. The H2
resonance is broad with no 31P coupling and displays a field
dependent width at -7.21 ppm (27 Hz, 300 MHz; 62 Hz, 500
MHz), integrating as 2 H relative to PC2H4P. On cooling to
168 K, the H2 signal greatly broadens (540 Hz, 300 MHz). Mo-
(H2)(CO)(dppe)2 does not display this temperature dependence,
although the signal is already very broad at 298 K (∼300 Hz,
300 MHz). The more electron-rich complex, Mo(H2)(CO)(Bz2-
PC2H4PBz2)2, shows very similar behavior to the Mn complex.1
A single temperature independent resonance was observed for
31P{1H} NMR consistent with H2 trans to CO and no observable
dihydrogen-dihydride equilibrium. The HD complex prepared
from HD gas gives an HD coupling constant of 32 Hz in accord
with a short H-H distance.1
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(N2)(CO)(dppe)2 and below the usual range1b (2060-2160
cm-1) observed for stable H2 binding to the analogous metal
fragment.
The H-H distance was determined by solid-state NMR
measurements13 to be 0.89(2) Å in [Mn(H2)(CO)(dppe)2]BAr′4
prepared by exposing solid toluene-free 1 to H2. This compares
favorably to H-H NMR distances observed in Mo(H2)(CO)3-
We are continuing to explore coordination to 1, which is
unique among first-row metals in giving isolable agostic, H2,
and N2 adducts. Preliminary results show that organosilanes
also interact but more weakly than in Mo(η2-SiRnH4-n)(CO)-
(dppe)2.18 We will examine the effects of using other low-
interacting anions and varying co-ligands, e.g., depe versus dppe.
Acknowledgment. This work was supported by the Division of
Chemical Sciences, Office of Basic Energy Sciences, U.S. Department
of Energy.
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(PCy3)2 (0.87 Å)13a and FeH(H2)(dppe)2 (0.86 Å)14 and
correlates with the large JHD. The distance in MnH3(dmpe)2
was estimated from solution NMR T1 to be 0.90 or 1.13 Å
depending on fast or slow H2 rotation.3 Remarkably, the H-H
length in Mo(H2)(CO)(dppe)2 (0.88 Å)13b is nearly identical to
the Mn system despite the fact that backbonding from the first
row Mn cation should be less than from the second row neutral
Mo. This should give a shorter H-H in the Mn system because
of less donation of electron density to the σ* H2 orbital. The
similarity of H-H distances in the Mn and Mo systems may
Supporting Information Available: X-ray diffraction data (15
pages). Ordering information is given on any current masthead page.
JA960499Q
(15) 1H NMR data for [Mn(N2)(CO)(dppe)2]BAr′4 system in CD2Cl2:
200 K: d 7.73 (s, C6H3(3,5-CF3)2, 8H, ortho); 7.53, (s, C6H3(3,5-CF3)2,
4H, para); 7.4-7.0 (m), 6.47 (m) (C6H5, toluene and phosphine, 43.5H);
2.57 (m, PCH2CH2P, 4H); 2.42 (m, PCH2CH2P, 4H); 2.28 (CH3, toluene,
2H). 298 K: d 7.73 (s, C6H3(3,5-CF3)2, 8H, ortho);7.56, (s, C6H3(3,5-CF3),
4H, para); 7.3-7.1 (m), 6.67(m) (C6H5, toluene and phosphine, 43.5H);
2.67 (m, PCH2CH2P, 8H); 2.34 (CH3, toluene, 2H).
(16) The orange-yellow precipitate was found to be a mixture of [Mn(N2)-
(CO)(dppe)2]BAr′4 and 1 by IR of a Nujol mull.
(17) Luo, X.-L.; Kubas, G. J.; Burns, C. J.; Butcher, R. J.; Bryan, J. C.
Inorg. Chem. 1995, 34, 6538. νNN is absent in the IR for µ-N2 complexes.
(18) (a) Luo, X.-L.; Kubas, G. J.; Bryan, J. C.; Burns, C. J.; Unkefer, C.
J. J. Am. Chem. Soc. 1994, 116, 10312. (b) Luo, X.-L.; Kubas, G. J.; Burns,
C. J.; Bryan, J. C.; Unkefer, C. J. J. Am. Chem. Soc. 1995, 117, 1159.
(11) Heinekey, D. M.; Schomber, B. M.; Radzewich C. E. J. Am. Chem.
Soc. 1994, 116, 4515.
(12) [Mn(H2)(CO)(dppe)2]BAr′4: 1H NMR (CD2Cl2) d 7.72 (s, C6H3-
(3,5-CF3)2, 8H, ortho); 7.55, (s, C6H3(3,5-CF3)2, 4H, para); 7.4-7.0 (m),
(C6H5 of toluene and phosphine, 40H); 2.52 (m, PCH2CH2P, 4H); 2.24 (m,
PCH2CH2P, 4H); -7.23 (2H, 60Hz FWHM 500 MHz); 31P (CD2Cl2) d
85.4. IR (CH2Cl2, cm-1) ν(CO) 1896.
(13) (a) Zilm, K. W.; Millar, J. M. AdV. Magn. Opt. Reson. 1990, 15,
163. (b) Zilm, K. W.; Merrill, R. A.; Kummer, M. W.; Kubas G. J. J. Am.
Chem. Soc. 1986, 108, 7837.
(14) Ricci, J. S.; Koetzle, T. F.; Bautista, M. T.; Hofstede, T. M.; Morris,
R. H.; Sawyer, J. F. J. Am. Chem. Soc. 1989, 11, 8823.