Manganese-tin triple bonds: A new synthetic route to the manganese stannylidyne complex cation trans -[H(dmpe)2Mn-Sn(C6H 3-2,6-Mes2)]+ (dmpe = Me2PCH 2CH2PMe2, Mes = 2,4,6-Trimethylphenyl)

Article abstract of DOI:10.1021/ja406290t

A new approach to the first complex featuring a manganese-tin triple bond that takes advantage of the propensity of dihydrogen complexes to eliminate H₂ is reported. Reaction of the 18-valence-electron manganese dihydrogen hydride complex [MnH(η²-H₂)(dmpe) ₂] (1) (dmpe = Me₂PCH₂CH₂PMe ₂) with the organotin(II) chloride SnCl(C₆H ₃-2,6-Mes₂) (Mes = 2,4,6-trimethylphenyl) selectively afforded by H₂ elimination the chlorostannylidene complex trans-[H(dmpe)₂Mn-Sn(Cl)(C₆H₃-2,6-Mes ₂)] (2), which upon treatment with Na[B(C₆H ₃-3,5-(CF₃)₂)₄] and Li[Al(OC(CF ₃)₃)₄] was transformed quantitatively into the stannylidyne complex salts trans-[H(dmpe)₂Mn-Sn(C₆H ₃-2,6-Mes₂)]A [A = B(C₆H₃-3,5- (CF₃)₂)₄ (3a), Al(OC(CF₃) ₃)₄ (3b)]. Complexes 2 and 3a/3b were fully characterized, and the structures of 2 and 3a were determined by single-crystal X-ray diffraction. Complex 2 features the shortest Mn-Sn double bond reported to date, a large Mn-Sn-C_aryl bond angle, and a long Sn-Cl bond of the trigonal-planar-coordinated tin center. These bonding features can be rationalized in valence-bond terms by a strong contribution of the triply bonded resonance structure [L_nMn-SnR]Cl and were verified by a natural resonance theory (NRT) analysis of the electron density of the DFT-minimized structure of 2. Complex 3a features the shortest Mn-Sn bond reported to date and a linearly coordinated tin atom. Natural bond order and NRT analyses of the electronic structure of the complex cation in 3a/3b suggested a highly polar Mn-Sn triple bond with a 65% ionic contribution to the NRT Mn-Sn bond order of 2.25. Complex 3a undergoes reversible one-electron reduction, suggesting that open-shell stannylidyne complexes might be accessible using strong reducing agents.

Full text of DOI:10.1021/ja406290t

Communication
pubs.acs.org/JACS
Manganese−Tin Triple Bonds: A New Synthetic Route to the
Manganese Stannylidyne Complex Cation trans-
[H(dmpe)2MnSn(C6H3‑2,6-Mes2)]+ (dmpe = Me2PCH2CH2PMe2, Mes
= 2,4,6-Trimethylphenyl)
Alexander C. Filippou,* Priyabrata Ghana, Uttam Chakraborty, and Gregor Schnakenburg
Institut fur Anorganische Chemie, Universitat Bonn, Gerhard-Domagk-Str. 1, 53121 Bonn, Germany
̈
̈
S
* Supporting Information
series of closed-shell Mo and W complexes containing linearly
coordinated, triply bonded Ge−Pb atoms have been isolated by
our group using a very efficient N2/PMe3 elimination method.3
Recent studies in our group showed that this method could also
be successfully employed to prepare the rhenium germylidyne
complexes mer-[X2(PMe3)3ReGe(C6H3-2,6-Trip2)] (X = Cl,
I, H; Trip = C6H2-2,4,6-iPr3)4 and cationic germylidyne and
ABSTRACT: A new approach to the first complex
featuring a manganese−tin triple bond that takes
advantage of the propensity of dihydrogen complexes to
eliminate H2 is reported. Reaction of the 18-valence-
electron manganese dihydrogen hydride complex [MnH-
(η2-H2)(dmpe)2] (1) (dmpe = Me2PCH2CH2PMe2) with
the organotin(II) chloride SnCl(C6H3-2,6-Mes2) (Mes =
2,4,6-trimethylphenyl) selectively afforded by H2 elimi-
nation the chlorostannylidene complex trans-[H-
(dmpe)2MnSn(Cl)(C6H3-2,6-Mes2)] (2), which upon
treatment with Na[B(C6H3-3,5-(CF3)2)4] and Li[Al(OC-
(CF3)3)4] was transformed quantitatively into the
stannylidyne complex salts trans-[H(dmpe)2MnSn-
(C6H3-2,6-Mes2)]A [A = B(C6H3-3,5-(CF3)2)4 (3a),
Al(OC(CF3)3)4 (3b)]. Complexes 2 and 3a/3b were
fully characterized, and the structures of 2 and 3a were
determined by single-crystal X-ray diffraction. Complex 2
features the shortest Mn−Sn double bond reported to
date, a large Mn−Sn−Caryl bond angle, and a long Sn−Cl
bond of the trigonal-planar-coordinated tin center. These
bonding features can be rationalized in valence-bond terms
by a strong contribution of the triply bonded resonance
structure [LnMnSnR]Cl and were verified by a natural
resonance theory (NRT) analysis of the electron density of
the DFT-minimized structure of 2. Complex 3a features
the shortest Mn−Sn bond reported to date and a linearly
coordinated tin atom. Natural bond order and NRT
analyses of the electronic structure of the complex cation
in 3a/3b suggested a highly polar Mn−Sn triple bond with
a 65% ionic contribution to the NRT Mn−Sn bond order
of 2.25. Complex 3a undergoes reversible one-electron
reduction, suggesting that open-shell stannylidyne com-
plexes might be accessible using strong reducing agents.
s t a n n y l i d y n e c o m p l e x e s o f g r o u p
8 m e t a l s ,
[(R2PCH2CH2PR2)2ME(C6H3-2,6-R′2)]A (M = Fe, Ru; E =
Ge, Sn; R = Me, Et; R′ = Mes, Trip; A = singly charged anion).5
Lately we also accomplished the synthesis of the first silylidyne
complex, [(η5-C5H5)(CO)2MoSi(C6H3-2,6-Trip2)].6 To
date, the vast majority of these compounds involve second-
and third-row transition metals, whereas examples involving 3d
metals are extremely rare.2b,5a This is not surprising, since
shrinkage of the d orbitals is predicted to reduce the M−E σ- and
π-orbital overlap, weakening the ME triple bond.7 In this
context, there may be great benefit in pursuing new routes
employing transition-metal complexes with labile ligands other
than those used so far. In recent years, transition-metal
polyhydrides8 and dihydrogen complexes9 have attracted
ample attention because of their fascinating chemistry toward
C−H activation10 and potential applications in hydrogen
storage.11 Taking advantage of the propensity of these
compounds to eliminate dihydrogen, we present here a new
stepwise approach to the first manganese complex featuring a
Mn−Sn triple bond.
The 18-valence-electron (18VE) manganese dihydrogen
hydride complex [MnH(η2-H2) (dmpe)2] (1) was chosen as a
promising starting material for an entry into Mn−Sn multiple-
bond chemistry, since 1 has been shown to lose H2 easily to give a
16VE intermediate that can be trapped quantitatively by a variety
of ligands.12 In fact, treatment of 1 with 1 equiv of SnCl(C6H3-
2,6-Mes2) in toluene was accompanied by a rapid color change
from yellow to dark red-brown with concomitant evolution of
H2. Monitoring of the reaction by 31P NMR spectroscopy
revealed the selective formation of a new complex that was
isolated as an air-sensitive, red-brown solid in 93% yield and
shown by elemental analysis, IR and multinuclear NMR
spectroscopy, and single-crystal X-ray diffraction (scXRD) to
be the chlorostannylidene complex trans-[H(dmpe)2Mn
Sn(Cl)(C6H3-2,6-Mes2)] (2) (Scheme 1). Complex 2 shows
ompounds featuring triple bonds between transition metals
Cand the heavier group 14 congeners of carbon are of
fundamental importance for the understanding of chemical
bonding and outline a challenging research field with many new
perspectives for the molecular chemistry of transition metals and
main-group elements stemming from the electronic structure
and polarity of the ME triple bond (M = transition metal; E =
Si−Pb).1 Following the first report of a molybdenum
germylidyne complex in 1996 by Power and co-workers,2 a
Received: June 25, 2013
Published: July 19, 2013
© 2013 American Chemical Society
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Communication
remarkable thermal stability and does not melt or decompose
upon heating to 240 °C.
large steric demand of the m-terphenyl substituent, whereas the
Caryl−Sn−Cl angle is lowered to 87.69(5)°, indicating that the tin
center uses only p orbitals for σ bonding to its substituents.
Finally, the Sn−Cl bond of 2 [2.5566(5) Å] is considerably
elongated relative to the Sn−Cl bond lengths of two- or three-
coordinate SnII centers in organotin chlorides [SnCl(C6H3-2,6-
Trip2), 2.4088(8) Å; SnCl(C6H3-2,6-Trip2)(py): 2.448(2) Å].15
The same features (short ME bond, large M−E−Caryl angle,
long E−Cl bond) are also shown by other chlorotetrelylidene
complexes of electron-rich metal centers16 recently isolated in
our group4,5 and can be rationalized in valence-bond terms by a
contribution of the ylidyne resonance form d to the bonding
(Figure 2).
Scheme 1. Synthesis of Manganese Chlorostannylidene and
Stannylidyne Complexes
Figure 2. Resonance forms for the metal−tetrel bond in electron-rich
metal chlorotetrelylidene complexes.
The molecular structure of 2 (Figure 1) shows a distorted
octahedral complex with a trans arrangement of the stannylidene
This was verified by a natural resonance theory (NRT) analysis
of the density functional theory (DFT) electron density of 2,17a
which showed a major triply bonded resonance structure similar
to d with a weight of 23% followed by several doubly bonded
(overall weight 50%) and singly bonded (overall weight 21%)
resonance structures, leading to an NRT bond order of 2.08
(Table 1).17b The Mn−Sn double bond of 2 has low covalent
character, as evidenced by the low covalent (0.69) and high ionic
(1.39) contributions to the natural bond order (NBO) (Table
1).17b The high polarity of the Mn−Sn double bond is also
reflected in the high opposite natural population analysis (NPA)
charges (Mn, −0.87; Sn, +1.46). An NBO analysis of 2 indicated
that the σ and π components of the MnSn bond are strongly
polarized toward the Sn and Mn atoms, respectively, as in the
MC bonds of Fischer-type carbene complexes.18
The IR and NMR spectra corroborate the solid-state structure
of 2. The 31P{1H} NMR spectrum displays a singlet at 79.1 ppm
indicating the trans configuration of complex 2. The singlet signal
is accompanied by tin satellites [2J(119Sn,P) = 210 Hz] and
appears at slightly higher field than that of 1 (82.73 ppm).12 The
1H and 13C{1H} NMR spectra suggest an overall Cs-symmetric
complex in solution in which the m-terphenyl substituent is fixed
in the orthogonal conformation observed in the solid state.19 In
combination with the hindered rotation about the C−mesityl
bond, this renders the ortho and meta positions of the
enantiotopic mesityl substituents diastereotopic, giving rise to a
Figure 1. DIAMOND plot of the molecular structure of 2. Thermal
ellipsoids are set at 30% probability. H atoms have been omitted except
for H37 bonded to Mn. Selected bond lengths (Å) and angles (deg):
Mn−Sn, 2.3997(3); Mn−H37, 1.51(2); Sn−Cl, 2.5566(5); Sn−C1,
2.209(2); Mn−Sn−C1, 158.53(4); Mn−Sn−Cl, 113.66(1); C1−Sn−
Cl, 87.69(5); Sn−Mn−H37, 176.6(9); Sn−Mn−P1, 96.24(2); Sn−
Mn−P2, 98.50(2); Sn−Mn−P3, 96.02(2); Sn−Mn−P4, 101.50(2).
1
ligand and the hydrido ligand, which could be located in the
difference Fourier map and refined isotropically to a Mn−H
distance of 1.51(2) Å. The P atoms are inclined toward the
hydrido ligand, as evidenced by the Sn−Mn−P bond angles of
96−102°, to minimize the steric pressure exerted by the bulky
chlorostannylidene ligand. Complex 2 reveals some remarkable
structural features. The Mn−Sn bond of 2 [2.3997(3) Å] is
shorter than all previously reported Mn−Sn double bonds
(2.44−2.52 Å)13 and is also considerably shorter than Mn−Sn
single bonds (median value 2.625 Å).14 This suggests a strong
Mn−Sn bonding interaction in 2. Furthermore, the angles at the
trigonal-planar-coordinated tin atom (sum of angles at Sn =
359.9°) differ markedly. Thus, the Mn−Sn−Caryl angle is
considerably widened to 158.53(4)°, mainly because of the
double set of resonances for the ortho and meta H and 13C
nuclei (Figure S1 in the Supporting Information). The 1H NMR
spectrum of 2 also displays a characteristic high-field signal
(−13.73 ppm) for the hydrido ligand, which is split into a quintet
by coupling to the four equivalent 31P nuclei [2J(P,H) = 59 Hz].
The presence of a hydrido ligand is also verified by a
characteristic IR absorption band at 1760 cm−1 originating
from the ν(Mn−H) stretching vibration (Figures S5 and S7).
Interestingly, the 119Sn{1H} NMR spectrum of 2 displays a very
broad signal at 540 ppm.20
The strong polarization of the Sn−Cl bond of complex 2 (vide
supra) led us to assume that the chloride group might be easily
removed. In fact, treatment of 2 with a nonoxidizing halide-
abstraction agent such as Na[B(C6H3-3,5-(CF3)2)4] or Li[Al-
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Communication
a
Table 1. Selected B3LYP-Calculated and Experimental Bond Lengths and Angles and Selected Results of the NBO and NRT
Analyses of 2 and 3
b
c
bond lengths (pm)
bond angles (deg)
NPA charges
Sn
NBO analysis
occ.
NRT analysis
Mn−Sn
Mn−H
Mn−Sn−C1 H−Mn−Sn
Mn
H
bond
pol.
bond: BO (cov./ionic)
2 calc
2.486
1.566
158.2
173.8
−0.87
+1.46
−0.18
σ
π
1.92
1.82
72.6% Sn
Mn−Sn: 2.08 (0.69/1.39)
Mn−H37: 0.35 (0.04/0.31)
83.3% Mn
2 exp
3 calc
2.3997(3)
2.373
1.51(2)
1.552
158.53(4)
180.0
176.6(9)
180.0
−0.87
+1.35
−0.14
σ
π
1.93
1.44
73.0% Sn
Mn−Sn: 2.25 (0.80/1.45)
Mn−H75: 0.50 (0.39/0.11)
76.6% Mn
3 exp
2.3434(5)
1.44(3)
176.85(9)
177.1(14)
a
b
Basis sets: TZVPP for Mn, Sn, P, Cl, Hhydride; 6-31G* for all other atoms. NBO analysis: bond (Mn−Sn σ or Mn−Sn π), occupancy (occ.), and
c
bond polarization (pol.). NRT analysis: bond: total bond order (covalent bond order/ionic bond order).
(OC(CF3)3)4] in fluorobenzene was accompanied by rapid
precipitation of MCl (M = Li, Na), selectively affording the
stannylidyne complex salts 3a and 3b, respectively (Scheme 1).
Complexes 3a and 3b were isolated in 90% yield as purple,
extremely air-sensitive solids. The thermal stabilities of 3a and 3b
are remarkable; 3a melted at 195 °C, whereas 3b neither melted
nor decomposed upon heating to 240 °C.
(CO)3(CNR)2SnCl] (R = C6H3-2,6-Dipp2; Dipp = 2,6-
diisopropylphenyl), which contains a singly bonded, V-shaped
SnCl ligand,23 and compares very well with that predicted for a
Mn−Sn triple bond [d(MnSn) = 2.35 Å] using the
theoretically calculated triple-bond radii of Mn (1.03 Å) and
tin (1.32 Å).24
NRT analysis of the electron density of the DFT-minimized
structure of the complex cation of 3a/3b suggests a highly polar
MnSn bond, as evidenced by the low covalent (0.85) and high
ionic (1.45) contributions to the total Mn−Sn NBO of 2.25,
which is slightly larger than that of 2, rationalizing the observed
shorter Mn−Sn bond in 3a/3b than in 2. The polarity of the
MnSn bond is also reflected in the high opposite NPA charges
of Mn (−0.87) and Sn (+1.35). A look at the frontier Kohn−
Sham orbitals of the complex cation of 3a/3b shows that the
LUMO+1 and LUMO correspond to the πin* and πout* Mn−Sn
bonds and that two π-type molecular orbitals (HOMO, πout;
HOMO−2, πin) in combination with the Mn−Sn σ-type orbital
(HOMO−8) contribute to the formation of the Mn−Sn triple
bond (Figure 4). The composition and structure of 3a and 3b
were further confirmed by elemental analyses and IR and NMR
spectroscopy. Analogous to 2, the 31P{1H} NMR spectra of 3a
and 3b show a singlet flanked by tin satellites [2J(119Sn,P) = 200−
215 Hz] at slightly higher field (71.0 ppm) than in 2, and the 1H
NMR spectra show a distinctive quintet signal [2J(P,H) = 66 Hz]
for the hydrido ligand, which appears at slightly higher field
(−14.59 ppm) than that of 2. Notably, the position of the
ν(Mn−H) absorption band in the IR spectrum is dependent on
the counteranion (3a, 1792 cm−1; 3b, 1778 cm−1). The
stannylidyne complex cation has a time-averaged C2v-symmetric
structure in solution, which renders the mesityl substituents of
the m-terphenyl group homotopic. Therefore, only a single set of
signals is observed for the ortho and meta 1H and 13C nuclei of
the mesityl groups, respectively (Figure S3). Furthermore, the
13C{1H} NMR spectra of 3a and 3b display a distinctive low-field
signal for the tin-bonded Caryl atom at 188.2 ppm, which is typical
for stannylidyne complexes.3b,7d Finally, 3a is distinguished by a
broad 119Sn NMR signal appearing at lower field (761 ppm) than
that of 2.
The solid-state structure determined by scXRD (Figure 3)
shows that 3a is composed of well-separated cations and anions,
Figure 3. DIAMOND plot of the molecular structure of the complex
cation in 3a. Thermal ellipsoids are set at 30% probability. H atoms have
been omitted except for H75 bonded to Mn. Selected bond lengths (Å)
and angles (deg): Mn−Sn, 2.3434(5); Sn−C1, 2.159(3); Mn−H75,
1.44(3); Mn−Sn−C1, 176.85(9); Sn−Mn−H75, 177.1(14); Sn−Mn−
P1, 97.40(3); Sn−Mn−P2, 100.94(3); Sn−Mn−P3, 98.16(3); Sn−
Mn−P4, 99.17(3).
excluding any bonding contact between the electrophilic tin
center and the counteranion.21,22 The trans-configured,
distorted-octahedral 18VE stannylidyne complex cation features
a nearly linearly coordinated tin atom [Mn−Sn−C1 =
176.85(9)°] and the shortest Mn−Sn bond reported to date
[2.3434(5) Å]. Notably, the Mn−Sn bond is 0.3 Å shorter than
that found in the manganostannylene mer,trans-[Mn-
Figure 4. Frontier Kohn−Sham orbitals of the complex cation of 3a/3b (isosurface value, 0.04 e Å−3; energies in eV).
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Journal of the American Chemical Society
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3339. (e) Pandey, K. K.; Lledos
(f) Pandey, K. K.; Patidar, P. Polyhedron 2012, 37, 85 and references
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, A. Inorg. Chem. 2009, 48, 2748.
The electrochemical properties of complex 3a were studied by
cyclic voltammetry. Remarkably, 3a undergoes a reversible one-
electron reduction at a half-wave potential of −1.58 V vs
[Fe(C5Me5)2]+1/0 (−2.03 V vs [Fe(C5H5)2]+1/0) (Figure S8).
This suggests that open-shell stannylidyne complexes might be
accessible using strong reducing agents such as [Fe(C5Me5)-
(C6Me6)].25 Studies in this direction are currently underway.
The presented mild and selective approach to the first
manganese complex featuring a Mn−Sn triple bond illustrates
the synthetic potential of dihydrogen complexes in metal−tetrel
multiple-bond chemistry. Further investigations to explore this
synthetic potential for building up metal complexes with
nonclassical π-acceptor ligands of the heavier group 14 elements
are currently in progress.
(11) (a) Schlapback, L.; Zuttel, A. Nature 2001, 414, 353. (b) Grochala,
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W.; Edwards, P. P. Chem. Rev. 2004, 104, 1283. (c) van den Berg, A. W.
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(13) Only a very few complexes featuring π bonds between Mn and
trigonal-planar-coordinated Sn have been structurally characterized to
date. See: (a) Herrmann, W. A.; Kneuper, H.-J.; Herdtweck, E. Chem.
Ber. 1989, 122, 437. (b) Ettel, F.; Huttner, G.; Zsolnai, L.; Emmerich, C.
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M.; Herber, R. H.; Nowik, I.; Ross, D. H.; Russell, C. A.; Wass, D. F.
Inorg. Chem. 2011, 50, 2252.
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental section and additional data. This material is
available free of charge via the Internet at http://pubs.acs.org.
(14) A Cambridge Structural Database survey (5/06/2013) gave 43
hits of Mn complexes containing single bonds to four-coordinate Sn
atoms. The Mn−Sn single-bond lengths ranged from 2.455 to 2.758 Å
with mean and median values of 2.624 and 2.625 Å, respectively. For the
shortest and longest reported Mn−Sn single bonds, respectively, see:
(a) Schiemenz, B.; Ettel, F.; Huttner, G.; Zsolnai, L. J. Organomet. Chem.
1993, 458, 159. (b) Haupt, H.-J.; Preut, H.; Wolfes, W. Z. Anorg. Allg.
Chem. 1978, 446, 105.
AUTHOR INFORMATION
Corresponding Author
filippou@uni-bonn.de
■
Notes
The authors declare no competing financial interest.
(15) Eichler, B. E.; Pu, L.; Stender, M.; Power, P. P. Polyhedron 2001,
20, 551.
(16) For N2 complexes of electron-rich metal centers, see:
(a) Pombeiro, A. J. L.; Richards, R. L. Coord. Chem. Rev. 1990, 104,
13. (b) Pombeiro, A. J. L.; Guedes da Silva, M. F. C.; Michelin, R. A.
Coord. Chem. Rev. 2001, 218, 43.
(17) (a) Glendening, E. D.; Weinhold, F. J. Comput. Chem. 1998, 19,
593. (b) Glendening, E. D.; Weinhold, F. J. Comput. Chem. 1998, 19,
610.
ACKNOWLEDGMENTS
■
We thank the Deutsche Forschungsgemeinschaft (SFB 813) for
generous financial support and C. Schmidt, K. Prochnicki, H.
Spitz, Dr. B. Lewall, and A. Martens for contributions to the
experimental work.
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(19) The orthogonal orientation of the m-terphenyl substituent in 2 is
best evidenced by the dihedral angle of 87.64(8)° between the
stannylidene ligand plane passing through the Mn, Sn, Cl, and C1 atoms
and the central aryl ring plane (C1−C6).
(20) It is noteworthy that detection of 119Sn NMR signals is not easy
and has been often hampered by signal broadening resulting from the
effect of chemical shift anisotropy on relaxation. See: (a) Grindley, T. B.;
Curtis, R. D.; Thangasara, R.; Wasylishen, R. E. Can. J. Chem. 1990, 68,
2102. (b) Reference 23.
(21) The closest interionic contact (2.399 Å) exists between a F atom
of the counteranion and a m-CH group of the central phenyl ring of the
m-terphenyl substituent. No contacts to the Sn atom shorter than van
der Waals contacts were found.
(22) The solid-state structure of 3b was also determined using scXRD
and showed the same structural motif and very similar bonding
parameters for the complex cation as in 3a. However, because of the low
quality of two data sets for two different samples, the structure of 3b was
not further considered.
(23) Stewart, M. A.; Moore, C. E.; Ditri, T. B.; Labios, L. A.; Rheingold,
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(5) (a) Blom, B. Reactivity of Ylenes at Late Transition Metal Centers.
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(7) For selected quantum-chemical studies of the ME bond, see: (a)
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Chem. Soc. 2003, 125, 1660. (c) Takagi, N.; Yamazaki, K.; Nagase, S.
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