Carbon disulfide binding at dinuclear and mononuclear nickel complexes ligated by a redox-active ligand: Iminopyridine serving as an accumulator of redox equivalents for the activation of heteroallenes

Article abstract of DOI:10.1039/c2cc34307j

The dinuclear complex Ni₂L¹(η²-CS ₂)₂ (2), featuring iminopyridine ligation, is prepared by COD substitution from Ni₂L¹(COD)₂ (1). Spectroscopic, structural, and theoretical data reveals significant activation of the metal-bound C-S bonds, as well as the different oxidation states of the iminopyridine in 1 (1-) and 2 (0).

Full text of DOI:10.1039/c2cc34307j

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COMMUNICATION
Carbon disulfide binding at dinuclear and mononuclear nickel complexes
ligated by a redox-active ligand: iminopyridine serving as an accumulator
of redox equivalents for the activation of heteroallenesw
Amarnath Bheemaraju, Jeffrey W. Beattie, Richard L. Lord, Philip D. Martin and
Stanislav Groysman*
Received 15th June 2012, Accepted 7th August 2012
DOI: 10.1039/c2cc34307j
The dinuclear complex Ni2L1(g2-CS2)2 (2), featuring iminopyridine
ligation, is prepared by COD substitution from Ni2L1(COD)2 (1).
Spectroscopic, structural, and theoretical data reveals significant
activation of the metal-bound C–S bonds, as well as the different
oxidation states of the iminopyridine in 1 (1ꢀ) and 2 (0).
change to purple-brown and precipitate formation. Unlike 1,
Ni2L1(CS2)2 (2, Fig. 1) has poor solubility in THF, enabling its
easy isolation in high yield as a dark purple solid.z Compound
2 has modest solubility in DMF or DMSO. The 1H NMR
spectrum of 2 is consistent with two identical iminopyridine
units. We prepared a 13C-enriched (99% 13C) sample of 2 and
collected its 13C{1H} NMR spectrum. The 13CS2 signal of 2
appears at 268 ppm (see Fig. S3 in the ESIw), indicating its
substantial activation.7,9 The IR spectrum of Ni2L1(CS2)2
shows two prominent signals at ca. 1138 cmꢀ1 and 648 cmꢀ1
that shift to 1095 cmꢀ1 and 633 cmꢀ1 upon substitution by
13CS2 (Fig. S24–S27, ESIw). These signals are assigned as CQS
and C–S stretches, respectively.9,13 ESI-MS displays a molecular
ion peak at m/z = 581.8, and the signal attributed to CS2 loss is
observed at m/z = 505.9 (Fig. S22 and S23, ESIw).
Metal-assisted activation of CS2 has often resulted in its
further reactivity, culminating in disproportionation or C–C
coupling.5,6,14 Compound 2 is sparingly soluble, can be isolated in
high yield, and demonstrates no decomposition in solution over 24
hours as judged by its 1H NMR spectrum. We have also prepared
a mononuclear analogue of 2, compound 4. Compound 4 was
obtained by the reaction of 3 (prepared from N-benzylimino-
pyridine15 and Ni(COD)2, see ESIw for details) with 1 equiv. of
CS2. Spectroscopic data for 3 and 4 are similar to the spectro-
scopic data for 1 and 2 (Fig. S4–S6, ESIw). Compound 4 has
significantly higher solubility in THF, DMF and DMSO.
Surprisingly, 4 is not stable in solution for prolonged periods
Contemporary interest in the chemistry of carbon dioxide is
motivated by its detrimental environmental effects combined with
a potential to serve as an inexpensive and plentiful C1 building
block in the synthesis of chemicals or fuels.1,2 Development of the
basic chemistry of carbon dioxide is impeded by its low reactivity
with metal centers.3,4 Carbon disulfide is a useful model for
carbon dioxide reactivity: it is a liquid, it is slightly more reactive,
and it often displays similar binding and activation modes.5–9
Thus, carbon disulfide chemistry can help shed light on the
chemistry of its relatively unreactive congener.
Dinuclear metal complexes tailored for cooperative reactivity10
present a promising strategy towards activation of small molecules,
in particular carbon dioxide.11 Our synthetic endeavors are focused
on the preparation and reactivity studies of dimetallic platforms
featuring redox-active chelating units with carbon dioxide and
related heteroallenes. We have recently reported dinuclear
complexes of Ni(0) with flexible bis(iminopyridine) ligands.12
The coordination chemistry of these ligands was found
to depend on the imino-carbon atom substituent. For the
H-substituent, dinuclear complex Ni2L1(COD)2 (1) was obtained,
whereas a Me-imino carbon atom substituent led to the formation
of bis(homoleptic) complexes of (NiL2)2 constitution. Herein, we
report the synthesis and the characterization of the dinuclear
Ni2L1(CS2)2 complex obtained by reaction of 1 and CS2, and
compare it with its mononuclear analogue.
Treatment of a blue-violet THF solution of 1 with two
equivalents of carbon disulfide results in an immediate color
Department of Chemistry, Wayne State University, Detroit,
Michigan 48202, USA. E-mail: groysman@chem.wayne.edu;
Tel: +1 313 577 2689
w Electronic supplementary information (ESI) available: Full
experimental procedures, NMR spectra, and X-ray data. CCDC
887355–887357. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c2cc34307j
Fig. 1 Reactivity of 1 and 3 with CS2.
c
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Fig. 2 (a): X-ray structure of 2; (b): X-ray structure of 3. (c): X-ray
structure of 4. The structures are drawn with 50% probability
ellipsoids. Relevant bond distances are shown. The structure of 4
contains two crystallographically independent molecules showing
different CS bond metrics.
Fig. 3 Frontier orbital diagram of 4. Doubly occupied and unoccupied
orbitals represented by bold and normal lines, respectively.
of time, undergoing facile decomposition to unidentified products,
as evidenced by 1H NMR spectroscopy (see ESIw).
2 can be recrystallized from DMSO–ether. The X-ray
structure of Ni2L1(CS2)2 is presented in Fig. 2.y This structure
is one of the few examples of a group 10 carbon-disulfide
complex.5,6,9,16 The first example, Pt(PPh3)2(CS2), was reported
by Wilkinson and coworkers,16a and the structure of
mononickel complex 4 and is presented in the ESIw in Fig. S28).
The LUMO (81) shows the completely unoccupied p* orbital of
the iminopyridine ligand, and the LUMO + 3 (84) shows the
antibonding orbital between Ni–dx2–y2 and the activated CS2. In
addition, there are two doubly occupied orbitals, 78 and 80,
which correspond to in- and out-of-phase combinations
between dz2 and a sulfur p-orbital and therefore only count as
one metal-based orbital, confirming the NiII oxidation state. We
were not able to able to obtain the structure of 1, however, we
crystallized its mononuclear analogue, 3 (Fig. 2).** A comparison
between the structures of 2/4a/4b and 3 is instructive: whereas in
the CS2 complex the iminopyridine chelate is fully oxidized, in 3 it
is in (1ꢀ) state (C1–C2 = 1.42(1) A, C1–N1 = 1.31(1) A),
consistent with the electronic structure of 3 that we reported
earlier. The iminopyridine chelate serves as a redox accumulator,
storing the electrons in the iminopyridine backbone in 1 and
transferring them to CS2 in 2. Related metal-mediated
transformations (including atom and group transfer and
bond activation) enabled by the redox-active nature of the
iminopyridine ligands have been recently reported.17–19 The
properties and reactivity of redox-active nitrogen-based ligands
has been recently reviewed.20
Ni(dtbpe)(CS2) (dtbpe
= 1,2-bis(di-tert-butylphosphino)-
ethane) was recently reported by Hillhouse and co-workers.9
The structures of 2 and 4 provide the first examples involving a
non-phosphine ligated group 10 metal CS2 complex. The
metal coordination geometry is nearly square-planar. Side-on
coordination of CS2 leads to a significant difference in the
bound vs. unbound CS2 bonds (C3–S2
= 1.59(1) A,
C3–S1 = 1.70(1) A). Inspection of the relevant C–C bonds in
the imino-pyridine ligand backbone (C1–C2 = 1.45(1) A;
C1–N1 = 1.27(1) A) reveals fully oxidized iminopyridine
ligand. The structure of the mononuclear analogue 4z (ESIw)
confirms the overall structural features of CS2 ligation and the
iminopyridine ligand redox state. The unit cell contains two
independent molecules of 4 (4a and 4b) showing a variability in
the C–S/CQS metrics: 1.69(1)/1.62(1) A are observed for 4a
and 1.74(1)/1.55(1) A are detected for 4b. However, relevant
iminopyridine metrics are almost identical for 4a, 4b and 2, and
are consistent with the fully oxidized iminopyridine: C1–C2 =
1.47(1) A; C1–N1 = 1.27(1) A for 4a and C1–C2 = 1.45(1) A;
C1–N1 = 1.27(1) A for 4b. Density functional theory calcula-
tions for 48 are consistent with C–S/CQS bond lengths of
1.706/1.634 A and iminopyridine metrics of C1–C2 = 1.458 A
and C1–N1 = 1.303 A. Unlike the electronic structure we
reported for the iminopyridine/COD and bis(iminopyridine)
complexes,12 4 has NiII and an oxidized iminopyridine ligand
as evidenced by the orbital diagram in Fig. 3 (the electronic
structure for the dinickel complex 2 is the same as for the
We have carried out preliminary reactivity studies involving
2. Treatment of 2 with PPh3 at RT for 24 h does not lead to
S-atom transfer to the phosphine (Fig. S19 and S20, ESIw).
Treatment of 13C-labelled 2 (2-13CS2) with unlabelled CS2 has
resulted in exchange, forming free 13CS2 (192 ppm in DMSO-d6)
and 2-CS2 (no 13C NMR signal at 268 ppm). Cyclic voltammetry of
2 has demonstrated several irreversible features, most notably an
irreversible oxidation around ꢀ0.25 V (vs. FeCp2/FeCp2+, see
Fig. S29, ESIw). For comparison, this feature was absent in the
voltammogram of 1 (Fig. S30, ESIw). The presence of this oxidation
wave prompted us to investigate chemical oxidation of 2.
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Treatment of a deep-purple solution of 2 in DMSO-d6 with
1 M. Arresta and A. Dibenedetto, in Carbon Dioxide Recovery
and Utilization, Kluwer Academic Publishers, Dordrecht, The
Netherlands, 2010, p. 211.
2 T. Sakakura, J.-C. Choi and H. Yasuda, Chem. Rev., 2007,
107, 2365.
2 equiv. of [FeCp2](PF6) led to an immediate color change to
1
yellow, followed by a slow decay to purple. H NMR of the
resulting solution indicated formation of FeCp2, along with a
trace amount of 2. Formation of free 13CS2 was observed by
13C NMR spectrum. Addition of two more equivalents of
[FeCp2](PF6) has led to a significant increase in the free 13CS2
as indicated by 13C NMR spectrum. Based on these experi-
ments, we propose that the oxidation of 2 releases CS2 instead
of its functionalization. Compound 4 displays similar behavior,
releasing CS2 upon treatment with [FeCp2](PF6).
3 J. Mascetti, in Carbon Dioxide as a Chemical Feedstock, ed. M. Aresta,
Wiley-VCH, Weinheim, Germany, 2010, p. 55.
4 For selected examples of structurally characterized M-CO2 complexes,
see: (a) M. Aresta, C. F. Nobile, V. G. Albano, E. Forni and
M. Manassero, J. Chem. Soc., Chem. Commun., 1975, 636;
(b) G. Fachinetti, C. Floriani and P. F. Zanazzi, J. Am. Chem. Soc.,
1978, 100, 7405; (c) J. S. Field, R. J. Haines, J. Sundermeyer and
S. F. Woollam, J. Chem. Soc., Chem. Commun., 1990, 985;
(d) T.-F. Wang, C.-C. Hwu, C.-W. Tsai and K.-J. Lin, Organometallics,
1997, 16, 3089; (e) C. H. Lee, D. S. Laitar, P. Mueller and J. P. Sadighi,
J. Am. Chem. Soc., 2007, 129, 13802; (f) C. C. Lu, C. T. Sauoma,
M. W. Day and J. C. Peters, J. Am. Chem. Soc., 2007, 129, 4;
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Soc., 1983, 105, 5914.
5 (a) J. A. Ibers, Chem. Soc. Rev., 1982, 11, 57; (b) K. K. Pandey,
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6 W. Poppitz, Z. Anorg. Allg. Chem., 1982, 489, 67.
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Chem. Soc., 2011, 133, 4948.
9 J. S. Anderson, V. M. Iluc and G. L. Hillhouse, Inorg. Chem., 2010,
49, 10203.
10 For selected recent references, see: (a) D. E. Herbert and
O. V. Ozerov, Organometallics, 2011, 30, 6641; (b) A. R. Fout,
Q. Zhao, D. J. Xiao and T. A. Betley, J. Am. Chem. Soc., 2011,
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Int. Ed., 2011, 50, 12205.
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31, 470; (b) J. P. Krogman, B. M. Foxman and C. M. Thomas,
J. Am. Chem. Soc., 2011, 133, 14582.
In summary, we report synthesis and characterization of the
dinuclear L1Ni2(CS2)2 complex featuring square planar Ni
centers ligated by iminopyridine chelates. Spectroscopic,
theoretical and structural investigation of the dinuclear CS2
complex 2 and its mononuclear analogue 4, and their COD-
ligated counterparts 1 and 3, reveal that the iminopyridine
serves as a redox accumulator, moderating redox changes in
Ni upon binding of a heteroallene. Our preliminary experi-
ments targeting oxidative modification of bound CS2 have
resulted in CS2 release in both 2 and 4. We are currently
preparing related CO2 complexes, and investigating their
reactivity. In addition, other dinucleating ligands are being
designed, to enable more reactive heteroallene adducts.
We thank Wayne State University for funding. SG thanks
Prof. H. B. Schlegel for helpful discussions. AB thanks Bashar
Ksebati and Lew Hryhorczuk for the experimental assistance.
The computational resources were provided by the WSU Grid.
Notes and references
12 A. Bhemaraju, R. L. Lord, P. Muller and S. Groysman, Organo-
¨
metallics, 2012, 31, 2120.
13 DFT calculations agree with these assignments.
14 (a) M. G. Mason, P. N. Swepston and J. A. Ibers, Inorg. Chem.,
1983, 22, 411; (b) C. Bianchini, C. Mealli, A. Meli and M. Sabat,
Inorg. Chem., 1984, 23, 4125; (c) J. J. Maj, A. D. Rae and
L. F. Dahl, J. Am. Chem. Soc., 1982, 104, 4278.
z Synthesis of 2. A 1.5 M THF solution of CS2 (0.15 mL, 0.231 mmol)
was added to a 5 mL violet-blue THF solution of Ni2(L1)(COD)2
(1, 75 mg, 0.115 mmol). A precipitate was obtained after the addition
of CS2. The reaction was stirred for 15 min, and the solid was
separated from the reaction mixture and washed with ether. Removal
of the supernatant followed by drying resulted a purple brown solid
(60 mg, 88%). The 13C-labeled sample was prepared in an identical
fashion. 1H NMR (DMSO-d6, 400 MHz) d 9.44 (d, J = 4.8, 2H),
8.90 (s, 2H), 8.23 (t, J = 7.6 Hz, 2H), 7.96 (d, J = 8.0, 2H), 7.90
(t, J = 6.4, 2H), 7.51 (s, 4H), 5.23 (s, 4H); 13C NMR (DMSO-d6,
75 MHz) d 267.87 (13CS2). MS (ESI) calcd for [Ni2(L1)(CS2)2]+ 581.9,
15 E. C. Volpe, P. T. Wolczanski and E. B. Lobkovsky, Organometallics,
2010, 29, 364.
16 For the structures of Group 10 CS2 complexes, see: (a) M. Baird,
G. Hartwell, Jr., R. Mason, A. I. M. Rae and G. Wilkinson, Chem.
Commun., 1967, 92; (b) C. Bianchini, D. Masi, C. Mealli and
A. Meli, Inorg. Chem., 1984, 23, 2838; (c) D. H. Farrar,
R. R. Gukathasan and S. A. Morris, Inorg. Chem., 1984,
23, 3258; (d) P. Leoni, M. Pasquali, L. Fadini, A. Albinati,
P. Hofmann and M. Metz, J. Am. Chem. Soc., 1997, 119, 8625;
(e) T. Kashiwagi, N. Yasuoka, T. Ueki, N. Kasai, M. Kakudo,
S. Takahashi and N. Hagihara, Bull. Chem. Soc. Jpn., 1968,
41, 296.
found 581.8; [Ni2(L1)(CS2)]+ 505.9, found 505.8. IR (cmꢀ1 12CS/13CS
,
signals): 1138/1095 (s), 648/633 (s). Anal. calcd for C22H18N4Ni2S4: C,
45.24; H, 3.11; N, 9.59. Found: C, 44.97; H, 3.27 N, 9.32.
y Crystal data for 2. C11H9N2NiS2, M = 292.03, monoclinic, space
group P21/n, a = 7.2464(7) A, b = 10.7305(9) A, c = 14.446(1) A,
b = 94.265(5)1, V = 1120.14(17) A, Dc = 1.732 g cmꢀ1, Z = 4, m =
2.073 mmꢀ1, T = 100(2), 1920 unique reflections, R1(I > 2s(I)) =
0.0695, wR2(I > 2s(I)) = 0.1731, GOF = 1.071.
17 (a) C. C. Lu, E. Bill, T. Weyhermuller, E. Bothe and K. Wieghardt,
¨
J. Am. Chem. Soc., 2008, 130, 3181–3197; (b) C. C. Lu, S. DeBeer
z Crystal data for 4. C28H24N4Ni2S4, M = 662.17, triclinic, space
%
George, T. Wehermuller, E. Bill, E. Bothe and K. Wieghardt,
¨
Angew. Chem., 2008, 120, 6484.
group P1, a = 8.7358(7) A, b = 9.5434(7) A, c = 17.008(1) A, a =
98.210(5)1, b = 98.742(5)1, g = 96.563(5)1, V = 1373.60(19) A, Dc =
1.601 g cmꢀ1, Z = 2, m = 1.701 mmꢀ1, T = 100(2), 5945 unique
reflections, R1(I > 2s(I)) = 0.0712, wR2(I > 2s(I)) = 0.2004, GOF =
1.030.
18 (a) T. W. Myers and L. A. Berben, J. Am. Chem. Soc., 2011,
133, 11865; (b) T. W. Myers, N. Kazem, S. Stoll, R. D. Britt,
M. Shanmugam and L. A. Berben, J. Am. Chem. Soc., 2011,
133, 8662; (c) T. W. Myers and L. A. Berben, Inorg. Chem.,
2012, 51, 1480.
19 N. A. Ketterer, H. Fan, K. J. Blackmore, X. Yang, J. W. Ziller,
M.-H. Baik and A. F. Heyduk, J. Am. Chem. Soc., 2008, 130, 4364.
20 (a) K. G. Caulton, Eur. J. Inorg. Chem., 2012, 435; (b) P. J. Chiric,
Forum on Redox-Active Ligands, Inorg. Chem., 2011, 50, 9737.
8 See ESIw for calculations details.
** Crystal data for 3. C21H24N2Ni, M = 363.13, monoclinic, space
group C2/c, a = 25.286(3) A, b = 7.3278(6) A, c = 21.348(2) A, b =
120.423(8)1, V = 3411.0(6) A, Dc = 1.414 g cmꢀ1, Z = 8, m =
1.141 mmꢀ1, T = 100(2), 6713 unique reflections, R1(I > 2s(I)) =
0.0258, wR2(I > 2s(I)) = 0.0710, GOF = 1.046.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 9595–9597 9597