Zerovalent titanium-sulfur complexes. Novel dithiocarbamato derivatives of Ti(CO)6: [Ti(CO)4(S2CNR2)] -

Article abstract of DOI:10.1039/b700808b

Oxidation of [Ti(CO)₆]^2- by thiuram disulfides, (R₂NCS₂)₂, affords the first isolable mononuclear six-coordinate titanium(0) carbonyls, [Ti(CO)₄(S ₂CNR₂)]^-,

Full text of DOI:10.1039/b700808b

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Zerovalent titanium–sulfur complexes. Novel dithiocarbamato
derivatives of Ti(CO)6: [Ti(CO)4(S2CNR2)]2{{§
Robert E. Jilek, Giovanna Tripepi, Eugenijus Urnezius, William W. Brennessel, Victor G. Young, Jr. and
John E. Ellis*
Received (in Berkeley, CA, USA) 18th January 2007, Accepted 14th March 2007
First published as an Advance Article on the web 3rd April 2007
DOI: 10.1039/b700808b
Oxidation of [Ti(CO)6]22 by thiuram disulfides, (R2NCS2)2,
gave an analogous titanate product, 3, but it could not be
separated completely from the byproduct, [C5H10NCS2]2.
affords the first isolable mononuclear six-coordinate tita-
nium(0) carbonyls, [Ti(CO)4(S2CNR2)]2, which have unusual
However, a pure bis(triphenylphosphine)iminium or PPN salt of
3 was readily obtained by mixing [PPN][Ti(CO)4(g3-BH4)], where
the latter anion is an exceptionally labile titanium carbonyl,11 with
trigonal prismatic geometries and chemical and spectral
properties that are remarkably similar to those of the 18-
electron and seven-coordinate anion [Ti(CO)4(g5-C5H5)]2.
K[C5H10NCS2] in thf. Poor solubility of the byproduct, KBH4,
facilitated the purification of 3, which was isolated as a green PPN
salt in 58% yield, Scheme 2."
Metal carbonyl chemistry is well-developed for most d-block
elements,2 but for titanium, particularly in the absence of
cyclopentadienyl or other dienyl ligands, it remains rather poorly
explored.3 Thus, despite the rich history of metal carbonyls with
sulfur-containing ligands,4 none have been described for titanium,
or the heavier group 4 elements. We report herein on the synthesis,
isolation and characterization of the initial examples: dithiocarba-
matocarbonyltitanates(12), [Ti(CO)4(S2CNR2)]2, for R = Me (1),
Et (2) and C5H10 or pentamethylene (3). These species are of
additional interest because they are unprecedented examples of
isolable mononuclear six-coordinate Ti(0) carbonyls and possess
some unusual properties (see below). The only previously reported
compound of this type is the exceedingly unstable 16-electron
Ti(CO)6, which reportedly decomposes at about 40 K in a
CO/inert gas matrix.5 All prior isolable and well-established
mononuclear Ti(0) carbonyls have been 18-electron seven-
coordinate complexes, such as [Ti(CO)4(g5-C5H5)]2 (4),6
[Ti(CO)5(SnPh3)2]22,7 and [Ti(CO)6(AuPEt3)]2.8
These initial examples of zerovalent titanium dithiocarbamates9
were prepared by the oxidation of dark red [K(15-crown-5)2]2-
[Ti(CO)6]10 with thiuram disulfides, (R2NCS2)2, in tetrahydro-
furan, thf. Satisfactorily pure green microcrystals of composition
[K(15-crown-5)2][Ti(CO)4(S2CNR2)], were thereby obtained in
about 20–30% isolated yields for 1 and 2. Difficulties in separating
these materials from the potassium dithiocarbamate byproducts
led to the relatively low yields, Scheme 1, (see ESI§). Similar
reactions of dipentamethylenethiuram disulfide, (C5H10NCS2)2,
Spectroscopic properties of 1–3 were independent of cation and
entirely consistent with the proposed formulations. IR spectra in
the carbonyl stretching frequency region showed essentially
identical two band patterns in thf, n(CO) 1925m, 1785s cm21
which were nearly superimposable on corresponding spectra in thf
observed for 4, n(CO) 1921m, 1775s cm21 6 13
C NMR spectra of
,
.
1–3 had single very downfield carbonyl carbon resonances at
about 292 ppm in pyridine or thf (see ESI§), which were close to
the position reported for 4, in dmso, dC = 289 ppm.6 In contrast,
normal 18-electron cis-disubstituted octahedral carbonyl com-
plexes, such as [Mo(CO)4(S2CNEt2)]2,12 have strikingly different
IR spectra in the n(CO) region, showing generally four major
peaks, and a 13C NMR spectrum consisting of two different
carbonyl carbon resonances of equal intensity.
Despite the formal 16-electron natures of 1–3, they showed no
tendency to combine with CO, PMe3 or PPh3 in thf or neat
pyridine at room temperature.13 In this respect, they resemble the
18-electron titanate 4, but are significantly less reactive than the
previously known formally unsaturated dithiocarbamato carbo-
nyls, M(CO)2(S2CNR2)2, M = Mo, W; R = alkyl. Unlike 3, the
latter species are CO carriers and readily add a variety of donors to
afford isolable seven-coordinate complexes, M(CO)2L(S2CNR2)2,
L = CO, PPh3, pyridine, and related ligands.14
Single-crystal X-ray analyses of 1 and 3 were carried out to
confirm their formulations. Unlike all previously reported mono-
nuclear six-coordinate zerovalent metal complexes, they exhibited
nearly trigonal prismatic geometry about the metals (Fig. 1)I.
Because their Ti(CO)4S2 core structures are almost identical, only
the molecular structure of 3 will be described herein (See ESI§ for
1).15 The asymmetric unit contains a normal PPN cation, well
separated from anion 3. To emphasize the unusual character of 3,
Scheme 1
Department of Chemistry, University of Minnesota, 207 Pleasant Street
SE, Minneapolis, MN 55455, USA. E-mail: ellis@chem.umn.edu;
Fax: +1 (612) 626-7541
{ Highly Reduced Organometallics, Part 61; for Part 60 see ref. 1
{ Dedicated to the memory of Professor F. Albert Cotton.
§ Electronic supplementary information (ESI) available: Experimental
procedures and characterizations of compounds 1, 2 and X-ray crystal
data for 3. See DOI: 10.1039/b700808b
Scheme 2
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Chemical Society for financial support of this research and the
Scuola Normale Superiore, Pisa, Italy for a graduate fellowship to
G. T. We are very grateful to Ms Christine Lundby for expert
assistance in the preparation of this manuscript.
Notes and references
" Satisfactory elemental analyses (C, H, N) were obtained for all new
compounds. [PPN[[3]: A solution of K[S2CNC5H10] (0.281 mmol) in thf
(20 mL, 0 uC), prepared in situ from a potassium naphthalene reduction of
the thiuram disulfide, was added to
a red solution of
[PPN][Ti(CO)4(BH4)]11 (0.193 g, 0.271 mmol) in thf (10 mL, 0 uC). The
resulting green solution was stirred at 0 uC for 1 h, filtered, and evaporated
in vacuo. Purification was effected by washing the residue with diethyl ether,
pentane, and then drying in vacuo to afford a satisfactorily pure deep green
solid (0.139 g, 58% yield). IR (thf): n(CO) 1925m, 1785s cm21 13C{1H}
.
Fig. 1 Molecular structure of 3; cation is not shown. Thermal ellipsoids
(75 MHz, [2H8]thf, 20 uC, SiMe4, resonances due to PPN omitted, labeling
of C as per Fig. 1): d 25.2 (C7,9 or C8), 26.6 (C8 or C7,9), 49.4 (C6,10), 205.3
(C5), 293.0 (CO). Green needles for the X-ray crystal study were grown
from a diethyl ether–thf layered solution at 230 uC.
are set at the 50% probability level, with hydrogens omitted for clarity.
˚
Selected bond lengths (A) and angles (u): Ti–C(1) 2.018(2), Ti–C(2)
2.021(2), Ti–C(3) 1.996(2), Ti–C(4) 1.996(2), Ti–S(1) 2.5258(6), Ti–S(2)
2.5313(6), S(1)–C(5) 1.726(2), S(2)–C(5) 1.726(2), N(1)–C(5) 1.324(3),
C(1)–O(1) 1.160(2), C(2)–O(2) 1.161(2), C(3)–O(3) 1.169(2), C(4)–O(4)
1.169(2); S(1)–Ti–S(2) 70.44(2), S(1)–C(5)–S(2) 115.3(1), av Ti–C–O
178(1).
I Crystal data for [PPN][3]: C46H40N2O4P2S2Ti, M = 858.76, monoclinic,
˚
space group P21/n, a = 9.8175(8), b = 22.0718(17), c = 19.6526(16) A, V =
3
4257.6(6) A , T = 173 K, Z = 4, m(Mo-Ka) = 0.420 mm21, 49057
˚
reflections collected, 9768 unique (Rint = 0.0357). Refinement on F2, final
R1 = 0.0413 (for 7916 reflections with I . 2s(I)). wR2 = 0.0928 (for all
data). CCDC 637410. For crystallographic data in CIF or other electronic
format see DOI: 10.1039/b700808b
salient features of its structure will be compared to those of the
conventional 18-electron W(0) complex, [W(CO)4(S2CNC5H10)]2,
5, which has an identical ligand set.12 Whereas anion 5 contains a
normal M(CO)4 core structure appropriate for a cis-octahedral
fragment, the corresponding unit in 3 has a strikingly different
square pyramidal geometry, resulting in four equivalent carbonyl
groups. Remarkably, the Ti(CO)4 group in 3 is nearly super-
imposable on that of the seven-coordinate complex 4.16 This
feature helps to explain why the IR and NMR spectra of 3 are so
similar to those of 4, but very different from those of 5, in the
carbonyl region. Only minor structural deviations are present in
the essentially planar bidentate dithiocarbamato, or dtc, S2CN
1 M. V. Barybin, W. W. Brennessel, B. E. Kucera, M. E. Minyaev,
V. J. Sussman, V. G. Young, Jr. and J. E. Ellis, J. Am. Chem. Soc.,
2007, 129, 1141.
2 J. E. Ellis and W. Beck, Angew. Chem., Int. Ed. Engl., 1995, 35, 2489;
F. Calderazzo, Carbonyl complexes of the Transition Metals, in
Encyclopedia of Inorganic Chemistry, ed. R. B. King, J. Wiley,
Chichester, UK, 1994, vol. 2.
3 D. J. Sikora, D. W. Macomber and M. D. Rausch, Adv. Organomet.
Chem., 1986, 25, 317; F. G. N. Cloke, Titanium Complexes in Oxidation
States Zero and Below, inComprehensive Organometallic Chemistry II,
ed. E. W. Abel, F. G. A. Stone and G. Wilkinson, Pergamon Press,
London, 1995, vol. 4.
4 F. A. Cotton, G. Wilkinson, C. A. Murillo and M. Bochmann,
Advanced Inorganic Chemistry, J. Wiley, New York, 6th edn, 1999.
5 R. Busby, W. Klotzbu¨cher and G. A. Ozin, Inorg. Chem., 1977, 16, 822.
6 B. A. Kelsey and J. E. Ellis, J. Am. Chem. Soc., 1986, 108, 1344;
J. E. Ellis, S. R. Frerichs and B. K. Stein, Organometallics, 1993, 12,
1048.
units in 3 and 5. For example, the average C–S distance in 3,
17
1.726(2) A, is slightly longer than that in 5, 1.707(7) A. More
˚
˚
important are differences in the metal–sulfur distances in the two
18
anions, where the average M–S distance in 3, 2.529(3) A, is
˚
˚
nearly 0.06 A shorter than the corresponding distance in 5,
7 K.-M. Chi, S. R. Frerichs and J. E. Ellis, J. Chem. Soc., Chem.
Commun., 1988, 1013.
2.586(2) A.12 These data indicate that the metal–sulfur interactions
˚
8 P. J. Fischer, V. G. Young, Jr. and J. E. Ellis, Chem. Commun., 1997,
1249.
9 Only Ti(IV) dithiocarbamate complexes have been well established
previously. Also, corresponding M(0) complexes have only been
reported for Cr, Mo and W. See: G. Hogarth, Prog. Inorg. Chem.,
2005, 53, 71.
in 3 are significantly stronger than those in 5, particularly because
˚
the atomic radius of titanium is about 0.06 A larger than that of
tungsten.19 Indeed, the spectral, chemical and structural properties
of 1–3 strongly suggest that the dtc ligands function as both s-
and p-donors and thereby become electronically equivalent to the
g5-cyclopentadienyl group in 4. On this basis, 1–3 should be
regarded as 18 electron complexes, and are strikingly robust
examples of transition metal compounds in which ‘‘p-stabilized
unsaturation’’ plays a key role in defining their physicochemical
properties.20 Structural properties of 1–3 are also in accord with
independent theoretical analyses by Templeton and Ward21 and
Kubacek and Hoffmann22 on closely related six-coordinate low-
10 K.-M. Chi, S. R. Frerichs, S. B. Philson and J. E. Ellis, J. Am. Chem.
Soc., 1988, 110, 303.
11 P. J. Fischer, V. G. Young, Jr. and J. E. Ellis, Angew. Chem., Int. Ed.,
2000, 39, 189; P. J. Fischer, PhD Thesis, University of Minnesota, 1998.
12 B. Zhuang, L. Huang, L. He, Y. Yang and J. Lu, Inorg. Chim. Acta,
1988, 145, 225; K.-H. Yih, S.-C. Chen, Y. C. Lin, G.-H. Lee and
Y. Wang, J. Organomet. Chem., 1995, 494, 149.
13 Thus, in these cases no significant change in the spectra of 1–3 were
observed. Interestingly, below 210 uC solutions of 1 in neat pyridine
changed from green to brown, but IR and NMR spectra remained
nearly unchanged indicating the interaction of 1 with the solvent was
quite weak. On warming above 210 uC the original color reappeared.
Carbonyl exchange and related reactions with 1–3 will be described
elsewhere.
spin d4 complexes, including Mo(CO)2(S2CNPr2 )2. Like 1–3, the
i
latter species has a nearly trigonal prismatic geometry, and both
studies showed that optimization of cooperative p-donor (SR)–
p-acceptor (CO) interactions with the metal centers was achieved
in this unusual coordination environment.
14 R. Colton, G. R. Scollary and I. B. Tomkins, Aust. J. Chem., 1968, 21,
15; J. L. Templeton and B. C. Ward, Inorg. Chem., 1980, 19, 1753;
J. A. Broomhead and C. G. Young, Aust. J. Chem., 1982, 35, 277, and
references therein.
We thank the US National Science Foundation and the
Petroleum Research Fund, administered by the American
2640 | Chem. Commun., 2007, 2639–2641
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15 For example, two trigonal faces in 3, defined by atoms S1, C1, C4 and
S2, C2, C3, have an average twist angle, w, of 1 ¡ 1u, which is
experimentally indistinguishable from a trigonal prism, for which w = 0u.
For ideal octahedral geometry, w = 60u. See: F. A. Cotton, G. Wilkinson,
C. A. Murillo and M. Bochmann, Advanced Inorganic Chemistry,
J. Wiley, New York, 6th edn, 1999, pp. 6–7. Dihedral angles between
selected least-squares planes are also consistent with a slightly distorted
trigonal prismatic environment about titanium in 3, see Table S1 and
Fig. S1 in ESI§.
18 The average Ti–S distance in 3 falls within the range of prior
˚
values reported for Ti–dtc complexes, where the longest is 2.611 A
in TiCp(S2CNMe2)3 (W. L. Steffen, H. K. Chun and R. C. Fey,
˚
Inorg. Chem., 1978, 17, 3498) and the shortest is 2.512 A in
TiCl(S2CNMe2)3 (D. F. Lewis and R. C. Fey, J. Am. Chem. Soc.,
1974, 96, 3843).
19 A. F. Wells, Structural Inorganic Chemistry, Clarendon Press, Oxford,
UK, 5th edn, 1984, p. 1288.
20 K. G. Caulton, New J. Chem., 1994, 18, 25; D. J. Darensbourg,
J. D. Draper, B. J. Frost and J. H. Reibenspies, Inorg. Chem., 1999, 38,
4705.
16 For example, average Ti–C and C–O distances in 3, 2.01(1) and 1.165(5)
˚
A, respectively, are statistically identical to corresponding values in 4,
1.994(6) and 1.164(6), respectively. Also, the average cis and trans C–Ti–
C angles of the Ti(CO)4 units in 3 (cis 71(2); trans 110(1)u) and 4 (cis,
72(2); trans, 113(2)u) are essentially the same6.
21 J. L. Templeton and B. C. Ward, J. Am. Chem. Soc., 1980, 102, 6568;
J. L. Templeton, P. B. Winston and B. C. Ward, J. Am. Chem. Soc.,
1981, 103, 7713.
22 P. Kubacek and R. Hoffmann, J. Am. Chem. Soc., 1981, 103,
4320.
17 The N–C distance in 3, 1.324(3) s, is marginally shorter than the
12
corresponding distance in 5, 1.335(9) A .
˚
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