Synthesis and structural characterization of tris(2-seleno-1- mesitylimidazolyl) hydroborato complexes: A new type of strongly electron donating tripodal selenium ligand

Article abstract of DOI:10.1039/b608078b

A new tripodal ligand that features three selenium donors, namely the tris(2-seleno-1-mesitylimidazolyl)hydroborato ligand, [Tse^Mes], has been constructed via the reaction of KBH₄ with 1-mesitylimidazole-2- selone; comparison of the IR spectroscopic data of [Tse^Mes]Re(CO) ₃ with those of a variety of related LRe(CO)₃ complexes demonstrates that the [Tse^Mes] ligand is more strongly electron donating than Cp, Cp*, [Tp], [Tp^Me₂] and [Tm ^Mes] ligands. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b608078b

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Synthesis and structural characterization of tris(2-seleno-1-
mesitylimidazolyl) hydroborato complexes: A new type of strongly
electron donating tripodal selenium ligand{{
Mao Minoura, Victoria K. Landry, Jonathan G. Melnick, Keliang Pang, Luciano Marchio` and
Gerard Parkin*
Received (in Berkeley, CA, USA) 6th June 2006, Accepted 19th July 2006
First published as an Advance Article on the web 10th August 2006
DOI: 10.1039/b608078b
A new tripodal ligand that features three selenium donors,
namely
the
tris(2-seleno-1-mesitylimidazolyl)hydroborato
ligand, [TseMes], has been constructed via the reaction of
KBH4 with 1-mesitylimidazole-2-selone; comparison of the IR
spectroscopic data of [TseMes]Re(CO)3 with those of a variety of
related LRe(CO)3 complexes demonstrates that the [TseMes
]
ligand is more strongly electron donating than Cp, Cp*, [Tp],
[TpMe ] and [TmMes] ligands.
2
Scheme 1
Tripodal ligands, a prominent feature in coordination chemistry,
provide three donor atoms for facial coordination to a metal
center. The widespread use of this ligand system derives mainly
from the ability to manipulate both steric and electronic properties.
For example, electronic properties are strongly influenced by
whether the tripod ligand is a neutral L3 donor or ‘‘anionic’’ L2X
or X3 donor,1 as illustrated by the series of [N3]-donors:
tris(pyrazolyl)methane (L3),2 tris(pyrazolyl)hydroborato (L2X),3
and tris[(amido)methyl]ethane (X3).4,5 Tripodal ligands belonging
to the L2X class, in particular, have found widespread applications
because of the large variety of donor atoms that have been
incorporated. Thus, the donor set of the L2X class of ligands may
be comprised of either a homonuclear or heteronuclear array, with
examples of the former including [C3],6 [N3],3 [P3],7 and [S3]8,9
donors. In this paper, we introduce a new class of tripodal L2X
ligand that features an [Se3] donor array.
Whereas [S3] tripodal ligands are ubiquitous,8–10 analogous
ligands that feature an [Se3] donor array are uncommon, and the
only examples of which we are aware belong to the neutral L3
tris(selenoether) MeC(CH2SeR)3 (R = Me, Ph) system.11,12 Since
the tris(2-mercapto-1-R-imidazolyl)hydroborato ligand, [TmR],
introduced by Reglinski and Spicer8a has proven to be versatile,13
with a large variety of [TmR] derivatives having been prepared,8 we
envisioned that a similar series of tripodal ligands that feature an
L2X [Se3] donor array should be accessible and thereby provide a
set of ligands with modified electronic properties.
derivatives of both the main group metals and transition metals
(Scheme 2).16 For example, [TseMes]K reacts with 1 equivalent of
MI2 (M = Co, Zn, Cd, Hg) to give the corresponding [TseMes]MX
derivative, with the structure of the cobalt complex being
illustrated in Fig. 1. Similarly, [TseMes]K reacts with ‘‘Zn(SPh)2’’
to give [TseMes]ZnSPh. In addition to these 1 : 1 complexes, the 2 :
1 complex [TseMes]2Zn may be obtained by treatment of ZnI2 with
2 equivalents of [TseMes]K. The molecular structure of [TseMes]2Zn
has been determined by X-ray diffraction, thereby demonstrating
that the [TseMes] ligands coordinate in a k2-Se,Se manner such that
the zinc is effectively tetrahedral.17
Treatment of the trivalent metal halides GaCl3 and InCl3 with
[TseMes]K yields {[TseMes]2M}[MCl4] (M = Ga, In). Despite the
fact that the cations {[TseMes]2M}+ (M = Ga, In) have a similar
2 : 1 composition to that of the zinc complex, [TseMes]2Zn, a
significant difference resides with the fact that the [TseMes] ligands
Indeed, the tris(2-seleno-1-mesitylimidazolyl)hydroborato ligand
may be obtained as its potassium derivative, [TseMes]K, via the
reaction of KBH4 with 1-mesitylimidazole-2-selone,14 as illustrated
in Scheme 1.15 [TseMes]K is a convenient reagent for a variety of
Department of Chemistry, Columbia University, New York,
New York 10027, USA. E-mail: parkin@columbia.edu
{ Electronic supplementary information (ESI) available: experimental
details and crystallographic data. See DOI: 10.1039/b608078b
{ The HTML version of this article has been enhanced with colour
images.
Scheme 2
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Fig. 1 Molecular structure of [TseMes]CoI.
of {[TseMes]2M}+ (M = Ga, In) coordinate in a k3-Se,Se,Se
manner such that the metal centers are octahedral.16
In addition to k2-Se,Se and k3-Se,Se,Se coordination modes to
a single metal center, the [TseMes] can also bridge two metals. Thus,
the [TseMes] ligand in {[TseMes]Cu}2, obtained from the reaction of
[TseMes]K with CuCl (Scheme 3), bridges the two metals such that
each ligand coordinates in a k2-Se,Se mode to one copper and a
k1-Se mode to the other, with the result that each copper is
trigonally coordinated (Fig. 2). This type of motif is also
observed in the structure of the sulfur counterpart {[TmMes]Cu}2
and the related tris(thioxotriazolyl)hydroborato complexes
{[TrEt,Me]Cu}2,18 {[TrMe,o-Py]Cu}2,19 and {[TrMes,Me]Cu}2.19
An important issue concerned with the application of the
[TseMes] ligand relates to its steric and electronic properties and, in
particular, how they compare with those of the [TmMes] sulfur
counterpart. In this regard, the first comprehensive evaluation of
the steric and electronic properties of ligands was provided by
Tolman, who obtained data for PR3 ligands by analyzing a large
series of nickel carbonyl complexes of the type Ni(PR3)3(CO).20
Nickel, however, is not an ideal choice of metal for the evaluation
of the steric and electronic properties of multidentate ligands such
as [TseR], [TmR], [TpRR9] and [CpR], because a corresponding
series of nickel carbonyl complexes is unknown. Fortunately, a
large number of [TpRR9]Re(CO)3 and [CpR]Re(CO)3 complexes
Fig. 2 Structure of {[TseMes]Cu}2.
are known and provide a good basis for evaluating the steric and
electronic properties of such ligands.21 For this reason, we have
synthesized and structurally characterized the rhenium tricarbonyl
compounds, [TseMes]Re(CO)3 (Fig. 3 and Scheme 3)22 and
[TmMes]Re(CO)3.23
With respect to the steric properties, X-ray diffraction studies
˚
indicate that the average Re–Se bond length (2.64 A) is 0.11 A
˚
longer than the corresponding Re–S bond length (2.53 A),
˚
24
thereby causing the mesityl substituents to be displaced further
from the metal center. Consequently, the [TseMes] ligand is less
sterically demanding than the [TmMes] ligand, as quantified by
their respective cone angles: [TseMes] (211u) and [TmMes] (219u);
see Table 1.25
A convenient means of assessing the electron donating ability of
a ligand is via measurement of the nCO stretching frequency of a
metal carbonyl derivative.26 On this basis, the lower nCO stretching
frequencies of [TseMes]Re(CO)3 (1999 and 1886 cm21) compared
to [TmMes]Re(CO)3 (2007 and 1890 cm21) indicate that the
[TseMes] ligand is more electron donating than the [TmMes
]
ligand.27,28 In addition to [TseMes] being more electron donating
than [TmMes], it is important to note that it is also more electron
donating than Cp, Cp*, [Tp] and [TpMe ] ligands (Table 1).29 Thus,
2
[TseMes] belongs to a new class of strongly electron donating
tripodal ligand.
Scheme 3
Fig. 3 Molecular structure of [TseMes]Re(CO)3.
Chem. Commun., 2006, 3990–3992 | 3991
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11 D. J. Gulliver, E. G. Hope, W. Levason, G. L. Marshall, S. G. Murray
and D. M. Potter, J. Chem. Soc., Perkin Trans. 2, 1984, 429.
12 (a) A. J. Barton, A. R. J. Genge, N. J. Hill, W. Levason, S. D. Orchard,
B. Patel, G. Reid and A. J. Ward, Heteroat. Chem., 2002, 13, 550; (b)
W. Levason, S. D. Orchard and G. Reid, Coord. Chem. Rev., 2002, 225,
159.
Table 1 Electron donating and steric properties of various ligands as
evaluated by nCO values and cone angles (H)
LRe(CO)3
nCO (cm21 c
)
nCO(av) (cm21
)
H (u)
[Tp]a
Cpa
2020, 1896
2019, 1897
2017, 1893
2007, 1890
1999, 1892
1999, 1886
1958
1958
1955
1949
1946
1943
211
141
255
219
170
211
13 For example, we have employed the [TmR] system to mimic aspects of
the sulfur-rich active sites in zinc enzymes and proteins. See, for
example: M. M. Morlok, K. E. Janak, G. Zhu, D. A. Quarless and
G. Parkin, J. Am. Chem. Soc., 2005, 127, 14039.
a
[TpMe
2
]
[TmMes b
]
Cp*a
[TseMes b
]
14 1-Mesitylimidazole-2-selone is obtained using a method analogous to
that for the methyl derivative. See: L. J. Guziec and F. S. Guziec, Jr.,
J. Org. Chem., 1994, 59, 4691.
a
b
Reference 21a. This work. KBr disk.
c
15 The nature of [TseMes]K has not been determined by X-ray diffraction
and the connectivity shown in Scheme 1 is only intended to be
illustrative.
In summary, a new tripodal ligand that features three selenium
donors, namely [TseMes], has been constructed via the reaction of
KBH4 with 1-mesitylimidazole-2-selone. Reactivity studies indicate
that [TseMes] is an effective ligand for both main group metals and
transition metals, while a comparison of [TseMes]Re(CO)3 and a
variety of related LRe(CO)3 complexes demonstrates that the
[TseMes] ligand is more strongly electron donating than Cp, Cp*,
16 The molecular structures of [TseMes]MI (M = Co, Zn, Cd, Hg),
[TseMes]ZnSPh, [TseMes]2Zn, {[TseMes]2M}[MCl4] (M = Ga, In),
{[TseMes]Cu}2, [TseMes]Re(CO)3 and [TmMes]Re(CO)3 have been
determined by X-ray diffraction (see Electronic Supplementary
Information{). Thermal parameters for the structures illustrated in
Fig. 1–3 are at the 20% probability level.
17 As expected for a tetrahedral zinc center, the 3-center-2-electron Zn–H–
B interaction is largely insignificant. See, for example, reference 8e and
(a) C. Kimblin, B. M. Bridgewater, T. Hascall and G. Parkin, J. Chem.
Soc., Dalton Trans., 2000, 891; (b) H. M. Alvarez, T. B. Tran,
M. A. Richter, D. M. Alyounes, D. Rabinovich, J. M. Tanski and
M. Krawiec, Inorg. Chem., 2003, 42, 2149; (c) C. Kimblin,
B. M. Bridgewater, D. G. Churchill, T. Hascall and G. Parkin, Inorg.
Chem., 2000, 39, 4240.
[Tp], [TpMe ] and [TmMes] ligands. As such, [TseR] ligands offer
2
considerable potential in coordination chemistry.
We thank the National Science Foundation (CHE-03-50498) for
support of this research.
18 M. Careri, L. Elviri, M. Lanfranchi, L. Marchio`, C. Mora and
M. A. Pellinghelli, Inorg. Chem., 2003, 42, 2109.
Notes and references
19 R. Cammi, M. Gennari, M. Giannetto, M. Lanfranchi, L. Marchio`,
G. Mori, C. Paiola and M. A. Pellinghelli, Inorg. Chem., 2005, 44, 4333.
20 (a) C. A. Tolman, J. Am. Chem. Soc., 1970, 92, 2956; (b) C. A. Tolman,
Chem. Rev., 1977, 77, 313.
1 M. L. H. Green, J. Organomet. Chem., 1995, 500, 127.
2 (a) H. R. Bigmore, S. C. Lawrence, P. Mountford and C. S. Tredget,
Dalton Trans., 2005, 635; (b) D. L. Reger, Comments Inorg. Chem.,
1999, 21, 1.
21 (a) D. M. Tellers, S. J. Skoog, R. G. Bergman, T. B. Gunnoe and
W. D. Harman, Organometallics, 2000, 19, 2428; (b) R. G. Bergman,
T. R. Cundari, A. M. Gillespie, T. B. Gunnoe, W. D. Harman,
T. R. Klinckman, M. D. Temple and D. P. White, Organometallics,
2003, 22, 2331.
3 Tris(pyrazolyl)hydroborato ligands are commonly abbreviated as
[TpRR9]. See: S. Trofimenko, Scorpionates – The Coordination
Chemistry of Polypyrazolylborate Ligands, Imperial College Press,
London, 1999.
4 L. H. Gade, Acc. Chem. Res., 2002, 35, 575.
22 For comparison with a cationic analogue, [{MeC(CH2SeMe)3}
Re(CO)3]+, see: J. Connolly, A. R. J. Genge, W. Levason, S. D.
Orchard, S. J. A. Pope and G. Reid, J. Chem. Soc., Dalton Trans., 1999,
2343.
5 An important related class of LX3 are tris(amidoethyl)amine and its
various derivatives, i.e. [N(CH2CH2NR)3]. See, for example: (a)
R. R. Schrock, Acc. Chem. Res., 1997, 30, 9; (b) R. R. Schrock, Pure
Appl. Chem., 1997, 69, 2197; (c) J. G. Verkade, Acc. Chem. Res., 1993,
26, 483; (d) J. G. Verkade, Coord. Chem. Rev., 1994, 137, 233.
6 See, for example: (a) R. Fra¨nkel, U. Kernbach, M. Bakola-
Christianopoulou, U. Plaia, M. Suter, H. Ponikwar, H. No¨th,
C. Moinet and W. P. Fehlhammer, J. Organomet. Chem., 2001, 617–
618, 530; (b) R. Fra¨nkel, C. Birg, U. Kernbach, T. Habereder, H. No¨th
and W. P. Fehlhammer, Angew. Chem., Int. Ed., 2001, 40, 1907; (c)
I. Nieto, F. Cervantes-Lee and J. M. Smith, Chem. Commun., 2005,
3811.
7 For example, [PhB(CH2PR2)3] ligands. See: (a) I. R. Shapiro, D. M.
Jenkins, J. C. Thomas, M. W. Day and J. C. Peters, Chem. Commun.,
2001, 2152; (b) A. A. Barney, A. F. Heyduk and D. G. Nocera, Chem.
Commun., 1999, 2379; (c) J. C. Peters, J. D. Feldman and T. D. Tilley,
J. Am. Chem. Soc., 1999, 121, 9871.
23 For some [RTmMe]Re(CO)3 derivatives, see: (a) R. Garcia, A. Paulo,
A. Domingos and I. Santos, J. Organomet. Chem., 2001, 632, 41; (b)
R. Garcia, A. Paulo, A. Domingos and I. Santos, Dalton Trans., 2003,
2757.
24 This difference is comparable to the difference in covalent radii of sulfur
˚
˚
(1.03 A) and selenium (1.19 A). See: S. S. Batsanov, Russ. Chem. Bull.,
1995, 44, 2245.
25 Note that these cone angles refer specifically for coordination to Re.
26 See, for example, reference 21a and C. E. Zachmanoglou, A. Docrat,
B. M. Bridgewater, G. Parkin, C. G. Brandow, J. E. Bercaw,
C. N. Jardine, M. Lyall, J. C. Green and J. B. Keister, J. Am. Chem.
Soc., 2002, 124, 9525.
27 Methyl substituted [RTmMe] ligands are more electron donating than
the mesityl derivative as judged by the nCO frequencies of
[TmMe]Re(CO)3 (1987 & 1863 cm21),23a [PhTmMe]Re(CO)3 (1895 &
8 For example, [TmR] ligands. See: (a) M. Garner, J. Reglinski, I. Cassidy,
M. D. Spicer and A. R. Kennedy, Chem. Commun., 1996, 1975; (b)
J. Reglinski, M. Garner, I. D. Cassidy, P. A. Slavin, M. D. Spicer and
D. R. Armstrong, J. Chem. Soc., Dalton Trans., 1999, 2119; (c)
C. Santini, G. G. Lobbia, C. Pettinari, M. Pellei, G. Valle and
S. Calogero, Inorg. Chem., 1998, 37, 890; (d) C. Kimblin,
B. M. Bridgewater, D. G. Churchill and G. Parkin, Chem. Commun.,
1999, 2301; (e) M. Tesmer, M. Shu and H. Vahrenkamp, Inorg. Chem.,
2001, 40, 4022; (f) S. Bakbak, V. K. Bhatia, C. D. Incarvito,
A. L. Rheingold and D. Rabinovich, Polyhedron, 2001, 20, 3343.
9 For example, [TtR] ligands. See: (a) P. J. Schebler, C. G. Riordan,
I. A. Guzei and A. L. Rheingold, Inorg. Chem., 1998, 37, 4754; (b)
K. Fujita, A. L. Rheingold and C. G. Riordan, Dalton Trans., 2003,
2004.
1865 cm21 23b and [MeTmMe]Re(CO)3 (1895 & 1860 cm21 23b
) .
)
28 This observation is in accord with comparisons between thioethers and
selenoethers which indicate that the selenoether is a better donor because
of a reduced electronegativity (references 11 and 12) Furthermore,
selenolate ligands are also more electron donating than thiolate ligands,
as illustrated by lower energy nCO stretching frequency of
Ru(SeC6Me4H)4(CO) (2019 cm21) compared to Ru(SC6Me4H)4(CO)
(2040 cm21). See: M. M. Millar, T. O’Sullivan, N. de Vries and
S. A. Koch, J. Am. Chem. Soc., 1985, 107, 3714.
29 For other studies which indicate that [TmR] ligands are more strongly
electron donating than [TpRR9] ligands, see: (a) L. A. Graham,
A. R. Fout, K. R. Kuehne, J. L. White, B. Mookherji, F. M. Marks,
G. P. A. Yap, L. N. Zakharaov, A. L. Rheingold and D. Rabinovich,
Dalton Trans., 2005, 171; (b) M. Garner, M.-A. Lehmann, J. Reglinski
and M. D. Spicer, Organometallics, 2001, 20, 5233.
10 W. Levason and G. Reid, in Comprehensive Coordination Chemistry II,
ed. J. A. McCleverty and T. J. Meyer, Elsevier, New York, 2004, ch. 1.17
and 1.18.
3992 | Chem. Commun., 2006, 3990–3992
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