Dihydrobis(4-cyanopyrazol-1-yl)borate, [Bp4CN], a functionalized heteroscorpionate ligand with cross-linking potential

Article abstract of DOI:10.1021/ic000482f

Dihydrobis(4-cyanopyrazol-1-yl)borate, Bp^4CN, the first example of a polypyrazolylborate ligand capable of intermolecular cross-linking, was synthesized. It forms chelates as a bidentate ligand but retains additional coordinating ability through the two 4-CN groups, leading to polymeric structures in M[Bp^4CN]₂ complexes or in the heteroleptic Co[Bp^4CN][Tp^Cy]. These -CN cross-links are broken by DMF, and the octahedral complexes M[Bp^4CN]₂(DMF)₂ (M = Zn, Co) were structurally characterized. No -CN coordination was found in Rh[BP^4CN](COD) or in the sterically hindered heteroleptic complex Co[Bp^4CN][Tp^Np], which contained five-coordinate cobalt.

Full text of DOI:10.1021/ic000482f

Inorg. Chem. 2000, 39, 5569-5571
5569
Dihydrobis(4-cyanopyrazol-1-yl)borate, [Bp4CN], a
Functionalized Heteroscorpionate Ligand with
Cross-Linking Potential†
Arnold L. Rheingold, Christopher D. Incarvito, and
Swiatoslaw Trofimenko*
Department of Chemistry and Biochemistry, University of
Delaware, Newark, Delaware 19716
ReceiVed May 4, 2000
Among the numerous scorpionate ligands known,1 differing
in the number and nature of substituents (R, Ar, RF, and halogen)
on the pyrazolyl carbons and on boron, there are very few
examples of scorpionates containing organic functional groups
as substituents. The only homoscorpionate example is Tpcpd
(cpd ) carboxypyrrolidido), which is capable of both N3O3
hexadenticity and N2O2 tetradenticity.2 Other heteroscorpionate
examples include the octahedral complex Ni[(pz4CN)Bp]2, which
was prepared from 4-cyanopyrazole and the square-planar Ni-
(Bp)2.3 Both cyano substituents in Ni[(pz4CN)Bp]2 were con-
verted by alkaline hydrolysis to the dicarboxylic acid and to its
further derivatives. Additional examples of scorpionates contain-
ing cyano groups on pyrazolyl carbon atoms are the 2,6-dicyano-
4,4,8,8-R4-pyrazaboles, R2B(µ-pz4CN)2BR2, with R being H or
Et, which could be regarded as complexes of the [R2B(4-
CNpz)2]- ligand with a [R2B]+ cation.4,5 In fact, asymmetric
pyrazaboles were prepared in precisely this fashion: [R2B-
(pz)2]- + R′2BX, where X- was a good leaving group.6
Since the powerfully electron-withdrawing 4-CN group in
pyrazoles can not only coordinate to metal ions, provided its
coordinating ability is not compromised by the presence of
sterically interfering 3- and 5-substituents, but can also be
transformed into other functionalities, such as -COOH or CH2-
NH2, the effect of a 4-CN substituent in scorpionate ligands on
their coordination chemistry was explored. We are reporting now
the synthesis of dihydrobis(4-cyanopyrazol-1-yl)borate, [Bp4CN],
the simplest member of this ligand family, and of several of its
complexes, the structures of which were established by X-ray
crystallography.
soluble in dichloromethane and chloroform, and insoluble in hydro-
carbons. Mp 228-229 °C. IR (cm-1): BH2, 2468, 2384; CN, 2236.
1H NMR (in DMSO-d6): 7.78, 8.06 ppm. Anal. Calcd for C8H6BN6Tl:
C, 23.9; H, 1.50; N, 21.0. Found: C, 24.1; H, 1.63; N, 20.6.
Rh[Bp4CN](COD). This complex was prepared by stirring an
equimolar mixture of Tl[Bp4CN] and [Rh(COD)Cl]2 in methylene
chloride for 1 h, filtering off the TlCl, and evaporating the filtrate.
The yellow residue was obtained in 78% yield, and it was recrystallized
from a heptane/toluene mixture. Mp 236-237 °C, dec. IR (cm-1): BH2,
2451, 2422; CN, 2237. Anal. Calcd for C16H18BN6Rh: C, 47.1; H,
4.41; N, 20.6. Found: C, 47.5; H, 4.58; N, 20.2.
Co[Bp4CN][TpNp]. An equimolar mixture of Co[TpNp]Cl, prepared
by the literature method,8 and Tl[Bp4CN] was stirred in methylene
chloride until the blue color changed to wine-red. The slurry was filtered
and chromatographed on alumina, producing a wine-red eluate, which
produced a red solid on evaporation. It was recrystallized from a toluene/
heptane mixture yielding dark garnet-colored crystals. Mp: 192-194
°C, dec. IR (cm-1): BH2, 2468, 2384; CN, 2236. Anal. Calcd for
C32H46B2CoN12: C, 56.9; H, 6.73; N, 24.6. Found: C, 57.3; H, 6.92;
N, 24.2.
Co[Bp4CN][TpCy]. This complex was prepared as above, but instead
of Co[TpNp]Cl, Co[TpCy]Cl was used.9 The red solution yielded a yellow
solid on evaporation. Mp 254-258 °C, dec, with prior turning red from
about 146 °C but without melting. IR (cm-1): BH2, 2453; CN, 2235,
2262. Anal. Calcd for C35H46B2CoN12: C, 58.7; H, 6.43; N, 23.5.
Found: C, 59.1; H, 6.62; N, 23.1.
Upon the coordination of DMF, the IR spectrum had the following
peaks: BH2, 2455; CN, 2231; and CO, 1649 cm-1. This compound
decomposed at 157-160 °C.
Experimental Section
Tl[Bp4CN]. A mixture of 4-cyanopyrazole7 and KBH4 in 2.5:1 mole
ratio was stirred and refluxed in DMF, with the hydrogen evolved being
measured by a wet-test-meter. After the theoretical amount had been
evolved, the solution was cooled under nitrogen and concentrated under
vacuum. The concentrated solution was added with rapid stirring to an
excess of aqueous TlNO3, and the precipitated Tl[Bp4CN] was filtered
off and washed with water and with a small amount of acetone to
remove excess 4-cyanopyrazole. It was obtained, when dried, in 73%
yield. This compound is very soluble in polar solvents such as DMF
and MeCN, moderately soluble in methanol and acetone, very sparingly
Zn[Bp4CN]2(DMF)2. To a solution of Tl[Bp4CN] in methylene chloride
containing enough DMF to dissolve all of the Tl salt was added half
an equivalent of Zn(OAc)2. After stirring for 1 h, Celite was added
and the slurry was filtered. The colorless filtrate was evaporated,
resulting in the growth of large colorless crystals. Mp 121-123 °C.
IR (cm-1): BH2, 2427; CN, 2230; DMF, 1633. Calcd for C22H26B2N14O2-
Zn: C, 43.6; H, 4.30; N, 32.4. Found: C, 43.8; H, 4.62; N, 32.0.
Co[Bp4CN]2(DMF)2. This complex was prepared as above, using
CoCl2 instead of Zn(OAc)2, and it was isolated as large orange crystals.
Mp 194-198 °C dec. IR (cm-1): BH2, 2425; CN, 2232; DMF, 1649.
Calcd for C22H26B2CoN14O2: C, 44.1; H, 4.34; N, 32.7. Found: C,
44.5; H, 4.62; N, 32.3.
† Dedicated to Professor Heinrich Vahrenkamp on the occasion of his
60th birthday.
(1) Trofimenko, S. Scorpionates: The Coordination Chemistry of Poly-
pyrazolylborate Ligands; Imperial College Press: London, U.K., 1999.
(2) Rheingold, A. L.; Incarvito, C. D.; Trofimenko, S. J. Chem. Soc.,
Dalton Trans. 2000, 1333.
(3) Trofimenko, S. Acc. Chem. Res. 1971, 4, 17.
(4) Trofimenko, S. J. Am. Chem. Soc. 1967, 89, 3165.
(5) Trofimenko, S. J. Am. Chem. Soc. 1967, 89, 4948.
(6) Trofimenko, S. Inorg. Chem. 1969, 8, 1714.
(7) Trofimenko, S. J. Org. Chem. 1963, 28, 2755.
The related Fe(II) and Ni(II) complexes were obtained in similar
fashion, as colorless and pale-violet crystals, respectively, and they had
IR spectra essentially identical to those of the above cobalt and zinc
complexes.
(8) Calabrese, J. C.; Trofimenko, S. Inorg. Chem. 1992, 31, 4810.
(9) Rheingold, A. L.; Haggerty, B. S.; Trofimenko, S. Angew. Chem.,
Int. Ed. Engl. 1994, 33, 1983.
10.1021/ic000482f CCC: $19.00 © 2000 American Chemical Society
Published on Web 10/21/2000

5570 Inorganic Chemistry, Vol. 39, No. 24, 2000
Notes
Table 1. Crystallographic Data for Zn[Bp4CN]2(DMF)2‚2DMF (2),
Rh[Bp4CN](COD) (4), and Co[Bp4CN][TpNp] (5)
(2)‚2DMF
(4)
(5)
formula
fw
space group
a, Å
b, Å
c, Å
C28H40B2N16O4Zn C16H18BN6Rh C32H46B2CoN12
751.75
C2/c
408.08
P21/c
679.36
C2/c
12.9031(15)
15.6057(18)
19.498(2)
104.763(2)
3796.6(8)
4, 1/2
16.606(11)
7.615(5)
14.990(9)
114.70(4)
1722(2)
4, 1
33.801(6)
10.9292(18)
20.488(3)
103.545(3)
7358(4)
8, 1
â, deg
V, Å3
Z, Z′
D(calc), g cm-3 1.315
1.574
1.226
µ(MoKR), cm-1 7.02
10.01
243(2)
5.06
173(2)
temp, K
173(2)
radiation
Mo KR (λ ) 0.71073 Å)
R(F),%a
6.72
19.00
4.23
7.05
17.97
R(wF2), %a
12.78
2
2
a Quantity minimized ) R(wF2) ) ∑[w(Fo - Fc2)2]/∑[(wFo )2]1/2
;
Figure 1. The structure of Zn[Bp4CN]2(DMF)2, 2. Bond lengths (Å)
and angles (deg): Zn-O(1), 2.167(3); Zn-N(1), 2.147(3); Zn-N(4),
2.142(3); 2.164; O(1)-Zn-N(1), 93.96(12); O(1)-Zn-N(4), 90.04-
(13); N(1)-Zn-N(4), 87.55(12);
2
R ) ∑∆/∑(Fo), ∆ ) |(Fo - Fc)|, w ) 1/[σ2(Fo ) + (aP)2 + bP], P )
[2Fc2 + max(Fo,0)]/3.
Results and Discussion
The Bp4CN ligand was prepared by the reaction of 4-cyano-
pyrazole with KBH4, and it was isolated as the Tl salt, the low
solubility of which in halocarbon solvents, in contrast to other
Tl scorpionate salts, suggested a polymeric structure.
The special nature of the Bp4CN ligand became dramatically
apparent when we tried to prepare the simple monomeric bis-
complexes, M[Bp4CN]2, with first-row transition metals by the
reaction of Tl[Bp4CN], 1, with MX2 (X ) Cl, OAc, ClO4) in
methylene chloride. Instead of the typical organic-soluble deep
purple tetrahedral cobalt(II) complexes, or orange-red square-
planar nickel(II) ones, which were obtained with all known Bpx
ligands, except for those containing coordinating substituents
in the 3-position such as pyridyl,10 bipyridyl,11 or 2,4-(dimethoxy)-
phenyl,12 insoluble materials were obtained, with colors char-
acteristic of octahedral structures. However, these products were
readily extracted with DMF, from which large crystals were
obtained, the structures of which turned out to be M[Bp4CN]2-
(DMF)2, as was shown by X-ray crystallography for the zinc
complex, 2 (Figure 1). The analogous cobalt complex, 3, had
the same structure. Identical products were also obtained when
DMF was added to methylene chloride before the reaction. In
these structures, the two DMF molecules were cis to one
another, the remaining coordination sites being occupied by the
two [Bp4CN] ligands. The infrared spectra of these, as well as
of the nickel(II) and iron(II) analogues, were essentially
superimposable. While the preference for octahedral coordina-
tion by cobalt and nickel may be understandable, it was
surprising that zinc, which is often content with being four-
coordinate and tetrahedral, still opted for the octahedral, bis-
DMF structure. The M[Bp4CN]2 complexes prior to addition of
DMF had two nitrile bands at 2232 and 2262 cm-1, as did Co-
[TpCy][Bp4CN], being assigned to the uncoordinated and coor-
dinated 4-CN, respectively. This is in agreement with previous
observations that the CN stretch in coordinated benzonitrile
appears at a higher frequency than that of uncoordinated
benzonitrile.13-15 In the M[Bp4CN]2(DMF)2 complexes, as well
Figure 2. The structure of Rh[Bp4CN](COD), 4. Bond lengths (Å) and
angles (deg): Rh-N(1), 2.127(5); Rh-N(4), 2.107(4); Rh-C(9), 2.112-
(5); Rh-C(10), 2.128(5); Rh-C(13), 2.131(6); Rh-C(14), 2.118(6);
N(1)-Rh-N(4), 87.98(15); N(1)-Rh-C(9), 92.33(19); N(4)-Rh-
C(9), 155.79(19); N(1)-Rh-C(10), 91.71(17); N(4)-Rh-C(10), 166.25-
(18).
as in Rh[Bp4CN](COD) and in Co[Bp4CN][TpNp], only one peak
in the 2232-2237 cm-1 range was observed.
These results show clearly the intermolecular coordinating
ability of the 4-CN group which, in the absence of competing
donors, leads to octahedral, cross-linked structures, as the two
open coordination sites are occupied by the 4-CN donors from
a neighboring M[Bp4CN]2 complex. At the same time, such bonds
are relatively labile, permitting their cleavage by DMF.
To avoid any complication from additional coordination by
the 4-CN groups, the complex Rh[Bp4CN](COD), 4, was
prepared by the reaction of 1 with [Rh(COD)Cl]2. Its structure
(Figure 2) shows the presence of a symmetry plane, and the
distance to Rh of the pseudoaxial B-H is about 2.9 Å and thus
not in the agostic bonding range. This structure shows clearly
that the CN groups are quite unhindered and should be capable
to coordinate to other metals. Although Rh(COD) complexes
with Bp and Bp* have been reported before16,17 and several
(10) Bardwell, D. A.; Jeffery, J. C.; Jones, P. L.; McCleverty, J. A.; Psillakis,
E.; Reeves, Z.; Ward, M. D. J. Chem. Soc., Dalton Trans. 1997, 2079.
(11) Fleming, J. S.; Psillakis, E.; Couchman, S. M.; Jeffery, J. C.;
McCleverty, J. A.; Ward, M. D. J. Chem. Soc., Dalton Trans. 1998,
537.
(12) Rheingold, A. L.; Haggerty, B. S.; Liable-Sands, L. M.; Trofimenko,
S. Inorg. Chem. 1999, 38, 6306.
(14) Walton, R. A. Can. J. Chem. 1966, 44, 1480.
(15) Clarke, R. E.; Ford, P. C. Inorg. Chem. 1970, 9, 227.
(16) King, R. B.; Bond, A. J. Organomet. Chem. 1974, 73, 115.
(17) Cocivera, M.; Ferguson, G.; Kaitner, B.; Lalor, F. J.; O’Sullivan, D.
J. Organometallics 1982, 1, 1125.
(13) Walton, R. A. Spectrochim. Acta 1965, 21, 1795.

Notes
Inorganic Chemistry, Vol. 39, No. 24, 2000 5571
distances for the BpPh ligand averaged 2.131 Å.22 In 3, the
Co-N bonds averaged 2.125 Å, consisting in each Bp4CN ligand
of one short and one long Co-N bond, the short bond being
trans to the coordinated DMF.
A different situation prevailed with the rather similar het-
eroleptic complex Co[TpCy][Bp4CN], synthesized from Co[TpCy]-
Cl and 1. Its solution had the same wine-red color as that of 5.
However, upon evaporation of the solvent, the crystallizing solid
was yellow. Upon the dissolving of the solid, the wine-red
solution was regenerated. This pointed to the existence of a five-
coordinate structure in solution but of a six-coordinate one in
the solid state. It also indicated that the intermolecular CN-
Co bond, present in the solid state, was rather weak. Accord-
ingly, this compound added DMF readily to yield an octahedral
complex characterized by a strong CO band at around 1650
Figure 3. The structure of Co[TpNp][Bp4CN], 5. Bond lengths (Å) and
angles (deg): Co-N(1), 2.117(4); Co-N(11), 2.073(3); Co-N(21),
2.077(3); Co-N(31), 2.075(4); Co-N(41), 2.072(4); Co-H(2), 3.104;
N(1)-Co-N(11), 84.89(13); N(1)-Co-N(21), 90.96(14); N(1)-Co-
(N31), 170.75(14); N(1)-Co-N(41), 92.51(14); N(11)-Co-N(21),
91.66(14); N(11)-Co-N(31), 92.19(14); N(11)-Co-N(41), 166.79-
(14); N(21)-Co-N(31), 97.91(14); N(21)-Co-N(41), 101.35(15);
N(31)-Co-N(41), 88.34(14).
cm-1
.
The Bp4CN ligand is fairly flexible, as can be seen from the
angle between the pyrazolyl planes, expanding or contracting
to accommodate the rest of the molecule. This angle was 121.5°
in 5, 128.0° in 2, and 112.1° in 4. The coordination behavior of
Bp4CN depends on the presence of other ligands. In complexes
with divalent metals, M[Bp4CN]2, the presence of two CN donor
functionalities, plus two open coordination sites, leads to
intractable polymeric structures. However, the CN-M bonds
can be broken up by DMF, leading to well-defined, isolable
M[Bp4CN]2(DMF)2 species. In heteroleptic complexes which are
not too hindered, such as Co[TpCy][Bp4CN], the CN groups can
still coordinate to the metal ion of a neighboring molecule,
leading to oligomeric or polymeric structures in the solid state,
but such bonds are so weak that only a five-coordinate species
exists in solution. In more sterically hindered heteroleptic
complexes, such as 5, the five-coordinate structure is also
retained in the solid state. Finally, in heteroleptic complexes,
such as 4, where the metal is content to remain four-coordinate,
the CN groups of the Bp4CN ligand are not involved in
coordination at all.
homoscorpionate Rh(COD) complexes have been synthesized
and had their structures determined,18-20 complex 4 seems to
be the first instance of a structurally characterized Rh[Bpx]-
(COD) species.
We also synthesized heteroleptic complexes of structure Co-
[Tpx][Bp4CN] by the reaction of Co[Tpx]Cl with 1 and found
that CN coordination to cobalt depended on the structure of
Tpx. Thus, Co[TpNp][Bp4CN], 5, prepared from Co[TpNp]Cl and
1, had a wine-red color, typical of five-coordinate cobalt
scorpionate complexes, in solution and in the solid state. Its
structure (Figure 3) showed that the cobalt ion was indeed five-
coordinate, with the pseudoaxial B-H-Co distance of 3.103
Å and thus out of bonding range. The Co-N distances for
[Bp4CN] averaged 2.074 Å, being about 0.1 Å longer than in
Co[Bp]2, where they averaged 1.967 Å,21 but shorter than in
the related complex, Co[TpiPr,4Br][BpPh], where an agostic
B-H-Co bond (2.26 Å) was present and where the C-N
Supporting Information Available: An X-ray crystallographic file
in CIF format for the structure determination of 2, 4, and 5. This
material is available free of charge via the Internet at http://pubs.acs.org.
IC000482F
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J.; Parvez, M.; Ruhl, B. Organometallics 1982, 1, 1132.
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2000, 39, 1333.
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J. P. Inorg. Chem. 1973, 12, 508.
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Inorg. Chem. 1997, 36, 5097.
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