Cyanide lability and linkage isomerism of hexacyanochromate(III) induced by the Co(II) ion

Article abstract of DOI:10.1021/ic901681e

Reactions between the [M^III(CN)₆]^3- (M=Fe and Co) anions and the mononuclear complex [Co^II(dppe) ₂(H₂O)][BF₄]₂ result in the formation of two isostructural trinuclear clusters {[Co^II(dppe) ₂]₂[M^III(CN)₆}(BF₄). Surprisingly, reactions of [Co(dppe)₂(H₂O)](BF ₄)₂ and [Co(triphos)(CH₃CN)₂] (BF₄)₂ with [Cr(CN)₆]^3- yield the mononuclear complexes [Co(dppe)₂(CN)](BF₄) and [Co(triphos)(CN)₂], respectively. In the former case, an unusual pentanuclear intermediate complex {[Co₃^II(dppe) ₄(MeCN)][Cr^III(CN)₆]₂} was isolated. The reaction was probed by solution IR spectroscopy, which revealed a gradual conversion of the v(C≡N) stretches of the starting materials to those of the CN-bridged intermediate and eventually to the single v(C≡N) stretch of the final mononuclear product. The loss of carbon-bound CN^- ligands from [Cr(CN)₆]^3- occurs on a sufficiently slow time-scale for observation of varying degrees of cyanide linkage isomerism in the trigonal bipyramidal complex {[Co(tmphen)₂]₃[Cr(CN) ₆]₂}; the study was aided by the use of different Co(II) starting materials. Results obtained by a combination of X-ray crystallography, infrared spectroscopy, and magnetometry provide unequivocal evidence that the presence of certain Lewis acids (e.g., Co(II) in this work and Fe(II) ions and BPh₃ in previously reported studies) promote the process of cyanide linkage isomerism, which, in the case of Co(II) species, leads to facile labilization of cyanide ligands from the [Cr(CN)₆]^3- anion.

Full text of DOI:10.1021/ic901681e

Inorg. Chem. 2010, 49, 583–594 583
DOI: 10.1021/ic901681e
Cyanide Lability and Linkage Isomerism of Hexacyanochromate(III) Induced by the
Co(II) Ion
Carolina Avendano,† Ferdi Karadas,† Matthew Hilfiger,† Michael Shatruk,‡ and Kim R. Dunbar*,†
†Department of Chemistry, Texas A&M University, College Station, Texas 77842-3012, and ‡Department of
Chemistry & Biochemistry, Florida State University, Tallahassee, Florida 32306
Received September 11, 2009
Reactions between the [MIII(CN)6]3- (M=Fe and Co) anions and the mononuclear complex [CoII(dppe)2(H2O)][BF4]2
result in the formation of two isostructural trinuclear clusters {[CoII(dppe)2]2[MIII(CN)6]}(BF4). Surprisingly, reactions of
[Co(dppe)2(H2O)](BF4)2 and [Co(triphos)(CH3CN)2](BF4)2 with [Cr(CN)6]3- yield the mononuclear complexes
[Co(dppe)2(CN)](BF4) and [Co(triphos)(CN)2], respectively. In the former case, an unusual pentanuclear intermediate
complex {[CoII3(dppe)4(MeCN)][CrIII(CN)6]2} was isolated. The reaction was probed by solution IR spectroscopy, which
revealed a gradual conversion of the ν(CtN) stretches of the starting materials to those of the CN-bridged intermediate
and eventually to the single ν(CtN) stretch of the final mononuclear product. The loss of carbon-bound CN- ligands from
[Cr(CN)6]3- occurs on a sufficiently slow time-scale for observation of varying degrees of cyanide linkage isomerism in the
trigonal bipyramidal complex {[Co(tmphen)2]3[Cr(CN)6]2}; the study was aided by the use of different Co(II) starting
materials. Results obtained by a combination of X-ray crystallography, infrared spectroscopy, and magnetometry provide
unequivocal evidence that the presence of certain Lewis acids (e.g., Co(II) in this work and Fe(II) ions and BPh3 in
previously reported studies) promote the process of cyanide linkage isomerism, which, in the case of Co(II) species, leads
to facile labilization of cyanide ligands from the [Cr(CN)6]3- anion.
Introduction
with novel magnetic properties have been prepared by such
an approach.7
Transition metal cyanide chemistry is in a renaissance
period that began over a decade ago and is now one of the
central themes of molecular magnetism research. The in-
creased activity is due, in large measure, to the discovery
that certain analogues of the Prussian blue (PB) family
exhibit spontaneous magnetization at temperatures as high
as 376 K.1-3 These findings helped to fuel interest in the topic
of paramagnetic cyanide complexes, particularly mixed
metal/transition metal clusters.4-6 The preparation of
finite complexes rather than face-centered cubic PB phases
relies on the use of capping ligands, typically multidentate
organic molecules that block a number of coordination
sites on metal ions and prevent the growth of extended
structures. Many multinuclear cyanide-bridged complexes
In our quest for unusual cyanide compounds with various
coligands, we turned to the tripodal phosphine ligand 1,1,1-
tris(diphenylphosphinomethyl)ethane (triphos) for the pre-
paration of molecular cubes {[MCl]4[Re(triphos)(CN)3]4}8-10
and squares {[MIICl2]2[CoII(triphos)(CN)2]2}11,12 (M = Mn,
Fe, Co, Ni, Zn; Scheme 1). The formation of these species is
dictated by the geometry of mononuclear building blocks,
[Re(triphos)(CN)3]13 and [Co(triphos)(CN)2],14 with three
and two orthogonal CN- ligands respectively that serve as
bridges in the resulting multinuclear complexes.
(7) Shatruk, M.; Avendano, C.; Dunbar, K. R. Prog. Inorg. Chem. 2009,
56, 155–334.
(8) Schelter, E. J.; Karadas, F.; Avendano, C.; Prosvirin, A. V.;
Wernsdorfer, W.; Dunbar, K. R. J. Am. Chem. Soc. 2007, 129, 8139–8149.
(9) Schelter, E. J.; Prosvirin, A. V.; Dunbar, K. R. J. Am. Chem. Soc.
2004, 126, 15004–15005.
(10) Schelter, E. J.; Prosvirin, A. V.; Reiff, W. M.; Dunbar, K. R. Angew.
Chem., Int. Ed. 2004, 43, 4912–4915.
(11) Karadas, F.; Schelter, E. J.; Prosvirin, A. V.; Bacsa, J.; Dunbar, K. R.
Chem. Commun. 2005, 1414–1416.
(12) Karadas, F.; Schelter, E. J.; Shatruk, M.; Prosvirin, A. V.; Bacsa, J.;
Smirnov, D.; Ozarowski, A.; Krzystek, J.; Telser, J.; Dunbar, K. R. Inorg.
Chem. 2008, 47, 2074–2082.
(13) Schelter, E. J.; Bera, J. K.; Bacsa, J.; Galan-Mascaros, J. R.; Dunbar,
K. R. Inorg. Chem. 2003, 42, 4256–4258.
*To whom correspondence should be addressed. E-mail: dunbar@
mail.chem.tamu.edu.
(1) Mallah, T.; Thiebaut, S.; Verdaguer, M.; Veillet, P. Science 1993, 262,
1554–1557.
(2) Entley, W. R.; Girolami, G. S. Science 1995, 268, 397–400.
(3) Holmes, S. M.; Girolami, G. S. J. Am. Chem. Soc. 1999, 121, 5593–
5594.
(4) Rebilly, J. N.; Mallah, T. Struct. Bonding 2006, 122, 103–131.
(5) Sieklucka, B.; Podgajny, R.; Przychodzen, P.; Korzeniak, T. Coord.
Chem. Rev. 2005, 249, 2203–2221.
(6) Przychodzen, P.; Korzeniak, T.; Podgajny, R.; Sieklucka, B. Coord.
Chem. Rev. 2006, 250, 2234–2260.
(14) Rupp, R.; Huttner, G.; Kircher, P.; Soltek, R.; Buchner, M. Eur. J.
Inorg. Chem. 2000, 1745–1757.
r
2009 American Chemical Society
Published on Web 12/10/2009
pubs.acs.org/IC

584 Inorganic Chemistry, Vol. 49, No. 2, 2010
Avendano et al.
Scheme 1. Formation of (a) Molecular Squares {[MIICl2]2[CoII(triphos)(CN)2]2} (MII
=
Mn, Fe, Co, Ni, and Zn) and (b) Trimers
{[CoII(dppe)2]2[MIII(CN)6}þ (MIII = Fe, Co)
Of relevance to the present report, the tendency of the
CoII ion to form pentacoordinate complexes with phos-
phine ligands is well-known.15,16 Our successful use of
[Co(triphos)(CN)2] for the preparation of molecular squares
prompted us to synthesize other cyanide-bridged complexes
with phosphine groups. Trinuclear species are the simplest
model systems for the study of magnetic properties as a
function of the nature of metal ions and their coordination
geometries; hence, we sought to identify starting materials
that would lead to products with three metal ions. The
bidentate ligand 1,2-bis(diphenylphosphino)ethane (dppe)
affords the pentacoordinate building block [Co(dppe)2-
(H2O)]2þ which, when combined with a hexacyanometallate
anion in a 2:1 ratio, leads to the desired trinuclear complex
(Scheme 1b). This approach, while convenient for CoII,
does not translate well to other first row transition metals,
as they are not prone to forming pentacoordinate precursors
with dppe or other phosphine ligands.17
We successfully used the outlined synthetic strategy
to prepare cyanide-bridged molecular trimers with
[FeIII(CN)6]3- and [CoIII(CN)6]3- anions, but an unexpected
result occurred with [CrIII(CN)6]3-, namely, the formation of
a trigonal-bipyramidal (TBP) cyanide-bridged cluster. In this
molecule, three equatorial positions are occupied by CoII ions
and the two axial positions are filled by [CrIII(CN)6]3- ions. A
study of the formation and structure of this complex revealed
that the CoII ions are bound to the C atoms of the bridging
CN- ligands as a consequence of cyanide linkage isomerism.
Reversal of the orientation of the cyanide ligand has been
observed for several other cyanide-bridged compounds con-
taining CrIII.18,19 For example, many years ago, Brown and
Shriver observed that a CoII3[CrIII(CN)6]2 Prussian blue
analogue undergoes a cyanide flip process when heated in
an inert atmosphere to 100 °C,20 and recently, Coronado and
et al. discovered that the cyanide linkage isomerism of the
Prussian blue analog K0.4Fe4[Cr(CN)6]2.8 16H2O can be
3
reversed with the application of pressure.19 Our group
reported facile, albeit partial isomerization of the CN moiety
for the {[CoII(tmphen)2][CrIII(CN)6]2} cluster with a TBP
core.21 A complete reversal of cyanide bridges was estab-
lished for the related complex {[FeII(tmphen)2][CrIII-
(CN)6]2},22 as well as for the adduct of [CrIII(CN)6]3- with
triphenylboron, [CrIII(NCBPh3)6]3- 23
.
Herein, we report a detailed investigation of cyanide
linkage isomerism in the aforementioned CoII-CrIII com-
plexes with the aim of demonstrating the generality of this
phenomenon for [CrIII(CN)6]3- in the presence of Lewis
acids such as FeII and CoII ions and BPh3.
Experimental Section
Starting Materials. K3[Cr(CN)6] (Aldrich), K3[Co(CN)6]
(Pfaltz and Bauer), (TBA)3[Fe(CN)6] (Fluka; TBA = tetra-
butylammonium), 18-crown-6 (Aldrich), 1,1,1-tris(diphenyl-
phosphinomethyl)ethane (triphos; Aldrich), and Co(BF4)2
3
6H2O (Aldrich) were used as received. The ligand 1,2-bis-
(diphenylphosphino)ethane (dppe; Aldrich) was recrystallized
from ethanol before use. [Co(triphos)(CH3CN)2](BF4)2 was
prepared as previously reported.14 Acetonitrile and methanol
˚
were dried over 3 A molecular sieves and distilled prior to use.
Unless stated otherwise, all compounds were prepared under
anaerobic conditions.
[CoII(dppe)2(H2O)][BF4]2 (1). A quantity of dppe (2.34 g, 5.6
mmol) was added to a solution of Co(BF4)2 6H2O (1.0 g, 2.8
3
mmol) in 70 mL of acetonitrile. The peach-colored mixture was
stirred for 12 h, during which time a bright yellow precipitate
was observed to form. The solid was collected by filtration in the
air, washed with dichloromethane (3 ꢀ 10 mL) and diethyl ether
(3 ꢀ 5 mL), and air-dried. Yield=2.58 g (88%). Elem anal. calcd
for CoP4F8OC52B2H50: C, 59.63; H, 4.81; F, 14.51. Found: C,
60.89; H, 4.71; F, 14.27%.
(15) Jacob, V.; Huttner, G.; Kaifer, E.; Kircher, P.; Rutsch, P. Eur. J.
Inorg. Chem. 2001, 2783–2795.
(16) Jacob, V.; Mann, S.; Huttner, G.; Walter, O.; Zsolnai, L.; Kaifer, E.;
Rutsch, P.; Kircher, P.; Bill, E. Eur. J. Inorg. Chem. 2001, 2625–2640.
(17) DuBois, D. L.; Miedaner, A. Inorg. Chem. 1986, 25, 4642–4650.
(18) Harris, T. D.; Long, J. R. Chem. Commun. 2007, 1360–1362.
(19) Coronado, E.; Gimenez-Lopez Mari, C.; Levchenko, G.; Romero
Francisco, M.; Garcia-Baonza, V.; Milner, A.; Paz-Pasternak, M. J. Am.
Chem. Soc. 2005, 127, 4580–4581.
(21) Shatruk, M.; Chambers, K. E.; Prosvirin, A. V.; Dunbar, K. R.
Inorg. Chem. 2007, 46, 5155–5165.
(22) Shatruk, M.; Dragulescu-Andrasi, A.; Chambers, K. E.; Stoian, S.
A.; Bominaar, E. L.; Achim, C.; Dunbar, K. R. J. Am. Chem. Soc. 2007, 129,
6104–6116.
(23) Schelter, E. J.; Shatruk, M.; Heintz, R. A.; Galan-Mascaros, J. R.;
Dunbar, K. R. Chem. Commun. 2005, 1417–1419.
(20) Brown, D. B.; Shriver, D. F. Inorg. Chem. 1969, 8, 37–42.

Article
Inorganic Chemistry, Vol. 49, No. 2, 2010 585
Table 1. Reaction Conditions Used along with Colors and IR Data for Different Batches of {[Co(tmphen)2]3[Cr(CN)6]2}
sample
solvent
Co(II) source
time
temperature
color
ν(CtN),a cm-1
8ab
8b
MeCN
MeCN
MeCN
MeCN
MeOH
CoCl2
Co(BF4)2 6H2O
24 h
24 h
5 min
1 min
24 h
25 °C
25 °C
25 °C
0 °C
brown
yellow/brown
yellow
peach-yellow
peach
2157(b), 2133(t), 2105(i)
2155, 2147(b), 2135, 2126 (t), 2103(i)
2155, 2147(b), 2123(t), 2103(i)
2155, 2147(b), 2123(t)
3
8cc
8dc
8e
Co(BF4)2 6H2O
Co(BF4)2 6H2O
3
3
CoCl2 6H2O
3
25 °C
2157(b), 2125(t)
a t = terminal, b = bridging normal, i = bridiging isomerized. b The sample was prepared under anaerobic conditions. c Reactions were performed
with stirring.
{[CoII(dppe)2]2[CoIII(CN)6]}(BF4) (2). A solution of [(18-
crown-6)K]3[Co(CN)6] was prepared by stirring 56.5 mg
(0.170 mmol) of K3[Co(CN)6] and 113 mg (0.430 mmol) of 18-
crown-6 in 13 mL of methanol for 6 h. The resulting solution was
filtered to remove excess K3[Co(CN)6]. A second solution was
prepared by stirring 300 mg (0.28 mmol) of 1 in 7 mL of
acetonitrile to give a dark orange solution. The [(18-crown-
6)K]3[Co(CN)6] solution was slowly added to the dark orange
solution, and the mixture was left to stand undisturbed for
3 days. The crop of orange-red crystals that formed was filtered
in the air and washed with copious amounts of methanol. Yield
=225 mg (78%). Elemental analysis indicated the presence of
interstitial water and methanol molecules. Elem anal. calcd for
Co3P8F4O5N6C111BH108 (2 CH3OH 4H2O): C, 62.93; H, 5.14;
N, 3.97; F, 3.58. Found: C, 63.69; H, 5.10; N, 4.29; F, 3.03%.
IR(Nujol): ν(CtN) 2126, 2148 cm-1. ESIþ-MS (CH3CN): m/z
1926 ([M]þ), 964 ([M - H]2þ). UV-vis(CH3CN), λmax, nm
(ε, M-1 cm-1): 255 (5.8 ꢀ 104), 315 (2.1 ꢀ 104), 383 (5951).
{[CoII(dppe)2]2[FeIII(CN)6]}(BF4) (3). Compound 3 was pre-
pared in a fashion analogous to that described above for
compound 2. The acetonitrile soluble salt (TBA)3[Fe(CN)6]
(62 mg per 10 mL of solvent) was used as the source of
[Fe(CN)6]3- anions. Yield=205 mg (71%). Elem anal. calcd
for Co2FeP8F4N6C110BH96: C, 65.28; H, 4.94; N, 4.11; F, 3.72.
Found: C, 65.37; H, 4.80; N, 4.23; F, 3.52%. IR(Nujol): ν(CtN)
2108, 2129 cm-1. ESIþ-MS (CH3CN): m/z 1923 ([M]þ), 962
([M - H]2þ). UV-vis(CH3CN), λmax, nm (ε, M-1 cm-1): 253
(5.4 ꢀ 104), 320 (2.7 ꢀ 104), 394 (6857).
block-shaped crystals which were collected by filtration and
washed with copious amounts of methanol followed by 5 mL of
diethyl ether. Yield = 98 mg (33%). Elem anal. calcd for
CoP4F4N2C55BH52O: C, 64.31; H, 5.10; F, 7.40; N, 2.72. Found:
C, 64.64 ; H, 4.94 ; F, 7.38 ; N, 2.72%. IR(Nujol): ν(CtN) 2114
cm-1. ESIþ-MS (CH3CN): m/z 908 ([M]þ). UV-vis(CH3CN),
λ
max, nm (ε, M-1 cm-1): 299 (3.3 ꢀ 104), 353 (3.5 ꢀ 104).
CoII(triphos)(CN)2 (6). This compound was synthesized using
a different route than the previously reported procedure.15
A
solution of [(18-crown-6)K]3[Cr(CN)6] was prepared by stirring
81.3 mg (0.250 mmol) of K3[Cr(CN)6] and 159 mg (0.6 mmol) of
18-crown-6 in 10 mL of methanol for 6 h. The resulting solution
was filtered to remove excess K3[Cr(CN)6]. A second solu-
tion was prepared by stirring 290 mg (0.3 mmol) of
[Co(triphos)(CH3CN)2](ClO4)2 in 10 mL of acetonitrile to give
a dark green solution. The [(18-crown-6)K]3[Cr(CN)6] solution
was slowly added to the dark green solution to yield a dark
orange solution. The mixture was left to stand undisturbed for
3 days, during which time dark-red block-shaped crystals
formed. The crystals were collected in the air and washed
with diethyl ether. Yield = 59 mg (27%). Elem anal. calcd
for CoP3N2C43H39: C, 70.21; H, 5.34; N, 3.81. Found: C,
69.95; H, 5.55; N, 3.77%. IR(Nujol): ν(CtN) 2096(s),
3
3
2101(w) cm-1
.
{[CoII3(dppe)4(MeCN)][CrIII(CN)6]2} (7). A solution of [(18-
crown-6)K]3[Cr(CN)6] was prepared by stirring 55.2 mg (0.170
mmol) of K3[Cr(CN)6] and 113 mg (0.430 mmol) of 18-crown-6
in 7 mL of acetonitrile for 6 h and was filtered to remove excess
K3[Cr(CN)6]. This solution was slowly added to a dark orange
solution of 300 mg (0.28 mmol) of 1 in 3 mL of acetonitrile.
Crystals were observed after 30 min. Surprisingly, the few
orange crystals obtained were of sufficient quality for single-
crystal X-ray analysis.
[CoII(dppe)2(CN)][BF4] (4). This compound was synthesized
using a different route than the previously reported procedure.24
A solution of [(18-crown-6)K]3[Cr(CN)6] was prepared by
stirring 81.3 mg (0.250 mmol) of K3[Cr(CN)6] and 159 mg
(0.6 mmol) of 18-crown-6 in 10 mL of methanol for 6 h. The
resulting solution was filtered to remove excess K3[Cr(CN)6]. A
second solution was prepared by stirring 321 mg (0.3 mmol) of
1 in 10 mL of acetonitrile to give a dark orange solution. The
[(18-crown-6)K]3[Cr(CN)6] solution was slowly added to the
dark orange solution, and the mixture was left to stand un-
disturbed. After 3 days, dark red-brown block crystals were
present. The crystals were collected in the air and washed with
acetonitrile/diethyl ether. Yield = 193 mg (42%). Elem anal.
calcd for CoP4F4NC53BH48: C, 65.72; H, 4.99; F, 7.85; N, 1.45.
Found: C, 65.20; H, 4.95; F, 7.78; N, 1.65%. IR(Nujol): ν(CtN)
2096 cm-1. ESIþ-MS (CH3CN): m/z 882 ([M]þ). UV-vis
(CH3CN), λmax, nm (ε, M-1 cm-1): 502 (327).
{[CoII(tmphen)2]3[CrIII(CN)6]2} (8). Five different samples of
this complex were prepared according to the general method
reported earlier21 by varying the solvent, temperature, and
reaction time (Table 1). In all reactions, a 4 mM solution of
Co(II) salt was treated with 2 equiv of tmphen, and the solution
was stirred for ∼30 min, after which time a 4 mM solution of
[K(18-crown-6)]3[Cr(CN)6] was added. The product that
formed was recovered by filtration, washed with the same
solvent as used for the reaction, and dried in vacuo. No
structural characterization of these samples was undertaken,
but, as established in our previous reports,21 they contain
varying amounts of interstitial solvent, which exchanges for
water with the exposure of solid samples to humid laboratory air.
Physical Measurements. Elemental analyses were performed
by Atlantic Microlab, Inc. IR spectra were measured as Nujol
mulls or solutions placed between KBr plates on a Nicolet 740
FT-IR spectrometer. Electrospray mass spectra were acquired
on an MDS Sciex API QStar Pulsar mass spectrometer using an
electrospray ionization source. All spectra were acquired in the
positive ion mode in an acetonitrile solution at an approximately
50 μM analyte concentration. The spray voltage was ∼4800 V,
and the nozzle skimmer potential was adjusted to 10 V
to minimize fragmentation. Solution UV-visible absorption
spectra were obtained by using a Shimadzu UV-1601PC
[CoIII(dppe)2(CN)2][BF4] (5). A solution of [(18-crown-6)K]3-
[Cr(CN)6] was prepared by stirring 81.3 mg (0.250 mmol) of
K3[Cr(CN)6] and 159 mg (0.6 mmol) of 18-crown-6 in 10 mL of
methanol for 6 h. The resulting solution was filtered to remove
excess K3[Cr(CN)6]. A second solution was prepared by stirring
321 mg (0.3 mmol) of 1 in 10 mL of acetonitrile to give a dark
orange solution and the [(18-crown-6)K]3[Cr(CN)6] solution
was slowly added to it. The resulting mixture was left to
stand undisturbed for 3 days, after which time the solution was
filtered in air. After one week, the filtrate had produced yellow
(24) Rigo, P.; Bressan, M. Inorg. Nucl. Chem. Lett. 1973, 9, 527–532.

586 Inorganic Chemistry, Vol. 49, No. 2, 2010
Avendano et al.
Table 2. Crystal Structural Data and Refinement Parameters for Compounds 2-5 and 7
formula
Co3P8F4O2N6C112BH104 (2 2MeOH)
3
Co2FeP8F4O2N6C112BH104 (3 2MeOH)
3
CoP4F4NC53BH48 (4)
P21/c
space group
unit cell
P21/n
a = 12.653(1) A
P21/n
a = 12.718(4) A
˚
˚
˚
a = 12.572(2) A
˚
b = 21.718(2) A
˚
c = 19.395(1) A
˚
b = 21.726(7) A
˚
c = 19.360(6) A
˚
b = 25.590(3) A
˚
c = 14.669(2) A
β = 92.207(1)°
5325.7(6)A
2
β = 91.837(5) °
5347(3) A
2
β = 106.029(2))°
4536.0(10)A
1
3
3
3
˚
˚
˚
unit cell volume, V
Z
density, Fcalcd
1.335 g/cm3
0.644 mm-1
orange-red needle
0.18 ꢀ 0.10 ꢀ 0.08 mm
110 K
1.353 g/cm3
0.627 mm-1
dark-green needle
0.16 ꢀ 0.11 ꢀ 0.07 mm
110 K
1.418 g/cm3
0.575 mm-1
red-brown block
0.11 ꢀ 0.10 ꢀ 0.09 mm
110 K
absorption coefficient, μ
cryst color and habit
cryst size
temp
radiation, λ
˚
˚
˚
Mo KR, 0.71073 A
Mo KR, 0.71073 A
Mo KR, 0.71073 A
min. and max. θ
reflns collected
independent reflns
data/params/restraints
R [Fo > 4σ(Fo)]
1.86 to 25.00°
44963 [Rint =0.0320]
9010
9010/724/237
R1 = 0.086
wR2 = 0.203
1.280
1.41 to 28.43°
32303 [Rint =0.2242]
9158
9158/641 /746
R1 = 0.092
wR2 = 0.194
1.084
1.59 to 28.30°
25866 [Rint = 0.0342]
10270
10270/577/1
R1 = 0.060
wR2 = 0.146
1.087
Goodness-of-fit on F2
max./min. residual densities, e A
-3
˚
1.63, -0.78
1.23, -0.92
1.96, -0.51
3
Co3Cr2P8O4.5N15.5C127.5H124.5
(7 4.5MeOH 2.5MeCN)
formula
CoP4F4N3C56BH43 (5)
P1
3
3
space group
unit cell
Aba2
a = 51.547(10) A
˚
˚
a = 10.498(1) A
˚
b = 13.092(2) A
˚
b = 26.188(5) A
˚
c = 19.705(4) A
˚
c = 19.029(2) A
R=97.372(2)°
β = 92.307(2)°
γ = 111.080(1)°
3
3
˚
˚
26600(9) A
8
unit cell volume, V
Z
2409.9(5) A
2
density, Fcalcd
abs. coeff., μ
cryst color and habit
cryst size
temperature
1.416 g/cm3
0.547 mm-1
yellow block
0.20 ꢀ 0.20 ꢀ 0.10 mm
110 K
1.236 g/cm3
0.676 mm-1
orange plate
0.15 ꢀ 0.11 ꢀ 0.07 mm
110 K
˚
˚
radiation, λ
Mo KR, 0.71073 A
Mo KR, 0.71073 A
min. and max. θ
reflns collected
independent reflns
data/params/restraints
R [Fo > 4σ(Fo)]
1.69 to 28.44°
25826[Rint =0.0297]
10940
10940/626 /0
R1 = 0.042
wR2 = 0.1116
1.034
1.35 to 26.37°
101233 [Rint =0.1948]
23377
23377 /1397/4
R1 = 0.126
wR2 =0.283
1.027
GoF on F2
max./min residual densities, e A
-3
˚
1.14, -0.68
1.91, -0.76
3
spectrophotometer in the 200-1000 nm range. Magnetic mea-
surements were performed on crushed microcrystalline samples
with the use of a Quantum Design MPMS-XL SQUID magnet-
ometer. DC magnetic susceptibility measurements were per-
formed in the range of 1.8-300 K in an applied field of 0.1 T.
Magnetization data were collected in the 0-7 T range starting at
zero field at 1.8 K. Data were corrected for the diamagnetic
contributions calculated from the Pascal constants.25 The mole-
cular weight of each compound was adjusted according to the
interstitial solvent content, as determined from thermal gravi-
metric analysis data.
Bruker SAINT26 software package. The absorption correction
(SADABS27) was based on fitting a function to the empirical
transmission surface as sampled by multiple equivalent mea-
surements. Solution and refinement of the crystal structures was
carried out using the SHELX28 suite of programs and the
graphical interface X-SEED.29 Structure solution by direct
methods resolved positions of all metal and P atoms as well as
most of the C and N atoms. The remaining non-hydrogen atoms
were located by alternating cycles of least-squares refinements
and difference Fourier maps. Hydrogen atoms were placed at
calculated positions. In the case of compounds with [BF4]-
counterions, these were disordered and were restrained to mean-
ingful geometries. The final refinement was performed with
anisotropic thermal parameters for all non-hydrogen atoms.
A summary of pertinent information relating to unit cell para-
meters, data collection, and refinements is provided in Table 2.
Selected metal-ligand bond distances are provided in Table 3
X-Ray Crystallography. In a typical experiment, a crystal
selected for study was suspended in polybutene oil (Aldrich) and
mounted on a cryoloop which was placed in a N2 cold stream.
Single-crystal X-ray data were collected on a Bruker APEX or
Bruker SMART 1000 diffractometer equipped with a CCD
detector at 110 K. The data sets were recorded as three ω scans
of 606 frames each, at a 0.3° step width, and integrated with the
(27) Sheldrick, G. M. SADABS; University of Gottinggen: Gottingen,
Germany, 1996.
(28) Sheldrick George, M. Acta Crystallogr., Sect. A 2008, 64, 112–122.
(29) Barbour, L. J. J. Supramol. Chem. 2003, 1, 189–191.
(25) Bain, G. A.; Berry, J. F. J. Chem. Educ. 2008, 85, 532–536.
(26) SMART; SAINT; Siemens Analytical X-ray Instruments Inc.: Madison,
WI, 1996.

Article
Inorganic Chemistry, Vol. 49, No. 2, 2010 587
a
˚
Table 3. Metal-Ligand Bond Distances (A) and Bond Angles (deg) in the Crystal Structures of Compounds 2-5
{[Co(dppe)2]2[Co(CN)6]}(BF4) (2)
{[Co(dppe)2]2[Fe(CN)6]}(BF4) (3)
Co(1)-NtC
Co(2)-CtNbr
Co(2)-CtNter
(CtN)br
(CtN)ter
Co(1)-Pavg
1.983(5)
1.925(7)
2.08(1)
1.099(9)
1.08(1)
2.27(1)
Co(1)-NtC
178.5(5)
176.4(6)
176.1(9)
81.83(6)
97.72(6)
81.16(6)
99.03(6)
Co(1)-NtC
Fe-CtNbr
Fe-CtNter
(CtN)br
(CtN)ter
Co(1)-Pavg
1.982(4)
1.997(5)
2.102(7)
1.069(7)
1.031(8)
2.279(1)
Co(1)-NtC
Fe-CtNbr
176.9(5)
175.9(5)
175.4(6)
81.00(6)
99.21(6)
81.62(7)
97.95(6)
Co(2)-CtNbr
Co(2)-CtNter
P(1)-Co-P(2)
P(2)-Co-P(3)
P(3)-Co-P(4)
P(4)-Co-P(1)
Fe-CtNter
P(1)-Co-P(2)
P(2)-Co-P(3)
P(3)-Co-P(4)
P(4)-Co-P(1)
[Co(dppe)2(CN)][BF4] (4)
[Co(dppe)2(CN)2][BF4] (5)
Co-CtNter
Co(1)-Pavg
1.986(3)
2.278(1)
Co-CtNter
178.3(3)
86.00(3)
106.35(3)
89.12(3)
167.95(3)
Co-CtNter
Co(1)-Pavg
1.889(2)
2.317(1)
Co-CtNter
175.2(2)
82.99(2)
97.01(2)
82.99(2)
97.01(2)
P(1)-Co-P(2)
P(2)-Co-P(3)
P(3)-Co-P(4)
P(4)-Co-P(1)
P(1)-Co-P(2)
P(2)-Co-P(3)
P(3)-Co-P(4)
P(4)-Co-P(1)
a ter = terminal, br = bridging.
˚
Table 4. Metal-to-Ligand Bond Distances (A) and Bond Angles (deg) in the Crystal Structures of Compound 7
{[Co3(dppe)4(MeCN)][Cr(CN)6]2} (7)
Cr(1)-(CtN)terminal
Cr(1)-N(1)tC(1)-Co(1)
2.047(17) Cr(1)-N(1)tC(1)-Co(1) 174.8(12) Co(2)-P(4)
Cr(1)-N(3)tC(3)-Co(2) 171.1(10) Co(2)-P(6)
2.042(15)
2.310(5) C(3)-Co(2)-C(6)
2.230(4) C(3)-Co(2)-P(5)
87.4(5)
90.4(4)
Cr(1)- N(3)tC(3)-Co(2) 2.023(12) Cr(1)-N(2)tC(2)-Co(3) 175.3(10) Co(2)-P(5)
Cr(1)- N(2)tC(2)-Co(3) 2.021(16)
2.228(4) C(6)-Co(2)-P(5)
155.9(4)
162.4(5)
Co(2)-C(3)tN-Cr(1) 1.830(14) C(3)-Co(2)-P(6)
Cr(2)- (CtN)terminal
2.032(16) Cr(2)-N(4)tC(4)-Co(1) 169.4(10) Co(2)-C(6)tN-Cr(2) 1.867(14) C(6)-Co(2)-P(6)
92.3(4)
82.79(15)
95.8(4)
98.2(4)
105.85(17)
Cr(2)- N(4)tC(4)-Co(1) 2.005(12) Cr(2)-N(6)tC(6)-Co(2) 175.0(10)
Cr(2)- N(6)tC(6)-Co(2) 2.036(13) Cr(2)-N(5)tC(5)-Co(3) 174.1(11)
Cr(2)- N(5)tC(5)-Co(3) 2.034(15)
P(5)-Co(2)-P(6)
C(3)-Co(2)-P(4)
C(6)-Co(2)-P(4)
P(5)-Co(2)-P(4)
Co(1)-P(1)
2.242(4) C(4)-Co(1)-C(1)
90.2(5)
P(6)-Co(2)-P(4)
101.61(16)
88.7(6)
Co(1)-P(2)
Co(1)-P(3)
2.230(4) C(4)-Co(1)-P(2)
2.269(4) C(1)-Co(1)-P(2)
173.6(4)
93.1(4)
88.7(4)
103.9(3)
Co(3)-P(8)
2.225(4) C(5)-Co(3)-C(2)
2.242(4) C(5)-Co(3)-N(13) 96.4(5)
2.035(11) C(2)-Co(3)-N(13) 98.1(5)
Co(3)-P(7)
Co(1)-C(1)tN(1)-Cr(1) 1.910(12) C(4)-Co(1)-P(1)
Co(1)-C(4)tN(4)-Cr(2) 1.865(16) C(1)-Co(1)-P(1)
P(2)-Co(1)-P(1)
Co(3)-N(13)-CMe
Co(3)-C(2)tN-Cr(1) 1.916(15) C(5)-Co(3)-P(8)
89.6(4)
170.5(4)
91.4(3)
171.0(4)
93.3(4)
92.1(4)
85.21(15) Co(3)-C(5)tN-Cr(2) 1.868(14) C(2)-Co(3)-P(8)
84.4(4)
97.0(3)
100.60(16)
158.01(16)
C(4)-Co(1)-P(3)
N1(3)-Co(3)-P(8)
C(5)-Co(3)-P(7)
C(2)-Co(3)-P(7)
N(13)-Co(3)-P(7)
P(8)-Co(3)-P(7)
C(1)-Co(1)-P(3)
P(2)-Co(1)-P(3)
P(1)-Co(1)-P(3)
87.08(16)
for compounds 2-5 and in Table 4 for compound 7. Complete
listings of atomic and thermal parameters, bond distances,
and bond angles are available in the cif files (Supporting
Information).
An orange solution of [CoII(dppe)2(H2O)][BF4]2 (1) in
acetonitrile was reacted with solutions of [MIII(CN)6]3-
(M=Co and Fe) in methanol to form the isostructural
linear cyanide-bridged trimers 2 and 3 (Figure 1). The
compounds are stable in solution and in the solid state
and are soluble in MeCN, CH2Cl2, and DMF. In both 2
and 3, the ligand environment and geometries of each
metal ion are the same as in the starting material, with the
P atoms of dppe occupying the equatorial plane and the
N-bound bridging cyanide from the hexacyanometallate
fragment coordinating to the apical position of the square
pyramid.
Results and Discussion
Syntheses and Single-Crystal X-ray Structures. The
CoII center in [CoII(dppe)2(X)]2þ, where X = MeCN
or H2O, is in a square-pyramidal geometry, which is
typical for most phosphine complexes of CoII, includ-
ing those with chelating and monodentate phosphine
ligands.14,30-32 The presence of two dppe capping ligands
and a single open site available for further chemistry
renders this compound a convenient precursor for the
assembly of linear trinuclear molecules.
Surprisingly, the reaction of an acetonitrile solution of
1 and a methanol solution of [CrIII(CN)6]3- led to the
formation of red crystals of the mononuclear complex
[CoII(dppe)2(CN)][BF4] (4). After removal of the cry-
stals and exposure of the mother liquor to air for a week,
yellow crystals of [CoIII(dppe)2(CN)2][BF4] (5) were
obtained.
(30) Mangani, S.; Orioli, P.; Ciampolini, M.; Nardi, N.; Zanobini, F.
Inorg. Chim. Acta 1984, 85, 65–72.
(31) Stalick, J. K.; Corfields, P. W. R.; Meek, D. W. Inorg. Chem. 1973,
12, 1668–1675.
(32) Ciancanelli, R.; Noll, B. C.; DuBois, D. L.; DuBois, M. R. J. Am.
Chem. Soc. 2002, 124, 2984–2992.
In the structure of 4, the geometry around the CoII
ion is substantially altered, as the dppe ligands have

588 Inorganic Chemistry, Vol. 49, No. 2, 2010
Avendano et al.
Figure 1. (a) Molecular structure of the asymmetric unit of {[CoII(dppe)2]2[CoIII(CN)6]}þ and (b) the thermal ellipsoid plot of the trinuclear cation
{[CoII(dppe)2]2[CoIII(CN)6]}þ in 2. Ellipsoids projected at the 50% probability level.
Figure 2. Thermal ellipsoid plots of the cation (a) [Co(dppe)(CN)]þ in 4 and (b) [Co(dppe)(CN)2]þ in 5. Ellipsoids projected at the 50% probability level.
rearranged around the cobalt ion and the CN- ligand is
positioned in the equatorial plane of the square pyramid
(Figure 2a). The source of this cyanide is obviously the
[CrIII(CN)6]3- ion as it is the only cyanide-containing
starting material used in the reaction. Moreover,
[CrIII(CN)6]3- has been shown to undergo cyanide link-
age isomerism, leading to new cyano complexes that
contain an N-bound CrIII ion.19,22,23 The observed pro-
ducts invoke two possible mechanisms, one in which a
cyanide ligand of [CrIII(CN)6]3- is labilized, which frees it
up to coordinate to the [Co(dppe)2(solv)]2þ complex, and
another in which an intermediate bridging state is formed
which assists in the cyanide flip and eventual formation of 4.
The latter mechanism is strongly supported by the present
experimental evidence, as we were able to isolate an inter-
mediate compound for the cyanide-bridged complex. The
reaction of [CoII(dppe)2(H2O)]2þ and [CrIII(CN)6]3- in
acetonitrile led to the isolation of crystals of the
kinetic product {[CoII3(dppe)4(MeCN)][CrIII(CN)6]2} (7).
Compound 7 exhibits a TBP structure in which the
[CrIII(CN)6]3- ions reside in the apical sites of the bipyramid
and the equatorial sites are occupied by the CoII ions
(Figure 3a). Each Co center is coordinated to one chelating
dppe ligand and two bridging cyanides from the apical hexa-
cyanochromate fragments. The fifth site on the Co(2) and
Co(3) ions is occupied by a P atom of a shared bridging dppe
ligand, while the fifth site on Co(1) is occupied by an
acetonitrile molecule (Figure 3b).
assume a typical chelating mode to a single CoII ion
(Scheme 2). It is proposed that compound 7 is an unstable
intermediate that crystallizes during the course of the
reaction. The isolation of this molecule provides crucial
insight into the mechanism of linkage isomerism.
Further characterization of the {[CoII3(dppe)4(Me-
CN)][CrIII(CN)6]2} species was hampered by an insuffi-
cient amount of material due to its facile conversion to
compound 4. Attempts to isolate pure samples of 7 by
using other solvents were unsuccessful. In a similar
manner, we found that, when an acetonitrile solution
of [Co(triphos)(CH3CN)2]2þ is treated with a methanol
solution of [Cr(CN)6]3- in a 2:1 ratio, the result is not the
expected cyanide-bridged species but rather the pre-
viously reported mononuclear precursor Co(triphos)-
(CN)2.14 Unfortunately, we were not able to isolate any
intermediates in this reaction.
The isomerization and loss of cyanide ligands from
[CrIII(CN)6]3- is not particularly surprising, as we had
previously noted the cyanide flip issue in the reactions
used to prepare the trigonal-bipyramidal complexes
{[M(tmphen)2]3[Cr(CN)6]2} (M =Fe, Co). Whereas the
Fe-containing complex exhibited fast and essentially
complete reversal of the bridging CN- ligands,22 the
Co-containing TBP showed varying degrees of linkage
isomerism depending on the experimental conditions.21
To carry out a more detailed analysis of the isomerization
process and to probe its ramifications on the properties of
{[Co(tmphen)2]3[Cr(CN)6]2} (8), we prepared several
samples of the material by varying the solvent, time,
and temperature of the reaction (Table 1).
As the reaction progresses, it leads to the decomposi-
tion of the TBP cluster and subsequent formation of the
final product 4, with the result that all of the dppe ligands

Article
Inorganic Chemistry, Vol. 49, No. 2, 2010 589
Figure 3. (a) Side view and (b) top view of the structure of {[CoII3(dppe)4(MeCN)][CrIII(CN)6]2}. (Phenyl rings omitted for the sake of clarity.)
Scheme 2. Conversion of Mononuclear Complex 1 to Mononuclear Complex 4 via the Formation of the Pentanuclear TBP Intermediate 7
Table 5. ν(CtN) Stretching Frequencies in the Complexes {[CoII(dp-
The first suspicion of cyanide linkage isomerism during
III
pe)2]2[M0 (CN)6]} (M0 = Co, Fe), Corresponding PB Analogues, and Free
the preparation of 8 came from the observation of the
Hexacyanometallate Anions
mixed colors of the product which first crystallized as a
yellow solid but, after some time, reverted to an admix-
ture of a yellow and brown (sample 8b in Table 1). When
ν(CtN), cm-1
complex
bridging
terminal
[M0(CN)6]3- aniona
the reaction was carried out with stirring for a short time
(5 min) and immediately filtered, the product was recov-
ered as a yellow solid (8c). Lowering the reaction tem-
perature to 0 °C and shortening the reaction time to 1 min
lead to the isolation of a peach-yellow product (8d). In
order to slow down the reaction and favor the formation
of the brown product observed in 8b, we used anhydrous
CoCl2 as a starting material because the chloride ligands
tend to be less labile than water. Indeed, this reaction (8a)
resulted in the crystallization of a brown product. When
the same reaction is performed in the more polar solvent
methanol (8e), which enhances dissociation of the CoCl2
starting material, rapid precipitation of a peach solid
ensues. The color of the product did not change even after
storage in the mother liquor for 24 h. These collective
observations suggest the occurrence of cyanide linkage
isomerism that can be controlled by reaction conditions.
This hypothesis has been confirmed by a combination of IR
spectroscopic and magnetic measurements (see below).
(2) CoII-FeIII-CoII
(3) CoII-CoIII-CoII
2129
2148
2108
2126
2101
2126
a As measured for (TMA)3[Fe(CN)6] and [(18-crown-6)K]3[Co(CN)6]
(TMA = tetramethylammonium).
Infrared Spectroscopy. Compounds 2 and 3 exhibit
characteristic bands in the ν(CtN) stretching region. A
comparison of the observed IR data to the ν(CtN)
stretches reported for extended Prussian blue phases
and the free hexacyanometallate anions aids in the assign-
ment of the bands in the new compounds to bridging or
terminal CN- ligands. As the data in Table 5 indicate, the
IR spectra of 2 and 3 exhibit lower-frequency stretches
that are only slightly shifted as compared to the corre-
sponding modes of the [MIII(CN)6]3- ions (M=Co, Fe)
and, therefore, are reasonably assigned to the ter-
minal cyanides. Upon formation of the MIII-CtN-
CoII bridge, the CN stretching frequency increases.7

590 Inorganic Chemistry, Vol. 49, No. 2, 2010
Avendano et al.
Table 6. ν(CtN) Stretching Frequencies for Complexes 4 and 5 and a Summary
of the Solution IR Study of the Reaction of [CoII(dppe)2(H2O)]2þ with
[CrIII(CN)6]3- in Acetonitrile
ν(CtN), cm-1
complex
bridging
terminal
(4) [CoII(dppe)2(CN)][BF4]
(5) [CoIII(dppe)2(CN)2][BF4]
2096
2114
[Co(dppe)2(H2O)]2þ þ [Cr(CN)6]3- in Acetonitrile
[CrIII(CN)6]3-
initial
30 min
1 h
2115a
2113
2113, 2090
2098
2113, 2098
2140
2140, 2130
2130
3 days
a As measured for [(18-crown-6)K]3[Cr(CN)6].
Consequently, the IR stretches of complexes 2 and 3 that
appear at ∼20 cm-1 higher than the ν(CtN) of the free
[MIII(CN)6]3- ions are assigned to the bridging CN-
ligands.
For compounds 4 and 5, the expected characteristic IR
bands were observed (Table 6).14,24,33 It should be noted
at this point that the stretch for complex 4 located at 2096
cm-1 is significantly lower than the ν(CtN) mode for the
free [CrIII(CN)6]3- ion (2116 cm-1) or of any possible
cyanide-bridged species that could result from the reac-
tion of [CoII(dppe)2(H2O)]2þ and [CrIII(CN)6]3-; as such,
its appearance can be easily monitored by solution IR
spectroscopy. The slow formation of 4 from the reaction
of [CoII(dppe)2(H2O)]2þ and [CrIII(CN)6]3- in acetoni-
trile prompted us to perform a detailed solution IR study
of this reaction in order to obtain insight into the mechan-
Figure 4. Solution IR spectra from samples taken from the reaction of
[CoII(dppe)2(H2O)]2þ and [CrIII(CN)6]3- in acetonitrile.
breadth of the signals; nevertheless, we can conclude that
the dominant species in solution after 3 days are
[CrIII(CN)6]3- and [CoII(dppe)2(CN)]þ.
The solution IR data clearly indicate the formation of
an initial bridging complex followed by the formation of
4. The structure of the bridging pentanuclear intermedi-
ate 7 that we accidentally isolated suggests that some
CN- ligands of [CrIII(CN)6]3- are labilized and bind
through the carbon ends to the CoII centers. This is a
reasonable explanation for the displacement of the che-
lating dppe ligands observed in the structure of 7. As
monitored by solution IR spectroscopy, the reaction
progresses to the stage where there are two dominant
ν(CtN) stretches which correspond to free [CrIII-
(CN)6]3- and [CoII(dppe)2(CN)]þ, the latter of which
crystallizes from solution as a [BF4]- salt. Both solution
and solid-state IR data confirm the assignment of the
ν(CtN) stretch to this material.
ism of the dissociation of [CrIII(CN)6]3-
.
The solution IR study was carried out by taking a ∼1
mL aliquot from the reaction mixture at various intervals
(Table 6 and Figure 4). The first aliquot was collected
immediately after mixing the solutions of [CrIII(CN)6]3-
and [CoII(dppe)2(H2O)]2þ. The IR spectrum of this first
aliquot contains two ν(CtN) stretches at 2140 and 2113
cm-1. The latter feature corresponds to the ν(CtN)
stretch observed for [CrIII(CN)6]3- in acetonitrile
(2115 cm-1), and the former, which is ∼30 cm-1 higher
in energy, is attributed to a CN-bridged species. A second
aliquot, collected after 30 min from the beginning of the
reaction, shows the same two ν(CtN) stretches as the
first (initial) aliquot, but with two shoulders at ∼2130 and
2090 cm-1. A third aliquot collected after 1 h shows an IR
stretch at 2130 cm-1 which can be attributed to a CN-
bridged species and, more importantly, a ν(CtN) stretch
at 2098 cm-1 which corresponds to the ν(CtN) stretch of
a bona fide sample of [CoII(dppe)2(CN)]þ. After the
In addition, solution IR studies of the reaction of
[Co(triphos)(CH3CN)2]2þ and [MIII(CN)6]3- (M = Cr,
Fe, and Co) in MeCN were also performed. The IR
spectrum of [Co(triphos)(CH3CN)2]2þ in acetonitrile
exhibits two ν(CtN) stretches at 2317 and 2300 cm-1
(Table 7), values that are in reasonable accord with those
reported for [CoII(CH3CN)6][BF4]2 (2292 and 2316
34
reaction had proceeded for
3 days, crystals of
cm-1). The first aliquot of the [Co(triphos)(CH3-
CN)2]2þ/[CrIII(CN)6]3- (2:1) reaction mixture collected
after 5 min contains peaks at 2115, 2300, and 2317 cm-1
associated with the reactants, along with three new
ν(CtN) stretches at 2146, 2164, and 2170 cm-1 that are
in the region of cyanide-bridged species. Aliquots col-
lected after 1 and 3 h exhibit the same features, the only
significant difference being a decrease in the intensity of
[CoII(dppe)2(CN)][BF4] (4) were obtained, and a fourth
aliquot was sampled from the mother liquor. An IR
spectrum of this aliquot exhibits two ν(CtN) stretches
at 2112 and 2098 cm-1, with no discernible features at
higher frequencies. The feature at 2112 cm-1 corresponds
to the ν(CtN) stretch of [CrIII(CN)6]3- in acetonitrile,
and the ν(CtN) stretch at 2098 cm-1 is assigned to
[CoII(dppe)2(CN)]þ. The presence of a bridging species
in solution cannot be completely ruled out due to the
(34) Hathaway, B. J.; Holah, D. G.; Underhill, A. E. J. Chem. Soc. 1962,
2444–2448.
(33) Rigo, P.; Longato, B.; Favero, G. Inorg. Chem. 1972, 11, 300–303.

Article
Inorganic Chemistry, Vol. 49, No. 2, 2010 591
Table 7. Summary of the ν(CtN) Stretches for Reaction Solutions of
[CoII(triphos)(CN)2]2þ with [MIII(CN)6]3- (M = Cr, Fe, Co) in Acetonitrilea
ν(CtN), cm-1
[Co(triphos)(CH3CN)2]2þ þ
[MIII(CN)6]3-
bridging
terminal
[Co(triphos)(CH3CN)2]2þ
2300, 2317
[CrIII(CN)6]3-
initial
1 h
2 days
[Co(triphos)(CN)2]
2115b
2146, 2164, 2170
2146, 2164, 2170
2300, 2317, 2115
2300, 2317, 2115
2115, 2101, 2097
2101, 2096
[FeIII(CN)6]3-
initial
1 h
2116b
2116
2116
2116
2141
2141, 2152(sh)c
2141, 2152(sh)c
2 days
[CoIII(CN)6]3-
initial
1 h
2127b
2145, 2165, 2174(sh) 2127
2145, 2165, 2174(sh) 2127
2 days
2145, 2165, 2174
2127
a IR stretches in bold correspond to new species formed in solution.
b As measured for acetonitrile solutions of [(18-crown-6)K]3[Cr(CN)6],
[TMA]3[Fe(CN)6] and [(18-crown-6)K]3[Co(CN)6]. c sh = shoulder.
the ν(CtN) stretches of the reactants as compared to
those of the product cyanide-bridged species. The last
aliquot (collected after two days) exhibits stretches corre-
sponding to the starting materials as well as two new
stretches at 2097 and 2101 cm-1 and no evidence for the
previously observed ν(CtN) stretches assigned to the CN-
bridged species. The new stretches at 2097 and 2101 cm-1
are due to the presence of Co(triphos)(CN)2 (6).
Figure 5. Cyanide stretching bands in the IR spectra of complex 8
prepared by varying the Co(II) starting material, solvent, time, and
temperature of the reaction (8a: CoCl2 in MeCN, 24 h, 25 °C. 8b:
Co(BF4)2 6H2O in MeCN, 24 h, 25 °C. 8c: Co(BF4)2 6H2O in MeCN,
3
3
5 min, 25 °C. 8d: Co(BF4)2 6H2O in MeCN, 1 min, 0 °C. 8e: CoCl2 6H2O
3
3
in MeOH, 24 h, 25 °C).
Co-NtC-Cr bridging mode, exhibits very low intensity
in this sample. In sample 8b, the intensity of the higher-
frequency band increases at the expense of the lower-
frequency band at 2103 cm-1, in accord with the faster
crystallization of this sample as compared to 8a and a
lower degree of cyanide reversal. This trend continues for
sample 8c, the rapid precipitation of which was enhanced
by stirring the mixture. For samples 8d and 8e, the band at
2103 cm-1 is absent, a confirmation that a lower reaction
temperature or the use of methanol as solvent results in a
slower rate of CN- bridge reversal. The magnetic data, how-
ever, reveal that appreciable cyanide linkage isomerism
has taken place even in these two samples. It must be
pointed out that sample 8e precipitated instantaneously
upon mixing methanolic solutions of the reactants and
was kept under the mother liquor for 24 h before being
recovered by filtration. Nevertheless, the IR spectrum of
this sample is essentially the same as that of sample 8d,
which was isolated from the reaction mixture after stir-
ring for only 1 min at 0 °C. Obviously, the cyanide “flip”
occurs rapidly in solution upon formation of the complex.
After precipitation of the material, the cyanide bridges
remain intact and do not reverse in the solid state.
Magnetic Properties. DC magnetic measurements
were performed on freshly prepared crushed polycrystal-
line samples in the temperature range of 2-300 K in an
applied magnetic field of 1000 Oe. Compound 1 exhibits a
χT value of 0.68 emu ol-1 at 300 K, which is substantially
higher than expected for a single S=1/2 center (0.375 emu
mol-1 K). Upon lowering temperature, the susceptibility
decreases linearly to 0.53 emu mol-1 K at 2 K (Figure 6a),
indicating significant temperature-independent para-
magnetism (TIP) of the Van Vleck type, which is due to
The solution IR study revealed that the reaction bet-
ween [Co(triphos)(CH3CN)2]2þ and [CrIII(CN)6]3- re-
sults in the formation of intermediate cyanide-bridged
species. The latter react further to form 6 via labiliza-
tion of cyanide ligands from [Cr(CN)6]3-, which are
sequestered by the low-spin CoII complex. Lability of
the cyanide ligand is observed only in the case of
[Cr(CN)6]3-. Reactions of [Co(triphos)(CH3CN)2]2þ
with [Fe(CN)6]3- and [Co(CN)6]3- result in cyanide-
bridged species whose ν(CtN) stretches are persistent
in the IR spectra of the aliquots collected after two days
(2141 and 2152 cm-1 and 2146, 2164, and 2173 cm-1
,
respectively). Attempts to crystallize these cyanide-
bridged species were unsuccessful.
We also performed an IR study of different forms of the
TBP complex 8 obtained under different reaction condi-
tions (Table 1). In a previous study, we investigated a
whole family of such complexes formed with different
transition metals, and the majority of them exhibit two
ν(CtN) stretches, one that is very close in energy to the
free hexacyanometallate anion and the other which is
higher in energy by ∼20-30 cm-1. These bands are
assigned to terminal and bridging CN- ligands, respec-
tively. All samples of 8 reveal the presence of these two
bands (Figure 5).
Note that three of the five samples exhibit a third band
at ∼2103 cm-1. The latter feature is assigned to the
Co-CtN-Cr configuration of the CN- bridge. Indeed,
this band is the most intense in sample 8a, which, accord-
ing to the preparation conditions, should exhibit the
highest degree of cyanide linkage isomerism. Moreover,
the absorption at 2057 cm-1, assigned to the normal

592 Inorganic Chemistry, Vol. 49, No. 2, 2010
Avendano et al.
Figure 6. Temperature dependence of χT for (a) compound 1 (S = 1/2, g = 2.41, TIP = 4.8 ꢀ 10-4 emu mol-1, zJ = -0.1 cm-1) and (b) compound 4
(S = 1/2, g = 2.20, TIP = 1.5 ꢀ 10-4 emu mol-1). Inset: Field-dependent magnetization curve at 1.8 K. Experimental points are depicted with open circles,
and solid lines correspond to the best-fit curves.
low-lying excited states. Both the temperature depen-
dence of χT and the field-dependent magnetization data
were satisfactorily fitted to an S=1/2 system with g=2.41
and TIP=4.8 ꢀ 10-4 emu mol-1 (Figure 6a). In order to
account for the slight decrease of χT at low temperatures,
a molecular field correction (zJ=-0.10 cm-1) was added
to include intermolecular interactions.35 The 300 K
χT value of 4 is slightly lower, namely 0.47 emu mol-1
K and decreases linearly to reach 0.43 emu mol-1 K at 2 K
(Figure 6b). Thus, the TIP contribution is lower (1.5 ꢀ
10-4 emu mol-1) compared to that of complex 1. Such
behavior is in agreement with a greater separation bet-
ween the ground and excited states for 4, as expected for
the stronger ligand field created by the presence of the
carbon-bound cyanide in this complex. The temperature
Figure 7. Temperature dependence of χT for 2 (two S = 1/2 centers,
g = 2.11, TIP = 2.1 ꢀ 10-4 emu mol-1, J = -0.4 cm-1). Inset: Field-
dependence of χT and field-dependent magnetization
data fit the behavior of an S=1/2 system with g=2.20
(Figure 6b, inset).
dependent magnetization curve at 1.8 K. Experimental points are shown
with open circles. Solid lines correspond to theoretical simulations
obtained with MAGPACK.36
Compound 2 exhibits a value of χT=0.95 emu mol-1
K
at 300 K, higher than the spin-only value expected for two
noninteracting low-spin S = 1/2 CoII ions (0.75 emu
mol-1 K). The susceptibility value decreases linearly as
the temperature is lowered (Figure 7), once again due to
the TIP contribution of CoII ions (2.1 ꢀ 10-4 emu mol-1
per ion). The sharp decrease in the susceptibility at very
low temperatures is due to intramolecular antiferromag-
netic exchange between S=1/2 CoII centers through the
diamagnetic CoIII ion (J=-0.4 cm-1). The same model
satisfactorily describes the field-dependent magnetization
data (Figure 7, inset).
For compound 3, the χT value at 300 K is 1.72 emu
mol-1 K, which is higher than the spin-only value of 1.13
emu mol-1 K expected for two noninteracting low-spin
S=1/2 CoII ions and one low-spin S=1/2 FeIII ion. Such a
deviation is mainly due to the significant orbital contri-
bution characteristic of the orbitally degenerate 2T2
ground state of the low-spin FeIII ion and the TIP
contribution of CoII ions, as observed in the isostruc-
tural complex 2. The value of χT decreases upon cooling
until T ∼ 60 K, and then abruptly increases to a maxi-
mum of 1.87 emu mol-1 K at ∼ 6 K (Figure 8), which is
indicative of the stabilization of an S=3/2 ground state,
suggesting a ferromagnetic superexchange between the
CoII and FeIII centers rather than antiferromagnetic
coupling, which would result in an S=1/2 ground state.
It must be noted that the gradual decrease of χT from
room temperature cannot be taken as an indication of
antiferromagnetic coupling because spin-orbit coupling
of the low-spin FeIII ion and the TIP of the CoII centers
are important contributors. For comparison, Figure 8
depicts the sum of χT plots for compound 2 and
(TBA)3[Fe(CN)6]. The χT versus T dependence for 3
shows an obvious deviation to higher values as compared
to the combined curve. This deviation increases with
decreasing temperature, thus confirming that remote
ferromagnetic coupling is occurring between the CoII
and FeIII centers in 3. Stabilization of the high-spin
ground state was further corroborated by field-dependent
magnetization data (Figure 8, inset), which saturate at a
value of 3.05 μB as expected for a ferromagnetic S=3/2
ground state. The observed decrease in χT below 6 K and
the slight deviation of the magnetization data from the
Brillouin function calculated for S=3/2 are attributed to
the weak intermolecular antiferromagnetic interactions
and zero-field splitting effects.
Due to the varying degrees of cyanide linkage isomerism
observed in samples of compound 8 (Table 1), an accurate
(35) Carlin, R. L. Magnetochemistry 1986, 328.

Article
Inorganic Chemistry, Vol. 49, No. 2, 2010 593
Figure 8. Temperature dependence of χT for 3. For comparison, the
dashed line shows a combined contribution from isostructural complex 2
and TBA3[Fe(CN)6] (see the text). Inset: Field-dependent magnetization
at 1.8 K, with the solid line depicting the Brillouin function for S = 3/2,
g = 2.00.
Figure 9. Temperature dependence of χT for different samples of 8. The
solid lines are guides for the eye. The dashed line represents a combined
contribution from two model isostructural complexes, namely,
{[M(tmphen)2]3[M0(CN)6]2} (M = Co/Co and Zn/Cr; see the text).
treatment of its magnetic behavior is not possible. Never-
theless, the magnetic properties of the samples follow the
qualitative trend expected for the mixture of isomerized and
nonisomerized TBP clusters. First of all, we note that, in
the nonisomerized form, each CoII ion is equipped with
two tmphen ligands and two N-bound cyanides. As
demonstrated previously by our study of the model
compound {[CoII(tmphen)2]3[CoIII(CN)6]2}, such a co-
ordination results in a high-spin S = 3/2 state for the
CoII centers.21 After the cyanide flipping event, how-
ever, the equatorial CoII ions are coordinated to the
strong-field C-bound end of the CN- ligands and are in
the low-spin S = 1/2 state.37 (Of course, intermediate
forms of the cluster can exist, in which cyanide bridges
are only partially reversed, but the overall trend for the
CoII centers to assume the low-spin electronic config-
uration upon increasing the extent of cyanide linkage
isomerism is evident.)
(Figure 9), for which the extent of cyanide flip is the
greatest; this observation is in accord with stronger
ferromagnetic exchange.
Conclusions
Contrary to the commonly held notion that CrIII is a
rather inert ion due to the half-filling of its t2g orbitals, we
found that the presence of certain Lewis acids (e.g., CoII
and FeII ions, BPh3) leads to facile labilization of cyanide
ligands from the [Cr(CN)6]3- anion. Results of the detailed
study presented herein obtained by a combination of
X-ray crystallography, infrared spectroscopy, and mag-
netic measurements provide unequivocal evidence for this
phenomenon during reactions of various CoII complexes
with the hexacyanochromate(III) anion. The final pro-
ducts of such reactions are mono- or polynuclear com-
plexes in which the CN- ligand is carbon-bound to the
CoII ion rather than to the CrIII center. Reactions of
[Co(dppe)2(H2O)](BF4)2 and [Co(triphos)(CH3CN)2]-
(BF4)2 with [Cr(CN)6]3- yield the mononuclear complexes
[Co(dppe)2(CN)](BF4) and [Co(triphos)(CN)2], respec-
tively. In the former case, we were able to isolate
the pentanuclear intermediate {[CoII3(dppe)4(MeCN)]-
[CrIII(CN)6]2}. A study of this reaction by solution IR
spectroscopy reveals a gradual conversion of the ν(CtN)
stretches of the starting materials to those of the CN-
bridged intermediate and finally to the single ν(CtN)
stretch of the mononuclear product. The loss of carbon-
bound CN- ligands from [Cr(CN)6]3- is on a sufficiently
slow time-scale for observation of varying degrees of
cyanide linkage isomerism in the trigonal-bipyramidal
complex {[Co(tmphen)2]3[Cr(CN)6]2}, aided by the use of
different CoII starting materials. The varying extent of
the cyanide flip in this complex leads to a variable ratio
of the N-coordinated high-spin S = 3/2 form to the
C-coordinated low-spin S = 1/2 CoII form, a fact that
is reflected in the magnetic behavior of the mixtures. These
results are in contrast to the fast and complete rever-
sal of cyanide bridges that we observed in the chemistry
The conversion of the equatorial CoII ions from the
high-spin S=3/2 to the low-spin S=1/2 state results in a
spin-pairing of electrons in the t2g orbitals and has two
major ramifications on the magnetic behavior of 8: (1)
The room-temperature value of χT decreases as the
degree of cyanide flip increases. As a means of compar-
ison, the dashed line in Figure 9 shows a combined
contribution from three equatorial high-spin S=3/2 CoII
and two axial S=3/2 CrIII ions in the absence of magnetic
coupling. This was calculated by adding the χT versus T
dependences of the model compounds {[CoII(tmphen)2]3-
[CoIII(CN)6]2} and {[ZnII(tmphen)2]3[CrIII(CN)6]2},21
which contain diamagnetic metal ions in the axial and
equatorial positions, respectively. (2) The ferromagnetic
coupling between the CoII and CrIII centers becomes
stronger as the cyanide linkage isomerism occurs. This
fact has been explained by the removal of the antiferro-
magnetic contribution to the CoII-CrIII magnetic super-
2
exchange upon going from the high-spin t2g5eg to the
low-spin t2g6eg1 CoII ion.21 Indeed, the high-temperature
part of the χT versus T curve is much steeper for sample 8a
(36) Borras-Almenar, J. J.; Clemente-Juan, J. M.; Coronado, E.;
Tsukerblat, B. S. J. Comput. Chem. 2001, 22, 985–991.
(37) Jian, F. F.; Xiao, H. L.; Li, L.; Sun, P. P. J. Coord. Chem. 2004, 57,
1131–1137.

594 Inorganic Chemistry, Vol. 49, No. 2, 2010
Avendano et al.
leading to {[Fe(tmphen)2]3[Cr(CN)6]2} and (Et4N)3-
[Cr(NCBPh3)].
acknowledged financial support from the Welch Founda-
tion (A-1449). Funding of the CCD diffractometer
(CHE-9807975) and the SQUID magnetometer (NSF
9974899) by the NSF is also gratefully acknowledged.
Acknowledgment. The authors would like to thank
Dr. Joseph H. Reibenspies for his help with single-crystal
X-ray crystallography. This research was supported by
the NSF (PI grant CHE-0610019) and partially by the
DOE (grant DE-FG03-02ER45999). K.R.D. gratefully
Supporting Information Available: Crystallographic files in
CIF format. This material is available free of charge via the
Internet at http://pubs.acs.org.