Spin-state tuning at pseudotetrahedral d7 ions: Examining the structural and magnetic phenomena of four-coordinate [BP3]Co II-X systems

Published on Web 04/20/2005

Spin-State Tuning at Pseudotetrahedral d⁷Ions: Examining

the Structural and Magnetic Phenomena of Four-Coordinate

[BP₃]Co^II-X Systems

David M. Jenkins and Jonas C. Peters*

Contribution from the DiVision of Chemistry and Chemical Engineering, Arnold and Mabel

Beckman Laboratories of Chemical Synthesis, California Institute of Technology,

Pasadena, California 91125

Received August 3, 2004; E-mail: jpeters@caltech.edu

Abstract: Electronic structure, spin-state, and geometrical relationships for a series of pseudotetrahedral

Co(II) aryloxide, siloxide, arylthiolate, and silylthiolate complexes supported by the tris(phosphino)borate

[BP₃] ligands [PhBP₃] and [PhBP^iPr₃] ([PhB(CH₂PPh₂)₃]^-and [PhB(CH₂PⁱPr₂)₃]^-, respectively) are described.

Standard ¹H NMR, optical, electrochemical, and solution magnetic data, in addition to low-temperature

EPR and variable temperature SQUID magnetization data, are presented for the new cobalt(II) complexes

[PhBP₃]CoOSiPh₃(2), [PhBP₃]CoO(4-^tBu-Ph) (3), [PhBP₃]CoO(C₆F₅) (4), [PhBP₃]CoSPh (5), [PhBP₃]CoS-

(2,6-Me₂-Ph) (6), [PhBP₃]CoS(2,4,6-ⁱPr₃-Ph) (7), [PhBP₃]CoS(2,4,6-^tBu₃-Ph) (8), [PhBP₃]CoSSiPh₃(9),

[PhBP₃]CoOSi(4-NMe₂-Ph)₃(10), [PhBP₃]CoOSi(4-CF₃-Ph)₃(11), [PhBP₃]CoOCPh₃(12), [PhBP^iPr₃]-

CoOSiPh₃(14), and [PhBP^iPr₃]CoSSiPh₃(15). The low-temperature solid-state crystal structures of 2, 3,

5-10, 12, and 15 are also described. These pseudotetrahedral cobalt(II) complexes are classified as

featuring one of two limiting distortions, either umbrella or off-axis. Magnetic and spectroscopic data

1

3

demonstrate that both S ) /₂and S ) /₂ground-state electronic configurations are accessible for the

umbrella distorted structure type, depending on the nature of the X-type ligand, its denticity (η¹versus η³),

1

and the tripodal phosphine ligand employed. Off-axis distorted complexes populate an S )

/ ground-

2

1

state exclusively. For those four-coordinate complexes that populate S )

/ ground states, X-ray data

2

show two Co-P bond distances that are invariably shorter than a third Co-P bond. The pseudotetrahedral

siloxides 2, 10, and 11 are exceptional in that they display gradual spin crossover in the solid state. The

diamagnetic cobalt(III) complex {[PhBP₃]CoOSiPh₃}{BAr₄} ({16}{BAr₄}) (Ar ) Ph or 3,5-(CF₃)₂-C₆H₃)

has also been prepared and structurally characterized. Accompanying electronic structure calculations (DFT)

for complexes 2, 6, and {16}⁺support the notion of a close electronic structure relationship between these

four-coordinate systems and octahedral, sandwich, and half-sandwich coordination complexes.

I. Introduction

within complex metalloenzyme active sites is often achieved

by the interpretation of spectroscopic data.^3,4Moreover, chemical

reactivity can be dramatically effected by subtle spin-state/

stereochemical relationships, as in numerous biocatalytic trans-

formations.⁵Inorganic complexes that expose new insights

regarding the relationship between stereochemistry and elec-

tronic structure are therefore of broad concern.

Stereochemical and electronic structure phenomena of the

first-row transition ions are central issues in coordination

chemistry. These properties are strongly coupled and the

experimental determination of one property often intimates a

great deal about the other. For instance, knowledge of a

complex’s solid-state crystal structure can point to its electronic

ground-state configuration. The four-coordinate first-row transi-

tion ions Fe(II), Co(II), and Ni(II) are exemplary. Each is high

spin when approximately tetrahedral, occupying S ) 2, S )

³/₂, and S ) 1 ground states, respectively. By contrast, when

these same ions feature square planar structures, low-spin (Co-

(II) and Ni(II))¹or intermediate-spin (Fe(II))²ground states are

manifest. Knowledge of the interplay between stereochemistry

and electronic structure thus lies at the heart of our ability to

anticipate magnetic phenomena from key structural parameters.

Indeed, the assignment of local stereochemical environments

From the perspective of ligand field theory (LFT) one of the

best-studied transition ions is Co^{2+ 6}

. The most prominent

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R. A.; Spartalian, K.; Ibers, J. A. J. Am. Chem. Soc. 1985, 107, 4207-

4215. (b) Goedken, V. L.; Pluth, J. J.; Peng, S.; Bursten, B. J. Am. Chem.

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C. J. Am. Chem. Soc. 1983, 105, 5116-5118. (d) Khalifah, R. G.; Rogers,

J. I.; Harmon, P. J.; Carroll, S. B. Biochemistry 1984, 23, 3129-3136. (e)

Briganti, F.; Pierattelli, R.; Scozzafava, A.; Supuran, C. T. Eur. J. Med.

Chem. 1996, 31, 1001-1010.

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Enzymol. 1993, 226, 52-71.

(5) (a) Lippard, S. J.; Berg, J. M. Principles of Bioinorganic Chemistry;

University Science Books: Mill Valley, CA 1994; Chapter 12. (b) Bren,

K. L.; Pecoraro, V. L.; Gray, H. B. Inorg. Chem. 2004, 43, 7894-7896.

(1) (a) Nishida, Y.; Kida, S. Bull. Chem. Soc. Jpn. 1978, 51, 143-149. (b)

Gray, H. B. Chemical Bonds: An Introduction to Atomic and Molecular

Structure; University Science Books: Mill Valley, CA 1994.

9

7148

J. AM. CHEM. SOC. 2005, 127, 7148-7165

Phenomena of Four-Coordinate [BP₃]Co^II

−X Systems

A R T I C L E S

coordination numbers encountered for this ion are four through

six. Six-coordinate pseudooctahedral species typically populate

high-spin configurations, although several low-spin systems

under the influence of unusually strong ligand fields have been

characterized.⁷A host of six-coordinate systems also exhibit

spin-crossover phenomena in the solid state.⁸Sandwich and

mixed-sandwich complexes (e.g., Cp₂Co, CpCoL₃⁺, TpCoCp;

Tp ) tris(pyrazolyl)borate) constitute a spectroscopically and

magnetically rich subset of the octahedral Co(II) family.⁹Five-

coordinate Co²⁺ions exhibit both trigonal bipyramidal and

square pyramidal limiting structures. In contrast to the octahedral

systems, these five-coordinate ions are most commonly low

spin,¹⁰although again both spin forms are well documented, as

are systems that exhibit spin crossover in the solid state.^11,12

Four-coordinate Co(II) systems are nominally either pseudo-

tetrahedral or square planar, although a great many species are

known to adopt structures that are highly distorted from these

limiting structure types.¹³Nevertheless, prior to recent studies

undertaken by our laboratory^14-16all of the four-coordinate

cobalt(II) systems that were known to exhibit low-spin ground-

state configurations were classified as square planar. Ions of

approximate tetrahedral geometries, whether nearly perfect T_d

symmetry (e.g., CoCl₄^2-) or species better described as pseudo-

tetrahedral, distorted tetrahedral, or trigonal pyramidal had been

without exception classified as high spin.^17-20

Figure 1. Chemical structures of [PhBP₃] and [PhBP^iPr₃].

X). [BP₃] is the generic abbreviation used to denote these anionic

tris(phosphino)borate ligands, and [PhBP₃] and [PhBP^iPr

]

3

designate the [PhB(CH₂PPh₂)₃]^-and [PhB(CH₂PⁱPr₂)₃]^-anions,

respectively (Figure 1). These [BP₃]M-X systems are striking

in their propensity to populate low-spin electronic configura-

tions. For example, we have characterized a series of L₃Fe-N_x

species that can accommodate low-spin ground-state configura-

tions for cases where (i) the iron center is either di-,²¹tri-,^22,23

or tetravalent²⁴(i.e. d,⁶d,⁵or d⁴), and (ii) a favorable interaction

exists with the N_x-type ligand that is characterized by one σ

and two π bonds (e.g. S ) 0, {[PhBP₃]Fe^IItNR}^-;²¹S ) /₂,

1

Our group has been exploring the nature of highly covalent

pseudotetrahedral first-row transition ions (L₃M-X) supported

by relatively strong field tris(phosphino)borate ligands ([BP₃]M-

[BP₃]Fe^IIItNR;^22,23S ) 0, [PhBP^iPr₃]FetN).²⁴When the degree

of π-bonding is attenuated, as is the situation for the divalent

halides [BP₃]Fe-X,^22,25amides [PhBP₃]Fe-NRR′,²⁶alkyls

[PhBP^iPr₃]Fe-R,²⁷and diazenidos [PhBP^iPr₃]Fe-NdNR,²³rig-

orously high-spin (S ) 2) ground states are invariably populated.

(6) (a) Lever, A. B. P. Inorganic Electronic Spectroscopy, 2nd ed.; Elsevier:

New York, 1984; pp 480-505. (b) Cotton, F. A.; Wilkinson, G. AdVanced

Inorganic Chemistry, 5th ed.; Wiley: New York, 1988; p 729.

(7) For examples of low-spin Co(II) octahedral systems see: (a) Faus, J.; Julve,

M.; Lloret, F.; Mun˜oz, M. C. Inorg. Chem. 1993, 32, 2013-2017. (b)

Santra, B. K.; Lahiri, G. K. J. Chem. Soc., Dalton Trans. 1998, 139-145.

(c) Iwatsuki, S.; Obeyama, K.; Koshino, N.; Funahashi, S.; Kashiwabara,

K.; Suzuki, T.; Takagi, H. D. Can. J. Chem. 2001, 79, 1344-1351. (d)

Reinen, D.; Ozarowski, A.; Jakob, B.; Pebler, J.; Stratemeier, H.; Wieghardt,

K.; Tolksdorf, I. Inorg. Chem. 1987, 26, 4010-4017.

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Hayami, S.; Inoue, K.; Maeda, Y. Chem. Lett. 2003, 32, 882-883. (c)

Sieber, R.; Decurtins, S.; Stoeckli-Evans, H.; Wilson, C.; Yufit, D.; Howard,

J. A. K.; Capelli, S. C.; Hauser, A. Chem. Eur. J. 2000, 6, 361-368. (d)

Gaspar, A. B.; Mun˜oz, M. C.; Niel, V.; Real, J. A. Inorg. Chem. 2001, 40,

9-10. (e) Faus, J.; Julve, M.; Lloret, F.; Real, J. A.; Sletten, J. Inorg. Chem.

1994, 33, 5535-5540. (f) Kremer, S.; Henke, W.; Reinen, D. Inorg. Chem.

1982, 21, 3013-3022.

(9) For Cp₂Co see: (a) Gordon, K. R.; Warren, K. D. Inorg. Chem. 1978, 17,

987-994. (b) Ko¨nig, E.; Schnakig, R.; Kremer, S.; Kanellakopulos, B.;

Klenze, R. Chem. Phys. 1978, 27, 331-344. (c) Warren, K. D. Struct.

Bonding 1976, 27, 45-159. For mixed sandwich complexes see: (d)

Brunker, T. J.; Barlow, S.; O’Hare, D. Chem. Commun. 2001, 2052-2053.

(e) Brunker, T. J.; Crowley, A. R.; O’Hare, D. Organometallics 2002, 21,

3123-3138. (f) Brunker, T. J.; Green, J. C.; O’Hare, D. Inorg. Chem. 2003,

42, 4366-4381.

By contrast, several divalent cobalt ions supported by these

[BP₃] platforms populate low-spin (S ) ¹/₂) ground-state

electronic configurations, even in the absence of a strongly

π-bonding X ligand. For example, we have reported that in

solution the simple halides [PhBP₃]CoX (X ) I, Br, Cl) each

exhibit a doublet ground state that is predominantly populated

at room temperature.^14,15The observed ground spin states of

these complexes contrasts not only the numerous tetrahedral

and distorted tetrahedral complexes studied previously but also

tripodal borate Co(II) systems that are structurally very similar.

These systems include Theopold’s²⁸and Moro-oka’s²⁹S ) /₂

3

[Tp′′]CoX ([Tp′′] ) hydrotris(3-isopropyl-5-methylpyrazoly1)-

3

borate species and Riordan’s³⁰S )

/

[PhTt^tert-butyl]CoX

2

([PhTt^tert-butyl] ) PhB(CH₂S^tBu)₃) derivatives.

(10) (a) Sacconi, L. Coord. Chem. ReV. 1972, 8, 351-367. (b) Morassi, R.;

Bertini, I.; Sacconi, L. Coord. Chem. ReV. 1973, 11, 343-402.

(11) (a) Kennedy, B. J.; Murray, K. S. Inorg. Chim. Acta 1987, 134, 249-254.

(b) Kennedy, B. J.; Fallon, G. D.; Gatehouse, B. M. K. C.; Murray, K. S.

Inorg. Chem. 1984, 23, 580-588.

(12) (a) Thue´ry, P.; Zarembowitch, J. Inorg. Chem. 1986, 25, 2001-2008. (b)

Zarembowitch, J.; Kahn, O. Inorg. Chem. 1984, 23, 589-593.

(13) Cirera, J.; Alemany, P.; Alvarez, S. Chem. Eur. J. 2004, 10, 190-207.

(14) Shapiro, I. R.; Jenkins, D. M.; Thomas, J. C.; Day, M. W.; Peters, J. C.

Chem. Commun. 2001, 2152-2153.

(15) Jenkins, D. M.; Di Bilio, A. J.; Allen, M. J.; Betley, T. A.; Peters, J. C. J.

Am. Chem. Soc. 2002, 124, 15336-15350.

(21) The complex {[PhBP₃]FetN(1-Ad)}{NBu₄} has been thoroughly charac-

terized (including by XRD analysis) and features a singlet ground state.

Brown, S. D.; Peters, J. C. J. Am. Chem. Soc. 2005, 127, 1913-1923.

(22) Brown, S. D.; Betley, T. A.; Peters, J. C. J. Am. Chem. Soc. 2003, 125,

322-323.

(23) Betley, T. A.; Peters, J. C. J. Am. Chem. Soc. 2003, 125, 10782-10783.

(24) Betley, T. A.; Peters, J. C. J. Am. Chem. Soc. 2004, 126, 6252-6254.

(25) Betley, T. A.; Peters, J. C. Inorg. Chem. 2003, 43, 5074-5084.

(26) Brown, S. D.; Peters, J. C. J. Am. Chem. Soc. 2004, 126, 4538-4539.

(27) Daida, E. J.; Peters, J. C. Inorg. Chem. 2004, 43, 7474-7485.

(28) (a) Reinaud, O. M.; Rheingold, A. L.; Theopold, K. H. Inorg. Chem. 1994,

33, 2306-2308. (b) Detrich, J. L.; Konecˇny´, R.; Vetter, W. M.; Doren,

D.; Rheingold, A. L.; Theopold, K. H. J. Am. Chem. Soc. 1996, 118, 1703-

1712. (c) Jewson, J. D.; Liable-Sands, L. M.; Yap, G. P. A.; Rheingold, A.

L.; Theopold, K. H. Organometallics 1999, 18, 300-305.

(29) (a) Shirasawa, N.; Akita, M.; Hikichi, S.; Moro-oka, Y. Chem. Commun.

1999, 417-418. (b) Shirasawa, N.; Nguyet, T. T.; Hikichi, S.; Moro-oka,

Y.; Akita, M. Organometalics 2001, 20, 3582-3598.

(30) (a) Schebler, P. J.; Riordan, C. G.; Guzei, I. A.; Rheingold, A. L. Inorg.

Chem. 1998, 37, 4754-4755. (b) Schebler, P. J.; Mandimutsira, B. S.;

Riordan, C. G.; Laible-Sands, L. M.; Incarvito, C. D.; Rheingold, A. L. J.

Am. Chem. Soc. 2001, 123, 331-332.

(16) Jenkins, D. M.; Peters, J. C. J. Am. Chem. Soc. 2003, 125, 11162-11163.

(17) (a) Ray, M.; Hammes, B. S.; Yap, G. P. A.; Rheingold, A. L.; Borovik, A.

S. Inorg. Chem. 1998, 37, 1527-1532. (b) Sacconi, L.; Orlandini, A.;

Midollini, S. Inorg. Chem. 1974, 13, 2850-2859. (c) Mani, F.; Mealli, C.

Inorg. Chim. Acta 1981, 54, L77-L79.

(18) Holm, R. H. Acc. Chem. Res. 1969, 2, 307-316.

(19) Jaynes, B. S.; Doerrer, L. H.; Liu, S.; Lippard, S. J. Inorg. Chem. 1995,

34, 5735-5744.

(20) (a) Everett, G. W.; Holm, R. H. J. Am. Chem. Soc. 1965, 87, 5266-5267.

(b) Cotton, F. A.; Soderberg, R. H. J. Am. Chem. Soc. 1962, 84, 872-873.

9

J. AM. CHEM. SOC. VOL. 127, NO. 19, 2005 7149

A R T I C L E S

Jenkins and Peters

Figure 2. Qualitative stereochemical structures and d-orbital splitting diagrams relevant to the four-coordinate structures discussed in this report.

Several other puzzling observations have already been

reported for the [BP₃]Co^II-X family. For instance, a complex

featuring an aryloxide X-type ligand, [PhBP₃]CoO(2,6-Me₂-Ph),

exhibits a quartet rather than a doublet ground state.¹⁵In

addition, iodide and chloride complexes of the [PhBP^iPr₃] anion

([PhBP^iPr₃]CoI and [PhBP^iPr₃]CoCl) appear to populate rigor-

ously high-spin ground states.²⁵Each of these observations is

somewhat counterintuitive. The [PhBP^iPr₃] anion is more

electron-releasing than [PhBP₃],²⁵and on the basis of electronic

considerations its Co²⁺complexes should be more likely to

populate the low-spin configuration than [PhBP₃]Co(II) systems.

The same is true of the aryloxide ligand. A more strongly

π-donating aryloxide linkage might be expected to more

favorably confer a low-spin ground-state configuration than a

simple halide ligand.

Perceiving a need to broaden our appreciation of how the

interplay between local stereochemistry, the L₃donor ligand

field strength, and the nature of the X-type ligand work to confer

a specific electronic configuration, we set out to systematically

characterize a host of pseudotetrahedral d⁷[BP₃]Co-X ions

amenable to structure/spin-state correlations. Herein we present

the results of this study. Previous studies that have attempted

to correlate steric factors with spin-state preferences have

emphasized d⁶octahedral systems (e.g., (L₃)₂Fe(II)).³¹The

systems described in this contribution afford a unique op-

portunity to examine spin-state preferences in four-coordinate

pseudotetrahedral L₃MX structures by correlating an observed

spin-state to the identity of a single X-type ligand, or the identity

of an L₃donor scaffold.

(S ) ³/₂) ground-state configurations. An intramolecular distor-

tion that interconverts A and B is denoted as the “classic case”

in Figure 2. This phenomenon is well-known for Co(II) ions.

Configurational and consequently spin-state equilibria in solution

between Co(II) ions of these two limiting structure types is a

phenomenon that was lucidly described by Holm and Everett

nearly four decades ago.^18,20aAlso, stereochemical tuning of

Co(II) complexes using macrocyclic tetradentate ligands that

can dictate one configuration versus another, and therefore

different ground spin-states, was demonstrated by Lippard and

co-workers using tropocoronand ligands.¹⁹

The interconversion between structures A and B is severe

and is likely difficult to access in the crystalline state. A gentler

distortion, denoted as the “new case” in Figure 2, is one of axial

character and instead produces the pseudotetrahedral structure

type C. Under three-fold symmetry (C_3V) a distortion of this

type stabilizes an orbital of a₁symmetry and provides a d-orbital

splitting diagram comprised of 1a₁+ 2e. This is a familiar

orbital arrangement that has been frequently used to describe

the electronic structures of sandwich (Cp^RM) and mixed-

2

sandwich (Cp^RML₃) complexes.^9,32For sandwich complexes,

the a₁orbital is most typically placed slightly above a lowest-

lying degenerate e-set, although the relative positioning of the

lowest three orbitals (a₁+ e) has been debated.^9cAn important

point to underscore is that, to a first approximation, pseudo-

tetrahedral complexes of structure type C (i.e. those typically

supported by tripodal L₃donor sets) are electronically best

described using a crude “two-over-three” d-orbital splitting

diagram akin to that of sandwich complexes such as Cp₂Fe. A

tetrahedral splitting diagram is less appropriate. Therefore, while

ligands that favor monomeric L₃MX structures are quite often

referred to as “tetrahedral enforcers” owing to the pseudotet-

rahedral stereochemistry they confer, from an electronic structure

perspective these ligands might be more appropriately regarded

as “octahedral enforcers.” The tripodal ligand enforces the

requisite axial distortion that gives rise to an approximate two-

A qualitative sketch of the d-orbital splitting diagrams

anticipated for the various limiting stereochemical structures is

shown in Figure 2. Structures A and B illustrate the most

familiar coordination geometries for four-coordinate Co²⁺ions.

These structure types are square planar (A) and tetrahedral (B)

1

and give rise respectively to low-spin (S ) /₂) and high-spin

(31) (a) Constable, E. C.; Baum, G.; Bill, E.; Dyson, R.; van Eldik, R.; Fenske,

D.; Kaderli, S.; Morris, D.; Neubrand, A.; Neuburger, M.; Smith, D. R.;

Wieghardt, K.; Zehnder, M.; Zuberbu¨hler, A. D. Chem. Eur. J. 1999, 5,

498-508 and references therein. (b) Sohrin, Y.; Kokusen, H.; Matsui, M.

Inorg. Chem. 1995, 34, 3928-3934.

(32) (a) Bally, T.; Borden, W. T. ReV. Comput. Chem. 1999, 13, 1-97. (b)

Boone, A. J.; Chang, C. H.; Greene, S. N.; Herz, T.; Richards, N. G. J.

Coord. Chem. ReV. 2003, 238-239, 291-314. (c) Carreo´n-Macedo, J.;

Harvey, J. N. J. Am. Chem. Soc. 2004, 126, 5789-5797.

9

7150 J. AM. CHEM. SOC. VOL. 127, NO. 19, 2005

Phenomena of Four-Coordinate [BP₃]Co^II

−X Systems

A R T I C L E S

Table 1. Summary of Physical Properties

Evans’ method

SQUID ø_mT

(cm³K mol-¹)

at 20 K, 300 K

(

µ_effin BM, C₆D₆,

Co^II/Co^III, Co^I/Co^II

(mV)^a

complex

color

298 K)

[PhBP₃]CoI, 1

green

2.8

3.4

3.8

2.4

2.3

2.8

3.9

2.5

3.5

3.9

3.8

4.1

4.3

4.0

diamagnetic

0.82, 1.01

10, -920

[PhBP₃]CoOSiPh₃, 2

purple

red-brown

olive green

red

red-brown

red

green

red

blue

blue-green

green

0.47, 1.45

0.90, 1.90

1.32, 2.13 (at 240 K)

0.40, 0.49

0.50, 0.51

0.46, 0.63

2.05, 2.21

0.47, 0.44

0.46, 1.56

1.22, 1.95

2.08, 2.28

1.83, 1.90

2.19, 2.36

0.47, 1.23

diamagnetic

-360, -1290

[PhBP₃]CoO(4-^tBu-Ph), 3

[PhBP₃]CoO(C₆F₅), 4

[PhBP₃]CoSPh, 5

-390, -1330

NA, NA

-160, -1120

[PhBP₃]CoS(2,6-Me₂-Ph), 6

[PhBP₃]CoS(2,4,6-ⁱPr₃-Ph), 7

[PhBP₃]CoS(2,4,6-^tBu₃-Ph), 8

[PhBP₃]CoSSiPh₃, 9

-170, -1100

-80, -1190

-60, -1080

-210 (irreversible), -1010

-360, -1300

[PhBP₃]CoOSi(4-NMe₂-Ph)₃, 10

[PhBP₃]CoOSi(4-CF₃-Ph)₃, 11

[PhBP₃]CoOCPh₃, 12

[PhBPⁱPr₃]CoI, 13

-60 (irreversible), -1080

-300, -1310

60, -1250

[PhBPⁱPr₃]CoOSiPh₃, 14

[PhBPⁱPr₃]CoSSiPh₃, 15

{[PhBP₃]CoOSiPh₃}

purple

green

100 (irreversible), -1690

-140 (irreversible), -1330

oxidation product of 2

{B(3,5-(CF₃)₂-Ph)₄},

{16} {B(3,5-(CF₃)₂-Ph)₄}

^aElectrochemical data were collected in THF with a glassy carbon working electrode, platinum wire counter electrode, and Ag/AgNO₃reference electrode.

All measurements were reported versus an external standard of ferrocene. Either [TBA][PF₆] or [TBA][ClO₄] was used as the supporting electrolyte, as

specified in the Experimental Section.

over-three splitting of the d-orbitals under idealized three-fold

symmetry. Thus, the ground-state electronic structures of three-

fold symmetric [Tp′′]Co^II-X complexes are appropriately

A tripodal amine donor ligand system that supports complexes

with a single aryloxide ligand bound to cobalt has also been

described.³⁸

assigned as A_2g,^28,29,33but as discussed further below, these

The choice of [BP₃]Co^II-X complexes that feature O-atom

and S-atom X-type linkages was due to the relative ease with

which steric and electronic parameters could be tuned in a

systematic fashion, and to the ease with which their Co(II)

complexes could be generated and purified. Other X-type

linkages were considered, for example alkyls and amides, but

these types of complexes have proven to be synthetically

problematic within the [BP₃]Co-X family. Attempts to prepare

them has led to problematic side reactions indicative of

undesirable redox chemistry rather than clean metalation.

The family of complexes shown in Table 1 is conveniently

accessible via the use of soft thallium reagents TlEAr (eq 1). The

typical method for preparation of these thallium reagents in-

volves a metathesis reaction between commercially available

thallium ethoxide and the desired phenol, arylthiol, silanol, or

silylthiol.³⁹The types of alcohols and thiols amenable to this

method of preparation are restricted to those that have pK_avalues

lower than that of the ethanol byproduct (pK_a) 15.9). The

addition of one equivalent of the desired thallium reagent as a

THF solution to a THF solution of [BP₃]CoX (X ) I or Cl)

affords the substituted product in high crude yield with TlX as

an easily separable byproduct. Filtration of the crude reaction

mixture followed by crystallization, typically by vapor diffusion

of petroleum ether in benzene, provides each of the desired com-

plexes in crystalline form in modest to high yields (Table 1).

4

ground states bear a closer electronic relationship to high-spin

octahedral complexes, such as [Tp]₂Co,³⁴than to high-spin

tetrahedral complexes, such as [Cs]₂[CoCl₄].³⁵As will be shown,

these general ideas help to account for the relative ease with

which complexes of the high-spin structure type C can cross

over to a related but low-spin structure type D, given appropriate

choice of the donor ligand set.

II. Results and Discussion

Synthesis and Routine Solution Characterization of [BP₃]-

Co^II-X Complexes. To more thoroughly examine structure/

spin-state relationships within the [BP₃]Co^IIX system we

prepared a series of [BP₃]Co^II-X complexes that feature O-atom

and S-atom X-type linkages. Table 1 lists each of the [BP₃]Co

complexes featured in the present study, along with their

numerical designations (1-16) and standard magnetic and

electrochemical characterization data. A fair number of alkoxide,

aryloxide, thiolate, and arylthiolate derivatives of cobalt have

been described previously, and several L₃MX systems that are

structurally related to the present cobalt derivatives warrant

specific mention. For instance, tris(pyrazolyl)borate (Tp) deriva-

tives of cobalt(II)^28,29,33that feature X-type linkages related to

the present systems have been reported. There are also several

neutral tris(phosphine) Co(I) complexes, for example (PPh₃)₃-

CoOPh,^36,37although we are unaware of any four-coordinate

P₃Co^IIX species other than those supported by [BP₃] ligands.

[BP₃]Co-X ^TI-ER8 [BP₃]Co-ER

(1)

-TIX

X ) I or Cl;

(33) (a) Thompson, J. S.; Sorrell, T.; Marks, T. J.; Ibers, J. A. J. Am. Chem.

Soc. 1979, 101, 4193-4201. (b) Yoshimitsu, S.; Hikichi, S.; Akita, M.

Organometallics 2002, 21, 3762-3773.

ER ) aryloxide, siloxide, thiolate, silylthiolate

(34) (a) Jesson, J. P.; Trofimenko, S.; Eaton, D. R. J. Am. Chem. Soc. 1967, 89,

3148-3158. (b) De Alwis, D.; Chanaka, L.; Schultz, F. A. Inorg. Chem.

2003, 42, 3616-3622.

Despite the paramagnetic nature of these Co(II) derivatives,

¹H NMR spectroscopy aids in their characterization. The

(35) (a) Katzin, L. I. J. Am. Chem. Soc. 1954, 76, 3089-3090. (b) Katzin, L.

I.; Gebert, E. J. Am. Chem. Soc. 1953, 75, 2830-2832. (c) Schmidtke, H.;

Nover, J. Inorg. Chem. Acta 1995, 240, 231-237.

(36) Hayashi, Y.; Yamamoto, T.; Yamamoto, A.; Komiya, S.; Kushi, Y. J. Am.

Chem. Soc. 1986, 108, 385-391.

(37) (a) Kownacki, I.; Kubicki, M.; Marciniec, B. Polyhedron 2001, 20, 3015-

3018. (b) Osakada, K.; Takizawa, T.; Tanaka, M.; Yamamoto, T. J.

Organomet. Chem. 1994, 473, 359-369. (c) Tran, D. T. T.; Taylor, N. J.;

Corrigan, J. F. Angew. Chem., Int. Ed. 2000, 39, 935-937.

9

J. AM. CHEM. SOC. VOL. 127, NO. 19, 2005 7151

A R T I C L E S

Jenkins and Peters

species [PhBP₃]CoOSi(4-CF₃-Ph)₃11. For this complex the Co-

(I/II) redox event is anodically shifted by ca. 300 mV, and the

Co(II/III) redox event is irreversible. These differences are likely

due to the electron-withdrawing CF₃substituent, which desta-

bilizes the higher-valent Co(III) state but renders the lower-

valent Co(I) state more readily accessible.

The [PhBP₃]Co(II) arylthiolate complexes 5-9 also exhibit

well-behaved Co(II/III) and Co(I/II) redox events. These thiolate

species are, as might be expected, easier to reduce and more

difficult to oxidize than their aryloxide relatives. Again, only a

small degree of variance is observed for the potentials among

the arylthiolate family of complexes. Two subtle differences

worth noting are that (i) complexes [PhBP₃]CoS(2,4,6-ⁱPr₃-Ph),

7, and [PhBP₃]CoS(2,4,6-^tBu-Ph), 8, are approximately 100 mV

more difficult to oxidize than [PhBP₃]CoSPh, 5, and [PhBP₃]-

CoS(2,6-Me₂-Ph), 6, presumably reflecting the difference in

electron-releasing character between the arylthiolate substituents

and (ii) it is ca. 100 mV more difficult to reduce 7 than 8, an

observation that is difficult to rationalize in simple terms. The

triphenylsilylthiolato derivative [PhBP₃]CoSSiPh₃(9) is the most

easily reduced species (-1010 mV) but displays an irreversible

oxidation event around -210 mV. The reduction potential

recorded for complex 9 can be compared with that of its

[PhBP^iPr₃]-supported congener [PhBP^iPr₃]CoSSiPh₃(15). The

latter complex 15 is ca. 300 mV more difficult to reduce (-1330

mV) due to its more electron-releasing P₃donor scaffold, but

still displays an irreversible Co(II/III) process. Comparison of

the redox processes observed between [PhBP₃]CoOSiPh₃, 2, and

[PhBP^iPr₃]CoOSiPh₃, 14, reveals a another curiosity of note.

While 14 is appreciably more difficult to reduce (by ca. 400

mV), as should be expected, its oxidation to Co(III) is

electrochemically irreversible. By contrast, {[PhBP₃]Co^III-

OSiPh₃}⁺is electrochemically accessible and stable (vide infra).

This is difficult to rationalize, except to suggest that a putative

{[PhBP^iPr₃]Co^IIIOSiPh₃}⁺species may be more prone to loss

of the triphenylsilyl substituent in the presence of a fluorinated

counteranion from the electrolyte.

Finally, it is interesting to compare the electrochemical data

recorded for these cobalt systems with that of a related series

of recently reported [BP₃]Ni-X systems.⁴¹For example, a

reversible Ni(II/I) reduction event is observed for the complex

[PhBP₃]NiOSiPh₃at -1.47 V, which is ca. 180 mV more

negative than the Co(II/I) event of 2. More striking is how

difficult it is to oxidize the nickel systems to the Ni(III) state.

For the complex [PhBP₃]NiOSiPh₃, the first oxidative process

is encountered at a potential that is ca. 700 mV more positive

than for its cobalt analogue 2. This large difference presumably

reflects the relative instability of a d⁷versus a d⁶electronic

configuration within the [BP₃]Ni-X and [BP₃]Co-X platforms,

respectively.

Figure 3. ¹H NMR spectra of 1 and 13 in C₆D₆. Assignments are based

on the correlation between single scan integration values and T₁relaxation

times for each resonance. For 1: (i) PhB(CH₂PPh₂)₃, (ii) m-P(C₆H₅)₂, (iii)

o-B(C₆H₅), (iv) m- and p-B(C₆H₅), (v) o-P(C₆H₅)₂, (vi) p-P(C₆H₅)₂. For

13: (i and ii) PhB(CH₂PⁱPr₂)₃and P(CH(CH₃)₂)₂, (iii and vii) P(CH(CH₃)₂)₂,

(iv) o-B(C₆H₅), (v) p-B(C₆H₅), (vi) m-B(C₆H₅). s indicates a residual solvent

resonance.

signature proton resonances are listed for each isolated complex

in the Experimental Section. We examined the paramagnetically

1

shifted H NMR spectra of the iodides, [PhBP₃]CoI (1) and

[PhBP^iPr₃]CoI (13), in some detail as representative of this family

of complexes. Both complexes exhibit solution spectra (see

Figure 3) consistent with approximate C₃symmetry at room

temperature, as only a single set of resonances arises from the

phosphine donor arms. T₁relaxation times can be used as a

guide to determine the relative distances of ligand-based protons

from a coordinated metal center containing unpaired spin.⁴⁰By

measuring a T₁relaxation time for each proton resonance shown

in Figure 3 (top and bottom), and correlating these relaxation

times with the integrated number of protons corresponding to

each resonance, we are able to assign the spectrum of 1 with a

high degree of confidence as indicated. The spectrum of 13

suffers from some ambiguity due to certain resonances having

similar T₁relaxation times and integration values. Notably, the

chemical shift range of the resonances observed for 13 is much

broader than that of 1, likely due to their different respective

spin states (vide infra).

Electrochemical Data. The electrochemical response of each

complex featured in this study was examined by cyclic

voltammetry in THF solution using either [ⁿBu₄N][PF₆] or

[ⁿBu₄N][ClO₄] as the supporting electrolyte, a glassy carbon

working electrode, a platinum wire counter electrode, and a Ag/

AgNO₃reference electrode. The potentials of well-defined redox

processes were recorded versus an external ferrocene standard

and are listed in Table 1.

The electrochemical data are generally unremarkable, but a

few comparative comments are warranted. For those [PhBP₃]-

Co(II) complexes featuring a Co-OR linkage, specifically

complexes 2, 3, 4, 10, 11, and 12, fully reversible Co(II/III)

and Co(I/II) redox processes are observed. Relatively little shift

in the potential of either redox event is apparent within this

family, with the exception of the p-CF₃-substituted silyloxide

Chemical Oxidation of 2 To Produce {[PhBP₃]CoOSiPh₃}-

{BAr₄}, {16}{BAr₄}. Reversible oxidation waves for these

[BP₃]Co^II-X derivatives suggest that their chemical oxidation

might afford the corresponding trivalent {[BP₃]Co^III-X}⁺

products, which would comprise a structurally unusual class of

pseudotetrahedral Co(III) complexes (type E in Figure 2). We

have prepared and thoroughly characterized one such example

pertinent to the present study: {[PhBP₃]CoOSiPh₃}{BAr₄},

(38) Archibald, S. J.; Foxon, S. P.; Freeman, J. D.; Hobson, J. E.; Pernutz, R.

N.; Walton, P. H. J. Chem. Soc., Dalton Trans. 2002, 2797-2799.

(39) The preparation was modified from this reference. See Experimental Section.

Jazdzewski, B. A.; Holland, P. L.; Pink, M.; Young, V. G., Jr.; Spencer,

D. J. E.; Tolman, W. B. Inorg. Chem. 2001, 40, 6097-6107.

(40) Ming, L. Physical Methods in Bioinorganic Chemistry; Que, L., Jr., Ed.;

University Science Books: Sausalito, CA, 2000; pp 375-464.

(41) MacBeth, C. E.; Thomas, J. C.; Betley, T. A.; Peters, J. C. Inorg. Chem.

2004, 43, 4645-4662.

9

7152 J. AM. CHEM. SOC. VOL. 127, NO. 19, 2005

Phenomena of Four-Coordinate [BP₃]Co^II

−X Systems

A R T I C L E S

Figure 4. Displacement ellipsoid representations (50%) of the core structures of [PhBP₃]CoSPh (5); [PhBP₃]CoS(2,6-Me₂-Ph) (6); [PhBP₃]CoS(2,4,6-ⁱPr₃-

Ph) (7); [PhBP₃]CoS(2,4,6-^tBu₃-Ph) (8); [PhBP₃]CoO(4-^tBu-Ph) (3); [PhBP₃]CoOSiPh₃(2); [PhBP₃]CoSSiPh₃(9); [PhBP₃]CoOSi(4-NMe₂-Ph)₃(10); and

[PhBP₃]CoOCPh₃(12); [PhBP^iPr₃]CoSSiPh₃(15). The 3 and 3′ structures show the disorder in the -O(4-^tBu-Ph) ligand which is bound either η¹(left) or

η³(right) to the cobalt center. See Table 2 for bond lengths and angles, Table 3 for experimental parameters, and the Supporting Information for complete

crystallographic details pertaining to each structure.

{16}{BAr₄} (Ar ) C₆H₅, 3,5-(CF₃)₂-C₆H₃). The addition of

THF to a solid mixture of 2 and [Cp₂Fe][BAr₄] effects a rapid

oxidation process to generate the diamagnetic, green product

{16}{BAr₄} (eq 2). Cationic {16}{BAr₄} exhibits a sharp singlet

in its ³¹P NMR spectrum at 64.6 ppm and also a sharp singlet

in the ¹⁹F NMR spectrum at -58.5 for the tetra(3,5-bis-

(trifluoromethyl)phenyl)borate salt derivative. The combined ¹H

and ³¹P NMR spectra obtained for this system reveal that it is

three-fold symmetric in solution on the NMR time scale. The

diamagnetic ground state of {16}⁺suggests that its electronic

configuration is likely related to the diamagnetic imide [PhBP₃]-

CotN-p-tolyl, which is an S ) 0 Co(III) species featuring a

bona fide Co-N triple bond linkage.⁴²However, whereas {16}⁺

can be reduced at a potential of ca. -360 mV, the imide species

[PhBP₃]CotN-p-tolyl is stable to reduction at potentials as low

Figure 5. Displacement ellipsoid representations (50%) of {[PhBP₃]-

CoOSiPh₃}{BPh₄}, {16}{BPh₄}. Hydrogen atoms and solvent molecules

have been omitted for clarity. See Table 2 for bond lengths and angles,

as ca. - 3.0 V, reflecting both the difference in charge and the

weaker strength of the π-bonding in {16}⁺.

Table 3 for experimental parameters, and the Supporting Information for

details.

[Cp₂Fe][BAr₄]

{[PhBP₃]CoOSiPh₃}{BAr₄}

[PhBP₃]CoOSiPh₃

8

THF, -Cp₂Fe

{16}{BAr₄}

2

structures have been determined for many of the cobalt

complexes listed in Table 1. These results are summarized by

their core structure representations, collected in Figures 4 and

5, and by a list of salient bond distances and angles, collected

in Table 2. Table 3 lists pertinent crystallographic information.

In each structure, the tris(phosphino)borate is κ³-bound to a

monomeric cobalt center. The pseudotetrahedral structures can

be broadly divided into two separate classes based on the

average length of the Co-P bonds. As will be corroborated by

diamagnetic green crystals

(2)

Ar ) Ph or 3,5-(CF₃)₂-C₆H₃

Structural Characterization and Stereochemical Clas-

sification of [BP₃]Co^II-X Derivatives. Solid-state crystal

(42) Jenkins, D. M.; Betley, T. A.; Peters, J. C. J. Am. Chem. Soc. 2002, 124,

11238-11239.

9

J. AM. CHEM. SOC. VOL. 127, NO. 19, 2005 7153

A R T I C L E S

Jenkins and Peters

Table 2. X-ray Diffraction Table Showing Key Bond Lengths (Å) and Angles (deg) for 2, 3, 5, 6, 7, 8, 9, 10, 12, 15, and {16}{BPh₄}

Co

−

E^a

E

−

Z^b

Co−P1

Co−P2

Co−P3

Co

−E

−X

P1−Co

−P2

P1

−Co

−P3

P2

−Co

−P3

P1

−Co

−E

P2−Co

−E

P3−Co−E

[PhBP₃]CoOSiPh₃,

1.799(2) 1.612(2) 2.156(1) 2.284(1) 2.169(1) 172.5(1)

1.832(7) 1.327(8) 2.247(1) 2.230(1) 2.227(1) 110.3(4)

91.38(3)

91.48(4)

85.88(3)

90.96(4)

94.60(3)

96.35(4)

129.45(7) 119.42(7) 125.82(7)

2

[PhBP₃]CoO(4-^tBu-Ph),

133.4(3)

97.6(2)

106.9(2)

110.8(1)

127.5(3)

151.2(1)

3^c

[PhBP₃]CoO(4-^tBu-Ph),

1.885(4) 1.330(6) 2.247(1) 2.230(1) 2.227(1)

88.4(3)

3′^c

[PhBP₃]CoSPh, 5^d

2.153(1) 1.742(4) 2.175(1) 2.178(1) 2.249(1) 104.3(1)

2.167(1) 1.781(2) 2.251(1) 2.199(1) 2.208(1) 114.6(1)

89.44(4)

89.44(2)

97.52(4)

100.23(2)

90.70(4)

86.73(2)

121.14(4) 143.46(4) 103.24(4)

96.48(2) 147.37(2) 123.30(2)

[PhBP₃]CoS(2,6-Me₂-Ph),

6

[PhBP₃]CoS(2,4,6-ⁱPr₃-Ph),

2.155(1) 1.802(2) 2.201(1) 2.280(1) 2.205(1) 123.2(1)

2.207(1) 1.784(5) 2.354(1) 2.351(1) 2.387(1) 111.3(2)

2.190(1) 2.120(1) 2.206(1) 2.167(1) 2.243(1) 128.0(1)

89.26(2)

96.11(5)

88.14(2)

92.81(5)

98.04(2)

96.30(5)

143.37(2) 107.92(2) 119.75(2)

109.46(5) 125.24(5) 128.37(6)

7^d

[PhBP₃]CoS(2,4,6-^tBu₃-Ph)

8

[PhBP₃]CoSSiPh₃, 9

87.79(3)

90.43(2)

99.63(3)

94.56(2)

87.65(3)

86.47(2)

108.15(3) 139.91(3) 123.62(3)

117.36(5) 127.61(5) 129.77(5)

[PhBP₃]CoOSi(4-NMe₂-Ph)₃, 1.809(1) 1.618(2) 2.265(1) 2.144(1) 2.194(1) 165.7(1)

10

[PhBP₃]CoOCPh₃, 12^d

1.839(1) 1.398(2) 2.349(1) 2.361(1) 2.387(1) 138.0(1)

2.178(1) 2.113(1) 2.179(1) 2.179(1) 2.357(1) 131.5(1)

1.766(3) 1.652(3) 2.187(1) 2.182(1) 2.184(1) 178.6(2)

92.56(2)

90.30(2)

90.67(5)

95.67(2)

92.75(2)

90.18(5)

92.40(2)

93.27(2)

90.50(5)

126.83(5) 102.99(5) 133.18(5)

136.69(2) 126.30(2) 105.64(2)

[PhBP^iPr₃]CoSSiPh₃, 15

{[PhBP₃]CoOSiPh₃}

{BPh₄},

{16}{BPh₄}

125.6(1)

124.4(1)

124.8(1)

^aE represents the fourth, non-phosphine atom directly bound to the Co center, either O or S. ^bZ represents the non-cobalt atom bound to E, either C or

Si. ^cThis complex features an aryloxide ligand distorted over two positions. 3 shows the bond distances and angles for the η¹conformation. 3′ shows the

bond angles and distances for the η³conformation. ^dThere are two crystallographically independent molecules in the unit cell. Structural parameters for the

nearly isomorphic second structure can be found in the Supporting Information.

the SQUID and EPR data discussed below, complexes featuring

average Co-P bond distances between 2.15 and 2.25 Å (e.g.,

6 and 10) are characteristic of low-spin ground states (type D

in Figure 2), whereas those with average Co-P bond distances

between 2.30 and 2.35 Å (e.g., 8 and 12) are characteristic of

high-spin ground states (type C in Figure 2).

tetrahedron therefore has an S(T_d) ) 0, and a perfect square

plane has an S(D_4h) ) 0. Construction of a 2-D plot of S(G)

values can then be used to show that a perfect tetrahedron has

an S(D_4h) ) 33.3, and a perfect square plane has an S(T_d) )

33.3.¹³As should be obvious, a trigonal pyramidal structure is

geometrically much closer to a tetrahedron than a square plane,

and this is reflected by its respective S(G) values; it features a

relatively small S(T_d) by comparison to a large S(D_4h) value

(S(T_d) ) 3.57; S(D_4h) ) 34.87).

The basic stereochemical structures observed in the solid state

can be further classified, at least qualitatively, as having one of

two general structure types that we will refer to throughout as

either umbrella distorted or off-axis distorted (Figure 6).¹³In

the former limiting description, the X-type ligand is regarded

as axial and trisects the three Co-P linkages, coincident with

the B-Co vector. In the latter limiting description the X-type

ligand cants away from the imaginary vector running through

the B and Co atoms to such an appreciable extent that it is better

regarded as an equatorial ligand rather than an axial ligand.

Distortions of an umbrella type are common for four-coordinate

complexes supported by tripodal ligands, and it may be said

that such ligands in fact enforce the umbrella distortion.^28,30

Four-coordinate complexes that exhibit an off-axis distortion

are less frequently encountered and appear to arise from the

population of a low-spin ground state, as discussed in greater

detail below.

Rigorously distinguishing between structures that arise from

these two limiting distortions is not readily apparent by

inspection. An elegant method known as the continuous

symmetry measure^13,43proves very useful in this regard because

it allows one to quantitatively discuss how close a given

molecular geometry is to an idealized structure type. For

example, Alvarez and co-workers have used this approach to

quantitatively compare observed geometric structures to those

of idealized tetrahedra or square planes.¹³Under a continuous

symmetry measurement, the distance (i.e., deviation) of a given

molecule from an idealized polyhedron of a symmetry point

group defined as G is numerically defined as S(G). A perfect

Plotting the data for the X-ray structures shown in Figures 4

and 5 on a 2-D continuous symmetry map allows us to see the

deviations. Comparing the species on a S(T_d) versus S(D_4h) map

shows that the complexes we have prepared are all reasonably

close to an ideal tetrahedron (Figure 7). Complexes on the upper

left portion of the graph exhibit a typical umbrella distortion

and are high spin (vide infra). This class includes complexes

[PhBP₃]CoS(2,4,6-^tBu₃-Ph) (8), [PhBP₃]CoOCPh₃(12), and

[PhBP^iPr₃]CoI (13). The ambient temperature solid-state structure

of [PhBP₃]CoOSiPh₃(2) is also in this class (see Supporting

Information).¹⁶The upper-right box features low-spin umbrella

complexes including the siloxides 2 (98 K structure), [PhBP₃]-

CoOSi(4-NMe₂-Ph)₃(10), and {[PhBP₃]CoOSiPh₃}{BPh₄} ({16}-

{BPh₄}) and the iodide complex we previously reported,

[PhBP₃]CoI (1).¹⁵The complexes we denote as off-axis distorted

structure types are somewhat distinct from the [PhBP₃]Co^II-X

complexes we have described in previous studies (lower right

box in Figure 7).^14,15These off-axis complexes all incorporate

a thiolate as the fourth ligand. Furthermore, all of these off-

axis distorted species (5, 6, 7, 9, and 15) are low spin at 98 K.

These five complexes feature one elongated Co-P bond in an

axial position and two shorter Co-P bonds in the equatorial

positions.

Given that all of these P₃Co^IIX species show a modest

distortion from an ideal tetrahedron, it is worth examining

whether some of the species are better described as trigonal

pyramidal. A continuous symmetry plot mapping the deviations

from an ideal trigonal pyramid and a tetrahedron is shown in

Figure 8. Almost all of the complexes fall within the middle

portion of the graph, implying roughly equal distortions from

(43) (a) Avnir, D.; Katzenelson, O.; Keinan, S.; Pinsky, M.; Pinto, Y.; Salomon,

Y.; Zabrodsky, H. Concepts in Chemistry: A Contemporary Challenge;

Rouvray, D. H., Ed.; Research Studies Press: Taunton (UK), 1996. (b)

Alvarez, S.; Avnir, D.; Llunell, M.; Pinsky, M. New J. Chem. 2002, 26,

996-1009.

9

7154 J. AM. CHEM. SOC. VOL. 127, NO. 19, 2005

Phenomena of Four-Coordinate [BP₃]Co^II

−X Systems

A R T I C L E S

Table 3. Crystallographic Details for [PhBP₃]CoOSiPh₃(2), [PhBP₃]CoO(4-^tBu-Ph) (3), [PhBP₃]CoSPh (5), [PhBP₃]CoS(2,6-Me₂-Ph) (6),

[PhBP₃]CoS(2,4,6-ⁱPr₃-Ph) (7), [PhBP₃]CoS(2,4,6-^tBu₃-Ph) (8), [PhBP₃]CoSSiPh₃(9), [PhBP₃]CoOSi(4-NMe₂-Ph)₃(10), [PhBP₃]CoOCPh₃

(12), [PhBP^iPr₃]CoSSiPh₃(15), and {[PhBP₃]CoOSiPh₃}{BPh₄}, {16}{BPh₄}

[PhBP₃]CoS(2,6-Me₂-Ph),

[PhBP₃]CoOSiPh₃, (2)

[PhBP₃]CoO(4-^tBu-Ph), (3)

[PhBP₃]CoSPh, (5)

(6)

chemical formula

formula weight

T (K)

C₆₃H₅₆BOP₃SiCo‚1.5(C₆H₆)

C₅₅H₅₄BCoOP₃

893.63

98

0.71073

38.2238(8)

12.4091(5)

90

C₅₁H₄₁BCoP₃S

853.59

C₅₃H₅₀BCoP₃S

881.64

1136.98

98

λ (Å)

0.71073

13.1013(14)

14.4428(16)

16.9894(19)

77.984(2)

67.962(1)

89.536(2)

2905.8(6)

P1h

0.71073

16.8066(16)

14.0767(13)

19.2736(18)

90

113.801(2)

90

0.71073

12.0158(9)

12.5469(9)

15.5078(12)

77.780(1)

77.546(1)

77.919(1)

2198.2(3)

P1h

a (Å)

b (Å)

c (Å)

R (deg)

â (deg)

γ (deg)

V (Å³)

90

18130.4(9)

I₁/a

4172.0(7)

P2₁

space group

Z

2

16

4

2

D

_calcd(g/cm³)

1.299

1.310

1.359

1.332

µ(cm^-1

)

4.44

5.25

6.10

5.80

R1, wR2 (I > 2σ(I))^a

0.0336, 0.0673

0.0560, 0.0808

0.0463, 0.0661

0.0405, 0.0743

[PhBP₃]CoS(2,4,6-ⁱPr₃-Ph),

[PhBP₃]CoS(2,4,6-^tBu₃-Ph),

[PhBP₃]CoSSiPh₃,

[PhBP₃]CoOSi(4-NMe₂-Ph)₃,

(7)

(8)

(9)

(10)

chemical formula

formula weight

T (K)

C₆₀H₆₄BCoP₃S

979.82

C₆₃H₇₀BCoP₃S‚C₆H₆

1100.01

C₆₃H₅₆BCoP₃SSi‚2 C₆H₆

1192.09

C₆₉H₇₁BCoN₃OP₃Si

1149.03

98

λ (Å)

0.71073

15.3738(11)

17.3171(13)

20.7953(15)

65.655(1)

87.329(2)

88.642(2)

5038.5(6)

P1h

0.71073

a (Å)

13.8333(10)

13.9539(10)

17.5922(13)

99.145(1)

106.587(1)

107.029(1)

3000.8(4)

P1h

11.0416(8)

12.8426(9)

22.3975(15)

87.228(1)

81.435(1)

73.024(1)

3003.7(4)

P1h

12.1092(11)

12.3919(11)

21.3227(19)

80.696(2)

75.993(2)

72.962(1)

2954.0(5)

P1h

b (Å)

c (Å)

R (deg)

â (deg)

γ (deg)

V (Å³)

space group

Z

4

2

D

_calcd(g/cm³)

1.292

1.217

1.318

1.292

µ(cm-¹)

5.20

4.40

4.70

4.40

R1, wR2 (I > 2σ(I))^a

0.0482, 0.0836

0.0663, 0.1133

0.0529, 0.0777

0.0485, 0.0712

{

[PhBP₃]CoOSiPh₃

}

[PhBP₃]CoOCPh₃, (12)

[PhBP^iPr₃]CoSSiPh₃, (15)

{

BPh₄16}{BPh₄}

}

,

{

chemical formula

formula weight

T (K)

C₆₄H₅₆BCoOP₃‚C₆H₆

1081.85

C₄₅H₆₈BCoP₃SSi

831.79

98

0.71073

10.3161(4)

14.4678(6)

29.963(1)

90

4472.0(3)

P2₁2₁2₁

4

C₈₇H₇₆B₂CoOP₃Si‚11/2 CH₂Cl₂

1466.50

98

λ (Å)

0.71073

13.592(2)

14.814(2)

19.251(3)

72.567(3)

87.945(3)

82.565(2)

3667.0(9)

P1h

a (Å)

12.9420(10)

20.8485(15)

21.5457(16)

77.529(1)

82.143(1)

88.626(1)

5622.9(7)

P1h

b (Å)

c (Å)

R (deg)

â (deg)

γ (deg)

V (Å³)

space group

Z

4

2

D

_calcd(g/cm³)

1.278

1.235

6.00

0.0431, 0.0606

1.328

µ(cm^-1

R1, wR2 (I > 2σ(I))^a

)

4.40

4.70

0.0457, 0.0867

0.0643, 0.1156

both idealized geometries. Complexes 2, 10, 13, and {16}⁺

exhibit less distortion from a tetrahedron, whereas complex 6,

while distorted from both idealized geometries, is slightly closer

to a trigonal pyramid. It is admittedly difficult to tease out a

definitive difference between these two ideal geometries for the

structures described. The dotted line shown in Figure 8

qualitatively draws the same distinction illustrated by the

previous graph (Figure 7). The complexes on the left side feature

typical umbrella distortions and the complexes on the right

feature off-axis distortions. The single discrepancy between this

plot and the plot in Figure 7 is that this symmetry map suggests

that complex 1 belongs to the off-axis class instead of the

umbrella class. None of the complexes we have prepared is truly

close to a trigonal pyramidal geometry since the equatorial

L-Co-L angles are inequivalent. Known examples of trigonal

pyramidal Co(II) species have been assigned as high spin and

often feature a tetradentate ligand with three equivalent tripodal

Figure 6. Limiting distortions relevant to the pseudotetrahedral structure

types discussed in this report.

9

J. AM. CHEM. SOC. VOL. 127, NO. 19, 2005 7155

A R T I C L E S

Jenkins and Peters

knowledge, an η³-binding mode of an aryloxide ligand is

unprecedented for cobalt complexes. In the η³-bonding mode

(3′), the Co-O bond distance is 1.885(4) Å and the Co-C bond

distances are 2.277(5) and 2.341(5) for C(46) and C(47),

respectively. Furthermore, the Co-O-C(46) bond angle is

highly bent (88.4(3)°) to accommodate favorable π-bonding to

the aryl ring.

The solid-state structure determined for diamagnetic {16}-

{BPh₄} shows an umbrella distortion (Figure 5). The complex

is markedly three-fold symmetric and represents an ideal

example of structure type E, as shown in Figure 2. The Co-P

distances display a variance of only 0.005 Å, and the average

of the three Co-P distances is short at 2.185 Å (Table 2). The

Co-O bond distance (1.766(3) Å) is only 0.03 Å shorter than

in 2, and the Co-O-Si bond angle is almost perfectly linear

(178.6(2)°). [PhBP^iPr₃]CoSSiPh₃(15) features an off axis

distortion. However, the axial elongation is much more pro-

nounced for complex 15 than the other off axis complexes, and

in this case, the axial Co-P bond is greater than 0.17 Å longer

than the equatorial Co-P bonds.

Magnetic Characterization (SQUID and EPR) of [BP₃]-

Co^II-X Derivatives. As mentioned in the Introduction, experi-

mental evidence for the preferred low-spin ground-state con-

figuration in a L₃Co^II-X structure type was first provided by

the complex [PhBP₃]CoI (1).¹⁵In the solid state this complex

displays the distorted structure type represented as D in Figure

2. The related chloride and bromide complexes [PhBP₃]Co-X

are also low-spin species in their monomeric form in solution,

but they give rise to dimeric structures in the crystalline state

and are therefore of little utility to the present discussion. In

contrast to these low-spin [PhBP₃]Co(II) halides, we have

previously assigned quartet ground states to the complexes

[PhBP^iPr₃]CoI (13), [PhBP^iPr₃]CoCl, and [PhBP₃]CoO(2,6-Me₂-

Ph) (type C in Figure 2). These assignments, considered

collectively, suggested to us the possibility that the low- and

high-spin ground states of pseudotetrahedral L₃Co^IIX structure

types may in fact lie closer in energy (i.e., ∆H_HS/LS) k_BT) than

had been previously anticipated.^15,25For the collection of

[PhBP₃]Co^IIX complexes presented in this paper (see Table 1)

it is clear that a low-spin ground state is most typically, although

not always, preferred. Moreover, this spin preference is in

contrast to Co(II) species supported by the [PhBP^iPr₃] ligand,

where the high-spin configuration more typically dominates.

Figure 7. Calculated continuous symmetry deviation for each molecule is

plotted on a continuous symmetry map of S(T_d) (tetrahedral) versus S(D_4h)

(square planar). The complexes can be assigned to one of two classes on

the basis of their deviations from these two idealized structure types. The

umbrella class can be subdivided into high-spin and low-spin complexes.

Complexes with two components on the symmetry map (a and b) have

two asymmetric molecules within the unit cell. The crystal structure of

[PhBP₃]CoOSiPh₃(2) was solved at two different temperatures.

SQUID and EPR data have been collected for all of these

[BP₃]Co(II) complexes. Rigorously high-spin species include

complexes 8, 12, 13, and 14. Each of these complexes adopts

a structure that falls into the upper left portion of the symmetry

plot shown in Figure 7, exhibiting a typical umbrella distortion.

Complexes 1, 5, 6, 7, and 9 provide examples of rigorously

low-spin species. The complexes 2, 10, 11, and 15 display spin-

crossover phenomena in the solid state. The low-spin Co(II)

derivatives, type D in Figure 2, give rise to stereochemical

structures exhibiting both umbrella and off-axis distortions. The

interpretation of the magnetic data for 3 and 4 is more complex

due to the potential for η³interactions from the X-type ligand.

Figure 8. Calculated continuous symmetry deviation for each molecule is

plotted on a continuous symmetry map of S(T_d) (tetrahedral) versus S(C_3V)

(trigonal pyramidal). Complexes to the left of the dashed line can be assigned

as umbrella distorted, while those on the right can be assigned as off-axis

distorted. Complexes with two components on the symmetry map (a and

b) have two asymmetric molecules within the unit cell. The crystal structure

of [PhBP₃]CoOSiPh₃(2) was solved at two different temperatures.

arms and one axial donor ligand giving equivalent L-Co-L

angles near 120°.¹⁷

Complex 3 does not follow the generalized descriptions

discussed above since the aryloxide ligand is disordered over

two positions, one that exhibits η¹-bonding to the cobalt center

(3) and one that displays η³-bonding (3′) (Figure 4).⁴⁴To our

In examining the [PhBP₃] supported thiolates (5-9), it is

2

apparent that the preferred ground state is E. Each complex,

excluding 8, has values of ø_mT_av(cm³K mol^-1) from 10 to 300

K slightly above the spin only value of ø_mT ) 0.375 for a single

unpaired electron: 5, 0.41; 6, 0.50; 7, 0.51; 9, 0.45 (see

(44) The atoms in the aryloxide ligand were refined isotropically. See Supporting

Information for details.

9

7156 J. AM. CHEM. SOC. VOL. 127, NO. 19, 2005

Phenomena of Four-Coordinate [BP₃]Co^II

−X Systems

A R T I C L E S

Supporting Information for details). The solid-state magnetic

moment of complexes 5 and 7 very gently increases as the

temperature of each sample is raised above 250 K. We collected

data from 4 to 300 K, and then back to 4 K for complex 7, and

this gentle curvature at higher temperatures was reproducible.

It thus appears likely that partial population of an S ) ³/₂state

is present at higher temperatures. A similar curvature is also

observed for [PhBP₃]CoI (1).¹⁵Recall that the low-spin thiolates

5, 6, and 9 each exhibit an off-axis distortion in the solid-state

and short average Co-P bond distances. The EPR data for these

three species corroborates their doublet assignments, exhibiting

axial (g_|> g ) EPR signals centered near g ) 2 (see Supporting

Information for details). On the other hand, complex 8 exhibits

a ø_mT_av(10-300 K) value of 2.11 cm³K mol^-1between 10

and 300 K, an amount that is slightly greater than the spin only

3

value for an S ) /₂system (1.88 cm³K mol^-1). Its low-

Figure 9. SQUID magnetization plot of ø_mT versus T: [PhBP₃]CoOSiPh₃

(2) ([), [PhBP₃]CoOSi(4-CF₃-Ph)₃(11) (b), [PhBP₃]CoOCPh₃(12) (2),

[PhBP₃]CoSSiPh₃(9) (0) [PhBP₃]CoOSi(4-NMe₂-Ph)₃(10) as the temper-

ature was raised (4) and lowered (9).

temperature EPR spectrum confirms this assignment by showing

two signals, one at g ≈ 2 and a second signal at low field

centered at g ≈ 5.8. The S ) ³/₂ground state of 8 must be due

to high steric crowding by the bulky X-type thiolate ligand.

Population of the quartet spin state expands the average Co-P

bond distances, thereby alleviating unfavorable steric contacts.

Many pseudotetrahedral Co(II) complexes supported by [Tp]

ligands have been prepared, and all of them are high spin.^28,29

Given the preference we have found for [BP₃]Co-thiolates to

populate low-spin ground states, we were curious as to whether

a thiolate ligand might confer the low-spin configuration to a

[Tp]Co(II) system. [Tp]Co thiolates have been prepared previ-

ously, and while magnetic data are not available for these

species, they are presumed to be high spin.^33aWe prepared one

example of such a complex, [Tp^3,5-Me2]CoS(2,6-Me₂-Ph) (17),

to obtain low-temperature magnetic and structural data for

comparison with the [BP₃]CoX systems described herein. The

X-ray structure for 17, which is pseudotetrahedral, can be found

in the Supporting Information. SQUID data for this sample

unequivocally show that 17 populates a high-spin ground state,

with øT_av(10-300 K) ) 2.41 cm³K mol^-1. The electronic

distinction between [BP₃] and [Tp] ligands is thus evident.

Figure 10. Glassy toluene EPR spectra (20 K) for (A) [PhBP₃]CoOSiPh₃

(2) (black line), (B) [PhBP₃]CoOSi(4-NMe₂-Ph)₃(10) (red line), (C) [PhBP₃]-

CoOSi(4-CF₃-Ph)₃(11) (blue line), (D) [PhBP₃]CoOCPh₃(12) (purple line).

Instrumental parameters for the spectra can be found in the Supporting

Information.

Whereas thiolate ligands typically confer low-spin ground

states in the case of [BP₃]Co(II) derivatives, weaker-field

siloxide ligands form complexes that exhibit spin crossover, as

evinced by the complexes [PhBP₃]CoOSiPh₃(2), [PhBP₃]-

CoOSi(4-NMe₂-Ph)₃(10), and [PhBP₃]CoOSi(4-CF₃-Ph)₃(11).

Each of these complexes exhibits an umbrella distortion at low

temperature. The structure of 2 has been examined both at 98

K (see Figure 4) and at 298 K (see Supporting Information),

and while the average of the Co-P bond distances is expanded

at 298 K (reflecting population of the high-spin state, vide infra)

both X-ray data sets confirm the umbrella distortion (Figure

7).¹⁶Changing the substituents at the para position on the

siloxide aryl rings dramatically effects the spin state population

at a given temperature (Figure 9). The presence of the electron

withdrawing CF₃dramatically lowers the T_cof the spin crossover

event. As shown in Figure 9, there is a strong temperature

dependence of the moment of 10, and a relatively well-defined

partial hysteresis is evident centered around 150 K. A more

gradual and fully reversible change in ø_mT is observed above

170 K. This magnetic behavior is distinct from the data collected

for 2, which shows gradual crossover (Figure 9). EPR spectra

were collected at 20 K for 2, 10, and 11, and the spectra are

consistent with the low-temperature SQUID data obtained for

each sample.⁴⁵The EPR spectrum of 2 shown in Figure 10A

exhibits an axial signal (g₁) 2.21, g₂) 2.05, g₃) 2.03)

featuring well-defined hyperfine coupling (I_Co) ⁷/₂) as well as

1

superhyperfine coupling to phosphorus (3 × P, I_P) /₂). The

observation of phosphorus coupling reflects the highly covalent

character of these systems.⁴⁶The octet pattern (A_1(Co)) 105

G) in the g₁region of the spectrum confirms a monomeric

species in solution in each case (see Supporting Information

for simulated versus experimental spectra for 2).¹⁶Noticeably

absent from the spectra of 2 and 10 are any low field signals

that would signify the presence of a high-spin Co(II) compo-

nent.⁴⁷The EPR spectrum of 11 (Figure 10C) is more interest-

ing. A broad but discernible signal at low field (near g ) 5.5)

is present at 20 K, suggesting the presence of a high-spin

(45) For a discussion of EPR of Co(II) see: Banci, L.; Bencini, A.; Benelli, C.;

Gatteschi, D.; Zanchini, C. Struct. Bonding 1982, 52, 37-86.

(46) Stelzer, O.; Sheldrick, W. S.; Subramanian, J. J. Chem. Soc., Dalton Trans.

1976, 966-970.

(47) Palmer, G. Physical Methods in Bioinorganic Chemistry; Que, L., Jr., Ed.;

University Science Books: Sausalito, CA, 2000.

9

J. AM. CHEM. SOC. VOL. 127, NO. 19, 2005 7157

A R T I C L E S

Jenkins and Peters

Figure 12. SQUID magnetization plot of ø_mT versus T for (A) [PhBP^iPr₃]-

CoOSiPh₃(14) (2), and (B) [PhBP^iPr₃]CoSSiPh₃(15) as temperature was

raised ([) and then lowered (0).

Figure 11. SQUID magnetization plot of ø_mT versus T for [PhBP₃]CoO(4-

^tBu-Ph) (3) as the temperature was raised ([) and then lowered (0), and

data for [PhBP₃]CoO(C₆F₅) (4) (2) as temperature was raised.

and the fact that five-coordinate cobalt(II) systems supported

by phosphine ligands invariably populate low-spin ground

states.¹⁰The fluorinated aryloxide complex [PhBP₃]CoO(C₆F₅)

(4) is less likely to exhibit π-bonding to the aryl ring due to its

electron-withdrawing nature, although interactions with the ortho

fluorines of the aryl group cannot be discounted. A similar four-

coordinate/five-coordinate equilibrium may explain the change

in spin state that is observed in the SQUID data (Figure 11).

component, as is also evident from the solid-state SQUID data.

The hyperfine coupling in the g ) 2 region of the spectrum

can be attributed to the presence of the low-spin component of

11 in the glass, in analogy to the spectra of 2 and 10. The effect

of solvent in the crystal lattice on the spin-crossover process

was measured for 2 in the solid state and was found to be

minimal (see Supporting Information). Interestingly, the trityl-

oxide complex [PhBP₃]CoOCPh₃(12) exhibits a highly bent

Co-O-C bond angle of 138.0(1)°, compared with the angle

of 172.5(1)° for 2 and 165.7(1) for related 10. Moreover, each

of the Co-P bond distances is expanded (Co-P_av) 2.37 Å),

suggestive of a high-spin ground state in accord with its solid-

state SQUID (Figure 9) and glassy toluene EPR data (Figure

10D). The disparate spin state preference between 2 and 12

underscores the electronic sensitivity of the [PhBP₃]Co system

to modest modifications, even in the secondary coordination

sphere.

The magnetic data are complicated for 3 and 4 by the

possibility of η³interactions of the axial ligand. The solid-state

crystal structure obtained for 3 at 98 K reveals the presence of

two distinct conformational isomers. One of these isomers is a

four-coordinate pseudotetrahedral species with a Co-O-C_ipso

angle of 110.3(4)°. The other isomer is nominally five-

coordinate and displays an η³-binding mode of the aryloxide

ligand that provides an acute Co-O-C_ipsoangle of 88.4(3)°.

SQUID magnetization data for 3 are shown in Figure 11. It is

clear that the sample predominantly populates a doublet state

at low temperature, although a weak signal at low field is

discernible in the glassy toluene EPR spectrum of the sample

(see Supporting Information). As the sample is warmed the ø_mT

value gradually rises and at 300 K almost complete crossover

to the high-spin component is evident. Perhaps the simplest

explanation of these data is that the pseudotetrahedral isomer

of 3 is high spin at all temperatures, and that the isomer that

exhibits an η³-bonding mode is low spin at all temperatures.

The magnetic data would then reflect variable populations of

the two conformational isomers as a function of temperature in

both solid and solution. Consistent with this explanation is the

fact that the two other pseudotetrahedral aryloxide and alkoxide

complexes we have examined, [PhBP₃]CoO(2,6-Me₂-Ph)¹⁵and

[PhBP₃]CoOCPh₃(12), both exhibit high-spin ground states,

The complexes supported by the [PhBP^iPr₃] ligand feature

two high-spin complexes, [PhBP^iPr₃]CoI (13) and [PhBP^iPr₃]-

CoOSiPh₃(14), that exhibit the umbrella distortion, and one

thiolate complex, [PhBP^iPr₃]CoSSiPh₃(15), that exhibits an off-

axis distortion and is low spin (vide infra) at low temperature.

Structural and magnetic data have been reported previously for

[PhBP^iPr₃]CoI (13).²⁵This complex exhibits a high-spin ground-

state configuration and is therefore distinct from its low-spin

analogue [PhBP₃]CoI (1).¹⁵SQUID data collected on a poly-

crystalline sample of 14 are plotted in Figure 12A. The sample

is clearly an S ) ³/₂system (ø_mT_av(10 K - 300 K) ) 2.30 cm³

K mol^-1) and obeys the Curie-Weiss law. Similar magnetic

behavior was reported for iodide 13.²⁵Magnetization data for

the thiolate derivative 15 are plotted in Figure 12B. The

temperature dependence of its magnetic moment is more

complex. Below 100 K the sample appears to be low spin (ø_mT_av

(10 K - 100 K) ) 0.50 cm³K mol^-1). A gradual rise in ø_mT

is observed above 100 K and a maximum value of 1.23 cm³K

mol^-1is reached at 300 K, the highest temperature at which

the data could be recorded. The magnetic behavior of the sample

is fully reversible. The appearance of a gradual spin-crossover

phenomenon is similar to the rise in magnetic moment that was

observed for thiolates 5 and 7 near room temperature. An

interesting observation is that the solution moment of 15 at room

temperature is 4.0 µ_B, consistent with a fully high-spin popula-

tion. This moment is different from that determined at low

temperature by SQUID analysis of the polycrystalline sample,

and we therefore elected to further analyze 15 by EPR

spectroscopy as a powder and as a frozen glass. These data,

along with the glassy toluene EPR spectrum of 14, are shown

in Figure 13. The 20 K glassy toluene spectra of 14 and 15

(Figure 13A and 13B) each show spectra characteristic of S )

³/₂species. The low field signal present in the glassy spectrum

9

7158 J. AM. CHEM. SOC. VOL. 127, NO. 19, 2005

Phenomena of Four-Coordinate [BP₃]Co^II

−X Systems

A R T I C L E S

Figure 13. Glassy toluene EPR spectra (20 K) of (A) [PhBP^iPr₃]CoOSiPh₃

(14) (black line), and (B) [PhBP^iPr₃]CoSSiPh₃(15) (red line). (C) Powder

sample EPR spectrum (20 K) of [PhBP^iPr₃]CoSSiPh₃(15) (blue line).

Instrumental parameters for the spectra can be found in the Supporting

Information.

of 15 is absent in its powder spectrum at 20 K (Figure 13C).

There appears therefore to be a stronger preference to populate

the high-spin configuration in solution for this thiolate complex.

A different solution conformation of 15 may exist than the one

that is observed in its solid-state structure at 98 K. Differences

in spin behavior between solution and solid samples are not

uncommon for spin-crossover systems.⁴⁸Even with the stronger

donor ligand [PhBP^iPr₃] these complexes favor the high-spin

state unless there is a fourth ligand that is an unusually strong

donor such as a thiolate ligand. The extreme Co-P axial bond

elongation in 15 likely reflects a steric compensation that allows

the doublet ground state to be populated.

Theoretical Analysis of [BP₃]Co^II-X Derivatives. To more

thoroughly consider the case of their d⁷electronic structures

we have undertaken the theoretical DFT examination of one

representative complex from each structural subgroup. For this

study we chose the siloxide complex [PhBP₃]CoOSiPh₃(2) as

representative of the umbrella subgroup, and thiolate [PhBP₃]-

CoS(2,6-Me₂-Ph) (6) as representative of the off-axis subgroup.

Single-point electronic structure calculations (DFT) were per-

formed using (i) the experimentally determined X-ray coordi-

nates for each complex as the ground-state geometrical structure

under (ii) the assumption of a doublet electronic ground state.

Each of the structures was subsequently allowed to relax into a

theoretically determined global minimum in the absence of

geometric constraints, but still under the assumption of a doublet

ground-state electronic configuration. For each complex, the

frontier molecular orbitals obtained by both methods of analysis

are qualitatively quite similar. There are, however, noteworthy

structural differences between the experimentally and theoreti-

cally determined structures.

Figure 14. Molecular orbitals derived from a single-point energy DFT

calculation of [PhBP₃]CoOSiPh₃(2) assuming a doublet ground state and

the crystallographically determined atomic coordinates. The DFT minimized

structure of 2 and its related MOs are provided in the Supporting

Information.

For pseudotetrahedral 2 the predicted frontier molecular

orbitals from the geometry-restricted calculation are shown in

Figure 14. Orbitals calculated for the DFT-optimized structure

are provided in the Supporting Information. The singly occupied

molecular orbital (SOMO) is energetically well-separated from

the lowest unoccupied orbital (LUMO). The lobal representa-

tions of the frontier orbitals containing significant d-orbital

character map well with those we have sketched qualitatively

in Figure 2, although the SOMO is predicted by DFT to lie

much closer in energy to the lower set of orbitals than to the

LUMO. The SOMO and LUMO orbitals are nearly orthogonal

to one another and align reasonably well along the plane

containing the Co-O-Si vector. The lowest-lying three orbitals

(48) Gu¨tlich, P.; Garcia, Y.; Goodwin, H. A. Chem. Soc. ReV. 2000, 29, 419-

2

427.

consist of one orbital of d_z-parentage (HOMO-2) and two

9

J. AM. CHEM. SOC. VOL. 127, NO. 19, 2005 7159

A R T I C L E S

Jenkins and Peters

calcd

Table 4. Experimental and Calculated Bond Lengths and Angles for 2, 6, and {16}

2

exptl

calcd

6

exptl

calcd

{16}

exptl

Bond Lengths (Å)

2.167

Co-O

Co-P1

Co-P2

Co-P3

1.799

2.156

2.284

2.169

1.849

2.429

2.224

2.251

Co-S

2.223

2.418

2.271

2.286

Co-O

1.766

2.187

2.182

2.184

1.776

2.236

2.241

2.234

Co-P1

Co-P2

Co-P3

2.251

2.199

2.208

Co-P1

Co-P2

Co-P3

Bond Angles (deg)

114.6

Co-O-Si

P1-Co-P2

P1-Co-P3

P2-Co-P3

P1-Co-O

P2-Co-O

P3-Co-O

172.5

91.38

85.88

162.8

88.9

94.8

Co-S-C46

P1-Co-P2

P1-Co-P3

P2-Co-P3

P1-Co-S

P2-Co-S

P3-Co-S

119.1

89.4

95.9

89.5

99.9

Co-O-Si

P1-Co-P2

P1-Co-P3

P2-Co-P3

P1-Co-O

P2-Co-O

P3-Co-O

178.6

90.67

90.18

90.50

125.6

124.4

124.8

178.7

90.8

89.44

100.23

86.73

96.48

147.37

123.30

94.60

88.8

90.5

129.45

119.42

125.82

116.7

138.2

118.1

124.4

126.4

123.5

145.4

122.1

orbitals that are canted away from the Co-O-Si vector. While

the low symmetry of the structure inevitably gives rise to

d-orbital mixing, the lowest-lying pair of orbitals can be crudely

described as d_xyand d_{x -y}type orbitals, where the z-axis is

assumed to be coincident with the Co-O-Si vector. Interest-

ingly, there are two high-lying [BP₃]-centered orbitals (HOMO,

HOMO-1) comprised within the frontier manifold that would

have been difficult to anticipate in the absence of the calculation.

interesting for comparison in that it provides an experimentally

determined ground-state structure featuring one Co-P bond that

is strikingly elongated by comparison to the other two. In effect,

the crystal structure of this complex is a better match for the

theoretical structure obtained for 6, and related d⁷[BP₃]Ni(NR)

species,⁴¹by virtue of the exaggerated distortion.⁴⁹Because the

solution magnetic (Evans) and EPR data for 15 are indicative

of a high-spin component, it is reasonable to suggest that an

equilibrium mixture between κ²- and κ³-binding modes might

exist in solution. Nevertheless, it seems more likely to us that

the equilibrium is between an umbrella distorted high-spin

structure, similar to 8, and an off-axis distorted low-spin

structure in the solid state.

Our inability to faithfully reproduce the crystallographically

determined structures of 2 and 6 detracts from our confidence

to use DFT methods to theoretically predict the ground spin-

state of these systems. To illustrate this point, when we calculate

the total energy of the DFT-minimized structures of iodide 1

assuming a doublet and a quartet state, respectively, the quartet

state is energetically favored by 8.8 kcal/mol. This result is in

obvious contradiction to the experimentally observed low-spin

state preference. A general problem associated with open-shell

DFT calculations is that there is, as yet, no universally applicable

method and basis-set that can be confidentially applied to a given

system.³²

The geometries and electronic structures of closed-shell

coordination complexes are more reliably predicted by current

DFT methods, and we therefore examined diamagnetic {16}⁺.

The nature of the SOMO orbital of 2 suggests that the removal

of one unpaired electron should relieve its distorted structure.

This is in fact observed, both crystallographically and theoreti-

cally. Recall that XRD analysis of {16}{BPh₄} revealed an

extremely symmetric structure containing a P₃Co subunit

defining one-half of an octahedron almost perfectly trisected

by the Co-O-Si bond vector (Figure 5). {16}⁺can be

consequently classified by structure type E from Figure 2 and

its molecular orbital diagram is therefore anticipated to reflect

the two-over-three splitting diagram of an octahedral complex.

The presence of degenerate π-binding should give rise to a

sizable separation between the upper two and lower three

d-orbitals. This picture is evident from the single-point electronic

2

DFT-minimization of the geometry of 2 gives rise to a

conformationally similar structure (2-DFT), with the noteworthy

distinction that distortion of one of its Co-P bond distances is

grossly exaggerated (2.22 Å, 2.25 Å, 2.43 Å). Despite this

structural distinction, the calculation still points to a LUMO

that is energetically well-separated from a lower-lying set of

d-type orbitals that includes the SOMO. This conclusion is in

accord with DFT studies for 6. Table 4 compares the core bond

lengths and angles of the calculated to the DFT determined

structures.

Lobal representations of the frontier molecular orbitals of the

off-axis thiolate complex 6 based upon a single-point electronic

structure calculation are displayed in Figure 15. As can be

gleaned, the LUMO is again energetically well-separated from

a set of low-lying d-type orbitals. Certain comparative differ-

ences do arise with respect to the MO structure of 2. The LUMO

now appears to be coincident with the trigonal plane defined

by two Co-P vectors and the Co-S vector, and the SOMO is

directed with a lobe that is pointed toward the axial P donor

ligand. The shape of the SOMO explains why the axial

phosphine ligand is appreciably elongated in the crystal structure

of 6. The DFT-minimized structure, 6-DFT, dramatically

exaggerates this elongation, as was also observed for the case

of 2. The HOMO and HOMO-1 orbitals are no longer ligand

centered but now comprise d-type orbitals with additional

contributions from the equatorial thiolate ligand. The phasing

suggests that the interaction is of Co-S π* character in each

case. The lowest-lying orbital is difficult to distinguish from

the HOMO and HOMO-1 orbitals, and the high degree of

mixing due to the low symmetry of the system is evident.

The transition state for dissociation of one phosphine ligand

should look similar to the highly distorted structures we have

calculated for 2 and 6. In this context it is interesting to note

that the d⁷low-spin half-sandwich complex TpCoCp* has been

characterized and dissociation of one of its pyrazolyl donor arms

is indeed observed in the solid state.^9e,fThe authors have

suggested that in solution an equilibrium between the κ²- and

κ³-binding modes is present.^9fThe crystal structure of 15 is

(49) We very recently obtained the solid-state crystal structure of the low-spin

half-sandwich complex [PhBP₃]CoCp. While a thorough discussion of this

and related [BP₃]-supported half-sandwich complexes is beyond the scope

of this report, we note that in the crystalline state [PhBP₃]CoCp features a

κ³-bonding mode of the [PhBP₃] ligand and features one Co-P bond

distance that is extremely distorted (2.479 Å) by comparison to the other

two Co-P bond distances (2.242 and 2.231 Å).

9

7160 J. AM. CHEM. SOC. VOL. 127, NO. 19, 2005

Phenomena of Four-Coordinate [BP₃]Co^II

−X Systems

A R T I C L E S

Figure 15. Molecular orbitals derived from a single-point energy DFT

calculation of [PhBP₃]CoS(2,6-Me₂-Ph) (6) assuming a doublet ground state

and the crystallographically determined atomic coordinates. The DFT

minimized structure of 6 and its related MOs are provided in the Supporting

Information.

Figure 16. Molecular orbitals consisting of significant d-orbital contribu-

tions for the frontier region of {16}⁺. Orbitals were derived from a single-

point electronic structure calculation assuming a singlet ground state and

the crystallographically determined atomic coordinates.

sandwich complexes.^9f,50The DFT-optimized structure for {16}⁺

was found to be in good agreement with the crystal structure

of {16}{BPh₄} (see Supporting Information and Table 4).

structure calculation performed for {16}⁺. Lobal representations

for the orbitals of dominant d-type contributions are shown in

Figure 16. As for the MO structure of 2, a number of filled

ligand centered orbitals lie at relatively high energy, in this case

falling between the lower-lying (a₁+ e) filled d-orbital set

composed of d_z, d_xy, and d_{x -y}and the upper-lying, empty

d-orbital e set comprising d_xzand d_yz. These frontier d-orbitals

reflect the unmistakable analogy between pseudotetrahedral

{16}⁺and the electronic structure of sandwich and half-

Concluding Remarks

It is evident from the present study that magnetic phenomena

for distorted tetrahedral d⁷ions can be much richer then had

2

(50) Sohn, Y. S.; Hendrickson, D. N.; Gray, H. B. J. Am. Chem. Soc. 1971, 93,

3603-3612.

9

J. AM. CHEM. SOC. VOL. 127, NO. 19, 2005 7161

A R T I C L E S

Jenkins and Peters

been appreciated previously. For highly covalent [BP₃]Co(II)

complexes, low-spin, high-spin, and spin-crossover complexes

are readily accessible for a variety of related geometries best

described as pseudotetrahedral with an umbrella or off-axis

distortion. The observation of an S ) ¹/₂ground state for four-

coordinate [BP₃]Co^IIX complexes appears at this stage to be

neither exceptional nor uncommon - a host of such complexes

have now been thoroughly characterized. Given this situation,

it is of obvious interest to re-examine other d⁷L₃CoX scaffolds

glassy toluene EPR spectra of the two species. We have also

found that the spin population of a d⁷L₃CoX system can be

fine-tuned by adjusting the electron-donor character of the

X-type linkage at a site even further removed from the cobalt

center. This is evident from the SQUID magnetization and EPR

data for [PhBP₃]CoOSi(4-NMe₂-Ph)₃(10) and [PhBP₃]CoOSi-

(4-CF₃-Ph)₃(11), and moreover suggests that truly cooperative

spin-crossover d⁷L₃CoX platforms might be realized if X-type

linkages can be appropriately tailored. This possibility represents

1

an exciting opportunity, as it would enable the reactivity patterns

to determine whether access to the S ) /₂ground state will

1

of structurally related S )

/

and S ) ³/₂, coordinatively

prove more ubiquitous than was previously thought. Four-

coordinate L₃Co^II-SR thiolate complexes should offer a good

starting point in this regard, although for the single [Tp]Co(II)

2

unsaturated d⁷ions to be probed as a function of their spin

populations.^32c,51

3

thiolate we have examined herein the more typical S ) /₂

Experimental Section

ground state is preferred.

All manipulations were carried out using standard Schlenk or

glovebox techniques under a dinitrogen atmosphere. Unless otherwise

noted, solvents were deoxygenated and dried by thorough sparging with

N₂gas, followed by passage through an activated alumina column.

Diethyl ether, tetrahydrofuran, petroleum ether, benzene, and toluene

were periodically tested with a standard purple solution of sodium

benzophenone ketyl in tetrahydrofuran to confirm that oxygen and

moisture had been effectively removed. The preparation of [PhBP₃]-

CoI (1),^14,15{[PhBP₃]CoCl},¹⁵[PhBP₃]CoOSiPh₃(2),¹⁶and [PhBP^iPr₃]-

CoI (13)²⁵has been previously reported. The reagents TlOSiPh₃,⁵²TlO-

p-^tBu-Ph,³⁹TlSPh,⁵³HS(2,4,6-ⁱPr₃-Ph),⁵⁴HS(2,4,6-^tBu₃-Ph),⁵⁵HOSi(p-

NMe₂-Ph)₃,⁵⁶HOSi(p-CF₃-Ph)₃,⁵⁷[Cp₂Fe][BPh₄],⁵⁸and [Cp₂Fe][B(3,5-

(CF₃)₂-Ph)₄]⁵⁹were prepared according to literature procedures. TlO(C₆F₅),

TlS(2,6-Me₂-Ph), TlS(2,4,6-ⁱPr₃-Ph), TlS(2,4,6-^tBu₃-Ph), TlSSiPh₃,

TlOCPh₃, TlOSi(p-NMe₂-Ph)₃, and TlOSi(p-CF₃-Ph)₃were prepared

via a modification of a general synthetic method reported by Tolman

(see below).³⁹The reagents HO(p-^tBu-Ph), HO(C₆F₅), HSPh, HS(2,6-

Me₂-Ph), HOCPh₃, HOSiPh₃, HSSiPh₃, CoBr₂, and [K][Tp^3,5-Me2] were

purchased from commercial vendors and used without further purifica-

tion. Thallium ethoxide was purchased from Aldrich, filtered through

a pad of Celite to remove insoluble black material, and then stored at

-35 °C. Elemental analyses were performed by Desert Analytics,

Tucson, AZ. A Varian Mercury-300 NMR spectrometer or a Varian

Inova-500 NMR spectrometer was used to record ¹H, ³¹P, and ¹⁹F NMR

spectra at ambient temperature. ¹H chemical shifts were referenced to

residual solvent. ³¹P NMR chemical shifts are referenced to an external

standard of H₃PO₄with the ³¹P signal being set at 0 ppm. ¹⁹F NMR

chemical shifts are referenced to an external standard of neat hexafluo-

robenzene with the ¹⁹F signal being set at -163 ppm. Deuterated

toluene, benzene, and tetrahydrofuran were purchased from Cambridge

Isotope Laboratories, Inc. and were degassed and dried over activated

3 Å molecular sieves prior to use. UV-vis measurements were obtained

using a Varian Cary 50 Bio using a quartz crystal cell equipped with

a Teflon cap. UV-vis-NIR measurements were taken in C₆D₆on a

Cary 500 spectrophotometer using a 1 mm path length quartz crystal

Our data establish that ground spin-state assignments for these

types of d⁷ions can be readily made by inspection of their low-

temperature solid-state structural, SQUID, and EPR data. The

halide structures [PhBP₃]CoI and [PhBP^iPr₃]CoCl represent the

simplest limiting cases.^15,25For example, the crystal structure

1

of [PhBP₃]CoI reveals its S ) /₂ground state by virtue of its

three relatively short Co-P distances, with one bond longer

than the other two. This contrasts with the structure of [PhBP^iPr₃]-

CoCl, in which each Co-P distance is elongated but essentially

equidistant and a three-fold axis is more readily discerned. The

collection of low-temperature data collected for complexes 3

and 4 serve as a reminder that SQUID or EPR data need to be

interpreted cautiously in the absence of structural data. In

particular, slippage of a monodentate X-type ligand to a higher

coordination mode (e.g., from η¹to η³) can confer a spin-state

change.

Curious and perhaps still counterintuitive is that the stronger-

field donor ligand [PhBP^iPr₃] tends to confer the high-spin

configuration. Such is the case not only for [PhBP^iPr₃]CoCl, but

also for [PhBP^iPr₃]CoI (13) and [PhBP^iPr₃]CoOSiPh₃(14). To

account for this, we maintain that conformational constraints

imposed by the [PhBP^iPr₃] ligand will disfavor short Co-P

contacts so as to minimize steric repulsion by the isopropyl

groups of this bulky ligand. In the absence of overriding factors,

such as strong π-bonding at the X-linkage, a high-spin popula-

tion is energetically preferred. By choosing a more strongly

π-donating X-type linkage, as is the case for the complex

[PhBP^iPr₃]CoSSiPh₃(15), an S ) ¹/₂ground state can be realized

(at least in the solid state), but now the requisite distortion that

relieves the σ* (and appreciably π*) interaction of the SOMO

is far more pronounced than for the case of low-spin [PhBP₃]

systems, presumably alleviating repulsive steric interactions.

While these examples illustrate the effect that rather dramatic

structural and electronic differences can have on the observed

ground spin-states of these systems, more subtle differences can

have equally striking consequences. For instance, the complex

[PhBP₃]CoS(2,4,6-ⁱPr₃-Ph) (7) is an off-axis low-spin species,

whereas [PhBP₃]CoS(2,4,6-^tBu₃-Ph) (8) adopts a distinctly

different umbrella distortion and populates a high-spin ground

state. Even more subtle changes can have profound electronic

consequences. Replacement of the trityl C-atom in [PhBP₃]-

CoOCPh₃(12) by the Si-atom in [PhBP₃]CoOSiPh₃(2) alters

(51) Poli, R. Chem. ReV. 1996, 96, 2135-2204.

(52) Harvey, S.; Lappert, M. F.; Raston, C. L.; Skelton, B. W.; Srivastava, G.;

White, A. H. J. Chem. Soc., Chem. Commun. 1988, 17, 1216-1217.

(53) Detty, M. R.; Wood, G. P. J. Org. Chem. 1980, 45, 80-89.

(54) (a) Oae, S.; Togo, H. Bull. Chem. Soc. Jpn. 1983, 56, 3802-3812. (b)

Blower, P. J.; Dilworth, J. R.; Hutchinson, J. P.; Zubieta, J. A. J. Chem.

Soc., Dalton Trans. 1975, 1533-1541.

(55) Bochmann, M.; Webb, K. J. Inorg. Synth. 1997, 31, 158-162.

(56) Gilman, H.; Plunkett, M. A.; Dunn, G. E. J. Am. Chem. Soc. 1951, 73,

1686-1688. Trichlorosiliane was substituted for tetrachlorosilane.

(57) HOSi(p-CF₃-Ph) was prepared by the method of: Pauling, H.; Andrews,

D. A.; Hindley, N. C. HelV. Chim. Acta 1976, 59, 1233-1243.

(58) (a) Connelly, N. G.; Geiger, W. E. Chem. ReV. 1996, 96, 877-910. (b)

Jordan, R. F.; LaPointe, R. E.; Bajgur, C. S.; Echols, S. F.; Willett, R. J.

Am. Chem. Soc. 1987, 109, 4111-4113. (c) Aggarwal, R. P.; Connelly, N.

G.; Crespo, M. C.; Dunne, B. J.; Hopkins, P. M.; Orpen, A. G. J. Chem.

Soc., Dalton Trans. 1992, 655-662.

3

1

the system’s ground electronic state from S ) /₂to S ) /₂,

respectively. This secondary sphere effect is striking, and is

immediately evident by comparison of the low-temperature

(59) Le Bras, J.; Jiao, H.; Meyer, W. E.; Hampel, F.; Gladysz, J. A. J.

Organomet. Chem. 2000, 616, 54-66.

9

7162 J. AM. CHEM. SOC. VOL. 127, NO. 19, 2005

Phenomena of Four-Coordinate [BP₃]Co^II

−X Systems

A R T I C L E S

cell equipped with a Teflon cap. X-ray diffraction experiments were

carried out by the Beckman Institute Crystallographic Facility on a

Bruker Smart 1000 CCD diffractometer. Cyclic voltammetry measure-

ments were made inside a dry glovebox using a BAS 100 electrochemi-

cal analyzer with a glassy carbon working electrode, platinum wire

counter electrode, and Ag/AgNO₃reference electrode. All potentials

were measured versus an external standard of ferrocene.

Magnetic Measurements. Measurements were recorded using a

Quantum Designs SQUID magnetometer running MPMSR2 software

(Magnetic Property Measurement System revision 2). Data were

recorded at 5000 G. Samples were suspended in the magnetometer in

plastic straws sealed under nitrogen with Lilly no. 4 gel caps. Loaded

samples were centered within the magnetometer using the DC centering

scan at 35 K and 5000 gauss. Data were acquired at 4-30 K (one data

point every 2 K), 30-300 K (one data point every 5 K).

in the dark to minimize thallium ethoxide degradation. The precipitates

were collected on a medium frit and washed with petroleum ether (2

× 10 mL) and then dried. The thallium reagents were used without

further purification.

1

Additional H NMR data for [PhBP₃]CoI, 1: ¹H NMR (C₆D₆,

300 MHz): δ 22.3 (6 H, T₁) 2.4 ms, PhB(CH₂PPh₂)₃), 10.8 (12 H, T₁

) 23.6 ms, m-P(C₆H₅)₂), 7.7 (2 H, T₁) 40.5 ms, o-B(C₆H₅)), 7.5 (3

H, T₁) 205 ms, m- and p-B(C₆H₅)), 4.3 (12 H, T₁) 1.2 ms,

o-P(C₆H₅)₂), 2.2 (6 H, T₁) 46.7 ms, p-P(C₆H₅)₂).

Synthesis of [PhBP₃]CoOSiPh₃, 2. A THF (4 mL) solution of

TlOSiPh₃(0.173 g, 0.361 mmol) was added dropwise to a stirring

solution of [PhBP₃]CoI (1) (0.315 g, 0.361 mmol) in THF (8 mL).

The resulting solution was allowed to stir for 10 h. An orange precipitate

formed (TlI) which was filtered away over diatomaceous earth. The

THF was then removed in vacuo, and the resulting solid was dissolved

in benzene (4 mL). Crystals were grown from vapor diffusion of

petroleum ether into a benzene solution. The purple crystals were dried

and weighed (0.300 g, 81% yield). The crystals were recrystallized

two additional times (from benzene/petroleum ether) before measure-

ments were taken on the samples (95% yield for each recrystallization).

¹H NMR (C₆D₆, 300 MHz): δ 15.6, 10.0, 9.8, 8.6, 8.3, 7.4, 1.1 (br),

øM

mG

ø_m)

(3)

(4)

µ_eff

)

7.997ø T

x

m

The magnetic susceptibility was adjusted for diamagnetic contribu-

tions using the constitutive corrections of Pascal’s constants.⁶⁰The molar

magnetic susceptibility (ø_m) was calculated by converting the calculated

magnetization (ø) obtained from the magnetometer to a molar suscep-

tibility (using the multiplication factor {(molecular weight)/[(sample

weight)(field strength)]}). Data were analyzed using eqs 3 and 4.

Solution magnetic moments were measured by the method of Evans⁶¹

and were adjusted for diamagnetic contributions using the constitutive

corrections of Pascal’s constants.

EPR Measurements. X-band EPR spectra were obtained on a

Bruker EMX spectrometer equipped with a rectangular cavity working

in the TE₁₀₂mode. Variable-temperature measurements were conducted

with an Oxford continuous-flow helium cryostat (temperature range

3.6-300 K). Accurate frequency values were provided by a frequency

counter built in the microwave bridge. Solution spectra were acquired

in toluene. Sample preparation was performed under a nitrogen

atmosphere. Specific experimental parameters for each spectra presented

can be found in the Supporting Information.

Computational Methods. All calculations were performed using

the hybrid DFT functional B3LYP as implemented in the Jaguar 5.0

program package.⁶²This DFT functional utilizes the Becke three-

parameter functional⁶³(B3) combined with the correlation functional

of Lee, Yang, and Parr⁶⁴(LYP). LACVP** was used as the basis set.

Input coordinates were derived as described in the text from crystal-

lographically determined structures. Spin-states and molecular charges

were explicitly stated, and no molecular symmetry was imposed. Default

values for geometry and SCF iteration cutoffs were used. All structures

converged under these criteria except for the geometry minimization

of 2. In this case, multiple additional cycles showed no more than 1

kcal/mol difference in energy.

The continuous symmetry measurements were determined with the

program SHAPE developed at the Universitat de Barcelona, Spain.⁶⁵

General Method for the Preparation of Thallium Reagents

(modified from Tolman et al.).³⁹The appropriate phenol, thiol, silanol,

or silylthiol (about 200 mg) was dissolved in petroleum ether (10 mL)

and a minimal amount of THF (if necessary). One equivalent of thallium

ethoxide was added as a petroleum ether solution (4 mL), and the

reaction mixture was stirred for 30 min. The reaction was preformed

1

-2.2. H NMR (d₈-toluene, 300 MHz): δ 15.4, 9.9, 9.7, 8.6, 8.1, 7.4

(m), 7.1 (m), 1.1 (br), -2.0. UV-vis (C₆H₆) λ_max, nm (ꢀ): 557 (700),

763 (310). UV-vis (C₆D₆) λ_max, nm (ꢀ): 557 (670), 757 (280), 1136

(320). UV-vis (toluene) λ_max, nm (ꢀ): 557 (650), 763 (290). UV-vis

(THF) λ_max, nm (ꢀ): 557 (670), 761 (300). Evans’ Method (C₆D₆, 295

K): 3.4 µ_B; (d₈-toluene, 295 K): 3.5 µ_B; (d₈-THF, 295 K): 3.5 µ_B.

EPR (toluene, 20 K): g_x) 2.03, a_x(Co)) 65 G, a_x(P)) 34 G; g_y)

2.05, a_y(Co)) 12 G, a_y(P)) 27 G; g_z) 2.21, a_z(Co)) 105 G, a_z(P)) 28

G. Electrochemistry (vs ferrocene in THF with [TBA][ClO₄] as

supporting electrolyte): Co^III/Co^II, -360 mV; Co^II/Co^I, -1290 mV.

Anal. Calcd for C₆₃H₅₆BCoOP₃Si: C, 74.19; H, 5.53. Found: C, 74.27;

H, 5.42.

Synthesis of [PhBP₃]CoO(4-^tBu-Ph), 3. Followed protocol for 2.

Used TlO-p-^tBu-Ph (85.8 mg, 0.243 mmol) and 1 (212 mg, 0.243

mmol). Red-brown crystals of 3 were isolated (115 mg, 53% yield).

¹H NMR (C₆D₆, 300 MHz): δ 12.3 (br), 9.2, 7.7, 2.1, 1.2, 0.9, 0.5

(br). UV-vis (C₆H₆) λ_max, nm (ꢀ): 428 (2300), 567 (1600), 715 (700).

Evans’ Method (C₆D₆): 3.4 µ_B. Electrochemistry (vs ferrocene in THF

with [TBA][PF₆] as supporting electrolyte): Co^III/Co^II, -390 mV; Co^II/

Co^I, -1330 mV. Anal. Calcd for C₅₅H₅₄BCoOP₃: C, 73.92; H, 6.09.

Found: C, 73.58; H, 6.01.

Synthesis of [PhBP₃]CoO(C₆F₅), 4. Followed protocol for 2. Used

TlO(C₆F₅) (109 mg, 0.281 mmol) and 1, (245 mg, 0.281 mmol). Olive

green crystals of 4 were isolated (159 mg, 59% yield). ¹H NMR (C₆D₆,

300 MHz): δ 16.9, 10.9, 8.5, 7.5, -1.0, -3.4, -61.8. ¹⁹F NMR (C₆D₆,

282 MHz): δ -73.1, -181.0. UV-vis (C₆H₆) λ_max, nm (ꢀ): 582 (740),

712 (560). Evans’ Method (C₆D₆): 3.8 µ_B. Anal. Calcd for C₅₁H₄₁-

BCoF₅OP₃: C, 66.04; H, 4.46. Found: C, 65.19; H, 4.39.

Synthesis of [PhBP₃]CoSPh, 5. Followed protocol for 2. Used

TlSPh (94.0 mg, 0.300 mmol) and 1 (261 mg, 0.300 mmol). Red

crystals of 5 were isolated (193 mg, 75% yield). ¹H NMR (C₆D₆, 300

MHz): δ 24.7 (br), 16.7, 10.7, 8.3, 7.7, 7.6, 2.2 (br), 1.4, -3.1 (br),

-6.0. UV-vis (C₆D₆) λ_max, nm (ꢀ): 460 (3500), 597 (1800), 1204 (270).

Evans’ Method (C₆D₆): 2.4 µ_B. Electrochemistry (vs ferrocene in THF

with [TBA][PF₆] as supporting electrolyte): Co^III/Co^II, -160 mV; Co^II/

Co^I, -1120 mV. Anal. Calcd for C₅₁H₄₆BCoP₃S: C, 71.76; H, 5.43.

Found: C, 71.94; H, 5.42.

Synthesis of [PhBP₃]CoS(2,6-Me₂-Ph), 6. Followed protocol for

2. Used TlS(2,6-Me₂-Ph) (102 mg, 0.297 mmol) and 1 (259 mg, 0.297

mmol). Red crystals of 6 were isolated (172 mg, 66% yield). ¹H NMR

(C₆D₆, 300 MHz): δ 79.8, 26.7, 19.6, 13.1, 9.9, 8.2, 8.0, 0.5 (br), -1.1,

-21.1. UV-vis (C₆H₆) λ_max, nm (ꢀ): 396 (4600), 486 (3200), 599

(2300), 741 (900). Evans’ Method (C₆D₆): 2.3 µ_B. Electrochemistry

(vs ferrocene in THF with [TBA][ClO₄] as supporting electrolyte):

(60) Kahn, O. Molecular Magnetism; VCH Publishers: New York, 1993; pp

1-86.

(61) (a) Sur, S. K. J. Magn. Reson. 1989, 82, 169-173. (b) Evans, D. F. J.

Chem. Soc. 1959, 2003-2005.

(62) Jaguar 5.0, Schrodinger, LLC, Portland, Oregon, 2002.

(63) Becke, A. D. J. Chem. Phys. 1993, 98, 5648-5652.

(64) Lee, C.; Yang, W.; Parr R. G. Phys. ReV. B 1988, 37, 785-789.

(65) Llunell, M.; Casanova, D.; Cirera, J.; Bofill, J. M.; Alemany, P.; Alvarez,

S.; Pinsky, M.; Avnir, D. SHAPE, version 1.1; Barcelona, 2003.

9

J. AM. CHEM. SOC. VOL. 127, NO. 19, 2005 7163

A R T I C L E S

Jenkins and Peters

Co^III/Co^II, -170 mV; Co^II/Co^I, -1100 mV. Anal. Calcd for C₅₃H₅₀-

BCoP₃S: C, 72.20; H, 5.72. Found: C, 72.42; H, 5.69.

3.8 µ_B. Electrochemistry (vs ferrocene in THF with [TBA][PF₆] as

supporting electrolyte): Co^III/Co^II, -300 mV; Co^II/Co^I, -1310 mV.

Anal. Calcd for C₆₄H₅₆BCoOP₃: C, 76.58; H, 5.62. Found: C, 76.23;

H, 5.88.

Synthesis of [PhBP₃]CoS(2,4,6-ⁱPr₃-Ph), 7. Followed protocol for

2. Used TlS(2,4,6-ⁱPr₃-Ph) (117 mg, 0.265 mmol) and {[PhBP₃]CoCl}

(207 mg, 0.265 mmol). Red-brown crystals of 7 were isolated (178

Additional ¹H NMR data for [PhBP^iPr₃]CoI, 13: ¹H NMR (C₆D₆,

300 MHz): δ 115.6 (6 H, T₁) 2.6 ms, PhB(CH₂PⁱPr₂)₃or

P(CH(CH₃)₂)₂), 41.6 (6 H, T₁) 0.9 ms, PhB(CH₂PⁱPr₂)₃or P(CH-

(CH₃)₂)₂), 24.1 (18 H, T₁) 5.6 ms, P(CH(CH₃)₂)₂), 12.8 (2 H, T₁)

17.5 ms, o-B(C₆H₅)), 9.0 (1 H, T₁) 204 ms, p-B(C₆H₅)), 7.3 (2 H, T₁

) 244 ms, m-B(C₆H₅)), 3.3 (18 H, T₁) 1.9 ms, P(CH(CH₃)₂)₂).

Synthesis of [PhBP^iPr₃]CoOSiPh₃, 14. A THF solution (2 mL) of

the thallium reagent TlOSiPh₃(58 mg, 0.12 mmol) was added to a

stirring THF solution (5 mL) of [PhBP^iPr₃]CoI, 13, (81 mg, 0.12 mmol).

The solution was stirred for 1 h, and a yellow precipitate formed (TlI).

The precipitate was removed by filtration over diatomaceous earth. The

remaining reaction volatiles were removed in vacuo, and the purple

powder was redissolved in benzene (5 mL). The benzene solution was

filtered over diatomaceous earth to remove any residual TlI and then

frozen and lyophilized to remove any trace of THF. Vapor diffusion

of petroleum ether into a benzene solution (1 mL) afforded purple

crystalline product (39 mg, 41% yield). ¹H NMR (C₆D₆, 300 MHz): δ

32.8, 21.6, 12.1, 11.0, 9.2, 8.9, 7.5, 7.4, 1.7, -60.3. UV-vis (C₆H₆)

1

mg, 68% yield). H NMR (C₆D₆, 300 MHz): δ 46.4 (br), 21.7, 11.6,

9.1, 8.9, 7.8, 7.7, 4.8, 1.3, 0.9. UV-vis (C₆H₆) λ_max, nm (ꢀ): 384 (6600),

471 (4200), 609 (3000), 743 (900). Evans’ Method (C₆D₆): 2.8 µ_B.

Electrochemistry (vs ferrocene in THF with [TBA][PF₆] as supporting

electrolyte): Co^III/Co^II, -80 mV; Co^II/Co^I, -1190 mV. Anal. Calcd

for C₆₀H₆₄BCoP₃S: C, 73.54; H, 6.58. Found: C, 73.18; H, 6.57.

Synthesis of [PhBP₃]CoS(2,4,6-^tBu₃-Ph), 8. Followed protocol for

2. Used TlS(2,4,6-^tBu₃-Ph) (145 mg, 0.302 mmol) and 1 (263 mg, 0.302

mmol). Red crystals of 8 were isolated (113 mg, 36% yield). ¹H NMR

(C₆D₆, 300 MHz): δ 50.8, 16.7, 12.8, 10.6 (br), 9.2, 8.1, 2.3, -3.2

(br), -5.4, -27.0. UV-vis (C₆D₆) λ_max, nm (ꢀ): 479 (3600), 640

(1900), 754 (1800), 1190 (430). Evans’ Method (C₆D₆): 3.9 µ_B.

Electrochemistry (vs ferrocene in THF with [TBA][PF₆] as supporting

electrolyte): Co^III/Co^II, -60 mV; Co^II/Co^I, -1080 mV. Anal. Calcd

for C₆₃H₇₀BCoP₃S: C, 74.04; H, 6.90. Found: C, 73.95; H, 6.98.

Synthesis of [PhBP₃]CoSSiPh₃, 9. Followed protocol for 2. Used

TlSSiPh₃(187 mg, 0.376 mmol) and 1 (328 mg, 0.376 mmol). Green

crystals of 9 were isolated (321 mg, 83% yield). ¹H NMR (C₆D₆, 300

MHz): δ 46.4, 11.9, 8.7-6.6, 1.3. UV-vis (C₆H₆) λ_max, nm (ꢀ): 622

(1300), 747 (670). Evans’ Method (C₆D₆): 2.5 µ_B. Electrochemistry

(vs ferrocene in THF with [TBA][PF₆] as supporting electrolyte): Co^III/

Co^II, -210 mV (irreversible), Co^II/Co^I, -1010 mV. Anal. Calcd for

C₆₃H₅₆BCoP₃SSi: C, 73.04; H, 5.45. Found: C, 73.06; H, 5.49.

Synthesis of [PhBP₃]CoOSi(4-NMe₂-Ph)₃, 10. Followed protocol

for 2. Used TlOSi(p-NMe₂-Ph)₃(228 mg, 0.374 mmol) and {[PhBP₃]-

CoCl} (292 mg, 0.374 mmol). Layering of petroleum ether (14 mL)

onto a toluene solution (4 mL) afforded red crystalline product (173

mg). The recrystalization of the supernatant leads to additional product

λ_max, nm (ꢀ): 553 (600), 657 (380), 788 (320). Evans’ Method (C₆D₆):

4.3 µ_B. Electrochemistry (vs ferrocene in THF with [TBA][PF₆] as

supporting electrolyte): Co^III/Co^II, 100 mV (irreversible); Co^II/Co^I,

-1690 mV. Anal. Calcd for C₄₅H₆₈BCoOP₃Si: C, 66.25; H, 8.40.

Found: C, 66.20; H, 8.14.

Synthesis of [PhBP^iPr₃]CoSSiPh₃, 15. Followed protocol for 14.

Used TlSSiPh₃(161 mg, 0.325 mmol) and 13 (216 mg, 0.325 mmol).

Green crystals of 15 were isolated (167 mg, 62% yield). ¹H NMR (C₆D₆,

300 MHz): δ 35.7, 20.7, 11.4, 9.3, 8.7, 7.8, 7.1, 6.2, 5.0, -52.6. UV-

vis (C₆H₆) λ_max, nm (ꢀ): 604 (420), 664 (520), 747 (1300). Evans’

Method (C₆D₆): 4.0 µ_B. Electrochemistry (vs ferrocene in THF with

[TBA][PF₆] as supporting electrolyte): Co^III/Co^II, -140 mV (irrevers-

ible), Co^II/Co^I, -1330 mV. Anal. Calcd for C₄₅H₆₈BCoP₃SSi: C, 64.97;

H, 8.24. Found: C, 64.77; H, 8.28.

1

(115 mg) to give a total yield of 67%. H NMR (C₆D₆, 300 MHz): δ

103.7, 14.6, 10.1 (br), 9.5, 8.1, 7.8, 7.4, 2.7, 2.1, -1.0. UV-vis (C₆H₆)

λ_max, nm (ꢀ): 555 (940), 699 (440), 772 (420). Evans’ Method (C₆D₆):

3.5 µ_B. Electrochemistry (vs ferrocene in THF with [TBA][ClO₄] as

supporting electrolyte): Co^III/Co^II, -360 mV; Co^II/Co^I, -1300 mV.

Anal. Calcd for C₆₉H₇₁BCoN₃OP₃Si: C, 72.12; H, 6.23; N, 3.66.

Found: C, 71.77; H, 6.40; N, 3.52.

Synthesis of {[PhBP₃]CoOSiPh₃}{BAr₄}, {16}{BAr₄}. Solid

[PhBP₃]CoOSiPh₃, 2, (98 mg, 0.096 mmol) and [Cp₂Fe][B(3,5-(CF₃)₂-

Ph)₄] (101 mg, 0.096 mmol) were added to a 20-mL vial, and then

THF (8 mL) was added. The solution immediately went from purple

to green and was stirred for 10 min. The reaction mixture was then

dried in vacuo to leave a green powder which was washed with

petroleum ether (3 × 10 mL). The green powder was dried to leave

the pure product (140 mg, 77% yield) which was stored at -35 °C. A

similar procedure was used with [Cp₂Fe][BPh₄] as the oxidant giving

the less soluble counteranion (for X-ray crystallography). In this case

the dried product was washed with a petroleum ether/benzene mixture

(7:3) (3 × 10 mL) to give the final product (51% yield). A single crystal

was grown at -35 °C in CH₂Cl₂using the {BPh₄} counteranion. For

{16}{B(3,5-(CF₃)₂-Ph)₄}: ¹H NMR (C₆D₆, 300 MHz): δ 8.43 (s, 8 H,

o-B(3,5-(CF₃)₂-C₆H₃)₄), 7.81 (d, J ) 6.0 Hz, 8 H, o-Si(C₆H₅)₃

and o-B(C₆H₅)), 7.67 (t, J ) 7.5 Hz, 2 H, m-B(C₆H₅)), 7.60 (s, 4 H,

p-B(3,5-(CF₃)₂-C₆H₃)₄), 7.47 (t, J ) 7.5 Hz, 1 H, p-B(C₆H₅)), 7.28 (m,

9 H, m- and p-Si(C₆H₅)₃), 7.10 (m, 12 H, o-P(C₆H₅)₂), 6.67 (t, J ) 7.2

Hz, 6 H, p-P(C₆H₅)₂), 6.40 (t, J ) 7.5 Hz, 12 H, m-P(C₆H₅)₂), 0.98 (br

s, 6 H, PhB(CH₂PPh₂)₃). ³¹P{¹H} NMR (C₆D₆, 121.4 MHz): δ 64.6.

¹⁹F{¹H} NMR (C₆D₆, 282 MHz): -58.5. UV-vis (C₆D₆) λ_max, nm (ꢀ):

624 (830). Anal. Calcd for C₉₅H₆₈B₂CoF₂₄OP₃Si: C, 60.59; H, 3.64.

Found: C, 60.59; H, 4.00. For {16}{BPh₄}: Identical UV-vis and

³¹P NMR spectra were found for the substituted anion.

Synthesis of [PhBP₃]CoOSi(4-CF₃-Ph)₃, 11. Followed protocol for

2. Used TlOSi(p-CF₃-Ph)₃(99 mg, 0.145 mmol) and {[PhBP₃]CoCl},

(113 mg, 0.145 mmol). The toluene solution (2 mL) was layered with

15 mL of petroleum ether and cooled to -35 °C until crystals formed.

The crystals were then dried in vacuo yielding the pure compound (117

1

mg, 66% yield). H NMR (C₆D₆, 300 MHz): δ 17.4, 10.7, 9.8 (br),

8.8, 8.5, 7.3, -0.9, -4.2, -81.6 (br). ¹⁹F NMR (C₆D₆, 282 MHz): δ

-58.1. UV-vis (C₆H₆) λ_max, nm (ꢀ): 560 (740), 757 (320). Evans’

Method (C₆D₆, 298 K): 3.9 µ_B. Electrochemistry (vs ferrocene in THF

with [TBA][ClO₄] as supporting electrolyte): Co^III/Co^II, -60 mV

(irreversible), Co^II/Co^I, -1080 mV. Anal. Calcd for C₆₆H₅₃BCoF₉OP₃-

Si: C, 64.77; H, 4.36. Found: C, 64.70; H, 4.55.

Synthesis of [PhBP₃]CoOCPh₃, 12. A THF solution (3 mL) of the

thallium reagent TlOCPh₃(124 mg, 0.267 mmol) was added to a stirring

THF solution (10 mL) of [PhBP₃]CoI, 1, (233 mg, 0.267 mmol). The

solution was stirred for 4 h, and a yellow precipitate formed (TlI). The

precipitate was removed by filtration over diatomaceous earth. The

remaining reaction volatiles were removed in vacuo, and the blue/green

powder was then washed with petroleum ether (2 × 10 mL) and dried.

The solid was then reconstituted in benzene (4 mL) and then triturated

with petroleum ether (15 mL). The supernatant was separated from

the brown solid via filtration. The blue/green solution was dried in

vacuo and then crystallized by vapor diffusion of petroleum ether into

Synthesis of [Tp^3,5-Me2]CoS(2,6-Me₂-Ph), 17. Following a related

procedure,^33aanhydrous CoBr₂(61.8 mg, 0.285 mmol) was suspended

in THF (13 mL) for 10 min. Solid [K][Tp^3,5-Me2] (0.113 g, 0.335 mmol)

was added over 30 min. To this solution, TlS(2,6-Me₂-Ph) (0.114 g,

0.335 mmol) was added as a THF (3 mL) slurry and stirred for 30

min. After the addition of the thallium reagent, the solution turned green,

1

a benzene solution (47 mg, 18% yield). H NMR (C₆D₆, 300 MHz):

δ 20.1, 16.7, 11.3, 9.5, 8.7, 7.9, 7.7, -1.4, -5.0, -83.7. UV-vis (C₆H₆)

λ_max, nm (ꢀ): 578 (550), 778 (270). Evans’ Method (C₆D₆, 298 K):

9

7164 J. AM. CHEM. SOC. VOL. 127, NO. 19, 2005

Phenomena of Four-Coordinate [BP₃]Co^II

−X Systems

A R T I C L E S

and white precipitate appeared (TlBr). The solution was filtered over

Celite and then dried in vacuo to leave a green powder. The green

powder was taken up in toluene (3 mL) and then crystallized by vapor

diffusion of petroleum ether, giving green crystals (0.043 g, 31% yield).

Additional crystallizations can be used to collect more product, giving

69% total yield. ¹H NMR (C₆D₆, 300 MHz): δ 68.1, 38.5, 29.4, 16.2,

-0.8, -11.4, -38.8. UV-vis (C₆H₆) λ_max, nm (ꢀ): 647 (1300). SQUID

(10-300 K): øT_av) 2.41 cm³K mol^-1. Anal. Calcd for C₂₃H₃₁-

BCoN₆S: C, 55.99; H, 6.33; N, 17.03. Found: C, 55.72; H, 6.23; N,

17.12.

Zewail lab provided access to a NIR spectrophotometer. Dr.

Michael Day and Larry Henling provided crystallographic

assistance, and Dr. Angel Di Bilio provided EPR assistance.

Finally, we are very grateful to Dr. Santiago Alvarez and Jordi

Cirera (Universitat de Barcelona, Spain) for help with the

continuous symmetry measurement analysis and a stimulating

scientific exchange.

Supporting Information Available: Additional SQUID mag-

netization data and EPR spectra, experimental parameters for

EPR spectra, theoretical details for 2, 6, and {16}⁺; detailed

crystallographic data in CIF format. This material is free of

charge via the Internet at http://pubs.acs.org.

Acknowledgment. Financial support from the NSF (CHE-

0132216), the DOE (PECASE), and the Alfred P. Sloan

Foundation is gratefully acknowledged. D.M.J. was supported

by an NSF predoctoral fellowship. The Beckman Institute

(Caltech) provided access to a SQUID magnetometer, and the

JA045310M

9

J. AM. CHEM. SOC. VOL. 127, NO. 19, 2005 7165

Article Doi

Spin-state tuning at pseudotetrahedral d7 ions: Examining the structural and magnetic phenomena of four-coordinate [BP3]Co II-X systems

DOI: 10.1021/ja045310m

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Article abstract of DOI:10.1021/ja045310m

Full text of DOI:10.1021/ja045310m

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