The use of electron spin resonance to follow reactions and equilibria of complexes in solution

Journal of Molecular Siructure, 47 (1978) 91-106 0 Else&r Scientific Publishing Company, Amsterdam

The Use of Electron

of Complexes

in Solution

of Chemistry,

The University,

solution

reactions

sociation

(&*z

(CN exchange

cis Pt(Me)z(PEt3)2 chemical(Nb(CN)s4-, (LCoOz)

of complexes to illustrate

+ base),

Fe3+

(Fe3' high

dis-

addition

of Cr(NO)(CN)S3-,

solutions),

Cu(hfacac)zPPh3).

and magnetic

in

substitution

and hydrolysis

+ -OBut,

England

spectroscopy

[Ni(cyclam)Brz]$

VO(acac)z

(R~No + ~a~ck. reactions

Examples

salt,

Leicester,

spin resonance

and equilibria

is reviewed.

Reactions

Raynor

The use of electron

to follow

in The Netherlands

to Follow

J. Barrie

Abstract

Printed

Spin Resonance

and Equilibria

Department

91 -

stereo-

oxidation

spin

- low spin)

are

given.

Once a new spectroscopic physicists

have

2 decimal

places

the chemist useful

worked

soon makes

by Zavoisky

33 years

since

electron

by 1976 had reached

which

about

compounds,

to him are important.

of papers

2000 annually,

to

to gain chemically

spin resonance

and the number

and the

measurements

irrelevant

use of the technique compounds

discovered

and made

of seemingly

about

in 1944,

has been

out the theory

on a range

information

It is now

technique

was discovered

devoted

of which

to e.s.r.

about

1300

92 could

be described

make-useful radicals

conclusions

and compounds

especially chemists

those have

structure

in organic

the structure

for detecting

in solution

This paper

concentrates

and monitor

classified

under

These

changes

the

following

dissociation

(b)

addition

(c)

substitution

(d)

stereochemical

(e)

oxidation

(f)

magnetic

in peak height

as it grows the nature

reactions examples.

Rates

or decreases

The power

monitored

may readily

in intensity. present

of the g-value

tool

is

paramagnetic

to follow

complexes

in

may be loosely

be obtained

line

in solution

changes

discerned

and ligand

is seen

by measuring

in the spectrum

Molecular

are readily

and metal

changes

by measuring

of e.s.r. -spectroscopy

and equilibria

e.s.r.

for this purpose.

and reactions

of the narrowest

of the species

measurements couplings.

are readily

spectra.

cases

and

headings.

(a)

phenomena

in the e.s.r.

In many

of inorganic

The type of equilibria

reactions,

the information

on the use of e.s.r.

solution.

in many

few inorganic

and characterising

equilibria

chemists

radical

relatively

and so is a powerful

many

and bonding

to monitor

in solution.

Whilst

to monitor

chemistry,

used the technique

technique

reactions

about

1

relevant.

and use e.s.r.

of complexes

the only species

as chemically

in

from

hyperfine

in studying in the following

93

(a)

Dissociation The unusual, yet classic case of a diamagnetic molecule

dissociating into a pair of radicals is that of potassium nitrosyldisulphonate

(1).

-03s \ y-0,

so 3-2

t-N<;;;

)

-03s /'

W

soj-

.

..I

(1)

Here, the equilibrium lies strongly to the right in solution, and there is no evidence for the dimer in solution.

An

example where only e.s.r. can readily detect the dis-

sociating species is with the recently prepared Ni(cyclam)Brz J%& which in solution loses the axial bromides according to the equilibrium.

2

[Ni(cyclam)~r3]*$

[Ni(cyclam)Br] 2+ +-Br-$[Ni(cyclam)]

3+ + 2Br-

. . .. Cyclam is the tetrademate

ligand

The e.s.r. spectra of a solution in dimethylsulphoxide shows four equally spaced lines arising from interaction of the uu-

(2)

paired electron, which is in a dz2 orbital, with one bromine atom.

This immediately characterises one of the species in

solution and shows that dissociation had taken place.

To

establish further the extent of the dissociation, rapidly frozen solutions of the complex with (1) added bromide and (2) added silver perchlorate gave the spectra shown in Figure 1.

Figure 1

The e.s.r. spectrum at 77 K of solutions containing (a) [NiESrz(cyclam)]+ (the

arrows

show

the seven parallel features), (b) a mixture of [NiEQ(cyclam)]+

and [NiEJr(cyclam)]2+.

The group of seven arrows show the parallel features of the [NiBrp(cyclam)]+ and the

group

of four arrows the parallel features of the [NiBr(cy~lam)]~+.

(c) [Ni(cyclam)]3+.

95

These

show that with

exhibits

seven

(11, added

lines

from

bromide,

two equivalent

the presence

of [Ni(cyclam)Brz]+,

perchlorate,

the spectrum

and is identical excess

silver

being

(b)

Thus

the bromide

that

the equilibrium

of the bromides

feature

and confirms

(21, added coupling

silver

to bromide

of iNi(cyclam)C12]+

suggesting

in an antibonding_ai

line with

with

shows no hyperfine

perchlorate,

The lability

bromides

whilst

to the spectrum

is [Ni(cyclam)]3+.

the parallel

with

the species

formed

(2) is established.

is due to the unpaired (c$?-) orbital

which

electron

is directly

in

ions.

Addition ??ree radical

adducts

which

bases

and its adducts

R2N0 + AlC13.

group

the e.s.r.,spectrum which

+ GaqCl.6

R2N0

+ GaCl3

e.s.r.

each of which MdiCal.

This

interconversion

spectra

of the

aluminium

trichloride

+

R2N0 + Ga2Ch

. . (4)

\\

R2N0 + &Cl3

. . (5)

equilibrium

in (4) and

of the two paramagnetic

itself

species

(3-S).

(5) are seen,

the unassociated

manifests

in

equilibria

. . (3)

with

The

in toluene,

2GaClg

of the adducts

stable

t&chloride,

of two species following

2,2,6,6-

known.

*

is in equilibrium latter

base

are well

with

the presence

to form

is particularly

halides

Gallium

are members

Ca2Cl6

Separate

three

with

shows

R2NO

(R2NO)

is, for example

However,

acids

The free radical

nitroxide

with

in solution

equilibrium

add to Lewis

are paramagnetic.

tetramethylpiperidine

product

readily

nitroxide

as a rapid

so that the

96

Stick diagram

Figure2

of a spectrum

representition

on

the limits of (a) slow and (b) fast exchange for two forms of a radical I = 1 (~1 resulting

with -I = %

and

spectrum.

-a*1

-1-1 I

I

1

1

1 I

I

II

I

I

1

II

I

I1

%a

I

1

I

“Ga nGa

I

1

I

1t11

Figme

3

E-s-r.

spectrum

in toluene

of TEMPO with gallium

showing the reconstruction Species

specba

of the two species.

f.+Cl6)

is in major abundances I

R2NO + GaC13-

trichloride of

the

1,
Species

2 is

II

97

signal

is the weighted

would

the two and showing marked

Had the interconversion

line width changes. spectrum

average-of

consist

This is represented

of two separate

diagramatically

sets of spectra are seen in Figure in terms of a quartet

(I

=

Yh

60

triplets

by the nitrogen

weaker,

bond in GazCl6 no assoication

of vanadyl

adduct lifetimes

(40 kJ mol-I).

for pyridine formation

is diffusion

in benzene

with

suggest that

controlled

and

if there is a direct encounter

correlation

reaction

site.

time of pyridine

than that of VO(acac)p,

to achieve

,kf is about one twentieth

enthalpy

phenomenon.

longer than for

and piperidine

then it has time to reorientate

different

if

the forward rate con-

at the VO(acac)a

the rotational

that for pyridine

Ga bond were

take place, whilst

are rather

is about 10 times shorter

2-picoline,

of the chlorine

(>50, 7.5 and 1.4 each x lo-* set 9

the two molecules

&I benzene

is weak

Csrlin and FLieger4 have shown that the

will only take place

since

The rapid association

VO(acac>2

From this may be deduced

the rate of adduct

However,

each split into

If the O+

with Ga2Cl6 would

for piperidine

stants, kf, which

between

of the nitroxide.

acetylacetonate,

and 2-picoline

reaction

is analysed

of the g- and -A (51V) values of the line-

added bases, Walker,

respectively).

respectively)

there would be no time dependent

From analysis

pyridine

2 and the two

The spectrum

3.

equal to the bond strength

it were stronger,

widths

in Figure

superimposed.

implies that the 0 -+ Ga bond strength

and approximately bridging

signals

of lines from each of 6gCa and 71Ga

% and 40% abundance

and dissociation

been slow, the

reaction.

Since for

(0.05 x log M-l set-l)

(log M-l set -l) then it must arise from a of activation

and a weaker

V-N bond strength.

98 Since

2-picoline

V-N bond,

has a larger p_$

the difference

introduced

by the methyl

Substitution

(c)

which

is undoubtedly

volving only

e-s-r.

obtained. metal

ligands

and the solvent

time,

chance

l3CN labelled

cyanide

the axial

compared

with

of - AS,f shows protonated greatly

thus supporting

cyanide

reflects

and

[C~(CN)SNO]~that

using

in acid set 'I

rate of 7 x 10B6 set-l AS,+ = 75

K-l.

J mol-'

The large

for axial

the exchange

K-l,

value

exchange

this mechanism.

the strong

is a

rate

is a is

The lability

trans-influence

of the

group.

Similarly, studied.

In acid solutions,

there

rate was 10m4

AFai = 140 kJ molB1,

complex

crystal

Spencer

and found

+ and AS _e = 21 J mol-l

species.

of the axial

with

exchange

signal

is considerable

(e) exchange

that the activated

increased,

nitrosyl

(a) cyanide

between

of the short

processes.

of cyanide

the equatorial

= 128 kJ mol-l

occurs

No e.s.r.

because

so

of transition

exchange

there

ions in solution

Furthermore,

to be given,

e.g. low spin d5 ions, exchange

the exchange

solution

at 346 K.

energy,

where

in-

the information

in solution.

solution

e.s.r.

studies

in the reactions that rapid

in fluid

In systems

interesting

have yielded

or ions

of following

Myers5followed

A!:

could

are so labile

stabilisation

better

strain

where

In the examples

Usually,intermediates

relaxation

due to steric

chemistry

to some

and mechanism.

can then he detected

field

itself

spectroscopy

complexes

to a stronger

reactions

has lent

kinetics

lead

group.

It is in this area of inorganic spectroscopy

would

6

the acid hydrolysis

'Ihe sequential

exchange

of [Cr(CN)gN0]3of cyanide

by water

may be may readily

99 be followed by e.s.r-, and the kinetics of the last two steps FOP

have been measured.

C~CN)~NO(H,O),

to CCP(CN)(NO)(H~O)~]+

(I) and for the latter to [Cr(NO)
= 1.3 x 10D3 set-l at 294 K with AEIi:- 80 kJ mol-l,

# + * ASI = -25 J mol-l K-l, AEII = 56 kJ mol-l, A_SII = -100 J mol-I K-1.

The first unequivocal demonstration of the occurrence of radical substitution reactions at a transition metal site was recently reported by Lappert -et al.

7

U.V. irradiation of cis-

Pt(Me)2(PEt3)2 in the presence of di-t-butyl peroxide in benzene yielded Pt(Me)(OBut)(PEt3)2 and methyl radicals.

The mechanism

was established as S xby employing the spin trapping technique. -H The complex + (ButON) (as thermal source of BuO') + ButNO (spin trap) in benzene yields, upon heatingTan e.s.r. signal comprising a triplet of quartets from the generated nitroxide radical Bu t . 'NOCH3'

A control experiment without the complex

yielded the products But,

But, NO-

NO.

and

ButO'

each of which

But'

comprises only a triplet of lines.

An S 1 mechanism is ruled -ij-

out since no radicals are generated when the complex + tetrahydrofuran are photolysed alone.

The contrast in the e.s.r. signals of aqueous solutions of Fe3+ having a very broad signal and solutions of Pe3* containing excess fluoride which have seven narrow (11Gf lines is noticeable.

In the absence of fluoride, the broad lines arise from a

large static zero field splitting which provides an effective

relaxation mechanism-

Species in solution which could have a

large zero field splitting are [F~(H~o)~oH]~+,

[FE?(H~o)~]~+A

(A = anion), b?e(H20)5A] **, [(H20)~F~~~:Fe(H*O)~]4+. However

in

the presence of fluoride, the ion has tightly bound

ligands and is bigblysymmetrical. yieldinga

smalllinewidth

which is caused by inefficient relaxation through collisions with other molecules. about 2 x lo-l2 sec.

(d)

The lifetime between collisions would be

8

Stereochemieal

The change in stereochemistry of a complex on going from the solid to solution, or the equilibrium between two stereochemical isomers may readily be detected by e.s.r. spectroscopy.

The salt K@b(CN)6.2H20

has a dodecahedral arrangement

of cyanide ligands (D2d) in the solid state. (in the diamagnetic izkorphous confirms this since gu

Its e.s.r. spectrum

host Kt,Mo(CN)6.2H20) readily

= 1.972 end gL = 1.990 which is appropriate

for an unpaired electron in a g~(cJlf~-y~)orbital.

However in

glycerol solution, the e.s.r. spectrum is quite different with EP

= 2.002 and_gL = 1.976 showing that the unpaired electron

is in an _a,($~~> orbital which is compatable with square antiprismatic symmetry (H djm9 The balance between these two different -4 stereochemistries is energetically very fine and dependent upon the different nature of the solvation and lattice energies.

There is the fascinating case of a copper complex which can exist in both square pyramidal and trigonal bipyramidal forms in solution.

10

The triphenylphosphine adduct of bisthexafluoro-

acetylacetonato)copper II in toluene exhibits the e.s.r. spectrum

101

Fim

4

E.s.r. spectra of Cu(hfacac)z.PPhj in frozen solution of (a) toluene where there is a mixture of "basal" and "axial" isomers, and (b) in methylcyclohexane where only the "axial" isomer exists.

(a) Figure 5

(b) The two forms of Cu(CF$OCHCOCF3)2PPh3 (a) "basal" square pyramid and (b) "axial" trigonal bipyramid.

102 shown

in Fig&me

spectra group

4_

comprising

lies in the basal

(Figure

5) and yielding

isomer,

where

bipyramid El

the PPh3

and yields

> 2 and indicates

square a large group

of a square

pyramidal

31P h.f.s.,

and an %xial"

lies on the axis

no h.f.s.

to 31P.

of a tiigonal

In each

the same electronic

configuration

ground

case

gl) >

state,

namely

In methylcyclohexane, only the "axial" trigonal bipyramidal isomer is detected.

,dxy.

(e)

Oxidation The reversible

vitamin

B12

(B1

oxidation is well

by molecular

established

oxygen

and proceeds

of reduced according

21to the equilibrium

(6).

Co"(B4

i- 02

,--'

co11102-

.I..

I (a) Figw?e6

(b) Relevant

orbitals

of the Co-O-O

(a) a-polarisation

by Tz

unit

unpaired

electron

of the O-Co u-bond.

(b1 T interaction between the T", orbital and the metal _&, orbital. -

(6)

103 W-~~~~rrxp~

ZTim

Ctr hk

species

possessing

one unpaired

on 02.

The e.s.r.

parameters

to cobalt

are readily

Co-02 bonding oxidation

occurs

compounds.

may readily

be monitored

enthalpy

carried

of the reaction

may be represented

although

under

certain

and (CN)~COO~CO(CN)~~-

studies

formation,

(CN)$o"'-

circumstances,

and

023-

it is possible

The spectra

are shown

The course

scheme.

+

12'13

Appropriate

12

in Figure

to prepare

of (CN15C002~-

7.

Magnetic There

equilibria

are numerous in which

electrons

in either

ths two forms being the solid. chemical

change,is

examples

a molecule

of spin paired

can exist

the spin paired in thermal

A beautiful

(BunPh2P)2NiX2 form

oxidations rate

derived.

by dimer

by the following

i- 02

of

hemoglobin

of Co II complexes.

complicated

(~)5C011102-Co111(CN)55-.

(f)

and numerous

data has also been

(CN)5C011

and with

of the cobalt

by e.s.r.

is often

of spin polarisation

II complexes

The course

coupling

The same sort of reversible

6).

cobalt

in a n* orbital

(15G) hyperfine

in terms

out on a range

and entropy

localised

and small

(Figure

in many

and related

have been

electron

explained

electrons

c33&3i~rez33TWifl&

dissolved

in

of this,

planar

benzene

is low spin and diagmagnetic,

in solution which

which

whilst

d-

free arrangement, and in

involves

- tetrahedral in

free

the metal

or the spin

equilibrium

example

the square

with

- spin

the

a stereo-

equilibrium square

the tetrahedral

planar

form

of

104

a

b

Figure

7

E.s.r.

spectra

of

and

(CN)&00z3-

f4,

&!N)~COO$O(CN~~~-.

Fz&_n?e 8

Tile

spectrum at

s2.s.r.

i.ug (a)

~FeCl~~cyelam)f+,

and Id

[FeBr~
belonging

to

and the low

the high

spin

77

K of

(b)

solutions contain[Fe(NCS)z(cycladf+

In (c?,

features

spin form are labelled

form, Is.

HJ.

105 has a high spin configuration it is not amenable

CFe(cyclam)Br2] (Figure

by e.s.r.

features

to the low spin

= 2 and gl=

The relative

with the spectrum

is lowered.

the Br- in the axial

of their

the high

spin

The e.s.r. spectrum

positions,

one observed, stronger

2.34 and 1.08

3++.

l.on ) along with high

of the two spectra

of the high

as the temperature

in terms

Fe

studied

at g = 3.25,

6 attributabletothe

intensities

form is the only

We have

in dimethylsulphoxide.

8) at 77 K shows

attributable

fll

+

to study

Unfortunately

and is paramagnetic.

spin

Fez+

varies

temperature,

in intensity

and Cl- replacing

the spectra and this

at

ion.

with

form decreasing

With NCS-

ligaud

spin

features

of the high

is readily

field which

spin

explained

would

stabilise

state.

References

1.

Based

2.

A. Desideri,

on Chemical

Dalton,

Abstracts

J.B. Raynor

and the UKCIS.

and C.K. Poon,

J. Chem.

Sot.

1977,

3.

C. Hambley

4.

F.A. Walker, R.L. Carlin and P.H. Rieger, J. Chem. Phys., 1966,

5.

J.B.

and J.B. Ray-nor, J. Chem.

Sot. Dalton,

1974,

604.

45, 4181. .. Spencer

and R.J.

Myers,

J. Amer.

Chem.

Sot.,

1964,

86, 522. .'A 6.

J. Burgess, 1968,

501.

D.A.

Goodman

and J.B.

Raynor,

J. Chem.

Sot. A,

106 7.

8.

D.J. Cardin,

M.F.

Chem.

1973,

Comm.,

H. Levanon,

Lappert

and P.W.

Lednor,

J. Chem.

Sot.

350.

G. Stein

and 2. Luz,

J. Amer.

Chem.

Sot.,

1968,

Sot. Dalton,

1975,

90, 5293. .m 9.

P.M. Kiernan

and W.P.

Griffith,

and V.K.

Kapur,

3. Chem.

2489.

10.

B.B.

l_l.

J.B. Ray-nor, Inorg.

12.

E.I.

13.

M. Mori, 1967,

Wayland

Ochiai,

Nucl.

J. Inorg.

J.A.

71, 103. -

Inorg.

Chem.

Nucl.

Weil and J.K.

Chem.,

Lett.,

Chem.,

1974,

1973,

Kinnaird,

1974,

13, 2517. w

10, 867. -

35, 3375. w

J. Phys.

Chem.,