Molecular orbital study of coordinated dioxygen

Structure, 65 (1980) 173-183 Elsevier Scientific Publishing Company, Amsterdam

Journal of Molecular

MOLECULAR

ORBITAL

STUDY

-

Printed in The Netherlands

OF COORDINATED

DIOXYGEN

I. Structure and bonding of model monomeric Co(II) complexes R. BOCA

Department of Inorganic (Czechoslovakia)

Chemistry,

Slovak

Technical

University.

880 37 Bratislava

(Received 14 August 1979)

ABSTRACT The CNDO-UHF type of MO-LCAO-SCF calculation is carried out for model systems of dioxygen fixation: 0, C!oCl,L’complexes in which L = none and L = NH,. A geometry variation is performed with respect to 5 internal coordinates describing the degrees of freedom of the Co-O, group. The calculated geometry, spin densities and atomic charges agree with available data based on X-ray and ESR measurements of real dioxygen carriers. Structure and bonding of complexes are discussed in more detail. INTRODUCTION

The nature of dioxygen bonding in transition metal adduct complexes has attracted attention for many years. This interest is stimulated largely by the existence of important natural dioxygen carriers such as hemoglobin and myoglobin. A variety of monomeric dioxygen adduct complexes have been made: beginning with the first adducts of cobalt Schiff base chelates [l] such as Co(acacen)LOz [ 21, continuing with the “picket-fence” [3] and “capped” [ 43 iron porphyrins or manganese porphyrin dioxygen carriers 151, and terminating with the synthetic hemopolymers [6] as models for heme proteins. The second area where dioxygen complexes play an important role is that of catalytic oxidation reactions. Not only tetradentate ligands such as Schiff bases or porphyrins are necessary for dioxygen fixation. Quite simple dioxygen complexes are also known, for example Co(CN), Oz3- and Co(NH3 ),02* ions or Vaska compounds Ir(PPh,)z(CO)X(02) (X = Cl, Br, I). X-ray structures have been determined for a number of these compounds. They may be categorized into two general classes: u bonded (or superoxo) complexes, I, and 7r bonded (or peroxo) complexes, II. O/O

CO-

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II 0 1980

Eisevier Scientific Publishing Company

174

However, Brown and Raymond [7] suggested in 1975 that “all of the Co-O, adducts reported to date have been of limited precision due to either poor crystal quality or unresolved disorder problems”. Both of the structural types (I and II) have a bonding model proposed by Pauling [8] favoring thea bonded structure and Griffith [9] the 7r bonded. These models have been accepted for the qualitative description of the bonding in monomeric dioxygen adduct complexes (see the reviews [lOZl] ). Quantitative theoretical studies of these compounds have been done using quantum chemical computational methods. Here, the molecular orbital theory seems to be the most convenient. Some of MO-LCAO-SCF calculations are mentioned below. A SCCC calculation has been performed for [(NH3)5CoOzCo(NH,),] ‘+ ion [22]. Fantucci and Valenti [23], on the INDO-UHF level of Hamiltonian approximation, have studied a model of Co(acacen)LOz (L = none, NH3) in the fixed geometry of the Co-O2 group. Veillard and co-workers [24-261 have performed an ab initio study (in the minimal basis set) of another model of Co(acacen)LO, where the ethylene bridge and the methyl groups have been substituted with hydrogens. Considering axial ligands such as L = none, H20, CN- and CO, MO-LCAOSCF calculations have been carried out for 4 geometric arrangements of the Co-O2 group: linear, perpendicular and two bent (with the Co-O,-Ob angle (Y= 153” as in Co( CN)S 0Z3- ion [7] and OL= 126” as in the Co(bzacen) PyOZ adduct [27] ). However, only 4 points on the multi-dimensional energy hyper-surface have been obtained with respect to the arrangement of the Co-O;? group. Thus, the limited geometry variation gives only limited conclusions. Strictly speaking, the molecular energy (the adiabatic potential hypersurface) is to be defined over 6 internal coordinates of the dioxygen. Because the Co-O,-O, plane is normal to the equatorial plane of ligands, these 6 internal coordinates can only be reduced to 5 (Fig. 1): the 0,-O, distance (r, ), the Co-O, distance (rZ), the Co--O,-Ob angle (cK),the angle /3 (the deviation of the Co-O, linkage from the normal vector g of the equatorial plane of ligands E) and the torsional angle up(which defines the various conformers). Thus the arrangement of the c0-0~ group describes the adiabatic potential of the form ET = & (rl, r2, a, 0, g). RESULTS

AND

DISCUSSION

In the present work the less time consuming, although semiempirical, CNCC-UHF method [28] is preferred; the method has been extended to the transition metals by Clack et al. [29] . Experience with this method verifies that it is also suitable for geometry calculations in the case of the transition metal compounds. The optimum geometry of the Co-O2 group is studied with respect to all degrees of freedom. Therefore, the equatorial plane of ligands has been chosen to be the simplest: in this preliminary study it is defined by 4 chlorines.

175

Fig. 1. Internal coordinates describing an arrangement of the M-O,

group.

The fifth, axial ligand is L = none or L = NHs, so that the formula of the studied low-spin complexes is O2 CoC14 L2-. The basis set considered corresponds to the valence orbitals of atoms; cobalt 4~ orbitals are also included in the basis set. The Co-Cl distance has been taken as 2.24 A, corresponding to the optimum value for the free square planar COC~,~- ion. The torsional angle has been fixed as IJJ = 45”, so that the staggered conformers have been considered first. The remaining geometric parameters (F1, r2, a, f3) were simultaneously varied and the total molecular energy was calculated at each geometry. The results of the geometry variation are listed in Table 1 (for 20 geometries) and Table 2 (for 30 geometries). The calculated optimum geometries of complexes are listed in Table 3. To open the discussion we are concerned primarily with the comparison of our calculated geometry with the experimental geometries of the real monomeric u bonded (superoxo) complexes. The summary of six X-ray structural results has been reported in [7] and [30]. The calculated O--O distance r, = 1.19 A is less than the majority of experimental values, but it is greater than the calculated value for the free dioxygen: r1 = 1.13 a and near to the calculated value for the free superoxide ion rl = 1.19 A. The calculated Co--O, distances fall within the range of experimental values. It is not sur-

176 TABLE

1

Geometry

variation for O,CoCl,z

-a Q

(A) [r, = 1.17, P = 0]

r2

95

100

105

1.90 1.85 1.80

7.88

7.89 7.90 7.77

7.87

a

P

(W

[rt = 1.17,

r2 = 1.851

0 10 20

85

90

95

100

7.88 8.09 7.89

7.90 8.06

7.79

8.05 7.91

105 7.87

rl

(C) [a = 95, p = lo]

rz

1.17

1.19

1.21

1.90 1.875 1.85

8.10 8.11 8.09

8.15 8. I 7 8.16

8.10 8.12 8.11

=The total molecular energy is ET = -3370.0 -E’ (eV) where E’ is listed in (A), (B), (C) for a pair of fixed parameters in [ ] and a pair of varied parameters. TABLE

2

Geometry

(A) lr,

variation for 0, CoCI, NH,2- a

= 1.17, p = O]

r2

90

100

110

1.90 1.85 1.80

5.63 5.51

5.72 5.70 5.53

5.61 5.64

a!

(B) Cc = 1.17, rr = 1.851

(C)

[a =

100,

p = lo]

P

90

95

100

105

110

0 10 20

5.51 5.66 4.39

5.70 5.75 5.29

5.69 5.70

5.64

5.74 5.32

1.15

1.17

1.19

1.21

5.58 5.40

5.75

5.80 5.81

5.75 5.76

1.90 1.85

177 TABLE

2 (continued) a!

(D) [r2 = 1.875,~

= 51

r1

90

95

100

105

1.21 1.19 1.17

5.79 5.81 5.7 3

5.84 5.87

5.81 5.86 5.81

5-81

=‘I’he total molecular energy is ET = TABLE

3760.0-

E’ (eV) w h ere E’ is listed iu A, B, C, D.

3

Calculated optimum

geometry of 0, CoCl, L*-

Parametera

L = none

L=NH,

Experimentalb

r, 64)

1.19 1.875 95 10

1.19 1.875 95 5

1.1-1.39 1.86-1.95 117-153 4, 6

rz (A) 0~ (deg.) P (deg.)

=Fi.xed parameters: Z?C,,-~~ = 2.24, RC,_+ = 1.80, symmetry C,. bFrom 6 structures 17, 30 ] _

experimental structure of NH,

[31] ,

prising that the calculated angle 01deviates from the experimental range. This quantity is very sensitive to the structural type of a complex. It is important to point out that the calculated angle p = ca. 5-10” is greater than zero: it is not too far from the experimental value fi = ca. 6” in (NEta) [COLON ]5 H,O [7] or/3 = ca. 4” in Co( 3-F-saltmen) (1-MeIm)02 - 2 (CH3 )* CO [ 301. The nature of this distortion follows from the dominant role of the highest occupied molecular orbital (HOMO). This is composed mainly of the dioxygen 7r* orbital and cobalt dzz orbital, so that when /3 > 0 the overlap of dz2 with n* increases and simultaneously the “destabilizing” overlap of dzl with the oxygen nonbonding orbital (n) decreases. Therefore, the bent structure with fl > 0 is stabilized (Fig. 2). One-electron energy levels (orbital energies) above -8 eV are shown in Fig. 3. They are assigned in terms of the irreducible representations of the C, and a = A” is point group symmetry: the representation s = A’ is symmetric antisymmetric with respect to the horizontal plane of symmetry. The dcharacter and 02-character of the individual MO’s is denoted with full and/or empty circles. It is interesting to describe the stabilization of the complex in terms of the main interactions between the cobalt d’ system and the dioxygen orbitals. The simplified MO diagram of the O2 CoC14*- ion is shown in Fig. 4. From this figure it follows that the spin-pairing of the & electron with one of the 7~* electrons occurs here. The electron at the n’* orbital remains unpaired (that orbital is perpendicular to the Co+&-Ob plane). Then, the large spin density

p=0

P’O

Fig. 2. Optimisation

of bent structure

with p > 0.

eV

-1L I

I ___----

I i

100 90 80 7a

-

-3 L

60

-r_:_-----

-+

I

3

3 IL5

___

13s

.P

l

12s

I -' i-

/ I

50 *g

I

2

10s 9s

-6 1 I I -7 i

I

Fig. 3. The highest occupied molecular of IMO’S; 0 : oxygen character of MO%.

orbitals

of O,COCI,‘-

(C, symmetry).

0 : d-character

is localized on the oxygen atoms. This qualitative conclusion is proved with the calculated values of the spin densities (Table 4): p (0,) = 0.34 and p (Ob) = 0.66. Calculated spin densities are in good agreement with the experimental ones for frozen Co(bzacen)Py02 [33] or values calculated by the

179

o,i3x1

Cold71

Fig. 4. Simplified MO diagram for bent structure of O,CoCl,L’TABLE

4

Cakulated

spin densities in 0,CoCI,L2UHF densities

L: Symmetry

None C,

Orbitaldensity d2 0.0000 d d& -0.0003 0.0004

Projected densities=

NH, C,

0.0000

0.0004 -0.0001

d YZ dXY

(C, symmetry).

0.0004 0.0000

Atomicdensity co -0.0015 Oa 0.3376 Ob 0.6630

0.0004 0.0000 -0.0010 0.3159

0.6847

None c 2”

0.9663

-0.0211 -0.0001 -0.0211 0.0043

0.9370

0.0267 0.0267

None C,

0.0000

0.0004 -0.0003 0.0004 0.0000 -0.0011

0.3372 0.6629

NH, C,

0.0000 0.0004 -0.0001 0.0004 0.0000

-0.0007 0.3154 0.6845

None C 2”

0.9663

-0.0173 -0.0001 -0.0173 0.0043

0.9443

0.0234 0.0234

aSingle annihilation procedure according to [ 321.

INDO-UHF method for Co(acacen)NHs02 1231. However, this result gives no indication of the extent of electron transfer to O2 (forming a Co(III)-O~~ adduct). Calculated atomic charges are: Q(0,) = -0.23 and Q(a) = -0.22 which are in agood agreement with the suggestions of Tovrog et al. [ 341 based on ESR results. These authors found the electron transfer into 0, ranging from AQ = ca. 0.1 to A Q = ca. 0.8 of an electron in various Co-O2 adducts. For example, their upper limit for electron transfer in Co(acacen)PyOa is AQ = ca. 0.4 e. The optimum geometry of the O2 CoC12- complex has been obtained from the position of the minimum on the multi-dimensional energy hypersurface. However, there are several equivalent minima on this hyper-surface corresponding to the different geometries; a saddle point must exist between these minima. We have considered that the transition state is probably repre-

180

sented with the sideways (perpendicular, or 7r) structure. Therefore, the geometry optimization has also been performed for this structure of G, symmetry (Table 5). The difference in the energy between the optimum bent structure (C, symmetry) and the optimized n structure (C,, symmetry) is LUZ= 0.5 eV. This quantity is much lower than the stabilization energy of the O,CoCl,‘complex of ca. 7.6 eV (calculated with respect to the planar CoCL2- ion and free triplet dioxygen). Since both these quantities are probably overestimated, their ratio seems to be acceptable. Thus, flipping of the coordinated dioxygen is indicated between various minima on the adiabatic potential hyper-surface at higher temperature (Fig. 5). This is in qualitative agreement with the interpretation of the ESR spectrum of Co(bzacen)PyO, in solution [ 353, where the oxygen atoms were found to be magnetically equivalent under the conditions of measurement. Also another type of motion of the coordinated dioxygen has been considered. It is the rotation between various conformers. The calculated rotational barrier between the staggered and eclipsed conformers is not too high: about 0.05 eV. Conclusions about the nature of stabilization of the bent structure, I, with respect to the 7~structure, II, follow from Table 6. The total molecular energy can be divided into monocentric and bicentric parts [ 36,371; bicentric parts of the energy are listed in this table. Thus a “softening” of the O-O interaction is apparent for the optimum bent structure with respect to the optimized ‘ITstructure (by 5 eV), while the sum of the Co-O portion of the energy increases (by 3 eV) and simultaneously the sum of the Co-Cl portion of the energy also increases (by 6 eV). The Wiberg index 1381 for the O-O bond decreases significantly, so that this quantity also indicates a softening of the O-O bond. Also, for the 7rstructure an unfavorable charge distribution is obtained and spin densities are significantly changed (Table 4). Thus, the TI structure corresponds to the singlet dioxygen (Fig. 6), while the optimum bent structure seems to be an activated doublet one. The former agrees with the suggestions of Griffith [9] since the bent structure is more favorable as shown by both the present calculations and the X-ray structural analysis of mononuclear Co (II) complexes [7,21]. TABLE

5

Geometry

variation for perpendicular

structure of 0,CoC1,2-

R

1.15

1.17

1.19

1.96 1.94 1.92

7.64 7.63 7.62

7.67 7.67 7.66

7.57 7.59 7.60

a

aThe total molecular energy is ET = - 3370.0 - E’ (eV) where E’ is listed in the table; R is the distance of the middle of the O-O bond from the Co atom; optimum R = 1.94 corresponds to r2 = ca. 2.026 (A)).

181

0117 P

;

0 -\ ;co;

I

\

-. )CO\

/'

.' ', ;,/

s, :;

'.

'

'?b' ;

,

Fig. 5. Possible flipping of coordinated

0

'203

dioxygen.

TABLE 6 Calculated properties of O,CoCI,L’L

L

Propert

None

None

Symmetry Total energy

C,, -3377.7

G

G

-3378.2

-3765.9

Bicentric part of energy 39.5 - wJ,-%) 13.0 --E(Co--o,) -

E(a--%

)

E( Co-Cl)

13.0 18.5

Propert

NH,

34.6 21.2 8.3 20.1

E( Co-N)

34.3 20.9 7.4 17.8 21.4

Wiberg index WCo-w

1.60 0.46

~(~O-Ob)

0.46

w(o,--"b)

W(Co-Cl)

0.64

1.18 0.72 0.18 0.67

W(Co-N) aFor optimum

geometry;

energies in eV.

1.17 0.71 0.15 0.60 0.57

Atomic Q(o,)

QW,) Q(Co1

Energy E&) +3) E&-l)

None

None

NH,

-0.22 -0.23

-0.22 -0.23

+ 0.79 -0.58

+ 0.52 -0.61 -0.07

charge +O.Ol -I-0.01 + 0.27 -0.58

of highest occupied MO -1.03 -1.23 -0.07 -1.18 -1.23 -0.07 -1.05 -1.61 -0.52

182 ------

d,=_ yp

6a2

2a2 +-

----

___3t__

Cold71

Fig. 6. Simplified MO diagram for optimized symmetry)_

9i’fi

O2 (‘A)

perpendicular, n-structure of O,CoCI,‘-

(C,,

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