Oxidative addition of dihydrogen to Ni(II) complexes.

Reaet.Kinet.Catal.Lett. Vol. 64, No. 2, 247-254 (1998)

Jointly published by Elsevier ScienceB.V., Amsterdam and Akad~miaiKiad6, Budapest

RKCL3198 OXIDATIVE ADDITION OF DIHYDROGEN TO Ni(II) COMPLEXES. A B INITIO MO/MP4 CALCULATIONS OF THE ELECTRONIC STRUCTURE OF NiH2CI2(PH3)2 COMPLEX LI. Zakharov, A.N. Startsev and G.M. Zhidomirov Boreskov Institute o f Catalysis, Novosibirsk 630090, Russia

Received April 22, 1997 Accepted June 11, 1997

Abstract Electronic state d 6 Ni(IV) in the complex t-NiH2CI2(PH3)2] was studied by means o f ab initio MO/MP4 calculations.

Keywords: Oxidative addition of H2, ab initio calculations, NiH2ChL2 complex

INTRODUCTION The oxidative addition of saturated hydrocarbons R-H is usually mediated by metal(0) complexes resulting in an increase of the formal oxidation state of the metal by two and in the formation of alkyl-hydride complexes [1]. Because alkyl and hydride ligands are very strongly reducing, stable reaction products can be formed only with metals in low oxidation states. In the case of a high oxidation state of the metal, reaction (1) starts as oxidative addition (la) and results in dissociative (electrophilic) substitution ( l c) [2]. Recently [3] an alkyl-hydride complex of Pt(IV) was synthesized. It allowed the authors [3] to suggest the possibility of the oxidative addition pathway (la ---> lb ). Quantum-chemical studies [4] show that the Corresponding Pt(IV) species, Pt(H)RClz(OH2)2 complex, is indeed stable, being only 19.2 kJ/mol above the PtC12 (OH2)2 + RH energy level. A new example of oxidative addition of alkane and arene C-H bonds to PdX2L2 complexes has been recently reported [5].

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ZAKHAROVet al.: OXIDATIVEADDITION

248

/R P V)

o/

/ a PtC12 + RH

~

\.

CI\ I~, cl/Pt., IH

(I)

o,,, c

/R

Pt(I1) o-n +

Scheme 1

The oxidative addition reaction (la, b) must be facilitated by substituting alkanes with molecular hydrogen (i.e. R = H) as a hydride bond is stronger than an alkyl one. The feasibility of the Ha oxidative addition to complexes of transition metals in the oxidation state +1 was shown by Vaska and Di Luzio [6]. An HE oxidative addition to an Os(II) complex has been recently demonstrated [7], a dichlorodihydridoosmium(IV) complex OsH2CI2(PH3)2 being synthesized and characterized. Dihydride [8] and dialkyl [9] complexes of Pt(IV) with the composition PtR2X2L2 (R=H, CH3) are also known. Today the H2 oxidative addition is considered to be one of the key reactions in homogeneous catalysis by group VIII metals [10, 11]. Thus, this reaction is well known to be the first stage in the formation of the active center of Wilkinson's olefin hydrogenation catalyst [12]. We have supposed [13] that a high oxidation state of Ni in Ni/MoS2 catalysts [14] might be formed in a hydrogen atmosphere via its oxidative addition to Ni(II) in a square planar coordination environment of the catalyst [15]. In the present communication we report the results of a quantum-chemical investigation of the possibility of the H2 oxidative addition to a square planar NiC12(PH3)2 complex with the formation of a stable 18-electron NiH2CI2(PH3)2 complex.

ZAKHAROVet al.: OXIDATIVEADDITION

249

RESULTS AND DISCUSSION All the calculations were carried out using the a b initio method of SCF MO LCAO with the LANL 1 effective core potential for inner shells of Ni, CI and P atoms [16] and basis set double- ~ (DZ) for valence shells [17]. For H atoms in H2, the basis set was extended by a polarization p-orbital with the exponent ~ = 1.1. For H atoms in PH3, a split-valence 3-21G basis set was used. The electron correlation was taken into account using the second order Moller-Plesset perturbation theory (MP2) [18]. Vibrational frequencies were calculated at the MP2 level to identify stationary points (minima or transition states). Activation energies (F-~c) and energies of oxidative addition reaction (AE) were determined using single-point MP4 calculations [19] at the MP2 - optimized geometry. On the basis of the calculations presented in Table 1, we conclude that a d 6 Ni(]V) species, NiH2CI2(PH3)2 complex (Fig. 1), may be thermodynamically stable (AE = -19.6 kJ/mol). Theoretical investigations of the oxidative addition of H2 to PtC12(PH3)2 and PdCIz(PH3)2 complexes give the values AE = -32.2 and -67.8 kJ/mol, respectively [20]. Pt(IV) and Pd(IV) oxidation states are well known in organometallic chemistry [21].

Y

(+0 01)HR, ~/ 9 \\/ \

H

/ |

O

~51.5 H(+0.21) //"3e H ii /o /

(+0.07)Hw'~ 103.8~~_-.~"9.0 r Z ~ ) . H ~ ~ " ' . . e"C_(!04.6) C1(-0.46) |

NII.0.45j, ziQ0.64nl.lA1.35A2.0 2.0 dx2-y2 2.0 dl.4 -,x-r, '-'z2 "~xz dyz ,xy )

Fig. 1. Fully op6miTed (at the MP2 level) geometric parameters of the d6transition metal complex, NirvH2C12(PH3)2. Bond distances and angles are in angstroms and degrees, respectively(C2v - symmetry). Values in parenthesesshow net atomic charges and Ni orbital populations(at the HF level)

250

ZAKHAROVet al.: OXIDATIVEADDITION Table 1

Calculated MP4 energies of complexes and transition states (TS) for the oxidative addition of H2 and reductive elimination of HCI ( H: + NiC12L2 --~ NiH2CI2L2 ~ Ni(H)CILz+ HC1 ) Complex

MP4 energy (a.u.)

Activation energies E= and energies of reaction AE

(~/mol) H2 + NiCI2(PHa)2 TS-HH NiH2CI2(PH3)z TS-HCI Ni(I-1)Ct(PH3)2 + HC1

-1.16450 } E=0.0 -84.96566 -86.09113 -86.13762 -86.08189 -70.77083

E~ = +102.5 AE~dd= - 19.6 E~ = +146.3 } AE~I-=+36.2

-15.34554

Let us consider the Ni electronic state in square planar d s NiCI2(PH3)2 and in octahedral d 6 NiH2CI2(PH3)2 complexes. Figure 2 shows a correlation diagram of MO orbitals between these complexes. Going from NiL4 to NiL4H2, the d a orbital is considerably destabilized, leading to LUMO of the d 6 NiL4H2 complex, but the 4p orbital becomes more stable in energy, changing to the M-H bonds. From these features, one can predict that the oxidative addition of H2 easily occurs for the d s NiL4 complexes. Calculated orbital populations of the Ni atom in the NiC12(PH3)2 complex are: Ni +~ (4s 0.4s 4p 0.6x3dS.Odxy0.62). The formally empty dxy orbital has 0.62 electron due to the coordinate-covalent bonding. Let us compare this with the electronic state of Ni in the NiH2C12(PH3)2 complex (Fig.l): Ni -~ (4s ~ 4p L1 3d 6"~ dz2135 dxyl4). The formally empty dz2 and dxy orbitals have 1.35 and 1.4 electrons, respectively, due to the coordinate-covalent bonding. One can see that the H2 oxidative addition results in an increase of the electron density on Ni 4p orbitals (due to the formation of Ni-H bonds) and a decrease in the population of 3d~ orbital, i . e . to the formation of the Ni(IV) d 6 electron configuration. The kinetic stability of the NiH2C12(PH3)z complex can be characterized by the activation energies of the reductive elimination of H2 and HC1. Structures of the transition states of these reactions optimized at the MP2 level are shown in Fig. 3 - TS-HH and TS-HC1. The activation barrier values (Eac= 120 - 160 kJ/mol) calculated at the MP4 level (Table 1) confirm the stability of the d 6 Ni(IV) state in the NiH2CI2(PH3)2 complex.

ZAKHAROVet al.: OXIDATIVEADDITION

251

C2v

Oh dxy

a2

dxy ~......................

(~*

eg

dz2

oOO b2

dyz

t2g

- dxz

dx2-y2 M-H

a1

M-H b2

M-L M-L

~ _ 8j_

M-L'

~

M-L'

~ - -

G

bl

~

- - " - ' a L - - --

H

H-H

CI

[+L j H

-~4~

_ _

M/

L

CI

L--M--L

/I

H

Fig. 2. Schematic correlation diagram for the H2 oxidative addition to the squaretransition metal complex NiaC12(PH3)2. The reaction coordinate (H2 + d 8

planar d s

ME4 --'9 d 6 ML4I-I2 ) maintains C2v symmetry

ZAKHAROVet

252

al.:

(+0.13)H O. 0"95 ~ H(+0.13)

Y

/

1.45 ~

T

1.45

c)(-o.5) |

OXIDATIVEADDITION

x

cbo.5)

Ni-0.14 4s 0 5 4 0 93 0.83 2.0 d 1-98 ( 9 p. dz2 dxz d~2z~ a) T S

1.93 dxy )

- HH

H(-0.01)

~//~\ o.8~ 9

\

- :..--164.0 -"'-Z""-""~"

,~ ~ ' -122.9 .

'2-'~/'/"

./

H( +o 17)

"_"x., \ \1.91

C1(-0.43)

c1(-o.58) @ Ni+O.17r~-~

2.0 d}z~ d 1"75 ~ , 968,~1.02 Uz2 dxz x2-y2

dxl~90)

b) T S - HC1 Fig. 3. OptimiTed structures (at the MP2 level) of the transition state (TS) for the reductive elimination of H2 (a) and HC1 (b) from the NiH2C12(PH3)2 complex. Side view of the complexes without the PH3-groups

ZAKHAROVet al.: OXIDATIVEADDITION

253

So far, it is believed that the Ni(IV) oxidation state is possible for the square planar Ni(SzCzR2)2 complex [22]. However, detailed quantum-chemical calculations of the electronic structure of Ni(S2CEH2)z show that a d 8 Ni(II) diamagnetic singlet is the ground state in this complex [23, 24]. Accounting for the tendency of the Ni(S2CzR2)2 complex to form stable dimeric structures, one can assume [25] that these are structures for which the unusual Ni(IV) oxidation state with electron configuration d 6 can occur. Our ab initio calculations of the H2S - Ni(SECEHz)z molecular adduct have confirmed stabilization of the Ni(IV) species [26]. Thus, though the organometallic chemistry of nickel is dominated by the d s Ni(II) electronic state, recent investigations provide some support and give encouragement for the study of new systems invoLving the d 6 Ni(IV) state [13].

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