Stereochemistry of Monohydrido- and Dihydrido-Dodecanickel

7 Stereochemistry of Monohydrido- and Dihydrido-Dodecanickel Carbonyl Clusters Containing a Hexagonal Close Packed Nickel Fragment

Downloaded by UNIV LAVAL on July 12, 2016 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/ba-1978-0167.ch007

A Model System for Hydrogen Interaction with a Close Packed Metal Lattice ROBERT W. BROACH and LAWRENCE F. DAHL—Department of Chemistry, University of Wisconsin-Madison, Madison, WI 53706 GIULIANO LONGONI and PAOLO CHINI—Instituto di Chimica Generale dell' Universita di Milano, 20133 Milano, Italy ARTHUR J. SCHULTZ and JACK M. WILLIAMS—Chemistry Division, Argonne National Laboratory, Argonne, IL 60439

+

Single-crystal x-ray diffraction studies of the [Ph As] , [PPN] , and [Ph P] salts (Compounds 1, 2, and 3) of the [Ni (CO) H ] dianion and of [Ph As]3 [Ni (CO) H] · (acetone) (Compound 4) and subsequent neutron diffraction investigations of Compounds 3 and 4 provided the first detailed crystallographic description of the interaction of interstitial hydrogens with a close-packed fragment of metal atoms. X-ray diffraction results revealed the common geometry of the anions as a 12-atom nickel fragment of a hcp metal lattice (containing two octahedral and six tetrahedral holes) surrounded by nine terminal and 12 doubly bridging carbonyl ligands. Neutron diffraction results showed hydrogen occupation of one octahedral hole in the trianion and both octahedral holes in each dianion. Stereochemical implications are discussed with respect to possible proton migration within these nickel clusters and to their observed reversible protonation-deprotonation reactions. 4

+

+

4

2-

12

X

-ray

21

+

2

4

d i f f r a c t i o n studies o n salts of the n i c k e l

3-

12

21

a n d p l a t i n u m car-

b o n y l dianions [ M ( C O ) ( M - C O ) 3 ] „ ~ ( M = N i , Pt) showed that these 3

3

2

2

0-8412-0390-3/78/33-167-093/$05.00/0 © American Chemical Society

Bau; Transition Metal Hydrides Advances in Chemistry; American Chemical Society: Washington, DC, 1978.

94

TRANSITION M E T A L

HYDRIDES

complexes are based on the stacking of a triangular M ( C O ) 3 ( M 2 - C O ) 3 b u i l d i n g 3

block (1-7).

T h e p l a t i n u m c a r b o n y l dianions, where the stacking of the t r i a n

gular t r i m e t a l units is i n a nearly eclipsed fashion, have been isolated (8) for η = 1 to 6 a n d η ~ 1 0 (and characterized (4, 5) structurally for η = 2, 3, 4, 5); the n i c k e l analogues, where the triangular t r i m e t a l units are staggered, have been obtained (9,10) (and structurally analyzed (3, 7)) only for η = 2 and η = 3.

These

latter dianions were found by C h i n i a n d L o n g o n i (9, 10) to give rise by a c i d i c hydrolysis to a new series of n i c k e l c a r b o n y l anions, for w h i c h a n a l y t i c a l data i n d i c a t e d 12 n i c k e l atoms i n each anion. hydrolysis of the [ N i C O ) ( u - C O ) ] 3

3

2

3

2 -

A t r i a n i o n , w h i c h was prepared by

d i a n i o n at a b u f f e r e d p H of ~ 5 to 6 i n

T H F , gave an ir spectrum containing carbonyl bands at 1990(s) and 1830(s) c m

- 1

w h i l e its * H N M R spectrum i n a c e t o n e - d exhibited a h i g h - f i e l d resonance for 6


h y d r o g e n at 3 4 r . (M -CO)3]2 2

A d i a n i o n , w h i c h was prepared f r o m either the [ N i ( C O ) 3

3

or [ N i ( C O ) 3 ( M 2 - C O ) 3 ] 3 ~ dianions by hydrolysis at a lower b u f f

2 _

2

3

ered p H of - 3 to 4, showed ir carbonyl bands i n T H F at 2020(s) and 1860(s) c m " a n d a N M R proton resonance i n a c e t o n e - d at 2 8 r .

1

R e l a t i v e integration of the

6

N M R peaks of the cation's p h e n y l hydrogens to the anion's hydrogens indicated that the trianion contained one hydrogen atom while the dianion contained two. F u r t h e r m o r e , it was f o u n d that the m o n o h y d r i d o t r i a n i o n c o u l d be protonated by acetic acid i n T H F to give the d i h y d r i d o dianion, and conversely the dianion could be deprotonated by potassium terJ-butoxide i n acetone or T H F to give the trianion. T h e presence of hydrogen atoms i n this new series of anions i n d i c a t e d that the anions' stereochemistry was m a r k e d l y different f r o m those of the previously characterized [ M 3 ( C O ) 3 ( M 2 - C O ) 3 ] ~ dianions, and, moreover, the highly shielded n

2

nature of the hydrogens i n d i c a t e d f r o m their N M R resonances suggested (11) the distinct possibility that they were interstitial.

T h i s i n d i c a t i o n of interstitial

m e t a l - h y d r o g e n clusters intensified our interest i n their stereochemistry since previously there have been only two substantiated examples of transition metal p o l y h e d r a l cluster compounds c o n t a i n i n g interstitial h y d r o g e n atoms.

Based

u p o n prior independent x-ray d i f f r a c t i o n analyses (12, 13) w h i c h showed that N b e l n contains [ M o 6 C l s ]

4+

type clusters (14) (with each of the other iodine atoms

b r i d g i n g two such [ N b l e ] groups) i n accord w i t h the c o m p o u n d ' s f o r m u l a t i o n 6

as [ N b l 8 ] ( I ~ ) 6 / 2 > p o w d e r neutron d i f f r a c t i o n measurements (15) of H N b I n 3+

6

6

a n d D N b o I n i n d i c a t e d that the h y d r o g e n atoms o c c u p y the octahedral sites of the [ N b I n ] groups. 6

T h e interstitial nature of the h y d r o g e n atoms i n the

[Rhi3(CO)24H5_ ] ~ (n = 2,3,4) anions (16) was clearly disclosed from an analysis n

n

(17) of their U N M R spectra. l

T o determine both the stoichiometry and atomic arrangement of these new nickel carbonyl cluster anions (including an assessment of the geometrical effects of the hydrogen atoms), x-ray diffraction studies of [ P h A s ] [ N i i ( C O ) i H ] ~ 4

2

+

2

2

2

2

(1), [ P P N r + [ N i ( C O ) 2 i H ] - (2) (where P P N denotes the bis(triphenylphos2

1 2

phine)iminium

2

cation),

2

[Ph P] [Nii2(CO)2iH ] ~ 4

2

+

2

2

(3),

and

[Ph As] 4

3

+

[ N i i 2 ( C O ) 2 j H ] ~ - M e 2 C O (4), were c a r r i e d out, followed b y neutron d i f f r a c t i o n 3


7.

B R O A C H E T AL.

Carbonyl Clusters Containing a hep Nickel Fragment

analyses of Compounds 3 and 4.

These investigations have ascertained the correct

f o r m u l a t i o n to be [ N i ( C O ) 2 1 H _ ] 1 2

95

4

n

n -

(n = 2,3) a n d have revealed a h i g h l y

unusual geometry for the dodecanickel framework based on a hep array of metal atoms.

T h e subsequent neutron d i f f r a c t i o n studies not only have substantiated

the proposed presence of the interstitial h y d r o g e n atoms but also have a l l o w e d the first detailed crystallographic e x a m i n a t i o n of the interaction of interstitial hydrogen atoms with a close-packed fragment of metal atoms.

This work, w h i c h

is of interest with respect to the behavior of hydrogen i n metals (18,19), was found to be of p a r t i c u l a r pertinence i n connection w i t h the observed structural d i f f e r ences between the interstitial face-centered n i c k e l - h y d r o g e n c o m p o u n d , N i H o . 6 (20), a n d ccp n i c k e l (21).


Experimental Since the crystallographic analyses of these four compounds were not at all straightforward, an outline of the approaches used to obtain their crystal structures is given below. Details of the x-ray and neutron diffraction work (22), i n c l u d i n g the extensive refinements, w i l l be published elsewhere. X - r a y D i f f r a c t i o n S t u d i e s . Initial attempts to solve the t r i c l i n i c crystal structures of the P P N + a n d the P h A s + salts of the [ N i i ( C O ) i H ] - dianions by trial-and-error models founded on a considerable n u m b e r of " h e a v y a t o m " interpretations of the calculated Patterson maps a n d by direct methods, v i a the use of M U L T A N (23), were unsuccessful. Several models based on 13-atom n i c k e l fragments geometrically similar to the 13-atom r h o d i u m fragment pre viously d e t e r m i n e d (16, 17) i n the [ R h i 3 ( C O ) 2 4 H ] ~ d i a n i o n gave l i m i t e d agreement, but no such m o d e l under least-squares refinements converged w i t h an u n w e i g h t e d R i ( F ) discrepancy index lower than 30%. T h e correct crystal structure c o n t a i n i n g the dodecanickel d i a n i o n was u l t i m a t e l y u n r a v e l e d for the P h A s salt 1 by " b r u t e force," i n v o l v i n g an initial phasing under P I symmetry of a F o u r i e r m a p w i t h two tentative arsenic positions, followed by a recognition on this m a p of low-density peaks as a possible dodecanickel framework together w i t h a disregarding of m u c h higher density peaks p r o d u c e d by pseudo-nickel imagery. This knowledge of the dianion s architecture then enabled one to obtain the correct solution of the crystal structure of the P P N + salt 2 f r o m a w r o n g one generated on an Ε-map b y M U L T A N (23). T h e crystallographic d e t e r m i n a t i o n of the t e t r a p h e n y l p h o s p h o n i u m salt 3, crystals of w h i c h were subsequently furnished by L o n g o n i a n d C h i n i , was necessitated f r o m its crystals b e i n g more suitable for a neutron d i f f r a c t i o n i n vestigation (vide infra) of the [ N i i 2 ( C O ) 2 i H 2 ] ~ d i a n i o n . T h e solution of this structural p r o b l e m was based on the i n i t i a l assumption that the space group of the c o m p o u n d conforms to t r i c l i n i c P I s y m m e t r y rather than to the true m o n o c l i n i c P2 /c s y m m e t r y . Since the choice of o r i g i n is a r b i t r a r y under noncentrosymmetric P I symmetry, a n i c k e l triangle of side 2.5Å was placed i n the unit cell i n the orientation d e t e r m i n e d f r o m the c o m p u t e d Patterson m a p w i t h the origin situated at one of the n i c k e l atoms. A F o u r i e r m a p phased on these three nickel atoms gave, under P I symmetry, four separate assemblages of peaks. Sets of 12 peaks (corresponding to the k n o w n geometry of the dodecanickel fragment d e t e r m i n e d f r o m C o m p o u n d 1) were chosen f r o m the distributions of peaks i n each assemblage i n accord with monoclinic P2\/c symmetry, and the coordinates for 12 independent nickel atoms were shifted to what was assumed to be the true 4

2

3

4

2

2

2

2

+

2

1



96

TRANSITION M E T A L HYDRIDES

Figure 1. Architecture of the [ N i i ( C O ) i H ] ~ trianion in Compound 4, as determined from the x-ray diffraction re finement, showing the anisotropic thermal ellipsoids of 20% probability and the atom labeling scheme common to the trianion and three dianions of Compounds I, 2, 3, and 4. All four anions are similarly constructed from a planar N i ( C O ) ( M - C O ) 6 fragment of D h - 6 2 m symmetry capped by two Ν ι ( Ο Ο ) ( μ - 0 Ο ) fragments through Ni-Ni interactions. 2

2

3

6

3

3

2

3

3

2

3

center of symmetry. F u r t h e r F o u r i e r syntheses on atoms f r o m one independent anion under P2\/c symmetry revealed positions for 38 atoms of the independent anion. A t this point it was discovered that the i n i t i a l choice for the center of symmetry was incorrect. O n e of the anions was related to the first by the Cb glide plane, but the other two were not related to the first by either the center of symmetry or 2\ screw axis. Phased on the 38 atoms previously found, a F o u r i e r synthesis calculated under Pc symmetry revealed two anions related by a center of symmetry at approximately 0,0, After the coordinates for the independent anion were shifted to correspond to the true center of s y m m e t r y at the o r i g i n , further F o u r i e r a n d difference F o u r i e r syntheses under P2\/c s y m m e t r y ex h i b i t e d positions for a l l independent n o n h y d r o g e n atoms. T h e crystal structure of C o m p o u n d 4, w h i c h was f o u n d to possess a centros y m m e t r i c t r i c l i n i c unit c e l l c o n t a i n i n g six tetraphenylarsonium cations, two [ N i i ( C O ) i H ] ~ trianions, a n d two solvent acetone molecules, also was solved b y the c o m b i n e d P a t t e r s o n - F o u r i e r method, based on the assumption of the dodecanickel core of the trianion b e i n g analogous to that of the d i a n i o n . Peaks corresponding to a l l of the n i c k e l a n d arsenic atoms were first located f r o m suc cessive Fourier syntheses i n the unit cell under noncentrosymmetric P I symmetry, after w h i c h i n i t i a l atomic coordinates for the 12 independent n i c k e l a n d three independent arsenic atoms were obtained b y an o r i g i n shift to an a p p r o x i m a t e center of s y m m e t r y relating pairs of these peaks to one another. 2

2

3



7.

B R O A C H E T AL.


97

Neutron Diffraction Studies. T h e t w o interstitial h y d r o g e n atoms i n the d i a n i o n of 3 a n d the one interstitial h y d r o g e n atom i n the t r i a n i o n of 4 were located f r o m single-crystal neutron d i f t r a c t i o n data collected at the C P - 5 reactor at A r g o n n e N a t i o n a l L a b o r a t o r y . I n each case, the i n i t i a l positions of the i n terstitial h y d r o g e n n u c l e i were obtained f r o m a difference F o u r i e r synthesis of nuclear scattering density based o n the coordinates for a l l n o n h y d r o g e n atoms determined from the x-ray refinements and for the phenyl hydrogen atoms fixed at Idealized positions. I n C o m p o u n d 4, the six m e t h y l h y d r o g e n atoms of the solvent acetone molecule were unresolved i n the difference m a p , p r e s u m a b l y because of large librations of the m e t h y l groups a n d because of errors introduced b y uncertainties i n the t h e r m a l parameters. A s a consequence of the unusually large n u m b e r of independent atoms i n both C o m p o u n d 3 a n d C o m p o u n d 4, only the positional a n d isotropic nuclear temperature factors for all atoms i n the anion were refined (with the coordinates for a l l atoms i n the cations fixed at their x-ray d e t e r m i n e d values a n d w i t h their isotropic nuclear temperature factors f i x e d at reasonable values). Results and

Discussion

General Description of the Structures. T h e x-ray structural determinations of the four compounds revealed discrete anions a n d cations w i t h n o r m a l i n t e r ionic separations a n d w i t h stoichiometries consistent w i t h the a n a l y t i c a l a n d spectral data.

S h o w n i n F i g u r e s 1 a n d 2 are O R T E P d r a w i n g s of

Figure 2. A view of the [ N t i ( C O ) i H ] ~ trianion of Compound 4 normal to the NiQ(CO)^2-CO)e plane, emphasizing the approximate threefold axis. The entire trianion ideally possesses C 3 - 3 m symmetry. 2

2

3

V


the


98

TRANSITION M E T A L

3

HYDRIDES

4

Figure 3. Analogous views with identical atom labelings and with 30% anisotropic thermal ellipsoids obtained from the x-ray diffraction refinements of the [ N t i 2 ( C O ) 2 i H ] ~ dianion for Compounds 1, 2, and 3, and [ N i i ( C O ) i H ] ~ trianion for Compound 4. The dianions ideally conform to D3h-62m symmetry, while the trianion ideally possess C 3 - 3 m symmetry. 2

2

2

3

2

V

[ N i i ( C O ) 2 i H ] ~ trianion (in C o m p o u n d 4) w h i c h consists of a planar N i 6 ( C O ) 3 3

2

( M 2 - C 0 ) fragment symmetrically attached to two equivalent N i ( C O ) 3 ( M 2 - C O ) 3 6

3

moieties by N i - N i interactions.

T h i s 12-atom n i c k e l fragment of a hep metal

lattice surrounded by nine t e r m i n a l a n d 12 d o u b l y b r i d g i n g c a r b o n y l ligands is c o m m o n to the anions of a l l four compounds, as shown i n F i g u r e 3.

Although

none of the anions has any crystallographic site s y m m e t r y constraints, the d i h y d r i d o dianions of C o m p o u n d s 1,2, and 3 ideally conform to D 3 / , - 6 2 m s y m m e t r y (where the horizontal m i r r o r plane contains the h e x a n i c k e l fragment a n d the threefold axis passes through the centroids of the h e x a n i c k e l a n d two t r i n i c k e l fragments) w h i l e the m o n o h y d r i d o trianion of C o m p o u n d 4 ideally conforms to C^ -Sm s y m m e t r y . v

T h e v i e w i n F i g u r e 2 of the trianion clearly reveals the

approximate threefold axis c o m m o n to all of the anions.

G e o m e t r i c a l deviations

of the dodecanickel f r a m e w o r k of the t r i a n i o n f r o m C$ s y m m e t r y a n d of the v

dianion from

s y m m e t r y are not appreciable i n the four compounds.

The

observed angular distortions of the outer two n i c k e l triangles w i t h respect to the


7.

B R O A C H E T AL.

99


central three nickel atoms differ by only 1° to 7° f r o m the regular staggered value of 60° w h i l e the calculated t i p p i n g of each outer t r i n i c k e l plane relative to the central hexanickel plane is slight, w i t h the values r a n g i n g f r o m 0.4° to 2.2°. These geometrical distortions c a n be attributed p r i m a r i l y to i n t r a - a n i o n steric effects, upon w h i c h are superimposed a s y m m e t r i c p a c k i n g forces.

T h e strong

steric interactions between the c a r b o n y l ligands of the central hexanickel f r a g ment and those of each of the trinickel fragments produce a considerable outward bending of the carbonyl groups i n the two outer trinickel fragments.

T h e relative

constancy of this c a r b o n y l b e n d i n g i n the anions of the four compounds a p p a r ently reflects the e q u i l i b r a t i o n achieved between n o n b o n d i n g repulsive effects of the c a r b o n y l ligands and i n t e r p l a n a r b o n d i n g interactions of the n i c k e l atoms.


Stereochemistry of the D i a n i o n a n d T r i a n i o n . CORE.

(a) T H E D O D E C A N I C K E L

A d r a w i n g of the dodecanickel framework for the dianion of C o m p o u n d

1, s h o w i n g the three kinds of equivalent nickels under assumed D / j s y m m e t r y , 3

is given i n F i g u r e 4.

T h e mean N i - N i bonding distances (Table I) obtained from

the refinements based on the x-ray d i f f r a c t i o n data are the averages for each of the five different types of distances—viz., those for the N i i - N i i bonds w i t h i n the two N i ( C O ) 3 ( M 2 - C O ) 3 fragments, for the c a r b o n y l - b r i d g e d N i 2 - N i 3

noncarbonyl-bridged

Ni -Ni 2

2

3

and

bonds w i t h i n the central N i ( C O ) 3 ( M 2 - C O ) 6 6

fragment, and for the N i i - N i a n d N i i - N i bonds between the central a n d outer 2

fragments.

3

A s t r i k i n g feature revealed i n T a b l e I is the close s i m i l a r i t y of the

corresponding N i - N i distances i n the four anions together w i t h the small, but nevertheless highly significant, differences i n the N i i - N i 3 and N11-N12 inter-ring distances between the two halves of the [ N i i ( C O ) 2 i H ] ~ trianion. 2

3

The simi-

larities of the larger values i n one of the two halves of the t r i a n i o n w i t h those i n both equivalent halves of the dianion i n Compounds 1,2, and 3 provided the first evidence that the hydrogen atom i n the trianion was localized i n only one of the two octahedral interstices (rather than disordered i n both octahedral interstices) and that the a d d i t i o n of the second proton to the trianion causes a similar i n t e r -

F i g u r e 4. The dodecanickel framework shouting the three kinds of nickel atoms under assumed D3h-62m symmetry. This system can be viewed as a hep array of nickel atoms (with an a.b.a stacking pattern) containing two octahedral and six tetrahedral interstices.


100

TRANSITION M E T A L HYDRIDES

T a b l e I.

M e a n N i - N i Distances (Å) i n the [ N i (CO)2iH .„]"*Anions 1 2

4

(n = 2 , 3) D e t e r m i n e d f r o m X - r a y D i f f r a c t i o n S t u d i e s Anion

[NiJCO) H Y21

Idealized

Geometry 1


2

3

2

3

2

c

C

3v

3

2

4 Differw/H; wo/H

Stereochemistry of Monohydrido- and Dihydrido-Dodecanickel

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