Crystallographic Investigations on Polyhydride Metal Complexes

6 Crystallographic Investigations on Polyhydride Metal Complexes

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 21, 2017 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/ba-1978-0167.ch006

1

ROBERT BAU, W. EAMON CARROLL, DONALD W. HART , and RAYMOND G. TELLER—Department of Chemistry, University of Southern California, Los Angeles, CA 90007 THOMAS F. KOETZLE—Department of Chemistry, Brookhaven National Laboratory, Upton, NY 11973

The molecular structures of four transition metal polyhydride complexes are reported: H Ir(PMe Ph) and H Re(PMe Ph) by x-ray diffraction analysis and H Os(PMe Ph) and H Re (PEt Ph) by neutron diffraction analysis. Although the hydride ligands were not located in HIr(PMePh), the arrangement of phosphorus atoms about the iridium atom suggests a distorted octahedral geometry for this molecule. X-ray analysis of H Re(PMe Ph) reveals a bent phosphorus-rhenium-phosphorus backbone for the molecule, that is consistent with a tricapped trigonal prism with phosphorus atoms in equatorial positions. Neutron diffraction analysis of H Os(PMe Ph) shows that the molecule is a distorted pentagonal bipyramid, with one equatorial and two axial phosphine ligands. The neutron diffraction analysis of the H Re (PEt Ph) dimer reveals a molecule with four terminal and four bridging hydride ligands. This molecule provides the first example of a metal-metal bond bridged by four hydrogen atoms. 3

2

3

7

4

2

2

2

3

2

3

2

8

2

4

3

7

2

2

4

8

2

2

2

3

4

D

u r i n g the 1960s, J . C h a t t , B . L . Shaw, a n d co-workers synthesized numerous m i x e d hydride-phosphine complexes of the third-row transition metals tungsten (1, 2, 3), r h e n i u m (4, 5, 6, 7), o s m i u m (8, 9,10, 11), a n d i r i d i u m (12,13,14,15) (see T a b l e I). These unusual covalent compounds, called p o l y h y d r i d e complexes, were found to be r e m a r k a b l y stable a n d m a y contain u p to seven h y d r i d e ligands per metal atom. T h e general method of preparation i n volves reaction of the metal chlorides w i t h tertiary phosphines under ref l u x i n g conditions (in alcohol) to y i e l d complexes of the type M C 1 L _ ( L = P R ) , f o l lowed b y reduction to the corresponding p o l y h y d r i d e w i t h L 1 A I H 4 (in T H F ) or X

6

X

3

1Present address: Department of Chemistry, University of Arkansas, Fayetteville, A R 72701.

0-8412-0390-3/78/33-167-073/$05.00/0 ©

American Chemical Society

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

74

TRANSITION

T a b l e I. H WL

Schematic of P o l y h y d r i d e H ReL H 0sL H ReL H 0sL H ReL H 0sL ( L = tertiary phosphine)

H4WL4 6

3

N a B H (in ethanol).

7

2

6

2

5

3

4

3

3

4

2

4

METAL

HYDRIDES

Complexes H IrL H IrL HIrL 5

2

3

3

4

T h e original number of phosphines usually remains constant

4

d u r i n g the hydrogénation step.

T h e synthetic approach to the r h e n i u m series

of p o l y h y d r i d e complexes (Scheme 1) (7) is a representative example.

T h e re-


sulting p o l y h y d r i d e has a l l c h l o r i d e ligands replaced b y a sufficient n u m b e r of h y d r o g e n atoms to achieve a closed-shell (18 electron) configuration.

The

compounds are generally colorless (or light yellow), d i a m a g n e t i c , a n d soluble i n m a n y organic solvents.

Most are air stable i n the solid state.

T h e fact that

phosphine ligands stabilize these molecules can be attributed to the softness (i.e., 7r-accepting ability) a n d steric b u l k of these ligands. T h e p o l y h y d r i d e series (Table I) later was extended by T e b b e to i n c l u d e H 5 T a ( M e 2 P C H C H P M e 2 ) 2 (16) a n d by Ginsberg to include the anionic species 2

2

[HgRe(PR )]~ (17). 3

(Abbreviations used i n this paper are as follows:

E t - e t h y l , Cp-cyclopentadienyl, and Ph-phenyl.)

Me-methyl,

T e c h n i c a l l y , the classic

[ R e H g ] anion (18) also can be considered a m e m b e r of this series. =

M a n y anal-

ogous compounds involving second-row and first-row transition metals are k n o w n (19);

however, these are generally less stable than those i n v o l v i n g t h i r d - r o w

transition metals.

H y d r i d e - p h o s p h i n e complexes of first-row transition elements

are l i m i t e d m a i n l y to compounds h a v i n g few hydrogen atoms, such as H C o L , 4

H F e L , and H C o L 2

4

3

(19).

3

T h e subject of p o l y h y d r i d e complexes was i n c l u d e d as part of a larger, comprehensive review of m e t a l h y d r i d e complexes by Kaesz a n d Saillant ( 19), a n d their interesting N M R behavior has been s u m m a r i z e d b y Jesson (20).

Scheme 1.

Synthesis of rhenium polyhydride complexes (4, 5, 6, 7, 17) Na

KRe0

Ï

[ReH ]

4

9

=

—

[ReH L] 8

_

HC! ReOCl L 3

4

L l A 1 H

L1AIH4.L

L

ReCl L

2

2

'>

H ReL, 7

H,ReL

H ReL 7

H Re L 8

2

4

4

2

( L = t e r t i a r y phosphine)


The

6.

BAU E T AL.

Crystallographic

75

Investigations

r h e n i u m h y d r i d e complexes, i n particular, have been r e v i e w e d b y Giusto (21 ). In this chapter, we w i l l focus on structural aspects; b a c k g r o u n d m a t e r i a l w i l l be r e v i e w e d briefly, a n d some recent results w i l l be presented. Prior Crystallographic

Work on Polyhydride

Complexes


T h e general historical background of structural work on h y d r i d e complexes has been covered very w e l l i n review articles by Ginsberg (22) and by F r e n z and Ibers (23). T h e latter article catalogs all crystallographic investigations of metal hydrides u p to 1970. Since m a n y of the problems i n v o l v e d i n structurally characterizing the h y d r i d e ligand have been discussed i n these reviews, they w i l l not be repeated here. Suffice it to say that f r o m x-ray data often it is not possible to locate hydrogen atoms attached to third-row transition metals, and that neutron

H Figure

1.

The tricapped trigonal prismatic of the [ReH ]= anion (IS)

geometry

Q

diffraction methods are preferable for the accurate d e t e r m i n a t i o n of h y d r o g e n positions i n such compounds. (There are, however, examples of t h i r d - r o w complexes i n w h i c h h y d r i d e ligands have been located a n d refined f r o m x-ray data, as have been reported b y C h u r c h i l l a n d C h a n g (24, 25) a n d by Ibers (26).) O n e of the first metal p o l y h y d r i d e complexes to be thoroughly studied was K ^ R e H g , the nature of w h i c h was established f i r m l y only after a neutron d i f fraction study reported i n 1964 by Abrahams, Ginsberg, and K n o x (18). A t that



76

TRANSITION

METAL

HYDRIDES

t i m e , this c o m p o u n d was the only k n o w n example of a b i n a r y transition m e t a l p o l y h y d r i d e anion (i.e., w i t h no phosphine ligands); somewhat later, the tech n e t i u m analog was prepared (27).

T h e geometry of the [ R e H g ] d i a n i o n (see =

F i g u r e 1) is that of a t r i c a p p e d trigonal p r i s m w i t h an average R e - Η distance of 1.68(1) Å. G i n s b e r g a n d co-workers also investigated the eight-coordinate H R e ( P P h ) 3 (28). 5

3

complex

D e c o m p o s i t i o n of the crystal d u r i n g x-ray data collection

prevented a precise structure d e t e r m i n a t i o n , but the r h e n i u m a n d three phos phorus atoms were unambiguously located.

F i g u r e 3. A representation decahedral structure of Me Ph) as derived from fraction (29) 2

4y

T h e ReP3 skeleton, w h i c h is n o n -

of the doH Mo(Px-ray dif 4


6.

BAU

ET

Crystallographic

AL.

investigations

planar, is illustrated i n F i g u r e 2 (top).

11

The phosphorus-rhenium-phosphorus

bond angles (134.3°, 107.2°, and 107.1°) are consistent w i t h coordination based on either the dodecahedron ( F i g u r e 2 (bottom)), truncated octahedron, or b i c a p p e d octahedron.

Since the h y d r i d e ligands c o u l d not be located, the exact

geometry of this complex remains i n doubt. A n o t h e r eight-coordinate p o l y h y d r i d e structure that was studied is that of H 4 M o ( P P h M e 2 ) 4 (29).

T h e x-ray data collected for this c o m p l e x were of suffi


cient q u a l i t y to p e r m i t location of the four h y d r i d e ligands.

T h e geometry

( F i g u r e 3) is that of a dodecahedron c o m p r i s i n g two distorted tetrahedra:

one

composed of four hydrogen atoms and the other of four phosphorus atoms.

The

average M o - H distance is 1.70(3) Å, w h i c h is i n agreement w i t h other reported M - H (terminal) distances. X - r a y structural data also exist for two seven-coordinate p o l y h y d r i d e s , H Re(DPPE)(PPh ) 3

3

2

and H R e ( D P P E ) 3

noethane, also k n o w n as diphos).

(30) ( D P P E is b i s d i p h e n y l p h o s p h i -

2

Analysis of the data for the latter c o m p o u n d

Figure 4. A sketch of postulated structure H Re(Ph PCH CH PPh2)2 (21, 30) 3

2

2

the of

2

revealed the following arrangement of phosphorus atoms about the metal:

two

axial phosphines [ P - R e - P angle = 167.4(5)°] and two equatorial phosphines w i t h a P - R e - P angle of 151.5(5)°.

A nonbonding interaction energy m a p calculated

about the r h e n i u m atom, assuming a R e - Η bond distance of 1.68 Å, showed three energy m i n i m a that were assigned to the three h y d r o g e n l i g a n d positions (30, 31 ).

T h e geometry of the resultant molecule is that of a distorted pentagonal

b i p y r a m i d (as illustrated i n F i g u r e 4) i n w h i c h the axial phosphine ligands are bent a w a y f r o m the two equatorial phosphorus atoms a n d t o w a r d the two cis h y d r o g e n ligands. T h e structure determination of mer-H Ir(PPh ) 3

3 3

is an example of a situation

i n w h i c h h y d r o g e n atoms b o n d e d to t h i r d - r o w transition elements c o u l d be lo-


78

TRANSITION

cated but not refined (32).

METAL

HYDRIDES

A series of difference electron density syntheses re

vealed three peaks that were assigned to the hydrogen positions.

T h e geometry

of this molecule is a p p r o x i m a t e l y octahedral w i t h p h o s p h o r u s - i r i d i u m - p h o s phorus angles of 153° (trans) a n d 103° (cis) (see F i g u r e 5).

T h e average i r i d

ium—hydrogen b o n d distance is 1.60 Å. A g a i n note the d e v i a t i o n f r o m perfect octahedral geometry that is caused b y the displacement of the phosphine ligands a w a y f r o m each other a n d t o w a r d the h y d r o g e n atoms.

Because of the small

size of h y d r o g e n atoms, it seems reasonable that steric interactions w o u l d cause


larger ligands to distort t o w a r d them.

H o w e v e r , E l i a n a n d H o f f m a n n have

presented theoretical evidence that this type of distortion also can be rationalized i n terms of electronic effects (33). A b o u t two years ago we began a systematic investigation of the structures of polyhydride complexes.

In this article we present some x-ray diffraction results

on / a c - H 3 Ï r ( P M e 2 P h ) a n d H R e ( P M e 2 P h ) 2 a n d neutron d i f f r a c t i o n results on 7

3

H O s ( P M e 2 P h ) and HsRe2(PEt2Ph) . 4

3

4

T h e latter compound, w h i c h is a pyrolysis

product of H7Re(PEt2Ph) , is the only d i m e r i c m e m b e r of the p o l y h y d r i d e series 2

T a b l e II. HsIr(PMe Phh

H Re(PMe Ph)

2

7

(X-ray)

Monoclinic P2jn

Triclinic PI

Crystal type Space group

6.454(2) Å C e l l constants a 16.061(7) Å b 13.303(6) Å c a 106.38(2)° β 103.48(2)° 78.13(2)° y C e l l vol 1272.0 Â N o . of molecules 2 in the unit cell 1.59 g c m Calc. density Obs. density Absorption coefficient 36.0 c m " [ M o Κα \ Wavelength used i n data \ x-rays > ( λ = 0.71069 Å) collection Room Temp D a t a collection Τ D a t a collection 0.54 Å" l i m i t (sin θ/λ) N o . of reflections 3417 used i n structure (I > 3σ) analysis R = 0.045 ' F i n a l agreement factors R = 0.064 3

- 3

—

1

1

F

0

* R

F

=

wF

Σ\Ρ -\F \\/ZFO;R F= 0

C

W

2

(X-ray)

\WW\F 0

19.038(17) Å 6.337(4) Å 15.234(13) Å 90.0 93.72(4)° 90.0 1834.0 Â 3

4 1.70 g c m " 1.70 g c m

3

- 3

71.6 c m " [ M o Κα l x-rays

1

U = 0.71069 Å Room Temp 0.54

Å"

1

1672 (I > 3σ) R = 0.050 R = 0.055 F

wF

\F \\ /^wF \ c

2

2

Q


x/2

2


6.

Crystallographic

BAU E T AL.

79

Investigations

Figure 5. The distorted octahe dral structure of mer-ΗsIr(PPhs)s (32) Crystal Data HsRe (PEt Ph) 2

2

H 0s(PMe Ph)

4

4

(X-ray)

(Neutron)

12.353(4) 23.137(7) ÅÅ

129.38(1)° 90.0 4369.8 Å 3

(Neutron)

- 3

60.4 c m " [Mo Κα J x-rays

124.63(2)° 89.93(2)° 1306.8 Å

1.64 g c m "

1.55 g c m " 1.51 g c m "

3

1

U = 0.71069Å

125.07(1)° 90.06(4)° 1283.8 Å 3

3

3

1.57 g c m "

3

3

53.6 c m " 2.01 c m " [ thermal \ [ M o Κα \ {neutrons } ( x-rays } (λ = 1.1598 A ) U = 0.71069 Å) Room Temp 80° K Room Temp 1

1

\ \ J

11.409(2)Å 90.0 19.438(5) 12.441(4) 11.098(2) Å ÅÅ

19.634(6) 12.276(4) 11.489(4) ÅÅ Å

129.51(2)° 90.0 4259.5 Å 3

1.59 g c m 1.60 g c m "

3

Triclinic PI

Monoclinic C2/c

23.309(7)Å

2

(X-ray)

(

2.23 c m " thermal \ neutrons } λ = 1.01939 Å 90° K 1

0.54 Å"

0.62 Å-

0.59 Å"

0.63 Å"

2367 (I > 2σ) R = 0.053 R F = 0.057

2729 (I > 2σ) R = 0.086 RwF = 0.049

3486 (I > 3σ) R = 0.055 R F = 0.060

3381 (I > 3σ) R = 0.044 R = 0.042

1

F

W

F

1

F

W

1

F

wF


12.388(2) A 90.0 11.103(4) Å 90.36(5)°

80

TRANSITION

METAL

HYDRIDES

k n o w n to exist. T h i s c o m p o u n d o r i g i n a l l y was referred to as an a g n o h y d r i d e complex by C h a t t and C o f f e y (7), who were at the t i m e unaware of the exact n u m b e r of hydrogen atoms i n the molecule. Experimental T h e compounds / a c - H I r ( P M e P h ) (13), H R e ( P M e P h ) (7), H O s ( P M e P h ) (JO), and H R e ( P E t P h ) (7) a l l were prepared using standard literature methods. T h e r h e n i u m complexes were recrystallized f r o m n-hexane and the others f r o m absolute ethanol. U n i t cell parameters, o r i g i n a l l y obtained photographically a n d later redetermined accurately from diffractometer settings, are listed w i t h other relevant crystal data in Table II. X - r a y diffraction data were collected on a Nonius C A D - 3 diffractometer i n the manner described i n an earlier p u b l i c a t i o n (34). Reflee3


2

3

8

2

2

2

3

7

2

2

4

F i g u r e 6. The structure of iaLC-HsIr(PMe Ph)s, viewed approximately along the noncrystallographic threefold axis of the molecule. This structure is also based on the distorted octahedron. The hydride ligands were not located. 2


4

6.

Crystallographic

BAU E T AL.

81

Investigations

T a b l e III. Selected Distances a n d Angles i n H O s ( P M e P h ) Distances (in Å) Os-H(l) 2.317(2) 1.663(3) Os-P(l) Os-H(2) 2.307(2) 1.648(3) Os-P(2) Os-H(3) 2.347(2) 1.644(3) Os-P(3) Os-H(4) 1.681(3) Angles (in degrees) H(l)-Os-H(2) 69.4(2) P(l)-Os-H(l) 92.9(1) H(2)-Os-H(3) 67.9(2) P(l)-Os-H(2) 83.7(1) H(3)-Os-H(4) 70.0(2) 83.9(1) P(l)-Os-H(3) H(l)-Os-H(3) 137.3(2) P(l)_Os-H(4) 91.3(1) H(2)-Os-H(4) 137.9(2) P(2)-Os-H(l) 89.6(1) H(l)-Os-H(4) 84.4(1) 152.7(2) P(2)-Os-H(2) P(3)-Os-H(l) 79.7(1) 84.9(1) P(2)-Os-H(3) P(3)-Os-H(2) P(2)-Os-H(4) 92.7(1) 149.1(2) P(3)-Os-H(3) 143.0(2) P(3)-Os-H(4) 73.0(1)


4

a

2

3

a

From neutron diffraction analysis.

tions were measured at r o o m temperature w i t h M o Κ α r a d i a t i o n to a (sin0/X) l i m i t of 0.54 Å . Analysis of the data y i e l d e d the positions of a l l the n o n h y drogen atoms i n the molecules. F u l l - m a t r i x , least-squares refinement resulted i n the f i n a l agreement factors g i v e n i n T a b l e II. N e u t r o n d i f f r a c t i o n data for H O s ( P M e 2 P h ) a n d H s R e ( P E t P h ) 4 were collected at the Brookhaven H i g h F l u x B e a m Reactor under operating conditions (35, 36) specified i n T a b l e II. N o n h y d r o g e n atom positions obtained f r o m a n x-ray analysis were used to phase the neutron data, a n d subsequent difference syntheses revealed the positions of a l l hydrogen atoms i n both molecules. Least-squares refinements were carried out w i t h anisotropic temperature factors i n c l u d e d for a l l atoms to y i e l d the agreement factors listed i n T a b l e II. Least-squares computations were performed w i t h a local version of O R F L S (37), w i t h the X - R A Y system (38), or w i t h the C R Y M system (39). M o l e c u l a r plots were produced w i t h O R T E P (40). Calculations for the neutron structures were carried out on C D C 7600 and C D C 6600 computers at Brookhaven National L a b o r a t o r y , m a k i n g use of programs described by B e r m a n et al. (41 ). -1

4

2

3

2

Results f a c - H 3 l r ( P M e 2 P h ) 3 . This six-coordinate molecule, w h i c h is the facial ana log of the k n o w n m e r - H I r ( P P h ) (32), exhibits noncrystallographic, threefold s y m m e t r y ( F i g u r e 6). A l t h o u g h the h y d r i d e ligands have not been located i n tnis study, the arrangement of phosphorus atoms [iridium-phosphorus distances are 2.296(3), 2.296(3), a n d 2.291(3) A ; p h o s p h o r u s - i r i d i u m - p h o s p h o r u s angles are 101.4(1)°, 102.1(1)°, a n d 99.5(1)°] leaves little doubt that the geometry of the molecule is that of a trigonally distorted octahedron w i t h a facial (cis) a r rangement of ligands. As i n other transition metal h y d r i d e complexes, this dis tortion is related to the modest steric requirements of the h y d r o g e n atoms. H O s ( P M e 2 P h ) 3 . A few years ago, Mason studied the P E t P n analog of this c o m p o u n d w i t h x-ray techniques a n d f o u n d its O s P skeleton to be a planar, distorted T-shaped unit (42). These u n p u b l i s h e d results have been m e n t i o n e d b r i e f l y by Aslanov et al. (43). In our neutron d i f f r a c t i o n analysis of H4OS( P M e P h ) , (44), we c o n f i r m e d this basic geometry a n d d e t e r m i n e d the positions 3

3

3

4

2

3

2

3


82

TRANSITION

METAL

HYDRIDES

of the four h y d r i d e ligands. Distances a n d angles about the o s m i u m atom i n this c o m p o u n d are given i n T a b l e III. T h e geometry that is illustrated i n F i g u r e 7 is reminiscent of that of H 3 R e ( D P P E ) 2 discussed earlier (see F i g u r e 4). It is a distorted pentagonal b i p y r a m i d w i t h two phosphine ligands i n axial positions. T h e equatorial H O s P fragment is planar w i t h i n ±0.01 A . O s m i u m - h y d r o g e n distances are 1.663(3), 1.648(3), 1.644(3), a n d 1.681(3) Â, a n d nonbonding H - H contact distances are 1.883(5), 1.840(6), a n d 1.909(5) Â. T h e h y d r o g e n - o s -


4

Journal of the American Chemical Society

Figure 7. (Top) The pentagonal bipyramidal geometry of H Os(PMe

Crystallographic Investigations on Polyhydride Metal Complexes

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