Bonding Energetics in Organometallic Compounds - American

Chapter 18

Gas-Phase Chemistry of First-Row Transition Metal Ions with Nitrogen-Containing Compounds Theoretical and Experimental Investigations

Downloaded by NORTH CAROLINA STATE UNIV on May 3, 2015 | http://pubs.acs.org Publication Date: June 25, 1990 | doi: 10.1021/bk-1990-0428.ch018

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A. Mavridis , K. Kunze , J. F. Harrison , and J. Allison 1

Chemistry Department, University of Athens, 13a Navarinou Street, Athens 10680 Greece Department of Chemistry, Michigan State University, East Lansing, MI 48824 2

The r i c h gas phase chemistry of f i r s t row t r a n s i t i o n metal (+1) ions with ammonia, and organic compounds (R-X where X=NH , CN, NO ) is discussed. Ongoing theoretical investigations into the Sc /NH system are presented, which provide some insights into the bonding and energetics of a variety of ΜΝΗ+ complexes. 2

2

+

3

x

T h e r e a r e a number o f mass s p e c t r o m e t r i c t e c h n i q u e s t h a t have been d e v e l o p e d f o r t h e study o f t h e low p r e s s u r e g a s phase r e a c t i o n s o f i o n i c s p e c i e s w i t h organic molecules. The e a r l i e s t e x p e r i m e n t s involving ion/molecule reactions involved chemical ionization mass s p e c t r o m e t r y , and t h e most r e c e n t u t i l i z e i o n beam and F o u r i e r t r a n s f o r m i o n c y c l o t r o n r e s o n a n c e m e t h o d s . The earliest s t u d i e s were p r e d o m i n a n t l y organic i n nature. More r e c e n t l y , t h e s e m e t h o d s h a v e b e e n u s e d t o s t u d y o r g a n o m e t a l l i c c h e m i s t r y i n t h e gas phase. The various ionization techniques available i n mass spectrometry a l l o w f o r the g e n e r a t i o n o f unique gas phase s p e c i e s i n c l u d i n g bare t r a n s i t i o n metal ions ( s u c h a s Co*, N i ) and l i g a t e d s p e c i e s ( s u c h a s C o C O , CoNO , N i P F , N i C H ) . Their chemistry with small m o l e c u l e s and a v a r i e t y o f l a r g e r m o l e c u l e s c o n t a i n i n g t h e f u n c t i o n a l groups o f o r g a n i c c h e m i s t r y h a s been e x t e n s i v e l y s t u d i e d i n t h e p a s t 15 y e a r s (1) . The e a r l i e r s t u d i e s were l a r g e l y m e c h a n i s t i c i n n a t u r e , t o g a i n a n u n d e r s t a n d i n g o f how p r o d u c t i o n s w e r e f o r m e d . R e c e n t l y , c o m b i n e d e x p e r i m e n t a l and t h e o r e t i c a l e f f o r t s have provided important insights into how t h e s e reactions occur. These insights will become, we believe, the basis for a fresh evaluation o f the r e a c t i v i t y o f i n o r g a n i c and o r g a n o m e t a l l i c s p e c i e s t h a t a r e s t u d i e d and u t i l i z e d i n c o n d e n s e d p h a s e s . +

+

+

+

3

+

s

s

0097-6156/90/0428-O263$06.00A) © 1990 American Chemical Society In Bonding Energetics in Organometallic Compounds; Marks, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

264

BONDING ENERGETICS IN ORGANOMETALLIC COMPOUNDS

B o t h p o l a r a n d n o n p o l a r o r g a n i c compounds e x h i b i t a r i c h chemistry with bare t r a n s i t i o n metal i o n s ( l ) . S m a l l p o l a r compounds r e a c t w i t h i o n s s u c h a s F e a n d Co , i n a s i n g l e , b i m o l e c u l a r step t o form a metalo l e f i n c o m p l e x , r e a c t i o n (1) . +


Co

+

+ C H X 3

-> C o C H

7

3

+ e

+ HX

(1)

S u c h r e a c t i o n s h a v e b e e n r e p o r t e d f o r X=C1, B r , OH, SH, OR a n d e v e n f o r X=H a n d R ( R = a l k y l s u b s t i t u e n t ) . The mechanism b y w h i c h t h e s e p r o d u c t s a r e f o r m e d was p r o p o s e d b y A l l i s o n a n d R i d g e i n 1 9 7 9 ( 2 ) , a n d i s shown i n r e a c t i o n (2) . The r e a c t a n t s f i r s t f o r m a c o m p l e x (a) , w h i c h may b e s i m p l y e l e c t r o s t a t i c a l l y bound. In (b) , t h e t r a n s i t i o n m e t a l i o n i n s e r t s i n t o t h e p o l a r C-X b o n d ( f o r m a l " o x i d a t i v e a d d i t i o n " ) , f o l l o w e d b y t h e shift of a H Co

+

+ C H X 3

7

(a)

C H X* · -Co 3

7

+

- C H 3

(b)

- Co

7

+

- X

(c)i +

(C H )Co (HX) 3

e

CoC H 3

+ e

(2)

+ HX

atom t h a t i s o n a C w h i c h i s β t o t h e m e t a l (0-H shift), across t h e m e t a l o n t o X, r e s u l t i n g i n t h e degradation o f t h e m o l e c u l e i n t o two s m a l l e r , s t a b l e s p e c i e s t h a t r e s i d e a s l i g a n d s on t h e m e t a l . The l a s t step (d) i s a c o m p e t i t i v e ligand loss. In this p r o c e s s , i t a p p e a r s t h a t t h e l i g a n d t h a t i s more w e a k l y bound t o t h e m e t a l i s p r e f e r e n t i a l l y l o s t (2) . I n some c a s e s , two p r o d u c t s r e s u l t from t h i s d i s s o c i a t i o n . F o r example, C o r e a c t s w i t h p r o p a n o l t o form b o t h CoH 0 and C o C H (2). W h i l e s t u d i e s o f s e r i e s o f , e.g., a l c o h o l s , a n d l a b e l i n g s t u d i e s p r o v i d e d some i n s i g h t s into t h e mechanisms that are operative i n the c h e m i s t r y , many i m p o r t a n t questions are d i f f i c u l t t o approach e x p e r i m e n t a l l y . To u n d e r s t a n d t h e c h e m i s t r y , i t i s c e r t a i n l y important t o understand t h e bonding o f the m o l e c u l e s and fragments i n v o l v e d t o t h e t r a n s i t i o n m e t a l and t h e s t r e n g t h s o f t h e s e bonds. Many f e a t u r e s such as t h e d i s t r i b u t i o n o f t h e charge i n t h e v a r i o u s i n t e r m e d i a t e s may a l s o be i m p o r t a n t i n c o n t r o l l i n g t h e t y p e s o f p r o d u c t s t h a t a r e formed. Thus, t h e o r e t i c a l studies are necessary t o understand the "driving f o r c e s " t h a t dominate t h e s e r e a c t i o n s and l e a d t o t h e r i c h chemistry. The c h e m i s t r y o f p o l a r compounds c o n t a i n i n g many t y p e s o f f u n c t i o n a l groups and m u l t i f u n c t i o n a l o r g a n i c compounds, w i t h b a r e t r a n s i t i o n m e t a l i o n s h a s b e e n +

2

3

e

In Bonding Energetics in Organometallic Compounds; Marks, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

18.

MAVRIDISETAL.

Gas-Phase Chemistry of Transition Metal Ions 265

reported. The r i c h e s t chemistry c e r t a i n l y involves t h o s e f u n c t i o n a l g r o u p s t h a t c o n t a i n n i t r o g e n , and a few e x a m p l e s w i l l be p r o v i d e d h e r e . S t e p n o w s k i and A l l i s o n ( A ) r e p o r t e d t h e chemistry of a v a r i e t y of t r a n s i t i o n metal ions with a s e r i e s of a l k y l cyanides. W h i l e i o n s s u c h as F e and C o insert i n t o t h e C-X b o n d i n many p o l a r compounds, t h e y do n o t a p p e a r t o do s o f o r -X = -CN. I n s t e a d , C-C b o n d s a r e c l e a v e d , a s shown f o r t h e C o / n - p r o p y l n i t r i l e system i n r e a c t i o n s ( 3 ) and ( 4 ) . +

+

+


Co

+

+ n-C H CN 3

7

-

CoCH CN

-

CoC H

+

+ C H

s

2

+ 4

2

(3)

4

+ CH CN

(4)

3

The r e a c t i v i t y o f n i t r i l e s may be d o m i n a t e d by a number o f a s p e c t s o f t h e c h e m i s t r y i n c l u d i n g t h e f a c t that C-CN bond dissociation energy (BDE) is s u b s t a n t i a l l y l a r g e r t h a n t h e BDE's f o r C - C l and C-OH bonds(4). A l s o , t h e r e i s evidence t h a t the geometry of t h e i n i t i a l c o m p l e x h a s a d r a m a t i c e f f e c t - an "end-on" i n t e r a c t i o n of the metal ions w i t h the n i t r i l e group may make i n s e r t i o n i n t o t h e C-CN bond g e o m e t r i c a l l y inaccessible(S). Another i n t e r e s t i n g f u n c t i o n a l group containing n i t r o g e n i s the n i t r o group. The c h e m i s t r y o f C o with a s e r i e s o f n i t r o a l k a n e s has b e e n r e p o r t e d by C a s s a d y et al.(£). Consider the Co / ~ N0 system. Fourteen d i f f e r e n t products were r e p o r t e d . Product i o n s s u c h a s C o ( C H ) and C o ( H N 0 ) s u g g e s t e d t h a t C o i n s e r t e d i n t o t h e C-N0 b o n d . Other product ions such as CoC H 0 and CoNO suggest that RN0 may be c o n v e r t e d , i n p a r t , t o R0- and NO- on t h e m e t a l c e n t e r . T h u s t h e -N0 group a c t i v e l y i n t e r a c t s w i t h the metal. The chemistry of organometallic anions with n i t r o a l k a n e s has a l s o been r e p o r t e d ( 7 ) . The c h e m i s t r y o f NO w i t h t r a n s i t i o n m e t a l i o n s h a s a l s o r e c e i v e d some a t t e n t i o n , and d e s e r v e s comment here. One o f t h e f i r s t ways i n w h i c h C o i o n s were g e n e r a t e d f o r mass s p e c t r o m e t r i c s t u d y was by e l e c t r o n i m p a c t i o n i z a t i o n o f t h e v o l a t i l e compound C o ( C O ) N O . In a d d i t i o n t o Co , i o n s s u c h a s CoCO and CoNO w e r e formed. W h i l e Co and C o C O i o n s f r e q u e n t l y r e a c t w i t h organic molecules in the gas phase, CoNO is unreactive(fi). The r e a s o n f o r t h i s c h a n g e i n c h e m i s t r y u p o n c h a n g e o f l i g a n d was u n c l e a r , s i n c e t h e r e was o n l y " t r a d i t i o n a l " bonding-schemes t o c o n s i d e r a t the time. However, t h e o r e t i c a l s t u d i e s show t h a t t h e CO l i g a n d i s n o t b o n d e d t o f i r s t row t r a n s i t i o n m e t a l i o n s v i a t h e D e w a r - C h a t t m o d e l , b u t i s e l e c t r o s t a t i c a l l y b o u n d (£,£) - t h u s i t ' s p r e s e n c e does not change t h e e l e c t r o n i c s t r u c t u r e of the metal. P r e s u m a b l y NO a c t s a s a 1 o r 3 - e l e c t r o n d o n o r w i t h Co* ( w h i c h h a s a 3 d g r o u n d s t a t e c o n f i g u r a t i o n ) , s o t h e CoNO i o n h a s a s i n g l e u n p a i r e d +

+

n

c

H

3

+

3

7

2

+

e

+

2

2

+

3

+

7

2

2

+

3

+

+

+

+

+

8

+


266


e l e c t r o n on t h e m e t a l , and s h o u l d n o t p a r t i c i p a t e i n r e a c t i o n s t h a t r e q u i r e an i n s e r t i o n s t e p . We a l s o n o t e t h a t t h e NO l i g a n d d o e s a p p e a r t o be r e a c t i v e when bound t o Co . J a c o b s o n r e p o r t e d the observation o f r e a c t i o n (5) i n v o l v i n g CO. +

2

Co NO

+

+ CO

2

-

Co N

+

+ CO

2

(5)

2

T h e r e h a v e b e e n a number o f s t u d i e s reported i n v o l v i n g m e t a l i o n r e a c t i o n s w i t h a m i n e s and ammonia. The f i r s t of these involved Co with a series of a m i n e s , r e p o r t e d by R a d e c k i and A l l i s o n i n 1 9 8 4 ( 1 1 ) . R e a c t i o n s (6-9) were observed f o r t h e Co /n-C H NH system·


+

+

3

Co

+

+ n-C H NH 3

7

2

-

CoC H N

+

-

CoC H N

+

-

CoCH N

-

C H N

3

3

+

+

8

2

(6)

2

+ 2 H

5

+ C H

5

3

+ H

7

7

2

(7)

2

(8)

4

+ CoH

(9)

These r e s u l t s were c e r t a i n l y s u r p r i s i n g since insertion into the C-NH bond was not observed, a l t h o u g h t h e r e a c t i o n (10) 2

Co*

+ C H NH 3

7

-

2

CoC H 3

+

+ NH

e

(10)

3

should be exothermic. I t was proposed that H e l i m i n a t i o n o c c u r r e d by i n i t i a l i n s e r t i o n i n t o t h e N-H bond. While Co d i d n o t i n s e r t i n t o t h e C-NH b o n d o f p r i m a r y amines, i t does appear t o i n s e r t i n t o the C-N b o n d i n t h e t e r t i a r y a m i n e , ( C H ) N , and in allyl a m i n e . To e x p l a i n t h e f a i l u r e t o o b s e r v e r e a c t i o n (10) i t was p r o p o s e d t h a t t h e r e was a b a r r i e r along the r e a c t i o n channel - t h a t the i n s e r t i o n intermediate was energetically inaccessible for thermal energy r e a c t i o n s , w h i c h c o u l d o n l y be due t o a weak C o - N H bond. The BDE w o u l d h a v e t o be u n u s u a l l y s m a l l , < 20 kcal/mol. I t has s i n c e b e e n shown, a s d i s c u s s e d b e l o w , t h a t t h i s i s not the case. A n o t h e r e x p l a n a t i o n may be t h a t Co d o e s r e a c t w i t h , e.g., p r o p y l a m i n e , t o f o r m p r o p e n e and ammonia, b u t t h e r e i s a b a r r i e r t o the d i s s o c i a t i o n s t e p o f ( C H ) C o ( N H ) - w h i c h c a n n o t be d i s t i n g u i s h e d i n t h e mass s p e c t r u m f r o m t h e c o m p l e x containing the i n t a c t molecule, Co (C H NH ). The gas p h a s e c h e m i s t r y o f a number o f m e t a l i o n s w i t h a m i n e s has been s t u d i e d . B a b i n e c and A l l i s o n ( 1 2 . ) r e p o r t e d t h e chemistry o f C r , Mn , Fe , N i , or and Zn with η-propyl a m i n e . O n l y F e c l e a r l y i n s e r t e d i n t o the C-N bond, as e v i d e n c e d by the observation of the two products FeNH and F e C H . Mn (with a h a l f - f i l l e d d-shell, 3d 4s g r o u n d s t a t e ) and Z n (with a f i l l e d 2

+

2

2

5

3

+

2

+

3

e

3

+

3

+

+

+

+

7

2

+

+

+

s

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3

l

+

e

+


18.

MAVRIDISETAL.

Gas-Phase Chemistry of Transition Metal Ions

1 0

267

1

d-shell, 3d 4s ground s t a t e ) were u n r e a c t i v e with p r o p y l amine. In contrast, Cr (3d g r o u n d s t a t e ) and Cu (3d ground s t a t e ) were b o t h o b s e r v e d t o i n d u c e H elimination only. S i q s w o r t h and C a s t l e m a n ( 1 2 ) s t u d i e d the reactions of Ag " and Cu with methyl amine, d i m e t h y l a m i n e and t r i m e t h y l a m i n e . In these studies, h y d r i d e a b s t r a c t i o n was o b s e r v e d t o y i e l d t h e ( a m i n e H) i o n and t h e MH n e u t r a l p r o d u c t . I n 1987, B u c k n e r and F r e i s e r ( H ) showed t h a t Co d o e s f o r m a s t r o n g b o n d t o NH , and d i d s o a s f o l l o w s . I n an FTMS i n s t r u m e n t t h e y f o r m e d CoOH , w h i c h r e a c t s w i t h ammonia, r e a c t i o n ( 1 1 ) . +

+

5

1 0

2

9

+

+

+

2


+

CoOH

+

+ NH

-

3

CoNH

+ 2

+ H0

(11)

2

They also found that the NH group can be d i s p l a c e d f r o m Co* by b e n z e n e , b u t n o t by a c e t o n i t r i l e . W i t h t h i s i n f o r m a t i o n , t h e y c o n c l u d e d t h a t t h e BDE f o r Co -NH i s 65 ± 8 k c a l / m o l . I n t h i s same w o r k t h e y reported the chemistry of MNH w i t h some h y d r o c a r b o n s . They a l s o f o u n d t h a t F e O r e a c t s w i t h ammonia t o f o r m F e N H , and reported some c h e m i s t r y for this ionic species. T h e i r w o r k s u g g e s t e d t h a t t h e BDE f o r FeNH was > 41 k c a l / m o l and < 81 k c a l / m o l . T h i s w o r k was e x t e n d e d i n a r e p o r t by B u c k n e r , G o r d and F r e i s e r i n 1988 (IS). They f o u n d t h a t a v a r i e t y o f m e t a l ions including Sc , Ti and V r e a c t w i t h ammonia i n an e x o t h e r m i c p r o c e s s by r e a c t i o n ( 1 2 ) , w h i c h s u g g e s t s t h a t t h e s e M -NH b o n d s a r e s t r o n g e r t h a n 93 kcal/mol. The BDE f o r V -NH was d e t e r m i n e d t o be 101 ±7 k c a l / m o l , and t h e BDE f o r F e - N H t o be l o w e r , 54 ± 14 k c a l / m o l . 2

2

+

2

+

+

+

+

+

+

+

+

M

+

+ NH

-

3

MNH

+

+ H

(12)

2

+

They a l s o r e p o r t e d a number o f r e a c t i o n s f o r MNH w i t h neutral molecules. A v a r i e t y of mechanisms for r e a c t i o n (12) was d i s c u s s e d , i n c l u d i n g d e h y d r o g e n a t i o n v i a d i r e c t d e c o m p o s i t i o n o f t h e i n t e r m e d i a t e s (H) M -NH a n d / o r H-M -NH . 2

+

2

The R e a c t i o n o f Sc+ w i t h NH Our i n t e n t i s t o g a i n t h e o r e t i c a l i n s i g h t s i n t o t h e i n i t i a l i n t e r a c t i o n of M with a p o l a r molecule, the insertion step and reductive elimination by a systematic study of the i n t e r a c t i o n of M w i t h NH . We begin here with S c b e c a u s e i t h a s o n l y two valence e l e c t r o n s and t h u s i t s t h e o r e t i c a l d e s c r i p t i o n and i t s c a l c u l a t e d i n t e r a c t i o n w i t h NH and i t s f r a g m e n t s w i l l be more r e l i a b l e than with the latter transition elements. We w i l l f i r s t d e s c r i b e t h e S c N core and then b u i l d up the various intermediates by adding hydrogens. S p e c i f i c f r a g m e n t s t o be s t u d i e d i n c l u d e ScNH , S c N H , HScNH and S c N H * . The c a l c u l a t i o n s w i l l i n c l u d e a l l e l e c t r o n s and w i l l u s e b o t h MCSCF and 3

+

+

3

+

3

+

+

+

2

+

2


268


CI t e c h n i q u e s . The b a s i s s e t s used a n d the approach have been d e s c r i b e d previously(16).

general

+

S£H . The r e l a t i v e e n e r g i e s o f t h e l o w l y i n g s t a t e s o f S c , Ν, NH and N H a r e c o l l e c t e d i n F i g u r e 1. B e c a u s e t h e e x c i t e d s t a t e s o f Ν a r e much h i g h e r i n e n e r g y t h a n t h o s e o f S c we w i l l c o n s i d e r t h e s t a t e s o f ScN which c o r r e l a t e t o the ground s t a t e o f Ν and the s d and d configuration of Sc (1£). As S c i n an s d c o n f i g u r a t i o n a p p r o a c h e s Ν we c a n i m a g i n e f o r m i n g a σ bond b e t w e e n t h e 4 s e l e c t r o n o n S c and a 2p σ e l e c t r o n on Ν and a π bond b e t w e e n t h e 3 d and 2 ρ e l e c t r o n s . This leaves an unpaired e l e c t r o n on Ν i n t h e 2 p o r b i t a l and r e s u l t s i n a Π s t a t e r e p r e s e n t e d b y 2

+

2

+

+


+

x z

χ

2

I t i s o n l y a t l a r g e ScN s e p a r a t i o n s t h a t the Sc e l e c t r o n i n t h e a bond i s p u r e 4 s . A s t h e bond f o r m s o t h e r o r b i t a l s o n S c o f a symmetry mix i n t o t h e b o n d i n g orbital. T h i s i s s e e n most v i v i d l y i n F i g u r e 2 a w h i c h shows t h e o c c u p a n c i e s o f v a r i o u s a t o m i c o r b i t a l s i n t h e Π state o f ScN as a function o f separation. At e q u i l i b r i u m t h e a t o m i c o r b i t a l s i n t h e σ bond have t h e occupancy 2

+

(4s°-° 4 p / - °

3d

a

°-

e e

)

(2

S c

l P < 7

-

2 8

)

N

and t h e 4 s o r b i t a l h a s gone f r o m a n o c c u p a n c y o f 1 t o 0. I n t e r e s t i n g l y t h e π bond occupancy i s e q u a l t o t h a t o f t h e a bond < xz°- >Sc< Px ' >N 3d

ei

l

2

while t h e unpaired l o c a l i z e d o n N.

2e

electron

remains

essentially

Pdy^-^JscUPy *' )» 0

1

+

The n e t r e s u l t i s t h a t S c l o o s e s 0.49 e l e c t r o n s t o Ν r e s u l t i n g i n the charge d i s t r i b u t i o n + 1

*

6 1

Sc

Ν

- 0

·

4 β

.

I f we b r e a k t h e η bond i n t h i s s t a t e and p l a c e t h e h i g h s p i n e l e c t r o n o n S c i n a d i o r b i t a l we w i l l f o r m a Δ^. s t a t e . +

4


y

18.

Gas-Phase Chemistry of Transition Metal Ions 269

MAVRIDIS ET A L

60

2p ( D) 3

2

(55)

50 _ 40 ο

(36)

σ π 2

2

(*Δ)

σπ

2

( A ) 2

t


s

(32)

^ 30 « ο UJ 20

Bonding Energetics in Organometallic Compounds - American

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