26 Borane Anion Ligands—New Bonding
Inorganic Compounds with Unusual Properties Downloaded from pubs.acs.org by UNIV OF BATH on 07/03/16. For personal use only.
Combinations with Metals DONALD F. GAINES, MARK B. FISCHER, STEVEN J. HILDEBRANDT, JEFFREY A. ULMAN, and JOHN W. LOTT University of Wisconsin, Madison Wis. 53706
This paper discusses new types of borane-transition metal complexes (metalloboranes). The borane anions investigated include B H -, B H -, and B H 4-; typical metalloborane complexes are(CO) MnB H ,(CO) ReB H ,(π-C H )(CO) FeB H ,(CO) MnB H ,and salts of (CO) MnB H Sig nificant variations in the bonding between the borane ligands and the transition metal in these complexes appear to depend largely on the size of the borane ligand. Low metal oxida tion states are favored, and the most stable complexes result when the borane functions as a tridentate ligand. Chemical studies have revealed, however, that reversible bidentate -tridentate borane ligand functionality is possible in some cases. X-ray and spectroscopic studies indicate that most of these metalloboranes obey conventional polyhedral skele tal electron-counting schemes although there are several interesting variations. 3
8
5
8
9
3
3
1
8
4
3
8
5
5
2
13-.
5
8
3
8
13
3
9
T i T e t a l c o m p l e x e s c o n t a i n i n g b o r a n e or b o r a n e a n i o n l i g a n d s h a v e o f t e n b e e n r e f e r r e d to as m e t a l l o b o r a n e s , e s p e c i a l l y w h e n the m e t a l c a n be c o n s i d e r e d fragment.
as o c c u p y i n g
a boron position i n a borane polyhedral
O u t l i n e d here are some of o u r recent e x p e r i m e n t a l
findings
i n the a r e a of m e t a l l o b o r a n e c h e m i s t r y . M u c h of t h e e a r l i e r w o r k w a s r e v i e w e d elsewhere BH~ 3
8
(1,2,3).
Complexes T h e b i d e n t a t e f u n c t i o n a l i t y of the B H " a n i o n has b e e n w e l l estab 3
8
l i s h e d f o r the c h r o m i u m g r o u p c o m p l e x e s [ ( C O ) M B H ] " ( M — C r , 4
311
3
8
312 Mo,
INORGANIC
W)
(4),
extended
COMPOUNDS WITH
a n d for the c o p p e r
complex
UNUSUAL PROPERTIES
(Ph P) CuB H 3
2
3
(5).
8
We
the c h r o m i u m w o r k b y p r e p a r i n g the i s o e l e c t r o n i c n e u t r a l
manganese g r o u p complexes ( C O ) M B H 4
3
8
( Μ — M n , R e ) as w e l l as a n
i r o n d e r i v a t i v e h - C p ( C O ) F e B H . T h e s e n e w species are p r e p a r e d b y 5
3
8
the g e n e r a l s u b s t i t u t i o n r e a c t i o n w h i c h is i l l u s t r a t e d b y R e a c t i o n 1
(6)
for the m a n g a n e s e c o m p l e x . (CO) MnBr + M e N B H ( C O ) M n B H 5
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The M-B H 3
8
4
B and Ή
n
3
8
4
3
8
+ CO + Me NBr
(1)
4
N M R spectra of these complexes i n d i c a t e t h a t the
b o n d i n g a n d g e o m e t r y are analogous to those e s t a b l i s h e d b y
X - r a y studies of t h e c h r o m i u m a n d c o p p e r complexes.
The
proposed
structure of ( C O ) M n B H , 1, illustrates the g e n e r a l s t r u c t u r a l features 4
3
8
Structure 1 t h a t a p p e a r to c h a r a c t e r i z e a l l the k n o w n b i d e n t a t e B H " 3
A v e r y i n t e r e s t i n g p r o p e r t y of ( C O ) M n B H 4
3
of a c a r b o n y l g r o u p to f o r m the n e u t r a l ( C O ) M n B H 3
(CO) MnB H 4
3
8
3
U V or Δ *± (CO) MnB H CO 3
complexes.
8
is its r e v e r s i b l e loss
8
3
8
(Reaction
2).
(2)
8
T h i s r e a c t i o n illustrates w h a t appears to b e the first e x a m p l e of r e v e r s i b l e bidentate-tridentate borane l i g a n d functionality. Spectroscopic (Ή
and
n
B N M R , a n d I R ) of ( C O ) M n B H 3
3
8
studies
i n d i c a t e t h a t the B H " 3
a n i o n f u n c t i o n s as a t r i d e n t a t e l i g a n d b o u n d to the m e t a l b y Mn-H-B
bridge hydrogen bonds
(one
from each boron).
s t r u c t u r e p r o p o s e d f o r this c o m p l e x is S t r u c t u r e 2. (CO) MnB H 3
3
8
The Ή
8
three
T h e static N M R of
s p e c t r u m consists of t w o resonances, a h i g h field g r o u p
of area three t h a t i n d i c a t e s three M n - H - B
protons, a n d a b r o a d
low
Inorganic Compounds with Unusual Properties Downloaded from pubs.acs.org by UNIV OF BATH on 07/03/16. For personal use only.
26.
Borane Anion Ligands
GAINES E T A L .
313
Structure 2 field g r o u p of area five t h a t i n d i c a t e s a n i n t r a m o l e c u l a r e x c h a n g e b e t w e e n B H a n d B H B protons.
U p o n decouplng
c o m e s h a r p singlets. T h e
n
B , these t w o resonances
be
B N M R s p e c t r u m consists of a s i n g l e reso
n
n a n c e w h i c h has t e m p e r a t u r e - d e p e n d e n t fine structure. U p o n d e c o u p l i n g a l l Ή , a v e r y n a r r o w resonance is o b s e r v e d w h i c h is i n d i c a t i v e of the m a g n e t i c e q u i v a l e n c e of a l l three b o r o n
atoms.
Selective decoupling
Μ η - ί ί - Β alone d e c o u p l e d ) c o m b i n e d w i t h F o u r i e r t r a n s f o r m l i n e
(e.g.
narrowing techniques (7)
gives f u r t h e r c r e d e n c e to o u r p o s t u l a t e t h a t
the h y d r o g e n s t h a t are n o t i n v o l v e d i n M n - H - B b o n d i n g are i n v o l v e d i n r a p i d t a u t o m e r i s m a r o u n d the p e r i p h e r y of the B t r i a n g l e . 3
BH~ 5
8
Ligands
It has b e e n f o u n d t h a t the s q u a r e p y r a m i d a l B H " l i g a n d acts as a 5
monodentate ligand.
8
I n a l l cases r e p o r t e d to d a t e , the l i g a n d is b o u n d
to the m e t a l b y a s i g m a b o n d i n v o l v i n g a b o r o n a t o m i n the base of t h e B
5
p y r a m i d [labeled B ( 2 ) ] .
T h e first e x a m p l e s w e r e p r e p a r e d b y n u c l e o -
p h i l i c d i s p l a c e m e n t of a h a l o g e n f r o m a h a l o p e n t a b o r a n e ( 9 ) carbonyl metallate anion (Reaction 3) ( 8 ) . 1- or 2 - C l B H 5
+
8
by a penta-
T h e isolated products always
NaM(CO) -» 2-[M(CO) ]B H ( M = M n or R e ) 5
5
5
8
+
NaCl
h a v e the B - M b o n d at the 2 - p o s i t i o n o n the base of the B
5
(3)
pyramid
regardless of the p o s i t i o n of the B - C l b o n d i n the s t a r t i n g m a t e r i a l . M o r e recently it was discovered that B H 5
to
IrCl(CO)(PMe ) 3
2
to
produce
9
and B r B H
2 - [ I r B r ( C O ) ( P M e ) ] B H , respectively (9). 2
3
2
5
8
5
8
add oxidatively
2-[IrClH(CO)(PMe ) ]B H 3
2
5
8
and
W e have discovered
a
314
INORGANIC
COMPOUNDS
WITH
UNUSUAL
PROPERTIES
t h i r d r e a c t i o n v a r i a t i o n , n a m e l y n u c l e o p h i l i c s u b s t i t u t i o n of the anion
for
a
halogen
in h - C p F e ( C O ) X 5
that p r o d u c e s
2
B H ~ 5
8
2-[/i -CpFe5
(CO) ]B H . 2
5
8
R e a c t i o n of the B H " a n i o n w i t h F e ( C O ) X 5
4
8
a m i x t u r e of p r o d u c t s
that p r o b a b l y contains
(X =
2
B r , I) produces
(CO) FeB H 3
4
(perhaps
8
t w o isomers ) a n d a c o m p l e x that is t e n t a t i v e l y i d e n t i f i e d as ( C O ) F e B H 9 . 3
It w a s d e m o n s t r a t e d
p r e v i o u s l y that r e a c t i o n of
p r o d u c e s the a p i c a l isomer of ( C O ) F e B H
8
the B H ~ a n i o n
5
3
5
8
ft -CpCoB H , Inorganic Compounds with Unusual Properties Downloaded from pubs.acs.org by UNIV OF BATH on 07/03/16. For personal use only.
5
8
Cp)-5-CoB H 0
BH ~ 8
CoCl
1 3
4
and N a C H
2
5
l,2-(ft -CpCo) B H
5
4
with
2
4
B H 5
9
with
5
Fe(CO)
5
( J O ) a n d t h a t r e a c t i o n of produces two
isomers
of
( a n d derivatives thereof) a n d 5 - ( h 5
c
(II).
Ligand
1S
T h e o n l y k n o w n c o m p l e x of the t r i d e n t a t e B H 8
cently
described
(CO) MnB Hi 3
8
3
1 3
" l i g a n d is the r e
w h o s e structure, as d e t e r m i n e d
X - r a y , is d e p i c t e d i n S t r u c t u r e 3 ( 1 2 ) .
The
n
B and Ή
by
N M R spectra
Structure 3 of this c o m p l e x i n d i c a t e the presence of three M n - H - B b r i d g e
hydrogen
atoms, b u t the d a t a do not d i s t i n g u i s h b e t w e e n a structure that i n c o r p o rates the M n a t o m i n the p o l y h e d r a l cage a n d the alternate c o r r e c t s t r u c ture, s h o w n i n 3, i n w h i c h the m e t a l is n o t i n c o r p o r a t e d i n t o the b o r a n e p o l y h e d r a l f r a m e w o r k b u t r a t h e r is b o n d e d to a t r i a n g u l a r b o r o n via three M n - H - B b r i d g e h y d r o g e n b o n d s . A B H i " 8
2
2
face
complex, ( E t P ) 3
2
P t B H i , has b e e n r e p o r t e d , a n d a l t h o u g h its structure is u n k n o w n , the 8
2
P t is p r o b a b l y i n c o r p o r a t e d i n t o the p o l y h e d r a l f r a m e w o r k of the b i - o r tridentate B
8
ligand
(13).
26.
GAINES
BH " 9
1S
ET
AL.
Borane
Anion
315
Ligands
Ligands
2
T h e s e complexes are p r e p a r e d b y s u b s t i t u t i o n reactions b e t w e e n t h e B H 9
1 4
" anion a n d metal carbonyl halides (Reaction 4)
(14).
Oxidation
R 0 2
B H 9
1 4
- + BrMn(CO)
6-(CO) -6-MnB H
5
3
9
1 3
~ + 2CO + H B r
of these complexes i n the presence of a p p r o p r i a t e L e w i s bases
(4)
produces
complexes of the l i g a n d B H i L ~ ( R e a c t i o n 5 ) . 2
Inorganic Compounds with Unusual Properties Downloaded from pubs.acs.org by UNIV OF BATH on 07/03/16. For personal use only.
9
6-(CO) -6-MnB H 3
9
1 3
- + HgCl
2-THF-6-(CO) -6-MnB H 3
9
+ T H F ->
2
+ H g + HC1 + C l "
1 2
T h e b o n d i n g of the t r i d e n t a t e B H i " 9
3
(5)
l i g a n d a n d its d e r i v a t i v e s ,
2
B 9 H 1 2 I / , to m a n g a n e s e g r o u p metals is i l l u s t r a t e d b y t h e x - r a y d e t e r 1
m i n e d s t r u c t u r e of 2 - T H F - 6 - ( C O ) - 6 - M n B H 3
9
1 2
, Structure 4 (15).
In all
Structure 4 of these complexes, the B H i " or B H i L 2
9
3
2
9
_ 1
l i g a n d s are t r i d e n t a t e a n d are
b o u n d to the m e t a l via t w o M n - H - B b r i d g e h y d r o g e n b o n d s a n d a B - M n s i g m a - t y p e b o n d . T h e m e t a l c a n b e c o n s i d e r e d as s u b s t i t u t i n g for a b o r o n p o s i t i o n i n a b o r a n e f r a m e w o r k , w h i c h differs f r o m t h e case of t h e t r i dentate B H i 8
_ 1 3
l i g a n d . A t t e m p t s to d i s p l a c e the T H F f r o m t h e c o m p l e x
p r o d u c e d i n R e a c t i o n 5 b y u s i n g E t N r e s u l t i n cleavage of the T H F at 3
a n a l p h a c a r b o n a t o m f o l l o w e d b y a c o m p l e x i n t e r n a l r e a r r a n g e m e n t to a
zwitterion,
1 0 - [ E t N ( C H ) O ] - 6 - ( C O ) - 6 - M n B H i , whose 3
2
4
3
d e t e r m i n e d s t r u c t u r e is d e p i c t e d i n S t r u c t u r e 5
9
(16).
2
x-ray-
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316
INORGANIC
COMPOUNDS
WITH
UNUSUAL
PROPERTIES
Structure 5 Skeletal Electron Counting in Metalloboranes T h e general polyhedral electron-counting 21, 22),
schemes (17,
18, 19,
t h a t w e r e d e v e l o p e d l a r g e l y b y W a d e , a l l o w one to
20,
correlate
g e n e r a l s t r u c t u r e types for m e t a l l o b o r a n e p o l y h e d r a a n d t h e i r fragments a n d to p r e d i c t p r o b a b l e f o r m u l a t i o n s a n d s t r u c t u r e types for other m e t a l l o b o r a n e systems that are as yet u n k n o w n .
T h e s k e l e t a l e l e c t r o n counts
for a n u m b e r of c h a r a c t e r i z e d m e t a l l o b o r a n e s a n d c o r r e s p o n d i n g
boranes
are t a b u l a t e d i n T a b l e I. T h e starred examples i n T a b l e I w a r r a n t s p e c i a l c o m m e n t . T h e structure of M n ( C O ) i o ( B H 3 ) H resembles t h a t of B H i 3
in which B T a b l e I. Structure Type Close Nido
2
and B
4
2
S k e l e t a l E l e c t r o n C o u n t i n g i n Boranes a n d M e t a l l o b o r a n e s Number of Vertices, η 6 4 5 6 9 10
Skeletal Electron Pairs
Compounds'
1
B H " , (7r-CpCo) B H (CO) MnB H ,B H (CO) FeB H ,7r-CpCoB H , Β Η (CO) FeB H ,B H (Et P).>PtB H ,B H 5-(7r-Cp)CoB H ,6-(CO) MnB H L ( L = E t 0 , T H F , Ο ( C H ) N E t , or H " ) (CO) MnB H , (COhCrB^" , 7 r - C p ( C O ) F e B H , (
