Multinuclear magnetic resonance study and spin-spin couplings · 2019. 9. 7. · and spin-spin couplings Armin Dorr, Dietrich Gudat" I Dieter Hiinssgen, Heribert Hens, Edith Stahlhut

Page 1

Bull Soc Chim Fr ( 1994) 131 , 674-682 © Elsevier, Paris

Multinuclear magnetic resonance study of sterically crowded stannylphosphines and stannylamines

stereochemical influences on chemical shielding and spin-spin couplings

Armin Dorr, Dietrich Gudat" I Dieter Hiinssgen, Heribert Hens, Edith Stahlhut

Anorgam.sch Chemuche.s /ruhhd der Untver.sitiit Bonn, Gerhard DomQgk Str I, D-59121 Bonn, Gennanll

(roceived 25 November 1993, accepted 31 March 1994)

Summary - Multinuclear (I H, I~N, 29Si, 31 P, 119Sn) NMR datn of steric:a1ly crowded acyclic: 6tannylphosphincs tBu3SnPilY (Y = H, SnMe3, SntBu3 , Sit..·!e3: 1-3, 10) , (tBu2RSn),PH (R = Me, CI; 4, 5), tBu3SnPY2 (Y = 5nMe3 , SiMe3: 6,11), PH(SntBu,PHSntBU3h (7) , SntBu2(PY2), (Y :::: H, SiMe" PHSntBllJ; 8, 9, 12), cyclic stan nylphosphincs (tBu2SnPY ) .. (n = 2, Y = H , C3HaCI, tBu , SnMe, : 13-16 : n = 3, Y = H; 18), ( MC"lSnPSntBu3h (17), and litannllamines tBul SnNI1Y (Y = H, SnMe3, SntBu3 : 19-21) were obtained by various 10- and 2D-techniqucs. IIp_ and 1 N-shicldings may be explained qualitatively in terms of t ..... o counteracting inftuenccs, vu electronegativity differences and steric requ irements of the substituents. In a similar manner, the trends in one-bond coupling I KSnP and I KSn!>! mny be rationalized using a simple model based on the deformation or bond angles by sterically demanding substituents. The signs of long-range couplings 2 KSnPH and I KPSnCCti could be determined, which may be userul for future structural studies. Temperature-dependent effects in the spectra of 13, 18 allow concl usions about the confonnational dynamics of the molecules.

tin. phosphorus compounds I tin-nitrogen eompounds I J I P chemical shins I 15N cnemical shifts I II!lSn chemical snifts I trends In I/(SnP and I K SnN I I I!lSnlI!lSn and 119Sn31 P long-range couplings I phosphorus inversion

Introduction

Organotin compounds with direct bonds between tin and the group 15 clements phosphorus and nitrogen have attracted considerable interest because of the intriguing rear.tivity of the tin-element bond , which makes these derivatives useful synthetic intermedi­ates and st arting materials in o rganic and element­organic chemistry [I]. In addition to the application of NMR as an anaJytical tool to determine constitu­tion or purity, informat ion about the nature of the tin­nitrogen bond was gained from analysis of trends in chemicaJ shifts and couplings for various tin-nitrogen compounds [2]. Even if the NMR investigation of tin­phosphorus compounds is much easier, systematic stud­ies have been confined to trimethylstannylphosphines (Me,Sn).PR,_. (R ~ H, alkyl, Ph) 12-41. Recently, by following a fa miliar concept in element orga nic chem­istry, as yet unknown structural types of Slannylphos­phines with tin-phosphorus chain or ring structures, respectively, were made accessible via kinetic stabiliza­tion by ster ically demanding ·Sn(tBuh- and -Sn(tBuh groups [5.8]. Initia l investigations of the chemical re­activities of these compounds [7, 8J gave evidence for a promising synthetic potentiaJ due to the presence of

• Correspondence and reprints

highly reactivc phosphorus-hydrogcn , phosphorus-tin, or phosphorus-silicon bonds.

In this work, a systematic multinuclear (' H, 31 P, 1198n, '295 i, 15N) NMR study of t hese novel, stericnlly crowded stannylphosphincs and some related stannyl­amines is presented. The dnta.. give evidence fur a marked influence of s tcrically induced bond deforma­tions and dynamic processes (pyramida l invers ion at phosphorus) o n the NMR parameters, thus en(1bling 11

qualitative discussion of trends in structure and bond­ing.

Experimental section

Compounds I, 15 151, 2·4, 6-8, 14, 16· 18 19[, 5 [81, 9, 11, 12 [7[, 10110[,1316], 19 \111 , and 20, 21 1121 were prepared following literature proced ures. N~"R spectra were recorded on Varian FT80 A (lip) and Bruker AMX 300 spectrometers (I H, I!>N, 29Si, lip, 119Sn) equipped with multinuclear units. Samples were measured in C60a (5-25% solut ions) in 5 mm o.d. tubes at 30"e if not stated other­wise. The spectra of 13 were recorded at IODC and thooc of 14, 18 at 70"C, respectively, in order to reduce dynamic broadening effects. Chemical shifts tSlH 16I H(C6D.5 H 7.15)1 and 629Si (=:205i :::: 19.867184 MHz) arc given relative to

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675

n n

., .. .J)) .,.,

6 lip (ppm)

Fig 1. Double quantum filtered 121.5 MHz lip {'H}-spectrum of (tBu2SnPH)J, 18, at 70"C (760 scans ; spectral width = 3300 Hz; 16 K data points; intcrpulse delays of 3 s; data processing with 3 Hz exponential line broadening) . The presence of magnetically active 117{II'Sn nuclei removes the magnetic equivalence of the 31 P nuclei, so that the satellite spectrum may be observed. Since the parent line is suppressed by the double quantum filter , the splitting due to the 3)

(PSn) coupling becomes visible.

external MI!.jSi ; 6u N (=:lsN = 10.136767 MHz) relative to external ncat MeNO, ; 631 P (:::31 P = 40.480747 MHz) rel­ative to external 85% H3P04 ; 6"9Sn (::: 1I95n==-37.290665 MHz) relative to external Me..,Sn . Heteronuclear cou­plings were obtained from 'H_(nJpH. " JSnll) or 31p {'H}-spectra (nJsnp) . 2Jpp betv.-een chemically equiva­lent nuclei was extracted from 11 9Sn_ or 29Si_satellites in normal or double quantum filtered 31p eH}-spectra. (fig I) , or by analysis of the AA'XX'-pattern of the PH-resonance (l31. 'JpP and 3JpH of 9 were calculated from the I P e H}-spectra of a mixture of isotopomers H.D2_~PSn(tBuhPHyD2 _ ~ {X,lI = ()'2) prepared by partial deuterolysis (C0300/H,O) [71 of 12. in order to facilitate the discussion of different hetero-

nuclear couplings, the values of the reduced coupling con­stants KAB = 411"' /( h"a'Yb) JAB are aJso included where ap­propriate. All J values are given in Hz ; reduced couplings arc a.ctuaUy presented as K x 10- 19 and arc given in 51 units (N A - 'm-3). mSn_ and 29Si_NMR spectra of phos­phine derivatives were in generaJ recorded using the OEPT­~uence based on 3 J sncCH(62-9S Hz}, ' J SnCH (50-55 Hz) , or JS;CH (&-6 Hz), yielding the appropriate chemical s hifts together with nJsnP, IJps;, 'JllvSnll1Sn and 2JsnSi (fig 2) . The values for' JIIVSnllTSn were converted into 2 JIIDSnIIOSn ; the accuracy of the couplings is ± l Hz for I JsnP and ±O.2 Hz in the other cases. in the case of 5, 14, 18, where line broad­ening resulting from dynamic effects or partially relaxed

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676

c c • C C • •

J I

50 .. " )0

'19 5 Sn (ppm)

Fig 2. Vertical expansion of the 111.9 MHz IIIISn {IH}_OEPT spectrum of t-Su3SnP(SnMelh, 6 (256 scans; spectral width = 62500 Hz ; 64 K data points ; inlerpulse delays of 4 s; defocusing delay 7 ms, 10" read pulse: data processing with zero filling to 128 K and gauss filtering) . Peaks marked with an asterisk are due to an impurity ; (C) denote 13C satellites. lI1/III1Sn satellites due to coupling between chemically non-equivalent tin nuclei are present on both signals. The II11S0 satellites show a characteristic phase dis&.ortion; the &Symmetric appearance results from their natufe of AB-type spectra. The SnMel re:sonance (upfield doublet) exhibits an additional single pair of satellites arising from the isotopomer tBu,Snp(IUISnMe3WI7SnMel) where the magnetic: equivalence of the SoMel groups is removed due to the different isotopic labelling.

couplings to quadrur,::lar 35/37 C)-nuclei occurred, the data were obtained (rom H-detected 2D-shi(t correlations, with couplings being accurate to ±IO Hz. In the same way, I~N NMR data in natural abundance were extracted (rom IH-detected I H/ 15 N-correiation experiments; in addition, I JS"N of 21 was obtained from the UIISn-spectrum under suppression of the signal of the uN-i.sotopomers with the DEPT-sequence 1131 .

Results and discussion

Relevant NMR data (1 H ' SN 29Si 31 P 1I9Sn) .,e given , , , , in tables I-III (acyclic stannylphosphines), IV (cyclic stannylphosphines), and V (stannylamines). The val­ues of coupling constants are generally shown without a sign. In those cases where signs are explicitly included, their determination is based on the extraction o( relative

signs of reduced couplings from analysis of 2D-spectra (fig 3) or higher order multiplets. The assignment of ab­solute signs is based on the following "key couplings" : 'KSnP < 0 14,141; 1KSnN < 0 [2, 14, 151; IKsiP < 0 1141 ; 2KsnCH < 0 [141; and 3KSnCcn > 0 [14J.

Chemico.l shifts

The 31 P and ISN resonances of acyclic stannylphos­phines 1-6 and stannylamines 19-21 appear at higher field than the parent hydrogen derivatives EH3 (E = N, P) and further display a marked increase with the number of stannyl groups attached to the phos­phorus or nitrogen atom, respectively. The same ef­fects are known for the derivatives (Mc3Sn)nP~_n [2-4], and their origin has been related to a low degree of

Page 4

Tab

le I

. N

MR

dat

il of

acy

clic

sta

nnyl

mon

opho

sphi

nes

H ..

P(S

nR,R

')m{S

nR

'R"'

h_n_

m'

611 P

6119 Sn

IJ

lloS"

up"

2 JIIO

S n"0

S n"

~'fI

(P

H)

1 Jpu

" 'J

IIO

SnU"

1 tB

ulSn

PH2

-304

.4

29.3

62

6 (3

43)

0.6

171.

3 (3

5.18

) 42

.2 (

9.37

)

2 tB

usSn

A(M

c3Sn

B)P

H

-316

38

.7 [

SnA

[ 89

6 (4

91)

388

(230

) nd

16

3.3

(33.

54)

nd

23.0

1Sn

BI

755

(414

)

3 (tBu~SnMehPH

-339

.6

62.1

87

5 (4

79)

376

(223

) 0.

36

161.

0 (3

3.07

) 41

.7 (

9.26

)

4 (t

Bu3

Snh

PII

-325

.4

42.0

10

44 (

572

.0)

515

(305

) 0

.30

171

(35

.1)

31.6

(7.

02)

5 (t

Bu2

SnC

lhP

H

-295

.8

152.

1 11

19 (

613.

1)

236

(140

) 1.3

1 16

1.4

(33.

15)

45.6

(10

.1)

6 tB

u3Sn

A(M

elSn

BhP

-3

26

.8

46.8

1SnA

I lI

SO

(630

.1)

351

(208

) rl

JSnA

SnBI

32

.1 IS

nB]

915

(501

) 31

2 (1

85)

[2J Sn

Bsn

BI

" re

duce

d co

uplin

gs K

(in

uni

ts o

f 10

1l~ N

A -2

m-l

) in

par

enth

eses

.

Tab

le I

I. N

MR

dat

a of

acy

clic

sta

nnyl

olig

opho

sphi

nes.

6ll p

2J

pp"

61111 5n

'J

" I"Sn

!lp

'J" ,I

I S,,'

lp

'J" II

OSftll

'Sn

~'H

2JII

'S"I

II"

J!lP

III

7 tB

u3Sn

Ap8

HSn

BtB

u2

-29

8.7

[pa)

7.

9 (4

.0)

41.2

[S

nAI

1022

(56

0.0)

2

.9 (

1.5)

SO

l (2

97)

0.78

[p

aHI

31.8

(7.

06)

169

[I Jp

AH

I

I [p

ap

bl

IP8 Sn

AI

[paS

nB]

ISnA

SnB

] 37

.2 (

8.26

) 16

6 [I

Jpbl

ll

pb

H

I -2

72

.5 [

pbl

3.2

(1.6

) 11

7.1

[SnB

I 1

107

(606

.5)

4.0

(2.

2)

489

(289

) 1.

21 [

pbH

I 37

.3 (

8.28

) 0.

8 [3

Jpllu

l tB

u3Sn

Ap

8 HSn

BtB

u2

[. J p

8paj

[p

llSnB

I [p

bSnA

I [S

nBS

nBI

0.9

[3Jp

blll

1074

(58

8.4)

[p

bS

nBI

8 tB

U:!S

nA-P

H

-298

.8

7.8

(4.0

) 41

.1 [

SnA

I +

10

24

(-

661.

0)

4.0

(2.2

) 53

3 (3

16)

0.8

6 37

.3 (

8.28

) 16

9 II

J pH

]

I 11

9.6

ISnB

I +1

113

(-60

9.8)

IS

nA

HI

SnB

tBu2

+0

.5Il JP

H]

I 31

.6 (

7.02

)

tBu3

SnA

-PH

IS

nB

HI

0 tB

u2Sn

(PH

2)2

-28

8.1

1.1

(0.5

6)

74.9

65

0 (3

56)

nd

174 ~

hil

i 2.

6 I J

Pltj

.. re

duce

d co

uplin

gs K

(In

uni

ts o

f 10

19 N

A -2

m-3

) in

par

enth

eses

.

'" .. ..

Page 5

0>

OJ

Tab

le I

II.

NM

R d

ata

of s

tnn

nyl-

sily

l-ph

osrh

ines

.

,._--

---

._---

-----

--..

.. -_ .

.. _-

/iJlp

611~Sn

IJll~Sn~II'B

1I1!JS

i J)

. 1""S

i '2 J.

\OS

nU

Si

oth

cn;"

10

tB

u3S

nPH

Sir..·

lcl

-276

.9

25.6

85

5 ('

G8

) 5

.15

:16.

6 37

.5

61

11 0

.533

[P

ili

lJ3

'1"

1I

IMIA

, 'J

uu

Sn

ll 3

1.0

, '}

SII

I 5,

2

II

tBu

JS

nP{S

iMc3

h

-27

1.<1

30

.1

102

8 (5

63.2

) '1.

65

38."

27

.9

12

tBu2

Sn

[p(S

ir· .. 1c

3hI2

-

152.

2 95

.1

l02!

l (5

03.8

) '1.

56

J7.7

26

.3

111'

1' 3

8.2

-3

.2 [J

J1'

5.1

--

--

---

-_ ..

--..

redu

ced

cou

pli

np

K (

in u

nils

of

to!!1

N A

-2 m

-J)

in p

nrt'n

lhcs

cs.

Tab

le I

V.

NM

R d

ata

of c

ycli

c st

anny

lpho

sphi

ncs

[tl3u

1S

n-P

R']

...

.sli

p

'2 J

pr"

611

95n

'J

" 1 LO

S ..

""

l Jl1

tsnu

r 'J

" ' "Sn

'\·S

.. 6

11t

(P

II)

1 JII

.Sn,,

-n

JP

II

------.~--

--

.--

~--

--.

-..

13

Ci

J-{t

Bul

Sn-

PH

]2 -2

59.1

30

.2 (

10.9

) 50

.1

571

(313

) 38

2 (2

2G)

HI7

47

.2 (

10.5

) 14

4.5

P JI"III

trar

u-It B

u 15

n-P

H 12

-26

3.0

30.5

(15

.5)

48.1

58

·1 (

320)

38

0 (2

25)

1.99

47

.fl (

10.6

) 14

4.5

~ J

plI

+

1.4

I J

PII]

14

[tB

u1S

n-P

(CH

2)J

C1]1

-1

22

.7

od

'.1

76

3 (·1

18)

325

(192

)

15

It

Bu1

Sn-

PtB

u]2

-50.

1 od

51

.3

9·11

(5

16)

325

(102

)

16

'r

o,*

-26

4.8

,18

.'1 (

211.

5)

108.

1 [S

nA

] 87

6 (4

80)

234

(139

)

ItB

u2

Sn

AP

Sn

BM

eJ]2

IS

nAS

nA

J

16.3

[S

nB

, 1

005

(583

.5)

12.6

41

5 (2

,16)

ISo

OP

I [S

nA

Sn

B]

17

'r

o,*

-22

3.6

o

d

84.3

{Sn

A]

789

(432

) 68

8 (4

07)

[Me1

SnA

PS

nB

tBu

Jh

[Sn

AS

nA

]

32.1

ISo

OI

1209

(G

6>..\

) 1'

l.0

1118

(247

)

ISo

BP

I [S

nAS

nB

]

18

[t

Bu

,Sn

-PH

[J

-33

5.6

G.O

(3.0

) 55

.2

101

18 (

57.

1.2)

7.

0 :J

57 (

'lll

) 0.

38

--11

.9

160,

6 l JP

IIJ

( +9.

30)

0.6

[ JP

II/

Q

redu

ced

coup

ling

s J(

(in

uni

ts o

f 10

19 N

1\ -

1 m

-J

) In

par

cnL

hcse

s.

Page 6

679

Table V . NMR data of stannylarnines.

611!~Sn 2 JIIOSnll'Sn" 61bN I JlIgSnl~N " 6' H (NH) 1 JUNI!! 2JII 'Sn I H" ,. tBu,SnNH'l -27.9 - 402.5 113.3 - 0.65 (248.0)

62.8 16.5 (3.66)

20 tBulSnANHSnBMcJ - 15.7 [SnA[ 430 (255) - 408.7 nd' -1.50 60.2 18 (4.0) 74 .5 [SnB I II (2.4)

21 (tBu3Snh NH - 16.1 381 (226) - 419.3 138.5 - 1.8 7 55.1 +35 ( -7.8) (303.2)

.. reduced couplings K (in units or IOlg N A _2 m- J ) in parentheses. ~ no unequivocal assignment possible ba:ause of low SIN level.

"..,.

."

•

~

" ... "~

."

•

."

" •• ,UN

, "'"' ."

I - ." , r ,

."

Fig 3. 300. 13 r.Hh I H-dctc<::ted I H/ i5 N heteronuclear HMQC-shift correlation of (tBu3Sn)NH , 21 . 256 experi­ments of 48 scalls and 2K data points were collected ; spec­tral width 1200 Hz in F2 and 608 Hz in Fl ; zero-filling to 512 W in FI , shifted sine multiplication in both dimen­sions. The spect rum is displayed in magnitude mode. The t ilt of the cross peaks attributable to the Ilt/lliSn satel· li tes givt.'S lK(1l7/ 119S nIH)tK(1l7/I19Snl !>N) > 0) . Since IK (II~Snl .)N) < 0 12,141. it follows that lK C17/ Il9Sn i H) < 0 and 2 JC 11/1195111 H) > O.

hybridization at phosphorus [3], and low electronega­tivity and high nuclear polarisability of the adjacent tin nuclei 141. In addition, the observed values of 6ll p (1-6 ) and 61 N (19-21 ) show a strong dependence upon the nature of R in the RJSn-groups, which can be un­derstood Qualitatively on the basis of two major influ­ences. Firstly, the group c1ect ronegativity of an ~Sn­moiety is enhanced by a -acceptor (R = CI) and aUen­uated by q·donor substituents (R = tBu, Me), leading to relative deshielding or shielding, respectively, of the adjacent phosphorus or nitrogen nuclei (see 611 P for (tBu,RSn),PH , -296 (R ~ CI, 5), - 340 (R ~ Me, 4)) . In the same sense, the strong electron-releasing proper­ties of the tBu-moieties may be held responsible for the fact that the increase in II P-shielding, which is observed as a consequence of formal replacement of hydrogens in EHl , by R3Sn-substituents, is significantly stronger for

R = tBu than for R = Me (o3 l p for PHJ : -238 [14]; R,SnPH, , -304 (R ~ tBu, 1), - 269 (R ~ Me [16\); tBu,Sn(R,Sn)PH , - 325 (R ~ tBu, 3), - 316 (R ~ Me, 2). Secondly, intramolecular interactions betv.-een bulky stannylligands can force enlargement of the valence a n­gles at the central atom, which results in a decrease of nuclear Shielding. A similar relation is known for tertiary phosphines [17]. In the case of the extremely large tBulSn-group, the effect of sterically induced dis­tortions on the phosphorus o r nitrogen shielding coun· teracts the electronegativity influence, which is seen as the reason for the observed sequence of chemical shifts fo, (tBu,RSn),PH W 'P ~ -340 (R ~ Me, 4), -326 (R = tBu, 3). The sterically induced bond angle vari· ation is certainly a cooperative Quality which is deter­mined by both the number and size of all non-hydrogen ligands present. Since its infiuence on the phosphorus shielding cannot be neglected in comparison to other effects, it was not possible to derive a simple increment system to predict chemical shifts of stannylphosphines in terms of additive substituent contributions.

Synchronism of both electroner;ativity and steric fac­tors sufficiently explains the 3 P-deshielding in t he chain-type stannylphosphines 7-9 and the silylated derivatives 10-12, in particular , the downfield shifts ob­served for 31 P in 12 and the central II P nucleus in 7 point to a hip;h degree of local steric hindrance. A sig­nificant infiuence of the ring size on phosphorus shield· ing is found for the heterocycles [PHSn(tBu2)Jn (n = 2 (13),3 (IS»). As compared to the chemical shift of the central phosphorus in 7 (631 p - 272.5), which displays a very similar substitution pattern as in the cyclic deriva­tives, the I I P resonance is shifted to lower field fo r the four-membered ring derivat.ive, 13 (OlI P = -261 (av­erage»), and to higher field for the six-membered ring compound, IS (6l1 P = - 336). The latter suggests that. the phosphorus bond angles in t.he six-membered het­erocycle may be somewhat smaller than in the open­chain derivatives.

The observed range of 11 9S0 chemical shifts indicates t.etra-coordination of the t.in nuclei in all cases. Even if a consistent theoretical discussion of 1198n shielding is extremely difficult [2J. some Qualitative trends emerge from the data in table I·V. The tin nuclei in the stan­nylamines 19-21 a re slightly dcshielded (6119Sn - 28 to - 16 ppm) with respect to comparable tetraalkyl­stannanes (tB u3SnMe: 61l9Sn -25.4 [2]), reflecting the

Page 7

680

higher electronegativity of the amino group. PhosphinyJ substituents induce pronounced downfield shifts compa­rable to those of tin-sulfur Of -selenium derivatives 121, which can be attributed to the availability of low lying u·(PSn) states. The rather large but non-overlapping ranges observed for the chemical shifts of 11950 nu­clei in "tBu3SnP (6 11950 25-50 ppm) and tBu2SnP2 fragments (611980 75-120 ppm), respectively, indicate a sensitive dependence of the shielding on the lower­ing of the local symmetry 1141 as well as on neighbor­ing effects associated with variations in the second co­ordination sphere. The I tgsn chemical shifts for com­pounds (R3Sn)nEH3_n increase with n for derivatives with E = P, N ; however, both systems exhibit a differ­ent behavior with respect to variation of R. For stan­nylamines, the values of bll9Sn increase upon changing from R -== Me to R ::::: tBu (see 20 and appropriate ex­amples in 12». The opposite effect is observed in the phosphine series : in the case of tBu3SnPHn(SnMe3h_n (n = 1 (2) , 2 (6)}, the 1l9Sn resonances of the nuclei in the tBu3Sn-groups appear at (ower field than those of the Me3Sn-group (tu5 :::::: 15 ppm). Chemical shifts for 1I9Sn nuclei in tBu2SnP2 fragments are generally higher in cyclic stannylphosphines as in acyclic deriva­tives, irrespective of the ring size. No simple explanation is present for the unique upfield shift of 14, however, a higher coordination number at tin resulting from weak intramolecular interactions between tin and the chlorine atoms in the side chains [18) may be of importance.

Coupling constants

Due to the dependence of the signs and values of JAB on the gyromagnetic ratios b} of the nuclei, compar­isons of couplings involving different elements or even different isotopes of the same element are best done by using the concept of reduced coupling constants KAB = 411'2/(h,),,,")b) JAB instead (values are given together with J in tables I-V where necessary). It is recognized that 1 KSnP is generally negative 12, 4, 14]; furthermore, 1 K SnN has been found to be negative in compounds of the type R3SnNY2 [13, IS]. For the fol­lowing discussion it is therefore assumed that 1 KSnE < o (E = P, N), even if the signs were not experimentally determined for each individual case.

Inspection of the data in tables I-V reveals that vari­ation of the substituents at phosphorus and nitrogen results in similar trends for 1 KSnP and 1 KSnN , respec­tively, as has been discussed for the 31 P and IsN nu­clear shieldings. Thus, 1 KSnE in 1-12 (E = P) and 19-21 (E = N) changes continua.lly to more nega­tive values with increasing number of stannyl- or silyl­substituents at E. Similar behavior has been previ­ously found for I KSnP in trimethylstannyl-phosphines (Me3Sn)nPR3_n (R = H. Ph, lBu) {3, 4, 16]. Compar­ison of the values for 1 KSnP between compounds with the same number of ~Sn-substituents indicates that the reduced coupling is also notably affected by the nature of the alkyl group R ; I KSnP is substantially more negative in a fragment tBu3SnP as compared to MC3SnP. The same trend emerges when the values of I KSnN in 19-21 (table V) arc compared with that of (Me3SnhN, 22 (IJSnN = - 84 Hz 115], I KSnN = - 184x

1019 N A -2 m-3). The value of 1 KSnP for the com­mon RaSnP·fragment in R3SnPHSnR; (R = tBu; R' = Me (2), tBu (6» and (R,Sn),SnPR' (R = Me, R' = Me (IJSnP = +832 Hz [3, 41, lKsnP = -457 x 1019 N A -2 m-3 ), tBu (3)), respectively, grows more negative when the alkyl substituents in adjacent stannyl groups are changed from R' = Me to R' = tBu. As has been discussed in the previous section, this neighboring­group cITed strongly suggests that the variations in I KSnE art! La IJ, greaL part at.Lriuutaule to :stt!ric effects which grow in importance with increasing bulk of the stannyl moieties. Again, the same situation has been found for tertiary phosphines R3P, where 1 Kpc also adopts more negative values with increasing size of R 117, 19}.

Even if it is generally recognized that a concise dis­cussion of substituent effects on 1 KSnE (E = P, N) is rather intricate {2, 13, 14], the observed trends indicate that sterically induced expansion of phosphorus or nj· trogen valence angles is of major importance. Whereas a moderate varia.tion of the bond angles is expected to have no effect on the hybridization (this seems appro­priate at least for E = P [20]), it will result in reduced s-orbital overlap PsnE and concomitantly lower bond energies (and therefore lower a-a" excitation energies). In terms of the Pople--Santry model [211. both factors work to enhance the negative value of the mutual po-larizability term and thus produce an overall algebraic decrease of 1 K .

In contrast to the sterk influences discussed so far, the electronegativity of R in the R3Sn ligands seems to exert rather small effects on 1 KSnP in 1-11. The only notable exception is 5, where the increase of all CQuplings to the tin nuclei (I Ksnp , 2 KSnPH • 3KsnCCH) can be attributed to the presence of the electronegative chlorine atom.

The bond angle influence on I KSnP is lucidly cor· roborated by the observed differences between endo­cyclic and exocyclic couplings in the four-membered ring systems 13-11, where endocyclic Sn- P-Sn bond angles have been found to be close to 9()<> 15, 61. Re­garding the low degree of hybridization in the phos­phorus valence shell 120], this allows maximum overlap of honding orbital!'>, giving Iect.c; negative values of f3snP as well as higher bond energies. As a result, a net al­gebraic increase in 1 K is expected, which is in accord with the less negative values observed for 1 KSnP.endo (see table IV) . Since the endocyclic bonds are fixed within the rigid ring structure, any sterk pressure re­sulting from introduction of .additional bulky stannyl groups in 16, 11 is expected to be released predom­inantly via distortion or the exocyclic phosphorus-tin bonds; consequently, 1 KSnP.uo is comparable to the values found for the acyclic derivatives 1-12. The mag­nitude of I KSnP for the six-membered ring system 18 (IKsnP 574 x 1019 N A- 2 m- 3) is only mar~inally smaller than for the open-chain analogues 1, 8 ( KsnP 588, (- )610 X 1019 N A - 2 m-3 in the PSnB fragment, table II), indicating that the Sn-P- Sn bond angles in 18 are significantly larger than in four-membered ring sys­tems and presumably come close to those in the open­chain structures.

The magnitude of the geminal couplings 2 KSnNSn in 20, 21 (226, 255 x 1019 N A - 2 m- 3 ) is in the same range

Page 8

as 2 KSnPSn in 2-18 (139-407 x IOHI N A - 2 m - 3); no sign information is as yet available. A general correla­t ion of the values of 2 K SnESn with structural parameters is not immediately evident, but it appears that changes in both sterk requirements and electronegativity of the substituents have effects on the coupling. Considering that quantitative understanding oC structural influences upon 2 J SnESn is generally difficult [21, no further discus­sion is attempted .

Data on Curther long-range couplings e K PSnP , 2 KSnPII , 3 K PSnPld are included in tables I-V. The signs of 2 K SnPIi « 0) and <I KpsncC Il (> 0) were determined from 2D-spect.ra and may prove valuable as a base for further relative sign determinations. The values oC 2 K PP arc generally small ; this is not unexpected regarding t he presence of highly electropositive stannyl-subst.ituents as well as steric crowding effect.s which tend to increase the energies of rotamers with gauche-orientations of phosphorus lone-pairs (this would place bulky stannyl­groups in positions gauche to each ot.her). The absolute values of both 2KsnPIl and 2 K psnp show a marked in­crease in the four-membered ring systems as compared to acyclic derivatives and the six-membered heterocy­cle 18. This effect may be attributed to changes in the bond structure discussed above, together with the pres­ence of an additional coupling pathwny. The rela tive magnitudes for the two isomers of 13 e Kps nP cu > 2Kps np t,.on~) are in accord with the different dihedral angles between the two phosphorus lone pairs [22J.

Conformation and stereoisomerism

All studied substrates except 13 display a single set of NMR signals, indicating that these molecules may be described in terms of a single stereoisomer on the NMR time scale. According to t heir I H-spectra, the two sets of signals in the case of 13 can be assigned to stercoiso­mers with cis- or trans-orientation of the phosphorus substituents relative to t he four-membered ring. From the integration of relative intensities, the trans-isomer is slightly energetically favo red (.6.G = 0.3 kJ mol- I at 298 K). The dynamic interconversion of both isomers a.t elevated temperatures has been reported previously [6J. In the case of 14, a similar isomerization process is sug­fested by the observed temperature dependence of the

I p_ and 119Sn_NMR spect.ra which show substantial dynamically induced line broadening at ambient tem­perature. However, owing to t he extremely low solu­bility at temperatures below O°C, no spectrum in the slow exchnnge limit could be obtained. No evidence for dynamic changes was detected for the remaining four­membered ring systems, 15-17, suggesting that t.hese derivatives exist in a stable conformation. T he num­ber and multiplicity of the signals in t he lH_ and 13e­

spectra is in accord with trans (C2h )-rather than cis (C2 ... )-substitution, in analogy to the solid state struc­t ures of 13 and 15 [5, 6J. It may be proposed that the cis-isomers are in this case destabilized owing to en­ergetically unfavorable interactions between the bulky phosphorus ligands.

Temperature-dependent line-broadening effects sim­ilar to that of 14 were also observed for t he six­membered heterocycle, 18. At the high temperature limit, only a single set of sharp signals is found for all

681

tBu- and PH-moieties, respectively, indicating a.n effec­tive molecular symmetry of D3h . Since no static molec­ular conformation of appropriate symmetry is possi­ble to give a pyramidal coordination geometry at the phosphorus centers, the observed symmetry must be explained by dynamic averaging between different con­formational isomers via configuration inversion. To be observed spectroscopically, t he dynamic process must involve diastereomers with different orientation of the PH-hydrogens relative to the ring plane (cis-trons-trans and all-cis stereoisomers), but the NMR data. a.lIow no conclusion if the six-membered ring has a planar or rapidly inverting, puckered structure.

Even if different diastereomers a re expected for the acyclic stannyl-phosphines 7-9 due to the presence of two chiral phosphorus centers, only a single set of NMR signals is observed, and no dynamic broadening effects are detectable in this case. This suggests that as in the cyclic systems, configuration inversion at phosphorus occurs very easily. T he comparison of the extent of the dynamic effects in t he spectra of cyclic (13, 18) and acyclic (7, 9) stannylphosphines indicates t hat the inversion process is fastest in the open chain derivatives and becomes consecutively slower with smaller ring sizes.

Acknowledgment

Financial support by the Fonds der Chemischen Indus­trie is gratefully acknowledged.

References

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