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alkene systems
ELSEVIER
In~q~J;.ulit'aChimica Acta 207 f l gt)SI 49-54
The kinetics and mechanism of ligand exchange in photogenerated ( r/'-bromobenzene )Ca(CO) 5/alkene systems Santino Ladogana, Suresh K. Nayak, Jaap P. Smit, Gerard R. Dobson * I)epartmem ~l'Chemi.w~3'. Ih~iver~ity r~/'Norlh T,'.~as, I)e~Ho~. TX 76203-5068. USA Received 18 No'~cmher 1996: revised h, Marcll 1997
Abstract The kinetic,,, and mechanisni of the reactio.) of pll~logcncratcd ( r/~-bromobei|zcne )Cr( CO ) ~ { bromobenzene = PhBr), whicb coordinate~, weakly ( = L,, ) to Cr lhn~ugh a "dative" B r - , C r bond, with retire strongly bonding alkcne "traps' (I,,: I-hexene: I-decene; 2.4,4-trimethyl° pentene), has been studied by emph~ying pulsed laser flash pht~tolysis with visible and inli'ared detection. The kinetics data are I,
( ?II "Pil Br )Cr( C O ),, --, ( ~:-i,, )Ca( CO )~
COllniMenl with a ,~ingle ultrcllanism, involving disst~ciation of PhBr I'l'Olli (tlb-PhBr)Cr(CO),~, followed by competitive reaction tff the [ Ca( CO )~ I intermediate thus produced with PhBr or I,,. Data taken 'neal' t~ very high I L, I, and in dilute ~,~lution in the "inert' st~lvents ( S: Iluomben,'ene, n-ileptane ) agree closely, indicating thai solvent efl'~cts are relatively uninlportant. Relative rates of attack ~tt [ Cr( CO )., ] increase i.t the oaler I -hexene < I odecene < 2,2,4-trimelhylpentene. Carbonyl stretching spectra for ( C,,FI~X~Cr(CO)~ contplexe.~ {X = H, F, CI. Br) are quite similar, and indicate thai tllese L,, all are weak ,r and 7r ht~nders, bu! do not enable one to distinguish between "edgc-~n' hil|di|lg of tile riug to the metal ( H. F) i'ronl dalive X -, Cr bonding, ( CI, Br). Fh~wever. il¢iiviltlon paralllUtCl'~,tor complexes cr, hibitin~ the tw~ btmtling lut~des arc diag,nt),,,lic Ita tacit litlkagc. ~C) I~)t)8 Elsevier Science S.A. A,'¢'vwOP~t,',,' Kill,'lit'r, dlld II|0,1fhiuliMII;t'hlOllli|llll ~,tHlll~k'X~'r,'. ('ilrhonyl ;;onll~J¢~¢,,,; AJkt:l|~. ¢t~lllpJt,'~,~.n
i. I n i r m l u ¢ i i o n
F:or some two decades, kil|eticists I'~cttsed their attelllion on ligandosubstitutiou reactions of 'stable' Group 6 nletal carbonyls and their substitution products, ( L , ) , M ( C O ) , ...... in which L~ strongly coordinates to the inetal ( through N, P, As, Sb, S. Se or Te donor ligands) I I I. More recently, analogous complexes containing much more weakly coordinating ligands (L,,,), containing CI 12-51 and O 161 functional grotlps, have been studied, often through use o1' Ilash photolysis techniques. This report expands these later studies through a detailed investigation of the displacement of bromobenzene (PhBr), which coordinates to Cr thn~ugh Br 17J. Ii'om its photogenerated pentacarbonylchrtuniuln(O) comple,~, governed by k ( Scheme I ). Experiinenlal evidence which indicates thai solvation accompanies vibrational deexcitation of [ C r ( C O ) ~ I * produced immediately after the Ilash has been presented 18 I. * Cort'eSl)onding :mthor. Tel.: + I-9,.1t}-565 27 13; fax: + 1-817-565 43 18: e-lnail: wyvern f~onramp.nct 0020-1693/98/$19.00 ~ 1998 Elsevier Science S.A. All righl~, reserved PIISO02()- 1693( 97 ) 0 5 5 5 0 - 3
k~lPhBrl+(11 ophBr)CI{CO),~ . Cn(CO)~. hv~ I~:r(CO)~l* ..........
kJ
ktdl,~lI (I.OCr(CO) ~ ,~ Scheme 1.
2. Experimental 2. I. Material.~' Cr(CO),, ( Pressure Chemical Co. ) was vacuum sublimed before use. The following reagents were fracLionally distilled under nitrogen from the listed drying agents ( given, together with the source, in parentheses): benzene ( EM Science. Na): Iluorobenzeue, chlorobenzene and bromobenzene ( A l d r i c h . P~O~): I-hexene. I-decene and 2,4A-trimethylpenteue (Johnson Matthey, anhydrous MgSO4); n-heptane (Sigma° Aldrich, Nat.
~l
S. l , Io.eun, el al I Im,tt~anica Chimica Acta 267 ( 1008140-54
Cill'boliyl sltelehillg ~peetra for ( I.~ ICI'| CO I, and ( L, )Crl CO)~ ctlmplexes in hydrocarl~n sol,venl Complex
I, ICO) (cnl
I)
A~" I/Z-Benzene )Crt CO ), Ii"-Fluorol~nzene lCrl CO t ~l'-Chlon~enzene) Cr(CO), 71'.Bromtfflenzene )Cr( CO )~ l¢-I-HexenelCllfO),
A',-"
E
1927(mi 1934tm1 1934(m) 19201m1
1054tvs) 1955(vs) 19581 vs) 1957(vs) 1963
0;" I-Decen¢ )Cri CO )
il;.2,4.4-Trimelhylf~ntene If'r( ("O i~ ( Aniline iCr{CO),~" (Triphenylpl~l~plli.¢)Crl CO ),"
1962 1958
2072.5 201~6( w )
1916,5
1936.5 1945(sl
"lld: I I¢,1. "Rei', 1171.
2,2. hh'nti/icalion q[ reaction inwrmediates aml products The identities of ('q'-PhBr)CrlCO)s and the trapping products. ( iT'-( l-hexene ) )Cr(CO) ~, ( r/-'-( I-decene t )Cr( CO ), and ( r/'-( 2.4,4-trimethylpentene ) )Cr( CO ) ~ were confirmed by employing a pulsed l a i r flash photolysis systenl based on a Tachisto model 850 photolyzing excimer laser IN: gas fill; 337 nm. I0 mJ, 10 ns fwhi), and a Tektronix 2440 digital oscilloscope, coupled to UV=Vis and IR analyzo ing trains. The latter employs a Spectra-Physic,~ CW lineroller,hie 1R diode laser analyzing ,~oui'ce. Tile .~yslenl ha~ been described in detail in Rel. 15 I. where a block diagranl Ior Ihe laser system i.~ giveil. Spectral e hange,~ were mt~nittwed in the 1R (1970~1915 cnl ~) by enlploying a I ntlu CitF~ IR cell with I fr{ CO ). I - 3 × 10 ~ M, £arbonyl ,~lrelehili~t ~lle¢lril wt.~re ohliiint<,d poinlobyopoilil I 1~2 eni i ini~rvill~l after palsed la,~er Ihl~h pbololy,qs of Cf( CO ),,I PhBr ( 0,5 M I / L 10,25 fVl i/lt~ti~ltlane solutions, wbi¢tl liiininiiled possible biuidobroadeiiiilg as a ¢onsequeuce of high I L, I ~. AUenlpts to observe tile very weak, higli energy A I " hands expecled at - 2070 cln ~ for ( PhBr)Cr( CO ) ~ and ( L, )Crl CO), were not .succes,sful 2.
2.3. ~lser.lklsh i~hoto,t,'~'is ki~lelics sltldies Because of superior si{~nal-toonoise ratio,, in th," visible spectral Ix,giun, kinetics studies were monitored at 490 inu, Solutions ,~ 3,5 × I0 ' M in Cr(CO),, also containing large excesses of solvent and trap were employed to insure pseudo first-order reaclion conditions. PhBr and L, were weighed so ' lilt ~';llt~llyl ~li~l~ltitlt~ ~Wt'titl 1 Niodcl 211.SXIt I;I'-IR ~pcci ~lili~iei I Iof '~laltk" ¢olltl~texe~. e~ I pilt'ridiilt" t('rl ('I) I., ~,,ell' ohlailled tl ii Ilep lall~/l~uCI ~o|llli'on.~ of t, tir~ill~ ¢!~11o21111~tllio11~,tilld il ~iti~ I'oulid thai ~i~tti1'o i~ini b~uld-k4~10uiiltl did ilol lilk~ l~lact, hi ~tdulioilx ,~ li,5 M m I. ii~ : 'S~lt.l~lllx' ( S ) il~ ilk)kit ¢l~llli~til Sl~,~i~2x ~.hit:h so l~¢al~l) ,~lktrdillttlc hi I ('rl ('O i, I Ihill Ihi2it I S tl.?i'l (~(1 t. c'Oillplext'~ tilt' ~letid~-~lt~lt" inlc~utcdlat~ on ltle lilll¢-~cal¢ ~t" I !.~ t('rl l?Ot. rcaelhm, ttow~vcl', ill "llt.,tll' s~lilti~. ~m~ I Phllr I -~ I Crl ('el. I. Phitr tuilcli~ilx t~#h ti.~ i.., t i:~ilrdiilaled hi [ ('1~1170 l, ] ) Olld tix S t ill II1,~ oulcr ~:~lordinalion ~ph.~l~¢ I.
that their concentrations wei~ known with accuracy. Temperatures were maintained within :1:0.05°C by employing a VWR 1140 external circulator and a jacketted quatlz ! cm cell. Values for k,,h, were obtained from averages of 3-10 traces of plots of In(A,-A~, ) versus time (A. and A<~ are me:zsured absorbances at time t and at inlinite time, respectively 1. The kinetics data were analyzed employiug ASYSTbased coinputer progranls developed in-house. All plols exhibited correlation coefticients > 0.99. For values of k,+,, ,~e¢ Section 5: error limits, given in parentheses as the uncero tainties of the lasl digit of file oiled value, are I .~tandard devialiou.
3, R~sulls 3, I, hh,nlff¢~ 'aliOtl ¢!I reat lalli ~~'.d/u
Tlle IX spectrulu For( ~l'~i)hlh)Cr( CO )~ was obliOllcd ill the absence of [.o,. Phlls of absorballCe versus linli: ¢~hihiled '~tep I~n¢tiOllS' over several IllS. indicating Ihat tile complex is Ullrea¢iive on this Iinie-seale, Similar spectral studies were also carried oul for ( L,, )Crl CO I~ ( I.,~ ~- Phil. PhF. PhCI I. ~o lhal the I I spectra for all four COiliplexes might be ¢olno pared. These are given in Table I. Thue-resolved spectra were taken for Cr( CO ) J PhBr ( 0.5 M ) / I ohexeile l 0.25 M ) / n=heplane solution ( Fig. I ) i see filotnole I }. C~mparisons of hires observed p~:,;l~;:'hllj 41}it nm (Vis) and 1927 cnl i lilt clearly i:~dic.l,: thai phologelleraled (llI.PhBi')Cr(CO)~
is COI;'.C,vI:"!
II, I Ihc
known
151
(il:-117~,l~lie)"
CrlCOi~ product, The dose shllilarilies of the kinetics observed for reactions taking place after pulsed laser flash photolysi~ of f r ( f O i . / P h B r solutions t'onlahlhlL2 I-d¢cen¢ and 2.4.4AriUl~lhyli~nleut~ as the il~ipphl I nucleophiles ¢on. lirnled thai analogous reticlions were taking place. 3,2, Kinetics ,xtudies
Neat solutions (containing only Cr(COh~. PhBr and 1.... but no inert dihieui. S ) ( see footnote 2 ) were found to afford
.%. lx./-~l~lll ,'t .I. / I.-rk, lmil'tl Chimh'., ,,,1el. 267 ~/~V,~J4~ -54
51
C lO.O
4, ~O
i
/
G. 40 "~
6. OO
dec
1,80
• 5[~1
1.20
I
I
I
I
I
I
I
I
I
I
IrhBrl/[L,l
FiQ. 2. Ph~l~ of I//~,,l,. ~'. I Phllr I ! I L. I at'let pul~ed laser flash ph~lol,,4~ ill' ('rl ('{) I,,!brllilluhen/¢nc' f1.. solLIlilms i.il =15.[I (" ( I., ~ l-Ilcxcn¢ I hcx I.
v(CO), cm -I I:ig, I, Tilile-rcmkcd sp¢¢llll, 1015 197l)¢n1 ', for tile reaclilln lakin~ plat'e after pulsed I:iser Ila.qi i~tilli~ll)'sin of a ('ri('Oh,/hnuli~lben/¢iie/ I-hcxcne/n-hu'plan¢ soluti~)n. Time afler the Iklsh: A. lJll: It. 75(i: ('. 2qlX)
ps.
belier si~lmls than did mhililms in which I PhBrl,I L,I << I 8 I. Conccninlilons fur both PhBr and 1., t I-hoxene. I-deccne, 7,4,4-1rJnlethyll~nlon~i wei'e v~il'ied w i d d ) l l P h l t r l , 14.$---1,7 M; I.. (i,74,4 M I; iit dilute .sohidon 1 S ~-~PhF, n-h~pl~lle ): I Phl]rl iilld I 1.. I WCl'~ also varied widely, with I I PhBrl + I l.. I ) =~ I M, Plots of I/k,,,,. vcr,~ilS I P h B r l / I L . I were IJnelir with ii corn)iron inlerCel)l iit hoth n01it and dilul~ ,~olutilln I FiT. 2 i. ¢onsi,~lenl wilh Ih~ principal nil~¢hlulisnl ollser~:od in sy~l¢lilS in which lliher I.~, are displilc~d hy I., [ 2,-.51 ' Ii
IPhlh'i('ri('()l,
--"
iI il I'lilll I
I'll
I('ri('Ol~l
I
....... I I . , i ( ' r ( ( , { ) l ~
SdlClllt'
Whelt' I('rt t ' l ) i , I i5 li ~l¢litl~ ~llil¢ iiil~rnlcdiiilt' and in terlil,~ of the psi.,ildll lir,~tollrdcr iIi1¢ ¢llli~lalit,, ~:,,,,. I i Phlh'l, I Io, I >~' I Crl f'(i i, I ). Ihe lilechiini~ :l I ,SchcniL. =l ) iifl'llrdk !he riile hiw, /,,h.=lJ,,ll..lfl/
,IPhBrl+/,:ll.,I)
(I)
w h i d l Call be r c a r l ' l i n l ¢ d to a f f o r d I h l c a r ' d O i l b l o n~¢ipro¢iil' plots. Ilk.l,, = I/kl+k
~lPhBrl/k~k:ll.,I
(2i
Thu.~ phll.~ ¢i1' I//.,,i,, VOl'SU,~ I P h l l r l / 1 1 . I )Jr{ ~Xl)~cled Ill lIC linear, with inlcreepts. I/lt~. and slope~, k , I k , k , , ;is i)b.~ervcd ( Pi I. 2 ). Frolll ihcs¢ di.llll Ih~ 'conlpclilion i'aiJo.s'. k,//~ i. can h0 ohlaJned ~i,~ h~icrcepllshtpe of the "dl)uble re¢iproclir i)lots, The inier~g~i.s of ihc rc¢ipi'l)¢al plol.~ arc r~hilcd exclusively Io the unJntol¢¢ular i'illcs of di,s,so¢ililiotl of Pkilr i'ronl ( l i i - P h l t r ) I('rl CO)~. :uld thu~ .shl)Lild be Ih¢ ' The rlitcs ill' cliinbhililion ,ll I. iiiid I ('r( ('0),, I lake I)hicc Oil the ps lime.~cal¢. Ia.qer Ihan n.iles ill diflu~i.n: lhtls [.., ~I.. cx.c'halilC L'~I1 involve iqll)' [. whidl iilC iil[catl~ pnlperly pllsililmt'd Ibr i'¢iit~lion, Ih¢ ill¢<:hlilli.~ln is Illu~ i~el'hlip~ fleet Ihlluthl id as I,i.
I .du, c~nc, tdec" I, },4.4-trillielh)
Ipcnlcne I pellt ) I.
S~illl¢ for all nudeophilos, also consislenl with ol')scrvalion ( Pi 7. 2 i. The ral¢ con,stalll~, ki/~:/k i. ki and /Q/k i. ~ir0 pl'eSerited in Tahl~ 2 4. Values o I ~ H ' and -%.~"' for I OCO ) <(~l'-.ltrPh hond dissociation are ~Jven in Table 3 Ivid¢ infra i ~ .t,t. 77w is!lhl('.(¢ ~!t 'ilWrl' dih.'.r~" <.l n/ : Studies were alno conducted Io exarrliri¢ tile ¢1"f¢clof solul i o n ¢ o n l l l t ) s i t i o l l oil i'~.Itc Lind illCdl;.lllJ~lll. This w~s d(mc Iir',l Io CX;.LlllJI1¢ Ilics~ ~.'11~111~ ~.1.",lh¢ soh.llions ~re ¢ o n v c r l e d f r o n l
"neat', conulinin~ only F)hBr .'rod L,. m "dih,tc', m which the Im.,d,mlin;mt O,'qlll'~oileltl i', ;.i "sol~..ent'. S. Thi,, ~,:ts of letted through dihltioli of equal con~'enlr[iliorls of PhI)r. I.,, ,.rift') S ( PhF, i].-hel~tan¢ ). j;~}l' the 'reversihh.' tli~,~wi;liiw." tlle~:h~misnl ~'l)ll~jSlt:lll wilh the' daln ( S¢}iem~' 2 ). t~,hele [ Phlh'l I I., I. lh¢ r~ttc h.l~, ( i!q, ( 2 ) ) l~ec~,ne~
Thus, A,,i,. ~hotlhl ;il'lpear ii~ Ih¢ inlcrccpl ill' a plol llf/,,,i,~ ~,¢r,Stl,~ (I Pkltrl ~ I1,.I ). alld tiny Jllllu¢llC'¢~ Oll ral¢ ill Ihc' dlaltg¢ froltl 'll¢~ll' IO dJhll¢ ,sohlliOll ( its I I,, I d¢¢r¢:i,~¢,~ I will hc ~×p~ci~d Io ilpp¢iir )is ii ,shape for Ihi~ phil, "I'h¢~¢ phlis, fl~r d~llli lit 31~<~C. ,~llown in Fig, 3. ¢xhihil lilrg¢ Jntcr¢¢pl,~ I k . . t. io~¢thcr with ,small slop0s, ~.~. Kinclics tlalli ill,SO w¢1'¢ tlikcn f o r ' d i h i l ¢ " ,~ohllJOll,~ ~ h ¢ i t " I P h B r l / I I.,I wan ~,,aried I S ~ (I PI~Brl + I!.,I ) ,= I M ). ~ il I'iillCiJl)ll o f tile i d c n i i l y
olS
( P h B r . I)hl '. li-h¢l~liin¢ ) lind I..
( I -hcxcne, I .decent. 2.4.4qrhlt¢ihylp¢lilen¢ ). vv'hich ~llould L AIh.'lllpt', tl~ obtaill IhL' 'lll'it|l;If~ ' olnll'~etitiorl lj|ill~. ~h//* , ( ,~4.'hL'H1L' I 1
i1~ cnlpIo)m~ ~idci F vatFl,~ I I,,, i, I I , I aml bl,h Vi~ ~md IR N~L'cinq~l~,~t¢lm¢|l'~f h~l%L' thll~ i~lf I~d It) illL'tHIL'hl~,i%'e r¢~)lIl~; ¢[l,~rt,, are ~,lliltifluJl|)~,
" ,%ilK'¢ mlerL*cpt~, ifl'the dllul'fle r¢¢ipr(i~.'al plot,, ~)f I/~.,~.. ,, Cl',,u,, I Pid]r [ / J I... I are relati~,ely ~,111411.riley cxhil'~il r¢l-~ltP,'ely larD.' crmr lira)L,,. IIm,,¢~.~ r, ~iilee [11¢~ ate delllon~,lra',¢d t~ b~ i11dcp¢ndcnl ()1 the id¢illily ,fl* J.. and ~l ~,, ermt lira)l,, h)r ,~11" ~nid .~.S". wer¢ derb.o.I ha,,¢d ~lt lhe eXl~eriment,d unu'¢rtainlie', I)t values o1/, t. will) tile stantl~jrd deviation ha,,,,:d (~rl ;.dl ~,dhi,:~ ur kj ( 4 seis ~d dala GIr ?, I..'. Table 2 ).
~2
,~ l+a+hL+ana{'l +d / lnorg+ n/ca Chimi{'a Acta 267 1 lqg,'¢J 40 54
T+ble 2 R+tle ¢on+~l;ml~ I'i+~rreactitms of photoge.¢rated I r/+-PhBr!CrlCO)+ with various Irapping nu¢leophilcn 11+. = I-hexcnc. I-decent. 2.4.4-trimcthylpentenc ) in x agitHl~ inert ,~l+,ems ( S = fluoroben/ene, n-hepl:mc ) and +arious temperatures S.I..,
TI+C)
25J(+) 30.0 35.0 45.0
PhBr, I .hexene
n+hcptane, I-h~xe.e PhBL I od¢cen¢
PhBr, 2,4,4-trim¢fl~ylpcnten¢
l.+,Ik
4.6( 7 ) 7.83( I I )
050( 8 ) 0,47(7)
10.6(6)
0,51(9)
24( 4 )
0.47( 7 )
35.0
45.0 25.t)
21 * 4 ) 4.418 )
25.0
4.31 7 )
0.6~113
35,0 4.+ 0 25,0
OAf 1"71
O.611121
35,0
45.0
2541
LI~X~
11.2914 2.13(3 4,371 7 8.541 I 1,8513 2,9416 5,76~ 12
0.45(t) ) 0.40( ? ) O.42(9)
105(H)
4.613t 9.6112) 211131
~)
2.29( 3 ) 3.70 ( 4 ) 5.33(8
0.43< 9 )
&
nohcplan¢. Iohcxcne
IO "k~k,/l,
~
5.019 ) 9.6(17)
25.O
PhF. I-hexen¢
10 ;I., (s ')
057(3)
II,i314
I ITIHI 1.17,(7) I .q~t( 181
5,4117 111+813I
211.1~11~I
10 k,., l', +)
(M
1.471 2 )
(1.(158(8 !
lO~ku in
')
~. 40
I+00
h~p
+i
J
l, .0
i, ~++0
lPlt~! ~ P~xl Ig 8 ~. Phil', ol 1,+, ~.,,,. I Phltlq ~ I ~exl allct p.b, ed la.,cr Ila.,h plmlol~,,i,, o | IUI+I('{1)+./hlOltlOl'~+21|/t+.n~+./I-h~xcltt..' ~.ul~.~:tll I ++N ) *.olullol).. at 2~(" +¢ + ,+heptane (h e p L llu.or,hcn~.e,¢ I Phl ;L) )
ol~:y Eq, t 2 ). The,~e plots are illustrated in Figs, 2 and 4, ttlld 01~ CtlrlSiM~211l w i t h |he "reversible dis,st)¢iation' Illek'hauisllt,
The nite ¢on+~lants, k~, Ior dissertation of PhBr lhmt photo+ generated t~.PhBr)CrtCO)+, k:/k ~, the "¢Oml~lition retires', and ktkflk ~, together with k,,,, and ku lbr dilution studies where I PhBrl ~:- I I+hexene I and where A,,rVl~p+,,,t,nt+, It1¢ influence of dilution upon rate ( Eq. t 3 ) t. aim+ ai~ ;ivcn i. Table 2 4, Dls,:u~i.. 4, P. Kinet6 '.~+and ,+c+'h{+m.v+++
~,QO
t},~O
0~10
i|'+V
il~Bd/ll~xl l'it~ 4+ l'1ot~ oF i I/..,.. v... [ l + h l h l / i h c x i for IL';.wiiliil.~ |+.ikia|j... llhi++'J,. allL'i
Imlwd i.t~cs ilash phuM+~ sis of (.'it ('O i+./bl~+m~4~en+cnc/i .Itexcat¢/..oh+¢,t ( *+-~ + sqdutit+it+,at ~.()++(' ( ~ + It+aL l|tmrohetl/¢lt+, llohcphl++¢ L
mechanism shown in l-ig. 2. Vatues of ~dq/!/, + + k:). whdhcr ohlah1+d directly froll| plol.~ of k,,.,, ver.~um ( i L, ] + I PhBr] ) or w, the reciprocals of ( ,,,lope + h'ilcrccpt ) of pit+In of I I/,,,,, v¢rsus i PhBrl / l L, l in d+Itlt¢ s,+ItithUl 151. Wt~t~, calctlhllcd tu lit, the maine, wilhhl cxperimelllal tlllC~2t~++till|~,
The data ab, o it|tile,.IlL' thal soivel|t +:l'l+~.'l..,ill lhe.~e syslelSl~ art" slight. +lild that ttal;.i taken i. 'lll.'~.lt" solutmns ¢olllainill~ only !+,, and I+,, Ibr x+hit'h signalqoqmise ratios of kinelics data arc Ol+thuizcd. well rclm:~¢nt the rates and mechanism of Ih+ I+,/i+, exchange process: the differences hi nile conslants t+lwo:n the 'neat" dala and Iho.,,¢ in dilule sohtlion arc les~ titan 2ll t.. T h o ~alue,,, f(+r the m l e r c c p l + o f p l o l s o f I ]k.l,,
"l'hevery cxteunive kmcticn data I~l~+rtcd h+l'~ al'~ interredly ¢~msixtcnt and lull). SUplX+rt tl)e 'r+zvcrsihle disstmiation"
ver~u+ I P h B r l / I L ! aw the ~am+. within exl~,rimenlal tllli;erttlilltyl thus /~,, the rate CIIII,~I++IIIII+or unimolect, lar
S. l++++h+~a++++~'l +eL / l++m'.+am~'+t ('hhttfi '~t At 'let 2fi 7 r / ~+V~i 49 .-54
Cr-Br bond lission, of fundamental importance, related as it is to the Cr-PhBr bond strength, is independent of the nalnre of the solution. The influence of the identity of l+~ upon rate is signilicant only for the "competition ratios', k,]k ~. obtamable as intercept/slope of plots of I ik,, h, versus I PhBrl / I L, I t Eq. ( 2 ) ). Since the rates or" interaction of PhBr with 1Cr(CO) s I arc expected to be independent of the identity of the incoming nueleophile. L.. changes in the "competition ratios" appear to be dominated hy changes in/,> Competition ratios vary as a function of tltc identity of L~ in the order l-hexene< I-decene < 2,4,4-trimethylpenlene. This is not an easy order Io inteq~rct; il is not related to the rclative rates of diffusion of L, through the solution 191, nor to steric effects in the vicinity of Ihe the olelinic linkage, which are expected to he grealest for 2,4,44riinelhylpenlene. It would seem reasonable Io suppose that chain 'agostic' C-H-Cr interactions play a nile+ perhaps Ihmugh a "chain-walk" reaction pathway 15, I01, to afford the rcaclion product coordinaled tlnou,,-,h the double bond. Another possible inllnence on the "¢onlpetition ratio" might bcv, hethcr the "agostic" hydrogens m'e secondatS or lerliary, Evidence in rel:fled W systems have indicated Ihal - ( ' - H - W bond strenglhs incre:ts¢ in Ihe ordcr terliary
4.2, B,,mlim,, It has bt'en sh~wvn IhHI ;li'ent.,s exhibit IV,,O dilferin~ m o d e s
of blinding Io ('r, For Phi I ',lUd PhF it hits been proposed Ill~,ll Ihi', I+ondinb., is tf,('.('. +edge otl'+ Ihrt~uFh im is~daletl ling double buml 17 l; ~uch buntlmt: ha,. hccn oh~cr~cd Io lake place m related s)slCmn i I~I l'.r Ph('l mid PhBr. l~nndin~! likely involves. °¢Oilvenlional' dalivc inlcraclion lilrolJBh a hme pair on ihc halogen 1141. Table 3 exhibits vahle~ l'or/,, i~+rdissocinlion of each of lile~e arei|c', Iroin Cr. h indi,.'alcs that lilt+,wt'aker "tdgeoon' bonding is disfavored kinclically vcrstts dative bonding tln'otlgh tile halogen, and that the Br+Cr bond is slightly stronger than is the CI-Cr bond. plcsttm. ably as a resnll of the greater hasicity of Br versus ('1.
.53
Presuming that the enthalpy of activation appmxhnales the Cr-Br bond strength l-5 l, the kdter is about 13 kcal sol + The enlropy of activation is slightl)+positive, not inconsistent with the dissociath~e medmnism
;+lclJvalJorl. alld nlor¢ posilive
eutrt)l~i¢~, of ;lClJv;llJoll
that for "e<.lge+or~" b o n d i n g ,
'l'al+le 3
I'~iLlL' +;OIl'+l.tlt|'. +.tilt[ dt-'li~, dllOll rrHliltliCiL'l ', lot I+++.HIO~.~ll'Pqillllqd I'tL'tl|d+,:+if'l'ltHl'.l t'htIH1)lt|tiH I)) ~+'+.sii~l+~l+,"~t TM,
,~|nh!.liltlt'ill
l~+.~.{I('l'+ ')
ItCh/erie Iqtauqdwnlcne ['hhmff~cn,'cilc Ihm~,ol+,c+~zcm: Aniline Tfil~hen,rll)hosphmc
2,57 ~Sl. IW I. ~L} ( 5 ) ,~ I().... 5 . ~ I IX}-, l()' 4,(+q ( ] I ) ," ItF 2?,4 > It) ' ~' lu ,t Ill " '
At 24+')"('. R¢I, i 15 I. ~' At 23 ~"('. RcI. I IOI. ' l'~slhnalcd l'rtml eMral'Jt~la+ionof data taken at I I()+ 13(1"('. L'
illtd Ht'li,:
Tahle + Bawd un IhJs illlelltr¢lalion, il thn,~ i,, nol ,,Ulpl
~II"
(k,:+dmol +)
~L4( I ) ')~+( 21
14 I}<41 13, I( 2x I 2h,4( I I lh.~,l Itlt
.~,' tualk +2..4< %I 22~ ~ I lfl I+ I~,~ ~h ~) ) Iq( ~ I 13(5 I
+n.,l 't
lor d;ll D,t'
54
S. lauh,,~,ma et al. t Im,rg,mi,',t Chimi,'a A,'/a 2671 i')08149-54
$. Supplementary material
Ii(}1 J.D. Simon and X. Xie, J. Phys. Chenk, 93 (Iq89) 291; X. Xie and
Tables of values of k,,, for the kinetics studies ( 7 pages) are available from the authors on request.
I I ! i Y. lshikawa. C.E. Brown, P.A. Hacker[ and D.M. Raynor, Chem. Phys.
Acknowledgements The support of this research by the Robert A. Welch Foundation under Grant No. B-434 is gratefully acknowledged. We thank Professor Michael B. Hall. Texa.s A&M University. for providing data in advance of publication.
References I I I J.A.S Howell and P.M. Burkinshaw. Chem. Rev.. 83 (1083) 557. 121 GR. Dobson and S. Zhang, J. Coord. Chem.. 21 1 It)~)) 155. 131 S. Zhang. G.R. L~,bson, V. Zang. H.C. Bajaj and R. van Eldik. Inorg. Chem,, 29 1 It)9(l) 3477. 141 S. Zhang and G.R. Dobson, Organometailic,,, I I ( Iq92 t 2447. I 51 S. l,adogana, S. Nayak, J.P. Smit and G.R. l~,~bson, lnorg. Chenl., 36 I 1997) 650. 161 S, Wieland and R. van Eldik, Organometallics. IO I 19ql ) 31 IO, 171 S, Zh~tng and G.R. Dobson, Polyhedron. 9 I 1990) 251 I. 181 l a) S.C. Yu, X. Xo, R. Lingl¢, Jr, and J.B. Hopkins. J. Am. (,Item. St,c,, 112 (It)Oil) 3 ~ 8 ; (h~ M. Lee and C.B, Hams. J. An1. Chem. S~.', I I ! ( 1980 ) 8063: I c ) A.(;. Joly and K.A, Ne/,,on, J. Phys. Cheul,. q3 I Iq8~)), 2870: Id) Chetll. Phy~., 152 I It)~)) hq: el. Ref. 1131 10J J,G, ('riven rml JN. Pills, Jr,, Phol,~'hemi~lry, Wiley. Net. York,
J.D. Simon, J. Am. Chem. Sot., 112 (1990) 1130. Lett., 150 (1988) 306: C.E. Brown, Y. Ishikawa, P.A. Hackett and D.A. Rayner, J. Am. Chem. Sot., 112 (1990) 2530. 1121 S. Zarid and M.B. Hall, J. Phys. Chem., in press. 1131 R.E. Rundle and J.M. Goring, J. Am, Chem. Sot., 72 (1951}) 3337: J. Browning, J.M. Green, B.R, Penlbld, J.L. Spencer and F,G.A. Stone, J. Chem. Soc.. Chem. Commun., (1973) 31; J. Browning and J.R. Penlold, J. Cryst. Mol. Struet., 4 (1974) 335: R.E. Cobbledick and F.W.B. Einstein, Acta Crystallogr., Sect. B, 34 (1978) 1849: J,R. Sweet and W.A.G. Graham, J. Am, Cheln. Sot., 105 (Iq83) 305: H, van der Heijden, A.G. Orpen and P. Passman, J. Chem. S~:., Chem. Commun.. (19,45) 1576: W.D. Harman, M. Sakine and H. Taube, J. Am. Chem. Soe.. I IO (1988) 5725: S.T. Belt. S.B. Duekeu and R. Perutz. J. Chem. S~:.. Cheln. Commun., (IqSt)) q28: W.D. Jones and I.. Dong. J. Am. Chem. Sot., I I I 1 1989) 8722. 1141 D.N. Lawson. J.A. Osborn and G. Wilkinson. J. Chem. Soc. A, I 19661 1733: W. F and K. Schloler, Z. Nalurfursch.. Tell B. 33 ( 19781 12 ! 4; F.I.. t~arcelo. F.A. Cotton. P. Lahuerta. M. Sanau, W. Schwotzer and M. I.Tl~eda.Organometallics. 61 19871 11115:C.H. Winter. A. Arif and J.A. Glady,~z. J. Am. Chem. Sot.. IOq 11987 ) 7560: R.J. Kulawiec and R.H. Crahtree. Organometallics. 7 I 10881 1891. 1151 R.J. I)Cnllenl~crg and D.J. Darenshourg. Inurg. Cheln.. I I I 1972 ) 72. II¢,I M.J. Wo~kulich and J.l). Atwood. J. Organomet. Chem.. 184 (IO81)) 77. 1171 I..E. Orgel. hlorg. Chenl.. I I 1962 ~ 25. 1181 F.A. Cotton and C.S. Kraihanzel, J. An1. Cheul. Sot.. 871 101'~5) 4432. I Iql G.R. Dohmm. I.W. Stole and R.K. Sheline, Ad~. Inorg. (Then). Radioehenl.. 8 t 1~)66 ) I. 12OI W.A.(i, (irallam. Inorg. ('hen1.. 7 ( Iq68~ 315, 12II R.A. Ita~vu alld (;R. Dol~ou. IIiorg. ('hiul. Ackl. ('~ I Iq721 1'~5.
In~q~J;.ulit'aChimica Acta 207 f l gt)SI 49-54
The kinetics and mechanism of ligand exchange in photogenerated ( r/'-bromobenzene )Ca(CO) 5/alkene systems Santino Ladogana, Suresh K. Nayak, Jaap P. Smit, Gerard R. Dobson * I)epartmem ~l'Chemi.w~3'. Ih~iver~ity r~/'Norlh T,'.~as, I)e~Ho~. TX 76203-5068. USA Received 18 No'~cmher 1996: revised h, Marcll 1997
Abstract The kinetic,,, and mechanisni of the reactio.) of pll~logcncratcd ( r/~-bromobei|zcne )Cr( CO ) ~ { bromobenzene = PhBr), whicb coordinate~, weakly ( = L,, ) to Cr lhn~ugh a "dative" B r - , C r bond, with retire strongly bonding alkcne "traps' (I,,: I-hexene: I-decene; 2.4,4-trimethyl° pentene), has been studied by emph~ying pulsed laser flash pht~tolysis with visible and inli'ared detection. The kinetics data are I,
( ?II "Pil Br )Cr( C O ),, --, ( ~:-i,, )Ca( CO )~
COllniMenl with a ,~ingle ultrcllanism, involving disst~ciation of PhBr I'l'Olli (tlb-PhBr)Cr(CO),~, followed by competitive reaction tff the [ Ca( CO )~ I intermediate thus produced with PhBr or I,,. Data taken 'neal' t~ very high I L, I, and in dilute ~,~lution in the "inert' st~lvents ( S: Iluomben,'ene, n-ileptane ) agree closely, indicating thai solvent efl'~cts are relatively uninlportant. Relative rates of attack ~tt [ Cr( CO )., ] increase i.t the oaler I -hexene < I odecene < 2,2,4-trimelhylpentene. Carbonyl stretching spectra for ( C,,FI~X~Cr(CO)~ contplexe.~ {X = H, F, CI. Br) are quite similar, and indicate thai tllese L,, all are weak ,r and 7r ht~nders, bu! do not enable one to distinguish between "edgc-~n' hil|di|lg of tile riug to the metal ( H. F) i'ronl dalive X -, Cr bonding, ( CI, Br). Fh~wever. il¢iiviltlon paralllUtCl'~,tor complexes cr, hibitin~ the tw~ btmtling lut~des arc diag,nt),,,lic Ita tacit litlkagc. ~C) I~)t)8 Elsevier Science S.A. A,'¢'vwOP~t,',,' Kill,'lit'r, dlld II|0,1fhiuliMII;t'hlOllli|llll ~,tHlll~k'X~'r,'. ('ilrhonyl ;;onll~J¢~¢,,,; AJkt:l|~. ¢t~lllpJt,'~,~.n
i. I n i r m l u ¢ i i o n
F:or some two decades, kil|eticists I'~cttsed their attelllion on ligandosubstitutiou reactions of 'stable' Group 6 nletal carbonyls and their substitution products, ( L , ) , M ( C O ) , ...... in which L~ strongly coordinates to the inetal ( through N, P, As, Sb, S. Se or Te donor ligands) I I I. More recently, analogous complexes containing much more weakly coordinating ligands (L,,,), containing CI 12-51 and O 161 functional grotlps, have been studied, often through use o1' Ilash photolysis techniques. This report expands these later studies through a detailed investigation of the displacement of bromobenzene (PhBr), which coordinates to Cr thn~ugh Br 17J. Ii'om its photogenerated pentacarbonylchrtuniuln(O) comple,~, governed by k ( Scheme I ). Experiinenlal evidence which indicates thai solvation accompanies vibrational deexcitation of [ C r ( C O ) ~ I * produced immediately after the Ilash has been presented 18 I. * Cort'eSl)onding :mthor. Tel.: + I-9,.1t}-565 27 13; fax: + 1-817-565 43 18: e-lnail: wyvern f~onramp.nct 0020-1693/98/$19.00 ~ 1998 Elsevier Science S.A. All righl~, reserved PIISO02()- 1693( 97 ) 0 5 5 5 0 - 3
k~lPhBrl+(11 ophBr)CI{CO),~ . Cn(CO)~. hv~ I~:r(CO)~l* ..........
kJ
ktdl,~lI (I.OCr(CO) ~ ,~ Scheme 1.
2. Experimental 2. I. Material.~' Cr(CO),, ( Pressure Chemical Co. ) was vacuum sublimed before use. The following reagents were fracLionally distilled under nitrogen from the listed drying agents ( given, together with the source, in parentheses): benzene ( EM Science. Na): Iluorobenzeue, chlorobenzene and bromobenzene ( A l d r i c h . P~O~): I-hexene. I-decene and 2,4A-trimethylpenteue (Johnson Matthey, anhydrous MgSO4); n-heptane (Sigma° Aldrich, Nat.
~l
S. l , Io.eun, el al I Im,tt~anica Chimica Acta 267 ( 1008140-54
Cill'boliyl sltelehillg ~peetra for ( I.~ ICI'| CO I, and ( L, )Crl CO)~ ctlmplexes in hydrocarl~n sol,venl Complex
I, ICO) (cnl
I)
A~" I/Z-Benzene )Crt CO ), Ii"-Fluorol~nzene lCrl CO t ~l'-Chlon~enzene) Cr(CO), 71'.Bromtfflenzene )Cr( CO )~ l¢-I-HexenelCllfO),
A',-"
E
1927(mi 1934tm1 1934(m) 19201m1
1054tvs) 1955(vs) 19581 vs) 1957(vs) 1963
0;" I-Decen¢ )Cri CO )
il;.2,4.4-Trimelhylf~ntene If'r( ("O i~ ( Aniline iCr{CO),~" (Triphenylpl~l~plli.¢)Crl CO ),"
1962 1958
2072.5 201~6( w )
1916,5
1936.5 1945(sl
"lld: I I¢,1. "Rei', 1171.
2,2. hh'nti/icalion q[ reaction inwrmediates aml products The identities of ('q'-PhBr)CrlCO)s and the trapping products. ( iT'-( l-hexene ) )Cr(CO) ~, ( r/-'-( I-decene t )Cr( CO ), and ( r/'-( 2.4,4-trimethylpentene ) )Cr( CO ) ~ were confirmed by employing a pulsed l a i r flash photolysis systenl based on a Tachisto model 850 photolyzing excimer laser IN: gas fill; 337 nm. I0 mJ, 10 ns fwhi), and a Tektronix 2440 digital oscilloscope, coupled to UV=Vis and IR analyzo ing trains. The latter employs a Spectra-Physic,~ CW lineroller,hie 1R diode laser analyzing ,~oui'ce. Tile .~yslenl ha~ been described in detail in Rel. 15 I. where a block diagranl Ior Ihe laser system i.~ giveil. Spectral e hange,~ were mt~nittwed in the 1R (1970~1915 cnl ~) by enlploying a I ntlu CitF~ IR cell with I fr{ CO ). I - 3 × 10 ~ M, £arbonyl ,~lrelehili~t ~lle¢lril wt.~re ohliiint<,d poinlobyopoilil I 1~2 eni i ini~rvill~l after palsed la,~er Ihl~h pbololy,qs of Cf( CO ),,I PhBr ( 0,5 M I / L 10,25 fVl i/lt~ti~ltlane solutions, wbi¢tl liiininiiled possible biuidobroadeiiiilg as a ¢onsequeuce of high I L, I ~. AUenlpts to observe tile very weak, higli energy A I " hands expecled at - 2070 cln ~ for ( PhBr)Cr( CO ) ~ and ( L, )Crl CO), were not .succes,sful 2.
2.3. ~lser.lklsh i~hoto,t,'~'is ki~lelics sltldies Because of superior si{~nal-toonoise ratio,, in th," visible spectral Ix,giun, kinetics studies were monitored at 490 inu, Solutions ,~ 3,5 × I0 ' M in Cr(CO),, also containing large excesses of solvent and trap were employed to insure pseudo first-order reaclion conditions. PhBr and L, were weighed so ' lilt ~';llt~llyl ~li~l~ltitlt~ ~Wt'titl 1 Niodcl 211.SXIt I;I'-IR ~pcci ~lili~iei I Iof '~laltk" ¢olltl~texe~. e~ I pilt'ridiilt" t('rl ('I) I., ~,,ell' ohlailled tl ii Ilep lall~/l~uCI ~o|llli'on.~ of t, tir~ill~ ¢!~11o21111~tllio11~,tilld il ~iti~ I'oulid thai ~i~tti1'o i~ini b~uld-k4~10uiiltl did ilol lilk~ l~lact, hi ~tdulioilx ,~ li,5 M m I. ii~ : 'S~lt.l~lllx' ( S ) il~ ilk)kit ¢l~llli~til Sl~,~i~2x ~.hit:h so l~¢al~l) ,~lktrdillttlc hi I ('rl ('O i, I Ihill Ihi2it I S tl.?i'l (~(1 t. c'Oillplext'~ tilt' ~letid~-~lt~lt" inlc~utcdlat~ on ltle lilll¢-~cal¢ ~t" I !.~ t('rl l?Ot. rcaelhm, ttow~vcl', ill "llt.,tll' s~lilti~. ~m~ I Phllr I -~ I Crl ('el. I. Phitr tuilcli~ilx t~#h ti.~ i.., t i:~ilrdiilaled hi [ ('1~1170 l, ] ) Olld tix S t ill II1,~ oulcr ~:~lordinalion ~ph.~l~¢ I.
that their concentrations wei~ known with accuracy. Temperatures were maintained within :1:0.05°C by employing a VWR 1140 external circulator and a jacketted quatlz ! cm cell. Values for k,,h, were obtained from averages of 3-10 traces of plots of In(A,-A~, ) versus time (A. and A<~ are me:zsured absorbances at time t and at inlinite time, respectively 1. The kinetics data were analyzed employiug ASYSTbased coinputer progranls developed in-house. All plols exhibited correlation coefticients > 0.99. For values of k,+,, ,~e¢ Section 5: error limits, given in parentheses as the uncero tainties of the lasl digit of file oiled value, are I .~tandard devialiou.
3, R~sulls 3, I, hh,nlff¢~ 'aliOtl ¢!I reat lalli ~~'.d/u
Tlle IX spectrulu For( ~l'~i)hlh)Cr( CO )~ was obliOllcd ill the absence of [.o,. Phlls of absorballCe versus linli: ¢~hihiled '~tep I~n¢tiOllS' over several IllS. indicating Ihat tile complex is Ullrea¢iive on this Iinie-seale, Similar spectral studies were also carried oul for ( L,, )Crl CO I~ ( I.,~ ~- Phil. PhF. PhCI I. ~o lhal the I I spectra for all four COiliplexes might be ¢olno pared. These are given in Table I. Thue-resolved spectra were taken for Cr( CO ) J PhBr ( 0.5 M ) / I ohexeile l 0.25 M ) / n=heplane solution ( Fig. I ) i see filotnole I }. C~mparisons of hires observed p~:,;l~;:'hllj 41}it nm (Vis) and 1927 cnl i lilt clearly i:~dic.l,: thai phologelleraled (llI.PhBi')Cr(CO)~
is COI;'.C,vI:"!
II, I Ihc
known
151
(il:-117~,l~lie)"
CrlCOi~ product, The dose shllilarilies of the kinetics observed for reactions taking place after pulsed laser flash photolysi~ of f r ( f O i . / P h B r solutions t'onlahlhlL2 I-d¢cen¢ and 2.4.4AriUl~lhyli~nleut~ as the il~ipphl I nucleophiles ¢on. lirnled thai analogous reticlions were taking place. 3,2, Kinetics ,xtudies
Neat solutions (containing only Cr(COh~. PhBr and 1.... but no inert dihieui. S ) ( see footnote 2 ) were found to afford
.%. lx./-~l~lll ,'t .I. / I.-rk, lmil'tl Chimh'., ,,,1el. 267 ~/~V,~J4~ -54
51
C lO.O
4, ~O
i
/
G. 40 "~
6. OO
dec
1,80
• 5[~1
1.20
I
I
I
I
I
I
I
I
I
I
IrhBrl/[L,l
FiQ. 2. Ph~l~ of I//~,,l,. ~'. I Phllr I ! I L. I at'let pul~ed laser flash ph~lol,,4~ ill' ('rl ('{) I,,!brllilluhen/¢nc' f1.. solLIlilms i.il =15.[I (" ( I., ~ l-Ilcxcn¢ I hcx I.
v(CO), cm -I I:ig, I, Tilile-rcmkcd sp¢¢llll, 1015 197l)¢n1 ', for tile reaclilln lakin~ plat'e after pulsed I:iser Ila.qi i~tilli~ll)'sin of a ('ri('Oh,/hnuli~lben/¢iie/ I-hcxcne/n-hu'plan¢ soluti~)n. Time afler the Iklsh: A. lJll: It. 75(i: ('. 2qlX)
ps.
belier si~lmls than did mhililms in which I PhBrl,I L,I << I 8 I. Conccninlilons fur both PhBr and 1., t I-hoxene. I-deccne, 7,4,4-1rJnlethyll~nlon~i wei'e v~il'ied w i d d ) l l P h l t r l , 14.$---1,7 M; I.. (i,74,4 M I; iit dilute .sohidon 1 S ~-~PhF, n-h~pl~lle ): I Phl]rl iilld I 1.. I WCl'~ also varied widely, with I I PhBrl + I l.. I ) =~ I M, Plots of I/k,,,,. vcr,~ilS I P h B r l / I L . I were IJnelir with ii corn)iron inlerCel)l iit hoth n01it and dilul~ ,~olutilln I FiT. 2 i. ¢onsi,~lenl wilh Ih~ principal nil~¢hlulisnl ollser~:od in sy~l¢lilS in which lliher I.~, are displilc~d hy I., [ 2,-.51 ' Ii
IPhlh'i('ri('()l,
--"
iI il I'lilll I
I'll
I('ri('Ol~l
I
....... I I . , i ( ' r ( ( , { ) l ~
SdlClllt'
Whelt' I('rt t ' l ) i , I i5 li ~l¢litl~ ~llil¢ iiil~rnlcdiiilt' and in terlil,~ of the psi.,ildll lir,~tollrdcr iIi1¢ ¢llli~lalit,, ~:,,,,. I i Phlh'l, I Io, I >~' I Crl f'(i i, I ). Ihe lilechiini~ :l I ,SchcniL. =l ) iifl'llrdk !he riile hiw, /,,h.=lJ,,ll..lfl/
,IPhBrl+/,:ll.,I)
(I)
w h i d l Call be r c a r l ' l i n l ¢ d to a f f o r d I h l c a r ' d O i l b l o n~¢ipro¢iil' plots. Ilk.l,, = I/kl+k
~lPhBrl/k~k:ll.,I
(2i
Thu.~ phll.~ ¢i1' I//.,,i,, VOl'SU,~ I P h l l r l / 1 1 . I )Jr{ ~Xl)~cled Ill lIC linear, with inlcreepts. I/lt~. and slope~, k , I k , k , , ;is i)b.~ervcd ( Pi I. 2 ). Frolll ihcs¢ di.llll Ih~ 'conlpclilion i'aiJo.s'. k,//~ i. can h0 ohlaJned ~i,~ h~icrcepllshtpe of the "dl)uble re¢iproclir i)lots, The inier~g~i.s of ihc rc¢ipi'l)¢al plol.~ arc r~hilcd exclusively Io the unJntol¢¢ular i'illcs of di,s,so¢ililiotl of Pkilr i'ronl ( l i i - P h l t r ) I('rl CO)~. :uld thu~ .shl)Lild be Ih¢ ' The rlitcs ill' cliinbhililion ,ll I. iiiid I ('r( ('0),, I lake I)hicc Oil the ps lime.~cal¢. Ia.qer Ihan n.iles ill diflu~i.n: lhtls [.., ~I.. cx.c'halilC L'~I1 involve iqll)' [. whidl iilC iil[catl~ pnlperly pllsililmt'd Ibr i'¢iit~lion, Ih¢ ill¢<:hlilli.~ln is Illu~ i~el'hlip~ fleet Ihlluthl id as I,i.
I .du, c~nc, tdec" I, },4.4-trillielh)
Ipcnlcne I pellt ) I.
S~illl¢ for all nudeophilos, also consislenl with ol')scrvalion ( Pi 7. 2 i. The ral¢ con,stalll~, ki/~:/k i. ki and /Q/k i. ~ir0 pl'eSerited in Tahl~ 2 4. Values o I ~ H ' and -%.~"' for I OCO ) <(~l'-.ltrPh hond dissociation are ~Jven in Table 3 Ivid¢ infra i ~ .t,t. 77w is!lhl('.(¢ ~!t 'ilWrl' dih.'.r~" <.l n/ : Studies were alno conducted Io exarrliri¢ tile ¢1"f¢clof solul i o n ¢ o n l l l t ) s i t i o l l oil i'~.Itc Lind illCdl;.lllJ~lll. This w~s d(mc Iir',l Io CX;.LlllJI1¢ Ilics~ ~.'11~111~ ~.1.",lh¢ soh.llions ~re ¢ o n v c r l e d f r o n l
"neat', conulinin~ only F)hBr .'rod L,. m "dih,tc', m which the Im.,d,mlin;mt O,'qlll'~oileltl i', ;.i "sol~..ent'. S. Thi,, ~,:ts of letted through dihltioli of equal con~'enlr[iliorls of PhI)r. I.,, ,.rift') S ( PhF, i].-hel~tan¢ ). j;~}l' the 'reversihh.' tli~,~wi;liiw." tlle~:h~misnl ~'l)ll~jSlt:lll wilh the' daln ( S¢}iem~' 2 ). t~,hele [ Phlh'l I I., I. lh¢ r~ttc h.l~, ( i!q, ( 2 ) ) l~ec~,ne~
Thus, A,,i,. ~hotlhl ;il'lpear ii~ Ih¢ inlcrccpl ill' a plol llf/,,,i,~ ~,¢r,Stl,~ (I Pkltrl ~ I1,.I ). alld tiny Jllllu¢llC'¢~ Oll ral¢ ill Ihc' dlaltg¢ froltl 'll¢~ll' IO dJhll¢ ,sohlliOll ( its I I,, I d¢¢r¢:i,~¢,~ I will hc ~×p~ci~d Io ilpp¢iir )is ii ,shape for Ihi~ phil, "I'h¢~¢ phlis, fl~r d~llli lit 31~<~C. ,~llown in Fig, 3. ¢xhihil lilrg¢ Jntcr¢¢pl,~ I k . . t. io~¢thcr with ,small slop0s, ~.~. Kinclics tlalli ill,SO w¢1'¢ tlikcn f o r ' d i h i l ¢ " ,~ohllJOll,~ ~ h ¢ i t " I P h B r l / I I.,I wan ~,,aried I S ~ (I PI~Brl + I!.,I ) ,= I M ). ~ il I'iillCiJl)ll o f tile i d c n i i l y
olS
( P h B r . I)hl '. li-h¢l~liin¢ ) lind I..
( I -hcxcne, I .decent. 2.4.4qrhlt¢ihylp¢lilen¢ ). vv'hich ~llould L AIh.'lllpt', tl~ obtaill IhL' 'lll'it|l;If~ ' olnll'~etitiorl lj|ill~. ~h//* , ( ,~4.'hL'H1L' I 1
i1~ cnlpIo)m~ ~idci F vatFl,~ I I,,, i, I I , I aml bl,h Vi~ ~md IR N~L'cinq~l~,~t¢lm¢|l'~f h~l%L' thll~ i~lf I~d It) illL'tHIL'hl~,i%'e r¢~)lIl~; ¢[l,~rt,, are ~,lliltifluJl|)~,
" ,%ilK'¢ mlerL*cpt~, ifl'the dllul'fle r¢¢ipr(i~.'al plot,, ~)f I/~.,~.. ,, Cl',,u,, I Pid]r [ / J I... I are relati~,ely ~,111411.riley cxhil'~il r¢l-~ltP,'ely larD.' crmr lira)L,,. IIm,,¢~.~ r, ~iilee [11¢~ ate delllon~,lra',¢d t~ b~ i11dcp¢ndcnl ()1 the id¢illily ,fl* J.. and ~l ~,, ermt lira)l,, h)r ,~11" ~nid .~.S". wer¢ derb.o.I ha,,¢d ~lt lhe eXl~eriment,d unu'¢rtainlie', I)t values o1/, t. will) tile stantl~jrd deviation ha,,,,:d (~rl ;.dl ~,dhi,:~ ur kj ( 4 seis ~d dala GIr ?, I..'. Table 2 ).
~2
,~ l+a+hL+ana{'l +d / lnorg+ n/ca Chimi{'a Acta 267 1 lqg,'¢J 40 54
T+ble 2 R+tle ¢on+~l;ml~ I'i+~rreactitms of photoge.¢rated I r/+-PhBr!CrlCO)+ with various Irapping nu¢leophilcn 11+. = I-hexcnc. I-decent. 2.4.4-trimcthylpentenc ) in x agitHl~ inert ,~l+,ems ( S = fluoroben/ene, n-hepl:mc ) and +arious temperatures S.I..,
TI+C)
25J(+) 30.0 35.0 45.0
PhBr, I .hexene
n+hcptane, I-h~xe.e PhBL I od¢cen¢
PhBr, 2,4,4-trim¢fl~ylpcnten¢
l.+,Ik
4.6( 7 ) 7.83( I I )
050( 8 ) 0,47(7)
10.6(6)
0,51(9)
24( 4 )
0.47( 7 )
35.0
45.0 25.t)
21 * 4 ) 4.418 )
25.0
4.31 7 )
0.6~113
35,0 4.+ 0 25,0
OAf 1"71
O.611121
35,0
45.0
2541
LI~X~
11.2914 2.13(3 4,371 7 8.541 I 1,8513 2,9416 5,76~ 12
0.45(t) ) 0.40( ? ) O.42(9)
105(H)
4.613t 9.6112) 211131
~)
2.29( 3 ) 3.70 ( 4 ) 5.33(8
0.43< 9 )
&
nohcplan¢. Iohcxcne
IO "k~k,/l,
~
5.019 ) 9.6(17)
25.O
PhF. I-hexen¢
10 ;I., (s ')
057(3)
II,i314
I ITIHI 1.17,(7) I .q~t( 181
5,4117 111+813I
211.1~11~I
10 k,., l', +)
(M
1.471 2 )
(1.(158(8 !
lO~ku in
')
~. 40
I+00
h~p
+i
J
l, .0
i, ~++0
lPlt~! ~ P~xl Ig 8 ~. Phil', ol 1,+, ~.,,,. I Phltlq ~ I ~exl allct p.b, ed la.,cr Ila.,h plmlol~,,i,, o | IUI+I('{1)+./hlOltlOl'~+21|/t+.n~+./I-h~xcltt..' ~.ul~.~:tll I ++N ) *.olullol).. at 2~(" +¢ + ,+heptane (h e p L llu.or,hcn~.e,¢ I Phl ;L) )
ol~:y Eq, t 2 ). The,~e plots are illustrated in Figs, 2 and 4, ttlld 01~ CtlrlSiM~211l w i t h |he "reversible dis,st)¢iation' Illek'hauisllt,
The nite ¢on+~lants, k~, Ior dissertation of PhBr lhmt photo+ generated t~.PhBr)CrtCO)+, k:/k ~, the "¢Oml~lition retires', and ktkflk ~, together with k,,,, and ku lbr dilution studies where I PhBrl ~:- I I+hexene I and where A,,rVl~p+,,,t,nt+, It1¢ influence of dilution upon rate ( Eq. t 3 ) t. aim+ ai~ ;ivcn i. Table 2 4, Dls,:u~i.. 4, P. Kinet6 '.~+and ,+c+'h{+m.v+++
~,QO
t},~O
0~10
i|'+V
il~Bd/ll~xl l'it~ 4+ l'1ot~ oF i I/..,.. v... [ l + h l h l / i h c x i for IL';.wiiliil.~ |+.ikia|j... llhi++'J,. allL'i
Imlwd i.t~cs ilash phuM+~ sis of (.'it ('O i+./bl~+m~4~en+cnc/i .Itexcat¢/..oh+¢,t ( *+-~ + sqdutit+it+,at ~.()++(' ( ~ + It+aL l|tmrohetl/¢lt+, llohcphl++¢ L
mechanism shown in l-ig. 2. Vatues of ~dq/!/, + + k:). whdhcr ohlah1+d directly froll| plol.~ of k,,.,, ver.~um ( i L, ] + I PhBr] ) or w, the reciprocals of ( ,,,lope + h'ilcrccpt ) of pit+In of I I/,,,,, v¢rsus i PhBrl / l L, l in d+Itlt¢ s,+ItithUl 151. Wt~t~, calctlhllcd tu lit, the maine, wilhhl cxperimelllal tlllC~2t~++till|~,
The data ab, o it|tile,.IlL' thal soivel|t +:l'l+~.'l..,ill lhe.~e syslelSl~ art" slight. +lild that ttal;.i taken i. 'lll.'~.lt" solutmns ¢olllainill~ only !+,, and I+,, Ibr x+hit'h signalqoqmise ratios of kinelics data arc Ol+thuizcd. well rclm:~¢nt the rates and mechanism of Ih+ I+,/i+, exchange process: the differences hi nile conslants t+lwo:n the 'neat" dala and Iho.,,¢ in dilule sohtlion arc les~ titan 2ll t.. T h o ~alue,,, f(+r the m l e r c c p l + o f p l o l s o f I ]k.l,,
"l'hevery cxteunive kmcticn data I~l~+rtcd h+l'~ al'~ interredly ¢~msixtcnt and lull). SUplX+rt tl)e 'r+zvcrsihle disstmiation"
ver~u+ I P h B r l / I L ! aw the ~am+. within exl~,rimenlal tllli;erttlilltyl thus /~,, the rate CIIII,~I++IIIII+or unimolect, lar
S. l++++h+~a++++~'l +eL / l++m'.+am~'+t ('hhttfi '~t At 'let 2fi 7 r / ~+V~i 49 .-54
Cr-Br bond lission, of fundamental importance, related as it is to the Cr-PhBr bond strength, is independent of the nalnre of the solution. The influence of the identity of l+~ upon rate is signilicant only for the "competition ratios', k,]k ~. obtamable as intercept/slope of plots of I ik,, h, versus I PhBrl / I L, I t Eq. ( 2 ) ). Since the rates or" interaction of PhBr with 1Cr(CO) s I arc expected to be independent of the identity of the incoming nueleophile. L.. changes in the "competition ratios" appear to be dominated hy changes in/,> Competition ratios vary as a function of tltc identity of L~ in the order l-hexene< I-decene < 2,4,4-trimethylpenlene. This is not an easy order Io inteq~rct; il is not related to the rclative rates of diffusion of L, through the solution 191, nor to steric effects in the vicinity of Ihe the olelinic linkage, which are expected to he grealest for 2,4,44riinelhylpenlene. It would seem reasonable Io suppose that chain 'agostic' C-H-Cr interactions play a nile+ perhaps Ihmugh a "chain-walk" reaction pathway 15, I01, to afford the rcaclion product coordinaled tlnou,,-,h the double bond. Another possible inllnence on the "¢onlpetition ratio" might bcv, hethcr the "agostic" hydrogens m'e secondatS or lerliary, Evidence in rel:fled W systems have indicated Ihal - ( ' - H - W bond strenglhs incre:ts¢ in Ihe ordcr terliary
4.2, B,,mlim,, It has bt'en sh~wvn IhHI ;li'ent.,s exhibit IV,,O dilferin~ m o d e s
of blinding Io ('r, For Phi I ',lUd PhF it hits been proposed Ill~,ll Ihi', I+ondinb., is tf,('.('. +edge otl'+ Ihrt~uFh im is~daletl ling double buml 17 l; ~uch buntlmt: ha,. hccn oh~cr~cd Io lake place m related s)slCmn i I~I l'.r Ph('l mid PhBr. l~nndin~! likely involves. °¢Oilvenlional' dalivc inlcraclion lilrolJBh a hme pair on ihc halogen 1141. Table 3 exhibits vahle~ l'or/,, i~+rdissocinlion of each of lile~e arei|c', Iroin Cr. h indi,.'alcs that lilt+,wt'aker "tdgeoon' bonding is disfavored kinclically vcrstts dative bonding tln'otlgh tile halogen, and that the Br+Cr bond is slightly stronger than is the CI-Cr bond. plcsttm. ably as a resnll of the greater hasicity of Br versus ('1.
.53
Presuming that the enthalpy of activation appmxhnales the Cr-Br bond strength l-5 l, the kdter is about 13 kcal sol + The enlropy of activation is slightl)+positive, not inconsistent with the dissociath~e medmnism
;+lclJvalJorl. alld nlor¢ posilive
eutrt)l~i¢~, of ;lClJv;llJoll
that for "e<.lge+or~" b o n d i n g ,
'l'al+le 3
I'~iLlL' +;OIl'+l.tlt|'. +.tilt[ dt-'li~, dllOll rrHliltliCiL'l ', lot I+++.HIO~.~ll'Pqillllqd I'tL'tl|d+,:+if'l'ltHl'.l t'htIH1)lt|tiH I)) ~+'+.sii~l+~l+,"~t TM,
,~|nh!.liltlt'ill
l~+.~.{I('l'+ ')
ItCh/erie Iqtauqdwnlcne ['hhmff~cn,'cilc Ihm~,ol+,c+~zcm: Aniline Tfil~hen,rll)hosphmc
2,57 ~Sl. IW I. ~L} ( 5 ) ,~ I().... 5 . ~ I IX}-, l()' 4,(+q ( ] I ) ," ItF 2?,4 > It) ' ~' lu ,t Ill " '
At 24+')"('. R¢I, i 15 I. ~' At 23 ~"('. RcI. I IOI. ' l'~slhnalcd l'rtml eMral'Jt~la+ionof data taken at I I()+ 13(1"('. L'
illtd Ht'li,:
Tahle + Bawd un IhJs illlelltr¢lalion, il thn,~ i,, nol ,,Ulpl
~II"
(k,:+dmol +)
~L4( I ) ')~+( 21
14 I}<41 13, I( 2x I 2h,4( I I lh.~,l Itlt
.~,' tualk +2..4< %I 22~ ~ I lfl I+ I~,~ ~h ~) ) Iq( ~ I 13(5 I
+n.,l 't
lor d;ll D,t'
54
S. lauh,,~,ma et al. t Im,rg,mi,',t Chimi,'a A,'/a 2671 i')08149-54
$. Supplementary material
Ii(}1 J.D. Simon and X. Xie, J. Phys. Chenk, 93 (Iq89) 291; X. Xie and
Tables of values of k,,, for the kinetics studies ( 7 pages) are available from the authors on request.
I I ! i Y. lshikawa. C.E. Brown, P.A. Hacker[ and D.M. Raynor, Chem. Phys.
Acknowledgements The support of this research by the Robert A. Welch Foundation under Grant No. B-434 is gratefully acknowledged. We thank Professor Michael B. Hall. Texa.s A&M University. for providing data in advance of publication.
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