Catalytic amine-borane dehydrogenation by a PCP-pincer palladium complex: A combined experimental and DFT analysis of the reaction mechanism

Dal on T ansac ions PAPER Ci e his: Dal on T ans., 2013, 42, 3533 Recei ed 27 h Sep embe 2012, Accep ed 4 h Decembe 2012 DOI: 10.1039/c2d 32273k www. sc.o g/dal on Ca aly ic amine-bo ane dehyd ogena ion by a PCP-pince palladium complex: a combined expe imen al and DFT analysis o he eac ion mechanism†‡ And ea Rossin,* a Gio anni Bo a i, b Ana M. Lozano-Vila, b Ma ga i a Paneque,* b Mau izio Pe uzzini,* a And ea Rossi a and Fab izio Zanobini a Ca aly ic dehyd ogena ion o ammonia-bo ane (NH 3 ·BH 3 , AB) and dime hylamine bo ane (NHMe 2 ·BH 3 , DMAB) by he Pd II complex [( Bu PCP)Pd(H 2 O)]PF 6 [ Bu PCP = 2,6-C 6 H 3 (CH 2 P Bu 2 ) 2 ] leads o oligome iza ion and o ma ion o spen uels o gene al o mula cyclo-[BH 2 –NR 2 ] n (n= 2,3; R = H, Me) as eac ion byp oduc s, while one equi alen o H 2 is eleased pe amine-bo ane equi alen . The p ocesses we e ollowed h ough mul inuclea ( 31 P, 1 H, 11 B) a iable empe a u e NMR spec oscopy; kine ic measu e- men s on he hyd ogen p oduc ion a e and he ela i e a e cons an s we e also ca ied ou . One non- hyd idic in e media e could be de ec ed a low empe a u e, whose chemical na u e was explo ed h ough a DFT modeling o he eac ion mechanism, a he M06//6-31+G(d,p) compu a ional le el. The compu a ional ou pu was o help o p opose a eliable mechanis ic pic u e o he p ocess. In oduc ion The inc easing ques o efficien and sa e hyd ogen ese oi s o p ac ical applica ions in uel-cells echnology wi hin he ame o he so-called “hyd ogen economy” 1 has led o an eno mous g ow h o he ela ed hyd ogen s o age esea ch ield in ecen yea s. 2 As a as chemical s o age is conce ned, one o he mos p omising ma e ials ha can sa is y all he a o emen ioned equi emen s is ammonia-bo ane (AB). 3 In ac , AB is an easy- o-handle solid, i is he mally s able and wi h a ela i ely high hyd ogen con en (19.3% w H). Homo- geneously-ca alyzed AB dehyd ogena ion/hyd olysis is he pa h ha he ino ganic chemis can ollow o exploi AB as hyd o- gen s o age ma e ial. A numbe o li e a u e examples a e known, whe e H 2 e olu ion is mainly ca alyzed by u henium, 4 hodium, 5 and i idium 6 o ganome allics. Pince - ype me al complexes in pa icula ha e been p o ed o be ex emely efficien as alkane dehyd ogena ion ca alys s. 7 Gi en he iso- lobal analogy be ween AB and e hane, some I -based pince sys ems ha e been also exploi ed o AB dehyd ogena ion. 6 G oup 10 ansi ion me al complexes ha e been sca cely exploi ed o achie e his a ge ; o he bes o ou knowledge, he only li e a u e s udies o da e a e ep esen ed by he Ni 0 -Ende s’NHC (1,3,4- iphenyl-4,5-dihyd o-1H-1,2,4- iazol-5- ylidene) complex 8 ha can elease mo e han 2.5 equi alen s o H 2 pe equi alen o AB wi hin 4 h a 60 °C and by he Pd II complex [Pd(MeCN) 4 ][BF 4 ] 2 , 9 eleasing up o 2 equi alen s o H 2 pe equi alen o AB wi hin 60 s a oom empe a u e. Some nanos uc u ed Pd 0 he e ogeneous sys ems ha e been also examined o AB dehyd ogena ion, 10 wi h compa able esul s. He e, amine-bo ane dehyd ogena ion/oligome iza ion by an ai -s able obus PCP-pince Pd II complex [( Bu PCP)Pd(H 2 O)]- PF 6 [2, Bu PCP = 2,6-C 6 H 3 (CH 2 P Bu 2 ) 2 ] is desc ibed. The expec ed compound has been gene a ed om he co espond- ing chlo o complex ( Bu PCP)Pd(Cl) (1) a e halide abs ac ion Scheme 1 Syn hesis o complex 2. †In memo y o D S e ano Midollini, in ecogni ion o his ema kable scien i ic con ibu ions o he g ow h o he I alian o ganome allic chemis y. ‡Elec onic supplemen a y in o ma ion (ESI) a ailable: Fig. S1–S5, comple e c ys allog aphic da a and ables o 2, Ca esian xyz coo dina es and ela ed absolu e G THF ene gy alues o 2 H _AB,2 H _AB2,2 H _BNBN,2 H _cyc and [( H PCP)- Pd(η 2 -H 2 )] + . CCDC 891104. Fo ESI and c ys allog aphic da a in CIF o o he elec- onic o ma see DOI: 10.1039/c2d 32273k a Consiglio Nazionale delle Rice che, Is i u o di Chimica dei Compos i O ganome allici (ICCOM-CNR), Via Madonna del Piano 10, 50019 Ses o Fio en ino (Fi enze), I aly. E-mail: [email p o ec ed] b Ins i u o de In es igaciones Químicas, CSIC and Uni e sidad de Se illa, A . Amé ico Vespucio 49, 41092 Se illa, Spain This jou nal is © The Royal Socie y o Chemis y 2013 Dal on T ans., 2013, 42, 3533–3541 | 3533 Published on 04 Decembe 2012. Downloaded by Cen o de In es igaciones Cien i icas Isla de la Ca uja (CICIC) on 23/05/2014 12:22:29. View A icle Online View Jou nal | View Issue wi h AgPF 6 in we THF (Scheme 1). The [( Bu PCP)Pd] + agmen , wi h a coo dina ion acancy on he me al cen e, is a he eac i e, and he emp y si e is easily occupied by wa e molecules, hus o ming he ela ed aquo complex. Expe imen al sec ion All eac ions we e pe o med using he s anda d Schlenk p o- cedu es unde a d y ni ogen a mosphe e, unless speci ied. All sol en s employed we e pu ged wi h ni ogen o 10 min be o e use. The Bu PCP(H) ligand (STREM Chemicals L d), sil e hexa luo ophospha e (STREM Chemicals L d), ammonium e achlo opallada e(II) (Ald ich), ammonia-bo ane complex (Ald ich), dime hylamine-bo ane complex (Ald ich) and i- e hylamine-bo ane complex (Ald ich) we e used as pu chased, wi hou u he ea men . Deu e a ed sol en s (Ald ich) we e degassed by h ee eeze–pump– haw cycles be o e use. NMR spec a we e eco ded on a BRUKER AVANCE II 300 MHz spec- ome e , equipped wi h a low- empe a u e measu emen ool. 1 H and 13 C{ 1 H} chemical shi s a e epo ed in pa s pe million (ppm) down ield o e ame hylsilane (TMS) and we e calib a ed agains he esidual p o ia ed esonance o he deu e a ed sol en ( 1 H) o he deu e a ed sol en esonance ( 13 C{ 1 H}), espec i ely, while 31 P{ 1 H} was e e enced o 85% H 3 PO 4 wi h down ield shi aken as posi i e. 11 B chemical shi s we e e e enced o BF 3 ·OE 2 . In a ed spec a we e eco ded wi h a Pe kin-Elme Spec um BX FT-IR spec o- pho ome e . The C, H elemen al analyses we e made a ICCOM-CNR using a The mo FlashEA 1112 Se ies CHNS-O elemen al analyze wi h an accep ed ole ance o ±0.4 uni s on ca bon (C) and hyd ogen (H). Syn hesis o ( Bu PCP)Pd(Cl) (1) This compound has al eady been epo ed in he li e a u e; 11 in he e, an imp o emen o he o iginal p epa a ion is epo ed. Ammonium e achlo opallada e(II) (NH 4 ) 2 PdCl 4 (0.14 g, 0.5 mmol) was dissol ed in 20 mL 2-me hoxye hanol a oom empe a u e. The ee ligand Bu PCP(H) (0.2 g, 0.5 mmol, 1 equi .) was added o his solu ion, and he esul ing mix u e was hea ed a 100 °C o 40 min. A e ha ime, he colou less solu ion was b ough o oom empe a u e, and a whi e c ys al- line p ecipi a e o pu e 1(0.2 g; yield: 80%) was ob ained a e concen a ion o ca. 5 mL and addi ion o 15 mL H 2 O. The p e- cipi a e was il e ed, washed wi h small po ions (3 × 10 mL) o an E OH–H 2 O mix u e (4 : 1) and inally d ied unde a b isk ni ogen s eam. 31 P{ 1 H}-NMR (121.49 MHz, ppm, CDCl 3 ): 72.1 (s). Syn hesis o [( Bu PCP)Pd(H 2 O)]PF 6 (2) 0.1 g (0.19 mmol) o 1we e dissol ed in 15 mL THF and 0.1 mL H 2 O. To his solu ion, 0.06 g (0.22 mmol, 1.2 equi .) o solid AgPF 6 we e added in one po ion. Immedia e o ma ion o a whi e AgCl p ecipi a e was obse ed. The suspension was le o s i igo ously o e nigh a ambien empe a u e, in he da k. A e ha ime, he colou less supe na an was il e ed and concen a ed o ca. 5 mL unde a b isk ni ogen lux. n-Pen ane addi ion (10 mL) led o p ecipi a ion o 2as a whi e c ys alline solid, ha was washed wi h small pen ane po ions (3 × 10 mL) and d ied unde a ni ogen s eam (0.12 g; quan i- a i e yield). Single c ys als sui able o X- ay analysis we e g own om THF–n-pen ane solu ions a ambien empe a u e. Anal. Calcd o 2,C 24 H 45 F 6 OP 3 Pd (662.91): C, 43.48; H, 6.84. Found: C, 43.25; H, 7.01. IR (298 K, KB , cm −1 ): 3312 w [ν(O– H)], 2960 m, 2900 m, 2870 m [ν(C–H)], 1627 w [ν(C C)], 1472 m, 1370 m, 1265 m, 1178 m, 1021 m, 963 w, 843 s [γ(C–H)], 557 s [ν(P–F)]. 1 H-NMR (300.13 MHz, ppm, THF-d 8 , 298 K): 6.95 (A H, 3H, m), 5.12 (H 2 O, 2H, s), 3.33 (PCH 2 , 4H, , * J H–P = 4.1 Hz), 1.40 (CH 3 , 36H, , * J H–P = 6.9 Hz). 13 C{ 1 H} NMR (75.46 MHz, ppm, THF-d 8 ,298K):149.7(C a -ipso , , * J C–P = 9.8 Hz), 146.4 (C a -p , s), 124.0 (C a -m , s), 121.0 (C a -o , , * J C–P = 10.4 Hz), 32.9 (PCH 2 , , * J C–P = 7.6 Hz), 29.4 [PC(CH 3 ) 3 , , * J C–P = 11.2 Hz], 26.6 (PC(CH 3 ) 3 , s). 31 P{ 1 H}-NMR (121.49 MHz, ppm, THF-d 8 , 298 K): 76.7 (PCP, s), −148.0 (PF 6 , sep ., J P–F = 710.5 Hz). X- ay da a collec ion X- ay da a o 2we e collec ed a low empe a u e (120 K) on an Ox o d Diff ac ion XCALIBUR 3 diff ac ome e equipped wi h a CCD a ea de ec o using Mo K α adia ion (λ= 0.7107 Å). The p og am used o he da a collec ions was C ysAlis CCD 1.171. 12 Da a educ ions we e ca ied ou wi h he p og am C ysAlis RED 1.171 13 and he abso p ion co ec ions we e applied wi h he p og am ABSPACK 1.17.13. Di ec me hods implemen ed in Si 97 14 we e used o sol e he s uc u es and he e inemen s we e pe o med by ull-ma ix leas -squa es agains F 2 implemen ed in SHELX97. 15 All he non-hyd ogen a oms we e e ined aniso opically while he hyd ogen a oms we e ixed in calcula ed posi ion and e ined iso opically wi h he he mal ac o depending on ha o he a om o which hey a e bound. A diffe en ea men was used o he hyd o- gen a oms o he aquo ligand: hey we e ound in he Fou ie densi y map, he coo dina es we e ee o e ine and hei he mal ac o s we e bound o ha o oxygen. Geome ical cal- cula ions we e pe o med by PARST97 16 and molecula plo s we e p oduced by he p og am ORTEP3. 17 VT-NMR moni o ing o he eac ion o 2 wi h excess BH 3 ·L (L = NH 3 , NHMe 2 , NE 3 ) Only he eac ion o 2wi h BH 3 ·NH 3 is desc ibed, as he o he bo ane adduc s we e eac ed wi h 2in an analogous manne . In a ypical expe imen , 2(0.040 g, 0.06 mmol) was placed in a 5 mm qua z NMR ube unde an ine a mosphe e. THF-d 8 (0.5 mL) was ans e ed in o he ube ia a cannula, and he esul ing solu ion was b ough o 190 K using a d y ice– ace one ba h. 5 equi . BH 3 ·NH 3 (0.0093 g, 0.3 mmol) we e dis- sol ed in he minimum amoun (0.5 mL) o THF-d 8 in o a sepa- a e Schlenk lask unde ni ogen, and he esul ing solu ion was sy inged in o he cold NMR ube. The ube was hen inse ed in o he NMR spec ome e p obe head p e-cooled a 190 K. Subsequen ly, he p obe was wa med slowly o oom empe a u e in s eps o 20 K each, and new se s o Pape Dal on T ansac ions 3534 |Dal on T ans., 2013, 42, 3533–3541 This jou nal is © The Royal Socie y o Chemis y 2013 Published on 04 Decembe 2012. Downloaded by Cen o de In es igaciones Cien i icas Isla de la Ca uja (CICIC) on 23/05/2014 12:22:29. View A icle Online mul inuclea NMR da a we e collec ed a each s ep. In he case o (nea ) liquid amine-bo ane eagen s, he equi ed numbe o equi alen s we e di ec ly sy inged in o he p e-cooled NMR ube, always kep a 190 K. Kine ic measu emen s (a) H 2 e olu ion a e. The kine ic uns we e ca ied ou wi h he Man o he Moon X102 ki (see Fig. S1‡ o de ails). Unde an ine a mosphe e, 0.010 g o AB (0.32 mmol) we e placed in a wo-necked ound-bo omed 30 mL lask connec ed o a swi chable h ee-way al e h ough a To ion sc ew. AB was dissol ed in 1 mL dioxane, s i ed a 303 K in an oil ba h, hen he al e was swi ched o he p essu e ansduce which is con- nec ed ia wi eless o he so wa e eco ding he kine ic p o ile on-line. A s ock solu ion o 2in he same sol en was added o each he desi ed 2: AB ela i e s oichiome y (see below) and he eac ion was s i ed o 18–24 hou s. The esul ing kine ic da a we e co ec ed by he injec ed olume o ca alys solu ion: he p essu e inc ease caused by he injec ion was measu ed on a blank solu ion and used o calcula e he co ec numbe o eleased H 2 equi alen s. A he end o he eac ion, he supe - na an was ans e ed o an NMR ube and analyzed h ough 31 P{ 1 H} and 11 B NMR spec oscopy, o iden i y he soluble eac- ion by-p oduc s. (b) Moni o ing con e sion o AB o DMAB o de e mi- na ion o he eac ion a e u ilizing 11 B NMR spec oscopy. The a ia ion o AB/DMAB concen a ion du ing he eac ion wi h 2a 303 K in THF-d 8 was moni o ed h ough he in e- g a ion o he ela ed 11 B NMR signals. A coaxial ex e nal s an- da d o BF 3 ·E 2 O was employed, o ha e a e e ence signal o he peaks in eg a ion. The NMR ubes we e p epa ed in he same manne as ha desc ibed abo e o he VT expe imen s. In o de o a oid sample decomposi ion, addi ion o he excess amine-bo ane (5 equi alen s) was made a 190 K and he mix u e wa med slowly o 303 K wi hin he p obe head o he NMR spec ome e . Compu a ional de ails Densi y Func ional Theo y (DFT) calcula ions we e pe o med o he AB case only o he sake o simplici y, using he Gaus- sian09 p og am ( e ision A.02). 18 Model s uc u es we e op i- mized wi h an M06 unc ional 19 (al eady employed success ully in o he cases o he ea men o molecules con- aining da i e bonds 20 ) using he LANL2DZ pseudopo en ial 21 and ela ed basis se 22 on he palladium and phospho us a oms, a 6-31G* basis se on all he o he pince a oms and a 6-31+G(d,p) basis se on BH 3 ·NH 3 . In oduc ion o diffuse unc ions is essen ial o well- ep oduce con o ma ional equili- b ia and expe imen al elec on affini ies. 23 An ex a p- ype pola iza ion unc ion o P and an ex a - ype unc ion o Pd we e added o he s anda d se . 22 Gibbs ene gy calcula ions o in e ela i e he modynamic s abili ies we e ca ied ou on a model sys em, wi h he e -bu yl g oups on he pince ligand eplaced by H a oms ( H PCP), in o de o each a sa is ac o y comp omise be ween model sys em accu acy and sho compu- a ional imes. E alua ion o he sol en effec s was pe o med h ough a con inuum modelling o he eac ion medium. Bulk sol en effec s (THF, ε= 7.42) we e exp essed h ough he SMD Con inuum Model, 24 wi h he same basis se used o he gas phase op imiza ions. Gibbs ene gy in solu ion was calcula ed acco ding o he ollowing simpli ied equa ion: GTHF ¼Ggas þðETHF EgasÞ Resul s and discussion Syn hesis and cha ac e iza ion o he ca aly ic species The syn hesis o he chlo o complex ( Bu PCP)Pd(Cl) 1was ca ied ou h ough he di ec eac ion o ammonium e a- chlo opallada e(II) (NH 4 ) 2 PdCl 4 and he comme cially a ailable Bu PCP(H) in 2-me hoxye hanol, wi h concomi an in si u C–H bond ac i a ion and HCl/NH 4 Cl elimina ion. The eac ion yield has been sligh ly imp o ed ( om 75 o 80%) wi h espec o he o iginal li e a u e epo , 11 whe e he benzoni ile adduc cis-PdCl 2 (PhCN) 2 as a Pd II sou ce was employed ins ead. Halide abs ac ion om 1wi h AgPF 6 in we THF led o he isola ion o he ai -s able compound [( Bu PCP)Pd(H 2 O)]- PF 6 (2); he aquo ligand hough is a he labile and i can be easily eplaced by o he small molecules ha en e he me al coo dina ion sphe e. Ne e heless, no eplacemen o he co- o dina ed wa e molecule wi h o he neu al coo dina ing species (ace oni ile, ca bon monoxide 25 ) was a emp ed, since hese species may be eac i e owa ds amine-bo anes, while wa e is kine ically and he modynamically ine unde he eac- ion condi ions employed in ou s udy ( ide in a); his has also been con i med by he compu a ional esul s. 26 2has been ho oughly cha ac e ized h ough mul inuclea ( 31 P, 1 H, 13 C) NMR and IR spec oscopy; single c ys als sui able o X- ay diff ac ion we e g own om THF–n-pen ane solu ions. The compound c ys allizes in he monoclinic P2 1 space g oup (Fig. 1); he coo dina ion geome y a he Pd II cen e is (dis- o ed) squa e plana {[α(P(1)–Pd–P(2)] = 166.02(3)°; α[P(1)–Pd– O(1)] = 96.86(9)°; α[P(1)–Pd–C(1)] = 83.48(9)°}; he s uc u al pa ame e s {d[Pd–C(1)] = 2.026(3) Å; d[Pd–P(1)] = 2.314(1) Å; Fig. 1 ORTEP diag am ( he mal ellipsoids a 50% p obabili y) o he asym- me ic uni in he c ys al la ice o 2. Hyd ogen a oms on he ligand omi ed o cla i y. Hyd ogen bonds depic ed as yellow do ed lines. See he ESI‡ o he comple e se o c ys allog aphic pa ame e s and ables. Dal on T ansac ions Pape This jou nal is © The Royal Socie y o Chemis y 2013 Dal on T ans., 2013, 42, 3533–3541 | 3535 Published on 04 Decembe 2012. Downloaded by Cen o de In es igaciones Cien i icas Isla de la Ca uja (CICIC) on 23/05/2014 12:22:29. View A icle Online d[Pd–O(1)] = 2.157(3) Å} a e e y simila o hose o o he species con aining he [( Bu PCP)Pd] + agmen . 27 The PF 6 − coun e ion is hyd ogen-bonded o he coo dina ed aquo ligand {d[F(2)⋯H(1B)] = 2.51(5) Å; d[F(3)⋯H(1A)] = 2.24(5) Å; α[F(2)- ⋯H(1B)–O(1)] = 109(3)°; α[F(3)⋯H(1A)–O(1)] = 142(4)°}. Va iable empe a u e mul inuclea NMR analysis The eac ion o 2wi h 5 equi alen s o AB in THF-d 8 was ol- lowed ia mul inuclea 31 P, 1 H and 11 B a iable- empe a u e (VT) NMR spec oscopy in he 190–298 K empe a u e ange. A 190 K he only species p esen in he 31 P{ 1 H} NMR spec a besides he s a ing ma e ial is a non-hyd idic in e media e a δ P = 81 ppm (Fig. 2); abo e 230 K, a slow con e sion o his species in o he hyd ide compound ( Bu PCP)Pd(H) (δ P = 94.4 ppm; δ H =−4.2 ppm, 1 J H–P = 13.7 Hz) 11,28 occu ed. Du ing he same eac ion ime, an off-whi e polyme ic p ecipi- a e was also o med a he bo om o he NMR ube. The soluble dehyd ogena ion by-p oduc s a e en a i ely assigned o oligome ic species o gene al o mula [BH 2 –NH 2 ] n (n=2–3), 29 as sugges ed by he 11 B-NMR peak cen e ed a δ B =−12.1 ppm eco ded in he supe na an solu ion (Fig. 3). I appea s as a e y b oad pseudo-qua e , esul ing om he o e lapping o wo iple s alling a close chemical shi s ( his has been con- i med by he 11 B{ 1 H} spec a, whe e wo b oad single s a e obse ed a δ B =−11.6 and δ B =−12.7 ppm. The p esence o wo peaks is likely o be ela ed o he diffe en chain size, since ea ly li e a u e epo s 29b s a ed ha he cyclic and linea isome s a e indis inguishable in he 11 B-NMR spec a). Un o - una ely, all he a emp s made o isola e he pu e p oduc s ailed. The solid p ecipi a e has been in es iga ed by IR spec- oscopy: i p obably consis s o a mix u e o uniden i ied longe -chain [BH 2 –NH 2 ] n oligome s o he same ype as hose p esen in solu ion [ he adso p ion bands ela ed o he ν(BH 2 ), ν(NH 2 ) and ν(B–N) no mal modes we e obse ed a 3284, 2425 and 1458 cm −1 , espec i ely; 30 he exac chemical composi ion hough could no be de e mined]. A e 18 h a ambien empe a u e, no all he excess o AB was con e ed in o oligome s; a comple e con e sion could no be achie ed, while he s a ing ca alys u ned in o he he modynamically s able hyd ide species ( Bu PCP)Pd(H) quan i a i ely. This is also in ag eemen wi h he kine ic esul s ( ide in a). I is likely ha he monohyd ide species ep esen s he “spen ca a- lys ”, as con i med by i s di ec eac i i y wi h AB (1 : 5), which did no lead o any H 2 p oduc ion and/o AB oligome iza ion. The eac ion wi h he N-subs i u ed amine-bo ane DMAB (in he same 1 : 5 s oichiome ic a io) p oceeded h ough a eac ion mechanism ha seems o be he same as ha o AB; again, a low empe a u es a peak alling a δ P = 82.0 ppm was de ec ed in he 31 P{ 1 H} NMR spec um, ha slowly disappea s u ning in o he ( Bu PCP)Pd(H) complex (Fig. S2‡); a he end o he ca alysis, he cyclic dime cyclo-[BH 2 –NMe 2 ] 2 was he main species obse ed in he 11 B NMR spec um as a iple a δ B = 5.9 ppm ( 1 J B–H = 113.4 Hz, Fig. S3‡). 31 No un eac ed DMAB was de ec ed a oom empe a u e a e 18 h; a odds wi h wha was ound o AB, comple e con e sion o DMAB o Fig. 2 31 P{ 1 H} VT-NMR spec a o he 2: AB mix u e (1 : 5 s oichiome y, THF- d 8 ). The ed box highligh s he signal o he eac ion in e media e de ec ed be ween 190 and 230 K. Fig. 3 11 B VT-NMR spec a o he 2: AB mix u e (1 : 5 s oichiome y, THF-d 8 ). The ed box highligh s he signal o he oligome ic p oduc ob ained du ing he dehyd ogena ion p ocess. Pape Dal on T ansac ions 3536 |Dal on T ans., 2013, 42, 3533–3541 This jou nal is © The Royal Socie y o Chemis y 2013 Published on 04 Decembe 2012. Downloaded by Cen o de In es igaciones Cien i icas Isla de la Ca uja (CICIC) on 23/05/2014 12:22:29. View A icle Online oligome s was achie ed. No e idence o o ma ion o he e hyl- ene analogue BH 2 NMe 2 was ound, ei he coo dina ed o he me al cen e o ee in solu ion (a odds wi h o he li e a u e cases 4d, ,5a,b,i,l ), p esumably because o he as dime iza ion p ocess. No eac ion occu ed be ween 2and ie hylamine bo ane NE 3 ·BH 3 (always in a 1 : 5 a io); only he un eac ed s a ing ma e ials we e obse ed in he NMR spec a, a all empe a- u es. This con i ms ha hyd ogen is di ec ly p oduced om he amine-bo ane dehyd ogena ion. Quan i a i e H 2 de e mina ion The kine ic p o iles a 303 K o he AB dehyd ogena ion eac- ion we e also de e mined, o diffe en (2: AB) s oichiome ic a ios (Fig. 4). In he (1 : 5) case, he eac ion was comple ed a e ca. 22 h, and one H 2 equi alen pe AB equi alen was eleased. Analysis h ough 11 B NMR o he supe na an solu- ion a he end o he eac ion did no show un eac ed AB, in con as wi h he obse a ion made in he VT NMR expe i- men ; his is p obably due o he diffe en hyd ogen concen- a ion in he wo expe imen s. Indeed, unde hese expe imen al condi ions, ca. 50% o 2is s ill p esen a he end o he p ocess. In ag eemen wi h ha , he inc ease o AB concen a ion in he eac ion causes a elease o less H 2 equi alen s and a mo e apid ca alys deac i a ion [i.e. o - ma ion o ( Bu PCP)Pd(H)]. Thus, wi h a (1 : 20) a io, a ound 0.9 H 2 equi alen s pe AB equi alen a e eleased be o e all he me al complex has been ans o med in o ( Bu PCP)Pd(H), wi h some AB being un eac ed in he inal solu ion. In con as , unning he eac ion wi h a (1 : 50) a io, he amoun o H 2 equi alen s eleased is ne e highe han 0.4–0.5, wi h un eac ed monome in he inal solu ion, his indica ing a quicke ca alys deac i a ion. The measu emen o he amoun o H 2 eleased in he eac- ion wi h DMAB ollowing his app oach was p o ed o be no easible. The ola ili y o he co esponding spen uel cyclo- [BH 2 –NMe 2 ] 232 p o ided an exceedingly high alue o he o al gas p essu e eco ded; i was no possible o quan i y he con- ibu ion gi en by each o he wo species. None heless, i is easonable o assume ha a maximum o one H 2 equi alen pe monome equi alen is eleased also in his case, acco d- ing o he 11 B NMR analysis o he spen uel (see p e ious pa ag aph and Fig. S3‡). Kine ic measu emen s and e alua ion o he eac ion a e h ough 11 B NMR spec oscopy Fo he sake o compa ison, he kine ic a e cons an (k obs ) alues o he dehyd ogena ion o AB and DMAB by 2a 303 K we e de e mined h ough 11 B NMR moni o ing. The com- pe i i e eac ion o nascen H 2 wi h he palladium agmen o p oduce he “spen o m”( Bu PCP)Pd(H) is esponsible o he a ia ion o he ac i e ca alys concen a ion in ime; conse- quen ly, o a mo e eliable e alua ion o he a e cons an s, only he da a poin s collec ed wi hin he ini ial 3 h we e aken in o accoun o he i ing, assuming ha he ca alys concen- a ion is cons an wi hin his ime ange. The esul s we e plo ed as a 1/[AB] s. g aph (Fig. 5), assuming a second o de kine ic a e law wi h espec o AB: −d[AB]/d =k obs [AB] 2 . The assump ion comes om he esul s o he DFT mechanis ic analysis ( ide in a), whe e he only eac ion s ep showing a posi i e ΔG alue is ha ela ed o he adduc be ween he [( H PCP)Pd] + agmen and wo molecules o AB. In o de o e i y his hypo hesis, an ini ial a e expe imen was Fig. 4 H 2 e olu ion in he AB dehyd ogena ion p ocess ca alyzed by 2(303 K, dioxane) o diffe en 2: AB a ios (◆,1:5;▲, 1 : 20). Fig. 5 (a) Linea [AB] s. and (b) no malized 1/[AB] s. plo s (second o de a e law) o he AB dehyd ogena ion eac ion ca alyzed by 2(303 K, THF-d 8 ). Dal on T ansac ions Pape This jou nal is © The Royal Socie y o Chemis y 2013 Dal on T ans., 2013, 42, 3533–3541 | 3537 Published on 04 Decembe 2012. Downloaded by Cen o de In es igaciones Cien i icas Isla de la Ca uja (CICIC) on 23/05/2014 12:22:29. View A icle Online pe o med on he eac ion be ween 2and AB (see ESI‡ o de ails). The esul s con i med he second o de in subs a e concen a ion. Following he second o de da a i ing, a inal k obs alue o 4.31(11) × 10 −4 M −1 s −1 is ob ained o AB. As o DMAB, he same app oxima ion led o a k obs alue o 2.67(5) × 10 −4 M −1 s −1 (Fig. S4‡). The eac ion a e is highe o AB han o DMAB, p obably because o a p edominan s e ic hin- d ance o he seconda y amine wi h espec o ammonia ha easonably makes dime iza ion slowe . DFT modeling o he AB dime iza ion mechanism The esul s coming om he VT-NMR s udies p omp ed us o cas ligh on he eac ion mechanism h ough DFT modeling, a he M06//6-31+G(d,p) compu a ional le el. Fo he sake o simplici y, only he o ma ion o cyclo-(BH 2 –NH 2 ) n (n=2)was aken in o accoun in he compu a ional modeling o he eac- ion mechanism. To his aim, a simpli ied s uc u e o 2was aken in o accoun o he he modynamics calcula ions, whe e he bulky e -bu yl g oups we e eplaced wi h hyd ogen a oms, o achie e he bes comp omise be ween model sys em accu acy and compu a ional ime. 33 The esul ing [( H PCP)Pd- (H 2 O)] + ca ion 2 H was assumed o be he s a ing complex. The o e all p oposed ca aly ic cycle o he AB dime iza ion eac- ion wi h o ma ion o he ino ganic cyclobu ane analogue [BH 2 –NH 2 ] 2 and wo equi alen s o H 2 (ΔG=−12.0 kcal mol −1 , Scheme 2) is epo ed in Scheme 3. Due o he complex na u e o he mechanis ic s eps in ol ed in his p ocess, no ansi ion s a es could be ound along he eac ion coo dina e; he e o e, only he ΔG alues ( he modynamics) o each s ep ha e been assessed in THF. The aquo complex 2 H eadily unde goes ligand exchange wi h one molecule o AB, o o m an η 1 -BH-bonded adduc 2 H _AB (Fig. 6). 26 The in e ac ion o AB wi h he [( H PCP)Pd] + agmen is weak, bu he exchange p ocess is he modynami- cally downhill, wi h a ΔGo −7.5 kcal mol −1 . No s abilizing in e ac ion o he ammonia-bo ane N–H bonds wi h he [( H PCP)Pd] + agmen was ound, in line wi h wha was al eady obse ed in o he cases o ca ionic ansi ion me al moie- ies. 9,6c I is p oposed ha he labile in e media e obse ed in he 31 P{ 1 H} NMR spec a in he a iable- empe a u e expe i- men s could be ep esen ed by he simple adduc be ween [( Bu PCP)Pd] + and AB, wi h he same s uc u e as 2 H _AB. In e ac ion o 2 H _AB wi h an addi ional AB molecule leads o he adduc 2 H _AB2 (Fig. 7), wi h a posi i e Gibbs ene gy a i- a ion o +4.9 kcal mol −1 . The en opy educ ion while passing om (2 H _AB + AB) o 2 H _AB2 is a plausible eason o his s ep o be endoe gonic. The p esence o he second AB molecule a o s he coo dina ion slippage o he Pd-bound AB om Scheme 2 Cyclo-dime iza ion o AB. Scheme 3 P oposed ca aly ic cycle o AB cyclo-dime iza ion in he p esence o 2. The dihyd ogen bonding be ween hyd idic and p o onic H a oms is shown in ed. Reac an s and p oduc s a e d awn in blue. Fig. 6 Op imized geome y o [( H PCP)Pd(η 1 -AB)] + (2 H _AB). Selec ed op imized bond leng hs epo ed (Å). H a oms on he pince ligand omi ed o cla i y. A om colo code: whi e, H; g ay, C; pu ple, P; blue, N; pink, B; ligh blue, Pd. Fig. 7 Op imized geome y o [( H PCP)Pd(σ-BH-AB)⋯AB] + (2 H _AB2). Selec ed op imized bond leng hs epo ed (Å). H a oms on he pince ligand omi ed o cla i y. A om colo code: see Fig. 6. N–H⋯H–B dihyd ogen bonds and σ-BH agos ic in e ac ions d awn as yellow do ed lines. Pape Dal on T ansac ions 3538 |Dal on T ans., 2013, 42, 3533–3541 This jou nal is © The Royal Socie y o Chemis y 2013 Published on 04 Decembe 2012. Downloaded by Cen o de In es igaciones Cien i icas Isla de la Ca uja (CICIC) on 23/05/2014 12:22:29. View A icle Online η 1 -BH o σ-BH agos ic. This e eals ha bo h coo dina ion modes a he Pd II ca ion a e possible, and he σ-agos ic geome- y can be conside ed a so o “ ansi ion s a e”be ween diffe en η 1 -BH modes o he luxional AB “ligand”. 34 Dihyd o- gen B–H⋯H–N bonding be ween he wo monome s is p esen [op imized d(B–H⋯H–N) = 1.83 Å]. A weak N–H in e ac ion o he non-coo dina ed monome wi h he me al cen e is also p esen [op imized d(N–H⋯Pd) = 2.67 Å]. A his s age, a me al-media ed AB dime iza ion may occu , wi h o ma ion o one equi alen o H 2 and he linea BH 3 – NH 2 –BH 2 –NH 3 dime ( he ino ganic bu ane analogue, which was also independen ly p epa ed and cha ac e ized) 35 coo di- na ed o palladium, i.e. he [( H PCP)Pd(η 1 -BH–BH 3 –NH 2 –BH 2 – NH 3 )] + adduc (2 H _BNBN, Fig. 8). The ΔG o his s ep equals −13.5 kcal mol −1 . Fu he in amolecula dehyd ogena ion (p omo ed by he exis ence o an in amolecula dihyd ogen bonding in 2 H _BNBN, as e idenced in Fig. 8) leads o [( H PCP)Pd(η 1 -BH- cyclo-(BH 2 –NH 2 ) 2 ] + (2 H _cyc, Fig. 9), whe e he s abilizing σ-BH agos ic in e ac ion p o ides ΔG=−1.1 kcal mol −1 . A his s age, eplacemen o he cyclic dime by ano he AB “monome ”(ΔG=−2.2 kcal mol −1 ) egene a es he s a ing ac i e species 2 H _AB o ano he ca aly ic un. Du ing he eac- ion cou se ca alys deac i a ion p og essi ely occu s, h ough di ec in e ac ion o 2 H wi h nascen H 2 and subsequen o - ma ion o he non-classical hyd ide [( H PCP)Pd(η 2 -H 2 )] + (Fig. S5‡). This eac ion is almos he moneu al (ΔG=+ 0.8 kcal mol −1 ). Despi e he lack o expe imen al e idence o he exis ence o such an uns able and eac i e dihyd ogen compound {only he pla inum(II) analogue [( Bu PCP)P (η 2 -H 2 )] + was expe imen ally syn hesized h ough p o ona ion o he classical ( Bu PCP)P (H) and de ec ed in he 1 H NMR spec um a e y low empe a u es (180 K) 36 } and he sca ci y o genuine non-classical Pd molecula hyd ogen complexes, i can be possibly in oked as he ansien p ecu so o he inal classi- cal monohyd ide. Conclusions In summa y, amine-bo ane dehyd ogena ion/oligome iza ion media ed by he obus palladium (PCP)-pince complex [( Bu PCP)Pd(H 2 O)]PF 6 has been s udied h ough a combina ion o expe imen al (VT-NMR, kine ic p o iles) and compu a ional (DFT) echniques, wi h he aim o un a eling eac ion in e - media es and he amoun o H 2 p oduced unde he applied expe imen al condi ions. A odds wi h o he Pd II -based sys ems, 9 he pince compound examined p oduces only one H 2 equi alen pe AB equi alen . This e eals a limi ed de- hyd ogena ion abili y ha leads o “alipha ic” a he han “a o- ma ic”(bo azine-like) oligome s as a spen uel. A he same ime, his inding can be seen as an ad an age om an engin- ee ing poin o iew, since in hyd ogen uel cells echnology signi ican effo s a e being made o educe o elimina e bo a- zine as a de imen al side-p oduc . Besides, hese mechanis ic s udies a e o undamen al impo ance in ligh o he de elop- men o be e -pe o ming homogeneous ca alys s o an efficien AB dehyd ogena ion. No el pince - ype ansi ion me al o ganome allics a e cu en ly being p epa ed in ou lab- o a o ies, wi h he aim o es ing hem as no el amine-bo ane dehyd ogena ion ca alys s. Acknowledgemen s The PIRODE p ojec o he I alian Minis y o he En i on- men , he EFOR p ojec o he I alian Na ional Resea ch Council, he Spanish Minis y o Science (p ojec s CTQ2010- 17476 and Consolide -Ingenio 2010 CSD2007-00006) and he Jun a de Andalucía (p ojec P09-FQM-4832) (FEDER suppo ) a e acknowledged o inancial suppo . The CNR–CSIC bila - e al ag eemen is acknowledged o mobili y unding h ough he “Chemical hyd ogen s o age: amino bo anes ac i a ion wi h ansi ion me al o ganome allics”p ojec (CSIC 2010IT0036, CNR 11544). A. M. L.-V. hanks he CSIC and ESF o suppo h ough he JAE-doc p og am. The use o he com- pu a ional acili ies o he Cen e de Se eis Cien í ics i Aca- dèmics de Ca alunya (CESCA) is g ea ly app ecia ed. Thanks a e also gi en o he p ojec Fi enze Hyd olab-2 suppo ed by ECRF. No es and e e ences 1(a) R. Lan, J. T. S. I ine and S. Tao, In . J. Hyd ogen Ene gy, 2012, 37, 1482–1494; (b) T. Umegaki, J.-M. Yan, Fig. 8 Op imized geome y o [( H PCP)Pd(σ-BH–BH 3 –NH 2 –BH 2 –NH 3 )] + (2 H _BNBN). Selec ed op imized bond leng hs epo ed (Å). H a oms on he pince ligand omi ed o cla i y. A om colo code: see Fig. 6. N–H⋯H–B dihyd o- gen bonds and σ-BH agos ic in e ac ions d awn as yellow do ed lines. Fig. 9 Op imized geome y o {( H PCP)Pd[σ-BH-cyclo-(BH 2 NH 2 ) 2 ]} + (2 H _cyc). Selec ed op imized bond leng hs epo ed (Å). H a oms on he pince ligand omi ed o cla i y. A om colo code: see Fig. 6. σ-BH agos ic in e ac ions d awn as yellow do ed lines. 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T uhla , J. Phys. Chem. B, 2009, 113, 6378–6396. Pape Dal on T ansac ions 3540 |Dal on T ans., 2013, 42, 3533–3541 This jou nal is © The Royal Socie y o Chemis y 2013 Published on 04 Decembe 2012. Downloaded by Cen o de In es igaciones Cien i icas Isla de la Ca uja (CICIC) on 23/05/2014 12:22:29. View A icle Online 25 Li e a u e examples con aining he [( Bu PCP)M(L)] + ca ionic agmen a e: (a)[( Bu PCP)Ni(MeCN)] + : V. A. Le ina, A. Rossin, N. V. Belko a, M. R. Chie o i, L. M. Eps ein, O. A. Filippo , R. Gobe o, L. Gonsal i, A. Lledòs, E. S. Shubina, F. Zanobini and M. Pe uzzini, Angew. Chem., In . Ed., 2011, 50, 1367–1370; (b)[( Bu PCP)P (CO)] + : D. Vuzman, E. Po e eno , Y. Diskin-Posne , G. Lei us, L. W. Y. Shimon and D. Mils ein, Dal on T ans., 2007, 5692–5700. 26 The di ec eac ion o AB wi h he coo dina ed wa e mol- ecule did no lead o easonable ΔG alues in some s eps o he (supposed) ca aly ic cycle, p o iding a e y un a o - able o e all he modynamics. Indeed, no expe imen al e i- dence o eac ion be ween AB o DMAB and H 2 Owas ound, unde he applied expe imen al condi ions [i.e. absence o 11 B NMR signals ypical o bo ic acid- like hyd olysis p oduc s ( o be expec ed a ound δ B = +18.0 ppm)]. F om all hese indings, his hypo hesis was disca ded. 27 [( Bu PCP)Pd(H 2 O)]BF 4 : B. F. M. Kimmich, W. J. Ma shall, P. J. Fagan, E. Haup man and R. M. Bullock, Ino g. Chim. Ac a, 2002, 330,52–58. ( Bu PCP)Pd(NO 2 )/( Bu PCP)Pd(NO 3 ): R. Johansson, L. Oh s om and O. F. Wend , C ys . G ow h Des., 2007, 7, 1974–1979. ( Bu PCP)Pd(CH 3 ): R. Johansson, M. Ja enma k and O. F. Wend , O ganome allics, 2005, 24, 4500–4502. ( Bu PCP)Pd(OCOCF 3 ): M. T. Johnson, M. Ce ina, K. Rissansen and O. F. Wend , Ac a C ys allog ., Sec . E: S uc . Rep. Online, 2010, 66, m675. 28 M. C. Denney, N. A. Smy he, K. L. Ce o, R. A. Kemp and K. I. Goldbe g, J. Am. Chem. Soc., 2006, 128, 2508–2509. 29 (a) W. J. Shaw, J. C. Linehan, N. K. Szymczak, D. J. Heldeb an , C. Yonke , D. M. Camaioni, R. T. Bake and T. Au ey, Angew. Chem., In . Ed., 2008, 47, 7493–7496; (b) J. S. Wang and R. A. Geanangel, Ino g. Chim. Ac a, 1988, 148, 185–190. 30 K. W. Böddeke , S. G. Sho e and R. K. Bun ing, J. Am. Chem. Soc., 1966, 88, 4396–4401. 31 (a) H. J. Cowley, M. S. Hol , R. L. Melen, J. M. Rawson and D. S. W igh , Chem. Commun., 2011, 47, 2682–2684; (b) T. Bewe ies, J. Thomas, M. Klahn, A. Schulz, D. Helle and U. Rosen hal, ChemCa Chem, 2011, 3, 1865–1868. 32 C. A. Jaska, K. Temple, A. J. Lough and I. Manne s, Chem. Commun., 2011, 962–963. 33 An assessmen o he compu a ional e o made when passing om he eal o he model sys em was made on he gas phase ΔE alues o he exchange eac ion [( R PCP) Pd(H 2 O)] + +AB→[( R PCP)Pd(AB)] + +H 2 O. The esul s we e he ollowing: R = Bu, ΔE=−8.3 kcal mol −1 ;R=H,ΔE= −11.5 kcal mol −1 . Thus, he same o de o magni ude o he he modynamic eac ion pa ame e s is calcula ed when eplacing he ( eal) e -bu yl subs i uen s on phospho us wi h hyd ogen a oms. 34 This kind o luxional coo dina ion hap ici y ( ha ans- la es in o magne ically equi alen H a oms in he 1 H NMR spec a) has also been obse ed o ansi ion me al bo ohy- d ide complexes, such as: (a) (PPh 3 ) 2 Cu(BH 4 ): I. E. Golub, O. A. Filippo , E. I. Gu sul, N. V. Belko a, L. M. Eps ein, A. Rossin, M. Pe uzzini and E. S. Shubina, Ino g. Chem., 2012, 51, 6486–6497; (b)( Bu PCP)Ni(BH 4 ): A. Rossin, M. Pe uzzini and F. Zanobini, Dal on T ans., 2011, 40, 4447–4452. 35 X. Chen, J.-C. Zhao and S. G. Sho e, J. Am. Chem. Soc., 2010, 132, 10658–10659. 36 B. F. M. Kimmich and R. M. Bullock, O ganome allics, 2002, 21, 1504–1507. Dal on T ansac ions Pape This jou nal is © The Royal Socie y o Chemis y 2013 Dal on T ans., 2013, 42, 3533–3541 | 3541 Published on 04 Decembe 2012. 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