Pd Complexes with O-donor ligands
I am curious why people use Pd Complexes bonded with EWG O-donor ligand AcO-, CF3CO2- or OxSO-. Less people use Pd complexes with EDG alkoxy ligands?? It seems to me d(pi)-p(pi) repulsion may destabilze the Pd complexes. People use EWG O-ligand just for reduce the extend of d(pi)-p(pi) repulsion? Or other reasons? Stability of the Pd complexes? Welcome for your commnets or EXP! :lol:lol:lol the main reason from my point of view is thatthe EWGs connected to the phosphine ligand decrease the electronic density of the Pd center. Sorry. Thx for your comment! I am concerned about M-OR complexes such as acetates. Not Pd-P(OR)3.
People seldom use e.g. Pd(OMe)2 as a catalyst, but quite many use Pd(OAc)2 or other EWG oxygen donor ligands. I guess too unstable of Pd(OMe)2???? In essence, I believe you guys are agreeing with each other.
In order to increase the stability of low-vanlent (electron rich) Pd complexes, The Pd center like Pi-acid ligands such as CO, phosphine, which may accept the electron density fom the Pd center. One of the examples is the zero valent Pd(0) cluster stabilized by carbonyl ligands. Compared with EDG-O ligand, EWG group in EWG-O ligand will benefit EWG-O ligand accept electron density from the metal center. Thus, EWG-O ligand complexed Pd is more popular. However, for CO or PR3 ligand, Pd(0) complexes is prefered. In my question, Pd(II) complex! In my opinion, niuniu123's comment is reasonable. For Pd(0), OR has almost no change to form Pd-O bond due to the high electron-rich metal centre. Pd(II) provids a possibility althgh the case is very rare. I only found one example in which the Pd-O bond is formed when O donates 2e to Pd to reduce the d-p repulsion(For more details, see the attachments). For pure OMe (O does not donate 2e), It may be impossible. Thx niuniu123 & M=C. :D:D
I am concerning only Pd(II) complexes with O-donor ligands. I found many people use weakly-coordinated O-ligand e.g. CH3CO2-, but seldom see Pd(OMe)2. I would like to know why?
The paper M=C gave is the Pd complex with one EWG O-donoe ligand and one EDG O-ligand(OMe). Definitely, it can occur beta-H elimination for Pd-(OEt)2 complexes.
To M=C, "Pd-O bond is formed when O donates 2e to Pd to reduce the d-p repulsion" should be wrong!
when O donates 2 [color=red]pi[/color] e to Pd to [color=red]increase[/color] the d-p repulsion.
That's why I guess people use weak-bound & EWG O-ligand (OAc, OTf) to stabilze Pd(OR)2 complexes?!?!
[quote]原帖由 [i]Metalcarbene[/i] 于 2006-7-9 16:37 发表
In my opinion, niuniu123's comment is reasonable. For Pd(0), OR has almost no change to form Pd-O bond due to the high electron-rich metal centre. Pd(II) provids a possibility althgh the case is very rare. I only found one example in which the Pd-O bond is formed when O donates 2e to Pd to reduce the d-p repulsion(For more details, see the attachments). For pure OMe (O does not donate 2e), It may be impossible. [/quote]
[[i] 本帖最后由 choscar 于 2006-7-9 17:09 编辑 [/i]] The favorate ligands for these low-valent Pd (0, I, or II) complexes are those that act as the electron acceptors while they donate electrons to the metal center. Since the Pd (II) center are electron rich, the high electron density in metal has the donation of electrons from O-ligands unfavorable. However, this may happen only if the electron density in center Pd metal decrease through back-donation of it to the empty orbital of the surrounding ligands. For Pd(0) complex, its metal center's electrons are so rich that very strong pi-acid ligand such as CO is necessary for stabilizing it. For Pt(II), it is still electron rich, although it is not so electron rich as Pd(0) center. Thus, Pd(II) complex also favors acid ligand that can decrease its metal center's electron density although the acid ligand is not necessary to be so strong like CO (EWG-O ligand is OK). As for EDG-O ligand, it cannot decrease the electron density in metal center by accepting electron density from Pd(II), which is why it is not favored by comparison to EWG-O ligand.
In short, I think my comment is fit for all the Low-valent Pd complexes. In other words, it is fit for all the electron rich Pd complexes including Pd(0), Pd(I) and Pd(II), etcs. :)
[quote]原帖由 [i]choscar[/i] 于 2006-7-9 17:07 发表
Thx niuniu123 & M=C. :D:D
I am concerning only Pd(II) complexes with O-donor ligands. I found many people use weakly-coordinated O-ligand e.g. CH3CO2-, but seldom see Pd(OMe)2. I would like t ... [/quote]
[[i] 本帖最后由 niuniu123 于 2006-7-9 22:34 编辑 [/i]] Thx a lot for MC's ref and structure. I just browse them. For this case, MeO-Pd is survived cause of the presence of very strong CF3-containing EWG-O ligands that benefit the decrease of electron density in Pd(II). Thx! I guess and understand what you mean, not exactly.
I ask this Q, because we can discuss some topic seriously. Also I saw 3 recent JACS commun. papers using late transition metals (Ir Pd, or Pt) containing OAc- lligand for C-H activation.
(1) Davies, J. Am. Chem. Soc.; 2006; 128(13); 4210-4211. (2) Fagnou J. Am. Chem. Soc.; 2006; 128(27); 8754-8756. (3) Periana et al J. Am. Chem. Soc.; 2006.128(27); 7404-7405.
They all use computaional method and propose a 6mr metathesis TS. I also study non-d0 complexes containing alkoxy ligand for activation of small molecules. I expect that using alkoxy ligands for very late transition metals should be more reactive (M-O is weakened by d(pi)-p(pi) repulsion. Thus, I don't why they do not use?? Instability of the Pd(OMe)2 or 6mr metathesis pathway for aetate ligand is more superior to 4mr metathesis for alkoxy ligand??? In my previous post, I attached the cif file only to show that there is a possibility to crystallize the Pd(II)-OMe complex although it is a dimer and contains very strong EWG-O ligands. In my opinion, it is just the two factors (dimer+EWG) that make it stable to get the crystal structure.
For the "reduce", maybe more explanation is neccesary.
OMe ligand needs to donate 2e to one of the metal center due to the square planar structure (16e). Therefore the electron density on O is decreased, which reduced the d-p repulsion [color=red]between O and another Pd[/color]. In other words, the cooperative effect between the two metal center made a contribution to its stability. otherwise what is the role of the two metal center? or why can I not find the crystal structure of monomer? I am not sure whether my understanding is right or not.
ps: sorry for the typo in my previous post. In the first row, "change"->"chance". Choscar, pl read this JACS paper that is related to your interest on this matter.
[url]http://pubs.acs.org/cgi-bin/asap.cgi/jacsat/asap/pdf/ja060915z.pdf[/url]
The authors indeed mention the stability of the title compound is affected by the ratio of Pd with (OAc). 1:4 is better? (pl check). This paper shows EDG-O ligand may coordinate to Pd in the presence of EWG-O ligand. Sounds like my comments make sense.
For MC, the reason why to form a dimer may be attributed to its inclination to achieve square planar geometry around the Pd metallic center and 16e structure. If it is monomer, it will not possess this favorate square planar geometry and 16e configuration (thus the monomer is not stable). This just likes the case of trimethyl aluminum, AlMe3. Due to Al's high electropositivity, the three coordinate AlMe3 monomer is seriously electron deficient, which is not stable. Thus, it is inclined to dimerize. As a result of this dimerization, each Al center is four coordinate and has a stable octet valence electron configuration. The two bridging Al-Me-Al hold two 3c-2e bonds.
[[i] 本帖最后由 niuniu123 于 2006-7-11 10:52 编辑 [/i]] I know the paper you gave me - a very recent theoretical work from Goddard's group. It is based on Stoltz (also at Berkerly).
[quote]原帖由 [i]niuniu123[/i] 于 2006-7-11 10:36 发表
Choscar, pl read this JACS paper that is related to your interest on this matter.
[url]http://pubs.acs.org/cgi-bin/asap.cgi/jacsat/asap/pdf/ja060915z.pdf[/url]
The authors indeed mention the stability of ... [/quote]
[[i] 本帖最后由 choscar 于 2006-7-11 19:03 编辑 [/i]]
nonsense
I find your guys' comments most nonsensical. Most people try to understand the problem from the view point of stability of the organometallic complex. Remember, Most of the time Palladium is used as a catalyst and typiclally 5-10%. YOu don't want to introduce alkoxylligand on palladium due to a) for tansmetallation, carboxylate is much easy to displace b) even if acetate ion reductive eliminate with the other group on Pd(II), it is much easy for Pd (0) to oxidative insert to bring back the original species considering the microscopic reversalbility of the reaction. For Alkoxide, it is a different matter. Once it reductive eliminate, it is hard to come back. You don't want see 5-10% ALKOXIDE in your product, do you?[color=Red]Niuniu123's comments[/color]:[size=3] [b][color=Blue]大家在此讨论问题，目的是交流学习。参与者并非该领域专家，观点可能根本不对。这又有何妨。望大家都展开心门，积极讨论。 不要顾忌别人怎么看。若您是本方向大拿，尽可展示你的才学，对于不对的观点给予指出和指正。这才是我们在这讨论的目的。不要NONSENSE，NONSENSICAL挂在嘴上。太NAIVE(江总语)。如果你觉的这样能被别人高看一眼，我也不反对。不过请发贴前想清楚你发贴的目的何在？是为了帮助别人，还是为了教训一下吧友以显示你的才学。这身边的牛人海了去了，多你一个也不多，少你一个也不不少，对吧。:$ :$ :$[/color][/b][/size]
[[i] 本帖最后由 niuniu123 于 2006-11-8 10:33 编辑 [/i]] i think those EWG-O anions are easy to be kicked out during reaction by other ligands such as triphenylphosphine or NHC carbene ligands. And also, if you put a too good ligand on Pd, then reactant can't bind to Pd to happen reaction. i am not familar with other transition metal but for Pd generally even you put Pd(II) in the reaction, it will be reduced to Pd(0) first which is the actual active catalyst. I asked Dr. Fu (MIT) about this queston. He told me the base in the reaction or some electrophile could be reductant to reduce Pd(II) to P(0). 硫脲类化合物,特别是酰基硫脲类化合物的Pd配合物研究的比较多,可以应用于贵金属的萃取和分离 About title Q, personally, I think niuniu give a resonable answer. About reactivity of Pd(acac) complexes, it can be another topic. Really, many guys above shared very valueable points.
Herein, I have a question relative to back-donation bonding. As we know, in organometallics, CO, CN etc are good pi acids. In principle, as pi acids, the Lumo of ligands should be empty to accept electron density from metal d orbitals. My Q is there is any literature or paper about Home and Lumo of general ligands so that we can judge ligands are pi acids or not?!
Btw, maybe it is not a good question. Anyway, your answer are welcome! 第一次来，多多关照！ choscar提了一个很好的问题但在一定程度上不是很清楚引起了大家的一些误解. 我觉得首先要明确的是“why people use Pd Complexes bonded with EWG O-donor ligand AcO-, CF3CO2- or OxSO-. Less people use Pd complexes with EDG alkoxy ligands”中的Pd complex 指的是什么，是可以买来的钯试剂还是在反应催化循环中的活性钯物种。
如果是第一种，人们用很多 Pd(OAc)2, PdCl2, Pd(acac)2 却很少Pd(OMe)2 之类，我觉得是因为前者是commercial available 后者不是。这个的原因，我同意choscar的看法，是因为前者稳定而后者不稳定。解释Pd络合物的稳定性，大家就会发现前面几个催化剂Pd可以形成四配位的Pd，如果Pd(OMe)2存在的话很难形成类似的结够。
如果是指的是催化循环中的活性催化剂，则有一个关键的问题，这个循环中的活性中心是Pd(0)还是Pd(II)。 像通常的偶联反应实际是由零价钯通过氧化加成开始的，但在循环开始之前会有一个Pd(0)生成的过程。 本来活性物种是通过得到的零价钯，那么什么催化剂前体Pd(OAc)2, PdCl2, Pd(acac)2都应该是不重要的，但很多例子中不同的催化剂前体效率会差很多，这可能实在生成零价钯的过程中由于配体电性离去性等不同产生的差别，估计也很难解释。
另外一种情况是反应循环通过二价钯启动，比如一些C-H键活化的例子，是通过芳基对Pd催化剂的亲和取代启动了，这样Pd的配体的电性就直接影响了反应的启动。无论是只有二价钯的循环还是Pd二价四价的循环都存在这样的问题。
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