* Washington lifted sanctions on Deripaska firms
* Trustees supposed to stop him controlling the businesses
* One trustee is a law firm that acted for Deripaska companies
* Democratic senator sees potential conflict of interest (Updates throughout with new comments)
By Polina Ivanova, Polina Devitt and Nathan Layne
MOSCOW/NEW YORK, Jan 30 (Reuters) - A law firm appointed to act as a buffer between Russian metals magnate Oleg Deripaska and one of his businesses, under a deal that allowed Washington to lift sanctions on the firm, has worked closely with Deripaska for years.
Asked about the law firm, the U.S. Treasury Department said it would vigorously monitor arrangements to ensure Deripaska cannot influence board members of the businesses. A representative for Deripaska said there was no scope for the businessman to influence how the law firm acted.
But a senior U.S. senator said the connection between Deripaska and the law firm further bolstered the argument, being made by U.S. President Donald Trump's Democratic opponents, that the sanctions should not have been lifted.
The U.S. Treasury Department agreed to lift sanctions on three businesses controlled by Deripaska on condition that the tycoon - who will still be under sanctions himself - reduces his ownership in the businesses and severs his control.
As part of the deal, some of Deripaska's voting rights as a shareholder in his main company, En+, were handed over to a "voting trust obligated to vote in the same manner as the majority of shares held by shareholders other than Deripaska," according to the Treasury Department.
On Monday, announcing names of this and other trustees, En+ said that one of the trustees exercising the voting rights was called "Ogier Global Nominee (Jersey) Limited."
A source close to En+, when asked about the nature of the trustee, said Ogier Global was "a very respectable Jersey law firm".
Jersey-based Ogier Global has handled multiple deals involving Deripaska's firms.
Ogier's Cayman office represented Deripaska in a 2014 case against equity fund Pericles Emerging Market Partners, court documents showed. Pericles was run by Paul Manafort, who was briefly campaign chairman for Donald Trump during the 2016 presidential campaign.
Ogier also acted on behalf of aluminium giant Rusal , which is controlled by En+, in 2017 when it placed the first "Panda Bond" offering by a Russian company, the name given to overseas debt sold in the Chinese market, Ogier said in a statement at the time.
"Rusal is a long-standing client and we are particularly pleased to have worked with them again," a partner in Ogier's Jersey office was cited by the law firm as saying at the time.
One of its Jersey lawyers "has worked on several headline deals including the $1.5 billion IPO of En+ Group," Ogier's website said.
Like Ogier, Rusal is registered in Jersey. As of Jan. 28, its registered address was the same as that of the Ogier law firm: Ogier House, 44 Esplanade, St. Helier, Jersey.
According to the Jersey Financial Services Registry, Ogier Global Nominee (Jersey) Limited is at that address, as is Ogier Global, and Ogier Global is listed as providing an office for Ogier Global Nominee.
Deripaska's representative told Reuters the deal left the businessman with no opportunity to influence Ogier and that the trustees have been vetted by the Treasury Department's Office of Foreign Assets Control (OFAC), which administers U.S. sanctions.
Ogier has, in any case, no discretion on how to exercise the voting rights because it must vote in line with the majority of shares other than those held by Deripaska, the representative said.
En+ declined to comment. Ogier did not immediately reply to a request for comment.
A Treasury spokesperson said OFAC has and will continue to ensure that the companies covered by the deal take the necessary steps to ensure that Deripaska doesn't have influence or control over board members.
"As we've made clear from the beginning, OFAC will vigorously monitor and ensure compliance with all the requirements" of the deal reached between OFAC and the Deripaska businesses, the spokesperson said.
U.S. congressional Democrats have kept up the pressure on the Trump administration over the decision to lift the sanctions, this week demanding additional briefings and floating the idea of legislation to make sure Rusal and En+ are in compliance.
Senator Mark Warner, the top Democrat on the Senate Intelligence Committee, has been among the most vocal critics of the deal.
"My concern all along was that Deripaska would, directly or indirectly, still retain control over his companies and benefit personally from this deal," Warner said in response to a question from Reuters about Ogier's role as trustee.
"Potential conflicts of interest in the new governance structure are another data point demonstrating that the Senate should have overruled the Trump administration and kept the sanctions in place when we had the opportunity," he said. (Additional reporting by Clara Denina in LONDON Editing by Giles Elgood)
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