By Edward Taylor

FRANKFURT, Feb 16 (Reuters) - Deutsche Boerse's DB1Gn.DE chief executive said insider trading allegations against him would prove unfounded, pointing out that he did not determine the timing of his share purchases ahead of the announcement of merger plans with the London Stock Exchange LSE.L .

Deutsche Boerse's supervisory board created a unique share purchase plan tailored for CEO Carsten Kengeter which allowed him to invest 4.5 million euros ($4.8 million) in Deutsche Boerse shares at 75 euros apiece, while Boerse granted him a further 4.5 million euros in shares, he told a news conference on Thursday.

German police and prosecutors this month searched Kengeter's office and apartment as they investigate whether secret merger talks with LSE were already under way at the time the package was granted.

"When I purchased the shares using my own funds, I did not do so at a time of my own choosing," Kengeter said on Thursday.

"I did so between 1 and 21 December 2015 within a time-frame fixed by the supervisory board," he said, adding that the shares were subject to a holding period until the end of 2019.

He declined to say whether it was already clear in December 2015 that a merger with LSE would be attempted, saying he could not comment on the ongoing investigation.

"Insider trading is against my innermost conviction," he said, adding that he and Deutsche Boerse were fully cooperating with the public prosecutor.

Separately, Kengeter said a failure to complete the merger with LSE to create Europe's biggest stock market would weaken Germany's main financial centre, Frankfurt.

"Any concerns that Frankfurt as a financial centre might be disadvantaged by the proposed business combination fail to recognise one thing: the biggest risk to Frankfurt... is doing nothing," he said.

Deutsche Boerse and LSE are proposing to locate the company headquarters in London, despite calls by regulators in the German state of Hesse to transfer more responsibility to Germany. The Brexit vote has amplified those demands.

Kengeter said that Deutsche Boerse would create an additional 300 jobs in Frankfurt, adding that he was engaged in constructive talks with policymakers in Hesse, Deutsche Boerse's home state, and with antitrust regulators in Brussels.

Regardless of where the combined holding company will be headquartered, the regulatory laws ensure that Frankfurt retains key responsibilities over the Frankfurt Stock Exchange and Eurex Deutschland, Kengeter said.

"The two exchanges as well as post trading and the market data business will remain in Frankfurt, even after the merger," Kengeter said. ($1 = 0.9407 euros)

(Editing by Maria Sheahan/Keith Weir) ((Edward.Taylor@thomsonreuters.com; +49 69 7565 1187))